Differential effect of acetyltransferase expression on the genotoxicity of heterocyclic amines in CHO cells

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.

Role of Human N-Acetyltransferase 2 Genetic Polymorphism on Aromatic Amine Carcinogen-Induced DNA Damage and Mutagenicity in a Chinese Hamster Ovary Cell Mutation Assay

Carcinogenic aromatic amines such as 4-aminobiphenyl (ABP) and 2-aminofluorene (AF) require metabolic activation to form electrophilic intermediates that mutate DNA leading to carcinogenesis. Bioactivation of these carcinogens includes N-hydroxylation catalyzed by CYP1A2 followed by O-acetylation catalyzed by arylamine N-acetyltransferase 2 (NAT2). To better understand the role of NAT2 genetic polymorphism in ABP- and AF-induced mutagenesis and DNA damage, nucleotide excision repair-deficient (UV5) Chinese hamster ovary (CHO) cells were stably transfected with human CYP1A2 and either NAT2*4 (rapid acetylator) or NAT2*5B (slow acetylator) alleles. ABP and AF both caused significantly (P < 0.001) greater mutagenesis measured at the hypoxanthine phosphoribosyl transferase (hprt) locus in the UV5/CYP1A2/NAT2*4 acetylator cell line compared to the UV5, UV5/CYP1A2, and UV5/CYP1A2/NAT2*5B cell lines. ABP- and AF-induced hprt mutant cDNAs were sequenced and over 80% of the single-base substitutions were at G:C base pairs. DNA damage also was quantified by γH2AX in-cell western assays and by identification and quantification of the two predominant DNA adducts, N-(deoxyguanosin-8-yl)-4-aminobiphenyl (dG-C8-ABP) and N-(deoxyguanosin-8-yl)-2-aminofluorene (dG-C8-AF) by liquid chromatography-mass spectrometry. DNA damage and adduct levels were dose-dependent, correlated highly with levels of hprt mutants, and were significantly (P < 0.0001) greater in the UV5/CYP1A2/NAT2*4 rapid acetylator cell line following treatment with ABP or AF as compared to all other cell lines. Our findings provide further clarity on the importance of O-acetylation in CHO mutagenesis assays for aromatic amines. They provide evidence that NAT2 genetic polymorphism modifies aromatic amine-induced DNA damage and mutagenesis that should be considered in human risk assessments following aromatic amine exposures. Environ. Mol. Mutagen. 61:235-245, 2020. © 2019 Wiley Periodicals, Inc.

Genotoxicity of heterocyclic amines (HCAs) on freshly isolated human peripheral blood mononuclear cells (PBMC) and prevention by phenolic extracts derived from olive, olive oil and olive leaves

In this study we investigated the genotoxic potential of 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine, (PhIP); 2-amino-3-methyl-3H-imidazo[4,5-f]quinoline, (IQ); 2-amino-3,8-dimethyl-imidazo[4,5-f]quinoxaline, (MeIQx) and 2-amino-3,4,8-trimethyl-3H-imidazo[4,5-f]quinoxaline (DiMeIQx) on human freshly isolated peripheral blood mononuclear cells (PBMC) by the comet assay. The preventive ability of three different phenolic extracts derived from olive (O-PE), virgin olive oil (OO-PE) and olive leaf (OL-PE) on PhIP induced DNA damage was also investigated. PhIP and IQ induced a significant DNA damage at the lowest concentration tested (100 μM), while the genotoxic effect of MeIQx and DiMeIQx become apparent only in the presence of DNA repair inhibitors Cytosine b-D-arabinofuranoside and Hydroxyurea (AraC/HU). The inclusion of metabolic activation (S9-mix) in the culture medium increased the genotoxicity of all HCAs tested. All three phenolic extracts showed an evident DNA damage preventive activity in a very low concentration range (0.1-1.0 μM of phenols) which could be easily reached in human tissues "in vivo" under a regular intake of virgin olive oil. These data further support the observation that consumption of olive and virgin olive oil may prevent the initiation step of carcinogenesis. The leaf waste could be an economic and simple source of phenolic compounds to be used as food additives or supplements.

Impact of DNA repair on the dose-response of colorectal cancer formation induced by dietary carcinogens

Colorectal cancer (CRC) is one of the most frequently diagnosed cancers, which is causally linked to dietary habits, notably the intake of processed and red meat. Processed and red meat contain dietary carcinogens, including heterocyclic aromatic amines (HCAs) and N-nitroso compounds (NOC). NOC are agents that induce various N-methylated DNA adducts and O⁶-methylguanine (O⁶-MeG), which are removed by base excision repair (BER) and O⁶-methylguanine-DNA methyltransferase (MGMT), respectively. HCAs such as the highly mutagenic 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) cause bulky DNA adducts, which are removed from DNA by nucleotide excision repair (NER). Both O⁶-MeG and HCA-induced DNA adducts are linked to the occurrence of KRAS and APC mutations in colorectal tumors of rodents and humans, thereby driving CRC initiation and progression. In this review, we focus on DNA repair pathways removing DNA lesions induced by NOC and HCA and assess their role in protecting against mutagenicity and carcinogenicity in the large intestine. We further discuss the impact of DNA repair on the dose-response relationship in colorectal carcinogenesis in view of recent studies, demonstrating the existence of 'no effect' point of departures (PoDs), i.e. thresholds for genotoxicity and carcinogenicity. The available data support the threshold concept for NOC with DNA repair being causally involved.

Curcumin inhibits PhIP induced cytotoxicity in breast epithelial cells through multiple molecular targets

Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), found in cooked meat, is a known food carcinogen that causes several types of cancer, including breast cancer, as PhIP metabolites produce DNA adduct and DNA strand breaks. Curcumin, obtained from the rhizome of Curcuma longa, has potent anticancer activity. To date, no study has examined the interaction of PhIP with curcumin in breast epithelial cells. The present study demonstrates the mechanisms by which curcumin inhibits PhIP-induced cytotoxicity in normal breast epithelial cells (MCF-10A). Curcumin significantly inhibited PhIP-induced DNA adduct formation and DNA double stand breaks with a concomitant decrease in reactive oxygen species (ROS) production. The expression of Nrf2, FOXO targets; DNA repair genes BRCA-1, H2AFX and PARP-1; and tumor suppressor P16 was studied to evaluate the influence on these core signaling pathways. PhIP induced the expression of various antioxidant and DNA repair genes. However, co-treatment with curcumin inhibited this expression. PhIP suppressed the expression of the tumor suppressor P16 gene, whereas curcumin co-treatment increased its expression. Caspase-3 and -9 were slightly suppressed by curcumin with a consequent inhibition of cell death. These results suggest that curcumin appears to be an effective anti-PhIP food additive likely acting through multiple molecular targets.

