Differential expression of Tie-2 receptors and angiopoietins in response to in vivo hypoxia in rats

The angiopoietin-Tie2 pathway regulates Purkinje cell dendritic morphogenesis in a cell-autonomous manner

Neuro-vascular communication is essential to synchronize central nervous system development. Here, we identify angiopoietin/Tie2 as a neuro-vascular signaling axis involved in regulating dendritic morphogenesis of Purkinje cells (PCs). We show that in the developing cerebellum Tie2 expression is not restricted to blood vessels, but it is also present in PCs. Its ligands angiopoietin-1 (Ang1) and angiopoietin-2 (Ang2) are expressed in neural cells and endothelial cells (ECs), respectively. PC-specific deletion of Tie2 results in reduced dendritic arborization, which is recapitulated in neural-specific Ang1-knockout and Ang2 full-knockout mice. Mechanistically, RNA sequencing reveals that Tie2-deficient PCs present alterations in gene expression of multiple genes involved in cytoskeleton organization, dendritic formation, growth, and branching. Functionally, mice with deletion of Tie2 in PCs present alterations in PC network functionality. Altogether, our data propose Ang/Tie2 signaling as a mediator of intercellular communication between neural cells, ECs, and PCs, required for proper PC dendritic morphogenesis and function.

Role of Angiopoietins in Development of Cancer and Neoplasia Associated with Viral Infection

Angiopoietin/tyrosine protein kinase receptor Tie-2 signaling in endothelial cells plays an essential role in angiogenesis and wound healing. Angiopoietin-1 (Ang-1) is crucial for blood vessel maturation while angiopoietin-2 (Ang-2), in collaboration with vascular endothelial growth factor (VEGF), initiates angiogenesis by destabilizing existing blood vessels. In healthy people, the Ang-1 level is sustained while Ang-2 expression is restricted. In cancer patients, Ang-2 level is elevated, which correlates with poor prognosis. Ang-2 not only drives tumor angiogenesis but also attracts infiltration of myeloid cells. The latter rapidly differentiate into tumor stromal cells that foster tumor angiogenesis and progression, and weaken the host’s anti-tumor immunity. Moreover, through integrin signaling, Ang-2 induces expression of matrix metallopeptidases (MMPs) to promote tumor cell invasion and metastasis. Many oncogenic viruses induce expression of Ang-2 to promote development of neoplasia associated with viral infection. Multiple Ang-2 inhibitors exhibit remarkable anti-tumor activities, further highlighting the importance of Ang-2 in cancer development.

[Morphological characteristics of fetal testes in chronic intrauterine hypoxia in different gestation periods]

OBJECTIVE

To study the morphological characteristics and expression of vascular endothelial growth factor (VEGF) in the fetal testes exposed to chronic intrauterine hypoxia during pathological pregnancy in different gestation periods.

MATERIAL AND METHODS

The testes from 48 male fetuses that had died in the antenatal or early neonatal period in mothers with pathological pregnancy were morphologically evaluated.

RESULTS

Chronic intrauterine hypoxia was shown to be a powerful damaging factor and leads to delayed gonadal development. Histological examination of testicular tissue showed a significant reduction in the number of tubular cells per vision field, a decrease in tubular diameter and area, with the simultaneously increased area of the stroma and a larger number of vessels. Immunohistochemical study revealed the pronounced cytoplasmic expression of VEGF in testicular tissue in different gestation periods in the spermatogenic epitheliocytes, vessels, Leydig interstitial cells, while the maximal expression of this receptor was observed at 19-25 weeks' gestation, the degree of expression decreased at 26-29 weeks' gestation.

CONCLUSION

Intrauterine hypoxia has a destabilizing effect on the processes of proliferation and differentiation of the spermatogenic epithelium, interstitial endocrinocytes, activates the processes of angiogenesis and the growth of connective tissue. All this can involve not only gonadal dysgenesis, but also future reproductive dysfunction. Hypoxia stimulates the expression of VEGF, whose receptors are present in almost all testicular cell populations. It can be assumed that VEGF can act as a paracrine regulator of Leydig cell activity, also as an inducer of angiogenesis, and thus play a certain role in the development of male fertility.

Hypoxia Mediates Tumor Malignancy and Therapy Resistance

Hypoxia is a hallmark of the tumor microenvironment and contributes to tumor malignant phenotypes. Hypoxia-inducible factor (HIF) is a master regulator of intratumoral hypoxia and controls hypoxia-mediated pathological processes in tumors, including angiogenesis, metabolic reprogramming, epigenetic reprogramming, immune evasion, pH homeostasis, cell migration/invasion, stem cell pluripotency, and therapy resistance. In this book chapter, we reviewed the causes and types of intratumoral hypoxia, hypoxia detection methods, and the oncogenic role of HIF in tumorigenesis and chemo- and radio-therapy resistance.

Angiopoietin-4-dependent venous maturation and fluid drainage in the peripheral retina

The maintenance of fluid homeostasis is necessary for function of the neural retina; however, little is known about the significance of potential fluid management mechanisms. Here, we investigated angiopoietin-4 (Angpt4, also known as Ang3), a poorly characterized ligand for endothelial receptor tyrosine kinase Tie2, in mouse retina model. By using genetic reporter, fate mapping, and in situ hybridization, we found Angpt4 expression in a specific sub-population of astrocytes at the site where venous morphogenesis occurs and that lower oxygen tension, which distinguishes peripheral and venous locations, enhances Angpt4 expression. Correlating with its spatiotemporal expression, deletion of Angpt4 resulted in defective venous development causing impaired venous drainage and defects in neuronal cells. In vitro characterization of angiopoietin-4 proteins revealed both ligand-specific and redundant functions among the angiopoietins. Our study identifies Angpt4 as the first growth factor for venous-specific development and its importance in venous remodeling, retinal fluid clearance and neuronal function.

Therapeutic targeting of the angiopoietin-TIE pathway

The endothelial angiopoietin (ANG)-TIE growth factor receptor pathway regulates vascular permeability and pathological vascular remodelling during inflammation, tumour angiogenesis and metastasis. Drugs that target the ANG-TIE pathway are in clinical development for oncological and ophthalmological applications. The aim is to complement current vascular endothelial growth factor (VEGF)-based anti-angiogenic therapies in cancer, wet age-related macular degeneration and macular oedema. The unique function of the ANG-TIE pathway in vascular stabilization also renders this pathway an attractive target in sepsis, organ transplantation, atherosclerosis and vascular complications of diabetes. This Review covers key aspects of the function of the ANG-TIE pathway in vascular disease and describes the recent development of novel therapeutics that target this pathway.

Effects and mechanisms of docosahexaenoic acid on the generation of angiopoietin-2 by rat brain microvascular endothelial cells under an oxygen- and glucose-deprivation environment

Objective

The aim of this study was to investigate the effects of docosahexaenoic acid (DHA) on the generation of angiopoietin-2 (Ang-2) by rat brain microvascular endothelial cells under an oxygen- and glucose-deprivation environment (OGD), and its relationship, if any, with cyclooxygenase 2 (COX-2) expression.

Methods

Annexin V and propidium iodide apoptosis assay was used to detect apoptosis. Enzyme linked immunosorbent assay was used to detect Ang-2, vascular endothelial growth factor (VEGF), prostaglandin E2 (PGE2), and prostaglandin I2 (PGI2) content. Reverse transcription polymerase chain reaction (RT-PCR) was used to detect Ang-2 and VEGF mRNA expression. Western blot was used to detect expression of COX-2 protein.

