Combined depletion of O6-alkylguanine-DNA alkyltransferase and glutathione to modulate nitrosourea resistance in breast cancer

MCF-7 human breast cancer cells possess high levels of O6-alkylguanine-DNA alkyltransferase and moderate levels of glutathione, and are more resistant to chloroethylnitrosoureas (CNUs) than cells with low levels of either molecule. The role of each as a component of CNU resistance was assessed using O6-benzylguanine (O6-bG) or O6-methylguanine (O6-mG) to deplete the alkyltransferase and l -buthionine sulfoxamine ( l -BSO) to deplete glutathione. O6-bG and O6-mG potentiated 1,3-bis(2-chloroethyl)-l-nitrosourea (BCNU) cytotoxicity, resulting in a dose modification factor of 5.4 and 2.3, respectively, which reflected the more potent inhibitory effect of O6-bG. l -BSO alone had little effect on the survival of MCF-7 cells following BCNU exposure, but when combined with O6-mG, BCNU cytotoxicity was additive, yielding a dose modification factor of 3.2. O6-bG or O6-mG and l -BSO acted independently, as neither class of inhibitor affected the other's mechanism of CNU resistance. Furthermore, MCF-7 cells overexpressing GSTμ were not more resistant to BCNU than the parent cell line in either the presence or absence of O6-bG or l -BSO. These results indicate that on a relative basis in MCF-7 cells, the alkyltransferase is the cell's first line of defense against CNUs. This suggests that therapeutic trials based on O6-bG-induced biochemical modulation of CNU resistance may increase the efficacy of these chemotherapeutic agents against human malignant cells and that l -BSO may have little additive effect when used with these agents.