Mapping the charged residues in the second immunoglobulin-like domain of the vascular endothelial growth factor/placenta growth factor receptor Flt-1 required for binding and structural stability

Effect of Placental Derived Nucleoproteins on liver regeneration in DEN-induced liver fibrosis model

BACKGROUND

Placental Derived Nucleoproteins (PDNs) is commonly associated with the process of angiogenesis, and doesn't affect the healthy vasculature. PDNs are clinically estimated for the treatment of cancer cases and severe hepatic injuries. Thus, the pathophysiological effects of PDNs targeting liver fibrosis is a concern.

OBJECTIVES

To assess the molecular, histopathological, and chemical impact of PDNs on liver regeneration in Diethylnitrosamine (DEN)-induced mice liver fibrosis.

METHODS

Normal untreated reference group of ten mice and two groups of induced liver cirrhosis using the recommended weekly dose of Diethylnitrosamine in total of eleven doses, initially 20 mg/kg body weight, and then 30 mg/kg in the third week, followed by 50 mg/kg for the last eight weeks, one of them combined treatment aligned with injection with total dose of extracted PDNs 25 mg/kg, in comparison to PDNs only treated group. An autopsy was performed after 22 weeks of the initial dose of DEN in each group. Molecular characterization of Alpha smooth muscle actin, TGFβ and NF-κB biomarkers for liver then liver function panel were analyzed and finally hepatopathological changes were observed using H&E stain and Sirius red stain.

RESULTS

Liver enzymes, total bilirubin and total proteins in tissue in PDNs-DEN treated models were controlled in the direction of normal group and 50 % reduction of fibrosis in comparing to DEN-treated models. The cellular arrangement of fibrosis in the DEN entire groups were differentiated with high significant impact on the survival of mice. Increased levels of the biochemical markers in liver homogenate, loss of tissue architecture, and proliferation were observed in induced groups and down regulation of alpha smooth muscle actin, TGFβ and NF-κB.

CONCLUSION

This finding demonstrates an improvement of Liver tissue induced fibrosis using DEN combined with PDNs. This strategy is to generate an animal model with a lower occurrence of fibrosis in a short time treatment regarding liver regeneration.

VEGF-A/VEGFR-1 signalling and chemotherapy-induced neuropathic pain: therapeutic potential of a novel anti-VEGFR-1 monoclonal antibody

BACKGROUND

Neuropathic pain is a clinically relevant adverse effect of several anticancer drugs that markedly impairs patients' quality of life and frequently leads to dose reduction or therapy discontinuation. The poor knowledge about the mechanisms involved in neuropathy development and pain chronicization, and the lack of effective therapies, make treatment of chemotherapy-induced neuropathic pain an unmet medical need. In this context, the vascular endothelial growth factor A (VEGF-A) has emerged as a candidate neuropathy hallmark and its decrease has been related to pain relief. In the present study, we have investigated the role of VEGF-A and its receptors, VEGFR-1 and VEGFR-2, in pain signalling and in chemotherapy-induced neuropathy establishment as well as the therapeutic potential of receptor blockade in the management of pain.

METHODS

Behavioural and electrophysiological analyses were performed in an in vivo murine model, by using selective receptor agonists, blocking monoclonal antibodies or siRNA-mediated silencing of VEGF-A and VEGFRs. Expression of VEGF-A and VEGFR-1 in astrocytes and neurons was detected by immunofluorescence staining and confocal microscopy analysis.

RESULTS

In mice, the intrathecal infusion of VEGF-A (VEGF165 isoforms) induced a dose-dependent noxious hypersensitivity and this effect was mediated by VEGFR-1. Consistently, electrophysiological studies indicated that VEGF-A strongly stimulated the spinal nociceptive neurons activity through VEGFR-1. In the dorsal horn of the spinal cord of animals affected by oxaliplatin-induced neuropathy, VEGF-A expression was increased in astrocytes while VEGFR-1 was mainly detected in neurons, suggesting a VEGF-A/VEGFR-1-mediated astrocyte-neuron cross-talk in neuropathic pain pathophysiology. Accordingly, the selective knockdown of astrocytic VEGF-A by intraspinal injection of shRNAmir blocked the development of oxaliplatin-induced neuropathic hyperalgesia and allodynia. Interestingly, both intrathecal and systemic administration of the novel anti-VEGFR-1 monoclonal antibody D16F7, endowed with anti-angiogenic and antitumor properties, reverted oxaliplatin-induced neuropathic pain. Besides, D16F7 effectively relieved hypersensitivity induced by other neurotoxic chemotherapeutic agents, such as paclitaxel and vincristine.

CONCLUSIONS

These data strongly support the role of the VEGF-A/VEGFR-1 system in mediating chemotherapy-induced neuropathic pain at the central nervous system level. Thus, treatment with the anti-VEGFR-1 mAb D16F7, besides exerting antitumor activity, might result in the additional advantage of attenuating neuropathic pain when combined with neurotoxic anticancer agents.

Therapeutic implication of vascular endothelial growth factor receptor-1 (VEGFR-1) targeting in cancer cells and tumor microenvironment by competitive and non-competitive inhibitors

