Hemostatic properties of the SV-40 transfected human microvascular endothelial cell line (HMEC-1). A representative in vitro model for microvascular endothelium

Inhibition of thiol isomerase activity diminishes endothelial activation of plasminogen, but not of protein C

INTRODUCTION

Cell surface thiol isomerase enzymes, principally protein disulphide isomerase (PDI), have emerged as important regulators of platelet function and tissue factor activation via their action on allosteric disulphide bonds. Allosteric disulphides are present in other haemostasis-related proteins, and we have therefore investigated whether thiol isomerase inhibition has any influence on two endothelial activities relevant to haemostatic regulation, namely activation of protein C and activation of plasminogen, with subsequent fibrinolysis.

MATERIALS AND METHODS

The study was performed using the human microvascular endothelial cell line HMEC-1. Thiol isomerase gene expression was measured by RT-PCR and activation of protein C and plasminogen by cell-based assays using chromogenic substrates S2366 and S2251, respectively. Cell mediated fibrinolysis was measured by monitoring absorbance at 405 nm following fibrin clot formation on the surface of HMEC-1 monolayers.

RESULTS AND CONCLUSIONS

A variety of thiol isomerase enzymes, including PDI, were expressed by HMEC-1 cells and thiol reductase activity detectable on the cell surface was inhibited by both RL90 anti-PDI antibody and by the PDI inhibitor quercetin-3-rutinoside (rutin). In cell-based assays, activation of plasminogen, but not of protein C, was inhibited by RL90 antibody and, to a lesser extent, by rutin. Fibrin clot lysis occurring on a HMEC-1 monolayer was also significantly slowed by RL90 antibody and by rutin, but RL90-mediated inhibition was abolished in the presence of exogenous tissue plasminogen activator (tPA). We conclude that thiol isomerases, including PDI, are involved in fibrinolytic regulation at the endothelial surface, although not via a direct action on tPA. These findings broaden understanding of haemostatic regulation by PDI, and may aid in development of novel anti-thrombotic therapeutic strategies targeted via the fibrinolysis system.

Modeling human endothelial cell transformation in vascular neoplasias

Endothelial cell (EC)-derived neoplasias range from benign hemangioma to aggressive metastatic angiosarcoma, which responds poorly to current treatments and has a very high mortality rate. The development of treatments that are more effective for these disorders will be expedited by insight into the processes that promote abnormal proliferation and malignant transformation of human ECs. The study of primary endothelial malignancy has been limited by the rarity of the disease; however, there is potential for carefully characterized EC lines and animal models to play a central role in the discovery, development and testing of molecular targeted therapies for vascular neoplasias. This review describes molecular alterations that have been identified in EC-derived neoplasias, as well as the processes that underpin the immortalization and tumorigenic conversion of ECs. Human EC lines, established through the introduction of defined genetic elements or by culture of primary tumor tissue, are catalogued and discussed in relation to their relevance as models of vascular neoplasia.

Endothelial differentiation of adipose tissue-derived mesenchymal stromal cells in glioma tumors: implications for cell-based therapy

Multipotent human adipose tissue mesenchymal stromal cells (hAMSCs) are promising therapy vehicles with tumor-homing capacity that can be easily modified to deliver cytotoxicity activating systems in the proximity of tumors. In a previous work, we observed that hAMSCs are very effective delivering cytotoxicity to glioma tumors. However, these results were difficult to reconcile with the relatively few hAMSCs surviving implantation. We use a bioluminescence imaging (BLI) platform to analyze the behavior of bioluminescent hAMSCs expressing HSV-tTK in a U87 glioma model and gain insight into the therapeutic mechanisms. Tumor-implanted hAMSCs express the endothelial marker PECAM1(CD31), integrate in tumor vessels and associate with CD133-expressing glioma stem cells (GSC). Inhibition of endothelial lineage differentiation in hAMSCs by Notch1 shRNA had no effect on their tumor homing and growth-promoting capacity but abolished the association of hAMSCs with tumor vessels and CD133+ tumor cells and significantly reduced their tumor-killing capacity. The current strategy allowed the study of tumor/stroma interactions, showed that tumor promotion and tumor-killing capacities of hAMSCs are based on different mechanisms. Our data strongly suggest that the therapeutic effectiveness of hAMSCs results from their association with special tumor vascular structures that also contain GSCs.

