Calcium‐activated DNA fragmentation kills immature thymocytes

Glucocorticoid hormones kill immature thymocytes by activating a self‐destructive process that involves extensive DNA fragmentation. It has been demonstrated that thymocyte suicide is dependent on an early, sustained increase in cytosolic Ca 2+ concentration, and new protein synthesis, but the biochemical lesion that leads to cell death has not been established. To determine whether endonuclease activation or activation of another Ca 2+ ‐de‐pendent process could mediate cell killing, we treated thymocytes with the glucocorticoid methylprednisolone in the presence of inhibitors of various Ca 2+ ‐dependent degradative enzymes. The role of poly(ADP‐ribose) polymerase, an enzyme known to be activated by DNA damage, was also assessed. Glucocorticoid‐induced chromatin cleavage and cell killing were blocked by the endonuclease inhibitor aurintricarboxylic acid, whereas inhibitors of other Ca 2+ ‐dependent degradative processes or of poly(ADP‐ribose) polymerase did not abrogate cell death. In addition, stimulation of thymocyte DNA fragmentation by the Ca 2+ ionophore A23187 resulted in cell killing that could be blocked by the endonuclease inhibitor. Together, our results suggest that thymocyte suicide is caused by extensive Ca 2+ ‐stimulated DNA fragmentation.— M c C onkey , D. J.; H artzell , P.; N icotera , P.; O rrenius , S. Calcium‐activated DNA fragmentation kills immature thymocytes. FASEB J. 3: 1843‐1849; 1989.