Angiogenesis Flashcards
What is angiogenesis?
Formation of a new blood vessel from pre-existing blood vessels
What is vasculogenesis?
Early stages of blood vessel formation during development (bone marrow progenitor cells have a big contribution)
What is the first organ system that forms in embryo?
Blood vessels
What is the most common way in which new blood vessels are formed?
Angiogenesis e.g. wound healing or menstrual cycle
What is arteriogenesis?
Collateral growth that is dependent on shear stress and external factors like macrophages
What is normally the stimulus for angiogenesis?
Need for new blood vessels- mainly hypoxia
What happens in response to need for new blood vessels?
Growth factors are released that activate endothelial cells in pre-existing capillaries (small vessels)
What happens to the endothelial cells after being activated?
They undergo a conformation change where they go from being part of a very organised monolayer to sending out filopodia and begin to migrate towards growth factors
What needs to happen of endothelial cells to be able to send out filopodia and migrate?
Cytoskeleton of tip cell must be modified and it needs to control the interaction with neighbouring cells at cell-cell junctions
What will cause tip cells to stop moving?
They will keep moving until they find another tip cell with which they will fuse
How do tip cells move?
They themselves don’t divide, they require their neighbouring cells to divide behind them to push the tip cells towards the growth factor
What animal has helped advance knowledge of angiogenesis?
Zebrafish embryos
What activates of angiogenesis are there?
VEGFs FGFs PDGFB EGF LPA
What inhibitors of angiogenesis are there?
Thrombospondin-1
Statins
What happens one allele of VEGF is lost?
It is incompatible with life
What is HIF?
Hypoxia induced transcription factor which is important for regulation of genes involved in angiogenesis
What happens to HIF under normal conditions?
HIF is inhibited by Von Hippel-Lindau (a tumour suppressor gene) so doesn’t drive expression of angiogenesis
What happens to HIF under hypoxic conditions?
Von Hippel-Lindau doesn’t bind to HIF so HIF is mobilised and it can translocate into the nucleus and drive expression of genes involved in angiogenesis
What does VEGF mean?
Vascular endothelial growth factor- by far the best known pro-angiogenic growth factor
How many members of the VEGF family are there?
5 VEGF-A VEGF-B VEGF-C VEGF-D PIGF
How many tyrosine kinase receptors for VEGF are there?
3
VEGFR-1
VEGFR-2
VEGFR-3
How many coreceptors for VEGF are there?
Neuropilin-1
Neuropilin-2
What is the major mediator of VEGF-dependent angiogenesis?
VEGFR-2 - it activates signalling pathways that regulate endothelial cell migration, survival and proliferation
What leads the outgrowth of blood vessel sprouts towards gradients of VEGF?
Specialised endothelial tip cells
By what pathway are tip cells selected?
Notch
Where is the notch signalling pathway found?
In other tissues other than endothelia
How does the notch ligand binding activate the notch receptor?
By cleaving the intracellular domain (NICD)
What does NICD do once cleaved?
It translocates to the nucleus where it binds to the transcription factor RBP-J and regulates transcription
What happens when a tip cell is chosen in terms of notch?
It begins to express notch ligand which binds to the stalk cells’ notch receptors and tells them that it’s the tip cell and they are stalk cells
What do stalk cells do once they know they are stalk cells?
They divide and push tip cell towards the growth factor
What is the notch ligand also known as?
Delta-like ligand 4 (DLL4)
What does VEGF do?
Activate endothelial cells in a capillary and increases expression of DLL4
What does DLL4 do?
Drives notch signalling and inhibits expression of VEGFR2 in adjacent cells which means cells on either side recognise role as stalk cells and push tip cell forward
How do macrophages have an important role in vessel anastomosis (physiologically and pathologically)?
They have been shown to carve tunnels in the ECM thereby providing avenues for subsequent capillary infiltration and tissue resident macrophages are associated with angiogenic tip cells during anastomosis so help stabilise newly formed vessels by promoting tip cell fusion
Once the new vessel has formed due to tip cells fusing and stalk cells separating to form a patent tube, how does the vessel stabilise?
