Angiopoietin-1, unlike angiopoietin-2, is incorporated into the extracellular matrix via its linker peptide region

Cationic LNP-formulated mRNA expressing Tie2-agonist in the lung endothelium prevents pulmonary vascular leakage

Dysfunction of endothelial cells (ECs) lining the inner surface of blood vessels are causative for a number of diseases. Hence, the ability to therapeutically modulate gene expression within ECs is of high therapeutic value in treating diseases such as those associated with lung edema. mRNAs formulated with lipid nanoparticles (LNPs) have emerged as a new drug modality to induce transient protein expression for modulating disease-relevant signal transduction pathways. In the study presented here, we tested the effect of a novel synthetic, nucleoside-modified mRNA encoding COMP-Ang1 (mRNA-76) formulated into a cationic LNP on attenuating inflammation-induced vascular leakage. After intravenous injection, the respective mRNA was found to be delivered almost exclusively to the ECs of the lung, while sparing other vascular beds and bypassing the liver. The mode of action of mRNA-76, such as its activation of the Tie2 signal transduction pathway, was tested by pharmacological studies in vitro and in vivo in respective mouse models. mRNA-76 was found to prevent lung vascular leakage/lung edema as well as neutrophil infiltration in a lipopolysaccharide-challenging model.

Angiopoietin-2 binds to multiple interactive sites within von Willebrand factor

Background

Biosynthesis of von Willebrand factor (VWF) in endothelial cells drives the formation of storage-organelles known as Weibel-Palade bodies (WPBs). WPBs also contain several other proteins, including angiopoietin-2 (Ang-2).

Objectives

At present, the molecular basis of the VWF-Ang-2 interaction is poorly understood. Here, we used immunosorbent-binding assays and specific recombinant VWF fragments to analyze VWF-Ang-2 interactions.

Results

We found that VWF bound to immobilized Ang-2 most efficiently (half-maximal binding at 0.5 ± 0.1 μg/mL) under conditions of high CaCl2 (10 mM) and slightly acidic pH (6.4-7.0). Interestingly, several isolated recombinant VWF domains (A1/Fc, A2/Fc, D4/Fc, and D'D3-HPC4) displayed dose-dependent binding to immobilized Ang-2. Binding appeared specific, as antibodies against D'D3, A1, and A2 significantly reduced the binding of these domains to Ang-2. Complexes between VWF and Ang-2 in plasma could be detected by immunoprecipitation- and immunosorbent assays. Unexpectedly, control experiments also revealed complexes between VWF and angiopoietin-1 (Ang-1), a protein structurally homologous to Ang-2. Furthermore, direct binding studies showed dose-dependent binding of VWF to immobilized Ang-1 (half-maximal binding at 1.8 ± 1.0 μg/mL). Interestingly, rather than competing for Ang-1 binding, Ang-2 enhanced the binding of VWF to Ang-1 about 3-fold. Competition experiments further revealed that binding to VWF does not prevent Ang-1 and Ang-2 from binding to Tie-2.

Conclusion

Our data show that both Ang-1 and Ang-2 bind to VWF, seemingly using different interactive sites. Ang-2 modulates the binding of VWF to Ang-1, the (patho)-physiological consequences of which remain to be investigated.

Intracameral Injection of AAV-DJ.COMP-ANG1 Reduces the IOP of Mice by Reshaping the Trabecular Outflow Pathway

Purpose

The angiopoietin-1 (ANG1)-TIE signaling pathway orchestrates the development and maintenance of the Schlemm's canal (SC). In this study, we investigated the impact of adeno-associated virus (AAV)-mediated gene therapy with cartilage oligomeric matrix protein-ANG1 (COMP-ANG1) on trabecular outflow pathway.

Methods

Different serotypes of AAVs were compared for transduction specificity and efficiency in the anterior segment. The selected AAVs encoding COMP-ANG1 or ZsGreen1 (control) were delivered into the anterior chambers of wild-type C57BL/6J mice. The IOP and ocular surface were monitored regularly. Ocular perfusion was performed to measure the outflow facility and label flow patterns of the trabecular drainage pathway. Structural features of SC as well as limbal, retinal, and skin vessels were visualized by immunostaining. Ultrastructural changes in the SC and trabecular meshwork were observed under transmission electron microscopy.

Results

AAV-DJ could effectively infect the anterior segment. Intracameral injection of AAV-DJ.COMP-ANG1 lowered IOP in wild-type C57BL/6J mice. No signs of inflammation or angiogenesis were noticed. Four weeks after AAV injection, the conventional outflow facility and effective filtration area were increased significantly (P = 0.005 and P = 0.04, respectively). Consistently, the area of the SC was enlarged (P < 0.001) with increased density of giant vacuoles in the inner wall (P = 0.006). In addition, the SC endothelia lay on a more discontinuous basement membrane (P = 0.046) and a more porous juxtacanalicular tissue (P = 0.005) in the COMP-ANG1 group.

Conclusions

Intracamerally injected AAV-DJ.COMP-ANG1 offers a significant IOP-lowering effect by remodeling the trabecular outflow pathway of mouse eyes.

Inflammation-mediated matrix remodeling of extracellular matrix-mimicking biomaterials in tissue engineering and regenerative medicine

Extracellular matrix (ECM)-mimicking biomaterials are considered effective tissue-engineered scaffolds for regenerative medicine because of their biocompatibility, biodegradability, and bioactivity. ECM-mimicking biomaterials preserve natural microstructures and matrix-related bioactive components and undergo continuous matrix remodeling upon transplantation. The interaction between host immune cells and transplanted ECM-mimicking biomaterials has attracted considerable attention in recent years. Transplantation of biomaterials may initiate injuries and early pro-inflammation reactions characterized by infiltration of neutrophils and M1 macrophages. Pro-inflammation reactions may lead to degradation of the transplanted biomaterial and drive the matrix into a fetal-like state. ECM degradation leads to the release of matrix-related bioactive components that act as signals for cell migration, proliferation, and differentiation. In late stages, pro-inflammatory cells fade away, and anti-inflammatory cells emerge, which involves macrophage polarization to the M2 phenotype and leukocyte activation to T helper 2 (Th2) cells. These anti-inflammatory cells interact with each other to facilitate matrix deposition and tissue reconstruction. Deposited ECM molecules serve as vital components of the mature tissue and influence tissue homeostasis. However, dysregulation of matrix remodeling results in several pathological conditions, such as aggressive inflammation, difficult healing, and non-functional fibrosis. In this review, we summarize the characteristics of inflammatory responses in matrix remodeling after transplantation of ECM-mimicking biomaterials. Additionally, we discuss the intrinsic linkages between matrix remodeling and tissue regeneration. STATEMENT OF SIGNIFICANCE: Extracellular matrix (ECM)-mimicking biomaterials are effectively used as scaffolds in tissue engineering and regenerative medicine. However, dysregulation of matrix remodeling can cause various pathological conditions. Here, the review describes the characteristics of inflammatory responses in matrix remodeling after transplantation of ECM-mimicking biomaterials. Additionally, we discuss the intrinsic linkages between matrix remodeling and tissue regeneration. We believe that understanding host immune responses to matrix remodeling of transplanted biomaterials is important for directing effective tissue regeneration of ECM-mimicking biomaterials. Considering the close relationship between immune response and matrix remodeling results, we highlight the need for studies of the effects of clinical characteristics on matrix remodeling of transplanted biomaterials.