The infection of microvascular endothelial cells with ExoU-producing Pseudomonas aeruginosa triggers the release of von Willebrand factor and platelet adhesion

An increased plasma concentration of von Willebrand factor (vWF) is detected in individuals with many infectious diseases and is accepted as a marker of endothelium activation and prothrombotic condition. To determine whether ExoU, a Pseudomonas aeruginosa cytotoxin with proinflammatory activity, enhances the release of vWF, microvascular endothelial cells were infected with the ExoU-producing PA103 P. aeruginosa strain or an exoU-deficient mutant. Significantly increased vWF concentrations were detected in conditioned medium and subendothelial extracellular matrix from cultures infected with the wild-type bacteria, as determined by enzyme-linked immunoassays. PA103-infected cells also released higher concentrations of procoagulant microparticles containing increased amounts of membrane-associated vWF, as determined by flow cytometric analyses of cell culture supernatants. Both flow cytometry and confocal microscopy showed that increased amounts of vWF were associated with cytoplasmic membranes from cells infected with the ExoU-producing bacteria. PA103-infected cultures exposed to platelet suspensions exhibited increased percentages of cells with platelet adhesion. Because no modulation of the vWF mRNA levels was detected by reverse transcription-polymerase chain reaction assays in PA103-infected cells, ExoU is likely to have induced the release of vWF from cytoplasmic stores rather than vWF gene transcription. Such release is likely to modify the thromboresistance of microvascular endothelial cells.

A Matrigel-based tube formation assay to assess the vasculogenic activity of tumor cells

Over the past several decades, a tube formation assay using growth factor-reduced Matrigel has been typically employed to demonstrate the angiogenic activity of vascular endothelial cells in vitro. However, recently growing evidence has shown that this assay is not limited to test vascular behavior for endothelial cells. Instead, it also has been used to test the ability of a number of tumor cells to develop a vascular phenotype. This capability was consistent with their vasculogenic behavior identified in xenotransplanted animals, a process known as vasculogenic mimicry (VM). There is a multitude of evidence demonstrating that tumor cell-mediated VM plays a vital role in the tumor development, independent of endothelial cell angiogenesis. For example, tumor cells were found to participate in the blood perfused, vascular channel formation in tissue samples from melanoma and glioblastoma patients. Here, we described this tubular network assay as a useful tool in evaluation of vasculogenic activity of tumor cells. We found that some tumor cell lines such as melanoma B16F1 cells, glioblastoma U87 cells, and breast cancer MDA-MB-435 cells are able to form vascular tubules; but some do not such as colon cancer HCT116 cells. Furthermore, this vascular phenotype is dependent on cell numbers plated on the Matrigel. Therefore, this assay may serve as powerful utility to screen the vascular potential of a variety of cell types including vascular cells, tumor cells as well as other cells.

Defibrotide prevents the activation of macrovascular and microvascular endothelia caused by soluble factors released to blood by autologous hematopoietic stem cell transplantation

Endothelial activation and damage occur in association with autologous hematopoietic stem cell transplantation (HSCT). Several of the early complications associated with HSCT seem to have a microvascular location. Through the present study, we have characterized the activation and damage of endothelial cells of both macro (HUVEC) and microvascular (HMEC) origin, occurring early after autologous HSCT, and the potential protective effect of defibrotide (DF). Sera samples from patients were collected before conditioning (Pre), at the time of transplantation (day 0), and at days 7, 14, and 21 after autologous HSCT. Changes in the expression of endothelial cell receptors at the surface, presence and reactivity of extracellular adhesive proteins, and the signaling pathways involved were analyzed. The expression of ICAM-1 at the cell surface increased progressively in both HUVEC and HMEC. However, a more prothrombotic profile was denoted for HMEC, in particular at the time of transplantation (day 0), reflecting the deleterious effect of the conditioning treatment on the endothelium, especially at a microvascular location. Interestingly, this observation correlated with a higher increase in the expression of both tissue factor and von Willebrand factor on the extracellular matrix, together with activation of intracellular p38 MAPK and Akt. Previous exposure and continuous incubation of cells with DF prevented the signs of activation and damage induced by the autologous sera. These observations corroborate that conditioning treatment in autologous HSCT induces a proinflammatory and a prothrombotic phenotype, especially at a microvascular location, and indicate that DF has protective antiinflammatory and antithrombotic effects in this setting.

