VEGF signaling inside vascular endothelial cells and beyond

Stealing VEGF from thy neighbor

The distribution and patterning of blood vessels is controlled by vascular endothelial growth factor (VEGF), which is precisely regulated throughout its life cycle. Okabe et al. show that VEGF is titrated away from the endothelium by adjacent neurons via endocytosis, regulating density and trajectory of blood vessels.

The role of molecular imaging in the evaluation of myocardial and peripheral angiogenesis

Angiogenesis, or the formation of new microvasculature, is a physiological process that may occur in the setting of chronic tissue ischemia and can play an important role in improving tissue perfusion and blood flow following myocardial infarction or in the presence of peripheral vascular disease (PVD). Molecular imaging of angiogenesis within the cardiovascular system is a developing field of study. Targeted imaging of angiogenesis has the potential for non-invasive assessment of the underlying molecular signaling events associated with the angiogenic process and, when applied in conjunction with physiological perfusion imaging, may be utilized to predict and evaluate clinical outcomes in the setting of ischemic heart disease or PVD. This review discusses the developing radiotracer-based imaging techniques and technology currently in use that possess potential for clinical translation, with specific focus on PET and SPECT imaging of myocardial and peripheral angiogenesis.

Angiogenesis versus arteriogenesis: neuropilin 1 modulation of VEGF signaling

In development and disease, vascular endothelial growth factor (VEGF) regulates the expansion of the vascular tree. In response to hypoxia, VEGF promotes new capillary formation through the process of angiogenesis by inducing endothelial cell sprouting, proliferation, and migration. Wound healing, tissue regeneration, and tumor growth depend on angiogenesis for adequate nutrient and oxygen delivery. Under different conditions, VEGF promotes arterial growth, modulates lumen expansion, and induces collateral vessel formation, events collectively referred to as arteriogenesis. Induction of arteriogenesis after cardiac or cerebral arterial occlusion can reduce ischemia and improve disease outcome. Endothelial VEGF receptor 2 (VEGFR2) signaling governs both processes. However, modulation of downstream VEGF signaling effectors, such as extracellular-signal-regulated kinase (ERK) activation, differs in order to achieve angiogenic versus arteriogenic outcomes. Recent reports show that neuropilin 1 (NRP1), a VEGF receptor, can instill VEGF signaling outcomes that specifically regulate either angiogenesis or arteriogenesis. Here, we discuss how NRP1 functions as a VEGFR2 co-receptor in angiogenesis and a modulator of VEGFR2 trafficking in arteriogenesis. The unique role played by neuropilin in different endothelial processes makes it an exciting therapeutic target to specifically enhance angiogenesis or arteriogenesis in disease settings.

Capture of endothelial cells under flow using immobilized vascular endothelial growth factor

We demonstrate the ability of immobilized vascular endothelial growth factor (VEGF) to capture endothelial cells (EC) with high specificity under fluid flow. To this end, we engineered a surface consisting of heparin bound to poly-L-lysine to permit immobilization of VEGF through the C-terminal heparin-binding domain. The immobilized growth factor retained its biological activity as shown by proliferation of EC and prolonged activation of KDR signaling. Using a microfluidic device we assessed the ability to capture EC under a range of shear stresses from low (0.5 dyne/cm²) to physiological (15 dyne/cm²). Capture was significant for all shear stresses tested. Immobilized VEGF was highly selective for EC as evidenced by significant capture of human umbilical vein and ovine pulmonary artery EC but no capture of human dermal fibroblasts, human hair follicle derived mesenchymal stem cells, or mouse fibroblasts. Further, VEGF could capture EC from mixtures with non-EC under low and high shear conditions as well as from complex fluids like whole human blood under high shear. Our findings may have far reaching implications, as they suggest that VEGF could be used to promote endothelialization of vascular grafts or neovascularization of implanted tissues by rare but continuously circulating EC.

Inhibition of protein kinase C enhances angiogenesis induced by platelet-derived growth factor C in hyperglycemic endothelial cells

Background

Diabetes is a risk factor for the development of cardiovascular diseases with impaired angiogenesis. We have previously shown that platelet-derived growth factor C (PDGF-C) and its receptor, PDGF receptor α (PDGFR-α) were downregulated in ischemic limbs of diabetic mice, although the underlying mechanisms remained elusive. Protein kinase C (PKC) is a family of serine/threonine kinases and is known to be involved in angiogenesis. The purpose of this study is to elucidate the mechanisms of how PDGF-C/PDGFR-α axis is impaired in diabetes.

Methods

Human umbilical vein endothelial cells (HUVECs) and human cardiac microvascular endothelial cells (HMVECs) cultured in normoglycemic or hyperglycemic conditions were examined. We also examined the effects of PKC inhibition on the PDGF-C/PDGFR-α axis in endothelial cells exposed to hyperglycemia.

Results

Hyperglycemia inhibited proliferation and decreased viability of both HUVECs and HMVECs. Hyperglycemic endothelial cells exhibited decreased PDGFR-α expression both at messenger RNA (mRNA) and protein levels, while there was no significant change in expression of PDGF-C. We also found that expression of PKC-α, one of the PKC isoforms, was increased in hyperglycemic endothelial cells and that inhibition of PKC upregulated PDGFR-α expression in these cells. Phosphorylation of extracellular signal-regulated kinase (ERK) and Akt induced by PDGF-C was significantly attenuated in hyperglycemic endothelial cells, whereas inhibition of PKC effectively reversed these inhibitory effects. Moreover, inhibition of PKC also promoted angiogenesis induced by PDGF-C in hyperglycemic endothelial cells, which was not observed in vascular endothelial growth factor-A (VEGF-A)-induced angiogenesis.

Conclusions

These findings suggest that downregulation of the PDGF-C/PDGFR-α axis is involved in impaired angiogenesis of hyperglycemia through upregulation of PKC. Targeting PKC to restore PDGF-C signaling might be a novel therapeutic strategy for the treatment of vascular complications in diabetes.

Electronic supplementary material

The online version of this article (doi:10.1186/s12933-015-0180-9) contains supplementary material, which is available to authorized users.

CD146 acts as a novel receptor for netrin-1 in promoting angiogenesis and vascular development

Angiogenesis, a process that newly-formed blood vessels sprout from pre-existing ones, is vital for vertebrate development and adult homeostasis. Previous studies have demonstrated that the neuronal guidance molecule netrin-1 participates in angiogenesis and morphogenesis of the vascular system. Netrin-1 exhibits dual activities in angiogenesis: either promoting or inhibiting angiogenesis. The anti-angiogenic activity of netrin-1 is mediated by UNC5B receptor. However, how netrin-1 promotes angiogenesis remained unclear. Here we report that CD146, an endothelial transmembrane protein of the immunoglobulin superfamily, is a receptor for netrin-1. Netrin-1 binds to CD146 with high affinity, inducing endothelial cell activation and downstream signaling in a CD146-dependent manner. Conditional knockout of the cd146 gene in the murine endothelium or disruption of netrin-CD146 interaction by a specific anti-CD146 antibody blocks or reduces netrin-1-induced angiogenesis. In zebrafish embryos, downregulating either netrin-1a or CD146 results in vascular defects with striking similarity. Moreover, knocking down CD146 blocks ectopic vascular sprouting induced by netrin-1 overexpression. Together, our data uncover CD146 as a previously unknown receptor for netrin-1 and also reveal a functional ligand for CD146 in angiogenesis, demonstrating the involvement of netrin-CD146 signaling in angiogenesis during vertebrate development.

Pazopanib for the treatment of renal cell carcinoma

The incidence and mortality from renal cell cancer (RCC) is increasing. RCC tumors are particularly vascular in nature as a result of disruption of the VHL gene and/or its upstream pathway leading to upregulation of the hypoxia-inducible factor transcription factor. The hypoxia-inducible factor pathway drives angiogenesis by upregulating VEGF and bFGF, amongst other proangiogenic downstream target genes. Therapies which target angiogenesis have been successful in treating metastatic RCC (mRCC) and the receptor tyrosine kinase inhibitor, pazopanib, is licensed for first line treatment of mRCC. This review details the past, current and future roles of pazopanib in the treatment of mRCC.

