Chemotactic properties of angiopoietin-1 and -2, ligands for the endothelial-specific receptor tyrosine kinase Tie2

Current status and perspective of antiangiogenic therapy for cancer: hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is well known as a typical angiogenic tumor, especially in the moderately to poorly differentiated type. Such clinicopathological characteristics are not only useful for imaging diagnosis but are also applicable to the treatment of HCC. In addition, recent molecular studies have revealed that angiogenesis is closely related to hepatocarcinogenesis. In this review, the molecular mechanism of HCC angiogenesis and the antiangiogenic prevention of HCC are reviewed to introduce the latest trends in antiangiogenic treatment of cancers, including HCC.

Activation of Tie2 by angiopoietin-1 and angiopoietin-2 results in their release and receptor internalization

The receptor tyrosine kinase Tie2 is highly expressed in endothelial cells and is crucial for angiogenesis and vascular maintenance. The ligands for Tie2 are the angiopoietins, of which angiopoietin-1 and angiopoietin-2 have been the most studied. Angiopoietin-1 has been characterized as the primary activating ligand for Tie2 whereas the role of angiopoietin-2 remains controversial; activating Tie2 in some studies and inhibiting Tie2 in others. Our studies were aimed at understanding the regulation of Tie2 in endothelial cells by angiopoietin-1 and angiopoietin-2 and revealed that both ligands activated Tie2 in a concentration-dependent manner. Angiopoietin-2 was considerably weaker at activating Tie2 compared with angiopoietin-1 suggesting that angiopoietin-2 may be a partial agonist. Activation of Tie2 by these ligands resulted in differential turnover of the receptor where binding of angiopoietin-1, and to a lesser extent angiopoietin-2, induced rapid internalization and degradation of Tie2. Furthermore, our binding studies demonstrate that both ligands are differentially released from the endothelial cell surface after receptor activation and accumulate in the surrounding medium. Altogether, these data begin our understanding of the regulation of Tie2 and the activity of the angiopoietins after engaging the endothelial cell surface.

The angiopoietin pathway is modulated by PAR-1 activation on human endothelial progenitor cells

OBJECTIVES

The importance of protease-activated receptor-1 (PAR-1) in blood vessel development has been shown in knock-out mice. As endothelial progenitor cells (EPCs) express functional PAR-1, we examined whether PAR-1 stimulation by the peptide SFLLRN interfered with the angiopoietin pathway, that is EPC commitment, proliferation and migration.

METHODS AND RESULTS

Given the strong PAR-1 expression on CD34+ cells, we tested the effect of SFLLRN 75 micromol L(-1) on the emergence of EPCs from cord blood. PAR-1 activation did not modify the number of colonies or the day of emergence, in keeping with the lack of induction of angiopoietin 1 gene expression. Conversely, SFLLRN treatment of EPCs induced angiopoietin 2 gene expression and protein synthesis. Experiments with polyclonal blocking antibodies showed that angiopoietin 2 was involved in the proliferative effect of PAR-1 activation. PAR-1 activation also enhanced migration toward angiopoietin 1 in a Boyden chamber assay.

CONCLUSIONS

Our study demonstrates that PAR-1-induced proliferation of EPCs involves angiopoietin 2. PAR-1 also enhances EPC migration toward angiopoietin 1. These findings might explain the role of thrombin in neovascularization via the angiopoietin pathway.

Sildenafil induces angiogenic response in human coronary arteriolar endothelial cells through the expression of thioredoxin, hemeoxygenase and vascular endothelial growth factor

This study was undertaken to investigate the effect of phosphodiesterase-5 (PDE5) inhibitor, sildenafil, on angiogenic response in human coronary arteriolar endothelial cells (HCAEC). The cells exposed to sildenafil (1-20 microM) demonstrated significantly accelerated tubular morphogenesis with the induction of thioredoxin-1 (Trx-1), hemeoxygenase-1 (HO-1) and VEGF. Sildenafil induced VEGF and angiopoietin specific receptors such as KDR, Tie-1 and Tie-2. This angiogenic response was repressed by tinprotoporphyrin IX (SnPP), an inhibitor of HO-1 enzyme activity. Sildenafil below 1 muM has no angiogenic effect as evidenced by reduced tuborogenesis. Sildenafil along with SnPP inhibited both VEGF and Angiopoietin-1 (Ang-1) protein expression. Therefore our results demonstrated for the first time that sildenafil is a very potent pro-angiogenic factor.

Role of hematopoietic lineage cells as accessory components in blood vessel formation

In adults, the vasculature is normally quiescent, due to the dominant influence of endogenous angiogenesis inhibitors over angiogenic stimuli. However, blood vessels in adults retain the capacity for brisk initiation of angiogenesis, the growth of new vessels from pre-existing vessels, during tissue repair and in numerous diseases, including inflammation and cancer. Because of the role of angiogenesis in tumor growth, many new cancer therapies are being conducted against tumor angiogenesis. It is thought that these anti-angiogenic therapies destroy the tumor vessels, thereby depriving the tumor of oxygen and nutrients. Therefore, a better understanding of the molecular mechanisms in the process of sprouting angiogenesis may lead to more effective therapies not only for cancer but also for diseases involving abnormal vasculature. It is widely believed that after birth, endothelial cells (EC) in new blood vessels are derived from resident EC of pre-existing vessels. However, evidence is now emerging that cells derived from the bone marrow may also contribute to postnatal angiogenesis. Most studies have focused initially on the contribution of endothelial progenitor cells in this process. However, we have proposed a concept in which cells of the hematopoietic lineage are mobilized and then entrapped in peripheral tissues, where they function as accessory cells that promote the sprouting of resident EC by releasing angiogenic signals. Most recently we found that hematopoietic cells play major roles in tumor angiogenesis by initiating sprouting angiogenesis and also in maturation of blood vessels in the fibrous cap of tumors. Therefore, manipulating these entrapment signals may offer therapeutic opportunities to stimulate or inhibit angiogenesis.

Regulation of Ang2 release by PTEN/PI3-kinase/Akt in lung microvascular endothelial cells

Angiopoietin-2 (Ang2) is a Tie-2 ligand that destabilizes vascular structures, allowing for neovascularization or vessel regression depending on local vascular endothelial cell growth factor (VEGF) concentrations. Although various stimuli have been shown to affect Ang2 expression, information on the underlying mechanisms involved in Ang2 production in endothelial cells (EC) is just beginning to emerge. In the present study, we have used adenovirus-mediated gene transfer and pharmacological inhibitors to examine the role of the PTEN/PI3-K/Akt pathway on Ang2 release. Inhibition of PI3-kinase with wortmannin led to a stimulation of basal Ang2 release in EC, while overexpression of an active form of Akt reduced Ang2. In addition, adenovirus-mediated gene transfer of the phosphatase PTEN stimulated Ang2 release. Incubation of the cells with Ang1, an agent that activates the PI3-K/Akt pathway in EC, reduced Ang2 release. This effect of Ang1 could be prevented by wortmannin and LY-294002 pretreatment. Similarly, in VEGF-treated EC the increase in Ang2 production observed was greater in the presence of a PI3-K inhibitor. Our observations that PTEN acts as a positive modulator of Ang2 release, while activation of the PI3-K/Akt pathway downregulates Ang2, reveal an additional mechanism through which the PTEN/PI3-K/Akt pathway could affect the angiogenic process.

Signaling and functions of angiopoietin-1 in vascular protection

Angiopoietin-1 (Ang1) has powerful vascular protective effects: suppressing plasma leakage, inhibiting vascular inflammation, and preventing endothelial death. Preclinical studies indicate that Ang1 may be therapeutically useful in a number of situations, including treatment of edema, endotoxemia, and transplant arteriosclerosis. However, the ligand has also been implicated in vessel remodeling, induction of angiogenesis and pulmonary hypertension, indicating that strategies to minimize any deleterious effects while optimizing vessel protection are likely to be needed. This review surveys the published data on vascular protective effects of Ang1 and highlights the therapeutic potential of this ligand, as well as possible limitations to its use. We also consider the data on Ang1 receptors and speculate on how to maximize therapeutic benefit by targeting the Tie receptors.

