Vascular Endothelial Growth Factors (VEGFs) are a group of homodimeric polypeptides, and are master regulators of vascular development, maintenance and angiogenesis (Ferrara and Davis-Smyth, 1997). VEGF is also a potent growth factor, and is produced by macrophages and CD4+ and CD8+ T cells (Melter et al, 2000; Freeman et al, 1995). Members of the VEGF family include 5 structurally related proteins (VEGFA-D, and placental growth factor (PIGF), which are further individually subdivided based on alternative splicing (Ferrara, 2010; He et al, 1999). VEGF receptor activation leads to multiple complex signalling pathways, primarily inducing angiogenesis, and for this reason VEGF is a prime anti-tumour therapeutic target.
VEGF is upregulated in response to hypoxic and metabolic perturbations, such as dysregulated glucose levels, primarily by the activation of the transcription factor NF-kB (Tong et al, 2006; Schweiki et al, 1995; Schweiki et al, 1992). VEGFs bind with high affinity to one of the three tyrosine kinase receptors; VEGFR1, VEGFR2, and VEGFR3 (Ferrara and Davis-Smyth, 1997), which are expressed highly on endothelial cells and monocytes. It has also been demonstrated that VEGF can bind to several non-VEGF receptors, such as neuropilin receptors (NRP), and heparin sulphate peptidoglycans (HSPG), which act as co-receptors and facilitate complex downstream signalling (Teran and Nugent, 2015). Canonical VEGF signalling occurs mostly between a homodimeric VEGF molecule with a homodimeric VEGF receptor, and is induced upon ligation of VEGF to its membrane bound receptor VEGFR, leading to autophosphorylation of the cytoplasmic domains on the receptor and subsequent downstream signalling, initiated by the binding of adaptor molecules to the tyrosine residues of the VEGFR (Koch et al, 2011). However, heterodimeric VEGF receptor signalling has more recently been described (MacGabhann and Popel, 2007).
VEGF ELISA kits
Canonical VEGF receptor signalling
Canonical VEGF signalling is typically mediated in a paracrine manner, however, recent studies have identified an autocrine method of VEGF-VEGFR stimulation in endothelial cell, although the precise metabolic signalling involved is still unclear (Domingan et al, 2015; Lee et al, 2007). VEGFR activation is tightly regulated on multiple mechanisms; levels of ligand and receptor expression, rate of receptor internalization by endocytosis, and by intracellular interactions with other signalling pathways, with evidence for cross-talk between VEGF and Integrin signalling pathways (Somanath et al, 2009). Activation of the distinct VEGF receptors leads to differential biological outcomes. The primary angiogenic signalling is initiated by the activation of VEGFR2, via stimulation with VEGFA. Activation of VEGF2 leads to downstream signalling via phospholipase-C (PLC) and protein kinase C (PKC) to activate mitogen-associated protein kinase (MAPK) pathways, inducing proliferation (Takahashi et al, 1999). VEGFR2 endocytosis occurs by clatherin-mediated endocytosis (Ballmer-Hofer et al, 2011). Interestingly, VEGFR2 stimulation and association with its co-receptor NRP1 has been demonstrated in vitro to promote adhesion of cells expressing one of either receptor (Koch et al, 2014). More recent research has identified a link between inflammatory mediators and VEGF expression, with activated T cells inducing VEGF in a CD40-dependent manner (Reinders et al, 2003). VEGF can also induce chemokine transcription, and it has been shown to upregulate the chemoattractant-inducting molecule IL-8 (Lee et al, 2002).
Non-canonical VEGF receptor signalling
Non-canonical VEGF signalling describes an alternative receptor activation, based on VEGF-independent VEGFR signalling; which can occur in a ligand-free environment via the phosphorylation of VEGFR form external stresses; or by the binding of non-VEGF receptors (Lemmon et al, 2010).
The role of VEGF in disease
VEGF is a proliferative and vascular inducing molecule and plays a vital role in the development and maintenance of tumours. Tumour-infiltrating lymphocytes (TILs) can induce the upregulation of VEGF, thereby promoting vascularity in the tumour microenvironment and promoting tumourigenesis, as VEGF can induce further polarization of T cells to the Th1 subset (Mor et al, 2004; Reinders et al, 2003). Moreover, autocrine VEGF signalling has generated a lot of interest in the triple negative breast cancer field, with a focus on VEGF - integrin signalling crosstalk (Goel et al, 2013). Additionally, VEGF is of key importance in the central nervous system, and mediates neuroprotection and neurogenesis, with VEGF levels dramatically decreased upon aging (Ahluwalia et al, 2014). The role of VEGF in the CNS is somewhat polarized, as VEGF can induce blood-brain barrier leakage upon injury, stroke and multiple neurodegenerative diseases (Lane et al, 2016; Zlokovic et al, 2011). Furthermore, ligand-independent activation of VEFR2 contributes to the pathogenesis of diabetes, via upregulated reactive oxygen species (Warren et al, 2014).
