Gene/Protein Disease Symptom Drug Enzyme Compound
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Query: UMLS:C0596263 (carcinogenesis)
 64,820 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

An acetyltransferase-overexpressing strain of Salmonella typhimurium (NM2009) has been used to investigate roles of human liver microsomal cytochrome P450 (P450) enzymes in the activation of carcinogenic nitrosamine derivatives, including N-nitrosodialkylamines and tobacco-smoke-related nitrosamines, to genotoxic products. Studies employing correlation of activities with several P450-dependent monooxygenase reactions in different human liver samples, inhibition of microsomal activities by antibodies raised against human P450 enzymes and by specific P450 inhibitors, and reconstitution of activities with purified P450 enzymes suggest that the tobacco-smoke-related nitrosamines 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL) and N-nitrosonornicotine (NNN) as well as N-nitrosodimethylamine (NDMA) and N-nitrosodiethylamine (NDEA) are oxidized to genotoxic products by different P450 enzymes, particularly P450 2E1 and 2A6. The activation of NDMA and NNN by liver microsomes was suggested to be catalyzed more actively by P450 2E1 than by other P450 enzymes because the activities were well correlated with NDMA N-demethylation and aniline p-hydroxylation in different human samples, and purified P450 2E1 had the highest activities in reconstituted monooxygenase systems. The relatively high contribution of P450 2A6 to the activation of NDEA and NNK was supported by the correlation seen with coumarin 7-hydroxylation in human liver microsomes, and antibodies raised against P450 2A6 inhibited both activities by approximately 50%. P450 3A4, 2D6 and 2C enzymes appear not to be extensively involved in the activation of these nitrosamines as judged by several criteria examined. Thus, this work indicates that several P450 enzymes, particularly P450 2E1 and 2A6, catalyze metabolic activation of nitrosamine derivatives including N-nitrosodialkylamines and tobacco-smoke-related nitrosamines in human liver microsomes.
Carcinogenesis 1992 Oct
PMID:Cytochrome P450 2E1 and 2A6 enzymes as major catalysts for metabolic activation of N-nitrosodialkylamines and tobacco-related nitrosamines in human liver microsomes. 142 39

Rabbit nasal olfactory and respiratory microsomes were found to catalyze the alpha-hydroxylation of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) with specific activities of 262 and 136 pmol/min/mg protein in the formation of keto aldehyde, and of 318 and 190 pmol/min/mg protein in the formation of keto alcohol respectively. The formation of NNK-N-oxide was observed in experiments with rabbit olfactory and respiratory microsomes, but not with rat nasal microsomes. However, the rat nasal microsomes had higher activity in catalyzing the alpha-hydroxylation of NNK. In a reconstituted system, rabbit P450NMa, a major constitutive P450 isozyme in nasal microsomes, displayed high activities in the formation of the keto aldehyde and the keto alcohol with apparent Km values of 15 and 9 microM respectively. In comparison, rabbit olfactory specific P450NMb had a low activity in catalyzing the formation of keto aldehyde (Km = 186 microM) and no activity in the formation of keto alcohol. The P450NMa-catalyzed oxidation of NNK was inhibited by nicotine and diallyl sulfide. Kinetic studies indicated that nicotine is a competitive inhibitor. These results demonstrate that enzymes in rabbit nasal microsomes, especially P450NMa, efficiently catalyze the bioactivation of NNK.
Carcinogenesis 1992 Nov
PMID:Metabolism of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), a tobacco-specific carcinogen, by rabbit nasal microsomes and cytochrome P450s NMa and NMb. 142 86

