HYPERTHERMIC MODULATION OF SN-38-INDUCED TOPOISOMERASE-I DNA CROSS-LINKING AND SN-38 CYTOTOXICITY THROUGH ALTERED TOPOISOMERASE-I ACTIVITY

The effect of different temperatures (37-42.5 degrees C) on SN-38 (the active metabolite of CPT-I I) cytotoxicity was examined in the human lung carcinoma cell lines H460 and Calu-6 as well as the murine fibros arcoma cell line L929. The cytotoxicity of SN-38, determined by MTT ce ll survival assay, was significantly increased in each cell line in co mbination with 41.8 degrees C hyperthermia (x60-120 min); the combinat ion of SN-38 with 40.5 degrees C and 42.5 degrees C, however, was unch anged compared to 37 degrees C, Determination of topoisomerase (Topo) I DNA cross-linking in Calu-6 cells and L929 cells after treatment wit h SN-38 showed the same temperature profile as seen in the cell-surviv al assays with increased Topo I DNA cross-linking after treatment with the combination of SN-38 and 41.8 degrees C hyperthermia and unchange d Topo I DNA cross-linking at 40.5 degrees C and 42.5 degrees C, To te st the hypothesis that increased Topo I DNA cross-linking and SN-38 cy totoxicity at 41.8 degrees C is caused by hyperthermia-modulated chang es in Topo I activity, catalytic activity of Topo I extracted from eac h cell line and of purified human Topo I was determined at 20-42.5 deg rees C, Topo I activity was found to be gradually increased with risin g temperatures, resulting in significantly higher activity at 41.8 deg rees 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 de grees C. Our data are used to explain a series of events resulting in hyperthermic enhancement of Topo I DNA cross-linking and SN-38 cytotox icity in combination with 41.8 degrees C hyperthermia via increased To po I activity. (C) 1999 Wiley-Liss, Inc.