Placenta growth factor. Potentiation of vascular endothelial growth factor bioactivity, in vitro and in vivo, and high affinity binding to Flt-1 but not to Flk-1/KDR

Regulation of flt-1 expression during mouse embryogenesis suggests a role in the establishment of vascular endothelium

Flt-1 is a high affinity binding receptor for the vascular endothelial cell growth factor (VEGF) and is primarily expressed in endothelial cells. In this study we have investigated the temporal and spatial regulation of its expression by establishing mouse lines containing the lacZ gene targeted into the flt-1 locus through homologous recombination in embryonic stem (ES) cells. In the yolk sac as well as in the embryo proper, lacZ expression faithfully reflected the endogenous expression pattern of the flt-1 gene. LacZ staining of heterozygous embryos led to the following observations: (1) the onset of flt-1 expression is detected at the early primitive streak stage in the extraembryonic mesoderm, and is strongly up-regulated thereafter, reaching a maximum by early to midsomite stages and declining subsequently; (2) while flt-1 is widely expressed within the developing vascular endothelium, its expression level is differentially regulated both spatially and temporally. The pattern of flt-1 expression suggests that it may play an important role in the initiation of endothelium development; and (3) flt-1 is expressed in essentially all the cells in early blood islands, but later its expression is gradually restricted to the endothelial lineage. Our results indicate that flt-1 is a marker for hemangioblasts, the presumed progenitor for both hematopoietic and angioblastic lineage. The flt-1 expression pattern also suggests that it may play important roles in both vasculogenesis and angiogenesis.

Genomic organization of the mouse and human genes for vascular endothelial growth factor B (VEGF-B) and characterization of a second splice isoform

A second isoform and the genomic structures of mouse and human vascular endothelial growth factor B are described. Both genes consist of seven coding exons and span about 4 kilobases of DNA. The two identified isoforms of vascular endothelial growth factor B are generated by alternative splicing where different splice acceptor sites in exon 6 introduce a frameshift and a partial use of different but overlapping reading frames. Consequently, the COOH-terminal domains in the two isoforms show no resemblance. Mouse and human cDNA clones for the novel isoform of vascular endothelial growth factor B encoded a secreted protein of 186 amino acid residues. Expression in transfected cells generated a protein of 25 kDa which upon secretion was modified by O-linked glycosylation and displayed a molecular mass of 32 kDa under reducing conditions. The protein was expressed as a disulfide-linked homodimer, and heterodimers were generated when coexpressed with vascular endothelial growth factor. The entirely different COOH-terminal domains in the two isoforms of vascular endothelial growth factor B imply that some functional properties of the two proteins are distinct.

The vascular endothelial growth factor receptor Flt-1 mediates biological activities. Implications for a functional role of placenta growth factor in monocyte activation and chemotaxis

Two distinct receptors for vascular endothelial growth factor (VEGF), the tyrosine kinase receptors Flt-1 and Flk-1/KDR, have been described. In this study we show that monocytes, in contrast to endothelium, express only the VEGF receptor Flt-1, and that this receptor specifically binds also the VEGF homolog placenta growth factor (PlGF). Both VEGF and PlGF stimulate tissue factor production and chemotaxis in monocytes at equivalent doses. In contrast, endothelial cells expressing both the Flt-1 and the Flk-1/KDR receptors produce more tissue factor upon stimulation with VEGF than after stimulation with PlGF. Neutralizing antibodies to the KDR receptor reduce the VEGF-stimulated tissue factor induction in endothelial cells to levels obtained by stimulation with PlGF alone, but do not affect PlGF-induced tissue factor induction in endothelial cells nor the VEGF-dependent tissue factor production in monocytes. These findings strongly suggest Flt-1 as a functional receptor for VEGF and PlGF in monocytes and endothelial cells and identify this receptor as a mediator of monocyte recruitment and procoagulant activity.

