MCP-1 promotes mural cell recruitment during angiogenesis in the aortic ring model

Rings of rat or mouse aorta embedded in collagen gels produce angiogenic outgrowths in response to the injury of the dissection procedure. Aortic outgrowths are composed of branching endothelial tubes and surrounding mural cells. Mural cells emerge following endothelial sprouting and gradually increase during the maturation of the neovessels. Treatment of aortic cultures with angiopoietin-1 (Ang-1), an angiogenic factor implicated in vascular maturation and remodeling, stimulates the mural cell recruitment process. Ang-1 induces expression of many cytokines and chemokines including monocyte chemotactic protein-1 (MCP-1). Inhibition of p38 MAP kinase, a signaling molecule required for mural cell recruitment, blocks Ang1-induced MCP-1 expression. Recombinant MCP-1 dose-dependently increases mural cell number while an anti-MCP-1 blocking antibody reduces it. In addition, antibody mediated neutralization of MCP-1 abrogates the stimulatory effect of Ang-1 on mural cell recruitment. Aortic rings from genetically modified mice deficient in MCP-1 or its receptor CCR2 have fewer mural cells than controls. MCP-1 deficiency also impairs the mural cell recruitment activity of Ang-1. Our studies indicate that spontaneous and Ang1-induced mural cell recruitment in the aortic ring of model of angiogenesis are in part mediated by MCP-1. These results implicate MCP-1 as one of the mediators of mural cell recruitment in the aortic ring model, and suggest that chemokine pathways may contribute to the assembly of the vessel wall during the angiogenesis response to injury.     

Requisite role of p38 MAPK in mural cell recruitment during angiogenesis in the rat aorta model

During the early stage of angiogenesis, neovascular sprouts are composed primarily of endothelial cells. As they mature, microvessels acquire a coating of mural cells, which are critical for the development and maintenance of a functional vasculature. Though growth factor regulation of mural cell recruitment has been extensively investigated, the intracellular signaling events involved in this process remain poorly understood. Among the intracellular kinases implicated in angiogenesis, the p38 MAPK has been shown to transduce signals critical for vascular remodeling and maturation. The rat aorta model of angiogenesis was used to further investigate the role of this signaling pathway in the recruitment of mural cells during angiogenesis. The p38 MAPK inhibitor SB203580 selectively blocked mural cell recruitment, resulting in the formation of naked endothelial tubes without mural cells. SB203580 inhibited angiopoietin-1-induced mural cell recruitment without influencing angiopoietin-1-stimulated endothelial sprouting. Adenoviral vector-mediated expression of a dominant negative form of p38 MAPK significantly reduced mural cell recruitment, whereas overexpression of a constitutively activated form of MKK6, an upstream activator of p38 MAPK, increased mural cell number. These results indicate that the p38 MAPK signaling pathway plays a critical role in mural cell recruitment during neovascularization and may represent a therapeutic target in angiogenesis-related disorders.     

From vessel sprouting to normalization: role of the prolyl hydroxylase domain protein/hypoxia-inducible factor oxygen-sensing machinery

The accepted model of vessel branching distinguishes several endothelial cell fates. At the forefront of a vessel sprout, "tip cells" guide the sprouting vessel toward an angiogenic stimulus. Behind the tip, "stalk cells" proliferate to elongate the vessel branch and create a lumen. In mature vessels, endothelial cells acquire a streamlined shape to optimally conduct blood flow. For this purpose, endothelial cells switch to the "phalanx" cell fate, which is characterized by quiescent and nonproliferating cells aligned in a tight cobblestonelike layer. Vessel maturation also requires the recruitment of mural cells (ie, smooth muscle cells and pericytes). These cell fates are often altered in pathological conditions, most prominently during the formation of tumor vasculature. Given the essential role of hypoxia as the driving force for initiating angiogenesis, it is not surprising that the hypoxia-sensing machinery controls key steps in physiological and pathological angiogenesis.     

