A single d(GpG) cisplatin adduct on the estrogen response element decreases the binding of the estrogen receptor

Both cisplatin and the estrogen receptor (ER) are known to bend DNA. The influence of the bending of sequences by the d(GpG)cisPt adduct binding of ER to estrogen response element (ERE)-like sequences was examined. Three ERE-like oligonucleotides with different affinities for ER and which include a GG in the linker sequence were designed in order to form a single central d(GpG)cisPt adduct. Using electrophoretic mobility shift assay and Scatchard analysis, it was shown that the presence of a single d(GpG)cisPt adduct in the linker sequence decreases the ER affinity for DNA. These results do not support a critical role of a DNA bend in the initial recognition of ERE by ER. Then, the platination of DNA outside of the ERE half-sites decreases the interaction of ER with ERE.

Phenobarbital increases DNA adduct and metabolites formed by ochratoxin A: role of CYP 2C9 and microsomal glutathione-S-transferase

Ochratoxin A (OTA), a mycotoxin that induces nephrotoxicity and urinary tract tumors, is genotoxic and can be metabolized not only by different cytochromes P450 (CYP) but also by peroxidases involved in the arachidonic cascade, although the exact nature of the metabolites involved in the genotoxic process is still unknown. In order to establish the relation between OTA genotoxicity and the formation of metabolites, we chose three experimental models: kidney microsomes from rabbit, human bronchial epithelial cells, and microsomes from yeast that specifically express the human cytochrome P450 2C9 or 2B6 genes. OTA-DNA adducts were analyzed by (32)P postlabeling and the OTA derivatives formed were isolated by HPLC after incubation of OTA in the presence of: (1) kidney microsomes from rabbit pretreated or not with phenobarbital (PB); (2) human pulmonary epithelial cells simultaneously pretreated (or not) with PB alone or in the presence of ethacrynic acid (EA); (3) microsomes expressing CYP 2B6 and 2C9. PB pretreatment significantly increased DNA adducts formed after OTA treatment, both in the presence of kidney microsomes and bronchial epithelial cells, and induced the formation of new adducts. Ethacrynic acid, which inhibits microsomal glutathione-S-transferase, reduced DNA adduct level. DNA adducts were detected when OTA were incubated with microsomes expressing human CYP 2C9 but not with those expressing CYP 2B6. Several metabolites detected by HPLC were increased after PB treatment. Some of them could be related to DNA-adduct formation. In conclusion, OTA biotransformation, enhanced by PB pretreatment, increased DNA-adduct formation through pathways involving microsomal glutathion-S-transferase and CYP 2C9.

Frequent allelic loss at chromosome 3p distinct from genetic alterations of the 8-oxoguanine DNA glycosylase 1 gene in head and neck cancer

Cigarette smoking is the major known risk factor for head and neck cancer. Tobacco promotes oxidative stress and enhances tissue levels of 8-hydroxyguanine (8-OH-G) in smokers. The presence of 8-OH-G does not impede replication but leads to an accumulation of G-->T transversions. Recently, the gene for human 8-oxoguanine DNA glycosylase 1 (hOGG1), an enzyme involved in the repair of 8-OH-G in humans, was cloned and mapped to chromosome 3p. In head and neck tumors, the hOGG1 gene locus is often targeted by loss of heterozygosity (LOH), and the spectrum of mutations in the p53 gene shows a bias in favor of G:C-->T:A transversions, as would be expected if HOGG1 repair functions were disabled. To test the involvement of hOGG1 in head and neck carcinogenesis, we had previously screened 56 tumors for LOH at 3p. From these tumors and two others, we selected 33 tumors demonstrating LOH for further mutational analysis of this gene. No somatic inactivating mutation was found in hOGG1. Polymorphisms involving intron 4 and exon 7 were present in 30% of the patients. A new polymorphism was identified in one patient in exon 6 and led to the amino-acid change G308E. Similar repair activities were found for the wild-type and exon 6-variant enzymes. Therefore, the involvement of hOGG1 in head and neck carcinogenesis is not strongly supported by this work.

GSTM1, smoking and lung cancer: a case-control study

BACKGROUND

We conducted a case-control study to examine the risk of lung cancer in relation to GSTM1 polymorphism and cigarette smoking (primarily of black tobacco) in a French population.

METHODS

The 611 subjects were 301 incident lung cancer cases and 310 hospital controls. We were able to constitute a DNA bank for 547 subjects (89.5%) and gather detailed information on smoking history for all of them. Results presented here concern 247 cases and 254 controls.

RESULTS

Taking non- or light smokers as the reference category, we estimated odds ratios (OR) of 4.2 (95% CI: 2.6-6.7) and 5.2 (95% CI: 3.3-8.3) for the medium and heavy smokers respectively. On the other hand we estimated that the crude OR associating GSTM1 with lung cancer was 1.3 (95% CI: 0.9-1.8). Furthermore our data do not depart significantly from a multiplicative model of the combined effects of smoking and GSTM1 deficiency.

CONCLUSIONS

We conclude that smoking and the GSTM1 gene are each a risk factor for lung cancer, and that their combined effect does not differ significantly from that of a multiplicative model.

Cytochrome P-450 3A4 and 2C8 are involved in zopiclone metabolism

Zopiclone is a widely prescribed, nonbenzodiazepine hypnotic that is extensively metabolized by the liver in humans. The aim of the present study was to identify the human cytochrome P-450 (CYP) isoforms involved in zopiclone metabolism in vitro. Zopiclone metabolism was studied with different human liver microsomes and a panel of heterologously expressed human CYPs (CYP1A2, 2C8, 2C9, 2C18, 2C19, 2D6, 2E1, and 3A4). In human liver microsomes, zopiclone was metabolized into N-desmethyl-zopiclone (ND-Z) and N-oxide-zopiclone (NO-Z) with the following K(m) and V(m) of 78 +/- 5 and 84 +/- 19 microM, 45 +/- 1 and 54 +/- 5 pmol/min/mg for ND-Z and NO-Z generation, respectively. Ketoconazole (CYP3A inhibitor) inhibited approximately 40% of the generation of both metabolites, sulfaphenazole (CYP2C inhibitor) inhibited the formation of ND-Z, whereas alpha-naphtoflavone (CYP1A), quinidine (CYP2D6), and chlorzoxazone (CYP2E1) did not affect zopiclone metabolism. The generation of ND-Z and NO-Z were highly correlated to testosterone 6beta-hydroxylation (CYP3A activity, r = 0.95 and 0.92, respectively; p =.0001), and ND-Z was highly correlated to CYP2C8 activity (paclitaxel 6alpha-hydroxylase; r = 0.76, p =.004). Recombinant CYP2C8 had the highest enzymatic activity toward zopiclone metabolism into both its metabolites, followed by CYP2C9 and 3A4. CYP3A4 is the major enzyme involved in zopiclone metabolism in vitro, and CYP2C8 contributes significantly to ND-Z formation.

Microsatellite analysis and response to chemotherapy in head-and-neck squamous-cell carcinoma

Molecular studies have revealed that microsatellite instability and loss of heterozygosity occurred in head-and-neck cancer, suggesting the involvement both of suppressor and of mutator pathways in head-and-neck carcinogenesis. There is evidence for relations between tumor phenotype and clinical parameters. Indeed, replication-error phenotype, characterized by microsatellite instability, was associated with decreased sensitivity to chemotherapeutic agents in cell lines. Loss of heterozygosity is a frequent mechanism of inactivation of tumor-suppressor genes, which might be implicated in resistance to chemotherapy. In head-and-neck cancer, chemosensitivity is inconstant, and no marker is available to predict response to treatment. In order to evaluate the role of tumor phenotype on resistance to chemotherapy, we analyzed 56 primary head-and-neck squamous-cell carcinomas collected at time of diagnosis and a sub-group of 23 resistant tumors collected after chemotherapy at 22 microsatellite loci. At time of diagnosis, only one tumor showed MSI-H phenotype. Loss of heterozygosity (LOH) was observed in 75% of tumors, indicating the dominant role of the suppressor in comparison with the mutator pathway in HNSCC carcinogenesis. No change in microsatellite patterns was observed after treatment, suggesting that chemotherapy did not select mismatch-repair-deficient clones. Univariate analyses showed that LOH at 9p or 17p was significantly associated with drug resistance. In a multivariate analysis, only LOH at 17p remains predictive of low response to chemotherapy, with a relative risk of 3.7 and 95% CI of 1.1-13, indicating that p53 alterations could play a role in chemotherapy resistance in HNSCC. Int. J. Cancer (Pred. Oncol.) 84:410-415, 1999.

