Vascular endothelial growth factor: a new member of the platelet-derived growth factor gene family

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.

The role of angiogenesis in the tumor growth of Syrian hamster pancreatic cancer cell line HPD-NR

BACKGROUND/AIMS

New therapeutic approach is required for pancreatic cancer, one of the most intractable malignancies. The role of angiogenesis in the tumor growth of a Syrian hamster pancreatic cancer cell line HPD-NR, which closely resembles its human counterpart, was investigated.

METHODS

Angiogenic activity was measured as stimulation of growth of human umbilical vein endothelial cells (HUVEC), and angiogenic factors produced by HPD-NR cells were identified by reverse-transcription polymerase chain reaction and Northern blot analysis. Then in vitro and in vivo antitumor effects of a potent angiogenesis inhibitor, O-(chloroacetylcarbamoyl)fumagillol (AGM-1470), were examined.

RESULTS

The conditioned medium of HPD-NR cells stimulated the growth of HUVEC, and four hamster angiogenic factors were detected with an overexpression of transforming growth factor alpha and vascular endothelial growth factor messenger RNAs. AGM-1470 specifically inhibited the growth of HUVEC and that of HPD-NR tumors in vivo with decreased vascularity of the tumors but not the growth of HPD-NR cells in vitro.

CONCLUSIONS

The results suggest that angiogenesis plays an important role in tumor growth of HPD-NR cells and can be a new target of medical therapy for pancreatic cancer.

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 role of vascular permeability factor and basic fibroblast growth factor in tumor angiogenesis

In the last decade a considerable amount of research has been dedicated to studying the process of angiogenesis. In the field of tumor biology angiogenesis is a relevant subject of investigation as well, since newly formed blood vessels are required for the growth of tumors and provide an exit route for metastasizing tumor cells. In this review we discuss some aspects of tumor angiogenesis with emphasis on the role that growth factors bFGF and VPF play in this process. A number of biochemical characteristics and biological properties of the two factors and their receptors are reviewed, and the expression of bFGF and VPF in both normal tissues and in tumors is discussed. Finally, we speculate on the use of bFGF and VPF expression as a diagnostic parameter and on possible clinical applications.

Differential expression of vascular endothelial growth factor (vascular permeability factor) forms in rat tissues

Vascular endothelial growth factor (VEGF)/vascular permeability factor (VPF), exists as multiple forms due to alternative splicing of mRNA. VEGF165/164 (human/rodent homologue) is often assumed to be the predominant form, although truly quantitative assessments are lacking. We have used the RNase protection assay to directly quantitate the relative abundance of VEGF mRNA forms in five rat tissues (brain, kidney, lung, spleen, and heart) and two rat glioma cell lines (C6 and 9L). The three major forms, which code for proteins of 188, 164, and 120 amino acids, were observed in all of the tissues and cells examined. However, the relative abundance differed among the samples. VEGF188 was the predominant form (> 50% of total VEGF mRNA) in heart and lung, but was the least abundant form (6-15%) in all other samples. VEGF164 was lower (approximately 25%) in heart and lung, but was predominant (> 50%) in brain and kidney. VEGF164 and VEGF120 were present in equimolar amounts (each form approximately 46% of total) in the spleen, C6, and 9L. VEGF120 was also present in kidney (38%) and lung (27%) and was least abundant (approximately 15%) in brain and heart. A rat homologue of VEGF206 was not observed. VEGF mRNA splicing occurs in a tissue-specific manner. The assumption that the predominant physiologic form of VEGF is a VEGF165/164 homodimer should be viewed with caution.

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.

Human keratinocytes express the three major splice forms of vascular endothelial growth factor

Vascular endothelial growth factor is a powerful mitogen for endothelial cells, recently reported to be produced by keratinocytes. In the present work, we examined human keratinocytes in primary culture for the splice variants of vascular endothelial growth factor. In situ hybridization revealed that 100% of cultured human keratinocytes expressed mRNA for this cytokine, and analysis by reverse transcriptase-polymerase chain reaction indicated that three species of mRNA were produced. Southern hybridization and size calculations of PCR products revealed mRNA species corresponding to 121, 165, and 189 amino-acid forms of this cytokine. Using a rabbit anti-vascular endothelial growth factor antiserum, we radioimmunoprecipitated two molecular weight forms (approximately 45 and 58 kDa, non-reducing conditions) from keratinocyte culture supernatants. Under reducing conditions, three bands of approximately 15, 20, and 24 kDa appeared, corresponding with the predominant forms of vascular endothelial growth factor described. We propose that secretion of vascular endothelial growth factor by human keratinocytes in vivo sustains angiogenesis during physiologic tissue repair and in pathologic states accompanied by neovascularization.

