Identification of a soluble form of the angiopoietin receptor TIE-2 released from endothelial cells and present in human blood

TIE1 and TEK signalling, intraocular pressure, and primary open-angle glaucoma: a Mendelian randomization study

BACKGROUND

In primary open-angle glaucoma (POAG), lowering intraocular pressure (IOP) is the only proven way of slowing vision loss. Schlemm's canal (SC) is a hybrid vascular and lymphatic vessel that mediates aqueous humour drainage from the anterior ocular chamber. Animal studies support the importance of SC endothelial angiopoietin-TEK signalling, and more recently TIE1 signalling, in maintaining normal IOP. However, human genetic support for a causal role of TIE1 and TEK signalling in lowering IOP is currently lacking.

METHODS

GWAS summary statistics were obtained for plasma soluble TIE1 (sTIE1) protein levels (N = 35,559), soluble TEK (sTEK) protein levels (N = 35,559), IOP (N = 139,555) and POAG (Ncases = 16,677, Ncontrols = 199,580). Mendelian randomization (MR) was performed to estimate the association of genetically proxied TIE1 and TEK protein levels with IOP and POAG liability. Where significant MR estimates were obtained, genetic colocalization was performed to assess the probability of a shared causal variant (PPshared) versus distinct (PPdistinct) causal variants underlying TIE1/TEK signalling and the outcome. Publicly available single-nucleus RNA-sequencing data were leveraged to investigate differential expression of TIE1 and TEK in the human ocular anterior segment.

RESULTS

Increased genetically proxied TIE1 signalling and TEK signalling associated with a reduction in IOP (- 0.21 mmHg per SD increase in sTIE1, 95% CI = - 0.09 to - 0.33 mmHg, P = 6.57 × 10-4, and - 0.14 mmHg per SD decrease in sTEK, 95% CI = - 0.03 to - 0.25 mmHg, P = 0.011), but not with POAG liability. Colocalization analysis found that the probability of a shared causal variant was greater for TIE1 and IOP than for TEK and IOP (PPshared/(PPdistinct + PPshared) = 0.98 for TIE1 and 0.30 for TEK). In the anterior segment, TIE1 and TEK were preferentially expressed in SC, lymphatic, and vascular endothelium.

CONCLUSIONS

This study provides novel human genetic support for a causal role of both TIE1 and TEK signalling in regulating IOP. Here, combined evidence from cis-MR and colocalization analyses provide stronger support for TIE1 than TEK as a potential IOP-lowering therapeutic target.

Validation of eGFR for Detecting Associations Between Serum Protein Biomarkers and Subsequent GFR Decline

SIGNIFICANCE STATEMENT

eGFR from creatinine, cystatin C, or both has been primarily used in search of biomarkers for GFR decline. Whether the relationships between biomarkers and eGFR decline are similar to associations with measured GFR (mGFR) decline has not been investigated. This study revealed that some biomarkers showed statistically significant different associations with eGFR decline compared with mGFR decline, particularly for eGFR from cystatin C. The findings indicate that non-GFR-related factors, such as age, sex, and body mass index, influence the relationship between biomarkers and eGFR decline. Therefore, the results of biomarker studies using eGFR, particularly eGFRcys, should be interpreted with caution and perhaps validated with mGFR.

BACKGROUND

Several serum protein biomarkers have been proposed as risk factors for GFR decline using eGFR from creatinine or cystatin C. We investigated whether eGFR can be used as a surrogate end point for measured GFR (mGFR) when searching for biomarkers associated with GFR decline.

METHODS

In the Renal Iohexol Clearance Survey, GFR was measured with plasma iohexol clearance in 1627 individuals without diabetes, kidney, or cardiovascular disease at baseline. After 11 years of follow-up, 1409 participants had one or more follow-up GFR measurements. Using logistic regression and interval-censored Cox regression, we analyzed the association between baseline levels of 12 serum protein biomarkers with the risk of accelerated GFR decline and incident CKD for both mGFR and eGFR.

RESULTS

Several biomarkers exhibited different associations with eGFR decline compared with their association with mGFR decline. More biomarkers showed different associations with eGFRcys decline than with eGFRcre decline. Most of the different associations of eGFR decline versus mGFR decline remained statistically significant after adjustment for age, sex, and body mass index, but several were attenuated and not significant after adjusting for the corresponding baseline mGFR or eGFR.

CONCLUSIONS

In studies of some serum protein biomarkers, eGFR decline may not be an appropriate surrogate outcome for mGFR decline. Although the differences from mGFR decline are attenuated by adjustment for confounding factors in most cases, some persist. Therefore, proposed biomarkers from studies using eGFR should preferably be validated with mGFR.

An overview of the molecular and clinical significance of the angiopoietin system in leukemia

The angiogenesis efficacy in solid tumors and hematological malignancies has been identified for more than twenty years. Although the exact role of angiogenesis in leukemia as a common hematological malignancy has not yet been extensively studied, its effect is demonstrated on the initiation and maintenance of a favorable microenvironment for leukemia cell proliferation. The angiopoietin family is a defined molecular mediator for angiogenesis, which contributes to vascular permeability and angiogenesis initiation. They participate in the angiogenesis process by binding to tyrosine kinase receptors (Tie) on endothelial cells. Considering the role of angiogenesis in leukemia development and the crucial effects of the Ang-Tie system in angiogenesis regulation, many studies have focused on the correlation between the Ang-Tie system and leukemia diagnosis, monitoring, and treatment. In this study, we reviewed the Ang-Tie system's potential diagnostic and therapeutic effects in different types of leukemia in the gene expression level analysis approach. The angiopoietin family context-dependent manner prevents us from defining its actual function in leukemia, emphasizing the need for more comprehensive studies.

Role of Angiopoietin-Tie axis in vascular and lymphatic systems and therapeutic interventions

The Angiopoietin (Ang)-Tyrosine kinase with immunoglobulin-like and EGF-like domains (Tie) axis is an endothelial cell-specific ligand-receptor signaling pathway necessary for vascular and lymphatic development. The Ang-Tie axis is involved in regulating angiogenesis, vascular remodeling, vascular permeability, and inflammation to maintain vascular quiescence. Disruptions in the Ang-Tie axis are involved in many vascular and lymphatic system diseases and play an important role in physiological and pathological vascular conditions. Given recent advances in the Ang-Tie axis in the vascular and lymphatic systems, this review focuses on the multiple functions of the Ang-Tie axis in inflammation-induced vascular permeability, vascular remodeling, atherosclerosis, ocular angiogenesis, tumor angiogenesis, and metastasis. A summary of relevant therapeutic approaches to the Ang-Tie axis, including therapeutic antibodies, recombinant proteins and small molecule drugs are also discussed. The purpose of this review is to provide new hypotheses and identify potential therapeutic strategies based on the Ang-Tie signaling axis for the treatment of vascular and lymphatic-related diseases.

Implications of microvascular dysfunction and nitric oxide mediated inflammation in severe COVID-19 infection

Infection with COVID-19 has resulted in over 276,000 deaths in the United States and over 1.5 million deaths globally, with upwards of 15% of patients requiring hospitalization. Severe COVID-19 infection is, in essence, a microvascular disease. This contention has been emphasized throughout the course of the pandemic, particularly due to the clinical manifestation of severe infection. In fact, it has been hypothesized and shown in particular instances that microvascular function is a significant prognosticator for morbidity and mortality. Initially thought to be isolated to the pulmonary system and resulting in ARDS, patients with COVID-19 have been observed to have acute cardiac, renal, and thrombolytic complications. Therefore, severe COVID-19 is a vascular disease that has systemic implications. The objective of this review is to provide a mechanistic background for the microvascular nature of severe COVID-19 infection, with a particular emphasis on dysfunction of the endothelial glycocalyx and nitric oxide mediated pathogenesis.

