VEGFR Flashcards
VEGF roles and ligand isoforms
A-F VEGF found (E and F are virally encoded from snake venom). Also is placental growth factor (PIGF)
VEGF-A is most potent angiogenic stimulator. Secreted by endothelial cells, fibroblasts, SM, platelets, neutrophils, macrophages, and from 60% of solid tumours (unregulated). VEGF-A levels regulated by hypoxia, ischaemia, and inflammatory mediators.
Endogenous roles of VEGF ligands are angiogenesis, endothelial proliferaton, migration, survival and vascular permeability. Roles in exercise, wound healing, menstruatoion, granulation formation, and neuroprotection.
VEGF-A structure and alternative splicing
consists of 7 B-strands that are highly twisted, 2 a-helixes. 3 intramolecular disulphide bonds. Links to other monomers of VEGF by intermolecular disulphide bonds. Hydrophobic interactions perpendicular to the plane of the B-strands enhances monomer association.
There are 16 isoforms of VEGF-A that arise from alternative splicing. They differ in length, bioavailability, and their ability to act at VEGFRs. Major splicing sites include Exon 6/7 and exon 8a/8b.
VEGFxxxa (e.g., xxx=165, 121, or 189) are isoforms of VEGF that are proangiogenic.
VEGFxxxb (e.g., xxx=165,121,189) are antiangiogenic by regulating the activity of the pro-angiogenic VEGF isoforms.
What regulates the alternative splicing of VEGF-A
Involves RNA binding proteins such as serine/arginine (SR) rich splicing factors (SRSF1, SRSF2, SRSF5, SRSF6). These are activated by phosphorylation by SR protein kinase 1 (SRPK1).
The phospho-SRs translocate to the nucleus and bind to regulatory sites at the exon boundaries - triggers exon removal.
Cell type, other growth factors, and which SRSF binds influence the splicing favoured.
VEGFR1 role and which ligands bind
PIGF, VEGF-B, and VEGF-A bind.
Found on vascular endothelial cells. Also found on monocyte/macrophage-lineage cells that promote their mobilisation from the bone marrow. Some potential roles in embryonic vascularisation and chemo-induced neuropathic pain.
VEGF165a is more selective for it than VEGFR2.
It has weaker RTK activity than VEGFR2.
It can also have its EC domain cleaved, so it acts as a ‘VEGF trap’. It will still bind VEGF, activity. Negatively affects the VEGF activity at VEGFR2.
VEGFR2 role and ligands binding
VEGF-A, -D, -E, -F.
prototypical VEGFR.
vasular and lymphatic endothelial cells, particularly in the leading edge of the tip cells in angiogenic sprouting.
Also interacts with neuropilin-1.
VEGFR3 role and ligands binding
binds VEGF-C, -D
Preferentially found on lymphatic endothelial cells. develops retina and involved in cancer angiogenesis + lymphatic node metastasis.
VEGF-C signalling favours Akt-ERK signalling, promoting lymphangiogenesis
VEGF-D found to contribute to lymphatic metastasis in cancer
Different factors needed in activation of VEGFRs + tytosine residues responsible
ATP and Mg2+ needed for the intrinsic activity of VEGFRs.
ATP binds in the cleft between the 2 domains of the glycine rich loop.
Mg2+ is chelated by a DFG motif in the activation loop.
Y1054/Y1059 Tyr residues is where initial auto-phosphorylation occurs
VEGFR signalling cascades, and dimerisation role in it
Signals through PLCgamma, Grb2-Ras-Raf-MAPK, and PI3K
Can form dimers: VEGF1 homo, VEGFR1/2, VEGFR2 homo, VEGFR2/3, and VEGFR3 homo.
VEGFR1/2 negatively regulates VEGFR2, possibly by PI3K activity.
VEGFR2/3 has role in angiogenic sprouting.
Co-receptors for VEGFRs features
Neuropilin1 and neuropilin2 (NRP1/NRP2)
NRP1 expressed in arterial endothelial cells. unregulated in some cancers - correlating with aggressiveness of cancer. role in axonal guidance.
NRP2 expressed in lymphatic and venal endothelial cells. Overexpression increases tumour lymphangiogenesis.
NRP1 binds VEGF165a, VEGF189a, VEGF206a, VEGF-B, VEGF-E and PIGF. Cannot bind shorter VEGF-A isoforms.
NRP2 binds VEGF-A (low affinity), VEGF-C and VEGF-D.
Role of NRP co-receptors on VEGFR
NRP1 can mediate the trafficking of VEGFRs on the same cell into the cytoplasm (down regulation). If the NRP1 is on an adjacent cell to the VEGFR, it inhibits endocytosis of the VEGFR.
Therapeutic targeting of VEGF system examples
Monoclonal antibodies. High selectivity, but long-term efficacy issues. low stability, expensive, and have side effects (e.g., hypertension, and proteinuria). have suffix -ab
e.g., bevacuzimab (anti-VEGFA) and ramucirumab (anti-VEGFR2)
RTKIs are cheaper and effective, but much less selective. high side effects profile, and resistance can arise. RTKIs have suffix -ib.
e.g, axitinib (type 1), sorafenib (type 2), and vatalinib (covalent)
Combination therapies of both classes above have been used.
Some drugs are also being developed that target NRPs.
