The 24-kD
apoptotic protease (AP24) is a
serine protease that is activated during apoptosis and has the capacity to activate internucleosomal DNA fragmentation in isolated nuclei. This study examined the following: (a) the functional relationship between AP24 and the CPP32-like
proteases of the
caspase family; and (b) whether activation of CPP32-like
proteases is sufficient to commit irreversibly a cell to apoptotic death. In three different
leukemia cell lines, we showed that agents that directly (carbobenzoxy-
Ala-Ala-borophe (
DK120) or indirectly inhibit activation of AP24 (
protein kinase inhibitors,
basic fibroblast growth factor, tosylphenylalaninechloromethylketone, and
caspase inhibitors) protected cells from apoptosis induced by TNF or UV light. Only the
caspase inhibitors, however, prevented activation of CPP32-like activity as revealed by cleavage of the synthetic substrate,
DEVD-pNa, by cell cytosols, and also by in vivo cleavage of poly (
ADP-ribosyl) polymerase, a known substrate of CPP32. Activation of
DEVD-pNa cleaving activity without apoptosis was also demonstrated in two variants derived from the U937 monocytic
leukemia in the absence of exogenous inhibitors. Cell-permeable
peptide inhibitors selective for CPP32-like
proteases suppressed AP24 activation and apoptotic death. These findings indicate that CPP32-like activity is one of several upstream signals required for AP24 activation. Furthermore, activation of CPP32-like
proteases alone is not sufficient to commit irreversibly a cell to apoptotic death under conditions where activation of AP24 is inhibited.