High affinity VEGF binding and developmental expression suggest Flk-1 as a major regulator of vasculogenesis and angiogenesis

Vascular endothelial growth factor and glioma angiogenesis: coordinate induction of VEGF receptors, distribution of VEGF protein and possible in vivo regulatory mechanisms

We have previously suggested that tumor angiogenesis in human gliomas is regulated by a paracrine mechanism involving vascular endothelial growth factor (VEGF) and flt-1 (VEGF-receptor 1). VEGF, an endothelial-cell-specific mitogen, is abundantly expressed in glioma cells which reside along necrotic areas, whereas flt-1, a tyrosine-kinase receptor for VEGF, is expressed in tumor endothelial cells, but not in endothelial cells in normal adult brain. Recently, a second tyrosine-kinase receptor which binds VEGF with high affinity, designated KDR or flk-1, has been described. We performed in situ hybridization for VEGF mRNA, flt-1 mRNA and KDR mRNA on serial sections of normal brain, low-grade and high-grade glioma specimens. We show that KDR mRNA is co-expressed with flt-1 in vascular cells in glioblastoma but not in low-grade glioma. Since flt-1 and KDR are not expressed in endothelial cells in the normal adult brain, the coordinate up-regulation of 2 receptors for VEGF appears to be a critical event which controls tumor angiogenesis. Immunocytochemistry with a monoclonal anti-VEGF antibody revealed significant amounts of VEGF protein in the same glioma cells that expressed VEGF mRNA. The largest amount of VEGF immunoreactivity, however, was detected on the vasculature of glioblastomas, the site where VEGF exerts its biological functions. These findings suggest that VEGF is produced and secreted by glioma cells and acts on tumor endothelial cells which express VEGF receptors. To further characterize VEGF-producer cells in vivo, we investigated cellular proliferation, immunoreactivity to the p53 tumor-suppressor gene product and epidermal-growth-factor-receptor (EGFR) expression on serial sections by immunocytochemistry. VEGF-producer cells did not show increased cellular proliferation, p53 immunoreactivity or EGFR immunoreactivity as compared with glioma cells which did not express VEGF. Our studies therefore do not demonstrate evidence for a growth advantage of VEGF-producer cells in vivo or VEGF induction by p53 mutation or EGFR over-expression.

The Fps/Fes protein-tyrosine kinase promotes angiogenesis in transgenic mice

The fps/fes proto-oncogene encodes a cytoplasmic protein-tyrosine kinase known to be highly expressed in hematopoietic cells. To investigate fps/fes biological function, an activating mutation was introduced into the human fps/fes gene which directs amino-terminal myristylation of the Fps/Fes protein. This mutant, myristylated protein induced transformation of Rat-2 fibroblasts. The mutant fps/fes allele was incorporated into the mouse germ line and was found to be appropriately expressed in transgenic mice, in a tissue-specific pattern indistinguishable from that of the endogenous mouse gene. These mice displayed widespread hypervascularity, progressing to multifocal hemangiomas. High levels of both the transgenic human and endogenous murine fps/fes transcripts were detected in vascular tumors by using RNase protection, and fps/fes transcripts were localized to endothelial cells of both the vascular tumors and normal blood vessels by in situ RNA hybridization. Primary human umbilical vein endothelial cultures were also shown to express fps/fes transcripts and the Fps/Fes tyrosine kinase. These results indicate that fps/fes expression is intrinsic to cells of the vascular endothelial lineage and suggest a direct role of the Fps/Fes protein-tyrosine kinase in the regulation of angiogenesis.

Association of vascular endothelial growth factor expression with tumor angiogenesis and with early relapse in primary breast cancer

Angiogenesis is an independent prognostic indicator in breast cancer. In this report, the relationship between expression of vascular endothelial growth factor (VEGF; a selective mitogen for endothelial cells) and the microvessel density was examined in 103 primary breast cancers. The expression of VEGF was evaluated by immunocytochemical staining using anti-VEGF antibody. The microvessel density, which was determined by immunostaining for factor VIII antigen, in VEGF-rich tumors was clearly higher than that in VEGF-poor tumors (P < 0.01). There was a good correlation between VEGF expression and the increment of microvessel density. Furthermore, postoperative survey demonstrated that the relapse-free survival rate of VEGF-rich tumors was significantly worse than that of VEGF-poor tumors. It was suggested that the expression of VEGF is closely associated with the promotion of angiogenesis and with early relapse in primary breast cancer.