Human CYP2B6: expression, inducibility and catalytic activities

Human cytochrome (CYP)2B6 cDNA was cloned and expressed in bacteria and in yeast. Its expression in Saccharomyces cerevisiae enabled us to obtain, at a high level, an active yeast-expressed CYP2B6 protein, so as to assess its role in the metabolism of ethoxyresorufin, pentoxyresorufin, benzyloxyresorufin, ethoxycoumarin, testosterone and cyclophosphamide. Kinetic analysis showed that human CYP2B6 preferentially metabolized benzyloxyresorufin and pentoxyresorufin, although other CYPs also metabolized these substrates in human liver microsomes. CYP2B6 also manifested a strong 4-hydroxycyclophosphamide activity. Its expression in Escherichia coli enabled us to produce a very specific anti-human CYP2B6 antibody. No cross reactivity of this antibody was observed with CYPs1A1, 1A2, 3A4, 3A5, 2C8, 2C9, 2C18, 2C19, 2D6 or 2E1. This antibody enabled us to study the hepatic and extrahepatic expression of CYP2B6 in man, as well as its expression and inducibility in primary cultured human hepatocytes and in different human cell lines. Immunoblot analysis revealed that the CYP2B6 protein was expressed in 43 of the 48 human liver samples tested, with levels ranging from 0.4 to 8 pmol/mg of microsomal protein with a mean of 1.7 pmol/mg protein. CYP2B was also expressed in human brain, intestine and kidney, and at a lower level in the lung. CYP2B mRNA was detected in human liver, kidney, lung, trachea and intestine. We also found that CYP2B6 is induced at protein and mRNA levels by phenobarbital (2 mM) and cyclophosphamide (1 mM), an anticancer drug known to be metabolized by CYP2B6. No expression or inducibility of CYP2B6 was observed in any of the human cell lines tested.

Decrease in hepatic cytochrome P450 after interleukin-2 immunotherapy

Interleukin-2 (IL-2) has been shown to decrease cytochrome P450 (CYP) mRNAs and proteins in cultured rat hepatocytes, and IL-2 administration decreases CYPs in rats. Although high doses of IL-2 are administered to cancer patients, the effect on human CYPs has not yet been determined. Patients with hepatic metastases from colon or rectum carcinomas were randomly allocated to various daily doses of human recombinant IL-2 (from 0 to 12.10(6) units/m(2)). IL-2 was infused from day 7 to day 3 before hepatectomy and the conservation of a non-tumorous liver fragment in liquid nitrogen. Hepatic CYPs and monooxygenase activities were not significantly decreased in 5 patients receiving daily doses of 3 or 6 10(6) IL-2 units/m2, compared to 7 patients who did not receive IL-2. In contrast, in 6 patients receiving daily doses of 9 or 12 x 10(6) IL-2 units/m2, the mean values for immunoreactive CYP1A2, CYP2C, CYP2E1, and CYP3A4 were 37, 45, 60 and 39%, respectively, of those in controls; total CYP was significantly decreased by 34%, methoxyresorufin O-demethylation by 62%, and erythromycin N-demethylation by 50%. These observations suggest that high doses of IL-2 may decrease total CYP and monooxygenase activities in man.

In vitro metabolism of quinidine: the (3S)-3-hydroxylation of quinidine is a specific marker reaction for cytochrome P-4503A4 activity in human liver microsomes

The aim of this study was to evaluate the (3S)-3-hydroxylation and the N-oxidation of quinidine as biomarkers for cytochrome P-450 (CYP)3A4 activity in human liver microsome preparations. An HPLC method was developed to assay the metabolites (3S)-3-hydroxyquinidine (3-OH-Q) and quinidine N-oxide (Q-N-OX) formed during incubation with microsomes from human liver and from Saccharomyces cerevisiae strains expressing 10 human CYPs. 3-OH-Q formation complied with Michaelis-Menten kinetics (mean values of Vmax and Km: 74.4 nmol/mg/h and 74.2 microM, respectively). Q-N-OX formation followed two-site kinetics with mean values of Vmax, Km and Vmax/Km for the low affinity isozyme of 15.9 nmol/mg/h, 76.1 microM and 0.03 ml/mg/h, respectively. 3-OH-Q and Q-N-OX formations were potently inhibited by ketoconazole, itraconazole, and triacetyloleandomycin. Isozyme specific inhibitors of CYP1A2, -2C9, -2C19, -2D6, and -2E1 did not inhibit 3-OH-Q or Q-N-OX formation, with Ki values comparable with previously reported values. Statistically significant correlations were observed between CYP3A4 content and formations of 3-OH-Q and Q-N-OX in 12 human liver microsome preparations. Studies with yeast-expressed isozymes revealed that only CYP3A4 actively catalyzed the (3S)-3-hydroxylation. CYP3A4 was the most active enzyme in Q-N-OX formation, but CYP2C9 and 2E1 also catalyzed minor proportions of the N-oxidation. In conclusion, our studies demonstrate that only CYP3A4 is actively involved in the formation of 3-OH-Q. Hence, the (3S)-3-hydroxylation of quinidine is a specific probe for CYP3A4 activity in human liver microsome preparations, whereas the N-oxidation of quinidine is a somewhat less specific marker reaction for CYP3A4 activity, because the presence of a low affinity enzyme is demonstrated by different approaches.

Interindividual variability in P450-dependent generation of neoantigens in halothane hepatitis

Halothane hepatitis occurs because susceptible patients mount immune responses to trifluoroacetylated protein antigens, formed following cytochrome P450-mediated bioactivation of halothane to trifluoroacetyl chloride. In the present study, an in vitro approach has been used to investigate the cytochrome P450 isozyme(s) which catalyze neoantigen formation and to explore the protective role of non-protein thiols (cysteine and reduced glutathione). Significant levels of trifluoroacetyl protein antigens were generated when human liver microsomes, and also microsomes from livers of rats pre-treated with isoniazid, phenobarbital or beta-naphtoflavone, were incubated with halothane plus a nicotinamide adenine dinucleotidephosphate (NADPH) generating system. Immunoblotting studies revealed that the major trifluoroacetyl antigens expressed in vitro exhibited molecular masses of 50-55 kDa and included 60 and 80 kDa neoantigens recognized by antibodies from patients with halothane hepatitis. Much lower concentrations of halothane were required to produce maximal antigen generation in isoniazid-induced rat microsomes, as compared with phenobarbital or isosafrole-induced microsomes (0.5 vs 12.5 microl/ml). In isoniazid-induced microsomes, antigen generation was inhibited > 90% by the nucleophiles cysteine and glutathione and by the CYP2E1-selective inhibitors diallylsulfide and p-nitrophenol, but was unaffected by inhibitors of other P450 isozymes (furafylline, sulfaphenazole or triacetyloleandomycin). Neoantigen formation in six human liver microsomal preparations was inhibited in the presence of diallylsulfide, but not by furafylline, sulfaphenazole or triacetyloleandomycin, and exhibited marked variability which correlated with CYP2E1 levels. These results suggest that the balance between metabolic bioactivation by CYP2E1 and detoxication of reactive metabolites by cellular nucleophiles could be an important metabolic risk factor in halothane hepatitis.

Effects of the bacterial status of rats on the changes in some liver cytochrome P450 (EC 1.14.14.1) apoproteins consequent to a glucosinolate-rich diet

The aim of the present work was to investigate the influence of the intestinal microflora on the changes in hepatic cytochrome P450 apoproteins induced by dietary glucosinolates. Ten rats harbouring a conventional digestive microflora were offered either a diet containing 390 g myrosinase-free rapeseed meal/kg (n 5) or a control diet devoid of glucosinolates (n 5). A similar trial was performed using germ-free rats. After 4 weeks of exposure to the dietary regimens, animals were slaughtered and their livers removed for preparation of microsomes and analysis of cytochrome P450 (EC 1.14.14.1). The glucosinolate-rich diet decreased the concentration of total cytochrome P450 in conventional rats only (-34%). The bacterial status did not modify the concentration of apoproteins CYP1A2 and CYP2B1/B2, but greatly decreased the concentration of the male constitutive isoform CYP2C11 (-53 and -45% respectively in conventional and germ-free rats). Germ-free rats fed on the glucosinolate-rich diet had a greater concentration of CYP3A (+139%) and a lower concentration of CYP2E1 (-32%) than their counterparts fed on the control diet. However, these differences were absent in conventional animals. On the whole, the influence of the intestinal microflora on the changes in hepatic cytochrome P450 due to the consumption of cruciferous vegetables is very complex and obviously involves different mechanisms according to the apoprotein.

