Dissection of the impact of various intracellular signaling pathways on stable cell aggregate formation of rat thymocytes after initial lectin-dependent cell association using a plant lectin as model and target-selective inhibitors

Bivalent lectins as bridging molecules between cells or cell surface lectins as docking points are involved in mediation of cell adhesion by specific recognition of suitable glycoconjugates on an opposing surface. The initial contact formation by a lectin can lead to intracellular post-binding events which effect stable cell association even in the presence of the haptenic sugar. To delineate the participation of intracellular signaling pathways in the cascade of reactions to establish firm association, reagents with proven inhibitory capacity on certain biochemical targets provide suitable tools. Using this approach with rat thymocytes and the galactoside-binding lectin from mistletoe (Viscum album L. agglutinin, VAA) as a model, a panel of 27 inhibitors with impact on e.g. several types of kinases, tyrosine phosphatases, NO synthases, G proteins, enzymes of arachidonate and cyclic nucleotide metabolism and calmodulin was systematically tested with respect to their capacity to impair the formation of lactose-resistant cell aggregates. In addition to the recently reported effectiveness of N-ethyimaleimide, nordihydroguaiaretic acid, and trifluoperazine the agents diacyigiycerol kinase inhibitor II, emodin, D609, DPI, KT5720, KT5926, MK-886, bisindolyimaleimide I, and (±)methoxyverapamil were able to reduce aggregate stability in the presence of the haptenic sugar. Thus, various types of kinases including p56 tyrosine kinase, lipoxygenases, phosphatidylcholine-specific phospholipase C as well as calmodulin and Ca²⁺-currents, but not modulators of the metabolism of cyclic nucleotides, NO synthases, MAP kinases, tyrosine phosphatases and phospholipase A (preferentially group II) and C can play a role in eliciting contact stability. More than one principal signaling pathway appears to be linked to the measurable parameter, since inhibitory substances show additive properties in co-incubation assays and differentially affect two lectin-elicited cellular activities, i.e. intracellular movement of Ca²⁺-ions and H₂O₂-generation, which can accompany cell adhesion and aggregation. Pronounced differences in the extent of modulation of H₂O₂-generation in human neutrophils by the same set of substances emphasizes that general conclusions on the post-binding effects for a certain lectin in different cell types are definitely precluded. In aggregate, the approach to employ inhibitors with target selectivity intimates an involvement of protein kinases A, C, Ca²⁺/calmodulin-dependent protein kinase II, p56 tyrosine kinase, leukotrienes and/or hydroxyeicosatetraenoic acids, phosphatidylcholine-specific phospholipase C and Ca²⁺-fluxes in events following initial binding of a galactoside-specific plant lectin to rat thymocytes which establish firm cell contacts.