Targeting the Angiopoietin/Tie Pathway: Prospects for Treatment of Retinal and Respiratory Disorders

Anti-angiogenic approaches have significantly advanced the treatment of vascular-related pathologies. The ephemeral outcome and known side effects of the current vascular endothelial growth factor (VEGF)-based anti-angiogenic treatments have intensified research on other growth factors. The angiopoietin/Tie (Ang/Tie) family has an established role in vascular physiology and regulates angiogenesis, vascular permeability, and inflammatory responses. The Ang/Tie family consists of angiopoietins 1-4, their receptors, tie1 and 2 and the vascular endothelial-protein tyrosine phosphatase (VE-PTP). Modulation of Tie2 activation has provided a promising outcome in preclinical models and has led to clinical trials of Ang/Tie-targeting drug candidates for retinal disorders. Although less is known about the role of Ang/Tie in pulmonary disorders, several studies have revealed great potential of the Ang/Tie family members as drug targets for pulmonary vascular disorders as well. In this review, we summarize the functions of the Ang/Tie pathway in retinal and pulmonary vascular physiology and relevant disorders and highlight promising drug candidates targeting this pathway currently being or expected to be under clinical evaluation for retinal and pulmonary vascular disorders.

Receptor Tyrosine Kinase Signaling and Targeting in Glioblastoma Multiforme

Glioblastoma multiforme (GBM) is amongst the deadliest of human cancers, with a median survival rate of just over one year following diagnosis. Characterized by rapid proliferation and diffuse infiltration into the brain, GBM is notoriously difficult to treat, with tumor cells showing limited response to existing therapies and eventually developing resistance to these interventions. As such, there is intense interest in better understanding the molecular alterations in GBM to guide the development of more efficient targeted therapies. GBM tumors can be classified into several molecular subtypes which have distinct genetic signatures, and they show aberrant activation of numerous signal transduction pathways, particularly those connected to receptor tyrosine kinases (RTKs) which control glioma cell growth, survival, migration, invasion, and angiogenesis. There are also non-canonical modes of RTK signaling found in GBM, which involve G-protein-coupled receptors and calcium channels. This review uses The Cancer Genome Atlas (TCGA) GBM dataset in combination with a data-mining approach to summarize disease characteristics, with a focus on select molecular pathways that drive GBM pathogenesis. We also present a unique genomic survey of RTKs that are frequently altered in GBM subtypes, as well as catalog the GBM disease association scores for all RTKs. Lastly, we discuss current RTK targeted therapies and highlight emerging directions in GBM research.

Quantile-specific heritability of serum growth factor concentrations

BACKGROUND

"Quantile-dependent expressivity" occurs when the effect size of a genetic variant depends upon whether the phenotype (e.g. growth factor concentration) is high or low relative to its distribution.

METHODS

Quantile-regression analysis was applied to family sets from the Framingham Heart Study to determine whether the heritability (h²) of vascular endothelial growth factor (VEGF), hepatocyte growth factor (HGF), angiopoietin-2, and angiopoietin-2 (sTie-2) and VEGFR1 (sFlt-1) receptor concentrations were quantile-specific.

RESULTS

Quantile-specific h² (±SE) increased with increasing percentiles of the age- and sex-adjusted VEGF (P_trend<10^-16), HGF (P_trend=0.0004), angiopoietin-2 (P_trend=0.0002), sTie-2 (P_trend=1.2 × 10^-5), and sFlt-1 distributions (P_trend=0.04).

CONCLUSION

Heritabilities of VEGF, HGF, angiopoitein-2, sTie-2 and sFlt-1 concentrations are quantile dependent. This may explain reported interactions of genetic loci (rs10738760, rs9472159, rs833061, rs3025039, rs2280789, rs1570360, rs2010963) with metabolic syndrome, diet, recurrent miscarriage, hepatocellular carcinoma, erysipelas, diabetic retinopathy, and bevacizumab treatment in their effect on VEGF concentrations.

Microvascular dysfunction in COVID-19: the MYSTIC study

RATIONALE

Pre-clinical and autopsy studies have fueled the hypothesis that a dysregulated vascular endothelium might play a central role in the pathogenesis of ARDS and multi-organ failure in COVID-19.

OBJECTIVES

To comprehensively characterize and quantify microvascular alterations in patients with COVID-19.

METHODS

Hospitalized adult patients with moderate-to-severe or critical COVID-19 (n = 23) were enrolled non-consecutively in this prospective, observational, cross-sectional, multi-center study. Fifteen healthy volunteers served as controls. All participants underwent intravital microscopy by sidestream dark field imaging to quantify vascular density, red blood cell velocity (V_RBC), and glycocalyx dimensions (perfused boundary region, PBR) in sublingual microvessels. Circulating levels of endothelial and glycocalyx-associated markers were measured by multiplex proximity extension assay and enzyme-linked immunosorbent assay.

MEASUREMENTS AND MAIN RESULTS

COVID-19 patients showed an up to 90% reduction in vascular density, almost exclusively limited to small capillaries (diameter 4-6 µm), and also significant reductions of V_RBC. Especially, patients on mechanical ventilation showed severe glycocalyx damage as indicated by higher PBR values (i.e., thinner glycocalyx) and increased blood levels of shed glycocalyx constituents. Several markers of endothelial dysfunction were increased and correlated with disease severity in COVID-19. PBR (AUC 0.75, p = 0.01), ADAMTS13 (von Willebrand factor-cleaving protease; AUC 0.74, p = 0.02), and vascular endothelial growth factor A (VEGF-A; AUC 0.73, p = 0.04) showed the best discriminatory ability to predict 60-day in-hospital mortality.

CONCLUSIONS

Our data clearly show severe alterations of the microcirculation and the endothelial glycocalyx in patients with COVID-19. Future therapeutic approaches should consider the importance of systemic vascular involvement in COVID-19.

Identification of specific Tie2 cleavage sites and therapeutic modulation in experimental sepsis

Endothelial Tie2 signaling plays a pivotal role in vascular barrier maintenance at baseline and after injury. We previously demonstrated that a sharp drop in Tie2 expression observed across various murine models of critical illnesses is associated with increased vascular permeability and mortality. Matrix metalloprotease (MMP)-14-mediated Tie2 ectodomain shedding has recently been recognized as a possible mechanism for Tie2 downregulation in sepsis. Here, we identified the exact MMP14-mediated Tie2 ectodomain cleavage sites and could show that pharmacological MMP14 blockade in experimental murine sepsis exerts barrier protective and anti-inflammatory effects predominantly through the attenuation of Tie2 cleavage to improve survival both in a pre-treatment and rescue approach. Overall, we show that protecting Tie2 shedding might offer a new therapeutic opportunity for the treatment of septic vascular leakage.

The Angiopoietin-Tie2 Pathway in Critical Illness

Lethal features of sepsis and acute respiratory distress syndrome (ARDS) relate to the health of small blood vessels. For example, alveolar infiltration with proteinaceous fluid is often driven by breach of the microvascular barrier. Spontaneous thrombus formation within inflamed microvessels exacerbates organ ischemia, and in its final stages, erupts into overt disseminated intravascular coagulation. Disruption of an endothelial signaling axis, the Angiopoietin-Tie2 pathway, may mediate the abrupt transition from microvascular integrity to pathologic disruption. This review summarizes preclinical and clinical results that implicate the Tie2 pathway as a promising target to restore microvascular health in sepsis and ARDS.

Angiopoietin-Tie Signaling Pathway in Endothelial Cells: A Computational Model

The angiopoietin-Tie signaling pathway is an important vascular signaling pathway involved in angiogenesis, vascular stability, and quiescence. Dysregulation in the pathway is linked to the impairments in vascular function associated with many diseases, including cancer, ocular diseases, systemic inflammation, and cardiovascular diseases. The present study uses a computational signaling pathway model validated against experimental data to quantitatively study various mechanistic aspects of the angiopoietin-Tie signaling pathway, including receptor activation, trafficking, turnover, and molecular mechanisms of its regulation. The model provides mechanistic insights into the controversial role of Ang2 and its regulators vascular endothelial protein tyrosine phosphatase (VE-PTP) and Tie1 and predicts synergistic effects of inhibition of VE-PTP, Tie1, and Tie2 cleavage on enhancing the vascular protective actions of Tie2.

