Specific binding of human fibrinogen to cultured human fibroblasts. Evidence for the involvement of the E domain

Plasma-activated fibrinogen coatings onto poly(vinylidene fluoride) surface for improving biocompatibility with tissues

CO2, N2, and N2/H2 radiofrequency plasma exposure was used for functionalization of poly(vinylidene fluoride) surface aiming the fibrinogen immobilization. Fibrinogen was immobilized onto poly(vinylidene fluoride) surface using both simple plasma activation and covalent coupling. The modified surfaces have been characterized by X-ray photoelectron spectroscopy, attenuated total reflectance–Fourier transform infrared spectroscopy, near infrared–chemical imaging, atomic force microscopy, and wettability measurements, and the obtained materials were tested as supports for fibroblast cell cultures. The plasma type and the immobilization procedure have influenced the fibrinogen attachment onto the poly(vinylidene fluoride) surface, which was achieved mainly through amide bonds when using coupling agents. Covalent immobilization of fibrinogen onto poly(vinylidene fluoride) surface leads to a more stable protein-modified polymer surface. Non-cytotoxic plasma-based coating technology has the ability to covalently immobilize bioactive molecules for surface modification of some biomaterials that mainly could be achieved by the immobilization of proteins such as fibrinogen that triggers desirable cellular responses. The fibrinogen-modified poly(vinylidene fluoride) materials showed increased cell viability of fibroblasts. Cell viability was enhanced by plasma-activated fibrinogen coatings onto poly(vinylidene fluoride) surface, this being more significant if coating was linked further by a coupling reaction. Hence, they could be good candidates for biomedical applications.

Enhanced proliferation and migration of fibroblasts on the surface of fibroblast growth factor-2-loaded fibrin microthreads

Fibrin microthreads are discrete biopolymer fibers, 50-100 μm in diameter, produced from the natural extracellular matrix protein of the provisional matrix that promotes tissue regeneration in the in vivo wound healing environment. The goals of this study were to investigate the feasibility of creating fibrin microthreads containing fibroblast growth factor-2 (FGF-2), and to study the potential of a fibrin matrix to bind signaling proteins known to promote wound healing and regulate cell function in localized cellular microenvironments on scaffold surfaces. FGF-2 was loaded into fibrin microthreads in concentrations ranging from 0 to 200 ng/mL, to investigate the effect of the material on fibroblast attachment, proliferation, cellular outgrowth, and alignment. Although FGF-2-loaded microthreads did not affect fibroblast attachment, they significantly increased cellular outgrowth and proliferation relative to unloaded microthreads. The most pronounced effects were observed at day 7 of cell culture. Further, all of the fibrin microthreads promoted the alignment of fibroblasts and their cytoskeletal components along the long axis of threads, independent of the FGF-2 concentration. Ultimately, we anticipate that these fibrin microthreads will be a promising biopolymer material to promote the regeneration of injured tissues because of their mechanical stability and their matrix signaling capabilities, particularly when loaded with matrix-bound growth factors such as FGF-2.

Fibrin-induced skin fibrosis in mice deficient in tissue plasminogen activator

The deposition of fibrin is an integral part of the tissue repair process, but its persistence is also associated with a number of fibrotic conditions. This study addressed the hypothesis that reduced fibrinolysis and fibrin persistence are associated with an enhanced accumulation of collagen and the development of skin fibrosis. Decreased fibrinolysis was confirmed in fibrin gel cultures that contained human dermal fibroblasts plus the specific plasmin inhibitor alpha(2)-antiplasmin or dermal fibroblasts isolated from plasminogen activator (PA)-deficient mice. Collagen accumulation was significantly increased in the presence of inhibitor and in tPA-deficient, but not uPA-deficient, fibroblasts compared with controls. These findings were also confirmed using a skin fibrosis model in which multiple injections of fibrin were given subcutaneously to PA-deficient mice. Injection sites from tPA-deficient mice displayed significantly increased collagen levels compared with uPA-deficient mice and wild-type controls. Up-regulation of fibroblast procollagen gene expression and reduced activation of pro-MMP-1 appeared to mediate the increase in collagen by human dermal fibroblasts in the presence of alpha2-antiplasmin. These findings suggest that persistent fibrin is associated with enhanced collagen accumulation that may result in the development of fibrotic skin disorders in which reduced fibrinolysis is a feature.