Upregulated periostin promotes angiogenesis in keloids through activation of the ERK 1/2 and focal adhesion kinase pathways, as well as the upregulated expression of VEGF and angiopoietin‑1

Periostin, a secreted extracellular matrix protein, is highly expressed in wound healing and in various types of human cancer and is involved in angiogenesis. Keloids, considered dermal benign tumors, are granulomatous lesions characterized by capillary proliferation. However, the underlying regulatory mechanism of angiogenesis in keloids remains to be elucidated. The present study aimed to examine the effect of periostin on angiogenesis in keloids. The expression of periostin was upregulated and the vessel density was higher in human keloids compared with normal tissue, observed following staining with CD31 and CD105. Periostin demonstrated a markedly positive correlation with blood vessel density, which was assessed using CD31 staining (r=0.711; P<0.01) and a weak correlation was observed using CD105 staining (r=0.251; P<0.01). Conditioned medium from keloid fibroblasts (KFs) promoted the migration and tube formation of human umbilical vein endothelial cells (HUVECs) compared with normal fibroblasts and this effect may have been abrogated by the short hairpin RNA knockdown of periostin. Treatment with recombinant human periostin promoted the migration and tube formation of HUVECs by activating the extracellular signal‑regulated kinase 1/2 and focal adhesion kinase signaling pathway. In addition, periostin increased the secretion of vascular endothelial growth factor and angiopoietin‑1 in the KFs. In conclusion, these data suggested that upregulation in the level of periostin may promote angiogenesis directly and indirectly in keloids and may be a key factor in keloid development. Periostin may, therefore, be a promising therapeutic target in the treatment of keloids and other angioproliferative diseases.

KRIT1 protein depletion modifies endothelial cell behavior via increased vascular endothelial growth factor (VEGF) signaling

Disruption of endothelial cell-cell contact is a key event in many cardiovascular diseases and a characteristic of pathologically activated vascular endothelium. The CCM (cerebral cavernous malformation) family of proteins (KRIT1 (Krev-interaction trapped 1), PDCD10, and CCM2) are critical regulators of endothelial cell-cell contact and vascular homeostasis. Here we show novel regulation of vascular endothelial growth factor (VEGF) signaling in KRIT1-depleted endothelial cells. Loss of KRIT1 and PDCD10, but not CCM2, increases nuclear β-catenin signaling and up-regulates VEGF-A protein expression. In KRIT1-depleted cells, increased VEGF-A levels led to increased VEGF receptor 2 (VEGFR2) activation and subsequent alteration of cytoskeletal organization, migration, and barrier function and to in vivo endothelial permeability in KRIT1-deficient animals. VEGFR2 activation also increases β-catenin phosphorylation but is only partially responsible for KRIT1 depletion-dependent disruption of cell-cell contacts. Thus, VEGF signaling contributes to modifying endothelial function in KRIT1-deficient cells and microvessel permeability in Krit1(+/-) mice; however, VEGF signaling is likely not the only contributor to disrupted endothelial cell-cell contacts in the absence of KRIT1.

Tumor vessel normalization by chloroquine independent of autophagy

Chloroquine (CQ) has been evaluated as an autophagy blocker for cancer treatment, but it is unknown if it acts solely by inhibiting cancer cell autophagy. We report that CQ reduced tumor growth but improved the tumor milieu. By normalizing tumor vessel structure and function and increasing perfusion, CQ reduced hypoxia, cancer cell invasion, and metastasis, while improving chemotherapy delivery and response. Inhibiting autophagy in cancer cells or endothelial cells (ECs) failed to induce such effects. CQ's vessel normalization activity relied mainly on alterations of endosomal Notch1 trafficking and signaling in ECs and was abrogated by Notch1 deletion in ECs in vivo. Thus, autophagy-independent vessel normalization by CQ restrains tumor invasion and metastasis while improving chemotherapy, supporting the use of CQ for anticancer treatment.

CCN1 secretion and cleavage regulate the lung epithelial cell functions after cigarette smoke

Despite extensive research, the pathogenesis of cigarette smoking (CS)-associated emphysema remains incompletely understood, thereby impeding development of novel therapeutics, diagnostics, and biomarkers. Here, we report a novel paradigm potentially involved in the development of epithelial death and tissue loss in CS-associated emphysema. After prolonged exposure of CS, CCN1 cleavage was detected both in vitro and in vivo. Full-length CCN1 (flCCN1) was secreted in an exosome-shuttled manner, and secreted plasmin converted flCCN1 to cleaved CCN1 (cCCN1) in extracellular matrix. Interestingly, exosome-shuttled flCCN1 facilitated the interleukin (IL)-8 and vascular endothelial growth factor (VEGF) release in response to cigarette smoke extract (CSE). Therefore, flCCN1 potentially promoted CS-induced inflammation via IL-8-mediated neutrophil recruitment and also maintained the lung homeostasis via VEGF secretion. Interestingly, cCCN1 abolished these functions. Furthermore, cCCN1 promoted protease and matrix metalloproteinase (MMP)-1 production after CSE. These effects were mainly mediated by the COOH-terminal fragments of CCN1 after cleavage. Both the decrease of VEGF and the elevation of MMPs favor the development of emphysema. cCCN1, therefore, likely contributes to the epithelial cell damage after CS. Additionally, CSE and cCCN1 both stimulated integrin-α7 expressions in lung epithelial cells. The integrin-α7 appeared to be the binding receptors of cCCN1 and, subsequently, mediated its cellular function by promoting MMP1. Consistent with our observation on the functional roles of cCCN1 in vitro, elevated cCCN1 level was found in the bronchoalveolar lavage fluid from mice with emphysematous changes after 6 mo CS exposure. Taken together, we hypothesize that cCCN1 promoted the epithelial cell death and tissue loss after prolonged CS exposure.

Receptor tyrosine kinases endocytosis in endothelium: biology and signaling

Receptor tyrosine kinases are involved in regulation of key processes in endothelial biology, including proliferation, migration, and angiogenesis. It is now generally accepted that receptor tyrosine kinase signaling occurs intracellularly and on the plasma membrane, although many important details remain to be worked out. Endocytosis and subsequent intracellular trafficking spatiotemporally regulate receptor tyrosine kinase signaling, whereas signaling endosomes provide a platform for the compartmentalization of signaling events. This review summarizes recent advances in our understanding of endothelial receptor tyrosine kinase endocytosis and signaling using vascular endothelial growth factor receptor-2 as a paradigm.

Laminin promotes vascular network formation in 3D in vitro collagen scaffolds by regulating VEGF uptake

Angiogenesis is an essential neovascularisation process, which if recapitulated in 3D in vitro, will provide better understanding of endothelial cell (EC) behaviour. Various cell types and growth factors are involved, with vascular endothelial growth factor (VEGF) and its receptors VEGFR1 and VEGFR2 key components. We were able to control the aggregation pattern of ECs in 3D collagen hydrogels, by varying the matrix composition and/or having a source of cells signalling angiogenic proteins. These aggregation patterns reflect the different developmental pathways that ECs take to form different sized tubular structures. Cultures with added laminin and thus increased expression of α6 integrin showed a significant increase (p<0.05) in VEGFR2 positive ECs and increased VEGF uptake. This resulted in the end-to-end network aggregation of ECs. In cultures without laminin and therefore low α6 integrin expression, VEGFR2 levels and VEGF uptake were significantly lower (p<0.05). These ECs formed contiguous sheets, analogous to the 'wrapping' pathway in development. We have identified a key linkage between integrin expression on ECs and their uptake of VEGF, regulated by VEGFR2, resulting in different aggregation patterns in 3D.

