Angiopoietins: a link between angiogenesis and inflammation

Oligomeric procyanidins inhibit cell migration and modulate the expression of migration and proliferation associated genes in human umbilical vascular endothelial cells

The consumption of flavan-3-ols has been associated with reduced risk of cardiovascular diseases and improvements in vascular function. However, the nature of the flavan-3-ols responsible and the mechanisms underlying the vascular responses are not fully understood. We used microarrays to search for molecular changes in response to the exposure to (-)-epicatechin (EC), procyanidin dimer B2, and a mixture of oligomeric procyanidins in human umbilical vein endothelial cells (HUVECs). No gene expression changes were detected in HUVECs exposed to EC or dimer B2, however, the oligomeric procyanidins induced significant gene expression changes in both resting and TNF-alpha-stimulated cells. In particular, the expression of genes such as ADAMTS1, THBS1, ANGPT2, CYR61, ET-1, EDG3, and PDE4B involved in endothelial cell migration and proliferation, were substantially over-represented. Also, exposure to the oligomers arrested the cells at the G(0)/G(1 )phase and inhibited cell migration. These data show that human endothelial cells respond to oligomeric procyanidins by exhibiting a less migratory phenotype and by a general modulation of the expression of genes that are associated with key events in the angiogenic process. The molecular changes associated with procyanidin treatment identified in this study are consistent with the beneficial effects of flavan-3-ols on vascular function.

Regulation of vascular integrity

The integrity of blood vessels is critical to vascular homeostasis. Maintenance of vascular integrity has been conventionally regarded as a passive process that is largely dependent on continuous blood flow. Recent studies, however, have begun unveiling molecular processes essential for maintenance of vascular integrity and homeostasis under physiological conditions, leading to the notion that maintenance of the vasculature is an active biological process that requires continuous, basal cellular signaling. Failure of this system results in serious consequences such as hemorrhage, edema, inflammation, and tissue ischemia. In this review, we will discuss the emerging concepts in regulation of vascular integrity with the emphasis on structural components of blood vessels that are essential for vascular maintenance.

Serum levels of angiogenic molecules in autoimmune thyroid diseases and their correlation with laboratory and clinical features

CONTEXT

Because angiogenesis has a role in the pathogenesis of inflammatory conditions, we studied angiogenesis soluble markers in autoimmune thyroid diseases.

OBJECTIVE

The aim of the study was to measure concentrations of angiopoietins, Tie-2, and vascular endothelial growth factor in sera from autoimmune thyroid disease patients.

DESIGN

Soluble Tie-2 (sTie-2), angiopoietin-1, angiopoietin-2, and vascular endothelial growth factor were quantified by ELISA in sera from 44 untreated Graves' disease (GD) patients, 25 untreated Hashimoto's thyroiditis (HT) patients, 13 non-GD hyperthyroid patients, and 22 age-matched controls. Subgroups of patients with active and non-active Graves' ophthalmopathy (GO) were analyzed. Correlations among these markers and clinical parameters were assessed by bivariate and multivariate analyses.

RESULTS

STIE-2 levels were higher in GD or HT patients compared to controls (P < 0.01). In addition, serum Ang-2 concentrations were higher in untreated GD patients compared to controls, HT patients, or non-GD hyperthyroid patients (P < 0.01), and no difference was observed between HT patients and controls. Significant correlations were found between free T(4)/sTie-2 and free T(4)/Ang-2 levels (r = 0.464, P < 0.01; and r = 0.463, P < 0.01, respectively) as well as between sTie-2/anti-TSH receptor antibody (r = 0.527; P < 0.01) and sTie-2/Ang-2 (r = 0.563; P = 0.001). Furthermore, sTie-2 levels were significantly higher in patients with active GO when compared to those with inactive GO (P < 0.05). Interestingly, Ang-2 levels decreased significantly after treatment with antithyroid drugs (P < 0.01).

CONCLUSIONS

Ang-2 and sTie-2 could participate in the pathogenesis of GD and potentially be used as markers of GO activity. Antithyroid drugs affect the angiogenic pattern in GD.

Serum angiopoietin-1 and -2 levels discriminate cerebral malaria from uncomplicated malaria and predict clinical outcome in African children

BACKGROUND

Limited tools exist to identify which individuals infected with Plasmodium falciparum are at risk of developing serious complications such as cerebral malaria (CM). The objective of this study was to assess serum biomarkers that differentiate between CM and non-CM, with the long-term goal of developing a clinically informative prognostic test for severe malaria.

METHODOLOGY/PRINCIPAL FINDINGS

Based on the hypothesis that endothelial activation and blood-brain-barrier dysfunction contribute to CM pathogenesis, we examined the endothelial regulators, angiopoietin-1 (ANG-1) and angiopoietin-2 (ANG-2), in serum samples from P. falciparum-infected patients with uncomplicated malaria (UM) or CM, from two diverse populations--Thai adults and Ugandan children. Angiopoietin levels were compared to tumour necrosis factor (TNF). In both populations, ANG-1 levels were significantly decreased and ANG-2 levels were significantly increased in CM versus UM and healthy controls (p<0.001). TNF was significantly elevated in CM in the Thai adult population (p<0.001), but did not discriminate well between CM and UM in African children. Receiver operating characteristic curve analysis showed that ANG-1 and the ratio of ANG-2:ANG-1 accurately discriminated CM patients from UM in both populations. Applied as a diagnostic test, ANG-1 had a sensitivity and specificity of 100% for distinguishing CM from UM in Thai adults and 70% and 75%, respectively, for Ugandan children. Across both populations the likelihood ratio of CM given a positive test (ANG-1<15 ng/mL) was 4.1 (2.7-6.5) and the likelihood ratio of CM given a negative test was 0.29 (0.20-0.42). Moreover, low ANG-1 levels at presentation predicted subsequent mortality in children with CM (p = 0.027).

CONCLUSIONS/SIGNIFICANCE

ANG-1 and the ANG-2/1 ratio are promising clinically informative biomarkers for CM. Additional studies should address their utility as prognostic biomarkers and potential therapeutic targets in severe malaria.

Bench-to-bedside review: Angiopoietin signalling in critical illness - a future target?

Multiple organ dysfunction syndrome (MODS) occurs in response to major insults such as sepsis, severe haemorrhage, trauma, major surgery and pancreatitis. The mortality rate is high despite intensive supportive care. The pathophysiological mechanism underlying MODS are not entirely clear, although several have been proposed. Overwhelming inflammation, immunoparesis, occult oxygen debt and other mechanisms have been investigated, and – despite many unanswered questions – therapies targeting these mechanisms have been developed. Unfortunately, only a few interventions, usually those targeting multiple mechanisms at the same time, have appeared to be beneficial. We clearly need to understand better the mechanisms that underlie MODS. The endothelium certainly plays an active role in MODS. It functions at the intersection of several systems, including inflammation, coagulation, haemodynamics, fluid and electrolyte balance, and cell migration. An important regulator of these systems is the angiopoietin/Tie2 signalling system. In this review we describe this signalling system, giving special attention to what is known about it in critically ill patients and its potential as a target for therapy.

