VEGF receptor 2 endocytic trafficking regulates arterial morphogenesis

The interaction of heparan sulfate proteoglycans with endothelial transglutaminase-2 limits VEGF165-induced angiogenesis

Sprouting angiogenesis is stimulated by vascular endothelial growth factor (VEGF165) that is localized in the extracellular matrix (ECM) and binds to heparan sulfate (HS)-bearing proteins known as heparan sulfate proteoglycans (HSPGs). VEGF165 presentation by HSPGs enhances VEGF receptor-2 (VEGFR2) signaling. We investigated the effect of TG2, which binds to HSPGs, on the interaction between VEGF165 and HS and angiogenesis. Mice with tg2 deficiency showed transiently enhanced retina vessel formation and increased vascularization of VEGF165-containing Matrigel implants. In addition, endothelial cells in which TG2 was knocked down exhibited enhanced VEGF165-induced sprouting and migration, which was associated with increased phosphorylation of VEGFR2 at Tyr(951) and its targets Src and Akt. TG2 knockdown did not affect the phosphorylation of VEGFR2 at Tyr(1175) or cell proliferation in response to VEGF165 and sprouting or signaling in response to VEGF121. Decreased phosphorylation of VEGFR2 at Tyr(951) was due to ECM-localized TG2, which reduced the binding of VEGF165 to endothelial ECM in a manner that required its ability to bind to HS but not its catalytic activity. Surface plasmon resonance assays demonstrated that TG2 impeded the interaction between VEGF165 and HS. These results show that TG2 controls the formation of VEGF165-HSPG complexes and suggest that this regulation could be pharmacologically targeted to modulate developmental and therapeutic angiogenesis.

AIP1 Expression in Tumor Niche Suppresses Tumor Progression and Metastasis

Studies from tumor cells suggest that tumor-suppressor AIP1 inhibits epithelial-mesenchymal transition (EMT). However, the role of AIP1 in the tumor microenvironment has not been examined. We show that a global or vascular endothelial cell (EC)-specific deletion of the AIP1 gene in mice augments tumor growth and metastasis in melanoma and breast cancer models. AIP1-deficient vascular environment not only enhances tumor neovascularization and increases premetastatic niche formation, but also secretes tumor EMT-promoting factors. These effects from AIP1 loss are associated with increased VEGFR2 signaling in the vascular EC and could be abrogated by systemic administration of VEGFR2 kinase inhibitors. Mechanistically, AIP1 blocks VEGFR2-dependent signaling by directly binding to the phosphotyrosine residues within the activation loop of VEGFR2. Our data reveal that AIP1, by inhibiting VEGFR2-dependent signaling in tumor niche, suppresses tumor EMT switch, tumor angiogenesis, and tumor premetastatic niche formation to limit tumor growth and metastasis.

Site-Specific Phosphorylation of VEGFR2 Is Mediated by Receptor Trafficking: Insights from a Computational Model

Matrix-binding isoforms and non-matrix-binding isoforms of vascular endothelial growth factor (VEGF) are both capable of stimulating vascular remodeling, but the resulting blood vessel networks are structurally and functionally different. Here, we develop and validate a computational model of the binding of soluble and immobilized ligands to VEGF receptor 2 (VEGFR2), the endosomal trafficking of VEGFR2, and site-specific VEGFR2 tyrosine phosphorylation to study differences in induced signaling between these VEGF isoforms. In capturing essential features of VEGFR2 signaling and trafficking, our model suggests that VEGFR2 trafficking parameters are largely consistent across multiple endothelial cell lines. Simulations demonstrate distinct localization of VEGFR2 phosphorylated on Y1175 and Y1214. This is the first model to clearly show that differences in site-specific VEGFR2 activation when stimulated with immobilized VEGF compared to soluble VEGF can be accounted for by altered trafficking of VEGFR2 without an intrinsic difference in receptor activation. The model predicts that Neuropilin-1 can induce differences in the surface-to-internal distribution of VEGFR2. Simulations also show that ligated VEGFR2 and phosphorylated VEGFR2 levels diverge over time following stimulation. Using this model, we identify multiple key levers that alter how VEGF binding to VEGFR2 results in different coordinated patterns of multiple downstream signaling pathways. Specifically, simulations predict that VEGF immobilization, interactions with Neuropilin-1, perturbations of VEGFR2 trafficking, and changes in expression or activity of phosphatases acting on VEGFR2 all affect the magnitude, duration, and relative strength of VEGFR2 phosphorylation on tyrosines 1175 and 1214, and they do so predictably within our single consistent model framework.

Vascular endothelial growth factor (VEGF) is an important regulator of blood vessel growth. To date, therapies attempting to harness the VEGF system to promote blood vessel growth (e.g. for wound healing or ischemic disease) have achieved only limited success. To improve VEGF-based therapies, we need to better understand how VEGF promotes development of functional blood vessels. We have developed a computational model of VEGF binding to the receptor VEGFR2, trafficking of VEGFR2 through endosomal compartments in the cell, and activation of VEGFR2 on several tyrosine residues. The pattern of tyrosines activated on VEGFR2 influences cell behavior, promoting cell survival, proliferation, or migration. The combination of these cues influences the diameter of vessels, degree of branching, and leakiness of the resultant vessel network. Our model shows that changes in VEGFR2 trafficking as a result of VEGF immobilization to the extracellular matrix are sufficient to describe observed changes in the pattern of VEGFR2 activation compared to stimulation with purely soluble VEGF. This model can be used to predict how VEGF immobilization, interactions with co-receptors or proteins that deactivate VEGFR2, and changes to VEGFR2 trafficking can be tuned to promote development of functional blood vessel networks for tissue engineering applications.

MET Suppresses Epithelial VEGFR2 via Intracrine VEGF-induced Endoplasmic Reticulum-associated Degradation

Hepatocyte growth factor (HGF) and vascular endothelial growth factor (VEGF) drive cancer through their respective receptors, MET and VEGF receptor 2 (VEGFR2). VEGFR2 inhibits MET by promoting MET dephosphorylation. However, whether MET conversely regulates VEGFR2 remains unknown. Here we show that MET suppresses VEGFR2 protein by inducing its endoplasmic-reticulum-associated degradation (ERAD), via intracrine VEGF action. HGF-MET signaling in epithelial cancer cells promoted VEGF biosynthesis through PI3-kinase. In turn, VEGF and VEGFR2 associated within the ER, activating inositol-requiring enzyme 1α, and thereby facilitating ERAD-mediated depletion of VEGFR2. MET disruption upregulated VEGFR2, inducing compensatory tumor growth via VEGFR2 and MEK. However, concurrent disruption of MET and either VEGF or MEK circumvented this, enabling more profound tumor inhibition. Our findings uncover unique cross-regulation between MET and VEGFR2-two RTKs that play significant roles in tumor malignancy. Furthermore, these results suggest rational combinatorial strategies for targeting RTK signaling pathways more effectively, which has potentially important implications for cancer therapy.