Secretion of VEGF-165 has unique characteristics, including shedding from the plasma membrane

VEGF secretion is studied using VEGF165-GFP chimera. Efficient secretion requires Sar1- and Arf1-dependent steps and glycosylation in the Golgi. VEGF is retained in the outer surface of the plasma membrane, and shedding with other membrane components is an important step in the secretion process.

Vascular endothelial growth factor (VEGF) is a critical regulator of endothelial cell differentiation and vasculogenesis during both development and tumor vascularization. VEGF-165 is a major form that is secreted from the cells via a poorly characterized pathway. Here we use green fluorescent protein– and epitope-tagged VEGF-165 and find that its early trafficking between the endoplasmic reticulum and the Golgi requires the small GTP-binding proteins Sar1 and Arf1 and that its glycosylation in the Golgi compartment is necessary for efficient post-Golgi transport and secretion from the cells. The relative temperature insensitivity of VEGF secretion and its Sar1 and Arf1 inhibitory profiles distinguish it from other cargoes using the “constitutive” secretory pathway. Prominent features of VEGF secretion are the retention of the protein on the outer surface of the plasma membrane and the stimulation of its secretion by Ca²⁺ and protein kinase C. Of importance, shedding of VEGF-165 from the cell surface together with other membrane components appears to be a unique feature by which some VEGF is delivered to the surroundings to exert its known biological actions. Understanding VEGF trafficking can reveal additional means by which tumor vascularization can be inhibited by pharmacological interventions.

Pathogenesis of infantile haemangioma

Haemangioma is a vascular tumour of infancy that is well known for its rapid growth during the first weeks to months of a child's life, followed by a spontaneous but slow involution. During the proliferative phase, the vessels are disorganized and composed of immature endothelial cells. When the tumour involutes, the vessels mature and enlarge but are reduced in number. Fat, fibroblasts and connective tissue replace the vascular tissue, with few, large, feeding and draining vessels evident. Both angiogenesis and vasculogenesis have been proposed as mechanisms contributing to the neovascularization in haemangioma tumours. In recent years, several of the 'building blocks', the cells comprising the haemangioma, have been isolated. Among them are haemangioma progenitor/stem cells, endothelial cells and pericytes. This review focuses on these cell types, and the molecular pathways within these cells that have been implicated in driving the pathogenesis of infantile haemangioma.

The endocrine role between β cells and intra-islet endothelial cells

The islets of Langerhans is the endocrine function region of pancreas, which exist in five cell types. The majority of endocrine cells are insulin-secreting β cells, mixed up with glucagon-secreting α-cells. The islets of Langerhans are highly vascularized, and the capillary network around the islet is about five times denser than that in the exocrine tissues. It guarantees endocrine cells adequately contact with the capillary networks. Above mentioned is the basis of deep study the interaction between β cells and capillary. Increasing number of studies contribute to the consensus that endothelial cells have positive effects in the islet microenvironment. Endothelial cells can act as endocrine cells which release many active substances, such as hepatocyte growth factors (HGF), thrombospondin-1(TSP-1), laminins, and collagens by means of different molecule pathways, inducing β cells differentiation, proliferation, survivor, and insulin release next to the vessels. Apart from the effect of endothelial cells on β cells by paracrine fashion, the islets can utilize VEGF-A, angiopoietin-1 and insulin signaling to increase the interaction with endothelial cells. As the endocrine role of endothelial cells to β cells, it may be a novel target to stimulate β cells regeneration, promote vascularization post islet transplantation strategy in the treatment of diabetes mellitus.

Extracellular regulation of VEGF: isoforms, proteolysis, and vascular patterning

The regulation of vascular endothelial growth factor A (VEGF) is critical to neovascularization in numerous tissues under physiological and pathological conditions. VEGF has multiple isoforms, created by alternative splicing or proteolytic cleavage, and characterized by different receptor-binding and matrix-binding properties. These isoforms are known to give rise to a spectrum of angiogenesis patterns marked by differences in branching, which has functional implications for tissues. In this review, we detail the extensive extracellular regulation of VEGF and the ability of VEGF to dictate the vascular phenotype. We explore the role of VEGF-releasing proteases and soluble carrier molecules on VEGF activity. While proteases such as MMP9 can 'release' matrix-bound VEGF and promote angiogenesis, for example as a key step in carcinogenesis, proteases can also suppress VEGF's angiogenic effects. We explore what dictates pro- or anti-angiogenic behavior. We also seek to understand the phenomenon of VEGF gradient formation. Strong VEGF gradients are thought to be due to decreased rates of diffusion from reversible matrix binding, however theoretical studies show that this scenario cannot give rise to lasting VEGF gradients in vivo. We propose that gradients are formed through degradation of sequestered VEGF. Finally, we review how different aspects of the VEGF signal, such as its concentration, gradient, matrix-binding, and NRP1-binding can differentially affect angiogenesis. We explore how this allows VEGF to regulate the formation of vascular networks across a spectrum of high to low branching densities, and from normal to pathological angiogenesis. A better understanding of the control of angiogenesis is necessary to improve upon limitations of current angiogenic therapies.

Vascular endothelial growth factor-A signaling in bone marrow-derived endothelial progenitor cells exposed to hypoxic stress

Bone marrow-derived endothelial progenitor cells (BM-EPCs) are stimulated by vascular endothelial growth factor-A (VEGF-A) and other potent proangiogenic factors. During angiogenesis, an increase in VEGF-A expression stimulates BM-EPCs to enhance endothelial tube formation and contribute to an increase in microvessel density. Hypoxia is known to produce an enhanced angiogenic response and heightened levels of VEGF-A have been seen in oxygen deprived epithelial and endothelial cells, yet the pathways for VEGF-A signaling in BM-EPCs have not been described. This study explores the influence of hypoxia on VEGF-A signaling in rat BM-EPCs utilizing a novel proteomic strategy to directly identify interacting downstream components of the combined VEGF receptor(s) signaling pathways, gene expression analysis, and functional phenotyping. VEGF-A signaling network analysis following liquid chromatographic separation and tandem mass spectrometry revealed proteins related to inositol/calcium signaling, nitric oxide signaling, cell survival, cell migration, and inflammatory responses. Alterations in BM-EPC expression of common angiogenic genes and tube formation in response to VEGF-A during hypoxia were measured and combined with the proteomic analysis to enhance and support the signaling pathways detected. BM-EPC tube formation assays in response to VEGF-A exhibited little tube formation; however, a cell projection/migratory phenotype supported the signaling data. Additionally, a novel assay measuring BM-EPC incorporation into preformed endothelial cell tubes indicated a significant increase of incorporated BM-EPCs after pretreatment with VEGF-A during hypoxia. This study verifies known VEGF-A pathway components and reveals several unidentified mechanisms of VEGF-A signaling in BM-EPCs during hypoxia that may be important for migration to sites of vascular regeneration.

Endothelial cell-dependent regulation of arteriogenesis

RATIONALE

Arteriogenesis is the process of formation of arterial conduits. Its promotion is an attractive therapeutic strategy in occlusive atherosclerotic diseases. Despite the functional and clinical importance of arteriogenesis, the biology of the process is poorly understood. Synectin, a gene previously implicated in the regulation of vascular endothelial cell growth factor signaling, offers a unique opportunity to determine relative contributions of various cell types to arteriogenesis.

OBJECTIVE

We investigated the cell-autonomous effects of a synectin knockout in arterial morphogenesis.

METHODS AND RESULTS

A floxed synectin knockin mouse line was crossbred with endothelial-specific (Tie2, Cdh5, Pdgfb) and smooth muscle myosin heavy chain-specific Cre driver mouse lines to produce cell type-specific deletions. Ablation of synectin expression in endothelial, but not smooth muscle cells resulted in the presence of developmental arterial morphogenetic defects (smaller size of the arterial tree, reduced number of arterial branches and collaterals) and impaired arteriogenesis in adult mice.

CONCLUSIONS

Synectin modulates developmental and adult arteriogenesis in an endothelial cell-autonomous fashion. These findings show for the first time that endothelial cells are central to both developmental and adult arteriogenesis and provide a model for future studies of factors involved in this process.

