The Tie-2 ligand angiopoietin-2 destabilizes quiescent endothelium through an internal autocrine loop mechanism

Distinct activities of Bartonella henselae type IV secretion effector proteins modulate capillary-like sprout formation

The zoonotic pathogen Bartonella henselae (Bh) can lead to vasoproliferative tumour lesions in the skin and inner organs known as bacillary angiomatosis and bacillary peliosis. The knowledge on the molecular and cellular mechanisms involved in this pathogen-triggered angiogenic process is confined by the lack of a suitable animal model and a physiologically relevant cell culture model of angiogenesis. Here we employed a three-dimensional in vitro angiogenesis assay of collagen gel-embedded endothelial cell (EC) spheroids to study the angiogenic properties of Bh. Spheroids generated from Bh-infected ECs displayed a high capacity to form sprouts, which represent capillary-like projections into the collagen gel. The VirB/VirD4 type IV secretion system and a subset of its translocated Bartonella effector proteins (Beps) were found to profoundly modulate this Bh-induced sprouting activity. BepA, known to protect ECs from apoptosis, strongly promoted sprout formation. In contrast, BepG, triggering cytoskeletal rearrangements, potently inhibited sprouting. Hence, the here established in vitro model of Bartonella- induced angiogenesis revealed distinct and opposing activities of type IV secretion system effector proteins, which together with a VirB/VirD4-independent effect may control the angiogenic activity of Bh during chronic infection of the vasculature.

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.

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.

The angiopoietin-Tie2 system as a therapeutic target in sepsis and acute lung injury

BACKGROUND

Sepsis and acute lung injury (ALI)/acute respiratory distress syndrome (ARDS) are life-threatening syndromes characterised by inflammation and increased vascular permeability. Amongst other factors, the angiopoietin-tyrosine kinase with immunoglobulin-like and EGF-like domains 2 (Tie2) system is involved.

OBJECTIVE

To explore whether the angiopoietin-Tie2 system provides suitable targets for the treatment of sepsis and ALI/ARDS.

METHODS

Original experimental and patient studies on angiopoietins and sepsis/endotoxemia, inflammation, lung injury, hyperpermeability, apoptosis, organ functions and vital outcomes were reviewed.

RESULTS/CONCLUSION

The angiopoietin-Tie2 system controls the responsiveness of the endothelium to inflammatory, hyperpermeability, apoptosis and vasoreactive stimuli. Angiopoietin-2 provokes inflammation and vascular hyperpermeability, while angiopoietin-1 has a protective effect. Targeted angiopoietin-2 inhibition with RNA aptamers or blocking antibodies is a potential anti-inflammatory and anti-vascular hyperpermeability strategy in the treatment of sepsis and ALI/ARDS.

Angiopoietin-2: development of inhibitors for cancer therapy

Angiopoietin-2 (Ang2) is a member of the Ang family, which plays an important role in angiogenesis during the development and growth of human cancers. Ang2's role in angiogenesis generally is considered as an antagonist for Ang1, inhibiting Ang1-promoted Tie2 signaling, which is critical for blood vessel maturation and stabilization. Ang2 modulates angiogenesis in a cooperative manner with another important angiogenic factor, vascular endothelial growth factor A. Genetic studies have revealed that Ang2 also is critical in lymphangiogenesis during development. However, new evidence suggests more complicated roles for Ang2 in angiogenesis in physiologic processes and invasive phenotypes of cancer cells during progression of human cancers. This article discusses recent studies of Ang2 in angiogenesis and the implication of Ang2 as a therapeutic target as well as a potential inhibitor for antiangiogenesis treatment for cancer patients.

Angiopoietin 2 is a partial agonist/antagonist of Tie2 signaling in the endothelium

Angiopoietin 2 (Ang2) was originally shown to be a competitive antagonist for Ang1 of the receptor tyrosine kinase Tie2 in endothelial cells (ECs). Since then, reports have conflicted on whether Ang2 is an agonist or antagonist of Tie2. Here we show that Ang2 functions as an agonist when Ang1 is absent but as a dose-dependent antagonist when Ang1 is present. Exogenous Ang2 activates Tie2 and the promigratory, prosurvival PI3K/Akt pathway in ECs but with less potency and lower affinity than exogenous Ang1. ECs produce Ang2 but not Ang1. This endogenous Ang2 maintains Tie2, phosphatidylinositol 3-kinase, and Akt activities, and it promotes EC survival, migration, and tube formation. However, when ECs are stimulated with Ang1 and Ang2, Ang2 dose-dependently inhibits Ang1-induced Tie2 phosphorylation, Akt activation, and EC survival. We conclude that Ang2 is both an agonist and an antagonist of Tie2. Although Ang2 is a weaker agonist than Ang1, endogenous Ang2 maintains a level of Tie2 activation that is critical to a spectrum of EC functions. These findings may reconcile disparate reports of Ang2's effect on Tie2, impact our understanding of endogenous receptor tyrosine kinase signal transduction mechanisms, and affect how Ang2 and Tie2 are targeted under conditions such as sepsis and cancer.

Host-derived angiopoietin-2 affects early stages of tumor development and vessel maturation but is dispensable for later stages of tumor growth

The angiopoietin/Tie2 system has been identified as the second vascular-specific receptor tyrosine kinase system controlling vessel assembly, maturation, and quiescence. Angiopoietin-2 (Ang-2) is prominently up-regulated in the host-derived vasculature of most tumors, making it an attractive candidate for antiangiogenic intervention. Yet, the net outcome of Ang-2 functions on tumor angiogenesis is believed to be contextual depending on the local cytokine milieu. Correspondingly, Ang-2 manipulatory therapies have been shown to exert protumorigenic as well as antitumorigenic effects. To clarify the role of Ang-2 for angiogenesis and tumor growth in a definite genetic experimental setting, the present study was aimed at comparatively studying the growth of different tumors in wild-type and Ang-2-deficient mice. Lewis lung carcinomas, MT-ret melanomas, and B16F10 melanomas all grew slower in Ang-2-deficient mice. Yet, tumor growth in wild-type and Ang-2-deficient mice dissociated during early stages of tumor development, whereas tumor growth rates during later stages of primary tumor progression were similar. Analysis of the intratumoral vascular architecture revealed no major differences in microvessel density and perfusion characteristics. However, diameters of intratumoral microvessels were smaller in tumors grown in Ang-2-deficient mice, and the vasculature had an altered pattern of pericyte recruitment and maturation. Ang-2-deficient tumor vessels had higher pericyte coverage indices. Recruited pericytes were desmin and NG2 positive and predominately alpha-smooth muscle actin negative, indicative of a more mature pericyte phenotype. Collectively, the experiments define the role of Ang-2 during tumor angiogenesis and establish a better rationale for combination therapies involving Ang-2 manipulatory therapies.

