The effect of fibrin on cultured vascular endothelial cells

Coagulation and the vessel wall in pulmonary embolism

Venous thromboembolism comprises deep-vein thrombosis, thrombus in transit, acute pulmonary embolism, and chronic thromboembolic pulmonary hypertension (CTEPH). Pulmonary thromboemboli commonly resolve, with restoration of normal pulmonary hemodynamics. When they fail to resorb, permanent occlusion of the deep veins and/or CTEPH are the consequences. Apart from endogenous fibrinolysis, venous thrombi resolve by a process of mechanical fragmentation, through organization of the thromboembolus by invasion of endothelial cells, leukocytes, and fibroblasts leading to recanalization. Recent data utilizing various models have contributed to a better understanding of venous thrombosis and the resolution process that is directed at maintaining vascular patency. This review summarizes the plasmatic and cellular components of venous thrombus formation and resolution.

Utilizing sphingosine-1-phosphate to stimulate sprouting angiogenesis

In vitro models are useful for dissecting cell behavior under controlled conditions. Angiogenesis is a multistep process where endothelial cells (ECs) are activated by pro-angiogenic factors to degrade the basement membrane, migrate into the surrounding matrix, and form sprouting structures connecting neighboring vessels. Sphingosine-1-phosphate (S1P), a biologically active sphingolipid, promotes vessel morphogenesis and angiogenesis during embryonic development and in adults under normal and pathological conditions via its actions on ECs. Here, we describe an in vitro endothelial morphogenic assay that is significantly enhanced by S1P. This method allows for testing whether molecules and their related signaling pathways regulate the initiation of angiogenic sprouts stimulated by S1P, as well as whether individual compounds have pro- or anti-angiogenic properties.

Fibulin-1 and fibrinogen in human atherosclerotic lesions

Fibulin-1 is a fibrinogen-binding blood protein and a component of many extracellular matrices (ECM) including those of blood vessels. In this study, the deposition patterns of fibulin-1 and fibrinogen were examined in human coronary artery atherosclerotic lesions excised by atherectomy from 20 patients. Fibulin-1 deposition was found to be closely overlapping with fibrinogen located within the atherosclerotic lesions and in regions containing fresh thrombi. Pronounced intracellular fibulin-1 immunostaining was apparent in lesion areas rich in macrophages and foam cells, although THP-1 macrophages and foam cells were found not to express fibulin-1. Strong ECM deposition of fibulin-1 was observed in acellular atheromatous and myxomatous regions. By contrast, fibulin-1 was present at relatively low levels in the ECM associated with smooth muscle cells within and outside of lesions and was not detected in sclerotic regions. These results reveal the pattern of fibulin-1 within human atherosclerotic lesions and highlight the potential for fibulin-1, perhaps derived from the blood and acting in conjunction with fibrinogen, to play a role in the etiology and cardiovascular disease progression, particularly with respect to thrombotic aspects of atherosclerosis.

In vitro analysis of the interactions between preadipocytes and endothelial cells in a 3D fibrin matrix

The volume-persistent survival of transplanted adipose tissue in vivo relies on early vascularization, due to an otherwise early induction of apoptosis of the centrally located cells. Thus, one way to enable the early formation of a capillary network resulting in a sufficient perfusion of the transplanted construct might be the co-transplantation of autologous preadipocytes with endothelial cells. To investigate preadipocyte-endothelial cell interaction, three-dimensional proliferation- and angiogenesis assays were performed in vitro. Proliferation rates of co-cultured endothelial cells and preadipocytes suspended in a fibrin matrix were elucidated by Alamarblue assays. The spheroid angiogenesis model was applied for analyzing the effects of vascular endothelial cell growth factor (VEGF) and basic fibroblast growth factor (bFGF) (produced by preadipocytes) as well as the impact of cell-cell interaction between preadipocytes and endothelial cells and fibrin matrix on endothelial cell migration. Preadipocytes proliferated in fibrin glue, whereas endothelial cells underwent apoptosis. By co-culturing, both cell types demonstrated an increased proliferation rate. Preadipocytes provoked migration of endothelial cells. Blocking bFGF and/or VEGF led to a significant decrease of migration. Changes in fibrin structure were followed by migration of single cells instead of sprouting. An appropriate fibrin matrix as well as already differentiated endothelial cells are necessary for preadipocytes to develop their angiogenic activity via bFGF and VEGF.

Elevated levels of soluble P-selectin in mice alter blood-brain barrier function, exacerbate stroke, and promote atherosclerosis

Cerebrovascular and cardiovascular diseases are a major cause of morbidity and mortality. Soluble P-selectin (sP-selectin) is a biomarker for platelet/endothelial activation and is considered a risk factor for vascular disease. sP-selectin enhances procoagulant activity by inducing leukocyte-derived microparticle production and promotes activation of leukocyte integrins. However, it is not known whether it directly contributes to vascular complications. We investigated the effect of increased levels of sP-selectin on blood-brain barrier (BBB) function, stroke outcome, and atherosclerosis by comparing wild-type mice with P-sel(DeltaCT/DeltaCT) mice in which the endogenous P-selectin gene was replaced with a mutant that produces abnormally high plasma levels of sP-selectin. P-sel(DeltaCT/DeltaCT) mice presented several abnormalities, including (1) higher BBB permeability, with 25% of the animals showing differential permeability between the right and left hemispheres; (2) altered social behavior with increased aggression; (3) larger infarcts in the middle cerebral artery occlusion ischemic stroke model; and (4) increased susceptibility to atherosclerotic, macrophage-rich lesion development in both male and female mice on the apoE(-/-) genetic background. Thus, elevated sP-selectin is not only a biomarker for vascular disease, but also may contribute directly to atherosclerosis and cerebrovascular complications.

Identification of a binding site on human FGF-2 for fibrinogen

Endothelial cell adhesive interactions are mediated by both fibrinogen and fibrin, and growth is stimulated by fibroblast growth factor 2 (FGF-2). We have shown previously that FGF-2 binds specifically and with high affinity to fibrinogen and fibrin and that fibrinogen potentiates the proliferative capacity of FGF-2 and also protects it from proteolytic degradation. To further characterize this interaction we have performed FGF-2 mutagenesis to identify the interactive site. Because FGF-1 has a similar structure to FGF-2 but does not bind to fibrinogen, we used a strategy of cassette and site-directed mutagenesis, exchanging residues from FGF-1 and FGF-2 and correlating structural changes with fibrinogen binding. Two cassette interchange mutants, 2212 and 2211, contained either the third cassette or both the third and fourth cassettes from FGF-1, and neither exhibited any affinity for fibrinogen. Exchange of 5 residues (Phe95, Ser100, Asn102, Arg107, and Arg109) from FGF-2 into the corresponding sites in the third cassette of FGF-1 imparted high-affinity binding with apparent dissociation constants (Kd) of 5.3 nM and 8.6 nM, respectively, compared with 1.3 nM for wild-type FGF-2. We conclude that these 5 residues define a high-affinity binding site in FGF-2 for fibrinogen.

