Farnesoid X receptor responds to bile acids and represses cholesterol 7 alpha-hydroxylase gene (CYP7A1) transcription

Cholesterol 7 alpha-hydroxylase gene (CYP7A1) transcription is repressed by bile acids. The goal of this study is to elucidate the mechanism of CYP7A1 transcription by bile acid-activated farnesoid X receptor (FXR) in its nat ive promoter and cellular context and to identify FXR response elements in the gene. In Chinese hamster ovary cells transfected with retinoid X recept or alpha (RXR alpha)/FXR, only chenodeoxycholic acid (CDCA) and deoxycholic acid (DCA) were able to stimulate a heterologous promoter/reporter contain ing an ecdysone response element. In HepG2 cells, all bile acids (25 mu M) were able to repress CYP7A1/luciferase reporter activity, and only CDCA and DCA further repressed reporter activity when cotransfected with RXR alpha/ FXR, The concentration of CDCA required to inhibit 50% of reporter activity (IC50) was determined to be approximately 25 mu M without FXR and 10 mu M with FXR. Deletion analysis revealed that the bile acid response element lo cated between nucleotides -148 and -128 was the FXR response element, but R XR alpha/FXR did not bind to this sequence. These results suggest that bile acid-activated FXR exerts its inhibitory effect on CYP7A1 transcription by an indirect mechanism, in contrast to the stimulation and binding of FXR t o intestinal bile acid-binding protein gene promoter. Results also reveal t hat bile acid receptors other than FXR are present in HepG2 cells.