Effects of atorvastatin metabolites on induction of drug-metabolizing enzymes and membrane transporters through human pregnane X receptor

BACKGROUND AND PURPOSE

Atorvastatin metabolites differ in their potential for drug interaction because of differential inhibition of drug-metabolizing enzymes and transporters. We here investigate whether they exert differential effects on the induction of these genes via activation of pregnane X receptor (PXR) and constitutive androstane receptor (CAR).

EXPERIMENTAL APPROACH

Ligand binding to PXR or CAR was analysed by mammalian two-hybrid assembly and promoter/reporter gene assays. Additionally, surface plasmon resonance was used to analyse ligand binding to CAR. Primary human hepatocytes were treated with atorvastatin metabolites, and mRNA and protein expression of PXR-regulated genes was measured. Two-hybrid co-activator interaction and co-repressor release assays were utilized to elucidate the molecular mechanism of PXR activation.

KEY RESULTS

All atorvastatin metabolites induced the assembly of PXR and activated CYP3A4 promoter activity. Ligand binding to CAR could not be proven. In primary human hepatocytes, the para-hydroxy metabolite markedly reduced or abolished induction of cytochrome P450 and transporter genes. While significant differences in co-activator recruitment were not observed, para-hydroxy atorvastatin demonstrated only 50% release of co-repressors.

CONCLUSIONS AND IMPLICATIONS

Atorvastatin metabolites are ligands of PXR but not of CAR. Atorvastatin metabolites demonstrate differential induction of PXR target genes, which results from impaired release of co-repressors. Consequently, the properties of drug metabolites have to be taken into account when analysing PXR-dependent induction of drug metabolism and transport. The drug interaction potential of the active metabolite, para-hydroxy atorvastatin, might be lower than that of the parent compound.     

孕甾烷x受体在药物代谢中的作用及研究进展

孕甾烷X受体（Pregnane X receptor,PXR）为核受体超家族中NRII亚家族成员,于肝脏和肠道中广泛表达。该受体作为药物代谢的关键调控因子广泛参与药物的吸收、分布、代谢及排泄过程。本文从药物代谢角度分别对PXR参与调节的Ⅰ、Ⅱ相代谢酶及转运体进行介绍,为临床药物相互作用及针对PXR为靶点的药物研发提供参考。     

Dexamethasone induces pregnane X receptor and retinoid X receptor-alpha expression in human hepatocytes: synergistic increase of CYP3A4 induction by pregnane X receptor activators

In this report we show that submicromolar concentrations of dexamethasone enhance pregnane X receptor (PXR) activator-mediated CYP3A4 gene expression in cultured human hepatocytes. Because this result is only observed after 24 h of cotreatment and is inhibited by pretreatment with cycloheximide, we further investigated which factor(s), induced by dexamethasone, might be responsible for this effect. We report that dexamethasone increases both retinoid X receptor-alpha (RXRalpha) and PXR mRNA expression in cultured human hepatocytes, whereas PXR activators such as rifampicin and clotrimazole do not. Accumulation of RXRalpha and PXR mRNA reaches a maximum at a concentration of 100 nM dexamethasone after treatment for 6 to 12 h and is greatly diminished by RU486. A similar pattern of expression is observed with tyrosine aminotransferase mRNA. Moreover, the effect of dexamethasone on PXR mRNA accumulation seems to be through direct action on the glucocorticoid receptor (GR) because the addition of cycloheximide has no effect, and dexamethasone does not affect the degradation of PXR mRNA. Furthermore, dexamethasone induces the accumulation of a RXRalpha-immunoreactive protein and increases the nuclear level of RXRalpha:PXR heterodimer as shown by gel shift assays with a CYP3A4 ER6 PXRE probe. This accumulation of latent PXR and RXRalpha in the nucleus of hepatocytes explains the synergistic effect observed with dexamethasone and PXR activators together on CYP3A4 induction. These results reveal the existence of functional cross talk between the GR and PXR, and may explain some controversial aspects of the role of the GR in CYP3A4 induction.     