Results

DHA reduced the apoptosis rate (P = 0.026) and decreased the secretion of Ang-2, VEGF, PGE2, and PGI2 (P = 0.006, P = 0.000, P = 0.002, P = 0.004 respectively). The relative expression of Ang2 and Vegf mRNA, as well as COX-2 expression, also decreased (P = 0.000, P = 0.005, P = 0.007 respectively). These effects were antagonized by GW9662 (peroxisome proliferator-activated receptor-γ antagonist). COX-2 protein expression levels were positively correlated with Ang2 and Vegf mRNA expression levels (γ = 0.69, P = 0.038 and γ = 0.76, P = 0.032, respectively). Ang-2 and VEGF mRNA levels were positively correlated with Ang-2 (γ = 0.84, P = 0.012) and VEGF (γ = 0.71, P = 0.036) secretion levels respectively.

Conclusion

DHA reduced apoptosis induced by an OGD environment, thus decreasing Ang-2 and VEGF synthesis. This phenomenon was associated with a decrease in COX-2 protein expression, PGE2 and PGI2 secretion, and generation regulation via intracellular transcriptional pathways.

Cross Talk between Proliferative, Angiogenic, and Cellular Mechanisms Orchestred by HIF-1α in Psoriasis

Psoriasis is a chronic inflammatory skin disease where the altered regulation in angiogenesis, inflammation, and proliferation of keratinocytes are the possible causes of the disease, and the transcription factor “hypoxia-inducible factor 1-alpha” (HIF-1α) is involved in the homeostasis of these three biological phenomena. In this review, the role of HIF-1α in the cross talk between the cytokines and cells of the immunological system involved in the pathogenesis of psoriasis is discussed.

Short hairpin RNA-mediated knockdown of VEGFA in Müller cells reduces intravitreal neovascularization in a rat model of retinopathy of prematurity

Vascular endothelial growth factor (VEGF) A is implicated in aberrant angiogenesis and intravitreous neovascularization (IVNV) in retinopathy of prematurity (ROP). However, VEGFA also regulates retinal vascular development and functions as a retinal neural survival factor. By using a relevant ROP model, the 50/10 oxygen-induced retinopathy (OIR) model, we previously found that broad inhibition of VEGFA bioactivity using a neutralizing antibody to rat VEGF significantly reduced IVNV area compared with control IgG but also significantly reduced body weight gain in the pups, suggesting an adverse effect. Therefore, we propose that knockdown of up-regulated VEGFA in cells that overexpress it under pathological conditions would reduce IVNV without affecting physiological retinal vascular development or overall pup growth. Herein, we determined first that the VEGFA mRNA signal was located within the inner nuclear layer corresponding to CRALBP-labeled Müller cells of pups in the 50/10 OIR model. We then developed a lentiviral-delivered miR-30eembedded shRNA against VEGFA that targeted Müller cells. Reduction of VEGFA by lentivector VEGFA-shRNAetargeting Müller cells efficiently reduced 50/10 OIR up-regulated VEGFA and IVNV in the model, without adversely affecting physiological retinal vascular development or pup weight gain. Knockdown of VEGFA in rat Müller cells by lentivector VEGFA-shRNA significantly reduced VEGFR2 phosphorylation in retinal vascular endothelial cells. Our results suggest that targeted knockdown of overexpressed VEGFA in Müller cells safely reduces IVNV in a relevant ROP model.

Physiological and pathological angiogenesis in the adult pulmonary circulation

Angiogenesis occurs during growth and physiological adaptation in many systemic organs, for example, exercise-induced skeletal and cardiac muscle hypertrophy, ovulation, and tissue repair. Disordered angiogenesis contributes to chronic inflammatory disease processes and to tumor growth and metastasis. Although it was previously thought that the adult pulmonary circulation was incapable of supporting new vessel growth, over that past 10 years new data have shown that angiogenesis within this circulation occurs both during physiological adaptive processes and as part of the pathogenic mechanisms of lung diseases. Here we review the expression of vascular growth factors in the adult lung, their essential role in pulmonary vascular homeostasis and the changes in their expression that occur in response to physiological challenges and in disease. We consider the evidence for adaptive neovascularization in the pulmonary circulation in response to alveolar hypoxia and during lung growth following pneumonectomy in the adult lung. In addition, we review the role of disordered angiogenesis in specific lung diseases including idiopathic pulmonary fibrosis, acute adult distress syndrome and both primary and metastatic tumors of the lung. Finally, we examine recent experimental data showing that therapeutic enhancement of pulmonary angiogenesis has the potential to treat lung diseases characterized by vessel loss.

Prolonged mechanical ventilation alters the expression pattern of angio-neogenetic factors in a pre-clinical rat model

OBJECTIVE

Mechanical ventilation (MV) is a life saving intervention for patients with respiratory failure. Even after 6 hours of MV, diaphragm atrophy and dysfunction (collectively referred to as ventilator-induced diaphragmatic dysfunction, VIDD) occurs in concert with a blunted blood flow and oxygen delivery. The regulation of hypoxia sensitive factors (i.e. hypoxia inducible factor 1α, 2α (HIF-1α,-2α), vascular endothelial growth factor (VEGF)) and angio-neogenetic factors (angiopoietin 1-3, Ang) might contribute to reactive and compensatory alterations in diaphragm muscle.

METHODS

Male Wistar rats (n = 8) were ventilated for 24 hours or directly sacrificed (n = 8), diaphragm and mixed gastrocnemius muscle tissue was removed. Quantitative real time PCR and western blot analyses were performed to detect changes in angio-neogenetic factors and inflammatory markers. Tissues were stained using Isolectin (IB 4) to determine capillarity and calculate the capillary/fiber ratio.

RESULTS

MV resulted in up-regulation of Ang 2 and HIF-1α mRNA in both diaphragm and gastrocnemius, while VEGF mRNA was down-regulated in both tissues. HIF-2α mRNA was reduced in both tissues, while GLUT 4 mRNA was increased in gastrocnemius and reduced in diaphragm samples. Protein levels of VEGF, HIF-1α, -2α and 4 did not change significantly. Additionally, inflammatory cytokine mRNA (Interleukin (IL)-6, IL-1β and TNF α) were elevated in diaphragm tissue.

CONCLUSION

The results demonstrate that 24 hrs of MV and the associated limb disuse induce an up-regulation of angio-neogenetic factors that are connected to HIF-1α. Changes in HIF-1α expression may be due to several interactions occurring during MV.

The hypoxic testicle: physiology and pathophysiology

Mammalian spermatogenesis is a complex biological process occurring in the seminiferous tubules in the testis. This process represents a delicate balance between cell proliferation, differentiation, and apoptosis. In most mammals, the testicles are kept in the scrotum 2 to 7°C below body core temperature, and the spermatogenic process proceeds with a blood and oxygen supply that is fairly independent of changes in other vascular beds in the body. Despite this apparently well-controlled local environment, pathologies such as varicocele or testicular torsion and environmental exposure to low oxygen (hypoxia) can result in changes in blood flow, nutrients, and oxygen supply along with an increased local temperature that may induce adverse effects on Leydig cell function and spermatogenesis. These conditions may lead to male subfertility or infertility. Our literature analyses and our own results suggest that conditions such as germ cell apoptosis and DNA damage are common features in hypoxia and varicocele and testicular torsion. Furthermore, oxidative damage seems to be present in these conditions during the initiation stages of germ cell damage and apoptosis. Other mechanisms like membrane-bound metalloproteinases and phospholipase A2 activation could also be part of the pathophysiological consequences of testicular hypoxia.