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.

Neuropilin regulation of angiogenesis, arteriogenesis, and vascular permeability

The formation of the cardiovasculature, consisting of both the heart and blood vessels, is a critical step in embryonic development and relies on three processes termed vasculogenesis, angiogenesis, and vascular remodeling. The transmembrane protein NRP1 is an essential modulator of embryonic angiogenesis with additional roles in vessel remodeling and arteriogenesis. NRP1 also enhances arteriogenesis in adults to alleviate pathological tissue ischemia. However, in certain circumstances, vascular NRP1 signaling can be detrimental, as it may promote cancer by enhancing tumor angiogenesis or contribute to tissue edema by increasing vascular permeability. Understanding the mechanisms of NRP1 signaling is, therefore, of profound importance for the design of therapies aiming to control vascular functions. Previous work has shown that vascular NRP1 can variably serve as a receptor for two secreted glycoproteins, the VEGF-A and SEMA3A, but it also has a poorly understood role as an adhesion receptor. Here, we review current knowledge of NRP1 function during blood vessel growth and homeostasis, with special emphasis on the vascular roles of its multiple ligands and signaling partners.

Plasminogen activator inhibitor-1 inhibits angiogenic signaling by uncoupling vascular endothelial growth factor receptor-2-αVβ3 integrin cross talk

OBJECTIVE

Plasminogen activator inhibitor-1 (PAI-1) regulates angiogenesis via effects on extracellular matrix proteolysis and cell adhesion. However, no previous study has implicated PAI-1 in controlling vascular endothelial growth factor (VEGF) signaling. We tested the hypothesis that PAI-1 downregulates VEGF receptor-2 (VEGFR-2) activation by inhibiting a vitronectin-dependent cooperative binding interaction between VEGFR-2 and αVβ3.

APPROACH AND RESULTS

We studied effects of PAI-1 on VEGF signaling in human umbilical vein endothelial cells. PAI-1 inhibited VEGF-induced phosphorylation of VEGFR-2 in human umbilical vein endothelial cells grown on vitronectin, but not on fibronectin or collagen. PAI-1 inhibited the binding of VEGFR-2 to β3 integrin, VEGFR-2 endocytosis, and intracellular signaling pathways downstream of VEGFR-2. The anti-VEGF effect of PAI-1 was mediated by 2 distinct pathways, one requiring binding to vitronectin and another requiring binding to very low-density lipoprotein receptor. PAI-1 inhibited VEGF-induced angiogenesis in vitro and in vivo, and pharmacological inhibition of PAI-1 promoted collateral arteriole development and recovery of hindlimb perfusion after femoral artery interruption.

CONCLUSIONS

PAI-1 inhibits activation of VEGFR-2 by VEGF by disrupting a vitronectin-dependent proangiogenic binding interaction involving αVβ3 and VEGFR-2. These results broaden our understanding of the roles of PAI-1, vitronectin, and endocytic receptors in regulating VEGFR-2 activation and suggest novel therapeutic strategies for regulating VEGF signaling.

Molecular controls of lymphatic VEGFR3 signaling

OBJECTIVES

Vascular endothelial growth factor receptor 3 (VEGFR3) plays important roles both in lymphangiogenesis and angiogenesis. On stimulation by its ligand VEGF-C, VEGFR3 is able to form both homodimers as well as heterodimers with VEGFR2 and activates several downstream signal pathways, including extracellular signal-regulated kinases (ERK)1/2 and protein kinase B (AKT). Despite certain similarities with VEGFR2, molecular features of VEGFR3 signaling are still largely unknown.

APPROACH AND RESULTS

Human dermal lymphatic endothelial cells were used to examine VEGF-C-driven activation of signaling. Compared with VEGF-A activation of VEGFR2, VEGF-C-induced VEGFR3 activation led to a more extensive AKT activation, whereas activation of ERK1/2 displayed a distinctly different kinetics. Furthermore, VEGF-C, but not VEGF-A, induced formation of VEGFR3/VEGFR2 complexes. Silencing VEGFR2 or its partner neuropilin 1 specifically abolished VEGF-C-induced AKT but not ERK activation, whereas silencing of neuropilin 2 had little effect on either signaling pathway. Finally, suppression of vascular endothelial phosphotyrosine phosphatase but not other phosphotyrosine phosphatases enhanced VEGF-C-induced activation of both ERK and AKT pathways. Functionally, both ERK and AKT pathways are important for lymphatic endothelial cells migration.

CONCLUSIONS

VEGF-C activates AKT signaling via formation of VEGFR3/VEGFR2 complex, whereas ERK is activated by VEGFR3 homodimer. Neuropilin 1 and vascular endothelial phosphotyrosine phosphatase are involved in regulation of VEGFR3 signaling.

Canonical and noncanonical vascular endothelial growth factor pathways: new developments in biology and signal transduction

The past 5 years have witnessed a significant expansion in our understanding of vascular endothelial growth factor (VEGF) signaling. In particular, the process of canonical activation of VEGF receptor tyrosine kinases by homodimeric VEGF molecules has now been broadened by the realization that heterodimeric ligands and receptors are also active participants in the signaling process. Although heterodimer receptors were described 2 decades ago, their impact, along with the effect of additional cell surface partners and novel autocrine VEGF signaling pathways, are only now starting to be clarified. Furthermore, ligand-independent signaling (noncanonical) has been identified through galectin and gremlin binding and upon rise of intracellular levels of reactive oxygen species. Activation of the VEGF receptors in the absence of ligand holds immediate implications for therapeutic approaches that exclusively target VEGF. The present review provides a concise summary of the recent developments in both canonical and noncanonical VEGF signaling and places these findings in perspective to their potential clinical and biological ramifications.

