Identification and quantitation of dibenzo[a,l]pyrene--DNA adducts formed by rat liver microsomes in vitro: preponderance of depurinating adducts

Diet derived polycyclic aromatic hydrocarbons and its pathogenic roles in colorectal carcinogenesis

Polycyclic aromatic hydrocarbon (PAHs) are molecules that contaminate meat products during the high-temperature cooking of meat. This study reviewed the pathogenic roles of meat derived polycyclic aromatic hydrocarbons in the carcinogenesis of colorectal cancer (CRC). Ingested PAHs undergo xenobiotic metabolism resulting in the activation of genotoxic metabolites that can induce DNA damage in the colorectum. Genetic polymorphisms in PAH xenobiotic enzymes are linked to the risk of CRC and suggest a role for PAH-meat ingestion in carcinogenesis of colorectal malignancies. Furthermore, PAH specific DNA adducts have been identified in colorectal cancer tissue and linked to high meat intake. DNA adduct resolution is mediated by the nucleotide excision repair, and polymorphisms within genes of this repair pathway and high meat intake are associated with increased CRC risk. In the literature, there is evidence from metabolic enzyme gene variants, DNA repair genes, PAH metabolites, and epidemiological studies suggesting PAH involvement in CRC.

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.

Phytotherapeutic approach: a new hope for polycyclic aromatic hydrocarbons induced cellular disorders, autophagic and apoptotic cell death

Polycyclic aromatic hydrocarbons (PAHs) comprise the major class of cancer-causing chemicals and are ranked ninth among the chemical compounds threatening to humans. Moreover, interest in PAHs has been mainly due to their genotoxic, teratogenic, mutagenic and carcinogenic property. Polymorphism in cytochrome P450 (CYP450) and aryl hydrocarbon receptor (AhR) has the capacity to convert procarcinogens into carcinogens, which is an imperative factor contributing to individual susceptibility to cancer development. The carcinogenicity potential of PAHs is related to their ability to bind to DNA, thereby enhances DNA cross-linking, causing a series of disruptive effects which can result in tumor initiation. They induce cellular toxicity by regulating the generation of reactive oxygen species (ROS), which arbitrate apoptosis. Additionally, cellular toxicity-mediated apoptotic and autophagic cell death and immune suppression by industrial pollutants PAH, provide fertile ground for the proliferation of mutated cells, which results in cancer growth and progression. PAHs play a foremost role in angiogenesis necessary for tumor metastasization by promoting the upregulation of metalloproteinase-9 (MMP-9), vascular endothelial growth factor (VEGF) and hypoxia inducible factor (HIF) in human cancer cells. This review sheds light on the molecular mechanisms of PAHs induced cancer development as well as autophagic and apoptotic cell death. Besides that authors have unraveled how phytotherapeutics is an alternate potential therapeutics acting as a savior from the toxic effects of PAHs for safer and cost effective perspectives.

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.

Systems toxicology approach to understand the kinetics of benzo(a)pyrene uptake, biotransformation, and DNA adduct formation in a liver cell model

Cell-based models are important for deriving mechanistic information about stress response pathways that have evolved to protect cells from toxic insult, such as exposure to environmental pollutants. One determinant of the stress response is the amount of chemical entering the cell and the cell's ability to detoxify and remove the chemical. If the stress response is overwhelmed, an adverse outcome will ensue. It was the goal of our study to quantify uptake and elimination rates of benzo(a)pyrene (BaP), a ubiquitous environmental pollutant, in a murine liver cell line. We evaluated the kinetic behavior in the context of BaP uptake, biotransformation, DNA adduct formation and repair along with the transcriptional and cell proliferation response. A low (50 nM) and a high (5 μM) BaP concentration were chosen in order to differentiate the role of exposure concentration in the time-resolved interaction of BaP with cells. While rates of uptake and the initial transcriptional response were similar for both BaP concentrations, cells exposed to 50 nM BaP completely recovered from exposure within 24 h, whereas cells exposed to 5 μM BaP did not. Biotransformation proceeded faster on 50 nM BaP, and the few DNA adducts formed were completely repaired after transient cell cycle arrest. In contrast, DNA adducts greatly accumulated in cells exposed to 5 μM BaP, despite significant biotransformation; complete cell cycle arrest and toxicity evolved. On the basis of the kinetic rate constants and cellular response, we conclude that at least short-term, pulsed exposures to 50 nM BaP, which we consider environmentally relevant, can be handled by cells without adverse outcome. Further studies are needed to determine the ability of cells to recover from repeated exposure. Our study emphasizes the importance of quantifying chemical uptake and fate in cell models to differentiate a stress response from an adverse outcome for better risk assessment.

Mechanism of DNA depurination by carcinogens in relation to cancer initiation

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

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

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

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

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

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.

