Extracellular matrix assembly of cell-derived and plasma-derived fibronectins by substrate-attached fibroblasts

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.     

Localization of cell surface sites involved in fibronectin fibrillogenesis

Fibronectin binding sites on cultured human fibroblasts were localized by high voltage electron microscopy using either 5- or 18-nm colloidal gold beads (Au5 or Au18) bound to intact fibronectin, the 70-kD amino- terminal fragment of fibronectin that blocks incorporation of exogenous fibronectin into extracellular matrix, or 160-180-kD fragments of fibronectin with cell adhesion and heparin-binding activities. Binding sites for Au18-fibronectin on the cell surface were localized to specific regions along the edge of the fibroblast and on retraction fibers. Au18-fibronectin complexes at these sites were initially localized in clusters that co-aligned with intracellular microfilament bundles. With longer incubations, Au18-fibronectin complexes were arranged into long fibrillar networks on the cell surface and in the extracellular space. The appearance of Au18-fibronectin in these fibrillar networks and disappearance of clusters of Au18-fibronectin suggest that Au18-fibronectin complexes are arranged into matrix at specific regions of the cell surface. Au18-70-kD fragment complexes initially had a similar distribution to Au18-fibronectin complexes. With longer incubations, Au18-70-kD fragment complexes were found in long linear arrangements on the cell surface. Double labeling experiments using Au18-70-kD fragment and Au5-160-180-kD fragments showed that the 70-kD fragment and the 160-180-kD fragments bind to different regions of the cell.     

Multiple domains are involved in the interaction of endothelial cell thrombospondin with fibronectin

Thrombospondin is a large multifunctional glycoprotein synthesized, secreted and incorporated into the extracellular matrix by several cell types in culture. It is also present in the blood platelet and is secreted following platelet activation. We have previously shown that thrombospondin co-distributes with fibronectin in the extracellular matrix and that it can bind directly to purified fibronectin. In order to elucidate the chemical aspects of thrombospondin incorporation into the extracellular matrix, we studied the interaction of endothelial cell thrombospondin and fibronectin. We find that endothelial cell thrombospondin has two distinct binding domains for fibronectin. One domain is on the 70-kDa core fragment, probably similar to that of platelet thrombospondin. The other domain is on the 27-kDa N-terminal fragment and is unique to endothelial cell thrombospondin. The dissociation constant of the intact endothelial-cell-derived molecule is 0.7 +/- 0.2 x 10(-7) M. Following fragmentation, the separate domains bind with somewhat lower affinity: the core domain binds with a Kd of 3.4 +/- 1.5 x 10(-7) M and the N-terminal domain binds with a Kd of 8.8 +/- 1.8 x 10(-7) M. Binding of the intact molecule is Ca2+-independent. By contrast, following tryptic fragmentation, binding of the 70-kDa fragment is practically lost. It can be restored, however, by removal of Ca2+, indicating that the binding site on this domain is either sequestered or becomes so following fragmentation. Heparin, which also binds to both fragments, competed with fibronectin binding to the 27-kDa fragment but not to the 70-kDa domain. The fact that heparin also competitively inhibits fibronectin binding of the intact molecule further supports sequestration of the fibronectin-binding domain on the 70-kDa core fragment. Our data suggest that endothelial-cell thrombospondin possesses two distinct binding sites for fibronectin, a low-affinity constitutively available one and a high-affinity one, possibly sequestered on the intact unbound molecule.     

Transforming growth factor beta increases cell surface binding and assembly of exogenous (plasma) fibronectin by normal human fibroblasts.

