Monoclonal antibodies to murine thrombospondin-1 and thrombospondin-2 reveal differential expression patterns in cancer and low antigen expression in normal tissues.

There is a considerable interest for the discovery and characterization of tumor-associated antigens, which may facilitate antibody-based pharmacodelivery strategies. Thrombospondin-1 and thrombospondin-2 are homologous secreted proteins, which have previously been reported to be overexpressed during remodeling typical for wound healing and tumor progression and to possibly play a functional role in cell proliferation, migration and apoptosis. To our knowledge, a complete immunohistochemical characterization of thrombospondins levels in normal rodent tissues has not been reported so far. Using antibody phage technology, we have generated and characterized monoclonal antibodies specific to murine thrombospondin-1 and thrombospondin-2, two antigens which share 62% aminoacid identity. An immunofluorescence analysis revealed that both antigens are virtually undetectable in normal mouse tissues, except for a weak staining of heart tissue by antibodies specific to thrombospondin-1. The analysis also showed that thrombospondin-1 was strongly expressed in 5/7 human tumors xenografted in nude mice, while it was only barely detectable in 3/8 murine tumors grafted in immunocompetent mice. By contrast, a high-affinity antibody to thrombospondin-2 revealed a much lower level of expression of this antigen in cancer specimens. Our analysis resolves ambiguities related to conflicting reports on thrombosponding expression in health and disease. Based on our findings, thrombospondin-1 (and not thrombospondin-2) may be considered as a target for antibody-based pharmacodelivery strategies, in consideration of its low expression in normal tissues and its upregulation in cancer.

Tetraspanin CD63 promotes vascular endothelial growth factor receptor 2-ß1 integrin complex formation, thereby regulating activation and downstream signaling in endothelial cells in vitro and in vivo.

CD63 is a member of the transmembrane-4 glycoprotein superfamily (tetraspanins) implicated in the regulation of membrane protein trafficking, leukocyte recruitment, and adhesion processes. We have investigated the involvement of CD63 in endothelial cell (EC) signaling downstream of ß1 integrin and VEGF. We report that silencing of CD63 in primary ECs arrested capillary sprouting and tube formation in vitro because of impaired adhesion and migration of ECs. Mechanistically, CD63 associated with both ß1 integrin and the main VEGF receptor on ECs, VEGFR2. Our data suggest that CD63 serves to bridge between ß1 integrin and VEGFR2 because CD63 silencing disrupted VEGFR2-ß1 integrin complex formation identified using proximity ligation assays. Signaling downstream of ß1 integrin and VEGFR2 was attenuated in CD63-silenced cells, although their cell surface expression levels remained unaffected. CD63 was furthermore required for efficient internalization of VEGFR2 in response to VEGF. Importantly, systemic delivery of VEGF failed to potently induce VEGFR2 phosphorylation and downstream signaling in CD63-deficient mouse lungs. Taken together, our findings demonstrate a previously unrecognized role for CD63 in coordinated integrin and receptor tyrosine kinase signaling in vitro and in vivo.

Ascorbic acid rescues cardiomyocyte development in Fgfr1(-/-) murine embryonic stem cells.

Fibroblast growth factor receptor 1 (Fgfr1) gene knockout impairs cardiomyocyte differentiation in murine embryonic stem cells (mESC). Here, various chemical compounds able to enhance cardiomyocyte differentiation in mESC [including dimethylsulfoxide, ascorbic acid (vitC), free radicals and reactive oxygen species] were tested for their ability to rescue the cardiomyogenic potential of Fgfr1(-/-) mESC. Among them, only the reduced form of vitC, l-ascorbic acid, was able to recover beating cell differentiation in Fgfr1(-/-) mESC. The appearance of contracting cells was paralleled by the expression of early and late cardiac gene markers, thus suggesting their identity as cardiomyocytes. In the attempt to elucidate the mechanism of action of vitC on Fgfr1(-/-) mESC, we analyzed several parameters related to the intracellular redox state, such as reactive oxygen species content, Nox4 expression, and superoxide dismutase activity. The results did not show any relationship between the antioxidant capacity of vitC and cardiomyocyte differentiation in Fgfr1(-/-) mESC. No correlation was found also for the ability of vitC to modulate the expression of pluripotency genes. Then, we tested the hypothesis that vitC was acting as a prolyl hydroxylase cofactor by maintaining iron in a reduced state. We first analyze hypoxia inducible factor (HIF)-1α mRNA and protein levels that were found to be slightly upregulated in Fgfr1(-/-) cells. We treated mESC with Fe(2+) or the HIF inhibitor CAY10585 during the first phases of the differentiation process and, similar to vitC, the two compounds were able to rescue cardiomyocyte formation in Fgfr1(-/-) mESC, thus implicating HIF-1α modulation in Fgfr1-dependent cardiomyogenesis.