孕烷X受体的研究进展及其在药物代谢中的作用

孕烷X受体(pregnane X receptor,PXR)是机体对有毒物质适应性防卫机制的一个重要组成部分,PXR被大量的外源性和内源性化学物质激活,这些物质包括类固醇、抗生素、抗真菌的物质和胆汁酸等.PXR配体结合区域三维结构显示它具有一个特殊球形配体结合腔,这种结构允许PXR与广泛的疏水性化学物质结合.PXR与9-顺式维甲酸受体(9-cis retinoic aid receptor,RXR)以异型二聚体的形式与细胞色素氧化酶P450 3A家族和其他参与药物代谢的Ⅱ相药物代谢酶以及药物转运蛋白的DNA响应元件结合,通过外源物刺激诱导多个基因的表达.分析PXR的结构与功能对药物设计和筛选具有重要的应用价值.     

Indirect activation of pregnane X receptor in the induction of hepatic CYP3A11 by high-dose rifampicin in mice

Rifampicin (RIF), a typical ligand of human pregnane X receptor (PXR), powerfully induces the expression of cytochrome P450 3A4 (CYP3A4) in humans. Although it is thought that RIF is not a ligand of rodent PXR, treatment with high-dose RIF (e.g. more than 20?mg/kg) increases the expression of CYP3A in the mouse liver. In this study, we investigated whether the induction of CYP3A by high-dose RIF in the mouse liver is mediated via indirect activation of mouse PXR (mPXR). The results showed that high-dose RIF increased the expression of CYP3A11 and other PXR-target genes in the liver of wild-type mice but not PXR-knockout mice. However, the results of reporter gene and ligand-dependent assembly assays showed that RIF does not activate mPXR in a ligand-dependent manner. In addition, high-dose RIF stimulated nuclear accumulation of mPXR in the mouse liver, and geldanamycin and okadaic acid attenuated the induction of Cyp3a11 and other PXR-target genes in primary hepatocytes, suggesting that high-dose RIF triggers nuclear translocation of mPXR. In conclusion, the present study suggests that high-dose RIF stimulates nuclear translocation of mPXR in the liver of mice by indirect activation, resulting in the transactivation of Cyp3a11 and other PXR-target genes.     

Induction of cytochrome P450 3A by paclitaxel in mice: pivotal role of the nuclear xenobiotic receptor, pregnane X receptor.

Paclitaxel, a taxane anti-microtubule agent, is known to induce CYP3A in rat and human hepatocytes. Recent studies suggest that a member of the nuclear receptor family, pregnane X Receptor (PXR), is a key regulator of the expression of CYP3A in different species. We investigated the role of PXR activation, in vitro and in vivo, in mediating Cyp3a induction by paclitaxel. Pregnenolone 16 alpha-carbonitrile (PCN), an antiglucocorticoid, was employed as a positive control for mouse PXR (mPXR) activation in vitro, and Cyp3a induction in vivo. In cell based reporter gene assays paclitaxel and PCN activated mPXR with an EC(50) of 5.6 and 0.27 microM, respectively. Employing PXR wild-type and transgenic mice lacking functional PXR (-/-), we evaluated the expression and activity of CYP3A following treatment with paclitaxel and PCN. Paclitaxel significantly induced CYP3A11 mRNA and immunoreactive CYP3A protein in PXR wild-type mice. Consistent with kinetics of CYP3A induction, the V(max) of testosterone 6 beta-hydroxylation in microsomal fraction increased 15- and 30-fold in paclitaxel- and PCN-treated mice, respectively. The Cyp3a induction response was completely abolished in paclitaxel- and PCN-treated PXR-null mice. This suggests that paclitaxel-mediated CYP3A induction in vivo requires an intact PXR-signaling mechanism. Our study validates the use of PXR activation assays in screening newer taxanes for potential drug interactions that may be related to PXR-target gene induction.     

