Recognition of distinct adhesive sites on fibrinogen by related integrins on platelets and endothelial cells

Thrombus formation with rehydrated, lyophilized platelets

Stored human platelets are frequently used in hemorrhagic emergencies, but have limited immediate utility for controlling bleeding due to storage lesion and are frequently contaminated with microorganisms. The development of paraformaldehyde-treated, lyophilized and rehydrated (RL) platelets, which are sterile and have a prolonged shelf life (years), ameliorate the efficacy and sterility problems with stored platelets. RL platelets have been shown to have many native functions of fresh platelets in vitro and to mediate hemostasis in vivo in large animal models of hemorrhagic shock and cardiopulmonary bypass induced platelet dysfunction. To further evaluate the functional properties of this transfusion product, we studied the role of RL platelets in three aspects of thrombus formation and lysis. First, the interaction between RL platelets and fibrinogen was investigated. The surface density of unligated GPIIb-IIIa on RL and fresh platelets were, respectively 30000 and 70000 molecules per cell as detected with the monoclonal antibody 10E-5. Freezing, lyophilization and rehydration steps in the preparation of RL platelets resulted in the surface presentation of 120000 molecules of fibrinogen per cell from alpha granule sources. After ADP activation, RL platelets bound exogenous 125I-labeled fibrinogen in a dose-dependent manner with an affinity that is similar to that of fresh platelets and was inhibited by RGD peptides. 125I-Labeled fibrinogen binding to RL and fresh platelets, respectively, saturated at 14000 and 32000 molecules per cell. Scanning electron microscopic ultrastructural analysis showed that fibrin strands interacted with the surface of RL platelets in a normal manner. The second set of studies investigated the ability of RL platelets to catalyze and amplify the clot formation process in an activation-dependent manner. We showed that RL platelets undergo degranulation in fibrin in clots and functioned as thrombogenic surfaces for the generation of activated coagulation factors and fibrin generation. A final set of studies was performed to investigate fibrin of clots that contained RL platelets. RL platelet clots were lysed in the presence of tissue plasminogen activator with a similar time course as clots without platelets, and lysis occurred faster than when fresh platelets were included in the fibrin mass. The results of these three studies demonstrate that RL platelets are capable of mediating thrombus formation and do not inhibit lysis. Our results help explain how RL platelets restore hemostasis in vivo, and indicate that these cells might be a viable alternative to fresh stored platelets in transfusion medicine.

Tissue engineering of heart valves: formation of a three-dimensional tissue using porcine heart valve cells

Tissue engineering is a promising approach to obtaining lifetime durability of heart valves. The goal of this study was to develop a heart valve-like tissue and to compare the ultrastructure with normal valves. Myofibroblasts and endothelial cells were seeded on a type I collagen scaffold. The histologic organization and extracellular matrix were compared in light and electron micrographs. Radiolabeled proteoglycans were characterized by enzymatic degradation experiments. In tissue engineered specimens, cross sectional evaluation revealed that the scaffold (300 microm) was consistently infiltrated with myofibroblasts. Both sides were covered with a multicellular layer of myofibroblasts and overlaid by endothelial cells (50 microm). A newly formed extracellular matrix containing collagen fibrils and proteoglycans was found in the interstitial space. Collagen fibrils with a 60 nm banding pattern were found in both specimens. Small sized proteoglycans (65 nm) were associated and aligned at intervals of 60 nm with collagen fibrils. Large sized proteoglycans (180 nm) were located outside the collagen bundles in amorphous compartments of the extracellular matrix. The majority of glycosaminoglycans were chondroitin/dermatan sulfate, and a minority were heparan sulfate. The morphology and topography of cells and the organization of extracellular matrix in artificial tissues strongly resembles those of native valve tissues.

Topographical and physicochemical modification of material surface to enable patterning of living cells

Precise control of the architecture of multiple cells in culture and in vivo via precise engineering of the material surface properties is described as cell patterning. Substrate patterning by control of the surface physicochemical and topographic features enables selective localization and phenotypic and genotypic control of living cells. In culture, control over spatial and temporal dynamics of cells and heterotypic interactions draws inspiration from in vivo embryogenesis and haptotaxis. Patterned arrays of single or multiple cell types in culture serve as model systems for exploration of cell-cell and cell-matrix interactions. More recently, the patterned arrays and assemblies of tissues have found practical applications in the fields of Biosensors and cell-based assays for Drug Discovery. Although the field of cell patterning has its origins early in this century, an improved understanding of cell-substrate interactions and the use of microfabrication techniques borrowed from the microelectronics industry have enabled significant recent progress. This review presents the important early discoveries and emphasizes results of recent state-of-the-art cell patterning methods. The review concludes by illustrating the growing impact of cell patterning in the areas of bioelectronic devices and cell-based assays for drug discovery.

Identification and characterization of two cation binding sites in the integrin beta 3 subunit

The midsegment of the beta(3) subunit has been implicated in the ligand and cation binding functions of the beta(3) integrins. This region may contain a metal ion-dependent adhesion site (MIDAS) and fold into an I domain-like structure. Two recombinant fragments, beta(3)-(95-373) and beta(3)-(95-301), were expressed and found to bind fibrinogen. Whereas 0.1 mm Ca(2+) supported ligand binding to both recombinant fragments, 1.0 mm Ca(2+) suppressed binding to the longer but not the shorter fragment. These properties suggest that beta(3)-(95-373) contains both the ligand-competent (LC) and inhibitory (I) cation binding sites, and beta(3)-(95-301) lacks the I site. In equilibrium dialysis experiments, beta(3)-(95-373) contained two divalent cation binding sites, one reactive with either Mg(2+) or Ca(2+) and one Ca(2+)-specific, whereas beta(3)-(95-301) lacked the Ca(2+)-specific site. Mutant forms of beta(3)-(95-373) suggested that the LC site is a MIDAS motif involving Asp(119), Ser(121), Ser(123), Asp(217), and/or Glu(220) as coordination sites, and the I site was dependent upon residues within beta(3)-(301-323). In a molecular model of beta(3)-(95-373), a second Ca(2+) could be docked onto a flexible loop in close proximity to the MIDAS. These results indicate that the ligand competent and Ca(2+)-specific inhibitory cation binding sites are distinct and reside in beta(3)-(95-373).

Galectin-1 is overexpressed in nasal polyps under budesonide and inhibits eosinophil migration

Because of the importance of galectins for various cellular activities, the influence of the glucocorticoid budesonide on the level of expression of galectins-1 and -3 was investigated in human nasal polyposis. Ten nasal polyps obtained from surgical resection were maintained for 24 hours in the presence of various concentrations of budesonide. As quantitatively demonstrated by means of computer-assisted microscopy, 250 ng/ml (the highest dose tested) induced a pronounced increase of galectin-1 expression. This feature was observed in nasal polyps from allergic patients but not in those from nonallergic patients. Since eosinophils represent the main inflammatory cell population in nasal polyps, we investigated the effect of galectin-1 on their migration levels by means of quantitative phase-contrast computer-assisted videomicroscopy. Our results show that galectin-1 (coated on plastic supports) markedly reduced the migration levels of eosinophils in comparison to P-selectin. On the cellular level, marked modifications in the polymerization/depolymerization dynamics of the actin cytoskeleton (as revealed by means of computer-assisted fluorescence microscopy) and, to a much lesser extent, an increase in the adhesiveness of eosinophils to tested substrata were detectable. The present study therefore reveals a new galectin-1-mediated mechanism of action for glucocorticoid-mediated anti-inflammatory effects.

Platelet integrin alphaIIbbeta3-ligand interactions: what can we learn from the structure?

Upon vascular injury, platelets initiate interaction with exposed subendothelial matrices through various receptors such as glycoprotein (GP) Ib/IX/V complex, alpha2beta1 integrin, and GPVI/FcRgamma. Although these interactions cannot sustain stable platelet thrombus formation by themselves, they ultimately lead to the activation of alphaIIbbeta3 integrin (GPIIb-IIIa complex [GPIIb-IIIa]), the most abundant receptor in platelets. The alphaIIbbeta3 integrin plays a central role in primary hemostasis by serving as a receptor for fibrinogen and von Willebrand factor (vWf). It establishes a stable interaction with vWf bound to the extracellular matrices and uses fibrinogen as a bridging molecule in platelet aggregate formation. The alphaIIbbeta3 integrin also plays an important role in the pathogenesis of thrombosis. Over the past decades, a tremendous amount of effort has been made to elucidate the ligand-binding mechanisms of alphaIIbbeta3, in part because of its clinical significance. Most of the studies have relied on biochemical analyses of purified alphaIIbbeta3 or recombinant proteins generated in vitro. With the lack of actual 3-dimensional structure, molecular modeling has provided a useful framework for interpreting such experimental data on structure-function correlation of integrin molecules. However, it has also generated disagreement between different models. The aim of this minireview is to summarize the past efforts as well as the recent accomplishments in elucidating the structure/function of alphaIIbbeta3. Finally, we will try to explain all those experimental data using the recently published crystal structure of the extracellular domains of the alphaVbeta3 heterodimeric complex.

