SMAD and p38 MAPK signaling pathways independently regulate α1(I) collagen gene expression in unstimulated and transforming growth factor-β-stimulated hepatic stellate cells

Abstract

The hepatic stellate cell (HSC) is the predominant cell type responsible for excess collagen deposition during liver fibrosis. Both transforming growth factor-β (TGF-β), the most potent fibrogenic cytokine for HSCs, which classically activates Smad signaling, and p38 MAPK signaling have been shown to influence collagen gene expression; however, the relative contribution and mechanisms that these two signaling pathways have in regulating collagen gene expression have not been investigated. The aim of this study was to investigate the relative roles and mechanisms of both Smad and p38 MAPK signaling in α1(I) collagen gene expression in HSCs. Inhibiting either p38 MAPK or Smad signaling reduced α1(I) collagen mRNA expression in untreated or TGF-β-treated HSCs, and when both signaling pathways were simultaneously inhibited, α1(I) collagen gene expression was essentially blocked. Both signaling pathways were found to independently and additively increase α1(I) collagen gene expression by transcriptional mechanisms. TGF-β treatment increased α1(I) collagen mRNA half-life, mediated by increased stability of α1(I) collagen mRNA through p38 MAPK signaling but not through Smad signaling. In conclusion, both p38 MAPK and Smad signaling independently and additively regulate α1(I) collagen gene expression by transcriptional activation, whereas p38 MAPK and not Smad signaling increased α1(I) collagen mRNA stability. © 2005 by The American Society for Biochemistry and Molecular Biology, Inc.