Primary monocytes regulate endothelial cell survival through secretion of angiopoietin-1 and activation of endothelial Tie2

Angiocrine signaling in sinusoidal homeostasis and liver diseases

The hepatic sinusoids are composed of liver sinusoidal endothelial cells (LSECs), which are surrounded by hepatic stellate cells (HSCs) and contain liver-resident macrophages called Kupffer cells, and other patrolling immune cells. All these cells communicate with each other and with hepatocytes to maintain sinusoidal homeostasis and a spectrum of hepatic functions under healthy conditions. Sinusoidal homeostasis is disrupted by metabolites, toxins, viruses, and other pathological factors, leading to liver injury, chronic liver diseases, and cirrhosis. Alterations in hepatic sinusoids are linked to fibrosis progression and portal hypertension. LSECs are crucial regulators of cellular crosstalk within their microenvironment via angiocrine signaling. This review discusses the mechanisms by which angiocrine signaling orchestrates sinusoidal homeostasis, as well as the development of liver diseases. Here, we summarise the crosstalk between LSECs, HSCs, hepatocytes, cholangiocytes, and immune cells in health and disease and comment on potential novel therapeutic methods for treating liver diseases.

Monocytes Release Pro-Cathepsin D to Drive Blood-to-Brain Transcytosis in Diabetes

BACKGROUND

Microvascular complications are the major outcome of type 2 diabetes progression, and the underlying mechanism remains to be determined.

METHODS

High-throughput RNA sequencing was performed using human monocyte samples from controls and diabetes. The transgenic mice expressing human CTSD (cathepsin D) in the monocytes was constructed using CD68 promoter. In vivo 2-photon imaging, behavioral tests, immunofluorescence, transmission electron microscopy, Western blot analysis, vascular leakage assay, and single-cell RNA sequencing were performed to clarify the phenotype and elucidate the molecular mechanism.

RESULTS

Monocytes expressed high-level CTSD in patients with type 2 diabetes. The transgenic mice expressing human CTSD in the monocytes showed increased brain microvascular permeability resembling the diabetic microvascular phenotype, accompanied by cognitive deficit. Mechanistically, the monocytes release nonenzymatic pro-CTSD to upregulate caveolin expression in brain endothelium triggering caveolae-mediated transcytosis, without affecting the paracellular route of brain microvasculature. The circulating pro-CTSD activated the caveolae-mediated transcytosis in brain endothelial cells via its binding with low-density LRP1 (lipoprotein receptor-related protein 1). Importantly, genetic ablation of CTSD in the monocytes exhibited a protective effect against the diabetes-enhanced brain microvascular transcytosis and the diabetes-induced cognitive impairment.

CONCLUSIONS

These findings uncover the novel role of circulatory pro-CTSD from monocytes in the pathogenesis of cerebral microvascular lesions in diabetes. The circulatory pro-CTSD is a potential target for the intervention of microvascular complications in diabetes.

Intermediate Monocytes and Circulating Endothelial Cells: Interplay with Severity of Atherosclerosis in Patients with Coronary Artery Disease and Type 2 Diabetes Mellitus

The aim was to investigate the association of monocyte heterogeneity and presence of circulating endothelial cells with the severity of coronary atherosclerosis in patients with coronary artery disease (CAD) and type 2 diabetes mellitus (T2DM). We recruited 62 patients with CAD, including 22 patients with DM2. The severity of atherosclerosis was evaluated using Gensini Score. Numbers of classical (CD14++CD16-), intermediate (CD14++CD16+), and non-classical (CD14+CD16++) monocyte subsets; circulating endothelial progenitor cells; and the presence of circulating endothelial cells were evaluated. Counts and frequencies of intermediate monocytes, but not glycaemia parameters, were associated with the severity of atherosclerosis in diabetic CAD patients (rs = 0.689; p = 0.001 and rs = 0.632; p = 0.002, respectively). Frequency of Tie2+ cells was lower in classical than in non-classical monocytes in CAD patients (p = 0.007), while in patients with association of CAD and T2DM, differences between Tie2+ monocytes subsets disappeared (p = 0.080). Circulating endothelial cells were determined in 100% of CAD+T2DM patients, and counts of CD14++CD16+ monocytes and concentration of TGF-β predicted the presence of circulating endothelial cells (sensitivity 92.3%; specificity 90.9%; AUC = 0.930). Thus, intermediate monocytes represent one of the key determinants of the appearance of circulating endothelial cells in all the patients with CAD, but are associated with the severity of atherosclerosis only in patients with association of CAD and T2DM.

