4. External factors controlling division and behaviour of normal and cancerous cells Flashcards
Recall the 3 broad categories of external factors influencing cell division
- Growth factors
- Cell-EM adhesion
- Cell-cell adhesion
Describe the theory of “density-dependent cell division”
At high density, ells compete for growth factors - supercedes theory of contact inhibition somewhat
By what mechanism do growth factors trigger cell division
Trigger ERK cascade
How is a strong and sustained stimulus for the ERK cascade maintained?
GFs and cell-ECM adhesion will both trigger it, so they work synergistically
Why are signalling pathways involving GFs often implicated in cancer?
Cancerous cells lose:
- density-dependency
- Contact-inhibition
- Anchorage-dependency
- Cell-cell contacts
Describe the concept of anchorage-dependency
Cells cannot respond to GFs unless they are both adhered and spread on a surface
Summarise cell-anchorage
Settles on surface Spreads across Acquires polarity Becomes motile All has ATP requirement
What characteristic is essential for cells to spread on? What molecule most frequently performs this function?
Must be adhesive
Fibronectin will stick to most cells
What is the key characteristic of a cell-ECM adhesion molecule
Mechanically continuous with cytoplasm so an intracellular signalling pathway can be initiated by association with cytoskeleton
What is the key adhesion molecule in cell-signalling?
Integrins
Describe the structure of the integrin family
Heterodimers
alpha and beta subunits
20 known combos
How do integrins usually interact in their extracellular domain?
Recognise short, specific peptide sequences eg RGD
RGD is found in many molecules including fibrinogen and fibronectin
Summarise the interaction of integrins in their intracellular domain
With just one exception, interact with actin cytoskeleton via ABPs
Binding causes integrin complex to cluster and form focal adhesions
Which integrin does NOT interact with the actin cytoskeleton?
Cytokeratin-binding integrin (epithelial hemidesmosomes)
In inflammation, what may integrins bind to extracellularly?
ICAM-1
Summarise outside-in signalling
ECM:integrin complex triggers signal inside cell triggering conformational change
May influence phenotype and differentiation of the cell
Describe an experiment that can show the effect of outside-in signalling
When mammary epithelium is cultured on T1 collagen it does not become secretory, but on BM matrix the cells organise into “organoids” and produce milk
Summarise !inside-out” signalling
Signal produced inside cell affects affinity of integrin for ECM binding
Give an example of inside-out signalling
Hormone acting on an intracellular receptor
Removal of what factor is most likely to lead to cell-cell junction breakdown?
Ca++
What is the result of sustained interactions between cells?
Cell-cell junction formed
Describe the relationship between proliferation rate and cell-cell junctions
Junctions:
Present = low proliferation (MAPK inactive)
Absent = high proliferation (MAPK activated)
In what sort of cells is contact-inhibition demonstrated?
Non-epithelial
What is the function of adherens junctions?
Master junctions - control formation of other junctions
What are adherens junctions composed of?
Cadherin and catenins
What is cadherin?
Ca++-dependent cell-adhesion molecule
Recall the pathway that leads to a signal being generated by a master junction being formed
Cadherin binds intracellularly to beta-catenin, which in turn binds alpha-catenin and links the actin cytoskeleton to generate a signal
What is the function of beta-catenin?
Intracellular 2nd messenger for adherens junctions
Describe the metabolism of beta-catenin
Rapidly degraded in cytoplasm
What is the consequence of beta-catenin buildup?
When bound to cadherin it is unavailable for LEF-1 binding
However
When it builds up in the cytoplasm, it can bind to LEF-1 –> TF production and subsequent proliferation
Describe the 2 mechanisms of contact-inhibition
- Beta-catenin bound to cadherin so unavailable for LEF-1 binding, cell not stimulated to proliferate
- Cadherin clustering activates Ras superfamily, which influence proliferation - cell-cell contacts can inhibit activation