Mass Spectrometric Characterization of Human Serum Albumin Adducts Formed with N-Oxidized Metabolites of 2-Amino-1-methylphenylimidazo[4,5-b]pyridine in Human Plasma and Hepatocytes

2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), a carcinogenic heterocyclic aromatic amine formed in cooked meats, is metabolically activated to electrophilic intermediates that form covalent adducts with DNA and protein. We previously identified an adduct of PhIP formed at the Cys(34) residue of human serum albumin following reaction of albumin with the genotoxic metabolite 2-hydroxyamino-1-methyl-6-phenylimidazo[4,5-b]pyridine (HONH-PhIP). The major adducted peptide recovered from a tryptic/chymotryptic digest was identified as the missed-cleavage peptide LQQC*([SO2PhIP])PFEDHVK, a [cysteine-S-yl-PhIP]-S-dioxide linked adduct. In this investigation, we have characterized the albumin adduction products of N-sulfooxy-2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (N-sulfooxy-PhIP), which is thought to be a major genotoxic metabolite of PhIP formed in vivo. Targeted and data-dependent scanning methods showed that N-sulfooxy-PhIP adducted to the Cys(34) of albumin in human plasma to form LQQC*([SO2PhIP])PFEDHVK at levels that were 8-10-fold greater than the adduct levels formed with N-(acetyloxy)-2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (N-acetoxy-PhIP) or HONH-PhIP. We also discovered that N-sulfooxy-PhIP forms an adduct at the sole tryptophan (Trp(214)) residue of albumin in the sequence AW*([PhIP])AVAR. However, stable adducts of PhIP with albumin were not detected in human hepatocytes. Instead, PhIP and 2-amino-1-methyl-6-(5-hydroxy)phenylimidazo[4,5-b]pyridine (5-HO-PhIP), a solvolysis product of the proposed nitrenium ion of PhIP, were recovered during the proteolysis, suggesting a labile sulfenamide linkage had formed between an N-oxidized intermediate of PhIP and Cys(34) of albumin. A stable adduct was formed at the Tyr(411) residue of albumin in hepatocytes and identified as a deaminated product of PhIP, Y(*[desaminoPhIP])TK, where the 4-HO-tyrosine group bound to the C-2 imidazole atom of PhIP.

Effects of 2-amino-1-methyl-6-phenylimidazo [4, 5-b] pyridine (PhIP) on histopathology, oxidative stress, and expression of c-fos, c-jun and p16 in rat stomachs

2-Amino-1-methyl-6-phenylimidazo [4, 5-b] pyridine (PhIP) is one of the most abundant heterocyclic amines (HCAs) generated from overcooking meat at high temperatures. To understand the possible mechanism of PhIP-associated stomach cancer, the effects of PhIP on morphology, oxidative stress, gene expression of c-fos, c-jun and p16 in rat stomachs were investigated. The results showed that (1) 15mg/kg body weight PhIP induced obvious histopathological changes in gastric mucosa; (2) PhIP (10 and/or 15mg/kg) significantly decreased superoxide dismutase (SOD) and glutathioneperoxidase (GPx) activities, while increased catalase (CAT) activity compared with the control. With the elevated doses of PhIP, malondialdehyde (MDA) contents, protein carbonyl (PCO) contents and DNA-protein crosslinks (DPC) coefficients were significantly raised in a dose-dependent manner; (3) PhIP at the doses of 10mg/kg and/or 15mg/kg significantly inhibited p16 mRNA and protein expression, whereas enhanced c-fos and c-jun expression relative to control. The data indicated that PhIP could cause stomach injury, oxidative stress in rat stomachs as well as the activation of c-fos and c-jun and inactivation of p16, which may play a role in the pathogenesis of PhIP-associated stomach cancer.

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.

NATb/NAT1*4 promotes greater arylamine N-acetyltransferase 1 mediated DNA adducts and mutations than NATa/NAT1*4 following exposure to 4-aminobiphenyl

N-acetyltransferase 1 (NAT1) is a phase II metabolic enzyme responsible for the biotransformation of aromatic and heterocyclic amine carcinogens such as 4-aminobiphenyl (ABP). NAT1 catalyzes N-acetylation of arylamines as well as the O-acetylation of N-hydroxylated arylamines. O-acetylation leads to the formation of electrophilic intermediates that result in DNA adducts and mutations. NAT1 is transcribed from a major promoter, NATb, and an alternative promoter, NATa, resulting in mRNAs with distinct 5'-untranslated regions (UTR). NATa mRNA is expressed primarily in the kidney, liver, trachea, and lung while NATb mRNA has been detected in all tissues studied. To determine if differences in 5'-UTR have functional effect upon NAT1 activity and DNA adducts or mutations following exposure to ABP, pcDNA5/FRT plasmid constructs were prepared for transfection of full-length human mRNAs including the 5'-UTR derived from NATa or NATb, the open reading frame, and 888 nucleotides of the 3'-UTR. Following stable transfection of NATb/NAT1*4 or NATa/NAT1*4 into nucleotide excision repair (NER) deficient Chinese hamster ovary cells, N-acetyltransferase activity (in vitro and in situ), mRNA, and protein expression were higher in NATb/NAT1*4 than NATa/NAT1*4 transfected cells (P < 0.05). Consistent with NAT1 expression and activity, ABP-induced DNA adducts and hypoxanthine phosphoribosyl transferase mutants were significantly higher (P < 0.05) in NATb/NAT1*4 than in NATa/NAT1*4 transfected cells following exposure to ABP. These differences observed between NATa and NATb suggest that the 5'-UTRs are differentially regulated.

Role of human CYP1A1 and NAT2 in 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine-induced mutagenicity and DNA adducts

2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) is carcinogenic in multiple organs and numerous species. Bioactivation of PhIP is initiated by PhIP N(2)-hydroxylation catalysed by cytochrome P450s. Following N-hydroxylation, O-acetylation catalysed by N-acetyltransferase 2 (NAT2) is considered a further possible activation pathway. Genetic polymorphisms in NAT2 may modify cancer risk following exposure. Nucleotide excision repair-deficient Chinese hamster ovary (CHO) cells stably transfected with human cytochrome P4501A1 (CYP1A1) and a single copy of either NAT2*4 (rapid acetylator) or NAT2*5B (slow acetylator) alleles were used to test the effect of CYP1A1 and NAT2 polymorphism on PhIP genotoxicity. Cells transfected with NAT2*4 had significantly higher levels of N-hydroxy-PhIP O-acetyltransferase (p = 0.0150) activity than cells transfected with NAT2*5B. Following PhIP treatment, CHO cell lines transfected with CYP1A1, CYP1A1/NAT2*4 and CYP1A1/NAT2*5B each showed concentration-dependent cytotoxicity and hypoxanthine phosphoribosyl transferase (hprt) mutagenesis not observed in untransfected CHO cells. dG-C8-PhIP was the primary DNA adduct formed and levels were dose dependent in transfected CHO cells in the order: CYP1A1 < CYP1A1 and NAT2*5B < CYP1A1 and NAT2*4, although levels did not differ significantly (p > 0.05) following one-way analysis of variance. These results strongly support activation of PhIP by CYP1A1 with little effect of human NAT2 genetic polymorphism on mutagenesis and DNA damage.

