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

Human CYP1A1-activated aneugenicity of aflatoxin B1 in mammalian cells and its combined effect with benzo(a)pyrene

Aflatoxin B1 (AFB1) is the most toxic mycotoxin and a proven human carcinogen that requires metabolic activation, known by cytochrome P450 (CYP) 1A2 and 3A4. Previous evidence showed that AFB1 is activated by human recombinant CYP1A1 expressed in budding yeast. Yet, the toxicity, in particular the genotoxicity of the reactive metabolites formed from AFB1 remains unclear. Humans could be exposed to both AFB1 and benzo(a)pyrene (BaP) simultaneously, thus we were interested in their combined genotoxic effects subsequent to metabolic activation by CYP1A1. In this study, molecular docking of AFB1 to human CYP1A1 indicated that AFB1 is valid as a substrate. In the incubations with AFB1 in human CYP1A1-expressed microsomes, AFM1 as a marking metabolite of AFB1 was detected. Moreover, AFB1 induced micronucleus formation in a Chinese hamster V79-derived cell line and in a human lung epithelial BEAS-2B cell line, both expressing recombinant human CYP1A1, V79-hCYP1A1 and 2B-hCYP1A1 cells, respectively. Immunofluorescence of centromere protein B stained micronuclei was dominant in AFB1-treated BEAS-2B cells exposed to AFB1, suggesting an aneugenic effect. Moreover, AFB1 elevated the levels of ROS, 8-OHdG, AFB1-DNA adduct, and DNA breaks in 2B-hCYP1A1 cells, compared with those in the parental BEAS-2B cells. Meanwhile, AFB1 increased CYP1A1, RAD51, and γ-H2AX protein levels in 2B-hCYP1A1 cells, which were attenuated by the CYP1A1 inhibitor bergamottin. Co-exposure of AFB1 with BaP increased 8-OHdG, RAD51, and γ-H2AX levels (indicating DNA damage). In conclusion, AFB1 could be activated by human CYP1A1 for potent aneugenicity, which may be further enhanced by co-exposure to BaP.

Yeast-based genotoxicity tests for assessing DNA alterations and DNA stress responses: a 40-year overview

By damaging DNA molecules, genotoxicants cause genetic mutations and also increase human susceptibility to cancers and genetic diseases. Over the past four decades, several assays have been developed in the budding yeast Saccharomyces cerevisiae to screen potential genotoxic substances and provide alternatives to animal-based genotoxicity tests. These yeast-based genotoxicity tests are either DNA alteration-based or DNA stress-response reporter-based. The former, which came first, were developed from the genetic studies conducted on various types of DNA alterations in yeast cells. Despite their limited throughput capabilities, some of these tests have been used as short-term genotoxicity tests in addition to bacteria- or mammalian cell-based tests. In contrast, the latter tests are based on the emergent transcriptional induction of DNA repair-related genes via activation of the DNA damage checkpoint kinase cascade triggered by DNA damage. Some of these reporter assays have been linked to DNA damage-responsive promoters to assess chemical carcinogenicity and ecotoxicity in environmental samples. Yeast-mediated genotoxicity tests are being continuously improved by increasing the permeability of yeast cell walls, by the ectopic expression of mammalian cytochrome P450 systems, by the use of DNA repair-deficient host strains, and by integrating them into high-throughput formats or microfluidic devices. Notably, yeast-based reporter assays linked with the newer toxicogenomic approaches are becoming powerful short-term genotoxicity tests for large numbers of compounds. These tests can also be used to detect polluted environmental samples, and as effective screening tools during anticancer drug development.

An in vitro system for measuring genotoxicity mediated by human CYP3A4 in Saccharomyces cerevisiae

