Effect of heat on dsb repair in G1-and S-phase studied in the human HeLaS3cell line

Purpose: The aim of this study was to investigate the relation between doub le-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-held gel electrophoresis and apoptotic cell dea th 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 d egrees C did not affect the number of radiation-induced dsb but was found t o modify their repair as measured for a X-ray dose of 40 Gy. In both cell c ycle phases dsb repair kinetics measured after irradiation alone could be d escribed by a fast and a slow component with the majority of dsb being repa ired with fast kinetics. Prior heating at 44 degrees C was found to have on ly a minor effect on these half-times but mainly to affect the number of sl owly rejoined dsb. In GI-phase cells the number of slowly rejoined dsb meas ured 300 min after irradiation was enhanced by a factor of 1.8 and in S-pha se 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 trea tment, which was especially pronounced for S-phase cells. Conclusions: The pronounced thermal radiosensitization found for S-phase ce lls was attributed to the heat-mediated increase in the number of slowly re joined dsb and partly also to the enhanced fraction of apoptotically dying cells when compared to GI-phase cells.