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.

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.

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.

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.

The opening act: vasculogenesis and the origins of circulation

Previous studies on cellular and molecular mechanisms that regulate vascular development identified key signaling pathways and transcription factors. These findings supported the notion that the formation of vasculature is predominantly regulated by genetic programs, which is generally accepted. However, recent progress in understanding nongenetic factors that can modify the preprogrammed genetic mechanisms added another layer of complexity to our current understanding of vascular development. Here, we briefly summarize historic viewpoints and evolutionary perspectives on vascular development. We also review the cellular and molecular mechanisms that govern the emergence of the endothelial lineage and the subsequent process of vasculogenesis during development, with an emphasis on vascular endothelial growth factor and angiopoietin signaling cascades. Finally, we discuss epigenetic factors such as hemodynamic forces and hypoxic responses that can modulate and override the predetermined genetic mechanisms of vascular development.

Spatiotemporal analysis of the protein expression of angiogenic factors and their related receptors during folliculogenesis in rats with and without hormonal treatment

This study investigated the protein expression and cellular localization of ANGPT1, ANGPT2, and their receptor TEK, as well as vascular endothelial growth factor A (VEGFA) and its receptor KDR (VEGFR2) during folliculogenesis. To obtain follicles at different stages for immunochemistry and western analyses, we used prepubertal untreated, diethylstilbestrol- and equine chorionic gonadotropin-treated rats. To confirm that these hormonal treatments reflect physiological change, we used non-treated adult rats. No expression of ANGPT1 was observed in granulosa cells (Gc) from immature hormone-treated and non-treated rats at any follicular stage. By contrast, ANGPT1 expression in theca cells (Tc) increased with follicular maturation. ANGPT2 protein was either absent or weakly expressed in Gc at all follicular stages. In Tc, minimal expression of ANGPT2 protein was detected in the preantral follicle (PF), whereas it was stronger in the early antral follicle (EAF) and preovulatory follicle (POF). TEK staining was absent in Gc but was intense in Tc at every follicular stage. Staining for VEGFA was either absent or weakly present in Gc and Tc in PF and EAF, although in POF it was stronger in Gc and Tc. Staining for KDR was absent in Gc and very low in Tc from PF. Gc and Tc of EAF showed positive staining for KDR and in POF the staining was stronger. These results were confirmed by western immunoblot. A similar pattern of expression of these proteins was observed in cycling rats. In conclusion, we observed that the protein expression of ANGPT1, ANGPT2, VEGFA and their receptors increased during follicular development in rats.

VEGF-mediated fusion in the generation of uniluminal vascular spheroids

Embryonic mouse allantoic tissue (E8.5) was cultured in hanging drops to generate a three-dimensional vascular micro-tissue. The resulting tissue spheroids had an inner network of small diameter vessels expressing platelet endothelial cell adhesion molecule-1 (PECAM-1) and an outer layer of cells expressing SMalphaA, SM22-alpha, and SM-MHC. In a subsequent phase of culture, the fusion-promoting activity of vascular endothelial growth factor (VEGF) was used to transform the inner network of small diameter endothelial tubes into a contiguous layer of cells expressing PECAM-1, CD34, and VE-cadherin that circumscribed a central lumen-like cavity. The blood vessel-like character of the VEGF-treated spheroids was further demonstrated by their physiologically relevant vasodilatory and contractile responses, including contraction induced by KCl and relaxation stimulated by high-density lipoproteins and acetylcholine-induced nitric oxide production.

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.

Novel anti-myeloma agents and angiogenesis

During the last decade several novel agents have been used in the management of patients with multiple myeloma. Immunomodulatory drugs and proteasome inhibitors exert their efficacy both directly by inducing apoptosis of myeloma cells and indirectly through the interruption of the interactions between myeloma and stromal cells in the bone marrow (BM) microenvironment. These interactions are crucial for myeloma cell growth and survival. The adherence of myeloma cells to BM stromal cells leads to the overproduction of several cytokines with angiogenic properties that enhance the survival and growth of myeloma cells through paracrine and autocrine loops. The correlation of these molecules with clinical features and survival of myeloma patients supports the importance of angiogenesis in the pathogenesis of the disease and reveals these cytokines as suitable targets for the development of novel anti-myeloma therapies. This review summarises all available preclinical and clinical data for the effect of novel agents that are used in myeloma therapy, such as thalidomide, lenalidomide, bortezomib and VEGF inhibitors, on angiogenesis, which is at least partially responsible for their remarkable anti-myeloma efficacy.

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.

Defective remodeling and maturation of the lymphatic vasculature in Angiopoietin-2 deficient mice

Molecular mechanisms regulating the remodeling of the lymphatic vasculature from an immature plexus of vessels to a hierarchal network of initial and collecting lymphatics are not well understood. One gene thought to be important for this process is Angiopoietin-2 (Ang-2). Ang2(-/-) mice have previously been reported to exhibit an abnormal lymphatic phenotype but the precise nature of the lymphatic defects and the underlying mechanisms have yet to be defined. Here we demonstrate by whole-mount immunofluorescence staining of ear skin and mesentery that lymphatic vessels in Ang2(-/-) mice fail to mature and do not exhibit a collecting vessel phenotype. Furthermore, dermal lymphatic vessels in Ang2(-/-) pups prematurely recruit smooth muscle cells and do not undergo proper postnatal remodeling. In contrast, Ang2 knock-out Ang1 knock-in mice do develop a hierarchal lymphatic vasculature, suggesting that activation of Tie-2 is required for normal lymphatic development. Taken together, this work pinpoints a specific lymphatic defect of Ang2(-/-) mice and further defines the sequential steps in lymphatic vessel remodeling.