Impact of the phosphatidylinositide 3-kinase signaling pathway on the cardioprotection induced by intermittent hypoxia

BACKGROUND

Exposure to intermittent hypoxia (IH) may enhance cardiac function and protects heart against ischemia-reperfusion (I/R) injury. To elucidate the underlying mechanisms, we developed a cardioprotective IH model that was characterized at hemodynamic, biochemical and molecular levels.

METHODS

Mice were exposed to 4 daily IH cycles (each composed of 2-min at 6-8% O2 followed by 3-min reoxygenation for 5 times) for 14 days, with normoxic mice as controls. Mice were then anesthetized and subdivided in various subgroups for analysis of contractility (pressure-volume loop), morphology, biochemistry or resistance to I/R (30-min occlusion of the left anterior descending coronary artery (LAD) followed by reperfusion and measurement of the area at risk and infarct size). In some mice, the phosphatidylinositide 3-kinase (PI3K) inhibitor wortmannin was administered (24 µg/kg ip) 15 min before LAD.

RESULTS

We found that IH did not induce myocardial hypertrophy; rather both contractility and cardiac function improved with greater number of capillaries per unit volume and greater expression of VEGF-R2, but not of VEGF. Besides increasing the phosphorylation of protein kinase B (Akt) and the endothelial isoform of NO synthase with respect to control, IH reduced the infarct size and post-LAD proteins carbonylation, index of oxidative damage. Administration of wortmannin reduced the level of Akt phosphorylation and worsened the infarct size.

CONCLUSION

We conclude that the PI3K/Akt pathway is crucial for IH-induced cardioprotection and may represent a viable target to reduce myocardial I/R injury.

Intersectin-1s: an important regulator of cellular and molecular pathways in lung injury

Abstract Acute lung injury (ALI) and acute respiratory distress syndrome (ARDS) are severe syndromes resulting from the diffuse damage of the pulmonary parenchyma. ALI and ARDS are induced by a plethora of local or systemic insults, leading to the activation of multiple pathways responsible for injury, resolution, and repair or scarring of the lungs. Despite the large efforts aimed at exploring the roles of different pathways in humans and animal models and the great strides made in understanding the pathogenesis of ALI/ARDS, the only viable treatment options are still dependent on ventilator and cardiovascular support. Investigation of the pathophysiological mechanisms responsible for initiation and resolution or advancement toward lung scarring in ALI/ARDS animal models led to a better understanding of the disease's complexity and helped in elucidating the links between ALI and systemic multiorgan failure. Although animal models of ALI/ARDS have pointed out a variety of new ideas for study, there are still limited data regarding the initiating factors, the critical steps in the progression of the disease, and the central mechanisms dictating its resolution or progression to lung scarring. Recent studies link deficiency of intersectin-1s (ITSN-1s), a prosurvival protein of lung endothelial cells, to endothelial barrier dysfunction and pulmonary edema as well as to the repair/recovery from ALI. This review discusses the effects of ITSN-1s deficiency on pulmonary endothelium and its significance in the pathology of ALI/ARDS.

Computational model of VEGFR2 pathway to ERK activation and modulation through receptor trafficking

Vascular Endothelial Growth Factor (VEGF) signal transduction is central to angiogenesis in development and in pathological conditions such as cancer, retinopathy and ischemic diseases. We constructed and validated a computational model of VEGFR2 trafficking and signaling, to study the role of receptor trafficking kinetics in modulating ERK phosphorylation in VEGF-stimulated endothelial cells. Trafficking parameters were optimized and validated against four previously published in vitro experiments. Based on these parameters, model simulations demonstrated interesting behaviors that may be highly relevant to understanding VEGF signaling in endothelial cells. First, at moderate VEGF doses, VEGFR2 phosphorylation and ERK phosphorylation are related in a log-linear fashion, with a stable duration of ERK activation; but with higher VEGF stimulation, phosphoERK becomes saturated, and its duration increases. Second, a large endosomal fraction of VEGFR2 makes the ERK activation reaction network less sensitive to perturbations in VEGF dosage. Third, extracellular-matrix-bound VEGF binds and activates VEGFR2, but by internalizing at a slower rate, matrix-bound VEGF-induced intracellular ERK phosphorylation is predicted to be greater in magnitude and more sustained, in agreement with experimental evidence. Fourth, different endothelial cell types appear to have different trafficking rates, which result in different levels of endosomal receptor localization and different ERK response profiles.

Recent molecular discoveries in angiogenesis and antiangiogenic therapies in cancer

Four decades ago, angiogenesis was recognized as a therapeutic target for blocking cancer growth. Because of its importance, VEGF has been at the center stage of antiangiogenic therapy. Now, several years after FDA approval of an anti-VEGF antibody as the first antiangiogenic agent, many patients with cancer and ocular neovascularization have benefited from VEGF-targeted therapy; however, this anticancer strategy is challenged by insufficient efficacy, intrinsic refractoriness, and resistance. Here, we examine recent discoveries of new mechanisms underlying angiogenesis, discuss successes and challenges of current antiangiogenic therapy, and highlight emerging antiangiogenic paradigms.

Quantitative mass spectrometry-based proteomics in angiogenesis

The process of new blood vessel formation from pre-existing ones is called angiogenesis. Beyond playing a critical role in the physiological development of the vascular system, angiogenesis is a well-recognised hallmark of cancer. Unbiased system-wide approaches are required to complement the current knowledge, and intimately understand the molecular mechanisms regulating this process in physiological and pathological conditions. In this review we describe the cellular and molecular dynamics regulating the physiological growth of vessels and their deregulation in cancer, survey in vitro and in vivo models currently exploited to investigate various aspects of angiogenesis and describe state-of-the-art and most widespread methods and technologies in MS shotgun proteomics. Finally, we focus on current applications of MS to better understand endothelial cell behaviour and propose how modern proteomics can impact on angiogenesis research.

Platelet-derived growth factor C promotes revascularization in ischemic limbs of diabetic mice

BACKGROUND

Platelet-derived growth factor C (PDGF-C) has been reported to promote angiogenesis independently of vascular endothelial growth factor (VEGF), although its significance in postnatal angiogenesis in vivo remains poorly understood. VEGF has been employed as a major molecular tool to induce therapeutic angiogenesis. However, VEGF therapy is not very effective in models of cardiovascular diseases associated with diabetes, and the mechanisms of this phenomenon still remain to be elucidated.

METHODS

We used a murine model of hind limb ischemia and of streptozotocin-induced diabetes.

RESULTS

Expression of PDGF-C and its receptor PDGFR-α were markedly upregulated in ischemic limbs. Treatment with a neutralizing antibody against PDGF-C significantly impaired blood flow recovery and neovascularization after ischemia almost to the same extent as a VEGF-neutralizing antibody. Mice deficient in PDGF-C exhibited reduced blood flow recovery after ischemia compared with wild-type mice, confirming a strong proangiogenic activity of PDGF-C. Next, we injected an expression vector encoding PDGF-C into ischemic limbs. Blood flow recovery and neovascularization after ischemia were significantly improved in the groups treated with PDGF-C compared with controls. Attenuation of angiogenic responses to ischemia has been reported in patients with diabetes even after VEGF treatment, although a precise mechanism remains unknown. We hypothesized that PDGF-C might relate to the impaired angiogenesis of diabetes. We tested this hypothesis by inducing diabetes by intraperitoneal injection of streptozotocin. Expression levels of PDGF-C at baseline and after ischemia were significantly lower in limb tissues of diabetic mice than in those of control mice, whereas expression levels of other members of the PDGF family and VEGF were not changed or were even higher in diabetic mice. Introduction of VEGF complementary DNA expression plasmid vector into ischemic limbs did not improve blood flow recovery. However, these changes were effectively reversed by additional introduction of the PDGF-C complementary DNA plasmid vector.

