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Formation of lactose-resistant aggregates of human platelets induced by the mistletoe lectin and differential signaling responses to cell contact formation by the lectin or thrombin.

Human platelets afford a suitable and physiologically relevant model to study receptor-dependent cell aggregation and ensuing biosignaling reactions. Since cell surface glycoconjugates can serve as ligands in recognitive protein--carbohydrate interactions, it is of interest to investigate the reactivity of such epitopes for a plant lectin and the elicited intracellular responses. Therefore, the galactose-specific lectin (Viscum album agglutinin, VAA) was employed as a tool for this purpose. It was found that VAA induced platelet aggregation at a concentration of 2.5 microgram/ml using 2.5. 108 cells/ml, composed of the formation of both lactose-sensitive (Lac+) and lactose-resistant (Lac-) intercellular contacts. Lac- aggregates were formed only by metabolically active platelets of about 70% of the samples from the group of studied volunteers. The requirement of metabolic activity for formation of these contacts which no longer depend on lectin--ligand recognition was underscored by the lack of their appearance in the presence of metabolic inhibitors such as nordihydroguaiaretic acid, trifluoperazine, N-ethylmaleimide and menadione. With respect to biosignaling, the effective aggregation of platelets did not affect the basal level of Ca2+ in cells and reduced the rate of the menadione-dependent generation of H2O2. In parallel series platelet aggregation induced by bovine thrombin (0.03 U/ml) triggered an increase in the cytoplasmic Ca2+ level and an enhancement of the H2O2 generation. Overall, these results imply metabolically controlled post-binding reactions which strengthen the lectin-induced cell association and demonstrate differential responses with respect to the Ca2+ level and H2O2-generation between lectin- or thrombin-mediated aggregation of human platelets.