Alternative adhesion sites in human fibrinogen for vascular endothelial cells

Fibrinogen and tumors

Elevated plasma fibrinogen (Fg) levels consistently correlate with an unfavorable prognosis in various tumor patient cohorts. Within the tumor microenvironment, aberrant deposition and expression of Fg have been consistently observed, interacting with multiple cellular receptors and thereby accentuating its role as a regulator of inflammatory processes. Specifically, Fg serves to stimulate and recruit immune cells and pro-inflammatory cytokines, thereby contributing to the promotion of tumor progression. Additionally, Fg and its fragments exhibit dichotomous effects on tumor angiogenesis. Notably, Fg also facilitates tumor migration through both platelet-dependent and platelet-independent mechanisms. Recent studies have illuminated several tumor-related signaling pathways influenced by Fg. This review provides a comprehensive summary of the intricate involvement of Fg in tumor biology, elucidating its multifaceted role and the underlying mechanisms.

Forces of interaction of red blood cells and endothelial cells at different concentrations of fibrinogen: Measurements with laser tweezers in vitro

Blood microrheology depends on the constituents of blood plasma, the interaction between blood cells resulting in red blood cell (RBC) and platelets aggregation, and adhesion of RBC, platelets and leukocytes to vascular endothelium. The main plasma protein molecule -actuator of RBC aggregation is fibrinogen. In this paper the effect of interaction between the endothelium and RBC at different fibrinogen concentrations on the RBC microrheological properties was investigated in vitro. Laser tweezers were used to measure the RBC-endothelium interaction forces. It was shown for the first time that the interaction forces between RBC and endothelium are comparable with the RBC aggregation forces, they increase with fibrinogen concentration and reach the saturation level of about 4 pN at the concentration of 4 mg/ml. These results are important for better understanding the mechanisms of RBC and endothelium interaction and developing the novel therapeutic protocols of the microrheology correction in different pathologies.

Congenital (hypo-)dysfibrinogenemia and bleeding: A systematic literature review

Ranging from bleeding to thrombosis, the clinical features of congenital fibrinogen qualitative disorders, including dysfibrinogenemia and hypodysfibrinogenemia, are highly heterogeneous. Although the associations between some specific fibrinogen mutations and the thrombotic phenotypes have been well elucidated, the underlying mechanism between fibrinogen variants and bleeding events remains underestimated. After systematically reviewing the literature of (hypo-)dysfibrinogenemia patients with bleeding phenotypes, we identified several well-characterized bleeding-related fibrinogen variants in those patients. Several possible pathomechanisms are proposed to explain the genotype-phenotype associations: 1, mutations in the NH₂-terminal portion of the Aα chain hamper fibrinogen fitting into the active site cleft of thrombin and drastically slow the conversion of fibrinogen into monomeric fibrin; 2, mutations adding new N-linked glycosylation sites introduce bulky and negatively charged carbohydrate side chains and undermine the alignment of fibrin monomers during polymerization; 3, mutations generating unpaired cysteine form extra disulfide bonds between the abnormal fibrinogen chains and produce highly branched and fragile fibrin networks; 4, truncation mutations in the fibrinogen αC regions impair the lateral fibril aggregation, as well as factor XIII crosslinking, endothelial cell and platelet binding. These established relationships between specific variants and the bleeding tendency will help manage (hypo-)dysfibrinogenemia patients to avoid adverse bleeding outcomes.

The Role of Fibrin(ogen) in Wound Healing and Infection Control

Fibrinogen, one of the most abundant plasma proteins playing a key role in hemostasis, is an important modulator of wound healing and host defense against microbes. In the current review, we address the role of fibrin(ogen) throughout the process of wound healing and subsequent tissue repair. Initially fibrin(ogen) acts as a provisional matrix supporting incoming leukocytes and acting as reservoir for growth factors. It later goes on to support re-epithelialization, angiogenesis, and fibroplasia. Importantly, removal of fibrin(ogen) from the wound is essential for wound healing to progress. We also discuss how fibrin(ogen) functions through several mechanisms to protect the host against bacterial infection by providing a physical barrier, entrapment of bacteria in fibrin(ogen) networks, and by directing immune cell function. The central role of fibrin(ogen) in defense against bacterial infection has made it a target of bacterial proteins, evolved to interact with fibrin(ogen) to manipulate clot formation and degradation for the purpose of promoting microbial virulence and survival. Further understanding of the dual roles of fibrin(ogen) in wound healing and infection could provide novel means of therapy to improve recovery from surgical or chronic wounds and help to prevent infection from highly virulent bacterial strains, including those resistant to antibiotics.

The 95RGD97 sequence on the Aα chain of fibrinogen is essential for binding to its erythrocyte receptor

Background

Erythrocyte aggregation, a cardiovascular risk factor, is increased by high plasma fibrinogen levels. Here, the effect of different fibrinogen mutations on binding to its human erythrocyte receptor was assessed in order to identify the interaction sites.

Methods

Three fibrinogen variants were tested, specifically mutated in their putative integrin recognition sites on the Aα chain (mutants D97E, D574E and D97E/D574E) and compared with wild-type fibrinogen.

Results

Atomic force microscopy-based force spectroscopy measurements showed a significant decrease both on the fibrinogen-erythrocyte binding force and on its frequency for fibrinogen with the D97E mutation, indicating that the corresponding arginine-glycine-aspartate sequence (residues 95-97) is involved in this interaction, and supporting that the fibrinogen receptor on erythrocytes has a β₃ subunit. Changes in the fibrin clot network structure obtained with the D97E mutant were observed by scanning electron microscopy.

Conclusion

These findings may lead to innovative perspectives on the development of new therapeutic approaches to overcome the risks of fibrinogen-driven erythrocyte hyperaggregation.

Atomic force microscopy as a tool to evaluate the risk of cardiovascular diseases in patients

The availability of biomarkers to evaluate the risk of cardiovascular diseases is limited. High fibrinogen levels have been identified as a relevant cardiovascular risk factor, but the biological mechanisms remain unclear. Increased aggregation of erythrocytes (red blood cells) has been linked to high plasma fibrinogen concentration. Here, we show, using atomic force microscopy, that the interaction between fibrinogen and erythrocytes is modified in chronic heart failure patients. Ischaemic patients showed increased fibrinogen-erythrocyte binding forces compared with non-ischaemic patients. Cell stiffness in both patient groups was also altered. A 12-month follow-up shows that patients with higher fibrinogen-erythrocyte binding forces initially were subsequently hospitalized more frequently. Our results show that atomic force microscopy can be a promising tool to identify patients with increased risk for cardiovascular diseases.