Increased plasminogen activator inhibitor-1 in keloid fibroblasts may account for their elevated collagen accumulation in fibrin gel cultures

Proteolytic degradation of the provisional fibrin matrix and subsequent substitution by fibroblast-produced collagen are essential features of injury repair. Immunohistochemical studies revealed that although dermal fibroblasts of normal scars and keloids expressed both urokinase type plasminogen activator (uPA) and plasminogen activator inhibitor-1 (PAI-1), keloid fibroblasts had a much higher PAI-1 expression. In long-term three-dimensional fibrin gel cultures (the in vitro fibroplasia model), normal fibroblasts expressed moderate and modulated activity levels of uPA and PAI-1. In contrast, keloid fibroblasts expressed a persistently high level of PAI-1 and a low level of uPA. The high PAI-1 activity of keloid fibroblasts correlated with their elevated collagen accumulation in fibrin gel cultures. Substituting collagen for fibrin in the gel matrix resulted in increased uPA activity and reduced collagen accumulation of keloid fibroblasts. Furthermore, decreasing PAI-1 activity of keloid fibroblasts in fibrin gel cultures with anti-PAI-1-neutralizing antibodies also resulted in a reduction in collagen accumulation by keloid fibroblasts. Cumulatively, these results suggest that PAI-1 overexpression is a consistent feature of keloid fibroblasts both in vitro and in vivo, and PAI-1 may play a causative role in elevated collagen accumulation of keloid fibroblasts.

Thrombin Cleavage-Independent Deposition of Fibrinogen in Extracellular Matrices

Lung epithelial cells (A549) synthesize and secrete fibrinogen (FBG) in vitro when stimulated with interleukin-6 and dexamethasone. This FBG secretion is polarized in the basolateral direction, suggesting that FBG is a component of the extracellular matrix (ECM). Immunofluorescent staining of A549 cells showed a fibrillar pattern of FBG, similar to the staining detected using antibodies against the matrix constituents, collagen type IV and fibronectin (FN). The same pattern of staining was detected using antibodies against fibrinopeptides A and B, as well as with the T2G1 monoclonal antibody against the fibrin-specific epitope, β15-21. Matrix staining was unaltered in the presence of the thrombin inhibitor, hirudin, or the plasmin inhibitor, aprotinin, consistent with the interpretation that matrix deposition of FBG does not require such enzymatic action. Metabolic labeling studies confirmed that FBG secreted from A549 cells or deposited into the ECM showed no evidence of thrombin or plasmin proteolytic processing or of transglutaminase-mediated covalent cross-linking (γ-γ dimers or α-polymers). Incubation of either A549 cell-derived or purified plasma FBG with cultures of human foreskin fibroblasts resulted in FBG deposition in the ECM that colocalized with matrix fibrils containing endogenously produced FN and laminin (LN). Binding of FBG to this exogenously produced matrix was unaltered by inhibition of thrombin and plasmin action, yet also exhibited exposure of the fibrin-specific epitope, β15-21. The majority (∼70%) of newly synthesized and secreted FBG is bound to the cell surface as determined by its trypsin-sensitivity. Cell surface-bound FBG is initially deoxycholate-soluble, which, over time, becomes incorporated in the deoxycholate-insoluble ECM in a similar fashion to FN. These data suggest that matrix incorporation requires the binding of secreted FBG to cell-associated matrix assembly sites. However, unlike FN, FBG in the ECM is composed of the dimeric protamer (Aα/Bβ/γγ) and not high molecular weight polymers indicative of fibrin. This study provides evidence that deposition of FBG in both endogenous and exogenously produced matrices results in conformational changes that occur independently of thrombin cleavage. This matrix-bound FBG, on which unique cell-reactive domains are likely exposed, could augment cellular response mechanisms evoked during injury and inflammation.