Interaction with the phosphotyrosine binding domain/phosphotyrosine interacting domain of SHC is required for the transforming activity of the FLT4/VEGFR3 receptor tyrosine kinase

The FLT4 gene encodes two isoforms of a tyrosine kinase receptor, which belongs to the family of receptors for vascular endothelial growth factor. As the result of an alternative processing of primary mRNA transcripts, the long isoform differs from the short isoform by an additional stretch of 65 amino acid residues located at the C terminus and containing three tyrosine residues, Tyr1333, Tyr1337, and Tyr1363. Only the long isoform is endowed with a transforming capacity in fibroblasts. We show that this activity is related to the capacity of the tyrosine 1337-containing sequence to interact with the phosphotyrosine binding domain of the SHC protein. This demonstrates that a functional property of this newly described domain includes relay of mitogenic signals. In addition, it shows that the same receptor can mediate different functions through the optional binding of the phosphotyrosine binding domain and that the alternative use of this domain is sufficient to direct the signal toward different pathways.

Heterozygous embryonic lethality induced by targeted inactivation of the VEGF gene

Angiogenesis is required for a wide variety of physiological and pathological processes. The endothelial cell-specific mitogen vascular endothelial growth factor (VEGF) is a major mediator of pathological angiogenesis. Also, the expression of VEGF and its two receptors, Flt-1 and Flk-1/KDR, is related to the formation of blood vessels in mouse and rat embryos. Mice homozygous for mutations that inactivate either receptor die in utero between days 8.5 and 9.5. However, ligand(s) other than VEGF might activate such receptors. To assess the role of VEGF directly, we disrupted the VEGF gene in embryonic stem cells. Here we report the unexpected finding that loss of a single VEGF allele is lethal in the mouse embryo between days 11 and 12. Angiogenesis and blood-island formation were impaired, resulting in several developmental anomalies. Furthermore, VEGF-null embryonic stem cells exhibit a dramatically reduced ability to form tumours in nude mice.

Abnormal blood vessel development and lethality in embryos lacking a single VEGF allele

The endothelial cell-specific vascular endothelial growth factor (VEGF) and its cellular receptors Flt-1 and Flk-1 have been implicated in the formation of the embryonic vasculature. This is suggested by their colocalized expression during embryogenesis and the impaired vessel formation in Flk-1 and Flt-1 deficient embryos. However, because Flt-1 also binds placental growth factor, a VEGF homologue, the precise role of VEGF was unknown. Here we report that formation of blood vessels was abnormal, but not abolished, in heterozygous VEGF-deficient (VEGF+/-) embryos, generated by aggregation of embryonic stem (ES) cells with tetraploid embryos (T-ES) and even more impaired in homozygous VEGF-deficient (VEGF-/-) T-ES embryos, resulting in death at mid-gestation. Similar phenotypes were observed in F1-VEGF+/- embryos, generated by germline transmission. We believe that this heterozygous lethal phenotype, which differs from the homozygous lethality in VEGF-receptor-deficient embryos, is unprecedented for a targeted autosomal gene inactivation, and is indicative of a tight dose-dependent regulation of embryonic vessel development by VEGF.

Embryonic angiogenesis: a review

Supply with nutrients is essential from early embryonic stages onwards. Therefore, circulatory organs form the first functioning organ system. With the exception of the heart, this system is at first formed by only one cell type, the endothelial cell. Emergence, behavior, and differentiation of endothelial cells are discussed in this review. At first, endothelial cells develop from angioblasts (primary angiogenesis/angioblastic development), later they develop from preexisting endothelial cells (secondary angiogenesis/angiotrophic growth). The composition of the extracellular matrix may promote or inhibit angiogenesis. Various growth factors which can be bound to the extracellular matrix may have been found, but only two of them (VEGF, P1GF) seem to influence endothelial cell behavior directly. Heterogeneity and organ-typical differentiation of endothelial cells seem to be dependent on cell-cell signaling within each organ.

The carboxyl-terminal domain (111-165) of vascular endothelial growth factor is critical for its mitogenic potency

