Retinoid acid related orphan receptor α (RORα ) regulates circadian rhythm and fasting induction of sterol 12α -hydroxylase (CYP8B1) in bile acid synthesis
Preeti Pathak1, Tiangang Li1,2, John Chiang1
1Department of Integrative Medical Sciences, Northeast Ohio Medical University, Roofstown, OH; 2Pharmacology, Toxicology and Therpeutics' University of Kansas Medical Center, Kansas City, KS
Sterol 12α-hydroxylase (CYP8B1) is involved in cholic acid synthesis and plays a key role in intestinal cholesterol absorption and pathogenesis of cholesterol gallstone disease, dyslipidemia, fatty liver disease and atherosclerosis. In this study, we investigated the underlying mechanism of a fasting-induced and cholesterol activated nuclear receptor and core clock gene RORa in regulation of circadian rhythm and fasting induction of CYP8B1 expression. In free fed mice, CYP8B1 mRNA expression was reduced to the lowest level at the onset of the dark cycle when RORα mRNA expression was the lowest. Fasting stimulated, while re-feeding reduced expression of CYP8B1 mRNA and protein. Interestingly, fasting and feeding had little effect on the diurnal rhythm of RORα mRNA expression, but fasting and cAMP increased RORα protein stability by protein kinase A-mediated phosphorylation. Adenovirus-mediated transduction of RORα to mice strongly induced CYP8B1 gene expression, increased 12α-hydroxylated bile acids, and serum and liver cholesterol. Reporter assay, mutagenesis and EMSA identified a functional RORα response element in CYP8B1 promoter. Mammalian two-hybrid assay shows RORα interacts with cAMP response element binding protein-binding protein (CBP). The interaction was stimulated by cAMP but inhibited by 7ahydroxycholesterol, an antagonist of RORα. ChlP assay showed that RORα recruited CBP to the CYP8B1 promoter to stimulate histone acetylation and induce CYP8B1. In conclusion, RORα is a key regulator of circadian expression and fasting induction of CYP8B1 to increase 12a-hydroxylated-bile acids and regulate bile acid composition, which plays a role in intestinal lipid absorption and maintaining lipid homeostasis. Glucagon/cAMP signaling phosphorylates and stabilizes RORα protein to induce CYP8B1 in response to fasting and circadian rhythm to regulate cholic acid synthesis and bile acid composition, which regulates hepatic metabolic homeostasis, dyslipidemia, diabetes and obesity. Antagonizing RORα activity may be a therapeutic strategy for treating inflammatory diseases such as non-alcoholic fatty liver disease and type 2 diabetes.
The following people have nothing to disclose: Preeti Pathak, Tiangang Li, John Chiang