Enhanced bleomycin-induced DNA damage and cytotoxicity with calmodulin antagonists

A wide variety of structurally different calmodulin antagonists enhanced the cytotoxicity of bleomycin A2 to leukemic L1210 cells. This potentiation occurred with nontoxic concentrations of calmodulin antagonists. The most potent blockers of L1210 calmodulin activity, melittin and mastoparan, were the most potent potentiators of bleomycin A2 cytotoxicity. Less potent agents such as pimozide, a diphenylbutylpiperidine, trifluoperazine and chlorpromazine, phenothiazines, and W-7, a naphthalene sulfonamide, required higher concentrations for potentiation of bleomycin A2-induced cytotoxicity, while homologs that lack anticalmodulin activity failed to increase the cytotoxicity seen with bleomycin A2. The potentiation of bleomycin A2 cytotoxicity was not due to an elevated cellular content of bleomycin A2 or to inhibition of bleomycin A2 inactivation. Using alkaline elution techniques, we found that pimozide increased bleomycin A2-induced DNA damage in intact L1210 cells. Pimozide did not, however, directly increase the formation of reactive species by bleomycin as measured by single or double strand breakage of covalently closed circular DNA. Thus, the potentiation of bleomycin cytotoxicity by these agents appears to be mediated by an increased damage to cellular DNA; this may be due to inhibition of DNA repair. The hypothesized calmodulin-dependent mechanism was not shared by all agents that caused breaks in DNA because no potentiation in cytotoxicity was observed when calmodulin antagonists were combined with either etoposide or X-irradiation.