Inhibition of tumor angiogenesis using a soluble receptor establishes a role for Tie2 in pathologic vascular growth

Anticancer Potential of Indole Phytoalexins and Their Analogues

Indole phytoalexins, found in economically significant Cruciferae family plants, are synthesized in response to pathogen attacks or stress, serving as crucial components of plant defense mechanisms against bacterial and fungal infections. Furthermore, recent research indicates that these compounds hold promise for improving human health, particularly in terms of potential anticancer effects that have been observed in various studies. Since our last comprehensive overview in 2016 focusing on the antiproliferative effects of these substances, brassinin and camalexin have been the most extensively studied. This review analyses the multifaceted pharmacological effects of brassinin and camalexin, highlighting their anticancer potential. In this article, we also provide an overview of the antiproliferative activity of new synthetic analogs of indole phytoalexins, which were synthesized and tested at our university with the aim of enhancing efficacy compared to the parent compound.

Beyond VEGF: Angiopoietin-Tie Signaling Pathway in Diabetic Retinopathy

Complications from diabetic retinopathy such as diabetic macular edema (DME) and proliferative diabetic retinopathy (PDR) constitute leading causes of preventable vision loss in working-age patients. Since vascular endothelial growth factor (VEGF) plays a major role in the pathogenesis of these complications, VEGF inhibitors have been the cornerstone of their treatment. Anti-VEGF monotherapy is an effective but burdensome treatment for DME. However, due to the intensive and burdensome treatment, most patients in routine clinical practice are undertreated, and therefore, their outcomes are compromised. Even in adequately treated patients, persistent DME is reported anywhere from 30% to 60% depending on the drug used. PDR is currently treated by anti-VEGF, panretinal photocoagulation (PRP) or a combination of both. Similarly, a number of eyes, despite these treatments, continue to progress to tractional retinal detachment and vitreous hemorrhage. Clearly there are other molecular pathways other than VEGF involved in the pathogenesis of DME and PDR. One of these pathways is the angiopoietin-Tie signaling pathway. Angiopoietin 1 (Ang1) plays a major role in maintaining vascular quiescence and stability. It acts as a molecular brake against vascular destabilization and inflammation that is usually promoted by angiopoietin 2 (Ang2). Several pathological conditions including chronic hyperglycemia lead to Ang2 upregulation. Recent regulatory approval of the bi-specific antibody, faricimab, may improve long term outcomes in DME. It targets both the Ang/Tie and VEGF pathways. The YOSEMITE and RHINE were multicenter, double-masked, randomized non-inferiority phase 3 clinical trials that compared faricimab to aflibercept in eyes with center-involved DME. At 12 months of follow-up, faricimab demonstrated non-inferior vision gains, improved anatomic outcomes and a potential for extended dosing when compared to aflibercept. The 2-year results of the YOSEMITE and RHINE trials demonstrated that the anatomic and functional results obtained at the 1 year follow-up were maintained. Short term outcomes of previously treated and treatment-naive eyes with DME that were treated with faricimab during routine clinical practice suggest a beneficial effect of faricimab over other agents. Targeting of Ang2 has been reported by several other means including VE-PTP inhibitors, integrin binding peptide and surrobodies.

Overfit deep neural network for predicting drug-target interactions

Drug-target interactions (DTIs) prediction is an important step in drug discovery. As traditional biological experiments or high-throughput screening are high cost and time-consuming, many deep learning models have been developed. Overfitting must be avoided when training deep learning models. We propose a simple framework, called OverfitDTI, for DTI prediction. In OverfitDTI, a deep neural network (DNN) model is overfit to sufficiently learn the features of the chemical space of drugs and the biological space of targets. The weights of trained DNN model form an implicit representation of the nonlinear relationship between drugs and targets. Performance of OverfitDTI on three public datasets showed that the overfit DNN models fit the nonlinear relationship with high accuracy. We identified fifteen compounds that interacted with TEK, a receptor tyrosine kinase contributing to vascular homeostasis, and the predicted AT9283 and dorsomorphin were experimentally demonstrated as inhibitors of TEK in human umbilical vein endothelial cells (HUVECs).

Brassinin Promotes the Degradation of Tie2 and FGFR1 in Endothelial Cells and Inhibits Triple-Negative Breast Cancer Angiogenesis

Simple Summary

Brassinin is a natural compound enriched in several commonly consumed vegetables, such as broccoli and cabbages. It shows potent anti-cancer activity against several cancers. However, its effects on triple-negative breast cancer (TNBC), an aggressive subtype with limited treatment options, remain elusive so far. Therefore, we investigated the effects of brassinin on TNBC angiogenesis and growth. Our results demonstrate that brassinin inhibits TNBC growth preferentially through inhibiting the angiogenic activity of endothelial cells (ECs). Additional in-vitro analyses revealed that this effect may be mediated by brassinin-stimulated degradation of two pivotal angiogenesis-related receptors in ECs: Tie2 and fibroblast growth factor receptor 1. These findings provide novel insights into the cellular and molecular mechanisms underlying the anti-cancer activity of brassinin and indicate that this phytochemical may be a promising lead compound or drug candidate for TNBC treatment.

Abstract

Brassinin, a phytoalexin derived from cruciferous vegetables, has been reported to exhibit anti-cancer activity in multiple cancer types. However, its effects on triple-negative breast cancer (TNBC) development and the underlying mechanisms have not been elucidated so far. In this study, we demonstrated in vitro that brassinin preferentially reduces the viability of endothelial cells (ECs) when compared to other cell types of the tumor microenvironment, including TNBC cells, pericytes, and fibroblasts. Moreover, brassinin at non-cytotoxic doses significantly suppressed the proliferation, migration, tube formation, and spheroid sprouting of ECs. It also efficiently inhibited angiogenesis in an ex-vivo aortic ring assay and an in-vivo Matrigel plug assay. Daily intraperitoneal injection of brassinin significantly reduced tumor size, microvessel density, as well as the perfusion of tumor microvessels in a dorsal skinfold chamber model of TNBC. Mechanistic analyses showed that brassinin selectively stimulates the degradation of Tie2 and fibroblast growth factor receptor 1 in ECs, leading to the down-regulation of the AKT and extracellular signal-regulated kinase pathways. These findings demonstrate a preferential and potent anti-angiogenic activity of brassinin, which may be the main mechanism of its anti-tumor action. Accordingly, this phytochemical represents a promising candidate for the future anti-angiogenic treatment of TNBC.

