Heterocyclic aromatic amine metabolism, DNA adduct formation, mutagenesis, and carcinogenesis

METHODS FOR AROMATIC AND HETEROCYCLIC AMINE CARCINOGEN-DNA ADDUCT ANALYSIS BY LIQUID CHROMATOGRAPHY-TANDEM MASS SPECTROMETRY

Xenobiotic-DNA adducts are used as biomarkers to assess the genotoxic effects of carcinogens. Rats were dosed with 4-aminobiphenyl (4-ABP), 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx), or 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP). DNA was isolated from the colons of vehicle and carcinogen-treated rats and digested using different nucleases and alkaline phosphatase. Deoxyribonucleoside adducts were quantified by capillary liquid chromatography-tandem mass spectrometry (LC-MS/MS) using isotope dilution methods with deuterated internal standards. Major adducts were those bound to the C8 position of deoxyguanosine. 3'- and 5'-Exonucleases were the most efficient nucleases at isolating dG-C8-ABP adducts. However, bulky adducts such as dG-C8-MeIQx and dG-C8-PhIP were better isolated using nuclease P1 rather than a combination of micrococcal nuclease and spleen phosphodiesterase. The use of DNase I enhanced the detection of all three adducts. We describe LC-MS/MS methods for DNA adduct detection and support the testing of different nucleases that increase DNA digestion efficiency and make available more DNA adducts for detection.

Tissue differences in the modulation of rat cytochromes P450 and phase II conjugation systems by dietary doses of phenethyl isothiocyanate

Rats were fed diets supplemented with phenethyl isothiocyanate (PEITC) at 0.06 (low dose, dietary intake level), 0.6 (medium dose) and 6.0 micromole/g (high dose), and xenobiotic-metabolising enzymes were monitored in liver, lung and kidney. At the low dose, inhibition of the hepatic O-dealkylation of ethoxy- and methoxyresorufin was noted, whereas at the high dose increases in the O-depentylation of pentoxyresorufin and O-debenzylation of benzyloxyquinoline were observed, whereas p-nitrophenol hydroxylase was inhibited. Hepatic bioactivation of 2-amino-3-methylimidazo-[4,5-f]quinoline to mutagens was not influenced by the PEITC-treatment. In the lung, at the high dose, ethoxyresorufin dealkylation was elevated and that of pentoxyresorufin suppressed; no significant changes were seen in the kidney. Quinone reductase was markedly elevated at all doses in liver, but the lung enzyme was refractive whereas in the kidney a modest rise was observed at the high dose. Hepatic glutathione S-transferase activity was stimulated by PEITC-treatment, but no effect was evident in the lung or kidney. It is concluded that the effects of PEITC on xenobiotic-metabolising systems are dose- and tissue-dependent, with the liver being the most sensitive and the lung generally resistant. Increased detoxication rather than cytochrome P450 inhibition is the likely mechanism of the chemopreventive activity of PEITC.

Electrochemical biosensor featuring a two-enzyme pathway and DNA for screening toxic reactive metabolites of arylamines

We demonstrate for the first time a biosensor featuring a sequential two-enzyme pathway suitable to screen potentially toxic reactive metabolites generated during metabolism.

UGT1A1 and UGT1A9 functional variants, meat intake, and colon cancer, among Caucasians and African-Americans

Glucuronidation by the UDP-glucuronosyltransferase enzymes (UGTs) is one of the primary detoxification pathways of dietary heterocyclic amines (HCAs) and polycyclic aromatic hydrocarbons (PAHs). In a population-based case-control study of 537 cases and 866 controls, we investigated whether colon cancer was associated with genetic variations in UGT1A1 and UGT1A9 genes and we determined if those variations modify the association between colon cancer and dietary HCA and PAH exposure. We measured functional UGT1A1 polymorphisms at positions -53 (28; A(TA)6TAA to A(TA)7TAA), -3156 (G>A), -3279 (T>G) and the UGT1A9-275(T>A) polymorphism, and found no association with colon cancer overall. However, when stratified by race, the UGT1A1-3279 GG/TG intermediate/low activity genotypes were associated with an increased risk of colon cancer (odds ratio (OR)=1.5, 95% confidence interval (CI)=1.1-2.0) in Caucasians. This finding is also supported by haplotype analyses where the UGT1A1-3279G-allele-bearing haplotype is overrepresented in case group. Overall, UGT1A1-53 and -3156 genotypes modified the association between dietary benzo(a)pyrene (BaP) and colon cancer (P for interaction=0.02 and 0.03, respectively). The strongest association was observed for those with <7.7 ng/day BaP exposure and the low activity genotypes, for both UGT1A1 28/28 (OR=1.8, 95% CI=1.1-2.9) and -3156AA (OR=1.7, 95% CI=1.0-3.0), compared to >or=7.7 ng/day and combined high/intermediate genotypes. These data support a hypothesis that UGTs modify the association between meat-derived PAH exposure and colon cancer by their role in the elimination of dietary carcinogens.

Mutagenic and carcinogenic hazards of settled house dust. II: Salmonella mutagenicity

Settled house dust (SHD) is a complex mixture that contains numerous chemical contaminants. Very little is known about the hazards of SHD as compared to other complex matrices such as air and soil. In this study, the mutagenic hazards associated with the extracts of sieved dust from 52 homes were examined using the Salmonella Mutagenicity Test. All of the SHD samples displayed mutagenic activity and the mean mutagenic potencies ranged from 2300to 23 600 revertants per gram. Testing with various Salmonella strains revealed a predominance of frameshift mutagens in the dust samples. Analyses showed that polycyclic aromatic hydrocarbons (PAHs) were likely responsible for a quarter of the mutagenic activity of the SHD samples. In an effort to identify factors that influenced dust mutagenicity, the relationships between SHD mutagenicity and household activities were investigated. Mutagenicity was positively correlated with parameters such as the time since last vacuuming (r2 = 0.11, p < 0.05) and the number of people living in the home (r2 = 0.11-0.43, p < 0.05). However, the causative factors responsible for these relationships remain unclear.

Mechanisms of colorectal and lung cancer prevention by vegetables: a genomic approach

Colorectal cancer (CRC) and lung cancer (LC) occur at high incidence, and both can be effectively prevented by dietary vegetable consumption. This makes these two types of cancer highly suitable for elucidating the underlying molecular mechanisms of cancer chemoprevention. Numerous studies have shown that vegetables exert their beneficial effects through various different mechanisms, but effects on the genome level remain mostly unclear. This review evaluates current knowledge on the mechanisms of CRC and LC prevention by vegetables, thereby focusing on the modulation of gene and protein expressions. The majority of the effects found in the colon are changes in the expression of genes and proteins involved in apoptosis, cell cycle, cell proliferation and intracellular defense, in favor of reduced CRC risk. Furthermore, vegetables and vegetable components changed the expression of many more genes and proteins involved in other pathways for which biologic meaning is less clear. The number of studies investigating gene and protein expression changes in the lungs is limited to only a few in vitro and animal studies. Data from these studies show that mostly genes involved in biotransformation, apoptosis and cell cycle regulation are affected. In both colon and lungs, genomewide analyses of gene and protein expression changes by new genomics and proteomics technologies, as well as the investigation of whole vegetables, are few in number. Further studies applying these 'omics' approaches are needed to provide more insights on affected genetic/biologic pathways and, thus, in molecular mechanisms by which different chemopreventive compounds can protect against carcinogenesis. Particularly studies with combinations of phytochemicals and whole vegetables are needed to establish gene expression changes in the colon, but especially in the lungs.

