RECEPTOR-BINDING OF OSTEOBLAST-SPECIFIC FACTOR-1 (OSF-1 HB-GAM) TO HUMAN OSTEOSARCOMA CELLS PROMOTES CELL ATTACHMENT/

OSF-1/HB-GAM is a member of developmentally regulated growth factors a nd cytokines. High expression levels of this factor are found in diffe rent tissues, e.g., in brain and in bone. We have analyzed the biologi cal function and binding properties of natural OSF-1 to human osteobla sts. Using antibodies raised against the entire OSF-1 molecule or a sy nthetic carboxy-terminal peptide (amino acids (aa) 110-140) we have in vestigated the binding sites of rat OSF-1 on human osteoblast-like ost eosarcoma cell lines: HOS(TE85) and MG-63. Immunofluorescence microsco pic studies and flow cytometric data revealed that OSF-1 is specifical ly bound to the surface of these cells. Further characterization of th e binding sites showed that both osteosarcoma cells express two differ ent kinds of binding sites: Besides binding to a specific OSF-1 recept or, OSF-1 also significantly binds to cell surface heparan sulfates. U sing the peptide specific polyclonal antibody we show that the carboxy -terminal domain, aa 110 to 140 of OSF-1, seems to be involved in liga nd binding. Studies on the biological function of OSF-1 revealed a str ong cell attachment promoting activity in vitro. This activity is not diminished after digestion of cell surface heparan sulfates by heparin ase I and heparinase I, demonstrating that the OSF-1 receptor mediates the cell attachment of osteoblasts. Our results indicate that one bio logical function of OSF-1 is the promotion of osteoblast attachment to the extracellular bone matrix.