Differential Metabolome in Rheumatoid Arthritis: a Brief Perspective

PURPOSE OF REVIEW

Rheumatoid arthritis (RA) is a chronic autoimmune, inflammatory disease of the synovium that affects the movable joints. It develops due to the infiltration and invasion of the synovial joints by immune cells. Metabolism is anabolic or catabolic chemical reactions occurring in a cell. The biochemical pathways in synovial and immune cells are altered affecting the downstream metabolite formation. Changes in the metabolite levels alter signaling cascades which further intensify the disease. Despite current knowledge of metabolomics, there remain certain features that need to be elucidated to correlate the differential metabolite levels with RA.

RECENT FINDINGS

Metabolite profiling can be used to find altered patterns of metabolites in RA. Glucose, lipid, amino acid, and estrogen metabolism are the key pathways that are altered and contribute to the aggravation of RA. The altered metabolic pathways involved in different cells in RA results in complex interactions between metabolites and biomacromolecules; thus, it generates autoantigens. Moreover, understanding the correlation between differential metabolites and disease severity might help reveal potential new biomarkers and therapeutic targets for RA pathogenesis. So, considering the multi-faceted role of altered metabolites in the pathogenesis of RA, metabolic pathways of different cells are needed to be studied for a better understanding of their functions in the disease and thus, improving the present therapeutic strategies.

Regulation of endothelial cell differentiation in embryonic vascular development and its therapeutic potential in cardiovascular diseases

During vertebrate development, the cardiovascular system begins operating earlier than any other organ in the embryo. Endothelial cell (EC) forms the inner lining of blood vessels, and its extensive proliferation and migration are requisite for vasculogenesis and angiogenesis. Many aspects of cellular biology are involved in vasculogenesis and angiogenesis, including the tip versus stalk cell specification. Recently, epigenetics has attracted growing attention in regulating embryonic vascular development and controlling EC differentiation. Some proteins that regulate chromatin structure have been shown to be directly implicated in human cardiovascular diseases. Additionally, the roles of important EC signaling such as vascular endothelial growth factor and its receptors, angiopoietin-1 and tyrosine kinase containing immunoglobulin and epidermal growth factor homology domain-2, and transforming growth factor-β in EC differentiation during embryonic vasculature development are briefly discussed in this review. Recently, the transplantation of human induced pluripotent stem cell (iPSC)-ECs are promising approaches for the treatment of ischemic cardiovascular disease including myocardial infarction. Patient-specific iPSC-derived EC is a potential new target to study differences in gene expression or response to drugs. However, clinical application of the iPSC-ECs in regenerative medicine is often limited by the challenges of maintaining cell viability and function. Therefore, novel insights into the molecular mechanisms underlying EC differentiation might provide a better understanding of embryonic vascular development and bring out more effective EC-based therapeutic strategies for cardiovascular diseases.

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.

EphrinB2/EphB4 signaling regulates non-sprouting angiogenesis by VEGF

Vascular endothelial growth factor (VEGF) is the master regulator of angiogenesis, whose best-understood mechanism is sprouting. However, therapeutic VEGF delivery to ischemic muscle induces angiogenesis by the alternative process of intussusception, or vascular splitting, whose molecular regulation is essentially unknown. Here, we identify ephrinB2/EphB4 signaling as a key regulator of intussusceptive angiogenesis and its outcome under therapeutically relevant conditions. EphB4 signaling fine-tunes the degree of endothelial proliferation induced by specific VEGF doses during the initial stage of circumferential enlargement of vessels, thereby limiting their size and subsequently enabling successful splitting into normal capillary networks. Mechanistically, EphB4 neither inhibits VEGF-R2 activation by VEGF nor its internalization, but it modulates VEGF-R2 downstream signaling through phospho-ERK1/2. In vivo inhibitor experiments show that ERK1/2 activity is required for EphB4 regulation of VEGF-induced intussusceptive angiogenesis. Lastly, after clinically relevant VEGF gene delivery with adenoviral vectors, pharmacological stimulation of EphB4 normalizes dysfunctional vascular growth in both normoxic and ischemic muscle. These results identify EphB4 as a druggable target to modulate the outcome of VEGF gene delivery and support further investigation of its therapeutic potential.

Utilizing combinatorial engineering to develop Tie2 targeting antagonistic angiopoetin-2 ligands as candidates for anti-angiogenesis therapy

In many human cancers, the receptor tyrosine kinase (RTK) Tie2 plays important roles in mediating proliferation, survival, migration and angiogenesis. Thus, molecules that could potently inhibit activation of the Tie2 receptor would have a significant impact on cancer therapy. Nevertheless, attempts to develop Tie2-targeted inhibitors have met with little success, and there is currently no FDA-approved therapeutic selectively targeting Tie2. We used a combinatorial protein engineering approach to develop a new generation of angiopoietin (Ang)2-derived Tie2 antagonists as potential cancer therapeutics and as tools to study angiogenesis. The construct for designing a yeast surface display (YSD) library of potential antagonists was an Ang2 binding domain (Ang2-BD) that retains Tie2 binding ability but prevents ligand multimerization and receptor dimerization and activation. This mutant library was then screened by quantitative high-throughput flow cytometric sorting to identify Ang2-BD variants with increased expression, stability and affinity to Tie2. The selected variants were recombinantly expressed and showed high affinity to soluble and cellular Tie2 and strongly inhibited both Tie2 phosphorylation and endothelial capillary tube formation and cell invasion compared to the parental Ang2-BD. The significance of the study lies in the insight it provides into the sequence-structure-function relationships and mechanism of action of the antagonistic Ang mutants. The approach of using a natural protein ligand as a molecular scaffold for engineering high-affinity agents can be applied to other ligands to create functional protein antagonists against additional biomedical targets.

Transport of drugs from blood vessels to tumour tissue

The effectiveness of anticancer drugs in treating a solid tumour is dependent on delivery of the drug to virtually all cancer cells in the tumour. The distribution of drug in tumour tissue depends on the plasma pharmacokinetics, the structure and function of the tumour vasculature and the transport properties of the drug as it moves through microvessel walls and in the extravascular tissue. The aim of this Review is to provide a broad, balanced perspective on the current understanding of drug transport to tumour cells and on the progress in developing methods to enhance drug delivery. First, the fundamental processes of solute transport in blood and tissue by convection and diffusion are reviewed, including the dependence of penetration distance from vessels into tissue on solute binding or uptake in tissue. The effects of the abnormal characteristics of tumour vasculature and extravascular tissue on these transport properties are then discussed. Finally, methods for overcoming limitations in drug transport and thereby achieving improved therapeutic results are surveyed.

