Genotoxicity and carcinogenicity in rats and mice of 2-amino-3,6-dihydro-3-methyl-7H-imidazolo[4,5-f]quinolin-7- one: an intestinal bacterial metabolite of 2-amino-3-methyl-3H-imidazo[4,5-f]quinoline

The Human Fecal Microbiota Metabolizes Foodborne Heterocyclic Aromatic Amines by Reuterin Conjugation and Further Transformations

SCOPE

Heterocyclic aromatic amines (HAAs) are process-induced food contaminants with high mutagenic and/or carcinogenic potential. Although the human gut microbiota is known to affect the metabolism of dietary constituents, its impact on HAA metabolism and toxicity has been little studied. Here, the glycerol-dependent metabolism of seven foodborne HAAs (AαC, Trp-P-1, harman, norharman, PhIP, MeIQx, and MeIQ) by the human fecal microbiota is investigated.

METHODS AND RESULTS

As analyzed by HPLC-DAD/FLD, the extent of conversion is strongly dependent on glycerol supplementation and HAA structure. AαC (60-100%) and the 2-aminoimidazoazarenes (up to 58%) are especially prone to microbial conversion. Based on high-resolution MS and/or NMR spectroscopy data, 70 fecal metabolites are identified in total, mainly formed by chemical reactions with one or two molecules of microbially derived reuterin. Moreover, it has been demonstrated that the human fecal microbiota can further transform reuterin adducts by reduction and/or hydroxylation reactions. Upon isolation, some reuterin-induced HAA metabolites appear to be partially unstable, complicating structural identification.

CONCLUSION

The formation of microbial metabolites needs to be incorporated into risk assessment considerations for HAAs in human health. In this study, several HAA metabolites, mainly reuterin-dependent, are identified in vitro, providing the basis for future human studies investigating microbial HAA metabolism.

Synthesis, characterization, and determination of genotoxic effect of a novel dimeric 8-hydroxyquinoline Cd(II) SCN complex

In this study, a Cd(II) complex was synthesized using 8-hydroxyquinoline and thiocyanate as the ligands and structurally characterized with the combination of FTIR, ¹H-NMR, ¹³C-NMR, UV-vis, and MS spectral data. Then, genotoxic effects of the prepared complex were investigated. Genotoxic properties of the dimeric 8-hydroxyquinolinthiocyanatoCd(II) [Cd₂(8Q)₂(SCN)₂] complex synthesized as drug raw material were analyzed in human peripheral blood lymphocytes. Concentrations of 1, 2, 4, 6, and 8 μg/mL [Cd₂(8Q)₂(SCN)₂] were used for 24 and 48  h durations. [Cd₂(8Q)₂(SCN)₂] significantly increased chromosomal aberrations (CAs) at 4, 6, and 8 μg/mL concentrations after a 24- h period and 2 and 4 μg/mL after a 48-h period. [Cd₂(8Q)₂(SCN)₂] significantly decreased the mitotic index (MI) at all concentrations, both at 24 and 48 h. Micronuclei frequency (MN) was not affected by [Cd₂(8Q)₂(SCN)₂] treatment compared with the control. After application for a 48 h period, 6 and 8 μg/mL concentrations showed toxic effects both in chromosomal abnormality and in micronucleus tests. It also decreased the cytokinesis-block proliferation index (CBPI), but this result was statistically significant only at 6 and 8 μg/mL concentrations. In the comet assay (single-cell gel electrophoresis (SCGE)), significant increases in comet tail length, tail moment, and tail intensity were observed at all concentrations. [Cd₂(8Q)₂(SCN)₂] displays clastogenic effect in the concentrations used in human peripheral lymphocytes at chromosomal abnormality, micronucleus tests, and cytokinesis-block proliferation index parameters. Further studies should be conducted in other test systems to evaluate the complete genotoxic potential of [Cd₂(8Q)₂(SCN)₂].