Vascular permeability factor in brain metastases: correlation with vasogenic brain edema and tumor angiogenesis

Metastatic brain tumors are almost always associated with vasogenic brain edema, which in turn plays a pivotal role in the evolution of neurological morbidity associated with these lesions. Attention has recently focused on a group of proteinaceous vascular permeability factors (VPF's) that are capable of inducing angiogenesis and promoting increased capillary permeability. To test the hypothesis that metastatic brain tumors expressing VPF's are associated with peritumoral cerebral edema, a rabbit polyclonal immunoglobulin (Ig) G anti-VPF was used to immunostain pathological specimens of metastatic cerebral tumors obtained from 22 patients who underwent surgery at Yale-New Haven Hospital. Magnetic resonance (MR) imaging was used to correlate VPF staining in tumor tissue with the occurrence of peritumoral brain edema. A histological study of the microvasculature was then conducted by immunostaining the specimens for endothelial cell factor VIII surface antigen, using two gliosis specimens as controls. Results revealed 21 of 22 tumors stained positively for VPF's; the negative-VPF tumor was a melanoma that exhibited no peritumoral edema. Twenty of 22 tumors had MR imaging-evident vasogenic edema. The presence and intensity of VPF immunostaining of microvascular features were noted and compared. Factor VIII staining demonstrated tumor vascularity to be most abundant in VPF-rich regions of tumor. The authors therefore report a high correlation between the presence of VPF's and the occurrence of peritumoral brain edema associated with cerebral metastases.

Peripheral proliferative retinopathies: an update on angiogenesis, etiologies and management

Many clinical entities may be associated with the development of peripheral retinal neovascularization. In this paper, we review the mechanisms of normal and abnormal angiogenesis in the retina. Specific disease entities associated with peripheral proliferative retinopathies are discussed. These include vascular disease with ischemia, inflammatory diseases with possible ischemia and a variety of miscellaneous causes, including hereditary diseases and tumors. Basic principles for the clinical evaluation of patients with retinal neovascularization are described. Finally, the treatments for retinal neovascularization, including cryopexy and local and panretinal photocoagulation are reviewed, and techniques and possible mechanisms of the beneficial results of treatment are described.

Expression and subcellular distribution of basic fibroblast growth factor are regulated during migration of endothelial cells

Migration of endothelial cells is involved in normal and pathological angiogenesis and in re-endothelialization after vascular injury or rupture of atherosclerotic plaques. Several types of endothelial cells are known to synthesize basic fibroblast growth factor (bFGF); in some of these, migration is increased by exogenous bFGF and inhibited by anti-bFGF antibodies. Using immunocytochemical techniques and RNase protection analysis, we studied endothelial cells from bovine coronary arteries and veins as well as from adrenal microvessels. We found that bFGF mRNA and peptide were present in confluent endothelial cells and were upregulated during migration stimulated by removal of some cells from the monolayer. During migration, extracellular matrix stores of bFGF were depleted, and bFGF immunoreactivity began to accumulate in the cytoplasm of endothelial cells between 2 and 6 hours. After migration had begun, but before the initiation of DNA synthesis, bFGF immunoreactivity increased in the nuclei and nucleoli. Exogenous bFGF stimulated endothelial migration, and antibodies to bFGF markedly inhibited migration, suggesting that an intracrine function of nuclear bFGF is not sufficient for cell migration. In all three types of endothelial cells studied, bFGF was identified as an endogenous regulator, but not as the sole regulator, or migration. Moreover, bFGF expression and subcellular localization were found to be regulated during endothelial cell migration.

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.

Homologs of vascular endothelial growth factor are encoded by the poxvirus orf virus

A gene encoding a polypeptide with homology to mammalian vascular endothelial growth factors (VEGFs) has been discovered in the genome of orf virus (OV), a parapoxvirus that affects sheep and goats and, occasionally, humans. The gene is transcribed abundantly early in infection and is found immediately outside the inverted terminal repeat at the right end of the genome. In the NZ2 strain of OV (OV NZ2), the gene encodes a polypeptide with a molecular size of 14.7 kDa, while in another strain, OV NZ7, there is a variant gene that encodes a polypeptide of 16 kDa. The OV NZ2 and OV NZ7 polypeptides show 22 to 27% and 16 to 23% identity, respectively, to the mammalian VEGFs. The viral polypeptides are only 41.1% identical to each other, and there is little homology between the two genes at the nucleotide level. Another unusual feature of these genes is their G+C content, particularly that of OV NZ7. In a genome that is otherwise 63% G+C, the OV NZ2 gene is 57.2% G+C and the OV NZ7 gene is 39.7% G+C. The OV NZ2 gene, but not the OV NZ7 gene, is homologous to the mammalian VEGF genes at the DNA level, suggesting that the gene has been acquired from a mammalian host and is undergoing genetic drift. The lesions induced in sheep and humans after infection with OV show extensive dermal vascular endothelial proliferation and dilatation, and it is likely that this is a direct effect of the expression of the VEGF-like gene.