C/EBP-δ positively regulates MDSC expansion and endothelial VEGFR2 expression in tumor development

Vascular endothelial cells and Gr-1+CD11b+ myeloid derived suppressor cells (MDSCs) are two important components that constitute the tumor microenvironment. Targeting these cells offers the potential to halt tumor growth. In this study, we report a common mediator in C/EBP-δ that regulates both components and aids in tumor development. C/EBP-δ is elevated in tumor derived MDSCs. Interestingly, genetic deletion of C/EBP-δ in mice significantly impaired MDSC expansion in response to tumor progression, but it had no effect on Gr-1+CD11b+ cell production in normal development. It suggests a specific role of C/EBP-δ in emergency myelopoiesis under tumor conditions. Consistent with the pro tumor functions of MDSCs, loss of C/EBP-δ resulted in reduced tumor angiogenesis and tumor growth. Moreover, we found expression of C/EBP-δ in vascular endothelial cells. C/EBP-δ regulated cell motility, endothelial network formation and vascular sprouting. Notably, inactivation of C/EBP-δ in endothelial cells specifically inhibited the expression of VEGFR2 but not VEGFR1. Ectopic expression of C/EBP-δ increased and knockdown of the gene decreased VEGFR2 expression. C/EBP-δ is recruited to the promoter region of VEGFR2, indicative of transcriptional regulation. Collectively, this study has identified a positive mediator in C/EBP-δ, which regulates tumor induced MDSC expansion and VEGFR2 expression in endothelium. Considering the importance of MDSCs and endothelial cells in tumor progression, targeting C/EBP-δ may provide an interesting means for cancer therapy, killing two birds with one stone.

TIE-2 expressing monocytes in human cancers

Tumor-associated macrophages (TAM) are well known as a key player in the tumor microenvironment, which support cancer progression. More recently, a lineage of monocytes characterized by the expression of the TIE-2/Tek angiopoietin receptor identified a subset of circulating and tumor-associated monocytes endowed with proangiogenic activity. TIE-2 expressing monocytes (TEM) were found both in humans and mice. Here, we review the phenotypes and functions of TEM reported so far in human cancer and their potential use as markers of cancer progression and metastasis. Finally, we discuss the therapeutic approaches currently used or proposed to target TEM.

Differential regulation of blood flow-induced neovascularization and mural cell recruitment by vascular endothelial growth factor and angiopoietin signalling

KEY POINTS

Combining nitric oxide (NO)-mediated increased blood flow with angiopoietin-1-Tie2 receptor signalling induces arteriolargenesis - the formation of arterioles from capillaries - in a model of physiological angiogenesis. This NO-Tie-mediated arteriolargenesis requires endogenous vascular endothelial growth factor (VEGF) signalling. Inhibition of VEGF signalling increases pericyte coverage in microvessels. Together these findings indicate that generation of functional neovasculature requires close titration of NO-Tie2 signalling and localized VEGF induction, suggesting that the use of exogenous VEGF expression as a therapeutic for neovascularization may not be successful.

ABSTRACT

Signalling through vascular endothelial growth factor (VEGF) receptors and the tyrosine kinase with IgG and EGF domains-2 (Tie2) receptor by angiopoietins is required in combination with blood flow for the formation of a functional vascular network. We tested the hypothesis that VEGF and angiopoietin-1 (Ang1) contribute differentially to neovascularization induced by nitric oxide (NO)-mediated vasodilatation, by comparing the phenotype of new microvessels in the mesentery during induction of vascular remodelling by over-expression of endothelial nitric oxide synthase in the fat pad of the adult rat mesentery during inhibition of angiopoietin signalling with soluble Tie2 (sTie2) and VEGF signalling with soluble Fms-like tyrosine kinase receptor-1 (sFlt1). We found that NO-mediated angiogenesis was blocked by inhibition of VEGF with sFlt1 (from 881 ± 98% increase in functional vessel area to 279 ± 72%) and by inhibition of angiopoietin with sTie2 (to 337 ± 67%). Exogenous angiopoietin-1 was required to induce arteriolargenesis (8.6 ± 1.3% of vessels with recruitment of vascular smooth muscle cells; VSMCs) in the presence of enhanced flow. sTie2 and sFlt1 both inhibited VSMC recruitment (both 0%), and VEGF inhibition increased pericyte recruitment to newly formed vessels (from 27 ± 2 to 54 ± 3% pericyte ensheathment). We demonstrate that a fine balance of VEGF and angiopoietin signalling is required for the formation of a functional vascular network. Endogenous VEGF signalling prevents excess neovessel pericyte coverage, and is required for VSMC recruitment during increased nitric oxide-mediated vasodilatation and angiopoietin signalling (NO-Tie-mediated arteriogenesis). Therapeutic vascular remodelling paradigms may therefore require treatments that modulate blood flow to utilize endogenous VEGF, in combination with exogenous Ang1, for effective neovascularization.

Noninvasive estimation of liver fibrosis in biopsy-proven hepatitis C virus-infected patients: angiogenic fibrogenic link

BACKGROUND/AIM

The assessment of liver fibrosis provides useful information not only for diagnosis but also for therapeutic decisions. This study aimed to develop and evaluate a predictive score named the angiogenic index (Angio-Index) for liver fibrosis staging and to compare Angio-Index by King, Gotebörg University Cirrhosis Index, Lok, FIB-4, and aspartate aminotranferase/alanine aminotranferase scores in hepatitis C virus-infected patients.

PATIENTS AND METHODS

Serum levels of angiopoietin-2, basic fibroblast growth factor, hepatocyte growth factor, and endostatin were assayed using an enzyme-linked immunosorbent assay in 122 HCV patients represented in two sets (estimation group and validation group). Stepwise linear discriminant analysis and area under receiver-operating characteristic curves (AUCs) were utilized to produce a predictive score comprising significant angiogenic biomarkers.

RESULTS

A novel score named the Angio-Index score was created on the basis of a combination of angiopoietin-2, basic fibroblast growth factor, hepatocyte growth factor, and endostatin. Angio-Index produces an AUC of 0.90 for significant fibrosis, 0.865 for advanced fibrosis, and 0.857 for cirrhosis. The Angio-Index score correctly classified 71% of the significant fibrosis (F2-F4) with a sensitivity of 93% and a specificity of 91%. The Angio-Index had a similar AUC in the validation study. The above six scores showed lower AUCs than Angio-Index.