The characterization of (±)-anti-benzo[a]pyrene diolepoxide–DNA adducts and (±)-anti-dibenzo[a,l]pyrene diolepoxide–DNA adducts in the same DNA sample using solid-matrix phosphorescence

Solid-matrix phosphorescence (SMP) spectra and lifetimes were used to characterize the (+/-)-anti-benzo[a]pyrene diolepoxide [(+/-)-anti-B[a]PDE] and (+/-)-anti-dibenzo[a,l]pyrene diolepoxide [(+/-)-anti-DB[a,l]PDE] bonded to the same sample of DNA. SMP spectra and lifetimes were also acquired for two samples of DNA that had only (+/-)-anti-B[a]PDE or (+/-)-anti-DB[a,l]PDE bonded to the individual samples of DNA. A detailed comparison of the SMP properties was made among the three samples of DNA. The SMP excitation spectra for the (+/-)-anti-B[a]PDE-DNA and the (+/-)-anti-DB[a,l]PDE-DNA adducts were very similar. However, the SMP emission spectra of the two DNA adduct systems were very dissimilar with a major emission band for the (+/-)-anti-B[a]PDE-DNA adducts appearing at 613 nm and for the (+/-)-anti-DB[a,l]PDE-DNA adducts a major band was at 558 nm. It was possible to selectively use SMP emission wavelengths and obtain a SMP excitation of spectrum of the (+/-)-anti-DB[a,l]PDE-DNA adducts in the dual adducted DNA sample without the (+/-)-anti-B[a]PDE-DNA adducts emitting SMP. In addition, it was shown that the SMP emission spectrum of the dual adducted DNA sample could be used to detect both adduct systems in the modified DNA sample. It was demonstrated that the SMP lifetimes could be effectively employed to characterize the dual adducted DNA sample. For example, the SMP decay curve for the (+/-)-anti-DB[a,l]PDE-DNA adducts could be acquired without any SMP emission from the (+/-)-anti-B[a]PDE-DNA adducts. Also, ln(SMP intensity) versus time plots were very useful in characterizing the dual adducted DNA sample.

On the species-specific biotransformation of dibenzo[a,l]pyrene

We were aimed at investigating the activation of the carcinogenic polycyclic aromatic hydrocarbon (PAH) dibenzo[a,l]pyrene (DB[a,l]P) in Chinese hamster V79 cells that express single human, rat or fish cytochrome P450 (CYP) enzymes. DB[a,l]P is detectable in environmental samples and has been characterized as the most potent carcinogenic species among all PAHs as yet tested in rodent bioassays. Metabolite profiles and metabolite-dependent cytotoxic and clastogenic activities were monitored. The total turnover of CYP-mediated transformation of DB[a,l]P was as follows: human CYP1B1>fish CYP1A1 approximately human CYP1A1>>rat CYP1A2>rat CYP1A1. By contrast, enzyme forms that are not classified as being members of family CYP1, such as CYP2A6, 2E1, 2B1, and 3A4, failed to catalyze any detectable conversion of this substrate. All CYP1A1 enzymes tested formed both the K-region trans-8,9- and the trans-11,12-dihydrodiol, whereas human CYP1B1 failed to catalyze K-region activation. In cells expressing human or fish CYP1A1, human CYP1B1, and rat CYP1A2, the (-)-trans-11,12-dihydrodiol was formed enantiospecifically. DB[a,l]P-dependent cytotoxicities (EC(50)) were found in the following order: human CYP1A1 (12 nM)>fish CYP1A1 (30 nM)>human CYP1B1 (45 nM)>>other forms. In addition, an appreciable micronuclei formation was detected in human CYP1A1- and 1B1-expressing cells during exposure to DB[a,l]P. Our study demonstrates that human CYP1A1, 1B1 and fish CYP1A1 are able to transform DB[a,l]P into genotoxic derivatives in appreciable amounts. In contrast, CYP enzymes from rat predominantly target the K-region of DB[a,l]P and thus are serving more a rather protective route of biotransformation. Together our data suggest that humans might be more susceptible to DB[a,l]P-induced carcinogenicity than rats.

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.

Convenient syntheses of dibenzo[c,p]chrysene and its possible proximate and ultimate carcinogens: in vitro metabolism and DNA adduction studies

