Transcriptional regulation of the gene encoding cholesterol 7 alpha-hydroxylase in the rat

The cytochrome P450 enzyme, cholesterol 7 alpha-hydroxylase (CYP7A), catalyses the first and rate-limiting step in the conversion of cholesterol to bile acids. Expression of the CYP7A gene is under complex physiological control, encompassing amongst others a feedback down-regulation by bile acids. Using the CYP7A cDNA of the rat as a probe, we isolated a rat genomic clone containing the 5' part of the gene, including approximately 3.6 kb of upstream sequences. Sequence analysis revealed the presence of several putative regulatory elements. Transient expression analyses of transfected primary hepatocytes demonstrated that the major transcription-activating region is located in the proximal 145 nucleotide (nt). Upon addition of taurocholate to the culture, a significant reduction of the transcriptional activity was observed, suggesting the presence of a bile acid-responsive element in the proximal region of the CYP7A promoter. In addition, evidence was obtained for the presence of a thyroxine-responsive site further upstream. After addition of taurocholate, steady-state CYP7A mRNA levels, as judged by Northern analysis of hepatocyte RNA, are eightfold reduced. On the other hand, the transcriptional activity of CYP7A, as shown both in CAT assays and run-on experiments, revealed only a threefold decrease. These experiments suggest that both transcriptional control and regulation of CYP7A mRNA stability play an important part in the feedback regulation of CYP7A activity in the rat.