Human umbilical vein endothelial cells express high affinity receptors for factor Xa

Protective, repairing and fibrinolytic effects of rivaroxaban on vascular endothelium

AIMS

Rivaroxaban, a direct inhibitor of activated factor X (FXa), is the only new oral anticoagulant approved for secondary prevention after acute coronary syndrome. Our objective was to identify the possible molecular mechanisms of rivaroxaban that contribute to endothelial function.

METHODS

Cell viability and growth of human umbilical vein endothelial cells (HUVEC) were registered. Gene expression studies comparing the effects of rivaroxaban and FXa were conducted by a selective RNA array and confirmed by protein quantification. Wound-healing experiments on HUVEC, platelet adhesion, enzymatic activity, and cell-based assays for fibrin formation were performed with rivaroxaban.

RESULTS

Rivaroxaban (50 nM) only altered (>2 fold change) the expression of matrix metallopeptidase 2 and urokinase plasminogen activator (u-PA), but counteracted the FXa (9 nM)-induced upregulation of several pro-inflammatory genes (P < 0.05) and FXa-enhanced platelet adhesion over HUVEC. Rivaroxaban increased u-PA protein expression in HUVEC supernatants and enhanced u-PA activity (up to 4 IU ng^-1 of u-PA). Rivaroxaban (1 nM-1 μM) showed a significant and dose-dependent positive effect on HUVEC growth that was inhibited by BC-11-hydroxibromide, an inhibitor of u-PA. Healing properties after a wound on HUVEC cultures, and fibrinolytic properties were also shown by rivaroxaban. Both effects were reversed by BC-11-hydroxibromide.

CONCLUSIONS

Rivaroxaban enhanced viability, growth and migration of HUVEC, mainly by u-PA activation and upregulation, which also participate in the rivaroxaban-induced fibrinolytic activity at endothelial level. Rivaroxaban also protected from the pro-inflammatory effects of FXa on HUVEC. Altogether may improve endothelial functionality and could contribute to the cardiovascular benefits of rivaroxaban.

Effects of factor Xa on the expression of proteins in femoral arteries from type 2 diabetic patients

AIM

Further to its pivotal role in haemostasis, factor Xa (FXa) promotes effects on the vascular wall. The purpose of the study was to evaluate if FXa modifies the expression level of energy metabolism and oxidative stress-related proteins in femoral arteries obtained from type 2 diabetic patients with end-stage vasculopathy.

METHODS

Femoral arteries were obtained from 12 type 2 diabetic patients who underwent leg amputation. Segments from the femoral arteries were incubated in vitro alone and in the presence of 25 nmol l(-1) FXa and 25 nmol l(-1) FXa + 50 nmol l(-1) rivaroxaban.

RESULTS

In the femoral arteries, FXa increased triosephosphate isomerase and glyceraldehyde-3-phosphate dehydrogenase isotype 1 expression but decreased pyruvate dehydrogenase expression. These facts were accompanied by an increased content of acetyl-CoA. Aconitase activity was reduced in FXa-incubated femoral arteries as compared with control. Moreover, FXa increased the protein expression level of oxidative stress-related proteins which was accompanied by an increased malonyldialdehyde arterial content. The FXa inhibitor, rivaroxaban, failed to prevent the reduced expression of pyruvate dehydrogenase induced by FXa but reduced acetyl-CoA content and reverted the decreased aconitase activity observed with FXa alone. Rivaroxaban + FXa but not FXa alone increased the expression level of carnitine palmitoyltransferase I and II, two mitochondrial long chain fatty acid transporters. Rivaroxaban also prevented the increased expression of oxidative stress-related proteins induced by FXa alone.

CONCLUSIONS

In femoral isolated arteries from type 2 diabetic patients with end-stage vasculopathy, FXa promoted disruption of the aerobic mitochondrial metabolism. Rivaroxaban prevented such effects and even seemed to favour long chain fatty acid transport into mitochondria.

Active site-labeled prothrombin inhibits prothrombinase in vitro and thrombosis in vivo

