PHD2 regulates arteriogenic macrophages through TIE2 signalling

HTATIP2 regulates arteriogenic activity in monocytes from patients with limb ischemia

Use of autologous cells isolated from elderly patients with multiple comorbidities may account for the modest efficacy of cell therapy in patients with chronic limb threatening ischemia (CLTI). We aimed to determine whether proarteriogenic monocyte/macrophages (Mo/MΦs) from patients with CLTI were functionally impaired and to demonstrate the mechanisms related to any impairment. Proarteriogenic Mo/MΦs isolated from patients with CLTI were found to have an impaired capacity to promote neovascularization in vitro and in vivo compared with those isolated from healthy controls. This was associated with increased expression of human HIV-1 TAT interactive protein-2 (HTATIP2), a transcription factor known to suppress angiogenesis/arteriogenesis. Silencing HTATIP2 restored the functional capacity of CLTI Mo/MΦs, which was associated with increased expression of arteriogenic regulators Neuropilin-1 and Angiopoietin-1, and their ability to enhance angiogenic (endothelial tubule formation) and arteriogenic (smooth muscle proliferation) processes in vitro. In support of the translational relevance of our findings, silencing HTATIP2 in proarteriogenic Mo/MΦs isolated from patients with CLTI rescued their capacity to enhance limb perfusion in the ischemic hindlimb by effecting greater angiogenesis and arteriogenesis. Ex vivo modulation of HTATIP2 may offer a strategy for rescuing the functional impairment of pro-angio/arteriogenic Mo/MΦs prior to autologous delivery and increase the likelihood of clinical efficacy.

Clinical Significance of Tie-2-Expressing Monocytes/Macrophages and Angiopoietins in the Progression of Ovarian Cancer-State-of-the-Art

Tumour growth and metastasis are specific to advanced stages of epithelial ovarian cancer (EOC). Tumour angiogenesis is an essential part of these processes. It is responsible for providing tumours with nutrients, metabolites, and cytokines and facilitates tumour and immune cell relocation. Destabilised vasculature, a distinctive feature of tumours, is also responsible for compromising drug delivery into the bulk. Angiogenesis is a complex process that largely depends on how the tumour microenvironment (TME) is composed and how a specific organ is formed. There are contrary reports on whether Tie-2-expressing monocytes/macrophages (TEMs) reported as the proangiogenic population of monocytes have any impact on tumour development. The aim of this paper is to summarise knowledge about ovarian-cancer-specific angiogenesis and the unique role of Tie-2-expressing monocytes/macrophages in this process. The significance of this cell subpopulation for the pathophysiology of EOC remains to be investigated.

Monocyte proinflammatory phenotypic control by ephrin type A receptor 4 mediates neural tissue damage

Circulating monocytes have emerged as key regulators of the neuroinflammatory milieu in a number of neuropathological disorders. Ephrin type A receptor 4 (Epha4) receptor tyrosine kinase, a prominent axon guidance molecule, has recently been implicated in the regulation of neuroinflammation. Using a mouse model of brain injury and a GFP BM chimeric approach, we found neuroprotection and a lack of significant motor deficits marked by reduced monocyte/macrophage cortical infiltration and an increased number of arginase-1+ cells in the absence of BM-derived Epha4. This was accompanied by a shift in monocyte gene profile from pro- to antiinflammatory that included increased Tek (Tie2 receptor) expression. Inhibition of Tie2 attenuated enhanced expression of M2-like genes in cultured Epha4-null monocytes/macrophages. In Epha4-BM-deficient mice, cortical-isolated GFP+ monocytes/macrophages displayed a phenotypic shift from a classical to an intermediate subtype, which displayed reduced Ly6chi concomitant with increased Ly6clo- and Tie2-expressing populations. Furthermore, clodronate liposome-mediated monocyte depletion mimicked these effects in WT mice but resulted in attenuation of phenotype in Epha4-BM-deficient mice. This demonstrates that monocyte polarization not overall recruitment dictates neural tissue damage. Thus, coordination of monocyte proinflammatory phenotypic state by Epha4 is a key regulatory step mediating brain injury.