Fibrinogen and fibrin are anti-adhesive for keratinocytes: a mechanism for fibrin eschar slough during wound repair

During cutaneous wound repair the epidermis avoids the fibrin-rich clot; rather it migrates down the collagen-rich dermal wound margin and over fibronectin-rich granulation tissue. The mechanism(s) underlying keratinocyte movement in this precise pathway has not been previously addressed. Here we demonstrate that cultured human keratinocytes do not express functional fibrinogen/fibrin receptors, specifically alpha v beta 3. Biologic modifiers known to induce integrin expression or activation did not induce adhesion to fibrin, fibrinogen, or its fragments. Epidermal explant outgrowth and single epidermal cell migration failed to occur on either fibrin or fibrinogen. Surprisingly, fibrin and fibrinogen mixed at physiologic molar ratios with fibronectin abrogated keratinocyte attachment to fibronectin. Keratinocytes transduced with the beta 3 integrin subunit cDNA, expressed alpha v beta 3 on their surface and attached to and spread on fibrinogen and fibrin. beta-gal cDNA-transduced keratinocytes did not demonstrate this activity. Furthermore, beta 3 cDNA-transduced keratinocyte adhesion to fibrin was inhibited by LM609 monoclonal antibody to alpha v beta 3 in a concentration-dependent fashion. From these data, we conclude that normal human keratinocytes cannot interact with fibrinogen and its derivatives due to the lack of alpha v beta 3. Thus, fibrinogen and fibrin are authentic anti-adhesive for keratinocytes. This may be a fundamental reason why the migrating epidermis dissects the fibrin eschar from wounds.

The role of the coagulation system in tumour angiogenesis

The coagulation system, which is activated in most cancer patients, has an important role in tumour biology. It may make a substantial contribution to tumour angiogenesis, which represents an imbalance in the normal mechanisms that allow organised healing after injury. The recently recognised, but steadily growing, knowledge of the relationship between the coagulation and angiogenesis pathways has research and clinical implications. Manipulation of these systems may minimise both the neoangiogenesis essential for tumour growth and associated thromboembolic complications. However, since surgery is the primary treatment for most cancers, the angiogenesis of wound healing and haemostatic competence must be maintained. In this article, we summarise the complex interactions between the coagulation system and the angiogenic process that occur in cancer growth. We focus upon the contributions of the vascular endothelium, platelets, and coagulation factors to the angiogenic process and explore the coagulation system as a therapeutic target.

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.

Conformational changes upon conversion of fibrinogen into fibrin. The mechanisms of exposure of cryptic sites

Conformational changes upon conversion of fibrinogen to fibrin result in the exposure of multiple binding sites that provide its interaction with various proteins and cells and, thus, its participation in a number of physiological and pathological processes. Here we focus on conformational changes in the fibrinogen D regions (domains) and alpha C-domains that are directly involved in intermolecular interactions upon fibrin assembly. According to the current view, two alpha C-domains that interact intramolecularly in fibrinogen undergo an intra- to intermolecular switch to form alpha C-polymers in fibrin. The availability of recombinant fragments that correspond to the alpha C-domain made it possible to further clarify this mechanism and to reveal novel cryptic sites in this domain for plasminogen and its activator tPA, whose exposure may play an important role in the regulation of fibrinolysis. To elucidate the mechanism of exposure of cryptic sites in the D regions, we tested the accessibility of their fibrin specific epitopes (A alpha 148-160 and gamma 312-324) that are also involved in binding of plasminogen and tPA, in several fragments derived from fibrinogen (fragment D), and crosslinked fibrin (fragment D-D and its non-covalent complex with the E1 fragment, D-D:E1). Neither D nor D-D bound tPA, plasminogen, or anti-A alpha 148-160 and anti-gamma 312-324 monoclonal antibodies. At the same time both epitopes became accessible in the D-D:E1 complex. Melting of D and D-D revealed that their domains have the same stability while in the D-D:E1 complex they became more stable. These results indicate that upon fibrin assembly, driven primarily by the interaction between complementary binding sites of the E and two D regions, the latter undergo conformational changes that cause the exposure of their cryptic sites. They also suggest that the fibrin specific conformation of the D regions is preserved in the D-D:E1 complex.

Platelet-fibrinogen interactions

Binding of fibrinogen to GPIIb-IIIa on agonist-stimulated platelets results in platelet aggregation, presumably by crosslinking adjacent activated platelets. Although unactivated platelets express numerous copies of GPIIb-IIIa on their surface, spontaneous, and potentially deleterious, platelet aggregation is prevented by tightly regulating the fibrinogen binding activity of GPIIb-IIIa. Preliminary evidence suggests that it is the submembranous actin or actin-associated proteins that constrains GPIIb-IIIa in a low affinity state and that relief of this constraint by initiating actin filament turnover enables GPIIb-IIIa to bind fibrinogen. Two regions of the fibrinogen alpha chain that contain an RGD motif, as well as the carboxyl-terminus of the fibrinogen gamma chain, represent potential binding sites for GPIIb-IIIa in the fibrinogen molecule. However, ultrastructural studies using purified fibrinogen and GPIIb-IIIa, and studies using recombinant fibrinogen in which the RGD and relevant gamma chain motifs were mutated indicate that sequences located at the carboxyl-terminal end of the gamma chain mediates fibrinogen binding to GPIIb-IIIa. There is evidence that fibrinogen itself binds to regions in the amino terminal portions of both GPIIb and GPIIIa and that the sites interacting with the fibrinogen gamma chain and with RGD-containing peptides are spatially distinct. Nonetheless, there appears to be allosteric linkage between these sites, accounting for the ability of RGD-containing peptides to inhibit platelet aggregation and arterial thrombosis.

Interaction of fibrin with VE-cadherin

The conversion of fibrinogen into fibrin and the association of fibrin(ogen) with activated platelets play a fundamental role in hemostasis because their interaction with the injured vessel prevents blood extravasation. Platelet aggregates and fibrin also participate in the occlusion of the vascular lumen in pathological conditions. Fibrin II also promotes the formation of new blood vessels, for example, during wound healing and tumor growth. Using an in vitro assay, we have studied the mechanism by which fibrin II induces formation of capillaries. Generation of fibrin II on top of an endothelial cell monolayer rapidly rearranged the ECs into a capillary network. In contrast, neither fibrin I nor fibrin 325 induced these morphogenetic changes, indicating that exposure of the N-terminal peptide beta 15-42 is involved in this process. Binding studies, using the N-terminal fragment of fibrin (NDSK II), showed that NDSK II binds to EC with high affinity, but neither NDSK nor NDSK325 bound specifically. Binding of NDSK II to endothelial cells was blocked with an antibody to VE-cadherin. Direct association of NDSK II and VE-cadherin was also demonstrated in a VE-cadherin antibody capture assay. NDSK II bound specifically with the captured VE-cadherin but NDSK or NDSK 325 did not associate with VE-cadherin. Moreover, fibrin II associated with EC VE-cadherin and this interaction triggered the formation of capillary-like structures. A better understanding of the cellular responses to fibrin, identification of the fibrin binding site within VE-cadherin and the intracellular signaling that follows this interaction, could yield important information that may translate into better control of the angiogenic process.

Fibrin and wound healing

Although hemostasis is the major role of fibrin in wound repair, once the clot is present the wound cells must deal with it. The invasion and clearing of fibrin by these cells involves multiple complex processes that may go array XXX and delay wound repair. A good example, of the latter is leg ulcers. These chronic wounds contain a plethora of proteases that digest fibronectin and growth factors in the fibrin clot resulting in a corrupt provisional matrix that no longer supports reepithelialization or granulation tissue formation. Every good wound care provider knows that these wounds will not heal unless the corrupt matrix is removed by vigorous debridement that stimulates the accumulation of a competent provisional matrix.

Integrin activation is required for VEGF and FGF receptor protein presence on human microvascular endothelial cells

Endothelial cell proliferation and migration is initiated by growth factors including FGF and VEGF that bind to specific transmembrane receptor tyrosine kinases. Mechanisms that regulate in vivo expression of fibroblast growth factor receptors (FGFR) and vascular endothelial growth factor receptors (VEGFR) are not well understood. Since it is well known that different matrices influence the proliferation and migration of endothelial cells in culture, we hypothesized that changes in the extracellular matrix environment can regulate growth factor receptors on endothelial cells. We cultured human microvascular endothelial cells on different matrices (vitronectin, laminin, fibronectin, fibrin, and collagen IV) and examined for the presence of growth factor receptors (FGFR-1, FGFR-2, VEGFR-1, and VEGFR-2). We show that vitronectin increased the presence of all four growth factor receptors and most notably, VEGFR-1. In contrast, fibrin decreased all four receptors, especially FGFR-1 and FGFR-2. Inhibiting phosphotyrosine signaling abolished immunostaining for all four receptors, regardless of the matrix, but was not dependent on activating the Fyn-Shc pathway. Cells plated on vitronectin in the presence of blocking antibodies to integrins alphavbeta3 and alphavbeta5 similarly decreased presence of these growth factor receptors. Our data suggests a possible mechanism of how matrix-integrin interactions regulate endothelial cell responsiveness to growth factors and anchorage-dependent cell growth.