Genetic variants in the angiopoietin-2 gene are associated with increased risk of ARDS

OBJECTIVE

Angiopoietin-2 (Ang-2) is a potent regulator of vascular permeability and inflammation in acute lung injury and acute respiratory distress syndrome (ARDS). Genetic variants in the Ang-2 gene may lead to altered activities of Ang-2 (or ANGPT2) gene. The aim of this study was to assess if genetic variants of Ang-2 are associated with the risk of ARDS.

DESIGN

Unmatched, case-control study nested within a prospectively enrolled cohort.

SETTING

Intensive care units (ICU) of an academic medical center.

PATIENTS

About 1,529 critically ill patients with risk factors for ARDS consecutively admitted to the ICUs from 1999 to 2006. Cases were 449 patients who developed ARDS and controls were 1,080 subjects who did not developed ARDS.

INTERVENTION

None.

MEASUREMENTS AND RESULTS

Nine tagging SNPs (tSNPs) spanning the entire Ang-2 gene were genotyped in all patients. The results were analyzed using logistic regression models, adjusting for covariates. The variant T allele of one tSNP (rs2515475) was significantly associated with increased risk of ARDS (OR(adjusted) = 1.28; P = 0.042). This association was stronger in subjects with extrapulmonary injuries (OR(adjusted) = 1.79; P = 0.004). Haplotype TT in block 2 containing the T allele of the rs2515475 was also significantly associated with higher risk of ARDS (OR(adjusted) = 1.42; P = 0.009), particularly in subjects with extrapulmonary injuries (OR(adjusted) = 1.90; P = 0.004).

CONCLUSION

Common genetic variation in the Ang-2 gene may be associated with increased risk of ARDS, especially among patients with extrapulmonary injuries.

Control of vascular morphogenesis and homeostasis through the angiopoietin-Tie system

Angiogenesis, the growth of blood vessels, is a fundamental biological process that controls embryonic development and is also involved in numerous life-threatening human diseases. Much work in the field of angiogenesis research has centred on the vascular endothelial growth factor (VEGF)-VEGF receptor system. The Tie receptors and their angiopoietin (Ang) ligands have been identified as the second vascular tissue-specific receptor Tyr kinase system. Ang-Tie signalling is essential during embryonic vessel assembly and maturation, and functions as a key regulator of adult vascular homeostasis. The structural characteristics and the spatio-temporal regulation of the expression of receptors and ligands provide unique insights into the functions of this vascular signalling system.

Angiopoietin 2 and cardiovascular disease in dialysis and kidney transplantation

BACKGROUND

Accelerated atherosclerosis in patients with chronic kidney disease (CKD) is still incompletely understood. Angiopoietin 1 (Ang-1) and Ang-2 are 55-kDa antagonistic nonredundant gatekeepers of endothelial activation and thus are potential important factors in accelerated atherosclerosis. We aimed to study: (1) angiopoietin levels in patients treated by means of dialysis and kidney transplantation, (2) the association of altered angiopoietin levels with atherosclerosis, and (3) changes in altered levels after renal transplantation.

STUDY DESIGN

Cross-sectional and longitudinal observational study.

SETTING & PARTICIPANTS

117 patients with CKD (61 hemodialysis [HD] patients, 24 peritoneal dialysis [PD] patients, and 32 renal transplant recipients) and 22 healthy controls.

PREDICTOR

Treatment by means of HD or PD or renal transplantation versus healthy controls.

OUTCOME

Serum Ang-1 and Ang-2 levels and ratio and changes in levels before and 3 months after transplantation. Correlations of angiopoietin levels with the presence and severity of coronary heart disease and peripheral arterial disease.

MEASUREMENTS

Ang-1 and Ang-2 were measured in sera by using an immunoradiometric sandwich assay and enzyme-linked immunosorbent assay, respectively. Coronary heart disease was scored by using coronary angiography, and peripheral arterial disease, by using ultrasonography.

RESULTS

Ang-1 level was decreased in HD patients compared with controls (29.1 +/- 12 versus 45.3 +/- 11.5 ng/mL; P < 0.001). In contrast, Ang-2 level was increased (HD, 8.7 +/- 0.64; PD, 6.48 +/- 8.1 ng/mL versus controls, 0.88 +/- 0.43 ng/mL; P < 0.001). Ang levels in renal transplant recipients were not different from healthy controls. Longitudinally, individual Ang-2 levels decreased after kidney transplantation (P = 0.01). In addition, in patients with CKD, Ang-2 level correlated significantly with scores of coronary heart disease (r = 0.486; P < 0.001) and peripheral arterial disease (r = 0.648; P < 0.001).

LIMITATIONS

Cross-sectional study design.

CONCLUSIONS

Circulating Ang-2 level was increased in patients treated with dialysis, although the mechanism is unknown. Kidney transplantation normalized circulating Ang-2 levels after 3 months. In addition, Ang-2 might be a mediator (and thus a marker) that accounts for accelerated atherosclerosis in dialysis patients.

Angiopoietin-2 levels are associated with disease progression in metastatic malignant melanoma

PURPOSE

The blood vessel-destabilizing Tie2 ligand angiopoietin-2 (Ang-2) acts in concert with the vascular endothelial growth factor/vascular endothelial growth factor receptor system to control vessel assembly during tumor progression. We hypothesized that circulating soluble Ang-2 (sAng-2) may be involved in melanoma progression.

EXPERIMENTAL DESIGN

Serum samples (n=98) from melanoma patients (American Joint Committee on Cancer stages I-IV), biopsies of corresponding patients, and human melanoma cell lines were analyzed for expression of Ang-2 and S100beta. Multiple sera of a subcohort of 33 patients were tested during progression from stage III to IV. Small interfering RNA-based loss-of-function experiments were done to assess effects of Ang-2 on melanoma cells.

RESULTS

Circulating levels of sAng-2 correlate with tumor progression in melanoma patients (P<0.0001) and patient survival (P=0.007). Analysis of serum samples during the transition from stage III to IV identified an increase of sAng-2 up to 400%. Comparative analyses revealed a 56% superiority of sAng-2 as predictive marker over the established marker S100beta. Immunohistochemistry and reverse transcription-PCR confirmed the prominent expression of Ang-2 by tumor-associated endothelial cells but identified Ang-2 also as a secreted product of melanoma cells themselves. Corresponding cellular experiments revealed that human melanoma-isolated tumor cells were Tie2 positive and that Ang-2 acted as an autocrine regulator of melanoma cell migration and invasion.

CONCLUSIONS

The experiments establish sAng-2 as a biomarker of melanoma progression and metastasis correlating with tumor load and overall survival. The identification of an autocrine angiopoietin/Tie loop controlling melanoma migration and invasion warrants further functional experiments and validate the angiopoietin/Tie system as a promising therapeutic target for human melanomas.

A designed angiopoietin-2 variant, pentameric COMP-Ang2, strongly activates Tie2 receptor and stimulates angiogenesis

Despite that angiopoietin-2 (Ang2) produces more versatile and dynamic functions than angiopoietin-1 (Ang1) in angiogenesis and inflammation, the molecular mechanism that underlies this difference is still unknown. To define the role of oligomerization of Ang2 in activation of its receptor, Tie2, we designed and generated different oligomeric forms of Ang2 by replacement of the amino-terminal domains of Ang2 with dimeric, tetrameric, and pentameric short coiled-coil domains derived from GCN4, matrillin-1, and COMP. COMP-Ang2 strongly binds and activates Tie2, whereas GCN4-Ang2 and MAT-Ang2 weakly to moderately bind and activate Tie2. Although native Ang2 strongly binds to Tie2, it does not activate Tie2. Accordingly, COMP-Ang2 strongly promotes endothelial cell survival, migration, and tube formation in a Tie2-dependent manner, and the potency of COMP-Ang2 is almost identical to that of COMP-Ang1. Furthermore, the potency of COMP-Ang2-induced enhanced angiogenesis in the wound healing region is almost identical to the potency of COMP-Ang1-induced enhanced angiogenesis. Overall, there is no obvious difference between COMP-Ang2 and COMP-Ang1 in in vitro and in vivo angiogenesis. Our results provide compelling evidence that proper oligomerization of Ang2 is a critical determinant of its binding and activation of Tie2.