Vascular endothelial growth factor (VEGF) is a potent and specific mitogen for endothelial cells. VEGF is synthesized and secreted by many differentiated cells in response to a variety of stimuli including hypoxia. VEGF is expressed in a variety of tissues as multiple homodimeric forms (121, 165, 189, and 206 amino acids/monomer) resulting from alternative RNA splicing. VEGF121 is a soluble mitogen that does not bind heparin; the longer forms of VEGF bind heparin with progressively higher affinity. The higher molecular weight forms of VEGF can be cleaved by plasmin to release a diffusible form(s) of VEGF. We characterized the proteolysis of VEGF by plasmin and other proteases. Thrombin, elastase, and collagenase did not cleave VEGF, whereas trypsin generated a series of smaller fragments. The isolated plasmin fragments of VEGF were compared with respect to heparin binding, interaction with soluble VEGF receptors, and ability to promote endothelial cell mitogenesis. Plasmin yields two fragments of VEGF as indicated by reverse phase high performance liquid chromatography and SDS-polyacrylamide gel electrophoresis: an amino-terminal homodimeric protein containing receptor binding determinants and a carboxyl-terminal polypeptide which bound heparin. Amino-terminal sequencing of the carboxyl-terminal peptide identified the plasmin cleavage site as Arg110-Ala111. A heterodimeric form of VEGF165/110, was isolated from partial plasmin digests of VEGF165. The carboxyl-terminal polypeptide (111-165) displayed no affinity for soluble kinase domain region (KDR) or Fms-like tyrosine kinase (FLT-1) receptors. The various isoforms of VEGF (165, 165/110, and 121) bound soluble kinase domain region receptor with similar affinity (approximately 30 pM). In contrast, soluble FLT-1 receptor differentiated VEGF isoforms (165, 165/110, 110, and 121) with apparent affinities of 10, 30, 120, and 200 pM, respectively. Endothelial cell mitogenic potencies of VEGF110 and VEGF121 were decreased more than 100-fold compared to that of VEGF165. The present findings indicate that removal of the carboxyl-terminal domain, whether it is due to alternative splicing of mRNA or to proteolysis, is associated with a significant loss in bioactivity.

Novel human vascular endothelial growth factor genes VEGF-B and VEGF-C localize to chromosomes 11q13 and 4q34, respectively

BACKGROUND

Vascular endothelial growth factor (VEGF) is an important regulator of endothelial cell proliferation, migration, and permeability during embryonic vasculogenesis as well as in physiological and pathological angiogenesis. The recently isolated VEGF-B and VEGF-C cDNAs encode novel growth factor genes of the VEGF family.

METHODS AND RESULTS

Southern blotting and polymerase chain reaction analysis of somatic cell hybrids and fluorescence in situ hybridization (FISH) of metaphase chromosomes were used to assess the chromosomal localization of VEGF-B and VEGF-C genes. The VEGF-B gene was found on chromosome 11q13, proximal to the cyclin D1 gene, which is amplified in a number of human carcinomas. However, VEGF-B was not amplified in several mammary carcinoma cell lines containing amplified cyclin D1. The VEGF-C gene was located on chromosome 4q34, close to the human aspartylglucosaminidase gene previously mapped to 4q34-35.

CONCLUSIONS

The VEGF-B locus in 11q13 and the VEGF-C locus in 4q34 are candidate targets for mutations that lead to vascular malformations or cardiovascular diseases.

Identification of vascular endothelial growth factor determinants for binding KDR and FLT-1 receptors. Generation of receptor-selective VEGF variants by site-directed mutagenesis

Vascular endothelial growth factor (VEGF) expression in various cell types is induced by hypoxia and other stimuli. VEGF mediates endothelial cell proliferation, angiogenesis, vascular growth, and vascular permeability via the endothelial cell receptors, kinase insert domain-containing receptor (KDR)/fetal liver kinase 1 (Flk-1) and FLT-1. Alanine-scanning mutagenesis was used to identify a positively charged surface in VEGF that mediates binding to KDR/Flk-1. Arg82, Lys84 and His86, located in a hairpin loop, were found to be critical for binding KDR/Flk-1, while negatively charged residues, Asp63, Glu64, and Glu67, were associated with FLT-1 binding. A VEGF model based on PDGFb indicated these positively and negatively charged regions are distal in the monomer but are spatially close in the dimer. Mutations within the KDR site had minimal effect on FLT-1 binding, and mutants deficient in FLT-1 binding did not affect KDR binding. Endothelial cell mitogenesis was abolished in mutants lacking KDR affinity; however, FLT-1 deficient mutants induced normal proliferation. These results suggest dual sets of determinants in the VEGF dimer that cross-link cell surface receptors, triggering endothelial cell growth and angiogenesis. Furthermore, this mutational analysis implicates KDR, but not FLT-1, in VEGF induction of endothelial cell proliferation.

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.