Differential expression of the two VEGF receptors flt and KDR in placenta and vascular endothelial cells

Vascular endothelial growth factor (VEGF) is a newly identified growth and permeability factor with a unique specificity for endothelial cells. Recently the flt-encoded tyrosine kinase was characterized as a receptor for VEGF. A novel tyrosine kinase receptor encoded by the KDR gene was also found to bind VEGF with high affinity when expressed in CMT-3 cells. Screening for flt and KDR expression in a variety of species and tissue-derived endothelial cells demonstrates that flt is predominantly expressed in human placenta and human vascular endothelial cells. Placenta growth factor (PIGF), a growth factor significantly related to VEGF, is coexpressed with flt in placenta and human vascular endothelial cells. KDR is more widely distributed and expressed in all vessel-derived endothelial cells. These data demonstrate that cultured human endothelial cells isolated from different tissues express both VEGF receptors in relative high levels and, additionally, that all investigated nonhuman endothelial cells in culture are also positive for KDR gene expression.

Inhibition of vascular endothelial cell growth factor activity by an endogenously encoded soluble receptor

Vascular endothelial cell growth factor, a mitogen selective for vascular endothelial cells in vitro that promotes angiogenesis in vivo, functions through distinct membrane-spanning tyrosine kinase receptors. The cDNA encoding a soluble truncated form of one such receptor, fms-like tyrosine kinase receptor, has been cloned from a human vascular endothelial cell library. The mRNA coding region distinctive to this cDNA has been confirmed to be present in vascular endothelial cells. Soluble fms-like tyrosine kinase receptor mRNA, generated by alternative splicing of the same pre-mRNA used to produce the full-length membrane-spanning receptor, encodes the six N-terminal immunoglobulin-like extracellular ligand-binding domains but does not encode the last such domain, transmembrane-spanning region, and intracellular tyrosine kinase domains. The recombinant soluble human receptor binds vascular endothelial cell growth factor with high affinity and inhibits its mitogenic activity for vascular endothelial cells; thus this soluble receptor could act as an efficient specific antagonist of vascular endothelial cell growth factor in vivo.

Human mesangial cells and peripheral blood mononuclear cells produce vascular permeability factor

Vascular permeability factor, or vascular endothelial growth factor (VPF/VEGF) is a disulfide-linked dimeric glycoprotein of about 40 kD that promotes fluid and protein leakage from blood vessels. Various human tumor cell lines and cells including fetal vascular smooth muscle cells produce VPF/VEGF. Since glomerular mesangial cells (MC) are closely related to vascular smooth muscle cells, we examined whether cultured human MC produce VPF/VEGF. Northern blotting analysis revealed that cultured human MC expressed a 3.7 kilobases (kb) VPF/VEGF mRNA. Human peripheral blood mononuclear cells (PBMC) also expressed VPF/VEGF transcripts of 8.6 and 3.8 kb. Although the sizes of the transcripts suggested the existence of unique molecular species of VPF/VEGF mRNA in PBMC, RT-PCR analysis revealed that PBMC as well as human MC expressed 121, 165, and 189 amino acid-containing isoforms of VPF/VEGF, implying that there are no unique alternative splicing products of VPF/VEGF mRNA in PBMC. Fetal calf serum and 12-o-tetradecanoyl- phorbol-13-acetate (TPA) transiently enhanced VPF/VEGF mRNA expression in cultured human MC. Transforming growth factor-beta 1 enhanced VPF/VEGF mRNA expression in cultured human MC at least within 24 hours. Dexamethasone (DEX) inhibited the TPA-induced increase in VPF/VEGF mRNA expression, whereas DEX did not change the basal level. The DEX depressed the TPA-induced increase in VPF/VEGF mRNA expression is therefore probably a result of transcriptional control. VPF/VEGF protein was detected in cultured human MC with immunoperoxidase staining using anti-VPF/VEGF antibody. TPA increased VPF/VEGF protein levels as well as those of VPF/VEGF mRNA in cultured human MC.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

In vivo effects of vascular endothelial growth factor on the chicken chorioallantoic membrane

The effect of vascular endothelial growth factor (VEGF165) on the chorioallantoic membrane (CAM) of 13-day-old chick embryos was studied. The factor was applied in doses of 0.5-4 micrograms for a period of up to 4 days. Macroscopical, histological and immunohistological studies were carried out. The localization of the factor was examined with an anti-VEGF antibody. The mitogenicity of VEGF165 and basic fibroblast growth factor (bFGF) were studied by means of the BrdU-anti-BrdU method. Furthermore, the effect of heparin alone and in combination with VEGF165 was investigated. VEGF165 specifically induces angiogenesis in doses of 0.5 microgram and more. A brush-like formation of blood vessels can be seen in the region of the precapillary vessels. Angiogenesis also takes place in the region of the capillaries and the venules. Histologically we found indications of sprouting as well as of intussusceptive capillary growth. The presence of the factor in the application area could be demonstrated with the anti-VEGF antibody for a period of 3 days. The factor is located in the chorionic epithelium and the intraepithelial capillaries. The BrdU-studies show that VEGF165 induces strong endothelial cell proliferation, whereas bFGF elicits fibrocyte proliferation and minor endothelial cell proliferation. Heparin induces squamous metaplasia of the chorionic and allantoic epithelium in combination with an aggregation of fibrocytes. We could not detect any enhancement of VEGF165 by heparin.