Possible involvement of multiple human cytochrome P450 isoforms in the liver metabolism of propofol

Previous studies of propofol (2,6-diisopropylphenol) pharmacology have shown that this widely used anaesthetic drug is extensively cleared from the body by conjugation of the parent molecule or its quinol metabolite. On the basis of potential influence of propofol on the metabolism of co-administered agents, many investigators have evaluated the effects of propofol on cytochrome P450 (CYP) activities. CYP isoforms involved in propofol metabolism are not defined. In this study, our objective was to elucidate further the CYP isoforms responsible for the hydroxylation of propofol. Using microsomes from 12 different human livers, we investigated CYP isoforms involved in propofol hydroxylase activity, using selective chemical inhibitors of CYP isoforms, correlation with immunoquantified specific CYP isoform content, immunoinhibition, and 11 functionally active human CYP isoforms expressed in a heterologous system (yeast and human B-lymphoblastoid cells). We found a low variability in the production of the hydroxylated metabolite of propofol, 2,6-diisopropyl-1,4-quinol. This activity was mediated by CYP and followed Michaelis-Menten kinetics with apparent K(M) and Vmax values of 18 microM (95% Cl 15.1-20.1) and 2.6 nmol min-1 mg-1 (95% Cl 2.45-2.68) respectively. Part of the propofol hydroxylase activity was mediated by CYP2C9 in human liver, especially at low substrate concentration. Moreover, propofol was likely to be metabolized by additional isoforms such as CYP2A6, 2C8, 2C18, 2C19 and 1A2, especially when substrate concentrations are high. This low specificity among CYP isoforms may contribute to the low interindividual variability (two-fold) and may contribute to the low level of metabolic drug interactions observed with propofol.

Covalent binding of carbamazepine reactive metabolites to P450 isoforms present in the skin

Carbamazepine is an anticonvulsant associated with a high risk for severe cutaneous reactions. Upon metabolism by cytochrome P450, carbamazepine may produce reactive metabolites. We evaluated in vitro the covalent binding of carbamazepine reactive metabolites on human P450s and then the presence of these P450s in human epidermis. Carbamazepine reactive metabolites covalent binding to human liver microsomes involved P450 subfamilies 1A, 2C and 3A. Specific covalent binding to yeasts expressing different P450s showed that carbamazepine reactive metabolites bound specifically to P450 1A2 and 3A4. We confirmed the constitutive presence of P450 3A in human epidermis and after induction with coaltar of P450 1A. Consequently, the production in epidermis of carbamazepine reactive metabolites is theoretically possible with formation of P450 adduct metabolites.

Use of heterologously expressed human cytochrome P450 1A2 to predict tacrine-fluvoxamine drug interaction in man

The aim of the present study was to evaluate the use of recombinant human cytochrome P-450 1A2 (rH-CYP1A2) in studies performed in vitro in order to predict metabolic drug-drug interactions occurring in man. In vitro metabolism of tacrine (a CYP1A2 probe) in the presence and absence of fluvoxamine, a CYP1A2 inhibitor, was investigated in human liver mircrosomes and with different rH-CYP. Vmax, Km and Ki determined with human liver microsomes were compared with those observed using rH-CYP1A2, assuming that 1 mg of liver microsomes contains, on average, 69 pmol of CYP1A2. The extent of tacrine metabolism inhibition procured by fluvoxamine with rH-CYP1A2, was compared with previous results observed in man. The Vax and Km for 1-hydroxytacrine formation rates obtained with rH-CYP1A2 were in good agreement with those observed in human liver microsomes (175+/-9 versus 140+/-60 pmol/min/mg for Vmax and 14+/-2 versus 16+/-2 microM for Km, respectively. The Ki of fluvoxamine on 1-hydroxytacrine formation rate observed with rH-CYP1A2 was similar to that observed with human liver microsome (0.35+/-0.05 versus 0.20+/-0.20 microM, respectively). Using the Km, Vmax and Ki determined with rH-CYP1A2, we calculated that fluvoxamine produced an inhibition of 1-, 2- and 4-hydroxytacrine formation rate of 91, 87 and 88%, respectively, in the range of tacrine and fluvoxamine concentrations observed in man. These percentages of inhibition calculated in vitro were in agreement with the percentage of fluvoxamine-dependent decrease in tacrine apparent oral clearance previously observed in man (83+/-13%). We conclude that human CYP1A2 expressed in yeast is a powerful tool to predict and to quantify drug-drug interactions in man.

Cytochrome P450 CYP2D6 gene polymorphism and lung cancer susceptibility in Caucasians

Many studies have been performed in an attempt to establish a link between the polymorphism of the cytochrome P450 CYP2D6 gene and the incidence of lung cancer. Nevertheless, whether or not this genetic polymorphism has a role in the development of the disease remains unclear. Recently, new advances in our knowledge of the CYP2D6 gene and its locus (CYP2D) have been achieved. In particular, CYP2D6 was found to be highly polymorphic and multiple novel mutations and allelic variants of the gene have been identified. In addition, a number of CYP2D rearrangements, including those with amplification of the gene, have been demonstrated. Taking this new information into account, we have reconsidered the potential influence of CYP2D6 polymorphism in lung cancer susceptibility by performing a comparative analysis of the overall mutational spectrum of CYP2D6 and of the rearrangements of CYP2D in 249 patients with lung cancer and in 265 control individuals matched on age, sex, hospital and residence area. For this purpose, a strategy based on SSCP analysis of the entire coding sequence of CYP2D6 and on RFLP analysis of the gene locus was carried out in DNA samples from each individual. Forty mutations occurring in various combinations on 42 alleles of the gene and 82 different genotypes were identified. No significant difference in the distribution of the mutations, alleles or genotypes was observed between the two groups, except a particular genotype (CYP2D6*1A/*2), which was more common in the sub-group of moderate smokers (< 30 pack-years) suffering from small cell carcinoma (Odds Ratio (OR) 3.6, 95% CI 1.1-11.9). When the phenotype was predicted according to genotype, only a trend toward a higher frequency of ultrarapid metabolizers in patients was obtained. In spite of a complete analysis of the CYP2D6 gene and its locus, this case-control study provides elements against an influence of the CYP2D6 polymorphism on lung cancer susceptibility.

Role of CYP2D6 in the N-hydroxylation of procainamide

Sequential oxidations at the arylamine moiety of the procainamide molecule leading to the formation of N-hydroxyprocainamide and its nitroso derivative may be responsible for lupus erythematosus observed in patients treated with the drug. The objective of the present study was to characterize major cytochrome P450 isozyme(s) involved in the N-hydroxylation of procainamide. Firstly, incubations were performed with microsomes from either lymphoblastoid cells or yeast transfected with cDNA encoding for specific human cytochrome P450 isozymes. Experiments performed with these enzyme expression systems indicated that the highest formation rate of N-hydroxyprocainamide was observed in the presence of CYP2D6 enriched microsomes. Additional experiments demonstrated that the formation rate of N-hydroxyprocainamide by CYP2D6 enriched microsomes was decreased from 45 +/- 4% to 93 +/- 1% by quinidine at concentrations ranging from 30 nM to 100 microM (all p < 0.05 vs control) and by approximately 75% by antibodies directed against CYP2D6. Secondly, incubations were performed with microsomes prepared from 15 human liver samples. Using this approach, an excellent correlation was observed between the formation rate of N-hydroxyprocainamide and dextromethorphan O-demethylase activity (CYP2D6; r = 0.9305; p < 0.0001). In contrast, no correlation could be established between N-hydroxyprocainamide formation rate and caffeine N3-demethylase (CYP1A2), coumarin 7-hydroxylase (CYP2A6), S-mephenytoin N-demethylase (CYP2B6), tolbutamide methlhydroxylase (CYP2C9), S-mephenytoin 4'-hydroxylase (CYP2C19), chlorzoxazone 6-hydroxylase (CYP2E1), dextromethorphan N-demethylase (CYP3A4), testosterone 6 beta-hydroxylase (CYP3A4/5) or lauric acid 12-hydroxylase (CYP4A11) activities. Furthermore, formation rate of N-hydroxyprocainamide was decreased in a concentration-dependent manner by quinidine (300 nM to 100 microM) and by antibodies directed against CYP2D6 but not by furafylline 20 microM (CYP1A2), ketoconazole 1 microM (CYP3A4), sulfaphenazole 10 microM (CYP2C9) or antibodies directed against CYP1A1/1A2, CYP2C, CYP2A6, CYP2E1 or CYP3A4/3A5. In conclusion, the results obtained in the present study demonstrate that CYP2D6 is the major human cytochrome P450 isozyme involved in the formation of the reactive metabolite of procainamide, namely N-hydroxyprocainamide.