Mouse and human prothrombin (ProT) active site specifically labeled with D-Phe-Pro-Arg-CH(2)Cl (FPR-ProT) inhibited tissue factor-initiated thrombin generation in platelet-rich and platelet-poor mouse and human plasmas. FPR-prethrombin 1 (Pre 1), fragment 1 (F1), fragment 1.2 (F1.2), and FPR-thrombin produced no significant inhibition, demonstrating the requirement for all three ProT domains. Kinetics of inhibition of ProT activation by the inactive ProT(S195A) mutant were compatible with competitive inhibition as an alternate nonproductive substrate, although FPR-ProT deviated from this mechanism, implicating a more complex process. FPR-ProT exhibited ∼10-fold more potent anticoagulant activity compared with ProT(S195A) as a result of conformational changes in the ProT catalytic domain that induce a more proteinase-like conformation upon FPR labeling. Unlike ProT and ProT(S195A), the pathway of FPR-ProT cleavage by prothrombinase was redirected from meizothrombin toward formation of the FPR-prethrombin 2 (Pre 2)·F1.2 inhibitory intermediate. Localization of ProT labeled with Alexa Fluor® 660 tethered through FPR-CH(2)Cl ([AF660]FPR-ProT) during laser-induced thrombus formation in vivo in murine arterioles was examined in real time wide-field and confocal fluorescence microscopy. [AF660]FPR-ProT bound rapidly to the vessel wall at the site of injury, preceding platelet accumulation, and subsequently to the thrombus proximal, but not distal, to the vessel wall. [AF660]FPR-ProT inhibited thrombus growth, whereas [AF660]FPR-Pre 1, lacking the F1 membrane-binding domain did not bind or inhibit. Labeled F1.2 localized similarly to [AF660]FPR-ProT, indicating binding to phosphatidylserine-rich membranes, but did not inhibit thrombosis. The studies provide new insight into the mechanism of ProT activation in vivo and in vitro, and the properties of a unique exosite-directed prothrombinase inhibitor.

ADAMTS13 bound to endothelial cells exhibits enhanced cleavage of von Willebrand factor

ADAMTS13 is a plasma metalloprotease that cleaves ultralarge von Willebrand factor multimers to generate less thrombogenic fragments. Although this cleavage can occur at the surface of endothelial cells, it is currently unknown whether this process involves binding of the ADAMTS13 to the endothelial cell plasma membrane. Using different assay systems, we present evidence that ADAMTS13 binds to endothelial cells in a specific, reversible, and time-dependent manner with a K(d) of 58 nm. This binding requires the COOH-terminal thrombospondin type 1 repeats of the protease. Binding is inhibited in the presence of heparin and by trypsin treatment of the cells. ADAMTS13 that was prebound to endothelial cells exhibited increased proteolysis of VWF as compared with ADAMTS13 present only in solution. These data support the notion that cleavage of VWF occurs mainly at the endothelial cell surface.

Regulatory mechanism of matrix metalloprotease-2 enzymatic activity by factor Xa and thrombin

Matrix metalloprotease (MMP)-2 plays a key role in many biological and pathological processes related to cell migration, invasion, and mitogenesis. MMP-2 is synthesized as a zymogen that is activated through either a conformational change or proteolysis of the propeptide. Several activating enzymes for pro-MMP-2 have been proposed, including metalloproteases and serine proteases. The mechanism of pro-MMP-2 activation by metalloproteases is well established, and the most studied activation mechanism involves cleavage of the propeptide by membrane type 1-MMP (MT1-MMP). In contrast, serine protease activation has not been thoroughly studied, although studies suggest that MT1-MMP may be involved in activation by thrombin and plasmin. Here, we demonstrate that factor Xa mediates MT1-MMP-independent processing of pro-MMP-2 in vascular smooth muscle cells and endothelial cells. Factor Xa and thrombin directly cleaved the propeptide on the carboxyl terminal sides of the Arg(98) and Arg(101) residues, whereas plasmin only cleaved the propeptide downstream of Arg(101). Moreover, processed MMP-2 showed enzymatic activity that was enhanced by intermolecular autoproteolytic processing at the Asn(109)-Tyr peptide bond. In addition to its role in activation, factor Xa rapidly degraded MMP-2, thereby restricting excessive MMP-2 activity. Thrombin also degraded MMP-2, but the degradation was reduced greatly under cell-associated conditions, resulting in an increase in processed MMP-2. Overall, factor Xa and thrombin regulate MMP-2 enzymatic activity through its activation and degradation. Thus, the net enzymatic activity results from a balance between MMP-2 activation and degradation.

Identification of a surface protein on human brain microvascular endothelial cells as vimentin interacting with Escherichia coli invasion protein IbeA

Escherichia coli K1 is the most common gram-negative bacteria that cause meningitis during the neonatal period. The ibeA gene product in E. coli K1 has been characterized as a virulence factor that contributes to the binding to and invasion of brain microvascular endothelial cells (BMEC). Here, we identified a surface protein on human BMEC, vimentin, that interacts with the E. coli invasion protein IbeA. The binding sites of the IbeA-vimentin interaction are located in the 271-370 residue region of IbeA and the vimentin head domain. The regulatory protease factor Xa is able to cleave IbeA between R297 and K298 residues, and this cleavage abolishes the IbeA-vimentin interaction.