CONCLUSION

Whereas liver biopsy is invasive, costly, and associated with complications, Angio-Index is simple, noninvasive, and more accurate; it may decrease the need for liver biopsy in Egyptian patients.

Efficacy of trebananib (AMG 386) in treating epithelial ovarian cancer

INTRODUCTION

Epithelial ovarian cancer (EOC) is the leading cause of death among gynecologic cancers. The majority of women are diagnosed with advanced stage disease. It is considered a chemosensitive cancer with a high initial response rate to first-line platinum and taxane-based chemotherapy. However, most patients with advanced EOC will relapse with subsequent resistance to conventional chemotherapy and ultimately succumb to their disease. Therefore, new therapeutic agents and strategies are desperately needed to improve the outcomes in patients with advanced EOC.

AREAS COVERED

This review focuses on the use of Trebananib (a non-VEGF-dependent angiogenesis pathway inhibitor) in EOC. Angiogenesis has been recognized as an important process promoting EOC growth and metastasis. Targeting angiogenesis in EOC have been developed and studied with demonstrated clinical efficacy. Bevacizumab, a humanized monoclonal antibody, that targets vascular endothelial growth factor A (VEGF-A), has been the most well evaluated molecular targeted therapy in the treatment of advanced and recurrent EOC with proven clinical efficacy. However, VEGF-dependent angiogenesis pathway inhibitors are often associated with serious toxicities and drug resistance ultimately develops. Hence, new therapeutic approach targeting the angiopoietin-Tie-2 complex pathway (a non-VEGF-dependent angiogenesis pathway) has gained interest over the past few years as an alternative strategy to overcome VEGF-dependent anti-angiogenesis-related toxicity and resistance.

EXPERT OPINION

Targeting angiopoietin-Tie-2 pathway represents a promising alternative approach to tumor anti-angiogenesis with a distinct toxicity profile from the VEGF-dependent pathway inhibitors. However, there are still many questions to be answered regarding the optimal treatment schedules, maintenance regimens, duration of maintenance therapy, and the best combination strategy. Currently there is no reliable surrogate molecular, cellular, or genetic marker that would definitively predict response to anti-angiogenic therapy. Identification of certain relevant and predictive biomarkers in the future may optimize treatment's efficacy by distinguishing the subset group of patients with EOC that would derive the most benefit from existing antiangiogenic treatment regimens.

Advances in anti-angiogenic agents for ovarian cancer treatment: The role of trebananib (AMG 386)

Ovarian cancer is a multifaceted and genomically complex disease and has emerged as leading cause of death among gynecological malignancies. Gold-standard treatment consisted of cytoreductive surgery and paclitaxel-carboplatin chemotherapy. Recently, promising results of randomized trials have definitively confirmed the importance of angiogenesis in oncogenesis and ovarian cancer behavior, by showing a significant prolongation of progression-free survival with the addiction of an angiogenesis inhibitor to standard treatment in the first and second line setting. Research over the years has sequentially provided a rapidly broadening signaling landscape and many drugs targeting different signaling pathways of angiogenesis have been developed and investigated. Recently accumulating scientific evidence has started to shed light on the efficacy of AMG 386, a new peptibody reported to neutralize the interaction between angiopoietins (Ang1/2) and their Tie2 receptors, thus representing a promising alternative, both in terms of efficacy and toxicity profile and is considerably under investigation. The aim of this review is to summarize the recent researches and clinical progresses of AMG 386 as a novel target agent in ovarian cancer.

Emerging Role of Sphingosine-1-phosphate in Inflammation, Cancer, and Lymphangiogenesis

The main function of the lymphatic system is to control and maintain fluid homeostasis, lipid transport, and immune cell trafficking. In recent years, the pathological roles of lymphangiogenesis, the generation of new lymphatic vessels from preexisting ones, in inflammatory diseases and cancer progression are beginning to be elucidated. Sphingosine-1-phosphate (S1P), a bioactive lipid, mediates multiple cellular events, such as cell proliferation, differentiation, and trafficking, and is now known as an important mediator of inflammation and cancer. In this review, we will discuss recent findings showing the emerging role of S1P in lymphangiogenesis, in inflammation, and in cancer.

Gr-1+CD11b+ myeloid cells efficiently home to site of injury after intravenous administration and enhance diabetic wound healing by neoangiogenesis

Vascularization is an important factor that affects diabetic wound healing. There is increasing evidence that myeloid cell lineages play a role in neovascularization. In this study, the efficiency of Gr-1+CD11b+ myeloid cells to home to the site of injury and enhance diabetic wound healing by neoangiogenesis after intravenous administration was investigated. Gr-1+CD11b+ myeloid cells were injected into tail vein after establishment of dorsal window chamber, hindlimb ischaemia and ear-punch injury in diabetic or non-diabetic mice. The Gr-1+CD11b+ myeloid cells efficiently homed to the site of injury after intravenous administration and increased neoangiogenesis. The chemokine receptor type 4 (CXCR4) is robustly expressed by Gr-1+CD11b+ myeloid cells. Inhibition of CXCR4 decreases the homing ability of Gr-1+CD11b+ myeloid cells to the site of injury, which indicates that the CXCR4/SDF-1 axis plays an important role in the homing of Gr-1+CD11b+ myeloid cells to the site of injury. In addition, Gr-1+CD11b+ myeloid cells were found to improve blood flow recovery of ischaemic limb and enhance wound healing in diabetic mice by neoangiogenesis after intravenous administration. Taken together, the results of this study suggest that Gr-1+CD11b+ myeloid cells may serve as a potential cell therapy for diabetic wound healing.