Thrombin Cleavage-Independent Deposition of Fibrinogen in Extracellular Matrices

Lung epithelial cells (A549) synthesize and secrete fibrinogen (FBG) in vitro when stimulated with interleukin-6 and dexamethasone. This FBG secretion is polarized in the basolateral direction, suggesting that FBG is a component of the extracellular matrix (ECM). Immunofluorescent staining of A549 cells showed a fibrillar pattern of FBG, similar to the staining detected using antibodies against the matrix constituents, collagen type IV and fibronectin (FN). The same pattern of staining was detected using antibodies against fibrinopeptides A and B, as well as with the T2G1 monoclonal antibody against the fibrin-specific epitope, β15-21. Matrix staining was unaltered in the presence of the thrombin inhibitor, hirudin, or the plasmin inhibitor, aprotinin, consistent with the interpretation that matrix deposition of FBG does not require such enzymatic action. Metabolic labeling studies confirmed that FBG secreted from A549 cells or deposited into the ECM showed no evidence of thrombin or plasmin proteolytic processing or of transglutaminase-mediated covalent cross-linking (γ-γ dimers or α-polymers). Incubation of either A549 cell-derived or purified plasma FBG with cultures of human foreskin fibroblasts resulted in FBG deposition in the ECM that colocalized with matrix fibrils containing endogenously produced FN and laminin (LN). Binding of FBG to this exogenously produced matrix was unaltered by inhibition of thrombin and plasmin action, yet also exhibited exposure of the fibrin-specific epitope, β15-21. The majority (∼70%) of newly synthesized and secreted FBG is bound to the cell surface as determined by its trypsin-sensitivity. Cell surface-bound FBG is initially deoxycholate-soluble, which, over time, becomes incorporated in the deoxycholate-insoluble ECM in a similar fashion to FN. These data suggest that matrix incorporation requires the binding of secreted FBG to cell-associated matrix assembly sites. However, unlike FN, FBG in the ECM is composed of the dimeric protamer (Aα/Bβ/γγ) and not high molecular weight polymers indicative of fibrin. This study provides evidence that deposition of FBG in both endogenous and exogenously produced matrices results in conformational changes that occur independently of thrombin cleavage. This matrix-bound FBG, on which unique cell-reactive domains are likely exposed, could augment cellular response mechanisms evoked during injury and inflammation.

Polarized secretion of fibrinogen by lung epithelial cells

The lung epithelium has recently been identified as a novel site of fibrinogen (FBG) biosynthesis. A coordinated upregulation of A alpha, B beta, and gamma chain FBG gene transcription occurs upon stimulation of A549 lung epithelial cells with dexamethasone (DEX) and the proinflammatory mediator interleukin-6 (IL-6). Subsequently, the cells synthesize and secrete fully assembled FBG. This study addresses the polarity of such FBG secretion by A549 cells cultured on polycarbonate membrane filters. After induction with IL-6 and DEX, cells were metabolically labeled, and FBG was immunopurified from the apical and basolateral chambers. Analysis by gel electrophoresis revealed that A549 cells secreted newly synthesized FBG in a polarized manner, with the majority (80%) of FBG secreted basolaterally. Consistent with this observation, immunoelectron microscopy using Protein A-gold labeling showed FBG within secretory vesicles in close proximity to the basolateral aspect of the A549 cell membrane. Polarized secretion was microtubule-dependent since depolymerization using colchicine significantly reduced the basolateral component of secretion, causing intracellular retention of FBG. These data provide evidence that FBG is secreted by lung alveolar epithelial cells vectorially toward the basement membrane, which may reflect in vivo processes associated with local injury, inflammation, and repair mechanisms.

