Mutagenicity of heterocyclic amines when activated by pancreas tissue

Application of the Margin of Exposure (MOE) approach to substances in food that are genotoxic and carcinogenic: example: CAS No: 105650-23-5 PhIP (2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine)

PhIP (2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine) is a heterocyclic amine and genotoxic carcinogen contaminant that occurs during the grilling and frying of meat and fish, through an interaction between sugars and creatine via the Maillard reaction. PhIP causes cancers of the prostate, mammary gland and colon in rodents. Dose-response modelling of the data for these three tumour types gives BMDL10 values of 0.48 mg/kg/day for the prostate tumours, 0.74 mg/kg/day for mammary tumours and 2.71 mg/kg/day for colon tumours. The lowest MOEs for prostate, mammary and colon tumours were 20,000, 40,000 and 150,000, respectively.

Heterocyclic amines: Chemistry and health

Heterocyclic amines (HAs) occur at the ppb range in foods. Most of them demonstrate potent mutagenicity in bacteria mutagenicity test, and some of them have been classified by the International Agency for Research on Cancer as probable/possible human carcinogens. Their capability of formation even during ordinary cooking practices implies frequent exposure by the general public. Over the past 30 years, numerous studies have been stimulated aiming to alleviate human health risk associated with HAs. These studies contribute to the understanding of their formation, characterization, and quantification in foods; their mutagenesis/carcinogenesis, mechanisms of antimutagenesis by chemical or phytogenic modulators; and strategies to inhibit their formation. The chemistry of HAs, their implications in human health, factors influencing their formation, and feasible ways of suppression will be briefly reviewed. Their occurrence in trace amounts in foods necessitates continuous development and amelioration of analytical techniques. Various inhibitory strategies, ranging from modifying cooking conditions to incorporation of different modulators, have been developed. This will remain one of the foremost areas of research in the field of food chemistry and safety.

Mutant yields and mutational spectra of the heterocyclic amines MeIQ and PhIP at the S1 locus of human-hamster AL cells with activation by chick embryo liver (CELC) co-cultures

Cooking meat and fish at high temperature creates heterocyclic amines (HA) including 2-amino-3,4-dimethylimidazo[4,5-f]quinoline (MeIQ) and 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP). Several HA are mutagens in the Ames' S9/Salmonella assay. While PhIP is a substantial Ames' test mutagen, it is 1000-fold less active than the extraordinarily potent MeIQ. In contrast, MeIQ is significantly less mutagenic than PhIP in several mammalian cell assays, especially in repair-deficient Chinese hamster ovary (CHO) cells. HA are suspect human carcinogens on the basis of (i) epidemiological evidence, (ii) induction of tumors in rodents and monkeys, (iii) DNA adduct formation and (iv) mutagenic capacity. In this study, MeIQ and PhIP were significant mutagens at the S1 locus of co-cultivated human/hamster hybrid AL cells following metabolic activation by beta-napthoflavone (betaNF)-induced chick embryonic liver cultures (CELC). MeIQ was more mutagenic than PhIP in the CELC+AL cell assay. The mutant response curves increase with dose and then plateau (PhIP), or decrease (MeIQ). The inflections in these response curves coincide with dose-dependent decreases in cytochrome CYP1A1 activity. Molecular analysis of S1- mutants indicates that a substantial fraction, >65%, of the mutations induced by PhIP are deletions of 4.2 to 133 (Mbp); half are larger than 21 Mbp. Mutations induced by MeIQ were smaller, most (56%) being less than 5.7 Mbp. When appropriate metabolic activation is combined with a target locus, which can detect both small and large chromosomal mutations, both MeIQ and PhIP are significant mutagens and clastogens in repair proficient mammalian cells.

DNA adducts of heterocyclic amine food mutagens: implications for mutagenesis and carcinogenesis