Dual delivery of IL-10 and AT-RvD1 from PEG hydrogels polarize immune cells towards pro-regenerative phenotypes

Inflammation after traumatic injury or surgical intervention is both a protective tissue response leading to regeneration and a potential cause of wound complications. One potentially successful strategy to harness to pro-regenerative roles of host inflammation is the localized delivery of bioactive materials to induce immune suppressive cellular responses by cells responding to injury. In this study, we designed a fully synthetic poly (ethylene) glycol (PEG)-based hydrogel to release the specialized pro-resolving lipid mediator aspirin-triggered resolvin-D1 (AT-RvD1) and recombinant human interleukin 10 (IL-10). We utilized a unique side-by-side internally controlled implant design wherein bioactive hydrogels were implanted adjacent to control hydrogels devoid of immune modulatory factors in the dorsal skinfold window chamber. We also explored single-immune cell data with unsupervised approaches such as SPADE. First, we show that RGD-presenting hydrogel delivery results in enhanced immune cell recruitment to the site of injury. We then use intra-vital imaging to assess cellular recruitment and microvascular remodeling to show an increase in the caliber and density of local microvessels. Finally, we show that the recruitment and re-education of mononuclear phagocytes by combined delivery IL-10 and AT-RvD1 localizes immune suppressive subsets to the hydrogel, including CD206+ macrophages (M2a/c) and IL-10 expressing dendritic cells in the context of chronic inflammation following surgical tissue disruption. These data demonstrate the potential of combined delivery on the recruitment of regenerative cell subsets involved in wound healing complications.

The Role of Macrophages in Vascular Repair and Regeneration after Ischemic Injury

Macrophage is one of the important players in immune response which perform many different functions during tissue injury, repair, and regeneration. Studies using animal models of cardiovascular diseases have provided a clear picture describing the effect of macrophages and their phenotype during injury and regeneration of various vascular beds. Many data have been generated to demonstrate that macrophages secrete many important factors including cytokines and growth factors to regulate angiogenesis and arteriogenesis, acting directly or indirectly on the vascular cells. Different subsets of macrophages may participate at different stages of vascular repair. Recent findings also suggest a direct interaction between macrophages and other cell types during the generation and repair of vasculature. In this short review, we focused our discussion on how macrophages adapt to the surrounding microenvironment and their potential interaction with other cells, in the context of vascular repair supported by evidences mostly from studies using hindlimb ischemia as a model for studying post-ischemic vascular repair.

Endothelial Scaffolding Protein ENH (Enigma Homolog Protein) Promotes PHLPP2 (Pleckstrin Homology Domain and Leucine-Rich Repeat Protein Phosphatase 2)-Mediated Dephosphorylation of AKT1 and eNOS (Endothelial NO Synthase) Promoting Vascular Remodeling

OBJECTIVE

A decrease in nitric oxide, leading to vascular smooth muscle cell proliferation, is a common pathological feature of vascular proliferative diseases. Nitric oxide synthesis by eNOS (endothelial nitric oxide synthase) is precisely regulated by protein kinases including AKT1. ENH (enigma homolog protein) is a scaffolding protein for multiple protein kinases, but whether it regulates eNOS activation and vascular remodeling remains unknown. Approach and Results: ENH was upregulated in injured mouse arteries and human atherosclerotic plaques and was associated with coronary artery disease. Neointima formation in carotid arteries, induced by ligation or wire injury, was greatly decreased in endothelium-specific ENH-knockout mice. Vascular ligation reduced AKT and eNOS phosphorylation and nitric oxide production in the endothelium of control but not ENH-knockout mice. ENH was found to interact with AKT1 and its phosphatase PHLPP2 (pleckstrin homology domain and leucine-rich repeat protein phosphatase 2). AKT and eNOS activation were prolonged in VEGF (vascular endothelial growth factor)-induced ENH- or PHLPP2-deficient endothelial cells. Inhibitors of either AKT or eNOS effectively restored ligation-induced neointima formation in ENH-knockout mice. Moreover, endothelium-specific PHLPP2-knockout mice displayed reduced ligation-induced neointima formation. Finally, PHLPP2 was increased in the endothelia of human atherosclerotic plaques and blood cells from patients with coronary artery disease.

CONCLUSIONS

ENH forms a complex with AKT1 and its phosphatase PHLPP2 to negatively regulate AKT1 activation in the artery endothelium. AKT1 deactivation, a decrease in nitric oxide generation, and subsequent neointima formation induced by vascular injury are mediated by ENH and PHLPP2. ENH and PHLPP2 are thus new proatherosclerotic factors that could be therapeutically targeted.

Tooth Regeneration: Insights from Tooth Development and Spatial-Temporal Control of Bioactive Drug Release

Tooth defect and tooth loss are common clinical diseases in stomatology. Compared with the traditional oral restoration treatment, tooth regeneration has unique advantages and is currently the focus of oral biomedical research. It is known that dozens of cytokines/growth factors and other bioactive factors are expressed in a spatial-temporal pattern during tooth development. On the other hand, the technology for spatial-temporal control of drug release has been intensively studied and well developed recently, making control release of these bioactive factors mimicking spatial-temporal pattern more feasible than ever for the purpose of tooth regeneration. This article reviews the research progress on the tooth development and discusses the future of tooth regeneration in the context of spatial-temporal release of developmental factors.