The Fps/Fes protein-tyrosine kinase promotes angiogenesis in transgenic mice

The fps/fes proto-oncogene encodes a cytoplasmic protein-tyrosine kinase known to be highly expressed in hematopoietic cells. To investigate fps/fes biological function, an activating mutation was introduced into the human fps/fes gene which directs amino-terminal myristylation of the Fps/Fes protein. This mutant, myristylated protein induced transformation of Rat-2 fibroblasts. The mutant fps/fes allele was incorporated into the mouse germ line and was found to be appropriately expressed in transgenic mice, in a tissue-specific pattern indistinguishable from that of the endogenous mouse gene. These mice displayed widespread hypervascularity, progressing to multifocal hemangiomas. High levels of both the transgenic human and endogenous murine fps/fes transcripts were detected in vascular tumors by using RNase protection, and fps/fes transcripts were localized to endothelial cells of both the vascular tumors and normal blood vessels by in situ RNA hybridization. Primary human umbilical vein endothelial cultures were also shown to express fps/fes transcripts and the Fps/Fes tyrosine kinase. These results indicate that fps/fes expression is intrinsic to cells of the vascular endothelial lineage and suggest a direct role of the Fps/Fes protein-tyrosine kinase in the regulation of angiogenesis.

Platelet endothelial cell adhesion molecule-1 (PECAM-1/CD31): alternatively spliced, functionally distinct isoforms expressed during mammalian cardiovascular development

The establishment of the cardiovascular system represents an early, critical event essential for normal embryonic development. An important component of vascular ontogeny is the differentiation and development of the endothelial and endocardial cell populations. This involves, at least in part, the expression and function of specific cell surface receptors required to mediate cell-cell and cell-matrix adhesion. Platelet endothelial cell adhesion molecule-1 (PECAM-1, CD31) may well serve such a function. It is a member of the immunoglobulin superfamily expressed by the entire vascular endothelium in the adult. It is capable of mediating adhesion by a heterophilic mechanism requiring glycosaminoglycans, as well as by a homophilic, glycosaminoglycan independent, mechanism. It has been shown to regulate the expression of other adhesion molecules on naive T cells. This report documents by RT-PCR and immunohistochemical analysis the expression of PECAM-1 during early post implantation mouse embryo development. PECAM-1 was expressed by early endothelial precursors first within the yolk sac and subsequently within the embryo itself. Interestingly, embryonic PECAM-1 was expressed as multiple isoforms in which one or more clusters of polypeptides were missing from the cytoplasmic domain. The sequence and location of the deleted polypeptides corresponded to exons found in the human PECAM-1 gene. The alternatively spliced isoforms were capable of mediating cell-cell adhesion when transfected into L-cells. The isoforms differed, however, in their sensitivity to a panel of anti-PECAM-1 monoclonal antibodies. These data suggest that changes in the cytoplasmic domain of PECAM-1 may affect its function during cardiovascular development, and are consistent with our earlier report that systematic truncation of the cytoplasmic domain of human PECAM-1 resulted in changes in its ligand specificity, divalent cation and glycosaminoglycan dependence, as well as its susceptibility to adhesion blocking monoclonal antibodies. This is the first report of naturally occurring alternatively spliced forms of PECAM-1 having possible functional implications.

Detection and quantification of vascular endothelial growth factor/vascular permeability factor in brain tumor tissue and cyst fluid: the key to angiogenesis?

In primary malignant brain tumors increased vascularity and marked edema strongly suggest a possible role of the vascular endothelial growth factor/vascular permeability factor (VEGF/VPF). This was confirmed by earlier in situ hybridization studies, by analysis of the expression of the mitogen in different subsets of glioblastoma cells, and by the fact that the VEGF/VPF receptor flt-1 (fms-like tyrosine kinase) is up-regulated in tumor cells in vivo. To assess and quantify the expression of the VEGF/VPF gene and of the receptor gene, 26 surgical specimens of brain tumor tissue from 24 patients were analyzed. In most malignant gliomas, the expression level of the VEGF/VPF gene is elevated and can be increased up to 20- to 50-fold in comparison with low-grade tumors. Using polymerase chain reaction-based amplification, it could be shown that the messenger RNAs of three different VEGF/VPF forms are synthesized in tumor tissue samples. Northern blot studies revealed that in some samples a significant expression of the gene coding for placenta growth factor, a growth factor closely related to VEGF/VPF, was observed. In addition, using a radioreceptor assay it was possible to detect high VEGF/VPF-like activity in the cyst fluids of brain tumors, indicating the accumulation of the mitogen and permeability factor in brain tumor cysts. Further investigations revealed that astrocytoma and glioblastoma cells in culture express the VEGF/VPF gene and secrete the VEGF/VPF protein, whereas gene expression of the two known VEGF/VPF receptors, kinase insert domain-containing receptor and flt-1, could not be detected. These data support previous reports, which stated that VEGF/VPF acts as a paracrine growth and permeability factor in brain tumors and may contribute to tumor growth by initiating tumor angiogenesis.

