Recent studies have shown that protein kinase C (PKC) δ is proteolytically activated at the onset of apoptosis induced by DNA-damaging agents, tumor necrosis factor, and anti-Fas antibody. However, the relationship of PKCδ cleavage to induction of apoptosis is unknown. The present studies demonstrate that full-length PKCδ is cleaved at DMQD330N to a catalytically active fragment by the cysteine protease CPP32. The results also demonstrate that overexpression of the catalytic kinase fragment in cells is associated with chromatin condensation, nuclear fragmentation, induction of sub-G1 phase DNA and lethality. By contrast, overexpression of full-length PKCδ or a kinase inactive PKCδ fragment had no detectable effect. The findings suggest that proteolytic activation of PKCδ by a CPP32-like protease contributes to phenotypic changes associated with apoptosis.

The c-Abl protein tyrosine kinase is activated by certain DNA-damaging agents, and its overexpression causes arrest in the G1 phase of the cell cycle by a mechanism dependent on the tumour-suppressor protein p53 (refs 2-4). Here we investigate the possible role of c-Abl in growth arrest induced by DNA damage. Transient transfection experiments using wild-type or inactivated c-Abl show that both induce expression of p21, an effector of p53, but only wild-type c-Abl downregulates the activity of the cyclin-dependent kinase Cdk2 and causes growth arrest. Exposure to ionizing radiation of cells that stably express active or inactive c-Abl is associated with induction of c-Abl/p53 complexes and p21 expression. However, cells expressing the dominant-negative c-Abl mutant and cells lacking the c-abl gene are impaired in their ability to downregulate Cdk2 or undergo G1 arrest in response to ionizing radiation. We also show that expression of c-Abl kinase in p21(-1-), but not in p53(-1-), cells results in downregulation of Cdk2. Our results suggest that c-Abl kinase contributes to the regulation of growth arrest induced by ionizing radiation by a p53-dependent, p21-independent mechanism.