DIFFERENTIAL MODULATION OF CELL-ADHESION BY INTERACTION BETWEEN ADHESIVE AND COUNTER-ADHESIVE PROTEINS - CHARACTERIZATION OF THE BINDING OFVITRONECTIN TO OSTEONECTIN (BM40, SPARC)

Heparin-binding forms of vitronectin, a multifunctional adhesive glyco protein, are associated with the extracellular matrix (ECM) at differe nt locations in the body and serve to promote cell adhesion and the re gulation of pericellular proteolysis at sites of angiogenesis. In the present study we characterized the interactions of vitronectin with th e counter-adhesive protein osteonectin (also termed SPARC or BM40). Os teonectin and vitronectin were both found associated with the ECM of c ultured endothelial cells and were localized in vessel wall sections o f kidney tissue. In vitro, the heparin-binding multimeric isoform of v itronectin bound to immobilized osteonectin in a saturable manner with half-maximal binding at 30-40 nM. Preincubation of plasma vitronectin with plasminogen activator inhibitor 1 (PAI-1), which provoked multim er formation, induced the binding of vitronectin to osteonectin. Bindi ng was optimal at physiological ionic strength, and binary complexes w ere stabilized by tissue transglutaminase-mediated cross-linking. In a concentration-dependent fashion, PAI-1, CaCl2, heparin and heparan su lphate, but not other glycosaminoglycans, interfered with the binding of vitronectin to osteonectin. Using vitronectin-derived synthetic pep tides as well as mutant forms of recombinant osteonectin, we found tha t the heparin-binding region of vitronectin interacted with the C-term inal region of osteonectin that contains a high-affinity Ca2+-binding site with counter-adhesive properties. Adhesion of cultured endothelia l cells was partly abrogated by osteonectin and was correspondingly re versed by vitronectin in a concentration-dependent manner. These resul ts indicate that specific interactions between vitronectin and osteone ctin modulate cell adhesion and might thereby regulate endothelial cel l function during angiogenesis.