Regulation of xenobiotic and bile acid metabolism by the nuclear pregnane X receptor

The nuclear pregnane X receptor (PXR; NR1I2) is an integral component of the body's defense mechanism against chemical insult (chemoprotection). PXR is activated by a diverse array of lipophilic chemicals, including xenobiotics and endogenous substances, and regulates the expression of cytochromes P450, conjugating enzymes, and transporters involved in the metabolism and elimination of these potentially harmful chemicals from the body. Among the chemicals that bind and activate PXR is the toxic bile acid lithocholic acid; activation of PXR, in turn, protects against the severe liver damage caused by this bile acid.Thus, PXR serves as a physiological sensor of lithocholic acid and perhaps other bile acids and coordinately regulates genes involved in their detoxification. Interestingly, both the antibiotic rifampicin and the herbal antidepressant St. John's wort activate PXR and have anticholestatic properties, which suggests that more potent, selective PXR agonists may be useful in the treatment of biliary cholestasis or other diseases characterized by the accumulation of bile acids or other toxins in the liver.     

Rat pregnane X receptor: molecular cloning, tissue distribution, and xenobiotic regulation.

An orphan nuclear receptor, termed the pregnane X receptor (PXR), has recently been cloned from mouse and human and defines a novel steroid signaling pathway (Cell 92, 73-82, 1998; Proc. Natl. Acad. Sci. USA 95, 12208-122313, 1998). Transient cotransfection experiments demonstrate that the PXR responds to structurally dissimilar compounds and confers the induction of cytochrome P4503A (CYP3A), a subfamily of enzymes that involve the metabolism of two-thirds of drugs and other xenobiotics. In this report, we describe the molecular cloning, tissue distribution, and xenobiotic regulation of a rat PXR designated rPXR-1. rPXR-1 exhibits a 95% sequence identity with the mouse PXR, but only 79% identity with the human PXR, providing the molecular basis that rats and mice have a similar CYP3A induction profile but differ from humans. rPXR-1 gene was expressed abundantly in liver, intestine, and, to a lesser extent, kidney, lung, and stomach. The tissue distribution and the relative abundance of rPXR-1 mRNA among these tissues resemble those of CYP3A, suggesting that PXR is important not only for induction but also for constitutive expression of these enzymes. Xenobiotics known to induce liver microsomal enzymes showed differential effects on the rPXR-1 expression as determined by Northern blot analysis. Dexamethasone, for example, increased the accumulation of rPXR-1 mRNA, whereas troleandomycin slightly suppressed it. Compounds that increase PXR expression (inducers) and compounds that interact with PXR (ligands) likely have synergistic effects on CYP3A induction, which provides a novel molecular explanation for drug-drug interactions.     

孕烷X受体研究进展

孕烷X受体(pregnane X receptor,PXR)是核受体超家族成员之一,以同名内源性配体孕烷命名,主要表达在肝脏、小肠、胃、肾脏等组织,在机体异源性/内源性物质的代谢及排泄过程中起重要的调节作用。近年的研究表明,PXR还可以通过调节其下游靶基因的表达,直接参与机体脂质、胆固醇和糖代谢,维持机体内环境的稳态。本文将就PXR的研究进展做一综述。     

孕烷X受体的研究进展

孕烷X受体(pregnane X receptor,PXR)属于核受体超家族(NRs)中的NR1I亚家族,主要在肝脏、小肠、胃、肾脏等组织中高表达。自该受体发现以来已相继克隆出小鼠、大鼠、猴及人等哺乳动物的PXR。PXR作为内源及外源物激活受体在机体的防御机制中发挥重要的生物调节作用和"解毒"功能。此外,大量研究还表明PXR通过调节下游靶基因的表达而广泛参与机体的物质及能量代谢,并在某些疾病的发生发展中发挥重要作用。本文就PXR的新近研究进展做一综述。     

Cytoplasmic localization of pregnane X receptor and ligand-dependent nuclear translocation in mouse liver