Dominant-negative and targeted null mutations in the endothelial receptor tyrosine kinase, tek, reveal a critical role in vasculogenesis of the embryo

The receptor tyrosine kinases (RTKs) expressed on the surface of endothelial cells are likely to play key roles in initiating the program of endothelial cell growth during development and subsequent vascularization during wound healing and tumorigenesis. Expression of the Tek RTK during mouse development is restricted primarily to endothelial cells and their progenitors, the angioblasts, suggesting that Tek is a key participant in vasculogenesis. To investigate the role that Tek plays within the endothelial cell lineage, we have disrupted the Tek signaling pathway using two different genetic approaches. First, we constructed transgenic mice expressing a dominant-negative form of the Tek receptor. Second, we created a null allele of the tek gene by homologous recombination in embryonic stem (ES) cells. Transgenic mice expressing dominant-negative alleles of Tek or homozygous for a null allele of the tek locus both died in utero with similar defects in the integrity of their endothelium. By crossing transgenic mice that express the lacZ reporter gene under the transcriptional control of the endothelial cell-specific tek promoter, we found that the extraembryonic and embryonic vasculature was patterned correctly. However, homozygous tek embryos had approximately 30% and 75% fewer endothelial cells at day 8.5 and 9.0, respectively. Homozygous null embryos also displayed abnormalities in heart development, consistent with the conclusion that Tek is necessary for endocardial/myocardial interactions during development. On the basis of the analysis of mice carrying either dominant-negative or null mutations of the tek gene, these observations demonstrate that the Tek signaling pathway plays a critical role in the differentiation, proliferation, and survival of endothelial cells in the mouse embryo.

Basic fibroblast growth factor improves myocardial function in chronically ischemic porcine hearts

The effect of basic fibroblast growth factor (bFGF) administration on regional myocardial function and blood flow in chronically ischemic hearts was studied in 26 pigs instrumented with proximal circumflex coronary artery (LCX) ameroid constrictors. In 13 animals bFGF was administered extraluminally to the proximal left anterior descending (LAD) and LCX arteries with heparin-alginate beads and 13 other animal served as controls. bFGF-treated pigs showed a fourfold reduction in left ventricular infarct size compared to untreated controls (infarct size: 1.2 +/- 0.4% vs. 5.1 +/- 1.3% of LV mass, mean +/- SEM, P < 0.05). Percent fractional shortening (% FS) in the LCX area at rest was reduced compared with the LAD region in both bFGF and control pigs. However, there was better recovery in the LCX area after rapid pacing in bFGF-treated pigs (% FSLCX/% FSLAD, 22.9 +/- 7.3%-->30.5 +/- 8.5%, P < 0.05 vs. prepacing) than in controls (16.0 +/- 7.8%-->14.3 +/- 7.0%, P = NS). Furthermore, LV end-diastolic pressure rise with rapid pacing was less in bFGF-treated than control pigs (pre-pacing; pacing; post-pacing, 10 +/- 1; 17 +/- 3; 11 +/- 1* mmHg vs 10 +/- 1; 24 +/- 4; 15 +/- 1 mmHg, *P < 0.05 vs. control). Coronary blood flow in the LCX territory (normalized for LAD flow) was also better during pacing in bFGF-treated pigs than in controls. Thus, periadventitial administration of bFGF in a gradual coronary occlusion model in pigs results in improvement of coronary flow and reduction in infarct size in the compromised territory as well as in prevention of pacing-induced hemodynamic deterioration.

Localization of transcription factor GATA-4 to regions of the mouse embryo involved in cardiac development

To clarify the role of transcription factor GATA-4 in mammalian development, we have determined the pattern of expression of GATA-4 in early postimplantation mouse embryos. Using in situ hybridization and immunohistochemistry, we find that GATA-4 RNA and protein are expressed in cells associated with heart development. Intraembryonic expression of GATA-4 RNA is first apparent in coelomic epithelial cells of the primitive streak embryo (approximately 7.0-7.5 days postcoitum). During formation and bending of the heart tube (approximately 8 days postcoitum), GATA-4 RNA and protein are expressed in endocardium, myocardium, and embryonic structures containing precardiac mesoderm such as the septum transversum and intraembryonic coelomic epithelium. By the onset of cardiac septation (approximately 9 days postcoitum), abundant GATA-4 RNA expression is evident in endocardium, endocardial cushion tissue, and myocardium. Expression of GATA-4 by the myocardium continues through gestation and after birth. The temporal and spacial patterns of GATA-4 expression support a role for this factor in the regulation of cardiac differentiation, analogous to the established role of transcription factor GATA-1 in the regulation of hematopoiesis.

