Hyperthermic modulation of SN-38-induced topoisomerase I DNA cross-linking and SN-38 cytotoxicity through altered topoisomerase I activity.

Abstract

The effect of different temperatures (37-42.5 degrees C) on SN-38 (the active metabolite of CPT-11) cytotoxicity was examined in the human lung carcinoma cell lines H460 and Calu-6 as well as the murine fibrosarcoma cell line L929. The cytotoxicity of SN-38, determined by MTT cell survival assay, was significantly increased in each cell line in combination with 41.8 degrees C hyperthermia (x60-120 min); the combination of SN-38 with 40.5 degrees C and 42.5 degrees C, however, was unchanged compared to 37 degrees C. Determination of topoisomerase (Topo) I DNA cross-linking in Calu-6 cells and L929 cells after treatment with SN-38 showed the same temperature profile as seen in the cell-survival assays with increased Topo I DNA cross-linking after treatment with the combination of SN-38 and 41.8 degrees C hyperthermia and unchanged Topo I DNA cross-linking at 40.5 degrees C and 42.5 degrees C. To test the hypothesis that increased Topo I DNA cross-linking and SN-38 cytotoxicity at 41.8 degrees C is caused by hyperthermia-modulated changes in Topo I activity, catalytic activity of Topo I extracted from each cell line and of purified human Topo I was determined at 20-42.5 degrees C. Topo I activity was found to be gradually increased with rising temperatures, resulting in significantly higher activity at 41.8 degrees C compared to 37 degrees C; further increase of temperature past 41.8 degrees C decreased Topo I activity back to levels found at 37 degrees C. Our data are used to explain a series of events resulting in hyperthermic enhancement of Topo I DNA cross-linking and SN-38 cytotoxicity in combination with 41.8 degrees C hyperthermia via increased Topo I activity.