Phenethyl isothiocyanate (PEITC), a constituent of cruciferous vegetables, has been shown to inhibit chemical carcinogenesis, possibly due to its ability to block the activation or to enhance the detoxification of chemical carcinogens. The present study was conducted to elucidate the biochemical mechanisms involved by characterizing the effects of PEITC on phase I and phase II xenobiotic-metabolizing enzymes. A single dose of PEITC to F344 rats (1 mmol/kg) decreased the liver N-nitrosodimethylamine demethylase (NDMAd) activity (mainly due to P450 2E1) by 80% at 2 h and the activity of NDMAd remained decreased by 40% at 48 h after treatment. The liver pentoxyresorufin O-dealkylase (PROD) activity and P450 2B1 protein level were elevated 10- and 7-fold at 24 h after treatment respectively. The liver microsomal ethoxyresorufin O-dealkylase (EROD) (mainly due to P450 1A) and erythromycin N-demethylase (mainly due to P450 3A) activities were decreased at 2-12 h after treatment and recovered afterwards. The lung microsomal PROD and EROD activities were not significantly affected; whereas, the nasal microsomal PROD and EROD activities were decreased by 40-50%. After a treatment with PEITC, the rates of oxidative metabolism of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) were decreased in liver microsomes by 40-60% at 2 h and recovered gradually; the rates in lung microsomes were markedly decreased by 60-70% at 2 h and remained at the decreased level at 24 h; and the rates in nasal mucosa microsomes were decreased gradually with the lowest activities observed at 18 h (50%) followed by a gradual recovery. Furthermore, the treatment with PEITC resulted in a maximal 5-fold increase of NAD(P)H:quinone oxidoreductase and 1.5-fold increase of glutathione S-transferase activities in the liver, but the activities of these two enzymes were not significantly affected in the lung and nasal mucosa. The sulfotransferase activity in the liver was decreased by 32-48% at 24-48 h after treatment; the nasal activity was increased by 1.8- to 2.5-fold, but the lung activity was not significantly changed. The hepatic UDP glucuronosyltransferase activity was slightly decreased at 2 h but slightly increased at 48 h after treatment, but no changes were observed for the lung and nasal activities. The study demonstrates that PEITC selectively affects xenobiotic-metabolizing enzymes in the liver, lung and nasal mucosa and it is especially effective in inhibiting the P450-dependent oxidation of NNK in the lung and of NDMA in the liver.
Carcinogenesis 1992 Dec
PMID:Effects of phenethyl isothiocyanate, a carcinogenesis inhibitor, on xenobiotic-metabolizing enzymes and nitrosamine metabolism in rats. 147 25

2-Amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx), one of the most abundant of the heterocyclic aromatic amines formed during the cooking of meat, is genotoxic and carcinogenic in rodents. MeIQx requires metabolic activation by P450 before it can exert these effects. Whilst there is indirect evidence that the mutagenic product is N-hydroxy-MeIQx (N-OHMeIQx), we have now identified this unequivocally following incubation of the amine with human hepatic microsomal fraction. A mixture of unlabelled MeIQx, [13C,15N2]MeIQx and [14C]MeIQx was used as substrate and the products analysed by HPLC-thermospray mass spectrometry. Characteristic doublet ions, 3 mass units apart, were found at m/z 214/217 ([M+H]+) from the parent compound, MeIQx and at 230/233 ([M+H]+) from N-OHMeIQx. The presence of a doublet ion at m/z 214/217 with the doublet at 230/233 [M+H+] provided additional evidence that this was N-OHMeIQx, as facile loss of 'O' is characteristic of N-hydroxylamines. Further evidence for the identity of the major metabolite, which accounted for approximately 90% of all microsomal metabolism, was obtained by comparing the mutagenicity of the HPLC eluate using Salmonella typhimurium YG1024, which is particularly sensitive to N-hydroxylamines, and TA98/1,8-DNP6 which is resistant to most N-hydroxylamines. Ninety-five per cent of direct-acting mutagenicity present in the reaction mixture was associated with a single peak, which co-eluted with N-OHMeIQx, as indicated by mass spectrometry. In the presence of a metabolic activation system, only one additional mutagenic peak, corresponding to unchanged MeIQx, could be detected. MeIQx (5 microM) was N-hydroxylated at a rate of 77 +/- 11 pmol/mg/min (mean +/- SEM, n = 4) by human liver microsomes. The specific inhibitor of human CYP1A2, furafylline (5 microM) inhibited the N-hydroxylation of MeIQx by > 90%. These data show that N-OHMeIQx is both the major oxidation product and the major genotoxic product of MeIQx generated by microsomal fractions of human liver and that the reaction is catalysed almost exclusively by CYP1A2.
Carcinogenesis 1992 Dec
PMID:N-hydroxy-MeIQx is the major microsomal oxidation product of the dietary carcinogen MeIQx with human liver. 147 28