Isolation of tyrosine kinase related genes expressed in the early hematopoietic system

Transmembrane tyrosine kinase receptors are involved in cellular interactions which promote proliferation and differentiation of many cell types. To identify receptor tyrosine kinases important in embryonic hematopoietic cell development we have utilized the polymerase chain reaction (PCR) and degenerate oligonucleotides for isolation of such genes from mouse yolk sac and fetal liver. Sequence analysis of PCR amplified cDNAs from these hematopoietic sites of day 8 and 14 embryos, resulted in the isolation of nine tyrosine kinase and three serine/threonine kinase related clones. Two of these receptors, tek and flk-1, are expressed in both yolk sac and fetal liver and have been shown previously to be important for endothelial cell development. Two other clones, 9B4 and 9A2 appeared novel upon isolation but have been recently described as ryk and SK2 (rat homologue). Here we describe the twelve isolated kinases, the specific expression patterns of flk-1, tek and ryk kinases and their potential relationship to the development of the hematopoietic system.

Molecular mechanisms of developmental and tumor angiogenesis

Angiogenesis, the sprouting of capillaries from preexisting vessels, is of fundamental importance during embryonic development and is the principal process by which the brain and certain other organs become vascularized. Angiogenesis occurs during embryonic development but is almost absent in adult tissues. Transient and tightly controlled (physiological) angiogenesis in adult tissues occurs during the female reproductive cycle and during wound healing. In contrast, pathological angiogenesis is characterized by the persistent proliferation of endothelial cells, and is a prominent feature of diseases such as proliferative retinopathy, rheumathoid arthritis, and psoriasis. In addition, many tumors are able to attract blood vessels from neighbouring tissues. Tumor-induced angiogenesis requires a constitutive activation of endothelial cells. These endothelial cells dissolve their surrounding extracellular matrix, migrate toward the tumor, proliferate, and form a new vascular network, thus supplying the tumor with nutrients and oxygen and removing waste products. The onset of angiogenesis in human gliomas is characterized by the expression of genes encoding angiogenic growth factors such as vascular endothelial growth factor (VEGF), platelet-derived growth factor (PDGF) in tumor cells, and coordinate induction of genes in endothelial cells which encode the respective growth factor receptors. Developmental and tumor angiogenesis appear to be regulated by a paracrine mechanism involving VEGF and VEGF receptor-1 and -2.

Localization of two angiogenic growth factors (PDECGF and VEGF) in human placentae throughout gestation

The spatiotemporal distribution of two angiogenic growth factors, platelet-derived endothelial cell growth factor (PDECGF) and vascular endothelial growth factor (VEGF) were determined using immunohistochemistry on sections of human placentae from each trimester of pregnancy. In the first trimester PDECGF was detected in trophoblast and in a band in the centre of the villous core. During gestation staining spread throughout the stroma but began to weaken in trophoblast until, by term, it was found only in stroma and in some endothelial cells. VEGF was detected exclusively in cytotrophoblast during the first trimester and then in syncytiotrophoblast throughout the remainder of pregnancy. Western blot analysis revealed that PDECGF antisera bound to three bands approximately 27, 47 and 94 kDa. The lowest band was not detected in platelet lysate and may represent an alternatively processed form of this peptide in placenta. VEGF antisera bound strongly to bands approximately 36, 46, 54, 56 and 64 kDa. The intensity of most bands increased between the first and second trimesters, consistent with an increased level of angiogenesis as the placenta develops. The presence of both factors in trophoblast in early pregnancy may be indicative of the trophoblast playing an active role in influencing the development of the villous vascular network.

Induction of vascular endothelial growth factor expression by prostaglandin E2 and E1 in osteoblasts

PGE1 and PGE2 are potent stimulators of bone formation. Osteogenesis is strongly dependent on angiogenesis. Vascular endothelial growth factor (VEFG), a secreted endothelial cell-specific mitogen, has been implicated in physiological and pathological angiogenesis. The aim of this study was to examine the possible role of VEGF in PG stimulation of bone formation. We found that in rat calvaria-derived osteoblast-enriched cells and in the osteoblastic RCT-3 cell line PGE2 and E1 increased VEGF mRNA and protein levels. The increased expression of VEGF mRNA produced by PGE2 was rapid (maximal at 1 h), transient (declined by 3 h), potentiated by cycloheximide, and abolished by actinomycin D. PGE2 had no effect on VEGF mRNA stability, suggesting transcriptional regulation of VEGF expression by PGF2. Rp-cAMP, a cAMP antagonist, suppressed VEGF mRNA induced by PGE2, indicating cAMP mediation. The upregulation of VEGF expression by PGE2 in the preosteoblastic RCT-1 cells was potentiated by treatment with retinoic acid, which induces the differentiation of these cells. The upregulation of VEGF mRNA by PGE2 was inhibited by dexamethasone treatment. In addition, Northern blot analysis showed that VEGF mRNA is expressed in adult rat tibia. In summary, we documented, for the first time, the expression of VEGF in osteoblasts and in bone tissue. Stimulation of VEGF expression by PGs and its suppression by glucocorticoids, which, respectively, stimulate and suppress bone formation, strongly implicate the involvement of VEGF in bone metabolism.

