Gene/Protein Disease Symptom Drug Enzyme Compound
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Query: UMLS:C0406810 (NAME)
 13,345 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

During the past several years, important advances have been made in our understanding of the mechanisms that regulate the expression of genes that determine drug clearance, including phase I and phase II drug-metabolising enzymes and drug transporters. Orphan nuclear receptors have been recognised as key mediators of drug-induced changes in both metabolism and efflux mechanisms. In this review, we summarise recent findings regarding the function of nuclear receptors in regulating drug-metabolising and transport systems, and the relevance of these receptors to clinical drug-drug interactions and the development of new drugs. Emphasis is given to two newly recognised 'orphan' receptors (the pregnane X receptor [PXR] and the constitutive androstane receptor [CAR]) and their regulation of cytochrome P450 enzymes, such as CYP3A4, CYP2Cs and CYP2B6; and transporters, such as P-glycoprotein (MDR1), multidrug resistance-associated proteins (MRPs) and organic anion transporter peptide 2 (OATP2). Although 'cross-talk' occurs between these two receptors and their target sequences, significant species differences exist between ligand-binding and activation profiles for both receptors, and PXR appears to be the predominant or 'master' regulator of hepatic drug disposition in humans. Several important physiological processes, such as cholesterol synthesis and bile acid metabolism, are also tightly controlled by certain ligand-activated orphan nuclear receptors (farnesoid X receptor [FXR] and liver X receptor [LXR]). In general, their ability to bind a broad range of ligands and regulate an extensive array of genes that are involved in drug clearance and disposition makes these orphan receptors attractive targets for drug development. Drugs have the capacity to alter nuclear receptor expression (modulators) and/or serve as ligands for the receptors (agonists or antagonists), and thus can have synergistic or antagonistic effects on the expression of drug-metabolising enzymes and transporters. Coadministration of drugs that are nuclear receptor agonists or antagonists can lead to severe toxicity, a loss of therapeutic efficacy or an imbalance in physiological substrates, providing a novel molecular mechanism for drug-drug interactions.
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PMID:Role of orphan nuclear receptors in the regulation of drug-metabolising enzymes. 1467 87

The human constitutive androstane receptor (CAR, NR1I3) is a member of the orphan nuclear receptor superfamily that plays an important role in the control of drug metabolism and disposition. In this study, we sequenced all the coding exons of the NR1I3 gene for 334 Japanese subjects. We identified three novel single nucleotide polymorphisms (SNPs) that induce non-synonymous alterations of amino acids (His246Arg, Leu308Pro, and Asn323Ser) residing in the ligand-binding domain of CAR, in addition to the Val133Gly variant, which was another CAR variant identified in our previous study. We performed functional analysis of these four naturally occurring CAR variants in COS-7 cells using a CYP3A4 promoter/enhancer reporter gene that includes the CAR responsive elements. The His246Arg variant caused marked reductions in both transactivation of the reporter gene and in the response to 6-(4-chlorophenyl)imidazo[2,1-b][1,3]thiazole-5-carbaldehyde O-(3,4-dichlorobenzyl)oxime (CITCO), which is a human CAR-specific agonist. The transactivation ability of the Leu308Pro variant was also significantly decreased, but its responsiveness to CITCO was not abrogated. The transactivation ability and CITCO response of the Val133Gly and Asn323Ser variants did not change as compared to the wild-type CAR. These data suggest that the His246Arg and Leu308Pro variants, especially His246Arg, may influence the expression of drug-metabolizing enzymes and transporters that are transactivated by CAR.
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PMID:Functional analysis of four naturally occurring variants of human constitutive androstane receptor. 1599 Mar 49