Monocyte-endothelial cell interactions in vascular and tissue remodeling

Monocytes are circulating leukocytes of innate immunity derived from the bone marrow that interact with endothelial cells under physiological or pathophysiological conditions to orchestrate inflammation, angiogenesis, or tissue remodeling. Monocytes are attracted by chemokines and specific receptors to precise areas in vessels or tissues and transdifferentiate into macrophages with tissue damage or infection. Adherent monocytes and infiltrated monocyte-derived macrophages locally release a myriad of cytokines, vasoactive agents, matrix metalloproteinases, and growth factors to induce vascular and tissue remodeling or for propagation of inflammatory responses. Infiltrated macrophages cooperate with tissue-resident macrophages during all the phases of tissue injury, repair, and regeneration. Substances released by infiltrated and resident macrophages serve not only to coordinate vessel and tissue growth but cellular interactions as well by attracting more circulating monocytes (e.g. MCP-1) and stimulating nearby endothelial cells (e.g. TNF-α) to expose monocyte adhesion molecules. Prolonged tissue accumulation and activation of infiltrated monocytes may result in alterations in extracellular matrix turnover, tissue functions, and vascular leakage. In this review, we highlight the link between interactions of infiltrating monocytes and endothelial cells to regulate vascular and tissue remodeling with a special focus on how these interactions contribute to pathophysiological conditions such as cardiovascular and chronic liver diseases.

Extracellular Vesicles as Mediators of Nickel-Induced Cancer Progression

Emerging evidence suggests that extracellular vesicles (EVs), which represent a crucial mode of intercellular communication, play important roles in cancer progression by transferring oncogenic materials. Nickel (Ni) has been identified as a human group I carcinogen; however, the underlying mechanisms governing Ni-induced carcinogenesis are still being elucidated. Here, we present data demonstrating that Ni exposure generates EVs that contribute to Ni-mediated carcinogenesis and cancer progression. Human bronchial epithelial (BEAS-2B) cells and human embryonic kidney-293 (HEK293) cells were chronically exposed to Ni to generate Ni-treated cells (Ni-6W), Ni-transformed BEAS-2B cells (Ni-3) and Ni-transformed HEK293 cells (HNi-4). The signatures of EVs isolated from Ni-6W, Ni-3, HNi-4, BEAS-2B, and HEK293 were analyzed. Compared to their respective untreated cells, Ni-6W, Ni-3, and HNi-4 released more EVs. This change in EV release coincided with increased transcription of the EV biogenesis markers CD82, CD63, and flotillin-1 (FLOT). Additionally, EVs from Ni-transformed cells had enriched protein and RNA, a phenotype also observed in other studies characterizing EVs from cancer cells. Interestingly, both epithelial cells and human umbilical vein endothelial (HUVEC) cells showed a preference for taking up Ni-altered EVs compared to EVs released from the untreated cells. Moreover, these Ni-altered EVs induced inflammatory responses in both epithelial and endothelial cells and increased the expression of coagulation markers in endothelial cells. Prolonged treatment of Ni-alerted EVs for two weeks induced the epithelial-to-mesenchymal transition (EMT) in BEAS-2B cells. This study is the first to characterize the effect of Ni on EVs and suggests the potential role of EVs in Ni-induced cancer progression.