The pregnane X receptor (PXR) plays an important role in the response to xenobiotics and endogenous toxins. We have used a specific anti-PXR antibody in the Western blotting of mouse liver nuclear extracts to show that PXR is accumulated in the nucleus after treatment with 5-pregnen-3beta-ol-20-one-16alpha-carbonitrile (PCN), followed by an increase in Cyp3a11 mRNA. Expression of wild type PXR and various mutants as green fluorescent fusion proteins in mouse livers showed that PXR was retained in the cytoplasm from where PCN treatment translocated PXR into the nucleus. Furthermore, the xenochemical response signal, the nuclear translocation signal, and the activation function 2 domain were all required for the nuclear translocation to occur. Immunoprecipitation experiments using the hsp90 antibody demonstrated the presence of PXR in a complex with the endogenous cytoplasmic constitutive active/androstane receptor retention protein (CCRP) in HepG2 cells. Fluorescence resonance energy transfer analysis of mouse liver sections after co-expression of cyan fluorescent protein-CCRP and yellow fluorescent protein-PXR also indicated that CCRP and PXR were closely associated in vivo. Overexpression of exogenous CCRP increased the cytoplasmic level of the PXR.CCRP.hsp90 complex, whereas a decrease in endogenous CCRP by treatment with small interfering RNA for CCRP repressed the PXR-mediated reporter activity in HepG2 cells. We conclude that the CCRP mediates the retention of PXR in the cytosol and modulates the activation of PXR in response to PCN treatment.     

Orphan nuclear receptor pregnane X receptor sensitizes oxidative stress responses in transgenic mice and cancerous cells

Efficient handling of oxidative stress is critical for the survival of organisms. The orphan nuclear receptor pregnane X receptor (PXR) is important in xenobiotic detoxification through its regulation of phase I and phase II drug-metabolizing/detoxifying enzymes and transporters. In this study we unexpectedly found that the expression of an activated human PXR in transgenic female mice resulted in a heightened sensitivity to paraquat, an oxidative xenobiotic toxicant. Heightened paraquat sensitivity was also seen in wild-type mice treated with the mouse PXR agonist pregnenolone-16alpha-carbonitrile. The PXR-induced paraquat sensitivity was associated with decreased activities of superoxide dismutase and catalase, enzymes that scavenge superoxide and hydrogen peroxide, respectively. Paradoxically, the general expression and activity of glutathione S-transferases, a family of phase II enzymes that detoxify electrophilic and cytotoxic substrates, was also induced in the transgenic mice. PXR regulates glutathione S-transferase expression in an isozyme-, tissue-, and sex-specific manner, and this regulation is independent of the nuclear factor-erythroid 2 p45-related factor 2/Kelch-like Ech-associated protein 1 pathway. In cell cultures, expression of activated human PXR sensitizes the cancerous colon and liver cells to the cytotoxic effect of paraquat, which is associated with an increased production of the reactive oxygen species. The current study reveals a novel function of PXR in the mammalian oxidative stress response, and this regulatory pathway may be implicated in carcinogenesis by sensitizing normal and cancerous tissues to oxidative cellular damage.     

Evolution of pharmacologic specificity in the pregnane X receptor

Background  

The pregnane X receptor (PXR) shows the highest degree of cross-species sequence diversity of any of the vertebrate nuclear hormone receptors. In this study, we determined the pharmacophores for activation of human, mouse, rat, rabbit, chicken, and zebrafish PXRs, using a common set of sixteen ligands. In addition, we compared in detail the selectivity of human and zebrafish PXRs for steroidal compounds and xenobiotics. The ligand activation properties of the Western clawed frog (Xenopus tropicalis) PXR and that of a putative vitamin D receptor (VDR)/PXR cloned in this study from the chordate invertebrate sea squirt (Ciona intestinalis) were also investigated.     

