EORTC Receptor and Biomarker Study Group Report: A Sandwich Enzyme-Linked Immunosorbent Assay for Vascular Endothelial Growth Factor in Blood and Tumor Tissue Extracts

A four-antibody sandwich enzyme-linked immunosorbent assay (ELISA) for vascular endothelial growth factor (VEGF) for application in blood (serum and plasma) and tumor tissue extracts was set up within the framework of the EORTC Receptor and Biomarker Study Group (RBSG). Polyclonal antibodies against VEGF165 were raised in chickens and rabbits, and used in a previously described assay format. The assay was validated and characterized for use in serum, plasma and tumor tissue extracts. The resulting VEGF ELISA was found to be specific for VEGF165 and VEGF121, the main isoforms of VEGF. The assay showed good precision and parallelism in serial dilutions of samples. The assay was not susceptible to interference by heterophilic antibodies because avian antibodies (duck anti-chicken and chicken anti-VEGF) were used in the pre-analyte stage and mammalian antibodies (rabbit anti-VEGF and goat anti-rabbit) in the post-analyte stage. In conclusion, a sensitive, robust and specific VEGF ELISA has been developed. Research into the prognostic value of VEGF employing this assay is currently underway.

A three-step purification method of large quantities of human recombinant alpha endothelial cellular growth factor for clinical use

The endothelial cellular growth factor alpha-ECGF is a candidate drug for the induction of therapeutic neoangiogenesis. Its use in extensive experimental and clinical trials is hampered by the fact that currently published purification procedures allow only small yields, and the absence of pyrogenic impurities is not demonstrated. The rh alpha-ECGF was expressed in E. coli. Isolation of rh alpha-ECGF from E. coli lysates to apparent homogenicity was achieved by a three step purification procedure involving ionic exchange, heparin-sepharose and polymyxin B chromatography. By this method, 200 mg of rh alpha-ECGF was purified from 15 g wet weight E. coli bacteria. The isolated protein of 18 kDa appeared as a single band after SDS gel electrophoresis and subsequent silver-staining. The biological activity was expressed in the chorion-allantois-membrane assay and in the 3H-thymidine proliferation in baby hamster kidney cells. Drug trials with rabbits revealed no increase in body temperature after intravenous injections with 1 mg rh-ECGF.

Src kinase becomes preferentially associated with the VEGFR, KDR/Flk-1, following VEGF stimulation of vascular endothelial cells

BACKGROUND

The cytoplasmic tyrosine kinase, Src, has been found to play a crucial role in VEGF (vascular endothelial growth factor) - dependent vascular permeability involved in angiogenesis. The two main VEGFRs present on vascular endothelial cells are KDR/Flk-1 (kinase insert domain-containing receptor/fetal liver kinase-1) and Flt-1 (Fms-like tyrosine kinase-1). However, to date, it has not been determined which VEGF receptor (VEGFR) is involved in binding to and activating Src kinase following VEGF stimulation of the receptors.

RESULTS

In this report, we demonstrate that Src preferentially associates with KDR/Flk-1 rather than Flt-1 in human umbilical vein endothelial cells (HUVECs), and that VEGF stimulation resulted in an increase of Src activity associated with activated KDR/Flk-1. These findings were determined through immunoprecipitation-kinase experiments and coimmunoprecipitation studies, and were further confirmed by GST-pull-down assays and Far Western studies. However, Fyn and Yes, unlike Src, were found to associate preferentially with Flt-1.

CONCLUSIONS

Thus, Src preferentially associates with KDR/Flk-1, rather than with Flt-1, upon VEGF stimulation in endothelial cells. Our findings further highlight the potential significance of upregulated KDR/Flk-1-associated Src activity in the process of angiogenesis, and help to elucidate more clearly the specific roles and mechanisms involving Src family tyrosine kinase in VEGF-stimulated signal transduction events.

[Cloning, purification and biological activity of human vascular endothelial growth factor fragment in E. coli]

OBJECTIVE

To observe the effect of human vascular endothelial growth factor (VEGF) fragment (3 approximately 4 exon) in E. coli on anti-angiogenesis.

METHODS

Through RT-PCR amplification, endonuclease cut and DNA sequence analysis identification, hVEGF fragment cDNA was inserted into E. coli expression vector pTrcHis2A. The prokaryotic expression plasmid pTrcHis2A/VEGF(3 approximately 4) was constructed and transformed into TOP10F.

RESULTS

After 8hr isopropy-beta-D-thiogalactoside (IPTG) induction, VEGF fragment was expressed in 15% of total proteins through sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). The expressed protein was highly antigenic and specific. The VEGF fragment was further purified by affinity, which could inhibit HUVEC proliferation and neovascularization of the chick chorioallantoic membrane.

CONCLUSION

VEGF fragment is anti-angiogenetic, which may potentially be used in oncologico-biological targeting therapy.

Expression of human vascular endothelial growth factor (VEGF165) in Pichia pastoris and its biological activity

OBJECTIVE

To express human vascular endothelial growth factor (hVEGF165) cDNA in Pichia pastroris, purify the expressed product and detect the biological activity of it.

METHODS

By inserting hVEGF165 cDNA coding 165 amino acid residues into Pichia pastoris expression vector pPIC9K containing AOX1 promoter and the sequences of alpha secreting signal peptides, a recombinant expression plasmid pPIC9K/hVEGF165 was constructed and transformed to yeast host strain KM71, then multiple-copy insert transformants were screened out and cultured in flasks, and hVEGF165 was expressed under the induction of 1% methanol.

RESULTS

SDS-PAGE showed that after being induced with 1% methanol for 4 d, the expressed product existed in supernatant in the form of soluble molecule and contained 60% of total protein expressed. Western blot showed good antigenicity and specificity of expressed product. After being purified by Heparin-Sepharose CL6B affinity chromatography, the purity of expressed product reached above 90%. Biological assays proved that the expressed product could stimulate the proliferation of HUVEC.

CONCLUSION

hVEGF165 was successfully expressed. The study opened up a wide prospect for the application of VEGF165 in the prevention and treatment of ischemic heart disease and other tissue ischemic diseases such as secondary arterial occlusion in limbs.

