DEGRADATION OF NUCLEAR MATRIX AND DNA CLEAVAGE IN APOPTOTIC THYMOCYTES

In dexamethasone-treated thymocyte cultures an increase in nuclear pro teolytic activity paralleled chromatin fragmentation and the appearanc e of small apoptotic cells. The elevation of nuclear proteolytic activ ity was accompanied by site-specific degradation of nuclear mitotic ap paratus protein and lamin B, two essential components of the nuclear m atrix, Nuclear mitotic apparatus protein phosphorylation and cleavage into 200 and 48 kDa fragments occured within 30 minutes of dexamethaso ne treatment, Cleavage of lamin B, which generated a fragment of 46 kD a consistent with the central rod domain of the protein, was also dete cted after 30 minutes of exposure to the steroid hormone, The level of lamin B phosphorylation did not change as a result of the dexamethaso ne treatment and the lamina did not solubilize until the later stages of apoptosis. Initial DNA breaks, detected by the terminal transferase -mediated dUTP-biotin nick end labeling assay, occurred throughout the nuclei and solubilization of lamina was not required for this process to commence, The data presented in this paper support a model of apop totic nuclear destruction brought about by the site-specific proteolys is of key structural proteins, Both the nuclear mitotic apparatus prot ein and lamin B were specifically targeted by protease(s) at early sta ges of the cell death pathway, which possibly initiate the cascade of degradative events in apoptosis.