Identification of novel molecular markers for prognosis estimation of acute myeloid leukemia: over-expression of PDCD7, FIS1 and Ang2 may indicate poor prognosis in pretreatment patients with acute myeloid leukemia

Numerous factors impact on the prognosis of acute myeloid leukemia (AML), among which molecular genetic abnormalities are developed increasingly, however, accurate prediction for newly diagnosed AML patients remains unsatisfied. For further improving the prognosis evaluation system, we investigated the transcripts levels of PDCD7, FIS1, FAM3A, CA6, APP, KLRF1, ATCAY, GGT5 and Ang2 in 97 AML patients and 30 non-malignant controls, and validated using the published microarray data from 225 cytogenetically normal AML (CN-AML) patients treated according to the German AMLCG-1999 protocol. Real-time quantitative polymerase chain reaction and western blot were carried out, and clinical data were collected and analyzed. High Ang2 and FIS1 expression discriminated the CR rate of AML patients (62.5% versus 82.9% for Ang2, P = 0.011; 61.4% versus 82.2% for FIS1, P = 0.029). In CN-AML, patients with high FIS1 expression were more likely to be resistant to two courses of induction (P = 0.035). Overall survival (OS) and relapse-free survival (RFS) were shorter in CN-AML patients with high PDCD7 expression (P<0.001; P = 0.006), and PDCD7 was revealed to be an independent risk factor for OS in CN-AML (P = 0.004). In the analysis of published data from 225 CN-AML patients, PDCD7 remained independently predicting OS in CN-AML (P = 0.039). As a conclusion, Ang2 and FIS1 seem related to decreased CR rate of AML patients, and PDCD7 is associated with shorter OS and RFS in CN-AML. Hence, PDCD7, Ang2 and FIS1 may indicate a more aggressive form and poor prognosis of AML.

Dysregulation of the angiopoietin-Tie-2 axis in sepsis and ARDS

Dynamic changes in microvascular endothelial structure and function are pivotal in the acute inflammatory response, the body's rapid, coordinated effort to localize, sequester, and eliminate microbial invaders at their portal of entry. To achieve this, the endothelium becomes leaky and inflamed, providing innate immune cells and humoral effector molecules access to the site of infection. During sepsis this locally adaptive response becomes manifest throughout the body, leading to dangerous host consequences. Increased leakiness in the pulmonary circulation contributes to acute respiratory distress syndrome (ARDS), a complication of sepsis associated with 40% mortality. Understanding the molecular governance of vascular leak and inflammation has major diagnostic, prognostic, and potentially therapeutic implications for this common and pernicious disease. This review summarizes results from cell-based experiments, animal models, and observational human studies; together, these studies suggest that an endothelial receptor called Tie2 and its ligands, called angiopoietins, form a signaling axis key to the vascular dyshomeostasis that underlies sepsis.

A novel angiopoietin-derived peptide displays anti-angiogenic activity and inhibits tumour-induced and retinal neovascularization

BACKGROUND AND PURPOSE

Pathological angiogenesis is associated with various human diseases, such as cancer, autoimmune diseases and retinopathy. The angiopoietin (Ang)-Tie2 system plays critical roles in several steps of angiogenic remodelling. Here, we have investigated the anti-angiogenic effect of a novel angiopoietin-derived peptide.

EXPERIMENTAL APPROACH

Using computational methods, we identified peptides from helical segments within angiopoietins, which were predicted to inhibit their activity. These peptides were tested using biochemical methods and models of angiogenesis. The peptide with best efficacy, A11, was selected for further characterization as an anti-angiogenic compound.

KEY RESULTS

The potent anti-angiogenic activity of A11 was demonstrated in a multicellular assay of angiogenesis and in the chorioallantoic membrane model. A11 bound to angiopoietins and reduced the binding of Ang-2 to Tie2. A11 was also significantly reduced vascular density in a model of tumour-induced angiogenesis. Its ability to inhibit Ang-2 but not Ang-1-induced endothelial cell migration, and to down-regulate Tie2 levels in tumour microvessels, suggests that A11 targets the Ang-Tie2 pathway. In a rat model of oxygen-induced retinopathy, A11 strongly inhibited retinal angiogenesis. Moreover, combination of A11 with an anti-VEGF antibody showed a trend for further inhibition of angiogenesis, suggesting an additive effect.

CONCLUSIONS AND IMPLICATIONS

Our results indicate that A11 is a potent anti-angiogenic compound, through modulation of the Ang-Tie2 system, underlining its potential as a therapeutic agent for the treatment of ocular and tumour neovascularization, as well as other pathological conditions that are dependent on angiogenesis.

Synthesis and biological profile of the pan-vascular endothelial growth factor receptor/tyrosine kinase with immunoglobulin and epidermal growth factor-like homology domains 2 (VEGF-R/TIE-2) inhibitor 11-(2-methylpropyl)-12,13-dihydro-2-methyl-8-(pyrimidin-2-ylamino)-4H-indazolo[5,4-a]pyrrolo[3,4-c]carbazol-4-one (CEP-11981): a novel oncology therapeutic agent

A substantial body of evidence supports the utility of antiangiogenesis inhibitors as a strategy to block or attenuate tumor-induced angiogenesis and inhibition of primary and metastatic tumor growth in a variety of solid and hematopoietic tumors. Given the requirement of tumors for different cytokine and growth factors at distinct stages of their growth and dissemination, optimal antiangiogenic therapy necessitates inhibition of multiple, complementary, and nonredundant angiogenic targets. 11-(2-Methylpropyl)-12,13-dihydro-2-methyl-8-(pyrimidin-2-ylamino)-4H-indazolo[5,4-a]pyrrolo[3,4-c]carbazol-4-one (11b, CEP-11981) is a potent orally active inhibitor of multiple targets (TIE-2, VEGF-R1, 2, and 3, and FGF-R1) having essential and nonredundant roles in tumor angiogenesis and vascular maintenance. Outlined in this article are the design strategy, synthesis, and biochemical and pharmacological profile for 11b, which completed Phase I clinical assessing safety and pharmacokinetics allowing for the initiation of proof of concept studies.

Lack of efficacy of combined antiangiogenic therapies in xenografted human melanoma

Antiangiogenic therapy is theoretically a promising anticancer approach but does not always produce significant tumor control. Combinations of antiangiogenic therapies are therefore being investigated as strategies to enhance clinical benefit. Common targets for angiogenic blockade include endothelial specific receptors, such as Tie2/Tek, which signal blood vessel stabilization via recruitment and maturation of pericytes. Here, we report on the effects of targeted Tie2 antiangiogenic therapy (TekdeltaFc) in combination with nontargeted metronomic cyclophosphamide (LDM CTX) (reported to also act in antiangiogenic fashion) in xenografted human melanoma. Individually, these therapies showed transient antitumor activity, but, in combination, there was no significant reduction in tumor growth. In addition, while TekdeltaFc caused the expected increased pericyte coverage in treated blood vessels, LDM CTX alone or in combination with TekdeltaFc resulted in increased levels of VEGF production. Collectively, our data highlight the complexity of molecular interactions that may take place when antiangiogenic regimens are combined.