Activation of pregnane X receptor disrupts glucocorticoid and mineralocorticoid homeostasis

The pregnane X receptor (PXR) was isolated as a xenobiotic receptor that regulates responses to various xenobiotic agents. In this study, we show that PXR plays an important endobiotic role in adrenal steroid homeostasis. Activation of PXR by genetic (transgene) or pharmacological (ligand, such as rifampicin) markedly increased plasma concentrations of corticosterone and aldosterone, the respective primary glucocorticoid and mineralocorticoid in rodents. The increased levels of corticosterone and aldosterone were associated with activation of adrenal steroidogenic enzymes, including cytochrome P450 (CYP)11a1, CYP11b1, CYP11b2, and 3beta-hydroxysteroid dehydrogenase. The PXR-activating transgenic mice also exhibited hypertrophy of the adrenal cortex, loss of glucocorticoid circadian rhythm, and lack of glucocorticoid responses to psychogenic stress. Interestingly, the transgenic mice had normal pituitary secretion of ACTH and the corticosterone-suppressing effect of dexamethasone was intact, suggesting a functional hypothalamus-pituitary-adrenal axis despite a severe disruption of adrenal steroid homeostasis. The ACTH-independent hypercortisolism in the PXR-activating transgenic mice is reminiscent of the pseudo-Cushing's syndrome in patients. The glucocorticoid effect appears to be PXR specific, as the activation of constitutive androstane receptor in transgenic mice had little effect. We propose that PXR is a potential endocrine disrupting factor that may have broad implications in steroid homeostasis and drug-hormone interactions.     

CYP3A4 and pregnane X receptor humanized mice

Marked species differences exist in P450 expression and activities. In order to produce mouse models that can be used to more accurately predict human drug and carcinogen metabolism, P450- and xenobiotic receptor humanized mice are being prepared using bacterial artificial chromosomes (BAC) and P1 phage artificial chromosomes (PAC) genomic clones. In some cases, transgenic mice carrying the human genes are bred with null-mice to produce fully humanized mice. Mice expressing human CYP1A1, CYP1A2, CYP2E1, CYP2D6, CYP3A4, and CYP3A7 were generated and characterized. Studies with the CYP3A4-humanized (hCYP3A4) mouse line revealed new information on the physiological function of this P450 and its role in drug metabolism in vivo. With this mouse line, CYP3A4, under certain circumstances, was found to alter the serum levels of estrogen resulting in deficient lactation and low pup survival as a result of underdeveloped mammary glands. This hCYP3A4 mouse established the importance of intestinal CYP3A4 in the pharmacokinetics of orally administered drugs. The hCYP3A4 mice were also used to establish the mechanisms of potential gender differences in CYP3A4 expression (adult female > adult male) that could account for human gender differences in drug metabolism and response. The pregnane X receptor (PXR) is also involved in induction of drug metabolism through its target genes including CYP3A4. Since species differences exist in ligand specificity between human and mice, a PXR-humanized mouse (hPXR) was produced that responds to human PXR activators such as rifampicin but does not respond to the rodent activator pregnenalone 16alpha-carbonitrile.     

Regulation of CYP3A4 by pregnane X receptor: The role of nuclear receptors competing for response element binding

Induction of the major drug metabolizing enzyme CYP3A4 by xenobiotics contributes to the pronounced interindividual variability of its expression and often results in clinically relevant drug-drug interactions. It is mainly mediated by PXR, which regulates CYP3A4 expression by binding to several specific elements in the 5′ upstream regulatory region of the gene. Induction itself shows a marked interindividual variability, whose underlying determinants are only partly understood. In this study, we investigated the role of nuclear receptor binding to PXR response elements in CYP3A4, as a potential non-genetic mechanism contributing to interindividual variability of induction. By in vitro DNA binding experiments, we showed that several nuclear receptors bind efficiently to the proximal promoter ER6 and distal xenobiotic-responsive enhancer module DR3 motifs. TRα1, TRβ1, COUP-TFI, and COUP-TFII further demonstrated dose-dependent repression of PXR-mediated CYP3A4 enhancer/promoter reporter activity in transient transfection in the presence and absence of the PXR inducer rifampin, while VDR showed this effect only in the absence of treatment. By combining functional in vitro characterization with hepatic expression analysis, we predict that TRα1, TRβ1, COUP-TFI, and COUP-TFII show a strong potential for the repression of PXR-mediated activation of CYP3A4 in vivo. In summary, our results demonstrate that nuclear receptor binding to PXR response elements interferes with PXR-mediated expression and induction of CYP3A4 and thereby contributes to the interindividual variability of induction.