Evidence that error-prone DNA repair converts dibenzo[a,l]pyrene-induced depurinating lesions into mutations: formation, clonal proliferation and regression of initiated cells carrying H-ras oncogene mutations in early preneoplasia

A base-repair based electrochemiluminescent genotoxicity sensor that detects abasic sites in double-stranded DNA films

A novel genotoxicity sensor was developed based on the base repair process associated with the electrochemiluminescence (ECL) detection of abasic sites in a double-stranded DNA monolayer. This is the first time that an ECL sensor with the ability to identify the removed nucleobases in a DNA duplex has been studied. The successful detection of abasic sites created by DNA glycosylase indicates its further applications for examining some other specific types of DNA damage.

Critical depurinating DNA adducts: Estrogen adducts in the etiology and prevention of cancer and dopamine adducts in the etiology and prevention of Parkinson's disease

Endogenous estrogens become carcinogens when dangerous metabolites, the catechol estrogen quinones, are formed. In particular, the catechol estrogen-3,4-quinones can react with DNA to produce an excess of specific depurinating estrogen-DNA adducts. Loss of these adducts leaves apurinic sites in the DNA, generating subsequent cancer-initiating mutations. Unbalanced estrogen metabolism yields excessive catechol estrogen-3,4-quinones, increasing formation of depurinating estrogen-DNA adducts and the risk of initiating cancer. Evidence for this mechanism of cancer initiation comes from various types of studies. High levels of depurinating estrogen-DNA adducts have been observed in women with breast, ovarian or thyroid cancer, as well as in men with prostate cancer or non-Hodgkin lymphoma. Observation of high levels of depurinating estrogen-DNA adducts in high risk women before the presence of breast cancer indicates that adduct formation is a critical factor in breast cancer initiation. Formation of analogous depurinating dopamine-DNA adducts is hypothesized to initiate Parkinson's disease by affecting dopaminergic neurons. Two dietary supplements, N-acetylcysteine and resveratrol complement each other in reducing formation of catechol estrogen-3,4-quinones and inhibiting formation of estrogen-DNA adducts in cultured human and mouse breast epithelial cells. They also inhibit malignant transformation of these cells. In addition, formation of adducts was reduced in women who followed a Healthy Breast Protocol that includes N-acetylcysteine and resveratrol. When initiation of cancer is blocked, promotion, progression and development of the disease cannot occur. These results suggest that reducing formation of depurinating estrogen-DNA adducts can reduce the risk of developing a variety of types of human cancer.

Genotoxic Effects of Benzo[a]pyrene and Dibenzo[a,l]pyrene in a Human Lung Cell Line

Several studies have shown that polycyclic aromatic hydrocarbons (PAHs) produce genotoxic effects in assays performed in vivo and in vitro. This study was undertaken to investigate the ability of benzo[ a]pyrene (BP) and dibenzo[ a,l]pyrene (DBP) to induce DNA damage in a human lung fibroblast cell line (MRC-5), using sister-chromatid exchanges test (SCEs), the comet assay, and evaluating point mutations in codon 12 of the K- ras protooncogene by polymerase chain reaction–single-strand conformation polymorphisms (PCR-SSCPs) and restriction fragment length polymorphisms (RFLP)-enriched PCR methods. Sister-chromatid exchanges frequencies were significantly increased in cells exposed to benzo[ a]pyrene and dibenzo[ a,l]pyrene in relation to controls ( p < .001). Using the standard alkaline comet assay, significant differences between groups were found for the variable comet moment (CM) when cells were exposed to BP ( p < .001) and DBP ( p < .001). Nevertheless, PCR-SSCP and RFLP-enriched PCR methods did not show any association between treatments with BP and DBP and K- ras point mutations. The data presented in this study indicated that BP and DBP induced both DNA strand breaks and sister-chromatid exchanges but not significant point mutations at codon 12 of K- ras gene in the MRC-5 cell line.

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.

Mechanistic Models Fit to ED001 Data on >40,000 Trout Exposed to Dibenzo[A,L]pyrene Indicate Mutations Do Not Drive Increased Tumor Risk

ED001-study data on increased liver and stomach tumor risks in >40,000 trout fed dibenzo[a,l]pyrene (DBP), one of the most potently mutagenic chemical carcinogens known, provide the greatest low-dose dose-response resolution of any experimentally induced tumor data set to date. Although multistage somatic mutation/clonal-expansion cancer theory predicts that genotoxic carcinogens increase tumor risk in linear no-threshold proportion to dose at low doses, ED001 tumor data curiously exhibit substantial low-dose nonlinearity. To explore the role that nongenotoxic mechanisms may have played to yield such nonlinearity, the liver and stomach tumor data sets were each fit by two models that each assume a genotoxic and a nongenotoxic pathway to increased tumor risk: the stochastic 2-stage (MVK) cancer model, and a model implementing the more recent dysregulated adaptive hyperplasia (DAH) theory of tumorigenesis. MVK and DAH fits to the data sets were each excellent, but unexpectedly each MVK fit implies that DBP acts to increase tumor risk by entirely non-mutagenic mechanisms. Given that DBP is such a potent mutagen, the MVK-model fits obtained appear to be biologically implausible, whereas the DAH-model fits reflect that model's assumption that chemical-induced tumorigenesis typically is driven by elevated repair-cell populations rather than mutations per se.

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.

Self-catalyzed site-specific depurination of G residues mediated by cruciform extrusion in closed circular DNA plasmids

A major variety of "spontaneous" genomic damage is endogenous generation of apurinic sites. Depurination rates vary widely across genomes, occurring with higher frequency at "depurination hot spots." Recently, we discovered a site-specific self-catalyzed depurinating activity in short (14-18 nucleotides) DNA stem-loop-forming sequences with a 5'-G(T/A)GG-3' loop and T·A or G·C as the first base pair at the base of the loop; the 5'-G residue of the loop self-depurinates at least 10(5)-fold faster than random "spontaneous" depurination at pH 5. Formation of the catalytic intermediate for self-depurination in double-stranded DNA requires a stem-loop to extrude as part of a cruciform. In this study, evidence is presented for self-catalyzed depurination mediated by cruciform formation in plasmid DNA in vitro. Cruciform extrusion was confirmed, and its extent was quantitated by digestion of the plasmid with single strand-specific mung bean endonuclease, followed by restriction digestion and sequencing of resulting mung bean-generated fragments. Appearance of the apurinic site in the self-depurinating stem-loop was confirmed by digestion of plasmid DNA with apurinic endonuclease IV, followed by primer extension and/or PCR amplification to detect the endonuclease-generated strand break and identify its location. Self-catalyzed depurination was contingent on the plasmid being supercoiled and was not observed in linearized plasmids, consistent with the presence of the extruded cruciform in the supercoiled plasmid and not in the linear one. These results indicate that self-catalyzed depurination is not unique to single-stranded DNA; rather, it can occur in stem-loop structures extruding from double-stranded DNA and therefore could, in principle, occur in vivo.

Cigarette smoke condensate-induced oxidative DNA damage and its removal in human cervical cancer cells

Exposure to cigarette smoke is well documented to increase oxidative stress and could account for higher risk of cervical cancer in smokers. Cervical pre-cancerous lesions that are initiated by human papillomavirus (HPV) infection generally regress in the absence of known risk factors such as smoking. 8-oxodeoxyguanosine (8-oxodG) is a highly mutagenic oxidative DNA lesion that is formed by the oxidation of deoxyguanosine. In the present study, we examined: a) the effect of cigarette smoke condensate (CSC) on 8-oxodG formation in and its removal from HPV-transfected (ECT1/E6 E7), HPV-positive (CaSki) and HPV-negative (C33A) human cervical cancer cells, and b) the cell cycle progression and apoptosis in CSC-treated ECT1/E6 E7 cells. CSC induced 8-oxodG in a dose- (p=0.03) and time (p=0.002)-dependent fashion in ECT1/E6 E7 cells as determined by flow cytometry. A 2.4-fold higher level of 8-oxodG was observed in HPV-positive compared with HPV-negative cells. However, 8-oxodG lesions were almost completely removed 72 h post-exposure in all cell lines as determined by ImageStream analysis. This observation correlates with the 2- and 5-fold increase in the p53 levels in ECT1/E6 E7 and CaSki cells with no significant change in C33A cells. We conclude that: a) cigarette smoke constituents induce oxidative stress with higher burden in HPV-positive cervical cancer cells and b) the significant increase observed in p53 levels in wild-type cervical cells (ECT1/E6 E7 and CaSki) may be attributed to the p53-dependent DNA repair pathway while a p53-independent pathway in C33A cells cannot be ruled out.