Angiopoietin-2 levels are associated with retinopathy and predict mortality in Malawian children with cerebral malaria: a retrospective case-control study*

OBJECTIVE

To investigate the relationship among the angiopoietin-Tie-2 system, retinopathy, and mortality in children with cerebral malaria.

DESIGN

A case-control study of retinopathy-positive vs. retinopathy-negative children with clinically defined cerebral malaria.

SETTING

Queen Elizabeth Central Hospital in Blantyre, Malawi.

SUBJECTS

One hundred fifty-five children presenting with severe malaria and meeting a strict definition of clinical cerebral malaria (Blantyre Coma Score ≤ 2, Plasmodium falciparum parasitemia, no other identifiable cause for coma) were included in the study.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

Clinical and laboratory parameters were recorded at admission and funduscopic examinations were performed. Admission levels of angiopoietin-1, angiopoietin-2, and a soluble version of their cognate receptor were measured by enzyme-linked immunosorbent assay. We show that angiopoietin-1 levels are decreased and angiopoietin-2 and soluble Tie-2 levels are increased in children with cerebral malaria who had retinopathy compared with those who did not. Angiopoietin-2 and soluble Tie-2 were independent predictors of retinopathy (adjusted odds ratio [95% CI], angiopoietin-2, 4.3 [1.3-14.6], p = .019; soluble Tie-2, 9.7 [2.1-45.8], p = .004). Angiopoietin-2 and soluble Tie-2 were positively correlated with the number of hemorrhages, the severity or retinal whitening, and the extent of capillary whitening observed on funduscopic examination (p < .05 after adjustment for multiple comparisons). Angiopoietin-2 and soluble Tie-2 levels were elevated in children with cerebral malaria who subsequently died and angiopoetin-2 was an independent predictor of death (adjusted odds ratio: 3.9 [1.2-12.7], p = .024). When combined with clinical parameters, angiopoetin-2 improved prediction of mortality using logistic regression models and classification trees.

CONCLUSIONS

These results provide insights into mechanisms of endothelial activation in cerebral malaria and indicate that the angiopoietin-Tie-2 axis is associated with retinopathy and mortality in pediatric cerebral malaria.

Expression and functional roles of angiopoietin-2 in skeletal muscles

Background

Angiopoietin-1 (ANGPT1) and angiopoietin-2 (ANGPT2) are angiogenesis factors that modulate endothelial cell differentiation, survival and stability. Recent studies have suggested that skeletal muscle precursor cells constitutively express ANGPT1 and adhere to recombinant ANGPT1 and ANGPT2 proteins. It remains unclear whether or not they also express ANGPT2, or if ANGPT2 regulates the myogenesis program of muscle precursors. In this study, ANGPT2 regulatory factors and the effects of ANGPT2 on proliferation, migration, differentiation and survival were identified in cultured primary skeletal myoblasts. The cellular networks involved in the actions of ANGPT2 on skeletal muscle cells were also analyzed.

Methodology/Principal Findings

Primary skeletal myoblasts were isolated from human and mouse muscles. Skeletal myoblast survival, proliferation, migration and differentiation were measured in-vitro in response to recombinant ANGPT2 protein and to enhanced ANGPT2 expression delivered with adenoviruses. Real-time PCR and ELISA measurements revealed the presence of constitutive ANGPT2 expression in these cells. This expression increased significantly during myoblast differentiation into myotubes. In human myoblasts, ANGPT2 expression was induced by H₂O₂, but not by TNFα, IL1β or IL6. ANGPT2 significantly enhanced myoblast differentiation and survival, but had no influence on proliferation or migration. ANGPT2-induced survival was mediated through activation of the ERK1/2 and PI-3 kinase/AKT pathways. Microarray analysis revealed that ANGPT2 upregulates genes involved in the regulation of cell survival, protein synthesis, glucose uptake and free fatty oxidation.

Conclusion/Significance

Skeletal muscle precursors constitutively express ANGPT2 and this expression is upregulated during differentiation into myotubes. Reactive oxygen species exert a strong stimulatory influence on muscle ANGPT2 expression while pro-inflammatory cytokines do not. ANGPT2 promotes skeletal myoblast survival and differentiation. These results suggest that muscle-derived ANGPT2 production may play a positive role in skeletal muscle fiber repair.

Angiopoietin-1 protects heart against ischemia/reperfusion injury through VE-cadherin dephosphorylation and myocardiac integrin-β1/ERK/caspase-9 phosphorylation cascade

Early reperfusion after myocardial ischemia that is essential for tissue salvage also causes myocardial and vascular injury. Cardioprotection during reperfusion therapy is an essential aspect of treating myocardial infarction. Angiopoietin-1 is an endothelial-specific angiogenic factor. The potential effects of angiopoietin-1 on cardiomyocytes and vascular cells undergoing reperfusion have not been investigated. We propose a protective mechanism whereby angiopoietin-1 increases the integrity of the endothelial lining and exerts a direct survival effect on cardiomyocytes under myocardial ischemia followed by reperfusion. First, we found that angiopoietin-1 prevents vascular leakage through regulating vascular endothelial (VE)-cadherin phosphorylation. The membrane expression of VE-cadherin was markedly decreased on hypoxia/reoxygenation but was restored by angiopoietin-1 treatment. Interestingly, these effects were mediated by the facilitated binding between SH2 domain-containing tyrosine phosphatase (SHP2) or receptor protein tyrosine phosphatase μ (PTPμ) and VE-cadherin, leading to dephosphorylation of VE-cadherin. siRNA against SHP2 or PTPμ abolished the effect of angiopoietin-1 on VE-cadherin dephosphorylation and thereby decreased levels of membrane-localized VE-cadherin. Second, we found that angiopoietin-1 prevented cardiomyocyte death, although cardiomyocytes lack the angiopoietin-1 receptor Tie2. Angiopoietin-1 increased cardiomyocyte survival through integrin-β1-mediated extracellular signal-regulated kinase (ERK) phosphorylation, which inhibited caspase-9 through phosphorylation at Thr¹²⁵ and subsequently reduced active caspase-3. Neutralizing antibody against integrin-β1 blocked these protective effects. In a mouse myocardial ischemia/reperfusion model, angiopoietin-1 enhanced cardiac function and reduction in left ventricular-end systolic dimension (LV-ESD) and left ventricular-end diastolic dimension (LV-EDD) with an increase in ejection fraction (EF) and fractional shortening (FS). Our findings suggest the novel cardioprotective mechanisms of angiopoietin-1 that are achieved by reducing both vascular leakage and cardiomyocyte death after ischemia/reperfusion injury.