Laser scanning cytometry: a novel method for the detection of platelet--endothelial cell adhesion

BACKGROUND

Adherence of platelets to endothelial cells may be a significant event in the development of vascular thrombosis. Existing models, which examine platelet-endothelial cell interactions, compromise endothelial cell integrity or use radioactivity to identify platelets that adhere to endothelial cells. We report a novel method for in vitro detection of platelet-endothelial cell adhesion that allows endothelial cells to remain as an intact monolayer and for visualization of individual platelets.

METHODS

Fluorescently labeled platelets were incubated with a confluent monolayer of endothelial cells. Laser scanning cytometry (LSC) identified platelets bound to endothelial cells based on their fluorescent signals.

RESULTS

LSC detection of platelets reliably reproduced well-described findings of thrombin-induced platelet-endothelial cell adhesion. Results demonstrating reduced adhesion with a glycoprotein IIb-IIIa-specific blocking monoclonal antibody confirmed the specificity of the LSC detection of platelet-endothelial cell adhesion.

CONCLUSIONS

LSC is a novel method for detecting platelet--endothelial cell adhesion. Its advantages over other methods are: (a) endothelial cells remain undisturbed and adherent throughout; (b) the ability to detect individual bound platelets and subpopulations; (c) the ability to store images and slides and then relocate, revisualize, and reanalyze individual cells or cell populations of interest; and (d) no radioactivity.

Galectin-1 is highly expressed in human gliomas with relevance for modulation of invasion of tumor astrocytes into the brain parenchyma

Protein (lectin)-carbohydrate interaction is supposed to be relevant for tumor cell behavior. The aims of the present work are to investigate whether galectin-1 modulates migration/invasion features in human gliomas in vitro, whether it can be detected in human gliomas immunohistochemically, and whether its expression is attributable to certain glioma subgroups with respect to invasion and prognosis. For this purpose, we quantitatively determined (by computer-assisted microscopy) the immunohistochemical expression of galectin-1 in 220 gliomas, including 151 astrocytic, 38 oligodendroglial, and 31 ependymal tumors obtained from surgical resection. We also xenografted three human glioblastoma cell lines (the H4, U87, and U373 models) into the brains of nude mice in order to characterize the in vivo galectin-1 expression pattern in relation to tumor invasion of the normal brain parenchyma. In addition, we characterized the role in vitro of galectin-1 in U373 tumor astrocyte migration and kinetics. Our data reveal expression of galectin-1 in all human glioma types with no striking differences between astrocytic, oligodendroglial, and ependymal tumors. The level of galectin-1 expression correlated with the grade in the group of astrocytic tumors only. Furthermore, immunopositivity of high-grade astrocytic tumors from patients with short-term survival periods was stronger than that of tumors from patients with long-term survivals. In human glioblastoma xenografts, galectin-1 was preferentially expressed in the more invasive parts of these xenografts. In vitro experiments revealed that galectin-1 stimulates migration of U373 astrocytes.

Angiogenesis in wound healing

During wound healing, angiogenic capillary sprouts invade the fibrin/fibronectin-rich wound clot and within a few days organize into a microvascular network throughout the granulation tissue. As collagen accumulates in the granulation tissue to produce scar, the density of blood vessels diminishes. A dynamic interaction occurs among endothelial cells, angiogenic cytokines, such as FGF, VEGF, TGF-beta, angiopoietin, and mast cell tryptase, and the extracellular matrix (ECM) environment. Specific endothelial cell ECM receptors are critical for these morphogenetic changes in blood vessels during wound repair. In particular, alpha(v)beta3, the integrin receptor for fibrin and fibronectin, appears to be required for wound angiogenesis: alpha(v)beta3 is expressed on the tips of angiogenic capillary sprouts invading the wound clot, and functional inhibitors of alpha(v)beta3 transiently inhibit granulation tissue formation. Recent investigations have shown that the wound ECM can regulate angiogenesis in part by modulating integrin receptor expression. mRNA levels of alpha(v)beta3 in human dermal microvascular endothelial cells either plated on fibronectin or overlaid by fibrin gel were higher than in cells plated on collagen or overlaid by collagen gel. Wound angiogenesis also appears to be regulated by endothelial cell interaction with the specific three-dimensional ECM environment in the wound space. In an in vitro model of human sprout angiogenesis, three-dimensional fibrin gel, simulating early wound clot, but not collagen gel, simulating late granulation tissue, supported capillary sprout formation. Understanding the molecular mechanisms that regulate wound angiogenesis, particularly how ECM modulates ECM receptor and angiogenic factor requirements, may provide new approaches for treating chronic wounds.

Src homology domain 2-containing tyrosine phosphatase 2 associates with intercellular adhesion molecule 1 to regulate cell survival

Intercellular adhesion molecule-1 (ICAM-1) binds to the plasma protein fibrinogen (Fg) to mediate leukocyte/endothelial cell interactions. In our studies, the ligation of Fg to ICAM-1 on tumor necrosis factor-alpha-stimulated endothelial cells resulted in the tyrosine phosphorylation of Src homology domain 2 (SH2)-containing phosphatase-2 (SHP-2). The ICAM-1 cytoplasmic sequence IKKYRLQ conforms poorly to the concensus immunoreceptor tyrosine-based inhibition motifs found in receptors that bind SHP-2. Nevertheless, the tyrosine phosphorylated sequence (IKKpYRLQ) bound specifically to the SH2 domain proximal to the NH(2)-terminal of SHP-2 (SHP-2-N) but not to the SH2 domain proximal on the COOH-terminal side (SHP-2-C). Phosphorylated ICAM-1 bound SHP-2-N. In immunoprecipitation experiments, SHP-2 associated with phosphorylated ICAM-1. Cells expressing truncated ICAM-1 that lacked the cytoplasmic sequence (ICAM-1(TR)) failed to associate with SHP-2. ICAM-1 containing the tyrosine to alanine substitution at position 485 (ICAM-1(Y485A)) associated weakly with SHP-2. Cells expressing ICAM-1(TR) and ICAM-1(Y485A) underwent apoptosis upon adhesion to Fg, whereas the wild type ICAM-1 maintained cell survival. These results indicate that ICAM-1 interactions with SHP-2 allow better cellular survival mediated through Fg-ICAM-1 ligation.

Fibrinogen interactions with ICAM-1 (CD54) regulate endothelial cell survival

We tested hypothesis that the interaction of fibrinogen (Fg) with intercellular adhesion molecule 1 (ICAM-1) mediates cellular adhesion and cell proliferation. Our results demonstrate that Fg : ICAM-1 ligation mediates endothelial cell survival and has an anti-apoptotic effect via activation of the MAP kinase pathway. Fg : ICAM-1 ligation in endothelial cells treated with tumor necrosis factor (TNF)alpha resulted in the hyperphosphorylation of extracellular signal-regulated kinase (ERK)-1/2 (eightfold to 10-fold) at 5-30 min. The specificity of ERK-1/2 phosphorylation was verified using the recognition peptides Fg-gamma-(117-133) and ICAM-1(8-22). ERK-1/2 hyperphosphorylation was dependent on intact cytoskeleton, as treatment with cytochalasin B and nocodazole blocked this activity. The attachment of TNFalpha-treated endothelial cells to fibrinogen or Fg-gamma-(117-133) resulted in cell survival, as assessed by an annexin V binding assay. ICAM-1(8-22) blocked the survival process. The MEK-1 inhibitor PD 98059 blocked ERK-1/2 phosphorylation, and treatment of endothelial cells with PD 98059 resulted in apoptosis even upon Fg : ICAM-1 ligation. Cells transfected with dominant-negative ERK-1/2 underwent apoptosis upon Fg : ICAM-1 ligation. Cell survival factor A1 was specifically upregulated upon adhesion of TNFalpha-stimulated endothelial cells to Fg. A1 expression was blocked by ICAM-1(8-22) and PD 98059. The Fg : ICAM-1 endothelial cell survival pathway appears to be mediated via the activation and upregulation of ERK-1/2 and A1.

A Leu262Pro mutation in the integrin β3 subunit results in an αIIb-β3 complex that binds fibrin but not fibrinogen

Platelet retraction of a fibrin clot is mediated by the platelet fibrinogen receptor, IIbβ3. In certain forms of the inherited platelet disorder, Glanzmann thrombasthenia (GT), mutant IIbβ3 may interact normally with fibrin yet fail to support fibrinogen-dependent aggregation. We describe a patient (LD) with such a form of GT. Platelets from LD supported normal clot retraction but failed to bind fibrinogen. Platelet analysis using flow cytometry and immunoblotting showed reduced but clearly detectable IIbβ3, findings consistent with type II GT. Genotyping of LD revealed 2 novel β3 mutations: a deletion of nucleotides 867 to 868, resulting in a premature stop codon at amino acid residue 267, and a T883C missense mutation, resulting in a leucine (Leu) 262-to-proline (Pro) substitution. Leu262 is highly conserved among β integrin subunits and lies within an intrachain loop implicated in subunit association. Leu262Proβ3 cotransfected with wild-type IIb into COS-7 cells showed delayed intracellular maturation and reduced surface expression of easily dissociable complexes. In human embryonic kidney 293 cells, Leu262Proβ3 formed a complex with endogenous av and retracted fibrin clots similarly to wild-type β3. The same cells, however, were unable to bind immobilized fibrinogen. The molecular requirements for IIbβ3 to interact with fibrin compared with fibrinogen, therefore, appear to differ. The region surrounding β3 Leu262 may maintain β3 in a fibrinogen-binding, competent form, but it appears not to be required for receptor interactions with fibrin.