FGF-2 but not FGF-1 binds fibrin and supports prolonged endothelial cell growth

Endothelial cell viability and growth are dependent on both polypeptide growth factors, and integrin-mediated matrix interactions. We have now examined the ability of fibrin-binding and non-binding growth factors to support long-term endothelial cell growth in the presence or absence of the soluble form. Endothelial cells were cultured on a fibrin surface, with or without FGF-1 or FGF-2, and proliferation was determined by (3)H-thymidine incorporation. Cells cultured on fibrin with no growth factor showed minimal proliferation up to 96 h. In contrast, when FGF-2 was incorporated into fibrin, proliferation was increased 6.5 +/- 0.6-fold, equal to growth on a fibrin surface with FGF-2 continually present in the medium. Thymidine incorporation was similar when cells were cultured on a fibrin surface that had been incubated with FGF-2 and then the growth factor removed (8.6 +/- 0.5-fold). In contrast to results with FGF-2, a surface of fibrin exposed to FGF-1 supported minimal growth, whereas growth was comparable to either FGF-1 or FGF-2 present in the medium. Comparable results were observed when proliferation was quantitated by cell counting at times up to 48 h. Binding studies demonstrated no high-affinity interaction of FGF-1 with fibrinogen or fibrin. We conclude that FGF-2 bound to fibrin supports prolonged endothelial cell growth as well as soluble FGF-2, whereas FGF-1 does not bind to fibrin and can support endothelial cell growth only if continually present in soluble form. Fibrin may serve as a matrix reservoir for FGF-2 to support cell growth at sites of injury or thrombosis.

Software-supported image quantification of angiogenesis in an in vitro culture system: application to studies of biocompatibility

Healing of soft tissue trauma and bone discontinuities following implantation involves acute inflammatory reactions and the formation of blood vessels (angiogenesis). During angiogenesis new capillary vessels arise from the existing vasculature. Endothelial cells (EC) are the major cell type involved in angiogenesis. Corrosion of orthopaedic metallic implant materials (e.g. CoCr alloys) can cause locally high concentrations of heavy metal ions in the peri-implant tissues. Some divalent metal ions (Co2+, Ni2+, Zn2+) lead to the activation of EC in vitro. Upon exposure to these ions. EC release cytokines and chemokines and increase the expression of cell surface adhesion molecules, which represents the pro-inflammatory phenotype. In this study we have examined whether metal ions influence the other endothelial aspect of wound healing, the angiogenic response. Therefore, we utilized an in vitro model of angiogenesis and examined the effects of divalent cobalt ions on the in vitro vessel formation. The quantification of the cobalt/ion-exerted effects on angiogenesis in vitro was performed using a contrast-rich vital staining and analysed by software-supported image quantification.

Angiogenesis, thrombospondin, and ductal carcinoma in situ of the breast

Angiogenesis, the growth of new vessels from existing vasculature, plays an essential role in tumour development. The process involves interaction between a variety of cells, growth factors, and components of the extracellular matrix, regulated by pro-angiogenic and anti-angiogenic factors. This review profiles these factors, outlines the available methods for measuring new vessel formation, and discusses the importance of angiogenesis in breast cancer, with emphasis on ductal carcinoma in situ.

Inflammation and atherothrombosis

An immune and inflammatory response involving endothelial and smooth muscle cells accompanies the accumulation of lipids and fibrous materials in atheromatous arteries. The inflammatory response involves not only the intrinsic cells of the artery wall, but also circulating leukocytes. Lymphocytes as well as macrophages participate importantly in this disease process. Plaque composition and vulnerability have emerged as more critical determinants of plaque rupture than the degree of lumenal stenosis. Thus, plaque biology has proven more important than gross morphology in determining the clinical consequences of the disease. Rupture of the atherosclerotic plaque, its thrombotic complications, and their sequelae have gained increased recognition as the proximate causes of disability and death due to related syndromes such as acute myocardial infarction. Arterial thromboses occur much more readily in arteries damaged or distorted by the atherosclerotic process. The consequences of a given plaque disruption and subsequent thrombus formation will depend on both "systemic" or fluid phase determinants and local "solid state" factors. The complex heterogeneous structure of thrombi includes fibrin, platelets, erythrocytes, and leukocytes. The local balance of proteases and inhibitors on the fibrin and cell surfaces will determine thrombus stability and persistence. Enhanced understanding of the processes involved in the development and progression of atherosclerosis and its complications will surely provide areas that can be targeted in the treatment of the disease.

Vascular endothelial growth factor–stimulated endothelial cells promote adhesion and activation of platelets

Coagulation abnormalities, including an increased platelet turnover, are frequently found in patients with cancer. Because platelets secrete angiogenic factors on activation, this study tested the hypothesis that platelets contribute to angiogenesis. Stimulation with vascular endothelial growth factor (VEGF, 25 ng/mL) of human umbilical vein endothelial cells (HUVECs) promoted adhesion of nonactivated platelets 2.5-fold. In contrast, stimulation of HUVECs with basic fibroblast growth factor (bFGF) did not promote platelet adhesion. By blocking tissue factor (TF) activity, platelet adhesion was prevented and antibodies against fibrin(ogen) and the platelet-specific integrin, αIIbβ3, inhibited platelet adhesion for 70% to 90%. These results indicate that VEGF-induced platelet adhesion to endothelial cells is dependent on activation of TF. The involvement of fibrin(ogen) and the αIIbβ3 integrin, which exposes a high-affinity binding site for fibrin(ogen) on platelet activation, indicates that these adhering platelets are activated. This was supported by the finding that the activity of thrombin, a product of TF-activated coagulation and a potent platelet activator, was required for platelet adhesion. Finally, platelets at physiologic concentrations stimulated proliferation of HUVECs, indicative of proangiogenic activity in vivo. These results support the hypothesis that platelets contribute to tumor-induced angiogenesis. In addition, they may explain the clinical observation of an increased platelet turnover in cancer patients. Platelets may also play an important role in other angiogenesis-dependent diseases in which VEGF is involved, such as diabetes and autoimmune diseases.

