Chapter 10: Angiogenesis (Main) Flashcards
What is the definition of angiogenesis
Formation of capillary vessels from pre-existing blood vessels
Which cells form the inner lining of all blood vessels?
Endothelial cells, they are the key players in angiogenesis
Angiogenesis is a multistep endothelial cell process. What are the different steps (5)?
- Quiescent/resting stage
- Vessel dilation and pericyte detachment (= unstable blood vessel)
- Degradation of matrix and basement membrane
- Cellular migration and proliferation
- Lumen formation, basement membrane formation and pericyte attachment. Fusion of blood vessel sprouts and further vessel maturation (=functional blood vessel)
Red = endothelial cell
Purple = basal membrane
Green = supporting cells / parasites
What are the different reasons why we would need angiogenesis? (and also label these ‘good’ and ‘bad’)
- “Good” reasons:
- Pregnancy, embryogenesis
- Wound healing
- Inflammation
- “Bad” reasons:
- Cardiovascular disorders
- Inflammatory diseases
- Cancer
Why is angiogenesis so important in cancer?
Because tumors (primary and metastases) require blood vessels to grow beyond 2-3mm^2 in size
What is dormancy?
Dormancy is a stage in cancer progression where the cells cease dividing but survive in a quiescent state while waiting for appropriate environmental conditions to begin proliferation again.
How is the balance between activators and inhibitors that dictate the angiogenic activity called?
The angiogenic balance/switch
What is the most important angiogenesis activator?
Vascular Endothelial Growth Factor A (VEGF-A)
Fill in: VEGF-A is frequently highly/poorly expressed by tumor cells
VEGF-A is frequently highly expressed by tumor cells
Which combination of VEGFR will lead to vasculogenesis and angiogenesis?
VEGFR-2 (with VEGF-A)
Which two types of signaling are activated upon activation of the VEGFR?
The AKT signaling and the RAS-RAF signaling
What do the AKT and RAS-RAF signaling induce?
The AKT-signaling pathway inhibits apoptosis and activates vascular permeability. The RAS-RAF-signaling pathway activates migration and proliferation
What three types of phenotypes can an endothelial cell take on due induction of VEGF?
The phalanx cells, tip cells and stalk cells
What are the phenotypes/functions of the phalanx cells, tip cells and stalk cells?
- Phalanx cells: the resting/quiescent endothelical cell.
- Tip cells: the migratory endothelial cell at the forefront of the sprout.
- Stalk cells: the proliferative endothelial cells just behind the tip cell.
Which pathway regulates the tip/stalk phenotype?
VEGF/DLL4/Notch signaling
Explain the process of VEGF/DLL4/Notch signaling
Cells with high VEGFR-2 will turn into tip cells and increase expression of secretion of DLL4. The underlying cells have more Notch and more sensitive to DLL4. Activation of Notch will lead to activation of proliferation thereby the stalk cell. At the same time stalk cells will express more VEGFR-1 which makes them less sensitive to VEGF thereby preventing these cells to switch to a tip cell phenotype
What drives VEGF expression?
Hypoxia (lack of oxygen). This can be due to a too high consumption or too low delivery of oxygen
Which transcription factor is activated when there is hypoxia?
Hypoxia inducible factor-1 (HIF-1)
Note: this TF activates 100 genes, of which VEGF
What is the opposite term (antonym) of hypoxia?
Normoxia (=normal levels of oxygen)
Explain how in normoxic conditions, angiogenesis is inhibited
In presence of oxygen, propyl 4-hydroxylase will hydrolyse the HIF-1-alfa. This hydroxylated state can recruit VHL (amongst other proteins) which will ubiquitinate HIF-1-alfa. HIF-1-alfa will then be degraded, so no transcription of angiogenesis is induced.
Explain how in hypoxic conditions, angiogenesis is induced
There are low levels of oxygen -> HIF-1-alfa is not hydroxylated -> no ubiquitination of HIF-1-alfa -> transcription of angiogenesis is induced
Normally, if angiogenesis has occurred, there is more oxygen and thus a feedback loop exists. This is important so that there is not continuous angiogenesis. A tumor will evade this process. How?
