The Localization of Protein Kinase Cδ in Different Subcellular Sites Affects Its Proapoptotic and Antiapoptotic Functions and the Activation of Distinct Downstream Signaling Pathways

Protein kinase Cδ (PKCδ) regulates cell apoptosis and survival in diverse cellular systems. PKCδ translocates to different subcellular sites in response to apoptotic stimuli; however, the role of its subcellular localization in its proapoptotic and antiapoptotic functions is just beginning to be understood. Here, we used a PKCδ constitutively active mutant targeted to the cytosol, nucleus, mitochondria, and endoplasmic reticulum (ER) and examined whether the subcellular localization of PKCδ affects its apoptotic and survival functions. PKCδ-Cyto, PKCδ-Mito, and PKCδ-Nuc induced cell apoptosis, whereas no apoptosis was observed with the PKCδ-ER. PKCδ-Cyto and PKCδ-Mito underwent cleavage, whereas no cleavage was observed in the PKCδ-Nuc and PKCδ-ER. Similarly, caspase-3 activity was increased in cells overexpressing PKCδ-Cyto and PKCδ-Mito. In contrast to the apoptotic effects of the PKCδ-Cyto, PKCδ-Mito, and PKCδ-Nuc, the PKCδ-ER protected the cells from tumor necrosis factor–related apoptosis-inducing ligand–induced and etoposide-induced apoptosis. Moreover, overexpression of a PKCδ kinase-dead mutant targeted to the ER abrogated the protective effect of the endogenous PKCδ and increased tumor necrosis factor–related apoptosis-inducing ligand–induced apoptosis. The localization of PKCδ differentially affected the activation of downstream signaling pathways. PKCδ-Cyto increased the phosphorylation of p38 and decreased the phosphorylation of AKT and the expression of X-linked inhibitor of apoptosis protein, whereas PKCδ-Nuc increased c-Jun NH 2 -terminal kinase phosphorylation. Moreover, p38 phosphorylation and the decrease in X-linked inhibitor of apoptosis protein expression played a role in the apoptotic effect of PKCδ-Cyto, whereas c-Jun NH 2 -terminal kinase activation mediated the apoptotic effect of PKCδ-Nuc. Our results indicate that the subcellular localization of PKCδ plays important roles in its proapoptotic and antiapoptotic functions and in the activation of downstream signaling pathways. (Mol Cancer Res 2007;5(6):627–39)