Depurinating naphthalene-DNA adducts in mouse skin related to cancer initiation

Naphthalene has been shown to be a weak carcinogen in rats. To investigate its mechanism of metabolic activation and cancer initiation, mice were topically treated with naphthalene or one of its metabolites, 1-naphthol, 1,2-dihydrodiolnaphthalene (1,2-DDN), 1,2-dihydroxynaphthalene (1,2-DHN), and 1,2-naphthoquinone (1,2-NQ). After 4 h, the mice were sacrificed, the treated skin was excised, and the depurinating and stable DNA adducts were analyzed. The depurinating adducts were identified and quantified by ultraperformance liquid chromatography/tandem mass spectrometry, whereas the stable adducts were quantified by (32)P-postlabeling. For comparison, the stable adducts formed when a mixture of the four deoxyribonucleoside monophosphates was treated with 1,2-NQ or enzyme-activated naphthalene were also analyzed. The depurinating adducts 1,2-DHN-1-N3Ade and 1,2-DHN-1-N7Gua arise from reaction of 1,2-NQ with DNA. Similarly, the major stable adducts appear to derive from the 1,2-NQ. The depurinating DNA adducts are, in general, the most abundant. Therefore, naphthalene undergoes metabolic activation to the electrophilic ortho-quinone, 1,2-NQ, which reacts with DNA to form depurinating adducts. This is the same mechanism as other weak carcinogens, such as the natural and synthetic estrogens, and benzene.

Genomic analysis of cancer tissue reveals that somatic mutations commonly occur in a specific motif

Somatic mutations are hallmarks of cancer progression. We sequenced 26 matched human prostate tumor and constitutional DNA samples for somatic alterations in the SRD5A2, HPRT, and HSD3B2 genes, and identified 71 nucleotide substitutions. Of these substitutions, 79% (56/71) occur within a WKVnRRRnVWK sequence (a novel motif we call THEMIS [from the ancient Greek goddess of prophecy]: W=A/T, K=G/T, V=G/A/C, R=purine (A/G), and n=any nucleotide), with one mismatch allowed. Literature searches identified this motif with one mismatch allowed in 66% (37/56) of the somatic prostate cancer mutations and in 74% (90/122) of the somatic breast cancer mutations found in all human genes analyzed. We also found the THEMIS motif with one allowed mismatch in 88% (23/26) of the ras1 gene somatic mutations formed in the sensitive to skin carcinogenesis (SENCAR) mouse model, after induction of error-prone DNA repair following mutagenic treatment. The high prevalence of the motif in each of the above mentioned cases cannot be explained by chance (P<0.046). We further identified 27 somatic mutations in the error-prone DNA polymerase genes pol eta, pol kappa, and pol beta in these prostate cancer patients. The data suggest that most somatic nucleotide substitutions in human cancer may occur in sites that conform to the THEMIS motif. These mutations may be caused by "mutator" mutations in error-prone DNA polymerase genes.

C/EBPbeta represses p53 to promote cell survival downstream of DNA damage independent of oncogenic Ras and p19(Arf)

CCAAT/enhancer-binding protein-beta (C/EBPbeta) is a mediator of cell survival and tumorigenesis. When C/EBPbeta(-/-) mice are treated with carcinogens that produce oncogenic Ras mutations in keratinocytes, they respond with abnormally elevated keratinocyte apoptosis and a block in skin tumorigenesis. Although this aberrant carcinogen-induced apoptosis results from abnormal upregulation of p53, it is not known whether upregulated p53 results from oncogenic Ras and its ability to induce p19(Arf) and/or activate DNA-damage response pathways or from direct carcinogen-induced DNA damage. We report that p19(Arf) is dramatically elevated in C/EBPbeta(-/-) epidermis and that C/EBPbeta represses a p19(Arf) promoter reporter. To determine whether p19(Arf) is responsible for the proapoptotic phenotype in C/EBPbeta(-/-) mice, C/EBPbeta(-/-);p19(Arf-/-) mice were generated. C/EBPbeta(-/-);p19(Arf-/-) mice responded to carcinogen treatment with increased p53 and apoptosis, indicating p19(Arf) is not essential. To ascertain whether oncogenic Ras activation induces aberrant p53 and apoptosis in C/EBPbeta(-/-) epidermis, we generated K14-ER:Ras;C/EBPbeta(-/-) mice. Oncogenic Ras activation induced by 4-hydroxytamoxifen did not produce increased p53 or apoptosis. Finally, when C/EBPbeta(-/-) mice were treated with differing types of DNA-damaging agents, including alkylating chemotherapeutic agents, they displayed aberrant levels of p53 and apoptosis. These results indicate that C/EBPbeta represses p53 to promote cell survival downstream of DNA damage and suggest that inhibition of C/EBPbeta may be a target for cancer cotherapy to increase the efficacy of alkylating chemotherapeutic agents.

Mechanistic insights into the Michael addition of deoxyguanosine to catechol estrogen-3,4-quinones

The reaction of catechol estrogen quinones with DNA to produce the depurinating adducts, 4-OHE 2(E 1)-1-N7Gua and 4-OHE 2(E 1)-1-N3Ade, has been linked to the initiation of breast and other human cancers. A better understanding into the mechanism of how these adducts are formed would be useful to studies aimed at correlating adduct formation to DNA damage. Possible reaction intermediates, produced as a result of Michael addition of deoxyguanosine (dG) to catechol estrogen-3,4-quninones, have been modeled using density functional theory to determine likely intermediates on the potential energy surface (PES) of this reaction. Specifically, the sequence of elimination events, glycosidic bond cleavage and rearomatization of the estrogen A ring, was explored. Consistent with known experimental procedures, B3LYP calculations indicate that a proton source is needed to effect the Michael addition. Calculations also indicate that a catalytic mechanism, where one catechol estrogen quinone could adduct multiple purine bases, is unlikely. Experimental investigation toward an observed cationic reaction intermediate was also consistent with a stoichiometric reaction between estrone-3,4-quinone (E 1-3,4-Q) and dG. HPLC-MS analysis indicates that the cationic reaction intermediate contains the 2'-deoxyribose moiety. Assay of 4-OHE 1-1-N7Gua adduct formation and 2'-deoxyribose formation at different times during the reaction of E 1-3,4-Q with dG indicates that equimolar amounts of each are produced, further supporting a stoichiometric process with respect the catechol estrogen quinone. Differences in the UV spectroscopy of cationic reaction intermediate and the 4-OHE 1-1-N7Gua adduct allowed for kinetic analysis of the glycosidic bond cleavage process. Kinetic scanning analysis indicates that the decomposition of the cationic reaction intermediate is a first-order process with a t 1/2 of 40 min at 30 degrees C. Measurement of the unimolecular rate constant k at different temperatures afforded an Arrhenius plot, which provided values for Delta H, Delta S, and Delta G of 24.7 kcal/mol, 7.2 eu, and 26.8 kcal/mol, respectively. The computational data in conjunction with experimental results are consistent with a mechanism that involves a proton-assisted Michael addition to form an alpha-ketoenol ring system, followed by slow loss of the proton at C1 to restore the aromatic A ring, then fast cleavage of the glycosidic bond to form the 4-OHE 1-1-N7Gua adduct.

Reduction of estrogen-induced transformation of mouse mammary epithelial cells by N-acetylcysteine

A growing number of studies indicate that breast cancer initiation is related to abnormal estrogen oxidation to form an excess of estrogen-3,4-quinones, which react with DNA to form depurinating adducts and induce mutations. This mechanism is often called estrogen genotoxicity. 4-Catechol estrogens, precursors of the estrogen-3,4-quinones, were previously shown to account for most of the transforming and tumorigenic activity. We examined whether estrogen-induced transformation can be reduced by inhibiting the oxidation of a 4-catechol estrogen to its quinone. We demonstrate that E6 cells (a normal mouse epithelial cell line) can be transformed by a single treatment with a catechol estrogen or its quinone. The transforming activities of 4-hydroxyestradiol and estradiol-3,4-quinone were comparable. N-Acetylcysteine, a common antioxidant, inhibited the oxidation of 4-hydroxyestradiol to the quinone and consequent formation of DNA adducts. It also drastically reduced estrogen-induced transformation of E6 cells. These results strongly implicate estrogen genotoxicity in mammary cell transformation. Since N-acetylcysteine is well tolerated in clinical studies, it may be a promising candidate for breast cancer prevention.