Combination of in vivo angiopoietin-1 gene transfer and autologous bone marrow cell implantation for functional therapeutic angiogenesis

OBJECTIVE

Autologous bone marrow mononuclear cell (BM-MNC) implantation into ischemic tissues promotes angiogenesis, but a large amount of marrow aspiration is required, which is a major clinical limitation. Angiopoietin-1 (Ang-1) is requisite for vascular maturation during angiogenesis. We examined the impacts of combinatorial Ang-1 gene transfer and low-dose autologous BM-MNC implantation on therapeutic angiogenesis in a rabbit model of hind limb ischemia.

METHODS AND RESULTS

Rabbits were divided into 4 groups: phosphate-buffered saline (control), 500 microg Ang-1 plasmid (Ang-1), 1 x 10(6) autologous BM-MNCs (BMC), and Ang-1 plasmid plus BM-MNCs (combination). The Ang-1 group had a greater angiographic score and capillary density compared with the control (P<0.05), but the Ang-1 gene therapy alone did not improve transcutaneous oxygen pressure (TcO2) and skin ulcer score. However, the combination group showed a significant improvement in not only angiographic score and capillary density (P<0.05) but also TcO2 (P<0.05) and skin ulcer score. These efficacies were greater in the combination group compared with the BMC group.

CONCLUSIONS

This Ang-1 gene and BM-MNC combination therapy enhances not only quantitative but also qualitative angiogenesis in ischemic tissues. Moreover, the combination therapy will enable a reduction in the amount of BM aspiration required for significant therapeutic angiogenesis.

Gene expressions of canine angiopoietin-1 and -2 in normal tissues and spontaneous tumours

The angiopoietin (Ang) family of proteins are central to the regulation of angiogenesis. The purposes of this study were to determine cDNA sequences of canine Ang-1 and Ang-2 and investigate their expressions in normal tissues and spontaneous tumours. The cDNA sequences of canine Ang-1 and Ang-2 were 1,494 and 1,488 bp, and the deduced amino acid sequences were 497 and 495 residues, respectively. The cDNA sequences of canine Ang-1 and Ang-2 showed high homology with those of the other mammalian species. Canine Ang-1 and Ang-2 mRNA were detectable in all 22 normal tissues and spontaneous tumours. Higher mRNA expression level of canine Ang-2 was demonstrated in mammary simple carcinomas, haemangiosarcoma and hepatocellular carcinoma in comparison with normal tissues.

Id1 gene transfer confers angiogenic property on fully differentiated endothelial cells and contributes to therapeutic angiogenesis

BACKGROUND

Transplantation of endothelial progenitor cells has been proposed as a potential strategy for therapeutic revascularization. However, the limited endogenous cell pool and the related technical difficulties constitute clinically important disadvantages to autologous transplantation. In this study we investigated whether fully differentiated endothelial cells (ECs) modified with gene transfer of Id1, a helix-loop-helix transcription factor involved in angiogenesis, have the potential to contribute to therapeutic angiogenesis.

METHODS AND RESULTS

The Id1 gene was transferred into human umbilical vein ECs (HUVECs) via a Sendai virus vector. Id1 stimulated migration, proliferation, and capillary-like tube/cord formation of HUVECs. In addition, Id1 reduced serum deprivation-induced HUVEC apoptosis, as shown by FACS analysis with annexin V and TUNEL staining. Transplantation of Id1-overexpressing HUVECs accelerated recovery of blood flow as evaluated by laser-Doppler perfusion imaging, increased capillary density, and improved the rate of limb salvage compared with the transplantation of control HUVECs. Histochemical analysis revealed that the regenerated vascular networks of limbs transplanted with Id1-overexpressing HUVECs contained numerous HUVECs, some of which were in a proliferative state. Untransfected HUVECs were also incorporated with Id1-transfected HUVECs, suggesting the noncell autonomous effect of Id1. Finally, angiopoietin-1 was upregulated in Id1-overexpressing HUVECs and functionally contributed to the in vitro angiogenic effect of Id1.

CONCLUSIONS

Id1 gene transfer conferred HUVECs with an angiogenic property, contributing to neovascularization after transplantation into ischemic lesions. Transplantation of Id1-overexpressing mature ECs may serve as a novel and useful strategy for therapeutic angiogenesis.

Expression, localization and hormonal control of angiopoietin-1 in the rhesus macaque endometrium: potential role in spiral artery growth

Angiopoietin-1 (Ang-1) is an important angiogenic factor that has not been thoroughly studied in the primate endometrium. We evaluated the endometrial expression of Ang-1 and its receptor, Tie2, during induced menstrual cycles in rhesus macaques. Tie2 expression was confined to the vascular endothelium without marked change during the cycle. However, Ang-1 expression varied considerably during the cycle. In the proliferative phase, Ang-1 was only expressed in the basal zone glands, and this expression was estradiol (E2) dependent. In the early- to mid-secretory phase, Ang-1 expression spread to the upper glands, luminal epithelium and the vascular smooth muscle cells (VSMC) of spiral arteries. In the late secretory phase, the signal disappeared from the glands but remained elevated in the VSMC of spiral arteries. Notably, there was a significant correlation between VSMC proliferation and Ang-1 expression in the VSMC of the spiral arteries. Progesterone (P) withdrawal in the early secretory phase induced a decline in Ang-1 expression in the glands and VSMC of spiral arteries along with a complete suppression of VSMC proliferation. These data suggest, for the first time, that Ang-1 may play a key role in the P-dependent growth of the unique spiral arteries in the primate endometrium.

Cardiovascular genomics: a current overview of in vivo and in vitro studies

The cardiovascular system is the first system that is developed in the embryo. The cardiovascular development is a complex process involving the coordination, differentiation, and interaction of distinct cell lineages to form the heart and the diverse array of arteries, veins, and capillaries required to supply oxygen and nutrients to all tissues. Embryonic stem cells have been proposed as an interesting model system to investigate molecular and cellular mechanisms involved in mammalian development. The present review is focused on extrinsic soluble factors, intrinsic transcription factors, receptors, signal transduction pathways, and genes regulating the development of cardiovascular system in vivo and in vitro. Special emphasis has been given to cardiovascular genomics including gene expression studies on the cardiovascular system under developmental and pathophysiological conditions.

Angiopoietin-2 stimulates migration of endothelial progenitors and their interaction with endothelium

The factors controlling recruitment of endogenous and transplanted endothelial progenitor cells (EPC) to areas of neovascularization are largely unknown. In this study, we have examined the possibility that EPC migration and adhesion could be regulated by angiopoietin-2 (Ang2), a soluble ligand expressed by endothelial cells at sites of vessel remodelling and angiogenesis. We show for the first time that Ang2 causes a marked stimulation of EPC migration. This was specific for EPC as the ligand failed to affect endothelial cell migration. Ang2-stimulated EPC migration was inhibited by soluble Tie2 ectodomain. Furthermore, the ligand stimulated adhesion between EPC and endothelial monolayers.

Sequential induction of angiogenic growth factors by TNF-alpha in choroidal endothelial cells

Inflammatory mediators have been proposed to play a critical role in the pathogenesis of choroidal neovascularization, a blinding complication of age-related macular degeneration. We evaluated the expression of TNF-alpha in human choroidal neovascular membranes and found that it colocalized with cells expressing VEGF, angiopoietin (Ang)-1 and Ang2. In cultured choroidal endothelial cells we found that TNF-alpha increased Ang2 mRNA (increased transcription) and protein levels prior to those of Ang1 and VEGF. The results raise the possibility that during neovascularization, TNF-alpha may modulate endothelial plasticity and survival by sequential inactivation of Tie2 followed by activation of Tie2 and VEGF receptors.

Endothelial/pericyte interactions

Interactions between endothelial cells and mural cells (pericytes and vascular smooth muscle cells) in the blood vessel wall have recently come into focus as central processes in the regulation of vascular formation, stabilization, remodeling, and function. Failure of the interactions between the 2 cell types, as seen in numerous genetic mouse models, results in severe and often lethal cardiovascular defects. Abnormal interactions between the 2 cell types are also implicated in a number of human pathological conditions, including tumor angiogenesis, diabetic microangiopathy, ectopic tissue calcification, and stroke and dementia syndrome CADASIL. In the present review, we summarize current knowledge concerning the identity, characteristics, diversity, ontogeny, and plasticity of pericytes. We focus on the advancement in recent years of the understanding of intercellular communication between endothelial and mural cells with a focus on transforming growth factor beta, angiopoietins, platelet-derived growth factor, spingosine-1-phosphate, and Notch ligands and their respective receptors. We finally highlight recent important data contributing to the understanding of the role of pericytes in tumor angiogenesis, diabetic retinopathy, and hereditary lymphedema.