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.

Hepatic stellate cells display a functional vascular smooth muscle cell phenotype in a three-dimensional co-culture model with endothelial cells

Hepatic stellate cells (HSCs) are pericytes of liver sinusoidal endothelial cells (LSECs) and activation of HSC into a myofibroblast-like phenotype (called transdifferentiation) is involved in several hepatic disease processes including neovascularization during liver metastasis, chronic and acute liver injury. While early smooth muscle cell (SMC) differentiation markers including SM alpha-actin and SM22alpha are expressed in a variety of non-SMC, expression of late-stage markers is far more restricted. Here, we found that in addition to early SMC markers, activated rat HSC express a large panel of characteristic late vascular SMC markers including SM myosin heavy chain, h1-calponin and h-caldesmon. Furthermore, myocardin, which is present exclusively in SMCs and cardiomyocytes and controls the transcription of a subset of early and late SMC markers, is highly expressed in activated HSC. We further studied activated HSC in a functional three-dimensional spheroidal co-culture system together with endothelial cells (EC). Co-culture spheroids of EC and SMC differentiate spontaneously and organize into a core of SMC and a surface layer of EC representing an inside-outside model of the physiological assembly of blood vessels. Replacing SMC by in vitro activated HSC resulted in a similar organized spheroid with differentiated, von-Willebrand factor producing, surface lining quiescent human umbilical vein endothelial cell and a core of HSC. In an in vitro angiogenesis assay, activated HSC induced quiescence in vascular EC-the hallmark of vascular SMC function. Co-spheroids of LSEC and activated HSC formed capillary-like sprouts in gel angiogenesis assays expressing the vascular EC marker VE-cadherin. Our findings indicate that activated HSC are capable to adapt a functional SMC phenotype and to induce formation of tubular sprouts by LSEC and vascular endothelial cells. Since tumors and tumor metastasis induce HSC activation, HSC may take part in tumor-induced neoangiogenesis by adapting SMC-like functions.

Angiopoietin modulation of vascular endothelial growth factor: Effects on retinal endothelial cell permeability

PURPOSE

Vascular permeability is important at many sites, but particularly so in diabetic retinopathy where macular oedema is the major cause of blindness. Angiopoietin-2 (Ang-2) and vascular endothelial growth factor (VEGF) are important factors involved in neovascularization and vascular leakage, but there is little data on their interaction to promote increased vascular permeability.

METHODS

Porcine retinal endothelial cells (PREC) were seeded into permeable inserts and cultured in 24-well plates that permit measurement of permeability using fluorescent dextrans. Cell purity was assessed immunohistochemically. At confluency, PREC were treated with increasing concentrations of VEGF (20-100ng/ml) and Ang-2 (15-75ng/ml). The effect on tight junctions was assessed by visualization with an anti-ZO-1 antibody.

RESULTS

Immunohistochemistry showed high purity of isolated PREC. Permeability of untreated PREC monolayers was low. The increase in permeability in Ang-2 treated cells (25-30% compared with non-treated cells) was less than that for cells treated with VEGF only (20-100% compared with untreated cells). Highest permeability was seen with a combination of Ang-2 and VEGF (100-400% compared with untreated cells). Permeability increased with time after growth factor application. Preliminary ZO-1 immunohistochemistry appeared to demonstrate the presence of tight junctions between untreated PREC, and loss of tight junctions after treatment with VEGF and Ang-2.

CONCLUSIONS

VEGF alone is twice as potent in interrupting tight junctions in an endothelial cell monolayer as Ang-2. However, both growth factors acting together increase permeability three times as much as VEGF alone. Treatments designed to reduce vascular permeability in diabetic macular oedema should consider that crosstalk between growth factors including VEGF and the Ang-2/Tie-2 system can multiply their effects.

A modular tissue engineering construct containing smooth muscle cells and endothelial cells

Human umbilical vein endothelial cells (HUVEC) were seeded on sub-mm sized collagen cylinders containing embedded umbilical vein smooth muscle cells (UVSMC). These cylindrical "modules" are intended to be used as a vascularized construct, in which HUVEC lined channels are created by the random packing of the modules in situ or within a larger container. Embedding UVSMC cultured in medium containing 10% FBS had an adverse effect on subsequently seeded HUVEC junction morphology; HUVEC/UVSMC co-culturing was done in HUVEC medium (2% FBS with the addition of 0.03 mg/mL endothelial cell growth supplement) as compared to HUVEC seeded on collagen-only modules. In contrast, embedding UVSMC cultured in serum-free medium prior to embedding improved EC junction morphology. Such serum-free culturing, also prevented the UVSMC induced contraction of the collagen modules. On the other hand, embedding serum-free cultured UVSMC promoted HUVEC proliferation and NO secretion compared to those modules embedded with 10% serum cultured UVSMC. These results suggest, not surprisingly, that embedded UVSMC phenotype plays an important role in the seeded HUVEC phenotype, and that the response can be modulated by the UVSMC culture medium serum concentration. These studies were undertaken with a view to using the UVSMC to modulate the thrombogenicity of the HUVEC. Exploration of this outcome awaits further studies directed to understanding the mechanism of the cellular interactions.

