胰岛素抵抗，肝脏脂肪沉积和糖异生的关联

代谢相关性脂肪性肝病(metabolic associated fatty liver disease, MAFLD)并非是一种静止的病变，会逐渐引起炎症细胞浸润、坏死、纤维化，甚至发展成为肝癌。胎球蛋白作为肝细胞因子，在人类代谢疾病、炎症等方面都具有重要作用。糖脂代谢异常可导致代谢相关性脂肪性肝病、糖尿病等代谢紊乱性疾病。近年有大量关于胎球蛋白A、胎球蛋白B、糖脂代谢与代谢相关性脂肪性肝病的研究。这篇综述将总结目前的研究进展，以便更好地了解胎球蛋白与糖脂代谢对代谢相关性脂肪性肝病的影响，为诊断、防治代谢相关性脂肪性肝病提供思路。

目前我国高血压、高血糖、肥胖、脂代谢异常等代谢性疾病发病率随社会发展持续上升，显著增加心血管疾病、2型糖尿病、代谢相关脂肪性疾病、代谢综合征等慢性非传染性疾病发病风险。胰岛素抵抗(insulin resistance, IR)作为核心病理机制，其早期识别对疾病防控至关重要。甘油三酯–葡萄糖指数(triglyceride-glucose index, TyG index)通过空腹甘油三酯与血糖乘积的对数值计算，作为新型非侵入性生物标志物，在评估胰岛素抵抗及预测代谢性疾病风险中展现独特优势。本综述将对TyG指数与胰岛素抵抗的作用机制作基本概括，重点阐述其在代谢性疾病诊断、风险预测及临床管理中的应用价值，以期为疾病的预防和治疗及临床综合管理提供一种思路和研究方向。

全球代谢相关脂肪性肝病(MASLD)的患病率逐年增高，且与2型糖尿病(T2DM)发病关系密切。流行病学证据表明，MASLD患者发生T2DM的风险是健康人的2倍以上，MASLD是T2DM的驱动因素，可作为T2DM的“预警窗口期”。本文基于“见肝之病，知肝传脾，当先实脾”理论，结合现代医学研究进展，从肝胰循环、肝与线粒体氧化应激、肝与肠道菌群和肝与胆汁酸代谢四个层面，论述MASLD继发T2DM的病机及发病机制，从“脾”论治以发挥中医“治未病”的优势，对MASLD的治疗及T2DM的预防具有重要意义。

代谢功能障碍相关脂肪性肝病(MASLD)是一种与多种代谢异常密切相关的慢性肝脏疾病，其核心病理特征为脂肪在肝细胞内异常堆积，且排除其他已知致病因素，如酒精性肝病、病毒性肝炎及自身免疫性疾病等。MASLD与代谢综合征和2型糖尿病之间相互影响，共同促进动脉粥样硬化性心脏病、慢性肾脏病及肝细胞癌等恶性肿瘤等多种并发症的发生。这种复杂的代谢网络使得MASLD成为日益严峻的公共卫生问题。2024年，瑞美替罗成为首个获美国食品药品监督管理局批准用于治疗MASH的药物，然而针对MASLD的早期阶段仍缺乏获批药物。在此背景下，胰高血糖素样肽-1 (GLP-1)受体激动剂因其独特的多靶点调控作用，在MASLD治疗中展现出显著的临床应用潜力。本研究旨在系统分析GLP-1受体激动剂在MASLD治疗中的作用机制，并结合循证医学证据评估其疗效与安全性，为临床实践提供理论依据。

非酒精性脂肪性肝病(Non-Alcoholic Fatty Liver Disease, NAFLD)是世界范围内最常见的慢性肝病，其全球患病率估计高达35%，已成为一个重大的公共卫生问题。NAFLD包括单纯性脂肪肝，脂肪性肝炎(NASH)，以及潜在的肝硬化和肝癌。NAFLD与胰岛素抵抗、肥胖、肠道微生物失调和遗传危险因素密切相关。肥胖的流行和2型糖尿病患者的增多极大地促进了NAFLD负担的增加。NAFLD的发病机制、确定治疗靶点和推进药物开发方面取得了稳步进展，但仍存在重大挑战，尚未有药物被批准用于该疾病。一旦诊断确定，减肥、饮食调整和治疗潜在代谢综合征仍然是治疗的主要内容。指南建议在特定患者中使用吡格列酮和维生素E。本文综述了NAFLD和NASH的发病机制及危险因素，为治疗策略的发展和有用的诊断工具提供见解。

司美格鲁肽作为新兴的胰高血糖素样肽-1受体激动剂(GLP-1RA)，是该药物类别中最近批准的药物，也是目前唯一可用于皮下注射和口服制剂的GLP-1RA，主要用于2型糖尿病的治疗。其药物特性可在血糖控制、胰岛素抵抗改善、体重管理、肝脏疾病上体现出显著的效果。在心脑血管疾病上也可体现出一定的治疗效果，成为当前医疗行业的热点。然而有利必有弊，司美格鲁肽的过度依赖也会伴随一些疾病的风险，如延缓胃排空导致的胃肠疾病、胰腺炎、胆道疾病等风险。本文通过分析司美格鲁肽的安全性和危险性，介绍该药物在降血糖、减重、治疗心血管疾病等多方面的进展，为临床应用提供参考。

代谢功能障碍相关脂肪性肝病(MAFLD)已成为全球慢性肝病的主要病因，其发生发展与脂质代谢紊乱密切相关。本文系统综述了脂质代谢异常在MAFLD发病机制中的作用，重点阐述甘油三酯、游离脂肪酸、胆固醇及脂毒性介导肝脏损伤的分子机制，并总结了靶向脂质代谢通路的潜在治疗策略。通过整合现有研究进展，本文旨在为MAFLD的早期无创诊断及基于脂质代谢通路的治疗策略提供理论依据和新的研究视角。

甲状腺激素作为重要的代谢调节因子，通过促进脂质输出和氧化、调控脂肪从头合成、调节肝脏胰岛素敏感性以及抑制肝脏糖异生等多种途径，在维持能量代谢平衡、脂肪代谢稳态以及肝脏功能方面发挥着至关重要的作用。因此，甲状腺功能减退作为代谢相关性脂肪性肝病(MAFLD)的一个重要危险因素，两者的相关性一直被广为讨论，但针对甲状腺功能减退是否会导致MAFLD的发病风险增加目前尚无统一结论。本文将从病理生理学基础、流行病学证据以及治疗干预三个方面，系统综述甲状腺功能减退与MAFLD发病的相关性，并探讨甲状腺激素及其类似物对MAFLD的潜在治疗作用，以期为MAFLD的防治提供新的思路和理论依据。

目的：基于网络药理学探讨GLP-1RAs治疗非酒精性脂肪肝合并糖尿病的作用机制。方法：通过Pharmmapper、TargetNET、SEA和the Binding Database数据库查找筛选4种GLP-1RAs的作用靶点。通过GeneCards、DisGeNET以及OMIM查找并筛选NAFLD和T2DM的致病基因。使用DAVID数据库进行GO和KEGG富集分析，Cytoscape3.10.0绘制GLP-1RAs治疗NAFLD和T2DM的“药物–靶点–疾病的网络关系图”，利用MCODE插件进行聚类分析，Hubba插件筛选核心靶点，并绘制“核心靶点–通路的网络图”。结果：通过网络药理学筛选出GLP-1RAs治疗NAFLD合并T2DM的35个潜在靶点，筛选后获得13个核心靶点，它们分别是CASP3、ESR1、SIRT1、IGF1R、CCND1、GSK3B、AR、PRKACA、PARP1、NOS3、REN、MAPK8、ACE1。GO和KEGG富集分析结果表明，RAS系统相关通路和胰岛素抵抗相关通路是研究的关键通路。结论：GLP-1RAs有可能是通过激活RAS系统中ACE2/Ang(1-7)/Mas轴，对抗ACE/AngII/AT1R轴发挥其作用，以及根据血糖水平调节胰岛素的释放，增加胰岛素的生物合成和分泌等途径改善胰岛素抵抗对NAFLD和T2DM表现出有益疗效。

目的：通过网络药理学筛选茵陈–荷叶治疗非酒精性脂肪性肝病的有效成分和靶点。方法：应用TCMSP和SWISS数据库查找茵陈、荷叶的活性成分及其靶点，通过OMIM和GeneCards数据库检索非酒精性脂肪性肝病的相关靶点，并获取交集靶点。利用Cytoscape 3.10.3构建茵陈–荷叶与非酒精性脂肪性肝病之间的“药物–成分–疾病–靶点”网络图，筛选出关键靶点，并进行GO和KEGG富集分析。结果：得到茵陈–荷叶的有效活性成分共24个，交集靶点为183个。通过蛋白互作分析(PPI)，识别出核心靶点包括TP53、AKT1、JUN、HSP90AA1、STAT3、TNF、IL6、MAPK1、HSP90AB1、BCL2等。KEGG和GO分析结果显示，主要影响的生物过程包括耐受性诱导、T细胞耐受性诱导的调节以及肽类激素处理等10种类型；涉及的细胞成分包括轴丝动力蛋白复合体、质膜和细胞质区域等10种类型；主要的分子功能包括细胞因子活性、相同蛋白质结合等10种功能。研究表明，这些成分主要通过脂质代谢与动脉粥样硬化、内分泌抵抗等通路对非酒精性脂肪性肝病发挥作用。结论：茵陈–荷叶改善非酒精性脂肪性肝病的作用机制可能通过内分泌抵抗、脂质与动脉粥样硬化等多条通路实现，其特点在于具有多个靶点和多个成分。

目的：基于生物信息学探讨大黄–栀子药对治疗非酒精性脂肪肝的作用机制。方法：通过中药系统药理数据库(TCMSP)收集大黄和栀子的主要活性成分，并按照口服生物利用度(OB) ≥ 30%，类药性(DL) ≥ 0.18进行选择，通过中药系统药理数据库在线靶标预测平台数据库预测并且进行筛选大黄栀子两味药的相关化合物靶点基因，疾病靶点选用GeneCards数据库以及OMIM数据库进行筛选，获取NAFLD相关疾病的潜在作用靶点。通过构建化合物–疾病靶点进行去重之后，在韦恩图中进行筛选大黄和栀子治疗NAFLD关键靶点。通过String数据库构建蛋白质–蛋白质相互作用利用Cytoscape建立药物–活性成分–靶点–疾病的网络图，使用DAVID数据库将韦恩图中所筛选出来的交集靶点进行基因本体(GO)功能富集分析和基因组百科全书(KEGG)通路富集分析。结果：总共得到相关药效成分31个，药物与疾病的交集靶点69个，GO功能富集分析得到了627条信息，KEGG通路分析总共发现144条通路，以疾病相关程度和P值进行筛选前十的数据，结果发现主要通过PI3K-Akt信号通路(PI3K-Akt signaling pathway)、化学致癌(Chemical carcinogemesis)和乙型肝炎(Hepatitis B)等通路发挥作用，得出“大黄–栀子”可能通过AKT1、TNF、IL6、IL1B等重要靶点，主要参与细胞因子介导的信号通路、RNA聚合酶Ⅱ启动转录的阳性调节等生物学过程，从而调节NAFLD等通路达到治疗作用。结论：基于生物信息学，初步探讨出“大黄–栀子”治疗非酒精性脂肪肝的作用机制，为接下来实验论证给予一定的基础。

随着全球肥胖发病率的上升，非酒精性脂肪性肝病(Nonalcoholic fatty liver disease, NAFLD)已经成为儿童及青少年慢性肝病的主要表现。NAFLD代表一个广泛的疾病谱，包括非酒精性单纯性脂肪肝(nonalcoholic fatty liver, NAFL)、非酒精性脂肪性肝炎(non-alcoholic steatohepatitis, NASH)、肝硬化及肝癌。虽然目前普遍认为NAFLD与肥胖以及胰岛素抵抗密切相关，但其具体的发病机制尚未明确。在本文中，我们将对儿童青少年非酒精性脂肪性肝病的发病机制进行综述。

目的：系统评价中西医结合治疗2型糖尿病合并非酒精性脂肪性肝病(NAFLD)的临床疗效。方法：对PubMed、Cochrane Library、Web of science、知网、维普、万方数据库进行检索，筛选出相关的随机对照试验，用RevMan5.4软件进行Meta分析。结局指标包括丙氨酸氨基转移酶(ALT)、天冬氨酸氨基转移酶(AST)、空腹血糖(FBG)、餐后2小时血糖(2hPG)、稳态模型评估胰岛素抵抗指数(HOMA-IR)、甘油三酯(TG)、总胆固醇(TC)、低密度脂蛋白胆固醇(LDL-C)、高密度脂蛋白胆固醇(HDL-C)。结果：涉及2494名患者的35个随机对照试验被纳入meta分析。meta分析表明，中西医结合治疗2型糖尿病合并非酒精性脂肪肝病的干预措施对结局指标有显著影响。ALT加权均数差((WMD) = −6.30，95%置信区间(confidence interval, CI) [−7.51, −5.09]，p < 0.00001)、AST (WMD = −5.26, 95% CI [−6.21, −4.32], p < 0.00001)、FBG (WMD = −0.68, 95% CI [−0.83, −0.53], p < 0.00001)、2hPG (WMD = −1.16, 95% CI [−1.44, −0.88], p < 0.00001)、HOMA-IR (WMD = −0.99, 95% CI [−1.25, −0.74], p < 0.00001)、TG (WMD = −0.56, 95% CI [−0.69, −0.43], p < 0.00001)、TC (WMD = −0.65, 95% CI [−0.84, −0.46], p < 0.00001)、LDL-C (WMD = −0.44, 95% CI [−0.56, −0.32], p < 0.00001)、HDL-C (WMD = 0.19, 95% CI [0.09, 0.29], p < 0.00001)。结论：当前证据显示，中药联合西药对T2DM合并NAFLD患者在改善糖脂代谢、肝功能和胰岛素抵抗方面更有利。

Short term high fat feeding in rats results specifically in hepatic fat accumulation and provides a model of non-alcoholic fatty liver disease in which to study the mechanism of hepatic insulin resistance. Short term fat feeding (FF) caused a approximately 3-fold increase in liver triglyceride and total fatty acyl-CoA content without any significant increase in visceral or skeletal muscle fat content. Suppression of endogenous glucose production (EGP) by insulin was diminished in the FF group, despite normal basal EGP and insulin-stimulated peripheral glucose disposal. Hepatic insulin resistance could be attributed to impaired insulin-stimulated IRS-1 and IRS-2 tyrosine phosphorylation. These changes were associated with activation of PKC-epsilon and JNK1. Ultimately, hepatic fat accumulation decreased insulin activation of glycogen synthase and increased gluconeogenesis. Treatment of the FF group with low dose 2,4-dinitrophenol to increase energy expenditure abrogated the development of fatty liver, hepatic insulin resistance, activation of PKC-epsilon and JNK1, and defects in insulin signaling. In conclusion, these data support the hypothesis hepatic steatosis leads to hepatic insulin resistance by stimulating gluconeogenesis and activating PKC-epsilon and JNK1, which may interfere with tyrosine phosphorylation of IRS-1 and IRS-2 and impair the ability of insulin to activate glycogen synthase.

Defective liver gluconeogenesis is the main mechanism leading to fasting hyperglycemia in type 2 diabetes, and, in concert with steatosis, it is the hallmark of hepatic insulin resistance. Experimental obesity results, at least in part, from hypothalamic inflammation, which leads to leptin resistance and defective regulation of energy homeostasis. Pharmacological or genetic disruption of hypothalamic inflammation restores leptin sensitivity and reduces adiposity. Here, we evaluate the effect of a hypothalamic anti-inflammatory approach to regulating hepatic responsiveness to insulin. Obese rodents were treated by intracerebroventricular injections, with immunoneutralizing antibodies against Toll-like receptor (TLR)4 or tumor necrosis factor (TNF)α, and insulin signal transduction, hepatic steatosis, and gluconeogenesis were evaluated. The inhibition of either TLR4 or TNFα reduced hypothalamic inflammation, which was accompanied by the reduction of hypothalamic resistance to leptin and improved insulin signal transduction in the liver. This was accompanied by reduced liver steatosis and reduced hepatic expression of markers of steatosis. Furthermore, the inhibition of hypothalamic inflammation restored defective liver glucose production. All these beneficial effects were abrogated by vagotomy. Thus, the inhibition of hypothalamic inflammation in obesity results in improved hepatic insulin signal transduction, leading to reduced steatosis and reduced gluconeogenesis. All these effects are mediated by parasympathetic signals delivered by the vagus nerve.

Metabolic dyslipidemia is characterized by high circulating triglyceride (TG) and low HDL cholesterol levels and is frequently accompanied by hepatic steatosis. Increased hepatic lipogenesis contributes to both of these problems. Because insulin fails to suppress gluconeogenesis but continues to stimulate lipogenesis in both obese and lipodystrophic insulin-resistant mice, it has been proposed that a selective postreceptor defect in hepatic insulin action is central to the pathogenesis of fatty liver and hypertriglyceridemia in these mice. Here we show that humans with generalized insulin resistance caused by either mutations in the insulin receptor gene or inhibitory antibodies specific for the insulin receptor uniformly exhibited low serum TG and normal HDL cholesterol levels. This was due at least in part to surprisingly low rates of de novo lipogenesis and was associated with low liver fat content and the production of TG-depleted VLDL cholesterol particles. In contrast, humans with a selective postreceptor defect in AKT2 manifest increased lipogenesis, elevated liver fat content, TG-enriched VLDL, hypertriglyceridemia, and low HDL cholesterol levels. People with lipodystrophy, a disorder characterized by particularly severe insulin resistance and dyslipidemia, demonstrated similar abnormalities. Collectively these data from humans with molecularly characterized forms of insulin resistance suggest that partial postreceptor hepatic insulin resistance is a key element in the development of metabolic dyslipidemia and hepatic steatosis.

No abstract

Endoplasmic reticulum (ER) stress has been implicated in the pathophysiology of human type 2 diabetes (T2DM). Although SIRT1 has a therapeutic effect on metabolic deterioration in T2DM, the precise mechanisms by which SIRT1 improves insulin resistance remain unclear. Here, we demonstrate that adenovirus-mediated overexpression of SIRT1 in the liver of diet-induced insulin-resistant low-density lipoprotein receptor-deficient mice and of genetically obese ob/ob mice attenuates hepatic steatosis and ameliorates systemic insulin resistance. These beneficial effects were associated with decreased mammalian target of rapamycin complex 1 (mTORC1) activity, inhibited the unfolded protein response (UPR), and enhanced insulin receptor signaling in the liver, leading to decreased hepatic gluconeogenesis and improved glucose tolerance. The tunicamycin-induced splicing of X-box binding protein-1 and expression of GRP78 and CHOP were reduced by resveratrol in cultured cells in a SIRT1-dependent manner. Conversely, SIRT1-deficient mouse embryonic fibroblasts challenged with tunicamycin exhibited markedly increased mTORC1 activity and impaired ER homeostasi and insulin signaling. These effects were abolished by mTORC1 inhibition by rapamycin in human HepG2 cells. These studies indicate that SIRT1 serves as a negative regulator of UPR signaling in T2DM and that SIRT1 attenuates hepatic steatosis, ameliorates insulin resistance, and restores glucose homeostasis, largely through the inhibition of mTORC1 and ER stress.

No abstract

Lately, the world has faced tremendous progress in the understanding of non-alcoholic fatty liver disease (NAFLD) pathogenesis due to rising obesity rates. Peroxisome proliferator-activated receptors (PPARs) are transcription factors that modulate the expression of genes involved in lipid metabolism, energy homeostasis and inflammation, being altered in diet-induced obesity. Experimental evidences show that PPAR-alpha is the master regulator of hepatic beta-oxidation (mitochondrial and peroxisomal) and microsomal omega-oxidation, being markedly decreased by high-fat (HF) intake. PPAR-beta/delta is crucial to the regulation of forkhead box-containing protein O subfamily-1 expression and, hence, the modulation of enzymes that trigger hepatic gluconeogenesis. In addition, PPAR-beta/delta can activate hepatic stellate cells aiming to the hepatic recovery from chronic insult. On the contrary, PPAR-gamma upregulation by HF diets maximizes NAFLD through the induction of lipogenic factors, which are implicated in the fatty acid synthesis. Excessive dietary sugars also upregulate PPAR-gamma, triggering de novo lipogenesis and the consequent lipid droplets deposition within hepatocytes. Targeting PPARs to treat NAFLD seems a fruitful approach as PPAR-alpha agonist elicits expressive decrease in hepatic steatosis by increasing mitochondrial beta-oxidation, besides reduced lipogenesis. PPAR-beta/delta ameliorates hepatic insulin resistance by decreasing hepatic gluconeogenesis at postprandial stage. Total PPAR-gamma activation can exert noxious effects by stimulating hepatic lipogenesis. However, partial PPAR-gamma activation leads to benefits, mainly mediated by increased adiponectin expression and decreased insulin resistance. Further studies are necessary aiming at translational approaches useful to treat NAFLD in humans worldwide by targeting PPARs.

No abstract

Nonalcoholic fatty liver (NAFL) is a common liver disease, associated with insulin resistance. Betaine has been tested as a treatment for NAFL in animal models and in small clinical trials, with mixed results. The present study aims to determine whether betaine treatment would prevent or treat NAFL in mice and to understand how betaine reverses hepatic insulin resistance. Male mice were fed a moderate high-fat diet (mHF) containing 20% of calories from fat for 7 (mHF) or 8 (mHF8) mo without betaine, with betaine (mHFB), or with betaine for the last 6 wk (mHF8B). Control mice were fed standard chow containing 9% of calories from fat for 7 mo (SF) or 8 mo (SF8). HepG2 cells were made insulin resistant and then studied with or without betaine. mHF mice had higher body weight, fasting glucose, insulin, and triglycerides and greater hepatic fat than SF mice. Betaine reduced fasting glucose, insulin, triglycerides, and hepatic fat. In the mHF8B group, betaine treatment significantly improved insulin resistance and hepatic steatosis. Hepatic betaine content significantly decreased in mHF and increased significantly in mHFB. Betaine treatment reversed the inhibition of hepatic insulin signaling in mHF and in insulin-resistant HepG2 cells, including normalization of insulin receptor substrate 1 (IRS1) phosphorylation and of downstream signaling pathways for gluconeogenesis and glycogen synthesis. Betaine treatment prevents and treats fatty liver in a moderate high-dietary-fat model of NAFL in mice. Betaine also reverses hepatic insulin resistance in part by increasing the activation of IRS1, with resultant improvement in downstream signaling pathways.

Mammalian genomes encode a huge number of long noncoding RNAs (lncRNAs) with unknown functions. This study determined the role and mechanism of a new lncRNA, lncRNA suppressor of hepatic gluconeogenesis and lipogenesis (lncSHGL), in regulating hepatic glucose/lipid metabolism. In the livers of obese mice and patients with nonalcoholic fatty liver disease, the expression levels of mouse lncSHGL and its human homologous lncRNA B4GALT1-AS1 were reduced. Hepatic lncSHGL restoration improved hyperglycemia, insulin resistance, and steatosis in obese diabetic mice, whereas hepatic lncSHGL inhibition promoted fasting hyperglycemia and lipid deposition in normal mice. lncSHGL overexpression increased Akt phosphorylation and repressed gluconeogenic and lipogenic gene expression in obese mouse livers, whereas lncSHGL inhibition exerted the opposite effects in normal mouse livers. Mechanistically, lncSHGL recruited heterogeneous nuclear ribonucleoprotein A1 (hnRNPA1) to enhance the translation efficiency of CALM mRNAs to increase calmodulin (CaM) protein level without affecting their transcription, leading to the activation of the phosphatidyl inositol 3-kinase (PI3K)/Akt pathway and repression of the mTOR/SREBP-1C pathway independent of insulin and calcium in hepatocytes. Hepatic hnRNPA1 overexpression also activated the CaM/Akt pathway and repressed the mTOR/SREBP-1C pathway to ameliorate hyperglycemia and steatosis in obese mice. In conclusion, lncSHGL is a novel insulin-independent suppressor of hepatic gluconeogenesis and lipogenesis. Activating the lncSHGL/hnRNPA1 axis represents a potential strategy for the treatment of type 2 diabetes and steatosis.

No abstract

No abstract

The pregnane X receptor (PXR), along with its sister receptor constitutive androstane receptor (CAR), was initially characterized as a xenobiotic receptor that regulates drug metabolism. In this study, we have uncovered an unexpected endobiotic role of PXR in obesity and type 2 diabetes. PXR ablation inhibited high-fat diet (HFD)-induced obesity, hepatic steatosis, and insulin resistance, which were accounted for by increased oxygen consumption, increased mitochondrial β-oxidation, inhibition of hepatic lipogenesis and inflammation, and sensitization of insulin signaling. In an independent model, introducing the PXR(-/-) allele into the ob/ob background also improved body composition and relieved the diabetic phenotype. The ob/ob mice deficient of PXR showed increased oxygen consumption and energy expenditure, as well as inhibition of gluconeogenesis and increased rate of glucose disposal during euglycemic clamp. Mechanistically, the metabolic benefits of PXR ablation were associated with the inhibition of c-Jun NH2-terminal kinase activation and downregulation of lipin-1, a novel PXR target gene. The metabolic benefit of PXR ablation was opposite to the reported prodiabetic effect of CAR ablation. Our results may help to establish PXR as a novel therapeutic target, and PXR antagonists may be used for the prevention and treatment of obesity and type 2 diabetes.

No abstract

Non-alcoholic fatty liver disease (NAFLD) is causally linked to type 2 diabetes, insulin resistance and dyslipidemia. In a normal liver, insulin suppresses gluconeogenesis and promotes lipogenesis. In type 2 diabetes, the liver exhibits selective insulin resistance by failing to inhibit hepatic glucose production while maintaining triglyceride synthesis. Evidence suggests that the insulin pathway bifurcates downstream of Akt to regulate these two processes. Specifically, mTORC1 has been implicated in lipogenesis, but its role on hepatic steatosis has not been examined. Here, we generated mice with hepatocyte-specific deletion of Tsc1 to study the effects of constitutive mTORC1 activation in the liver. These mice developed normally but displayed mild hepatomegaly and insulin resistance without obesity. Unexpectedly, the Tsc1-null livers showed minimal signs of steatosis even under high-fat diet condition. This 'resistant' phenotype was reversed by rapamycin and could be overcome by the expression of Myr-Akt. Moreover, rapamycin failed to reduce hepatic triglyceride levels in models of steatosis secondary to Pten ablation in hepatocytes or high-fat diet in wild-type mice. These observations suggest that mTORC1 is neither necessary nor sufficient for steatosis. Instead, Akt and mTORC1 have opposing effects on hepatic lipid accumulation such that mTORC1 protects against diet-induced steatosis. Specifically, mTORC1 activity induces a metabolic shift towards fat utilization and glucose production in the liver. These findings provide novel insights into the role of mTORC1 in hepatic lipid metabolism.

No abstract

No abstract

No abstract

Irisin, a hormone proteolytically processed from fibronectin type III domain-containing protein 5 (FNDC5), has been reported to induce the browning of sc adipocytes by increasing the level of uncoupling protein 1. In this study, we showed that activation of the nuclear receptor constitutive androstane receptor induced FNDC5 mRNA expression in the liver and increased the circulating level of irisin in mice. FNDC5/irisin is a direct transcriptional target of constitutive androstane receptor. Hepatic-released irisin functioned as a paracrine/autocrine factor that inhibited lipogenesis and gluconeogenesis via the Adenosine 5'-monophosphate (AMP)-activated protein kinase pathway. Adenovirus-overexpressed irisin improved hepatic steatosis and insulin resistance in genetic-induced obese mice. Irisin transgenic mice were also protected against high-fat diet-induced obesity and insulin resistance. In conclusion, our results reveal a novel pathway in regulating FNDC5/irisin expression and identify a physiological role for this hepatic hormone in glucose and lipid homeostasis.

BACKGROUNDPostreceptor insulin resistance (IR) is associated with hyperglycemia and hepatic steatosis. However, receptor-level IR (e.g., insulin receptor pathogenic variants, INSR) causes hyperglycemia without steatosis. We examined 4 pathologic conditions of IR in humans to examine pathways controlling lipid metabolism and gluconeogenesis.METHODSCross-sectional study of severe receptor IR (INSR, n = 7) versus postreceptor IR that was severe (lipodystrophy, n = 14), moderate (type 2 diabetes, n = 9), or mild (obesity, n = 8). Lipolysis (glycerol turnover), hepatic glucose production (HGP), gluconeogenesis (deuterium incorporation from body water into glucose), hepatic triglyceride (magnetic resonance spectroscopy), and hepatic fat oxidation (plasma β-hydroxybutyrate) were measured.RESULTSLipolysis was 2- to 3-fold higher in INSR versus all other groups, and HGP was 2-fold higher in INSR and lipodystrophy versus type 2 diabetes and obesity (P < 0.001), suggesting severe adipose and hepatic IR. INSR subjects had a higher contribution of gluconeogenesis to HGP, approximately 77%, versus 52% to 59% in other groups (P = 0.0001). Despite high lipolysis, INSR subjects had low hepatic triglycerides (0.5% [interquartile range 0.1%-0.5%]), in contrast to lipodystrophy (10.6% [interquartile range 2.8%-17.1%], P < 0.0001). β-hydroxybutyrate was 2- to 7-fold higher in INSR versus all other groups (P < 0.0001), consistent with higher hepatic fat oxidation.CONCLUSIONThese data support a key pathogenic role of adipose tissue IR to increase glycerol and FFA availability to the liver in both receptor and postreceptor IR. However, the fate of FFA diverges in these populations. In receptor-level IR, FFA oxidation drives gluconeogenesis rather than being reesterified to triglyceride. In contrast, in postreceptor IR, FFA contributes to both gluconeogenesis and hepatic steatosis.TRIAL REGISTRATIONClinicalTrials.gov NCT01778556, NCT00001987, and NCT02457897.FUNDINGNational Institute of Diabetes and Digestive and Kidney Diseases, US Department of Agriculture/Agricultural Research Service 58-3092-5-001.