Ang2 inhibitors and Tie2 activators: potential therapeutics in perioperative treatment of early stage cancer

Anti-angiogenic drugs targeting the VEGF pathway are most effective in advanced metastatic disease settings of certain types of cancers, whereas they have been unsuccessful as adjuvant therapies of micrometastatic disease in numerous phase III trials involving early-stage (resectable) cancers. Newer investigational anti-angiogenic drugs have been designed to inhibit the Angiopoietin (Ang)-Tie pathway. Acting through Tie2 receptors, the Ang1 ligand is a gatekeeper of endothelial quiescence. Ang2 is a dynamically expressed pro-angiogenic destabilizer of endothelium, and its upregulation is associated with poor prognosis in cancer. Besides using Ang2 blockers as inhibitors of tumor angiogenesis, little attention has been paid to their use as stabilizers of blood vessels to suppress tumor cell extravasation and metastasis. In clinical trials, Ang2 blockers have shown limited efficacy in advanced metastatic disease settings. This review summarizes preclinical evidence suggesting the potential utility of Ang2 inhibitors or Tie2 activators as neoadjuvant or adjuvant therapies in the prevention or treatment of early-stage micrometastatic disease. We further discuss the rationale and potential of combining these strategies with immunotherapy, including immune checkpoint targeting antibodies.

New soluble angiopoietin analog of Hepta-ANG1 prevents pathological vascular leakage

Vascular leak is a key driver of organ injury in diseases, and strategies that reduce enhanced permeability and vascular inflammation are promising therapeutic targets. Activation of the angiopoietin-1 (ANG1)-Tie2 tyrosine kinase signaling pathway is an important regulator of vascular quiescence. Here we describe the design and construction of a new soluble ANG1 mimetic that is a potent activator of endothelial Tie2 in vitro and in vivo. Using a chimeric fusion strategy, we replaced the extracellular matrix (ECM) binding and oligomerization domain of ANG1 with a heptameric scaffold derived from the C-terminus of serum complement protein C4-binding protein α. We refer to this new fusion protein biologic as Hepta-ANG1, which forms a stable heptamer and induces Tie2 phosphorylation in cultured cells, and in the lung following intravenous injection of mice. Injection of Hepta-ANG1 ameliorates vascular endothelial growth factor- and lipopolysaccharide-induced vascular leakage, in keeping with the known functions of Angpt1-Tie2 in maintaining quiescent vascular stability. The new Hepta-ANG1 fusion is easy to produce and displays remarkable stability with high multimericity that can potently activate Tie2. It could be a new candidate ANG1 mimetic therapy for treatments of inflammatory vascular leak, such as acute respiratory distress syndrome and sepsis.

Role of Extracellular Matrix in Gastrointestinal Cancer-Associated Angiogenesis

Gastrointestinal tumors are responsible for more cancer-related fatalities than any other type of tumors, and colorectal and gastric malignancies account for a large part of these diseases. Thus, there is an urgent need to develop new therapeutic approaches to improve the patients' outcome and the tumor microenvironment is a promising arena for the development of such treatments. In fact, the nature of the microenvironment in the different gastrointestinal tracts may significantly influence not only tumor development but also the therapy response. In particular, an important microenvironmental component and a potential therapeutic target is the vasculature. In this context, the extracellular matrix is a key component exerting an active effect in all the hallmarks of cancer, including angiogenesis. Here, we summarized the current knowledge on the role of extracellular matrix in affecting endothelial cell function and intratumoral vascularization in the context of colorectal and gastric cancer. The extracellular matrix acts both directly on endothelial cells and indirectly through its remodeling and the consequent release of growth factors. We envision that a deeper understanding of the role of extracellular matrix and of its remodeling during cancer progression is of chief importance for the development of new, more efficacious, targeted therapies.

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.

Glomerular Function and Structural Integrity Depend on Hyaluronan Synthesis by Glomerular Endothelium

BACKGROUND

A glycocalyx envelope consisting of proteoglycans and adhering proteins covers endothelial cells, both the luminal and abluminal surface. We previously demonstrated that short-term loss of integrity of the luminal glycocalyx layer resulted in perturbed glomerular filtration barrier function.

METHODS

To explore the role of the glycocalyx layer of the endothelial extracellular matrix in renal function, we generated mice with an endothelium-specific and inducible deletion of hyaluronan synthase 2 (Has2), the enzyme that produces hyaluronan, the main structural component of the endothelial glycocalyx layer. We also investigated the presence of endothelial hyaluronan in human kidney tissue from patients with varying degrees of diabetic nephropathy.

RESULTS

Endothelial deletion of Has2 in adult mice led to substantial loss of the glycocalyx structure, and analysis of their kidneys and kidney function showed vascular destabilization, characterized by mesangiolysis, capillary ballooning, and albuminuria. This process develops over time into glomerular capillary rarefaction and glomerulosclerosis, recapitulating the phenotype of progressive human diabetic nephropathy. Using a hyaluronan-specific probe, we found loss of glomerular endothelial hyaluronan in association with lesion formation in tissue from patients with diabetic nephropathy. We also demonstrated that loss of hyaluronan, which harbors a specific binding site for angiopoietin and a key regulator of endothelial quiescence and maintenance of EC barrier function results in disturbed angiopoietin 1 Tie2.

CONCLUSIONS

Endothelial loss of hyaluronan results in disturbed glomerular endothelial stabilization. Glomerular endothelial hyaluronan is a previously unrecognized key component of the extracelluar matrix that is required for glomerular structure and function and lost in diabetic nephropathy.

Extracellular matrix scaffolding in angiogenesis and capillary homeostasis

The extracellular matrix (ECM) of blood vessels, which is composed of both the vascular basement membrane (BM) and the interstitial ECM is identified as a crucial component of the vasculature. We here focus on the unique molecular composition and scaffolding of the capillary ECM, which provides structural support to blood vessels and regulates properties of endothelial cells and pericytes. The major components of the BM are collagen IV, laminins, heparan sulfate proteoglycans and nidogen and also associated proteins such as collagen XVIII and fibronectin. Their organization and scaffolding in the BM is required for proper capillary morphogenesis and maintenance of vascular homeostasis. The BM also regulates vascular mechanosensing. A better understanding of the mechanical and structural properties of the vascular BM and interstitial ECM therefore opens new perspectives to control physiological and pathological angiogenesis and vascular homeostasis. The overall aim of this review is to explain how ECM scaffolding influences angiogenesis and capillary integrity.

miR-153 inhibits the migration and the tube formation of endothelial cells by blocking the paracrine of angiopoietin 1 in breast cancer cells

The sprouting of endothelial cells is the first step of tumor angiogenesis. Our previous study suggests that miR-153 suppresses breast tumor angiogenesis partially through targeting hypoxia-induced factor (HIF1α). In this study, we demonstrated that miR-153 also suppresses the migration and the tube formation of endothelial cells through directly targeting angiopoietin 1 (ANG1) in breast cancer cells. There was a negative correlation between miR-153 and ANG1 levels in breast cancer. miR-153 blocked the expression and secretion of ANG1 in breast cancer cells through binding to ANG1 mRNA. Conditioned medium from the breast cancer cell, MCF7, treated with miR-153 had no effect on the proliferation of HUVECs, but significantly inhibited the migration and tube formation of HUVECs, which could be rescued by overexpression of ANG1. In addition, miR-153 also directly inhibited the proliferation and migration of MCF7 through downregulation of ANG1. These findings suggest that miR-153 suppresses the activity of tumor cells and the migration and tube formation of endothelial cells by silencing ANG1.

