Blood HEMOSTASIS, THROMBOSIS, AND VASCULAR BIOLOGY

Inflammation induces hemorrhage in thrombocytopenia

2008-05-08

The role of platelets in hemostasis is to produce a plug to arrest bleeding. During thrombocytopenia, spontaneous bleeding is seen in some patients but not in others; the reason for this is unknown. Here, we subjected thrombocytopenic mice to models of dermatitis, stroke, and lung inflammation. The mice showed massive hemorrhage that was limited to the area of inflammation and was not observed in uninflamed thrombocytopenic mice. Endotoxin-induced lung inflammation during thrombocytopenia triggered substantial intra-alveolar hemorrhage leading to profound anemia and respiratory distress. By imaging the cutaneous Arthus reaction through a skin window, we observed in real time the loss of vascular integrity and the kinetics of skin hemorrhage in thrombocytopenic mice. Bleeding—observed mostly from venules—occurred as early as 20 minutes after challenge, pointing to a continuous need for platelets to maintain vascular integrity in inflamed microcirculation. Inflammatory hemorrhage was not seen in genetically engineered mice lacking major platelet adhesion receptors or their activators (IIbβ3, glycoprotein Ib [GPIb], GPVI, and calcium and diacylglycerol-regulated guanine nucleotide exchange factor I [CalDAG-GEFI]), thus indicating that firm platelet adhesion was not necessary for their supporting role. While platelets were previously shown to promote endothelial activation and recruitment of inflammatory cells, they also appear indispensable to maintain vascular integrity in inflamed tissue. Based on our observations, we propose that inflammation may cause life-threatening hemorrhage during thrombocytopenia.

Calcitonin receptor-like receptor guides arterial differentiation in zebrafish

Nicoli, S., Tobia, C., Gualandi, L., De Sena, G., Presta, M. — 2008-05-08

The calcitonin receptor-like receptor (crlr) is a major endothelial cell receptor for adrenomedullin, a peptide vasodilator involved in cardiovascular development, homeostasis, and disease. Here, we used the zebrafish (Danio rerio) model to characterize the role of crlr in vascular development. Crlr is expressed within somites from the 4- to the 13-somite stage and by arterial progenitors and axial vessels during zebrafish development. Loss of crlr results in profound alterations in vascular development and angiogenesis, including atrophic trunk dorsal aorta and interruption of anterior aortic bifurcation, delay in intersomitic vessel development, and lack of blood circulation. Remarkably, crlr morphants are characterized by the loss of arterial endothelial cell identity in dorsal aorta, as shown by the lack of expression of the arterial markers ephrin-B2a, DeltaC, and notch5. Down-regulation of crlr affects vascular endothelial growth factor (vegf) expression, whereas vegf overexpression is sufficient to rescue arterial differentiation in crlr morphants. Finally, genetic and biochemical evidences indicate that somitic crlr expression is under the control of sonic hedgehog. These data demonstrate that crlr plays a nonredundant role in arterial differentiation, representing a novel element of the sonic hedgehog–vegf-notch signaling cascade that controls arterial/venous fate.

Protein Z-dependent protease inhibitor deficiency produces a more severe murine phenotype than protein Z deficiency

Zhang, J., Tu, Y., Lu, L., Lasky, N., Broze, G. J. — 2008-05-08

Protein Z (PZ) is a plasma vitamin K–dependent protein that functions as a cofactor to dramatically enhance the inhibition of coagulation factor Xa by the serpin, protein Z–dependent protease inhibitor (ZPI). In vitro, ZPI not only inhibits factor Xa in a calcium ion–, phospholipid-, and PZ-dependent fashion, but also directly inhibits coagulation factor XIa. In murine gene-deletion models, PZ and ZPI deficiency enhances thrombosis following arterial injury and increases mortality from pulmonary thromboembolism following collagen/epinephrine infusion. On a factor V_Leiden genetic background, ZPI deficiency produces a significantly more severe phenotype than PZ deficiency, implying that factor XIa inhibition by ZPI is physiologically relevant. The studies in mice suggest that human PZ and ZPI deficiency would be associated with a modest thrombotic risk with ZPI deficiency producing a more severe phenotype.

Identification of type 1 von Willebrand disease patients with reduced von Willebrand factor survival by assay of the VWF propeptide in the European study: Molecular and Clinical Markers for the Diagnosis and Management of Type 1 VWD (MCMDM-1VWD)

2008-05-08

The decreased survival of von Willebrand factor (VWF) in plasma has been implicated as a mechanism in a subset of type 1 von Willebrand disease (VWD) patients. We have previously reported that the ratio of plasma levels of VWF and its propeptide (VWFpp) can be used to identify patients with reduced VWF survival. In this study, we report the assay of VWFpp and VWF:Ag in 19 individuals recruited from 6 European centers within the MCMDM-1VWD study. Eight individuals had a VWF:Ag level less than 30 IU/dL. Seven of these patients had a robust desmopressin response and significantly reduced VWF half-life that was predicted by a markedly increased steady-state plasma VWFpp/VWF:Ag ratio. VWF mutations previously associated with reduced VWF survival were identified in each of the 7 individuals. Thus, a substantially increased ratio of steady-state VWFpp/VWF:Ag predicted a reduced VWF half-life in patients with markedly decreased VWF:Ag levels. These data indicate that a reduced VWF survival is found in a subpopulation of patients with type 1 VWD. The systematic assay of both plasma VWF and the VWF propeptide in moderately severe type 1 VWD patients may identify patients with a reduced VWF survival phenotype.