No abstract

No abstract

Pancreatic-derived factor (PANDER) is a cytokine-like protein that is highly expressed in pancreatic islets. In vitro, PANDER pretreatment or viral-mediated overexpression promotes apoptosis of islet β cells. Under conditions of insulin resistance, chronic hyperglycemia potently activates PANDER expression and stimulates the cosecretion of insulin and PANDER in β cells. PANDER binds to the liver cell membrane and induces insulin resistance, resulting in increased gluconeogenesis. Recently, PANDER was found to be expressed in rodent and human liver, and its expression is increased in the liver of diabetic mice and rats. Hepatic overexpression of PANDER promotes lipogenesis in the liver and induces insulin resistance in C57BL/6 mice, whereas the inactivation of hepatic PANDER markedly reduces steatosis, insulin resistance, and hyperglycemia in db/db mice. PANDER deficiency protects mice from high-fat-diet-induced hyperglycemia by decreasing gluconeogenesis in the liver. In summary, PANDER plays an important role in the progression of type 2 diabetes by negatively regulating islet β-cell function and insulin sensitivity in the liver.

No abstract

Type 2 diabetes mellitus (T2DM) has been one of the most prevalent metabolic disorders. Nonetheless, the commonly used anti-T2DM drugs failed to substant to treat T2DM when anti-T2DM was withdrawn. Here we put forward a superior and sustainable anti-diabetic strategy using intraperitoneal administration of amino-acid-functionalized gadofullerene nanoparticles (GFNPs) in db/db diabetic mice. Highly accumulated in the pancreas and liver, GFNPs could prominently decrease hyperglycemia, along with permanently maintaining normal blood sugar levels in T2DM mice and even stopping administration. Importantly, GFNPs reversed the pancreas islets dysfunctions by reducing oxidative stress and inflammation responses and fundamentally normalized the insulin secretory function of the pancreas islets. Mechanistically, GFNPs improved hepatic insulin resistance by regulating glucose and lipid metabolism through the activation of IRS2/PI3K/AKT signal pathways, resulting in inhibiting gluconeogenesis and increasing glycogenesis in the liver. Additionally, GFNPs relieved hepatic steatosis in the liver, ultimately maintaining systemic glucose and lipid metabolic homeostasis without obvious toxicity. Together, GFNPs reverse the dysfunctions of the pancreas and improve hepatic insulin resistance, providing a promising approach for T2DM treatment.

No abstract

Black women, compared with White women, have high rates of whole-body insulin resistance but a lower prevalence of fasting hyperglycemia and hepatic steatosis. This dissociation of whole-body insulin resistance from fasting hyperglycemia may be explained by racial differences in gluconeogenesis, hepatic fat, or tissue-specific insulin sensitivity. Two groups of premenopausal federally employed women, without diabetes were studied. Using stable isotope tracers, [2H2O] and [6,62-H2]glucose, basal glucose production was partitioned into its components (gluconeogenesis and glycogenolysis) and basal whole-body lipolysis ([2H5]glycerol) was measured. Indices of insulin sensitivity, whole-body (SI), hepatic (HISIGPR), and adipose tissue, were calculated. Hepatic fat was measured by proton magnetic resonance spectroscopy. Black women had less hepatic fat and lower fractional and absolute gluconeogenesis. Whole-body SI, HISIGPR, and adipose tissue sensitivity were similar by race, but at any given level of whole-body SI, Black women had higher HISIGPR. Therefore, fasting hyperglycemia may be a less common early pathological feature of prediabetes in Black women compared with White women, because gluconeogenesis remains lower despite similar whole-body SI.

Elevated levels of free fatty acids (FFAs) in the liver, resulting from either increased lipolysis or imbalanced FFAs flux, is a key pathogenic factor of hepatic steatosis. This study was conducted to examine the therapeutic effect of tetrahydrocurcumin (THC), a naturally occurring curcuminoid and a metabolite of curcumin, on oleic acid (OA)-induced steatosis in human hepatocellular carcinoma cells and to elucidate the underlying mechanism. HepG2 cells were incubated with OA to induce steatosis, and then treated with various concentrations of THC. The results showed that THC treatment significantly decreased lipid accumulation in OA-treated HepG2 cells, possibly, by inhibiting the expression of the lipogenic proteins, sterol regulatory element-binding protein 1 (SREBP-1c), peroxisome proliferator-activated receptor gamma (PPARγ), fatty acid synthase (FAS), and fatty acid-binding protein 4 (FABP4). Moreover, THC attenuated OA-induced hepatic lipogenesis in an adenosine monophosphate–activated protein kinase (AMPK)-dependent manner, which was reversed by pretreatment with an AMPK inhibitor. THC promoted lipolysis and upregulated the expression of genes involved in β-oxidation. Glucose uptake and insulin signaling impaired in HepG2 cells incubated with OA were abated by THC treatment, including phosphorylation of the insulin receptor substrate 1 (IRS-1)/phosphoinositide 3-kinase (PI3K)/Akt and downstream signaling pathways, forkhead box protein O1 (FOXO1) and glycogen synthase kinase 3 β (GSK3β), which are involved in gluconeogenesis and glycogen synthesis, respectively. Altogether, these results demonstrated the novel therapeutic benefit of THC against hepatic steatosis and, consequently, a potential treatment for non-alcoholic fatty liver disease (NAFLD). Keywords: Tetrahydrocurcumin, Steatosis, HepG2, AMPK, Insulin resistance

The present study aimed to investigate the molecular mechanisms underlying the anti-obesity effect of flavonoid eriodictyol (ED) supplementation in mice fed with a high-fat diet (HFD). C57BL/6N mice were fed with normal diet (ND), HFD (40 kcal% fat), or HFD + 0.005% (<i>w</i>/<i>w</i>) ED for 16 weeks. In HFD-induced obese mice, dietary ED supplementation significantly alleviated dyslipidemia and adiposity by downregulating the expression of lipogenesis-related genes in white adipose tissue (WAT), while enhancing fecal lipid excretion. ED additionally improved hepatic steatosis and decreased the production of pro-inflammatory cytokines by downregulating the expression of hepatic enzymes and the genes involved in lipogenesis and upregulating the expression of hepatic fatty acid oxidation-related enzymes and genes. In addition, ED improved insulin resistance (IR) by suppressing hepatic gluconeogenesis, enhancing glucose utilization, and modulating the production and release of two incretin hormones, namely gastric inhibitory polypeptide (GIP) and glucagon-like peptide-1 (GLP-1). Taken together, the current findings indicated that ED can protect against diet-induced obesity and related metabolic disturbances, including dyslipidemia, inflammation, fatty liver disease, and IR in diet-induced obese mice.

The incorporation of excess saturated free fatty acids (SFAs) into membrane phospholipids within the ER promotes ER stress, insulin resistance, and hepatic gluconeogenesis. Thioesterase superfamily member 2 (Them2) is a mitochondria-associated long-chain fatty acyl-CoA thioesterase that is activated upon binding phosphatidylcholine transfer protein (PC-TP). Under fasting conditions, the Them2/PC-TP complex directs saturated fatty acyl-CoA toward β-oxidation. Here, we showed that during either chronic overnutrition or acute induction of ER stress, Them2 and PC-TP play critical roles in trafficking SFAs into the glycerolipid biosynthetic pathway to form saturated phospholipids, which ultimately reduce ER membrane fluidity. The Them2/PC-TP complex activated ER stress pathways by enhancing translocon-mediated efflux of ER calcium. The increased cytosolic calcium, in turn, led to the phosphorylation of calcium/calmodulin-dependent protein kinase II, which promoted both hepatic insulin resistance and gluconeogenesis. These findings delineate a mechanistic link between obesity and insulin resistance and establish the Them2/PC-TP complex as an attractive target for the management of hepatic steatosis and insulin resistance.

Hepatic insulin resistance is recognized as a driver of type 2 diabetes and fatty liver disease but specific therapies are lacking. Here we explore the potential of human induced pluripotent stem cells (iPSCs) for modeling hepatic insulin resistance in vitro, with a focus on resolving the controversy about the impact of inflammation in the absence of steatosis. For this, we establish the complex insulin signaling cascade and the multiple inter-dependent functions constituting hepatic glucose metabolism in iPSC-derived hepatocytes (iPSC-Heps). Co-culture of these insulin-sensitive iPSC-Heps with isogenic iPSC-derived pro-inflammatory macrophages induces glucose output by preventing insulin from inhibiting gluconeogenesis and glycogenolysis and activating glycolysis. Screening identifies TNFα and IL1β as the mediators of insulin resistance in iPSC-Heps. Neutralizing these cytokines together restores insulin sensitivity in iPSC-Heps more effectively than individual inhibition, reflecting specific effects on insulin signaling and glucose metabolism mediated by NF-κB or JNK. These results show that inflammation is sufficient to induce hepatic insulin resistance and establish a human iPSC-based in vitro model to mechanistically dissect and therapeutically target this metabolic disease driver.

Neuregulins (NRGs) are emerging as an important family of signaling ligands that regulate glucose and lipid homeostasis. NRG1 lowers blood glucose levels in obese mice, whereas the brown fat-enriched secreted factor NRG4 protects mice from high-fat diet-induced insulin resistance and hepatic steatosis. However, the therapeutic potential of NRGs remains elusive, given the poor plasma half-life of the native ligands. Here, we engineered a fusion protein using human NRG1 and the Fc domain of human IgG1 (NRG1-Fc) that exhibited extended half-life in circulation and improved potency in receptor signaling. We evaluated its efficacy in improving metabolic parameters and dissected the mechanisms of action. NRG1-Fc treatment triggered potent AKT activation in the liver, lowered blood glucose, improved insulin sensitivity, and suppressed food intake in obese mice. NRG1-Fc acted as a potent secretagogue for the metabolic hormone FGF21; however, the latter was largely dispensable for its metabolic effects. NRG1-Fc directly targeted the hypothalamic POMC neurons to promote membrane depolarization and increase firing rate. Together, NRG1-Fc exhibits improved pharmacokinetic properties and exerts metabolic benefits through dual inhibition of hepatic gluconeogenesis and caloric intake.

No abstract

The livers of insulin-resistant, diabetic mice manifest selective insulin resistance, suggesting a bifurcation in the insulin signaling pathway: Insulin loses its ability to block glucose production (i.e., it fails to suppress PEPCK and other genes of gluconeogenesis), yet it retains its ability to stimulate fatty acid synthesis (i.e., continued enhancement of genes of lipogenesis). Enhanced lipogenesis is accompanied by an insulin-stimulated increase in the mRNA encoding SREBP-1c, a transcription factor that activates the entire lipogenic program. Here, we report a branch point in the insulin signaling pathway that may account for selective insulin resistance. Exposure of rat hepatocytes to insulin produced a 25-fold increase in SREBP-1c mRNA and a 95% decrease in PEPCK mRNA. Insulin-mediated changes in both mRNAs were blocked by inhibitors of PI3K and Akt, indicating that these kinases are required for both pathways. In contrast, subnanomolar concentrations of rapamycin, an inhibitor of the mTORC1 kinase, blocked insulin induction of SREBP-1c, but had no effect on insulin suppression of PEPCK. We observed a similar selective effect of rapamycin in livers of rats and mice that experienced an insulin surge in response to a fasting-refeeding protocol. A specific inhibitor of S6 kinase, a downstream target of mTORC1, did not block insulin induction of SREBP-1c, suggesting a downstream pathway distinct from S6 kinase. These results establish mTORC1 as an essential component in the insulin-regulated pathway for hepatic lipogenesis but not gluconeogenesis, and may help to resolve the paradox of selective insulin resistance in livers of diabetic rodents.

Diet-induced obesity is associated with fatty liver, insulin resistance, leptin resistance, and changes in plasma lipid profile. Endocannabinoids have been implicated in the development of these associated phenotypes, because mice deficient for the cannabinoid receptor CB1 (CB1-/-) do not display these changes in association with diet-induced obesity. The target tissues that mediate these effects, however, remain unknown. We therefore investigated the relative role of hepatic versus extrahepatic CB1 receptors in the metabolic consequences of a high-fat diet, using liver-specific CB1 knockout (LCB1-/-) mice. LCB1(-/-) mice fed a high-fat diet developed a similar degree of obesity as that of wild-type mice, but, similar to CB1(-/-) mice, had less steatosis, hyperglycemia, dyslipidemia, and insulin and leptin resistance than did wild-type mice fed a high-fat diet. CB1 agonist-induced increase in de novo hepatic lipogenesis and decrease in the activity of carnitine palmitoyltransferase-1 and total energy expenditure were absent in both CB1(-/-) and LCB1(-/-) mice. We conclude that endocannabinoid activation of hepatic CB1 receptors contributes to the diet-induced steatosis and associated hormonal and metabolic changes, but not to the increase in adiposity, observed with high-fat diet feeding. Theses studies suggest that peripheral CB1 receptors could be selectively targeted for the treatment of fatty liver, impaired glucose homeostasis, and dyslipidemia in order to minimize the neuropsychiatric side effects of nonselective CB1 blockade during treatment of obesity-associated conditions.

No abstract

Nonalcoholic fatty liver disease (NAFLD) is a major contributing factor to hepatic insulin resistance in type 2 diabetes. Diacylglycerol acyltransferase (Dgat), of which there are two isoforms (Dgat1 and Dgat2), catalyzes the final step in triglyceride synthesis. We evaluated the metabolic impact of pharmacological reduction of DGAT1 and -2 expression in liver and fat using antisense oligonucleotides (ASOs) in rats with diet-induced NAFLD. Dgat1 and Dgat2 ASO treatment selectively reduced DGAT1 and DGAT2 mRNA levels in liver and fat, but only Dgat2 ASO treatment significantly reduced hepatic lipids (diacylglycerol and triglyceride but not long chain acyl CoAs) and improved hepatic insulin sensitivity. Because Dgat catalyzes triglyceride synthesis from diacylglycerol, and because we have hypothesized that diacylglycerol accumulation triggers fat-induced hepatic insulin resistance through protein kinase C epsilon activation, we next sought to understand the paradoxical reduction in diacylglycerol in Dgat2 ASO-treated rats. Within 3 days of starting Dgat2 ASO therapy in high fat-fed rats, plasma fatty acids increased, whereas hepatic lysophosphatidic acid and diacylglycerol levels were similar to those of control rats. These changes were associated with reduced expression of lipogenic genes (SREBP1c, ACC1, SCD1, and mtGPAT) and increased expression of oxidative/thermogenic genes (CPT1 and UCP2). Taken together, these data suggest that knocking down Dgat2 protects against fat-induced hepatic insulin resistance by paradoxically lowering hepatic diacylglycerol content and protein kinase C epsilon activation through decreased SREBP1c-mediated lipogenesis and increased hepatic fatty acid oxidation.

No abstract

A central paradox in type 2 diabetes is the apparent selective nature of hepatic insulin resistance--wherein insulin fails to suppress hepatic glucose production yet continues to stimulate lipogenesis, resulting in hyperglycemia, hyperlipidemia, and hepatic steatosis. Although efforts to explain this have focused on finding a branch point in insulin signaling where hepatic glucose and lipid metabolism diverge, we hypothesized that hepatic triglyceride synthesis could be driven by substrate, independent of changes in hepatic insulin signaling. We tested this hypothesis in rats by infusing [U-(13)C] palmitate to measure rates of fatty acid esterification into hepatic triglyceride while varying plasma fatty acid and insulin concentrations independently. These experiments were performed in normal rats, high fat-fed insulin-resistant rats, and insulin receptor 2'-O-methoxyethyl chimeric antisense oligonucleotide-treated rats. Rates of fatty acid esterification into hepatic triglyceride were found to be dependent on plasma fatty acid infusion rates, independent of changes in plasma insulin concentrations and independent of hepatocellular insulin signaling. Taken together, these results obviate a paradox of selective insulin resistance, because the major source of hepatic lipid synthesis, esterification of preformed fatty acids, is primarily dependent on substrate delivery and largely independent of hepatic insulin action.

Obese, insulin-resistant states are characterized by a paradoxical pathogenic condition in which the liver appears to be selectively insulin resistant. Specifically, insulin fails to suppress glucose production, yet successfully stimulates de novo lipogenesis. The mechanisms underlying this dysregulation remain controversial. Here, we hypothesized that carbohydrate-responsive element-binding protein (ChREBP), a transcriptional activator of glycolytic and lipogenic genes, plays a central role in this paradox. Administration of fructose increased hepatic hexose-phosphate levels, activated ChREBP, and caused glucose intolerance, hyperinsulinemia, hypertriglyceridemia, and hepatic steatosis in mice. Activation of ChREBP was required for the increased expression of glycolytic and lipogenic genes as well as glucose-6-phosphatase (G6pc) that was associated with the effects of fructose administration. We found that fructose-induced G6PC activity is a major determinant of hepatic glucose production and reduces hepatic glucose-6-phosphate levels to complete a homeostatic loop. Moreover, fructose activated ChREBP and induced G6pc in the absence of Foxo1a, indicating that carbohydrate-induced activation of ChREBP and G6PC dominates over the suppressive effects of insulin to enhance glucose production. This ChREBP/G6PC signaling axis is conserved in humans. Together, these findings support a carbohydrate-mediated, ChREBP-driven mechanism that contributes to hepatic insulin resistance.

Nonalcoholic fatty liver disease is associated with hepatic insulin resistance and may result primarily from increased hepatic de novo lipogenesis (PRIM) or secondarily from adipose tissue lipolysis (SEC). We studied mice with hepatocyte- or adipocyte-specific SREBP-1c overexpression as models of PRIM and SEC. PRIM mice featured increased lipogenic gene expression in the liver and adipose tissue. Their selective, liver-specific insulin resistance was associated with increased C18:1-diacylglycerol content and protein kinase Cε translocation. SEC mice had decreased lipogenesis mediated by hepatic cholesterol responsive element-binding protein and featured portal/lobular inflammation along with total, whole-body insulin resistance. Hepatic mitochondrial respiration transiently increased and declined with aging along with higher muscle reactive oxygen species production. In conclusion, hepatic insulin resistance originates from lipotoxicity but not from lower mitochondrial capacity, which can even transiently adapt to increased peripheral lipolysis. Peripheral insulin resistance is prevented during increased hepatic lipogenesis only if adipose tissue lipid storage capacity is preserved.

No abstract

No abstract

No abstract

Dysregulation in adipokine biosynthesis and function contributes to obesity-induced metabolic diseases. However, the identities and functions of many of the obesity-induced secretory molecules remain unknown. Here, we report the identification of leucine-rich alpha-2-glycoprotein 1 (LRG1) as an obesity-associated adipokine that exacerbates high fat diet-induced hepatosteatosis and insulin resistance. Serum levels of LRG1 were markedly elevated in obese humans and mice compared with their respective controls. LRG1 deficiency in mice greatly alleviated diet-induced hepatosteatosis, obesity, and insulin resistance. Mechanistically, LRG1 bound with high selectivity to the liver and promoted hepatosteatosis by increasing de novo lipogenesis and suppressing fatty acid β-oxidation. LRG1 also inhibited hepatic insulin signaling by downregulating insulin receptor substrates 1 and 2. Our study identified LRG1 as a key molecule that mediates the crosstalk between adipocytes and hepatocytes in diet-induced hepatosteatosis and insulin resistance. Suppressing LRG1 expression and function may be a promising strategy for the treatment of obesity-related metabolic diseases.

In type 2 Diabetes (T2D) free fatty acids (FFAs) in plasma are increased and hepatic insulin resistance is "selective", in the sense that the insulin-mediated decrease of glucose production is blunted while insulin's effect on stimulating lipogenesis is maintained. We investigated the molecular mechanisms underlying this pathogenic paradox. Primary rat hepatocytes were exposed to palmitate for twenty hours. To establish the physiological relevance of the in vitro findings, we also studied insulin-resistant Zucker Diabetic Fatty (ZDF) rats. While insulin-receptor phosphorylation was unaffected, activation of Akt and inactivation of the downstream targets Glycogen synthase kinase 3α (Gsk3α and Forkhead box O1 (FoxO1) was inhibited in palmitate-exposed cells. Accordingly, dose-response curves for insulin-mediated suppression of the FoxO1-induced gluconeogenic genes and for de novo glucose production were right shifted, and insulin-stimulated glucose oxidation and glycogen synthesis were impaired. In contrast, similar to findings in human T2D, the ability of insulin to induce triglyceride (TG) accumulation and transcription of the enzymes that catalyze de novo lipogenesis and TG assembly was unaffected. Insulin-induction of these genes could, however, be blocked by inhibition of the atypical PKCs (aPKCs). The activity of the Akt-inactivating Protein Phosphatase 2A (PP2A) was increased in the insulin-resistant cells. Furthermore, inhibition of PP2A by specific inhibitors increased insulin-stimulated activation of Akt and phosphorylation of FoxO1 and Gsk3α. Finally, PP2A mRNA levels were increased in liver, muscle and adipose tissue, while PP2A activity was increased in liver and muscle tissue in insulin-resistant ZDF rats. In conclusion, our findings indicate that FFAs may cause a selective impairment of insulin action upon hepatic glucose metabolism by increasing PP2A activity.

The c-Jun N-terminal kinase (JNK) is a member of an evolutionarily conserved subfamily of mitogen-activated protein kinases (MAPKs). JNK regulates various cellular responses such as differentiation, proliferation, migration, immune reaction, and cell death in response to a diverse range of extracellular stimuli.1 Gene deletion and pharmacological interventions have revealed that JNK signaling is required for acute hepatocellular injury, liver regeneration, and carcinogenesis.2-7 In addition, JNK plays a central role in obesity and insulin resistance.8 Therefore, JNK has attracted attention as a key regulator in the pathogenesis of nonalcoholic fatty liver disease (NAFLD). AP-1, activator protein-1; ASO, antisense oligonucleotide; ER, endoplasmic reticulum; FFAs, free fatty acids; IRS, insulin receptor substrate; JNK, c-Jun N-terminal kinase; MAPK, mitogen-activated protein kinase; NAFLD, nonalcoholic fatty liver disease; NASH, nonalcoholic steatohepatitis; NF-κB, nuclear factor-κB; TNF-α, tumor necrosis factor-α. NAFLD is a spectrum of liver disorders ranging from simple steatosis to nonalcoholic steatohepatitis (NASH) and liver fibrosis, and is commonly associated with clinical features of the metabolic syndrome such as obesity, type II diabetes, and dyslipidemia. A "two-hit" model has been proposed for the development of NALFD.9 The "first hit" is the initial hepatic lipid accumulation, but a "second hit" is required for liver injury and inflammation. JNK has been implicated to play a role in both of these "hits". First, increased JNK activity can promote the insulin resistance that underlies the development of metabolic syndrome, including hepatic steatosis. Second, hepatic oxidative stress, which is a major candidate for the "second hit", could cause cellular injury and trigger inflammation through JNK activation. Thus, JNK might play a pivotal role in each step of the pathogenesis of NAFLD. Indeed, recent studies have demonstrated that JNK is activated in the livers of patients with NASH, and that genetic deletion of JNK isoforms attenuates hepatic steatosis in experimental animal models.10-12 Given the potential for JNK inhibition as a possible therapy for insulin resistance/diabetes,13 an in-depth understanding of the role of JNK in the pathogenesis of NAFLD is of critical importance. However, because of the ubiquitous expression of two JNK isoforms (JNK1 and JNK2) and their functional redundancy, compensation, or individuality, it has been difficult to interpret the distinctive roles for JNK isoforms in liver disease. In this issue of HEPATOLOGY, by using the combination of gene knockout and knockdown techniques in mice, Singh and coworkers14 elegantly illustrated the differential contributions of JNK1 and JNK2 in multiple steps of the pathogenesis of NAFLD, and expanded the possibilities of selective inhibition of a JNK isoform as a future therapy for NAFLD. Obesity and insulin resistance are the two major risk factors for NAFLD. In short, systemic insulin resistance, especially at the level of adipocytes, enhances the flux of free fatty acids (FFAs) to the liver. In addition, increased de novo lipogenesis due to hyperinsulinemia induces hepatic steatosis, a "first hit" of NAFLD. This is the differential or selective insulin resistance. JNK plays a central role in both systemic and hepatic insulin resistance through serine phosphorylation of the insulin receptor substrate (IRS)-1 and IRS-2, leading to down-modulation of tyrosine phosphorylation of these molecules, which is required for normal insulin signaling.8, 15 In fact, JNK activation induced by various stimuli such as obesity-induced inflammation, FFAs, oxidative stress, or endoplasmic reticulum (ER) stress, mediates insulin resistance and subsequent hepatic steatosis. Singh and coworkers investigated the distinct roles for JNK1 and JNK2 in developing or established insulin resistance and hepatic steatosis by using genetic knockout or antisense oligonucleotide (ASO) knockdown techniques in mice fed a high-fat diet, respectively. In addition to reproducing the previously reported decreased incidence of insulin resistance in jnk1−/− but not in jnk2−/− mice, they found that ASO knockdown of either JNK1 or JNK2 is able to reverse the established insulin resistance, mimicking a gene targeting therapy. Given that ASOs are preferentially delivered to the liver, JNK ablation in the liver may improve not only hepatic but also systemic insulin sensitivity. Furthermore, they demonstrated that ablation of JNK1 but not JNK2 improved hepatic steatosis. The differential effect of JNK2 knockdown, which is effective on insulin resistance but not on hepatic steatosis, highlights that hepatic steatosis is more than a mere consequence of insulin resistance. JNK1 might have a direct effect on hepatic lipogenesis that is independent of insulin resistance. A previous article supports these results, in that ASO treatment against JNK1 reduces hepatocyte lipid production in vitro and hepatic steatosis in vivo (Fig. 1).16 Multiple roles of JNK isoforms in the pathogenesis of NAFLD. In the setting of systemic insulin resistance, hyperinsulinemia and hyperlipidemia contribute to the development of hepatic steatosis by de novo lipogenesis and increased FFA flux to the liver, respectively. Steatosis-induced oxidative stress, ER stress, and lipid peroxidation activate JNK. Both JNK1 and JNK2 cause hepatic insulin resistance by serine phosphorylation of IRS-1 and IRS-2, whereas only JNK1 induces steatosis. Oxidative stress and proinflammatory cytokines mediate hepatocyte injury through JNK activation. JNK1 and JNK2 indirectly activate caspase-8. JNK2 also plays a cytoprotective function by inhibiting the Bim-dependent mitochondrial pathway of apoptosis. JNK promotes the development of inflammation through AP-1–dependent transcription. JNK1 in inflammatory cells promotes inflammation. According to the "two-hit" hypothesis for NAFLD progression, a "second-hit' is required for the actual hepatocellular injury. A major candidate for the "second hit" is hepatic oxidative stress, which is induced by excess FFAs, associated mitochondrial dysfunction, or lipid peroxidation. Although early and transient activation of JNK promotes cell survival, sustained activation induced by reactive oxygen species via inactivation of the mitogen-activated protein kinase phosphatases contributes to cell death.17 Many factors other than oxidative stress, such as FFAs, ER stress, and inflammatory cytokines including tumor necrosis factor-α directly activate JNK and accelerate cellular injury in NAFLD. The distinct roles for JNK1 and JNK2 in hepatocyte death are still controversial. JNK1 phosphorylates and activates Itch, which induces ubiquitination/degradation of c-FLIP (cellular FLICE-inhibitory protein) and subsequent caspase-8–dependent apoptosis, whereas JNK2 may activate caspase-8 more directly.3, 4 In a cell culture model of NAFLD, FFA-induced JNK activation in primary hepatocytes activates proapoptotic Bcl-2 (B-cell lymphoma-2) family members Bim and Bax and induces cytotoxicity at the mitochondrion level.6 On the other hand, Singh and coworkers found that JNK2 ablation in mice fed a high-fat diet augments hepatocellular injury independent of the level of hepatic steatosis. They propose that JNK2 plays a cytoprotective function by inhibiting Bim-dependent apoptosis through phosphorylation and degradation of Bim. Together with a cytoprotective function of JNK1 through phosphorylation and stabilization of antiapoptotic Bcl-2 family member Mcl-1 (Y.K. and D.A.B., unpublished data), each JNK isoform exhibits both proapoptotic and antiapoptotic effect in the hepatocyte depending on stimulation, time-course, or other circumstances of the cell (Fig. 1). Another important aspect of NAFLD is chronic hepatic inflammation, which is a key event for the progression of the disease. In the steatotic liver, many factors such as oxidative stress, ER stress, and lipid peroxidation can directly activate the inhibitor of nuclear factor-κB (NF-κB) kinase or JNK to activate transcription of proinflammatory cytokines through NF-κB or activator protein-1 (AP-1), respectively. In addition to obesity-induced inflammation derived from adipose tissue, increased serum FFAs and increased portal vein lipopolysaccharide concentrations from small intestinal bacterial overgrowth may activate NF-κB and JNK through Toll-like receptor signaling. This hepatic and systemic inflammation enhances insulin resistance, hepatic steatosis, and liver injury, leading to the progression of steatohepatitis to liver fibrosis. Although JNK1 deletion diminishes hepatic inflammation in animal models, it is unclear whether this is a direct effect of JNK1 deletion on inflammatory signaling or secondary effect through less obesity and hepatic steatosis.11, 12 Solinas et al. demonstrated that selective JNK1 deletion in hematopoietic cells prevents systemic and hepatic inflammation and subsequent development of insulin resistance induced by a high-fat diet without affecting the level of obesity or hepatic steatosis.18 Similarly, JNK1 deletion in hematopoietic cells prevents hepatic steatosis-induced liver inflammation and fibrosis despite a similar level of steatosis, induced by choline-deficient L-amino acid–defined diet (Y.K. and D.A.B., unpublished data). Taken together, JNK1 contributes to the development of inflammation in NAFLD not only through insulin target cells such as adipocytes and hepatocytes, but also through activating inflammatory signaling in hematopoietic cells (most likely Kupffer cells) (Fig. 1). Thus, both JNK isoforms play critical roles in each step of insulin resistance, hepatic steatosis, liver injury, and inflammation in the pathogenesis of NAFLD. The article by Singh et al. provides new insights into the functions of JNK isoforms in multiple steps of NAFLD pathogenesis by utilizing an ASO knockdown system in addition to knockout mice. In particular, the finding that JNK1 ablation is effective in treating established insulin resistance and hepatic steatosis, whereas JNK2 ablation exacerbates liver injury, is extremely important in exploring future therapy. This raises an alarm over indiscriminate JNK inhibition and underscores that the selective inhibition of the JNK1 isoform could be a very effective treatment of NAFLD. At the same time, new questions arise from the research of Singh and coworkers. How do JNK isoforms regulate lipogenesis in the liver? Do the isoforms compensate each other when one of them is knocked down? Is cell lineage–specific JNK1 ablation required for NAFLD therapy? Further development of time-specific and cell lineage–specific knockdown systems will provide answers for these questions and further extend the possibility of JNK isoform targeting therapy. We thank Dr. Jerrold M. Olefsky, University of California, San Diego, for helpful discussions.