Specific effects of fibrinogen and the γA and γ'-chain fibrinogen variants on angiogenesis and wound healing

In a newly formed wound, the natural fibrin network provides the first temporary matrix for tissue repair. Topical application of fibrin to a new wound may improve wound healing. A matrix of the common natural γ' fibrin variant may further improve wound healing because it is expected to have a different architecture and this will influence angiogenesis, because it possesses increased thrombin and factor XIII binding and decreased platelet binding, when compared with the common γA fibrin matrix. Our objective was to determine the effect of fibrinogen and its γA and γ' variants on angiogenesis and wound healing. We used in vitro angiogenesis models and an in vivo rat full-thickness excisional wound healing model. When comparing γA and γ' fibrin in vitro, more tube-like structures were formed on day 7 in γA fibrin than in γ' fibrin (13.83±6.12 AU vs. 6.1±1.46 AU). Wounds treated with fibrin demonstrated improved healing in vivo with more perfusion (47%±3% vs. 26%±4%, p<0.01 in placebo) and higher CD34 density score (2.0±0.4 vs. 2.8±0.1, p<0.01) on day 21 with fibrin matrices when compared with placebo-treated wounds. Increased perfusion was observed in γA fibrin-treated wounds on day 21 (53%±10% vs. 41%±7% for γ' fibrin). The other parameters showed slightly improved (not significant) wound healing with γA fibrin compared with γ' fibrin matrices. In conclusion, the use of fibrin and fibrin variant matrices offers an interesting methodology to stimulate the wound healing process.

Molecular weight fibrinogen variants alter gene expression and functional characteristics of human endothelial cells

BACKGROUND

Fibrin is a temporary matrix that not only seals a wound, but also provides a temporary matrix structure for invading cells during wound healing. Two naturally occurring fibrinogen variants, high molecular weight (HMW) and low molecular weight (LMW) fibrinogen, display different properties in supporting angiogenesis in vivo and in vitro.

OBJECTIVES

This study was aimed at investigating the functional characteristics and molecular mechanisms of human microvascular endothelial cells (HMVECs) cultured on HMW and LMW fibrin matrices.

METHODS AND RESULTS

HMVECs on HMW fibrin matrices showed increased proliferation and tube formation as compared with their counterparts on unfractionated and LMW fibrin. Degradation of HMW fibrin was markedly enhanced by the presence of HMVECs, that of LMW fibrin was enhanced only slightly. However, the expression levels of fibrinolysis-regulating proteins and integrins were similar. Subsequent microarray analysis revealed that the expression of 377 genes differed significantly between HMVECs cultured on HMW fibrin and those cultured on LMW fibrin. Among these genes, UNC5B, DLL4 and the DLL4-Notch downstream targets Hey1, Hey2 and Hes1 showed increased expression in HMVECs on LMW fibrin. However, pharmacologic and genetic (DLL4 small interfering RNA) inhibition of DLL4-Notch signaling blunted rather than enhanced proliferation and tube formation by HMVECs on both fibrin variants.

CONCLUSIONS

Heterogeneity in naturally occurring fibrinogen strongly influences endothelial cell proliferation and tube formation, and causes alterations in gene expression, including that of DLL4-Notch. The higher fibrinolytic sensitivity of HMW fibrin in the presence of HMVECs contributes to increased tube formation. Although the expression of DLL4-Notch was altered, it did not explain the enhanced tube formation in HMW fibrin. This study provides new perspectives for biological and tissue engineering applications.

Fibrin matrices with affinity-based delivery systems and neurotrophic factors promote functional nerve regeneration

Glial-derived neurotrophic factor (GDNF) and nerve growth factor (NGF) have both been shown to enhance peripheral nerve regeneration following injury and target different neuronal populations. The delivery of either growth factor at the site of injury may, therefore, result in quantitative differences in motor nerve regeneration and functional recovery. In this study we evaluated the effect of affinity-based delivery of GDNF or NGF from fibrin-filled nerve guidance conduits (NGCs) on motor nerve regeneration and functional recovery in a 13 mm rat sciatic nerve defect. Seven experimental groups were evaluated consisting of GDNF or NGF and the affinity-based delivery system (DS) within NGCs, control groups excluding the DS and/or growth factor, and nerve isografts. Groups with growth factor in the conduit demonstrated equivalent or superior performance in behavioral tests and relative muscle mass measurements compared to isografts at 12 weeks. Additionally, groups with GDNF demonstrated greater specific twitch and tetanic force production in extensor digitorum longus (EDL) muscle than the isograft control, while groups with NGF produced demonstrated similar force production compared to the isograft control. Assessment of motor axon regeneration by retrograde labeling further revealed that the number of ventral horn neurons regenerating across NGCs containing GDNF and NGF DS was similar to the isograft group and these counts were greater than the groups without growth factor. Overall, the GDNF DS group demonstrated superior functional recovery and equivalent motor nerve regeneration compared to the isograft control, suggesting it has potential as a treatment for motor nerve injury.

Heparin-binding-affinity-based delivery systems releasing nerve growth factor enhance sciatic nerve regeneration

The controlled delivery of nerve growth factor (NGF) to the peripheral nervous system has been shown to enhance nerve regeneration following injury, although the effect of release rate has not been previously studied with an affinity-based delivery system (DS). The goal of this research was to determine if the binding site affinity of the DS affected nerve regeneration in vivo using nerve guidance conduits (NGCs) in a 13-mm rat sciatic nerve defect. These DSs consisted of bi-domain peptides that varied in heparin-binding affinity, heparin and NGF, which binds to heparin with moderate affinity. Eight experimental groups were evaluated consisting of NGF with DS, control groups excluding one or more components of the DS within silicone conduits and nerve isografts. Nerves were harvested 6 weeks after treatment for analysis by histomorphometry. These DSs with NGF resulted in a higher frequency of nerve regeneration compared to control groups and were similar to the nerve isograft group in measures of nerve fiber density and percent neural tissue, but not in total nerve fiber count. In addition, these DSs with NGF contained a significantly greater percentage of larger diameter nerve fibers, suggesting more mature regenerating nerve content. While there were no differences in nerve regeneration due to varying peptide affinity with these DSs, their use with NGF enhanced peripheral nerve regeneration through a NGC across a critical nerve gap.

