Cancer 7: External factors controlling division and behaviour of normal and cancerous cells Flashcards
Define cell behaviour
The way cells interact with their external environment and their reactions to this, particularly proliferative and motile responses of cells.
What external influences are detected by cells?
Chemical: hormones, growth factors, ion concs, ECM, molecules on other cells, nutrients and dissolved gas (O2/CO2) concs.
Physical: mechanical stresses, temperature, the topography or “layout” of the ECM and other cells
What is a lamellipod?
Cytoskeletal protein actin projection on the leading edge of the cell
What is the basic behaviour of cells in culture?
Cell spreading - energy requiring process required to modulate cell adhesion and the cytoskeleton during spreading
What is cell-ECM adhesion?
Cells need to be bound to the ECM to be fully competent for responding to soluble growth factors.
- They need to be attached to ECM to begin protein synthesis and proliferation.
- Attached to ECM for survival.
Anchorage dependence
What external factors can influence cell division?
Growth factors
Cell-cell adhesion
Cell-ECM adhesion
What can the cell phenotype be determined by?
The cell phenotype can be determined by the composition of the matrix.
Example: Cultured mammary epithelium
(A) in interstitial matrix (type 1 collagen), mammary epithelium does not differentiate to secretory cells;
(B) in basal lamina (basement membrane) matrix, mammary cells organise into “organoids” and produce milk proteins.
How do cells receive information about its surrounding from its adhesion to ECM?
Cells have receptors on their cell surface which bind specifically to ECM molecules. These molecules are often linked, at their cytoplasmic domains, to the cytoskeleton. This arrangement means that there is mechanical continuity between ECM and the cell interior
What are integrins?
These are heterodimer complexes of alpha and beta subunits that associate extracellularly by their “head” regions. Each of the “leg” regions spans the plasma membrane.
Ligand-binding occurs at the junction of the head regions
See slide for diagram
How many different combinations of alpha and beta subunits are there?
More than 20. They bind specifically to short peptide sequences on ECM proteins.
For example alpha5beta1 fibronectin receptor binds arg-gly-asp (RGD)
Peptide sequences such as RGD are found in more than one ECM molecule.
How do integrins link to the actin cytoskeleton?
Most integrins link to the actin cytoskeleton via actin-binding proteins.
Integrin complexes cluster to form focal adhesions (most) or hemidesmosomes.
These clusters are involved in signal transduction
How does signalling happen from the the ECM to the inside of the cell?
ECM receptors (e.g. integrins) can act to transduce signals
e. g. ECM binding to an integrin complex can stimulate the complex to produce a signal inside the cell,
i. e. “outside-in” integrin signalling
How do cell-ECM adhesion and signals be switched on and off?
Integrins complexes can adopt flexed and extended molecular confirmations.
Switching between these confirmations affects their ability to bind their ligands, and their signalling. In this way, cell-ECM adhesion, and signals, can be switched on and off.
How can the the ECM alter the phenotype of the cell?
The composition of the ECM will determine which integrin complexes bind and which signals it receives.
What do focal adhesions do?
They sense the mechanical properties of their surroundings - Cells can exert force on a environment causing it to stretch/deform.
The amount of force generated is dependent on the force generated by the cytoskeleton and the stiffness of the ECM
What do integrins recruit?
Cytoplasmic proteins which promote both signalling and actin assembly.
What is inside-out integrin signalling?
A signal generated inside the cell (e.g. as the result of hormone binding to receptor) can act on an integrin complex to alter the affinity of an integrin (i.e. alter its affinity for its ECM binding)
(e.g. in inflammation or blood-clotting, switching on adhesion of circulating leukocytes)
Summarise the conformation changes to the integrin complexes during activation and signalling
Low-affinity = bent confirmation, weak or no binding to ligand
High-affinity = extended confirmation, strong binding to ligand
Signal from inside the cell acts on the integrin complex to promote the switch to the extended high-affinity conformation: “inside-out” activation; switches adhesion on.
ECM ligand binds and causes the further opening of the legs. This exposes the binding sites for the recruitment of cytoplasmic signalling molecules:
What is density-dependence of cell division
The higher the cell density the less cell proliferation - cells compete for growth factors
GF –> ERK MAP Kinase Cascade
What signals control proliferation of tissue cells?
Growth factors (density dependence) ECM (anchorage dependence)
Both signals needed for efficient stimulation of proliferation –> activate MAPK
What are the different contact interactions between cells?
short-term: transient interactions between cells which do not form stable cell-cell junctions
long term: stable interactions resulting in formation of cell-cell junctions
What happens when there is cell contact between non-epithelial cells?
When most non-epithelial cells “collide”, they do not form stable cell-cell contacts. They actually “repel” one another by paralysing motility at the contact site, promoting the 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.
Which cells form long terms contacts?
Epithelial cells and endothelial cells, which form layers, and neurones forming synapses.
strongly adhere and form specific cell-cell junctions (adherens junctions, desmosomes, tight junctions, gap junctions)
How does cell-cell adhesion affect cell proliferation?
No cell-cell junctions, activated MAPK,
decreased p27KIP1, high proliferation
cell-cell junctions form, inactive MAPK,
increased p27KIP1, low proliferation
High calcium = no junctions produced. +ve Adhesion blocking
Low calcium = increased junctions formed. -ve adhesion blocking