Vascular endothelial tissue factor pathway inhibitor kinetics in culture following exposure to DX-9065a--a selective and direct factor Xa inhibitor

BACKGROUND

Tissue factor (TF), a membrane-bound glycoprotein that initiates blood coagulation by allosteric activation of factor (f) VII, is regulated predominantly by tissue factor pathway inhibitor (TFPI). Because vascular endothelial cells synthesize and constitutively secrete TFPI and fXa may directly influence its cellular clearance, we sought to determine the effects of DX-9065a, a direct and selective fXa inhibitor, on TFPI kinetics in culture.

METHODS/RESULTS

Human umbilical vein endothelial cells were grown to confluence and incubated with unfractionated heparin (1.0 U/mL), enoxaparin (1.5 U/mL), or DX-9065a at low (10 ng/ml), moderate (30 ng/ml), or high (90 ng/ml) concentrations. Compared to control, increases in TFPI were seen with both unfractionated heparin (182% higher, p < 0.001) and enoxaparin (194% higher, p < 0.001). Low concentration DX-9065a did not increase TFPI levels above control (0.8% higher, p = 0.91). In contrast, moderate and high concentrations produced 124% higher (p < 0.001) and 198% higher (p < 0.001) TFPI concentrations than control, respectively.

CONCLUSIONS

DX-9065a increases vascular endothelial cell TFPI concentrations in human tissue culture. Although the mechanism has yet to be established, decreased fXa availability may limit fXa-TFPI complex formation and its subsequent cellular uptake. Whether increased surface TFPI contributes to the overall anticoagulant profile of DX-9065a will require further investigation.

Role of coagulation factor Xa and protease-activated receptor 2 in human mesangial cell proliferation

BACKGROUND

Fibrin deposition and mesangial cell proliferation are frequently observed in the active type of mesangioproliferative glomerulonephritis. Coagulation factors, such as factor V and factor Xa are colocalized with fibrin in the mesangial areas in active type of IgA nephropathy with mesangial cell proliferation. In this study, therefore, we studied the role of factor Xa and its receptor, protease-activated receptor 2 (PAR2) in mesangial cell proliferation and fibrin deposition, and examined ant-proliferative effects of a specific factor Xa inhibitor, DX-9065a, in cultured human mesangial cells.

METHODS

To examine the effect of DX-9065a on the factor Xa-induced proliferation of cultured human mesangial cells, we measured thymidine incorporation and cell numbers. We also examined the effect of DX-9065a on extracellular regulated kinase (ERK) activation and fibrin production induced by factor Xa in human mesangial cells.

RESULTS

Factor Xa increased [(3)H]-thymidine incorporation and cell numbers in a dose-dependent manner in mesangial cells, which was inhibited by DX-9065a. DX-9065a also suppressed factor Xa-triggered fibrin deposition on mesangial cell surface. Factor Xa induced the activation of ERK in mesangial cells and this activation was also completely inhibited by DX-9065a, but not inhibited by PAR1 antagonist. Factor Xa-induced cell proliferation and ERK activation were inhibited by PD98059.

CONCLUSION

There results suggest that factor Xa can induce mesangial cell proliferation through the activation of ERK via PAR2 in mesangial cells and that PAR2 may play a crucial role in the cell proliferation induced by factor Xa. Our results implicate that DX-9065a may be a promising agent to regulate proliferation of mesangial cellss and inhibit the coagulation process in mesangium.

Antithrombotic agents in the treatment of severe sepsis

Sepsis, a systemic inflammatory syndrome, is a response to infection and when associated with multiple organ dysfunction is termed severe sepsis. It remains a leading cause of mortality in the critically ill. The response to the invading microorganisms may be considered as a balance between a pro-inflammatory and an anti-inflammatory reaction. While an inadequate pro-inflammatory reaction and a strong anti-inflammatory response could lead to overwhelming infection and the death of the patient, a strong and uncontrolled pro-inflammatory response, manifested by the release of pro-inflammatory mediators may lead to microvascular thrombosis and multiple organ failure. Endotoxin triggers sepsis via the release of various mediators such as tumour necrosis factor-alpha and interleukin-1 (IL-1). These cytokines activate the complement and coagulation systems, release adhesion molecules, prostaglandins, leukotrienes, reactive oxygen species and nitric oxide. Other mediators involved in the sepsis syndrome include IL-1, -6 and -8; arachidonic acid metabolites; platelet activating factor; histamine; bradykinin; angiotensin; complement components and vasoactive intestinal peptide. These pro-inflammatory responses are counteracted by IL-10. Most of the trials targeting the different mediators of the pro-inflammatory response have failed due to a lack of correct definition of sepsis. Understanding the exact pathophysiology of the disease will enable more advanced treatment options. Targeting the coagulation system with various anticoagulant agents including, activated protein C, and tissue factor pathway inhibitor (TFPI) is a rational approach. Many clinical trials have been conducted to evaluate these agents in severe sepsis. While trials on antithrombin and TFPI were not so successful, the double-blind, placebo-controlled, Phase III trial of recombinant human activated Protein C Worldwide Evaluation in Severe Sepsis (PROWESS) was successful, creating a significant decrease in mortality when compared to the placebo group. A better understanding of the pathophysiologic mechanism of severe sepsis will provide better treatment options, and combination antithrombotic treatment may provide a multipronged approach for the treatment of severe sepsis.