Transforming growth factor beta (TGF-beta) enhances the cell surface binding of 125I-fibronectin by cultured human fibroblasts. The effect of TGF-beta on cell surface binding was maximal after 2 h of exposure to TFG-beta and did not require epidermal growth factor or protein synthesis. The enhancement was dose dependent and was found with the 125I-labeled 70-kilodalton amino-terminal fragment of fibronectin as well as with 125I-fibronectin. Treatment of cultures with TGF-beta for 6 h resulted in a threefold increase in the estimated number of fibronectin binding sites. The increase in number of binding sites was accompanied by an increased accumulation of labeled fibronectin in detergent-insoluble extracellular matrix. The effect of TGF-beta was biphasic; after 6 h of exposure, less labeled fibronectin bound to treated cultures than to control cultures. Exposure of cells to TGF-beta for greater than 6 h caused a two- to threefold increase in the accumulation of cellular fibronectin in culture medium as detected by a quantitative enzyme-linked immunosorbent assay. The second phase of the biphasic effect and the increase in soluble cellular fibronectin were blocked by cycloheximide. Immunofluorescence staining of fibroblast cultures with antifibronectin revealed that TGF-beta caused a striking increase in fibronectin fibrils. The 70-kilodalton amino-terminal fragment of fibronectin, which blocks incorporation of fibronectin into extracellular matrix, blocked anchorage-independent growth of NRK-49F cells in the presence of epidermal growth factor. Our results show that an increase in the binding and rate of assembly of exogenous fibronectin is an early event preceding the increase in expression of extracellular matrix proteins. Such an early increase in cell surface binding of exogenous fibronectin may be a mechanism whereby TGF-beta can modify extracellular matrix characteristics rapidly after tissue injury or during embryonic morphogenesis.     

Fibronectin's amino-terminal matrix assembly site is located within the 29-kDa amino-terminal domain containing five type I repeats

Fibronectin is organized into disulfide cross-linked, insoluble pericellular matrix fibrils by fibroblasts in vitro. Two sites, the Arg-Gly-Asp-Ser-containing cell attachment domain and a site located in the first 70 kDa of fibronectin, are required for matrix assembly. The first 70 kDa of fibronectin contain two structural motifs termed type I and type II homologies, which are repeated nine and two times, respectively. Previous work has implicated the amino-terminal region and the carboxyl terminus containing three type I repeats in matrix assembly, suggesting that type I repeats possess binding activity essential for fibronectin matrix assembly. To test this hypothesis, we developed a sensitive capture immunoassay to quantify insoluble matrix fibronectin and tested a panel of fibronectin fragments, containing all of the type I repeats found in the intact protein, for their ability to inhibit matrix assembly. Only fragments containing the first five type I repeats inhibited fibronectin matrix assembly, although sequences carboxyl-terminal to this domain enhanced this activity. Additional evidence for the specific recognition of the amino-terminal type I repeats by matrix assembling cells was found when the reversible, detergent-sensitive binding of a 125I-labeled fragment containing the first five type I repeats (29 kDa) to cell monolayers was studied. Only monolayers of cell lines that incorporate fibronectin into a fibrillar matrix specifically bound 125I-labeled 29 kDa. Binding of the radiolabeled amino-terminal fragment to matrix-forming cells was inhibited by unlabeled fragments containing the first five type I repeats but not by unlabeled fragments containing the remaining seven type I repeats. Matrix assembly is therefore not a generalized property of type I repeats. Rather, a critical site is located within the first 29 kDa of fibronectin.     

Fibronectin fibril formation involves cell interactions with two fibronectin domains

Fibronectin fragments and domain-specific antibodies have been used to study the mechanism by which cells reorganize exogenous fibronectin substrata into fibrils. Fibroblasts prevented from protein synthesis, and hence not secreting endogenous fibronectin or other matrix components, reorganized exogenous fibronectin substrata into arrays resembling the matrix of normally cultured cells. Cells also formed fibrils from substrata containing mixtures of cell- and either of two different heparin-binding fibronectin fragments but not from either fragment alone. The gelatin-binding fragment alone or in conjunction with the cell-binding fragment did not promote fibril formation. Antibodies recognizing cell- and either heparin- or the gelatin-binding domains labeled fibrils formed by cells under normal culture conditions or when a substratum of intact fibronectin was used as the sole exogenous source. However, only antibodies recognizing the cell- or either heparin-binding fragment reduced fibrillogenesis from intact fibronectin substrates when added during cell spreading. These data suggest that formation of fibronectin fibrils can occur at the cell surface and that membrane components recognizing the cell- and the heparin-binding domains in fibronectin may cooperate in the assembly process     

Fibrillar organization of fibronectin is expressed coordinately with cell surface gangliosides in a variant murine fibroblast