Role of intestinal microflora in xenobiotic-induced toxicity

In addition to its role in digestion of food in the gastrointestinal tract, the intestinal microflora is also capable of biotransforming numerous drugs. Likewise, the intestinal microflora may significantly modulate xenobiotic-induced toxicity by either activating or inactivating xenobiotics via metabolism. To date, most investigations of xenobiotic metabolism have focused not only on metabolism in host tissues, but the modulation of the pharmacological activity of drugs by the intestinal microflora. Despite its importance, the presumed role of intestinal microflora metabolism in xenobiotic-induced toxicity has been understudied. Therefore, it is appropriate to briefly review our current situation, and state which research in xenobiotic metabolism by intestinal microflora, particularly in the field of toxicology, is needed.

Evaluation of epidemiological studies of intestinal bacteria that affected occurrence of colorectal cancer: studies of prevention of colorectal tumors by dairy products and lactic acid bacteria

Enviromental factors have been consistently associated with colon cancer risk. In particular, consumption of Western-style diet including red meat is the most widely accepted etiologic risk factor. It has been reported that dietary factors change the proportion of intestinal flora, and it also affects the composition of fecal bile acids and the intestinal activity of some mutagens. In addition, it was suggested that modulating the composition of intestinal flora may reduce the occurrence of colorectal cancer. In this review, we present the clinical studies on the association between intestinal flora and the risk of colorectal cancer that have been carried out to date. The clinical studies of intestinal bacteria related to colorectal cancer risk have not shown consistent results so far, compared with the accomplishments of some basic studies. On the other hand, it was suggested in some clinical studies that lactic acid bacteria reduce the occurrence of colorectal cancer.

Polymorphisms of cytochrome P450 1A2 and N-acetyltransferase genes, meat consumption, and risk of colorectal cancer

PURPOSE

Polymorphic cytochrome P-450 1A2, N-acetyltransferase 1, and 2 are important enzymes involved in the biotransformation of aromatic and heterocyclic amines known as carcinogens for colorectal cancer. A hospital-based study was designed to investigate the association between colorectal cancer and cytochrome P-450 1A2, N-acetyltransferase 1, and N-acetyltransferase 2, with the interaction of meat consumption.

METHODS

We genotyped these polymorphisms for 727 colorectal cancer cases and 736 healthy controls. Information on sociodemographic characteristics and diet were ascertained using a structured questionnaire.

RESULTS

The colorectal cancer risk was significantly increased in rapid N-acetyltransferase 1 carriers with high white meat consumption (almost every day) compared to those carrying the slow N-acetyltransferase 1 genotype with low white meat consumption (less than once a week, odds ratio, 3.00; 95 percent confidence interval, 1.83-4.92). Furthermore, a gene-gene interaction between cytochrome P-450 1A2*1C and N-acetyltransferase 1 was found and modulated by white meat consumption.

CONCLUSIONS

N-acetyltransferase 1 might compete with cytochrome P-450 1A2*1C to increase the colorectal cancer risk in intermediate white meat consumers, whereas the rapid N-acetyltransferase 1 genotype may exert a harmful effect on individuals with high carcinogen exposure.

The use of cultured primary bovine colon epithelial cells as a screening model to detect genotoxic effects of heterocyclic aromatic amines in the comet assay

Isolated epithelial cells from the bovine colon were maintained in dividing monolayer cultures and used as a model system for colon tissue in in vitro toxicological studies. The cytotoxic effects of the heterocyclic aromatic amines 2-amino-3-methylimidazo [4,5-f]quinoline (IQ) and 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhiP) were investigated in these cells and IC(50) values were determined by inhibition of neutral red uptake into the cultured cells. Although PhiP was not cytotoxic up to concentrations of 500 microM, IQ was cytotoxic above 300 microM. The induction of DNA strand breaks in cultured bovine epithelial colon cells was determined using the alkaline single-cell gel electrophoresis (Comet assay) technique, and subsequently, the DNA damage was used as a determinant of genotoxic effects of the heterocyclic aromatic amines in order to establish this system for detection of adverse effects of chemicals in a model system for the colon. In the absence of an external enzymatic metabolizing system (S9 mix) both amines did not induce DNA strand breaks. When S9 mix was used, PhiP induced DNA strand breaks above 10 microM whereas IQ did not show any significant effect at 300 microM. This cell culture system was found to be a useful screening system for testing of compounds that are considered to affect colonic tissue.

