Regulation of alpha1(I) collagen in hepatic stellate cells

Liver fibrosis is characterized by an increased deposition of extracellular matrix proteins, including type I collagen, that disrupts the normal architecture and function of the liver. The primary source of type I collagen in the fibrotic liver is the hepatic stellate cell (HSC). HSCs undergo an activation process from a quiescent, non-collagen producing cell with retinoid stores, to a myofibroblast-like cell that synthesizes high levels of collagen. The increase in type I collagen gene expression in the activated HSC occurs by a three-fold increase in gene transcription, and a 16-fold increase in the stability of α1(I) collagen mRNA resulting in a 50-fold increase in steady state mRNA levels.; Posttranscriptional regulation is frequently mediated by sequences in the 3′ untranslated region (UTR) of the mRNA. The α1(I) collagen mRNA molecule contains a poly(C) rich region in the 3′ UTR to which αCP (hnRNP E), a RNA binding protein, binds in activated, but not quiescent HSC cytoplasmic extracts. In vitro decay assays (IVDAs) demonstrate that the presence of an αCP binding site in RNA transcripts is sufficient to confer increased stability when compared to transcripts lacking the αCP binding region in NIH extracts and activated HSC extracts. siRNA reduction of αCP binding activity in activated HSCS resulted in a reduced steady state level of α1(I) collagen, confirming that αCP is required for the observed increased levels of α1(I) collagen mRNA in activated HSCs.; We sought to determine if the phosphatidylinositol-3 kinase (PI3-K) was involved in the regulation of HSC fibrogenesis due to its involvement in the HSC proliferative response. Inhibition of PI3-K activity greatly reduced type I collagen secretion and steady state mRNA levels of α1(I) collagen. We demonstrated that inhibiting PI3-K activity decreased collagen reporter gene activity, and reduced the α1(I) collagen mRNA half-life. Therefore, PI3-K regulates the steady state levels of α1(I) collagen mRNA at two levels; gene transcription and mRNA stability. These data show that the PI3-K pathway is a critical regulator of HSC fibrogenic and proliferative response and identifies this signaling pathway as a prime target for therapeutic intervention of liver fibrosis.