Dosage-dependent requirement of BMP type II receptor for maintenance of vascular integrity

Germ-line mutations in bone morphogenic protein type II receptor (Bmpr2) confer susceptibility to pulmonary arterial hypertension (PAH), which is characterized by obstructive vascular lesions in small arteries. The molecular and cellular mechanisms that account for the etiology of this disorder remain elusive, as does the role of Bmpr2 in postnatal tissue homeostasis. Here we show that in adult mice, stably silencing Bmpr2 expression by RNA interference does not increase pulmonary arterial resistance but results in severe mucosal hemorrhage, incomplete mural cell coverage on vessel walls, and gastrointestinal hyperplasia. We present evidence that BMP receptor signaling regulates vascular remodeling during angiogenesis by maintaining the expression of endothelial guidance molecules that promote vessel patterning and maturation and by counteracting growth factor–induced AKT activation. Attenuation of this function may cause vascular dysmorphogenesis and predisposition to angioproliferative diseases. Our findings provide a mechanistic link between PAH and other diseases associated with the BMP/TGF-β pathways, such as hereditary hemorrhagic telangiectasia and juvenile polyposis syndrome.

Essential role of endothelial Smad4 in vascular remodeling and integrity

New blood vessels are formed through the assembly or sprouting of endothelial cells (ECs) and become stabilized by the formation of perivascular matrix and the association with supporting mural cells. To investigate the role of endothelial Smad4 in vascular development, we deleted the Smad4 gene specifically in ECs using the Cre-LoxP system. EC-specific Smad4 mutant mice died at embryonic day 10.5 due to cardiovascular defects, including attenuated vessels sprouting and remodeling, collapsed dorsal aortas, enlarged hearts with reduced trabeculae, and failed endocardial cushion formation. Noticeably, Smad4-deficient ECs demonstrated an intrinsic defect in tube formation in vitro. Furthermore, the mutant vascular ECs dissociated away from the surrounding cells and suffered from impaired development of vascular smooth muscle cells. The disturbed vascular integrity and maturation was associated with aberrant expression of angiopoietins and a gap junction component, connexin43. Collectively, we have provided direct functional evidence that Smad4 activity in the developing ECs is essential for blood vessel remodeling, maturation, and integrity.

Angiopoietin: a TIE(d) balance in tumor angiogenesis

Angiopoietins (ANG-1 and ANG-2) and their TIE-2 receptor tyrosine kinase have wide-ranging effects on tumor malignancy that includes angiogenesis, inflammation, and vascular extravasation. These multifaceted pathways present a valuable opportunity in developing novel inhibition strategies for cancer treatment. However, the regulatory role of ANG-1 and ANG-2 in tumor angiogenesis remains controversial. There is a complex interplay between complementary yet conflicting roles of both the ANGs in shaping the outcome of angiogenesis. Embryonic vascular development suggests that ANG-1 is crucial in engaging interaction between endothelial and perivascular cells. However, recruitment of perivascular cells by ANG-1 has recently been implicated in its antiangiogenic effect on tumor growth. It is becoming clear that TIE-2 signaling may function in a paracrine and autocrine manner directly on tumor cells because the receptor has been increasingly found in tumor cells. In addition, alpha(5)beta(1) and alpha(v)beta(5) integrins were recently recognized as functional receptors for ANG-1 and ANG-2. Therefore, both the ligands may have wide-ranging functions in cellular activities that affect overall tumor development. Collectively, these TIE-2-dependent and TIE-2-independent activities may account for the conflicting findings of ANG-1 and ANG-2 in tumor angiogenesis. These uncertainties have impeded development of a clear strategy to target this important angiogenic pathway. A better understanding of the molecular basis of ANG-1 and ANG-2 activity in the pathophysiologic regulation of angiogenesis may set the stage for novel therapy targeting this pathway.

Circulating angiopoietin-2 is a strong prognostic factor in acute myeloid leukemia

Angiogenesis plays an important role in solid tumors and hematologic malignancies. The angiopoietins act as essential regulators in this process. We investigated the impact of circulating angiopoietin-1 (Ang-1), angiopoietin-2 (Ang-2) and soluble Tie2 (sTie2) on overall survival in patients with acute myeloid leukemia (AML). Ang-1, Ang-2 and sTie2 were measured in plasma samples from 68 AML patients and 11 controls using enzyme-linked immunosorbent assay. Circulating levels of Ang-2 and sTie2 (median (range): 1098.0 (361.4-4147.6) pg/ml and 3.40 (1.21-10.00) ng/ml, respectively) were significantly elevated in AML patients as compared to controls (307.9 (199.7-1225.0) pg/ml and 2.88 (1.71-3.29) ng/ml; P<0.001 and P=0.014). In a univariate Cox proportional hazards model, higher levels of Ang-2 and sTie2 were predictive of poor survival. In multivariate analyses, Ang-2 and cytogenetics proved to be independent prognostic factors, with a relative risk of 4.07 (95% confidence interval (CI) 1.88-8.81) and 2.70 (95% CI 1.25-5.81), respectively. The 3-year survival rate for AML patients with Ang-2 levels>/=1495.6 pg/ml was only 14.7% compared to 64.7% for those with Ang-2 levels<1495.6 pg/ml. These data provide evidence that circulating Ang-2 represents an independent prognostic factor in AML and may be used as a prognostic tool in the risk-adapted management of AML.

Angiogenesis in diabetic nephropathy

Angiogenesis, the formation of new blood vessels from pre-existing vasculature, plays a key role in both physiologic and pathologic events, including wound healing, cancer, and diabetes. Neovascularization has been implicated in the genesis of diverse diabetic complications such as retinopathy, impaired wound healing, neuropathy, and, most recently, diabetic nephropathy. Diabetic nephropathy is one of the major microvascular-associated complications in diabetes and is the leading cause of end-stage renal disease worldwide. In this review we describe the major factors involved in the pathologic glomerular microvascular alterations in response to hyperglycemia and the possible use of anti-angiogenic therapies for the treatment of diabetic nephropathy.

