4. External factors controlling division and behaviour of normal and cancerous cells

Question 1

Recall the 3 broad categories of external factors influencing cell division

Answer 1

1. Growth factors
2. Cell-EM adhesion
3. Cell-cell adhesion

Question 2

Describe the theory of “density-dependent cell division”

Answer 2

At high density, ells compete for growth factors - supercedes theory of contact inhibition somewhat

Question 3

By what mechanism do growth factors trigger cell division

Answer 3

Trigger ERK cascade

Question 4

How is a strong and sustained stimulus for the ERK cascade maintained?

Answer 4

GFs and cell-ECM adhesion will both trigger it, so they work synergistically

Question 5

Why are signalling pathways involving GFs often implicated in cancer?

Answer 5

Cancerous cells lose:

1. density-dependency
2. Contact-inhibition
3. Anchorage-dependency
4. Cell-cell contacts

Question 6

Describe the concept of anchorage-dependency

Answer 6

Cells cannot respond to GFs unless they are both adhered and spread on a surface

Question 7

Summarise cell-anchorage

Answer 7

Settles on surface
Spreads across
Acquires polarity
Becomes motile
All has ATP requirement

Question 8

What characteristic is essential for cells to spread on? What molecule most frequently performs this function?

Answer 8

Must be adhesive

Fibronectin will stick to most cells

Question 9

What is the key characteristic of a cell-ECM adhesion molecule

Answer 9

Mechanically continuous with cytoplasm so an intracellular signalling pathway can be initiated by association with cytoskeleton

Question 10

What is the key adhesion molecule in cell-signalling?

Answer 10

Integrins

Question 11

Describe the structure of the integrin family

Answer 11

Heterodimers
alpha and beta subunits
20 known combos

Question 12

How do integrins usually interact in their extracellular domain?

Answer 12

Recognise short, specific peptide sequences eg RGD

RGD is found in many molecules including fibrinogen and fibronectin

Question 13

Summarise the interaction of integrins in their intracellular domain

Answer 13

With just one exception, interact with actin cytoskeleton via ABPs
Binding causes integrin complex to cluster and form focal adhesions

Question 14

Which integrin does NOT interact with the actin cytoskeleton?

Answer 14

Cytokeratin-binding integrin (epithelial hemidesmosomes)

Question 15

In inflammation, what may integrins bind to extracellularly?

Answer 15

ICAM-1

Question 16

Summarise outside-in signalling

Answer 16

ECM:integrin complex triggers signal inside cell triggering conformational change
May influence phenotype and differentiation of the cell

Question 17

Describe an experiment that can show the effect of outside-in signalling

Answer 17

When mammary epithelium is cultured on T1 collagen it does not become secretory, but on BM matrix the cells organise into “organoids” and produce milk

Question 18

Summarise !inside-out” signalling

Answer 18

Signal produced inside cell affects affinity of integrin for ECM binding

Question 19

Give an example of inside-out signalling

Answer 19

Hormone acting on an intracellular receptor

Question 20

Removal of what factor is most likely to lead to cell-cell junction breakdown?

Answer 20

Ca++

Question 21

What is the result of sustained interactions between cells?

Answer 21

Cell-cell junction formed

Question 22

Describe the relationship between proliferation rate and cell-cell junctions

Answer 22

Junctions:
Present = low proliferation (MAPK inactive)
Absent = high proliferation (MAPK activated)

Question 23

In what sort of cells is contact-inhibition demonstrated?

Answer 23

Non-epithelial

Question 24

What is the function of adherens junctions?

Answer 24

Master junctions - control formation of other junctions

Question 25

What are adherens junctions composed of?

Answer 25

Cadherin and catenins

Question 26

What is cadherin?

Answer 26

Ca++-dependent cell-adhesion molecule

Question 27

Recall the pathway that leads to a signal being generated by a master junction being formed

Answer 27

Cadherin binds intracellularly to beta-catenin, which in turn binds alpha-catenin and links the actin cytoskeleton to generate a signal

Question 28

What is the function of beta-catenin?

Answer 28

Intracellular 2nd messenger for adherens junctions

Question 29

Describe the metabolism of beta-catenin

Answer 29

Rapidly degraded in cytoplasm

Question 30

What is the consequence of beta-catenin buildup?

Answer 30

When bound to cadherin it is unavailable for LEF-1 binding
However
When it builds up in the cytoplasm, it can bind to LEF-1 –> TF production and subsequent proliferation

Question 31

Describe the 2 mechanisms of contact-inhibition

Answer 31

1. Beta-catenin bound to cadherin so unavailable for LEF-1 binding, cell not stimulated to proliferate
2. Cadherin clustering activates Ras superfamily, which influence proliferation - cell-cell contacts can inhibit activation