Interaction of non‑parenchymal hepatocytes in the process of hepatic fibrosis (Review)

Hepatic fibrosis (HF) is the process of fibrous scar formation caused by chronic liver injury of different etiologies. Previous studies have hypothesized that the activation of hepatic stellate cells (HSCs) is the central process in HF. The interaction between HSCs and surrounding cells is also crucial. Additionally, hepatic sinusoids capillarization, inflammation, angiogenesis and fibrosis develop during HF. The process involves multiple cell types that are highly connected and work in unison to maintain the homeostasis of the hepatic microenvironment, which serves a key role in the initiation and progression of HF. The current review provides novel insight into the intercellular interaction among liver sinusoidal endothelial cells, HSCs and Kupffer cells, as well as the hepatic microenvironment in the development of HF.

Insights into Macrophage/Monocyte-Endothelial Cell Crosstalk in the Liver: A Role for Trem-2

Liver disease accounts for millions of deaths worldwide annually being a major cause of global morbidity. Hepatotoxic insults elicit a multilayered response involving tissue damage, inflammation, scar formation, and tissue regeneration. Liver cell populations act coordinately to maintain tissue homeostasis and providing a barrier to external aggressors. However, upon hepatic damage, this tight regulation is disrupted, leading to liver pathology which spans from simple steatosis to cirrhosis. Inflammation is a hallmark of liver pathology, where macrophages and endothelial cells are pivotal players in promoting and sustaining disease progression. Understanding the drivers and mediators of these interactions will provide valuable information on what may contribute to liver resilience against disease. Here, we summarize the current knowledge on the role of macrophages and liver sinusoidal endothelial cells (LSEC) in homeostasis and liver pathology. Moreover, we discuss the expanding body of evidence on cell-to-cell communication between these two cell compartments and present triggering receptor expressed on myeloid cells-2 (Trem-2) as a plausible mediator of this cellular interlink. This review consolidates relevant knowledge that might be useful to guide the pursue of successful therapeutic targets and pharmacological strategies for controlling liver pathogenesis.

The New Zealand white rabbit animal model of acute radiation syndrome: hematopoietic and coagulation-based parameters by radiation dose following supportive care

PURPOSE

Animal models that accurately reflect human responses to radiation injury are needed for advanced mechanistic investigation and development of effective therapeutics. The rabbit is an established animal model accepted by the FDA for studies of cardiovascular disease, lipid metabolism, the development of anticoagulants, testing of bone implants, and the development of treatments for infectious diseases such as HIV. The purpose of this study was to investigate the New Zealand White (NZW) Rabbit model as a model of acute radiation exposure because of its established similarity to human vascular, immune, and coagulation responses.

MATERIALS AND METHODS

Two sequential studies were performed in a total of 81 male NZW rabbits, 16-20 weeks of age. All animals underwent clinical observations and peripheral blood analyses following a single dose of 0, 6, 7, 8, 8.5, 9, or 10 Gy of total body irradiation via a 6 MV Linear accelerator photon source on day 0. Animals were treated with timed release fentanyl patch (days 0-30), subcutaneous hydration (day 1, Study 2 only), and oral sulfamethoxazole/trimethoprim 30 mg/kg once daily (days 3-30) and were followed for 30 days or to time of mortality.

RESULTS

Study 1 revealed the estimated LD30, -50, -70, and -90 with 95% confidence intervals (CI) at 30 days to be 6.7 (CI: 5.9-7.4), 7.3 (CI: 6.7-7.8), 7.9 (CI: 7.3-8.4), and 8.8 (CI: 7.9-9.7) Gy, respectively. In study 2, a survey of blood coagulation and biochemical parameters were performed over time and necropsy. Complete blood counts taken from animals exposed to 7, 8, or 10 Gy, demonstrated dose-dependent depletion of lymphocytes, neutrophils, and platelets. Platelet counts recovered to baseline levels in survivors by day 30, whereas lymphocyte and neutrophil counts did not. Decedent animals demonstrated grade 3 or 4 neutropenia and lymphopenia at time of death; 64% of the decedents experienced a 30% or greater drop in hematocrit. Decedent animals demonstrated more than 100% increases from serum baseline levels of blood urea nitrogen, creatinine, aspartate aminotransferase, and triglyceride levels at the time of death whereas survivors on average demonstrated modest or no elevation.

