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

Abstract

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.

Cytotoxicity and mutagenicity of dibenzo[a,l]pyrene and (±)-dibenzo[a,l]pyrene-11,12-dihydrodiol in V79MZ cells co-expressing either hCYP1A1 or hCYP1B1 together with human glutathione-S-transferase A1

“…Human cytochrome P450-1A1 (hCYP1A1) and cytochrome P450-1B1 (hCYP1B1), and to a lesser extent other CYPs, are known to catalyze the stereospecific biotransformation of DBP to yield predominantly two of the four possible stereoisomers, (+)-syn-DBPDE (S,R,S,R) and (−)-anti-DBPDE (R,S,S,R) [15,16]. The (−)-anti-DBPDE stereoisomer was responsible for the majority of DBPDE-DNA adducts, and was produced as a greater proportion of the total metabolites by hCYP1B1 as compared to hCYP1A1 [17,18]. Interestingly, however, activation via hCYP1A1-mediated metabolism of DBP elicited stronger cytotoxic effects [19], possibly due to polar DNA adducts of metabolites produced only in cells expressing hCYP1A1.…”

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

“…DB[a,l]P is highly mutagenic in MCL-5 cells, a cell line derived from human B-lymphoblastoid cells that expresses several transfected cytochrome P450 genes and thus is capable of metabolizing PAH (15). Low doses of DB[a,l]P are mutagenic in the V79 Chinese hamster lung cell line cocultured with MCF-7 human mammary carcinoma cells to provide metabolic activation capacity (16). In cells that are able to metabolize PAH, DB[a,l]P is activated to electrophilic metabolites, which react with nucleophiles such as DNA (16).…”

Integrating Field Analyses with Laboratory Exposures to Assess Ecosystems Health

“…It was mutagenic in human B lymphoblastoid h1A1v2 cells (thymidine kinase locus) that express CYP1A1 (Durant et al, 1996) and in human B-lymphoblastoid MCL-5 cells that contain five human liver CYPs and microsomal epoxide hydrolase (Busby et al, 1995). Chinese hamster V79 cells (6-thioguanine resistance) co-cultivated with irradiated human mammary carcinoma MCF-7 cells were mutated by dibenzo [a,l]pyrene (Ralston et al, 1997). Dibenzo [a,l]pyrene induced morphological cell transformation in mouse embryo fibroblast C3H10T½Cl8 cells and was more active than benzo [a]pyrene .…”

“…(v) DNA damage and repair of dibenzo [a,l]pyrene In comparison with benzo [a]pyrene, dibenzo [a,l]pyrene induces 10-100 times more DNA adducts (Binkova et al, 2000;Melendez-Colon et al, 2000;Binkova & Sram, 2004), which are more resistant to nucleotide excision repair, probably because they distort DNA to a lesser degree (Dreij et al, 2005). Dibenzo [a,l]pyrene-11,12,-diol-13,14-oxide reacts preferably with deoxyadenosine while benzo [a]pyrene reacts more readily with deoxyguanosine (Ralston et al, 1997;Smith et al, 2001). These differences also apply more generally to fjord-region and bay-region diol epoxides (see references in Dreij et al, 2005).…”

“…Chinese hamster V79 cells (6-thioguanine resistance) co-cultivated with irradiated human mammary carcinoma MCF-7 cells were mutated by racemic dibenzo [a,l]pyrene-11,12-diol and (-)-dibenzo [a,l]pyrene-11R,12R-diol, but not by (+)-dibenzo [a,l]pyrene-11S,12S-diol (Ralston et al, 1997). Chinese hamster V79 cells (6-thioguanine resistance) in the presence of an Aroclor 1254-induced rat liver preparation were mutated by racemic dibenzo [a,l]pyrene-11,12-diol and each enantiomer.…”