rhBMP-2, rhVEGF(165), rhPTN and thrombin-related peptide, TP508 induce chemotaxis of human osteoblasts and microvascular endothelial cells

Osteogenesis and angiogenesis are inter-linked and tightly regulated processes involved in growth, repair, and bone remodeling. Bone morphogenic protein 2 (BMP-2), vascular endothelial growth factor (VEGF), pleiotrophin (PTN) and thrombin-related peptide, TP508 have all been found to have the ability to promote bone fracture healing by enhancing both the osteogenesis and angiogenesis processes. One of the underlying mechanisms proposed is that mediators for osteogenesis may also be involved in mediating angiogenesis and vice versa. The aim of this study was to examine the chemotactic effects of rhBMP-2, rhVEGF(165), rhPTN and TP508 on human osteoblasts and endothelial cells. Using a direct-viewing chemotaxis assay system, we report for the first time, the direct quantitative observation of chemotaxis of both human osteoblastc cells and microvascular endothelial cells towards sources of rhBMP-2, rhVEGF(165), rhPTN and TP508. This study confirmed that rhBMP-2, rhVEGF(165), rhPTN and TP508 have chemotactic effects on both human osteoblastic and endothelial cells, indicating that these factors are directly involved in promoting angiogenesis and osteogenesis by recruiting osteoblasts and endothelial cells via chemotaxis.

Effects of TNF-α and curcumin on the expression of thrombomodulin and endothelial protein C receptor in human endothelial cells

The objective of this study was to elucidate the effects of tumor necrosis factor-alpha (TNF-alpha) on the expression of thrombomodulin (TM) and endothelial protein C receptor (EPCR) in human endothelial cells as well as the effect of curcumin, a spice and coloring food compound, as a potential therapeutic agent. Human umbilical vein endothelial cells (HUVECs) treated with TNF-alpha (2.0 ng/ml) showed reduced TM mRNA levels by 80%, 97%, 94%, and 97% at 3, 6, 12, and 24 h, respectively (P<0.05), by real-time PCR analysis. Dose-dependent study showed that TM mRNA levels of HUVECs were decreased by 86%, 89%, 91%, and 94% after treatment of TNF-alpha (0, 0.25, 0.5, 1, and 2 ng/ml) for 6 h, respectively (P<0.05). TM protein levels in HUVECs were significantly reduced by 69% in TNF-alpha-treated cells as compared to controls (P<0.05) by Western blot analysis. Secreted protein and activity of TM of HUVEC cultures were also significantly reduced in TNF-alpha-treated cells. In addition, EPCR mRNA levels of HUVECs were significantly reduced in TNF-alpha-treated group as compared to controls (P<0.05). Furthermore, these effects were observed in other types of endothelial cells from human coronary arteries, lung, and skin. Curcumin effectively blocked these effects of TNF-alpha on downregulation of TM and EPCR. These data demonstrate that TNF-alpha significantly decreases expression of TM and EPCR at both mRNA and protein levels in several human endothelial cells. Curcumin can effectively block TNF-alpha-induced endothelial dysfunction. This study suggests a new molecular mechanism of inflammation-induced thrombosis and a new therapeutic strategy to prevent this clinical problem.

Human microvascular endothelial cells immortalized with human telomerase catalytic protein: a model for the study of in vitro angiogenesis

Human microvascular endothelial cell-1 (HMEC-1) generated by transfection with SV40 large T antigen has been the prevailing model for in vitro studies on endothelium. However, the transduction of SV40 may lead to unwanted cell behaviors which are absent in primary cells. Thus, establishing a new microvascular endothelial cell line, which is capable of maintaining inherent features of primary endothelial cells, appears to be extremely important. Here, we immortalized primary human microvascular endothelial cells (pHMECs) by engineering the human telomerase catalytic protein (hTERT) into the cells. Endothelial cell-specific markers were examined and the angiogenic responses were characterized in these cells (termed as HMVECs, for human microvascular endothelial cells). We found that VEGF receptor 2 (Flk-1/KDR), tie1, and tie2 expression is preserved in HMVEC, whereas Flk-1/KDR is absent in HMEC-1. In addition, HMVEC showed similar angiogenic responses to VEGF as HMEC-1. Furthermore, the HMVEC line was found to generate a prominent angiogenic response to periostin, a potent angiogenic factor identified recently. The data indicate that HMVEC may serve as a suitable in vitro endothelium model.