DNA-damaging activity and mutagenicity of 16 newly synthesized thiazolo[5,4-a]acridine derivatives with high photo-inducible cytotoxicity

The discovery of the potent anticancer properties of natural alkaloids in the pyrido-thiazolo-acridine series has suggested that thiazolo-acridine derivatives could be of great interest. In a continuous attempt to develop DNA-binding molecules and DNA photo-cleavers, 16 new thiazolo[5,4-a]acridines were synthesized and studied for their photo-inducible DNA-intercalative, cytotoxic and mutagenic activities, by use of the DNA methyl-green bioassay, the Alamar Blue viability assay and the Salmonella mutagenicity test using strains TA97a and TA98 with and without metabolic activation and photo-activation. Without photo-activation, one compound showed a DNA-intercalative activity in the DNA major groove while three compounds displayed intercalating properties after photo-activation. In the dark, four molecules possessed cytotoxic activities against a THP1 acute monocytic leukemia cell line while 15 derivatives displayed photo-inducible cytotoxic activity against this cell line. All compounds were mutagenic in strain TA97a with metabolic activation (+S9mix) and 15 molecules were mutagenic in strain TA98 without activation (-S9mix). Study of the quantitative structure-activity relationships (QSAR) from the Salmonella mutagenicity data revealed that several descriptors could describe cytotoxic and mutagenic activities after photo-activation. From the results of the mutagenicity test, four compounds with elevated mutagenic activities were selected for additional experiments. Their capacities to induce single-strand breaks (SSB) and chromosome-damaging effects were monitored by the comet and the micronucleus assays in normal human keratinocytes. Comparison of the minimal genotoxic concentrations showed that two compounds possessed higher capacities to induce SSB after photo-activation. In the micronucleus assay, three molecules were able to induce high numbers of micronuclei following photo-activation. Overall, the results of this study confirm that acridines are predominantly genotoxic via a DNA-intercalating mechanism in the dark, while DNA-adducts were probably induced following photo-activation.

Activation of aminoflavone (NSC 686288) by a sulfotransferase is required for the antiproliferative effect of the drug and for induction of histone gamma-H2AX

Aminoflavone (AF) is entering clinical trials. We recently reported that AF induces DNA-protein cross-links (DPC) and gamma-H2AX in MCF-7 human breast cancer cells. To elucidate the mechanism of action of AF and provide biomarkers indicative of AF activity, we correlated AF activity profile (GI(50)) with gene expression patterns in the NCI-60 cell lines. Sulfotransferases (SULT) showed the highest positive correlation coefficients among approximately 14,000 probe sets analyzed (r = 0.537, P < 0.001). Stable transfection of SULT1A1 into AF-resistant MDA-MB-231 cells sensitized these cells to AF. AF produced DPCs, gamma-H2AX foci, and S-phase arrest in the SULT1A1-transfected but not in the parent MDA-MB-231 cells. Conversely, cells in which SULT1A1 was knocked down by small interfering RNA failed to induce gamma-H2AX. Inhibition of SULTs and cytochrome P450 (CYP) enzymes by natural flavonoids blocked the antiproliferative activity of AF and the formation of AF-DNA adducts. AF also induces SULT1A1 and CYP expression in MCF-7 cells, suggesting the existence of an aryl hydrocarbon receptor-mediated positive feedback for AF activation by CYP and SULT1A1. Metabolism studies showed that AF can be oxidized by CYP at two amino groups to form N-hydroxyl metabolites that are substrates for bioactivation by SULTs. We propose that both N-sulfoxy-groups can be further converted to nitrenium ions that form adducts with DNA and proteins. The results reported here show the importance of SULT1A1 and CYP for AF activation and anticancer activity. They also suggest using SULT1A1 and gamma-H2AX as biomarkers for prediction of AF activity during patient selection and monitoring of clinical trials.

Synthesis of oligonucleotides containing the N2-deoxyguanosine adduct of the dietary carcinogen 2-amino-3-methylimidazo[4,5-f]quinoline

2-amino-3-methylimidazo[4,5-f]quinoline (IQ) is a highly mutagenic heterocyclic amine formed in all cooked meats. IQ has been found to be a potent inducer of frameshift mutations in bacteria and carcinogenic in laboratory animals. Upon metabolic activation, IQ forms covalent adducts at the C8- and N2-positions of deoxyguanosine with a relative ratio of up to approximately 4:1. We have previously incorporated the major dGuo-C8-IQ adduct into oligonucleotides through the corresponding phosphoramidite reagent. We report here the sequence-specific synthesis of oligonucleotides containing the minor dGuo-N2-IQ adduct. Thermal melting analysis revealed that the dGuo-N2-IQ adduct significantly destabilizes duplex DNA.

Influence of structural and functional modifications of selected genotoxic carcinogens on metabolism and mutagenicity - a review

Alterations in molecular structure are responsible for the differential biological response(s) of a chemical inside a biosystem. Structural and functional parameters that govern a chemical's metabolic course and determine its ultimate outcome in terms of mutagenic/carcinogenic potential are extensively reviewed here. A large number of environmentally-significant organic chemicals are addressed under one or more broadly classified groups each representing one or more characteristic structural feature. Numerous examples are cited to illustrate the influence of key structural and functional parameters on the metabolism and DNA adduction properties of different chemicals. It is hoped that, in the event of limited experimental data on a chemical's bioactivity, such knowledge of the likely roles played by key molecular features should provide preliminary information regarding its bioactivation, detoxification and/or mutagenic potential and aid the process of screening and prioritising chemicals for further testing.

Prediction of rodent carcinogenic potential of naturally occurring chemicals in the human diet using high-throughput QSAR predictive modeling

Consistent with the U.S. Food and Drug Administration (FDA) Critical Path Initiative, predictive toxicology software programs employing quantitative structure-activity relationship (QSAR) models are currently under evaluation for regulatory risk assessment and scientific decision support for highly sensitive endpoints such as carcinogenicity, mutagenicity and reproductive toxicity. At the FDA's Center for Food Safety and Applied Nutrition's Office of Food Additive Safety and the Center for Drug Evaluation and Research's Informatics and Computational Safety Analysis Staff (ICSAS), the use of computational SAR tools for both qualitative and quantitative risk assessment applications are being developed and evaluated. One tool of current interest is MDL-QSAR predictive discriminant analysis modeling of rodent carcinogenicity, which has been previously evaluated for pharmaceutical applications by the FDA ICSAS. The study described in this paper aims to evaluate the utility of this software to estimate the carcinogenic potential of small, organic, naturally occurring chemicals found in the human diet. In addition, a group of 19 known synthetic dietary constituents that were positive in rodent carcinogenicity studies served as a control group. In the test group of naturally occurring chemicals, 101 were found to be suitable for predictive modeling using this software's discriminant analysis modeling approach. Predictions performed on these compounds were compared to published experimental evidence of each compound's carcinogenic potential. Experimental evidence included relevant toxicological studies such as rodent cancer bioassays, rodent anti-carcinogenicity studies, genotoxic studies, and the presence of chemical structural alerts. Statistical indices of predictive performance were calculated to assess the utility of the predictive modeling method. Results revealed good predictive performance using this software's rodent carcinogenicity module of over 1200 chemicals, comprised primarily of pharmaceutical, industrial and some natural products developed under an FDA-MDL cooperative research and development agreement (CRADA). The predictive performance for this group of dietary natural products and the control group was 97% sensitivity and 80% concordance. Specificity was marginal at 53%. This study finds that the in silico QSAR analysis employing this software's rodent carcinogenicity database is capable of identifying the rodent carcinogenic potential of naturally occurring organic molecules found in the human diet with a high degree of sensitivity. It is the first study to demonstrate successful QSAR predictive modeling of naturally occurring carcinogens found in the human diet using an external validation test. Further test validation of this software and expansion of the training data set for dietary chemicals will help to support the future use of such QSAR methods for screening and prioritizing the risk of dietary chemicals when actual animal data are inadequate, equivocal, or absent.

