Cyclic AMP-Dependent Protein Kinase Mediates Glycoprotein Ibα Shedding from Platelet.

Extracellular domain of platelet glycoprotein (GP) Ibα contains ligand binding sites for von Willebrand factor (VWF) and α-thrombin. GPIbα binding to VWF exposed at the injured vessel initiates thrombus formation, thus it plays key roles in thrombosis and hemostasis. While a lot of research has been performed to elucidate the critical roles for GPIbα in platelet activation, little is known about the negative regulatory mechanisms of this adhesion receptor. Here we show that inhibition of cAMP-dependent protein kinase (PKA) resulted in the shedding of GPIbα from platelet. GPIbα was shed after platelets were incubated with PKA inhibitors (H89, PKI) in a dose-dependent manner. PKA mediated GPIbα shedding was inhibited by calpain inhibitors (MDL 28170, E64d, calpain inhibitor-I, calpain inhibitor-II) in a dose-dependent manor, suggesting that shedding of GPIbα is a result of calpain cleavage. Time course experiment revealed that PKA mediated GPIbα shedding occurred as a late event, 10 minutes after platelet activation. Flow cytometry and western-blot data suggested that the cleavage site was at N-terminal of residues 484 and 485 on GPIbα. These residues are responsible for disulfide bond linkage to GPIbβ. Though the size of GPIbα shed fragments from platelet treated with H89 was the same as platelet treated with calcium ionophore A23187 or thrombin, yet the intensity of platelet activation, the amount of GPIbα shedding, and redistribution of GPIbα were different, suggesting that the mechanism of PKA inhibition-initiated GPIbα shedding is different from the shedding caused by A23187 or thrombin. Platelets treated with the PKA inhibitor, H89, presented significant decrease in ristocetin induced platelet aggregation and platelet adhesion on VWF under shear stress. In conclusion, these data provide new evidence that inhibition of PKA results in calpain mediated GPIbα shedding which may play a role in limiting thrombus infinite formation after platelet activation.