The mechanism of metabolic activation of the very potent carcinogen dibenzo(a,l)pyrene

Sherry Lynn Ralston, Purdue University

Abstract

Dibenzo (a,l) pyrene (DB (a,l) P) is an environmental polycyclic aromatic hydrocarbon (PAH) and the most potent carcinogen tested in rodent bioassays. As with other PAHs, DB (a,l) P requires metabolic activation in order to exert its biological activity. The reactive metabolites of PAHs are capable of covalently binding with DNA, which is an important event in the initiation of carcinogenesis. To determine whether DB (a,l) P binds to DNA and could play a role in the induction of human cancers, the pathway of metabolic activation of DB (a,l) P was determined by examining the stable DNA adducts formed in cell culture systems and mouse epidermis. This was accomplished by treating various cells and mouse skin with DB (a,l) P and examining the DNA adducts by $\sp{35}$S- and $\sp{33}$P-postlabeling techniques, immobilized boronate chromatography (IBC), thin-layer chromatography (TLC) and high-performance liquid chromatography (HPLC). The human mammary carcinoma cell line (MCF-7), SENCAR mouse embryo (SME) cells and mouse keratinocyte (MK) cells formed three major and three minor DB (a,l) P-DNA adducts that were detectable and resolvable by HPLC. In hamster embryo (HE) and mouse epidermis, DB (a,l) P formed one major and three minor adducts. It was determined that the reactive metabolite that formed in these cells and that bound to the DNA was the fjord-region DB (a,l) P-11,12-diol-13,14-epoxide (DB (a,l) PDE). DB (a,l) P was stereoselectively metabolized in cells to the ($-$)-anti(11R,12S,13S,14R)-DB (a,l) PDE and (+)-syn(11S,12R,13S,14R)-DB (a,l) PDE which reacted extensively with deoxyadenosine in DNA. The formation of high proportions of deoxyadenosine adducts may be related to the high carcinogenic activity of DB (a,l) P. The presence or absence and the proportion of specific DB (a,l) P-DNA adducts in HE and MCF-7 cells was dependent upon both dose and length of time of exposure. Studies of the mutagenic potency of DB (a,l) P and DB (a,l) P-11,12-diols in the MCF-7-mediated V79 mutation assay demonstrated that DB (a,l) P and ($-$)-DB (a,l) P-11,12-diol are highly mutagenic: the potency of ($-$)-DB (a,l) P-11,12-diol exceeds that of DB (a,l) P. The (+)-DB (a,l) P-11,12-diol showed no mutagenic activity in the assay. Analysis of the DB (a,l) P-DNA adducts demonstrated high stereoselectivity for activation of the ($-$)-DB (a,l) P-11,12-diol. These results demonstrate that DB (a,l) P can be activated to ultimate carcinogenic ($-$)-anti- and (+)-syn-DB (a,l) PDE in cells derived from humans as well as rodents and suggest that exposure to DB (a,l) P could pose a carcinogenic risk for humans.

Degree

Ph.D.

Advisors

Baird, Purdue University.

Subject Area

Biochemistry|Pharmacology|Toxicology

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