Down-regulation of Cholesterol 7α-Hydroxylase (CYP7A1) Gene Expression by Bile Acids in Primary Rat Hepatocytes Is Mediated by the c-Jun N-terminal Kinase Pathway

Cholesterol 7alpha-hydroxylase (CYP7A1), the rate-limiting enzyme in the neutral pathway of bile acid biosynthesis, is feedback-inhibited at the transcriptional level by hydrophobic bile acids. Recent studies show that bile acids are physiological ligands for farnesoid X receptor (FXR). Activated FXR indirectly represses CYP7A1 transcription through induction of small heterodimer protein (SHP-1). In this study, we provide evidence that bile acids rapidly down-regulate CYP7A1 transcription via activation of the JNK/c-Jun pathway. Furthermore, we demonstrate that SHP-1 is also a direct target of activated c-Jun. In primary rat hepatocyte cultures, taurocholate (TCA) strongly activated JNK in a time- and concentration-dependent manner. Tumor necrosis factor-alpha, a potent activator of JNK, also rapidly activated JNK and down-regulated CYP7A1 mRNA levels. Overexpression of dominant-negative JNK1 or a transactivating domain mutant of c-Jun significantly blocked the ability of TCA to down-regulate CYP7A1 mRNA. In contrast, overexpression of wild-type c-Jun (c-Jun(wt)) enhanced the repression of CYP7A1 by TCA. Moreover, overexpression of c-Jun(wt) resulted in increased SHP-1 promoter activity. Mutation of a putative AP-1 (c-Jun) element suppressed c-Jun-mediated activation of the SHP-1 promoter construct. These results indicate that the bile acid-activated JNK pathway plays a pivotal role in regulating CYP7A1 levels in primary rat hepatocytes.