TGF-beta and CTGF regulation of extracellular matrix synthesis in tissue fibrosis: Analysis of in vitro and in vivo models

Transforming growth factor-beta (TGF-β) and connective tissue growth factor (CTGF) are profibrogenic growth factors associated with the pathogenesis of most fibrotic diseases. The goal of this work was to characterize the common and distinct profibrogenic effects of these growth factors using in vitro and in vivo models of systemic sclerosis (SSc) and renal fibrosis. Both growth factors potently induce extracellular matrix (ECM) synthesis in SSc fibroblasts but via distinct mechanisms. In vitro, TGF-β stimulates matrix synthesis in normal and SSc fibroblasts, whereas CTGF selectively induces ECM expression in SSc fibroblasts only in cooperation with insulin. Furthermore, SSc fibroblasts express increased TGF-βRI levels. This altered TGF-β receptor ratio significantly correlates with unresponsiveness to inhibition of collagen basal levels by blocking TGF-βRII signal transduction and may suggest an alteration in autocrine TGF-β signaling in these cells. In the cell types contributing to pathologic ECM deposition in renal fibrosis, TGF-β and CTGF induce similar profibrotic effects in mesangial cells but have divergent functions in epithelial cells. Only TGF-β stimulates collagen whereas both TGF-β and CTGF induce tenascin-C, an ECM protein associated with epithelial-mesenchymal transdifferentiation (EMT), which is a process implicated in the generation of increased interstitial fibroblasts associated with renal fibrosis. Thus, CTGF's profibrogenic actions may distinctly facilitate EMT with a less pronounced effect on collagen deposition, whereas TGF-β is a known mediator of both EMT and excessive ECM deposition in tubular epithelial cells. TGF-β receptor ratios are also modulated in vivo in the remnant kidney model of renal fibrosis. TGF-βRII levels remain elevated with disease progression, whereas increases in TGF-β and TGF-βRI levels are transient. CTGF levels are induced subsequent to TGF-β, remaining elevated upon the onset of fibrosis with kinetics similar to TGF-βRII. Collectively, our studies suggest that alterations in TGF-β receptor levels may be a common mechanism in tissue fibrosis by which TGF-β upegulates its signaling pathway to achieve its profibrotic functions. In addition, these studies suggest that CTGF is a potent inducer of ECM but likely in the context of other profibrotic signaling pathways, such as TGF-β or insulin/insulin like growth factors (IGFs), and may serve to perpetuate the fibrotic response initiated by TGF-β.