Cell cycle dependent apoptosis and cell cycle blocks induced by hyperthermia in HL-60 cells.

The effects of heat are strongly dependent on the time of heating at a given temperature. The relationship between treatment time and temperature for a biological isoeffect (the Arrhenius plot) has been confirmed for a variety of normal tissues and tumours. A marked change of slope occurs somewhere between 42-43 degrees C. Above this transition temperature the slope is constant for a variety of cells and tissues. Therefore, when defining thermal doses in hyperthermia studies, both the time and temperature of heating are equally important determinants. In this study, cell cycle progression and apoptosis were analysed in HL-60 cells after heating from 5-60 min at 45.0 degrees C and also heating with five different iso-dose time-temperature heat treatments. A heat shock of 5-15 min at 45.0 degrees C caused the accumulation of cells in G1 and G2/M phases after 12 h at 37 degrees C, whereas a heat shock of 20-60 min at 45.0 degrees C reduced the number of non-apoptotic cells in all phases because the number of apoptotic cells increased. The fraction of apoptotic cells followed a sigmoid curve as the heating time increased from 5-60 min at 45.0 degrees C. Cell cycle analysis showed that apoptosis occurred predominantly in S-phase cells for shorter heating times but in all phases at longer times. An isodose heat shock lower than 44.0 degrees C (42.0-43.0 degrees C) gave the same apoptotic index, while heat shock from 44.0-46.0 degrees C caused a greater than expected apoptotic index. Thus, there was a transition at 44.0 degrees C in HL-60 cells, above which apoptosis increased rapidly. These results indicate that isodose analysis based on clonogenic survival in fibroblast cells may not be relevant for cell types which readily undergo apoptosis. Clonogenic survival was also compared with apoptosis for HL-60 cells and an apoptotic-resistant derivative cell line, HWC-2, heated for various times at 45.0 degrees C. Survival based on a clonogenic assay was much lower than survival based only on apoptotic index at all times for HL-60 cells. HWC-2 cells did not undergo apoptosis and also had a higher clonogenic survival than HL-60 cells.