Tumorigenicity of 7,12-dimethylbenz[a]anthracene, its hydroxymethylated derivatives and selected dihydrodiols in the newborn mouse

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.

Functional characterization of human cytochrome P450 2S1 using a synthetic gene-expressed protein in Escherichia coli

Human cytochrome P450 2S1 was recently identified and shown to be inducible by 2,3,7,8-tetrachlorodibenzo-p-dioxin and hypoxia. It is highly expressed in epithelial cells of tissues that are exposed to the environment and in many tumors of epithelial origin. The biological function of CYP2S1 has not yet been determined, although its possible role in carcinogen metabolism has been suggested. In this report, we investigated its ability to metabolize carcinogens. To obtain a large quantity of active enzyme for substrate screening, we overexpressed CYP2S1 in Escherichia coli (200 nM culture), using a synthetic gene approach. High-level expression allowed us to achieve purification of CYP2S1 with high specific content and purity (16 nmol/mg). Despite high-level expression, we found that CYP2S1 was not readily reduced by cytochrome P450 reductase, and thus no activity was found using NADPH. However, the oxidative activity of CYP2S1 was supported by cumene hydroperoxide or H(2)O(2), such that CYP2S1 oxidized many important environmental carcinogens, including benzo[a]pyrene, 9,10-dihydro-benzo[a]pyrene, 7,12-dimethylbenz[a]anthracene, benzo[a]pyrene-7,8-dihydrodiol, aflatoxin B1, naphthalene, and styrene, with high turnover. Most substrates tested were converted to detoxification products, except in the case of benzo[a]pyrene-7,8-dihydrodiol, which was converted into the very potent carcinogenic metabolite 7,8-dihydrodiol-trans-9,10-epoxide at a relatively efficient rate (K(m) = 12.4 +/- 2 microM, turnover = 2.3 min(-1)). This metabolite formation was also supported both in vitro and in vivo by fatty acid hydroperoxides described in the accompanying report (p. 1044). Together, these data indicate that CYP2S1 contributes to the metabolism of environmental carcinogens via an NADPH independent activity.

Detection of benzylic adducts in DNA and nucleotides from 7-sulfooxymethyl-12-methylbenz[a]anthracene and related compounds by 32P-postlabeling using new TLC systems

7,12-Dimethylbenz[a]anthracene (DMBA) is a highly potent experimental carcinogen, that must be transformed to its ultimate carcinogenic form in vivo. The meso-region theory of aromatic hydrocarbon carcinogenesis predicts that 7-hydroxymethyl sulfate (7-HMBA) ester plays a major role in the metabolic activation, benzylic DNA adduct formation and complete carcinogenicity of HMBA and DMBA. This study was undertaken to detect highly lipophilic benzylic DNA adducts resulting from the reaction between 7-hydroxymethy sulfate ester of HMBA (7-SMBA) and DNA as well as determine their DNA base selectivity. Synthetic 7-SMBA was incubated with DNA (800 microg/ml) and individual deoxynucleoside 3'-monophosphates (600 microg/ml) and benzylic adducts were analyzed by 32P-postlabeling/TLC following their enrichment with butanol extraction. Dilute ammonium hydroxide-based solvents were developed to detect the highly lipophilic aralkyl adducts. The reaction with DNA, dGp and dAp gave rise to multiple adducts; dCp and dTp showed no significant adducts. Chromatographic comparison revealed that the major DNA adduct was derived from dG. The methodology developed was also found applicable for highly lipophilic adducts resulting from sulfate esters of structurally-related metabolites of DMBA.

7-Sulfooxymethyl-12-methylbenz[a]anthracene is an exceptionally reactive electrophilic mutagen and ultimate carcinogen

The hypothesis was tested that an ultimate carcinogen of 7-hydroxymethyl-12-methylbenz[a]anthracene (HMBA), a major metabolite of 7,12-dimethylbenz[a]anthracene (DMBA), is a benzylic carbonium ion generated from an exceptionally reactive aralkylating metabolite, such as an electrophilic sulfate ester. In conformity with this hypothesis, sarcomas were rapidly induced in rats following repeated subcutaneous injection of HMBA (67%) or its electrophilic sulfate ester, sodium 7-sulfooxymethyl-12-methylbenz[a]anthracene (SMBA) (100%). It would appear from the results summarized here that the search for a carcinogenic metabolite of DMBA has been successful. In addition, an aralkylating electrophilic mutagen and carcinogen has been prepared from HMBA, which is itself either an ultimate carcinogen or a direct precursor of an ultimate carcinogen, i.e., a benzylic carbonium ion.