Effect of rapid human N-acetyltransferase 2 haplotype on DNA damage and mutagenesis induced by 2-amino-3-methylimidazo-[4,5-f]quinoline (IQ) and 2-amino-3,8-dimethylimidazo-[4,5-f]quinoxaline (MeIQx)

Heterocyclic amines such as 2-amino-3-methylimidazo-[4,5-f]quinoline (IQ) and 2-amino-3,8-dimethylimidazo-[4,5-f]quinoxaline (MeIQx) are dietary carcinogens generated when meats are cooked well-done. Bioactivation includes N-hydroxylation catalyzed by cytochrome P4501A2 (CYP1A2) followed by O-acetylation catalyzed by N-acetyltransferase 2 (NAT2). Nucleotide excision repair-deficient Chinese hamster ovary (CHO) cells stably transfected with human CYP1A2 and either NAT2*4 (rapid acetylator) or NAT2*5B (slow acetylator) alleles were treated with IQ or MeIQx to examine the effect of NAT2 genetic polymorphism on IQ- or MeIQx-induced DNA adducts and mutagenesis. MeIQx and IQ both induced decreases in cell survival and significantly (p<0.001) greater number of endogenous hypoxanthine phosphoribosyl transferase (hprt) mutants in the CYP1A2/NAT2*4 than the CYP1A2/NAT2*5B cell line. IQ- and MeIQx-induced hprt mutant cDNAs were sequenced and over 85% of the mutations were single-base substitutions with the remainder exon deletions likely caused by splice-site mutations. For the single-base substitutions, over 85% were at G:C base pairs. Deoxyguanosine (dG)-C8-IQ and dG-C8-MeIQx adducts were significantly (p<0.001) greater in the CYP1A2/NAT2*4 than the CYP1A2/NAT2*5B cell line. DNA adduct levels correlated very highly with hprt mutants for both IQ and MeIQx. These results suggest substantially increased risk for IQ- and MeIQx-induced DNA damage and mutagenesis in rapid NAT2 acetylators.

The impact of NAT2 acetylator genotype on mutagenesis and DNA adducts from 2-amino-9H-pyrido[2,3-b]indole

2-amino-9H-pyrido[2,3-b]indole (AalphaC) is a carcinogenic heterocyclic aromatic amine (HAA) that is produced in high quantities in tobacco smoke and that also forms in charred meats. The bioactivation of AalphaC occurs by cytochrome P450-mediated (P450 1A2) N-oxidation of the exocyclic amine group, to form 2-hydroxyamino-9H-pyrido[2,3-b]indole (HONH-AalphaC). The HONH-AalphaC metabolite can then undergo further activation by phase II enzymes to form the penultimate ester species, which bind to DNA. Some epidemiological studies suggest a role for NAT2 genetic polymorphisms in human susceptibilities to various cancers from tobacco smoke and from consumption of well-done meats, where the exposures to AalphaC can be substantial. In this investigation, we have measured the genotoxicity of AalphaC in nucleotide excision repair-deficient Chinese hamster ovary (CHO) cells stably transfected with human P450 1A2 and either the NAT2*4 (rapid, wild-type acetylator) or the NAT2*5B (the most common slow acetylator) allele, to determine the role of NAT2 phenotype in the biological effects of AalphaC. Mutations at the hypoxanthine phosphoribosyl transferase (hprt) locus were induced in a dose-dependent manner by AalphaC and were found to be highest in cells transfected with P450 1A2 and NAT2*4, followed by cells transfected with P450 1A2 and NAT2*5B. The level of formation of the deoxyguanosine (dG) adduct N-(deoxyguanosin-8-yl)-2-amino-9H-pyrido[2,3-b]indole (dG-C8-AalphaC) paralleled the mutagenic potency in these cell lines. However, AalphaC did not form DNA adducts or induce mutations in untransfected CHO cells or in cells only expressing P450 1A2. These findings clearly demonstrate that NAT2 genetic polymorphism plays a major role in the genotoxic potency of AalphaC.

Quantitation of 13 heterocyclic aromatic amines in cooked beef, pork, and chicken by liquid chromatography-electrospray ionization/tandem mass spectrometry

The concentrations of heterocyclic aromatic amines (HAAs) were determined, by liquid chromatography-electrospray ionization/tandem mass spectrometry (LC-ESI-MS/MS), in 26 samples of beef, pork, and chicken cooked to various levels of doneness. The HAAs identified were 2-amino-3-methylimidazo[4,5- f]quinoline, 2-amino-1-methylimidazo[4,5- b]quinoline, 2-amino-1-methylimidazo[4,5- g]quinoxaline (I gQx), 2-amino-3-methylimidazo[4,5- f]quinoxaline, 2-amino-1,7-dimethylimidazo[4,5- g]quinoxaline (7-MeI gQx), 2-amino-3,8-dimethylimidazo[4,5- f]quinoxaline, 2-amino-1,6-dimethyl-furo[3,2- e]imidazo[4,5- b]pyridine, 2-amino-1,6,7-trimethylimidazo[4,5- g]quinoxaline, 2-amino-3,4,8-trimethylimidazo[4,5- f]quinoxaline, 2-amino-1,7,9-trimethylimidazo[4,5- g]quinoxaline, 2-amino-1-methyl-6-phenylimidazo[4,5- b]pyridine (PhIP), 2-amino-9 H-pyrido[2,3- b]indole, and 2-amino-3-methyl-9 H-pyrido[2,3- b]indole. The concentrations of these compounds ranged from <0.03 to 305 parts per billion (micrograms per kilogram). PhIP was the most abundant HAA formed in very well done barbecued chicken (up to 305 microg/kg), broiled bacon (16 microg/kg), and pan-fried bacon (4.9 microg/kg). 7-MeI gQx was the most abundant HAA formed in very well done pan-fried beef and steak, and in beef gravy, at concentrations up to 30 microg/kg. Several other linear tricyclic ring HAAs containing the I gQx skeleton are formed at concentrations in cooked meats that are relatively high in comparison to the concentrations of their angular tricyclic ring isomers, the latter of which are known experimental animal carcinogens and potential human carcinogens. The toxicological properties of these recently discovered I gQx derivatives warrant further investigation and assessment.