GM-CSF ameliorates microvascular barrier integrity via pericyte-derived Ang-1 in wound healing

Skin wound healing involves complex coordinated interactions of cells, tissues, and mediators. Maintaining microvascular barrier integrity is one of the key events for endothelial homeostasis during wound healing. Vasodilation is observed after vasoconstriction, which causes blood vessels to become porous, facilitates leukocyte infiltration and aids angiogenesis at the wound-area, postinjury. Eventually, vessel integrity has to be reestablished for vascular maturation. Numerous studies have found that granulocyte macrophage colony-stimulating factor (GM-CSF) accelerates wound healing by inducing recruitment of repair cells into the injury area and releases of cytokines. However, whether GM-CSF is involving in the maintaining of microvascular barrier integrity and the underlying mechanism remain still unclear. Aim of this study was to investigate the effects of GM-CSF on modulation of microvascular permeability in wound healing and underlying mechanisms. Wound closure and microvascular leakage was investigated using a full-thickness skin wound mouse model after GM-CSF intervention. The endothelial permeability was measured by Evans blue assay in vivo and in vitro endothelium/pericyte co-culture system using a FITC-Dextran permeability assay. To identify the source of angiopoietin-1 (Ang-1), double staining is used in vivo and ELISA and qPCR are used in vitro. To determine the specific effect of Ang-1 on GM-CSF maintaining microvascular stabilization, Ang-1 siRNA was applied to inhibit Ang-1 production in vivo and in vitro. Wound closure was significantly accelerated and microvascular leakage was ameliorated after GM-CSF treatment in mouse wound sites. GM-CSF decreased endothelial permeability through tightening endothelial junctions and increased Ang-1 protein level that was derived by perictye. Furthermore, applications of siRNAAng-1 inhibited GM-CSF mediated protection of microvascular barrier integrity both in vivo and in vitro. Our data indicate that GM-CSF ameliorates microvascular barrier integrity via pericyte-derived Ang-1 during wound healing.

Model of vascular desmoplastic multispecies tumor growth

We present a three-dimensional nonlinear tumor growth model composed of heterogeneous cell types in a multicomponent-multispecies system, including viable, dead, healthy host, and extra-cellular matrix (ECM) tissue species. The model includes the capability for abnormal ECM dynamics noted in tumor development, as exemplified by pancreatic ductal adenocarcinoma, including dense desmoplasia typically characterized by a significant increase of interstitial connective tissue. An elastic energy is implemented to provide elasticity to the connective tissue. Cancer-associated fibroblasts (myofibroblasts) are modeled as key contributors to this ECM remodeling. The tumor growth is driven by growth factors released by these stromal cells as well as by oxygen and glucose provided by blood vasculature which along with lymphatics are stimulated to proliferate in and around the tumor based on pro-angiogenic factors released by hypoxic tissue regions. Cellular metabolic processes are simulated, including respiration and glycolysis with lactate fermentation. The bicarbonate buffering system is included for cellular pH regulation. This model system may be of use to simulate the complex interactions between tumor and stromal cells as well as the associated ECM and vascular remodeling that typically characterize malignant cancers notorious for poor therapeutic response.

Vascular dysfunction as a target for adjuvant therapy in cerebral malaria

Cerebral malaria (CM) is a life-threatening complication of Plasmodium falciparum malaria that continues to be a major global health problem. Brain vascular dysfunction is a main factor underlying the pathogenesis of CM and can be a target for the development of adjuvant therapies for the disease. Vascular occlusion by parasitised red blood cells and vasoconstriction/vascular dysfunction results in impaired cerebral blood flow, ischaemia, hypoxia, acidosis and death. In this review, we discuss the mechanisms of vascular dysfunction in CM and the roles of low nitric oxide bioavailability, high levels of endothelin-1 and dysfunction of the angiopoietin-Tie2 axis. We also discuss the usefulness and relevance of the murine experimental model of CM by Plasmodium berghei ANKA to identify mechanisms of disease and to screen potential therapeutic interventions.

A review of therapies for diabetic macular oedema and rationale for combination therapy

Diabetic macular oedema (DMO) is responsible for significant visual impairment in diabetic patients. The primary cause of DMO is fluid leakage resulting from increased vascular permeability through contributory anatomical and biochemical changes. These include endothelial cell (EC) death or dysfunction, pericyte loss or dysfunction, thickened basement membrane, loss or dysfunction of glial cells, and loss/change of EC Glycocalyx. The molecular changes include increased reactive oxygen species, pro-inflammatory changes: advanced glycation end products, intracellular adhesion molecule-1, Complement 5-9 deposition and cytokines, which result in increased paracellular permeability, tight junction disruption, and increased transcellular permeability. Laser photocoagulation has been the mainstay of treatment until recently when pharmacological treatments were introduced. The current treatments for DMO target reducing vascular leak in the macula once it has occurred, they do not attempt to treat the underlying pathology. These pharmacological treatments are aimed at antagonising vascular endothelial growth factor (VEGF) or non-VEGF inflammatory pathways, and include intravitreal injections of anti-VEGFs (ranibizumab, aflibercept or bevacizumab) or steroids (fluocinolone, dexamethasone or triamcinolone) as single therapies. The available evidence suggests that each individual treatment modality in DMO does not result in a completely dry macula in most cases. The ideal treatment for DMO should improve vision and improve morphological changes in the macular (eg, reduce macular oedema) for a significant duration, reduced adverse events, reduced treatment burden and costs, and be well tolerated by patients. This review evaluates the individual treatments available as monotherapies, and discusses the rationale and potential for combination therapy in DMO. A comprehensive review of clinical trials related to DMO and their outcomes was completed. Where phase III randomised control trials were available, these were referenced, if not available, phase II trials have been included.

The angiopoietin-Tie2 signaling axis in the vascular leakage of systemic inflammation

The ability of small blood vessels to undergo rapid, reversible morphological changes is essential for the adaptive response to tissue injury or local infection. A canonical feature of this response is transient hyperpermeability. However, when leakiness is profound or persistent, adverse consequences accrue to the host, including organ dysfunction and shock. A growing body of literature identifies the Tie2 receptor, a transmembrane tyrosine kinase highly enriched in the endothelium, as an important regulator of vascular barrier function in health and in disease. The principal ligands of Tie2, Angiopoietins 1 and 2, exert opposite effects on this receptor in the context of inflammation. This review will focus on recent studies that have illuminated novel aspects of the exquisitely controlled Tie2 signaling axis while proposing unanswered questions and future directions for this field of study.

