Download Cell Behaviour 1 – External Factors Controlling Cell division Anil

Cell Behaviour 1 – External Factors Controlling Cell division
Anil Chopra
1. Describe using examples the role of external growth factors in controlling the
decision of a cell to divide.
2. Describe using examples how mutation of a proto-oncogene can perturb the
normal controls on cell division.
3. Explain why signalling pathways involving growth factors are often implicated in
the uncontrolled division of cancerous cells.
4. Explain the role of contact inhibition and anchorage dependence in limiting the
division of normal cells within their tissues.
5. 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.
The α6β4 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)
o 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
o e.g. ECM binding to an integrin complex can stimulate the complex to
produce a signal inside the cell,
o i.e. “outside-in” signalling”
o In this type of signalling, cells receive information about their
surroundings from adhesion to ECM.
o 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 longtern 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
Molecular organisation of adherens junctions
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
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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.