Dibenzo[c,p]chrysene (DB[c,p]C) is the only hexacyclic polycyclic aromatic hydrocarbon having two fjord regions, both in different chemical environments. Its environmental presence and relative tumorigenic potency are not known due to the lack of synthetic standards. We report here the synthesis of dibenzo[c,p]chrysene (1), its proximate carcinogens, i.e., trans-1,2-dihydroxy-1,2-dihydro-DB[c,p]C (2) and trans-11,12-dihydroxy-11,12-dihydro-DB[c,p]C (3), and possible ultimate carcinogens, i.e., anti-trans-1,2-dihydroxy-3,4-epoxy-1,2,3,4-tetrahydro-DB[c,p]C (4) and anti-trans-11,12-dihydroxy-13,14-epoxy-11,12,13,14-tetrahydro-DB[c,p]C (5). The syntheses of 1 and the appropriately methoxy-substituted DB[c,p]C (12 and 27), key intermediates for the synthesis of its proximate and ultimate metabolites, were tried first using a Suzuki cross-coupling reaction. However, the cyclization of olefins (10 and 11) gave poor yields of the desired products. An alternate method was thus developed employing a photochemical approach. The in vitro metabolism of DB[c,p]C was established with the S9 fraction of liver homogenate from phenobarbital/beta-naphthoflavone-induced Sprague-Dawley rats. The major dihydrodiol formed was identified as the fjord region 11,12-dihydroxy-11,12-dihydro-DB[c,p]C, while the major and minor phenols were identified as 11-hydroxy-DB[c,p]C and 12-hydroxy-DB[c,p]C, respectively. Further, the DNA adduction studies with the calf thymus DNA led to a mixture of dA and dG adducts for both fjord region diol epoxides (4 and 5). Interestingly, the dA to dG ratio for 1,2-dihydroxy-3,4-epoxide was much higher (3.2) compared to that of 11,12-dihydroxy-13,14-epoxide (0.5).

Metabolic activation of polycyclic and heterocyclic aromatic hydrocarbons and DNA damage: a review

Polycyclic aromatic hydrocarbons (PAHs) and heterocyclic aromatic compounds (HACs) constitute a major class of chemical carcinogens present in the environment. These compounds require activation to electrophilic metabolites to exert their mutagenic or carcinogenic effects. There are three principal pathways currently proposed for metabolic activation of PAH and HAC: the pathway via bay region dihydrodiol epoxide by cytochrome P450 enzymes (CYPs), the pathway via radical cation by one-electron oxidation, and the ortho-quinone pathway by dihydrodiol dehydrogenase (DD). In addition to these major pathways, a brief description of a minor metabolic activation pathway, sulfonation, for PAHs that contain a primary benzylic alcoholic group or secondary hydroxyl group(s) is included in this review. The DNA damages caused through the reactive metabolites of PAH/HAC are described involving the DNA covalent binding to form stable or depurinating adducts, the formation of apurinic sites, and the oxidative damage. The review emphasizes the chemical/biochemical reactions involved in the metabolic processes and the chemical structures of metabolites and DNA adducts.

Solid-matrix fluorescence quenching of benzo[e]pyrene and (+/-)-anti-dibenzo[a, l]pyrene diolepoxide-DNA adducts

The solid-matrix fluorescence (SMF) quenching of benzo[e]pyrene and (+/-)-anti-dibenzo[a, l]pyrene-11,12-diol-13,14-epoxide ((+/-)-antiDB[a, l]PDE)-DNA adducts with thallium nitrate (TlNO(3)) and sodium iodide (NaI) was examined and several SMF quenching models were developed. The SMF quenching data for B[e]P with either TlNO(3) or NaI fit a two-independent-binding-site model. However, the SMF quenching of (+/-)-anti-DB[a, l]PDE-DNA adducts with TlNO(3) fits a sphere of action model, but quenching with NaI was modeled with the two-independent-binding-site model. The data were compared with earlier SMF quenching data for 7,8,9,10-tetrahydroxytetrahydro-benzo[a]pyrene (tetrol I-1) and (+/-)-anti-benzo[a]pyrene-trans-7,8-dihydrodiol-9,10-epoxide ((+/-)-anti-BPDE)DNA adducts. The interpretation of the SMF quenching data for the (+/-)-anti-DB[a, l]PDE-DNA adducts was distinctively different than the interpretation of the SMF quenching data for the (+/-)-antiBPDE-DNA adducts. This initial study shows that SMF quenching has the potential to characterize polycyclic aromatic hydrocarbonDNA adducts with different numbers of aromatic rings. In addition, the data indicated that external and intercalated DNA adducts interacted with heavy-atom salts in dissimilar fashions. The new SMF methodology developed is useful for the characterization of both polycyclic aromatic hydrocarbon-DNA adducts and metabolites from polycyclic aromatic hydrocarbons.

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.

Effect of chemopreventive agents on DNA adduction induced by the potent mammary carcinogen dibenzo[a,l]pyrene in the human breast cells MCF-7

Over 1500 structurally diverse chemicals have been identified which have potential cancer chemopreventive properties. The efficacy and mechanisms of this growing list of chemoprotective agents may be studied using short-term bioassays that employ relevant end-points of the carcinogenic process. In this study, we have examined the effects of eight potential chemopreventive agents, N-acetylcysteine (NAC), benzylisocyanate (BIC), chlorophyllin, curcumin, 1,2-dithiole-3-thione (D3T), ellagic acid, genistein, and oltipraz, on DNA adduction of the potent mammary carcinogen dibenzo[a,l]pyrene (DBP) using the human breast cell line MCF-7. Bioactivation of DBP by MCF-7 cells resulted in the formation of one predominant (55%) dA-derived and several other dA- or dG-derived DNA adducts. Three test agents, oltipraz, D3T, and chlorophyllin substantially (>65%) inhibited DBP-DNA adduction at the highest dose tested (30 microM). These agents also significantly inhibited DBP adduct levels at a lower dose of 15 microM, while oltipraz was effective even at the lowest dose of 5 microM. Two other agents, genistein and ellagic acid were moderate (45%) DBP-DNA adduct inhibitors at the highest dose tested, while NAC, curcumin, and BIC were ineffective. These studies indicate that the MCF-7 cell line is an applicable model to study the efficacy of cancer chemopreventive agents in a human setting. Moreover, this model may also provide information regarding the effect of the test agents on carcinogen bioactivation and detoxification enzymes.