CONCLUSION

This NZW rabbit model demonstrates dose-dependent depletion of hematopoietic parameters. The LD_50/30 of 7.8 Gy (95% CI: 6.6-8.4) with supportive care appears to be close to the ranges reported for rhesus macaques (5.25-7.44 Gy) and humans (6-8 Gy) with supportive care. These findings support the utility of the NZW rabbit model for further mechanistic investigation of acute radiation exposure and medical countermeasure testing.

Dissection of heterocellular cross-talk in vascularized cardiac tissue mimetics

Cellular specialization and interaction with other cell types in cardiac tissue is essential for the coordinated function of cell populations in the heart. The complex interplay between cardiomyocytes, endothelial cells and fibroblasts is necessary for adaptation but can also lead to pathophysiological remodeling. To understand this complex interplay, we developed 3D vascularized cardiac tissue mimetics (CTM) to study heterocellular cross-talk in hypertrophic, hypoxic and fibrogenic environments. This 3D platform responds to physiologic and pathologic stressors and mimics the microenvironment of diseased tissue. In combination with endothelial cell fluorescence reporters, these cardiac tissue mimetics can be used to precisely visualize and quantify cellular and functional responses upon stress stimulation. Utilizing this platform, we demonstrate that stimulation of α/β-adrenergic receptors with phenylephrine (PE) promotes cardiomyocyte hypertrophy, metabolic maturation and vascularization of CTMs. Increased vascularization was promoted by conditioned medium of PE-stimulated cardiomyocytes and blocked by inhibiting VEGF or upon β-adrenergic receptor antagonist treatment, demonstrating cardiomyocyte-endothelial cross-talk. Pathophysiological stressors such as severe hypoxia reduced angiogenic sprouting and increased cell death, while TGF β2 stimulation increased collagen deposition concomitant to endothelial-to-mesenchymal transition. In sum, we have developed a cardiac 3D culture system that reflects native cardiac tissue function, metabolism and morphology - and for the first time enables the tracking and analysis of cardiac vascularization dynamics in physiology and pathology.

Focus on Early Events: Pathogenesis of Pulmonary Arterial Hypertension Development

Significance: Pulmonary arterial hypertension (PAH) is a progressive disease of the lung vasculature characterized by the proliferation of all vascular wall cell types, including endothelial, smooth muscle, and fibroblasts. The disease rapidly advances into a form with extensive pulmonary vascular remodeling, leading to a rapid increase in pulmonary vascular resistance, which results in right heart failure. Recent Advances: Most current research in the PAH field has been focused on the late stage of the disease, largely due to an urgent need for patient treatment options in clinics. Further, the pathobiology of PAH is multifaceted in the advanced disease, and there has been promising recent progress in identifying various pathological pathways related to the late clinical picture. Critical Issues: Early stage PAH still requires additional attention from the scientific community, and although the survival of patients with early diagnosis is comparatively higher, the disease develops in patients asymptomatically, making it difficult to identify and treat early. Future Directions: There are several reasons to focus on the early stage of PAH. First, the complexity of late stage disease, owing to multiple pathways being activated in a complex system with intra- and intercellular signaling, leads to an unclear picture of the key contributors to the pathobiology. Second, an understanding of early pathophysiological events can increase the ability to identify PAH patients earlier than what is currently possible. Third, the prompt diagnosis of PAH would allow for the therapy to start earlier, which has proved to be a more successful strategy, and it ensures better survival in PAH patients.