DNA sequence modulates the conformation of the food mutagen 2-amino-3-methylimidazo[4,5-f]quinoline in the recognition sequence of the NarI restriction enzyme

The conformations of C8-dG adducts of 2-amino-3-methylimidazo[4,5-f]quinoline (IQ) positioned in the C-X1-G, G-X2-C, and C-X3-C contexts in the C-G1-G2-C-G3-C-C recognition sequence of the NarI restriction enzyme were compared, using the oligodeoxynucleotides 5'-d(CTCXGCGCCATC)-3'.5'-d(GATGGCGCCGAG)-3', 5'-d(CTCGXCGCCATC)-3'.5'-d(GATGGCGCCGAG)-3', and 5'-d(CTCGGCXCCATC)-3'.5'-d(GATGGCGCCGAG)-3' (X is the C8-dG adduct of IQ). These were the NarIIQ1, NarIIQ2, and NarIIQ3 duplexes, respectively. In each instance, the glycosyl torsion angle chi for the IQ-modified dG was in the syn conformation. The orientations of the IQ moieties were dependent upon the conformations of torsion angles alpha' [N9-C8-N(IQ)-C2(IQ)] and beta' [C8-N(IQ)-C2(IQ)-N3(IQ)], which were monitored by the patterns of 1H NOEs between the IQ moieties and the DNA in the three sequence contexts. The conformational states of IQ torsion angles alpha' and beta' were predicted from the refined structures of the three adducts obtained from restrained molecular dynamics calculations, utilizing simulated annealing protocols. For the NarIIQ1 and NarIIQ2 duplexes, the alpha' torsion angles were predicted to be -176 +/- 8 degrees and -160 +/- 8 degrees , respectively, whereas for the NarIIQ3 duplex, torsion angle alpha' was predicted to be 159 +/- 7 degrees . Likewise, for the NarIIQ1 and NarIIQ2 duplexes, the beta' torsion angles were predicted to be -152 +/- 8 degrees and -164 +/- 7 degrees , respectively, whereas for the NarIIQ3 duplex, torsion angle beta' was predicted to be -23 +/- 8 degrees . Consequently, the conformations of the IQ adduct in the NarIIQ1 and NarIIQ2 duplexes were similar, with the IQ methyl protons and IQ H4 and H5 protons facing outward in the minor groove, whereas in the NarIIQ3 duplex, the IQ methyl protons and the IQ H4 and H5 protons faced into the DNA duplex, facilitating the base-displaced intercalated orientation of the IQ moiety [Wang, F., Elmquist, C. E., Stover, J. S., Rizzo, C. J., and Stone, M. P. (2006) J. Am. Chem. Soc. 128, 10085-10095]. In contrast, for the NarIIQ1 and NarIIQ2 duplexes, the IQ moiety remained in the minor groove. These sequence-dependent differences suggest that base-displaced intercalation of the IQ adduct is favored when both the 5'- and 3'-flanking nucleotides in the complementary strand are guanines. These conformational differences may correlate with sequence-dependent differences in translesion replication.

Translesion synthesis past the C8- and N2-deoxyguanosine adducts of the dietary mutagen 2-Amino-3-methylimidazo[4,5-f]quinoline in the NarI recognition sequence by prokaryotic DNA polymerases

2-Amino-3-methylimidazo[4,5-f]quinoline (IQ) is found in cooked meats and forms DNA adducts at the C8- and N2-positions of dGuo after appropriate activation. IQ is a potent inducer of frameshift mutations in bacteria and is carcinogenic in laboratory animals. We have incorporated both IQ-adducts into the G1- and G3-positions of the NarI recognition sequence (5'-G1G2CG3CC-3'), which is a hotspot for arylamine modification. The in vitro replication of the oligonucleotides was examined with Escherichia coli pol I Klenow fragment exo-, E. coli pol II exo-, and Sulfolobus solfataricus P2 DNA polymerase IV (Dpo4), and the extension products were sequenced by tandem mass spectrometry. Replication of the C8-adduct at the G3-position resulted in two-base deletions with all three polymerases, whereas error-free bypass and extension was observed at the G1-position. The N2-adduct was bypassed and extended by all three polymerases when positioned at the G1-position, and the error-free product was observed. The N2-adduct at the G3-position was more blocking and was bypassed and extended only by Dpo4 to produce an error-free product. These results indicate that the replication of the IQ-adducts of dGuo is strongly influenced by the local sequence and the regioisomer of the adduct. These results also suggest a possible role for pol II and IV in the error-prone bypass of the C8-IQ-adduct leading to frameshift mutations in reiterated sequences, whereas noniterated sequences result in error-free bypass.

Conformational differences of the C8-deoxyguanosine adduct of 2-amino-3-methylimidazo[4,5-f]quinoline (IQ) within the NarI recognition sequence

2-amino-3-methylimidazo[4,5-f]quinoline (IQ) is a highly mutagenic heterocyclic amine found in cooked meats. The major DNA adduct of IQ is at the C8-position of dGuo. We have previously reported the incorporation of the C8-IQ adduct into oligonucleotides, namely, the G1-position of codon 12 of the N-ras oncogene sequence (G1G2T) and the G3-position of the NarI recognition sequence (G1G2CG3CC) (Elmquist et al. (2004) J. Am. Chem. Soc. 126, 11189-11201). Ultraviolet spectroscopy and circular dichroism studies indicated that the conformation of the adduct in the two oligonucleotides was different, and they were assigned as groove-bound and base-displaced intercalated, respectively. The conformation of the latter was subsequently confirmed through NMR and restrained molecular dynamics studies (Wang et al. (2006) J. Am. Chem. Soc. 128, 10085-10095). We report here the incorporation of the C8-IQ adduct into the G1- and G2-positions of the NarI sequence. A complete analysis of the UV, CD, and NMR chemical shift data for the IQ protons are consistent with the IQ adduct adopting a minor groove-bound conformation at the G1- and G2-positions of the NarI sequence. To further correlate the spectroscopic data with the adduct conformation, the C8-aminofluorene (AF) adduct of dGuo was also incorporated into the NarI sequence; previous NMR studies demonstrated that the AF-modified oligonucleotides were in a sequence-dependent conformational exchange between major groove-bound and base-displaced intercalated conformations. The spectroscopic data for the IQ- and AF-modified oligonucleotides are compared. The sequence-dependent conformational preferences are likely to play a key role in the repair and mutagenicity of C8-arylamine adducts.

