Angiopoietin-1 activates both anti- and proapoptotic mitogen-activated protein kinases

Primary immunodeficiency-related genes and varicella-zoster virus reactivation syndrome: a Mendelian randomization study

Background

Currently, evidence regarding the causal relationship between primary immunodeficiency-related genes and varicella-zoster virus reactivation syndrome is limited and inconsistent. Therefore, this study employs Mendelian randomization (MR) methodology to investigate the causal relationship between the two.

Methods

This study selected 110 single-nucleotide polymorphisms (SNPs) of primary immunodeficiency-related genes as instrumental variables (IVs). Genetic associations of primary immunodeficiency-related genes were derived from recent genome-wide association studies (GWAS) data on human plasma protein levels and circulating immune cells. Data on genes associated with varicella-zoster virus reactivation syndrome were obtained from the GWAS Catalog and FINNGEN database, primarily analyzed using inverse variance weighting (IVW) and sensitivity analysis.

Results

Through MR analysis, we identified 9 primary immunodeficiency-related genes causally associated with herpes zoster and its subsequent neuralgia; determined causal associations of 20 primary immunodeficiency-related genes with three vascular lesions (stroke, cerebral aneurysm, giant cell arteritis); revealed causal associations of 10 primary immunodeficiency-related genes with two ocular diseases (retinopathy, keratitis); additionally, three primary immunodeficiency-related genes each were associated with encephalitis, cranial nerve palsy, and gastrointestinal infections.

Conclusions

This study discovers a certain association between primary immunodeficiency-related genes and varicella-zoster virus reactivation syndrome, yet further investigations are warranted to explore the specific mechanisms underlying these connections.

Novel Targets and Potential Mechanisms of Mizuhopecten yessoensis-Derived Tripeptide NCW as Antihypertensive Peptides

SCOPE

Mizuhopecten yessoensis-derived tripeptide Asn-Cys-Trp (NCW) exhibits a potent antihypertensive effect in vivo. However, a lack of knowledge of the antihypertensive mechanism of tripeptide NCW limits its application for functional foods industrialization. The purpose of this study is to elucidate the corresponding targets and mechanisms of tripeptide NCW in hypertension regulation.

METHODS AND RESULTS

Administration of tripeptide NCW for 3 weeks, the blood pressure of spontaneously hypertensive rats (SHRs) is significantly decreased. After sacrifice, the serum sample is analyzed using tandem mass tag (TMT)-based liquid chromatography with tandem mass spectrometry to identify differentially expressed proteins. The proteomic analysis indicates that tripeptide NCW administration alters serum protein profiles in SHR rats, significantly upregulating 106 proteins and downregulating 30 proteins. These proteins enhance the glycolysis, glucose, and TCA cycle, improve amino metabolism, trigger the cAMP/PKA, cGMP/PKG, PI3K/AKT, and AMPK signal pathways, and inhibit Ras-regulated JNK activation, TGF-β/MAPK, and TGF-β/ RhoA/ROCK pathways.

CONCLUSION

Tripeptide NCW supplementation is demonstrated to regulate signal pathways involved in the control of blood pressure and regulate the energy and amino acids metabolic processes in serum, providing important insights into the protective effects of tripeptide NCW on hypertension.

Loss-of-function mutations of the TIE1 receptor tyrosine kinase cause late-onset primary lymphedema

Primary lymphedema (PL), characterized by tissue swelling, fat accumulation, and fibrosis, results from defects in lymphatic vessels or valves caused by mutations in genes involved in development, maturation, and function of the lymphatic vascular system. Pathogenic variants in various genes have been identified in about 30% of PL cases. By screening of a cohort of 755 individuals with PL, we identified two TIE1 (tyrosine kinase with immunoglobulin- and epidermal growth factor-like domains 1) missense variants and one truncating variant, all predicted to be pathogenic by bioinformatic algorithms. The TIE1 receptor, in complex with TIE2, binds angiopoietins to regulate the formation and remodeling of blood and lymphatic vessels. The premature stop codon mutant encoded an inactive truncated extracellular TIE1 fragment with decreased mRNA stability, and the amino acid substitutions led to decreased TIE1 signaling activity. By reproducing the two missense variants in mouse Tie1 via CRISPR/Cas9, we showed that both cause edema and are lethal in homozygous mice. Thus, our results indicate that TIE1 loss-of-function variants can cause lymphatic dysfunction in patients. Together with our earlier demonstration that ANGPT2 loss-of-function mutations can also cause PL, our results emphasize the important role of the ANGPT2/TIE1 pathway in lymphatic function.

An optogenetic-phosphoproteomic study reveals dynamic Akt1 signaling profiles in endothelial cells

The serine/threonine kinase AKT is a central node in cell signaling. While aberrant AKT activation underlies the development of a variety of human diseases, how different patterns of AKT-dependent phosphorylation dictate downstream signaling and phenotypic outcomes remains largely enigmatic. Herein, we perform a systems-level analysis that integrates methodological advances in optogenetics, mass spectrometry-based phosphoproteomics, and bioinformatics to elucidate how different intensity, duration, and pattern of Akt1 stimulation lead to distinct temporal phosphorylation profiles in vascular endothelial cells. Through the analysis of ~35,000 phosphorylation sites across multiple conditions precisely controlled by light stimulation, we identify a series of signaling circuits activated downstream of Akt1 and interrogate how Akt1 signaling integrates with growth factor signaling in endothelial cells. Furthermore, our results categorize kinase substrates that are preferably activated by oscillating, transient, and sustained Akt1 signals. We validate a list of phosphorylation sites that covaried with Akt1 phosphorylation across experimental conditions as potential Akt1 substrates. Our resulting dataset provides a rich resource for future studies on AKT signaling and dynamics.

ETS1, ELK1, and ETV4 Transcription Factors Regulate Angiopoietin-1 Signaling and the Angiogenic Response in Endothelial Cells

Background

Angiopoietin-1 (Ang-1) is the main ligand of Tie-2 receptors. It promotes endothelial cell (EC) survival, migration, and differentiation. Little is known about the transcription factors (TFs) in ECs that are downstream from Tie-2 receptors.

Objective

The main objective of this study is to identify the roles of the ETS family of TFs in Ang-1 signaling and the angiogenic response.

Methods

In silico enrichment analyses that were designed to predict TF binding sites of the promotors of eighty-six Ang-1-upregulated genes showed significant enrichment of ETS1, ELK1, and ETV4 binding sites in ECs. Human umbilical vein endothelial cells (HUVECs) were exposed for different time periods to recombinant Ang-1 protein and mRNA levels of ETS1, ELK1, and ETV4 were measured with qPCR and intracellular localization of these transcription factors was assessed with immunofluorescence. Electrophoretic mobility shift assays and reporter assays were used to assess activation of ETS1, ELK1, and ETV4 in response to Ang-1 exposure. The functional roles of these TFs in Ang-1-induced endothelial cell survival, migration, differentiation, and gene regulation were evaluated by using a loss-of-function approach (transfection with siRNA oligos).

Results

Ang-1 exposure increased ETS1 mRNA levels but had no effect on ELK1 or ETV4 levels. Immunostaining revealed that in control ECs, ETS1 has nuclear localization whereas ELK1 and ETV4 are localized to the nucleus and the cytosol. Ang-1 exposure increased nuclear intensity of ETS1 protein and enhanced nuclear mobilization of ELK1 and ETV4. Selective siRNA knockdown of ETS1, ELK1, and ETV4 showed that these TFs are required for Ang-1-induced EC survival and differentiation of cells, while ETS1 and ETV4 are required for Ang-1-induced EC migration. Moreover, ETS1, ELK1, and ETV4 knockdown inhibited Ang-1-induced upregulation of thirteen, eight, and nine pro-angiogenesis genes, respectively.

