7. External factors controlling division and behaviour of normal and cancerous cells Flashcards
What is the term ‘cell behaviour’ used to describe?
The way cells interact with their external environment and their reactions to this (particularly proliferative and motile responses of cells)
What external influences do cells detect?
Chemical influences • hormones • growth factors • ion concentration • ECM (density, composition) • molecules on other cells • nutrients • dissolved gas concentrations
Physical influences
• mechanical stresses
• temperature
• topography of the ECM
What external factors can influence cell division (in relation to cancer)?
- Growth factors
- Cell-cell adhesion
- Cell-ECM adhesion
How does a cell change when it is moved from a suspension to a culture surface?
- Spherical => spreads
- Gains motility as it acquires a polarity (guided by lamellipodia and filopodia)
- Energy required to modulate cell adhesion and the cytoskeleton during spreading (not gravity dependent)
- When the culture is turned upside down, the cells will stills spread
What is blebbing and when can it happen?
- Cell detaches its cytoskeleton from the membrane
- Causes membrane to swell into spherical bubbles
- Can happen if there is a cell on top of another, so it has no contact with the ECM substratum
What is necessary for a cell to be able to respond to growth factors (in terms of ECM)?
- Cells have to be adhered to matrix in order to respond to external GFs => proliferation
- Cell has to be able to fully spread out as well
- ‘Anchorage dependence’
How does cell survival differ in suspension and attachment to ECM?
- Suspension - do not significantly synthesise protein or DNA (no S phase)
- ECM - combined with spreading, allows protein synthesis and proliferation to occur
How is the cell attached to the ECM?
- Cells have receptors which bind specifically to ECM molecules
- Linkage at cytoplasmic domains allows mechanical continuity between cell interior and ECM
Describe the results in a test where mammary epithelial cells were mixed with gels made up of 2 type of ECM?
Interstitial matrix (type 1 collagen)
• cells cluster together (loose and undifferentiated)
• hormones present in culture but no effect on the cells
• not secretory cells
Basal lamina matrix gel
- cells form a spherical cyst (hollow ball)
- these organoids switch on the production of milk proteins
Describe the structure of integrins
(one of the groups of cell-ECM adhesion complexes)
• Heterodimer complexes of alpha and beta subunits
• Associate extracellularly by their ‘head’ region
• Beta unit has a slightly longrr cytoplasmic tail
• Lingand-binding (to ECM) occurs at the junction of the ‘head’ regions
• Most are linked to the actin cytoskeleton via actin-binding proteins
What is the most important ECM receptor?
Integrin
How many different known combinations of integrin are there?
- 20 combinations
* About 10 alpha chains and 8 beta chains known
What do integrins recognise?
• Each combination specifically binds a particular, short, peptide sequence within a matrix molecule
• Such peptide sequences can be found in more than one ECM molecule
e.g. a5b1 receptor binds to arg-gly-asp (RGD), found in fibronectin, vitronectin, fibrinogen etc.
Are integrins rigid or flexible?
- Flexible
- Can adopt different combinations (folded, intermediate or extended)
- Important for its function
What is the only integrin that we know of that isn’t associated with the actin cytoskeleton?
a6b4 integrin complex found in epithelial hemidesmosomes, linked to the cytokeratin (intermediate filament) network
What types of clusters do integrins form and what are they involved in?
- Focal adhesions (most)
- Hemidesmosomes (a6b4)
- Involved in signal transduction
Apart from epithelial cells, what other cells can integrins bind to (and via which protein)?
- White blood cells - PECAM-1 (CD31)
* Endothelial cells in inflammation - ICAM-1
What are focal adhesions?
- Large, dynamic protein complexes through which the cytoskeleton of a cell connects to the ECM
- Found at the ends of bundles of filamentous actin (microfilaments of the cytoskeleton)
What is outside-in signalling?
- Cell binds to the matrix via the integrin
- Stimulates an intracellular signal
- Cell can receive information about its surroundings from this
- Composition of ECM determines which integrin complexes bind and which signals it receives
- This can alter the phenotype of the cell
- However, not all cells express the same integrins
How do focal adhesions sense the mechanical properties of their surroundings, and how do molecules take advantage of different forces?
• Amount of force generated at focal adhesions depends on:
- the force generated by the cytoskeleton (F cell)
- stiffness of the ECM
• Some of the ECM can be coiled, and when it is stretched, it is exposed to allow other molecules to bind for specific function
Do integrins have any enzymatic activity and how do they signal?
• No enzymatic activity
• Therefore, when integrins cluster, they recruit:
- signalling molecules
- molecules that associate with the actin cytoskeleton
How do integrins change configuration when switched on and off?
- On - extended
* Off - flexed/bent
What is inside-out signalling and when is it used?
• Signal generated inside the cell (due to hormone binding to receptor)
• Affinity of integrin is altered - extended conformation
e.g. inflammation, blood clotting, adhesion of circulating leukocytes
• Allows cells to be able to stick when necessary e.g. over-adhesion of platelets could cause clots
(• Ligand binding can then cause a further change to the molecule)
When normal tissues cells are plated in a culture dish, when do they stop proliferating and why??
Once the culture surface area is covered, forming a confluent monolayer
• Contact inhibition
• Competition for external growth factors
