Identification and quantification of stable DNA adducts formed from dibenzo[a,l]pyrene or its metabolites in vitro and in mouse skin and rat mammary gland

A look beyond the priority: A systematic review of the genotoxic, mutagenic, and carcinogenic endpoints of non-priority PAHs

Knowledge of the toxic potential of polycyclic aromatic hydrocarbons (PAHs) has increased over time. Much of this knowledge is about the 16 United States - Environmental Protection Agency (US - EPA) priority PAHs; however, there are other US - EPA non-priority PAHs in the environment, whose toxic potential is underestimated. We conducted a systematic review of in vitro, in vivo, and in silico studies to assess the genotoxicity, mutagenicity, and carcinogenicity of 13 US - EPA non-priority parental PAHs present in the environment. Electronic databases, such as Science Direct, PubMed, Scopus, Google Scholar, and Web of Science, were used to search for research with selected terms without time restrictions. After analysis, following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) protocol, 249 articles, published between 1946 and 2020, were selected and the quality assessment of these studies was performed. The results showed that 5-methylchrysene (5-MC), 7,12-dimethylbenz[a]anthracene (7,12-DMBA), cyclopenta[cd]pyrene (CPP), and dibenzo[al]pyrene (Db[al]P) were the most studied PAHs. Moreover, 5-MC, 7,12-DMBA, benz[j]aceanthrylene (B[j]A), CPP, anthanthrene (ANT), dibenzo[ae]pyrene (Db[ae]P), and Db[al]P have been reported to cause mutagenic effects and have been being associated with a risk of carcinogenicity. Retene (RET) and benzo[c]fluorene (B[c]F), the least studied compounds, showed evidence of a strong influence on the mutagenicity and carcinogenicity endpoints. Overall, this systematic review provided evidence of the genotoxic, mutagenic, and carcinogenic endpoints of US - EPA non-priority PAHs. However, further studies are needed to improve the future protocols of environmental analysis and risk assessment in severely exposed populations.

Comparative Tumorigenicity and DNA Damage Induced by Dibenzo[ def,p]chrysene and Its Metabolites in the Mouse Ovary

Ovarian cancer ranked second in incidence among gynecologic cancers, but it causes more deaths than any other gynecologic cancer; at present there is no curative treatment beyond surgery. Animal models that employ carcinogens found in the human environment can provide a realistic platform to understand the mechanistic basis for disease development and to design rational chemopreventive/therapeutic strategies. We and others have shown that the administration of the environmental pollutant and tobacco smoke constituent dibenzo[ def,p]chrysene (DBP) to mice by several routes of exposure can induce tumors in multiple sites including the ovary. In the present study we compared, for the first time, the tumorigenicity and DNA damage induced by DBP and its metabolites DBP-dihydrodiol (DBPDHD) and DBP-dihydrodiol epoxide (DBPDE) in the mouse ovary. Compounds were dissolved in dimethyl sulfoxide (DMSO) as the vehicle and administered by topical application into the mouse oral cavity three times per week for 38 weeks. No tumors were observed in mice treated with DMSO. At equal dose (24 nmol/30 μL DMSO), the incidence of ovarian tumors induced by DBPDHD was higher (60.7%), although not significantly, than that induced by DBP (44.8%). Similarly the levels of DNA damage induced by DBPDHD in the ovary were higher than those observed with DBP. We did not observe any histological abnormality in the ovary of mice treated with DBPDE, which is consistent with lack of DNA damage. Our results suggested that both DBP and DBPDHD can be metabolized in the mouse ovary leading to the formation of DBPDE that can damage DNA, which is a prerequisite step in the initiation stage of carcinogenesis.

Depurinating estrogen-DNA adducts, generators of cancer initiation: their minimization leads to cancer prevention