Conclusion

We conclude that ETS1, ELK1, and ETV4 transcription factors play significant angiogenic roles in Ang-1 signaling in ECs.

MiR-34b-3p Impaired HUVECs Viability and Migration via Targeting PDK1 in an In Vitro Model of Gestational Diabetes Mellitus

Gestational diabetes mellitus (GDM) leads to poor pregnancy outcomes. The methods for GDM early diagnosis and treatment are still unknown. This study aimed to investigate the expression and diagnostic potential of miR-34b-3p in GDM patients and further analyzed the effects of miR-34b-3p on HUVECs viability and migration. The expression of miR-34b-3p was detected in HUVECs of GDM and normal pregnant women by qRT-PCR. Then the HUVECs were isolated from normal pregnant women. High glucose (HG) was used to treat the HUVECs to mimic the GDM in vitro. The cell viability and migration were determined by MTT, wound healing assay, and transwell assay. The interaction between miR-34b-3p and PDK1 was evaluated by luciferase activity assay. Our results showed that miR-34b-3p was up-regulated in HUVECs of GDM patients. Then the HUVECs were isolated from normal pregnant women and they were treated with HG to mimic the GDM in vitro. Interestingly, knockdown of miR-34b-3p restored the impairment of HG treatment-induced effects in HUVECs. More importantly, PDK1 was proved to be a potential target of miR-34b-3p. Finally, the rescue experiments confirmed that miR-34b-3p impaired cell viability and migration ability in HUVECs by targeting PDK1. These findings concluded that miR-34b-3p impaired HUVECs viability and migration in GDM by targeting PDK1, which might provide a novel perspective for the pathogenesis and underlying therapeutic target for GDM.

Roles of miR-640 and Zinc Finger Protein 91 (ZFP91) in Angiopoietin-1-Induced In Vitro Angiogenesis

Angiopoietin-1 (Ang-1) is a ligand of Tie-2 receptors that promotes angiogenesis. It has been established that regulatory loops exist between angiogenic growth factors and distinct pro or anti-angiogenic miRNAs, but the nature and the roles of Ang-1-regulated miRNAs remain unclear. In this study, we assessed the role of miR-640 in Ang-1-induced angiogenesis in human umbilical vein endothelial cells (HUVECs). Exposure to Ang-1 (300 ng/mL) from 6 to 72 h significantly decreased expression of mature miR-640, a response that was mediated by Tie-2 receptors and was also observed in response to Ang-2, the vascular endothelial growth factor, and transforming growth factor β. Increasing miR-640 levels using a mimic inhibited Ang-1-induced cell migration and capillary-like tube formation whereas inhibition of miR-640 enhanced these responses. Pull down assays of biotinylated miR-640 revealed that miR-640 directly targets Zinc Finger Protein 91 (ZFP91), an atypical E3-ubiquitin ligase. Ang-1 exposure induced ZFP91 expression through down-regulation of miR-640. Silencing of ZFP91 significantly inhibited Ang-1-induced cell migration and tube formation. We conclude that Ang-1 upregulates ZFP91 expression through transcriptional down-regulation of miR-640 and that ZFP91 plays important roles in the promotion of Ang-1-induced endothelial cell migration and differentiation.

Inhibition of microRNA-451 is associated with increased expression of Macrophage Migration Inhibitory Factor and mitgation of the cardio-pulmonary phenotype in a murine model of Bronchopulmonary Dysplasia

Background

Macrophage migration inhibitory factor (MIF) has been implicated as a protective factor in the development of bronchopulmonary dysplasia (BPD) and is known to be regulated by MicroRNA-451 (miR-451). The aim of this study was to evaluate the role of miR-451 and the MIF signaling pathway in in vitro and in vivo models of BPD.

Methods

Studies were conducted in mouse lung endothelial cells (MLECs) exposed to hyperoxia and in a newborn mouse model of hyperoxia-induced BPD. Lung and cardiac morphometry as well as vascular markers were evaluated.

Results

Increased expression of miR-451 was noted in MLECs exposed to hyperoxia and in lungs of BPD mice. Administration of a miR-451 inhibitor to MLECs exposed to hyperoxia was associated with increased expression of MIF and decreased expression of angiopoietin (Ang) 2. Treatment with the miR-451 inhibitor was associated with improved lung morphometry indices, significant reduction in right ventricular hypertrophy, decreased mean arterial wall thickness and improvement in vascular density in BPD mice. Western blot analysis demonstrated preservation of MIF expression in BPD animals treated with a miR-451 inhibitor and increased expression of vascular endothelial growth factor-A (VEGF-A), Ang1, Ang2 and the Ang receptor, Tie2.

Conclusion

We demonstrated that inhibition of miR-451 is associated with mitigation of the cardio-pulmonary phenotype, preservation of MIF expression and increased expression of several vascular growth factors.

Ionizing radiation induces endothelial transdifferentiation of glioblastoma stem-like cells through the Tie2 signaling pathway

Glioblastomas (GBM) are brain tumors with a poor prognosis despite treatment that combines surgical resection and radio-chemotherapy. These tumors are characterized by abundant vascularization and significant cellular heterogeneity including GBM stem-like cells (GSC) which contribute to tumor aggressiveness, resistance, and recurrence. Recent data has demonstrated that GSC are directly involved in the formation of new vessels via their transdifferentiation into Tumor Derived Endothelial Cells (TDEC). We postulate that cellular stress such as ionizing radiation (IR) could enhance the transdifferentiation of GSC into TDEC. GSC neurospheres isolated from 3 different patients were irradiated or not and were then transdifferentiated into TDEC. In fact, TDEC obtained from irradiated GSC (TDEC IR+) migrate more towards VEGF, form more pseudotubes in Matrigel^TM in vitro and develop more functional blood vessels in Matrigel^TM plugs implanted in Nude mice than TDEC obtained from non-irradiated GSC. Transcriptomic analysis allows us to highlight an overexpression of Tie2 in TDEC IR+. All IR-induced effects on TDEC were abolished by using a Tie2 kinase inhibitor, which confirms the role of the Tie2 signaling pathway in this process. Finally, by analyzing Tie2 expression in patient GBMs by immunohistochemistry, we demonstrated that the number of Tie2+ vessels increases in recurrent GBM compared with matched untreated tumors. In conclusion, we demonstrate that IR potentiates proangiogenic features of TDEC through the Tie2 signaling pathway, which indicates a new pathway of treatment-induced tumor adaptation. New therapeutic strategies that associate standard treatment and a Tie2 signaling pathway inhibitor should be considered for future trials.

The Angiopoietin ligands and Tie receptors: potential diagnostic biomarkers of vascular disease

The Angiopoietin-1 (Angpt1)/Tie2 signaling pathway is important in regulating vascular function. Angpt1-induced Tie2 activation promotes vascular endothelial cell survival and reduces vascular leakage. Angiopoietin-2 (Angpt2), a weak agonist/antagonist of Tie2, opposes and regulates Angpt1 action. The Tie family of receptor tyrosine kinases, Tie2 and Tie1, exist as either homo-or heterodimers. The molecular complex between the receptors is also crucial in controlling Angpt1 signaling; hence, the molecular balance between Angpt1:Angpt2 and Tie2:Tie1 is important in determining endothelial integrity and vascular stability. This review presents evidence of the change observed in the Angiopoietin/Tie molecules in various pathophysiological conditions and discusses the potential clinical applications of these molecules in vascular complications.