P450 activity is required to metabolically activate many chemical carcinogens, rendering them highly genotoxic. CYP3A4 is the most abundantly expressed P450 enzyme in the liver, accounting for most drug metabolism and constituting 50% of all hepatic P450 activity. CYP3A4 is also expressed in extrahepatic tissues, including the intestine. However, the role of CYP3A4 in activating chemical carcinogens into potent genotoxins is unclear. To facilitate efforts to determine whether CYP3A4, per se, can activate carcinogens into potent genotoxins, we expressed human CYP3A4 in the DNA-repair mutant (rad4 rad51) strain of budding yeast Saccharomyces cerevisiae and tested the novel, recombinant yeast strain for ability to report CYP3A4-mediated genotoxicity of a well-known genotoxin, aflatoxin B1 (AFB1 ). Yeast microsomes containing human CYP3A4, but not those that do not contain CYP3A4, were active in hydroxylation of diclofenac, a known CYP3A4 substrate drug, a result confirming CYP3A4 activity in the recombinant yeast strain. In cells exposed to AFB1 , the expression of CYP3A4 supported DNA adduct formation, chromosome rearrangements, cell death, and expression of the large subunit of ribonucleotide reductase, Rnr3, a marker of DNA damage. Expression of CYP3A4 also conferred sensitivity in rad4 rad51 mutants exposed to colon carcinogen, 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx). These data confirm the ability of human CYP3A4 to mediate the genotoxicity of AFB1 , and illustrate the usefulness of the CYP3A4-expressing, DNA-repair mutant yeast strain for screening other chemical compounds that are CYP3A4 substrates, for potential genotoxicity. Environ. Mol. Mutagen. 58:217-227, 2017. © 2017 Wiley Periodicals, Inc.

CYP1A1 I462V polymorphism is associated with reduced genotoxicity in yeast despite positive association with increased cancer risk

CYP1A1 functions in detoxifying xenobiotics but occasionally converts compounds into potent genotoxins. CYP1A1 activates polyaromatic hydrocarbons, such as benzo[a]pyrene 7,8 dihydrodiol (BaP-DHD), rendering them genotoxic. Particular alleles of CYP1A1, such as CYP1A1 I462V have been correlated with a higher incidence of breast and lung cancer, but it is unknown whether these variants express enzymes in vivo that are more potent in generating genotoxins. We individually expressed CYP1A1 (CYP1A1.1), CYP1A1 T461N (CYP1A1.4) and I462V (CYP1A1.2) alleles in wild-type and DNA repair deficient mutant strains of Saccharomyces cerevisiae (budding yeast) and asked which yeast strains exhibited the highest levels of carcinogen-associated genotoxicity after exposure to BaP-DHD, aflatoxin B1 (AFB₁), and heterocyclic aromatic amines (HAAs). We measured carcinogen-associated recombination, Rad51 foci, and carcinogen-associated toxicity in a DNA repair mutant deficient in both nucleotide excision repair and recombinational repair. CYP1A1 activity was confirmed by measuring ethoxyresorufin-O-deethylation (EROD) activities. Our data indicate that CYP1A1 I462V allele confers the least carcinogen-associated genotoxicity, compared to CYP1A1; however, results vary depending on the chemical carcinogen and the genotoxic endpoint. We speculate that the cancer-associated risk of CYP1A1 I462V may be caused by exposure to other xenobiotics.

Scientific concepts and applications of integrated discrete multiple organ co-culture technology

Over several decades, animals have been used as models to investigate the human-specific drug toxicity, but the outcomes are not always reliably extrapolated to the humans in vivo. Appropriate in vitro human-based experimental system that includes in vivo parameters is required for the evaluation of multiple organ interaction, multiple organ/organ-specific toxicity, and metabolism of xenobiotic compounds to avoid the use of animals for toxicity testing. One such versatile in vitro technology in which human primary cells could be used is integrated discrete multiple organ co-culture (IdMOC). IdMOC system adopts wells-within-well concept that facilitates co-culture of cells from different organs in a discrete manner, separately in the respective media in the smaller inner wells which are then interconnected by an overlay of a universal medium in the large containing well. This novel in vitro approach mimics the in vivo situation to a great extent, and employs cells from multiple organs that are physically separated but interconnected by a medium that mimics the systemic circulation and provides for multiple organ interaction. Applications of IdMOC include assessment of multiple organ toxicity, drug distribution, organ-specific toxicity, screening of anticancer drugs, metabolic cytotoxicity, etc.