Critical assessment of the genetic toxicity of naphthalene

Studies demonstrating that naphthalene produces respiratory tract tumors in mice and rats raised the question of whether humans are at risk for cancer, at environmental or workplace concentrations of naphthalene. Arguments in favor of a threshold-dependent mode of action for tumor induction have been based on the facts that naphthalene does not appear to bind to DNA in vivo and that the rodent tumors occurred at high dose levels associated with substantial target site toxicity. A summary of more than 45 publications describing results for naphthalene in genetic toxicology test methods shows that 80% of the studies reported found no evidence of genotoxicity for naphthalene and that some of the studies which reported positive finding were technically unsuited to study this class of chemicals and, therefore, generated unreliable data. The remaining positive findings for naphthalene were all consistent with secondary DNA effects produced by toxicity from naphthalene alone or one of its metabolites. Based on the data reviewed in this report, it is not apparent that genetic lesions produced by naphthalene or any of its metabolites drive the tumorigenic activity.

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.

Cigarette smoke condensate-induced level of adenomatous polyposis coli blocks long-patch base excision repair in breast epithelial cells

Our previous studies have shown that treatment with cigarette smoke condensate (CSC) transforms normal breast epithelial cell line, MCF-10A. In the present study, the mechanism of CSC-induced transformation of breast epithelial cells was examined. We first determined whether benzo[a]pyrene (B[a]P)- and CSC-induced levels of APC are capable of inhibiting long-patch base excision repair (LP-BER) since our earlier studies had shown that an interaction of APC with DNA polymerase beta (pol-beta) blocks strand-displacement synthesis. With the use of a novel in vivo LP-BER assay, it was demonstrated that increased and decreased APC levels in different breast cancer cell lines were associated with a decrease or increase in LP-BER activity, respectively. The effect of APC on LP-BER in malignant and pre-malignant breast epithelial cell lines was produced by either overexpression or knockdown of APC. Furthermore, it was shown that the decreased LP-BER in B[a]P- or CSC-treated pre-malignant breast epithelial cells is associated with an increased level of APC and decreased cell growth. Our results suggest that the decreased growth allows cells to repair the damaged DNA before mitosis, and failure to repair damaged DNA has the potential to transform pre-malignant breast epithelial cells.

Formation of depurinating N3adenine and N7guanine adducts after reaction of 1,2-naphthoquinone or enzyme-activated 1,2-dihydroxynaphthalene with DNA. Implications for the mechanism of tumor initiation by naphthalene

Naphthalene is considered by the US Environmental Protection Agency to be a carcinogenic compound based on inhalation studies in rats. The primary metabolite of naphthalene is naphthalene 1,2-arene oxide. This unstable intermediate can lead to formation of 1-naphthol and naphthalene-1,2-dihydrodiol. Secondary metabolites include 1,2-dihydroxynaphthalene (1,2-DHN), which can be further oxidized to 1,2-naphthoquinone (1,2-NQ). Based on the metabolism of naphthalene and its similarity to the metabolic activation of carcinogenic natural estrogens, synthetic estrogens and benzene, we hypothesize that naphthalene is activated to initiate cancer by reaction of 1,2-NQ with DNA to form the depurinating adducts 1,2-DHN-4-N3Ade and 1,2-DHN-4-N7Gua. These adducts were synthesized by reaction of 1,2-NQ with Ade or dG in acetic acid/water/DMF (1:1:1). 1,2-NQ was reacted with DNA, and the depurinating 1,2-DHN-4-N3Ade and 1,2-DHN-4-N7Gua adducts were analyzed by ultraperformance liquid chromatography/tandem mass spectrometry and HPLC with electrochemical detection. After the reaction of 1,2-NQ with DNA, the N3Ade and N7Gua adducts were found. Similarly, when 1,2-DHN was activated by tyrosinase in the presence of DNA, higher amounts of the N3Ade and N7Gua adducts were detected. These same adducts were also formed when 1,2-DHN was activated by prostaglandin H synthase or 3-methylcholanthrene-induced rat liver microsomes in the presence of DNA. These depurinating adducts are analogous to those obtained from the ortho-quinones of natural estrogens, synthetic estrogens and benzene. These results suggest that reaction of ortho-quinones with DNA by 1,4-Michael addition is a general mechanism of weak carcinogenesis that occurs with naphthalene and a number of other aromatic compounds.

Androgens and Estrogens in the Etiology and Prevention of Breast Cancer

Breast cancer is the most common cause of cancer death in women worldwide. Rates vary about fivefold around the world, but they are increasing in regions that until recently had low rates of disease. Despite the numerous uncertainties surrounding the etiology of breast cancer, intensive epidemiological, clinical, and genetic studies have identified a number of biologic and social traits as risk factors associated with breast cancer. Principal among them are the evidence of BRCA1 and BRCA2 susceptibility genes, familial history of breast cancer, age, higher socioeconomic status, ionizing radiation, tallness in adult life, alcohol consumption, and a variety of hormone and metabolic factors. Among the hormonal influences, a relevant etiological function has been ascribed to elevated levels of estrogens, their active metabolites, and androgens. These endocrine factors may represent future targets for breast cancer prevention.

Induction of A.T to G.C mutations by erroneous repair of depurinated DNA following estrogen treatment of the mammary gland of ACI rats

Evidence suggests that the genotoxic mechanism of estrogens (estrone/estradiol) in breast cancer involves their oxidation to 3,4-quinones and reaction with DNA to form depurinating N3Ade and N7Gua adducts. We examined whether estrogen genotoxicity is mutagenic in the mammary gland of the female ACI rat, a model for estrogen-dependent breast cancer. Mutagenesis was studied by PCR amplification of the H-ras1 gene (exons 1-2), cloning in pUC18, transforming Escherichia coli, and sequencing the inserts in plasmids from individual colonies. Mammary glands of both estrogen-responsive (ACI and DA) and resistant (Sprague-Dawley) rats contained pre-existing mutations at frequencies of (39.8-58.8)x10(-5), the majority (62.5-100%) of which were A.T to G.C transitions. Estradiol-3,4-quinone (200 nmol) treatment of ACI rats caused rapid (6h to 1 day) mutagenesis (frequency (83.3-156.1)x10(-5); A.T to G.C 70-73.3%). The estrogen-induced A.T to G.C mutations were detected as G.T heteroduplexes, as would be expected if N3Ade depurinations caused Gua misincorporations by erroneous repair. These heteroduplexes were identified by the T.G-DNA glycosylase (TDG) assay. TDG converts G.T heteroduplexes to G.abasic sites, rendering DNA templates refractory to PCR amplification. Consequently, A.T to G.C mutations present as G.T heteroduplexes in the DNA are eliminated from the spectra. TDG treatment of mammary DNA from estradiol-3,4-quinone-treated ACI rats brought A.T to G.C mutations down to pre-existing frequencies. Our results demonstrate that treatment with estradiol-3,4-quinone, an important metabolite of estrogens, produced A.T to G.C mutations in the DNA of the mammary gland of ACI rats.

Free-energy perturbation methods to study structure and energetics of DNA adducts: results for the major N2-dG adduct of benzo[a]pyrene in two conformations and different sequence contexts