Excess circulating angiopoietin-2 levels in sepsis: harbinger of death in the intensive care unit?

The early recognition and management of sepsis remain the greatest challenges in the field of critical care medicine. Endothelial injury is one of the hallmarks of sepsis, leading to capillary leak, microcirculatory dysfunction, organ failure, and eventual death in many critically ill patients. The angiogenic growth factors, angiopoietin (angpt)-1 and angpt-2, act upon the Tie-2 receptor in opposing roles. Angpt-2 has been found in abundance in septic patients when compared with healthy controls. In the study by Kümpers and colleagues in the previous issue of Critical Care, angpt-2 levels correlated with markers of tissue hypoxia, disease severity, and mortality in septic adults. However, the temporal kinetics of the angiopoietins were not assessed. It remains to be seen whether angpt-2 levels will function solely as an early marker of sepsis or whether the manipulation of the angpt/Tie-2 system will become a rational therapeutic target for the management of sepsis.

Circulating angiopoietin-2 is a marker and potential mediator of endothelial cell detachment in ANCA-associated vasculitis with renal involvement

BACKGROUND

The endothelial-specific angiopoietin (Ang)-Tie ligand-receptor system has been identified as a non-redundant regulator of endothelial cell detachment in vitro. Binding of circulating angiopoietin-2 (Ang-2) to the Tie2 receptor antagonizes Tie2 signalling and leads to disassembly of cell-cell junctions. Here, we ask whether circulating Ang-2 correlates with the severity of endothelial damage in ANCA-associated vasculitis (AAV) with renal involvement.

METHODS

Ang-2 was measured in sera obtained from 45 patients with AAV and 20 healthy controls by in-house ELISA. The disease activity was monitored by BVAS and the enumeration of circulating endothelial cells (CECs).

RESULTS

Ang-2 was significantly elevated in active AAV with renal involvement compared to controls and patients in remission. In contrast, Ang-2 was normal in patients with active granulomatous disease limited to the respiratory tract. Linear regression analysis demonstrated a strong association of Ang-2 with BVAS (r(s)(2) = 0.49 P < 0.0001) and the number of CECs (r(s)(2) = 0.48 P < 0.001). An Ang-2 cut-off value >4.15 ng/ml for a positive result yielded 100% specificity and 65% sensitivity for active systemic vasculitis. The positive predictive value was 99% and the negative predictive value 84%.

CONCLUSIONS

Circulating Ang-2 is elevated and closely correlates with BVAS and CEC numbers in AAV with renal involvement. These data indicate that Ang-2 might be a potential mediator of endothelial cell detachment in AAV.

Antiangiogenic activity of Tripterygium wilfordii and its terpenoids

ETHNOPHARMACOLOGICAL RELEVANCE

Tripterygium wilfordii Hook. f. (Celastraceae) has been traditionally used as folk medicine for centuries in China for the treatment of immune-inflammatory diseases.

AIM OF THE STUDY

This study aimed to assess the antiangiogenic activities which support the therapeutic use of Tripterygium wilfordii and its terpenoids for angiogenesis disease such as cancer.

MATERIALS AND METHODS

The ethanol extract of Tripterygium wilfordii and subsequent fractions were evaluated on an in vivo antiangiogenic zebrafish embryo model.

RESULTS

Three antiangiogenic terpenoids were isolated by bioassay-guided purification, namely, celastrol (4), cangoronine (5) and triptolide (7). Among them, triptolide manifested the most potent antiangiogenic activity against vessel formation by nearly 50% at 1.2 microM. Semi-quantitative RT-PCR analysis revealed that triptolide dose- and time-dependently reduced the mRNA expression of angiopoietin (angpt)2 and tie2 in zebrafish, indicating the involvement of angpt2/tie2 signaling pathway in the antiangiogenic action of triptolide.

CONCLUSIONS

The discovery of an alternative pathway further confirms the value of ethnopharmacological investigations into traditional botanicals for leads for potential drug development.

Thyroid hormone-induced angiogenesis

A series of reports in the past decade have ascribed pro-angiogenic activity to several thyroid hormone analogues, including L-thyroxine (T(4)), 3,5,3-triiodo-L-thyronine (T(3)) and diiodothyropropionic acid (DITPA). Model systems of angiogenesis have demonstrated that thyroid hormone-induced neovascularization is initiated at a cell surface receptor for the hormone on an integrin. The hormone signal is transduced within the cell by extracellular regulated kinase 1/2 (ERK1/2) into secretion of basic fibroblast growth factor (bFGF) and other vascular growth factors and consequent angiogenesis. Intact animal studies have shown that endogenous thyroid hormone supports blood vessel density in heart and brain and that thyroid hormone administration can induce angiogenesis in ischemic limbs.

Angiogenesis in gliomas

Angiogenesis, the sprouting of new blood vessels from preexisting blood vessels, is a hallmark of glioma progression. Malignant gliomas are among the most lethal tumors with a very dismal prognosis, despite advances in standard therapy, including surgery, radiation, and chemotherapy. The median survival of patients with malignant gliomas has changed little in the last few years and is still measured in months. In an attempt to develop new therapeutic strategies and identify the molecular mechanism involved in glioma growth and progression, there has been extraordinary scientific interest in the past 2 decades in angiogenic responses associated with gliomas. This chapter focuses on the molecular mechanism of glioma angiogenesis and summarizes some of the therapeutic approaches based on antiangiogenesis.

Angiopoietin-2 exocytosis is stimulated by sphingosine-1-phosphate in human blood and lymphatic endothelial cells

OBJECTIVE

Although diverse functions of angiopoietin-2 (Ang2) have been revealed, little is known about upstream signaling molecules regulating Ang2 exocytosis. We therefore investigated the mechanism of Ang2 exocytosis in human blood and lymphatic endothelial cells (BECs and LECs) by stimulation with sphingosine-1-phosphate (S1P).

METHODS AND RESULTS

By immunostaining and ELISA analyses using our newly developed human Ang2-specific antibodies, Ang2 exocytosis from human endothelial cells was examined. Both exogenous and endogenous S1P trigger rapid Ang2 exocytosis in time- and dose-dependent manners. Intriguingly, S1P-induced Ang2 exocytosis is higher in LECs than BECs. These effects of S1P are mainly mediated by the endothelial differentiation gene receptor 1, which subsequently activates its downstream phospholipase C and intracellular calcium mobilization to trigger Ang2 exocytosis. Consistently, S1P also dramatically stimulates Ang2 exocytosis from the ECs of ex vivo-incubated blood vessels.

CONCLUSION

These results imply that the rapid secretion of Ang2 by exocytosis from endothelial cells is another possible mechanism underlying S1P-induced angiogenesis and inflammation.