Fibrin independent proinflammatory effects of tissue factor in experimental crescentic glomerulonephritis

BACKGROUND

Tissue factor initiated glomerular fibrin deposition is an important mediator of injury in crescentic glomerulonephritis. Recent data have suggested noncoagulant roles for tissue factor in inflammation.

METHODS

To test the hypothesis that in addition to its effects in initiating coagulation, tissue factor has proinflammatory effects in glomerulonephritis, rabbits given crescentic anti-glomerular basement membrane (GBM) antibody-induced glomerulonephritis were defibrinogenated with ancrod. One group of defibrinogenated rabbits was also given anti-tissue factor antibodies. Comparisons were made between these groups, as well as a third group that was neither defibrinogenated with ancrod nor given anti-tissue factor antibodies.

RESULTS

Defibrinogenation alone abolished glomerular fibrin deposition, reduced crescent formation, and limited renal impairment (ancrod-treated, serum creatinine 274 +/- 37 micromol/L; untreated 415 +/- 51 micromol/L; P < 0.01). Tissue factor inhibition in defibrinogenated rabbits resulted in further protection of renal function (creatinine 140 +/- 19 micromol/L, P < 0.01) and reduced proteinuria (0.4 +/- 0.2g/day, untreated 2.6 +/- 0.4 g/day, P <0.01), which was significantly increased by defibrinogenation alone (ancrod-treated, 5.6 +/- 1.2 g/day). Anti-tissue factor antibodies (but not defibrinogenation alone) attenuated glomerular T-cell and macrophage recruitment, and major histocompatibility complex (MHC) class II expression.

CONCLUSION

These results demonstrate important proinflammatory effects of tissue factor in crescentic glomerulonephritis that are fibrin independent and provide in vivo evidence for tissue factor's proinflammatory effects on MHC class II expression and leukocyte accumulation.

Factor Xa induces mitogenesis of vascular smooth muscle cells via autocrine production of epiregulin

Factor Xa has been reported to elicit smooth muscle cell proliferation via autocrine release of platelet-derived growth factor. However, this study has shown that factor Xa-induced mitogenesis of rat aortic smooth muscle cell is independent of platelet-derived growth factor. We also could not observe any platelet-derived growth factor isoforms in the cultured medium of factor Xa-stimulated cells. Our finding that the cultured medium of factor Xa-stimulated cells strongly induces rat aortic smooth muscle cell mitogenesis in the absence of factor Xa activity led us to explore the existence of a novel autocrine pathway. The autocrine growth factor was purified from the cultured medium and was identified to be epiregulin. Recombinant epiregulin was also able to induce the mitogenesis. The secretion of epiregulin from factor Xa-stimulated rat aortic smooth muscle cell required mRNA expression and protein synthesis of the growth factor. The mitogenic effect of factor Xa on rat aortic smooth muscle cell was significantly reduced by anti-epiregulin antibody or by antisense oligodeoxynucleotide to epiregulin. Several lines of experimental evidence clearly indicate that the autocrine production of epiregulin, an epidermal growth factor-related ligand, is induced in the factor Xa-stimulated mitogenic process of rat aortic smooth muscle cell.

Factor Xa induces mitogenesis of coronary artery smooth muscle cell via activation of PAR-2

Factor Xa-induced stimulation of coronary artery smooth muscle cells (CASMC) was investigated by analyzing [(3)H]thymidine incorporation, cell proliferation, and ERK-1/2 activation. Exposure of the cells to factor Xa evoked a time-dependent activation of ERK-1/2 with increased [(3)H]thymidine incorporation and cell proliferation. The factor Xa-induced ERK-1/2 activation was not desensitized by preincubation of the cells with thrombin. However, ERK-1/2 activation was markedly attenuated by prior exposure of the cells to protease-activated receptor-2 (PAR-2) activating peptide, SLIGKV. The mitogenic effect of factor Xa was significantly reduced in the presence of anti-PAR-2 monoclonal antibody. Several lines of experimental evidence indicate that factor Xa-induced mitogenesis of CASMC is a cellular process mediated by PAR-2 activation.