Natural and synthetic organic compounds in the environment-a symposium report

In March 2000, an international two-day symposium was organized in Noordwijkerhout, The Netherlands, on 'Natural and synthetic organic compounds in the environment'. The emphasis of the symposium was on the following classes of compounds: polycyclic aromatic hydrocarbons, xeno-estrogens, phyto-estrogens, and veterinary drugs. Sources, environmental distribution, uptake, biotransformation and toxic effects from the molecular to the population level were discussed. Other important aspects were the development of biomarkers, analytical methods, bioassays, molecular modelling and other research tools. Finally, the implications of the findings for government policies were discussed. In this paper, a summary is given of the most important facts and views presented at the symposium.

Estrogens as endogenous genotoxic agents--DNA adducts and mutations

Estrogens induce tumors in laboratory animals and have been associated with breast and uterine cancers in humans. In relation to the role of estrogens in the induction of cancer, we examine formation of DNA adducts by reactive electrophilic estrogen metabolites, formation of reactive oxygen species by estrogens and the resulting indirect DNA damage by these oxidants, and, finally, genomic and gene mutations induced by estrogens. Quinone intermediates derived by oxidation of the catechol estrogens 4-hydroxyestradiol or 4-hydroxyestrone may react with purine bases of DNA to form depurinating adducts that generate highly mutagenic apurinic sites. In contrast, quinones of 2-hydroxylated estrogens produce less harmful, stable DNA adducts. The catechol estrogen metabolites may also generate potentially mutagenic oxygen radicals by metabolic redox cycling or other mechanisms. Several types of indirect DNA damage are caused by estrogen-induced oxidants, such as oxidized DNA bases, DNA strand breakage, and adduct formation by reactive aldehydes derived from lipid hydroperoxides. Estradiol and the synthetic estrogen diethylstilbestrol also induce numerical and structural chromosomal aberrations and several types of gene mutations in cells in culture and in vivo. In conclusion, estrogens, including the natural hormones estradiol and estrone, must be considered genotoxic carcinogens on the basis of the evidence outlined in this chapter.

Fluorescence line-narrowing detection in chromatography and electrophoresis

A review of the basic aspects of fluorescence line-narrowing spectroscopy (FLNS) and its coupling with thin-layer chromatography (TLC) and polyacrylamide gel electrophoresis (PAGE) for off-line high-resolution low temperature spectral characterization is discussed. This is followed by a description of the on-line interfacing of capillary electrophoresis (CE) and capillary electrochromatography (CEC) with FLN detection. CE/ CEC-FLNS instrumentation and its applications for spectral identification of closely related analytes are also presented. Future prospects of micro and capillary high performance liquid chromatography (HPLC) with on-line high-resolution low temperature spectroscopic identification are considered.

Cancer initiation by polycyclic aromatic hydrocarbons results from formation of stable DNA adducts rather than apurinic sites

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous environmental pollutants with high carcinogenic potencies that have been linked to the etiology of human cancers through their presence in cigarette smoke and environmental mixtures. They are metabolically activated in cells by cytochrome P450 enzymes and/or peroxidases to reactive intermediates that damage DNA. One pathway of activation forms dihydrodiol epoxides that covalently bind to exocyclic amino groups of purines in DNA to form stable adducts. Another pathway involves formation of radical cations that bind to the N7 or C8 of purines to form unstable adducts that depurinate to leave apurinic (AP) sites in DNA. In the present study the proportions of stable DNA adducts and AP sites formed by the carcinogenic PAHs dibenzo[a,l]-pyrene (DB[a,l]P), 7,12-dimethylbenz[a]anthracene (DMBA), and benzo[a]pyrene (B[a]P) have been investigated in a target tissue for carcinogenesis, mouse epidermis. After topical application of the PAHs on the skin of female SENCAR mice epidermal DNA was isolated and the formation of stable DNA adducts was measured by (33)P-postlabeling and HPLC analysis. AP sites in DNA were measured with an aldehyde reactive probe in a slot-blot assay. At both 4 and 24 h after exposure, DB[a,l]P formed significantly higher amounts of stable DNA adducts than DMBA, and B[a]P exhibited the lowest level of binding. In contrast, the number of AP sites present in mice treated with these PAHs was in the order: DMBA > B[a]P >> DB[a,l]P. The level of AP sites was significantly lower than the level of stable adducts for each PAH. The most potent carcinogen, DB[a,l]P, induced the highest level of stable adducts and the lowest level of AP sites in epidermal DNA. These results indicate that stable DNA adducts rather than AP sites are responsible for tumor initiation by carcinogenic PAHs.