Human fibroblasts bind directly to fibrinogen at RGD sites through integrin alpha(v)beta3

Fibroblast migration into the blood clot initially filling a wound requires close interaction between fibroblasts and the matrix of the fibrin clot. However, very little is known about the specific receptor-ligand interactions that mediate fibroblast attachment to fibrin. Using an attachment assay developed to measure even relatively weak interactions, we demonstrate here that normal human dermal fibroblasts can attach to substrates coated with fibrinogen, fibrin, or the fibrinogen breakdown product I-9D. Fibroblast attachment to these ligands did not require the presence of fibronectin on the cell surface or as a component of the substrate. Cells treated with cycloheximide and monensin, to limit the synthesis and secretion of endogenous fibronectin, attached as well as untreated cells. The synthetic peptide GRGDS inhibited adhesion to fibrinogen, fibrin, and fibrinogen I-9D by about 60%, while the control peptide GRGES had no substantial effect. We conclude that attachment to these ligands is mediated at least partially by direct interactions between the substrates and one specific receptor, the integrin alpha(v)beta3. Affinity chromatography demonstrated that alpha(v)beta3 from detergent lysates of fibroblasts bound to a fibrinogen matrix and was eluted with EDTA. Furthermore, antibodies against the alpha(v)beta3 complex or against the alpha(v) subunit inhibited fibroblast attachment to fibrinogen and fibrin by 50-70%. An inhibitory antibody against the integrin beta1 subunit had no effect. The observation that integrin antagonists could not produce complete inhibition suggests that there may be other fibroblast cell surface proteins that can bind directly to fibrinogen.

Human fibroblast adhesion to fibrinogen

Fibrinogen and fibrin mediate the adhesion of many cell types. In this report, the adhesion sites for human dermal fibroblasts on fibrinogen are identified and characterized. Fibroblasts showed a time- and dose-dependent adhesion to fibrinogen. Using a combination of synthetic peptide mimetics, monoclonal antibodies, and recombinant fibrinogens, two major classes of adhesive sites were identified. One class was RGD-dependent and involved the RGD sites in the alpha chain of fibrinogen. alpha V integrins present on fibroblasts appeared to mediate this adhesion. Inhibition studies showed that the RGD-independent site was blocked by an ICAM-1 antagonist peptide. Furthermore, the inhibition was additive with RGD peptide inhibition and accounted for essentially all of the fibroblast adhesion. Together, these results suggest that fibroblast adhesion to fibrinogen is mediated by both alpha V integrins and ICAM-1.

Angiogenic activity of fibrin degradation products is located in fibrin fragment E

The source of angiogenic activity of fibrin degradation products has been sought in a series of experiments, applying degradation products from different types of fibrin and fibrinogen to the chick chorioallantoic membrane. The presence of platelets or fibronectin during clotting was not essential for activity, and neither was crosslinking. Fibrinogen degradation products were non-stimulatory, as was serum. Molecular sieve column chromatography indicated a range of active fragments. Admixture of active fibrin degradation products with antifibrin fragment E, but not D, antiserum neutralized activity. Preparations containing only fibrin fragment E retained activity. A commercial preparation of fibrinogen fragment E was inactive until treated with thrombin. These experiments point to fibrin fragment E being the source of angiogenic activity, with thrombin cleavage being the essential step in generating activity.

Specific binding of human fibrinogen fragment D to Aspergillus fumigatus conidia

The interaction of purified human fibrinogen with Aspergillus fumigatus conidia was investigated by immunofluorescence and electron microscopy and binding assays with radiolabeled proteins. We described the localization of the binding sites on the A. fumigatus conidia and on the fibrinogen molecule and determined the binding characteristics. Immunofluorescence revealed that the fixation of purified fibrinogen was selectively associated with conidia and suggested a role for the D domains of the fibrinogen molecule. Binding assays performed with 125I-radiolabeled proteins confirmed that binding sites were located specifically in the D domains. No reaction could be detected with fragment E. The binding of 125I-fragment D to conidia was time dependent, saturable, and specific. Scatchard analysis of the data revealed an average of 1,200 binding sites per conidium, and an apparent dissociation constant (Kd) of 2.2 x 10(-9) M was estimated. Pretreatment of the cells with proteolytic enzymes or heat abolished binding, demonstrating the protein nature of the binding sites. Ultrastructural localization of the fungal receptors was determined by transmission electron microscopy. Labeling appeared to be associated with the outer electron-dense layer of the conidial wall and progressively decreased during the germination process. Labeling of thin sections with fragment D and an antifibrinogen immune serum revealed that binding sites also lay in the inner part of the wall and in vacuoles. These results indicate the presence at the conidial surface of specific receptors for fibrinogen which could act as mediators of conidial adherence to host tissues.