Targeted in vivo extracellular matrix formation promotes neovascularization in a rodent model of myocardial infarction

Background

The extracellular matrix plays an important role in tissue regeneration. We investigated whether extracellular matrix protein fragments could be targeted with antibodies to ischemically injured myocardium to promote angiogenesis and myocardial repair.

Methodology/Principal Findings

Four peptides, 2 derived from fibronectin and 2 derived from Type IV Collagen, were assessed for in vitro and in vivo tendencies for angiogenesis. Three of the four peptides—Hep I, Hep III, RGD—were identified and shown to increase endothelial cell attachment, proliferation, migration and cell activation in vitro. By chemically conjugating these peptides to an anti-myosin heavy chain antibody, the peptides could be administered intravenously and specifically targeted to the site of the myocardial infarction. When administered into Sprague-Dawley rats that underwent ischemia-reperfusion myocardial infarction, these peptides produced statistically significantly higher levels of angiogenesis and arteriogenesis 6 weeks post treatment.

Conclusions/Significance

We demonstrated that antibody-targeted ECM-derived peptides alone can be used to sufficiently alter the extracellular matrix microenvironment to induce a dramatic angiogenic response in the myocardial infarct area. Our results indicate a potentially new non-invasive strategy for repairing damaged tissue, as well as a novel tool for investigating in vivo cell biology.

Genetic Link Between Cancer and Thrombosis

From the beginning of their lives, cancer cells exert a procoagulant activity in their microenvironment, which can extend systemically and become clinically evident as Trousseau's syndrome, the well-known association between tumor and thrombosis. It is becoming clear that the genetic mechanisms responsible for neoplastic transformation (activation of oncogenes such as RAS or MET, and inactivation of tumor suppressor genes such as p53 or PTEN) directly induce the expression of genes controlling hemostasis. Activation of blood coagulation results in a selective advantage for cancer cells, as fibrin provides a scaffold for anchorage and invasion, and coagulation proteins induce receptor-mediated intracellular signals promoting invasive growth. Targeting the tumor procoagulant activity can fight not only a dangerous tumor adverse effect, but also the core mechanisms of cancer onset and progression.

Single and combined effects of alphavbeta3- and alpha5beta1-integrins on capillary tube formation in a human fibrinous matrix

The fibrinous exudate of a wound or tumor stroma facilitates angiogenesis. We studied the involvement of RGD-binding integrins during tube formation in human plasma-derived fibrin clots and human purified fibrin matrices. Capillary-like tube formation by human microvascular endothelial cells in a 3D plasma-derived fibrinous matrix was induced by FGF-2 and TNF-α and depended largely on cell-bound u-PA and plasmin activities. While tube formation was minimally affected by the addition of either the αvβ3-integrin inhibiting mAb LM609 or the α5-integrin inhibiting mAb IIA1, the general RGD-antagonist echistatin completely inhibited this process. Remarkably, when αvβ3- and α5β1-integrins were inhibited simultaneously, tube formation was reduced by 78%. It was accompanied by a 44% reduction of u-PA antigen accumulation and 41% less production of fibrin degradation products. αvβ5-integrin-blocking antibodies further enhanced the inhibition by mAb LM609 and mAb IIA1 to 94%, but had no effect by themselves. αv-specific cRGD only inhibited angiogenesis when α5β1-integrin was simultaneously blocked. Endostatin mimicked the effect of α5β1-integrin and inhibited tube formation only in the presence of LM609 or cRGD (73 and 80%, respectively). Comparable results were obtained when purified fibrin matrices were used instead of the plasma-derived fibrinous matrices. These data show that blocking of tube formation in a fibrinous exudate requires the simultaneous inhibition of αvβ3- and α5β1-integrins. This may bear impact on attempts to influence angiogenesis in a fibrinous environment.

Affinity-based release of glial-derived neurotrophic factor from fibrin matrices enhances sciatic nerve regeneration

Glial-derived neurotrophic factor (GDNF) promotes both sensory and motor neuron survival. The delivery of GDNF to the peripheral nervous system has been shown to enhance regeneration following injury. In this study, we evaluated the effect of affinity-based delivery of GDNF from a fibrin matrix in a nerve guidance conduit on nerve regeneration in a 13 mm rat sciatic nerve defect. Seven experimental groups were evaluated which received GDNF or nerve growth factor (NGF) with the delivery system within the conduit, control groups excluding one or more components of the delivery system, and nerve isografts. Nerves were harvested 6 weeks after treatment for analysis by histomorphometry and electron microscopy. The use of the delivery system (DS) with either GDNF or NGF resulted in a higher frequency of nerve regeneration vs. control groups, as evidenced by a neural structure spanning the 13 mm gap. The GDNF DS and NGF DS groups were also similar to the nerve isograft group in measures of nerve fiber density, percent neural tissue and myelinated area measurements, but not in terms of total fiber counts. In addition, both groups contained a significantly greater percentage of larger diameter fibers, with GDNF DS having the largest in comparison to all groups, suggesting more mature neural content. The delivery of GDNF via the affinity-based delivery system can enhance peripheral nerve regeneration through a silicone conduit across a critical nerve gap and offers insight into potential future alternatives to the treatment of peripheral nerve injuries.

Dietary supplement with fermented soybeans, natto, improved the neurobehavioral deficits after sciatic nerve injury in rats

Clearance of fibrin and associated inflammatory cytokines by tissue-type plasminogen activator (t-PA) is related to improved regeneration in neurological disorder. The biological activity of fermented soybean (natto) is very similar to that of t-PA. We investigated the effect of the dietary supplement of natto on peripheral nerve regeneration. The peripheral nerve injury was produced by crushing the left sciatic nerve with a vessel clamp in Sprague-Dawley rats. The injured animals were fed orally either with saline or natto (16 mg/day) for seven consecutive days after injury. Increased functional outcome such as sciatic nerve functional index, angle of ankle, compound muscle action potential and conduction latency were observed in natto-treated group. Histological examination demonstrated that natto treatment improved injury-induced vacuole formation, S-100 and vessel immunoreactivities and axon loss. Oral intake of natto prolonged prothrombin time and reduced fibrinogen but did not change activated partial thromboplastin time and bleeding time. Furthermore, natto decreased injury-induced fibrin deposition, indicating a tolerant fibrinolytic activity. The treatment of natto significantly improved injury-induced disruption of blood-nerve barrier and loss of matrix component such as laminin and fibronectin. Sciatic nerve crush injury induced elevation of tumor necrosis factor alpha (TNF-alpha) production and caused apoptosis. The increased production of TNF-alpha and apoptosis were attenuated by natto treatment. These findings indicate that oral intake of natto has the potential to augment regeneration in peripheral nerve injury, possibly mediated by the clearance of fibrin and decreased production of TNF-alpha.

