Repression of Smad7 mediated by DNMT1 determines hepatic stellate cell activation and liver fibrosis in rats.

Conversion of hepatic stellate cells (HSCs) into hepatic myofibroblasts is a necessary event during the development of liver fibrosis. DNA methyltransferase 1 (DNMT1), which catalyzes DNA methylation and subsequently leads to the transcriptional repression of profibrotic genes, is selectively induced in myofibroblasts from diseased livers. Treatment of HSC with the DNA methylation inhibitor, 5-aza-2'-deoxycytidine (5-azadC), prevented TGF-β1-induced proliferation and alpha-smooth muscle actin (α-SMA) and collagen expression. 5-AzadC also rescued TGF-β1-induced suppression of Smad7 expression which occurs during HSC activation. Similarly, silencing the expression of the DNMT1 gene ameliorated the suppression of Smad7 expression by TGF-β1. In addition, DNMT1 inhibition, by 5-azadC or DNMT1 silencing, prevented the phosphorylation of Smad2 and Smad3. These studies suggest that epigenetic repression of Smad7 promotes the phosphorylation of Smad2 and Smad3 that may be an important molecular mechanism for perpetuated HSC activation and liver fibrosis.