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

A. B. Samal^1*, A. V. Timoshenko¹, E. N. Loiko¹, H. Kaltner², and H.-J. Gabius²

¹Department of Biophysics, Belarussian State University, pr. F. Skariny 4, Minsk, 220050 Belarus; fax: (017) 220-7465; E-mail: timoshenko@phys.bsu.unibel.by

²Institut für Physiologische Chemie, Tierärztliche Fakultät, Ludwig-Maximilians-Universität, D-80539 München, Germany

^* To whom correspondence should be addressed.

Received April 4, 1997; Revision received September 23, 1997

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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 µg/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.

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KEY WORDS: lectins, Viscum album L., platelets, aggregation, disaggregation, thrombin, intracellular Ca²⁺

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