Prostaglandin-H synthase mediated metabolism and mutagenic activation of 2-amino-3-methylimidazo [4,5-f] quinoline (IQ)

Mutagenicity of carcinogenic heterocyclic amines in Salmonella typhimurium YG strains and transgenic rodents including gpt delta

Chemical carcinogens to humans have been usually identified by epidemiological studies on the relationships between occupational or environmental exposure to the agents and specific cancer induction. In contrast, carcinogenic heterocyclic amines were identified under the principle that mutagens in bacterial in the Ames test are possible human carcinogens. In the 1970s to 1990s, more than 10 heterocyclic amines were isolated from pyrolysates of amino acids, proteins, meat or fish as mutagens in the Ames test, and they were demonstrated as carcinogens in rodents. In the 1980s and 1990s, we have developed derivatives of the Ames tester strains that overexpressed acetyltransferase of Salmonella typhimurium. These strains such as Salmonella typhimurium YG1024 exhibited a high sensitivity to the mutagenicity of the carcinogenic heterocyclic amines. Because of the high sensitivity, YG1024 and other YG strains were used for various purposes, e.g., identification of novel heterocyclic amines, mechanisms of metabolic activation, comparison of mutagenic potencies of various heterocyclic amines, and the co-mutagenic effects. In the 1990s and 2000s, we developed transgenic mice and rats for the detection of mutagenicity of chemicals in vivo. The transgenics were generated by the introduction of reporter genes for mutations into fertilized eggs of mice and rats. We named the transgenics as gpt delta because the gpt gene of Escherichia coli was used for detection of point mutations such as base substitutions and frameshifts and the red/gam genes of λ phage were employed to detect deletion mutations. The transgenic rodents gpt delta and other transgenics with lacI or lacZ as reporter genes have been utilized for characterization of mutagenicity of heterocyclic amines in vivo. In this review, we summarized the in vitro mutagenicity of heterocyclic amines in Salmonella typhimurium YG strains and the in vivo mutagenicity in transgenic rodents. We discussed the relationships between in vitro and in vivo mutagenicity of the heterocyclic amines and their relations to the carcinogenicity.

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.

Dose-dependent reduction of 3,2'-dimethyl-4-aminobiphenyl-derived DNA adducts in colon and liver of rats administered celecoxib

Colon cancer is second leading cause of cancer-related deaths in Western countries. Diet and smoking, which contain aromatic and heterocyclic amines, are major risk factors for colon cancer. Colorectal cancers have a natural history of long latency and therefore provide ample opportunities for effective chemoprevention. 3,2'-Dimethyl-4-aminobiphenyl (DMABP) is an experimental aromatic amine that causes cancer in rat colon and serves as an experimental model for arylamine and heterocyclic amine mutagens derived from diet and smoking. In this study, we investigated the effects of celecoxib, a selective cyclooxygenase-2 (COX-2) inhibitor on DMABP-induced DNA adduct formation in rat liver and colon. Male F-344 rats (5-week old) were provided free access to modified AIN-76A rat chow containing 0 (control), 500, 1000, or 1500 ppm celecoxib. Two weeks later, the rats received a subcutaneous injection of 100mg/kg DMABP in peanut oil. Two days after DMABP treatment, the rats were killed and DMABP-derived adducts were analyzed in colon and liver DNA by butanol extraction-mediated (32)P-postlabeling. Two major DNA adducts, identified as dG-C8-DMABP and dG-N(2)-DMABP, were detected in liver and colon of rats treated with DMABP. These DNA adducts were diminished approximately 35-40% with 500 ppm and 65-70% with 1,000 ppm celecoxib. In the colon, no further decline in DNA adducts was observed at 1500 ppm. The same DMABP-DNA adducts also were detected in the liver and were also diminished by celecoxib treatment. The reduction in DMABP-DNA adduct levels in celecoxib-treated animals provides further support for celecoxib as a chemopreventive agent for colorectal cancer.

Cytochrome P450 activation of arylamines and heterocyclic amines

Arylamines and heterocyclic arylamines (HAAs) are of particular interest because of demonstrated carcinogenicity in animals and humans and the broad exposure to many of these compounds. The activation of these, and also some arylamine drugs, involves N-hydroxylation, usually by cytochrome P450 (P450). P450 1A2 plays a prominent role in these reactions. However, P450 1A1 and 1B1 and other P450s are also important in humans as well as experimental animals. Some arylamines (including drugs) are N-hydroxylated predominantly by P450s other than those in Family 1. Other oxygenases can also have roles. An important issue is extrapolation between species in predicting cancer risks, as shown by the low rates of HAA activation by rat P450 1A2 and low levels of P450 1A2 expression in some nonhuman primates.

Effects of polyunsaturated fatty acids on prostaglandin synthesis and cyclooxygenase-mediated DNA adduct formation by heterocyclic aromatic amines in human adenocarcinoma colon cells