Activation of aflatoxin B1 by expression of human CYP1A2 polymorphisms in Saccharomyces cerevisiae

Human susceptibility to environmental carcinogens is highly variable and depends on multiple genetic factors, including polymorphisms in cytochrome P450 genes. Although epidemiological studies have identified individual polymorphisms in cytochrome P450 genes that may alter cancer risk, there is often conflicting data about whether such polymorphisms alter the genotoxicity of environmental carcinogens. This is particularly true of the CYP1A2 polymorphisms that confer differential activation of multiple human carcinogens. To determine whether a single cytochrome P450 polymorphism confers higher levels of carcinogen-associated genotoxicity, we chose an organism that lack enzymes to metabolically activate aflatoxins and expressed individual human P450 genes in budding yeast. We measured the frequencies of recombination, Rad51 foci formation, 7-methoxyresorufin O-demethylase activities, and the concentrations of carcinogen-associated DNA adducts in DNA repair proficient yeast expressing P450 polymorphisms after exposure to aflatoxin B1 (AFB1).We measured growth of rad4 rad51 cells expressing CYP1A2 polymorphisms while exposed to AFB1. We observed that there was significantly less AFB1-associated genotoxicity in yeast expressing CYP1A2 I386F, while yeast expressing CYP1A2 C406Y exhibited intermediate levels of genotoxicity compared to yeast expressing CYP1A2 D348N or wild type. We conclude that differences in carcinogen genotoxicity can be observed in yeast expressing different CYP1A2 alleles. This is the first report that carcinogen-associated P450 polymorphisms can be studied in yeast.

Definition of metabolism-dependent xenobiotic toxicity with co-cultures of human hepatocytes and mouse 3T3 fibroblasts in the novel integrated discrete multiple organ co-culture (IdMOC) experimental system: results with model toxicants aflatoxin B1, cyclophosphamide and tamoxifen

The integrated discrete multiple organ co-culture system (IdMOC) allows the co-culturing of multiple cell types as physically separated cells interconnected by a common overlying medium. We report here the application of IdMOC with two cell types: the metabolically competent primary human hepatocytes, and a metabolically incompetent cell line, mouse 3T3 fibroblasts, in the definition of the role of hepatic metabolism on the cytotoxicity of three model toxicants: cyclophosphamide (CPA), aflatoxin B1 (AFB) and tamoxifen (TMX). The presence of hepatic metabolism in IdMOC with human hepatocytes was demonstrated by the metabolism of the P450 isoform 3A4 substrate, luciferin-IPA. The three model toxicants showed three distinct patterns of cytotoxic profile: TMX was cytotoxic to 3T3 cells in the absence of hepatocytes, with slightly lower cytotoxicity towards both 3T3 cells and hepatocytes in the IdMOC. AFB was selective toxic towards the human hepatocytes and relatively noncytotoxic towards 3T3 cells both in the presence and absence of the hepatocytes. CPA cytotoxicity to the 3T3 cells was found to be significantly enhanced by the presence of the hepatocytes, with the cytotoxicity dependent of the number of hepatocytes, and with the cytotoxicity attenuated by the presence of a non-specific P450 inhibitor, 1-aminobenzotriazole. We propose here the following classification of toxicants based on the role of hepatic metabolism as defined by the human hepatocyte-3T3 cell IdMOC assay: type I: direct-acting cytotoxicants represented by TMX as indicated by cytotoxicity in 3T3 cells in the absence of hepatocytes; type II: metabolism-dependent cytotoxicity represented by AFB1 with effects localized within the site of metabolic activation (i. e. hepatocytes); and type III: metabolism-dependent cytotoxicity with metabolites that can diffuse out of the hepatocytes to cause toxicity in cells distal from the site of metabolism, as exemplified by CPA.

Effects of furanocoumarins from apiaceous vegetables on the catalytic activity of recombinant human cytochrome P-450 1A2

Inhibition of cytochrome P-450 1A2 (CYP1A2)-mediated activation of procarcinogens may be an important chemopreventive mechanism. Consumption of apiaceous vegetables (rich in furanocoumarins) inhibits CYP1A2 in humans. Because many furanocoumarins are potent inhibitors of several CYPs, we characterized the effects of three furanocoumarins from apiaceous vegetables on human CYP1A2 (hCYP1A2). We assessed hCYP1A2 methoxyresorufin O-demethylase (MROD) activity using microsomes from Saccharomyces cerevisiae expressing hCYP1A2. Isopimpinellin exhibited mechanism-based inactivation (MBI) of hCYP1A2 (K(i) = 1.2 μM, k (inact) = 0.34 min⁻¹, and partition ratio = 8). Imperatorin and trioxsalen were characterized as mixed inhibitors with K(i) values of 0.007 and 0.10 μM, respectively. These results indicate that even if present at low levels in apiaceous vegetables, imperatorin, trioxsalen and isopimpinellin may contribute significantly to CYP1A2 inhibition and potentially decreased procarcinogen activation. Moreover, the in vivo effect of isopimpinellin on CYP1A2 may be longer lasting compared to reversible inhibitors.