A comprehensive investigation of 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) metabolism in the mouse using a multivariate data analysis approach

2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) is a potent rodent carcinogen and a potential human carcinogen because of its existence in the normal human diet. N2-OH-PhIP, a major PhIP metabolite, has been identified as a precursor of genotoxic species. In vitro data supported the view that CYP1A2 is the major enzyme responsible for the formation of N2-OH-PhIP. However, disruption of the CYP1A2 gene in mouse failed to inhibit PhIP-induced carcinogenesis. To investigate the mechanism underlying this observation, the metabolism of PhIP in wild-type, Cyp1a2-null, and CYP1A2-humanized mice was examined in detail using a metabolomic approach. Following data acquisition in a high-resolution LC-MS system, urinary metabolomes of the control and PhIP-treated mice were characterized in a principal component analysis (PCA) model. Comprehensive metabolite profiles of PhIP in high dose (10 mg/kg) and low dose (100 microg/kg) were established through analyzing urinary ions contributing to the separation of three mouse lines in the multivariate model and by measuring radiolabled PhIP metabolite in a radio-HPLC assay, respectively. The genotoxicity of PhIP to three mouse lines was evaluated by measuring DNA adduction levels in liver, lung, colon, and mammary gland. On the basis of the chemical identities of 17 urinary PhIP metabolites, including eight novel metabolites, multivariate data analysis revealed the role of CYP1A2 in PhIP metabolism and a human-mouse interspecies difference in the catalytic activity of CYP1A2. In addition, the results also showed that Cyp1a2-null mice still possess significant N2-hydroxylation and DNA adduction activities, which may be partially attributed to mouse CYP2C enzymes according to the results from in vitro microsome and Supersome incubations and antibody inhibition experiments.

Chemical and electrochemical oxidation of C8-arylamine adducts of 2'-deoxyguanosine

The electrochemical and chemical oxidation of a series of C8-arylamine adducts of 2'-deoxyguanosine has been examined. The oxidations were found to be reversible by cyclic and square-wave voltammetry in both aqueous buffer and aprotic organic solvent. The mechanism of the oxidation in protic media was either one- or two-electron, depending on the aryl group. The chemical oxidation resulted in guanidinohydantoin and spiroiminodihydantoin rearrangement products similar to those observed for 8-oxo-7,8-dihydro-2'-deoxyguanosine.

Role of nucleotide excision repair deficiency in intestinal tumorigenesis in multiple intestinal neoplasia (Min) mice

Mice deficient in the Xeroderma pigmentosum group A (Xpa) gene are defective in nucleotide excision repair (NER) and highly susceptible to skin carcinogenesis after dermal exposure to UV light or chemicals. Min (multiple intestinal neoplasia) mice, heterozygous for a germline nonsense mutation in the tumor suppressor gene adenomatous polyposis coli (Apc), develop intestinal tumors spontaneously and show additional intestinal tumors after exposure to the food mutagen 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP). In this study, we investigated the impact of loss of XPA function on PhIP-induced intestinal tumorigenesis in F1 offspring of Min/+ (Apc(+/-)) mice crossed with Xpa gene-deficient mice. Apc(+/-) mice lacking both alleles of Xpa had higher susceptibility towards toxicity of PhIP, higher levels of PhIP-DNA adducts in the middle and distal small intestines, as well as in liver, and a higher number of small intestinal tumors at 11 weeks, compared with Apc(+/-) mice with one or two intact Xpa alleles. Localization of tumors was not affected, being highest in middle and distal small intestines in all genotypes. At 11 weeks of age, the number of spontaneous intestinal tumors was not significantly increased by homozygous loss of Xpa, but untreated Apc(+/-)/Xpa(-/-) mice had significantly shorter life-spans than their XPA-proficient littermates. Heterozygous loss of Xpa did not affect any of the measured end points. In conclusion, the Xpa gene and the NER pathway are involved in repair of bulky PhIP-DNA adducts in the intestines and the liver, and most probably of DNA lesions leading to spontaneous intestinal tumors. These results confirm a role of the NER pathway also in protection against cancer in internal organs, additional to its well-known importance in protection against skin cancer. An effect of Apc(+/-) on adduct levels, additional to that of Xpa(-/-), indicates that the truncated APC protein may affect a repair pathway other than NER.

The Urinary Metabolite Profile of the Dietary Carcinogen 2-Amino-1-Methyl-6-Phenylimidazo[4,5-b]Pyridine Is Predictive of Colon DNA Adducts after a Low-Dose Exposure in Humans

Epidemiologic evidence indicates that exposure to heterocyclic amines in the diet is an important risk factor for the development of colon cancer. Well-done cooked meats contain significant levels of heterocyclic amines, which have been shown to cause cancer in laboratory animals. To better understand the mechanisms of heterocyclic amine bioactivation in humans, the most mass abundant heterocyclic amine, 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), was used to assess the relationship between PhIP metabolism and DNA adduct formation. Ten human volunteers where administered a dietary relevant dose of [(14)C]PhIP 48 to 72 hours before surgery to remove colon tumors. Urine was collected for 24 hours after dosing for metabolite analysis, and DNA was extracted from colon tissue and analyzed by accelerator mass spectrometry for DNA adducts. All 10 subjects were phenotyped for cytochrome P4501A2 (CYP1A2), N-acetyltransferase 2, and sulfotransferase 1A1 enzyme activity. Twelve PhIP metabolites were detected in the urine samples. The most abundant metabolite in all volunteers was N-hydroxy-PhIP-N(2)-glucuronide. Metabolite levels varied significantly between the volunteers. Interindividual differences in colon DNA adducts levels were observed between each individual. The data showed that individuals with a rapid CYP1A2 phenotype and high levels of urinary N-hydroxy-PhIP-N(2)-glucuronide had the lowest level of colon PhIP-DNA adducts. This suggests that glucuronidation plays a significant role in detoxifying N-hydroxy-PhIP. The levels of urinary N-hydroxy-PhIP-N(2)-glucuronide were negatively correlated to colon DNA adduct levels. Although it is difficult to make definite conclusions from a small data set, the results from this pilot study have encouraged further investigations using a much larger study group.

Effects of single nucleotide polymorphisms in human N-acetyltransferase 2 on metabolic activation (O-acetylation) of heterocyclic amine carcinogens

N-Acetyltransferase 2 (NAT2) catalyzes the O-acetylation of N-hydroxy heterocyclic amines such as N-hydroxy-2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (N--OH--MeIQx) and N-hydroxy-2-amino-1-methyl-6-phenylimidazo[4,5-b] pyridine (N--OH --PhIP) to DNA binding metabolites that initiate mutagenesis and carcinogenesis. NAT2 acetylator phenotype is associated with increased cancer risk. Single nucleotide polymorphisms (SNPs) have been identified in the NAT2 coding region. Although the effects of these SNPs on N-acetyltransferase activity have been reported, very little is known regarding their effects on O-acetylation activity. To investigate the functional consequences of SNPs in the NAT2 coding region on the O-acetylation of N-hydroxy heterocyclic amines, reference NAT2*4 and NAT2 variant alleles possessing one were cloned and expressed in yeast (Schizosaccaromyces pombe). T111C, C282T, C481T, C759T, and A803G (K268R) SNPs did not significantly (p > 0.05) modify O-acetylation catalysis with N--OH--PhIP or N--OH--MeIQx. C190T (R64W), G191A (R64Q), T341C (I114T), A434C (E145P), G590A (R197Q) and A845C (K282T) significantly (p < 0.01) reduced the O-acetylation of both N--OH--PhIP and N--OH--MeIQx, whereas G857A (G286E) significantly (p < 0.05) decreased catalytic activity towards the O-acetylation of N--OH--MeIQx but not N--OH--PhIP. These results have important implications towards the interpretation of molecular epidemiological studies of NAT2 genotype and cancer risk.