Identification of a major GpVI-binding locus in human type III collagen

2008-05-08

We have analyzed the adhesion of human and murine platelets, and of recombinant human and murine GpVI ectodomains, to synthetic triple-helical collagen-like peptides. These included 57 peptides derived from the sequence of human type III collagen and 9 peptides derived from the cyanogen bromide fragment of bovine type III collagen, 1(III)CB4. We have identified several peptides that interact with GpVI, in particular a peptide designated III-30 with the sequence GAOGLRGGAGPOGPEGGKGAAGPOGPO. Both human and murine platelets bound to peptide III-30 in a GpVI-dependent manner. III-30 also supported binding of recombinant GpVI ectodomains. Cross-linked III-30 induced aggregation of human and murine platelets, although with a lower potency than collagen-related peptide. Modifications of the peptide sequence indicated that the hydroxyproline residues play a significant role in supporting its GpVI reactivity. However, many peptides containing OGP/GPO motifs did not support adhesion to GpVI. These data indicate that the ability of a triple-helical peptide to bind GpVI is not solely determined by the presence or spatial arrangement of these OGP/GPO motifs within the peptides.

TNF primes endothelial cells for angiogenic sprouting by inducing a tip cell phenotype

2008-05-08

Pathological angiogenesis associated with wound healing often occurs subsequent to an inflammatory response that includes the secretion of cytokines such as tumor necrosis factor (TNF). Controversy exists on the angiogenic actions of TNF, with it being generally proangiogenic in vivo, but antiangiogenic in vitro. We find that whereas continuous administration of TNF in vitro or in vivo inhibits angiogenic sprouting, a 2- to 3-day pulse stimulates angiogenesis by inducing an endothelial "tip cell" phenotype. TNF induces the known tip cell genes platelet-derived growth factor B (PDGFB) and vascular endothelial cell growth factor receptor-2 (VEGFR2), while at the same time blocking signaling through VEGFR2, thus delaying the VEGF-driven angiogenic response. Notch signaling regulates tip cell function, and we find that TNF also induces the notch ligand jagged-1, through an NFB-dependent mechanism. Enrichment of jagged-1 in tip cells was confirmed by immunofluorescent staining as well as by laser capture microdissection/quantitative reverse-transcription–polymerase chain reaction (qRT-PCR) of tip cells sprouting in vitro. Thus, in angiogenesis, the temporal expression of TNF is critical: it delays angiogenesis initially by blocking signaling through VEGFR2, but in addition by inducing a tip cell phenotype through an NFB-dependent pathway, it concomitantly primes endothelial cells (ECs) for sprouting once the initial inflammatory wave has passed.

Angiogenic endothelium shows lactadherin-dependent phagocytosis of aged erythrocytes and apoptotic cells

2008-04-25

Angiogenic endothelium plays a crucial role in tumor growth. During angiogenesis, complex alterations in the microenvironment occur. In response, the endothelium undergoes phenotypic changes, for example overexpression of _v-integrins. Here, we show that the overexpression of _v-integrins on angiogenic endothelial cells is engaged in phagocytic actions involving binding ("tethering") and uptake ("tickling") of lactadherin (also termed MFG-E8)–opsonized particles. Phosphatidylserine (PS)–exposing multilamellar vesicles, "aged" erythrocytes, and apoptotic melanoma cells incubated with lactadherin were all phagocytosed by angiogenic endothelial cells in vitro. Furthermore, we demonstrated lactadherin expression in and around tumor blood vessels making opsonization in situ plausible. By engineering the surface of erythrocytes with covalently coupled cyclic Arg-Gly-Asp (RGD) peptides—mimicking lactadherin opsonization—we could induce phagocytosis by angiogenic endothelial cells both in vitro and in vivo. In vitro, this was confirmed by cytochalasin D preincubation. When RGD-erythrocytes were administered intravenously in tumor-bearing mice, blood vessel congestion followed by tumor core necrosis was seen. Moreover, RGD-erythrocytes could delay tumor growth in a murine melanoma model, possibly through induction of tumor infarctions. These results reveal that angiogenic endothelial cells have phagocytic properties for lactadherin-opsonized large particles and apoptotic cells. Implications of our findings for diagnostic and therapy of angiogenesis-driven diseases are discussed.

Bone marrow-derived mesenchymal stem cells facilitate engineering of long-lasting functional vasculature

Au, P., Tam, J., Fukumura, D., Jain, R. K. — 2008-04-25

Vascular tissue engineering requires a ready source of endothelial cells and perivascular cells. Here, we evaluated human bone marrow–derived mesenchymal stem cells (hMSCs) for use as vascular progenitor cells in tissue engineering and regenerative medicine. hMSCs expressed a panel of smooth muscle markers in vitro including the cardiac/smooth muscle–specific transcription coactivator, myocardin. Cell-cell contact between endothelial cells and hMSCs up-regulated the transcription of myocardin. hMSCs efficiently stabilized nascent blood vessels in vivo by functioning as perivascular precursor cells. The engineered blood vessels derived from human umbilical cord vein endothelial cells and hMSCs remained stable and functional for more than 130 days in vivo. On the other hand, we could not detect differentiation of hMSCs to endothelial cells in vitro, and hMSCs by themselves could not form conduit for blood flow in vivo. Similar to normal perivascular cells, hMSC-derived perivascular cells contracted in response to endothelin-1 in vivo. In conclusion, hMSCs are perivascular cell precursors and may serve as an attractive source of cells for use in vascular tissue engineering and for the study of perivascular cell differentiation.

