Comparison of the morphological transforming activities of dibenzo[a,l]pyrene and benzo[a]pyrene in C3H10T1/2CL8 cells and characterization of the dibenzo[a,l]pyrene-DNA adducts

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

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

Carcinogenesis: Failure of resolution of inflammation?

Inflammation in the tumor microenvironment is a hallmark of cancer and is recognized as a key characteristic of carcinogens. However, the failure of resolution of inflammation in cancer is only recently being understood. Products of arachidonic acid and related fatty acid metabolism called eicosanoids, including prostaglandins, leukotrienes, lipoxins, and epoxyeicosanoids, critically regulate inflammation, as well as its resolution. The resolution of inflammation is now appreciated to be an active biochemical process regulated by endogenous specialized pro-resolving lipid autacoid mediators which combat infections and stimulate tissue repair/regeneration. Environmental and chemical human carcinogens, including aflatoxins, asbestos, nitrosamines, alcohol, and tobacco, induce tumor-promoting inflammation and can disrupt the resolution of inflammation contributing to a devastating global cancer burden. While mechanisms of carcinogenesis have focused on genotoxic activity to induce mutations, nongenotoxic mechanisms such as inflammation and oxidative stress promote genotoxicity, proliferation, and mutations. Moreover, carcinogens initiate oxidative stress to synergize with inflammation and DNA damage to fuel a vicious feedback loop of cell death, tissue damage, and carcinogenesis. In contrast, stimulation of resolution of inflammation may prevent carcinogenesis by clearance of cellular debris via macrophage phagocytosis and inhibition of an eicosanoid/cytokine storm of pro-inflammatory mediators. Controlling the host inflammatory response and its resolution in carcinogen-induced cancers will be critical to reducing carcinogen-induced morbidity and mortality. Here we review the recent evidence that stimulation of resolution of inflammation, including pro-resolution lipid mediators and soluble epoxide hydrolase inhibitors, may be a new chemopreventive approach to prevent carcinogen-induced cancer that should be evaluated in humans.

Resolution of eicosanoid/cytokine storm prevents carcinogen and inflammation-initiated hepatocellular cancer progression

Toxic environmental carcinogens promote cancer via genotoxic and nongenotoxic pathways, but nongenetic mechanisms remain poorly characterized. Carcinogen-induced apoptosis may trigger escape from dormancy of microtumors by interfering with inflammation resolution and triggering an endoplasmic reticulum (ER) stress response. While eicosanoid and cytokine storms are well-characterized in infection and inflammation, they are poorly characterized in cancer. Here, we demonstrate that carcinogens, such as aflatoxin B₁ (AFB₁), induce apoptotic cell death and the resulting cell debris stimulates hepatocellular carcinoma (HCC) tumor growth via an "eicosanoid and cytokine storm." AFB₁-generated debris up-regulates cyclooxygenase-2 (COX-2), soluble epoxide hydrolase (sEH), ER stress-response genes including BiP, CHOP, and PDI in macrophages. Thus, selective cytokine or eicosanoid blockade is unlikely to prevent carcinogen-induced cancer progression. Pharmacological abrogation of both the COX-2 and sEH pathways by PTUPB prevented the debris-stimulated eicosanoid and cytokine storm, down-regulated ER stress genes, and promoted macrophage phagocytosis of debris, resulting in suppression of HCC tumor growth. Thus, inflammation resolution via dual COX-2/sEH inhibition is an approach to prevent carcinogen-induced cancer.

