Phase II UDP-glucuronosyltransferases by PXR

描述（由申请人提供）：拟议研究的长期目标是研究外源核受体PXR对II期药物代谢和清除酶的转录调控。II期偶联酶（如udp -葡萄糖醛基转移酶（UGTs））与氧化I期细胞色素P450酶（CYPs）协同作用，消除诸如处方药和非处方药等外源性药物，以及胆红素、类固醇激素和胆汁酸等内源性药物。核受体PXR最初被认为是一种异种传感器，通过与哺乳动物CYP基因启动子中发现的PXR反应元件结合来调节CYP基因的表达。PXR作为一种物种特异性异种传感器的身份已被先前的转基因和基因敲除研究所确立。已经确定CYP基因作为PXR的靶点，在编码II期UGT基因产物的基因中存在候选PXR应答元件，这提高了PXR在外源应答和清除方面更广泛的生理功能的潜力。然而，ugt是否由PXR诱导尚不清楚。为了研究UGTs作为PXR的潜在转录靶点，我们提出：(1)克隆UGT1A1启动子并对其进行PXR调控；(2)在具有增强（VP-hPXR转基因）、受损（PXR敲除）或“人源化”（PXR敲除/hPXR转基因）受体活性的小鼠模型中检测肝脏UGTs的调节；(3)检测PXR活性改变对胆红素稳态的影响；(4) PXR对肠道ugt的调控作用。如果PXR对ugt等II期酶的调控被证明是正确的，我们将把PXR建立为哺乳动物外源反应的主要转录调节剂。这些研究结果将为理解UGT的转录调控提供新的元素。预计利用“人源化”小鼠阐明UGT调控的分子基础将对人类生理和疾病具有重要意义。这些包括胆红素和激素稳态、药物代谢和化学致癌作用。

DESCRIPTION (provided by applicant): The long-term goal of the proposed research is to study the transcriptional regulation of the phase II drug-metabolizing and clearing enzymes by the xenobiotic nuclear receptor PXR. The phase II conjugating enzymes such as the UDP-glucuronosyltransferases (UGTs) function in concert with the oxidative phase I cytochrome P450 enzymes (CYPs) to eliminate xenobiotics such as prescription drugs and over-the-counter medications, and endobiotics such as bilirubin, steroid hormones and bile acids. The nuclear receptor PXR was initially identified as a xenosensor to regulate the expression of CYP genes via its binding to the PXR-response elements found within the mammalian CYP gene promoters. The identity of PXR as a species-specific xenosensor has been established by previous transgenic and gene knockout studies. Having established CYP genes as PXR targets, the presence of candidate PXR response elements in genes encoding the phase II UGT gene products raises the potential for a broader physiological function of PXR in xenobiotic response and clearance. However, whether UGTs are induced by PXR is unclear. To investigate UGTs as potential transcriptional targets of PXR, we propose to: (1) clone the UGT1A1 promoter and characterize its regulation by PXR; (2) examine the regulation of hepatic UGTs in mouse models bearing heightened (VP-hPXR transgenic), compromised (PXR knockout), or "humanized" (PXR knockout/hPXR transgenic) receptor activity; (3) examine the effect of altered PXR activity on bilirubin homeostasis; (4) examine the regulation of intestinal UGTs by PXR. If the regulation of phase II enzymes such as the UGTs by PXR proven to be true, we are toward establishing PXR as a master transcriptional regulator of the mammalian xenobiotic response. The results of these studies will provide novel elements in understanding the transcriptional regulation of UGT. It is anticipated that elucidation of the molecular basis of UGT regulation using the "humanized" mice will have great implication in human physiology and diseases. These include bilirubin and hormonal homeostasis, drug metabolism, and chemical carcinogenesis.