A Leu262Pro mutation in the integrin β3 subunit results in an αIIb-β3 complex that binds fibrin but not fibrinogen

Platelet retraction of a fibrin clot is mediated by the platelet fibrinogen receptor, IIbβ3. In certain forms of the inherited platelet disorder, Glanzmann thrombasthenia (GT), mutant IIbβ3 may interact normally with fibrin yet fail to support fibrinogen-dependent aggregation. We describe a patient (LD) with such a form of GT. Platelets from LD supported normal clot retraction but failed to bind fibrinogen. Platelet analysis using flow cytometry and immunoblotting showed reduced but clearly detectable IIbβ3, findings consistent with type II GT. Genotyping of LD revealed 2 novel β3 mutations: a deletion of nucleotides 867 to 868, resulting in a premature stop codon at amino acid residue 267, and a T883C missense mutation, resulting in a leucine (Leu) 262-to-proline (Pro) substitution. Leu262 is highly conserved among β integrin subunits and lies within an intrachain loop implicated in subunit association. Leu262Proβ3 cotransfected with wild-type IIb into COS-7 cells showed delayed intracellular maturation and reduced surface expression of easily dissociable complexes. In human embryonic kidney 293 cells, Leu262Proβ3 formed a complex with endogenous av and retracted fibrin clots similarly to wild-type β3. The same cells, however, were unable to bind immobilized fibrinogen. The molecular requirements for IIbβ3 to interact with fibrin compared with fibrinogen, therefore, appear to differ. The region surrounding β3 Leu262 may maintain β3 in a fibrinogen-binding, competent form, but it appears not to be required for receptor interactions with fibrin.

Identification of amino acid sequences in fibrinogen gamma -chain and tenascin C C-terminal domains critical for binding to integrin alpha vbeta 3

Integrin alpha(v)beta(3) recognizes fibrinogen gamma and alpha(E) chain C-terminal domains (gammaC and alpha(E)C) but does not require the gammaC dodecapeptide sequence HHLGGAKQAGDV(400-411) for binding to gammaC. We have localized the alpha(v)beta(3) binding sites in gammaC using gammaC-derived synthetic peptides. We found that two peptides GWTVFQKRLDGSV(190-202) and GVYYQGGTYSKAS(346-358) block the alpha(v)beta(3) binding to gammaC or alpha(E)C, block the alpha(v)beta(3)-mediated clot retraction, and induce the ligand-induced binding site 2 (LIBS2) epitope in alpha(v)beta(3). Neither peptide affects fibrinogen binding to alpha(IIb)beta(3). Scrambled or inverted peptides were not effective. These results suggest that the two gammaC-derived peptides directly interact with alpha(v)beta(3) and specifically block alpha(v)beta(3)-gammaC or alpha(E)C interaction. The two sequences are located next to each other in the gammaC crystal structure, although they are separate in the primary structure. Asp-199, Ser-201, Gln-350, Thr-353, Lys-356, Ala-357, and Ser-358 residues are exposed to the surface. This suggests that the two sequences are part of alpha(v)beta(3) binding sites in fibrinogen gammaC domain. We also found that tenascin C C-terminal fibrinogen-like domain specifically binds to alpha(v)beta(3). Notably, a peptide WYRNCHRVNLMGRYGDNNHSQGVNWFHWKG from this domain that includes the sequence corresponding to gammaC GVYYQGGTYSKAS(346-358) specifically binds to alpha(v)beta(3), suggesting that fibrinogen and tenascin C C-terminal domains interact with alpha(v)beta(3) in a similar manner.

alpha(v)beta(3) Integrin antagonists as inhibitors of bone resorption

The alpha(v)beta(3) integrin is a non-covalent, heterodimeric, cell-surface protein that is expressed with varying density on numerous cell types, including osteoclasts, vascular smooth muscle cells, endothelial cells and a variety of tumour cells. Functionally, alpha(v)beta(3) mediates a diverse range of biological events including the adhesion of osteoclasts to bone matrix, smooth muscle cell migration and angiogenesis. Specifically, there has been significant attention focused on the preparation of inhibitors of alpha(v)beta(3) for use as inhibitors of bone resorption, in recognition of the medical need for improved prevention and treatment of osteoporosis. Herein, we summarise the pertinent chemistry and biological advances in the medicinal design and biological evaluation of peptide and small molecule alpha(v)beta(3) antagonists as inhibitors of bone resorption.

The crystal structure of modified bovine fibrinogen

Here we report the crystal structure at approximately 4-A resolution of a selectively proteolyzed bovine fibrinogen. This key component in hemostasis is an elongated 340-kDa glycoprotein in the plasma that upon activation by thrombin self-assembles to form the fibrin clot. The crystals are unusual because they are made up of end-to-end bonded molecules that form flexible filaments. We have visualized the entire coiled-coil region of the molecule, which has a planar sigmoidal shape. The primary polymerization receptor pockets at the ends of the molecule face the same way throughout the end-to-end bonded filaments, and based on this conformation, we have developed an improved model of the two-stranded protofibril that is the basic building block in fibrin. Near the middle of the coiled-coil region, the plasmin-sensitive segment is a hinge about which the molecule adopts different conformations. This segment also includes the boundary between the three- and four-stranded portions of the coiled coil, indicating the location on the backbone that anchors the extended flexible Aalpha arm. We suggest that a flexible branch point in the molecule may help accommodate variability in the structure of the fibrin clot.

Platelets induce alterations of chemotactic and adhesive properties of endothelial cells mediated through an interleukin-1-dependent mechanism. Implications for atherogenesis

Platelets and alterations of chemotactic and adhesive properties of endothelium play an important role in the pathophysiology of atherosclerosis. We investigated the effect of platelets on secretion of monocyte chemotactic protein-1 (MCP-1) and on surface expression of intercellular adhesion molecule-1 (ICAM-1) of cultured endothelium. Pretreatment of cultured monolayers of endothelial cells with alpha-thrombin-activated platelets significantly enhanced secretion of MCP-1 and ICAM-1 surface expression (P<0.01) that could be inhibited by interleukin-1 (IL-1) antagonists by approximately 40%. Activation of transcription factor nuclear factor-kappaB (NF-kappaB) which regulates transcription of early inflammatory response genes such as MCP-1, was significantly increased in endothelial cells treated with activated platelets via an IL-1 mediated mechanism as determined by electrophoretic mobility shift assay (EMSA) and kappaB-dependent transcriptional activity. In trans-well experiments, alpha-thrombin-activated platelets enhanced IL-1-dependent surface expression of vitronectin receptor (alpha(v)beta(3)) on the luminal aspect of endothelial monolayers and promoted alpha(v)beta(3)-mediated platelet/endothelium adhesion that could be inhibited by the antiadhesive peptides GRGDSP and c(RGDfV). We conclude that activated platelets induce significant changes in chemotactic (secretion of MCP-1) and adhesive (surface expression of ICAM-1 and alpha(v)beta(3)) properties of cultured endothelium. These findings imply a potential pathophysiological mechanism of platelets in an early stage of atherogenesis.

Fibrin and collagen differentially regulate human dermal microvascular endothelial cell integrins: stabilization of alphav/beta3 mRNA by fibrin1

Integrin alphavbeta3 is specifically but transiently expressed on the tips of capillary sprouts as they invade the fibrin clot during angiogenesis of cutaneous wound repair. Specific blocking of alphavbeta3 function inhibits granulation tissue formation in cutaneous wounds. The mechanisms of regulation of alphavbeta3 expression on human dermal microvascular endothelial cells, however, have not been fully delineated. As alphavbeta3 was highly expressed on capillary sprouts in 5 d wounds rich in fibrin, but was almost undetectable on blood vessels in 7 d wounds rich in collagen, we hypothesized that the extracellular matrix environment could regulate human dermal micro- vascular endothelial cell alphavbeta3 expression. To address this, human dermal microvascular endothelial cells were cultured on surfaces coated with collagen, fibronectin, and gelatin, and mRNA levels of integrin alphav/beta3 were determined. Compared with human dermal microvascular endothelial cells on collagen, mRNA levels of alphav/beta3 were higher in human dermal microvascular endothelial cells on fibronectin and on gelatin. To simulate the in vivo environment better, human dermal microvascular endothelial cells cultured on collagen were overlaid by fibrin or collagen gels prior to assessment of alphav/beta3 mRNA levels. alphav/beta3 mRNA levels were higher in human dermal microvascular endothelial cells surrounded by a three-dimensional fibrin gel compared with a collagen gel, whether angiogenic factors were present or absent. As modulation of mRNA stability is a potential regulatory mechanism for integrin expression, integrin subunit mRNA stability was assessed. beta3 mRNA decayed much faster than alphav, alpha2, and beta1 mRNA. Three-dimensional fibrin gels enhanced alphav/beta3 mRNA stability compared with collagen gels. We propose that the provisional matrix molecules in the wound clot regulate angiogenesis associated with cutaneous wound repair through their modulation of integrin receptor expression.

