Expression of soluble VEGF receptor 2 and characterization of its binding by surface plasmon resonance

Quantifying the phosphorylation timescales of receptor-ligand complexes: a Markovian matrix-analytic approach

Cells interact with the extracellular environment by means of receptor molecules on their surface. Receptors can bind different ligands, leading to the formation of receptor-ligand complexes. For a subset of receptors, called receptor tyrosine kinases, binding to ligand enables sequential phosphorylation of intra-cellular residues, which initiates a signalling cascade that regulates cellular function and fate. Most mathematical modelling approaches employed to analyse receptor signalling are deterministic, especially when studying scenarios of high ligand concentration or large receptor numbers. There exist, however, biological scenarios where low copy numbers of ligands and/or receptors need to be considered, or where signalling by a few bound receptor-ligand complexes is enough to initiate a cellular response. Under these conditions stochastic approaches are appropriate, and in fact, different attempts have been made in the literature to measure the timescales of receptor signalling initiation in receptor-ligand systems. However, these approaches have made use of numerical simulations or approximations, such as moment-closure techniques. In this paper, we study, from an analytical perspective, the stochastic times to reach a given signalling threshold for two receptor-ligand models. We identify this time as an extinction time for a conveniently defined auxiliary absorbing continuous time Markov process, since receptor-ligand association/dissociation events can be analysed in terms of quasi-birth-and-death processes. We implement algorithmic techniques to compute the different order moments of this time, as well as the steady-state probability distribution of the system. A novel feature of the approach introduced here is that it allows one to quantify the role played by each kinetic rate in the timescales of signal initiation, and in the steady-state probability distribution of the system. Finally, we illustrate our approach by carrying out numerical studies for the vascular endothelial growth factor and one of its receptors, the vascular endothelial growth factor receptor of human endothelial cells.

Gain of affinity for VEGF165 binding within the VEGFR2/NRP1 cellular complex detected by an HTRF-based binding assay

Neuroplin 1 (NRP1), a transmembrane protein interacting with Vascular Endothelial Growth Factor VEGF-A₁₆₅ (called here VEGF165) and the tyrosine kinase Receptor 2 (VEGFR2) promote angiogenesis and vascular homeostasis. In a pathophysiological context, several studies suggested that VEGFR2 and NRP1 mediate tumor development and progression. Given the involvement of the VEGF165 network in promoting tumor angiogenesis, NRP1, VEGFR2 and VEGF165 have been identified as targets for anti-angiogenic therapy. No binding assay exists to monitor specifically the binding of VEGF165 to the VEGFR2/NRP1 complex in intact cells. We established a binding assay based on the homogenous time-resolved fluorescence (HTRF®) technology. This unique binding assay enables to assess the interaction of VEGF165 with VEGFR2 or NRP1 within the VEGFR2/NRP1 complex. Ligand binding saturation experiments revealed that VEGF165 binds the VEGFR2/NRP1 complex at the cell surface with a ten to twenty-fold higher affinity compared to SNAP-VEGFR2 or SNAP-NRP1 receptors alone not engaged in the heteromeric complex. The assay allows characterizing the impact of NRP1 ligands on VEGF165 to the complex. It shows high specificity, reproducibility and robustness, making it compatible with high throughput screening (HTS) applications for identifying new VEGF165 antagonists selective for NRP1 or the VEGFR2/NRP1 complex.

