VEGFA-dependent activation of VEGFR2 causes autophosphorylation and activation of the Axl receptor tyrosine kinase via Src-1:SH2D2A-dependent reaction. Phosphorylation of Axl tyrosine residues 779 and 821 (773 and 815 in mouse) is required for VEGFA-dependent binding of the p85-subunit of PI3K and activation of PI3K (Ruan & Kazlauskas, 2012).
Axl with its two phosphorylated YxxM motifs associates with the p85 subunit of PI3K and mediates VEGFA mediated activation of PI3K/AKT pathway (Ruan & Kazlauskas, 2012).
Binding of VEGFA to VEGFR2 induces receptor dimerization and autophosphorylation, leading to the recruitment of downstream signalling molecules. Once the two VEGFR2 receptors are cross-linked to each other, via simultaneous interaction with VEGFA dimer, their membrane-proximal Ig-like domain 7s are held in close proximity so that low-affinity homotypic interactions between these domains further stabilise the receptor dimers. This allows for the exact positioning of the intracellular kinase domains resulting in VEGFR2 autophosphorylation (Ruch et al. 2007, Holmes at al. 2007). The major tyrosine residues known to be autophosphorylated are Y801 and Y951 in the kinase-insert domain, Y1054 and Y1059 within the kinase domain, and Y1175 and Y1214 in the C-terminal tail of VEGFR (Dougher-Vermazen et al. 1994, Cunningham et al. 2007, Kendall et al. 1999, Matsumoto et al. 2005). The Y1175 (mice Y1173) is crucial for endothelial and haemopoietic cell development. Mice with muatation Y1173F die between E8.5 and E9.5 from lack of endothelial and haemopoietic development (Sakurai et al. 2005).
AXL/UFO (Tyrosine-protein kinase receptor UFO) is a member of the TAM (Tyro3/Axl/Mer) family of receptor tyrosine kinases (RTKs). AXL has been implicated in angiogenesis because of its ability to promote angiogenically related cellular responses in endothelial cells (Holland et al, 2005). AXL is required for VEGFA-dependent activation of PI3K. Activated Src family kinases recruit AXL via its juxtamembrane domain and thereby trigger ligand-independent autophosphorylation of AXL that promotes association with PI3K and activation (Ruan & Kazlauskas 2012).