Chromosome instability in human hepatocellular carcinoma depends on p53 status and aflatoxin exposure

Hepatocellular carcinoma (HCC) is a heterogeneous disease triggered by various risk factors and frequently characterized by chromosome instability. This instability is considered to be caused primarily by Hepatitis B virus (HBV), although aflatoxin B1 (AFB1), a potent fungal mutagen is also suspected to influence chromosomal repair. We studied 90 HCCs from Italy, the country with the highest incidence of hepatocellular carcinoma in Europe, 81 samples from France and 52 specimens from Shanghai, in a region where intake of AFB1 via the diet is known to be high. All 223 tumours were characterized for 15 different genomic targets, including allelic loss at 13 chromosome arms and mutations of beta-catenin and p53 genes. Despite disparity in risk-factor distribution, Italian and French cases did not significantly differ for 14 of the 15 targets tested. beta-Catenin and p53 displayed moderate and similar mutation rates (18-29% of cases) in European series. By contrast, tumours from Shanghai were significantly different, with a lower mutation rate for beta-catenin (4% vs. 26%, p<0.0003) and a higher mutation rate for p53 (48% vs. 22%, p<0.0001) when compared with tumours of European origin. The Arg249Ser mutation, hallmark of exposure to AFB1, represented half of the changes in p53 in Shanghai. Furthermore, when stratified for the presence of HBV or p53 mutations, chromosome instability was always higher in Chinese than in European patients. This difference was particularly strong in p53-wildtype tumours (fractional allelic loss, 29.4% vs. 16.7%, p<0.0001). We suggest that AFB1-associated mutagenesis represents a plausible cause for the higher chromosome instability observed in Chinese HCCs, when compared with European primary liver carcinomas.

Apiaceous vegetable constituents inhibit human cytochrome P-450 1A2 (hCYP1A2) activity and hCYP1A2-mediated mutagenicity of aflatoxin B1

In humans, apiaceous vegetables (carrots, parsnips, celery, parsley, etc.) inhibit cytochrome P-450 1A2, a biotransformation enzyme known to activate several procarcinogens, including aflatoxin B1 (AFB). We evaluated eight phytochemicals from apiaceous vegetables for effects on human cytochrome P-450 1A2 (hCYP1A2) activity using a methoxyresorufin O-demethylase (MROD) assay and a trp-recombination assay. Saccharomyces cerevisiae was used for heterologous CYP1A2 expression and this yeast strain is also diploid and auxotrophic for tryptophan due to mutations in the trp5 alleles. When these two alleles undergo AFB-induced mitotic recombination, gene conversion occurs, allowing yeast to grow in the absence of tryptophan. The apiaceous constituents psoralen, 5-methoxypsoralen (5-MOP), 8-methoxypsoralen (8-MOP), and apigenin were potent inhibitors of hCYP1A2-mediated MROD activity in yeast microsomes, whereas quercetin was a modest hCYP1A2 inhibitor. Naringenin, caffeic acid, and chlorogenic acid did not inhibit hCYP1A2-mediated MROD activity. The 2-h pretreatment of intact yeast cells with psoralen, 5-MOP, and 8-MOP significantly improved cell survival after subsequent 4-h AFB treatment and reduced hCYP1A2-mediated mutagenicity of AFB. Apigenin also significantly decreased mutagenicity. These results suggest that in vivo CYP1A2 inhibition by apiaceous vegetables may be due to the phytochemicals present and imply that apiaceous vegetable intake may be chemopreventive by inhibiting CYP1A2-mediated carcinogen activation.