Angiopoietins promote ovarian cancer progression by establishing a procancer microenvironment

Despite decades of research, the survival rate of ovarian cancer patients is largely unchanged. Current chemotherapeutic drugs are effective only transiently because patients with advanced disease eventually develop resistance. Thus, there is a pressing need for identifying novel therapeutic targets in ovarian cancer. Mounting evidence suggests that angiopoietins (Angpts) may play an essential role in cancer progression; however, the expression profiles and biological effects of Angpts on ovarian cancer remain largely unknown. Here, we show that, compared with their normal counterparts, expressions of Angpt1, Angpt2, and Angpt4 are increased in ovarian cancer cells and tissues and that human ovarian cancer cells also express the Angpt receptor Tie-2-receptor tyrosine kinase. We show that increased expression of Angpt1, Angpt2, or Angpt4 promotes intraperitoneal growth of ovarian cancers and shortens survival of the experimental mice. We further show, for the first time, that Angpts promote accumulation of cancer-associated fibroblasts and tumor angiogenesis in the ovarian cancer microenvironment, as well as enhance ovarian cancer cell proliferation and invasion in vivo. In addition, we establish a novel function of Angpts in promoting proliferation and invasion and inducing Tie-2 and extracellular signal-regulated kinase 1/2 activation in ovarian cancer-associated fibroblasts. Taken together, these data suggest that the Angpt-Tie-2 functional axis is an important player in ovarian cancer progression and an attractive target for ovarian cancer therapy.

Molecular control of capillary morphogenesis and maturation by recognition and remodeling of the extracellular matrix: functional roles of endothelial cells and pericytes in health and disease

This review addresses fundamental mechanisms underlying how capillaries form in three-dimensional extracellular matrices and how endothelial cells (ECs) and pericytes co-assemble to form capillary networks. In addition to playing a critical role in supplying oxygen and nutrients to tissues, recent work suggests that blood vessels supply important signals to facilitate tissue development. Here, we hypothesize that another major function of capillaries is to supply signals to suppress major disease mechanisms including inflammation, infection, thrombosis, hemorrhage, edema, ischemic injury, fibrosis, autoimmune disease and tumor growth/progression. Capillary dysfunction plays a key pathogenic role in many human diseases, and thus, this suppressing function may be attenuated and central toward the initiation and progression of disease. We describe how capillaries form through creation of EC-lined tube networks and vascular guidance tunnels in 3D extracellular matrices. Pericytes recruit to the abluminal EC tube surface within these tunnel spaces, and work together to assemble the vascular basement membrane matrix. These processes occur under serum-free conditions in 3D collagen or fibrin matrices and in response to five key growth factors which are stem cell factor, interleukin-3, stromal-derived factor-1α, fibroblast growth factor-2 and insulin. In addition, we identified a key role for EC-derived platelet-derived growth factor-BB and heparin-binding epidermal growth factor in pericyte recruitment and proliferation to promote EC-pericyte tube co-assembly and vascular basement membrane matrix deposition. A molecular understanding of capillary morphogenesis and maturation should lead to novel therapeutic strategies to repair capillary dysfunction in major human disease contexts including cancer and diabetes.

Effect of aging and cardiovascular risk factors on receptor Tie1 expression in human erectile tissue

INTRODUCTION

Erectile dysfunction is highly prevalent in patients with advanced age or cardiovascular disease risk factors (CVDRFs). These conditions interfere on expression of vascular growth factors and respective receptors causing disturbance in endothelial function.

AIM

This study aims to assess the effect of aging and CVDRF on the expression of tyrosine kinase with immunoglobulin-like and EGF-like domains (Tie) 1 in human corpus cavernosum (CC).

METHODS

CC fragments obtained from programmed surgeries or organ donors were divided into three groups: young, healthy aged, and aged with CVDRF. Angiopoietin (Ang) 1, Ang2, Tie1, and Tie2 mRNA and protein levels were assessed by real-time polymerase chain reaction and Western blotting, respectively. Dual-immunolabeling of Tie1 with specific markers of endothelium and smooth muscle and Ang1 and Ang2 was performed.

MAIN OUTCOME MEASURES

To characterize the expression of Tie1 in human CC and elucidate its potential inhibitory effect in Ang-Tie2 system.

RESULTS

Analysis of mRNAs demonstrated a decrease in Tie1 expression in CVDRF individuals compared with aged or young healthy individuals. No variation for Tie2, Ang1, or Ang2 expression was observed among the studied groups. In all analyzed CC fragments, a 125 kDa band, Tie1, was detected. This protein presented a significant age-related decrease, specially in individuals with CVDRF. Immunofluorescence study revealed Tie1 expression in the endothelium of samples of all experimental groups.

CONCLUSIONS

Employing different methodological approaches, we show for the first time that Tie1 is expressed in human CC endothelium, and its level of expression diminishes in aged individuals, particularly those with CVDRF. This finding reinforces the view that delivery of Ang1 to the CC of erectile dysfunction affected CVDRF patients is able to activate a beneficial Tie2 response.

The effect of modified ultrafiltration on angiopoietins in pediatric cardiothoracic operations

BACKGROUND

Cardiopulmonary bypass subjects patients' blood to hemodilution and nonphysiologic conditions, resulting in a systemic inflammatory response. Modified ultrafiltration (MUF) counteracts hemodilution and has also been postulated to improve outcomes by proinflammatory cytokine removal. The objective of this study was to investigate whether the benefits of MUF include the removal of proinflammatory mediators, such as angiopoietin-2 (angpt-2). We hypothesize that some of the clinical benefits of MUF are related to the preferential removal of angpt-2.

METHODS

We performed a prospective cohort study in children 18 years old or younger undergoing cardiopulmonary bypass. Serum samples were obtained from each patient preoperatively, after cardiopulmonary bypass, and on intensive care unit admission. A fluid sample from the MUF effluent was also analyzed. Angpt-1, angpt-2, interleukin-8, and interleukin-10 levels were determined by enzyme-linked immunosorbent assay.

RESULTS

Thirty-one patients were enrolled. Angpt-1 levels significantly decreased across all time points (p<0.01). Angpt-2 concentrations were significantly elevated at intensive care unit admission when compared with both preoperative and post-cardiopulmonary bypass levels (p<0.01). The angpt-2:1 ratio significantly increased after cardiopulmonary bypass to intensive care unit admission (p<0.01). There was no significant difference between the angpt-2 or angpt-1 percentage of extraction within MUF effluent. Interleukin-8 and interleukin-10 significantly increased from preoperative to intensive care unit admission (both p<0.01).

CONCLUSIONS

The results of this study demonstrate that MUF removes both proinflammatory and antiinflammatory mediators equally. This study suggests that the clinical benefits of MUF cannot be attributed to the removal of larger quantities of proinflammatory mediators such as angpt-2 and interleukin-8.