Base-displaced intercalated structure of the food mutagen 2-amino-3-methylimidazo[4,5-f]quinoline in the recognition sequence of the NarI restriction enzyme, a hotspot for -2 bp deletions

The solution structure of the oligodeoxynucleotide 5'-d(CTCGGCXCCATC)-3'.5'-d(GATGGCGCCGAG)-3' containing the heterocyclic amine 8-[(3-methyl-3H-imidazo[4,5-f]quinolin-2-yl)amino]-2'-deoxyguanosine adduct (IQ) at the third guanine in the NarI restriction sequence, a hot spot for -2 bp frameshifts, is reported. Molecular dynamics calculations restrained by distances derived from 24 (1)H NOEs between IQ and DNA, and torsion angles derived from (3)J couplings, yielded ensembles of structures in which the adducted guanine was displaced into the major groove with its glycosyl torsion angle in the syn conformation. One proton of its exocyclic amine was approximately 2.8 A from an oxygen of the 5' phosphodiester linkage, suggesting formation of a hydrogen bond. The carcinogen-guanine linkage was defined by torsion angles alpha' [N9-C8-N(IQ)-C2(IQ)] of 159 +/- 7 degrees and beta' [C8-N(IQ)-C2(IQ)-N3(IQ)] of -23 +/- 8 degrees . The complementary cytosine was also displaced into the major groove. This allowed IQ to intercalate between the flanking C.G base pairs. The disruption of Watson-Crick hydrogen bonding was corroborated by chemical-shift perturbations for base aromatic protons in the complementary strand opposite to the modified guanine. Chemical-shift perturbations were also observed for (31)P resonances corresponding to phosphodiester linkages flanking the adduct. The results confirmed that IQ adopted a base-displaced intercalated conformation in this sequence context but did not corroborate the formation of a hydrogen bond between the IQ quinoline nitrogen and the complementary dC [Elmquist, C. E.; Stover, J. S.; Wang, Z.; Rizzo, C. J. J. Am. Chem. Soc. 2004, 126, 11189-11201].

Biochemical basis of genotoxicity of heterocyclic arylamine food mutagens: Human DNA polymerase eta selectively produces a two-base deletion in copying the N2-guanyl adduct of 2-amino-3-methylimidazo[4,5-f]quinoline but not the C8 adduct at the NarI G3 site

Heterocyclic arylamines are highly mutagenic and cause tumors in animal models. The mutagenicity is attributed to the C8- and N2-G adducts, the latter of which accumulates due to slower repair. The C8- and N 2-G adducts derived from 2-amino-3-methylimidazo[4,5-f]quinoline (IQ) were placed at the G1 and G3 sites of the NarI sequence, in which the G3 site is an established hot spot for frameshift mutation with the model arylamine derivative 2-acetylaminofluorene but G1 is not. Human DNA polymerase (pol) eta extended primers beyond template G-IQ adducts better than did pol kappa and much better than pol iota or delta. In 1-base incorporation studies, pol eta inserted C and A, pol iota inserted T, and pol kappa inserted G. Steady-state kinetic parameters were measured for these dNTPs opposite the C8- and N 2-IQ adducts at both sites, being most favorable for pol eta. Mass spectrometry of pol eta extension products revealed a single major product in each of four cases; with the G1 and G3 C8-IQ adducts, incorporation was largely error-free. With the G3 N 2-IQ adduct, a -2 deletion occurred at the site of the adduct. With the G1 N 2-IQ adduct, the product was error-free at the site opposite the base and then stalled. Thus, the pol eta products yielded frame-shifts with the N 2 but not the C8 IQ adducts. We show a role for pol eta and the complexity of different chemical adducts of IQ, DNA position, and DNA polymerases.

Urinary metabolite profiling reveals CYP1A2-mediated metabolism of NSC686288 (aminoflavone)

NSC686288 [aminoflavone (AF)], a candidate chemotherapeutic agent, possesses a unique antiproliferative profile against tumor cells. Metabolic bioactivation of AF by drug-metabolizing enzymes, especially CYP1A monooxygenases, has been implicated as an underlying mechanism for its selective cytotoxicity in several cell culture-based studies. However, in vivo metabolism of AF has not been investigated in detail. In this study, the structural identities of 13 AF metabolites (12 of which are novel) in mouse urine or from microsomal incubations, including three monohydroxy-AFs, two dihydroxy-AFs and their sulfate and glucuronide conjugates, as well as one N-glucuronide, were determined by accurate mass measurements and liquid chromatography-tandem mass spectrometry fragmentation patterns, and a comprehensive map of the AF metabolic pathways was constructed. Significant differences between wild-type and Cyp1a2-null mice, within the relative composition of urinary metabolites of AF, demonstrated that CYP1A2-mediated regioselective oxidation was a major contributor to the metabolism of AF. Comparisons between wild-type and CYP1A2-humanized mice further revealed interspecies differences in CYP1A2-mediated catalytic activity. Incubation of AF with liver microsomes from all three mouse lines and with pooled human liver microsomes confirmed the observations from urinary metabolite profiling. Results from enzyme kinetic analysis further indicated that in addition to CYP1A P450s, CYP2C P450s may also play some role in the metabolism of AF.

2-Dodecylcyclobutanone, a radiolytic product of palmitic acid, is genotoxic in primary human colon cells and in cells from preneoplastic lesions

The irradiation of fat results in the formation of 2-alkylcyclobutanones, a new class of food contaminants. Results of previous in vitro studies with primary human colon cells and in vivo experiments with rats fed with 2-alkylcyclobutanones indicated that these radiolytic derivatives may be genotoxic and enhance the progression of colon tumors. The underlying mechanisms of these effects, however, are not clearly understood. Therefore we performed additional investigations to elucidate the genotoxic potential of 2-dodecylcyclobutanone (2dDCB) that is generated from palmitic acid. In particular, we explored the relative sensitivities of human colon cells, representing different stages of tumor development and healthy colon tissues, respectively. HT29clone19A cells, LT97 adenoma cells and primary human epithelial cells were exposed to 2dDCB (150-2097 microM). We determined cytotoxic effects using trypan blue exclusion. Genotoxicity, reflected as strand breaks, was assessed using the alkaline version of the comet assay and chromosomal abnormalities were investigated by 24-color fluorescence-in-situ-hybridization. 2dDCB was cytotoxic in a time- and dose-dependent manner in LT97 adenoma cells and in freshly isolated primary cells but not in the human colon tumor cell line. Associated with this was a marked induction of DNA damage by 2dDCB in LT97 adenoma cells and in freshly isolated colonocytes, whereas in the HT29clone19A cells no strand breaks were detectable. A long-term incubation of LT97 adenoma cells with lower concentrations of 2dDCB revealed cytogenetic effects. In summary, 2dDCB was clearly genotoxic in healthy human colon epithelial cells and in cells representing preneoplastic colon adenoma. These findings provide additional evidence that this compound may be regarded as a possible risk factor for processes in colon carcinogenesis related to initiation and progression.

