Interaction of bleomycin, hyperthermia and a calmodulin inhibitor (trifluoperazine) in mouse tumour cells: II. DNA damage, repair and chromatin changes.

We have reported in the preceding paper that the treatment of plateau phase mouse EMT6 tumour cells with a combination of hyperthermia (HT; 44 degrees C) and trifluoperazine (TFP; 30 micrograms ml-1; an inhibitor of calmodulin) greatly enhances the cytotoxicity of the antitumour drug belomycin (BLM). The cytotoxic action of BLM is thought to arise from the induction of DNA damage in a manner which reflects chromatin accessibility. Thus we have studied the effects of the two modifiers (HT and TFP) on chromatin structure and BLM-induced DNA damage. Co-treatment of cells with HT and TFP altered chromatin organisation by the formation and slow resolution of new DNA attachment sites at the nuclear matrix. HT increased drug-induced DNA damage (strand breaks and alkali-labile lesions) by the general depression of repair rather than through the generation of new sites for drug action. TFP produced a more discrete block in the repair of alkali-labile lesions in DNA. Both processes appear to occur for the combination of BLM, HT and TFP, and we propose that the novel chromatin configuration permits the accumulation of potentially lethal DNA strand breaks. Our study indicates the potential value of chromatin/DNA repair modifying regimens for overcoming the poor responsiveness of some tumour cells to chemotherapeutic drugs and provides a rational basis for the use of calmodulin inhibitors in thermochemotherapy.