Roles of tyrosine phosphorylation and cleavage of protein kinase Cδ in its protective effect against tumor necrosis factor-related apoptosis inducing ligand-induced apoptosis

Protein kinase Cδ (PKCδ) regulates cell apoptosis in a cell- and stimulus-specific manner. Here, we studied the role of PKCδ in the apoptotic effect of TRAIL in glioma cells. We found that transfection of the cells with a PKCδ kinase-dead mutant (K376R) or with a small interfering RNA targeting the PKCδ mRNA increased the apoptotic effect of tumor necrosis factor-related apoptosis inducing ligand (TRAIL), whereas overexpression of PKCδ decreased it. PKCδ acted downstream of caspase 8 and upstream of cytochrome c release from the mitochondria. TRAIL induced cleavage of PKCδ within 2-3 h of treatment, which was abolished by caspase 3, 8, and 9 inhibitors. The cleavage of PKCδ was essential for its protective effect because overexpression of a caspase-resistant mutant (PKCδD327A) did not protect glioma cells from TRAIL-induced apoptosis but rather increased it. TRAIL induced translocation of PKCδ to the perinuclear region and the endoplasmic reticulum and phosphorylation of PKCδ on tyrosine 155. Using a PKCδY155F mutant, we found that the phosphorylation of PKCδ on tyrosine 155 was essential for the cleavage of PKCδ in response to TRAIL and for its translocation to the endoplasmic reticulum. In addition, phosphorylation of PKCδ on tyrosine 155 was necessary for the activation of AKT in response to TRAIL. Our results indicate that PKCδ protects glioma cells from the apoptosis induced by TRAIL and implicate the phosphorylation of PKCδ on tyrosine 155 and its cleavage as essential factors in the anti-apoptotic effect of PKCδ.