Degradation of the nuclear matrix is a common element during radiation-induced apoptosis and necrosis

Human promyelocytic leukemia (HL60) cells were irradiated with 10 or 50 Gy of X rays and studied for up to 72 h postirradiation to determine the mode of death and assess changes in the nuclear matrix. After 50 Gy irradiation, cells were found to die early, primarily by apoptosis, while cells irradia ted with 10 Gy died predominantly by necrosis, Disassembly of the nuclear l amina and degradation of the nuclear matrix protein lamin B occurred in cel ls undergoing radiation-induced apoptosis or necrosis. However, using Weste rn blotting and a recently developed flow cytometry assay to detect changes in nuclear matrix protein content, we found that the kinetics and mechanis ms of disassembly of the nuclear lamina are different for each mode of cell death, During radiation-induced apoptosis, cleavage and degradation of lam in B to a similar to 28-kDa fragment was detected in most cells within 4-12 h after irradiation. Measurements of dual-labeled apoptotic cells revealed that nonrandom DNA fragmentation was evident prior to or concomitant with breakdown of the nuclear lamina, Disassembly of the nuclear lamina during r adiation-induced necrosis occurred much later (between 30-60 h after irradi ation), and a different cleavage pattern of lamin B was observed. Degradati on of the nuclear lamina was also inhibited in apoptosis-resistant BCL2-ove rexpressing HL60 cells exposed to 50 Gy until approximately 48 h after irra diation. These data indicate that breakdown of the nuclear matrix may be a common element in radiation-induced apoptosis and necrosis, but that the me chanisms and temporal patterns of breakdown of the nuclear lamina during ap optosis are distinct from those of necrosis. (C) 1999 by Radiation Research Society.