Dysregulation of circulating sTie2 and sHER2 in HIV-infected women with preeclampsia

OBJECTIVE

This study assesses whether circulating sTie2 and sHER2 are altered in HIV-negative and HIV-positive pregnant normotensive and preeclamptic women.

METHODS

Serum samples were obtained from 80 pregnant women, stratified into four groups, namely, HIV-negative normotensives (20); HIV-positive normotensives (20); HIV-negative preeclamptics (20); and HIV-positive preeclamptics (20). The concentration of sTie2 and sHER2 was analyzed by Bio-Plex multiplex immunoassay and generated from a standard curve.

RESULTS

sTie2 differed significantly by pregnancy type (p = 0.0403) but not by HIV status (p = 0.5214). sHER2 did not show a significant difference between normotensive and preeclampsia (p = 0.3677) and by HIV status (p = 0.5249).

CONCLUSION

Irrespective of HIV status, reduced concentrations of sTie2 were evident in preeclampsia (PE) reflecting a dysregulation of the angiogenic process. sHER2 was similar between pregnancy types, attributable to the oxidative stressed microenvironment which promotes dysregulation of the MAPK and P13K/Akt signaling. HIV status did not influence sTie2 and sHER2 expression, reflecting the immune reconstitution of highly active antiretroviral therapy. sTie2 and sHER2 were not influenced by PE comorbid with HIV infection.

Gamma-secretase-dependent signaling of receptor tyrosine kinases

Human genome harbors 55 receptor tyrosine kinases (RTK). At least half of the RTKs have been reported to be cleaved by gamma-secretase-mediated regulated intramembrane proteolysis. The two-step process involves releasing the RTK ectodomain to the extracellular space by proteolytic cleavage called shedding, followed by cleavage in the RTK transmembrane domain by the gamma-secretase complex resulting in release of a soluble RTK intracellular domain. This intracellular domain, including the tyrosine kinase domain, can in turn translocate to various cellular compartments, such as the nucleus or proteasome. The soluble intracellular domain may interact with transcriptional regulators and other proteins to induce specific effects on cell survival, proliferation, and differentiation, establishing an additional signaling mode for the cleavable RTKs. On the other hand, the same process can facilitate RTK turnover and proteasomal degradation. In this review we focus on the regulation of RTK shedding and gamma-secretase cleavage, as well as signaling promoted by the soluble RTK ICDs. In addition, therapeutic implications of increased knowledge on RTK cleavage on cancer drug development are discussed.

Endothelial Activation: The Ang/Tie Axis in Sepsis

Sepsis, a dysregulated host response to infection that causes life-threatening organ dysfunction, is a highly heterogeneous syndrome with no specific treatment. Although sepsis can be caused by a wide variety of pathogenic organisms, endothelial dysfunction leading to vascular leak is a common mechanism of injury that contributes to the morbidity and mortality associated with the syndrome. Perturbations to the angiopoietin (Ang)/Tie2 axis cause endothelial cell activation and contribute to the pathogenesis of sepsis. In this review, we summarize how the Ang/Tie2 pathway is implicated in sepsis and describe its prognostic as well as therapeutic utility in life-threatening infections.

Molecular Regulation of Sprouting Angiogenesis

The term angiogenesis arose in the 18th century. Several studies over the next 100 years laid the groundwork for initial studies performed by the Folkman laboratory, which were at first met with some opposition. Once overcome, the angiogenesis field has flourished due to studies on tumor angiogenesis and various developmental models that can be genetically manipulated, including mice and zebrafish. In addition, new discoveries have been aided by the ability to isolate primary endothelial cells, which has allowed dissection of various steps within angiogenesis. This review will summarize the molecular events that control angiogenesis downstream of biochemical factors such as growth factors, cytokines, chemokines, hypoxia-inducible factors (HIFs), and lipids. These and other stimuli have been linked to regulation of junctional molecules and cell surface receptors. In addition, the contribution of cytoskeletal elements and regulatory proteins has revealed an intricate role for mobilization of actin, microtubules, and intermediate filaments in response to cues that activate the endothelium. Activating stimuli also affect various focal adhesion proteins, scaffold proteins, intracellular kinases, and second messengers. Finally, metalloproteinases, which facilitate matrix degradation and the formation of new blood vessels, are discussed, along with our knowledge of crosstalk between the various subclasses of these molecules throughout the text. Compr Physiol 8:153-235, 2018.

Regulation of Angiopoietin Signalling by Soluble Tie2 Ectodomain and Engineered Ligand Trap

Angiopoietin-1 (Angpt1) is a glycoprotein ligand important for maintaining the vascular system. It signals via a receptor tyrosine kinase expressed on the surface on endothelial cells, Tie2. This receptor can undergo regulated ectodomain cleavage that releases the ligand-binding domain (sTie2) into the circulation. The concentration of sTie2 is increased in a range of conditions, including peripheral arterial disease and myocardial infarction, where it has been suggested to bind and block Angpt1 resulting in vascular dysfunction. Here we use a joint mathematical modelling and experimental approach to assess the potential impact of sTie2 on the ability of Angpt1 to signal. We find that the concentrations of sTie2 relative to Angpt1 required to suppress signalling by the ligand are more than ten–fold higher than those ever seen in normal or disease conditions. In contrast to the endogenous sTie2, an engineered form of sTie2, which presents dimeric ligand binding sites, inhibits Angpt1 signalling at seventy-fold lower concentrations. While loss of Tie2 ectodomain can suppress Angpt1 signalling locally in the cells in which the receptor is lost, our study shows that the resulting increase in circulating sTie2 is unlikely to affect Angpt1 activity elsewhere in the body.

Tie2 as a novel key factor of microangiopathy in systemic sclerosis

BACKGROUND

The angiopoietin(Ang)/Tie2 system is a key regulator of vascular biology. The expression of membrane bound (mb) Tie2 and Ang-1 ensures vessel stability, whereas Ang-2, inducible by vascular endothelial growth factor (VEGF), hypoxia, and inflammation, acts as an antagonist. Tie2 signalling is also attenuated by soluble Tie2 (sTie2), the extracellular domain of the receptor, which is shed upon stimulation with VEGF. Herein, we investigate the role of Ang/Tie2 in the peripheral vasculopathy in systemic sclerosis (SSc) including animal models.

METHODS

The expression of Ang-1/-2 and Tie2 in skin/serum of SSc patients was compared with healthy controls by immunohistochemistry (IHC)/ELISA. Expression of Ang/Tie2 was analysed in different animal models: VEGF transgenic (tg) mice, hypoxia model, bleomycin-induced skin fibrosis, and tight skin 1 (TSK1) mice.

RESULTS

In SSc, dermal microvessels abundantly expressed Ang-2, but not Ang-1 compared with healthy controls. The percentage of mbTie2+ microvessels was profoundly decreased whereas the levels of sTie2 were increased already in early disease. Both in skin and sera of SSc patients, the Ang1/2 ratio was reduced, being lowest in patients with digital ulcers indicating vessel destabilizing conditions. We next studied potential influencing factors in animal models. The VEGF tg mouse model, the hypoxia, and the inflammation-dependent bleomycin model all showed a similar dysregulation of Ang/Tie2 as in SSc, which did not apply for the non-inflammatory TSK1 model.

CONCLUSION

Peripheral microvasculopathy in SSc results from a complex dysregulation of angiogenic signalling networks including the VEGF and the Ang/Tie2 system. The profoundly disturbed Ang-/Tie-2 balance might represent an important target for vascular therapeutic approaches in SSc.