Angiopoietin 1 attenuates interleukin-6-induced endothelial cell permeability through SHP-1

The regulation of endothelial cell (EC) permeability is critical for the physiological homeostasis of blood vessels and tissues. The elevation of pro-inflammatory cytokines is highly associated with lesions, such as the increased vascular permeability of diabetic retinas. We have previously reported that interleukin-6 (IL-6) increases EC permeability through the downregulation of tight junction protein expression. Angiopoietin 1 (Ang1) has an anti-permeability function, but the effect of Ang1 on vascular permeability induced by inflammatory cytokines is unclear. In the present study, we investigated the effect of Ang1 on IL-6-induced EC permeability and its underlying molecular mechanisms. We demonstrated that Ang1 inhibited the IL-6-induced increase in EC permeability by inhibiting the reductions in the levels of tight junction protein ZO-1 and occludin, which was related to the decrease in vascular endothelial growth factor (VEGF) secretion through the inhibition of STAT3 activation by Ang1. Mechanistically, Ang1 induced the dissociation of the tyrosine phosphatase SHP-1 from the Tie2 receptor and increased the binding of SHP-1 to JAK1, JAK2, and STAT3, which are IL-6 downstream signaling proteins. We conclude that SHP-1 plays an important role in the Ang1-induced inhibition of JAK/STAT3 signaling. These results provide evidence for a potential beneficial role of Ang1 in suppressing the vascular permeability induced by the pro-inflammatory cytokine IL-6 in diabetic retinopathy.

Multiplexed, high-throughput measurements of cell contraction and endothelial barrier function

Vascular leakage, protein exudation, and edema formation are events commonly triggered by inflammation and facilitated by gaps that form between adjacent endothelial cells (ECs) of the vasculature. In such paracellular gap formation, the role of EC contraction is widely implicated, and even therapeutically targeted. However, related measurement approaches remain slow, tedious, and complex to perform. Here, we have developed a multiplexed, high-throughput screen to simultaneously quantify paracellular gaps, EC contractile forces, and to visualize F-actin stress fibers, and VE-cadherin. As proof-of-principle, we examined barrier-protective mechanisms of the Rho-associated kinase inhibitor, Y-27632, and the canonical agonist of the Tie2 receptor, Angiopoietin-1 (Angpt-1). Y-27632 reduced EC contraction and actin stress fiber formation, whereas Angpt-1 did not. Yet both agents reduced thrombin-, LPS-, and TNFα-induced paracellular gap formation. This unexpected result suggests that Angpt-1 can achieve barrier defense without reducing EC contraction, a mechanism that has not been previously described. This insight was enabled by the multiplex nature of the force-based platform. The high-throughput format we describe should accelerate both mechanistic studies and the screening of pharmacological modulators of endothelial barrier function.

Differential roles of p38 MAPK and ERK1/2 in angiopoietin-2-mediated rat pulmonary microvascular endothelial cell apoptosis induced by lipopolysaccharide

Angiopoietin-2 (Ang-2) is a Tie-2 ligand that destabilizes vascular structures, enhances vascular permeability and induces vascular regression and endothelial cell apoptosis. Although there is evidence for the involvement of the Ang/Tie2 axis in acute lung injury (ALI), the underlying mechanisms involved in Ang-2-induced cell apoptosis are not well understood. In this study, whether Ang-2 contributes to microvascular endothelial cell injury and mediates lipopolysaccharide (LPS)-induced endothelial cell apoptosis and its associated signaling pathways was investigated. Exposure of rat pulmonary microvascular endothelial cells (RPMVECs) to LPS, Ang-2 and related inhibitors was performed to measure the expression levels of Ang-2, the activation of mitogen-activated protein kinases (MAPKs), the phosphorylation of extracellular signal-regulated kinase (ERK)1/2, and expression of the apoptosis-related proteins Bax and Bcl-2 using western blotting, reverse transcription-quantitative polymerase chain reaction, flow cytometry and fluorescence microscopy. The expression of Ang-2 in the RPMVECs was increased by LPS independent of time. The phosphorylation of p38 MAPK and ERK1/2 was significantly upregulated and the activation of apoptosis-related proteins Bax and Bcl was mediated by Ang-2. In addition, inhibition of the p38 pathway by SB203580 attenuated the Ang-2-mediated cell apoptosis, but inhibition of the ERK1/2 pathway by PD98059 exerted an anti-apoptotic effect against Ang-2. In conclusion, LPS-induced apoptosis is partly mediated via stimulation of p38 and ERK1/2 signaling pathways, where Ang-2 acts an inflammation-related factor to participate in the course of cell apoptosis in RPMVECs.

Negative regulation of angiogenesis by novel micro RNAs

Angiopoietin-1 (Ang-1) is a ligand of Tie-2 receptors that promotes survival, migration, and differentiation of endothelial cells (ECs). Recent studies have identified several microRNA (miRNA) families that either promote or inhibit angiogenesis. To date, the nature and functional importance of miRNAs in Ang-1-induced angiogenesis are unknown. Microarray screening of known miRNAs in human umbilical vein endothelial cells (HUVECs) revealed that the expressions of miR-103b, miR-330-5p, miR-557, miR-575, miR-1287-5p, and miR-1468-5p significantly decrease following exposure to Ang-1 for 24 h. Exposure to the angiogenesis factors angiopoietin-2 (Ang-2), vascular endothelial growth factor, fibroblast growth factor 2, and transforming growth factor β also inhibits miR-103b expression, but exerts varying effects on the other miRNAs. By overexpressing miR-103b, miR-330-5p, miR-557, miR-575, miR-1287-5p, and miR-1468-5p with selective mimics, we demonstrated that the pro-survival effects of Ang-1 are eliminated, Caspase-3 activity increases, and cell migration, proliferation, and capillary-like tube formation decreases. Conversely, transfection with selective miRNA inhibitors increases cell survival, inhibits Caspase-3 activity, and stimulates migration, proliferation and tube formation. miRNet miRNA-target gene network analyses revealed that miR-103, miR-330-5p, miR-557, miR-575, miR-1287-5p, and miR-1468-5p directly interact with 47, 95, 165, 108, 49, and 16 gene targets, respectively. Since many of these genes are positive regulators of angiogenic processes, we conclude that these miRNAs function as anti-angiogenic miRNAs and that their downregulation may be essential for Ang-1-induced angiogenesis to occur.

High Glucose-Mediated Tyrosine Nitration of PI3-Kinase: A Molecular Switch of Survival and Apoptosis in Endothelial Cells

Diabetes and hyperglycemia are associated with increased retinal oxidative and nitrative stress and vascular cell death. Paradoxically, high glucose stimulates expression of survival and angiogenic growth factors. Therefore, we examined the hypothesis that high glucose-mediated tyrosine nitration causes inhibition of the survival protein PI3-kinase, and in particular, its regulatory p85 subunit in retinal endothelial cell (EC) cultures. Retinal EC were cultured in high glucose (HG, 25 mM) for 3 days or peroxynitrite (PN, 100 µM) overnight in the presence or absence of a peroxynitrite decomposition catalyst (FeTPPs, 2.5 µM), or the selective nitration inhibitor epicatechin (100 µM). Apoptosis of ECs was assessed using TUNEL assay and caspase-3 activity. Immunoprecipitation and Western blot were used to assess protein expression and tyrosine nitration of p85 subunit and its interaction with the p110 subunit. HG or PN accelerated apoptosis of retinal ECs compared to normal glucose (NG, 5 mM) controls. HG- or PN-treated cells also showed significant increases in tyrosine nitration on the p85 subunit of PI3-kinase that inhibited its association with the catalytic p110 subunit and impaired PI3-kinase/Akt kinase activity. Decomposing peroxynitrite or blocking tyrosine nitration of p85 restored the activity of PI3-kinase, and prevented apoptosis and activation of p38 MAPK. Inhibiting p38 MAPK or overexpression of the constitutively activated Myr-Akt construct prevented HG- or peroxynitrite-mediated apoptosis. In conclusion, HG impairs pro-survival signals and causes accelerated EC apoptosis, at least in part via tyrosine nitration and inhibition of PI3-kinase. Inhibitors of nitration can be used in adjuvant therapy to delay diabetic retinopathy and microvascular complication.