The vascular endothelial growth factor receptor-1 (VEGFR-1) is a tyrosine kinase receptor for VEGF-A, VEGF-B, and placental growth factor (PlGF) ligands that is expressed in endothelial, myelomonocytic and tumor cells. VEGF-B and PlGF exclusively bind to VEGFR-1, whereas VEGF-A also binds to VEGFR-2. At variance with VEGFR-2, VEGFR-1 does not play a relevant role in physiological angiogenesis in the adult, while it is important in tumor-associated angiogenesis. VEGFR-1 and PlGF are expressed in a variety of tumors, promote invasiveness and contribute to resistance to anti-VEGF-A therapy. The currently approved antiangiogenic therapies for the treatment of a variety of solid tumors hamper VEGF-A signaling mediated by both VEGFR-2 and VEGFR-1 [i.e., the monoclonal antibody (mAb) anti-VEGF-A bevacizumab, the chimeric molecule aflibercept and several small molecule tyrosine kinase inhibitors] or exclusively by VEGFR-2 (i.e., the mAb anti-VEGFR-2 ramucirumab). However, molecules that interfere with VEGF-A/VEGFR-2 signaling determine severe adverse effects due to inhibition of physiological angiogenesis and their efficacy is hampered by tumor infiltration of protumoral myeloid cells. Blockade of VEGFR-1 may exert anti-tumor activity by multiple mechanisms: a) inhibition of tumor-associated angiogenesis; b) reduction of myeloid progenitor mobilization and tumor infiltration by VEGFR-1 expressing M2 macrophages, which contribute to tumor progression and spreading; c) inhibition of invasiveness, vasculogenic mimicry and survival of VEGFR-1 positive tumor cells. As a consequence of these properties, molecules targeting VEGFR-1 are expected to produce less adverse effects and to counteract resistance towards anti-VEGF-A therapies. More interestingly, selective VEGFR-1 inhibition might enhance the efficacy of immunotherapy with immune checkpoint inhibitors. In this review, we will examine the experimental evidence available so far that supports targeting VEGFR-1 signal transduction pathway for cancer treatment by competitive inhibitors that prevent growth factor interaction with the receptor and non-competitive inhibitors that hamper receptor activation without affecting ligand binding.

The Anti-Vascular Endothelial Growth Factor Receptor-1 Monoclonal Antibody D16F7 Inhibits Glioma Growth and Angiogenesis In Vivo

The vascular endothelial growth factor (VEGF) receptor-1 (VEGFR-1) is a tyrosine kinase receptor that does not play a relevant role in physiologic angiogenesis in adults, whereas it is important in tumor angiogenesis. In high-grade glioma VEGFR-1 expression by tumor endothelium and neoplastic cells contributes to the aggressive phenotype. We recently generated an anti-VEGFR-1 monoclonal antibody (D16F7 mAb) characterized by a novel mechanism of action, since it hampers receptor activation without interfering with ligand binding. The mAb is able to inhibit chemotaxis and extracellular matrix invasion of glioma cells in vitro stimulated by VEGF-A and by the VEGFR-1-selective ligand placental growth factor (PlGF). In this study, we have investigated the influence of D16F7 on glioma growth and angiogenesis in vivo using C6 glioma cells transfected with the human VEGFR-1. D16F7 was able to inhibit receptor activation and migration and extracellular matrix invasion of C6 cells overexpressing the receptor in response to PlGF and VEGF-A. In nude mice, treatment with 10 and 20 mg/kg D16F7 as a single agent was well tolerated and significantly inhibited glioma growth (P < 0.001). Strikingly, in an intracranial orthotopic model, mice dosed with 20 mg/kg D16F7 demonstrated a 65% increase in median survival time compared with vehicle-treated controls (P < 0.001) with a high percentage of long-term survivors (46%). These effects were associated with glioma cell apoptosis and decreased tumor-associated vessel formation. Overall, these results highlight the therapeutic potential of D16F7 in glioma treatment, deserving further investigation after a humanization process as single agent or in combination therapies.

Arg-Leu-Tyr-Glu tetrapeptide inhibits tumor progression by suppressing angiogenesis and vascular permeability via VEGF receptor-2 antagonism

The tetrapeptide Arg-Leu-Tyr-Glu (RLYE) is known to inhibit vascular endothelial growth factor-A (VEGF-A)-induced angiogenesis in vitro. Herein, we examined its underlying mechanism and antitumor activity associated with vascular remodeling. RLYE inhibited VEGF-A-induced angiogenesis in a mouse model and suppressed VEGF-A-induced angiogenic signal cascades in human endothelial cells. However, RLYE showed no inhibitory effect on VEGF-A-induced proliferation and migration of multiple myeloma cells expressing VEGF receptor (VEGFR)-1, but not VEGFR-2. In addition, RLYE showed no inhibitory effect on angiogenic activities induced by VEGF-B, basic fibroblast growth factor, epithermal growth factor, sphingosine-1-phosphate, and placental growth factor. RLYE bound specifically to VEGFR-2 at the VEGF-A binding site, thereby blocking VEGF-A-VEGFR-2 binding and VEGF-A-induced VEGFR-2 internalization. The RLYE peptide inhibited tumor growth and metastasis via suppression of tumor angiogenesis in tumor-bearing mice. Moreover, RLYE showed a synergistic effect of the cytotoxic agent irinotecan on tumor cell apoptosis and tumor progression via tumor vessel normalization due to stabilization of VE-cadherin-mediated adherens junction, improvement of pericyte coverage, and inhibition of vascular leakage in tumors. Our results suggest that RLYE can be used as an antiangiogenic and tumor blood vessel remodeling agent for inhibition of tumor growth and metastasis by antagonizing VEGFR-2, with the synergistic anti-cancer effect via enhancement of drug delivery and therapeutic efficacy.

Vascular endothelial growth factor A: just one of multiple mechanisms for sex-specific vascular development within the testis?

Testis development from an indifferent gonad is a critical step in embryogenesis. A hallmark of testis differentiation is sex-specific vascularization that occurs as endothelial cells migrate from the adjacent mesonephros into the testis to surround Sertoli-germ cell aggregates and induce seminiferous cord formation. Many in vitro experiments have demonstrated that vascular endothelial growth factor A (VEGFA) is a critical regulator of this process. Both inhibitors to VEGFA signal transduction and excess VEGFA isoforms in testis organ cultures impaired vascular development and seminiferous cord formation. However, in vivo models using mice which selectively eliminated all VEGFA isoforms: in Sertoli and germ cells (pDmrt1-Cre;Vegfa(-/-)); Sertoli and Leydig cells (Amhr2-Cre;Vegfa(-/-)) or Sertoli cells (Amh-Cre;Vegfa(-/-) and Sry-Cre;Vegfa(-/-)) displayed testes with observably normal cords and vasculature at postnatal day 0 and onwards. Embryonic testis development may be delayed in these mice; however, the postnatal data indicate that VEGFA isoforms secreted from Sertoli, Leydig or germ cells are not required for testis morphogenesis within the mouse. A Vegfa signal transduction array was employed on postnatal testes from Sry-Cre;Vegfa(-/-) versus controls. Ptgs1 (Cox1) was the only upregulated gene (fivefold). COX1 stimulates angiogenesis and upregulates, VEGFA, Prostaglandin E2 (PGE2) and PGD2. Thus, other gene pathways may compensate for VEGFA loss, similar to multiple independent mechanisms to maintain SOX9 expression. Multiple independent mechanism that induce vascular development in the testis may contribute to and safeguard the sex-specific vasculature development responsible for inducing seminiferous cord formation, thus ensuring appropriate testis morphogenesis in the male.