Temporal expression in rats of receptor tyrosine kinase Tie2 during early wound healing after tooth extraction

We examined the role of Tie2 in regulating wound healing after tooth extraction. Wistar rats underwent maxillary incisor tooth extraction, and immunodetection techniques were used to determine Tie2 expression in the healing wound. The wound was initially filled with blood coagulum containing densely aggregated erythrocytes, leukocytes, fibrin, and endothelial progenitor cells, indicating that blood vessel formation started in the socket. Tie2 was detected on monocytic cell membranes. On day 3, fibroblastic cells proliferated in the coagulum, small vessels appeared by day 5, and new bone formed in the vessel-rich area. Robust woven bone trabeculae were present around vessels by day 7, and woven bone and osteoclast-like giant cells were present on day 10. Woven bone surrounded sinusoidal capillary-like vessels. Full-length (140-160 kDa) Tie2 was not detected at any time, although Tie2 fragments were present in the healing wound. N-terminus- and C-terminus-specific Tie2 antibodies detected 40-kDa and 60-kDa fragments or 70-kDa and 50-kDa fragments, respectively. The levels of these fragments decreased during the first 3 days and started to increase by day 5-10. The Tie2 extracellular domain initially inhibited angiogenesis, and its degradation relieved inhibition of new vessel formation. The onset of vessel formation in the wound may be induced by scattered endothelial progenitor cells.

Common and specific effects of TIE2 mutations causing venous malformations

Venous malformations (VMs) are localized defects in vascular morphogenesis frequently caused by mutations in the gene for the endothelial tyrosine kinase receptor TIE2. Here, we report the analysis of a comprehensive collection of 22 TIE2 mutations identified in patients with VM, either as single amino acid substitutions or as double-mutations on the same allele. Using endothelial cell (EC) cultures, mouse models and ultrastructural analysis of tissue biopsies from patients, we demonstrate common as well as mutation-specific cellular and molecular features, on the basis of which mutations cluster into categories that correlate with data from genetic studies. Comparisons of double-mutants with their constituent single-mutant forms identified the pathogenic contributions of individual changes, and their compound effects. We find that defective receptor trafficking and subcellular localization of different TIE2 mutant forms occur via a variety of mechanisms, resulting in attenuated response to ligand. We also demonstrate, for the first time, that TIE2 mutations cause chronic activation of the MAPK pathway resulting in loss of normal EC monolayer due to extracellular matrix (ECM) fibronectin deficiency and leading to upregulation of plasminogen/plasmin proteolytic pathway. Corresponding EC and ECM irregularities are observed in affected tissues from mouse models and patients. Importantly, an imbalance between plasminogen activators versus inhibitors would also account for high d-dimer levels, a major feature of unknown cause that distinguishes VMs from other vascular anomalies.

Effect of laser photocoagulation on plasma levels of VEGF-A, VEGFR-2, and Tie2 in infants with retinopathy of prematurity

PURPOSE

To report the baseline plasma levels of vascular endothelial growth factor-A (VEGF-A), soluble vascular endothelial growth factor receptor-2 (sVEGFR-2) and soluble Tie2 in infants with treatment-requiring retinopathy of prematurity (ROP) and to investigate the effect of laser treatment on the plasma levels.

METHODS

Blood samples were collected from infants with prethreshold type 1 ROP before, 1 day after, and 1 week after confluent laser photocoagulation. Enzyme-linked immunosorbent assay procedure was used to determine plasma VEGF-A, sVEGFR-2, and sTie2 levels.

RESULTS

A total of 48 eyes of 30 infants were included. The mean postconceptional age at which laser photocoagulation was performed was 37.5 ± 2.9 weeks. The mean number of laser spots applied to each eye was 1,480 ± 420. Regression of active retinopathy occurred within 1 week of treatment in all cases. Baseline plasma VEGF-A, sVEGFR-2, and sTie2 levels did not differ between unilateral and bilateral and also zone I and zone II disease. A significantly progressive decrease was observed in plasma VEGF-A, sVEGFR-2, and sTie2 levels at 1 day and 1 week after laser photocoagulation.

CONCLUSIONS

Plasma levels of VEGF-A, sVEGFR-2, and sTie2 decreased significantly in our patients after laser treatment; however, because of the variability in the measurements, further studies evaluating the clinical value of these angiogenic factors in patients with treatment-requiring ROP are necessary.

Evaluation of first trimester serum soluble endothelial cell-specific tyrosine kinase receptor in normal and affected pregnancies

AIMS

To assess soluble endothelial cell-specific tyrosine kinase receptor (sTie-2) levels in the first trimester of pregnancy and its association with adverse pregnancy outcomes; and examine the predictive accuracy.

STUDY DESIGN

In this nested case-control study, serum sTie-2 levels were measured in 2616 women with singleton pregnancies attending first trimester screening in New South Wales, Australia. Multivariate logistic regression models were used to assess the association and predictive accuracy of serum sTie-2 with subsequent adverse pregnancy outcomes.

RESULTS

Median (interquartile range) sTie-2 for the total population was 19.6 ng/ml (13.6-26.4). Maternal age, weight, and smoking status significantly affected sTie-2 levels. There was no difference in serum sTie-2 between unaffected and women with adverse pregnancy outcomes. After adjusting for maternal and clinical risk factors, low sTie-2 (<25th centile) was associated with preeclampsia (Adjusted odds ratio: 1.61; 95% CI: 1.01-2.57), however, the accuracy of sTie-2 in predicting preeclampsia was not different from chance (AUC = 0.54; p = 0.08) and does not add valuable predictive information to maternal and clinical risk factors.

CONCLUSIONS

Our findings suggest that low sTie-2 levels are associated with preeclampsia, however, it does not add valuable information to clinical and maternal risk factor information in predicting preeclampsia or any other adverse pregnancy outcomes.

Increased endothelial inflammation, sTie-2 and arginase activity in umbilical cords obtained from gestational diabetic mothers

Objective

The aim of this study was to determine subclinical inflammation in umbilical vein derived endothelial cells (HUVECs) obtained from Asian Indian subjects with gestational diabetes (GDM) and to determine levels of angiogenic factors and arginase activity in their cord blood.

Methods

This case-control study included 38 control and 30 GDM subjects. Subjects were confirmed as GDM based on 75g oral glucose tolerance test (OGTT) conducted in the second trimester of pregnancy. Angiogenic markers and arginase activity were measured in cord blood by ELISA and colorimetric methods respectively. Endothelial inflammation was assessed through adhesion of PKH26-labelled leukocytes onto HUVEC monolayer obtained from the study groups. Gene and surface expression of adhesion molecules were confirmed via reverse transcription polymerase chain reaction (RT-PCR) and flow cytometry respectively.

Results

The study revealed increased adhesion of leukocytes to HUVECs isolated from GDM subjects compared to controls. HUVECs of babies born to GDM mothers had increased surface and mRNA expression of E-selectin. sTie2 levels were significantly higher in the cord blood for GDM subjects (3869 ± 370 ng/L) compared to controls (3045 ± 296 ng/L). Furthermore, arginase activity was higher in cord blood of GDM mothers as opposed to the control group (7.75 ± 2.4 µmoles of urea/ml/hour vs 2.88 ±0.49 µmoles of urea/ml/hour; p-value= 0.019). Spearman’s correlation analysis revealed positive correlation of cord blood arginase activity with glucose intolerance (ρ=0.596, p=0.004) and post load glucose values (ρ=0.472, p=0.031) of mothers observed during the second trimester of pregnancy.

Conclusions

HUVECs derived from Asian Indian GDM mothers, exhibit signs of sub-clinical endothelial inflammation along with increased levels of sTie2 and arginase activity in their cord blood serum.