Analysis of cellular responses to aflatoxin B(1) in yeast expressing human cytochrome P450 1A2 using cDNA microarrays

Aflatoxin B1 (AFB(1)) is a potent human hepatotoxin and hepatocarcinogen produced by the mold Aspergillus flavus. In human, AFB(1) is bioactivated by cytochrome P450 (CYP450) enzymes, primarily CYP1A2, to the genotoxic epoxide that forms N(7)-guanine DNA adducts. To characterize the transcriptional responses to genotoxic insults from AFB(1), a strain of Saccharomyces cerevisiae engineered to express human CYP1A2 was exposed to doses of AFB(1) that resulted in minimal lethality, but substantial genotoxicity. Flow cytometric analysis demonstrated a dose and time dependent S phase delay under the same treatment conditions, indicating a checkpoint response to DNA damage. Replicate cDNA microarray analyses of AFB(1) treated cells showed that about 200 genes were significantly affected by the exposure. The genes activated by AFB(1)-treatment included RAD51, DUN1 and other members of the DNA damage response signature reported in a previous study with methylmethane sulfonate and ionizing radiation [A.P. Gasch, M. Huang, S. Metzner, D. Botstein, S.J. Elledge, P.O. Brown, Genomic expression responses to DNA-damaging agents and the regulatory role of the yeast ATR homolog Mec1p, Mol. Biol. Cell 12 (2001) 2987-3003]. However, unlike previous studies using highly cytotoxic doses, environmental stress response genes [A.P. Gasch, P.T. Spellman, C.M. Kao, O. Carmel-Harel, M.B. Eisen, G. Storz, D. Botstein, P.O. Brown, Genomic expression programs in the response of yeast cells to environmental changes, Mol. Biol. Cell 11 (2000) 4241-4257] were largely unaffected by our dosing regimen. About half of the transcripts affected are also known to be cell cycle regulated. The most strongly repressed transcripts were those encoding the histone genes and a group of genes that are cell cycle regulated and peak in M phase and early G1. These include most of the known daughter-specific genes. The rapid and coordinated repression of histones and M/G1-specific transcripts cannot be explained by cell cycle arrest, and suggested that there are additional signaling pathways that directly repress these genes in cells under genotoxic stress.

Interindividual differences in response to chemoprotection against aflatoxin-induced hepatocarcinogenesis: implications for human biotransformation enzyme polymorphisms

It is now evident that most, if not all, of the remarkable species differences in susceptibility to AFB hepatocarcinogenesis is due in large part, if not exclusively, to differences in biotransformation. Certainly the relative rate of oxidative formation of the proximate carcinogen, AFB-8,9-exo-epoxide, is an important determinant of species and interindividual differences in susceptibility to AFB. However, mice produce relatively large amounts of exo-AFBO, yet are highly resistant to AFB-hepatocarcinogenesis because they express a particular form of GST with remarkably high catalytic activity toward the exo-epoxide of AFB. Rats, which are highly susceptible to AFB hepatocarcinogenesis,can be made resistant through dietary induction of an orthologous form of GST that is normally expressed in only very small amounts. Based on these findings in laboratory animal models, there is great interest in identifying chemicals and/or specific dietary constituents that could offer protection against AFB-hepatocarcinogenesis to humans. Current experimental strategies have focused on the antiparasitic drug, oltipraz, which induces protection in rats and has also shown some promise in humans. The mechanism of protection in rats appears to be via induction of an alpha class GST with high catalytic activity toward AFBO (rGSTA5-5). vet human alpha class GST proteins that are constitutively expressed in the liver (hGSTA1 and hGSTA2) have little, if any activity toward AFBO. Rather, it appears that mu class GSTs may be responsible for the very low, but potentially significant, detoxification activity toward AFBO. Oltipraz and certain dietary constituents may induce mu class GSTs in human liver, and this could afford some protection against the genotoxic effects of AFBO. However, it also appears that oltipraz, and perhaps certain dietary constituents, act as competitive inhibitors of human CYP1A2. As CYP1A2 appears to mediate most of the activation of AFB to exo-AFBO in human liver at low dietary concentrations of AFB encountered in the human diet, much of the putative protective effects of oltipraz could be mediated via inhibition of CYP1A2 rather than induction of GSTs. There is now evidence that human microsomal epoxide hydrolase (mEH) could play a role in protecting human DNA from the genotoxic effects of AFB, although the importance of this detoxification pathway, relative to mu class GSTs, remains to be elucidated. Oltipraz is an effective inducer of mEH in rats (Lamb Franklin, 2000), and thus induction of this pathway in humans could also potentially contribute to the protective effects of this drug toward AFB genotoxicity. Because the dihydrodiol of AFB may contribute indirectly to the carcinogenic effects of AFB via protein adduction and subsequent hepatotoxicity, the recently characterized human aflatoxin aldehyde reductase (AFAR) may also offer some protection against AFB-induced carcinogenicity in humans. Current and future dietary and/or chemointervention strategies aimed at reducing the carcinogenic effects of AFB in humans should consider all of the possible mechanistic approaches for modifying AFB-induced genotoxicity.