Modulation of polymorphonuclear leukocyte microbicidal activity and oxidative metabolism by fibrinogen degradation products D and E

Fibrinogen degradation products (FDP) D and E are typically present in blood of patients with disseminated intravascular coagulation and related conditions in which granulocyte (PMN) defense against bacterial infection may be compromised. This study was intended to determine whether FDP modify PMN functions critical to their bactericidal activity. Incubation of human PMN and Escherichia coli with 50-100 micrograms/ml FDP did not affect phagocytosis, but reduced by greater than 90% the cells' ability to inhibit bacterial colony growth compared with control PMN incubated with albumin or fibrinogen. FDP (10-100 micrograms/ml) inhibited PMN O2- release and chemotaxis stimulated by FMLP by 17-50% (P less than 0.005) and 41% (P less than 0.01), respectively. Fragment E3, and not fragment D1, was primarily responsible for inhibition of FMLP-induced PMN O2- release. Phorbol myristate acetate (10 ng/ml), 1-oleoyl-2-acetylglycerol (10(-6) M), AA (4.2 x 10(-5) M), and zymosan-activated serum-stimulated PMN O2- release were also decreased 37-63% by FDP compared with control protein. There are at least two mechanisms by which FDP may impair PMN responses. With respect to FMLP, FDP (16-100 micrograms/ml) inhibited specific binding to the cell surface over a ligand concentration range of 1.4-85 nM [3H]FMLP. In contrast, FDP did not effect the extent of phorbol ester binding to PMN but blocked activation of protein kinase C. These data suggest that elevated plasma FDP inhibit several PMN functions critical to the bactericidal role of these inflammatory cells.

Fibrinogen is chemotactic for vascular smooth muscle cells

We studied the effect of fibrinogen on the migration of bovine aortic smooth muscle cells in culture, using a Neuro Probe 48-well micro chemotaxis chamber. Fibrinogen stimulated the migration of the cells dose-dependently at concentrations from 30 to 1000 micrograms/ml. A modified checkerboard analysis of the response demonstrated that the effect was largely chemotactic in nature. The present results suggest that fibrinogen may play an important role in the pathogenesis of arterial intimal thickening and atherosclerosis.

Fibrin induces release of von Willebrand factor from endothelial cells

Addition of fibrinogen to human umbilical vein endothelial cells in culture resulted in release of von Willebrand factor (vWf) from Weibel-Palade bodies that was temporally related to formation of fibrin in the medium. Whereas no release occurred before gelation, the formation of fibrin was associated with disappearance of Weibel-Palade bodies and development of extracellular patches of immunofluorescence typical of vWf release. Release also occurred within 10 min of exposure to preformed fibrin but did not occur after exposure to washed red cells, clot liquor, or structurally different fibrin prepared with reptilase. Metabolically labeled vWf was immunopurified from the medium after release by fibrin and shown to consist of highly processed protein lacking pro-vWf subunits. The contribution of residual thrombin to release stimulated by fibrin was minimized by preparing fibrin clots with nonstimulatory concentrations of thrombin and by inhibiting residual thrombin with hirudin or heating. We conclude that fibrin formed at sites of vessel injury may function as a physiologic secretagogue for endothelial cells causing rapid release of stored vWf.