The potent mutagen/carcinogen benzo[a]pyrene (B[a]P) is activated to (+)-anti-B[a]PDE, which induces a variety of mutations (e.g., G --> T, G --> A, etc.) via its major adduct [+ta]-B[a]P-N2-dG. One hypothesis is that adducts (such as [+ta]-B[a]P-N2-dG) induce different mutations via different conformations, probably when replicated by different lesion-bypass DNA polymerases (DNAPs). We showed that Escherichia coli DNAP V was responsible for G --> T mutations with [+ta]-B[a]P-N2-dG in a 5'-TGT sequence (Yin et al., (2004) DNA Repair 3, 323), so we wish to study conformations of this adduct/sequence context by molecular modeling. The development of a CHARMM-based molecular dynamics (MD) simulations protocol with free-energy calculations in the presence of solvent and counterions is described. A representative base-pairing and base-displaced conformation of [+ta]-B[a]P-N2-dG in the 5'-TGT sequence are used: (1) BPmi5, which has the B[a]P moiety in the minor groove pointing toward the base on the 5'-side of the adduct, and (2) Gma5, which has the B[a]P moiety stacked with the surrounding base pairs and the dG moiety displaced into the major groove. The MD output structures are reasonable when compared to known NMR structures. Changes in DNA sequence context dramatically affect the biological consequences (e.g., mutagenesis) of [+ta]-B[a]P-N2-dG. Consequently, we also developed a MD-based free-energy perturbation (FEP) protocol to study DNA sequence changes. FEP involves the gradual "fading-out" of atoms in a starting structure (A) and "fading-in" of atoms in a final structure (B), which allows a realistic assessment of the energetic and structural changes when two structures A and B are closely related. Two DNA sequence changes are described: (1) 5'-TGT --> 5'-TGG, which involves two steps [T:A --> T:C --> G:C], and (2) 5'-TGT --> 5'-TGC, which involves three steps [T:A --> T:2AP --> C:2AP --> C:G], where 2AP (2-aminopurine) is included, because T:2AP and C:2AP retain more-or-less normal pairing orientations between complementary bases. FEP is also used to evaluate the impact that a 5'-TGT to 5'-UGT sequence change might have on mutagenesis with [+ta]-B[a]P-N2-dG. In summary, we developed (1) a CHARMM-based molecular dynamics (MD) simulations protocol with free-energy calculations in the presence of solvent and counterions to study B[a]P-N2-dG adducts in DNA duplexes, and (2) a MD-based free-energy perturbation (FEP) protocol to study DNA sequence context changes around B[a]P-N2-dG adducts.

Determination of apurinic/apyrimidinic lesions in DNA with high-performance liquid chromatography and tandem mass spectrometry

A new method has been developed to accurately measure apurinic and apyrimidinic (AP) DNA damage sites, which are lesions in DNA formed by loss of a nucleobase from oxidative stress or carcinogen adducts. If AP sites are left unrepaired (or if improperly repaired), these sites can lead to DNA mutations that may ultimately result in the formation of cancer. Hence, detection of AP sites may provide a useful indicator of exposure and susceptibility to chemical carcinogens and oxidative stress. AP detection is currently accomplished by immunodetection methods using an aldehyde reactive probe [Nakamura, J., Walker, V. E., Upton, P. B., Chiang, S.-Y., Kow, Y. W., and Swenberg, J. A. (1998) Cancer Res. 58, 222-225; Atamna, H., Cheung, I., and Ames, B. N. (2000) Proc. Natl. Acad. Sci. U.S.A. 97, 686-691]; however, these approaches lack the specificity required for unequivocal identification of the AP site. Therefore, we have developed an accurate method based on mass spectrometry detection of AP sites from AP DNA that have been prelabeled with O-4-nitrobenzylhydroxylamine (NBHA). Once labeled and once the excess labeling agent has been removed, enzymatic digestion of DNA to monomeric subunits can be accomplished, followed by isolation and detection with high-performance liquid chromatography coupled to electrospray ionization tandem mass spectrometry (HPLC-ESI-MS/MS). Optimization and validation of the experimental procedures and detection limits have been established using a model DNA oligomer (11-mer) containing uracil. Enzymatic removal of uracil with uracil glycosylase generates well-defined AP sites in both single- and double-stranded DNA. The addition of NBHA labels the AP site in the oligomer, creating a labeled 11-mer. HPLC-ESI-MS/MS in the negative ionization mode was used to monitor and confirm binding of NBHA to the AP oligomer. The NBHA-tagged oligomer underwent endo- and exonuclease digestion to the 5'-deoxyribose monophosphate (5'-dRp) level, thereby releasing free 5'-dRp-NBHA. The 5'-dRp-NBHA product was partially purified by solid phase extraction and quantified by LC-MS/MS using several transitions of the deprotonated molecule ([M - H]- at m/z 363) and isotopically labeled 5'-dRp-NBHA as an internal standard. Further experiments with 5',3'-bisphosphate-deoxyribose and heat/acid-treated calf thymus DNA showed similar labeling, digestion, and detection results. Initial results show a quantification limit with 100 mug of DNA to be 100 fmol (three abasic sites per 10(7) bases).

Catechol estrogen quinones as initiators of breast and other human cancers: implications for biomarkers of susceptibility and cancer prevention

Exposure to estrogens is associated with increased risk of breast and other types of human cancer. Estrogens are converted to metabolites, particularly the catechol estrogen-3,4-quinones (CE-3,4-Q), that can react with DNA to form depurinating adducts. These adducts are released from DNA to generate apurinic sites. Error-prone base excision repair of this damage may lead to the mutations that can initiate breast, prostate and other types of cancer. The reaction of CE-3,4-Q with DNA forms the depurinating adducts 4-hydroxyestrone(estradiol) [4-OHE1(E2)-1-N3Ade and 4-OHE1(E2)-1-N7Gua. These two adducts constitute more than 99% of the total DNA adducts formed. Increased levels of these quinones and their reaction with DNA occur when estrogen metabolism is unbalanced. Such an imbalance is the result of overexpression of estrogen activating enzymes and/or deficient expression of the deactivating (protective) enzymes. This unbalanced metabolism has been observed in breast biopsy tissue from women with breast cancer, compared to control women. Recently, the depurinating adduct 4-OHE1(E2)-1-N3Ade has been detected in the urine of prostate cancer patients, but not in urine from healthy men. Mutagenesis by CE-3,4-Q has been approached from two different perspectives: one is mutagenic activity in the lacI reporter gene in Fisher 344 rats and the other is study of the reporter Harvey-ras gene in mouse skin and rat mammary gland. A-->G and G-->A mutations have been observed in the mammary tissue of rats implanted with the CE-3,4-Q precursor, 4-OHE2. Mutations have also been observed in the Harvey-ras gene in mouse skin and rat mammary gland within 6-12 h after treatment with E2-3,4-Q, suggesting that these mutations arise by error-prone base excision repair of the apurinic sites generated by the depurinating adducts. Treatment of MCF-10F cells, which are estrogen receptor-alpha-negative immortalized human breast epithelial cells, with E2, 4-OHE2 or 2-OHE2 induces their neoplastic transformation in vitro, even in the presence of the antiestrogen ICI-182,780. This suggests that transformation is independent of the estrogen receptor. The transformed cells exhibit specific mutations in several genes. Poorly differentiated adenocarcinomas develop when aggressively transformed MCF-10F cells are selected and injected into severe combined immune depressed (SCID) mice. These results represent the first in vitro/in vivo model of estrogen-induced carcinogenesis in human breast epithelial cells. In other studies, the development of mammary tumors in estrogen receptor-alpha knockout mice expressing the Wnt-1 oncogene (ERKO/Wnt-1) provides direct evidence that estrogens may cause breast cancer through a genotoxic, non-estrogen receptor-alpha-mediated mechanism. In summary, this evidence strongly indicates that estrogens can become endogenous tumor initiators when CE-3,4-Q react with DNA to form specific depurinating adducts. Initiated cells may be promoted by a number of processes, including hormone receptor stimulated proliferation. These results lay the groundwork for assessing risk and preventing disease.

Homology modeling of four Y-family, lesion-bypass DNA polymerases: the case that E. coli Pol IV and human Pol kappa are orthologs, and E. coli Pol V and human Pol eta are orthologs

Y-family DNA polymerases (DNAPs) are a superfamily of evolutionarily related proteins that exist in cells to bypass DNA damage caused by both radiation and chemicals. Cells have multiple Y-family DNAPs, presumably to conduct translesion synthesis (TLS) on DNA lesions of varying structure and conformation. The potent, ubiquitous environmental mutagen/carcinogen benzo[a]pyrene (B[a]P) induces all classes of mutations with G-->T base substitutions predominating. We recently showed that a G-->T mutagenesis pathway for the major adduct of B[a]P ([+ta]-B[a]P-N2-dG) in Escherichia coli depends on Y-family member DNAP V. Since no X-ray crystal study for DNAP V has been reported, no structure is available to help in understanding the structural basis for dATP insertion associated with G-->T mutations from [+ta]-B[a]P-N2-dG. Herein, we do homology modeling to construct a model for UmuC, which is the polymerase subunit of DNAP V. The sequences of eight Y-family DNAPs were aligned based on the positioning of conserved amino acids and an analysis of conserved predicted secondary structure, as well as insights gained from published X-ray structures of five Y-family members. Starting coordinates for UmuC were generated from the backbone coordinates for the Y-family polymerase Dpo4 for reasons discussed, and were refined using molecular dynamics with CHARMM 27. A survey of the literature revealed that E. coli DNAP V and human DNAP eta show a similar pattern of dNTP insertion opposite a variety of DNA lesions. Furthermore, E. coli DNAP IV and human DNAP kappa show a similar dNTP insertional pattern with these same DNA lesions, although the insertional pattern for DNAP IV/kappa differs from the pattern for DNAPs V/eta. These comparisons prompted us to construct and refine models for E. coli DNAP IV and human DNAPs eta and kappa as well. The dNTP/template binding pocket of all four DNAPs was inspected, focusing on the array of seven amino acids that contact the base of the incoming dNTP, as well as the template base. DNAPs V and eta show similarities in this array, and DNAPs IV and kappa also show similarities, although the arrays are different for the two pairs of DNAPs. Thus, there is a correlation between structural similarities and insertional similarities for the pairs DNAPs V/eta and DNAPs IV/kappa. Although the significance of this correlation remains to be elucidated, these observations point the way for future experimental studies.

