Expression of hGSTA1 in stably transfected V79MZ cells co-expressing hCYP1B1 or hCYP1A1 confers variable resistance to cytotoxicity of dibenzo[a,l]pyrene (DB[a,l]P) or its dihydrodiol metabolite Free

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Cytochrome p450s (CYP) are phase I enzymes that catalyze activation of a wide range of carcinogens to more reactive electrophilic metabolites. Glutathione-S-transferases (GSTs) are phase II enzymes that can conjugate glutathione (GSH) to intermediate metabolites, thus detoxifying these reactive compounds. Either human CYP1A1 or CYP1B1 can activate DB[a,l]P to dibenzo[a,l]pyrene-11,12-dihydrodiol-13,14-epoxide (DB[a,l]PDE); however, DB[a,l]P has previously shown higher cytotoxicity in cells expressing human P4501A1. Further, human GSTA1-1 is the most effective of the GST isoforms in conjugating (-)-anti-DB[a,l]PDE with GSH. We developed V79MZ hamster lung fibroblasts stably transfected with either human P4501B1 (V79MZh1B1), or human P4501A1 (V79MZh1A1), alone or in combination with human GST alpha-1 (hGSTA1-1) and used them to examine GST protection against DB[a,l]P cytotoxicity.As expected, DB[a,l]P had higher cytotoxicity in V79MZh1A1 (IC₅₀ = 2.7 nM, compared to 6.0 nM in V79MZh1B1). Expression of hGSTA1-1 conferred 2-fold resistance to cytotoxicity in the hCYP1A1-expressing cells, relative to control V79MZ cells expressing hCYP1A1 alone. In V79MZh1B1 cells (expressing hCYP1B1), added expression of hGSTA1-1 conferred a 5-fold protection relative to cells expressing hCYP1B1 alone. When cells were exposed to (+/−)DB[a,l]P-11, 12-dihydrodiol, again V79MZh1A1 was more sensitive to cytotoxicity than V79MZh1B1 (IC₅₀ = 0.7 nM compared to 4.8 nM respectively). The expression of hGSTA1-1 conferred a slight (1.3-fold) protection in the cells expressing human P4501A1, but in the V79MZ cells expressing hCYP1B1, the GSTA1-1 conferred a robust 9-fold protection (both relative to the respective controls expressing hCYP1A1 or hCYP1B1). These findings demonstrate that: a) the relative protection by hGSTA1 against cellular toxicity of DB[a,l]P is much more effective when it is activated by hCYP1B1 than by hCYP1A1; and b) in cells expressing hCYP1B1, the fold-resistance conferred by hGSTA1 expression against the toxicity of DB[a,l]P-diol is twice that conferred against the parent compound DB[a,lP. In view of the known variations in expression of these genes among human tissues, these findings have implications for differential tissue-specific protection by GSTA1 against DB[a,l]P and possibly other polycyclic aromatic hydrocarbon carcinogens a