Immunologic relationship between platelet membrane glycoprotein GPIIb/IIIa and cell surface molecules expressed by a variety of cells

A polyclonal antiserum to platelet membrane glycoprotein GPIIb/IIIa was used to detect antigenically related molecules on a diverse panel of human cells. Umbilical vein endothelial cells, erythroleukemic HEL cells, and diploid fetal lung GM1380 fibroblasts expressed GPIIb/IIIa-related molecules, as judged by immunofluorescence and immunoprecipitation of surface-labeled proteins. The GPIIb and GPIIIa subunits were both present and were of similar molecular weight in these cell types. These molecules were synthetic products of the cells, as shown by immunoprecipitation of intrinsically labeled proteins. Promyeloid U937 cells could be induced by 4 beta-phorbol 12-myristate 13-acetate to synthesize and express GPIIb/IIIa-related molecules on their cell surface. The GPIIb/IIIa-related molecules were not precisely identical in the various cell types, based on slight differences in electrophoretic mobility and their failure to react with monoclonal antibodies specific for each subunit of platelet GPIIb/IIIa. These results suggest the existence of a widely distributed family of GPIIb/IIIa-related molecules. This family of "cytoadhesins" may share a common function in cellular adhesive reactions.

A model for the role of hyaluronic acid and fibrin in the early events during the inflammatory response and wound healing

A model is presented outlining the molecular and cellular events that occur during the early stages of the wound healing process. The underlying theme is that there is a specific binding interaction between fibrin, the major clot protein, and hyaluronic acid (HA), a constituent of the wound extracellular matrix. This binding interaction, which could also be stabilized by other cross-linking components, provides the driving force to organize a three-dimensional HA matrix attached to and interdigitated with the initial fibrin matrix. The HA-fibrin matrix plays a major role in the subsequent tissue reconstruction processes. We suggest that HA and fibrin have both structural and regulatory functions at different times during the wound healing process. The concentration of HA in blood and in the initial clot is very low. This is consistent with the proposed interaction between HA and fibrin(ogen), which could interfere with either fibrinogen activation or fibrin assembly and cross-linking. We propose that an activator (e.g. derived from a plasma precursor, platelets or surrounding cells) is produced during the clotting reaction and then stimulates one or more blood cell types to synthesize and secrete HA into the fibrin matrix of the clot. We predict that HA controls the stability of the matrix by regulating the degradation of fibrin. The new HA-fibrin matrix increases or stabilizes the volume and porosity of the clot and then serves as a physical support, a scaffold through which cells trapped in the clot or cells infiltrating from the peripheral edge of the wound can migrate. The HA-fibrin matrix also actively stimulates or induces cell motility and activates and regulates many functions of blood cells, which are involved in the inflammatory response, including phagocytosis and chemotaxis. The secondary HA-fibrin matrix itself is then modified as cells continue to migrate into the wound, secreting hyaluronidase and plasminogen activator to degrade the HA and fibrin. At the same time these cells secrete collagen and glycosaminoglycans to make a more differentiated matrix. The degradation products derived from both fibrin and HA are, in turn, important regulatory molecules which control cellular functions involved in the inflammatory response and new blood vessel formation in the healing wound. The proposed model generates a number of testable experimental predictions.

Fibrinogen and fibrin: biochemistry and pathophysiology

Fibrinogen is a thrombin-coagulable glycoprotein occurring in the blood of vertebrates. The primary structure of the alpha, beta, and gamma polypeptide chains of human fibrinogen is known from amino acid and nucleic acid sequencing. The intact molecule has a trinodular, dimeric structure and is functionally bivalent. Thrombin cleaves short peptides from the amino termini of the alpha and beta chains exposing polymerization sites that are responsible for the formation of fibrin fibers and appearance of a clot. The major physiological function of fibrinogen is the formation of fibrin that binds together platelets and some plasma proteins in a hemostatic plug. In pathological situations, the network entraps large numbers of erythrocytes and leukocytes forming a thrombus that may occlude a blood vessel. Fibrinogen and fibrin are multifunctional proteins. Fibrinogen is indispensable for platelet aggregation; it also binds to several plasma proteins, however, the biological function of this interaction is not completely understood. Fibrin is an essential matrix for regulation of fibrinolysis and for facilitation of cell attachment in wound healing.

