ACTIVATION AND POSSIBLE INVOLVEMENT OF CALPAIN, A CALCIUM-ACTIVATED CYSTEINE PROTEASE, IN DOWN-REGULATION OF APOPTOSIS OF HUMAN MONOBLAST U937 CELLS

An active form of calpain mu, a low-Ca2+-requiring intracellular cyste ine protease, was detected using a cleavage site-directed antibody in apoptotic human monoblast U937 cells treated with tumor necrosis facto r-alpha and interferon-gamma. Membrane-permeable calpain inhibitors ac celerated apoptosis of U937 cells thus induced and suppressed the acti vation of procalpain. These findings suggest that calpain down-regulat es apoptosis by shutting off the intracellular signals for cell death.