Cytochrome P450 mediated bioactivation of methyleugenol to 1'-hydroxymethyleugenol in Fischer 344 rat and human liver microsomes

Cytochrome P450 mediated metabolism of methyleugenol to the proximate carcinogen 1'-hydroxymethyleugenol has been investigated in vitro. Kinetic studies undertaken in liver microsomes from control male Fischer 344 rats revealed that this reaction is catalyzed by high affinity (Km of 74.9 +/- 9.0 microM, Vmax of 1.42 +/- 0.17 nmol/min/nmol P450) and low affinity (apparent Km several mM) enzymic components. Studies undertaken at low substrate concentration (20 microM) with microsomes from livers of rats treated with the enzyme inducers phenobarbital, dexamethasone, isosafrole and isoniazid indicated that a number of cytochrome P450 isozymes can catalyze the high affinity component. In control rat liver microsomes, 1'-hydroxylation of methyleugenol (assayed at 20 microM substrate) was inhibited significantly (P < 0.05) by diallylsulfide (40%), p-nitrophenol (55%), tolbutamide (30%) and alpha-naphthoflavone (25%) but not by troleandomycin, furafylline, quinine or cimetidine. These results suggested that the reaction is catalyzed by CYP 2E1 and by another as yet unidentified isozyme(s) (most probably CYP 2C6), but not by CYP 3A, CYP 1A2, CYP 2D1 or CYP 2C11. Administration of methyleugenol (0-300 mg/kg/day for 5 days) to rats in vivo caused dose-dependent auto-induction of 1'-hydroxylation of methyleugenol in vitro which could be attributed to induction of various cytochrome P450 isozymes, including CYP 2B and CYP 1A2. Consequently, high dose rodent carcinogenicity studies are likely to over-estimate the risk to human health posed by methyleugenol. The rate of 1'-hydroxylation of methyleugenol in vitro in 13 human liver samples varied markedly (by 37-fold), with the highest activities being similar to the activity evident in control rat liver microsomes. This suggests that the risk posed by dietary ingestion of methyleugenol could vary markedly in the human population.

Possible involvement of multiple cytochrome P450S in fentanyl and sufentanil metabolism as opposed to alfentanil

Fentanyl, sufentanil, and alfentanil are commonly used as opioid analgesics. Alfentanil clearance has previously been shown to exhibit an important interindividual variability, which was not observed for fentanyl or sufentanil. Differences in pharmacokinetic parameters of alfentanil have previously been associated with the wide distribution of CYP3A4, the only known hepatic cytochrome P450 monooxygenase (CYP) involved in the conversion of alfentanil to noralfentanil. Little is known about the involvement of CYP enzymes in the oxidative metabolism of fentanyl and sufentanil. Microsomes prepared from different human liver samples were compared for their abilities to metabolize fentanyl, sufentanil and alfentanil, and it was found that disappearance of the three substrates was well correlated with immunoreactive CYP3A4 contents but not with other CYPs, including CYP1A2, CYP2A6, CYP2C8, CYP2C9, CYP2D6 and CYP2E1. Specific known inhibitors of CYP enzymes gave similar results, whereas the use of recombinant human CYP enzymes expressed in yeast provided information about the possible involvement of other CYPs than CYP3A4 in the biotransformation of fentanyl and sufentanil. The possible in vivo interaction of fentanyl and sufentanil with other drugs catalyzed by CYP3A4 is also discussed.

Expression and localization of CYP3A4 and CYP3A5 in human lung

Expression in the lung of procarcinogen-metabolizing P450 enzymes in the CYP3A subfamily may contribute to the initiation of pulmonary carcinogenesis by agents that require metabolic activation, such as tobacco-derived polycyclic aromatic hydrocarbons. Expression and localization of CYP3A4 and CYP3A5 proteins in human lung were determined by immunohistochemistry with three antibodies, one specific for members of the CYP3A subfamily and two antipeptide antibodies specific for CYP3A4 and CYP3A5, respectively. Positive immunostaining in one or several cell types of the lung was observed in all patients with anti-CYP3A4 and anti-CYP3A5 antibodies. With the anti-CYP3A4 antibody epithelial staining was observed in five cases and staining of alveolar macrophages in 12 of 27 cases. To determine which CYP3A genes are transcribed in lung tissue, analysis by reverse-transcriptase-polymerase chain reaction with gene-specific primers for CYP3A4, CYP3A5, and CYP3A7 was performed. CYP3A5 mRNA was detected in all eight samples studied, CYP3A4 mRNA in one sample, and CYP3A7 mRNA in none of the samples. CYP3A5 was localized by immunohistochemistry in the ciliated and mucous cells of the bronchial wall, bronchial glands, bronchiolar columnar and terminal cuboidal epithelium, type I and type II alveolar epithelium, vascular and capillary endothelium, and alveolar macrophages, whereas CYP3A4 was found in bronchial glands, bronchiolar columnar and terminal epithelium, type II alveolar epithelium, and alveolar macrophages. These data establish that CYP3A5 is the predominant CYP3A form in human lung, that CYP3A4 is expressed in about 20% of individuals, and considerable variation of pulmonary expression occurs in both CYPs between individuals.

Inhibition by omeprazole of proguanil metabolism: mechanism of the interaction in vitro and prediction of in vivo results from the in vitro experiments

Both the antimalarial prodrug proguanil and the gastric proton pump inhibitor omeprazole are substrates for cytochrome P450 (CYP)2C19 and CYP3A. However, the relative contribution of each enzyme to proguanil bioactivation to cycloguanil and to the metabolism of omeprazole, as well as their potential to interact, remains to be examined. The bioactivation of proguanil to its active metabolite cycloguanil was studied in vitro in human liver microsomes and in vivo in 12 healthy subjects, in the absence and in the presence of omeprazole. The formation of cycloguanil from proguanil exhibited biphasic kinetic behavior in four of six human livers, indicating that at least two enzymes are responsible for this metabolic step. Cycloguanil formation activity did not correlate with immunoreactive CYP3A4 content or with CYP3A4 activity, as measured by testosterone 6beta-hydroxylation, suggesting that CYP3A4 plays a limited role in cycloguanil formation. Furthermore, troleandomycin (10 microM) inhibited only 10 to 17% of cycloguanil formation at proguanil concentrations of 100 and 500 microM. At a proguanil concentration of 20 microM, omeprazole at 10 microM inhibited cycloguanil formation in vitro by 47 +/- 59%. These in vitro results were consistent with the results of our in vivo study in healthy subjects, which showed a 32 +/- 11% decrease in proguanil apparent oral clearance and a 65 +/- 8% decrease in proguanil partial metabolic clearance to cycloguanil in the presence of omeprazole (both P < .001). We conclude that in vitro studies of proguanil metabolism and interactions are predictive of in vivo situations, that CYP2C19 is the main enzyme responsible for proguanil bioactivation to cycloguanil and that omeprazole inhibits this biotransformation in vitro and in vivo by inhibiting this enzyme.

Pregnenolone-7beta-hydroxylating activity of human cytochrome P450-1A1

In many human and murine tissues, both pregnenolone and dehydroepiandrosterone are hydroxylated at the 7alpha and 7beta positions by a cytochrome P450-containing microsomal complex. The 7alpha- and 7beta-hydroxysteroids produced were shown to activate an immune response in mice. Based upon identification by crystallization to constant specific activity and gas chromatography-mass spectrometry analysis, we ascertained that a yeast-expressed human cytochrome P450-1A1 was able to 7beta-hydroxylate pregnenolone (K(M) from 3.2 +/- 0.5 to 4.1 +/- 0.4 microM, turnover number from 117 +/- 15 to 135 +/- 13 pmol/min/nmol of cytochrome P450-1A1). The other human cytochromes P450 tested did not produce identifiable quantities of 7alpha- or 7beta-hydroxylated derivatives of pregnenolone or dehydroepiandrosterone. These findings indicate that cytochrome P450-1A1 involvement in the 7beta-hydroxylation of pregnenolone may contribute to the production of the 7-hydroxylated steroids necessary for activation of the immune defences.

Contribution of human cytochrome P450 to benzo[a]pyrene and benzo[a]pyrene-7,8-dihydrodiol metabolism, as predicted from heterologous expression in yeast

The metabolism of benzo[a]pyrene (B[a]P) and its proximate mutagen B[a]P-7,8-dihydrodiol (7,8-diol) was investigated in the presence of human microsomal epoxide hydrolase and P450 1A1, 1A2, 2C8, 2C9, 2C18, 2C19, 2D6 and 3A4 expressed in the yeast Saccharomyces cerevisiae. P450 1A1 had the highest turnover numbers for the formation of all B[a]P metabolites, including phenols and dihydrodiols. P450 1A2, 2C8, 2C9, 2C18, 2C19 and 3A4, which are well represented in the liver, gave rise to the formation of appreciable amounts of 3-hydroxy-B[a]P and of some dihydrodiols from B[a]P. When 7,8-diol was used as substrate, P450 1A1 also exhibited the highest turnover numbers for the formation of tetrols, the hydrolysis products of the diolepoxides, whereas P450 1A2, 2C8, 2C19 and 3A4 showed moderate activities. In order to test the validity of the yeast system, the contribution of each P450 isoform to B[a]P and 7,8-diol metabolism was evaluated as the product of the turnover numbers of recombinant P450s by specific contents of each P450 in human liver microsomes. Calculated formation rates for each B[a]P and 7,8-diol metabolite globally matched experimental values. There is evidence that P450 3A4 and 2C9 play a major role in the formation of 3-hydroxy-B[a]P from B[a]P. Accumulation of the proximate mutagen 7,8-diol was predicted to be mainly driven by P450 1A2, 2C9 and 2C19, while formation of the genotoxic diolepoxides from 7,8-diol appeared to be dependent on P450 1A2 and 3A4 in the liver.

