An endogenous calcium-dependent, caspase-independent intranuclear degradation pathway in thymocyte nuclei: Antagonism by physiological concentrations of K+ ions

Calcium ions have been implicated in apoptosis for many years, however the precise role of this ion in the cell death process remains incomplete. We have extensively examined the role of Ca2+ on nuclear degradation in vitro using highly purified nuclei isolated from non-apoptotic rat thymocytes. We show that these nuclei are devoid of CAD (caspase-activated DNase), and DNA degradation occurs independent of caspase activity. Serine proteases rather than caspase-3 appear necessary for this Ca2+-dependent DNA degradation in nuclei. We analyzed nuclei treated with various concentrations of Ca2+ in the presence of both a physiological (140 mM) and apoptotic (40 mM) concentration of KCl. Our results show that a 5-fold increase in Ca2+ is required to induce DNA degradation at the physiological KCl concentration compared to the lower, apoptotic concentration of the cation. Ca2+-induced internucleosomal DNA degradation was also accompanied by the release of histones, however the apoptotic-specific phosphorylation of histone H2B does not occur in these isolated nuclei. Interestingly, physiological concentrations of K+ inhibit both Ca2+-dependent DNA degradation and histone release suggesting that a reduction of intracellular K+ is necessary for this apoptosis-associated nuclear degradation in cells. Together, these data define an inherent caspase-independent catabolic pathway in thymocyte nuclei that is sensitive to physiological concentrations of intracellular cations.