7. External factors controlling division and behaviour of normal and cancerous cells Flashcards
(46 cards)
What is cell behaviour?
The term used to describe the ways in which cells interact with their external environment and their reaction to this, particularly proliferative and motile responses of cells
What external influences are detected by cells?
Chemical - hormones, growth factors, ion concentrations, ECM, molecules on other cells, nutrients and dissolved gas concentrations
Physical - mechanical stresses, temperature, the topography or ‘layout’ of the ECM and other cells
What external factors can influence cell division?
All external factors can potentially influence cell division but the ones that are best understood are:
Growth factor
Cell-cell adhesion
Cell-ECM adhesion
How is the basic behaviour of cells understood?
Much of the basic behaviour of cells was understood by examining isolated cells in culture as you can remove a lot of the variables and focus on a single variable that you are investigating
How do cells behave when put into a culture medium?
Initially you get an isolated cell on the culture medium, which will then settle down on the culture surface due to gravity
It will then spread across the culture medium and it will usually obtain some polarity - it will have a front and a back
The front is usually the motile part and is a broad region known as a lamellipod
This is NOT a passive process
Energy is required to modulate cell adhesion and the cytoskeleton during spreading
How does Cell-ECM Adhesion influence cell proliferation?
An experiment was carried out observing the degree to which the cells proliferated
If cells were suspended in agar (non-adhesive) few cells entered S phase
If they were able to stick to a small adhesive patch that did NOT allow them to spread out fully, a small proportion of cells proliferated
If the cells were allowed to stick to a larger adhesive patch, which allowed them to spread out fully - almost all the cells started proliferating
What is ANCHORAGE DEPENDENCE?
The requirement of attachment to ECM for survival
Why is cell spreading important?
Most cells will stick to fibronectin (matrix molecule that is also found in the blood)
If you have a defined small patch of fibronectin and put a cell on it, the cell would stick but it cannot spread and it will die of apoptosis
If you take the same amount of fibronectin and distribute it over a number of small spots, the cell will be able to spread and it will survive and grow
So it’s not just adhesion that is required for the cells to proliferate, the cells also need to be able to spread to enable the cells to respond to growth factors and proliferate
How is the phenotype of the cell mainly determined?
By the composition of the matrix that they are growing in
What are cell-ECM adhesion molecules?
Cells have receptors on their cell surface, which bind specifically to ECM molecules
These molecules are often transmembrane molecules and are linked, at their cytoplasmic domains, to the cytoskeleton
This means that there is mechanical continuity between the ECM and the cell interior
What are integrins?
Integrins are the most important of the matrix receptors
Integrins are heterodimer complexes consisting of alpha and beta subunits
They bind to the ECM via their heads
Each of their tail regions crosses the plasma membrane and projects into the cell
The integrins look like they have a head and two legs
Outline the structure of integrins
There are about 10 alpha and 8 beta subunits that form more than 20 known combinations
Each combination specifically binds a short, specific peptide sequence(e.g. alpha-5 beta-1 integrin complex is a fibronectin receptor and binds arg-gly-asp (RGD))
Such peptide sequences are often found in more than one ECM molecule e.g. RGD is found in fibronectin, vitronectin, fibrinogen and others
Intracellularly, the integrins are linked, via actin-binding proteins, to the actin cytoskeleton (most integrins do this)
What is special about the a6b4 integrin complex
The a6 b4 integrin complex is found in epithelial hemidesmosomes - these are linked through the cytokeratin (intermediate filament) cytoskeleton rather than through the actin cytoskeleton
What are the roles of integrins?
Integrin complexes cluster to form local adhesions (most) or hemidesmosomes (alpha-6 beta-4)
These clusters are often involved in SIGNAL TRANSDUCTION - the integrins are not just an adhesive patch for adhesion of cells, it is also a platform for signalling
This dual function of integrins allows the cells to interpret the matrix composition of the environment
Many integrins are also designed to bind to specific adhesion molecules on other cells rather than binding to the matrix (e.g. a5b3 binds to PECAM-1 (CD31) and a2bb2 binds to ICAM-1 on endothelial cells in inflammation)
This is particularly important in the immune system and blood clotting
In which direction do integrins transmit signals?
ECM receptors (e.g. integrins) can act to transduce signals in both directions
E.g. ECM binding to an integrin complex can stimulate the complex to produce a signal inside the cell
The signals can be from ‘outside-in’
The signals can be from the ‘inside-out’
What is inside-out signalling?
A signal generated inside the cell (e.g. as a result of the hormone binding to the receptor) can act on an integrin complex to alter the affinity of an integrin (i.e. alter its affinity for ECM binding)
How do inside-out signals work?
Integrin complexes are folded over, in which case they have a low affinity for matrix molecules so they don’t stick particularly well
There are signals generated within the cell that makes them unfold and go into a high affinity conformation and become sticky
This is important in the immune system and blood clotting
Give an example of where inside-out signalling is used
Platelets have integrins on their surface but they are inactive - this is good because you don’t want them to stick to everything
When we do need to activate the platelets, inside-out signals will activate the integrins so they become high affinity and start to stick
What is outside-in signalling?
A cell can receive information about its surroundings from its adhesion to ECM
E.g. the composition of ECM will determine which integrin complexes bind and which signals it receives
This can alter the phenotype of the cells
How does integrin activation and signalling occur?
Integrins are initially in the low affinity state (bent)
They can be switched on, into the high affinity conformation, by an inside-out signal (this can be due to an external factor such as a growth factor or hormone). This changes the cell’s properties so it is now adhesive
Once they bind to the matrix, you get other changes taking place
The ligand binding also causes a change in conformation
The legs separate and cytoplasmic signalling molecules can then bind and that binding will then allow signalling to take place - this is outside-in
When the integrins cluster, the molecules act on one another and you get signalling
What is density dependence of cell division and what causes it?
As cells become densely packed, the rate of proliferation starts to slow down
When they fill up the space provided, they tend to stop division or it becomes very minimal
It used to be thought that this cessation of proliferation was due to the cells running out of space
Then an experiment was conducted where a fresh medium containing all the necessary factors for growth was spread across part of the culture surface and the cells that were exposed to these extra growth factors continued to proliferate
This showed that it wasn’t the contact with neighbouring cells that was preventing cell division, it was actually the availability of growth factors
The density of cells was too high so there wasn’t enough growth factor available for cell division - DENSITY-DEPENDENCE OF CELL DIVISION
What are the two signals that interact to produce proliferation?
Growth factor (density dependence) ECM (anchorage dependence)
Why are two different signals needed in the proliferation of a cell?
Signalling from growth factors and signalling from ECM come together to produce proliferation
So growth factor receptors and integrin signalling complexes can each activate identical signalling pathways (e.g. MAPK)
Individually, this activation is weak and/or transient
Together, this activation is strong and sustained
These separate pathways act synergistically (together)
What are the two types of contact interactions between cells
Short-term: transient interactions between cells that do NOT form stable cell-cell junctions. Cells make brief contact with one another
Long-term: stable interactions resulting in the formation of stable cell-cell junctions
