Fibronectin and certain polypeptide regions of this adhesive glycoprotein mediate cell attachment and spreading on various substrates. We explored the theoretical prediction that this adhesive protein could become a competitive inhibitor of fibronectin-mediated processes if present in solution at appropriately high concentrations. Fibronectin function was inhibited by purified plasma fibronectin at 5-10 mg/ml, by a 75,000-dalton cell-interaction fragment of the protein at 0.5-1 mg/ml, and even by two synthetic peptides containing a conserved, hydrophilic amino acid sequence at 0.1-0.5 mg/ml . Inhibition of fibronectindependent cell spreading was dose dependent, noncytotoxic, and reversible . It was competitive in nature, since increased quantities of substrate-adsorbed fibronectin or longer incubation periods decreased the inhibition. A peptide inhibitory for fibronectin-mediated cell spreading also inhibited fibronectin-mediated attachment of cells to type I Collagen, but it did not affect concanavalin A-mediated spreading .These results demonstrate the potential of a cell adhesion molecule and its biologically active peptide fragments to act as competitive inhibitors, and they suggest that fibronectin may act by binding to a saturable cell surface receptor.Cell adhesion to other cells or to substrates is a complex process which is still poorly understood at the molecular level (e.g., references 1 and 2). Adhesive events mediated by fibronectin and other extracellular attachment proteins provide experimental systems for analyzing polypeptide domains that mediate binding and adhesive functions (3-9). For example, a region offibronectintermed the "cell-binding" region, which interacts with the cell surface to mediate cell attachment and spreading on substrates, has been identified and localized to polypeptide fragments (10, 11). Even a synthetic peptide from this region of fibronectin can mediate cell attachment to a plastic substrate (12).Theoretically, a cell adhesion protein might become a specific inhibitor of its own function if it were bound in excess to a cellular receptor. Saturation of receptors by soluble adhesive molecules might prevent receptor interactions with substrate-or cell-attached adhesive molecules . A rough analogy might be the prozone effect in immunoprecipitation, in which an excess ofantibody can saturate sites on antigens and prevent the formation ofaggregates.THE JOURNAL OF CELL BIOLOGY " VOLUME 99 JULY 1984 [29][30][31][32][33][34][35][36] We tested this theoretical model with soluble plasma fibronectin and certain peptide fragments of this glycoprotein . These molecules were found to be competitive, reversible inhibitors of fibronectin function . Our results suggest that adhesive proteins can have a dualistic nature, mediating positive or negative effects depending on their concentrations and whether they are in solution or attached to a substrate . MATERIALS AND METHODSFibronectin and Fibronectin Fragments : Plasma fibronectin was isolated from human plasma (National Institu...

We have examined the hypothesis that glycolipids might serve as receptors for the cell surface glycoprotein fibronectin using three different biological assay systems. We find that purified solubilized gangliosides inhibit fibronectin-mediated hemagglutination, cell spreading, and restoration of a normal morphologic phenotype to transformed cells. The inhibition is dose-dependent and competitive; hemagglutination by 2 micrograms/ml fibronectin is half-maximally inhibited by less than 1 microM gangliosides. The most effective ganglioside inhibitors generally contain the most sialic acid residues. The isolated oligosaccharide portions of gangliosides retain this inhibitory activity and the oligosaccharides with more sialic acid are more effective inhibitors. A series of other lipids or ganglioside constituents are either less effective or without detectable activity. The more active of these lipids are the more negatively charged phospholipids such as phosphatidyl serine and phosphatidyl inositol. Our results support the hypothesis that the "receptors" for fibronectin on the cell surface either consist of or contain gangliosides or other negatively charged lipids.

No abstract

No abstract

Fibronectin and other cell attachment proteins provide molecular models for beginning to unravel the complex interactions of the cell surface with the extracellular matrix. This area has been reviewed in considerable detail previously [I-lo]. Our brief review will therefore be selective rather than comprehensive, and it will focus on some recent generalizations about this class of proteins, as well as on recent advances in the molecular analysis of the functions of these proteins and their receptors. We shall also present various popular or provocative hypotheses and speculations about future work in the field. CLASSES AND SPECIFICITY OF ATTACHMENT PROTEINS of 1)An emerging generalization about this class of proteins is that they are composed separable functional regions, each specialized for specific binding activities (Fig. . Each appears to have one or more regions essential for binding to the cell surface. Fibronectin (Fig. 2) interacts with the fibroblast cell surface primarily through a region termed the cell-binding or cell-recognition site; recent molecular analysis of the function of this site will be discussed below. Although this site appears to be required for cell interactions with fibronectin [ 11,121, interactions at a heparin-binding domain may provide substantial strengthening of this interaction [ 131. In addition, neuronal cells may be capable of interacting with a heparin-binding domain elsewhere in the molecule in the process of axonal elongation; fibronectin-independent interactions with extracellular materials such as heparan-sulfate-containing molecules appear to be important in some aspects of this process [ 14,151.