Estrogens can initiate cancer by reacting with DNA. Specific metabolites of endogenous estrogens, the catechol estrogen-3,4-quinones, react with DNA to form depurinating estrogen-DNA adducts. Loss of these adducts leaves apurinic sites in the DNA, generating mutations that can lead to the initiation of cancer. A variety of endogenous and exogenous factors can disrupt estrogen homeostasis, which is the normal balance between estrogen activating and protective enzymes. In fact, if estrogen metabolism becomes unbalanced and generates excessive catechol estrogen 3,4-quinones, formation of depurinating estrogen-DNA adducts increases and the risk of initiating cancer is greater. The levels of depurinating estrogen-DNA adducts are high in women diagnosed with breast cancer and those at high risk for the disease. High levels of depurinating estrogen-DNA adducts before the presence of breast cancer indicates that adduct formation is a critical factor in breast cancer initiation. Women with thyroid or ovarian cancer also have high levels of estrogen-DNA adducts, as do men with prostate cancer or non-Hodgkin lymphoma. Depurinating estrogen-DNA adducts are initiators of many prevalent types of human cancer. These findings and other discoveries led to the recognition that reducing the levels of estrogen-DNA adducts could prevent the initiation of human cancer. The dietary supplements N-acetylcysteine and resveratrol inhibit formation of estrogen-DNA adducts in cultured human breast cells and in women. These results suggest that the two supplements offer an approach to reducing the risk of developing various prevalent types of human cancer. Graphical abstract Major metabolic pathway in cancer initiation by estrogens.

Simultaneous detection of deoxyadenosine and deoxyguanosine adducts in the tongue and other oral tissues of mice treated with Dibenzo[a,l]pyrene

We were the first to demonstrate that direct application of the environmental pollutant and tobacco smoke constituent dibenzo[a,l]pyrene (DB[a,l]P) into the oral cavity of mice induced squamous cell carcinoma (SCC) in oral tissues but not in the tongue; however, the mechanisms that can account for the varied carcinogenicity remain to be determined. Furthermore, we also showed that not only dA adducts, but also dG adducts can account for the mutagenic activity of DB[a,l]P in the oral tissues in vivo. In this study, we initially focused on DB[a,l]P-induced genotoxic effects in both oral and tongue tissues. Therefore, to fully assess the contribution of these DNA adducts in the initiation stage of carcinogenesis induced by DB[a,l]P, an LC-MS/MS method to simultaneously detect and quantify DB[a,l]PDE-dG and -dA adducts was developed. Mice were orally administered with DB[a,l]P (24 nmole, 3 times per week for 5 weeks) or its fjord region diol epoxide, (±)-anti-11,12-dihydroxy-13,14-epoxy-11,12,13,14-tetrahydrodibenzo[a,l]pyrene (DB[a,l]PDE, 12 nmole, single application); animals were sacrificed at 2, 7, 14, and 28 days after the last dose of carcinogen administration. Oral and tongue tissues were obtained and DNA were isolated followed by enzymatic hydrolysis. Following the development of an isotope dilution LC-MS/MS method, we successfully detected (−)-anti-cis- and (−)-anti-trans-DB[a,l]PDE-N²-dG, as well as (−)-anti-cis- and (−)-anti-trans-DB[a,l]PDE-N⁶-dA in oral and tongue tissues of mice treated with DB[a,l]P. Levels of (−)-anti-trans-DB[a,l]PDE-N⁶-dA were ≥2 folds higher than (−)-anti-cis-DB[a,l]PDE-N⁶-dA adduct and those of dG adducts in the oral tissues and tongue at all time points selected after the cessation of DB[a,l]P treatment. Levels of dG adducts were comparable in both tissues. Collectively, our results support that DB[a,l]P is predominantly metabolized to (−)-anti-DB[a,l]PDE, and the levels and persistence of (−)-anti-trans-DB[a,l]PDE-N⁶-dA may, in part, explain the carcinogenicity of DB[a,l]P in the oral tissues but not in the tongue.

The molecular etiology and prevention of estrogen-initiated cancers: Ockham's Razor: Pluralitas non est ponenda sine necessitate. Plurality should not be posited without necessity