The complex TIE between macrophages and angiogenesis

Macrophages are primarily known as phagocytic immune cells, but they also play a role in diverse processes, such as morphogenesis, homeostasis and regeneration. In this review, we discuss the influence of macrophages on angiogenesis, the process of new blood vessel formation from the pre-existing vasculature. Macrophages play crucial roles at each step of the angiogenic cascade, starting from new blood vessel sprouting to the remodelling of the vascular plexus and vessel maturation. Macrophages form promising targets for both pro- and anti-angiogenic treatments. However, to target macrophages, we will first need to understand the mechanisms that control the functional plasticity of macrophages during each of the steps of the angiogenic cascade. Here, we review recent insights in this topic. Special attention will be given to the TIE2-expressing macrophage (TEM), which is a subtype of highly angiogenic macrophages that is able to influence angiogenesis via the angiopoietin-TIE pathway.

Immunity, Hypoxia, and Metabolism-the Ménage à Trois of Cancer: Implications for Immunotherapy

It is generally accepted that metabolism is able to shape the immune response. Only recently we are gaining awareness that the metabolic crosstalk between different tumor compartments strongly contributes to the harsh tumor microenvironment (TME) and ultimately impairs immune cell fitness and effector functions. The major aims of this review are to provide an overview on the immune system in cancer; to position oxygen shortage and metabolic competition as the ground of a restrictive TME and as important players in the anti-tumor immune response; to define how immunotherapies affect hypoxia/oxygen delivery and the metabolic landscape of the tumor; and vice versa, how oxygen and metabolites within the TME impinge on the success of immunotherapies. By analyzing preclinical and clinical endeavors, we will discuss how a metabolic characterization of the TME can identify novel targets and signatures that could be exploited in combination with standard immunotherapies and can help to predict the benefit of new and traditional immunotherapeutic drugs.

Acute and chronic hypoxia differentially predispose lungs for metastases

Oscillations in oxygen levels affect malignant cell growth, survival, and metastasis, but also somatic cell behaviour. In this work, we studied the effect of the differential expression of the two primary hypoxia inducible transcription factor isoforms, HIF-1α and HIF-2α, and pulmonary hypoxia to investigate how the hypoxia response of the vascular endothelium remodels the lung pre-metastatic niche. Molecular responses to acute versus chronic tissue hypoxia have been proposed to involve dynamic HIF stabilization, but the downstream consequences and the extent to which differential lengths of exposure to hypoxia can affect HIF-isoform activation and secondary organ pre-disposition for metastasis is unknown. We used primary pulmonary endothelial cells and mouse models with pulmonary endothelium-specific deletion of HIF-1α or HIF-2α, to characterise their roles in vascular integrity, inflammation and metastatic take after acute and chronic hypoxia. We found that acute hypoxic response results in increased lung metastatic tumours, caused by HIF-1α-dependent endothelial cell death and increased microvascular permeability, in turn facilitating extravasation. This is potentiated by the recruitment and retention of specific myeloid cells that further support a pro-metastatic environment. We also found that chronic hypoxia delays tumour growth to levels similar to those seen in normoxia, and in a HIF-2α-specific fashion, correlating with increased endothelial cell viability and vascular integrity. Deletion of endothelial HIF-2α rendered the lung environment more vulnerable to tumour cell seeding and growth. These results demonstrate that the nature of the hypoxic challenge strongly influences the nature of the endothelial cell response, and affects critical parameters of the pulmonary microenvironment, significantly impacting metastatic burden. Additionally, this work establishes endothelial cells as important players in lung remodelling and metastatic progression.

PDE5 Inhibition Stimulates Tie2-Expressing Monocytes and Angiopoietin-1 Restoring Angiogenic Homeostasis in Diabetes

CONTEXT

Vascular dysfunction is a common feature in end-organ complications of type 2 diabetes mellitus (T2DM). The endothelium-specific receptor tyrosine kinase Tie2 and its ligand, angiopoietin-1 (Ang1), participate in the processes of vessel repair, renewal, and maturation. However, their dysregulation in T2DM has seldom been investigated.

OBJECTIVES

To examine the relationship between angiogenic Tie2-expressing monocytes (TEMs) and Ang1, and their pharmacological modulation by the phosphodiesterase type 5 inhibitor (PDE5i) sildenafil, in T2DM and in db/db mouse model.

DESIGN AND SETTING

Randomized, double-blind, placebo-controlled study.

PATIENTS AND INTERVENTION

db/db male mice were randomly assigned to receive 8 weeks of sildenafil or vehicle. Diabetic men were randomly assigned to receive 4 weeks of sildenafil or placebo.

