Regulation of the CYP3A4 gene by hydrocortisone and xenobiotics: role of the glucocorticoid and pregnane X receptors.

The molecular mechanisms of regulation of the CYP3A4 gene have been examined in an in vitro reporter gene system, containing -1 kb of the CYP3A4 promoter, in a HepG2 cell line. This system allows for the separate and combined transfection of expression plasmids encoding the human glucocorticoid receptor (hGR) and the human pregnane X receptor (hPXR), and, therefore, the opportunity to assess the role of these receptors in the induction process. Hydrocortisone produces a dose-dependent increase in CYP3A4 activation, a response that is increased in the presence of either receptor. Moreover, transfection of the hPXR decreased the EC(50) for hydrocortisone-dependent induction by a factor of 3.3, a response that was not changed by simultaneous cotransfection of the hGR. In addition, the hydrocortisone dose-response curve falls within the physiological blood level concentration of this steroid, implicating a regulatory role for hydrocortisone in the basal level of CYP3A4 expression. Although the responses to dexamethasone and rifampicin were both increased by both receptors, dexamethasone activation of CYP3A4 was similar for both the hGR and the hPXR, whereas rifampicin-dependent activation favored the hPXR. We conclude that regulation of the CYP3A4 gene is receptor-dependent and that hydrocortisone may function as a regulator of basal expression via the hPXR and the hGR. The implications of this latter conclusion for possible regulatory interactions between hydrocortisone and xenobiotic inducers remain to be clarified.     

A high-throughput cell-based reporter gene system for measurement of CYP1A1 induction

INTRODUCTION: Enzyme induction is undesirable in new drug discovery process, with consequences spanning from auto-induction to toxicity. Cytochrome P450 (CYP) 1A1 has long been known to be one of the metabolic enzymes involved in activating many procarcinogens, the first step toward tumor formation during chemical carcinogenesis. Induction of CYP1A1 during drug treatment may predispose the patients to some risk of chemical carcinogenesis. METHODS: Based on the signal-transduction mechanism of CYP1A1 induction, a high-throughput reporter-gene system was established by stable transformation of H4IIE cells to incorporate the luciferase gene under control of CYP1A1 promoter. This stable cell line was validated with known CYP1A1 inducers, such as 3-methylcholanthrene (3-MC), beta-naphthoflavone (beta-NF), alpha-naphthoflavone (alpha-NF) and 3-indocarbinol. Thirty in-house new chemical entities (NCEs) were then screened with this reporter-gene system, and also administered to rats to evaluate in vivo CYP1A1 induction. RESULTS: CYP1A1 reporter gene system can be used to identify strong inducers, such as 3-MC, beta-NF and alpha-NF, and weak inducers, such as 3-indocarbinol. In vitro induction of 30 in-house compounds in reporter gene system did not correlate with in vivo induction in rat liver microsome measured by ethoxyresorufin-O-dealkylation (EROD) activity, but had a reasonable correlation with Western blot signals. DISCUSSION: This reporter-gene system may be useful in eliminating compounds that can cause CYP1A1 induction at an early stage of drug discovery.     

CYP3A4 induction by drugs: correlation between a pregnane X receptor reporter gene assay and CYP3A4 expression in human hepatocytes.