Endothelial nitric oxide synthase is critical for ischemic remodeling, mural cell recruitment, and blood flow reserve

The genetic loss of endothelial-derived nitric oxide synthase (eNOS) in mice impairs vascular endothelial growth factor (VEGF) and ischemia-initiated blood flow recovery resulting in critical limb ischemia. This result may occur through impaired arteriogenesis, angiogenesis, or mobilization of stem and progenitor cells. Here, we show that after ischemic challenge, eNOS knockout mice [eNOS (-/-)] have defects in arteriogenesis and functional blood flow reserve after muscle stimulation and pericyte recruitment, but no impairment in endothelial progenitor cell recruitment. More importantly, the defects in blood flow recovery, clinical manifestations of ischemia, ischemic reserve capacity, and pericyte recruitment into the growing neovasculature can be rescued by local intramuscular delivery of an adenovirus encoding a constitutively active allele of eNOS, eNOS S1179D, but not a control virus. Collectively, our data suggest that endogenous eNOS-derived NO exerts direct effects in preserving blood flow, thereby promoting arteriogenesis, angiogenesis, and mural cell recruitment to immature angiogenic sprouts.     

PDGF-D induces macrophage recruitment, increased interstitial pressure, and blood vessel maturation during angiogenesis

Platelet-derived growth factor-D (PDGF-D) is a recently characterized member of the PDGF family with unknown in vivo functions. We investigated the effects of PDGF-D in transgenic mice by expressing it in basal epidermal cells and then analyzed skin histology, interstitial fluid pressure, and wound healing. When compared with control mice, PDGF-D transgenic mice displayed increased numbers of macrophages and elevated interstitial fluid pressure in the dermis. Wound healing in the transgenic mice was characterized by increased cell density and enhanced recruitment of macrophages. Macrophage recruitment was also the characteristic response when PDGF-D was expressed in skeletal muscle or ear by an adeno-associated virus vector. Combined expression of PDGF-D with vascular endothelial growth factor-E (VEGF-E) led to increased pericyte/smooth muscle cell coating of the VEGF-E-induced vessels and inhibition of the vascular leakiness that accompanies VEGF-E-induced angiogenesis. These results show that full-length PDGF-D is activated in tissues and is capable of increasing interstitial fluid pressure and macrophage recruitment and the maturation of blood vessels in angiogenic processes.     

MCP-1 mediates TGF-beta-induced angiogenesis by stimulating vascular smooth muscle cell migration

Transforming growth factor-beta (TGF-beta) and its signaling mediators play crucial roles in vascular formation. Our previous microarray analysis identified monocyte chemoattractant protein-1 (MCP-1) as a TGF-beta target gene in endothelial cells (ECs). Here, we report that MCP-1 mediates the angiogenic effect of TGF-beta by recruiting vascular smooth muscle cells (VSMCs) and mesenchymal cells toward ECs. By using a chick chorioallantoic membrane assay, we show that TGF-beta promotes the formation of new blood vessels and this promotion is attenuated when MCP-1 activity is blocked by its neutralizing antibody. Wound healing and transwell assays established that MCP-1 functions as a chemoattractant to stimulate migration of VSMCs and mesenchymal 10T1/2 cells toward ECs. Furthermore, the conditioned media from TGF-beta-treated ECs stimulate VSMC migration, and inhibition of MCP-1 activity attenuates TGF-beta-induced VSMC migration toward ECs. Finally, we found that MCP-1 is a direct gene target of TGF-beta via Smad3/4. Taken together, our findings suggest that MCP-1 mediates TGF-beta-stimulated angiogenesis by enhancing migration of mural cells toward ECs and thus promoting the maturation of new blood vessels.     

