Degradation of lamin B₁ precedes oligonucleosomal DNA fragmentation in apoptotic thymocytes and isolated thymocyte nuclei

Chromatin condensation and nuclear envelope breakdown are characteristic features of apoptotic cell death, but the mechanisms underlying these phenomena have not been identified. Solubilization of nuclear lamin is responsible for both events in mitosis. In this work, we report that glucocorticoids stimulate rapid degradation of lamin B₁ that occurs before oligonucleosomal DNA fragmentation in apoptotic thymocytes. Protease inhibitors and the Ca²⁺ buffering agent BAPTA-AM block lamin degradation and DNA fragmentation, indicating that the processes are regulated by similar or identical mechanisms. Incubation of isolated thymocyte nuclei with Ca²⁺ stimulates lamin degradation before the detection of oligonucleosomal DNA fragments. However, in contrast to lamin dissolution during mitosis and some other forms of apoptosis, glucocorticoid-induced degradation of lamin B₁ in thymocytes is not accompanied by dephosphorylation-mediated activation of cdc2. Our results demonstrate that lamin degradation is an early feature of apoptosis in thymocytes and suggest that chromatin condensation and breakdown of the nuclear envelope may occur as a result of disruption of nuclear lamina architecture.