Clustering of vitronectin and RGD peptides on microspheres leads to engagement of integrins on the luminal aspect of endothelial cell membrane

Abstract

In previous work [Conforti et al, Blood 80:437, 1992], we have shown that integrins in endothelial cells (EC) are not polarized to the basal cell membrane, but are also exposed on the apical cell surface, in contact with blood. Therefore, endothelial integrins might be available for binding circulating plasma proteins. However soluble plasma vitronectin (vn) bound very poorly to EC apical surface and this interaction was unaffected by Arg- Gly-Asp (RGD) peptides or an anti-αvβ3 serum. In contrast, beads (diameter, 4.5 μm) coupled with plasma vn associated to EC apical surface in a time- and concentration-dependent way. Addition of antibodies directed to vn, αvβ3, and RGD-containing peptides blocked the interaction of vn beads with EC. In contrast, heparin and antibodies directed to αvβ5 and β1 integrin chain had no effect. Beads coupled with Gly-Arg-Gly-Asp-Ser-Pro bound to the EC surface, but not those coupled with Gly-Arg-Gly-Glu-Ser-Pro. This interaction was blocked by αvβ3 antibodies and RGD peptides, but not by αvβ3 antibody. Overall, these results indicate that luminal αvβ3 retains its binding capacity for surface-linked vn and RGD-containing ligands, but binding is observed only when the ligand is offered in a clustered, multivalent form. We propose that when vn or RGD-containing proteins are bound to circulating cells, they can act as bridging molecules by promoting adhesion of the cells to the endothelium via apical integrins.