Elucidation of estrogen carcinogenesis required a few fundamental discoveries made by studying the mechanism of carcinogenesis of polycyclic aromatic hydrocarbons (PAH). The two major mechanisms of metabolic activation of PAH involve formation of radical cations and diol epoxides as ultimate carcinogenic metabolites. These intermediates react with DNA to yield two types of adducts: stable adducts that remain in DNA unless removed by repair and depurinating adducts that are lost from DNA by cleavage of the glycosyl bond between the purine base and deoxyribose. The potent carcinogenic PAH benzo[a]pyrene, dibenzo[a,l]pyrene, 7,12-dimethylbenz[a]anthracene and 3-methylcholanthrene predominantly form depurinating DNA adducts, leaving apurinic sites in the DNA that generate cancer-initiating mutations. This was discovered by correlation between the depurinating adducts formed in mouse skin by treatment with benzo[a]pyrene, dibenzo[a,l]pyrene or 7,12-dimethylbenz[a]anthracene and the site of mutations in the Harvey-ras oncogene in mouse skin papillomas initiated by one of these PAH. By applying some of these fundamental discoveries in PAH studies to estrogen carcinogenesis, the natural estrogens estrone (E1) and estradiol (E2) were found to be mutagenic and carcinogenic through formation of the depurinating estrogen-DNA adducts 4-OHE1(E2)-1-N3Ade and 4-OHE1(E2)-1-N7Gua. These adducts are generated by reaction of catechol estrogen quinones with DNA, analogously to the DNA adducts obtained from the catechol quinones of benzene, naphthalene, and the synthetic estrogens diethylstilbestrol and hexestrol. This is a weak mechanism of cancer initiation. Normally, estrogen metabolism is balanced and few estrogen-DNA adducts are formed. When estrogen metabolism becomes unbalanced, more catechol estrogen quinones are generated, resulting in higher levels of estrogen-DNA adducts, which can be used as biomarkers of unbalanced estrogen metabolism and, thus, cancer risk. The ratio of estrogen-DNA adducts to estrogen metabolites and conjugates has repeatedly been found to be significantly higher in women at high risk for breast cancer, compared to women at normal risk. These results indicate that formation of estrogen-DNA adducts is a critical factor in the etiology of breast cancer. Significantly higher adduct ratios have been observed in women with breast, thyroid or ovarian cancer. In the women with ovarian cancer, single nucleotide polymorphisms in the genes for two enzymes involved in estrogen metabolism indicate risk for ovarian cancer. When polymorphisms produce high activity cytochrome P450 1B1, an activating enzyme, and low activity catechol-O-methyltransferase, a protective enzyme, in the same woman, she is almost six times more likely to have ovarian cancer. These results indicate that formation of estrogen-DNA adducts is a critical factor in the etiology of ovarian cancer. Significantly higher ratios of estrogen-DNA adducts to estrogen metabolites and conjugates have also been observed in men with prostate cancer or non-Hodgkin lymphoma, compared to healthy men without cancer. These results also support a critical role of estrogen-DNA adducts in the initiation of cancer. Starting from the perspective that unbalanced estrogen metabolism can lead to increased formation of catechol estrogen quinones, their reaction with DNA to form adducts, and generation of cancer-initiating mutations, inhibition of estrogen-DNA adduct formation would be an effective approach to preventing a variety of human cancers. The dietary supplements resveratrol and N-acetylcysteine can act as preventing cancer agents by keeping estrogen metabolism balanced. These two compounds can reduce the formation of catechol estrogen quinones and/or their reaction with DNA. Therefore, resveratrol and N-acetylcysteine provide a widely applicable, inexpensive approach to preventing many of the prevalent types of human cancer.

Biodegradation of a mixture of PAHs by non-ligninolytic fungal strains isolated from crude oil-contaminated soil

Nine native non-ligninolytic fungal strains were isolated from Maya crude oil-contaminated soil and selected based on their ability to grow and use crude oil and several polycyclic aromatic hydrocarbons (PAHs) as carbon source, for their application to PAH removal in soil. The fungi were identified by PCR amplification of intergenic transcribed sequences regions and microbiological techniques, and results showed them to be part of the genera Fusarium, Neurospora, Aspergillus, Scedosporium, Penicillium, Neosartorya and Talaromyces. A primary selection of fungi was made in minimal medium plates, considering the tolerance to different concentrations of PAHs for each strain. The radial extension rate exhibited significant differences (p < 0.05) from 200 to 1,000 mg of PAHs mixture l⁻¹. A secondary selection of Aspergillus terreus, Talaromyces spectabilis, and Fusarium sp. was achieved based on their tolerance to 2,000 mg of a mixture of Phenanathrene and Pyrene kg⁻¹ of soil in a solid-state microcosm system for 2 weeks. The percentage of PAH removal obtained by the three strains was approximately 21 % of the mixture.