Angiogenic inhibition reduces germinal matrix hemorrhage

The germinal matrix of premature infants is selectively vulnerable to hemorrhage within the first 48 h of life. To assess the role of vascular immaturity in germinal matrix hemorrhage (GMH), we evaluated germinal matrix angiogenesis in human fetuses and premature infants, as well as in premature rabbit pups, and noted active vessel remodeling in all three. Vascular endothelial growth factor (VEGF), angiopoietin-2 and endothelial cell proliferation were present at consistently higher levels in the germinal matrix relative to the white matter anlagen and cortical mantle. On that basis, we asked whether prenatal treatment with either of two angiogenic inhibitors, the COX-2 inhibitor celecoxib, or the VEGFR2 inhibitor ZD6474, could suppress the incidence of GMH in premature rabbit pups. Celecoxib treatment decreased angiopoietin-2 and VEGF levels as well as germinal matrix endothelial proliferation. Furthermore, treatment with celecoxib or ZD6474 substantially decreased the incidence of GMH. Thus, by suppressing germinal matrix angiogenesis, prenatal celecoxib or ZD6474 treatment may be able to reduce both the incidence and severity of GMH in susceptible premature infants.

Pro-angiogenic cytokines and their role in tumor angiogenesis

The development of solid tumors depends upon an adequate supply of blood. This can be achieved by way of co-option of preexisting blood vessels and by the induction of angiogenesis. During the past 30 years, tumor angiogenesis had been found to play a crucial role in the progression of solid tumors. Tumor angiogenesis was found to be induced by a variety of pro-angiogenic cytokines of which the best characterized is vascular endothelial growth factor (VEGF). Indeed, the first FDA approved anti-angiogenic drug for the treatment of cancer is Avastin, a neutralizing antibody directed against VEGF. This review focuses on cytokines which have been reported to induce tumor angiogenesis.

Synthesis, structural analysis, and SAR studies of triazine derivatives as potent, selective Tie-2 inhibitors

A novel class of selective Tie-2 inhibitors was derived from a multi-kinase inhibitor 1. By reversing the amide connectivity and incorporating aminotriazine or aminopyridine hinge-binding moieties, excellent Tie-2 potency and KDR selectivity could be achieved with 3-substituted terminal aryl rings. X-ray co-crystal structure analysis aided inhibitor design. This series was evaluated on the basis of potency, selectivity, and rat pharmacokinetic parameters.

Alternative vascularization mechanisms in cancer: Pathology and therapeutic implications

Although cancer cells are not generally controlled by normal regulatory mechanisms, tumor growth is highly dependent on the supply of oxygen, nutrients, and host-derived regulators. It is now established that tumor vasculature is not necessarily derived from endothelial cell sprouting; instead, cancer tissue can acquire its vasculature by co-option of pre-existing vessels, intussusceptive microvascular growth, postnatal vasculogenesis, glomeruloid angiogenesis, or vasculogenic mimicry. The best-known molecular pathway driving tumor vascularization is the hypoxia-adaptation mechanism. However, a broad and diverse spectrum of genetic aberrations is associated with the development of the "angiogenic phenotype." Based on this knowledge, novel forms of antivascular modalities have been developed in the past decade. When applying these targeted therapies, the stage of tumor progression, the type of vascularization of the given cancer tissue, and the molecular machinery behind the vascularization process all need to be considered. A further challenge is finding the most appropriate combinations of antivascular therapies and standard radio- and chemotherapies. This review intends to integrate our recent knowledge in this field into a rational strategy that could be the basis for developing effective clinical modalities using antivascular therapy for cancer.

Evolution of a Highly Selective and Potent 2-(Pyridin-2-yl)-1,3,5-triazine Tie-2 Kinase Inhibitor

Inhibition of angiogenesis is a promising and clinically validated approach for limiting tumor growth and survival. The receptor tyrosine kinase Tie-2 is expressed almost exclusively in the vascular endothelium and is required for developmental angiogenesis and vessel maturation. However, the significance of Tie-2 signaling in tumor angiogenesis is not well understood. In order to evaluate the therapeutic utility of inhibiting Tie-2 signaling, we developed a series of potent and orally bioavailable small molecule Tie-2 kinase inhibitors with selectivity over other kinases, especially those that are believed to be important for tumor angiogenesis. Our earlier work provided pyridinyl pyrimidine 6 as a potent, nonselective Tie-2 inhibitor that was designed on the basis of X-ray cocrystal structures of KDR inhibitors 34 (triazine) and 35 (nicotinamide). Lead optimization resulted in pyridinyl triazine 63, which exhibited >30-fold selectivity over a panel of kinases, good oral exposure, and in vivo inhibition of Tie-2 phosphorylation.

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.

Localization of Ang-1, -2, Tie-2, and VEGF expression at endothelial-pericyte interdigitation in rat angiogenesis

Endothelial cells and pericytes play critical role in angiogenesis, which is controlled, in part, by the angiopoietin (Ang)/Tie-2 system and vascular endothelial growth factor (VEGF). Here, we investigated Ang, Tie-2, and VEGF expression within endothelial cells and pericyte interdigitations (EPI), which consist of cytoplasmic projections of pericytes and corresponding endothelial indentations. After subcutaneous implantation of a thermoreversible gelation polymer disc in rats, the capillary density was low on day 5, increased to a peak on day 7, and then decreased on days 10-20. A small number of EPI were observed on day 5, then increased sharply to a peak on day 10, but had decreased on day 20. Light and electron microscopy immunohistochemical and RNA in situ hybridization analyses revealed that Tie-2 localized at endothelial cells, and Ang-2 localized at endothelial cells and pericytes, while Ang-1 and VEGF localized at pericytes, and Ang-1 was most intensely observed at EPI of pericytes. Conventional quantitative RT-PCR and Western blot analyses revealed that the level of Ang-1 was low on days 5-7, then increased on days 10-20, while the level of VEGF was high on days 5-10, but had decreased on day 20. The level of Ang-2 remained high and Tie-2 remained at the level of the control on days 5-20. The present study showed that the angiogenic phase might be initiated by increases in Ang-2 and VEGF, while the microvessel maturation phase might be initiated by a relative increase in Ang-1 and a decrease in VEGF. Moreover, EPI might serve as a pathway for the Ang-1/Tie-2 system, with VEGF promoting pericyte recruitment for microvascular integrity.