Vascular endothelial growth factor–stimulated endothelial cells promote adhesion and activation of platelets

Coagulation abnormalities, including an increased platelet turnover, are frequently found in patients with cancer. Because platelets secrete angiogenic factors on activation, this study tested the hypothesis that platelets contribute to angiogenesis. Stimulation with vascular endothelial growth factor (VEGF, 25 ng/mL) of human umbilical vein endothelial cells (HUVECs) promoted adhesion of nonactivated platelets 2.5-fold. In contrast, stimulation of HUVECs with basic fibroblast growth factor (bFGF) did not promote platelet adhesion. By blocking tissue factor (TF) activity, platelet adhesion was prevented and antibodies against fibrin(ogen) and the platelet-specific integrin, αIIbβ3, inhibited platelet adhesion for 70% to 90%. These results indicate that VEGF-induced platelet adhesion to endothelial cells is dependent on activation of TF. The involvement of fibrin(ogen) and the αIIbβ3 integrin, which exposes a high-affinity binding site for fibrin(ogen) on platelet activation, indicates that these adhering platelets are activated. This was supported by the finding that the activity of thrombin, a product of TF-activated coagulation and a potent platelet activator, was required for platelet adhesion. Finally, platelets at physiologic concentrations stimulated proliferation of HUVECs, indicative of proangiogenic activity in vivo. These results support the hypothesis that platelets contribute to tumor-induced angiogenesis. In addition, they may explain the clinical observation of an increased platelet turnover in cancer patients. Platelets may also play an important role in other angiogenesis-dependent diseases in which VEGF is involved, such as diabetes and autoimmune diseases.

The effects of thrombus, thrombectomy and thrombolysis on endothelial function

OBJECTIVE

this study was undertaken to examine and compare the effects of thrombus, thrombectomy, and thrombolysis on endothelial function as measured by endothelium-dependent vasorelaxation (EDR).

METHODS

adult, male New Zealand white rabbits underwent ligation of the left common iliac to femoral artery to induce thrombosis and were then randomly assigned to one of five groups, n=6 in each. Group A consisted of ligation and thrombosis for 4 h. Group B underwent similar ligation for 4 h, but without intraluminal thrombus present. Following 4 h of ligation and thrombosis, Group C underwent thrombectomy while group D was treated with urokinase (UK), 4000 U/min for 30 min. Group E underwent UK infusion alone. The right external iliac artery served as control vessel in each group. All arteries were removed and endothelial function was determined by measuring EDR.

RESULTS

the presence of thrombus reduced EDR by 50% (group A) compared to control. Vessels with interrupted flow, but not exposed to thrombus, retained normal EDR (group B). Thrombectomy decreased EDR significantly (group C) compared to thrombolysis (group D) and control. UK did not significantly alter EDR (groups D, E).

CONCLUSIONS

exposure of endothelium to thrombus significantly decreases EDR. EDR was not affected by interruption of blood flow in the absence of thrombus. Thrombectomy appeared to cause a further additive insult to the endothelium. In contrast, thrombolysis with UK preserved residual endothelial function. These data suggest that it is important to differentiate the effects of thrombus on endothelium from effects due to thrombectomy or thrombolysis when evaluating treatment modalities for arterial thrombosis.

Integrin alphavbeta3-RGDS interaction mediates fibrin-induced morphological changes of glomerular endothelial cells

BACKGROUND

In our previous studies, we found that intraglomerular deposition of fibrin and its metabolites was related to glomerular sclerosis and reduced renal function. It has been reported that both overlying and underlying fibrin may induce specific morphological changes of cultured endothelial cells from large blood vessels. The dependency of these morphological changes on the integrin alphavbeta3-arginyl-glycyl-aspartyl-serine (RGDS) interaction is still controversial. We hypothesized that glomerular endothelial cells (GECs) stimulated by fibrin might undergo morphological changes through an integrin alphavbeta3-RGDS interaction. Methods. In vitro studies were performed to examine the growing status of GECs stimulated by overlying and underlying fibrin gels in the presence or absence of the following: 50 microg/ml anti-alphavbeta3 integrin monoclonal antibody 23C6 or nonimmune mouse IgG, 1 mg/ml synthetic RGDS or arginyl-glycyl-glycyl-serine (RGGS) peptide, 10 mg/ml sodium heparin, 100 microg/ml cycloheximide, and 10 microM actinomycin D. Fast protein liquid chromatography (FPLC)-purified fibrinogen and the third to fifth passages of human GECs were also used in this study.

RESULTS

GECs developed capillary tube structure after 60 hours of culturing on fibrin gels, and GECs cultured on gelatin-coated plates displayed a monolayer of cobblestone-like cells in the presence or absence of 23C6 and synthetic RGDS peptide. Fibrin-induced capillary tube formation was promoted by 23C6 and inhibited by RGDS peptide, cycloheximide, and actinomycin D. Disorganization of the GEC monolayer was induced by overlying fibrin, but was not induced by overlying agarose gels and glass cover slips or culturing in fibrinogen, 0.05 NIH U/ml thrombin, fibrin supernatants, as well as in fibrin degradation products. Disorganization of GEC monolayer can be induced by both des-AA-fibrin and des-AABB-fibrin and was unaffected by heparin. Furthermore, both 23C6 and synthetic RGDS peptide prevented disorganization of GECs induced by overlying fibrin, whereas nonimmune mouse IgG, synthetic RGGS peptide, cycloheximide, and actinomycin D had no similar effect.

CONCLUSIONS

GECs cultured on fibrin gels may develop capillary structure spontaneously, and GECs covered by fibrin gels may undergo disorganization. Our data suggest that these GEC morphological changes are mediated by an integrin alphavbeta3-RGDS interaction.

Urinary tissue factor: a potential marker of disease

Tissue factor (TF) is the main physiological initiator of blood coagulation and may be important in the biology of a variety of solid malignancies, particularly where angiogenesis is a critical factor. TF is frequently encrypted in the plasma membrane of cells in contact with blood, and is exposed only after stimulation by certain agonists. Cancer cells variably express TF and cancer cell lines which exhibit multidrug resistance contain more TF than parental cells. TF is increased in both tumour-associated macrophages and blood monocytes and has been implicated in abnormal coagulation activation seen in patients with inflammatory conditions and cancer. TF is also found in urine (uTF) in a lipid-associated form, probably of kidney origin. uTF levels can be assayed in a cost-effective manner and may be clinically important, particularly in patients with renal disorders and malignancy. uTF levels are not significantly affected by age, gender or cigarette smoking.

Elevated expression of urokinase-like plasminogen activator and plasminogen activator inhibitor type 1 during the vascular remodeling associated with pulmonary thromboembolism

Information is lacking on the mechanisms involved in the organization, resolution, and repair of the vascular lumen after acute pulmonary thromboembolism. Because recent data suggest that the balance between plasminogen activators (PAs) and type 1 plasminogen activator inhibitor (PAI-1) plays a role in regulating cell migration within the extracellular matrix, we investigated the expression of these molecules by immunohistochemical and in situ hybridization analysis of pulmonary artery specimens from patients suffering fatal pulmonary embolism. The data were compared with the expression of these molecules in both patients' noninvolved pulmonary arteries and organ donor pulmonary arteries. Regions of initial organization and vascular remodeling were identified by a modified trichrome stain and by the presence of proliferating cell nuclear antigen (PCNA), a cell marker of proliferation. Staining for tissue-type PA antigen was low to undetectable in endothelial cells directly in contact with the fibrin-platelet thromboembolus and in areas in which the endothelial cell lining was replaced by cell growth into the thrombus. Urokinase-like PA (u-PA) expression was detected in mononuclear cells within the thrombus in the initial phase of thromboembolism and within cells migrating into the thrombus during the later stages of organization. PAI-1 expression was elevated in the monolayer of endothelial cells underlying the fresh platelet-fibrin thromboembolus and in a PCNA-positive cell population present between the pulmonary arterial intima and the thromboembolus that represents early organization. Increased expression of PAI-1 may play a role in inhibiting proteolysis and fostering the localization of the acute fibrin-platelet thrombus to the vascular wall, which is followed by the upregulation of u-PA in migrating cells during the reorganization process.