Nuclear magnetic resonance solution structure of an N(2)-guanine DNA adduct derived from the potent tumorigen dibenzo[a,l]pyrene: intercalation from the minor groove with ruptured Watson-Crick base pairing

The most potent tumorigen identified among the polycyclic aromatic hydrocarbons (PAH) is the nonplanar fjord region dibenzo[a,l]pyrene (DB[a,l]P). It is metabolically activated in vivo through the widely studied diol epoxide (DE) pathway to form covalent adducts with DNA bases, predominantly guanine and adenine. The (+)-11S,12R,13R,14S DE enantiomer forms adducts via its C14 position with the exocyclic amino group of guanine. Here, we present the first nuclear magnetic resonance solution structure of a DB[a,l]P-derived adduct, the 14R-(+)-trans-anti-DB[a,l]P-N(2)-dG (DB[a,l]P-dG) lesion in double-stranded DNA. In contrast to the stereochemically identical benzo[a]pyrene-derived N(2)-dG adduct (B[a]P-dG) in which the B[a]P rings reside in the B-DNA minor groove on the 3'-side of the modifed deoxyguanosine, in the DB[a,l]P-derived adduct the DB[a,l]P rings intercalate into the duplex on the 3'-side of the modified base from the sterically crowded minor groove. Watson-Crick base pairing of the modified guanine with the partner cytosine is broken, but these bases retain some stacking with the bulky DB[a,l]P ring system. This new theme in PAH DE-DNA adduct conformation differs from (1) the classical intercalation motif in which Watson-Crick base pairing is intact at the lesion site and (2) the base-displaced intercalation motif in which the damaged base and its partner are extruded from the helix. The structural considerations that lead to the intercalated conformation of the DB[a,l]P-dG lesion in contrast to the minor groove alignment of the B[a]P-dG adduct, and the implications of the DB[a,l]P-dG conformational motif for the recognition of such DNA lesions by the human nucleotide excision repair apparatus, are discussed.

The etiology and prevention of breast cancer

Metabolism of estrogens via the catechol estrogen pathway is characterized by a balanced set of activating and protective enzymes (homeostasis). Disruption of homeostasis, with excessive production of catechol estrogen quinones, can lead to reaction of these quinones with DNA to form depurinating estrogen-DNA adducts. Some of the mutations generated by these events can lead to initiation of breast cancer. A wealth of evidence, from studies of metabolism, mutagenicity, cell transformation and carcinogenicity, demonstrates that estrogens are genotoxic. Women at high risk for breast cancer, or diagnosed with the disease, have relatively high levels of depurinating estrogen-DNA adducts compared to normal-risk women. The dietary supplements N-acetylcysteine and resveratrol can inhibit formation of catechol estrogen quinones and their reaction with DNA to form estrogen-DNA adducts, thereby preventing initiation of breast cancer.

Mechanism of DNA depurination by carcinogens in relation to cancer initiation

Depurinating DNA adducts formed by aromatic hydrocarbons and catechol estrogen quinones play a major role in cancer initiation. Most of these adducts depurinate instantaneously, but some guanine adducts depurinate from DNA with half-lives of hours. We report here, that after 10 h at 37 °C, reaction of estradiol-3,4-quinone (E(2)-3,4-Q) with ds-DNA to yield N7Gua and N3Ade adducts was complete and more efficient than with ss-DNA. When E(2)-3,4-Q reacted with t-RNA, no adducts were detected after 10 h, and the level of N3Ade and N7Gua adducts after 10 days was less than half that with ss-DNA after 10 h. Reaction of E(2)-3,4-Q and dG yielded 4-OHE(2)-1-N7dG, which spontaneously depurinated to yield 4-OHE(2)-1-N7Gua. To investigate the mechanism of depurination, E(2)-3,4-Q was reacted with carbocyclicdeoxyguanosine, in which the ring oxygen of the deoxyribose moiety is substituted with CH(2) , and depurination was observed. The results from this experiment demonstrate that the oxocarbenium ion mechanism plays the major role in depurination and provides the first experimental evidence for this mechanism. A newly discovered β-elimination mechanism also plays a minor role in depurination. Understanding why the depurinating estrogen-DNA adducts come from DNA, and not from RNA, underscores the critical role that these adducts play in initiating cancer.