MAIN OUTCOMES AND MEASURES

Peripheral blood cells were investigated by flow cytometry to quantify inflammatory myeloid CD11b+ Gr1+ cells and proangiogenic TEMs in mice and classical CD14++CD16neg monocytes and proangiogenic TEMs in humans at baseline and after treatment. In vitro human tube formation assay was used to test serum angiogenic potential.

RESULTS

We show that TEMs and Ang1 are defective in mouse and human models of diabetes and are normalized by PDE5i treatment. Serum angiogenic properties are impaired in diabetes because they do not support the in vitro formation of capillary-like structures, but they are reestablished by in vivo PDE5i treatment.

CONCLUSIONS

Restoring a more physiological Tie2-Ang1 axis with sildenafil reestablishes serum angiogenic properties in diabetes, promoting angiogenic homeostasis.

The Role of Metabolic Remodeling in Macrophage Polarization and Its Effect on Skeletal Muscle Regeneration

Significance: Macrophages are crucial for tissue homeostasis. Based on their activation, they might display classical/M1 or alternative/M2 phenotypes. M1 macrophages produce pro-inflammatory cytokines, reactive oxygen species (ROS), and nitric oxide (NO). M2 macrophages upregulate arginase-1 and reduce NO and ROS levels; they also release anti-inflammatory cytokines, growth factors, and polyamines, thus promoting angiogenesis and tissue healing. Moreover, M1 and M2 display key metabolic differences; M1 polarization is characterized by an enhancement in glycolysis and in the pentose phosphate pathway (PPP) along with a decreased oxidative phosphorylation (OxPhos), whereas M2 are characterized by an efficient OxPhos and reduced PPP. Recent Advances: The glutamine-related metabolism has been discovered as crucial for M2 polarization. Vice versa, flux discontinuities in the Krebs cycle are considered additional M1 features; they lead to increased levels of immunoresponsive gene 1 and itaconic acid, to isocitrate dehydrogenase 1-downregulation and to succinate, citrate, and isocitrate over-expression. Critical Issues: A macrophage classification problem, particularly in vivo, originating from a gap in the knowledge of the several intermediate polarization statuses between the M1 and M2 extremes, characterizes this field. Moreover, the detailed features of metabolic reprogramming crucial for macrophage polarization are largely unknown; in particular, the role of β-oxidation is highly controversial. Future Directions: Manipulating the metabolism to redirect macrophage polarization might be useful in various pathologies, including an efficient skeletal muscle regeneration. Unraveling the complexity pertaining to metabolic signatures that are specific for the different macrophage subsets is crucial for identifying new compounds that are able to trigger macrophage polarization and that might be used for therapeutical purposes.

Angiopoietin 1 influences ischemic reperfusion renal injury via modulating endothelium survival and regeneration

Background

Damage to the endothelium due to ischemia reperfusion injury (IRI) leads to a disruption of the microvasculature, which could be influenced by angiopoietin 1 via its effects on endothelium. We investigated the physiological and therapeutic roles of angiopoietin 1 in renal IRI using angiopoietin 1 knockout and over-expression mice.

Methods

Renal IRI was induced by clamping the right renal artery seven days after left uninephrectomy for 25 min followed by reperfusion. A whole body angiopoietin 1 knockout was achieved by induction with tamoxifen. The renal tubule over-expression of angiopoietin 1 was induced by doxycycline.

Results

In the normal mice, the renal expression of angiopoietin 1 increased 7 days to 14 days after IRI. The angiopoietin 1 knockout caused a delay in the recovery of renal function, less tubular regeneration and more residual tubular necrosis. The endothelial density was lower and the VE-cadherin protein loss was greater in the knockout mice. The over-expression of angiopoietin 1 attenuated the tubular necrosis and renal function impairment 1 and 3 days after IRI. The loss of the endothelium was ameliorated in the over-expression mice. This protective effect was associated with the up-regulation of the gene expression of epidermal growth factor, hepatocyte growth factor, and insulin like growth factor-1 and less tubular apoptosis. The over-expression of angiopoietin 1 stimulated tumor necrosis factor-α, C-C chemokine receptor type 2 and CX3C chemokine receptor 1 inflammatory gene expression, but did not influence macrophage infiltration.

Conclusions

Altogether, the augmentation and downregulation of angiopoietin 1 attenuated renal damage and impaired renal recovery, respectively, by influencing the survival/regeneration of the endothelium. The manipulation of angiopoietin 1 represents a novel therapeutic approach for the treatment of ischemic kidney injury.

Electronic supplementary material

The online version of this article (10.1186/s10020-019-0072-7) contains supplementary material, which is available to authorized users.