Serum levels of angiopoietin-1 in patients with pulmonary hypertension due to mitral stenosis

The molecular basis and pathophysiology of pulmonary hypertension (PH) are rapidly evolving areas. Recently discovered angiopoietins (Ang) constitute a family of growth factors, and whether they play a causal or protective role in pulmonary hypertension has not been fully elucidated. Since left heart disease probably represents the most frequent cause of PH, we sought to determine whether there was a relationship between serum Ang-1 levels and pulmonary hypertension caused by mitral stenosis (MS). The study population was composed of 49 patients with isolated MS. These patients were then divided into group 1 [31 patients with severe MS: mitral valve area (MVA) ≤1.1 cm(2)] and group 2 (18 patients with mild-moderate MS: MVA 1.2-2.0 cm(2)). Twenty-one healthy volunteers comprised the control group (group 3). All of the subjects underwent complete transthoracic echocardiography with determination of systolic pulmonary artery pressure (PAPs). Ang-1 levels were determined in serum. Serum levels of Ang-1 were significantly higher in the control group compared to patients with severe (group 1) and mild-moderate (group 2) MS (p < 0.001). Ang-1 levels were found to have moderate inverse correlation with PAPs and left atrial (LA) diameter (r: -0.620, p < 0.001 and r: -0.489, p < 0.001, respectively). The AUC for the ROC curve for predicting PAPs <50 mmHg by serum Ang-1 level was 0.824 (95% CI 0.722-0.926, p < 0.001). A serum level of Ang-1 above 34,656 pg/ml has 74% sensitivity and 80% specificity for predicting that PH is not severe (PAPs <50 mmHg). In conclusion, the findings of this study are distinctive in the sense that they clearly demonstrate a negative correlation between serum Ang-1 levels and the degree of PH.

The angiopietin-1-Tie2 pathway prevents rather than promotes pulmonary arterial hypertension in transgenic mice

The role of the angiopoietin-1 (Ang1)-Tie2 pathway in the pathogenesis of pulmonary arterial hypertension (PAH) is controversial. Although Ang1 is well known to prevent endothelial activation and injury in systemic vascular beds, this pathway has been suggested to mediate pulmonary vascular remodeling in PAH. Therefore, we used transgenic models to determine the effect of increased or decreased Tie2 activity on the development of PAH. We now report modest spontaneous elevation in right ventricular systolic pressure in Tie2-deficient mice (Tie2(+/-)) compared with wild-type (WT) littermate controls, which was exacerbated upon chronic exposure to the clinically relevant PAH triggers, serotonin (5-HT) or interleukin-6 (IL-6). Moreover, overexpression of Ang1 in transgenic mice had no deleterious effect on pulmonary hemodynamics and, if anything, blunted the response to 5-HT. Exposure to 5-HT or IL-6 also decreased lung Ang1 expression, further reducing Tie2 activity and inducing pulmonary apoptosis in the Tie2(+/-) group only. Similarly, cultured pulmonary artery endothelial cells subjected to Tie2 silencing demonstrated increased susceptibility to apoptosis after 5-HT treatment. Finally, treatment of Tie2-deficient mice with Z-VAD, a pan-caspase inhibitor, prevented the pulmonary hypertensive response to 5-HT. Thus, these findings firmly establish that endothelial survival signaling via the Ang1-Tie2 pathway is protective in PAH.

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.

Roles of angiopoietins in kidney development and disease

Angiopoietins are a family of growth factors, the best studied being angiopoietin 1 (Ang-1), which binds to and tyrosine-phosphorylates endothelial Tie-2, causing enhanced survival and cell-cell stabilization. Ang-2 and Tie-1 downregulate Ang-1-induced Tie-2 signaling, and angiopoietin actions are further modified by vascular endothelial growth factor A and integrins. Metanephric capillaries express Tie genes, whereas metanephric mesenchyme, maturing tubules, and mature podocytes express Ang-1. Ang-1 null embryos begin to form blood vessels, but subsequent vascular remodeling fails, and analyses of chimeric wild-type/Tie null mutant embryos show that Tie genes are needed for renal endothelial survival. Ang-2 is transiently expressed in renal arterial smooth muscle and mesangial cells, and tubules around adult vasa rectae express Ang-2. Ang-2 null mice have increased pericytes around kidney cortical peritubular capillaries, perhaps an indirect consequence of upregulated Tie-2 signaling. Ang-1 therapies attenuate peritubular capillary loss in adult models of tubulointerstitial disease, although, in one study, this was accompanied by enhanced inflammation and fibrosis. Podocyte-directed Ang-2 transgenic overexpression causes glomerular endothelial apoptosis, downregulated nephrin expression, and increased albuminuria, and glomerular Ang-2 is upregulated in hyperglycemic and immune-mediated glomerulopathies. Thus, angiopoietins affect podocyte as well as glomerular endothelial biology, and imbalanced angiopoietin signaling contributes to glomerular pathobiology.

Regulation of angiopoietin expression by bacterial lipopolysaccharide

Angiopoietins are ligands for Tie-2 receptors and play important roles in angiogenesis and inflammation. While angiopoietin-1 (Ang-1) inhibits inflammatory responses, angiopoietin-2 (Ang-2) promotes cytokine production and vascular leakage. In this study, we evaluated in vivo and in vitro effects of Escherichia coli lipopolysaccharides (LPS) on angiopoietin expression. Wild-type C57/BL6 mice were injected with saline (control) or E. coli LPS (20 mg/ml ip) and killed 6, 12, and 24 h later. The diaphragm, lung, and liver were excised and assayed for mRNA and protein expression of Ang-1, Ang-2, and Tie-2 protein and tyrosine phosphorylation. LPS injection elicited a severalfold rise in Ang-2 mRNA and protein levels in the three organs. By comparison, both Ang-1 and Tie-2 levels in the diaphragm, liver, and lung were significantly attenuated by LPS administration. In addition, Tie-2 tyrosine phosphorylation in the lung was significantly reduced in response to LPS injection. In vitro exposure to E. coli LPS elicited cell-specific changes in Ang-1 expression, with significant induction in Ang-1 expression being observed in cultured human epithelial cells, whereas significant attenuation of Ang-1 expression was observed in response to E. coli LPS exposure in primary human skeletal myoblasts. In both cell types, E. coli LPS elicited substantial induction of Ang-2 mRNA, a response that was mediated in part through NF-kappaB. We conclude that in vivo endotoxemia triggers functional inhibition of the Ang-1/Tie-2 receptor pathway by reducing Ang-1 and Tie-2 expression and inducing Ang-2 levels and that this response may contribute to enhanced vascular leakage in sepsis.

Downregulation of angiopoietin-1 and Tie2 in chronic hypoxic pulmonary hypertension

BACKGROUND

Angiopoietins, newly discovered vascular-specific growth factors, and vascular endothelial growth factors (VEGF) play distinct and complementary roles in angiogenesis and vascular maturation. However, the exact roles of angiogenic factors in the adult pulmonary vasculature remain unclear.

OBJECTIVE

To elucidate possible roles of angiopoietins and VEGF in the development of hypoxic pulmonary hypertension (PH), changes in the expression of angiogenic factors were examined.

METHODS

The cellular distribution and expression of angiopoietins and their receptor Tie2 and VEGF were investigated by RT-PCR, immunoblot, and immunohistochemical methods in rat lung under normal and hypoxic conditions.

RESULTS

During the development of PH with vascular remodeling characterized by a decrease in vessel density of intrapulmonary arteries, protein expression of angiopoietin-1 (Ang-1), Tie2, and VEGF significantly decreased in the pulmonary arteries, and Tie2 receptor was inactivated in the lung. The expression of angiopoietin-3 (Ang-3), an endogenous antagonist of Ang-1, significantly increased in the intima under hypoxic conditions.

CONCLUSIONS

Since both Ang-1/Tie2 and VEGF promote angiogenesis and vascular survival, and play protective roles in the adaptation of microvascular changes during the onset of PH, the downregulation of both Ang-1/Tie2 and VEGF and upregulation of Ang-3 appear to be associated with vascular rarefaction and the development of hypoxic PH.