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

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

Repair-Resistant DNA Lesions

The eukaryotic global genomic nucleotide excision repair (GG-NER) pathway is the major mechanism that removes most bulky and some nonbulky lesions from cellular DNA. There is growing evidence that certain DNA lesions are repaired slowly or are entirely resistant to repair in cells, tissues, and in cell extract model assay systems. It is well established that the eukaryotic DNA lesion-sensing proteins do not detect the damaged nucleotide, but recognize the distortions/destabilizations in the native DNA structure caused by the damaged nucleotides. In this article, the nature of the structural features of certain bulky DNA lesions that render them resistant to NER, or cause them to be repaired slowly, is compared to that of those that are good-to-excellent NER substrates. Understanding the structural features that distinguish NER-resistant DNA lesions from good NER substrates may be useful for interpreting the biological significance of biomarkers of exposure of human populations to genotoxic environmental chemicals. NER-resistant lesions can survive to replication and cause mutations that can initiate cancer and other diseases. Furthermore, NER diminishes the efficacy of certain chemotherapeutic drugs, and the design of more potent pharmaceuticals that resist repair can be advanced through a better understanding of the structural properties of DNA lesions that engender repair-resistance.

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.

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

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

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

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

Identification and quantification of DNA adducts in the oral tissues of mice treated with the environmental carcinogen dibenzo[a,l]pyrene by HPLC-MS/MS

Tobacco smoking is one of the leading causes for oral cancer. Dibenzo[a,l]pyrene (DB[a,l]P), an environmental pollutant and a tobacco smoke constituent, is the most carcinogenic polycyclic aromatic hydrocarbon (PAH) tested to date in several animal models (target organs: skin, lung, ovary, and mammary tissues). We have recently demonstrated that DB[a,l]P is also capable of inducing oral cancer in mice; however, its metabolic activation to the ultimate genotoxic metabolite dibenzo[a,l]pyrene-11,12-dihydrodiol-13,14-epoxide (DB[a,l]PDE) in mouse oral cavity has not been examined. Here we developed a liquid chromatography-tandem mass spectrometry (LC-MS/MS) method to detect and quantify (±)-anti-DB[a,l]PDE-dA adducts in oral tissues of mice treated with DB[a,l]P. [(15)N(5)]-(±)-anti-DB[a,l]PDE-N(6)-dA adducts were synthesized as internal standards. The stereoisomeric adducts were characterized by MS, NMR, and CD analysis. The detection limit of the method is 8 fmol with 100 μg of digested DNA as the matrix. Two adducts were detected and identified as (-)-anti-cis and (-)-anti-trans-DB[a,l]PDE-dA in the oral tissues of mice following the direct application of DB[a,l]P (240 nmol per day, for 2 days) into the oral cavity, indicating that DB[a,l]P is predominantly metabolized into (-)-anti-DB[a,l]PDE in this target organ. We also compared the formation and removal of adducts as a function of time, following the direct application of DB[a,l]P (24 nmol, 3 times per week for 5 weeks) into the oral cavity of mice. Adducts were quantified at 48 h, 1, 2, and 4 weeks after the last dose. Maximal levels of adducts occurred at 48 h, followed by a gradual decrease. The levels (fmol/μg DNA) of (-)-anti-trans adducts (4.03 ± 0.27 to 1.77 ± 0.25) are significantly higher than (-)-anti-cis-DB[a,l]PDE-dA adduct (1.63 ± 0.42 to 0.72 ± 0.04) at each time point (p < 0.005). The results presented here indicate that the formation and persistence of (-)-anti-DB[a,l]PDE-dA adducts may, in part, contribute to the initiation of DB[a,l]P-induced oral carcinogenesis.

Lack of contribution of covalent benzo[a]pyrene-7,8-quinone-DNA adducts in benzo[a]pyrene-induced mouse lung tumorigenesis