The role of Src & ERK1/2 kinases in inspiratory resistive breathing induced acute lung injury and inflammation

Background

Inspiratory resistive breathing (IRB), a hallmark of obstructive airway diseases, is associated with large negative intrathoracic pressures, due to strenuous contractions of the inspiratory muscles. IRB is shown to induce lung injury in previously healthy animals. Src is a multifunctional kinase that is activated in the lung by mechanical stress. ERK1/2 kinase is a downstream target of Src. We hypothesized that Src is activated in the lung during IRB, mediates ERK1/2 activation and IRB-induced lung injury.

Methods

Anaesthetized, tracheostomized adult rats breathed spontaneously through a 2-way non-rebreathing valve. Resistance was added to the inspiratory port to provide a peak tidal inspiratory pressure of 50% of maximum (inspiratory resistive breathing). Activation of Src and ERK1/2 in the lung was estimated during IRB. Following 6 h of IRB, respiratory system mechanics were measured by the forced oscillation technique and bronchoalveolar lavage (BAL) was performed to measure total and differential cell count and total protein levels. IL-1b and MIP-2a protein levels were measured in lung tissue samples. Wet lung weight to total body weight was measured and Evans blue dye extravasation was estimated to measure lung permeability. Lung injury was evaluated by histology. The Src inhibitor, PP-2 or the inhibitor of ERK1/2 activation, PD98059 was administrated 30 min prior to IRB.

Results

Src kinase was activated 30 min after the initiation of IRB. Src inhibition ameliorated the increase in BAL cellularity after 6 h IRB, but not the increase of IL-1β and MIP-2a in the lung. The increase in BAL total protein and lung injury score were not affected. The increase in tissue elasticity was partly inhibited. Src inhibition blocked ERK1/2 activation at 3 but not at 6 h of IRB. ERK1/2 inhibition ameliorated the increase in BAL cellularity after 6 h of IRB, blocked the increase of IL-1β and returned Evans blue extravasation and wet lung weight to control values. BAL total protein and the increase in elasticity were partially affected. ERK1/2 inhibition did not significantly change total lung injury score compared to 6 h IRB.

Conclusions

Src and ERK1/2 are activated in the lung following IRB and participate in IRB-induced lung injury.

Angiopoietin-1 attenuates angiotensin II-induced ER stress in glomerular endothelial cells via a Tie2 receptor/ERK1/2-p38 MAPK-dependent mechanism

Research has indicated that endoplasmic reticulum (ER) stress in endothelial cells affects vascular pathologies and induces cellular dysfunction and apoptosis. Angiopoietin1 (Angpt1) has been shown to have therapeutic potential in some vascular diseases, including chronic kidney disease. This study showed that Angpt1 is a powerful factor that attenuated ER stress-induced cellular dysfunction and apoptosis in glomerular endothelial cells (GEnCs). Furthermore, Angpt1 significantly decreased the angiotensin II (Ang II)-induced expression of the ER stress response proteins GRP78, GRP94, p-PERK and CHOP. These results suggest that the Angpt1-mediated cellular protection may occur downstream of the ER stress response. In addition, both specific inhibitors and siRNAs for Tie2 reversed these changes, implying the importance of Tie2 receptor activation in the signalling pathways that prevent ER stress. The protective effects of Angpt1 are related to the activation of two downstream signalling pathways, ERK1/2 and p38 MAPK. The inhibition of these pathways with specific inhibitors, PD98059 and SB203580, respectively, partially increased the expression of chaperones that assist in folding proteins in the ER and reduce the protective effects of Angpt1. In conclusion, Angpt1 attenuated ER stress-induced cellular dysfunction and apoptosis via the Tie2 receptor/ERK1/2-p38 MAPK pathways in GEnCs. This study may provide insights into a novel underlying mechanism and a strategy for alleviating ER stress-induced injury.

Fibulin-5 Regulates Angiopoietin-1/Tie-2 Receptor Signaling in Endothelial Cells

Background

Fibulin-5 is an extracellular matrix glycoprotein that plays critical roles in vasculogenesis and embryonic development. Deletion of Fibulin-5 in mice results in enhanced skin vascularization and upregulation of the angiogenesis factor angiopoietin-1 (Ang-1), suggesting that Fibulin-5 functions as an angiogenesis inhibitor. In this study, we investigate the inhibitory effects of Fibulin-5 on Ang-1/TIE-2 receptor pathway signaling and cell survival in human endothelial cells.

Methodology/Principal Findings

Recombinant wild-type and RGE-mutant Fibulin-5 proteins were generated through stable transfection of HEK293 and CHO cells, respectively. In vitro solid phase binding assays using pure proteins revealed that wild-type Fibulin-5 does not bind to Ang-1 or TIE-2 proteins but strongly binds to heparin. Binding assays using human umbilical vein endothelial cells (HUVECs) indicated that wild-type Fibulin-5 strongly binds to cells but RGE-mutant Fibulin-5, which is incapable of binding to integrins, does not. Pre-incubation of HUVECs for 1 hr with Fibulin-5 significantly increased caspase 3/7 activity, ERK1/2 phosphorylation, and expressions of the transcription factor early growth response 1 (EGR1) and the dual-specificity phosphatase 5 (DUSP5). Fibulin-5 also strongly attenuated Ang-1-induced TIE-2 and AKT phosphorylation, decreased Ang-1-induced expressions of the transcription factors Inhibitor of DNA Binding 1 (ID1) and Kruppel-like Factor 2 (KLF2), and reversed the inhibitory effect of Ang-1 on serum deprivation-induced cytotoxicity and caspase 3/7 activity.

Conclusion/Significance

We conclude that Fibulin-5 strongly binds to the endothelial cell surface through heparin-sulfate proteoglycans and possibly integrins and that it exerts strong anti-angiogenic effects by reducing endothelial cell viability and interfering with the signaling pathways of the Ang-1/TIE-2 receptor axis.

Comparative Phosphoproteomics Analysis of VEGF and Angiopoietin-1 Signaling Reveals ZO-1 as a Critical Regulator of Endothelial Cell Proliferation

VEGF and angiopoietin-1 (Ang-1) are essential factors to promote angiogenesis through regulation of a plethora of signaling events in endothelial cells (ECs). Although pathways activated by VEGF and Ang-1 are being established, the unique signaling nodes conferring specific responses to each factor remain poorly defined. Thus, we conducted a large-scale comparative phosphoproteomic analysis of signaling pathways activated by VEGF and Ang-1 in ECs using mass spectrometry. Analysis of VEGF and Ang-1 networks of regulated phosphoproteins revealed that the junctional proteins ZO-1, ZO-2, JUP and p120-catenin are part of a cluster of proteins phosphorylated following VEGF stimulation that are linked to MAPK1 activation. Down-regulation of these junctional proteins led to MAPK1 activation and accordingly, increased proliferation of ECs stimulated specifically by VEGF, but not by Ang-1. We identified ZO-1 as the central regulator of this effect and showed that modulation of cellular ZO-1 levels is necessary for EC proliferation during vascular development of the mouse postnatal retina. In conclusion, we uncovered ZO-1 as part of a signaling node activated by VEGF, but not Ang-1, that specifically modulates EC proliferation during angiogenesis.