Previously, cytochrome P450 3A7 (CYP3A7), which constitutes the major CYP enzyme in fetal livers, has been considered a fetus-specific enzyme. However, CYP3A7 mRNA has recently been shown to be expressed at significant levels in a subset of adult human livers, several of which carry the CYP3A7*1C allele that contains the proximal PXR/CAR element of CYP3A4. The objective of this study was to investigate CYP3A7 expression at the protein level by developing a CYP3A7-specific antibody to allow its quantification. Based on results from 59 adult human liver samples, we found significant CYP3A7 protein expression in approximately one in 10 adult livers amounting for 24-90 pmol/mg microsomal protein, thereby contributing 9-36% to total CYP3A levels in these livers. CYP3A7 protein was detected in five of seven livers carrying the CYP3A7*1C allele (two of which only had trace amounts), whereas an additional three livers expressing CYP3A7 were apparently homozygous for CYP3A7*1. The mean protein expression level of CYP3A7 was 42 pmol/mg within the group of livers expressing CYP3A7 and 4 pmol/mg in all liver samples. CYP3A7 expression was thus higher than that of the polymorphically expressed CYP3A5 in adult human livers, based on a comparison with a previous study using our CYP3A5 peptide-specific antibody. The relatively high level of CYP3A7 protein expression detected in a subset of adult livers may be relevant with respect to the metabolism of exogenous and endogenous substrates, such as retinoic acid and dehydroepiandrosterone.
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PMID:CYP3A7 protein expression is high in a fraction of adult human livers and partially associated with the CYP3A7*1C allele. 1604 Dec 41

Both the human pregnane X receptor (hPXR) and constitutive androstane receptor (hCAR) are capable of regulating CYP3A4 and CYP2B6 gene expression. However, the majority of currently identified CYP3A4 and CYP2B6 inducers are confirmed activators of hPXR but not hCAR. To compare these receptors with respect to their chemical selectivities, 16 drugs known to induce CYP3A4 and/or CYP2B expression were evaluated for relative activation of hPXR versus hCAR. Because of the high basal but low chemical-induced activation of hCAR in immortalized cells, alternative methods were used to evaluate hCAR activation potential. Thirteen of the 16 compounds were classified as moderate to strong hPXR activators. In contrast, carbamazepine (CMZ), efavirenz (EFV), and nevirapine (NVP) were classified as negligible or weak hPXR activators at concentrations associated with efficacious CYP2B6 reporter or endogenous gene induction in primary human hepatocytes, suggesting potential activation of hCAR. Subsequent experiments demonstrated that these three drugs efficiently induced nuclear accumulation of in vivo-transfected enhanced yellow fluorescent protein-hCAR and significantly increased expression of a CYP2B6 reporter gene when hCAR was expressed in CAR-/- mice. In addition, using a recently identified, chemically responsive splice variant of hCAR (hCAR3), the hCAR activation profiles of the 16 compounds were evaluated. By combining results from the hPXR- and hCAR3-based reporter gene assays, these inducers were classified as hPXR, hCAR, or hPXR/hCAR dual activators. Our results demonstrate that CMZ, EFV, and NVP induce CYP2B6 and CYP3A4 preferentially through hCAR and that hCAR3 represents a sensitive tool for in vitro prediction of chemical-mediated human CAR activation.
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PMID:Relative activation of human pregnane X receptor versus constitutive androstane receptor defines distinct classes of CYP2B6 and CYP3A4 inducers. 1704 Oct 8

Oligonucleotide microarrays were used to study the variability of pharmacokinetics and drug metabolism (PKDM)-related gene expression in 75 normal human livers. The objective was to define and use absorption, distribution, metabolism and excretion (ADME) gene expression variability to discern co-regulated genes and potential surrogate biomarkers of inducible gene expression. RNA was prepared from donor tissue and hybridized on Agilent microarrays against an RNA mass balanced pool from all donors. Clustering of PKDM gene sets revealed donors with distinct patterns of gene expression that grouped genes known to be regulated by the nuclear receptor, pregnane X-receptor (PXR). Fold range metrics and frequency distributions from the heterogeneous human population were used to define the variability of individual PKDM genes in the 75 human livers and were placed in context by comparing expression data with basal ADME gene expression variability in an inbred and diet/environment controlled population of 27 Rhesus livers. The most variable genes in the hepatic transcriptome were mainly related to drug metabolism, intermediary metabolism, inflammation and cell cycle control. Unique patterns of expression across 75 individuals of inducible ADME gene expression allowed their expression to be correlated with the expression of many other genes. Correlated genes for AhR, CAR and PXR responsive genes (CYP1A2, CYP2B6 and CYP3A4) were identified that may be co-regulated and, therefore, provide clues to the identity of surrogate gene or protein markers for CYP induction. In conclusion, microarrays were used to define the variable expression of hepatic ADME genes in a diverse human population, the expression variability of ADME genes was compared with the expression variability in an inbred population of Rhesus monkeys, and genes were defined that may be co-regulated with important inducible CYP genes.
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PMID:Compendium of gene expression profiles comprising a baseline model of the human liver drug metabolism transcriptome. 1711 15