Specificity of cDNA-expressed human and rodent cytochrome P450s in the oxidative metabolism of the potent carcinogen 7,12-dimethylbenz[a]anthracene

7,12-Dimethylbenz[a]anthracene (DMBA), a potent carcinogen, requires metabolic activation by cytochrome P450s (P450s) to electrophilic metabolites that result in DNA modification, mutagenicity, and carcinogenicity. In this study, we used eight human forms, four rodent forms, and one rabbit form of P450 expressed from recombinant vaccinia or baculovirus vectors to define their specificity for metabolizing DMBA. Of the eight human P450s, 1A1 was the most active (specific activity = 14.7 nmol/min/nmol of P450) in total metabolism of DMBA and showed approximately 6- to 33-fold more activity than other P450s, 2B6, 2C9, and 1A2 were also capable of metabolizing DMBA (2.0-2.5 nmol/min/nmol of P450), whereas 2C8, 2E1, 3A4, and 3A5 exhibited relatively low activities. Among animal P450s, mouse 1A1 exhibited activity similar to that of human 1A1 and had 5.0- to 37-fold more activity than other rodent and rabbit P450s. In regard to enzyme regioselectivity, most human and rodent P450s predominantly formed the 8,9-diol, but human 2B6 and rat 2B1 preferentially formed the 5,6-diol. In the production of monohydroxymethyl metabolites, all the enzymes yielded more 7-hydroxymethyl-12-methylbenz[a]anthracene (7HOM12MBA) than 12-hydroxymethyl-7-methylbenz[a]anthracene (7M12HOMBA), except for human 1A1, which presented the reverse selectivity. Human liver microsomes from 10 organ donors were shown to metabolize DMBA and in most circumstances generated the metabolic profile DMBA trans-8,9-dihydrodiol > 7HOM12MBA > or = DMBA trans-5,6-dihydrodiol > or = 7,12-dihydroxymethylbenz[a]anthracene > 7M12HOMBA > DMBA trans-3,4-dihydrodiol. Thus, the combined activity of hepatic microsomal 2C9, 1A2, and 2B6 may contribute to the metabolic activation and the metabolism of DMBA in normal human liver.

beta-Carotene-mediated inhibition of a DNA adduct induced by 7,12-dimethylbenz(a)anthracene and 7-hydroxymethyl-12-methylbenz(a)anthracene in mouse mammary gland in vitro

The influence of beta-carotene on the formation of DNA-adducts induced by 7,12-dimethylbenz(a)anthracene (DMBA) and 7-hydroxymethyl-12-methylbenz(a)anthracene (7-OHM-12-MBA) during transformation of mouse mammary cells in organ culture was analysed. Treatment with beta-carotene (10(-8)-10(-5) mol/l) caused inhibition (48.8-94.4%) of an adduct (VI), which was detectable in DNA samples from DMBA-treated mammary glands. Out of six adducts, derived from further analysis of DNA samples from 7-OHM-12-MBA-treated glands, adduct f eluted in the same fraction as adduct (VI), indicating these adducts were analogous. Likewise, adduct f was also inhibited by beta-carotene. Boronate chromatographic analysis revealed this particular adduct was a syn-dihydrodiol epoxide product. Adduct inhibition was detectable both at the start and after DMBA treatment. alpha-Tocopherol and canthaxanthin were ineffective in inhibiting adducts. It is reasonable to conclude that beta-carotene-mediated modification of adducts is associated with the inhibition of a syn-adduct, which is derived from further metabolism of a 7-OHM-12-MBA intermediate.

Metabolism and toxicity of xenobiotics in the adrenal cortex, with particular reference to 7,12-dimethylbenz(a)anthracene

The adrenal cortex contains high amounts of detoxifying enzymes, as well as generators and protectors of reactive oxygen species. The high content of cytochrome P-450 enzymes in the adrenal cortex together with its remarkable tendency to accumulate hydrophobic substances probably contributes to the extraordinary vulnerability of the gland to a number of xenobiotics. The best studied adrenocorticolytic compounds are the potent carcinogen 7,12-dimethylbenz(a)anthracene (DMBA) and its liver metabolite 7-hydroxymethyl-12-methylbenz(a)anthracene (7-OHM-12-MBA). Adrenocorticolysis generated by these agents in vivo as well as in vitro demonstrates high regioselective requirements and is strongly influenced by the presence of ACTH, steroids, cytochrome P-450 inhibitors and antioxidants. Furthermore, 7-OHM-12-MBA has been demonstrated to uniquely generate selective and massive oxidation of mitochondrial glutathione in cultured rat adrenal cells. The DMBA-induced adrenocorticolysis is thoroughly discussed in this review with particular emphasis on the metabolism of DMBA and the influence of various effectors. A working hypothesis involving a possible peroxidative mechanism is also presented.

