No abstract

L iver fibrosis is characterized by an increase in the synthesis and deposition of extracellular matrix proteins, including type I collagen. The activated hepatic stellate cell (HSC) is responsible for increased type I collagen synthesis during liver fibrosis. After a fibrogenic stimulus, the HSC undergoes a complex phenotypic change from a quiescent vitamin A-storing cell to that of an activated, myofibroblast-like cell (reviewed in Friedman 1 and Eng et al. 2 ). Numerous changes occur during HSC activation, including the loss of retinoid stores, the appearance of smooth muscle ␣-actin, and a dramatic increase in the synthesis of type I collagen.Type I collagen is the product of 2 genes, the ␣1(I) and the ␣2(I) genes. Although located on different chromosomes, these genes are coordinately regulated in a developmental, tissue-specific, and inducible manner. [3][4][5][6] Previous studies have shown that ␣1(I) collagen gene expression is increased during HSC activation at both the transcriptional and posttranscriptional levels. 7-9 Transcriptional regulatory elements have been identified in the 5Ј-flanking region, the promoter region, first intron, and 3Ј-flanking region of both type I collagen genes in several species. [10][11][12][13][14][15][16][17][18][19][20][21][22][23] Several deoxyribonuclease (DNase) I-hypersensitive sites (HSs) have been identified in the 5Ј-flanking region of the murine ␣1(I) collagen gene that are present in collagen-producing, but not in non-collagenproducing cells, suggesting that these elements are in-

No abstract

Transforming growth factor (TGF)␤

During fibrosis the hepatic stellate cell (HSC) undergoes a complex activation process characterized by increased proliferation and extracellular matrix deposition. The 70-kDa ribosomal S6 kinase (p70S6K) is activated by mitogens, growth factors, and hormones in a phosphatidylinositol 3-kinase-dependent manner. p70S6K regulates protein synthesis, proliferation, and cell cycle control. Because these processes are involved in HSC activation, we investigated the role of p70S6K in HSC proliferation, cell cycle control, and type I collagen expression. Platelet-derived growth factor (PDGF) stimulated p70S6K phosphorylation, which was blocked by LY294002, an inhibitor of phosphatidylinositol 3-kinase. Rapamycin blocked phosphorylation of p70S6K but had no affect on PDGF-induced Akt phosphorylation, positioning p70S6K downstream of Akt. Transforming growth factor-beta, which inhibits HSC proliferation, did not affect PDGF-induced p70S6K phosphorylation. Rapamycin treatment did not affect alpha1(I) collagen mRNA but reduced type I collagen protein secretion. Expression of smooth muscle alpha-actin was not affected by rapamycin treatment, indicating that HSC activation was not altered. Rapamycin inhibited serum-induced DNA synthesis approximately 2-fold. Moreover, rapamycin decreased expression of cyclins D1, D3, and E but not cyclin D2, Rb-Ser780, and Rb-Ser795. Together, p70S6K plays a crucial role in HSC proliferation, collagen expression, and cell cycle control, thus representing a potential therapeutic target for liver fibrosis.