NCTC 2071A cells, a line of transformed murine fibroblasts, grow in serum-free medium, are deficient in gangliosides, synthesize fibronectin, but do not retain and organize it on the cell surface. When the cells are exposed to exogenous gangliosides, fibrillar strands of fibronectin become attached to the cell surface. A morphologically distinct variant of NCTC 2071A cells was observed to both retain cell surface fibronectin and organize it into a fibrillar network when the cells were stained with anti-fibronectin antibodies and a fluorescent second antibody. A revertant cell type appeared to resemble the parental NCTC 2071A cells in terms of morphology and fibronectin organization. All three cell types were subjected to mild NaIO4 oxidation and reduction with KB3H4 of very high specific radioactivity in order to label the sialic acid residues of surface gangliosides. The variant had much more surface gangliosides than the parental, particularly more complex gangliosides corresponding to GM1 and GD1a. The surface gangliosides of the revertant were intermediate between the parental and the variant. By using sialidase, which hydrolyzes GD1a to GM1, and 125I-labeled cholera toxin, which binds specifically to GM1, the identity and levels of these gangliosides were confirmed in the three cell types. When variant cells were exposed to sialidase for 2 d, there appeared to be little change in fibronectin organization. Concomitant treatment of the cells with the B subunit of cholera toxin, which bound to all the surface GM1 including that generated by the sialidase, however, eliminated the fibrillar network of fibronectin. In addition, exposure of the variant cells to a 70,000-mol-wt fragment of fibronectin, which lacks the cell attachment domain but contains a matrix assembly domain, inhibited the formation of fibers. Finally, all three cell types were assayed for their ability to attach to and spread on fibronectin-coated surfaces; no significant differences were found. Our results further establish that the ability of a cell to organize fibronectin into an extracellular matrix is dependent on certain gangliosides, but they also indicate that cell adhesion to fibronectin is independent of these gangliosides. We suggest that matrix organization and cell attachment and spreading are based on separate mechanisms and that these functions are associated with different cell surface "receptors."     

Adhesion of glycosaminoglycan-deficient chinese hamster ovary cell mutants to fibronectin substrata

We have examined the role of cell surface glycosaminoglycans in fibronectin-mediated cell adhesion by analyzing the adhesive properties of Chinese hamster ovary cell mutants deficient in glycosaminoglycans. The results of our study suggest that the absence of glycosaminoglycans does not affect the initial attachment and subsequent spreading of these cells on substrata composed of intact fibronectin or a fibronectin fragment containing the primary cell-binding domain. However, in contrast to wild-type cells, the glycosaminoglycan- deficient cells did not attach to substrate composed of a heparin- binding fibronectin fragment. Furthermore, the wild-type but not the glycosaminoglycan-deficient cells formed F-actin-containing stress fibers and focal adhesions on substrata composed of intact fibronectin. We propose, therefore, that cell surface proteoglycan(s) participate in the transmembrane linking of intracellular cytoskeletal components to extracellular matrix components which occurs in focal adhesions.     

A 49-residue peptide from adhesin F1 of Streptococcus pyogenes inhibits fibronectin matrix assembly

F1 is an adhesin of Streptococcus pyogenes which binds the N-terminal 70-kDa region of fibronectin with high affinity. The fibronectin binding region of F1 is comprised of a 43-residue upstream domain and a repeat domain comprised of five tandem 37-residue sequences. We investigated the effects of these domains on the assembly of fibronectin matrix by human dermal fibroblasts, MG63 osteosarcoma cells, or fibroblasts derived from fibronectin-null stem cells. Subequimolar or equimolar concentrations of recombinant proteins containing both the upstream and repeat domains or just the repeat domain enhanced binding of fibronectin or its N-terminal 70-kDa fragment to cell layers; higher concentrations of these recombinant proteins inhibited binding. The enhanced binding did not result in greater matrix assembly and was caused by increased ligand binding to substratum. In contrast, recombinant or synthetic protein containing the 43 residues of the upstream domain and the first 6 residues from the repeat domain exhibited monophasic inhibition with an IC(50) of approximately 10 nm. Truncation of the 49-residue sequence at its N or C terminus caused loss of inhibitory activity. The 49-residue upstream sequence blocked incorporation of both endogenous cellular fibronectin and exogenous plasma fibronectin into extracellular matrix and inhibited binding of 70-kDa fragment to fibronectin-null cells in a fibronectin-free system. Inhibition of matrix assembly by the 49-mer had no effect on cell adhesion to substratum, cell growth, formation of focal contacts, or formation of stress fibers. These results indicate that the 49-residue upstream sequence of F1 binds in an inhibitory mode to N-terminal parts of exogenous and endogenous fibronectin which are critical for fibronectin fibrillogenesis.     