Meat consumption and cancer risk

The authors review the key studies on the association between meat intake and cancer risk, including a new prospective cohort study by Amanda Cross and colleagues published inPLoS Medicine.

Meat, meat cooking methods and preservation, and risk for colorectal adenoma

Cooking meat at high temperatures produces heterocyclic amines (HCAs) and polycyclic aromatic hydrocarbons (PAHs). Processed meats contain N-nitroso compounds. Meat intake may increase cancer risk as HCAs, PAHs, and N-nitroso compounds are carcinogenic in animal models. We investigated meat, processed meat, HCAs, and the PAH benzo(a)pyrene and the risk of colorectal adenoma in 3,696 left-sided (descending and sigmoid colon and rectum) adenoma cases and 34,817 endoscopy-negative controls. Dietary intake was assessed using a 137-item food frequency questionnaire, with additional questions on meats and meat cooking practices. The questionnaire was linked to a previously developed database to determine exposure to HCAs and PAHs. Intake of red meat, with known doneness/cooking methods, was associated with an increased risk of adenoma in the descending and sigmoid colon [odds ratio (OR), 1.26; 95% confidence interval (95% CI), 1.05-1.50 comparing extreme quintiles of intake] but not rectal adenoma. Well-done red meat was associated with increased risk of colorectal adenoma (OR, 1.21; 95% CI, 1.06-1.37). Increased risks for adenoma of the descending colon and sigmoid colon were observed for the two HCAs: 2-amino-3,8-dimethylimidazo[4,5]quinoxaline and 2-amino-1-methyl-6-phenylimidazo[4,5]pyridine (OR, 1.18; 95% CI, 1.01-1.38 and OR, 1.17, 95% CI, 1.01-1.35, respectively) as well as benzo(a)pyrene (OR, 1.18; 95% CI, 1.02-1.35). Greater intake of bacon and sausage was associated with increased colorectal adenoma risk (OR, 1.14; 95% CI, 1.00-1.30); however, total intake of processed meat was not (OR, 1.04; 95% CI, 0.90-1.19). Our study of screening-detected colorectal adenomas shows that red meat and meat cooked at high temperatures are associated with an increased risk of colorectal adenoma.

1H nuclear magnetic resonance spectroscopy-based studies of the metabolism of food-borne carcinogen 2-amino-3-methylimidazo[4,5-f]quinoline by human intestinal microbiota

2-Amino-3-methylimidazo[4,5-f]quinoline (IQ) is a mutagenic/carcinogenic compound formed from meat and fish during cooking. Following ingestion, IQ is metabolized mainly by liver xenobiotic-metabolizing enzymes, but intestinal bacteria may also contribute to its biotransformation. The aim of this study was to investigate the metabolism of IQ by the human intestinal microbiota. Following incubation of IQ (200 microM) under anoxic conditions with 100-fold dilutions of stools freshly collected from three healthy volunteers, we quantified residual IQ by high-pressure liquid chromatography (HPLC) analysis and characterized the production of IQ metabolites by in situ (1)H nuclear magnetic resonance ((1)H-NMR) spectroscopic analysis of crude incubation media. In addition, we looked for IQ-degrading bacteria by screening collection strains and by isolating new strains from the cecal contents of human-microbiota-associated rats gavaged with IQ on a regular basis. HPLC and (1)H-NMR analyses showed that the three human microbiota degraded IQ with different efficiencies (range, 50 to 91% after 72 h of incubation) and converted it into a unique derivative, namely, 7-hydroxy-IQ. We found 10 bacterial strains that were able to perform this reaction: Bacteroides thetaiotaomicron (n = 2), Clostridium clostridiiforme (n = 3), Clostridium perfringens (n = 1), and Escherichia coli (n = 4). On the whole, our results indicate that bacteria belonging to the predominant communities of the human intestine are able to produce 7-hydroxy-IQ from IQ. They also suggest interindividual differences in the ability to perform this reaction. Whether it is a metabolic activation is still a matter of debate, since 7-hydroxy-IQ has been shown to be a direct-acting mutagen in the Ames assay but not carcinogenic in laboratory rodents.