Combined cord blood stem cells and gene therapy enhances angiogenesis and improves cardiac performance in mouse after acute myocardial infarction

BACKGROUND

Gene and stem cell therapies hold promise for the treatment of ischaemic cardiovascular disease. However, combined stem cell and angiogenic growth factor gene therapy for acute ischaemic myocardium has not been previously reported. This study hypothesized that combined stem cell and gene therapy would not only augment new vessels formation but also improve myocardial function in acute ischaemic myocardium.

METHODS

Human angiopoietin-1 (Ang1) cDNA and VEGF(165) cDNA were ligated into AAV vector. The purified CD34(+) cells were obtained from human umbilical cord blood samples. Cord blood CD34(+) cells were transduced with AAV vector encoding either the human Ang1 (AAV-Ang1) or VEGF(165) (AAV-VEGF) cDNA alone, or both (AAV-Ang1 plus VEGF). Immediately after ligation of the left anterior descending coronary artery in male SCID mice, culture-expanded CD34(+) cells transduced with AAV-Ang1, AAV-VEGF or AAV-Ang1 plus VEGF were injected intramyocardially at the left anterior free wall.

RESULTS

Western blot showed that Ang1 and VEGF protein expressions were enhanced in the CD34(+)cells transduced with AAV-Ang1 and AAV-VEGF, respectively. Infarct size significantly decreased and capillary density significantly increased after treatment with CD34(+)/AAV-Ang1 plus VEGF when compared with treatment by CD34(+) only. Combined therapy with CD34(+) and AAV-Ang1, CD34(+) and AAV-VEGF, CD34(+) and AAV-Ang1 plus VEGF, all showed significantly higher cardiac performance in echocardiography than the therapy with CD34(+) alone 4 weeks after myocardial infarction.

CONCLUSIONS

Combined therapy with human umbilical cord blood CD34(+) cells and both Ang1 and VEGF genes reduced infarct size, attenuated the progression of cardiac dysfunction and increased capillary density in acute myocardial infarction in mice.

Angiopoietin affects neutrophil migration

OBJECTIVE

After an ischemic event vascular growth factors are involved in regulating leukocyte infiltration in inflammatory processes. This study focused on effects of 2 other angiogenic growth factors, angiopoietin-1 and angiopoietin-2, on human neutrophils and on the involvement of the angiopoietin receptor Tie-2.

METHODS

Neutrophils were from venous blood of healthy donors and cell migration was studied by micropore filter assays. Receptor expression was investigated by reverse transcriptase-polymerase chain reaction (PCR) for mRNA and fluorescence-activated cell-sorter scanner (FACS) analysis. Signaling mechanisms required for angiopoietin-dependent effects were tested functionally by using signaling enzyme blockers.

RESULTS

The angiopoietins were chemotactic for neutrophils. They showed antagonistic effects on each other and both inhibited VEGF-directed migration of neutrophils. The effects of both angiopoietins were Tie-2 dependent. Tie-2 receptor immunoreactivity was confirmed on neutrophils by FACS. De novo synthesis is suggested by Tie-2 receptor mRNA expression as demonstrated by reverse transcriptase PCR.

CONCLUSIONS

Data suggest that a Tie-2 receptor is expressed by human neutrophils whose active site ligation with either angiopoietin-1 or angiopoietin-2 exerts migratory effects on the one hand and arrests VEGF-mediated chemotaxis on the other. These effects suggest a role of angiopoietins in modulating neutrophilic inflammation.

Angiopoietin 1 is mitogenic for cultured endothelial cells

The angiopoietin (Ang)/Tie2 system is implicated in blood vessel formation and maturation. However, the mitogenic effects of angiopoietins remain to be elucidated. Here, we show that Ang1 is mitogenic for cultured endothelial cells. Ang1 dose-dependently induced the proliferation and increased the labeling index of a murine brain capillary endothelial cell line, IBE cells. Ang1 also increased the labeling index of human umbilical vein endothelial cells (HUVEC). Ang1 up-regulated the expression of cyclin D1 in both of these cells. Ang1 activated mitogen-activated protein kinase (MAPK) and phosphoinositide 3-kinase (PI3K) in IBE cells and HUVECs. Activated PI3K was associated with c-Fes protein tyrosine kinase in these cells, but not with Tie2. p70 S6 kinase (p70 S6K) was activated by Ang1-treatment, although this activation was blocked by a PI3K inhibitor, LY294002. Simultaneous treatment of cells with PD98059 (MAPK/extracellular regulated kinase kinase inhibitor) and rapamycin (mTOR inhibitor) completely blocked Ang1-induced mitogenic activity for IBE cells and HUVECs. Although Ang2 at high concentration weakly activated Tie2 and p70 S6K, it failed to activate Ras and MAPK, or to induce cell proliferation. Taken together, these findings indicate that Ang1 exerts mitogenic activity on endothelial cells, which requires activation of both MAPK and p70 S6K.

Cooperative interaction of Angiopoietin-like proteins 1 and 2 in zebrafish vascular development

Angiopoietin-like protein (Angptl) 1 and Angptl2, which are considered orphan ligands, are highly homologous, particularly in the fibrinogen-like domain containing the putative receptor binding site. This similarity suggests potentially cooperative functions between the two proteins. In this report, the function of Angptl1 and Angptl2 is analyzed by using morpholino antisense technology in zebrafish. Knockdown of both Angptl1 and Angptl2 produced severe vascular defects due to increased apoptosis of endothelial cells at the sprouting stage. In vitro studies showed that Angptl1 and Angptl2 have antiapoptotic activities through the phosphatidylinositol 3-kinase/Akt pathway, and coinjection of constitutively active Akt/protein kinase B mRNA rescued impaired vascular development seen in double knockdown embryos. These results provide a physiological demonstration of the cooperative interaction of Angptl1 and Angptl2 in endothelial cells through phosphatidylinositol 3-kinase/Akt mediated antiapoptotic activities.

Roles of reactive oxygen species in angiopoietin‐1/tie‐2 receptor signaling

In this study we identified the involvement of reactive oxygen species (ROS) in signaling and biological effects of the angiopoietin-1 (Ang-1)/tie-2 receptor pathway. Exposure of human umbilical vein endothelial cells to Ang-1 (50 ng/ml) induced rapid and transient production of ROS, particularly superoxide anions. ROS production was attenuated by preincubation with a peptide (gp91ds-tat) that inhibits the association of the gp91(phox) subunit with the p47(phox) subunit of NADPH oxidase and by the expression of a dominant-negative form of Rac-1 (Rac1N17). These results suggest that ROS production in response to Ang-1 exposure originates mainly from a Rac-1-dependent NADPH oxidase. Overexpression of antioxidants (superoxide dismutase and catalase) and Rac1N17, as well as preincubation with selective inhibitors of NADPH oxidase augmented basal p38 phosphorylation, inhibited Ang-1-induced PAK-1 phosphorylation and potentiated Ang-1-induced Erk1/2 phosphorylation but had no influence on AKT and SAPK/JNK phosphorylation by Ang-1. Exposure to Ang-1 (100 ng/ml) for 5 h induced a threefold increase in endothelial cell migration, a response that was strongly inhibited by overexpression of antioxidants, Rac1N17, and selective NADPH oxidase inhibitors. We conclude that activation of tie-2 receptors by Ang-1 triggers the production of ROS through activation of NADPH oxidase and that ROS generation by Ang-1 promotes endothelial cell migration while negatively regulating Erk1/2 phosphorylation.