Factor XIII cross-linking of fibronectin at cellular matrix assembly sites

We describe the effect of activated Factor XIII (Factor XIIIa, plasma transglutaminase) on the incorporation of plasma fibronectin into extracellular matrix by cultured human fibroblasts. In the absence of added Factor XIIIa, fibronectin binds to cultured fibroblast cell layers and is assembled into disulfide-bonded multimers of the extracellular matrix. When Factor XIIIa was included in the binding medium of skin fibroblasts, accumulation of 125I-fibronectin in the deoxycholate-insoluble matrix was increased. Fibronectin accumulating in the cell layer was cross-linked into nonreducible high molecular weight aggregates. The 70-kDa amino-terminal fragment of fibronectin inhibited the binding and cross-linking of 125I-fibronectin to cell layers, whereas fibrinogen had little effect. When 125I-fibronectin was incubated with isolated matrices or with cell layers pretreated with cytochalasin B, it did not bind and could not be cross-linked by Factor XIIIa into the matrix. HT-1080 human fibrosarcoma cells bound exogenous fibronectin following treatment with dexamethasone; Factor XIIIa cross-linked the bound fibronectin and caused its efficient transfer to the deoxycholate-insoluble matrix. These results indicate that exogenous fibronectin is susceptible to Factor XIIIa-catalyzed cross-linking at cellular sites of matrix assembly. Thus, Factor XIIIa-mediated fibronectin cross-linking complements disulfide-bonded multimer formation in the stabilization of assembling fibronectin molecules and thus enhances the formation of extracellular matrix.     

Induction of fibronectin matrix assembly in human fibrosarcoma cells by dexamethasone

Previous studies have suggested that the assembly of fibronectin into the extracellular matrix of cultured fibroblasts is mediated by specific matrix assembly receptors that recognize a binding site in the amino terminus of the fibronectin molecule (McKeown-Longo, P.J., and D.F. Mosher, 1985, J. Cell Biol., 100:364-374). In the presence of dexamethasone, human fibrosarcoma cells (HT-1080) acquired the ability to specifically bind exogenous plasma fibronectin and incorporate it into a detergent-insoluble extracellular matrix. Dexamethasone-induced fibronectin binding to HT-1080 cells was time dependent, dose dependent, and inhibited by cycloheximide. Saturation binding curves indicated that dexamethasone induced the appearance of 7.7 X 10(4) matrix assembly receptors per cell. The induced receptors exhibited a dissociation constant (KD) for soluble fibronectin of 5.0 X 10(-8) M. In parallel experiments, normal fibroblasts exhibited 4.1 X 10(5) receptors (KD = 5.3 X 10(-8) M) per cell. In the presence of cycloheximide, the induced fibronectin-binding activity on HT-1080 cells returned to uninduced levels within 12 h. In contrast, fibronectin-binding activity on normal fibroblasts was stable in the presence of cycloheximide for up to 54 h. The first-order rate constant (Kt = 2.07 X 10(-4) min-1) for the transfer of receptor-bound fibronectin to extracellular matrix was four- to fivefold less than that for normal fibroblasts (Kt = 1.32 X 10(-3) min-1). Lactoperoxidase-catalyzed iodination of HT-1080 monolayers indicated that a 48,000-mol-wt cell surface protein was enhanced with dexamethasone. The results from these experiments suggest that dexamethasone induces functional matrix assembly receptors on the surface of HT-1080 cells; however, the rate of incorporation of fibronectin into the matrix is much slower than that of normal fibroblasts.     