Integrin and growth factor receptor alliance in angiogenesis

A sequence of events in vascular and stromal cells maintained in a highly coordinated manner regulates angiogenesis and tissue remodeling. These processes are mediated by the ability of cells to respond to environmental cues and activate surface integrins. Physiological and pathological processes in vascular biology are dependent on the specificity of important signaling mechanisms that are activated through the association between growth factors, their receptors, integrins, and their specific extracellular matrix ligands. A large body of evidence from in vitro and in vivo models demonstrates the importance of coordination of signals from the extracellular environment that activates specific tyrosine kinase receptors and integrins in order to regulate angiogenic processes in vivo. In addition to complex formation between growth factor receptors and integrins, growth factors and cytokines also directly interact with integrins, depending upon their concentration levels in the environment, and differentially regulate integrin-related processes. Recent studies from a number of laboratories including ours have provided important novel insights into the involvement of many signaling events that improve our existing knowledge on the cross-talk between growth factor receptors and integrins in the regulation of angiogenesis. In this review, our focus will be on updating the recent developments in the field of integrin-growth factor receptor associations and their implications in the vascular processes.

Excess circulating angiopoietin-2 is a strong predictor of mortality in critically ill medical patients

Introduction

The endothelial specific angiopoietin (Ang)-Tie2 ligand-receptor system has been identified as a non-redundant mediator of endothelial activation in experimental sepsis. Binding of circulating Ang-1 to the Tie2 receptor protects the vasculature from inflammation and leakage, whereas binding of Ang-2 antagonises Tie2 signalling and disrupts endothelial barrier function. Here, we examine whether circulating Ang-1 and/or Ang-2 independently predict mortality in a cohort of critically ill medical patients.

Methods

Circulating vascular endothelial growth factor (VEGF), Ang-1 and Ang-2 were prospectively measured in sera from 29 healthy controls and 43 medical ICU patients by immunoradiometric assay (IRMA) and ELISA, respectively. Survival after 30 days was the primary outcome studied.

Results

Median serum Ang-2 concentrations were increasingly higher across the following groups: healthy controls, patients without sepsis, patients with sepsis and patients with septic shock. In contrast, Ang-1 and VEGF concentrations were significantly lower in all patient groups compared with healthy controls. Ang-2 correlated with partial pressure of oxygen in arterial blood (PaO₂)/fraction of inspired oxygen (FiO₂), tissue hypoxia, Sequential Organ Failure Assessment (SOFA) and Physiology and Chronic Health Evaluation II (APACHE II) score. Multivariate Cox regression analyses confirmed a strong independent prognostic impact of high Ang-2 as a novel marker of 30-day survival.

Conclusions

A marked imbalance of the Ang-Tie system in favour of Ang-2 is present in critically ill medical patients. Our findings highlight the independent prognostic impact of circulating Ang-2 in critical illness. Ang-2 may be used as a readily available powerful predictor of outcome and may open new perspectives to individualise treatment in the ICU.

Rod-Shaped monocytes patrol the brain vasculature and give rise to perivascular macrophages under the influence of proinflammatory cytokines and angiopoietin-2

The nervous system is constantly infiltrated by blood-derived sentinels known as perivascular macrophages. Their immediate precursors have not yet been identified in situ and the mechanism that governs their recruitment is mostly unknown. Here, we provide evidence that CD68(+)GR1(-) monocytes can give rise to perivascular macrophages in mice suffering from endotoxemia. After adhesion to the endothelium, these monocytes start to crawl, adopt a rod-shaped morphology when passing through capillaries, and can manifest the ability to proliferate and form a long cytoplasmic protuberance. They are attracted in greater numbers during endotoxemia by a combination of vasoregulatory molecules, including TNF (tumor necrosis factor), interleukin-1beta, and angiopoietin-2. After a period of several hours, some of them cross the endothelium to expand the population of perivascular macrophages. Depletion of adherent monocytes and perivascular macrophages can be achieved by injection of anti-angiopoietin-2 peptide-Fc fusion protein. This study extends our understanding of the behavior of monocytes at the blood-brain interface and provides a way to block their infiltration into the nervous tissue under inflammatory conditions.

Angiopoietin-2 is associated with decreased endothelial nitric oxide and poor clinical outcome in severe falciparum malaria

Adherence of parasitized erythrocytes to activated endothelium causes microvascular obstruction, tissue ischemia, and clinical complications in severe malaria (SM); however, the mechanisms leading to endothelial activation remain unclear. The angiogenic factors, angiopoietin-2 (Ang-2) and vascular endothelial growth factor (VEGF) are modulators of endothelial activation, with Ang-2 release from Weibel-Palade bodies (WPBs) being regulated by endothelial nitric oxide (NO). We explored the relationships between endothelial NO bioavailability, Ang-2, VEGF, tissue perfusion, and clinical outcomes in SM. We measured plasma Ang-2 and VEGF, together with biomarkers of severity from 146 adults with and without SM, in parallel with longitudinal measures of endothelial function by using reactive hyperemia peripheral arterial tonometry (a measure of endothelial NO bioavailability). Regression was used to relate concentrations of Ang-2/VEGF with malaria disease severity, biomarkers of perfusion, endothelial activation, and parasite biomass. The longitudinal relationship between Ang-2 and endothelial function was assessed by using a mixed-effects model. Ang-2 concentrations were elevated in SM and associated with increased venous lactate, plasma intercellular cell adhesion molecule-1 concentrations, parasite biomass, and mortality. In contrast, VEGF concentrations were inversely associated with these biomarkers. Ang-2 concentrations were significantly better predictors of death than venous lactate (P = 0.03). Recovery of endothelial function was associated with falling concentrations of Ang-2. Ang-2 release from endothelial cells with reduced NO bioavailability is likely to contribute to endothelial activation, sequestered parasite biomass, impaired perfusion, and poor outcome in severe falciparum malaria. Agents that improve endothelial NO, reduce WPB exocytosis, and/or antagonize Ang-2 may have therapeutic roles in SM.

The angiopoietin-2 gene of endothelial cells is up-regulated in hypoxia by a HIF binding site located in its first intron and by the central factors GATA-2 and Ets-1

Angiopoietins are ligands of the endothelial cell tyrosine kinase receptor Tie2. Angiopoietin-1 (Ang-1) is widely expressed in human normal adult tissues and promotes blood vessel maturation and stabilization by inducing Tie2 receptor phosphorylation. In contrast, the antagonistic ligand Angiopoietin-2 (Ang-2) is up-regulated by hypoxia, expressed only at sites of vascular remodeling and plays a crucial role in destabilizing vessels for normal or pathological angiogenesis. Ang-2 expression is tightly regulated at transcriptional and post-transcriptional levels. To characterize the regulatory sequences of the human Ang-2 gene we cloned a fragment of around 8.5 kb upstream of the Ang-2 coding sequence and analyzed the luciferase reporter activity of constructs of various lengths in endothelial and non-endothelial cells. We isolated a minimal promoter sequence sufficient to promote significant Ang-2 non-cell type specific transcription. Moreover, we identified sequences conferring endothelial specificity. Indeed, sequence analysis of the fragment revealed the presence of several potential binding sites for specific endothelial regulatory factors like GATA or Ets. Using GATA-2 and Ets-1 co-transfection and overexpression assay, we showed that these two factors are able to induce Ang-2 promoter activation. We also show that hypoxic regulation of Ang-2 is HIF-dependent and demonstrate that HIF-1alpha binds in human microvascular endothelial cells (HMVEC) to an evolutionary conserved Hypoxia-Responsive Element (HRE) located in the first intron of the Ang-2 gene. In conclusion, our study provides new elements in favor of HIF involvement in Ang-2 hypoxic regulation and identifies Ets-1 and particularly GATA-2 as central factors in endothelial specific Ang-2 expression.