Hemostatic/fibrinolytic predictors of allograft coronary artery disease after cardiac transplantation

Allograft coronary artery disease (CAD) remains the leading cause of morbidity and mortality affecting the long-term survival of patients after cardiac transplantation. Because there is increasing evidence that imbalances in hemostatic and fibrinolytic pathways are associated with graft failure, we hypothesized that atherothrombotic risk factors may contribute to allograft CAD. This study sought to determine if plasma hemostatic and fibrinolytic parameters are associated with the severity of allograft CAD. The extent of allograft CAD was investigated by angiography and intravascular ultrasound (IVUS) in 16 cardiac transplant recipients. Intimal thickening was quantified using IVUS by measuring the intimal index (li = intimal area/[intimal area + luminal area]) in two to five segments of the left anterior descending (LAD) coronary artery. The maximal li per patient was calculated and index to the time post-transplant (Mxli/Yr). Plasma fibrinogen (FGN), tissue-type plasminogen activator (t-PA), plasminogen activator inhibitor-1 (PAI-1), lipoprotein(a) (Lp(a)), and net fibrinolytic activity of plasma were assayed 6-24 months after transplant as indicators of the fibrinolytic system and then correlated with the IVUS measurements. The FGN level correlated with the severity of intimal thickening, Mxli/Yr (r2 = 0.41, p = 0.008), and was inversely correlated with angiographic tertiary vessel filling (r2 = 0.25, p = 0.051). In patients with lower plasma fibrinolytic activity (lytic zone less than 100 mm2), Mxli/Yr was increased eightfold (0.218 +/- 0.137 versus 0.025 +/- 0.021, p = 0.001). t-PA (r2 = 0.0004, p = 0.94), PAI-1 (r2 = 0.008, p = 0.75) and Lp(a) levels (r2 = 0.11, p = 0.21) did not predict Mxli/Yr. Thus, we demonstrate that plasma FGN and net fibrinolytic activity correlate with the degree of intimal thickening measured by IVUS after cardiac transplantation. These data suggest that fibrin deposition may play a role in allograft CAD after cardiac transplantation.

Fibrin Regulation of Interleukin-8 Gene Expression in Human Vascular Endothelial Cells

Recent studies in our laboratory, as well as others, have suggested that fibrin can regulate cell function in vitro and likely control inflammation in vivo by acting as a potent cell activator. This has led us to hypothesize that during tissue and vascular injury, fibrin can enhance leukocyte recruitment by inducing vascular endothelial cell expression of leukocyte chemotactic factors. To begin to test this hypothesis, we developed an in vitro model of in situ fibrin polymerization on human umbilical vein endothelial cell culture (HUVEC) and determined the ability of fibrin to induce HUVEC expression of the potent leukocyte chemotactic factor interleukin-8 (IL-8). Our initial studies showed that fibrin induced IL-8 expression in a time- and dose-dependent fashion. Fibrin-induced IL-8 expression in HUVEC could be seen as early as 2 hours post-fibrin stimulation. Additionally, fibrin concentrations as low as 30 μg/mL stimulated a detectable level of IL-8 antigen expression from HUVEC. We also showed that this fibrin induced IL-8 had the identical molecular weight and similar antigenic identity as recombinant and monocyte derived IL-8. Northern blot analysis showed that the IL-8 antigen increase seen in fibrin treated HUVEC was due to fibrin induced elevation of steady state mRNA expression in HUVEC. These data clearly support our hypothesis that fibrin is a potent vascular endothelial cell (VEC) activator that can directly contribute to leukocyte recruitment and activation by inducing leukocyte chemotactic factor expression from VEC.

Human fibroblast adhesion to fibrinogen

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

Fibrinogen and its relations to subclinical extracoronary and coronary atherosclerosis in hypercholesterolemic men

The association between plasma fibrinogen and the presence of carotid, femoral, and aortic plaque (high-resolution B-mode ultrasonography) and coronary calcium deposit (ultrafast computed tomography scanner) was determined in 693 hypercholesterolemic, never-treated men free of previous or current clinical symptoms of cardiovascular disease. The number of subjects with extracoronary disease sites and coronary calcification deposits was significantly higher in the upper than in the lower tertile of fibrinogen. Plasma fibrinogen increased according to the number of diseased sites. The odds ratio of the upper to lower fibrinogen tertile for the presence of arterial lesions was 2.6 (1.7 to 4) for carotid, 2.2 (1.5 to 3.2) for aorta, 2.2 (1.5 to 3.1) for femoral, 1.8 (1.3 to 2.6) for coronary, and 3.6 (2.3 to 6.1) for one of four diseased sites. Adjustment for age, total cholesterol, HDL cholesterol, triglycerides, current smoking, and systolic pressure slightly reduced the association between fibrinogen and atherosclerosis. A synergistic effect between fibrinogen and total cholesterol/ HDL cholesterol (TC/HDL) ratio seemed to be operating on atherosclerosis, because nearly all of the individuals (98%) had a diseased site when fibrinogen and TC/HDL tertiles were the highest. This result suggests that fibrinogen is involved in the subclinical phase of extracoronary and coronary atherosclerosis and may potentiate the atherogenic effect of hyperlipidemia.

Inhibitors of fibrinolysis are elevated in atherosclerotic plaque

The proteins of the fibrinolytic system have been examined in the human normal and atherosclerotic arterial wall by immunohistochemical techniques and by quantitative immunoassay of extracts. The concentration of plasminogen activator inhibitor-1 (PAI-1) increased significantly during the progression from normal vessels to fatty streaks to the developed atherosclerotic plaque. Staining for PAI-1 was strongly positive, particularly in the areas adjacent to the plaque. In these areas, PAI-1 appeared to colocalized with its binding protein vitronectin. Alpha2-antiplasmin (alpha2-AP) was present in the aorta at even higher concentrations than PAI-1; a small but significant increase was seen in some atherosclerotic compared with normal vessel walls. Tissue plasminogen activator (TPA) showed the opposite trend, being lowest in lesions with plaque. Thus, higher concentrations of the two principal inhibitors of fibrinolysis, PAI-1 and alpha2-AP, together with lower levels of TPA, are characteristic of advanced atheromatous lesions. Alteration in the balance of the fibrinolytic system, favoring its inhibition, may predispose to the development or maintenance of atherosclerotic plaque.