Lipid homeostasis in vertebrate cells is regulated by a family of membrane-bound transcription factors designated sterol regulatory element-binding proteins (SREBPs).SREBPs directly activate the expression of more than 30 genes dedicated to the synthesis and uptake of cholesterol, fatty acids, triglycerides, and phospholipids, as well as the NADPH cofactor required to synthesize these molecules (1-4).In the liver, three SREBPs regulate the production of lipids for export into the plasma as lipoproteins and into the bile as micelles.The complex, interdigitated roles of these three SREBPs have been dissected through the study of ten different lines of gene-manipulated mice.These studies form the subject of this review.

No abstract

Obesity is considered to be a metaflammatory condition. Ecklonia cava, brown algae rich in polyphenols, has shown strong antioxidant activity in vitro. This study investigated the effect of E. cava polyphenol extract (ECPE) on the regulation of fat metabolism, inflammation, and the antioxidant defense system in high fat diet-induced obese mice. After obesity was induced by a high-fat diet (HFD), the mice were administered ECPE by gavage for 5 days/12 weeks. ECPE supplementation reduced body weight gain, adipose tissue mass, plasma lipid profiles, hepatic fat deposition, insulin resistance, and the plasma leptin/adiponectin ratio derived from HFD-induced obesity. Moreover, ECPE supplementation selectively ameliorated hepatic protein levels associated with lipogenesis, inflammation, and the antioxidant defense system as well as activation of AMPK and SIRT1. Collectively, ECPE supplement might have potential antiobesity effects via regulation of AMPK and SIRT1 in HFD-induced obesity.

Nonalcoholic fatty liver disease (NAFLD) is characterized by the accumulation of excess liver triacylglycerol (TAG), inflammation, and liver damage. The goal of the present study was to directly quantify the biological sources of hepatic and plasma lipoprotein TAG in NAFLD. Patients (5 male and 4 female; 44 +/- 10 years of age) scheduled for a medically indicated liver biopsy were infused with and orally fed stable isotopes for 4 days to label and track serum nonesterified fatty acids (NEFAs), dietary fatty acids, and those derived from the de novo lipogenesis (DNL) pathway, present in liver tissue and lipoprotein TAG. Hepatic and lipoprotein TAG fatty acids were analyzed by gas chromatography/mass spectrometry. NAFLD patients were obese, with fasting hypertriglyceridemia and hyperinsulinemia. Of the TAG accounted for in liver, 59.0% +/- 9.9% of TAG arose from NEFAs; 26.1% +/- 6.7%, from DNL; and 14.9% +/- 7.0%, from the diet. The pattern of labeling in VLDL was similar to that in liver, and throughout the 4 days of labeling, the liver demonstrated reciprocal use of adipose and dietary fatty acids. DNL was elevated in the fasting state and demonstrated no diurnal variation. These quantitative metabolic data document that both elevated peripheral fatty acids and DNL contribute to the accumulation of hepatic and lipoprotein fat in NAFLD.

sterol regulatory element-binding protein sterol regulatory element sterol regulatory element-binding protein-1c or adipocyte differentiation and determination factor-1 basic helix-loop-helix bHLH leucine zipper low density lipoprotein cAMP-response element-binding protein vitamin D receptor-interacting protein activator-recruited cofactor fatty acid synthase polyunsaturated fatty acid(s) The sterol regulatory element-binding proteins (SREBPs)1 were first identified by two groups working independently on cholesterol metabolism (1Yokoyama C. Wang X. Briggs M.R. Admon A. Wu J. Hua X. Goldstein J.L. Brown M.S. Cell. 1993; 75: 185-197Crossref Google Scholar, 2Hua X. Yokoyama C. Wu J. Briggs M.R. Brown M.S. Goldstein J.L. Wang X. Proc. Natl. Acad. Sci. U. S. A. 1993; 90: 11603-11607Crossref PubMed Scopus (522) Google Scholar) and fat cell differentiation (3Tontonoz P. Kim J.B. Graves R.A. Spiegelman B.M. Mol. Cell. Biol. 1993; 13: 4753-4759Crossref PubMed Scopus (545) Google Scholar). Subsequent studies have demonstrated there are three major SREBP isoforms encoded by two different genes (4Brown M.S. Goldstein J.L. Cell. 1997; 89: 331-340Abstract Full Text Full Text PDF PubMed Scopus (3097) Google Scholar). These unique members of the basic helix-loop-helix leucine zipper (bHLHLZ) family of transcriptional regulatory proteins can be distinguished from other family members by two characteristics. The first is they are synthesized as precursors that are threaded into membranes of the endoplasmic reticulum and nuclear envelope in a hairpin orientation such that the amino and carboxyl tails both face the cytoplasm. The amino-terminal half of the precursor is clipped out of the membrane in two steps responding to regulatory cues that signal the need for increased cellular cholesterol (5Sakai J. Duncan E.A. Rawson R.B. Hua X. Brown M.S. Goldstein J.L. Cell. 1996; 85: 1037-1046Abstract Full Text Full Text PDF PubMed Scopus (442) Google Scholar). The released amino-terminal fragment, which contains the transcriptional activation and DNA binding domains, is targeted to the nucleus where it activates expression of SREBP target genes. The second distinguishing feature of the SREBPs is that they have a unique dual DNA binding specificity, which is discussed below. Two of the three major isoforms are produced from the SREBP-1 gene, which contains two promoters (6Miserez A.R. Cao G. Probst L. Hobbs H.H. Genomics. 1997; 40: 31-40Crossref PubMed Scopus (91) Google Scholar). Transcription from each promoter produces an mRNA with a unique first exon that encodes one of the alternative amino termini referred to as 1a and 1c, respectively (Fig.1). These alternate exons are attached during mRNA splicing to a common second exon in the same reading frame, and therefore, the remaining protein coding information of both isoforms is identical. There is alternative mRNA splicing at the 3′-end as well (7Hua X. Wu J. Goldstein J.L. Brown M.S. Hobbs H.H. Genomics. 1995; 25: 667-673Crossref PubMed Scopus (253) Google Scholar), but this does not appear to be conserved in all mammalian species and its functional significance remains unclear (8Shimomura L. Shimano H. Horton J.D. Goldstein J.L. Brown M.S. J. Clin. Invest. 1997; 99: 838-845Crossref PubMed Scopus (650) Google Scholar). In contrast, mRNAs produced through alternative promoter usage at the 5′-end yield proteins with significant differences in their capacity to activate gene expression (9Shimano H. Horton J.D. Shimomura L. Hammer R.E. Brown M.S. Goldstein J.L. J. Clin. Invest. 1997; 99: 846-854Crossref PubMed Scopus (704) Google Scholar). The longer amino-terminal region in SREBP-1a contains a high percentage of acidic amino acids that make it a potent transcriptional activation domain. The SREBP-1c isoform is a much weaker activator of gene expression because it lacks 29 acidic amino acids present in 1a. Using a nuclease mapping technique to evaluate the relative levels of SREBP-1a and SREBP-1c mRNA, the ratio was shown to vary over an ∼50–100-fold range in different tissues of the body. In liver and adipocytes, 1c mRNA is 9- and 3-fold, respectively, more abundant than SREBP-1a, whereas in spleen SREBP-1a is 10 times more abundant than 1c (8Shimomura L. Shimano H. Horton J.D. Goldstein J.L. Brown M.S. J. Clin. Invest. 1997; 99: 838-845Crossref PubMed Scopus (650) Google Scholar). Whether these ratios reflect similar differences in the levels of each protein remains to be firmly established. In all cultured cell lines examined, SREBP-1a was expressed at higher levels (8Shimomura L. Shimano H. Horton J.D. Goldstein J.L. Brown M.S. J. Clin. Invest. 1997; 99: 838-845Crossref PubMed Scopus (650) Google Scholar). 1c mRNA is the predominant isoform in adult liver and adipocytes, so it is likely to be the key protein involved in SREBP-1-dependent processes in these tissues. Why different tissues express different ratios of SREBP-1a and -1c is not clear. It is possible that the more active 1a isoform is preferentially expressed when there is a high demand for cholesterol and fatty acids such as when new membrane is required during periods of rapid cell division. SREBP-1a stimulates gene expression in vitro and in cultured cells by interacting with the transcriptional coactivators CBP and P300 (10Oliner J.D. Andresen J.M. Hansen S.K. Zhou S. Tjian R. Genes Dev. 1996; 10: 2903-2911Crossref PubMed Scopus (141) Google Scholar, 11Ericsson J. Edwards P.A. J. Biol. Chem. 1998; 273: 17865-17870Abstract Full Text Full Text PDF PubMed Scopus (51) Google Scholar) (Fig. 1). These are large, ubiquitous transcriptional coactivator proteins that are recruited to specific promoters through binding to activation domains of several DNA binding transcription factors in addition to SREBPs (12Kwok R.P. Lundblad J.R. Chrivia J.C. Richards J.P. Bachinger H.P. Brennan R.G. Roberts S.G. Green M.R. Goodman R.H. Nature. 1994; 370: 223-226Crossref PubMed Scopus (1306) Google Scholar). The shorter activation domain of SREBP-1c does not interact efficiently with CBP or P300, and how this isoform activates transcription is not clearly understood. 2K. A. Dooley and T. F. Osborne, unpublished data. The single SREBP-2 isoform similarly interacts with CBP and P300 to activate transcription (Fig. 1). The amino-terminal domain of SREBP-1a interacts also with a separate multisubunit complex alternately called vitamin D receptor-interacting protein (DRIP) or activator-recruited cofactor (ARC) (13Nåår A.M. Beauring P.A. Zhou S. Abraham S. Solomon W. Tjian R. Nature. 1999; 398: 828-832Crossref PubMed Scopus (376) Google Scholar). This heterogeneous complex increases transcription through interacting with activation domains of several other DNA-binding transcriptional regulatory proteins in addition to SREBP-1a. The DRIP/ARC interaction is independent of CBP/P300. As members of the bHLHLZ family of DNA-binding proteins, SREBPs form dimers that recognize the inverted repeat E-box 5′-CANNTG-3′ (where N represents any base). bHLH subfamilies can be classified according to their preference for specific bases at the middle positions of the E-box (14Murre C. Bain G. van Dijk M.A. Engel I. Furnari B.A. Massari M.E. Matthews J.R. Quong M.W. Rivera R.R. Stuiver M.H. Biochim. Biophys. Acta. 1994; 1218: 129-135Crossref PubMed Scopus (421) Google Scholar). SREBPs belong to the same subfamily as Myc/Max and USF, which all prefer the 5′-CACGTG-3′ E-box. However, SREBPs are further distinguished because they not only bind to this inverted repeat but also to the direct repeat sterol regulatory element (SRE) 5′-TCACNCCAC-3′ or to related sites (15Millinder-Vallett S. Sanchez H.B. Rosenfeld J.M. Osborne T.F. J. Biol. Chem. 1996; 271: 12247-12253Abstract Full Text Full Text PDF PubMed Scopus (205) Google Scholar). This flexibility is because of a unique tyrosine residue in the SREBP basic domain that corresponds to an arginine in all other E-box-binding bHLH proteins. Kim et al. (16Kim J.B. Spotts G.D. Halvorsen Y.-D. Shih H.-M. Ellenberger T. Towle H.C. Spiegelman B.M. Mol. Cell. Biol. 1995; 15: 2582-2588Crossref PubMed Scopus (299) Google Scholar) demonstrated the importance of the tyrosine residue by changing it to an arginine to mimic other bHLH proteins. The resulting protein bound only to E-boxes. The reciprocal mutation changing the arginine of the related USF1 protein to a tyrosine converted it into an E-box- and SRE-recognizing protein. When the x-ray structure of the DNA binding domain of SREBP-1 bound to the SRE element was compared with DNA-bound structures for other bHLH proteins it was revealed that the tyrosine permits the basic domain to adopt a slightly different conformation allowing it to recognize specifically the direct repeat site (17Parraga A. Bellsolell L. Ferre-D'Amare A.R. Burley S.K. Structure. 1998; 6: 661-672Abstract Full Text Full Text PDF PubMed Scopus (111) Google Scholar). This duality in DNA recognition has significant functional implications because all cholesterol-regulated SREBP-dependent promoters that have been carefully evaluated contain direct repeat SRE type sites and not E-boxes (18Athanikar J.N. Osborne T.F. Proc. Natl. Acad. Sci. U. S. A. 1998; 95: 4935-4940Crossref PubMed Scopus (54) Google Scholar). SREBPs are the only mammalian bHLH proteins that have been identified with the unique tyrosine residue in their DNA binding domain, and they are not present in the nucleus until a low sterol level activates their proteolytic release from their membrane tether. There are several other E-box-binding proteins in the nucleus independent of the cholesterol level of the cell. If cholesterol-regulated genes had E-box sites they could be activated by these other proteins before SREBP entered the nucleus. Thus, the net difference in target gene expression before and after cholesterol depletion would be small. However, direct repeat SRE sites would ensure that no other bHLH protein could activate target genes in the absence of nuclear SREBP. This would effectively maximize the regulatory response and amplify the difference between the uninduced and induced state. Several distinct genes of both cholesterol and fatty acid metabolism were directly activated by SREBPs in studies performed in cultured cells (Ref. 19Magaña M.M. Koo S.-H. Towle H.C. Osborne T.F. J. Biol. Chem. 2000; 275: 4726-4733Abstract Full Text Full Text PDF PubMed Scopus (132) Google Scholar, and references therein). Even genes of fatty acid metabolism appear to be activated through SRE recognition and not through E-boxes even though SREBPs are capable of binding and activating promoters containing E-boxes in transient transfection studies (3Tontonoz P. Kim J.B. Graves R.A. Spiegelman B.M. Mol. Cell. Biol. 1993; 13: 4753-4759Crossref PubMed Scopus (545) Google Scholar, 18Athanikar J.N. Osborne T.F. Proc. Natl. Acad. Sci. U. S. A. 1998; 95: 4935-4940Crossref PubMed Scopus (54) Google Scholar). As more promoters are analyzed in sufficient detail, it is possible some genes will be regulated through SREBP binding to E-boxes. This could link gene activation with different regulatory signals that are relayed through multiple E-box-binding proteins including SREBPs. Studies evaluating SREBP expression in response to dietary and genetic manipulation in animals have provided additional strong evidence that SREBPs are fundamentally involved in both lipogenesis and cholesterol homeostasis (20Shimano H. Horton J.D. Hammer R.E. Shimomura I. Brown M.S. Goldstein J.L. J. Clin. Invest. 1996; 98: 1575-1584Crossref PubMed Scopus (703) Google Scholar, 21Shimano H. Shimomura L. Hammer R.E. Herz J. Brown M.S. Goldstein J.L. Horton J.D. J. Clin. Invest. 1997; 100: 2115-2124Crossref PubMed Scopus (362) Google Scholar, 22Horton J.D. Shimomura I. Brown M.S. Hammer R.E. Goldstein J.L. Shimano H. J. Clin. Invest. 1998; 101: 2331-2339Crossref PubMed Google Scholar, 23Horton J.D. Bashmakov Y. Shimomura I. Shimano H. Proc. Natl. Acad. Sci. U. S. A. 1998; 95: 5987-5992Crossref PubMed Scopus (546) Google Scholar, 24Pai J. Guryev O. Brown M.S. Goldstein J.L. J. Biol. Chem. 1998; 273: 26138-26148Abstract Full Text Full Text PDF PubMed Scopus (199) Google Scholar, 25Kim J.B. Sarraf P. Wright M. Yao K.M. Mueller E. Solanes G. Lowell B.B. Spiegelman B.M. J. Clin. Invest. 1998; 101: 1-9Crossref PubMed Scopus (618) Google Scholar, 26Shimomura I. Shimano H. Korn B.S. Bashmakov Y. Horton J.D. J. Biol. Chem. 1998; 273: 35299-35306Abstract Full Text Full Text PDF PubMed Scopus (324) Google Scholar, 27Shimano H. Yahagi N. Amemiya-Kudo M. Hasty A.H. Osuga J. Tamura Y. Shionoiri F. Iizuka Y. Ohashi K. Harada K. Gotoda T. Ishibashi S. Yamada N. J. Biol. Chem. 1999; 274: 35832-35839Abstract Full Text Full Text PDF PubMed Scopus (589) Google Scholar) (Fig.2). Specific analyses of individual isoforms suggest SREBP-1 may be selectively involved in activation of genes involved in fatty acid metabolism and de novolipogenesis whereas SREBP-2 may be more selective for genes involved directly in cholesterol homeostasis (22Horton J.D. Shimomura I. Brown M.S. Hammer R.E. Goldstein J.L. Shimano H. J. Clin. Invest. 1998; 101: 2331-2339Crossref PubMed Google Scholar, 24Pai J. Guryev O. Brown M.S. Goldstein J.L. J. Biol. Chem. 1998; 273: 26138-26148Abstract Full Text Full Text PDF PubMed Scopus (199) Google Scholar). Moreover, aberrant expression of SREBPs in mice resulted in metabolic syndromes with physiologic effects similar to specific disorders of lipid metabolism in humans (28Shimomura I. Hammer R.E. Richardson J.A. Ikemoto S. Bashmakov Y. Goldstein J.L. Brown M.S. Genes Dev. 1998; 12: 3182-3194Crossref PubMed Scopus (691) Google Scholar). Also, overexpression of SREBP-1 and -2 has been documented in livers and adipose tissue of the leptin-deficient ob/ob mouse or obese Zucker rat, respectively (29Shimomura I. Bashmakov Y. Horton J.D. J. Biol. Chem. 1999; 274: 30028-30032Abstract Full Text Full Text PDF PubMed Scopus (591) Google Scholar, 30Boizard M. LeLiepvre X. Lemarchand P. Foufelle F. Ferre P. Dugail I. J. Biol. Chem. 1998; 273: 29164-29171Abstract Full Text Full Text PDF PubMed Scopus (111) Google Scholar). Addition of excess cholesterol resulted in the inhibition of processing for membrane-bound precursor forms of both SREBP-1 and -2 in experiments performed in both animals (31Shimomura I. Bashmakov Y. Shimano H. Horton J.D. Goldstein J.L. Brown M.S. Proc. Natl. Acad. Sci. U. S. A. 1997; 94: 12354-12359Crossref PubMed Scopus (127) Google Scholar) and cultured cells (32Hua X. Sakai J. Brown M.S. Goldstein J.L. J. Biol. Chem. 1996; 271: 10379-10384Abstract Full Text Full Text PDF PubMed Scopus (171) Google Scholar). However, when hamsters were fed a diet supplemented with a bile acid-binding resin and a 3-hydroxy-3-methylglutaryl-coenzyme A reductase inhibitor to short-circuit liver cholesterol regulation and set up a pseudo-low cholesterol environment, both expression and proteolytic activation of SREBP-2 were increased (33Sheng Z. Otani H. Brown M.S. Goldstein J.L. Proc. Natl. Acad. Sci. U. S. A. 1995; 92: 935-938Crossref PubMed Scopus (288) Google Scholar). In contrast, expression of SREBP-1 was not altered nor was processing of membrane-bound SREBP-1 accelerated in these animals. In fact, SREBP-1 processing was inhibited (33Sheng Z. Otani H. Brown M.S. Goldstein J.L. Proc. Natl. Acad. Sci. U. S. A. 1995; 92: 935-938Crossref PubMed Scopus (288) Google Scholar). Overexpression of SREBP-1a in cultured cells or animal livers resulted in activation of genes of cholesterol and fatty acid metabolism (20Shimano H. Horton J.D. Hammer R.E. Shimomura I. Brown M.S. Goldstein J.L. J. Clin. Invest. 1996; 98: 1575-1584Crossref PubMed Scopus (703) Google Scholar). Interestingly, in the animal studies this was associated with an increase in hepatic levels of cholesterol and triglycerides, but serum levels were largely unaffected. Crossing these animals with LDL receptor knockout animals resulted in a dramatic increase in circulating levels of cholesterol and triglycerides (34Horton J.D. Shimano H. Hamilton R.L. Brown M.S. Goldstein J.L. J. Clin. Invest. 1999; 103: 1067-1076Crossref PubMed Scopus (164) Google Scholar). Thus, lack of accumulation of serum lipids in the SREBP-1a overexpressing strain was likely due to unregulated expression of hepatic LDL receptors mediated by the overexpression of SREBP-1a. Similar overexpression of SREBP-2 also resulted in accumulation of both lipid classes, although there was more cholesterol relative to fatty acids in both cultured cells and livers than was found in SREBP-1a overexpressors (22Horton J.D. Shimomura I. Brown M.S. Hammer R.E. Goldstein J.L. Shimano H. J. Clin. Invest. 1998; 101: 2331-2339Crossref PubMed Google Scholar, 24Pai J. Guryev O. Brown M.S. Goldstein J.L. J. Biol. Chem. 1998; 273: 26138-26148Abstract Full Text Full Text PDF PubMed Scopus (199) Google Scholar). Because the activation potential of SREBP-1c is significantly lower than that of either SREBP-1a or SREBP-2 (9Shimano H. Horton J.D. Shimomura L. Hammer R.E. Brown M.S. Goldstein J.L. J. Clin. Invest. 1997; 99: 846-854Crossref PubMed Scopus (704) Google Scholar), its overexpression in liver resulted in a much lower level of activation for genes of both fatty acid and cholesterol metabolism and a correspondingly lower level of accumulation of fatty acids and cholesterol (22Horton J.D. Shimomura I. Brown M.S. Hammer R.E. Goldstein J.L. Shimano H. J. Clin. Invest. 1998; 101: 2331-2339Crossref PubMed Google Scholar, 24Pai J. Guryev O. Brown M.S. Goldstein J.L. J. Biol. Chem. 1998; 273: 26138-26148Abstract Full Text Full Text PDF PubMed Scopus (199) Google Scholar, 34Horton J.D. Shimano H. Hamilton R.L. Brown M.S. Goldstein J.L. J. Clin. Invest. 1999; 103: 1067-1076Crossref PubMed Scopus (164) Google Scholar). Selective overexpression of SREBP-1c in cultured preadipocytes activated genes involved in fat cell differentiation and lipid accumulation (35Kim J.B. Spiegelman B.M. Genes Dev. 1996; 10: 1096-1107Crossref PubMed Scopus (857) Google Scholar). A more specific role for SREBPs in glucose homeostasis and fat metabolism was first provided by studies in which SREBP-1c was overexpressed in adipose cells (28Shimomura I. Hammer R.E. Richardson J.A. Ikemoto S. Bashmakov Y. Goldstein J.L. Brown M.S. Genes Dev. 1998; 12: 3182-3194Crossref PubMed Scopus (691) Google Scholar). These animals developed insulin-resistant hyperglycemia and a fatty liver and accumulated high levels of serum triglycerides, signs reminiscent of the human disorder congenital generalized lipodystrophy. Two follow-up studies further support a key role for SREBP-1c in insulin action. In one report it was noted that these same lipodystrophic animals also had very low levels of serum leptin and leptin administration resulted in a reversal of the insulin resistance (36Shimomura I. Hammer R.E. Ikemoto S. Goldstein J.L. Brown M.S. Nature. 1999; 401: 73-76Crossref PubMed Scopus (878) Google Scholar). In a related study, it was demonstrated that SREBP-1c mRNA levels decreased in rats treated with streptozotocin to induce diabetes and insulin administration reversed this effect (37Shimomura I. Bashmakov Y. Ikemoto S. Horton J.D. Brown M.S. Goldstein J.L. Proc. Natl. Acad. Sci. U. S. A. 1999; 96: 13656-13661Crossref PubMed Scopus (638) Google Scholar). Feeding a high carbohydrate diet to rodents after a period of fasting resulted in a significant activation of the entire lipogenic program, which is a signature insulin response (38Towle H.C. Kaytor E.N. Shih H.M. Annu. Rev. Nutr. 1997; 17: 405-433Crossref PubMed Scopus (251) Google Scholar). SREBP-1c mRNA expression was activated during this fasting/refeeding regimen in normal animals (23Horton J.D. Bashmakov Y. Shimomura I. Shimano H. Proc. Natl. Acad. Sci. U. S. A. 1998; 95: 5987-5992Crossref PubMed Scopus (546) Google Scholar, 25Kim J.B. Sarraf P. Wright M. Yao K.M. Mueller E. Solanes G. Lowell B.B. Spiegelman B.M. J. Clin. Invest. 1998; 101: 1-9Crossref PubMed Scopus (618) Google Scholar). Also, overexpression of SREBP-1c in liver prevented the down-regulation of lipogenic genes during fasting (23Horton J.D. Bashmakov Y. Shimomura I. Shimano H. Proc. Natl. Acad. Sci. U. S. A. 1998; 95: 5987-5992Crossref PubMed Scopus (546) Google Scholar). Additionally, re-activation of the hepatic lipogenic program during the feeding phase was not observed in animals in which the SREBP-1 gene was disrupted even though the SREBP-2 gene was expressed at normal levels (27Shimano H. Yahagi N. Amemiya-Kudo M. Hasty A.H. Osuga J. Tamura Y. Shionoiri F. Iizuka Y. Ohashi K. Harada K. Gotoda T. Ishibashi S. Yamada N. J. Biol. Chem. 1999; 274: 35832-35839Abstract Full Text Full Text PDF PubMed Scopus (589) Google Scholar). These studies provide compelling evidence that SREBP-1c is a key transcriptional activator for early events in the initiation of lipogenesis. Interestingly, the SREBP-1 gene in these knockout animals produced a truncated mutant mRNA, which was still induced normally during the refeeding stage (27Shimano H. Yahagi N. Amemiya-Kudo M. Hasty A.H. Osuga J. Tamura Y. Shionoiri F. Iizuka Y. Ohashi K. Harada K. Gotoda T. Ishibashi S. Yamada N. J. Biol. Chem. 1999; 274: 35832-35839Abstract Full Text Full Text PDF PubMed Scopus (589) Google Scholar). The ubiquitous bHLHLZ proteins USF1 and USF2 were proposed to be involved in the fasting/refeeding response because FAS mRNA was expressed at reduced levels during the refeeding phase in animals where either USF1 or USF2 genes were inactivated by homologous recombination (39Casado M. Vallett V.S. Kahn A. S. J. Biol. Chem. 1999; 274: Full Text Full Text PDF PubMed Scopus Google Scholar). how SREBP-1c and proteins both be involved in the fasting and refeeding response is at but it is possible that as a SREBP-1 similar to in the LDL receptor promoter Osborne T.F. Proc. Natl. Acad. Sci. U. S. A. 2000; PubMed Scopus Google Scholar). SREBP-1 is a more likely target of insulin because it is to multiple levels of regulation by and that whereas levels are largely by these same (27Shimano H. Yahagi N. Amemiya-Kudo M. Hasty A.H. Osuga J. Tamura Y. Shionoiri F. Iizuka Y. Ohashi K. Harada K. Gotoda T. Ishibashi S. Yamada N. J. Biol. Chem. 1999; 274: 35832-35839Abstract Full Text Full Text PDF PubMed Scopus (589) Google Scholar). In SREBP-1c mRNA was activated by insulin M. C. Dugail I. Lemarchand P. C. LeLiepvre X. Spiegelman Kim J.B. Ferre P. Foufelle F. Mol. Cell. Biol. 1999; PubMed Scopus Google Scholar). However, of mRNA for a lipogenic gene such as FAS required addition of insulin and a high level of A a of SREBP-1c prevented of FAS mRNA by insulin and These also demonstrated that and SREBP-1c mRNA The effects of and on SREBP-1c mRNA levels are with the effects of the two on gene expression and Using similar et al. M. C. Ferre P. Foufelle F. Proc. Natl. Acad. Sci. U. S. A. 1999; 96: PubMed Scopus Google Scholar) also that the expression of the gene is mediated through a SREBP-1c as Thus, when these studies are with the from fasting/refeeding and studies all of the SREBP-1c is an early in the of insulin in Also, because the SREBP-1c mRNA was still increased by fasting and refeeding in the SREBP-1 knockout animals but the activation of the lipogenic program was a can be where insulin activates which activates metabolic It is also to that transcriptional regulatory protein that activates the SREBP-1c promoter is a direct target of Because insulin of increased SREBP-1c mRNA but activation of FAS gene expression required both insulin and high SREBP-1c is activated at both the transcriptional and levels by for activation of SREBP-1c protein by insulin was noted J.B. Sarraf P. Wright M. Yao K.M. Mueller E. Solanes G. Lowell B.B. Spiegelman B.M. J. Clin. Invest. 1998; 101: 1-9Crossref PubMed Scopus (618) Google Scholar), and SREBP was in cultured cells through a protein J. A. L. G. M. W. Biophys. 1998; PubMed Scopus Google Scholar). A of how SREBP expression and are by insulin and dietary is clearly an that further Because SREBPs are regulated directly by cholesterol and they are involved in both cholesterol and fatty acid it was to they are directly regulated by fatty acids as The addition of acid and other longer fatty acids inhibited sterol regulatory transcription and decreased processing of membrane-bound SREBP-1 and -2 in cultured cells T. Osborne T.F. J. Biol. Chem. 1998; 273: Full Text Full Text PDF PubMed Scopus Google Scholar, M. J. Biol. Chem. 1998; 273: Full Text Full Text PDF PubMed Scopus Google Scholar). A of that evaluated SREBP regulation by different of fatty acids in animal feeding studies SREBP-1 mRNA expression was significantly when were supplemented with specific polyunsaturated fatty acids or J. H.P. J. Biol. Chem. 274: Full Text Full Text PDF Scopus Google Scholar, M. O. J. Biol. Chem. 1999; 274: Full Text Full Text PDF PubMed Scopus Google Scholar, M. J. Biol. Chem. 1999; 274: Full Text Full Text PDF PubMed Scopus Google Scholar). of these studies provided further evidence that SREBP-1 mRNA was decreased in livers of animals J. H.P. J. Biol. Chem. 274: Full Text Full Text PDF Scopus Google Scholar). In a separate animal SREBP-1 mRNA was only slightly but the level of SREBP-1 protein was significantly reduced when were to the diet N. Shimano H. Hasty A.H. Amemiya-Kudo M. H. Tamura Y. Iizuka Y. Shionoiri F. Ohashi K. Osuga J. Harada K. Gotoda T. R. Ishibashi S. Yamada N. J. Biol. Chem. 1999; 274: Full Text Full Text PDF PubMed Scopus Google Scholar). SREBP-2 processing in these studies was not There were significant differences in feeding dietary than the fatty and of animal that are likely to for differences in these separate it is likely that SREBP-1 mRNA levels and protein processing are both by fatty their as bHLHLZ transcriptional regulatory proteins in the SREBPs have been shown to unique functional that as key of There is compelling evidence of a key role for regulation of fatty acid and cholesterol metabolism through SREBP proteins (Fig. SREBP-1c gene expression and protein are both directly to significant regulation by dietary and and SREBP-1c is likely an of insulin in the The role of SREBP-1a is but because it is a much more potent activator of gene expression than 1c, it is required to ensure that cells with a high need for cholesterol and fatty acids can activate to levels to up with the The evidence SREBP-2 may be selectively involved in cholesterol is how the different SREBPs activate genes of cholesterol or fatty acid metabolism in at the level of the individual The is likely to from studies that carefully how individual SREBP isoforms to activate key promoters in each SREBPs are of gene expression by and in a with more transcriptional proteins such as nuclear and Osborne T.F. Proc. Natl. Acad. Sci. U. S. A. 2000; PubMed Scopus Google Scholar). The of SREBP and the of their binding relative to the and of SREBP binding sites are from promoter to promoter (Ref. 19Magaña M.M. Koo S.-H. Towle H.C. Osborne T.F. J. Biol. Chem. 2000; 275: 4726-4733Abstract Full Text Full Text PDF PubMed Scopus (132) Google Scholar, and references therein). Additionally, the for specific in the same promoter can be distinct on the individual SREBP isoform that is activating gene expression M.M. Koo S.-H. Towle H.C. Osborne T.F. J. Biol. Chem. 2000; 275: 4726-4733Abstract Full Text Full Text PDF PubMed Scopus (132) Google Scholar). Thus, studies that the in the SREBP will provide significant into of both gene regulation and