Protective effect of acetaminophen against colon cancer initiation effects of 3,2'-dimethyl-4-aminobiphenyl in rats

A previous investigation demonstrated the anticarcinogenicity of acetaminophen (APAP) against colon carcinogenesis in rats induced by 3,2'-dimethyl-4-aminobiphenyl (DMAB). DMAB was selected as a structurally related surrogate for heterocyclic amines, formed during cooking of protein, which are believed to be involved in human colon cancer. The objective of the present study was to ascertain whether the early initiating effects of this colon carcinogen are inhibited by APAP. Six groups of male F344 rats were treated over a 6-week period as follows: (1) vehicle (corn oil) for 6 weeks; (2) APAP in the diet at 1000 ppm daily for 6 weeks; (3) 50 mg/kg DMAB by gavage once a week for the last 4 weeks; (4) 5 mg/kg DMAB as for (3); (5) 1000 ppm APAP for 6 weeks and 50 mg/kg DMAB for the last 4 weeks; and (6) 1000 ppm APAP and 5 mg/kg DMAB as for (5). Colonic tissue was within normal limits in the control and APAP groups. In the APAP only group, apical enterocytic hypertrophy and hyperaemia over the entire surface epithelium was present. In the high-dose DMAB group, in the lower third of the crypts, foci of enlarged glands with hypertrophic cells exhibiting karyomegaly and anisokaryosis (FHE) of 3+ degree of severity were evident in 100% of the animals. Also, there were increases in periglandular fibrocytes, matrix and mononuclear cells (PF). In the low-dose DMAB group both FHE and PF changes with the same degree of severity were reduced. In rats given the low dose of DMAB plus APAP, FHE and PF with the same degree of severity (3+) was absent. Both DMAB exposures increased significantly the replicating fraction of colonic enterocytes in an exposure-related fashion and the replicating fractions were significantly reduced by APAP. In 32P-postlabelling of colon, liver and urinary bladder DNA, high-dose DMAB produced 2-6 distinct dose-related spots reflecting DNA adducts. These spots were reduced or were no longer detectable in all three tissues when APAP was given 2 weeks before and during DMAB exposure. Using immunohistochemical detection of DMAB adducts in the colon, a dose-related colour intensity was present for both doses of DMAB. APAP reduced this by 94-fold. Thus, APAP produced a marked protective effect in colonic enterocytes against several parameters of neoplastic development by the carcinogen.

Impact of bacteria in dairy products and of the intestinal microflora on the genotoxic and carcinogenic effects of heterocyclic aromatic amines

This article gives a short overview on the present state of knowledge of the effects of the intestinal microflora on the health hazards of heterocyclic aromatic amines (HAs). Results of single cell gel electrophoresis assays with conventional, germ free and human flora associated rats indicate that the presence of intestinal microorganisms strongly enhances the induction of DNA-damage in colon and liver cells by IQ. Furthermore, it was found that supplementation of the feed with Lactobacilli attenuates the induction of colon cancer by this same amine. These recent findings suggest that the intestinal microflora and lactic acid bacilli in dairy products strongly affect the health risks of HAs. Nevertheless, most previous experiments with HAs focused on the involvement of mammalian enzymes in the biotransformation of these compounds and only a few articles are available which concern interactions of bacteria with HAs. Some of these studies suggested that the formation of directly mutagenic hydroxy-metabolites of the amines by fecal bacteria might be an important activation pathway but it turned out that the hydroxy-derivative of IQ is not genotoxic in mammalian cells and does not cause colon cancer in laboratory rodents. There is some evidence that hydrolysis of HA-metabolites by bacterial ss-glucuronidase might play a role in the activation of HAs but experimental data are scarce and no firm conclusions can be drawn at present. The most important detoxification mechanism appears to be the direct binding of the HAs to the cell walls of certain bacterial strains contained in fermented foods. It was shown that these effects do also take place under physiologically relevant conditions. Overall, it seems that intestinal bacteria play a key role in the activation and detoxification of HAs which has been an area of research long ignored. The elucidation of these mechanisms may enable the development of biomarkers for colon cancer risk and nutritional strategies of protection.

