Protein kinase C (PKC) has been implicated in signaling induced by diverse sets of stimuli regulating growth, differentiation, and apoptosis. The present study focused on the fate of PKC isotype proteins during Fas‐mediated apoptosis of human leukemic cell lines. Among the PKC isotypes expressed in different cell types, such as Jurkat, HPB‐ALL, U937, and HL60, all the nPKC isotypes including nPKCδ, nPKCε, and nPKCθ, but not cPKCα and βII and aPKCζ (n, c, and a represent novel, conventional and atypical, respectively), showed limited proteolytic cleavage during Fas‐mediated apoptosis. The limited proteolysis of nPKC isotypes means the disappearance of the intact protein band concomitant with the appearance of two fragments, most likely containing the kinase and regulatory domains, in contrast to the so‐called down‐regulation known for both cPKC and nPKC isotypes following exposure to stimuli such as 12‐O‐tetradecanoyl‐phorbol 13‐acetate (TPA). The time course of Fas‐mediated apoptosis in Jurkat cells parallels that of the activation of a 32‐kDa cysteine protease (CPP32)‐like protease and also closely parallels the proteolytic cleavage of nPKC isotypes. A peptide inhibitor of the CPP32‐like protease, Ac‐DEVD‐CHO, blocked the proteolytic cleavage of nPKC isotypes as well as apoptosis mediated by Fas. Transfection of recombinant protein coding for the catalytic fragment of nPKCδ to COS1 cells resulted in the apoptotic morphology of cells and nuclei.

The effect of TPA on apoptosis depends on the cell type. TPA significantly suppressed Fas‐mediated apoptosis in Jurkat, whereas TPA alone caused apoptosis in HPB‐ALL, U937, and HL60, only slight apoptosis in Jurkat. The proteolytic fragmentation of nPKC isotypes again closely correlated with the degree of apoptosis even in apoptosis induced by TPA. Separation of TPA‐treated cells into apoptotic and non‐apoptotic differentiating cells revealed that the proteolytic fragmentation of nPKC isotypes occurs only in apoptotic cells and, in adherent differentiating cells, nPKC isotypes as well as cPKCα were down‐regulated without the generation of nPKC fragments. These results are consistent with the idea that nPKC isotypes meet two different fates, down‐regulation and proteolytic cleavage generating kinase and regulatory fragments, and that the proteolytic cleavage of nPKC isotypes is a step in the signaling pathway involved in Fas‐mediated and TPA‐induced apoptosis.