Autoantibodies to the C-terminal subunit of RLIP76 induce oxidative stress and endothelial cell apoptosis in immune-mediated vascular diseases and atherosclerosis

2008-04-25

Although detection of autoantibodies in the peripheral blood from patients with immune-mediated endothelial dysfunctions has so far failed to provide tools of diagnostic or pathogenetic value, putative bioindicators include anti-endothelial cell antibodies, a heterogeneous family of antibodies that react with autoantigens expressed by endothelial cells. In this study, to identify endothelial autoantigens involved in the autoimmune processes causing endothelial damage, we screened a human microvascular endothelial cell cDNA library with sera from patients with Behçet's disease. We identified antibodies to the C-terminus of Ral binding protein1 (RLIP76), a protein that catalyzes the ATP-dependent transport of glutathione (GSH) conjugates including GSH-4-hydroxy-t-2,3-nonenal, in the serum of a significant percentage of patients with various diseases characterized by immune-mediated endothelial dysfunction, including Behçet disease, systemic sclerosis, systemic lupus erythematosus and carotid atherosclerosis. These autoantibodies increased intracellular levels of 4-hydroxy-t-2,3-nonenal, decreased levels of GSH and activated C-Jun NH2 Kinase signaling (JNK), thus inducing oxidative stress-mediated endothelial cell apoptosis. The dietary antioxidant alpha-tocopherol counteracted endothelial cell demise. These findings suggest that autoantibodies to RLIP76 play a pathogenetic role in immune-mediated vascular diseases and represent a valuable peripheral blood bioindicator of atherosclerosis and immune-mediated vascular diseases.

Inhibition of endogenous TGF-{beta} signaling enhances lymphangiogenesis

2008-04-25

Lymphangiogenesis is induced by various growth factors, including VEGF-C. Although TGF-β plays crucial roles in angiogenesis, the roles of TGF-β signaling in lymphangiogenesis are unknown. We show here that TGF-β transduced signals in human dermal lymphatic microvascular endothelial cells (HDLECs) and inhibited the proliferation, cord formation, and migration toward VEGF-C of HDLECs. Expression of lymphatic endothelial cell (LEC) markers, including LYVE-1 and Prox1 in HDLECs, as well as early lymph vessel development in mouse embryonic stem cells in the presence of VEGF-A and C, were repressed by TGF-β but were induced by TGF-β type I receptor (TβR-I) inhibitor. Moreover, inhibition of endogenous TGF-β signaling by TβR-I inhibitor accelerated lymphangiogenesis in a mouse model of chronic peritonitis. Lymphangiogenesis was also induced by TβR-I inhibitor in the presence of VEGF-C in pancreatic adenocarcinoma xenograft models inoculated in nude mice. These findings suggest that TGF-β transduces signals in LECs and plays an important role in the regulation of lymphangiogenesis in vivo.

A functional 14-3-3{zeta}-independent association of PI3-kinase with glycoprotein Ib{alpha}, the major ligand-binding subunit of the platelet glycoprotein Ib-IX-V complex

2008-04-25

Engagement of the adhesion receptor glycoprotein (GP) Ib-IX-V by von Willebrand factor (VWF) mediates platelet adhesion to damaged vessels and triggers platelet activation and thrombus formation in heart attack and stroke. GPIb-IX-V contains distinct 14-3-3–binding sites at the GPIb C-terminus involving phosphorylation of Ser609, an upstream site involving phosphorylated Ser587/Ser590, and a protein kinase A (PKA)–dependent site on GPIbβ involving Ser166. 14-3-3 regulates the VWF-binding affinity of GPIb-IX-V and inhibiting 14-3-3 association blocks receptor signaling, suggesting a key functional role for 14-3-3. We used deletion mutants of GPIb expressed in Chinese hamster ovary (CHO) cells to define the relationship of 14-3-3 binding to another GPIb-IX-V–associated signaling protein, phosphoinositide 3-kinase (PI3-kinase). Pull-down experiments involving glutathione S-transferase (GST)–PI3-kinase/p85-subunit and GST–14-3-3 indicated that both proteins interacted with contiguous GPIb sequences 580 to 590/591 to 610. Deleting these, but not upstream sequences of GPIb expressed in CHO cells, inhibited VWF/ristocetin-dependent Akt phosphorylation, relative to wild-type receptor, confirming this region encompassed a functional PI3-kinase–binding site. Pull-down experiments with GST-p85 truncates indicated the GPIb-binding region involved the p85 breakpoint cluster region (BCR) domain, containing RSXSXP. However, pull-down of GPIb-IX was unaltered by mutation/deletion/phosphorylation of this potential 14-3-3–binding sequence in mutant constructs of GST-p85, suggesting PI3-kinase bound GPIb independently of 14-3-3; 14-3-3 inhibitor peptide R18 also blocked pull-down of receptor by GST-14-3-3 but not GST-p85, and GST-p85 pull-downs were unaffected by excess 14-3-3. Together, these data suggest the GPIb C-terminus regulates signaling through independent association of 14-3-3 and PI3-kinase.

Immunologic ignorance of vascular endothelial cells expressing minor histocompatibility antigen

2008-04-25

Endothelial cells (ECs) presenting minor histocompatibility antigen (mhAg) are major target cells for alloreactive effector CD8⁺ T cells during chronic transplant rejection and graft-versus-host disease (GVHD). The contribution of ECs to T-cell activation, however, is still a controversial issue. In this study, we have assessed the antigen-presenting capacity of ECs in vivo using a transgenic mouse model with beta-galactosidase (β-gal) expression confined to the vascular endothelium (Tie2-LacZ mice). In a GVHD-like setting with adoptive transfer of β-gal–specific T-cell receptor–transgenic T cells, β-gal expression by ECs was not sufficient to either activate or tolerize CD8⁺ T cells. Likewise, transplantation of fully vascularized heart or liver grafts from Tie2-LacZ mice into nontransgenic recipients did not suffice to activate β-gal–specific CD8⁺ T cells, indicating that CD8⁺ T-cell responses against mhAg cannot be initiated by ECs. Moreover, we could show that spontaneous activation of β-gal–specific CD8⁺ T cells in Tie2-LacZ mice was exclusively dependent on CD11c⁺ dendritic cells (DCs), demonstrating that mhAgs presented by ECs remain immunologically ignored unless presentation by DCs is granted.