Elucidation of the role of breathless, a Drosophila FGF receptor homolog, in tracheal cell migration

DFGF-R1 (breathless), a Drosophila FGF receptor homolog, is required for the migration of tracheal cells and the posterior midline glial cells during embryonic development. To define the role of this receptor in cell migration, we have monitored the biological effects of a deregulated receptor containing the extracellular and transmembrane regions of the torso dominant allele and the cytoplasmic domain of DFGF-R1. Ubiquitous expression of the chimeric receptor at the time of tracheal cell migration did not disrupt migration in wild-type embryos. However, induction of the chimeric receptor corrected the tracheal defects of breathless (btl) mutant embryos, allowing the tracheal cells to migrate along their normal tracts. This result indicates that the normal activity of DFGF-R1 in promoting cell migration does not require spatially restricted cues. Late inductions of the chimeric construct, after the normal initiation of tracheal migration, allowed the definition of a broad time window during which the external signals guiding migration persist and the tracheal cells retain the capacity to respond to these cues. Rescue of tracheal migration in btl mutant embryos by the chimeric construct provides a sensitive biological assay for the activity of other Drosophila receptor tyrosine kinases (RTKs). Deregulated receptors containing the cytoplasmic domains of DFGF-R2, DER, torso, and sevenless were all able to partially rescue the migration defects. Consistent with the notion that these RTKs share a common signaling pathway, constructs containing the activated downstream elements Dras1 and Draf were also able to rescue tracheal migration, demonstrating that these two proteins are key players in the DFGF-R1 signaling pathway.

Glioblastoma growth inhibited in vivo by a dominant-negative Flk-1 mutant

Angiogenesis, the sprouting of capillaries from pre-existing blood vessels, is a fundamental process in the formation of the vascular system during embryonic development. In adulthood, angiogenesis takes place during corpus luteum formation and in pathological conditions such as wound healing, diabetic retinopathy, and tumor-igenesis. Vascularization is essential for solid tumour growth and is thought to be regulated by tumour cell-produced factors, which have a chemotactic and mitogenic effect on endothelial cells. Vascular endothelial growth factor (VEGF), a homodimeric glycoprotein of relative molecular mass 45,000, is the only mitogen, however, that specifically acts on endothelial cells, and it may be a major regulator of tumour angiogenesis in vivo. Its expression has been shown to be upregulated by hypoxia, and its cell-surface receptor, Flk-1, is exclusively expressed in endothelial cells. Here we investigate the biological relevance of the VEGF/Flk-1 receptor/ligand system for angiogenesis using a retrovirus encoding a dominant-negative mutant of the Flk-1/VEGF receptor to infect endothelial target cells in vivo, and find that tumour growth is prevented in nude mice. Our results emphasize the central role of the Flk-1/VEGF system in angiogenesis in general and in the development of solid tumours in particular.

Endothelial cell transformation by polyomavirus middle T antigen in mice lacking Src-related kinases

BACKGROUND

Expression of polyomavirus middle T antigen (PymT) rapidly induces endothelial tumors (hemangiomas) in mice, with an apparent single rate-limiting step. Because activation of Src-like kinases is thought to be an important component of PymT-induced transformation, we have analyzed the functional requirement for individual kinases in this process. This type of analysis has only recently become possible, with the generation of 'gene knock-out' mice lacking each of the kinase genes src, fyn and yes.

RESULTS

Hemangiomas develop efficiently in newborn mice lacking either src, fyn or yes after inoculation with a PymT-transducing retrovirus. In src- and fyn-deficient mice, the kinetics of induction and the histological properties of the tumors were indistinguishable from those in wild-type mice. In contrast, a reduced number of tumors arose in yes-deficient mice, with a significantly longer latency period. Transformed endothelial cell lines derived from the induced hemangiomas, however, did not differ in their morphological and tumorigenic properties from cell lines established previously from wild-type mice. Biochemical analysis of complexes between PymT and the Src-related kinases in these cell lines suggests that the Yes kinase is responsible for a significant amount of the PymT-associated kinase activity in transformed endothelial cells.

CONCLUSION

We have demonstrated that inactivation of a single tyrosine kinase of the Src family in endothelial cells does not abrogate PymT-induced hemangioma formation. As the remaining kinases do not compensate for the absence of a family member by elevated kinase activity, the loss--which affects the transformation process to varying degrees--can be studied in this model system. Our studies suggest that the PymT-Yes kinase complex plays a major role in the tumor-initiating action of PymT.