The dynamics of angiogenic factors and their soluble receptors in relation to organ dysfunction in disseminated intravascular coagulation associated with sepsis

We prospectively studied (1) the relationships between angiogenic factors, their soluble receptors and organ dysfunction and (2) the effects of disseminated intravascular coagulation (DIC)-induced platelet consumption, thrombin generation, and tissue hypoxia on the expression of the factors and receptors. Fifty patients with sepsis were classified into two subgroups: 37 patients with DIC and 13 patients without DIC. DIC patients showed higher Sequential Organ Failure Assessment (SOFA) scores, the prevalence of multiple organ dysfunction syndrome (MODS) and more increased soluble fibrin and lactate levels. We observed lower levels of vascular endothelial growth factor (VEGF), soluble VEGF receptor 2 (sVEGFR2), angiopoietin 1 (Ang1) and Ang1/Ang2, and higher sVEGFR1 and Ang2 levels in DIC patients, but not significant differences in soluble Tie2 expression during the study period. The levels of VEGF, sVEGFR1, and Ang2 in DIC patients correlated with the SOFA scores. Clear differences were observed in the levels of Ang2 in the DIC patients between survivors and nonsurvivors and between those with and without MODS. The area under receiver operating characteristic curves for predicting death and MODS by Ang2 were 0.710 and 0.784, respectively. The VEGF levels showed a marked correlation with the platelet counts. Soluble fibrin and lactate levels independently predicted increases in the levels of VEGF, sVEGFR1, and Ang2 in DIC patients. In conclusion, VEGF, sVEGFR1, Ang2, and Ang1/Ang2, especially Ang2, may have roles in the development of MODS in sepsis associated with DIC, and VEGF, sVEGFR1, and Ang2 serum levels correlated with the extent of DIC-induced platelet consumption, thrombin generation, and blood lactate levels.

ADAM12 is expressed in the tumour vasculature and mediates ectodomain shedding of several membrane-anchored endothelial proteins

ADAM (a disintegrin and metalloproteinase) 12 is a metalloprotease implicated in cancer progression. ADAM12 can activate membrane-anchored proteins, such as sonic hedgehog, Delta-like 1 and certain epidermal growth factor receptor ligands, through a process called ectodomain shedding. We screened several membrane-anchored proteins to further dissect the substrate profile of ADAM12-mediated ectodomain shedding, and found shedding of five previously unreported substrates [Kitl1, VE-cadherin (vascular endothelial cadherin), Flk-1 (fetal liver kinase 1), Tie-2, and VCAM-1 (vascular cell adhesion molecule 1)], of which the latter four are specifically expressed by endothelial cells. We also observed that ADAM12 expression was increased in the tumour vasculature of infiltrating ductal carcinoma of the human breast as compared with little to no expression in normal breast tissue vasculature, suggesting a role for ADAM12 in tumour vessels. These results prompted us to further evaluate ADAM12-mediated shedding of two endothelial cell proteins, VE-cadherin and Tie-2. Endogenous ADAM12 expression was very low in cultured endothelial cells, but was significantly increased by cytokine stimulation. In parallel, the shed form of VE-cadherin was elevated in such cytokine-stimulated endothelial cells, and ADAM12 siRNA (small interfering RNA) knockdown reduced cytokine-induced shedding of VE-cadherin. In conclusion, the results of the present study demonstrate a role for ADAM12 in ectodomain shedding of several membrane-anchored endothelial proteins. We speculate that this process may have importance in tumour neovascularization or/and tumour cell extravasation.

The role of angiogenic factors and their soluble receptors in acute lung injury (ALI)/ acute respiratory distress syndrome (ARDS) associated with critical illness

Background

Acute lung injury (ALI) and acute respiratory distress syndrome (ARDS) are characterized by a disruption of the endothelium and alveolar epithelial barriers involving increased microvascular permeability, thus resulting in the set of protein-rich pulmonary edema. Angiogenic factors and their receptors, including vascular endothelial growth factor (VEGF)/VEGF-receptor (VEGFR) and the angiopoietin (Ang)/Tie2 signaling pathways, play pivotal roles in both angiogenesis and microvascular permeability. The aim of the study was to assess the relationship between angiogenic factors, their soluble receptors and ALI/ARDS associated with critically ill patients, including sepsis, severe trauma, and post-cardiac arrest syndrome (PCAS).

Methods

One hundred fifty-nine critically ill patients, including 50 patients with sepsis, 57 patients with severe trauma and 52 resuscitated after out-of-hospital cardiac arrest, were divided into three subgroups: including 25 ALI patients, 101 ARDS patients and 22 non-ALI/ARDS patients. The serum levels of angiogenic factors were measured at the time of admission (day 1), as well as day 3 and day 5 and then were compared among the ALI, ARDS and non-ALI/ARDS groups. Their predictive values for developing ALI/ARDS and 28-day mortality were evaluated.

Results

Higher levels of sVEGFR1 and Ang2 were observed in the ALI and ARDS patients than in the non-ALI/ARDS patients during the entire study period. The Ang2/Ang1 ratio in the ARDS group was also significantly higher than that in the non-ALI/ADRS group. The sVEGFR2 levels in the ARDS group on day 1 were significantly lower than those of the non-ALI/ADRS group. In addition, significant positive correlations were seen between the sVEGFR1, Ang2, Ang2/Ang1, and the development of ALI/ARDS in critical illness. There were also significant negative correlations between the minimal value of sVEGFR2, the maximal value of Ang1 and the ALI/ARDS group. In particular, sVEGFR2 and Ang2 were independent predictors of developing ALI/ARDS. Moreover, Ang2 and sVEGFR2 also independently predicted the mortality in ALI/ARDS patients.

Conclusions

Angiogenic factors and their soluble receptors, particularly sVEGFR2 and Ang2, are thus considered to be valuable predictive biomarkers in the development of ALI/ARDS associated with critical illness and mortality in ALI/ARDS patients.

Using angiogenic factors and their soluble receptors to predict organ dysfunction in patients with disseminated intravascular coagulation associated with severe trauma

Introduction

Disseminated intravascular coagulation (DIC) is characterized by the concomitant activation of coagulofibrinolytic disorders and systemic inflammation associated with endothelial dysfunction-induced microvascular permeability. Angiogenic factors, including vascular endothelial growth factor (VEGF), angiopoietin (Ang), and their receptors, play crucial roles in angiogenesis and microvascular permeability. The aim of the study was to assess the relationship between angiogenic factors, their soluble receptors and organ dysfunction associated with DIC after severe trauma.

Materials and methods

A total of 57 patients with severe trauma were divided into two subgroups; 30 DIC patients and 27 non-DIC patients. The DIC was diagnosed based on the Japanese Association for Acute Medicine (JAAM) DIC and the International Society on Thrombosis and Haemostasis (ISTH) overt DIC criteria. The serum levels of angiogenic factors were measured at the time of admission (Day 1), Day 3 and Day 5. This study compared levels of these angiogenic factors between the two DIC groups, and evaluated their predictive value for organ dysfunction.

Results

DIC patients, especially those with ISTH DIC, showed higher Sequential Organ Failure Assessment (SOFA) scores and lactate levels. There were lower levels of VEGF, Ang1 and the soluble Tie2 in the ISTH DIC patients than the non-DIC patients. The levels of soluble VEGF receptor-1 (sVEGFR1), Ang2 and the Ang2/Ang1 ratio in the ISTH DIC patients were higher than in non-DIC patients. The relationship between the presence of massive transfusion and angiogenic factors indicated the same results. The levels of sVEGFR1, Ang2 and the Ang2/Ang1 ratio correlated with the SOFA scores. In particular, sVEGFR1 and Ang2 were independent predictors of an increase in the SOFA score. The lactate levels independently predicted increases in the levels of sVEGFR1 and Ang2. The decrease in the platelet counts also independently predicted the increase in Ang2 levels in DIC patients.

Conclusions

Angiogenic factors and their soluble receptors, particularly sVEGFR1 and Ang2, are considered to play pivotal roles in the development of organ dysfunction in DIC associated with severe trauma. DIC-induced tissue hypoxia and platelet consumption may play crucial roles in inducing sVEGFR1 and Ang2, and in determining the prognosis of the severity of organ dysfunction.

Significant decrease in maternal serum concentrations of angiopoietin-1 and -2 after delivery

OBJECTIVE

To characterize the physiological distribution of angiopoietins (Ang)-1 and Ang-2 and soluble endothelial cell-specific tyrosine kinase receptor-2 (Tie-2) at term and following delivery.