Endothelial repair is dependent on CD11c+ leukocytes to establish regrowing endothelial sheets with high cellular density

Endothelial injury makes the vessel wall vulnerable to cardiovascular diseases. Injured endothelium regenerates by collective sheet migration, that is, the endothelial cells coordinate their motion and regrow as a sheet of cells with retained cell-cell contacts into the wounded area. Leukocytes appear to be involved in endothelial repair in vivo; however, little is known about their identity and role in the reparative sheet migration process. To address these questions, we developed a high-quality en face technique that enables visualizing of leukocytes and endothelial cells simultaneously following an endoluminal scratch wound injury of the mouse carotid artery. We discovered that regrowing endothelium forms a broad proliferative front accompanied by CD11c⁺ leukocytes. Functionally, the leukocytes were dispensable for the initial migratory response of the regrowing endothelial sheet, but critical for the subsequent formation and maintenance of a front zone with high cellular density. Marker expression analyses, genetic fate mapping, phagocyte targeting experiments, and mouse knock-out experiments indicate that the CD11c⁺  leukocytes were mononuclear phagocytes with an origin from both Ly6C^high and Ly6C^low monocytes. In conclusion, CD11c⁺ mononuclear phagocytes are essential for a proper endothelial regrowth following arterial endoluminal scratch injury. Promoting the endothelial-preserving function of CD11c⁺  leukocytes may be a strategy to enhance endothelial repair following surgical and endovascular procedures.

Effect of THP-1 Cells on the Formation of Vascular Tubes by Endothelial EA.hy926 Cells in the Presence of Placenta Secretory Products

We studied the effect of THP-1 cells on the formation of vessel-like structures by endothelial cells in the presence of placenta-conditioned media. Addition of THP-1 cells to endothelial cells cultured in the presence of media conditioned by first-trimester placentas led to an increase in the length of cell tubes and reduced their number in comparison with endothelial cell monoculture. In the presence of media conditioned by third-trimester placentas, THP-1 cells did not affect the length and number of cell tubes formed by endothelial cells. When evaluating the formation of vessel-like structures by endothelial cells in co-culture, marked decrease in the length of cell tubes in the presence of media conditioned by first-trimester placentas vs. third-trimester placentas was noted. No differences in the length and number of cell tubes formed by endothelial cells co-cultured with THP-1 cells in the presence of placental factors from women with preeclampsia and uncomplicated pregnancy were found. These findings can reflect the peculiarities of the influence of macrophages on the formation of blood vessels by endothelial cells in the placenta.

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.

Monocyte-endothelial cell interactions in the regulation of vascular sprouting and liver regeneration in mouse

BACKGROUND & AIMS

Regeneration of the hepatic mass is crucial to liver repair. Proliferation of hepatic parenchyma is intimately dependent on angiogenesis and resident macrophage-derived cytokines. However the role of circulating monocyte interactions in vascular and hepatic regeneration is not well-defined. We investigated the role of these interactions in regeneration in the presence and absence of intact monocyte adhesion.

METHODS

Partial hepatectomy was performed in wild-type mice and those lacking the monocyte adhesion molecule CD11b. Vascular architecture, angiogenesis and macrophage location were analyzed in the whole livers using simultaneous angiography and macrophage staining with fluorescent multiphoton microscopy. Monocyte adhesion molecule expression and sprouting-related pathways were evaluated.

RESULTS

Resident macrophages (Kupffer cells) did not migrate to interact with vessels whereas infiltrating monocytes were found adjacent to sprouting points. Infiltrated monocytes colocalized with Wnt5a, angiopoietin 1 and Notch-1 in contact points and commensurate with phosphorylation and disruption of VE-cadherin. Mice deficient in CD11b showed a severe reduction in angiogenesis, liver mass regeneration and survival following partial hepatectomy, and developed unstable and leaky vessels that eventually produced an aberrant hepatic vascular network and Kupffer cell distribution.

CONCLUSIONS

Direct vascular interactions of infiltrating monocytes are required for an ordered vascular growth and liver regeneration. These outcomes provide insight into hepatic repair and new strategies for hepatic regeneration.