The emerging role of class-3 semaphorins and their neuropilin receptors in oncology

The semaphorins, discovered over 20 years ago, are a large family of secreted or transmembrane and glycophosphatidylinositol -anchored proteins initially identified as axon guidance molecules crucial for the development of the nervous system. It has now been established that they also play important roles in organ development and function, especially involving the immune, respiratory, and cardiovascular systems, and in pathological disorders, including cancer. During tumor progression, semaphorins can have both pro- and anti-tumor functions, and this has created complexities in our understanding of these systems. Semaphorins may affect tumor growth and metastases by directly targeting tumor cells, as well as indirectly by interacting with and influencing cells from the micro-environment and vasculature. Mechanistically, semaphorins, through binding to their receptors, neuropilins and plexins, affect pathways involved in cell adhesion, migration, invasion, proliferation, and survival. Importantly, neuropilins also act as co-receptors for several growth factors and enhance their signaling activities, while class 3 semaphorins may interfere with this. In this review, we focus on the secreted class 3 semaphorins and their neuropilin co-receptors in cancer, including aspects of their signaling that may be clinically relevant.

Chemokine-coupled β2 integrin-induced macrophage Rac2-Myosin IIA interaction regulates VEGF-A mRNA stability and arteriogenesis

Myeloid cells are important contributors to arteriogenesis, but their key molecular triggers and cellular effectors are largely unknown. We report, in inflammatory monocytes, that the combination of chemokine receptor (CCR2) and adhesion receptor (β2 integrin) engagement leads to an interaction between activated Rac2 and Myosin 9 (Myh9), the heavy chain of Myosin IIA, resulting in augmented vascular endothelial growth factor A (VEGF-A) expression and induction of arteriogenesis. In human monocytes, CCL2 stimulation coupled to ICAM-1 adhesion led to rapid nuclear-to-cytosolic translocation of the RNA-binding protein HuR. This activation of HuR and its stabilization of VEGF-A mRNA were Rac2-dependent, and proteomic analysis for Rac2 interactors identified the 226 kD protein Myh9. The level of induced Rac2-Myh9 interaction strongly correlated with the degree of HuR translocation. CCL2-coupled ICAM-1 adhesion-driven HuR translocation and consequent VEGF-A mRNA stabilization were absent in Myh9(-/-) macrophages. Macrophage VEGF-A production, ischemic tissue VEGF-A levels, and flow recovery to hind limb ischemia were impaired in myeloid-specific Myh9(-/-) mice, despite preserved macrophage recruitment to the ischemic muscle. Micro-CT arteriography determined the impairment to be defective induced arteriogenesis, whereas developmental vasculogenesis was unaffected. These results place the macrophage at the center of ischemia-induced arteriogenesis, and they establish a novel role for Myosin IIA in signal transduction events modulating VEGF-A expression in tissue.

KIF13B regulates angiogenesis through Golgi to plasma membrane trafficking of VEGFR2

Although the trafficking of newly synthesized VEGFR2 to the plasma membrane is a key determinant of angiogenesis, the molecular mechanisms of Golgi to plasma membrane trafficking are unknown. Here, we have identified a key role of the kinesin family plus-end molecular motor KIF13B in delivering VEGFR2 cargo from the Golgi to the endothelial cell surface. KIF13B is shown to interact directly with VEGFR2 on microtubules. We also observed that overexpression of truncated versions of KIF13B containing the binding domains that interact with VEGFR2 inhibited VEGF-induced capillary tube formation. KIF13B depletion prevented VEGF-mediated endothelial migration, capillary tube formation and neo-vascularization in mice. Impairment in trafficking induced by knockdown of KIF13B shunted VEGFR2 towards the lysosomal degradation pathway. Thus, KIF13B is an essential molecular motor required for the trafficking of VEGFR2 from the Golgi, and its delivery to the endothelial cell surface mediates angiogenesis.

PTP1b is a physiologic regulator of vascular endothelial growth factor signaling in endothelial cells

BACKGROUND

Regulation of vascular endothelial growth factor receptor-2 (VEGFR2) signaling is a control point that determines the extent of vascular tree formation. Recent studies demonstrated an important role played by VEGFR2 endothelial trafficking in control of its activity and suggested the involvement of a phosphotyrosine phosphatase 1b (PTP1b) in this process. This study was designed to define the role of PTP1b in endothelial VEGFR2 signaling and its role in regulation of angiogenesis and arteriogenesis.

METHODS AND RESULTS

We generated mice carrying an endothelial-specific deletion of PTP1b and examined the effect of this knockout on VEGF signaling, angiogenesis, and arteriogenesis in vitro and in vivo. PTP1b knockout endothelial cells had increased VEGF-dependent activation of extracellular signal-regulated kinase signaling, sprouting, migration, and proliferation compared with controls. Endothelial PTP1b null mice had increased retinal and Matrigel implant angiogenesis and accelerated wound healing, pointing to enhanced angiogenesis. Increased arteriogenesis was demonstrated by observations of faster recovery of arterial blood flow and large numbers of newly formed arterioles in the hindlimb ischemia mouse model. PTP1b endothelial knockout also rescued impaired blood flow recovery after common femoral artery ligation in synectin null mice.

CONCLUSIONS

PTP1b is a key regulator of endothelial VEGFR2 signaling and plays an important role in regulation of the extent of vascular tree formation.

Conformational remodeling of the fibronectin matrix selectively regulates VEGF signaling

The fibronectin matrix plays a crucial role in the regulation of angiogenesis during development, tissue repair and pathogenesis. Previous work has identified a fibronectin-derived homophilic binding peptide, anastellin, as an effective inhibitor of angiogenesis; however, its mechanism of action is not well understood. In the present study, we demonstrate that anastellin selectively inhibits microvessel cell signaling in response to the VEGF165 isoform, but not VEGF121, by preventing the assembly of the complex containing the VEGF receptor and neuropilin-1. Anastellin treatment resulted in the inactivation of α5β1 integrins but was not accompanied by a change in either adhesion complexes or adhesion-based signaling. Integrin inactivation was associated with a masking of the fibronectin synergy site within the extracellular matrix (ECM), indicating that α5β1 inactivation resulted from a decrease in available ligand. These data demonstrate that anastellin influences the microvessel cell response to growth factors by controlling the repertoire of ligated integrins and point to anastellin as an effective regulator of fibronectin matrix organization. These studies further suggest that homophilic fibronectin binding peptides might have novel applications in the field of tissue regeneration as tools to regulate neovascularization.

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.

Imatinib inhibits VEGF-independent angiogenesis by targeting neuropilin 1-dependent ABL1 activation in endothelial cells

Neuropilin 1 regulates angiogenesis in a VEGF-independent manner via association with ABL1, suggesting that Imatinib represents a novel opportunity for anti-angiogenic therapy.