A pregnane X receptor agonist with unique species-dependent stereoselectivity and its implications in drug development

Pregnane X receptor (PXR) is an orphan nuclear receptor that regulates the expression of genes encoding drug-metabolizing enzymes and transporters. In addition to affecting drug metabolism, potent and selective PXR agonists may also have therapeutic potential by removing endogenous and exogenous toxins. In this article, we report the synthesis and identification of novel PXR agonists from a library of peptide isosteres. Compound S20, a C-cyclopropylalkylamide, was found to be a PXR agonist with both enantiomer- and species-specific selectivity. S20 has three chiral carbons and was resolved into its two enantiomers. The individual S20 enantiomers exhibited striking mouse/human-specific PXR activation, whereby enantiomer (+)-S20 preferentially activated hPXR, and enantiomer (-)-S20 was a better activator for mPXR. As a human PXR (hPXR) agonist, (+)-S20 was more potent and efficacious than rifampicin. Mutagenesis studies revealed that the ligand binding domain residue Phe305 is critical for the preference for the (-)-S20 enantiomer by the rodent PXR. Treatment of S20 induced the expression of drug-metabolizing enzymes and transporters in reporter gene assays, in primary human hepatocytes, and in "humanized" hPXR transgenic mice. To our knowledge, S20 represents the first compound whose enantiomers have opposite species preference in activating a xenobiotic receptor. The stereoselectivity may be used to guide the development of safer drugs to avoid drug-drug interactions or to achieve human-specific therapeutic effects when a xenobiotic receptor is being used as a drug target.     

The pregnane X receptor: from bench to bedside

BACKGROUND: The pregnane X receptor (PXR; NR1I2), a member of the nuclear receptor superfamily, regulates the expression of metabolic enzymes and transporters involved in the response of mammals to their chemical environment. OBJECTIVE: To summarize the functions and clinical implications of PXR. METHODS: In the current review, the clinical implications of PXR are discussed, and the use of genetically engineered PXR mouse models is highlighted. RESULTS/CONCLUSION: Recent advances in mouse models, including Pxr-null and PXR-humanized mice, provide in vivo tools for evaluating the physiological functions of PXR and its role in controlling xenobiotic metabolism and transport. By using the PXR knockout and humanized mouse models, PXR was found to influence drug-drug interactions, hepatic steatosis, and the homeostasis of vitamin D, bile acids, and steroid hormones. PXR was also shown to influence inflammatory bowel diseases.     

The role of stress in the regulation of drug metabolizing enzymes in mice.

The role of stress in the regulation of several enzymatic systems which are involved in the biotransformation of xenobiotics in the liver was investigated in this study using restraint stress as a stress model. The results demonstrated that stress suppressed total basal P450 content (35%) and basal ethoxyresorufin 7-dealkylase (EROD) activity (33%), while slightly increasing basal methoxyresorufin 7-dealkylase (MROD) activity (20%). Basal pentoxyresorufin 7- dealkylase (PROD) and coumarin 7-hydroxylase (COH) activities were not affected. On the other hand, restraint stress increased total P450 content in 1,4-bis[2-(3,5- dichloropyridyloxy)]benzene (TCPOBOP)-treated mice (35%), while slightly suppressing PROD activity (26%). In addition, CYP2E1 dependent p-nitrophenol hydroxylation (PNP), was suppressed (40%) by stress in TCPOBOP-treated animals and cytosolic aldehyde dehydrogenases were not affected. Although stress had no effect on basal P4502A5 activity, the inducibility of this hepatic activity increased 2-fold after stress exposure. A pronounced suppression (7-fold) in glutathione content was observed in lungs of TCPOBOP treated mice after stress, whereas basal levels remained unaffected. In addition, only a slight suppression (20%) in liver glutathione content was found in both treatment groups. Northern blot analysis revealed that restraint stress had a relatively suppressive effect on control CYP1A2 expression in the liver. In contrast, stress markedly enhanced the expression of liver CYP2A5 in TCPOBOP-treated mice, but did so to a lesser extent in controls. Stress also increased CYP2A5 mRNA in TCPOBOP-treated mice to a greater degree than the activity of the corresponding cytochrome. On the other hand, liver P4502A5 activity was found to be induced by TCPOBOP by about 2.5-fold. However, the drug does not appear to be involved in the expression of CYP2A5. Finally, although the activity of liver P4502A5 cytochrome was found to be increased 3, 8 and 27 h after stress, after which it gradually declined up to 75 h, CYP2A5 liver expression appeared to be suppressed 3, 8, 27 and 51 h after stress, while 75 h later it apparently reached normal levels. In conclusion, the results of this study showed that restraint stress significantly alters several enzymatic systems differently at a basal level than under conditions of TCPOBOP induction. In addition, stress was found to significantly interfere with the expression processes of CYP1A2 and CYP2A5.     

