Angiogenesis Flashcards
What are the three ways of making blood vessels?
Vasculogenesis – formation of new blood vessels from bone marrow progenitor cells
Angiogensis – formation of new blood vessels by sprouting from pre-existing vessels
Arteriogenesis – collateral growth of blood vessels that is dependent on shear stress and external factors like macrophages
What is the main trigger for angiogenesis?
Hypoxia
What is the most essential pro-angiogenic factor?
VEGF (Vascular Endothelial Growth Factor)
State the 5 steps describing the process of sprouting angiogenesis
- Endothelial activation => tip/stalk cell selection
- Tip cell navigation and stalk cell proliferation
- Branching coordination
- Stalk elongation, tip cell fusion, and lumen formation
- Perfusion and vessel maturation.
Explain the mechanism by which hypoxia triggers angiogenesis.
HIF (hypoxia-inducible factor) is a transcription factor that is responsible for the expression of genes involved in angiogenesis
- In normoxic conditions, Von Hippel Lindau protein (encoded by a tumour suppressor gene) binds to HIF-α => hydroxyproline and ubiquitin attachment => degredation by proteasome => NO ANGIOGENESIS
- In hypoxic conditions, HIF-α doesn’t bind to von Hippel Lindau so it can associate with HIF-β => expression of genes (e.g. VEGF) => ANGIOGENESIS
How many members are there in the VEGF family? List them.
5
VEGF-A, B, C, D, and PIGF (placental growth factor)
How many tyrosine kinase receptors are there for VEGF? List them.
3
VEGFR 1, 2 and 3
Note: VEGFR3 has disulphide bridge in its ligand binding domain
How many coreceptors are there for VEGF? List them.
2
Neuropilin 1 and 2 (Nrp 1/2)
Which receptor is the major mediator of VEGF-dependent angiogenesis, activating signalling pathways that regulate endothelial cell migration, survival, proliferation.
VEGFR2
What pathway is crucial for the selection of tip cells?
Canonical Notch signalling pathway between adjacent endothelial cells at the angiogenic front
Describe the notch signalling pathway
- Notch receptors and ligands are membrane-bound proteins that interact through their extracellular domains.
- This interaction triggers a series of proteolytic cleavages of the Notch receptor
- The final one, catalysed by the g-secretase complex, releases the active Notch intercellular domain (NICD) from the cell membrane
- NICD translocates to the nucleus and binds to the transcription factor RBP-J => inhibition of expression of VEGF receptors in the cell
What is another name for the notch ligand?
Delta-like ligand (Dll4)
What effect does VEGF have on notch signalling?
It increases expression of Dll4
Dll4 then drives Notch signalling, which inhibits expression of VEGFR2 in the adjacent cell
Dll4 expressing tip cells develop a motile, invasive and sprouting phenotype
Adjacent stalk cells form the base of the emerging sprout and proliferate to support sprout elongation
The tip cells are those cells which receive the ‘highest dose’ of VEGF and so responds by migrating according to the VEGF gradient
Note that initially, Dll4 and Notch signaling are thought to be balanced between adjacent endothelial cells until presumptive tip cells eventually increase Dll4 expression
After appropriate stimulation, what else is required for tip cell formation?
Tip-cell formation requires degradation of the basement membrane, pericyte detachment and loosening of endothelial cell junctions
State how the tip cells are able to navigate and migrate
Tip cells navigate in response to guidance signals (such as semaphorins and ephrins) and adhere to the extracellular matrix (mediated by integrins) to migrate.
How do proliferating stalk cells become stabilised?
Proliferating stalk cells attract pericytes and deposit basement membranes to become stabilized
State the recruited immune cells involved in the angiogenic process and state their role
Recruited myeloid cells such as tumour-associated macrophages (TAMs) and TIE-2-expressing monocytes (TEMs) associated with tip cells can produce pro-angiogenic factors or proteolytically liberate angiogenic growth factors from the ECM.
Macrophages carve out tunnels in the extra cellular matrix (ECM), providing avenues for capillary infiltration.