DESIGN

A prospective, descriptive study.

SETTING

Helsinki University Central Hospital.

POPULATION

Twenty healthy term pregnant women undergoing elective cesarean delivery and their newborns.

METHODS

The concentrations were analysed by enzyme-linked immunosorbent assay in maternal antepartum and the first postpartum day sera, umbilical serum, amniotic fluid and maternal and newborn urine.

MAIN OUTCOME MEASURES

Concentrations of Ang-1, Ang-1 and Tie-2. Results. Concentrations of maternal serum Ang-1 and Ang-2 decreased after delivery {[median (range)]: Ang-1, from 33 (25-51) to 30 (18-49) ng/mL, p= 0.017; and Ang-2, from 5.4 (1.8-18) to 1.4 (0.7-4.6) ng/mL, p < 0.0001}, whereas Tie-2 concentrations remained stable [23 (13-41) vs. 25 (14-29) ng/mL, p= 0.107]. Compared with maternal antepartum serum, umbilical serum concentrations of Ang-1 [46 (28-59) ng/mL, p < 0.0001] and Tie-2 [45 (21-71) ng/mL, p < 0.0001] were higher and those of Ang-2 similar [5.4 (1.8-18) vs. 4.2 (2.9-6.0) ng/mL; p= 0.067]. Low concentrations of Ang-1 [1.2 (0.1-2.2) ng/mL], Ang-2 [1.1 (0.3-4.1) ng/mL] and Tie-2 [0.4 (0.08-0.9) ng/mL] were observed in amniotic fluid, but they were undetectable in newborn urine and in most of the maternal urine samples.

CONCLUSIONS

Maternal Ang-1 and Ang-2 concentrations decreased following delivery. Umbilical concentrations of Ang-1 and Tie-2 were higher than the maternal concentrations.

Time-dependent changes of plasma concentrations of angiopoietins, vascular endothelial growth factor, and soluble forms of their receptors in nonsmall cell lung cancer patients following surgical resection

Even when patients with nonsmall cell lung cancer undergo surgical resection at an early stage, recurrent disease often impairs the clinical outcome. There are numerous causes potentially responsible for a relapse of the disease, one of them being extensive angiogenesis. The balance of at least two systems, VEGF VEGFR and Ang Tie, regulates vessel formation. The aim of this study was to determine the impact of surgery on the plasma levels of the main angiogenic factors during the first month after surgery in nonsmall cell lung cancer patients. The study group consisted of 37 patients with stage I nonsmall cell lung cancer. Plasma concentrations of Ang1, Ang2, sTie2, VEGF, and sVEGF R1 were evaluated by ELISA three times: before surgical resection and on postoperative days 7 and 30. The median of Ang2 and VEGF concentrations increased on postoperative day 7 and decreased on day 30. On the other hand, the concentration of sTie2 decreased on the 7th day after resection and did not change statistically later on. The concentrations of Ang1 and sVEGF R1 did not change after the surgery. Lung cancer resection results in proangiogenic plasma protein changes that may stimulate tumor recurrences and metastases after early resection.

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.

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.

The molecular balance between receptor tyrosine kinases Tie1 and Tie2 is dynamically controlled by VEGF and TNFα and regulates angiopoietin signalling

Angiopoietin-1 (Ang1) signals via the receptor tyrosine kinase Tie2 which exists in complex with the related protein Tie1 at the endothelial cell surface. Tie1 undergoes regulated ectodomain cleavage in response to phorbol esters, vascular endothelial growth factor (VEGF) and tumour necrosis factor-α (TNFα). Recently phorbol esters and VEGF were found also to stimulate ectodomain cleavage of Tie2. Here we investigate for the first time the effects of factors activating ectodomain cleavage on both Tie1 and Tie2 within the same population of cells, and their impact on angiopoietin signalling. We find that phorbol ester and VEGF activated Tie1 cleavage within minutes followed by restoration to control levels by 24 h. However, several hours of PMA and VEGF treatment were needed to elicit a detectable decrease in cellular Tie2, with complete loss seen at 24 h of PMA treatment. TNFα stimulated Tie1 cleavage, and induced a sustained decrease in cellular Tie1 over 24 h whilst increasing cellular Tie2. These differential effects of agonists on Tie1 and Tie2 result in dynamic modulation of the cellular Tie2∶Tie1 ratio. To assess the impact of this on Ang1 signalling cells were stimulated with VEGF and TNFα for differing times and Ang1-induced Tie2 phosphorylation examined. Elevated Tie2∶Tie1, in response to acute VEGF treatment or chronic TNFα, was associated with increased Ang1-activated Tie2 in cells. These data demonstrate cellular levels of Tie1 and Tie2 are differentially regulated by pathophysiologically relevant agonists resulting in dynamic control of the cellular Tie2∶Tie1 balance and modulation of Ang1 signalling. These findings highlight the importance of regulation of signalling at the level of the receptor. Such control may be an important adaptation to allow modulation of cellular signalling responses in systems in which the activating ligand is normally present in excess or where the ligand provides a constitutive maintenance signal.

Airway angiopoietin-2 in ventilated very preterm infants: association with prenatal factors and neonatal outcome

OBJECTIVE

Pulmonary angiogenesis is a prerequisite for lung development. Angiopoietin-2 (Ang2) destabilizes endothelial cells through its endothelial receptor TIE-2, enabling vascular sprouting. Ang1 stabilizes new blood vessels. Soluble TIE-2 (sTIE-2) modulates these effects. We hypothesized that histological funisitis is associated with alterations of Ang2 in airways and of the systemic angiopoietin-TIE-2 homeostasis in very low birth weight (VLBW) infants, contributing to pulmonary morbidity and mortality.

METHODS

We measured Ang2 in tracheobronchial aspirate fluid (TAF) of 42 VLBW <30 weeks of gestation from day 1 through 15 and Ang1, Ang2, and sTIE-2 in umbilical cord serum of 28 infants by enzyme-linked immunosorbent assay. Histological examination distinguished three groups: funisitis, chorioamnionitis, and controls.

RESULTS

Funisitis was associated with lower Ang2 values in TAF but not with changes of Ang1, Ang2, and sTIE-2 in umbilical cord serum. Infants who developed bronchopulmonary dysplasia (BPD) or died had a persistently decreased ratio of previously measured Ang1 to Ang2 in TAF on days 1-5 and increased cord serum concentrations of sTIE-2. Moderate BPD/death was associated with an increase of Ang2 in TAF on day 10 and decreased Ang1/Ang2 ratio from day 3-15. Small for gestational age (SGA) infants had increased Ang2 in TAF on day 1-7 and a lower Ang1/Ang2 ratio on days 5-7.

CONCLUSIONS

The predominance of Ang2 in airway fluid of infants with BPD/death and SGA infants suggests a link between disrupted placental and fetal pulmonary angiogenesis. Histological funisitis with reduced Ang2 in TAF was of minor relevance for outcome in our cohort.

Circulating angiopoietin-2 and soluble Tie-2 in type 2 diabetes mellitus: a cross-sectional study

BACKGROUND

Type 2 diabetes is associated with increased levels of Angiopoietin-2 (Ang-2) and soluble Tie-2 (sTie-2), but its impact on vascular disease is still unknown. This study aimed to further explore the associations of Ang-2 and sTie-2 with metabolic control and diabetic complications.

METHODS

In a cross-sectional designed study, levels of Ang-2 and sTie-2 as well as their relationships to cardiometabolic parameters were determined in 80 type 2 diabetic subjects (age 65 ± 7 years, female 47.4%).