Effects of naphthalene quinonoids on the induction of oxidative DNA damage and cytotoxicity in calf thymus DNA and in human cultured cells

The purpose of this study was to examine the differences in the induction of DNA damage and cytotoxic effects by quinonoid derivatives of naphthalene in calf thymus DNA (ct-DNA) and in human T47D breast cancer cells. Results indicated that copper(II) and NADPH were essential for causing oxidant-mediated aldehydic DNA lesions (ADLs), including abasic sites and aldehydic base/sugar lesions, in ct-DNA exposed to 1,2-naphthalenediol (NCAT), 1,4-naphthalenediol (NHQ), 1,2-naphthoquinone (1,2-NQ), and 1,4-naphthoquinone (1,4-NQ). The ADLs induced by naphthalene quinonoids in ct-DNA decrease in the rank order NCAT congruent with 1,2-NQ > NHQ >> 1,4-NQ. Results from the analyses in cells indicated that after 1.5-5 h of exposure all naphthalene quinonoids induced a cytotoxic response in T47D cells at concentrations 10-100 microM or above, where NHQ and 1,4-NQ were approximately 5-10 times more efficient than NCAT and 1,2-NQ in the induction of cell death. In addition, NHQ, 1,2-NQ, and 1,4-NQ were not able to produce measurable levels of ADLs in cells at concentrations up to 1.25 mM, whereas NCAT (0.75-1.25 mM) induced a significant increase in the number of ADLs in T47D cells after 1.5 h of exposure when compared to control. The specific type of ADLs induced by NCAT is resistant to cellular excision repair pathway. Results from the measurements of reactive oxygen species (ROS) indicated that all naphthalene quinonoids induced increases in ROS formation in T47D cells. The induction of ROS formation in cells by naphthalene quinonoids decreases in the rank order 1,4-NQ congruent with 1,2-NQ > NHQ > NCAT. Overall, results from our investigation suggest that naphthalene quinonoids cause cell death at concentrations well below the concentrations at which they induce the formation of ADLs, perhaps by altering intracellular redox status.

Mutagenesis studies with four stereoisomeric N2-dG benzo[a]pyrene adducts in the identical 5′-CGC sequence used in NMR studies: G→T mutations dominate in each case

Benzo[a]pyrene (B[a]P) is a polycyclic aromatic hydrocarbon (PAH) and a potent mutagen/carcinogen found ubiquitously in the environment. B[a]P is primarily metabolized to diol epoxides, which react principally at N2-dG in DNA. B[a]P-N2-dG adducts have been shown to induce a variety of mutations, notably G-->T, G-->A, G-->C and -1 frameshifts. Four stereoisomers of B[a]P-N2-dG (designated: [+ta]-;, [+ca]-, [-ta] and [-ca]) were studied by NMR in duplex 11mers in a 5'-CGC sequence context, and each adopted a different adduct conformation (Geacintov, et al. (1997) Chem. Res. Toxicol., 10, 111). Herein these four identical B[a]P-containing 11mers are built into duplex plasmid genomes and mutagenesis studied in Escherichia coli following SOS-induction. In nucleotide excision repair (NER) proficient E.coli, no adduct-derived mutants are detected. In NER deficient E.coli, G-->T mutations dominate for all four stereoisomers [+ta]-, [+ca]-, [-ta] and [-ca]-B[a]P-N(2)-dG, and mutation frequency is similar. Thus, the mutagenic pattern for these four B[a]P-N2-dG stereoisomers is the same, in spite of the fact that they adopt dramatically different conformations in ds-oligonucleotides as determined by NMR. These findings suggest that adduct conformation must be fluid enough in the 5'-CGC sequence that the duplex DNA conformation can interconvert to mutagenic and non-mutagenic conformations during lesion-bypass. A comparison of all published studies with these four B[a]P-N2-dG stereoisomers in E.coli reveals that B[a]P-N2-dG adduct stereochemistry tends to have a lesser impact on mutagenic pattern (e.g. G-->T versus G-->A mutations) than does DNA sequence context, which is discussed.

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.

Harvey-ras gene expression and epidermal cell proliferation in dibenzo[a,l]pyrene-treated early preneoplastic SENCAR mouse skin

Topical application of dibenzo[a,l]pyrene (DB[a,l]P) to the dorsal skin of SENCAR mice induces codon 61 (CAA Gln to CTA Leu) mutations in the Harvey (H)-ras gene within 12 h after treatment. Between days 1 and 3, the frequency of these mutations increases rapidly, suggesting that skin cells carrying the codon 61 mutations proliferate in this period. We have investigated DB[a,l]P-treated mouse skin (12 h-7 d) for further evidence of H-ras expression and epidermal cell proliferation. Two waves of cell proliferation were observed: the first wave (1-2 d) correlated with the clonal proliferation of codon 61-mutated cells, and the second wave (3-7 d) correlated with DB[a,l]P-induced hyperplasia. DB[a,l]P-induced early preneoplastic cell proliferation correlated with H-ras and specific G1 cyclin expression. Total H-ras protein and cyclin D1 were found to increase during DB[a,l]P-induced hyperplasia, but the levels of guanosine triphosphate-bound (active) H-ras protein and cyclin E were increased during the putative clonal proliferation of codon 61-mutated cells. These results suggest that DB[a,l]P-induced oncogenically mutated cells proliferate in early preneoplastic skin. As this proliferation occurs in the absence of any promoting treatment, we propose that this phenomenon is a tumor initiation event.

Identification and quantification of the depurinating DNA adducts formed in mouse skin treated with dibenzo[a,l]pyrene (DB[a,l]P) or its metabolites and in rat mammary gland treated with DB[a,l]P

Dibenzo[a,l]pyrene (DB[a,l]P) is the most potent carcinogenic polycyclic aromatic hydrocarbon and has been identified in the environment. Comparative tumorigenicity studies in mouse skin and rat mammary gland indicate that DB[a,l]P is slightly more potent than DB[a,l]P-11,12-dihydrodiol and much more potent than (+/-)-syn-DB[a,l]P-11,12-dihydrodiol-13,14-epoxide {(+/-)-syn-DB[a,l]PDE} and (+/-)-anti-DB[a,l]PDE. We report here the identification and quantification of the depurinating adducts formed in mouse skin treated with DB[a,l]P, DB[a,l]P-11,12-dihydrodiol, (+/-)-syn-DB[a,l]PDE, or (+/-)-anti-DB[a,l]PDE and rat mammary gland treated with DB[a,l]P. The biologically formed adducts were compared with standard adducts by their retention times on HPLC and their spectra obtained by fluorescence line-narrowing spectroscopy at low temperature. In mouse skin treated with DB[a,l]P, depurinating adducts comprised 99% of the total adducts. Most of the depurinating adducts were formed by one-electron oxidation, with 63% at Ade and 12% at Gua. The remainder were formed by the diol epoxide, with 18% at Ade and 6% at Gua. When mouse skin was treated with DB[a,l]P-11,12-dihydrodiol, depurinating adducts comprised 80% of the total, and the predominant one was with Ade (69%). Treatment of skin with (+/-)-syn-DB[a,l]PDE resulted in 32% depurinating adducts, primarily at Ade (25%), whereas treatment with (+/-)-anti-DB[a,l]PDE produced 97% stable adducts. The formation of depurinating adducts following treatment of rat mammary gland with DB[a,l]P resulted in approximately 98% depurinating adducts, with the major adducts formed by one-electron oxidation. Only one depurinating diol epoxide adduct was formed. Tumorigenicity, mutations, and DNA adduct data suggest that depurinating Ade adducts play a major role in the initiation of tumors by DB[a,l]P.