Describe the stabilisation and quiescence after fusion of neighbouring branches
Lumen formation allows perfusion of the neovessel, which resumes quiescence by promoting a phalanx phenotype, re-establishment of junctions, deposition of basement membrane, maturation of pericytes and production of vascular maintenance signals.
Which cell adhesion molecules are essential for vessel stabilisation and quiescence? Why?
VE-Cadherin (Constitutively expressed at junctions)
- Controls contact inhibition of cell growth
- Promotes survival of EC
What growth factor do pericytes produce that is important for stabilisation of new blood vessels?
Angiopoietin 1 (Ang-1)
How does Ang-1 carry out its function?
Ang-1 is constitutively released by pericytes and binds to Tie-2 receptors almost exclusively expressed by endothelial cells.
=> Promotes vessel stability and inhibits inflammatory gene expression
Where is Angiopoietin 2 released from and what triggers its release? How does it affect Ang-1 signalling?
Ang-2 is stored in Weibel–Palade bodies (in endothelial cells) and is rapidly secreted upon inflammatory stimulus.
=> Antagonises Ang-1 signalling, promotes vascular instability and VEGF-dependent angiogenesis
State three diseases in which Ang-2 plasma levels are raised
Congestive heart failure
Sepsis
Chronic Kidney Disease
How do tumours less than 1mm^3 receive oxygen and nutrients? How do larger tumours overcome this problem?
Receive oxygen and nutrients by diffusion from host vasculature.
Larger tumors require new vessel network so secrete angiogenic factors
What is the name given to the point at which a tumour begins to initiate signals to generate new vasculature? What does its timing in the tumour-progression pathway depend on? What does the switch begin with?
Angiogenic switch
It can occur at different stages in the tumour-progression pathway, depending on the nature of the tumour and its microenvironment.
The switch begins with perivascular detachment and vessel dilation
What are some of the characteristics of tumour blood vessels?
- Irregularly shaped, dilated, tortuous
- Not organised into definitive venules, arterioles and capillaries
- Leaky and haemorrhagic, partly due to the overproduction of VEGF
- Perivascular cells often become loosely associated
Note that pericytes loosely associated with tumour-associated blood vessels favours chronic leakage in tumours; this is enhanced by angiopoietin 2.
What do Cancer-associated fibroblasts (CAFs) do to promote tumour angiogenesis? Where do they come from?
Under the influence of transforming growth factor-β (TGFβ) and other tumour-derived factors, peri-tumoural fibroblasts differentiate into CAFs
=> They secrete extracellular matrix and pro-angiogenic growth factors
Describe the role of platelets in tumour angiogenesis
Tumours cause platelet activation, aggregation and degranulation;
Activated platelets release pro-angiogenic mediators and proteases that support the proliferation and activation of CAFs, such as PDGFB and TGFβ
Also, angiostatic molecules: thrombospondin 1, plasminogen activator inhibitor 1 (PAI1), endostatin
What is the aim of anti-angiogenic therapy in cancer?
To normalise tumour blood vessels to reduce hypoxia and improve efficiency of drug delivery
What are the consequences of being too aggressive with anti-angiogenic therapy?
This can make the tumour blood supply inadequate for the delivery of chemotherapeutic drugs
What is avastin?
Anti-VEGF humanised mouse antibody
Also called bevacizumab
What are the side effects of avastin?
GI perforation Hypertension Proteinuria Venous thrombosis Haemorrhage
What are the two main methods of unconventional resistance to VEGF blockade?
Tumour adopts evasive strategy to bypass the angiogenic blockade
Intrinsic or pre-existing difference – a tumour may not have been particularly sensitive to VEGF in the first place
What did avastin start getting used for other than cancer?
Diabetic retinopathy
Age-related macular degeneration (AMD); Abnormal growth of choroidal blood vessels => “Leaky” vessels cause oedema => Visual impairment
What slightly modified form of avastin became licensed by the FDA to treat AMD?
Lucentis (ranibizumab)
But Avastin is much cheaper
What can pro-angiogenic therapies potentially be used for?
Ischaemic diseases; MI, peripheral ischaemic disease