Molecular dynamics of a food carcinogen-DNA adduct in a replicative DNA polymerase suggest hindered nucleotide incorporation and extension

2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) is the most abundant of the carcinogenic heterocyclic aromatic amines in the human diet, and the major mutagenic effect of dietary PhIP is G-->T transversions. The major PhIP-derived DNA adduct is to C8 of guanine. We have investigated this adduct in a PhIP-induced mutational hotspot 5'-GGGA-3' of the Apc tumor suppressor gene, frequently mutated in mammalian colon tumors. We have carried out a molecular dynamics study to elucidate on a structural level nucleotide incorporation and extension opposite this major adduct during replication. The PhIP adduct was modeled into the ternary complex closed conformation of DNA polymerase RB69, at incorporation and extension positions, with normal cytosine or mismatched partner adenine. RB69 polymerase is a member of the B family as are most replicative eukaryotic DNA polymerases such as DNA polymerase alpha. These systems were subjected to molecular dynamics simulations with AMBER. Our results show that the adduct can reside on the major groove side of the modified DNA template opposite an incoming dCTP or dATP. In the case of the normal partner, disturbance to the active site is observed at the incorporation step, but there is less perturbance in the extension simulation. In the case of the mismatched partner, a less disturbed active site is observed during the incorporation step, but extension appears to be more difficult. Disturbances include adverse impacts on Watson-Crick hydrogen bonding in the nascent base pair, on the distance between the alpha-phosphate of the incoming dNTP and the primer terminus 3'-OH, and on critical protein interactions with the dNTP. However, in all of these cases, a near reaction ready distance (within 3.5 angstroms) between the 3'-terminal oxygen of the primer and the Palpha of the incoming nucleotide triphosphate is sampled occasionally (0.4-23.5% of the time). Thus, error-free bypass or the induction of a G-->T transversion mutation could occur at times and contribute to an extent to the mutagenic effect of PhIP. Polymerase stalling would be the more common outcome and in vivo could lead to switch to an error-prone bypass polymerase.

Age at exposure and Apc status influence the levels of 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP)-DNA adducts in mouse intestine and liver

We have previously shown that C57BL/6J-Min/+ (multiple intestinal neoplasia) mice, heterozygous for the Min mutation in the adenomatous polyposis coli gene, were more susceptible to intestinal tumorigenesis and had higher intestinal PhIP-DNA adduct levels after exposure to the food mutagen 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) on day 12 than on day 36 after birth [I.-L. Steffensen, H.A.J. Schut, J.E. Paulsen, A. Andreassen, J. Alexander, Intestinal tumorigenesis in multiple intestinal neoplasia mice induced by the food mutagen 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine: perinatal susceptibility, regional variation, and correlation with DNA adducts, Cancer Res. 61 (200l) 8689-8696]. In the present study, we have evaluated further whether this difference in susceptibility is related to adduct formation/removal, cell proliferation, apoptosis or expression of the nucleotide excision repair protein Xeroderma pigmentosum group A (XPA) in the intestines. Min/+ and +/+ (wild-type) mice were given a subcutaneous injection of 50 mg/kgbw PhIP on day 12 or 36, and the levels of PhIP-DNA adducts after 8, 12, 24 h, 3 or 7 days were quantified by use of 32P-postlabelling. In Min/+ mice, adduct levels were significantly higher after exposure on day 12 than on day 36 in the middle (1.5- to 8.5-fold) and distal (1.3- to 6.5-fold) small intestine from 8h to 3 days after administration of PhIP, but not in the colon and proximal small intestine. In the liver - a non-target organ for PhIP - adduct levels were 2.0- to 7.5-fold higher after exposure on day 12 than on day 36 from 8 to 24h after exposure. Adduct levels were generally higher in the middle (1.1- to 1.8-fold) and distal (1.1- to 2.0-fold) small intestines of Min/+ compared with +/+ mice after PhIP exposure on day 12, i.e. in the area of the intestines previously found also to have the highest number of tumors in Min/+ mice. PhIP increased cell proliferation and the number of apoptotic cells in the intestine and liver. However, the higher susceptibility to intestinal tumorigenesis in Min/+ mice exposed to PhIP at early age, or in Min/+ mice compared with +/+ mice, could not be explained by differences in cell proliferation, apoptosis or expression of the XPA repair protein.

Cytochrome P450 activation of arylamines and heterocyclic amines

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

DNA adduct formation of 14 heterocyclic aromatic amines in mouse tissue after oral administration and characterization of the DNA adduct formed by 2-amino-9H-pyrido[2,3-b]indole (AalphaC), analysed by 32P_HPLC

Heterocyclic aromatic amines (HAAs) are produced during cooking of proteinaceous food such as meat and fish. Humans eating a normal diet are regularly exposed to these food-borne substances. HAAs have proved to be carcinogenic in animals and to induce early lesions in the development of cancer. DNA adduct levels in mouse liver have been measured by 32P-HPLC after oral administration each of 14 different HAAs. The highest DNA adduct levels were detected for 3-amino-1-methyl-5H-pyrido[4,3-b]-indole (Trp-P-2), 3-amino-1,4-dimethyl-5H-pyrido[4,3-b]indole (Trp-P-1) and 2-amino-9H-pyrido[2,3-b]indole (AalphaC), respectively. To assess a relative risk in a human population, a relative risk index was calculated by combining the DNA adduct levels in mouse liver with human daily intake of heterocyclic amines in a US and in a Swedish population. Such calculations suggest that AalphaC presents the highest risk for humans, e.g. nine-fold higher compared with the most abundant amines in food, 2-amino-1-methyl-6-phenylimidazo[4,5-b]-pyridine (PhIP). Therefore, the distribution of DNA adducts in different tissues of mouse was investigated after oral administration of AalphaC. The highest AalphaC-DNA adduct levels were found in liver (137 adducts/10(8) normal nucleotides) followed by heart, kidney, lung, large intestine, small intestine, stomach and spleen, in descending order. To characterize the chemical structure of the major DNA adduct, chemical synthesis was performed. The major DNA adduct from the in vivo experiments was characterized by five different methods. On the basis of these results, the adduct was characterized as N2-(deoxyguanin-8-yl)-2-amino-9H-pyrido [2,3-b]indole. Considering the abundance of AalphaC not only in grilled meat, but also in other products like grilled chicken, vegetables and cigarette smoke and in light of the results of the present study, it is suggested that the human cancer risk for AalphaC might be underestimated.

