Mechanisms of Action of Acetaldehyde in the Up-Regulation of the Human α2(I) Collagen Gene in Hepatic Stellate Cells: Key Roles of Ski, SMAD3, SMAD4, and SMAD7

Abstract

Alcohol-induced liver fibrosis and eventually cirrhosis is a leading cause of death. Acetaldehyde, the first metabolite of ethanol, up-regulates expression of the human α2(I) collagen gene (COL1A2). Early acetaldehyde-mediated effects involve phosphorylation and nuclear translocation of SMAD3/4-containing complexes that bind to COL1A2 promoter to induce fibrogenesis. We used human and mouse hepatic stellate cells to elucidate the mechanisms whereby acetaldehyde up-regulates COL1A2 by modulating the role of Ski and the expression of SMADs 3, 4, and 7. Acetaldehyde induced up-regulation of COL1A2 by 3.5-fold, with concomitant increases in the mRNA (threefold) and protein (4.2- and 3.5-fold) levels of SMAD3 and SMAD4, respectively. It also caused a 60% decrease in SMAD7 expression. Ski, a member of the Ski/Sno oncogene family, is colocalized in the nucleus with SMAD4. Acetaldehyde induces translocation of Ski and SMAD4 to the cytoplasm, where Ski undergoes proteasomal degradation, as confirmed by the ability of the proteasomal inhibitor lactacystin to blunt up-regulation of acetaldehyde-dependent COL1A2, but not of the nonspecific fibronectin gene (FN1). We conclude that acetaldehyde up-regulates COL1A2 by enhancing expression of the transactivators SMAD3 and SMAD4 while inhibiting the repressor SMAD7, along with promoting Ski translocation from the nucleus to cytoplasm. We speculate that drugs that prevent proteasomal degradation of repressors targeting COL1A2 may have antifibrogenic properties.