Angiopoietin/Tie2系统在血管新生与成熟过程中的表达及意义

血管的新生和成熟需要经过血管内皮细胞与血管周围细胞间相互作用等一系列复杂的生物学过程,这一过程受到多种细胞因子的调控。促血管生成素(angiopoietin,Ang)是近年发现的一个与血管新生与成熟密切相关的家族,包括4个成员,即Ang1、Ang2、Ang3和Ang4,其共同的特异性受体为Tie2。Ang及受体Tie2在生理性血管形成过程中对新生血管的形成、重构及成熟发挥重要的调控作用。另有研究显示,Ang/Tie2系统也参与了缺血后再灌、肿瘤及视网膜病变等多种病理性血管的形成过程。研究Ang/Tie2系统的生物学效应以及其在血管新生与成熟过程中的表达及意义,有助于深入了解相关疾病的发病机制并为疾病的治疗提供新线索。     

Mid-luteal angiogenesis and function in the primate is dependent on vascular endothelial growth factor

Vascular endothelial growth factor (VEGF) is essential for the angiogenesis required for the formation of the corpus luteum; however, its role in ongoing luteal angiogenesis and in the maintenance of the established vascular network is unknown. The aim of this study was to determine whether VEGF inhibition could intervene in ongoing luteal angiogenesis using immunoneutralisation of VEGF starting in the mid-luteal phase. In addition, the effects on endothelial cell survival and the recruitment of periendothelial support cells were examined. Treatment with a monoclonal antibody to VEGF, or mouse gamma globulin for control animals, commenced on day 7 after ovulation and continued for 3 days. Bromodeoxyuridine (BrdU), used to label proliferating cells to obtain a proliferation index, was administered one hour before collecting ovaries from control and treated animals. Ovarian sections were stained using antibodies to BrdU, the endothelial cell marker, CD31, the pericyte marker, alpha-smooth muscle actin, and 3' end DNA fragments as a marker for apoptosis. VEGF immunoneutralisation significantly suppressed endothelial cell proliferation and the area occupied by endothelial cells while increasing pericyte coverage and the incidence of endothelial cell apoptosis. Luteal function was markedly compromised by anti-VEGF treatment as judged by a 50% reduction in plasma progesterone concentration. It is concluded that ongoing angiogenesis in the mid-luteal phase is primarily driven by VEGF, and that a proportion of endothelial cells of the mid-luteal phase vasculature are dependent on VEGF support.     

Pathological angiogenesis is reduced by targeting pericytes via the NG2 proteoglycan

The NG2 proteoglycan is expressed by nascent pericytes during the early stages of angiogenesis. To investigate the functional role of NG2 in neovascularization, we have compared pathological retinal and corneal angiogenesis in wild type and NG2 null mice. During ischemic retinal neovascularization, ectopic vessels protruding into the vitreous occur twice as frequently in wild type retinas as in NG2 null retinas. In the NG2 knock-out retina, proliferation of both pericytes and endothelial cells is significantly reduced, and the pericyte:endothelial cell ratio falls to 0.24 from the wild type value of 0.86. Similarly, bFGF-induced angiogenesis is reduced more than four-fold in the NG2 null cornea compared to that seen in the wild type retina. Significantly, NG2 antibody is effective in reducing angiogenesis in the wild type cornea, suggesting that the proteoglycan can be an effective target for anti-angiogenic therapy. These experiments therefore demonstrate both the functional importance of NG2 in pericyte development and the feasibility of using pericytes as anti-angiogenic targets.     