The morphologic change of endothelial cells by ancrod-generated fibrin is triggered by alpha v beta 3 integrin binding and the subsequent activation of a G-protein coupled phospholipase C

The mechanism of morphologic change of human cultured umbilical vein endothelial cells (HUVECs) caused by fibrin was investigated. Ancrod, a thrombin-like enzyme, did not cause morphologic alteration of HUVEC by itself at concentrations ranging from 0.01 to 10 U/ml. However, when 0.02 U/ml of ancrod was added to cultured HUVEC monolayers in the presence of citrated plasma, it caused pronounced morphologic change of HUVEC after 6-10 h incubation period. Gly-Pro-Arg-Pro (4 mg/ml), an inhibitor of fibrin polymerization, prevented the morphologic alteration, indicating that the morphologic alteration was caused by the polymerized fibrin. The morphologic change of HUVEC caused by ancrod-generated fibrin was not observed in the presence of an intracellular calcium mobilization inhibitor TMB-8 (50 microM), and the morphologic alteration was also less pronounced with BAPTA(15 microM)-loaded HUVECs and HUVECs pretreated with EGTA (1.2 mM). Ancrod (in Medium 199) itself did not stimulate phosphoinositide breakdown of HUVEC. However, when ancrod was present in plasma, it caused an increase of [3H]IP1 of HUVECs preloaded with [3H]myoinositol. This IP1 increment was inhibited by Gly-Pro-Arg-Pro. The increase of IP1 was significantly inhibited by the pretreatment of monoclonal antibodies 23C6 and 7E3 directed against alpha v beta 3 integrin. Neomycin (1 mM) and pertussis toxin (100 ng/ml), but not aspirin or mepacrine, blocked this enhanced phosphoinositide breakdown. The morphologic change was also prevented by the monoclonal antibodies, 23C6 and 7E3. These results suggest that both intra- and extra-cellular calcium participate in the event of morphologic change of HUVEC caused by ancrod-generated fibrin, and the morphologic change is mediated, at least in part, by fibrin binding to integrin alpha v beta 3 on HUVECs, causing the subsequent activation of the endogenous G-protein coupled phospholipase C.

Endothelium, fibrinolysis, cardiac risk factors, and prostaglandins: a unified model of atherogenesis

A model of atherogenesis is described in which it is proposed that a state of relative impairment of intravascular fibrinolytic function is the primary defect which makes possible both the initiation and the continued progression of arterial plaques. The key mechanism by which impaired fibrinolysis is atherogenic centers on the unique disruptive effect which fibrin has on the contiguous endothelium of the vascular intimal surface. From this perspective, in areas of spontaneous endothelial injury, impaired fibrinolysis maintains and promotes the gradual enlargement of the area of injury by causing persistently increased intimal permeability and by allowing enhanced fibrin and platelet deposition. This hypothesis thus represents a modification of the response-to-injury hypothesis in which the emphasis has been shifted from the initial endothelial injury to a state of interference with the normal process of healing endothelial injuries. Consistent with this viewpoint, it is noted that all positive risk factors for vascular disease are associated with impairment of fibrinolytic function and, conversely, negative cardiac risk factors enhance fibrinolysis. It is further proposed that one or more prostaglandins, or closely related metabolites, represent the mediators of primary physiologic importance with regard to in vivo regulation of fibrinolysis. By this hypothesis, adequate dietary intake of essential fatty acids, as well as maintenance of unimpaired eicosanoid metabolism, become centrally important in both preventing and reversing arteriosclerosis. This two-tiered model can be used to organize and potentially explain the interrelationship between diverse and apparently divergent sets of epidemiological data which previous models have been unable to accommodate.

Lack of effect of thrombin on fibrin(ogen)-endothelial cell interaction

In the present study we investigate the fibrin(ogen)-endothelial cell binding and the effect of thrombin on the endothelial cells in relation to fibrin(ogen) binding capacity. Endothelial cell fibrinogen binding was concentration and time-dependent, reaching saturation at 1.4 μM of added ligand. At equilibrium, the number of fibrinogen molecules bound per endothelial cell in the monolayer was 5.8±0.7×10(6). When endothelial cells were activated by different concentrations of thrombin (0-0.1 NIH units ml(-1)), no increase in fibrinogen binding capacity was observed at all the thrombin concentration tested. Whereas disruption of endothelial cell monolayers was observed at thrombin concentrations higher than 0.05 NIH units ml(-1), no increase in the amount of fibrinogen bound was observed. Therefore, resting and thrombin-activated endothelial cells show the same fibrinogen binding capacity.The adhesion of endothelial cells in suspension on immobilized fibrinogen or fibrin was studied to ascertain whether the behavior of fibrin is similar to that of fibrinogen. The extent of endothelial cell attachment to immobilized fibrinogen and fibrin was similar (4275±130 cells cm(-2) for fibrinogen and 4350±235 cells cm(-2) for fibrin) and represent approximately 40% of the added endothelial cells. However, endothelial cell adhesion to immobilized fibrin was significantly faster than endothelial cell adhesion to immobilized fibrinogen. The maximum binding rate was 66±9 and 46±8 cells cm(-2) min(-1) for fibrin and fibrinogen, respectively. Therefore, the fibrinopeptides released by thrombin from fibrinogen induce qualitative changes which enhance the fibrin interaction with the endothelial cells.

Contraction of human brain endothelial cells induced by thrombogenic and fibrinolytic factors. An in vitro cell culture model

BACKGROUND AND PURPOSE

Vasogenic brain edema is a frequent complication of ischemic stroke. The mechanism of the blood-brain barrier opening that underlies the edema formation is poorly understood. In the present study we examined the response of endothelial cells cultured from adult human brain to thrombogenic and fibrinolytic factors that possibly accumulate in the occluded vascular segments in ischemic stroke.

METHODS

The changes in the morphology of cultured human brain microvascular endothelial cells were observed by phase-contrast light microscopy and quantified with computerized morphometry.

RESULTS

Active proteases (eg, thrombin, plasmin, urokinase) as well as heparin and protamine, but not fibrinogen and antithrombin III, produced significant changes in endothelial cell morphology. Two shape patterns of contraction were observed: protamine treatment resulted in rounded cells with a decrease in both cell perimeter and area, whereas all other agents induced spiderlike cell morphology with increased perimeter and reduced area. The rate of contraction was dose dependent, and at comparable enzyme concentrations plasmin produced faster contraction than thrombin. The observed changes were reversed 3 hours after abrogating the treatment.

CONCLUSIONS

In an in vitro model we have demonstrated that factors involved in thrombus formation and dissolution induce endothelial cell contraction, which could affect focally the permeability of the blood-brain barrier by opening paracellular avenues between endothelial cells in vivo. Thus, the genesis of brain edema in thromboembolic stroke or occasionally during fibrinolytic therapy can be attributed in part to the contact of these factors with the microvascular endothelium.

