Mammalian cell mutagenicity and metabolism of heterocyclic aromatic amines

Mutational specificity of 2-amino-3-methylimidazo-[4,5-f]quinoline in the hprt locus of CHO-K1 cells

2-Amino-3-methylimidazo[4,5-f]quinoline (IQ), a food carcinogen formed in cooked meats, can induce gene mutation at the hprt locus of CHO-K1 cells in the presence of hepatic S9 mix. To elucidate the molecular nature of IQ-induced mutation, we characterized the entire coding region of the hypoxanthine phosphoribosyl-transferase gene of 23 independent mutants derived from IQ-treated CHO cells by direct sequencing of polymerase chain reaction-amplified cDNA. Ten of the 23 IQ-induced mutants examined contained single base substitutions; one mutant had three single-base substitutions. Among the base substitutions, G.C-->C.G (six of 13) and A.T-->C.G (three of 13) transversions predominated. Most of the base-substitution mutations occurred preferentially at a middle G and had a dA in their 3' ends. Of the 13 other mutations (56.5%), 12 missing one or more complete exons were splice-site mutations, and one mutant had a partial deletion of an exon. A high frequency of complete exon deletion (11 of 12) in exons 2-5 was observed. Interestingly, 75% of the mutants (nine of 12) with splice-site mutations were induced by IQ only at higher concentrations (300-500 microM). This was probably due to the occurrence of GC base-substitution mutations that affected hprt mRNA splicing, especially at the intron-exon boundaries.

Modifying actions of solvent extracts from fruit and vegetable residues on 2-amino-3-methylimidazo[4,5-f]quinoline (IQ) and 2-amino-3,4-dimethylimidazo[4,5-f]quinoxaline (MeIQx) induced mutagenesis in Salmonella typhimurium TA 98

The edible parts of 13 fruits--apples, apricots, bananas, blackberries, sweet cherries, red currants, white grapes, oranges, peaches, pears, plums, raspberries, and strawberries--and of 12 vegetables--asparagus, green beans, broccoli, brussels sprouts, red and white cabbage, carrots, cauliflower, onions, green peppers, spinach, and tomatoes--were squeezed in order to separate juices and residues. The residues were washed, lyophilized, and extracted sequentially with n-hexane, dichloromethane, acetone, and 2-propanol. Solvent extracted materials were tested in Salmonella typhymurium TA 98 for antimutagenicity against IQ and MeIQx. We found antimutagenic activities in 96% of the n-hexane extracts, 64% of the dichloromethane extracts, 44% of the acetone extracts, and 36% of the 2-propanol extracts. Since no or only minor differences were seen between the mutagens IQ and MeIQx investigations were continued with IQ only. Additional antimutagenic activities were detected in a total of 29.6% of extracts tested when an enzyme preparation with glycosidase-activities (fecalase) was included in the assay. These activities were found in originally inactive or less active dichloromethane, acetone, and 2-propanol extracts, and are therefore strongly suggestive for the liberation of antimutagenic aglycones from inactive glycosides. The existence of possibly a multitude of antimutagenic factors in fruits and vegetables was further substantiated by: (1) solvent partitioning of the n-hexane extracts of cauliflower, peaches, and spinach; (2) separation of the n-hexane and dichloromethane extracts of cauliflower, peaches, and spinach into acidid, neutral, and basic compounds; (3) chromatographic analysis of the n-hexane and dichloromethane extracts of spinach. Taken together, antimutagenic activities were present in 32 of 36 subfractions, corresponding to 88.9%. In the green vegetables beans, broccoli, and spinach the known antimutagen chlorophyll was proven to contribute considerably to antimutagenic potency. Other important contributions may be caused by various fibers: (I) antimutagenicity of fruit and vegetable solvent extracts was extensively heat stable; (II) heating surprisingly caused an increase of antimutagenic potencies or generated new antimutagenic activities in several solvent fractions, especially of broccoli, white and red cabbage. Indeed, mutagenicity induced by IQ was strongly reduced by lignin, weakly by alginic acid and pectin A, while cellulose, gum arabic, gum guar, and xylan were ineffective. With respect to the mechanisms of antimutagenicity binding of IQ by various fibers and inhibition of cytochrome P-450-dependent monooxygenases might be of major importance since no solvent fraction of any fruit or vegetable was able to reduce mutagenic activity induced by N-OH-IQ in S. typhimurium TA 98NR.