The CD40-TRAF6 axis is the key regulator of the CD40/CD40L system in neointima formation and arterial remodeling

2008-04-25

We investigated the role of CD40 and CD40L in neointima formation and identified the downstream CD40-signaling intermediates (tumor necrosis factor [TNF]–receptor associated factors [TRAF]) involved. Neointima formation was induced in wild-type, CD40^–/–, CD40L^–/–, and in CD40^–/– mice that contained a CD40 transgene with or without mutations at the CD40-TRAF2,3&5, TRAF6, or TRAF2,3,5&6 binding sites. Compared with wild-type mice, CD40^–/– mice showed a significant decrease in neointima formation with increased collagen deposition and decreased inflammatory cell infiltration. Neointima formation was also impaired in wild-type mice reconstituted with CD40^–/– bone marrow. In vitro, the capacity of CD40^–/– leukocytes to adhere to the endothelium was reduced. Ligated carotid arteries of CD40^–/– mice showed a smaller total vessel volume and an impaired remodeling capacity, reflected by decreased gelatinolytic/collagenolytic activity. Comparable results were found in mice with defects in CD40-TRAF6 and CD40-TRAF 2/3/5&6 binding, but not in mice with defects in CD40-TRAF2/3&5 binding. Neointima formation and vascular remodeling in CD40-receptor–deficient mice is impaired, due to a decreased inflammatory cell infiltration and matrix-degrading protease activity, with CD40-TRAF6 signaling as the key regulator. This identifies the CD40-TRAF6 axis as a potential therapeutic target in vascular disease.

Visualization of microtubule growth in living platelets reveals a dynamic marginal band with multiple microtubules

2008-04-25

The marginal band of microtubules maintains the discoid shape of resting blood platelets. Although studies of platelet microtubule coil structure conclude that it is composed of a single microtubule, no investigations of its dynamics exist. In contrast to previous studies, permeabilized platelets incubated with GTP-rhodamine-tubulin revealed tubulin incorporation at 7.9 (± 1.9) points throughout the coil, and anti-EB1 antibodies stained 8.7 (± 2.0) sites, indicative of multiple free microtubules. To pursue this result, we expressed the microtubule plus-end marker EB3-GFP in megakaryocytes and examined its behavior in living platelets released from these cells. Time-lapse microscopy of EB3-GFP in resting platelets revealed multiple assembly sites within the coil and a bidirectional pattern of assembly. Consistent with these findings, tyrosinated tubulin, a marker of newly assembled microtubules, localized to resting platelet microtubule coils. These results suggest that the resting platelet marginal band contains multiple highly dynamic microtubules of mixed polarity. Analysis of microtubule coil diameters in newly formed resting platelets indicates that microtubule coil shrinkage occurs with aging. In addition, activated EB3-GFP–expressing platelets exhibited a dramatic increase in polymerizing microtubules, which travel outward and into filopodia. Thus, the dynamic microtubules associated with the marginal band likely function during both resting and activated platelet states.

Gas6 promotes inflammation by enhancing interactions between endothelial cells, platelets, and leukocytes

2008-04-08

The role of Gas6 in endothelial cell (EC) function remains incompletely characterized. Here we report that Gas6 amplifies EC activation in response to inflammatory stimuli in vitro. In vivo, Gas6 promotes and accelerates the sequestration of circulating platelets and leukocytes on activated endothelium as well as the formation and endothelial sequestration of circulating platelet-leukocyte conjugates. In addition, Gas6 promotes leukocyte extravasation, inflammation, and thrombosis in mouse models of inflammation (endotoxinemia, vasculitis, heart transplantation). Thus, Gas6 amplifies EC activation, thereby playing a key role in enhancing the interactions between ECs, platelets, and leukocytes during inflammation.

CYP4F2 genetic variant alters required warfarin dose

2008-04-08

Warfarin is an effective, commonly prescribed anticoagulant used to treat and prevent thrombotic events. Because of historically high rates of drug-associated adverse events, warfarin remains underprescribed. Further, interindividual variability in therapeutic dose mandates frequent monitoring until target anticoagulation is achieved. Genetic polymorphisms involved in warfarin metabolism and sensitivity have been implicated in variability of dose. Here, we describe a novel variant that influences warfarin requirements. To identify additional genetic variants that contribute to warfarin requirements, screening of DNA variants in additional genes that code for drug-metabolizing enzymes and drug transport proteins was undertaken using the Affymetrix drug-metabolizing enzymes and transporters panel. A DNA variant (rs2108622; V433M) in cytochrome P450 4F2 (CYP4F2) was associated with warfarin dose in 3 independent white cohorts of patients stabilized on warfarin representing diverse geographic regions in the United States and accounted for a difference in warfarin dose of approximately 1 mg/day between CC and TT subjects. Genetic variation of CYP4F2 was associated with a clinically relevant effect on warfarin requirement.

Reduced incidence of ischemic stroke in patients with severe factor XI deficiency

Salomon, O., Steinberg, D. M., Koren-Morag, N., Tanne, D., Seligsohn, U. — 2008-04-08

Inherited disorders of hemostasis are natural models for investigating mechanisms of thrombosis and development of antithrombotic therapy. Because mice with total factor XI deficiency are protected against ischemic stroke and do not manifest excessive bleeding, we investigated the incidence of ischemic stroke in patients with severe inherited factor XI deficiency. Incidence of ischemic stroke in 115 patients aged 45 years or more with severe factor XI deficiency (activity less than 15 U/dL) was compared with incidence in the Israeli population as estimated from a stroke survey of 1528 patients. Adjustment for major risk factors of stroke (hypertension, diabetes mellitus, hypercholesterolemia, current smoking) was based on comparison of their prevalence in the stroke survey to an Israeli health survey of 9509 subjects. Incidence of myocardial infarction in the factor XI cohort was also recorded. After adjustment for the 4 major risk factors of ischemic stroke, the expected incidence of ischemic stroke was 8.56 compared with one observed (P = .003). The reduced 1:115 incidence of ischemic stroke contrasted with a 19:115 incidence of myocardial infarction, similar to the expected incidence. Thus, severe factor XI deficiency probably is protective against ischemic stroke but not against acute myocardial infarction.