Interindividual variability in catalytic activity and immunoreactivity of three major human liver cytochrome P450 isozymes

OBJECTIVE

Interindividual variations in immunoreactivity and function of three major human drug metabolising P450 monooxygenases has been investigated in liver microsomes from 42 Caucasians (kidney donors or liver biopsies).

METHODS

Diclofenac 4'-hydroxylation, dextromethorphan O-demethylation and midazolam 1'-hydroxylation, measured by HPLC in incubates, were used as probes to determine CYP2C9, CYP2D6 and CYP3A4 function kinetics, respectively. Immunoquantification of the three isoforms was achieved by Western blotting, using rabbit polyclonal antibodies raised against human CYP2C9 and human CYP3A4, and mouse monoclonal antibody raised against human CYP2D6.

RESULTS

Diclofenac 4'-hydroxylation exhibited Michaelis-Menten kinetics with kM = 3.4 mumol.l-1 and Vmax = 45 nmole.mg-1 P.h-1. Relative immunoreactivity of CYP2C9 was correlated with Vmax and CL(int). Dextromethorphan O-demethylation in EM (extensive metabolisers) liver microsomes also showed Michaelis-Menten kinetics, with kM = 4.4 mumol.l-1 and Vmax = 5.0 nmol.mg-1 P.h-1. Relative immunoreactivity of CYP2D6 was correlated with Vmax and CL(int). Midazolam 1'-hydroxylation also exhibited Michaelis-Menten kinetics with kM = 3.3 mumol.l-1 and Vmax = 35 nmol.mg-1 P.h-1. Relative immunoreactivity of CYP3A4 was correlated with Vmax and CL(int). Immunoreactivity and function were correlated for each isozyme, but there was no cross correlation between isozymes.

CONCLUSION

The velocity of metabolite formation (Vmax) by the three major human drug metabolising P450 monoxygenases is correlated with their immunoreactivity in liver microsomes. Interindividual variation was much larger for Vmax than kM. Interindividual variability was more pronounced for CYP2D6, probably due to the presence of several different functional alleles in the population of extensive metabolisers.

Stable expression of human cytochrome P450 3A4 in conjunction with human NADPH-cytochrome P450 oxidoreductase in V79 Chinese hamster cells

V79 Chinese hamster cells were constructed for stable expression of human cytochrome P450 3A4 with and without coexpression of human NADPH-cytochrome P450 oxidoreductase. Expression of the cDNAs was shown by Northern and Western analyses. Activity was tested by 6 beta-hydroxylation of testosterone for cytochrome P450 3A4 and by cytochrome c reduction for NADPH-cytochrome P450 reductase. Five V79 cell lines were obtained expressing cytochrome P450 3A4, human NADPH-cytochrome P450 oxidoreductase, and both. Cytochrome P450 3A4 activity depended highly on cytochrome P450 reductase activity, with lowest activity when only the parental Chinese hamster cytochrome P450 reductase was present, 5- and 10-fold higher when coexpressed with the human NADPH-cytochrome P450 reductase. Correspondingly, cytotoxic and genotoxic potency of aflatoxin B1 was increased by orders of magnitudes when human cytochrome P450 3A4 was coexpressed with the human NADPH-cytochrome P450 reductase. The effect of NADPH-cytochrome P450 reductase coexpression on cytochrome P450 3A4 activity was also tested by nifedipine oxidation and midazolam hydroxylation. Nifedipine oxidation was increased about 10-fold, 1-hydroxylation of midazolam and 4-hydroxylation of midazolam were increased 15-fold.

The biotransformation of clomipramine in vitro, identification of the cytochrome P450s responsible for the separate metabolic pathways

The aim of the study was to identify the cytochrome P450s (CYPs) that catalyze the biotransformation of clomipramine in vitro. A high-performance liquid chromatography method was developed to assay N-desmethylclomipramine, 8-hydroxyclomipramine, 2-hydroxyclomipramine, 8-hydroxydesmethhylclomipramine, didesmethylclomipramine and 2-hydroxydesmethylclomipramine formed by microsomes prepared from human liver and yeast expressing human CYP1A1, 1A2, 2C8, 2C9, 2C18, 2C19, 2D6 and 3A4. There was a statistically significant correlation between the formation rate of desmethylclomipramine and the immunoquantified concentration of CYP3A4 in 12 human liver microsome preparations (rs = 0.664, P = .028). Ketoconazole was a very potent inhibitor of desmethylclomipramine formation (Ki = 0.054 microM) and microsomes from yeast expressing CYP3A4 were also active in forming the metabolite (formation rate: 25.6 nmol/nmol of CYP per hr). Thus, the results are consistent with the assumption that the N-demethylation of clomipramine is catalyzed by CYP3A4. As expected from in vivo panel studies, CYP2C19 in yeast was also very active in the N-demethylation (formation rate, 145 nmol/nmol of CYP per hr). Fluvoxamine was a potent inhibitor of desmethylclomipramine formation (Ki, 0.15 microM), suggesting that CYP1A2 is a third CYP involved in the N-demethylation. CYP2D6 in yeast microsomes catalyzed the 8-hydroxylation of clomipramine and desmethylclomipramine (formation rates, 65 and 75 nmol/nmol of CYP per hr) and quinidine was a very potent inhibitor (Ki, 0.10 and 0.16 microM). Both results confirm that CYP2D6 catalyzes the 8-hydroxylation in agreement with the results obtained in previous in vivo studies. Besides quinidine, paroxetine, fluoxetine and norfluoxetine, all were potent inhibitors of the 8-hydroxylations (Ki, 0.24-1.5 microM) and sertraline was a less potent inhibitor (Ki, 16 and 27 microM, respectively).

Regulation of sucrase-isomaltase and hexose transporters in Caco-2 cells: a role for cytochrome P-4501A1?

Involvement of cytochrome P-4501A1 (CYP1A1) in the regulation of sucrase-isomaltase and hexose transporters was analyzed in low (TC7)- and high (PF11)-glucose-consuming Caco-2 clones. CYP1A1 mRNA is elevated in exponentially growing cells concomitantly with high rates of glucose consumption and high levels of GLUT-1 and GLUT-3 mRNA. After confluency, CYP1A1 is not detectable in TC7 cells; this is associated with a decreased glucose consumption, a downregulation of GLUT-1 and GLUT-3, and an upregulation of sucrase-isomaltase, SGLT-1, GLUT-2, and GLUT-5. In PF11 cells CYP1A1 mRNA remains elevated, along with high glucose consumption, high levels of GLUT-1 and GLUT-3, and minimal expression of sucrase-isomaltase, SGLT-1, GLUT-2, and GLUT-5. Exposure of TC7 cells to inducers of CYP1A1 results in high levels of CYP1A1 mRNA, a 10-fold increase of glucose consumption after confluency, an upregulation of GLUT-1 and GLUT-3, and a downregulation of sucrase-isomaltase, GLUT-2, and, to a lesser extent, SGLT-1 and GLUT-5. These results suggest that activation of CYP1A1, whether spontaneous or drug induced, is involved in the variations of glucose utilization and in the associated modifications of expression of sucrase-isomaltase and hexose transporters.

Human cytochrome P450 2E1 (CYP2E1): from genotype to phenotype

CYP2E1 is involved in the activation of various carcinogens, including N-nitrosamines, which are believed to be important in human carcinogenesis. Humans exhibit wide interindividual variability in levels of CYP2E1 mRNA and protein, which might explain interindividual differences in susceptibility to carcinogens activated by CYP2E1. Such variability could be due either to genetic polymorphisms observed in the CYP2E1 gene (Rsa I in the 5'-flanking region, Dra I in intron 6 and Taq I in intron 7) or to varying inducibility by xenobiotics. The aim of the present study was to establish whether, in a Caucasian population (n = 93), there existed a relationship between allelic forms of the CYP2E1 gene and the phenotype determined in vitro by hepatic ability to 6-hydroxylate chlorzoxazone. Rates of chlorzoxazone-6-hydroxylation were significantly correlated with levels of immunochemically measured CYP2E1 (p < 0.001). CYP1A2, 2C8, 2C9, 2C18, 2D6, 3A4 and 3A5 did not appear to be significantly involved in chlorzoxazone metabolism, whereas the participation of CYP1A1 could not be excluded. Frequencies of the rare alleles for the three polymorphism sites were 2.2% for RsaI, 7.5% for DraI and 8.5% for TaqI. Despite substantial interindividual variations in chlorzoxazone hydroxylase activity, no relationship between any of the three polymorphisms and CYP2E1 activity was established. Therefore, in humans, interindividual variability in CYP2E1 levels is probably due to differing induction levels as a result of environmental factors, or to genetic factors other than those studied in this work.