Further studies on the interaction of migrating keratinocytes with fibrinogen

If glass implants placed under one edge of a skin wound in the adult newt are coated with fibrinogen (FGN), keratinocytes from the wound periphery migrate onto the implant. To learn more about the site(s) in FGN that permits this migration, we exposed keratinocytes to implants coated with forms of FGN containing modifications or deletions in the 3 most commonly studied cell binding sites; the RGDF sequence at A alpha 95-98, RGDS at A alpha 572-575 and the carboxy terminal 12 amino acids in the gamma A chain. Recombinant FGN with either RGD sequence altered to RGE supported migration as well as unmodified FGN did. Replacement of the carboxy terminal 4 amino acids in the gamma A chain by a 20 amino acid sequence that disrupts the ability of the gamma terminus to mediate platelet aggregation (the gamma' variant) likewise had no effect. Nor did simultaneous antibody blockade of the RGDS, RGDF, and gamma A sites have any effect. At its best, Dhem1, a fragment containing the RGDS and gamma A sites, produced only about half as much migration as the maximum obtained on intact FGN. Dhem2, a fragment differing from Dhem1 only by having a gamma' variant in place of gamma A, was even less active. Two other D fragments, both of which were missing a large part of the A alpha chain, and one of which contained none of the three major binding sites, supported considerable migration, suggesting that loss of the A alpha chain COOH terminus reveals a site that was not exposed in Dhem1 and 2. A alpha chain fragments containing the RGDF or RGDS sequence were active, but a much larger fragment without RGD was inactive. A soluble peptide consisting of the sequence, RGDS, was a potent inhibitor of migration on FGN but RGDF and the gamma A pentapeptide, KQAGD, were minimally effective. Longer versions of these peptides decreased the effectiveness in all cases. These results suggest that under certain circumstances, newt keratinocytes may interact with each of the 3 major binding sites in FGN as well as a site outside these sequences.

Recent observations on the role of hemostatic determinants in the development of the atherothrombotic plaque

Recent evidence suggests that hemostatic determinants play a major role in the evolution of the atherothrombotic plaque. Platelets can serve as cholesterol donors for macrophages, thereby facilitating foam cell formation. Lipoprotein(a) inhibits fibrinolysis and may also contribute to atherogenesis by serving as a ligand for the scavenger receptor. By complexing with fibrin(ogen) in atheromatous lesions, lipoprotein(a) attenuates clearance of this protein, promoting atherogenesis and vascular dysfunction. These observations suggest that thrombotic determinants are critical for the development of the atheromatous plaque, and may guide the appropriate selection of potential therapeutic options in the future.

Patency and healing of microvascular prostheses: a review of 10 years of experimental work in Groningen

From 1982 onwards, in Groningen, The Netherlands, we have worked on the experimental evaluation and development of microvascular prostheses in rats and rabbits. In this review article a systematic overview of this experimental work is presented and the results are discussed with regard to the literature to come to a current state of the art on (experimental) microvascular grafting with prosthetic conduits.

A new protective solution for hypothermic storage of free vein grafts in cardiovascular surgery

In order to reduce the operative injury of the endothelium in free reversed vein grafts, cultured human endothelial cells were used to test the optimal concentration of the constituents of a flushing solution for improved protection of the endothelium. The following solution proved to be the most suitable when tested at 20 degrees C; mannitol 160 mmol l-1, glucose 15 mmol l-1, NaCl 30 mmol l-1, KHCO3 5 mmol l-1, K2SO4 10 mmol l-1, KH2PO4 4 mmol l-1, MgSO4 20 mmol l-1, CaCl2 1.5 mmol l-1, potassium citrate 1.0 mmol l-1, Pluronic F-68 20 mg l-1, HEPES 4 mmol l-1, HEPES-Na 6 mmol l-1, pH 7.25, osmolality 325 mosmol kg-1 H2O. When endothelial cell injury was measured by a 51Cr-release assay, the new solution protected human endothelial cells in culture during hypothermic incubation better than isotonic NaCl, St Thomas' cardioplegic solution or Krebs-Henseleit's buffer. Transmission and scanning electron microscopy showed that the endothelium of human saphenous vein grafts was well preserved following 6 h of incubation at 20 degrees C with the new solution. As determined by morphometry using scanning electron microscopy, the endothelium of free porcine vein grafts was better preserved after incubation for 2 h at 20 degrees C with the new solution than with either isotonic NaCl (p = 0.02) or diluted, heparinized blood (p = 0.02) as the incubation medium, all cases observed following 2 h of subsequent arterial flow. The present study indicates that the endothelium of free vein grafts can be well protected against hypothermia when the flushing and irrigation fluid has a composition favouring endothelial protection. It appears likely that such treatment of vein grafts will reduce the frequency of vein graft narrowing and occlusion, post-operatively.

Fibrin contact increases endothelial permeability to albumin

We studied the effects of contact of bovine pulmonary artery endothelial cell monolayers with fibrin on the endothelial barrier function. Fibrin formed by clotting purified fibrinogen (0.5 to 3.0 mg/ml) with alpha-thrombin (1 U/ml) was added to endothelial monolayers and permeability measurements were made after fibrin removal. Fibrin incubation for 3 hours resulted in 2- to 5-fold increases in transendothelial 125I-albumin permeability. Permeability returned to baseline value within 3 hours after fibrin removal. Direct contact with fibrin was necessary for the response, since fibrin separated from the endothelium did not increase permeability. Contact with agarose (2 mg/ml) or fibrinogen (0.5 to 3.0 mg/ml) also did not increase endothelial permeability. Transmission electron microscopic examination indicated normal appearance of interendothelial junctions at a time when albumin permeability was increased and no overt evidence of endothelial injury. Incubation of fibrin with endothelial monolayers at 4 degrees C prevented the increase in albumin permeability. We examined the possibility that increased albumin transcytosis was responsible for fibrin's effect using 14C-sucrose (Mr = 342D), a lipid insoluble tracer. Fibrin increased sucrose flux by 1.5-fold compared to 2- to 5-fold increases in albumin flux. The results indicate that fibrin contact with the endothelial cell increases endothelial permeability. The effect of fibrin may involve activation of temperature-sensitive bulk phase transcytosis of albumin.

