Inhibitory effects of MLDG-containing heterodimeric disintegrins reveal distinct structural requirements for interaction of the integrin α9ß1 with VCAM-1, tenascin-C, and osteopontin

The integrin alpha 9 beta 1 is expressed on epithelial cells, smooth muscle cells, skeletal muscle, and neutrophils and recognizes at least three distinct ligands: vascular cell adhesion molecule 1 (VCAM-1), tenascin-C, and osteopontin. The alpha 9 subunit is structurally similar to the integrin alpha 4 subunit, and alpha 9 beta 1 and alpha 4 beta 1 both recognize VCAM-1 as a ligand. We therefore examined whether the disintegrin EC3, which we have recently shown specifically inhibits the binding of alpha 4 integrins to ligands, would also be a functional inhibitor of alpha 9 beta 1. EC3 and a novel heterodimeric disintegrin that we identified, EC6, both were potent inhibitors of alpha 9 beta 1-mediated adhesion to VCAM-1 and of neutrophil migration across tumor necrosis factor-activated endothelial cells. A peptide containing a novel MLDG motif shared by both of these disintegrins also inhibited alpha 9 beta 1- and alpha 4 beta 1-mediated adhesion to VCAM-1. Surprisingly though, concentrations of EC3 that completely inhibited adhesion of alpha 9-transfected cells to VCAM-1 had little or no effect on adhesion to either of the other alpha 9 beta 1 ligands, osteopontin and tenascin-C. Furthermore, peptides AEIDGIEL and SV-VYGLR, which we have previously shown inhibit binding of alpha 9 beta 1-expressing cells to tenascin-C and osteopontin, respectively, had no effect on adhesion to VCAM-1. These data suggest that there are structurally distinct requirements for interactions of the alpha 9 beta 1 integrin with VCAM-1 and the extracellular matrix ligands osteopontin and tenascin-C.