8. Angiogenesis Flashcards
1
Q
Define angiogenesis.
A
Angiogenesis is the formation of a new blood vessel from pre-existing blood vessels
2
Q
What are the different ways of making blood vessels?
A
- Angiogenesis (sprouting) - used in wound healing or the menstrual cycle
- Vasculogenesis (bone marrow progenitor cell) - used in development in the embryo
- Arteriogenesis (collateral growth) - dependent on shear stress and external factors like macrophages
3
Q
Describe the process of angiogensis.
A
- To begin with, there is a need for new blood vessels (usually the result of hypoxia)
- Growth factors are released that activate endothelial cells in the pre-existing capillaries (this happens in small vessels)
- The endothelial cells undergo a conformational change where they go from being part of a very organised monolayer, to sending out filopodia and begin to migrate towards the growth factors
- To allows the endothelial cells to do this, the cytoskeleton of the tip cell must be modified and it need to control the interaction with neighbouring cells at cell-cell junctions
- The tip cells will keep on moving until they find another tip cell, with which they will fuse
- The tip cell themselves don’t divide, they require their neighbouring cells to divide behind them to push the tip cells towards the growth factor
- Eventually, the tip cell will meet another tip cell and it will fuse and stabilise
4
Q
Explain the regulation of angiogenesis.
A
- When you form a new blood vessel, you need to destabilise the pre-existing blood vessel and then restabilise it
- There are activators and inhibitors of angiogenesis - a balance of these two groups regulates angiogenesis
- There are some proteins/regulators that are absolutely essential e.g. VEGF - a loss of one allele of VEGF is incompatible with life
- The loss of other, less important, regulators of angiogenesis could still mean that vasculature develops but it wont be quite normal
5
Q
Explain how angiogenesis is triggered and supressed during hypoxia and normoxia.
A
- HYPOXIA is the main trigger for angiogenesis
- There is a transcription factor called HIF (hypoxia-inducible transcription factor), which is important for the regulation of genes involved in angiogenesis.
- Under normal conditions, HIF in inhibited by Von Hippel-Lindau (a tumour suppressor gene)
- As HIF is inhibited in normoxia, it does not drive the expression of angiogenesis genes.
- When there is hypoxia - Von Hippel-Landau does NOT bind to HIF so HIF is mobilised and it can translocate into the nucleus and drive the expression of genes involved in angiogenesis
- One of the targets of HIF is the expression of VEGF (vascular endothelial growth factor)
6
Q
Name the VEGF family members and the receptors.
A
- VEGF is, by far, the best known pro-angiogenic growth factor
- There are 5 members of the VEGF family:
- VEGF-A
- VEGF-B
- VEGF-C
- VEGF-D
- PIGF (placental growth factor)
- There are 3 tyrpsone kinase receptors for VEGF:
- VEGFR-1
- VEGFR-2
- VEGFR-3
- There are 2 coreceptors for VEGF:
- Neuropilin-1 (Nrp1)
- Neuropilin -2 (Nrp2)
- VEGFR-2 is the major mediator of VEGF-dependen angionesis - it activates signalling pathways that regulate endothelial cell migration, survival and proliferation
7
Q
Describe sprouting angiogenesis.
A
- In sprouting angiogenesis, specialised endothelial tip cells lead the outgrowth of blood-vessel sprouts towards gradients of VEGF
- Once a tip cell has been selected, it seems to control the behaviour of the cells around it via cell-cell communication
- There is a pathway called Notch that is crucial for the selection of tip cells
8
Q
Describe the canonical notch signalling pathway.
A
- This is crucial for the selection of tip cells.
- Binding of the notch ligand to the notch receptor activates the receptor by cleaving the intracellular domain (NICD)
- NICD then translocates to the nucleus where it binds to the transcription factor RBP-J and regulates transcription
- When a tip cell is chosen, it begins to express notch ligand which binds to the stalk cells’ notch receptors and tells them that ‘I am the tip cell, you are the stalk cells’
- The stalk cells then begin to divide and push the tip cell towards the growth factor
- Notch ligand = Delta-like ligand 4 (Dll4)
- VEGF will activate endothelial cells in a capillary and increase expression of Dll4
- Dll4 then drives the notch signalling, and inhibits the expression of VEGFR2 in the adjacent cell
- By doing this, the cells on either side of the tip cell will then recognise their role as stalk cells that have to divide and push the tip cell forward
9
Q
How to the sprouts progress forward once tip and stalk cells have been identified?
A
- The cells will interact with the ECM and there will be guidance systems in place
- Macrophages also have an important role in vessel anastomosis (both physiological and pathological)
- Macrophages have been shown to carve out tunnels in the ECM, thereby providing avenues for subsequent capillary infiltration
- Tissue-resident macrophages were shown to be associated with angiogenic tip cells during anastomosis
- So macrophages appear to help stabilise newly formed vessels (by promoting tip cell fusion)
10
Q
What is involved in stabilisation of the new vessel?
A
- Once the tip cells have fused and the stalk cells are separating for form a patent tube, the new vessel needs to stabilise
- Stabilisation involves reforming the endothelial monolayer barrier and recruiting neural cells (pericytes) and switching off the active angiogenesis process
- The endothelial cells bind in a homophilic way via proteins on the membranes
- Cadherin is an important protein that lines the junctions of endothelial cells
- VE-Cadherin is essential for vessel stabilisation and quiescence
- The homophilic interaction between the cadherins on the endothelial cells mediates the adhesion between endothelial cells and is important in intracellular signaling
- The cadherin interactions are also important in contact inhibition of cell growth
- The cadherins also promote survival of the endothelial cells
- Pericytes are eventually recruited, which produce a load of proteins that are involved in stabilising the vessel.
11
Q
A
12
Q
Define mural cells.
A
Generally refers to smooth muscle cells and pericytes, both of which are involved in the formation of normal vasculature and are responsive to VEGF
13
Q
Why are pericytes important in the stabilisation of new blood vessels?
A
- Pericytes are important in the stabilisation of new blood vessels because they produce proteins such as Angiopoietin 1, that goes on to control junctional systems e.g. Notch system
- The angiopoietin/Tie-2 system is specific to the endothelium
14
Q
Describe the angiopoietin-Tie2 Ligand receptor system.
A
- The angiopoietin-Tie2 system is required to modulate the activation and return to quiescence of endothelial cells
- Tie2 is a receptor that can bind to Angiopoietin 1
- Angiopoietin 1, when it binds to Tie2, promotes quiescence in the vasculature
- Angiopoietin 2, is the one that gets released when you need to form new blood vessels or when you need to respond to inflammation or when the vasulature needs to be destabilised
- So Angiotensin-2 antagonises Ang-1 signalling and has pro-angiogenic effects
15
Q
Define angiogenic switch.
A
Angiogenic Switch- there’s a point at which the tumour gets to a certain size where diffusion is no longer sufficient, so some cells within the tumour become hypoxic and send angiogenic signals.
16
Q
Describe tumour blood vessels.
A
- These are NOT properly formed because the signals are not physiological - there is an imbalance in the signals that are regulating angiogenesis
- So in tumours haemorrhage is common
- Tumour vessels can be:
- 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 the bone marrow
19
Q
What are the 2 main modes of unconventional resistance to VEGF blockage?
A
- The tumour adopts an evasive strategyand adapts to bypass the specific angiogenic blockade
- Intrinsic or pre-existing difference - the idea that a particular tumour in particular place in a particular person, was not very sensitive to VEGF anyway, so knocking out VEGF made little difference
20
Q
In what other diseases can angiogenic therapy be used?
A
