Morphological transforming activity and metabolism of cyclopenta-fused isomers of benz[a]anthracene in mammalian cells

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

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

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.

Mouse skin tumor-initiating activity of benz[j]aceanthrylene in SENCAR mice

Benz[j]aceanthrylene (B[j]A), a cyclopenta-fused derivative of benz[a]anthracene, has been reported to be an active bacterial cell and mammalian cell gene mutagen, a morphological transforming agent in C3H10T1/2CL8 mouse embryo fibroblasts and a mouse lung tumorigen in strain A/J mice. B[j]A was evaluated as a skin tumor initiator in female SENCAR mice and was found to induce papilloma formation in the range of 40-400 micrograms/mouse. B[j]A was found to be extremely active, inducing 8.7 papillomas/mouse after an initiating dose of 40 micrograms/mouse. At this dose, 100% of the mice bore tumors. Comparison with four other cyclopenta-fused polycyclic aromatic hydrocarbons suggests that B[j]A is extremely potent.

Activation and metabolism of benz[j]aceanthrylene-9,10-dihydrodiol, the precursor to bay-region metabolism of the genotoxic cyclopenta-PAH benz[j]aceanthrylene

Benz[j]aceanthrylene, a cyclopentafused polycylic aromatic hydrocarbon produced in combustion emissions, possesses a bay region and an etheno bridge which may both contribute to the overall genotoxicity of the compound. In order to assess the role of activation at the bay region, the precursor epoxide benz[j]aceanthrylene 9,10-oxide, its dehydration product 10-hydroxybenz[j]aceanthrylene, the key dihydrodiol 9,10-dihydroxy-9,10-dihydrobenz[j]aceanthrylene and the bay-region diol-epoxide 7,8-epoxy-9,10-dihydroxy-7,8,9,10- tetrahydrobenz[j]aceanthrylene were evaluated in the bacterial histidine-reversion plate incorporation assay (Ames assay) with Salmonella typhimurium strain TA98. The diol-epoxide alone showed direct-acting mutagenicity (10 revertants per nmole), which was decreased by addition of exogenous metabolic activation (Aroclor 1254-treated rat-liver S9), whereas all the other compounds tested were activated by increasing concentrations of S9. The potency of the diol-epoxide was not sufficient to account for the activity of the parent compound. Identification by proton nuclear magnetic resonance and mass spectrometry of the major products of further metabolism by Aroclor 1254-treated rat-liver S9 of the bay region precursor dihydrodiol 9,10-dihydroxy-9,10-dihydrobenz[j]aceanthrylene indicated that oxidation occurred predominantly at the etheno bridge, to give 9,10-dihydroxy-9,10-dihydrobenz[j]aceanthrylene-2(1H)-one, arising by (non-enzymic) rearrangement of the etheno bridge epoxide and the tetrol 1,2,9,10-tetrahydroxy-1,2,9,10- tetrahydrobenz[j]aceanthrylene. The bay region tetrol 7,8,9,10-tetrahydroxy-7,8,9,10-tetrahydrobenz[j] aceanthrylene was observed, implying further bay-region metabolism; re-aromatization of the benzo ring to benz[j]aceanthrylene-9,10-diol also occurred. Thus oxidation at the etheno bridge accounts for the majority of the activity of benz[j]aceanthrylene and its derivatives when Aroclor 1254-treated rat-liver S9 is used for exogenous metabolic activation.

Ki-ras oncogene mutations in tumors and DNA adducts formed by benz[j]aceanthrylene and benzo[a]pyrene in the lungs of strain A/J mice

Strain A/J mice received intraperitoneal injections of benz[j]aceanthrylene (B[j]A) or benzo[a]pyrene (B[a]P). At 24, 48, and 72 h, lung tissues were removed for analysis of B[a]P- or B[j]A-derived DNA adduct formation during the first 3 d of exposure. One group of mice exposed to these hydrocarbons was kept for 8 mo to determine lung tumor multiplicity, the occurrence of mutations in codons 12 and 61 of the Ki-ras gene in the tumors that arose, the relationship between Ki-ras oncogene mutations in tumors, and the presence and quantity of genomic DNA adducts. The major DNA adduct in the lungs of mice exposed to B[a]P was N2-(10 beta-[+B, 7 alpha, 9 alpha-trihydroxy-7,8,9,10-tetrahydrobenzo[a]pyrene]yl)-deoxyguanosine (BPDE-I-dGuo) arising from bay-region diolepoxide activation of B[a]P and was consistent with the occurrence of tumors with mutations GGT-->TGT (56%), GGT-->GTT (25%), and GGT-->GAT (19%) in codon 12, all involving mutations of a guanine. B[j]A, a demethylated analogue of 3-methylcholanthrene (3-MCA) with an unsaturated cyclopenta ring, produced 16-to 60-fold more tumors at equivalent doses than did B[a]P; the mutations in tumors were GGT-->TGT (4%), GGT-->GTT (30%), and GGT-->CGT (65%). Analysis of adduction patterns in DNA suggested that B[j]A was activated to form DNA-binding derivatives in A/J mouse lungs primarily at the cyclopenta ring even though B[j]A contains a bay region. As reported in the published literature, the mutation spectrum induced by 3-MCA in Ki-ras codon 12 of mouse cells is similar to that of B[a]P but not to that of its close relative B[j]A. In contrast to B[j]A, 3-MCA is activated mostly via a bay-region diol-epoxide since its cyclopenta ring is saturated and not easily epoxidates. Therefore, we propose that the GGT-->CGT mutations produced by B[j]A in Ki-ras codon 12 were mostly the result of cyclopenta-ring-derived adducts.