As the first step in the process of carcinogenesis, most chemical carcinogens require metabolic activation by cytochromes P450 for conversion to highly reactive electrophiles that bind covalently to DNA. Studies in rodents suggest that low or high levels of expression of a single P450 can determine susceptibility or resistance to chemically induced cancer. Although rodent systems have been used to explore the molecular basis of chemical carcinogenesis and to identify chemicals capable of damaging genes and causing cancer, it has been understood that marked species differences exist in the expression, regulation, and catalytic activities of different P450s. Thus, large efforts are underway to study the catalytic activities of human P450s directly by expression of their cDNAs in cultured cells. Two systems are being used: a) transient high-level P450 production in HepG2 cells for analysis of catalytic activities, and b) stable expression in human B-lymphoblastoid cells to study promutagen and procarcinogen activation. These studies define the relative contributions of individual P450 forms to the activation of various chemical carcinogens. The B-lymphoblastoid cDNA expression system can also be used to determine whether a chemical will be hazardous or toxic to humans. The most intriguing aspects of P450s are the occurrence of human genetic polymorphisms in P450 expression, which could be a risk factor for chemical carcinogenesis. The best-studied P450 genetic polymorphism is the debrisoquine/sparteine polymorphism which is due to mutant CYP2D6 alleles. Four mutant alleles have been characterized that account for most of the defective CYP2D6 genes in Caucasians. These can be detected by polymerase chain reaction assays.(ABSTRACT TRUNCATED AT 250 WORDS)
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PMID:Human cytochromes P450: evolution and cDNA-directed expression. 148 67

The aim of this work was to optimize the ionic strength (tau) in the liver microsomal assay (LMA) in performing short-term genotoxicity tests. tau optimization would increase the sensitivity (i.e. decrease false negatives) and at the same time increase the specificity (decrease false positives). Such optimization depends upon the relative activities and stabilities of the liver polysubstrate cytochrome P450- and FAD-containing monooxygenase-dependent metabolizing enzymes present in the incubation mixtures. With regard to phase-I pathway, the expression of various P450-like activities (IA1, IA2, IIB1, IIE1, IIIA P450 classes) and thiobenzamide s-oxidase (as FAD-MFO marker), were examined in terms of their exact incubation conditions for the LMA during a period of preincubation (1 h) over the tau range 0.06-1.40. As a comparison with the phase-II pathway, the behaviour of glutathione S-transferases (total and pi class), glutathione S-epoxide transferase, epoxide hydrolase and UDP-glucuronosyl transferase were studied. Lipid peroxidation (LP) was also determined. Experiments were performed on S9 fractions derived from sodium phenobarbital, beta-naphthoflavone, isosafrol, ethanol and pregnenolone 16-alpha carbonitrile super-induced mouse liver. The maximal value of the mean specific activity (Asp), up to a 46% increase, was found at tau = 0.864 for oxidative reactions considered. On the contrary, a slight modulation of Asp for post-oxidative reactions was seen. LP was not changed appreciably by varying tau. In vitro DNA binding of the well-known premutagenic agent [14C]dimethylnitrosamine ([14C]DMNA), mediated by mouse hepatic microsomal enzymes, showed a significant increase of specific activity at tau = 0.864 (2.25-fold) compared to the usual tau (0.06) used. Additional confirmation of these results stems from mutagenesis experiments using DMNA on the diploid D7 strain of Saccharomyces cerevisiae as a biological test system. Indeed, a significant enhancement of mitotic gene conversion (up to 1.8-fold), mitotic crossing-over (2.6-fold) and reverse point mutation (2.6-fold) frequencies was achieved at tau = 0.86 compared to tau = 0.06 (traditional). These data show that tau = 0.86 can provide more convenient conditions for in vitro bioactivation (as exemplified by an increased Asp phase-I/Asp phase-II ratio), as well as DNA binding and genotoxic response.
Carcinogenesis 1992 Aug
PMID:Strategies for advancement of short-term mutagenicity tests: on the optimal ionic strength for the liver microsomal assay. 149 90