Angiogenesis and multiple myeloma

The bone marrow microenvironment in multiple myeloma is characterized by an increased microvessel density. The production of pro-angiogenic molecules is increased and the production of angiogenic inhibitors is suppressed, leading to an "angiogenic switch". Here we present an overview of the role of angiogenesis in multiple myeloma, the pro-angiogenic factors produced by myeloma cells and the microenvironment, and the mechanisms involved in the myeloma-induced angiogenic switch. Current data suggest that the increased bone marrow angiogenesis in multiple myeloma is due to the aberrant expression of angiogenic factors by myeloma cells, the subsequent increase in pro-angiogenic activity of normal plasma cells as a result of myeloma cell angiogenic activity, and the increased number of plasma cells overall. Hypoxia also contributes to the angiogenic properties of the myeloma marrow microenvironment. The transcription factor hypoxia-inducible factor-1α is overexpressed by myeloma cells and affects their transcriptional and angiogenic profiles. In addition, potential roles of the tumor suppressor gene inhibitor of growth family member 4 and homeobox B7 have also been recently highlighted as repressors of angiogenesis and pro-angiogenic related genes, respectively. This complex pathogenetic model of myeloma-induced angiogenesis suggests that several pro-angiogenic molecules and related genes in myeloma cells and the microenvironment are potential therapeutic targets.

Blood vessel maturation, vascular phenotype and angiogenic potential in malignant melanoma: one step forward for overcoming anti-angiogenic drug resistance?

Angiogenesis is a pivotal process for growth, invasion and spread of the majority of solid tumors including melanoma. Anti-angiogenic agents have not been systematically tested in patients with advanced melanoma. Clinical efficacy of angiogenesis inhibitors targeting endothelial cells has not been as affirmative as initially hoped and improved clinical outcomes have been observed in combination with chemotherapy or additional drugs for many types of human cancer. However, angiogenesis is not only dependent on endothelial cell invasion and proliferation, it also requires pericyte coverage of vascular sprouts for stabilization and maturation of vascular walls. Recent data suggest that pericytes might be able to confer resistance to anti-vascular endothelial growth factor (VEGF) therapy. This review will focus on the significance of the vascular phenotype but also on the impact of pericyte-mediated vessel maturation for the susceptibility to anti-angiogenic therapy, including malignant melanoma, which we identified as crucial factor regarding therapeutic efficacy.

Angiopoietin-1/Tie2 signal augments basal Notch signal controlling vascular quiescence by inducing delta-like 4 expression through AKT-mediated activation of beta-catenin

Angiopoietin-1 (Ang1) regulates both vascular quiescence and angiogenesis through the receptor tyrosine kinase Tie2. We and another group previously showed that Ang1 and Tie2 form distinct signaling complexes at cell-cell and cell-matrix contacts. We further demonstrated that the former up-regulates Notch ligand delta-like 4 (Dll4) only in the presence of cell-cell contacts. Because Dll4/Notch signal restricts sprouting angiogenesis and promotes vascular stabilization, we investigated the mechanism of how the Ang1/Tie2 signal induces Dll4 expression to clarify the role of the Dll4/Notch signal in Ang1/Tie2 signal-mediated vascular quiescence. Under confluent endothelial cells, the basal Notch signal was observed. Ang1, moreover, induced Dll4 expression and production of the Notch intracellular domain (NICD). Ang1 stimulated transcriptional activity of β-catenin through phosphoinositide 3-kinase (PI3K)/AKT-mediated phosphorylation of glycogen synthase kinase 3β (GSK3β). Correspondingly, the GSK3β inhibitor up-regulated Dll4, whereas depletion of β-catenin by siRNA blocked Ang1-induced Dll4 expression, indicating the indispensability of β-catenin in Ang1-mediated up-regulation of Dll4. In addition, Dll4 expression by the GSK3β inhibitor was only observed in confluent cells, and was impeded by DAPT, a γ-secretase inhibitor, implying requirement of the Notch signal in β-catenin-dependent Dll4 expression. Consistently, we found that either Ang1 or NICD up-regulates Dll4 through the RBP-J binding site within intron 3 of the DLL4 gene and that β-catenin forms a complex with NICD/RBP-J to enhance Dll4 expression. Ang1 induced the deposition of extracellular matrix that is preferable for basement membrane formation through Dll4/Notch signaling. Collectively, the Ang1/Tie2 signal potentiates basal Notch signal controlling vascular quiescence by up-regulating Dll4 through AKT-mediated activation of β-catenin.

Significance of AEG-1 expression in correlation with VEGF, microvessel density and clinicopathological characteristics in triple-negative breast cancer

PURPOSE

Our study is to examine astrocyte-elevated gene-1 (AEG-1) expression in triple-negative breast cancer and to determine whether it is associated with vascular endothelial growth factor (VEGF), microvessel density (MVD), clinicopathological parameters and poor survival.

METHODS

Specimens from 125 patients with triple-negative breast cancers were investigated by immunohistochemistry for MVD, AEG-1 and VEGF expression. Correlations between the expression of AEG-1, VEGF, MVD, and various clinicopathological factors including survival status were studied.

RESULTS

AEG-1 and VEGF were highly expressed in 56.8% and 52.8% of triple-negative breast cancer patients, respectively. The intensity of AEG-1 was gradually up-regulated from VEGF-MVD-low, VEGF-high, or MVD-high to VEGF-MVD-high tissues using Western blot analysis. Statistically significant correlation was found among AEG-1 and VEGF, and MVD. Moreover, AEG-1 expression was correlated with clinical stage, lymphatic venous invasion, lymph nodal metastasis, tumor size, Ki67, and recurrence. Patients with AEG-1 high-expression showed far lower disease-free survival (DFS) and overall survival (OS) rates than those with AEG-1 low-expression. For VEGF and MVD, there were similar results in these patients. Only AEG-1 expression and tumor size were independent prognostic factors for both DFS and OS by multivariate analysis. However, the prognostic impact of tumor size was not as strong as that of AEG-1.

CONCLUSIONS

AEG-1 expression may be related with tumor angiogenesis and progression and is a valuable prognostic factor in patients with triple-negative breast cancer.