A novel role for VEGF in endocardial cushion formation and its potential contribution to congenital heart defects

Normal cardiovascular development is exquisitely dependent on the correct dosage of the angiogenic growth factor and vascular morphogen vascular endothelial growth factor (VEGF). However, cardiac expression of VEGF is also robustly augmented during hypoxic insults, potentially mediating the well-established teratogenic effects of hypoxia on heart development. We report that during normal heart morphogenesis VEGF is specifically upregulated in the atrioventricular (AV) field of the heart tube soon after the onset of endocardial cushion formation (i.e. the endocardium-derived structures that build the heart septa and valves). To model hypoxia-dependent induction of VEGF in vivo, we conditionally induced VEGF expression in the myocardium using a tetracycline-regulated transgenic system. Premature induction of myocardial VEGF in E9.5 embryos to levels comparable with those induced by hypoxia prevented formation of endocardial cushions. When added to explanted embryonic AV tissue, VEGF fully inhibited endocardial-to-mesenchymal transformation. Transformation was also abrogated in AV explants subjected to experimental hypoxia but fully restored in the presence of an inhibitory soluble VEGF receptor 1 chimeric protein. Together, these results suggest a novel developmental role for VEGF as a negative regulator of endocardial-to-mesenchymal transformation that underlies the formation of endocardial cushions. Moreover, ischemia-induced VEGF may be the molecular link between hypoxia and congenital defects in heart septation.

Primary study of vascular endothelial growth factor immunohistochemical staining in the diagnosis of early acute myocardial ischemia

The expression of vascular endothelial growth factor (VEGF) in the model of rat early acute myocardial ischemia was studied by Strept-Avidin-Biotin-Peroxidase Complex (SABC) immunohistochemical staining. After ligating the anterior descending branch of the left coronary artery, an initial rapid (30min) positive expression of VEGF in myocardial ischemic areas was observed, the intensity of positive expression of VEGF increased with the continuation of myocardial ischemia. After 5h infarction, the strongly positive myocytes of SABC-VEGF staining were predominantly limited to perimyocardial infarction areas. No positive expression of VEGF was found in the control group. These findings suggested that SABC-VEGF method could give a sensitive, specific, simple and objective morphologic evidence to the diagnosis of sudden cardiac death caused by acute early myocardial ischemia.

Folding screening assayed by proteolysis: application to various cystine deletion mutants of vascular endothelial growth factor

The production of recombinant proteins in Escherichia coli often leads to the formation of inclusion bodies. Although this has a number of advantages, a major disadvantage is the need to develop folding protocols for the renaturing of the proteins. However, the systematic screening of folding conditions is often hampered by the lack of convenient assays to detect correctly folded proteins. To address this problem we present a simple protocol, which combines folding screens and limited proteolysis to rapidly assess and optimize folding conditions. The efficacy of this method, termed FSAP (folding screening assayed by proteolysis), is demonstrated by the large-scale folding, purification and crystallization of various cystine deletion mutants of the cystine knot family member: vascular endothelial growth factor (VEGF). These mutants are particularly difficult to fold as the cystine knot is believed to make major contributions to the stability of the protein and this family of proteins lacks extensive hydrophobic core regions.

Distribution of transforming growth factor-beta isoforms TGF-beta 1, TGF-beta 2 and TGF-beta 3 and vascular endothelial growth factor in vulvar lichen sclerosus

OBJECTIVE

To study the distribution of transforming growth factor beta (TGF-beta) isoforms TGF-beta 1, TGF-beta 2 and TGF-beta 3 and vascular endothelial growth factor (VEGF) in vulvar lichen sclerosus.

STUDY DESIGN

Biopsies were obtained from 10 patients with vulvar lichen sclerosus, snap frozen and stained with polyclonal antibodies to TGF-beta 1, TGF-beta 2, TGF-beta 3 and VEGF. Control tissues used were uninvolved thigh tissue from two of the lichen sclerosus patients and normal vulvar tissue obtained from eight patients during gynecologic procedures. Two specimens of morphea were also examined.

RESULTS

Weak TGF-beta 1 staining was demonstrated in the epidermis of all the lichen sclerosus, morphea, thigh and five of the eight normal vulvar specimens. Slight increase in TGF-beta 1 staining was seen in the upper and middermis in 6 of the 10 lichen sclerosus specimens and in the morphea specimens as compared to the control tissue, and this staining was localized within cells. TGF-beta 2 staining was present throughout the epidermis in all the normal thigh, normal vulva, lichen sclerosus and morphea specimens. TGF-beta 2 staining was increased within cells in the upper and middermis of the lichen sclerosus and morphea specimens. TGF-beta 3 staining occurred in the basal half of the epidermis of all the control, lichen sclerosus and morphea specimens, and only slight upper dermal staining of a few individual cells was seen in 3 of the 10 lichen sclerosus specimens. VEGF staining was similar in the normal tissues, lichen sclerosus and morphea.

CONCLUSION

These results suggest that TGF-beta may.

Developmental expression of vascular endothelial growth factor in the masseter muscle of rats

Developmental changes in vascular endothelial growth factor (VEGF) in rat masseter after birth were investigated. VEGF was extracted efficiently and reproducibly from muscle homogenate with low concentrations of non-ionic detergents, such as Triton X-100, Nonidet P-40, and Tween 20. The amount of VEGF measured by enzyme-linked immunosorbent assay (ELISA) increased markedly by approximately 9-fold, from day 8 to 35 after birth. The increase in VEGF was closely correlated with the development of the capillary network, as shown by the capillary to muscle fibre ratio (C/F ratio). Immunoblotting revealed that the predominant molecular species of VEGF concentrated with heparin-sepharose beads was VEGF(188). These results suggest that VEGF plays an important part in the development and maintenance of the capillary network in the rat masseter.