CONCLUSIONS

These results indicate that downregulation of PDGF-C expression in limb tissues of diabetic mice contributes to impaired angiogenesis and suggest that introduction of PDGF-C might be a novel strategy for therapeutic angiogenesis, especially in the diabetic state.

CLINICAL RELEVANCE

Angiogenesis and arteriogenesis after ischemia are attenuated in most diabetic patients, although the precise mechanisms remain unclear. Platelet-derived growth factors (PDGFs) have a variety of functions on many cell types, and PDGF-C stimulates angiogenesis and revascularizes ischemic tissues. This study indicates the role for PDGF-C as a critical regulator of impaired angiogenesis of diabetes and suggests that PDGF-C might be a novel target for the treatment of ischemic cardiovascular diseases in diabetes.

The neuropilin 1 cytoplasmic domain is required for VEGF-A-dependent arteriogenesis

Neuropilin 1 (NRP1) plays an important but ill-defined role in VEGF-A signaling and vascular morphogenesis. We show that mice with a knockin mutation that ablates the NRP1 cytoplasmic tail (Nrp1^cyto) have normal angiogenesis but impaired developmental and adult arteriogenesis. The arteriogenic defect was traced to the absence of a PDZ-dependent interaction between NRP1 and VEGF receptor 2 (VEGFR2) complex and synectin, which delayed trafficking of endocytosed VEGFR2 from Rab5+ to EAA1+ endosomes. This led to increased PTPN1 (PTP1b)-mediated dephosphorylation of VEGFR2 at Y¹¹⁷⁵, the site involved in activating ERK signaling. The Nrp1^cyto mutation also impaired endothelial tubulogenesis in vitro, which could be rescued by expressing full-length NRP1 or constitutively active ERK. These results demonstrate that the NRP1 cytoplasmic domain promotes VEGFR2 trafficking in a PDZ-dependent manner to regulate arteriogenic ERK signaling and establish a role for NRP1 in VEGF-A signaling during vascular morphogenesis.

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The NRP1 cytoplasmic domain promotes VEGF receptor (VEGFR) 2 endocytic trafficking

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In its absence, VEGR2 trafficking is delayed in sorting endosomes

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PTP1b binds to Rab5+ sorting endosomes and dephosphorylates the Y¹¹⁷⁵ site of VEGFR2

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Loss of the NRP1 cytoplasmic domain impairs developmental and adult arteriogenesis

Effects of VEGF-loaded chitosan coatings

Vascular endothelial growth factor (VEGF) is a powerful growth factor that promotes vascularization as well as osteoblastic differentiation and bone regeneration, all of which are key processes in the osseointegration of dental implants. Strategies to increase vascularization through delivery of VEGF may improve osseointegration, especially in patients with reduced bone healing potential. The aim of this study was to determine the potential of chitosan coatings on titanium to deliver VEGF and to support growth and matrix production of osteoblastic cells in vitro. Chitosan was chemically bonded to titanium coupons via silane-glutaraldehyde linker molecules and loaded with 0, 20, 50, or 100 ng of VEGF. Protein was released during a three day period with around 75% of VEGF (4.44, 11.37, and 22.10 ng/mL/cm(2) from the 20, 50, and 100 ng loaded levels, respectively) released during the first 12 h, and 90-95% of the VEGF released from the coatings by day 3. Saos-2 bone cells continued to proliferate over the 28-day period on the VEGF-loaded chitosan coatings in contrast to cells seeded on uncoated titanium, which plateaued after 14 days. Cells on uncoated titanium exhibited a peak in alkaline phosphatase expression at approximately 14 days, concomitant with the plateau in growth. While osteoblast-like cells on all chitosan coatings exhibited up to a 2-fold enhancement of the alkaline phosphatase activity and 10-fold increase in calcium deposition compared to uncoated controls, the incorporation of VEGF into the coatings did not enhance osteoblast matrix production over plain chitosan coatings throughout this study.

Control of angiogenesis by AIBP-mediated cholesterol efflux

Cholesterol is a structural component of the cell, indispensable for normal cellular function, but its excess often leads to abnormal proliferation, migration, inflammatory responses and/or cell death. To prevent cholesterol overload, ATP-binding cassette (ABC) transporters mediate cholesterol efflux from the cells to apolipoprotein A-I (ApoA-I) and to the ApoA-I-containing high-density lipoprotein (HDL)^1-3. Maintaining efficient cholesterol efflux is essential for normal cellular function^4-6. However, the role of cholesterol efflux in angiogenesis and the identity of its local regulators are poorly understood. Here we show that ApoA-I binding protein (AIBP) accelerates cholesterol efflux from endothelial cells (EC) to HDL and thereby regulates angiogenesis. AIBP/HDL-mediated cholesterol depletion reduces lipid rafts, interferes with VEGFR2 dimerization and signaling, and inhibits VEGF-induced angiogenesis in vitro and mouse aortic neovascularization ex vivo. Remarkably, Aibp regulates the membrane lipid order in embryonic zebrafish vasculature and functions as a non-cell autonomous regulator of zebrafish angiogenesis. Aibp knockdown results in dysregulated sprouting/branching angiogenesis, while forced Aibp expression inhibits angiogenesis. Dysregulated angiogenesis is phenocopied in Abca1/Abcg1-deficient embryos, and cholesterol levels are increased in Aibp-deficient and Abca1/Abcg1-deficient embryos. Our findings demonstrate that secreted AIBP positively regulates cholesterol efflux from EC and that effective cholesterol efflux is critical for proper angiogenesis.

Substrate-specific conformational regulation of the receptor tyrosine kinase VEGFR2 catalytic domain

The contributions of the phosphoacceptor and the catalytic domain context to protein kinase biology and inhibitor potency are routinely overlooked, which can lead to mischaracterization of inhibitor and receptor functions. The receptor tyrosine kinase vascular endothelial growth factor receptor-2 (VEGFR2) is studied as a model system using a series of phosphoacceptor substrates (k(cat)/K(m) 684-116,000 M(-1) s(-1)) to assess effects on catalysis and inhibitor binding. ATP-competitive inhibitor potency toward the VEGFR2 catalytic domain (VEGFR2-CD) varies with different phosphoacceptor substrates, which is unexpected because the phosphoacceptors do not affect K(m,ATP) values. Indazole-based inhibitors are up to 60-fold more potent with two substrates (gastrin, minigastrin) relative to the others. Thus there is a component of uncompetitive inhibition because a specific phosphoacceptor enhances potency but is not strictly required. This substrate-specific inhibitory potency enhancement correlates with phosphoacceptor active site saturation and is not observed with other related kinases. The effect is confined to a specific catalytic domain conformation because autophosphorylation eliminates the potency enhancement as does the addition of the juxtamembrane domain (20 amino acids). Indazole inhibitor structure-activity analysis reveals that the magnitude of potency enhancement correlates with the size of the substituent that binds in a regulatory region of the active site. VEGFR drugs profiled with VEGFR2-CD using minigastrin have potency well-correlated with inhibition of full-length, cellular VEGFR2 autophosphorylation, an indication that the minigastrin-induced conformation is biologically relevant. These findings raise the possibility that inhibitors directed toward a common target can have different biological effects based on the kinase-substrate complexes present in different cellular contexts.

Targeted treatments for metastatic esophageal squamous cell cancer

Squamous cell carcinoma, one of the two major sub-types of esophageal carcinomas, constitutes the great majority of tumors in the upper and middle third of the organ. Declining in incidence in western countries, it continues to be a significant public health problem in the far east. Targeted treatments are novel therapies introduced in the clinical therapeutic armamentarium of oncology in the last 10-15 years. They represent a rational way of treating various cancers based on their molecular lesions. Although no such agent has been approved so far for the treatment of esophageal squamous cell carcinomas (ESCC), several are in clinical trials and several others have displayed pre-clinical activity that would justify the efforts and risks of pursuing their clinical development in this disease. This paper discusses some of these targeted agents in more advanced development in metastatic ESCC, as well as some promising drugs with pre-clinical or initial clinical data in the disease.