Platelet granule secretion continuously prevents intratumor hemorrhage

Cancer is associated with a prothrombogenic state capable of platelet activation. Platelets, on the other hand, can support angiogenesis, a process involved in the progression of tumor growth and metastasis. However, it is unclear whether platelet/tumor interactions substantially contribute to tumor physiology. We investigated whether platelets stabilize tumor vessels and studied the underlying mechanisms. We induced severe acute thrombocytopenia in mice bearing s.c. Lewis lung carcinoma or B16F10 melanoma. Intravital microscopy revealed that platelet depletion led to a rapid destabilization of tumor vessels with intratumor hemorrhage starting as soon as 30 min after induction of thrombocytopenia. Using an inhibitor of glycoprotein Ibalpha (GPIbalpha) and genetically engineered mice with platelet adhesion defects, we investigated the role of platelet adhesion receptors in stabilizing tumor vessels. We found that a single defect in either GPIbalpha, von Willebrand factor, P-selectin, or platelet integrin activation did not lead to intratumor hemorrhage. We then compared the ability of transfused resting and degranulated platelets to prevent intratumor hemorrhage. Whereas resting platelets prevented thrombocytopenia-induced tumor bleeding, circulating degranulated platelets did not. This suggests that the prevention of intratumor hemorrhage by platelets relies on the secretion of the content of platelet granules. Supporting this hypothesis, we further found that thrombocytopenia dramatically impairs the balance between propermeability and antipermeability factors in tumor-bearing animals, in particular depleting blood of angiopoietin-1 and serotonin. Our results show a crucial contribution of platelets to tumor homeostasis through continuous prevention of severe intratumor hemorrhage and consequent cell death. The study also suggests platelet function as a reasonable target for specific destabilization of tumor vessels.

Circulating angiopoietin 2 correlates with mortality in a surgical population with acute lung injury/adult respiratory distress syndrome

There are few blood biomarkers predictive of mortality in adult respiratory distress syndrome (ARDS), and none that currently serve as therapeutic targets. Here, we ask whether a circulating protein angiopoietin 2 (Ang2) correlates with severity of lung injury and mortality in a surgical intensive care unit cohort with acute lung injury (ALI)/ARDS. Tie 2 is a tyrosine kinase receptor expressed on endothelial cells. One ligand, angiopoietin 1, phosphorylates Tie 2 and stabilizes adult vasculature. An alternate ligand, Ang2, serves as a context-dependent antagonist and disrupts barrier function. Previously, our laboratory detected high circulating Ang2 levels in septic patients and a correlation with low Pa(O2)/F(IO2). In this study, daily plasma was collected in 63 surgical intensive care unit patients. Eighteen patients met clinical criteria for ALI or ARDS. The median Ang2 at admission in patients who never developed ALI/ARDS was 3.7 ng/mL (interquartile range [IQR], 5.6; n = 45). The Ang2 on the day a patient met criteria for ALI/ARDS was 5.3 ng/mL (IQR, 6.7) for survivors (n = 11) and 19.8 ng/mL (IQR, 19.2) for nonsurvivors (n = 7; P= 0.004). To explore the mechanism of high Ang 2 leading to increased permeability, plasma from patients with ALI was applied to cultured lung endothelial cells and found to disrupt normal junctional architecture. This effect can be rescued with the Tie 2 agonist angiopoietin 1. A patient's convalescent (low Ang2) plasma did not disrupt junctional architecture. Although further studies with larger sample sizes will be needed to confirm these results, high Ang2 in critically ill patients with ALI/ARDS is associated with a poor outcome. These data, coupled with our cell culture experiments, suggest that antagonism of Ang2 may provide a future novel therapeutic target for ARDS.

Angiopoietin-2 predicts disease-free survival after allogeneic stem cell transplantation in patients with high-risk myeloid malignancies

Emerging data suggest a critical role for bone marrow angiogenesis in hematologic malignancies. The angiopoietin/Tie ligand-receptor system is an essential regulator of this process. We evaluated whether circulating angiopoietin-2 (Ang-2) is a predictor for the probability of disease-free survival (DFS) in allogeneic hematopoietic stem cell transplantation (allo-HSCT) for high-risk acute myeloid leukemia or myelodysplastic syndrome. Ang-2 was measured by enzyme-linked immunosorbent assay in serum from 20 healthy controls and 90 patients with acute myeloid leukemia or myelodysplastic syndrome before conditioning for HSCT. Circulating Ang-2 was elevated in patients (median, 2.21 ng/mL; range, 0.18-48.84 ng/mL) compared with controls (median, 0.87 ng/mL; range, 0.27-4.51 ng/mL; P < .001). Multivariate analyses confirmed the independent prognostic impact of Ang-2 (hazard ratio [HR] = 2.46; 95% confidence interval [CI], 1.27-4.76, P = .005), percentage of bone marrow infiltration (HR = 1.14; 95% CI, 1.01-1.29, P = .033), and chemotherapy cycles before HSCT (HR = 1.38; 95% CI, 1.01-1.08, P = .048). Regression tree analysis detected optimal cutoff values for Ang-2 and recursively identified bone marrow blasts and Ang-2 as the best predictors for DFS. Because few predictors for DFS exist in the setting of allo-HSCT, Ang-2 may be used as a readily available powerful biomarker to pre-estimate DFS and may open new perspectives for risk-adapted treatment of high-risk myeloid malignancies.

Circulating angiopoietin-1 and angiopoietin-2 in critically ill patients: development and clinical application of two new immunoassays

Introduction

In critically ill patients, the massive release of angiopoietin-2 (Ang-2) from endothelial Weibel–Palade bodies interferes with constitutive angiopoietin-1 (Ang-1)/Tie2 signaling in endothelial cells, thus leading to vascular barrier breakdown followed by leukocyte transmigration and capillary leakage. The use of circulating Ang-1 and Ang-2 as novel biomarkers of endothelial integrity has therefore gained much attention. The preclinical characteristics and clinical applicability of angiopoietin immunoassays, however, remain elusive.

Methods

We developed sandwich immunoassays for human Ang-1 (immunoradiometric sandwich assay/immunoluminometric sandwich assay) and Ang-2 (ELISA), assessed preanalytic characteristics, and determined circulating Ang-1 and Ang-2 concentrations in 30 healthy control individuals and in 94 critically ill patients. In addition, Ang-1 and Ang-2 concentrations were measured in 10 patients during a 24-hour time course with respect to interference by intravenous antibiotic treatment and by extended daily dialysis.