Solution conformation of the (+)-trans-anti-benzo[g]chrysene-dA adduct opposite dT in a DNA duplex

The solution structure of the adduct derived from the covalent bonding of the fjord region (+)-(11S, 12R, 13R, 14S) stereoisomer of anti -11,12-dihydroxy-13,14-epoxy-11,12,13, 14-tetrahydrobenzo[g]chrysene, (+)- anti -B[g]CDE, to the exocyclic N(6)amino group of the adenine residue dA6, (designated (+)- trans-anti -(B[g]C)dA6), positioned opposite a thymine residue dT17 in the DNA sequence context d(C1-T2-C3-T4-C5-(B[g]C)A6-C7-T8-T9-C10-C11). d(G12-G13-A14-A15-G16-T17-G18-A19-G20++ +-A21-G22) (designated (B[g]C)dA. dT 11-mer duplex), has been studied using structural information derived from NMR data in combination with molecular dynamics (MD) calculations. The solution structure of the (+)- trans-anti -(B[g]C)dA.dT 11-mer duplex has been determined using an MD protocol where both interproton distance and dihedral angle restraints deduced from NOESY and COSY spectra are used during the refinement process, followed by additional relaxation matrix refinement to the observed NOESY intensities to account for spin diffusion effects. The results established that the covalently attached benzo[g]chrysene ring intercalates into the DNA helix directed towards the 5'-side of the modified strand and stacks predominantly with dT17 when intercalated between dC5.dG18 and (B[g]C)dA6.dT17 base-pairs. All base-pairs, including the modified (B[g]C)dA6.dT17 base-pair, are aligned through Watson-Crick pairing as in normal B -DNA. In addition, the potential strain associated with the highly sterically hindered fjord region of the aromatic portion of the benzo[g]chrysenyl ring is relieved through the adoption of a non-planar, propeller-like geometry within the chrysenyl ring system. This conformation shares common structural features with the related (+)- trans-anti -(B[c]Ph)dA adduct in the identical base sequence context, derived from the fjord region (+)-(1S,2R,3R,4S)-3, 4-dihydroxy-1,2-epoxy-1,2,3,4-tetrahydrobenzo[c]phenanthrene stereoisomer, in which intercalation is also observed towards the 5'-side of the modified dA6.dT17 base-pair.

On-line identification of diastereomeric dibenzo[a,l]pyrene diol epoxide-derived deoxyadenosine adducts by capillary electrophoresis-fluorescence line-narrowing and non-line narrowing spectroscopy

A capillary electrophoretic method for the separation and on-line identification of closely related analytes using low-temperature fluorescence spectroscopy is reported for the eight diastereomeric deoxyadenosine (dA) adducts derived from dibenzo[a,l]pyrene diol epoxide (DB[a,l]PDE). Electrophoretic separation of stereoisomers was accomplished by application of a mixed surfactant buffer [dioctyl sulfosuccinate (DOSS) and Brij-S], which was below the critical micelle concentration (CMC) due to the high concentration (approximately 25%) of organic solvent. Addition of multiple surfactant additives to the separation buffer provided electrophoretic resolution, which was unattainable under single surfactant conditions. It is shown that the CE-separated analyte zones could be identified on-line via low-temperature (4.2 K) fluorescence non-line narrowing and fluorescence line-narrowing (FLN) spectroscopy. In addition, it was determined that in CE buffer trans-syn-,cis-syn- and cis-anti-DB[a,l]PDE-14-N6dA diastereomeric adducts exist mostly with the -dA and DB[a,l]P moiety in an "open"-type conformation while the trans-anti-DB[a,l]PDE-14-N6dA adducts exist in two different conformations whose relative distribution depends on matrix composition. The above conformations have also been revealed by selective laser excitation. Thus, the low-temperature methodology not only provides fingerprint structure via vibrationally resolved 4.2 K fluorescence spectra for adduct identification, but also provides conformational information on the spatial relationship of the carcinogen and dA moiety. These results, taken together with those for DB[a,l]P-DNA adducts formed in standard glasses and mouse epidermis exposed to DB[a,l]P, support our earlier findings that DB[a,l]P-derived adducts exist in different conformations [Jankowiak et al., Chem. Res. Toxicol. 11 (1998) 674]. Therefore, the combination of the separation power of CE and spectral selectivity of low-temperature fluorescence spectroscopy at NLN and FLN conditions provides a powerful methodology which should prove useful for identification of closely related DNA adducts formed at low levels in biological systems.

