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PubMed1991

Hydrophobic properties of porcine fibronectin and its functional domains.

The relations between surface hydrophobicities and binding properties of the functional domains of porcine plasma fibronectin were investigated. Porcine plasma fibronectin as well as human plasma fibronectin was adsorbed on a hydrophobic column with butyl or phenyl ligands in the presence of 0.5 M ammonium sulfate, and recovered in a single peak by decreasing the concentration of ammonium sulfate to 0 M, indicating that both fibronectins have very high surface hydrophobicities. On digestion with thermolysin, porcine plasma fibronectin yielded five fragments (140-150, 43, 25, 17, and 14 kDa) similar to those reported for human fibronectin, although porcine fibronectin was more resistant to the digestion than human fibronectin. The three heparin-binding fragments were found to have a wide range of surface hydrophobicities, the 140-150 kDa fragment having the lowest, the 25 kDa fragment a higher, and the 14 kDa fragment the highest among all the fragments. The 43 kDa collagen-binding and 17 kDa fragments had surface hydrophobicities as high as that of fibronectin. It is noteworthy that the 43 kDa collagen-binding fragment contributes to the high surface hydrophobicity of intact fibronectin in spite of the high content of carbohydrates.

Adsorption↗

PubMed1991

[The role of fibronectin and possible participation of its proteolytic fragments in the changes in cell behavior].

The paper deals with a potential role of fibronectin proteolysis associated with plasma cell membrane receptors in the control of cell behaviour. The molecule of fibronectin contains at least 5 adhesive domains providing its interaction with cell receptors and at least 2 domains interacting with other molecules of an extracellular matrix (ECM). Different cells in various states (steady state, motion, proliferation) interact with all or some of the adhesive domains of fibronectin. Limited fibronectin proteolysis as a linking between the cell and ECM results in a change in the cell status. Limited proteolysis of cell-bound fibronectin may occur with several proteinases: 1) uPA having a receptor in the focal contact of a cell; 2) plasmin resulted from plasminogen under the action of uPA; 3) stromelysin whose synthesis is induced by fibronectin proteolytic fragments; 4) metalloproteinases secreted by some cells and involving in the hapatotactic motion of a cell over fibronectin. Proteolysis of fibronectin and other ECM molecules may be inhibited itself due to proteolysis-induced release of inhibitors via binding to fibronectin (proteasonexin) and via binding to other ECM molecules (PAI-1). The fact that there is a direct and inverse correlation in the proteolytic process associated with a fibronectin cell (and other ECM molecules) indicate that the behavior of a cell can be controlled by the mechanism of proteolytic impairment of the cell-EMC and cell-cell bonds.

Cell Adhesion↗

PubMed1990

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.

Cells, Cultured↗

PubMed1990

Immunohistochemical study of fibronectin in experimental myocardial infarction.

Light microscopic immunohistochemical studies were performed to evaluate the distribution of fibronectin in paraffin sections of p-formaldehyde-fixed normal rat hearts and the hearts of rats that had undergone ligation of the left coronary artery. A peroxidase-labeled antibody technique was used, together with appropriate immunohistochemical control procedures, for the localization of fibronectin in normal hearts and in the hearts of sham-operated animals. Fibronectin was localized in the interstitial space between myocytes, and beneath arterial, venous, and capillary endothelium. At 4 hours after coronary ligation, fibronectin was localized in a patchy fashion in the cytoplasm and interstitial space of some of the myocytes in the area supplied by the ligated vessel. At 24 hours, there was more intense, homogeneous staining in necrotic myocytes in the infarcted area and in the capillary endothelium in the border zone. At 48 hours, the intensity of staining for fibronectin was maximal in and between the necrotic myocytes in the center of the infarct and in proliferating and migrating capillaries and fibroblasts in the border zone. Similar patterns of localization were observed at 3 and 7 days after coronary ligation, but with progressive decreases in the intensity of staining. Two sources of fibronectin appeared to have contributed to these changes: plasma fibronectin diffusing through damaged blood vessels would account for the early staining observed in necrotic myocytes in the center of the infarct, whereas de novo synthesis of fibronectin by connective tissue cells and endothelial cells in sprouting capillaries would be responsible for the subsequent staining observed in viable capillaries in the border zone of the infarct. Known properties of fibronectin in vitro, combined with these in vivo observations, indicate that fibronectin may influence the thrombotic, inflammatory, angiogenic, and fibrotic processes involved in infarct healing.

Animals↗

PubMed1988

Regulation of the fibronectin receptor affinity by divalent cations.

