Tumour angiogenesis Flashcards
What are the characteristics of malignant tumours?
- Growth
- Invasiveness
- Metastasis
What happens in the growth of malignant tumours?
• Unlimited growth (not self-limited as in benign tumours)
• Relies on adequate blood supply to prevent hypoxia and nutrients
-Hypoxia drives new blood vessel growth around tumour
What is invasiveness?
○ Migration of tumour cells into surrounding stroma where they are free to disseminate via vascular or lymphatic channels to distant organs
What is metastasis?
○ Spread of tumour cells from primary site to form secondary tumours at other sites in the body
Steps involved in metastasis
- Increased growth from one originating clonal cell
- Tumour needs nutrients and oxygen so induces new blood vessels to surround tumour
- Cells start to lose their characteristics
○ Become less epithelial like and change morphology/shape/mobility to become more motile - Invade into new blood vessels that have formed around tumour
- Circulating cells (aggregates/multiple cells) will lodge into distant sites
- Move out of capillary
- Aggregate of cells start to proliferate again and metastasise and so cycle begins again
What does the process of . cancer metastasis consist of?
• Process of cancer metastasis consists of sequential, interlinked and selective steps with some stochastic elements
What is the outcome of each step in metastasis influenced by?
• Outcome of each step is influenced by interation of metstatic cellular subpopulations with homeostatic factors
What is each of the metastatic cascade potentially?
Each step of the metastatic cascade is potentially rate limiting such that failure of a tumor cell to complete any step effectively impedes that portion of the process
What are key steps in cancer progression?
- Extensive mutagenic and epigenetic changes followed by clonal selection
- Angiogenesis (overcomes limitations imposed by hypoxia)
- Epithelial to mesenchymal transition (invasive properties allowing intravasation and extravasation)
- Colonisation of target organs (ability to expand from micrometastases)
- Release of metastatic cells that have acquired the ability to colonise
What is angiogenesis?
Angiogenesis is the formation of new blood vessels from pre-existing vessels
What is vasculogenesis?
Vasculogenesis is the formation of new blood vessels form new progenitors
What are the types of angiogenesis?
- Developmental/vasculogenesis
- Organ growth
- Normal angiogenesis
- Pathological angiogenesis
What happens in normal angiogenesis?
○ Wound repair
○ Placenta during pregnancy
○ Cycling ovary
What happens in pathological angiogenesis?
○ Tumour angiogenesis
○ Ocular and inflammatory disorders
Steps involved in tumour angiogenesis
• Tumour divides a number of times before it gets to a certain size
• Outgrows existing nutrients
○ Tumour becomes hypoxic
• Hypoxia is a stimulus for tumour to start secreting angiogenic factors
• Factors released by tumours that act on nearby capillaries
• Nearby capillary starts to have tip formation in the vessel (own proliferation get upregulated) so new blood vessels form around tumour
• When cells shed off they can escape through local blood supply and develop micro-metastases
Neovascularization of tumours
- Tumours will not grow beyond a size of about 2mm without their own blood supply as cells cannot survive the lack of oxygen (hypoxia)
- Angiogenesis (development of a new blood supply) is promoted by hypoxia
What is a strong stimulus for tumour angiogenesis?
• Hypoxia is a strong stimulus for tumour angiogenesis
What is hypoxia?
• Hypoxia – low oxygen tension <1% O2
How does hypoxia increase?
• Increases with increasing distance from capillaries
What does hypoxia activate?
• Activates transcription of genes involved in angiogenesis, tumour cell migration and metastasis
-TF gets stabilised
Examples of 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)
How are angiogenic factors mainly stored and released by what enzyme?
• These factors are mainly stored bound to components of the extracellular matrix and may be released by enzymes called matrix metalloproteinases
Steps involved in vascular endothelial growth factor(VEGF) signalling
• Cancer becomes hypoxic
• Hypoxia results in VEGF secretion out of tumour
• VEGF is not an autocrine GF for the tumour, it is a GF for blood vessels
• VEGF binds to VGEF receptor (tyrosine kinase receptor) in endothelial cells
○ Switches on downstream signalling
○ These pathways are upregulated in tumour cells and drive growth of endothelial cells
○ Binding results in increased survival of endothelial cells (do not apoptose) – activate PKB/AKT pathway, gene transcription is also switched on, vessels become leaky and permeable to allow cells to move in between tumour and endothelial cell compartment
What are mechanisms of tumour cell invasion?
