CASPASE-MEDIATED PROTEOLYSIS DURING APOPTOSIS - INSIGHTS FROM APOPTOTIC NEUTROPHILS

Apoptosis is initiated by activation of caspases (interleukin 1 beta-c onverting enzyme homologues), which cause coordinated cleavage of seve ral death substrates that function in structural or homeostatic pathwa ys. The relationship between substrate cleavage and apoptosis is not y et known, nor is it clear whether cleavage of specific substrates is a critical requirement for apoptosis. The human neutrophil provides nov el insights into the roles of proteolysis of specific substrates durin g apoptosis, since only a subset of caspase substrates are present in mature neutrophils. Of the death substrates we screened, PARP, the nuc lear mitotic apparatus protein (NuMA), the 70 kDa subunit of the U1 sm all ribonucleoprotein (U1-70kDa) and the catalytic subunit of DNA-depe ndent protein kinase (DNA-PKcs) mere not detected in non-apoptotic neu trophils; in contrast, lamin B and fodrin mere present in amounts simi lar to those found in other cells. Caspase-3 activity was absent in fr eshly isolated neutrophils, but was detected when neutrophils were age d in vitro, coincident with the onset of morphologic and biochemical a poptosis. The absence of PARP, NuMA, U1-70kDa and DNA-PKcs in non-apop totic neutrophils suggests that these are not critical anti-apoptotic proteins, and that their fragments are not required components of the neutrophil apoptotic pathway. These studies highlight the conserved ro le of caspase activation in the apoptotic mechanism, and focus attenti on on several conserved structural substrates as potential transducers of the proteolytic signal in apoptosis. (C) 1998 Federation of Europe an Biochemical Societies.