Angiopoietin-2 causes inflammation in vivo by promoting vascular leakage

Angiopoietins (Angs) are endothelium-selective ligands that exert most of their actions through the Tie-2 receptor. It is widely accepted that Ang-1 promotes the structural integrity of blood vessels and exhibits anti-inflammatory properties. In contrast, the role of Ang-2 remains less clear because it has been shown to behave as a Tie-2 agonist or antagonist under different experimental conditions. To define the role of Ang-2 in acute inflammation, we studied the effects of recombinant Ang-2 administration in vivo. We show herein that Ang-2, but not Ang-1, induces edema formation in the mouse paw in a dose-dependent manner; the edema seems to be fast-peaking (maximum at 30 min) and resolves within 4 h. The effect of Ang-2 is blocked by the coadministration with a soluble form of the Tie-2 receptor or Ang-1. NO and prostaglandin E(2) levels in mouse paw following the injection of Ang-2 remained unaltered, suggesting that the action of Ang-2 does not involve these mediators. In addition, Ang-2 exerted a weak stimulatory effect on leukocyte migration in the mouse paw. Similarly, Ang-2 injected into the mouse air pouch produced only a modest effect on cell extravasation that peaked at 30 min. However, when cell migration was elicited using zymosan, Ang-2 significantly inhibited leukocyte migration. We conclude that Ang-2 by itself stimulates the extravasation of cell-poor fluid, but in the presence of ongoing inflammation it reduces cellular infiltration in tissues.

Direct chemotactic action of angiopoietin-1 on mesenchymal cells in the presence of VEGF

Angiopoietin-1 (Ang1) and its receptor, Tie2, play an important role in angiogenesis and vessel maturation. We previously reported that overexpression of Ang1 in MCF7 xenograft tumors facilitated vessel stabilization by mural cells, and that cultured SMC express Tie2. Here, we investigated whether Ang1 directly acts as a chemoattractant on mural cells or their precursors. In a Matrigel plug assay, neither Ang1 nor VEGF alone induced angiogenesis but together stimulated infiltration of non-endothelial cells that were CD31-negative, vimentin-positive and also positive for VEGFR-1 and Tie2. While negative for smooth muscle actin, reactivity for desmin suggests that the cells are mural cell precursors. VEGF treatment of cultured smooth muscle cells (SMC) upregulated Tie2 and allowed for Ang1-mediated phosphorylation of Tie2 and the AKT serine-threonine kinase. The combination of Ang1 and VEGF stimulated SMC migration in a Boyden chamber-type assay. In the presence of VEGF, Tie2 is upregulated on mural cells, allowing for a migratory response to Ang1. These findings support the view that Ang1, in concert with VEGF, can act directly on mural cells or their precursors to facilitate their recruitment to new blood vessels. This action may play an important role in vascular stabilization.

In utero angiopoietin-2 gene delivery remodels placental blood vessel phenotype: a murine model for studying placental angiogenesis

Angiopoietin (Ang)-2, the natural antagonist of the Ang1/Tie2 receptor is a complex regulator of blood vessel plasticity that plays a pivotal role in both vessel sprouting [in the presence of vascular endothelial growth factor (VEGF)-A] and vessel regression (in the absence of VEGF-A). Based on the spatial and temporal expression of Ang2 throughout human gestation, we recently suggested that the Ang2 may play a pivotal role in placental angiogenesis. Further, to examine this tenet we have developed a novel murine model system in which in utero Ang2 gene delivery via a non-replicating adenoviral expression vector has the potential to manipulate the blood vessel phenotype in vivo during pregnancy. Ang2 overexpression selectively and rapidly remodels the labyrinth perivascular extracellular matrix, subsequently promoting plasticity of the maternal and fetal vessels, which appear honeycombed due to a 2-fold increase in blood vessel luminal area. High levels of Ang2 impair endothelial cell adhesiveness, leading to vascular leakiness with perivascular oedema, which increases placental weight. These observations suggest that the Ang2 overexpression may play a key role in placental vascular remodelling. Furthermore, we suggest a novel new model to study the pathobiology of placental vascularization and the effect of placental blood vessels on fetal phenotype.

Expression and significance of Tie-1 and Tie-2 receptors, and angiopoietins-1, 2 and 4 in colorectal adenocarcinoma: Immunohistochemical analysis and correlation with clinicopathological factors

AIM: There is strong evidence that tyrosine kinases are involved in the regulation of tumor progression, cellular growth and differentiation. Recently, many kinds of tyrosine kinase receptors have been reported, among them Tie-1 and Tie-2 receptors constitute a major class. Angiopoietin (Ang)-1 is known as a ligand of Tie-2 tyrosine kinase receptor. The objective of this study was to establish a comprehensive Tie-1 and Tie-2 and Ang-1, 2 and 4 expression profile in human colorectal adenocarcinomas.

METHODS: We examined 96 cases of surgically resected human colorectal adenocarcinoma by immunohistochemistry and investigated the statistical correlation between the expressions of Ties and Angs and clinicopathological factors.

RESULTS: Among the 96 cases of adenocarcinoma, 87 (90.6%), 92 (95.8%), 83 (86.5%), 89 (92.7%), and 76 cases (79.2%) showed positive staining in the cytoplasm of carcinoma cells for the Tie-1 and Tie-2 and Ang-1, 2 and 4 proteins, respectively. Histologically, the expressions of Ties and Angs were variable. The expressions of Ties and Angs were correlated with several clinicopathological factors, but did not correlate with the presence of lymph node metastasis. Ties and Angs were highly expressed in human colorectal adenocarcinoma cells.

CONCLUSION: These findings suggest that the Tie-Ang receptor-ligand complex is one of the factors involved in the cellular differentiation and progression of human colorectal adenocarcinoma.

Angiopoietin-1 and angiopoietin-2 in diabetes mellitus: relationship to VEGF, glycaemic control, endothelial damage/dysfunction and atherosclerosis

BACKGROUND

microvascular complications in diabetes identify those at risk of cardiovascular disease (CVD), suggesting a link between abnormal neovascularisation and CVD. This may be related to high plasma vascular endothelial growth factor (VEGF). We hypothesised increased angiopoietins (Ang)-1 and -2 in patients with diabetes that are related to VEGF, medium-term glycaemic control, endothelial damage/dysfunction and atherosclerosis.

METHODS AND PATIENTS

we measured plasma Ang-1 and Ang-2 alongside VEGF (all by ELISA) in 96 patients with type-2 diabetes mellitus (41 with and 56 without overt CVD) who were compared to 35 age- and sex-comparable healthy controls. Common carotid intima-media thickness (CC-IMT) was used to assess carotid atherosclerosis, plasma von Willebrand factor (vWf) and urine albumin:creatinine ratio (UACr) to quantify and endothelial damage/dysfunction, and HbA1c to mark medium-term hypergylcaemia.

RESULTS

Ang-2 (but not Ang-1) was higher in patients with diabetes compared to controls (p<0.01), with no significant difference between patients with and without CVD. As expected, CC-IMT, UACr, HbA1c, vWf, and VEGF were also abnormal in the patients. Within the patient group alone, and in the entire cohort, VEGF and Ang-2 correlated strongly (both p<0.001) and with several other markers. However, in multivariate analysis, the only significant relationship that remained after adjustments was between VEGF and HbA1c (p<0.001).

CONCLUSIONS

Angiogenic growth factor Ang-2, like VEGF, is raised in diabetes regardless of vascular disease. Both growth factors correlated with HbA1c and with each other, not with endothelial injury or atherosclerosis, but after multiple adjustment, only that between HbA1c and VEGF significant remained. VEGF is likely to have a more prominent role in diabetes than Ang-2.

Angiopoietin-1 promotes tumor angiogenesis in a rat glioma model

Angiopoietins have been implicated in playing an important role in blood vessel formation, remodeling, maturation, and maintenance. However, the role of angiopoietins in tumor angiogenesis remains uncertain. In this study, expression of human angiopoietin-1 (hAng-1) and angiopoietin (hAng-2) was amplified in the rat glioma cell line GS9L by stable transfection using an inducible tet-off system. Transfected cells were implanted intracerebrally into syngenic Fischer 344 rats. We demonstrated by means of magnetic resonance imaging that increased hAng-1 expression promoted a significant in vivo growth of intracerebral gliomas in rats. Overexpression of hAng-1 resulted in more numerous, more highly branched vessels, which were covered by pericytes. On the other hand, tumors derived from hAng-2-overexpressing cells were smaller than empty-plasmid control tumors. The tumor vasculature in these tumors was composed of aberrant small vascular cords, which were associated with few mural cells. Our results indicate that in the presence of hAng-1, tumors induce a more functional vascular network, which led to better tumor perfusion and growth. On the other hand, overexpression of hAng-2 led to less intact tumor vessels, inhibited capillary sprouting, and impaired tumor growth.

