External factors controlling division and behaviour of normal and cancerous cells Flashcards
What chemical external influences are detected by cells
Hormones Growth factors Ion concs ECM Molecules on other cells Nutrients Dissolved gas (O2/CO2) concs.
What physical external influences are detected by cells
Mechanical stresses
Temperature
The topography or “layout” of the ECM and other cells
Which external factors influence cell division/proliferation
Growth factors
Cell-cell adhesion
Cell-ECM adhesion
Describe the adhesion of tissue cells in culture (cell-ECM adhesion)
In suspension, cells do not significantly synthesise protein or DNA
Cell proliferation
Anchorage dependence
How does cell-ECM adhesion influence cell proliferation
Cells must be spread out and bound to ECM in order to be competent in responding to soluble growth factors (for protein/DNA synthesis).
This principle is known as Anchorage dependence.
Describe what happens to a cell when it is placed on a culture medium
It will begin to settle and spread across the surface
It will gain some sort of polarity
It will become motile
NOTE: this is an active process, it is not because of gravity. Energy is required to modulate cell adhesion and changes in the cytoskeleton during spreading
Describe the cell-ECM adhesion molecules
Receptors on cell surfaces which bind specifically to ECM molecules
These are often linked to the cytoskeleton at their cytoplasmic domains
This ensures mechanical continuity between ECM and the cell interior
What are integrins
ECM receptors
Heterodimer complexes of alpha and beta subunits
Describe the head and leg regions of integrins
integrins associate extracellularly by their “head” regions.
Ligand-binding occurs at the junction of the head regions
Each of the “leg” regions spans the plasma membrane
What do the extracellular parts of integrins bind to and give an example
Short, specific peptide sequences (e.g. arg-gly-asp binding to RGD sequence)
What do most integrins bind to intracellularly and how
Actin cytoskeleton via actin-binding proteins at focal adhesions
What is an exception to this generalisation of integrin binding?
The alpha6beta4 integrin is found in hemidesmosomes in epithelia and it binds to cytokeratin instead
What do integrins form when they cluster?
Most integrins – local adhesions
alpha6beta4 integrin - hemidesmosomes
Give examples of which specific adhesion molecules on other cells that integrins can bind to
αvβ3 binds to PECAM-1(CD31) on endothelial cells in inflammation
WBC integrin binds to ICAM on endothelial cells – important in inflammation
What is the other important purpose of integrins other than cell adhesion?
It is a platform for signal transduction
What are the two signalling actions of ECM receptors
Transduce signals (from outside > inside cell) Generate signals (from inside > outside cell)
Describe outside-in signalling of integrins
Binding of integrin complexes to ECM can stimulate a signal inside the cell
The ligand binds and opens the legs of the complex, allowing cytoplasmic signalling molecules to bind
Describe inside-out signalling of integrins and give an example of when it occurs
Growth factors can generate signals inside the cell, which can act on the integrin complex and alter its affinity
Occurs often in inflammation or blood-clotting - “switching on” of circulating leukocytes
What does Inside-out signaling activate
Folded integrin complex within the plasma membrane, acting to extend the complex.
This then allows cytoplasmic signaling molecules to bind, promoting signaling and actin assembly
What is proliferation dependent on (for integrin complexes and GF)
Density and Anchorage
GF - density
Integrin/ECM - anchorage
Describe how density influences proliferation
Proliferation seems to cease when the density of growth factors (which are signaling for proliferation) is too great At high density, cells compete for growth factors = Density-dependence of cell division.
What is anchorage dependence
Cells must be spread out and bound to extracellular matrix to respond to soluble growth factors and proliferate
When cells are dividing on a culture medium, they will stop dividing once they reach the edges of the medium. What was originally thought to be the reason behind this? and what is the actual reason for this
Contact inhibition of cell division
Competition for external growth factors (density-dependence)
Describe the ERK MAP kinase cascade
Chain of proteins in the cell that communicates a signal from a receptor on the surface of the cell to the DNA in the nucleus of the cell
The pathway includes many proteins, including MAPK (originally called ERK), which communicate by adding phosphate groups to a neighboring protein, which acts as an “on” or “off” switch.
Describe the stimulation of proliferation by the ERK MAP kinase cascade
Growth factor receptors and integrin complexes can both activate the same signalling pathways (like MAPK).
Individually GFs or integrin signaling complexes will only result in weak/transient activation of the pathway
Together, their activation of the cascade is strong and sustained, leading to more profound proliferation
What are the two types of contact interactions between cells
Short-term - transient interactions between non-epithelial cells which do not form cell-cell junctions
Long term - stable interactions resulting in formation of cell-cell junctions
What happens to non-epithelial cells when they come into contact with one another?
repel one another by paralysing motility at the contact site
Promoting the formation of a motile front at another site on the cell
Causes it to move off in the
opposite direction
This is contact inhibition of locomotion and is partly responsible for preventing the multilayering of cells
What are some long-term cell-cell contacts and what are the junctions in the epithelia arranged as
Adherens junctions
Desmosomes
Tight junctions and gap junctions
Continuous belts (zonula) or discrete spots (macula)
What happens when epithelial cells become in contact with one-another
mutual spreading, extending the area of contact and helping to lead to a more stable layer
What effect does high calcium levels have on an epithelium?
Formation of cell-cell junctions
MAP Kinase pathway (inactive) - increase p27kIP1
Results in low proliferation
What effects do antibodies blocking adherens junctions have on an epithelium?
Formation of cell-cell junctions will not occur in high levels of ABA (high proliferation)
What are adherens junctions mediated by and what does it associate with intracellularly
Cadherins
transmembrane (Ca2+ dependent) cell adhesion molecule
Associates with β-catenin intracellularly.
Describe adherens junctions structure
β-catenin associates with an α-catenin molecule, which associates with an actin filament.
Give an example of a pathology that involves cadherins
Adenomatous polyposis coli (APC), a form of hereditary colon cancer
The affected protein is involved in degrading β-catenin (cell-cell junctions)
Results in an overgrowth of the colon epithelium and thousands of polyps (proliferation)
Other than beta-catenins, what are some other mechanisms of cell-cell contact inhbition
Clustering of cadherins after cell-cell contact is known to alter the activation of small GTPases
e.g. Rac is activated, Rho is inhibited: this can influence proliferation
What are the consequences of cells losing their behavioural constraints
Proliferate uncontrollably (lose density dependence of proliferation)
Be less adherent and will multilayer (lose contact inhibition of locomotion and anchorage dependence)
Cause epithelial breakdown cell-cell contacts
Be not Hayflick limited, express telomerase
What are the consequences of loss of contact inhibition of locomotion for the progression of cancer
Promoting formation of solid tumours
Invasion and metastasis