Vascular dermatan sulfate regulates the antithrombotic activity of heparin cofactor II

He, L., Giri, T. K., Vicente, C. P., Tollefsen, D. M. — 2008-04-08

Heparin cofactor II (HCII)–deficient mice form occlusive thrombi more rapidly than do wild-type mice following injury to the carotid arterial endothelium. Dermatan sulfate (DS) and heparan sulfate (HS) increase the rate of inhibition of thrombin by HCII in vitro, but it is unknown whether vascular glycosaminoglycans play a role in the antithrombotic effect of HCII in vivo. In this study, we found that intravenous injection of either wild-type recombinant HCII or a variant with low affinity for HS (K173H) corrected the abnormally short thrombosis time of HCII-deficient mice, while a variant with low affinity for DS (R189H) had no effect. When HCII was incubated with frozen sections of the mouse carotid artery, it bound specifically to DS in the adventitia. HCII was undetectable in the wall of the uninjured carotid artery, but it became concentrated in the adventitia following endothelial injury. These results support the hypothesis that HCII interacts with DS in the vessel wall after disruption of the endothelium and that this interaction regulates thrombus formation in vivo.

Sulfated polysaccharides identified as inducers of neuropilin-1 internalization and functional inhibition of VEGF165 and semaphorin3A

Narazaki, M., Segarra, M., Tosato, G. — 2008-04-08

Neuropilin-1 (NRP1) and NRP2 are cell surface receptors shared by class 3 semaphorins and vascular endothelial growth factor (VEGF). Ligand interaction with NRPs selects the specific signal transducer, plexins for semaphorins or VEGF receptors for VEGF, and promotes NRP internalization, which effectively shuts down receptor-mediated signaling by a second ligand. Here, we show that the sulfated polysaccharides dextran sulfate and fucoidan, but not others, reduce endothelial cell-surface levels of NRP1, NRP2, and to a lesser extent VEGFR-1 and VEGFR-2, and block the binding and in vitro function of semaphorin3A and VEGF₁₆₅. Administration of fucoidan to mice reduces VEGF₁₆₅-induced angiogenesis and tumor neovascularization in vivo. We find that dextran sulfate and fucoidan can bridge the extracellular domain of NRP1 to that of the scavenger receptor expressed by endothelial cells I (SREC-I), and induce NRP1 and SREC-I coordinate internalization and trafficking to the lysosomes. Overexpression of SREC-I in SREC-I–negative cells specifically reduces cell-surface levels of NRP1, indicating that SREC-I mediates NRP1 internalization. These results demonstrate that engineered receptor internalization is an effective strategy for reducing levels and function of cell-surface receptors, and identify certain sulfated polysaccharides as "internalization inducers."

Mice lacking the extracellular matrix protein MAGP1 display delayed thrombotic occlusion following vessel injury

2008-04-08

Mice lacking the extracellular matrix protein microfibril-associated glycoprotein-1 (MAGP1) display delayed thrombotic occlusion of the carotid artery following injury as well as prolonged bleeding from a tail vein incision. Normal occlusion times were restored when recombinant MAGP1 was infused into deficient animals prior to vessel wounding. Blood coagulation was normal in these animals as assessed by activated partial thromboplastin time and prothrombin time. Platelet number was lower in MAGP1-deficient mice, but the platelets showed normal aggregation properties in response to various agonists. MAGP1 was not found in normal platelets or in the plasma of wild-type mice. In ligand blot assays, MAGP1 bound to fibronectin, fibrinogen, and von Willebrand factor, but von Willebrand factor was the only protein of the 3 that bound to MAGP1 in surface plasmon resonance studies. These findings show that MAGP1, a component of microfibrils and vascular elastic fibers, plays a role in hemostasis and thrombosis.

Angiopoietin-1 promotes endothelial cell proliferation and migration through AP-1-dependent autocrine production of interleukin-8

2008-04-08

Angiopoietin-1 (Ang-1), ligand for the endothelial cell–specific Tie-2 receptors, promotes migration and proliferation of endothelial cells, however, whether these effects are promoted through the release of a secondary mediator remains unclear. In this study, we assessed whether Ang-1 promotes endothelial cell migration and proliferation through the release of interleukin-8 (IL-8). Ang-1 elicited in human umbilical vein endothelial cells (HUVECs) a dose- and time-dependent increase in IL-8 production as a result of induction of mRNA and enhanced mRNA stability of IL-8 transcripts. IL-8 production is also elevated in HUVECs transduced with retroviruses expressing Ang-1. Neutralization of IL-8 in these cells with a specific antibody significantly attenuated proliferation and migration and induced caspase-3 activation. Exposure to Ang-1 triggered a significant increase in DNA binding of activator protein-1 (AP-1) to a relatively short fragment of IL-8 promoter. Upstream from the AP-1 complex, up-regulation of IL-8 transcription by Ang-1 was mediated through the Erk1/2, SAPK/JNK, and PI-3 kinase pathways, which triggered c-Jun phosphorylation on Ser63 and Ser73. These results suggest that promotion of endothelial migration and proliferation by Ang-1 is mediated, in part, through the production of IL-8, which acts in an autocrine fashion to suppress apoptosis and facilitate cell proliferation and migration.

Regenerative repair after endoluminal injury in mice with specific antagonism of protease activated receptors on CD34+ vascular progenitors

2008-04-08

Tissue factor (TF) and thrombin are involved in intimal hyperplasia (IH) and remodelling following vascular injury. Because many neointimal smooth muscle cells (VSMCs) derive from circulating vascular progenitors (VPs), we investigated how thrombin influences VP phenotype and function. Following wire-induced carotid artery injury in mice, the majority of circulating VPs expressed TF, were capable of initiating clotting in vitro, and had protease-activated receptors (PAR)–1, –2, and –4. Thrombin, through PAR-1, inhibited apoptosis and caused proliferation, resulting in the outgrowth of VP coexpressing markers of activated endothelial cells and VSMCs, even in the presence of growth factors. These mixed-phenotype VPs circulated as a minority population after injury and shared a similar phenotype with many neointimal cells. Labeled CD34⁺ cells, injected up to 2 weeks after injury, could be detected in the injured vessel wall, suggesting that continued recruitment may contribute to progressive IH. Finally, CD34⁺ cells incubated with thrombin prior to injection promoted florid neointimal lesions, whereas those incubated with PAR antagonists inhibited IH and promoted regenerative repair characterized by the development of a quiescent endothelium. We conclude that IH after vascular injury is due to the direct actions of thrombin on mobilized VPs.