Results

The assays had detection limits of 0.12 ng/ml (Ang-1) and 0.2 ng/ml (Ang-2). Inter-assay and intra-assay imprecision was ≤8.8% and 3.7% for Ang-1 and was ≤4.6% and 5.2% for Ang-2, respectively. Angiopoietins were stable for 24 hours and were resistant to four freeze–thaw cycles. Angiopoietin concentrations were not associated with age, body mass index or renal function in healthy individuals. Ang-1 and Ang-2 concentrations correlated with severity of illness in critically ill patients. Angiopoietin concentrations were not influenced by antibiotic treatment or by extended daily dialysis.

Conclusion

Ang-1 and Ang-2 might serve as a novel class of biomarker in critically ill patients. According to preclinical and clinical validation, circulating Ang-1 and Ang-2 can be reliably assessed by novel immunoassays in the intensive care unit setting.

The periphery of the human fetal adrenal gland is a site of angiogenesis: zonal differential expression and regulation of angiogenic factors

CONTEXT

Although the inner fetal zone (FZ) of the mid-gestation human fetal adrenal (HFA) produces dehydroepiandrosterone sulfate, the function of the outer definitive zone (DZ) remains less clear. We have proposed that the DZ phenotype is that of a pool of progenitor cells, many of which are mitotically active. Recently, we studied HFA expression of a family of vascular endothelial cell-specific angiogenic factors, the angiopoietins (Angs), and demonstrated that Ang2 was localized predominantly in the periphery of the gland. Ang1 stabilizes, whereas Ang2 destabilizes, vessels, increasing responsiveness to angiogenic stimuli such as vascular endothelial growth factor (VEGF)-A and fibroblast growth factor (FGF)-2.

OBJECTIVE

Our objective was to test the hypothesis that the periphery of the HFA is a site of angiogenesis.

DESIGN

Studies were conducted involving RNA, frozen sections, and primary cell cultures from midgestation HFAs.

MAIN OUTCOME MEASURES

Immunofluorescence, laser capture microdissection, and real-time quantitative RT-PCR were used.

RESULTS

Double immunostaining demonstrated that proliferating endothelial cells were limited to the DZ and DZ/FZ border. Ang2 mRNA was primarily expressed in the DZ, whereas Ang1 mRNA was primarily in the FZ. VEGF-A and FGF-2 mRNA levels were higher in the DZ. FGF-2 (10 ng/ml) induced Ang2 mRNA by 4-fold in both zones of cells (P < 0.01, at 24 h), but not Ang1 or VEGF-A mRNA.

CONCLUSION

Data suggest that angiogenesis occurs at the periphery of the HFA. The DZ-predominant expression of Ang2 may be explained, in part, by the parallel pattern of FGF-2 expression.

Angiopoietin-1 mediates the proangiogenic activity of the bone morphogenic protein antagonist Drm

Recent observations have shown that Drm, a member the Dan family of bone morphogenic protein (BMP) antagonists, induces endothelial cell (EC) sprouting in vitro and angiogenesis in vivo by interacting with signaling EC receptors in a BMP-independent manner. Here, recombinant Drm (rDrm) up-regulates angiopoientin-1 (Ang-1) expression in EC without affecting Ang-2 and Tie-2 receptor expression. Ang-1 up-regulation is mediated by the activation of the transcription factor NF-kappaB. Specific inhibition of Ang-1 activity by anti-Ang-1 antibodies, soluble Tie-2 receptor, or Ang-1 siRNA transfection significantly reduced the rDrm-mediated sprouting of EC in three-dimensional fibrin and type I collagen gels. In addition, Ang-1 antagonists inhibited the angiogenic activity exerted by rDrm in the chick embryo chorioallantoic membrane. Taken together, the data indicate that the proangiogenic activity of Drm is mediated by the activation of an Ang-1-dependent autocrine loop of stimulation in EC.

Adipose tissue gene expression profiles in ob/ob mice treated with leptin

Leptin plays a critical role in regulating body weight, lipid metabolism, apoptosis and microvasculature of adipose tissue. To explore multiple signaling pathways of leptin action on adipose tissue, real-time PCR utilizing TaqMan low-density arrays was performed to compare mRNA expression in adipose tissue of ob/ob mice treated with vehicle or leptin (2.5 microg/d or 10 microg/d) for 14 days via subcutaneous osmotic minipumps. Of the 24 target genes selected for characterization, many were differentially expressed between control ob/ob mice and leptin-treated ob/ob mice. Increases in mRNA expression were found for hormone sensitive lipase (HSL), uncoupling protein 2 (UCP2), adrenergic receptor 3 (ADR3), mitofusin 2 (Mfn2), sirtuin 3 (Sirt3), transcription factor sterol regulatory element binding factor 1 (SREBF1), Bcl-2, Bax, Caspase 3, tumor necrosis factor alpha (TNFalpha), adiponectin and angiopoietin 2 (Ang-2). Decreases in expression were found for stearoyl-coenzyme A desaturase 1 (SCD1), fatty acid synthase (FAS), and retinol binding protein 4 (RBP4). There were no changes in expression of transcription factors involved in adipocyte differentiation (C/EBPalpha, PPARalpha, and PPARgamma). These results confirm that alterations in the expression of specific adipose tissue genes including those associated with the promotion of lipid mobilization, energy dissipation, and apoptosis may mediate leptin-induced fat loss in ob/ob mice.

Ethanol stimulates endothelial cell angiogenic activity via a Notch- and angiopoietin-1-dependent pathway

AIMS

Our aims were to determine the effect of alcohol (EtOH) on endothelial angiogenic activity and to delineate the cell signalling mechanisms involved.

METHODS AND RESULTS

Treatment of human umbilical vein endothelial cells (HUVECs) with EtOH (1-100 mM, 24 h) dose-dependently increased their network formation on Matrigel (an index of angiogenesis) with a maximum response (2.5- to 3-fold increase) at 25 mM. Ethanol also stimulated the proliferation (by cell count and proliferating cell nuclear antigen expression) and migration (by scratch wound assay) of HUVECs. In parallel cultures, EtOH stimulated Notch receptor (1 and 4) and Notch target gene (hrt-1, -2, and -3) mRNA and protein expression and enhanced CBF-1/RBP-Jk promoter activity. EtOH also stimulated, at the mRNA and protein level, the expression of angiopoietin-1 (Ang1) and its Tie2 receptor in these cells. Knockdown of Notch 1 or 4 by siRNA or inhibition of Notch-mediated, CBF-1/RBP-Jk-regulated gene expression by the Epstein-Barr virus-encoded protein RPMS-1 inhibited both ethanol-induced Ang1/Tie2 expression in HUVECs and their network formation on Matrigel. Moreover, knockdown of Ang1 or Tie2 by siRNA inhibited ethanol-induced endothelial network formation.

CONCLUSION

These data demonstrate that ethanol, at levels consistent with moderate consumption, enhances endothelial angiogenic activity in vitro by stimulating a novel Notch/CBF-1/RBP-JK-Ang1/Tie2-dependent pathway. These actions of ethanol may be relevant to the cardiovascular effects of alcohol consumption purported by epidemiological studies.