To enable new blood vessel growth, endothelial cells (ECs) express neuropilin 1 (NRP1), and NRP1 associates with the receptor tyrosine kinase VEGFR2 after binding the vascular endothelial growth factor A (VEGF) to enhance arteriogenesis. We report that NRP1 contributes to angiogenesis through a novel mechanism. In human and mouse ECs, the integrin ligand fibronectin (FN) stimulated actin remodeling and phosphorylation of the focal adhesion component paxillin (PXN) in a VEGF/VEGFR2-independent but NRP1-dependent manner. NRP1 formed a complex with ABL1 that was responsible for FN-dependent PXN activation and actin remodeling. This complex promoted EC motility in vitro and during angiogenesis on FN substrates in vivo. Accordingly, both physiological and pathological angiogenesis in the retina were inhibited by treatment with Imatinib, a small molecule inhibitor of ABL1 which is widely used to prevent the proliferation of tumor cells that express BCR-ABL fusion proteins. The finding that NRP1 regulates angiogenesis in a VEGF- and VEGFR2-independent fashion via ABL1 suggests that ABL1 inhibition provides a novel opportunity for anti-angiogenic therapy to complement VEGF or VEGFR2 blockade in eye disease or solid tumor growth.

Molecular pathways governing development of vascular endothelial cells from ES/iPS cells

Assembly of complex vascular networks occurs in numerous biological systems through morphogenetic processes such as vasculogenesis, angiogenesis and vascular remodeling. Pluripotent stem cells such as embryonic stem (ES) and induced pluripotent stem (iPS) cells can differentiate into any cell type, including endothelial cells (ECs), and have been extensively used as in vitro models to analyze molecular mechanisms underlying EC generation and differentiation. The emergence of these promising new approaches suggests that ECs could be used in clinical therapy. Much evidence suggests that ES/iPS cell differentiation into ECs in vitro mimics the in vivo vascular morphogenic process. Through sequential steps of maturation, ECs derived from ES/iPS cells can be further differentiated into arterial, venous, capillary and lymphatic ECs, as well as smooth muscle cells. Here, we review EC development from ES/iPS cells with special attention to molecular pathways functioning in EC specification.

Prediction of substrate sites for protein phosphatases 1B, SHP-1, and SHP-2 based on sequence features

Tyrosine phosphorylation plays crucial roles in numerous physiological processes. The level of phosphorylation state depends on the combined action of protein tyrosine kinases and protein tyrosine phosphatases. Detection of possible phosphorylation and dephosphorylation sites can provide useful information to the functional studies of relevant proteins. Several studies have focused on the identification of protein tyrosine kinase substrates. However, compared with protein tyrosine kinases, the prediction of protein tyrosine phosphatase substrates involved in the balance of protein phosphorylation level falls behind. This paper described a method that utilized the k-nearest neighbor algorithm to identity the substrate sites of three protein tyrosine phosphatases based on the sequence features of manually collected dephosphorylation sites. In the performance evaluation, both sensitivities and specificities could reach above 75% for all three protein tyrosine phosphatases. Finally, the method was applied on a set of known tyrosine phosphorylation sites to search for candidate substrates.

Enhanced angiogenesis and increased cardiac perfusion after myocardial infarction in protein tyrosine phosphatase 1B-deficient mice

The protein tyrosine phosphatase 1B (PTP1B) modulates tyrosine kinase receptors, among which is the vascular endothelial growth factor receptor type 2 (VEGFR2), a key component of angiogenesis. Because PTP1B deficiency in mice improves left ventricular (LV) function 2 mo after myocardial infarction (MI), we hypothesized that enhanced angiogenesis early after MI via activated VEGFR2 contributes to this improvement. At 3 d after MI, capillary density was increased at the infarct border of PTP1B(-/-) mice [+7±2% vs. wild-type (WT), P = 0.05]. This was associated with increased extracellular signal-regulated kinase 2 phosphorylation and VEGFR2 activation (i.e., phosphorylated-Src/Src/VEGFR2 and dissociation of endothelial VEGFR2/VE-cadherin), together with higher infiltration of proangiogenic M2 macrophages within unchanged overall infiltration. In vitro, we showed that PTP1B inhibition or silencing using RNA interference increased VEGF-induced migration and proliferation of mouse heart microvascular endothelial cells as well as fibroblast growth factor (FGF)-induced proliferation of rat aortic smooth muscle cells. At 8 d after MI in PTP1B(-/-) mice, increased LV capillary density (+21±3% vs. WT; P<0.05) and an increased number of small diameter arteries (15-50 μm) were likely to participate in increased LV perfusion assessed by magnetic resonance imaging and improved LV compliance, indicating reduced diastolic dysfunction. In conclusion, PTP1B deficiency reduces MI-induced heart failure promptly after ischemia by enhancing angiogenesis, myocardial perfusion, and diastolic function.

Glycosylation-dependent lectin-receptor interactions preserve angiogenesis in anti-VEGF refractory tumors

The clinical benefit conferred by vascular endothelial growth factors (VEGF)-targeted therapies is variable, and tumors from treated patients eventually reinitiate growth. Here, we identify a glycosylation-dependent pathway that compensates for the absence of cognate ligand and preserves angiogenesis in response to VEGF blockade. Remodeling of the endothelial cell (EC) surface glycome selectively regulated binding of galectin-1 (Gal1), which upon recognition of complex N-glycans on VEGFR2, activated VEGF-like signaling. Vessels within anti-VEGF-sensitive tumors exhibited high levels of α2-6-linked sialic acid, which prevented Gal1 binding. In contrast, anti-VEGF refractory tumors secreted increased Gal1 and their associated vasculature displayed glycosylation patterns that facilitated Gal1-EC interactions. Interruption of β1-6GlcNAc branching in ECs or silencing of tumor-derived Gal1 converted refractory into anti-VEGF-sensitive tumors, whereas elimination of α2-6-linked sialic acid conferred resistance to anti-VEGF. Disruption of the Gal1-N-glycan axis promoted vascular remodeling, immune cell influx and tumor growth inhibition. Thus, targeting glycosylation-dependent lectin-receptor interactions may increase the efficacy of anti-VEGF treatment.