Glucocorticoid effects on angiogenesis are associated with mTOR pathway activity

Glucocorticoids (GC) often are administered during pregnancy, but despite their widespread use in clinical practice, it remains uncertain how GC exposure affects pro-angiogenic factors and their receptors. We investigated the effects of GC on vascular endothelial growth factor (VEGF), placental growth factor (PIGF), vascular endothelial growth factor receptor 1 (VEGFR1) and vascular endothelial growth factor receptor 2 (VEGFR2) protein and mRNA expressions and investigated the possible association of GC with the Akt/mTOR pathway. We incubated human umbilical vein endothelial cells (HUVECs) with a synthetic GC, triamcinolone acetonide (TA). TA administration caused decreased cellular and soluble VEGF and VEGFR1 protein expressions and increased soluble VEGFR2 expression. VEGF, VEGFR1 and VEGFR2 mRNA expressions were altered in a time and dose dependent manner. PIGF protein expression was unaffected by TA treatment, but PIGF mRNA expression decreased in a dose dependent manner after incubation for 48 and 72 h. Phospho-mTOR and phospho-Akt expressions were unaffected. Phospho-p70S6K and phospho-4EBP1 protein expressions and the vascular network forming capacity of HUVECs decreased in a dose dependent manner. We found that GC exert detrimental effects on angiogenesis by altering cellular and soluble angiogenic protein and mRNA levels, and vascular network forming capacities by the Akt/mTOR pathway.

Angiogenic growth factors interactome and drug discovery: The contribution of surface plasmon resonance

Angiogenesis is implicated in several pathological conditions, including cancer, and in regenerative processes, including the formation of collateral blood vessels after stroke. Physiological angiogenesis is the outcome of a fine balance between the action of angiogenic growth factors (AGFs) and anti-angiogenic molecules, while pathological angiogenesis occurs when this balance is pushed toward AGFs. AGFs interact with multiple endothelial cell (EC) surface receptors inducing cell proliferation, migration and proteases upregulation. On the contrary, free or extracellular matrix-associated molecules inhibit angiogenesis by sequestering AGFs (thus hampering EC stimulation) or by interacting with specific EC receptors inducing apoptosis or decreasing responsiveness to AGFs. Thus, angiogenesis results from an intricate network of interactions among pro- and anti-angiogenic molecules, EC receptors and various modulators. All these interactions represent targets for the development of pro- or anti-angiogenic therapies. These aims call for suitable technologies to study the countless interactions occurring during neovascularization. Surface plasmon resonance (SPR) is a label-free optical technique to study biomolecular interactions in real time. It has become the golden standard technology for interaction analysis in biomedical research, including angiogenesis. From a survey of the literature it emerges that SPR has already contributed substantially to the better understanding of the neovascularization process, laying the basis for the decoding of the angiogenesis "interactome" and the identification of "hub molecules" that may represent preferential targets for an efficacious modulation of angiogenesis. Here, the still unexploited full potential of SPR is enlightened, pointing to improvements in its use for a deeper understanding of the mechanisms of neovascularization and the identification of novel anti-angiogenic drugs.

Real-time monitoring biomarker expression of carcinoma cells by surface plasmon resonance biosensors

A novel surface plasmon resonance (SPR) biosensor which is capable of monitoring proteomic biomarker secretion from living cells is reported here. Vascular endothelial growth factor (VEGF) secretion from living SKOV-3 ovarian cancer cells was measured for concept demonstration.

Unraveling the influence of endothelial cell density on VEGF-A signaling

Vascular endothelial growth factor-A (VEGF) is the master determinant for the activation of the angiogenic program leading to the formation of new blood vessels to sustain solid tumor growth and metastasis. VEGF specific binding to VEGF receptor-2 (VEGFR-2) triggers different signaling pathways, including phospholipase C-γ (PLC-γ) and Akt cascades, crucial for endothelial proliferation, permeability, and survival. By combining biologic experiments, theoretical insights, and mathematical modeling, we found that: (1) cell density influences VEGFR-2 protein level, as receptor number is 2-fold higher in long-confluent than in sparse cells; (2) cell density affects VEGFR-2 activation by reducing its affinity for VEGF in long-confluent cells; (3) despite reduced ligand-receptor affinity, high VEGF concentrations provide long-confluent cells with a larger amount of active receptors; (4) PLC-γ and Akt are not directly sensitive to cell density but simply transduce downstream the upstream difference in VEGFR-2 protein level and activation; and (5) the mathematical model correctly predicts the existence of at least one protein tyrosine phosphatase directly targeting PLC-γ and counteracting the receptor-mediated signal. Our data-based mathematical model quantitatively describes VEGF signaling in quiescent and angiogenic endothelium and is suitable to identify new molecular determinants and therapeutic targets.