Metabolism of 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline in human hepatocytes: 2-amino-3-methylimidazo[4,5-f]quinoxaline-8-carboxylic acid is a major detoxification pathway catalyzed by cytochrome P450 1A2

Metabolic pathways of the mutagen 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx) remain incompletely characterized in humans. In this study, the metabolism of MeIQx was investigated in primary human hepatocytes. Six metabolites were characterized by UV and mass spectroscopy. Novel metabolites were additionally characterized by 1H NMR spectroscopy. The carcinogenic metabolite, 2-(hydroxyamino)-3,8-dimethylimidazo[4,5-f]quinoxaline, which is formed by cytochrome P450 1A2 (P450 1A2), was found to be transformed into the N(2)-glucuronide conjugate, N(2)-(beta-1-glucosiduronyl)-2-(hydroxyamino)-3,8-dimethylimidazo[4,5-f]quinoxaline. The phase II conjugates N(2)-(3,8-dimethylimidazo[4,5-f]quinoxalin-2-yl)sulfamic acid and N(2)-(beta-1-glucosiduronyl)-2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline, as well as the 7-oxo derivatives of MeIQx and N-desmethyl-MeIQx, 2-amino-3,8-dimethyl-6-hydro-7H-imidazo[4,5-f]quinoxalin-7-one (7-oxo-MeIQx), and 2-amino-6-hydro-8-methyl-7H-imidazo[4,5-f]quinoxalin-7-one (N-desmethyl-7-oxo-MeIQx), thought to be formed exclusively by the intestinal flora, were also identified. A novel metabolite was characterized as 2-amino-3-methylimidazo[4,5-f]quinoxaline-8-carboxylic acid (IQx-8-COOH), and it was the predominant metabolite formed in hepatocytes exposed to MeIQx at levels approaching human exposure. IQx-8-COOH formation is catalyzed by P450 1A2. This metabolite is a detoxication product and does not induce umuC gene expression in Salmonella typhimurium strain NM2009. IQx-8-COOH is also the principal oxidation product of MeIQx excreted in human urine [Turesky, R., et al. (1998) Chem. Res. Toxicol. 11, 217-225]. Thus, P450 1A2 is involved in both the metabolic activation and detoxication of this procarcinogen in humans. Analogous metabolism experiments were conducted with hepatocytes of untreated rats and rats pretreated with the P450 inducer 3-methylcholanthrene. Unlike human hepatocytes, the rat cell preparations did not produce IQx-8-COOH but catalyzed the formation of 2-amino-3,8-dimethyl-5-hydroxyimidazo[4,5-f]quinoxaline as a major P450-mediated detoxication product. In conclusion, our results provide evidence of a novel MeIQx metabolism pathway in humans through P450 1A2-mediated C(8)-oxidation of MeIQx to form IQx-8-COOH. This biotransformation pathway has not been detected in experimental animal species. Considerable interspecies differences exist in the metabolism of MeIQx by P450s, which may affect the biological activity of this mutagen and must be considered when assessing human health risk.

The comet assay with multiple mouse organs: comparison of comet assay results and carcinogenicity with 208 chemicals selected from the IARC monographs and U.S. NTP Carcinogenicity Database