药物过量时激活核受体孕烷X受体促进毒物消除的解毒机制

目的考察喹硫平(QTP)在正常剂量与中毒剂量下对大鼠脑与肝内孕烷X受体(PXR)、细胞色素P4503A4(CYP3A4)、P-糖蛋白(P-gp)的mRNA表达的影响,并考察QTP中毒情况下给予PXR激活剂(地塞米松)对大鼠上述指标的影响,探索药物过量时激活PXR信号通路促进毒物消除的分子机制。方法正常组大鼠腹腔注射正常剂量QTP 10 mg·kg^-1。模型组大鼠腹腔注射中毒剂量QTP 100 mg·kg^-1诱导QTP中毒模型。实验组大鼠连续4 d腹腔注射地塞米松(DEX)2.5 mg·kg^-1·d^-1诱导PXR激活模型后,腹腔注射中毒剂量QTP 100 mg·kg^-1。按体重将SD大鼠随机分为4组,每组18只:空白组、正常组、模型组和实验组。空白组大鼠连续4 d腹腔注射大豆油,第4天腹腔注射甘油-水溶液;正常组大鼠连续4 d腹腔注射大豆油,第4天腹腔注射正常剂量喹硫平10 mg·kg^-1;模型组大鼠连续4 d腹腔注射大豆油,第4天腹腔中毒剂量注射喹硫平100 mg·kg^-1;实验组连续4 d腹腔注射地塞米松2.5 mg·kg^-1,第4天腹腔注射中毒剂量喹硫平100 mg·kg^-1。于第4天处置12,24,48 h后麻醉,采集大鼠的肝、海马及前额叶皮质。用实时定量荧光PCR技术测定各组大鼠肝、海马及前额叶皮质中PXR、CYP3A4、P-gp的mRNA表达。结果给药24h后,空白组、正常组、模型组和实验组大鼠肝PXR mRNA的表达分别为(15.8±0.8)×10^-3,(27.8±2.4)×,(33.3±1.4)×10^-3,(49.2±2.0)×10^-3,正常组与空白组比较,差异有统计学意义(P<0.05);模型组与正常组比较,差异有统计学意义(P<0.05);实验组与10^-3模型组比较,差异有统计学意义(P<0.05)。这4组在肝CYP3A4及P-gp的结果与在肝PXR的结果一致。各组在海马及前额叶皮质的结果与在肝的结果一致。QTP可诱导大鼠脑与肝内PXR、CYP3A4和P-gp的mRNA表达并呈剂量依赖性;合用DEX可加速中毒剂量QTP对PXR、CYP3A4和P-gp的诱导作用。结论QTP可自我诱导PXR-CYP3A4/P-gp这一信号通路,当QTP过量时这种诱导效应更明显,合用PXR激动剂可加速该诱导过程。药物过量时。激活PXR信号通路可快速有效激活解毒系统,从而可能达到快速解毒以及保护器官的目的。     

孕烷X受体介导的代谢酶和转运体转录调控及其在化疗药物耐药中的作用

孕烷X受体(PXR,NR1I2)是生物体内药物代谢酶和转运体基因表达的主要调控因子之一.近来研究发现,PXR介导的药物代谢酶和转运体的过表达,与化疗药物多药耐药的产生密切相关.鉴于PXR在药物代谢酶和转运体调控中的重要性和PXR转录调控的多样性,有必要对其导致的多药耐药形成机制进行更深入的研究.本文综述了PXR介导的代谢酶和转运体基因表达调控机制,及其引起化疗药物多药耐药的相关研究进展,为提高化疗药物敏感性、逆转化疗药物的多药耐药提供有效的治疗策略.     

孕烷X受体及组成性雄甾烷受体的研究新进展

孕烷受体(pregnane X receptor,PXR)和组成性雄甾烷受体(constitutive androstane receptor,CAR)是核受体(nuclear receptor,NR)亚家族的重要成员;为配体活化的转录因子,能调控大量的靶基因。本文主要对其基本结构、机制及参与转录活化的辅助因子作简要介绍,重点讲述了它们在调节药物代谢与转运、糖异生及生酮作用、脂质代谢以及炎症反应等方面的意义。通过对PXR及CAR的研究,可以有效预测和防止药物相互作用;为寻找疾病治疗新靶标提供方向。