RESULTS

After controlling for age and BMI, Ang-2 levels were associated with levels of sTie-2, diastolic blood pressure, plasma insulin, homeostasis model assessment of insulin resistance (HOMA-IR), creatinine, glomerular filtration rate (GFR), and gamma-glutamyl transferase (GGT) (all p < 0.02). Presence of diabetic macrovascular complications, polyneuropathy and insulin therapy were associated with higher Ang-2 levels (p < 0.05). Conversely, sTie-2 levels were associated with glycated hemoglobin (HbA1c), fasting plasma glucose and insulin, HOMA-IR, triglyceride, and liver function parameters (all p < 0.03). Multiple linear regression analysis showed that Ang-2 remained significantly associated only with levels of GGT (p < 0.04), whereas sTie-2 remained significantly associated with HbA(1c), insulin levels, and HOMA-IR (p < 0.03). No differences in Ang-2 and sTie-2 levels were observed with regard to gender of participants.

CONCLUSIONS

Ang-2 is independently associated with levels of GGT while sTie-2 is independently associated with levels of HbA(1c), plasma insulin and HOMA-IR in type 2 diabetic subjects. Therefore we suggest that the associations of Ang-2 and sTie-2 with type 2 diabetes are based on different patho-physiological mechanisms.

Myoferlin gene silencing decreases Tie-2 expression in vitro and angiogenesis in vivo

Angiogenesis consists in the growth of new blood vessels from pre-existing ones. Although anti-angiogenesis interventions have been shown to have therapeutic properties in human diseases such as cancer, their effect is only partial and the identification of novel modulators of angiogenesis is warranted. Recently, we reported the unexpected proteomic identification in endothelial cells (EC) of Myoferlin, a member of the Ferlin family of transmembrane proteins. Ferlins are well known to regulate the fusion of lipid vesicles at the plasma membrane in muscle cells, and we showed that Myoferlin gene knockdown not only decreases lipid vesicle fusion in EC but also attenuates Vascular Endothelial Growth Factor (VEGF) Receptor-2 (VEGFR-2) expression. Herein, we show that Myoferlin gene silencing in cultured EC also results in attenuated expression of a second tyrosine kinase receptor, Tie-2, which is another well-described angiogenic receptor. Most importantly, we provide evidence that delivery of a low-volume Myoferlin siRNA preparation in mouse tissues results in attenuated angiogenesis and edema formation. This provides the first evidence that acute Myoferlin knockdown has anti-angiogenic effects and validates Myoferlin as an anti-angiogenesis target. Furthermore, this supports the unexpected but increasingly accepted concept that proper tyrosine kinase receptors expression at the plasma membrane requires Myoferlin.

Epithin/PRSS14 proteolytically regulates angiopoietin receptor Tie2 during transendothelial migration

Epithin/PRSS14, a type II transmembrane serine protease, is involved in normal epithelial development and tumor progression. Here we report, as an interacting substrate of epithin, a receptor tyrosine kinase Tie2 that is well known for important roles in the vessel stability. Epithin interacts with and degrades the Tie2 extracellular portion that contains the ligand-binding domain. Epithin is located in the neighbor of Tie2-expressing vessels in normal tissue. Furthermore, epithin can cleave and degrade Tie2 not only in the same cell but also from neighboring cells nearby, resulting in the degradation of the Tie2 ectodomain. The remaining Tie2 fragment was highly phosphorylated and was able to recruit a downstream effector, phosphatidylinositol 3-kinase. Knocking down epithin expression using short hairpin RNA in thymoma cell severely impaired the migration through endothelial cells that show the actin rearrangement during the process. The diminution of epithin protein expression in 4T1 breast cancer cells caused the significant decrease in the number of transendothelial migrating cells in vitro as well as in those of metastasizing tumor nodules in vivo, Therefore, we propose that epithin, which regulates endothelial Tie2 functions, plays a critical role in the fine tuning of transendothelial migration for normal and cancer cells.

Angiopoietins, abdominal aortic aneurysm and atherosclerosis

Abdominal aortic aneurysm (AAA) and atherosclerosis are common causes of mortality and morbidity in an aging population. Angiogenesis is believed to contribute to the development and progression of these diseases. Angiopoietins (angpts) are known to be important regulators of angiogenesis. Angpts can also influence inflammation and have been shown to possess both pro-atherosclerotic and atheroprotective effects. This review explores the potential roles that the angpts play in the development and progression of AAA and atherosclerosis.

Amniotic fluid angiopoietin-1, angiopoietin-2, and soluble receptor tunica interna endothelial cell kinase-2 levels and regulation in normal pregnancy and intraamniotic inflammation-induced preterm birth

BACKGROUND

Angiopoietin-1 (Ang-1) and Ang-2 act selectively on endothelial cells by engaging the Tunica interna endothelial cell kinase-2 (Tie2) receptor. A soluble form of Tie2 (sTie2) blocks angiopoietin bioactivity.

OBJECTIVE

The aim of the study was to characterize changes and expression patterns of Ang-1, Ang-2, and sTie2 in amniotic fluid (AF) and placenta during human pregnancy and intraamniotic inflammation (IAI)-induced preterm birth.

DESIGN AND SETTING

We conducted a cross-sectional study at a tertiary university hospital.

PATIENTS

AF levels of Ang-1, Ang-2, and sTie2 were evaluated in 176 women during second trimester (n = 40), third trimester (n = 37), and preterm labor (positive IAI, n = 50; negative IAI, n = 49). Placenta and cord blood of select women were analyzed.

MAIN OUTCOME MEASURES

Ang-1, Ang-2, sTie2, and IL-6 were evaluated by ELISA. Real-time PCR measured Ang-1, Ang-2, and Tie2 placental mRNA levels. Placenta was immunostained for Ang-1 and Ang-2. Placental explant cultures were stimulated with lipopolysaccharide, Pam3Cys, and modulators of protein synthesis/secretion (cycloheximide, monensin, and brefeldin A).

RESULTS

In normal pregnancy, the levels and ratios of AF Ang-1, Ang-2, and sTie2 varied with gestational age (GA) (P < 0.001). PCR revealed corresponding changes in placental Ang-1 and Ang-2, but not Tie2, mRNA. IAI raised AF Ang-1, Ang-2, and sTie2 above the expected level for GA without affecting their placental mRNA. Ang-2 immunoreactivity appeared enhanced in areas of villous edema. AF Ang-2/Ang-1 ratio was an important determinant of cord blood IL-6 (P < 0.001). Ex-vivo, sTie2 release was increased by Golgi disrupting but not bacterial mimic agents.

CONCLUSIONS

Ang-1, Ang-2, and sTie2 are physiological constituents of AF that are GA and IAI regulated. Ang-2/Ang-1 ratio may play a role in modulating the fetal inflammatory response to IAI. Placental sTie2 shedding likely involves a Golgi-mediated mechanism.

The interaction of soluble Tie2 with angiopoietins and pulmonary vascular permeability in septic and nonseptic critically ill patients

Circulating angiopoietin (Ang) 1 may inhibit and Ang-2 may enhance pulmonary vascular permeability in septic and nonseptic patients with or at risk for acute lung injury or acute respiratory distress syndrome. We hypothesized that the soluble form of the Ang-binding Tie2 receptor (sTie2), whose shedding may be induced by vascular endothelial growth factor (VEGF) levels, may bind circulating Angs and thereby inhibit their effects on pulmonary vascular permeability. In 24 septic and 40 nonseptic mechanically ventilated patients, sTie2, Ang-1, Ang-2, and VEGF plasma levels were measured together with the pulmonary leak index (PLI) for (67)Gallium-labeled transferrin as a measure of pulmonary vascular permeability. Soluble Tie2 and VEGF levels correlated (r = 0.53, P = 0.001). Soluble Tie2 was higher in septic than in nonseptic patients (7.43 [6.57 - 8.40] vs. 5.03 [4.57 - 5.54] ng/mL; P < 0.001). Soluble Tie2 was associated with the PLI (standardized regression coefficient [beta] = 0.26; P = 0.006) but lost its association with the PLI when the Angs were included in a multivariate model. Soluble Tie2 did not affect the association between Ang-1 or Ang-2 and the PLI (beta = -0.39, P < 0.001; beta = 0.52, P < 0.001, respectively), independently of underlying disease. Although limited to correlations and associations, the clinical data support in vivo shedding of sTie2 through VEGF signaling upon pulmonary vascular injury. However, this shedding may not prevent a direct role of Angs in pulmonary vascular permeability.