Muscle cell derived angiopoietin-1 contributes to both myogenesis and angiogenesis in the ischemic environment

Recent strategies to treat peripheral arterial disease (PAD) have focused on stem cell based therapies, which are believed to result in local secretion of vascular growth factors. Little is known, however, about the role of ischemic endogenous cells in this context. We hypothesized that ischemic muscle cells (MC) are capable of secreting growth factors that act as potent effectors of the local cellular regenerative environment. Both muscle and endothelial cells (ECs) were subjected to experimental ischemia, and conditioned medium (CM) from each was collected and analyzed to assess myogenic and/or angiogenic potential. In muscle progenitors, mRNA expression of VEGF and its cognate receptors (Nrp1, Flt, Flk) was present and decreased during myotube formation in vitro, and EC CM or VEGF increased myoblast proliferation. Angiopoietin-1 (Ang-1), Tie1, and Tie2 mRNA increased during MC differentiation in vitro. Exogenous Ang-1 enhanced myogenic (MyoD and Myogenin) mRNA in differentiating myoblasts and increased myosin heavy chain protein. Myotube formation was enhanced by MC CM and inhibited by EC CM. Ang-1 protein was present in CM from MCs isolated from both the genetically ischemia-susceptible BALB/c and ischemia-resistant C57BL/6 mouse strains, and chimeric Tie2 receptor trapping in situ ablated Ang-1's myogenic effects in vitro. Ang-1 or MC CM enhanced myotube formation in a mixed isolate of muscle progenitors as well as a myoblast co-culture with pluripotent mesenchymal cells (10T1/2) and this effect was abrogated by viral expression of the extracellular domain of Tie2 (AdsTie2). Furthermore, mesh/tube formation by HUVECs was enhanced by Ang-1 or MC CM and abrogated by Tie2 chimeric receptor trapping. Our results demonstrate the ability of muscle and endothelial cell-derived vascular growth factors, particularly Ang-1, to serve as multi-functional stimuli regulating crosstalk between blood vessels and muscle cells during regeneration from ischemic myopathy.

Angiopoietin-1 inhibits toll-like receptor 4 signalling in cultured endothelial cells: role of miR-146b-5p

AIMS

Bacterial lipopolysaccharides (LPS) induce innate immune inflammatory responses in endothelial cells by activating toll-like receptor 4 (TLR4) signalling. Here, we investigate the effects of angiopoietin-1 (Ang-1) on LPS-induced TLR4 signalling and the role of the miR-146 family of micro RNAs in the effects of Ang-1 on TRL4 signalling.

METHODS AND RESULTS

Leucocyte adhesion to human umbilical vein endothelial cells (HUVECs) was detected using fluorescence microscopy. Adhesion molecule, pro-inflammatory cytokine, miR-146a, and miR-146b-5p expressions in HUVECs were quantified using real-time PCR. TLR4 signalling protein levels were measured using immunoblotting. Exposure of HUVECs to LPS for 4-6 h induces robust inflammatory responses, including enhanced leucocyte adhesion, up-regulation of adhesion molecule expression (VCAM1, ICAM1, E-SELECTIN), enhanced cytokine production (TNFα, IL1β, IL6, and IL8), and increased NFκB luciferase reporter activity. Addition of Ang-1 to the culture medium for 24 h prior to LPS exposure significantly attenuates these responses. Prolonged Ang-1 exposure significantly decreases IRAK1 and TRAF6 protein levels but has no effect on TLR4, MYD88, IRAK4, or TAK1 expressions. Ang-1 triggers significant up-regulation of miR-146b-5p levels but has no effect on miR-146a or miR-146b-3p expressions. Transfection of HUVECs with a miR-146b-5p mimic significantly attenuates LPS-induced inflammatory responses and IRAK1 and TRAF6 expressions. In HUVECs transfected with a miR-146b-5p inhibitor, Ang-1 has no effect on LPS-induced inflammatory responses or IRAK1 and TRAF6 expressions.

CONCLUSION

Ang-1 disrupts TLR4 signalling, resulting in inhibition of LPS-induced inflammatory responses in endothelial cells. This inhibition occurs through selective targeting of IRAK1 and TRAF6 proteins by miR-146b-5p.

Angiopoietin-1 upregulates de novo expression of IL-1β and Il1-Ra, and the exclusive release of Il1-Ra from human neutrophils

The expression of the angiopoietin (Ang) receptor, Tie2, on both endothelial and inflammatory cells supports the idea that Ang signaling may play a fundamental role in initiating and maintaining the inflammatory response. We have previously shown that Ang1 and/or Ang2 alter the innate immune response by enhancing human neutrophil survival, chemotaxis and production of inflammatory cytokine interleukin-8 (IL-8) in vitro. Thus, we hypothesized that Ang1 and Ang2 could modulate other inflammatory signals in neutrophils, a possibility we explored through a gene-based assay looking at changes in the mRNA expression of 84 inflammatory cytokines and their receptors. We observed that Ang1 (10(-8) M), but not Ang2, increased mRNA expression of prominent pro-inflammatory cytokine IL-1β and its natural antagonist IL-1RA, by up to 32.6- and 10.0-fold respectively, compared to PBS-control. The effects of Ang1 extended to the proteins, as Ang1 increased intracellular levels of precursor and mature IL-1β, and extracellular levels of IL-1RA proteins, by up to 4.2-, 5.0- and 4.4-fold respectively, compared to PBS-control. Interestingly, Ang1 failed at inducing IL-1β protein release or at increasing intracellular IL-1RA, but the ratio of IL-1RA to mature IL-1β remained above 100-fold molar excess inside and outside the cells. The above-noted effects of Ang1 were mediated by MAP kinases, whereby inhibiting MEK1/2 lead to up to 70% effect reduction, whereas the blockade of p38MAPK activity doubled Ang1's effect. These findings suggest that Ang1 selectively alters the balance of neutrophil-derived inflammatory cytokines, favoring the blockade of IL-1 activity, a consideration for future therapies of inflammatory diseases.

Differential expression of miRNA in rat myocardial tissues under psychological and physical stress

In the current study, microRNA (miRNA) microarrays were used to detect differentially expressed miRNAs in the myocardial tissues of rat models under stress, to screen target miRNA candidates for miRNA therapy of stress-induced myocardial injury. Rats were bound and suspended in order to induce acute stress (AS) and chronic stress (CS) models. miRNA microarrays were used to detect differentially expressed miRNA in the myocardial tissues of the stressed and control groups. In comparison to the normal control, there were 68 differentially expressed miRNAs in the AS model, of which 32 were upregulated and 36 were downregulated. There were 55 differentially expressed miRNAs in the CS model, of which 20 were upregulated and 35 were downregulated. Of the 123 miRNAs, 15 miRNAs were differentially expressed between the AS and CS groups, of which four were significantly upregulated (rno-miR-296, rno-miR-141, rno-miR-382 and rno-miR-219-5p) and 11 were downregulated (significantly downregulated, rno-miR-135a and rno-miR-466b). The stress of being bound and suspended induces myocardial injury in the rats. Myocardial injury may cause the differential expression of certain miRNAs. Psychological stress may lead to the significant upregulation of rno-miR-296, rno-miR-141, rno-miR-382 and rno-miR-219-5p in addition to the significant downregulation of miR-135a and miR-466b.

Regulation of angiopoietin-1/Tie-2 receptor signaling in endothelial cells by dual-specificity phosphatases 1, 4, and 5

Background

Angiopoietin‐1 (Ang‐1) promotes survival and migration of endothelial cells, in part through the activation of mitogen‐activated protein kinase (MAPK) pathways downstream of Tie‐2 receptors. Dual‐specificity phosphatases (DUSPs) dephosphorylate phosphotyrosine and phosphoserine/phosphothreonine residues on target MAPKs. The mechanisms by which DUSPs modulate MAPK activation in Ang‐1/Tie‐2 receptor signaling are unknown in endothelial cells.

Methods and Results

Expression of various DUSPs in human umbilical vein endothelial cells exposed to Ang‐1 was measured. The functional roles of DUSPs in Ang‐1‐induced regulation of MAPK activation, endothelial cell survival, migration, differentiation, and permeability were measured using selective siRNA oligos. Ang‐1 differentially induces DUSP1, DUSP4, and DUSP5 in human umbilical vein endothelial cells through activation of the PI‐3 kinase, ERK1/2, p38, and SAPK/JNK pathways. Lack‐of‐function siRNA screening revealed that DUSP1 preferentially dephosphorylates p38 protein and is involved in Ang‐1‐induced cell migration and differentiation. DUSP4 preferentially dephosphorylates ERK1/2, p38, and SAPK/JNK proteins and, under conditions of serum deprivation, is involved in Ang‐1‐induced cell migration, several antiapoptotic effects, and differentiation. DUSP5 preferentially dephosphorylates ERK1/2 proteins and is involved in cell survival and inhibition of permeability.