Recanalization of arterial thrombus, and inhibition with β-radiation in a new murine carotid occlusion model: mRNA expression of angiopoietins, metalloproteinases, and their inhibitors

BACKGROUND

Recanalization is an important physiologic phenomenon because it can efficiently reestablish circulation after thrombosis. We attempted to characterize molecular events related to recanalization or organization of arterial thrombus in a new murine model by studying genes reported to be involved in angiogenesis or neointima formation.

METHODS

Platinum coils, radioactive phosphorus 32 coils or not, were implanted in the carotid artery in mice to cause thrombotic occlusion. The outcome of the occlusion was followed up with transmyocardial angiography and pathologic analysis at 2, 6, or 15 days. Angiographic results were compared with the Pearson chi2 test. Messenger RNA expression of von Willebrand factor (vWF); smooth muscle alpha-actin (SMA+); platelet endothelial cell adhesion molecule-1 (PECAM-1); vascular endothelium cadherin (VE-Cad); endothelial nitric oxide synthase (eNOS); vascular cell adhesion molecule-1 (VCAM-1); tumor necrosis factor alpha (TNF-alpha); matrix metalloproteinase (MMP-9, MMP-12, and MMP-14), and tissue inhibitors of MMPs (TIMPs: TIMP-1, TIMP-2, TIMP-3, TIMP-4); angiopoietins (Ang-1, Ang-2); and receptors Tie-1 and Tie-2, were analyzed with reverse transcriptase polymerase chain reaction 2, 6, and 15 days after surgery. Levels of mRNA expression were compared with analysis of variance and the Student t test.

RESULTS

Carotid arteries implanted with nonradioactive 0.015-caliber coils were occluded in 84% of arteries on day 2, but in only 57% of arteries on day 15, which confirms that recanalization occurred in this model. Arteries implanted with 0.015-caliber 32P coils did not become recanalized, and 100% were occluded on day 15 (n = 13; P = .006). Recanalization was associated with endothelial-like cell-lined channels, whereas persistent occlusion was caused by complete filling of the lumen with conjunctive tissue. Coil occlusion, with or without recanalization, was followed by decreased expression of vWf, VE-Cad, eNOS, VCAM-1, MMP-2, TIMP-1, and TIMP-2; stable expression of PECAM-1, SMA+, and TIMP-3; and overexpression of Ang-1 and Ang-2, MMP-9, MMP-14, and TIMP-4. Statistically significant differences when arteries were implanted with 32P coils included decreased expression of TIMP-4 (P = .011) and increased expression of MMP-9 (P = .02).

CONCLUSION

Recanalization and organization of arterial thrombus is associated with expression of genes involved in angiogenesis and neointima formation. Recanalization can be prevented with beta-radiation, but molecular mechanisms remain to be refined.

CLINICAL RELEVANCE

A better understanding of molecular mechanisms involved in angiogenesis has permitted its regulation as a new option in treatment of various diseases. Inhibition of angiogenesis may help control diseases such as cancer, arthritis, or diabetes retinopathy. On the other hand, stimulation of angiogenesis may palliate conditions associated with insufficient blood supply, such as ischemic heart disease or critical limb ischemia. Yet little is known regarding recanalization (to be differentiated from thrombolysis), a cellular process that occurs concurrently with thrombus "organization." Recanalization is an important physiologic phenomenon because it can efficiently reestablish antegrade circulation after thrombosis both in veins and in arteries, and could be modulated for therapeutic purposes. Thus our efforts at better understanding of mechanisms involved in recanalization could be used, in addition to its promotion to recover flow after thrombotic occlusions, to prevent its occurrence after endovascular interventions designed to permanently occlude aneurysms.

Plasma vascular endothelial growth factor, angiopoietin-1, and angiopoietin-2 in diabetes: implications for cardiovascular risk and effects of multifactorial intervention

OBJECTIVE

Vascular endothelial growth factor (VEGF) and angiopoietin (Ang)-1 and Ang-2 are mediators of angiogenesis. More recent data suggest that the balance between these growth factors may affect vascular endothelial integrity. Because diabetes is closely associated with endothelial perturbation, we studied plasma levels of these angiogenic growth factors in patients with diabetes; their relationship with glycemia, inflammation, and endothelial damage/dysfunction; and the effect of intensified cardiovascular risk management.

RESEARCH DESIGN AND METHODS

We measured plasma VEGF, Ang-1, and Ang-2 alongside plasma von Willebrand factor (vWf) and urine albumin-to-creatinine ratio (marking endothelial damage/dysfunction) and interleukin (IL)-6 in 94 patients (38 with overt cardiovascular disease [CVD]) with diabetes and 34 normal control subjects.

RESULTS

Plasma vWf (P=0.009), IL-6 (P <0.001), VEGF (P=0.001), and Ang-2 (P=0.001), but not Ang-1 (P=0.635), were higher in diabetic patients with and without CVD than in control subjects. On multivariate analysis, HbA1c was an independent predictor of plasma VEGF (P=0.032) and Ang-2 (P=0.015). Of the 94 patients, a subgroup of 33 patients with and 31 patients without CVD participated in a year of intensified cardiovascular risk management. HbA1c and LDL cholesterol reduced significantly with treatment, along with associated reductions in plasma vWf and VEGF in both groups (P <0.001). Ang-2 decreased (P <0.001) only in patients without CVD. There were no significant changes in plasma IL-6 levels in both groups.

CONCLUSIONS

Plasma Ang-2 (but not Ang-1), like VEGF levels, are selectively elevated in patients with diabetes and are associated with indexes of endothelial damage/dysfunction, regardless of vascular disease. Intensive multifactorial intervention is associated with reductions in plasma VEGF, vWf, and (in patients without CVD) Ang-2 levels, possibly reflecting an improved vascular profile with treatment.

Expression of Tie-1 and 2 receptors, and angiopoietin-1, 2 and 4 in gastric carcinoma; immunohistochemical analyses and correlation with clinicopathological factors

AIMS

There is strong evidence that tyrosine kinases are involved in the regulation of tumour progression, cellular growth and differentiation. Recently, many kinds of tyrosine kinase receptors have been reported, and among them Tie-1 and 2 constitute a major class. Angiopoietin (Ang)-1 is known as a ligand of the Tie-2 tyrosine kinase receptor. The aim of this study was to determine the expression profile of Tie-1 and 2 and Ang-1, 2 and 4 in gastric adenocarcinoma.

METHODS AND RESULTS

Eighty-nine cases of surgically resected human gastric adenocarcinoma were studied by immunohistochemistry. Of these, 60 (67.4%), 61 (68.5%), 69 (77.5%), 75 (84.3%), and 47 cases (52.8%) showed positive staining in the cytoplasm of carcinoma cells for the Tie-1 and 2 and Ang-1, 2 and 4 proteins, respectively. The expression of Ties and Angs was significantly correlated with several type of histological differentiation and several clinicopathological factors.

CONCLUSIONS

Ties and Angs were highly expressed in human gastric adenocarcinoma cells. These findings suggest that the Tie-Ang receptor-ligand complex is one of the factors involved in the cellular differentiation and progression of human gastric adenocarcinoma.

Functional inhibition of secreted angiopoietin: a novel role for angiopoietin 1 in coronary vessel patterning

The angiopoietins are a family of growth factors critical for development and maintenance of the vasculature. The primary amino acid sequence of the angiopoietins predicts that they are comprised of a coiled-coiled and a fibrinogen-like domain. The coiled-coiled domain mediates ligand multimerization, whereas the fibrinogen domain engages the receptor. This multimerization is required to elicit a ligand-mediated biological effect via activation of their receptor Tie2. In vitro and in vivo knockout studies have suggested that the angiopoietins are chemotactic for endothelial cells. We were interested in ascertaining whether the angiopoietins have this activity within the animal proper. To accomplish this we engineered a dominant-interfering form of angiopoietin (Ang) 1, called Ang1cc. Ang1cc contains the coiled-coiled domain, which can heterodimerize with other angiopoietins produced in the same cell. We show that Ang1cc can inhibit Tie2 activation and can inhibit Ang1 activity in vitro and in vivo.

Pre-administration of angiopoietin-1 followed by VEGF induces functional and mature vascular formation in a rabbit ischemic model

BACKGROUND

Angiopoietin-1 (Ang1) and vascular endothelial growth factor (VEGF) play important roles in vascular formation and maturation, suggesting that the combination of these two would be a promising therapy for ischemia. However, it remains unclear what the best schedule of administration of these cytokines might be.