Liver carcinogen aflatoxin B1 as an inducer of mitotic recombination in a human cell line

The mycotoxin aflatoxin B1 (AFB1) is one of the most potent rodent and human liver carcinogens. Upon cytochrome P450-specific metabolism, it induces mutations as well as mitotic recombination events in in vitro systems. We have found that in the lower eukaryote yeast, the recombinagenic activity of AFB1 surpasses its mutagenic activity, and we speculated on possible consequences in terms of the mechanism of liver carcinogenesis. In this study we investigated whether the recombinagenic activity of AFB1 also would be identified in human cells. To address this question, we followed the fate of a heterozygous thymidine kinase (tk) allele in the human lymphoblastoid cell line TK6 upon exposure to AFB1. Individual mutants that had lost tk activity were subjected to loss of heterozygosity analysis of the tk locus and its flanking markers. Fluorescence in situ hybridization analysis on chromosome 17 also was performed. In parallel, a similar analysis was performed on TK6 cells exposed to the alkylating agent N-nitrosomethylurea, a well-known classic point mutagen. Our analysis showed a difference in the molecular mechanism leading to inactivation of the tk allele upon exposure to these two mutagens. In AFB1-exposed cells the fraction of recombination-derived mutants predominated, whereas in N-nitrosomethylurea-exposed cells the fraction of point mutants was higher. Thus, the recombinagenic activity of AFB1 previously identified in a lower eukaryote also was found in the human cell line TK6. Our data support the hypothesis that mitotic recombination represents a central mechanism of action in AFB1-induced liver carcinogenesis.

A new sensitive test based on yeast cells for studying environmental pollution

Different tests based on yeast cells were developed for determination of mutagenic/carcinogenic action; however, they all showed lower sensitivity compared to bacterial tests, the main reason for this being the limited permeability of yeast cells. We found that general permeability of Saccharomyces cerevisiae cells can be increased by mutation and on this basis we developed a more sensitive test. The aim of this study was to prove the applicability of our test, called D7ts1, in environmental studies. Soil, water and air samples were taken during 1998 from regions in Bulgaria with declared low, average or high pollution levels and investigated for presence of mutagenic/carcinogenic activities in the bacterial test of Ames, the yeast D7 test of Zimmermann and our new D7ts1 test. Results obtained evidenced the following conclusions: (1) the usage of D7ts1 test instead of D7 test permits a clearer measurement of positive samples and detects mutagenic/carcinogenic activities undetectable by D7 test; (2) all samples with positive Ames test were positive in the D7ts1 test; however, some samples, clearly positive in the D7ts1, were negative in the Ames test; therefore, the simultaneous usage of D7ts1 and Ames tests in environmental studies is advantageous because it detects dangers for the human health activities to which bacterial cells do not respond; and (3) regions in Bulgaria declared clean were found to be polluted; particularly troubled are the whole-year positive data in the three tests for air samples from a 'clean' region.

Codon 249 of the human TP53 tumor suppressor gene is no hot spot for aflatoxin B1 in a heterologous background

Mutations in the TP53 tumor suppressor gene are the most common alteration in cancer, and human primary liver cancers related to previous dietary exposure to the mycotoxin aflatoxin B1 (AFB1) exhibit a specific hot spot mutation at TP53 codon 249. We have asked whether the 249 hot spot is related to a particular susceptibility to AFB1 of this TP53 region or whether it is related to a phenotype of the 249S p53 mutant protein. This was addressed by constructing a metabolically competent variant of Saccharomyces cerevisiae strain yIG397 expressing human cytochrome P450 1A2 and P450-reductase and isolating AFB1-induced mutants that failed to express the genomic ADE2 reporter gene. Molecular analysis revealed that only 8/40 mutants had a mutation in the TP53 target gene, whereas 32/40 mutants were due to a recombination event eliminating the ADE2 reporter gene. None of 19 mutations identified in the eight mutant TP53 plasmids altered codon 249, thus this codon was no hot spot if the TP53 gene was in the heterologous background yeast. The genotoxic action of AFB1 was completely different from that of the alkylating agent ethyl-methane-sulfonate, where 28/30 induced mutations were linked to the TP53 target gene.

