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.
Samal
A B
AB
Department of Biophysics, Belarussian State University, Minsk, 220050, Belarus. timoshenko@phys.bsu.unibel.by
Timoshenko
A V
AV
Loiko
E N
EN
Kaltner
H
H
Gabius
H J
HJ
D000818:Animals; D020013:Calcium Signaling; D002417:Cattle; D006801:Humans; D006861:Hydrogen Peroxide / Q000378:metabolism; D066298:In Vitro Techniques; D007785:Lactose / Q000494:pharmacology*; D037102:Lectins / Q000494:pharmacology*; D008954:Models, Biological; D028321:Plant Preparations; D010940:Plant Proteins; D010974:Platelet Aggregation / Q000187:drug effects* / Q000502:physiology*; D010975:Platelet Aggregation Inhibitors / Q000494:pharmacology; D054790:Ribosome Inactivating Proteins, Type 2; D015398:Signal Transduction; D013917:Thrombin / Q000494:pharmacology*; D014118:Toxins, Biological / Q000494:pharmacology*
D037102:Lectins; D028321:Plant Preparations; D010940:Plant Proteins; D010975:Platelet Aggregation Inhibitors; D054790:Ribosome Inactivating Proteins, Type 2; D014118:Toxins, Biological; C009674:ribosome inactivating protein, Viscum; D006861:Hydrogen Peroxide; D013917:Thrombin; D007785:Lactose