Interdependence of calcium signaling and protein tyrosine phosphorylation in human endothelial cells

The signal transduction cascade which initiates transmembraneous influx of Ca2+ into endothelial cells in response to the discharge of intracellular Ca2+ stores is thought to involve a step sensitive to tyrosine kinase inhibition. We investigated the interrelationship between Ca2+ signaling and protein tyrosine phosphorylation following cell stimulation with either the receptor-dependent agonist, bradykinin, or the protein-tyrosine phosphatase inhibitor, phenylarsine oxide. In cultured human endothelial cells phenylarsine oxide instigated a concentration-dependent increase in the intracellular concentration of free Ca2+ ([Ca2+]i). This increase in [Ca2+]i was not associated with the tyrosine phosphorylation of phospholipase C gamma, enhanced formation of inositol 1,4,5-trisphosphate, or the rapid depletion of intracellularly stored Ca2+ but was coincident with the enhanced and prolonged tyrosine phosphorylation of a number of cytoskeletal proteins. In bradykinin-stimulated cells the tyrosine phosphorylation of the same cytoskeletal proteins (most notably 85- and 100-kDa proteins) was transient when cells were stimulated in the presence of extracellular Ca2+, was maintained under Ca2+-free conditions, and was reversed following readdition of extracellular Ca2+. These data suggest that the tyrosine phosphorylation of 2 cytoskeletal proteins is determined by the level of Ca2+ present in intracellular stores thus indicating a critical role for tyrosine phosphorylation in the control of capacitative Ca2+ entry in endothelial cells.