Conclusions

DUSP1, DUSP4, and DUSP5 differentially modulate MAPK signaling pathways downstream of Tie‐2 receptors, thus highlighting the importance of these phosphatases to Ang‐1‐induced angiogenesis.

Angiopoietin-1 elicits pro-inflammatory responses in monocytes and differentiating macrophages

The angiopoietin/Tie2 system is an important regulator of angiogenesis and inflammation. In addition to its functions in endothelial cells, Tie2 expression on non-endothelial cells allows for angiopoietin ligands to stimulate the cells. Although Ang1 is a strong Tie2 receptor agonist, little is known regarding the effect of Ang1 on non-endothelial cells, such as monocytes and macrophages. In this study, we found that Ang1 functionally binds to and stimulates monocytes via p38 and Erk1/2 phosphorylation. Ang1-mediated monocyte stimulation is associated with proinflammatory cytokine TNF-α expression. We also determined that Ang1 switched macrophage differentiation toward a pro-inflammatory phenotype, even in the presence of an anti-inflammatory mediator. These findings suggest that Ang1 plays a role in stimulating pro-inflammatory responses and could provide a new strategy by which to manage inflammatory responses.

The Tie2 receptor antagonist angiopoietin-2 in systemic lupus erythematosus: its correlation with various disease activity parameters

BACKGROUND

Systemic lupus erythematosus is one of the autoimmune diseases characterized by multisystem involvement associated with autoantibody and immune complex vasculitis along with endothelial cell damage.

OBJECTIVE

to study the possible role of Angiopoietin- 2 (Ang-2) as a recently highlighted inflammatory and angiogenic mediator in the pathogenesis of SLE and its correlation with the state of another inflammatory marker, P-Selectin, as well as with various markers of the disease activity.

PATIENTS AND METHODS

The present study included 3 main groups: active SLE patients (group I), inactive SLE patients (group II) and healthy normal control subjects (group III). Groups I and II were subjected to disease activity assessment using the SLEDAI scoring system and measurement of plasma Ang-2 and P-Selectin by ELISA in addition to various laboratory investigations to assess disease activity as: Complete blood count, ESR, serum creatinine, C3, C4 and 24-h urinary proteins.

RESULTS

The mean level of Plasma Ang-2 and P-selectin showed a high significant increase in active group compared to inactive SLE patients and control subjects (p < 0.001).There was a significant positive correlation between Ang-2, P-Selectin, and each of SLEDAI score and 24-h urinary proteins in all SLE patients as well as in the active group, and Ang-2 was a significant independent marker for proteinuria. A significant negative correlation was found between Ang-2, P-Selectin and each of C3, C4. Ang-2 and P-Selectin showed a high sensitivity and specificity in the patients with SLE.

CONCLUSION

Our study suggests that Ang-2 may be a more useful marker than P-Selectin, C3 and C4 in the assessment of disease activity.

Vascular anomalies: from genetics toward models for therapeutic trials

Vascular anomalies are localized abnormalities that occur during vascular development. Several causative genes have been identified not only for inherited but also for some sporadic forms, and the molecular pathways involved are becoming understood. This gives us the opportunity to generate animals carrying the causative genetic defects, which we hope model the phenotype seen in human patients. These models would enable us not only to test known antiangiogenic drugs, but also to develop novel approaches for treatment, directly targeting the mutated protein or molecules implicated in the pathophysiological signaling pathways.

Angiopoietin-1 and vascular endothelial growth factor regulation of leukocyte adhesion to endothelial cells: role of nuclear receptor-77

OBJECTIVE

Vascular endothelial growth factor (VEGF) promotes leukocyte adhesion to endothelial cells (ECs). Angiopoietin-1 (Ang-1) inhibits this response. Nuclear receptor-77 (Nur77) is a proangiogenic nuclear receptor. In the present study, we assessed the influence of Ang-1 and VEGF on Nur77 expression in ECs, and evaluated its role in Ang-1/VEGF-mediated leukocyte adhesion.

METHODS AND RESULTS

Expression of Nur77 was evaluated with real-time polymerase chain reaction and immunoblotting. Adhesion of leukocytes to ECs was monitored with inverted microscopy. Nur77 expression or activity was inhibited using adenoviruses expressing dominant-negative form of Nur77, retroviruses expressing Nur77 in the antisense direction, and small interfering RNA oligos. Both Ang-1 and VEGF induce Nur77 expression, by >5- and 30-fold, respectively. When combined, Ang-1 potentiates VEGF-induced Nur77 expression. Ang-1 induces Nur77 through the phosphoinositide 3-kinase and extracellular signal-regulated protein kinase 1/2 pathways. VEGF induces Nur77 expression through the protein kinase D/histone deacetylase 7/myocyte enhancer factor 2 and extracellular signal-regulated protein kinase 1/2 pathways. VEGF induces nuclear factor-kappaB transcription factor, vascular cell adhesion molecule-1, and E-selectin expressions, and promotes leukocyte adhesion to ECs. Ang-1 inhibits these responses. This inhibitory effect of Ang-1 disappears when Nur77 expression is disrupted, restoring the inductive effects of VEGF on adhesion molecule expression, and increased leukocyte adhesion to ECs.

CONCLUSIONS

Nur77 promotes anti-inflammatory effects of Ang-1, and functions as a negative feedback inhibitor of VEGF-induced EC activation.

Molecular control of angiopoietin signalling

The angiopoietins act through the endothelial receptor tyrosine kinase Tie2 to regulate vessel maturation in angiogenesis and control quiescence and stability of established vessels. The activating ligand, Ang1 (angiopoietin-1), is constitutively expressed by perivascular cells, and the ability of endothelial cells to respond to the ligand is controlled at the level of the Ang1 receptor. This receptor interacts with the related protein Tie1 on the cell surface, and Tie1 inhibits Ang1 signalling through Tie2. The responsiveness of endothelium to Ang1 is determined by the relative levels of Tie2 and the inhibitory co-receptor Tie1 in the cells. Tie1 undergoes regulated ectodomain cleavage which is stimulated by a range of factors including VEGF (vascular endothelial growth factor), inflammatory cytokines and changes in shear stress. Ectodomain cleavage of Tie1 relieves inhibition of Tie2 and enhances Ang1 signalling. This mechanism regulates Ang1 signalling without requiring changes in the level of the ligand and allows Ang1 signalling to be co-ordinated with other signals in the cellular environment. Regulation of signalling at the level of receptor responsiveness may be an important adaptation in systems in which an activating ligand is normally present in excess or where the ligand provides a constitutive maintenance signal.

Vascular disruption and the role of angiogenic proteins after spinal cord injury

Spinal cord injuries (SCI) can result in devastating paralysis, for which there is currently no robustly efficacious neuroprotective/neuroregenerative treatment. When the spinal cord is subjected to a traumatic injury, the local vasculature is disrupted and the blood–spinal cord barrier is compromised. Subsequent inflammation and ischemia may then contribute to further secondary damage, exacerbating neurological deficits. Therefore, understanding the vascular response to SCI and the molecular elements that regulate angiogenesis has considerable relevance from a therapeutic standpoint. In this paper, we review the nature of vascular damage after traumatic SCI and what is known about the role that angiogenic proteins—angiopoietin 1 (Ang1), angiopoietin 2 (Ang2) and angiogenin—may play in the subsequent response. To this, we add recent work that we have conducted in measuring these proteins in the cerebrospinal fluid (CSF) and serum after acute SCI in human patients. Intrathecal catheters were installed in 15 acute SCI patients within 48 h of injury. CSF and serum samples were collected over the following 3–5 days and analysed for Ang1, Ang2 and angiogenin protein levels using a standard ELISA technique. This represents the first description of the endogenous expression of these proteins in an acute human SCI setting.

