Stela Gengrinovitch

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)

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.

Glypican-1 Is a VEGF165 Binding Proteoglycan That Acts as an Extracellular Chaperone for VEGF165

Glypican-1 is a member of a family of glycosylphosphatidylinositol anchored cell surface heparan sulfate proteoglycans implicated in the control of cellular growth and differentiation. The 165-amino acid form of vascular endothelial growth factor (VEGF165) is a mitogen for endothelial cells and a potent angiogenic factor in vivo. Heparin binds to VEGF165 and enhances its binding to VEGF receptors. However, native HSPGs that bind VEGF165 and modulate its receptor binding have not been identified. Among the glypicans, glypican-1 is the only member that is expressed in the vascular system. We have therefore examined whether glypican-1 can interact with VEGF165. Glypican-1 from rat myoblasts binds specifically to VEGF165 but not to VEGF121. The binding has an apparent dissociation constant of 3 × 10−10 m. The binding of glypican-1 to VEGF165 is mediated by the heparan sulfate chains of glypican-1, because heparinase treatment abolishes this interaction. Only an excess of heparin or heparan sulfates but not other types of glycosaminoglycans inhibited this interaction. VEGF165 interacts specifically not only with rat myoblast glypican-1 but also with human endothelial cell-derived glypican-1. The binding of125I-VEGF165 to heparinase-treated human vascular endothelial cells is reduced following heparinase treatment, and addition of glypican-1 restores the binding. Glypican-1 also potentiates the binding of 125I-VEGF165 to a soluble extracellular domain of the VEGF receptor KDR/flk-1. Furthermore, we show that glypican-1 acts as an extracellular chaperone that can restore the receptor binding ability of VEGF165, which has been damaged by oxidation. Taken together, these results suggest that glypican-1 may play an important role in the control of angiogenesis by regulating the activity of VEGF165, a regulation that may be critical under conditions such as wound repair, in which oxidizing agents that can impair the activity of VEGF are produced, and in situations were the concentrations of active VEGF are limiting.