Does glycosylation of melanoma cells influence their interactions with fibronectin?
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Fibronectins
Fibronectins isolated fro early-passage and late-passage (in vitro aged) human fibroblasts were shown to differ in their ability to support cell adhesion and to influence cell morphology. Because fibroblast adhesion requires interactions between fibronectin, the cell surface, and the component of the extracellular matrix, we examined those functions in isolated cellular fibronectin. In comparison to fibronectin isolated from early-passage cells, fibronectin from late-passage cells bound poorly to native collagen types I and II. No differences were observed in the binding of the two fibronectins to denatured collagen. The binding of both fibronectins to native collagen was similarly promoted by heparin. Cell binding activity was evaluated by using a Boyden chamber assay to measure chemotaxis in response to either fibronectin. No differences were detected in cell binding. Comparisons of molecular weights by NaDodSO4/polyacrylamide gel electrophoresis reveals that fibronectin from late-passage cells is larger than that from early-passage cells. That difference is observed both in fibronectins isolated from conditioned media and in fibronectins isolated from the cell layer. These data support the hypothesis that late-passage cells produce a structurally and functionally distinct fibronectin. The defective binding to native collagen may account for some aspects of the aged phenotype.
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Abstract Using a previously described model system for the incorporation of plasma fibronectin into the extracellular matrix (McKeown‐Longo, P.J. and Mosher, D.F., 1985. J. Cell Biol., 100 : 364–374), we compared the binding of cell‐derived and plasma‐derived fibronectins to human fibroblast cell layers. Binding was measured in time course experiments using metabolically labeled cell‐derived, iodinated cell‐derived, and iodinated plasma‐derived fibronectins. The kinetics of matrix assembly of cell‐ and plasma‐derived fibronectins were the same. Competitive binding curves using intact fibronectin or the 70‐kD amino‐terminal fragment of fibronectin suggested that cell surface binding sites have equal affinity for cell‐ and plasma‐derived fibronectins. Iodinated fibronectins did not bind to isolated matrices containing collagen type I, fibronectin, and thrombospondin. These results suggest that fibroblasts do not distinguish between cell‐derived and plasma‐derived fibronectins when assembling exogenous fibronectin into extracellular matrix.
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Fibronectins isolated from the conditioned medium produced by cultures of undifferentiated (monolayer) and differentiated (nodular) swine vascular smooth muscle cells are similar but not identical. In general, the nodular-cell fibronectin has a smaller molecular mass than monolayer-cell fibronectin and appears to lack the COOH-terminal interchain disulfide linkage. We studied the incorporation of cellular and plasma fibronectins into the cell layer. Smooth muscle cells bound 2.5 times more monolayer-cell fibronectin than nodular-cell fibronectin. Polypeptide fragments of human plasma fibronectin were used as a model system to investigate fibronectin incorporation into the cell layer. Only intact molecules were incorporated into the cell layer and subsequently organized into fibers. Polypeptide fragments of molecular mass 205 kDa and 185 kDa were not incorporated even though they retained the collagen-, cell-, and heparin-binding regions. Incorporation appears to require an activity associated with either the NH2-terminal or COOH-terminal domains. We propose that fibronectin activity is lost during differentiation of smooth muscle cells.
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Fibronectin molecules are dimers composed of subunits whose primary structures may differ. This is due to alternative splicing in at least two regions (ED and IIICS) of the pre-mRNA. Using two monoclonal antibodies specific for two different epitopes of domain 5 (high affinity for heparin), we have quantitatively analyzed the expression of the IIICS sequence in human fibronectins from different sources. The results demonstrated that the percentage of fibronectin subunits containing the IIICS is higher in fibronectins from tumor-derived or simian virus 40-transformed human cells than in fibronectins from human plasma or normal human fibroblasts. Furthermore, we observed that 45-65% of fibronectin subunits from transformed cells or normal embryonic fibroblasts are sialylated on the heparin-binding domain 5, whereas this occurs in only 24-28% of fibronectin subunits from normal adult fibroblasts. On the contrary, no sialylation was observed on domain 5 in fibronectin from human plasma.
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Fibronectin, a structural glycoprotein on cell surfaces, appears in significant concentrations too in human blood plasma. Fibronectin influences cell interactions and cell growth and is an essential part of the extracellular matrix. The following review summarizes the biochemistry and function of fibronectin--or rather of fibronectins, bearing in mind the difference in protein structure between plasma fibronectin and cell surface fibronectin. Fibronectin is identical with one opsonin (alpha 2-opsonic glycoprotein). During shock and septicemia it undergoes a marked decrease. The diminished concentration in the plasma indicates a reduced function of the reticulo-endothelial system. A report is given on initial substitution trials.
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Exogenous plasma and endogenous cellular fibronectins on the surface of cultured fibroblasts and in extracellular matrix fibrils were colocalized by fluorescent and high voltage immunoelectron microscopy. Fibroblast cultures grown in the presence or absence of cycloheximide were incubated with exogenous plasma fibronectin labeled with fluorescein isothiocyanate. A monoclonal antibody specific for the EIIIA sequence of cellular fibronectin was used to detect cellular fibronectin. A rabbit antifluorescein antibody identified fluoresceinated plasma fibronectin. In cultures incubated in the presence of cycloheximide, plasma fibronectin was bound to the cell surface and was assembled into extracellular fibrils. In cultures grown in the absence of cycloheximide, plasma and cellular fibronectins were observed in the same matrix fibrils and in the same locations on the cell surface. There was not, however, random admixture of the two proteins.
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Immunoelectron microscopy
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Fibronectins are a group of closely related proteins that are found in body fluids and tissue extracellular matrices. Fibronectins play important roles in the maintenance of hemostasis and in the organization of developing tissues. The interaction of cells with fibronectin probably involves specific cell-sunface molecules. Two distinct receptors for fibronectin havebeen postulated. One receptor, a 140-kilodalton complex, has been identified in a variety of cell types and is believed to mediate the attachment of cells to fibronectin-coated substrata. The existence of a second receptor has been proposed but it has not been identified. This receptor may function in the assembly of soluble fibronectin into the extracellular matrix. In this paper some of the recent developments in the identification and characterization of fibronectin-binding molecules on the surfaces of eukaryotic cells are outlined.
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Cell surface receptor
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