Profibrogenic TGF-beta/ALK5 signaling via CTGF expression in hepatocytes

Abstract

Connective Tissue Growth Factor (CTGF) plays an important role in TGF-β induced liver fibrogenesis. Hepatic stellate cells and bile duct epithelial cells were recognized as its cellular source in liver. Here, we demonstrate induction of CTGF expression in hepatocytes of damaged liver and identify a molecular mechanism responsible. Strong CTGF expression was found in bile duct epithelial cells, hepatic stellate cells and hepatocytes in fibrotic liver tissue from patients with chronic hepatitis B infection. Similarly, CTGF expression was induced in hepatocytes of CCl4 treated FVB mice. CTGF expression and secretion was detected spontaneously in medium of hepatocytes after 3 days of culture and was enhanced by stimulation with TGF-β. TGF-β-induced CTGF expression was mediated through the ALK5/Smad3 pathway, while ALK1 activitation antagonised this effect. CTGF expression in liver tissue of TGF-β transgenic mice correlated with serum TGF-β levels. Smad7 overexpression in cultured hepatocytes abrogated TGF-β-dependent and intrinsic CTGF expression. In line with these data, hepatocyte specific Smad7 expression reduced CTGF expression in CCl4 treated transgenic mice, whereas mice bearing a disrupted Smad7 gene displayed enhanced expression. Furthermore, IFN-g treatment of patients with chronic HBV infection induced Smad7 expression in hepatocytes leading to decreased CTGF expression and fibrogenesis. In conclusion, our data provide strong in vitro and in vivo evidence for pro-fibrogenic activity of TGF-β directed to hepatocytes mediated via upregulation of CTGF. Further, we identify ALK5 dependent Smad3 signaling as responsible pathway inducing CTGF expression, which can be interfered by an activated ALK1 pathway and completely inhibited by TGF-β antagonist Smad7.