Evaluation of caffeine as a test drug for CYP1A2, NAT2 and CYP2E1 phenotyping in man by in vivo versus in vitro correlations

Caffeine is used to phenotype subjects in vivo for the cytochrome P450 isoforms CYP1A2 and CYP2E1, and for N-acetyltransferase type 2 (NAT2). However, how much of the variation in phenotyping parameters may be attributed to variations in CYP1A2 and CYP2E1 activities has not been determined. Therefore, this study intraindividually compared enzyme activities and/or content in liver samples with pharmacokinetic parameters of caffeine in vivo after administration of a test dose in 25 patients undergoing hepatectomy. Parameters measured in vitro were the high affinity components of caffeine 3-demethylation and phenacetin 0-deethylation, microsomal CYP1A2 and CYP2E1 immunoreactivity, and cytosolic sulfamethazine N-acetylation. Caffeine parameters in vivo included caffeine clearance from plasma and/or saliva, paraxanthine/caffeine ratios in plasma and saliva, plasma theophylline/caffeine ratio, and several metabolite ratios from spot urine sampled 6 h postdose. Correlations between parameters were determined using weighted linear regression analyses. Caffeine clearance and paraxanthine/caffeine ratios correlated most highly to intrinsic clearance of caffeine 3-demethylation and to CYP1A2 immunoreactivity (r= 0.584-0.82), whereas urinary CYP1A2 ratios correlated less strongly with CYP1A2 parameters in vitro. Assignment of acetylator phenotype by urinary NAT2 ratios was concordant with sulfamethazine-N-acetylation in vitro. In contrast to CYP1A2 parameters in vitro, CYP2E1 immunoreactivity was not related to the theophylline/caffeine plasma ratio. CYP1A2 activity, thus, is the major determinant of caffeine clearance and the paraxanthine/caffeine ratios in vivo, of which the saliva ratio 6 h postdose appears as the most advantageous parameter. The results confirm that phenotyping using caffeine provides valid estimates of CYP1A2 and NAT2 activity.

Simulation of human benzo[a]pyrene metabolism deduced from the analysis of individual kinetic steps in recombinant yeast

Human cytochrome P450 1A1 (1A1) and microsomal epoxide hydrolase (mEH)-dependent metabolic activation of benzo[a]pyrene (BP) have been reconstituted with microsomes from yeast cells expressing the two enzymes. The formation of the postulated ultimate mutagen 7 beta, 8 alpha-dihydroxy-9 alpha, 10 alpha-epoxy-7,8,9,10-tetrahydro-BP, the so-called diol epoxide-2 (DE2) derived from the reoxidation of BP-7,8-dihydrodiol by 1A1, was estimated by HPLC measurement of its hydrolysis product 7 beta, 8 alpha, 9 alpha, 10 beta-tetrahydrotetrol-BP (T2-tetrol). The 1A1/mEH coupled system was analyzed by varying the incubation time, initial substrate concentration, and molar ratio of the two enzymes. A minimum kinetic model of BP metabolism by 1A1 and mEH was constructed on the basis of the overall kinetic parameters (Vmax, Km) for a number of individual steps determined with human 1A1 and mEH expressed in yeast. The model was converted into a set of differential equations including 30 independent kinetic constants, 15 chemical species, and 8 enzymes and enzyme/substrate complexes. Numerical simulation of the model enabled us to satisfactorily reproduce the experimental kinetics of formation of BP-phenols, -dihydrodiols, and -tetrols for all tested conditions. Such a validated model was used to investigate the kinetics of unstable genotoxic species such as BP-epoxides and diol epoxides, which were not directly measurable. Based on numerical simulation, BP-7,8-oxide and -9,10-oxide appear to accumulate rapidly to reach a plateau after 2 min, while maximal accumulation of DE2 occurs after about a half-hour and declines during the following 2 h. A contribution of BP-9,10-dihydrodiol metabolism to T2-tetrol formation via the formation of a BP-7,8-oxide-9,10-dihydrodiol is predicted to be detectable after 2 hours due to the preferential accumulation of BP-9,10-dihydrodiol following 1A1-dependent consumption of initially accumulated BP-7,8-dihydrodiol.

Involvement of cytochrome P450 3A4 enzyme in the N-demethylation of methadone in human liver microsomes

Methadone has become one of the most widely used drugs for opiate dependency treatment. This drug is extensively metabolized by the cytochrome P450 hepatic enzyme family in man, yielding an N-demethylated metabolite that cyclizes spontaneously into 2-ethylidene-1,5-dimethyl-3,3-diphenylpyrrolidine. The specific forms of cytochrome P450 involved in this oxidative N-demethylation were examined in a panel of 20 human liver microsomal preparations previously characterized with respect to their P450 enzyme contents. Methadone was demethylated with an apparent Km of 545 +/- 258 microM (n = 3). The metabolic rates were 745 +/- 574 pmol/(min.mg of protein). This metabolic pathway was strongly correlated with estradiol 2-hydroxylation, testosterone 6 beta-hydroxylation, nifedipine oxidation, erythromycin N-demethylation, and toremifene N-demethylation, all of these monooxygenase activities being supported by P450 3A4. Furthermore, the total P450 3A content of liver microsomal samples, determined by immuno-quantification using a monoclonal anti-human P450 3A4 antibody, was correlated with methadone demethylation (r = 0.72; p < 0.003). Methadone metabolism was 60-72% inhibited either by three mechanism-based inhibitors of P450 3A4 (gestodene, TAO, and erythralosamine) or by four reversible inhibitors of P450 3A (ketoconazole, dihydroergotamine, quercetin, and diazepam with an apparent Ki of 50 microM) and by two nonspecific inhibitors (metyrapone and SKF-525A). Conversely, quinidine (inhibitor of P450 2D6), 7,8-benzoflavone (inhibitor of P450 1A), or sulfaphenazole (inhibitor of P450 2C) did not significantly inhibit, and may even have activated, methadone metabolism. Four heterologously expressed P450 proteins were able to catalyze the N-demethylation of methadone, namely, P450 2C8, P450 2C18, P450 2D6, and P450 3A4. However, referring to their relative liver content, it can be asserted that P450 3A4 is the major enzyme involved in the N-demethylation of methadone on average. Accordingly, caution should be advised in the clinical use of methadone when other drugs are also administered that induce or inhibit P450 3A4, such as rifampicin or diazepam, respectively.

Hepatic cytochrome P450 and UDP-glucuronosyl transferase are affected by five sources of dietary fiber in germ-free rats

The influence of dietary fiber on xenobiotic-metabolizing enzymes (XME) was assessed using germ-free rats fed inulin and other sources of fiber (wheat bran, carrot, cocoa and oat). The consumption of cocoa fiber greatly modified the hepatic cytochrome P450 isoenzymatic profile, causing a strong enhancement of 1A2 and 2B1/B2 forms, concomitant with a significant decrease of the constitutive form 2C11, compared with all of the other types of fiber. Moreover, rats fed the cocoa fiber diet had a higher specific activity of hepatic UDP-glucuronosyl transferase than their carrot fiber- and wheat bran-fed counterparts. Intestinal UDP-glucuronosyl transferase was unaffected by the type of ingested fiber. Diet composition also did not alter the specific activity of glutathione-S-transferase in the liver, small intestine, or colon. Using earlier results obtained in heteroxenic rats, we show that intestinal microflora plays a key role in some of the effects of fiber on XME, although this is not a necessary prerequisite for all of the liver alterations.