Alternative mRNA is favored by the A3 haplotype of the EPCR gene PROCR and generates a novel soluble form of EPCR in plasma

2008-03-24

The endothelial cell protein C receptor also exists in soluble form in plasma (sEPCR), resulting from ADAM17 cleavage. Elevated sEPCR levels are observed in subjects carrying the A3 haplotype, which is characterized by a Ser219Gly substitution in the transmembrane domain, rendering the receptor more sensitive to cleavage. Because sEPCR production is not completely blocked by metalloprotease inhibition, we looked for another mechanism. Comparing mRNA expression patterns and levels in A3 and non-A3 cells from 32 human umbilical cord veins, we detected a truncated mRNA in addition to the full-length mRNA. This truncated mRNA was 16 times more abundant in A3 human umbilical vein endothelial cells than in non-A3 human umbilical vein endothelial cells and encoded a protein lacking the transmembrane domain. We stably expressed a recombinant form of this protein (rEPCRisoform) and a protein mimicking the plasma sEPCR (rEPCRsol). Functional studies of the purified recombinant proteins revealed that the rEPCRisoform bound to recombinant protein C with similar affinity than rEPCRsol and that it also inhibited the anticoagulant activity of APC. Trace amounts of the EPCR isoform were found in the plasma of A3 subjects. These results suggest that the sEPCRisoform could contribute to the regulatory effect of sEPCR in plasma.

The combined roles of ADAMTS13 and VWF in murine models of TTP, endotoxemia, and thrombosis

Chauhan, A. K., Walsh, M. T., Zhu, G., Ginsburg, D., Wagner, D. D., Motto, D. G. — 2008-03-24

Ultralarge von Willebrand factor (UL-VWF) multimers are thought to play a central role in pathogenesis of the disease thrombotic thrombocytopenic purpura (TTP); however, experimental evidence in support of this hypothesis has been difficult to establish. Therefore, to examine directly the requirement for VWF in TTP pathogenesis, we generated ADAMTS13-deficient mice on a TTP-susceptible genetic background that were also either haploinsufficient (Vwf^+/–) or completely deficient (Vwf^–/–) in VWF. Absence of VWF resulted in complete protection from shigatoxin (Stx)–induced thrombocytopenia, demonstrating an absolute requirement for VWF in this model (Stx has been shown previously to trigger TTP in ADAMTS13-deficient mice). We next investigated the requirements for ADAMTS13 and VWF in a murine model of endotoxemia. Unlike Stx-induced TTP findings, LPS-induced thrombocytopenia and mortality were not affected by either VWF or ADAMTS13 deficiency, suggesting divergent mechanisms of thrombocytopenia between these 2 disorders. Finally, we show that VWF deficiency abrogates the ADAMTS13-deficient prothrombotic state, suggesting VWF as the only relevant ADAMTS13 substrate under these conditions. Together, these findings shed new light on the potential roles played by ADAMTS13 and VWF in TTP, endotoxemia, and normal hemostasis.

Serglycin proteoglycan deletion induces defects in platelet aggregation and thrombus formation in mice

2008-03-24

Serglycin (SG), the hematopoietic cell secretory granule proteoglycan, is crucial for storage of specific secretory proteins in mast cells, neutrophils, and cytotoxic T lymphocytes. We addressed the role of SG in platelets using SG^–/– mice. Wild-type (WT) but not SG^–/– platelets contained chondroitin sulfate proteoglycans. Electron microscopy revealed normal -granule structure in SG^–/– platelets. However, SG^–/– platelets and megakaryocytes contained unusual scroll-like membranous inclusions, and SG^–/– megakaryocytes showed extensive emperipolesis of neutrophils. SG^–/– platelets had reduced ability to aggregate in response to low concentrations of collagen or PAR4 thrombin receptor agonist AYPGKF, and reduced fibrinogen binding after AYPGKF, but aggregated normally to ADP. ³H-serotonin and ATP secretion were greatly reduced in SG^–/– platelets. The -granule proteins platelet factor 4, β-thromboglobulin, and platelet-derived growth factor were profoundly reduced in SG^–/– platelets. Exposure of P-selectin and IIb after thrombin treatment was similar in WT and SG^–/– platelets. SG^–/– mice exhibited reduced carotid artery thrombus formation after exposure to FeCl₃. This study demonstrates that SG is crucial for platelet function and thrombus formation. We propose that SG^–/– platelet function deficiencies are related to inadequate packaging and secretion of selected -granule proteins and reduced secretion of dense granule contents critical for platelet activation.

Identification of 31 novel mutations in the F8 gene in Spanish hemophilia A patients: structural analysis of 20 missense mutations suggests new intermolecular binding sites

2008-03-24

Hemophilia A (HA) is an X-linked bleeding disorder caused by a wide variety of mutations in the factor 8 (F8) gene, leading to absent or deficient factor VIII (FVIII). We analyzed the F8 gene of 267 unrelated Spanish patients with HA. After excluding patients with the common intron-1 and intron-22 inversions and large deletions, we detected 137 individuals with small mutations, 31 of which had not been reported previously. Eleven of these were nonsense, frameshift, and splicing mutations, whereas 20 were missense changes. We assessed the impact of the 20 substitutions based on currently available information about FV and FVIII structure and function relationship, including previously reported results of replacements at these and topologically equivalent positions. Although most changes are likely to cause gross structural perturbations and concomitant cofactor instability, p.Ala375Ser is predicted to affect cofactor activation. Finally, 3 further mutations (p.Pro64Arg, p.Gly494Val, and p.Asp2267Gly) appear to affect cofactor interactions with its carrier protein, von Willebrand factor, with the scavenger receptor low-density lipoprotein receptor–related protein (LRP), and/or with the substrate of the FVIIIapi•FIXa (Xase) complex, factor X. Characterization of these novel mutations is important for adequate genetic counseling in HA families, but also contributes to a better understanding of FVIII structure-function relationship.