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.

flk-1, an flt-related receptor tyrosine kinase is an early marker for endothelial cell precursors

We have used RT-PCR to screen pluripotent murine embryonic stem cells to identify receptor tyrosine kinases (RTKs) potentially involved in the determination or differentiation of cell lineages during early mouse development. Fourteen different tyrosine kinase sequences were identified. The expression patterns of four RTKs have been examined and all are expressed in the mouse embryo during, or shortly after, gastrulation. We report here the detailed expression pattern of one such RTK, the flt-related gene flk-1. In situ hybridization analysis of the late primitive streak stage embryo revealed that flk-1 was expressed in the proximal-lateral embryonic mesoderm; tissue fated to become heart. By headfold stages, staining was confined to the endocardial cells of the heart primordia as well as to the blood islands of the visceral yolk sac and the developing allantois. Patchy, speckled staining was detected in the endothelium of all the major embryonic and extraembryonic blood vessels as they formed. During early organogenesis, expression was detected in the blood vessels of highly vascularized tissues such as the brain, liver, lungs and placenta. Since flk-1 was expressed in early mesodermal cells prior to any morphological evidence for endothelial cell differentiation (vasculogenesis), as well as in cells that form blood vessels from preexisting ones (angiogenesis), it appears to be a very early marker of endothelial cell precursors. We have previously reported that another novel RTK, designated tek, was expressed in differentiating endothelial cells. We show here that flk-1 transcripts are expressed one full embryonic day earlier than the first tek transcripts. The expression of these two RTKs appear to correlate with the specification and early differentiation of the endothelial cell lineage respectively, and therefore may play important roles in the establishment of this lineage.

Transcriptional regulation of vascular endothelial growth factor gene expression in ovarian bovine granulosa cells

The expression of vascular endothelial growth factor (VEGF) in cultured bovine granulosa cells has been studied. As shown by northern blot analysis, granulosa cells express the VEGF gene. Analysis of the VEGF transcripts by the polymerase chain reaction technique shows that granulosa cells express predominantly the smallest VEGF coding forms (VEGF121 and VEGF164). Since in the promoter region of the VEGF gene there are four potential AP-1 sites and two potential AP-2 sites we have studied if TPA and forskolin could regulate VEGF gene expression. TPA induces VEGF transcription in a time- and dose-dependent fashion. Maximal VEGF mRNA levels are detected 6 h after TPA treatment. Induction apparently requires de novo protein synthesis since it does not occur when translation is inhibited by cycloheximide. Forskolin, a naturally occurring diterpene that activates adenylylcyclase, also increases VEGF mRNA content in a time-dependent manner. Induction does not require de novo protein synthesis and, in contrast to TPA, induction is strongly potentiated by cycloheximide. Luteotrophic hormone, a known activator of adenylylcyclase, also induces VEGF transcription. These results imply that granulosa cells may be a source of VEGF which could play a role in the angiogenic process associated with ovulation and corpus luteum formation.

Expression of the vascular permeability factor/vascular endothelial growth factor gene in central nervous system neoplasms

Expression of the vascular permeability factor/vascular endothelial growth factor (VEGPF) gene was investigated in human central nervous system (CNS) neoplasms and normal brain. Adsorption of capillary permeability activity from human glioblastoma multiforme (GBM) cell conditioned medium and GBM cyst fluids by anti-VEGPF antibodies demonstrated that VEGPF is secreted by GBM cells and is present in sufficient quantities in vivo to induce vascular permeability. Cloning and sequencing of polymerase chain reaction-amplified GBM and normal brain cDNA demonstrated three forms of the VEGPF coding region (567, 495, and 363 nucleotides), corresponding to mature polypeptides of 189, 165, and 121 amino acids, respectively. VEGPF mRNA levels in CNS tumors vs. normal brain were investigated by the RNase protection assay. Significant elevation of VEGPF gene expression was observed in 81% (22/27) of the highly vascular and edema-associated CNS neoplasms (6/8 GBM, 8/8 capillary hemangioblastomas, 6/7 meningiomas, and 2/4 cerebral metastases). In contrast, only 13% (2/15) of those CNS tumors that are not commonly associated with significant neovascularity or cerebral edema (2/10 pituitary adenomas and 0/5 nonastrocytic gliomas) had significantly increased levels of VEGPF mRNA. The relative abundance of the forms of VEGPF mRNA was consistent in tumor and normal brain: VEGPF495 > VEGPF363 > VEGPF567. In situ hybridization confirmed the presence of VEGPF mRNA in tumor cells and its increased abundance in capillary hemangioblastomas. Our results suggest a significant role for VEGPF in the development of CNS tumor neovascularity and peritumoral edema.