Surface plasmon resonance biosensor for the detection of VEGFR-1--a protein marker of myelodysplastic syndromes

The surface plasmon resonance (SPR) biosensor system with dispersionless microfluidics for the direct and label-free detection of a soluble vascular endothelial growth factor receptor (sVEGFR-1) is described. The detection approach takes advantage of an affinity interaction between sVEGFR-1 and its ligand, vascular endothelial growth factor (VEGF-A), which is covalently immobilized on the surface of the SPR sensor. The ability of the immobilized VEGF-A to specifically bind the sVEGFR-1 receptor is demonstrated in a buffer. The detection of sVEGFR-1 in 2% human blood plasma is carried out by using the sequential injection approach. The detection limit of 25 ng/mL is achieved. In addition, we demonstrate that the functional surface of the sensor can be regenerated for repeated use.

AT1 receptors activation enhances the expression of MMP-2, MMP-13 and VEGF but not MMP-9 in B16F10 melanoma cells

Melanoma is one of the most aggressive cancers of all solid tumors. The effect of angiotensin II on expression of three Matrix Metalloproteinases (MMPs) and Vascular Endothelial Growth Factor (VEGF) in B16F10 melanoma cells was evaluated. Also the blocking effect of losartan on angiotensin II induced effects was assessed. B16F10 murine melanoma cells were cultured in RPMI-1640 medium supplemented with 10% Fetal Bovine Serum (FBS) and 24 h prior to experiment the serum free medium was used. Angiotensin II (0 M, 10(-10) M, 10(-9) M or 10(-8) M) alone or in combination with Losartan (10(-6) M) in RPMI-1640 replaced the medium for experiments. After the incubation time (0, 1, 2, 6 and 12 h) cells were harvested using 0.05% (w/v) Trypsin and then recovered by centrifugation. The expression of MMP-2, MMP-13, MMP-9 and VEGF in B16F10 cell lysate was assessed by immunoblotting. Angiotensin II significantly enhanced the expression of MMP-2, MMP-13 and VEGF by concentrations as low as 0.1 nM. But angiotensin II could not stimulate any significant increase in MMP-9 expression by angiotensin II in B16F10 cells. Losartan abolished the enhancing effect of every concentration of angiotensin II on MMP-2, MMP-13 and VEGF expression completely and in all incubation times. As a result, angiotensin II through activation of AT1 receptors can stimulate the expression of MMP-2, MMP-13 and VEGF in B16F10 melanoma cells. This is an important conclusion because of the importance of these factors in melanoma invasiveness and the possible important role that angiotensin receptor blockers may play as cancer medications.

Antagonistic VEGF variants engineered to simultaneously bind to and inhibit VEGFR2 and alphavbeta3 integrin

Significant cross-talk exists between receptors that mediate angiogenesis, such as VEGF receptor-2 (VEGFR2) and α(v)β(3) integrin. Thus, agents that inhibit both receptors would have important therapeutic potential. Here, we used an antagonistic VEGF ligand as a molecular scaffold to engineer dual-specific proteins that bound to VEGFR2 and α(v)β(3) integrin with antibody-like affinities and inhibited angiogenic processes in vitro and in vivo. Mutations were introduced into a single-chain VEGF (scVEGF) ligand that retained VEGFR2 binding, but prevented receptor dimerization and activation. Yeast-displayed scVEGF mutant libraries were created and screened by high-throughput flow cytometric sorting to identify several variants that bound with high affinity to both VEGFR2 and α(v)β(3) integrin. These engineered scVEGF mutants were specific for α(v)β(3) integrin and did not bind to the related integrins α(v)β(5), α(iib)β(3), or α(5)β(1). In addition, surface plasmon resonance and cell binding assays showed that dual-specific scVEGF proteins can simultaneously engage both receptors. Compared to monospecific scVEGF mutants that bind VEGFR2 or α(v)β(3) integrin, dual-specific scVEGF proteins more strongly inhibited VEGF-mediated receptor phosphorylation, capillary tube formation, and proliferation of endothelial cells cultured on Matrigel or vitronectin-coated surfaces. Moreover, dual specificity conferred strong inhibition of VEGF-mediated blood vessel formation in Matrigel plugs in vivo, whereas monospecific scVEGF mutants that bind VEGFR2 or α(v)β(3) integrin were only marginally effective. Instead of relying on antibody associating domains or physical linkage, this work highlights an approach to creating dual-specific proteins where additional functionality is introduced into a protein ligand to complement its existing biological properties.