The comet assay is a microgel electrophoresis technique for detecting DNA damage at the level of the single cell. When this technique is applied to detect genotoxicity in experimental animals, the most important advantage is that DNA lesions can be measured in any organ, regardless of the extent of mitotic activity. The purpose of this article is to summarize the in vivo genotoxicity in eight organs of the mouse of 208 chemicals selected from International Agency for Research on Cancer (IARC) Groups 1, 2A, 2B, 3, and 4, and from the U.S. National Toxicology Program (NTP) Carcinogenicity Database, and to discuss the utility of the comet assay in genetic toxicology. Alkylating agents, amides, aromatic amines, azo compounds, cyclic nitro compounds, hydrazines, halides having reactive halogens, and polycyclic aromatic hydrocarbons were chemicals showing high positive effects in this assay. The responses detected reflected the ability of this assay to detect the fragmentation of DNA molecules produced by DNA single strand breaks induced chemically and those derived from alkali-labile sites developed from alkylated bases and bulky base adducts. The mouse or rat organs exhibiting increased levels of DNA damage were not necessarily the target organs for carcinogenicity. It was rare, in contrast, for the target organs not to show DNA damage. Therefore, organ-specific genotoxicity was necessary but not sufficient for the prediction of organ-specific carcinogenicity. It would be expected that DNA crosslinkers would be difficult to detect by this assay, because of the resulting inhibition of DNA unwinding. The proportion of 10 DNA crosslinkers that was positive, however, was high in the gastrointestinal mucosa, stomach, and colon, but less than 50% in the liver and lung. It was interesting that the genotoxicity of DNA crosslinkers could be detected in the gastrointestinal organs even though the agents were administered intraperitoneally. Chemical carcinogens can be classified as genotoxic (Ames test-positive) and putative nongenotoxic (Ames test-negative) carcinogens. The Ames test is generally used as a first screening method to assess chemical genotoxicity and has provided extensive information on DNA reactivity. Out of 208 chemicals studied, 117 are Ames test-positive rodent carcinogens, 43 are Ames test-negative rodent carcinogens, and 30 are rodent noncarcinogens (which include both Ames test-positive and negative noncarcinogens). High positive response ratio (110/117) for rodent genotoxic carcinogens and a high negative response ratio (6/30) for rodent noncarcinogens were shown in the comet assay. For Ames test-negative rodent carcinogens, less than 50% were positive in the comet assay, suggesting that the assay, which detects DNA lesions, is not suitable for identifying nongenotoxic carcinogens. In the safety evaluation of chemicals, it is important to demonstrate that Ames test-positive agents are not genotoxic in vivo. This assay had a high positive response ratio for rodent genotoxic carcinogens and a high negative response ratio for rodent genotoxic noncarcinogens, suggesting that the comet assay can be used to evaluate the in vivo genotoxicity of in vitro genotoxic chemicals. For chemicals whose in vivo genotoxicity has been tested in multiple organs by the comet assay, published data are summarized with unpublished data and compared with relevant genotoxicity and carcinogenicity data. Because it is clear that no single test is capable of detecting all relevant genotoxic agents, the usual approach should be to carry out a battery of in vitro and in vivo tests for genotoxicity. The conventional micronucleus test in the hematopoietic system is a simple method to assess in vivo clastogenicity of chemicals. Its performance is related to whether a chemical reaches the hematopoietic system. Among 208 chemicals studied (including 165 rodent carcinogens), 54 rodents carcinogens do not induce micronuclei in mouse hematopoietic system despite the positive finding with one or two in vitro tests. Forty-nine of 54 rodent carcinogens that do not induce micronuclei were positive in the comet assay, suggesting that the comet assay can be used as a further in vivo test apart from the cytogenetic assays in hematopoietic cells. In this review, we provide one recommendation for the in vivo comet assay protocol based on our own data.

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.

Colon-specific genotoxicity of heterocyclic amines detected by the modified alkaline single cell gel electrophoresis assay of multiple mouse organs

The in vivo genotoxicity of five heterocyclic amines-Trp-P-2 (13 mg/kg), IQ (13 mg/kg), MeIQ (13 mg/kg), MeIQx (13 mg/kg), and PhIP (40 mg/kg)-in the mucosa of gastrointestinal and urinary tract organs (stomach, duodenum, jejunum, ileum, colon, and bladder) was studied by the alkaline single cell gel electrophoresis (SCG) (Comet) assay. Male CD-1 mice were sacrificed 1, 3, and 8 h after intraperitoneal injection. All the heterocyclic amines studied yielded statistically significant DNA damage in the colon but not the small intestine (duodenum, jejunum, and ileum) or urinary bladder. In this study, five heterocyclic amines were injected intraperitoneally to avoid the consequences of ingestion. Thus, the extensive damage to colon DNA was concluded to be due, at least in part, to a systemic effect.