Tie2-R849W mutant in venous malformations chronically activates a functional STAT1 to modulate gene expression

Tie2 is an endothelial receptor tyrosine kinase. An amino-acid substitution of tryptophan for arginine at residue 849 (Tie2-R849W) leads to a ligand-independent activation of its kinase activity. This mutation has been associated with familial venous malformations (VMs), manifested by variable thickness or lack of smooth-muscle cells in the veins of patient lesions. The underlying mechanism for Tie2-R849W action in endothelial cells remains elusive. In this study, we used adenoviral infection to differentiate the effects of ectopic Tie2 (wild type, kinase-dead K855A, or constitutively active R849W) expression on endothelial cellular behaviors and Tie2-mediated downstream targets. Ectopic Tie2 reduced endothelial cell proliferation and serum withdrawal-induced apoptosis, while stimulating migration. When comparing R849W with K855A and its wild-type counterpart, a functional tyrosine kinase activity was required only for migration, and constitutively active Tie2-R849W conferred highest resistance to serum-induced apoptosis, but lowest ability to maintain tube-like structures formed on Matrigel. We further demonstrated that Tie2-R849W chronically induced STAT1 tyrosine phosphorylation and the promoter activity of STAT1-responsive IFN-regulatory factor 1 (IRF1). Although STAT1 phosphorylation required JNK and p38MAPK activation, only JNK activation was essential for IRF1 promoter activation by Tie2-R849W. Additional studies are needed to study the role of STAT1 activation in VMs.

Angiopoietin-2, marker and mediator of endothelial activation with prognostic significance early after trauma?

OBJECTIVE

To measure plasma levels of angiopoietin-1 (Ang-1), angiopoietin-2 (Ang-2), and vascular endothelial growth factor (VEGF) early after trauma and to determine their clinical significance.

BACKGROUND

Angiopoietins and VEGF play a central role in the physiology and pathophysiology of endothelial cells. Ang-2 has recently been shown to have pathogenetic significance in sepsis and acute lung injury. Little is known about the role of angiopoietins and VEGF early after trauma.

METHODS

Blood specimens from consecutive major trauma patients were obtained immediately upon arrival in the emergency department and plasma samples assayed for Ang-1, Ang-2, VEGF, markers of endothelial activation, protein C pathway, fibrinolytic system, and complement. Base deficit was used as a measure of tissue hypoperfusion. Data were collected prospectively.

RESULTS

Blood samples were obtained from 208 adult trauma patients within 30 minutes after injury before any significant fluid resuscitation. Plasma levels of Ang-2, but not Ang-1 and VEGF were increased and correlated independently with severity of injury and tissue hypoperfusion. Furthermore, plasma levels of Ang-2 correlated with markers of endothelial activation, coagulation abnormalities, and activation of the complement cascade and were associated with worse clinical outcome.

CONCLUSIONS

Ang-2 is released early after trauma with the degree proportional to both injury severity and systemic hypoperfusion. High levels of Ang-2 were associated with an activated endothelium, coagulation abnormalities, complement activation, and worse clinical outcome. These data indicate that Ang-2 is a marker and possibly a direct mediator of endothelial activation and dysfunction after severe trauma.

Inflammation induces hemorrhage in thrombocytopenia

The role of platelets in hemostasis is to produce a plug to arrest bleeding. During thrombocytopenia, spontaneous bleeding is seen in some patients but not in others; the reason for this is unknown. Here, we subjected thrombocytopenic mice to models of dermatitis, stroke, and lung inflammation. The mice showed massive hemorrhage that was limited to the area of inflammation and was not observed in uninflamed thrombocytopenic mice. Endotoxin-induced lung inflammation during thrombocytopenia triggered substantial intra-alveolar hemorrhage leading to profound anemia and respiratory distress. By imaging the cutaneous Arthus reaction through a skin window, we observed in real time the loss of vascular integrity and the kinetics of skin hemorrhage in thrombocytopenic mice. Bleeding-observed mostly from venules-occurred as early as 20 minutes after challenge, pointing to a continuous need for platelets to maintain vascular integrity in inflamed microcirculation. Inflammatory hemorrhage was not seen in genetically engineered mice lacking major platelet adhesion receptors or their activators (alphaIIbbeta3, glycoprotein Ibalpha [GPIbalpha], GPVI, and calcium and diacylglycerol-regulated guanine nucleotide exchange factor I [CalDAG-GEFI]), thus indicating that firm platelet adhesion was not necessary for their supporting role. While platelets were previously shown to promote endothelial activation and recruitment of inflammatory cells, they also appear indispensable to maintain vascular integrity in inflamed tissue. Based on our observations, we propose that inflammation may cause life-threatening hemorrhage during thrombocytopenia.

Angiopoietin 2 concentrations in infants developing bronchopulmonary dysplasia: attenuation by dexamethasone

OBJECTIVES

To study the association between angiopoietin 2 (Ang2) concentrations in tracheal aspirates (TAs) and adverse outcome (bronchopulmonary dysplasia (BPD)/death) in ventilated premature infants (VPIs) and modulation of Ang2 concentrations with dexamethasone (Dex) use.

STUDY DESIGN

Serial TA samples were collected on days 1, 3, 5 and 7, and Ang2 concentrations were measured. Ang2 TA concentrations were compared prior to and after 48 to 72 h of using Dex.

RESULT

A total of 151 TA samples were collected from 60 VPIs. BPD was defined as the oxygen requirement at 36 weeks postmenstrual age (PMA). Twelve infants (mean+/-s.d.) (gestational age (GA) 26.5+/-2.1 weeks, birth weight (BW) 913+/-230 g) had no BPD, 32 infants (GA 25.8+/-1.4 weeks, BW 768+/-157 g) developed BPD and 16 infants (GA 24.5+/-1.1 weeks, BW 710+/-143 g) died before 36 weeks PMA. Ang2 concentrations were significantly lower in infants with no BPD (median, 25th and 75th percentile) (157, 16 and 218 pg mg(-1)) compared with those who developed BPD (234, 138 and 338 pg mg(-1), P=0.03) or BPD and/or death (234, 157 and 347 pg mg(-1), P=0.017), in the first week of life. Twenty-six VPIs (BW 719+/-136 g, GA 25.1+/-1.3 weeks) received 27 courses of Dex. Ang2 concentrations before starting Dex were 202, 137 and 278 pg mg(-1) and significantly decreased to 144, 0 and 224 pg mg(-1) after therapy (P=0.007).

CONCLUSIONS

Higher Ang2 concentrations in TAs are associated with the development of BPD or death in VPIs. Dex use suppressed Ang2 concentrations.

Balance between angiopoietin-1 and angiopoietin-2 is in favor of angiopoietin-2 in atherosclerotic plaques with high microvessel density

INTRODUCTION

Atherosclerotic plaque microvessels are associated with plaque hemorrhage and rupture. The mechanisms underlying plaque angiogenesis are largely unknown. Angiopoietin (Ang)-1 and -2 are ligands of the endothelial receptor Tie-2. Ang-1 induces formation of stable vessels, whereas Ang-2 destabilizes the interaction between endothelial cells and their support cells. We studied the expression patterns of Ang-1 and -2 in relation to plaque microvessels.

METHODS AND RESULTS

Carotid endarterectomy specimens were studied (n = 100). Microvessel density (MVD) was correlated with the presence of macrophages and with a (fibro)atheromatous plaque phenotype. A negative correlation was observed between Ang-1 expression and MVD. A positive correlation was observed between the ratio of Ang-2/Ang-1 and MVD. Ang-2 expression was correlated with matrix metalloproteinase-2 (MMP-2) activity. Immunohistochemical staining of Ang-1 was observed in smooth muscle cells, whereas Ang-2 was detected in endothelial cells, smooth muscle cells and macrophages.

