ACTIVATION OF A CRMA-INSENSITIVE, P35-SENSITIVE PATHWAY IN IONIZING RADIATION-INDUCED APOPTOSIS

The response of eukaryotic cells to ionizing radiation (IR) includes i nduction of apoptosis. However, the signals that regulate this respons e are unknown. The present studies demonstrate that IR treatment of U- 937 cells is associated with: (i) internucleosomal DNA fragmentation; (ii) cleavage of poly(ADP-ribose) polymerase; (iii) cleavage of protei n kinase C delta; and (iv) induction of an Ac-DEVD-p-nitroanilide clea ving activity. Overexpression of the cowpox protein CrmA blocked tumor necrosis factor (TNF)-induced apoptosis but had no effect on IR-induc ed DNA fragmentation or cleavage of poly(ADP-ribose) polymerase and pr otein kinase C delta. By contrast, overexpression of the baculovirus p 35 protein blocked both IR and TNF induced apoptosis. The results furt her demonstrate that the IR-induced proteolytic activity is directly i nhibited by the addition of purified recombinant p35, but not by CrmA. We show that the CPP32 protease is sensitive to p35 and not CrmA. We also show that IR induces activation of CPP32 and that this event, lik e induction of apoptosis, is sensitive to overexpression of p35 and no t CrmA. These findings indicate that IR induced apoptosis involves act ivation of CPP32 and that this CrmA-insensitive apoptotic pathway is d istinct from those induced by TNF and certain other stimuli.