PKC alpha protein but not kinase activity is critical for glioma cell proliferation and survival

Protein kinase C alpha (PKCalpha) has been implicated in tumor development with high levels of PKCalpha expression being associated with various malignancies including glioblastomas and tumors of the breast and prostate. To account for its upregulation in these cancers, studies have suggested that PKCalpha plays a role in promoting cell survival. Here we show by siRNA depletion in U87MG glioma cells that a critical threshold level of PKCalpha protein expression is essential for their growth in the presence of serum and for their survival following serum deprivation. Derivation of PKCalpha wt and KO mouse embryo fibroblast cell lines confirms a role for PKCalpha in protecting cells from apoptosis induced by serum deprivation. Notably, PKCalpha was found to mediate chemo-protection in these fibroblastic cell lines. In U87MG cells PKCalpha does not confer chemoprotection though this likely reflects growth arrest associated with its depletion. To determine the requirements for catalytic function, comparison was made between distinct classes of PKC inhibitors. In contrast to loss of PKCalpha protein, inhibition of PKC kinase activity in glioma cell lines does not significantly inhibit growth or survival. Conversely, inhibition with calphostin C, which targets the regulatory domain of PKC, potently inhibits proliferation and induces apoptosis. Evidence is presented that it is the fully phosphorylated, folded form of PKCalpha that confers this activity-independent behaviour. These results indicate an essential pro-proliferative and pro-survival role for PKCalpha in glioma but question the use of ATP competitive inhibitors as therapeutics, either alone, or in combination with chemotoxic agents.