External factors controlling division and behaviour of normal and cancerous cells Flashcards Preview

Y2 MCD Cancer Fwong COPY > External factors controlling division and behaviour of normal and cancerous cells > Flashcards

Flashcards in External factors controlling division and behaviour of normal and cancerous cells Deck (33)
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What are the three best-known external factors that influence cell division?

Growth factor
Cell-cell adhesion
ECM-cell adhesion


Describe what happens to a cell when it is placed on a culture medium

It will begin to settle and spread across the surface
It will gain some sort of polarity
It will become motile
NOTE: this is an active process, it is not just happening because of gravity. Energy is required to modulate cell adhesion and changes inthe cytoskeleton during spreading


Describe what happens to cells placed on:
a. Non-adhesive agar
b. Small adhesive patch
c. Large adhesive patch

a. Non-adhesive agar
Very few cells enter S phase
b. Small adhesive patch
A small proportion of cells will proliferation
c. Large adhesive patch
Almost all the cells will start proliferating


What is the difference in proliferation when a cell is placed on:
a. A small patch of fibronectin
b. The same amount of fibronectin spread over a larger area

a. A small patch of fibronectin
The cell can stick but it can’t spread so it will probably die via apoptosis
b. The same amount of fibronectin spread over a larger area
The cell is able to stick AND spread so it will survive and grow
NOTE: this shows that adhesion AND spreading is important for cell survival and proliferation


Cells need to be attached to ECM and they need a certain degree of spreading to be able to respond to soluble growth factors. What is the term given to the requirement of ECM binding for growth?

Anchorage dependence


Describe the structure of integrins.

There are heterodimer complexes of alpha and beta subunits
They associate extracellularly via their head and each of the tail regions spans the plasma membrane


How many different alpha and beta subunits are there?

10 alpha and 8 beta
There are over 20 known combinations


What do the extracellular parts of integrins bind to?

Short, specific peptide sequences (e.g. arg-gly-asp (RGD sequence))


What do most integrins bind to intracellularly?

Actin cytoskeleton


What is an exception to this generalisation?

The alpha6beta4 integrin is found in hemidesmosomes in epithelia and it binds to cytokeratin instead


What do integrins form when they cluster?

Most integrins – local adhesions
al[ha6beta4 integrin - hemidesmosomes


What is the other important purpose of integrins other than cell adhesion?

It is a platform for signal transduction


Describe inside-out signalling of integrins.

Growth factors can generate signals inside the cell, which can act on the integrin complex and alter its affinity (this is important in bloodclotting)


Describe outside-in signalling of integrins.

A cell can receive information about its surrounding via adhesion to the ECM
The ligand binds and opens the legs of the complex, allowing cytoplasmic signalling molecules to bind


Describe how the experiment with cultures of mammary epithelium demonstrated the profound effect of ECM on the phenotype of cells.

When the mammary cells were placed on a culture medium with interstitial matrix (type 1 collagen), they formed clumps and were loosely associated
When placed on a culture medium with basement membrane matrix (e.g. laminin), the cells formed a very ordered system (organoid) and even began producing milk proteins


When cells are dividing on a culture medium, they will stop dividing once they reach the edges of the medium. What was originally thought to be the reason behind this?

Contact inhibition of cell division


What is the actual reason for this?

Density-dependence – increased competition for growth factors


In summary, what two pathways work synergistically to trigger proliferation in cells?

Anchorage dependence (ECM dependent)
Density dependence (growth factor dependent)


What happens to most non-epithelial cells when they make contact with each other?

They will move away from each other
The motility on the side that made contact will become paralysed meaning that the cells can then move away from each other
This prevents multi-layering


Which types of cells form stable cell-cell junctions when they come into contact?

Epithelial cells
Endothelial cells


What are the two types of cell-cell junction?

Zonulae (belts)
Maculae (spots)


What happens to epithelial cells when they come into contact with one another?

Contact-induced spreading of epithelial cells
Contact between epithelial cells leads to mutual induction of spreading, so that the total spread area of the contacted cells is greater than the sum of the two separated cells. This could result in a stable monolayer.


What effect does low calcium levels have on an epithelium?

Many cell-cell junctions are calcium dependent In the absence of calcium/low calcium, the junctions will break down
This leads to:
 Increased MAPK activation
 Decrease activity of p27KIP1 (Cdk inhibitor)
When calcium returned to normal and the junctions were reformed:
 Decreased MAPK activation
 Increased activity of p27KIP1 (Cdk inhibitor)


What effects do antibodies blocking adherens junctions have on an epithelium?

The same results were achieved
This showed that cell-adhesion affects proliferation


Describe the structure of an adherens junction.

There is a cadherin domain that is transmembrane and projects extracellularly
Cadherins are homophilic and associate with similar structures on adjacent cells
Intracellularly, the cadherin is bound to beta-catenin, which is bound to alpha-catenin, which, in turn, is bound to the actin cytoskeleton


Describe the action of beta-catenin when it isn’t sequestered by cadherin at the plasma membrane.

Normally, beta-catenin is rapidly degraded by APC so it doesn’t tend to achieve high concentrations in the cytoplasm
Free beta-catenin in the cytoplasm (that is not broken down by APC) can bind to LEF-1 to form a complex that acts as a transcription factor
This complex can then regulate gene expression and promote proliferation


Explain how the APC mutation causes adenomatous polyposis coli.

The APC mutation means that the APC protein can no longer degrade beta-catenin as efficiently
So beta-catenin accumulates in the cytoplasm, associated with LEF-1 and triggers increased proliferation


What is contact inhibition of proliferation?

When bound to cadherin at the plasma membrane, beta-catenin is NOT available to bind to LEF-1 and cause nuclear effects
Cytoplasmic levels of beta-catenin can rise if there is:
 Inhibition of degradation
 Loss of cadherin-mediated adhesion
This can lead to the formation of beta-catenin/LEF-1 complexes that promote proliferation


Describe some other cadherin-associated signalling pathways that are known to influence contact-induced inhibition of proliferation.

When cadherins cluster together you can get changes in the activation of some of the small GTPases including Rac and Rho
Changes in the small GTPases can induce proliferation


Describe ways in which cells can lose their social skills.

Proliferate uncontrollably (loss of density dependence)
Become less adherent and multi-layer (loss of contact inhibition of locomotion and loss of anchorage dependence)
Epithelia break down cell-cell contacts
Not hayflick limited (express telomerase and become immortal)