The duality of angiogenesis: implications for therapy of human disease

Angiogenesis, the development of a microvasculature to a neoplastic, inflammatory, or infectious disease process, is a promising therapeutic target for disease therapy that has not been fully exploited. To further understand angiogenesis and its potential for therapy of dermatologic disorders, one must understand the many dualities of pathologic angiogenesis. These dualities are direct versus indirect angiogenesis inhibition, the differing origins of endothelial cells, which may arise either locally or through bone marrow stem cells, and regulation of vascular endothelial growth factor (VEGF) by hypoxia-dependent and/or independent pathways. The future development of therapy directed at pathologic angiogenesis is dependent upon an understanding of the factors that regulate angiogenesis. The presence of both direct and indirect inhibition of angiogenesis, the multiple sources of endothelial cells, and the regulation of VEGF by hypoxia-independent and/or-dependent pathways must taken into consideration if the promise of effective therapy of human disease is to be realized.

Angiopoietins: a link between angiogenesis and inflammation

The angiopoietin (Ang)-Tie ligand-receptor system has a key regulatory role in regulating vascular integrity and quiescence. Besides its role in angiogenesis, it is an important regulator in numerous diseases including inflammation. Ang-1-mediated Tie2 activation is required to maintain the quiescent resting state of the endothelium. Agonistic Ang-1 functions are antagonized by Ang-2, which is believed to inhibit Ang-1-Tie2 signaling. Ang-2 destabilizes the quiescent endothelium and primes it to respond to exogenous stimuli, thereby facilitating the activities of inflammatory (tumor necrosis factor and interleukin-1) and angiogenic (vascular endothelial growth factor) cytokines. Intriguingly, Ang-2 is expressed weakly by the resting endothelium but becomes strongly upregulated following endothelial activation. Moreover, endothelial cells store Ang-2 in Weibel-Palade bodies from where it can be made available quickly following stimulation, suggesting a role of Ang-2 in controlling rapid vascular adaptive processes. This suggests that Ang-2 is the dynamic regulator of the Ang-Tie2 axis, thereby functioning as a built-in switch controlling the transition of the resting quiescent endothelium towards the activated responsive endothelium.

Angiopoietin-2 functions as an autocrine protective factor in stressed endothelial cells

Angiopoietin (Ang)-2, a context-dependent agonist/antagonist for the vascular-specific Tie2 receptor, is highly expressed by endothelial cells at sites of normal and pathologic angiogenesis. One prevailing model suggests that in these settings, Ang-2 acts as an autocrine Tie2 blocker, inhibiting the stabilizing influence of the Tie2 activator Ang-1, thereby promoting vascular remodeling. However, the effects of endogenous Ang-2 on cells that are actively producing it have not been studied in detail. Here, we demonstrate that Ang-2 expression is rapidly induced in endothelial cells by the transcription factor FOXO1 after inhibition of the phosphatidylinositol 3-kinase/Akt pathway. We employ RNAi and blocking antibodies to show that in this setting, Ang-2 unexpectedly functions as a Tie2 agonist, bolstering Akt activity so as to provide negative feedback on FOXO1-regulated transcription and apoptosis. In addition, we show that Ang-2, like Ang-1, activates Tie2/Akt signaling in vivo, thereby inhibiting the expression of FOXO1 target genes. Consistent with a role for Ang-2 as a Tie2 activator, we demonstrate that Ang-2 inhibits vascular leak. Our data suggests a model in which Ang-2 expression is induced in stressed endothelial cells, where it acts as an autocrine Tie2 agonist and protective factor.

Cellular mechanisms and signalling pathways activated by high glucose and AGE-albumin in the aortic endothelium

This review summarizes evidence on the effect of excess circulating glucose concentration and AGE-albumin on the aortic endothelial cells (ECs) phenotype, transport function, and expression of signalling molecules. The recent reports on the ECs dysfunction in diabetes are briefly reviewed, to provide a broader view on the link between ECs structural changes, functional alterations, and the underlying biochemical mechanisms. The original results emerging from streptozotocin-injected mice and human aortic endothelial cells grown in high (25 mM) glucose concentration are presented. Compared to physiological condition, in diabetes aortic ECs switch to a biosynthetic phenotype, present an increased number of caveolae, and enhance (by approximately 20%) transcytosis of AGE-albumin (AGE-Alb). In cultured ECs, 25 mM glucose induces approximately 2.6 fold increase in pSTAT-3 and pERK1 and approximately 1.8 fold increase in pERK2; further exposure to 5 microM AGE-Alb causes approximately 4.3 fold increase in pERK1/2 (vs. 5 mM glucose). Together, these data may explain the phenotypic change, enhanced permeability, and proliferation of aortic ECs in diabetic conditions.

Emerging roles of the Angiopoietin-Tie and the ephrin-Eph systems as regulators of cell trafficking

Vascular receptor tyrosine kinases (RTK) have been identified as critical regulatory signaling molecules of developmental and adult vascular morphogenic processes [vascular endothelial growth factor (VEGF) receptors=sprouting; EphB receptors=assembly; Tie2 receptor=maturation and quiescence]. It is intriguing that the same molecules that control the growth of blood and lymphatic vessels play critical roles in the adult to regulate maintenance functions related to vascular homeostasis. VEGF is among the most potent inducers of vascular permeability. The second vascular RTK system, the interaction of paracrine-acting Angiopoietin-1 with its cognate receptor Tie2, acts as an endothelial maintenance and survival-mediating molecular system, which stabilizes the vessel wall and controls endothelial cell quiescence. The third vascular RTK system, the interaction of Eph receptors with their Eph family receptor-interacting protein (ephrin) ligands, transduces positional guidance cues on outgrowing vascular sprouts, which are critical for proper arteriovenous assembly and establishment of blood flow. As such, Eph-ephrin interactions act as an important regulator of cell-cell interactions, exerting propulsive and repulsive functions on neighboring cells and mediating adhesive functions. This review summarizes recent findings related to the roles of the Angiopoietin-Tie and the Eph-ephrin systems as regulators of cell trafficking in the vascular system. The recognition of vascular homeostatic functions of vascular RTKs marks an important change of paradigm in the field of angiogenesis research as it relates angiogenesis-inducing molecules to vascular maintenance functions in the adult. This may also broaden the scope of vascular RTK-targeted therapies.