Stereoselective fungal metabolism of 7,12-dimethylbenz[a]anthracene: identification and enantiomeric resolution of a K-region dihydrodiol

Syncephalastrum racemosum UT-70 and Cunninghamella elegans ATCC 36112 metabolized 7,12-dimethylbenz[a]anthracene (7,12-DMBA) to hydroxymethyl metabolites as well as 7-hydroxymethyl-12-methylbenz[a]anthracene trans-3,4-, -5,6-, -8,9-, and -10,11-dihydrodiols. The 7,12-DMBA metabolites were isolated by reversed-phase high-performance liquid chromatography and identified by their UV-visible absorption, mass, and nuclear magnetic resonance spectral characteristics. A comparison of the circular dichroism spectra of the K-region (5,6-position) dihydrodiol of both fungal strains with those of the 7,12-DMBA 5S,6S-dihydrodiol formed from 7,12-DMBA by rat liver microsomes indicated that the major enantiomer of the 7-hydroxymethyl-12-methylbenz[a]anthracene trans-5,6-dihydrodiol formed by both fungal strains had a 5R,6R absolute stereochemistry. Direct resolution of the fungal trans-5,6-dihydrodiols by chiral stationary-phase high-performance liquid chromatography indicated that the ratios of the R,R and S,S enantiomers were 88:12 and 77:23 for S. racemosum and C. elegans, respectively. These results indicate that the fungal metabolism of 7,12-DMBA at the K region (5,6-position) is highly stereoselective and different from that reported for mammalian enzyme systems.

Three stage carcinogenesis in mouse skin--recent results and present status of an advanced model system of chemical carcinogenesis

In the three stage model of carcinogenesis in mouse skin, to a certain compartment of normal cells new and adverse biological properties are imposed by the initiator DMBA yielding 'potential tumor cells,' or 'initiated cells.' These initiated cells exhibit a selective advantage of growth over surrounding normal cells if exposed to DTE promoters. The new properties may result from a genomic mutation associated with an increase of density, affinity or cooperativity of certain membrane receptors as compared to normal cells. Complete promoters, such as TPA, but not incomplete promoters such as RPA may impose a second additional genomic or epigenomic insult onto initiated cells ('conversion'). As a consequence, initiated cells become ready to respond to unspecific mitogenic stimuli ('propagation'), as provided by hyperplasiogenic agents. The initiation/promotion model of mouse skin presents new and attractive possibilities to apply the oncogene approach for further in-depth analysis of the molecular mechanisms of its stages. It remains to be seen to what extent molecular events demonstrated causative for the stages in the mouse skin model are applicable also to stages in multistage models of chemical carcinogenesis operational in other target tissues.

Chemical carcinogens. A review and analysis of the literature of selected chemicals and the establishment of the Gene-Tox Carcinogen Data Base. A report of the U.S. Environmental Protection Agency Gene-Tox Program

The literature on 506 selected chemicals has been evaluated for evidence that these chemicals induce tumors in experimental animals and this assessment comprises the Gene-Tox Carcinogen Data Base. Three major sources of information were used to create this evaluated data base: all 185 chemicals determined by the International Agency for Research on Cancer to have Sufficient evidence of carcinogenic activity in experimental animals, 28 selected chemicals bioassayed for carcinogenic activity by the National Toxicology Program/National Cancer Institute and found to induce tumors in mice and rats, and 293 selected chemicals which had been evaluated in genetic toxicology and related bioassays as determined from previous Gene-Tox reports. The literature data on the 239 chemicals were analyzed by the Gene-Tox Carcinogenesis Panel in an organized, rational and consistent manner. Criteria were established to assess individual studies employing single chemicals and 4 categories of response were developed: Positive, Negative, Inconclusive (Equivocal) and Inconclusive. After evaluating each of the individual studies on the 293 chemicals, the Panel placed each of the 506 chemicals in an overall classification category based on the strength of the evidence indicating the presence or absence of carcinogenic effects. An 8-category decision scheme was established using a modified version of the International Agency for Research on Cancer approach. This scheme included two categories of Positive (Sufficient and Limited), two categories of Negative (Sufficient and Limited), a category of Equivocal (the evidence of carcinogenicity from well-conducted and well-reported lifetime studies had uncertain significance and was neither clearly positive nor negative), and three categories of Inadequate (the evidence of carcinogenicity was insufficient to make a decision, however, the data suggested a positive or negative indication). Of the 506 chemicals in the Gene-Tox Carcinogen Data Base, 252 were evaluated as Sufficient Positive, 99 as Limited Positive, 40 as Sufficient Negative, 21 as Limited Negative, 1 as Equivocal, 13 as Inadequate with the data suggesting a positive indication, 32 as Inadequate with the data suggesting a negative indication, and 48 Inadequate with the data not suggesting any indication of activity. This data base was analyzed and examined according to chemical class, using a 29 chemical class scheme.(ABSTRACT TRUNCATED AT 400 WORDS)

