EFFECT OF CELL-CYCLE STAGE, DOSE-RATE AND REPAIR OF SUBLETHAL DAMAGE ON RADIATION-INDUCED APOPTOSIS IN F9 TERATOCARCINOMA CELLS

There are at least two different modes of cell death after treatment w ith ionizing radiation. The first is a failure to undergo sustained ce ll division despite metabolic survival, and we refer to this end point as ''classical reproductive cell death.'' The second is a process tha t results in loss of cell integrity. This second category includes cel lular necrosis as well as apoptosis. Earlier studies in our laboratory showed that the predominant mechanism of cell death for irradiated F9 cells is apoptosis, and there is no indication that these cells die b y necrosis. We have therefore used cells of this cell line to reassess basic radiobiological principles with respect to apoptosis. Classical reproductive cell death was determined by staining colonies derived f rom irradiated cells and scoring colonies of less than 50 cells as rep roductively dead and colonies of more than 50 cells as survivors. Cell s that failed to produce either type of colony (detached from the plat e or disintegrated) were scored as having undergone apoptosis. Using t hese criteria we found that the fraction of the radiation-killed F9 ce lls that died by apoptosis did not vary when cells were irradiated at different stages of the cell cycle despite large variations in overall survival. This suggests that the factors that influence radiation sen sitivity throughout the cell cycle have an equal impact on apoptosis a nd classical reproductive cell death. There was no difference in cell survival between split doses and single doses of X rays, suggesting th at sublethal damage repair is not a factor in radiation-induced apopto sis of F9 cells. Apoptosis was not affected by changes in dose rate in the range of 0.038-4.96 Gy/min. (C) 1995 by Radiation Research Societ y