METHODS

Six experimental groups were used to prepare the rabbit ischemic hindlimb model following naked plasmid intramuscular administration as follows: empty vector (C), single gene (Ang1, A; VEGF, V), Ang-1 followed by VEGF (A - V), co-administration of Ang1 and VEGF (A + V), and VEGF followed by Ang1 (V - A).

RESULTS

Thirty days after gene administration, A - V showed a significantly increased blood pressure and blood-flow recovery in the ischemic limb compared with the control group. Histological findings by alpha-smooth muscle-actin (alpha-SMA) staining revealed that the two combination groups had more mature vessels as compared with the control group. Significantly, A - V revealed the highest density of alpha-SMA-positive vessels compared with VEGF alone or Ang1 alone. Angiographic assessment revealed that A - V had a greater increased arterial diameter compared with VEGF alone. Edema, one of the major adverse effects induced by VEGF, was not found in A - V throughout the experiments, while VEGF alone and V - A showed severe edema induced by VEGF.

CONCLUSIONS

The pre-administration of Ang1 followed by VEGF resulted in an improvement of hemodynamic status, an increased number of vessels covered with alpha-actin-positive mural cells, and prevention of VEGF-mediated edema. Thus, priming by Ang1 gene administration would be beneficial for therapeutic angiogenesis in VEGF gene therapy.

A mathematical model of tumour angiogenesis, regulated by vascular endothelial growth factor and the angiopoietins

Angiogenesis--the growth of new blood vessels from existing ones--is a prerequisite for the growth of solid tumours beyond a diameter of approximately 2 mm. In recent years, the angiopoietins have emerged as important regulators of angiogenesis. They mediate a delicate balance between vascular quiescence, regression and new growth, but their mechanism of action is not fully understood. This work attempts to provide a mathematical description of the role of the angiopoietins in angiogenesis. The model is formulated within the framework of reinforced random walks, which allows easy transition between the continuum (macroscopic) and discrete (microscopic) forms. Model predictions are in qualitative agreement with experimental observations, and may have implications for anti-cancer therapies based on the prevention of angiogenesis.

Angiopoietin 1 inhibits ocular neovascularization and breakdown of the blood-retinal barrier

Several retinal and choroidal diseases are potentially treatable by intraocular delivery of genes whose products may counter or neutralize abnormal gene expression that occurs as part of the diseases. However, prior to considering a transgene, it is necessary to thoroughly investigate the effects of its expression in normal and diseased eyes. An efficient way to do this is to combine tissue-specific promoters with inducible promoter systems in transgenic mice. In this study, we used this approach to evaluate the effects of ectopic expression of angiopoietin-1 (Ang1) in normal eyes and those with ocular neovascularization. Adult mice with induced expression of Ang1 ubiquitously, or specifically in the retina, appeared normal and had no identifiable changes in retinal or choroidal blood vessels or in retinal function as assessed by electroretinography. Increased expression of Ang1 in eyes with severe retinal ischemia or in eyes with rupture of Bruch's membrane significantly suppressed the development of retinal or choroidal neovascularization, respectively. This inhibition of ocular neovascularization is particularly interesting and noteworthy, because overexpression of Ang1 in skin stimulates neovascularization. Ang1 also significantly reduced VEGF-induced retinal vascular permeability. These data suggest that intraocular delivery of ang1 has potential for treatment of ocular neovascularization and macular edema.

The role of angiopoietins in the development of endothelial cells from cord blood CD34+ progenitors

Circulating endothelial progenitors contribute to neovascularization at sites of injury and tumorigenesis in postnatal life. Yet, the molecular mechanisms initiating the endothelial developmental program of these precursors remain elusive. Here we provide evidence that endothelial development from progenitors circulating in human cord blood requires angiopoietins, a set of growth factors also involved in vascular branching during embryogenesis. We show that cord blood cells with the potential for endothelial development reside in a CD34(+)CD11b+ subset capable of autonomously producing and binding angiopoietins. Functionally, endogenous angiopoietin-1 regulates initial endothelial cell commitment, whereas angiopoietin-2 enhances expansion of the endothelial cell progeny. These findings suggest a role for angiopoietins as regulators of endothelial development from circulating progenitors and imply a function of angiopoietins at distinct developmental steps in postnatal angiogenesis.

Expression of the angopoietin-1, angopoietin-2, Tie2, and vascular endothelial growth factor gene in epithelial ovarian cancer

OBJECTIVES

Angiopoietin/Tie2 system with vascular endothelial growth factor (VEFG) is known to be important for the initiation of angiogenesis in tumors. The aim was to evaluate whether angiopoietin/Tie2 system with VEFG affects prognosis in patients of epithelial ovarian cancer.

METHODS

Angiopoietin-1 (Ang-1), Angiopoietin-2 (Ang-2), Tie2, and VEGF gene expression were analyzed by real-time quantitative reverse transcription-polymerase chain reaction (RT-PCR) in 85 epithelial ovarian cancer surgical specimens. These gene expressions were correlated with clinical-pathological parameters, microvessel density (MVD), and patients' survival.

RESULTS

Ang-1/Ang-2 gene expression ratio, VEGF, and Tie2 gene expression significantly associated with MVD, respectively (P < 0.0001, P = 0.024, P = 0.005). The patients with low Ang-1/Ang-2 gene expression ratio and high VEGF gene expression were found to have a significantly higher MVD when compared to others (P = 0.0003). Moreover, there was a significant difference between the values of MVD in patients with low Ang-1/Ang-2 gene expression ratio and high VEGF and high Tie2 gene expression and those in others (P = 0.0025). FIGO stage (P = 0.014), residual disease (P = 0.042), histological grade (P = 0.028), Ang-1/Ang-2 gene expression ratio (P = 0.010), and combination of Ang-1/Ang-2 gene expression ratio and VEGF gene expression (P = 0.019), were found to be significantly associated with a poor prognosis in univariate Cox regression analysis. Multivariate Cox regression analysis revealed that FIGO stage is an independent prognostic factor (P = 0.035). Low Ang-1/Ang-2 gene expression ratio had a tendency to be an independent prognostic factor (P = 0.061).

CONCLUSIONS

Angiogenesis occurred by angiopoietin/Tie2 system in concert with VEGF in epithelial ovarian cancer did not affect patients' survival. However, gene expression of Ang-1 and Ang-2 might present a pertinent diagnostic tool to select a high-risk group of patients independent of clinical-pathological parameters and a new insight to understand the biology of epithelial ovarian cancer.

Overexpression of angiopoietin-1 and angiopoietin-2 in hepatocellular carcinoma

BACKGROUND/AIMS

Hepatocellular carcinoma (HCC) is a highly vascular tumor. Angiopoietin-1 and Angiopoietin-2 have been shown to be involved in tumor angiogenesis. We investigated the expression of Angiopoietin-1 and Angiopoietin-2 in HCC.

METHODS

The expression of Angiopoietin-1 and Angiopoietin-2 mRNAs in cultured hepatoma cells under hypoxic conditions and in HCC and noncancerous liver tissue was evaluated by real-time PCR. The expression of Angiopoietin-1, Angiopoietin-2, and their receptor Tie-2 in HCC was assessed by immunohistochemistry. The changes in Angiopoietin-1 and Angiopoietin-2 expression were evaluated in relation to tumor differentiation and changes in tumor vascularity.

RESULTS

Hypoxic conditions did not up-regulate the expression of Angiopoietin-1 and Angiopoietin-2 mRNAs in hepatoma cells. Increased expression of Angiopoietin-1 and Angiopoietin-2 mRNAs was detected in HCC. Angiopoietin-1 and Angiopoietin-2 were detected in hepatoma cells, hepatic stellate cells, and smooth muscle cells, whereas Tie-2 was detected in endothelial cells, hepatic stellate cells and smooth muscle cells. Increased expression of Angiopoietin-2 and Angiopoietin-2 mRNA was associated with tumor dedifferentiation. The expression of Angiopoietin-1 and Angiopoietin-2 correlated with HCC vascularity.

CONCLUSIONS

Our findings indicate that the increased expression of Angiopoietin-1 and Angiopoietin-2 play a critical role in the process of vascular development in HCC.

