Protein C supports platelet binding and activation under flow: role of glycoprotein Ib and apolipoprotein E receptor 2.

Journal of thrombosis and haemostasis : JTH

PubMedID: 18489431

White TC, Berny MA, Tucker EI, Urbanus RT, de Groot PG, Fernández JA, Griffin JH, Gruber A, McCarty OJ. Protein C supports platelet binding and activation under flow: role of glycoprotein Ib and apolipoprotein E receptor 2. J Thromb Haemost. 2008;6(6):995-1002.
BACKGROUND
Activated protein C (APC) regulates thrombin generation and inhibits apoptosis. Endothelial protein C receptor (EPCR)-bound protein C is activated by thrombomodulin-bound thrombin. APC inactivates coagulation factors (F)Va/VIIIa and generates cytoprotective signaling downstream of protease-activated receptor-1 (PAR-1). Binding of APC to EPCR both modifies and induces PAR-1 signaling, but it is unknown if protein C interacts with cells in an alternative manner.

AIM
To determine whether platelets possess receptors for protein C that can generate intracellular signals.

RESULTS
Immobilized protein C or APC supported platelet adhesion, lamellipodia formation and elevation of intracellular Ca(2+). Adhesion of platelets to protein C or APC was inhibited by soluble recombinant apolipoprotein E receptor 2' (ApoER2') and by receptor-associated protein (RAP), an inhibitor of the low-density lipoprotein receptor family. Under shear, surface-bound protein C supported platelet adhesion and aggregation in a glycoprotein (GP)Ibalpha-dependent manner, and adhesion of platelets to immobilized protein C was abrogated by the addition of soluble forms of ApoER2' or RAP. APC bound to purified recombinant ApoER2' or GPIbalpha.

CONCLUSIONS
Our data demonstrate that activation of platelets with rapid intracellular signaling caused by binding to immobilized protein C or APC occurs via mechanisms that require ApoER2 and GPIbalpha and that APC directly binds to purified ectodomains of the receptors ApoER2 and GPIbalpha. These findings imply that protein C and APC may directly promote cell signaling in other cells by binding to ApoER2 and/or GPIbalpha.