External factors controlling division and behaviour of normal and cancerous cells Flashcards
Cell behaviour
Describes the way cells interact with their external environment and their reactions to this, particularly proliferative and motile responses of cells
External influences detected by the cell
CHEMICAL -hormones -growth factors -ion concentrations -extracellular matrix -molecules on other cells -nutrients -dissolved gas (oxygen and carbon dioxide) concentrations PHYSICAL -mechanical stresses -temperature -topography (layout) of the extracellular matrix and other cells
External factors influencing cell division in relation to cancer cell behaviour
- Growth factors
- Cell-cell adhesion
- Cell-ECM adhesion
-all external factors may influence cell proliferation
Adhesion of tissue cells in culture
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Basic behaviour of cells in culture
-Cell-spreading=not passive, gravity-dependent event. Energy required to modulate cell adhesion and cytoskeleton during spreading
Cell-ECM adhesion influencing cell proliferation
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The importance of cell spreading
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Cell-ECM adhesion
- in suspension, cells do not significantly synthesise protein or DNA
- cells require to be attached to ECM (and a degree of spreading required) to begin protein synthesis and proliferation (DNA synthesis)
- attachment to ECM may be required for cell survival
MOST CELLS NEED MATRIX
Cell-ECM adhesion molecules
- cells have receptors on their cell surface which bind specifically to ECM molecules=molecules often linked, at their cytoplasmic domains, to the cytoskeleton
- arrangement means there is mechanical continuity between ECM and cell interior
Integrins
- heterodimer complexes of alpha and beta subunits
- associate extracellularly by head regions
- leg regions each span plasma membrane
- ligand-binding occurs at junction of head regions
- more than 20 combinations of alpha and beta subunits known which each bind specifically to short peptide sequence on extracellular matrix proteins (eg: alpha5beta1 fibronectin receptor binds arg-gly-asp=RGD)
- peptide sequences such as RGD are found in more than one ECM molecule (eg: RGD in fibronectin, vitronectin, fibrinogen etc)
Oncogenes
Mutant gene which promotes uncontrolled cell proliferation
Proto-oncogene
Normal cellular gene corresponding to the oncogene
Ras mutation
-in approximately 30% of all cancers
Ras (signalling intermediate)
Oncogenes:
- V12Ras (Gly12Val mutation)
- L61Ras (Gln16Leu mutation)
c-Raf (signalling intermediate)
Oncogene:
-v-Raf (deletion of regulatory domain)
c-Jun (transcription factor)
-v-Jun (deletion of regulatory domain)
Signals controlling proliferation of tissue cells
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Uncontrolled proliferation of tissue cells
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Benign and malignant tumours
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Local invasion and metastasis
- in addition to deregulated proliferation, a major feature of cancerous tumours is ability to spread
- most human cancers are carcinomas
- in order to spread to other sites (metastasis), cells must break away from primary tumour, travel to blood or lymph vessel, enter vessel, lodge at distant site, leave vessel and ultimately establish secondary tumour
Primary carcinoma cell metastasis
- cell-cell adhesion must be downregulated (eg: cadherin levels reduced)
- cells must be motile
- degradation of extracellular matrix must take place=matrix metaloproteinase levels (MMP) increased in order to migrate through basal lamina and interstitial extracellular matrix
- degree of carcinoma cell-cell adhesion is indicator of how differentiated primary tumour is and indicates invasiveness and prognosis
Integrins extra
- most integrins link to actin cytoskeleton via actin-binding proteins (exception is alpha6beta4 integrin complex in epithelial hemidesmosomes which is linked to the cytokeratin)
- integrin complexes cluster to form focal adhesions (most) or hemidesmosomes (alpha6beta4 integrin)
- clusters involved in signal transduction
- some integrins also bind to specific adhesion molecules on other cells (eg: alphavbeta3 binds to PECAM-1(CD31) and alphaIIbbeta2 to ICAM-1 on endothelial cells in inflammation)
Clustered integrins form focal adhesions
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Signalling to and from ECM receptors
-ECM receptors (eg: integrins) can act to transduce signals
Integrin signalling involves conformational changes to the complex
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Outside-in signalling
- cell can receive information about its surroundings from its extracellular matrix adhesion
- can alter cell phenotype
Focal adhesions sense mechanical properties of surroundings
-the amount of force that is generated at a focal adhesion depends on both the force generated by the cytoskeleton and the extracellular matrix stiffness
Integrins recruit cytoplasmic proteins which promote signalling and actin assembly
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Anchorage dependence mechanisms