Acute changes of coagulation and fibrinolysis parameters after experimental thromboembolic stroke and thrombolytic therapy

Thrombolysis is the only effective pharmaceutical therapy in acute ischemic stroke in humans but has a high risk of intracerebral hemorrhage. We aimed to establish an animal model to study changes of coagulation and fibrinolytic parameters during thromboembolic ischemic stroke and thrombolysis with recombinant tissue plasminogen activator (rt-PA). We used a thromboembolic stroke model in the rat. Animals were treated with rt-PA thrombolysis (n=10) and compared with untreated (n=10), sham operated (n=10) and control animals (n=20). Coagulation parameters (APTT, PT, TT, fibrinogen, AT III, TAT) and fibrinolytic parameters (t-PA antigen concentration, t-PA activity, PAI-1 concentration, PAI activity, plasminogen, antiplasmin) were measured at two time points (2.5 and 5h after stroke induction) with a battery of commercially available test kits. We observed an (1) initiation of coagulation and inhibition of fibrinolysis by the operation procedure itself, (2) simultaneous activation of fibrinolysis and its inhibitors after stroke induction and (3) potent initiation of fibrinolysis and consumption of fibrinolysis inhibitors after rt-PA therapy of stroke. We established a model system to monitor coagulation and fibrinolysis during thrombolytic therapy of stroke in the rat. This model may be used to study the influence of these parameters on hemorrhagic stroke transformation and outcome in experimental stroke in future.

Fibrin structure and wound healing

Fibrinogen and fibrin play an important role in blood clotting, fibrinolysis, cellular and matrix interactions, inflammation, wound healing, angiogenesis, and neoplasia. The contribution of fibrin(ogen) to these processes largely depends not only on the characteristics of the fibrin(ogen) itself, but also on interactions between specific-binding sites on fibrin(ogen), pro-enzymes, clotting factors, enzyme inhibitors, and cell receptors. In this review, the molecular and cellular biology of fibrin(ogen) is reviewed in the context of cutaneous wound repair. The outcome of wound healing depends largely on the fibrin structure, such as the thickness of the fibers, the number of branch points, the porosity, and the permeability. The binding of fibrin(ogen) to hemostasis proteins and platelets as well as to several different cells such as endothelial cells, smooth muscle cells, fibroblasts, leukocytes, and keratinocytes is indispensable during the process of wound repair. High-molecular-weight and low-molecular-weight fibrinogen, two naturally occurring variants of fibrin, are important determinants of angiogenesis and differ in their cell growth stimulation, clotting rate, and fibrin polymerization characteristics. Fibrin sealants have been investigated as matrices to promote wound healing. These sealants may also be an ideal delivery vehicle to deliver extra cells for the treatment of chronic wounds.

Transglutaminase-mediated oligomerization of the fibrin(ogen) alphaC domains promotes integrin-dependent cell adhesion and signaling

Interactions of endothelial cells with fibrin(ogen) are implicated in inflammation, angiogenesis, and wound healing. Cross-linking of the fibrinogen alphaC domains with factor XIIIa generates ordered alphaC oligomers mimicking polymeric arrangement of the alphaC domains in fibrin. These oligomers and those prepared with tissue transglutaminase were used to establish a mechanism of the alphaC domain-mediated interaction of fibrin with endothelial cells. Cell adhesion and chemical cross-linking experiments revealed that oligomerization of the alphaC domains by both transglutaminases significantly increases their RGD (arginyl-glycyl-aspartate)-dependent interaction with endothelial alphaVbeta3 and to a lesser extent with alphaVbeta5 and alpha5beta1 integrins. The oligomerization promotes integrin clustering, thereby increasing cell adhesion, spreading, formation of prominent peripheral focal contacts, and integrin-mediated activation of focal adhesion kinase (FAK) and extracellular signal-regulated kinase (ERK) signaling pathways. The enhanced integrin clustering is likely caused by ordered juxtaposition of RGD-containing integrin-binding sites upon oligomerization of the alphaC domains and increased affinity of these domains for integrins. Our findings provide new insights into the mechanism of the alphaC domain-mediated interaction of endothelial cells with fibrin and imply its potential involvement in cell migration. They also suggest a new role for transglutaminases in regulation of integrin-mediated adhesion and signaling via covalent modification of integrin ligands.

Injectable fibrin scaffold improves cell transplant survival, reduces infarct expansion, and induces neovasculature formation in ischemic myocardium

OBJECTIVES

In this study, we determined whether fibrin glue improves cell transplant retention and survival, reduces infarct expansion, and induces neovasculature formation.

BACKGROUND

Current efforts in restoring the myocardium after myocardial infarction (MI) include the delivery of viable cells to replace necrotic cardiomyocytes. Cellular transplantation techniques are, however, limited by transplanted cell retention and survival within the ischemic tissue.

METHODS

The left coronary artery of rats was occluded for 17 min followed by reperfusion. One week later, bovine serum albumin (BSA), fibrin glue, skeletal myoblasts in BSA, or skeletal myoblasts in fibrin glue were injected into the infarcted area of the left ventricle. The animals were euthanized five weeks after injection, and their hearts were excised, fresh frozen, and sectioned for histology and immunohistochemistry.

RESULTS

After five weeks, the mean area covered by skeletal myoblasts in fibrin glue was significantly greater than the area covered by myoblasts injected in BSA. Myoblasts within the infarct were often concentrated around arterioles. The infarct scar size and myoblasts in the fibrin group were significantly smaller than those in the control and BSA groups. Fibrin glue also significantly increased the arteriole density in the infarct scar as compared with the control group.

CONCLUSIONS

This study indicates that fibrin glue increases cell transplant survival, decreases infarct size, and increases blood flow to ischemic myocardium. Therefore, fibrin glue may have potential as a biomaterial scaffold to improve cellular cardiomyoplasty treat and MIs.

Pathophysiologic roles of the fibrinogen gamma chain

PURPOSE OF REVIEW

Fibrinogen binds through its gamma chains to cell surface receptors, growth factors, and coagulation factors to perform its key roles in fibrin clot formation, platelet aggregation, and wound healing. However, these binding interactions can also contribute to pathophysiologic processes, including inflammation and thrombosis. This review summarizes the latest findings on the role of the fibrinogen gamma chain in these processes, and illustrates the potential for therapeutic intervention.

RECENT FINDINGS

Novel gamma chain epitopes that bind platelet integrin alpha IIbbeta3 and leukocyte integrin alphaMbeta2 have been characterized, leading to the revision of former dogma regarding the processes of platelet aggregation, clot retraction, inflammation, and thrombosis. A series of studies has shown that the gamma chain serves as a depot for fibroblast growth factor-2 (FGF-2), which is likely to play an important role in wound healing. Inhibition of gamma chain function with the monoclonal antibody 7E9 has been shown to interfere with multiple fibrinogen activities, including factor XIIIa crosslinking, platelet adhesion, and platelet-mediated clot retraction. The role of the enigmatic variant fibrinogen gamma chain has also become clearer. Studies have shown that gamma chain binding to thrombin and factor XIII results in clots that are mechanically stiffer and resistant to fibrinolysis, which may explain the association between gammaA/gamma' fibrinogen levels and cardiovascular disease.

SUMMARY

The identification of new interactions with gamma chains has revealed novel targets for the treatment of inflammation and thrombosis. In addition, several exciting studies have shown new functions for the variant gamma chain that may contribute to cardiovascular disease.

Annexin II regulates fibrin homeostasis and neoangiogenesis in vivo

A central tenet of fibrinolysis is that tissue plasminogen activator-dependent (t-PA- dependent) conversion of plasminogen to active plasmin requires the presence of the cofactor/substrate fibrin. However, previous in vitro studies have suggested that the endothelial cell surface protein annexin II can stimulate t-PA-mediated plasminogen activation in the complete absence of fibrin. Here, homozygous annexin II-null mice displayed deposition of fibrin in the microvasculature and incomplete clearance of injury-induced arterial thrombi. While these animals demonstrated normal lysis of a fibrin-containing plasma clot, t-PA-dependent plasmin generation at the endothelial cell surface was markedly deficient. Directed migration of annexin II-null endothelial cells through fibrin and collagen lattices in vitro was also reduced, and an annexin II peptide mimicking sequences necessary for t-PA binding blocked endothelial cell invasion of Matrigel implants in wild-type mice. In addition, annexin II-deficient mice displayed markedly diminished neovascularization of fibroblast growth factor-stimulated cornea and of oxygen-primed neonatal retina. Capillary sprouting from annexin II-deficient aortic ring explants was markedly reduced in association with severe impairment of activation of metalloproteinase-9 and -13. These data establish annexin II as a regulator of cell surface plasmin generation and reveal that impaired endothelial cell fibrinolytic activity constitutes a barrier to effective neoangiogenesis.

Annexin II regulates fibrin homeostasis and neoangiogenesis in vivo

A central tenet of fibrinolysis is that tissue plasminogen activator-dependent (t-PA- dependent) conversion of plasminogen to active plasmin requires the presence of the cofactor/substrate fibrin. However, previous in vitro studies have suggested that the endothelial cell surface protein annexin II can stimulate t-PA-mediated plasminogen activation in the complete absence of fibrin. Here, homozygous annexin II-null mice displayed deposition of fibrin in the microvasculature and incomplete clearance of injury-induced arterial thrombi. While these animals demonstrated normal lysis of a fibrin-containing plasma clot, t-PA-dependent plasmin generation at the endothelial cell surface was markedly deficient. Directed migration of annexin II-null endothelial cells through fibrin and collagen lattices in vitro was also reduced, and an annexin II peptide mimicking sequences necessary for t-PA binding blocked endothelial cell invasion of Matrigel implants in wild-type mice. In addition, annexin II-deficient mice displayed markedly diminished neovascularization of fibroblast growth factor-stimulated cornea and of oxygen-primed neonatal retina. Capillary sprouting from annexin II-deficient aortic ring explants was markedly reduced in association with severe impairment of activation of metalloproteinase-9 and -13. These data establish annexin II as a regulator of cell surface plasmin generation and reveal that impaired endothelial cell fibrinolytic activity constitutes a barrier to effective neoangiogenesis.

New cell attachment peptide sequences from conserved epitopes in the carboxy termini of fibrinogen

Fibrinogen seems to contribute significantly to cell binding and recruitment into wounds besides its major role in clot formation. We describe 19- to 21-mer cell-binding (haptotactic) peptides from the C-termini of fibrinogen beta-chain (Cbeta), the extended alphaE chain, and near the C-terminal of the gamma-chain. When these peptides were covalently bound to a biologically inert matrix such as Sepharose beads (SB), they elicited beads attachment to cells, mostly of mesenchymal origin (including fibroblasts, endothelial cells, and smooth muscle cells) as well as some transformed cell lines. Based on such haptotactic activity, these peptides were termed "haptides." By contrast, peptides homologous to fibrinogen C-termini alpha- and gamma-chains elicited no such activity. The haptide Cbeta could not block the interaction of fibroblasts with antibodies directed against integrins beta(1), alpha(v), alpha(v)beta(1), alpha(v)beta(3), and alphaIIbeta(3). Moreover, GRGDS peptide could not inhibit enhanced cell binding to SB-Cbeta, as expected from an integrin-mediated process. In soluble form the haptides were accumulated in cells with nonsaturable kinetics without any toxic or proproliferative effects in concentrations up to 80 microM. These findings suggest that the conserved haptidic sequences within fibrin(ogen) can be associated with the adhesion and migration of cells into fibrin clots and may have a significant role in normal wound healing and in various pathological conditions.

Fibrin is a regulator of Schwann cell migration after sciatic nerve injury in mice

Fibrin, derived from the blood protein fibrinogen, is deposited in the sciatic nerve after injury and retards functional regeneration. Since Schwann cell migration is critical for remyelination of injured nerves, we investigated the effects of fibrin in this process. In vivo experiments showed that fibrin co-localizes with fibronectin deposition in the injured sciatic nerve. In vitro migration assays demonstrated that fibrin alone is not a permissive substrate for Schwann cell migration. Furthermore, migration assays of Schwann cells on mixed fibrin/fibronectin substrates showed that fibrin has a dose dependent inhibitory effect on Schwann cell migration on fibronectin. Our results show that fibrin, deposited in the sciatic nerve after injury, changes the composition of the extracellular matrix and inhibits Schwann cell migration. This negative effect of fibrin should be considered in the therapeutic application of biomaterials based on fibrin matrices.

Fibrinogen induces chemotactic activity in endothelial cells

Chemotaxis of blood monocytes into the vessel wall together with the change of the relative content of the extracellular matrix (ECM) proteins at sites of predilection is an early cellular marker of atherogenesis. To examine the influence of ECM proteins on secretion of chemoattractants by endothelial cells (EC), porcine EC were seeded on gelatin (G), fibronectin (Fn) and fibrinogen (Fg). After 24 h cells seeded on G and Fn showed the histiotypic 'cobblestone'-morphology whereas cells seeded on Fg did not. Chemotactic activity for monocytes in supernatants from cells seeded on Fg was more than two-fold higher compared with G and was independent of soluble Fn or Fg in the supernatant. Quantification of monocyte chemoattracting protein-1, PDGF-AB and IL-8 in EC supernatants showed that Fg led to a significant increase in secretion of all three proteins compared with cells cultured on G. Preincubation of porcine EC with the tripeptide arginine-glycine-aspartic acid, as inhibitor of binding of Fg to integrin receptors, but not with the control tripeptide arginine-glycine-glutamic acid showed a decrease in chemotactic activity for cells cultured on Fg but not on Fn or G. Inhibition of protein kinase C (PKC) activity in EC by GF109203 resulted in a decrease of fibrinogen-induced chemotactic activity. Also the tyrosine-kinase inhibitor herbimycin inhibited fibrinogen mediated secretion of chemokines. The role of the PKC pathway for matrix mediated signal transduction is further corroborated by Fg-dependent induction of the PKC isoform delta. These data indicate an integrin-dependent signal transduction pathway leading to induction of chemotactic activity by the ECM protein fibrinogen. This mechanism may contribute to induction of chemokines in early atherosclerotic lesions.

Fibrin inhibits peripheral nerve remyelination by regulating Schwann cell differentiation

Remyelination is a critical step for functional nerve regeneration. Here we show that fibrin deposition in the peripheral nervous system after injury is a key regulator of remyelination. After sciatic nerve crush, fibrin is deposited and its clearance correlates with remyelination. Fibrin induces phosphorylation of ERK1/2 and production of p75 NGF low-affinity receptor in Schwann cells and maintains them in a nonmyelinating state, suppresses fibronectin production, and prevents synthesis of myelin proteins. In mice depleted of fibrin(ogen), remyelination of myelinated axons is accelerated due to the faster transition of the Schwann cells to a myelinating state. Regulation of fibrin clearance and/or deposition could be a key regulatory mechanism for Schwann differentiation after nerve damage.

The hemostatic system and angiogenesis in malignancy

Coagulopathy and angiogenesis are among the most consistent host responses associated with cancer. These two respective processes, hitherto viewed as distinct, may in fact be functionally inseparable as blood coagulation and fibrinolysis, in their own right, influence tumor angiogenesis and thereby contribute to malignant growth. In addition, tumor angiogenesis appears to be controlled through both standard and non-standard functions of such elements of the hemostatic system as tissue factor, thrombin, fibrin, plasminogen activators, plasminogen, and platelets. "Cryptic" domains can be released from hemostatic proteins through proteolytic cleavage, and act systemically as angiogenesis inhibitors (e.g., angiostatin, antiangiogenic antithrombin III aaATIII). Various components of the hemostatic system either promote or inhibit angiogenesis and likely act by changing the net angiogenic balance. However, their complex influences are far from being fully understood. Targeted pharmacological and/or genetic inhibition of pro-angiogenic activities of the hemostatic system and exploitation of endogenous angiogenesis inhibitors of the angiostatin and aaATIII variety are under study as prospective anti-cancer treatments.

Role of fibrin matrix in angiogenesis

Angiogenesis, the formation of new blood vessels from existing vessels, plays an important role during development. In the adult, it is limited to the female reproductive system and to tissue repair and pathological conditions. Repair associated angiogenesis is usually accompanied by the presence of inflammatory cells, vascular leakage, and fibrin deposition. The temporary fibrin matrix acts, not only as a sealing matrix, but also as a scaffolding for invading leukocytes and endothelial cells during tissue repair. We have used a three-dimensional fibrin matrix to study the outgrowth of human microvascular endothelial cells in capillary-like tubular structures. This process is induced by the simultaneous addition of an angiogenic growth factor (bFGF or VEGF) and the cytokine TNF alpha, and is enhanced by hypoxia. It involves proteolytic activities, in particular cell bound urokinase/plasmin and matrix metalloproteinase activities. Modulation of the fibrin structure markedly affects the extent and stability of capillary tube formation in vitro. Preparation of fibrin at different pH (7.0-7.8) or crosslinking of the fibrin matrix induces differences in fibrin matrix rigidity and structure. This is accompanied by a change in capillary ingrowth. Heparins, in particular low molecular weight heparins, modulate the fibrin structure and by this action affect angiogenesis in vitro. A mutant fibrinogenNieuwegein, which lacks the terminal part of the A alpha chain of fibrin harboring an RGD sequence and the transglutaminase sequence, provided additional evidence that the structure of fibrin is an important determinant for angiogenesis. These findings may have impact on improving wound healing and on influencing angiogenesis in malignancies with a fibrinous stroma.

Aberrant fibrin formation and cross-linking of fibrinogen Nieuwegein, a variant with a shortened Aalpha-chain, alters endothelial capillary tube formation

A congenital dysfibrinogenemia, fibrinogen(Nieuwegein), was discovered in a young man without any thromboembolic complications or bleeding. A homozygous insertion of a single nucleotide (C) in codon Aalpha 453 (Pro) introduced a stop codon at position 454, which resulted in the deletion of the carboxyl-terminal segment Aalpha 454-610. The ensuing unpaired cysteine at Aalpha 442 generated fibrinogen-albumin complexes of different molecular weights. The molecular abnormalities of fibrinogen(Nieuwegein) led to a delayed clotting and a fibrin network with a low turbidity. Electron microscopy confirmed that thin fibrin bundles were organized in a fine network. The use of fibrinogen(Nieuwegein)-derived fibrin (fibrin(Nieuwegein)) in an in vitro angiogenesis model resulted in a strong reduction of tube formation. The ingrowth of human microvascular endothelial cells (hMVEC) was independent of alpha(v)beta(3), indicating that the reduced ingrowth is not due to the absence of the RGD-adhesion site at position Aalpha 572-574. Rather, the altered structure of fibrin(Nieuwegein) is the cause, since partial normalization of the fibrin network by lowering the pH during polymerization resulted in an increased tube formation. Whereas factor XIIIa further decreased the ingrowth of hMVEC in fibrin(Nieuwegein), tissue transglutaminase (TG), which is released in areas of vessel injury, did not. This is in line with the absence of the cross-linking site for TG in the alpha-chains of fibrinogen(Nieuwegein). In conclusion, this newly discovered congenital dysfibrinogenemia has a delayed clotting time and leads to the formation of an altered fibrin structure, which could not be cross-linked by TG and which is less supportive for ingrowth of endothelial cells.