Soluble VEGFR-2: an antilymphangiogenic variant of VEGF receptors

The vascular endothelial growth factor (VEGF) family of secreted proteins and their receptors are major regulators of blood vessel development (hemangiogenesis) and lymphatic vessel development (lymphangiogenesis). VEGF acts through a complex system of receptor tyrosine kinases, which can be membrane bound or soluble. New data concerning the receptor system are still emerging, thus contributing to the complexity of the system. Very recently a soluble form of VEGFR-2, termed sVEGFR-2, which is a result of alternative splicing, has been discovered. Earlier, it has been shown that a secreted/soluble form of VEGFR-1, termed sVEGFR-1, is produced by alternative splicing and exerts an antihemangiogenic effect by binding VEGF-A. The newly discovered spliced variant of sVEGFR-2 binds the lymphangiogenic growth factor VEGF-C and thus inhibits VEGF-C-induced activation of VEGFR-3, consequently inhibiting lymphatic endothelial cell proliferation. Its inactivation in murine embryos permits hyperplasia of dermal lymphatics and invasion of lymphatics into the cornea. Tumor lymphangiogenesis seems to influence the metastatic behavior of malignant cells. A correlation has been found between the downregulation of  sVEGFR-2 and the malignant progression of neuroblastoma, which is characterized by lymphogenic metastases in progressed stages. Data show that lymphangiogenesis is regulated by both activators and inhibitors, and its balance is crucial in health and disease.

r84, a novel therapeutic antibody against mouse and human VEGF with potent anti-tumor activity and limited toxicity induction

Vascular endothelial growth factor (VEGF) is critical for physiological and pathological angiogenesis. Within the tumor microenvironment, VEGF functions as an endothelial cell survival factor, permeability factor, mitogen, and chemotactic agent. The majority of these functions are mediated by VEGF-induced activation of VEGF receptor 2 (VEGFR2), a high affinity receptor tyrosine kinase expressed by endothelial cells and other cell types in the tumor microenvironment. VEGF can also ligate other cell surface receptors including VEGFR1 and neuropilin-1 and -2. However, the importance of VEGF-induced activation of these receptors in tumorigenesis is still unclear. We report the development and characterization of r84, a fully human monoclonal antibody that binds human and mouse VEGF and selectively blocks VEGF from interacting with VEGFR2 but does not interfere with VEGF:VEGFR1 interaction. Selective blockade of VEGF binding to VEGFR2 by r84 is shown through ELISA, receptor binding assays, receptor activation assays, and cell-based functional assays. Furthermore, we show that r84 has potent anti-tumor activity and does not alter tissue histology or blood and urine chemistry after chronic high dose therapy in mice. In addition, chronic r84 therapy does not induce elevated blood pressure levels in some models. The ability of r84 to specifically block VEGF:VEGFR2 binding provides a valuable tool for the characterization of VEGF receptor pathway activation during tumor progression and highlights the utility and safety of selective blockade of VEGF-induced VEGFR2 signaling in tumors.

Preparation of extracellular domain 3 of human VEGF receptor-2 and the monitoring of its real-time binding to VEGF by biosensors