Thermal conformational changes of bovine fibrinogen by differential scanning calorimetry and circular dichroism

The thermal denaturation of bovine fibrinogen has been investigated using differential scanning calorimetry (DSC) and circular dichroism (CD) spectroscopy. Differential scanning calorimetry measurements were carried out while changing the scan-rate. The transition at 57 degrees C was found to be irreversible and highly scan-rate dependent, suggesting that the denaturation is, at least in part, under kinetic control. The secondary structural changes at various temperatures were monitored by far-ultraviolet CD spectroscopy. These results show that the DSC transition for the thermal denaturation of bovine fibrinogen can be interpreted in terms of a kinetic process, N --> F, where k is a first-order kinetic constant that changes with temperature according to the Arrhenius equation. An important transition peak was observed at 78.8 degrees C which is attributed to the C-terminal parts of the Aalpha chains of fibrinogen.

Glycoprotein Ibalpha can mediate endothelial cell migration on von Willebrand factor-containing substrata

We have previously shown that, in addition to the vitronectin receptor (VNR, alpha(v)beta(3)), the GP Ib complex can participate in endothelial cell (EC) attachment to von Willebrand Factor (vWF) (D. A. Beacham, M. S. Cruz, and R. I. Handin, 1995, Thromb. Haemostas. 73, 309-317; D. A. Beacham, L.-P. Tran, and S. S. Shapiro, 1997, Blood 89, 4071-4077). In this study we have investigated the functional roles of these vWF receptors in the migration of untreated and TNFalpha-treated EC on vWF, a mixture of vWF and type I collagen, and on vitronectin (VN). In agreement with previous studies (D. I. Leavesley, M. A. Schwartz, M. Rosenfeld, and D. A. Cheresh, 1993, J. Cell Biol. 121, 163-170), the migration of untreated and TNFalpha-treated EC on VN was dependent entirely on the VNR. Migration of untreated EC on vWF was inhibited 10-15% by recombinant vWF-A1, the GP Ibalpha-binding domain on vWF which abrogates the platelet GP Ibalpha-vWF interaction. In contrast, migration of TNFalpha-treated EC on vWF was inhibited 50-60% by vWF-A1 or the anti-GP Ibalpha mAb AS-7 but only 20% by the anti-VNR mAb LM609. On a mixed vWF-collagen substratum, vWF-A1 inhibited untreated EC migration by 45%, and TNFalpha-treated EC migration by 75%. The possible role of EC proliferation was eliminated, since hydroxyurea completely inhibited EC proliferation without reducing migration significantly. The anti-GP Ibalpha mAb Ib1 inhibited EC migration by 50%, but reduced proliferation by only 15%. Taken together, our data demonstrate that EC migration on vWF-containing substrata involves the GP Ib complex as well as the VNR and raises the possibility that the VNR and GP Ib act cooperatively in supporting EC migration.

Activation-dependent adhesion of human platelets to Cyr61 and Fisp12/mouse connective tissue growth factor is mediated through integrin alpha(IIb)beta(3)

Cyr61 and connective tissue growth factor (CTGF), members of a newly identified family of extracellular matrix-associated signaling molecules, are found to mediate cell adhesion, promote cell migration and enhance growth factor-induced cell proliferation in vitro, and induce angiogenesis in vivo. We previously showed that vascular endothelial cell adhesion and migration to Cyr61 and Fisp12 (mouse CTGF) are mediated through integrin alpha(v)beta(3). Both Cyr61 and Fisp12/mCTGF are present in normal blood vessel walls, and it has been demonstrated that CTGF is overexpressed in advanced atherosclerotic lesions. In the present study, we examined whether Cyr61 and Fisp12/mCTGF could serve as substrates for platelet adhesion. Agonist (ADP, thrombin, or U46619)-stimulated but not resting platelets adhered to both Cyr61 and Fisp12/mCTGF, and this process was completely inhibited by prostaglandin I(2), which prevents platelet activation. The specificity of Cyr61- and Fisp12/mCTGF-mediated platelet adhesion was demonstrated by specific inhibition of this process with polyclonal anti-Cyr61 and anti-Fisp12/mCTGF antibodies, respectively. The adhesion of ADP-activated platelets to both proteins was divalent cation-dependent and was blocked by RGDS, HHLGGAKQAGDV, or echistatin, but not by RGES. Furthermore, this process was specifically inhibited by the monoclonal antibody AP-2 (anti-alpha(IIb)beta(3)), but not by LM609 (anti-alpha(v)beta(3)), indicating that the interaction is mediated through integrin alpha(IIb)beta(3). In a solid phase binding assay, activated alpha(IIb)beta(3), purified by RGD affinity chromatography, bound to immobilized Cyr61 and Fisp12/mCTGF in a dose-dependent and RGD-inhibitable manner. In contrast, unactivated alpha(IIb)beta(3) failed to bind to either protein. Collectively, these findings identify Cyr61 and Fisp12/mCTGF as two novel activation-dependent adhesive ligands for the integrin alpha(IIb)beta(3) on human platelets, and implicate a functional role for these proteins in hemostasis and thrombosis.

Potentiation of endothelial cell proliferation by fibrin(ogen)-bound fibroblast growth factor-2

Endothelial cell growth is stimulated by fibroblast growth factor-2 (FGF-2), and both adhesion and proliferation are modulated by interactions with fibrinogen and fibrin. Previous evidence indicates that FGF-2 binds specifically and with high affinity to fibrinogen and fibrin, suggesting that their effects on endothelial cells may be coordinated. In this study, we have, therefore, investigated the ability of FGF-2 bound to fibrinogen and fibrin to stimulate proliferation of endothelial cells. Human umbilical vein endothelial cells were cultured in the presence of FGF-2 with or without fibrinogen, and proliferation was assessed by microscopic examination of cultures, incorporation of [3H]thymidine and by cell counting. Cells cultured in the presence of both FGF-2 and fibrinogen proliferated more rapidly than those with FGF-2 alone and exhibited a decreased population doubling time. At concentrations of FGF-2 up to 150 ng/ml, there was greater endothelial cell proliferation in the presence of fibrinogen than in its absence with the most pronounced effect below 1 ng/ml. The maximum effect of fibrinogen was observed at a molar ratio of fibrinogen to FGF-2 of 2:1, corresponding to the maximum molar binding ratio. Endothelial cells proliferated when plated on fibrin or surface-immobilized fibrinogen with FGF-2, indicating that FGF-2 bound to surface-associated fibrin(ogen) retained activity. We conclude that fibrinogen- or fibrin-bound FGF-2 is able to support endothelial cell proliferation and that fibrinogen potentiates the proliferative capacity of FGF-2.

Endothelial cell VE-cadherin functions as a receptor for the beta15-42 sequence of fibrin

The contact of fibrin with the apical surface of human umbilical vein endothelial cells (HUVEC) can induce capillary tube formation via the interaction of fibrin beta15-42 with a putative cell receptor (Chalupowicz, D. G., Chowdhury, Z. A., Bach, T. L., Barsigian, C., and Martinez, J. (1995) J. Cell Biol. 130, 207-215). To characterize this interaction, we studied the binding of the thrombin-cleaved N-terminal disulfide knot of fibrin (NDSK II), a dimeric fragment with exposed beta15-42, to HUVEC in three separate assay systems. Time-course binding of 125I-NDSK II to HUVEC monolayers or suspensions revealed that binding was specific at 50-60%, as determined by the addition of unlabeled NDSK II. Specific binding of 125I-NDSK II to HUVEC was 70% reversible by dilution or by competition, and was found to be divalent cation-independent. Binding plateaued after 10 min at a saturation of 15-20 nM. Scatchard analysis using the LIGAND computer program defined a single population of receptors with a KD of 7.7 +/- 1.6 nM and approximately 21,000 +/- 7000 binding sites/cell. N-terminal disulfide knot derivatives in which beta15-42 was absent (NDSK 325) or unexposed (NDSK, NDSK I) did not show specific binding. Specific binding of 125I-NDSK II could not be inhibited by RGDS or by antibodies to the alphavbeta3 or beta1 integrins, PECAM-1, ICAM-1, or N-cadherin. In contrast, a synthetic beta15-42/ovalbumin conjugate inhibited total 125I-NDSK II binding by 47 +/- 19% (corresponding to 95% of specific 125I-NDSK II bound) and a monoclonal antibody to vascular endothelial cadherin (VE-cadherin) inhibited binding by 35 +/- 8% (corresponding to 70% of specific 125I-NDSK II bound). Another assay was based on the capture of cadherins from HUVEC lysates by a polyclonal pan-cadherin antibody immobilized on plastic dishes. Binding of NDSK II to the captured cadherins was 89 +/- 5% specific, while specific binding of NDSK 325 and NDSK was negligible. An immortalized line of human adipose-derived microvascular endothelial cells, which express N-cadherin but not VE-cadherin, demonstrated no specific binding of NDSK II by the capture assay. These data define a novel interaction of fibrin with VE-cadherin, which is mediated by the fibrin N-terminal beta15-42 sequence, and may contribute to the mechanism through which fibrin induces angiogenesis.