Fibronectin receptors of mononuclear phagocytes: Binding characteristics and biochemical isolation

Fibronectin receptors on mononuclear phagocytes are involved in the localization of monocytes at inflammatory sites and in the subsequent expression of macrophage-like phenotypes. In this study, we have investigated the hypothesis that proteolytically derived fragments of fibronectin may interfere with binding of fibronectin to monocytes in the extracellular matrix. We report on the reactivity of U937 cells with an 80-kDa tryptic fragment of fibronectin which contains the cell-binding domain but lacks the gelatin/collagen-binding domain. U937 cells attached to surfaces coated with the 80-kDa fragment as well as with intact fibronectin. Preincubation of the cells with the 80-kDa fragment inhibited attachment to both surfaces while intact fibronectin had little or no inhibitory effect. The Ki for inhibition of attachment (0.5 microM) was consistent with the Kd for binding of the 3H-labeled 80-kDa fragment (0.34 microM) to U937 cells in suspension. There were 4-5 x 10(5) 80-kDa binding sites per cell. The relatively high affinity of the 80-kDa fragment for the monocyte surface permitted the isolation and characterization of fibronectin-binding proteins from U937 cells and peripheral blood monocytes by affinity chromatography. When octylglucoside lysates of lactoperoxidase iodinated cells were applied to 80-kDa-Sepharose columns, a polypeptide complex of 152/125 kDa was eluted with the synthetic peptide GRGDSPC, but not with GRGESP. This complex resolved into a single diffuse band of 144 kDa upon reduction. Binding of the protein complex to the affinity column required divalent cations. The complex bound to wheat germ agglutinin and could be specifically eluted by N-acetylglucosamine. Similar cell-surface proteins were isolated from peripheral blood monocytes.     

Influence of tumor promoter on binding and release of fibronectin added to human lung fibroblasts

The tumor promoter 12-O-tetradecanoylphorbol 13-acetate (TPA) did not alter the binding and release kinetics of externally added 125I-labeled plasma fibronectin to human lung fibroblasts in culture. Cell layer-bound plasma fibronectin was found to be chased into the medium at the same rate in tumor-promoter-treated as in non-treated cells. Unlabeled fibronectin accumulated to a much higher degree in the medium when tumor promoter was present. We conclude that TPA does not interfere with the fibronectin receptor on substrate-attached fibroblasts, but may influence intracellular fibronectin before it is bound to the extracellular matrix.     

Interactions between thrombospondin and the small proteoglycan decorin: interference with cell attachment.

Decorin, a ubiquitous small interstitial dermatan sulfate proteoglycan, interacts with several extracellular matrix components, e.g., with type I collagen and fibronectin. Using a solid phase assay it is shown that the intact proteoglycan as well as its glycosaminoglycan-free core protein exhibits with KD values of about 5 nM and 2 nM, respectively, high affinity binding also to thrombospondin. However, the polysaccharide chain was required for an interaction with Sepharose-bound thrombospondin and served itself as ligand. In light of the results of binding studies with an N-terminal heparin-binding fragment of thrombospondin it is concluded that several structural features of thrombospondin and of decorin contribute to the mutual interaction of the two macromolecules. Thrombospondin substrata allowed attachment but prevented spreading of human skin fibroblasts. The addition of decorin or of its glycosaminoglycan-free core protein led to a considerable delay of cell attachment on a thrombospondin substrate. The strength of cell attachment appeared to be reduced. These data support the antiadhesive role of decorin regardless of whether subsequent cell spreading is supported or not.     

The core protein of the matrix-associated heparan sulfate proteoglycan binds to fibronectin