Regulation of Blood and Lymphatic Vessels by Immune Cells in Tumors and Metastasis

Research over the last decades has provided strong evidence for the pivotal role of the tumor-associated blood and lymphatic vasculature in supporting immunoevasion and in subverting T cell-mediated immunosurveillance. Conversely, tumor blood and lymphatic vessel growth is in part regulated by the immune system, with infiltrating innate as well as adaptive immune cells providing both immunosuppressive and various angiogenic signals. Thus, tumor angiogenesis and escape of immunosurveillance are two cancer hallmarks that are tightly linked and interregulated by cell constituents from compartments secreting both chemokines and cytokines. In this review, we discuss the implication and regulation of innate and adaptive immune cells in regulating blood and lymphatic angiogenesis in tumor progression and metastases. Moreover, we also highlight novel therapeutic approaches that target the tumor vasculature as well as the immune compartment to sustain and improve therapeutic efficacy in cancer.

A MicroRNA93-Interferon Regulatory Factor-9-Immunoresponsive Gene-1-Itaconic Acid Pathway Modulates M2-Like Macrophage Polarization to Revascularize Ischemic Muscle

BACKGROUND

Currently, no therapies exist for treating and improving outcomes in patients with severe peripheral artery disease (PAD). MicroRNA93 (miR93) has been shown to favorably modulate angiogenesis and to reduce tissue loss in genetic PAD models. However, the cell-specific function, downstream mechanisms, or signaling involved in miR93-mediated ischemic muscle neovascularization is not clear. Macrophages were best known to modulate arteriogenic response in PAD, and the extent of arteriogenic response induced by macrophages is dependent on greater M2 to M1 activation/polarization state. In the present study, we identified a novel mechanism by which miR93 regulates macrophage polarization to promote angiogenesis and arteriogenesis to revascularize ischemic muscle in experimental PAD.

METHODS

In vitro (macrophages, endothelial cells, skeletal muscle cells under normal and hypoxia serum starvation conditions) and in vivo experiments in preclinical PAD models (unilateral femoral artery ligation and resection) were conducted to examine the role of miR93-interferon regulatory factor-9-immunoresponsive gene-1 (IRG1)-itaconic acid pathway in macrophage polarization, angiogenesis, arteriogenesis, and perfusion recovery.

RESULTS

In vivo, compared with wild-type controls, miR106b-93-25 cluster-deficient mice (miR106b-93-25-/-) showed decreased angiogenesis and arteriogenesis correlating with increased M1-like macrophages after experimental PAD. Intramuscular delivery of miR93 in miR106b-93-25-/- PAD mice increased angiogenesis, arteriogenesis, and the extent of perfusion, which correlated with more M2-like macrophages in the proximal and distal hind-limb muscles. In vitro, miR93 promotes and sustains M2-like polarization even under M1-like polarizing conditions (hypoxia serum starvation). Delivery of bone marrow-derived macrophages from miR106b-93-25-/- to wild-type ischemic muscle decreased angiogenesis, arteriogenesis, and perfusion, whereas transfer of wild-type macrophages to miR106b-93-25-/- had the opposite effect. Systematic analysis of top differentially upregulated genes from RNA sequencing between miR106b-93-25-/- and wild-type ischemic muscle showed that miR93 regulates IRG1 function to modulate itaconic acid production and macrophage polarization. The 3' untranslated region luciferase assays performed to determine whether IRG1 is a direct target of miR93 revealed that IRG1 is not an miR93 target but that interferon regulatory factor-9, which can regulate IRG1 expression, is an miR93 target. In vitro, increased expression of interferon regulatory factor-9 and IRG1 and itaconic acid treatment significantly decreased endothelial angiogenic potential.

CONCLUSIONS

miR93 inhibits interferon regulatory factor-9 to decrease IRG1-itaconic acid production to induce M2-like polarization in ischemic muscle to enhance angiogenesis, arteriogenesis, and perfusion recovery in experimental PAD.

Elementary immunology: Na+ as a regulator of immunity

The skin can serve as an interstitial Na+ reservoir. Local tissue Na+ accumulation increases with age, inflammation and infection. This increased local Na+ availability favors pro-inflammatory immune cell function and dampens their anti-inflammatory capacity. In this review, we summarize available data on how NaCl affects various immune cells. We particularly focus on how salt promotes pro-inflammatory macrophage and T cell function and simultaneously curtails their regulatory and anti-inflammatory potential. Overall, these findings demonstrate that local Na+ availability is a promising novel regulator of immunity. Hence, the modulation of tissue Na+ levels bears broad therapeutic potential: increasing local Na+ availability may help in treating infections, while lowering tissue Na+ levels may be used to treat, for example, autoimmune and cardiovascular diseases.

