Activation of Syk by protein kinase C-δ regulates thrombin-induced intercellular adhesion molecule-1 expression in endothelial cells via tyrosine phosphorylation of RelA/p65

Protein kinase C-delta (PKC-delta) plays a pivotal role in mediating thrombin-induced NF-kappa B activation and ICAM-1 expression in endothelial cells. However, the downstream mechanisms mediating its function are unclear. In this study, we show that PKC-delta-mediated activation of protein-tyrosine kinase Syk plays an important role in thrombin signaling of NF-kappa B activation and intercellular adhesion molecule-1 (ICAM-1) expression in endothelial cells. Stimulation of human vascular endothelial cells with thrombin resulted in a time-dependent phosphorylation of Syk on tyrosine 525 and 526, an indication of Syk activation. Inhibition of PKC-delta by pharmacological and genetic approaches prevented Syk activation by thrombin. These results place Syk downstream of PKC-delta in transmitting thrombin-activated signaling in endothelial cells. Consistent with this, thrombin-induced NF-kappa B activity and ICAM-1 expression were prevented by the expression of a kinase-defective mutant or RNA interference knockdown of Syk. Similarly, inhibiting Syk also impaired NF-kappa B activity and ICAM-1 expression induced by a constitutively active mutant of PKC-delta. Analysis of the NF-kappa B pathway showed that Syk contributes to thrombin-induced NF-kappa B activation by controlling its transactivation potential and that this response is associated with tyrosine phosphorylation of RelA/p65. Thus, these data unveil a novel pathway in which Syk signals downstream of PKC-delta to mediate thrombin-induced ICAM-1 expression in endothelial cells by increasing transcriptional capacity of NF-kappa B via a mechanism that relies on tyrosine phosphorylation of RelA/p65.