The docking protein FRS2α is a critical regulator of VEGF receptors signaling

Vascular endothelial growth factors (VEGFs) signal via their cognate receptor tyrosine kinases designated VEGFR1-3. We report that the docking protein fibroblast growth factor receptor substrate 2 (FRS2α) plays a critical role in cell signaling via these receptors. In vitro FRS2α regulates VEGF-A and VEGF-C-dependent activation of extracellular signal-regulated receptor kinase signaling and blood and lymphatic endothelial cells migration and proliferation. In vivo endothelial-specific deletion of FRS2α results in the profound impairment of postnatal vascular development and adult angiogenesis, lymphangiogenesis, and arteriogenesis. We conclude that FRS2α is a previously unidentified component of VEGF receptors signaling.

NRP1 presented in trans to the endothelium arrests VEGFR2 endocytosis, preventing angiogenic signaling and tumor initiation

Neuropilin 1 (NRP1) modulates angiogenesis by binding vascular endothelial growth factor (VEGF) and its receptor, VEGFR2. We examined the consequences when VEGFR2 and NRP1 were expressed on the same cell (cis) or on different cells (trans). In cis, VEGF induced rapid VEGFR2/NRP1 complex formation and internalization. In trans, complex formation was delayed and phosphorylation of phospholipase Cγ (PLCγ) and extracellular regulated kinase 2 (ERK2) was prolonged, whereas ERK1 phosphorylation was reduced. Trans complex formation suppressed initiation and vascularization of NRP1-expressing mouse fibrosarcoma and melanoma. Suppression in trans required high-affinity, steady-state binding of VEGF to NRP1, which was dependent on the NRP1 C-terminal domain. Compatible with a trans effect of NRP1, quiescent vasculature in the developing retina showed continuous high NRP1 expression, whereas angiogenic sprouting occurred where NRP1 levels fluctuated between adjacent endothelial cells. Therefore, through communication in trans, NRP1 can modulate VEGFR2 signaling and suppress angiogenesis.

Reversible acetylation regulates vascular endothelial growth factor receptor-2 activity

The tyrosine kinase receptor vascular endothelial growth factor receptor 2 (VEGFR2) is a key regulator of angiogenesis. Here we show that VEGFR2 is acetylated in endothelial cells both at four lysine residues forming a dense cluster in the kinase insert domain and at a single lysine located in the receptor activation loop. These modifications are under dynamic control of the acetyltransferase p300 and two deacetylases HDAC5 and HDAC6. We demonstrate that VEGFR2 acetylation essentially regulates receptor phosphorylation. In particular, VEGFR2 acetylation significantly alters the kinetics of receptor phosphorylation after ligand binding, allowing receptor phosphorylation and intracellular signaling upon prolonged stimulation with VEGF. Molecular dynamics simulations indicate that acetylation of the lysine in the activation loop contributes to the transition to an open active state, in which tyrosine phosphorylation is favored by better exposure of the kinase target residues. These findings indicate that post-translational modification by acetylation is a critical mechanism that directly affects VEGFR2 function.

Dynamin 2 regulation of integrin endocytosis, but not VEGF signaling, is crucial for developmental angiogenesis

Here we show that dynamin 2 (Dnm2) is essential for angiogenesis in vitro and in vivo. In cultured endothelial cells lacking Dnm2, vascular endothelial growth factor (VEGF) signaling and receptor levels are augmented whereas cell migration and morphogenesis are impaired. Mechanistically, the loss of Dnm2 increases focal adhesion size and the surface levels of multiple integrins and reduces the activation state of β1 integrin. In vivo, the constitutive or inducible loss of Dnm2 in endothelium impairs branching morphogenesis and promotes the accumulation of β1 integrin at sites of failed angiogenic sprouting. Collectively, our data show that Dnm2 uncouples VEGF signaling from function and coordinates the endocytic turnover of integrins in a manner that is crucially important for angiogenesis in vitro and in vivo.

Ephrin-B2 controls PDGFRβ internalization and signaling

Ephrin-B2 is essential for supporting mural cells; namely, pericytes and vascular smooth muscle cells (VSMCs). Nakayama et al. find that ephrin-B2 controls platelet-derived growth factor receptor β (PDGFRβ) distribution in the VSMC plasma membrane, endocytosis, and signaling. VSMCs lacking ephrin-B2 exhibited a redistribution of PDGFRβ from caveolin-positive to clathrin-associated membrane fractions and enhanced PDGF-B-induced PDGFRβ internalization. Mice lacking ephrin-B2 in vascular smooth muscle developed vessel wall defects and aortic aneurysms. These results suggest that ephrin-B2 is an important regulator of PDGFRβ endocytosis in mural cells.

B-class ephrins, ligands for EphB receptor tyrosine kinases, are critical regulators of growth and patterning processes in many organs and species. In the endothelium of the developing vasculature, ephrin-B2 controls endothelial sprouting and proliferation, which has been linked to vascular endothelial growth factor (VEGF) receptor endocytosis and signaling. Ephrin-B2 also has essential roles in supporting mural cells (namely, pericytes and vascular smooth muscle cells [VSMCs]), but the underlying mechanism is not understood. Here, we show that ephrin-B2 controls platelet-derived growth factor receptor β (PDGFRβ) distribution in the VSMC plasma membrane, endocytosis, and signaling in a fashion that is highly distinct from its role in the endothelium. Absence of ephrin-B2 in cultured VSMCs led to the redistribution of PDGFRβ from caveolin-positive to clathrin-associated membrane fractions, enhanced PDGF-B-induced PDGFRβ internalization, and augmented downstream mitogen-activated protein (MAP) kinase and c-Jun N-terminal kinase (JNK) activation but impaired Tiam1–Rac1 signaling and proliferation. Accordingly, mutant mice lacking ephrin-B2 expression in vascular smooth muscle developed vessel wall defects and aortic aneurysms, which were associated with impaired Tiam1 expression and excessive activation of MAP kinase and JNK. Our results establish that ephrin-B2 is an important regulator of PDGFRβ endocytosis and thereby acts as a molecular switch controlling the downstream signaling activity of this receptor in mural cells.

A ligand-independent VEGFR2 signaling pathway limits angiogenic responses in diabetes

Although vascular complications are a hallmark of diabetes, the molecular mechanisms that underlie endothelial dysfunction are unclear. We showed that reactive oxygen species generated from hyperglycemia promoted ligand-independent phosphorylation of vascular endothelial growth factor receptor 2 (VEGFR2). This VEGFR2 signaling occurred within the Golgi compartment and resulted in progressively decreased availability of VEGFR2 at the cell surface. Consequently, the responses of endothelial cells to exogenous VEGF in a mouse model of diabetes were impaired because of a specific deficiency of VEGFR2 at the cell surface, despite a lack of change in transcript abundance. Hyperglycemia-induced phosphorylation of VEGFR2 did not require intrinsic receptor kinase activity and was instead mediated by Src family kinases. The reduced cell surface abundance of VEGFR2 in diabetic mice was reversed by treatment with the antioxidant N-acetyl-L-cysteine, suggesting a causative role for oxidative stress. These findings uncover a mode of ligand-independent VEGFR2 signaling that can progressively lead to continuously muted responses to exogenous VEGF and limit angiogenic events.