The heterocyclic amines (HCAs) are a family of mutagenic/carcinogenic compounds produced during the pyrolysis of creatine, amino acids and proteins. The major subclass of HCAs found in the human diet comprise the aminoimidazoazaarenes (AIAs) 2-amino-3-methylimidazo[4,5-f]quinoline (IQ), 2-amino-3,4-dimethylimidazo[4,5-f]quinoline (MeIQ), 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx), 2-amino-3,4,8-trimethylimidazo[4,5-f]quinoxaline (DiMeIQx) and 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP). All, except DiMeIQx, have been shown to be carcinogenic in animals. These compounds are present in cooked muscle meats at the p.p.b. level. Since the discovery of the HCAs in the late 1970s, many studies have examined the DNA adducts of these compounds. This review compiles the literature on AIA-DNA adducts including their identification and characterization, pathways of formation, mutagenesis in vitro and in vivo, and their association with carcinogenesis in animal models. It is now known that metabolic activation leading to the formation of DNA adducts is critical for mutagenicity and carcinogenicity of these compounds. All of the AIAs studied adduct to the guanine base, the major adduct being formed at the C8 position. Two AIAs, IQ and MeIQx, also form minor adducts at the N2 position of guanine. A growing body of literature has reported on the mutation spectra induced by AIA-guanine adducts. Studies of animal tumors induced by AIAs have begun to relate AIA-DNA adduct-induced mutagenic events with the mutations found in critical genes associated with oncogenesis. Several studies have demonstrated the feasibility of chemoprevention of AIA tumorigenesis. Only a few studies have reported on the detection of AIA-DNA adducts in human tissues; difficulties persist in the routine detection of AIA-DNA adducts in humans for the purpose of biomonitoring of exposure to AIAs. The AIAs are nevertheless regarded as possible human carcinogens, and future research on AIA-DNA adducts is likely to help address the role of AIAs in human cancer.

Mutagenicity of carcinogenic nitrosamines when activated by hamster and human pancreatic duct epithelial cells

We have measured the ability of pancreatic duct epithelial cells (DEC) from Syrian hamsters and humans and CK cells, immortalized hamster DEC, to metabolize chemical carcinogens to species that were mutagenic in S. typhimurium TA98 and in V79 cells. The chemicals were N-nitrosobis(2-oxopropyl)amine (BOP), 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) and 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP). The ability of ethanol (EtOH) to modify the metabolizing efficiency was also measured. When an S9 preparation from EtOH-treated CK cells was used to metabolize NNK the number of revertants was 271 +/- 73 compared with 17 +/- 2 when the S9 from control CK cells was used. When hamster DEC were used there was no increase in the mutation frequency for BOP in V79 cells (64 +/- 20 mutants/10(6) survivors per mumol) when EtOH-DEC were used. However, the mutation frequencies of NNK and PhIP rose when the EtOH-treated DEC were used from 62 +/- 31 to 198 +/- 28 mutants/10(6) survivors per mumol for NNK and from 94 +/- 25 to 166 +/- 25 mutants/10(6) survivors per mumol for PhIP. A similar result was obtained when human DEC were used, i.e. no change in BOP mutagenicity and a slight increase in PhIP mutagenicity, from 34 +/- 14 to 65 +/- 12 mutants/10(6) survivors per mumol. There were large increases in the mutagenicity of NNK with each of the three samples of human DEC that were used, from 75 +/- 0 to 213 +/- 38, 75 +/- 13 to 175 +/- 25 and 38 +/- 13 to 285 +/- 25 mutants/10(6) survivors per mumol. The EtOH treatment regimen that was used more closely mimicked chronic exposure at low concentrations in vivo. These data show that hamster DEC are capable of metabolizing NNK, which is carcinogenic in these cells in vivo. Furthermore, human DEC metabolized NNK as efficiently as hamster DEC.

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.

Mutagenicity of 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) when activated by hamster pancreatic duct epithelial cells: a chemopreventive role for glutathione

We have shown a role for glutathione (GSH) in the detoxification of 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) using mutagenicity in V79 cells as the end-point. Immortalized hamster pancreas duct epithelial cells (CK cells) were used to metabolize PhIP in this assay. Intracellular GSH concentrations were lowered by treatment with buthionine sulfoximine (BSO) and were raised by treatment with sodium sulfite. BSO treatment (10 mM, 4 h) reduced the GSH concentration in V79 cells from 18 +/- 1 to 6 +/- 1 nmol/mg protein, 4 h after treatment. The mutation frequency of PhIP in these V79 cells rose from 15 +/- 2 to 34 +/- 4 mutants/10(6) survivors in BSO-treated V79 cells. In a related experiment both CK and V79 cells were treated with sulfite. Sulfite treatment (2 mM, 4 h) produced a greater reduction in PhIP mutagenicity when the V79 cells were treated with sulfite (from 15 +/- 2 to 3 +/- 1 mutants/10(6) survivors) than when the CK cells were treated (from 15 +/- 2 to 7 +/- 2 mutants/10(6) survivors). These data show a relationship between intracellular GSH concentration and the mutagenicity of PhIP.