Platelet aggregation in flow: differential roles for adhesive receptors and ligands

This article addresses the flow-dependent differential roles of the platelet receptors, glycoprotein (GP) GPIb and GPIIb-IIIa, in platelet aggregation mediated by ristocetin and soluble von Willebrand factor (vWF), by adenosine diphosphate (ADP) and soluble fibrinogen (Fg), and by thrombin and ADP in absence of exogenous ligands. Platelet-rich plasma or "activated" washed platelets were sheared in a coaxial cylinder at 100 to 1000 sec(-1) or in tubular flow, with surface ligands monitored by flow cytometry, with fluorescently labeled soluble ligands or monoclonal antibodies against specific adhesive domains on receptors or ligands. Aggregation was quantitated by monitoring the change in particle concentration with time by particle counting, and expressed as Capture efficiencies (CE) = Experimental/calculated initial rates of aggregation. The contributions of adhesive domains on putative ligands or receptors mediating aggregation at any given flow condition were evaluated with monoclonal antibodies or peptides known to block these adhesive sites. Surprisingly, ristocetin, which "chemically activates" GPIb/vWF to mediate spontaneous binding of the ligand to its receptor, at low concentrations yielding <2000 platelet-bound vWF monomers, gave efficient aggregation even at 1000 sec(-1) (CE = 0.34 +/- 0.02, n = 11) with only GPIb required. The physiologic activators ADP and thrombin both supported efficient aggregation of washed platelets with no exogenous ligands at 1000 sec(-1) by surface-secreted vWF (CE = 0.08 +/- 0.01, n = 6), in contrast to poorer ADP and soluble Fg-mediated aggregation in the absence of secretion (CE = 0.05). The secreted, platelet-bound, vWF-mediated aggregation completely depends on GPIIb-IIIa but partially and increasingly requires GPIb with increasing shear. Molecular models for these interactions are presented in terms of "rolling" and "firm" capture. Flow conditions will be critically important in designing and selecting anti-thrombotic drugs directed against the appropriate adhesive domains on receptors and ligands, which likely include other members such as thrombospondin and P-selectin.

Glycoprotein IIb Leu214Pro Mutation Produces Glanzmann Thrombasthenia With Both Quantitative and Qualitative Abnormalities in GPIIb/IIIa

Glanzmann thrombasthenia is an inherited bleeding disorder due to a functional reduction or absence of platelet GPIIb/IIIa (αIIbβ3) integrin receptors. Based on a prolonged bleeding time and absence of platelet aggregation in response to physiologic agonists, a 55-year-old white man was diagnosed as having Glanzmann thrombasthenia. The patient's platelet fibrinogen level was ≈5% of normal. As judged by complex-dependent monoclonal antibody (MoAb) binding, surface expression of platelet GPIIb/IIIa receptors was less than 5.5% of normal, whereas the binding of an anti-GPIIIa specific MoAb (7H2) was ≈12% of normal. Immunoblot analysis of the patient's platelet lysates showed ≈35% of normal levels of GPIIIa, ≈30% of normal levels of GPIIb, and an abnormally migrating fragment of GPIIb. Biotinylation of the surface proteins on the patient's platelets followed by immunoprecipitation and sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis showed only GPIIb and GPIIIa subunits of normal size. Surface expression of platelet αvβ3 receptors was 192% of normal, suggesting that the patient's' defect was in GPIIb. Sequence analysis of the patient's GPIIb cDNA identified a T to C transition at nucleotide 643, predicting a Leu214Pro substitution. Direct sequencing of GPIIb exon 6 indicated that the patient is homozygous for the mutation. The nature of the Leu214Pro mutation was analyzed by expression in Chinese hamster ovary (CHO) cells. As judged by subunit-specific MoAb binding, surface expression of mutant receptors was ≈60% of normal, but these receptors were not recognized by the complex-dependent monoclonal antibodies, 10E5 and 7E3. In addition, mutant receptors pretreated with the ligand-induced binding site MoAb AP5 were not recognized by the activation-dependent MoAb PAC-1 and mutant expressing CHO cells did not adhere to immobilized fibrinogen. These data suggest that the Leu214Pro mutation in GPIIb disrupts the structural conformation, and either directly or indirectly, the ligand binding properties of the heterodimeric complex. This is in accord with studies from other integrins that have implicated a β-turn in a homologous region as important in ligand binding. Thus, the Leu214Pro mutation appears to produce the Glanzmann thrombasthenia phenotype by both qualitative and quantitative abnormalities. In addition, the mutation appears to confer susceptibility of the GPIIb subunit to proteolysis.

Antipeptide Monoclonal Antibodies to Defined Fibrinogen Aα Chain Regions: Anti-Aα 487-498, a Structural Probe for Fibrinogenolysis

The fibrinogen αC domain (Aα 220-610) is one of the earliest targets attacked by plasmin following fibrinolytic system activation. Monoclonal antibodies (MoAbs) to defined sequences within the αC domain provide the opportunity to explore the structure-function relationships involved in plasmin's interaction with its Aα chain substrate at greater resolution and can serve as reagents with potential clinical use for detecting fibrinogenolysis in vivo. The MoAb F-104 was raised against a multiple antigenic peptide derivative modelled after the hydrophilic 12-residue sequence corresponding to Aα 487-498 within the αC domain. A sensitive solution phase competitive enzyme-linked immunosorbent assay (ELISA) was developed for MoAb F-104 that can be applied for the direct measurement of intact fibrinogen (purified or plasma; ED50%≈5 pmol Aα chain equivalents/mL), with negligible cross-reactive interference from peptide cleavage products released by plasmin from the COOH-terminal end of the Aα chain (<3%). Immunoblotting and ELISA studies to characterize the fate of the F-104 epitope during fibrinogenolysis in vitro indicated a rapid loss of fibrinogen-associated immunoreactivity that reflected the heterogeneity of plasmin cleavage sites within the αC domain; cleavage at the 493-494 arg-his bond destroyed the F-104 epitope, while cleavage at other sites released it in an altered, inaccessible, conformation within the structure of 35- to 40-kD and 17.5- to 18-kD Aα chain degradation products. Application of the F-104 ELISA to monitor the course of Aα chain proteolysis in a small study population of patients undergoing thrombolytic therapy for myocardial infarction (n = 14) showed that the loss of fibrinogen-associated F-104 immunoreactivity was a very early marker (within 15 to 30 minutes) of in vivo fibrinogenolysis. Additional data obtained suggest that MoAb F-104 may have promise as a reagent for evaluating the creation of an effective lytic state early during therapy, information that could help determine the need for further clinical intervention. Thus, these studies illustrate a rational, targeted, approach towards the development of a novel antifibrinogen MoAb whose application as a structural probe for the region Aα 487-498 in vitro and in vivo can provide new insights into the various molecular forms of fibrinogen that circulate under physiologic conditions and in disease.

Antipeptide Monoclonal Antibodies to Defined Fibrinogen Aα Chain Regions: Anti-Aα 487-498, a Structural Probe for Fibrinogenolysis

The fibrinogen αC domain (Aα 220-610) is one of the earliest targets attacked by plasmin following fibrinolytic system activation. Monoclonal antibodies (MoAbs) to defined sequences within the αC domain provide the opportunity to explore the structure-function relationships involved in plasmin's interaction with its Aα chain substrate at greater resolution and can serve as reagents with potential clinical use for detecting fibrinogenolysis in vivo. The MoAb F-104 was raised against a multiple antigenic peptide derivative modelled after the hydrophilic 12-residue sequence corresponding to Aα 487-498 within the αC domain. A sensitive solution phase competitive enzyme-linked immunosorbent assay (ELISA) was developed for MoAb F-104 that can be applied for the direct measurement of intact fibrinogen (purified or plasma; ED50%≈5 pmol Aα chain equivalents/mL), with negligible cross-reactive interference from peptide cleavage products released by plasmin from the COOH-terminal end of the Aα chain (<3%). Immunoblotting and ELISA studies to characterize the fate of the F-104 epitope during fibrinogenolysis in vitro indicated a rapid loss of fibrinogen-associated immunoreactivity that reflected the heterogeneity of plasmin cleavage sites within the αC domain; cleavage at the 493-494 arg-his bond destroyed the F-104 epitope, while cleavage at other sites released it in an altered, inaccessible, conformation within the structure of 35- to 40-kD and 17.5- to 18-kD Aα chain degradation products. Application of the F-104 ELISA to monitor the course of Aα chain proteolysis in a small study population of patients undergoing thrombolytic therapy for myocardial infarction (n = 14) showed that the loss of fibrinogen-associated F-104 immunoreactivity was a very early marker (within 15 to 30 minutes) of in vivo fibrinogenolysis. Additional data obtained suggest that MoAb F-104 may have promise as a reagent for evaluating the creation of an effective lytic state early during therapy, information that could help determine the need for further clinical intervention. Thus, these studies illustrate a rational, targeted, approach towards the development of a novel antifibrinogen MoAb whose application as a structural probe for the region Aα 487-498 in vitro and in vivo can provide new insights into the various molecular forms of fibrinogen that circulate under physiologic conditions and in disease.

