Bioactivation of 2-amino-3-methylimidazo[4,5-f] quinoline (IQ) by prostaglandin H synthase

Metabolism and biomarkers of heterocyclic aromatic amines in molecular epidemiology studies: lessons learned from aromatic amines

Aromatic amines and heterocyclic aromatic amines (HAAs) are structurally related classes of carcinogens that are formed during the combustion of tobacco or during the high-temperature cooking of meats. Both classes of procarcinogens undergo metabolic activation by N-hydroxylation of the exocyclic amine group to produce a common proposed intermediate, the arylnitrenium ion, which is the critical metabolite implicated in toxicity and DNA damage. However, the biochemistry and chemical properties of these compounds are distinct, and different biomarkers of aromatic amines and HAAs have been developed for human biomonitoring studies. Hemoglobin adducts have been extensively used as biomarkers to monitor occupational and environmental exposures to a number of aromatic amines; however, HAAs do not form hemoglobin adducts at appreciable levels, and other biomarkers have been sought. A number of epidemiologic studies that have investigated dietary consumption of well-done meat in relation to various tumor sites reported a positive association between cancer risk and well-done meat consumption, although some studies have shown no associations between well-done meat and cancer risk. A major limiting factor in most epidemiological studies is the uncertainty in quantitative estimates of chronic exposure to HAAs, and thus, the association of HAAs formed in cooked meat and cancer risk has been difficult to establish. There is a critical need to establish long-term biomarkers of HAAs that can be implemented in molecular epidemioIogy studies. In this review, we highlight and contrast the biochemistry of several prototypical carcinogenic aromatic amines and HAAs to which humans are chronically exposed. The biochemical properties and the impact of polymorphisms of the major xenobiotic-metabolizing enzymes on the biological effects of these chemicals are examined. Lastly, the analytical approaches that have been successfully employed to biomonitor aromatic amines and HAAs, and emerging biomarkers of HAAs that may be implemented in molecular epidemiology studies are discussed.

DNA adducts of 2-amino-3-methylimidazo[4,5-f]quinoline (IQ) in fetal tissues of patas monkeys after transplacental exposure

Transplacental genotoxicity of the heterocyclic amine food-derived mutagen/carcinogen 2-amino-3-methylimidazo[4,5-f]quinoline (IQ) has been investigated by (32)P-postlabeling assay for IQ-DNA adducts in maternal liver, placenta, and several fetal tissues of patas monkeys, after exposure to 15, 35, or 50 mg/kg IQ near the end of gestation or to the highest dose in the first or second trimester. Dose-dependent adduct formation occurred in all tissues, with the highest levels occurring in maternal liver. Adduct amounts were similar among fetal tissues and placenta, except for lower levels in fetal brain and slightly more adducts in fetal liver. Adducts in placenta, fetal liver, lung, kidney, skin, and adrenal gland, but not in maternal liver or fetal brain, increased significantly as gestation progressed. Pretreatment with phenobarbital, which induces CYP enzymes that detoxify IQ, decreased adducts in maternal liver and possibly placenta, but not in fetal tissues. The CYP inducer beta-naphthoflavone caused a significant increase in IQ-DNA adducts in fetal lungs. Regression analysis suggested that IQ activation in maternal and fetal liver and possibly placenta contributed to adduct formation in fetal tissues; adducts in placenta and/or fetal liver were strong predictors for those in most fetal tissues. The results indicate that exposure of pregnant primates to IQ results in DNA adduct formation in most fetal tissues, especially late in gestation; that upregulation of maternal detoxification does not provide fetal protection; and that adducts in placenta indicate adduct levels in fetal tissues.

Bioactivation of the food mutagen 2-amino-3-methyl-imidazo[4, 5-f]quinoline (IQ) by prostaglandin-H synthase and by monooxygenases: DNA adduct analysis

2-Amino-3-methylimidazo[4,5-f]quinoline (IQ) is a known multisite carcinogen in rodents and a potent mutagen in acetyltransferase-proficient Salmonella typhimurium strains on activation by either monooxygenases (MFO) or by prostaglandin H synthase (PHS). The primary metabolites formed by MFO- or PHS-mediated IQ-oxidation are different ([Wolz]), but secondary metabolism could ultimately result in the same DNA-binding intermediates. For further investigations, the DNA adduct pattern was now studied by means of (32)P-postlabelling analysis in vitro on PHS-activation and compared to that formed on MFO-mediated activation of IQ in hepatocytes. The C8-dG-IQ-adduct N-(deoxyguanosin-8-yl)-IQ was the major adduct in all samples, that is, in DNA isolated from S. typhimurium YG1024 treated with PHS-oxidized IQ or its nitro-derivative, from ovine seminal vesicle cells, and from hepatocytes exposed to IQ or nitro-IQ. This speaks for the formation of a common DNA-reactive species, presumably an arylnitrenium ion, generated by different pathways in these cellular model systems. The similarity of critical biochemical DNA lesions suggests that PHS can contribute to the bioactivation of IQ in vivo: this is of particular interest in extrahepatic tissues since expression of cytochrome P450 isoenzymes known to be involved in the N-oxidation of IQ is largely confined to the liver.

Activation of heterocyclic amines by combinations of prostaglandin H synthase-1 and -2 with N-acetyltransferase 1 and 2

Cooking of meats produces several heterocyclic amines which are mutagenic and potentially carcinogenic. We found that metabolic activation of one of these heterocyclic amines, the quinoline derivative 2-amino-3-methylimidazo[4,5-f]quinoline (IQ), can be catalyzed by prostaglandin H synthase (PHS) as well as by CYP1A2. N-Acetyltransferase (NAT) increased IQ-DNA adduct formation by either of these pathways. In sonicate from transiently transfected COS cells, NAT1 increased CYP1A2 catalyzed adduct formation 4-fold while NAT2 increased adduct formation 12-fold. Both expressed human and purified ovine PHS-1 and PHS-2 catalyzed IQ-DNA adduct formation. The presence of NAT1 and NAT2 increased PHS-1 catalyzed adduct formation 2.5- and 4-fold, respectively. PHS-2 catalyzed IQ adduct formation was also enhanced by either NAT. The pyridine derivative, 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine, also produced by protein pyrolysis, did not form detectable DNA adducts during incubation with PHS. These results indicate that IQ is a substrate for both PHS-1 and PHS-2 and that NAT increases the ability of the resulting IQ metabolites to cause DNA damage. PHS activity, constitutive and induced, as well as NAT polymorphisms should be considered as factors in environmental carcinogenesis.