Stereoselectivity of activation of 7,12-dimethylbenz[a]anthracene- 3,4-dihydrodiol to the anti-diol epoxide metabolite in a human mammary carcinoma MCF-7 cell-mediated V79 cell mutation assay

Low-dose synergistic immunosuppression of T-dependent antibody responses by polycyclic aromatic hydrocarbons and arsenic in C57BL/6J murine spleen cells

Polycyclic aromatic hydrocarbons (PAHs) and arsenic are both environmental agents that are known to have significant immunotoxicity. Previous studies have shown that PAH exposure of spleen cells in vitro produces significant immune suppression of humoral immunity, especially when P450 activation products are examined. Exposure to arsenic, particularly sodium arsenite, has also been found to be suppressive to antibody responses in vitro and in vivo. The purpose of the present studies was to examine the immunotoxicity of PAHs and arsenite following coexposures with the theory being that the agents may exert synergistic actions, which might be based on their different mechanisms of action. Spleen cells were isolated from male C57BL/6J wild-type mice and treated with PAHs and/or arsenic (arsenite or arsenate). Immunotoxicity assays were used to assess the T-dependent antibody response (TDAR) to sheep red blood cells (SRBCs), measured by a direct plaque-forming cell (PFC) assay. Cell viability was measured by trypan blue staining. Spleen cell viability was not altered following 4 days of PAH and/or arsenic treatment. However, the TDAR demonstrated suppression by both PAHs and arsenic in a concentration-dependent manner. p53 was also induced by NaAsO(2) (As(3)(+)) and PAHs alone or in combination. The PAHs and their metabolites investigated included benzo[a]pyrene (BaP), BaP-7,8-diol, BaP-7,8-diol-9,10-epoxide (BPDE), 7,12-dimethylbenz[a]anthracene (DMBA), DMBA-3,4-diol, dibenzo[a,l]pyrene (DB[a,l]P). PAH metabolites were found to be more potent than parent compounds in producing immunosuppression and inducing p53 expression. Interestingly, DB[a,l]P, a potent carcinogenic PAH not previously characterized for immunotoxicity, was also found to be strongly immunosuppressive. Arsenite (NaAsO(2), As(3)(+)) was found to produce immunosuppression at concentrations as low as 0.5 microM and was immunosuppressive at a 10-fold lower concentration than sodium arsenate (Na(2)HAsO(4), As(5)(+)). Coexposure of spleen cell cultures to PAHs and As(3)(+), both at individual low-effect concentrations, was found to produce profound suppression of the TDAR demonstrating synergy between these two chemical classes of agents.

Naturally occurring coumarins inhibit human cytochromes P450 and block benzo[a]pyrene and 7,12-dimethylbenz[a]anthracene DNA adduct formation in MCF-7 cells

Naturally occurring coumarins (NOCs) inhibit polycyclic aromatic hydrocarbon-induced skin tumor initiation in mice by blocking cytochrome P450 (P450)-mediated bioactivation of benzo[a]pyrene (B[a]P) and 7,12-dimethylbenz[a]anthracene (DMBA). Bergamottin selectively inhibits tumor initiation by B[a]P, whereas imperatorin and isopimpinellin inhibit tumor initiation with both carcinogens. The goals of the current study were to examine the ability of NOCs to inhibit human P450s in vitro and to establish whether NOCs, which are anticarcinogenic in mice, can block carcinogen bioactivation in cultured human cells. For the initial experiments, incubations containing 5 microM P450, P450 substrate, an NADPH generating system, and NOCs were used to determine the concentrations of each inhibitor that blocked 50% of P450 activity (IC(50)). These results confirmed that NOCs are capable of inhibiting multiple human P450s and that they exhibit selectivity for certain isoforms of human P450s. In subsequent experiments, we examined the effects of bergamottin, imperatorin, and isopimpinellin on DMBA and B[a]P DNA adduct formation in the human breast MCF-7 adenocarcinoma cell line. Coincubation of cells with the three different NOCs significantly inhibited DMBA DNA adduct formation by 29-82% at doses ranging from 2 to 10 microM and significantly inhibited B[a]P DNA adduct formation by 37-80% at doses ranging from 20 to 80 microM. HPLC analysis of the DNA hydrolysates demonstrated that inhibition of DNA adducts corresponded to inhibition of the major B[a]P and DMBA diol-epoxide-derived adducts. Although bergamottin was not effective at blocking DMBA bioactivation in the mouse skin model, it was similar in effectiveness to imperatorin and isopimpinellin in MCF-7 cells. These results demonstrate that NOCs, which are present in citrus fruits and other components of the human diet, are capable of inhibiting carcinogen metabolizing enzymes and blocking bioactivation of both B[a]P and DMBA in MCF-7 cells.

