Signaling of the Pregnane X Receptor

描述(由申请人提供)：孕烷X受体(PXR)在新陈代谢、内生信号和能量平衡的动态平衡中起着不可或缺的作用。关于内质网应激的研究表明，PXR的表达下调。内质网应激是代谢性疾病的常见主题。在下调之前，肝细胞核因子4�(HNF4�)表达下调。PxR最小启动子被HNF4�反式激活，但被激活转录因子(ATF)和CHOP(C/EBP同源蛋白)抑制。ATFS和CHOP是内质网应激蛋白。对细胞色素P450 3A4(CYP3A4)诱导作用的研究表明，利福平和鹅去氧胆酸(CDCA)具有功能性拮抗作用。利福平是一种PXR激活剂，通常用于治疗胆汁淤积症。另一方面，CDCA是法尼类X受体(FXR)的有效激活剂。有趣的是，在没有内质网应激活性的浓度下，CDCA可显著降低利福平对细胞色素P3A4的诱导作用。PXR介导的利福平诱导的细胞色素P3A4是治疗胆汁淤积症的重要基础。该项目的中心假设是，在广泛的代谢紊乱中，PXR的功能是疾病-药物相互作用的关键决定因素。具体目的是：(1)确定PXR下调的功能后果；(2)表征胆汁酸-利福平的相互作用。为了确定PXR表达下调在内质网应激相关疾病中是否普遍存在，将对大量代谢异常的病变肝脏进行PXR表达检测。HNF4ATF-�-CHOP网络在内质网应激和病变肝脏中抑制PXR的分子作用将被明确。为了明确内质网应激期间PXR下调的功能变化，我们将测试从表达PXR的细胞衍生的异种移植细胞在内质网应激下对PXR激活剂反应的逆转。将监测PXR靶基因的表达，如羧酸酯酶-2(CES2)。此外，CES2激活的前药在异种移植瘤中的抗癌潜力将在有或没有利福平的内质网应激下进行测定。为了阐明利福平和CDCA之间的分子作用，将在有或没有FXR激活的情况下确定PXR导向的聚合酶II的募集。特定物种的PXR激动剂(人和鼠)将被评估其不同的抗胆汁淤积活性。总之，这些研究将描述PxR、FxR以及HNF4�-ATF/CHOP网络在内质网应激条件下和代谢紊乱中PxR的调节表达和反式激活活性的分子相互作用。这些研究将获得与PXR相关的转录网络、疾病-药物相互作用和治疗优化的重要新信息。因此，该项目不仅将对临床实践产生直接影响，还将对PXR作为主要转录调控因子的基本理解产生直接影响。

DESCRIPTION (provided by applicant): The pregnane X receptor (PXR) is established to play an integral role in metabolism, endobiotic signaling and homeostasis of energy balance. Studies on the effect of endoplasmic reticulum (ER) stress, a common theme of metabolic diseases, have shown that the expression of PXR was down-regulated. The downregulation was preceded by the downregulation of hepatocyte nuclear factor-4� (HNF4�). A PXR minimal promoter was transactivated by HNF4� but repressed by activating transcription factors (ATFs) and CHOP (C/EBP homologous protein). ATFs and CHOP are ER-stress proteins. Studies on the induction of cytochrome P450 3A4 (CYP3A4) have revealed functional antagonism between rifampicin and chenodeoxycholic acid (CDCA). Rifampicin is a PXR activator and commonly used to treat cholestasis. CDCA, on the other hand, is a potent activator of the farnesoid X receptor (FXR). Interestingly, the induction of CYP3A4 by rifampicin was significantly reduced by CDCA at a concentration with no ER stress activity. PXR-directed induction of CYP3A4 by rifampicin serves as a major foundation for treating cholestasis. The central hypothesis of this project is that the functionality of PXR is a critical determinant of disease-drug interactions in wide spectrum of metabolic disorders. The specific aims are: (1) to ascertain functional consequences of PXR downregulation; and (2) to characterize bile acid-rifampicin interactions. In order to determine whether downregulation of PXR represents a common phenomenon in ER stress-related diseases, a large number of diseased livers with metabolic abnormalities will be tested for the expression of PXR. The molecular action of the HNF4�-ATF- CHOP network on PXR suppression will be specified under ER stress and in diseased livers. To specify functional changes of PXR downregulation during ER stress, xenografts derived from cells expressing PXR will be tested for their reversal in responding to PXR activators under ER stress. The expression of PXR target genes such as carboxylesterase-2 (CES2) will be monitored. In addition, the anticancer potential of CES2 activated prodrug in xenografts will be determined under ER stress in the presence or absence of rifampicin. To elucidate the molecular actions between rifampicin and CDCA, the PXR-directed recruitment of polymerase II will be determined with or without FXR activation. Species-specific PXR activators (human versus mouse) will be evaluated for their differential anticholestatic activities. Overall, these studieswill characterize molecular interplays among PXR, FXR along with the HNF4�-ATF/CHOP network in terms of regulated expression and transactivation activity of PXR during ER stress condition and in metabolic disorders. These studies will gain important new information on transcriptional networking, disease-drug interactions and therapeutic optimization related to PXR. Therefore, this project will have direct impact not only on clinical practice but also on the basic understanding of PXR as a master transcriptional regulator.