Extracellular matrix modulates angiogenesis in physiological and pathological conditions

Angiogenesis is a multistep process driven by a wide range of positive and negative regulatory factors. Extracellular matrix (ECM) plays a crucial role in the regulation of this process. The degradation of ECM, occurring in response to an angiogenic stimulus, leads to degradation or partial modification of matrix molecules, release of soluble factors, and exposure of cryptic sites with pro- and/or antiangiogenic activity. ECM molecules and fragments, resulting from proteolysis, can also act directly as inflammatory stimuli, and this can explain the exacerbated angiogenesis that drives and maintains several inflammatory diseases. In this review we have summarized some of the more recent literature data concerning the molecular control of ECM in angiogenesis in both physiological and pathological conditions.

Coexpression of ang1 and tie2 in odontoblasts of mouse developing and mature teeth-a new insight into dentinogenesis

Angiopoietin-1 regulates vascular angiogenesis and stabilization, and is reported to promote bone formation by facilitating angiogenesis. To estimate the role of Ang1 in odontogenesis, we explored the distribution of Ang1 and the receptor, Tie2 in the mouse developing and mature first molar of the mandible. At embryonic day 18, when differentiation of odontoblasts begins, immunosignals for Ang1 were intensely detected in the basement membrane and the distal side, which faced the basement membrane of odontoblasts. In situ hybridization revealed that Ang1 was expressed in odontoblasts and ameloblasts facing the basement membrane. Tie2 was localized in the distal side of odontoblasts. After birth, Ang1 was detected in the predentin, whereas both Ang1 and Tie2 were colocalized in odontoblasts and odontoblast processes. These distributions were retained up to 8 weeks. In contrast to odontoblasts, ameloblasts, cementoblasts and osteoblasts expressed Ang1 but did not express Tie2. Colocalization of Ang1 and Tie2 in odontoblasts and selective expression of Tie2 in odontoblasts among cells responsible for calcified tissue formation suggested the involvement of autocrine signals of Ang1-Tie2 in dentinogenesis.

Extracellular matrix structure and tissue stiffness control postnatal lung development through the lipoprotein receptor-related protein 5/Tie2 signaling system

Physical properties of the tissues and remodeling of extracellular matrix (ECM) play an important role in organ development. Recently, we have reported that low-density lipoprotein receptor-related protein (LRP) 5/Tie2 signaling controls postnatal lung development by modulating angiogenesis. Here we show that tissue stiffness modulated by the ECM cross-linking enzyme, lysyl oxidase (LOX), regulates postnatal lung development through LRP5-Tie2 signaling. The expression of LRP5 and Tie2 is up-regulated twofold in lung microvascular endothelial cells when cultured on stiff matrix compared to those cultured on soft matrix in vitro. LOX inhibitor, β-aminopropionitrile, disrupts lung ECM (collagen I, III, and VI, and elastin) structures, softens neonatal mouse lung tissue by 20%, and down-regulates the expression of LRP5 and Tie2 by 20 and 60%, respectively, which leads to the inhibition of postnatal lung development (30% increase in mean linear intercept, 1.5-fold increase in air space area). Importantly, hyperoxia treatment (Postnatal Days 1-10) disrupts ECM structure and stiffens mouse lung tissue by up-regulating LOX activity, thereby increasing LRP5 and Tie2 expression and deregulating alveolar morphogenesis in neonatal mice, which is attenuated by inhibiting LOX activity. These findings suggest that appropriate physical properties of lung tissue are necessary for physiological postnatal lung development, and deregulation of this mechanism contributes to postnatal lung developmental disorders, such as bronchopulmonary dysplasia.

Angiopoietin-1 and angiopoietin-2 are altered in polycystic ovarian syndrome (PCOS) during controlled ovarian stimulation

Polycystic ovarian syndrome (PCOS) ovaries are characterized by increased angiogenesis and hypervascularity. While angiopoietin-1 (Ang-1) and its antagonist, angiopoietin-2 (Ang-2), are essential for ovarian function and angiogenesis, the levels of Ang-1 and Ang-2 in PCOS are unknown. This was a prospective cohort study of 14 PCOS women and 14 matched controls undergoing controlled ovarian stimulation (COS). Serum was collected on day 3, hCG and retrieval days. Follicular fluid (FF) was collected on retrieval day. Serum Ang-1 and Ang-2 levels were constant throughout COS, but serum Ang-1 levels were increased at all time points in PCOS women compared with controls (p < 0.05). No differences between groups were found in serum Ang-2 levels or FF Ang-1 levels. However, FF Ang-2 levels were increased almost 2-fold in PCOS women compared with controls (p < 0.01), and correlated positively with number of oocytes retrieved (r = 0.65, p < 0.0001). This study is the first to provide evidence of an alteration in the Ang-1/Ang-2 system in PCOS women. The biological role of Ang-2 in promoting capillary leakage, the increased Ang-2 FF level in PCOS, and its correlation with number of oocytes suggest that Ang-2 may play an important role in the increased risk of ovarian hyperstimulation in PCOS.

The angiopoietin:Tie 2 interaction: a potential target for future therapies in human vascular disease

Angiopoietin-1 and -2 are endogenous ligands for the vascular endothelial receptor tyrosine kinase Tie2. Signalling by angiopoietin-1 promotes vascular endothelial cell survival and the sprouting and reorganisation of blood vessels, as well as inhibiting activation of the vascular endothelial barrier to reduce leakage and leucocyte migration into tissues. Angiopoietin-2 generally has an opposing action, and is released naturally at times of vascular growth and inflammation. There is a significant body of emerging evidence that promoting the actions of angiopoietin-1 through Tie2 is of benefit in pathologies of vascular activation, such as sepsis, stroke, diabetic retinopathy and asthma. Similarly, methods to inhibit the actions of angiopoietin-2 are emerging and have been demonstrated to be of preclinical and clinical benefit in reducing tumour angiogenesis. Here the author reviews the evidence for potential benefits of modulation of the interaction of angiopoietins with Tie2, and the potential applications. Additionally, methods for delivery of the complex protein angiopoietin-1 are discussed, as well as potentially deleterious consequences of administering angiopoietin-1.

Vascular endothelial growth factor-angiopoietin chimera with improved properties for therapeutic angiogenesis

BACKGROUND

There is an unmet need for proangiogenic therapeutic molecules for the treatment of tissue ischemia in cardiovascular diseases. However, major inducers of angiogenesis such as vascular endothelial growth factor (VEGF/VEGF-A) have side effects that limit their therapeutic utility in vivo, especially at high concentrations. Angiopoietin-1 has been considered to be a blood vessel stabilization factor that can inhibit the intrinsic property of VEGF to promote vessel leakiness. In this study, we have designed and tested the angiogenic properties of chimeric molecules consisting of receptor-binding parts of VEGF and angiopoietin-1. We aimed at combining the activities of both factors into 1 molecule for easy delivery and expression in target tissues.

METHODS AND RESULTS

The VEGF-angiopoietin-1 (VA1) chimeric protein bound to both VEGF receptor-2 and Tie2 and induced the activation of both receptors. Detailed analysis of VA1 versus VEGF revealed differences in the kinetics of VEGF receptor-2 activation and endocytosis, downstream kinase activation, and VE-cadherin internalization. The delivery of a VA1 transgene into mouse skeletal muscle led to increased blood flow and enhanced angiogenesis. VA1 was also very efficient in rescuing ischemic limb perfusion. However, VA1 induced less plasma protein leakage and myeloid inflammatory cell recruitment than VEGF. Furthermore, angioma-like structures associated with VEGF expression were not observed with VA1.