Heterodimers of placenta growth factor/vascular endothelial growth factor. Endothelial activity, tumor cell expression, and high affinity binding to Flk-1/KDR

Here we show that the Escherichia coli expressed monomers of placenta growth factor (PLGF)129 and vascular endothelial growth factor (VEGF)165 can be re-folded in vitro to form PLGF/VEGF heterodimers. The purified recombinant PLGF/VEGF heterodimers and VEGF homodimers have potent mitogenic and chemotactic effects on endothelial cells. However, PLGF/VEGF heterodimers display 20-50-fold less mitogenic activity than VEGF165 homodimers. In contrast, PLGF129 homodimers have little or no effect in these in vitro assays. We also demonstrate the presence of natural PLGF/VEGF heterodimers in the conditioned media of various human tumor cell lines. While PLGF/VEGF heterodimers bind with high affinity to a soluble Flk-1/KDR receptor, PLGF129 homodimers fail to bind to this receptor. Cross-linking of 125I-ligands to human umbilical vein endothelial cells reveals that PLGF/VEGF heterodimers and VEGF165 homodimers, but not PLGF129 homodimers, form complexes with membrane receptors. VEGF165 homodimers and PLGF/VEGF heterodimers stimulate tyrosine phosphorylation of a 220-kDa protein, the expected size for the KDR receptor in human umbilical vein endothelial cells, whereas PLGF129 homodimers are unable to induce tyrosine phosphorylation of this protein. These data indicate that PLGF may modulate VEGF-induced angiogenesis by the formation of PLGF/VEGF heterodimers in cells producing both factors.

Immunoadhesins: principles and applications

The prototypic immunoadhesin is an antibody-like molecule that fuses the Fc region of an immunoglobulin and the ligand-binding region of a receptor or adhesion molecule. In this article, we review some important structural and functional principles of immunoadhesins. In addition, we highlight some unique advantages of immunoadhesins as experimental tools in biology, as well as some of their exciting potential applications in medicine.

Differential endothelial migration and proliferation to basic fibroblast growth factor and vascular endothelial growth factor

Neovascularization is a feature of a variety of pathological processes. We compared the characteristics of vascular endothelial growth factor (VEGF) and basic fibroblast growth factor (bFGF) on migration and proliferation of human umbilical vein endothelium (HUVEC). Both VEGF and bFGF induced endothelial cell migration at similar concentrations (1/2 max. VEGF = approximately 1.0 ng/ml, bFGF = approximately 5.0 ng/ml). However, VEGF-stimulated migration was two-fold greater than bFGF at 1 and 10 ng/ml (p < 0.05). In contrast, bFGF induced proliferation four-fold more effectively than VEGF (1/2 max. 1 ng/ml and 1.4 ng/ml respectively). Checkerboard migration assays for bFGF showed a predominantly chemokinetic pattern, whereas VEGF was predominantly chemotactic. VEGF and bFGF were not synergistic in monolayer proliferation and migration assays. Three angiogenesis inhibitors, alpha-interferon, TNP-470, and platelet factor-4, inhibited VEGF and bFGF induced cell migration. These results indicate that VEGF and bFGF are chemoattractants that stimulate endothelial migration by different mechanisms and that both can be inhibited by known angiogenesis inhibitors.

The placenta growth factor gene of the mouse

Placenta growth factor (PlGF) and vascular endothelial growth factor (VEGF) are angiogenic factors containing the 8-cysteine motif of platelet-derived growth factor (PDGF). Both PlGF and VEGF are mitogens for endothelial cells in vitro and promote neoangiogenesis in vivo. In addition, PlGF strongly potentiates the proliferative and the permeabilization effects exerted by VEGF on the vascular endothelium. We have now isolated the cDNA coding for mouse Plgf by screening a mouse heart cDNA library with the human PlGF sequence as probe. The human PlGF protein has two forms, PlGF-1 and PlGF-2, that arise from alternative splicing of a single gene mapping on Chromosome (Chr) 14; the isolated mouse Plgf cDNA encodes the longer of these two forms (PlGF-2). We show that the mouse Plgf-2 mRNA is the only transcript present in the normal tissues analyzed. Mouse Plgf-2 is a 158-amino-acid-long protein that shows 78% similarity (65% identity) to the human PlGF-2. Computer analysis reveals a putative signal peptide and three probable N-glycosylation sites, two of which are also conserved in human PlGF. The mouse Plgf gene was isolated and characterized; the gene is encoded by 7 exons spanning a 13-kb DNA interval. Finally, we have mapped the mouse Plgf gene to Chr 12, one cM from D12Mit5, and the human PlGF gene to 14q24, using both FISH and genetic crosses.

