Received for publication August 16, 2007.
Revised February 9, 2008.
Accepted for publication February 12, 2008.

Unique Transcription Start Sites and Distinct Promoter Regions differentiate the Pregnane X Receptor (PXR) isoforms PXR 1 and PXR 2

Abstract

The pregnane X receptor (PXR) is known as the xenosensing receptor responsible for coordinated regulation of metabolic genes in response to diverse xenobiotic challenges. In particular, the ability of PXR to regulate CYP3A4, the enzyme capable of metabolizing more than 60% of all pharmaceuticals, defines its metabolic importance. Currently the list of PXR ligands and target genes is extensive, yet investigations into the regulation and expression of PXR are few. After an initial review of available sequence data, we discovered discrepancies in the 5' untranslated region (UTR) and transcriptional start site (TSS) characterizations of the human PXR gene, and subsequently endeavored to define TSSs and proximal promoters for isoforms PXR 1 and PXR 2. Reverse transcriptase polymerase chain reaction and primer extension experiments performed on RNA from human liver identified two TSSs for each receptor isoform. These results extended the 5'UTR sequence of each isoform and defined new proximal promoters for both. Candidate response elements for liver-enriched transcription factors and other receptors were found in both proximal promoters. Quantitative PCR from human liver illustrated a highly variable expression profile for total PXR; yet PXR 2 expression represented a consistent 2-5% of total PXR expression, despite the observed variability. Transfection experiments demonstrated comparable ability of PXR 1 and PXR 2 to transcriptionally activate the CYP3A4 promoter. Collectively, comparable function, consistent expression, and independent regulation suggest that PXR 2 is capable of contributing to the cumulative function of PXR and should be included in the larger investigations of PXR expression and regulation.

Key words: CYP induction, CYP3A, cytochrome P450 regulation, gene regulation, induction, PXR, SXR, transcriptional regulation