Tompkins Leslie M, Sit Tim L, Wallace Andrew D
Department of Environmental and Molecular Toxicology, North Carolina State University, Raleigh, NC 27695-7633, USA.
Drug Metab Dispos. 2008 May;36(5):923-9. doi: 10.1124/dmd.107.018317. Epub 2008 Feb 14.
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 the 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 PXRs 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 PXRs; yet PXR 2 expression represented a consistent 2 to 5% of total PXR expression, despite the observed variability. Transfection experiments demonstrated that PXR 1 and PXR 2 had comparable abilities 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 PXRs and should be included in the larger investigations of PXR expression and regulation.
孕烷X受体（PXR）被认为是一种异源物感应受体，负责在面对各种外源性物质挑战时协调代谢基因的调控。特别是，PXR调节CYP3A4的能力，这种酶能够代谢超过60%的所有药物，决定了其在代谢方面的重要性。目前，PXR配体和靶基因的清单很广泛，但对PXR调控和表达的研究却很少。在初步审查可用序列数据后，我们发现人类PXR基因的5'非翻译区（UTR）和转录起始位点（TSS）特征存在差异，随后努力确定PXR 1和PXR 2亚型的TSS和近端启动子。对来自人类肝脏的RNA进行逆转录-聚合酶链反应和引物延伸实验，确定了每个受体亚型的两个TSS。这些结果扩展了每个亚型的5'UTR序列，并为两者定义了新的近端启动子。在两个近端启动子中都发现了肝脏富集转录因子和其他受体的候选反应元件。来自人类肝脏的定量PCR显示，总PXR的表达谱高度可变；然而，尽管观察到有变化，但PXR 2的表达占总PXR表达的2%至5%，保持一致。转染实验表明，PXR 1和PXR 2在转录激活CYP3A4启动子方面具有相当的能力。总体而言，相当的功能、一致的表达和独立的调控表明，PXR 2能够对PXR的累积功能做出贡献，应纳入对PXR表达和调控的更广泛研究中。
