Involvement of hepatocyte nuclear factor 4 alpha in transcriptional regulation of the human pregnane X receptor gene in the human liver

Pregnane X receptor (PXR; NR1I2), a key transcriptional factor that regulates genes encoding drug-metabolizing enzymes and drug transporters, is abundantly expressed in the human liver. However, studies on the molecular mechanism of human PXR gene regulation are limited. In this study, we examined the involvement of hepatocyte nuclear factor 4alpha (HNF4alpha; NR2A1) in the transcriptional regulation of the human PXR gene in the human liver. The activities of the human PXR promoter containing the direct repeat 1 (DR1) element located at -88/-76 of the promoter were significantly increased by co-expression of HNF4alpha in the human hepatocellular carcinoma cell line. In addition, introduction of mutation into the DR1 element abolished the transcriptional activation of the human PXR promoter by exogenous HNF4alpha. The results of gel mobility shift assays and chromatin immunoprecipitation assays showed that HNF4alpha was bound to the promoter region containing the DR1 element. A knock-down of HNF4alpha by siRNA significantly decreased expression levels of endogenous PXR mRNA in HepG2 cells. Furthermore, expression levels of PXR mRNA positively correlated with those of HNF4alpha mRNA in 18 human liver samples. These results suggested that HNF4alpha transactivated the human PXR gene by binding to the DR1 element located at -88/-76 of the promoter and was involved in the expression of PXR in the human liver.