2-Amino-3,8-dimethylimidazo-[4,5-f]quinoxaline-induced DNA adduct formation and mutagenesis in DNA repair-deficient Chinese hamster ovary cells expressing human cytochrome P4501A1 and rapid or slow acetylator N-acetyltransferase 2

2-Amino-3,8-dimethylimidazo-[4,5-f]quinoxaline (MeIQx) is one of the most potent and abundant mutagens in the western diet. Bioactivation includes N-hydroxylation catalyzed by cytochrome P450s followed by O-acetylation catalyzed by N-acetyltransferase 2 (NAT2). In humans, NAT2*4 allele is associated with rapid acetylator phenotype, whereas NAT2*5B allele is associated with slow acetylator phenotype. We hypothesized that rapid acetylator phenotype predisposes humans to DNA damage and mutagenesis from MeIQx. Nucleotide excision repair-deficient Chinese hamster ovary cells were constructed by stable transfection of human cytochrome P4501A1 (CYP1A1) and a single copy of either NAT2*4 (rapid acetylator) or NAT2*5B (slow acetylator) alleles. CYP1A1 and NAT2 catalytic activities were undetectable in untransfected Chinese hamster ovary cell lines. CYP1A1 activity did not differ significantly (P > 0.05) among the CYP1A1-transfected cell lines. Cells transfected with NAT2*4 had 20-fold significantly higher levels of sulfamethazine N-acetyltransferase (P = 0.0001) and 6-fold higher levels of N-hydroxy-MeIQx O-acetyltransferase (P = 0.0093) catalytic activity than cells transfected with NAT2*5B. Only cells transfected with both CYP1A1 and NAT2*4 showed concentration-dependent cytotoxicity and hypoxanthine phosphoribosyl transferase mutagenesis following MeIQx treatment. Deoxyguanosine-C8-MeIQx was the primary DNA adduct formed and levels were dose dependent in each cell line and in the following order: untransfected < transfected with CYP1A1 < transfected with CYP1A1 and NAT2*5B < transfected with CYP1A1 and NAT2*4. MeIQx DNA adduct levels were significantly higher (P < 0.001) in CYP1A1/NAT2*4 than CYP1A1/NAT2*5B cells at all concentrations of MeIQx tested. MeIQx-induced DNA adduct levels correlated very highly (r2 = 0.88) with MeIQx-induced mutants. These results strongly support extrahepatic activation of MeIQx by CYP1A1 and a robust effect of human NAT2 genetic polymorphism on MeIQx-induced DNA adducts and mutagenesis. The results provide laboratory-based support for epidemiologic studies reporting higher frequency of heterocyclic amine-related cancers in rapid NAT2 acetylators.

2-amino-1-methyl-6-phenylimidazo [4,5-b] pyridine-induced DNA adducts and genotoxicity in chinese hamster ovary (CHO) cells expressing human CYP1A2 and rapid or slow acetylator N-acetyltransferase 2

Heterocyclic amine carcinogens such as 2-amino-1-methyl-6-phenylimidazo [4,5-b] pyridine (PhIP) are present in diet and cigarette smoke. Bioactivation in humans includes N-hydroxylation catalyzed by cytochrome P4501A2 possibly followed by O-acetylation catalyzed by N-acetyltransferase 2 (NAT2). Nucleotide excision repair-deficient Chinese hamster ovary (CHO) cells were stably transfected with human CYP1A2 and either NAT2*4 (rapid acetylator) or NAT2*5B (slow acetylator) alleles. CYP1A2 and NAT2 catalytic activities were undetectable in untransfected CHO cell lines. CYP1A2 catalytic activity levels did not differ significantly (P > 0.05) among the CYP1A2-transfected cell lines. Cells transfected with NAT2*4 had significantly higher levels of N-acetyltransferase (P = 0.0001) and N-hydroxy-PhIP O-acetyltransferase (P = 0.0170) catalytic activity than cells transfected with NAT2*5B. PhIP caused dose-dependent decreases in cell survival and significant (P < 0.001) increases in mutagenesis measured at the hypoxanthine phosphoribosyl transferase (hprt) locus in all the CYP1A2-transfected cell lines. Transfection with NAT2*4 or NAT2*5B did not further increase hprt mutagenesis. PhIP-induced hprt mutant cDNAs were sequenced, and 80% of the mutations were single base substitutions at G:C base pairs. dG-C8-PhIP DNA adduct levels were dose-dependent in the order: untransfected < transfected with CYP1A2 < transfected with CYP1A2 and NAT2*5B < transfected with CYP1A2 and NAT2*4. Following incubation with 1.2 microM PhIP, DNA adduct levels were significantly (P < 0.05) higher in CHO cells transfected with CYP1A2/NAT2*4 versus CYP1A2/NAT2*5B. These results strongly support an activation role for CYP1A2 in PhIP-induced mutagenesis and DNA damage and suggest a modest effect of human NAT2 and its genetic polymorphism on PhIP DNA adduct levels.

Formation and biochemistry of carcinogenic heterocyclic aromatic amines in cooked meats

Heteroyclic aromatic amines (HAAs) are a class of hazardous chemicals that are receiving heightened attention as a risk factor for human cancer. HAAs arise during the cooking of meats, fish, and poultry, and several HAAs also occur in tobacco smoke condensate and diesel exhaust. Many HAAs are carcinogenic and induce tumors at multiple sites in rodents. A number of epidemiologic studies have reported that frequent consumption of well-done cooked meats containing HAAs can result in elevated risks for colon, prostate, and mammary cancers. Moreover, DNA adducts of HAAs have been detected in human tissues, demonstrating that HAAs induce genetic damage even though the concentrations of these compounds in cooked meats are generally in the low parts-per-billion (ppb) range. With recent improvements in sensitivity of mass spectrometry instrumentation, HAAs, their metabolites, and DNA adducts can be detected at trace amounts in biological fluids and tissues of humans. The incorporation of HAA biomarkers in epidemologic studies will help to clarify the role of these dietary genotoxicants in the etiology of human cancer.