Vascular Endothelial Growth Factor-B in Physiology and Disease

Vascular endothelial growth factor-B (VEGF-B), discovered over 15 years ago, has long been seen as one of the more ambiguous members of the VEGF family. VEGF-B is produced as two isoforms: one that binds strongly to heparan sulfate in the pericellular matrix and a soluble form that can acquire binding via proteolytic processing. Both forms of VEGF-B bind to VEGF-receptor 1 (VEGFR-1) and the neuropilin-1 (NRP-1) coreceptor, which are expressed mainly in blood vascular endothelial cells. VEGF-B-deficient mice and rats are viable without any overt phenotype, and the ability of VEGF-B to induce angiogenesis in most tissues is weak. This has been a puzzle, as the related placenta growth factor (PlGF) binds to the same receptors and induces angiogenesis and arteriogenesis in a variety of tissues. However, it seems that VEGF-B is a vascular growth factor that is more tissue specific and can have trophic and metabolic effects, and its binding to VEGFR-1 shows subtle but important differences compared with that of PlGF. VEGF-B has the potential to induce coronary vessel growth and cardiac hypertrophy, which can protect the heart from ischemic damage as well as heart failure. In addition, VEGF-B is abundantly expressed in tissues with highly active energy metabolism, where it could support significant metabolic functions. VEGF-B also has a role in neuroprotection, but unlike other members of the VEGF family, it does not have a clear role in tumor progression. Here we review what is hitherto known about the functions of this growth factor in physiology and disease.

Platelet-derived growth factors and their receptors: structural and functional perspectives

The four types of platelet-derived growth factors (PDGFs) and the two types of PDGF receptors (PDGFRs, which belong to class III receptor tyrosine kinases) have important functions in the development of connective tissue cells. Recent structural studies have revealed novel mechanisms of PDGFs in propeptide loading and receptor recognition/activation. The detailed structural understanding of PDGF-PDGFR signaling has provided a template that can aid therapeutic intervention to counteract the aberrant signaling of this normally silent pathway, especially in proliferative diseases such as cancer. This review summarizes the advances in the PDGF system with a focus on relating the structural and functional understandings, and discusses the basic aspects of PDGFs and PDGFRs, the mechanisms of activation, and the insights into the therapeutic antagonism of PDGFRs. This article is part of a Special Issue entitled: Emerging recognition and activation mechanisms of receptor tyrosine kinases.

The discovery of placenta growth factor and its biological activity

Angiogenesis is a complex biological phenomenon crucial for a correct embryonic development and for post-natal growth. In adult life, it is a tightly regulated process confined to the uterus and ovary during the different phases of the menstrual cycle and to the heart and skeletal muscles after prolonged and sustained physical exercise. Conversly, angiogenesis is one of the major pathological changes associated with several complex diseases like cancer, atherosclerosis, arthritis, diabetic retinopathy and age-related macular degeneration. Among the several molecular players involved in angiogenesis, some members of VEGF family, VEGF-A, VEGF-B and placenta growth factor (PlGF), and the related receptors VEGF receptor 1 (VEGFR-1, also known as Flt-1) and VEGF receptor 2 (VEGFR-2, also known as Flk-1 in mice and KDR in human) have a decisive role. In this review, we describe the discovery and molecular characteristics of PlGF, and discuss the biological role of this growth factor in physiological and pathological conditions.

Tying the knot: the cystine signature and molecular-recognition processes of the vascular endothelial growth factor family of angiogenic cytokines

The cystine-knot motif, made up of three intertwined disulfide bridges, is a unique feature of several toxins, cyclotides and growth factors, and occurs in a variety of species, including fungi, insects, molluscs and mammals. Growth factor molecules containing the cystine-knot motif serve as ligands for a diverse range of receptors and play an important role in extracellular signalling. This superfamily of polypeptides comprises several homodimeric and heterodimeric molecules that are central characters in both health and disease. Amongst these molecules are a group of proteins that belong to the vascular endothelial growth factor (VEGF) subfamily. The members of this family are known angiogenic factors that regulate processes leading to blood vessel formation in physiological and pathological conditions. The focus of the present review is on the structural characteristics of proteins that belong to the VEGF family and on signal-transduction pathways that become initiated via the VEGF receptors.

Pilot study of extracorporeal removal of soluble fms-like tyrosine kinase 1 in preeclampsia

BACKGROUND

Targeted therapies to stabilize the clinical manifestations and prolong pregnancy in preeclampsia do not exist. Soluble fms-like tyrosine kinase 1 (sFlt-1), an alternatively spliced variant of the vascular endothelial growth factor receptor 1, induces a preeclampsia-like phenotype in experimental models and circulates at elevated levels in human preeclampsia. Removing sFlt-1 may benefit women with very preterm (<32 weeks) preeclampsia.

