Role of neuroblastoma tumor gangliosides in modulating alpha2beta1 integrin avidity and dynamic adhesion to type-I collagen

Integrins are an important family of alphabeta heterodimeric cell adhesion molecules mediating not only cell-cell and cell-matrix interactions but also signaling. The factors that regulate integrin affinity and avidity are poorly characterized. Gangliosides are candidates for these effects. Several lines of evidence support the possibility that changes in the composition of the local membrane environment, including the ganglioside composition, may play a role in determining the activation state of integrins. Here we investigated the effect of pathological gangliosides on platelet and tumor cell integrin alpha2beta1. Platelets adhesion to collagen was increased by atherosclerotic gangliosides in a concentration dependent manner and was found to be alpha2beta1 mediated. This enhanced binding was not due to release of preformed stores of integrin onto the surface but by clustering of the already expressed alpha2beta1 molecules resulting in increased avidity. Since platelet adhesion in flowing blood involves shear forces, we next determined if ganglioside modulation of integrin activity was operative under dynamic forces. Shear forces of 5 dynes·cm-2 simulating venous and 50 dynes·cm-2 simulating arterial flow were chosen. High shear force increased platelet adhesion 2.5 fold compared to low shear force. Pre-incubation of platelets with gangliosides increased adhesion at low shear force to a level comparable to high shear alone. The addition of gangliosides to platelets perfused at high shear force did not further increase adhesion. LAN1 cells are adherent to collagen in static assays. When cells were perfused at low and high shear force, no effect of either shear or gangliosides was observed. However when tumor cells were perfused in the presence of platelets, an increase in binding of tumor cells was observed at both 5 and 50 dynes·cm-2 compared to cells alone.;Based on these results the following model is proposed. Shed gangliosides enter circulation where they interact with and incorporate into platelet membranes resulting in integrin clustering. This promotes binding to collagen and activation of intracellular signaling resulting in platelet secretion, aggregation and platelet-cell interaction, events that promote tumor cell adhesion in blood and metastasis.