Potent synergism between vascular endothelial growth factor and basic fibroblast growth factor in the induction of angiogenesis in vitro

Vascular endothelial growth factor (VEGF), also known as vascular permeability factor or vasculotropin, is a recently characterized endothelial-specific mitogen which is angiogenic in vivo. Here we demonstrate that VEGF is angiogenic in vitro: when added to microvascular endothelial cells grown on the surface of three-dimensional collagen gels, VEGF induces the cells to invade the underlying matrix and to form capillary-like tubules, with an optimal effect at approximately 2.2nM (100ng/ml). When compared to basic fibroblast growth factor (bFGF) at equimolar (0.5nM) concentrations, VEGF was about half as potent. The most striking effect was seen in combination with bFGF: when added simultaneously, VEGF and bFGF induced an in vitro angiogenic response which was far greater than additive, and which occurred with greater rapidity than the response to either cytokine alone. These results demonstrate that like bFGF, VEGF induces an angiogenic response via a direct effect on endothelial cells, and that by acting in concert, these two cytokines have a potent synergistic effect on the induction of angiogenesis in vitro. We suggest that the synergism between VEGF and bFGF plays an important role in the control of angiogenesis in vivo.

Platelet-derived growth factor and alternative splicing: a review

The mitogenic and chemotactic potency of platelet-derived growth factor (PDGF) has linked this polypeptide to the pathogenesis of several disease states including atherosclerosis and neoplasia. We have reviewed the recent literature on aspects relating to the structure, distribution and biology of PDGF and its high-affinity cell-surface and intracellular receptors. In addition to platelets, several normal and tumor cells secrete the mitogen in one or more of three possible dimeric configurations. Alternative splicing of exon 6 in PDGF A-chain RNA results in the formation of two protein species with different carboxy-termini. Initially, it was thought that the longer A-chain variant was processed only by transformed cells. However, recent evidence indicates that alternative splicing occurs in several cells which express the A-chain, including early Xenopus embryos. The functional significance of the exon 6 product, a highly basic region spanned by 18 amino acid residues (A194-211), is not precisely clear. We have summarized recent findings which implicate roles for A194-211 in the processing, secretion, and mitogenesis of the A-chain homodimer, nuclear transport signalling, and heparin binding. Thus, alternative splicing could play an important role in the modulation of the functional properties of the PDGF A-chain variants per se and in the complex interactive network of polypeptide growth factors and cytokines.

AIDS-associated Kaposi's sarcoma cells in culture express vascular endothelial growth factor

PCR cloning and cDNA sequencing have been used to identify mRNAs of two splice products of the vascular endothelial growth factor (VEGF) gene, VEGF121 and VEGF165, in cells isolated from Kaposi's sarcomas (KS) of AIDS patients (AIDS-KS). As demonstrated by Northern blot analysis, AIDS-KS cells as well as tumor cells show a high expression level of the VEGF gene as compared to primary human vascular cells like smooth muscle cells or endothelial cells. In addition to the lower expression of the gene, vascular cells express a 3.9 kb band together with a 3.2 kb band instead of a 3.9 kb and a 4.3 kb band in AIDS-KS cells. Our data suggest that the angiogenic properties of AIDS-KS cells might be mediated by the secretion of this growth factor and that this factor alone or in combination with other endothelial mitogens may be involved in endothelial proliferation associated with Kaposi's sarcoma.

The fms-like tyrosine kinase, a receptor for vascular endothelial growth factor

The fms-like tyrosine kinase (Flt) is a transmembrane receptor in the tyrosine kinase family. Expression of flt complementary DNA in COS cells conferred specific, high-affinity binding of vascular endothelial growth factor, also known as vascular permeability factor (VEGF-VPF), a factor that induces vascular permeability when injected in the guinea pig skin and stimulates endothelial cell proliferation. Expression of Flt in Xenopus laevis oocytes caused the oocytes to release calcium in response to VEGF-VPF. These findings show that flt encodes a receptor for VEGF-VPF.

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.

Vascular permeability factor (vascular endothelial growth factor) gene is expressed differentially in normal tissues, macrophages, and tumors

Vascular permeability factor (VPF), also known as vascular endothelial growth factor (VEGF), increases microvascular permeability and is a specific mitogen for endothelial cells. Expression of VPF/VEGF previously was demonstrated in a variety of tumor cells, in cultures of pituitary-derived cells, and in corpus luteum. Here we present evidence, by Northern analysis and in situ hybridization, that the VPF/VEGF gene is expressed in many adult organs, including lung, kidney, adrenal gland, heart, liver, and stomach mucosa, as well as in elicited peritoneal macrophages. The highest levels of VPF/VEGF transcripts were found in epithelial cells of lung alveoli, renal glomeruli and adrenal cortex, and in cardiac myocytes. The prominence of VPF/VEGF mRNA in these tissues suggests a possible role for VPF/VEGF in regulating baseline microvascular permeability, which is essential for tissue nutrition and waste removal. We also demonstrate particularly high VPF/VEGF mRNA levels in several human tumors, where it may be involved in promoting tumor angiogenesis and stroma generation, both as an endothelial cell mitogen and indirectly by its permeability enhancing effect that leads to the deposition of a provisional fibrin gel matrix.