Cardioprotective effects of adipokine apelin on myocardial infarction

Angiogenesis plays an important role in myocardial infarction. Apelin and its natural receptor (angiotensin II receptor-like 1, AGTRL-1 or APLNR) induce sprouting of endothelial cells in an autocrine or paracrine manner. The aim of this study is to investigate whether apelin can improve the cardiac function after myocardial infarction by increasing angiogenesis in infarcted myocardium. Left ventricular end-diastolic pressure (LVEDP), left ventricular end systolic pressure (LVESP), left ventricular developed pressure (LVDP), maximal left ventricular pressure development (±LVdp/dtmax), infarct size, and angiogenesis were evaluated to analyze the cardioprotective effects of apelin on ischemic myocardium. Assays of 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide, 5-bromo-2′-deoxyuridine incorporation, wound healing, transwells, and tube formation were used to detect the effects of apelin on proliferation, migration, and chemotaxis of cardiac microvascular endothelial cells. Fluorescein isothiocyanate-labeled bovine serum albumin penetrating through monolayered cardiac microvascular endothelial cells was measured to evaluate the effects of apelin on permeability of microvascular endothelial cells. In vivo results showed that apelin increased ±LV dp/dtmax and LVESP values, decreased LVEDP values (all p < 0.05), and promoted angiogenesis in rat heart after ligation of the left anterior descending coronary artery. In vitro results showed that apelin dose-dependently enhanced proliferation, migration, chemotaxis, and tube formation, but not permeability of cardiac microvascular endothelial cells. Apelin also increased the expression of vascular endothelial growth factor receptors-2 (VEGFR2) and the endothelium-specific receptor tyrosine kinase (Tie-2) in cardiac microvascular endothelial cells. These results indicated that apelin played a protective role in myocardial infarction through promoting angiogenesis and decreasing permeability of microvascular endothelial cells via upregulating the expression of VEGFR2 and Tie-2 in cardiac microvascular endothelial cells.     

NG2 proteoglycan promotes tumor vascularization via integrin-dependent effects on pericyte function

The NG2 proteoglycan stimulates the proliferation and migration of various immature cell types, including pericytes. However, the role of NG2 in mediating pericyte/endothelial cell interaction has been less clear. In this study, we show that pericyte-specific NG2 ablation causes several structural deficits in blood vessels in intracranial B16F10 melanomas, including decreased pericyte ensheathment of endothelial cells, diminished formation of endothelial junctions, and reduced assembly of the vascular basal lamina. These deficits result in decreased tumor vessel patency, increased vessel leakiness, and increased intratumoral hypoxia. NG2-dependent mechanisms of pericyte interaction with endothelial cells are further explored in pericyte/endothelial cell co-cultures. siRNA-mediated NG2 knockdown in pericytes leads to reduced formation of pericyte/endothelial networks, reduced formation of ZO-1 positive endothelial cell junctions, and increased permeability of endothelial cell monolayers. We also show that NG2 knockdown results in loss of β1 integrin activation in endothelial cells, revealing a mechanism for NG2-dependent cross talk between pericytes and endothelial cells.     

Examining the role of Rac1 in tumor angiogenesis and growth: a clinically relevant RNAi-mediated approach

Angiogenesis, the sprouting of new blood vessels from the pre-existing vasculature, is a well established target in anti-cancer therapy. It is thought that the Rho GTPase Rac1 is required during vascular endothelial growth factor (VEGF)-mediated angiogenesis. In the present study, we have used a clinically relevant RNA interference approach to silence Rac1 expression. Human umbilical vein endothelial cells were transiently transfected with non-specific control siRNA (siNS) or Rac1 siRNA (siRac1) using electroporation or Lipofectamine 2000. Functional assays with transfected endothelial cells were performed to determine the effect of Rac1 knockdown on angiogenesis in vitro. Silencing of Rac1 inhibited VEGF-mediated tube formation, cell migration, invasion and proliferation. In addition, treatment with Rac1 siRNA inhibited angiogenesis in an in vivo Matrigel plug assay. Intratumoral injections of siRac1 almost completely inhibited the growth of grafted Neuro2a tumors and reduced tumor angiogenesis. Together, these data indicate that Rac1 is an important regulator of VEGF-mediated angiogenesis. Knockdown of Rac1 may represent an attractive approach to inhibit tumor angiogenesis and growth.     

RGS5 expression is a quantitative measure of pericyte coverage of blood vessels

Pericytes play a key role in the process of vascular maturation and stabilization however, the current methods for quantifying pericyte coverage of the neovasculature are laborious and subjective in nature. In this study, we have developed an objective, sensitive, and high-throughput method for quantifying pericyte coverage of angiogenic vessels by analyzing the expression of the pericyte-specific gene, the regulator of G-protein signaling 5 (RGS5). We determined that RGS5 expression was up-regulated during a defined developmental time period in which nascent vessel sprouts acquired a pericyte covering. Furthermore, RGS5 expression was dramatically reduced in vessels with poor pericyte coverage compared to normal angiogenic vasculature. Finally, we determined that the susceptibility of nascent vessels to regression by vascular endothelial growth factor (VEGF) inhibition was significantly reduced following RGS5 up-regulation, further implicating RGS5 in pericyte-endothelial cell interactions and the vascular maturation process. These studies establish the use of RGS5 gene expression as a quantitative and robust measure of pericyte coverage of neovasculature. This method provides a tool for vascular biologists studying pericyte-endothelial cell interactions and vascular maturation in both normal and pathological conditions, such as diabetic retinopathy and cancer.     