Effect of Monocyte-Like THP-1 Cells on the Formation of Vascular Tubes by EA.Hy926s Endothelial Cells in the Presence of Cytokines

The interaction of endothelial cells with cells of the microenvironment, including monocytes/ macrophages, and extracellular matrix during angiogenesis is controlled by cytokines. The stimulating effect bFGF, IL-8, and VEGF on the formation of capillary-like structures by endothelial cells was demonstrated in both monoculture and in co-culture with THP-1 cells; in the latter case, the effects of bFGF and VEGF were more pronounced. IL-8 reduced branching of vascular tubes in co-culture in comparison with monoculture of endothelial cells. Placental growth factor PlGF had no effect of tube formation by endothelial cells in monoculture, but in co-culture with THP-1 cells this cytokine in high concentrations exhibited proangiogenic activity. TGFb inhibited the formation of vascular tubes by endothelial cells and its antiangiogenic potential was more pronounced in co-culture with THP-1 cells.

Distinct CD11b+-monocyte subsets accelerate endothelial cell recovery after acute and chronic endothelial cell damage

BACKGROUND

Endothelial cell recovery requires replenishment of primary cells from the endothelial lineage. However, recent evidence suggests that cells of the innate immune system enhance endothelial regeneration.

METHODS AND RESULTS

Focusing on mature CD11b+-monocytes, we analyzed the fate and the effect of transfused CD11b+-monocytes after endothelial injury in vivo. CD11b-diphtheria-toxin-receptor-mice--a mouse model in which administration of diphtheria toxin selectively eliminates endogenous monocytes and macrophages--were treated with WT-derived CD11b+-monocytes from age-matched mice. CD11b+-monocytes improved endothelium-dependent vasoreactivity after 7 days while transfusion of WT-derived CD11b--cells had no beneficial effect on endothelial function. In ApoE-/--CD11b-DTR-mice with a hypercholesterolemia-induced chronic endothelial injury transfusion of WT-derived CD11b+-monocytes stimulated by interferon-γ (IFNγ) decreased endothelial function, whereas interleukin-4-stimulated (IL4) monocytes had no detectable effect on vascular function. Bioluminescent imaging revealed restriction of transfused CD11b+-monocytes to the endothelial injury site in CD11b-DTR-mice depleted of endogenous monocytes. In vitro co-culture experiments revealed significantly enhanced regeneration properties of human endothelial outgrowth cells (EOCs) when cultured with preconditioned-media (PCM) or monocytes of IL4-stimulated-subsets compared to the effects of IFNγ-stimulated monocytes.

CONCLUSION

CD11b+-monocytes play an important role in endothelial cell recovery after endothelial injury by homing to the site of vascular injury, enhancing reendothelialization and improving endothelial function. In vitro experiments suggest that IL4-stimulated monocytes enhance EOC regeneration properties most likely by paracrine induction of proliferation and cellular promotion of differentiation. These results underline novel insights in the biology of endothelial regeneration and provide additional information for the treatment of vascular dysfunction.

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

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

The diverse identity of angiogenic monocytes

BACKGROUND

The role of bone marrow-derived cells in stimulating angiogenesis, vascular repair or remodelling has been well established, but the nature of the circulating angiogenic cells is still controversial.

DESIGN

The existing literature on different cell types that contribute to angiogenesis in multiple pathologies, most notably ischaemic and tumour angiogenesis, is reviewed, with a focus on subtypes of angiogenic mononuclear cells and their local recruitment and activation.

RESULTS

A large number of different cells of myeloid origin support angiogenesis without incorporating permanently into the newly formed vessel, which distinguishes these circulating angiogenic cells (CAC) from endothelial progenitor cells (EPC). Although CAC frequently express individual endothelial markers, they all share multiple characteristics of monocytes and only express a limited set of discriminative surface markers in the circulation. When cultured ex vivo, or surrounding the angiogenic vessel in vivo, however, many of them acquire similar additional markers, making their discrimination in situ difficult.

CONCLUSION

Different subsets of monocytes show angiogenic properties, but the distinct microenvironment, in vitro or in vivo, is needed for the development of their pro-angiogenic function.

