Platelets Stimulate Thromboplastin Synthesis in Human Endothelial Cells

Endothelial dysfunction in patients with polycythaemia vera

Patients with polycythaemia vera (PV) are at increased risk of developing arterial and venous thromboembolic complications. We investigated whether endothelium-dependent, flow-mediated vasodilatation (FMD) is impaired in PV patients without clinical evidence of artery disease as observed in patients with conventional cardiovascular risk factors. FMD and endothelium-independent, nitroglycerine-induced vasodilatation (NMD) were assessed using high-resolution ultrasound in the brachial artery of 20 patients with PV and 20 sex- and age-matched control subjects (CTL). FMD was markedly impaired in PV patients compared with CTL (7.6 +/- 2.9% versus 11.6 +/- 5.7%, P = 0.009) whereas NMD was similar in both study groups. The impairment of FMD was independently related to the presence of PV (r = -0.434, P = 0.009) and vessel size (r = -0.107, P = 0.038) but was not related to haematocrit values and platelet counts. The results demonstrate that PV is associated with endothelial dysfunction in the pre-clinical phase of arterial disease. However, the precise mechanisms by which PV leads to this altered vascular reactivity remain unclear.

Polar expression of tissue factor in human umbilical vein endothelial cells

Endothelial cells grown on filters developed junctional complexes that reduced diffusional transport and increased electrical resistance over the cell layer. Induction of tissue factor by recombinant interleukin-1 beta led to a highly polarized tissue factor expression on the apical cell surface only. After prolonged growth to allow deposition of matrix, removal of the endothelial cells by collagenase or by 0.1 mol/L NH4OH left behind some cellular material as well as tissue factor, which was only detectable in the upper compartment. A human bladder carcinoma cell line, which does not form tight junctions and expresses tissue factor constitutively, showed essentially no polarity. Endothelial cell secretory compounds like von Willebrand factor, tissue plasminogen activator, and plasminogen activator inhibitor-1 were constitutively released to both sides. The added secretion due to recombinant interleukin-1 beta stimulation of the endothelial cells observed for von Willebrand factor and tissue plasminogen activator was, however, localized to the apical surface. The availability of tissue factor on the luminal surface of endothelial cells, ie, allowing contact with factor VII in the flowing blood, has potentially very significant pathophysiological consequences.

Annexin V inhibits the procoagulant activity of matrices of TNF-stimulated endothelium under blood flow conditions

A human ex vivo thrombosis model was used to investigate whether recombinant annexin V (rANV) can prevent thrombus formation under venous and arterial blood flow conditions. In this model, blood from an antecubital vein of healthy donors was allowed to flow directly over the extracellular matrix of tumor necrosis factor-stimulated endothelial cells (TNF-ECMs). TNF-ECMs were preincubated with rANV (2.9 mumol/L) for 30 minutes. With this rANV concentration all binding sites present on TNF-ECMs (1.6 +/- 0.5 x 10(12)/cm2) are occupied, and a maximal inhibition was observed in a tissue factor-dependent clotting assay. Fibrin deposition and platelet and leukocyte adhesion were measured on the rANV-treated and nontreated TNF-ECMs. Nontreated TNF-ECMs were used as controls. rANV inhibited fibrin deposition by 81% at a wall shear rate of 100 s-1. A nonsignificant inhibition was also observed at 650 s-1. Platelet-matrix adhesion, which is more prominent at higher shear rates, was significantly decreased by 60% at 100 s-1 but not at 650 s-1. The average leukocyte adherence was nonsignificantly lowered at 100 s-1. Virtually no leukocytes adhered at 650 s-1. The results demonstrated that rANV can inhibit blood coagulation under venous blood flow conditions and may serve as an antithrombotic drug.

Interaction of platelets with endothelial cells: activation of a novel neutral protease

The activation of a new neutral protease of MW 85,000 was demonstrated on interaction between porcine aortic endothelial cells and human and porcine platelets in culture. The activity of this enzyme, PECAP (platelet endothelial cell activated protease), was detected by electrophoresis on polyacrylamide gels impregnated with the substrate casein. Our results showed that the platelet-endothelial cell interaction did not involve induction of synthesis of de novo enzyme, but rather an activation of a latent enzyme. PECAP cleaved casein and fibrinogen, but had no activity against gelatin or elastin. It was not inhibited by inhibitors of metalloproteases (EDTA, 1.10 phenanthroline), serine proteases (phenylmethylsulfonyl fluoride, elastatinal), or cysteine proteases (iodoacetate, N-ethylmaleimide) and it seems to be unrelated to the previously known proteases of mesenchymal and hematopoietic cells.

Augmentation of procoagulant activity in monokine stimulated human endothelial cells by calmodulin/protein kinase C inhibitors

The incubation of human umbilical cord endothelial cell cultures with inflammatory mediators results in the induction of procoagulant activity. As many of these mediators activate protein kinase C, the effect of calmodulin and protein kinase C inhibitors on IL-1, TNF, phorbol ester and LPS stimulated procoagulant activity was determined. Incubation of endothelial cell cultures with these inflammatory agents in the presence of phenothiazine derivatives or other classes of calmodulin and protein kinase C antagonists resulted in a 2-4 fold increase in procoagulant activity compared to parallel stimulated cultures in the absence of antagonists. The augmented response of IL-1 stimulated endothelial cells to these antagonists was actinomycin D sensitive.