The constitutive androstane receptor (CAR; NR1I3) regulates the expression of genes involved in xenobiotic metabolism. Alternative splicing of the human CAR gene yields an array of mRNAs that encode structurally diverse proteins. One form of CAR, termed CAR2, contains an additional four amino acids (SPTV) that are predicted to reshape the ligand-binding pocket. The current studies show a marked, ligand-independent, CAR2-mediated transactivation of reporters containing optimal DR-3, DR-4, and DR-5 response elements, and reporters derived from the natural CYP2B6 and CYP3A4 gene promoters. Overexpression of the RXRalpha ligand binding domain was critical for achieving these effects. CAR2 interaction with SRC-1 was similarly dependent on the coexpression of RXRalpha. Mutagenesis of Ser233 (SPTV) to an alanine residue yielded a receptor possessing higher constitutive activity. Alternatively, mutating Ser233 to an aspartate residue drastically reduced the transactivation capacity of CAR2. The respective abilities of these mutagenized forms of CAR2 to transactivate a DR-4 x 3 reporter element correlated with their ability to interact with RxRalpha and to recruit SRC-1 in a ligand-regulated manner. Together, these results demonstrate a robust RXRalpha-dependent recruitment of coactivators and transactivation by CAR2. In addition, CAR2 displays novel dose responses to clotrimazole and androstanol compared with the reference form of the receptor while at the same time retaining the ability to bind CITCO. This result supports a hypothesis whereby the four-amino-acid insertion in CAR2 structurally modifies its ligand binding pocket, suggesting that CAR2 is regulated by a set of ligands distinct from those governing the activity of reference CAR.
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PMID:CAR2 displays unique ligand binding and RXRalpha heterodimerization characteristics. 1719 15

CYP3A4 is the most abundantly expressed drug-metabolizing P450 enzyme in human liver and contributes to the metabolism of a large number of drugs in use today. CYP3A4 is constitutively expressed in adult hepatocytes but it can also be transcriptionally induced by a variety of structurally diverse xenochemicals. CYP3A4 strongly contributes to the important variability in the therapeutic and toxic effects of drugs owing to the major role it plays in xenobiotic metabolism and the large intra- and inter-individual variability to which it is subjected. The functional examination of up to 13 kb of the CYP3A4 5'-flanking region has revealed that the regulation of this gene is a complex issue, with numerous transcription factors interacting with multiple promoter/enhancer elements. This also suggests that a high degree of human variability in the hepatic CYP3A4 expression could result from regulatory polymorphisms. Several transcription factors and nuclear receptors contribute to the hepatic-specific expression of CYP3A4, including: C/EBPalpha, C/EBPbeta, HNF4alpha, HNF3gamma, CAR and PXR. The induction phenomenon and the down-regulation of CYP3A4 in pathophysiological conditions, such as inflammatory situations, are key processes involved in the toxic vs. therapeutic effects of many drugs. Since CYP3A4 variation may affect the efficacy and toxicity of new drugs, development of reliable hepatic models for the assessment and prediction of the role of CYP3A4 in drug metabolism are important for drug development. Cultured human hepatocytes are the closest model to the human liver as far as CYP3A4 regulation and induction are concerned. However, other hepatic models should be considered in drug development for screening purposes owing to the limited availability of human hepatocytes.
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PMID:Transcriptional regulation and expression of CYP3A4 in hepatocytes. 1730 97

Erratic or unpredictable response to drugs remains a challenge of modern drug therapy. An important determinant of such interindividual differences in drug response is variability in the expression of drug-metabolizing enzymes and/or transporters at sites of absorption and/or tissue distribution. Variable drug-metabolizing enzyme and transporter expression can result in unpredictable exposure and tissue distribution of drugs and may manifest as adverse effects or therapeutic failure. In the past decade, important new insights have been made relating to the regulatory mechanisms governing the expression of drug-metabolizing enzymes and transporters by ligand-activated nuclear receptors. Specifically, there is compelling evidence to demonstrate that PXR, CAR, FXR, LXR, VDR, HNF4alpha, and AhR form a battery of nuclear receptors that regulate the expression of many important drug-metabolizing enzyme and transporters. In this review, the authors focus on clinically important drug-metabolizing enzymes such as CYP3A4, CYP2B6, CYP2C9, CYP2C19, UGT1A1, SULT2A1, and glutathione S-transferases and their regulation by nuclear receptors. They also review the nuclear receptor-mediated regulation of drug transporters such as MDR1, MRP2, MRP4, BSEP, BCRP, NTCP, OATP1B3, and OATP1A2. Finally, they outline how the drug development process has been affected by the current understanding of the involvement of nuclear receptors in the regulation of drug disposition genes.
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PMID:Nuclear receptors and the regulation of drug-metabolizing enzymes and drug transporters: implications for interindividual variability in response to drugs. 1744 83

