These studies demonstrate that treatment of human U‐937 cells with ionizing radiation (IR) is associated with activation of a cytoplasmic myelin basic protein (MBP) kinase. Characterization of the kinase by gel filtration and in‐gel kinase assays support activation of a 40 kDa protein. Substrate and inhibitor studies further support the induction of protein kinase C (PKC)‐like activity. The results of N‐terminal amino acid sequencing of the purified protein demonstrate identity of the kinase with an internal region of PKC delta. Immunoblot analysis was used to confirm proteolytic cleavage of intact 78 kDa PKC delta in control cells to the 40 kDa C‐terminal fragment after IR exposure. The finding that both IR‐induced proteolytic activation of PKC delta and endonucleolytic DNA fragmentation are blocked by Bcl‐2 and Bcl‐xL supports an association with physiological cell death (PCD). Moreover, cleavage of PKC delta occurs adjacent to aspartic acid at a site (QDN) similar to that involved in proteolytic activation of interleukin‐1 beta converting enzyme (ICE). The specific tetrapeptide ICE inhibitor (YVAD) blocked both proteolytic activation of PKC delta and internucleosomal DNA fragmentation in IR‐treated cells. These findings demonstrate that PCD is associated with proteolytic activation of PKC delta by an ICE‐like protease.

The MUC1 transforming protein is overexpressed by most human carcinomas. The present studies demonstrate that the MUC1 C-terminal subunit (MUC1 C-ter) localizes to mitochondria in HCT116/MUC1 colon carcinoma cells and that heregulin stimulates mitochondrial targeting of MUC1 C-ter. We also show that MUC1 attenuates cisplatin-induced (1) release of mitochondrial apoptogenic factors, (2) activation of caspase-3, and (3) induction of apoptosis. Moreover, knockdown of MUC1 expression in A549 lung and ZR-75-1 breast carcinoma cells by MUC1siRNA was associated with increased sensitivity to genotoxic drugs in vitro and in vivo. These findings indicate that MUC1 attenuates the apoptotic response to DNA damage and that this oncoprotein confers resistance to genotoxic anticancer agents.

The product of the c-abl gene is a non-receptor tyrosine kinase that is localized to the nucleus and cytoplasm. The precise function of c-Abl is unknown. Here we show that ionizing radiation activates c-Abl. Similar results were obtained with the alkylating agents cis-platinum and mitomycin C. We also demonstrate that cells deficient in c-Abl fail to activate Jun kinase (JNK/SAP kinase) after ionizing radiation or alkylating agent exposure and that reconstitution of c-Abl in these cells restores that response. In contrast, the stress response to tumour-necrosis factor is stimulated by a c-Abl-independent mechanism. These findings indicate that c-abl is involved in the stress response to DNA-damaging agents.

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.