Protein kinase C (PKC) δ regulates PKCα activity in a syndecan-4-dependent manner

The phosphorylation state of Ser(183) in the cytoplasmic tail of syndecan-4 determines the binding affinity of the cytoplasmic tail to phosphatidylinositol 4,5-bisphosphate (PIP2), the capacity of the tail to multimerize, and its ability to activate protein kinase C (PKC) alpha. We sought to identify the kinase responsible for this phosphorylation and to determine its downstream effects on PKCalpha activity and on endothelial cell function. Among several PKC isoenzymes tested, only PKCalpha and -delta were able to specifically phosphorylate Ser(183) in vitro. However, studies in cultured endothelial cells showed that the phosphorylation level of syndecan-4 was significantly reduced in endothelial cells expressing a dominant negative (DN) PKCS but not a DN PKCalpha mutant. Syndecan-4/plp(2)-dependent PKCa activity was significantly increased in PKCdelta DN cells, while PKCdelta overexpression was accompanied by decreased PKCa activity. PKCdelta-overexpressing cells exhibited a significantly lower proliferation rate and an impaired tube formation in response to FGF2, which were mirrored by similar observations in PKCa DN endothelial cells. These findings suggest that PKCdelta is the kinase responsible for syndecan-4 phosphorylation, which, in turn, attenuates the cellular response to FGF2 by reducing PKCalpha activity. The reduced PKCa activity then leads to impaired endothelial cell function. We conclude that PKCdelta regulates PKCalpha activity in a syndecan-4-dependent manner.