Human endothelial cell attachment and proliferation on a novel vascular graft prototype

A new vascular prosthesis prototype was assessed for its ability to support an endothelial cell layer in vitro. A coiled tubular structure, constructed from polymer-coated metallic wires, with an internal diameter of 690 microm, was used. Addition of heparin to the surface coating of the coil strongly enhanced the blood compatibility of the device. A series of coils with five different coatings, increasing in hydrophilicity, was studied. Heparin was added to one series, another series did not contain this anticoagulant drug. Upon contact with blood, a vascular prosthesis will instantaneously adsorb plasma proteins on its surface, and these proteins will influence the behavior of cells binding to the device. When coils were treated with human plasma proteins, mimicking the in vivo situation, human microvascular endothelial cells grew well on all coils studied, irrespective of the hydrophilicity of the underlying coating or the addition of heparin. For control coils, only endothelial cell growth on the most hydrophobic surfaces, and a moderate enhancing effect for heparin, were observed. This novel vascular graft prototype seems well suited for the support of an endothelial cell layer, especially when plasma proteins are adsorbed to its surface, and shows promise for in vivo testing.

Endoglin expression on human microvascular endothelial cells association with betaglycan and formation of higher order complexes with TGF-beta signalling receptors

Transforming growth factor-beta (TGF-beta) plays an important role in angiogenesis and vascular function. Endoglin, a transmembrane TGF-beta binding protein, is highly expressed on vascular endothelial cells and is the target gene for the hereditary haemorrhagic telangiectasia type I (HHT1), a dominantly inherited vascular disorder. The specific function of endoglin responsible for HHT1 is believed to involve alterations in TGF-beta responses. The initial interactions on the cell surface between endoglin and TGF-beta receptors may be an important mechanism by which endoglin modulates TGF-beta signalling, and thereby responses. Here it is shown that on human microvascular endothelial cells, endoglin is co-expressed and is associated with betaglycan, a TGF-beta accessory receptor with which endoglin shares limited amino acid homology. This complex formation may occur in either a ligand-dependent or a ligand-independent manner. In addition, the occurrence of three higher order complexes containing endoglin, type II and/or type I TGF-beta receptors, on these cells is demonstrated. Our findings suggest that endoglin may modify TGF-beta signalling by interacting with both betaglycan and the TGF-beta signalling receptors at physiological receptor concentrations and ratios.

Regulation of endothelin-1 gene expression in human microvascular endothelial cells by hypoxia and cobalt: role of hypoxia responsive element

Endothelin-1 (Et-1) is a vasoconstrictor peptide that plays an important role in the pathophysiology of hypertension, myocardial ischemia, and other diseases. We examined the mechanism of regulation the Et-1 mRNA expression in human microvascular endothelial cells (HMEC-1) in response to hypoxia and cobalt. To determine whether the 5'-flanking region of Et-1 gene mediate transcriptional responses to cellular hypoxia, we constructed reporter plasmids in which Et-1 5'-flanking sequences of Et-1 gene were fused to luciferase coding sequences. Constructs, which contain native Et-1 sequence 5'-AACGTGCA-3', located between -118 and -125 in the opposite orientation as the transcriptional unit, mediate transcriptional response to hypoxia and cobalt. This responsiveness was inhibited by genistein, a tyrosine kinase selective inhibitor. Both hypoxia and cobalt induced binding of HIF-1 (hypoxia inducible-1 factor) to this Et-1 hypoxia responsive element in gel shift assays. Mutation in this sequence eliminated both the hypoxia-induced HIF-1 binding and luciferase expression. Using the supershift assay we have shown that this hypoxia responsive element binds HIF-1alpha and HIF-1beta proteins. Interestingly, genistein only slightly affected HIF-1 binding. These results indicate that the Et-1 gene contains HIF-1 binding hypoxia responsive elements which mediate transcriptional responses to hypoxia and cobalt in microvascular endothelial cells. Genistein appears to inhibit this response by affecting the transcriptional activity of the HIF-1 complex, without significantly affecting its DNA-binding properties.