VE-cadherin and endothelial adherens junctions: active guardians of vascular integrity

VE-cadherin is a component of endothelial cell-to-cell adherens junctions, and it has a key role in the maintenance of vascular integrity. During embryo development, VE-cadherin is required for the organization of a stable vascular system, and in the adult it controls vascular permeability and inhibits unrestrained vascular growth. The mechanisms of action of VE-cadherin are complex and include reshaping and organization of the endothelial cell cytoskeleton and modulation of gene transcription. Here we review some of the most important pathways through which VE-cadherin modulates vascular homeostasis and discuss the emerging concepts in the overall biological role of this protein.

Genetic reduction of vascular endothelial growth factor receptor 2 rescues aberrant angiogenesis caused by epsin deficiency

OBJECTIVE

We previously showed that endothelial epsin deficiency caused elevated vascular endothelial growth factor receptor 2 (VEGFR2) and enhanced VEGF signaling, resulting in aberrant tumor angiogenesis and reduced tumor growth in adult mice. However, direct evidence demonstrating that endothelial epsins regulate angiogenesis specifically through VEGFR2 downregulation is still lacking. In addition, whether the lack of epsins causes abnormal angiogenesis during embryonic development remains unclear.

APPROACH AND RESULTS

A novel strain of endothelial epsin-deleted mice that are heterozygous for VEGFR2 (Epn1(fl/fl); Epn2(-/-); Flk(fl/+); iCDH5 Cre mice) was created. Analysis of embryos at different developmental stages showed that deletion of epsins caused defective embryonic angiogenesis and retarded embryo development. In vitro angiogenesis assays using isolated primary endothelial cells (ECs) from Epn1(fl/fl); Epn2(-/-); iCDH5 Cre (EC-iDKO) and Epn1(fl/fl); Epn2(-/-); Flk(fl/+); iCDH5 Cre (EC-iDKO-Flk(fl/+)) mice demonstrated that VEGFR2 reduction in epsin-depleted cells was sufficient to restore normal VEGF signaling, EC proliferation, EC migration, and EC network formation. These findings were complemented by in vivo wound healing, inflammatory angiogenesis, and tumor angiogenesis assays in which reduction of VEGFR2 was sufficient to rescue abnormal angiogenesis in endothelial epsin-deleted mice.

CONCLUSIONS

Our results provide the first genetic demonstration that epsins function specifically to downregulate VEGFR2 by mediating activated VEGFR2 internalization and degradation and that genetic reduction of VEGFR2 level protects against excessive angiogenesis caused by epsin loss. Our findings indicate that epsins may be a potential therapeutic target in conditions in which tightly regulated angiogenesis is crucial, such as in diabetic wound healing and tumors.

Vascular remodeling after ischemic stroke: mechanisms and therapeutic potentials

The brain vasculature has been increasingly recognized as a key player that directs brain development, regulates homeostasis, and contributes to pathological processes. Following ischemic stroke, the reduction of blood flow elicits a cascade of changes and leads to vascular remodeling. However, the temporal profile of vascular changes after stroke is not well understood. Growing evidence suggests that the early phase of cerebral blood volume (CBV) increase is likely due to the improvement in collateral flow, also known as arteriogenesis, whereas the late phase of CBV increase is attributed to the surge of angiogenesis. Arteriogenesis is triggered by shear fluid stress followed by activation of endothelium and inflammatory processes, while angiogenesis induces a number of pro-angiogenic factors and circulating endothelial progenitor cells (EPCs). The status of collaterals in acute stroke has been shown to have several prognostic implications, while the causal relationship between angiogenesis and improved functional recovery has yet to be established in patients. A number of interventions aimed at enhancing cerebral blood flow including increasing collateral recruitment are under clinical investigation. Transplantation of EPCs to improve angiogenesis is also underway. Knowledge in the underlying physiological mechanisms for improved arteriogenesis and angiogenesis shall lead to more effective therapies for ischemic stroke.

Transmembrane protein ESDN promotes endothelial VEGF signaling and regulates angiogenesis

Aberrant blood vessel formation contributes to a wide variety of pathologies, and factors that regulate angiogenesis are attractive therapeutic targets. Endothelial and smooth muscle cell-derived neuropilin-like protein (ESDN) is a neuropilin-related transmembrane protein expressed in ECs; however, its potential effect on VEGF responses remains undefined. Here, we generated global and EC-specific Esdn knockout mice and demonstrated that ESDN promotes VEGF-induced human and murine EC proliferation and migration. Deletion of Esdn in the mouse interfered with adult and developmental angiogenesis, and knockdown of the Esdn homolog (dcbld2) in zebrafish impaired normal vascular development. Loss of ESDN in ECs blunted VEGF responses in vivo and attenuated VEGF-induced VEGFR-2 signaling without altering VEGF receptor or neuropilin expression. Finally, we found that ESDN associates with VEGFR-2 and regulates its complex formation with negative regulators of VEGF signaling, protein tyrosine phosphatases PTP1B and TC-PTP, and VE-cadherin. These findings establish ESDN as a regulator of VEGF responses in ECs that acts through a mechanism distinct from neuropilins. As such, ESDN may serve as a therapeutic target for angiogenesis regulation.

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.

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.

Syndecan-4 signaling at a glance

Syndecan-4, a ubiquitous cell surface proteoglycan, mediates numerous cellular processes through signaling pathways that affect cellular proliferation, migration, mechanotransduction and endocytosis. These effects are achieved through syndecan-4 functioning as both a co-receptor for the fibroblast growth factor receptors (FGFR1-FGFR4) and its ability to independently activate signaling pathways upon ligand binding. As an FGFR co-receptor, syndecan-4 strengthens the duration and intensity of downstream signaling upon ligand binding; this is particularly evident with regard to mitogen-activated protein kinase (MAPK) signaling. In contrast, syndecan-4 also functions as an independent receptor for heparin-binding growth factors, such as fibroblast growth factors (FGFs), vascular endothelial growth factors (VEGFs) and platelet-derived growth factors (PDGFs). These signaling cascades affect canonical signaling components, such as the mammalian target of rapamycin (mTOR), AKT1 and the Rho family of GTPases. In combination with the integrin family of proteins, syndecan-4 is also able to form physical connections between the extracellular matrix (ECM) and cytoskeletal signaling proteins, and it has a key role in regulation of integrin turnover. This unique versatility of the interactions of syndecan-4 is characterized in this Cell Science at a Glance article and illustrated in the accompanying poster.

