The Role of Akt in the Signaling Pathway of the Glycoprotein Ib-IX Induced Platelet Activation

The platelet von Willebrand factor (vWF) receptor, glycoprotein Ib-IX (GPIb-IX), mediates platelet adhesion and induces signaling leading to integrin activation. Phosphoinositol 3-kinase (PI3K) is important in GPIb-IX-mediated signaling. PI3K-dependent signaling mechanisms, however, are unclear. We show that GPIb-IX-induced platelet aggregation and stable adhesion under flow were impaired in mouse platelets deficient in PI3K effectors, Akt1 and Akt2, and in human platelets treated with an Akt inhibitor, SH-6. Akt1 and Akt2 play important roles in early GPIb-IX signaling independent of Syk, adenosine diphosphate (ADP), or thromboxane A2 (TXA2), in addition to their recognized roles in ADP- and TXA2-dependent secondary amplification pathways. Knockout of Akt1 or Akt2 diminished platelet spreading on vWF but not on immobilized fibrinogen. Thus, Akt1 and Akt2 are both required only in the GPIb-IX-mediated integrin activation (inside-out signaling). In contrast, PI3K inhibitors abolished platelet spreading on both vWF and fibrinogen, indicating a role for PI3K in integrin outside-in signaling distinct from that in GPIb-IX-mediated inside-out signaling. Furthermore, Akt1- or Akt2-deficiency diminished vWF-induced cGMP elevation, and their inhibitory effects on GPIb-IX-dependent platelet adhesion were reversed by exogenous cGMP. Thus, Akt1 and Akt2 mediate GPIb-IX signaling via the cGMP-dependent signaling pathway.