CD39 modulates IL-1 release from activated endothelial cells

The activation of endothelial cells (EC) and monocyte-macrophages (M&phi;) by lipopolysaccharide (LPS) is considered an important element of the vascular injury observed in endotoxemia. Interleukin-1 (IL-1) beta release from M&phi; in response to LPS, appears to be mediated by the autocrine/paracrine release of ATP via P2X7 receptor activation. In EC, similar nucleotide-mediated signaling pathways may be influenced by high levels of expression of CD39, the vascular nucleoside triphosphate diphosphohydrolase (NTPDase; ENTPD I). To determine whether CD39 modulates ATP-mediated release of IL-1 from EC, we stimulated human EC with LPS and measured levels of ATP secretion and IL-1 release. LPS triggered ATP secretion from EC that was soon followed by IL-1alpha release. Overexpression of CD39 following infection with recombinant CD39 adenoviral vectors (AdCD39) abrogated the initial phase of ATP secretion and inhibited IL-1alpha release; comparable results were obtained with soluble NTPDase. These data demonstrate that CD39/NTPDase modulates IL-1alpha release from LPS stimulated human EC.

Identification of endoglycan, a member of the CD34/podocalyxin family of sialomucins

CD34 and podocalyxin are structurally related sialomucins, which are expressed in multiple tissues including vascular endothelium and hematopoietic progenitors. These glycoproteins have been proposed to be involved in processes as diverse as glomerular filtration, inhibition of stem cell differentiation, and leukocyte-endothelial adhesion. Using homologies present in the cytoplasmic tails of these proteins, we have identified a novel member of this family, which we designate endoglycan. This protein shares a similar overall domain structure with the other family members including a sialomucin domain, but also possesses an extremely acidic amino-terminal region. In addition, endoglycan contains several potential glycosaminoglycan attachment sites and is modified with chondroitin sulfate. Endoglycan mRNA and protein were detected in both endothelial cells and CD34(+) bone marrow cells. Thus, CD34, podocalyxin, and endoglycan comprise a family of sialomucins sharing both structural similarity and sequence homology, which are expressed by both endothelium and multipotent hematopoietic progenitors. While the members of this family may perform overlapping functions at these sites, the unique structural features of endoglycan suggest distinct functions for this molecule.

Anti-endothelial cell antibodies in primary antiphospholipid syndrome and SLE: patterns of reactivity with membrane antigens on microvascular and umbilical venous cell membranes

Although there has been recent emphasis on autoantibodies to epitopes on beta-2-glycoprotein I and prothrombin in the pathogenesis of antiphospholipid syndrome (APS), antibodies other than those directed toward epitopes on phospholipid binding proteins are present. These include those reactive with antigens on platelet membrane glycoproteins, and with vascular endothelial cell membrane. As the pathogenesis of the thrombotic manifestations of APS remains unexplained, further characterization of these antibodies may be informative. We have confirmed anti-endothelial cell binding to a range of cell membrane antigens in systemic lupus erythematosus (SLE) and primary APS. Furthermore, differences in both the pattern of antibody binding and band intensity between human umbilical vein (HUVEC) and human microvascular endothelial cells (HMEC-1) were demonstrated. Of 17 primary APS sera, antibody binding to HUVEC cell membranes was found in nine and to HMEC-1 membranes in seven. Binding at 72-79 kD was confined to HUVEC. In 32 SLE sera, binding to HUVEC and HMEC-1 membranes was detected in 17 and 22 respectively, binding at 135-155 kD being confined to HMEC-1. These results are consistent with the phenotypic variation in endothelial cells of different origins and confirm the frequent presence of autoantibodies reactive with vascular endothelium in both SLE and PAPS. Whether these antibodies could be involved in the pathogenesis of thrombosis, through induction of endothelial cell apoptosis or damage, remains to be determined.

