Gene/Protein Disease Symptom Drug Enzyme Compound
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Query: EC:1.14.13.97 (CYP3A4)
 6,365 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Phenobarbital (PB) response elements are composed of various nuclear receptor (NR)-binding sites. A 51-bp distal element PB-responsive enhancer module (PBREM) conserved in the PB-inducible CYP2B genes contains two NR-binding direct repeat (DR)-4 motifs. Responding to PB exposure in liver, the NR constitutive active receptor (CAR) translocates to the nucleus, forms a dimer with the retinoid X receptor (RXR), and activates PBREM via binding to DR-4 motifs. For CYP3A genes, a common NR site [DR-3 or everted repeat (ER)-6] is present in proximal promoter regions. In addition, the distal element called the xenobiotic responsive module (XREM) is found in human CYP3A4 genes, which contain both DR-3 and ER-6 motifs. Pregnane X receptor (PXR) could bind to all of these sites and, upon PB induction, a PXR:RXR heterodimer could transactivate XREM. These response elements and NRs are functionally versatile, and capable of responding to distinct but overlapping groups of xenochemicals.
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PMID:Phenobarbital response elements of cytochrome P450 genes and nuclear receptors. 1126 53

Intestinal P-glycoprotein, which is encoded by the MDR1 gene, plays an important role in the absorption and presystemic elimination of many xenobiotics. Hence, an understanding of the factors regulating its expression and function is of substantial interest. In addition to genetic factors, exposure to drugs such as rifampin can profoundly affect its expression. So far, the mechanisms by which rifampin induces MDR1 expression are poorly understood. Recent studies demonstrate that the nuclear receptor PXR (pregnane X receptor) is involved in xenobiotic induction of CYP3A4. Because CYP3A4 and MDR1 are often co-induced, we investigated whether a similar mechanism is also involved in MDR1 induction. The human colon carcinoma cell line LS174T was used as an intestinal model to study induction because in these cells the endogenous MDR1 gene is highly inducible by rifampin. The 5'-upstream region of human MDR1 was examined for the presence of potential PXR response elements. Several binding sites were identified that form a complex regulatory cluster at about -8 kilobase pairs. Only one DR4 motif within this cluster is necessary for induction by rifampin. We conclude that induction of MDR1 is mediated by a DR4 motif in the upstream enhancer at about -8 kilobase pairs, to which PXR binds.
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PMID:Nuclear receptor response elements mediate induction of intestinal MDR1 by rifampin. 1129 22

Although CYP3A induction by dexamethasone has been extensively documented, its mechanism is still unclear because both the role of the glucocorticoid receptor and the ability of dexamethasone to activate the human pregnane X receptor have been questioned. In an attempt to resolve this problem, we investigated the response of CYP3A4 to dexamethasone (10 nm-100 microm) in primary human hepatocytes and HepG2 cells, using a variety of methods: kinetic analysis of CYP3A4 and tyrosine aminotransferase expression, effects of RU486 and cycloheximide, ligand binding assay, cotransfection of HepG2 cells with CYP3A4 reporter gene constructs and vectors expressing the glucocorticoid receptor, pregnane X receptor or constitutively activated receptor. In contrast to rifampicin (monophasic induction), dexamethasone produces a biphasic induction of CYP3A4 mRNA consisting of a low-dexamethasone component (nmol concentrations) of low amplitude (factor of 3-4) followed by a high-dexamethasone component (supramicromolar concentrations) of high amplitude (factor of 15-30). We show that the low-dexamethasone component results from the glucocorticoid receptor-mediated expression of pregnane X receptor and/or constitutively activated receptor which, in turn, are able to transactivate CYP3A4 in a xenobiotic-independent manner. At supramicromolar concentrations (>10 microm), dexamethasone binds to and activates pregnane X receptor thus producing the high-dexamethasone component of CYP3A4 induction. We conclude that, in contrast to the other xenobiotic inducers of CYP3A4, glucocorticoids play a dual role in CYP3A4 expression, first by controlling the expression of PXR and CAR under physiological conditions (submicromolar concentrations) through the classical glucocorticoid receptor pathway, and second by activating the pregnane X receptor under bolus or stress conditions (supramicromolar concentrations).
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PMID:Dual effect of dexamethasone on CYP3A4 gene expression in human hepatocytes. Sequential role of glucocorticoid receptor and pregnane X receptor. 1173 89