Endothelial cell growth: biology and pharmacology in relation to angiogenesis.

The vascular system is lined by a monolayer of endothelial cells which proliferate very slowly under normal conditions. The formation of new capillary vessels is associated with some physiological circumstances and several pathological conditions. Angiogenesis requires migration, differentiation and proliferation of endothelial cells. The mechanism of tube formation is still poorly understood. Tumour growth is angiogenesis-dependent and angiogenesis is directly or indirectly induced by the tumour. Induction of angiogenesis is an important step in carcinogenesis and in metastatic development. Angiogenesis is induced during the transition from hyperplasia to neoplasia. Numerous angiogenic factors have been identified, most are mitogenic for endothelial cells and some are only responsible for tube formation. However, it is difficult to recognize which factor is the most important in vivo. Since angiogenesis is necessary for tumour growth, any natural or synthetic antiangiogenic compound may have an antineoplastic potential. Inhibition of tumour angiogenesis under the control of a tumour suppression gene could play an important role. Pharmacological compounds, such as heparin, heparin fragments and corticosteroids, have been shown to be antiangiogenic substances. More recently two new inhibitors of capillary endothelial cell proliferation and/or angiogenesis have been described: they are a cartilage-derived inhibitor and platelet factor 4.     

Pericytes and vessel maturation during tumor angiogenesis and metastasis

Despite promising results in preclinical and clinical studies, the therapeutic efficacy of antiangiogenic therapies has been restricted by a narrow focus on inhibiting the growth of endothelial cells. Other cell types in the tumor stroma are also critical to the progression of cancer, including mural cells. Mural cells are vascular support cells that range in phenotype from pericytes to vascular smooth muscle cells. Although the role of pericytes and pericyte‐like cells in the pathophysiology of cancer is still unclear, evidence indicates that aberrations in pericyte–endothelial cell signaling networks could contribute to tumor angiogenesis and metastasis. The purpose of this review is to evaluate critically recent evidence on the role of pericytes in tumor biology and discuss potential therapeutic targets for anticancer intervention. Am. J. Hematol. 85:593–598, 2010. © 2010 Wiley‐Liss, Inc.     

Spatio-temporal analysis of vascular endothelial growth factor expression and blood vessel remodelling in pig ovarian follicles during the periovulatory period

Vascular endothelial growth factor (VEGF) expression pattern and blood vessel remodelling were evaluated during the transition from the preovulatory follicle to the corpus luteum (CL). To this end, prepubertal gilts were treated with equine chorionic gonadotrophin (eCG) to collect preovulatory follicles (60 h after eCG) and with human chorionic gonadotrophin (hCG) to obtain periovulatory follicles 18 h and 36 h later. The VEGF mRNA content was analysed by in situ hybridization, while protein localization in follicular fluid (FF) and in granulosa and theca compartments was evaluated by ELISA, immunohistochemistry or western blot. Blood vessel architecture and vascular area (VA) were investigated using immunohistochemistry for von Willenbrand Factor, a specific endothelial marker. Vascular remodelling was finally tested using Ki-67 immunocytochemistry as a proliferation marker, or alpha-smooth muscle actin (alpha-SMA) as a specific mural cell marker. eCG-treated follicles showed high VEGF levels and two concentric blood vessel networks composed of proliferating endothelial cells without any association with mural components. hCG injection inhibited VEGF synthesis in the granulosa compartment and, as a consequence, the protein fell within the FF. In parallel, endothelial cell proliferation stopped and the VA decreased. Close to ovulation, VEGF production restarted in both follicular compartments and VEGF mRNA content significantly increased in the theca layer. Changes in follicular VEGF secretion were observed; the protein disappeared from FF and was observed in the extracellular matrix. An active angiogenesis characterized the follicle; endothelial cell proliferation was associated with a recruitment of alpha-SMA-positive mural cells. The data presented in this work showed that, in the phases preceding ovulation, a complete vascular remodelling occurs, characterized by both an evident neovascularization and the appearance of blood vessels presenting smooth musculature which could be involved in CL formation after ovulation.     

