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)-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 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 GST mu 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.
Gerson
S L
SL
Department of Medicine, University Hospitals of Cleveland, OH.
Berger
S J
SJ
Varnes
M E
ME
Donovan
C
C
D001943:Breast Neoplasms / Q000378:metabolism*; D002330:Carmustine / Q000494:pharmacology; D002460:Cell Line; D002470:Cell Survival; D004351:Drug Resistance; D005038:Ethylnitrosourea / Q000031:analogs & derivatives* / Q000494:pharmacology; D005978:Glutathione / Q000037:antagonists & inhibitors*; D005982:Glutathione Transferase / Q000032:analysis; D006147:Guanine / Q000031:analogs & derivatives / Q000494:pharmacology; D006801:Humans; D008780:Methyltransferases / Q000037:antagonists & inhibitors*; D019853:O(6)-Methylguanine-DNA Methyltransferase
C064976:O(6)-benzylguanine; C017139:1-(2-chloroethyl)-1-nitrosourea; D006147:Guanine; C008449:O-(6)-methylguanine; D008780:Methyltransferases; D019853:O(6)-Methylguanine-DNA Methyltransferase; D005982:Glutathione Transferase; D005978:Glutathione; D005038:Ethylnitrosourea; D002330:Carmustine