Characterization of a major aromatic DNA adduct detected in human breast tissues

A bulky DNA adduct (Spot 1) was previously detected in normal adjacent breast tissues of 41% (36/87) of women with breast cancer and in none (0/29) of the noncancer controls by (32)P-postlabeling. To characterize this adduct, it was chromatographically compared with DNA adduct profiles generated in several in vitro and in vivo experimental systems. First, MCF-7 cells were exposed to a number of chemical carcinogens, that is, benzo[a]pyrene (B[a]P), 4-OH-B[a]P, 9-OH-B[a]P, 11-OH-B[a]P, B[a]P-trans-4,5-dihydrodiol, 1-nitropyrene, 6-nitrochrysene, dibenzo[a,l]pyrene, benzo[c]phenanthrene, dibenzo[a,h]anthracene, 3-methylcholanthrene, and 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine. Spot 1 was detected as a minor adduct in cells treated with B[a]P but not other compounds. Second, to determine whether Spot 1 is derived from lipid peroxidation products or estrogen metabolites, it was compared with adduct profiles of cells or DNAs exposed to 17beta-estradiol, 4-hydroxy estradiol, 4-hydroxynonenal, or oxidized oat oil. Spot 1 was not detectable in these samples. In addition, Spot 1 did not comigrate with the 1,N(2)-ethenodeoxyguanosine adduct standard. Third, to explore the mechanism of Spot 1 formation, it was compared with adduct profiles detected in DNA or mononucleotides reacted with BPDE, 1-OH-7,8-dihydrodiol of B[a]P, and 3-OH-7,8-dihydrodiol of B[a]P as well as in rats orally treated with B[a]P. Spot 1 comigrated with a minor adduct in BPDE-treated DNA during anion exchange rechromatography but these two adducts were separated by partition chromatography. Spot 1 also behaved in a manner that was very similar to that of the polar B[a]P adducts detected in rat liver, but the two adducts were separated by HPLC. Fourth, Spot 1 was compared with CD1 mice exposed to 7H-benzo[c]fluorene (B[c]F). Spot 1 from some patients comigrated with a major adduct induced by B[c]F. Finally, we found that the presence of Spot 1 in human breast tissues was not related to smoking status but, rather, with CYP1A1 MspI polymorphism. The CYP1A1 mutant carriers had a significantly higher frequency of this adduct than did the wild-type genotypes. Furthermore, individuals with Spot 1 had a significantly higher staining intensity for BPDE-PAH adducts in their tissue sections than those without it. These results demonstrate that this major bulky DNA adduct detected in human breast tissues is related to PAH exposure.