Induction of cytochrome P450 3A4 (CYP3A4) is determined typically by employing primary culture of human hepatocytes and measuring CYP3A4 mRNA, protein and microsomal activity. Recently a pregnane X receptor (PXR) reporter gene assay was established to screen CYP3A4 inducers. To evaluate results from the PXR reporter gene assay with those from the aforementioned conventional assays, 14 drugs were evaluated for their ability to induce CYP3A4 and activate PXR. Sandwiched primary cultures of human hepatocytes from six donors were used and CYP3A4 activity was assessed by measuring microsomal testosterone 6beta-hydroxylase activity. Hepatic CYP3A4 mRNA and protein levels were also analyzed using branched DNA technology/Northern blotting and Western blotting, respectively. In general, PXR activation correlated with the induction potential observed in human hepatocyte cultures. Clotrimazole, phenobarbital, rifampin, and sulfinpyrazone highly activated PXR and increased CYP3A4 activity; carbamazepine, dexamethasone, dexamethasone-t-butylacetate, phenytoin, sulfadimidine, and taxol weakly activated PXR and induced CYP3A4 activity, and methotrexate and probenecid showed no marked activation in either system. Ritonavir and troleandomycin showed marked PXR activation but no increase (in the case of troleandomycin) or a significant decrease (in the case of ritonavir) in microsomal CYP3A4 activity. It is concluded that the PXR reporter gene assay is a reliable and complementary method to assess the CYP3A4 induction potential of drugs and other xenobiotics.     

Regulation of CYP2B6 in primary human hepatocytes by prototypical inducers.

The objectives of this study were to evaluate the ability of 14 compounds, which differentially activate human pregnane X receptor (hPXR), to induce CYP2B6 expression and to compare CYP2B6 and CYP3A4 concentration- and time-dependent induction by select inducers. Three primary human hepatocyte preparations were treated daily for 3 days with three concentrations of all compounds. Additional concentration- and/or time-response studies were conducted with clotrimazole, phenytoin, phenobarbital, and rifampin in six preparations. CYP2B6 and CYP3A4 protein and activities were assessed by Western blotting, bupropion hydroxylation, and testosterone 6beta-hydroxylation, respectively. To evaluate hPXR activation by the 14 compounds, reporter gene assays were conducted using Huh7 cells cotransfected with hPXR and a CYP2B6 (NR1)5-LUC reporter plasmid. Clotrimazole, phenobarbital, rifampin, and ritonavir strongly induced CYP2B6 and activated hPXR; dexamethasone t-butylacetate and sulfinpyrazone induced CYP2B6 weakly and activated hPXR moderately; paclitaxel strongly activated hPXR but did not increase CYP2B6 expression; carbamazepine and phenytoin moderately or strongly increased CYP2B6 expression but weakly activated hPXR; and dexamethasone, methotrexate, probenecid, sulfadimidine, and troleandomycin demonstrated weak or negligible effects on CYP2B6 and hPXR. EC50 values for CYP2B6 and CYP3A4 induction by clotrimazole, phenobarbital, phenytoin, and rifampin were strongly correlated (r2 = 0.99) and were statistically indistinguishable for clotrimazole, phenytoin, and rifampin. Kinetic constants governing time-dependent induction by phenobarbital and rifampin were also similar between CYP2B6 and CYP3A4. These results indicate that CYP2B6 is highly inducible by known CYP3A4 inducers and suggest that hPXR is a major determinant of CYP2B6-inducible expression for many, but not all, compounds evaluated in this study.     

In vivo analysis of glucocorticoid-induced reporter gene expression using gene gun DNA delivery

Glucocorticoid regulates various physiological processes via the activation and repression of gene expression. The anti-inflammatory effects and the adverse effects are believed to be dependent on the repression and the activation of genes, respectively. Reporter gene assay is a useful technique to separately evaluate these two functions and has been used for in vitro screening of novel ligands for the glucocorticoid receptor (GR). We report here the application of a reporter gene assay for the in vivo determination of the GR-mediated gene activation. A reporter plasmid containing glucocorticoid response elements was introduced to abdominal mouse skin using a gene gun. Administration of prednisolone induced the expression of the reporter gene, only when the GR expression plasmid was co-transfected with the reporter plasmid. Endogenous levels of corticosterone appeared to be negligible in this protocol. The dose response for this induction was comparable to those for the decreases in thymus weight and serum corticosterone. These results suggest that gene gun-mediated skin transfection enables the in vivo reporter gene assay and that this technique can be used to predict the potency of ligands for the GR-mediated gene activation.     