Factor Xa-inhibitor (DX-9065a) modulates the leukocyte-endothelial cell interaction in endotoxemic rat

Abnormalities of vascular endothelial function and coagulation play important roles in the development of septic organ dysfunction. DX-9065a is a novel Factor Xa inhibitor that is expected to modulate both coagulation and endothelial function. The purpose of this study is to examine the effect of DX-9065a on leukocyte-endothelial interaction. Rats were injected with 1.0 mg/kg of endotoxin simultaneously with saline, (placebo group), 0.3 mg/kg DX-9065a (low-dose group), or 3.0 mg/kg DX-9065a (high-dose group; n = 6 in each group). At 1 and 3 h after injection, the mesenteric microcirculation was observed under intravital microscopy. In addition, TNF, IL-6, alanine aminotransferase (ALT), blood urea nitrogen (BUN), and lactate levels were measured. The number of leukocytes adhering to the endothelium was significantly reduced in both the high-dose and low-dose groups (P < 0.05 for both, compared to the control group). A comparison of the cytokine levels showed that the peak levels in the treatment groups tended to be lower. Markers of organ damage also showed less increase in the treatment groups (P < 0.05 for both treatment groups compared to the control group). In summary, the Factor Xa inhibitor DX-9065a showed a protective effect on the microcirculation of endotoxemic rats by attenuating leukocyte-endothelial interaction. Although the mechanism for this effect could not be fully elucidated, suppression of both excessive coagulation and cytokine production appear to play a role.

Inhibition of factor Xa suppresses the expression of tissue factor in human monocytes and lipopolysaccharide-induced endotoxemia in rats

BACKGROUND

Activated factor X (FXa) is involved in hemostasis, thrombogenesis, inflammation, and cellular immune response. Tissue factor (TF) is an initiator of blood coagulation. We investigated whether FXa induces TF expression in human peripheral monocytes and whether treatment with FXa inhibitor reduces TF expression in an experimental model of rat endotoxemia.

METHODS

Human peripheral mononuclear cells were used to determine TF expression induced by FXa. Experimental rat endotoxemia was induced by intravenous bolus injection of lipopolysaccharide (LPS). A specific FXa inhibitor, DX-9065a, was administered subcutaneously immediately after LPS injection.

RESULTS

FXa induced TF expression in monocytes without intervention of thrombin and the expression was suppressed by FXa inhibitor. In the experimental model of rat endotoxemia, TF and TF mRNA expression levels in the liver were reduced by DX-9065a. Moreover, administration of DX-9065a suppressed the rise in plasma concentrations of thrombin-antithrombin III complex (TAT) and monocyte chemoattractant protein-1 (MCP-1).

CONCLUSIONS

Our results indicated that FXa can induce TF expression in human peripheral monocytes and that inhibition of FXa reduces TF expression in the liver of rat endotoxemia. These results suggest that FXa is an important factor for TF expression in sepsis.

Factor Xa activates endothelial cells by a receptor cascade between EPR-1 and PAR-2

In addition to its pivotal role in hemostasis, factor Xa binds to human umbilical vein endothelial cells through the recognition of a protein called effector cell protease receptor (EPR-1). This interaction is associated with signal transduction, generation of intracellular second messengers, and modulation of cytokine gene expression. Inhibitors of factor Xa catalytic activity block these responses, thus indicating that the factor Xa-dependent event of local proteolysis is absolutely required for cell activation. Because EPR-1 does not contain proteolysis-sensitive sites, we investigated the possibility that signal transduction by factor Xa requires proteolytic activation of a member of the protease-activated receptor (PAR) gene family. Catalytic inactivation of factor Xa by DX9065 suppressed factor Xa-induced increase in cytosolic free Ca(2+) in endothelial cells (IC(50)=0.23 micromol/L) but failed to reduce ligand binding to EPR-1. In desensitization experiments, trypsin or the PAR-2-specific activator peptide, SLIGKV, ablated the Ca(2+) signaling response induced by factor Xa. Conversely, pretreatment of endothelial cells with factor Xa blocked the PAR-2-dependent increase in cytosolic Ca(2+) signaling, whereas PAR-1-dependent responses were unaffected. Direct cleavage of PAR-2 by factor Xa on endothelial cells was demonstrated by cleavage of a synthetic peptide duplicating the PAR-2 cleavage site and by immunofluorescence with an antibody to a peptide containing the 40-amino acid PAR-2 extracellular extension. These data suggest that factor Xa induces endothelial cell activation via a novel cascade of receptor activation involving docking to EPR-1 and local proteolytic cleavage of PAR-2.

The elusive factor Xa receptor: failure to detect transcripts that correspond to the published sequence of EPR-1

The coagulation protease factor Xa induces cellular responses implicated in cardiovascular and inflammatory disease. Effector-cell protease receptor 1 (EPR-1) is a functionally characterized receptor of factor Xa, and the EPR-1complementary DNA (cDNA) was published. Remarkably, the cDNA encoding an inhibitor of apoptosis, survivin, is reportedly identical to that ofEPR-1 except for a few nucleotide differences and its orientation opposite to EPR-1. To isolate the EPR-1cDNA and gene, we surveyed gene databases for expressed sequence tags (ESTs) that could be derived from EPR-1. All ESTs with strong homology to EPR-1/survivin were derived from survivinand could not encode EPR-1. By polymerase chain reaction and Southern blot hybridization, EPR-1 was not detectable in the human or murine genome, but survivin was. Our data suggest that EPR-1 is either highly cell-specific or the published EPR-1 cDNA includes sequences from clones derived from survivin messenger RNA. The means by which factor Xa mediates its cellular effects requires further evaluation.