A comparison of genotoxicity between three common heterocyclic amines and acrylamide

Heterocyclic amines (HCAs), a group of genotoxic compounds formed during the heating of proteinaceous food items, have been known since the late 1970s. However, the genotoxic effect of these compounds in the low dose region has not yet been thoroughly studied. Here we used a sensitive flow cytometer-based micronucleus assay in mice to determine the frequency of micronucleated erythrocytes (fMPCE) of the three common HCAs, 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx), and 2-amino-3-methylimidazo[4,5-f]quinoline (IQ), in the low dose region. We especially looked for any deviation from linearity of the dose-response curves. Male Balb/C mice were intra peritoneally injected with different doses of either PhIP (0-36 mg/kg b.w.), MeIQx (0-90 mg/kg b.w.) or IQ (0-40 mg/kg b.w.). In the case of PhIP, we found a significant dose-response relationship, while MeIQx and IQ did not display an increased fMPCE level. This flow cytometer method allows for determination of the DNA content of micronuclei. All three HCAs tested here yielded a low DNA content of micronuclei, indicating that they do not possess aneugenic effects. A comparison between the HCAs and acrylamide (AA), another heat induced genotoxic compound, revealed that the slope of the dose-response curve is about 10 times steeper for PhIP than AA. In spite of this, AA probably constitutes a higher human risk than HCAs since the intake is about a 100- to 1000-fold higher than the intake of HCAs.

Modulation of hepatic cytochromes P450 and phase II enzymes by dietary doses of sulforaphane in rats: Implications for its chemopreventive activity

The principal objectives of our study were to ascertain whether sulforaphane, at dietary levels of intake, modulates rat hepatic cytochrome P450 and phase II enzyme systems and to evaluate the impact of such changes in the chemopreventive activity of this isothiocyanate. Animals were exposed to sulforaphane in their drinking water for 10 days, equivalent to daily doses of 3 and 12 mg/kg. Depentylation of pentoxyresorufin decreased and was paralleled by a decline in CYP2B apoprotein levels. At the higher dose, erythromycin N-demethylase activity declined and was accompanied by a similar decrease in CYP3A2 apoprotein levels. However, sulforaphane treatment upregulated CYP1A2 levels, determined immunologically, but the dealkylations of methoxy- and ethoxyresorufin were not similarly increased. Hepatic S9 preparations from sulforaphane-treated rats were less effective than control preparations in converting IQ (2-amino-3-methylimidazo-[4,5-f]quinoline) to mutagenic intermediates in the Ames test. To clarify the underlying mechanism, in vitro studies were undertaken. In beta-naphthoflavone-treated rats, the inhibition by sulforaphane of the O-dealkylations of methoxy- and ethoxyresorufin was enhanced if the isothiocyanate was preincubated in the presence of NADPH. It may be inferred that sulforaphane induces hepatic CYP1A2 but the enzyme is not catalytically competent because of bound sulforaphane metabolite(s). Finally, sulforaphane stimulated, in a dose-dependent fashion, quinone reductase but failed to influence glutathione S-transferase, epoxide hydrolase and glucuronosyl transferase activities. It is concluded that, even at dietary doses, sulforaphane can modulate the xenobiotic-metabolising enzyme systems, shifting the balance of carcinogen metabolism toward deactivation, and this may be an important mechanism of its chemopreventive activity.

Effect of Black Tea Intake on the Excretion of Mutagens in the Urine of Volunteers Taking a Beef Meal

The objective of this study was to investigate in a crossover study conducted in human volunteers whether black tea intake modulates the metabolism of heterocyclic amines, consumed in the form of well-cooked beefburgers, as exemplified by the excretion of mutagens in the urine. Mutagens were extracted from urine with blue rayon, and mutagenic activity was determined in the Ames test, in the presence of an activation system derived from Aroclor 1254–induced rats, and employing the Salmonella typhimurium O-acetylase over-expressing YG1024 bacterial strain. Volunteers consumed three well-cooked beefburgers, whereas a concurrently cooked fourth burger was analyzed for mutagenic activity. Following intake of the burgers, an increase in urinary mutagenic activity was observed, and mutagenic activity was completely excreted within 24 hours. A good correlation was obtained between the intake and excretion of mutagenic activity. The volunteers consumed the same burger meal on two different occasions, once following intake of 10 cups of strong black tea, and the second following intake of a corresponding volume of water. Urine was collected by each volunteer for 24 hours after the meal, and compliance was ascertained utilizing the excretion of p-aminobenzoic acid. The mutagenic ratio, defined as the ratio of urinary mutagenicity over the intake of mutagenicity, was not altered by the black tea, being 0.142 and 0.135 during the water and tea intake, respectively. It is inferred that short-term intake of high levels of black tea, as part of an otherwise normal diet, does not modulate the human metabolism of heterocyclic amines.

Relationship between Content and Activity of Cytochrome P450 and Induction of Heterocyclic Amine DNA Adducts in Human Liver Samples In vivo and In vitro

This study was designed to estimate a correlation between metabolic activation phenotypes and formation of DNA adducts by heterocyclic amines (HCA) in 15 liver samples from healthy donors. The correlation between the amount of endogenous DNA adducts and the content of cytochrome P450 in human liver samples in vivo was statistically significant at r2 = 0.71 and P &lt; 0.005. Furthermore, the isolated human liver microsomes were treated in vitro with two HCAs, 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) and 2-amino-9H-pyrido[2,3-b]indole (AαC), which have been recognized to induce two DNA adducts: 3′,5′-diphosphate-N-(2′-deoxyguanosin-8-yl)-PhIP (3′,5′-pdGp-C8-PhIP) and 3′,5′-diphosphate-N-(2′-deoxyguanosin-8-yl)-AαC (3′,5′-pdGp-C8-AαC). The correlations between the amount of DNA adducts induced by both compounds in vitro and the content of cytochrome P450 in human microsomes are statistically significant at r2 = 0.69 and r2 = 0.62 (P &lt; 0.001), respectively. Furthermore, the level of DNA adducts after treatment with PhIP and AαC correlated with the activities of three isozymes of cytochrome P450: CYP1A1, CYP1A2, and CYP3A4. Therefore, three chemical inhibitors were used in the experiments: ellipticine against CYP1A1, furafylline against CYP1A2, and troleandomycin against CYP3A4. The highest inhibition levels in the formation of 3′,5′-pdGp-C8-PhIP and 3′,5′-pdGp-C8-AαC adducts were estimated to occur in the presence of furafylline at 56% and 69%, respectively. Ellipticine was involved in the inhibition of 40% of 3′,5′-pdG-C8-PhIP adducts and in only 18% of the inhibition of 3′,5′-pdGp-C8-AαC adducts. Troleandomycin did not significantly inhibit the formation of 3′,5′-pdGp-C8-PhIP adducts under these conditions, but it inhibited the formation of 31% of the 3′,5′-pdGpC8-AαC adducts. We conclude that the formation of DNA adducts can be used as a relevant marker of interindividual variability in the metabolic activation of HCAs in humans.

An improved method for the extraction of mutagens from human urine and cooked meat using blue rayon

A reproducible method has been developed and validated that allows the detection of mutagenic material in human urine following the intake of a meal containing pan-fried beef patties. The mutagens are extracted from the urine with blue rayon and eluted with methanol/ammonia (100:1). Using 14C-2-amino-3-methylimidazo[4,5-f]quinoline (14C-IQ) as a tracer, the extraction efficiency of heterocyclic amines was consistently found to exceed 90%. It was observed that the critical factor in attaining high extraction efficiencies was the concentration of blue rayon, which depended on the nature of the matrix. Constituents of human urine were demonstrated to compete with IQ for the binding sites on the blue rayon. For the determination of the mutagenic activity in the extracted urine, the O-acetylase over-expressing Salmonella typhimurium strain YG1024 was utilised, and the Ames test procedure was optimised with respect to S9 concentration and incorporation of a pre-incubation step. Using the developed procedure, a marked rise in mutagenic activity, determined both in the absence and presence of an activation system, was evident in human urine following the intake of a beef patty meal. Normal mutagenicity levels were restored within 24 h. Finally, a method is described, once again employing blue rayon, for the extraction of heterocyclic amines from cooked beef patties with a 92% efficiency, as exemplified by IQ.

