The glycoprotein fibronectin is often decreased on tumor cells and is involved in cell adhesion and migration. Its two major forms, termed plasma and cellular fibronectins, were shown to be very closely related structurally within a species, but much less between species, by immunological and peptide mapping criteria. This pattern indicates an origin from a single gene rather than a gene family. The site on these fibronectins that interacts with cells was explored using synthetic peptides from evolutionarily conserved regions. The sequence Gly-Arg-Gly-Asp-Ser was found to be a critical recognition signal in fibroblast adhesion assays. Variant peptides generally showed much less activity, with the sole exception of the short inverted tetrapeptide sequence Ser-Asp-Gly-Arg. Fibronectin peptides were also found to be functional in vivo and microinjection into amphibian or chicken embryos specifically blocked gastrulation and neural crest cell migration. These and other preliminary experiments suggest that these peptides may also inhibit tumor cell migration and invasion. Methods for directly quantitating fibronectin binding by cells were developed using tritiated fibronectin and cell-binding fragments. Fibronectin bound to cells with moderate affinity (Kd = 8 x 10-7 M) with up to 500,000 sites per cell. Synthetic peptides also competitively inhibited fibronectin binding to the cell surface, but with lower apparent affinity (Ki = 10 minus 3 to 10 minus 4 M). These results strongly suggest that other sequence information besides the critical pentapeptide sequence are needed for full binding activity. Our future objectives will be to define the elements of fibronectin primary structure besides the critical pentapeptide sequence that are involved in binding to the cell surface, to explore the generality of this adhesive recognition signal in a variety of cell-to-substrate and cell-to-cell adhesion systems, especially in malignant cells, and to establish whether in vivo treatment with fibronectin synthetic peptides can prevent tumor cell invasion and metastasis in model systems.
