Angiogenesis Flashcards
What basic mechanisms regulate angiogenesis?
Inhibitors - thrombospondin-1, statins Activators - VEGFs, FGFs, PDGFB, EGF, LPA Hypoxia-inducible factor Notch signalling Angiopoietin/Tie-2 system
What role does angiogenesis play in health and disease?
- Angiogenic switch drives progressive tumour growth as newly vascularised tumour no longer relies solely on diffusion from host vasculature
- Tumour BVs are improperly formed bc signals are not physiological - imbalanced –> haemorrhage is common
- Ang1/Tie2 binding inhibits inflammatory gene expression
What can anti-angiogenic therapy be used for?
Normalise vasculature: reduces hypoxia, increases efficacy of conventional drug delivery
Summarise the main cellular pathways involved in angiogenesis
- Trigger - e.g. hypoxia
- GFs released activate endothelial cells in pre-existing capillaries
- Endothelial cells undergo conformational change – part of organised monolayer –> send out filopodia and begin to migrate towards GFs
- To allow endothelial cell to do this, cytoskeleton of tip cell must be modified and needs to control interaction w/neighbouring cells at cell-cell junctions
- Tip cells keep on moving until they find another tip cell
- When tip cell meets another tip cell, they fuse and stabilise
How does hypoxia-inducible factor (HIF) contribute to the regulation of angiogenesis?
Transcription factor important in regulation of genes involved in angiogenesis
Normoxia - inhibited by Von Hippel-Lindau (TSG)
Hypoxia - VHL does not bind to HIF, HIF mobilised and can translocate into nucleus and drive gene expression (e.g. VEGF)
Summarise the main molecular pathways involved in angiogenesis
- In stable BVs, DII4 (notch ligand) and Notch signalling maintain quiescence
- Tip cell selection
- VEGF activation increases Dll4 expression in tip cell
- DII4 drives Notch signalling, which inhibits expression of VEGFR2 in adjacent cell
- DII4-expressing tip cell acquires a motile, invasive, sprouting phenotype
- Adjacent stalk cells form base of emerging sprout, proliferate to support sprout elongation
How does binding of Notch ligand activate the Notch receptor (DII4)?
Cleaving of intracellular domain (NICD)
NICD translocates to nucleus
NICD binds to TF RBP-J, regulates transcription
Once the tip cell and stalk cells have been identified, how do the cells progress forwards?
Cells interact w/ECM - guidance systems in place
Macrophages carve out tunnels in ECM to provide avenues for subsequent capillary infiltration
What happens once the tip cells fuse?
Stalk cells separate to form a patent tube
New vessel stabilisation:
Reform endothelial monolayer barrier - VE-cadherin important
BM deposition
Recruit pericytes
Switch off active angiogenic process
Why are pericytes important in the stabilisation of new blood vessels?
Produce proteins, e.g. angiopoietin 1
Go on to control junctional systems, e.g. Notch
Angiopoietin 1, when bound to Tie2, promotes quiescence in vasculature
How does the angiopoietin/Tie-2 ligand-receptor system control stability of BVs?
Angiopoietin 1 (released from pericytes), when bound to Tie-2, promotes quiescence in vasculature = stabilisation
Angiopoietin 2 is released when you need to form new BVs or respond to inflammation or vasculature needs to be destabilised
Ang-2 antagonises ang-1 signalling and has pro-angiogenic effects
Do tip cells divide?
NO, they require the stalk cells to divide behind them to push tip cells towards GF
What problems often occur in tumour blood vessels?
- Irregularly shaped, dilated, tortuous
- Not organised into definitive venules, arterioles and capillaries
- Leaky and haemorrhagic, partly due to the over-production of VEGF
- Perivascular cells often become loosely associated
- Some endothelial cells may recruit endothelial progenitor cells from bone marrow
How are anti-angiogenic therapies likely to be used in cancer treatment in the future?
Anti-angiogenic therapy in combination w/other anti-cancer therapies
Resistance: combinatorial strategies involving angiogenesis inhibition and drugs targeting resistance mechanisms
Novel non-VEGF targets
What is angiogenesis?
The formation of new blood vessels from pre-existing blood vessels
Why do problems often occur in tumour BVs?
Angiogenic signals are not physiological
Imbalance in signals regulating angiogenesis
How can anti-angiogenic therapy be used in cancer?
Normalise tumour blood vessels to reduce hypoxia and improve efficiency of drug delivery
What is the problem with anti-angiogenic therapy being used in cancer?
If the therapy is aggressive, the ability of blood vessels to deliver other drugs to the tumour could be damaged
What could pro-angiogenic therapy be used for in the future?
Ischaemic disease
Vascular repair
Tissue engineering
What is angiogenic switch?
The point at which a tumour gets to a certain size (>1mm3) where diffusion of oxygen and nutrients from host vasculature is no longer sufficient, so some cells within the tumour become hypoxic and release angiogenic factors
It drives progressive tumour growth as newly vascularised tumour no longer relies solely on diffusion from host vasculature