METHODS AND RESULTS

We first show that negatively charged dextran sulfate cellulose columns adsorb sFlt-1 in vitro. In 5 women with very preterm preeclampsia and elevated circulating sFlt-1 levels, we next demonstrate that a single dextran sulfate cellulose apheresis treatment reduces circulating sFlt-1 levels in a dose-dependent fashion. Finally, we performed multiple apheresis treatments in 3 additional women with very preterm (gestational age at admission 28, 30, and 27+4 weeks) preeclampsia and elevated circulating sFlt-1 levels. Dextran sulfate apheresis lowered circulating sFlt-1, reduced proteinuria, and stabilized blood pressure without apparent adverse events to mother and fetus. Pregnancy lasted for 15 and 19 days in women treated twice and 23 days in a woman treated 4 times. In each, there was evidence of fetal growth.

CONCLUSIONS

This pilot study supports the hypothesis that extracorporeal apheresis can lower circulating sFlt-1 in very preterm preeclampsia. Further studies are warranted to determine whether this intervention safely and effectively prolongs pregnancy and improves maternal and fetal outcomes in this setting.

VEGF inhibitor (Iressa) arrests histone deacetylase expression: single-cell cotransfection imaging cytometry for multi-target-multi-drug analysis

Multi-target-multi-drug approaches are needed to accelerate the process of drug discovery screening and to design efficient therapeutic strategies against diseases that involve alterations in multiple cellular targets. Herein we report single-cell cotransfection imaging cytometry to quantitatively screen drug-induced off-target effects. Vascular endothelial growth factor (VEGF) and histone deacetylase (HDAC) genes amplified from the genomic DNA were cloned in fluorescently tagged gene constructs (RFP-HDAC/YFP-VEGF). These gene constructs were cotransfected in HEK-293 cells to explore the possibility of off-target effects of 4-phenylbutyrate and Iressa on the expression of VEGF and HDAC through single-cell imaging cytometry. Iressa (10 µM) treatment at the time of cotransfection or 48 h after cotransfection of RFP-HDAC/YFP-VEGF plasmids in HEK-293 cells resulted in off-target effects on HDAC expression. These results suggest possible applications of Iressa in the treatment of diseases in which expression of both HDAC and VEGF should be inhibited. 4-Phenylbutyrate (2.0 mM) did not show any off-target effects on VEGF expression. The developed quantitative multicolor live single-cell cotransfection imaging can be employed to select better drug combinations for faster screening and greater accuracy in multi-target-multi-drug analysis by increasing the on-target/desired off-target effects and eliminating the undesirable off-target effects.

Effective suppression of vascular network formation by combination of antibodies blocking VEGFR ligand binding and receptor dimerization

Antibodies that block vascular endothelial growth factor (VEGF) have become an integral part of antiangiogenic tumor therapy, and antibodies targeting other VEGFs and receptors (VEGFRs) are in clinical trials. Typically receptor-blocking antibodies are targeted to the VEGFR ligand-binding site. Here we describe a monoclonal antibody that inhibits VEGFR-3 homodimer and VEGFR-3/VEGFR-2 heterodimer formation, signal transduction, as well as ligand-induced migration and sprouting of microvascular endothelial cells. Importantly, we show that combined use of antibodies blocking ligand binding and receptor dimerization improves VEGFR inhibition and results in stronger inhibition of endothelial sprouting and vascular network formation in vivo. These results suggest that receptor dimerization inhibitors could be used to enhance antiangiogenic activity of antibodies blocking ligand binding in tumor therapy.

Structural insights into the binding of vascular endothelial growth factor-B by VEGFR-1(D2): recognition and specificity

The formation of blood vessels (angiogenesis) is a highly orchestrated sequence of events involving crucial receptor-ligand interactions. Angiogenesis is critical for physiological processes such as development, wound healing, reproduction, tissue regeneration, and remodeling. It also plays a major role in sustaining tumor progression and chronic inflammation. Vascular endothelial growth factor (VEGF)-B, a member of the VEGF family of angiogenic growth factors, effects blood vessel formation by binding to a tyrosine kinase receptor, VEGFR-1. There is growing evidence of the important role played by VEGF-B in physiological and pathological vasculogenesis. Development of VEGF-B antagonists, which inhibit the interaction of this molecule with its cognate receptor, would be important for the treatment of pathologies associated specifically with this growth factor. In this study, we present the crystal structure of the complex of VEGF-B with domain 2 of VEGFR-1 at 2.7 A resolution. Our analysis reveals that each molecule of the ligand engages two receptor molecules using two symmetrical binding sites. Based on these interactions, we identify the receptor-binding determinants on VEGF-B and shed light on the differences in specificity towards VEGFR-1 among the different VEGF homologs.

Structural determinants of growth factor binding and specificity by VEGF receptor 2

Vascular endothelial growth factors (VEGFs) regulate blood and lymph vessel formation through activation of three receptor tyrosine kinases, VEGFR-1, -2, and -3. The extracellular domain of VEGF receptors consists of seven immunoglobulin homology domains, which, upon ligand binding, promote receptor dimerization. Dimerization initiates transmembrane signaling, which activates the intracellular tyrosine kinase domain of the receptor. VEGF-C stimulates lymphangiogenesis and contributes to pathological angiogenesis via VEGFR-3. However, proteolytically processed VEGF-C also stimulates VEGFR-2, the predominant transducer of signals required for physiological and pathological angiogenesis. Here we present the crystal structure of VEGF-C bound to the VEGFR-2 high-affinity-binding site, which consists of immunoglobulin homology domains D2 and D3. This structure reveals a symmetrical 22 complex, in which left-handed twisted receptor domains wrap around the 2-fold axis of VEGF-C. In the VEGFs, receptor specificity is determined by an N-terminal alpha helix and three peptide loops. Our structure shows that two of these loops in VEGF-C bind to VEGFR-2 subdomains D2 and D3, while one interacts primarily with D3. Additionally, the N-terminal helix of VEGF-C interacts with D2, and the groove separating the two VEGF-C monomers binds to the D2/D3 linker. VEGF-C, unlike VEGF-A, does not bind VEGFR-1. We therefore created VEGFR-1/VEGFR-2 chimeric proteins to further study receptor specificity. This biochemical analysis, together with our structural data, defined VEGFR-2 residues critical for the binding of VEGF-A and VEGF-C. Our results provide significant insights into the structural features that determine the high affinity and specificity of VEGF/VEGFR interactions.

