Aggretin, a C-Type Lectin Protein, Induces Platelet Aggregation via Integrin α2β1 and GPIb in a Phosphatidylinositol 3-Kinase Independent Pathway

Abstract

Aggretin purified from Calloselasma rhodostoma venom was previously identified as α2β1 agonist in triggering platelet aggregation, and exists as a heterodimer sharing a great homologous sequence to GPIb binding proteins. We show here that binding to GPIb is also required in aggregation-inducing activity of aggretin. A2-IIE10, an anti-integrin α2 monoclonal antibody, delayed platelet aggregation while agkistin, a GPIb antagonist, only slightly inhibited platelet aggregation caused by aggretin. However, the aggretin-induced platelet aggregation was completely abolished by a combination of A2-IIE10 and agkistin. Either A2-IIE10 or agkistin significantly inhibited the binding of FITC-aggretin toward fixed platelets. Aggretin and collagen induced a similar signal transduction in platelets involving a time-dependent tyrosine phosphorylation of p125FAK and PLCγ2, but aggretin caused a much-delayed tyrosine-phosphorylation of PI 3-kinase compared with collagen. LY294002, a PI 3-kinase inhibitor, showed a significant inhibitory effect on collagen, but not aggretin-stimulated platelet aggregation. These findings indicate aggretin induces platelet aggregation via binding of α2β1 and GPIb, causing phosphorylation of p125FAK and PLCγ2 leading to platelet activation without the involvement of PI 3-kinase activation.