Pancreatic DNA adducts formed in vitro and in vivo by the food mutagens 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) and 2-amino-3-methyl-9H-pyrido[2,3-b]indole (MeA alphaC)

Genotoxic heterocyclic amines have been detected in grilled or fried meat and tobacco smoke. Among these, 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) and 2-amino-3-methyl-9H-pyrido[2,3-b]indole (MeA alphaC) have been shown to induce tumours in rodents in several organs. Here we report on the DNA adduct formation by PhIP and MeA alphaC in vitro and in vivo, both in rat hepatic and rat pancreatic tissues or cells. Using 32P-postlabelling analysis both compounds were shown to induce a dose-dependent DNA modification in primary rat hepatocytes that was correlated with cytotoxicity in these cells. In explanted rat pancreas maintained in dynamic short-term organ culture MeA alphaC was shown to induce covalent DNA adducts. No DNA adducts were observed with PhIP in this assay. DNA adducts were observed in the liver and the pancreas of F344 rats treated with PhIP, with a 36-times higher level of adducts in the pancreas, confirming data reported earlier. DNA adduct levels induced by feeding 32, 160 or 800 ppm MeA alphaC in the diet were dose-dependent and higher in the liver compared with other organs including pancreas. While for PhIP the N2-(desoxyguanin-8-yl)-derivative was accounting for more than 90% of DNA adducts detected, in the case of MeA alphaC the N2-(desoxyguanin-8-yl) adduct was predominant in vitro and determined in vivo as one of up to 5 DNA adducts. MeA alphaC had been reported to induce preneoplastic foci and tumours in the liver and tumours and atrophy in the pancreas. In the case of MeA alphaC, the DNA adduct formation and cytotoxicity observed by us in vitro and in vivo correlate with the organ specificity of the reported pathological lesions. In the case of PhIP our in vitro data in pancreas and liver and the low adduct levels in liver in vivo also reflect the reported lack of pathological effects in these organs. In contrast, in pancreas, in vivo extraordinarily high adduct levels induced by PhIP were observed confirming studies published earlier, in spite of the fact that this compound does not cause pancreatic lesions. This enigmatic observation is discussed and the relevant literature is reviewed.

Induction of DNA damage by risk factors of colon cancer in human colon cells derived from biopsies

In order to increase the understanding of the factors responsible for causing human colon cancer, a technique was developed to detect genotoxic effects of chemicals in human colon cells. Risk factors suspected to be associated with the aetiology of human colon cancer were subsequently investigated: the method is based on the measurement of DNA damage in primary cells freshly isolated from human colon biopsies with the single cell microgel ectrophoresis technique ('Comet Assay'). 2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), 2-amino-3-methyl-3H-imidazo[4,5f]quinoline (IQ), N-methyl-N-nitro-N-nitrosoguanidine (MNNG), dinitrosocaffeidine (DNC) lithocholic acid (LCA), hydrogen peroxide (H2O2) and benzo[a]pyrene (B[a]P) were investigated for their genotoxic and cytotoxic effects following 30 min incubation with colon cells of human, and for comparative purposes also of the rat colon. The nitrosamides (MNNG, DNC) were very genotoxic in human colon cells. MNNG was more genotoxic in human than in rat colon cells. In contrast, the rat colon carcinogens PhIP and IQ were not genotoxic in human colon cells. PhIP did induce DNA damage in rat colon cells, which correlates to its capacity of inducing tumors in this animal tissue. LCA was toxic (rat > human) and concomitantly caused DNA damage in higher concentrations. The widespread contaminant B[a]P was not genotoxic in colon cells of either species using this system. H2O2 was found to be a potent genotoxic agent to both rat and human colon cells (human > rat). In summary, those compounds chosen as representatives of endogenously formed risk factors (MNNG, H2O2, LCA) have a higher toxic and/or genotoxic potency in human colon tissue than in rat colon. They are also more effective in this system than the contaminants tested so far (B[a]P, PhIP, IQ). The newly developed technique is rapid and yields relevant results. It is a novel and useful approach to assess different chemical compounds for genotoxic activities in tumour target tissues of the human.