The bulk of characterized xenobiotic defense and disposition is conferred by the abundant enzymes cytochrome P450 3A4 and P-glycoprotein. Although expressed in many tissues, these enzymes are most abundant in the liver and intestine and seem to share most substrates and inhibitors, with the apparent synergy between these two promiscuous enzymes asserted because of their extensive overlap of substrates and shared tissue location. Since the broad-spectrum tolerance to lipophilic compounds of various sizes naturally results in a similar pattern of substrate/inhibitor recognition, the cause or mechanism of many drug/drug and drug/herb interactions can be difficult to determine. These two seemingly indiscriminate enzymes, however, do not share some unique inhibitor selectivity. Particularly, we show various potent CYP3A4 inhibitors that do not affect P-gp active transport function. Remarkably, we have also identified several compounds-valinomycin, norverapamil, reserpine, nobiletin, emetine, gallopamil, fluphenazine-that uniquely inhibit P-gp function with affinities comparable to benchmark P-gp inhibitors despite a lack of effect on CYP3A4 function at physiologically relevant concentrations. Indeed, valinomycin inhibits P-gp with an IC(50) similar to cyclosporin A yet apparently does not affect CYP3A4 function, and emetine and nobiletin are also specific for interaction with P-gp. Additionally, norverapamil and reserpine have, respectively, a 60- and 40-fold preference for inhibition of P-gp over CYP3A4. Some striking structural analogies among these compounds are discussed. These distinguishing qualities of substrate recognition between CYP3A4 and P-gp should reveal nuances of active-site architecture unique to each and could serve as tools to probe for the specific discernment of P-gp-mediated drug/drug or drug/herb interactions. Learning more about binding distinctions and quantitative activity relationships of substrate/inhibitor interactions with these two enzymes and the differences between them may indicate how they recognize such a wide variety of molecules as substrates (and/or inhibitors). Moreover, identification of specific inhibitors will allow the determination of which enzyme is responsible for drug interactions and/or the extent of contribution in a multiple exposure situation.
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PMID:Quantitative distinctions of active site molecular recognition by P-glycoprotein and cytochrome P450 3A4. 1174 42

Environmental chemicals are one of the risk factors in breast cancer genesis. Cytochrome P450 (CYP) enzymes play a major role in the activation of these chemicals. Using highly specific and sensitive reverse transcriptase-polymerase chain reaction (RT-PCR) analysis. the expression profile of all major xenobiotic metabolizing CYP forms was screened in breast tumour and surrounding tumour free (control) breast tissue in a series of 20 sample pairs obtained from females with infiltrating ductal carcinoma. The levels of CYPIAI mRNA were very low in both tumour and normal tissue. CYP1B1, CYP2B6, CYP2C, CYP2D6, CYP2E1, CYP4B1, and CYP11A1 expressions were positive in both tumours and control tissue. CYP2A6, CYP2A7, CYP2A13, CYP2F1, CYP3A4, CYP3A5. and CYP3A7 mRNAs were expressed neither in tumours nor in control tissue. These results show that several CYPs. responsible for the activation of a quite large number of procarcinogens and genotoxic estrogen metabolites. are expressed in breast tissue with a lack of qualitative differences in CYP expression at mRNA level between breast tumours and surrounding normal breast.
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PMID:The expression of cytochrome P450 enzymes in human breast tumours and normal breast tissue. 1176 4

The pregnane X receptor (PXR) is a promiscuous nuclear receptor that responds to a wide variety of drugs, xenobiotics and endogenous compounds, and plays a critical role in mediating drug-drug interactions in humans. PXR is the master regulator of the expression of the CYP3A4 gene, which encodes for the most abundant and promiscuous drug-metabolizing enzyme in humans. PXR also regulates the expression of other genes involved in xenobiotic metabolism, including CYP2C8, CYP2C9, CYP2B6, GSTA2 and MDR1, as well as genes critical to bile acid metabolism. While PXR functions as a xenobiotic sensor in numerous vertebrates, its relatively low sequence conservation across species causes the PXRs from different organisms to respond to distinct subsets of xenobiotics. Thus, PXR promiscuity is directed and not random. The recent determination of crystal structures of the ligand binding domain of human PXR has provided the first detailed molecular view of this promiscuous receptor, and has advanced our understanding of its varied biological functions. We review the evidence establishing the binding promiscuity of PXR and its directed specificity in different species, and analyze the structural determinants of these characteristics. In addition, we examine the relationship between the interaction of PXR with ligands and the manner in which CYP3A4 is thought to bind to substrate molecules. The accumulating structural and functional data on PXR may facilitate the development of improved methods for in vitro, in vivo and in silico screening for PXR activation.
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PMID:Structural insights into the promiscuity and function of the human pregnane X receptor. 1186 69

Many xenobiotics are metabolized and detoxified by cytochrome P-450s (CYP). The xenobiotics metabolizing CYPs are induced by various kinds of receptors. To induce CYP1A1, the Ah receptor requires a ligand for its activation as a transcription factor. On the other hand, benzimidazole compounds can induce CYP1A1 without binding to the Ah receptor as a ligand (ligand-independent pathway). In response to phenobarbital (PB) and other PB-type inducers, the nuclear receptor CAR (the NR-constitutive active receptor) translocates to the nucleus, forms a dimer with the retinoid X receptor (RXR), and activates the PB-responsive enhancer module (PBREM) in the PB-inducible CYP2B genes. For human CYP3A4 genes, pregnane X receptor (PXR) binds to the xenobiotic-responsive enhancer module (XREM) and upon induction by rifampicin, a PXR:RXR heterodimer could transactivate XREM.
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PMID:[Regulation of cytochrome P-450 (CYP) genes by nuclear receptors]. 1186 91