The level of DNA modification by (+)-syn-(11S,12R,13S,14R)- and (-)-anti-(11R,12S,13S,14R)-dihydrodiol epoxides of dibenzo[a,l]pyrene determined the effect on the proteins p53 and p21WAF1 in the human mammary carcinoma cell line MCF-7

The polycyclic aromatic hydrocarbon (PAH) dibenzo[a,l]pyrene (DB[a,l]P), the most carcinogenic PAH tested in rodent bioassays, exerts its pathobiological activity via metabolic formation of electrophilically reactive DNA-binding fjord region (+)-syn-(11S,12R,13S,14R)- or (-)-anti-(11R,12S,13S,14R)-DB[a,l]P-dihydrodiol epoxides (DB[a,l]-PDEs). DB[a,l]P is metabolized to these DB[a,l]PDEs which bind to DNA in human mammary carcinoma MCF-7 cells. The molecular response of MCF-7 cells to DNA damage caused by DB[a,l]PDEs was investigated by analyzing effects on the expression of the tumor suppressor protein p53 and one of its target gene products, the cyclin-dependent kinase inhibitor p21WAF1. Treatment of MCF-7 cells with (+)-syn- and (-)-anti-DB[a,l]PDE at a concentration range of 0.001-0.1 microM resulted in DB[a,l]PDE-DNA adduct levels between 2 and 30, and 3 and 80 pmol/mg DNA, respectively, 8 h after exposure. (-)-anti-DB[a,l]PDE exhibited a higher binding efficiency that correlated with a significantly stronger p53 response at low concentrations of the dihydrodiol epoxides. The level of p53 increased by 6-8 h after treatment. The p21WAF1 protein amount exceeded control levels by 12 h and remained elevated for 96 h. At a dose of 0.01 microM (+)-syn-DB[a,l]PDE, an increase in p21WAF1 was observed in the absence of a detectable change in p53 levels. The results indicate that the increase in p53 induced by DB[a,l]PDEs in MCF-7 cells requires an adduct level of approximately 15 pmot/mg DNA and suggest that the level of adducts rather than the specific structure of the DB[a,l]PDE-DNA adduct formed triggers the p53 response. The PAH-DNA adduct level formed may determine whether p53 and p21VAF1 pathways respond, resulting in cell-cycle arrest, or fail to respond and increase the risk of mutation induction by these DNA lesions.

Tumor-initiating activity of the (+)-(S,S)- and (-)-(R,R)- enantiomers of trans-11,12-dihydroxy-11,12-dihydrodibenzo[a,l]pyrene in mouse skin

A single administration of enantiomerically pure 11,12-dihydrodiols of dibenzo[a,l]pyrene (DB[a,l]P) on the back of NMRI mice and subsequent chronic treatment with 12-O-tetradecanoylphorbol 13-acetate (TPA) (initiation/promotion assay) revealed strikingly different carcinogenic activities of both enantiomers. Tumor-initiating activity of (-)-(11R,12R)-DB[a,l]P-dihydrodiol, which is the metabolic precursor of the (-)-anti-(11R,12S)-dihydrodiol (13S,14R)-epoxide, was exceptionally higher than the corresponding effect of (+)-(11S,12S)-DB[a,l]P-dihydrodiol, the metabolic precursor of (+)-syn-(11S,12R)-dihydrodiol (13S,14R)-epoxide. After topical application of 10 nmol (-)-11,12-dihydrodiol and promotion with TPA twice weekly for a further 18 weeks 93% of treated animals exhibited four to five tumors. In contrast, no neoplasms were observed after treatment with 10 nmol (+)-11,12-dihydrodiol, whereas in the group exposed to 20 nmol of this enantiomer only 13% of mice developed neoplasms (0.1 tumors/survivor). For DB[a,l]P, considered as the most potent carcinogenic polycyclic aromatic hydrocarbon to date, stereoselective formation of (+)-syn- and (-)-anti-11,12-dihydrodiol 13,14-epoxides via the corresponding enantiomeric 11,12-dihydrodiols has been found to be the principal metabolic activation pathway leading to DNA adducts and mutagenicity. Our study demonstrates that the striking difference in carcinogenic activity in mouse skin of (+)-(11S,12S)- and (-)-(11R,12R)-DB[a,l]P-dihydrodiol convincingly reflects the different genotoxicity, i.e. DNA binding and mutagenicity, of both enantiomers observed earlier.

Metabolism and activation of cyclopenta polycyclic aromatic hydrocarbons in liver tissue from rats and humans