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.

Activated protein C enhances human keratinocyte barrier integrity via sequential activation of epidermal growth factor receptor and Tie2

Keratinocytes play a critical role in maintaining epidermal barrier function. Activated protein C (APC), a natural anticoagulant with anti-inflammatory and endothelial barrier protective properties, significantly increased the barrier impedance of keratinocyte monolayers, measured by electric cell substrate impedance sensing and FITC-dextran flux. In response to APC, Tie2, a tyrosine kinase receptor, was rapidly activated within 30 min, and relocated to cell-cell contacts. APC also increased junction proteins zona occludens, claudin-1 and VE-cadherin. Inhibition of Tie2 by its peptide inhibitor or small interfering RNA abolished the barrier protective effect of APC. Interestingly, APC did not activate Tie2 through its major ligand, angiopoietin-1, but instead acted by binding to endothelial protein C receptor, cleaving protease-activated receptor-1 and transactivating EGF receptor. Furthermore, when activation of Akt, but not ERK, was inhibited, the barrier protective effect of APC on keratinocytes was abolished. Thus, APC activates Tie2, via a mechanism requiring, in sequential order, the receptors, endothelial protein C receptor, protease-activated receptor-1, and EGF receptor, which selectively enhances the PI3K/Akt signaling to enhance junctional complexes and reduce keratinocyte permeability.

The angiogenic factor angiopoietin-1 is a proneurogenic peptide on subventricular zone stem/progenitor cells

In the adult mammalian brain, the subventricular zone (SVZ) hosts stem cells constantly generating new neurons. Angiopoietin-1 (Ang-1) is an endothelial growth factor with a critical role in division, survival, and adhesion of endothelial cells via Tie-2 receptor activity. Expression of Tie-2 in nonendothelial cells, especially neurons and stem cells, suggests that Ang-1 may be involved in neurogenesis. In the present work, we investigated the putative role of Ang-1 on SVZ neurogenesis. Immature cells from SVZ-derived neurospheres express Ang-1 and Tie-2 mRNA, suggesting a role for the Ang-1/Tie-2 system in the neurogenic niche. Moreover, we also found that Tie-2 protein expression is retained on differentiation in neurons and glial cells. Ang-1 triggered proliferation via activation of the ERK1/2 (extracellular signal-regulated kinase 1/2) mitogen-activated protein kinase (MAPK) kinase pathway but did not induce cell death. Accordingly, coincubation with an anti-Tie-2 neutralizing antibody prevented the pro-proliferative effect of Ang-1. Furthermore, Ang-1 increased the number of NeuN (neuronal nuclear protein)-positive neurons in cultures treated for 7 d, as well as the number of functional neurons, as assessed by monitoring [Ca(2+)](i) rises after application of specific stimuli for neurons and immature cells. The proneurogenic effect of Ang-1 is mediated by Tie-2 activation and subsequent mTOR (mammalian target of rapamycin kinase) mobilization. In agreement, neuronal differentiation significantly decreased after exposure to an anti-Tie-2 neutralizing antibody and to rapamycin. Moreover, Ang-1 elicited the activation of the SAPK (stress-activated protein kinase)/JNK (c-Jun N-terminal kinase) MAPK, involved in axonogenesis. Our work shows a proneurogenic effect of Ang-1, highlighting the relevance of blood vessel/stem cell cross talk in health and disease.

High glucose and elevated fatty acids suppress signaling by the endothelium protective ligand angiopoietin-1

Pre-diabetes is characterized by hyperglycemia and dyslipidemia; it is associated with increased cardiovascular disease and endothelial dysfunction. Angiopoietin-1 (Ang1), a ligand for endothelial receptor, is a potent vascular protective factor important in maintaining normal endothelial function. The aim of the study was to examine the influence of elevated glucose and fatty acid concentrations on angiopoietin signaling in human cardiac microvascular endothelial cells. Incubation with 30 mM glucose caused 50% suppression in the ability of Ang1 to activate Tie2-receptor phosphorylation without any decrease in Tie2 expression or increased internalization in microvascular endothelial cells. Examination of downstream signaling revealed inhibition of Ang1-dependent Akt phosphorylation. By contrast, Ang1 activation of Erk1/2 signaling was not affected by hyperglycemia. Similar suppression of Ang1-dependent activation of Akt by hyperglycemia was observed in large vessel human endothelial cells. Incubation of microvascular endothelial cells with 200 microM palmitic acid significantly inhibited Ang1-dependent Akt phosphorylation without affecting phosphorylation of the Tie-2 receptor or of ERK1/2. Therefore, contrary to hyperglycemia, palmitate acted exclusively downstream of the receptor. The present findings suggest a mechanism by which increased glucose or fatty acids may suppress vascular protection by Ang1 and predispose to endothelial dysfunction and vascular disease.

Activated protein C utilizes the angiopoietin/Tie2 axis to promote endothelial barrier function

Activated protein C (APC) is an anticoagulant, approved as a treatment for severe sepsis, that can prevent apoptosis, inflammation, and vascular leakage. The aim of this study was to investigate whether APC protects endothelial barrier function through the angiopoietin (Ang)/Tie2 axis. APC significantly up-regulated gene and protein expression of Tie2 and Ang1 in a dose (0.01-10 microg/ml)- and time (0.5-24 h)-dependent manner in human umbilical vein endothelial cells (HUVECs). Interestingly, it markedly inhibited Ang2 with an IC(50) of approximately 0.1 microg/ml. HUVEC permeability, measured using Evans blue dye transfer, was significantly reduced in the presence of APC, and, in concordance, the tight junction associated protein zona occludens (ZO)-1 was up-regulated and localized peripherally around cells, compared with controls. Smooth muscle cell migration toward APC-stimulated HUVECs was elevated compared with unstimulated cells. Blocking antibodies and small interfering (si) RNA treatment, compared with isotype (IgG) or scrambled siRNA controls, showed that APC requires 3 receptors, the endothelial protein C receptor, protease-activated receptor 1, and Tie2 to perform all these barrier stabilization functions. In summary, this study demonstrates that APC has novel effects on the Ang/Tie2 axis, which enhance endothelial barrier function and are likely to contribute to its therapeutic effect in sepsis and other diseases associated with vascular leakage.-Minhas, N., Xue, M., Fukudome, K., Jackson, C. J. Activated protein C utilizes the angiopoietin/Tie2 axis to promote endothelial barrier function.

An Ang1-Tie2-PI3K axis in neural progenitor cells initiates survival responses against oxygen and glucose deprivation

Neural progenitor cells (NPCs) have the potential to survive brain ischemia and participate in neurogenesis after stroke. However, it is not clear how survival responses are initiated in NPCs. Using embryonic mouse NPCs and the in vitro oxygen and glucose deprivation (OGD) model, we found that angiopoietin-1 (Ang1) could prevent NPCs from OGD-induced apoptosis, as evidenced by terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling and annexin V labeling. Ang1 significantly elevated tunica intima endothelial kinase 2 (Tie2) autophosphorylation level, suggesting the existence of functional Tie2 receptors on NPCs. NPCs under OGD conditions exhibited reduction of Akt phosphorylation, decrease of the Bcl-2/Bax ratio, activation of caspase-3, cleavage of PARP, and downregulation of beta-catenin and nestin. Ang1 reversed the above changes concomitantly with significant rising of survival rates of NPCs under OGD, but all these effects of Ang1 could be blocked by either soluble extracellular domain of Tie2 Fc fusion protein (sTie2Fc) or the phosphoinositide 3-kinase (PI3K) inhibitor 2-(4-morpholinyl)-8-phenyl-1(4H)-benzopyran-4-one (LY294002). Our findings suggest the existence of an Ang1-Tie2-PI3K signaling axis that is essential in initiation of survival responses in NPCs against cerebral ischemia and hypoxia.

