Induction of apoptosis is driven by nuclear retention of protein kinase Cδ

Abstract

Protein kinase Cδ(PKCδ) mediates apoptosis downstream of many apoptotic stimuli. Because of its ubiquitous expression, tight regulation of the proapoptotic function of PKCδ is critical for cell survival. Full-length PKCδ is found in all cells, whereas the catalytic fragment of PKCδ, generated by caspase cleavage, is only present in cells undergoing apoptosis. Here we show that full-length PKCδ transiently accumulates in the nucleus in response to etoposide and that nuclear translocation precedes caspase cleavage of PKCδ. Nuclear PKCδ is either cleaved by caspase 3, resulting in accumulation of the catalytic fragment in the nucleus, or rapidly exported by a Crm1-sensitive pathway, thereby assuring that sustained nuclear accumulation of PKCδ is coupled to caspase activation. Nuclear accumulation of PKCδ is necessary for caspase cleavage, as mutants of PKCδ that do not translocate to the nucleus are not cleaved. However, caspase cleavage of PKCδ per se is not required for apoptosis, as an uncleavable form of PKCδ induces apoptosis when retained in the nucleus by the addition of an SV-40 nuclear localization signal. Finally, we show that kinase negative full-length PKCδ does not translocate to the nucleus in apoptotic cells but instead inhibits apoptosis by blocking nuclear import of endogenous PKCδ. These studies demonstrate that generation of the PKCδ catalytic fragment is a critical step for commitment to apoptosis and that nuclear import and export of PKCδ plays a key role in regulating the survival/death pathway. © 2007 by The American Society for Biochemistry and Molecular Biology, Inc.