Angiopoietin-like 3 (ANGPTL3) is a regulator of plasma triglyceride (TRG) levels due to its inhibitory action on the activity of lipoprotein lipase (LPL). ANGPTL3 is proteolytically cleaved by proprotein convertases to generate an active N-terminal domain, which forms a complex with ANGPTL8 orchestrating LPL inhibition. ANGPTL3-4-8 mouse model studies indicate that these three ANGPTL family members play a significant role in partitioning the circulating TRG to specific tissues according to nutritional states. Recent data indicate a positive correlation of ANGPTL3 with plasma glucose, insulin, and homeostatic model assessment of insulin resistance (HOMA-IR) in insulin-resistant states. The aim of this review is to critically present the metabolic effects of ANGPTL3, focusing on the possible mechanisms involved in the dysregulation of carbohydrate homeostasis by this protein. Heterozygous and homozygous carriers of ANGPTL3 loss-of-function mutations have reduced risk for type 2 diabetes mellitus. Suggested mechanisms for the implication of ANGPTL3 in carbohydrate metabolism include the (i) increment of free fatty acids (FFAs) owing to the enhancement of lipolysis in adipose tissue, which can induce peripheral as well as hepatic insulin resistance; (ii) promotion of FFA flux to white adipose tissue during feeding, leading to the attenuation of de novo lipogenesis and decreased glucose uptake and insulin sensitivity; (iii) induction of hypothalamic LPL activity in mice, which is highly expressed throughout the brain and is associated with enhanced brain lipid sensing, reduction of food intake, and inhibition of glucose production (however, the effects of ANGPTL3 on hypothalamic LPL in humans need more clarification); and (iv) upregulation of ANGPTL4 expression (owing to the plasma FFA increase), which possibly enhances insulin resistance due to the selective inhibition of LPL in white adipose tissue leading to ectopic lipid accumulation and insulin resistance. Future trials will reveal if ANGPTL3 inhibition could be considered an alternative therapeutic target for dyslipidemia and dysglycemia.

No abstract

The liver plays a central role in regulating glucose and lipid metabolism. Aberrant insulin action in the liver is a major driver of selective insulin resistance, in which insulin fails to suppress glucose production but continues to activate lipogenesis in the liver, resulting in hyperglycemia and hypertriglyceridemia. The underlying mechanisms of selective insulin resistance are not fully understood. Here It is shown that hepatic membrane phospholipid composition controlled by lysophosphatidylcholine acyltransferase 3 (LPCAT3) regulates insulin signaling and systemic glucose and lipid metabolism. Hyperinsulinemia induced by high-fat diet (HFD) feeding augments hepatic Lpcat3 expression and membrane unsaturation. Loss of Lpcat3 in the liver improves insulin resistance and blunts lipogenesis in both HFD-fed and genetic ob/ob mouse models. Mechanistically, Lpcat3 deficiency directly facilitates insulin receptor endocytosis, signal transduction, and hepatic glucose production suppression and indirectly enhances fibroblast growth factor 21 (FGF21) secretion, energy expenditure, and glucose uptake in adipose tissue. These findings identify hepatic LPCAT3 and membrane phospholipid composition as a novel regulator of insulin sensitivity and provide insights into the pathogenesis of selective insulin resistance.

A long-standing paradox in the pathophysiology of metabolic diseases is the selective insulin resistance of the liver. It is characterized by a blunted action of insulin to reduce glucose production, contributing to hyperglycemia, while de novo lipogenesis remains insulin sensitive, participating in turn to hepatic steatosis onset. The underlying molecular bases of this conundrum are not yet fully understood. Here, we established a model of selective insulin resistance in mice by silencing an inhibitor of insulin receptor catalytic activity, the growth factor receptor binding protein 14 (Grb14) in liver. Indeed, Grb14 knockdown enhanced hepatic insulin signaling but also dramatically inhibited de novo fatty acid synthesis. In the liver of obese and insulin-resistant mice, downregulation of Grb14 markedly decreased blood glucose and improved liver steatosis. Mechanistic analyses showed that upon Grb14 knockdown, the release of p62/sqstm1, a partner of Grb14, activated the transcription factor nuclear factor erythroid-2-related factor 2 (Nrf2), which in turn repressed the lipogenic nuclear liver X receptor (LXR). Our study reveals that Grb14 acts as a new signaling node that regulates lipogenesis and modulates insulin sensitivity in the liver by acting at a crossroad between the insulin receptor and the p62-Nrf2-LXR signaling pathways.

Stearoyl-CoA desaturase-1 (SCD1) catalyzes the synthesis of monounsaturated fatty acids from saturated fatty acids. Mice with a targeted disruption of Scd1 gene locus are lean and display increased insulin sensitivity. To examine whether Scd1 activity is required for the development of diet-induced hepatic insulin resistance, we used a sequence-specific antisense oligodeoxynucleotide (ASO) to lower hepatic Scd1 expression in rats and mice with diet-induced insulin resistance. Treatment of rats with Scd1 ASO markedly decreased liver Scd1 expression (approximately 80%) and total Scd activity (approximately 50%) compared with that in rats treated with scrambled ASO (control). Insulin clamp studies revealed severe hepatic insulin resistance in high-fat-fed rats and mice that was completely reversed by 5 days of treatment with Scd1 ASO. The latter treatment decreased glucose production (by approximately 75%), gluconeogenesis, and glycogenolysis. Downregulation of Scd1 also led to increased Akt phosphorylation and marked decreases in the expression of glucose-6-phosphatase (Glc-6-Pase) and phosphoenolpyruvate carboxykinase (PEPCK). Thus, Scd1 is required for the onset of diet-induced hepatic insulin resistance.

Visceral fat (VF) excess has been associated with decreased peripheral insulin sensitivity and has been suggested to contribute to hepatic insulin resistance. However, the mechanisms by which VF impacts on hepatic glucose metabolism and the quantitative role of VF in glycemic control have not been investigated. In the present study 63 type 2 diabetic subjects (age, 55 +/- 1 yr; fasting plasma glucose, 5.5-14.4 mmol/liter; hemoglobin A(1c), 6.1-11.7%) underwent measurement of 1) fat-free mass ((3)H(2)O technique), 2) sc and visceral abdominal fat area (magnetic resonance imaging), 3) insulin sensitivity (euglycemic insulin clamp), 4) endogenous glucose output ([(3)H]glucose infusion technique), and 5) gluconeogenesis ((2)H(2)O method). After adjustment for sex, age, body mass index, diabetes duration, ethnicity, and sc fat area, VF area was positively related to fasting hyperglycemia (partial r = 0.46; P = 0.001) as well as to hemoglobin A(1c) (partial r = 0.50; P = 0.0003). Insulin sensitivity was reciprocally related to VF independently of body mass index (partial r = 0.33; P = 0.01). In contrast, the relation of basal endogenous glucose output to VF was not statistically significant. This lack of association was explained by the fact that VF was positively associated with gluconeogenesis flux (confounder-adjusted, partial r = 0.45; P = 0.003), but was reciprocally associated with glycogenolysis (partial r = 0.31; P < 0.05). We conclude that in patients with established type 2 diabetes, VF accumulation has a significant negative impact on glycemic control through a decrease in peripheral insulin sensitivity and an enhancement of gluconeogenesis.

Increased glucose production and reduced hepatic glycogen storage contribute to metabolic abnormalities in diabetes. Irisin, a newly identified myokine, induces the browning of white adipose tissue, but its effects on gluconeogenesis and glycogenesis are unknown. In the present study, we investigated the effects and underlying mechanisms of irisin on gluconeogenesis and glycogenesis in hepatocytes with insulin resistance, and its therapeutic role in type 2 diabetic mice. Insulin resistance was induced by glucosamine (GlcN) or palmitate in human hepatocellular carcinoma (HepG2) cells and mouse primary hepatocytes. Type 2 diabetes was induced by streptozotocin/high-fat diet (STZ/HFD) in mice. In HepG2 cells, irisin ameliorated the GlcN-induced increases in glucose production, phosphoenolpyruvate carboxykinase (PEPCK) and glucose-6-phosphatase (G6Pase) expression, and glycogen synthase (GS) phosphorylation; it prevented GlcN-induced decreases in glycogen content and the phosphoinositide 3-kinase (PI3K) p110α subunit level, and the phosphorylation of Akt/protein kinase B, forkhead box transcription factor O1 (FOXO1) and glycogen synthase kinase-3 (GSK3). These effects of irisin were abolished by the inhibition of PI3K or Akt. The effects of irisin were confirmed in mouse primary hepatocytes with GlcN-induced insulin resistance and in human HepG2 cells with palmitate-induced insulin resistance. In diabetic mice, persistent subcutaneous perfusion of irisin improved the insulin sensitivity, reduced fasting blood glucose, increased GSK3 and Akt phosphorylation, glycogen content and irisin level, and suppressed GS phosphorylation and PEPCK and G6Pase expression in the liver. Irisin improves glucose homoeostasis by reducing gluconeogenesis via PI3K/Akt/FOXO1-mediated PEPCK and G6Pase down-regulation and increasing glycogenesis via PI3K/Akt/GSK3-mediated GS activation. Irisin may be regarded as a novel therapeutic strategy for insulin resistance and type 2 diabetes.

The hormone FGF21 regulates carbohydrate and lipid homeostasis as well as body weight, and increasing FGF21 improves metabolic abnormalities associated with obesity and diabetes. FGF21 is thought to act on its target tissues, including liver and adipose tissue, to improve insulin sensitivity and reduce adiposity. Here, we used mice with selective hepatic inactivation of the IR (LIRKO) to determine whether insulin sensitization in liver mediates FGF21 metabolic actions. Remarkably, hyperglycemia was completely normalized following FGF21 treatment in LIRKO mice, even though FGF21 did not reduce gluconeogenesis in these animals. Improvements in blood sugar were due in part to increased glucose uptake in brown fat, browning of white fat, and overall increased energy expenditure. These effects were preserved even after removal of the main interscapular brown fat pad. In contrast to its retained effects on reducing glucose levels, the effects of FGF21 on reducing circulating cholesterol and hepatic triglycerides and regulating the expression of key genes involved in cholesterol and lipid metabolism in liver were disrupted in LIRKO mice. Thus, FGF21 corrects hyperglycemia in diabetic mice independently of insulin action in the liver by increasing energy metabolism via activation of brown fat and browning of white fat, but intact liver insulin action is required for FGF21 to control hepatic lipid metabolism.

Berberine (BBR) is a compound originally identified in a Chinese herbal medicine Huanglian (Coptis chinensis French). It improves glucose metabolism in type 2 diabetic patients. The mechanisms involve in activation of adenosine monophosphate activated protein kinase (AMPK) and improvement of insulin sensitivity. However, it is not clear if BBR reduces blood glucose through other mechanism. In this study, we addressed this issue by examining liver response to BBR in diabetic rats, in which hyperglycemia was induced in Sprague-Dawley rats by high fat diet. We observed that BBR decreased fasting glucose significantly. Gluconeogenic genes, Phosphoenolpyruvate carboxykinase (PEPCK) and Glucose-6-phosphatase (G6Pase), were decreased in liver by BBR. Hepatic steatosis was also reduced by BBR and expression of fatty acid synthase (FAS) was inhibited in liver. Activities of transcription factors including Forkhead transcription factor O1 (FoxO1), sterol regulatory element-binding protein 1c (SREBP1) and carbohydrate responsive element-binding protein (ChREBP) were decreased. Insulin signaling pathway was not altered in the liver. In cultured hepatocytes, BBR inhibited oxygen consumption and reduced intracellular adenosine triphosphate (ATP) level. The data suggest that BBR improves fasting blood glucose by direct inhibition of gluconeogenesis in liver. This activity is not dependent on insulin action. The gluconeogenic inhibition is likely a result of mitochondria inhibition by BBR. The observation supports that BBR improves glucose metabolism through an insulin-independent pathway.

Type 2 diabetes is characterized by insulin resistance and impaired insulin secretion. Considerable evidence implicates altered fat topography and defects in adipocyte metabolism in the pathogenesis of type 2 diabetes. In individuals who develop type 2 diabetes, fat cells tend to be enlarged. Enlarged fat cells are resistant to the antilipolytic effects of insulin, leading to day-long elevated plasma free fatty acid (FFA) levels. Chronically increased plasma FFA stimulates gluconeogenesis, induces hepatic and muscle insulin resistance, and impairs insulin secretion in genetically predisposed individuals. These FFA-induced disturbances are referred to as lipotoxicity. Enlarged fat cells also have diminished capacity to store fat. When adipocyte storage capacity is exceeded, lipid 'overflows' into muscle and liver, and possibly the beta-cells of the pancreas, exacerbating insulin resistance and further impairing insulin secretion. In addition, dysfunctional fat cells produce excessive amounts of insulin resistance-inducing, inflammatory and atherosclerosis-provoking cytokines, and fail to secrete normal amounts of insulin-sensitizing cytokines. As more evidence emerges, there is a stronger case for targeting adipose tissue in the treatment of type 2 diabetes. Peroxisome-proliferator activated receptor gamma (PPARgamma) agonists, for example the thiazolidinediones, redistribute fat within the body (decrease visceral and hepatic fat; increase subcutaneous fat) and have been shown to enhance adipocyte insulin sensitivity, inhibit lipolysis, reduce plasma FFA and favourably influence the production of adipocytokines. This article examines in detail the role of adipose tissue in the pathogenesis of type 2 diabetes and highlights the potential of PPAR agonists to improve the management of patients with the condition.

Bile acids (BAs), the end products of cholesterol catabolism, are essential for the absorption of lipids and fat-soluble vitamins; but they have also emerged as novel signaling molecules that act as metabolic regulators. It has been well described that the enterohe-patic circulation, a nuclear (FXR) and a cytoplasmic (TGR5/M-BAR) receptor aid in controlling hepatic bile acid synthesis. Modulating bile acid synthesis greatly impacts in metabolism, because these receptors also are implicated in glucose, lipid, and energy expenditure. Recent studies had revealed the way these receptors participate in regulating gluconeogenesis, peripheral insulin sensitivity, glycogen synthesis, glucagon like peptide 1 (GLP-1) and insulin secretion. Nowadays, it is demonstrated that enhancing bile acid signaling in the intestine contributes to the metabolic benefits of bile acid sequestrants and bariatric surgery on glucose homeos-tasis. This paper discusses the role of bile acid as regulators of glucose metabolism and their potential as therapeutic targets for diabetes.

No abstract

FoxO1 plays an important role in mediating the effect of insulin on hepatic metabolism. Increased FoxO1 activity is associated with reduced ability of insulin to regulate hepatic glucose production. However, the underlying mechanism and physiology remain unknown. We studied the effect of FoxO1 on the ability of insulin to regulate hepatic metabolism in normal vs. insulin-resistant liver under fed and fasting conditions. FoxO1 gain of function, as a result of adenovirus-mediated or transgenic expression, augmented hepatic gluconeogenesis, accompanied by decreased glycogen content and increased fat deposition in liver. Mice with excessive FoxO1 activity exhibited impaired glucose tolerance. Conversely, FoxO1 loss of function, caused by hepatic production of its dominant-negative variant, suppressed hepatic gluconeogenesis, resulting in enhanced glucose disposal and improved insulin sensitivity in db/db mice. FoxO1 expression becomes deregulated, culminating in increased nuclear localization and accounting for its increased transcription activity in livers of both high fat-induced obese mice and diabetic db/db mice. Increased FoxO1 activity resulted in up-regulation of hepatic peroxisome proliferator-activated receptor-gamma coactivator-1beta, fatty acid synthase, and acetyl CoA carboxylase expression, accounting for increased hepatic fat infiltration. These data indicate that hepatic FoxO1 deregulation impairs the ability of insulin to regulate hepatic metabolism, contributing to the development of hepatic steatosis and abnormal metabolism in diabetes.

No abstract

The flavonoid luteolin has various pharmacological activities. However, few studies exist on the in vivo mechanism underlying the actions of luteolin in hepatic steatosis and obesity. The aim of the current study was to elucidate the action of luteolin on obesity and its comorbidity by analyzing its transcriptional and metabolic responses, in particular the luteolin-mediated cross-talk between liver and adipose tissue in diet-induced obese mice. C57BL/6J mice were fed a normal, high-fat, and high-fat + 0.005% (weight for weight) luteolin diet for 16 weeks. In high fat-fed mice, luteolin improved hepatic steatosis by suppressing hepatic lipogenesis and lipid absorption. In adipose tissue, luteolin increased PPARγ protein expression to attenuate hepatic lipotoxicity, which may be linked to the improvement in circulating fatty acid (FA) levels by enhancing FA uptake genes and lipogenic genes and proteins in adipose tissue. Interestingly, luteolin also upregulated the expression of genes controlling lipolysis and the tricarboxylic acid (TCA) cycle prior to lipid droplet formation, thereby reducing adiposity. Moreover, luteolin improved hepatic insulin sensitivity by suppressing SREBP1 expression that modulates Irs2 expression through its negative feedback and gluconeogenesis. Luteolin ameliorates the deleterious effects of diet-induced obesity and its comorbidity via the interplay between liver and adipose tissue.

The transcription factor nuclear factor-κB (NF-κB) mediates inflammation and stress signals in cells. To test NF-κB in the control of hepatic insulin sensitivity, we inactivated NF-κB in the livers of C57BL/6 mice through deletion of the p65 gene, which was achieved by crossing floxed-p65 and Alb-cre mice to generate L-p65-knockout (KO) mice. KO mice did not exhibit any alterations in growth, reproduction, and body weight while on a chow diet. However, the mice on a high-fat diet (HFD) exhibited an improvement in systemic insulin sensitivity. Hepatic insulin sensitivity was enhanced as indicated by increased pyruvate tolerance, Akt phosphorylation, and decreased gene expression in hepatic gluconeogenesis. In the liver, a decrease in intracellular cAMP was observed with decreased CREB phosphorylation. Cyclic nucleotide phosphodiesterase-3B (PDE3B), a cAMP-degrading enzyme, was increased in mRNA and protein as a result of the absence of NF-κB activity. NF-κB was found to inhibit PDE3B transcription through three DNA-binding sites in the gene promoter in response to tumor necrosis factor-α. Body composition, food intake, energy expenditure, and systemic and hepatic inflammation were not significantly altered in KO mice on HFD. These data suggest that NF-κB inhibits hepatic insulin sensitivity by upregulating cAMP through suppression of PDE3B gene transcription.

Ferulic acid (FA) is a phenolic phytochemical known for its antidiabetic property The present study is designed to evaluate the mechanism behind its antidiabetic property in high-fat and fructose-induced type 2 diabetic adult male rats. Animals were divided into 5 groups: (i) control, (ii) diabetic control, (iii) diabetic animals treated with FA (50 mg/(kg body weight · day)(-1), orally) for 30 days, (iv) diabetic animals treated with metformin (50 mg/(kg body weight · day)(-1), orally) for 30 days, and (v) control rats treated with FA. FA treatment to diabetic animals restored blood glucose, serum insulin, glucose tolerance, and insulin tolerance to normal range. Hepatic glycogen concentration, activity of glycogen synthase, and glucokinase were significantly decreased, whereas activity of glycogen phosphorylase and enzymes of gluconeogenesis (phosphoenolpyruvate carboxykinase (PEPCK) and glucose-6-phosphatase (G6Pase)) were increased in diabetic animals and FA restored these to normal levels similar to that of metformin. FA improved the insulin signalling molecules and reduced the negative regulators of insulin signalling. The messenger RNA of gluconeogenic enzyme genes (PEPCK and G6Pase) and the interaction between forkhead transcription factor-O1 and promoters of gluconeogenic enzyme genes (PEPCK and G6Pase) was reduced significantly by ferulic acid. It is concluded from the present study that FA treatment to type 2 diabetic rats improves insulin sensitivity and hepatic glycogenesis but inhibits gluconeogenesis and negative regulators of insulin signalling to maintain normal glucose homeostasis.

In insulin resistance and type II diabetes, there is an elevation of hepatic gluconeogenesis, which contributes to hyperglycaemia. Studies in experimental animals have provided evidence that consumption of high fat (HF) diets by female rats programs the progeny for glucose intolerance in adulthood, but the mechanisms behind the in utero programming remain poorly understood. The present study analysed the effect of a maternal HF diet on fetal gluconeogenic gene expression and potential regulation mechanism related to histone modifications. Dams were fed either a Control (C, 16% kcal fat) or a high-fat (HF, 45% kcal fat) diet throughout gestation. Livers of the offspring were collected on gestational day 21 and analysed to determine the consequences of a maternal HF diet on molecular markers of fetal liver gluconeogenesis. We demonstrated that offspring of HF-fed dams were significantly heavier and had significantly higher blood glucose levels at the time of delivery than offspring of dams fed the C diet. While maternal gluconeogenesis and plasma glucose were not affected by the HF diet, offspring of HF-fed dams had significantly higher mRNA contents of gluconeogenic genes in addition to the elevated plasma glucose. In addition to increased transcription rate, a gestational HF diet resulted in modifications of the Pck1 histone code in livers of offspring. Our results demonstrate that in utero exposure to HF diet has the potential to program the gluconeogenic capacity of offspring through epigenetic modifications, which could potentially lead to excessive glucose production and altered insulin sensitivity in adulthood.

Compounds of the trace element vanadium exert various insulin-like effects in in vitro and in vivo systems. These include their ability to improve glucose homeostasis and insulin resistance in animal models of Type 1 and Type 2 diabetes mellitus. In addition to animal studies, several reports have documented improvements in liver and muscle insulin sensitivity in a limited number of patients with Type 2 diabetes. These effects are, however, not as dramatic as those observed in animal experiments, probably because lower doses of vanadium were used and the duration of therapy was short in human studies as compared with animal work. The ability of these compounds to stimulate glucose uptake, glycogen and lipid synthesis in muscle, adipose and hepatic tissues and to inhibit gluconeogenesis, and the activities of the gluconeogenic enzymes: phosphoenol pyruvate carboxykinase and glucose-6-phosphatase in the liver and kidney as well as lipolysis in fat cells contributes as potential mechanisms to their anti-diabetic insulin-like effects. At the cellular level, vanadium activates several key elements of the insulin signal transduction pathway, such as the tyrosine phosphorylation of insulin receptor substrate-1, and extracellular signal-regulated kinase 1 and 2, phosphatidylinositol 3-kinase and protein kinase B activation. These pathways are believed to mediate the metabolic actions of insulin. Because protein tyrosine phosphatases (PTPases) are considered to be negative regulators of the insulin-signalling pathway, it is suggested that vanadium can enhance insulin signalling and action by virtue of its capacity to inhibit PTPase activity and increase tyrosine phosphorylation of substrate proteins. There are some concerns about the potential toxicity of available inorganic vanadium salts at higher doses and during long-term therapy. Therefore, new organo-vanadium compounds with higher potency and less toxicity need to be evaluated for their efficacy as potential treatment of human diabetes.