Angiogenesis in malignant glioma--a target for antitumor therapy?

The prognosis of malignant gliomas is still dismal despite aggressive treatment attempts. Thus, alternative therapy strategies are needed. Malignant gliomas are upon the best vascularized tumors in humans and their proliferation is hallmarked by a distinct proliferative vascular component. Hence it seems to be a logical consequence to apply anti-angiogenic treatment strategies to malignant gliomas. These treatment strategies have shown promising effects in animal models and some experimental clinical studies. This review gives a short introduction into the molecules involved in angiogenesis of malignant gliomas, it provides an overview of the latest experimental developments of glioma angiogenesis inhibition and discusses the results of clinical anti-angiogenic trials in patients with high grade glioma. Additionally the problem of monitoring the treatment success of an anti-angiogenic therapy is addressed.

Angiopoietin-2 levels are elevated in exudative pleural effusions

OBJECTIVE

To examine the pleural fluid (PF) and serum levels of angiopoietin (Ang)-1, Ang-2, and vascular endothelial growth factor (VEGF) in patients with pleural effusions (PEs).

METHODS

One hundred fifteen patients, 16 with transudative PEs due to heart failure and 99 with exudative PEs (malignant, 40; para-pneumonic, 24; tuberculous, 13; miscellaneous etiologies, 22) were included in the study. PF and serum levels of the growth factors were measured using enzyme-linked immunosorbent assay.

RESULTS

PF Ang-2 and VEGF levels but not Ang-1 levels were higher (p < 0.001) in exudates than in transudates. PF Ang-2 levels were higher in tuberculous PEs than in PEs of any other etiology and were lower in heart failure PEs than in PEs of any other etiology. The highest PF VEGF levels were observed in patients with malignant and parapneumonic PEs. The lowest PF VEGF levels were observed in patients with transudates. In PEs, Ang-2 levels correlate with VEGF levels (p < 0.001), RBC count (p = 0.002), nucleated cell count (p < 0.001), total protein levels (p < 0.001), and lactate dehydrogenase levels (p < 0.001). PF Ang-1 levels were lower than serum Ang-1 levels both in patients with exudates (p < 0.001) and in those with transudates (p = 0.001). PF Ang-2 levels were higher than serum Ang-2 levels both in patients with exudates (p < 0.001) and in those with transudates (p = 0.045). PF VEGF levels were higher than serum VEGF levels in patients with malignant PEs (p < 0.001) and parapneumonic PEs (p = 0.003), but lower than serum VEGF levels in heart failure PEs (p < 0.001). In patients with tuberculous PEs and exudative PEs of miscellaneous etiology, PF and serum VEGF levels did not differ significantly.

CONCLUSION

Ang-2 levels but not Ang-1 levels are elevated in exudative PEs, and they correlate with levels of VEGF and markers of pleural inflammation. It is thus possible that Ang-2 along with VEGF participate in pleural inflammation and the pathogenesis of exudative PEs.

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.

Heterogeneity of Tie2 expression in tumor microcirculation: influence of cancer type, implantation site, and response to therapy

To evaluate the expression of the Tie2/Tek tyrosine kinase receptor in tumor blood vessels, we examined Tie2lacZ(+)/RAG1(-) mice. There was considerable heterogeneity (Tie2-negative, Tie2-positive, or Tie2-composite blood vessels) in subcutaneous xenografts of human colorectal carcinoma (HCT116; 97.5% Tie2-positive vessels) versus human melanoma (WM115; 75.9% Tie2-positive vessels). Similar patterns of Tie2 expression occurred in abdominal metastases derived from the same cell lines. Immunostaining for endothelial markers and Tie2 revealed that endogenous protein levels corresponded with transgene activity. Endothelial cells were confirmed to be of mouse origin through triple immunofluorescence staining with mouse antiserum to human nuclei, isolectin GS-IB(4), and anti-Tie2. Similar Tie2 heterogeneity was observed in clinical specimens from a variety of human cancers, including malignant melanoma and colorectal carcinoma. We also examined the effect of Tek-Delta Fc anti-angiogenic therapy on tumor growth and Tie2 expression patterns in HCT116 and WM115 subcutaneous xenografts. Tek-Delta induced extensive tumor regression in HCT116 tumors and concomitant reductions in Tie2-expressing blood vessels. However, no significant responses were seen in Tek-Delta-treated WM115 tumors. Thus, vascular heterogeneity of Tie2 expression is cancer-type specific, suggesting that the tumor microenvironment and/or direct cancer cell interactions influence Tie2 endothelial expression.

Angiopoietin-2 sensitizes endothelial cells to TNF-alpha and has a crucial role in the induction of inflammation

The angiopoietins Ang-1 and Ang-2 have been identified as ligands of the receptor tyrosine kinase Tie-2 (refs. 1,2). Paracrine Ang-1-mediated activation of Tie-2 acts as a regulator of vessel maturation and vascular quiescence. In turn, the antagonistic ligand Ang-2 acts by an autocrine mechanism and is stored in endothelial Weibel-Palade bodies from where it can be rapidly released upon stimulation. The rapid release of Ang-2 implies functions of the angiopoietin-Tie system beyond its established role during vascular morphogenesis as a regulator of rapid vascular responses. Here we show that mice deficient in Ang-2 (encoded by the gene Angpt2) cannot elicit an inflammatory response in thioglycollate-induced or Staphylococcus aureus-induced peritonitis, or in the dorsal skinfold chamber model. Recombinant Ang-2 restores the inflammation defect in Angpt2(-/-) mice. Intravital microscopy showed normal TNF-alpha-induced leukocyte rolling in the vasculature of Angpt2(-/-)mice, but rolling cells did not firmly adhere to activated endothelium. Cellular experiments showed that Ang-2 promotes adhesion by sensitizing endothelial cells toward TNF-alpha and modulating TNF-alpha-induced expression of endothelial cell adhesion molecules. Together, these findings identify Ang-2 as an autocrine regulator of endothelial cell inflammatory responses. Ang-2 thereby acts as a switch of vascular responsiveness exerting a permissive role for the activities of proinflammatory cytokines.

