Tumour Angiogenesis, Invasion & Metastasis Flashcards
Characteristics of malignant tumours
Growth
Invasiveness
Metastasis
Growth of malignant tumours
Unlimited growth (not self-limited as in benign tumours)- as long as an adequate blood supply is available
Invasiveness of malignant tumours
Migration of tumour cells into the surrounding stroma where they are free to disseminate via vascular or lymphatic channels to distant organs
Metastasis of malignant tumours
Spread of tumour cells from the primary site to form secondary tumours at other sites in the body
The sequential process of metastasis
1) Transformation (mutagenic and epigenetic changes) of normal cell, becoming tumorigenic and starts proliferating
2) After cells get to a certain size, tumour angiogenesis is initiated (new blood vessels migrate and develop around that growing tumour).
3) Tumour cells become motile and invade into capillaries, venules and lymphatic vessels (intravasation) after epithelial-mesenchymal transition, to spread to other regions/organs
4) In order for them to form a micrometastasis, they need to move out of the vessel via extravasation into the organ parenchyma
5) Will eventually lodge and proliferate/colonise in a distant organ
Angiogenesis
formation of new blood vessels from pre-existing vessels
Vasculogenesis
formation of new blood vessels from progenitors (in early embryo development)
Types of angiogenesis
Developmental/Vasculogenesis
-for organ growth
Normal Angiogenesis
- wound repair
- placenta during pregnancy
- cycling ovary
Pathological Angiogenesis
- tumour angiogenesis
- ocular and inflammatory disorders
Tumour growth without its own blood supply
Tumours will generally not grow beyond a size of about 1-2mm3 without their own blood supply.
What stimulates tumour angiogenesis?
tumour hypoxia
-low oxygen tension <1% O2
tumour hypoxia activates transcription of genes involved in angiogenesis, tumour cell migration and metastasis
Tumour hypoxia increases with…
increasing distance from the capillaries
Process of tumour angiogenesis
1) Small tumour is at first self-sustaining
2) Tumour then becomes hypoxic when it reaches a certain size and starts to secrete angiogenic growth factors (e.g. VEGF) which stimulate new blood vessel growth
3) Angiogenic growth factors initiate endothelial cells within nearby capillary to proliferate and migrate
4) When enough endothelial cells are being stimulated this way, new vessels start to form from the nearby capillary and develop around the tumour, resulting in a blood vessel network around the tumour
5) This allows the tumour to grow in different angles
6) A cell can also then escape from the primary tumour through the vascular network to other regions of the body, providing a route of metastatic spread
Tumour angiogenic factors
Some tumour cells produce factors that stimulate the directional growth of endothelial cells:
· Vascular Endothelial Growth Factor (VEGF)
· Fibroblast Growth Factor-2 (FGF-2)
· Transforming Growth Factor β (TGF-β)
· Hepatocyte Growth Factor/Scatter Factor (HGF/SF)
Fate of tumour angiogenic factors
Secreted by tumour cells
OR
Stored bound to components of the extracellular matrix and may be released by enzymes called matrix metalloproteinases
Vascular Endothelial Growth Factor (VEGF) Signalling
The VEGF receptor is a tyrosine kinase receptor which dimerizes upon ligand binding and activates the:
- Ras/Raf/MEK pathway
- PI3 Kinase/AKT pathway
- Phospholipase C pathway
Effect of mechanisms of tumour cell motility & invasion
· Increased mechanical pressure caused by rapid cellular proliferation
· Increased motility of the malignant cells (epithelial to mesenchymal transition)
· Increased production of degradative enzymes by both tumour cells and stromal cells
Process of epithelial-mesenchymal transition
Epithelial cancer cells have certain expression of cell-cell adhesion molecules and other types of cell apparatus that enables them to keep that morphology:
· genes that make them an epithelial cell become downregulated
· epithelial cells lose expression of cell-cell adhesion molecules (e.g. E-cadherins)
At the same time, in order for them to change into mesenchymal cancer cells, they need to upregulate a number of cell signalling pathways that stimulate this process and also cause them to become more motile. These pathways drive and upregulate the expression of specific genes that provide the characteristics of a fibroblast cell:
>the phenotype of the cell starts to change (morphology and polarity changes, becoming more invasive)
>transition allows the cell to become motile and to invade
What is lost in epithelial-mesenchymal transition?
