10 - Invasion and Metastasis Flashcards
Neoplasia
An abnormal mass of tissue, the growth of which exceeds and is uncoordinated with that of normal tissues, and persists in the same excessive manner after apparent cessation of the stimuli which evoked the change
Benign
Neoplasm lacking ability to invade and metastasize
Benign macroscopic features
Generally rounded, well circumscribed, encapsulated tumours
Benign microscopic features
Banal features (uniform cells, well differentiated, low mitotic rate)
Malignant
- Neoplasm with capacity for local invasion and metastasis
- Very inefficient and therefore few circulating tumor cells initiate successful metastatic colonies
Malignant macroscopic features
Generally infiltrative (crab like), not encapsulated, hard, fibrous
Malignant microscopic features
- Pleomorphism (different appearance between cells)
- Anaplasia (look nothing like cells they’re derived from)
- Hyperchromasia
- Mitoses (numerous and abnormal forms)
Invasion
Process whereby tumour cells disobey organ boundaries and cross into foreign tissue
Metastasis
Secondary tumour established at site distant from primary tumour
Steps of invasion
- Altered cell-cell interactions
- Matrix dissolution
- Altered cell-ECM interactions
- Migration/locomotion
Steps of metastasis
- Intravasation
- Vascular dissemination
- Extravasation
- Colonisation
Cell-cell interactions in normal tissue
- Tight intercellular adhesions (e.g. E-cadherin homodimers)
- Tight adhesions lead to transduction of signals (maintain terminal differentiation, regulate growth, prevent cytoskeletal remodelling)
- “Contact inhibition”
Altered cell-cell interactions in cancer
- Tumour cells down regulate adhesion molecules and dissociate from one-another
- Loss of “contact inhibition”
- De-differentiation
- Up-regulated growth
- Increased cytoskeletal
remodelling - E-cadherin is a frequently disrupted molecule in some
carcinomas
Matrix dissolution in normal tissue
- Tissue compartments are separated by the “extracellular
matrix unit (ECM)” - ECM consists of basement membrane (BM) and interstitial matrix
- BM is a dense matrix of collagens, glycoproteins (e.g. laminins) and proteoglycans
- Interstitial matrix is loose matrix of fibrous structural proteins (e.g. collagens, elastins), adhesive glycoproteins and proteoglycans and water
- These structural elements must be degraded for invasion to occur
Matrix dissolution in cancer
- Tumour cells either secrete or induce ECM cells (e.g. fibroblasts/inflammatory cells) to secrete proteases
- MMPs breakdown ECM components for physical passage of tumour cells
- Elaborates growth factors from ECM
- Strong correlation between MMP expression and invasive and metastatic potential
Proteases secreted by tumour cells/ECM cells
- Matrix metalloproteinases (MMPs)
- Heparanases
- Serine proteases
MMPs
- Family of zinc-dependent protease molecules capable of
degrading all ECM components - .21 structurally related MMPs
- Divided based on substrate specificity (e.g. collageneases, gelatinases)
Physiological function of MMPs
- Embryogenesis and growth
- Uterine cycling and postpartum involution
- Tissue repair
Regulation of MMPs activity
- Regulated by tissue inhibitors (TIMPs)
- 4 structurally related proteins (TIMP1-4)
- Block MMP activity by binding to their conserved zinc binding site
- Synthesised and secreted by ECM cells as a host response to limit tumour invasion
Overexpression of TIMPs
- Correlated with reduced invasive capacity
- Complex interplay between MMPs and TIMPs determines the degree of ECM degradation
Normal cell matrix interactions
- Cells have receptors (e.g. integrins) for ECM constituents (e.g. laminin) along their basal surface
- Adhesions lead to transduction of inhibitory signals (contact inhibition) that maintains terminal differentiation, regulates growth and prevents cytoskeletal remodelling
Physical effects of altered cell matrix interactions
Regulate the shape, orientation and movement of cells
Integrins
- Large family of transmembrane glycoprotein heterodimers with an alpha and beta subunit attached to the intracellular cytoskeleton
- The binding target and the intracellular signal upon
binding is determined by the particular combination of
the α and β chains
Altered cell-matrix interactions in cancer
- Some integrins e.g. αVβ3, almost universally promotes tumour invasion and metastasis
- Others e.g. α2β1 are growth promotory in some cancers
(melanoma, gastric, prostate) but inhibitory in others (breast)
MIgration/Locomotion
- Final step of invasion
- Complex process involving many families of receptors and signalling proteins which have ultimate effect of altering the cytoskeleton
- Leading edge attaches to ECM constituents, detaches at the trailing edge and contract the cytoskeleton to propel forwards
- Movements potentiated and directed by the mileu of
promigratory factors at the leading edge
Mesenchymal migration
Tumours invading as single cells
Collective migration
Tumours invading as groups of cells
Promigatory factors