Immune endothelial-cell injury in heparin-associated thrombocytopenia

We studied the possibility that immune injury to endothelial cells may have a role in the development of thrombosis in some patients with heparin-associated thrombocytopenia. Serum samples from each of 27 patients who had this clinical diagnosis contained heparin-dependent platelet antibodies and deposited more than normal amounts of IgG, IgA, or IgM on endothelial cells, stimulating the production of tissue factor. Binding of immunoglobulins to endothelial cells was no longer detected when the patients were studied after heparin was withdrawn, but reappeared within several days upon reexposure to heparin in a patient who experienced a clinical recurrence. Binding of immunoglobulin to endothelial cells was partially reduced by the preadsorption of serum samples with heparin or heparan sulfate bound to Sepharose or by enzymatic cleavage of cell-bound heparan sulfate, and was augmented by the addition of heparan sulfate. Thus, serum from some patients with heparin-associated thrombocytopenia may contain antibodies that react with heparin bound to endothelial cells or with heparan sulfate synthesized by endothelial cells. Immune injury to both platelets and endothelial cells may play a part in the development of thrombosis in some patients after heparin therapy.

Cell biology of endothelial cells

Endothelial cells are a source of physiologically important molecules synthesized therein and secreted to the blood and/or to the subendothelial extracellular matrix. These molecules participate in formation of platelet and fibrin thrombi (e.g., von Willebrand factor and tissue factor) and contribute to antithrombotic properties of the endothelium (e.g., prostacyclin, thrombomodulin, and heparan sulfate). Endothelial cells synthesize and secrete plasminogen activator and inhibitors. They are the source of molecules regulating the growth of other cells; they synthesize angiotensin-converting enzyme, and bind lipoproteins and hormones. Finally, they are the target for, and participant in, immune reactions. Thus, endothelial cells constitute not only the first barrier between the blood and the extravascular space but also serve as a source of molecules influencing the structural and functional integrity of the circulation.

[Significance of the endothelium of the vascular wall for maintaining hemostasis]

The endothelial lining contributes in many respects to the patency of the vasculature. The production of heparan sulphate, release of prostacyclin and expression of the membrane cofactor thrombomodulin that is essentially required for the activation of protein C represent important mechanisms that warrant thromboresistance. If the integrity of the vessel wall is lost, the exposed subendothelium that has been built up by the endothelial cells serves as a highly reactive surface for platelets whose adherence is facilitated by another endothelial cell product, the von Willebrand Factor. Induction of tissue factor production after exposure to endotoxin also emphasizes an important role für the endothelium in the pathogenesis of disseminated intravascular coagulation. Once thrombosis has occurred the release of plasminogen activator of tissue-type from the endothelium leads to dissolution of the clot and a functional restoration of the blood vessel.

In vitro stimulation of monocyte tissue factor activity by autologous platelets

Washed human platelets were found to enhance phytohemagglutinin (PHA)-stimulated tissue factor (TF) synthesis when incubated with autologous mononuclear cell cultures. Furthermore, platelets increased TF synthesis even when no other stimulator was present during the incubation. Experiments utilizing similar cultures derived from blood of patients with Glanzmann's disease, von Willebrand's disease, and platelet storage pool disease indicated that platelets with each of these genetic defects possessed the capacity to enhance the synthesis of this initiator of coagulation by unstimulated cells as did normal platelets. The degree of platelet enhancement varied between individuals, but for any given donor, the extent of the effect depended on the concentration of platelets used. The effect was demonstrable at platelet/monocyte ratios ranging from a low of approximately 15 to the highest ratio used, about 300. For comparison the ratio of these two cellular elements in normal human blood is estimated to be approximately 1,000. These findings may reflect a relationship between these two cell types that can occur in vivo.

Inhibition of the thromboplastin response of endothelial cells in vitro

Confluent monolayers of human umbilical vein endothelial cells in culture responded with a 5-fold increase in thromboplastin (tissue factor) synthesis when exposed to 12-O-tetradecanoyl-phorbol-13-acetate (TPA) (50 ng/ml) or endotoxin (ETX) (25 micrograms/ml) for 16 hr. This induced thromboplastin synthesis was markedly inhibited by exposure of the cells to two different phosphodiesterase inhibitors, methylisobutylxanthine (MIX) and rac-4(3-butoxy-4-methoxybenzyl)-2-imidazole idinone (RO-20-1724) and to the transmethylation inhibitors 3-deazaadenosine (DZA) and 1-homocysteine thiolactone (Hcy) in combination. It was slightly (TPA) or not at all (ETX) inhibited upon exposure of the cells to the intracellular calcium antagonist 8-(N,N-diethylamino)-octyl 3,4, 5-trimethoxybenzoate hydrochloride (TMB-8). However, in the presence of MIX TMB-8 had a moderate additional inhibitory effect on TPA-induced thromboplastin response. The thromboplastin response of endothelial cells in vitro thus probably depends on transmethylation events for its full expression. It is also strongly modulated by the intracellular level of cAMP.