Endothelial cells modulate the proliferation of mural cell precursors via platelet-derived growth factor-BB and heterotypic cell contact

Embryological data suggest that endothelial cells (ECs) direct the recruitment and differentiation of mural cell precursors. We have developed in vitro coculture systems to model some of these events and have shown that ECs direct the migration of undifferentiated mesenchymal cells (10T1/2 cells) and induce their differentiation toward a smooth muscle cell/pericyte lineage. The present study was undertaken to investigate cell proliferation in these cocultures. ECs and 10T1/2 cells were cocultured in an underagarose assay in the absence of contact. There was a 2-fold increase in bromodeoxyuridine labeling of 10T1/2 cells in response to ECs, which was completely inhibited by the inclusion of neutralizing antiserum against platelet-derived growth factor (PDGF)-B. Antisera against PDGF-A, basic fibroblast growth factor, or transforming growth factor (TGF)-beta had no effect on EC-stimulated 10T1/2 cell proliferation. EC proliferation was not influenced by coculture with 10T1/2 cells in the absence of contact. The cells were then cocultured so that contact was permitted. Double labeling and fluorescence-activated cell sorter analysis revealed that ECs and 10T1/2 cells were growth-inhibited by 43% and 47%, respectively. Conditioned media from contacting EC-10T1/2 cell cocultures inhibited the growth of both cell types by 61% and 48%, respectively. Although we have previously shown a role for TGF-beta in coculture-induced mural cell differentiation, growth inhibition resulting from contacting cocultures or conditioned media was not suppressed by the presence of neutralizing antiserum against TGF-beta. Furthermore, the decreased proliferation of 10T1/2 cells in the direct cocultures could not be attributed to downregulation of the PDGF-B in ECs or the PDGF receptor-beta in the 10T1/2 cells. Our data suggest that modulation of proliferation occurs during EC recruitment of mesenchymal cells and that heterotypic cell-cell contact and soluble factors play a role in growth control during vessel assembly.     

Mesenchymal stem cells from adipose and bone marrow promote angiogenesis via distinct cytokine and protease expression mechanisms

Using a fibrin-based angiogenesis model, we have established that there is no canonical mechanism used by endothelial cells (ECs) to degrade the surrounding extracellular matrix (ECM), but rather the set of proteases used is dependent on the mural cells providing the angiogenic cues. Mesenchymal stem cells (MSCs) originating from different tissues, which are thought to be phenotypically similar, promote angiogenesis through distinct mechanisms. Specifically, adipose-derived stem cells (ASCs) promote utilization of the plasminogen activator-plasmin axis by ECs as the primary means of vessel invasion and elongation in fibrin. Matrix metalloproteinases (MMPs) serve a purpose in regulating capillary diameter and possibly in stabilizing the nascent vessels. These proteolytic mechanisms are more akin to those involved in fibroblast-mediated angiogenesis than to those in bone marrow-derived stem cell (BMSC)-mediated angiogenesis. In addition, expression patterns of angiogenic factors such as urokinase plasminogen activator (uPA), hepatocyte growth factor (HGF), and tumor necrosis factor alpha (TNFα) were similar for ASC and fibroblast-mediated angiogenesis, and in direct contrast to BMSC-mediated angiogenesis. The present study illustrates that the nature of the heterotypic interactions between mural cells and endothelial cells depend on the identity of the mural cell used. Even MSCs which are shown to behave phenotypically similar do not stimulate angiogenesis via the same mechanisms.