Vascular endothelial growth factor receptor-2 (VEGFR-2) plays an important role in stimulating the proliferation of endothelial cells and improving the permeability of blood vessels, which is involved in tumor angiogenesis, a process that is essential for tumor growth and metastasis. In this study, we describe a method for high yield of recombinant extracellular domain 3 (KDR3) of human VEGFR-2 in an Escherichia coli system with further purification by cation exchange chromatography and immobilized metal affinity chromatography (IMAC). The biological activity of recombinant KDR3 was performed by sequestering VEGF in HUVEC proliferation assay. The real-time binding of human VEGF to immobilized KDR3 was monitored by a label-free biosensor, Optical waveguide lightmode spectroscopy (OWLS). Under the given experimental conditions, the association rate constant k(a) was 4.2 x 10(3) M(-1) s(-1) and the dissociation rate k(d) was 5.1 x 10(-3) s(-1). The dissociation constant K(D) was then calculated to be 1.2 x 10(-6) M. The obtained values will serve as baseline parameters for the design of improved versions of recombinant soluble VEGF receptors and the evaluation of developed anti-KDR antibodies. In addition, such a scenario established by the use of OWLS will potentiate the kinetic study of ligand/receptor and antigen/antibody. The receptor discussed here, which block VEGF binding to cell membrane KDR, have potential clinical application in the treatment of cancer and other diseases where pathological angiogenesis is involved.

A humanized anti-VEGF rabbit monoclonal antibody inhibits angiogenesis and blocks tumor growth in xenograft models

Rabbit antibodies have been widely used in research and diagnostics due to their high antigen specificity and affinity. Though these properties are also highly desirable for therapeutic applications, rabbit antibodies have remained untapped for human disease therapy. To evaluate the therapeutic potential of rabbit monoclonal antibodies (RabMAbs), we generated a panel of neutralizing RabMAbs against human vascular endothelial growth factor-A (VEGF). These neutralizing RabMAbs are specific to VEGF and do not cross-react to other members of the VEGF protein family. Guided by sequence and lineage analysis of a panel of neutralizing RabMAbs, we humanized the lead candidate by substituting non-critical residues with human residues within both the frameworks and the CDR regions. We showed that the humanized RabMAb retained its parental biological properties and showed potent inhibition of the growth of H460 lung carcinoma and A673 rhabdomyosarcoma xenografts in mice. These studies provide proof of principle for the feasibility of developing humanized RabMAbs as therapeutics.

Exploiting Surface Plasmon Resonance (SPR) Technology for the Identification of Fibroblast Growth Factor-2 (FGF2) Antagonists Endowed with Antiangiogenic Activity

Angiogenesis, the process of new blood vessel formation, is implicated in various physiological/pathological conditions, including embryonic development, inflammation and tumor growth. Fibroblast growth factor-2 (FGF2) is a heparin-binding angiogenic growth factor involved in various physiopathological processes, including tumor neovascularization. Accordingly, FGF2 is considered a target for antiangiogenic therapies. Thus, numerous natural/synthetic compounds have been tested for their capacity to bind and sequester FGF2 in the extracellular environment preventing its interaction with cellular receptors. We have exploited surface plasmon resonance (SPR) technique in search for antiangiogenic FGF2 binders/antagonists. In this review we will summarize our experience in SPR-based angiogenesis research, with the aim to validate SPR as a first line screening for the identification of antiangiogenic compounds.

Alternatively spliced vascular endothelial growth factor receptor-2 is an essential endogenous inhibitor of lymphatic vessel growth

Disruption of the precise balance of positive and negative molecular regulators of blood and lymphatic vessels can lead to myriad diseases that affect one in four people worldwide. Although dozens of natural inhibitors of hemangiogenesis have been identified, an endogenous selective inhibitor of lymphatic vessels has not yet been described. We report the existence of a secreted, splice variant of vascular endothelial growth factor receptor-2 (sVegfr-2) that inhibits developmental and reparative lymphangiogenesis by blocking Vegf-c. Tissue-specific loss of sVegfr-2 in mice induced, at birth, spontaneous lymphatic invasion of the normally alymphatic cornea and hyperplasia of skin lymphatics without accompanying changes in blood vasculature. sVegfr-2 inhibited lymphangiogenesis but not hemangiogenesis induced by corneal suture injury or transplantation, enhanced corneal allograft survival, and suppressed lymphangioma cellular proliferation. Naturally occurring sVegfr-2 is a molecular uncoupler of blood and lymphatic vessels whose modulation might have a therapeutic role in lymphatic vascular malformations, transplantation, and potentially in tumor lymphangiogenesis and lymphedema.