The cell surface receptor for fibronectin is a heterodimeric membrane protein that recognizes an Arg-Gly-Asp sequence in fibronectin and that requires cations such as Mg2+ or Ca2+ for binding to fibronectin. The divalent cation requirements of this receptor were analyzed by measuring attachment of receptor liposomes to ligand-coated surfaces in the presence of different cations. The most striking effect observed was produced by Mn2+, which increased the binding of the receptor liposomes to fibronectin 2-3-fold over their binding in buffers containing Ca2+ and Mg2+. The binding activities of two related adhesion receptors, the vitronectin receptor and platelet GP IIb-IIIa, were supported but not enhanced by Mn2+. Two observations suggest that Mn2+ can compete with Ca2+ for the same cation-binding sites of the receptor. First, Mn2+ could still enhance fibronectin receptor binding activity even in the presence of 10-fold higher concentrations of Ca2+ or Mg2+. Second, Mn2+ inhibited the binding of radioactive Ca2+ to the alpha subunit of the receptor. The increased fibronectin receptor activity in the presence of Mn2+ appeared to be due to an increase in the affinity of the receptor for the Arg-Gly-Asp sequence because a 110-kDa cell attachment fragment and a synthetic hexapeptide containing the Arg-Gly-Asp sequence inhibited liposome binding more effectively in the presence of Mn2+ than in the presence of Ca2+/Mg2+. The affinity for the peptide was affected more than the affinity for the fragment, indicating that Mn2+ also induces a change in receptor specificity. Increased receptor binding in the presence of Mn2+ was also apparent in affinity chromatography of the fibronectin receptor on the 110-kDa fibronectin fragment; Mn2+ improved the yield of the receptor 4-fold. Mn2+ similarly increased the number of receptor-fibronectin complexes in preparations analyzed by electron microscopy. These results show that exogenous influences can modulate the affinity and specificity with which the fibronectin receptor binds to its ligands.

Calcium↗

PubMed1989

Effect of plasma fibronectin on the adhesive properties of human peripheral lymphocytes.

The adhesive properties of human peripheral lymphocytes (HPL) towards immunobilized substrate in vitro have been investigated. The role of plasma fibronectin was studied in two different systems: a) immobilized fibronectin as a model of cell adhesion in the tissues; b) soluble fibronectin as a model for cell adhesion in blood circulation. Collagen (Type I) or collagen-fibronectin-coated surfaces were used as substrate for lymphocyte adhesion in system b. A very small amount of HPL (5.1 +/- 1.2%) was attached to the collagen surface. The adhesion to fibronectin or to collagen-fibronectin surfaces, however, was substantially higher, reaching a value of 34.1 +/- 3.8% and 32.7 +/- 5.8%, respectively. The addition of soluble fibronectin to system a leads to a competitive inhibition of adhesion. On the contrary, in the presence of soluble fibronectin, the augmentation of the lymphocyte adhesion to collagen was found to be dose-dependent and nearly 4 times higher as compared to the control level. This effect was found to be maximal at fibronectin concentrations of 250-300 micrograms/ml. The data demonstrate the important role of plasma fibronectin in the adhesive interactions of lymphocytes.

Cell Adhesion↗

PubMed1986

The cellular origin of fibronectin in the basement membrane zone of developing tooth.

The cellular source of fibronectin in the dental epitheliomesenchymal interface was studied in interspecies combinations of mouse and quail tissue. Species-specific fibronectin antibodies were produced by immunizing rabbits with purified mouse or chicken fibronectin and by absorbing both antisera with purified heterologous fibronectin and insoluble tissue extract. The absorbed antisera to mouse and chicken fibronectin showed fluorescent staining only in mouse and chicken tissue sections, respectively, but not vice versa. When the mouse mesenchymal dental papilla was combined and cultured either with the mouse enamel organ or with the quail pharyngeal epithelium, mesenchymal cell differentiation was initiated and typical alignment of mesenchymal cells along the basement membrane was seen. Examination with transmission electron microscope revealed a typical bilaminar basal lamina with adherent fibrillar matrix on its mesenchymal aspect. Immunofluorescent localization of fibronectin with the mouse-specific fibronectin antiserum showed a brilliant staining in the mesenchymal tissue and in the basement membrane zone. When the chicken-specific fibronectin antiserum was used, no staining was detected in either tissue recombinations. We have suggested earlier that fibronectin in the dental basement membrane plays an important role during the differentiation of mesenchymal cells into odontoblasts. The present study demonstrates that fibronectin in the basement membrane of the developing tooth is produced exclusively by the differentiating mesenchymal cells.