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

The stable adducts of dibenzo[a,l]pyrene (DB[a,l]P) formed by rat liver microsomes in vitro were previously quantified, whereas the depurinating adducts were both identified and quantified [Li, et al. (1995) Biochemistry 34, 8043]. In this article, we report the identification and quantification of the stable DNA adducts obtained from DB[a,l]P and DB[a,l]P-11,12-dihydrodiol activated by rat liver microsomes and from reaction of (+/-)-anti-DB[a,l]P-11,12-dihydrodiol-13,14-epoxide (DB[a,l]PDE) and (+/-)-syn-DB[a,l]PDE with DNA in vitro. In addition, the stable DNA adducts were identified and quantified following treatment of mouse skin with DB[a,l]P, DB[a,l]P-11,12-dihydrodiol, (+/-)-anti-DB[a,l]PDE, or (+/-)-syn-DB[a,l]PDE in vivo and treatment of rat mammary gland with DB[a,l]P in vivo. The DNA adducts were analyzed by the (32)P-postlabeling method, and the major adducts were identified by comparison with standards. The six stable adducts of DB[a,l]P formed by rat liver microsomes in vitro were either guanine or adenine adducts of anti-DB[a,l]PDE or syn-DB[a,l]PDE. About 43% of the detected stable adducts from microsomes were with guanine and 44% were with adenine. The pattern of adducts formed from DB[a,l]P-11,12-dihydrodiol with microsomes was very similar to that from DB[a,l]P. Reaction of (+/-)-anti-DB[a,l]PDE with DNA in vitro formed higher levels of stable adducts (55% from guanine and 39% from adenine) than (+/-)-syn-DB[a,l]PDE did (about 44% with guanine and 47% with adenine). In mouse skin treated with DB[a,l]P, 1% of the total adducts detected were stable adducts, comprised of 51% guanine adducts and 46% from adenine; with DB[a,l]P-11,12-dihydrodiol, 54% of the total were stable adducts, with a pattern of adducts similar to those formed from DB[a,l]P. Treatment of mouse skin with (+/-)-syn-DB[a,l]PDE formed 68% stable adducts, mostly at guanine. With (+/-)-anti-DB[a,l]PDE, mouse skin contained almost exclusively (97%) stable adducts: 61% guanine adducts and 33% adenine adducts. In rat mammary gland treated with DB[a,l]P, 2% of the total adducts were stable, with 42% guanine adducts and 55% adenine adducts. Approximately equal to or greater amounts of stable guanine adducts were formed in all systems, except for rat mammary gland. In contrast, the majority of depurinating adducts were adenine adducts. The carcinogenic potencies of these compounds in mouse skin, published earlier, do not qualitatively or quantitatively correlate with stable adducts, but rather with depurinating adducts.

The effects of 3-methylcholanthrene on lymphocyte proliferation in the common carp (Cyprinus carpio L.)

The sensitivity of lymphocyte proliferation as bioindicator of pollution stress was evaluated in the common carp (Cyrinus carpio L.). The time course response of peripheral blood leukocyte proliferation in response or not to mitogens was measured from 1 to 7 days after peritoneal injection of 3-methylcholantrene (3-MC), and compared to the time course response of a highly sensitive biomarker, induction of cytochrome P450. 3-Methylcholanthrene (40 mg kg(-1)) inhibited both B- and T-lymphocyte proliferation in response to lipopolysaccharide (LPS) and concanavalin A (Con A). Studies with alpha-naphtofiavone, suggest the lack of metabolic processes. 3-Methylcholanthrene alone strongly stimulated resting peripheral blood leukocytes (PBLs) proliferation. This effect was not transient. The induction of lymphocyte proliferation paralleled the increase in cytochrome P450 content in the liver. The specificity of polycyclic aromatic hydrocarbon (PAH)-induced lymphocyte proliferation suggests that this immune activity may be an early marker of exposure to PAHs in aquatic environments. The capacity of 3-MC to induce rapid lymphocyte proliferation may be related to PAH-induced rapid clonal expansion in mammals. These results strongly suggested that the underlying mechanism might be the same in both models. More studies are needed in fish to explain this phenomenon and may be helpful in understanding the occurrence of neoplastic epizootics in fish associated with PAH exposition.

Tumor formation in the neonatal mouse bioassay indicates that the potent carcinogen dibenzo[def,p]chrysene (dibenzo[a,l]pyrene) is activated in vivo via its trans-11,12-dihydrodiol

The hexacyclic aromatic hydrocarbon dibenzo[def,p]chrysene, better known as dibenzo[a,l]pyrene (DBP) in the field of chemical carcinogenesis, is present in the environment as a combustion product of organic matter. This compound is probably the strongest chemical carcinogen ever tested. As ultimate genotoxic metabolites of DBP two electrophilically reactive species are discussed: (i) radical cations generated by one-electron oxidation, and (ii) fjord region dihydrodiol epoxides formed via the trans-11,12-dihydroxy 11,12-dihydro derivative of DBP (11,12-dihydrodiol). In order to delineate the metabolic pathway(s) involved in tumor formation by DBP, newborn Crl:CD-1(ICR)BR mice were intraperitoneally treated with the parent compound, its 11,12-dihydrodiol, and the two diastereomeric fjord region dihydrodiol epoxides. Due to severe acute and chronic toxicity, the total dose of DBP and of the 11,12-dihydrodiol was limited to 40 nmol. For the same reason the dihydrodiol epoxides could only be applied in doses up to 0.4 nmol. The tumor incidence was determined 55 +/- 1 weeks after treatment. Under these conditions, DBP and its 11,12-dihydrodiol induced lung tumors (incidence: 86.5% versus 92.0%; yield: 2.88 versus 7.44 tumors per mouse), liver (incidence: 57.7% versus 60.0%; yield: 3.63 versus 5.28 tumors per mouse) and other organs (incidence: 36.5% versus 32.0%; yield: 0.56 versus 0.52 tumors per mouse). By contrast, only lung tumors at low incidence were detected in mice treated with solvent only (incidence: 28.8%; yield: 0.58 tumors per mouse). As with the parent hydrocarbon, mice treated with low doses of diastereomeric syn- and anti-dihydrodiol epoxides of DBP showed increased tumor incidences in liver (incidence: 19.0 and 46.7%; yield: 0.36 and 1.47 tumors per mouse, respectively), and in various other organs (incidence: 7.1 and 20.0%; yield: 0.07 and 0.20 tumors per mouse, respectively). In consideration of the 100-fold differences in the doses of compounds applied in this study, the tumor-inducing potency increases in the order DBP < 11,12-dihydrodiol < anti-dihydrodiol epoxide. This result provides strong evidence that the potent carcinogen DBP is activated in vivo in the mouse via its 11,12-dihydrodiol and not preferentially through alternative pathways.

Genotoxicity of steroidal estrogens

The molecular mechanisms underlying the development of breast cancer in general, and estrogen-associated breast carcinogenesis in particular, are not completely understood. There are three mechanisms considered responsible for the carcinogenicity of estrogens in the human breast: (i) receptor-mediated hormonal activity, which stimulates cellular proliferation, resulting in more opportunities for accumulation of the genetic damage that leads to carcinogenesis; (ii) a cytochrome P450-mediated metabolic activation, which elicits direct genotoxic effects by increasing mutation rates; and (iii) the induction of aneuploidy by estrogen. In this article, we concentrate on discussing the role of estrogen receptors and the metabolic activation of 17beta-estradiol (E(2)) as mechanisms of breast cancer initiation.