Signal transduction by vascular endothelial growth factor receptors

Vascular endothelial growth factors (VEGFs) are master regulators of vascular development and of blood and lymphatic vessel function during health and disease in the adult. It is therefore important to understand the mechanism of action of this family of five mammalian ligands, which act through three receptor tyrosine kinases (RTKs). In addition, coreceptors like neuropilins (NRPs) and integrins associate with the ligand/receptor signaling complex and modulate the output. Therapeutics to block several of the VEGF signaling components have been developed with the aim to halt blood vessel formation, angiogenesis, in diseases that involve tissue growth and inflammation, such as cancer. In this review, we outline the current information on VEGF signal transduction in relation to blood and lymphatic vessel biology.

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

Computational Model of Gab1/2-Dependent VEGFR2 Pathway to Akt Activation

Vascular endothelial growth factor (VEGF) signal transduction is central to angiogenesis in development and in pathological conditions such as cancer, retinopathy and ischemic diseases. However, no detailed mass-action models of VEGF receptor signaling have been developed. We constructed and validated the first computational model of VEGFR2 trafficking and signaling, to study the opposing roles of Gab1 and Gab2 in regulation of Akt phosphorylation in VEGF-stimulated endothelial cells. Trafficking parameters were optimized against 5 previously published in vitro experiments, and the model was validated against six independent published datasets. The model showed agreement at several key nodes, involving scaffolding proteins Gab1, Gab2 and their complexes with Shp2. VEGFR2 recruitment of Gab1 is greater in magnitude, slower, and more sustained than that of Gab2. As Gab2 binds VEGFR2 complexes more transiently than Gab1, VEGFR2 complexes can recycle and continue to participate in other signaling pathways. Correspondingly, the simulation results show a log-linear relationship between a decrease in Akt phosphorylation and Gab1 knockdown while a linear relationship was observed between an increase in Akt phosphorylation and Gab2 knockdown. Global sensitivity analysis demonstrated the importance of initial-concentration ratios of antagonistic molecular species (Gab1/Gab2 and PI3K/Shp2) in determining Akt phosphorylation profiles. It also showed that kinetic parameters responsible for transient Gab2 binding affect the system at specific nodes. This model can be expanded to study multiple signaling contexts and receptor crosstalk and can form a basis for investigation of therapeutic approaches, such as tyrosine kinase inhibitors (TKIs), overexpression of key signaling proteins or knockdown experiments.

Emerging drugs for the treatment of diabetic ulcers

INTRODUCTION

Diabetic ulcers are chronic nonhealing ulcerations that despite the available medical tools still result in high amputation rates. Growing evidence suggests that alteration of the biochemical milieu of the chronic wound plays a significant role in impaired diabetic wound healing.

AREAS COVERED

The basic pathophysiology and the conventional treatment strategy of diabetic foot ulcers have been reviewed in the first section. In the second part, the most up-to-date bench and translational research in the field are described. The third section focuses on the drugs currently under development and the ongoing clinical trials evaluating their safety and efficacy. Finally, the major drug development issues and the possible scientific approaches to overcome them are analyzed.

EXPERT OPINION

Significant strides in understanding the chronic wound development have led to the development of topical therapies to address aberrant expression of growth factors and overexpression of inflammatory cytokines. Current research in the laboratory suggests that while decreased growth factor expression occurs at the local wound level, increased systemic serum levels of growth factors suggest growth factor resistance.

Navigation rules for vessels and neurons: cooperative signaling between VEGF and neural guidance cues

Many organs, such as lungs, nerves, blood and lymphatic vessels, consist of complex networks that carry flows of information, gases, and nutrients within the body. The morphogenetic patterning that generates these organs involves the coordinated action of developmental signaling cues that guide migration of specialized cells. Precision guidance of endothelial tip cells by vascular endothelial growth factors (VEGFs) is well established, and several families of neural guidance molecules have been identified to exert guidance function in both the nervous and the vascular systems. This review discusses recent advances in VEGF research, focusing on the emerging role of neural guidance molecules as key regulators of VEGF function during vascular development and on the novel role of VEGFs in neural cell migration and nerve wiring.

Endocytosis of receptor tyrosine kinases

Endocytosis is the major regulator of signaling from receptor tyrosine kinases (RTKs). The canonical model of RTK endocytosis involves rapid internalization of an RTK activated by ligand binding at the cell surface and subsequent sorting of internalized ligand-RTK complexes to lysosomes for degradation. Activation of the intrinsic tyrosine kinase activity of RTKs results in autophosphorylation, which is mechanistically coupled to the recruitment of adaptor proteins and conjugation of ubiquitin to RTKs. Ubiquitination serves to mediate interactions of RTKs with sorting machineries both at the cell surface and on endosomes. The pathways and kinetics of RTK endocytic trafficking, molecular mechanisms underlying sorting processes, and examples of deviations from the standard trafficking itinerary in the RTK family are discussed in this work.

Endothelial RAF1/ERK activation regulates arterial morphogenesis

Arterial morphogenesis is one of the most critical events during embryonic vascular development. Although arterial fate specification is mainly controlled by the Notch signaling pathway, arterial-venous patterning is modulated by a number of guidance factors. How these pathways are regulated is still largely unknown. Here, we demonstrate that endothelial activation of RAF1/extracellular signal-regulated kinase (ERK) pathway regulates arterial morphogenesis and arterial-venous patterning via Δ/Notch and semaphorin signaling. Introduction of a single amino acid RAF1 mutant (RAF1 Ser259Ala), which renders it resistant to inhibition by phosphorylation, into endothelial cells in vitro induced expression of virtually the entire embryonic arteriogenic program and activated semaphorin 6A-dependent endothelial cell-cell repulsion. In vivo, endothelial-specific expression of RAF1(S259A) during development induced extensive arterial morphogenesis both in the yolk sac and the embryo proper and disrupted arterial-venous patterning. Our results suggest that endothelial ERK signaling is critical for both arteriogenesis and arterial-venous patterning and that RAF1 Ser(259) phosphorylation plays a critical role in preventing unopposed ERK activation.