Dimerization of VEGF receptors and implications for signal transduction: a computational study

Vascular endothelial growth factor (VEGF) is a potent cytokine involved in the induction of neovascularization. Secreted as a cysteine-linked dimer, it has two binding sites at opposite poles through which it may bind VEGF receptors (VEGFRs), receptor tyrosine kinases found on the surface of endothelial and other cells. The binding of a VEGF molecule to two VEGFR molecules induces transphosphorylation of the intracellular domains of the receptors, leading to signal transduction. The dominant mechanism of receptor dimerization is not clear: the receptors may be present in an inactive pre-dimerized form, VEGF binding first to one of the receptors, the second receptor then ideally located for dimerization; or VEGF may bind receptor monomers on the cell surface, which then diffuse and bind to available unligated receptor monomers to complete the activation. Both processes take place and one or other may dominate on different cell types. We demonstrate the impact of dimerization mechanism on the binding of VEGF to the cell surface and on the formation of active signaling receptor complexes. We describe two methods to determine which process dominates, based on binding and phosphorylation assays. The presence of two VEGF receptor populations, VEGFR1 and VEGFR2, can result in receptor heterodimer formation. Our simulations predict that heterodimers will comprise 10-50% of the active, signaling VEGF receptor complexes, and that heterodimers will form at the expense of homodimers of VEGFR1 when VEGFR2 populations are larger. These results have significant implications for VEGF signal transduction and interpretation of experimental studies. These results may be applicable to other ligand-receptor pairs, in particular PDGF.

Inhibition of Ovarian Tumor Growth by Gene Therapy with Recombinant Soluble Vascular Endothelial Growth Factor Receptor 2

The growth and persistence of solid tumors and their metastases are angiogenesis dependent. Targeting angiogenesis represents a new strategy for the development of antitumor therapies. The extracellular immunoglobulin- like domain of VEGFR-2 (KDR/Flk-1), soluble VEGFR-2, may form a heterodimeric complex with a wild-type VEGF receptor and function as a dominant negative receptor. We assessed the effects of sFlk-1 on SKOV3 cell growth and proliferation in vitro. Furthermore, we investigated the effectiveness of recombinant soluble Flk-1 adenovirus on inhibition of tumor growth in an ovarian tumor (SKOV3) nude murine model, combined with cis-diamminedichloroplatinum (DDP). Nude mice bearing SKOV3 tumors received adsFlk- 1 (recombinant soluble Flk-1 adenovirus) and DDP, respectively or in combination, and tumor growth inhibition, microvessel density, and apoptosis in tumor tissue were assessed by immunohistochemical analysis. Our data revealed that sFlk-1 had little effect on tumor cell growth in vitro, whereas ad-sFlk-1 administration could inhibit tumor growth significantly (p < 0.05) in the nude murine model, accompanied by angiogenesis suppression and apoptosis induction, and augmented efficiency was observed in combination with DDP as well. The present findings suggest that gene therapy with ad-sFlk-1 is an efficient antiangiogenesis strategy, which may be important in further exploration and possible translation into a clinical trial.

Differential binding of VEGF isoforms to VEGF receptor 2 in the presence of neuropilin-1: a computational model

A comprehensive, biophysically accurate, computational model of vascular endothelial growth factor (VEGF) family member interactions with endothelial cell surface receptors was developed to study angiogenesis. Neuropilin-1 (NRP1) and the signaling VEGF receptor, VEGFR2, do not interact directly but are bridged by one VEGF isoform, VEGF165. Using the model and published experimental data, we estimated the kinetic rate of this VEGFR2-NRP1 coupling in vitro. With the use of this rate, our model gives predictions in good quantitative agreement with several independent in vitro experiments involving VEGF121and VEGF165isoforms, confirming that VEGFR2-NRP1 coupling through VEGF165can fully explain the observed differences in receptor binding and phosphorylation in response to these isoforms. Model predictions also determine the mechanism of action of a commonly used NRP1 antibody and predict the results of potential future experiments. This is the first model to include VEGF isoforms or NRPs, and it is a necessary step toward a quantitative molecular level description of VEGF that can be extended to in vivo situations. The model has applications for both proangiogenic and antiangiogenic therapies, such as for heart disease and cancer, as well as in tissue engineering.