Plasma angiopoietin-1, angiopoietin-2, and angiopoietin receptor tie-2 levels in congestive heart failure

OBJECTIVES

The goal of this research was to test the hypothesis that plasma angiopoietin (Ang-1), its soluble receptor tie-2, and Ang-2 levels would be abnormal in patients with acute and chronic congestive heart failure (CHF) when compared with healthy controls.

BACKGROUND

Increased plasma vascular endothelial growth factor (VEGF) in CHF is suggestive of excess angiogenesis-possibly driven by tissue hypoxia. However, other growth factors also have a major role in angiogenesis, such as those of the angiopoietin family (e.g., Ang-1, which exerts its activity via its receptor, tie-2, and Ang-2).

METHODS

We recruited 39 patients with acute CHF (mean age 67 +/- 10 years), 40 patients with chronic CHF (mean age 63 +/- 9 years), and 17 healthy controls (mean age 67 +/- 7 years), all in sinus rhythm. Citrated plasma was analyzed for Ang-1, Ang-2, tie-2, and VEGF by enzyme-linked immunosorbent assay.

RESULTS

Angiopoietin-2 (p < 0.001), tie-2 (p < 0.05), and VEGF (p < 0.05) levels were all higher in acute CHF compared with controls. The Ang-2 levels were higher in acute CHF compared with chronic CHF (p < 0.001), but there were no significant differences in Ang-1 levels between the groups. The principal significant correlations were between Ang-2 and tie-2 (Spearman, r = 0.407; p < 0.0001) and between Ang-2 and ejection fraction (r = -0.241, p = 0.043). Although only marginally raised, levels of VEGF correlated with both Ang-2 (r = 0.468, p < 0.001) and tie-2 (r = 0.569, p < 0.001).

CONCLUSIONS

We have demonstrated abnormal levels of Ang-2 and tie-2, but normal Ang-1, in both CHF patients. These abnormalities may, alongside VEGF, indicate a role for these angiogenic factors in the pathophysiology of CHF.

Ischemic preconditioning mediated angiogenic response in the heart

Angiogenesis represents a major focus for novel therapeutic approaches to the prevention and treatment of multiple diseases, most notably ischemic cardiovascular disease and cancer. Therapeutic angiogenesis achieved either through the use of discreet angiogenic proteins or by gene therapy is fast emerging as a highly attractive treatment modality for ischemic heart disease. The purpose of this review is to address this important clinical issue through the identification of potential signaling mechanisms by which a short episode of sublethal ischemia known as ischemic preconditioning causes angiogenesis and subsequently improves myocardial salvage following coronary artery occlusion.

Autocrine/paracrine role of the angiopoietin-1 and -2/Tie2 system in cell proliferation and chemotaxis of cultured fibroblastic synoviocytes in rheumatoid arthritis

Hypervascularity is a characteristic synovial feature of rheumatoid arthritis (RA). We previously reported that Tie1 and Tie2, endothelium-specific tyrosine kinase receptors essential for angiogenesis, are expressed not only by vascular cells, but also by a subpopulation of synovial lining and stromal cells in the inflamed RA synovium. The present study used immunohistochemistry and in situ hybridization to examine whether angiopoietin-1 and -2 (Ang1 and Ang2), ligands for Tie2, are also expressed in the RA synovium. Ang1 and Ang2 were expressed in all of 15 RA synovial samples, and their distribution pattern was similar to that of Tie2. Intense staining was noted in synovial lining and stromal cells, as well as in small vessels in areas of papillary projection and high cell density. Double immunohistochemistry revealed coexpression of Ang1, Ang2, and Tie2 in synovial components exhibiting proliferating cell nuclear antigen immunoreactivity. In addition, Ang1 and Ang2 were preferentially expressed in vimentin-positive fibroblastic cells. To address the functional role of Ang/Tie signaling in RA pathophysiology, we carried out [(3)H]thymidine incorporation and transwell chemotaxis assays using cultured fibroblastic synoviocytes obtained from 2 RA patients. Incubation with various concentrations of recombinant Ang1 or Ang2 did not alter DNA synthesis, but the ligands enhanced chemotactic migration of RA fibroblastic synoviocytes. Our results suggest that the autocrine/paracrine signaling of the Ang/Tie2 system is important for the up-regulated angiogenesis in the RA synovium, as well as for synoviocyte behavior, by regulating chemotactic cell movement.

Gene expression patterns of epithelial cells modulated by pathogenicity factors of Yersinia enterocolitica

Epithelial cells express genes whose products signal the presence of pathogenic microorganisms to the immune system. Pathogenicity factors of enteric bacteria modulate host cell gene expression. Using microarray technology we have profiled epithelial cell gene expression upon interaction with Yersinia enterocolitica. Yersinia enterocolitica wild-type and isogenic mutant strains were used to identify host genes modulated by invasin protein (Inv), which is involved in enteroinvasion, and Yersinia outer protein P (YopP) which inhibits innate immune responses. Among 22 283 probesets (14,239 unique genes), we found 193 probesets (165 genes) to be regulated by Yersinia infection. The majority of these genes were induced by Inv, whose recognition leads to expression of NF-kappa B-regulated factors such as cytokines and adhesion molecules. Yersinia virulence plasmid (pYV)-encoded factors counter regulated Inv-induced gene expression. Thus, YopP repressed Inv-induced NF-kappa B regulated genes at 2 h post infection whereas other pYV-encoded factors repressed host cell genes at 4 and 8 h post infection. Chromosomally encoded factors of Yersinia, other than Inv, induced expression of genes known to be induced by TGF-beta receptor signalling. These genes were also repressed by pYV-encoded factors. Only a few host genes were exclusively induced by pYV-encoded factors. We hypothesize that some of these genes may contribute to pYV-mediated silencing of host cells. In conclusion, the data demonstrates that epithelial cells express a limited number of genes upon interaction with enteric Yersinia. Both Inv and YopP appear to modulate gene expression in order to subvert epithelial cell functions involved in innate immunity.

Not just angiogenesis--wider roles for the angiopoietins

Since the discovery of the angiopoietins, much interest has been focused on their biological actions and their potential use as therapeutic targets. It is generally accepted that the angiopoietins play an important role in angiogenesis and hence are described as angiogenic factors. However, it is becoming increasingly clear that this is not their only role and it is likely that the angiopoietins have important roles in a wider range of biological and pathological functions.

A short synthetic peptide inhibits signal transduction, migration and angiogenesis mediated by Tie2 receptor

Tie2, an endothelial cell-specific receptor kinase, has an important role in tumour angiogenesis. In an attempt to identify peptides that specifically interact with and block the Tie2 pathway, a phage-displayed peptide library was screened on a recombinant Tie2 receptor. One peptide, NLLMAAS, completely abolished the binding to Tie2 of both angiopoietin 2 and angiopoietin 1 (Ang1). We further show that NLLMAAS specifically suppresses both Ang1-induced ERK activity and migration in human umbilical endothelial cells. Moreover, in vivo, this peptide inhibits angiogenesis in the chick chorioallantoic membrane assay. NLLMAAS is the first peptide described to interact with Tie2. Our results demonstrate that it is an efficient and specific antagonist of the binding of Tie2 ligands, and suggest that this peptide or its derivates may have potential applications in the treatment of angiogenesis diseases. It also represents a potent tool to dissect the molecular mechanisms involved in the Tie2 pathway.

Opposing effect of angiopoietin-1 on VEGF-mediated disruption of endothelial cell-cell interactions requires activation of PKC beta

Angiopoietin-1 (Ang1) and vascular endothelial growth factor (VEGF) cooperate in migration and survival of endothelial cells by activation of phosphatidylinositol-3 (PI-3) kinase and mitogen activating protein (MAP) kinase pathways. However, Ang1 opposes the effect of VEGF on vascular permeability. We found that Ang1 also blocks VEGF-mediated diffusion of fluoresin isothiocyanate (FITC)-labeled albumin across an endothelial cell monolayer. VEGF-mediated vascular permeability has been attributed, in part, to activation of phospholipase A(2) and subsequent formation of platelet activating factor. However, Ang1 had no effect on VEGF-induced activation of phospholipase A(2) or the release of arachidonic acid. VEGF-mediated permeability was associated with disruption of endothelial cell junctional complexes, dissociation of beta-catenin from VE-cadherin, and accumulation of beta-catenin in the cytosol. In contrast, Ang1 enhanced the interaction of beta-catenin with VE-cadherin and impaired VEGF-mediated dissociation of this complex. Ang1 also blocked VEGF-induced translocation of protein kinase C (PKC) and beta2 to the membrane, but had no effect on activation of PKC alpha. In addition, staurosporine and a PKC beta inhibitor, LY379196, blocked VEGF-mediated dissociation of beta-catenin from VE-cadherin, diffusion of albumin across the endothelial cell monolayer, and translocation of PKC beta isoforms. These data indicate that VEGF-mediated disruption of endothelial cell-cell interactions requires activation of PKC beta isoforms and that this pathway is blocked by Ang1.

