The effect of peroxisome-proliferator-activated receptor-alpha on the activity of the cholesterol 7 alpha-hydroxylase gene

Cholesterol 7 alpha -hydroxylase (Cyp7a1) plays a central role in the regul ation of bile acid and cholesterol metabolism, and transcription of the gen e is controlled by bile acids and hormones acting through a complex interac tion with a number of potential steroid-hormone-binding sites. Transcriptio nal activity of the human CYP7A1 gene promoter transfected into HepG2 cells was decreased in a concentration-dependent manner by co-transfection with an expression vector for peroxisome-proliferator-activated receptor-alpha ( PPAR alpha). This effect was augmented by 9-cis-retinoic acid receptor-alph a (RXR alpha) and activators of PPAR alpha to give a maximum inhibition of approx. 80 %. The region responsible for this inhibition contained a site k nown to bind hepatocyte nuclear factor 3 (HNF4), and mutation of this site greatly decreased the effect. Co-expression of HNF4 increased promoter acti vity and decreased the effect of PPAR alpha. Gel-mobility-shift assays fail ed to detect any binding of PPAR alpha /RXR alpha dimers to any regions of the promoter containing potential binding sites. Also the hepatic abundance of Cyp7a1 mRNA in mice in which the PPAR alpha gene was disrupted was the same as in normal mice, both during the dark phase, when the animals were f eeding, and during the light phase, when mRNA abundance was greatly increas ed. Cholesterol feeding produced the same increase in hepatic Cyp7a1 mRNA a bundance in PPAR alpha -null animals as in normals. It is concluded that, w hereas PPAR alpha can affect CYP7A1 gene transcription in vitro through an indirect action, probably by competing for cofactors, this is unlikely to b e a major influence on Cyp7a1 activity under normal physiological condition s.