Prolyl hydroxylase domain enzymes and their role in cell signaling and cancer metabolism

The prolyl hydroxylase domain (PHD) enzymes regulate the stability of the hypoxia-inducible factor (HIF) in response to oxygen availability. During oxygen limitation, the inhibition of PHD permits the stabilization of HIF, allowing the cellular adaptation to hypoxia. This adaptation is especially important for solid tumors, which are often exposed to a hypoxic environment. However, and despite their original role as the oxygen sensors of the cell, PHD are currently known to display HIF-independent and hydroxylase-independent functions in the control of different cellular pathways, including mTOR pathway, NF-kB pathway, apoptosis and cellular metabolism. In this review, we summarize the recent advances in the regulation and functions of PHD in cancer signaling and cell metabolism.

CD40 in coronary artery disease: a matter of macrophages?

Coronary artery disease (CAD), also known as ischemic heart disease (IHD), is the leading cause of mortality in the western world, with developing countries showing a similar trend. With the increased understanding of the role of the immune system and inflammation in coronary artery disease, it was shown that macrophages play a major role in this disease. Costimulatory molecules are important regulators of inflammation, and especially, the CD40L-CD40 axis is of importance in the pathogenesis of cardiovascular disease. Although it was shown that CD40 can mediate macrophage function, its exact role in macrophage biology has not gained much attention in cardiovascular disease. Therefore, the goal of this review is to give an overview on the role of macrophage-specific CD40 in cardiovascular disease, with a focus on coronary artery disease. We will discuss the function of CD40 on the macrophage and its (proposed) role in the reduction of atherosclerosis, the reduction of neointima formation, and the stimulation of arteriogenesis.

PHD2: from hypoxia regulation to disease progression

Oxygen represents one of the major molecules required for the development and maintenance of life. An adequate response to hypoxia is therefore required for the functioning of the majority of living organisms and relies on the activation of the hypoxia-inducible factor (HIF) pathway. HIF prolyl hydroxylase domain-2 (PHD2) has long been recognized as the major regulator of this response, controlling a myriad of outcomes that range from cell death to proliferation. However, this enzyme has been associated with more pathways, making the role of this protein remarkably complex under distinct pathologies. While a protective role seems to exist in physiological conditions such as erythropoiesis; the picture is more complex during pathologies such as cancer. Since the regulation of this enzyme and its closest family members is currently considered as a possible therapy for various diseases, understanding the different particular roles of this protein is essential.

Targeting vascular and leukocyte communication in angiogenesis, inflammation and fibrosis

Regulation of vascular permeability, recruitment of leukocytes from blood to tissue and angiogenesis are all processes that occur at the level of the microvasculature during both physiological and pathological conditions. The interplay between microvascular cells and leukocytes during inflammation, together with the emerging roles of leukocytes in the modulation of the angiogenic process, make leukocyte-vascular interactions prime targets for therapeutics to potentially treat a wide range of diseases, including pathological and dysfunctional vessel growth, chronic inflammation and fibrosis. In this Review, we discuss how the different cell types that are present in and around microvessels interact, cooperate and instruct each other, and in this context we highlight drug targets as well as emerging druggable processes that can be exploited to restore tissue homeostasis.

Molecular controls of arterial morphogenesis

Formation of arterial vasculature, here termed arteriogenesis, is a central process in embryonic vascular development as well as in adult tissues. Although the process of capillary formation, angiogenesis, is relatively well understood, much remains to be learned about arteriogenesis. Recent discoveries point to the key role played by vascular endothelial growth factor receptor 2 in control of this process and to newly identified control circuits that dramatically influence its activity. The latter can present particularly attractive targets for a new class of therapeutic agents capable of activation of this signaling cascade in a ligand-independent manner, thereby promoting arteriogenesis in diseased tissues.

An immature B cell population from peripheral blood serves as surrogate marker for monitoring tumor angiogenesis and anti-angiogenic therapy in mouse models

Tumor growth depends on the formation of new blood vessels (tumor angiogenesis) either from preexisting vessels or by the recruitment of bone marrow-derived cells. Despite encouraging results obtained with preclinical cancer models, the therapeutic targeting of tumor angiogenesis has thus far failed to deliver an enduring clinical response in cancer patients. One major obstacle for improving anti-angiogenic therapy is the lack of validated biomarkers, which allow patient stratification for suitable treatment and a rapid assessment of therapy response. Toward these goals, we have employed several mouse models of tumor angiogenesis to identify cell populations circulating in their blood that correlated with the extent of tumor angiogenesis and therapy response. Flow cytometry analyses of different combinations of cell surface markers that define subsets of bone marrow-derived cells were performed on peripheral blood mononuclear cells from tumor-bearing and healthy mice. We identified one cell population, CD45(dim)VEGFR1(-)CD31(low), that was increased in levels during active tumor angiogenesis in a variety of transgenic and syngeneic transplantation mouse models of cancer. Treatment with various anti-angiogenic drugs did not affect CD45(dim)VEGFR1(-)CD31(low) cells in healthy mice, whereas in tumor-bearing mice, a consistent reduction in their levels was observed. Gene expression profiling of CD45(dim)VEGFR1(-)CD31(low) cells characterized these cells as an immature B cell population. These immature B cells were then directly validated as surrogate marker for tumor angiogenesis and of pharmacologic responses to anti-angiogenic therapies in various mouse models of cancer.