Induction of anchorage-independent growth in human diploid fibroblasts by the cyclopenta-polycyclic aromatic hydrocarbon, benz[l]aceanthrylene

Cyclopenta-fused polycyclic aromatic hydrocarbons are a class of environmental PAH that have been recently identified. Many of these chemicals have been found to be more active than benzo[a]pyrene in tests for genetic toxicity using bacterial and rodent cells. Benz[l]aceanthrylene, a cyclopenta-polycyclic aromatic hydrocarbon related to benz[a]anthracene, and benzo[a]pyrene were compared for their activity to induce cytotoxicity and anchorage-independent growth with normal human diploid fibroblasts. Both benz[l]aceanthrylene and benzo[a]pyrene were relatively non-cytotoxic to normal human diploid fibroblasts. However, benz[l]aceanthrylene was twice as active compared to benzo[a]pyrene over the concentration range examined as an inducer of anchorage-independent growth. The ability of benz[l]aceanthrylene to induce anchorage-independent colony growth in normal human cells, in combination with its demonstrated ability as a mouse-skin tumorigen, suggests this PAH to be a potential multi-species carcinogen.

DNA adduct formation, metabolism, and morphological transforming activity of aceanthrylene in C3H10T1/2CL8 cells

Aceanthrylene (ACE), a cyclopenta-fused polycyclic aromatic hydrocarbon (CP-PAH) related to anthracene, has been studied for its ability to be metabolized, to form DNA adducts, and to morphologically transform C3H10T1/2CL8 mouse embryo fibroblasts in culture. Although ACE has been previously shown to be a strong mutagen in Salmonella typhimurium strains TA89 and TA100, it did not transform C3H10T1/2 cells (0.4-16 micrograms/ml) under 2 treatment protocols: treatment (for 24 h) 1 day after seeding the cells; treatment (for 24 h) 5 days after seeding the cells. Both protocols are effective in detecting the morphological transforming activity of PAH and CP-PAH and the latter protocol has been shown to be effective in detecting chemicals which are active in the first protocol only with the additional treatment of the cells with a tumor promoter. ACE is metabolized by C3H10T1/2 cells to ACE-1,2-dihydrodiol (the cyclopenta-ring dihydrodiol) at a rate of 450 pmoles ACE-1,2-dihydrodiol formed/h/10(6) cells. ACE-7,8-dihydrodiol and ACE-9,10-dihydrodiol, identified as major Aroclor-1254-induced rat liver microsomal metabolites from their UV, NMR, and mass spectral data, were not identified in incubations of C3H10T1/2 cells with ACE. ACE-DNA adducts in C3H10T1/2 cells were isolated, separated, identified, and quantitated using the 32P-postlabeling method. ACE forms 4 major adducts and each was identified as an ACE-1,2-oxide/2'-deoxyguanosine adduct. The level of adduction was 2.18 pmoles ACE adducts/mg DNA after a 24-h incubation of ACE (16 micrograms/ml) with C3H10T1/2 cells. ACE-DNA adduct persistence and repair were evaluated in C3H10T1/2 cells using a hydroxyurea block after ACE treatment. ACE-DNA adducts were not repaired under the conditions used in the morphological transformation studies. Thus, ACE provides an interesting example of a mutagenic PAH which is metabolized by C3H10T1/2 cells to active intermediates, forms relatively stable and persistent 2'-deoxyguanosine adducts in C3H10T1/2 cells, and yet induces no detectable morphological transforming activity under the experimental conditions used.

Metabolism of benz[j]aceanthrylene (cholanthrylene) and benz[l]aceanthrylene by induced rat liver S9

The metabolites of benz[j]aceanthrylene (B[j]A) produced by incubation with liver S9 proteins from rats induced with Aroclor-1254 and phenobarbital have been identified as: trans-B[j]A-1,2-dihydrodiol, B[j]A-9,10-dihydrodiol, B[j]A-11,12-dihydrodiol, and 10-hydroxy-B[j]A. The major metabolite formed (58-60%) by both induced S9 preparations was trans-B[j]A-1,2-dihydrodiol, the cyclopenta-ring dihydrodiol while oxidation at the k-region or the proximal-bay region was minor. There were no statistical differences in individual or total B[j]A metabolite rates between the 2 induced S9 preparations. B[l]A was metabolized by Aroclor-1254 and phenobarbital induced rat liver S9 preparations to trans-B[l]A-1,2-dihydrodiol, B[l]A-7,8-dihydrodiol, and B[l]A-4,5-dihydrodiol. The major B[l]A metabolite formed (28-40%) by both induced S9 preparations was B[l]A-7,8-dihydrodiol, the k-region dihydrodiol. Cyclopenta-ring oxidation to trans-B[l]A-1,2-dihydrodiol was approximately 50% of that observed for k-region oxidation. Both induced S9s produced similar rates of B[l]A metabolites except for B[l]A-7,8-dihydrodiol formation which was higher for Aroclor-1254-induced S9.