REDUCTION OF ETOPOSIDE INDUCED CELL-KILLING BY HYPERTHERMIA CAN OCCURWITHOUT CHANGES IN ETOPOSIDE TRANSPORT OR DNA TOPOISOMERASE-II ACTIVITY

We investigated the modification of etoposide (i.e. VP-16)-induced cel l killing by hyperthermia in a radioresistant human melanoma (Sk-Mel-3 ) and a human normal (AG1522) cell line. VP-16, a DNA topo II poison, was given as a Ih exposure at variable doses up to 35 mu M; hypertherm ia was given either before or following VP-16 treatment. Hyperthermic treatment comprised one of the following: 41 degrees C for 8 h, 42 deg rees C for 2 h or 45 degrees C for 15 min. Hyperthermia preceding VP-1 6 treatment reduced the cytotoxicity of the latter; the reduction of V P-16 cytotoxicity was directly proportional to the severity of the hyp erthermic treatment. For a particular combination of hyperthermic dose and VP-16 concentration, generally similar responses were seen in bot h cell lines. There were no effects on VP-16 cytotoxicity when both Sk -Mel-3 and AG1522 cells were heated at 41 degrees C for Sh following t reatment with VP-16. However, heating both cell lines at 45 degrees C for 15 min following VP-16 treatment again reduced the amount of cytot oxicity associated with VP-16. In addition, we found that a preceding exposure to 45 degrees C, 15 min heating did not affect either cellula r accumulation or efflux of [H-3]VP-16 in both cell lines. This sugges ted that the reduction in VP-16 cytotoxicity observed under those cond itions was not due to a modification of VP-16 transport. We found no d ifferences between the catalytic activities of topo II extracted from nuclei of Sk-Mel-3 and AG1522 cells that were either heated at 45 degr ees C for 15 min or that were not subjected to such treatment. These r esults therefore suggested that the substantial reduction of cytotoxic ity seen when 45 degrees C, 15 min heating preceded VP-16 treatment wa s also not due to an effect on topo II catalytic activity. Our results therefore demonstrate that hyperthermia, given either before or after VP-16, can actually reduce the amount of VP-16 cytotoxicity and that this can occur without any overt changes in VP-16 accumulation and eff lux or in topo II catalytic activity.