VEGF receptor 2/-3 heterodimers detected in situ by proximity ligation on angiogenic sprouts.

The vascular endothelial growth factors VEGFA and VEGFC are crucial regulators of vascular development. They exert their effects by dimerization and activation of the cognate receptors VEGFR2 and VEGFR3. Here, we have used in situ proximity ligation to detect receptor complexes in intact endothelial cells. We show that both VEGFA and VEGFC potently induce formation of VEGFR2/-3 heterodimers. Receptor heterodimers were found in both developing blood vessels and immature lymphatic structures in embryoid bodies. We present evidence that heterodimers frequently localize to tip cell filopodia. Interestingly, in the presence of VEGFC, heterodimers were enriched in the leading tip cells as compared with trailing stalk cells of growing sprouts. Neutralization of VEGFR3 to prevent heterodimer formation in response to VEGFA decreased the extent of angiogenic sprouting. We conclude that VEGFR2/-3 heterodimers on angiogenic sprouts induced by VEGFA or VEGFC may serve to positively regulate angiogenic sprouting.

Signal transduction by vascular endothelial growth factor receptors.

VEGFs (vascular endothelial growth factors) control vascular development during embryogenesis and the function of blood vessels and lymphatic vessels in the adult. There are five related mammalian ligands, which act through three receptor tyrosine kinases. Signalling is modulated through neuropilins, which act as VEGF co-receptors. Heparan sulfate and integrins are also important modulators of VEGF signalling. Therapeutic agents that interfere with VEGF signalling have been developed with the aim of decreasing angiogenesis in diseases that involve tissue growth and inflammation, such as cancer. The present review will outline the current understanding and consequent biology of VEGF receptor signalling.

Calcitonin receptor-like receptor guides arterial differentiation in zebrafish.

The calcitonin receptor-like receptor (crlr) is a major endothelial cell receptor for adrenomedullin, a peptide vasodilator involved in cardiovascular development, homeostasis, and disease. Here, we used the zebrafish (Danio rerio) model to characterize the role of crlr in vascular development. Crlr is expressed within somites from the 4- to the 13-somite stage and by arterial progenitors and axial vessels during zebrafish development. Loss of crlr results in profound alterations in vascular development and angiogenesis, including atrophic trunk dorsal aorta and interruption of anterior aortic bifurcation, delay in intersomitic vessel development, and lack of blood circulation. Remarkably, crlr morphants are characterized by the loss of arterial endothelial cell identity in dorsal aorta, as shown by the lack of expression of the arterial markers ephrin-B2a, DeltaC, and notch5. Down-regulation of crlr affects vascular endothelial growth factor (vegf) expression, whereas vegf overexpression is sufficient to rescue arterial differentiation in crlr morphants. Finally, genetic and biochemical evidences indicate that somitic crlr expression is under the control of sonic hedgehog. These data demonstrate that crlr plays a nonredundant role in arterial differentiation, representing a novel element of the sonic hedgehog-vegf-notch signaling cascade that controls arterial/venous fate.

Fibroblast growth factor receptor-1 phosphorylation requirement for cardiomyocyte differentiation in murine embryonic stem cells.

Fibroblast growth factor receptor-1 (Fgfr1) gene knockout impairs cardiac and haematopoietic development in murine embryonic stem cells (mESC). In FGFR1, tyrosine residues Y653 and Y654 are responsible for its tyrosine kinase (TK) activity whereas phosphorylated Y463 and Y766 represent docking sites for intracellular substrates. Aim of this study was the characterization of FGFR1 signalling requirements necessary for cardiomyocyte differentiation in mESC. To this purpose, fgfr1(-/-) mESC were infected with lentiviral vectors harbouring human wild-type hFGFR1 or the Y653/654F, Y463F and Y766F hFGFR1 mutants. The resulting embryonic stem (ES) cell lines were differentiated as embryoid bodies (EBs) and beating foci formation was evaluated. In order to appraise the presence of cells belonging to cardiovascular and haematopoietic lineages, specific markers were analysed by quantitative PCR, whole mount in situ hybridization and immunofluorescence. Transduction with TK(+) hFGFR1 or the TK(+) Y766F-hFGFR1 mutant rescued cardiomyocyte beating foci formation in fgfr1(-/-) EBs whereas the TK(-) Y653/654F-hFGFR1 mutant and the TK(+) Y463F-hFGFR1 mutant were both ineffective. Analysis of the expression of early and late cardiac markers in differentiating EBs confirmed these observations. At variance with cardiomyocyte differentiation, all the transduced TK(+) FGFR1 forms were able to rescue haematopoietic differentiation in EBs originated by infected fgfr1(-/-) mESC, only the TK(-) Y653/654F-hFGFR1 mutant being ineffective. In keeping with these observations, treatment with different signalling pathway inhibitors indicates that protein kinase C and ERK activation are essential for cardiomyocyte but not for haematopoietic differentiation in EBs generated by fgfr1(+/-) approximately mESC. In conclusion, our results suggest that, although FGFR1 kinase activity is necessary for both cardiac and haematopoietic lineage maturation in mESC, phosphorylation of Y463 in the intracellular domain of the receptor is a specific requirement for cardiomyocyte differentiation.

