Angiogenesis Flashcards
what are the physiological reasons for angiogenesis?
embryonic development
wound healing
menstrual cycle
how can angiogenesis turn out to be in pathology?
o Insufficient – e.g. baldness, MI.
o Involved in vascular malformations – e.g. Angiodysplasia (HHT & VWD).
o Excessive – e.g. Retinal disease, cancers, atherosclerosis.
what are the methods of creating new blood vessels and describe their action?
- vasculogenesis= use of bone marrow progenitor cells
- arteriogenesis= collateral growths to accommodate occlusions
- angiogenesis= sprouting dependent on endothelial cells
what is the outline of sprouting angiogenesis?
- Selection of sprouting ECs (tip cells and stalk cells)
- Sprout outgrowth and guidance.
- Sprout fusion and lumen formation.
- Perfusion and maturation.
name the initial trigger of angiogenesis
hypoxia
name the regulator of gene expression to promote angiogenesis
Hypoxia-Inducible Transcription Factor (HIF)
leads to the expression of Hypoxia inducible Genes e.g. VEGF
what controls the levels of HIF?
protein Von Hippel-Lindau
- tumour suppressor gene (pVHL)
what happens with pVHL and HIF in the absence of oxygen
1) in hypoxia pVHL does not bind to HIF
2) HIF alpha and HIF beta associated to create a transcription factor
3) allows HIF TF to translocate to the nucleus and bind to the HIF-region
this induces the translation of hypoxic factors e.g. VEGF that enable angiogenesis
what happens with pVHL and HIF in the presence of oxygen?
hypoxic trigger removed
1) so pVHL adds a hydroxyproline group to HIF (ubiquination)
2) HIF is degraded by a proteasome
no angiogenesis
pVHL is anti-angiogenesis
what does VEGF stand for?
vascular endothelial growth factor
what are the 5 members of the VEGF family?
VEGF-A/B/C/D and Placental GF (PlGF)
what are the 3 receptors VEGF can bind to? what are the co-receptors they need?
VEGFR-1/2/3
these require the co-receptors Nrp1 and 2
which of the VEGF receptor is the main mediator of VEGF-dependant angiogenesis?
VEGFR-2
is the major mediator of VEGF-dependant angiogenesis.
what is the first step of angiogenesis?
the selection of the endothelial tip cell occurs which lead the outgrowth of vessels towards gradients of VEGF
how is the tip cell selected?
the cell that receives the highest concentration of VEGF becomes the tip cell
by Canonical Notch Signalling between adjacent endothelial cells
which cell would have the notch receptor?
the stalk cell (support cell)
adjacent to the tip cell with the delta/jagged ligand
what happens to the notch receptor once it binds to the jagged ligand of the tip cell?
the intracellular domain of Notch called NICD translocates to the nucleus and binds to the TF RBP-J
how is quiescence in the endothelial layer maintained when tip selection occurs?
stable state: the cells signal with DII4 and Notch to each other when VEGF comes to bind
when does the induction sprouting occur?
in unstable state: VEGF activation increases the expression of DII4 (on tip cell)
this drives Notch signalling in the adjacent cell
when the tip cell has been selected and starts to produce more DII4 (notch ligand) , what effect does this have?
this drives Notch signalling in the adjacent cell so it will reduce the expression of VEGFR-2 on the adjacent cell so VEGF can’t bind to it
the adjacent cells has now been given its role of stalk cell
what is the role of the stalk cells?
while the tip cell is motile, invasive and sprouting the stalk cells adjacent form the base of the sprout and proliferate to support sprout elongation
what cells are recruited to guide and support the growing sprout?
myeloid cells (stimulated by angiopoietin 2)
which cells play a role in angiogenesis anastomosis? what do they do?
macrophages
carve out tunnels in the ECM for subsequent capillary infiltration and support the structure during anastomosis
what facilities the stabilisation of the new vessels?
- a barrier formed made with tight junctions and adherins
- pericytes
what molecules are associated with the barrier formation during stabilisation of the vessels?
VE-Cadherins and Angiopoietin 1
what receptor does the angiopoietin-1 bind to?
TIE-2 receptors on the endothelial cells
what are the effects of Angiopoietin 1 binding to TIE-2?