Therapeutic angiogenesis. A single intraarterial bolus of vascular endothelial growth factor augments revascularization in a rabbit ischemic hind limb model

Vascular endothelial growth factor (VEGF) is a heparin-binding, endothelial cell-specific mitogen. Previous studies have suggested that VEGF is a regulator of naturally occurring physiologic and pathologic angiogenesis. In this study we investigated the hypothesis that the angiogenic potential of VEGF is sufficient to constitute a therapeutic effect. The soluble 165-amino acid isoform of VEGF was administered as a single intra-arterial bolus to the internal iliac artery of rabbits in which the ipsilateral femoral artery was excised to induce severe, unilateral hind limb ischemia. Doses of 500-1,000 micrograms of VEGF produced statistically significant augmentation of collateral vessel development by angiography as well as the number of capillaries by histology; consequent amelioration of the hemodynamic deficit in the ischemic limb was significantly greater in animals receiving VEGF than in nontreated controls (calf blood pressure ratio, 0.75 +/- 0.14 vs. 0.48 +/- 0.19, P < 0.05). Serial angiograms disclosed progressive linear extension of the collateral artery of origin (stem artery) to the distal point of parent vessel (reentry artery) reconstitution in seven of nine VEGF-treated animals. These findings establish proof of principle for the concept that the angiogenic activity of VEGF is sufficiently potent to achieve therapeutic benefit. Such a strategy might ultimately be applicable to patients with severe limb ischemia secondary to arterial occlusive disease.

Transfected MCF-7 cells as a model for breast-cancer progression

The MCF-7 human breast carcinoma cell line has been used as a recipient for eukaryotic plasmid expression vectors to determine the effects of growth factor and growth factor receptor overexpression on the estrogen-dependent, antiestrogen sensitive and poorly metastatic phenotypes exhibited by this line. Overexpression of some members of the erbB family of ligands and receptors were found to have some effects on these phenotypes. However, only when two members of the fibroblast growth factor family, FGF-1 and FGF-4, were overexpressed was progressive in vivo growth observed is either ovariectomized nude mice without estrogen supplementation or in mice that received tamoxifen treatment. FGF transfected cells also exhibited an increased ability to form micrometastases. The implications of these results with regard to the possible role of the paracrine and autocrine effects of angiogenic growth factor production in breast cancer progression are discussed.

VEGF receptor subtypes KDR and FLT1 show different sensitivities to heparin and placenta growth factor

Vascular endothelial growth factor (VEGF) is an angiogenic growth factor which binds to two structurally related tyrosine kinase receptors denoted KDR and FLT1. To compare the interaction of VEGF with each receptor, cell lines which express individual receptor subtypes were identified using Northern blot hybridization. Bovine aortic endothelial (ABAE) cells and WM35 melanoma cells were found to express KDR, while FLT1 was primarily expressed on SK-MEL-37. Both receptor subtypes were detected on another melanoma cell line (WM9). Heparin augmented VEGF binding to KDR-expressing cells (ABAE and WM35), but inhibited VEGF binding to FLT1-expressing cells (SK-MEL-37 and WM9). The concentration of heparin required for half maximal stimulation of VEGF binding to KDR-expressing cells (500 ng/ml) was 25 times greater than that required for half maximal inhibition of binding to FLT1-expressing cells (20 ng/ml). In WM9 cells, the effect of heparin was bimodal; low concentration inhibited, while higher concentrations stimulated binding of 125I-VEGF. Placenta growth factor (PIGF-1) is a recently described growth factor structurally similar to VEGF. PIGF-1 had a negligible or no effect on 125I-VEGF binding to KDR-expressing cells (ABAE, WM35), but did complete for binding to FLT1-expressing cells (SK-MEL-37 and WM9). Addition of heparin had no effect on its ability to compete for binding with 125I-VEGF. The data indicate differential regulation of the two VEGF receptors by heparin and extended specificity of FLT1 receptor, but not KDR, for binding PIGF-1 growth factor.

