Platelet agonists generate intracellular signals which lead to activation of the platelet membrane glycoprotein IIb-IIIa (integrin alpha IIb beta 3). The resulting occupancy of alpha IIb beta 3 by ligands also generates signals into the cell (outside-in signaling). We reported previously that unlike platelet agonists, the F(ab')2 fragments of an anti-alpha IIb beta 3 monoclonal antibody, PMA4, induced fibrinogen binding to alpha IIb beta 3 without causing intracellular activation. In this study, in order to determine whether outside-in signaling occurs in the absence of agonist-induced intracellular signals, we used PMA4 F(ab')2 as an inducer of fibrinogen binding to alpha IIb beta 3. PMA4 F(ab')2-induced fibrinogen binding and subsequent platelet aggregation triggered tyrosine phosphorylation of several proteins including pp72syk but not pp125FAK. No Ca2+ influx or mobilization, thromboxane B2 synthesis, phosphorylation of pleckstrin or the myosin light chain, cytoplasmic alkalinization, or platelet shape changes, were detected. These findings suggest that, in the absence of agonist-induced signaling, alpha IIb beta 3 occupied by soluble fibrinogen generates only a limited outside-in signal.
Antibodies, Monoclonal Blood Platelets