RADIATION-INDUCED APOPTOSIS AND NECROSIS IN MOLT-4 CELLS - A STUDY OF DOSE-EFFECT RELATIONSHIPS AND THEIR MODIFICATION

Cell death induced by various radiation doses in Molt-4 and L5178Y cells was studied. Molt-4 cells have a low threshold for induction of interphase death while L5178Y cells have a high threshold. Interphase death in Molt-4 cells at doses > 100 Gy and in L5178Y cells at doses > 300 Gy showed typical necrosis. In contrast, cell death in Molt-4 cells after 2-30 Gy had features of both apoptosis and necrosis; the appearance of orcein-stained cells in the light microscope was suggestive of apoptosis, showing chromatin margination and apoptotic bodies. The concomitant appearance of non-viable cells as estimated by either orcein staining or eosin dye staining correlated well with this. Early cell swelling and a diffuse smear pattern in DNA gel electrophoresis were however suggestive of necrosis. Detailed electron microscopic observations revealed that events leading to the interphase death of Molt-4 cells after 2-30 Gy involved complicated pathways which included both apoptotic changes in the nucleus and necrotic changes in the cytoplasm. TPA not only had a protective effect on the interphase death of Molt-4 cells, but also converted the mode of cell death from interphase death to mitotic death after low doses of radiation. Cycloheximide and H-7, known to inhibit apoptosis in irradiated thymocytes, had no protective effect on the interphase death of Molt-4 cells, suggesting that the events leading to interphase death in Molt-4 cells might be different from those in thymocytes. BrdUrd-labelling had a sensitizing effect on the radiation response, but no effect on interphase death in Molt-4 cells thus suggesting that DNA damage may not be responsible for interphase death.