Phenotypic behavior of C2C12 myoblasts upon expression of the dystrophy-related caveolin-3 P104L and TFT mutants.

Caveolin-3 (Cav-3) is the main scaffolding protein present in myofiber caveolae. We transfected C2C12 myoblasts with dominant negative forms of Cav-3, P104L or DeltaTFT, respectively, which cause the limb-girdle muscular dystrophy 1-C. Both these forms triggered Cav-3 loss during C2C12 cell differentiation. The P104L mutation reduced myofiber formation by impaired AKT signalling, accompanied by dramatic expression of the E3 ubiquitin ligase Atrogin. On the other hand, the DeltaTFT mutation triggered hypertrophic myotubes sustained by prolonged AKT activation, but independent of increased levels of follistatin and interleukin 4 expression. These data suggest that separated mutations within the same dystrophy-related gene may cause muscle degeneration through different mechanisms.

A highly functional synthetic phage display library containing over 40 billion human antibody clones.

Several synthetic antibody phage display libraries have been created and used for the isolation of human monoclonal antibodies. The performance of antibody libraries, which is usually measured in terms of their ability to yield high-affinity binding specificities against target proteins of interest, depends both on technical aspects (such as library size and quality of cloning) and on design features (which influence the percentage of functional clones in the library and their ability to be used for practical applications). Here, we describe the design, construction and characterization of a combinatorial phage display library, comprising over 40 billion human antibody clones in single-chain fragment variable (scFv) format. The library was designed with the aim to obtain highly stable antibody clones, which can be affinity-purified on protein A supports, even when used in scFv format. The library was found to be highly functional, as >90% of randomly selected clones expressed the corresponding antibody. When selected against more than 15 antigens from various sources, the library always yielded specific and potent binders, at a higher frequency compared to previous antibody libraries. To demonstrate library performance in practical biomedical research projects, we isolated the human antibody G5, which reacts both against human and murine forms of the alternatively spliced BCD segment of tenascin-C, an extracellular matrix component frequently over-expressed in cancer and in chronic inflammation. The new library represents a useful source of binding specificities, both for academic research and for the development of antibody-based therapeutics.

VEGFR2 induces c-Src signaling and vascular permeability in vivo via the adaptor protein TSAd.

Regulation of vascular endothelial (VE) growth factor (VEGF)-induced permeability is critical in physiological and pathological processes. We show that tyrosine phosphorylation of VEGF receptor 2 (VEGFR2) at Y951 facilitates binding of VEGFR2 to the Rous sarcoma (Src) homology 2-domain of T cell-specific adaptor (TSAd), which in turn regulates VEGF-induced activation of the c-Src tyrosine kinase and vascular permeability. c-Src was activated in vivo and in vitro in a VEGF/TSAd-dependent manner, and was regulated via increased phosphorylation at pY418 and reduced phosphorylation at pY527. Tsad silencing blocked VEGF-induced c-Src activation, but did not affect pathways involving phospholipase Cγ, extracellular regulated kinase, and endothelial nitric oxide. VEGF-induced rearrangement of VE-cadherin-positive junctions in endothelial cells isolated from mouse lungs, or in mouse cremaster vessels, was dependent on TSAd expression, and TSAd formed a complex with VE-cadherin, VEGFR2, and c-Src at endothelial junctions. Vessels in tsad(-/-) mice showed undisturbed flow and pressure, but impaired VEGF-induced permeability, as measured by extravasation of Evans blue, dextran, and microspheres in the skin and the trachea. Histamine-induced extravasation was not affected by TSAd deficiency. We conclude that TSAd is required for VEGF-induced, c-Src-mediated regulation of endothelial cell junctions and for vascular permeability.

Vascular endothelial growth factors and receptors: anti-angiogenic therapy in the treatment of cancer.

Vascular endothelial growth factors (VEGFs) are critical regulators of vascular and lymphatic function during development, in health and in disease. There are five mammalian VEGF ligands and three VEGF receptor tyrosine kinases. In addition, several VEGF co-receptors that lack intrinsic catalytic activity, but that indirectly modulate the responsiveness to VEGF contribute to the final biological effect. This review describes the molecular features of VEGFs, VEGFRs and co-receptors with focus on their role in the treatment of cancer.