8. Angiogenesis Flashcards
What is angiogenesis physiologically essential for?
- Embryonic development
- Wound healing
- Menstrual cycle
What is vasculogenesis?
- Formation of blood vessels from scratch (angioblasts => endothelial cells)
- Involving bone marrow progenitors during development
What is angiogenesis?
Formation of new blood vessels, sprouting from a pre-existing vessel
What is arteriogenesis?
- Collateral growth
* May bypass a blockage
What is the name of something that triggers the activation of endothelial cells in angiogenesis?
Pro-angiogenic stimulus e.g. hypoxia
What happens to the cytoskeleton of specific selected endothelial cells in angiogenesis?
- Changes polarity
- Allows it to sense the outside world - allowing direction of blood vessel formation
- Cell needs to be in touch with nearby endothelial cells, to instruct them to also divide
What activates the endothelial cells in pre-existing capillaries to grow?
Growth factors => specific cells undergo a conformational change => tip cell
How does the organisation of the specific endothelial cells change once activated by growth factors?
From a very organised monolayer, to sending out filopodia
What molecules do the endothelial cells grow towards in angiogenesis?
Growth factors (VEGF)
Describe how tip cells move until they fuse?
- Tip cells don’t divide, they require neighbouring (stalk) cells to divide behind them and push them
- They keep on moving until they eventually fuse
What does it mean by the fact that regulators must be ‘balanced’ in angiogenesis?
- To form a new vessel, you need to destabilise then re-stabilise the pre-existing vessel
- Activators and inhibitors are involved in this
- A balance of the 2 regulates angiogenesis
What is VEGF and what does a loss of one allele of it lead to?
- Vascular endothelial growth factor (VEGF) - an activator
* Loss of one allele - incompatible with life
Describe the whole process of angiogenesis, starting with hypoxia
- Enough oxygen => Hypoxia-inducible factors (TF) is bound by pVHL (TSG)
- pVHL induces ubiquitination (inactivation), degrading HIF
- HIF is also inhibited by Von Hippel-Lindau (TSG)
1) If there is not enough oxygen, pVHL doesn’t bind to HIF
• HIF is also not inhibited by Von Hippel-Lindau
2) HIF is not degraded and enters the nucleus to find HIF-beta
3) This drives the transcription of genes that promote angiogenesis e.g. VEGF
4) VEGF activation increases expression of Dll4 on tip cell
5) Dll4 (notch ligand) on tip cell interacts with notch receptor on the stalk cell
6) Binding (through extracellular domains) activates it, cleaving the intracellular domain of the notch receptor (NICD)
7) NICD translocates to the nucleus and binds to the transcription factor RBP-J
8) This inhibits expression of VEGFR2 in the cell - so it can’t be the tip cell and becomes the stalk cell
9) Dll4-expressing tip cells acquire a sprouting phenotype
10) Adjacent cells (Stalk cells) form the base of the emerging sprout, and proliferate
What is the best-know pro-angiogenic growth factor?
VEGF
What are the 5 members of the VEGF family?
- VEGF-A
- VEGF-B
- VEGF-C
- VEGF-D
- PIGF (placental GF)
What are the 3 tyrosine kinase receptors for VEGF?
- VEGFR-1
- VEGFR-2
- VEGFR-3
What are the 2 co-receptors for VEGF?
- Neuropilin-1 (Nrp1)
* Neuropilin-2 (Nrp2)
Which receptor is the major mediator of VEGF-dependent angiogenesis?
VEGF-2
• activates signalling pathways that regulate endothelial cell migration, survival and proliferation
What is the first molecule that is targeted in anti-angiogenic therapy?
VEGF
What is ‘notch signalling’?
- A pathway, crucial for the selection of tip cells
- Promotes the distinction between the leading tip cell and the growing stalk cell
- Signalling occurs between adjacent endothelial cells at the angiogenic front - prevents all the cells in the area becoming tip cells
- Not specific to endothelial cells
What is the most important trigger for the transformation of endothelial cells from their quiescent state to rapidly proliferating stalk/tip cells?
VEGF
What is vascular anastomosis?
Connection between 2 blood vessels
What is the role of macrophages in vascular anastomosis?