Formation of stable DNA adducts and apurinic sites upon metabolic activation of bay and fjord region polycyclic aromatic hydrocarbons in human cell cultures

Carcinogenic polycyclic aromatic hydrocarbons (PAH), such as benzo[a]pyrene (B[a]P), 7,12-dimethylbenz[a]anthracene (DMBA), and dibenzo[a,l]pyrene (DB[a,l]P), are metabolically activated to electrophilically reactive bay or fjord region diol epoxides that bind to the exocyclic amino groups of purine bases in DNA to form stable adducts. In addition, it has been reported that these PAH can be enzymatically oxidized to yield radical cations that form apurinic (AP) sites in DNA via depurinating adducts. The formation of stable adducts and AP sites in DNA of human cells exposed to PAH was examined in cytochrome P450 (P450)-expressing mammary carcinoma MCF-7 cells and in leukemia HL-60 cells, which display a high peroxidase but no P450-mediated activity, after exposure to these PAH. Stable DNA adducts were assessed by (33)P-postlabeling/HPLC analysis, and the induction of AP sites in DNA was analyzed by an aldehyde reactive probe (ARP) and a slot blot method. After exposure for 4 h, the levels of stable DNA adducts were comparable in MCF-7 cells treated with B[a]P and DMBA, but significantly lower than those observed in MCF-7 cells treated with the stronger carcinogen DB[a,l]P. While the levels of stable adducts increased more than 10-fold (B[a]P and DMBA) or 100-fold (DB[a,l]P) after exposure for 24 h, the levels of AP sites remained low after both treatment periods. Thus, the levels of stable adducts were approximately 5-fold higher than the levels of AP sites after treatment with B[a]P or DMBA and more than 100-fold higher in cells exposed to DB[a,l]P for 24 h. None of these carcinogenic PAH formed detectable levels of stable DNA adducts or AP sites in HL-60 cells. The results demonstrate that metabolic activation of B[a]P, DMBA, and DB[a,l]P is catalyzed by P450 enzymes leading to diol epoxides that form predominantly stable DNA adducts but only low levels of AP sites.

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.

Targeted disruption of the microsomal epoxide hydrolase gene. Microsomal epoxide hydrolase is required for the carcinogenic activity of 7,12-dimethylbenz[a]anthracene

Microsomal epoxide hydrolase (mEH) is a conserved enzyme that is known to hydrolyze many drugs and carcinogens, and a few endogenous steroids and bile acids. mEH-null mice were produced and found to be fertile and have no phenotypic abnormalities thus indicating that mEH is not critical for reproduction and physiological homeostasis. mEH has also been implicated in participating in the metabolic activation of polycyclic aromatic hydrocarbon carcinogens. Embryonic fibroblast derived from the mEH-null mice were unable to produce the proximate carcinogenic metabolite of 7,12-dimethylbenz[a]anthracene (DMBA), a widely studied experimental prototype for the polycylic aromatic hydrocarbon class of chemical carcinogens. They were also resistant to DMBA-mediated toxicity. Using the two-stage initiation-promotion skin cancer bioassay, the mEH-null mice were found to be highly resistant to DMBA-induced carcinogenesis. In a complete carcinogenesis bioassay, the mEH mice were totally resistant to tumorigenesis. These data establish in an intact animal model that mEH is a key genetic determinant in DMBA carcinogenesis through its role in production of the ultimate carcinogenic metabolite of DMBA, the 3,4-diol-1,2-epoxide.

Comparison of HepG2 feeder cells generated by exposure to gamma-rays, X-rays, UV-C light or mitomycin C for ability to activate 7,12-dimethylbenz[a]anthracene in a cell-mediated Chinese hamster V79/HGPRT mutation assay