Heterocyclic aromatic amines in domestically prepared chicken and fish from Singapore Chinese households

Chicken and fish samples prepared by 42 Singapore Chinese in their homes were obtained. Researchers were present to collect data on raw sample weight, cooking time, maximum cooking surface temperature, and cooked sample weight. Each participant prepared one pan-fried fish sample and two pan-fried chicken samples, one marinated, one not marinated. The cooked samples were analyzed for five heterocyclic aromatic amine (HAA) mutagens, including MeIQx (2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline); 4,8-DiMeIQx (2-amino-3,4,8-trimethylimidazo[4,5-f]quinoxaline); 7,8-DiMeIQx (2-amino-3,7,8-trimethylimidazo[4,5-f]quinoxaline); PhIP (2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine), and IFP (2-amino-(1,6-dimethylfuro[3,2-e]imidazo[4,5-b])pyridine). A paired Student's t-test showed that marinated chicken had lower concentrations of PhIP (p<0.05), but higher concentrations of MeIQx (p<0.05) and 4,8-DiMeIQx (p<0.001) than non-marinated chicken, and also that weight loss due to cooking was less in marinated chicken than in non-marinated chicken (p<0.001). Interestingly, the maximum cooking surface temperature was higher for fish than for either marinated or non-marinated chicken (p<0.001), yet fish was lower in 4,8-DiMeIQx per gram than marinated or non-marinated chicken (p<0.001), lower in PhIP than non-marinated chicken (p<0.05), and lost less weight due to cooking than either marinated or non-marinated chicken (p<0.001). Fish was also lower in MeIQx and 7,8-DiMeIQx than marinated chicken (p<0.05). This study provides new information on HAA content in the Singapore Chinese diet.

The effect of UDP-glucuronosyltransferase 1A1 expression on the mutagenicity and metabolism of the cooked-food carcinogen 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine in CHO cells

UDP-glucuronosyltransferase proteins (UGT) catalyze the glucuronidation of both endogenous and xenobiotic compounds. In previous studies, UGT1A1 has been implicated in the detoxification of certain food-borne carcinogenic-heterocyclic amines. To determine the importance of UDP-glucuronosyltransferase 1A1 (UGT1A1) in the biotransformation of the cooked-food carcinogen 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), genetically modified CHO cells that are nucleotide excision repair-deficient, and express cytochrome P4501A2 (UV5P3 cell line) were transfected with a cDNA plasmid of human UGT1A1 to establish the UDP-glucuronosyltransferase 1A1 expressing 5P3hUGT1A1 cell line. Expression of the UGT1A1 gene was verified by screening neo gene expressing clonal isolates (G-418 resistant) for their sensitivity to cell killing from PhIP exposure. Five of 11 clones were chosen for further analysis due to their resistance to cell killing. Western blot analysis was used to confirm the presence of the UGT1A1 and CYP1A2 proteins. All five clones displayed a 52-kDa protein band, which corresponded to a UGT1A1 control protein. Only four of the clones had a protein band that corresponded to the CYP1A2 control protein. Correct fragment size of the cDNAs in the remaining four clones was confirmed by RT-PCR and quantification of the mRNA product was accomplished by real-time RT-PCR. Expression of UGT1A1 in the transfected cells was 10(4)-10(5)-fold higher relative to the UV5P3 parental cells. One clone (#14) had a 10-fold higher increase in expression at 1.47 x 10(5) over the other three clones. This clone was also the most active in converting N-hydroxy-PhIP to N-hydroxy-PhIP glucuronide conjugates in microsomal metabolism assays. Based on the D50 values, the cytotoxic effect of PhIP was decreased approximately 350-fold in the 5P3hUGT1A1 cells compared to the UV5P3 control cells. In addition, no significant increase in mutation frequency was observed in the transfected cells. These results clearly indicate that UGT1A1 plays a critical role in PhIP biotransformation, providing protection against PhIP-mediated cytotoxicity and mutagenicity.

Development and characterization of CHO repair-proficient cell lines for comparative mutagenicity and metabolism of heterocyclic amines from cooked food

In order to understand the role of repair and metabolism in the mutagenicity of heterocyclic amines from cooked foods, we previously developed the nucleotide excision repair-deficient CHO 5P3NAT2 cell line engineered to coexpress the mouse CYP1A2 and human N-acetyltransferase genes. In the present study, we have made a matched repair-competent cell line by mutagenizing 5P3NAT2 cells with ethyl methanesulfonate and selecting for resistance to cytotoxicity by 2-amino-3-methylimidazo[4,5-f]quinoline (IQ). In the differential cytotoxicity (DC) assay, 4 out of 15 clones showed no cytotoxic effect with IQ at the highest dose (30 microg/ml) tested, in contrast to repair-deficient 5P3NAT2 cells, which showed approximately 100% cytotoxicity at 0.3 microg/ml. Subsequently, these IQ-resistant clones were examined for resistance to killing by UV irradiation. All four IQ-resistant clones, which show resistance to UV similar to that of repair-proficient AA8 cells, still express both the CYP1A2 and N-acetyltransferase genes. Sequence analysis of CXPD cDNA from the 5P3NAT2R9 clone revealed an A:T-->G:C reversion event at the site of the UV5 mutation. This base change results in reversion of the codon 116 tyrosine in UV5 cells back to the original cysteine in AA8 cells, thereby restoring wild-type CXPD activity and repair function. In contrast to 5P3NAT2 cells, the repair-proficient 5P3NAT2R9 revertant cell line shows little IQ-induced cell killing, and dramatically lower levels of induced mutation at the adenine phosphoribosyltransferase (Aprt) gene locus over the range of 2-40 microg/ml IQ. This matched pair of repair-proficient/deficient cell lines can provide insight not only into the genotoxicity of heterocyclic amine dietary carcinogens such as IQ and PhIP, but also into the effects of nucleotide excision repair on the ultimate mutagenicity of these compounds.

Protection by beverages, fruits, vegetables, herbs, and flavonoids against genotoxicity of 2-acetylaminofluorene and 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) in metabolically competent V79 cells

Chinese hamster lung fibroblasts, genetically engineered for the expression of rat cytochrome P450 dependent monooxygenase 1A2 and rat sulfotransferase 1C1 (V79-rCYP1A2-rSULT1C1 cells), were utilized to check for possible protective effects of beverages of plant origin, fruits, vegetables, and spices against genotoxicity induced by 2-acetylaminofluorene (AAF) or 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP). Antigenotoxic activities of juices from spinach and red beets against AAF could be monitored with similar effectivity by the HPRT-mutagenicity test (IC(50)=0.64%; 2.57%) and alkaline single cell gel electrophoresis (comet assay; IC(50)=0.12%; 0.89%) which detects DNA strand breaks and abasic sites. Applying the comet assay, genotoxicity of PhIP could, however, be demonstrated only in the presence of hydroxyurea and 1-[beta-D-arabinofuranosyl]cytosine, known inhibitors of DNA repair synthesis. As expected, AAF and PhIP were unable to induce any genotoxic effects in the parent V79 cells. Genotoxic activity of PhIP was strongly reduced in a dose-related manner by green tea and red wine, by blueberries, blackberries, red grapes, kiwi, watermelon, parsley, and spinach, while two brands of beer, coffee, black tea, rooibos tea, morellos, black-currants, plums, red beets, broccoli (raw and cooked), and chives were somewhat less active. One brand of beer was only moderately active while white wine, bananas, white grapes, and strawberries were inactive. Similarly, genotoxicity of AAF was strongly reduced by green, black, and rooibos tea, red wine, morellos, black-currants, kiwi, watermelon, and spinach while plums, red beets, and broccoli (raw) were less potent. Broccoli cooked exerted only moderate and white wine weak antigenotoxic activity. With respect to the possible mechanism(s) of inhibition of genotoxicity, benzo[a]pyrene-7,8-dihydrodiol (BaP-7,8-OH) and N-OH-PhIP were applied as substrates for the CYP1A family and for rSULT 1C1, respectively. Morellos, black-currants, and black tea strongly reduced the genotoxicity of BaP-7,8-OH, onions, rooibos tea, and red wine were less potent while red beets and spinach were inactive. On the other hand, red beets and spinach strongly inhibited the genotoxicity of N-OH-PhIP, rooibos tea was weakly active while all other items were inactive. These results are suggestive for enzyme inhibition as mechanism of protection by complex mixtures of plant origin. Taken together, our results demonstrate that protection by beverages, fruits, and vegetables against genotoxicity of heterocyclic aromatic amines may take place within metabolically competent mammalian cells as well as under the conditions of the Salmonella/reversion assay.