Loss of:
- Epithelial shape and cell polarity
- Cytokeratin intermediate filament expression
- Epithelial adherens junction protein (E-cadherin)
What is gained in epithelial-mesenchymal transition?
Acquisition of:
- Fibroblast-like shape and motility
- Invasiveness
- Vimentin intermediate filament expression
- Mesenchymal gene expression (fibronectin, PDGF receptor, avb6 integrin)
- Protease secretion (MMP-2, MMP-9)
What is E-cadherin?
a homotypic adhesion molecule (adhesion of cells with the same cadherin)
What is E cadherin dependent on?
calcium
Function of E-cadherin
binds β-catenin and inhibits invasiveness
-hence it is lost and downregulated in cancer cells during the epithelail-mesenchymal transition
Significance of E-cadherin downregulation in cancer cells
E-cadherins facilitate contact inhibition. Once cells start to touch each other contact inhibition comes into play, where cells recognise closeby cells and then stop proliferating.
If E-cadherins are lost or get mutated (e.g. in cancer cells), cells can’t recognise when they are close to each other and you lose that contact inhibition. Therefore, proliferation doesn’t stop and cells grow on top of each other, resulting in disrupted cell-cell adhesion.
Integrins
transmembrane receptor adhesion molecules acquired in cancer cells
-heterodimers (⍺ & β subunits)
What do integrins adhere to? And how?
Adhesion to extracellular matrix
-via collagen, fibronectin, laminin
Function of integrins
Cell Migration:
-alpha and beta subunits/adhesion molecules will bind to extracellular matrix and signal into the cell. The cell can acquire some invasive properties and gets more adhesive and can move through the matrix
How do stromal cells contribute to tumour progression?
Factors released by stromal cells (macrophages, mast cells, fibroblasts) surrounding cancer cells include angiogenic factors, growth factors, cytokines, proteases.
>E.g. Urokinase-type plasminogen activator (uPA) activated by tumour cells, resulting in plasmin production
Significance of plasmin in tumour progression
Plasmin activates matrix metalloproteinases (MMPs) which permit invasion by degrading extracellular matrix (ECM) thus releasing matrix-bound angiogenic factors.
Activity of Urokinase-type plasminogen activator (uPA)
1) Stromal cell releases inactive pro-uPA
2) Pro-uPA will bind to receptor on cancer cell called uPAR (urokinase plasminogen activator receptor) and uPA is activated
3) Active uPA then converts plasminogen to plasmin
4) Plasmin is a proteolytic enzyme important in fibrin dissolution, but also important in mediating cell migration.
5) Plasmin can activate pro-MMPs to active MMPs to facilitate ECM degradation, and also activate latent growth factors contained in the extracellular matrix
Cancer Dissemination
Tumours don’t decide for themselves where they are going to end up, but we do see that certain tumours home to certain distant sites (unclear why).
The overall process of metastases is highly inefficiency:
Tumour cells can extravasate successfully (>80%) but the last two steps are very inefficient (<0.02% of cells actually form micrometastases).
What determines the pattern of tumour spread?
Mechanical Hypothesis
· Anatomical considerations: blood and lymphatic systems, entrapment in capillary beds (20-30um carcinoma cell, ~8um capillary)
Seed and Soil Hypothesis
· There are specific adhesions between tumour cells and endothelial cells in the target organ, creating a favourable environment in the target organ for colonisation
· Genetic alterations acquired during progression allow tumour cells to metastasize
Targeting tumour angiogenesis to inhibit cancer
Success with targeted therapy to angiogenic factors like vascular endothelial growth factor (VEGF)
-e.g. avastin
Targeting tumour cell motility to inhibit cancer
No success with targeting cell-cell adhesion molecules or integrins
Targeting tumour invasion to inhibit cancer
All clinical trials with matrix metalloproteinases have been unsuccessful in reducing tumour burden
Avastin
Monoclonal antibody which binds VEGF
- prevents VEGF binding to VEGF tyrosine kinase receptors on endothelial cells
- no downstream signalling pathways, inhibiting angiogenesis
*approved for colorectal, lung, kidney and ovarian cancers and eye diseases