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

Prostaglandin-H synthase (PHS), a mammalian peroxidase of interest for the extrahepatic formation of reactive intermediates of carcinogens, catalyzes in vitro the metabolic activation of the mutagen and carcinogen 2-amino-3-methylimidazo-[4,5-f]quinoline (IQ). Incubation of 14C-labeled IQ with ram seminal vesicle microsomes (RSVM), a rich source of PHS, resulted in protein binding and generated products mutagenic in S. typhimurium YG1024. The mutagenic activity produced in IQ/PHS incubations was stable and extractable with ethyl acetate. Upon fractionation of such extracts by HPLC and subsequent analysis, two metabolites were identified as 2,2'-azo-bis-3-methylimidazo[4,5-f]quinoline (azo-IQ) and 3-methyl-2-nitro-imidazo[4,5-f]quinoline (nitro-IQ) confirmed by comparison of HPLC retention times, UV/VIS-, 1H-NMR-spectroscopy, and mass spectrometry of synthesized standards. Azo-IQ was obtained by chemical oxidation of IQ with meta-sodium periodate. It was the major metabolite in PHS incubations, but has not been detected in monooxygenase incubations. Azo-IQ, without metabolic activation, was much less mutagenic in S. typhimurium YG1024 (308 rev/nmol) than nitro-IQ and 3-methyl-2-nitroso-imidazo[4,5-f]quinoline (nitroso-IQ), two other S9-independent mutagens which have been synthesized by chemical oxidation of IQ with sodium nitrite. Nitro-IQ was formed only in trace amounts but due to its potent mutagenicity in S. typhimurium YG1024 (2 x 10(6) rev/nmol) it accounted for most of the mutagenic activity of the incubations. These data show that PHS-mediated in vitro metabolism of IQ results in its metabolic activation; thus PHS may contribute to the genotoxicity of IQ in extrahepatic tissues.

Non-covalent DNA groove-binding by 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine

The cooked meat mutagen 2-amino-1-methyl-6-phenyl-imidazo[4,5-b]pyridine (PhIP) is metabolized in vivo to electrophilic intermediates that covalently bind to DNA guanines. Here we address the mechanism of PhIP's non-covalent interaction with DNA by using spectroscopic and computational methodologies. NMR methodologies indicated that upon addition of DNA, PhIP aromatic protons underwent a small, 0.11-0.12 p.p.m. upfield shift. DNA phosphorus resonances of non-covalent PhIP-DNA complexes broadened and slightly shifted upfield, while DNA base imino proton resonances shifted slightly downfield relative to DNA alone. UV and fluorescence spectra of PhIP titrated with DNA showed no detectable shifting and hypochromism of absorbance or fluorescence bands. In the presence of DNA, PhIP fluorescence was efficiently quenched by acrylamide, but not by silver ion. Further, the NMR spectra suggest that PhIP is in fast exchange with the DNA, and is slightly specific for adenine-thymine (A-T) sequences. Finally, structural arguments based on quantum chemistry calculations suggested that PhIP and its metabolites are unlikely to intercalate into DNA. These data collectively indicate that PhIP non-covalently binds in a groove of DNA.

DNA recombination induced by aflatoxin B1 activated by cytochrome P450 1A enzymes

Mutations in tumor suppressor genes are intricately associated with the etiology of neoplasia. Often, such mutations are followed by the loss of the second, functional alleles of tumor suppressor genes, a phenomenon known as loss of heterozygosity. Loss of heterozygosity may occur by different molecular mechanisms, including mitotic recombination, and it is conceivable that these molecular events are influenced by endogenous as well as exogenous factors. To test whether mitotic recombination is induced by certain carcinogens, we genetically engineered a Saccharomyces cerevisiae tester strain so that it metabolizes two important classes of carcinogens, polycyclic aromatic hydrocarbons and heterocyclic arylamines. This was accomplished by expressing human cDNA's coding for the cytochrome P450 (CYP) enzymes CYP1A1 or CYP1A2 in combination with NADPH-CYP oxidoreductase in a strain heterozygous for two mutations in the trp5 gene. Microsomes isolated from the transformed yeast strains activated various xenobiotics to powerful mutagens that were detected in the Ames test. Of these, the mycotoxin aflatoxin B1, when activated intracellularly in the strains containing either human CYP enzyme, significantly induced mitotic recombination. These results are discussed in light of possible mechanisms that are involved in aflatoxin B1-mediated hepatocarcinogenesis. Similarly, benzo[a]pyrene-trans-7,8-dihydrodiol and 3-amino-1-methyl-5H-pyrido[4,3-b]indole were activated to recombinagenic products, whereas benzo[a]pyrene and 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline were negative in this assay. Our results argue that the constructed yeast strains may be a valuable tool for the investigation of drug-induced mitotic recombination.

Some perspectives on the nutritional aspects of breast cancer research. Food-derived heterocyclic amines as etiologic agents in human mammary cancer

Epidemiologic and experimental evidence indicates that dietary factors influence the incidence of mammary gland cancer. The dietary causes of this cancer, however, remain largely unknown. This paper reviews the experimental studies implicating the food-derived heterocyclic amines (HAs) in human breast cancer. Heterocyclic amines are formed at the parts-per-billion levels in meats, such as beef, chicken, pork, and fish, cooked by ordinary methods. 2-Amino-1-methyl-6-phenylimidazo [4,5-b]pyridine (PhIP) is among the most prevalent of the HAs in fried and barbecued beef, a staple of the American diet. Chronic administration of PhIP in the diet has been shown to cause mammary gland cancer in rats. Two other food-derived HAs, 2-amino-3-methylimidazo[4,5-f]quinoline and 2-amino-3,4-dimethylimidazo[4,5-f]quinoline, also have been shown to be mammary carcinogens in rodent models. In rats, heterocyclic amines produce DNA adducts in the mammary gland after metabolic activation. Studies examining human urine for HAs and metabolites confirm that humans who consume cooked meats are exposed to HAs. Studies also reveal that humans can activate HAs metabolically. Therefore, the experimental evidence suggests that the food-derived HAs may be etiologic agents in human breast cancer. Humans, however, are exposed to a complex mixture of carcinogenic and anticarcinogenic agents through their diets. Experimental studies examining the interaction between HAs and other dietary factors with respect to mammary carcinogenesis are warranted.

Mutagenicity of coal-dust and smokeless-tobacco extracts in Salmonella typhimurium strains with differing levels of O-acetyltransferase activities

Epidemiological studies have indicated an increased incidence of gastric neoplasia in coal miners. Because smokeless tobacco use is prevalent in the mining industry, nitrites or other components of these products may be etiologically associated with these gastric neoplasms. In this study both nitrosated and non-nitrosated coal-dust (from West Virginia and New Mexico) as well as smokeless-tobacco (snuff and chewing tobacco) extracts were examined for the presence of aromatic amines and nitroarenes by comparing the activities of these extracts in the pre-incubation variant of the Ames assay. Salmonella strains with differing O-acetyltransferase activities (TA98 and YG1024) were utilized in this investigation. The results of the examination of the coal-dust extracts indicated positive activity only in the nitrosated extracts. Both nitrosated extracts elicited an increased number of revertants (2-4-fold) on YG1024 without S9 in comparison to TA98, suggesting the presence of nitroarenes in these extracts. Additionally, the nitrosated West Virginia coal extract showed higher levels of activity on YG1024 with S9, indicating the possible presence of aromatic amines in this complex mixture. The non-nitrosated smokeless-tobacco extracts showed activity only on YG1024 in the presence of S9, with the highest amount of activity occurring in the snuff sample. Except for the chewing-tobacco extract on TA98 without S9, positive activity was found in both nitrosated tobacco extracts on YG1024 and TA98. As with the coal extracts, the presence of nitroarenes was inferred for these nitrosated materials. A comparative study of the non-nitrosated snuff extract across 5 tester strains with varying sensitivities to aromatic amines and nitroarenes (TA98NR, TA98/1,8-DNP6, TA98, YG1021 and YG1024) indicated that aromatic amines were a probable source of the mutagenic activity. The curing process and/or the addition of certain flavorants are potential sources of the mutagenic aromatic amines suggested to be present in the non-nitrosated snuff extract. These findings are consistent with an etiologic role supplementary to the nitroso compounds for mutagenic nitroarenes and aromatic amines in the development of gastric neoplasia in coal miners.