VEGF-C and VEGF-D expression in neuroendocrine cells and their receptor, VEGFR-3, in fenestrated blood vessels in human tissues

Recently, vascular endothelial growth factor receptor 3 (VEGFR-3) has been shown to provide a specific marker for lymphatic endothelia in certain human tissues. In this study, we have investigated the expression of VEGFR-3 and its ligands VEGF-C and VEGF-D in fetal and adult tissues. VEGFR-3 was consistently detected in the endothelium of lymphatic vessels such as the thoracic duct, but fenestrated capillaries of several organs including the bone marrow, splenic and hepatic sinusoids, kidney glomeruli and endocrine glands also expressed this receptor. VEGF-C and VEGF-D, which bind both VEGFR-2 and VEGFR-3 were expressed in vascular smooth muscle cells. In addition, intense cytoplasmic staining for VEGF-C was observed in neuroendocrine cells such as the alpha cells of the islets of Langerhans, prolactin secreting cells of the anterior pituitary, adrenal medullary cells, and dispersed neuroendocrine cells of the gastrointestinal tract. VEGF-D was observed in the innermost zone of the adrenal cortex and in certain dispersed neuroendocrine cells. These results suggest that VEGF-C and VEGF-D have a paracrine function and perhaps a role in peptide release from secretory granules of certain neuroendocrine cells to surrounding capillaries.

Purification and refolding of vascular endothelial growth factor-B

Vascular endothelial growth factor (VEGF)-A interacts with the receptor tyrosine kinases VEGF-R1 and R2, and the importance of this interaction in endothelial cell (EC) function and blood vessel development has been well documented. Other ligands that interact differentially with these receptors and that are structurally related to VEGF-A include VEGF-B, VEGF-C, VEGF-D, and placenta growth factor (PLGF). Compared with VEGF-A, relatively little is known about the biological role of the VEGF-R1 specific ligand, VEGF-B. Two splice variant isoforms that differ at the COOH-terminus and which retain unique solubility characteristics are widely expressed throughout embryonic and postnatal development. Recent analysis of mice with a targeted deletion of the VEGF-B gene has revealed a defect in heart development and function consistent with an important role in vascularization of the myocardium (Bellomo D et al., 2000, Circ Res 86:E29-E35). To facilitate further characterization of VEGF-B, we have developed a protocol for expression and purification of refolded recombinant protein from Escherichia coli inclusion bodies (IBs). The approach developed resolves a number of significant issues associated with VEGF-B, including the ability to heterodimerize with endogenous VEGF-A when co-expressed in mammalian cells, a complex secondary structure incorporating inter- and intrachain disulfide bonds and hydrophobic characteristics that preclude the use of standard chromatographic resins. The resulting purified disulfide-linked homodimer was demonstrated to bind to VEGF-R1 and to compete with VEGF-A for binding to this receptor.

Matrix metalloproteinase-9 triggers the angiogenic switch during carcinogenesis

During carcinogenesis of pancreatic islets in transgenic mice, an angiogenic switch activates the quiescent vasculature. Paradoxically, vascular endothelial growth factor (VEGF) and its receptors are expressed constitutively. Nevertheless, a synthetic inhibitor (SU5416) of VEGF signalling impairs angiogenic switching and tumour growth. Two metalloproteinases, MMP-2/gelatinase-A and MMP-9/gelatinase-B, are upregulated in angiogenic lesions. MMP-9 can render normal islets angiogenic, releasing VEGF. MMP inhibitors reduce angiogenic switching, and tumour number and growth, as does genetic ablation of MMP-9. Absence of MMP-2 does not impair induction of angiogenesis, but retards tumour growth, whereas lack of urokinase has no effect. Our results show that MMP-9 is a component of the angiogenic switch.

The N-myc oncogene in human neuroblastoma cells: down-regulation of an angiogenesis inhibitor identified as activin A

Angiogenesis, the formation of new blood vessels, is seen during embryonic development and tumor progression, but the mechanisms have remained unclear. Recent data indicate that developmental and tumor angiogenesis can be induced by cellular oncogenes, leading to the enhanced activity of molecules stimulating angiogenesis. However, activated oncogenes might also facilitate angiogenesis by down-regulating endogenous inhibitors of angiogenesis. We report here that enhanced expression of the N-myc oncogene in human neuroblastoma cells down-regulates an inhibitor of endothelial cell proliferation, identified by amino acid sequencing as being identical with activin A, a developmentally regulated protein. Down-regulation appears to involve interaction of the N-Myc protein with the activin A promoter. In addition, activin A inhibits both endothelial cell proliferation in vitro and angiogenesis in vivo, and it induces hemorrhage in vivo. We suggest that the N-myc-induced down-regulation of activin A could contribute to developmental and tumor angiogenesis.

Growth suppressing factor for endothelial cells exhibits tumor regressing activity

Endothelium growth suppressing and tumor-regressing activities were copurified from the conditioned medium of P388D1 culture in the presence of 100 microg/ml carboxymethylated curdlan by a procedure including ammonium sulfate fractionation and six column chromatographies of Ceramic hydroxyapatite, Q-Sepharose, Sephacryl S-300 HR, Matrex PBA-30, PBE94, and anti-bovine serum albumin (anti-BSA) agarose. The intravenous administration of the purified growth suppressing factor for endothelial cells to sarcoma 180-bearing mouse caused a rapid decrease in the number of viable tumor cells in tumor lumps within 16 h. Immunohistochemical study showed that the intravenous injection of the purified factor to sarcoma 180-bearing mouse resulted in hemorrhagic disorder all over the tissue in the tumor lamp. Thus, the purified factor exhibited not only growth suppressing activity for endothelial cells but also tumor regressing activity at a concentration as low as about 15 ng/mouse. The purified factor significantly inhibited in vitro tubulogenesis of bovine artery, human umbilical vein, and adult human darmal microvascular endothelial cells on collagen gel at a concentration of about 5 ng/ml. After the tube formation of endothelial cells was completed on a collagen gel, the purified factor disrupted the tubes at a concentration of about 5 ng/ml within 48 h. These findings demonstrate that endothelium growth suppressing factor is a potent inhibitor of angiogenesis as well as the growth of endothelial cells, and may bring about the regression of a solid tumor by inhibiting angiogenesis.

A novel regulatory function of proteolytically cleaved VEGF-2 for vascular endothelial and smooth muscle cells

By high throughput sequencing, we have identified a cDNA encoding a polypeptide related to vascular endothelial growth factor (VEGF) and placenta growth factor (PlGF) in the VEGF/PDGF gene family. It is designated vascular endothelial growth factor 2 (VEGF-2). Similar to VEGF, expression of VEGF-2 mRNA is abundant in vascular smooth muscle cells and several highly vascularized tissues. VEGF-2 protein is expressed as a secreted 52 kDa precursor as well as the 30 kDa amino-terminal and 27 kDa carboxy-terminal cleavage products. The latter two cleavage products are linked via a disulfide bridge (or bridges) and can be copurified. Using copurified 30 and 27 kDa proteins, the effect of VEGF-2 on growth of several cell types, including vascular endothelial and smooth muscle cells, was determined. Our results demonstrate that VEGF-2 protein stimulates the growth of human vascular endothelial cells but inhibits growth of human aortic smooth muscle cells induced by platelet-derived growth factor. These studies establish VEGF-2 as a novel regulator for growth of vascular endothelial and smooth muscle cells.

Synthesis and physiological activity of heterodimers comprising different splice forms of vascular endothelial growth factor and placenta growth factor

Vascular endothelial growth factor (VEGF) and placenta growth factor (PIGF) are members of a dimeric-growth-factor family with angiogenic properties. VEGF is a highly potent and specific mitogen for endothelial cells, playing a vital role in angiogenesis in vivo. The role of PIGF is less clear. We expressed the monomeric splice forms VEGF-165, VEGF-121, PIGF-1 and PIGF-2 as unfused genes in Escherichia coli using the pCYTEXP expression system. In vitro dimerization experiments revealed that both homo- and hetero-dimers can be formed from these monomeric proteins. The dimers were tested for their ability to promote capillary growth in vivo and stimulate DNA synthesis in cultured human vascular endothelial cells. Heterodimers comprising different VEGF splice forms, or combinations of VEGF/PIGF splice forms, showed mitogenic activity. The results demonstrate that four different heterodimeric growth factors are likely to have as yet uncharacterized functions in vivo.

Selective binding of VEGF121 to one of the three vascular endothelial growth factor receptors of vascular endothelial cells

VEGF121 and VEGF165 are vascular endothelial growth factor splice variants that promote the proliferation of endothelial cells and angiogenesis. VEGF165 contains the 44 additional amino acids encoded by exon 7 of the VEGF gene. These amino acids confer upon VEGF165 a heparin binding capability which VEGF121 lacks. 125I-VEGF165 bound to three vascular endothelial growth factor (VEGF) receptors on endothelial cells, while 125I-VEGF121 bound selectively only to the flk-1 VEGF receptor which corresponds to the larger of the three VEGF receptors. The binding of 125I-VEGF121 to flk-1 was not affected by the removal of cell surface heparan sulfates or by heparin. Both VEGF165 and VEGF121 inhibited the binding of 125I-VEGF121 to a soluble extracellular domain of the flk-1 VEGF receptor in the absence of heparin. However, heparin potentiated the inhibitory effect of VEGF165 by 2-3-fold. These results contrast with previous observations which have indicated that the binding of 125I-VEGF165 to the flk-1 receptor is strongly dependent on heparin-like molecules. Further experiments showed that the receptor binding ability of VEGF165 is susceptible to oxidative damage caused by oxidants such as H2O2 or chloramine-T. VEGF121 was also damaged by oxidants but to a lesser extent. Heparin or cell surface heparan sulfates restored the flk-1 binding ability of damaged VEGF165 but not the receptor binding ability of damaged VEGF121. These observations suggest that alternative splicing can generate a diversity in growth factor signaling by determining receptor recognition patterns. They also indicate that the heparin binding ability of VEGF165 may enable the restoration of damaged VEGF165 function in processes such as inflammation or wound healing.

Characterization of novel vascular endothelial growth factor (VEGF) receptors on tumor cells that bind VEGF165 via its exon 7-encoded domain

Vascular endothelial growth factor (VEGF), a potent angiogenic factor, uses two receptor tyrosine kinases, FLK/KDR and FLT, to mediate its activities. We have cross-linked 125I-VEGF165 to the cell surface of various tumor cell lines and of human umbilical vein endothelial cells. High molecular mass (220 and 240 kDa) and/or lower molecular mass (165 and 175 kDa) labeled complexes were detected depending on the cell type. The 220- and 240-kDa labeled complexes were shown to contain FLT and FLK/KDR receptors, respectively. On the other hand, the 165- and 175-kDa complexes did not seem to contain FLK/KDR or FLT but instead appeared to contain novel VEGF receptors with relatively low molecular masses of approximately 120 and 130 kDa. These receptors were further characterized in breast cancer MDA MB 231 cells (231), which did not form the high molecular mass complexes and which did not express detectable amounts of flk/kdr or flt mRNA. The 231 cells displayed one VEGF165 binding site, with a Kd of 2.8 x 10(-10) M and 0.95 1.1 x 10(5) binding sites per cell. By comparison, human umbilical vein endothelial cells had two binding sites, one with a Kd of 7.5 x 10(-12) M, presumably FLK/KDR, and the other with a Kd of 2 x 10(-10) M, a value similar to the VEGF binding sites on 231 cells. These lower affinity/molecular mass receptors on 231 cells cross-linked 125I-VEGF165 but not 125I-VEGF121. Accordingly, exon 7 of VEGF, which encodes the 44 amino acids present in VEGF165 that are absent in VEGF121, was fused to glutathione S-transferase (GST). The GST-VEGF-exon 7 fusion protein bound to heparin-Sepharose with a similar affinity as VEGF165 and inhibited the binding of 125I-VEGF165 to 231 cells. Cross-linking of 125I-GST-VEGF-exon 7 to 231 cells resulted in the formation of 150- and 160-kDa labeled complexes that presumably contained the 120- and 130-kDa lower affinity/molecular mass VEGF165 receptors. It was concluded that certain tumor-derived cell lines express novel surface-associated receptors that selectively bind VEGF165 via the exon 7-encoded domain, which is absent in VEGF121.