Animals↗

PubMed1985

The interaction of plasma fibronectin with fibroblastic cells in suspension.

We have examined the interaction of soluble plasma [3H]fibronectin with fibroblastic cells in suspension. Fibronectin labeled by reductive methylation binds to baby hamster kidney cells in serum-free medium in a time-dependent manner at 4, 22, and 37 degrees C, with half-maximal binding occurring in 12-15 min at 22 degrees C. The binding is saturable and reversible. At least 90% of the cell-associated fibronectin is external to the plasma membrane, as judged by trypsin susceptibility of the bound radioactivity. Scatchard analysis of the concentration dependence of binding indicates the presence of a single class of binding sites, even at low input concentrations of fibronectin. There are approximately 5 +/- 1 X 10(5) sites/cell with an apparent dissociation constant of 8.0 +/- 0.5 X 10(-7) M; thus, the binding of soluble fibronectin to these cells is of moderate affinity. This putative fibroblast fibronectin receptor is resistant to trypsin in the presence of physiological concentrations of divalent cations but is susceptible to trypsin in the presence of 5 mM EDTA. Binding of 0.1 mg/ml [3H]fibronectin is 60-80% inhibited by 8 mg/ml unlabeled fibronectin and 95% inhibited by 1 mg/ml purified 75-kDa fibronectin cell-binding domain, but is unaffected by 1 mg/ml 44-kDa collagen-binding domain or 5 mg/ml ovalbumin. The binding parameters determined in this study further define the fibroblast cell-surface fibronectin receptor.

Animals↗

PubMed1985

Fibronectin binding to thrombin-stimulated platelets: evidence for fibrin(ogen) independent and dependent pathways.

Plasma fibronectin binds in a specific and saturable manner to thrombin-stimulated platelets. gamma-Thrombin stimulated 80% as much fibronectin binding to platelets as alpha-thrombin with conversion of less than or equal to 1% of platelet fibrinogen to fibrin. Afibrinogenemic and normal platelets bound similar quantities of fibronectin in the presence of calcium or magnesium-ethylene glycol tetra-acetic acid (EGTA). These observations indicate that fibronectin can interact with platelets without involvement of fibrin or fibrinogen. Nevertheless, two different effects of fibrin(ogen) on fibronectin binding were observed. First, exogenous fibrinogen inhibited fibronectin binding to thrombin-stimulated platelets. This inhibition was unidirectional, as fibronectin did not inhibit fibrinogen binding to ADP or thrombin-stimulated cells. Second, formaldehyde-fixed cells with surface-associated fibrin bound significant quantities of fibronectin. This interaction required calcium and did not occur on fixed cells with or without surface-bound fibrinogen. A portion of the ligand bound to fixed cells with surface-associated fibrin was modified to form a derivative with a molecular weight identical to that of the fibronectin subunit cross-linked to the alpha-chain of fibrin. This high mol wt derivative was also observed to a variable extent with living cells in the presence of magnesium or calcium but not in the presence of magnesium-EGTA. Thus, fibronectin binds to platelets by at least two mechanisms: (1) a fibrin(ogen)-independent pathway that requires divalent ions and is inhibited by exogenous fibrinogen; and (2) a fibrin-dependent pathway with an absolute calcium requirement. With nonaggregated, thrombin-stimulated platelets, the former pathway appears to predominate.

Adenosine Diphosphate↗

PubMed1982

The cooperative role of the transformation-sensitive glycoproteins, GP140 and fibronectin, in cell attachment and spreading.

