TRANSCRIPTIONAL REGULATION OF THE GENE ENCODING CHOLESTEROL 7-ALPHA-HYDROXYLASE IN THE RAT

The cytochrome P450 enzyme, cholesterol 7alpha-hydroxylase (CYP7A), ca talyses the first and rate-limiting step in the conversion of choleste rol to bile acids. Expression of the CYP7A gene is under complex physi ological control, encompassing amongst others a feedback down-regulati on by bile acids. Using the CYP7A cDNA of the rat as a probe, we isola ted 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 expre ssion analyses of transfected primary hepatocytes demonstrated that th e major transcription-activating region is located in the proximal 145 nucleotide (nt). Upon addition of taurocholate to the culture, a sign ificant reduction of the transcriptional activity was observed, sugges ting the presence of a bile acid-responsive element in the proximal re gion of the CYP7A promoter. In addition, evidence was obtained for the presence of a thyroxine-responsive site further upstream. After addit ion of taurocholate, steady-state CYP7A mRNA levels, as judged by Nort hern analysis of hepatocyte RNA, are eightfold reduced. On the other h and, the transcriptional activity of CYP7A, as shown both in CAT assay s and run-on experiments, revealed only a threefold decrease. These ex periments 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.