Molecular biology of blood-brain barrier ontogenesis and function

The vascular system of the central nervous system is derived from capillary endothelial cells, which have invaded the early embryonic neuroectoderm. This process is called angiogenesis and is probably regulated by brain-derived factors. Vascular endothelial cell growth factor (VEGF) is an angiogenic growth factor whose expression correlates with embryonic brain angiogenesis, i.e. expression is high in the embryonic brain when angiogenesis occurs and low in the adult brain when angiogenesis is shut off under normal physiological conditions. VEGF is also a vascular permeability factor (VPF) and, therefore, its expression is also consistent with the formation of the blood-brain barrier by brain endothelial cells, i.e. capillaries are leaky in the embryonic brain but are tight in the postnatal and adult brain. Thus, VEGF/VPF may be a key factor regulating endothelial cell growth and permeability. This notion is further supported by the observation that VEGF expression is induced and strongly upregulated in human malignant glioblastoma. This tumor is characterized by vascular proliferations, vascular leakage and edema. The differentiation of blood-brain barrier endothelial cells is probably regulated by astrocytes which form foot processes apposed to the abluminal vascular basement membrane. Blood-brain barrier endothelial cells express a set of cell surface proteins that are absent from permeable capillaries. We have characterized one such novel transmembrane glycoprotein which is a new member of the immunoglobulin superfamily. This protein and the analysis of the in vitro characteristics of brain endothelial cells may help to define the molecular mechanisms that are involved in blood-brain barrier induction and permeability.

Vascular endothelial growth factor and its receptors

Vascular endothelial growth factor (VEGF) is a highly specific mitogen for vascular endothelial cells and an angiogenic factor that is structurally related to platelet derived growth factor (PDGF). It is also known as the vascular permeability factor (VPF) because it efficiently potentiates the permeabilization of blood vessels. Five types of VEGF mRNA encoding VEGF species which differ in their molecular mass and in their biological properties are transcribed from a single gene as a result of alternative splicing. VEGFs are produced and secreted by several normal cell types including smooth muscle, luteal and adrenal cortex cells. VEGFs are also produced by different tumorigenic cells, and appear to play a major role in tumour angiogenesis. Antibodies directed against VEGF can inhibit the growth of a variety of VEGF producing tumours. Of the various VEGF species, the best characterized is the 165 amino acid long form (VEGF165). VEGF165 is a heparin binding growth factor, and its interaction with VEGF receptors on the cell surface of vascular endothelial cells depends on the presence of heparin-like molecules. Several cell types which do not proliferate in response to VEGF such as bovine corneal endothelial cells, HeLa cells and human melanoma cells also express cell surface VEGF receptors, but the function of the VEGF receptors in these cells is unclear. Recently, the tyrosine-kinase receptors encoded by the flt and KDR/flk-1 genes were found to function as VEGF165 receptors.

Expression of protein tyrosine kinases in islet cells: possible role of the Flk-1 receptor for beta-cell maturation from duct cells

To elucidate the expression of genes of importance for beta-cell replication and the production of insulin, single-stranded cDNAs from different preparations of insulin producing cells were used as template for the polymerase chain reaction (PCR) using primers specific for protein tyrosine kinases (PTKs). In RINm5F cells, as well as in fetal rat islets, the receptor PTK fetal liver kinase-1 (Flk-1) was expressed among other receptor and cytoplasmic tyrosine kinases. To elucidate the putative effects of stimulation of the Flk-1 receptor, fetal rat islet-like structures were cultured in the presence of the ligand for this receptor, vascular endothelial growth factor (VEGF). VEGF was found to stimulate both the insulin content/islet DNA ratio and the accumulation of insulin in the culture medium without affecting the rates of beta-cell replication. To investigate the localization of expression of the Flk-1 receptor in the pancreas, serial sections of fetal pancreata were immunostained for Flk-1 and insulin. Expression of Flk-1 was detected in endothelial-like cells and cells lining pancreatic ducts. The latter are considered to contain precursor cells for the endocrine pancreas. In conclusion, specific protein tyrosine kinases are expressed in islet cells, and are presumably participating in the regulation of islet function. Specifically, the receptor PTK Flk-1 may play a role of beta-cell maturation from pancreatic duct cells.

The related FLT4, FLT1, and KDR receptor tyrosine kinases show distinct expression patterns in human fetal endothelial cells

The growth factor receptors expressed on endothelial cells are of special interest because of their potential to program endothelial cell growth and differentiation during development and neovascularization in various pathological states, such as wound healing and angiogenesis associated with tumorigenesis. Vascular endothelial growth factor ([VEGF] also known as vascular permeability factor) is a potent mitogen and permeability factor, which has been suggested to play a role in embryonic and tumor angiogenesis. The newly cloned FLT4 receptor tyrosine kinase gene encodes a protein related to the VEGF receptors FLT1 and KDR/FLK-1. We have here studied the expression of FLT4 and the other two members of this receptor family in human fetal tissues by Northern and in situ hybridization. These results were also compared with the sites of expression of VEGF and the related placenta growth factor (PlGF). Our results reveal FLT4 mRNA expression in vascular endothelial cells in developing vessels of several organs. A comparison of FLT4, FLT1 and KDR/FLK-1 receptor mRNA signals shows overlapping, but distinct expression patterns in the tissues studied. Certain endothelia lack one or two of the three receptor mRNAs. These data suggest that the receptor tyrosine kinases encoded by the FLT gene family may have distinct functions in the regulation of the growth/differentiation of blood vessels.