Autocrine role of angiopoietins during megakaryocytic differentiation

The tyrosine kinase Tie-2 and its ligands Angiopoietins (Angs) transduce critical signals for angiogenesis in endothelial cells. This receptor and Ang-1 are coexpressed in hematopoietic stem cells and in a subset of megakaryocytes, though a possible role of angiopoietins in megakaryocytic differentiation/proliferation remains to be demonstrated. To investigate a possible effect of Ang-1/Ang-2 on megakaryocytic proliferation/differentiation we have used both normal CD34(+) cells induced to megakaryocytic differentiation and the UT7 cells engineered to express the thrombopoietin receptor (TPOR, also known as c-mpl, UT7/mpl). Our results indicate that Ang-1/Ang-2 may have a role in megakaryopoiesis. Particularly, Ang-2 is predominantly produced and released by immature normal megakaryocytic cells and by undifferentiated UT7/mpl cells and slightly stimulated TPO-induced cell proliferation. Ang-1 production is markedly induced during megakaryocytic differentiation/maturation and potentiated TPO-driven megakaryocytic differentiation. Blocking endogenously released angiopoietins partially inhibited megakaryocytic differentiation, particularly for that concerns the process of polyploidization. According to these data it is suggested that an autocrine angiopoietin/Tie-2 loop controls megakaryocytic proliferation and differentiation.

Matrix rigidity controls endothelial differentiation and morphogenesis of cardiac precursors

Tissue development and regeneration involve tightly coordinated and integrated processes: selective proliferation of resident stem and precursor cells, differentiation into target somatic cell type, and spatial morphological organization. The role of the mechanical environment in the coordination of these processes is poorly understood. We show that multipotent cells derived from native cardiac tissue continually monitored cell substratum rigidity and showed enhanced proliferation, endothelial differentiation, and morphogenesis when the cell substratum rigidity closely matched that of myocardium. Mechanoregulation of these diverse processes required p190RhoGAP, a guanosine triphosphatase-activating protein for RhoA, acting through RhoA-dependent and -independent mechanisms. Natural or induced decreases in the abundance of p190RhoGAP triggered a series of developmental events by coupling cell-cell and cell-substratum interactions to genetic circuits controlling differentiation.

Molecular mechanisms of tumor angiogenesis

Tumors have been recently recognized as aberrant organs composed of a complex mixture of highly interactive cells that in addition to the cancer cell include stroma (fibroblasts, adipocytes, and myofibroblasts), inflammatory (innate and adaptive immune cells), and vascular cells (endothelial and mural cells). While initially cancer cells co-opt tissue-resident vessels, the tumor eventually recruits its own vascular supply. The process of tumor neovascularization proceeds through the combined output of inductive signals from the entire cellular constituency of the tumor. During the last two decades, the identification and mechanistic outcome of signaling pathways that mediate tumor angiogenesis have been elucidated. Interestingly, many of the genes and signaling pathways activated in tumor angiogenesis are identical to those operational during developmental vascular growth, but they lack feedback regulatory control and are highly affected by inflammatory cells and hypoxia. Consequently, tumor vessels are abnormal, fragile, and hyperpermeable. The lack of hierarchy and inconsistent investment of mural cells dampen the ability of the vessels to effectively perfuse the tumor, and the resulting hypoxia installs a vicious cycle that continuously perpetuates a state of vascular inefficiency. Pharmacological targeting of blood vessels, mainly through the VEGF signaling pathway, has proven effective in normalizing tumor vessels. This normalization improves perfusion and distribution of chemotherapeutic drugs with resulting tumor suppression and moderate increase in overall survival. However, resistance to antiangiogenic therapy occurs frequently and constitutes a critical barrier in the inhibition of tumor growth. A concrete understanding of the chief signaling pathways that stimulate vascular growth in tumors and their cross-talk will continue to be essential to further refine and effectively abort the angiogenic response in cancer.

