TGF-β Drives Lung Fibrosis and Collagen Biosynthesis via a Smad3-Independent Signalling Pathway

We have shown that Smad3, an intracellular signal transducer for transforming growth factor‐β1 (TGF‐β1), is required to elicit the full histological manifestations of obliterative airway disease in a tracheal transplant model. This suggests that chronic allograft rejection results in TGF‐β1‐induced Smad3 activation that leads to airway obliteration through fibroproliferation and increased matrix deposition. In other systems, these latter events are causally related to the transdifferentiation of fibroblasts into myofibroblasts, but their role in obliterative bronchiolitis (OB) after lung transplantation is unknown. We confirmed the presence of myofibroblasts inside affected airways associated with experimental OB using immunohistochemistry. Studying airway fibroblasts in vitro, we observed increased myofibroblast transdifferentiation in response to TGF‐β1, evidenced by increased α‐smooth muscle actin mRNA and protein expression. In Smad3‐null fibroblasts, TGF‐β1 induction of myofibroblast transdifferentiation was greatly diminished but not abolished, suggesting the presence of Smad3‐independent pathways. Further studies revealed that small molecule inhibitors of p38 (SB203580) and MEK/ERK (U1026) further reduced the remaining effect of TGF‐β1 in Smad3‐deficient fibroblasts. Together, these studies suggest that in chronic allograft rejection, TGF‐β1 stimulates myofibroblast transdifferentiation through Smad3‐dependent and ‐independent signals, contributing to the excessive matrix deposition that characterizes obliterative bronchiolitis.

… Throughout the last 20 years, lung transplantation has evolved into an accepted … lung disease because of emphysema, pulmonary fibrosis, pulmonary hypertension, and cystic fibrosis. …

Idiopathic pulmonary fibrosis is a progressive and fatal fibrotic disease of the lungs with unclear etiology. Prior efforts to treat idiopathic pulmonary fibrosis that focused on anti-inflammatory therapy have not proven to be effective. Recent insight suggests that the pathogenesis is mediated through foci of dysregulated fibroblasts driven by profibrotic cytokine signaling. TGF-β and PDGF are 2 of the most potent of these cytokines. In the current study, we investigated the role of TGF-β–induced fibrosis mediated by activation of the Abelson (Abl) tyrosine kinase. Our data indicate that fibroblasts respond to TGF-β by stimulating c-Abl kinase activity independently of Smad2/3 phosphorylation or PDGFR activation. Moreover, inhibition of c-Abl by imatinib prevented TGF-β–induced ECM gene expression, morphologic transformation, and cell proliferation independently of any effect on Smad signaling. Further, using a mouse model of bleomycin-induced pulmonary fibrosis, we found a significant inhibition of lung fibrosis by imatinib. Thus, Abl family members represent common targets for the modulation of profibrotic cytokine signaling.

… IL-13 and TNF-a are TGF-β/Smad3-independent mediators of the fibrotic pathway, and mRNA expression remained elevated throughout from 0 to 72 hours following radiation exposure …

… is necessary for the development of pulmonary fibrosis in mice. Here, we … lung from patients with scleroderma showed increased Egr-1 levels, which were highest in early diffuse disease…

… Idiopathic pulmonary fibrosis (IPF) is a chronic lung disease … in response to TGF-beta in human normal lung epithelial cells. … There are likely other SMAD3-independent pathways that …

Transforming growth factor (TGF)-β uses several intracellular signaling pathways besides canonical ALK5-Smad2/3 signaling to regulate a diverse array of cellular functions. Several of these so-called non-canonical (non-Smad2/3) pathways have been implicated in the pathogenesis of fibrosis and may therefore represent targets for therapeutic intervention. This review summarizes our current knowledge on the mechanisms of non-canonical TGF-β signaling in fibrosis, the potential molecular targets and the use of agonists/antagonists for therapeutic intervention.

Transforming growth factor-β (TGF-β) signaling plays a central role in lung tissue homeostasis, coordinating epithelial repair, immune resolution, and stromal remodeling following injury. However, persistent or dysregulated TGF-β activation is a hallmark of both idiopathic pulmonary fibrosis (IPF) and lung cancer, two devastating pulmonary diseases that are traditionally studied as distinct entities. Emerging evidence suggests that this dichotomous view may obscure shared pathogenic mechanisms driven by aberrant TGF-β signaling dynamics. In this review, we synthesize experimental, translational, and clinical findings to propose a unifying framework in which IPF and lung cancer represent endpoints along a shared TGF-β-driven pathological continuum. We highlight how the duration and intensity of TGF-β signaling determine divergent cellular outcomes across epithelial cells, fibroblasts, and immune compartments-ranging from physiological wound repair to irreversible fibrotic remodeling and the establishment of a pro-tumorigenic niche. Particular emphasis is placed on the temporal transition from acute injury responses to chronic signaling states that promote epithelial plasticity, fibroblast fixation, immune suppression, and genomic instability. By integrating fibrosis and tumorigenesis into a single pathophysiological model, this review reframes TGF-β signaling as a time-dependent disease modifier rather than a disease-specific factor. This perspective provides a conceptual basis for therapeutic strategies targeting TGF-β signaling windows to intercept disease progression before irreversible fibrosis or malignant transformation occurs.

