ACTIVATION OF CPP32-LIKE PROTEASES IS NOT SUFFICIENT TO TRIGGER APOPTOSIS - INHIBITION OF APOPTOSIS BY AGENTS THAT SUPPRESS ACTIVATION OF AP24, BUT NOT CPP32-LIKE ACTIVITY

The 24-kD apoptotic protease (AP24) is a serine protease that is activ ated during apoptosis and has the capacity to activate internucleosoma l DNA fragmentation in isolated nuclei. This study examined the follow ing: (a) the functional relationship between AP24 and the CPP32-like p roteases of the caspase family; and (b) whether activation of CPP32-li ke proteases is sufficient to commit irreversibly a cell to apoptotic death. In three different leukemia cell lines, we showed that agents t hat directly (carbobenzoxy-Ala-Ala-borophe (DK120) or indirectly inhib it activation of AP24 (protein kinase inhibitors, basic fibroblast gro wth factor, tosylphenylalanine-chloromethylketone, and caspase inhibit ors) protected cells from apoptosis induced by TNF or UV light. Only t he caspase inhibitors, however, prevented activation of CPP32-like act ivity as revealed by cleavage of the synthetic substrate, DEVD-pNa, by cell cytosols, and also by in vivo cleavage of poly (ADP-ribosyl) pol ymerase, a known substrate of CPP32, Activation of DEVD-pNa cleaving a ctivity without apoptosis was also demonstrated in two variants derive d from the U937 monocytic leukemia in the absence of exogenous inhibit ors. Cell-permeable peptide inhibitors selective for CPP32-like protea ses suppressed AP24 activation and apoptotic death. These findings ind icate that CPP32-like activity is one of several upstream signals requ ired for AP24 activation. Furthermore, activation of CPP32-like protea ses alone is not sufficient to commit irreversibly a cell to apoptotic death under conditions where activation of AP24 is inhibited.