Benzo[a]pyrene (B[a]P) is a potent human and rodent lung carcinogen. This activity has been ascribed in part to the formation of anti-trans-7,8-dihydroxy-7,8-dihydroB[a]P-9,10-epoxide (BPDE)-DNA adducts. Other carcinogenic mechanisms have been proposed: (1) the induction of apurinic sites from radical cation processes, and (2) the metabolic formation of B[a]P-7,8-quinone (BPQ) that can form covalent DNA adducts or reactive oxygen species which can damage DNA. The studies presented here sought to examine the role of stable BPQ-DNA adducts in B[a]P-induced mouse lung tumorigenesis. Male strain A/J mice were injected intraperitoneally once with BPQ or trans-7,8-dihydroxy-7,8-dihydroB[a]P (BP-7,8-diol) at 30, 10, 3, or 0mg/kg. Lungs and livers were harvested after 24h, the DNA extracted and subjected to (32)P-postlabeling analysis. Additional groups of mice were dosed once with BPQ or BP-7,8-diol each at 30 mg/kg and tissues harvested 48 and 72 h later, or with B[a]P (50mg/kg, a tumorigenic dose) and tissues harvested 72 h later. No BPQ or any other DNA adducts were observed in lung or liver tissues 24, 48, or 72 h after the treatment with 30 mg/kg BPQ. BP-7,8-diol gave BPDE-DNA adducts at all time points in both tissues and B[a]P treatment gave BPDE-DNA adducts in the lung. In each case, no BPQ-DNA adducts were detected. Mouse body weights significantly decreased over time after BPQ or BP-7,8-diol treatments suggesting that systemic toxicity was induced by both agents. Model studies with BPQ and N-acetylcysteine suggested that BPQ is rapidly inactivated by sulfhydryl-containing compounds and not available for DNA adduction. We conclude that under these treatment conditions BPQ does not form stable covalent DNA adducts in the lungs or livers of strain A/J mice, suggesting that stable BPQ-covalent adducts are not a part of the complex of mechanisms involved in B[a]P-induced mouse lung tumorigenesis.

On the impact of the molecule structure in chemical carcinogenesis

Cancer is as a highly complex and multifactorial disease responsible for the death of hundreds of thousands of people in the western countries every year. Since cancer is clonal and due to changes at the level of the genetic material, viruses, chemical mutagens and other exogenous factors such as short-waved electromagnetic radiation that alter the structure of DNA are among the principal causes. The focus of this present review lies on the influence of the molecular structure of two well-investigated chemical carcinogens from the group of polycyclic aromatic hydrocarbons (PAHs), benzo[a]pyrene (BP) and dibenzo[a,l]pyrene (DBP). Although there is only one additional benzo ring present in the latter compound, DBP exerts much stronger genotoxic and carcinogenic effects in certain tumor models as compared to BP. Actually, DBP has been identified as the most potent tumorigen among all carcinogenic PAHs tested to date. The genotoxic effects of both compounds investigated in mammalian cells in culture or in animal models are described. Comparison of enzymatic activation, DNA binding levels of reactive diol-epoxide metabolites, efficiency of DNA adduct repair and mutagenicity provides some clues on why this compound is about 100-fold more potent in inducing tumors than BP. The data published during the past 20 years support and strengthen the idea that compound-inherent physicochemical parameters, along with inefficient repair of certain kinds of DNA lesions formed upon metabolic activation, can be considered as strong determinants for high carcinogenic potency of a chemical.

Characterization of naphtho[1,2-a]pyrene and naphtho[1,2-e]pyrene DNA adducts in C3H10T1/2 fibroblasts