Vascular endothelial growth factor-angiopoietin chimera with improved properties for therapeutic angiogenesis

BACKGROUND

There is an unmet need for proangiogenic therapeutic molecules for the treatment of tissue ischemia in cardiovascular diseases. However, major inducers of angiogenesis such as vascular endothelial growth factor (VEGF/VEGF-A) have side effects that limit their therapeutic utility in vivo, especially at high concentrations. Angiopoietin-1 has been considered to be a blood vessel stabilization factor that can inhibit the intrinsic property of VEGF to promote vessel leakiness. In this study, we have designed and tested the angiogenic properties of chimeric molecules consisting of receptor-binding parts of VEGF and angiopoietin-1. We aimed at combining the activities of both factors into 1 molecule for easy delivery and expression in target tissues.

METHODS AND RESULTS

The VEGF-angiopoietin-1 (VA1) chimeric protein bound to both VEGF receptor-2 and Tie2 and induced the activation of both receptors. Detailed analysis of VA1 versus VEGF revealed differences in the kinetics of VEGF receptor-2 activation and endocytosis, downstream kinase activation, and VE-cadherin internalization. The delivery of a VA1 transgene into mouse skeletal muscle led to increased blood flow and enhanced angiogenesis. VA1 was also very efficient in rescuing ischemic limb perfusion. However, VA1 induced less plasma protein leakage and myeloid inflammatory cell recruitment than VEGF. Furthermore, angioma-like structures associated with VEGF expression were not observed with VA1.

CONCLUSIONS

The VEGF-angiopoietin-1 chimera is a potent angiogenic factor that triggers a novel mode of VEGF receptor-2 activation, promoting less vessel leakiness, less tissue inflammation, and better perfusion in ischemic muscle than VEGF. These properties of VA1 make it an attractive therapeutic tool.

The effect of bevacizumab on human malignant melanoma cells with functional VEGF/VEGFR2 autocrine and intracrine signaling loops

Receptors for the angiogenic factor VEGF are expressed by tumor cancer cells including melanoma, although their functionality remains unclear. Paired human melanoma cell lines WM115 and WM239 were used to investigate differences in expression and functionality of VEGF and VEGFR2 in vitro and in vivo with the anti-VEGF antibody bevacizumab. Both WM115 and WM239 cells expressed VEGF and VEGFR2, the levels of which were modulated by hypoxia. Detection of native and phosphorylated VEGFR2 in subcellular fractions under serum-free conditions showed the presence of a functional autocrine as well as intracrine VEGF/VEGFR2 signaling loops. Interestingly, treatment of WM115 and WM239 cells with increasing doses of bevacizumab (0-300 µg/ml) in vitro did not show any significant inhibition of VEGFR2 phosphorylation. Small-molecule tyrosine kinase inhibitor, sunitinib, caused an inhibition of VEGFR2 phosphorylation in WM239 but not in WM115 cells. An increase in cell proliferation was observed in WM115 cells treated with bevacizumab, whereas sunitinib inhibited proliferation. When xenografted to immune-deficient mice, we found bevacizumab to be an effective antiangiogenic but not antitumorigenic agent for both cell lines. Because bevacizumab is unable to neutralize murine VEGF, this supports a paracrine angiogenic response. We propose that the failure of bevacizumab to generate an antitumorigenic effect may be related to its generation of enhanced autocrine/intracrine signaling in the cancer cells themselves. Collectively, these results suggest that, for cancers with intracrine VEGF/ VEGFR2 signaling loops, small-molecule inhibitors of VEGFR2 may be more effective than neutralizing antibodies at disease control.

Coordination of VEGF receptor trafficking and signaling by coreceptors

During development, regeneration and in certain pathological settings, the vasculature is expanded and remodeled substantially. Proper morphogenesis and function of blood vessels are essential in multicellular organisms. Upon stimulation with growth factors including vascular endothelial growth factors (VEGFs), the activation, internalization and sorting of receptor tyrosine kinases (RTKs) orchestrate developmental processes and the homeostatic maintenance of all organs including the vasculature. Previously, RTK signaling was thought to occur exclusively at the plasma membrane, a process that was subsequently terminated by endocytosis and receptor degradation. However, this model turned out to be an oversimplification and there is now a substantial amount of reports indicating that receptor internalization and trafficking to intracellular compartments depends on coreceptors leading to the activation of specific signaling pathways. Here we review the latest findings concerning endocytosis and intracellular trafficking of VEGFRs. The body of evidence is compelling that VEGF receptor trafficking is coordinated with other proteins such as Neuropilin-1, ephrin-B2, VE-cadherin and protein phosphatases.

Regulation of angiogenesis by PI3K signaling networks

Phosphoinositide 3-kinases (PI3Ks) are an evolutionary conserved family of lipid kinases that control cell growth, metabolism and survival. By generating lipid second messengers that interact with specialized lipid-binding domains found in a wide spectrum of signaling molecules, PI3Ks instigate signaling through a network of downstream effector pathways. Genetic studies in zebrafish and mice revealed the critical importance of intact PI3K signaling in the endothelium and provided first insights into how individual PI3K isoforms are utilized to control vascular development and function. Here, we review the myriad roles of PI3Ks in the endothelium and the mechanisms through which they couple environmental signals with specific steps of angiogenic vessel growth.

VEGFA and tumour angiogenesis

In this review we summarize the current understanding of signal transduction downstream of vascular endothelial growth factor A (VEGFA) and its receptor VEGFR2, and the relationship between these signal transduction pathways and the hallmark responses of VEGFA, angiogenesis and vascular permeability. These physiological responses involve a number of effectors, including extracellular signal-regulated kinases (ERKs), Src, phosphoinositide 3 kinase (PI3K)/Akt, focal adhesion kinase (FAK), Rho family GTPases, endothelial NO and p38 mitogen-activated protein kinase (MAPK). Several of these factors are involved in the regulation of both angiogenesis and vascular permeability. Tumour angiogenesis primarily relies on VEGFA-driven responses, which to a large extent result in a dysfunctional vasculature. The reason for this remains unclear, although it appears that certain aspects of the VEGFA-stimulated angiogenic milieu (high level of microvascular density and permeability) promote tumour expansion. The high degree of redundancy and complexity of VEGFA-driven tumour angiogenesis may explain why tumours commonly develop resistance to anti-angiogenic therapy targeting VEGFA signal transduction.