CONCLUSIONS

The VEGF-angiopoietin-1 chimera is a potent angiogenic factor that triggers a novel mode of VEGF receptor-2 activation, promoting less vessel leakiness, less tissue inflammation, and better perfusion in ischemic muscle than VEGF. These properties of VA1 make it an attractive therapeutic tool.

A clinicopathological correlation of the expression of the angiopoietin-Tie-2 receptor pathway in the brain of adults with Plasmodium falciparum malaria

Background

Plasma angiopoietin (Ang)-2 is associated with disease severity and mortality in adults and children with falciparum malaria. However the mechanism of action of the angiopoietins in fatal malaria is unclear. This study aimed to determine whether the expression of Ang-1 and Ang-2 and their receptor Tie-2 in cerebral endothelial or parenchymal cells was specific to cerebral malaria (CM), correlated with coma or other severe clinical features, and whether plasma and CSF levels of these markers correlated with the clinical and neuropathological features of severe and fatal malaria in Vietnamese adults.

Methods

Immunohistochemistry was performed for Ang-1, Ang-2 and Tie-2 on post-mortem brain tissue from fatal malaria cases and controls. Quantitative ELISA for plasma and cerebrospinal fluid levels of Ang-1, Ang-2 and Tie-2 was done to compare fatal cases with surviving patients from the same study.

Results

Immunohistochemistry revealed significant differences in expression in endothelial and parenchymal cells compared to controls. However there was no significant difference in expression of these markers on endothelial cells, astroglial cells or neurons between CM and non-cerebral malaria cases. Immunostaining of Ang-1, Ang-2 and Tie-2 was also not associated with Plasmodium falciparum-infected erythrocyte sequestration in the brain. However Ang-1 and Ang-2 expression in neurons was significantly correlated with the incidence of microscopic haemorrhages. Plasma levels of Ang-2 and Ang-2/Ang-1 ratio were associated with the number of severe malaria complications and were significant and independent predictors of metabolic acidosis and fatal outcome.

Conclusions

The independent prognostic significance of Ang-2 and the Ang-2/Ang-1 ratio in severe malaria was confirmed, although immunohistochemistry in fatal cases did not reveal increased expression on brain endothelium in cerebral versus non-cerebral cases. Activation of the Ang-Tie-2 pathway in severe malaria is therefore related to acidosis, number of severity criteria and outcome, but is not a specific event in the brain during cerebral malaria.

The complex role of angiopoietin-2 in the angiopoietin-tie signaling pathway

The angiopoietin-Tie signaling system is a vascular-specific receptor tyrosine kinase pathway that is essential for normal vascular development. Although the basic functioning of the pathway is understood, many uncertainties remain about the role of certain members of the pathway, particularly angiopoietin-2 (Ang2), in pathological vascular remodeling and angiogenesis. We summarize the components of the angiopoietin-Tie pathway and then focus on studies that highlight the role of Ang2 in disease settings, including cancer and inflammation. The expression of Ang2 is elevated in many cancers and types of inflammation, which prompted the development of specific reagents to block its interaction with the Tie2 receptor. The application of these reagents in preclinical models of inflammation and cancer has begun to elucidate the role of Ang2 in vascular remodeling and disease pathogenesis and has led to emerging clinical tests of Ang2 inhibitors.

Angiopoietins in angiogenesis

Tie-1 and Tie-2 tyrosine kinase receptors are expressed specifically on vascular endothelial cells and on a certain subtype of macrophages implicated in angiogenesis, thus, they have been a major focus of angiogenesis research. Tie-1 and Tie-2 are essential for vascular maturation during developmental, physiological and pathological angiogenesis. Angiopoietin 1-4 (Ang-1-4) have been identified as bona fide ligands of the Tie-2 receptor, while Tie-1 remains an orphan receptor which is able to heterodimerize with Tie-2 and to modulate Tie-2 signal transduction. The most exhaustively studied angiopoietins are Ang-1 and Ang-2. Ang-1 is a critical player in vessel maturation and it mediates migration, adhesion and survival of endothelial cells. Ang-2 disrupts the connections between the endothelium and perivascular cells and promotes cell death and vascular regression. Yet, in conjunction with VEGF, Ang-2 promotes neo-vascularization. Hence, angiopoietins exert crucial roles in the angiogenic switch during tumor progression, and increased expression of Ang-2 relative to Ang-1 in tumors correlates with poor prognosis. Its central role in the regulation of physiological and pathological angiogenesis makes the angiopoietin/Tie signaling pathway a therapeutically attractive target for the treatment of vascular disease and cancer.

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.

Angiopoietin-4 promotes glioblastoma progression by enhancing tumor cell viability and angiogenesis

Glioblastoma multiforme (GBM) is a highly invasive and vascularized aggressive brain tumor. Less than 10% of GBM patients survive >5 years after diagnosis. Angiogenesis plays an important role in GBM growth, and antiangiogenesis-based therapies have shown clinical efficacy for GBM patients. Unfortunately, therapeutic resistance often develops in these patients, suggesting that GBM cells are capable of switching their dependency on one proangiogenic signaling pathway to an alternative one. Therefore, it is important to identify novel angiogenic factors that play essential roles in tumor angiogenesis and GBM progression. Angiopoietins (Ang-1, Ang-2, and Ang-4) are the ligands of the Tie-2 receptor tyrosine kinase (RTK). The roles of Ang-1 and Ang-2 in tumor angiogenesis have been established. However, little is known about how Ang-4 affects tumor angiogenesis and GBM progression and the mechanism underlying its effects. In our current study, we establish that Ang-4 is upregulated in human GBM tissues and cells. We show that, like endothelial cells, human GBM cells express Tie-2 RTK. We first establish that Ang-4 promotes in vivo growth of human GBM cells by promoting tumor angiogenesis and directly activating extracellular signal-regulated kinase 1/2 (Erk1/2) in GBM cells. Our results establish the novel effects of Ang-4 on tumor angiogenesis and GBM progression and suggest that this pro-GBM effect of Ang-4 is mediated by promoting tumor angiogenesis and activating Erk1/2 kinase in GBM cells. Together, our results suggest that the Ang-4-Tie-2 functional axis is an attractive therapeutic target for GBM.

Impacts of angiopoietin-like proteins on lipoprotein metabolism and cardiovascular events

PURPOSE OF REVIEW

Angiopoietin-like proteins (Angptls) comprise a family of secreted glycoproteins with high homology to angiopoietins which are important regulators of angiogenesis. Angptl3 and Angptl4 inhibit lipoprotein lipase in mice. This article reviews recent human studies on Angptls and their effects on lipoprotein profiles and cardiovascular disease.