The elusive factor Xa receptor: failure to detect transcripts that correspond to the published sequence of EPR-1

The coagulation protease factor Xa induces cellular responses implicated in cardiovascular and inflammatory disease. Effector-cell protease receptor 1 (EPR-1) is a functionally characterized receptor of factor Xa, and the EPR-1complementary DNA (cDNA) was published. Remarkably, the cDNA encoding an inhibitor of apoptosis, survivin, is reportedly identical to that ofEPR-1 except for a few nucleotide differences and its orientation opposite to EPR-1. To isolate the EPR-1cDNA and gene, we surveyed gene databases for expressed sequence tags (ESTs) that could be derived from EPR-1. All ESTs with strong homology to EPR-1/survivin were derived from survivinand could not encode EPR-1. By polymerase chain reaction and Southern blot hybridization, EPR-1 was not detectable in the human or murine genome, but survivin was. Our data suggest that EPR-1 is either highly cell-specific or the published EPR-1 cDNA includes sequences from clones derived from survivin messenger RNA. The means by which factor Xa mediates its cellular effects requires further evaluation.

A synthetic inhibitor of factor Xa, DX-9065a, reduces proliferation of vascular smooth muscle cells in vivo in rats

The effect of factor Xa inactivation on the proliferation of vascular smooth muscle cells in vivo was investigated in an experimental restenosis model in rats by using the direct factor Xa inhibitor DX-9065a. In the left common carotid artery, an injury of the vascular endothelium was produced by four external vessel clamps for 60 minutes. After 14 days, 3H-labeled methyl thymidine and 5-bromo-2'-deoxyuridine, respectively, were injected intraperitoneally. After 24 hours, both the left (damaged) and right (nondamaged) carotid arteries were removed, and the incorporation of 3H-methyl thymidine/microg protein was determined. For morphological analysis, the cells were labeled with hematoxylin as well as 5-bromo-2'-deoxyuridine. Stained vascular smooth muscle cell nuclei were counted, and the proliferation index (percentage of 5-bromo-2'-deoxyuridine-positive nuclei to total nuclei stained with hematoxylin) was determined. An external damage of the carotid artery induced proliferation of vascular smooth muscle cells and formation of a neointima within 2 weeks after vessel injury. As compared with control animals, single subcutaneous injection of DX-9065a (2.5, 5, and 10 mg/kg) given 30 minutes before vessel injury significantly reduced the incorporation of 3H-methyl thymidine/microg protein and the total cell number, as well as the proliferation index. The antiproliferative action of DX-9065a was not dose dependent in the range from 2.5 to 10 mg/kg s.c. A combination of bolus injection (5 mg/kg s.c.) with continuous administration (5 mg/kg/d s.c. for 7 and 14 days, respectively) did not increase the antiproliferative effect of DX-9065a. The results indicate a role of factor Xa in the complex pathogenesis of restenosis and the usefulness of a highly effective and selective inhibitor of factor Xa to inhibit proliferative processes.

Coagulation factors VIIa and Xa induce cell signaling leading to up-regulation of the egr-1 gene

Intracellular signaling induced by the coagulation factors (F) VIIa and Xa is poorly understood. We report here studies on these processes in a human keratinocyte line (HaCaT), which is a constitutive producer of tissue factor (TF) and responds to both FVIIa and FXa with elevation of cytosolic Ca(2+), phosphorylation of extracellular signal-regulated kinase (Erk) 1/2, p38(MAPK), and c-Jun N-terminal kinase, and up-regulation of transcription of the early growth response gene-1 (egr-1). Using egr-1 as end point, we observed with both agonists that phosphatidylinositol-specific phospholipase C and the mitogen-activated protein kinase/Erk kinase/Erk pathway were mediators of the responses. The responses to FVIIa were TF-dependent and up-regulation of egr-1 mRNA did not require presence of the TF cytoplasmic domain. Antibodies to EPR-1 and factor V had no effect on the response to FXa. We have provided evidence that TF is not the sole component of the FVIIa receptor. The requirement for proteolytic activity of both FVIIa and FXa suggests that protease-activated receptors may be involved. We now report evidence suggesting that protease-activated receptor 2 or a close homologue may be a necessary but not sufficient component of this particular signal transduction pathway. The up-regulation of egr-1 describes one way by which the initiation of blood coagulation may influence gene transcription. The ability of these coagulation proteases to induce intracellular signals at concentrations at or below the plasma concentrations of their zymogen precursors suggests that these processes may occur also in vivo.