The rat lung and nasal cavity are two target organs for carcinogenesis by 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK). In order to characterize further the enzymes involved in the bioactivation of NNK, detailed kinetic and inhibitory studies were conducted with rat lung and nasal mucosa microsomes, and the results were compared with previous studies. The enzymes in rat lung microsomes catalyzed the alpha-hydroxylation, pyridine N-oxidation and carbonyl reduction of NNK. The apparent Km for the formation of the NNK-derived keto aldehyde, NNK-N-oxide, the NNK-derived keto alcohol and 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol were 28.8, 10.4, 7.0 and 178.1 microM respectively. In rat nasal microsomes, alpha-hydroxylation was the predominant pathway and the rate was approximately 200 times higher than that in lung microsomes. The apparent Kms for keto aldehyde and keto alcohol formation in rat nasal microsomes were 9.6 and 10.1 microM respectively. The cytochrome P450 inhibitors metyrapone and carbon monoxide markedly inhibited the metabolism of NNK in both rat lung and nasal microsomes. In rat lung microsomes, alpha-naphthoflavone and monospecific antibodies against P450s 1A2, 2A1 and 2B1 inhibited the formation of keto aldehyde by 39, 46, 64 and 23% respectively. In rat nasal microsomes, alpha-naphthoflavone and antibodies against P450s 1A2, 2A1 and 3A inhibited the metabolism of NNK by 80, 35, 20 and 14% respectively. The results indicate that cytochromes P450 play a major role in the metabolic activation of NNK in rat lung and nasal microsomes, and that there are tissue-related differences in NNK metabolism.
Carcinogenesis 1992 Aug
PMID:Kinetics and enzyme involvement in the metabolism of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) in microsomes of rat lung and nasal mucosa. 149 91

Human hepatoma HEPG2 cells were infected with recombinant vaccinia virus vectors containing cDNAs encoding both known and variant rat cytochromes P450 (CYP). CYP2B1 and CYP2B2 cytochromes were equally well expressed (110-140 pmol/mg of microsomal protein) and catalyzed metabolism of 7,12-dimethylbenz[a]anthracene (DMBA). Their regioselectivity for DMBA metabolism paralleled that of the respective purified rat liver enzymes and reproduced previously reported regioselective differences between CYP2B1 and CYP2B2 [Wilson et al. (1984) Carcinogenesis 5, 1475-1483]. CYP2A1 and CYP2A2 expressed in HEPG2 microsomes exhibited nearly equal DMBA-metabolizing activities that closely matched that of purified CYP2A1. Although purified rat liver CYP2B1 was 3 times more active than purified rat liver CYP2B2, the expressed recombinant microsomal CYP2B1 (rCYP2B1) was 20 times less active than rCYP2B2, where activity matched that of the purified cytochrome. Microsomal suppression of rCYP2B1 catalytic activity was also observed for benzo[a]pyrene. Specific amino acid substitutions at equivalent positions of the completely homologous NH2-terminal halves of rCYP2B1 and rCYP2B2 changed this suppression effect. Thus, a L58----F, I114----F double mutant exhibited 3 times the normal activity for rCYP2B1 while remaining inhibitory for rCYP2B2. The single substitutions produced very different effects. The L58----F substitution prevented expression of rCYP2B1, while the I114----F substitution was inhibitory for both rCYP2B1 and rCYP2B2 (40 and 70%). A single E282----V mutation produced a stimulation of rCYP2B1 activity comparable to that of the L58----F, I114----F double substitution.(ABSTRACT TRUNCATED AT 250 WORDS)
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PMID:Selective suppression of the catalytic activity of cDNA-expressed cytochrome P4502B1 toward polycyclic hydrocarbons in the microsomal membrane: modification of this effect by specific amino acid substitutions. 154 25