A Naturally Occurring Soluble Form of Vascular Endothelial Growth Factor Receptor 2 Detected in Mouse and Human Plasma

Angiogenesis and vasculogenesis are regulated in large part by several different growth factors and their associated receptor tyrosine kinases (RTKs). Foremost among these is the vascular endothelial growth factor (VEGF) family including VEGF receptor (VEGFR)-2 and -1. VEGFR ligand binding and biological activity are regulated at many levels, one of which is by a soluble, circulating form of VEGFR-1 (sVEGFR-1). This sVEGFR-1 can act as a competitive inhibitor of its ligand, serve as a possible biomarker, and play important roles in cancer and other diseases such as preeclampsia. Recombinant forms of sVEGFR-2 have been shown to have antiangiogenic activity, but a naturally occurring sVEGFR-2 has not been described previously. Here, we report such an entity. Having a molecular weight of ∼160 kDa, sVEGFR-2 can be detected in mouse and human plasma with several different monoclonal and polyclonal anti-VEGFR-2 antibodies using both ELISA and immunoprecipitation techniques. In vitro studies have determined that the sVEGFR-2 fragment can be found in the conditioned media of mouse and human endothelial cells, thus suggesting that it may be secreted, similar to sVEGFR-1, or proteolytically cleaved from the cell. Potential biological activity of this protein was inferred from experiments in which mouse sVEGFR-2 could bind to VEGF-coated plates. Similar to sVEGFR-1 and other soluble circulating RTKs, sVEGFR-2 may have regulatory consequences with respect to VEGF-mediated angiogenesis as well as potential to serve as a quantitative biomarker of angiogenesis and antiangiogenic drug activity, particularly for drugs that target VEGF or VEGFR-2.

Evaluation of novel antimouse VEGFR2 antibodies as potential antiangiogenic or vascular targeting agents for tumor therapy

We generated a panel of eight rat IgG(2a) monoclonal antibodies with high affinity for mouse VEGFR2 (KDR/Flk-1), the main receptor that mediates the angiogenic effect of VEGF-A. The antibodies (termed RAFL, R at Anti Flk) bound to dividing endothelial cells more strongly than they did to nondividing cells. Most of the RAFL antibodies blocked [(125)I]VEGF(165) binding to VEGFR2. Three of eight antibodies localized to VEGFR2-positive tumor endothelium after intravenous injection into mice bearing orthotopic MDA-MB-231 breast carcinomas, as judged by indirect immunohistochemistry. An average of 60% of vessels in the tumors was stained. The majority (50-80%) of vessels were also stained in a variety of other human and murine tumors growing in mice. The antibodies did not bind detectably to the vascular endothelium in normal heart, lung, liver, and brain cortex, whereas the vascular endothelium in kidney glomerulus and pancreatic islets was stained. Treatment of mice bearing orthotopic MDA-MB-231 tumors with RAFL-1 antibody inhibited tumor growth by an average of 48% and reduced vascular density by 65%, compared to tumors in mice treated with control IgG. Vascular damage was not observed in normal organs, including kidneys and pancreas. These studies demonstrate that anti-VEGFR2 antibodies have potential for vascular targeting and imaging of tumors in vivo.

Model of competitive binding of vascular endothelial growth factor and placental growth factor to VEGF receptors on endothelial cells

