Cytokine regulation of human sterol 12α-hydroxylase (CYP8B1) gene

Sterol 12 alpha-hydroxylase (CYP8B1) catalyzes cholic acid synthesis in the liver and is feedback inhibited by bile acids. In addition to activating farnesoid X receptor ( nuclear receptor subfamily 1H4), bile acids also induce inflammatory cytokines in hepatocytes. The objective of this study was to investigate the mechanism by which inflammatory cytokines inhibit human CYP8B1 gene transcription. Real-time PCR assays revealed that both chenodeoxycholic acid ( CDCA) and interleukin-1 beta (IL-1 beta) markedly reduced CYP8B1, cholesterol 7 alpha-hydroxylase CYP7A1 and hepatic nuclear factor 4 alpha (HNF4 alpha) mRNA expression levels in human primary hepatocytes. However, CDCA induced, but IL-1 beta reduced, small heterodimer partner (SHP) mRNA expression. IL-1 beta inhibited human CYP8B1 reporter activity only in liver cells, and a c-Jun NH(2)-terminal kinase (JNK)-specific inhibitor-blocked IL-1 beta inhibition. Activated JNK1 or c-Jun inhibited, whereas their dominant negative forms blocked, IL-1 beta inhibition of CYP8B1 transcription. Mutagenesis analyses mapped an IL-1 beta response element to a previously identified bile acid response element, which contains an HNF4 alpha binding site. A dominant negative HNF4 alpha inhibited CYP8B1 gene transcription and ectopically expressed HNF4 alpha blocked IL-1 beta inhibition. Furthermore, IL-1 beta inhibited HNF4 alpha gene transcription, protein expression, and binding to the CYP8B1 gene. JNK1 phosphorylated HNF4 alpha and a JNK-specific inhibitor blocked the IL-1 beta inhibition of HNF4 alpha expression. These results suggest that IL-1 beta inhibits CYP8B1 gene transcription via a mitogen-activated protein kinase/ JNK pathway that inhibits HNF4 alpha gene expression and its DNA-binding ability. This mechanism may play an important role in the adaptive response to inflammatory cytokines and in the protection of the liver during cholestasis.