DNA Polymerase α and β Activities during the Cell Cycle and Their Role in Heat Radiosensitization in Chinese Hamster Ovary Cells

Abstract

The levels of DNA polymerase alpha and beta activities were measured during the cell cycle using a total cell homogenate technique. The results indicate a decrease in the levels of both enzyme activities during the G1 phase and a gradual increase as cells enter the S phase. The recovery of DNA polymerase activities was measured after heating for 10 min at 45.5 degrees C during the G1 phase (1.5 hr after mitotic release) or S phase (8-9 hr after mitotic release). This treatment reduced the levels of DNA polymerase beta activity to 20-30% and DNA polymerase alpha activity to 40-50% of their control activity for both G1 and S phase. The activity of DNA polymerase beta recovered fully during 20-25 hr after heating for both G1 phase or S phase cells. There was no recovery of the activity of DNA polymerase alpha during this time. Survival was measured when cells were irradiated (4 Gy) at various times after hyperthermia (10 min at 45.5 degrees C), and for both G1 and S phase the interaction between heat and X rays disappeared by 20-25 hr after heating and the same increase was observed for the recovery of DNA polymerase beta activity. Furthermore, treatment with cycloheximide inhibited protein synthesis and prevented recovery from heat damage assayed in terms of both cell survival and beta polymerase activity. These results, in addition to experiments with heat protection by glycerol or thermotolerance induced with sodium arsenite or fractionated heat doses, suggest that beta polymerase may be an important enzyme involved in repairing X-ray-induced damage that can result in cell lethality.