Placental growth factor (PlGF) competes with vascular endothelial growth factor (VEGF) for binding to VEGF receptor (VEGFR)-1 but does not bind VEGFR2. Experiments show that PlGF can augment the response to VEGF in pathological angiogenesis and in models of endothelial cell survival, migration, and proliferation. This synergy has been hypothesized to be due to a combination of the following: signaling by PlGF through VEGFR1 and displacement of VEGF from VEGFR1 to VEGFR2 by PlGF, causing increased signaling through VEGFR2. In this study, the relative contribution of PlGF-induced VEGF displacement to the synergy is quantified using a mathematical model of ligand-receptor binding to examine the effect on ligand-receptor complex formation of VEGF and PlGF acting together. Parameters specific to the VEGF-PlGF system are used based on existing data. The model is used to simulate in silico a specific in vitro experiment in which VEGF-PlGF synergy is observed. We show that, whereas a significant change in the formation of endothelial surface growth factor-VEGFR1 complexes is predicted in the presence of PlGF, the increase in the number of VEGFR2-containing signaling complexes is less significant; these results were shown to be robust to significant variation in the kinetic parameters of the model. Synergistic effects observed in that experiment thus appear unlikely to be due to VEGF displacement but to a shift from VEGF-VEGFR1 to PlGF-VEGFR1 complexes and an increase in total VEGFR1 complexes. These results suggest that VEGFR1 signaling can be functional in adult-derived endothelial cells.

Characterization of the vascular endothelial growth factor-receptor interaction and determination of the recombinant protein by an optical receptor sensor

Vascular endothelial growth factor (VEGF) is one of the most important factors controlling angiogenesis. It is a homodimeric glycoprotein belonging to the family of cysteine-knot proteins. The biological activity is transduced via membrane-spanning receptors of the tyrosine kinase receptor family. Each biologically active VEGF has two receptor binding sites leading to receptor dimerization as first step following ligand binding. The ligand-binding site of the receptor is localized on extracellular Ig-like domains. The extracellular part of the receptor Flt-1 (VEGFR-1) was expressed as soluble protein and was used as receptor in an optical affinity sensor system (BIAcore). Suitable conditions allowed the determination of the association and dissociation rate constants as k(a)=4+/-1.2 x 10(6) M(-1) s(-1) and k(d)=3+/-0.8 x 10(-5) s(-1), respectively, leading to an affinity constant of K(D)=7.5+/-3 pM, which is within the range published already from other investigations and methods. Increasing receptor loadings of the sensor surface decreased the binding efficiency, as the ratio of bound VEGF-molecules to theoretically available binding sites increased from 1:1.5 to 1:2.6. Increasing the surface loading further, allowed the establishment of a quantitative assay with the analytical performance being influenced by the receptor loading and the contact time between sample and immobilized receptor, i.e. sample volume. This assay was used for VEGF determination during the cultivation of a recombinant Pichia pastoris strain.

Ultrastructural localization of the vascular permeability factor/vascular endothelial growth factor (VPF/VEGF) receptor-2 (FLK-1, KDR) in normal mouse kidney and in the hyperpermeable vessels induced by VPF/VEGF-expressing tumors and adenoviral vectors

Vascular permeability factor/vascular endothelial growth factor (VPF/VEGF) interacts with two high-affinity tyrosine kinase receptors, VEGFR-1 and VEGFR-2, to increase microvascular permeability and induce angiogenesis. Both receptors are selectively expressed by vascular endothelial cells and are strikingly increased in tumor vessels. We used a specific antibody to localize VEGFR-2 (FLK-1, KDR) in microvascular endothelium of normal mouse kidneys and in the microvessels induced by the TA3/St mammary tumor or by infection with an adenoviral vector engineered to express VPF/VEGF. A pre-embedding method was employed at the light and electron microscopic levels using either nanogold or peroxidase as reporters. Equivalent staining was observed on both the luminal and abluminal surfaces of tumor- and adenovirus-induced vascular endothelium, but plasma membranes at interendothelial junctions were spared except at sites connected to vesiculovacuolar organelles (VVOs). VEGFR-2 was also localized to the membranes and stomatal diaphragms of some VVOs. This staining distribution is consistent with a model in which VPF/VEGF increases microvascular permeability by opening VVOs to allow the transendothelial cell passage of plasma and plasma proteins.

Survey of the 1998 optical biosensor literature

The utilization of optical biosensors to study molecular interactions continues to expand. In 1998, 384 articles relating to the use of commercial biosensors were published in 130 different journals. While significant strides in new applications and methodology were made, a majority of the biosensor literature is of rather poor quality. Basic information about experimental conditions is often not presented and many publications fail to display the experimental data, bringing into question the credibility of the results. This review provides suggestions on how to collect, analyze and report biosensor data.