Expression of sterol 12 alpha-hydroxylase alters bile acid pool composition in primary rat hepatocytes and in vivo

(Background & Aims) under bar: The rate of 12 alpha -hydroxylation of bile acid intermediates is believed to determine the ratio of cholic acid (CA) t o chenodeoxycholic acid (CDCA) biosynthesis and the overall hydrophobicity of the bile acid pool. The aim of this study was to determine the effects o f the level of expression of sterol 12 alpha -hydroxylase (CYP8b1) and chol esterol 7 alpha -hydroxylase (CYP7a1) on rates of CA biosynthesis and bile acid pool composition. (Methods) under bar: Expression of CYP8b1 and CYP7a1 was accomplished through infection of primary rat hepatocytes (PRH) or int act male SD rats with replication-defective recombinant adenoviruses encodi ng either CYP8b1 or CYP7a1, (Results) under bar: Increased expression of CY P7a1 over basal levels in PRH dramatically increased bile acid biosynthesis (586% +/- 82%, P < 0.001) but did not alter the ratio of CA to CDCA. Conve rsely, increased expression of CYP8b1 in vitro had no significant effect on the rates of total bile acid synthesis but significantly increased (4.1-fo ld) the rates of CA biosynthesis, resulting in an increase in the CA-CDCA r atio from 1:6.6 to 2.8:1, In whole rats, increased CYP8b1 expression over b asal levels markedly increased the CA in the bile acid pool from 36% +/- 3. 4% to 50% +/- 2.9% in 5 days. CDCA and its muricholic acid derivatives decr eased from 64% =/- 3.4% to 50% +/- 2.9%. <(Conclusions)under bar>: Increase d expression of CYP8b1 led to a marked increase in CA biosynthesis both in PRH and in whole animals. CYP8b1 is capable of 12 alpha -hydroxylating bile acid intermediates from both the classic and acidic pathways.