Expression of tie-2, a member of a novel family of receptor tyrosine kinases, in the endothelial cell lineage

We are interested in the molecular mechanisms that are involved in the development of the vascular system. In order to respond to morphogenetic and mitogenic signals, endothelial cells must express appropriate receptors. To characterize endothelial cell-specific receptors, we have concentrated on receptor tyrosine kinases, because several lines of evidence suggested the importance of controlled phosphotyrosine levels in endothelial cells. A strategy based on PCR amplification using degenerate oligonucleotides and mouse brain capillaries as mRNA source, led to the identification of a novel receptor tyrosine kinase, which we designated tie-2. In situ hybridization using a tie-2-specific probe revealed an interesting spatial and temporal expression pattern. The gene was expressed specifically in the endothelial lineage. tie-2 transcripts were present in endothelial cell precursors (angioblasts) and also in endothelial cells of sprouting blood vessels throughout development and in all organs and tissues so far examined. tie-2 was down-regulated in the adult. Because of the unusual combination of immunoglobulin, EGF-like and fibronectin type III domains in the extracellular portion of tie-2 which is shared by TEK and tie, these molecules may be considered members of a new family of receptor tyrosine kinases. Signal transduction via this new class of tyrosine kinases could lead to a better understanding of the molecular mechanisms of blood vessel formation.

Defects in heart and lung development in compound heterozygotes for two different targeted mutations at the N-myc locus

Two types of mutant allele, one leaky and one null, have been generated by gene targeting at the N-myc locus in embryonic stem cells and the phenotypes of mice homozygous for these mutations have been described. These mutations have shown that N-myc has a number of functions during development, including a role in branching morphogenesis in the lung, which manifests itself at birth in mice homozygous for the leaky allele, and roles in the development of the mesonephric tubules, the neuroepithelium, the sensory ganglia, the gut and the heart, which become evident at midgestation in embryos homozygous for the null allele. In an attempt to define roles for N-myc at other stages of development, we have combined the two types of N-myc mutant allele in a compound heterozygote that as a result contains approximately 15% of normal levels of N-Myc protein. Compound heterozygotes died during gestation at a time intermediate to the times of death of embryos homozygous for either mutation individually, and their death appeared to result from cardiac failure stemming from hypoplasia of the compact subepicardial layer of the myocardium. Investigation of the expression pattern of N-myc and various markers of differentiation in wild-type and compound heterozygote mutant hearts has suggested that N-myc may function in maintaining the proliferation and/or preventing the differentiation of compact layer myocytes. This study illustrates the importance of generating different mutations at a given locus to elucidate fully the function of a particular gene during development.

Vascular endothelial growth factor receptor expression during embryogenesis and tissue repair suggests a role in endothelial differentiation and blood vessel growth

Vascular endothelial growth factor (VEGF) is a polypeptide mitogen that stimulates the growth of endothelial cells in vitro and promotes the growth of blood vessels in vivo. We have recently shown that the fms-like receptor tyrosine kinase (flt) is a receptor for VEGF. Here we used in situ hybridization to show that, in adult mouse tissues, the pattern of flt expression was consistent with localization in endothelium. We also show that flt was expressed in endothelium during neovascularization of healing skin wounds and during early vascular development in mouse embryos. Moreover, flt was expressed in populations of embryonic cells from which endothelium is derived such as early yolk sac mesenchyme. The expression of flt in the endothelium of both developing and mature blood vessels suggests that VEGF might regulate endothelial differentiation, blood vessel growth, and vascular repair.

Fetal liver kinase 1 is a receptor for vascular endothelial growth factor and is selectively expressed in vascular endothelium

Vascular endothelial growth factor (VEGF), also known as vascular permeability factor, induces endothelial proliferation in vitro and vascular permeability in vivo. The human transmembrane c-fms-like tyrosine kinase Flt-1 has recently been identified as a VEGF receptor. Flt-1 kinase has seven immunoglobulin-like extracellular domains and a kinase insert sequence, features shared by two other human gene-encoded proteins, kinase insert domain-containing receptor (KDR) and FLT-4. In this study we show that the mouse homologue of KDR, Flk-1, is a second functional VEGF receptor. Flk-1 binds VEGF with high affinity, undergoes autophosphorylation, and mediates VEGF-dependent Ca2+ efflux in Xenopus oocytes injected with Flk-1 mRNA. We also demonstrate by in situ hybridization that Flk-1 protein expression in the mouse embryo is restricted to the vascular endothelium and the umbilical cord stroma. VEGF and its receptors Flk-1/KDR and Flt-1 may play a role in vascular development and regulation of vascular permeability.