Expression of cytochrome P 450 3A enzymes in human lung: a combined RT-PCR and immunohistochemical analysis of normal tissue and lung tumours

We have previously demonstrated expression of cytochrome P 450 3A (CYP3A) protein in pulmonary carcinomas and surrounding normal tissue, using immunohistochemistry. These results suggested that different CYP3A enzymes may be expressed in normal and tumour tissue. Therefore, the aim of the present study was to identify specific CYP3A enzymes expressed in normal human lung and lung tumours. Both normal lung tissue and tumour tissue from eight patients was analyzed for CYP3A4, CYP3A5 and CYP3A7 mRNA using a specific RT-PCR (reverse transcriptase-polymerase chain reaction) method. Identical samples were subjected to immunohistochemical analysis of CYP3A protein. CYP3A5 was the major enzyme of the CYP3A subfamily present at the mRNA level in both normal human lung and lung tumours. CYP3A5 mRNA was detected in normal lung tissue in all eight cases and in tumour tissue in four cases. CYP3A7 mRNA was detected in five cases in normal tissue and in one tumour. Notably, no CYP3A4 mRNA was found in any of the samples. Immunohistochemical staining for CYP3A protein was found in normal lung tissue in each case. Interestingly, all pulmonary carcinomas showed immunostaining for CYP3A, while mRNA for CYP3A enzymes was found in only four cases. In summary, our study indicates a specific expression pattern of the members of the CYP3A subfamily in normal human lung and lung tumours. These findings have potential clinical significance, since it has been recently shown that CYP3A5 catalyzes the activation of the anticancer pro-drugs cyclophosphamide and ifosfamide. Thus, local activation of these agents may take place in pulmonary carcinomas and surrounding normal tissues.

Inhibition of CYP1A2 and CYP3A4 by oltipraz results in reduction of aflatoxin B1 metabolism in human hepatocytes in primary culture

Dithiolethiones are thought to act as potent chemoprotective agents against aflatoxin B1 (AFB1)-induced hepatocarcinogenesis in the rat by inducing glutathione S-transferases (GSTs). To determine whether these antioxidants can be similarly effective in human beings, we have investigated metabolism of AFB1, in primary human hepatocytes with or without pretreatment by oltipraz (OPZ), a synthetic derivative of the natural 1,2-dithiole-3-thione. Aflatoxin M1 (AFM1), glutathione conjugates of AFB1 oxides (AFBSGs), and unchanged AFB1 were quantitated in cultures derived from eight human liver donors. Parenchymal cells obtained from the three GST M1-positive livers metabolized AFB1 to AFM1 and to AFBSGs derived from the isomeric exo-and endo-8,9-oxides, whereas no AFBSGs were formed in the GST M1-null cells. Pretreatment of the cells with 3-methylcholanthrene or rifampicin, inducers of CYP1A2 and CYP3A4, respectively, caused a significant increase in AFB1 metabolism. Although OPZ induced GST A2, and to a lesser extent GST A1 and GST M1, it decreased formation of AFM1 and AFBSG, which involves CYP1A2 and CYP3A4. Inhibition by OPZ of AFB1 metabolism by reducing CYP1A2 and CYP3A4 was also demonstrated by decreased activity of their monooxygenase activities toward ethoxyresorufin and nifedipine, respectively. The significant inhibition by OPZ of human recombinant yeast CYP1A2 and CYP3A4 was also shown. These results demonstrate that AFBSG can be formed by GST M1-positive human hepatocytes only, and suggest that chemoprotection with OPZ is due to an inhibition of activation of AFB1, in addition to a GST-dependent inactivation of the carcinogenic exo-epoxide.

Cytochromes of the P450 2C subfamily are the major enzymes involved in the O-demethylation of verapamil in humans

The calcium channel blocker verapamil [2,8-bis-(3,4-dimethoxyphenyl)-6-methyl-2-isopropyl-6-azaoctanitrile+ ++] undergoes extensive biotransformation in man. We have previously demonstrated cytochrome P450 (CYP) 3A4 and 1A2 to be the enzymes responsible for verapamil N-dealkylation (formation of D-617 [2-(3,4-dimethoxyphenyl)-5-methylamino-2-isopropylvaleronitrile], and verapamil N-demethylation (formation of norverapamil [2,8-bis-(3,4-dimethoxyphenyl)-2-isopropyl-6-azaoctanitrile]), while there was no involvement of CYP3A4 and CYP1A2 in the third initial metabolic step of verapamil, which is verapamil O-demethylation. This pathway yields formation of D-703 [2-(4-hydroxy-3-methoxyphenyl)-8-(3,4-dimethoxyphenyl)-6-methyl-2-isopro pyl-6-azaoctanitrile] and D-702 [2-(3,4-dimethoxyphenyl)-8-(4-hydroxy-3-methoxyphenyl)-6-methyl-2-isopro pyl-6-azaoctanitrile]. The enzymes catalyzing verapamil O-demethylation have not been characterized so far. We have therefore identified and characterized the enzymes involved in verapamil O-demethylation in humans by using the following in vitro approaches: (I) characterization of O-demethylation kinetics in the presence of the microsomal fraction of human liver, (II) inhibition of verapamil O-demethylation by specific antibodies and selective inhibitors and (III) investigation of metabolite formation in microsomes obtained from yeast strain Saccharomyces cerevisiae W(R), that was genetically engineered for stable expression of human CYP2C8, 2C9 and 2C18. In human liver microsomes (n=4), the intrinsic clearance (CLint), as derived from the ratio of Vmax/Km, was significantly higher for O-demethylation to D-703 compared to formation of D-702 following incubation with racemic verapamil (13.9 +/- 1.0 vs 2.4 +/- 0.6 ml*min-1*g-1, mean+/-SD; p<0.05), S-verapamil (16.8 +/- 3.3 vs 2.2 +/- 1.2 ml* min-1*g-1, p<0.05) and R-verapamil (12.1 +/- 2.9 vs 3.6 +/- 1.3 ml*min-1*g-1; p<0.05), thus indicating regioselectivity of verapamil O-demethylation process. The CLint of D-703 formation in human liver microsomes showed a modest but significant degree of stereoselectivity (p<0.05) with a S/R-ratio of 1.41 +/- 0.17. Anti-LKM2 (anti-liver/kidney microsome) autoantibodies (which inhibit CYP2C9 and 2C19) and sulfaphenazole (a specific CYP2C9 inhibitor) reduced the maximum rate of formation of D-703 by 81.5 +/- 4.5% and 45%, that of D-702 by 52.7 +/- 7.5% and 72.5%, respectively. Both D-703 and D-702 were formed by stably expressed CYP2C9 and CYP2C18, whereas incubation with CYP2C8 selectively yielded D-703. In conclusion, our results show that enzymes of the CYP2C subfamily are mainly involved in verapamil O-demethylation. Verapamil therefore has the potential to interact with other drugs which inhibit or induce these enzymes.

High levels of DNA adducts in human colon are associated with colorectal cancer

Colon cancer is one of the most frequent causes of cancer death in western countries. Epidemiological studies suggest that colorectal cancer can be attributed, at least in part, to carcinogens and mutagens present in the diet and/or the environment. The covalent binding of these xenobiotics or their reactive metabolites to DNA is believed to initiate this chemical carcinogenesis. In the present study, using a 32P-postlabeling method, we investigated DNA adduct levels in control colons from patients without colorectal adenocarcinoma and in nontumoral and tumoral tissues from patients with colorectal adenocarcinoma. Our results show that the DNA adduct level is significantly higher (P < 0.001) in nontumoral than in control or tumoral colon samples. For the first time, we demonstrate in humans that the presence of numerous adducts in colonic mucosa is associated with colorectal cancer, a finding in agreement with the importance of chemical factors in causing this disease; therefore, after confirmation of the link between DNA adducts and colorectal cancer, the measurement of DNA adduct levels in colon samples could constitute a useful approach to the early detection of colorectal cancer.

CYP2D6 genotype and lung cancer risk according to histologic type and tobacco exposure

Polymorphism for CYP2D6 was determined genetically as part of a hospital-based case-control study. The cases were males with a histologically confirmed lung cancer diagnosis, < 75 years old, and no previous cancer diagnosis. Male controls were matched for age, hospital and residence area. This study includes 301 cases and 310 controls. A DNA bank was established for 547 patients (89.5%), and genotypes for CYP2D6 were differentiated by the Heim and Meyer method for the DNA samples of 249 cases and 271 controls. Among the cases, the frequencies of homozygous for the wild-type (EM), heterozygous (HEM) and homozygous for the mutant alleles (PM) were 62%, 32% and 7%; among the controls: 57%, 37% and 6%. Using EM as the reference, and adjusting for age, hospital and residence, we estimated the odds ratios for the HEM group and the PM group at 0.8 (95% CI [0.5-1.2]) and 1.1 (95% CI [0.5-2.4]) respectively. The PM frequency among the cases of adenocarcinoma was twice as high as among the controls (OR = 1.8, 95% CI [0.7-4.9]). This result was not observed among squamous and small cell carcinoma (OR = 0.7, 95% CI [0.3-1.8]). Twelve different case-control studies on CYP2D6 and lung cancer have so far been performed; the ORs they estimate range from 0.1 to 2.0, with a median value of approximately 0.6. This result lends some support to the hypothesis that belonging to the PM group is associated with a slight protective effect against lung cancer, but does not take into account the possibility that results may vary according to histologic type. In this context, the suggestion of a positive relationship between CYP2D6 and adenocarcinoma seems to us to merit investigation.

