Vascular endothelial growth factor (VEGF,/vɛdʒˈɛf/), originally known asvascular permeability factor (VPF),[1] is a signal protein produced by many cells that stimulates the formation of blood vessels. To be specific, VEGF is a sub-family ofgrowth factors, theplatelet-derived growth factor family ofcystine-knot growth factors. They are important signalingproteins involved in bothvasculogenesis (thede novo formation of the embryoniccirculatory system) andangiogenesis (the growth of blood vessels from pre-existing vasculature).
It is part of the system that restores the oxygen supply to tissues when blood circulation is inadequate such as in hypoxic conditions.[2] Serum concentration of VEGF is high inbronchial asthma anddiabetes mellitus.[3]VEGF's normal function is to create new blood vessels duringembryonic development, new blood vessels after injury, muscle following exercise, and new vessels (collateral circulation) to bypass blocked vessels.It can contribute to disease. Solid cancers cannot grow beyond a limited size without an adequate blood supply; cancers that can express VEGF are able to grow and metastasize. Overexpression of VEGF can cause vascular disease in theretina of the eye and other parts of the body. Drugs such asaflibercept,bevacizumab,ranibizumab, andpegaptanib can inhibit VEGF and control or slow those diseases.
In 1970,Judah Folkmanet al. described a factor secreted by tumors causing angiogenesis and called ittumor angiogenesis factor.[4] In 1983 Sengeret al. identified avascular permeability factor secreted by tumors in guinea pigs and hamsters.[1] In 1989 Ferrara and Henzel described an identical factor in bovine pituitary follicular cells which they purified, cloned and named VEGF.[5] A similar VEGF alternative splicing was discovered by Tischeret al. in 1991.[6] Between 1996 and 1997, Christinger and De Vos obtained the crystal structure of VEGF, first at 2.5 Å resolution and later at 1.9 Å.[7][8][9]
Crystal structure ofVammin, aVEGF-F from a snake venom
In mammals, the VEGF family comprises five members:VEGF-A, placenta growth factor (PGF),VEGF-B,VEGF-C andVEGF-D. The latter members were discovered after VEGF-A; before their discovery, VEGF-A was known as VEGF. A number of VEGF-related proteins encoded by viruses (VEGF-E) and in the venom of some snakes (VEGF-F) have also been discovered.
Important for Vasculogenesis, Also needed for angiogenesis during ischemia, inflammation, wound healing, and cancer.[citation needed]
Activity of VEGF-A, as its name implies, has been studied mostly on cells of the vascularendothelium, although it does have effects on a number of other cell types (e.g., stimulationmonocyte/macrophage migration, neurons, cancer cells, kidney epithelial cells). In vitro, VEGF-A has been shown to stimulate endothelial cellmitogenesis andcell migration. VEGF-A is also a vasodilator and increases microvascular permeability and was originally referred to as vascular permeability factor.
Schematic representation of the different isoforms of human VEGF
There are multiple isoforms of VEGF-A that result fromalternative splicing ofmRNA from a single, 8-exonVEGFA gene. These are classified into two groups which are referred to according to their terminal exon (exon 8) splice site: the proximal splice site (denoted VEGFxxx) or distal splice site (VEGFxxxb). In addition, alternate splicing of exon 6 and 7 alters theirheparin-binding affinity and amino acid number (in humans: VEGF121, VEGF121b, VEGF145, VEGF165, VEGF165b, VEGF189, VEGF206; the rodent orthologs of these proteins contain one fewer amino acids). These domains have important functional consequences for the VEGF splice variants, as the terminal (exon 8) splice site determines whether the proteins are pro-angiogenic (proximal splice site, expressed during angiogenesis) or anti-angiogenic (distal splice site, expressed in normal tissues). In addition, inclusion or exclusion of exons 6 and 7 mediate interactions withheparan sulfateproteoglycans (HSPGs) andneuropilin co-receptors on the cell surface, enhancing their ability to bind and activate theVEGF receptors (VEGFRs).[18] Recently, VEGF-C has been shown to be an important inducer of neurogenesis in the murine subventricular zone, without exerting angiogenic effects.[19]
All members of the VEGF family stimulate cellular responses by binding totyrosine kinase receptors (theVEGFRs) on the cell surface, causing them to dimerize and become activated throughtransphosphorylation, although to different sites, times, and extents. The VEGF receptors have an extracellular portion consisting of 7 immunoglobulin-like domains, a single transmembrane spanning region, and an intracellular portion containing a splittyrosine-kinase domain. VEGF-A binds to VEGFR-1 (Flt-1) and VEGFR-2 (KDR/Flk-1).[21] VEGFR-2 appears to mediate almost all of the known cellular responses to VEGF. The function of VEGFR-1 is less well-defined, although it is thought to modulate VEGFR-2 signaling.[22] Another function of VEGFR-1 may be to act as a dummy/decoy receptor, sequestering VEGF from VEGFR-2 binding (this appears to be particularly important during vasculogenesis in the embryo). VEGF-C and VEGF-D, but not VEGF-A, are ligands for a third receptor (VEGFR-3/Flt4), which mediateslymphangiogenesis. The receptor (VEGFR3) is the site of binding of main ligands (VEGFC and VEGFD), which mediates perpetual action and function of ligands ontarget cells. Vascular endothelial growth factor-C can stimulate lymphangiogenesis (via VEGFR3) and angiogenesis via VEGFR2. Vascular endothelial growth factor-R3 has been detected in lymphatic endothelial cells in CL of many species, cattle, buffalo and primate.[23]