Localisation of placenta growth factor (PIGF) in human term placenta

Placenta growth factor (PlGF) is a growth factor which belongs to the vascular endothelial growth factor (VEGF) family and is known to bind to the fms-like tyrosine kinase receptor (flt-1). Using Western blot analysis a 50 kDa band was identified in placental protein extract which corresponded to PlGF homodimer. Immunoreactive PlGF was localised to the vasculosyncytial membrane and in the media of large blood vessels of the placental villi, while staining within the mesenchyme was weak and diffuse. There was moderate staining for PlGF in discrete cells in the chorion and no staining in the epithelial layer of the amnion. The maternal decidual cells showed strong staining for PlGF immunoreactive protein. PlGF mRNA was predominantly expressed by the vasculosyncytial membrane of villous trophoblast, whilst there was no apparent expression of PlGF mRNA within the villous mesenchyme. These results suggest that PlGF may be an important paracrine factor for vascular endothelial cells in placental angiogenesis and an autocrine mediator of trophoblast function.

Coordinate expression of vascular endothelial growth factor receptor-1 (flt-1) and its ligand suggests a paracrine regulation of murine vascular development

Vascular endothelial growth factor (VEGF) is a candidate regulator of blood vessel growth during embryonic development and in tumors. To evaluate the role of VEGF receptor-1/flt-1 (VEGFR1/flt-1) in the development of the vascular system, we have characterized the murine homolog of the human flt-1 gene and have analyzed its expression pattern during mouse embryogenesis. Receptor binding studies using transfected COS cells revealed that the murine flt-1 gene encodes a high affinity receptor for VEGF. The apparent Kd for VEGF binding, as determined by Scatchard analysis, was 114 pM, demonstrating that VEGFR1/flt-1 has a higher affinity to VEGF than VEGF receptor-2/flk-1 (VEGFR2/flk-1). By in situ hybridization, VEGFR1/flt-1 was detected in the yolk sac mesoderm already at the early stages of vascular development, while the receptor ligand was expressed in the entire endoderm of 7.5-day mouse embryos. A comparison with VEGFR2/flk-1 showed that the two receptors shared a common expression domain in the yolk sac mesoderm, but were expressed at different sites in the ectoplacental cone. The differential expression of the two VEGF receptors persisted in the developing placenta, where VEGFR1/flt-1 mRNA was detected in the spongiotrophoblast layer, whereas VEGFR2/flk-1 transcripts were present in the labyrinthine layer which is the site of VEGF expression. In the embryo proper, VEGFR1/flt-1 mRNA was specifically localized in blood vessels and capillaries of the developing organs, closely resembling the pattern of VEGFR2/flk-1 transcript distribution. In the developing brain, the expression of VEGF receptors in the perineural capillary plexus and in capillary sprouts which have invaded the neuro-ectoderm correlated with endothelial cell proliferation and brain angiogenesis. The data are consistent with the hypothesis that VEGF and its receptors have an important function both in the differentiation of the endothelial lineage and in the neovascularization of developing organs, and act in a paracrine fashion.

Synergistic effect of vascular endothelial growth factor and basic fibroblast growth factor on angiogenesis in vivo

BACKGROUND

Recent studies have suggested that vascular endothelial growth factor (VEGF) and basic fibroblast growth factor (bFGF) may have synergistic effects on the induction of angiogenesis in vitro. Therefore, we investigated the hypothesis that the simultaneous administration of VEGF and bFGF, each having been previously shown to independently enhance collateral development in an animal model of hind limb ischemia, could have a synergistic effect in vivo.