CONCLUSIONS

In plaques with high MVD, the local balance between Ang-1 and Ang-2 is in favor of Ang-2. Plaque Ang-2 levels are associated with MMP-2 activity. Ang-2-induced MMP-2 activity might play a role in the development of (unstable) plaque microvessels.

Angiogenic profiling and comparison of immortalized endothelial cells for functional genomics

Genomics efforts of the past decade have resulted in the identification of numerous genes with putative roles in disease processes, including tumor angiogenesis. To functionally validate these genes, cultured endothelial cells are indispensable tools, though these may not completely mimic the phenotype of tissue endothelial cells as the proper microenvironment is lacking. To obtain experimental data representative of normal physiology, the use of primary endothelial cells is preferred. However, these cells are usually limited in passage number, can be difficult to obtain and show great interindividual variety. Furthermore, transfection efficiency is very limited in primary cells, hampering applications in functional genomics and gene function analysis. The use of properly characterized alternative endothelial cell sources is therefore warranted. Here, we compared immortalized endothelial cells - HMEC, RF24 and EVLC2 - with primary HUVEC. We show that RF24, and to a slightly lesser extent HMEC, resembles primary HUVEC most on all facets examined. RF24, in contrast to EVLC2, express the endothelial markers CD31, CD34, CD105, vWF and VE-cadherin, and are capable of migration and tube formation in vitro. Furthermore, the expression levels of angiogenic growth factors and their receptors are comparable to that of primary EC. In addition, whereas primary HUVEC are resistant to transfection using common lipophilic transfection reagents, HMEC and RF24 could be readily transfected. Hence, these cells pose a valuable tool for functional genomics in angiogenesis research.

Evolving functions of endothelial cells in inflammation

Inflammation is usually analysed from the perspective of tissue-infiltrating leukocytes. Microvascular endothelial cells at a site of inflammation are both active participants in and regulators of inflammatory processes. The properties of endothelial cells change during the transition from acute to chronic inflammation and during the transition from innate to adaptive immunity. Mediators that act on endothelial cells also act on leukocytes and vice versa. Consequently, many anti-inflammatory therapies influence the behaviour of endothelial cells and vascular therapeutics influence inflammation. This Review describes the functions performed by endothelial cells at each stage of the inflammatory process, emphasizing the principal mediators and signalling pathways involved and the therapeutic implications.

Tie2-expressing monocytes: regulation of tumor angiogenesis and therapeutic implications

Tumor-infiltrating myeloid cells are involved in crucial processes during tumor development. A subset of monocytes that express the angiopoietin receptor Tie2 play an important role in tumor angiogenesis. Selective depletion of these Tie2-expressing monocytes (TEMs) in tumor-bearing mice inhibits tumor angiogenesis and growth, suggesting that they might regulate angiogenic processes in tumors by providing paracrine support to nascent blood vessels. TEMs have also been identified in human blood and tumors. We discuss here the therapeutic opportunities emanating from the discovery of TEMs, which include the identification of new antitumor targets, monitoring TEMs as surrogate markers for clinical responses in cancer patients, and the possible use of TEMs as cellular vehicles for gene delivery to tumors.

Angiogenesis and chronic inflammation: cause or consequence?

Evidence has been gathered regarding the association between angiogenesis and inflammation in pathological situations. These two phenomena have long been coupled together in many chronic inflammatory disorders with distinct etiopathogenic origin, including psoriasis, rheumatoid arthritis, Crohn's disease, diabetes, and cancer. Lately, this concept has further been substantiated by the finding that several previously established non-inflammatory disorders, such as osteoarthritis and obesity, display both inflammation and angiogenesis in an exacerbated manner. In addition, the interplay between inflammatory cells, endothelial cells and fibroblasts in chronic inflammation sites, together with the fact that inflammation and angiogenesis can actually be triggered by the same molecular events, further strengthen this association. Therefore, elucidating the underlying cellular and molecular mechanisms that gather together the two processes is mandatory in order to understand their synergistic effect, and to develop new therapeutic approaches for the management of these disorders that cause a great deal of discomfort, disability, and in some cases death.

Systemic and intracardiac vascular endothelial growth factor and angiopoietin-1 and -2 levels in coronary artery disease: effects of angioplasty

BACKGROUND

Vascular growth factors are involved in the pathophysiology of human atherosclerotic vascular disease and plaque destabilization. We hypothesized that in stable patients with coronary artery disease (CAD), plasma levels of vascular endothelial growth factor (VEGF) and angiopoietins 1 and 2 (as indices of angiogenesis) would be no higher in coronary sinus blood when compared to the aortic root, coronary ostium, and peripheral femoral vein. Secondly, we hypothesized that percutaneous coronary intervention (PCI; angioplasty+/-stenting) would increase intracardiac levels of these indices, perhaps by destabilizing coronary plaques.

METHODS

Patients undergoing elective diagnostic coronary angiography (n = 70; mean age 58.8+/-11.2 years) of which 37 proceeded to PCI were recruited. Blood samples were obtained from the aortic root, coronary ostium, coronary sinus, and femoral vein. Plasma VEGF, angiopoietin-1 and angiopoietin-2 levels were measured by immunoassays.

RESULTS

There were no significant differences in VEGF, angiopoietin-1 and angiopoietin-2 levels when aortic root, coronary ostium, coronary sinus, and femoral vein samples were compared (P = not significant (NS)). In patients undergoing PCI, peripheral angiopoietin-2 levels were increased significantly post PCI (P = 0.01). There was also a difference in intracardiac gradient (that is, aortic root-coronary sinus difference) in angiopoietin-1 (P = 0.02) following PCI. No significant changes in VEGF with PCI were noted.

CONCLUSION

There were no differences in indices of angiogenesis when aortic root, coronary ostium, coronary sinus, and femoral vein levels of VEGF and angiopoietins are compared, suggesting that peripheral blood measurements of these indices are comparable to intracardiac levels. Although no immediate effects were observed in soluble VEGF levels, PCI affected intracardiac angiopoietin-1 with a systemic release of angiopoietin-2. Further investigations are necessary to determine the relative systemic and intracardiac effects of the angiopoietins in vascular remodelling post PCI.

Role of IL-1-mediated inflammation in tumor angiogenesis

Angiogenesis, or generation of new blood vessels from pre-existing vessels, is an integral part of many physiological or pathological processes, including tumor growth. Physiological angiogenesis is a complex process controlled by different proangiogenic as well as antiangiogenic factors. For angiogenic induction, the balance between these pro- and antiangiogenic factors in the microenvironment has to shift in favor of proangiogenic factors, either by upregulation of these pro-angiogenic factors or by downregulation of angiogenic inhibitors. Proinflammatory cytokines, such as IL-1 and TNFa, were found to be major proangiogenic stimuli of both physiological and pathological angiogenesis. The IL-1 family consists of pleiotropic proinflammatory and immunoregulatory cytokines, namely, IL-1alpha and IL-1beta, and one antagonistic protein, the IL-1 receptor antagonist (IL-1Ra), which binds to IL-1 receptors without transmitting an activation signal and represents a physiological inhibitor of preformed IL-1. Previously, we described an important role for microenvironment IL-1, mainly IL-1beta, in tumor angiogenesis. In this chapter, we analyze the role of microenvironment host- and tumor cell-derived IL-1 on angiogenesis and the role of inflammation in pathological angiogenesis.