• Tumour has closely packed cells
○ Cells move away through downregulation of adhesion molecules
○ Cells break away from nearby network of cells and upregulate other proteases that allow cells to move away from where it has been proliferating
• 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
What is the epithelial-mesenchymal transition?
• Go from epithelial shaped cell to more mesenchymal
What is there a loss of in the epithelial-mesenchymal transition?
• Loss of
○ Epithelial shape and cell polarity
○ Cytokeratin intermediate filament expression
○ Epithelial adherens junction protein (E-cadherin)
§ No more tight junctions
§ Allows cells to break off
What is there an acquisition of in epithelial-mesenchymal transition?
○ Fibroblast-like shape and motility
§ Cell is more elongated
○ Invasiveness
○ Vimentin intermediate filament expression
○ Mesenchymal gene expression (fibronectin, PDGF receptor, αvβ6 integrin)
○ Protease secretion (MMP-2, MMP-9)
○ Important for degradation of extracellular matrix and move in
What do E-Cadherins allow?
○ Allows cell adhesion (Cell-cell contact)
What do E-cadherins form?
Formation of tight junctions
Why do we get rid of tight junctions?
§ Need to get rid of these for cells to break off
What type of dimers are E-cadherins?
Homodimers
What are E-Cadherins dependent on?
Calcium dependent
What does E-Cadherin inhibit?
Inhibits invasiveness
What do E-Cadherins bind?
Bind beta-Catenin
Normal cell adhesion vs Disrupted cell adhesion?
○ Normal cell = tight junction and these adhesion molecules are responsible for this
○ Disrupted cell-cell adhesion = cells grow on top of each other whereas before there would be a monolayer of normal cells
What are integrins?
Cell-matrix adhesion molecules
What type of dimers are integrins?
○ Heterodimers (α and β subunits)
What are integrins involved in?
- Adhesion to extracellular matrix
- Cell migration
What do factors released from stromal cells include and contribute to?
• Factors released by stromal cells (macrophages, mast cells, fibroblasts) include angiogenic factors, growth factors, cytokines, proteases and contribute to tumour progression
What do stromal proteins release?
• Stromal cells release proteases
-Example: Urokinase-type plasminogen activator (uPA); activated by tumour cells - resulting in plasmin production
Where are specific receptors for UPA found?
On tumour cells
What is activated when UPAr complex becomes activated?
• UPA and UPAr complex become activated and activate plasminogen
What does plasmin activated by UPAr complex activate and what does this lead to ?
• Plasmin activates downstream proteins i.e. matrix metalloproteinases (MMPs)
○ Permitting invasion by degrading extracellular matrix (ECM) thus releasing matrix-bound angiogenic factors
○ MMPs go on to degrade ECM
○ Increased plasmin also activates latent GFs
Steps involved in cancer dissemination?
- Primary tumour formation
- Localised invasion
- Intravasation
- Transport through circulation
- Arrest in microvessels of various organs
- Extravasion
- Formation of a micrometastasis
- Colonisation – formation of a macrometastasis
Comment on efficiency of cancer dissemination
Overall process is highly inefficient
What is the mechanical hypothesis in tumour spread?
○ Anatomical considerations: Blood and lymphatic systems, entrapment in capillary beds (20-30µm carcinoma cell, ~8µm capillary)
What is the seed and oil hypothesis of tumour spread?
○ 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.
What is tumour growth dependent on?
• Tumour growth dependent on new blood vessel growth
How can highly angiogenic tumours be targeted?
• Highly angiogenic tumours can be targeted by blocking blood vessels of the tumour causing regression to an extent
What is Avastin?
First specific anti-angiogenesis drug
Mechanism of Avastin
○ Monoclonal antibody
○ Binds to VEGF
○ Prevents VEGF binding to receptors on endothelial cells
○ Prevent intracellular phosphorylation of receptor so no dimerization and therefore no downstream signalling