METHODS AND RESULTS

Ten days after surgical induction of unilateral hind limb ischemia, New Zealand White rabbits were randomized to receive either VEGF 500 micrograms alone (n = 6), bFGF 10 micrograms alone (n = 7), VEGF 500 micrograms, immediately followed by 10 micrograms bFGF (n = 7), or vehicle only (control animals, n = 8) in each case administered intra-arterially via a catheter in the internal iliac artery of the ischemic limb. BP ratio (BPR, ischemic/healthy limb) at day 10 for the VEGF+bFGF group was 0.82 +/- 0.01, much superior (P < .0005) to that of either the VEGF group (0.52 +/- 0.02) or the bFGF group (0.57 +/- 0.02). This outcome persisted at day 30: BPR in the VEGF+bFGF group (0.91 +/- 0.02) exceeded that of the control group (0.49 +/- 0.05, P < .0001), the VEGF group (0.65 +/- 0.03, P < .0005), or the bFGF group (0.66 +/- 0.03, P < .0005). Serial angiography demonstrated a progressive increase in luminal diameter of the stem collateral artery and the number of opacified collaterals in the thigh of the ischemic limbs in all groups. Stem artery diameter with VEGF+bFGF (1.34 +/- 0.07 mm) on day 30 was significantly (P < .05) greater than with either VEGF (1.09 +/- 0.09) or bFGF (1.18 +/- 0.06) alone. Capillary density was significantly greater (P < .05) in VEGF+bFGF animals (275 +/- 20 mm2) compared with VEGF (201 +/- 8) or bFGF (209 +/- 15).

CONCLUSIONS

Combined administration of VEGF and bFGF stimulates significantly greater and more rapid augmentation of collateral circulation, resulting in superior hemodynamic improvement compared with either VEGF or bFGF alone. This synergism of two angiogenic mitogens with different target cell specificities may have important implications for the treatment of severe arterial insufficiency in patients whose disease is not amenable to direct revascularization.

Role of the Flt-1 receptor tyrosine kinase in regulating the assembly of vascular endothelium

The vascular endothelial growth factor (VEGF) and its high-affinity binding receptors, the tyrosine kinases Flt-1 and Flk-1, are thought to be important for the development of embryonic vasculature. Here we report that Flt-1 is essential for the organization of embryonic vasculature, but is not essential for endothelial cell differentiation. Mouse embryos homozygous for a targeted mutation in the flt-1 locus, flt-1lcz, formed endothelial cells in both embryonic and extra-embryonic regions, but assembled these cells into abnormal vascular channels and died in utero at mid-somite stages. At earlier stages, the blood islands of flt-1lcz homozygotes were abnormal, with angioblasts in the interior as well as on the periphery. We suggest that the Flt-1 signalling pathway may regulate normal endothelial cell-cell or cell-matrix interactions during vascular development.

VEGF121, a vascular endothelial growth factor (VEGF) isoform lacking heparin binding ability, requires cell-surface heparan sulfates for efficient binding to the VEGF receptors of human melanoma cells

Four vascular endothelial growth factor (VEGF) splice variants containing 121, 165, 189, and 206 amino acids are produced from a single human gene as a result of alternative splicing. VEGF121 is not a heparin-binding protein, while the other VEGF species possess heparin binding ability. YU-ZAZ6 human melanoma cells expressed the mRNA encoding the VEGF receptor flt-1, but not the mRNA encoding the VEGF receptor KDR/flk-1. Both VEGF121 and VEGF165 bound to the VEGF receptors of these cells. Unexpectedly, heparin inhibited the binding of VEGF121 as well as the binding of VEGF165 to the VEGF receptors of the melanoma cells. Digestion of the cells with heparinase also inhibited the binding of both VEGF variants. The VEGF165 binding ability of heparinase-digested cells could be partially restored by the addition of exogenous heparin to the binding reaction. In contrast, the addition of heparin to heparinase-digested cells did not restore VEGF121 binding. These results suggest that cell-surface heparan sulfates may regulate the binding ability of the VEGF receptors of the melanoma cells. They also indicate that heparin is not able to fully substitute for cell surface-associated heparan sulfates since VEGF121 binding to the VEGF receptors of heparinase-treated cells is not restored by heparin. These data suggest that changes in the composition of cell-surface heparin-like molecules may differentially affect the interaction of various VEGF isoforms with VEGF receptors.

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.