- Carve out tunnels in the ECM
- Provide avenues for subsequent capillary infiltration
- Tissue-resident macrophages are associated with tip cells
Once the tip cells have fused, what does stabilisation involve?
- Reforming the endothelial monolayer barrier
- Recruiting mural cells (pericytes)
- Switching off the active angiogenesis process
How are myeloid cells in the retina useful in angiogenesis?
Wrap around the endothelial cells to help stabilisation after fusion
Where is VE-cadherin expressed and what is it’s role in angiogenesis?
(vascular-endothelial cadherin)
• Expressed at junctions
• Mediates adhesion between endothelial cells
• Controls contact inhibition of cell growth
• Promotes survival of endothelial cell
• Essential for stabilisation and quiescence
What cells can be used to stabilise the neovessels and what do they do?
- Mural cells - pericytes (+ smooth muscle cells)
- Pericytes wrap around the capillaries
- They produce a GF - angiopoietin-1 - this is a stabilising factor
What are the 2 antagonistic ligands of the Tie2 receptor and what do they do?
Angiopoietin-1 • promotes stability and quiescence • survival • anti-inflammatory • homeostatic
Angiopoietin-2
• pro-angiogenic
• released and helps during inflammatory response
• helps destabilise the blood vessel in angiogenesis
• antagonises angiopoietin-1
What is the angiopoietin-Tie2 system for?
Modulates the activation and return to quiescence of endothelial cells
Which molcule is ang-2 dependent on to work?
VEGF
When do tumours require new vessel networks to receive enough oxygen and nutrients?
When they grow larger than 1mm^(3)
How do tumours obtain new vasculature?
• Angiogenic switch - once diffusion is no longer sufficient and tumours cells become hypoxic, they send angiogenic signals
- can occur at different stages in the tumour-progression pathway
• Tumours secrete angiogenic factors
• Facilitates its progressive growth and further neovessel formation
How are tumour blood vessels different to normal blood vessels?
• Not properly formed because the signals are not physiological
• Imbalance of signals that regulate angiogenesis creates blood vessels that are:
- irregular, dilated, tortuous
- not organised into definitive venules, arterioles and capillaries
- leaky and haemorrhagic (partly due to over-production of VEGF)
- with loosely associated perivascular cells
How can angiogenesis be targeted in cancer and what is the aim in anti-angiogenic therapy?
- Targeting the VEGF pathways
- There is an anti-VEGF antibody
- Anti-angiogenic therapy can help normalise the tumour blood vessels
- However, aggressive therapy can damage the ability to deliver other drugs
- Aim is to reduce hypoxia and improve the efficiency of drug delivery (the main purpose of the drug)
- Reduces the risk of haemorrhage
What is avastin? Outline its efficacy and side effects.
• Anti-VEGF humanised MAb (mouse antibody) aka bevacizumab
• Limited efficacy
• No overall survival advantage over chemotherapy
• No quality of life
• Side effects:
- GI perforation
- hypertension
- proteinuria
- venous thrombosis
• Benefits are sometimes transitory and followed by a restoration of tumour growth and progression
• Reserved for very advanced cancer
Why do anti-VEGF antibodies cause so many unwanted effects?
VEGF is essential for the homeostasis of the endothelium
What are the 2 main modes of resistance to VEGF blockade by the tumour?
- Evasive strategy to bypass the specific blockade
* Intrinsic or pre-existing indifference - tumour is not actually sensitive to VEGF
In what case (non-cancer related) has anti-angiogenic treatment been beneficial in?
Age-related macular degeneration (AMD)
• Abnormal growth of choroidal vessels
• Leaky vessels cause oedema
• Visual impairment - main cause of blindness
• Treated with a modified form of Avastin => Lucentis
• Many patients stopped responding to treatment >2 years
• studies show that Avastin works just as well and is much cheaper, however it is not FDA approved
What could pro-angiogenic therapy be useful for in the future?
- Therapy following occlusion of an artery e.g. MI or PVD
* There have been attempts to inject VEGF into the cardiac tissue soon after occlusion to stimulate neovascularisation
What are the applications of the tissue engineering of blood vessels?
- Vascular graft for coronary and bypass surgery
* Vascular networks for organ regeneration
What in vitro disease models can be used to improve tumour modelling and drug screening, with reference to tumour angiogenesis?
“tumour-on-a-chip”