Immunohistochemical localization of vascular endothelial growth factor in the human placenta

To understand the role of vascular endothelial growth factor (VEGF) in placental development, we examined immunohistochemically 56 placentae ranging from 6--41-weeks gestation using rabbit antibody to a synthetic multiple antigen peptide (MAP), composed of N-terminal amino acid residues 1--20 of human VEGF. In the present study, syncytiotrophoblast and invading extravillous trophoblasts ubiquitously expressed VEGF throughout gestation. However, the expression of VEGF in syncytiotrophoblasts was uneven in the first trimester and most intense at the sprouting sites. In addition, some stromal cells in the villi and decidual cells were also positive in the first trimester. A morphometrical analysis of the ratio of the VEGF-positive cell area to the capillary area in the terminal villi statistically revealed a critical point of change at 16 weeks' gestation. These results provide further evidence to support the hypothesis that VEGF, locally expressed by trophoblasts, stromal cells in villi and decidual cells, may play an important role in the physiological growth and function of the vascular system in the villous stroma and basal plate during placental development and maturation.

Both v-Ha-Ras and v-Raf stimulate expression of the vascular endothelial growth factor in NIH 3T3 cells

Stimulation of NIH 3T3 cells with platelet-derived growth factor (PDGF)-BB and 12-O-tetradecanoylphorbol-13-acetate (TPA) enhances vascular endothelial growth factor (VEGF) gene expression. To address the question of whether Ras and Raf are involved in the induction of VEGF gene expression by PDGF and TPA, we examined the effects of both factors on NIH 3T3 cells stably transfected with v-Ha-ras or v-raf. In serum-starved NIH 3T3 cells, only low levels of mRNA expression can be detected, whereas both ras and raf transformed cell lines express enhanced levels of a 4.3-kilobase VEGF transcript. Stimulation with PDGF or TPA resulted in increased VEGF mRNA in all cell lines, with highest levels found in the transformed cells. Immunofluorescence studies confirmed that the elevated VEGF mRNA expression correlated with enhanced protein levels. Positive immunofluorescence signals could be detected in v-Ha-ras or v-raf transformed cell lines but not in unstimulated NIH 3T3 cells. VEGF from conditioned medium of v-raf transformed NIH 3T3 cells was partially purified by chromatography on heparin-Sepharose. Biological activity of this VEGF protein was demonstrated by competition with binding of recombinant 125I-VEGF165 to human umbilical vein endothelial cells and by its ability to stimulate proliferation of these cells.

A novel method to purify recombinant vascular endothelial growth factor (VEGF121) expressed in yeast

VEGF is a potent mitogen for vascular endothelial cells in vitro and acts as an angiogenic factor in vivo. VEGF121 differs from the other isoforms in that it lacks the heparin-binding domain. To study the potential differences in biological functions of the VEGF isoforms, we cloned and expressed VEGF121 in a yeast expression system. VEGF121 was secreted from the yeast cells as a homodimer with a molecular weight of 34-36 kDa. By taking advantage of the consecutive histidine residues present at position 11 and 12 in VEGF, a novel method of purification using Nickel affinity chromatography was developed. Since all the isoforms of VEGF have an identical amino terminal end, this method can be used to purify not only VEGF121 but also the other forms of VEGF. The level of expression achieved using this system was as high as 40 mg/L. The recombinant protein was biologically active in stimulating the in vitro proliferation of vascular endothelial cells and positively reacted to an antiserum made against recombinant VEGF165.

Purification and characterization of a naturally occurring vascular endothelial growth factor.placenta growth factor heterodimer

Vascular endothelial growth factor (VEGF) is a potent and selective mitogen for endothelial cells that is angiogenic in vivo and induced by hypoxia. A homologous protein, placenta growth factor (PlGF), is also reported to be mitogenic for endothelial cells in culture. The rat GS-9L glioma cell line produces not only VEGF homodimers but also PlGF homodimers and a novel heterodimer composed of VEGF and PlGF subunits. All three dimeric forms were purified to apparent homogeneity, and their structures and mitogenic activities were compared. VEGF.PlGF heterodimers are vascular endothelial cell mitogens nearly as potent as VEGF homodimers. Therefore, some of the biological activities attributed to VEGF homodimers might be mediated by VEGF.PlGF heterodimers. In contrast, pure PlGF homodimers are mitogenic for endothelial cells only at high, possibly non-physiologic concentrations; thus the biological relevance of their mitogenic activity for these cells is not obvious. However, the existence of not only homodimers but also heterodimers clearly extends the similarity between the VEGF/PlGF and the homologous platelet-derived growth factor systems.

The shortest isoform of human vascular endothelial growth factor/vascular permeability factor (VEGF/VPF121) produced by Saccharomyces cerevisiae promotes both angiogenesis and vascular permeability

Vascular endothelial growth factor/vascular permeability factor (VEGF/VPF) is a multifunctional cytokine that is expressed as four isoforms having 206, 189, 165, and 121 amino acids in humans. We constructed a system that produces the shortest isoform of human VEGF/VPF in Saccharomyces cerevisiae (yVEGF/VPF121). Active yVEGF/VPF121 was secreted from the yeast cells as a glycosylated dimeric protein. Various biological activities of the purified yVEGF/VPF121 were examined. It bound to cell surface receptor(s) and stimulated the growth of human umbilical vein endothelial cells in culture at a dose similar to that of native VEGF/VPF. Purified yVEGF/VPF121 also induced angiogenesis in mice, and promoted the extravasation of plasma proteins from the blood vessels. These observations demonstrated that the shortest isoform of VEGF/VPF with an amino acid sequence of 121 residues contains enough information necessary to trigger both angiogenesis and the induction of vascular permeability upon binding to its cognate receptor(s).