Coexpression of angiopoietin-1 with VEGF increases the structural integrity of the blood-brain barrier and reduces atrophy volume

Vascular endothelial growth factor (VEGF)-induced neovasculature is immature and leaky. We tested if coexpression of angiopoietin-1 (ANG1) with VEGF improves blood-brain barrier (BBB) integrity and VEGF neuroprotective and neurorestorative effects using a permanent distal middle cerebral artery occlusion (pMCAO) model. Adult CD-1 mice were injected with 2 × 10(9) virus genomes of adeno-associated viral vectors expressing VEGF (AAV-VEGF) or ANG1 (AAV-ANG1) individually or together in a 1:1 ratio into the ischemic penumbra 1 hour after pMCAO. AAV-LacZ was used as vector control. Samples were collected 3 weeks later. Compared with AAV-LacZ, coinjection of AAV-VEGF and AAV-ANG1 reduced atrophy volume (46%, P=0.004); injection of AAV-VEGF or AAV-ANG1 individually reduced atrophy volume slightly (36%, P=0.08 and 33%, P=0.09, respectively). Overexpression of VEGF reduced tight junction protein expression and increased Evans blue extravasation. Compared with VEGF expression alone, coexpression of ANG1 with VEGF resulted in upregulation of tight junction protein expression and reduction of Evans blue leakage (AAV-ANG1/AAV-VEGF: 1.4 ± 0.3 versus AAV-VEGF: 2.8 ± 0.7, P=0.001). Coinjection of AAV-VEGF and AAV-ANG1 induced a similar degree of angiogenesis as injection of AAV-VEGF alone (P=0.85). Thus, coexpression of ANG1 with VEGF improved BBB integrity and resulted in better neuroprotection compared with VEGF expression alone.

Differential role of monocyte subsets in atherosclerosis

Endothelial dysfunction and inflammation of the arterial wall continuously drive the development of atherosclerosis. Details regarding the sequential involvement of different monocyte subsets in the pathology of this disease have recently emerged. This review concentrates on major monocyte subpopulations in mouse and men and specifically addresses their phenotype, function and recruitment during primary atherosclerosis as well as their contribution to angiogenesis and tissue regeneration secondary to plaque rupture.

Opposing effects of the angiopoietins on the thrombin-induced permeability of human pulmonary microvascular endothelial cells

Background

Angiopoietin-2 (Ang-2) is associated with lung injury in ALI/ARDS. As endothelial activation by thrombin plays a role in the permeability of acute lung injury and Ang-2 may modulate the kinetics of thrombin-induced permeability by impairing the organization of vascular endothelial (VE-)cadherin, and affecting small Rho GTPases in human pulmonary microvascular endothelial cells (HPMVECs), we hypothesized that Ang-2 acts as a sensitizer of thrombin-induced hyperpermeability of HPMVECs, opposed by Ang-1.

Methodology/Principal Findings

Permeability was assessed by measuring macromolecule passage and transendothelial electrical resistance (TEER). Angiopoietins did not affect basal permeability. Nevertheless, they had opposing effects on the thrombin-induced permeability, in particular in the initial phase. Ang-2 enhanced the initial permeability increase (passage, P = 0.010; TEER, P = 0.021) in parallel with impairment of VE-cadherin organization without affecting VE-cadherin Tyr685 phosphorylation or increasing RhoA activity. Ang-2 also increased intercellular gap formation. Ang-1 preincubation increased Rac1 activity, enforced the VE-cadherin organization, reduced the initial thrombin-induced permeability (TEER, P = 0.027), while Rac1 activity simultaneously normalized, and reduced RhoA activity at 15 min thrombin exposure (P = 0.039), but not at earlier time points. The simultaneous presence of Ang-2 largely prevented the effect of Ang-1 on TEER and macromolecule passage.

Conclusions/Significance

Ang-1 attenuated thrombin-induced permeability, which involved initial Rac1 activation-enforced cell-cell junctions, and later RhoA inhibition. In addition to antagonizing Ang-1, Ang-2 had also a direct effect itself. Ang-2 sensitized the initial thrombin-induced permeability accompanied by destabilization of VE-cadherin junctions and increased gap formation, in the absence of increased RhoA activity.