Histone deacetylase inhibitor stimulate<Emphasis Type="Italic">CYP3A4</Emphasis> proximal promoter activity in hepg2 cells

The expression of CYP3A4 gene is induced by a variety of structurally unrelated xenobiotics including the antibiotic rifampicin, pregnenolone 16-carbonitrile (PCN), and endogenous hormones, that might mediate through steroid and xenobiotic receptor (SXR) system. The molecular mechanisms underlying regulation of CYP3A4 gene expression have not been understood. In order to gain the insight of the molecular mechanism of CYP3A4 gene expression, study has been undertaken to investigate if the histone deacetylation is involved in the regulation of CYP3A4 gene expression by proximal promoter in human hepatoma HepG2 cells. Also we have investigated to see if SXR is involved in the regulation of CYP3A4 proximal promoter activity in human hepatoma HepG2 cells. HepG2 cells were transfected with a plasmid pCYP3A4-Luc containing approximately 1 kb of the CYP3A4 proximal promoter region (-863 to +64 bp) in front of a reporter gene, luciferase, in the presence or absence of pSAP-SXR. In HepG2 cells, CYP3A4 inducers, such as rifampicin, PCN and RU486 showed minimal stimulation of CYP3A4 proximal promoter activity in the absence of SXR and histone deacetylase (HDAC) inhibitors. 4-Dimethylamino-N-[4-(2-hydroxycarbamoylvinyl)benzyl]benzamide (IN2001), a new class HDAC inhibitor significantly increased CYP3A4 proximal promoter activity over untreated control cells and rifampicin concomitant treatment with IN2001 increased further CYP3A4 proximal promoter activity that was stimulated by IN2001. The results of this study demonstrated that both HDAC inhibitors and SXR are essential to increase of CYP3A4 proximal promoter activity by CYP3A4 inducers such as PCN, rifampicin, and RU486. Especially SXR seems to be important for the dose dependent response of CYP3A4 inducing chemicals to stimulate CYP3A4 proximal promoter activity. Also this data suggested that HDAC inhibitors seemed to facilitate the CYP3A4 proximal promoter to be activated by chemicals.     

Glucocorticoid receptor enhancement of pregnane X receptor-mediated CYP2B6 regulation in primary human hepatocytes.

Although the glucocorticoid receptor (GR) facilitates the xenobiotic-induced expression of CYP2B in rodents, its role in the regulation of human CYP2B6 is unclear. In this report, the role of human GR in the regulation of CYP2B6 was evaluated using primary human hepatocytes and transfection assays with Huh7 cells. CYP2B6 expression was not induced in primary hepatocytes treated with dexamethasone (DEX) concentrations (0.01-1 microM) known to activate GR. In contrast, treatment with 0.1 microM DEX enhanced CYP2B6 induction by different pregnane X receptor (PXR) activators, including rifampin, phenytoin, clotrimazole, and phenobarbital. In Huh7 cells, cotransfection of human (h)GR and hPXR with CYP2B6-phenobarbital-responsive enhancer module (PBREM) reporter constructs revealed that all hPXR ligands induce CYP2B6 reporter gene activity, and this ligand-dependent activation is greatly enhanced by activated hGR. CYP2B6 reporter gene expression was not induced in the presence of hPXR ligands when hGR alone was cotransfected with CYP2B6 reporter construct. In hGR and human constitutive androstane receptor (hCAR) cotransfection assays, activated hGR increased the constitutive activation of PBREM reporter constructs by hCAR in the absence of inducers. In the presence of activated hGR and known inducers of CYP2B6, only PB treatment caused a further 2-fold activation of hCAR compared with control. These studies show that hGR is involved synergistically in the xenobiotic-responsive regulation of human CYP2B6 by hPXR and hCAR. Moreover, the results suggest that the GR-enhanced expression of CYP2B6 is mediated through an indirect mechanism that does not require increased expression of nuclear receptor.     

