Synergistic adhesive interactions and signaling mechanisms operating between platelet glycoprotein Ib/IX and integrin αIIBβ3 -: 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 alpha (IIb)beta (3). These include the following: 1) examining the sufficiency of GPIb/IX and integrin alpha (IIb)beta (3) to mediate irreversible cell adhesion on immobilized von Willebrand factor (vWf) under flow; 2) the ability of the vWf-GPIb interaction to induce integrin alpha (IIb)beta (3) activation independent of endogenous platelet stimuli; and 3) the identification of key second messengers linking the vWf-GPIb/IX interaction to integrin alpha (IIb)beta (3) activation. By using Chinese hamster ovary cells transfected with GPIb/IX and integrin alpha (IIb)beta (3), we demonstrate that these receptors are both necessary and sufficient to mediate irreversible cell adhesion under flow, wherein GPIb/M mediates cell tethering and rolling on immobilized vWf, and integrin alpha (IIb)beta (3) mediates cell arrest, Moreover, we demonstrate direct signaling between GPIb/M and integrin alpha (IIb)beta (3). Studies on human platelets demonstrated that vWf binding to GPIb/M is able to induce integrin alpha (IIb)beta (3) activation independent of endogenous platelet stimuli under both static and physiological flow conditions (150-1800 s(-1)). Analysis of the key second messengers linking the vWf-GPIb interaction to integrin alpha (IIb)beta (3) 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 alpha (IIb)beta (3) activation.