The role of angiopoietins during angiogenesis in gliomas

The formation of new blood vessels plays an important role in human disease development and progression. For instance, it is well established that the growth of most cancers critically depends on the supply of nutrition and oxygen by newly recruited blood vessels. Similarly, malignant gliomas, the most common primary brain tumors occurring in humans are highly dependent on angiogenesis. In recent years, there has been tremendous effort to uncover the molecular mechanisms that drive blood vessel growth in adult tissues, especially during cancer progression. Vascular endothelial growth factor (VEGF) and other morphogens, such as angiopoietins and ephrins have been shown to be critically involved in the formation of new blood vessels during both developmental and pathological angiogenesis as evidenced by genetic studies in mice. In this review, we focus on angiopoietins, a family of growth factor ligands binding to Tie tyrosine kinase receptors with emphasis on their functional consequences during the growth and progression of experimental tumors and malignant human gliomas.

Molecular profile and proliferative responses of rat lymphatic endothelial cells in culture

BACKGROUND

Lymphangiogenesis plays an important role in metastasis of many solid tumors. To study lymphangiogenesis under controlled conditions, an in vitro model is needed. The goal of this work was to establish such an in vitro model by determining a molecular profile of rat mesenteric lymphatic endothelial cells (RMLEC) and characterizing their proliferative responses to angiogenic and lymphangiogenic factors, such as vascular endothelial growth factor A and C (VEGF-A and VEGF-C).

METHODS AND RESULTS

RMLEC strongly expressed most lymphatic-specific markers, including Prox-1, LYVE-1, and VEGFR-3. Proliferation of RMLEC was serum and heparin dependent. In the presence of low (2%) serum concentration, exogenously added VEGF-A and VEGFC stimulated RMLEC in a linear and dose-dependent manner. This effect was abrogated by anti-VEGF-A and VEGF-C antibodies, as well as by soluble Tie-2 and Flt-4 fusion proteins. Abrogation was reversed by VEGF-A, suggesting that this factor as an important regulator of lymphangiogenesis.

CONCLUSIONS

Cultured RMLEC preserved a molecular profile consistent with the phenotype of lymphatic endothelium in vivo and respond to either VEGF-A or VEGF-C factors. VEGFA was able to rescue RMLEC proliferation inhibited by a neutralizing VEGF-C antibody or soluble Tie-2 fusion protein. These results support the existence of cross-talk among angiogenic and lymphangiogenic factors. This work established experimental conditions that allow in vitro modeling of lymphatic endothelial responses to lymphangiogenic regulators. Preliminary results using this model suggest that VEGF-A, VEGF-C, and angiopoietins work in concert to promote lymphangiogenesis in vivo.

Transcriptional and posttranscriptional regulation of angiopoietin-2 expression mediated by IGF and PDGF in vascular smooth muscle cells

Angiopoietins play a significant role in vascular development and angiogenesis. Both angiopoietin-1 (Ang1) and angiopoietin-2 (Ang2) bind the receptor tyrosine kinase Tie2. However, while Ang1 signaling results in the stabilization of vessel structure, Ang2 has been linked to vascular instability. The ratio of these two Tie2 ligands is thus critical for vascular stability and remodeling. This study identifies a mechanism of growth factor-mediated reduction in Ang2 expression in vascular smooth muscle cells (VSMCs). In response to PDGF, VSMCs downregulated Ang2 mRNA levels by 75% within 4 h, with a subsequent decrease in Ang2 protein levels. Quantitation of endogenous transcription rates revealed that PDGF stimulation did not alter Ang2 transcription rates, but instead induced a posttranscriptional mechanism of rapid Ang2 mRNA destabilization. The Ang2 mRNA half-life was reduced by at least 50% after PDGF treatment. The PDGF-induced mRNA turnover mechanism was dependent on several MAPK pathways, including ERK and JNK. In contrast, IGF-I, which did not significantly activate ERK or JNK, stimulated increased Ang2 expression through transcriptional activation. These findings demonstrate that VSMCs adjust Ang2 expression through multiple mechanisms, including changes in transcription as well as posttranscriptional mRNA destabilization.

Multiple angiopoietin recombinant proteins activate the Tie1 receptor tyrosine kinase and promote its interaction with Tie2

The Tie1 receptor tyrosine kinase was isolated over a decade ago, but so far no ligand has been found to activate this receptor. Here, we have examined the potential of angiopoietins, ligands for the related Tie2 receptor, to mediate Tie1 activation. We show that a soluble Ang1 chimeric protein, COMP-Ang1, stimulates Tie1 phosphorylation in endothelial cells with similar kinetics and angiopoietin dose dependence when compared with Tie2. The phosphorylation of overexpressed Tie1 was weakly induced by COMP-Ang1 also in transfected cells that do not express Tie2. When cotransfected, Tie2 formed heteromeric complexes with Tie1, enhanced Tie1 activation, and induced phosphorylation of a kinase-inactive Tie1 in a ligand-dependent manner. Tie1 phosphorylation was also induced by native Ang1 and Ang4, although less efficiently than with COMP-Ang1. In conclusion, we show that Tie1 phosphorylation is induced by multiple angiopoietin proteins and that the activation is amplified via Tie2. These results should be important in dissecting the signal transduction pathways and biological functions of Tie1.