X-ray-induced cell death: apoptosis and necrosis.

X-ray-induced cell death in MOLT-4N1, a subclone of MOLT-4 cells, and M10 cells was studied with respect to their modes of cell death, apoptosis and necrosis. MOLT-4N1 cells showed radiosensitivity similar to that of M10 cells, a radiosensitive mutant of L5178Y, as determined by the colony formation assay. Analysis of cell size demonstrated that MOLT-4N1 cells increased in size at an early stage after irradiation and then decreased to a size smaller than that of control cells, whereas the size of irradiated M10 cells increased continuously. Apoptosis detected by morphological changes and DNA ladder formation (the cleavage of DNA into oligonucleosomal fragments) occurred in X-irradiated MOLT-4N1 cells but not in M10 cells. Pulsed-field gel electrophoresis showed that the ladder formation involved an intermediate-sized DNA (about 20 kbp). Most of the DNA was detected at the origin in both methods of electrophoresis in the case of M10 cells, though a trace amount of ladder formation was observed. Heat treatment of M10 cells induced apoptosis within 30 min after treatment, in contrast to MOLT-4N1 cells. The results suggest that apoptosis and necrosis are induced by X rays in a manner which is dependent on the cell line irrespective of the capability of the cells to develop apoptosis. DNA fragmentation was the earliest change observed in the development of apoptosis.