Abstract
Hepatocyte nuclear factor 4alpha (HNF4alpha) regulates many genes that are preferentially expressed in liver. Mice lacking hepatic expression of HNF4alpha (HNF4alphaDeltaL) exhibited markedly increased levels of serum bile acids (BAs) compared with HNF4alpha-floxed (HNF4alphaF/F) mice. The expression of genes involved in the hydroxylation and side chain beta-oxidation of cholesterol, including oxysterol 7alpha-hydroxylase, sterol 12alpha-hydroxylase (CYP8B1), and sterol carrier protein x, was markedly decreased in HNF4alphaDeltaL mice. Cholesterol 7alpha-hydroxylase mRNA and protein were diminished only during the dark cycle in HNF4alphaDeltaL mice, whereas expression in the light cycle was not different between HNF4alphaDeltaL and HNF4alphaF/F mice. Because CYP8B1 expression was reduced in HNF4alphaDeltaL mice, it was studied in more detail. In agreement with the mRNA levels, CYP8B1 enzyme activity was absent in HNF4alphaDeltaL mice. An HNF4alpha binding site was found in the mouse Cyp8b1 promoter that was able to direct HNF4alpha-dependent transcription. Surprisingly, cholic acid-derived BAs, produced as a result of CYP8B1 activity, were still observed in the serum and gallbladder of these mice. These studies reveal that HNF4alpha plays a central role in BA homeostasis by regulation of genes involved in BA biosynthesis, including hydroxylation and side chain beta-oxidation of cholesterol in vivo.
Highlights
Hepatocyte nuclear factor 4a (HNF4a) regulates many genes that are preferentially expressed in liver
Cholesterol is hydroxylated by cholesterol 7a-hydroxylase promoter factor (CYP7A1) and sterol 27-hydroxylase (CYP27A1), the first enzymes in the classic and neutral bile acid (BA) biosynthesis pathways, respectively [12, 13]
The expression of CYP7A1 mRNA and protein was unchanged during the light cycle between liver-specific hepatocyte nuclear factor 4a-null (HNF4aDL) and HNF4aF/F mice but higher during the dark cycle only in HNF4aF/F mice, indicating that HNF4a control of the Cyp7a1 gene may only be significant during the dark cycle
Summary
Hepatocyte nuclear factor 4a (HNF4a) regulates many genes that are preferentially expressed in liver. Cholic acid-derived BAs, produced as a result of CYP8B1 activity, were still observed in the serum and gallbladder of these mice These studies reveal that HNF4a plays a central role in BA homeostasis by regulation of genes involved in BA biosynthesis, including hydroxylation and side chain b-oxidation of cholesterol in vivo.— Inoue, Y., A-M. Increased levels of unconjugated and glycine-conjugated BAs in gallbladder were observed in HNF4aDL mice, which were associated with the reduced expression of the very long chain acyl-CoA synthase-related gene (VLACSR) and BA CoA:amino acid N-acyltransferase involved in BA conjugation [11] These results indicate that HNF4a plays an important role in the regulation of enterohepatic circulation of BA. The side chain b-oxidations are catalyzed by the enzymes trihydroxycoprostanoyl-CoA oxidase [acyl-coenzyme A oxidase 2 (ACOX2)], D-type peroxisomal bifunctional enzyme (D-PBE), and sterol carrier protein x (SCPx)
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