Heterologous expression of human N-acetyltransferases 1 and 2 and sulfotransferase 1A1 in Salmonella typhimurium for mutagenicity testing of heterocyclic amines

A variety of carcinogenic heterocylic amines (HAs) are found in cooked food. They can be metabolised to reactive intermediates via N-hydroxylation catalysed by cytochrome P450 1A2, followed by conjugation of the resulting N-hydroxyl group by N-acetyltransferase (NAT) or sulfotransferase (SULT). In order to compare the role of O-acetylation and O-sulfonation by human enzymes in the activation of HAs, we have introduced the cDNAs for wild-type forms of human NAT1, NAT2 and SULT1A1 in the acetyltransferase-deficient Salmonella typhimurium strain TA1538/1,8-DNP. Functional expression of recombinant proteins was demonstrated using immunoblot analysis and determination of enzyme activity with characteristic substrates. The established strains were used to study the mutagenicity of the N-hydroxy derivatives of 2-amino-3-methylimidazo[4,5-f]quinoline (IQ) and 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP). The results demonstrate that N-hydroxy-HAs are activated by different human enzymes. At the concentrations used in the mutagenicity assay, N-hydroxy-IQ was activated by human NAT2, but not by NAT1 or SULT1A1. In contrast, N-hydroxy-PhIP was activated specifically by human SULT1A1, but not by NAT1 or NAT2. Therefore, both O-acetylation and O-sulfonation by human enzymes have to be regarded as important determinants for HA genotoxicity in humans.

Human exposure to heterocyclic amine food mutagens/carcinogens: relevance to breast cancer

Heterocyclic amines produced from overcooked foods are extremely mutagenic in numerous in vitro and in vivo test systems. One of these mutagens, 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), induces breast tumors in rats and has been implicated in dietary epidemiology studies as raising the risk of breast cancer in humans. Efforts in our laboratory and others have centered on defining the exposure to PhIP and other dietary mutagens derived from cooked food. We accomplish this by analyzing the foods with a series of solid-phase extractions and HPLC. We have developed an LC/MS/MS method to analyze the four major human PhIP metabolites (sulfates and glucuronides) following a single meal containing 27 microg of cooking-produced PhIP in 200 g of grilled meat. Although the intake of PhIP was similar for each of eight women, the total amount excreted in the urine and the metabolite profiles differed among the subjects. It appears that adsorption (digestion) from the meat matrix, other foods in the diet, and genetic differences in metabolism may contribute to the variation. The four major metabolites that can be routinely assayed in the urine are N(2)-OH-PhIP-N(2)-glucuronide, PhIP-N(2)-glucuronide, 4'-PhIP-glucuronide, and N(2)-OH-PhIP-N3-glucuronide. This work is suited to investigate individual exposure and risk, especially for breast cancer, from these potent dietary mutagens.

Gel microdrop flow cytometry assay for low-dose studies of chemical and radiation cytotoxicity

Low-level cytotoxicity may affect low-dose dose-response relations for cancer and other endpoints. Conventional colony-forming assays are rarely sensitive enough to examine small changes in cell survival and growth. Automated image-analysis techniques are limited to ca. 10(4) cells/plate. An alternative method involves encapsulation of single proliferating cells into ca. 35-75-microm-diameter agarose gel microdrops (GMDs) that are randomly grouped, differential exposure of these groups, culture at 37 degrees C for 3-5 days, and finally GMD analysis by flow cytometry (FC) to determine the ratio of GMDs containing multiple versus single cells as a measure of clonogenic survival. This GMD/FC assay was used to examine low-dose cell killing induced by a cooked-meat mutagen/rodent-carcinogen (MeIQx) in DNA-repair-deficient/metabolically-sensitive CHO cells. Results of conventional colony-forming assays using up to 30 replicate plates indicate a shouldered, threshold-like dose-response; in contrast, those obtained using the GMD/FC assay suggest "hypersensitivity"-like nonlinearity in dose-response. The GMD/FC assay was also applied to human A549 lung cells after GMD-encapsulation and gamma radiation followed by culture for a total of 4 days, to examine survival after exposure to > or =100 cGy delivered at a relatively low dose rate (0.18 cGy/min). Dose-response for clonogenic growth was again observed to be reduced with apparent nonlinear suggesting hypersensitivity between 0 and 50 cGy, insofar as doses of 5 and 10 cGy appear to be ca. fivefold more effective per unit dose than the 50- or 100-cGy doses used. The GMD/FC assay may thus reveal low-dose dose-response relations for chemical and radiation effects on cell proliferation/killing with implications for low-dose risk assessment.

Creatine and creatinine metabolism

The goal of this review is to present a comprehensive survey of the many intriguing facets of creatine (Cr) and creatinine metabolism, encompassing the pathways and regulation of Cr biosynthesis and degradation, species and tissue distribution of the enzymes and metabolites involved, and of the inherent implications for physiology and human pathology. Very recently, a series of new discoveries have been made that are bound to have distinguished implications for bioenergetics, physiology, human pathology, and clinical diagnosis and that suggest that deregulation of the creatine kinase (CK) system is associated with a variety of diseases. Disturbances of the CK system have been observed in muscle, brain, cardiac, and renal diseases as well as in cancer. On the other hand, Cr and Cr analogs such as cyclocreatine were found to have antitumor, antiviral, and antidiabetic effects and to protect tissues from hypoxic, ischemic, neurodegenerative, or muscle damage. Oral Cr ingestion is used in sports as an ergogenic aid, and some data suggest that Cr and creatinine may be precursors of food mutagens and uremic toxins. These findings are discussed in depth, the interrelationships are outlined, and all is put into a broader context to provide a more detailed understanding of the biological functions of Cr and of the CK system.