Kinetics of strain-dependent differential gene expression in oxygen-induced retinopathy in the rat

Recent evidence suggests that retinopathy of prematurity, a potentially blinding condition of premature human neonates, has a genetically-determined component. Different inbred strains of rat exhibit differential susceptibility to oxygen-induced retinopathy (OIR), a well-established experimental model of retinopathy of prematurity. To explore the basis for this differential susceptibility, we quantified the retinal expression of 8 angiogenesis-related genes during early post-natal retinal development in rats with OIR. Inbred Fischer 344 (F344), Dark Agouti (DA) and Sprague Dawley (SPD) rat neonates were exposed to alternating cycles of 80% oxygen in air and normoxia for up to 14 days. After 14 days of cyclic hyperoxic exposure, some rats were exposed to normoxia for a further 4 days. Retinal mRNA for vascular endothelial growth factor (VEGF), VEGF receptor 2 (VEGFR2), pigment epithelium-derived factor (PEDF), angiopoietin-2 (Ang2), Tie2, cyclooxygenase-2 (COX2), insulin-like growth factor-1 (IGF1) and erythropoietin (EPO) were quantified by real-time reverse-transcriptase polymerase chain reaction at different time-points. Time-course analysis showed that expression of mRNA for VEGF, VEGFR2 and Ang2 was significantly greater in OIR-resistant (F344) retinae than in OIR-susceptible (DA) retinae during the first 9 days of cyclic hyperoxia. However, at post-natal days 14 and 18, retinal mRNAs for VEGF, EPO, VEGFR2, Ang2, IGF1, COX2 and PEDF were expressed to a significantly greater extent in OIR-susceptible (DA, SPD) than OIR-resistant (F344) retinae. The VEGF/PEDF ratio was greater in the F344 compared with the DA strain up to day 9, but was higher in the DA than the F344 strain at days 14 and 18. Thus, we found that retinal expression of angiogenesis-related genes was significantly higher in OIR-resistant rats than in OIR-susceptible rats during early retinal development, but the pattern reversed during the proliferative phase of OIR. We conclude that susceptibility to OIR correlates with differential gene expression very early in retinal microvascular development, during periods of cyclic hyperoxic exposure rather than during subsequent sustained hypoxia.

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.

Negative regulation of VEGF-induced vascular leakage by blockade of angiotensin II type 1 receptor

OBJECTIVE

Permeability of blood vessels is essential for tissue homeostasis. However, disorganized hyperpermeability leads to progression of diseases. Vascular endothelial growth factor-A (VEGF) is a key regulator for leakiness of blood vessels and it has been reported that VEGF-mediated hyperpermeability was suppressed by angiopoietin-1 (Ang1). We found that Angiotensin-converting enzyme (ACE) was downregulated in endothelial cells by Ang1. ACE converts angiotensin I to angiotensin II (AII). Here, we studied the relationship between VEGF and AII relative to vascular permeability.

METHODS AND RESULTS

We showed that VEGF-mediated vascular hyperpermeability was suppressed in mice given AII type 1 receptor (AT1R) blocker (ARB); the effect was also seen in AT1R-deficient mice. In this system, we found that ARB inhibited VEGF-induced gap formation. Furthermore, we ascertained that angioedema induced by overexpression of VEGF decreased noticeably in ARB-treated ischemic mice.

CONCLUSIONS

Because ARB suppressed VEGF-induced vascular hyperpermeability, we propose that ARB may be used to minimize the risk of edema in therapeutic angiogenesis using VEGF.

Excess circulating angiopoietin-2 may contribute to pulmonary vascular leak in sepsis in humans

BACKGROUND

Acute respiratory distress syndrome (ARDS) is a devastating complication of numerous underlying conditions, most notably sepsis. Although pathologic vascular leak has been implicated in the pathogenesis of ARDS and sepsis-associated lung injury, the mechanisms promoting leak are incompletely understood. Angiopoietin-2 (Ang-2), a known antagonist of the endothelial Tie-2 receptor, was originally described as a naturally occurring disruptor of normal embryonic vascular development otherwise mediated by the Tie-2 agonist angiopoietin-1 (Ang-1). We hypothesized that Ang-2 contributes to endothelial barrier disruption in sepsis-associated lung injury, a condition involving the mature vasculature.

METHODS AND FINDINGS

We describe complementary human, murine, and in vitro investigations that implicate Ang-2 as a mediator of this process. We show that circulating Ang-2 is significantly elevated in humans with sepsis who have impaired oxygenation. We then show that serum from these patients disrupts endothelial architecture. This effect of sepsis serum from humans correlates with measured Ang-2, abates with clinical improvement, and is reversed by Ang-1. Next, we found that endothelial barrier disruption can be provoked by Ang-2 alone. This signal is transduced through myosin light chain phosphorylation. Last, we show that excess systemic Ang-2 provokes pulmonary leak and congestion in otherwise healthy adult mice.

CONCLUSIONS

Our results identify a critical role for Ang-2 in disrupting normal pulmonary endothelial function.

Role of angiopoietin-1 in experimental and human pulmonary arterial hypertension

INTRODUCTION

The pulmonary microvasculature, consisting mainly of an endothelial cell (EC) monolayer and scant matrix support, is incompletely muscularized. Thus, the distal pulmonary arterioles may be predisposed to regression on exposure to environmental stresses (ie, hypoxia) and may be dependent on EC survival factors, like angiopoietin (Ang) 1, to attenuate the development of pulmonary arterial hypertension (PAH). In order to clarify the link between Ang1 expression and the development of PAH in patients, we also studied messenger RNA and protein expression in lung samples from healthy control subjects and patients with idiopathic PAH (IPAH) or PAH associated with other diseases (APAH).

METHODS

Ang/Tie2 gene expression was assessed in rats that had been exposed to hypoxia (ie, 10% O2) for 1, 3, or 7 days. In a separate experiment, the cell-based gene transfer of Ang1/Ang2 was performed, and the effects were evaluated in rats with hypoxia-induced PAH.

RESULTS

Hypoxia induced significant early increases in right ventricular systolic pressure (RVSP) and right ventricle/left ventricle-plus-septum mass ratio (RV/[LV + S]), with a significant decrease in Tie2 expression. Hypoxic rats receiving Ang1 demonstrated significant improvements in RVSP and RV/(LV + S), with a partial normalization in Tie2 protein levels. Robust Ang1 expression was observed in healthy human lungs. Furthermore, there were no significant changes in the levels of Ang1 or Ang2 in IPAH or APAH samples vs those in control subjects.

CONCLUSIONS

Decreased activity of the Tie2 pathway with hypoxia may contribute to PAH, possibly by loss of EC survival signaling, which can be overcome by Ang1 gene transfer.

