Substrate adhesion of rat hepatocytes: on the mechanism of attachment to fibronectin

Fibronectin: a versatile gene for a versatile protein

We have isolated cDNA and genomic clones for rat fibronectins. A single gene gives rise to three different mRNA species by alternative splicing at a complex intron-exon boundary within the coding region. The fibronectins encoded by these three mRNAs differ by the insertion of different protein segments in the C-terminal heparin-binding domain. The amino acid sequences of the cell-, heparin- and fibrin-binding domains in the C-terminal third of fibronectin were deduced from the DNA sequences. The cell- and heparin-binding regions each consist of several similar repeating sequences known as type III homologies, while the fibrin-binding region comprises three repeats of a different type (type I homologies). The sequences suggest several hypotheses for the structure-function relationships of these domains of fibronectin; these hypotheses are now being tested. Determination of intron-exon boundaries within the fibronectin gene is beginning to reveal the modular structure of the gene and its relation to the repeating structure of fibronectin.

Interaction of the 70,000-mol-wt amino-terminal fragment of fibronectin with the matrix-assembly receptor of fibroblasts

Plasma fibronectin binds saturably and reversibly to substrate-attached fibroblasts and is subsequently incorporated into the extracellular matrix (McKeown-Longo, P.J., and D. F. Mosher, 1983, J. Cell Biol., 97:466-472). We examined whether fragments of fibronectin are processed in a similar way. The amino-terminal 70,000-mol-wt catheptic D fragment of fibronectin bound reversibly to cell surfaces with the same affinity as intact fibronectin but did not become incorporated into extracellular matrix. The 70,000-mol-wt fragment blocked binding of intact fibronectin to cell surfaces and incorporation of intact fibronectin into extracellular matrix. Binding of the 70,000-mol-wt fragment to cells was partially abolished by cleavage into 27,000-mol-wt heparin-binding and 40,000-mol-wt gelatin-binding fragments and more completely abolished by reduction and alkylation of disulfide bonds. Binding of the 70,000-mol-wt fragment to cells was not blocked by gelatin or heparin. When coated onto plastic, the 70,000-mol-wt fragment did not mediate attachment and spreading of suspended fibroblasts. Conversely, fibronectin fragments that had attachment and spreading activity did not block binding of exogenous fibronectin to substrate-attached cells. These results indicate that there is a cell binding site in the 70,000-mol-wt fragment that is distinct from the previously described cell attachment site and is required for assembly of exogenous fibronectin into extracellular matrix.

Characterization of a membrane-associated glycoprotein complex implicated in cell adhesion to fibronectin

We have characterized a 140-kDa glycoprotein complex purified by a monoclonal antibody and implicated in cell adhesion to the extracellular molecule fibronectin. Three major polypeptide components were purified by monoclonal antibody JG22E, which had apparent molecular weights of 155,000 (band 1), 135,000 (band 2), and 120,000 (band 3). In two-dimensional gel electrophoresis, each subunit migrated as either a broad band or a series of spots at acidic isoelectric points. After treatment with neuraminidase, the spots became focused around pH 6.2 (band 1), pH 5.6 (band 2), and pH 5.3 (band 3). These three major bands were compared by two-dimensional peptide mapping in a series of pairwise combinations and were found to be distinct proteins. In sucrose gradients, these proteins co-migrated as a complex sedimenting at approximately 8.4 S either before or after affinity purification, whereas separated subunits migrated at 4.7 to 5.8 S. Amino acid analysis revealed no detectable hydroxyproline and a composition characterized by a substantial number of cysteine residues compared to the average protein. Our results suggest that a noncovalent complex of structurally distinct glycoproteins is involved in adhesive interactions of fibronectin with cells.

The cell attachment determinant in fibronectin

Fibronectin possesses a domain that interacts with cell surfaces. The ability of fibronectin to promote cell attachment can be duplicated with a short amino acid sequence, glycyl-L-arginyl-glycyl-L-aspartyl-L-serine, taken from that domain. The tripeptide Arg-Gly-Asp appears to be irreplaceable for maintenance of the activity of this peptide, whereas the serine residue can be replaced with some, but apparently not all, possible residues. This recognition sequence, or a closely related sequence, is present in a number of proteins other than fibronectin that interact with cells. These proteins include collagens, fibrinogen, thrombin, a bacterial surface protein, and two viral proteins, as well as discoidin-I, a protein implicated in the aggregation of Dictyostelium discoideum. A similar sequence is also repeated in some, but not all, fibronectin molecules, making it possible that some fibronectin molecules have more than a single cell attachment site. Synthetic peptides constructed from sequences taken from several of these other proteins have also been shown to promote cell attachment. The tripeptide sequence may, therefore, constitute an ancient cellular recognition mechanism common to many proteins.

Proteoglycans and cell adhesion. Their putative role during tumorigenesis

In this review, evidence that proteoglycans are involved in cell adhesion and related behavior is considered, together with their putative role(s) during tumorigenesis. Proteoglycans are large, carboxylated and/or sulfated structures that interact with specific binding sites on cell surfaces. Their distribution and synthesis in tissues alter with the onset of tumorigenesis so that hyaluronic acid is generally increased and heparan sulfate decreased in the developing tumor and surrounding tissue. However, the precise role of proteoglycans during the tumorigenic process is far from clarified. Data suggest any putative roles will be related to the adhesive properties that these molecules confer to cells. Hyaluronic acid and chondroitin sulfate appear to be weakly adhesive molecules that may promote 'transformed' characteristics when they occur on cells in large amounts. These characteristics include reduced cell spreading, increased cell motility, as well as reduced contact inhibition. Consistent with such properties, neither hyaluronic acid nor chondroitin sulfate are localized in specialized adhesion sites such as focal or close contacts. In contrast, heparan sulfate is associated with increased cell-substratum adhesion and is involved in the spreading of cells onto fibronectin and other substrata. Its presence is generally associated with reduced motility and with a well-spread morphology. Unlike hyaluronate and chondroitin sulfate, heparan sulfate is found in specialized contacts. These adhesive properties of proteoglycans predict an instructive role in tumor development, and recent experiments have defined an involvement of these molecules in metastatic arrest. Additional studies utilizing invasive and metastatic tumor variants including tumor cells that employ different mechanisms to invade are required to clarify the role of proteoglycans in tumor progression.