Use of genetically engineered Salmonella typhimurium OY1002/1A2 strain coexpressing human cytochrome P450 1A2 and NADPH-cytochrome P450 reductase and bacterial O-acetyltransferase in SOS/umu assay

The major pathway of bioactivation of procarcinogenic heterocyclic aromatic amines (HCAs) is cytochrome P450 1A2 (CYP1A2)-catalyzed N-hydroxylation and subsequent esterification by O-acetyltransferase (O-AT). We have previously reported that an umu tester strain, Salmonella typhimurium OY1001/1A2, endogenously coexpressing human CYP1A2 and NADPH-P450 reductase (reductase), is able to detect the genotoxicity of some aromatic amines [Aryal et al., 1999, Mutat Res 442:113-120]. To further enhance the sensitivity of the strain toward HCAs, we developed S. typhimurium OY1002/1A2 by introducing pCW"/1A2:hNPR (a bicistronic construct coexpressing human P450 1A2 and the reductase) and pOA102 (constructed by subcloning the Salmonella O-AT gene in the pOA101-expressing umuC"lacZ gene) in S. typhimurium TA1535. In addition, as an O-AT-deficient strain, we developed the OY1003/1A2 strain by introducing pCW"/1A2:hNPR and pOA101 into O-AT-deficient S. typhimurium TA1535/1,8-DNP. Strains OY1001/1A2, OY1002/1A2, and OY1003/1A2 expressed, respectively, about 150, 120, and 140 nmol CYP1A2/l culture (in whole cells), and respective cytosolic preparations acetylated 15, 125, and > or = 0 nmol isoniazid/min/mg protein as the O-AT activities of cytosolic preparations, respectively. We compared the induction of umuC gene expression as a measure of genotoxicity and observed that the OY1002/1A2 strain was more sensitive than OY1001/1A2 strain toward the genotoxicity of 2-amino-1,4-dimethylimidazo[4,5-f]quinol ine(MeIQ), 2-amino-3-methylimidazo[4,5-f]quinoline (IQ),2-amino-3, 8-dimethylimidazo[4,5-f]quinoxaline (MeIQx),2-aminoanthracene, 2-amino-6-methyldipyrido[1,2-a::3,2'-d]i midazole,3-amino-1, 4-dimethyl-5H-pyrido[4,3-b]indole, and 3-amino-1-methyl-5H-pyrido[4, 3-a]indole. However, the genotoxicity of MeIQ, IQ, and MeIQx was not detected with the OY1003/1A2 strain. These results indicate that the newly developed strain OY1002/1A2 can be employed in detecting potential genotoxic aromatic amines requiring bioactivation by CYP1A2 and O-acetyltransferase.

Genotoxic effects of heterocyclic aromatic amines in human derived hepatoma (HepG2) cells

In order to study the mutagenic effects of heterocyclic aromatic amines (HAAs) in cells of human origin, five compounds, namely 2-amino-3-methyl-imidazo[4,5-f]quinoline (IQ), 2-amino-3, 4-dimethyl-imidazo[4,5-f]quinoline (MeIQ), 2-amino-3, 8-dimethyl-imidazo[4,5-f]quinoxaline (MeIQx), the pyridoimidazo derivative 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) and 3-amino-1,4-dimethyl-5H-pyrido[4,3-b]indole (Trp-P-1), were tested in micronucleus (MN) assays with a human derived hepatoma (HepG2) cell line. All HAAs caused significant, dose-dependent effects. The activities of IQ, MeIQ, MeIQx and PhIP were similar (lowest effective concentrations 25-50 microM), whereas Trp-P-1 was effective at a dose of >/=2.1 microM. In addition, the HAAs were tested in MN assays with Chinese hamster ovary (CHO) cells and in Salmonella strain YG1024 using HepG2 cell homogenates as an activation mix. In the CHO experiments, positive results were obtained with Trp-P-1 and PhIP, whereas the other compounds were devoid of activity under all experimental conditions. The discrepancy in the responsivity of the two cell lines is probably due to differences in their acetylation capacity: enzyme measurements with 2-aminofluorene as a substrate revealed that the cytosolic acetyltransferase activity in the HepG2 cells is approximately 40-fold higher than that of the CHO cells. In the bacterial assays all five HAAs gave positive results but the ranking order was completely different from that seen in the HepG2/MN experiments (IQ > MeIQ > Trp-P-1 >/= MeIQx >> PhIP) and the mutagenic potencies of the various compounds varied over several orders of magnitude. The order obtained in bacterial tests with rat liver S9 mix was more or less identical to that seen in the tests with HepG2 cell homogenates but the concentrations of the amines required to give positive results were in general substantially lower (10(-5)-10(-1) microM). Overall, the results of the present study indicate that MN/HepG2 tests might reflect the mutagenic effects of HAAs more adequately than other in vitro mammalian cell systems due to the presence of enzymes involved in the metabolic conversion of the amines.

Heterocyclic aromatic amines efficiently induce mitotic recombination in metabolically competent Saccharomyces cerevisiae strains

Heterocyclic aromatic amines (HAs) represent a class of potent bacterial mutagens and rodent carcinogens which gain their biological activity upon metabolic conversion by phase I and phase II enzymes. Subsequent to cytochrome P450 (CYP)-dependent hydroxylation, mainly catalyzed by CYP1A2, acetylation mediated by the activity of N-acetyltransferase, NAT2, produces the ultimate electrophilic product that may react with DNA. In addition to point mutations observed in HA-exposed cells as genotoxic endpoint in vitro, loss of heterozygosity (LOH) has often been identified in HA-related rodent tumors as another endpoint in vivo. LOH may reflect a chromosomal deletion, a chromosome loss or a previous mitotic recombination event and it represents a prominent mechanism for the inactivation of tumor suppressor alleles. In this study we have investigated whether LOH observed in several HA-induced rodent tumors is related to a recombinogenic activity of HA compounds, and to address this question we have studied the genotoxic activity of several HAs in metabolically competent Saccharomyces cerevisiae strains. For this purpose expression vectors have been constructed providing simultaneous expression of three human enzymes, CYP1A2, NADPH-cytochrome P450 oxidoreductase and NAT2 in different genotoxicity tester strains. Evidence for functional expression of all three enzymes has been obtained. One strain allowed us to monitor HA-induced gene conversion, another one HA-induced chromosomal translocation. A third strain allowed us to study HA-induced forward mutations in the endogenous URA3 gene. It was found that 2-amino-3-methylimidazo-[4,5-f]quinoline and 2-amino-3, 8-dimethylimidazo-[4,5-f]quinoxaline produced a strong recombinogenic response in either recombination tester strain. The recombinogenic activity was comparable with the mutagenic activity of the compounds. The other HAs, 2-amino-3, 4-dimethyl-imidazo-[4, 5-f]quinoline, 2-amino-6-methyldipyrido-[1,2-a:3',2'-d]imidazole, 2-aminodipyrido-[1,2-a:3', 2'-d]imidazole, 3-amino-1-methyl-5H pyrido-[4,3-b]indole and 2-amino-1-methyl-6-phenyl-imidazo-[4, 5-b]pyridine, produced weak or no increases in the genotoxic endpoints of interest. The described strains may provide a suitable tool to characterize the genotoxic potential of HAs in more detail.