Expression of antisense to integrin subunit beta 3 inhibits microvascular endothelial cell capillary tube formation in fibrin

alpha(v)beta(3) antagonists are potent angiogenesis inhibitors, and several different classes of inhibitors have been developed, including monoclonal antibodies, synthetic peptides, and small organic molecules. However, each class of inhibitor works by the same principal, by blocking the binding of ligands to alpha(v)beta(3). In an effort to develop an alpha(v)beta(3) inhibitor that down-regulates the actual level of alpha(v)beta(3), we developed an antisense strategy to inhibit alpha(v)beta(3) expression in vitro. beta(3) antisense expressed in endothelial cells specifically down-regulated alpha(v)beta(3) and inhibited capillary tube formation, with the extent of down-regulation correlating with the extent of tube formation inhibition. This inhibition was matrix-specific, since tube formation was not inhibited in Matrigel. These findings support the notion that alpha(v)beta(3) is required for an essential step of angiogenesis in fibrin, namely capillary tube formation. These results suggest that pseudogenetic inhibition of beta(3) integrins using antisense techniques may ultimately provide a therapeutic means to inhibit angiogenesis in vivo.

Factor XIIIa supports microvascular endothelial cell adhesion and inhibits capillary tube formation in fibrin

Coagulation factor XIIIa is a transglutaminase that catalyzes covalent cross-link formation in fibrin clots. In this report, we demonstrate that factor XIIIa also mediates adhesion of endothelial cells and inhibits capillary tube formation in fibrin. The adhesive activity of factor XIIIa was not dependent on the transglutaminase activity, and did not involve the factor XIIIb-subunits. The adhesion was inhibited by 99% using a combination of monoclonal antibodies directed against integrin vβ3 and β1-containing integrins, and was dependent on Mg2+ or Mn2+. Soluble factor XIIIa also bound to endothelial cells in solution, as detected by flow cytometry. In addition, factor XIIIa inhibited endothelial cell capillary tube formation in fibrin in a dose-dependent manner. Furthermore, the extent of inhibition differed in 2 types of fibrin. The addition of 10 to 100 μg/mL factor XIIIa produced a dose-dependent reduction in capillary tube formation of 60% to 100% in γA/γA fibrin, but only a 10% to 37% decrease in γA/γ′ fibrin. These results show that factor XIIIa supports endothelial cell adhesion in an integrin-dependent manner and inhibits capillary tube formation.

Factor XIIIa supports microvascular endothelial cell adhesion and inhibits capillary tube formation in fibrin

Coagulation factor XIIIa is a transglutaminase that catalyzes covalent cross-link formation in fibrin clots. In this report, we demonstrate that factor XIIIa also mediates adhesion of endothelial cells and inhibits capillary tube formation in fibrin. The adhesive activity of factor XIIIa was not dependent on the transglutaminase activity, and did not involve the factor XIIIb-subunits. The adhesion was inhibited by 99% using a combination of monoclonal antibodies directed against integrin vβ3 and β1-containing integrins, and was dependent on Mg2+ or Mn2+. Soluble factor XIIIa also bound to endothelial cells in solution, as detected by flow cytometry. In addition, factor XIIIa inhibited endothelial cell capillary tube formation in fibrin in a dose-dependent manner. Furthermore, the extent of inhibition differed in 2 types of fibrin. The addition of 10 to 100 μg/mL factor XIIIa produced a dose-dependent reduction in capillary tube formation of 60% to 100% in γA/γA fibrin, but only a 10% to 37% decrease in γA/γ′ fibrin. These results show that factor XIIIa supports endothelial cell adhesion in an integrin-dependent manner and inhibits capillary tube formation.

Role and localization of urokinase receptor in the formation of new microvascular structures in fibrin matrices

Fibrin or a fibrinous exudate can facilitate angiogenesis in many pathological conditions. In vitro, the outgrowth of capillary-like structures in fibrin can be mimicked by exposing human microvascular endothelial cells (hMVECs) to an angiogenic growth factor and tumor necrosis factor (TNF)-alpha. Urokinase-type plasminogen activator (u-PA) and plasmin activities are required for this angiogenic process. This study focuses on the role and localization of the u-PA receptor (u-PAR) in newly formed microvascular structures. The u-PAR-blocking monoclonal antibody (MAb) H-2 completely inhibited the formation of capillary-like tubular structures induced by exposure of hMVECs to basic fibroblast growth factor and TNF-alpha. This was accompanied by a several-fold increase in u-PA accumulation in the conditioned medium. The effect of MAb H-2 was not caused by blocking cellular activation by u-PA/u-PAR interaction, as the amino-terminal fragment (ATF) of u-PA, which also activates u-PAR, prevented tube formation. In addition, the inhibition by MAb H-2 was not due to an effect of the antibody on u-PAR-vitronectin binding. These data show that inhibition of tube formation can be caused not only by inhibition of u-PA or plasmin activities but also by unavailability of the u-PAR for cell-bound proteolysis. Immunohistochemical analysis showed that in in vitro angiogenesis u-PAR and u-PA were localized on the invading, tube-forming hMVECs and not on the endothelial cells that are located on top of the fibrin matrix. u-PAR and u-PA were also prominently expressed on endothelial cells of neovessels present in an atherosclerotic plaque. These data may give more insight into the role of u-PAR in repair-associated angiogenesis.