The metabolism of radiolabelled benz(j)aceanthrylene (B(j)A) was studied by high performance liquid chromatography (HPLC) using suspensions of hepatocytes and liver microsomes from control- or Aroclor 1254 (PCB)-treated rats, or with human liver microsomes (five different donors) as activation systems. The major metabolites formed in hepatocytes were sulfate conjugates, indicating that sulfation is an important detoxication pathway for B(j)A. In incubations with B(j)A and rat or human liver microsomes, the major metabolite formed was B(j)A-1,2-diol. Studies with rat liver microsomes using antibodies (Ab) towards either P4501A1, 1A2 or 3A2, resulted in approximately 30% reduction in covalent binding with all Ab-using microsomes from control animals, whereas with microsomes from PCB-treated animals an 85% reduction was observed using Ab towards P4501A2, and only minor reductions were observed with 1A1 or 3A2. When compared to B(j)A and benzo(a)pyrene (B(a)P), benz(1)aceanthrylene (B(l)A) caused higher numbers of revertants in the Salmonella assay when plated with rat liver microsomes from control animals or human liver microsomes. The total DNA adduct levels in hepatocytes from control animals after 2 h exposure to 30 micrograms/ml (120 microM) B(j)A or B(l)A, as measured by the 32P-postlabelling technique, were 3.8 +/- 1.5 and 10.1 +/- 5.8 fmol/microgram DNA, respectively. PCB-treatment decreased the total level of B(j)A adducts slightly (1.8 +/- 0.5 fmol/microgram DNA), whereas in contrast the level of B(1)A adducts was increased (24.5 +/- 20.1 fmol/microgram DNA). The major DNA adduct formed in control hepatocytes exposed to B(j)A co-chromatographed with B(j)A-1,2-oxide, which also appeared to be the major adduct formed when rat or human liver microsomes were co-incubated with calf thymus DNA. The total DNA adduct levels in the modified calf thymus DNA after 30 min exposure to 30 micrograms/ml B(j)A, B(l)A or B(a)P using rat liver microsomes form control animals, were 3.6, 66.3 and 1.4 fmol/microgram DNA, respectively. These levels increased to 22.7, 93.3 and 7.4 fmol/microgram DNA, respectively, using microsomes from PCB-treated animals. With human liver microsomes, the total DNA adduct levels after exposure to B(j)A, B(l)A or B(a)P, ranged between 0.4-1.0, 0.3-4.3, and 0.1-0.3 fmol/microgram DNA, respectively. Overall, the present data supports the notion that oxidation at the cyclopenta-ring is an important activation pathway for B(j)A, and indicate that the activation mechanism for B(j)A is similar in rat and human liver tissue.

Advances in capillary electrophoresis

This review summarizes the advancement in operational modes and selected applications of the title technique over the past five years. Regarding operational modes particular emphasis is put upon increasing selectivity and resolution, hyphenation of capillary electrophoresis with techniques based on other than electromigration principles, the so-called chip technology and new ways of detection. In applications selected examples of chiral separation and separation of biopolymers (proteins, nucleic acids) are emphasized. It is demonstrated that capillary electrophoresis represents a complementary technique to high-performance column chromatography and in a number of cases it offers better separations than standard chromatographic procedures.

Effect of cancer chemopreventive agents on microsome-mediated DNA adduction of the breast carcinogen dibenzo[a,l]pyrene

Due to the large and expanding number of potential cancer chemopreventive agents, there is an increasing need for short term tests to study the efficacy and mechanisms of these agents. In this study, we have employed a microsome-mediated test system to study the effect of several suspected chemopreventive agents on the DNA adduct formation capacity of the potent mammary carcinogen, dibenzo[a,l]pyrene (DBP). Bioactivation of DBP by Aroclor 1254-induced rat liver microsomes in the presence of calf thymus DNA (300 microg/ml) resulted in the formation of one major and six other prominent DNA adducts (324 adducts/10(7) nucleotides). These adducts were previously determined to be deoxyadenosine (dA) and deoxyanosine (dG)-derivatives of both anti- and syn-DBP-11,12-diol-13,14-epoxides (DBPDE). Intervention with ellagic acid, chlorophyllin, benzyl isocyanate (BIC), oltipraz or genistein (150 microM) strongly diminished DBP-DNA adduction by > or = 75%. Linoleic acid, curcumin and butylated hydroxytoluene (BHT) also significantly inhibited DBP DNA adduction (26-46%) while N-acetylcysteine (NAC) had no effect. Moreover, nonenzymatic studies with anti- and syn-DBPDE isomers revealed that chlorophyllin, ellagic acid, BIC and BHT may be inhibiting DBP-DNA adduction in an enzymatic-independent manner since these agents diminished DBPDE-DNA adduction by 30-75%. Genistein, oltipraz and curcumin did not diminish DBPDE-DNA adduction and therefore most likely require the presence of the microsomal subcellular fraction to inhibit DBP-DNA adduction.

Formation of stable adducts and absence of depurinating DNA adducts in cells and DNA treated with the potent carcinogen dibenzo[a,l]pyrene or its diol epoxides