Association between mutation spectra and stable and unstable DNA adduct profiles in Salmonella for benzo[a]pyrene and dibenzo[a,l]pyrene

Benzo[a]pyrene (BP) and dibenzo[a,l]pyrene (DBP) are two polycyclic aromatic hydrocarbons (PAHs) that exhibit distinctly different mutagenicity and carcinogenicity profiles. Although some studies show that these PAHs produce unstable DNA adducts, conflicting data and arguments have been presented regarding the relative roles of these unstable adducts versus stable adducts, as well as oxidative damage, in the mutagenesis and tumor-mutation spectra of these PAHs. However, no study has determined the mutation spectra along with the stable and unstable DNA adducts in the same system with both PAHs. Thus, we determined the mutagenic potencies and mutation spectra of BP and DBP in strains TA98, TA100 and TA104 of Salmonella, and we also measured the levels of abasic sites (aldehydic-site assay) and characterized the stable DNA adducts ((32)P-postlabeling/HPLC) induced by these PAHs in TA104. Our results for the mutation spectra and site specificity of stable adducts were consistent with those from other systems, showing that DBP was more mutagenic than BP in TA98 and TA100. The mutation spectra of DBP and BP were significantly different in TA98 and TA104, with 24% of the mutations induced by BP in TA98 being complex frameshifts, whereas DBP produced hardly any of these mutations. In TA104, BP produced primarily GC to TA transversions, whereas DBP produced primarily AT to TA transversions. The majority (96%) of stable adducts induced by BP were at guanine, whereas the majority (80%) induced by DBP were at adenine. Although BP induced abasic sites, DBP did not. Most importantly, the proportion of mutations induced by DBP at adenine and guanine paralleled the proportion of stable DNA adducts induced by DBP at adenine and guanine; however, this was not the case for BP. Our results leave open a possible role for unstable DNA adducts in the mutational specificity of BP but not for DBP.

Intercalative conformations of the 14R (+)- and 14S (-)-trans-anti-DB[a,l]P-N⁶-dA adducts: molecular modeling and MD simulations

Among the polycyclic aromatic hydrocarbon class of chemical carcinogens, dibenzo[a,l]pyrene (DB[a,l]P) is the most potent tumorigen that has been identified to date. Structurally, it is bulky with six aromatic rings, and it contains the nonplanar fjord-region. The conformational properties of DB[a,l]P-derived DNA adducts responsible for its extraordinary carcinogenicity are hence of great interest. We have carried out molecular modeling and MD simulations for the 14R (+)- and 14S (-)-trans-anti-DB[a,l]P-N⁶-dA adducts derived from the reactions of the DB[a,l]P diol epoxides with adenine in double-stranded DNA. The structures are based on the classically intercalated NMR solution structures of the analogous fjord-region benzo[c]phenanthrene-derived-N⁶-dA adducts. One objective was to gain insight on the impact of the more bulky DB[a,l]P ring system on the structural characteristics of the intercalative adduct conformations. A further objective was to elucidate the effect of the flexible twist associated with the sterically hindered aromatic ring in the fjord-region on the intercalated conformations, for comparison with the intercalated but planar bay-region benzo[a]pyrene-derived-N⁶-dA adducts. For the DB[a,l]P-N⁶-dA adducts, our results show that the 14R (+)-adduct is more favorably intercalated on the 5'-side of the modified adenine than the stereoisomeric 14S (-)-adduct, intercalated on its 3'-side. The 14R (+)-adduct manifests better van der Waals stacking interactions with flanking base pairs, less perturbed Watson-Crick hydrogen bonding, less local groove enlargement, less unwinding, and a lower solvent exposure than the 14S (-)-adduct. These structural findings are consistent with observed thermodynamic melting data, UV absorption properties, and fluorescence quenching studies. By contrast, the NMR solution structures for the analogous but less bulky B[c]Ph-derived adducts reveal no such stereoisomeric effect, while the planar bay-region benzo[a]pyrene-derived-N⁶-dA adducts do. Differences in nucleotide excision repair susceptibilities of the fjord and bay region adducts stem from distinctions in their intercalative conformations, produced by the intrinsic topological variations in their polycyclic aromatic ring systems.

Depurinating estrogen-DNA adducts in the etiology and prevention of breast and other human cancers

Experiments on estrogen metabolism, formation of DNA adducts, mutagenicity, cell transformation and carcinogenicity have led to and supported the hypothesis that the reaction of specific estrogen metabolites, mostly the electrophilic catechol estrogen-3,4-quinones, with DNA can generate the critical mutations to initiate breast and other human cancers. Analysis of depurinating estrogen-DNA adducts in urine demonstrates that women at high risk of, or with breast cancer, have high levels of the adducts, indicating a critical role for adduct formation in breast cancer initiation. Men with prostate cancer or non-Hodgkin lymphoma also have high levels of estrogen-DNA adducts. This knowledge of the first step in cancer initiation suggests the use of specific antioxidants that can block formation of the adducts by chemical and biochemical mechanisms. Two antioxidants, N-acetylcysteine and resveratrol, are prime candidates to prevent breast and other human cancers because in various M in vitro and in vivo experiments, they reduce the formation of estrogen-DNA adducts.