Glycoprotein IIb Leu214Pro Mutation Produces Glanzmann Thrombasthenia With Both Quantitative and Qualitative Abnormalities in GPIIb/IIIa

Glanzmann thrombasthenia is an inherited bleeding disorder due to a functional reduction or absence of platelet GPIIb/IIIa (αIIbβ3) integrin receptors. Based on a prolonged bleeding time and absence of platelet aggregation in response to physiologic agonists, a 55-year-old white man was diagnosed as having Glanzmann thrombasthenia. The patient's platelet fibrinogen level was ≈5% of normal. As judged by complex-dependent monoclonal antibody (MoAb) binding, surface expression of platelet GPIIb/IIIa receptors was less than 5.5% of normal, whereas the binding of an anti-GPIIIa specific MoAb (7H2) was ≈12% of normal. Immunoblot analysis of the patient's platelet lysates showed ≈35% of normal levels of GPIIIa, ≈30% of normal levels of GPIIb, and an abnormally migrating fragment of GPIIb. Biotinylation of the surface proteins on the patient's platelets followed by immunoprecipitation and sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis showed only GPIIb and GPIIIa subunits of normal size. Surface expression of platelet αvβ3 receptors was 192% of normal, suggesting that the patient's' defect was in GPIIb. Sequence analysis of the patient's GPIIb cDNA identified a T to C transition at nucleotide 643, predicting a Leu214Pro substitution. Direct sequencing of GPIIb exon 6 indicated that the patient is homozygous for the mutation. The nature of the Leu214Pro mutation was analyzed by expression in Chinese hamster ovary (CHO) cells. As judged by subunit-specific MoAb binding, surface expression of mutant receptors was ≈60% of normal, but these receptors were not recognized by the complex-dependent monoclonal antibodies, 10E5 and 7E3. In addition, mutant receptors pretreated with the ligand-induced binding site MoAb AP5 were not recognized by the activation-dependent MoAb PAC-1 and mutant expressing CHO cells did not adhere to immobilized fibrinogen. These data suggest that the Leu214Pro mutation in GPIIb disrupts the structural conformation, and either directly or indirectly, the ligand binding properties of the heterodimeric complex. This is in accord with studies from other integrins that have implicated a β-turn in a homologous region as important in ligand binding. Thus, the Leu214Pro mutation appears to produce the Glanzmann thrombasthenia phenotype by both qualitative and quantitative abnormalities. In addition, the mutation appears to confer susceptibility of the GPIIb subunit to proteolysis.

Disruption of angiogenesis by PEX, a noncatalytic metalloproteinase fragment with integrin binding activity

Angiogenesis depends on both cell adhesion and proteolytic mechanisms. In fact, matrix metalloproteinase 2 (MMP-2) and integrin alphavbeta3 are functionally associated on the surface of angiogenic blood vessels. A fragment of MMP-2, which comprises the C-terminal hemopexin-like domain, termed PEX, prevents this enzyme binding to alphavbeta3 and blocks cell surface collagenolytic activity. PEX blocks MMP-2 activity on the chick chorioallantoic membrane where it disrupts angiogenesis and tumor growth. Importantly, a naturally occurring form of PEX can be detected in vivo in conjunction with alphavbeta3 expression in tumors and during developmental retinal neovascularization. Levels of PEX in these vascularized tissues suggest that it interacts with endothelial cell alphavbeta3 where it serves as a natural inhibitor of MMP-2 activity, thereby regulating the invasive behavior of new blood vessels.

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.

The AGDV residues on the gamma chain carboxyl terminus of platelet-bound fibrinogen are needed for platelet aggregation

It has been clear that only the carboxyl terminus of fibrinogen (Fg) gamma chain is required for the initial binding of Fg from solution to its GPIIbIIIa (glycoprotein IIb and IIIa) receptor on activated platelets, whereas the two RGD sites on the A alpha chain do not play any role. In this study, we examined the role of these three putative adhesive domains on Fg already bound to its receptors in mediating platelet aggregation. Activated platelets were first incubated with Fg to let the Fg bind, then with monoclonal antibodies (mAb) to block the putative adhesive domains, and the platelet suspension was then sheared or stirred to induce aggregation. The mAb 4A5, which recognizes the last four amino acid residues (AGDV) in a dodecapeptide (H12) on the carboxyl terminus of the Fg gamma chain, markedly inhibited platelet aggregation. Z69/8, a mAb whose epitope is also on the dodecapeptide but does not recognize the AGDV residues, did not have any inhibitory effect on aggregation. The anti-RGDS and anti-RGDF mAbs did not affect both macro- and micro-aggregation at all, whether tested singly or together. These results demonstrate that, similar to the situation for the initial binding of soluble Fg, only the gamma chain carboxyl terminus with the AGDV residues are needed for platelet-bound Fg to support aggregation, while the RGD sites on the A alpha chain do not seem to be required.

Identification of a region in the integrin beta3 subunit that confers ligand binding specificity

Many integrin adhesion receptors bind ligands containing the Arg-Gly-Asp (RGD) peptide motif. Most integrins exhibit considerable specificity for particular ligands and can distinguish among the many conformations of RGD. In this study we identify the domain of the integrin beta subunit involved in determining ligand binding specificity. Chimeras of beta3 and beta5, the most homologous integrin beta subunits, were expressed with alphav on the surface of human 293 cells. The ligand binding phenotype of each chimera was assessed using the ligands Fab-9 and fibrinogen, both of which have a binding preference for alphavbeta3. The results of the study show that when exons C and D of the beta3 subunit (residues 95-233) are substituted into beta5, the chimera gained the ability to bind Fab-9 with an affinity close to that of wild-type alphavbeta3. This chimera was able to mediate cell adhesion to fibrinogen. Furthermore, the swap of only a 39-residue segment of this larger domain, beta3 residues 164-202, into the backbone of beta5 enabled the chimeric integrin to bind soluble Fab-9. This small domain is highly divergent among the integrin beta subunits, suggesting that it may play a role in determining ligand selection by all integrins.

Vitronectin receptor (alpha(v)beta3) mediates platelet adhesion to the luminal aspect of endothelial cells: implications for reperfusion in acute myocardial infarction

BACKGROUND

Platelet interaction with endothelium plays an important role in the pathophysiology of coronary microcirculation. We assessed the role of the vitronectin receptor (integrin alpha(v)beta3) in platelet/endothelium adhesion.

METHODS AND RESULTS

We investigated the effect on platelet/endothelium adhesion of plasma obtained from patients with acute myocardial infarction during reperfusion (before and 8, 24, 48, and 72 hours and 5 to 7 days after direct angioplasty) and with pretreatment with alpha-thrombin (2 U/mL) and recombinant human interleukin-1beta. Platelet/endothelium adhesion was significantly enhanced by approximately 20% after pretreatment of endothelium with patient plasma for 4 hours (P<.05) compared with endothelium treated with pooled control plasma. Plasma-induced platelet/endothelium adhesion was, in part, RGD peptide dependent. Pretreatment of endothelial cells with alpha-thrombin or recombinant human interleukin-1beta enhanced platelet/endothelium adhesion and surface expression of alpha(v)beta3 on the luminal aspect of endothelium (P<.05). The adhesion of platelets, isolated platelet microparticles, and Chinese hamster ovary cells bearing human recombinant alpha(IIb)beta3 (platelet glycoprotein IIb-IIIa) to activated endothelial cells was inhibited by antiadhesive peptides GRGDSP and c(RGDfV) and monoclonal antibodies 4F10, LM609, and 7E3.

CONCLUSIONS

The expression of vitronectin receptor exposed on the luminal aspect of activated endothelium is enhanced and mediates platelet/endothelium adhesion. Vitronectin receptor-mediated platelet attachment to activated endothelium during reperfusion may contribute to reperfusion injury and could be a target for antiadhesive therapy.

The role of putative fibrinogen Aalpha-, Bbeta-, and GammaA-chain integrin binding sites in endothelial cell-mediated clot retraction

In this study, endothelial cell-mediated clot retraction was supported by fibrin generated from several purified fractions of plasma fibrinogen, purified proteolytic fragments of plasma fibrinogen, recombinant normal fibrinogen, and recombinant variant fibrinogen. These results were surprising because some of these fibrinogens lack domains that are known binding sites for the integrin receptors that support clot retraction. Specifically, fibrinogens lacking Aalpha-chain RGD residues at 572-574 or lacking the gamma-chain residues AGDV 408-411 supported endothelial cell-mediated clot retraction as well as intact fibrinogen. Thus, clot retraction mediated by endothelial cells is not dependent on either of these sites. A variety of monoclonal antibodies against the integrin alphavbeta3 partially inhibited the endothelial cell-mediated retraction of clots formed from plasma fibrinogen. As expected, an antibody to the platelet integrin alphaIIbbeta3 did not inhibit endothelial cell-mediated clot retraction. These results indicate that this retraction is mediated at least in part by alphavbeta3. These results support the conclusion that (a) neither of the two fibrinogen cell binding sites described above is required to support clot retraction or that (b) either site alone or in conjunction with other fibrin(ogen) region(s) can support clot retraction. Thus, endothelial cell-mediated clot retraction appears to be dependent on fibrinogen cell binding sites other than those required to support adhesion of resting platelets to immobilized fibrinogen and platelet aggregation.