VEGF as a therapeutic target
The main anti-cancer target in VEGF signalling is VEGFA-VEGFR2 association, which has long been shown to inhibit tumour growth (Kim et al, 1993). Multiple drugs have been developed to target this interaction. Bevacizumab, a major breakthrough in anti-VEGF therapeutics, is a monoclonal antibody which neutralizes VEGFA and has been approved for use in treatments for colon cancer, glioblastoma and breast cancer (Peak and Levin, 2010). Several other molecules targeting this pathway include VEGFR neutralizing antibodies, soluble VEGFR peptides, and an anti-PIGF antibody (Van de Veire et al, 2010; Wada et al, 2005). Anti-VEGF therapy has also been demonstrated to improve the pathogenesis of rheumatoid arthritis, due to the resulting decreased levels of IL-6 (Yoo et al, 2005). VEGF is also an attractive target for pro-angiogenic therapy, such as following cardiac ischaemia and traumatic brain injuries, in order to regenerate cells. However, inducing VEGFA carries complications with an enhanced immune response (Oosthuyse et al, 2001).
Figure 1: Therapeutic strategies targeting VEGFR2 signalling. VEGFA binds to the membrane-bound VEFR2 receptor (shown here as a homodimer for simplicity, however, can also signal as a heterodimer), which mediates activation through the cytoplasmic tyrosine kinase domain conferring signals downstream to phospholipase g (PLCg) and protein kinase C (PKC). Subsequent activation of MAPK and AKT signalling pathways induces cell proliferation, survival and angiogenesis. Therapeutic strategies targeting VEGF signalling include anti-VEGFA antibodies, for example bevacizumab, anti-VEGFR2 antibodies, and receptor tyrosine kinase inhibitors.
Ahluwalia A, Jones MK, Szabo S, Tarnawski AS. Aging impairs transcriptional regulation of vascular endothelial growth factor in human microvascular endothelial cells: implications for angiogenesis and cell survival. J Physiol Pharmacol. 2014. 65(2):209-15.
Ballmer-Hofer, K., Andersson, A. E., Ratcliffe, L. E.
& Berger, P. Neuropilin-1 promotes VEGFR-2 trafficking through Rab11 vesicles thereby specifying signal output. Blood. 2011. 118, 816–826.
Domigan CK, Ziyad S, Iruela-Arispe ML. Canonical and noncanonical vascular endothelial growth factor pathways: new developments in biology and signal transduction. Arterioscler Thromb Vasc Biol. 2015. 35(1):30-9.
Ferrara N, Davis-Smyth T. The biology of vascular endothelial growth factor. Endocr Rev. 1997. 18(1):4-25.
Ferrara N. Binding to the extracellular matrix and proteolytic processing: Two key mechanisms regulating vascular endothelial growth factor action. Mol Biol Cell. 2010. 21: 687–690.
Freeman MR, Schneck FX, Gagnon ML, Corless C, Soker S, Niknejad K, Peoples GE, Klagsbrun M. Peripheral blood T lymphocytes and lymphocytes infiltrating human cancers express vascular endothelial growth factor: a potential role for T cells in angiogenesis. Cancer Res. 1995. 55(18):4140-5.
Goel HL, Pursell B, Chang C, Shaw LM, Mao J, Simin K, Kumar P, Vander Kooi CW, Shultz LD, Greiner DL, Norum JH, Toftgard R, Kuperwasser C, Mercurio AM. GLI1 regulates a novel neuropilin-2/α6β1 integrin based autocrine pathway that contributes to breast cancer initiation. EMBO molecular medicine. 2013. 5:488–508.
He Y, Smith SK, Day KA, Clark DE, Licence DR, Charnock-Jones DS. Alternative splicing of vascular endothelial growth factor (VEGF)-R1 (FLT-1) pre-mRNA is important for the regulation of VEGF activity. Mol Endocrinol. 1999. 13(4):537-45.
Kim KJ, Li B, Winer J, et al. Inhibition of vascu- lar endothelial growth factor-induced angiogen- esis suppresses tumour growth in vivo. Nature. 1993. 362:841-4.
Koch S, Tugues S, Li X, Gualandi L, Claesson-Welsh L. Signal transduction by vascular endothelial growth factor receptors. The Biochemical journal. 2011. 437:169–183.
Koch S, van Meeteren LA, Morin E, Testini C, Weström S, Björkelund H, Le Jan S, Adler J, Berger P, Claesson-Welsh L. NRP1 Presented in trans to the Endothelium Arrests VEGFR2 Endocytosis, Preventing Angiogenic Signaling and Tumor Initiation. Developmental Cell. 2014. 28:633–646.