VEGF receptor signaling in vertebrate development

The secreted glycoprotein vascular endothelial growth factor A (VEGF or VEGFA) affects many different cell types and modifies a wide spectrum of cellular behaviors in tissue culture models, including proliferation, migration, differentiation and survival. The versatility of VEGF signaling is reflected in the complex composition of its cell surface receptors and their ability to activate a variety of different downstream signaling molecules. A major challenge for VEGF research is to determine which of the specific signaling pathways identified in vitro control development and homeostasis of tissues containing VEGF-responsive cell types in vivo.

Novel anti-VEGF chimeric molecules delivered by AAV vectors for inhibition of retinal neovascularization

Vascular endothelial growth factor (VEGF) is important in pathological neovascularization, which is a key component of diseases such as the wet form of age-related macular degeneration, proliferative diabetic retinopathy and cancer. One of the most potent naturally occurring VEGF binders is VEGF receptor Flt-1. We have generated two novel chimeric VEGF-binding molecules, sFLT01 and sFLT02, which consist of the second immunoglobulin (IgG)-like domain of Flt-1 fused either to a human IgG1 Fc or solely to the CH3 domain of IgG1 Fc through a polyglycine linker 9Gly. In vitro analysis showed that these novel molecules are high-affinity VEGF binders. We have demonstrated that adeno-associated virus serotype 2 (AAV2)-mediated intravitreal gene delivery of sFLT01 efficiently inhibits angiogenesis in the mouse oxygen-induced retinopathy model. There were no histological observations of toxicity upon persistent ocular expression of sFLT01 for up to 12 months following intravitreal AAV2-based delivery in the rodent eye. Our data suggest that AAV2-mediated intravitreal gene delivery of our novel molecules may be a safe and effective treatment for retinal neovascularization.

A proangiogenic peptide derived from vascular endothelial growth factor receptor-1 acts through α5β1 integrin

Vascular endothelial growth factor receptor-1 (VEGFR-1) is a tyrosine kinase receptor for growth factors of the VEGF family. Endothelial cells express a membrane-bound and a soluble variant of this protein, the latter being mainly considered as a negative regulator of VEGF-A signaling. We previously reported that the soluble form is deposited in the extracellular matrix produced by endothelial cells in culture and is able to promote cell adhesion and migration through binding to α5β1 integrin. In this study, we demonstrate that the Ig-like domain II of VEGFR-1, which contains the binding determinants for the growth factors, is involved in the interaction with α5β1 integrin. To identify domain regions involved in integrin binding, we designed 12 peptides putatively mimicking the domain II surface and tested their ability to inhibit α5β1-mediated endothelial cell adhesion to soluble VEGFR-1 and directly support cell adhesion. One peptide endowed with both these properties was identified and shown to inhibit endothelial cell migration toward soluble VEGFR-1 as well. This peptide directly binds α5β1 integrin, but not VEGF-A, inducing endothelial cell tubule formation in vitro and neoangiogenesis in vivo. Alanine scanning mutagenesis of the peptide defined which residues were responsible for its biologic activity and integrin binding.

Crystal Structure of the Orf Virus NZ2 Variant of Vascular Endothelial Growth Factor-E

Mammalian vascular endothelial growth factors constitute a family of polypeptides, vascular endothelial growth factor (VEGF)-A, -B, -C, -D and placenta growth factor (PlGF), that regulate blood and lymphatic vessel development. VEGFs bind to three types of receptor tyrosine kinases, VEGF receptors 1, 2, and 3, that are predominantly expressed on endothelial and some hematopoietic cells. Pox viruses of the Orf family encode highly related proteins called VEGF-E that show only 25-35% amino acid identity with VEGF-A but bind with comparable affinity to VEGFR-2. The crystal structure of VEGF-E NZ2 described here reveals high similarity to the known structural homologs VEGF-A, PlGF, and the snake venoms Vammin and VR-1, which are all homodimers and contain the characteristic cysteine knot motif. Distinct conformational differences are observed in loop L1 and particularly in L3, which contains a highly flexible GS-rich motif that differs from all other structural homologs. Based on our structure, we created chimeric proteins by exchanging selected segments in L1 and L3 with the corresponding sequences from PlGF. Single loop mutants did not bind to either receptor, whereas a VEGF-E mutant in which both L1 and L3 were replaced gained affinity for VEGFR-1, illustrating the possibility to engineer receptor-specific chimeric VEGF molecules. In addition, changing arginine 46 to isoleucine in L1 significantly increased the affinity of VEGF-E for both VEGF receptors.

Crystal structure of human vascular endothelial growth factor-B: identification of amino acids important for receptor binding

The development of blood vessels (angiogenesis) is critical throughout embryogenesis and in some normal postnatal physiological processes. Pathological angiogenesis has a pivotal role in sustaining tumour growth and chronic inflammation. Vascular endothelial growth factor-B (VEGF-B) is a member of the VEGF family of growth factors that regulate blood vessel and lymphatic angiogenesis. VEGF-B is closely related to VEGF-A and placenta growth factor (PlGF), but unlike VEGF-A, which binds to two receptor tyrosine kinases VEGFR-1 (Flt-1) and VEGFR-2 (Flk-1/KDR), VEGF-B and PlGF bind to VEGFR-1 and not VEGFR-2. There is growing evidence of a role for VEGF-B in physiological and pathological blood vessel angiogenesis. VEGF-B may provide novel therapeutic strategies for the treatment of vascular disease and be a potential therapeutic target in aberrant vessel formation. To help understand at the molecular level the differential receptor binding profile of the VEGF family of growth factors we have determined the crystal structure of human VEGF-B(10-108) at 2.48 Angstroms resolution. The overall structure is very similar to that of the previously determined cysteine-knot motif growth factors: VEGF-A, PlGF and platelet-derived growth factor-B (PDGF-B). We also present a predicted model for the association of VEGF-B with the second domain of its receptor, VEGFR-1. Based on this interaction and the present structural data of the native protein, we have identified several putative residues that could play an important role in receptor recognition and specificity.