Activation of human vascular endothelial cells by factor Xa: effect of specific inhibitors

Recently, human umbilical vein endothelial cells (HUVEC) have been shown to express functional high-affinity receptors for factor Xa, which may be of importance in the regulation of coagulation and homeostasis of the vascular wall. In this paper, we demonstrate that when added to cultured HUVEC, factor Xa was a potent mitogen, stimulating an increase in cell number at a 0.3 to 100 nM concentration. The same doses of factor Xa also increased intracellular free calcium levels and phosphoinositide turnover. When added to confluent HUVEC, factor Xa induced the expression of tissue factor and the release of tissue-type plasminogen activator and plasminogen activator inhibitor-1 without affecting urokinase expression. Indirect (antithrombin-pentasaccharide) and direct (DX9065) inhibitors of factor Xa affected all these activities of factor Xa in a dose-dependent manner. Taken together, these data show that the activities induced by factor Xa on HUVEC were dependent on its catalytic activity and could be inhibited by both direct and indirect factor Xa inhibitors.

Factor Xa Induces Cytokine Production and Expression of Adhesion Molecules by Human Umbilical Vein Endothelial Cells

Proinflammatory effects induced by the serine protease factor Xa were investigated in HUVEC. Exposure of cells to factor Xa (5–80 nM) concentration dependently stimulated the production of IL-6, IL-8, and monocyte chemotactic protein-1 (MCP-1) and the expression of E-selectin, ICAM-1, and VCAM-1, which was accompanied by polymorphonuclear leukocyte adhesion. The effects of factor Xa were blocked by antithrombin III, but not by the thrombin-specific inhibitor hirudin, suggesting that factor Xa elicits these responses directly and not via thrombin. IL-1α and TNF-α were not implicated, since neither the IL-1 receptor antagonist nor a TNF-neutralizing Ab could suppress the factor Xa responses. Active site-inhibited factor Xa and factor Xa depleted from γ-carboxyglutamic acid residues were completely inactive. The effector cell protease receptor-1 (EPR-1) seems not to be involved since anti-EPR-1 Abs failed to inhibit cytokine production. Moreover, neither the factor X peptide Leu83-Leu88, representing the inter-epidermal growth factor sequence in factor Xa that mediates ligand binding to EPR-1, nor the peptide AG1, corresponding to the EPR-1 sequence Ser123-Pro137 implicated in factor Xa binding, inhibited the factor Xa-induced cytokine production. In conclusion, these findings indicate that factor Xa evokes a proinflammatory response in endothelial cells, which requires both its catalytic and γ-carboxyglutamic acid-containing domain. The receptor system involved in these responses induced by factor Xa remains to be established.

Hypotension and inflammatory cytokine gene expression triggered by factor Xa-nitric oxide signaling

The signaling pathway initiated by factor Xa on vascular endothelial cells was investigated. Factor Xa stimulated a 5- to 10-fold increased release of nitric oxide (NO) in a dose-dependent reaction (0.1-2.5 microG/ml) unaffected by the thrombin inhibitor hirudin but abolished by active site inhibitors, tick anticoagulant peptide, or Glu-Gly-Arg-chloromethyl ketone. In contrast, the homologous clotting protease factor IXa or another endothelial cell ligand, fibrinogen, was ineffective. A factor Xa inter-epidermal growth factor synthetic peptide L (83)FTRKL(88) (G) blocking ligand binding to effector cell protease receptor-1 inhibited NO release by factor Xa in a dose-dependent manner, whereas a control scrambled peptide KFTGRLL was ineffective. Catalytically active factor Xa induced hypotension in rats and vasorelaxation in the isolated rat mesentery, which was blocked by the NO synthase inhibitor L-N(G)-nitroarginine methyl ester (L-NAME) but not by D-NAME. Factor Xa/NO signaling also produced a dose-dependent endothelial cell release of interleukin 6 (range 0.55-3.1 ng/ml) in a reaction inhibited by L-NAME and by the inter-epidermal growth factor peptide Leu(83)-Leu(88) but unaffected by hirudin. Maximal induction of interleukin 6 mRNA required a brief, 30-min stimulation with factor Xa, unaffected by subsequent addition of tissue factor pathway inhibitor. These data suggest that factor Xa-induced NO release modulates endothelial cell-dependent vasorelaxation and cytokine gene expression. This pathway requiring factor Xa binding to effector cell protease receptor-1 and a secondary step of ligand-dependent proteolysis may preserve an anti-thrombotic phenotype of endothelium but also trigger acute phase responses during activation of coagulation in vivo.

