Effect of heat on dsb repair in G1- and S-phase studied in the human HeLa S3 cell line.
OBJECTIVE
The aim of this study was to investigate the relation between double-strand breaks and thermal radiosensitization in dependence on cell-cycle position.


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.
Dikomey
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Institute of Biophysics and Radiobiology, University of Hamburg, Germany. dikomey@uke.uni-hamburg.de
Duen
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D017209:Apoptosis; D004249:DNA Damage; D004260:DNA Repair; D016193:G1 Phase; D006367:HeLa Cells; D006358:Hot Temperature; D006801:Humans; D011836:Radiation Tolerance; D016196:S Phase