降血糖药物对细胞色素P 450 3A 4的转录调节作用

观察几种降血糖药物是否能通过活化孕烷X受体(PXR)诱导细胞色素P450 3A4(CYP 3A4)的转录表达。在人肝肿瘤细胞株HepG2细胞中,用瞬时共转染报告基因实验进行几种降血糖中药提取物在不同浓度和不同处理时间下对PXR介导的CYP 3A4的转录调节作用研究。筛选出五味子、牛蒡子、桔络3种中药提取物以及非磺脲类促胰岛素降糖药米格列奈分别作用于HepG2细胞24 h后,均能诱导PXR介导的CYP3A4基因的转录表达,诱导能力随浓度增强,而其他待测药物无类似作用。在不同处理时间的研究中,五味子、牛蒡子、桔络以及米格列奈均能在12 h~36 h内增强CYP 3A4的转录表达,诱导能力随时间延长呈增强趋势。     

A reporter gene cellular model for evaluating induction of CYP3A4 by new chemical entities

A cell‐based reporter gene assay to study CYP3A4 induction was developed in the present study. The pregnane X receptor (PXR) gene or variant (PXR2) was cloned into the pSG5 vector and cotransfected into HepG2 cells, with a construct, pGL3‐3A4‐Luc, containing the promoter/enhancer region of the CYP3A4 gene. The two systems were validated using rifampicin (RIF). The variant, PXR2, did not mediate induction of CYP3A4 by rifampicin (10 µM) whereas PXR showed dose‐dependent induction of CYP3A4, with a fold change of 40–60, compared to the vehicle control, 0.1% dimethylsulfoxide (DMSO). Further validation of the PXR/3A4 system was performed using other inducers of CYP3A4, after which CYP3A4 inducibility by a Wyeth compound, Tanaproget (TNPR), and the synthetic steroid, 3‐ketodesogestrel (3‐KDG), were assessed. At the highest concentrations tested, troleandomycin (TROL) and phenobarbital (PB) induced CYP3A4 by 15–25‐fold, while carbamazepine (CMZ), dexamethasone (DEX), and erythromycin (ERY) produced fold induction of <3. Pregnenolone 16‐α carbonitrile (PCN), a rat‐specific CYP3A inducer, did not induce CYP3A4. TNPR did not induce CYP3A4 at concentrations up to 10 µM while 3‐KDG produced a concentration‐dependent induction, with a fold change of 14 at 20 µM. These data show that the variant PXR2 is not activated by rifampicin. The PXR/3A4 system described can be used to study CYP3A4 induction and provides a robust, specific, and reproducible in vitro system that can be used to assess CYP3A4 inducibility by compounds in the drug development process. Drug Dev. Res. 67:470–475, 2006. © 2006 Wiley‐Liss, Inc.     

豆腐果苷对人孕烷X受体介导的CYP3A4和MDR1的转录调节作用

目的:建立和验证人孕烷X受体(human pregnant X receptor,hPXR)介导的CYP3A4、MDR1药物诱导剂的体外筛选体系,考察豆腐果苷对hPXR介导的CYP3A4、MDR1的转录调节作用。方法:利用构建的双荧光素酶报告基因系统,将表达载体和报告载体共转染HepG2细胞,以10μmol/L利福平为阳性对照,用不同浓度(0.004、0.04、0.4μmol/L)豆腐果苷处理48h后裂解细胞进行双荧光素酶活性检测。结果:不同浓度的豆腐果苷均不能通过激活hPXR来介导CYP3A4和MDR1表达上调,各浓度处理组的双荧光素酶比活性值与DMSO溶媒组差异无统计学意义(P>0.05)。结论:成功构建了hPXR介导的CYP3A4和MDR1药物诱导剂的体外筛选体系,并发现豆腐果苷不能通过激活hPXR介导CYP3A4和MDR1的表达上调。