Vasculogenesis and angiogenesis

Two distinct mechanisms, vasculogenesis and angiogenesis implement the formation of the vascular network in the embryo. Vasculogenesis gives rise to the heart and the first primitive vascular plexus inside the embryo and in its surrounding membranes, as the yolk sac circulation. Angiogenesis is responsible for the remodeling and expansion of this network. While vasculogenesis refers to in situ differentiation and growth of blood vessels from mesodermal derived hemangioblasts, angiogenesis comprises two different mechanisms: endothelial sprouting and intussusceptive microvascular growth (IMG). The sprouting process is based on endothelial cell migration, proliferation and tube formation. IMG divides existing vessel lumens by formation and insertion of tissue folds and columns of interstitial tissue into the vessel lumen. The latter are termed interstitial or intervascular tissue structures (ITSs) and tissue pillars or posts. Intussusception also includes the establishment of new vessels by in situ loop formation in the wall of large veins. The molecular regulation of these distinct mechanisms is discussed in respect to the most important positive regulators, VEGF and its receptors flk-1 (KDR) and flt-1, the Angiopoietin/tie system and the ephrin-B/EpH-B system. The cellular mechanisms and the molecular regulation of angiogenesis in the pathological state are summarized and the differences of physiological and pathological angiogenesis elaborated.

Effects of adrenomedullin on endothelial cells in the multistep process of angiogenesis: Involvement of CRLR/RAMP2 and CRLR/RAMP3 receptors

Recently, we demonstrated that U87 glioblastoma xenograft tumors treated with anti-adrenomedullin (AM) antibody were less vascularized than control tumors, suggesting that AM might be involved in neovascularization and/or vessel stabilization. Angiogenesis, the sprouting of new capillaries from preexisting blood vessels, is a multistep process that involves migration and proliferation of endothelial cells, remodeling of the extracellular matrix and functional maturation of the newly assembled vessels. In our study, we analyzed the role of AM on human umbilical vein endothelial cell (HUVEC) phenotype related to different stages of angiogenesis. Here we report evidence that AM promoted HUVEC migration and invasion in a dose-dependent manner. The action of AM is specific and is mediated by the calcitonin receptor-like receptor/receptor activity-modifying protein-2 and -3 (CRLR/RAMP2; CRLR/RAMP3) receptors. Furthermore, AM was able to induce HUVEC differentiation into cord-like structures on Matrigel. Suboptimal concentrations of vascular endothelial growth factor (VEGF) and AM acted synergistically to induce angiogenic-related effects on endothelial cells in vitro. Blocking antibodies to VEGF did not significantly inhibit AM-induced capillary tube formation by human endothelial cells, indicating that AM does not function indirectly through upregulation of VEGF. These findings suggest that the proangiogenic action of AM on cultured endothelial cells via CRLR/RAMP2 and CRLR/RAMP3 receptors may translate in vivo into enhanced neovascularization and therefore identify AM and its receptors acting as potential new targets for antiangiogenic therapies.

Angiopoietin/tie-2 as mediators of angiogenesis: a role in congestive heart failure?

Angiogenic factors, in particular vascular endothelial growth factor (VEGF) and the angiopoietins, Ang-1 and -2, have recently generated significant interest, especially in oncology. The process of angiogenesis is also thought to occur in response to ischaemic conditions, which lie at the core of cardiovascular disease states such as coronary artery disease and congestive heart failure. However, current data do not conclusively show evidence of angiogenesis per se in these conditions, despite (for example) the presence of high levels of VEGF and Ang-2. High levels of these angiogenic factors in heart disease also have not translated into clinically significant new vessel formation, as in accelerated cancer growth or proliferative retinopathy. Indeed, we would hypothesize that these angiogenic markers--especially the angiopoietins--do not necessarily translate into new vessel formation in congestive heart failure (CHF), but may well reflect disturbances of endothelial integrity in CHF.

VE-statin, an endothelial repressor of smooth muscle cell migration

The recruitment and proliferation of smooth muscle cells and pericytes are two key events for the stabilization of newly formed capillaries during angiogenesis and, when out of control in the adult, are the main causes of arteriosclerosis. We have identified a novel gene, named VE-statin for vascular endothelial-statin, which is expressed specifically by endothelial cells of the developing mouse embryo and in the adult, and in early endothelial progenitors. The mouse and human VE-statin genes have been located on chromosome 2 and 9, respectively, they span >10 kbp and are transcribed in two major variants arising from independent initiation sites. The VE-statin transcripts code for a unique protein of 30 kDa that contains a signal peptide and two epidermal growth factor (EGF)-like modules. VE-statin is found in the cellular endoplasmic reticulum and secreted in the cell supernatant. Secreted VE-statin inhibits platelet-derived growth factor (PDGF)-BB-induced smooth muscle cell migration, but has no effects on endothelial cell migration. VE-statin is the first identified inhibitor of mural cell migration specifically produced by endothelial cells.

ABIN-2 protects endothelial cells from death and has a role in the antiapoptotic effect of angiopoietin-1

A20 binding inhibitor of NF-kappaB activation-2, ABIN-2, is a newly identified intracellular protein that interacts with the zinc finger protein A20. ABIN-2 inhibits nuclear factor-kappaB (NF-kappaB) activity and is a possible effector of A20 regulation of NF-kappaB. Although A20 is a potent inhibitor of endothelial apoptosis, the effect of ABIN-2 on apoptosis is not known. ABIN-2 also interacts with the endothelial receptor Tie2. This receptor is essential for blood vessel formation and promotes endothelial survival. Here we examine the effects of ABIN-2 on endothelial cell apoptosis and its potential involvement in Tie2-mediated endothelial survival. ABIN-2 was found to inhibit endothelial apoptosis and rescue cells from death following growth factor deprivation. The inhibitors of phosphatidylinositol-3 kinase, wortmannin and LY294002, suppressed ABIN-2 inhibition of endothelial cell death. Deletion of the carboxy-terminus of ABIN-2 removed its ability to inhibit apoptosis. Expression of truncated ABIN-2 prevented the Tie2-activating ligand angiopoietin-1 from inhibiting endothelial cell death.

Adaptor ShcA protein binds tyrosine kinase Tie2 receptor and regulates migration and sprouting but not survival of endothelial cells

Angiopoietin-1 can promote migration, sprouting, and survival of endothelial cells through activation of different signaling pathways triggered by the Tie2 tyrosine kinase receptor. ShcA adapter proteins are targets of activated tyrosine kinases and are implicated in the transmission of activation signals to the Ras/mitogen-activated protein kinase pathway. Here we report the identification of an interaction between the adapter protein ShcA and the cytoplasmic domain of Tie2 through in vitro co-immunoprecipitation analysis. Stimulation of endogenous Tie2 in endothelial cells with its ligand angiopoietin-1 increased its association with ShcA and phosphorylation of the adapter protein. The interaction requires the SH2 domain of ShcA and the tyrosine phosphorylation of Tie2 as shown by pull-down experiments. Furthermore, Tyr-1101 of Tie2 was identified as the primary binding site for the SH2 domain of ShcA. Overexpression of a dominant-negative form of ShcA affects angiopoietin-1-induced chemotaxis and sprouting, although it has no effect on survival of endothelial cells. Furthermore, this mutant partially reduces the tyrosine phosphorylation of the regulatory p85 subunit of phosphatidylinositol 3-kinase. Together, our results identified a novel interaction between Tie2 with the adapter molecule ShcA and suggested that this interaction may play a role in the regulation of migration and three-dimensional organization of endothelial cells induced by angiopoietin-1.