-growth factor receptors and integrin signalling complexes can each activate identical signalling pathways (eg: MAPK)=activation individually is weak and/or transient but together, activation is strong and sustained
SEPARATE SIGNALLING PATHWAYS ACT SYNERGISTICALLY
Losing control: many components of signal transduction pathways are proto-oncogenes
- if gene coding component of signalling pathway is mutated so that protein is constitutively active, pathway will permanently be on
- receptors, signalling intermediates and signalling targets (eg: transcription factors) are proto-oncogenes
- mechanism of short-circuiting leading to uncontrolled proliferation as a result of loss of growth factor dependence
Contact inhibition of locomotion
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Loss of contact inhibition in cancer cells
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Behavioural restraint loss of the cell
Under certain conditions, cells lose their behavioural restraints. Results in:
- uncontrollable proliferation (lose density dependence of proliferation)
- less adherent and will multilayer (lose contact inhibition of locomotion and anchorage dependence)
- epithelial breakdown cell-cell contracts
- not Hayflick limited, express telomerase
Contact inhibition of proliferation mechanism
- when bound to cadherin at the membrane, beta-catenin not available for LEF-1 binding and nuclear effects
- normally cytoplasmic beta-catenin rapidly degraded
- if beta-catenin cytoplasmic levels rise as a result of inhibition of degradation or loss of cadherin-mediated adhesion, beta-catenin/LEF-1 complex enters nucleus and influences gene expression leading to cell proliferation
Adhesion-associated signalling pathways known to influence contact-induced inhibition of proliferation
- clustering of cadherins after cell-cell contact is known to alter activation of small GTPases eg: Rac activated, Rho inhibited=can influence cell proliferation
- some growth factor receptors are associated with cell-cell junctions=reduces capacity to promote proliferation
Beta-catenin dynamics in cells
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Cross-talk between cell-cell and cell-ECM adhesion
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APC gene
Adenomatous polyposis coli (APC)=inherited colon cancer, there area number of familial forms
- APC gene is a tumour suppressor gene
- APC gene-product is a protein involved in degradation of beta-catenin (junction-associated molecule)
Molecular organisation of adherens junctions
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Cell-cell adhesion affects cell proliferation
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Contact-induced spreading of epithelial cells
-contact between epithelial cells leads to the mutual induction of spreading, so that the total spread area of contacted cells is greater than sum of the two separated cells=could result in a stable monolayer
Cell-cell junctions in epithelia
-junctions are usually arranged as continuous belts (zonula) or as discrete spots (macula)
Long-term cell-cell contacts
- upon contact, some cell types strongly adhere and form specific cell-cell junctions (adherens junctions, desmosomes, tight junctions, gap junctions etc)
- this is true of epithelial cells and endothelial cells which form layers, and neurones which form synapses
Cell-cell contact between non-epithelial cells
- when most non-epithelial cells collide, they do not form stable cell-cell contacts=actually repel one another by paralysing motility at the contact site, promoting formation of a motile front at another site on the cell, and moving off in the opposite direction
- this is contact inhibition of locomotion and is responsible for preventing multilayering of cells in culture and in vivo
Short term contact interaction between cells
Transient interactions between cells which do not form stable cell-cell junctions
Long term contact interaction between cells
Stable interactions resulting in formation of cell-cell junctions
Cell populations
AT HIGH DENSITY, CELLS COMPETE FOR GROWTH FACTORS
- when cells in culture form a confluent monolayer, they stop proliferating and slow down many other metabolic activities=contact inhibition of cell division
- another set of experiments suggest that it is competition for external growth factors and not cell-cell contract responsible=density-dependence of cell division
Conformational changes to the integrin complexes during activation and signalling
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Inside-out integrin signalling
- a signal generated inside the cell (eg: as the result of hormone binding to receptor) can act on an integrin complex to alter integrin affinity (eg: alter its affinity for its extracellular matrix binding)
- used in inflammation or blood clotting, switching on adhesion of circulating leukocytes
Cell phenotype can be determined by the composition of the matrix
-example with cultured mammary epithelium
-the matrix type has profound effects on the cell phenotype
Experiments in 3D matrix gel cultures highlight difference of:
