CONTRIBUTION OF O-6-METHYLGUANINE-DNA METHYLTRANSFERASE TO RESISTANCETO 1,3-(2-CHLOROETHYL)-1-NITROSOUREA IN HUMAN BRAIN TUMOR-DERIVED CELL-LINES

To assess the possible role of the DNA repair protein O-6-methylguanin e-DNA methyltransferase (MGMT) in resistance of brain neoplasms to the clinically important chloroethylating agent 1,3-bis(2-chloroethyl)-1- nitrosourea (BCNU), we quantitated MGMT activity, BCNU survival, and t he effect of ablating MGMT activity on the sensitivity of 14 human med ulloblastoma- and glioma-derived cell lines. BCNU resistance, measured as 10% survival dose (LD(10)), differed eightfold among the lines. El imination of measurable MGMT activity with the substrate analogue inhi bitor O-6-benzylguanine (O-6-BG) revealed a variable but limited contr ibution of MGMT to survival. In no case did O-6-BG reduce LD(10) by mo re than 3.4-fold. In contrast, O-6-BG reduced the LD(10) for N-methyl- N'-nitro-N-nitrosoguanidine up to 31-fold in the same cell lines (Bobo la MS, Blank A, Berger MS, Si(ber JR, Mol Carcinog 13:70-80, 1995). Va riability in BCNU survival, manifested as a sevenfold range of LD(10), persists after measurable MGMT was eliminated, indicating that anothe r mechanism or mechanisms is operating to limit cytotoxicity. Cells al kylated while suspended in growth medium are more resistant to BCNU an d display less dependence on MGMT than cells treated while proliferati ng on a plastic substratum. When alkylated in suspension, most of the lines are either unresponsive to O-6-BG or contain a subpopulation tha t did not respond to O-6-BG. Our results demonstrate that BCNU resista nce is multifactorial and that MGMT makes a modest contribution to res istance in our lines. (C) 1995 Wiley-Liss, Inc.