Glioblastoma cells incorporate into tumor vasculature and contribute to vascular radioresistance

Glioblastoma multiforme (GBM) remains the most devastating neoplasm of the central nervous system and has a dismal prognosis. Ionizing radiation represents an effective therapy for GBM, but radiotherapy remains only palliative because of radioresistance. In this study, we demonstrate that glioma cells participate in tumor vascularization and contribute to vascular radioresistance. Using a 3-dimensional coculture system, we observed an intimate interaction of glioma cells with endothelial cells whereby endothelial cells form vascular structures, followed by the recruitment and vascular patterning of glioma cells. In addition, tumor cells stabilize the vascular structures and render them radioresistant. Blocking initial endothelial vascular formation with endothelial-specific inhibitors prevented tumor cells from forming any structures. However, these inhibitors exhibited minimum effects on vascular structures formed by tumor cells, due to the absence of the targeted receptors on tumor cells. Consistent with the in vitro findings, we show that glioma cells form perfused blood vessels in xenograft tumor models. Together, these data suggest that glioma cells mimic endothelial cells and incorporate into tumor vasculature, which may contribute to radioresistance observed in GBM. Therefore, interventions aimed at the glioma vasculature should take into consideration the chimeric nature of the tumor vasculature.

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.

Tie1-Tie2 interactions mediate functional differences between angiopoietin ligands

The Tie family of endothelial-specific receptor tyrosine kinases is essential for cell proliferation, migration, and survival during angiogenesis. Despite considerable similarity, experiments with Tie1- or Tie2-deficient mice highlight distinct functions for these receptors in vivo. The Tie2 receptor is further unique with respect to its structurally homologous ligands. Angiopoietin-2 and -3 can function as agonists or antagonists; angiopoietin-1 and -4 are constitutive agonists. To address the role of Tie1 in angiopoietin-mediated Tie2 signaling and determine the basis for the behavior of the individual angiopoietins, we used an in vivo FRET-based proximity assay to monitor Tie1 and -2 localization and association. We provide evidence for Tie1-Tie2 complex formation on the cell surface and identify molecular surface areas essential for receptor-receptor recognition. We further demonstrate that the Tie1-Tie2 interactions are dynamic, inhibitory, and differentially modulated by angiopoietin-1 and -2. Based on the available data, we propose a unified model for angiopoietin-induced Tie2 signaling.

Tie2 signaling regulates osteoclastogenesis and osteolytic bone invasion of breast cancer

Breast to bone metastasis is a common occurrence in the majority of patients with advanced breast cancer. The metastases are often incurable and are associated with bone destruction and high rates of morbidity. Understanding the underlying mechanisms of how metastatic tumor cells induce bone destruction is critically important. We previously reported that Tie2, a receptor tyrosine kinase, is significantly increased in human breast cancer tissues compared with normal and benign breast tumors and regulates tumor angiogenesis. In this study, we identify a new function of Tie2 in osteoclastogenesis and osteolytic bone invasion of breast cancer. Tie2 is present in hematopoietic stem/precursor cells. Genetic deletion of Tie2 or neutralization of Tie2 function using soluble Tie2 receptor impaired osteoclastogenesis in an embryonic stem cell differentiation assay. In contrast, deletion of Tie2 has no effect on osteoblastogenesis. As CD11b myeloid cells have the potential to become osteoclasts and Tie2 is present in a certain population of these cells, we isolated Tie2(+) and Tie2(-) myeloid cells. We observed a significant reduction of osteoclastogenesis in Tie2(-) compared with Tie2(+) CD11b cells. Consistently, neutralization of Tie2 activity in vivo significantly inhibited osteolytic bone invasion and tumor growth in a mammary tumor model, which correlated with a significant reduction of osteoclasts and tumor angiogenesis. Collectively, these data reveal a direct and novel role of Tie2 signaling in osteoclast differentiation. These findings identify Tie2 as a therapeutic target for controlling not only tumor angiogenesis but also osteolytic bone metastasis in breast cancer.

Interleukin-32beta propagates vascular inflammation and exacerbates sepsis in a mouse model

Background

Inflammation is associated with most diseases, which makes understanding the mechanisms of inflammation vitally important.

Methodology/Principal Findings

Here, we demonstrate a critical function of interleukin-32β (IL-32β) in vascular inflammation. IL-32β is present in tissues from humans, but is absent in rodents. We found that the gene is highly expressed in endothelial cells. Three isoforms of IL-32, named IL-32α, β, and ε, were cloned from human endothelial cells, with IL-32β being the major isoform. Pro-inflammatory cytokines (TNFα and IL-1β) induced IL-32β expression through NF-κB. Conversely, IL-32β propagated vascular inflammation via induction of vascular cell adhesion molecules and inflammatory cytokines. Accordingly, IL-32β increased adhesion of inflammatory cells to activated endothelial cells, a paramount process in inflammation. These results illustrate a positive feedback regulation that intensifies and prolongs inflammation. Importantly, endothelial/hematopoietic expression of IL-32β in transgenic mice elevated inflammation and worsened sepsis. This was demonstrated by significant elevation of leukocyte infiltration and serum levels of TNFα and IL-1β, increased vascular permeability and lung damage, and accelerated animal death. Together, our results reveal an important function of IL-32 in vascular inflammation and sepsis development.

Conclusions/Significance

Our results reveal an important function of IL-32 in vascular inflammation and sepsis development.

Eukaryotic expression of extracellular ligand binding domains of murine Tie-2 and its anti-angiogenesis effect in SGC-7901 cell lines

BACKGROUND AND AIMS

Researches about blocking angiogenesis to treat tumor have become one of the most promising and active fields in anticancer research. This study aimed to investigate the eukaryotic expression of extracellular ligand binding domains of murine Tie-2 and its anti-angiogenesis effect.

METHODS

A eukaryotic expression vector pcDNA3.1(+) integrating with a DNA fragment which encode extracellular ligand binding domains of murine Tie-2 was transfected into SGC-7901 gastric cancer cell line. The protein expression was detected by western blot analysis and immunocytochemistry staining. Following the construction of nude mouse tumor xenograft model with and without transfected cells, tumor microvessel density was determined by counting per high power field in the sections stained with an antibody to CD31 to test its inhibition of angiogenesis.

RESULTS

The extracellular ligand binding domains of murine Tie-2 receptor was highly expressed in SGC-7901 gastric cancer cells with plasmid transfection. The mean tumor sizes of groups with and without transfection were 1.27 +/- 0.35 and 1.75 +/- 0.17 cm(3), respectively (P = 0.025). The mean inhibitory rate of tumor was 27.18 +/- 19.93%. The comparison between highest microvessel density of group with transfection (14.00 +/- 3.80) and that of group without transfection (22.30 +/- 5.91) was statistically significant at P = 0.030.

