Early response of α2(I) collagen to acetaldehyde in human hepatic stellate cells is TGF‐β independent†

Abstract

Acetaldehyde is fibrogenic and induces the expression of type I collagen genes in hepatic stellate cells. Some of these acetaldehyde‐dependent events are mediated by H₂O₂ and thus establish a direct connection between oxidative stress and collagen upregulation. We localized to the −378 to −183 region of the α2(I) collagen (COL1A2) promoter an acetaldehyde‐responsive element (AcRE) functional in human hepatic stellate cells (HHSCs) and investigated molecular mechanisms whereby acetaldehyde stimulates and modulates its transcriptional activity. Because the AcRE co‐localized with a previously described transforming growth factor β (TGF‐β)1–responsive element, and both acetaldehyde and this cytokine induce their effects through H₂O₂, we investigated whether all fibrogenic actions of acetaldehyde were mediated by this cytokine. Here we show that acetaldehyde‐induced COL1A2 upregulation in HHSCs recognizes two distinct but overlapping early and late stages that last from 1 to 6 hours and from 6 to 24 hours, respectively. We present several lines of evidence to show that early acetaldehyde‐mediated events are independent of TGF‐β1. These include significant time‐course differences in the expression of COL1A2 and TGF‐β1 mRNAs and inability of neutralizing antibodies to TGF‐β1 to inhibit acetaldehyde‐dependent collagen gene transcription and Smad 3 phosphorylation. We also show that although acetaldehyde‐dependent upregulation of collagen was PI3K dependent, that of TGF‐β1 was PI3K independent. In conclusion, acetaldehyde‐dependent mechanisms involved in COL1A2 upregulation are similar, but not identical, to those of TGF‐β1. We suggest that early acetaldehyde‐dependent events induce the late expression of TGF‐β1 and create an H₂O₂‐dependent autocrine loop that may sustain and amplify the fibrogenic response of this alcohol metabolite. (HEPATOLOGY 2005;42:343–352.)