Cell Behaviour 1 – External Factors Controlling Cell division
Anil Chopra
- Describe using examples the role of external growth factors in controlling the decision of a cell to divide.
- Describe using examples how mutation of a proto-oncogene can perturb the normal controls on cell division.
- Explain why signalling pathways involving growth factors are often implicated in the uncontrolled division of cancerous cells.
- Explain the role of contact inhibition and anchorage dependence in limiting the division of normal cells within their tissues.
- Discuss the factors which restrain cells within their normal tissue boundaries.
There are various external influences on cells. These include:
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.
An example of cell ECM (extracellular matrix) adhesion:
•in suspension, cells do not significantly synthesise protein or DNA
•cells require to be attached to ECM (and a degree of spreading is required) to begin protein synthesis and proliferation (DNA synthesis)
•attachment to ECM may be required for survival (e.g. epithelia, endothelia) i.e. anchorage dependence
•Receptors on the cells’ surface bind to the extracellular matrix 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
Integrins
Integrins are heterodimer complexes of α and β subunits that associate extracellularly by their “head” regions. Each of the “tail” regions spans the plasma membrane. They are very important ECM receptors that recognise short specific peptide sequences (e.g. a5b1 fibronectin receptor binds arg-gly-asp RGD). There are over 20 different combinations of α/β subunit, each binding to a specific peptide sequence.
They are linked via “actin binding proteins” to the actin cytoskeleton inside the cell.
Theα6β4 integrin complex found in epithelial hemidesmosomes is linked to the cytokeratin (intermediate filament) network. These complexes can form focal adhesions or hemidesmosomes. They can also bind to specific adhesion molecules on some cells.
Cell 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)
- this is “inside-out” signalling (e.g. in inflammation or blood-clotting, switching on adhesion of circulating leukocytes).
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” signalling”
- In this type of signalling, cells receive information about their surroundings from adhesion to ECM.
- The ECM composition therefore determines what signal the cell receives and therefore alters the cell phenotype.
Integrins recruit cytoplasmic proteins which can result in signal transduction and actin cytoskeleton formation.
When cells in culture form a confluent monolayer, they cease proliferating and slow down many other metabolic activities. Competition for external growth factors is responsible:- Density-dependence of cell division.
The Connection of the ERK/MAPK Cascade and ECM Signalling
Anchorage Dependence:both extracellular matrix signalling and growth factors initiate the ERK/MAPK cascade, however, when only one of them is present, the activation is weak. When both of them are present the activation is strong and sustained.
NB
Short – term transient connections between cells do not form cell-cell junctions
Long – term sustained connections between cells form cell-cell junctions.
When non-epithelial cells collide they repel one another by paralysing cytoskeleton movement at the contact end and inducing cytoskeleton movement producing a motile front at the opposite end contact inhibition of locomotion.
When epithelial cells collide, they attract one another and form long-tern sustained connections and cell-cell junctions such as adherens junctions, desmosomes, tight junctions, gap junctions e.t.c. This is how epithelial cells form layers and neurones from synapses. Contact between epithelial cells also leads to the mutual induction of spreading, so that the total spread area of the contacted cells is greater than that of the sum of the two separated cells. This could result in a stable monolayer.
Cell Adhesion also affects cell proliferation, in that when there are no cell-cell junctions, MAPK is activated and expression of p27KIP1 is decreased resulting in high proliferation. When cell-cell junctions are present, MAPK is inactivated and expression of p27KIP1 is increased resulting in low proliferation.
Molecules at the Cell-Junction Level
There is evidence that the β catenin molecule is very important link between cell adhesion and proliferation. Adenomatous polyposis coli (APC) is an inherited colon cancer resulting in the Degredation of β-catenin. The APC gene is a tumour suppressor.
The below diagram shows that the more β-catenin bound to the cadherin on the membrane the less is available to bind to LEF-1 and cause gene transcription. This is the mechanism for contact inhibition of proliferation.
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.Some growth factor receptors are also associated with cell-cell junctions.
Cancer
Loss of Anchorage Dependence
In certain conditions, the cells will lost their dependence of proliferation, lose their anchorage dependence, lose their contact inhibition and form multilayers. They also become no longer Hayflick limited. This is when cells express telomerase which results in their telomere’s not being broken away and so the cell continuing to divide uncontrollably.
Loss of Growth Factor Dependence
Any receptor, signalling intermediate, transcription factor or signalling target that induces mitosis or other cellular pathways is known as a proto-oncogene. If the gene coding for their production is mutated it becomes an oncogene; the pathway is constantly active which can lead to uncontrolled proliferation. The most commonly mutated protein is Ras.
Metastasis
•Most human cancers are carcinomas (i.e. of epithelial origin)
•In order to spread to other sites (metastasis), cells must break away from the primary tumour, travel to a blood or lymph vessel, enter the vessel, lodge at a distant site, leave the vessel, and ultimately establish a secondary tumour
•Cell-cell adhesion is down-regulated (e.g. cadherin levels reduced)
•The cells are motile
•Degradation of ECM must take place; matrix metaloproteinase (MMP) levels increased in order to migrate through basal lamina and interstitial ECM.
•The degree of carcinoma cell-cell adhesion is an indicator of how differentiated the primary tumour is, and indicates its invasiveness and the prognosis.