- stabilises the vessel
- inhibits inflammatory gene expression
no more angiogenesis required so reach stable state
what are the roles of VE-cadherins?
o Constitutively expressed at junctions.
o Controls contact inhibition of cell growth.
o Promotes the survival of the EC.
what cells allow the stabilisation of the vessels?
mural cells
pericytes+ smooth muscles cells = mural cell
using the Angiopoietin/TIE system
what are the actions of Angiopoietin 2 on Tie-2?
they antagonise the receptor
what are the effects of Angiopoietin-2 binding to Tie-2?
antagonises Angiopoietin-1 signalling therefore leads to vascular instability and VEGF-dependant angiogenesis
in what pathologies is Angiopoietin-2 levels increased?
sepsis
CKD
congestive heart failure
high levels destabilises the vascular system
what size tumours are not dependent on angiogenesis for survival?
tumours less than 1mm^3 receive oxygen and nutrients by diffusion from the host
what is required by tumours greater than 1 mm^3?
“Angiogenic switch”
diffusion is insufficient for the tumor, so the tumour starts to produce angiogenic factors like VEGF so it can continue to grow
what does the tumour do when it need vessels?
secretes angiogenic factors (e.g. VEGF, Angiopoietin 2)
the “angiogenic switch” is driven by hypoxia
what are the characteristics of new vasculature associated with a tumour?
- Irregular shape
- Dilated, torturous.
- Not organised into arteries/veins/capillaries
- Leaky (overproduction of VEGF)
- Perivascular cells loosely associated
- May recruit endothelial progenitor cells from bone marrow.
what are the 4 methods that can be used to target the VEGF pathway?
Anti-VEGF antibodies – Avastin/Bevacizumab.
Soluble VEGF Receptors – VEGF-Trap.
Anti-VEGFR antibodies – IMC-1121b.
Small-molecule VEGFR inhibitors –
Vatalanib, Sunitinib.
what is avastatin?
what types of cancers is Avastin (with other drugs) used for?
anti VEGF antibodies
colorectal Cervical glioblastoma NSCLC ovarian
what are the disadvantages of Avastin?
1) has many side effects:
- GI perforation
- hypertension
- proteinuria
- VTE
- haemorrhage
- wound healing complications.
2) a limited efficacy: No quality of life or survival advantage over chemo alone.
how can sustained anti-angiogenic therapy actually have pro-tumour effect?
too much ischaemia leads to the release of hypoxic factors that further induce angiogenesis
vasculature may be inadequate for further delivery of drugs/oxygen
what is the aim for the vascular health in tumours?
normalised vasculature
- reduce hypoxia to reduce hypoxic factors
- increase efficacy of conventional therapies
what are the mechanisms of resistance to anti-VEGF? [4]
- VEGF inhibition –>more hypoxia–> release of OTHER angiogenic factors or increases tumour invasiveness
- Tumour vessels may be less sensitive to VEGF inhibition due to tumour cells lining the vessels.
- Tumour cells that recruit pericytes may be less responsive.
- Tumour cell vasculogenic mimicry (VM)- de novo vessel synthesis independent of endothelial vessels to start
what happens in tumour cell vasculogenic mimicry? what impact does this have on treatment?
the tumour cells remodel themselves to resemble vessels which then once perfused by a single vessel allow adequate nutrient delivery to the whole tumour.
Anti-angiogenic therapy is useless against vasculogenic mimicry
what happens in age-related macular degeneration (AMD) that leads to visual impairment?
Abnormal growth of choroidal blood vessels leads to leaky vessels causing oedema
what can be used to treat AMD?
AMD is the main cause of blindness
can be treated by off-licence Avastin but can people become refractory to it
what is the problem with creating anti-angiogenic drugs and what is a solution?
Problem – tumours are complex 3D structures that function in association with host vasculature and so is difficult to mimic on a 2D-cell line monolayer
Solution: “Tumour-on-a-chip” platform: a small system that incorporates human cells on a 3D ECM supported by perfused human micro-vessels
what will the selected tip cell now express? what is the purpose of it?
Delta Like Ligand 4
this will bind to the notch receptor on the adjacent cell to give it its stalk cell role
what is the role of angiopoietin 1?
bind to Tie-2 to promote vessel stability and inhibit inflammatory gene expression
what is the role of angiopoietin 2?
antagonise the action of Angipoietin 1 by binding to Tie 2 to promote vessel instability and therefore the need for VEGF dependent angiogenesis
when is Angiopoietin 2 released?
- when new vessel formation is required
- in response to inflammation
- to destabilise vessels