RECENT FINDINGS

Population-based studies have indicated that loss-of-function mutations of Angptl3, Angptl4, and Angptl5 are associated with a low triglyceride concentration. Angptl3 concentration is positively correlated with HDL-cholesterol, but not with triglyceride, suggesting that Angptl3 regulates HDL metabolism by inhibiting endothelial lipase. Angptl4 concentration is correlated positively with triglycerides in patients with metabolic syndrome, although most studies have failed to find such a correlation. Angptl6 has no effects on lipoprotein profiles. Angptl3 is associated with atherosclerosis in coronary, carotid, and femoral arteries. Conflicting results have been obtained regarding whether the E40K variant (a loss-of-function mutation of Angptl4) is associated with an increased risk for cardiovascular disease, which may occur due to the lipid-independent actions of Angptl4.

SUMMARY

Angptl3, Angptl4, and possibly Angptl5 are regulators of lipoprotein metabolism in humans. Whether inhibition of these Angptls will be useful for dyslipidemia to prevent cardiovascular disease remains to be elucidated in future studies.

Angiopoietin-1 reduces H(2)O(2)-induced increases in reactive oxygen species and oxidative damage to skin cells

UV light-based damage to skin cells can cause photoaging and skin cancer. A major cause of UV light-induced damage to skin is increased free radicals, such as superoxides. Increased superoxides can cause oxidative and nitrative damage to cell components. Thus, agents that counteract these damages may have therapeutic value. Herein, we show that angiopoietin-1 (ang1) prevented and blocked H(2)O(2)-induced increases in superoxides in human spontaneously immortalized keratinocyte line, HaCaT, and primary melanocytes (HeMn). Ang1 prevented H(2)O(2)-induced increases in damage to DNA (8-hydroxy-2'-deoxyguanosine) and proteins (nitrotyrosinylation). Ang1 promoted skin cell metabolism/viability, adhesion, and akt and MAPK(p42/44) activations. Using multi-gene transcriptional profiling, we found that skin cells express integrin subunits {(beta(1), beta(4-6), beta(8), alpha(v), alpha(2), alpha(3), alpha(6) (HaCaT)), (beta(1), beta(3), beta(5), beta(8), alpha(v), alpha(3) (HeMn))} and lack tie2 receptor mRNA. Integrin antibodies (alpha(v), beta(1)) disrupted skin cell adhesion to ang1 and ang1-induced decreases in superoxides. Our findings show that ang1 blocks free radical damage to skin cells and may be clinically useful to prevent and/or reduce photoaging and skin cancer.

Effects of angiopoietin-1 on attachment and metastasis of human gastric cancer cell line BGC-823

AIM: To evaluate the effects of angiopoietin-1 (Ang-1) on adhesion of gastric cancer cell line BGC-823 and expression of integrin β1, CD44V6, urokinase-type plasminogen activator (uPA) and matrix metalloproteinase-2 (MMP-2).

METHODS: BGC-823 cells were transfected transiently with adenovirus-Ang-1 (Ad-Ang-1). Cells transfected transiently with adenovirus-green fluorescent protein (Ad-GFP) and untransfected cells were used as a negative and blank control group, respectively. The cell adhesion rate between cell and extracellular matrix (ECM) was determined by cell adhesion assay. To investigate whether Ang-1 could reinforce gastric carcinoma metastasis, we performed migration and invasion assays in BGC-823 cells. The mRNA and protein expression of integrin β1, CD44V6, uPA and MMP-2 were detected by reverse transcription polymerase chain reaction and Western blotting, respectively. The expression of integrin β1 and CD44V6 was measured by immunohistochemistry.

RESULTS: BGC-823 cells were transfected successfully. The adhesion rate increased significantly in the Ad-Ang-1 group (P < 0.05). The Ad-Ang-1-transfected group had a significant increase in migration and invasion compared with that of the mock-transfected and Ad-GFP groups. The mRNA and protein expression of integrin β1, CD44V6, uPA and MMP-2 in the Ad-Ang-1 group was higher than that in the Ad-GFP and blank control groups (P < 0.05). Compared with mock-transfected and Ad-GFP groups, integrin β1 and CD44V6 expression intensity greatly increased (P < 0.05).

CONCLUSION: Transfection of Ang-1 into human gastric cancer cell line BGC-823 can significantly increase expression of integrin β1 and CD44V6, by which cell adhesion and metastasis to the ECM are promoted.

Angiopoietin-1-induced ubiquitylation of Tie2 by c-Cbl is required for internalization and degradation

Tie2 [where ‘Tie’ is an acronym from tyrosine kinase with Ig and EGF (epidermal growth factor) homology domains] is a receptor tyrosine kinase expressed predominantly on the surface of endothelial cells. Activated by its ligands, the angiopoietins, Tie2 initiates signalling pathways that modulate vascular stability and angiogenesis. Deletion of either the Tie2 or Ang1 (angiopoietin-1) gene in mice results in lethal vascular defects, signifying their importance in vascular development. The mechanism employed by the Tie2 signalling machinery to attenuate or cause receptor trafficking is not well defined. Stimulation of Tie2-expressing cells with Ang1 results in its ubiquitylation, suggesting that this may provide the necessary signal for receptor turnover. Using a candidate molecule approach, we demonstrate that Tie2 co-immunoprecipitates with c-Cbl in an Ang1-dependent manner and its ubiquitylation can be inhibited by the dominant-interfering molecule v-Cbl (a viral form of c-Cbl that contains only the tyrosine kinase-binding domain region). Inhibition of the Tie2–Cbl interaction by overexpression of v-Cbl blocks ligand-induced Tie2 internalization and degradation. In summary, our results illustrate that c-Cbl interacts with the Tie2 signalling complex in a stimulation-dependent manner, and that this interaction is required for Tie2 ubiquitylation, internalization and degradation.

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

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

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

BACKGROUND

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

OBJECTIVE

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

METHODS

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

RESULTS/CONCLUSION

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

Integrin binding angiopoietin-1 monomers reduce cardiac hypertrophy

Angiopoietins were thought to be endothelial cell-specific via the tie2 receptor. We showed that angiopoietin-1 (ang1) also interacts with integrins on cardiac myocytes (CMs) to increase survival. Because ang1 monomers bind and activate integrins (not tie2), we determined their function in vivo. We examined monomer and multimer expressions during physiological and pathological cardiac remodeling and overexpressed ang1 monomers in phenylephrine-induced cardiac hypertrophy. Cardiac ang1 levels (mRNA, protein) increased during postnatal development and decreased with phenylephrine-induced cardiac hypertrophy, whereas tie2 phosphorylations were unchanged. We found that most or all of the changes during cardiac remodeling were in monomers, offering an explanation for unchanged tie2 activity. Heart tissue contains abundant ang1 monomers and few multimers (Western blotting). We generated plasmids that produce ang1 monomers (ang1-256), injected them into mice, and confirmed cardiac expression (immunohistochemistry, RT-PCR). Ang1 monomers localize to CMs, smooth muscle cells, and endothelial cells. In phenylephrine-induced cardiac hypertrophy, ang1-256 reduced left ventricle (LV)/tibia ratios, fetal gene expressions (atrial and brain natriuretic peptides, skeletal actin, beta-myosin heavy chain), and fibrosis (collagen III), and increased LV prosurvival signaling (akt, MAPK(p42/44)), and AMPK(T172). However, tie2 phosphorylations were unchanged. Ang1-256 increased integrin-linked kinase, a key regulator of integrin signaling and cardiac health. Collectively, these results suggest a role for ang1 monomers in cardiac remodeling.

