Inducible nitric oxide synthase neutralizes carbamoylating potential of 1,3-bis(2-chloroethyl)-1-nitrosourea in C6 glioma cells

Expression of iNOS in glioma and other tumors has been extensively document ed but the effects of NO derived from iNOS on tumor-killing mechanisms of c hemotherapy drugs remain to be fully defined. We note that increased NO syn thesis by cytokine exposure or iNOS overexpression neutralized the cytotoxi city of 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) and 1-(2-chloroethyl)-3 -cyclohexyl-1-nitrosourea (CCNU), but not cisplatin, in rat C6 glioma cells . Suppression of BCNU cytotoxicity associated with iNOS overexpression coul d be abolished by pharmacological inhibition of NOS or coexpression of an a ntisense RNA against iNOS. Both BCNU and CCNU are chloroethylnitrosoureas t hat kill tumor cells via carbamoylating and alkylating actions. Further stu dies using compounds that each carry these different activities indicate th at iNOS neutralized carbamoylating, but not alkylating, action of chloroeth ylnitrosoureas. Temozolomide, a novel chemotherapy drug recently available for treating brain tumors, carries only alkylating, but not carbamoylating, action. Overexpression of iNOS in C6 cells failed to neutralize temozolomi de cytotoxicity. Results from the present study demonstrate the ability of iNOS-derived NO to confer chemoresistance against the carbamoylating potent ial of chloroethylnitrosoureas in vitro. Further investigation is needed to test whether iNOS expression, frequently noted in malignant brain tumors, also enhances chemoresistance against chloroethylnitrosoureas in vivo.