The extracellular matrix of cultured human lung fibroblasts contains one major heparan sulfate proteoglycan. This proteoglycan contains a 400-kDa core protein and is structurally and immunochemically identical or closely related to the heparan sulfate proteoglycans that occur in basement membranes. Because heparitinase does not release the core protein from the matrix of cultured cells, we investigated the binding interactions of this heparan sulfate proteoglycan with other components of the fibroblast extracellular matrix. Both the intact proteoglycan and the heparitinase-resistant core protein were found to bind to fibronectin. The binding of 125I-labeled core protein to immobilized fibronectin was inhibited by soluble fibronectin and by soluble cold core protein but not by albumin or gelatin. A Scatchard plot indicates a Kd of about 2 x 10(-9) M. Binding of the core protein was also inhibited by high concentrations of heparin, heparan sulfate, or chrondroitin sulfate and was sensitive to high salt concentrations. Thermolysin fragmentation of the 125I-labeled proteoglycan yielded glycosamino-glycan-free core protein fragments of approximately 110 and 62 kDa which bound to both fibronectin and heparin columns. The core protein-binding capacity of fibronectin was very sensitive to proteolysis. Analysis of thermolytic and alpha-chymotryptic fragments of fibronectin showed binding of the intact proteoglycan and of its isolated core protein to a protease-sensitive fragment of 56 kDa which carried the gelatin-binding domain of fibronectin and to a protease-sensitive heparin-binding fragment of 140 kDa. Based on the NH2-terminal amino acid sequence analyses of the 56- and 140-kDa fragments, the core protein-binding domain in fibronectin was tentatively mapped in the area of overlap of the two fragments, carboxyl-terminally from the gelatin-binding domain, possibly in the second type III repeat of fibronectin. These data document a specific and high affinity interaction between fibronectin and the core protein of the matrix heparan sulfate proteoglycan which may anchor the proteoglycan in the matrix.     

Inhibition of binding of fibronectin to matrix assembly sites by anti- integrin (alpha 5 beta 1) antibodies

Fibroblasts have cell surface sites that mediate assembly of plasma and cellular fibronectin into the extracellular matrix. Cell adhesion to fibronectin can be mediated by the interaction of an integrin (alpha 5 beta 1) with the Arg-Gly-Asp-Ser (RGDS)-containing cell adhesion region of fibronectin. We have attempted to elucidate the role of the alpha 5 beta 1 fibronectin receptor in assembly of fibronectin in matrices. Rat monoclonal antibody mAb 13, which recognizes the integrin beta 1 subunit, completely blocked binding and matrix assembly of 125I-fibronectin as well as binding of the 125I-70-kD amino-terminal fragment of fibronectin (70 kD) to fibroblast cell layers. Fab fragments of the anti-beta 1 antibody were also inhibitory. Antibody mAb 16, which recognizes the integrin alpha 5 subunit, partially blocked binding of 125I-fibronectin and 125I-70-kD. When cell layers were coincubated with fluoresceinated fibronectin and either anti-beta 1 or anti-alpha 5, anti-beta 1 was a more effective inhibitor than anti-alpha 5 of binding of labeled fibronectin to the cell layer. Inhibition of 125I-fibronectin binding by anti-beta 1 IgG occurred within 20 min. Inhibition of 125I-fibronectin binding by anti-beta 1 Fab fragments or IgG could not be overcome with increasing concentrations of fibronectin, suggesting that anti-beta 1 and exogenous fibronectin may not compete for the same binding site. No beta 1-containing integrin bound to immobilized 70 kD. These data indicate that the beta 1 subunit plays an important role in binding and assembly of exogenous fibronectin, perhaps by participation in the organization, regeneration, or cycling of the assembly site rather than by a direct interaction with fibronectin.     

Binding of plasma fibronectin to cell layers of human skin fibroblasts

Human plasma fibronectin bound to confluent cell layers of cultured human-skin fibroblasts in two distinct pools. Initial binding of fibronectin occurred in a deoxycholate-soluble pool (Pool I). Binding in Pool I was reversible and reached a steady state after 3 h. After longer periods of incubation, fibronectin became bound in a deoxycholate-insoluble pool (Pool II). Binding in Pool II was irreversible and proceeded at a linear rate for 30 h. After 30 h of incubation, a significant proportion of fibronectin bound in Pool II was present as disulfide-bonded multimers. HT1080 cells, a human sarcoma cell line, did not bind fibronectin in either pool. Also, isolated cell matrices prepared by deoxycholate extraction did not bind fibronection. Binding of fibronectin in Pool I of normal fibroblasts occurred via specific, saturable receptors. There were 128,000 binding sites per cell, and KDiss was 3.6 X 10(-8) M. Fluorescence microscopic localization of fibronectin bound in Pool I and Pool II was performed using fluorescein-conjugated fibronectin. Fluorescent staining in Pool I was present in a punctate pattern and in short, fine fibrils. Pool II fluorescence was exclusively in coarse, dense fibrils. These data indicate that plasma fibronectin may become incorporated into the tissue extracellular matrix via specific cell-surface receptors.     