Monitoring of DNA Adducts in Humans and 32P-Postlabelling Methods. A Review

DNA adduct formation in humans is a promising biomarker for elucidating the molecular epidemiology of cancer. For detection of DNA adducts, the most widely used methods include mass spectroscopy, fluorescence spectroscopy, immunoassays and 32P-postlabelling. Among them, the 32P-postlabelling method appears to meet best the criteria of sensitivity and amount of DNA needed, and, therefore, is one of the most appropriate methods for biomonitoring of human DNA adducts. Most classes of carcinogens have been subjected to 32P-postlabelling analysis, ranging from bulky and/or aromatic compounds to small and/or aliphatic compounds; it has also been used, with modifications, to detect apurinic sites in DNA, oxidative damage to DNA, UV-induced photodimers and, to a lesser extent, DNA damage caused by cytotoxic drugs. It has been used in human biomonitoring studies to detect DNA damage from occupational exposure to carcinogens, and also from environmental (i.e. non-occupational) exposures. It has also led to the discovery of the presence of numerous modifications in DNA arising from endogenous processes. The principle of the method is the enzymatic digestion of DNA to nucleotides, 5'-labelling of these nucleotides with an isotopically labelled phosphate group, and the resolution, detection and quantitation of the labelled products. Since the development of the original procedure in the early 1980s, many methods have been developed to increase the sensitivity by enrichment of modified nucleotides prior to labelling. The review presents the individual 32P-postlabelling techniques (standard procedure, enrichment methods) and a critical evaluation of these assays, besides reviewing the applications of the method to different DNA modifications, and its utilization in human biomonitoring studies. A review with 179 references.

DNA adduct formation by 7H-dibenzo[c,g]carbazole and its tissue- and organ-specific derivatives in Chinese hamster V79 cell lines stably expressing cytochrome P450 enzymes

The cytochrome P4501A subfamily (CYP1A) is involved in the metabolic activation of 7H-dibenzo[c,g]carbazole (DBC) and its tissue- and organ-specific derivatives, N-methyldibenzo[c,g]carbazole (MeDBC)and 5,9-dimethyldibenzo[c,g]carbazole (diMeDBC). In this study, we have evaluated the relationship between the tissue specificity and (32)P-postlabeled adduct patterns produced by these compounds by using a panel of Chinese hamster V79 cell lines stably expressing human CYP1A1 and CYP1A2 and/or N-acetyltransferase. Treatment of the parental cell lines V79MZ and V79NH, which are devoid of any CYP activity, with DBC and its derivatives did not result in detectable adducts. The highest DNA adduct levels were found in CYP1A1-expressing V79MZh1A1 cells after DBC and MeDBC treatment (24.5 +/- 7.2 and 16.2 +/- 3.6 adducts/10(8) nucleotides, respectively). Exposure of this cell line to DBC resulted in five distinct spots, while six spots with different chromatographic mobilities were detected in MeDBC-treated cells. DiMeDBC produced only very low levels of DNA adducts in V79MZh1A1 cells. DBC and MeDBC formed relatively low levels of DNA adducts in CYP1A2-expressing V79MZh1A2 cells (0.7 +/- 0.2 and 2.1 +/- 1.2 adducts/10(8) nucleotides, respectively). DBC formed three weak spots and MeDBC five spots in V79MZh1A2 cells, and all the spots had different chromatographic mobilities. In contrast, diMeDBC did not induce any DNA adducts in these cells, although diMeDBC induced a significant dose-dependent increase in micronucleus frequency under similar treatment conditions (r = 0.76; P < 0.001). The significant increase in DNA damage in the Comet assay following incubation of exposed cells with a repair-specific endonuclease (Fpg protein) suggests that base modifications such as 8-oxodG or Fapy-adducts might be responsible for the genotoxicity of diMeDBC in V79MZh1A2 cells. The similarities between the DNA adduct patterns produced by DBC and MeDBC in V79MZh1A1 and V79MZh1A2 cells suggest that biotransformation mediated via CYP1A1 and CYP1A2 might depend on a PAH-type pathway involving the aromatic ring system.

Antimutagenic activity of tea: role of polyphenols

PURPOSE OF REVIEW

Tea is considered to be one of the most promising dietary chemopreventive agents and, consequently, it is being studied extensively worldwide. Despite the fact that tea has proved very efficient in affording protection against chemical-induced cancer in animal models of the disease, epidemiological studies do not always support the laboratory findings, so that the value of tea as a human anticarcinogen may be considered as 'not proven'. A major mechanism of the anticarcinogenic activity of tea in animals is impairment of the interaction of carcinogens with DNA leading to mutations. The antimutagenic activity of tea as well as the underlying mechanisms will be reviewed, and the role of polyphenols, the postulated bioactive components, and caffeine will be critically evaluated.

RECENT FINDINGS

In rats, exposure to tea modulated the disposition of heterocyclic amines, a major group of food-borne carcinogens, stimulating the pathways that lead to deactivation, and this is concordant with the established ability of tea to modulate the carcinogen-metabolizing enzyme systems. These observations provide a rational mechanism for the anticarcinogenic activity of tea in animals.

SUMMARY

The beneficial activities of tea have always been attributed to the polyphenols, as these are present in tea at substantial concentrations and are endowed with antioxidant activity. It is becoming increasingly evident, however, that the bioavailability of these compounds is poor as a result of limited absorption and presystemic metabolism by mammalian and microbial enzymes. We propose that the biological activity of tea may be mediated by caffeine and microbial metabolites of polyphenols.

Use of transgenic and mutant animal models in the study of heterocyclic amine-induced mutagenesis and carcinogenesis

Heterocyclic amines (HCAs) are potent mutagens generated during the cooking of meat and fish, and several of these compounds produce tumors in conventional experimental animals. During the past 5 years or so, HCAs have been tested in a number of novel in vivo murine models, including the following: lacZ, lacI, cII, c-myc/lacZ, rpsL, and gptDelta. transgenics, XPA-/-, XPC-/-, Msh2+/-, Msh2-/- and p53+/- knock-outs, Apc mutant mice (ApcDelta716, Apc1638N, Apcmin), and A33DeltaNbeta-cat knock-in mice. Several of these models have provided insights into the mutation spectra induced in vivo by HCAs in target and non-target organs for tumorigenesis, as well as demonstrating enhanced susceptibility to HCA-induced tumors and preneoplastic lesions. This review describes several of the more recent reports in which novel animal models were used to examine HCA-induced mutagenesis and carcinogenesis in vivo, including a number of studies which assessed the inhibitory activities of chemopreventive agents such as 1,2-dithiole-3-thione, conjugated linoleic acids, tea, curcumin, chlorophyllin-chitosan, and sulindac.