Compartmentalization of PDGF on extracellular binding sites dependent on exon-6-encoded sequences

The PDGFs are a family of molecules assembled as disulfide-bonded homo- and heterodimers from two distinct but highly homologous polypeptide chains (PDGF-A and PDGF-B). Two PDGF A-chain transcripts, which arise from alternative usage of the 69-bp exon 6 and exon 7, give rise to two forms of PDGF-A. In spite of the conservation of two PDGF A-chain forms over at least 350 million years, no differences in their biological activities have been identified. We have investigated the activity of the sequence encoded by the alternatively spliced exon 6 of the PDGF A-chain (peptide AL). Addition of peptide AL at 10(-5)-10(-9) M to cultured endothelium and smooth muscle induced a dose-dependent, 3-20-fold increase in PDGF in conditioned media within 30 min. Peptide AL had no detectable effect on A- or B-chain transcript levels, and decrease in culture temperature did not prevent rapid release of PDGF. In human umbilical vein endothelial cells treated with peptide AL, the PDGF release was principally PDGF-BB, while in smooth muscle cells it was primarily PDGF-AA. The capacity to induce release of PDGF is shared by the homologous peptide encoded by exon 6 of the B-chain of PDGF. Binding studies and cross-linking analysis are consistent with a charge-based association of exon 6 sequences with membrane- and matrix-associated heparan-sulfate proteoglycans. We hypothesize that translation of exon 6 of the A- or B-chain of PDGF results in compartmentalization of these forms of PDGF with HS-PG, whereas forms lacking this sequence would be soluble and diffuse.

The alternative-splice isoforms of the PDGF A-chain differ in their ability to associate with the extracellular matrix and to bind heparin in vitro

Platelet-derived growth factor (PDGF) consists of disulfide-linked homo- or heterodimers of A and B chains. mRNA encoding the A chain (PDGF-A) occurs in two versions that differ by the presence or absence of a single short exon. These alternatively-spliced mRNAs encode polypeptides that differ in length by fifteen amino acids. The longer isoform (PDGF-AL) possesses a highly basic carboxy-terminal extension that is responsible for retaining PDGF-AL homodomers at the cell surface after secretion, while homodimers of the shorter isoform (PDGF-AS) are released into the extracellular medium. We have investigated the mechanism by which PDGF-AL remains in association with the cells that produce it. We expressed epitope-tagged versions of PDGF-AL and PDGF-AS in Cos cells and compared their intra- and extracellular distributions by immunofluorescence microscopy. PDGF-AL, but not PDGF-AS, was detected on and around cells in a diffuse pattern suggesting associated with the extracellular matrix (ECM). Metabolically radiolabelled PDGF-AL, but not PDGF-AS, could be eluted from ECM preparations by washing in high salt. Moreover, PDGF-AL bound reversibly to heparin-Sepharose in vitro at physiological salt concentrations, eluting at a salt concentration around 0.5 M. PDGF-AS did not bind to heparin under the same conditions. Thus, PDGF dimers that contain PDGF-AL may remain immobilized near the cells that secrete them by virtue of binding to heparin-like constituents of the ECM.

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.

Vascular endothelial growth factor (VEGF) induces plasminogen activators and plasminogen activator inhibitor-1 in microvascular endothelial cells

Extracellular proteolysis is believed to be an essential component of the angiogenic process. The effects of VEGF, a recently described angiogenic factor, were assessed on PA activity and PA and PAI-1 mRNA levels in microvascular endothelial cells. u-PA and t-PA activity were increased by VEGF in a dose-dependent manner, with maximal induction at 30 ng/ml. u-PA and t-PA mRNAs were increased 7.5- and 8-fold respectively after 15 hours, and PAI-1 mRNA 4.5-fold after 4 hours exposure to VEGF. At equimolar concentrations (0.5 nM), VEGF was a more potent inducer of t-PA mRNA than bFGF, while bFGF was a more potent inducer of u-PA and PAI-1 mRNAs. In addition, VEGF induced u-PA and PAI-1 mRNAs with kinetics similar to those previously demonstrated for bFGF. These results demonstrate the regulation of PA and PAI-1 production by VEGF in microvascular endothelial cells and are in accord with the hypothesis that extracellular proteolysis, appropriately balanced by protease inhibitors, is required for normal capillary morphogenesis.

