Differential protection by human glutathione S-transferase P1 against cytotoxicity of benzo[a]pyrene, dibenzo[a,l]pyrene, or their dihydrodiol metabolites, in bi-transgenic cell lines that co-express rat versus human cytochrome P4501A1

Abstract

Polycyclic aromatic hydrocarbons (PAHs) are activated by cytochrome P450 (CYP) isozymes, and a subset of the reactive metabolites generated is detoxified via conjugation with glutathione (GSH) by specific glutathione S-transferases (GSTs). We have used V79MZ cells stably transfected with either human or rat cytochrome P4501A1 (CYP1A1), alone or in combination with human GSTP1 (hGSTP1), to examine the dynamics of activation versus detoxification of benzo[a]pyrene (B[a]P), dibenzo[a,l]pyrene (DB[a,l]P), and their dihydrodiol metabolites. The cytotoxicity of B[a]P or DB[a,l]P was 9–11-fold greater in cells expressing human, as compared to rat CYP1A1, despite similar enzymatic activities. Co-expression of the hGSTP1 with the hCYP1A1 conferred 16-fold resistance to B[a]P cytotoxicity, compared to only 2.5-fold resistance when hGSTP1 was co-expressed with rat CYP1A1. The lower B[a]P cytotoxicity in the cells expressing rat CYP1A1, and weaker protection by hGSTP1 co-expression in these cells, were attributable to the much lower fraction of B[a]P metabolism via formation of the 7,8-dihydrodiol intermediate by the rat CYP1A1 compared to hCYP1A1. Resistance to the DB[a,l]P cytotoxicity conferred by hGSTP1 expression was also greater in cells co-expressing hCYP1A1 (7-fold) as compared to cells co-expressing rCYP1A1 (<2-fold). Resistance to B[a]P conferred by hGSTP1 was closely correlated with the activity level in two clonal transfectant lines with a 3-fold difference in hGSTP1-1 specific activity. Depletion of GSH to 20% of control levels via pretreatment with the de novo GSH biosynthesis inhibitor buthionine sulfoximine reduced the protection against B[a]P cytotoxicity by hGSTP1 from 16-fold to 5-fold, indicating that catalysis of conjugation with GSH, rather than binding or other effects, is responsible for the resistance. The cytotoxicity of the dihydrodiol intermediates of B[a]P or DB[a,l]P was much greater, and similar in cell lines expressing either human or rat CYP1A1. Again, however, the protection conferred by hGSTP1 co-expression was 2–5-fold greater in cells with hCYP1A1 than with rCYP1A1 expression. These results indicate that GST expression can effectively limit cytotoxicity following activation of B[a]P by human or rat CYP1A1, but is less effective as a defense against exposure of cells to the intermediate metabolite B[a]P-7,8-dihydrodiol.