Zoya Poltorak

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)

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 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.