Polycyclic aromatic hydrocarbons (PAH) are widespread environmental contaminants, and some are potent carcinogens in rodents. Carcinogenic PAH are activated in cells to metabolites that react with DNA to form stable covalent DNA adducts. It has been proposed [Cavalieri, E. L. & Roger, E. G. (1995) Xenobiotica 25, 677-688] that unstable DNA adducts are also formed and that apurinic sites in the DNA resulting from unstable PAH adducts play a key role in the initiation of cancer. The potent carcinogen dibenzo[a,l]pyrene (DB[a, l]P) is activated in cells to (+)-syn- and (-)-anti-DB[a,l]P-11, 12-diol-13,14-epoxide (DB[a,l]PDE), which have been shown to form stable adducts with DNA. To evaluate the importance of unstable PAH adducts, we compared stable adduct formation to apurinic site formation. Stable DB[a,l]PDE adducts were determined by 33P-postlabeling and HPLC. To measure apurinic sites they were converted to strand breaks, and these were monitored by examining the integrity of a particular restriction fragment of the dihydrofolate reductase gene. The method easily detected apurinic sites resulting from methylation by treatment of cells or DNA with dimethyl sulfate or from reaction of DNA with DB[a,l]P in the presence of horseradish peroxidase. We estimate the method could detect 0.1 apurinic site in the 14-kb fragment examined. However, apurinic sites were below our limit of detection in DNA treated directly with (+)-syn- or (-)-anti-DB[a,l]PDE or in DNA from Chinese hamster ovary B11 cells so treated, although in these samples the frequency of stable adducts ranged from 3 to 10 per 14 kb. We also treated the human mammary carcinoma cell line MCF-7 with DB[a,l]P and again could not detect significant amounts of unstable adducts. These results indicate that the proportion of stable adducts formed by DB[a,l]P activated in cells and its diol epoxides is greater than 99% and suggest a predominant role for stable DNA adducts in the carcinogenic activity of DB[a,l]P.

Application of capillary electrophoresis-fluorescence line-narrowing spectroscopy for on-line spectral characterization of closely related analytes

Capillary electrophoresis (CE) interfaced with low-temperature (4.2 K) fluorescence line-narrowing spectroscopy (FLNS) is used for the separation and spectral characterization of closely related analytes. In this paper, the CE-FLNS system is applied to the analysis of a mixture of deuterated and protonated benzo[a]pyrene, a mixture of structurally similar benzo[a]pyrene and benzo[e]pyrene, and mixtures of dibenzo[a,l]pyrene-derived adenine DNA adducts. The CE-FLNS system provides on-line separation and high-resolution spectroscopic identification of CE-separated analytes, via fingerprint structure of vibrationally resolved FLN spectra at 4.2 K. The combination of the separation power of CE and the spectral selectivity of FLNS provide a methodology that has potential to become a powerful tool for molecular analyte characterization. The main applications of the CE-FLNS system, due to its selectivity, should be in the chemical analysis of structurally similar analytes and applications where analyte purity and detailed structural characterization are required.

Tissue distribution of DNA adducts in rats treated by intramammillary injection with dibenzo[a,l]pyrene, 7,12-dimethylbenz[a]anthracene and benzo[a]pyrene

Dibenzo[a,l]pyrene (DBP) has recently emerged as a potent environmental carcinogen having greater carcinogenicity in the rat mammary epithelial glands than 7,12-dimethylbenz[a]anthracene (DMBA), previously considered to be the most potent mammary carcinogen and benzo[a]pyrene (BP), a ubiquitous environmental carcinogen. Previous studies on the tumor-initiating potential of DBP, DMBA, and BP demonstrated that DBP was 2.5 times more potent in inducing the tumors in mouse skin and rat mammary glands than DMBA; BP was a weak mammary carcinogen in these animals. The present study was designed to investigate if the significantly increased mammary carcinogenicity of DBP over DMBA and BP was related to increased DNA adduction at the target site. Female Sprague-Dawley rats were treated by intramammillary injection with an equimolar dose of 0.25 micromol/gland of DBP, DMBA, and BP at the 3rd, 4th and 5th mammary glands on both sides. 32P-Postlabeling analysis of mammary epithelial DNA of rats treated with DBP produced two major (nos. 3 and 6) and at least 5 minor adducts. DMBA treatment resulted in one major and 4 minor DNA adducts while BP produced one major and two minor adducts. Quantitation of the adduct radioactivity revealed that DNA adduction was 6- and 9-fold greater in DBP-treated animals than in BP- and DMBA-treated animals, respectively. The adduct levels per 10(9) nucleotides in mammary epithelial cells for DBP, BP and DMBA were in the following descending order: 1828 +/- 378, 300 +/- 45 and 207 +/- 72, respectively. Tissue distribution of DNA adducts in non-target organs following DBP treatment showed similar adduct pattern as found in the mammary epithelial cells except the liver, which resulted in 4 additional adduct spots; vehicle-treated tissue DNA processed in parallel did not show any detectable adducts. DMBA- and BP-DNA adduct patterns in various tissues were similar to that found in mammary epithelial cells, however, significant quantitative differences were found; BP-DNA adducts were undetectable in the pancreas and bladder. Quantitation of adduct radioactivity showed a 15- to 60-fold lower DBP-DNA adduction in these tissues than the levels found in the mammary tissue; similarly 5-20 and 30-100 times lower DNA adduction was found following treatment with DMBA and BP, respectively. The significantly increased binding of DBP to the mammary epithelial DNA over BP and DMBA is in concordance with its known higher mutagenicity and tumorigenicity.