Birth represents a dramatic change of nutrition from a fetal diet rich in carbohydrates and poor in fat to a neonatal diet rich in fat and poor in carbohydrates. Gluconeogenesis and ketogenesis are absent or very low in the fetal liver when the mother is correctly fed, and these metabolic pathways emerge after birth to reach adult values after 24 h. Gluconeogenesis increases rapidly in the liver of the newborn in parallel with the appearance of phosphoenolpyruvate carboxykinase (PEPCK), the rate-limiting enzyme of this metabolic pathway. The rise in plasma glucagon, the fall in plasma insulin and the resulting increase in liver cAMP which occur immediately after birth are the factors which induce the activation of liver PEPCK gene transcription. The appearance of ketogenesis is also controlled by the changes of plasma insulin and glucagon that increase the capacity for liver fatty acid oxidation by decreasing lipogenesis and malonyl-CoA concentration, by reducing the sensitivity of carnitine palmitoyl-CoA I to the inhibitory influence of malonyl-CoA, and by activating hydroxymethylglutaryl-CoA synthase by desuccinylation. Once liver PEPCK has reached adult value, i.e. 12 h after birth, other factors are involved in the regulation of hepatic gluconeogenesis. Indeed, the supply of gluconeogenic substrates and of free fatty acid is of crucial importance to support a high rate of gluconeogenesis and to maintain normoglycemia in the newborn. In the liver, fatty acid oxidation provides essential co-factors (acetyl-CoA, NADH and ATP) to support gluconeogenesis, and in peripheral tissue fatty acid oxidation inhibits glucose oxidation and stimulates the production of gluconeogenic precursors (lactate, pyruvate and alanine). Similar mechanisms are operative in human newborn. A defective hepatic fatty acid oxidation is likely to explain the frequent hypoglycemia observed in small-for-date neonates. Administration of oral triglycerides is an efficient mean to prevent hypoglycemia in these newborns.

Nesfatin-1, derived from nucleobindin 2, was recently identified as an anorexigenic signal peptide. However, its neural role in glucose homeostasis and insulin sensitivity is unknown. To evaluate the metabolic impact and underlying mechanisms of central nesfatin-1 signaling, we infused nesfatin-1 in the third cerebral ventricle of high-fat diet (HFD)-fed rats. The effects of central nesfatin-1 on glucose metabolism and changes in transcription factors and signaling pathways were assessed during euglycemic-hyperinsulinemic clamping. The infusion of nesfatin-1 into the third cerebral ventricle markedly inhibited hepatic glucose production (HGP), promoted muscle glucose uptake, and was accompanied by decreases in hepatic mRNA and protein expression and enzymatic activity of PEPCK in both standard diet- and HFD-fed rats. In addition, central nesfatin-1 increased insulin receptor (InsR)/insulin receptor substrate-1 (IRS-1)/AMP-dependent protein kinase (AMPK)/Akt kinase (Akt)/target of rapamycin complex (TORC) 2 phosphorylation and resulted in an increase in Fos immunoreactivity in the hypothalamic nuclei that mediate glucose homeostasis. Taken together, these results reveal what we believe to be a novel site of action of nesfatin-1 on HGP and the PEPCK/InsR/IRS-1/AMPK/Akt/TORC2 pathway and suggest that hypothalamic nesfatin-1 action through a neural-mediated pathway can contribute to increased peripheral and hepatic insulin sensitivity by decreasing gluconeogenesis and promoting peripheral glucose uptake in vivo.

Metabolic syndrome, originally described in 1988 as "syndrome X" by Reaven et al. (1), has evolved in our collective thinking from a vague association of common chronic disease states to a formally defined cluster of clinical traits with adverse impact on cardiovascular risk (2). The cause is incompletely understood but represents a complex interaction among genetic, environmental, and metabolic factors, clearly including diet (3,4) and level of physical activity (4,5). These abnormalities are mediated by—and interconnected by—complex pathways that affect energy homeostasis at cellular, organ, and whole-body levels. This review focuses on obesity-initiated metabolic syndrome, first to provide a pathogenetic overview of extrarenal metabolic derangements; second to consider predisposing conditions shaped by genetic or environmental factors, including growth constraints in utero; and finally to consider the impact of metabolic syndrome on the kidney in its prediabetic phase. The pathogenesis of hypertension in the context of metabolic syndrome is considered separately in this series. Similarly, central nervous system pathways that contribute to disordered energy homeostasis is addressed in detail by others. The mechanisms of irreversible renal injury from hypertension and overt diabetes are well documented and are beyond the scope of this review; nonetheless, they loom large in the long-term renal future of the patient with metabolic syndrome. The current worldwide epidemic of obesity-initiated metabolic syndrome, with its potential for renal damage, mandates our commitment to early renal protection in the obese and to vigorous prevention of obesity in both pediatric and adult populations. Metabolic Syndrome Defined: A Work in Progress The Adult Treatment Panel III (ATPIII) of the National Cholesterol Education Program (NCEP) (2) defines metabolic syndrome clinically as any three of the following five traits (Table 1): abdominal obesity, impaired fasting glucose (reflecting insulin resistance), hypertension, hypertriglyceridemia, and low HDL cholesterol. In addition, the NCEP ATPIII recognizes prothrombotic and proinflammatory states as characteristic of metabolic syndrome (2). Importantly, as subsequent paragraphs detail, these simple clinical criteria for diagnosis belie the emerging complexity of the underlying metabolic derangements (6). Thus, insulin resistance is viewed as the essential common denominator of metabolic syndrome, regardless of cause. Abdominal obesity, now identified solely by waist circumference criteria (Table 1), is the single most common cause of insulin resistance, and key mechanisms that mediate this pathway are becoming clear (6). Hypertension [defined in this high-risk context as ≥130/85 (2)] and the typical pattern of atherogenic dyslipidemia—hypertriglyceridemia, low HDL cholesterol, and increase in small dense LDL particles (2)—are also likely downstream consequences of insulin resistance with identifiable contributions from specific organs. Clinical criteria also do not emphasize the role of disordered skeletal muscle metabolism in this syndrome or highlight for the clinician the therapeutic power of regular exercise to offset insulin resistance. Finally, concepts now evolving from research advances suggest that the current clinical definition identifies individuals at a relatively advanced stage, well beyond onset of irreversible organ/tissue injury. Consequently, one immediate research challenge is to define, in the temporal evolution of metabolic syndrome, where interventions can both reverse metabolic derangements and prevent the tissue damage that conveys long-term risk. Table 1: Clinical criteria for diagnosis of metabolic syndromePrevalence and Cardiovascular Risks of Metabolic Syndrome On the basis of the Third National Health and Nutrition Examination Survey (NHANES III; 1988 to 1994), the prevalence of metabolic syndrome in the U.S. population ≥20 yr of age is 23.7% (7), rising to >40% in those ≥60 yr of age and in those from specific geographic regions (e.g., south Texas) (8). This compares with a 30.5% prevalence of obesity (body mass index [BMI] ≥30) and a 64.5% prevalence of overweight (BMI ≥25) in the NHANES III U.S. population sample, reflecting marked increases of 7 to 10%, respectively, in the previous decade (9). Among non-U.S. populations, prevalence ranges from 49.4% of 1625 hypertensive individuals in Spain (10), 19.8% in Greece (11), and 17.8% in older Italians (12). The last cohort exhibited a stepwise increase in insulin resistance severity (by Homeostasis Model Assessment) with increasing numbers of metabolic syndrome features (12). Importantly, overt metabolic syndrome is not limited to adults (13). Cruz et al. (14) examined 126 overweight Hispanic children who were aged 8 to 13 and had a family history of type 2 diabetes; 62% exhibited abdominal obesity, and 30% met criteria for metabolic syndrome. As in adults, obesity in the pediatric population is increasing, with Hispanic and non-Hispanic black adolescents at greatest risk (15). The 2002 NCEP ATPIII panel rated metabolic syndrome equivalent to cigarette smoking in magnitude of risk for premature coronary heart disease (2). In epidemiologic studies, metabolic syndrome increases risk of developing overt diabetes (16), cardiovascular disease (17,18), and cardiovascular mortality (17). In a prospective Finnish cohort, both NCEP ATPIII and World Health Organization criteria for defining metabolic syndrome predicted a five- to ninefold increase in risk of new diabetes over 4 yr (16). Lakka et al. (17), in a cohort of 1209 disease-free Finnish men who were aged 42 to 60 yr and followed for >11 yr, found that the presence of metabolic syndrome conferred a three- to fourfold increased risk for death from coronary heart disease. Using NHANES III data, Ninomiya et al. (7) described an approximately twofold increase in myocardial infarction and stroke risk in the presence of metabolic syndrome. Pathogenesis of Obesity-Initiated Metabolic Syndrome The NCEP panel identifies the root causes of metabolic syndrome as overweight/obesity, physical inactivity, and genetic factors (2). Unraveling underlying mechanisms has been complicated by the unique multiorgan complexity of this trait cluster. Fundamentally, the metabolic syndrome reflects disordered energy homeostasis. Just as evolution prepared us well for surviving hypotension but poorly for combating hypertension, it has apparently equipped us for surviving the fast but not the feast. Unger (19,20) described metabolic syndrome as "a failure of the system of intracellular lipid homeostasis which prevents lipotoxicity in organs of overnourished individuals," a system that normally acts "by confining the lipid overload to cells specifically designed to store large quantities of surplus calories, the white adipocytes." Central to the breakdown of this system are (1) exogenous fuel overload, (2) ectopic accumulation of lipid in nonadipose cells (21), and (3) insulin resistance (3,16). To summarize concepts to be detailed below, the evolution of metabolic syndrome seems to proceed not as a linear sequence of events but along a matrix of interconnected pathways that mediate interactions among multiple organs and also link these organs as a functional unit to regulate total-body energy homeostasis (Figure 1). Each organ/cell type is typically both a target and an effector within this matrix. Furthermore, disturbance within this matrix of pathways can be initiated by stimuli acting at any one of multiple sites in the matrix (e.g., in adipocytes, in hepatocytes, in skeletal myocytes), each independently capable of disturbing whole-body fuel homeostasis. However, initiation of metabolic syndrome by obesity, in keeping with the now-recognized role of adipose tissue as an endocrine organ, is characterized by powerful systemic stimuli that together impair energy homeostasis in multiple organs simultaneously, leaving no room for protective compensation. This multiplicity of pathways and targets likely explains the efficacy of obesity as the major generator of metabolic syndrome. Figure 1. : Pathogenesis of obesity-initiated metabolic syndrome. Increased abdominal fat mass yields high circulating free fatty acids (FFA), which drives increased cellular FFA uptake. Reduced release of adiponectin from expanding abdominal white adipose tissue (WAT) reduces mitochondrial FA uptake/oxidation in multiple tissues. Despite increased release of leptin from WAT, which normally also enhances FA oxidation, tissue resistance to leptin further promotes cytosolic FA build-up. As a result, excess intracellular FA and its metabolites (fatty acyl CoA, diacylglyceride) accumulate, causing insulin resistance (see pathway, Figure 2). Organ-specific consequences include increased hepatic gluconeogenesis and reduced skeletal muscle glucose uptake; the latter raises plasma glucose content and stimulates pancreatic insulin release, and hyperinsulinemia ensues. The newly available glucose plus high insulin now comes back full circle to stimulate further WAT lipogenesis. Increasing fat cell size induces release of chemotactic molecules (e.g., monocyte chemoattractant protein-1) with macrophage infiltration plus TNF-α and IL-6 generation. These cytokines generate an inflammatory reaction and enhance adipocyte insulin resistance in WAT.Figure 2. : Intracellular pathways of insulin resistance. Accumulation of FA and its metabolites (fatty acyl CoA and diacylglycerol) induce protein kinase C isoforms, leading to serine/threonine phosphorylation of insulin receptor substrate-1 (IRS-1) on serine 302. This renders the IRS-1 resistant to tyrosine phosphorylation by the activated insulin receptor. As a result, downstream effects of insulin receptor activation—Akt activation and translocation of the glucose transporter Glut 4 to the plasma membrane—is reduced. Glucose uptake is thereby diminished, secondarily decreasing both glucose-derived glycogen synthesis and glucose-dependent lipogenesis. Activation of the JNK pathway by elevated cytoplasmic FA may provide an additional pathway for induction of insulin resistance. Adapted from reference 52.Generation of Obesity-Associated Metabolic Syndrome: Reversible Derangements of the Metabolic Matrix Obesity-initiated metabolic syndrome is consistently associated with specific metabolic abnormalities: high circulating free fatty acids (FFA) (22); increased intracellular lipid content of not only white adipose tissue (WAT) but also hepatocytes, skeletal myocytes, pancreatic β cells, cardiomyocytes, gastrointestinal enterocytes, and vascular endothelial cells (23,24); insulin resistance in (at least) the same list of tissues; and reduced functional activity of two insulin-sensitizing adipokines that promote tissue fuel oxidation, adiponectin (25–27), and leptin (27). As abdominal fat expands, adiponectin is progressively reduced (25) while leptin levels are progressively elevated (28), the latter reflecting tissue leptin resistance (20,29). Although not yet included in formal clinical definitions, additional features are increasingly considered integral to obesity-initiated metabolic syndrome: macrocytic infiltration of WAT (30,31), increase in local and circulating inflammatory markers [C-reactive protein (32), TNF-α (33), plasminogen activator inhibitor-1 (34,35), and IL-6 (36)] and hyperhomocysteinemia (37). How do we integrate these diverse elements into a coherent process that permits rational clinical interventions? New data suggest that excess visceral fat mass alone is sufficient to generate all elements of the metabolic syndrome. Role of Abdominal Obesity Most studies support the view that the metabolic syndrome that now confronts U.S. physicians in epidemic proportions is largely initiated by abdominal obesity. The four most crucial elements that link abdominal obesity to other features of the metabolic syndrome seem to be elevated FFA, reduction in circulating insulin-sensitizing adiponectin, peripheral-tissue resistance to the insulin-sensitizing actions of leptin, and enhanced macrophage infiltration in fat tissue with release of proinflammatory cytokines (Figure 1). Abdominal fat is unique in its metabolic features as compared with peripheral fat depots, exhibiting larger adipocytes that contain more triglyceride (TG) and exhibit greater insulin resistance than smaller adipocytes. Adipocyte resistance to the lipogenic effect of insulin yields higher basal rates of lipolysis with increased release of FFA into the portal venous system. This direct access to the liver (see below) may also contribute to the unique impact of visceral fat on energy homeostasis. Abdominal fat may also secrete less leptin than subcutaneous fat. Thus Cnop et al. (28), comparing lean-insulin resistant, lean insulin-sensitive, and obese insulin-resistant adults, found that leptin levels correlated with increasing subcutaneous—but not visceral—fat mass, proposing yet another metabolic distinction between these two compartments. Abdominal fat mass expansion is also coupled with reciprocally reduced release of adiponectin, a multifunctional collagen-like molecule with potent capacity to stimulate fuel oxidation in peripheral tissues (25). Abdominal fat additionally expresses higher levels of renin-angiotensin system components: increased angiotensinogen and increased angiotensin II (Ang II) AT1 receptors (38). Finally, epidemiologic studies confirm the unique significance of abdominal fat mass in predicting microalbuminuria, diabetes, and overall cardiovascular risk (39). It was this compelling evidence for a unique role of central or visceral obesity—in contradistinction to subcutaneous obesity—that prompted the NCEP ATPIII decision to specify abdominal obesity in the clinical definition of metabolic syndrome. These phenomena originating in abdominal adipose tissue generate the clinical picture that we recognize as metabolic syndrome. The roles of FFA excess and adiponectin deficiency are reviewed below in the context of their actions in individual organs; the role of leptin is addressed subsequently to compare and integrate prevailing views of how insulin resistance evolves. Liver Under conditions of normal energy homeostasis, the liver serves as a short-term energy reservoir, taking up absorbed dietary glucose and FFA, synthesizing/storing glycogen, synthesizing/storing TG, and packaging TG into VLDL. During fasting, the liver must sustain a continuous supply of plasma glucose, acutely by glycogenolysis and later in the fasting period by gluconeogenesis. Secreted VLDL provides ongoing TG and ultimately FA fuel to skeletal muscle, heart, and other peripheral tissues via lipoprotein lipase activity in the vascular space. Effect of Excess FFA on Liver Intracellular FFA content is a function of substrate delivery from the plasma and FFA utilization (efflux into mitochondria for oxidation or cytosolic synthesis of intracellular lipids). With abdominal obesity, the increased FFA released into the portal vein from excess visceral fat lipolysis have direct access to the liver. Because cellular FA uptake is substrate dependent, increased hepatocyte FFA uptake ensues (23). Elevated cytoplasmic FA content leads to hepatic insulin resistance. This process involves competition of FA and glucose for access to mitochondrial oxidative metabolism. The molecular mechanism was recently described by Shulman et al. (40,41) (Figure 2), wherein elevated intracellular fatty acyl CoA activates protein kinase Cθ (PKCθ), causing phosphorylation of serine-302 of insulin receptor substrate-1 (IRS-1). This renders IRS-1 unavailable for tyrosine phosphorylation by the activated insulin receptor and reduces all downstream actions of insulin. As a result, the fasting state is simulated and hepatocyte enzymatic machinery is shifted to favor enhanced hepatic gluconeogenesis at the expense of glycogen synthesis. The consequent increase in liver-derived glucose in plasma leads to hyperinsulinemia, a hallmark of metabolic syndrome in its earliest stage and a marker of insulin resistance. The capacity of the insulin-resistant liver to impair secondarily systemic energy homeostasis is illustrated by transgenic studies introducing an insulin-resistant form of the rate-limiting enzyme of liver gluconeogenesis: phosphoenolpyruvate carboxykinase (42). Creating isolated hepatic insulin resistance led to systemic hyperglycemia, hyperinsulinemia, and a moderate increase in fat mass (42). The last reflects WAT utilization of surplus circulating glucose for insulin-induced lipogenesis. In effect, this represents a redistribution of fuel away from the liver to adipose fat stores. These findings emphasize the potential for activating the abnormal metabolic matrix simply by inducing hepatic insulin resistance and also illustrate the dual role of the liver as target and effector in metabolic syndrome derangements. In dogs that were fed an isocaloric moderate-fat diet, striking visceral obesity was associated with marked reduction in the ability of insulin to suppress hepatic gluconeogenesis, even before any reduction in insulin-stimulated glucose uptake appeared; investigators concluded that hepatic insulin resistance plays a dominant role in the pathophysiologic cascade initiated by abdominal obesity (43). FFA overload also provides substrate for increased hepatic TG synthesis and for TG-rich VLDL assembly and secretion. Although details are beyond the scope of this review, the peripheral metabolism of these VLDL generate a small, dense form of highly atherogenic LDL [reviewed by Avramoglu et al. (44)] along with an increase in plasma TG. In addition, increased hepatic lipase activity in the insulin-resistant state reduces levels of protective HDL-2 cholesterol (45), which is essential to the transport of cholesterol from tissues back to the liver. Thus, hepatic insulin resistance, high plasma TG, and low plasma HDL are pathogenetically linked manifestations of altered lipid regulation in metabolic syndrome. Effect of Adiponectin Deficiency on Liver In addition to the effects of elevated FFA load, the energy-related functions of the liver are profoundly affected by the reduced circulating levels of the adipokine adiponectin. The actions of this multifunctional protein are organ specific and uniformly insulin sensitizing. Adiponectin normally promotes insulin sensitivity in liver in part by enhancing FA oxidation (46); this reduces accumulation of cytoplasmic FA, thereby reducing intracellular FA levels and enhancing insulin action via IRS-1 availability to the insulin receptor. Second, like insulin, adiponectin normally suppresses hepatic gluconeogenic enzymes and induces glycogenetic enzymes. Increase in 5′-AMP-activated kinase mediates these effects of adiponectin (46,47). Conversely, deficiency of adiponectin in states of abdominal obesity directly contributes to insulin resistance by further enhancing accumulation of intracellular FA and FA metabolites and by stimulating hepatic glucose output. The impact of insulin-sensitizing adipokines is apparent from transgenic mouse models that completely lack fat (and thus both adiponectin and leptin) (48). Animals are insulin resistant; the provision of physiologic levels of both adiponectin and leptin fully restores normal energy homeostasis, whereas either alone is only partially effective (48). These studies underscore the regulatory role of fat-derived adipokines and lend logic to the seeming paradox that either too little or too much adipose tissue can lead to insulin resistance (21). Skeletal Muscle Increased circulating FFA also have an impact on skeletal muscle energy homeostasis. Skeletal muscle is normally a major site of glucose and FA uptake, accounting for the bulk of total-body glucose utilization and deriving 60% of resting energy from FA. As in the hepatocyte, increase in intramyocellular FA in skeletal muscle has been to impair insulin receptor by serine phosphorylation of this leads to reduced IRS-1 availability for tyrosine reducing Glut 4 translocation to the plasma with consequent reduction in glucose uptake (6). and glycogen synthesis in skeletal are also reduced. elevated circulating FFA contribute to insulin resistance in both liver and skeletal As in the hepatocyte, reduced adiponectin to increased visceral fat mass insulin resistance in skeletal muscle, also in part via reducing FA oxidation further increasing intramyocellular FA content and insulin action (48). Using in insulin-resistant of with type 2 diabetes, et al. found evidence of a 30% reduction in mitochondrial oxidative phosphorylation together with impaired muscle FA oxidation and an increase in intramyocellular lipid diabetes has also been associated with impaired muscle oxidative capacity Finally, the insulin resistance of is associated with impaired mitochondrial FA oxidative capacity in skeletal muscle energy is a of insulin resistance in skeletal and prediabetic have impaired exercise reduced and uptake at initiation of to the and reduced physical activity typical of resistance stimulates skeletal muscle oxidative enzymes and activates mitochondrial be predicted to basal metabolic both by reducing muscle mass and by muscle energy The physical consequences of these skeletal muscle metabolic abnormalities have not yet been in metabolic syndrome but are likely to the of ongoing and The is the of insulin the at which overt diabetes in the of metabolic syndrome, hepatic gluconeogenesis stimulates the to insulin, Increased FFA uptake by pancreatic cells also increases insulin and of and Glut 2 transporter (23). In the obese hyperinsulinemia is and progressively as overt β cells targets of reduction in insulin-stimulated insulin increase in mitochondrial and lipid also below) with of β cell mass effects of hyperinsulinemia on organ and function in the of metabolic syndrome are not well defined but are to of It is of that interventions that can to overt diabetes evidence that the pathway glucose uptake in vascular of endothelial and a potent and endothelial with by adiponectin These actions mediate a of energy enhancing tissue to insulin resistance insulin-induced is one of mechanisms abdominal resistance and of endothelial for hypertension in metabolic syndrome are addressed in detail by in this Role of in In addition to FFA excess and adiponectin functional leptin deficiency in peripheral tissues is to a role in the evolution of obesity-initiated insulin resistance. is in to fat mass normal via central nervous system receptors to and to enhance the latter stimulating energy utilization and Increase in fat-derived plasma leptin with abdominal obesity states is coupled with poorly understood leptin resistance to central and to peripheral insulin-sensitizing effects of leptin (see thus a functional leptin Central pathways of are addressed separately in this series. Unger and on the basis of compelling that peripheral-tissue leptin resistance is a crucial leading to insulin resistance in metabolic syndrome that major role in normal energy homeostasis is not prevention of obesity, as originally but protection of the of intracellular lipid overload of excess activates cellular fuel by stimulating FA oxidation, reducing enhancing glucose and and fat in adipose tissue as well as in muscle and liver cells Accumulation of cytoplasmic FA thus functional leptin deficiency acting via impaired mitochondrial oxidative capacity and enhanced lipogenesis. The insulin resistance be viewed as a to prevent further accumulation of intracellular reduced glucose glucose-derived (21). The mechanisms of peripheral resistance to the actions of leptin are not yet In excess FFA in the leptin resistance, and adiponectin deficiency are likely acting in to generate intracellular FA their sequence and to be The pathways in these intracellular have been reviewed in detail by Unger and Shulman and Shulman (6). The crucial of and in actions of adiponectin and their in metabolic addressed separately in this series. to Although excess abdominal fat serves to energy homeostasis in multiple other it also a target It is first a target of excess glucose in the vascular space. Increased glucose availability from gluconeogenesis and from reduction in glucose uptake, together with hyperinsulinemia, promote in WAT (Figure 1). This the disturbing that abdominal obesity a Excess activity is to fat from peripheral and to central visceral and to of metabolic syndrome. It thus is not that excess has been in the cause of metabolic syndrome. increased activity and a twofold increased of β in adipose tissue of obese were This enzyme acts to to the thus at a tissue levels of were directly to waist circumference and insulin resistance the of in WAT, transgenic in adipocytes the metabolic syndrome whereas were resistant to a to fat tissue induces systemic metabolic WAT also the target of an inflammatory studies in and adipose tissue documented this process and the role of macrophage infiltration and inflammatory in obesity and metabolic syndrome et al. that increasing mass and increasing fat each with macrophage infiltration in WAT and with increased of proinflammatory (Figure et al. found that in WAT of obese mouse models were inflammatory linked to macrophage in obesity, this inflammatory process within WAT insulin resistance. TNF-α and IL-6 have been to induce insulin resistance in and to contribute to insulin resistance in mouse models of obesity This in inflammatory reaction within WAT may insulin resistance in the adipocytes full circle in of multiorgan energy Figure : infiltration in WAT with mass index and adipocyte content In subcutaneous

When fed with a high-fat safflower oil diet for 3 wk, wild-type mice develop hepatic insulin resistance, whereas mice lacking glycerol-3-phosphate acyltransferase-1 retain insulin sensitivity. We examined early changes in the development of insulin resistance via liver and plasma metabolome analyses that compared wild-type and glycerol-3-phosphate acyltransferase-deficient mice fed with either a low-fat or the safflower oil diet for 3 wk. We reasoned that diet-induced changes in metabolites that occurred only in the wild-type mice would reflect those metabolites that were specifically related to hepatic insulin resistance. Of the identifiable metabolites (from 322 metabolites) in liver, wild-type mice fed with the high-fat diet had increases in urea cycle intermediates, consistent with increased deamination of amino acids used for gluconeogenesis. Also increased were stearoylglycerol, gluconate, glucarate, 2-deoxyuridine, and pantothenate. Decreases were observed in S-adenosylhomocysteine, lactate, the bile acid taurocholate, and 1,5-anhydroglucitol, a previously identified marker of short-term glycemic control. Of the identifiable metabolites (from 258 metabolites) in plasma, wild-type mice fed with the high-fat diet had increases in plasma stearate and two pyrimidine-related metabolites, whereas decreases were found in plasma bradykinin, alpha-ketoglutarate, taurocholate, and the tryptophan metabolite, kynurenine. This study identified metabolites previously not known to be associated with insulin resistance and points to the utility of metabolomics analysis in identifying unrecognized biochemical pathways that may be important in understanding the pathophysiology of diabetes.

CTRP3 is a secreted plasma protein of the C1q family that helps regulate hepatic gluconeogenesis and is downregulated in a diet-induced obese state. However, the role of CTRP3 in regulating lipid metabolism has not been established. Here, we used a transgenic mouse model to address the potential function of CTRP3 in ameliorating high-fat diet-induced metabolic stress. Both transgenic and wild-type mice fed a high-fat diet showed similar body weight gain, food intake, and energy expenditure. Despite similar adiposity to wild-type mice upon diet-induced obesity (DIO), CTRP3 transgenic mice were strikingly resistant to the development of hepatic steatosis, had reduced serum TNF-α levels, and demonstrated a modest improvement in systemic insulin sensitivity. Additionally, reduced hepatic triglyceride levels were due to decreased expression of enzymes (GPAT, AGPAT, and DGAT) involved in triglyceride synthesis. Importantly, short-term daily administration of recombinant CTRP3 to DIO mice for 5 days was sufficient to improve the fatty liver phenotype, evident as reduced hepatic triglyceride content and expression of triglyceride synthesis genes. Consistent with a direct effect on liver cells, recombinant CTRP3 treatment reduced fatty acid synthesis and neutral lipid accumulation in cultured rat H4IIE hepatocytes. Together, these results establish a novel role for CTRP3 hormone in regulating hepatic lipid metabolism and highlight its protective function and therapeutic potential in attenuating hepatic steatosis.

The liver X receptors (LXRalpha and beta) are nuclear receptors that coordinate carbohydrate and lipid metabolism. Treatment of insulin-resistant mice with synthetic LXR ligands enhances glucose tolerance, inducing changes in gene expression expected to decrease hepatic gluconeogenesis (via indirect suppression of gluconeogenic enzymes) and increase peripheral glucose disposal (via direct up-regulation of glut4 in fat). To evaluate the relative contribution of each of these effects on whole-body insulin sensitivity, we performed hyperinsulinemic-euglycemic clamps in high-fat-fed insulin-resistant rats treated with an LXR agonist or a peroxisome proliferator-activated receptor gamma ligand. Both groups showed significant improvement in insulin action. Interestingly, rats treated with LXR ligand had lower body weight and smaller fat cells than controls. Insulin-stimulated suppression of the rate of glucose appearance (Ra) was pronounced in LXR-treated rats, but treatment failed to enhance peripheral glucose uptake (R'g), despite increased expression of glut4 in epididymal fat. To ascertain whether LXR ligands suppress hepatic gluconeogenesis directly, mice lacking LXRalpha (the primary isotype in liver) were treated with LXR ligand, and gluconeogenic gene expression was assessed. LXR activation decreased expression of gluconeogenic genes in wild-type and LXRbeta null mice, but failed to do so in animals lacking LXRalpha. Our observations indicate that despite inducing suggestive gene expression changes in adipose tissue in this model of diet-induced insulin resistance, the antidiabetic effect of LXR ligands is primarily due to effects in the liver that appear to require LXRalpha. These findings have important implications for clinical development of LXR agonists as insulin sensitizers.