Regulation of the expression of the VEGF/VPS and its receptors: role in tumor angiogenesis

Vascular endothelial growth factor (VEGF)/vascular permeability factor (VPS) plays a crucial role for the vascularization of tumors including breast cancers. Tumors produce ample amounts of VEGF, which stimulates the proliferation and migration of endothelial cells (ECs), thereby inducing tumor vascularization by a paracrine mechanism. VEGF receptors (VEGF-Rs) are highly expressed by the ECs in tumor blood vessels. VEGF expression can be induced in various cell types by a number of stimuli including hypoxia, differentiation, growth factors and tumor promoters of the phorbol ester class, such as TPA. The VEGF inductive pathways comprise kinases, oncogenes, tumor suppressor genes, and steroid hormone transcription factors, many of which seem to converge on the activator protein (AP-1) transcription factor. Much less is known about the regulation of VEGF-R expression, which is restricted to ECs. This expression is greatly enhanced in diseased tissue such as solid tumors. So far, it appears that growth factors, cytokines, and tumor promoters are involved in the control of VEGF-R expression. Here we review current knowledge about the regulation of the expression of VEGF and its receptors.

The role of vascular endothelial growth factor in pathological angiogenesis

Vascular endothelial growth factor (VEGF) is a diffusible endothelial cell-specific mitogen and angiogenic factor that can also increase vascular permeability. By alternative splicing of mRNA, VEGF may exist as one of four different isoforms that have similar biological activities but differ markedly in targeting and bioavailability. The VEGF receptors are specifically expressed in the cell surface of vascular endothelial cells. Recent studies point to VEGF as a major regulator of physiological angiogenesis, such as developmental and reproductive angiogenesis. Furthermore, VEGF appears to be a crucial mediator of blood vessel growth associated with tumors and proliferative retinopathies. The VEGF mRNA is up-regulated in the majority of human tumors and the VEGF protein is increased in the aqueous and vitreous humors of patients with proliferative retinopathies. Anti-VEGF antibodies have the ability to suppress the growth of a variety of tumor cell lines in nude mice and also can inhibit angiogenesis in animal models of intraocular neovascularization. Therefore, strategies aimed at antagonizing VEGF may form the basis for an effective treatment of tumors and retinopathies. Furthermore, VEGF-induced angiogenesis is sufficient to achieve a therapeutic endpoint in models of coronary or limb ischemia.

Vascular endothelial growth factor in ocular fluid of patients with diabetic retinopathy and other retinal disorders

BACKGROUND

Retinal ischemia induces intraocular neovascularization, which often leads to glaucoma, vitreous hemorrhage, and retinal detachment, presumably by stimulating the release of angiogenic molecules. Vascular endothelial growth factor (VEGF) is an endothelial-cell-specific angiogenic factor whose production is increased by hypoxia.

METHODS

We measured the concentration of VEGF in 210 specimens of ocular fluid obtained from 164 patients undergoing intraocular surgery, using both radioimmuno-assays and radioreceptor assays. Vitreous proliferative potential was measured with in vitro assays of the growth of retinal endothelial cells and with VEGF-neutralizing antibody.

RESULTS

VEGF was detected in 69 of 136 ocular-fluid samples from patients with diabetic retinopathy, 29 of 38 samples from patients with neovascularization of the iris, and 3 of 4 samples from patients with ischemic occlusion of the central retinal vein, as compared with 2 of 31 samples from patients with no neovascular disorders (P < 0.001, P < 0.001, and P = 0.006, respectively). The mean (+/- SD) VEGF concentration in 70 samples of ocular fluid from patients with active proliferative diabetic retinopathy (3.6 +/- 6.3 ng per milliliter) was higher than that in 25 samples from patients with nonproliferative diabetic retinopathy (0.1 +/- 0.1 ng per milliliter, P = 0.008), 41 samples from patients with quiescent proliferative diabetic retinopathy (0.2 +/- 0.6 ng per milliliter, P < 0.001), or 31 samples from nondiabetic patients (0.1 +/- 0.2 ng per milliliter, P = 0.003). Concentrations of VEGF in vitreous fluid (8.8 +/- 9.9 ng per milliliter) were higher than those in aqueous fluid (5.6 +/- 8.6 ng per milliliter, P = 0.033) in all 10 pairs of samples obtained simultaneously from the same patient; VEGF concentrations in vitreous fluid declined after successful laser photocoagulation. VEGF stimulated the growth of retinal endothelial cells in vitro, as did vitreous fluid containing measurable VEGF. Stimulation was inhibited by VEGF-neutralizing antibodies.

CONCLUSIONS

Our data suggest that VEGF plays a major part in mediating active intraocular neovascularization in patients with ischemic retinal diseases, such as diabetic retinopathy and retinal-vein occlusion.