The cell-mediated Chinese hamster V79/HGPRT mutagenicity assay is an established in vitro testing method. Although gamma-irradiated human HepG2 hepatoma cells have been used recently for chemical activation, an alternative is now needed due to scheduled retirement of the available gamma-source. X-irradiation, 254 nm UV-C light and mitomycin C were examined as possible HepG2 mitotic inhibitors, and treated cells compared for activation of 7, 12-dimethylbenz[a]anthracene (DMBA). In colony-forming assays, V79 and HepG2 cells differed in sensitivity to DMBA, with V79 survival declining sharply between 1-2.5 microM (LD50=1.75 microM) while HepG2 survival decreased gradually, beginning at 0.01 microM DMBA (LD50=0.045 microM). When HepG2 feeder cells generated by each method were included in V79/HGPRT mutation assays, activation of 1 microM DMBA was found to vary according to the mitotic inhibitor used, with mutation frequencies decreasing in the order 4000 rads gamma-rays>25 microg/ml mitomycin C>4000 rads X-rays>25 J/m2 UV-C light. Only assays containing gamma-irradiated HepG2 cells generated an increase (2-3-fold) in mutation frequency when DMBA exposure was extended from 24 to 48 h. The effect of HepG2 preincubation with either Aroclor 1254 or DMBA on feeder cell activation of DMBA was also assessed using concentrations of Aroclor 1254 (10 microg/ml) or DMBA (1.0 microM) which were found to produce optimum induction of ethoxyresorufin-O-deethylase (EROD) activity (3.1-fold and 2-fold increases, respectively). Compared to results obtained with uninduced HepG2 cells, assays incorporating HepG2 cells activated by either Aroclor 1254 or DMBA produced slightly increased V79/HGPRT mutation frequencies after 24 h of exposure to mutagen; however, a 48 h incubation with mutagen in the presence of HepG2 preincubated with either Aroclor 1254 or DMBA resulted in higher mutation frequencies regardless of the mitotic inhibitor treatment. EROD activity was also induced 1.4-fold following exposure of HepG2 cells to mitomycin C alone. Although gamma-irradiation remains the treatment of choice for producing metabolically active HepG2 feeder cells, comparison of the alternatives tested suggests that mitomycin C would be a convenient and suitable replacement.

Stereoselective activation of dibenzo[a,l]pyrene and its trans-11,12-dihydrodiol to fjord region 11,12-diol 13,14-epoxides in a human mammary carcinoma MCF-7 cell-mediated V79 cell mutation assay

Dibenzo[a,l]pyrene (DB[a,l]P) represents the most potent carcinogenic polycyclic aromatic hydrocarbon (PAH) yet discovered. Like other PAHs, DB[a,l]P requires metabolic activation to exert its mutagenic and/or carcinogenic activity. In the human mammary carcinoma cell line MCF-7, DB[a,l]P is stereoselectively metabolized to the (-)-anti- and (+)-syn-DB[a,l]P-11,12-diol 13,14-epoxides (DB[a,l]PDE) which both bind extensively to deoxyadenosine residues in DNA. To further characterize the underlying mechanism of its strong carcinogenicity, the relationship between DNA binding and mutagenicity of DB[a,l]P was determined. Racemic DB[a,l]P-11,12-dihydrodiol and the two individual (+)- and (-)-enantiomers, the metabolic precursors of the stereoisomeric fjord region dihydrodiol epoxides, were also investigated. Induction of mutations at the HPRT locus was measured in a MCF-7 cell-mediated Chinese hamster V79 cell mutation assay. The parent hydrocarbon, (+/-)-DB[a,l]P-11,12-dihydrodiol, and (-)-DB[a,l]P-11,12-dihydrodiol were highly mutagenic under the assay conditions. In contrast, (+)-DB[a,l]P-(11S,12S)-dihydrodiol was not mutagenic using MCF-7 cells as the metabolic activating system. Analysis of DNA adducts in the same experiments revealed that MCF-7 cells treated with (-)-DB[a,l]P-11,12-dihydrodiol formed exclusively (-)-anti-DB[a,l]-PDE adducts whereas cells treated with (+)-DB[a,l]P-11,12-dihydrodiol did not contain detectable levels of DNA adducts. These results suggest that specific cytochrome P450 enzymes may have high stereoselectivity for activation of the two DB[a,l]P-11,12-dihydrodiol enantiomers, and this may play an important role in the metabolic activation of the strong carcinogen DB[a,l]P in human cells.