INTERACTION OF WHEAT-GERM-AGGLUTININ AND CONCANAVALIN-A WITH PLATELETS - STIMULATION OF PLATELET FUNCTIONAL REACTIONS AND BINDING WITH MEMBRANE-GLYCOPROTEINS

Effects of two lectins, wheat germ agglutinin (WGA) and concanavalin A (Con A), on platelet functional reactions and interaction of lectins with the platelet membrane glycoproteins (GPs) have been studied. Both lectins stimulated platelet aggregation and secretion of serotonin fr om platelet dense granules. The effects of WGA and Con A were blocked by specific sugars, N-acetyl-D-glucosamine and alpha-methyl-D-mannopyr anoside, respectively, by adenylate cyclase activator prostaglandin E- 1, and by anti-GP IIb-IIIa monoclonal antibody (monAB), CRC64, that in hibits platelet interaction with fibrinogen. The data indicate that bo th lectins interacting with the carbohydrate moiety on the platelet su rface stimulated not passive agglutination but fibrinogen-GP IIb-IIIa- dependent platelet aggregation which is coupled with the secretion fro m granules and activation of the intracellular systems of signal trans duction. However, there were significant differences between the stimu latory effects of WGA and Con A. WGA induced more pronounced and quick platelet aggregation and stimulated several times higher serotonin se cretion than Con A. In addition, adhesion studies showed that plastic- adsorbed WGA appeared to be a nonadhesive substrate, whereas Con A eff ectively stimulated platelet adhesion. Unlike Con A-induced platelet a ggregation, adhesion to Con A substrate was not inhibited by monAB CRC 64, i.e., was not dependent on GP IIb-IIIa-fibrinogen interaction. Bin ding of lectins with major platelet GPs was studied using immobilized WGA and Con A and platelet lysate as a source of GPs. Platelet lysate was incubated with immobilized lectins and then binding of individual GPs was evaluated using specific mono- and polyclonal antibodies. WGA binds with GP Ib and P-selectin but not with other GPs tested. Interac tion of Con A with platelet GPs was less specific. This lectin binds w ith GP IIb-IIIa, CP Ib, GP IV, and P-selectin. Although GP Ib appeared to be the main protein which bound WGA on platelet surface, anti-GP I b antibodies failed to affect WGA-induced platelet aggregation, but in hibited WGA-induced agglutination of fixed platelets. Thus, interactio n of the WGA with GP Ib could not be considered as a major stimulus in itiating WGA-dependent platelet activation and aggregation.