Regio- and stereo-selective metabolism of 4-methylbenz[a]anthracene by the fungus Cunninghamella elegans

Metabolism of 4-methylbenz[a]anthracene by the fungus Cunninghamella elegans was studied. C. elegans metabolized 4-methylbenz[a]anthracene primarily at the methyl group, this being followed by further metabolism at the 8,9- and 10,11-positions to form trans-8,9-dihydro-8,9-dihydroxy-4-hydroxymethylbenz[a]anthracene and trans-10,11-dihydro-10,11-dihydroxy-4-hydroxymethylbenz[a]anthracene. There was no detectable trans-dihydrodiol formed at the methyl-substituted double bond (3,4-positions) or at the 'K' region (5,6-positions). The metabolites were isolated by reversed-phase high-pressure liquid chromatography and characterized by the application of u.v.-visible-absorption-, 1H-n.m.r.- and mass-spectral techniques. The 4-hydroxymethylbenz[a]anthracene trans-8,9- and -10,11-dihydrodiols were optically active. Comparison of the c.d. spectra of the trans-dihydrodiols formed from 4-methylbenz[a]anthracene by C. elegans with those of the corresponding benz[a]anthracene trans-dihydrodiols formed by rat liver microsomal fraction indicated that the major enantiomers of the 4-hydroxymethylbenz[a]anthracene trans-8,9-dihydrodiol and trans- 10,11-dihydrodiol formed by C. elegans have S,S absolute stereochemistries, which are opposite to those of the predominantly 8R,9R- and 10R,11R-dihydrodiols formed by the microsomal fraction. Incubation of C. elegans with 4-methylbenz[a]anthracene under 18O2 and subsequent mass-spectral analysis of the metabolites indicated that hydroxylation of the methyl group and the formation of trans-dihydrodiols are catalysed by cytochrome P-450 mono-oxygenase and epoxide hydrolase enzyme systems. The results indicate that the fungal mono-oxygenase-epoxide hydrolase enzyme systems are highly stereo- and regio-selective in the metabolism of 4-methylbenz[a]anthracene.

Effects of butylated hydroxyanisole and butylated hydroxytoluene on 7,12-dimethylbenz[a]anthracene-DNA adduct formation in mouse embryo cell cultures

Neither butylated hydroxyanisole (BHA) nor butylated hydroxytoluene (BHT) significantly reduced overall 7,12-dimethylbenz[a]anthracene (DMBA)--DNA adduct formation in mouse embryo cell cultures. However, analysis of DMBA--DNA adducts by Servacel DHB chromatography and high-pressure liquid chromatography showed that treatment of cells with BHA, but not with BHT, resulted in a decreased contribution from the syn bay region dihydrodiol epoxide to overall binding.

Differentiation of the mammary gland and susceptibility to carcinogenesis

It has been demonstrated that in humans certain factors such as early menarche, late pregnancy, and nulliparity are associated with a higher risk of developing breast cancer, while early pregnancy acts as a protective factor. Induction of mammary cancer in rats by administration of the chemical carcinogen 7,12-dimethylbenz(a)anthracene reveals that the same factors influencing human breast cancer risk also affect the susceptibility of the rat mammary gland to the chemical carcinogen. Nulliparous rats and rats undergoing pregnancy interruption are more susceptible to developing carcinomas. This fact has been attributed to the incomplete differentiation of the gland at the time of carcinogen administration. Parous rats are resistant to the carcinogenic effect of DMBA, which is explained by the complete development of the gland attained during pregnancy and lactation. This development is manifested by the differentiation of terminal end buds into secretory units, which have a smaller proliferative compartment; the epithelial cells of these secretory units have a longer cell cycle, less avidity for binding DMBA, and possess a more efficient DNA excision repair capacity.