Expression of biologically active human vascular endothelial growth factor in yeast

Vascular endothelial growth factor (VEGF) is a glycoprotein consisting of two identical polypeptide chains linked by a disulfide bond. The unique biological activities of VEGF include its potent mitogenic and permeability inducing properties specific for the vascular endothelium. VEGF is implicated in tumor angiogenesis, wound healing, and the stimulation of collateral vessel formation at the site of arterial occlusion. Therefore, in order to produce large quantities of biologically active VEGF, a splice variant (VEGF165) was cloned and expressed in a yeast expression system. The coding region of VEGF165 was isolated from U937 cells by RT-PCR, sequenced and then cloned into the yeast expression vector pHILS1. VEGF165 was secreted into the medium as a dimer. Recombinant VEGF reacted to antibodies raised against the N-terminal and C-terminal synthetic polypeptides of human VEGF. As much as 35-40 mg/L of purified VEGF could be obtained from the yeast expression system. The recombinant protein was biologically active in inducing vascular endothelial cell proliferation in vitro and permeability changes in vivo.

Vascular endothelial growth factor is induced in response to transforming growth factor-beta in fibroblastic and epithelial cells

Transforming growth factor beta (TGF-beta) is a multifunctional polypeptide that regulates the proliferation and differentiation of various cells and has an angiogenic effect in vivo, although it inhibits the growth of cultured endothelial cells. We report here that TGF-beta treatment of quiescent cultures of mouse embryo-derived AKR-2B cells, which are growth-stimulated by TGF-beta, and human lung adenocarcinoma A549 cells, which are growth-inhibited by TGF-beta, results in the induction of vascular endothelial growth factor (VEGF) mRNA and protein. Maximal VEGF mRNA levels occurred 4-8 h after stimulation with a decline to background levels in 24 h. In contrast, the related placenta growth factor mRNA was not induced by TGF-beta in these cells. No VEGF receptor mRNA was seen in AKR-2B cells. Also, TGF-beta treatment of endothelial cells, which express the FLT1 and KDR/FLK-1 receptors for VEGF, did not cause VEGF induction. Because VEGF is known to be a strong angiogenic factor for endothelial cells, the results suggest that the angiogenic effect of TGF-beta on endothelial cells in blood vessels may be mediated at least partly by a paracrine induction of VEGF in other surrounding cell types.

Crystallization and X-ray diffraction study of recombinant platelet-derived endothelial cell growth factor

Crystals of recombinant platelet-derived endothelial cell growth factor (PD-ECGF) were obtained by the hanging drop vapour diffusion technique. The crystals belong to the space group P2(1)2(1)2(1) with unit cell dimensions a = 63.7 A, b = 70.4 A, c = 219.6, alpha = beta = gamma = 90 degrees, and probably contain a single dimer in the asymmetric unit. Diffraction to a minimum Bragg spacing of 3.5 A has been obtained using a synchrotron X-ray source.

Synthesis and secretion of vascular permeability factor/vascular endothelial growth factor by human retinal pigment epithelial cells

The presence of the secreted angiogenic molecule, vascular permeability factor/vascular endothelial growth factor, was investigated in human retinal pigment epithelial cells. Northern and in situ hybridization analysis of cultured cells identified vascular permeability factor/vascular endothelial growth factor mRNA. Western analysis of cell lysates identified a 42 kD protein that comigrated with human recombinant vascular permeability factor/vascular endothelial growth factor. Immunoassay measurements detected vascular permeability factor/vascular endothelial growth factor protein in cell lysates and conditioned media in vitro and in cell lysates isolated directly from post-mortem eyes. These data demonstrate that human retinal pigment epithelial cells can synthesize the secreted angiogenic peptide vascular permeability factor/vascular endothelial growth factor in vitro and in situ. The production and secretion of this factor by human retinal pigment epithelial cells may be important in the pathogenesis of ocular neovascularization.

Vascular endothelial growth factor is inactivated by binding to alpha 2-macroglobulin and the binding is inhibited by heparin

Vascular endothelial growth factor (VEGF) is a mitogen for cultured endothelial cells, and a potent angiogenic factor in vivo. Incubation of 125I-VEGF with human or bovine serum led to the formation of 125I-VEGF containing complexes that had a molecular mass greater than 300 kDa. These complexes were specifically immunoprecipitated with anti-human alpha 2-macroglobulin (alpha 2M) antibodies. Similar high molecular weight complexes were formed when 125I-VEGF was incubated with commercially available alpha 2M. The 125I-VEGF.alpha 2M complexes were resistant to boiling in the presence of SDS. The formation of 125I-VEGF.alpha 2M complexes was inhibited by iodoacetic acid, indicating that free sulfhydryl groups are required for complex assembly. Tryptic digestion of alpha 2M did not affect its VEGF binding ability. Tryptic digestion of 125I-VEGF.alpha 2M complexes on the other hand, resulted in the degradation of bound 125I-VEGF, indicating that alpha 2M does not protect bound 125I-VEGF from proteolytic digestion. The binding of 125I-VEGF to alpha 2M was partially inhibited by an excess of basic fibroblast growth factor. Other growth factors which bind to alpha 2M, such as platelet-derived growth factor and insulin, did not inhibit the binding of 125I-VEGF. The binding of VEGF to alpha 2M inhibited its receptor binding ability, indicating that alpha 2M may function as a VEGF removal and inactivation factor. Heparin and heparan sulfate, but not other glycosaminoglycans such as chondroitin sulfate, efficiently inhibited the binding of 125I-VEGF to alpha 2M. It is possible that heparin-like molecules released from extracellular matrixes could prevent the inactivation of VEGF by alpha 2M resulting in the potentiation of processes such as tumor angiogenesis.

Dual regulation of vascular endothelial growth factor bioavailability by genetic and proteolytic mechanisms

The vascular endothelial growth factor (VEGF) family encompasses four polypeptides that result from alternative splicing of mRNA. We have previously demonstrated differences in the secretion pattern of these polypeptides. Stable cell lines expressing VEGFs were established in human embryonic kidney CEN4 cells. VEGF121, the shortest form, was secreted and freely soluble in tissue culture medium. VEGF189 was secreted, but was almost entirely bound to the cell surface or extracellular matrix. VEGF165 displayed an intermediary behavior. Suramin induced the release of VEGF189, permitting its characterization as a more basic protein with higher affinity for heparin than VEGF165 or VEGF121, but with similar endothelial cell mitogenic activity. Heparin, heparan sulfate, and heparinase all induced the release of VEGF165 and VEGF189, suggesting heparin-containing proteoglycans as candidate VEGF-binding sites. Finally, VEGF165 and VEGF189 were released from their bound states by treatment with plasmin. The released 34-kDa dimeric species are active as endothelial cell mitogens and as vascular permeability agents. We conclude that the bioavailability of VEGF may be regulated at the genetic level by alternative splicing that determines whether VEGF will be soluble or incorporated into a biological reservoir and also through proteolysis following plasminogen activation.

