Abstract 499: Rac1 Plays a Stimulatory Role in Glycoprotein Ib-IX-Dependent Platelet Activation

Abstract

The platelet receptor for von Willebrand factor (VWF), the glycoprotein Ib-IX (GPIb-IX) complex, mediates platelet adhesion at sites of vascular injury and transmits signals leading to platelet activation. Ligand occupancy of GPIb-IX stimulates the sequential activation of the Src family kinase (SFK) Lyn, phosphoinositide 3-kinase (PI3K), and Akt, leading to activation of integrin aIIbβ3, synthesis of thromboxane, granule secretion, and integrin-dependent stable platelet adhesion and aggregation. It remains unclear, however, as to how the PI3K/Akt pathway is activated following ligand binding to GPIb-IX. Using platelet-specific conditional Rac1 -/- mice and the Rac1 selective inhibitor NSC23766 , we examined the role of Rac1 in GPIb-IX-dependent platelet activation. Rac1-deficient platelets are defective in VWF/GPIb-induced aggregation, thromboxane synthesis and granule secretion, and stable adhesion to VWF under shear stress. Importantly, GPIb-IX-induced activation of integrin aIIbβ3 was also significantly impaired in Rac1 -/- or Rac1-inhibitor treated platelets, indicating that Rac1 is critical for GPIb-mediated integrin activation. However, Rac1 deficiency does not alter the VWF binding function of GPIb-IX, demonstrating that the functional defects observed in Rac1-deficient platelets derives from a defect in VWF-induced GPIb-IX signaling. To determine why Rac1 is important in GPIb-IX signaling, we further show that VWF/GPIb-induced activation of Rac1 was abolished in Lyn -/- mouse platelets or SFK-inhibitor treated human platelets but remained unaffected by the PI3K inhibitor LY-294002, suggesting that Rac1 is activated downstream of Lyn independent of PI3K. Moreover, VWF/GPIb-induced PI3K-dependent activation of Akt and P38 MAPK was abolished in Rac1-deficient platelets. Thus, our study reveals that Rac1 is an important new link in the GPIb-IX signaling pathway that mediates Lyn-dependent activation of the PI3K/Akt pathway leading to integrin activation.