Calpain and calpastatin regulate neutrophil apoptosis

The average polymorphonuclear neutrophil (PMN) lives only a day and then di es by apoptosis. We previously found that the calcium-dependent protease ca lpain is required for apoptosis in several mouse models of cell death. Here we identify calpain, and its endogenous inhibitor calpastatin, as regulato rs of human neutrophil apoptosis. Cell death triggered by the translation i nhibitor cycloheximide is calpain-dependent, as evidenced using either a ca lpain active site inhibitor (N-acetyl-leucyl-leucyl-norleucinal) or agents that target calpain's calcium binding sites (PD150606, PD151746). No signif icant effect on cycloheximide-triggered apoptosis was found by using inhibi tors of the proteasome or of other papain-like cysteine proteases, providin g further evidence that the active site calpain inhibitor prevents apoptosi s via its action on calpain. In addition, we find that potentiation of calp ain activity by depleting its endogenous inhibitor, calpastatin, is suffici ent to cause apoptosis of neutrophils. Nevertheless, apoptosis signalled vi a the Fas antigen proceeds regardless of the presence of calpain inhibitor. These experiments support a growing body of work, indicating an upstream r egulatory role for calpain in many, but not all, forms of apoptotic cell de ath. They also identify calpastatin as a participant in apoptotic cell deat h and suggest that for at least one cell type, a decrease in calpastatin is a sufficient stimulus to initiate calpain-dependent apoptosis. J. Cell. Ph ysiol. 178:311-319, 1999. (C) 1999 Wiley-Liss, Inc.