The role of VEGF receptors in angiogenesis; complex partnerships

Vascular endothelial growth factors (VEGFs) regulate blood and lymphatic vessel development and homeostasis but also have profound effects on neural cells. VEGFs are predominantly produced by endothelial, hematopoietic and stromal cells in response to hypoxia and upon stimulation with growth factors such as transforming growth factors, interleukins or platelet-derived growth factor. VEGFs bind to three variants of type III receptor tyrosine kinases, VEGF receptor 1, 2 and 3. Each VEGF isoform binds to a particular subset of these receptors giving rise to the formation of receptor homo- and heterodimers that activate discrete signaling pathways. Signal specificity of VEGF receptors is further modulated upon recruitment of coreceptors, such as neuropilins, heparan sulfate, integrins or cadherins. Here we summarize the knowledge accumulated since the discovery of these proteins more than 20 years ago with the emphasis on the signaling pathways activated by VEGF receptors in endothelial cells during cell migration, growth and differentiation.

Redundant roles of VEGF-B and PlGF during selective VEGF-A blockade in mice

Vascular endothelial growth factor-A (VEGF-A) and its 2 transmembrane tyrosine-kinase receptors, VEGFR-1 and VEGFR-2, constitute a ligand-receptor signaling system that is crucial for developmental angiogenesis. VEGF-B and placental growth factor (PlGF) activate VEGFR-1 selectively, however, mice lacking either ligand display only minor developmental defects. We hypothesized that the relative contributions of VEGF-B and PlGF to VEGFR-1 signaling may be masked in the presence of VEGF-A, which is abundantly expressed during postnatal development. To test this hypothesis, neonatal or adult mice were treated with a monoclonal antibody (G6-23-IgG) blocking murine VEGF-A or a soluble VEGFR-1 receptor IgG chimeric construct [mFlt(1-3)-IgG], which neutralizes VEGF-A, VEGF-B, and PlGF. Both compounds attenuated growth and survival of neonatal mice to similar extents and the pathophysiologic alterations, including a reduction in organ size and vascularization, changes in gene expression, and hematologic end points, were essentially indistinguishable. In adult mice, we observed only minor changes in response to treatment, which were similar between both anti-VEGF compounds. In conclusion, our findings suggest that PlGF and VEGF-B do not compensate during conditions of VEGF-A blockade, suggesting a minor role for compensatory VEGFR-1 signaling during postnatal development and vascular homeostasis in adults. The absence of compensatory VEGFR-1 signaling by VEGF-B and PlGF may have important implications for the development of anticancer strategies targeting the VEGF ligand/receptor system.

Identification of Placenta Growth Factor Determinants for Binding and Activation of Flt-1 Receptor

Placenta growth factor (PlGF) belongs to the vascular endothelial growth factor (VEGF) family and represents a key regulator of angiogenic events in pathological conditions. PlGF exerts its biological function through the binding and activation of the seven immunoglobulin-like domain receptor Flt-1, also known as VEGFR-1. Here, we report the first detailed mutagenesis studies that provide a basis for understanding molecular recognition between PlGF-1 and Flt-1, highlighting some of the residues that are critical for receptor recognition. Mutagenesis analysis, performed on the basis of a structural model of interaction between PlGF and the minimal binding domain of Flt-1, has led to the identification of several PlGF-1 residues involved in Flt-1 recognition. The two negatively charged residues, Asp-72 and Glu-73, located in the beta3-beta4 loop, are critical for Flt-1 binding. Other mutations, which bring about a significant decrease in PlGF binding activity, are Gln-27, located in the N-terminal alpha-helix, and Pro-98 and Tyr-100 on the beta6 strand. The mutation of one of the two glycosylated residues of PlGF, Asn-84, generates a PlGF variant with reduced binding activity. This indicates that, unlike in VEGF, glycosylation plays an important role in Flt-1 binding. The double mutation of residues Asp-72 and Glu-73 generates a PlGF variant unable to bind and activate the receptor molecules on the cell surface. This variant failed to induce in vitro capillary-like tube formation of primary endothelial cells or neo-angiogenesis in an in vivo chorioallantoic membrane assay.

Suppression of tumor growth and metastasis by a VEGFR-1 antagonizing peptide identified from a phage display library

Although the VEGF-Flk-1-pathway has been known as the major driving force of angiogenesis, new evidence has shown that VEGFR-1/Flt-1 plays important roles during the neovascularization under pathological conditions including tumor, atherosclerosis and arthritis. In search of Flt-1 receptor antagonizing peptides, we screened a phage display 12-mer-peptide library with recombinant Flt-1 protein. Seven candidate peptides were identified that specifically bound to VEGF receptor Flt-1, of which peptide F56 (WHSDMEWWYLLG) almost abolished VEGF binding to receptor Flt-1 in vitro. In vivo, F56 fused with DHFR (DHFR-F56) inhibited angiogenesis in a CAM assay. Moreover, DHFR-F56 significantly inhibited the growth of nodules of human gastric cancer cell line MGC-803 in BALB/c nude mice. Histological analyses showed that necrosis of the implanted tumor was markedly enhanced following treatment with DHFR-F56. In the severe combined immunodeficiency disease (SCID) mouse model for studying metastasis of the human breast cancer cell line BICR-H1, synthetic peptide F56 significantly inhibited tumor growth and lung metastases. Taken together, our results have demonstrated that peptide F56, as a Flt-1 receptor antagonist, fulfilled the antiangiogenic and antimetastatic effects by specifically interfering with the interaction between VEGF and receptor Flt-1. Thus, short peptide F56 may have clinical potential in tumor therapy.

