Synergistic adhesive interactions and signaling mechanisms operating between platelet glycoprotein Ib/IX and integrin α_IIbβ₃: Studies in human platelets and transfected Chinese hamster ovary cells

This study investigates three aspects of the adhesive interaction operating between platelet glycoprotein Ib/IX and integrin α_IIbβ₃. These include the following: 1) examining the sufficiency of GPIb/IX and integrin α_IIbβ₃ to mediate irreversible cell adhesion on immobilized von Willebrand factor (vWf) under flow; 2) the ability of the vWf-GPIb interaction to induce integrin α_IIbβ₃ activation independent of endogenous platelet stimuli; and 3) the identification of key second messengers linking the vWf-GPIb/IX interaction to integrin α_IIbβ₃ activation. By using Chinese hamster ovary cells transfected with GPIb/IX and integrin α_IIbβ₃, we demonstrate that these receptors are both necessary and sufficient to mediate irreversible cell adhesion under flow, wherein GPIb/IX mediates cell tethering and rolling on immobilized vWf, and integrin α_IIbβ₃ mediates cell arrest. Moreover, we demonstrate direct signaling between GPIb/IX and integrin α_IIbβ₃. Studies on human platelets demonstrated that vWf binding to GPIb/IX is able to induce integrin α_IIbβ₃ activation independent of endogenous platelet stimuli under both static and physiological flow conditions (150-1800 s^-1). Analysis of the key second messengers lingking the vWf-GPIb interaction to integrin α_IIbβ₃ activation demonstrated that the first step in the activation process involves calcium release from internal stores, whereas transmembrane calcium influx is a secondary event potentiating integrin α_IIbβ₃ activation.