Binding and endocytosis of heparin by human endothelial cells in culture

Binding of heparin and low molecular weight heparin fragments (CY 222, Mr range 1500-8000) to human vascular endothelial cells was studied. Primary culture of human umbilical vein endothelial cells and either 125I or 3H-labeled heparin or [125I]CY 222 were used. Slow, saturable and specific binding was found. No other tested glycosaminoglycan, excepting a highly sulfated heparan fraction, was able to compete for heparin binding. Two groups of binding sites for [3H]heparin could be distinguished: one with high affinity (Kd = 0.12 microM) and another with lower affinity (Kd = 1.37 microM) and a relative large capacity of binding (1.16 X 10(7) molecules/cell) was calculated. The Kd for unlabeled heparin, as calculated from competition experiments, was 0.23 microM. Much lower affinity was calculated for unlabeled low molecular weight heparin fragments CY 222 (Kd = 4.3 microM) from competition experiments with [125I]CY 222. The binding reversibility was only partial for unfractionated heparin. Even by chasing with unlabeled compound, a fraction of 25-30% was not dissociable from endothelial cells. This fraction was much lower if incubation was carried out at 4 degrees C. The addition of basic proteins (histones) to the incubation medium greatly enhanced the undissociable binding at 37 degrees C, but not at 4 degrees C. The undissociable fraction of heparin was not available to degradation by purified microbial heparinase. These results suggest that a fraction of bound heparin is internalized by the vascular endothelium.

Interaction between fibrinogen and cultured endothelial cells. Induction of migration and specific binding

It has been suggested that fibrinogen (fg) or its physiological derivatives influence the motility and growth of endothelial cells (ECs), but direct support for this concept is still lacking. In the present study, the capacity of fg to interact with ECs and induce the migration of ECs was examined. The capacity of fg to induce EC migration was studied by means of a modification of the Boyden chamber technique. fg in the lower compartment of the chamber caused a time- and concentration-dependent migration of ECs across filters. fg present in equal concentrations above and below the filter increased EC migration, but the maximal effect invariably occurred in the presence of a gradient between the lower and the upper compartments. Trypsin or plasmin digestion of fg and preincubation of fg with Fab fragments from specific antibody completely abolished fg-induced EC migration. Dialysis of fg to eliminate small peptides that might contaminate the preparation did not modify fg-induced migration. Plasma obtained from healthy donors induced EC migration, but plasma from an afibrinogenemic patient was completely ineffective. The addition of purified fg to afibrinogenemic plasma restored plasma-induced EC migration. Plasmin degradation fragments D and E, of 100,000 and 50,000 mol wt, respectively, did not induce EC migration. However, fragment E caused dose-related inhibition of fg-induced EC migration Direct interaction of highly purified radioiodinated human fg with cultured human and bovine Ecs was observed. The binding was time dependent and plateaued at 10 min. Nonlabeled fg in a large molar excess inhibited the interaction, but unrelated proteins, including fibronectin, ovalbumin, and myoglobin, did not. Monospecific Fab fragments directed to fg inhibited binding by 38% at a 50 to 1 molar ratio whereas nonimmune Fab caused only 2% inhibition at a similar concentration. The binding of 125I-fg with ECs was saturable, and an apparent dissociation constant of 0.23 x 10(-6) M was estimated from binding isotherms. After 30 min of incubation the interaction between 125I-fg and the cells was completely reversible and displaceable by a large molar excess of unlabeled fg. Autoradiography of the display of EC-bound 125I on polyacrylamide gel showed the constitutive B beta- and gamma-chains of the fg molecule, with a partial loss of the A alpha-chain. Purified fragment E and E were tested for their capacity to inhibit fg binding. At a 1 to 400 125I-fg-to-fragment molar ratio, fragment E, which also inhibited migration, competed for binding by 44%, but fragment D was completely ineffective. These data show that fg may specifically associate with ECs and induce migration of these cells; it also appears that the structural requirement of this activity is located in the N-terminal part of the molecule.