Vascular endothelial growth factor: basic science and clinical progress

Vascular endothelial growth factor (VEGF) is an endothelial cell-specific mitogen in vitro and an angiogenic inducer in a variety of in vivo models. Hypoxia has been shown to be a major inducer of VEGF gene transcription. The tyrosine kinases Flt-1 (VEGFR-1) and Flk-1/KDR (VEGFR-2) are high-affinity VEGF receptors. The role of VEGF in developmental angiogenesis is emphasized by the finding that loss of a single VEGF allele results in defective vascularization and early embryonic lethality. VEGF is critical also for reproductive and bone angiogenesis. Substantial evidence also implicates VEGF as a mediator of pathological angiogenesis. In situ hybridization studies demonstrate expression of VEGF mRNA in the majority of human tumors. Anti-VEGF monoclonal antibodies and other VEGF inhibitors block the growth of several tumor cell lines in nude mice. Clinical trials with various VEGF inhibitors in a variety of malignancies are ongoing. Very recently, an anti-VEGF monoclonal antibody (bevacizumab; Avastin) has been approved by the Food and Drug Administration as a first-line treatment for metastatic colorectal cancer in combination with chemotherapy. Furthermore, VEGF is implicated in intraocular neovascularization associated with diabetic retinopathy and age-related macular degeneration.

Role of PlGF in the intra- and intermolecular cross talk between the VEGF receptors Flt1 and Flk1

Therapeutic angiogenesis is likely to require the administration of factors that complement each other. Activation of the receptor tyrosine kinase (RTK) Flk1 by vascular endothelial growth factor (VEGF) is crucial, but molecular interactions of other factors with VEGF and Flk1 have been studied to a limited extent. Here we report that placental growth factor (PGF, also known as PlGF) regulates inter- and intramolecular cross talk between the VEGF RTKs Flt1 and Flk1. Activation of Flt1 by PGF resulted in intermolecular transphosphorylation of Flk1, thereby amplifying VEGF-driven angiogenesis through Flk1. Even though VEGF and PGF both bind Flt1, PGF uniquely stimulated the phosphorylation of specific Flt1 tyrosine residues and the expression of distinct downstream target genes. Furthermore, the VEGF/PGF heterodimer activated intramolecular VEGF receptor cross talk through formation of Flk1/Flt1 heterodimers. The inter- and intramolecular VEGF receptor cross talk is likely to have therapeutic implications, as treatment with VEGF/PGF heterodimer or a combination of VEGF plus PGF increased ischemic myocardial angiogenesis in a mouse model that was refractory to VEGF alone.

Placental growth factor and its receptor, vascular endothelial growth factor receptor-1: novel targets for stimulation of ischemic tissue revascularization and inhibition of angiogenic and inflammatory disorders

In contrast to VEGF and its receptor VEGFR-2, PlGF and its receptor VEGFR-1 have been largely neglected and therefore their potential for therapy has not been previously explored. In this review, we describe the molecular properties of PlGF and VEGFR-1 and how this translates into an important role for PlGF in the angiogenic switch in pathological angiogenesis, by interacting with VEGFR-1 and synergizing with VEGF. PlGF was effective in the growth of new and stable vessels in cardiac and limb ischemia, through its action on different cell types (i.e. endothelial, smooth muscle and inflammatory cells and their precursors) that play a cardinal role in blood vessel formation. Accordingly, blocking its receptor VEGFR-1 with monoclonal antibodies (anti-VEGFR-1 mAb), expressed on al these cell types, successfully attenuated blood vessel formation during cancer, ischemic retinopathy and rheumatoid arthritis. In addition, while blocking this receptor was effective in reducing inflammatory disorders like atherosclerosis and rheumatoid arthritis, blocking the anti-angiogenic receptor VEGFR-2 was without effect. This indicates that in the latter diseases the beneficial effects of anti-VEGFR1 mAb were mainly due to its effect on inflammatory cells. Importantly, VEGFR-1 was also present on hematopoietic stem/progenitor cells, the precursors of inflammatory cells. Thus, these preclinical studies show proof-of-principle that PlGF and VEGFR-1 are promising therapeutic targets to treat angiogenesis and inflammation related disorders. Clinical trials will reveal whether this is also true for patients.

The viral CC chemokine-binding protein vCCI inhibits monocyte chemoattractant protein-1 activity by masking its CCR2B-binding site

Monocyte chemoattractant protein-1 (MCP-1) is a chemotactic cytokine mainly acting on monocytes and T cells that elicits its biological effects by interacting with the seven-transmembrane helix receptor CCR2B. The vaccinia virus strain Lister and many other poxviruses express soluble proteins (vCCI) that bind MCP-1 and other CC chemokines and inhibit their function. In order to define the interaction site of MCP-1 with vCCI from vaccinia, surface exposed residues of MCP-1 were identified and mutated to alanine. The MCP-1 variants were expressed, purified, and their interaction with vCCI was characterized. The site on MCP-1 for vCCI binding is dominated by arginine 18 with important additional contributions from tyrosine 13 and arginine 24. These residues define a binding site that largely overlaps with the CCR2B receptor interaction site. The viral chemokine-binding protein vCCI thus inhibits the biological function of MCP-1 by directly masking its CCR2B receptor-binding site.

A novel myocyte-specific gene Midori promotes the differentiation of P19CL6 cells into cardiomyocytes

Although several cardiac-specific transcription factors have been shown to play vital roles in various steps during the heart formation, the precise mechanism of the early stage of cardiogenesis has yet to be elucidated. By differential display technique, we tried to identify molecules that are expressed earlier than cardiac transcription factors such as CSX/NKX2-5 and GATA-4 and are involved in cardiomyocyte differentiation using the P19CL6 cell line, which efficiently differentiates into cardiomyocytes when treated with dimethyl sulfoxide. We isolated a novel gene designated Midori. Its deduced amino acid sequence contained an ATP/GTP-binding site, Ig-like domain, and Kringle-like domain. Northern blot analysis revealed that expression of Midori was restricted to the fetal and adult heart and adult skeletal muscle in mice. In whole mount in situ hybridization, Midori was expressed in cardiac crescent and developing heart but not in somites. The MIDORI protein was localized in the nucleus and overexpression of Midori induced expression of endogenous Midori itself, suggesting that MIDORI may act as a transcriptional regulator. Permanent P19CL6 cell lines overexpressing Midori more efficiently differentiated into cardiomyocytes than did parental cells, whereas those overexpressing the antisense Midori less efficiently differentiated. These results suggest that Midori may promote the differentiation of P19CL6 into cardiomyocytes.