Shedding of the endothelial receptor tyrosine kinase Tie2 correlates with leukemic blast burden and outcome after allogeneic hematopoietic stem cell transplantation for AML

Angiogenesis plays an important role in the growth and viability of hematologic malignancies. Emerging data suggest a crucial involvement of the endothelial-specific Tie2 receptor and its antagonistic ligand Angiopoietin-2 (Ang-2) in this process. The purpose of this study was to elucidate whether the soluble domain of the Tie2 receptor (sTie2)predicts outcome in patients with acute myeloid leukemia(AML) undergoing allogeneic hematopoietic stem cell transplantation(HSCT). Serum levels of sTie2 and Ang-2 were measured by ELISA in 181 AML patients before conditioning for HSCT. The median follow-up time was 22 months after HSCT. Pre-HSCT sTie2 levels were significantly higher inpatients (median 2.2 (range 1.8-3.0) ng/mL) compared to healthy controls (1.3 (0.9-1.6); p<0.0001). Elevated sTie2 levels were independently associated with active AML but did not relate to cytogenetics/mutational status before transplantation. Logistic regression analysis identified elevated sTie2 (odds ratio (OR) 3.07 (95% confidence interval(CI; 1.56-6.04), p=0.001) as a strong predictor for disease relapse and poor overall survival after HSCT. In a multimarker approach the highest risk for relapse was observed inpatients with both elevated sTie2 and elevated Ang-2 (OR 4.07, (95% CI 1.79-9.25) p<0.0001), as well as patients with both elevated Ang-2 and elevated bone marrow blast count (OR 4.16, (95% CI 1.88-7.36) p<0.0001). Elevated serum sTie2 levels were related to active leukemia,correlated with the percentage of leukemic blasts in the bone marrow, and independently predicted relapse in AML patients after allogeneic HSCT. Furthermore, our data indicate that Tie2 shedding and Ang-2 release seem to reflect overlapping, but nevertheless distinctive features in leukemia-associated neoangiogenesis.

An Autocrine Linkage Between Matrix Metalloproteinase-14 and Tie-2 Via Ectodomain Shedding Modulates Angiopoietin-1–Dependent Function in Endothelial Cells

Objective— The angiopoietin (Ang)–Tie-2 system plays a critical role during fetal and adult angiogenesis. Herein, we explored the Tie-2 shedding–related molecular mechanisms and the pathophysiological significance.

Methods and Results— By using a mouse hindlimb ischemia model, we observed dissociated expression between the full-length Tie-2 (fTie-2) protein and Tie-2 mRNA in thigh muscles 1 day after an ischemic operation, suggesting that fTie-2 expression was modified through the posttranscriptional regulation in vivo. A soluble form of Tie-2 produced in human umbilical vein endothelial cells was dramatically suppressed by treatment with siRNA–matrix metalloproteinase (MMP) 14 or tissue inhibitor of metalloproteinase 3, resulting in an increase in cellular fTie-2 and thereby enhancing Ang-1–dependent Akt phosphorylation and Akt-dependent endothelial functions, such as Ang-2 downregulation or an increase of endothelial viability. Phorbol-12-myristate-13 acetate (PMA) upregulates MMP-14 mRNA via protein kinase C–extracellular signal–regulated kinase pathways, and enhanced soluble Tie-2 production in an MMP-14–dependent manner, resulting in a reduction of cellular fTie-2. In addition, the PMA-induced soluble Tie-2 was mediated by the protein kinase C–extracellular signal–regulated kinase signaling pathways. Finally, downregulation of tissue inhibitor of metalloproteinase 3 and upregulation of MMP-14 mRNA were confirmed in ischemic thigh muscles 1 day after the operation.

Conclusion— An autocrine linkage between the endothelial protein kinase C–MMP-14 axis and Tie-2 shedding was shown to be a novel regulatory mechanism for the Ang–Tie-2 system and may play a role in modulating endothelial function during angiogenesis.

Clinical and genetic correlates of circulating angiopoietin-2 and soluble Tie-2 in the community

BACKGROUND

Experimental studies suggest that endothelial growth factors play an important role in angiogenesis and vascular remodeling. The clinical and genetic correlates of circulating angiopoietin-2 (Ang-2) and its soluble receptor/regulator Tie-2 (sTie-2) have not been determined in a community-based sample.

METHODS AND RESULTS

Serum Ang-2 and sTie-2 were assayed in 3778 third-generation cohort participants of the Framingham Heart Study (mean age, 40+/-9 years; 53% women). Clinical correlates and heritability of both biomarkers were assessed using generalized estimating equations and variance-component analyses. Ang-2 levels were higher and sTie-2 levels were lower in women than in men. Ang-2 was positively related to age, smoking, systolic blood pressure, hypertension treatment, and diabetes (P<0.05 for all) but was inversely associated with total cholesterol and diastolic blood pressure (P<0.0001 for both), and sTie-2 was positively associated with body mass index, diabetes, and triglycerides but was inversely related to age, alcohol consumption, and glomerular filtration rate (P<0.05 for all). Both Ang-2 and sTie-2 were higher in participants with metabolic syndrome (P<0.005), with stronger associations of Ang-2 with blood pressure traits and of sTie-2 with obesity-dyslipidemia components. Heritability estimates for Ang-2 and sTie-2 were 27% and 56%, respectively (P<0.0001). A region on chromosome 9 was significantly linked to circulating sTie-2 levels (logarithm of the odds score, 8.31).

CONCLUSION

Circulating levels of Ang-2 and sTie-2 are heritable traits associated with cardiovascular disease risk factors, including the metabolic syndrome. These observations are consistent with the notion that angiogenesis and vascular remodeling are determined in part by genetic influences and associated with metabolic risk factors.

Angiogenesis and Anti-angiogenic Therapy in Myelofibrosis with Myeloid Metaplasia

Myelofibrosis with myeloid metaplasia (MMM) is a clonal stem cell disorder that is characterized by florid bone marrow stromal reaction including collagen fibrosis, osteosclerosis, and angiogenesis. Almost all patients with MMM display increased bone marrow microvessel density (MVD) and the extent is among the highest in hematological malignancies. This particular information has encouraged the therapeutic use of anti-angiogenic drugs in MMM. In the current review, we summarize the general concepts regarding angiogenesis, assessment of angiogenesis in hematological malignancies and then the current literature on angiogenesis and anti-angiogenic therapy in MMM.

The role of the Angiopoietins in vascular morphogenesis

The Angiopoietin/Tie system acts as a vascular specific ligand/receptor system to control endothelial cell survival and vascular maturation. The Angiopoietin family includes four ligands (Angiopoietin-1, Angiopoietin-2 and Angiopoietin-3/4) and two corresponding tyrosine kinase receptors (Tie1 and Tie2). Ang-1 and Ang-2 are specific ligands of Tie2 binding the receptor with similar affinity. Tie2 activation promotes vessel assembly and maturation by mediating survival signals for endothelial cells and regulating the recruitment of mural cells. Ang-1 acts in a paracrine agonistic manner inducing Tie2 phosphorylation and subsequent vessel stabilization. In contrast, Ang-2 is produced by endothelial cells and acts as an autocrine antagonist of Ang-1-mediated Tie2 activation. Ang-2 thereby primes the vascular endothelium to exogenous cytokines and induces vascular destabilization at higher concentrations. Ang-2 is strongly expressed in the vasculature of many tumors and it has been suggested that Ang-2 may act synergistically with other cytokines such as vascular endothelial growth factor to promote tumor-associated angiogenesis and tumor progression. The better mechanistic understanding of the Ang/Tie system is gradually paving the way toward the rationale exploitation of this vascular signaling system as a therapeutic target for neoplastic and non-neoplastic diseases.

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.