Selective Induction of Human Hepatic Cytochromes P450 2B6 and 3A4 by Metamizole
ABSTRACT The pyrazolone drug metamizole is a widely used analgesic. Analysis of liver microsomes from patients treated with metamizole revealed selectively higher expression of cytochromes P450, CYP2B6 and CYP3A4 (3.8- and 2.8-fold, respectively), and 2.9-fold higher bupropion hydroxylase activity compared with untreated subjects. Further investigation of metamizole and various derivatives on different potential target genes in human primary hepatocytes demonstrated time- and concentration-dependent induction by metamizole of CYP2B6 (7.8- and 3.1-fold for mRNA and protein, respectively, at 100 muM) and CYP3A4 (2.4- and 2.9-fold, respectively), whereas other genes (CYP2C9, CYP2C19, CYP2D6, NADPH:cytochrome P450 reductase, ABCB1, constitutive androstane receptor (CAR), pregnane X receptor (PXR)) were not substantially altered. Using reporter gene assays, we show that metamizole is not acting as a direct ligand to either PXR or CAR, suggesting a phenobarbital-like mechanism of induction. These data warrant further studies to elucidate the drug-interaction potential of metamizole, especially in patients with long-term treatment.
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ABSTRACT: Cytochrome P450 2B6 (CYP2B6) belongs to the minor drug metabolizing P450s in human liver. Expression is highly variable both between individuals and within individuals, owing to non-genetic factors, genetic polymorphisms, inducibility, and irreversible inhibition by many compounds. Drugs metabolized mainly by CYP2B6 include artemisinin, bupropion, cyclophosphamide, efavirenz, ketamine, and methadone. is one of the most polymorphic CYP genes in humans and variants have been shown to affect transcriptional regulation, splicing, mRNA and protein expression, and catalytic activity. Some variants appear to affect several functional levels simultaneously, thus, combined in haplotypes, leading to complex interactions between substrate-dependent and -independent mechanisms. The most common functionally deficient allele is [Q172H, K262R], which occurs at frequencies of 15 to over 60% in different populations. The allele leads to lower expression in liver due to erroneous splicing. Recent investigations suggest that the amino acid changes contribute complex substrate-dependent effects at the activity level, although data from recombinant systems used by different researchers are not well in agreement with each other. Another important variant, [I328T], occurs predominantly in Africans (4-12%) and does not express functional protein. A large number of uncharacterized variants are currently emerging from different ethnicities in the course of the 1000 Genomes Project. The polymorphism is clinically relevant for HIV-infected patients treated with the reverse transcriptase inhibitor efavirenz, but it is increasingly being recognized for other drug substrates. This review summarizes recent advances on the functional and clinical significance of CYP2B6 and its genetic polymorphism, with particular emphasis on the comparison of kinetic data obtained with different substrates for variants expressed in different recombinant expression systems.Frontiers in Genetics 03/2013; 4:24. DOI:10.3389/fgene.2013.00024
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ABSTRACT: Retinoic acid receptors (RARs), retinoid X receptors (RXRs) and thyroid hormone receptors (TRs) are nuclear receptors that are crucial transcriptional regulators of many cellular processes such as differentiation, development, apoptosis, carbohydrate and lipid metabolism, homeostasis etc. In addition, RXRs are common heterodimerization partners for several receptors including vitamin D receptor, pregnane X receptor (PXR), constitutive androstane receptor (CAR) etc. In the course of 90s', PXR and CAR were discovered as key xenosensors regulating drug-metabolizing enzymes. Since there exist various cross-talks between cell signaling pathways, this was not surprising that RXRs, RARs and TRs were identified as regulators of human drug-metabolizing cytochromes P450 and cytochromes P450 involved in metabolism of endogenous compounds. Hence, a link between regulation of xenobiotic metabolizing enzymes and regulatory pathways of intermediary metabolism was established. Additionally, several drug-metabolizing enzymes are involved in metabolism of retinoids, rexinoids and thyroid hormones. In the current paper, we summarize the knowledge on the role of RARs, RXRs and TRs in the regulation of drug metabolizing cytochromes P450, and vice versa on the role of P450s in homeostasis of retinoids, rexinoids and thyroid hormone.Current Drug Metabolism 03/2011; 12(2):71-88. DOI:10.2174/138920011795016881 · 3.49 Impact Factor
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ABSTRACT: CYP2B6 is a polymorphic human drug metabolizing cytochrome P450 with clinical relevance for several drug substrates including cyclophosphamide, bupropion, and efavirenz. The common allele CYP2B6*6 [c. 516G>T, Q172H, and c.785A>G, K262R] has previously been associated with lower expression in human liver and with increased plasma levels of efavirenz in human immunodeficiency virus patients, but the molecular mechanism has remained unclear. We present novel data showing that hepatic CYP2B6 mRNA levels are reduced in *6 carriers, suggesting a pretranslational mechanism resulting in decreased expression. As one possibility, we first analyzed the common promoter variant, -750T>C, but the results did not suggest a prominent role in phenotype determination. In contrast, analysis of liver mRNA splicing variants demonstrated that the most common form lacking exons 4 to 6 (SV1) was tightly associated with the *6 allele and apparently also with the rare variant c.777C>A(CYP2B6*3). Further investigation using minigene constructs transfected into eukaryotic cell lines COS-1 and Huh7 demonstrated that the single nucleotide polymorphism c.516G>T in allele CYP2B6*6 was alone responsible for aberrant splicing resulting in high-splice variant (SV) 1 and low-CYP2B6 expression phenotype. Minigenes carrying the single c.785A>G polymorphism or the rare c.777C>A variant resulted in normal and intermediate expression phenotypes, respectively. In conclusion, the mechanism of the common *6 allele involves predominantly aberrant splicing, thus leading to reduced functional mRNA, protein, and activity. These results establish the single nucleotide polymorphism 516G>Tasthe causal sequence variation for severely decreased expression and function associated with CYP2B6*6.Journal of Pharmacology and Experimental Therapeutics 04/2008; 325(1):284-92. DOI:10.1124/jpet.107.133306 · 3.86 Impact Factor