tumour angiogenesis, invasion and metastasis

Question 1

characteristics of malignant tumours

Answer 1

Growth
Unlimited growth (not self-limited as in benign tumours) - as long as an adequate blood supply is available

Invasiveness
Migration of tumour cells into the surrounding stroma where they are free to disseminate via vascular or lymphatic channels to distant organs

Metastasis
Spread of tumour cells from the primary site to form secondary tumours at other sites in the body

Question 2

key steps in cancer progression

Answer 2

❶ Transformation: extensive mutagenic and epigenetic changes followed by clonal selection
❷ Angiogenesis: new blood vessel formation (overcomes limitations imposed by hypoxia)
❸ Motility and invasion: epithelial to mesenchymal transition (invasive properties allowing intravasation into circulation and extravasation from circulation to tissues)
❹ Metastasis: colonisation of target organs (ability to expand from micrometastases)

Question 3

what is angiogenesis

Answer 3

formation of new blood vessels from pre-exisiting vessels

vasculogenesis is the formation of new blood vessels from progenitors

Question 4

describe types of angiogenesis

Answer 4

1. developmental/vasculogenesis = organ growth
2. normal angiogenesis = wound repair placenta during pregnancy cycling ovary
3. pathological angiogenesis = tumour angiogenesis, ocular and inflammatory disorders

Question 5

neovascularisation of tumours

Answer 5

numerous blood vessels have infiltrated into the tumour tissue

tumours will not grow beyond a size of about 1-2mm without their own b blood supply

Question 6

tumour hypoxia

Answer 6

Hypoxia is a strong stimulus for tumour angiogenesis

Hypoxia – low oxygen tension <1% O2

Increases with increasing distance from capillaries

Activates transcription of genes involved in angiogenesis, tumour cell migration and metastasis

Question 7

angiogenic factors

Answer 7

Some tumour cells produce factors that stimulate the directional growth of endothelial cells:

Vascular Endothelial Growth Factor (VEGF)
Fibroblast Growth Factor 2 (FGF 2)
Placental growth factor (PlGF)
Angiopoietin 2 (Ang 2)

These factors are secreted by tumour cells or are stored bound to components of the extracellular matrix and may be released by enzymes called matrix metalloproteinases:

Matrix metalloproteinase 2 (MMP-2)

Question 8

vascular endothelial growth factor signalling

Answer 8

VEGF binds to VEGF-R2 on endothelial cells ❶
VEGF/VEGF-R2 dimerizes at the plasma membrane and recruits cofactors ❷ that subsequently activate 3 major signal transduction pathways ❸
Ultimately, VEGF activates cell survival, vascular permeability, gene expression and cell proliferation
All of these pathways are essential for angiogenesis ❹

Question 9

mechanism of tumour cell motility and invasion

Answer 9

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

Question 10

epithelial mesenchymal transition

Answer 10

Loss of:
Epithelial shape and cell polarity (β-catenin, claudin-1)
Cytokeratin intermediate filament expression
Epithelial adherens junction protein (E-cadherin)
Acquisition of:
Fibroblast-like shape and motility
Invasiveness
Vimentin intermediate filament expression
Mesenchymal gene expression (fibronectin, PDGF receptor, αvβ6 integrin)
Protease secretion (MMP-2, MMP-9)

Question 11

cell adhesion molecules and invasion

Answer 11

E-Cadherins
Homotypic adhesion molecule (adhesion of cells with the same cadherin)
Calcium-dependent
Inhibits invasiveness
Binds β-catenin

Question 12

stromal cell contribution to tumour progression

Answer 12

Factors released by stromal cells (macrophages, mast cells, fibroblasts) include angiogenic factors, growth factors, cytokines, proteases
Example: Urokinase-type plasminogen activator (uPA); activated by tumour cells - resulting in plasmin production
Plasmin activates matrix metalloproteinases (MMPs), which permit invasion by degrading extracellular matrix (ECM) and releasing matrix-bound angiogenic factors such as transforming growth factor-β1 (TGF-β1)

Question 13

steps involved in cancer dissemination

Answer 13

primary tumour formation
localised invasion
intravasation
transport through circulation
arrest in micro vessels of various organs 
extravasation 

The overall process is highly inefficient:
Tumour cells can extravasate successfully (>80%) but the last two steps are very inefficient (<0.02% of cells actually form micrometastases).

Question 14

what determines the pattern of tumour spread

Answer 14

Mechanical Hypothesis
Anatomical considerations: Blood and lymphatic systems, entrapment in capillary beds (20-30µm carcinoma cell, ~8µm capillary)
Seed and Soil Hypothesis
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

Question 15

targeting tumour angiogenesis, cell motility and invasion to inhibit cancer

Answer 15

Tumour angiogenesis
Success with targeted therapy to angiogenic factors like vascular endothelial growth factor

Cell motility
No success with targeting cell-cell adhesion molecules

Invasion
All clinical trials with matrix metalloproteinases have been unsuccessful in reducing tumour burden!

Question 16

Avastin

Answer 16

First specific anti-angiogenesis drug
in 2013 was the second biggest selling oncology product
Approved for colorectal, lung, kidney and ovarian cancers and eye diseases

Question 17

mechanism of action of bevacizumab/avastin

Answer 17

monoclonal antibody
binds to VEGF
prevents VEGF binding to VEGF receptors on endothelial cells