No abstract

Type 2 diabetes is characterized by fasting hyperglycemia, secondary to hepatic insulin resistance and increased glucose production. Peroxisome proliferator-activated receptor-gamma coactivator-1alpha (PGC-1alpha) is a transcriptional coactivator that is thought to control adaptive responses to physiological stimuli. In liver, PGC-1alpha expression is induced by fasting, and this effect promotes gluconeogenesis. To examine whether PGC-1alpha is involved in the pathogenesis of hepatic insulin resistance, we generated transgenic (TG) mice with whole body overexpression of human PGC-1alpha and evaluated glucose homeostasis with a euglycemic-hyperinsulinemic clamp. PGC-1alpha was moderately (approximately 2-fold) overexpressed in liver, skeletal muscle, brain, and heart of TG mice. In liver, PGC-1alpha overexpression resulted in increased expression of hepatocyte nuclear factor-4alpha and the gluconeogenic enzymes phosphoenolpyruvate carboxykinase and glucose-6-phosphatase. PGC-1alpha overexpression caused hepatic insulin resistance, manifested by higher glucose production and diminished insulin suppression of gluconeogenesis. Paradoxically, PGC-1alpha overexpression improved muscle insulin sensitivity, as evidenced by elevated insulin-stimulated Akt phosphorylation and peripheral glucose disposal. Content of myoglobin and troponin I slow protein was increased in muscle of TG mice, indicating fiber-type switching. PGC-1alpha overexpression also led to lower reactive oxygen species production by mitochondria and reduced IKK/IkappaB signaling in muscle. Feeding a high-fat diet to TG mice eliminated the increased muscle insulin sensitivity. The dichotomous effect of PGC-1alpha overexpression in liver and muscle suggests that PGC-1alpha is a fuel gauge that couples energy demands (muscle) with the corresponding fuel supply (liver). Thus, under conditions of physiological stress (i.e., prolonged fast and exercise training), increased hepatic glucose production may help sustain glucose utilization in peripheral tissues.

d- chiro-Inositol (DCI) is a biologically active component found in tartary buckwheat, which can reduce hyperglycemia and ameliorate insulin resistance. However, the mechanism underlying the antidiabetic effects of DCI remains largely unclear. This study investigated the effects and underlying molecular mechanisms of DCI on hepatic gluconeogenesis in mice fed a high fat diet and saturated palmitic acid-treated hepatocytes. DCI attenuated free fatty acid uptake by the liver via lipid trafficking inhibition, reduced diacylglycerol deposition, and hepatic PKCε translocation. Thus, DCI could improve insulin sensitivity by suppressing hepatic gluconeogenesis. Subsequent analyses revealed that DCI decreased hepatic glucose output and the expression levels of PEPCK and G6 Pase in insulin resistant mice through PKCε-IRS/PI3K/AKT signaling pathway. Likewise, such effects of DCI were confirmed in HepG2 cells with palmitate-induced insulin resistance. These findings indicate a novel pathway by which DCI prevents hepatic gluconeogenesis, reduces lipid deposition, and ameliorates insulin resistance via regulation of PKCε-PI3K/AKT axis.

Altogether, these findings underscore the potential of hepatocyte PPARα as a drug target for NAFLD.

Obesity and dyslipidemia are risk factors for metabolic disorders including diabetes and cardiovascular disease. Sphingolipids such as ceramide and glucosylceramides, while being a relatively minor component of the lipid milieu in most tissues, may be among the most pathogenic lipids in the onset of the sequelae associated with excess adiposity. Circulating factors associated with obesity (e.g., saturated fatty acids, inflammatory cytokines) selectively induce enzymes that promote sphingolipid synthesis, and lipidomic profiling reveals relationships between tissue sphingolipid levels and certain metabolic diseases. Moreover, studies in cultured cells and isolated tissues implicate sphingolipids in certain cellular events associated with diabetes and cardiovascular disease, including insulin resistance, pancreatic beta-cell failure, cardiomyopathy, and vascular dysfunction. However, definitive evidence that sphingolipids contribute to insulin resistance, diabetes, and atherosclerosis has come only recently, as researchers have found that pharmacological inhibition or genetic ablation of enzymes controlling sphingolipid synthesis in rodents ameliorates each of these conditions. Herein we will review the role of ceramide and other sphingolipid metabolites in insulin resistance, beta-cell failure, cardiomyopathy, and vascular dysfunction, focusing on these in vivo studies that identify enzymes controlling sphingolipid metabolism as therapeutic targets for combating metabolic disease.

Insulin resistance is a multi-faceted disruption of the communication between insulin and the interior of a target cell. The underlying cause of insulin appears to be inflammation that can either be increased or decreased by the fatty acid composition of the diet. However, the molecular basis for insulin resistance can be quite different in various organs. This review deals with various types of inflammatory inputs mediated by fatty acids, which affect the extent of insulin resistance in various organs.

Chronic liver inflammation leads to fibrosis and cirrhosis, which is the 12th leading cause of death in the United States. Hepatocyte steatosis is a component of metabolic syndrome and insulin resistance. Hepatic steatosis may be benign or progress to hepatocyte injury and the initiation of inflammation, which activates immune cells. While Kupffer cells are the resident macrophage in the liver, inflammatory cells such as infiltrating macrophages, T lymphocytes, neutrophils, and DCs all contribute to liver inflammation. The inflammatory cells activate hepatic stellate cells, which are the major source of myofibroblasts in the liver. Here we review the initiation of inflammation in the liver, the liver inflammatory cells, and their crosstalk with myofibroblasts.

This review focuses on the mechanisms of lipid accumulation in the liver, with an emphasis on the metabolic fate of free fatty acids (FFAs) in NAFLD and presents an update on the relevant cellular processes/mechanisms that are involved in lipotoxicity. The changes in the levels of various lipid species that result from the imbalance between lipolysis/lipid uptake/lipogenesis and lipid oxidation/secretion can cause organellar dysfunction, e.g. ER stress, mitochondrial dysfunction, lysosomal dysfunction, JNK activation, secretion of extracellular vesicles (EVs) and aggravate (or be exacerbated by) hypoxia which ultimately lead to cell death. The aim of this review is to provide an overview of how abnormal lipid metabolism leads to lipotoxicity and the cellular mechanisms of lipotoxicity in the context of NAFLD.

Nonalcoholic fatty liver disease (NAFLD) has an estimated prevalence of 25% in the general population, and cirrhosis secondary to nonalcoholic steatohepatitis (NASH) is predicted to become the leading cause of liver transplantation, yet there is a lack of effective licensed treatments for these conditions. There is a close relationship between insulin resistance (IR) and NAFLD, with prevalence of NAFLD being 5-fold higher in patients with diabetes compared to those without. IR is implicated both in pathogenesis of NAFLD and in disease progression from steatosis to NASH. Thus, modulation of IR represents a potential strategy for NAFLD treatment. This review highlights key proposed mechanisms linking IR and NAFLD, such as changes in rates of adipose tissue lipolysis and de novo lipogenesis, impaired mitochondrial fatty acid β-oxidation (FAO), changes in fat distribution, alterations in the gut microbiome, and alterations in levels of adipokines and cytokines. Furthermore, this review will discuss the main pharmacological strategies used to treat IR in patients with NAFLD and their efficacy based on recently published experimental and clinical data. These include biguanides, glucagon-like peptide 1 receptor (GLP-1) agonists, dipeptidyl peptidase 4 (DPP-4) inhibitors, peroxisome proliferator-activated receptor (PPAR-γ/α/δ) agonists, sodium glucose cotransporter 2 (SGLT2) inhibitors, and farnesoid X receptor (FXR) agonists, with further novel treatments on the horizon. Ideally, treatment would improve IR, reduce cardiovascular risk, and produce demonstrable improvements in NASH histology-this is likely to be achieved with a combinatorial approach.

Many studies have reported that metabolic dysfunction is closely involved in the complex mechanism underlying the development of non-alcoholic fatty liver disease (NAFLD), which has prompted a movement to consider renaming NAFLD as metabolic dysfunction-associated fatty liver disease (MAFLD). Metabolic dysfunction in this context encompasses obesity, type 2 diabetes mellitus, hypertension, dyslipidemia, and metabolic syndrome, with insulin resistance as the common underlying pathophysiology. Imbalance between energy intake and expenditure results in insulin resistance in various tissues and alteration of the gut microbiota, resulting in fat accumulation in the liver. The role of genetics has also been revealed in hepatic fat accumulation and fibrosis. In the process of fat accumulation in the liver, intracellular damage as well as hepatic insulin resistance further potentiates inflammation, fibrosis, and carcinogenesis. Increased lipogenic substrate supply from other tissues, hepatic zonation of Irs1, and other factors, including ER stress, play crucial roles in increased hepatic de novo lipogenesis in MAFLD with hepatic insulin resistance. Herein, we provide an overview of the factors contributing to and the role of systemic and local insulin resistance in the development and progression of MAFLD.

Fibroblast growth factor 21 (FGF21) is a peptide hormone that is synthesized by several organs and regulates energy homeostasis. Excitement surrounding this relatively recently identified hormone is based on the documented metabolic beneficial effects of FGF21, which include weight loss and improved glycemia. The biology of FGF21 is intrinsically complicated owing to its diverse metabolic functions in multiple target organs and its ability to act as an autocrine, paracrine, and endocrine factor. In the liver, FGF21 plays an important role in the regulation of fatty acid oxidation both in the fasted state and in mice consuming a high-fat, low-carbohydrate ketogenic diet. FGF21 also regulates fatty acid metabolism in mice consuming a diet that promotes hepatic lipotoxicity. In white adipose tissue (WAT), FGF21 regulates aspects of glucose metabolism, and in susceptible WAT depots, it can cause browning. This peptide is highly expressed in the pancreas, where it appears to play an anti-inflammatory role in experimental pancreatitis. It also has an anti-inflammatory role in cardiac muscle. Although typically not expressed in skeletal muscle, FGF21 is induced in situations of muscle stress, particularly mitochondrial myopathies. FGF21 has been proposed as a novel therapeutic for metabolic complications such as diabetes and fatty liver disease. This review aims to interpret and delineate the ever-expanding complexity of FGF21 physiology.

No abstract

Type 2 diabetes (T2D) is closely linked with non-alcoholic fatty liver disease (NAFLD) and hepatic insulin resistance, but the involved mechanisms are still elusive. Using DNA methylome and transcriptome analyses of livers from obese individuals, we found that both hypomethylation at a CpG site in PDGFA (encoding platelet derived growth factor alpha) and PDGFA overexpression are associated with increased T2D risk, hyperinsulinemia, increased insulin resistance and increased steatohepatitis risk. Both genetic risk score studies and human cell modeling pointed to a causative impact of high insulin levels on PDGFA CpG site hypomethylation, PDGFA overexpression, and increased PDGF-AA secretion from liver. We found that PDGF-AA secretion further stimulates its own expression through protein kinase C activity and contributes to insulin resistance through decreased expression of both insulin receptor substrate 1 and of insulin receptor. Importantly, hepatocyte insulin sensitivity can be restored by PDGF-AA blocking antibodies, PDGF receptor inhibitors and by metformin opening therapeutic avenues. Conclusion: Therefore, in the liver of obese patients with T2D, the increased PDGF-AA signaling contributes to insulin resistance, opening new therapeutic avenues against T2D and NAFLD.

Understanding the insulin signaling cascade provides insights on the underlying mechanisms of biological phenomena such as insulin resistance, diabetes, Alzheimer's disease, and cancer. For this reason, previous studies utilized chemical reaction network theory to perform comparative analyses of reaction networks of insulin signaling in healthy (INSMS: INSulin Metabolic Signaling) and diabetic cells (INRES: INsulin RESistance). This study extends these analyses using various methods which give further insights regarding insulin signaling. Using embedded networks, we discuss evidence of the presence of a structural "bifurcation" in the signaling process between INSMS and INRES. Concordance profiles of INSMS and INRES show that both have a high propensity to remain monostationary. Moreover, the concordance properties allow us to present heuristic evidence that INRES has a higher level of stability beyond its monostationarity. Finally, we discuss a new way of analyzing reaction networks through network translation. This method gives rise to three new insights: (i) each stoichiometric class of INSMS and INRES contains a unique positive equilibrium; (ii) any positive equilibrium of INSMS is exponentially stable and is a global attractor in its stoichiometric class; and (iii) any positive equilibrium of INRES is locally asymptotically stable. These results open up opportunities for collaboration with experimental biologists to understand insulin signaling better.

Several studies have developed dynamical models to understand the underlying mechanisms of insulin signaling, a signaling cascade that leads to the translocation of glucose, the human body's main source of energy. Fortunately, reaction network analysis allows us to extract properties of dynamical systems without depending on their model parameter values. This study focuses on the comparison of insulin signaling in healthy state (INSMS or INSulin Metabolic Signaling) and in type 2 diabetes (INRES or INsulin RESistance) using reaction network analysis. The analysis uses network decomposition to identify the different subsystems involved in insulin signaling (e.g., insulin receptor binding and recycling, GLUT4 translocation, and ERK signaling pathway, among others). Furthermore, results show that INSMS and INRES are similar with respect to some network, structo-kinetic, and kinetic properties. Their differences, however, provide insights into what happens when insulin resistance occurs. First, the variation in the number of species involved in INSMS and INRES suggests that when irregularities occur in the insulin signaling pathway, other complexes (and, hence, other processes) get involved, characterizing insulin resistance. Second, the loss of concordance exhibited by INRES suggests less restrictive interplay between the species involved in insulin signaling, leading to unusual activities in the signaling cascade. Lastly, GLUT4 losing its absolute concentration robustness in INRES may signify that the transporter has lost its reliability in shuttling glucose to the cell, inhibiting efficient cellular energy production. This study also suggests possible applications of the equilibria parametrization and network decomposition, resulting from the analysis, to potentially establish absolute concentration robustness in a species.

Within a mathematical model, the process of interaction of the metabolic processes such as glycolysis and gluconeogenesis is studied. As a result of the running of two opposite processes in a cell, the conditions for their interaction and the self-organization in a single dissipative system are created.

Non-alcoholic fatty liver disease (NAFLD) is one of the most widespread liver disorders on a global scale, posing a significant threat of progressing to more severe conditions like nonalcoholic steatohepatitis (NASH), liver fibrosis, cirrhosis, and hepatocellular carcinoma. Diagnosing and staging NAFLD presents challenges due to its non-specific symptoms and the invasive nature of liver biopsies. Our research introduces a novel artificial intelligence cascade model employing ensemble learning and feature fusion techniques. We developed a non-invasive, robust, and reliable diagnostic artificial intelligence tool that utilizes anthropometric and laboratory parameters, facilitating early detection and intervention in NAFLD progression. Our novel artificial intelligence achieved an 86% accuracy rate for the NASH steatosis staging task (non-NASH, steatosis grade 1, steatosis grade 2, and steatosis grade 3) and an impressive 96% AUC-ROC for distinguishing between NASH (steatosis grade 1, grade 2, and grade3) and non-NASH cases, outperforming current state-of-the-art models. This notable improvement in diagnostic performance underscores the potential application of artificial intelligence in the early diagnosis and treatment of NAFLD, leading to better patient outcomes and a reduced healthcare burden associated with advanced liver disease.

Insulin resistance (IR) is a key precursor to diabetes and a significant risk factor for cardiovascular disease. Traditional IR assessment methods require multiple blood tests. We developed a simple AI model using only fasting blood glucose to predict IR in non-diabetic populations. Data from the NHANES (1999-2020) and CHARLS (2015) studies were used for model training and validation. Input features included age, gender, height, weight, blood pressure, waist circumference, and fasting blood glucose. The CatBoost algorithm achieved AUC values of 0.8596 (HOMA-IR) and 0.7777 (TyG index) in NHANES, with an external AUC of 0.7442 for TyG. For METS-IR prediction, the model achieved AUC values of 0.9731 (internal) and 0.9591 (external), with RMSE values of 3.2643 (internal) and 3.057 (external). SHAP analysis highlighted waist circumference as a key predictor of IR. This AI model offers a minimally invasive and effective tool for IR prediction, supporting early diabetes and cardiovascular disease prevention.

The functional diversity in factors identified in association with non-alcoholic fatty liver disease (NAFLD) necessitates the utilization of holistic approaches to investigate the network properties of NAFLD pathogenesis. We describe the generation of the first mouse-specific multi-tissue metabolic model, the SteatoNet, to investigate NAFLD-related steatosis, by utilising an object-oriented modelling approach and incorporating numerous hepatic metabolic pathways, their interaction with peripheral tissues and hierarchical feedback regulation at the transcriptional and post-translational level. The unique validated model is based on steady-state analysis of ordinary differential equations that allows investigation of systemic behaviour in the absence of estimated kinetic parameters. Model analysis indicated the importance of the metabolic flux distribution parameter in controlling metabolite concentration and identified critical focal points in the network that may play a pivotal role in initiating NAFLD. Albeit requiring experimental validation, the candidate triggers identified by SteatoNet provide insight into the network properties of NAFLD and emphasize the promising scope and potential of SteatoNet as a primary investigative tool for hypotheses generation and exploring related metabolic diseases.

The cellular energy sensor AMP-activated protein kinase (AMPK) is a metabolic regulator that mediates adaptation to nutritional variations in order to maintain a proper energy balance in cells. We show here that suckling-weaning and fasting-refeeding transitions in rodents are associated with changes in AMPK activation and the cellular energy state in the liver. These nutritional transitions were characterized by a metabolic switch from lipid to glucose utilization, orchestrated by modifications in glucose levels and the glucagon:insulin ratio in the bloodstream. We therefore investigated the respective roles of glucose and pancreatic hormones on AMPK activation in mouse primary hepatocytes. We found that glucose starvation transiently activates AMPK, whereas changes in glucagon and insulin levels had no impact on AMPK. Challenge of hepatocytes with metformin-induced metabolic stress strengthened both AMPK activation and cellular energy depletion limited-glucose conditions, whereas neither glucagon nor insulin altered AMPK activation. Although both insulin and glucagon induced AMPK$α$ phosphorylation at its Ser-485/491 residue, they did not affect its activity. Finally, the decrease in cellular ATP levels in response to an energy stress was additionally exacerbated under fasting conditions and by AMPK deficiency in hepatocytes, revealing metabolic inflexibility and emphasizing the importance of AMPK for maintaining hepatic energy charge. Our results suggest that nutritional changes (i.e. glucose availability), rather than the related hormonal changes (i.e. the glucagon:insulin ratio), sensitize AMPK activation to the energetic stress induced by the dietary transition during fasting. This effect is critical for preserving the cellular energy state in the liver.

The liver is an essential metabolic organ, and its metabolic function is controlled by insulin and other metabolic hormones. Glucose is converted into pyruvate through glycolysis in the cytoplasm, and pyruvate is subsequently oxidized in the mitochondria to generate ATP through the TCA cycle and oxidative phosphorylation. In the fed state, glycolytic products are used to synthesize fatty acids through de novo lipogenesis. Long-chain fatty acids are incorporated into triacylglycerol, phospholipids, and/or cholesterol esters in hepatocytes. These complex lipids are stored in lipid droplets and membrane structures, or secreted into the circulation as very low-density lipoprotein particles. In the fasted state, the liver secretes glucose through both glycogenolysis and gluconeogenesis. During pronged fasting, hepatic gluconeogenesis is the primary source for endogenous glucose production. Fasting also promotes lipolysis in adipose tissue, resulting in release of nonesterified fatty acids which are converted into ketone bodies in hepatic mitochondria though β-oxidation and ketogenesis. Ketone bodies provide a metabolic fuel for extrahepatic tissues. Liver energy metabolism is tightly regulated by neuronal and hormonal signals. The sympathetic system stimulates, whereas the parasympathetic system suppresses, hepatic gluconeogenesis. Insulin stimulates glycolysis and lipogenesis but suppresses gluconeogenesis, and glucagon counteracts insulin action. Numerous transcription factors and coactivators, including CREB, FOXO1, ChREBP, SREBP, PGC-1α, and CRTC2, control the expression of the enzymes which catalyze key steps of metabolic pathways, thus controlling liver energy metabolism. Aberrant energy metabolism in the liver promotes insulin resistance, diabetes, and nonalcoholic fatty liver diseases.

The liver is crucial for the maintenance of normal glucose homeostasis - it produces glucose during fasting and stores glucose postprandially. However, these hepatic processes are dysregulated in type 1 and type 2 diabetes mellitus, and this imbalance contributes to hyperglycaemia in the fasted and postprandial states. Net hepatic glucose production is the summation of glucose fluxes from gluconeogenesis, glycogenolysis, glycogen synthesis, glycolysis and other pathways. In this Review, we discuss the in vivo regulation of these hepatic glucose fluxes. In particular, we highlight the importance of indirect (extrahepatic) control of hepatic gluconeogenesis and direct (hepatic) control of hepatic glycogen metabolism. We also propose a mechanism for the progression of subclinical hepatic insulin resistance to overt fasting hyperglycaemia in type 2 diabetes mellitus. Insights into the control of hepatic gluconeogenesis by metformin and insulin and into the role of lipid-induced hepatic insulin resistance in modifying gluconeogenic and net hepatic glycogen synthetic flux are also discussed. Finally, we consider the therapeutic potential of strategies that target hepatosteatosis, hyperglucagonaemia and adipose lipolysis.

Insulin resistance arises when the nutrient storage pathways evolved to maximize efficient energy utilization are exposed to chronic energy surplus. Ectopic lipid accumulation in liver and skeletal muscle triggers pathways that impair insulin signaling, leading to reduced muscle glucose uptake and decreased hepatic glycogen synthesis. Muscle insulin resistance, due to ectopic lipid, precedes liver insulin resistance and diverts ingested glucose to the liver, resulting in increased hepatic de novo lipogenesis and hyperlipidemia. Subsequent macrophage infiltration into white adipose tissue (WAT) leads to increased lipolysis, which further increases hepatic triglyceride synthesis and hyperlipidemia due to increased fatty acid esterification. Macrophage-induced WAT lipolysis also stimulates hepatic gluconeogenesis, promoting fasting and postprandial hyperglycemia through increased fatty acid delivery to the liver, which results in increased hepatic acetyl-CoA content, a potent activator of pyruvate carboxylase, and increased glycerol conversion to glucose. These substrate-regulated processes are mostly independent of insulin signaling in the liver but are dependent on insulin signaling in WAT, which becomes defective with inflammation. Therapies that decrease ectopic lipid storage and diminish macrophage-induced WAT lipolysis will reverse the root causes of type 2 diabetes.

Hepatic insulin signaling suppresses gluconeogenesis but promotes de novo lipid synthesis. Paradoxically, hepatic insulin resistance (HIR) enhances both gluconeogenesis and de novo lipid synthesis. Elucidation of the etiology of this paradox, which participates in the pathogenesis of non-alcoholic fatty liver disease (NAFLD), cardiovascular disease, the metabolic syndrome and hepatocellular carcinoma, has not been fully achieved. This article briefly outlines the previously proposed hypotheses on the etiology of the HIR paradox. It then discusses literature consistent with an alternative hypothesis that excessive gluconeogenesis, the direct effect of HIR, is responsible for the aberrant lipogenesis. The mechanisms involved therein are explained, involving de novo synthesis of fructose and uric acid, promotion of glutamine anaplerosis, and induction of glucagon resistance. Thus, gluconeogenesis via lipogenesis promotes hepatic steatosis, a component of NAFLD, and dyslipidemia. Gluconeogenesis-centred mechanisms for the progression of NAFLD from simple steatosis to non-alcoholic steatohepatitis (NASH) and fibrosis are suggested. That NAFLD often precedes and predicts type 2 diabetes is explained by the ability of lipogenesis to cushion against blood glucose dysregulation in the earlier stages of NAFLD. HIR-induced excessive gluconeogenesis is a major cause of the HIR paradox and its sequelae. Such involvement of gluconeogenesis in lipid synthesis rationalizes the fact that several types of antidiabetic drugs ameliorate NAFLD. Thus, dietary, lifestyle and pharmacological targeting of HIR and hepatic gluconeogenesis may be a most viable approach for the prevention and management of the HIR-associated network of diseases.

White adipose tissues (WAT) play crucial roles in maintaining whole-body energy homeostasis, and their dysfunction can contribute to hepatic insulin resistance and type 2 diabetes mellitus (T2DM). However, the mechanisms underlying these alterations remain unknown. By analyzing the transcriptome landscape in human adipocytes based on available RNA-seq datasets from lean, obese, and T2DM patients, we reveal elevated mitochondrial reactive oxygen species (ROS) pathway and NF-κB signaling with altered fatty acid metabolism in T2DM adipocytes. Mice with adipose-specific deletion of mitochondrial redox Trx2 develop hyperglycemia, hepatic insulin resistance, and hepatic steatosis. Trx2-deficient WAT exhibited excessive mitophagy, increased inflammation, and lipolysis. Mechanistically, mitophagy was induced through increasing ROS generation and NF-κB-dependent accumulation of autophagy receptor p62/SQSTM1, which recruits damaged mitochondria with polyubiquitin chains. Importantly, administration of ROS scavenger or NF-κB inhibitor ameliorates glucose and lipid metabolic disorders and T2DM progression in mice. Taken together, this study reveals a previously unrecognized mechanism linking mitophagy-mediated adipose inflammation to T2DM with hepatic insulin resistance.

Production of amphiregulin (Areg) by regulatory T (Treg) cells promotes repair after acute tissue injury. Here, we examined the function of Treg cells in non-alcoholic steatohepatitis (NASH), a setting of chronic liver injury. Areg-producing Treg cells were enriched in the livers of mice and humans with NASH. Deletion of Areg in Treg cells, but not in myeloid cells, reduced NASH-induced liver fibrosis. Chronic liver damage induced transcriptional changes associated with Treg cell activation. Mechanistically, Treg cell-derived Areg activated pro-fibrotic transcriptional programs in hepatic stellate cells via epidermal growth factor receptor (EGFR) signaling. Deletion of Areg in Treg cells protected mice from NASH-dependent glucose intolerance, which also was dependent on EGFR signaling on hepatic stellate cells. Areg from Treg cells promoted hepatocyte gluconeogenesis through hepatocyte detection of hepatic stellate cell-derived interleukin-6. Our findings reveal a maladaptive role for Treg cell-mediated tissue repair functions in chronic liver disease and link liver damage to NASH-dependent glucose intolerance.

Excessive gluconeogenesis can lead to hyperglycemia and diabetes through as yet incompletely understood mechanisms. Herein, we show that hepatic ZBTB22 expression is increased in both diabetic clinical samples and mice, being affected by nutritional status and hormones. Hepatic ZBTB22 overexpression increases the expression of gluconeogenic and lipogenic genes, heightening glucose output and lipids accumulation in mouse primary hepatocytes (MPHs), while ZBTB22 knockdown elicits opposite effects. Hepatic ZBTB22 overexpression induces glucose intolerance and insulin resistance, accompanied by moderate hepatosteatosis, while ZBTB22-deficient mice display improved energy expenditure, glucose tolerance, and insulin sensitivity, and reduced hepatic steatosis. Moreover, hepatic ZBTB22 knockout beneficially regulates gluconeogenic and lipogenic genes, thereby alleviating glucose intolerance, insulin resistance, and liver steatosis in db/db mice. ZBTB22 directly binds to the promoter region of PCK1 to enhance its expression and increase gluconeogenesis. PCK1 silencing markedly abolishes the effects of ZBTB22 overexpression on glucose and lipid metabolism in both MPHs and mice, along with the corresponding changes in gene expression. In conclusion, targeting hepatic ZBTB22/PEPCK1 provides a potential therapeutic approach for diabetes.

Elevated liver de novo lipogenesis contributes to non-alcoholic steatohepatitis (NASH) and can be inhibited by targeting acetyl-CoA carboxylase (ACC). However, hypertriglyceridemia limits the use of pharmacological ACC inhibitors as a monotherapy. ATP-citrate lyase (ACLY) generates acetyl-CoA and oxaloacetate from citrate, but whether inhibition is effective for treating NASH is unknown. Here, we characterize a new mouse model that replicates many of the pathological and molecular drivers of NASH and find that genetically inhibiting ACLY in hepatocytes reduces liver malonyl-CoA, oxaloacetate, steatosis, and ballooning as well as blood glucose, triglycerides, and cholesterol. Pharmacological inhibition of ACLY mirrors genetic inhibition but has additional positive effects on hepatic stellate cells, liver inflammation, and fibrosis. Mendelian randomization of human variants that mimic reductions in ACLY also associate with lower circulating triglycerides and biomarkers of NASH. These data indicate that inhibiting liver ACLY may be an effective approach for treatment of NASH and dyslipidemia.

