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Following a fibrogenic stimulus, the hepatic stellate cell (HSC) undergoes a complex activation process associated with increased cell proliferation and excess deposition of type I collagen. The focal adhesion kinase (FAK)-phosphatidylinositol 3-kinase (PI3K)-Akt signaling pathway is activated by platelet-derived growth factor (PDGF) in several cell types. We investigated the role of the FAK-PI3K-Akt pathway in HSC activation. Inhibition of FAK activity blocked HSC migration, cell attachment, and PDGF-induced PI3K and Akt activation. Both serum- and PDGF-induced Akt phosphorylation was inhibited by LY294002, an inhibitor of PI3K. A constitutively active form of Akt stimulated HSC proliferation in serum-starved HSCs, whereas LY294002 and dominant-negative forms of Akt and FAK inhibited PDGF-induced proliferation. Transforming growth factor-beta, an inhibitor of HSC proliferation, did not block PDGF-induced Akt phosphorylation, suggesting that transforming growth factor-beta mediates its antiproliferative effect downstream of Akt. Expression of type I collagen protein and alpha1(I) collagen mRNA was increased by Akt activation and inhibited when PI3K activity was blocked. Therefore, FAK is important for HSC migration, cell attachment, and PDGF-induced cell proliferation. PI3K is positioned downstream of FAK. Signals for HSC proliferation are transduced through FAK, PI3K, and Akt. Finally, expression of type I collagen is regulated by the PI3K-Akt signaling pathway.

The regulation of collagen alpha1(I) expression in hepatic stellate cells (HSCs) occurs in a complex fashion that is just beginning to be determined. The presence of regulatory sequences in both the 5' and 3' regions of the mRNA appear to be critical to its regulation in HSCs and are involved in the increased expression of collagen in activated HSCs. The 3' UTR contains a C-rich site that binds alphaCP, a known RNA-binding protein that is responsible for the increased stability of the mRNA in activated HSCs. Given that alphaCP is present in both activated and quiescent HSCs, there must be a mechanism for modifying alpha(CP to bind to the RNA in activated but not quiescent HSCs. The 5' region contains an evolutionary conserved stem-loop region that encompasses the translation initiation codon. This stem-loop can bind protein(s) in activated HSCs in an RNA cap-dependent manner. Such binding, together with the binding of alphaCP to the 3' UTR, can facilitate translation of collagen alpha1(I) mRNA, resulting in increased mRNA steady-state levels and collagen synthesis. A role of alphaCP in activating translation initiation has also been demonstrated. These two mechanisms work together to upregulate collagen alpha1(I) production in activated but not quiescent HSCs.