Effector protease receptor 1 mediates the mitogenic activity of factor Xa for vascular smooth muscle cells in vitro and in vivo

The binding of 125I-factor Xa to human aortic smooth muscle cell (SMC) monolayers was studied. At 4 degreesC, 125I-factor Xa bound to a single class of binding sites with a dissociation constant value of 3.6+/-0.7 nM and a binding site density of 11,720+/-1,240 sites/cell (n = 9). 125I-factor Xa binding was not affected by factor X, thrombin, or by DX9065, a direct inhibitor of factor Xa, but was inhibited by factor Xa (IC50 = 5.4+/-0.2 nM; n = 9) and by antibodies specific for the effector cell protease receptor 1 (EPR-1), a well-known receptor of factor Xa on various cell types. A factor X peptide duplicating the inter-EGF sequence Leu83-Leu88-(Gly) blocked the binding of 125I-factor Xa to these cells in a dose-dependent manner (IC50 = 110+/-21 nM). Factor Xa increased phosphoinositide turnover in SMCs and when added to SMCs in culture was a potent mitogen. These effects were inhibited by DX9065 and by antibodies directed against EPR-1 and PDGF. Increased expression of EPR-1 was identified immunohistochemically on SMCs growing in culture and in SMCs from the rabbit carotid artery after vascular injury. When applied locally to air-injured rabbit carotid arteries, antibodies directed against EPR-1 (100 mug/ artery) strongly reduced myointimal proliferation 14 d after vascular injury (65-71% inhibition, P < 0.01). DX9065 (10 mg/kg, subcutaneous) inhibited myointimal proliferation significantly (43% inhibition, P < 0.05). These findings indicate that SMCs express functional high affinity receptors for factor Xa related to EPR-1, which may be of importance in the regulation of homeostasis of the vascular wall and after vascular injury.

Induction of vascular smooth muscle cell growth by selective activation of the proteinase activated receptor-2 (PAR-2)

The proteinase-activated receptor (PAR-2) which belongs to the family of proteolytically cleaved receptors is activated by trypsin and by a synthetic peptide (SLIGKV) derived from the new amino terminus. Here, we have studied the mitogenic effect of trypsin and of SLIGKV on human aortic smooth muscle cells (SMC). Like trypsin, SLIGKV was a potent mitogen for SMC and exhibited the same activity as that of SFLLRN, a peptide mimicking the new amino terminus created by cleavage of the thrombin receptor. SLIGKV stimulated the proliferation of growth-arrested SMCs with a half-maximum mitogenic response at 80 nM. Under the same experimental conditions, the retro analogue or SLIGKV (VKGILS) did not show any mitogenic activity. Two specific inhibitors of the enzymatic activity of trypsin, alpha 1-antitrypsin and aprotinin, specifically inhibited trypsin-induced SMC growth (IC50 = 0.87 +/- 0.09 and 0.74 +/- 0.11 microM, respectively) but remain without effect on the mitogenic effect of the agonist peptide. The mitogenic effect of trypsin and SLIGKV was due to the release of platelet derived growth factor as demonstrated by the inhibitory activity of a neutralizing monoclonal anti-PGDF-BB antibody. These results demonstrate that the mitogenic effect of trypsin for SMCs is intimately linked to its esterolytic activity and mediated by the activation of PAR-2.

Coagulation factor Xa induces endothelium-dependent relaxations in rat aorta

The relaxing effect of coagulation factor Xa on phenylephrine-contracted rat aortic rings was compared with the effect of thrombin and trypsin. All three proteases induced a dose-dependent relaxation in the presence of an intact endothelium. EC50 values were 3 +/- 1, 24 +/- 9, and 16 +/- 1 nmol/L for thrombin, trypsin, and factor Xa, respectively. Whereas thrombin induced rapid relaxations followed by partial recontraction, trypsin and factor Xa induced slower sustained effects. Factor Xa-induced relaxations were not affected by hirudin at high concentrations (1 mumol/L) but were abolished by DX9065A, a specific inhibitor of the catalytic activity of factor Xa. Furthermore, no relaxations to factor Xa could be elicited in the presence of the NO synthase inhibitor N omega-nitro-L-arginine methyl ester (100 mumol/L), whereas relaxations were not altered in the presence of the inactive enantiomer N omega-nitro-D-arginine methyl ester (100 mumol/L). Addition of factor Xa together with thrombin induced relaxations that were larger than those induced by thrombin alone, whereas factor Xa had no additional effects on trypsin-induced relaxations. Further-more, factor Xa relaxed thrombin-desensitized aortic rings but was ineffective in trypsin-desensitized tissues. These data suggest that factor Xa acts on a cleavable endothelial receptor that induces NO release, resulting in the relaxation of precontracted rat aortic rings. Factor Xa does not act through endothelial thrombin receptors but may activate another cleavable trypsin-sensitive receptor.