THE VITRONECTIN RECEPTOR ALPHA-V-BETA-3 BINDS FIBRONECTIN AND ACTS IN CONCERT WITH ALPHA-5-BETA-1 IN PROMOTING CELLULAR ATTACHMENT AND SPREADING ON FIBRONECTIN

The vitronectin receptor (alpha-v-beta-3) is a member of the integrin superfamily of adhesive protein receptors that mediate a wide spectrum of adhesive cellular interactions, including attachment of vitronectin, von Willebrand factor, fibrinogen, and thrombospondin. We have studied the binding of fibronectin to the purified vitronectin receptor, and the role of this receptor in the attachment of cells of fibronectin. A solid-phase microtiter assay was developed to investigate the binding properties of the vitronectin receptor. Purified alpha-v-beta-3 bound fibronectin with high affinity in a saturable, divalent cation-dependent manner. Binding was inhibited by soluble vitronectin, by RGD-containing peptides, and by LM609, a monoclonal antibody against the vitronectin receptor known to inhibit the binding the adhesive proteins to alpha-v-beta-3. Immunoinhibition experiments showed that M21 human melanoma cells, which express the fibronectin receptor, alpha-5-beta-1 as well as alpha-v-beta-3, used both of these integrins to attach the spread on fibronectin. In support of this finding, M21-L cells, a variant cell line that specifically lacks alpha-v-beta-3 but expresses alpha-v-beta-1, attached and spread poorly on fibronectin. In addition, alpha-v-beta-3 from surface-labeled M21 cells was retained, and selectively eluted by RGDS from a fibronectin affinity column. These results indicate that alpha-v-beta-3 acts in concert with alpha-5-beta-1 in promoting fibronectin recognition by these cells. We conclude that fibronectin binds to the alpha-v-beta3 vitronectin receptor specifically and with high affinity, and that this interaction is biologically relevant in supporting cell adhesion to matrix proteins.