Protein kinase C-E: regulates the apoptosis and survival of glioma cells

In this study, we examined the role of protein kinase C (PKC)-epsilon in the apoptosis and survival of glioma cells using tumor necrosis factor-related apoptosis inducing ligand (TRAIL)-stimulated cells and silencing of PKC epsilon expression. Treatment of glioma cells with TRAIL induced activation, caspase-dependent cleavage, and down-regulation of PKC epsilon within 3 to 5 hours of treatment. Overexpression of PKC epsilon inhibited the apoptosis induced by TRAIL, acting downstream of caspase 8 and upstream of Bid cleavage and cytochrome c release from the mitochondria. A caspase-resistant PKC epsilon mutant (D383A) was more protective than PKC epsilon, suggesting that both the cleavage of PKC epsilon and its down-regulation contributed to the apoptotic effect of TRAIL. To further study the role of PKC epsilon in glioma cell apoptosis, we employed short interfering RNAs directed against the mRNA of PKC epsilon and found that silencing of PKC epsilon expression induced apoptosis of various glioma cell lines and primary glioma cultures. To delineate the molecular mechanisms involved in the apoptosis induced by silencing of PKC epsilon, we examined the expression and phosphorylation of various apoptosis-related proteins. We found that knockdown of PKC epsilon did not affect the expression of BcI2 and Bax or the phosphorylation and expression of Erk1/2, c-Jun-NH2-kinase, p38, or STAT, whereas it selectively reduced the expression of AKT. Similarly, TRAIL reduced the expression of AKT in glioma cells and this decrease was abolished in cells over-expressing PKC epsilon. Our results suggest that the cleavage of PKC epsilon and its down-regulation play important roles in the apoptotic effect of TRAIL. Moreover, PKC epsilon regulates AKT expression and is essential for the survival of glioma cells.