Arsenite pretreatment attenuates benzo[a]pyrene cytotoxicity in a human lung adenocarcinoma cell line by decreasing cyclooxygenase-2 levels

Both simultaneous and sequential exposure to arsenite and benzo[a]pyrene (BaP) potentially occur in human populations drinking arsenic-contaminated water or burning arsenic-contaminated coal. Although arsenite and BaP are both well-documented hazardous substances and human carcinogens, interactions between these two agents have not been well defined. In this study, we demonstrated that posttreatment with arsenite synergistically enhanced the cytotoxicity of BaP for a human lung adenocarcinoma cell line, CL3. In contrast, pretreatment of CL3 cells with arsenite attenuated BaP cytotoxicity. Involvement of heat-shock protein 70 and heme oxygenase-1 in this arsenite-mediated attenuation of BaP cytotoxicity was ruled out. Our data also indicated that arsenite pretreatment did not affect the BaP-mediated induction of CYP1A1, the initial enzyme involved in its metabolic activation, but did result in a significant decrease in mRNA and protein levels of cyclooxygenase-2 (COX-2), which is required to convert the BaP metabolite BaP 7,8-dihydrodiol to the ultimate epoxide. In contrast to the high susceptibility of CL3 cells to BaP, the human lung carcinoma cells, H460, and CL3R15 cells (arsenic-resistant CL3 cells) showed normal CYP1A1 inducibility by BaP, had negligible amounts of COX-2, and were highly resistant to BaP. The involvement of COX-2 in BaP activation was confirmed by transfection of H460 cells with a recombinant adenovirus, Ad-pgk-Cox2, coding for COX-2, which resulted in a significant increase in the levels of the COX-2 product prostaglandin E-2 in the medium and in the susceptibility of H460 cells to BaP. The present study confirms the importance of COX-2 in BaP activation and demonstrates that the arsenite-mediated attenuation of BaP cytotoxicity is mediated by a reduction in COX-2 levels.