The Contribution of the Three Hypothesized Integrin-Binding Sites in Fibrinogen to Platelet-Mediated Clot Retraction

Fibrinogen is a plasma protein that interacts with integrin αIIbβ3 to mediate a variety of platelet responses including adhesion, aggregation, and clot retraction. Three sites on fibrinogen have been hypothesized to be critical for these interactions: the Ala-Gly-Asp-Val (AGDV) sequence at the C-terminus of the γ chain and two Arg-Gly-Asp (RGD) sequences in the Aα chain. Recent data showed that AGDV is critical for platelet adhesion and aggregation, but not retraction, suggesting that either one or both of the RGD sequences are involved in clot retraction. Here we provide evidence, using engineered recombinant fibrinogen, that no one of these sites is critical for clot retraction; fibrinogen lacking all three sites still sustains a relatively normal, albeit delayed, retraction response. Three fibrinogen variants with the following mutations were examined: a substitution of RGE for RGD at position Aα 95-97, a substitution of RGE for RGD at position Aα 572-574, and a triple substitution of RGE for RGD at both Aα positions and deletion of AGDV from the γ chain. Retraction rates and final clot sizes after a 20-minute incubation were indistinguishable when comparing the Aα D97E fibrinogen or Aα D574E fibrinogen with normal recombinant fibrinogen. However, with the triple mutant fibrinogen, clot retraction was delayed compared with normal recombinant fibrinogen. Nevertheless, the final clot size measured after 20 minutes was the same size as a clot formed with normal recombinant fibrinogen. Similar results were observed using platelets isolated from an afibrinogenemic patient, eliminating the possibility that the retraction was dependent on secretion of plasma fibrinogen from platelet α-granules. These findings indicate that clot retraction is a two-step process, such that one or more of the three putative platelet binding sites are important for an initial step in clot retraction, but not for a subsequent step. With the triple mutant fibrinogen, the second step of clot retraction, possibly the development of clot tension, proceeds with a rate similar to that observed with normal recombinant fibrinogen. These results are consistent with a mechanism where a novel site on fibrin is involved in the second step of clot retraction.

The Contribution of the Three Hypothesized Integrin-Binding Sites in Fibrinogen to Platelet-Mediated Clot Retraction

Fibrinogen is a plasma protein that interacts with integrin αIIbβ3 to mediate a variety of platelet responses including adhesion, aggregation, and clot retraction. Three sites on fibrinogen have been hypothesized to be critical for these interactions: the Ala-Gly-Asp-Val (AGDV) sequence at the C-terminus of the γ chain and two Arg-Gly-Asp (RGD) sequences in the Aα chain. Recent data showed that AGDV is critical for platelet adhesion and aggregation, but not retraction, suggesting that either one or both of the RGD sequences are involved in clot retraction. Here we provide evidence, using engineered recombinant fibrinogen, that no one of these sites is critical for clot retraction; fibrinogen lacking all three sites still sustains a relatively normal, albeit delayed, retraction response. Three fibrinogen variants with the following mutations were examined: a substitution of RGE for RGD at position Aα 95-97, a substitution of RGE for RGD at position Aα 572-574, and a triple substitution of RGE for RGD at both Aα positions and deletion of AGDV from the γ chain. Retraction rates and final clot sizes after a 20-minute incubation were indistinguishable when comparing the Aα D97E fibrinogen or Aα D574E fibrinogen with normal recombinant fibrinogen. However, with the triple mutant fibrinogen, clot retraction was delayed compared with normal recombinant fibrinogen. Nevertheless, the final clot size measured after 20 minutes was the same size as a clot formed with normal recombinant fibrinogen. Similar results were observed using platelets isolated from an afibrinogenemic patient, eliminating the possibility that the retraction was dependent on secretion of plasma fibrinogen from platelet α-granules. These findings indicate that clot retraction is a two-step process, such that one or more of the three putative platelet binding sites are important for an initial step in clot retraction, but not for a subsequent step. With the triple mutant fibrinogen, the second step of clot retraction, possibly the development of clot tension, proceeds with a rate similar to that observed with normal recombinant fibrinogen. These results are consistent with a mechanism where a novel site on fibrin is involved in the second step of clot retraction.

The tryptase, mouse mast cell protease 7, exhibits anticoagulant activity in vivo and in vitro due to its ability to degrade fibrinogen in the presence of the diverse array of protease inhibitors in plasma

Mouse mast cell protease (mMCP) 7 is a tryptase of unknown function expressed by a subpopulation of mast cells that reside in numerous connective tissue sites. Because enzymatically active mMCP-7 is selectively released into the plasma of V3 mastocytosis mice undergoing passive systemic anaphylaxis, we used this in vivo model system to identify a physiologic substrate of the tryptase. Plasma samples taken from V3 mastocytosis mice that had been sensitized with immunoglobulin (Ig) E and challenged with antigen were found to contain substantial amounts of four 34-55-kDa peptides, all of which were derived from fibrinogen. To confirm the substrate specificity of mMCP-7, a pseudozymogen form of the recombinant tryptase was generated that could be activated after its purification. The resulting recombinant mMCP-7 exhibited potent anticoagulant activity in the presence of normal plasma and selectively cleaved the alpha-chain of fibrinogen to fragments of similar size as that seen in the plasma of the IgE/antigen-treated V3 mastocytosis mouse. Subsequent analysis of a tryptase-specific, phage display peptide library revealed that recombinant mMCP-7 preferentially cleaves an amino acid sequence that is nearly identical to that in the middle of the alpha-chain of rat fibrinogen. Because fibrinogen is a physiologic substrate of mMCP-7, this tryptase can regulate clot formation and fibrinogen/integrin-dependent cellular responses during mast cell-mediated inflammatory reactions.

Characterization of the novel murine monoclonal anti-von Willebrand factor (vWf) antibody GUR76-23 which inhibits vWf interaction with alpha IIb beta 3 but not alpha v beta 3 integrin

von Willebrand factor (vWf) is known to interact with the two beta 3 integrins, alpha IIb beta 3 and alpha v beta 3, in an RGD-dependent manner. We characterized a novel murine monoclonal antibody to human vWf, GUR76-23, which recognized a site within the carboxy-terminal half of the molecule containing the RGD sequence. This antibody inhibited high shear-induced platelet aggregation and blocked adhesion of ADP plus epinephrine-stimulated platelets to vWf, indicating that it interferes with the interaction with alpha IIb beta 3. Unlike antibodies against the RGD site, however, the antibody was without effect on adhesion of cultured human umbilical vein endothelial cells to vWf, a phenomenon known to involve the interaction with alpha v beta 3. GUR76-23 binding was not displaced by anti-RGD antibodies. These results suggest that the adhesive interaction of vWf with these two beta 3 integrins may be differentially modulated by a site(s) other than the common RGD module.