Tzafra Cohen

Vascular endothelial growth factor (VEGF) and its receptors

Vascular endothelial growth factor (VEGF) is a highly specific mitogen for vascular endothelial cells. Five VEGF isoforms are generated as a result of alternative splicing from a single VEGF gene. These isoforms differ in their molecular mass and in biological properties such as their ability to bind to cell‐surface heparan‐sulfate proteoglycans. The expression of VEGF is potentiated in response to hypoxia, by activated oncogenes, and by a variety of cytokines. VEGF induces endothelial cell proliferation, promotes cell migration, and inhibits apoptosis. In vivo VEGF induces angiogenesis as well as permeabilization of blood vessels, and plays a central role in the regulation of vasculogenesis. Deregulated VEGF expression contributes to the development of solid tumors by promoting tumor angiogenesis and to the etiology of several additional diseases that are characterized by abnormal angiogenesis. Consequently, inhibition of VEGF signaling abrogates the development of a wide variety of tumors. The various VEGF forms bind to two tyrosine‐kinase receptors, VEGFR‐1 (flt‐1) and VEGFR‐2 (KDR/flk‐1), which are expressed almost exclusively in endothelial cells. Endothelial cells express in addition the neuropilin‐1 and neuropilin‐2 coreceptors, which bind selectively to the 165 amino acid form of VEGF (VEGF165). This review focuses on recent developments that have widened considerably our understanding of the mechanisms that control VEGF production and VEGF signal transduction and on recent studies that have shed light on the mechanisms by which VEGF regulates angiogenesis.—Neufeld, G., Cohen, T., Gengrinovitch, S., Poltorak, Z. Vascular endothelial growth factor (VEGF) and its receptors. FASEB J. 13, 9–22 (1999)

Interleukin 6 Induces the Expression of Vascular Endothelial Growth Factor

Angiogenesis, the formation of new blood vessels, is induced by various growth factors and cytokines that act either directly or indirectly. Vascular endothelial growth factor (VEGF) is a specific mitogen for vascular endothelial cells and therefore has a central role in physiological events of angiogenesis. Interleukin-6 (IL-6) expression on the other hand is elevated in tissues that undergo active angiogenesis but does not induce proliferation of endothelial cells. We demonstrate using Northern analysis that treatment of various cell lines with IL-6 for 6-48 h results in a significant induction of VEGF mRNA. The level of induction is comparable to the documented induction of VEGF mRNA by hypoxia or cobalt chloride, an activator of hypoxia-induced genes. In addition, it is demonstrated by transient transfection assays that the effect of IL-6 is mediated not only by DNA elements at the promoter region but also through specific motif(s) located in the 5′-untranslated region (5′-UTR) of VEGF mRNA. Our results imply that IL-6 may induce angiogenesis indirectly by inducing VEGF expression. It is also shown that the 5′-UTR is important for the expression of VEGF. The 5′-UTR of VEGF is exceptionally long (1038 base pairs) and very rich in G + C. This suggests that secondary structures in the 5′-UTR might be essential for VEGF expression through transcriptional and post-transcriptional control mechanisms.

VEGF145, a Secreted Vascular Endothelial Growth Factor Isoform That Binds to Extracellular Matrix

A vascular endothelial growth factor (VEGF) mRNA species containing exons 1-6 and 8 of the VEGF gene was found to be expressed as a major VEGF mRNA form in several cell lines derived from carcinomas of the female reproductive system. This mRNA is predicted to encode a VEGF form of 145 amino acids (VEGF145). Recombinant VEGF145 induced the proliferation of vascular endothelial cells and promoted angiogenesis in vivo VEGF145 was compared with previously characterized VEGF species with respect to interaction with heparin-like molecules, cellular distribution, VEGF receptor recognition, and extracellular matrix (ECM) binding ability. VEGF145 shares with VEGF165 the ability to bind to the KDR/flk-1 receptor of endothelial cells. It also binds to heparin with an affinity similar to that of VEGF165. However, VEGF145 does not bind to two additional endothelial cell surface receptors that are recognized by VEGF165 but not by VEGF121. VEGF145 is secreted from producing cells as are VEGF121 and VEGF165. However, VEGF121 and VEGF165 do not bind to the ECM produced by corneal endothelial cells, whereas VEGF145 binds efficiently to this ECM. Basic fibroblast growth factor (bFGF)-depleted ECM containing bound VEGF145 induces proliferation of endothelial cells, indicating that the bound VEGF145 is active. The mechanism by which VEGF145 binds to the ECM differs from that of bFGF. Digestion of the ECM by heparinase inhibited the binding of bFGF to the ECM and released prebound bFGF, whereas the binding of VEGF145 was not affected by heparinase digestion. It therefore seems that VEGF145 possesses a unique combination of biological properties distinct from those of previously characterized VEGF species.