We reported previously that glutathione (GSH) is oxidized by peroxidases to a thiyl radical that can react with a number of chemicals, including the penultimate carcinogenic metabolite benzo[a]pyrene-7,8-dihydrodiol (7,8-B[a]PD), to give GSH conjugates. Here, we report that phenolic metabolites of benzo[a]pyrene (B[a]P) enhance the peroxidase-mediated formation of glutathione conjugates of 7,8-B[a]PD. The GSH conjugation of 7,8-B[a]PD in a horseradish peroxidase/peroxide system was increased over control values as follows: 9-OH-B[a]P by 4-fold, 7-OH-B[a]P by 3-fold, 1-OH-B[a]P by 2-fold. In contrast 3-OH-B[a]P was ineffective. A phenolic derivative of another polycyclic aromatic hydrocarbon (PAH), benz[a]anthracene, also enhanced GSH conjugation of 7,8-B[a]PD. The enhancement was dependent upon the presence of the phenol, horseradish peroxidase and peroxide. The phenolic compounds, including 3-OH-B[a]P, were also efficient reducing cofactors for the peroxidase. With the exception of 3-OH-B[a]P, the phenolic metabolites of PAH enhanced peroxidase-mediated formation of thiyl radical as detected by electron spin resonance spectrometry. Since both phenols and dihydrodiols are metabolites of B[a]P catalyzed by the cytochromes P450 system, enhancement of peroxidase-dependent 7,8-B[a]PD-GSH conjugation by phenols suggests a possible interaction between peroxidases and cytochromes P450 systems. This interaction may contribute to the detoxication of the penultimate carcinogenic PAH-dihydrodiols and other chemicals.
Carcinogenesis 1992 Apr
PMID:Peroxidase-mediated glutathione conjugation of benzo[a]pyrene-7,8-dihydrodiol is enhanced by benzo[a]pyrene phenols in vitro. 157 1

The metabolism of 2-amino-1-methyl-6-phenylimidazo[4,5-b]-pyridine (PhIP), a heterocyclic amine carcinogen detected in cooked meats, was investigated in mice. In 3-methylcholanthrene-induced mice administered 0.1, 1.0 and 10 mg/kg [14C]PhIP (i.p.), urinary and fecal excretion over 24 h accounted for 16% and 42-56% of the dose respectively. Urinary excretion of unchanged parent compound accounted for only 0.5-0.8% of the administered dose. At all doses, the major urinary metabolite was identified as 4'-(2-amino-1-methylimidazo[4,5-b]pyrid-6-yl)phenyl sulfate and this metabolite comprised approximately 5% of the dose. Uninduced mice excreted greater than 13% of a 10 mg/kg dose as the sulfate conjugate. Urinary excretion of both 2-amino-1-methyl-6-(4'-hydroxy)-phenylimidazo[4,5-b]pyridine (4'-hydroxy-PhIP) and a glucuronide conjugate of 2-hydroxyamino-1-methyl-6-phenyl-imidazo[4,5-b]pyridine (N-hydroxy-PhIP) was also higher (4-fold) in uninduced versus induced mice. The decreased urinary excretion of P450-derived metabolites via induction contrasted with increased metabolite formation by hepatic microsomal preparations. 4'-Hydroxy-PhIP and N-hydroxy-PhIP were produced in amounts nearly 7- and 3-fold higher respectively by induced versus uninduced microsomal incubations at 50 microM [3H]PhIP. At concentrations less than 10 microM, PhIP was almost exclusively converted by the induced preparations to an unidentified metabolite that was not retained by the C18 column. This metabolite, which also was formed in incubations with either 4'-hydroxy-PhIP or N-hydroxy-PhIP, was produced by microsomes from uninduced animals at a much slower rate. Covalent binding to microsomal protein in incubations with [3H]PhIP was concentration-dependent and 2- to 4-fold higher in induced than uninduced preparations. Covalent binding in liver and kidney of induced mice administered [14C]PhIP was dose dependent. At 10 mg/kg PhIP, adducts were produced at 1.7-fold higher levels in livers of induced versus uninduced mice, but renal binding was higher in uninduced animals. These studies indicate the importance of cytochrome P450 and other xenobiotic enzymes in the metabolism, disposition and activation of PhIP.
Carcinogenesis 1992 Apr
PMID:Metabolism of 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) in mice. 157 15


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