COMBINED DEPLETION OF O-6-ALKYLGUANINE-DNA ALKYLTRANSFERASE AND GLUTATHIONE TO MODULATE NITROSOUREA RESISTANCE IN BREAST-CANCER

MCF-7 human breast cancer cells possess high levels of O-6-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 O-6-benzylguanine (O-6-bG) or O-6-methylguanine (O-6-mG) to deplete the alkyltransferase and L-buthionine sulfoxamine (L-BSO) to deplete glutathione. O-6-bG and O-6-mG potentiated 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) cytotoxicity, resulting in a dose modification factor of 5.4 and 2.3, respectively, which reflected the more potent inhibitory effect of O-6-bG. L-BSO alone had little effect on the survival of MCF-7 cells following BCNU exposure, but when combined with O-6-mG, BCNU cytotoxicity was additive, yielding a dose modification factor of 3.2. O-6-bG or O-6-mG and L-BSO acted independently, as neither class of inhibitor affected the other's mechanism of CNU resistance. Furthermore, MCF-7 cells overexpressing GST mu were not more resistant to BCNU than the parent cell line in either the presence or absence of O-6-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 O-6-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.