The detergent-insoluble matrix of cultured human fibroblasts contains cytoskeletal and nuclear components, as well as two major, noncollagenous glycoproteins, fibronectin and GP140. These glycoproteins are stabilized by extensive intermolecular disulfide bonding. GP140, in contrast to fibronectin, is resistant to digestion with trypsin and is not cross-reactive with antisera prepared against fibronectin (Carter, W. G., and Hakomori, S. (1981) J. Biol. Chem. 256, 6953-6960). GP140 was partially purified, under nonreducing conditions, by differential extraction of trypsinized cells with sodium trichloroacetate. Alternatively, a higher yield of GP140 could be obtained under reducing conditions by extraction with urea-dithiothreitol followed by molecular sieve chromatography in the presence of sodium dodecyl sulfate and dithiothreitol. The purified GP140 contained mannose, galactose, and N-acetylglucosamine residues, totaling 2.7% of the molecule. In addition, mild periodate oxidation of GP140 followed by reduction with NaB3H4 under conditions designed to label sialic acid also labeled the peptide portion of the molecule. Amino acid analysis of GP140 detected periodate-sensitive hydroxylysine residues, as well as hydroxylproline, accounting for the periodate/NaB3H4-induced label in the peptide. These hydroxylated amino acids are major components of collagens and collagen-like proteins. The GP140 isolated under nonreducing conditions was found to induce stable cell attachment and cell spreading when coated on plastic surfaces. The cell attachment could not be inhibited with affinity purified anti-fibronectin antibodies. However, trypsinization of cells under conditions that removed surface fibronectin reduced the ability of the cells to bind to the GP140-coated surface. Metabolic labeling of cells with radioactive glucosamine during 1-h cell attachment experiments incorporated label into both fibronectin and GP140, as well as four other carbohydrate-containing components as part of a stable detergent-insoluble matrix, indicating that the cells rapidly glycosylate both fibronectin and GP140 and incorporate them into the matrix. Long term labeling and chase experiments indicated that fibronectin and GP140 in the matrix are subject to very slow turnover. Neither fibronectin nor GP140 were detectable in the detergent-insoluble matrix of SV40-transformed human fibroblasts by either metabolic or cell surface labeling. These results are consistent with the conclusion that fibronectin and GP140 may function in a cooperative manner in cell adhesion and spreading.

Amino Acids↗

PubMed1984

Bacterial adherence to fibronectin and endothelial cells: a possible mechanism for bacterial tissue tropism.

In the pathophysiology of endocarditis, bacteria must initially adhere to the endothelial surface components of the cardiac valve before invasion and colonization. The attachment of bacteria to endothelial cell surfaces is dependent on surface characteristics of both the bacteria and the endothelial cell. Fibronectin, a glycoprotein produced by endothelial cells, binds to some bacteria but not all. This report examines bacterial adherence to human endothelial cells and to fibronectin-coated surfaces. Radiolabeled Staphylococcus aureus (Cowan I strain) and viridans streptococci avidly bound to fibronectin-coated surfaces and endothelial cell monolayers. In contrast, gram-negative organisms such as Escherichia coli bound poorly to both substrates. The adherence of S. aureus was dependent on time as well as on the concentration of fibronectin or the endothelial cell number. Bacterial adherence was specific for endothelial cells or fibronectin, since none occurred to plastic or to wells coated with collagen or albumin. The binding of S. aureus to fibronectin or endothelial cells appeared dependent on a bacterial surface protein, since treatment of the bacteria with proteases markedly decreased adherence. S. aureus was not dependent on the protein A or teichoic acid content of the cell wall, but adherence was markedly decreased in bacterial strains that had a carbohydrate capsule. S. aureus pretreated with serum or purified fibronectin manifested enhanced adherence to endothelial cells, suggesting fibronectin-fibronectin interactions. Bacteria specifically attach to endothelial cells and to fibronectin-coated surfaces, which suggests that the ability of a bacterium to attach to these substrates may reflect the propensity to invade and colonize vascular tissues such as cardiac valves.

Bacterial Physiological Phenomena↗

PubMed1984

[Comparative study of the proteolytic products of fibronectins in the serum of adult and embryonic animals formed under the effects of proteinases of the spleen].

The effects of two spleen proteinases, cathepsin D and thiol proteinase I active in neutral media--on the structural properties of fibronectins from blood plasma on adult animals and their embryos were investigated. Proteinase I and cathepsin D caused rapid fragmentation of all fibronectins under study. Fibronectin from calf embryonic serum was more sensitive to proteinase I than that from adult animal serum. The molecular weight and the correlation between the proteolytic products formed under the influence of each enzyme, on the embryonic and "adult" fibronectins, are very similar but not identical. Similar results were obtained in experiments with proteolytic products of chicken serum and embryo fibronectins. Fragmentation of embryonic fibronectin occurs more rapidly than that of chicken fibronectin; the fibronectin proteolytic products differ both qualitatively and quantitatively. However, the determination of structural differences between these fibronectins is considerably hampered by the presence of protein contaminations in chicken fibronectin preparations.

Animals↗

PubMed1984

Intracellular localization of factor VIII-related antigen and fibronectin in cultured human endothelial cells: evidence for divergent routes of intracellular translocation.