Lange, C.; Storkebaum, E.; de Almodóvar, C.R.; Dewerchin, M.; Carmeliet, P. Vascular endothelial growth factor: A neurovascular target in neurological diseases. Nat. Rev. Neurol. 2016. 12, 439–454.
Lee S, Chen TT, Barber CL, Jordan MC, Murdock J, Desai S, Ferrara N, Nagy A, Roos KP, Iruela- Arispe ML. Autocrine VEGF signaling is required for vascular homeostasis. Cell. 2007. 130:691– 703.
Lee TH, Avraham H, Lee SH, Avraham S. Vascular endothelial growth factor modulates neutrophil transendothelial migration via up-regulation of interleukin-8 in human brain microvascular endothelial cells. J Biol Chem. 2002. 277(12):10445-51.
Lemmon, M. A. & Schlessinger, J. Cell signaling by receptor tyrosine kinases. Cell. 2010. 141, 1117–1134
Mac Gabhann F, Popel AS. Dimerization of VEGF receptors and implications for signal transduction: a computational study. Biophys Chem. 2007. 128(2-3):125-39.
Melter M, Reinders ME, Sho M, Pal S, Geehan C, Denton MD, Mukhopadhyay D, Briscoe DM. Ligation of CD40 induces the expression of vascular endothelial growth factor by endothelial cells and monocytes and promotes angiogenesis in vivo. Blood. 2000. 96(12):3801-8.
Mor F, Quintana FJ, Cohen IR. Angiogenesis-inflammation cross-talk: vascular endothelial growth factor is secreted by activated T cells and induces Th1 polarization. J Immunol. 2004. 172(7):4618-23.
Oncogene. 1999. 18(13):2221-30.
Oosthuyse B, Moons L, Storkebaum E, et al. Deletion of the hypoxia-response element in the vascular endothelial growth factor promoter causes motor neuron degeneration. Nat Genet. 2001. 28:131-8.
Peak SJ, Levin VA. Role of bevacizumab therapy in the management of glioblastoma. Cancer Manag Res. 2010. 2:97-104.
Reinders ME, Sho M, Izawa A, Wang P, Mukhopadhyay D, Koss KE, Geehan CS, Luster AD, Sayegh MH, Briscoe DM. Proinflammatory functions of vascular endothelial growth factor in alloimmunity. J Clin Invest. 2003. 112(11):1655-65.
Shim JW, Madsen JR. VEGF Signaling in Neurological Disorders. Int J Mol Sci. 2018. 19(1). pii: E275.
Shweiki, D., Itin, A., Soffer, D., and Keshet, E. Vascular endothe- lial growth factor induced by hypoxia may mediate hypoxia-initiated angiogenesis. Nature. 1992. 359:843–845.
Shweiki, D., Neeman, M., Itin, A., and Keshet, E. Induction of vas- cular endothelial growth factor expression by hypoxia and by glucose deficiency in multicell spheroids: implications for tumor angiogenesis. Proc. Natl. Acad. Sci. U. S. A. 1995. 92:768–772.
Somanath PR, Malinin NL, Byzova TV. Cooperation between integrin alphavbeta3 and VEGFR2 in angiogenesis. Angiogenesis. 2009. 12:177–185.
Takahashi T, Ueno H, Shibuya M. VEGF activates protein kinase C-dependent, but Ras-independent Raf-MEK-MAP kinase pathway for DNA synthesis in primary endothelial cells.
Teran M, Nugent MA. Synergistic Binding of Vascular Endothelial Growth Factor-A and its Receptors to HeparinSelectively Modulates Complex Affinity. J Biol Chem. 2015. 290(26):16451-62.
Tong Q, Zheng L, Lin L, Li B, Wang D, Huang C, Li D. VEGF is upregulated by hypoxia-induced mitogenic factor via the PI-3K/Akt-NF-kappaB signaling pathway. Respir Res. 2006. 2;7:37.
Van de Veire S, Stalmans I, Heindryckx F, et al. Further pharmacological and genetic evidence for the efficacy of PlGF inhibition in cancer and eye disease. Cell. 2010. 141:178-90.
Wada S, Tsunoda T, Baba T, et al. Rationale for antiangiogenic cancer therapy with vaccination using epitope peptides derived from human vascular endothelial growth factor receptor 2. Cancer Res. 2005. 65:4939-46.
Yoo SA, Bae DG, Ryoo JW, Kim HR, Park GS, Cho CS, Chae CB, Kim WU. Arginine-rich anti-vascular endothelial growth factor (anti-VEGF) hexapeptide inhibits collagen-induced arthritis and VEGF-stimulated productions of TNF-alpha and IL-6 by human monocytes. J Immunol. 2005. 174(9):5846-55.
Zlokovic, B.V. Neurovascular pathways to neurodegeneration in Alzheimer’s disease and other disorders. Nat. Rev. Neurosci. 2011. 12, 723–738.