Endothelial cell spreading on fibrin requires fibrinopeptide B cleavage and amino acid residues 15-42 of the beta chain

Adhesion and spreading of cultured human umbilical vein endothelial cells on fibrin surfaces of varying structure were characterized to understand better the interactions occurring between endothelium and fibrin at sites of vascular injury. Fibrin prepared with reptilase, which cleaves only fibrinopeptide A from fibrinogen, and fibrin prepared with thrombin, which cleaves both fibrinopeptide A and fibrinopeptide B, equally supported endothelial cell adhesion. In contrast, only fibrin made with thrombin mediated endothelial cell spreading, as assessed by fluorescence microscopy of cells stained with rhodamine phalloidin to identify actin stress fibers or by scanning electron microscopy. Fibrin prepared with reptilase failed to support cell spreading. To further investigate the role of the amino terminus of the fibrin beta chain after fibrinopeptide B cleavage in promoting cell spreading, protease III from Crotalus atrox venom was used to specifically cleave the amino-terminal 42 residues of the fibrinogen B beta chain. After clotting with thrombin, this fibrin derivative lacking B beta 1-42 failed to support significant cell spreading. Spreading on fibrin was unaffected by depletion of Weibel-Palade bodies from endothelial cells, indicating that the spreading was independent of stimulated von Willebrand factor release. We conclude that endothelial cell spreading on fibrin requires fibrinopeptide B cleavage and involves residues 15-42 of the fibrin beta chain.

Enhanced healing of 30 microns Gore-Tex PTFE microarterial prostheses by alcohol-pretreatment

Polytetrafluoroethylene (PTFE) microvascular prostheses with a fibril length of 30 microns were pretreated with alcohol (n = 18), implanted into the abdominal aorta of rats and were evaluated at 1 day (n = 3), 1 week (n = 3), 3 weeks (n = 6) and 6 weeks (n = 6) to determine whether alcohol-pretreatment might improve their healing. Untreated PTFE microvascular prostheses (n = 18) functioned as controls (all prostheses: length 10 mm, I.D. 1.5 mm). The alcohol-pretreated PTFE microarterial prostheses were initially completely filled and covered with clot and showed fast and complete healing within 6 weeks: endothelial cells and smooth muscle cells on the luminal surface, and the interstices filled with fibrous-like tissue. In contrast, the untreated PTFE prostheses were initially not filled or covered with clot and showed only healing near the anastomotic sites and scarce tissue ingrowth into the wall. These results demonstrate that alcohol-pretreatment improves the healing characteristics of PTFE microarterial prostheses with a fibril length of 30 microns. The alcohol-pretreatment renders the PTFE material more accessible to clot and subsequently to cells.

The cell in shock: the origin of multiple organ failure

The immediate organ damage seen after multiple trauma and in shock is a typical example of non-bacterial inflammation triggered by activation of various mediators of both the humoral and cellular systems. Anaphylatoxins and the low-flow syndrome during the shock phase account for increased PMN* margination, which in turn causes pulmonary leukostasis and may provoke massive mediator release by PMN (oxygen radicals, proteinases, eicosanoids, PAF etc). This probably leads to severe endothelial cell damage, especially in the lung. Adherence of PMN to the endothelium appears to create the micro-environment where high concentrations of proteolytic enzymes and reactive oxygen radicals exert a deleterious effect on the cell membrane. Endothelial cell membrane injury leads to increased vascular permeability and cell edema. The development of the 'organ in shock' may require a few hours and initially cause minor or no functional impairment at all. Only when shock is severe is there early organ failure, which in this stage may still be an expression of non-bacterial inflammation. Numerous studies have reported the existence of shock-induced cardiodepressant substances in association with various forms of circulatory shock. We have determined a net negative inotropic effect of the low-molecular-weight plasma fraction in severe hypovolemic-traumatic shock and have isolated a cardiodepressant factor (CDF), which by blockade of the calcium inward current has a negative inotropic a chronotropic effect. The intestine as a shock organ appears to range first among the organs involved. The translocation of bacteria from the intestinal tract, the 'intestine in shock' represents the trigger reaction that eventually leads from the 'organ in shock', early organ failure to late (septic) organ failure. Here the most prominent factor is endotoxin (LPS) as a basic mediator of gram-negative bacteria, which also triggers the activation of humoral and cellular systems. The posttraumatic hyperdynamic phase commonly starts on days 3-5 and is mainly caused by bacteremia and/or endotoxemia. Macrophages have a major impact on the late phase of organ failure. At present, the most prominent cellular mediator of the lethal effect of endotoxin is thought to be cachectin, which is identical with the tumor necrotising factor (TNF). TNF is secreted by monocytes/macrophages (MO/MA) in response to LPS. Via macrophage derived cytokines and by LPS there is activation of endothelial cells, with increased adhesiveness for PMN. Both due to this increased adhesiveness and the presence of LPS and cytokines, PMN undergo massive activation, which causes mediator release and tissue damage.(ABSTRACT TRUNCATED AT 400 WORDS)

Angiogenesis in arteries: review

The inner parts of the walls of large blood vessels do not normally contain intrinsic vasculature. In pathologic conditions such as arteriosclerosis or thrombosis, angiogenesis occurs, and may have significant clinical consequences. This review attempts to relate the little that is known about the factors specific to vascular walls which regulate angiogenesis to more general knowledge of the phenomenon.

The antiatherogenic effects of components isolated from pollen typhae

Based on the results of previous investigations that pollen Typhae, a Traditional Chinese Medicine, Had antiatherogenic effects, several components were isolated successively from the drug and their effects on porcine aortic endothelial cell (EC) and smooth muscle cell (SMC) cultures as well as on platelet aggregation were examined. 12 components isolated from Pollen Typhae have been identified on their chemical structures and biological effects. 4 of them showed different evident antiatherogenic effects. 1) Isorhamnetin-3-O-rhamnosyl-glucoside could stimulate EC to produce tPA and PGI2; 2) Quercetin-3-O-neohesperidose could protect EC from injury by fibrin, as well as raise tPA activity; 3) beta-Sitosterol palmitate could inhibit SMC proliferation and 4) beta-Sitosterol glucoside showed an inhibitory effect on platelet aggregation. These results would provide some information for the search of new drugs in the treatment and prevention of atherosclerosis.

Measuring plasma fibrinogen to predict stroke and myocardial infarction

Epidemiological observations indicate that high plasma fibrinogen levels are strongly correlated with the frequency of two major thrombotic complications of atherosclerosis, stroke and myocardial infarction. Thrombosis is increasingly recognized as a central mechanism in stroke and myocardial infarction, and fibrinogen is involved in events thought to play a major role in thrombosis. Therefore, elucidation of the relationship between fibrinogen and thrombosis may strengthen the predictive value of this protein and suggest new treatment to prevent stroke and myocardial infarction. The current data relating fibrinogen to thrombosis are not easy to reconcile with the available epidemiological observations. In addition, advances in understanding the atherogenic potential of several risk factors for coronary heart disease have used information on the measurement of the risk factors in population-based studies. Thus, measuring plasma fibrinogen to predict stroke and myocardial infarction may be important in gaining insight into the thrombogenic potential of this protein and in inspiring new strategies against the thrombotic complications of atherosclerosis.