Insulin resistance (IR) is a pivotal metabolic disorder associated with type 2 diabetes and metabolic syndrome. This study investigated the potential of hypoxanthine (Hx), a purine metabolite and uric acid precursor, in ameliorating IR and regulating hepatic glucose and lipid metabolism. We utilized both in vitro IR-HepG2 cells and in vivo diet-induced IR mice to investigate the impact of Hx. The HepG2 cells were treated with Hx to evaluate its effects on glucose production and lipid deposition. Activity-based protein profiling (ABPP) was applied to identify Hx-target proteins and the underlying pathways. In vivo studies involved administration of Hx to IR mice, followed by assessments of IR-associated indices, with explores on the potential regulating mechanisms on hepatic glucose and lipid metabolism. Hx intervention significantly reduced glucose production and lipid deposition in a dose-dependent manner without affecting cell viability in IR-HepG2 cells. ABPP identified key Hx-target proteins engaged in fatty acid and pyruvate metabolism. In vivo, Hx treatment reduced IR severities, as evidenced by decreased HOMA-IR, fasting blood glucose, and serum lipid profiles. Histological assessments confirmed reduced liver lipid deposition. Mechanistic insights revealed that Hx suppresses hepatic gluconeogenesis and fatty acid synthesis, and promotes fatty acid oxidation via the AMPK/mTOR/PPARα pathway. This study delineates a novel role of Hx in regulating hepatic metabolism, offering a potential therapeutic strategy for IR and associated metabolic disorders. The findings provide a foundation for further investigation into the role of purine metabolites in metabolic regulation and their clinical implications.

NAFLD is closely linked with hepatic insulin resistance. Accumulation of hepatic diacylglycerol activates PKC-ε, impairing insulin receptor activation and insulin-stimulated glycogen synthesis. Peripheral insulin resistance indirectly influences hepatic glucose and lipid metabolism by increasing flux of substrates that promote lipogenesis (glucose and fatty acids) and gluconeogenesis (glycerol and fatty acid-derived acetyl-CoA, an allosteric activator of pyruvate carboxylase). Weight loss with diet or bariatric surgery effectively treats NAFLD, but drugs specifically approved for NAFLD are not available. Some new pharmacological strategies act broadly to alter energy balance or influence pathways that contribute to NAFLD (e.g., agonists for PPAR γ, PPAR α/δ, FXR and analogs for FGF-21, and GLP-1). Others specifically inhibit key enzymes involved in lipid synthesis (e.g., mitochondrial pyruvate carrier, acetyl-CoA carboxylase, stearoyl-CoA desaturase, and monoacyl- and diacyl-glycerol transferases). Finally, a novel class of liver-targeted mitochondrial uncoupling agents increases hepatocellular energy expenditure, reversing the metabolic and hepatic complications of NAFLD.

Non-alcoholic fatty liver disease (NAFLD) is one of the most universal liver diseases with complicated pathogenesis throughout the world. Insulin resistance is a leading risk factor that contributes to the development of NAFLD. Vascular endothelial growth factor B (VEGFB) was described by researchers as contributing to regulating lipid metabolic disorders. Here, we investigated VEGFB as a main target to regulate insulin resistance and metabolic syndrome. In this study, bioinformatics, transcriptomics, morphological experiments, and molecular biology were used to explore the role of VEGFB in regulating insulin resistance in NAFLD and its molecular mechanism based on human samples, animal models, and cell models. RNA-seq was performed to analyze the signal pathways associated with VEGFB and NAFLD; Palmitic acid and High-fat diet were used to induce insulin-resistant HepG2 cells model and NAFLD animal model. Intracellular glucolipid contents, glucose uptake, hepatic and serum glucose and lipid levels were examined by Microassay and Elisa. Hematoxylin-eosin staining, Oil Red O staining, and Periodic acid-schiff staining were used to analyze the hepatic steatosis, lipid droplet, and glycogen content in the liver. Western blot and quantitative real-time fluorescent PCR were used to verify the expression levels of the VEGFB and insulin resistance-related signals PI3K/AKT pathway. We observed that VEGFB is genetically associated with NAFLD and the PI3K/AKT signal pathway. After VEGFB knockout, glucolipids levels were increased, and glucose uptake ability was decreased in insulin-resistant HepG2 cells. Meanwhile, body weight, blood glucose, blood lipids, and hepatic glucose of NAFLD mice were increased, and hepatic glycogen, glucose tolerance, and insulin sensitivity were decreased. Moreover, VEGFB overexpression reduced glucolipids and insulin resistance levels in HepG2 cells. Specifically, VEGFB/VEGFR1 activates the PI3K/AKT signals by activating p-IRS1 Our studies suggest that VEGFB could present a novel strategy for treating NAFLD as a positive factor.

Excessive adiposity in obesity is a significant risk factor for development of type 2 diabetes (T2D), nonalcoholic fatty liver disease, and other cardiometabolic diseases. An unhealthy expansion of adipose tissue (AT) results in reduced adipogenesis, increased adipocyte hypertrophy, adipocyte hypoxia, chronic low-grade inflammation, increased macrophage infiltration, and insulin resistance. This ultimately culminates in AT dysfunction characterized by decreased secretion of antidiabetic adipokines such as adiponectin and adipsin and increased secretion of proinflammatory prodiabetic adipokines including RBP4 and resistin. This imbalance in adipokine secretion alters the physiological state of AT communication with target organs including pancreatic β-cells, heart, and liver. In the pancreatic β-cells, adipokines are known to have a direct effect on insulin secretion, gene expression, cell death, and/or dedifferentiation. For instance, impaired secretion of adipsin, which promotes insulin secretion and β-cell identity, results in β-cell failure and T2D, thus presenting a potential druggable target to improve and/or preserve β-cell function. The cardiac tissue is affected by both the classic white AT-secreted adipokines and the newly recognized brown AT (BAT)-secreted BATokines or lipokines that alter lipid deposition and ventricular function. In the liver, adipokines affect hepatic gluconeogenesis, lipid accumulation, and insulin sensitivity, underscoring the importance of adipose-liver communication in the pathogenesis of nonalcoholic fatty liver disease. In this perspective, we outline what is currently known about the effects of individual adipokines on pancreatic β-cells, liver, and the heart.

Hepatic insulin sensitivity is critical for systemic glucose and lipid homeostasis. The liver is spatially organized into zones in which hepatocytes express distinct metabolic enzymes; however, the functional significance of this zonation to metabolic dysregulation caused by insulin resistance is undetermined. Here, we used CreER mice to selectively disrupt insulin signaling in periportal (PP) and pericentral (PC) hepatocytes. PP-insulin resistance has been suggested to drive combined hyperglycemia and excess lipogenesis in individuals with type 2 diabetes. However, PP-insulin resistance in mice impaired lipogenesis and suppressed high-fat diet (HFD)-induced hepatosteatosis, despite elevated gluconeogenesis and insulin. In contrast, PC-insulin resistance reduced HFD-induced PC steatosis while preserving normal glucose homeostasis, in part by shifting glycolytic metabolism from the liver to the muscle. These results demonstrate distinct roles of insulin in PP versus PC hepatocytes and suggest that PC-insulin resistance might be therapeutically useful to combat hepatosteatosis without compromising glucose homeostasis.

BACKGROUNDWhile saturated fat intake leads to insulin resistance and nonalcoholic fatty liver, Mediterranean-like diets enriched in monounsaturated fatty acids (MUFA) may have beneficial effects. This study examined effects of MUFA on tissue-specific insulin sensitivity and energy metabolism.METHODSA randomized placebo-controlled cross-over study enrolled 16 glucose-tolerant volunteers to receive either oil (OIL, ~1.18 g/kg), rich in MUFA, or vehicle (VCL, water) on 2 occasions. Insulin sensitivity was assessed during preclamp and hyperinsulinemic-euglycemic clamp conditions. Ingestion of 2H2O/acetaminophen was combined with [6,6-2H2]glucose infusion and in vivo 13C/31P/1H/ex vivo 2H-magnet resonance spectroscopy to quantify hepatic glucose and energy fluxes.RESULTSOIL increased plasma triglycerides and oleic acid concentrations by 44% and 66% compared with VCL. Upon OIL intervention, preclamp hepatic and whole-body insulin sensitivity markedly decreased by 28% and 27%, respectively, along with 61% higher rates of hepatic gluconeogenesis and 32% lower rates of net glycogenolysis, while hepatic triglyceride and ATP concentrations did not differ from VCL. During insulin stimulation hepatic and whole-body insulin sensitivity were reduced by 21% and 25%, respectively, after OIL ingestion compared with that in controls.CONCLUSIONA single MUFA-load suffices to induce insulin resistance but affects neither hepatic triglycerides nor energy-rich phosphates. These data indicate that amount of ingested fat, rather than its composition, primarily determines the development of acute insulin resistance.TRIAL REGISTRATIONClinicalTrials.gov NCT01736202.FUNDINGGerman Diabetes Center, German Federal Ministry of Health, Ministry of Culture and Science of the state of North Rhine-Westphalia, German Federal Ministry of Education and Research, German Diabetes Association, German Center for Diabetes Research, Portugal Foundation for Science and Technology, European Regional Development Fund, and Rede Nacional de Ressonancia Magnética Nuclear.

Metabolic dysfunction-associated steatotic liver disease (MASLD) and steatohepatitis (MASH) are associated with a high prevalence of type 2 diabetes (T2D). Individuals with MASLD exhibit insulin resistance (IR) and hyperglycemia, but it is unclear whether hepatic glucose production (HGP) is increased with MASLD severity. We evaluated HGP in a cohort of histologically characterized individuals with MASL/MASH using stable isotope infusion (6,6-

Diet and obesity contribute to insulin resistance and type 2 diabetes, in part via the gut microbiome. To explore the role of gut-derived metabolites in this process, we assessed portal/peripheral blood metabolites in mice with different risks of obesity/diabetes, challenged with a high-fat diet (HFD) + antibiotics. In diabetes/obesity-prone C57BL/6J mice, 111 metabolites were portally enriched and 74 were peripherally enriched, many of which differed in metabolic-syndrome-resistant 129S1/129S6 mice. Vancomycin treatment of HFD-fed C57BL/6J mice modified the microbiome and the portal/peripheral ratio of many metabolites, including upregulating tricarboxylic acid (TCA) cycle-related metabolites, like mesaconate, in portal blood. Treatment of isolated hepatocytes with mesaconate, itaconate, or citraconate improved insulin signaling and transcriptionally regulated genes involved in gluconeogenesis, fatty acid oxidation, and lipogenesis in vitro and in vivo. In humans, citraconate levels are inversely correlated with plasma glucose. Thus, portal versus peripheral metabolites play important roles in mediating effects of the microbiome on hepatic metabolism and the pathogenesis of HFD-related insulin resistance.

CIDEB (cell death-inducing DFF45-like effector B) deficiency is associated with a reduced incidence of metabolic dysfunction-associated steatotic liver disease (MASLD) in humans; however, the underlying mechanism responsible for this protective effect remains unclear. C57BL/6J male mice were fed a high-fat diet (HFD) to recapitulate key aspects of MASLD and hepatic insulin resistance. Cideb knockdown (KD) was achieved using a 2'-O-methoxyethyl (MOE) antisense oligonucleotide (ASO). In vivo rates of hepatic mitochondrial gluconeogenesis and tricarboxylic acid (TCA) cycle flux were assessed by Q-Flux. The Comprehensive Lab Animal Monitoring System (CLAMS) was used to evaluate rates of whole-body energy expenditure. Hepatic and peripheric insulin sensitivity were evaluated using hyperinsulinaemic-euglycaemic clamp studies combined with radio-labelled isotopes. We showed that Cideb ASO treatment increased rates of whole-body energy expenditure by ~25% and decreased hepatic triacylglycerol by ~65% in a HFD mouse model of MASLD compared with the wild-type mice. Cideb KD reduced hepatic fat content, which could mostly be attributed to increased rates of hepatic mitochondrial oxidation, in combination with reduced hepatic lipogenesis. Additionally, Cideb KD ameliorated HFD-induced insulin resistance, which could be attributed to decreased plasma membrane sn-1,2-diacylglycerols (DAGs)-protein kinase C (PKC)ε-insulin receptor kinase (IRK) These findings demonstrate that Cideb KD enhances mitochondrial fat oxidation and reduces hepatic lipogenesis, which in turn mitigates HFD-induced hepatic steatosis and insulin resistance via the plasma membrane sn-1,2-DAGs-PKCε-IRK

Carnitine orotate complex (Godex) has been shown to decrease glycated hemoglobin levels and improve steatosis in patients with type 2 diabetes mellitus with non-alcoholic fatty liver disease. However, the mechanisms of Godex in glucose metabolism remain unclear. Male C57BL/6J mice were divided into four groups: normal-fat diet, high-fat diet, a high-fat diet supplemented with intraperitoneal injection of (500 mg or 2,000 mg/kg/day) Godex for 8 weeks. Computed tomography, indirect calorimetry, and histological analyses including electron microscopy of the liver were performed, and biochemical profiles and oral glucose tolerance test and insulin tolerance test were undertaken. Expressions of genes in the lipid and glucose metabolism, activities of oxidative phosphorylation enzymes, carnitine acetyltransferase, pyruvate dehydrogenase, and acetyl-coenzyme A (CoA)/CoA ratio were evaluated. Godex improved insulin sensitivity and significantly decreased fasting plasma glucose, homeostatic model assessment for insulin resistance, steatosis, and gluconeogenesis, with a marked increase in fatty acid oxidation as well as better use of glucose in high-fat diet-fed mice. It preserved mitochondrial function and ultrastructure, restored oxidative phosphorylation enzyme activities, decreased acetyl-CoA/CoA ratio, and increased carnitine acetyltransferase content and pyruvate dehydrogenase activity. Carnitine acetyltransferase knockdown partially reversed the effects of Godex in liver and in vitro. Godex improved insulin resistance and steatosis by regulating carnitine acetyltransferase in liver in high-fat diet-fed mice.

Insulin resistance (IR) is the central pathophysiological feature in the pathogenesis of metabolic syndrome, obesity, type 2 diabetes mellitus (T2DM), hypertension, and dyslipidemia. As the main active ingredient in Lithocarpus litseifolius [Hance] Chun, previous studies have shown that phlorizin (PHZ) can reduce insulin resistance in the liver. However, the effect of phlorizin on attenuating hepatic insulin resistance has not been fully investigated, and whether this effect is related to AMPK remains unclear. The present study aimed to further investigate the effect of phlorizin on attenuating insulin resistance and the potential action mechanism. Free fatty acids (FFA) were used to induce insulin resistance in HepG2 cells. The effects of phlorizin and FFA on cell viability were detected by MTT analysis. Glucose consumption, glycogen synthesis, intracellular malondialdehyde (MDA), superoxide dismutase (SOD), total cholesterol (TC), and triglyceride (TG) contents were quantified after phlorizin treatment. Glucose uptake and reactive oxygen species (ROS) levels in HepG2 cells were assayed by flow cytometry. Potential targets and signaling pathways for attenuating insulin resistance by phlorizin were predicted by network pharmacological analysis. Moreover, the expression levels of proteins related to the AMPK/PI3K/AKT signaling pathway were detected by western blot. Insulin resistance was successfully induced in HepG2 cells by co-treatment of 1 mM sodium oleate (OA) and 0.5 mM sodium palmitate (PA) for 24 h. Treatment with phlorizin promoted glucose consumption, glucose uptake, and glycogen synthesis and inhibited gluconeogenesis in IR-HepG2 cells. In addition, phlorizin inhibited oxidative stress and lipid accumulation in IR-HepG2 cells. Network pharmacological analysis showed that AKT1 was the active target of phlorizin, and the PI3K/AKT signaling pathway may be the potential action mechanism of phlorizin. Furthermore, western blot results showed that phlorizin ameliorated FFA-induced insulin resistance by activating the AMPK/PI3K/AKT signaling pathway. Phlorizin inhibited oxidative stress and lipid accumulation in IR-HepG2 cells and ameliorated hepatic insulin resistance by activating the AMPK/PI3K/AKT signaling pathway. Our study proved that phlorizin played a role in alleviating hepatic insulin resistance by activating AMPK, which provided experimental evidence for the use of phlorizin as a potential drug to improve insulin resistance.

Defective insulin signalling and dysfunction of the endoplasmic reticulum (ER), driven by excessive lipid accumulation in the liver, is a characteristic feature in the pathogenesis of non-alcoholic fatty liver disease (NAFLD). Thromboxane A TP receptor knockout (TP TXA The TXA

An increasing life expectancy in society has burdened healthcare systems substantially because of the rising prevalence of age-related metabolic diseases. This study compared the effects of animal protein hydrolysate (APH) and casein on metabolic diseases using aged mice. Eight-week-old and 50-week-old C57BL/6J mice were used as the non-aged (YC group) and aged controls (NC group), respectively. The aged mice were divided randomly into 3 groups (NC, low-APH [LP], and high-APH [HP] and fed each experimental diet for 12 weeks. In the LP and HP groups, casein in the AIN-93G diet was substituted with 16 kcal% and 24 kcal% APH, respectively. The mice were sacrificed when they were 63-week-old, and plasma and hepatic lipid, white adipose tissue weight, hepatic glucose, lipid, and antioxidant enzyme activities, immunohistochemistry staining, and mRNA expression related to the glucose metabolism on liver and muscle were analyzed. Supplementation of APH in aging mice resulted in a significant decrease in visceral fat (epididymal, perirenal, retroperitoneal, and mesenteric fat) compared to the negative control (NC) group. The intraperitoneal glucose tolerance test and area under the curve analysis revealed insulin resistance in the NC group, which was alleviated by APH supplementation. APH supplementation reduced hepatic gluconeogenesis and increased glucose utilization in the liver and muscle. Furthermore, APH supplementation improved hepatic steatosis by reducing the hepatic fatty acid and phosphatidate phosphatase activity while increasing the hepatic carnitine palmitoyltransferase activity. Furthermore, in the APH supplementation groups, the red blood cell (RBC) thiobarbituric acid reactive substances and hepatic H APH supplementation reduced visceral fat accumulation and alleviated obesity-related metabolic diseases, including insulin resistance and hepatic steatosis, in aged mice. Therefore, high-quality animal protein APH that reduces the molecular weight and enhances the protein digestibility-corrected amino acid score has potential as a dietary supplement for healthy aging.

The global prevalence of overweight and obesity has risen sharply over the past few decades as a result of excess calorie intake and sedentary lifestyles. Obesity increases the risk for various metabolic disorders, such as hyperlipidemia, fatty liver disease, and diabetes mellitus. Isothiocyanates, which are abundant in cruciferous vegetables, have been shown to exhibit anticancer, anti-inflammatory, and antioxidant properties. However, the efficacy of benzyl isothiocyanate (BITC) in preventing the adverse effects of obesity, such as hepatic steatosis and insulin resistance, remains uncertain. To address this knowledge gap, we assessed whether BITC protects against hepatic insulin resistance by using primary mouse hepatocytes and AML12 cells treated with palmitic acid (PA) and mice fed a high-fat diet supplemented with cholesterol and cholic acid (HFCCD). We found that the impairments in insulin sensitivity caused by PA, such as decreases in the phosphorylation of insulin receptor substrate (IRS) 1 (Tyr608), Akt, glycogen synthase kinase (GSK) 3β, and FOXO1 and increases in the expression of glucose-6-phosphatase (G6Pase) and phosphoenolpyruvate carboxykinase 1 (PEPCK) mRNA in hepatocytes, were mitigated by pretreatment with BITC. BITC also attenuated PA-induced hepatic lipid accumulation and reactive oxygen species production. In vivo, BITC significantly reduced blood glucose levels and the HOMA-IR and inhibited hepatic lipid accumulation, IRS1 phosphorylation at Ser307, and G6Pase and PEPCK expression compared with that in mice fed the HFCCD alone. These results show that BITC ameliorates the lipotoxicity associated with insulin resistance by activating the IR/IRS/Akt/FOXO1 and GSK3β pathways, which leads to decreased gluconeogenesis and increased glycogen synthesis.

Hepatic insulin resistance (IR) is often said to be "pathway-selective" with preserved insulin stimulation of

The paradoxical action of insulin on hepatic glucose metabolism and lipid metabolism in the insulin-resistant state has been of much research interest in recent years. Generally, insulin resistance would promote hepatic gluconeogenesis and demote hepatic

Both glucose and triglyceride production are increased in type 2 diabetes and nonalcoholic fatty liver disease (NAFLD). For decades, the leading hypothesis to explain these paradoxical observations has been selective hepatic insulin resistance wherein insulin drives de novo lipogenesis (DNL) while failing to suppress glucose production. Here, we aimed to test this hypothesis in humans. We recruited obese subjects who met criteria for bariatric surgery with ( Compared with subjects without NAFLD, those with NAFLD demonstrated impaired insulin-mediated suppression of glucose production and attenuated-not increased-glucose-stimulated/high-insulin lipogenesis. Fructose-stimulated/low-insulin lipogenesis was intact. Hepatocellular insulin signaling, assessed for the first time in humans, exhibited a proximal block in insulin-resistant subjects: Signaling was attenuated from the level of the insulin receptor through both glucose and lipogenesis pathways. The carbohydrate-regulated lipogenic transcription factor Acute increases in lipogenesis in humans with NAFLD are not explained by altered molecular regulation of lipogenesis through a paradoxical increase in lipogenic insulin action; rather, increases in lipogenic substrate availability may be the key.

Metabolic diseases are a worldwide health problem. Insulin resistance (IR) is their distinctive hallmark. For their study, animal models that provide reliable information are necessary, permitting the analysis of the cluster of abnormalities that conform to it, its progression, and time-dependent molecular modifications. We aimed to develop an IR model by exogenous insulin administration. The effective dose of insulin glargine to generate hyperinsulinemia but without hypoglycemia was established. Then, two groups (control and insulin) of male Wistar rats of 100 g weight were formed. The selected dose (4 U/kg) was administered for 15, 30, 45, and 60 days. Zoometry, a glucose tolerance test, insulin response, IR, and the serum lipid profile were assessed. We evaluated insulin signaling, glycogenesis and lipogenesis, redox balance, and inflammation in the liver. Results showed an impairment of glucose tolerance, dyslipidemia, hyperinsulinemia, and peripheral and time-dependent selective IR. At the hepatic level, insulin signaling was impaired, resulting in reduced hepatic glycogen levels and triglyceride accumulation, an increase in the ROS level with MAPK-ERK1/2 response, and mild pro-oxidative microenvironmental sustained by MT, GSH, and GR activity. Hepatic IR coincides with additions in MAPK-p38, NF-κB, and zoometric changes. In conclusion, daily insulin glargine administration generated a progressive IR model. At the hepatic level, the IR was combined with oxidative conditions but without inflammation.

Fasting hyperglycemia and hypertriglyceridemia are characteristic of insulin resistance (IR) and rodent work has suggested this may be due to selective hepatic IR, defined by increased hepatic gluconeogenesis and de novo lipogenesis (DNL), but this has not been shown in humans. Cross-sectional study in men and women across a range of adiposity. Medication-free participants (n = 177) were classified as normoinsulinemic (NI) or hyperinsulinemic (HI) and as having low (LF) or high (HF) liver fat content measured by magnetic resonance spectroscopy. Fractional gluconeogenesis (frGNG) and hepatic DNL were measured using stable isotope tracer methodology following an overnight fast. Although HI and HF groups had higher fasting plasma glucose and triglyceride concentrations when compared to NI and LF groups respectively, there was no difference in frGNG. However, HF participants tended to have lower frGNG than LF participants. HI participants had higher DNL compared to NI participants but there was no difference observed between liver fat groups. Taken together, we found no metabolic signature of selective hepatic IR in fasting humans. DNL may contribute to hypertriglyceridemia in individuals with HI but not those with HF. Glycogenolysis and systemic glucose clearance may have a larger contribution to fasting hyperglycemia than gluconeogenesis, especially in those with HF, and these pathways should be considered for therapeutic targeting.

Short-chain fatty acids (SCFAs) are the main products of dietary fiber fermentation and are believed to drive the fiber-related prevention of the metabolic syndrome. Here we show that dietary SCFAs induce a peroxisome proliferator-activated receptor-γ (PPARγ)-dependent switch from lipid synthesis to utilization. Dietary SCFA supplementation prevented and reversed high-fat diet-induced metabolic abnormalities in mice by decreasing PPARγ expression and activity. This increased the expression of mitochondrial uncoupling protein 2 and raised the AMP-to-ATP ratio, thereby stimulating oxidative metabolism in liver and adipose tissue via AMPK. The SCFA-induced reduction in body weight and stimulation of insulin sensitivity were absent in mice with adipose-specific disruption of PPARγ. Similarly, SCFA-induced reduction of hepatic steatosis was absent in mice lacking hepatic PPARγ. These results demonstrate that adipose and hepatic PPARγ are critical mediators of the beneficial effects of SCFAs on the metabolic syndrome, with clearly distinct and complementary roles. Our findings indicate that SCFAs may be used therapeutically as cheap and selective PPARγ modulators.

Pathogenesis of insulin resistance in leptin-deficient ob/ob mice is obscure. In another form of diet-dependent obesity, high-fat-fed mice, hepatic insulin resistance involves ceramide-induced activation of atypical protein kinase C (aPKC), which selectively impairs protein kinase B (Akt)-dependent forkhead box O1 protein (FoxO1) phosphorylation on scaffolding protein, 40 kDa WD(tryp-x-x-asp)-repeat propeller/FYVE protein (WD40/ProF), thereby increasing gluconeogenesis. Resultant hyperinsulinemia activates hepatic Akt and mammalian target of rapamycin C1, and further activates aPKC; consequently, lipogenic enzyme expression increases, and insulin signaling in muscle is secondarily impaired. Here, in obese minimally-diabetic ob/ob mice, hepatic ceramide and aPKC activity and its association with WD40/ProF were increased. Hepatic Akt activity was also increased, but Akt associated with WD40/ProF was diminished and accounted for reduced FoxO1 phosphorylation and increased gluconeogenic enzyme expression. Most importantly, liver-selective inhibition of aPKC decreased aPKC and increased Akt association with WD40/ProF, thereby restoring FoxO1 phosphorylation and reducing gluconeogenic enzyme expression. Additionally, lipogenic enzyme expression diminished, and insulin signaling in muscle, glucose tolerance, obesity, hepatosteatosis, and hyperlipidemia improved. In conclusion, hepatic ceramide accumulates in response to CNS-dependent dietary excess irrespective of fat content; hepatic insulin resistance is prominent in ob/ob mice and involves aPKC-dependent displacement of Akt fromWD40/ProF and subsequent impairment of FoxO1 phosphorylation and increased expression of hepatic gluconeogenic and lipogenic enzymes; and hepatic alterations diminish insulin signaling in muscle.

Liver X receptors (LXRs) regulate lipogenesis and inflammation, but their contribution to the metabolic syndrome is unclear. We show that LXRs modulate key aspects of the metabolic syndrome in mice. LXRαβ-deficient-ob/ob (LOKO) mice remain obese but show reduced hepatic steatosis and improved insulin sensitivity compared to ob/ob mice. Impaired hepatic lipogenesis in LOKO mice is accompanied by reciprocal increases in adipose lipid storage, reflecting tissue-selective effects on the SREBP, PPARγ, and ChREBP lipogenic pathways. LXRs are essential for obesity-driven SREBP-1c and ChREBP activity in liver, but not fat. Furthermore, loss of LXRs in obesity promotes adipose PPARγ and ChREBP-β activity, leading to improved insulin sensitivity. LOKO mice also exhibit defects in β cell mass and proliferation despite improved insulin sensitivity. Our data suggest that sterol sensing by LXRs in obesity is critically linked with lipid and glucose homeostasis and provide insight into the complex relationships between LXR and insulin signaling.

Leptin is essential for energy homeostasis and regulation of food intake. Patients with congenital generalized lipodystrophy (CGL) due to mutations in 1-acylglycerol-3-phosphate-O-acyltransferase 2 (AGPAT2) and the CGL murine model (Agpat2(-/-) mice) both have severe insulin resistance, diabetes mellitus, hepatic steatosis, and low plasma leptin levels. In this study, we show that continuous leptin treatment of Agpat2(-/-) mice for 28 days reduced plasma insulin and glucose levels and normalized hepatic steatosis and hypertriglyceridemia. Leptin also partially, but significantly, reversed the low plasma thyroxine and high corticosterone levels found in Agpat2(-/-) mice. Levels of carbohydrate response element binding protein (ChREBP) were reduced, whereas lipogenic gene expression were increased in the livers of Agpat2(-/-) mice, suggesting that deregulated ChREBP contributed to the development of fatty livers in these mice and that this transcription factor is a target of leptin's beneficial metabolic action. Leptin administration did not change hepatic fatty acid oxidation enzymes mRNA levels in Agpat2(-/-) mice. The selective deletion of leptin receptors only in hepatocytes did not prevent the positive metabolic actions of leptin in Agpat2(-/-) mice, supporting the notion that the majority of metabolic actions of leptin are dependent on its action in nonhepatocyte cells and/or the central nervous system.

