Recombinant P‐selectin glycoprotein‐ligand‐1 delays thrombin‐induced platelet aggregation: a new role for P‐selectin in early aggregation

1 P‐selectin is involved, with P‐selectin glycoprotein (GP)‐ligand‐1 (PSGL‐1), in platelet/leukocyte interactions during thrombo‐inflammatory reactions; it also stabilizes platelet aggregates. Its antagonism accelerates thrombolysis and enhances the anti‐aggregatory effects of GPIIb–IIIa inhibitors. This study was designed to investigate the mechanisms of P‐selectin‐mediated platelet aggregation. 2 In freshly isolated human platelets, P‐selectin translocation after thrombin stimulation increased rapidly to 48, 72, and 86% positive platelets after 60, 120, and 300 s, respectively. Platelet aggregation at 60 s post‐stimulation averaged 46.7±1.9% and its extent followed closely the kinetics of P‐selectin translocation. 3 Pre‐treatment of platelets with P‐selectin antagonists, a recombinant PSGL‐1 (rPSGL‐Ig) or a blocking monoclonal antibody, significantly delayed platelet aggregation in a dose‐dependent manner. At 100  μ g ml −1 of rPSGL‐Ig, platelet aggregation was completely inhibited up to 60 s post‐stimulation and increased thereafter to reach maximal aggregation at 5 min. The second phase of platelet aggregation, in the presence of rPSGL‐Ig, was completely prevented by the addition of a GPIIb–IIIa antagonist (Reopro) at 60 s, whereas its addition in the absence of rPSGL‐Ig was without any significant effect. 4 Combination of rPSGL‐Ig with Reopro or with an inhibitor of Pi3K (LY294002), which reduces GPIIb–IIIa activation, showed to be more effective in inhibiting platelet aggregation, in comparison to the effects observed individually. 5 rPSGL‐Ig blocks P‐selectin, whereas Reopro and LY294002 block GPIIb–IIIa and its activation, respectively, without a major effect on the percentage of platelets expressing P‐selectin. 6 In summary, platelet P‐selectin participates with GPIIb–IIIa in the initiation of platelet aggregation. Its inhibition, with rPSGL‐Ig, delays the aggregation process and increases the anti‐aggregatory potency of Reopro. Thus, combination of P‐selectin and GPIIb–IIIa antagonism may constitute a promising therapeutic option in the management of thrombotic disorders.British Journal of Pharmacology (2006) 148 , 299–305. doi: 10.1038/sj.bjp.0706734