Vascular endothelial growth factor. Regulation by cell differentiation and activated second messenger pathways

Vascular endothelial growth factor (VEGF) is an angiogenic polypeptide that has been isolated from a variety of tumorigenic and nontransformed cell lines. Because of the importance of blood vessel growth to cell and tissue development, we have examined VEGF gene expression in a variety of mouse tissues and rodent models of cellular differentiation. Using a cloned murine VEGF cDNA we show that VEGF mRNA is expressed at relatively low levels in many adult mouse tissues examined. However, this message is dramatically induced in two models of cell differentiation: 3T3-adipose conversion and C2C12 myogenic differentiation. VEGF protein secretion is also induced in adipocyte differentiation. VEGF mRNA is markedly regulated in a pheochromocytoma (PC12) cell model of transformation and differentiation. The transformed undifferentiated cells express moderate levels of VEGF mRNA and this expression is virtually extinguished when cells differentiate into non-malignant neuron-like cells. Experiments employing phorbol esters and cAMP analogues indicate that VEGF mRNA expression is stimulated in preadipocytes by both protein kinase C and protein kinase A-mediated pathways. These results suggest that VEGF mRNA levels are closely linked to the process of cellular differentiation; they also clearly demonstrate that expression of this angiogenic factor is specifically regulated in a transformed cell line, possibly via aberrant activation of cellular second messenger pathways.

The binding of vascular endothelial growth factor to its receptors is dependent on cell surface-associated heparin-like molecules

Vascular endothelial growth factor (VEGF) induces the proliferation of endothelial cells and is a potent angiogenic factor that binds to heparin. We have therefore studied the effect of heparin upon the interaction of VEGF with its receptors. Heparin, at concentrations ranging from 0.1 to 10 micrograms/ml, strongly potentiated the binding of 125I-VEGF to its receptors on endothelial cells. Scatchard analysis of 125I-VEGF binding indicates that 1 microgram/ml heparin induces an 8-fold increase in the apparent density of high affinity binding sites for VEGF, but does not significantly affect the dissociation constant of VEGF. Cross-linking experiments showed that heparin strongly potentiates the formation of the 170-, 195- and 225-kDa 125I-VEGF-receptor complexes on endothelial cells. At high 125I-VEGF concentrations (4 ng/ml), heparin preferentially enhanced the formation of the 170- and 195-kDa complexes. Preincubation of the cells with heparin, followed by extensive washes, produced a similar enhancement of subsequent 125I-VEGF binding. The binding of 125I-VEGF was completely inhibited following digestion of endothelial cells with heparinase and could be restored by the addition of exogenous heparin to the digested cells. The enhancing effect of heparin facilitated the detection of VEGF receptors on cell types that were not known previously to express such receptors. Our results suggest that cell surface-associated heparin-like molecules are required for the interaction of VEGF with its cell surface receptors.

Glycosylation of vascular endothelial growth factor is not required for its mitogenic activity

We have stably expressed the cDNA encoding the 165 amino-acid long form of human vascular endothelial growth factor (VEGF) in BHK-21 cells. VEGF was partially purified from the conditioned medium of transfected cells using heparin-sepharose affinity chromatography. The partially purified VEGF was mitogenic for various types of endothelial cells and inhibited the binding of pure [125I]VEGF to its receptors. Western blot analysis, using anti-VEGF antibodies, revealed a 47 kDa VEGF homodimer in the partially purified VEGF fraction. Preincubation of the transfected cells with the N-glycosylation inhibitor tunicamycin resulted in the conversion of the 47 kDa VEGF homodimer into a smaller, deglycosylated form of 42 kDa. Partially purified preparations of the deglycosylated VEGF displayed a mitogenic activity that was similar to that of the glycosylated form and efficiently inhibited the binding of native [125I]VEGF to the VEGF receptors of bovine aortic arch derived endothelial cells.

Identification of non heparin-binding endothelial cell growth factor from rat myofibroblasts

Myofibroblasts (Mfs) from rat fat tissues produced a potent endothelial cell growth factor (Mf-ECGF). The growth factor activity found in the conditioned media from primary cultures of Mfs, was labile to heat (80 degrees C for 10 min) and proteinase (trypsin), and did not bind to heparin in the presence of 0.2 M NaCl. Mf-ECGF was partially purified 4760-fold with a recovery of 25% from serum-free conditioned media by sequential carboxymethyl (CM) ion-exchange column chromatography and gel filtration. This Mf-ECGF activity was recovered from the 40 kD region of a non-reducing SDS-PAGE, and from the pH region between 6.5 and 7 of isoelectric focusing, with recoveries of 20% and 65%, respectively. These results indicated that a major portion of ECGF activity in the conditioned media was clearly distinct from other well-known endothelial cell growth factors including fibroblast growth factors (FGFs).

High levels of biologically active vascular endothelial growth factor (VEGF) are produced by the baculovirus expression system

Vascular endothelial growth factor (VEGF) is a recently discovered mitogen for endothelial cells in vitro, and a potent angiogenesis promoting factor in vivo. VEGF is secreted from producing cells as a homodimer, binds to specific receptors on the cell surface of endothelial cells, and is produced in four forms as a result of alternative splicing. We have expressed the cDNA encoding the 165 amino-acid long isoform of VEGF in insect cells using the baculovirus based expression vector. We show that infected insect cells secrete large amounts of VEGF. Antibodies directed against a synthetic peptide prepared from human VEGF identify the secreted factor. The baculovirus derived VEGF expressed in insect cells (inVEGF) binds directly to the VEGF receptors inVEGF competes with pure mammalian cells derived [125I]-VEGF for binding to the VEGF receptors that are present on the cell surface of endothelial cells. Furthermore, inVEGF is biologically active and induces the proliferation of human umbilical vein derived endothelial cells.