The role of endosomal signaling triggered by metastatic growth factors in tumor progression

Within tumor microenvironment, a lot of growth factors such as hepatocyte growth factor and epidermal growth factor may induce similar signal cascade downstream of receptor tyrosine kinase (RTK) and trigger tumor metastasis synergistically. In the past decades, the intimate relationship of RTK-mediated receptor endocytosis with signal transduction was well established. In general, most RTK undergoes clathrin-dependent endocytosis and/or clathrin-independent endocytosis. The internalized receptors may sustain the signaling within early endosome, recycling to plasma membrane for subsequent ligand engagement or sorting to late endosomes/lysosome for receptor degradation. Moreover, receptor endocytosis influences signal transduction in a temporal and spatial manner for periodical and polarized cellular processes such as cell migration. The endosomal signalings triggered by various metastatic factors are quite similar in some critical points, which are essential for triggering cell migration and tumor progression. There are common regulators for receptor endocytosis including dynamin, Rab4, Rab5, Rab11 and Cbl. Moreover, many critical regulators within the RTK signal pathway such as Grb2, p38, PKC and Src were also modulators of endocytosis. In the future, these may constitute a new category of targets for prevention of tumor metastasis.

Effects of hypoxia-inducible factor-1α silencing on the proliferation of CBRH-7919 hepatoma cells

AIM: To study the effects of hypoxia-inducible factor-1α (HIF-1α) silencing on the proliferation of hypoxic CBRH-7919 rat hepatoma cells.

METHODS: The CBRH-7919 rat hepatoma cell line was used in this study and the hypoxic model was constructed using CoCl₂. The HIF-1α-specific RNAi sequences were designed according to the gene coding sequence of rat HIF-1α obtained from GeneBank. The secondary structure of the HIF-1α gene sequence was analyzed using RNA draw software. The small interfering RNA (siRNA) transfection mixture was produced by mixing the siRNA and Lipofectamine2000^TM, and transfected into the hypoxic hepatoma cells. Real time reverse transcription-polymerase chain reaction (RT-PCR) and Western blotting assay were used to detect the expression levels of mRNA and protein. HIF-1α and vascular endothelial growth factor (VEGF) mRNA was determined using real time RT-PCR; the protein expression levels of AKT, p-AKT, p21 and cyclinD1 were determined using Western blotting. The proliferation of hepatoma cells was observed using the methyl thiazolyl tetrazolium (MTT) assay and the bromodeoxyuridine (BrdU) incorporation cell proliferation assay.

RESULTS: Under induced hypoxia, the viability of the hepatoma cells reached a minimum at 800 μmol/L CoCl₂; the viability of the cells was relatively high at CoCl₂ concentrations between 100 μmol/L and 200 μmol/L. Under hypoxia, the mRNA and protein expression levels of HIF-1α and VEGF were significantly higher than that of hepatoma cells that were cultured in normaxia. HIF-1α-specific RNAi sequences were successfully transfected into hepatoma cells. The transfection of specific siRNAs significantly inhibited the mRNA and protein expression levels of HIF-1α and VEGF, along with the protein expression levels of p-AKT and cyclinD1; the protein expression of p21 was significantly increased, and there was no significant difference in the expression of AKT. The MTT assay showed that the amount of hepatoma cells in S phase in the siRNA transfection group was obviously smaller than that in the control group; in the siRNA transfection group, the amount of hepatoma cells in G1 phase was more than that in the control group. The BrdU incorporation assay showed that the number of BrdU positive hepatoma cells in the siRNA transfection group was less than that in the control group. The data of the MTT assay and BrdU incorporation assay suggested that HIF-1α silencing using siRNAs significantly inhibited the proliferation of hepatoma cells.

CONCLUSION: Hypoxia increases the expression of HIF-1α, and HIF-1α silencing significantly inhibits the proliferation of hypoxic CBRH-7919 rat hepatoma cells.

EphrinB2 reverse signaling protects against capillary rarefaction and fibrosis after kidney injury

Microvascular disease, a characteristic of acute and chronic kidney diseases, leads to rarefaction of peritubular capillaries (PTCs), promoting secondary ischemic injury, which may be central to disease progression. Bidirectional signaling by EphB4 receptor and ephrinB2 ligand is critical for angiogenesis during murine development, suggesting that ephrinB2 reverse signaling may have a role in renal angiogenesis induced by injury or fibrosis. Here, we found that ephrinB2 reverse signaling is activated in the kidney only after injury. In mice lacking the PDZ intracellular signaling domain of ephrinB2 (ephrinB2 ΔV), angiogenesis was impaired and kidney injury led to increased PTC rarefaction and fibrosis. EphrinB2 ΔV primary kidney pericytes migrated more than wild-type pericytes and were less able to stabilize capillary tubes in three-dimensional culture and less able to stimulate synthesis of capillary basement membrane. EphrinB2 ΔV primary kidney microvascular endothelial cells migrated and proliferated less than wild-type microvascular endothelial cells in response to vascular endothelial growth factor A and showed less internalization and activation of vascular endothelial growth factor receptor-2. Taken together, these results suggest that PDZ domain-dependent ephrinB2 reverse signaling protects against PTC rarefaction by regulating angiogenesis and vascular stability during kidney injury. Furthermore, this signaling in kidney pericytes protects against pericyte-to-myofibroblast transition and myofibroblast activation, thereby limiting fibrogenesis.

Interactions between VEGFR and Notch signaling pathways in endothelial and neural cells

Notch cell interaction mechanism governs cell fate decisions in many different cell contexts throughout the lifetime of all Metazoan species. It links the fate of one cell to that of its neighbors through cell-to-cell contacts, and binding of Notch receptors expressed on one cell to their membrane bound ligands on an adjacent cell. Environmental cues, such as growth factors and extracellular matrix molecules, superimpose a dynamic regulation on this canonical Notch signaling pathway. In this review, we will focus on Notch signaling in the vertebrate vascular and nervous systems and examine its role in angiogenesis, neurogenesis, and neurovascular interactions. We will also highlight the molecular relationships of the Notch pathway with vascular endothelial growth factors (VEGFs) and their high-affinity tyrosine kinase VEGF receptors, key regulators of both angiogenesis and neurogenesis.

Endothelial cell-specific chemotaxis receptor (ECSCR) enhances vascular endothelial growth factor (VEGF) receptor-2/kinase insert domain receptor (KDR) activation and promotes proteolysis of internalized KDR

The endothelial cell-specific chemotaxis receptor (ECSCR) is a cell-surface protein selectively expressed by endothelial cells (ECs), with roles in EC migration, apoptosis and proliferation. Our previous study (Verma, A., Bhattacharya, R., Remadevi, I., Li, K., Pramanik, K., Samant, G. V., Horswill, M., Chun, C. Z., Zhao, B., Wang, E., Miao, R. Q., Mukhopadhyay, D., Ramchandran, R., and Wilkinson, G. A. (2010) Blood 115, 4614-4622) showed that loss of ECSCR in primary ECs reduced tyrosine phosphorylation of vascular endothelial growth factor (VEGF) receptor 2/kinase insert domain receptor (KDR) but not VEGF receptor 1/FLT1. Here, we show that ECSCR biochemically associates with KDR but not FLT1 and that the predicted ECSCR cytoplasmic and transmembrane regions can each confer association with KDR. Stimulation with VEGF165 rapidly and transiently increases ECSCR-KDR complex formation, a process blocked by the KDR tyrosine kinase inhibitor compound SU5416 or inhibitors of endosomal acidification. Triple labeling experiments show VEGF-stimulated KDR(+)/ECSCR(+) intracellular co-localization. Silencing of ECSCR disrupts VEGF-induced KDR activation and AKT and ERK phosphorylation and impairs VEGF-stimulated KDR degradation. In zebrafish, ecscr interacts with kdrl during intersomitic vessel sprouting. Human placenta and infantile hemangioma samples highly express ECSCR protein, suggesting a role for ECSCR-KDR interaction in these tissues.