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.

Genotoxicity of ethyl carbamate (urethane) in Salmonella, yeast and human lymphoblastoid cells

Ethyl carbamate is a known carcinogen occurring in fermented food and beverages and is therefore of interest for food safety assurance. We studied the genotoxicity of ethyl carbamate in Salmonella typhimurium, in Saccharomyces cerevisiae and in human lymphoblastoid TK6 cells. In absence of cytochrome P450 enzymes, no ethyl carbamate-mediated genotoxicity was observed in any of the three test systems in the non-cytotoxic range. In the presence of an activating system, ethyl carbamate was found to be mutagenic in Salmonella typhimurium strain TA100 but not in strains TA98 and TA102, indicating base-pair substitutions at G-C base pairs. In contrast, no significant mutagenicity of ethyl carbamate could be detected in human lymphoblastoid TK6 cells. However, applied in cytotoxic concentrations, ethyl carbamate was genotoxic for Saccharomyces cerevisiae in the absence of P450-mediated metabolic activation. Inhibitors of P450IIE1 (DMSO, ethanol and dithiodiethylcarbamate) diminished ethyl carbamate-mediated mutagenicity in Salmonella typhimurium strain TA100 in a dose dependent manner, suggesting that P450IIE1 is the activating enzyme.

Metabolism of promutagens catalyzed by Drosophila melanogaster CYP6A2 enzyme in Saccharomyces cerevisiae

The somatic mutation and recombination test (SMART) in Drosophila melanogaster allows screening of chemicals for genotoxicity in a multicellular organism. In order to correlate data obtained in the SMART with those from genotoxicity tests in rodents, it is important to learn more on the variety of drug-metabolizing enzymes present in this insect and to identify their substrate specificities. In this study we have concentrated on the phase I enzyme cytochrome P450 6A2, which is the first cytochrome P450 cloned from Drosophila. A genomic CYP6A2 DNA fragment and its corresponding cDNA were cloned and sequenced, revealing a previously unidentified intron with an inframe stop codon. This intron is invariantly present in an insecticide resistant [OR(R)] and a sensitive (flr3) strain. Developmental Northern analysis of CYP6A2 mRNA demonstrated a peak of expression in the third larval and pupal stage. CYP6A2 mRNA was found to be present in the insecticide-resistant strain at higher levels than in the insecticide-sensitive strain. Therefore, insecticide resistance might be correlated with enhanced CYP6A2 expression. The substrate specificity of CYP6A2 enzyme was investigated by coexpressing CYP6A2 cDNA with the cDNA for human NADPH-cytochrome P450 reductase in the yeast Saccharomyces cerevisiae. The transformed strain activated the mycotoxin aflatoxin B1 to a product that induced gene conversion, scored at the trp5 locus. Two other compounds, 7,12-dimethylbenz[a]anthracene (DMBA) and 3-amino-1-methyl-5H-pyrido[4,3-b]indole (Trp-P-2), were metabolized in the transformed strain to cytotoxic products.

Induction in the gene RNR3 in Saccharomyces cerevisiae upon exposure to different agents related to carcinogenesis

The induction of the gene RNR3 was investigated in yeast Saccharomyces cerevisiae using RNR31 lacZ fusion. Gene induction was monitored by measuring beta-galactosidase activity. Various drugs that cause DNA damage effectively induced RNR3 expression; alkylating agents (cisplatin, mitomycin C and N-methyl-N'-nitro-N-nitrosoguanidine), a radical producer (bleomycin), and an intercalator (actinomycin D) induced RNR3. When yeast expressing rat CYP1A1 was exposed to 2-aminofluorene, a concentration-dependent induction of RNR3 was observed. Aflatoxin B1 also induced the expression of RNR3 in the same yeast strain concomitant with inhibition of cell growth. In control yeast, no induction of RNR3 was observed upon exposure to 2-aminofluorene or aflatoxin B1. Exposure to 2-acetylaminofluorene or benzo[a]pyrene did not lead to induction of RNR3 in yeast expressing CYP1A1. These results indicate that DNA damage by chemicals related to carcinogenesis induces RNR3, and that activation of these procarcinogens was required for DNA damage-dependent induction of RNR3.