Radical cations of benzo[a]pyrene and 6-substituted derivatives: reaction with nucleophiles and DNA

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.

Quantitation of benzo[a]pyrene metabolic profiles in human bronchoalveolar (H358) cells by stable isotope dilution liquid chromatography-atmospheric pressure chemical ionization mass spectrometry

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous environmental pollutants and are carcinogenic in multiple organs and species. Benzo[a]pyrene (B[a]P) is a representative PAH and has been studied extensively for its carcinogenicity and toxicity. B[a]P itself is chemically inert and requires metabolic activation to exhibit its toxicity and carcinogenicity. Three major metabolic pathways have been well documented. The signature metabolites generated from the radical cation (peroxidase or monooxygenase mediated) pathway are B[a]P-1,6-dione and B[a]P-3,6-dione, the signature metabolite generated from the diol-epoxide (P450 mediated) pathway is B[a]P-r-7,t-8,t-9,c-10-tetrahydrotetrol (B[a]P-tetrol-1), and the signature metabolite generated from the o-quinone (aldo-keto reductase mediated) pathway is B[a]P-7,8-dione. The contributions of these different metabolic pathways to cancer initiation and the exploitation of this information for cancer prevention are still under debate. With the availability of a library of [(13)C(4)]-labeled B[a]P metabolite internal standards, we developed a sensitive stable isotope dilution atmospheric pressure chemical ionization tandem mass spectrometry method to address this issue by quantitating B[a]P metabolites from each metabolic pathway in human lung cells. This analytical method represents a 500-fold increased sensitivity compared with that of a method using HPLC-radiometric detection. The limit of quantitation (LOQ) was determined to be 6 fmol on column for 3-hydroxybenzo[a]pyrene (3-OH-B[a]P), the generally accepted biomarker for B[a]P exposure. This high level of sensitivity and robustness of the method was demonstrated in a study of B[a]P metabolic profiles in human bronchoalveolar H358 cells induced or uninduced with the AhR ligand, 2,3,7,8-tetrachlorodibenzodioxin (TCDD). All the signature metabolites were detected and successfully quantitated. Our results suggest that all three metabolic pathways contribute equally in the overall metabolism of B[a]P in H358 cells with or without TCDD induction. The sensitivity of the method should permit the identification of cell-type differences in B[a]P activation and detoxication and could also be used for biomonitoring human exposure to PAH.

Colorectal polyp type and the association with charred meat consumption, smoking, and microsomal epoxide hydrolase polymorphisms

We determined the association between charred meat consumption, cigarette smoking, microsomal epoxide hydrolase (mEH) polymorphisms (rs1051740 and rs2234922), and colorectal adenomas and hyperplastic polyps (HPs) and explored gene-environment interactions. Men and women with colorectal adenomas (n = 519), HPs (n = 691), or concurrently with both types of polyps (n = 227) and polyp-free controls (n = 772) receiving a colonoscopy from December 2004 to September 2007 were recruited. Participants completed telephone interviews and provided buccal cell samples; genotyping of mEH was completed using Taqman assays. We conducted polytomous regression and calculated odd ratios (OR) and 95% confidence intervals. Interactions were evaluated using Wald chi-square tests. Consumption of >3 servings of charred meat per week was associated with distal HPs (OR = 2.0, 1.2-3.4) but not adenomas nor either type of proximal polyp. Heavy cigarette smoking (≥ 22 pack-years) was associated with an increased risk for colorectal adenomas (OR = 1.7, 95% CI: 1.2-2.4), HPs (OR = 2.4, 95% CI: 1.7-3.3), and both types (OR = 2.8, 95% CI: 1.8-4.3) with the strongest association for distal polyps. There was no association between mEH genotype and colorectal polyps, nor were any statistically significant gene-environment interactions identified. Future investigation of BaP exposure and colorectal neoplasia should analyze whether associations are dependent upon anatomic location.