2-Hydroxyamino-1-methyl-6-phenylimidazo[4,5-b]pyridine induction of recombinational mutations in mammalian cell lines as detected by DNA fingerprinting

2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) is the most abundant mutagenic heterocyclic amine in cooked foods. Two mouse tumor cell lines, BMT11 and FM3A, were exposed to its proximate form, 2-hydroxyamino-1-methyl-6-phenylimidazo[4,5-b]pyridine (N-OH-PhIP). Fifty-six subclones of BMT11 and 39 subclones of FM3A were isolated and analyzed by DNA fingerprinting. Treatment with 10-20 microM N-OH-PhIP gave rise to extra bands or shifted bands, but treatment without N-OH-PhIP did not. This suggests that mutations resulting from recombination were induced. The mutation frequencies were 21-53% and 22-35% for BMT11 and FM3A, respectively. These findings suggest that PhIP induces recombinational mutations.

Activity of aromatic amines in the eye: w/w+ somatic assay of Drosophila melanogaster

Aromatic amines represent a category of classical environmental hazards which require biotransformation in order to exert their mutagenic and carcinogenic effects. The mutagenic activity of several aromatic amines was tested by means of the w/w+ somatic assay of Drosophila melanogaster employing a wild-type strain Leiden Standard (LS) and an insecticide-resistant stock Hikone-R (HK-R). Four monocyclicortho-anisidine (O-AN), ortho-toluidine (O-TO), 2,4-diaminoanisole (2,4-DAA) and 4-nitro-o-phenylenediamine (4-N-o-PDA), and one bicyclic- 4,4'-oxidianiline (4,4'-ODA) aromatic amines were tested by chronic exposure. Two separate experiments were run, and concurrent controls were treated with the solvent mixture. Results show that the ortho-anilines (O-AN and O-TO) were active in the assay, while the diamines containing a methoxy (2,4-DAA) or a nitro group (4-N-o-PDA) were inactive. The bicyclic aromatic amine, 4,4'-ODA, was the most potent. The genotoxic responsiveness of both strains was very similar.

Cooking procedures and food mutagens: a literature review

Commonly eaten meat products prepared from beef, pork, mutton and chicken show some level of mutagenic activity following normal frying. Food preparation methods have a significant influence on the formation of the mutagenic activity. The main food mutagens found in cooked meat products are heterocyclic amines. Several of them have been tested in long-term animal studies and shown to be carcinogenic in rodents. From a health point of view, it is desirable to reduce or prevent the formation of food mutagens. Therefore, a deeper understanding of the precursors and reaction conditions for mutagen formation during normal domestic cooking is very important. Modelling experiments are useful tools for studying the influence of different physical parameters and various precursors on the mutagenic activity. The identification of several thermic mutagens from the modelling experiments support the theory that creatine or creatinine, amino acids and sugars are precursors in the formation of thermic mutagens. Creatine is generally accepted to be a precursor of the mutagens and, interestingly, the conversion of creatine to creatinine has been shown to be blocked by an excess of sugars, which also caused the mutagenic activity to decrease. The mutagenic activity differed for different amino acids used in the model systems, and various thermic mutagens were produced from the amino acids. The incorporation of carbon atoms originating from glucose into food mutagen molecules has shown glucose to be a precursor. Sugar has also been shown to either enhance or inhibit the yield of mutagenic activity, depending on its molar ratio versus the other reactants, which suggests that the Maillard reaction may be used to control the formation of mutagens.

ras mutations in 2-amino-3-methylimidazo-[4,5-f]quinoline-induced tumors in the CDF1 mouse