Cultured human endothelial cells synthesize and secrete both fibronectin and factor VIII-related antigen (VIIIR:Ag). In immunofluorescence microscopy, intracellular fibronectin was seen diffusely perinuclearly whereas VIIIR:Ag was located both diffusely in the perinuclear cytoplasm and in distinct rod-shaped granules. These granules could, moreover, be visualized with fluorochrome-coupled Ricinus communis agglutinin I (RCA), which also stained the Golgi apparatus as a reticular juxtanuclear structure, and they were identified as Weibel-Palade bodies by immunoelectron microscopy. Puromycin treatment depleted intracellular fibronectin but did not affect the granular localization of VIIIR:Ag. A short exposure of the cells to monensin caused a juxtanuclear accumulation of fibronectin at the Golgi region whereas VIIIR:Ag only was seen in rounded cytoplasmic granules. A prolonged monensin treatment brought about a cytoplasmic accumulation of fibronectin-containing vesicles whereas VIIIR:Ag showed no accumulation and there was no codistribution between granules containing fibronectin or VIIIR:Ag. Type IV procollagen, on the other hand, was distinctly co-localized with fibronectin. In monensin-treated cells RCA mainly stained the VIIIR:Ag-containing vesicles whereas Concanavalin A (Con A) appeared to label the fibronectin-containing vesicles. Immunoelectron microscopy of these cells revealed VIIIR:Ag in some vacuolar structures and typical Weibel-Palade bodies could not be identified. Exposure of the cells to tunicamycin, on the other hand, caused a prominent cytoplasmic accumulation of VIIIR:Ag and, within 96 h, led to the disappearance of most of the VIIIR:Ag-positive granules but did not affect the intracellular distribution of fibronectin. These results, which show that metabolical inhibitors affect differently the intracellular compartmentalization of fibronectin and VIIIR:Ag, indicate, that the two glycoproteins have divergent intracellular pathways in cultured human endothelial cells.

Antigens↗

PubMed1983

Factors influencing the incorporation of fibronectin into synovial fluid cryoprotein.

Methods of synovial fluid collection and processing known to affect cryoprotein formation were examined to investigate the proposed role of fibronectin in synovial fluid cryoprecipitation. Fibronectin, a nonimmunoglobulin, noncomplement synovial fluid protein was present in all resolubilized synovial fluid cryoproteins studied. Radiolabeled fibronectin was precipitated from rheumatoid synovial fluid to a significantly greater extent (10%) than from noninflammatory (osteoarthritic) synovial fluid (2.8%), normal plasma (1.3%), or normal serum (0.5%) (p less than 0.01). Clotting of synovial fluid reduced fibronectin concentration 44% and resulted in a reduction in the amount and percent incorporation of fibronectin into cryoprotein, whereas heparinization and hyaluronidase treatment increased cryoprecipitable fibronectin. Affinity depletion of synovial fluid fibronectin resulted in loss of C1q and reduction in IgG in the cryoprotein; however, fibronectin, C1q, and IgG could not be co-eluted from affinity matrices of gelatin and protein A-Sepharose. Cryoprotein formation from pathologic synovial fluid depends in part on fibronectin and appears to involve interactions between fibronectin and fibrinogen as well as immunoglobulin complexes and complement components.

Arthritis, Rheumatoid↗

PubMed1982

The interaction of human plasma fibronectin with a subunit of the first component of complement, C1q.

Fibronectin is a normal plasma protein that enhances reticuloendothelial system functioning, and may participate in immune complex clearance. The interaction of 125I-fibronectin with human C1 and C1q in vitro was investigated by employing a highly reproducible solid-phase binding assay in microtiter wells. We demonstrated that although fibronectin does not bind to antigen-antibody complex (BSA-anti-BSA) or immune complexes containing C1, a 20-fold increase in binding was obtained when the complexes contained C1q alone. In the absence of antigen-antibody complexes, fibronectin binds to the C1q fixed to the wells in a dose-response fashion but not to intact C1. C1q in the fluid phase inhibits 85% of the fibronectin binding to immobilized C1q. The amount of fibronectin bound by immobilized C1q or gelatin is approximately equal. The binding of fibronectin to C1q could be inhibited by the restoration of C1r + C1s to the C1 macromolecular complex before the addition of fibronectin. The inhibition was dependent on the concentration of C1r + C1s and achieved a maximum of 70% at 100 micrograms/ml. This inhibition could be reversed by the removal of C1r and C1s subunits with EDTA or C1 inhibitor. Digestion of C1q with pepsin resulted in an 85% loss of fibronectin binding. It therefore appears that at least one site of fibronectin binding to C1q is in the globular portion of this complement component.