Fibrin and the vessel wall

Fibrin is a major component of atherosclerotic plaques, and there may also be situations in which intravascular fibrin is formed in contact with the endothelium. The studies to be presented describe the distribution of fibrinogen/fibrin I, fibrin II, and fragments D and D-dimer in normal vessels and atherosclerotic plaques of increasing severity and also describe some functional effects of fibrin on normal endothelium. Immunohistochemical studies using three specific monoclonal antibodies with the avidin-biotin complex immunoperoxidase technique demonstrated that little fibrinogen/fibrin I or fibrin II and no D/D-dimer were detected in normal aortas. In early lesions and in fibrous plaques, fibrinogen/fibrin I and fibrin II were distributed in long threads and around vessel wall cells. D/D-dimer was not seen in early lesions. In advanced plaques all three molecular forms were detected in areas of loose connective tissue, in thrombi, and around cholesterol crystals. Thus increased fibrin formation and degradation may be associated with progression of atherosclerotic disease. Additionally, the presence of fibrin II around vessel wall cells suggests that these cells may be involved in the fbgn to fibrin transition within the vessel wall. The second aspect of the work to be presented concerns effects of fibrin on vascular endothelium. Fibrin formed on the surface of cultured human umbilical vein endothelial cells stimulated production of prostacyclin and tissue plasminogen activator by the cells in a time- and dose-dependent manner. Stimulation of prostacyclin was completely inhibited by indomethacin and partially inhibited by actinomycin D, cycloheximide, and trifluoperazine, while stimulation of t-PA synthesis was completely inhibited by actinomycin D and cycloheximide and partially inhibited by cytochalasin D, vinblastine, and trifluoperazine.(ABSTRACT TRUNCATED AT 250 WORDS)

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

Endothelial cells and activated platelets express integrin-type receptors responsible for adhesion to fibrinogen. We have located distinct integrin-directed endothelial cell and platelet attachment sites on immobilized fibrinogen using a combination of synthetic peptides, fibrinogen fragments, and specific anti-peptide monoclonal antibodies. Endothelial cells exclusively recognize an Arg-Gly-Asp-containing site near the C-terminus of the alpha chain (alpha residues 572-574) but fail to recognize the Arg-Gly-Asp sequence in the N-terminal region of the same chain (alpha residues 95-97). In contrast, platelets do not require either Arg-Gly-Asp sequence for binding to intact fibrinogen and are capable of recognizing, in addition to the alpha 572-574 sequence, a site at the C-terminus of the gamma chain (gamma residues 400-411). These data suggest a molecular mechanism whereby platelets and endothelial cells interact with distinct sites on the fibrinogen molecule during hemostasis and wound healing.

Mediation of fibrin-induced release of von Willebrand factor from cultured endothelial cells by the fibrin beta chain

The exposure of endothelial cells (EC) to fibrin has been shown to stimulate the rapid release of von Willebrand factor (vWf) from storage sites in Weibel-Palade bodies. We have now investigated the fibrin structural features required for stimulation of release. The role of fibrinopeptide cleavage was examined by preparing fibrin with thrombin to remove both fibrinopeptide A (FPA) and fibrinopeptide B (FPB) and with reptilase or Agkistrodon contortrix procoagulant to selectively remove FPA or FPB, respectively. vWf release was found to require FPB cleavage, whereas removal of FPA and Factor XIIIa cross-linking of fibrin were without effect. The dependence of release on FPB cleavage suggested that a site involving the NH2 terminus of the beta chain could mediate vWf secretion. To test this hypothesis, B beta chain derivatives were prepared and examined for their capacity to induce release. Purified B beta chain had no effect on release at a concentration of 20 nM but stimulated release from 26 +/- 6% of cells at 200 nM, the maximum solubility. However, after thrombin cleavage of FPB, release occurred from 36 +/- 9% of cells at 20 nM and from 60 +/- 7% at 200 nM, both significantly greater than before cleavage. FPB and B beta 1-42 showed no activity, whereas beta 15-42, representing the NH2 terminus of the thrombin cleaved beta chain, stimulated significant release at concentrations of 0.1 and 1 mM. We conclude that FPB cleavage from fibrin is required for stimulation of vWf release from EC and that this is mediated by a site that includes the NH2 terminus of the beta chain.

Histopathological study of aorto-coronary bypass grafts with special reference to fibrin deposits on grafted saphenous veins

A histopathological study was performed on 36 patients (60 grafts) who had undergone aorto-coronary bypass graft (ACBG) surgery 0 to 99 months prior to death. The following pathologic changes were found: 1) The thickness of diffuse intimal proliferation in the ACBG progressed with time from graft surgery to death. 2) The media became atrophic and the adventitia was increased slightly in thickness. 3) Fibrin deposits were found in 20 patients on/in the intimal thickenings of the vein graft walls and 7 patients showed incorporated fibrin in the thickened intima even one month after surgery. 4) Atherosclerosis, identified as intimal foam cell accumulation or frank plaques, was seen in only 3 patients 4 years after surgery. Fibrointimal proliferation occurred with relatively greater frequency in patients with fibrin deposits (P less than 0.001). Although it is well known that mural thrombi in vein grafts manifest fibrointimal proliferation, our results suggest that fibrin deposits might be responsible for intimal thickening even one month after graft surgery.

Differing polarity of the constitutive and regulated secretory pathways for von Willebrand factor in endothelial cells

von Willebrand factor (vWf) is secreted from endothelial cells by one of two pathways-a constitutive pathway and a regulated pathway originating from the Weibel-Palade bodies. The molecular form of vWf from each of these pathways differs, with the most biologically potent molecules being released from Weibel-Palade bodies (Loesberg, C., M. D. Gonsalves, J. Zandbergen, C. Willems, W. G. Van Aken, H. V. Stel, J. A. Van Mourik, and P. G. DeGroot. 1983. Biochim. Biophys. Acta. 763:160-168; Sporn, L. A., V. J. Marder, and D. D. Wagner. 1987. Cell. 46:185-190). We investigated the polarity of the two secretory pathways using human umbilical vein endothelial cells cultured on polycarbonate membrane filters which allowed sampling of media from both the apical and basolateral compartments. After metabolic labeling of cells, vWf (constitutively secreted during a 10-min period or released during a 10-min treatment with a secretagogue) was purified from the apical and basolateral chambers and subjected to gel analysis. Approximately equal amounts of vWf were constitutively secreted into both chambers, and therefore this secretory pathway appeared to be nonpolarized. On the contrary, an average of 90% of vWf released from Weibel-Palade bodies after treatment with the calcium ionophore A23187 or PMA appeared in the basolateral chamber, indicating that the regulated pathway of secretion is highly polarized. Thrombin, a secretagogue which promotes disruption of the endothelial monolayer, led to release of vWf from cells with no apparent polarity. The presence of microtubule-depolymerizing agents nocodazol and colchicine inhibited the polarized release of vWf. Ammonium chloride treatment did not disrupt the polarity of the regulated secretory pathway, indicating that maintenance of low pH in intracellular compartments was not required for the polarized delivery of preformed Weibel-Palade bodies to the plasma membrane.