Polycyclic aromatic hydrocarbons (PAHs) are a class of carcinogenic chemicals that are ubiquitous in the environment. Fjord-region naphthopyrene isomers are structurally similar to the potent fjord-region PAH carcinogen dibenzo[a,l]pyrene and thus have the potential to be potent carcinogens. Naphtho[1,2-a]pyrene (N[1,2-a]P) exhibited similar bacterial mutagenicity and morphological cell transforming activity when compared to benzo[a]pyrene (B[a]P), whereas the structural isomer, naphtho[1,2-e]pyrene (N[1,2-e]P) was inactive is these bioassays. In this study, we examined the formation of DNA adducts in C3H10T1/2Cl8 (C3H10T1/2) mouse embryo fibroblasts exposed to N[1,2-a]P or N[1,2-e]P and their respective dihydrodiols. The DNA adducts were characterized by co-chromatography with reaction products from anti-N[1,2-a]P diol epoxide (DE) or anti-N[1,2-e]PDE and polydeoxyadenosine (dAdo) or oligodeoxyguanosine (dGuo). C3H10T1/2 fibroblasts exposed to N[1,2-a]P or N[1,2-a]P-9,10-diol produced both anti-N[1,2-a]P-DE-dAdo and -dGuo adducts with total DNA adduction levels of 22.2 to 33.3 pmol DNA adducts/mug DNA. C3H10T1/2 fibroblasts exposed to N[1,2-e]P produced 2 major and 1 minor adducts. C3H10T1/2 fibroblasts exposed to N[1,2-e]P-11,12-diol produced 2 major adducts. All of the identified adducts were anti-N[1,2-e]PDE-dGuo and -dAdo adducts. While the total DNA adduct level in N[1,2-e]P-11,12-diol-treated fibroblasts was extremely high, 105.9 pmol DNA adducts/mug DNA, the level in N[1,2-e]P-treated fibroblasts was 1.47 pmol DNA adducts/microg DNA. We conclude that lack of biological activity of N[1,2-e]P may be related to its inability to form sufficient amounts of N[1,2-e]P-11,12-diol, which would then be metabolized to sufficient amounts of anti-N[1,2-e]PDE needed to transform these fibroblasts.

Competitive inhibition of carcinogen-activating CYP1A1 and CYP1B1 enzymes by a standardized complex mixture of PAH extracted from coal tar

A complex mixture of polycyclic aromatic hydrocarbons (PAH) extracted from coal tar, the Standard Reference Material (SRM) 1597, was recently shown to decrease the levels of DNA binding of the 2 strong carcinogens benzo[a]pyrene (BP) and dibenzo[a,l]pyrene (DBP) in the human mammary carcinoma-derived cell line MCF-7 (Mahadevan et al., Chem Res Toxicol 2005;18:224-231). The present study was designed to further elucidate the biochemical mechanisms involved in this inhibition process. We examined the effects of SRM 1597 on the metabolic activation of BP and DBP toward DNA-binding derivatives in Chinese hamster cells expressing either human cytochrome P450 (CYP) 1A1 or CYP1B1. SRM 1597 inhibited BP-DNA adduct formation through the entire exposure time in cells expressing human CYP1A1, while it significantly inhibited adduct formation only up to 48 hr when co-treated with DBP. Conversely, human CYP1B1-expressing cells were unable to catalyze PAH-DNA adduct formation on treatment with SRM 1597 alone, and on co-treatment with BP or DBP. The data obtained from biochemical experiments revealed that SRM 1597 competitively inhibited the activity of both human enzymes as analyzed by 7-ethoxyresorufin O-deethylation assays. While the Michaelis-Menten constant (K(M)) was <0.4 microM in the absence of SRM 1597, this value increased up to 1.12 (CYP1A1) or 4.45 microM (CYP1B1) in the presence of 0.1 microg/ml SRM 1597. Hence the inhibitory effects of the complex mixture on human CYP1B1 were much stronger when compared to human CYP1A1. Taken together, the decreases in PAH-DNA adduct formation on co-treatment with SRM 1597 revealed inhibitory effects on the CYP enzymes that convert carcinogenic PAH into DNA-binding metabolites. The implications for the tumorigenicity of complex environmental PAH mixtures are discussed.

Benzo[a]pyrene-7,8-quinone-3'-mononucleotide adduct standards for 32P postlabeling analyses: detection of benzo[a]pyrene-7,8-quinone-calf thymus DNA adducts