It involves reforming the endothelial monolayer barrier and recruiting neural cells (pericytes) and switching off the active angiogenesis process
How do endothelial cells form a cohesive monolayer?
They have junctions between each other formed by proteins on membranes of both cells involved that bind in a homophillic way
What is cadherin?
An important protein that lines the junctions of endothelial cells
What is VE-cadherin essential for?
Vessel stabilisation and quiescence
What does the homophilic interaction between the cadherins on endothelial cell mediate and what is it important in?
It mediates the adhesion between endothelial cells and is important in intracellular signalling
What else are cadherin interactions important in?
Contact inhibition of cell growth and promoting survival of endothelial cells
What is eventually recruited to stabilise a new vessel?
Pericytes which produce a load of proteins that are involved in stabilising the vessel
What are mural cells?
It generally refers to smooth muscle cells and pericytes both of which are involved in the formation of normal vasculature and are responsive to VEGF
What is an important protein in stabilisation of new blood vessels that pericytes produce?
Angiopoietin 1 which goes on to control junctional systems
What is the aniopoietin-Tie2 system required for?
To modulate the activation and return to quiescence of endothelial cells
What is Tie2?
A receptor which can bind to angiopoietin 1
What does angiopoietin 1 do when bound to Tie2?
Promotes quiescence in the vasculature
When is angiopoietin 2 released?
When you need to form new blood vessels or when you need to respond to inflammation or when vasculature needs to be destabilised
What effect does Ang 2 have on Ang1 signalling?
It antagonises it and has pro-angiogenic effects
How do tumours <1mm3 receive oxygen and nutrients?
By diffusion from host vasculature- when larger than this they require new vessel networks
How do tumours stimulate formation of new blood vessels?
They secrete angiogenic factors
What is the key point in tumour development in terms of neovasculature?
Angiogenic switch- point at which tumour gets to a certain size where diffusion is no longer sufficient so some cells within the tumour become hypoxic and send angiogenic signals
Why are tumour blood vessels not properly formed?
Signals aren’t physiological- there is an imbalance in the signals that are regulating angiogenesis so in tumours haemorrhage is common
What can tumour blood vessels be like?
Irregularly shaped, dilated and tortuous
Not organised into definitive venues, arterioles and capillaries
Leaky and haemorrhagic activity, partly due to overproduction of VEGF
Perivascular cells often become loosely associated
Some may recruit endothelial progenitor cells from bone marrow
What effects does anti-angiogenic therapy have on tumours?
Can help normalise tumour blood vessels but if you go for very aggressive anti-angiogenic therapy, you could end up damaging the ability to deliver other drugs to the tumour
What is Avastin?
Anti-VEGF humanised MAb (also called Bevacizumab)
What are the problems with avastin?
It has relatively limited efficacy and it has many side effects: GI perforation Hypertension Proteinuria Venous thrombosis Haemorrhage Wound healing complications No overall survival advantage over chemo alone No quality of life or survival advantage
What causes these side effects of avastin?
VEGF is essential for homeostasis of endothelium
What are the two main modes of unconventional resistance to VEGF blockade?
Tumour adopts an evasive strategy and adapts to bypass specific angiogenic blockade
Intrinsic or pre-existing difference- idea that a particular tumour in a particular place in a person wasn’t sensitive to VEGF so knocking out VEGF made little difference
What patients were massively benefited by anti-angiogenic treatments?
Age-related macular degeneration
What is lucent is?
Anti-VEGF therapy for age-related macular degeneration (AMD)
What is the main cause of blindness?
AMD
What is the difference between avastin and lucentis?
Not a lot, lucentis is a slightly modified form of avastin but avastin works just as well as lucentis and is much cheaper
What is tissue engineering of blood vessels used for?
Vascular graft for coronary and bypass surgery
Vascular networks for organ regeneration
