Effect of heat on dsb repair in G1- and S-phase studied in the human HeLa S3 cell line

PURPOSE: The aim of this study was to investigate the relation between double-strand breaks and thermal radiosensitization in dependence on cell-cycle position. MATERIALS AND METHODS: The experiments were performed with the human tumour cell line HeLa S3. Cells synchronized in G1- and S-phase were exposed to X-rays alone or in combination with prior heating at 44 degrees C for 20 min. Cell kill was determined by means of colony forming assay, double-strand breaks (dsb) using constant-field gel electrophoresis and apoptotic cell death was scored using the fraction of detached cells. RESULTS: In both cell-cycle phases heating at 44 degrees C for 20 min prior to irradiation resulted in an increased cellular radiosensitivity, whereby the thermal enhancement ratio (TER) was significantly higher in S- than in G1-phase cells with TER=2.1 and 1.2, respectively. Prior heating at 44 degrees C did not affect the number of radiation-induced dsb but was found to modify their repair as measured for a X-ray dose of 40 Gy. In both cell cycle phases dsb repair kinetics measured after irradiation alone could be described by a fast and a slow component with the majority of dsb being repaired with fast kinetics. Prior heating at 44 degrees C was found to have only a minor effect on these half-times but mainly to affect the number of slowly rejoined dsb. In G1-phase cells the number of slowly rejoined dsb measured 300 min after irradiation was enhanced by a factor of 1.8 and in S-phase cells even by a factor of 3.2. Fraction of apoptotically dying cells was low after X-irradiation alone but was clearly enhanced after combined treatment, which was especially pronounced for S-phase cells. CONCLUSIONS: The pronounced thermal radiosensitization found for S-phase cells was attributed to the heat-mediated increase in the number of slowly rejoined dsb and partly also to the enhanced fraction of apoptotically dying cells when compared to G1-phase cells.