Divergent Transforming Growth Factor-β Signaling in Hepatic Stellate Cells after Liver Injury: Functional Effects on ECE-1 Regulation

Abstract

In liver wound healing, transforming growth factor-beta (TGF-beta) plays a critical role in stellate cell activation as well as signaling cascades in the fibrogenic response to injury. We postulate that the TGF-beta-dependent downstream signaling pathway may vary according to the mechanism of stellate cell activation; this study was undertaken to ascertain whether the downstream signaling pathways mediated by TGF-beta vary in different liver injury models. We measured Smad3 and MAP kinase activation after isolating stellate cells from rat livers injured by either bile duct ligation (BDL) or repeated carbon tetrachloride (CCl(4)) administration. Phospho-Smad3 was dramatically up-regulated in stellate cells after CCl(4) injury, but not after BDL-induced injury. TGF-beta signaling in stellate cells activated after BDL was mediated prominently through ERK activation, whereas activation induced by CCl(4) injury or culture led to a cross-signaling mechanism involving both Smad3 and p38. The divergent Smad signaling pathways observed appeared to be attributable to the differential regulation of the early growth response gene-1 (Egr-1), an apparent negative transcriptional factor for Smad3 in our system. In addition, inhibition of ERK activation in stellate cells from BDL-injured liver led to a decrease in expression of endothelin-converting enzyme-1, a critical regulator of endothelin-1. We speculate that TGF-beta signaling proceeds through differential signaling pathways depending on the mechanism of liver injury that leads to stellate cell activation.