Human CYP3A4 is the major cytochrome P450 isoenzyme in adult human liver and is known to metabolise many xenobiotic and endogenous compounds. There is substantial inter-individual variation in the hepatic levels of CYP3A4. Although, polymorphic mutations have been reported in the 5' regulatory region of the CYP3A4 gene, those that have been investigated so far do not appear to have any effect on gene expression. To determine whether other mutations exist in this region of the gene, we have performed a new population screen on a panel of 101 human DNA samples. A 1140 bp section of the 5' proximal regulatory region of the CYP3A4 gene, containing numerous regulatory motifs, was amplified from genomic DNA as three overlapping segments. The 300 bp distal enhancer region at -7.9kb containing additional regulatory motifs was also amplified. Mutation analysis of the resulting PCR products was carried out using non-radioactive single strand conformation polymorphism (SSCP) and confirmatory sequencing of both DNA strands in those samples showing extra SSCP bands. In addition to detection of the previously reported CYP3A4*1B allele in nine subjects, three novel alleles were found: CYP3A4*1E (having a T-->A transversion at -369 in one subject), CYP3A4*1F (having a C-->G tranversion at -747 in 17 subjects) and CYP3A4*15B containing a nine-nucleotide insertion between -845 and -844 linked to an A-->G transition at -392 and a G-->A transition in exon 6 (position 485 in the cDNA) in one subject. All the novel alleles were heterozygous. No mutations were found in the upstream distal enhancer region. Our results clearly indicate that this rapid and simple SSCP approach can reveal mutant alleles in drug metabolising enzyme genes. Detection and determination of the frequency of novel alleles in CYP3A4 will assist investigation of the relationship between genotype, xenobiotic metabolism and toxicity in the CYP3A family of isoenzymes.
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PMID:Mutation analysis of the human CYP3A4 gene 5' regulatory region: population screening using non-radioactive SSCP. 1189 Sep 39

Tamoxifen is a widely utilized antiestrogen in the treatment and chemoprevention of breast cancer. Clinical studies document that tamoxifen administration markedly enhances the systemic elimination of other drugs. Additionally, tamoxifen enhances its own clearance following repeated dosing. The mechanisms that underlie these clinically important events remain unresolved. Here, we report that tamoxifen and its metabolite 4-hydroxytamoxifen markedly induce cytochrome P450 3A4, a drug-metabolizing enzyme of central importance, in primary cultures of human hepatocytes. Tamoxifen and 4-hydroxytamoxifen (1-10 microM) significantly increased the CYP3A4 expression and activity (measured as the rate of testosterone 6beta-hydroxylation). Maximal induction was achieved at the 5 microM level. At this level, tamoxifen and 4-hydroxytamoxifen caused a 1.5- to 3.3-fold (mean, 2.1-fold) and 3.4- to 17-fold (mean, 7.5-fold) increase in the CYP3A4 activity, respectively. In comparison, rifampicin treatment resulted in a 6- to 16-fold (mean, 10.5-fold) increase. We also observed corresponding increase in the CYP3A4 immunoreactive protein and mRNA levels. Furthermore, tamoxifen and 4-hydroxytamoxifen efficaciously activated the human pregnane X receptor (hPXR; also known as the steroid xenobiotic receptor), a key regulator of CYP3A4 expression. The efficacy of tamoxifen and 4-hydroxytamoxifen relative to rifampicin for hPXR activation was approximately 30 and 60%, respectively. Our results indicate that the mechanism of tamoxifen-mediated alteration in drug clearance pathways in humans may involve CYP3A4 induction by the parent drug and/or its metabolite. Furthermore, the CYP3A4 induction may be a result of hPXR activation. These findings have important implications for optimizing the use of tamoxifen and in the development of newer antiestrogens.
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PMID:Induction of cytochrome P450 3A4 in primary human hepatocytes and activation of the human pregnane X receptor by tamoxifen and 4-hydroxytamoxifen. 1195 Jul 95

The fully active dihydroxylated metabolite of vitamin D(3) induces the expression of CYP3A4 and, to a lesser extent, CYP2B6 and CYP2C9 genes in normal differentiated primary human hepatocytes. Electrophoretic mobility shift assays and cotransfection in HepG2 cells using wild-type and mutated oligonucleotides revealed that the vitamin D receptor (VDR) binds and transactivates those xenobiotic-responsive elements (ER6, DR3, and DR4) previously identified in CYP3A4, CYP2B6, and CYP2C9 promoters and shown to be targeted by the pregnane X receptor (PXR) and/or the constitutive androstane receptor (CAR). Full VDR response of various CYP3A4 heterologous/homologous promoter-reporter constructs requires both the proximal ER6 and the distal DR3 motifs, as observed previously with rifampicin-activated PXR. Cotransfection of a CYP3A4 homologous promoter-reporter construct (including distal and proximal PXR-binding motifs) and of PXR or CAR expression vectors in HepG2 cells revealed the ability of these receptors to compete with VDR for transcriptional regulation of CYP3A4. In conclusion, this work suggests that VDR, PXR, and CAR control the basal and inducible expression of several CYP genes through competitive interaction with the same battery of responsive elements.
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PMID:Expression of CYP3A4, CYP2B6, and CYP2C9 is regulated by the vitamin D receptor pathway in primary human hepatocytes. 1199 50


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