Involvement of Na+/Ca2+ Exchanger in Inside-Out Signaling Through the Platelet Integrin IIbβ3

Abstract

The platelet integrin IIbβ3 has become a new target for the treatment of pathological thrombosis. It becomes apparent that the affinity of IIbβ3 for its ligands is dynamically regulated by inside-out signaling. However, the components that couple diverse intracellular signals to the cytoplasmic domains of IIbβ3 remain obscure. Employing a chymotrypsin-induced IIbβ3 activation model, we previously proposed the hypothesis that Na+/Ca2 +exchanger (NCX) may be involved in inside-out signaling (Shiraga et al:Blood 88:2594, 1996). In the present study, employing two unrelated Na+/Ca2+ exchange inhibitors, 3′,4′-dichlorobenzamil (DCB) and bepridil, we investigated the role of NCX in platelet activation induced by various agonists in detail. Both inhibitors abolished platelet aggregation induced by all agonists examined via the inhibition of IIbβ3 activation. Moreover, these inhibitors abolished IIbβ3 activation induced by phorbol 12-myristate 13-acetate or A23187. On the other hand, neither of these inhibitors showed apparent inhibitory effects on protein phosphorylation of pleckstrin or myosin light chain, or an increase in intracellular calcium ion concentrations evoked by 0.1 U/mL thrombin. These effects of the NCX inhibitors are in striking contrast to those of protein kinase C inhibitor, Ro31-8220. Biochemical and ultrastructural analyses showed that NCX inhibitors, particularly DCB, made platelets “thrombasthenic”. These findings suggest that the NCX is involved in the common steps of inside-out signaling through integrin IIbβ3.