Estrogen and its metabolites are carcinogenic agents in human breast epithelial cells

Estrogens play a crucial role in the development and evolution of human breast cancer. However, it is still unclear whether estrogens are carcinogenic to the human breast. There are three mechanisms that have been considered to be responsible for the carcinogenicity of estrogens: receptor-mediated hormonal activity, a cytochrome P450 (CYP)-mediated metabolic activation, which elicits direct genotoxic effects by increasing mutation rates, and the induction of aneuploidy by estrogen. To fully demonstrate that estrogens are carcinogenic in the human breast through one or more of the mechanisms explained above it will require an experimental system in which, estrogens by itself or one of the metabolites would induce transformation phenotypes indicative of neoplasia in HBEC in vitro and also induce genomic alterations similar to those observed in spontaneous malignancies. In order to mimic the intermittent exposure of HBEC to endogenous estrogens, MCF-10F cells that are ERalpha negative and ERbeta positive were first treated with 0, 0.007, 70 nM and 1 microM of 17beta-estradiol (E(2)), diethylstilbestrol (DES), benz(a)pyrene (BP), progesterone (P), 2-OH-E(2), 4-hydoxy estradiol (4-OH-E(2)) and 16-alpha-OH-E(2) at 72 h and 120 h post-plating. Treatment of HBEC with physiological doses of E(2), 2-OH-E(2), 4-OH-E(2) induce anchorage independent growth, colony formation in agar methocel, and reduced ductulogenic capacity in collagen gel, all phenotypes whose expression are indicative of neoplastic transformation, and that are induced by BP under the same culture conditions. The presence of ERbeta is the pathway used by E(2) to induce colony formation in agar methocel and loss of ductulogenic in collagen gel. This is supported by the fact that either tamoxifen or the pure antiestrogen ICI-182,780 (ICI) abrogated these phenotypes. However, the invasion phenotype, an important marker of tumorigenesis is not modified when the cells are treated in presence of tamoxifen or ICI, suggesting that other pathways may be involved. Although we cannot rule out the possibility, that 4-OH-E(2) may interact with other receptors still not identified, with the data presently available the direct effect of 4-OH-E(2) support the concept that metabolic activation of estrogens mediated by various cytochrome P450 complexes, generating through this pathway reactive intermediates that elicit direct genotoxic effects leading to transformation. This assumption was confirmed when we found that all the transformation phenotypes induced by 4-OH-E(2) were not abrogated when this compound was used in presence of the pure antiestrogen ICI. The novelty of these observations lies in the role of ERbeta in transformation and that this pathway can successfully bypassed by the estrogen metabolite 4-OH-E(2). Genomic DNA was analyzed for the detection of micro-satellite DNA polymorphism using 64 markers covering chromosomes (chr) 3, 11, 13 and 17. We have detected loss of heterozygosity (LOH) in ch13q12.2-12.3 (D13S893) and in ch17q21.1 (D17S800) in E(2), 2-OH-E(2), 4-OH-E(2), E(2) + ICI, E(2) + tamoxifen and BP-treated cells. LOH in ch17q21.1-21.2 (D17S806) was also observed in E(2), 4-OH-E(2), E(2)+ICI, E(2)+tamoxifen and BP-treated cells. MCF-10F cells treated with P or P+E(2) did not show LOH in the any of the markers studied. LOH was strongly associated with the invasion phenotype. Altogether our data indicate that E(2) and its metabolites induce in HBEC LOH in loci of chromosomes 13 and 17, that has been reported in primary breast cancer, that the changes are similar to those induced by the chemical carcinogen (BP) and that the genomic changes were not abrogated by antiestrogens.

Spectral characterization of catechol estrogen quinone (CEQ)-derived DNA adducts and their identification in human breast tissue extract

Estrogens, including the natural hormones estrone (E(1)) and estradiol (E(2)), are thought to be involved in tumor induction. Catechol estrogen quinones (CEQ) derived from 4-hydroxyestrone (4-OHE(1)) and 4-hydroxyestradiol (4-OHE(2)) react with DNA and form depurinating N7Gua and N3Ade adducts that might be responsible for tumor initiation (Cavalieri, E. L., et al. (2000) J. Natl. Cancer Inst. Monogr. 27, 75). Current detection limits for the CEQ-derived DNA adducts by high-performance liquid chromatography with multichannel electrochemical detection are in the picomole range. To improve the limit of detection (LOD) for CEQ-derived DNA adducts, spectrophotometric monitoring was investigated. Spectroscopic studies of 4-OHE(1)-1-N3Ade, 4-OHE(1)-1-N7Gua, 4-OHE(2)-1-N3Ade, and 4-OHE(2)-1-N7Gua adduct standards were performed at 77 and 300 K. Upon laser excitation at 257 nm, the 4-OHE(1)- and 4-OHE(2)-derived N7Gua and N3Ade adducts are strongly phosphorescent at T = 77 K. No phosphorescence was observed at 300 K. Both N3Ade and N7Gua adduct types have weak phosphorescence origin bands near 383 and 385 nm, respectively. The corresponding phosphorescence lifetimes are 1.11 +/- 0.05 and 0.37 +/- 0.05 s. The LOD, based on phosphorescence measurements, is in the low femtomole range. The concentration LOD is approximately 10(-9) M, i.e., similar to that recently obtained for CEQ-derived N-acetylcysteine conjugates (Jankowiak, R., et al. (2003) Chem. Res. Toxicol. 16, 304). The LOD in capillary electrophoresis (CE) with field-amplified sample stacking and absorbance detection is about 3 x 10(-8) M. To verify whether CEQ-derived DNA adducts are formed in humans or not, tissue extracts from two breast cancer patients were analyzed by CE interfaced with room temperature absorption and low temperature (laser-excited) phosphorescence spectroscopies. For the first time, formation of CEQ-derived DNA adducts is shown in humans. For example, the level of 4-OHE(1)-1-N3Ade in the breast tissue extract from a patient with breast carcinoma (8.40 +/- 0.05 pmol/g of tissue) is larger by a factor of about 30 than that in the breast tissue sample from a woman without breast cancer (0.25 +/- 0.05 pmol/g of tissue). In contrast, similar amounts of 4-OHE(2)-1-N3Ade were observed in both types of tissue. Although more breast tissue samples from women with and without breast cancer need to be studied, these results suggest that the N3Ade adducts could serve as biomarkers to predict the risk of breast cancer.

Molecular modeling of the major benzo[a]pyrene N2-dG adduct in cases where mutagenesis results are known in double stranded DNA

The potent mutagen/carcinogen benzo[a]pyrene (B[a]P) is metabolically activated to (+)-anti-B[a]PDE, which induces a full spectrum of mutations (e.g. GC-->TA, GC-->AT, etc.). One hypothesis for this complexity is that different mutations are induced by different conformations of its major adduct [+ta]-B[a]P-N2-dG when bypassed during DNA replication (probably by different DNA polymerases). Previous molecular modeling studies suggested that B[a]P-N2-dG adducts can in principle adopt at least 16 potential conformational classes in ds-DNA. Herein we report on molecular modeling studies with the eight conformations most likely to be relevant to base substitution mutagenesis in 10 cases where mutagenesis has been studied in ds-DNA plasmids in E. coli with B[a]P-N2-dG adducts of differing stereoisomers and DNA sequence contexts, as well as in five cases where the conformation is known by NMR. Of the approximately 11,000 structures generated in this study, the computed lowest energy structures are reported for 120 cases (i.e. eight conformations and 15 examples), and their conformations compared. Of the eight conformations, four are virtually always computed to be high in energy. The remaining four lower energy conformations include two with the BP moiety in the minor groove (designated: BPmi5 and BPmi3), and two base-displaced conformations, one with the dG moiety in the major groove (designated: Gma5) and one with the dG in the minor groove (designated: Gmi3). Interestingly, these four are the only conformations that have been observed for B[a]P-N2-dG adducts in NMR studies. Independent of sequence contexts and adduct stereochemistry, BPmi5 structures tend to look reasonably similar, as do BPmi3 structures, while the base-displaced structures Gma5 and BPmi3 tend to show greater variability in structure. A correlation was sought between modeling and mutagenesis results in the case of the low energy conformations BPmi5, BPmi3, Gma5 and Gma3. Plots of log[(G-->T)/(G-->A)] versus energy[(conformation X)-(conformation Y)] were constructed for all six pairwise combinations of these four conformations, and the only plot giving a straight line involved Gma5 and Gmi3. While this finding is striking, its significance is unclear (as discussed).

Catechol ortho-quinones: the electrophilic compounds that form depurinating DNA adducts and could initiate cancer and other diseases

Catechol estrogens and catecholamines are metabolized to quinones, and the metabolite catechol (1,2-dihydroxybenzene) of the leukemogenic benzene can also be oxidized to its quinone. We report here that quinones obtained by enzymatic oxidation of catechol and dopamine with horseradish peroxidase, tyrosinase or phenobarbital-induced rat liver microsomes react with DNA by 1,4-Michael addition to form predominantly depurinating adducts at the N-7 of guanine and the N-3 of adenine. These adducts are analogous to the ones formed with DNA by enzymatically oxidized 4-catechol estrogens (Cavalieri,E.L., et al. (1997) PROC: Natl Acad. Sci., 94, 10937). The adducts were identified by comparison with standard adducts synthesized by reaction of catechol quinone or dopamine quinone with deoxyguanosine or adenine. We hypothesize that mutations induced by apurinic sites, generated by the depurinating adducts, may initiate cancer by benzene and estrogens, and some neurodegenerative diseases (e.g. Parkinson's disease) by dopamine. These data suggest that there is a unifying molecular mechanism, namely, formation of specific depurinating DNA adducts at the N-7 of guanine and N-3 of adenine, that could initiate many cancers and neurodegenerative diseases.