Matrix assembly sites for exogenous fibronectin are decreased on human fibroblasts after treatment with agents which increase intracellular cAMP

Studies with cultured fibroblasts have shown that plasma as well as cellular fibronectin can be organized into fibrillar structures and that this organization is mediated by sites at the cell surface. Treatment of human skin fibroblasts with cholera toxin resulted in a prompt decrease in the number of binding sites for 125I-labeled plasma fibronectin and a 125I-labeled 70-kDa amino-terminal fragment of fibronectin. This decrease was accompanied by less incorporation of labeled fibronectin into deoxycholate-insoluble extracellular matrix. Binding of 125I-fibronectin was also decreased in cultures treated with epinephrine, isoproterenol, or forskolin. These results, therefore, indicate that G proteins and the adenylate cyclase system are involved in regulation of fibronectin matrix assembly sites may be one mechanism whereby hormones or growth factors can modify extracellular matrix characteristics.     

Alignment of biologically active domains in the fibronectin molecule

Gelatin-binding material was isolated from a human plasma cryoprecipitate by affinity chromatography on gelatin-Sepharose. Individual fragments of fibronectin with Mr = 170,000, 100,000, and 80,000 and a mixture of fragments with Mr = 205,000 and 190,000 (200K fraction) were isolated from this material. These fragments reacted with antifibronectin and with antibodies to a gelatin-binding Mr = 70,000 tryptic fragment of fibronectin. They all shared the same NH2-terminal amino acid sequence. The 205K and 190K fragments bound also to heparin-Sepharose, whereas the smaller fragments did not. The 200K fraction and the 170K fragment mediated cell attachment when used to coat plastic, whereas the 100K and 80K fragments were inactive in this assay. Further digestion of the 205K and 190K fragments with chymotrypsin yielded separate sets of smaller fragments that bound to either gelatin-Sepharose or heparin-Sepharose, as well as fragments that did not show either of these binding activities but mediated cell attachment. Since the NH2-terminal ends of the 205K, 190K, 100K, and 80K fragments are the same, the results define the order of the active sites in the fibronectin molecule as gelatin-binding site, cell attachment site, and heparin-binding site.     

Factor XIIIa-mediated cross-linking of fibronectin in fibroblast cell layers. Cross-linking of cellular and plasma fibronectin and of amino-terminal fibronectin fragments

Factor XIIIa cross-links plasma fibronectin as it is being assembled into the extracellular matrix of cultured human skin fibroblasts (Barry, E. L. R., and Mosher, D. F. (1988) J. Biol. Chem. 262, 10464-10469). We have further characterized this process. Fibroblasts were metabolically labeled with proline in the presence or absence of ascorbate and Factor XIIIa. Endogenous fibronectin in the extracellular matrix was cross-linked by Factor XIIIa. There was no evidence for cross-linking of collagenous proteins. Fibro-blast cell layers were incubated with iodinated 27-kDa heparin-binding or 70-kDa collagen- and heparin-binding amino-terminal fibronectin fragments. Factor XIIa cross-linked the fragments into high molecular weight aggregates. The amounts of cross-linked fragments reaches a steady state after 1 to 2 h, whereas intact fibronectin continues to be cross-linked for 24 h. When fibroblast cell layers were pulsed with iodinated fibronectin or amino-terminal fragments and Factor XIIIa was included in the chase media, the high molecular weight aggregates were formed in a step-wise manner. The smallest cross-linking steps were to high molecular weight extracellular matrix molecules forming approximately 270-, 300-, and 440-kDa complexes for the 27-kDa fragment, 70-kDa fragment, and intact fibronectin, respectively. When iodinated fibronectin was bound to fibroblast cell layers and chased into the matrix pool in the absence of Factor XIIIa, it could also be cross-linked into high molecular weight complexes when Factor XIIIa was added to the media. These results, therefore, indicate that both cellular and plasma fibronectin and amino-terminal fragments are cross-linked specifically by Factor XIIIa, that the cross-linking is probably to other fibronectin molecules rather than to collagenous proteins, and that both assembling and assembled fibronectin are substrates for Factor XIIIa.