Tie2 is tied at the cell-cell contacts and to extracellular matrix by angiopoietin-1

Angiopoietin-1 (Ang1) binds to and activates Tie2 receptor tyrosine kinase. Ang1-Tie2 signal has been proposed to exhibit two opposite roles in the controlling blood vessels. One is vascular stabilization and the other is vascular angiogenesis. There has been no answer to the question as to how Tie2 induces two opposite responses to the same ligand. Our group and Dr. Alitalos group have demonstrated that trans-associated Tie2 at cell-cell contacts and extracellular matrix (ECM)-anchored Tie2 play distinct roles in the endothelial cells. The complex formation depends on the presence or absence of cell-cell adhesion. Here, we review how Ang1-Tie2 signal regulates vascular maintenance and angiogenesis. We further point to the unanswered questions that must be clarified to extend our knowledge of vascular biology and to progress basic knowledge to the treatment of the diseases in which Ang1-Tie2-mediated signal is central.

Tie2 in tumor endothelial signaling and survival: implications for antiangiogenic therapy

Signaling through the Tie2 receptor on endothelial cells has been shown to play an important role in normal and pathologic vascular development. We generated K1735 murine melanoma tumor cells that inducibly express soluble Tie2 receptor (Tie2Ex) to study the effects of inhibiting Tie2 signaling on tumor vasculature. Tie2Ex induction rapidly decreased AKT activation but not extracellular signal-regulated kinase (ERK) activation in tumor endothelial cells as detected by immunostaining. This was accompanied by an increase in endothelial cell TUNEL staining but no change in Ki-67 expression. Together with a decrease in the percentage of perfused vessels, this suggested that tumor vessel regression and impaired vascular function rather than angiogenesis inhibition was responsible for the delay in tumor growth following Tie2Ex treatment. However, Tie2Ex failed to inhibit the growth of larger, more established K1735 tumors. These tumors were additionally treated with sorafenib, a multikinase inhibitor that inhibits tumor endothelial cell ERK activation but not AKT activation. Combining Tie2Ex and sorafenib decreased both endothelial cell AKT and ERK activation, decreased endothelial cell survival and proliferation, and significantly inhibited growth of the more established tumors. These studies indicate that activity of specific signaling pathways and prosurvival effects are brought about by Tie2 activation in tumor endothelial cells, and knowledge of the effects of Tie2 inhibition can lead to development of more effective therapeutic regimens for inhibiting tumor neovascularization.

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.

Tyrosine phosphatase beta regulates angiopoietin-Tie2 signaling in human endothelial cells

OBJECTIVES

The endothelial cell (EC)-selective receptor tyrosine kinase, Tie2, and its ligands angiopoietin Ang-1 and Ang-2, are essential for blood vessel maintenance and repair. Ang-1 is an agonist of Tie2 receptor activation, whereas Ang-2 is a context-dependent antagonist/agonist. Therefore, we investigated the role of the EC-selective phosphatase, human protein tyrosine phosphatase beta (HPTPbeta), in regulating Tie2 activity.

METHODS AND RESULTS

siRNA silencing of HPTPbeta enhanced Ang-1 and Ang-2-induced Tie2 phosphorylation at 10 min (2.5-fold, P < 0.001; and 1.8-fold, P < 0.05, respectively). The cell survival response to Ang-1, but not Ang-2, was enhanced by HPTPbeta silencing as measured by flow cytometry (0.85-fold to 0.66-fold, P < 0.05) and ELISA (0.88-fold to 0.53-fold, P < 0.01). Hypoxia, which upregulated HPTPbeta expression in endothelial cells, impaired Ang-1-induced Tie2 phosphorylation.

CONCLUSIONS

These results reveal a novel role for HPTPbeta in modulating Ang-1-Tie2 signaling and endothelial cell survival.

Early growth response-1 regulates angiopoietin-1-induced endothelial cell proliferation, migration, and differentiation

OBJECTIVE

Angiopoietin-1 (Ang-1) is an important regulator of angiogenesis in endothelial cells. It promotes migration, proliferation, and differentiation of cells, although the regulating factors involved in these processes remain unclear. In this study, we evaluated the contribution of the transcription factor early growth response-1 (Egr-1) to Ang-1-induced angiogenesis in human umbilical vein endothelial cells (HUVECs).

METHODS AND RESULTS

Expression of Egr-1 was evaluated with real-time PCR and immunoblotting, whereas Egr-1 DNA binding activity was monitored with electrophoretic mobility shift assays. Cell migration was measured with wound healing and Boyden chamber assays, whereas cell proliferation and differentiation of cells into capillary-like tube structures were monitored with cell counting, BrdU incorporation and Matrigels. To selectively inhibit Egr-1 expression, we used both siRNA oligonucleotides and specific DNAzymes. Egr-1 mRNA expression rose approximately 9-fold within 2 hours of Ang-1 exposure and declined thereafter. Upregulation of Egr-1 expression was accompanied by an increase in nuclear mobilization and augmented DNA binding. These processes were mediated through the Erk1/2, PI-3 kinase/AKT, and mTOR pathways. Knockdown of Egr-1 expression completely abrogated Ang-1-induced endothelial migration and significantly reduced proliferation and capillary-like tube formation of HUVECs that overexpress Ang-1.

CONCLUSIONS

Ang-1 triggers significant and transient induction of Egr-1, and Egr-1 contributes to Ang-1-induced endothelial cell migration and proliferation.

Angiopoietin-1 inhibits intrinsic apoptotic signaling and vascular hyperpermeability following hemorrhagic shock

Studies from our laboratory demonstrated the involvement of intrinsic apoptotic signaling in hyperpermeability following hemorrhagic shock (HS). Angiopoietin 1 (Ang-1), a potent inhibitor of hyperpermeability, was recently shown to inhibit apoptosis. The purpose of our study was to determine the effectiveness of Ang-1 in attenuating HS-induced hyperpermeability and its relationship to apoptotic signaling. HS was induced in rats by withdrawing blood to reduce the mean arterial pressure to 40 mmHg for 1 h, followed by reperfusion. Mesenteric postcapillary venules were examined for changes in hyperpermeability by intravital microscopy. Mitochondrial release of second mitochondrial derived activator of caspases (smac) and cytochrome c were determined by Western blot and ELISA, respectively. Caspase-3 activity was determined by fluorometric assay. Parallel studies were performed in rat lung microvascular endothelial cell (RLMEC) monolayers, utilizing HS serum and the proapoptotic Bcl-2 homologous antagonist/killer [BAK (BH3)] peptide as inducers of hyperpermeability. In rats, Ang-1 (200 ng/ml) attenuated HS-induced hyperpermeability versus the HS group ( P < 0.05). Ang-1 prevented HS-induced collapse of mitochondrial transmembrane potential (ΔΨm), smac and cytochrome c release, and caspase-3 activity ( P < 0.05). In RLMEC monolayers, HS serum and BAK (BH3) peptide both induced hyperpermeability that was inhibited by Ang-1 ( P < 0.05). Ang-1 attenuated HS and BAK (BH3) peptide-induced collapse of ΔΨm, smac release, cytochrome c release, activation of caspase-3, and vascular hyperpermeability. In vivo, BAK (BH3) induced vascular hyperpermeability that was attenuated by Ang-1 ( P < 0.05). These findings suggest that Ang-1's role in maintaining microvascular endothelial barrier integrity involves the intrinsic apoptotic signaling cascade.