Because of the ‘architecture’ and characteristics of the blood vessel network. It is immature, leaky, tortuous and has irregular flow. This creates continuous hypoxia, leading to distinct molecular characteristics
There are five different therapeutic strategies for angiogenesis, but we will only be discussing cell activation and vessel targeting. What are the three therapeutics for cell activation?
(for those interested, the other three are ECM degradation, cell migration and cell proliferation)
VEGF scavengers, VEGF receptor antagonists and VEGF receptor blockers
What do the VEGF scavengers, VEGF receptor antagonists and VEGF receptor blockers do?
- VEGF scavengers – so that it can no longer bind to the receptor
- VEGF receptor antagonist – block the VEGF binding site on the receptors
- VEGF receptor blockers – Tyrosin kinase inhibitors (TKIs); molecules that prevent activation of the kinase domain intracellular
There are five different therapeutic strategies for angiogenesis, but we will only be discussing cell activation and vessel targeting. How does vessel targeting work?
Because tumor vessels are distinct from normal vessels, and have specific ‘tumor endothelial cell markers’, which can be targeted to destroy the existing tumor vasculature.
Unfortunately, the benefit of the drugs are limited because resistance often occurs in the treated patients. Which two mechanisms are in play for this resistance?
- Recruitment of bone-marrow derived cells
- Vessel cooption and vasculogenic mimicry
Definitions:
Vessel co-option: Vessel co-option is a non-angiogenic process through which tumour cells utilize pre-existing tissue blood vessels to support tumour growth, survival and metastasis.
Vasculogenic mimcry: Vasculogenic mimicry is the formation of microvascular channels by aggressive, metastatic and genetically deregulated tumour cells. This process differs from angiogenesis in that it occurs de novo without the presence of endothelial cells (tumour cells line tumour vessels effectively mimicking a true vascular endothelium)
(other factors that are in play here are growth factors redundancy, vessel stabilization by local stromal cells and increased invasiveness and metastasis, but you don’t have to know that for the exam)
How is the recruitment of bone-marrow derived cells achieved (a form of resistance of angiostatic drugs)
Recruitment of bone-marrow derived cells
-
Myeloid derived suppressor cells.
- Express angiogenic factors (that are not VEGF)
- Induce angiogenic phenotype in immune cells
-
Endothelial progenitor cells.
- Vasculogenesis
How is the vessel cooption and vasculogenic mimicry achieved (a form of insensitivity/less sensitive of angiostatic drugs)
Vessel co-option: Vessel co-option is a non-angiogenic process through which tumour cells utilize pre-existing tissue blood vessels to support tumour growth, survival and metastasis.
Vasculogenic mimcry: Vasculogenic mimicry is the formation of microvascular channels by aggressive, metastatic and genetically deregulated tumour cells. This process differs from angiogenesis in that it occurs de novo without the presence of endothelial cells (tumour cells line tumour vessels effectively mimicking a true vascular endothelium)
Vessel cooption
- Migration and growth of tumor cells along pre-existing vasculature.
Vasculogenic mimicry
- Tumor cell adaptation towards endothelial phenotype which allows formation of tumor cell lined vessels
The total network of blood vessels is immense (about 84,000 km). By using blood vessels markers, tumor blood vessels can be found because these markers are different from ‘regular’ blood vessels. What are the two blood vessel markers that are used regularly?
There are about 7 different kinds, of which the two you have to know are Vimentin and Galectin-1
True/false: During angiogenesis, endothelial cells proliferate and migrate in the direction of an angiostimulatory signal.
True
Put in the correct order:
A. HIF1a is degraded.
B. HIF1a is ubiquitinated.
C. HIF1a is hydroxylated.
C-B-A
HIF1a is hydroxylated -> HIF1a is ubiquitinated -> HIF1a is degraded.