In addition to binding toVEGFRs, VEGF binds to receptor complexes consisting of bothneuropilins and VEGFRs. This receptor complex has increased VEGF signalling activity inendothelial cells (blood vessels).[12][24] Neuropilins (NRP) arepleiotropic receptors and therefore other molecules may interfere with the signalling of the NRP/VEGFR receptor complexes. For example, Class 3semaphorins compete with VEGF165 for NRP binding and could therefore regulate VEGF-mediatedangiogenesis.[25]
VEGF-A production can be induced in a cell that is not receiving enoughoxygen.[21] When a cell is deficient in oxygen, it produces HIF,hypoxia-inducible factor, a transcription factor. HIF stimulates the release of VEGF-A, among other functions (including modulation of erythropoiesis). Circulating VEGF-A then binds to VEGF receptors on endothelial cells, triggering atyrosine kinase pathway leading to angiogenesis.[clarification needed] The expression ofangiopoietin-2 in the absence of VEGF leads to endothelial cell death and vascular regression.[26] Conversely, a German study donein vivo found that VEGF concentrations actually decreased after a 25% reduction in oxygen intake for 30 minutes.[27] HIF1 alpha and HIF1 beta are constantly being produced but HIF1 alpha is highly O2 labile, so, in aerobic conditions, it is degraded. When the cell becomes hypoxic, HIF1 alpha persists and the HIF1alpha/beta complex stimulates VEGF release. the combined use of microvesicles and 5-FU resulted in enhanced chemosensitivity of squamous cell carcinoma cells more than the use of either 5-FU or microvesicle alone. In addition, down regulation of VEGF gene expression was associated with decreased CD1 gene expression.[28]
VEGF-A and the corresponding receptors are rapidly up-regulated after traumatic injury of thecentral nervous system (CNS). VEGF-A is highly expressed in the acute and sub-acute stages of CNS injury, but the protein expression declines over time. This time-span of VEGF-A expression corresponds with the endogenousre-vascularization capacity after injury.[25] This would suggest that VEGF-A / VEGF165 could be used as target to promote angiogenesis after traumatic CNS injuries. However, there are contradicting scientific reports about the effects of VEGF-A treatments in CNS injury models.[25]
Although it has not been associated as abiomarker for the diagnosis of acute ischemicstroke,[29] high levels of serum VEGF in the first 48 hours after an cerebral infarct have been associated with a poor prognosis after 6 months[30] and 2 years.[31]
VEGF-A has been implicated with poor prognosis inbreast cancer. Numerous studies show a decreased overall survival and disease-free survival in those tumors overexpressing VEGF. The overexpression of VEGF-A may be an early step in the process ofmetastasis, a step that is involved in the "angiogenic" switch. Although VEGF-A has been correlated with poor survival, its exact mechanism of action in the progression of tumors remains unclear.[32]
VEGF-A is also released inrheumatoid arthritis in response toTNF-α, increasing endothelial permeability and swelling and also stimulating angiogenesis (formation of capillaries).[33]
VEGF-A is also important indiabetic retinopathy (DR). The microcirculatory problems in the retina of people withdiabetes can cause retinal ischaemia, which results in the release of VEGF-A, and a switch in the balance of pro-angiogenic VEGFxxx isoforms over the normally expressed VEGFxxxb isoforms. VEGFxxx may then cause the creation of new blood vessels in the retina and elsewhere in the eye, heralding changes that may threaten the sight.
VEGF-A plays a role in the disease pathology of the wet formage-related macular degeneration (AMD), which is the leading cause of blindness for the elderly of the industrialized world. The vascular pathology of AMD shares certain similarities with diabetic retinopathy, although the cause of disease and the typical source of neovascularization differs between the two diseases.
VEGF-D serum levels are significantly elevated in patients withangiosarcoma.[34]
Once released, VEGF-A may elicit several responses. It may cause acell to survive, move, or further differentiate. Hence, VEGF is a potential target for the treatment ofcancer. The first anti-VEGF drug, amonoclonal antibody namedbevacizumab, was approved in 2004. Approximately 10–15% of patients benefit from bevacizumab therapy; however, biomarkers for bevacizumab efficacy are not yet known.
Current studies show that VEGFs are not the only promoters of angiogenesis. In particular,FGF2 and HGF are potent angiogenic factors.
Patients suffering from pulmonary emphysema have been found to have decreased levels of VEGF in the pulmonary arteries.
VEGF-D has also been shown to be over expressed inlymphangioleiomyomatosis and is currently used as a diagnostic biomarker in the treatment of this rare disease.[35]
In thekidney, increased expression of VEGF-A inglomeruli directly causes the glomerular hypertrophy that is associated with proteinuria.[36]
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