Formation and differential repair of covalent DNA adducts generated by treatment of human cells with (+/-)-anti-dibenzo[a,l]pyrene-11,12-diol-13,14-epoxide

Dibenzo[a,l]pyrene (DBP) is the most potent tumor initiating polycyclic aromatic hydrocarbon tested to date in rodent tumor models. To investigate how DBP adduct formation and removal might influence carcinogenesis, we have examined the effects of treatment of several nucleotide excision repair (NER)-proficient (NER(+)) and -deficient (NER(-)) cell lines with the carcinogenic metabolite (+/-)-anti-DBP-11,12-diol-13,14-epoxide (DBPDE). The treatment of NER(-) cells with (+/-)-anti-DBPDE for 0.5, 1, or 2 h yielded similar total adduct levels, indicating that adduct formation was essentially complete during a 2 h treatment period with no additional adducts produced after replacement of media. In all cell lines, treatment with (+/-)-anti-DBPDE generated five major and at least two minor adducts that were chromatographically identical to those formed by direct treatment of 3'-GMP and 3'-AMP with (+/-)-anti-DBPDE. When adduct levels were assessed in NER(-) cells, the number of adducts/10(9) nucleotides decreased over time, suggesting that DNA replication was ongoing, so we incorporated a normalization strategy based on DNA synthesis. This strategy indicated that DBPDE-DNA adduct levels in NER(-) cells are stable over time. After normalization for DNA synthesis in the NER(+) cells, our data indicated that three adducts showed biphasic repair kinetics. A faster rate of removal was observed during the first 6 h following DBPDE removal followed by a slower rate for up to 34 h. Importantly, two of the major guanine adducts were particularly refractory to removal in the NER(+) cells. Our results suggest that the extreme carcinogenicity of DBPDE may result from the ability of a substantial percentage of two structurally distinct DBPDE-DNA adducts to escape repair.

Mutations induced by benzo[a]pyrene and dibenzo[a,l]pyrene in lacI transgenic B6C3F1 mouse lung result from stable DNA adducts

Dibenzo[a,l]pyrene (DB[a,l]P) and benzo[a]pyrene (B[a]P) are carcinogenic polycyclic aromatic hydrocarbons (PAHs) that are each capable of forming a variety of covalent adducts with DNA. Some of the DNA adducts formed by these PAHs have been demonstrated to spontaneously depurinate, producing apurinic (AP) sites. The significance of the formation of AP sites as a key event in the production of mutations and tumours by PAHs has been a subject of ongoing investigations. Because cells have efficient and accurate mechanisms for repairing background levels of AP sites, the contribution of PAH-induced AP site mutagenesis is expected to be maximal in conditions where those induced AP sites are produced in significant excess of the endogenous AP sites. In this study, we investigated the effect of two dosing regimens on the mutagenicity of DB[a,l]P and B[a]P in vivo using the Big Blue(R) transgenic mouse system. We compared administration of a single highly tumorigenic dose of each PAH with a fractionated delivery of the same total dose administered over 5 days, with the expectation that PAH-induced AP sites would be produced at a greater margin above background levels in animals receiving the high single dose than in the animals receiving the fractionated doses. Treatment with DB[a,l]P yielded a 2.5-fold (single dose) to 3-fold (fractionated dose) increase in mutant frequencies relative to controls. Both single-dose and fractionated dose treatment regimens with B[a]P produced about a 15-fold increase in mutant frequencies compared to controls. The mutations induced by B[a]P and DB[a,l]P correlated with the stable covalent DNA adducts produced by each. These mutation results are consistent with the previously identified stable covalent DNA adducts being the promutagenic lesions produced by these two PAHs and do not support a major role for depurinating adducts, contributing to PAH-induced mutagenesis in mouse lung in vivo.

