Increasing dietary cholesterol induces different regulation of classic and alternative bile acid synthesis

We investigated the effect of increasing dietary cholesterol on bile acid pool sizes and the regulation of the two bile acid synthetic pathways (classic, via cholesterol 7 alpha-hydroxylase, and alternative, via sterol 27-hydroxylase) in New Zealand white rabbits fed 3 g cholesterol/per day for up to 15 days. Feeding cholesterol for one day increased hepatic cholesterol 75% and cholesterol 7 alpha-hydroxylase activity 1.6 times without significant change of bile acid pool size or sterol 27-hydroxylase activity. After three days of cholesterol feeding, the bile acid pool size increased 83% (P < 0.01), and further feeding produced 10%-20% increments, whereas cholesterol 7 alpha-hydroxylase activity declined progressively to 60% below baseline. In contrast, sterol 27-hydroxylase activity rose 58% after three days of cholesterol feeding and remained elevated with continued intake. Bile drainage depleted the bile acid pool and stimulated downregulated cholesterol 7 alpha-hydroxylase activity but did not affect sterol 27-hydroxylase activity. Thus, increasing hepatic cholesterol does not directly inhibit cholesterol 7 alpha-hydroxylase and initially favors enzyme induction, whereas increased bile acid pool is the most powerful inhibitor of cholesterol 7 alpha-hydroxylase. Sterol 27-hydroxylase is insensitive to the bile acid flux but is upregulated by increasing hepatic cholesterol.