Integrin αIIbβ3-Mediated Outside-in Signaling: Brake or Amplifier of Platelet Activation?

αIIbβ3 is the most prominent integrin in platelets, and binding to its ligands, in addition to supporting platelet aggregation, also results in the transmission of so-called αIIbβ3-mediated outside-in signals into the cell interior. While it is well accepted that integrin-mediated outside-in signaling functions as an amplifier of platelet activation, accumulating evidence suggests that outside-in signaling can, under certain conditions, function as an inhibitor of platelet activation. In this regard, previous studies have shown that ligand binding and platelet aggregation activate the inositol phosphatase SHIP-1, a negative regulator of the PI3K/Akt signaling pathway, to shift activated integrins back to their resting state, leading to dissociation of platelet aggregates. Because the PI3K/Akt signaling pathway is also involved in platelet granule secretion, we examined whether ligand binding to αIIbβ3 might transmit inhibitory signals that suppress platelet granule secretion. Interestingly, we found that antagonists of integrin αIIbβ3 promote both platelet dense- and α-granule secretion stimulated by low dose agonists. In support of this finding, both mouse and human platelets lacking expression of αIIbβ3 exhibited increased granule secretion compared to their wild-type counterparts. Conversely, Mn⁺⁺-induced fibrinogen binding to αIIbβ3 inhibited low-dose agonist-induced platelet granule secretion. Biochemical analysis revealed that blocking ligand binding to, or absence of, αIIbβ3, enhanced agonist-induced Akt phosphorylation, while at the same time prevented the activation of the inhibitory enzyme, SHIP-1. To further investigate the role of SHIP-1 in inhibitory signaling, we examined the effect on platelet secretion of 3AC, a specific inhibitor of SHIP-1. We found that 3AC not only restores ADP-induced platelet granule secretion, but also increases CRP- or TRAP-induced platelet granule secretion. Taken together, these data demonstrate that integrin αIIbβ3-mediated outside-in signaling act as a brake to restrict unnecessary platelet activation that occurs in the presence of low-dose agonist stimulation.