Benzo[a]pyrene-7,8-quinone (BPQ) is one of the reactive metabolites of the widely distributed archetypal polycyclic aromatic hydrocarbon, benzo[a]pyrene (B[a]P). The formation of BPQ from B[a]P through trans-7,8-dihydroxy-7,8-dihydroB[a]P by the mediation of aldo-keto reductases and its role in the genotoxicity and carcinogenesis of B[a]P currently are under extensive investigation. Toxicity pathways related to BPQ are believed to include both stable and unstable (depurinating) DNA adduct formation as well as reactive oxygen species. We previously reported the complete characterization of four novel stable BPQ-deoxyguanosine (dG) and two BPQ-deoxyadenosine (dA) adducts (Balu et al., Chem. Res. Toxicol. 17 (2004) 827-838). However, the identification of BPQ-DNA adducts by 32P postlabeling methods from in vitro and in vivo exposures required 3'-monophosphate derivatives of BPQ-dG, BPQ-dA, and BPQ-deoxycytidine (dC) as standards. Therefore, in the current study, BPQ adducts of dGMP(3'), dAMP(3'), and dCMP(3') were prepared. The syntheses of the BPQ-3'-mononucleotide standards were carried out in a manner similar to that reported previously for the nucleoside analogs. Reaction products were characterized by UV, LC/MS analyses, and one- and two-dimensional NMR techniques. The spectral studies indicated that all adducts existed as diastereomeric mixtures. Furthermore, the structural identities of the novel BPQ-dGMP, BPQ-dAMP, and BPQ-dCMP adducts were confirmed by acid phosphatase dephosphorylation of the BPQ-nucleotide adducts to the corresponding known BPQ-nucleoside adduct standards. The BPQ-dGMP, BPQ-dAMP, and BPQ-dCMP adduct standards were used in 32P postlabeling studies to identify BPQ adducts formed in vitro with calf thymus DNA and DNA homopolymers. 32P postlabeling analysis revealed the formation of 8 major and at least 10 minor calf thymus DNA adducts. Of these BPQ-DNA adducts, the following were identified: 1 BPQ-dGMP adduct, 2 BPQ-dAMP adducts, and 3 BPQ-dCMP adducts. This study represents the first reported example of the characterization of stable BPQ-DNA adducts in isolated mammalian DNA and is expected to contribute significantly to the future BPQ-DNA adduct studies in vivo and thereby to the contribution of BPQ in B[a]P carcinogenesis.

Induction of Cyp1a1 and Cyp1b1 and formation of DNA adducts in C57BL/6, Balb/c, and F1 mice following in utero exposure to 3-methylcholanthrene

Fetal mice are more sensitive to chemical carcinogens than are adults. Previous studies from our laboratory demonstrated differences in the mutational spectrum induced in the Ki-ras gene from lung tumors isolated from [D2 x B6D2F1]F2 mice and Balb/c mice treated in utero with 3-methylcholanthrene (MC). We thus determined if differences in metabolism, adduct formation, or adduct repair influence strain-specific responses to transplacental MC exposure in C57BL/6 (B6), Balb/c (BC), and reciprocal F1 crosses between these two strains of mice. The induction of Cyp1a1 and Cyp1b1 in fetal lung and liver tissue was determined by quantitative fluorescent real-time PCR. MC treatment caused maximal induction of Cyp1a1 and Cyp1b1 RNA 2-8 h after injection in both organs. RNA levels for both genes then declined in both fetal organs, but a small biphasic, secondary increase in Cyp1a1 was observed specifically in the fetal lung 24-48 h after MC exposure in all four strains. Cyp1a1 induction by MC at 4 h was 2-5 times greater in fetal liver (7000- to 16,000-fold) than fetal lung (2000- to 6000-fold). Cyp1b1 induction in both fetal lung and liver was similar and much lower than that observed for Cyp1a1, with induction ratios of 8- to 18-fold in fetal lung and 10- to 20-fold in fetal liver. The overall kinetics and patterns of induction were thus very similar across the four strains of mice. The only significant strain-specific effect appeared to be the relatively poor induction of Cyp1b1 in the parental strain of B6 mice, especially in fetal lung tissue. We also measured the levels of MC adducts and their disappearance from lung tissue by the P(32) post-labeling assay on gestation days 18 and 19 and postnatal days 1, 4, 11, and 18. Few differences were seen between the different strains of mice; the parental strain of B6 mice had nominally higher levels of DNA adducts 2 (gestation day 19) and 4 (postnatal day 1) days after injection, although this was not statistically significant. These results indicate that differences in Phase I metabolism of MC and formation of MC-DNA adducts are unlikely to account for the marked differences observed in the Ki-ras mutational spectrum seen in previous studies. Further, the results suggest that other genetic factors may interact with chemical carcinogens in determining individual susceptibility to these agents during development.