Benzo[a]pyrene-7,8-dihydrodiol promotes checkpoint activation and G2/M arrest in human bronchoalveolar carcinoma H358 cells

Polycyclic aromatic hydrocarbons (PAHs) are potent carcinogens that require metabolic activation inside cells. The proximate carcinogens PAH-diols can be converted to o-quinones by aldo-keto reductases (AKRs) or to diol-epoxides by cytochrome P450 (P450) enzymes. We assessed the effect of benzo[a]pyrene-7,8-dihydrodiol (BPD) on proliferation in p53-null bronchoalveolar carcinoma H358 cells. BPD treatment led to a significant inhibition of proliferation and arrest in G2/M in H358 cells. The relative contribution of the AKR and P450 pathways to cell cycle arrest was assessed. Overexpression of AKR1A1 did not affect cell proliferation or cell cycle progression, and benzo[a]pyrene-7,8-dione did not cause any noticeable effect on cell growth, suggesting that AKR1A1 metabolic products were not involved in the antiproliferative effect of BPD. On the other hand, blockade of P450 induction or inhibition of P450 activity greatly impaired the effect of BPD. Moreover, P450 induction by 2,3,7,8-tetrachlorodibenzo-p-dioxin significantly enhanced the antiproliferative effect of BPD. Mechanistic studies revealed that BPD caused a DNA damage response, Chk1 activation, and accumulation of phospho-Cdc2 (Tyr15) in H358 cells, effects that were impaired by an ataxia-telangectasia mutated (ATM)/ATM-related (ATR) inhibitor. Similar results were observed in human bronchoepithelial BEAS-2B cells, arguing for analogous mechanisms in tumorigenic and immortalized nontumorigenic cells lacking functional p53. Our data suggest that a p53-independent pathway operates in lung epithelial cells in response to BPD that involves P450 induction and subsequent activation of the ATR/ATM/Chk1 damage check-point pathway and cell cycle arrest in G2/M.

Enzymatic biomarker measurement and study of DNA adduct formation in benzo

The aim of this study was to improve the knowledge on the metabolic pathways involved in benzo[a]pyrene (B[a]P) activation and on the relationship between adduct levels and enzymatic biomarker activities. With this purpose, a model to assess pollutant exposure via food supply has been developed for the sentinel organism, Mytilus galloprovincialis. Mussels were fed for 4 weeks with B[a]P-contaminated feed (50 mg/kg dry weight mussel). Bioaccumulation was studied by determination of B[a]P concentration in whole mussel by GC/MS analysis. Different biomarkers of pollutant exposure were measured to assess the metabolic state of the exposed organisms. CYP1A-like immunopositive protein titration and B[a]P hydroxylase (BPH) activity were assessed as indicators of phase I biotransformation. Glutathione-S-transferase (GST) activity was measured as an indicator of the conjugation activities. Catalase (CAT) and DT-diaphorase (DTD) activities were assessed as potential biomarkers of oxidative stress, whereas acetylthiocholine esterase (AChE) activity was measured as an indication of possible neurotoxicity of B[a]P exposure. DNA adduct levels were determined in digestive gland DNA by applying the 32P-postlabeling technique with nuclease P1 enhancement. For the developed conditions of exposure, B[a]P concentration reached in whole mussel tissues was very high (>500 mg/kg d.w. mussel) and significant B[a]P-induced changes were recorded for each enzymatic biomarkers. BPH and CAT activities were significantly increased by B[a]P exposure, whereas GST in the gills, DTD and AChE were significantly depressed. On the other hand, no change in CYP1A-like immunopositive protein content was observed. Induction and increase with time of bulky B[a]P-related DNA adducts were demonstrated in the digestive gland, although at low levels (0.269+/-0.082 adduct/10e8 dNps at maximum) by the 32P-postlabeling assay. DNA adduct level was significantly correlated with whole mussel tissue B[a]P concentration, so were all the enzymatic biomarkers measured except to GST activity in both gill and digestive gland tissues. BPH, DTD, CAT and AChE displayed a strong correlation with adduct levels. These results demonstrate the neurotoxicity and the genotoxicity of B[a]P exposure in the mussel. The induction of bulky DNA adducts in mussels demonstrates the existence of activation pathways already identified in vertebrates. It validates also the suitability of this model for further studies on B[a]P metabolism in mussels. Our results support the proposal of BPH, AChE, DTD and CAT activities as suitable biomarkers of PAH exposure for these sentinel species.