Possible factors in the potentiation of 1-(2-chloroethy)-3-trans-4-methylcyclohexyl-1-nitrosourea cytotoxicity by α-difluoromethylornithine in 9L rat brain tumor cells

Depletion of intracellular levels of polyamines in 9L rat brain tumor cells by α -difluoromethylornithine (DFMO), an enzyme-activated irreversible inhibitor of ornithine decarboxylase, significantly enhanced the cytotoxicity of 1-(2-chloroethyl)-3-trans-4-methylcyclohexyl-1-nitrosourea (MeCCNU) in vitro as measured by a colony-forming efficiency assay. Administered as a single agent, DFMO was not cytotoxic to 9L cells. Treatment for 48 hr with 10, 1, 0.5 or 0.1 mM DFMO produced similar levels of polyamine depletion and similar potentiation of MeCCNU cytotoxicity. Restoration of intracellular polyamine levels by the addition of exogenous putrescine (1 mM) to treated cells prevented the potentiation of MeCCNU, which indicates that this phenomenon might be the result of polyamine depletion. DNA adduct formation in polyamine-depleted and control cell was studied with [14C]-MeCCNU; no difference in monoadduct formation was found between polyamine-depleted and control cells. Experiments to determine whether polyamine depletion has an effect on enzymes involved in the repair of alkylated bases showed that the activity of O⁶-methylguanine-DNA demethylase, 7-methylguanine-DNA glycosylase and 3-methyladenine-DNA glycosylace were unaffected by 48 hr of treatment with 10 mM DFMO. DFMO treatment causes a substantial increase in the intracellular content of decarboxylated S-adenosyl-l-methionine, which was reversed by addition of putrescine. The possibility that the elevation of decarboxylated S-adenosyl-l-methionine rather than the depletion of polyamines is responsible for the effects of DFMO is discussed.