Cytokine Treatment of Endothelial Cells Increases Glycoprotein Ibα-Dependent Adhesion to von Willebrand Factor

Endothelial cells (EC) possess at least two membrane receptors for von Willebrand factor (vWF ), the vitronectin receptor (VNR, αvβ3 ), which recognizes an Arg-Gly-Asp (RGD) sequence in the C-terminus of vWF, and glycoprotein Ibα (GP Ibα), which interacts with a region in the N-terminal A1 domain of vWF. In the absence of added cytokines, EC attachment to a vWF substratum is mediated largely through the αvβ3 , with a smaller contribution by GP Ibα. In the present study, we have examined the effect of cytokines on the receptor specificity of EC attachment to wild-type vWF (WT-vWF ) and to vWF, which had been mutated in the C-terminal RGDS sequence (RADS-vWF ). Exposure of human umbilical vein EC (HUVEC) to tumor necrosis factor-α (TNF-α) or to TNF-α in combination with interferon-γ (IFN-γ), but not to interleukin-1β (IL-1), increased attachment to RADS-vWF by about twofold. The TNF-α–induced increase in EC attachment was accompanied by an increase in cell surface GP Ibα expression; GP Ibα surface expression was not increased by IL-1. Attachment of untreated HUVEC to WT-vWF could be inhibited 60% to 70% by a monoclonal antibody (MoAb) (LM609) to the VNR and 30% to 40% by the A1 fragment of vWF (containing the GP Ibα binding domain). The pattern of inhibition of attachment to WT-vWF was largely unchanged after TNF-α treatment of HUVEC. In contrast, the attachment to WT-vWF of HUVEC, treated with TNF-α +IFN-γ was completely inhibited by vWF-A1 and inhibited only 35% by the anti-VNR antibody LM609. Two MoAbs to GP Ibα produced similar, but incomplete, inhibition. Pretreatment of HUVEC with the combination of TNF-α +IFN-γ produced a dramatic decrease in VNR expression, confirming previous findings of Defilippi et al. These results suggest that in the presence of the inflammatory cytokines TNF-α +IFN-γ, the endothelial GP Ib complex is a major determinant of HUVEC adhesion to surface-bound vWF.

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.

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.

Fibrinogen is a ligand for integrin alpha5beta1 on endothelial cells

Previous studies have shown that fibrinogen can associate with endothelial cells via an Arg-Gly-Asp (RGD) recognition specificity. In the present study, we have characterized the specificity of fibrinogen binding to endothelial cells under different cation conditions. Fibrinogen binding to suspended endothelial cells was selectively supported by Mn2+ and was suppressed by Ca2+. The Mn2+-supported interaction was completely inhibited by RGD peptides but not by alphavbeta3 blocking monoclonal antibodies. In contrast, the interaction was completely blocked by two alpha5beta1 monoclonal antibodies. This interaction was not mediated by fibronectin bound to the integrin; could be demonstrated with purified alpha5beta1; and also was observed with a second alpha5beta1-bearing cell type, platelets. The binding of fibrinogen to alpha5beta1 on endothelial cells in the presence of Mn2+ was time-dependent, specific, saturable, and of high affinity (Kd = 65 nM). By employing anti-peptide monoclonal antibodies, the carboxyl-terminal RGD sequence at Aalpha 572-574 was implicated in fibrinogen recognition by alpha5beta1. Two circumstances were identified in which alpha5beta1 interacted with fibrinogen in the presence of Ca2+: when the receptor was activated with monoclonal antibody (8A2) or when the fibrinogen was presented as an immobilized substratum. These results identify fibrinogen as a ligand for alpha5beta1 on endothelial and other cells, an interaction which may have broad biological implications.

Changes in the expression of alpha v integrins in oral squamous cell carcinomas

Integrins are cell surface adhesion molecules that regulate normal cellular interactions; aberrant integrin expression is believed to play a role in tumour invasion and metastasis. The alpha v subunit is capable of forming heterodimers with several beta subunits but not all the heterodimers expressed in oral epithelium have been investigated. We have examined the distribution of alpha v integrins in normal buccal mucosa and seventeen oral squamous cell carcinomas. Antibodies to the alpha v subunit and alpha v beta 5 heterodimer stained normal epithelium, with strong expression in the basal layers and weaker staining in the more superficial layers. The beta 3 and beta 6 subunits were not expressed in normal epithelium. Anti-alpha v and anti-alpha v beta 5 antibodies stained all the squamous cell carcinomas, but the pattern of expression was variable both within and between tumours. Poorly differentiated tumours showed the weakest staining and often had areas showing loss of expression. beta 6 was expressed in all of the carcinomas, indicating new expression of the alpha v beta 6 integrin in malignant oral epithelium. These results suggest that alterations in alpha v integrins may contribute to the behaviour of malignant epithelium and that alpha v beta 6 expression may play a role in tumour progression.

Changes in binding affinity of a monoclonal antibody to a platelet binding domain of fibrinogen adsorbed to biomaterials

Previously, we found that when fibrinogen-coated polyurethanes resided in a buffer for a period of time (the 'residence time') platelet adhesion to these materials decreased. Other changes in adsorbed fibrinogen such as decreases in polyclonal antibody binding and SDS elutability supported the conclusion that fibrinogen undergoes postadsorptive conformational changes. Subsequently we measured the binding of monoclonal antibodies to the three putative platelet binding sites on fibrinogen, using a single mid-range concentration of antibody. We found that binding of a monoclonal antibody to the platelet binding site at the C-terminus of the gamma chain of fibrinogen changed little with residence time, while binding of monoclonal antibodies to the other two putative binding sites on fibrinogen either increased with residence time (RGDF at A alpha 95-98), or first increased and then decreased with residence time (RGDS at A alpha 572-575). In the current study, we measured antibody binding affinity, Ka, by measuring antibody binding at a series of antibody concentrations. This is a more sensitive method for detecting changes in adsorbed fibrinogen than measuring antibody binding from a single antibody concentration. The Ka was determined for two antibodies, M1 (4A5), which binds to a platelet binding domain of fibrinogen (gamma 402-411) and R1 (155 B 1616), which binds to residues 87-100 of the A alpha chain (containing an RGDF site). A summary of the results for the M1 antibody are as follows. The Ka was higher for M1 binding to fibrinogen adsorbed to Immulon I than to Biomer, Biospan or poly(ethylene terephthalate), suggesting that fibrinogen adsorbed to Immulon I is more platelet adhesive than fibrinogen adsorbed to the other polymers. On Biospan, the Ka decreased from 2.8 x 10(9) to 1.0 x 10(9) M-1 after a 24 h 37 degrees C residence time, which correlated with the decrease in platelet adhesiveness of adsorbed fibrinogen observed previously under these conditions. The change in Ka was greater when adsorbed fibrinogen was kept under denaturing conditions. For example, the Ka decreased from 2.8 x 10(9) to 0.8 x 10(9) M-1 after a 1 h 70 degrees C residence time whereas it remained approximately the same, 2.9 x 10(9) M-1, after a 24 h 0 degree C residence time.

Heparin-binding domain of fibrin mediates its binding to endothelial cells

Spreading of human umbilical vein endothelial cells (ECs) on fibrin requires thrombin cleavage of fibrinopeptide B (FPB) and subsequent exposure of the new beta 15-42 N-terminus. To further understand the interactions between ECs and fibrin beta 15-42 sequences, binding of fibrin(ogen) to EC monolayers was measured with polyclonal anti-fibrinogen (FBG) in parallel with monoclonal anti-FBG (18C6, beta 1-21; J88B, gamma 63-78) and anti-fibrin (T2G1, beta 15-21) antibodies in an indirect enzyme-linked immunosorbent assay. To accomplish this, large, soluble fragments of fibrin were prepared by cyanogen bromide (CNBr) cleavage (fibrin-CNBr); CNBr-cleaved FBG (FBG-CNBr) served as the control ligand. N-terminal fibrin-CNBr bound to EC monolayers and cells in suspension in a dose-dependent and saturable manner. By contrast, FBG-CNBr bound only 50% as well to EC monolayers, with no significant binding of intact FBG, C-terminal FBG plasmic fragment D, or N-terminal plasmic fragment E, which lacks beta 1-53. ECs bound the peptide beta 15-42-bovine serum albumin (BSA) conjugate but neither a scrambled beta 15-42 peptide conjugate nor conjugates of beta 24-42, beta 18-27, or beta 18-31. Binding of fibrin-CNBr was inhibited 54% by the beta 15-42-BSA conjugate and 17% by the B beta 1-42-BSA conjugate but not by free beta 15-42 peptide or RGDS-cell binding peptide. Binding of fibrin-CNBr was inhibited > 95% by heparin in a concentration-dependent manner; the same concentrations of heparin inhibited binding of beta 15-42-BSA by > 75% but not the dose-dependent binding of fibronection to ECs. These data suggest that in their native conformation, FBG B beta 15-42 sequences are unavailable for binding to ECs and that thrombin-induced exposure of beta 15-42 is required for binding by a heparin-dependent, RGD-independent mechanism at the new N-terminus of fibrin.