TRANSCRIPTIONAL ACTIVATION OF THE CHOLESTEROL 7-ALPHA-HYDROXYLASE GENE (CYP7A) BY NUCLEAR HORMONE RECEPTORS

The gene encoding cholesterol 7 alpha-hydroxylase (CYP7A), the rate-li miting enzyme in bile acid synthesis, is transcriptionally regulated b y bile acids and hormones. Previously, we have identified two bile aci d response elements (BARE) in the promoter of the CYP7A gene, The BARE II is located in nt -149/-118 region and contains three hormone respo nse element (HRE)-like sequences that form two overlapping nuclear rec eptor binding sites. One is a direct repeat separated by one nucleotid e DR1 (-146-TGGACTtAGTTCA-134) and the other is a direct repeat separa ted by five nucleotides DR5 (-139-AGTTCAaggccGGG TAA-123). Mutagenesis of these HRE sequences resulted in lower transcriptional activity of the CYP7A promoter/reporter genes in transient transfection assay in H epG2 cells. The orphan nuclear receptor, hepatocyte nuclear factor 4 ( HNF-4)(1), binds to the DR1 sequence as assessed by electrophoretic mo bility shift assay, and activates the CYP7A promoter/reporter activity by about 9-fold. Cotransfection of HNF-4 plasmid with another orphan nuclear receptor, chicken ovalbumin upstream promoter-transcription fa ctor LI (COUP-TRI), synergistically activated the CYP7A transcription by 80-fold. The DR5 binds the RXR/RAR heterodimer, A hepatocyte nuclea r factor-3 (HNF-3) binding site (-175-TGTTTGTTCT-166) was identified. HNF-3 was required for both basal transcriptional activity and stimula tion of the rat CYP7A promoter activity by retinoic acid, Combinatoria l interactions and binding of these transcription factors to BAREs may modulate the promoter activity and also mediate bile acid repression of CYP7A gene transcription.