An approach to investigating the importance of high potency polycyclic aromatic hydrocarbons (PAHs) in the induction of lung cancer by air pollution

Evidence suggests that people living in urban areas have an increased risk of lung cancer due to higher levels of air pollution in these areas. Benzo[a]pyrene (B[a]P) is currently used as the main indicator of carcinogenic polycyclic aromatic hydrocarbons (PAHs) in air pollution, but there is concern that B[a]P may not be the ideal surrogate of choice for PAH mixtures since higher potency PAHs have recently been identified which could potentially contribute more and variably to the overall carcinogenicity. Dibenzo[a,h]anthracene (DBA) and dibenzo[a,l]pyrene (DB[a,l]P) are estimated to have carcinogenic potencies 10 or more times greater than B[a]P but data on their presence and formation in the environment are limited. Several occupational and environmental PAH biomonitoring studies are reviewed here, with particular focus on the specific exposure groups, study design, sample tissue, in particular the use of nasal tissues, and biomarkers used in each study. Consideration of these data is then used to propose a novel biomonitoring approach to evaluate exposure, uptake and the role of high potency PAHs in air pollution-related lung cancer. This is based upon an occupational study examining specific DNA adducts for DBA and DB[a,l]P in nasal cells to evaluate the extent to which these high potency PAHs might contribute to the increased risk of developing lung cancer from air pollution.

Comparison of the genotoxic activities of the K-region dihydrodiol of benzo[a]pyrene with benzo[a]pyrene in mammalian cells: morphological cell transformation; DNA damage; and stable covalent DNA adducts

Benzo[a]pyrene (B[a]P) is the most thoroughly studied polycyclic aromatic hydrocarbon (PAH). Many mechanisms have been suggested to explain its carcinogenic activity, yet many questions still remain. K-region dihydrodiols of PAHs are metabolic intermediates depending on the specific cytochrome P450 and had been thought to be detoxification products. However, K-region dihydrodiols of several PAHs have recently been shown to morphologically transform mouse embryo C3H10T1/2CL8 cells (C3H10T1/2 cells). Because K-region dihydrodiols are not metabolically formed from PAHs by C3H10T1/2 cells, these cells provide a useful tool to independently study the mechanisms of action of PAHs and their K-region dihydrodiols. Here, we compare the morphological cell transforming, DNA damaging, and DNA adducting activities of the K-region dihydrodiol of B[a]P, trans-B[a]P-4,5-diol with B[a]P. Both trans-B[a]P-4,5-diol and B[a]P morphologically transformed C3H10T1/2 cells by producing both Types II and III transformed foci. The morphological cell transforming and cytotoxicity dose response curves for trans-B[a]P-4,5-diol and B[a]P were indistinguishable. Since morphological cell transformation is strongly associated with mutation and/or larger scale DNA damage in C3H10T1/2 cells, the identification of DNA damage induced in these cells by trans-B[a]P-4,5-diol was sought. Both trans-B[a]P-4,5-diol and B[a]P exhibited significant DNA damaging activity without significant concurrent cytotoxicity using the comet assay, but with different dose responses and comet tail distributions. DNA adduct patterns from C3H10T1/2 cells were examined after trans-B[a]P-4,5-diol or B[a]P treatment using 32P-postlabeling techniques and improved TLC elution systems designed to separate polar DNA adducts. While B[a]P treatment produced one major DNA adduct identified as anti-trans-B[a]P-7,8-diol-9,10-epoxide-deoxyguanosine, no stable covalent DNA adducts were detected in the DNA of trans-B[a]P-4,5-diol-treated cells. In summary, this study provides evidence for the DNA damaging and morphological cell transforming activities of the K-region dihydrodiol of B[a]P, in the absence of covalent stable DNA adducts. While trans-B[a]P-4,5-diol and B[a]P both induce morphological cell transformation, their activities as DNA damaging agents differ, both qualitatively and quantitatively. In concert with the morphological cell transformation activities of other K-region dihydrodiols of PAHs, these data suggest a new mechanism/pathway for the morphological cell transforming activities of B[a]P and its metabolites.