Rickettsia rickettsii infection of the EA.hy 926 endothelial cell line: morphological response to infection and evidence for oxidative injury

EA.hy 926 is a permanent human cell line that expresses highly differentiated functions characteristic of human vascular endothelium. Rickettsia rickettsii can efficiently infect and cause a cytopathic effect in EA.hy 926 cells. R. rickettsii produced visible lytic plaques in EA.hy 926 cells at 10 d post-infection (p.i.) following application of a secondary agarose overlay containing 2 micrograms emetine ml-1 and 40 micrograms NaF ml-1 on day 2. Rickettsial growth in EA.hy 926 cells had a similar profile to that occurring in human umbilical vein endothelial cells (HUVEC) and rickettsiae catalysed polymerization of actin tails. Intracellular multiplication of R. rickettsii resulted in significant changes in the internal morphology of EA.hy 926 cells, most notably extensive dilatation of the membranes of the endoplasmic reticulum and outer nuclear envelope by 72 h p.i. These events correlated with significant alterations in the host-cell antioxidant system, including decreased levels of intracellular reduced glutathione and glutathione peroxidase activity and increased amounts of intracellular peroxide through to 96 h of infection. These findings are similar to the changes described previously for R. rickettsii-infected HUVEC and suggest that common mechanisms associated with rickettsia-induced oxidative injury occur in the two cell lines. EA.hy 926 cells were also used to investigate the influence of the antioxidant alpha-lipoic acid on rickettsial infection. Overnight pretreatment with 1-500 microM alpha-lipoic acid did not prevent cells from being destroyed following infection with rickettsiae. Supplementation of the culture medium with 1 and 10 microM alpha-lipoic acid 2 h after rickettsial inoculation also did not provide any protective effect. However, 100, 200 and 500 microM alpha-lipoic acid increased the viability of infected cells at 96 h to 45, 51 and 70%, respectively compared with 26% for untreated, infected samples. Thiol levels and glutathione peroxidase activity in treated, infected cells increased and peroxide content decreased proportionally to increasing alpha-lipoic acid concentrations. Furthermore, treatment with 500 microM alpha-lipoic acid for 72 h p.i. completely prevented ultrastructural changes in infected cells. In conclusion, the permanent endothelial cell line EA.hy 926 is susceptible to injury induced by R. rickettsii infection. Although the cellular changes resulting from infection are not identical in all aspects to that demonstrated previously in HUVEC, the increased reproducibility and convenience of EA.hy 926 cells make them suitable for biochemical and morphological studies.

Differential regulation of interleukin-8 and intercellular adhesion molecule-1 by H2O2 and tumor necrosis factor-alpha in endothelial and epithelial cells

The reactive oxygen intermediate H2O2 can function as a signaling molecule to activate gene expression. In this study, we demonstrate that oxidant stress induced by tumor necrosis factor alpha (TNFalpha) or H2O2 differentially regulates intercellular adhesion molecule-1 (ICAM-1) and interleukin-8 (IL-8) gene expression in endothelial and epithelial cells. Northern blot analysis revealed that TNFalpha induced both ICAM-1 and IL-8 expression in either the A549 lung epithelial cell line or the human microvessel endothelial cell line (HMEC-1). In contrast, H2O2 selectively induced only ICAM-1 in HMEC-1 and only IL-8 in A549. This cell type-specific pattern of IL-8 expression was also observed in several other endothelial and epithelial cells. TNFalpha induced greater IL-8 gene expression as compared with H2O2, but the kinetics of induction were similar. The induction of epithelial IL-8 message was accompanied by a corresponding increase in functional IL-8 protein secretion as determined by a neutrophil motility assay. The increased neutrophil motility stimulated by conditioned media from H2O2- or TNFalpha-exposed A549 cells was completely inhibited by an anti-IL-8 antibody. TNFalpha and H2O2 also induced a differential pattern of CC chemokine expression in A549. While TNFalpha induced both RANTES and MCP-1, H2O2 induced only MCP-1. These data suggest that epithelial cells under oxidant stress contribute to the inflammatory cytokine network by selective production of IL-8, MCP-1, and RANTES, which may critically influence the site-specific recruitment of leukocyte subsets.