A novel role of transforming growth factor β1 in transcriptional repression of human cholesterol 7α-hydroxylase gene

Aims: Inhibition of cholesterol 7 alpha-hydroxylase (CYP7A1) by bile acids and inflammatory cytokines provides an important mechanism to protect hepatocytes from bile acid toxicity during cholestasis. Transforming growth factor beta 1 (TGF beta 1) released by hepatic stellate cells during chronic liver injury plays a critical role in liver inflammation and fibrogenesis. The objective of this study is to investigate the role of TGF beta 1 in hepatic bile acid synthesis. Methods: mRNA expressions in primary human hepatocytes and HepG2 cells were measured by quantitative real-tune polymerase chain reaction. Reporter assay, glutathione-S-transferase pull-down assay, adenovirus-mediated gene transduction, and chromatin immunoprecipitation assay were used to study the mechanism of TGF beta 1 regulation of CYP7A1 gene transcription. Results: TGF beta 1 inhibited the mRNA expression of CYP7A1 and bile acid synthesis in HepG2 cells and primary human hepatocytes. Mothers against decapentaplegic homolog (Smad3) inhibited both CYP7A1 promoter activity and mRNA expression by inhibiting DNA-binding activity of hepatocyte nuclear factor 4 alpha (HNF4alpha). The histone deacetylase (HDAC) inhibitor Tricostatin A partially blocked the TGF beta 1 inhibition of CYP7A1 mRNA expression, whereas TGF beta 1 decreased histone 3 acetylation in the CYP7A1 chromatin. TGF beta 1 treatment and adenovirus Smad3 reduced HNF4 alpha binding but increased the recruitment of Smad3, HDAC1, and a repressor mSin3A to the CYP7A1 chromatin. Conclusions: This study provides the first evidence that TGF beta 1 represses CYP7A1 gene transcription in human hepatocytes by a mechanism involving Smad3-dependent inhibition of HNF4a and HDAC remodeling of CYP7A1 chromatin. The TGF beta 1/Smad3 signaling may reduce bile acid synthesis in the liver and prevent hepatocyte injury in cholestatic liver disease.