VEGF-induced BBB permeability is associated with an MMP-9 activity increase in cerebral ischemia: both effects decreased by Ang-1

After cerebral ischemia, angiogenesis, by supplying for the deficient perfusion, may be a beneficial process for limiting neuronal death and promoting tissue repair. In this study, we showed that the combination of Ang-1 and vascular endothelial growth factor (VEGF) provides a more adapted therapeutic strategy than the use of VEGF alone. Indeed, we showed on a focal ischemia model that an early administration of VEGF exacerbates ischemic damage, because of its effects on blood-brain barrier (BBB) permeability. In contrast, a coapplication of Ang-1 and VEGF leads to a significant reduction of the ischemic and edema volumes by 50% and 42%, respectively, in comparison with VEGF-treated mice. We proposed that Ang-1 blocks the BBB permeability effect of VEGF in association with a modulation of matrix metalloproteinase (MMP) activity. Indeed, we showed on both ischemic in vivo and BBB in vitro models that VEGF enhances BBB damage and MMP-9 activity and that Ang-1 counteracts both effects. However, we also showed a synergic angiogenic effect of Ang-1 and VEGF in the brain. Taken together, these results allow to propose that, in cerebral ischemia, the combination of Ang-1 and VEGF could be used early to promote the formation of mature neovessels without inducing side effects on BBB permeability.

Expression and regulation of murine macrophage angiopoietin-2

Our understanding of angiogenesis has increased significantly in the past few years with the discovery of angiopoietins (Ang). Specifically, Ang2 has been associated with pathologic as well as normal vascularization. While previous studies have shown that a major source of Ang2 has been endothelial cells and tumor cells, we reasoned that macrophages would also have the ability to express angiopoietins, specifically Ang2, due to that cell's role in wound healing, tumor angiogenesis, and a number of non-oncological diseases, such as rheumatoid arthritis and psoriasis. In this study, murine macrophages constitutively expressed both transcripts and protein for Ang2 but not Ang1 or Ang3. The secretion of Ang2 was enhanced by treatment with lipopolysaccharide, interferon-gamma, prostaglandin E2 and other cyclic AMP-elevating agents, as well as vascular endothelial growth factor (VEGF). Cyclic AMP-dependent protein kinase (PKA) played a major role in this enhancement since the PKA inhibitor, H89, blocked secretion of Ang2. Since stimulation of the PKA pathway can lead to macrophage production of VEGF, it is possible that enhancement of Ang2 production by macrophages may be due to autocrine responsiveness to VEGF. Adding anti-VEGF antibodies to the supernatants of stimulated macrophages blocked secretion of Ang2. This study is the first to show murine macrophage production of Ang2 and to provide evidence that it can be regulated. Understanding the regulation of macrophage Ang2 production is especially important in an effort to target the pathologic role of macrophages while preserving their role in immunity and homeostasis.

Preferential activity of Tie2 promoter in arteriolar endothelium

The tyrosine kinase Tie2/Tek (the receptor for angiopoietins) is considered one of the most reliable markers of the endothelial phenotype, across organisms, organs, and developmental stages. However, endothelium is intrinsically heterogeneous in origin, composition and function, presenting an arteriolar/venular asymmetry. In this regard, the expression of Tie2 along the vascular tree, although thought to be homogenous, has not been systematically investigated. Therefore we questioned whether the activity of Tie2 promoter is uniform in the microvascular endothelium. To this end, we analyzed in situ the expression of the markers beta-galactosidase [LacZ(Tie2)] and green fluorescent protein (GFP) [GFP(Tie2)], placed under the Tie2 promoter in transgenic mice, in whole mount tissue samples, which allow the simultaneous evaluation of its relative distribution in various microvascular compartments. In the mesenteries of LacZ(Tie2) and GFP(Tie2) mice, we found that the activity of Tie2 promoter is asymmetrically distributed, being much stronger in arteries and arterioles than on the venular side of the vascular tree. This observation was replicated in the diaphragm of LacZ(Tie2) mice. The capillaries presented a mosaic pattern of Tie2 promoter activity. Stimulation of angiogenesis either by wounding, or by intraperitoneal injection of Vascular Endothelial Growth Factor (VEGF), revealed that the arteriolar/venular asymmetry is established at endothelial cellular level early during new capillary formation, even before the starting of the microvascular blood flow. In conclusion, a strong Tie2 promoter activity qualifies as a novel marker of the arteriolar phenotype in microvascular endothelium.

Biological characterization of angiopoietin-3 and angiopoietin-4

The angiopoietin (Ang) family of growth factors includes Ang1, Ang2, Ang3, and Ang4, all of which bind to the endothelial receptor tyrosine kinase Tie2. Ang3 (mouse) and Ang4 (human) are interspecies orthologs. In experiments with human endothelial cell lines, Ang3 was identified as an antagonist of Tie2 and Ang4 was identified as an agonist of Tie2. However, the biological roles of Ang3 and Ang4 are unknown. We examined the biological effect of recombinant Ang3 and Ang4 proteins in primary cultured endothelial cells and in vivo in mice. Recombinant Ang3 and Ang4 formed disulfide-linked dimers. Ang4 (400 ng/mL) markedly increased Tie2 and Akt phosphorylation in primary cultured HUVECs whereas Ang3 (400 ng/mL) did not produce significant changes. Accordingly, Ang4, but not Ang3, induced survival and migration in primary cultured HUVECs. Unexpectedly, intravenously administered Ang3 (30 microg) was more potent than Ang4 (30 microg) in phosphorylating the Tie2 receptor in lung tissue from mice in vivo. Accordingly, Ang3 was more potent than Ang4 in phosphorylating Akt in primary cultured mouse lung microvascular endothelial cells. Ang3 and Ang4 both produced potent corneal angiogenesis extending from the limbus across the mouse cornea in vivo. Thus, Ang3 and Ang4 are agonists of Tie2, but mouse Ang3 has strong activity only on endothelial cells of its own species.

Angiopoietin-1 promotes cardiac and skeletal myocyte survival through integrins

Cardiac myocyte loss, regardless of insult, can trigger compensatory myocardial remodeling leading to heart failure. Identifying mediators of cardiac myocyte survival may advance clinical efforts toward myocardial preservation. Angiopoietin-1 limits ischemia-induced cardiac injury. This benefit is ascribed to angiogenesis because the receptor, tie2, is largely endothelial-specific. We propose that direct, non-tie2 interactions of angiopoietin-1 on cardiac myocytes contribute to this cardioprotection. We found that mouse C2C12 skeletal myocytes lack tie2, yet dose-dependently adhered to angiopoietin-1 and angiopoietin-2 similarly to laminin, fibronectin, vitronectin, and more than to collagen-I, -III, and -IV. Adhesion was divalent cation-mediated (Mn2+, Ca2+, not Mg2+), blocked with EDTA/EGTA, RGD-based peptides, and select integrin subunit antibodies. Similar findings were obtained with human skeletal myocytes (HSMs) and freshly isolated rat neonatal cardiac myocytes (NCMs). Furthermore, angiopoietin-1 conferred significant survival advantage exceeding that of most cell matrices, which was not fully explained by differences in cell adhesion. Angiopoietin-1 promoted survival of serum-starved C2C12, HSM, and NCM (MTT, trypan blue) and prevented taxol-induced apoptosis (caspase-3). Immobilized and soluble angiopoietin-1 phosphorylated Akt(S473) and MAPK(p42/44), (not FAK(Y397)) in C2C12 more than in endothelial cells and more than did angiopoietin-2 or cell matrices. EDTA, RGD-based peptides, and some integrin antibodies blocked these responses. Angiopoietin-1 activated HSM and NCM Akt(S473) and MAPK(p42/44) survival pathways. We propose that this novel function contributes to developmental and cardioprotective actions of angiopoietin-1 presently attributed to vascular effects alone. Angiopoietin-1 may prove therapeutically valuable in cardiac remodeling by supporting myocyte viability and preserving pump function. The full text of this article is available online at http://circres.ahajournals.org.