CONCLUSION

The protein of extracellular ligand binding domains of murine Tie-2 can be expressed at high level in the eukaryotic expression system, and the expressed protein may have the anti-angiogenesis effect.

Angiopoietins

The formation of new blood vessels plays an important role during the development and progression of a disease. In recent years, there has been a tremendous effort to uncover the molecular mechanisms that drive blood vessel growth in adult tissues. Angiopoietins belong to a family of growth factors that are critically involved in blood vessel formation during developmental and pathological angiogenesis. The importance of Angiopoietin signaling has been recognized in transgenic mouse models as the genetic ablation of Ang-1, and its primary receptor Tie2 has led to early embryonic lethality. Interesting and unusual for a family of ligands, Ang-2 has been identified as an antagonist of Ang-1 in endothelial cells as evidenced by a similar embryonic phenotype when Ang-2 was overexpressed in transgenic mice. In this review, we focus on the functional consequences of autocrine Angiopoietin signaling in endothelial cells.

Astrocyte elevated gene-1 (AEG-1) functions as an oncogene and regulates angiogenesis

Astrocyte-elevated gene-1 (AEG-1) expression is increased in multiple cancers and plays a central role in Ha-ras-mediated oncogenesis through the phosphatidylinositol 3-kinase (PI3K)/Akt signaling pathway. Additionally, overexpression of AEG-1 protects primary and transformed human and rat cells from serum starvation-induced apoptosis through activation of PI3K/Akt signaling. These findings suggest, but do not prove, that AEG-1 may function as an oncogene. We now provide definitive evidence that AEG-1 is indeed a transforming oncogene and show that stable expression of AEG-1 in normal immortal cloned rat embryo fibroblast (CREF) cells induces morphological transformation and enhances invasion and anchorage-independent growth in soft agar, two fundamental biological events associated with cellular transformation. Additionally, AEG-1-expressing CREF clones form aggressive tumors in nude mice. Immunohistochemistry analysis of tumor sections demonstrates that AEG-1-expressing tumors have increased microvessel density throughout the entire tumor sections. Overexpression of AEG-1 increases expression of molecular markers of angiogenesis, including angiopoietin-1, matrix metalloprotease-2, and hypoxia-inducible factor 1-alpha. In vitro angiogenesis studies further demonstrate that AEG-1 promotes tube formation in Matrigel and increases invasion of human umbilical vein endothelial cells via the PI3K/Akt signaling pathway. Tube formation induced by AEG-1 correlates with increased expression of angiogenesis markers, including Tie2 and hypoxia-inducible factor-alpha, and blocking AEG-1-induced Tie2 with Tie2 siRNA significantly inhibits AEG-1-induced tube formation in Matrigel. Overall, our findings demonstrate that aberrant AEG-1 expression plays a dominant positive role in regulating oncogenic transformation and angiogenesis. These findings suggest that AEG-1 may provide a viable target for directly suppressing the cancer phenotype.

Contrasting actions of selective inhibitors of angiopoietin-1 and angiopoietin-2 on the normalization of tumor blood vessels

Angiopoietin-1 (Ang1) and angiopoietin-2 (Ang2) have complex actions in angiogenesis and vascular remodeling due to their effects on Tie2 receptor signaling. Ang2 blocks Ang1-mediated activation of Tie2 in endothelial cells under certain conditions but is a Tie2 receptor agonist in others. We examined the effects of selective inhibitors of Ang1 (mL4-3) or Ang2 (L1-7[N]), alone or in combination, on the vasculature of human Colo205 tumors in mice. The Ang2 inhibitor decreased the overall abundance of tumor blood vessels by reducing tumor growth and keeping vascular density constant. After inhibition of Ang2, tumor vessels had many features of normal blood vessels (normalization), as evidenced by junctional accumulation of vascular endothelial-cadherin, junctional adhesion molecule-A, and platelet/endothelial cell adhesion molecule-1 in endothelial cells, increased pericyte coverage, reduced endothelial sprouting, and remodeling into smaller, more uniform vessels. The Ang1 inhibitor by itself had little noticeable effect on the tumor vasculature. However, when administered with the Ang2 inhibitor, the Ang1 inhibitor prevented tumor vessel normalization, but not the reduction in tumor vascularity produced by the Ang2 inhibitor. These findings are consistent with a model whereby inhibition of Ang2 leads to normalization of tumor blood vessels by permitting the unopposed action of Ang1, but decreases tumor vascularity primarily by blocking Ang2 actions.

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

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

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 axis mediates malignant pleural effusion formation

PURPOSE

Angiopoietins and their receptor, Tie2, participate in angiogenesis, regulation of vascular permeability, and inflammation, all central to the pathogenesis of malignant pleural effusions (MPEs). In the present study, we aimed to examine the role of the angiopoietin/Tie2 axis in MPE pathogenesis.

EXPERIMENTAL DESIGN

MPE was induced by intrapleural injection of murine adenocarcinoma cells in C57BL/6 mice. Animals were given twice-weekly intraperitoneal injections of 40 mg/kg MuTekdeltaFc or vehicle. MuTekdeltaFc is a soluble Tie2 (sTie2) receptor that binds murine angiopoietins thereby disrupting their interaction with Tie2 receptors expressed on tissues. Animals were killed on day 14.

RESULTS

Angiopoietin/Tie2 axis blockade significantly reduced pleural fluid volume and pleural tumor foci. The mean +/- SEM pleural fluid volumes were 617 +/- 48 microl and 316 +/- 62 microl for the control and treated groups, respectively (P = .001), whereas the mean +/- SEM tumor foci were 7.3 +/- 1.0 and 3.0 +/- 0.52 for the control and treated groups, respectively (P = .001). In addition, tumor-associated cachexia, tumor angiogenesis, pleural vascular permeability, recruitment of inflammatory cells to the pleural cavity, and local elaboration of vascular endothelial growth factor and interleukin 6 were also downregulated, and tumor cell apoptosis was induced in animals treated with the inhibitor.

CONCLUSIONS

Our results indicate that the angiopoietin/Tie2 axis is an important component of MPE pathogenesis. Further studies are required to determine whether therapeutic interventions targeting this pathway could be beneficial for patients with MPE.