Reactions of estradiol-2,3-quinone with deoxyribonucleosides: possible insights in the reactivity of estrogen quinones with DNA

Estrogen 2,3- and 3,4-quinones are reactive species toward nucleophiles and Michael acceptors. As such, they can bind to DNA and induce cellular damages. As an alkylation model, reactions of estradiol-2,3-quinone with deoxyribonucleosides were previously studied by mass spectrometry. In this work, estrogen-deoxyribonucleoside adducts were synthesized by reaction of 17beta-estradiol-2,3-quinone with deoxyguanosine or deoxyadenosine and analyzed by NMR and LC-MS(n)() in order to determine the structure and the stereochemistry of the resulting covalent adducts. Although estradiol- and estrone-2,3-quinones were previously thought to give mainly stable adducts, identification of depurinating adducts with both nucleosides, i.e., 2-OHE(2)-6(alpha,beta)-N7Gua and 2-OHE(2)-6(alpha,beta)-N7Ade, was unambiguously obtained. This is of particular interest since depurinating adducts are generated from DNA, and therefore, their amount should be correlated to the parallel formation of apurinic sites, which might play an important role in the cancer initiation process. Besides, a byproduct, i.e., 2-hydroxy-11-oxo-estradiol, corresponding to an unstable alkylation product of 2-hydroxyestradiol has been unambiguously identified and is indicative of a plausible addition process at the C9 position of catechol estrogens. The synthetic adducts will be useful as reference compounds to further elucidate the structure of adducts formed by reaction of estrogen metabolites with DNA or oligonucleotides.

A unified mechanism in the initiation of cancer

Estrogens are involved in the initiation of breast, prostate, and other kinds of human cancer. In this process, the endogenous estrogens, estrone and estradiol, are metabolized to 2-catechol estrogens (2-CE, major) and 4-CE (minor). If the 4-CEs are further oxidized to CE-3,4-quinones, they may react with DNA to form depurinating adducts at N-7 of guanine and N-3 of adenine, and generate apurinic sites. Similarly, the carcinogenic synthetic estrogen hexestrol, a hydrogenated derivative of diethylstilbestrol, is metabolized to its quinone, which reacts with DNA to form analogous depurinating adducts. This could be the primary critical event leading to oncogenic mutations and then initiation of cancer. Evidence supporting this hypothesis has been obtained from the human breast and animal models susceptible to estrogen-induced tumors, including the Syrian golden hamster kidney, ACI rat mammary gland, and Noble rat prostate. The oxidation of phenols to catechols and then to quinones is not only a mechanism of tumor initiation for natural and synthetic estrogens, but also for the leukemogen benzene. In fact, catechol, one of the metabolites of benzene, when oxidized to its quinone, reacts with DNA to form N7guanine and N3adenine depurinating adducts. Thus, a unifying mechanism, namely formation of catechol quinones and reaction with DNA, could initiate not only cancer by oxidation of specific endogenous estrogen metabolites, but also leukemia by oxidation of benzene.

Catechol estrogen metabolites and conjugates in different regions of the prostate of Noble rats treated with 4-hydroxyestradiol: implications for estrogen-induced initiation of prostate cancer

Prostate carcinomas arise in 100% of Noble rats treated with estradiol and testosterone. We hypothesize that estrogens initiate prostate cancer mainly by formation of 4-catechol estrogens (CE), followed by their oxidation to catechol estrogen-3,4-quinones (CE-3,4-Q), which can react with DNA. To avoid cancer initiation, CE can be detoxified by catechol-O-methyltransferase (COMT), and CE-3,4-Q by conjugation with glutathione (GSH) or by reduction to CE, catalyzed by quinone reductase and/or cytochrome P450 reductase. To investigate the prostatic metabolism of estrogens, Noble rats were treated with the CE 4-hydroxyestradiol (4-OHE2) or estradiol-3,4-quinone (E2-3,4-Q), and CE metabolites and conjugates were analyzed in the four regions of the prostate, which differ in susceptibility to carcinoma formation. Following treatment of rats with 4-OHE2 (6 micromol/100 g body weight in 200 microl of trioctanoin/dimethylsulfoxide (4:1) by intraperitoneal injection) for 90 min, the non-susceptible ventral (VP) and anterior (AP) prostate had higher levels of 4-methoxyCE and GSH conjugates than the susceptible dorsolateral prostate (DLP) and periurethral prostate (PUP). After treatment with the same molar amount of E2-3,4-Q, the VP and AP contained more GSH conjugates, 4-CE and 4-methoxyCE than the susceptible DLP and PUP. These results suggest that prostate areas susceptible to carcinoma induction have less protection by COMT, GSH, and quinone reductase and/or cytochrome P450 reductase, favoring reaction of CE-3,4-Q with DNA, presumably to initiate cancer.

Evidence that a burst of DNA depurination in SENCAR mouse skin induces error-prone repair and forms mutations in the H-ras gene

Treatment of SENCAR mouse skin with dibenzo[a,l]pyrene results in abundant formation of abasic sites that undergo error-prone excision repair, forming oncogenic H-ras mutations in the early preneoplastic period. To examine whether the abundance of abasic sites causes repair infidelity, we treated SENCAR mouse skin with estradiol-3,4-quinone (E(2)-3,4-Q) and determined adduct levels 1 h after treatment, as well as mutation spectra in the H-ras gene between 6 h and 3 days after treatment. E(2)-3,4-Q formed predominantly (> or =99%) the rapidly-depurinating 4-hydroxy estradiol (4-OHE(2))-1-N3Ade adduct and the slower-depurinating 4-OHE(2)-1-N7Gua adduct. Between 6 h and 3 days, E(2)-3,4-Q induced abundant A to G mutations in H-ras DNA, frequently in the context of a 3'-G residue. Using a T.G-DNA glycosylase (TDG)-PCR assay, we determined that the early A to G mutations (6 and 12 h) were in the form of G.T heteroduplexes, suggesting misrepair at A-specific depurination sites. Since G-specific mutations were infrequent in the spectra, it appears that the slow rate of depurination of the N7Gua adducts during active repair may not generate a threshold level of G-specific abasic sites to affect repair fidelity. These results also suggest that E(2)-3,4-Q, a suspected endogenous carcinogen, is a genotoxic compound and could cause mutations.

Imbalance of estrogen homeostasis in kidney and liver of hamsters treated with estradiol: implications for estrogen-induced initiation of renal tumors

Reaction of endogenous catechol estrogen quinones (CE-Q) with DNA may initiate cancer by generation of oncogenic mutations. Treatment of male Syrian golden hamsters with estrogens or 4-catechol estrogens (4-CE), but not 2-CE, induces kidney, but not liver, tumors. The hamster provides an excellent model for studying activation and deactivation (protection) of estrogen metabolites in relation to formation of CE-Q. Several factors can unbalance estrogen homeostasis, thereby increasing the oxidative pathway leading to the carcinogenic CE-3,4-Q. Hamsters were injected with 8 micromol of estradiol (E(2)), and liver and kidney extracts were analyzed for 31 estrogen metabolites, conjugates, and depurinating DNA adducts by HPLC with electrochemical detection. Neither liver nor kidney contained 4-methoxyCE, presumably due to the known inhibition of catechol-O-methyltransferase by 2-CE. More O-methylation of 2-CE was observed in the liver and more formation of CE-Q in the kidney. These results suggest less protective methylation of 2-CE and more pronounced oxidation of CE to CE-Q in the kidney. To investigate this further, hamsters were pretreated with L-buthionine(S,R)-sulfoximine to deplete glutathione levels and then treated with E(2). Compared to the liver, a very low level of CE and methoxyCE was observed in the kidney, suggesting little protective reductase activity. Most importantly, reaction of CE-3,4-Q with DNA to form the depurinating 4-hydroxyE(2)(E(1))-1-N7Gua adducts was detected in the kidney, but not in the liver. Therefore, tumor initiation in the kidney appears to arise from relatively poor methylation of 2-CE and poor reductase activity in the kidney, resulting in high levels of CE-Q. Thus, formation of depurinating DNA adducts by CE-3,4-Q may be the first critical event in the initiation of estrogen-induced kidney tumors.