**3. Signalling in the VEGF pathway**

Binding of VEGF to the extracellular domain of VEGFR2 causes receptor dimerisation and phosphorylation of the receptor on tyrosine residues within the

#### **Figure 1.**

*Signalling from VEGFR2. The signalling cascades downstream of VEGFR2 have been the best studied and are illustrated here. VEGF binding to VEGFR2 induces dimerisation of VEGFR2 and phosphorylation of tyrosine residues (indicated by the four-digit numbers in the illustration). Pathways activated include the Ras-Raf-MEK-ERK pathway, PLCγ/PKC pathway and the PI3K/Akt pathway. Activation of downstream signalling from VEGF receptors exerts control over multiple processes required for angiogenesis including endothelial cell proliferation, migration, tube formation and vascular permeability.*

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cell maturation [20].

*DOI: http://dx.doi.org/10.5772/intechopen.89667*

pathways [15].

leakage [17].

**5. VEGF immunomodulation**

**4. Sprouting angiogenesis**

*Angiogenesis and Its Role in the Tumour Microenvironment: A Target for Cancer Therapy*

intracellular domain (**Figure 1**) [11]. The Y1054 and Y1059 residues, which lie within the kinase domain, become phosphorylated in response to VEGF stimulation. These positively regulate the intrinsic kinase activity of the intracellular domain and signal to phospholipase-Cγ (PLCγ), which in turn leads to VEGFR2 internalisation [12]. The Y1175 and Y1214 residues lie in the carboxyl terminal tail. These residues become highly phosphorylated in response to VEGF. Y1214 signalling leads to endothelial cell migration and Y1175 signalling leads to PLCγ and extracellular related kinase 1/2 (ERK1/2) activation that is required for DNA synthesis and cell proliferation [13]. Activation of ERK1/2 requires the Ras-Raf-MEK-ERK1/2 signalling cascade but may also require the PLCγ/PKC/PKD pathway [14]. The roles of Y951 and Y996 residues, which lie in the kinase insert region, have not been definitively determined, but Y951 phosphorylation has been shown to increase endothelial cell migration and proliferation via both the PLC-γ and PI3K

According to the established dogma, VEGF released by tumours stimulates the growth of new vessels in the following way. The VEGF diffuses through the tissue and activates endothelial cells located in local blood vessels. Firstly, VEGF receptor activation induces the selection of sprouting endothelial cells. Proteinases such as urokinase-type plasminogen activator, uPA, and members of the matrix metalloproteinase (MMP) family mediates the dissolution of the vascular basement membrane and extracellular matrix to facilitate the infiltration of sprouting endothelial cells into the surrounding tissue [16]. Next, endothelial proliferation, migration and branching allows for the formation of new vessels. This is followed by sprout fusion and lumen formation where vessels fuse together to form a network. Finally, there is perfusion and maturation. This is where the stabilisation of new blood vessels forms a functionally perfused system, which is mediated by the recruitment of pericytes to surround the newly formed endothelial tubes; recruitment of pericytes prevents further endothelial cell proliferation and migration and also suppresses vessel

Multiple possible mechanisms exist regarding immunosuppressive effects of VEGF on the tumour microenvironment. Firstly, due to the effect of VEGF on tumour vasculature, T cell migration from lymph nodes to the microenvironment may be impaired. Furthermore, the ability of T cells to migrate through vessels is negatively affected by VEGF through the down regulation of vascular endothelial selectins, adhesion molecules and promotion of Fas ligand expression. Secondly, VEGF binding to its receptor on myeloid derived suppressor cells within the tumour microenvironment results in STAT 3 signalling, with subsequent promotion of Treg cells and the down regulation of tumour specific T cells [18]. Additionally, the binding of VEGF to VEGFR2 has effects including reduced activation of cytotoxic CD8+ and CD4+ T cells, as well as the upregulation of inhibitory receptors including PD1 and CTLA4 [19]. The interaction of VEGF with VEGFR may also upregulate the programmed death ligand 1 (PDL1) on dendritic cells (DCs) [20]. Furthermore, the binding of VEGF to VEGFR1 on dendritic cells has the effect of inhibiting dendritic

*DOI: http://dx.doi.org/10.5772/intechopen.89667 Angiogenesis and Its Role in the Tumour Microenvironment: A Target for Cancer Therapy*

intracellular domain (**Figure 1**) [11]. The Y1054 and Y1059 residues, which lie within the kinase domain, become phosphorylated in response to VEGF stimulation. These positively regulate the intrinsic kinase activity of the intracellular domain and signal to phospholipase-Cγ (PLCγ), which in turn leads to VEGFR2 internalisation [12]. The Y1175 and Y1214 residues lie in the carboxyl terminal tail. These residues become highly phosphorylated in response to VEGF. Y1214 signalling leads to endothelial cell migration and Y1175 signalling leads to PLCγ and extracellular related kinase 1/2 (ERK1/2) activation that is required for DNA synthesis and cell proliferation [13]. Activation of ERK1/2 requires the Ras-Raf-MEK-ERK1/2 signalling cascade but may also require the PLCγ/PKC/PKD pathway [14]. The roles of Y951 and Y996 residues, which lie in the kinase insert region, have not been definitively determined, but Y951 phosphorylation has been shown to increase endothelial cell migration and proliferation via both the PLC-γ and PI3K pathways [15].