A proangiogenic peptide derived from vascular endothelial growth factor receptor-1 acts through {alpha}5{beta}1 integrin

2008-03-24

Vascular endothelial growth factor receptor-1 (VEGFR-1) is a tyrosine kinase receptor for growth factors of the VEGF family. Endothelial cells express a membrane-bound and a soluble variant of this protein, the latter being mainly considered as a negative regulator of VEGF-A signaling. We previously reported that the soluble form is deposited in the extracellular matrix produced by endothelial cells in culture and is able to promote cell adhesion and migration through binding to 5β1 integrin. In this study, we demonstrate that the Ig-like domain II of VEGFR-1, which contains the binding determinants for the growth factors, is involved in the interaction with 5β1 integrin. To identify domain regions involved in integrin binding, we designed 12 peptides putatively mimicking the domain II surface and tested their ability to inhibit 5β1-mediated endothelial cell adhesion to soluble VEGFR-1 and directly support cell adhesion. One peptide endowed with both these properties was identified and shown to inhibit endothelial cell migration toward soluble VEGFR-1 as well. This peptide directly binds 5β1 integrin, but not VEGF-A, inducing endothelial cell tubule formation in vitro and neoangiogenesis in vivo. Alanine scanning mutagenesis of the peptide defined which residues were responsible for its biologic activity and integrin binding.

Basal and angiopoietin-1-mediated endothelial permeability is regulated by sphingosine kinase-1

2008-03-24

Endothelial cells (ECs) regulate the barrier function of blood vessels. Here we show that basal and angiopoietin-1 (Ang-1)–regulated control of EC permeability is mediated by 2 different functional states of sphingosine kinase-1 (SK-1). Mice depleted of SK-1 have increased vascular leakiness, whereas mice transgenic for SK-1 in ECs show attenuation of leakiness. Furthermore, Ang-1 rapidly and transiently stimulates SK-1 activity and phosphorylation, and induces an increase in intracellular sphingosine-1-phosphate (S1P) concentration. Overexpression of SK-1 resulted in inhibition of permeability similar to that seen for Ang-1, whereas knockdown of SK-1 by small interfering RNA blocked Ang-1-mediated inhibition of permeability. Transfection with SK^S225A, a nonphosphorylatable mutant of SK-1, inhibited basal leakiness, and both SK^S225A and a dominant-negative SK-1 mutant removed the capacity of Ang-1 to inhibit permeability. These effects were independent of extracellular S1P as knockdown or inhibition of S1P₁, S1P₂, or S1P₃, did not affect the Ang-1 response. Thus, SK-1 levels in ECs powerfully regulate basal permeability in vitro and in vivo. In addition, the Ang-1–induced inhibition of leakiness is mediated through activation of SK-1, defining a new signaling pathway in the Ang-1 regulation of permeability.

Transcription factor Erg regulates angiogenesis and endothelial apoptosis through VE-cadherin

2008-03-24

Tight regulation of the balance between apoptosis and survival is essential in angiogenesis. The ETS transcription factor Erg is required for endothelial tube formation in vitro. Inhibition of Erg expression in human umbilical vein endothelial cells (HUVECs), using antisense oligonucleotides, resulted in detachment of cell-cell contacts and increased cell death. Inhibition of Erg expression by antisense in HUVECs also lowered expression of the adhesion molecule vascular endothelial (VE)–cadherin, a key regulator of endothelial intercellular junctions and survival. Using chromatin immunoprecipitation, we showed that Erg binds to the VE-cadherin promoter. Furthermore, Erg was found to enhance VE-cadherin promoter activity in a transactivation assay. Apoptosis induced by inhibition of Erg was partly rescued by overexpression of VE-cadherin–GFP, suggesting that VE-cadherin is involved in the Erg-dependent survival signals. To show the role of Erg in angiogenesis in vivo, we used siRNA against Erg in a Matrigel plug model. Erg inhibition resulted in a significant decrease in vascularization, with increase in caspase-positive endothelial cells (ECs). These results identify a new pathway regulating angiogenesis and endothelial survival, via the transcription factor Erg and the adhesion molecule VE-cadherin.

Determination of surface tissue factor thresholds that trigger coagulation at venous and arterial shear rates: amplification of 100 fM circulating tissue factor requires flow

Okorie, U. M., Denney, W. S., Chatterjee, M. S., Neeves, K. B., Diamond, S. L. — 2008-03-24

Protein microarrays presenting spots of collagen and lipidated tissue factor (TF) allowed a determination of the critical surface concentration of TF required to trigger coagulation under flow. Whole blood supplemented with corn trypsin inhibitor (to inhibit factor XIIa) was perfused over microarrays for 5 minutes. Immunofluorescence staining of platelet glycoprotein GPIb and fibrin(ogen) revealed a critical TF concentration (EC50) of 3.6, 8.4, and 10.2 molecules-TF/µm² at wall shear rates of 100, 500, and 1000 s^–1, respectively. For collagen arrays where only the center lane of spots (in the direction of flow) contained TF, a downstream distance of 14 mm was required for the thrombus to widen enough to reach across a 300-micrometer gap to the adjacent TF-free lanes of collagen spots, in agreement with numerical simulation. To investigate the effect of low levels of circulating TF, whole blood (± 100 fM added TF) was tested under static and flow conditions. After 5 minutes, the addition of 100 fM TF to whole blood had negligible effect under static conditions, but caused a 2.5-fold increase in fibrin formation under flow. This report defines the threshold concentrations of surface TF required to trigger coagulation under flow.