8,9-dihydroxy-8,9-dihydrodibenzo[a,l]pyrene is a potent morphological cell-transforming agent in C3H10T(1)/(2)Cl8 mouse embryo fibroblasts in the absence of detectable stable covalent DNA adducts

The comparative genotoxic effects of racemic trans-8,9-dihydroxy-8, 9-dihydrodibenzo[a,l]pyrene (trans-DB[a,l]P-8,9-diol), the metabolic K-region dihydrodiol of dibenzo[a,l] pyrene (DB[a,l]P) (dibenzo[def, p]chrysene) and DB[a,l]P in transformable mouse embryo C3H10T(1)/(2)Cl8 (C3H10T(1)/(2)) fibroblasts was investigated. The C3H10T(1)/(2) mouse embryo morphological cell-transforming activities of these polycyclic aromatic hydrocarbons (PAHs) were assayed using concentration-response studies. At concentrations of 33 nM and above both trans-DB[a,l]P-8,9-diol and DB[a,l]P produced significant (and similar) numbers of type II and III foci per dish and numbers of dishes with type II and II foci. Concomitant cytotoxicity studies revealed a reduction in colony survival of approximately 25% up to 198 nM for both PAHs. DNA adducts of trans-DB[a,l]P-8,9-diol and DB[a,l]P in C3H10T(1)/(2) cells were analyzed by a (32)P-post-labeling TLC/HPLC method. No adducts were observed in the DNA of C3H10T(1)/(2) cells treated with trans-DB[a, l]P-8,9-diol at concentrations that induced morphological cell transformation. Under the same exposure and chromatographic conditions, DNA adducts of deoxyadenosine and deoxyguanosine derived from the fjord region anti-DB[a,l]P-11,12-diol-13,14-epoxide and syn-DB[a,l]P-11,12-diol-13,14-epoxide were observed in the DNA of DB[a,l]P-treated cells. These results indicate that trans-DB[a,l]P-8, 9-diol has intrinsic genotoxic activity equal to that of DB[a,l]P, based on morphological cell transformation of mouse embryo fibroblasts. The activity of trans-DB[a,l]P-8,9-diol is apparently not associated with the formation of observable stable covalent DNA adducts. These results suggest that under appropriate conditions, trans-DB[a,l]P-8,9-diol may serve as an intermediate in the genotoxicity of DB[a,l]P.

Estimating cancer risk from outdoor concentrations of hazardous air pollutants in 1990

A public health concern regarding hazardous air pollutants (HAPs) is their potential to cause cancer. It has been difficult to assess potential cancer risks from HAPs, due primarily to lack of ambient concentration data for the general population. The Environmental Protection Agency's Cumulative Exposure Project modeled 1990 outdoor concentrations of HAPs across the United States, which were combined with inhalation unit risk estimates to estimate the potential increase in excess cancer risk for individual carcinogenic HAPs. These were summed to provide an estimate of cancer risk from multiple HAPs. The analysis estimates a median excess cancer risk of 18 lifetime cancer cases per 100,000 people for all HAP concentrations. About 75% of estimated cancer risk was attributable to exposure to polycyclic organic matter, 1,3-butadiene, formaldehyde, benzene, and chromium. Consideration of some specific uncertainties, including underestimation of ambient concentrations, combining upper 95% confidence bound potency estimates, and changes to potency estimates, found that cancer risk may be underestimated by 15% or overestimated by 40-50%. Other unanalyzed uncertainties could make these under- or overestimates larger. This analysis used 1990 estimates of concentrations and can be used to track progress toward reducing cancer risk to the general population.