Heterocyclic aromatic amines induce DNA strand breaks and cell transformation

Heterocyclic aromatic amines (HAAs), formed during the cooking of foods, are known to induce tumours in rodent bioassays and may thus contribute to human cancer risk. We tested six HAAs in a morphological transformation assay and in three in vitro genotoxicity assays. The morphological transforming abilities of HAAs were tested, in the presence of rat-liver S9, in the C3H/M2 fibroblast cell line. Concentration levels of 50 microM 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (8-MeIQx), 100 microM 2-amino-3,4,8-trimethylimidazo-[4,5-f]quinoxaline (4,8-DiMeIQx), 50 microM 2-amino-3-methylimidazo[4,5-f]quinoline (IQ), 100 microM 2-amino-9H-pyrido[2,3-b]indole (AalphaC), 100 microM 2-amino-3-methyl-9H-pyrido[2,3-b]indole (MeAalphaC) and 15 microM 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) induced maximum transformation potencies of 5.5, 6.6, 6.3, 5.2, 7.3 and 9.2 transformed foci per 10(4) surviving cells, respectively. Bacterial mutagenic activity was determined in the presence of rat-liver S9 using the Salmonella typhimurium reverse-mutation assay employing strain YG1019. Mutagenic potencies of 3800 revertants (revs)/ng with 8-MeIQx, 2900 revs/ng with 4,8-DiMeIQx, 3480 revs/ng with IQ, 1.6 revs/ng with AalphaC, 2.9 revs/ng with MeAalphaC and 5 revs/ng with PhIP were observed. Clastogenic activity in vitro was analysed by the micronucleus assay in metabolically competent MCL-5 cells. Dose-dependent induction of micronuclei was observed for all HAAs tested with 1-5.4% of cells containing micronuclei at 10 ng/ml. Micronucleus induction was in the order 4,8-DiMeIQx > 8-MeIQx > IQ > MeAalphaC > PhIP > AalphaC. DNA strand-breaking activity in MCL-5 cells was measured by the alkaline single cell-gel (comet) assay. The lowest effect doses for significant increases (P < or = 0.0007, Mann-Whitney test) in comet tail length (microm) were 45.5 microg/ml (200 microM) for PhIP, 90.9 microg/ml (410-510 microM) for 4,8-DiMeIQx, IQ, MeAalphaC and AalphaC, and 454.5 microg/ml (2130 microM) for 8-MeIQx. It is not yet clear which of these assays most accurately reflects the genotoxic potential to humans of compounds of this class of environmental carcinogens.

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.

Polymorphisms of N-acetyltransferases, glutathione S-transferases, microsomal epoxide hydrolase and sulfotransferases: influence on cancer susceptibility

It has become clear that several polymorphisms of human drug-metabolizing enzymes influence an individual's susceptibility for chemical carcinogenesis. This review gives an overview on relevant polymorphisms of four families of drug-metabolizing enzymes. Rapid acetylators (with respect to N-acetyltransferase NAT2) were shown to have an increased risk of colon cancer, but a decreased risk of bladder cancer. In addition an association between a NAT1 variant allele (NAT*10, due to mutations in the polyadenylation site causing approximately two fold higher activity) and colorectal cancer among NAT2 rapid acetylators was observed, suggesting a possible interaction between NAT1 and NAT2. Glutathione S-transferases M1 and T1 (GSTM1 and GSTT1) are polymorphic due to large deletions in the structural gene. Meta-analysis of 12 case-control studies demonstrated a significant association between the homozygous deletion of GSTM1 (GSTM1-0) and lung cancer (odds ratio: 1.41; 95% CI: 1.23-1.61). Combination of GSTM1-0 with two allelic variants of cytochrome P4501A1 (CYP1A1), CYP1A1 m2/m2 and CYP1A1 Val/Val further increases the risk for lung cancer. Indirect mechanisms by which deletion of GSTM1 increases risk for lung cancer may include GSTM1-0 associated decreased expression of GST M3 and increased activity of CYP1A1 and 1A2. Combination of GST M1-0 and NAT2 slow acetylation was associated with markedly increased risk for lung cancer (odds ratio: 7.8; 95% CI: 1.4-78.7). In addition GSTM1-0 is clearly associated with bladder cancer and possibly also with colorectal, hepatocellular, gastric, esophageal (interaction with CYP1A1), head and neck as well as cutaneous cancer. In individuals with the GSTT1-0 genotype more chromosomal aberrations and sister chromatid exchanges (SCEs) were observed after exposure to 1,3-butadiene or various haloalkanes or haloalkenes. Evidence for an association between GSTT1-0 and myelodysplastic syndrome and acute lymphoblastic leukemia has been presented. A polymorphic site of GSTP1 (valine to isoleucine at codon 104) decreases activity to several carcinogenic diol epoxides and was associated with testicular, bladder and lung cancer. Microsomal expoxide hydrolase (mEH) is polymorphic due to amino acid variation at residues 113 and 139. Polymorphic variants of mEH were associated with hepatocellular cancer (His-113 allele), ovarian cancer (Tyr-113 allele) and chronic obstructive pulmonary disease (His-113 allele). Three human sulfotransferases (STs) are regulated by genetic polymorphisms (hDHEAST, hM-PST, TS PST). Since a large number of environmental mutagens are activated by STs an association with human cancer risk might be expected.

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.

Chemical and biological factors affecting mutagen potency

This review surveys the chemical and biological factors that are correlated with the mutagenic activity of the aromatic and heterocyclic amines. Particular attention is given to the predicted quantum chemical properties of the parent amines and their metabolites. A number of chemical properties have been found to correlate well with measured mutagenic potency, including log P, the energies of the frontier orbitals of the parent amines, and the thermodynamic stability of the nitrenium ion, possibly the ultimate DNA-binding species. These correlations are intriguing clues to the mutagenic activity of the aromatic amines; however, many factors still await final explanation, including the exact mechanisms of the metabolic enzymes, the identity(s) of the ultimate DNA-binding species, the reaction mechanism in the DNA-adduction, the role of sequence context in the covalent and non-covalent binding of the adducts, and the role of DNA repair.