The interleukin-2 receptor down-regulates the expression of cytochrome P450 in cultured rat hepatocytes

BACKGROUND & AIMS

Interleukin (IL) 2 is used in advanced cancers, but its effects on cytochrome P450 remain unknown. Other cytokines down-regulate hepatic cytochrome P450, but it is not known whether this involves cytokine receptors. The aim of this study was to determine whether the IL-2 receptor is expressed on hepatocytes and whether its activation by IL-2 depresses cytochrome P450 in cultured rat hepatocytes.

METHODS

A monoclonal antibody specific for the rat IL-2 receptor alpha chain was used to label the receptor, whereas effects on cytochrome P450 were determined after 24 hours of culture with human recombinant IL-2 (5000 U/mL).

RESULTS

The presence of the IL-2 receptor in hepatocytes was shown by immunoblots, flow cytometry, and scanning confocal microscopy. IL-2 caused a 46% decrease in total cytochrome P450; a 35%, 35%, 36%, 26%, and 56% decrease in immunoreactive cytochrome P4501A1, 2B, 2C11, 2D1, and 3A, respectively; and a marked decrease in cytochrome P4503A2 and 2C11 messenger RNAs. Addition to the culture medium of the anti-receptor antibody or the tyrosine kinase inhibitor genistein prevented the IL-2-mediated decrease in cytochrome P450.

CONCLUSIONS

IL-2 down-regulates the expression of cytochrome P450 genes in cultured rat hepatocytes by interacting with its receptor expressed on hepatocytes.

Monitoring of azathioprine-induced immunosuppression with thiopurine methyltransferase activity in kidney transplant recipients

Thiopurine methyltransferase (TPMT), one of the enzymes involved in azathioprine metabolism, exhibits a wide range of activity in the normal population. We prospectively evaluated the monitoring of erythrocyte TPMT activity (using a radiochemical method) in kidney transplant recipients with regard to the efficacy of azathioprine. Three patterns in TPMT activity variation were observed. In group 1 patients, TPMT activity rose as early as 8 days after transplantation and steadily until month 3. In group 3 patients, TPMT activity remained unchanged. In group 2 patients, TPMT activity rose at month 1 after transplantation. Interestingly, the incidence of acute rejection was significantly (P < 0.01) different among the 3 groups, with the lowest incidence in group 1 and the highest in group 3. We hypothesized that TPMT activity increase was induced by azathioprine in the patients with the lowest incidence of acute rejection. The inducibility of TPMT activity would then appear to be an interesting marker of azathioprine-induced immunosuppression.

[Homozygote deficiency of thiopurine methyltransferase. A contraindication to the use of azathioprine in kidney transplantation]

Azathioprine-induced myelosuppression is the most important side effect observed in kidney transplantation. We report a case of severe neutropenia after kidney transplantation due to a thiopurine methyltransferase deficiency. This cause of azathioprine-induced myelotoxicity is rare, but its infectious consequences may be severe. Thiopurine methyltransferase deficiency must therefore be suspected when early and severe leukopenia occurs during azathioprine therapy. Erythrocyte thiopurine methyltransferase activity measurement confirms the diagnosis. Azathioprine and 6-mercaptopurine must afterwards be definitively avoided.

Immunohistochemical localization of cytochrome P450 2E1 in human pulmonary carcinoma and normal bronchial tissue

Cytochrome P450 2E1 (CYP2E1) is a major xenobiotic-metabolizing enzyme but data concerning its extrahepatic expression are few. CYP2E1 can metabolically activate many procarcinogens and therefore its presence in the lung might play a role in bioactivation of procarcinogens, so we studied the expression and localization of CYP2E1 in primary pulmonary carcinomas and surrounding normal bronchial tissue from 28 patients. Seromucous glands showed expression of CYP2E1 in 19 and bronchial epithelium in 18 of the 28 samples of normal bronchial tissue. Thirteen of the corresponding cases of primary pulmonary carcinoma showed staining for CYP2E1. In 11 of these 13 cases, CYP2E1 was also present in normal bronchial tissue. There was no statistically significant difference in the expression of CYP2E1 between adenocarcinomas and squamous cell carcinomas. No association was observed between the expression of CYP2E1 in tumour tissue and normal bronchial tissue. However, there was a significant correlation between the expression of CYP2E1 in seromucous glands and bronchial epithelium (r = 0.61, P < 0.01) of normal tissue. We conclude that CYP2E1 can be present in both normal and neoplastic bronchial tissue.

Genetic polymorphism of induction of CYP1A1 (EROD) activity

CYP1A1 is a cytochrome P450 which is inducible by polycyclic aromatic hydrocarbons (PAH). This induction is mediated via the Ahr locus which encodes the cytosolic Aryl hydrocarbon receptor. The induced activity of CYP1A1 can be measured in vitro by the ethoxyresorufin-O-deethylase (EROD) activity in lymphocytes after induction by benz(a)anthracene (B(a)A). Our purpose was to determine, using this assay, the genetic polymorphism of CYP1A1 induction. With this aim, a population and family study was undertaken. Using the statistical SKUMIX method, a bimodal distribution (two peaks) of the induced EROD activity among 102 unrelated individuals was obtained. We were unable to discriminate three classes of CYP1A1 induction phenotype since a trimodal distribution did not significantly improve the fit to the data (chi 2(1) = 0.37, p > 0.9). Segregation analysis performed on 57 nuclear families gave evidence of a major gene effect together with a polygenic component. The frequency of the high induction allele is equal to 0.11 with dominance on the low induction allele. This is an accordance with two distributions, with individuals showing low and high CYP1A1 induction phenotypes in proportions of 89% and 21% respectively. However, some degree of overlap between the two distributions prevented a clear genotype classification on the basis of the phenotype measured with the EROD assay. Further analyses should not be made with a dichotomized phenotype (low and high inducers) but should use quantitative measurements.

Cytochrome P4502B follows a vesicular route to the plasma membrane in cultured rat hepatocytes

BACKGROUND/AIMS

Autoantibodies against cytochrome P450 are found in some forms of autoimmune hepatitis. Cytochrome P450 is synthesized and mainly located in the endoplasmic reticulum but may also be expressed on the plasma membrane of hepatocytes. Vesicles migrate from the endoplasmic reticulum to the Golgi apparatus and then to the plasma membrane along microtubules. We determined the route followed by cytochrome P4502B to reach the plasma membrane.

METHODS

Rat hepatocytes were cultured for 2 hours after plating with various inhibitors of cellular trafficking. Detached, uncut, nonpermeabilized hepatocytes were then exposed to a monoclonal antibody specific for cytochrome P4502B and studied by flow cytometry and confocal microscopy.

RESULTS

The plasma membrane expression of cytochrome P4502B was markedly decreased after 2 hours of culture with cycloheximide (an inhibitor of protein synthesis), caffeine at 20 degrees C (conditions that decrease vesicular transport from the endoplasmic reticulum to the Golgi apparatus), brefeldin A (which redistributes Golgi components back to the endoplasmic reticulum), monensin (an inhibitor of Golgi functions), and colchicine, vinblastine, or nocodazole (three microtubule inhibitors).

CONCLUSIONS

Part of cytochrome P4502B follows a microtubule-dependent vesicular route from the endoplasmic reticulum to the plasma membrane in cultured rat hepatocytes.

Induction of cytochrome P4501A by smoking or omeprazole in comparison with UDP-glucuronosyltransferase in biopsies of human duodenal mucosa

Drug-metabolizing enzymes were investigated in duodenal biopsy specimens. Cytochrome P4501A (CYP1A) activity was determined by measuring 7-ethoxyresorufin O-deethylase (EROD) activity in biopsies from 20 smokers (3-30 cigarettes per day), 21 nonsmokers, and 10 nonsmokers receiving omeprazole treatment (20-60 mg/day for at least 1 week). Omeprazole is known to act as a polycyclic aromatic hydrocarbon (PAH)-type inducer in humans. EROD activity was found to be significantly induced in smokers and omeprazole-treated patients, with medians of 2.1 and 1.1 pmol.min-1.mg protein-1, respectively, compared with 0.5 pmol.min-1.mg protein-1 in nonsmokers. Immunoblot analysis substantiated that EROD activity was correlated with CYP1A protein. In contrast, UDP-glucuronosyltransferase (UGT) activity towards 4-methylumbelliferone (an overlapping substrate of several constitutive and inducible UGTs) was not significantly affected. The results demonstrate CYP1A induction by omeprazole and by constituents of cigarette smoke in the human duodenum and support the utility of duodenal biopsies to monitor CYP1A induction by PAH-type inducers.