Pulmonary fibrosis induced by PM2.5 exposure is a significant public health concern. While the central role of TGF-β1 in fibrotic signaling is well established, the crosstalk among the various signaling pathways activated by PM2.5 is not yet fully understood. A literature search was conducted in the ScienceDirect and PubMed databases using keywords such as “pulmonary fibrosis,” “PM2.5,” and “TGF-β.” Recent and high-impact articles published in English were selected, and their findings were synthesized into a narrative review. The synthesized literature reveals that PM2.5 exposure drives pulmonary fibrosis through a complex signaling network. The canonical TGF-β/Smad pathway acts as a central driver, but its pro-fibrotic effects are significantly amplified and sustained by crosstalk with non-canonical pathways (e.g., MAPK, PI3K/Akt) and other key regulatory networks like Wnt/β-catenin and Hippo/YAP. Overall, PM2.5-induced pulmonary fibrosis results from an interconnected network of pro-fibrotic signals, underscoring the need for multitargeted therapeutic approaches in the future.

While our previous studies suggest that limiting bleomycin-induced complement activation suppresses TGF-β signaling, the specific hierarchical interactions between TGF-β and complement in lung fibrosis are unclear. Herein, we investigated the mechanisms underlying TGF-β-induced complement activation in the pathogenesis of lung fibrosis. C57-BL6 mice were given intratracheal instillations of adenoviral vectors overexpressing TGF-β (Ad-TGFβ) or the firefly gene-luciferase (Ad-Luc; control). Two weeks later, mice with fibrotic lungs were instilled RNAi specific to receptors for C3a or C5a-C3ar or C5ar, and sacrificed at day 28. Histopathological analyses revealed that genetic silencing of C3ar or C5ar arrested the progression of TGF-β-induced lung fibrosis, collagen deposition and content (hydroxyproline, col1a1/2); and significantly suppressed local complement activation. With genetic silencing of either C3ar or C5ar, in Ad-TGFβ-injured lungs: we detected the recovery of Smad7 (TGF-β inhibitor) and diminished local release of DAF (membrane-bound complement inhibitor); in vitro: TGF-β-mediated loss of DAF was prevented. Conversely, blockade of the TGF-β receptor prevented C3a-mediated loss of DAF in both normal primary human alveolar and small airway epithelial cells. Of the 52 miRNAs analyzed as part of the Affymetrix array, normal primary human SAECs exposed to C3a, C5a or TGF-β caused discrete and overlapping miRNA regulation related to epithelial proliferation or apoptosis (miR-891A, miR-4442, miR-548, miR-4633), cellular contractility (miR-1197) and lung fibrosis (miR-21, miR-200C, miR-31HG, miR-503). Our studies present potential mechanisms by which TGF-β activates complement and promotes lung fibrosis.

Galectin-9 is a β-galactoside-binding protein with two carbohydrate recognition domains. Recent studies have revealed that galectin-9 regulates cellular biological reactions and plays a pivotal role in fibrosis. The aim of this study was to determine the role of galectin-9 in the pathogenesis of bleomycin-induced systemic sclerosis (SSc). Human galectin-9 levels in the serum of patients with SSc and mouse sera galectin-9 levels were measured by a Bio-Plex immunoassay and enzyme-linked immunosorbent assay. Lung fibrosis was induced using bleomycin in galectin-9 wild-type and knockout mice. The effects of galectin-9 on the fibrosis markers and signaling molecules in the mouse lung tissues and primary lung fibroblast cells were assessed with western blotting and quantitative polymerase chain reaction. Galectin-9 levels in the serum were significantly higher (9-fold) in patients compared to those of healthy individuals. Galectin-9 deficiency in mice prominently ameliorated epithelial proliferation, collagen I accumulation, and α-smooth muscle actin expression. In addition, the galectin-9 knockout mice showed reduced protein expression levels of fibrosis markers such as Smad2/3, connective tissue growth factor, and endothelin-1. Differences between the wild-type and knockout groups were also observed in the AKT, mitogen-activated protein kinase, and c-Jun N-terminal kinase signaling pathways. Galectin-9 deficiency decreased the signal activation induced by transforming growth factor-beta in mouse primary fibroblasts, which plays a critical role in fibroblast activation and aberrant catabolism of the extracellular matrix. Our findings suggest that lack of galectin-9 protects against bleomycin-induced SSc. Moreover, galectin-9 might be involved in regulating the progression of fibrosis in multiple pathways.

… The cross-talks between TGF-β/Smads and other cell signaling pathways will be discussed … devoted to the non-Smad pathways that are activated by the TGF-β receptors through either …