The role of polycyclic aromatic hydrocarbon-DNA adducts in inducing mutations in mouse skin

Polycyclic aromatic hydrocarbons (PAH) form stable and depurinating DNA adducts in mouse skin to induce preneoplastic mutations. Some mutations transform cells, which then clonally expand to establish tumors. Strong clues about the mutagenic mechanism can be obtained if the PAH-DNA adducts can be correlated with both preneoplastic and tumor mutations. To this end, we studied mutagenesis in PAH-treated early preneoplastic skin (1 day after exposure) and in the induced papillomas in SENCAR mice. Papillomas were studied by PCR amplification of the H-ras gene and sequencing. For benzo[a]pyrene (BP), BP-7,8-dihydrodiol (BPDHD), 7,12-dimethylbenz[a]anthracene (DMBA) and dibenzo[a,l]pyrene (DB[a,l]P), the codon 13 (GGC to GTC) and codon 61 (CAA to CTA) mutations in papillomas corresponded to the relative levels of Gua and Ade-depurinating adducts, despite BP and BPDHD forming significant amounts of stable DNA adducts. Such a relationship was expected for DMBA and DB[a,l]P, as they formed primarily depurinating adducts. These results suggest that depurinating adducts play a major role in forming the tumorigenic mutations. To validate this correlation, preneoplastic skin mutations were studied by cloning H-ras PCR products and sequencing individual clones. DMBA- and DB[a,l]P-treated skin showed primarily A.T to G.C mutations, which correlated with the high ratio of the Ade/Gua-depurinating adducts. Incubation of skin DNA with T.G-DNA glycosylase eliminated most of these A.T to G.C mutations, indicating that they existed as G.T heteroduplexes, as would be expected if they were formed by errors in the repair of abasic sites generated by the depurinating adducts. BP and its metabolites induced mainly G.C to T.A mutations in preneoplastic skin. However, PCR over unrepaired anti-BPDE-N(2)dG adducts can generate similar mutations as artifacts of the study protocol, making it difficult to establish an adduct-mutation correlation for determining which BP-DNA adducts induce the early preneoplastic mutations. In conclusion, this study suggests that depurinating adducts play a major role in PAH mutagenesis.

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.

Improved measurement of dibenzo[a,l]pyrene-induced abasic sites by the aldehyde-reactive probe assay

Dibenzo[a,l]pyrene (DB[a,l]P) induces abundant amounts of depurinating adducts that spontaneously dissociate to form abasic sites in DNA. However, several previous studies that used the aldehyde-reactive probe (ARP) assay, could not verify abasic site formation by DB[a,l]P. Therefore, we examined whether a modification of the ARP assay would allow greater quantification of abasic sites. A previous study indicated that the abasic site quantification is improved by letting ARP trap the nascent abasic sites in cells, before extracting DNA for the assay. To test whether the addition of ARP to the DB[a,l]P-DNA adduct-forming reaction would improve abasic site quantification, we treated calf thymus DNA (0.625 mg/mL) with DB[a,l]P (80 microM) and 3-methylcholanthrene-treated rat liver microsomes with or without ARP (3 mM). The inclusion of ARP in the adduct-forming reaction resulted in significantly greater detection of abasic sites (62 lesions/10(6) bp versus 3.7 lesions/10(6) bp). DB[a,l]P also induces DNA strand breaks. The strand breaks may occur at abasic sites and by other mechanisms, such as oxidative damage. ARP/O-methoxyamine-abasic site conjugates are refractory to strand breakage, however, ARP or O-methoxyamine (3-10 mM) could only partially protect DB[a,l]P-induced DNA degradation, presumably by protecting the abasic sites, but not the other strand breaks. These results suggest that if DNA strand breakages occur at the abasic sites or at bases flanking them, and the fragments are lost during DNA extraction, abasic site estimation could be compromised. To obtain an independent line of evidence for abasic site formation in DB[a,l]P-treated cells, mouse Mbeta16 fibroblasts were treated with DB[a,l]P and O-methoxyamine. O-Methoxyamine is known to potentiate cytotoxicity of abasic site-inducing chemicals by forming abasic site conjugates, which partially inhibits their repair. O-Methoxyamine was found to increase DB[a,l]P cytotoxicity in these cells, supporting the idea that DB[a,l]P formed abasic sites. In summary, the inclusion of ARP in the DB[a,l]P-DNA adduct-forming reaction traps and protects the nascent abasic sites, allowing an improved quantification of abasic sites.