Apoptosis induced by X-irradiation of rec-myc cells is postmitotic and not predicted by the time after irradiation or behavior of sister cells.

Rat embryo cells expressing the c-myc oncogene (rec-myc) were studied by time-lapse microscopy to determine whether radiation-induced apoptosis occurred before or after mitosis. Following X-irradiation with 9.5 Gy, cells were imaged every 3 min for 6 days. Episodes of apoptotic blebbing were very consistent from cell to cell, lasting 30-60 min, followed by cessation of movement and cell death. In contrast, the time of initiation of apoptotic blebbing was unpredictable. At least 96% of the apoptotic episodes were postmitotic, after one to four cell divisions and 2-97 h after a given division. Sister cells often behaved differently from one another, with apoptosis in one sister occurring many h or several divisions after apoptosis in the other. Thus, the onset of radiation-induced apoptosis in rec-myc cells is not strictly programmed but may result from the segregation of chromosome aberrations in the postirradiation generations.