MET is required for the recruitment of anti-tumoural neutrophils

Mutations or amplification of the MET proto-oncogene are involved in the pathogenesis of several tumours^1-4, which rely on the constitutive engagement of this pathway for their growth and survival^1,5. However, MET is expressed not only by cancer cells but also by tumour-associated stromal cells although its precise role in this compartment is not well characterized^6-11. Here, we show that MET is required for neutrophil chemoattraction and cytotoxicity in response to its ligand HGF. Met deletion in neutrophils enhances tumour growth and metastasis. This phenotype correlates with reduced neutrophil infiltration to both primary tumour and metastatic site. Similarly, Met is necessary for neutrophil transudation during colitis, skin rash or peritonitis. Mechanistically, Met is induced by tumour-derived TNF-α or other inflammatory stimuli in both mouse and human neutrophils. This induction is instrumental for neutrophil transmigration across an activated endothelium and iNOS production upon HGF stimulation. Consequently, HGF/MET-dependent nitric oxide release by neutrophils promotes cancer cell killing, which abates tumour growth and metastasis. Following systemic administration of a MET kinase inhibitor, we prove that the therapeutic benefit of MET targeting in cancer cells is partly countered by the pro-tumoural effect rising from MET blockade in neutrophils. Our work identifies an unprecedented role of MET in neutrophils, suggests a potential “Achilles’ heel” of MET-targeted therapies in cancer, and supports the rationale for evaluating anti-MET drugs in certain inflammatory diseases.

Galectin-2 induces a proinflammatory, anti-arteriogenic phenotype in monocytes and macrophages

Galectin-2 is a monocyte-expressed carbohydrate-binding lectin, for which increased expression is genetically determined and associated with decreased collateral arteriogenesis in obstructive coronary artery disease patients. The inhibiting effect of galectin-2 on arteriogenesis was confirmed in vivo, but the mechanism is largely unknown. In this study we aimed to explore the effects of galectin-2 on monocyte/macrophage phenotype in vitro and vivo, and to identify the receptor by which galectin-2 exerts these effects. We now show that the binding of galectin-2 to different circulating human monocyte subsets is dependent on monocyte surface expression levels of CD14. The high affinity binding is blocked by an anti-CD14 antibody but not by carbohydrates, indicating a specific protein-protein interaction. Galectin-2 binding to human monocytes modulated their transcriptome by inducing proinflammatory cytokines and inhibiting pro-arteriogenic factors, while attenuating monocyte migration. Using specific knock-out mice, we show that galectin-2 acts through the CD14/toll-like receptor (TLR)-4 pathway. Furthermore, galectin-2 skews human macrophages to a M1-like proinflammatory phenotype, characterized by a reduced motility and expression of an anti-arteriogenic cytokine/growth factor repertoire. This is accompanied by a switch in surface protein expression to CD40-high and CD206-low (M1). In a murine model we show that galectin-2 administration, known to attenuate arteriogenesis, leads to increased numbers of CD40-positive (M1) and reduced numbers of CD206-positive (M2) macrophages surrounding actively remodeling collateral arteries. In conclusion galectin-2 is the first endogenous CD14/TLR4 ligand that induces a proinflammatory, non-arteriogenic phenotype in monocytes/macrophages. Interference with CD14-Galectin-2 interaction may provide a new intervention strategy to stimulate growth of collateral arteries in genetically compromised cardiovascular patients.

Collaterals: Implications in cerebral ischemic diseases and therapeutic interventions

Despite the tremendous progress made in the treatment of cerebrovascular occlusive diseases, many patients suffering from ischemic brain injury still experience dismal outcomes. Although rehabilitation contributes to post-stroke functional recovery, there is no doubt that interventions that promote the restoration of blood supply are proven to minimize ischemic injury and improve recovery. In response to the acutely decreased blood perfusion during arterial occlusion, arteriogenesis, the compensation of blood flow through the collateral circulation during arterial obstructive diseases can act not only in a timely fashion but also much more efficiently compared to angiogenesis, the sprouting of new capillaries, and a mechanism occurring in a delayed fashion while increases the total resistance of the vascular bed of the affected territory. Interestingly, despite the vast differences between the two vascular remodeling mechanisms, some crucial growth factors and cytokines involved in angiogenesis are also required for arteriogenesis. Understanding the mechanisms underlying vascular remodeling after ischemic brain injury is a critical step towards the development of effective therapies for ischemic stroke. The present article will discuss our current views in vascular remodeling acutely after brain ischemia, namely arteriogenesis, and some relevant clinical therapies available on the horizon in augmenting collateral flow that hold promise in treating ischemic brain injury. This article is part of a Special Issue entitled SI: Cell Interactions In Stroke.

