Regulation of sterol 12 alpha-hydroxylase and cholic acid biosynthesis in the rat

Background & Aims: Sterol 12 alpha-hydroxylase (CYP8b1) is required for the biosynthesis of cholic acid (CA) and hence helps determine the ratio of CA to chenodeoxycholic acid (CDCA) in bile. This study examined the in vivo r egulation of CYP8b1 in the rat by bile acids, cholesterol; and thyroxine. M ethods: The specific activities (SAs),: messenger RNA (mRNA) levels, and tr anscriptional activities of CYP8b1 were determined in intact rats and rats with biliary diversion. Results: CA, CDCA, and:deoxycholic acid (DCA), fed as a supplement to-the diet, down-regulated CYP8b1 SAs by 99% +/- 0%,: 72% +/- 10%, and 98% +/- 1%, respectively. Under these same conditions, mRNA le vels decreased by 93% +/- 7%, 60% +/- 11%, and 93% +/- 4%, respectively. in traduodenal infusion of taurocholate (36 mu mol/h.100 g rat(-1)) decreased SAs and mRNA levels by 63% +/- 8% and 74% +/- 8%, respectively. Ursodeoxych olic acid (UDC) and hyocholic acid (HC) feeding increased CYP8b1 SAs by 119 % +/- 21% and 65% +/- 18%, respectively. CA feeding decreased CYP8b1 transc riptional activity by 72%. Complete biliary diversion increased CYP8b1. SAs and mRNA levels by 150% +/- 30% and 287% +/- 51%, respectively. Cholestero l feeding decreased CYP8b1 mRNA by 39% +/- 8%. In intact rats, a single inj ection of thyroid hormone eliminated CYP8b1 activity. Conclusions: CYP8b1 i s transcriptionally down-regulated by hydrophobic but not hydrophilic bile acids. Cholesterol feeding and a single thyroid hormone injection repressed CYP8b1 in the face of induction of cholesterol 7 alpha-hydroxylase (CYP7a1 by the new nomenclature) SAs. These results suggest that cholesterol, thyr oid hormone, and hydrophobic bile acids are important regulators of CYP8b1 and consequently of the bile acid pool composition.