External Factors Controlling Division Flashcards
What is anchorage dependence?
- Cells need to be adhered to matrix in order to respond to external GFs — proliferation
1) Describe the structure of integrins - extracellular and transmembrane
2) Describe the intracellular structure of integrins and an exception
1)
- Heterodimeric alpha and beta dimers - head binds extracellularly and tail transmembrane x1 each
- Different combos of subunits - different integrins
2)
- Most integrins are linked, via actin-binding proteins, to the actin cytoskeleton
- a64b integrin in epithelial hemidesmosomes linked to cytokeratin network is the only integrin we know that isn’t associated with actin cytoskeleton
What do integrins recognise?
- Integrins recognise short peptide sequences on matrix molecules e.g. a5b1 fibronectin receptors binds arg-gly-asp (RGD)
What are focal adhesions?
- Clusters of integrin alpha-beta heterodimers with linker molecules linking them to ends of f-actin in cytoskeleton
How do focal adhesions detect mechanical properties of their surroundings?
- Sense the mechanical properties of their surroundings by force transmitted through focal adhesions and via linker molecules
- Force generated at the focal adhesions depends on force cytoskeleton generates and the ECM stiffness
The ECM and type of integrin binding affects cell phenotypically, describe an experiment which proves this
Cultured mammillary epithelium experiment
- Cultured cells in different matrices - one in type 1 collagen-rich and another basal lamina-esque (type 4 collagen and laminin)
- Hormones that are needed for milk production
- Cells in 1st matrix were unorganised
- Cells in 2nd matrix formed compact organoids that produced milk
What is inside-out signalling and describe it? Also when is it useful?
- Intracellular signal can cause change in integrin affinity for ECM binding
- Useful for example for inflammation / blood clotting when we want certain cells to be able to adhere to stuff
What is outside-in signalling?
- When integrins bind ECM proteins it alters the cell phenotype via signalling and also alters the integrin conformation - extended form of integrin
What are the different conformations of integrins and what 2 things change it?
- Flexed / intermediate / extended
- Extended conformation allows ligand (ECM protein binding) whereas flexed prevents it
- Ouside in signalling - ligand binding promotes extension
- Inside out signalling can also promote extension
What is the main basis of contact inhibition of proliferation?
- Density dependence of cell division - competition for GFs
What signalling can integrin-matrix binding cause and what does it work synergistically with?
- Matrix - integrin signalling pathways similar to GF - signalling pathways
- These act synergistically to promote cellular proliferation
What happens in contact inhibition of locomotion?
- When non-epithelial cells collide
- Repel each other by paralysing motility at the contact site
- Motile site formed at non-contact site - move away
You can either form short term cell-cell contacts or long term cell-cell contacts, what essentially happens in both of these?
- Short term - transient interactions which do not form cell-cell junctions
- Long-term - stable interactions resulting in cell-cell junctions such as adherens junctions, desmosome, tight junctions, gap junctions
What are cell-cell adhesions dependent upon and how do they impact proliferation?
- Cell-cell adhesions are calcium dependent
- When there are NO cell-cell junctions, MAPK is activated, p27KIP1 is decreased — HIGH PROLIFERATION
- If calcium is added, junctions re-form — inactive MAPK, p27KIP1 is increased — LOW PROLIFERATION
Describe the structure of cadherin junctions - going down intracellularly
- Cadherins — beta-catenin — alpha catenin — actin cytoskeleton
How do beta-catenins impact cellular proliferation?
- Cytoplasmic beta-catenin can bind LEF-1 and alter transcription to increase proliferation
Describe the pathophysiology of Adenomatous Polyposis Coli
- Cytoplasmic beta-catenin can bind LEF-1 and alter transcription to increase proliferation
- APC molecule degrades cytoplasmic beta-catenin to prevent this
- If APC gene is faulty — allows beta-catenin to go unchecked → XS cellular proliferation → Adenomatous Polyposis Coli
Detail 2 mechanisms for contact induced inhibition of proliferation aside from density dependence which occurs when cell-cell contacts are made
- Beta-catenin
- Cadherin is necessary for adhesion. Cadherin is attached to beta-catenin intracellularly which is attached to alpha-catenin, which is in turn attached to the actin cytoskeleton
- If beta-cetanin is recruited to cadherin when needed for adhesion it is no longer available cytoplasmically for proliferative effect - prevents proliferation
- GTPases - Rho and Rac
* Clustering of cadherin cell-cell contacts → activates small GTPases → Rac is activated and Rho is inhibited → inhibits proliferation
Detail 4 things cancer does differently in regards inhibitory mechanims to do with these extracellular factors that confer cancerous properties
- Lose contact inhibition of proliferation - promotes solid tumour formation
- Lose contact inhibition of locomotion - promotes spread
- Loses anchorage dependence - so does not need to be bound to ECM in order to respond to GF signals to proliferate
- Not Hayflick-limited - express telomerase
Detail 3 things that occur to allow metastasis
- Cell-cell adhesion downregulated e.g. cadherin levels
- Cells must be motile
- Degradation of ECM must occur; Matrix Metalloproteinases (MMPs) increase to allow migration through basal lamina and interstitial ECM
How can degree of cell-cell adhesion help you determine prognosis of cancer?
- The degree of carcinoma cell-cell adhesion is an indicator of how differentiated the primary tumour is, and indicates its invasiveness and therefore helps indicate the prognosis