Refined carbohydrate enhancement of aberrant crypt foci (ACF) in rat colon induced by the food-borne carcinogen 2-amino-3-methyl-imidazo[4,5-f]quinoline (IQ)

The aberrant crypt foci (ACF) bioassay has been used extensively to study the early effects of different dietary components on the colonic mucosa of laboratory rodents. ACF are proposed to represent preneoplastic lesions of colon cancer. Compared to the normally used initiators 1,2-dimethylhydrazine dihydrochloride (DMH) and azoxymethane (AOM), the use of a diet-related colon cancer initiator, such as the heterocyclic amine 2-amino-3-methyl-imidazo[4,5-f]quinoline (IQ) formed during meat cooking, would probably give a more relevant insight into diet-related colon carcinogenesis. In the present study it is shown that a feeding regimen with continuous low IQ doses (0.03% in the diet) throughout a study period of 10 weeks has a significant effect on the induction of ACF in the colon of male F344 rats. In addition, the study illustrates that the incidence of the IQ-induced ACF can be modulated by the amount of refined carbohydrates in the diet. Rats given a high sucrose/dextrin diet showed a significantly higher number of ACF compared to rats given a diet high in starches. The effect on tumor outcome will await the termination of a ongoing parallel study.

Evaluation of hamburgers and hot dogs for the presence of mutagens

Food products derived from heat-treated (fried, broiled, baked) meat may contain traces of mutagenic heterocyclic amine contaminants, some of which are proven carcinogens in rodents. To confirm their presence and range in Canadian foods, and estimate the average human intake of these types of mutagens from frequently consumed heat-processed foods, several commercially prepared fried-beef patties (hamburgers) and hot dogs (weiners) were analysed for their mutagenic capacity. The mutagenicity of the extracts was tested in the Salmonella/microsome assay using strain TA98 with metabolic activation. 16 samples of hamburgers and 14 samples of hot dogs, randomly obtained from 'fast food' commercial establishments or street vendors, were used in this survey. The mutagenic activity of these samples ranged from very low to 1042 revertants/g equivalent for the hamburgers and from non-detectable to 4875 revertants/g equivalent for the hot dogs. The average values were 199 and 424 revertants/g for the hamburgers and hot dogs, respectively. The wide range in mutagenicity was found even for the same type of product obtained from the same outlet at different times. This indicates possible inconsistency in cooking procedures during the preparation of these products. It also shows the difficulty in accurately assessing the intake of mutagenic heterocyclic amines from hamburgers and hot dogs prepared in 'fast food' outlets.

Specific 5'-GGGA-3'-->5'-GGA-3' mutation of the Apc gene in rat colon tumors induced by 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine

The APC gene plays a major role in human colon carcinogenesis. We determined the genomic structure of the rat Apc gene, and we analyzed mutations in colon tumors induced in F344 rats by 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) and 2-amino-3-methylimidazo[4,5-f]quinoline (IQ), potent carcinogens contained in ordinary daily human food. Of eight PhIP-induced tumors, one tumor had two Apc mutations, two tumors had a mutation with loss of the normal allele, and one had a mutation. Two of the above five mutations were at nucleotide 1903, one at 2605, and two at 4237, all being a deletion of a guanine base at the 5'-GGGA-3' site and resulting in truncation of the APC protein. Of 13 IQ-induced tumors, 2 had an Apc mutation with loss of the normal allele. The two mutations were a missense mutation (T-->C) at nucleotide 1567 and a nonsense mutation (C-->T) at 2761. Alteration of the Apc gene was shown to play a more important role in PhIP-induced than in IQ-induced rat colon carcinogenesis. PhIP-induced tumors are characterized by their specific and unique mutation, which may be useful for mutational fingerprinting of human cancers.