Expression of angiopoietins in renal epithelial and clear cell carcinoma cells: regulation by hypoxia and participation in angiogenesis

The hereditary von Hippel-Lindau (VHL) syndrome predisposes sufferers to highly vascularized tumors such as renal clear cell carcinoma (RCC) and central nervous system hemangioblastoma. In RCC4 and RCC786-0 VHL- cells with VHL mutations, the protein of hypoxia-inducible factor-1alpha (HIF-1alpha) is constitutively stabilized and the mRNA levels of HIF target genes, including vascular endothelial growth factor (VEGF), are elevated. However, the expression of angiopoietins in these cells and their involvement in angiogenesis are not well known. In this study, we compared the mRNA levels of angiopoietins in human kidney proximal tubule epithelial (RPTE) and RCC4 and RCC786-0 VHL- cells. In RPTE cells, angiopoietin-4 (Ang-4) expression was selectively induced by hypoxia or by expression of a hybrid form of HIF-1alpha. Under normoxic conditions, the mRNA levels of Ang-4 were higher in RCC4 and RCC786-0 VHL- than RPTE cells. Angiopoietin-1 expression was detectable in RCC4 and RCC786-0 VHL- cells but not RPTE cells. In RCC786-0 VHL+ cells, which were stably transfected with a wild-type copy of VHL, the mRNA levels of VEGF and Ang-4 were suppressed and the hypoxic response was restored. We also demonstrated that stimulation of endothelial tube formation by conditioned medium harvested from RCC4 cells was inhibited by a soluble Tie-2 receptor. These results suggest that the angiopoietin/Tie-2 system may participate in the angiogenic response to hypoxia in renal tissues and in tumor angiogenesis in renal carcinoma.

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

BACKGROUND/AIMS

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

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

OBJECTIVES

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Peritubular capillary loss after mouse acute nephrotoxicity correlates with down-regulation of vascular endothelial growth factor-A and hypoxia-inducible factor-1 alpha

Although the response of kidneys acutely damaged by ischemia or toxins is dominated by epithelial destruction and regeneration, other studies have begun to define abnormalities in the cell biology of the renal microcirculation, especially with regard to peritubular capillaries. We explored the integrity of peritubular capillaries in relation to expression of vascular endothelial growth factor (VEGF)-A, hypoxia-inducible factor (HIF)-alpha proteins, and von Hippel-Lindau protein (pVHL) in mouse folic acid nephropathy, a model in which acute tubular damage is followed by partial regeneration and progression to patchy chronic histological damage. Throughout a period of 14 days, in areas of cortical tubular atrophy and interstitial fibrosis, loss of VEGFR-2 and platelet endothelial cell adhesion molecule-expressing peritubular capillaries was preceded by marked decreases in VEGF-A transcript and protein levels. Nephrotoxicity was associated with tissue hypoxia, especially in regenerating tubules, as assessed by an established in situ method. Despite the hypoxia, levels of HIF-1 alpha, a protein known to up-regulate VEGF-A, were reduced. During the course of nephrotoxicity, levels of pVHL, a factor that destabilizes HIF-1 alpha, increased significantly. We speculate that that down-regulation of VEGF-A may be functionally-implicated in the progressive attrition of peritubular capillaries in areas of tubular atrophy and interstitial fibrosis; VEGF-A down-regulation correlates with a loss of HIF-1 alpha expression which itself occurs in the face of increased tissue hypoxia.

Hypoxia-inducible factor-1 mediates activation of cultured vascular endothelial cells by inducing multiple angiogenic factors

Hypoxia-inducible factor-1 (HIF-1) mediates transcriptional activation of vascular endothelial growth factor (VEGF) and other hypoxia-responsive genes. Transgenic expression of a constitutively stable HIF-1alpha mutant increases the number of vascular vessels without vascular leakage, tissue edema, or inflammation. This study aimed to investigate the molecular basis by which HIF-1 mediates the angiogenic response to hypoxia. In primary human endothelial cells, hypoxia, desferrioxamine, or infection with Ad2/HIF-1alpha/VP16, an adenoviral vector encoding a constitutively stable hybrid form of HIF-1alpha, increased the mRNA and protein levels of VEGF, angiopoietin-2 (Ang-2), and angiopoietin-4 (Ang-4). Infection with Ad2/CMVEV (a control vector expressing no transgene) had no effect. Angiopoietin-1 (Ang-1) expression was not detected in human endothelial cells. Ang-4 was also induced by hypoxia or Ad2/HIF-1alpha/VP16 in human cardiac cells, whereas Ang-1 expression remained unchanged. Recombinant Ang-4 protein protected endothelial cells against serum starvation-induced apoptosis and increased cultured endothelial cell migration and tube formation. Ad2/HIF-1alpha/VP16 stimulated endothelial cell proliferation and tube formation. Hypoxia- or Ad2/HIF-1alpha/VP16-induced tube formation was significantly reduced by a Tie-2 inhibitor. These results suggest that HIF-1 mediates the angiogenic response to hypoxia by upregulating the expression of multiple angiogenic factors. Ang-4 can function similarly as Ang-1 and substitute for Ang-1 to participate in hypoxia-induced angiogenesis. Activation of the angiopoietin/Tie-2 system may play a role in the ability of HIF-1 to induce hypervascularity without excessive permeability.

The hypoxic response of tumors is dependent on their microenvironment

To reveal the functional significance of hypoxia and angiogenesis in astrocytoma progression, we created genetically engineered transformed astrocytes from murine primary astrocytes and deleted the hypoxia-responsive transcription factor HIF-1alpha or its target gene, the angiogenic factor VEGF. Growth of HIF-1alpha- and VEGF-deficient transformed astrocytes in the vessel-poor subcutaneous environment results in severe necrosis, reduced growth, and vessel density, whereas when the same cells are placed in the vascular-rich brain parenchyma, the growth of HIF-1alpha knockout, but not VEGF knockout tumors, is reversed: tumors deficient in HIF-1alpha grow faster, and penetrate the brain more rapidly and extensively. These results demonstrate that HIF-1alpha has differential roles in tumor progression, which are greatly dependent on the extant microenvironment of the tumor.

Protective role of angiopoietin-1 in experimental pulmonary hypertension

Angiopoietin-1 (Ang-1), a newly discovered ligand of the endothelial-specific tyrosine kinase receptor Tie-2, has been found to promote cell survival, vascular maturation, and stabilization. We hypothesized that Ang-1 gene transfer to the pulmonary microcirculation would improve pulmonary hemodynamics and vascular remodeling in experimental pulmonary hypertension. Rat pulmonary artery smooth muscle cells were transfected with Ang-1 cDNA or null (pFLAG-CMV-1) vector. Syngeneic Fisher 344 rats were treated with monocrotaline (MCT) (75 mg/kg IP) with or without delivery of 5x10(5) Ang-1-transfected cells into the right jugular vein. After 28 days, plasmid-derived Ang-1 mRNA was consistently and robustly detected by reverse transcriptase-polymerase chain reaction in lungs from all animals receiving Ang-1 gene therapy. Tie-2 receptor expression was markedly downregulated in rats treated with MCT, and this was partially restored by gene therapy with Ang-1. Animals receiving MCT exhibited 77% mortality by 28 days. In contrast, in pAng-1-treated animals, the 28-day mortality was only 14% (P<0.0001). In addition, right ventricular systolic pressure was reduced from 52+/-1.3 mm Hg in the MCT-treated group to 38+/-1.3 mm Hg by Ang-1 gene transfer (P<0.01), whereas the measurement of right to left ventricular plus septal weight ratio was also reduced from 0.41+/-0.03 to 0.31+/-0.01 (P<0.05). Moreover, MCT resulted in increased apoptosis, mainly in the microvasculature, and reduced endothelial NO synthase mRNA expression, both of which were prevented by Ang-1 gene transfer. Thus, cell-based gene transfer with Ang-1 improved survival and pulmonary hemodynamics in experimental pulmonary hypertension by a mechanism involving the inhibition of apoptosis and protection of the pulmonary microvasculature.