Development of new targeted therapies for breast cancer

Clinical application of truly targeted therapy relies on identification of a specific molecular feature (the target) that is biologically relevant, reproducibly measurable and definably correlated with clinical benefit. Ideally the target should be crucial to the tumor's malignant phenotype. The target must be easily measurable in readily obtained clinical samples. Interruption, interference or inhibition of the target should yield a clinical response in a significant proportion of patients whose tumors express the target but in few patients whose tumors do not express the target. As such, targeted therapy offers the twin hopes of maximizing efficacy while minimizing toxicity. A critical review of the hallmarks of malignancy provides a framework for considering potential targets and novel therapeutic interventions. The hallmarks of malignancy include uncontrolled proliferation, insensitivity to negative growth regulation, evasion of apoptosis, lack of senescence, invasion and metastasis, angiogenesis, and genomic elasticity. Existing therapies predominantly target proliferation either with cytotoxic agents, ionizing radiation or inhibition of estrogen receptor and HER2 growth factor signaling pathways. Further improvements in therapy must attack the other hallmarks of malignancy and will undoubtedly be accompanied by better means of individual patient selection for such therapies. Indeed, each of these hallmarks presents a therapeutic opportunity. To believe otherwise would be to assume that a feature is both biologically crucial yet therapeutically unimportant, an unlikely paradox.

Differential function of Tie2 at cell-cell contacts and cell-substratum contacts regulated by angiopoietin-1

Tie2 belongs to the receptor tyrosine kinase family and functions as a receptor for Angiopoietin-1 (Ang1). Gene-targeting analyses of either Ang1 or Tie2 in mice reveal a critical role of Ang1-Tie2 signalling in developmental vascular formation. It remains elusive how the Tie2 signalling pathway plays distinct roles in both vascular quiescence and angiogenesis. We demonstrate here that Ang1 bridges Tie2 at cell-cell contacts, resulting in trans-association of Tie2 in the presence of cell-cell contacts. In clear contrast, in isolated cells, extracellular matrix-bound Ang1 locates Tie2 at cell-substratum contacts. Furthermore, Tie2 activated at cell-cell or cell-substratum contacts leads to preferential activation of Akt and Erk, respectively. Microarray analyses and real-time PCR validation clearly show the differential gene expression profile in vascular endothelial cells upon Ang1 stimulation in the presence or absence of cell-cell contacts, implying downstream signalling is dependent upon the spatial localization of Tie2.

Angiopoietins assemble distinct Tie2 signalling complexes in endothelial cell-cell and cell-matrix contacts

The receptor tyrosine kinase Tie2, and its activating ligand Angiopoietin-1 (Ang1), are required for vascular remodelling and vessel integrity, whereas Ang2 may counteract these functions. However, it is not known how Tie2 transduces these different signals. Here, we show that Ang1 induces unique Tie2 complexes in mobile and confluent endothelial cells. Matrix-bound Ang1 induced cell adhesion, motility and Tie2 activation in cell-matrix contacts that became translocated to the trailing edge in migrating endothelial cells. In contrast, in contacting cells Ang1 induced Tie2 translocation to cell-cell contacts and the formation of homotypic Tie2-Tie2 trans-associated complexes that included the vascular endothelial phosphotyrosine phosphatase, leading to inhibition of paracellular permeability. Distinct signalling proteins were preferentially activated by Tie2 in the cell-matrix and cell-cell contacts, where Ang2 inhibited Ang1-induced Tie2 activation. This novel type of cellular microenvironment-dependent receptor tyrosine kinase activation may explain some of the effects of angiopoietins in angiogenesis and vessel stabilization.

Microvascular maturity elicited in tissue treated with cytokine-loaded hyaluronan-based hydrogels

Hydrogels composed of crosslinked, chemically modified hyaluronic acid (HA), gelatin (Gtn) and heparin (Hp) were preloaded with vascular endothelial growth factor (VEGF), angiopoietin-1 (Ang-1), keratinocyte growth factor (KGF) or platelet derived growth factor (PDGF) either individually or in combination with VEGF and implanted into the Balb/c mouse ear pinna. At 7 and 14 days post-surgery, elicited vascular maturity levels were quantified using immunohistochemical (IHC) staining techniques and reported as a vascular maturity index (VMI). At both time points, it was discovered that the dual cytokine combinations elicited greater maturity levels than that of cytokine administered individually. For example, VEGF and KGF-containing HA:Hp implants at day 7 yielded VMI values of -0.1375 and -0.092, respectively, whereas their combination resulted in a VMI of 0.176 (p<0.007). At day 7, only one of the seven HA:Hp experimental cases yielded a positive VMI (VEGF+KGF), whereas four of the seven HA:Hp cases yielded positive VMI values at day 14, indicating a sustained maturity response. The same general trends were found to exist in tissue treated with HA:Hp:Gtn experimental implants. Differences in elicited maturity also existed between tissue treated with HA:Hp and HA-containing hydrogels (VMI=0.176 for HA:Hp-VEGF+KGF vs. -0.064 for HA-VEGF+KGF, p<0.012), and these differences are thought to result from differences in characteristic cytokine release rates. This result also suggests that the presentation of multiple growth factors (GFs) on immobilized Hp may actively contribute to cytokine related signal transduction, a characteristic that may be exploited in the future to improve the efficacy of cytokine-loaded implants towards tissue regeneration therapeutic strategies.

Intra and extravascular transmembrane signalling of angiopoietin-1-Tie2 receptor in health and disease

Angiopoietin-1 (Ang-1) is the primary agonist for Tie2 tyrosine kinase receptor (Tie2), and the effect of Ang-1-Tie2 signalling is context-dependent. Deficiency in either Ang-1 or Tie2 protein leads to severe microvascular defects and subsequent embryonic lethality in murine model. Tie2 receptors are expressed in several cell types, including endothelial cells, smooth muscle cells, fibroblasts, epithelial cells, monocytes, neutrophils, eosinophils and glial cells. Ang-1-Tie2 signalling induces a chemotactic effect in smooth muscle cells, neutrophils and eosinophils, and induces differentiation of mesenchymal cells to smooth muscle cells. Additionally, this signalling pathway induces the secretion of serotonin, matrix metalloproteinases (MMPs) and plasmin. Ang-1 inhibits the secretion of tissue inhibitor of matrix metalloproteinase (TIMPs). Aberrant expression and activity of Tie2 in vascular and non-vascular cells may result in the development of rheumatoid arthritis, cancer, hypertension and psoriasis. Ang-1 has an anti-inflammatory effect, when co-localized with vascular endothelial growth factor (VEGF) in the vasculature. Thus, Ang-1 could be potentially important in the therapy of various pathological conditions such as pulmonary hypertension, arteriosclerosis and diabetic retinopathy. In this article, we have summarized and critically reviewed the pathophysiological role of Ang-1-Tie2 signalling pathway.