St. John's wort (SJW, Hypericum perforatum) is a well-tolerated herbal medicine widely used for the treatment of mild and moderate depressions. In the last 5 years, SJW has been implicated in drug interactions, which are largely mediated by the induction of the drug metabolizing enzymes, especially CYP3A4. There is still some controversy regarding the exact mechanism of induction and the identity of the SJW constituents involved. We investigated in LS174T cells the induction of CYP3A4 by ten SJW extracts, six commercial preparations, and the purified SJW constituent hyperforin. The content of hyperforin among the commercial preparations of SJW varied 62-fold (range 0.49-30.57 mg/dose). The CYP3A4 induction was mediated by PXR, but not by CAR. The magnitude of the induction correlated statistically significantly with the content of hyperforin in commercial SJW preparations (R = 0.87, p = 0.004) and in dry extracts (R = 0.65, p = 0.03), but not with their content of flavonoids or hypericin. Most of the CYP3A4 induction response occurred in the hyperforin range encountered in the blood of patients treated with SJW preparations. A temperature-induced decrease in the hyperforin content of a selected dry SJW extract abolished the induction of CYP3A4. In conclusion, commercial SJW preparations still exhibit an enormous variability in CYP3A4 induction, which is mediated by hyperforin and PXR. SJW preparations with lower hyperforin content should reduce the frequency of clinical interactions involving this herbal drug.
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PMID:Variability in PXR-mediated induction of CYP3A4 by commercial preparations and dry extracts of St. John's wort. 1759 54

Early in vitro toxicity screening might improve the success rate of new chemical entities in pharmaceutical development. In previous studies, the advantage of cytotoxicity screening with the HepG2 cell line was shown. Cytotoxicity could be identified for 70% of the compounds in these assays as compared with known toxicity in either in vitro assays in primary hepatocytes, in in vivo assays in rats, or in (pre-)clinical development in humans. The low Phase I and II enzyme levels in HepG2 cells might have been responsible for the fact that 30% of the compounds scored negative. Therefore, we performed two follow-up studies in which Cytochrome P450 (CYP) enzymes and Phase II metabolism were examined. In the present study, the transcript levels of CYP1A1, 1A2, 2A6, 2B6, 2C8, 2C9, 2C19, 2D6, 2E1, and 3A4 were measured with quantitative PCR. Results showed that transcripts of all CYPs were present in HepG2 cells, however, mRNA levels of most CYPs were dramatically lower than in primary human hepatocytes. These results were confirmed with luminometric assays which were used to measure the enzyme activities of CYP1A1, 1A2, 2C9, and 3A4. Regulation of CYP1A1, 1A2, 2B6, 2C8, 2D6, 2E1, and 3A4 by the aryl hydrocarbon receptor, pregnane X receptor and constitutive androstane receptor was studied in HepG2 cells at the mRNA and/or enzyme level. Regulation of CYP1A1, 1A2, 2B6, and 3A4 mRNA levels was similar to the regulation in primary human hepatocytes. In contrast, CYP2C8 mRNA levels are inducible in primary human hepatocytes, but not in HepG2 cells, after treatment with PXR/CAR activators. Consistent with other studies, CYP2D6 and 2E1 transcript levels were not changed after treatment with AhR, PXR, and CAR activators. Moreover, CYP1A1 and 1A2 enzyme levels could be induced by AhR agonists and CYP3A4 by PXR agonists. As a consequence of the low levels of CYPs in HepG2 cells, cytotoxicity of several compounds might have been missed or underestimated as compared with cytotoxicity in primary human hepatocytes. Inducing HepG2 cells with particular receptor stimulators might lead to higher toxicity for several of the tested compounds. Compared to primary human hepatocytes, HepG2 cells are a relatively easy-to-handle tool to study the up-regulation of CYP1A1, 1A2, 2B6, and 3A4.
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PMID:Cytochrome P450 enzyme levels in HepG2 cells and cryopreserved primary human hepatocytes and their induction in HepG2 cells. 1763 4


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