Dietary heterocyclic aromatic amines (HCA) and polyunsaturated fatty acids (PUFA) are both believed to play a role in colon carcinogenesis, and are both substrate for the enzyme cyclooxygenase (COX). In HCA-7 cells, highly expressing isoform COX-2, we investigated the effects of PUFA on prostaglandin synthesis and DNA adduct formation by the HCA 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) and 2-amino-3-methylimidazo[4,5-f]quinoline (IQ). Furthermore, we studied the role of COX, COX-2 in particular, and cytochrome P4501A2 (CYP1A2) by using the enzyme inhibitors indomethacin (IM), NS-398, and phenethyl isothiocyanate (PEITC), respectively. COX-mediated formation of prostaglandin E2 (PGE2) from linoleic acid (LA) showed that HCA-7 cells can convert LA into arachidonic acid (AA). Alternatively, eicosapentaenoic acid (EPA) was found to compete with AA for COX. Strongly decreased PGE2 levels by addition of IM demonstrated involvement of COX in PUFA metabolism. Both IM and NS-398 inhibited adduct formation by HCA to nearly the same extent, indicating involvement of COX-2 rather than COX-1, while CYP1A2 activity in HCA-7 cells was demonstrated by addition of PEITC. Overall, inhibiting effects were stronger for PhIP than for IQ. HCA-DNA adduct formation was stimulated by addition of PUFA, although high PUFA concentrations partly reduced this stimulating effect. Finally, similar effects for n-3 and n-6 fatty acids suggested that adduct formation may not be the crucial mechanism behind the differential effects of PUFA on colon carcinogenesis that have been described. These results show that COX, and COX-2 in particular, can play a substantial role in HCA activation, especially in extrahepatic tissues like the colon. Furthermore, the obvious interactions between PUFA and HCA in COX-2 expressing cancer cells may be important in modulating colorectal cancer risk.

Generation of free radicals and induction of DNA adducts by activation of heterocyclic aromatic amines via different metabolic pathways in vitro

Food-derived heterocyclic aromatic amines (HCAs) have proved to be carcinogenic in both rodents and nonhuman primates. Two different metabolic pathways are suggested for the metabolic activation of HCA. The hepatic pathway proceeds via a two-step process involving N-hydroxylation by cytochrome P4501A2 and subsequent O-acetylation by N-acetyltransferase-2. An alternative pathway may be of particular interest in extrahepatic tissues and proceeds via one-electron oxidation catalyzed by prostaglandin H synthase (PHS), rendering free-radical metabolites. In this study, we investigated the metabolic activation of two HCAs, 2-amino-3-methylimidazo[4,5-f]quinoline (IQ) and 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), by two different enzyme systems in vitro, generating different primary and secondary reactive metabolites. Rat liver S9 mix and PHS were used as the activating system and represent the hepatic and extrahepatic pathways, respectively. Electron-spin resonance spectroscopy showed that both IQ and PhIP exerted inhibiting effects on PHS-mediated formation of hydroxyl radicals during the conversion of arachidonic acid to prostaglandins. Evidence for the formation of HCA free radicals was presented in an indirect way by the formation of glutathione-derived thiyl radicals, with purified PHS as the activating system. Activation by S9 mix did not result in the formation of detectable radical metabolites, showing that the two metabolic routes primarily led to the formation of different metabolites. In all electron-spin resonance experiments, IQ appeared to be more effective than PhIP. In contrasts, DNA adduct analysis by means of (32)P-postlabeling showed similar adduct patterns for S9 and PHS in single-stranded and double-stranded salmon testes DNA after incubation with PhIP, indicating the ultimate formation of a common reactive intermediate. For IQ, activation by PHS led to an additional adduct spot that was not present after S9 activation. Furthermore, activation of IQ resulted in higher adduct levels compared with PhIP for both activation pathways. Overall, adduct levels were higher in single-stranded DNA than double-stranded DNA. Our results showed that the hepatic and extrahepatic pathways resulted in different primary metabolites, while the ultimate formation of a similar reactive intermediate for PhIP, possibly an arylnitrenium ion, suggested that both pathways could play an important role in the onset of carcinogenesis.

Evaluation of fecal mutagenicity and colorectal cancer risk

Colorectal cancer is one of the most common internal malignancies in Western society. The cause of this disease appears to be multifactorial and involves genetic as well as environmental aspects. The human colon is continuously exposed to a complex mixture of compounds, which is either of direct dietary origin or the result of digestive, microbial and excretory processes. In order to establish the mutagenic burden of the colorectal mucosa, analysis of specific compounds in feces is usually preferred. Alternatively, the mutagenic potency of fecal extracts has been determined, but the interpretation of these more integrative measurements is hampered by methodological shortcomings. In this review, we focus on exposure of the large bowel to five different classes of fecal mutagens that have previously been related to colorectal cancer risk. These include heterocyclic aromatic amines (HCA) and polycyclic aromatic hydrocarbons (PAH), two exogenous factors that are predominantly ingested as pyrolysis products present in food and (partially) excreted in the feces. Additionally, we discuss N-nitroso-compounds, fecapentaenes and bile acids, all fecal constituents (mainly) of endogenous origin. The mutagenic and carcinogenic potency of the above mentioned compounds as well as their presence in feces, proposed mode of action and potential role in the initiation and promotion of human colorectal cancer are discussed. The combined results from in vitro and in vivo research unequivocally demonstrate that these classes of compounds comprise potent mutagens that induce many different forms of genetic damage and that particularly bile acids and fecapentaenes may also affect the carcinogenic process by epigenetic mechanisms. Large inter-individual differences in levels of exposures have been reported, including those in a range where considerable genetic damage can be expected based on evidence from animal studies. Particularly, however, exposure profiles of PAH and N-nitroso compounds (NOC) have to be more accurately established to come to a risk evaluation. Moreover, lack of human studies and inconsistency between epidemiological data make it impossible to describe colorectal cancer risk as a result of specific exposures in quantitative terms, or even to indicate the relative importance of the mutagens discussed. Particularly, the polymorphisms of genes involved in the metabolism of heterocyclic amines are important determinants of carcinogenic risk. However, the present knowledge of gene-environment interactions with regard to colorectal cancer risk is rather limited. We expect that the introduction of DNA chip technology in colorectal cancer epidemiology will offer new opportunities to identify combinations of exposures and genetic polymorphisms that relate to increased cancer risk. This knowledge will enable us to improve epidemiological study design and statistical power in future research.

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.