Studies on LXR- and FXR-mediated effects on cholesterol homeostasis in normal and cholic acid-depleted mice.

As previously reported by us, mice with targeted disruption of the CYP8B1 gene (CYP8B1-/-) fail to produce cholic acid (CA), upregulate their bile acid synthesis, reduce the absorption of dietary cholesterol and, after cholesterol feeding, accumulate less liver cholesterol than wild-type (CYP8B1+/+) mice. In the present study, cholesterol-enriched diet (0.5%) or administration of a synthetic liver X receptor (LXR) agonist strongly upregulated CYP7A1 expression in CYP8B1-/- mice, compared to CYP8B1+/+ mice. Cholesterol-fed CYP8B1-/- mice also showed a significant rise in HDL cholesterol and increased levels of liver ABCA1 mRNA. A combined CA (0.25%)/cholesterol (0.5%) diet enhanced absorption of intestinal cholesterol in both groups of mice, increased their liver cholesterol content, and reduced their expression of CYP7A1 mRNA. The ABCG5/G8 liver mRNA was increased in both groups of mice, but cholesterol crystals were only observed in bile from the CYP8B1+/+ mice. The results demonstrate the cholesterol-sparing effects of CA: enhanced absorption and reduced conversion into bile acids. Farnesoid X receptor (FXR)-mediated suppression of CYP7A1 in mice seems to be a predominant mechanism for regulation of bile acid synthesis under normal conditions and, as confirmed, able to override LXR-mediated mechanisms. Interaction between FXR- and LXR-mediated stimuli might also regulate expression of liver ABCG5/G8.