2-Amino-3-methylimidazo[4,5-f]quinoline (IQ) is a very potent mutagen that is carcinogenic in rodents and nonhuman primates. IQ-induced CDF1 mouse lung and liver tumors were examined for activated Ki-ras and Ha-ras genes, respectively. Polymerase chain reaction (PCR)-amplified target DNAs were analyzed for mutations of codons 12, 13, and 61 by single-strand conformation polymorphism (SSCP) and direct sequencing methods. All mutations were localized to codon 61 of the ras genes. Forty-nine of 54 lung tumors induced by IQ possessed activating Ki-ras mutations, as did 20 of 26 lung tumors from the vehicle-treated animals; 80% and 75% of these mutations, respectively, were A-->T transversions of the second nucleotide redundant. One lung adenoma from the IQ-treated group contained a tandem duplication of the sequence corresponding to codons 50-57 of the Ki-ras gene (unpublished observations). In addition, seven of 34 IQ-induced liver tumors harbored activating Ha-ras mutations: five were C-->A (G-->T) transversions at the first nucleotide, and two were A-->T transversions at the second nucleotide of codon 61. None of the 15 liver tumors collected from the vehicle-treated mice possessed Ha-ras mutations in codon 12, 13, or 61. These data indicate that IQ induces Ha-ras gene activation in CDF1 mouse liver tumors. The mechanisms of lung tumor induction by IQ, however, is obscured by the high frequency of Ki-ras A-->T mutations observed in both the IQ-induced and spontaneous lung tumors. The different ras mutational spectra in lung and liver tumors may suggest either that two different pathways of IQ metabolism exist in these organs or that IQ contributes to CDF1 lung tumorigenesis by a mechanism other than its direct interaction with the Ki-ras gene.

Formation of DNA adducts of 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) in male Fischer-344 rats

2-Amino-1-methyl-6-phenylimidazo[4,5-b]-pyridine (PhIP) is known to induce colon tumors in male Fischer-344 rats. Using 32P-postlabeling assays, we have examined PhIP-DNA adduct formation in various organs and white blood cells (WBCs) of the male Fischer-344 rat 24 h after a single oral dose of 0, 0.5, 5 or 50 mg PhIP/kg. Three PhIP-DNA adducts were detected in WBCs and in all organs, except in the liver and stomach which had only two adducts. The extent of adduct formation was dose-related, but at 0.5 mg/kg no adducts could be detected in any of the organs. At 50 mg/kg, adduct levels, expressed as relative adduct labeling values (RAL x 10(7), or adducts per 10(7) nucleotides assuming complete labeling) were highest in the large intestine (5.66), followed by WBCs (5.04), stomach (1.44), small intestine (1.32), kidney (1.16), liver (0.67) and lungs (0.52). It is concluded that orally administered PhIP forms high levels of specific DNA adducts in the large intestine, the target organ in PhIP carcinogenesis in the male Fischer-344 rat, and that the high level of adducts in WBCs indicates that significant amounts of the ultimate carcinogenic form of PhIP are present in the circulation.

Enhancing effects of heterocyclic amines and beta-carbolines on the induction of chromosome aberrations in cultured mammalian cells

The effects of post-treatment with heterocyclic amines and beta-carbolines on the induction of chromosome aberrations were studied in Chinese hamster CHO K-1 cells and SV40-transformed excision repair-deficient human XP2OSSV cells. The number of chromosome aberrations induced by UV and MMC were increased by post-treatment with Trp-P-1 and Trp-P-2, in both the presence and the absence of S9 mix. A alpha C, MeA alpha C, Glu-P-1, Glu-P-2, IQ, MeIQ, harman and harmine increased chromosome aberrations only in the presence of S9 mix. Glu-P-2, IQ, MeIQ, harman, and harmine did not induce chromosome aberrations by themselves at the concentrations used in this study. Trp-P-1, Trp-P-2, A alpha C, MeA alpha C and Glu-P-1 were weak clastogens by themselves, but at much higher concentrations than those at which they increased the induction of chromosome aberrations in cells pretreated with UV or MMC. Therefore, the increases in chromosome aberrations were not considered to be additive.

Mutagenicity of some heterocyclic amines in Salmonella typhimurium with metabolic activation by human red blood cell cytosol

Purified human red blood cell cytosol was used to activate the heterocyclic amines 2-amino-3-methylimidazo[4,5-f]quinoline (IQ), 2-amino-3,4-dimethylimidazo[4,5-f]quinoline (MeIQ), 3-amino-1,4-dimethyl-5H-pyrido[4,3-b]indole (Trp-P-1) and 3-amino-1-methyl-5H-pyrido[4,3-b]indole (Trp-P-2) into mutagenic intermediate(s) in the Salmonella test. The liquid preincubation method in the presence of strain TA98 was utilized. In order to understand the mechanism involved in this metabolic activation, some modulators were incorporated in the medium. The results suggest that an oxygenated hemoprotein, probably oxyhemoglobin, is involved in the activation into genotoxic intermediate(s).