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

Background & Aims: The rate of 12alpha-hydroxylation of bile acid intermediates is believed to determine the ratio of cholic acid (CA) to chenodeoxycholic acid (CDCA) biosynthesis and the overall hydrophobicity of the bile acid pool. The aim of this study was to determine the effects of the level of expression of sterol 12alpha-hydroxylase (CYP8b1) and cholesterol 7alpha-hydroxylase (CYP7a1) on rates of CA biosynthesis and bile acid pool composition.

Methods: Expression of CYP8b1 and CYP7a1 was accomplished through infection of primary rat hepatocytes (PRH) or intact male SD rats with replication-defective recombinant adenoviruses encoding either CYP8b1 or CYP7a1.

Results: Increased expression of CYP7a1 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. Conversely, increased expression of CYP8b1 in vitro had no significant effect on the rates of total bile acid synthesis but significantly increased (4.1-fold) the rates of CA biosynthesis, resulting in an increase in the CA-CDCA ratio from 1:6.6 to 2.8:1. In whole rats, increased CYP8b1 expression over basal 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 decreased from 64% +/- 3.4% to 50% +/- 2.9%.

Conclusions: Increased expression of CYP8b1 led to a marked increase in CA biosynthesis both in PRH and in whole animals. CYP8b1 is capable of 12alpha-hydroxylating bile acid intermediates from both the classic and acidic pathways.