[A basic study on omental transplantation. Vascular endothelial cell growth factor in human omentum]

Omentum tissue has potent angiogenic activity, so transplantation is often effective in treatment of moyamoya disease. Omentum was homogenized and fractionated to investigate the presence of an endothelial cell growth factor. The effectiveness of extracts was measured by the growth activity of bovine aortic endothelial cell incubated for 6 days with various omentum extracts. The lipid fraction had no growth promoting activity. However, the water soluble extracts were active. The activity could be increased by ammonium sulfate precipitation (60-80% saturation). Gel permeation chromatography of the ammonium sulfate fraction showed that the endothelial growth factor had an apparent molecular weight of 96,000. Heparin affinity chromatography revealed poor heparin affinity. The activity was stable at pH 3.5 and pH 9.5, but inactivated by heating at 70 degrees C for 10 minutes or 100 degrees C for 2 minutes. These properties clearly distinguish the omentum-derived growth factor (OmDGF) from the heparin binding growth factor. OmDGF is probably distinct from other vascular endothelial cell growth factors.

Glycosylation is essential for efficient secretion but not for permeability-enhancing activity of vascular permeability factor (vascular endothelial growth factor)

The hyperpermeability of the microvasculature supplying solid tumors is largely attributable to a heterodimeric Mr 34,000-43,000 tumor-secreted protein, vascular permeability factor. Upon reduction, the vascular permeability factor secreted by line 10 tumor cells is resolved by SDS-PAGE into 3 discrete bands of Mr 24,000, 19,500, and 15,000. We demonstrate here that line 10 vascular permeability factor is an N-linked glycoprotein. Nonglycosylated vascular permeability factor migrates on reduced SDS-PAGE as two bands of Mr 20,000 and 15,000. Pulse-chase studies demonstrated that all three chains of native vascular permeability factor were secreted rapidly following synthesis and at equal rates, with a cellular half-retention time of approximately 37 min. When glycosylation was prevented by tunicamycin, individual bands of nonglycosylated vascular permeability factor were also secreted at equivalent rates, but much more slowly (approximately 60 min) than native glycoprotein. Both glycosylated and nonglycosylated forms of vascular permeability factor were equally potent at increasing dermal vessel permeability.

Vascular endothelial cell growth factor (VEGF) produced by A-431 human epidermoid carcinoma cells and identification of VEGF membrane binding sites

A distinct family of endothelial cell mitogens that are homologous to platelet-derived growth factor has recently been identified. Unlike other known endothelial cell mitogens, these vascular endothelial cell growth factors (VEGFs) are secreted and appear to act specifically on endothelial cells. We have purified VEGF 2083-fold to apparent homogeneity from protein-free culture medium conditioned by A-431 human epidermoid carcinoma cells. This A-431-derived VEGF was characterized as a homodimer composed of 22-kDa subunits with an N-terminal sequence that was similar to VEGFs produced by human HL-60 leukemic and U-937 histiocytic lymphoma cells. A-431 VEGF was used to identify specific and saturable binding sites for VEGF on human umbilical vein endothelial cells (HUVEC). By affinity cross-linking, VEGF-binding site complexes of 230, 170, and 125 kDa were detected on HUVEC. VEGF specifically induced the tyrosine phosphorylation of a 190-kDa polypeptide, which was similar in mass to the largest binding site detected by affinity cross-linking.

Vascular permeability factor: a tumor-derived polypeptide that induces endothelial cell and monocyte procoagulant activity, and promotes monocyte migration

Systemic infusion of low concentrations of tumor necrosis factor/cachectin (TNF) into mice that bear TNF-sensitive tumors leads to activation of coagulation, fibrin formation, and occlusive thrombosis exclusively within the tumor vascular bed. To identify mechanisms underlying the localization of this vascular procoagulant response, a tumor-derived polypeptide has been purified to homogeneity from supernatants of murine methylcholanthrene A-induced fibrosarcomas that induces endothelial tissue factor synthesis and expression (half-maximal response at approximately 300 pM), and augments the procoagulant response to TNF in a synergistic fashion. This tumor-derived polypeptide was identified as the murine homologue of vascular permeability factor (VPF) based on similar mobility on SDS-PAGE, an homologous NH2-terminal amino acid sequence, and recognition by a monospecific antibody to guinea pig VPF. In addition, VPF was shown to induce monocyte activation, as evidenced by expression of tissue factor. Finally, VPF was shown to induce monocyte chemotaxis across collagen membranes and endothelial cell monolayers. Taken together, these results indicate that VPF can modulate the coagulant properties of endothelium and monocytes, and can promote monocyte migration into the tumor bed. This suggests one mechanism through which tumor-derived mediators can alter properties of the vessel wall.

Conditioned medium from mouse sarcoma 180 cells contains vascular endothelial growth factor

Medium conditioned by mouse sarcoma 180 cells stimulates the growth of capillary endothelial cells. The growth factor produced by mouse sarcoma 180 cells is heparin-binding, dithiothreitol-sensitive, endothelial cell specific, and secreted into the medium. The characteristics of this mouse sarcoma-derived growth factor are very similar to those of vascular endothelial growth factor (VEGF) first described by Ferrara and Henzel (1989). The N-terminal amino acid sequences of the two growth factors are similar. Since the amino acid sequence of vascular permeability factor (VPF) is essentially identical to that of VEGF, a Western blot of mouse sarcoma 180-derived endothelial growth factor was probed with a polyclonal antibody raised against human VPF. This antibody reacted with several proteins of approximately 23 kDa, suggesting the presence of multiple forms of a VEGF-like protein. A full length cDNA probe for bovine VEGF reacted strongly with RNA isolated from mouse sarcoma 180 cells. We conclude that an endothelial growth factor found in conditioned medium from mouse sarcoma 180 cells is VEGF.