Role of vascular endothelial growth factor in regulation of physiological angiogenesis

Evidence accumulating over the last decade has established the fundamental role of vascular endothelial growth factor (VEGF) as a key regulator of normal and abnormal angiogenesis. The biological effects of VEGF are mediated by two tyrosine kinase receptors, Flt-1 (VEGFR-1) and KDR (VEGFR-2). The signaling and biological properties of these two receptors are strikingly different. VEGF is essential for early development of the vasculature to the extent that inactivation of even a single allele of the VEGF gene results in embryonic lethality. VEGF is also required for female reproductive functions and endochondral bone formation. Substantial evidence also implicates VEGF as an angiogenic mediator in tumors and intraocular neovascular syndromes, and numerous clinical trials are presently testing the hypothesis that inhibition of VEGF may have therapeutic value.

Inhibition of tumor growth and metastasis by targeting tumor-associated angiogenesis with antagonists to the receptors of vascular endothelial growth factor

Angiogenesis, the formation of new blood vessels, is essential for both tumor growth and metastasis. Recent advances in our understanding of the molecular mechanisms underlying the angiogenesis process and its regulation have led to the discovery of a variety of pharmaceutical agents with anti-angiogenic activity. The potential application of these angiogenesis inhibitors is currently under intense clinical and pre-clinical investigation. Compelling evidence suggests that vascular endothelial growth factor (VEGF) and its receptors play critical roles in tumor-associated angiogenesis, and that they represent good targets for therapeutic intervention. This has been demonstrated in a variety of animal tumor models in which disabling the function of VEGF and its receptors was shown to inhibit both tumor growth and metastasis. We have produced a panel of antibodies directed against the VEGF receptor 2, KDR/F1k-1. These antibodies potently block VEGF/KDR/F1k-1 interaction, and inhibit VEGF-stimulated activation of the receptor and proliferation of human endothelial cells. Further, the antibodies significantly inhibited tumor-associated angiogenesis in several animal models. Antagonists of VEGF and/or its receptors may offer higher specificity towards tumors with reduced side effects, and may be less likely to elicit drug resistance compared to conventional therapy. Anti-angiogenesis therapy represents a novel strategy for the treatment of cancer and other human disorders where pathological angiogenesis is involved.

Characterization of the VEGF binding site on the Flt-1 receptor

The angiogenic growth factor VEGF binds to the receptor tyrosine kinases Flt-1 and KDR/Flk-1. Immunoglobulin (Ig)-like loop-2 of Flt-1 is involved in binding VEGF, but the contribution of other Flt-1 Ig-loops to VEGF binding remains unclear. We tested the ability of membrane-bound chimeras between the extracellular domain of Flt-1 and the cell adhesion molecule embigin to bind VEGF. VEGF bound as well to receptors containing Flt-1 loops 1-2 or 2-3 as it did to the entire Flt-1 extracellular domain. Chimeras containing only loop-2 of Flt-1 bound VEGF with 22-fold lower affinity. We conclude that high-affinity VEGF binding requires Ig-like loop-2 plus either loop-1 or loop-3. In addition, Flt-1 amino acid residues Arg-224 and Asp-231 were not essential for high-affinity binding of VEGF to membrane-bound Flt-1.

Role of vascular endothelial growth factor in the regulation of angiogenesis

Compelling evidence indicates that vascular endothelial growth factor (VEGF) is a fundamental regulator of normal and abnormal angiogenesis. The loss of a single VEGF allele results in defective vascularization and early embryonic lethality. VEGF plays also a critical role in kidney development, and its inactivation during early postnatal life results in the suppression of glomerular development and kidney failure. Recent evidence indicates that VEGF is also essential for angiogenesis in the female reproductive tract and for morphogenesis of the epiphyseal growth plate and endochondral bone formation. Substantial experimental evidence also implicates VEGF in pathological angiogenesis. Anti-VEGF monoclonal antibodies or other VEGF inhibitors block the growth of several human tumor cell lines in nude mice. Furthermore, the concentrations of VEGF are elevated in the aqueous and vitreous humors of patients with proliferative retinopathies such as the diabetic retinopathy. In addition, VEGF-induced angiogenesis results in a therapeutic benefit in several animal models of myocardial or limb ischemia. Currently, both therapeutic angiogenesis using recombinant VEGF or VEGF gene transfer and inhibition of VEGF-mediated pathological angiogenesis are being pursued clinically.

Placenta growth factor: potential role in pregnancy

PROBLEM

In spite of the known requirement for adequate vascularity during placentation, little is known regarding the regulation of angiogenic growth factor production by trophoblast. Placenta growth factor (PIGF) is a recently discovered angiogenic growth factor whose expression is relatively limited to trophoblast.

METHOD OF STUDY

Current literature of PIGF was reviewed, with emphasis on its expression, regulation, role in angiogenesis, and potential function(s) at the maternal-fetal interface.

RESULTS

PIGF is abundantly expressed by trophoblast, which implies that it could act in a paracrine manner to modulate vascular development, stability, and/or function within the decidua and placental villi. In addition, expression of the PIGF receptor, fms-like tyrosine kinase (flt-1) receptor, on trophoblast raises the potential for an autocrine role of PIGF in regulating trophoblast growth and/or function.

CONCLUSIONS

The potential for PIGF to influence both vascular endothelial cells and trophoblast suggests that aberrant trophoblast production of PIGF could compromise cellular function during gestation and contribute to the vascular and placental pathologies noted in many obstetric complications.