Angiopoietin-1-induced PI3-kinase activation prevents neuronal apoptosis

Although angiopoietin-1 (Ang-1) is recognized as an endothelial growth factor, its presence in brain following an ischemic event suggests a role in the evolution of neuronal damage. Using primary neuronal cultures, we showed that neurons express Ang-1 and possess the functional angiopoietin-receptor Tie-2, which is phosphorylated in the presence of Ang-1. We further investigated in vitro whether Ang-1 could protect neurons against either excitotoxic necrosis or apoptosis induced by serum deprivation (SD). A neuroprotective effect for Ang-1 was detected exclusively in the apoptotic paradigm. Treatment of cells with the phosphatidyl-inositol 3-kinase (PI3-K) inhibitor, LY294002, inhibited Ang-1-induced phosphorylation of Akt, restored the cleavage of the effector caspase-3, and reduced the protective effect of Ang-1 against SD-induced toxicity. These findings suggest that Ang-1 has a neuroprotective effect against apoptotic stress and that this effect is dependent on the PI3-K/Akt pathway and inhibition of caspase-3 cleavage. This study provides evidence that Ang-1 is not just angiogenic but also neuroprotective. The understanding of neuroprotective mechanisms induced by Ang-1 may promote strategies based on the pleiotropic effects of angiogenic factors. Such approaches could be useful for the treatment of brain diseases in which both neuronal death and angiogenesis are involved.

Signaling molecules in nonfamilial pulmonary hypertension

BACKGROUND

Biochemical, genetic, and clinical evidence indicates that smooth-muscle proliferation around small pulmonary vessels is an essential part of the pathogenesis of pulmonary hypertension. Mutations in the bone morphogenetic protein receptor type 2 (BMPR2) have been linked to familial cases of pulmonary hypertension, but the molecular basis of the common nonfamilial forms is unknown.

METHODS

We evaluated the pattern of expression of angiopoietin-1, a protein involved in the recruitment of smooth-muscle cells around blood vessels; TIE2, the endothelial-specific receptor for angiopoietin-1; and bone morphogenetic protein receptor type 1A (BMPR1A) and BMPR2 in lung-biopsy specimens from patients with pulmonary hypertension and from normotensive control patients. The effect of angiopoietin-1 on the modulation of BMPR expression was also evaluated in subcultures of human pulmonary arteriolar endothelial cells.

RESULTS

The expression of angiopoietin-1 messenger RNA and the protein itself and the phosphorylation of TIE2 were strongly up-regulated in the lungs of patients with various forms of pulmonary hypertension, correlating directly with the severity of disease. A mechanistic link between familial and acquired pulmonary hypertension was demonstrated by the finding that angiopoietin-1 shuts off the expression of BMPR1A, a transmembrane protein required for BMPR2 signaling, in pulmonary arteriolar endothelial cells. Similarly, we found that the expression of BMPR1A was severely reduced in the lungs of patients with various forms of acquired as well as primary nonfamilial pulmonary hypertension.

CONCLUSIONS

These findings suggest that all forms of pulmonary hypertension are linked by defects in the signaling pathway involving angiopoietin-1, TIE2, BMPR1A, and BMPR2 and consequently identify specific molecular targets for therapeutic intervention.

Angiopoietin-2 and rat brain capillary remodeling during adaptation and deadaptation to prolonged mild hypoxia

Angiogenesis is a crucial component of rat brain adaptation to prolonged hypoxia, but it is not known whether this structural change is permanent or reversed on return to normoxia. Also, the intrinsic mechanisms controlling brain microvascular plasticity in response to oxygen availability remains unclear. Our results indicate that capillary density in the rat cerebral cortex increased by 60% after 3 wk of hypoxia and that it progressively decreased to prehypoxic values after 3 wk of normoxic recovery (deadaptation). Angiopoietin-2 (Ang2) expression in the capillary endothelium was induced between 6 h and 14 days of hypoxia but fell to control levels at 21 days of hypoxia. During deadaptation, Ang2 levels were elevated at 1-14 days but decreased to baseline at 21 days. In contrast, the constitutive expression of Ang1 and Tie2 was not affected during hypoxia or deadaptation. TUNEL-positive endothelial cells and caspase-3 activation were observed at 7 and 14 days of deadaptation. These data suggest that Ang2 might modulate both angiogenesis and vascular regression in the rat brain and that capillary regression occurring during deadaptation involves activation of apoptosis.

Microvessel formation from mouse embryonic aortic explants is oxygen and VEGF dependent

To delineate the roles of O(2) and vascular endothelial growth factor (VEGF) in the process of angiogenesis from the embryonic aorta, we cultured mouse embryonic aorta explants (thoracic level to lateral vessels supplying the mesonephros and metanephros) in a three-dimensional type I collagen gel matrix. During 8 days of culture under 5% O(2), but not room air, the addition of VEGF to explants stimulated the formation of CD31-positive, Flk-1-positive, Gs-IB(4)-positive structures in a concentration-dependent manner. Electron microscopy showed the structures to be capillary-like. VEGF-induced capillary-like structure formation was inhibited by sequestration of VEGF via addition of soluble Flt-1 fusion protein or anti-VEGF antibodies. Expression of Flk-1, but not Flt-1, was increased in embryonic aorta cultured under 5% O(2) relative to room air. Our data suggest that low O(2) upregulates Flk-1 expression in embryonic aorta in vitro and renders it more responsive to VEGF.

Regulation of angiopoietin and Tie-2 receptor expression in non-reproductive tissues by estrogen

Estrogen promotes endothelial cell proliferation and survival in the vasculture of non-reproductive organs. The main mechanisms through which estrogen exerts its effects on endothelial cells remain unknown. Angiopoietins are newly described modulators of endothelial cell survival and they exert their effects through the activation of endothelial cell-specific Tie-2 receptors. In this study, we evaluated whether estrogen modulates the activity and expression of Tie-2 receptors, Ang-1 and its endogenous antagonist; angiopoietins-2 (Ang-2) in non-reproductive organs. Using RT-PCR, we found that daily administration of 17-beta-estradiol for 8 days in ovariectomized rats results in a significant reduction in tissue Ang-1 mRNA expression. By comparison, estrogen therapy produced a significant increase in Ang-2 mRNA in estrogen-treated rats with heart, kidney and lung Ang-2 mRNA levels reaching 169%, 152% and 224% of those of oil-treated animals, respectively. We also observed that tyrosine phosphorylation of Tie-2 receptors is significantly attenuated in ovariectomized rats treated with 17-beta-estradiol. Our results suggest that the effects of estrogen on the vasculature of non-reproductive organs require the inhibition of angiopoietin-1-Tie-2 receptor pathway and that this inhibition is achieved through simultaneous down-regulation of Ang-1 and Tie-2 expression and elevation in Ang-2 expression.