Animals↗

PubMed1983

Abnormal glycosylation of human fibronectin secreted in the presence of monensin.

Detailed studies of the effects of the ionophore monensin upon the glycosylation of secreted fibronectin have been carried out. Human fibroblasts in culture were incubated in 1 microM monensin for several hours, following which radiolabeled glucosamine or mannose was added to the cultures. Parallel incubation and labeling of control cultures were done. Labeled fibronectin was isolated from the culture media by gelatin-Sepharose chromatography, from cell surfaces by urea extraction, and from intracellular locations by cell lysis followed by immunoprecipitation. Detailed comparison of the glycopeptides released from fibronectin by pronase and of the oligosaccharides liberated by hydrazinolysis was carried out, particularly focusing on the secreted fibronectin, using gel filtration, high performance liquid chromatography, and concanavalin A chromatography, in conjunction with the use of endoglycosidase H and specific exoglycosidases. We demonstrate that fibronectin in the medium of monensin-treated cultures differs in its glycosylation pattern from the control fibronectin. High mannose oligosaccharides are abundant in the monensin-derived fibronectin, whereas the control protein contains primarily complex oligosaccharides. Monensin apparently does not alter the initial glycosylation of fibronectin since the high mannose oligosaccharides are present on both control and monensin-treated intracellular fibronectin. We suggest, therefore, that monensin, by impairing intracellular translocation through the Golgi region, allows incompletely processed forms of fibronectin to reach the cell surface and to be released into the culture medium.

Adult↗

PubMed1983

Alterations of fibronectin and reticuloendothelial phagocytic function during adaptation to experimental shock.

Fibronectin is a large molecular weight glycoprotein that participates in opsonization as well as in adhesive structural tissue support. The effect of repetitive low grade traumatic shock on circulating fibronectin, opsonic activity, and RES phagocytic function was studied in rats during the process of adaptation to trauma. Enhanced hepatic Kupffer cell phagocytic activity was observed in rats that were resistant to trauma. During the adaptation process, serum fibronectin levels, as measured by immunoassay, were altered markedly and manifested a pattern of acute deficiency early post-trauma followed by rebound and sustained elevation of fibronectin greatly in excess of control levels. At the time adaptation to trauma was achieved, the circulating fibronectin was within the normal range. In an in vitro system, livers from adapted rats exhibited increased phagocytic ability. The data suggest that Kupffer cell activation occurs during the development of adaptation to trauma and that increased hepatic RE activity accounts for the overall increase in RES clearance seen in the trauma-adapted animals. Since repetitive low grade trauma, which can lead to adaptation, will cause immunoreactive opsonic fibronectin levels to become substantially higher than normal throughout much of the duration of the adaptation protocol, it is possible that this excessive circulating fibronectin represents a means by which local conditioning of blood vessels to trauma may be achieved. Most likely, this would be due to incorporation of the circulating fibronectin into the insoluble tissue fibronectin pool, which has been recently shown to take place. The findings suggest that RES function and fibronectin may be factors influencing trauma tolerance.

Adaptation, Physiological↗

PubMed1982

Fibronectin enhances in vitro monocyte-macrophage-mediated tumoricidal activity.

Control of neoplastic proliferation reflects in part monocyte/macrophage destruction of target cells--destruction that evidently requires cell-cell interaction. We herein show it to involve the natural plasma opsonin, fibronectin. With two cultured human tumor lines--Malme melanoma and CAK-I renal carcinoma cells--addition of fibronectin, purified to homogeneity, enhances macrophage-mediated cytotoxicity 2--4-fold (p less than 0.01). Both fresh human monocytes or the U-937-cultured macrophage line become more lethal to tumor cells with added fibronectin. The fibronectin-enhanced monocyte and U-937 tumoricidal activity occurred in a dose-dependent fashion. Specificity of fibronectin's action was validated by use of affinity-purified rabbit antifibronectin antibody, which completely abated its enhancement of tumoricidal activity. Enhancement of tumoricidal activity did not occur when monocytes or U-937 were exposed to fibronectin-coated plates. However, the addition of soluble fibronectin to fibronectin-coated plates was then capable of enhancing cytocidal activity. These studies demonstrate that human fibronectin is capable of increasing both fresh and cultured human monocyte tumor-directed cytotoxicity. Fibronectin appears to be a potentially important circulating molecule that may favorably influence human monocyte tumor cell cytotoxicity.

Cell Communication↗