- Secreted by tumour cells, ECM cells, inflammatory cells and liberated from ECM itself
- E.g. cytokines, ECM cleavage products and GFs
Advantage of collective migration
- A multi-cellular contractile body capable of more efficient movement
- Protection of inner cells from immune attack
- Higher autocrine concentrations of promigratory factors
- Allows passive transport of otherwise non-motile tumour
cells
Amoeboid migration
- Alternate form of invasion
- 30x faster than mesenchymal migration
- Cells undergo shape changes to squeeze through the ECM rather than cut through it (protease
independent) - Tumour cells may be able to switch between types of migration in vivo
- May in part explain poor response of cancers to antiMMPs
Three main pathways of tumour dissemination
- Direct seeding of body cavity
- Lymphatic spread
- Haematogenous spread
Direct seeding
- Occurs when a cancer penetrates a body cavity
- Pleural, pericardial, joint space
Lymphatic spread
- Dissemination via lymphatics typical of carcinomas (malignant epithelial neoplasms)
- Tumour cells penetrate the thin walled lymphatic spaces
- Lymphatics drain into lymph nodes where tumour may arrest and form a deposit
- Patterns of nodal involvement follow the anatomical routes of lymphatic drainage
Haematogeneous spread
- Dissemination via blood vessels seen in carcinomas and
sarcomas (connective tissue neoplasms) - Tumours typically penetrate venous spaces (thinner than arteries)
- Tumour cells follow the venous flow of the neoplasm and often
deposit in the first capillary bed they encounter
Multitude of steps to successfully initiate a metastasis
- Invade
- Intravasate
- Vascular dissemination
- Extravasate
- Colonise
- Tumour cells require a specific genetic signature to negotiate these steps
Primary tumours
- Genetically unstable and become heterogeneous with multiple subclones of cells
- Emergence of a subpopulation with most of the necessary genetic alterations for metastasis
- Many of these genes are acquired at an early stage of tumour evolution
- Additional genetic alterations are required to complete the metastatic phenotype
Intravasation
- Invasion into blood vessels involves penetration through the BM then adhesion to endothelium
- Requires ECM integrin interactions and proteolytic enzymes
Vascular dissemination
- Few cancer cells survive once in bloodstream
- Survival advantage if aggregated with platelets and fibrin
What is vascular dissemination mediated by
- Tumour cell release of platelet activating and procoagulant
factors (e.g. ADP, tissue factor, thromboxane A2) - Expression of adhesion molecules (e.g CD44, surface mucins)
Tumour-platelet thrombi
- Shield tumour cells from the immune system
- Reduce tumour cell exposure to mechanical sheer forces
- Also facilitate extravasation
Extravasation
Tumour cell arrest and extravasation at distant site
involves adhesion to endothelium, then egress through basement membrane
What is extravasation facilitated by
Tumour-platelet thrombi as platelets express numerous endothelial and ECM adhesion molecules
What does extravasation require
ECM-integrin interactions and proteolytic enzymes
Colonisation
- Often first site of metastasis is the first capillary bed
- Certain cancers have a propensity to metastasize to certain sites
- Stimuli for angiogenesis are necessarily induced by the tumour for survival
- Lead to upregulation of angiogenic factors (e.g. VEGF
and bFGF) and down regulation of antiangiogenic inhibitors (two key mechanism are hypoxia and angiogenic switch)
Seed and soil hypothesis
- Certain tumour cells (the seed) have a specific affinity for the milieu of certain organs (the soul
- Metastases occurs when there is a match
Example of seed and soil hypothesis
CD44 expressing tumours have propensity for LN deposition as LN venules express high levels of hyaluronate (the CD44 receptor)
Another example of seed and soil hypothesis
Breast cancer cells overexpressing CXCR4 hone to
bone marrow where its ligand, SDF1, is highly expressed
What are sites of metastasis determined by
- The tumour cells
- The microenvironment of the host tissue
- Prevailing circulatory patterns
What is required for a tumour to exceed 1-2mm in size
- Vascularisation
- Necessary for delivery of oxygen and nutrients and
removal of waste products
Process of vascularisation
Angiogenesis
Angiogenesis
-Induced outgrowth of pre existing vessels
- Formation of new vessels from endothelial cells recruited from the bone marrow
Hypoxia
Results in HIF-1 and HIF-2 expression, leading to transcription of pro-angiogenic factors
Angiogenic switch
Genetic aberrations result in altered expression of pro- and anti-angiogenic factors
Three examples of angiogenic switch
- Activating mutations of RAS and MYC
- MMPs release angiogenic factors sequestered in the ECM, e.g. bFGF
- Mutant p53 does not induce synthesis of anti-angiogenic
molecules, e.g. thrombospondin
Angiogenesis inhibitors
- Bevacizumab (VEGF inhibitor)
- Associated with improved outcomes in brain, lung, colorectal and breast cancer