Modulation of angiogenesis by {omega}-3 polyunsaturated fatty acids is mediated by cyclooxygenases

Szymczak, M., Murray, M., Petrovic, N. — 2008-03-24

The potential role of dietary fats in cancer is attracting considerable interest within the community. Both epidemiologic and experimental findings suggest that omega-3 polyunsaturated fatty acids (-3 PUFAs), which are almost absent from typical Western diets, exert protective effects against cancer progression, although the precise mechanism of this suppression remains unknown. One of the potential targets for -3 PUFAs in cancer suppression is angiogenesis, a process of new blood vessel formation within rapidly growing tumors. Here, we demonstrate that -6 PUFAs stimulate and -3 PUFAs inhibit major proangiogenic processes in human endothelial cells, including the induction of angiopoietin-2 (Ang2) and matrix metalloprotease-9, endothelial invasion, and tube formation, that are usually activated by the major -6 PUFA arachidonic acid. The cyclooxygenase (COX)–mediated conversion of PUFAs to prostanoid derivatives participated in modulation of the expression of Ang2. Thus, the -6 PUFA–derived prostaglandin E2 augmented, whereas the -3 PUFA–derived prostaglandin E3 suppressed the induction of Ang2 by growth factors. Our findings are consistent with the suggestion that PUFAs undergo biotransformation by COX-2 to lipid mediators that modulate tumor angiogenesis, which provides new insight into the beneficial effects of -3 PUFAs.

GSK3{beta} is a negative regulator of platelet function and thrombosis

Li, D., August, S., Woulfe, D. S. — 2008-03-24

Glycogen synthase kinase (GSK)3β is a ser-thr kinase that is phosphorylated by the kinase Akt. Although Akt has been shown to regulate platelet function and arterial thrombosis, its effectors in platelets remain unknown. We show here that agonist-dependent phosphorylation of GSK3β in platelets is Akt dependent. To determine whether GSK3β regulates platelet function, platelets from mice lacking a single allele of GSK3β were compared with those of wild-type (WT) controls. GSK3β^+/– platelets demonstrated enhanced agonist-dependent aggregation, dense granule secretion, and fibrinogen binding, compared with WT platelets. Treatment of human platelets with GSK3 inhibitors renders them more sensitive to agonist-induced aggregation, suggesting that GSK3 suppresses platelet function in vitro. Finally, the effect of GSK3β on platelet function in vivo was evaluated using 2 thrombosis models in mice. In the first, 80% of GSK3β^+/– mice (n = 10) formed stable occlusive thrombi after ferric chloride carotid artery injury, whereas the majority of wild-type mice (67%) formed no thrombi (n = 15). In a disseminated thrombosis model, deletion of a single allele of GSK3β in mice conferred enhanced sensitivity to thrombotic insult. Taken together, these results suggest that GSK3β acts as a negative regulator of platelet function in vitro and in vivo.

Response to desmopressin is influenced by the genotype and phenotype in type 1 von Willebrand disease (VWD): results from the European Study MCMDM-1VWD

2008-03-24

We have prospectively evaluated the biologic response to desmopressin in 77 patients with type 1 von Willebrand disease (VWD) enrolled within the Molecular and Clinical Markers for the Diagnosis and Management of type 1 VWD project. Complete response to desmopressin was defined as an increase of both ristocetin cofactor activity (VWF:RCo) and factor VIII coagulant activity (FVIII:C) to 50 IU/dL or higher and partial response as VWF:RCo or FVIII:C lower than 50 IU/dL after infusion, but at least 3-fold the basal level. Complete response was observed in 83% of patients; partial in 13%; and no response in 4%. Patients with some abnormality of VWF multimeric pattern had significantly lower basal FVIII:C and VWF, lower VWF:RCo/Ag ratio, and less complete responses to desmopressin than patients with a normal multimeric pattern (P = .002). Patients with mutations at codons 1130 and 1205 in the D'-D3 domain had the greatest relative increase, but shortest FVIII and VWF half-lives after infusion. Most partial and nonresponsive patients had mutations in the A1-A3 domains. Response to desmopressin in these VWD patients seemed to be associated with the location of the causative mutation. The presence of subtle multimeric abnormalities did not hamper potential clinically useful responses, as in typical type 1 VWD.

A shorter von Willebrand factor survival in O blood group subjects explains how ABO determinants influence plasma von Willebrand factor

2008-03-24

ABO blood groups greatly influence circulating von Willebrand factor (VWF) levels, and O group subjects have lower VWF values. In this study, we investigated whether ABO groups affect VWF survival by monitoring the post-DDAVP (1-desamino-8-d arginine vasopressin) time courses of VWF antigen (VWF:Ag), VWF collagen binding (VWF:CB), and factor VIII (FVIII) in 47 healthy subjects (28 O and 19 non-O blood groups). The elimination half-life (T_1/2el) of VWF was found significantly shorter in O than in non-O subjects (10.0 ± 0.8 hours vs 25.5 ± 5.3 hours, respectively; P < .01), as was the T_1/2el of VWF:CB (7.9 ± 0.5 hours vs 20.9 ± 4.5 hours; P < .01). A direct linear correlation was found between basal VWF:Ag and T_1/2el, subjects with higher VWF levels having longer-surviving VWF. ABO blood groups appeared to strongly influence VWF clearance, but not its synthesis or release from endothelial cells. The VWF propeptide to VWF:Ag ratio, useful for predicting an increased VWF clearance, was found significantly higher in O than in non-O individuals (1.6 ± 0.1 vs 1.2 ± 0.5, P < .001), with values that correlated inversely with T_1/2el (P < .001). Based on these findings, we conclude that the lower VWF values in O group individuals is attributable to a shorter VWF survival and circulating VWF values are strongly influenced by its half-life.