Transcriptional regulation of the PXR gene: identification and characterization of a functional peroxisome proliferator-activated receptor alpha binding site within the proximal promoter of PXR.

The pregnane X receptor (PXR, NR1I2) is widely regarded as a central factor in the body's response to changes in the fluxome, the overall metabolite profile in the body. PXR expression is regulated by a number of chemicals at the transcriptional level; the majority of these chemicals are ligands for PXR and substrates for PXR target genes. However, transcriptional activators of PXR, such as clofibrate, do not seem to be PXR ligands or substrates for its target genes. Understanding the molecular mechanisms underlying both these expected and, more importantly, unexpected transcriptional activations is central to fully understanding the roles of PXR in the human body. We have carried out an in silico analysis of the human PXR proximal promoter, identifying putative protein/DNA interaction sites within the 2 kilobases (kb) 5' to the putative transcription start site. These sites included several for liver-enriched transcription factors, such as the hepatic nuclear factors and CAAT-enhancer binding protein alpha, and chicken ovalbumin upstream promoter transcription factor, commensurate with the high expression of PXR in liver. Furthermore, we identified putative binding sites for a number of ligand-activated transcription factors, suggesting that these factors may regulate PXR gene expression. Further analysis of this regulatory region has shown that transcriptional activation of PXR by peroxisome proliferator-activated receptor alpha (PPARalpha) is via a binding site located approximately 1.3 kb upstream of the putative transcription start site, with ablation of this site preventing PPARalpha-mediated activation of PXR gene expression. We present a model of how regulation of PXR gene expression by ligand-activated transcription factors may play a central role in the body's response to xenobiotic exposure.