A growth factor for cardiac myocytes is produced by cardiac nonmyocytes

Cardiac nonmyocytes, primarily fibroblasts, surround cardiac myocytes in vivo. We examined whether nonmyocytes could modulate myocyte growth by production of one or more growth factors. Cardiac myocyte hypertrophic growth was stimulated in cultures with increasing numbers of cardiac nonmyocytes. This effect of nonmyocytes on myocyte size was reproduced by serum-free medium conditioned by the cardiac nonmyocytes. The majority of the nonmyocyte-derived myocyte growth-promoting activity bound to heparin-Sepharose and was eluted with 0.75 M NaCl. Several known polypeptide growth factors found recently in cardiac tissue, namely acidic fibroblast growth factor (aFGF), basic FGF (bFGF), platelet-derived growth factor (PDGF), tumor necrosis factor alpha (TNF alpha), and transforming growth factor beta 1 (TGF beta 1), also caused hypertrophy of cardiac myocytes in a dose-dependent manner. However, the nonmyocyte-derived growth factor (tentatively named NMDGF) could be distinguished from these other growth factors by different heparin-Sepharose binding profiles (TNF alpha, aFGF, bFGF, and TGF beta 1) by neutralizing growth factor-specific antisera (PDGF, TNF alpha, aFGF, bFGF, and TGF beta 1), by the failure of NMDGF to stimulate phosphatidylinositol hydrolysis (PDGF and TGF beta 1), and, finally, by the apparent molecular weight of NMDGF (45-50 kDa). This nonmyocyte-derived heparin-binding growth factor may represent a novel paracrine growth mechanism in myocardium.

Vascular permeability factor: a unique regulator of blood vessel function

Vascular permeability factor (VPF), also known as vascular endothelial growth factor (VEGF), is a potent polypeptide regulator of blood vessel function. VPF promotes an array of responses in endothelium, including hyperpermeability, endothelial cell growth, angiogenesis, and enhanced glucose transport. VPF regulates the expression of tissue factor and the glucose transporter. All of the endothelial cell responses to VPF are evidently mediated by high affinity cell surface receptors. Thus, endothelial cells have a unique and specific spectrum of responses to VPF. Since each of the responses of endothelial cells to VPF are also elicited by agonists, such as bFGF, TNF, histamine and others, it remains a major challenge to determine how post-receptor signalling pathways maintain both specificity and redundancy in cellular responses to various agonists.

The vascular endothelial growth factor family of polypeptides

Vascular endothelial growth factor (VEGF) was identified as a heparin-binding polypeptide mitogen with a target cell specificity restricted to vascular endothelial cells. Molecular cloning reveals the existence of four species of VEGF having 121, 165, 189, and 206 amino acids. These have strikingly different secretion patterns, which suggests multiple physiological roles for this family of polypeptides. The two shorter forms are efficiently secreted, while the longer ones are mostly cell-associated. Alternative splicing of mRNA, rather that transcription from different genes, is the mechanism for their generation. In situ hybridization reveals that the VEGF mRNA is widely distributed in most tissues and organs and expressed at particularly high levels in areas of active vascular proliferation, like the ovarian corpus luteum. Ligand autoradiography on rat tissue sections demonstrates that VEGF binding sites are associated with vascular endothelial cells of both fenestrated and non-fenestrated capillaries and with the endothelium of large vessels, while no displaceable binding is evident on non-endothelial cell types. These findings support the hypothesis that VEGF plays a highly specific role in the maintenance and in the induction of growth of vascular endothelial cells.

Effects of a variety of cytokines and inducing agents on vascular permeability factor mRNA levels in U937 cells

Vascular permeability factor (VPF) is an approximately 40-kDa disulfide-linked dimeric glycoprotein that is active in increasing blood vessel permeability, endothelial cell growth and angiogenesis. Little is known about VPF gene regulation. In this study, we investigated the effects of a variety of cytokines and inducing agents on VPF mRNA levels in the monocyte-like U937 cell line. Transforming growth factor-beta 1 caused a 1.8-fold increase in VPF mRNA levels after 4 hours, followed by a decline to basal levels by 18 hours. Phorbol 12-myristate 13-acetate, a potent inducer of the differentiation of U937 cells, caused a 12.5-fold increase in VPF mRNA levels at 24 hours, coinciding with the differentiation of these monocyte-like cells into macrophage-like cells.

Interaction of vasculotropin/vascular endothelial cell growth factor with human umbilical vein endothelial cells: binding, internalization, degradation, and biological effects