Localization and signaling patterns of vascular endothelial growth factors and receptors in human periapical lesions

INTRODUCTION

Vascular endothelial growth factors (VEGFs) and their receptors (VEGFRs) are key players in vasculogenesis and are also involved in pathologic conditions with bone destruction. Vasculogenesis is critical for disease progression, and bone resorption is a hallmark of apical periodontitis. However, the localization of VEGFs and VEGFRs and their gene signaling pathways in human apical periodontitis have not been thoroughly investigated. The aim of this study was to localize VEGFs and VEGFRs and analyze their gene expression as well as signaling pathways in human periapical lesions.

METHODS

Tissue was collected after endodontic surgery from patients diagnosed with chronic apical periodontitis. Periodontal ligament samples from extracted healthy wisdom teeth was also collected and used as control tissue. In lesion cryosections, VEGFs/VEGFRs were identified by immunohistochemistry/double immunofluorescence by using specific antibodies. A human VEGF signaling polymerase chain reaction array system was used for gene expression analysis comparing lesions with periodontal ligament samples.

RESULTS

The histologic evaluation revealed heterogeneous morphology of the periapical lesions with various degrees of inflammatory infiltrates. In the lesions, all investigated factors and receptors were identified in blood vessels and various immune cells. No lymphatic vessels were detected. Gene expression analysis revealed up-regulation of VEGF-A and VEGFR-3, although not significant. Phosphatidylinositol-3-kinases, protein kinase C, mitogen-activated protein kinases, and phospholipases, all known to be involved in VEGF-mediated angiogenic activity, were significantly up-regulated.

CONCLUSIONS

The cellular and vascular expressions of VEGFs and VEGFRs in chronic apical periodontitis, along with significant alterations of genes mediating VEGF-induced angiogenic responses, suggest ongoing vascular remodeling in established chronic periapical lesions.

Essential role for endocytosis in the growth factor-stimulated activation of ERK1/2 in endothelial cells

Vascular endothelial growth factor (VEGF) stimulates angiogenesis by binding to VEGF receptor 2 (VEGFR2) on endothelial cells (ECs). Downstream activation of the extracellular related kinases 1/2 (ERK1/2) is important for angiogenesis to proceed. Receptor internalization has been implicated in VEGFR2 signaling, but its role in the activation of ERK1/2 is unclear. To explore this question we utilized pitstop and dynasore, two small molecule inhibitors of endocytosis. First, we confirmed that both inhibitors block the internalization of VEGFR2 in ECs. We then stimulated ECs with VEGF in the presence and absence of the inhibitors and examined VEGFR2 signaling to ERK1/2. Activation of VEGFR2 and C-Raf still occurred in the presence of the inhibitors, whereas the activation of MEK1/2 and ERK1/2 was abrogated. Therefore, although internalization is not required for activation of either VEGFR2 or C-Raf in ECs stimulated with VEGF, internalization is necessary to activate the more distal kinases in the cascade. Importantly, inhibition of internalization also prevented activation of ERK1/2 when ECs were stimulated with other pro-angiogenic growth factors, namely fibroblast growth factor 2 and hepatocyte growth factor. In contrast, the same inhibitors did not block ERK1/2 activation in fibroblasts or cancer cells stimulated with growth factors. Finally, we show that these small molecule inhibitors of endocytosis block angiogenesis in vitro and in vivo. Therefore, receptor internalization may be a generic requirement for pro-angiogenic growth factors to activate ERK1/2 signaling in human ECs, and targeting receptor trafficking may present a therapeutic opportunity to block tumor angiogenesis.

Rhodocetin-αβ-induced neuropilin-1-cMet association triggers restructuring of matrix contacts in endothelial cells

OBJECTIVE

The snake venom component rhodocetin-αβ (RCαβ) stimulates endothelial cell motility in an α2β1 integrin-independent manner. We aimed to elucidate its cellular and molecular mechanisms.

METHODS AND RESULTS

We identified neuropilin-1 (Nrp1) as a novel target of RCαβ by protein-chemical methods. RCαβ and vascular endothelial growth factor (VEGF)-A avidly bind to Nrp1. Instead of acting as VEGF receptor 2 coreceptor, Nrp1 associates upon RCαβ treatment with cMet. Furthermore, cell-based ELISAs and kinase inhibitor studies showed that RCαβ induces phosphorylation of tyrosines 1234/1235 [corrected] and thus activation of cMet. Consequently, paxillin is phosphorylated at Y31, which is redistributed from streak-like focal adhesions to spot-like focal contacts at the cell perimeter, along with α2β1 integrin, thereby regulating cell-matrix interactions. Cortactin is abundant in the cell perimeter, where it is involved in the branching of the cortical actin network of lamellipodia, whereas tensile force-bearing actin stress fibers radiating from focal adhesions disappear together with zyxin, a focal adhesion marker, on RCαβ treatment.

CONCLUSIONS

Our data demonstrate that (1) Nrp1 is a novel target for venom components, such as RCαβ; (2) Nrp1 coupled to cMet regulates the type of cell-matrix interactions in a manner involving paxillin phosphorylation; and (3) altered cell-matrix interactions determine endothelial cell migration and cellular force management.

An inside view: VEGF receptor trafficking and signaling

Vascular endothelial growth factors (VEGF) and their receptors play a central role in the development of cardiovascular system and in vasculature-related processes in the adult organism. Given the critical role of this signaling cascade, intricate control systems have evolved to regulate its function. A new layer of added complexity has been the demonstration of the importance of endocytosis and intracellular trafficking of VEGF receptors in the regulation of VEGF signaling. In this review, we consider an evolving link between VEGF receptor endocytosis, trafficking, and signaling and their biological function.

Robustness in angiogenesis: notch and BMP shaping waves

Vascular patterning involves sprouting of blood vessels, which is governed by orchestrated communication between cells in the surrounding tissue and endothelial cells (ECs) lining the blood vessels. Single ECs are selected for sprouting by hypoxia-induced stimuli and become the 'tip' or leader cell that guides new sprouts. The 'stalk' or trailing ECs proliferate for tube extension and lumenize the nascent vessel. Stalk and tip cells can dynamically switch their identities during this process in a Notch-dependent manner. Here, we review recent studies showing that bone morphogenetic protein (BMP) signaling coregulates Notch target genes in ECs. In particular, we focus on how Delta-like ligand 4 (DLL4)-Notch and BMP effector interplay may drive nonsynchronized oscillatory gene expression in ECs essential for setting sharp tip-stalk cell boundaries while sustaining a dynamic pool of nonsprouting ECs. Deeper knowledge about the coregulation of vessel plasticity in different vascular beds may result in refinement of anti-angiogenesis and vessel normalization therapies.

Targeting VEGF signalling via the neuropilin co-receptor

The blockade of tumour vascularisation and angiogenesis continues to be a focus for drug development in oncology and other pathologies. Historically, targeting vascular endothelial growth factor (VEGF) activity and its association with VEGF receptors (VEGFRs) has represented the most promising line of attack. More recently, the recognition that VEGFR co-receptors, neuropilin-1 and -2 (NRP1 and NRP2), are also engaged by specific VEGF isoforms in tandem with the VEGFRs has expanded the landscape for the development of modulators of VEGF-dependent signalling. Here, we review the recent structural characterisation of VEGF interactions with NRP subdomains and the impact this has had on drug development activity in this area.