The neuropilins: multifunctional semaphorin and VEGF receptors that modulate axon guidance and angiogenesis.

The neuropilin-1 (np1) and neuropilin-2 (np2) receptors function as receptors for the axon guidance factors belonging to the class-3 semaphorin subfamily. In addition, both neuropilins are able to bind to certain heparin-binding splice forms of vascular endothelial growth factor (VEGF), indicating that both neuropilins have roles in the cardiovascular system as well. Gene targeting experiments indicate that np1 does indeed function as an important modulator of VEGF function during vasculogenesis and angiogenesis, but the role of np2 in the cardiovascular system has not been studied in detail as yet. This review focuses on the neuropilins, their interactions, and their biological roles in the nervous and cardiovascular systems.

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 KDRflk-1 genes were found to function as VEGF165 receptors.

Selective Binding of VEGF to One of the Three Vascular Endothelial Growth Factor Receptors of Vascular Endothelial Cells

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

Effect of vascular endothelial growth factor on hepatic regenerative activity following partial hepatectomy in rats

BACKGROUND/AIMS Vascular endothelial growth factor (VEGF) is an angiogenic factor with a growth-promoting effect that is thought to be restricted to vascular endothelial cells. Its essential role during liver regeneration has yet to be determined. The aim of this study was to document the effect of exogenous VEGF administration on liver regeneration in rats undergoing submaximal hepatic resections. METHODS Adult male Sprague-Dawley rats (n = 4/group) undergoing 30% partial hepatectomy were administered 200 ng VEGF165 intravenously and were sacrificed at 24, 36, and 48 h postoperatively. Liver regeneration was monitored by measuring the restituted liver mass, proliferating cell nuclear antigen (PCNA) immunostaining, and hepatic PCNA protein by Western blot. RESULTS Changes in restituted liver mass 48 h postsurgery were more prominent, but did not differ statistically between VEGF-treated and control rats (47% vs. 29%; p<0.06). Nevertheless, PCNA immunostaining showed increased labeling index of hepatocytes, apparent at 36 and 48 h after partial hepatectomy (38% vs. 18% [p<0.041 and 42% vs. 11% [p<0.021], respectively). Hepatic PCNA proteins measured by Western blot showed a 3-fold increase in VEGF-treated rats 48 h postsurgery compared with controls (p<0.01). CONCLUSION Exogenous VEGF administration early after partial hepatectomy stimulates liver regeneration in rats. Whether or not VEGF165 is a direct mitogen for hepatocytes remains to be determined.

Phosphatidylserine containing ω-3 fatty acids may improve memory abilities in non-demented elderly with memory complaints: a double-blind placebo-controlled trial.

Background: Phosphatidylserine (PS) may have beneficial effects on cognitive functions. We evaluated the efficacy of a novel preparation of PS containing ω–3 long-chain polyunsaturated fatty acids attached to its backbone (PS-DHA) in non-demented elderly with memory complaints. Methods: 157 participants were randomized to receive either PS-DHA or placebo for 15 weeks. Efficacy measures, assessed at baseline and endpoint, included the Rey Auditory Verbal Learning Test, Rey Complex Figure Test, and a computerized cognitive battery. Clinicians’ Global Impression of Change was assessed following 7 and 15 weeks of treatment. Results: 131 participants completed the study although 9 were excluded from the efficacy analysis due to protocol violation. At endpoint, verbal immediate recall was significantly improved in the PS-DHA group compared to the placebo group. Post-hoc analysis revealed that a subset of participants with relatively good cognitive performance at baseline had significant treatment-associated improvements in immediate and delayed verbal recall, learning abilities, and time to copy complex figure. These favorable results were further supported by responder analysis. Conclusions: The results indicate that PS-DHA may improve cognitive performance in non-demented elderly with memory complaints. Post-hoc analysis of subgroups suggests that participants with higher baseline cognitive status were most likely to respond to PS-DHA. The results of this exploratory study should be followed up by additional studies aimed at confirming the present tentative conclusions.

