The effect of a complex mixture of polycyclic aromatic hydrocarbons on the metabolic activation, PAH-DNA binding and tumor initiation of benzo(a)pyrene and dibenzo(a,l)pyrene

Charis Park Marston, Purdue University

Abstract

Environmental contaminants such as coal tar, food pyrolysis products, and cigarette smoke contain complex mixtures of polycyclic aromatic hydrocarbons (PAH). Much is known about the bioactivation mechanisms and structural properties of individual carcinogenic PAH. However, simply knowing the total levels of carcinogenic PAH in environmental mixtures may not allow accurate human risk assessment, since weakly or non-carcinogenic hydrocarbons present in complex mixtures can modify the carcinogenic activity of PAH. Assessing carcinogenic PAH activation within complex mixtures may allow us to predict the ability of that mixture to increase or decrease activation of the carcinogenic PAH present and ultimately to assess the relative carcinogenic risk that the mixture poses to humans. The effect of a complex mixture of PAH isolated from coal tar, Standard Reference Material 1597 (SRM), on the metabolic activation, PAH-DNA binding, and tumorigenicity of benzo[a]pyrene (B[ a]P) and dibenzo[a,l]pyrene (DB[ a,l]P) were examined in mouse epidermis and cell culture. The SRM did not have an effect on tumor initiation by B[ a]P in mouse epidermis. DB[a,l]P tumor initiation, however, was significantly decreased by the SRM. PAH-DNA adduct analysis indicated that the SRM did not effect the B[a]P-DNA adduct levels but decreased the DB [a,l]P-DNA adduct levels found in mouse skin after topical treatment. These results are not necessarily due to the induction of different PAH-metabolizing enzymes, since immunoblot analysis of microsomal proteins from the epidermis demonstrated that both cytochrome P450 (CYP) 1A1 and 1B1 isozymes, were induced by the SRM. The results of experiments performed with human mammary carcinoma derived MCF-7 cells and Chinese hamster V79 cells expressing human CYP1A1 or CYP1B1 suggested that B[a]P and DB[a,l ]P metabolism to PAH-DNA adduct formation were both inhibited by the SRM. Although the SRM induced both human CYP1A1 and CYP1B1 in MCF-7 cells, PAH-DNA adduct studies utilizing the V79 cells found that only the CYP1A1-expressing cells actively metabolized the SRM. Enzyme activity assays found the SRM to competitively inhibit human CYP1A1 and CYP1B1 activity. These results suggest that the carcinogenic nature of complex mixtures of PAH is determined by the ability of individual components to competitively inhibit the metabolic activation of certain carcinogenic PAH.

Degree

Ph.D.

Advisors

Baird, Purdue University.

Subject Area

Biochemistry|Oncology|Toxicology

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