Vasculotropin/vascular endothelial cell growth factor (VAS/VEGF) is a newly purified growth factor with a unique specificity for vascular endothelial cells. We have investigated the interactions of VAS/VEGF with human umbilical vein endothelial cells (HUVE cells). 125I-VAS/VEGF was found to HUVE cells in a saturable manner with a half-maximum binding at 2.8 ng/ml. Scatchard analysis did show two classes of high-affinity binding sites. The first class displayed a dissociation constant of 9 pM with 500 sites/cell. The dissociation constant and the number of binding sites of the second binding class were variable for different HUVE cell cultures (KD = 179 +/- 101 pM, 5,850 +/- 2,950 sites/cell). Half-maximal inhibition of 125I-VAS/VEGF occurred with a threefold excess of unlabeled ligand. Basic fibroblast growth factor (bFGF) and heparin did not compete with 125I-VAS/VEGF binding. In contrast, suramin and protamin sulfate completely displaced 125I-VAS/VEGF binding from HUVE cells. VAS/VEGF was shown to be internalized in HUVE cells. Maximum internalization (55% of total cell-associated radioactivity) was observed after 30 min. 125I-VAS/VEGF was completely degraded 2-3 hr after binding. At 3 hr, the trichloroacetic acid (TCA)-soluble radioactivity accumulated in the medium was 60% of the total radioactivity released by HUVE cells. No degradation fragment of 125I-VAS/VEGF was observed. Chloroquine completely inhibited degradation. VAS/VEGF was able to induce angiogenesis in vitro in HUVE cells. However, it did not significantly modulate urokinase-type plasminogen activator (u-PA), tissue-type plasminogen activator (t-PA), plasminogen activator inhibitor (PAI-1), and tissue factor (TF). Prostacyclin production was only stimulated at very high VAS/VEGF concentrations. Taken together, these results indicate that VAS/VEGF might be a potent inducer of neovascularization resulting from a direct interaction with endothelial cells. The angiogenic activity seems to be independent of the plasminogen activator or inhibitor system.

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.

Connective tissue growth factor: a cysteine-rich mitogen secreted by human vascular endothelial cells is related to the SRC-induced immediate early gene product CEF-10

Human umbilical vein endothelial (HUVE) cells have been previously reported to express the genes for the A and B chains of PDGF and to secrete PDGF-related factors into culture media. Antihuman PDGF IgG affinity chromatography was used to purify PDGF-related activity from HUVE cell-conditioned media. Immunoblot analysis of the affinity-purified proteins with anti-PDGF IgG and antibodies specific for the A or B chain peptides of PDGF combined with chemotactic and mitogenic assays revealed that the major PDGF immunorelated molecule secreted by HUVE cells is a monomer of approximately 36-38 kD and that less than 10% of the purified biologically active molecules are PDGF A or B chain peptides. Screening of an HUVE cell cDNA library in the expression vector lambda gtl 1 with the anti-PDGF antibody resulted in the cloning and sequencing of a cDNA with an open reading frame encoding a 38-kD cysteine-rich secreted protein which we show to be the major PDGF-related mitogen secreted by human vascular endothelial cells. The protein has a 45% overall homology to the translation product of the v-src-induced CEF-10 mRNA from chick embryo fibroblasts. We have termed this new mitogen connective tissue growth factor.

Growth of distal fetal rat lung epithelial cells in a defined serum-free medium

Fetal rat distal lung epithelial cells, in contrast to adult type II pneumocytes, will divide readily in culture in the presence of 10% (vol:vol) fetal bovine serum. The presence of serum makes purification of uncontaminated cell-derived growth factors difficult and modifies cellular responses to oxidant injury. We report the development of a defined serum-free medium that will support growth of fetal distal lung epithelial cells in primary culture. Initial studies used a low-serum (2%; vol:vol) to determine the effect of basal media, substrata, and various additives. Subsequent studies demonstrated growth on a poly-D-lysine substratum under serum-free culture conditions in Dulbecco's modified minimal essential medium with insulin (50 micrograms/ml), endothelial cell growth supplement (20 micrograms/ml), bovine pituitary extract (100 micrograms/ml), bovine serum albumin (50 micrograms/ml), selenous acid (4 ng/ml), reduced glutathione (500 ng/ml), soybean trypsin inhibitor (100 micrograms/ml), transferrin (5 micrograms/ml), HEPES buffer (2.6 mg/ml), and cholera toxin (5 micrograms/ml). Growth was enhanced by reducing the gas phase oxygen concentration from 21 to 3%. The undefined components of this medium, bovine pituitary extract and endothelial cell growth supplement, could be replaced by platelet-derived growth factor (20 ng/ml) with prostaglandin E1 (25 nM). The response of fetal distal lung epithelial cells to known growth factors differs substantially from that observed with type II pneumocytes from adult lung and is similar in many, though not all, respects to the responses reported for proximal airway cells from adult lung.

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.

Specific binding of vascular permeability factor to endothelial cells

Vascular permeability factor (VPF), also known as vascular endothelial cell growth factor, has recently been purified from guinea pig, human, and bovine sources. We show that various fetal or adult endothelial cell strains originating from either capillary or large vessels possess specific high affinity and saturable binding sites for guinea pig tumor-derived [125I]VPF. Two classes of sites with KDs of approximately 10 pM and 1 nM were detected for all endothelial cell types examined. Guinea pig [125I]VPF binding to endothelial cells was inhibited by human VPF (ID50 = 0.8 ng/ml) and by suramin (ID50 = 75 micrograms/ml) but not by heparin. Cross-linking experiments revealed specific [125I]VPF-receptor complexes of two types. Most of the complexes migrated very slowing in SDS-PAGE, indicating that they were of very high molecular weight and probably highly cross-linked. A portion of the molecules migrated as 270 kDa complexes, indicating that the molecular weight of the endothelial cell VPF receptor is about 230 kDa.