Dorsal aorta formation: separate origins, lateral-to-medial migration, and remodeling

Blood vessel formation is a highly dynamic tissue-remodeling event that can be observed from early development in vertebrate embryos. Dorsal aortae, the first functional intra-embryonic blood vessels, arise as two separate bilateral vessels in the trunk and undergo lateral-to-medial translocation, eventually fusing into a single large vessel at the midline. After this dramatic remodeling, the dorsal aorta generates hematopoietic stem cells. The dorsal aorta is a good model to use to increase our understanding of the mechanisms controlling the establishment and remodeling of larger blood vessels in vivo. Because of the easy accessibility to the developing circulatory system, quail and chick embryos have been widely used for studies on blood vessel formation. In particular, the mapping of endothelial cell origins has been performed using quail-chick chimera analysis, revealing endothelial, vascular smooth muscle, and hematopoietic cell progenitors of the dorsal aorta. The avian embryo model also allows conditional gene activation/inactivation and direct observation of cell behaviors during dorsal aorta formation. This allows a better understanding of the molecular mechanisms underlying specific morphogenetic events during dynamic dorsal aorta formation from a cell behavior perspective.

Characterization and optimization of vascular endothelial growth factor(165) (rhVEGF(165)) expression in Escherichia coli

Vascular endothelial growth factors(165) (VEGF(165)) is the most potent and widely used pro-angiogenic factor. Here we determined optimal culture condition of recombinant human VEGF(165) (rhVEGF(165)) in Escherichia coli (E. coli). rhVEGF(165) expression was the highest in 0.25% of L-arabinose induction concentration, at 20 °C induction temperature, and for 5 h induction time under the control of araBAD promoter using pBADHisA vector. In biological activity test, rhVEGF(165) significantly increased the proliferative activity of CPAE cells (p<0.001) and upregulated the expressions of endothelial cell growth-related genes, such as platelet endothelial cell adhesion molecule (PECAM-1), endothelial-specific receptor tyrosine kinase (TEK), kinase insert domain protein receptor (KDR), and tyrosine kinase with immunoglobulin-like and EGF-like domains 1 (TIE1) in calf pulmonary artery endothelial (CPAE) cells.

Endothelial nuclear factor-κB-dependent regulation of arteriogenesis and branching

BACKGROUND

Arteriogenesis and collateral formation are complex processes requiring integration of multiple inputs to coordinate vessel branching, growth, maturation, and network size. Factors regulating these processes have not been determined.

METHODS AND RESULTS

We used an inhibitor of NFκB activation (IκBαSR) under control of an endothelial-specific inducible promoter to selectively suppress endothelial nuclear factor-κB activation during development, in the adult vasculature, or in vitro. Inhibition of nuclear factor-κB activation resulted in formation of an excessively branched arterial network that was composed of immature vessels and provided poor distal tissue perfusion. Molecular analysis demonstrated reduced adhesion molecule expression leading to decreased monocyte influx, reduced hypoxia-inducible factor-1α levels, and a marked decrease in δ-like ligand 4 expression with a consequent decrease in Notch signaling. The latter was the principal cause of increased vascular branching as treatment with Jagged-1 peptide reduced the size of the arterial network to baseline levels.

CONCLUSIONS

These findings identify nuclear factor-κB as a key regulator of adult and developmental arteriogenesis and collateral formation. Nuclear factor-κB achieves this by regulating hypoxia-inducible factor-1α-dependent expression of vascular endothelial growth factor-A and platelet-derived growth factor-BB, which are necessary for the development and maturation of the arterial collateral network, and by regulating δ-like ligand 4 expression, which in turn determines the size and complexity of the network.

Endothelial differentiation of mesenchymal stromal cells

Human mesenchymal stromal cells (hMSCs) are increasingly used in regenerative medicine for restoring worn-out or damaged tissue. Newly engineered tissues need to be properly vascularized and current candidates for in vitro tissue pre-vascularization are endothelial cells and endothelial progenitor cells. However, their use in therapy is hampered by their limited expansion capacity and lack of autologous sources. Our approach to engineering large grafts is to use hMSCs both as a source of cells for regeneration of targeted tissue and at the same time as the source of endothelial cells. Here we investigate how different stimuli influence endothelial differentiation of hMSCs. Although growth supplements together with shear force were not sufficient to differentiate hMSCs with respect to expression of endothelial markers such as CD31 and KDR, these conditions did prime the cells to differentiate into cells with an endothelial gene expression profile and morphology when seeded on Matrigel. In addition, we show that endothelial-like hMSCs are able to create a capillary network in 3D culture both in vitro and in vivo conditions. The expansion phase in the presence of growth supplements was crucial for the stability of the capillaries formed in vitro. To conclude, we established a robust protocol for endothelial differentiation of hMSCs, including an immortalized MSC line (iMSCs) which allows for reproducible in vitro analysis in further studies.

Angiogenesis impairment in diabetes: role of methylglyoxal-induced receptor for advanced glycation endproducts, autophagy and vascular endothelial growth factor receptor 2

Diabetes impairs physiological angiogenesis by molecular mechanisms that are not fully understood. Methylglyoxal (MGO), a metabolite of glycolysis, is increased in patients with diabetes. This study defined the role of MGO in angiogenesis impairment and tested the mechanism in diabetic animals. Endothelial cells and mouse aortas were subjected to Western blot analysis of vascular endothelial growth factor receptor 2 (VEGFR2) protein levels and angiogenesis evaluation by endothelial cell tube formation/migration and aortic ring assays. Incubation with MGO reduced VEGFR2 protein, but not mRNA, levels in a time and dose dependent manner. Genetic knockdown of the receptor for advanced glycation endproducts (RAGE) attenuated the reduction of VEGFR2. Overexpression of Glyoxalase 1, the enzyme that detoxifies MGO, reduced the MGO-protein adducts and prevented VEGFR2 reduction. The VEGFR2 reduction was associated with impaired angiogenesis. Suppression of autophagy either by inhibitors or siRNA, but not of the proteasome and caspase, normalized both the VEGFR2 protein levels and angiogenesis. Conversely, induction of autophagy either by rapamycin or overexpression of LC3 and Beclin-1 reduced VEGFR2 and angiogenesis. MGO increased endothelial LC3B and Beclin-1, markers of autophagy, which were accompanied by an increase of both autophagic flux (LC3 punctae) and co-immunoprecipitation of VEGFR2 with LC3. Pharmacological or genetic suppression of peroxynitrite (ONOO(-)) generation not only blocked the autophagy but also reversed the reduction of VEGFR2 and angiogenesis. Like MGO-treated aortas from normglycemic C57BL/6J mice, aortas from diabetic db/db and Akita mice presented reductions of angiogenesis or VEGFR2. Administration of either autophagy inhibitor ex vivo or superoxide scavenger in vivo abolished the reductions. Taken together, MGO reduces endothelial angiogenesis through RAGE-mediated, ONOO(-)dependent and autophagy-induced VEGFR2 degradation, which may represent a new mechanism for diabetic angiogenesis impairment.

Perinuclear mitochondrial clustering creates an oxidant-rich nuclear domain required for hypoxia-induced transcription

Mitochondria can govern local concentrations of second messengers, such as reactive oxygen species (ROS), and mitochondrial translocation to discrete subcellular regions may contribute to this signaling function. Here, we report that exposure of pulmonary artery endothelial cells to hypoxia triggered a retrograde mitochondrial movement that required microtubules and the microtubule motor protein dynein and resulted in the perinuclear clustering of mitochondria. This subcellular redistribution of mitochondria was accompanied by the accumulation of ROS in the nucleus, which was attenuated by suppressing perinuclear clustering of mitochondria with nocodazole to destabilize microtubules or with small interfering RNA-mediated knockdown of dynein. Although suppression of perinuclear mitochondrial clustering did not affect the hypoxia-induced increase in the nuclear abundance of hypoxia-inducible factor 1α (HIF-1α) or the binding of HIF-1α to an oligonucleotide corresponding to a hypoxia response element (HRE), it eliminated oxidative modifications of the VEGF (vascular endothelial growth factor) promoter. Furthermore, suppression of perinuclear mitochondrial clustering reduced HIF-1α binding to the VEGF promoter and decreased VEGF mRNA accumulation. These findings support a model for hypoxia-induced transcriptional regulation in which perinuclear mitochondrial clustering results in ROS accumulation in the nucleus and causes oxidative base modifications in the VEGF HRE that are important for transcriptional complex assembly and VEGF mRNA expression.