THE VEGF SPLICE VARIANTS: PROPERTIES, RECEPTORS, AND USAGE FOR THE TREATMENT OF ISCHEMIC DISEASES

Vascular endothelial growth factor (VEGF) was discovered 10 years ago as a growth factor that can regulate angiogenesis and in addition the permeability of blood vessels. Numerous studies have revealed that it is essential for normal embryonic development and that it plays a major role in physiological and pathological events of angiogenesis in adults. It is unique in that its expression is regulated directly by hypoxia. These properties are now being exploited in attempts aimed at the induction of new blood vessels in pathological situations such as ischemic heart disease. Five VEGF forms of 121 to 206 amino-acids are produced from a single gene by alternative splicing. Cells expressing VEGF usually express several forms simultaneously. VEGF121 does not contain exons 6 and 7 of the gene and consequently lacks a heparin binding ability. However, this form is fully active as an inducer of angiogenesis, and as a blood vessel permeabilizing agent. Exon 6 and 7 contain 2 independent heparin binding domains. The VEGF form containing exon 7 (VEGF165) and the vascular endothelial growth factor form containing exon 6 (VEGF145) display similar biological potencies raising the question of why so many VEGF forms are required. It was found that VEGF121 diffuses better because it does not bind to heparan-sulfate proteoglycans. In contrast, VEGF145 binds to extracellular matrix ad is released from it slowly. When the receptor binding properties of VEGF121 and VEGF165 were compared it was found that VEGF165 binds to a class of VEGF receptors that is not recognized by VEGF121. These receptors are encoded by the neuropilin-1 gene, and we have recently found that the related neuropilin-2 gene also encodes a VEGF165 receptor. We have recently found evidence indicating the neuropilins form complexes with another VEGF receptor, VEGFR-1. However, the biological function of this complex remains to be elucidated.ZusammenfassungDer “Vascular Endothelian Growth Vactor” (VEGF) wurde vor zehn Jahren als Wachstumsfaktor entdeckt, der an der Regulation der Angiogenese und Permeabilität der Blutgefäße beteiligt ist. Zahlreiche Untersuchungen haben gezeigt, daß er essentiell für die normale embryonale Entwicklung ist und daß er eine entscheidende Rolle bei physiologischen und pathophysiologischen Vorgängen der Angiogenese spielt. Einzigartig ist, daß seine Expression direkt durch Hypoxie reguliert wird. Diese Eigenschaften werden genutzt, um neue Blutgefäße, zum Beispiel bei ischämischer Herzkrankheit, zu induzieren. Ausgehend von einem Gen, werden fünf verschiedene VEGF-Isoformen von 121 bis 206 Aminosäuren durch alternatives “Splicing” gebildet. Zellen, die VEGF exprimieren, produzieren meistens mehrere Isoformen gleichzeitig. VEGF121 enthält nicht die Exons 6 und 7, und daher fehlt eine Heparinbindungsstelle. Diese Form wirkt trotzdem als Angiogenesefaktor und steigert die Blutgefäßpermeabilität. Exon 6 und 7 enthalten zwei unabhängige Heparinbindungsstellen. Die VEGF121-Isoform, die Exon 7 enthält (VEGF165), und die VEGF-Isoform mit dem Exon 6 (VEGF145) haben eine ähnliche biologische Wirkung, und es stellt sich die Frage, warum mehrere VEGF-Isoformen benötigt, werden. Es wurde gefunden, daß die VEGF121-Isoform besser diffundiert, da sie nicht an Heparansulfat-Proteoglykane bindet. VEGF145 bindet dagegen an die Extrazellulärmatrix und wird von ihr nur langsam wieder freigesetzt. Ein Vergleich der Rezeptorbindungseigenschaften von VEGF121 und VEGF165 zeigte, daß VEGF165 an eine Gruppe von vaskulären endothelialen Wachstumsfaktorrezeptoren bindet, die von VEGF121 nicht erkannt werden. Diese Rezeptoren werden vom Neutropilin-1-Gen kodiert, und wir konnten vor kurzem zeigen, daß das verwandte Neutropilin-2-Gen auch einen VEGF165-Rezeptor kodiert. Auch konnten wir nachweisen, daß die Neutropiline mit VEGFR-1, einem weiterem endothelialen Wachstumsfaktorrezeptor, Komplexe ausbilden. Die biologische Funktion dieses Komplexes ist noch nicht bekannt.

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.