Insulin is critical for the regulation of de novo fatty acid synthesis, which converts glucose to lipid in the liver. However, how insulin signals are transduced into the cell and then regulate lipogenesis remains to be fully understood. Here, we identified CREB/ATF bZIP transcription factor (CREBZF) of the activating transcription factor/cAMP response element-binding protein (ATF/CREB) gene family as a key regulator for lipogenesis through insulin-Akt signaling. Insulin-induced gene 2a (Insig-2a) decreases during refeeding, allowing sterol regulatory element binding protein 1c to be processed to promote lipogenesis; but the mechanism of reduction is unknown. We show that Insig-2a inhibition is mediated by insulin-induced CREBZF. CREBZF directly inhibits transcription of Insig-2a through association with activating transcription factor 4. Liver-specific knockout of CREBZF causes an induction of Insig-2a and Insig-1 and resulted in repressed lipogenic program in the liver of mice during refeeding or upon treatment with streptozotocin and insulin. Moreover, hepatic CREBZF deficiency attenuates hepatic steatosis in high-fat, high-sucrose diet-fed mice. Importantly, expression levels of CREBZF are increased in livers of diet-induced insulin resistance or genetically obese ob/ob mice and humans with hepatic steatosis, which may underscore the potential role of CREBZF in the development of sustained lipogenesis in the liver under selective insulin resistance conditions. These findings uncover an unexpected mechanism that couples changes in extracellular hormonal signals to hepatic lipid homeostasis; disrupting CREBZF function may have the therapeutic potential for treating fatty liver disease and insulin resistance. (Hepatology 2018).

The development of hepatic insulin resistance (IR) is a critical factor in developing type 2 diabetes (T2D), where insulin fails to inhibit hepatic glucose production but retains its capacity to promote hepatic de novo lipogenesis leading to hyperglycemia and hypertriglyceridemia. Improving insulin sensitivity can be effective in preventing and treating T2D. However, selective control of glucose and lipid synthesis has been difficult. It is known that excess white adipose tissue is detrimental to insulin sensitivity, whereas brown adipose tissue transplantation can restore it in diabetic mice. However, challenges remain in our understanding of liver-adipose communication because the confounding effects of hypothalamic regulation of metabolic function cannot be ruled out in previous studies. There is a lack of

The liver plays a central role in controlling glucose and lipid metabolism. IDH2, a mitochondrial protein, controls TCA cycle flux. However, its role in regulating metabolism in obesity is still unclear. This study intends to investigate the impact of hepatic IDH2 expression on overnutrition-regulated glucose and lipid metabolism. Hepatic IDH2 was knocked-out in mice by the approach of CRISPR-Cas9. Mice were subjected to starvation and refeeding for hepatic glucose and lipid studies in vivo. Primary hepatocytes and mouse normal liver cell line, AML12 cells were used for experiments in vitro. This study found that IDH2 protein levels were elevated in the livers of obese people and mice with high-fat diet consumption or hepatic steatosis. Liver IDH2-deletion mice (IDH2 Elevated hepatic IDH2 under over-nutrition state contributes to elevated gluconeogenesis and glycogen synthesis. Inhibition of IDH2 in the liver could be a potential therapeutic target for obesity and diabetes.

Neprilysin is a peptidase that cleaves glucoregulatory peptides, including glucagon-like peptide-1 (GLP-1) and cholecystokinin (CCK). Some studies suggest that its inhibition in diabetes and/or obesity improves glycemia, and that this is associated with enhanced insulin secretion, glucose tolerance and insulin sensitivity. Whether reduced neprilysin activity also improves hepatic glucose metabolism has not been explored. We sought to determine whether genetic deletion of neprilysin suppresses hepatic glucose production (HGP) in high fat-fed mice. Nep

Hepatic glucose production (HGP) is an important component of glucose homeostasis, and deregulated HGP, particularly through gluconeogenesis, contributes to hyperglycemia and pathology of type-2 diabetes (T2D). It has been shown that the gluconeogenic gene expression is governed primarily by the transcription factor cAMP-response element (CRE)-binding protein (CREB) and its coactivator, CREB-regulated transcriptional coactivator 2 (CRTC2). Recently, we have discovered that Sam68, an adaptor protein and Src kinase substrate, potently promotes hepatic gluconeogenesis by promoting CRTC2 stability; however, the detailed mechanisms remain unclear. Here we show that in response to glucagon, Sam68 increases CREB/CRTC2 transactivity by interacting with CRTC2 in the CREB/CRTC2 complex and occupying the CRE motif of promoters, leading to gluconeogenic gene expression and glucose production. In hepatocytes, glucagon promotes Sam68 nuclear import, whereas insulin elicits its nuclear export. Furthermore, ablation of Sam68 in hepatocytes protects mice from high-fat diet (HFD)-induced hyperglycemia and significantly increased hepatic and peripheral insulin sensitivities. Thus, hepatic Sam68 potentiates CREB/CRTC2-mediated glucose production, contributes to the pathogenesis of insulin resistance, and may serve as a therapeutic target for T2D.

Insulin resistance precedes metabolic syndrome which increases the risk of type 2 diabetes and cardiovascular disease. However, there is a lack of safe and long-lasting methods for the prevention and treatment of insulin resistance. Gut microbiota dysbiosis can lead to insulin resistance and associated glucose and lipid metabolic dysfunction. Thus, the role of gut microbiota in metabolic diseases has garnered growing interest. Curcumin, the active ingredient of tropical plant Curcuma longa, has excellent prospects for the prevention and treatment of metabolic diseases. However, due to the extremely low bioavailability of curcumin, the mechanisms by which curcumin increases insulin sensitivity remains to be elucidated. This study aimed to elucidate the role of gut microbiota in mediating the effects of curcumin on improving insulin sensitivity in high-fat diet (HFD)-fed mice. Glucose, insulin, and pyruvate tolerance were tested and hepatic triglycerides (TGs) content was measured in HFD-fed mice treated with curcumin (100 mg kg Curcumin ameliorated HFD-induced glucose intolerance, insulin resistance, pyruvate intolerance, and hepatic TGs accumulation, while these effects were mediated by gut microbiota. Curcumin induced insulin-stimulated Akt phosphorylation levels in insulin-regulated peripheral tissues. The inhibitory effects of curcumin on the expressions of genes involved in hepatic gluconeogenesis and de novo lipogenesis were dependent on gut microbiota. Meanwhile, curcumin upregulated the expression of fibroblast growth factor 15 (FGF15) through gut microbiota. The effects of curcumin on promoting insulin sensitivity were dependent on gut microbiota in HFD-fed mice. Moreover, curcumin at least partly exerted its effects on increasing insulin sensitivity via FGF15 upregulation. This study provided new ideas on nutritional manipulations of gut microbiota for the treatment of metabolic diseases.

Riligustilide (RG), one of the dimeric phthalides of Angelica sinensis and Ligusticum chuanxiong, was confirmed effective against many diseases. However, its effects on type 2 diabetes mellitus (T2DM) and the underlying molecular mechanisms have not been clearly elucidated yet. The current study was designed to investigate the hypoglycemic potential by which RG affects the pathogenesis of T2DM. Comprehensive insights into the effects and underlying molecular mechanisms of RG on attenuating aberrant metabolism of glucose were determined in high-fat diet-induced T2DM mice and insulin-resistant (IR) HepG2 cells. In high-fat diet-induced C57BL/6J mice, RG administration significantly reduced hyperglycemia, decreased hyperinsulinemia, and ameliorated glucose intolerance. Mechanistically, RG activated PPARγ and insulin signaling pathway to improve insulin sensitivity, and increase glucose uptake as well as glycogenesis. In addition, RG also upregulated AMPK-TORC2-FoxO1 axis to attenuate gluconeogenesis in vivo and in vitro. According to the findings, RG may be a promising candidate for the treatment of T2DM.

Because of the paucity of information regarding metabolic effects of advanced glycation end products (AGEs) on liver, we evaluated effects of AGEs chronic administration in (1) insulin sensitivity; (2) hepatic expression of genes involved in AGEs, glucose and fat metabolism, oxidative stress and inflammation and; (3) hepatic morphology and glycogen content. Rats received intraperitoneally albumin modified (AlbAGE) or not by advanced glycation for 12 weeks. AlbAGE induced whole-body insulin resistance concomitantly with increased hepatic insulin sensitivity, evidenced by activation of AKT, inactivation of GSK3, increased hepatic glycogen content, and decreased expression of gluconeogenesis genes. Additionally there was reduction in hepatic fat content, in expression of lipogenic, pro-inflamatory and pro-oxidative genes and increase in reactive oxygen species and in nuclear expression of NRF2, a transcription factor essential to cytoprotective response. Although considered toxic, AGEs become protective when administered chronically, stimulating AKT signaling, which is involved in cellular defense and insulin sensitivity.

Activation of the sympathetic nervous system (SNS) constitutes a putative mechanism of obesity-induced insulin resistance. Thus, we hypothesized that inhibiting the SNS by using renal denervation (RDN) will improve insulin sensitivity (S

Palmitic acid esters of hydroxy stearic acids (PAHSAs) are bioactive lipids with antiinflammatory and antidiabetic effects. PAHSAs reduce ambient glycemia and improve glucose tolerance and insulin sensitivity in insulin-resistant aged chow- and high-fat diet-fed (HFD-fed) mice. Here, we aimed to determine the mechanisms by which PAHSAs improve insulin sensitivity. Both acute and chronic PAHSA treatment enhanced the action of insulin to suppress endogenous glucose production (EGP) in chow- and HFD-fed mice. Moreover, chronic PAHSA treatment augmented insulin-stimulated glucose uptake in glycolytic muscle and heart in HFD-fed mice. The mechanisms by which PAHSAs enhanced hepatic insulin sensitivity included direct and indirect actions involving intertissue communication between adipose tissue and liver. PAHSAs inhibited lipolysis directly in WAT explants and enhanced the action of insulin to suppress lipolysis during the clamp in vivo. Preventing the reduction of free fatty acids during the clamp with Intralipid infusion reduced PAHSAs' effects on EGP in HFD-fed mice but not in chow-fed mice. Direct hepatic actions of PAHSAs may also be important, as PAHSAs inhibited basal and glucagon-stimulated EGP directly in isolated hepatocytes through a cAMP-dependent pathway involving Gαi protein-coupled receptors. Thus, this study advances our understanding of PAHSA biology and the physiologic mechanisms by which PAHSAs exert beneficial metabolic effects.

No abstract

Sesamol is the major bioactive constituent isolated from sesame seeds and has a variety of bioactivities. However, its role and mechanism in liver insulin resistance remain unknown. The current study was designed to investigate the underlying adipose-liver crosstalk mechanism of sesamol ameliorating hepatic insulin sensitivity. The therapeutic effect of sesamol was evaluated in high-fat diet (HFD)-fed C57BL/6 J mice (100 mg/kg for 8 weeks, XYGW-2021-75) and the mechanism was further explored in HepG2 cells with/without adiponectin and adenosine 5 '-monophosphate-activated protein kinase (AMPK) inhibitor administration. Our in vivo data showed that sesamol reduced hepatic insulin resistance in HFD-induced mice with obesity by modulating protein expression levels of glycogen synthase (GS), phosphoenolpyruvate carboxykinase (PEPCK) and protein kinase B (AKT). Moreover, sesamol not only increased the serum and adipose tissue adiponectin concentrations but also activated the phosphorylation of AMPK in the liver. Furthermore, in vitro studies using recombinant human adiponectin and an AMPK inhibitor revealed that adiponectin and sesamol have a synergic impact on increasing glycogenesis and reducing gluconeogenesis, of which the effects could be attenuated by the AMPK inhibitor. Taken together, our results suggested that sesamol stimulated adiponectin secretion from adipocytes, whereby exhibited a co-effect on activating the downstream signal of hepatic AMPK, resulting in the alleviation of hepatic insulin resistance. The novel findings of sesamol on hepatic effects provides prospective therapeutic approaches to treat insulin resistance.

Obesity and obesity-related insulin resistance have been a research hotspot. Pituitary adenylate cyclase activating polypeptide (PACAP) has emerged as playing a significant role in energy metabolism, holding promising potential for attenuating insulin resistance. However, the precise mechanism is not fully understood. Palmitic acid and a high-fat diet (HFD) were used to establish insulin resistance model in Alpha mouse liver 12 cell line and C57BL/6 mice, respectively. Subsequently, we assessed the effects of PACAP both in vivo and in vitro. Lentivirus vectors were used to explore the signaling pathway through which PACAP may ameliorate insulin resistance. PACAP was found to selectively bind to the PACAP type I receptor receptor and ameliorate insulin resistance, which was characterized by increased glycogen synthesis and the suppression of gluconeogenesis in the insulin-resistant cell model and HFD-fed mice. These effects were linked to the activation of the Fas apoptotic inhibitory molecule/rapamycin-insensitive companion of mammalian target of rapamycin/RAC-alpha serine/threonine-protein kinase (FAIM/Rictor/AKT) axis. Furthermore, PACAP ameliorated insulin resistance by increasing solute carrier family 2, facilitated glucose transporter members 2/4 and inhibiting gluconeogenesis-related proteins glucose 6-phosphatase catalytic subunit 1 and phosphoenolpyruvate carboxykinase 2 expression. Meanwhile, the phosphorylation of hepatic AKT/glycogen synthase kinase 3β was promoted both in vivo and in vitro by PACAP. Additionally, PACAP treatment decreased body weight, food intake and blood glucose levels in obese mice. Our study shows that PACAP ameliorated insulin resistance through the FAIM/Rictor/AKT axis, presenting it as a promising drug candidate for the treatment of obesity-related insulin resistance.

Our previous studies demonstrated that peroxisome proliferator-activated receptor α (PPARα) activation reduces weight gain and improves insulin sensitivity in obese mice. Since excess lipid accumulation in non-adipose tissues is suggested to be responsible for the development of insulin resistance, this study was undertaken to examine whether the lemon balm extract ALS-L1023 regulates hepatic lipid accumulation, obesity, and insulin resistance and to determine whether its mechanism of action involves PPARα. Administration of ALS-L1023 to high-fat-diet-induced obese mice caused reductions in body weight gain, visceral fat mass, and visceral adipocyte size without changes of food consumption profiles. ALS-L1023 improved hyperglycemia, hyperinsulinemia, glucose and insulin tolerance, and normalized insulin-positive β-cell area in obese mice. ALS-L1023 decreased hepatic lipid accumulation and concomitantly increased the expression of PPARα target genes responsible for fatty acid β-oxidation in livers. In accordance with the in vivo data, ALS-L1023 reduced lipid accumulation and stimulated PPARα reporter gene expression in HepG2 cells. These effects of ALS-L1023 were comparable to those of the PPARα ligand fenofibrate, while the PPARα antagonist GW6471 inhibited the actions of ALS-L1023 on lipid accumulation and PPARα luciferase activity in HepG2 cells. Higher phosphorylated protein kinase B (pAkt)/Akt ratios and lower expression of gluconeogenesis genes were observed in the livers of ALS-L1023-treated mice. These results indicate that ALS-L1023 may inhibit obesity and improve insulin sensitivity in part through inhibition of hepatic lipid accumulation via hepatic PPARα activation.

The wake-active orexin system plays a central role in the dynamic regulation of glucose homeostasis. Here we show orexin receptor type 1 and 2 are predominantly expressed in dorsal raphe nucleus-dorsal and -ventral, respectively. Serotonergic neurons in ventral median raphe nucleus and raphe pallidus selectively express orexin receptor type 1. Inactivation of orexin receptor type 1 in serotonin transporter-expressing cells of mice reduced insulin sensitivity in diet-induced obesity, mainly by decreasing glucose utilization in brown adipose tissue and skeletal muscle. Selective inactivation of orexin receptor type 2 improved glucose tolerance and insulin sensitivity in obese mice, mainly through a decrease in hepatic gluconeogenesis. Optogenetic activation of orexin neurons in lateral hypothalamus or orexinergic fibers innervating raphe pallidus impaired or improved glucose tolerance, respectively. Collectively, the present study assigns orexin signaling in serotonergic neurons critical, yet differential orexin receptor type 1- and 2-dependent functions in the regulation of systemic glucose homeostasis.

PTEN is a dual lipid/protein phosphatase, downregulated in steatotic livers with obesity or HCV infection. Liver-specific PTEN knockout (LPTEN KO) mice develop steatosis, inflammation/fibrosis and hepatocellular carcinoma with aging, but surprisingly also enhanced glucose tolerance. This study aimed at understanding the mechanisms by which hepatic PTEN deficiency improves glucose tolerance, while promoting fatty liver diseases. Control and LPTEN KO mice underwent glucose/pyruvate tolerance tests and euglycemic-hyperinsulinemic clamps. Body fat distribution was assessed by EchoMRI, CT-scan and dissection analyses. Primary/cultured hepatocytes and insulin-sensitive tissues were analysed ex vivo. PTEN deficiency in hepatocytes led to steatosis through increased fatty acid (FA) uptake and de novo lipogenesis. Although LPTEN KO mice exhibited hepatic steatosis, they displayed increased skeletal muscle insulin sensitivity and glucose uptake, as assessed by euglycemic-hyperinsulinemic clamps. Surprisingly, white adipose tissue (WAT) depots were also drastically reduced. Analyses of key enzymes involved in lipid metabolism further indicated that FA synthesis/esterification was decreased in WAT. In addition, Ucp1 expression and multilocular lipid droplet structures were observed in this tissue, indicating the presence of beige adipocytes. Consistent with a liver to muscle/adipocyte crosstalk, the expression of liver-derived circulating factors, known to impact on muscle insulin sensitivity and WAT homeostasis (e.g. FGF21), was modulated in LPTEN KO mice. Although steatosis develops in LPTEN KO mice, PTEN deficiency in hepatocytes promotes a crosstalk between liver and muscle, as well as adipose tissue, resulting in enhanced insulin sensitivity, improved glucose tolerance and decreased adiposity.

Insulin resistance (IR) is the key pathological basis of many metabolic disorders. Lack of asialoglycoprotein receptor 1 (ASGR1) decreased the serum lipid levels and reduced the risk of coronary artery disease. However, whether ASGR1 also participates in the regulatory network of insulin sensitivity and glucose metabolism remains unknown. The constructed ASGR1 knockout mice and ASGR1-/- HepG2 cell lines were used to establish the animal model of metabolic syndrome and the IR cell model by high-fat diet (HFD) or drug induction, respectively. Then we evaluated the glucose metabolism and insulin signaling in vivo and in vitro. ASGR1 deficiency ameliorated systemic IR in mice fed with HFD, evidenced by improved insulin intolerance, serum insulin, and homeostasis model assessment of IR index, mainly contributed from increased insulin signaling in the liver, but not in muscle or adipose tissues. Meanwhile, the insulin signal transduction was significantly enhanced in ASGR1-/- HepG2 cells. By transcriptome analyses and comparison, those differentially expressed genes between ASGR1 null and wild type were enriched in the insulin signal pathway, particularly in phosphoinositide 3-kinase-AKT signaling. Notably, ASGR1 deficiency significantly reduced hepatic gluconeogenesis and glycogenolysis. The ASGR1 deficiency was consequentially linked with improved hepatic insulin sensitivity under metabolic stress, hepatic IR was the core factor of systemic IR, and overcoming hepatic IR significantly relieved the systemic IR. It suggests that ASGR1 is a potential intervention target for improving systemic IR in metabolic disorders.

Prolyl hydroxylase domain (PHD) enzymes change HIF activity according to oxygen signal; whether it is regulated by other physiological conditions remains largely unknown. Here, we report that PHD3 is induced by fasting and regulates hepatic gluconeogenesis through interaction and hydroxylation of CRTC2. Pro129 and Pro615 hydroxylation of CRTC2 following PHD3 activation is necessary for its association with cAMP-response element binding protein (CREB) and nuclear translocation, and enhanced binding to promoters of gluconeogenic genes by fasting or forskolin. CRTC2 hydroxylation-stimulated gluconeogenic gene expression is independent of SIK-mediated phosphorylation of CRTC2. Liver-specific knockout of PHD3 (PHD3 LKO) or prolyl hydroxylase-deficient knockin mice (PHD3 KI) show attenuated fasting gluconeogenic genes, glycemia, and hepatic capacity to produce glucose during fasting or fed with high-fat, high-sucrose diet. Importantly, Pro615 hydroxylation of CRTC2 by PHD3 is increased in livers of fasted mice, diet-induced insulin resistance or genetically obese ob/ob mice, and humans with diabetes. These findings increase our understanding of molecular mechanisms linking protein hydroxylation to gluconeogenesis and may offer therapeutic potential for treating excessive gluconeogenesis, hyperglycemia, and type 2 diabetes.

Liver-specific thyroid hormone receptor-β (TRβ)-specific agonists are potent lipid-lowering drugs that also hold promise for treating nonalcoholic fatty liver disease and hepatic insulin resistance. We investigated the effect of two TRβ agonists (GC-1 and KB-2115) in high-fat-fed male Sprague-Dawley rats treated for 10 days. GC-1 treatment reduced hepatic triglyceride content by 75%, but the rats developed fasting hyperglycemia and hyperinsulinemia, attributable to increased endogenous glucose production (EGP) and diminished hepatic insulin sensitivity. GC-1 also increased white adipose tissue lipolysis; the resulting increase in glycerol flux may have contributed to the increase in EGP. KB-2115, a more TRβ- and liver-specific thyromimetic, also prevented hepatic steatosis but did not induce fasting hyperglycemia, increase basal EGP rate, or diminish hepatic insulin sensitivity. Surprisingly, insulin-stimulated peripheral glucose disposal was diminished because of a decrease in insulin-stimulated skeletal muscle glucose uptake. Skeletal muscle insulin signaling was unaffected. Instead, KB-2115 treatment was associated with a decrease in GLUT4 protein content. Thus, although both GC-1 and KB-2115 potently treat hepatic steatosis in fat-fed rats, they each worsen insulin action via specific and discrete mechanisms. The development of future TRβ agonists must consider the potential adverse effects on insulin sensitivity.

Hepatic gluconeogenesis makes a significant contribution to the pathogenesis of obesity and its related insulin resistance. Cystathionine γ-lyase (CSE; also cystathionase), a principal hydrogen sulfide (H

Components of the adipose tissue (AT) extracellular matrix (ECM) are recently discovered contributors to obesity-related cardiometabolic disease. We identified increased adipocyte expression of ECM-related clusterin (apolipoprotein J) in obese versus lean women by microarray. Our objective was to determine We validated increased clusterin expression in adipocytes from a separate group of 18 lean and 54 obese individuals. The relationship of clusterin gene expression and plasma clusterin with IR, cardiovascular biomarkers, and risk of cardiovascular disease (CVD) was then determined. Further investigations in human cultured cells and in aged LDLR SAd clusterin correlated with IR, multiple CVD biomarkers, and CVD risk, independent of traditional risk factors. Circulating human clusterin exhibited similar associations. In human adipocytes, palmitate enhanced clusterin secretion, and in human hepatocytes, clusterin attenuated insulin signaling and APOA1 expression and stimulated hepatic gluconeogenesis. LRP2 (megalin), a clusterin receptor, highly expressed in liver, mediated these effects, which were inhibited by LRP2 siRNA. In response to Western diet feeding, an increase in adipocyte clusterin expression was associated with a progressive increase in liver fat, steatohepatitis, and fibrosis in aged LDLR Adipocyte-derived clusterin is a novel ECM-related protein linking cardiometabolic disease and obesity through its actions in the liver.

Elevated free fatty acids (FFAs) are fundamental to the pathogenesis of hepatic insulin resistance. However, the molecular mechanisms of insulin resistance remain not completely understood. Transcriptional dysregulation, post-transcriptional modifications and protein degradation contribute to the pathogenesis of insulin resistance. Poly(C) binding proteins (PCBPs) are RNA-binding proteins that are involved in post-transcriptional control pathways. However, there are little studies about the roles of PCBPs in insulin resistance. PCBP2 is the member of the RNA-binding proteins and is thought to participate in regulating hypoxia inducible factor-1 (HIF-1α) and signal transducers and activators of transcription (STAT) pathway which are involved in regulating insulin signaling pathway. Here, we investigated the influence of PCBP2 on hepatic insulin resistance. We showed that the protein and mRNA levels of PCBP2 were down-regulated under insulin-resistant conditions. In addition, we showed that over-expression of PCBP2 ameliorates palmitate (PA)-induced insulin resistance, which was indicated by elevated phosphorylation of protein kinase B (AKT) and glycogen synthase kinase 3β (GSK3β). We also found that over-expression of PCBP2 inhibits HIF1α and STAT3 pathway. Furthermore, glucose uptake was found to display a similar tendency with the phosphorylation of Akt. The expressions of phosphoenolpyruvate carboxykinase (PEPCK) and glucose-6-phosphatase (G6Pase), two key gluconeogenic enzymes, were down-regulated following Over-expression of PCBP2. Accordingly, PA-induced intracellular lipid accumulation was suppressed in over-expression of PCBP2 HepG2 cells. In addition, we found that over-expression of PCBP2 inhibits HIF1α and STAT3 pathway. Our results demonstrate that PCBP2 was involved in hepatic insulin sensitivity might via HIF-1α and STAT3 pathway in HepG2 cells.

C57BL/6J and 129S6/Sv (B6 and 129) mice differ dramatically in their susceptibility to developing diabetes in response to diet- or genetically induced insulin resistance. A major locus contributing to this difference has been mapped to a region on mouse chromosome 14 that contains the gene encoding PKCδ. Here, we found that PKCδ expression in liver was 2-fold higher in B6 versus 129 mice from birth and was further increased in B6 but not 129 mice in response to a high-fat diet. PRKCD gene expression was also elevated in obese humans and was positively correlated with fasting glucose and circulating triglycerides. Mice with global or liver-specific inactivation of the Prkcd gene displayed increased hepatic insulin signaling and reduced expression of gluconeogenic and lipogenic enzymes. This resulted in increased insulin-induced suppression of hepatic gluconeogenesis, improved glucose tolerance, and reduced hepatosteatosis with aging. Conversely, mice with liver-specific overexpression of PKCδ developed hepatic insulin resistance characterized by decreased insulin signaling, enhanced lipogenic gene expression, and hepatosteatosis. Therefore, changes in the expression and regulation of PKCδ between strains of mice and in obese humans play an important role in the genetic risk of hepatic insulin resistance, glucose intolerance, and hepatosteatosis; and thus PKCδ may be a potential target in the treatment of metabolic syndrome.

Human carboxylesterase 2 (CES2) has triacylglycerol hydrolase (TGH) activities and plays an important role in lipolysis. In this study, we aim to determine the role of human CES2 in the progression or reversal of steatohepatitis in diet-induced or genetically obese mice. High-fat/high-cholesterol/high-fructose (HFCF) diet-fed C57BL/6 mice or db/db mice were intravenously injected with an adeno-associated virus expressing human CES2 under the control of an albumin promoter. Human CES2 protected against HFCF diet-induced nonalcoholic fatty liver disease (NAFLD) in C57BL/6J mice and reversed steatohepatitis in db/db mice. Human CES2 also improved glucose tolerance and insulin sensitivity. Mechanistically, human CES2 reduced hepatic triglyceride (T) and free fatty acid (FFA) levels by inducing lipolysis and fatty acid oxidation and inhibiting lipogenesis via suppression of sterol regulatory element-binding protein 1. Furthermore, human CES2 overexpression improved mitochondrial respiration and glycolytic function, and inhibited gluconeogenesis, lipid peroxidation, apoptosis, and inflammation. Our data suggest that hepatocyte-specific expression of human CES2 prevents and reverses steatohepatitis. Targeting hepatic CES2 may be an attractive strategy for treatment of NAFLD.

胰岛素抵抗(Insulin Resistance, IR)作为关键的病理生理机制，与2型糖尿病、代谢综合征、动脉粥样硬化性心血管疾病以及缺血性脑卒中等多种疾病的发生发展密切相关。尽管高胰岛素–正葡萄糖钳夹技术被公认为评估胰岛素抵抗的传统金标准，但其操作流程复杂、实施成本较高且属于有创检查，因此在常规临床实践与大规模流行病学研究中难以广泛应用。近年来，多种基于常规代谢指标的新型胰岛素抵抗替代评估指数不断涌现，包括但不限于甘油三酯–葡萄糖指数(TyG)、其联合体质量指数的衍生指标(TyG-BMI)、胰岛素抵抗代谢评分(METS-IR)，以及血浆致动脉粥样硬化指数(AIP)等。这些新型指数综合了血糖、血脂、肥胖程度及脂质构成等多方面信息，具备操作简便、无创、成本低廉以及易于获取等显著优势。目前，它们已在代谢性疾病的危险分层、临床评估与预后预测中体现出重要的应用价值。本文旨在系统综述上述新型胰岛素抵抗指数的构建原理、临床应用现状及研究进展，以期为胰岛素抵抗及其相关疾病的早期识别与综合防治提供参考依据。