The angiopoietin/TIE receptor system: Focusing its role for ischemia-reperfusion injury

Ischemia and reperfusion (I/R) are of fatal consequence for the affected organs, as they provoke a profound inflammatory reaction. This thoroughly destroys cells and tissues, inducing functional failure or even complete loss of organ function. Since I/R is primarily a vascular problem, the interaction between the endothelium and the surrounding environment is of great significance. The angiopoietins (ANG) and the TIE receptors are key players for the vascular homeostasis. This review summarizes biochemical and cellular mechanisms leading to I/R injury. After a brief introduction to the ANG/TIE system, a comprehensive overview of its role for the development of I/R syndrome is given. Finally, current therapeutic approaches to mitigate the consequences of I/R by modulating ANG/TIE signaling are reviewed in detail.

Prolyl hydroxylase domain enzymes: important regulators of cancer metabolism

The hypoxia-inducible factor (HIF) prolyl hydroxylase domain enzymes (PHDs) regulate the stability of HIF protein by post-translational hydroxylation of two conserved prolyl residues in its α subunit in an oxygen-dependent manner. Trans-4-prolyl hydroxylation of HIFα under normal oxygen (O₂) availability enables its association with the von Hippel-Lindau (VHL) tumor suppressor pVHL E3 ligase complex, leading to the degradation of HIFα via the ubiquitin-proteasome pathway. Due to the obligatory requirement of molecular O₂ as a co-substrate, the activity of PHDs is inhibited under hypoxic conditions, resulting in stabilized HIFα, which dimerizes with HIFβ and, together with transcriptional co-activators CBP/p300, activates the transcription of its target genes. As a key molecular regulator of adaptive response to hypoxia, HIF plays important roles in multiple cellular processes and its overexpression has been detected in various cancers. The HIF1α isoform in particular has a strong impact on cellular metabolism, most notably by promoting anaerobic, whilst inhibiting O₂-dependent, metabolism of glucose. The PHD enzymes also seem to have HIF-independent functions and are subject to regulation by factors other than O₂, such as by metabolic status, oxidative stress, and abnormal levels of endogenous metabolites (oncometabolites) that have been observed in some types of cancers. In this review, we aim to summarize current understandings of the function and regulation of PHDs in cancer with an emphasis on their roles in metabolism.

Macrophages in homeostatic immune function

Macrophages are not only involved in inflammatory and anti-infective processes, but also play an important role in maintaining tissue homeostasis. In this review, we summarize recent evidence investigating the role of macrophages in controlling angiogenesis, metabolism as well as salt and water balance. Particularly, we summarize the importance of macrophage tonicity enhancer binding protein (TonEBP, also termed nuclear factor of activated T-cells 5 [NFAT5]) expression in the regulation of salt and water homeostasis. Further understanding of homeostatic macrophage function may lead to new therapeutic approaches to treat ischemia, hypertension and metabolic disorders.

TIE2-expressing monocytes/macrophages regulate revascularization of the ischemic limb

A third of patients with critical limb ischemia (CLI) will eventually require limb amputation. Therapeutic neovascularization using unselected mononuclear cells to salvage ischemic limbs has produced modest results. The TIE2-expressing monocytes/macrophages (TEMs) are a myeloid cell subset known to be highly angiogenic in tumours. This study aimed to examine the kinetics of TEMs in patients with CLI and whether these cells promote neovascularization of the ischemic limb. Here we show that there are 10-fold more circulating TEMs in CLI patients, and removal of ischemia reduces their numbers to normal levels. TEM numbers in ischemic muscle are two-fold greater than normoxic muscle from the same patient. TEMs from patients with CLI display greater proangiogenic activity than TIE2-negative monocytes in vitro. Using a mouse model of hindlimb ischemia, lentiviral-based Tie2 knockdown in TEMs impaired recovery from ischemia, whereas delivery of mouse macrophages overexpressing TIE2, or human TEMs isolated from CLI patients, rescued limb ischemia. These data suggest that enhancing TEM recruitment to the ischemic muscle may have the potential to improve limb neovascularization in CLI patients.