Cell senescence and cancer Flashcards
1
Q
What is cell senescence?
A
- form of permanent arrest of cell proliferation
- major defence against cancer
- contributes to mechanisms of ageing
2
Q
What causes cell senescence?
A
- extended proliferation
- activation of an oncogene
- genotoxic (DNA damage) stresses
3
Q
How is cell senescence identified?
A
- morphological changes - big and flat, prominent nucleolus, stress fibres
4
Q
What are some molecular properties of senescent cells?
A
- expression of effectors (cell cycle inhibitors) - p16, p53, p21, ARF
- DNA damage signalling - possible universal = nuclear foci with components of DNA damage signalling, like 53BP1
- increased lysosomal content : popular stain = beta-galactosidase, often called SABG
- increased ROS (reactive oxygen species) levels
5
Q
What is SASP?
A
- senescence associated secretory phenotype - senescent cells secrete inflammatory factors : cytokines and their receptor - IL-6, IL-8
- proteases - MMPs
- angiogenic factors - VEGF
- other growth factor - IGF2
6
Q
What are the mechanisms of normal cell senescence?
A
- associated with telomeres - difficult to replicate = ‘end replication problem’
- the DNA right at the 3’ (lagging) end of each strand - where the last RNA binds, cannot be replicated y DNA polymerase, which works 3’ to 5’ starting from an RNA primer
- therefore a small stretch of DNA a the 3’ end cannot be replicated normally
7
Q
How do telomeres play a role?
A
- telomeres - structures at the ends of chromosomes, made of 0.5 - 20kb of repeats of the hexamer DNA seq - TTAGGG
- and a ‘cap’ of proteins (shelterin complexes) that bind to this sequence
- this cap protects the normal DNA end from being recognized as a DNA break by DNA repair enzymes, and so being joined to other free DNA ends`
8
Q
What is telomerase?
A
- enzyme telomerase, a protein RNA complex, can replicate telomeric DNA by reverse-transcribing DNA hexamers from its own RNA seq, and joining them to the new 3’ end, 5’ to 3’ end
- telomerase activity is highest in germ cells - which have longest telomeres
- telomeres have 2 main subunits - TERT and TERC
9
Q
Telomeres and cell senescence
A
- in humans most somatic cells lack telomerase activity, and therefore telomeres shorten as cells divide
- replicative senescence is triggered in normal cells when telomere get quite short
10
Q
How does senescence signalling from short telomeres work?
A
- normal telomeres bind cap protein
- short telomere - cap unstable and breaks
- end recognised as DNA break, by DNA damage complex (ATM, CHEK2)
- recruits and activated p53 by phosphorylation
- p53 signals growth arrest
11
Q
What is DDS?
A
- DNA damage signalling
- around each uncapped telomere, a focus of DNA damage signalling forms
- foci are big enough to label. in cell nuclei
12
Q
What is the role of p16?
A
- activates RB family
- p16 inhibits CDK4 prevents cyclin D from binding, so RB is not phosphorylated = active
- RB is able to bind to E2F = G1 arrest
- p16 expression increases with age
13
Q
What is cell immortality?
A
- when cells don’t senesce
14
Q
What are normal immortal cells?
A
- germ line
- early embryo and ESC
- these cells do not express telomerase subunit TERT - so they have telomerase activity and maintain full-length telomeres
- whereas most somatic cells express TERC, but v little TERT - so telomeres shorten at division
15
Q
What are somatic - ‘adult’ stem cells?
A
- all kinds of stem cells still present after birth - so in children too
- basal epidermis has some telomerase activity - not enough to make cells immortal
- so telomerase shorten less per division than in other somatic cells = gradual senesce
16
Q
How is cell senescense involved in tumour suppression?
A
- p53 and p16 are products of the 2 genes most commonly defective in advanced human cancers
- cell senescence is the only established function of p16, and an important function of p53
- reactivated telomerase activity is reported in 90% of human cancer cell lines
17
Q
Which viral oncogenes target p53 and p16(RB)?
A
- SV40 targets large T = inactivated p53 and RB
- HPV - E6 targets p53 and E7 targets RB
- adenovirus - E1B targets p53 and E1A targes RB
18
Q
How does normal cell senescence become disrupted?
A
- deficiency of p53 and RB = cells start to grow = 2nd arrest and telomeric crisis
19
Q
How can cells escape crisis?
A
- inc TERT expression = unlimited growth forming immortal cells
20
Q
What is the typical sequence of mutations in immortalisation in cancer progression?
A
- first have oncogene activation eg. RAS, RAF
- inactivate normal senescence - p16 and p53 pathways
- telomere extension - cells re-express TERT
21
Q
How do cells escape from crisis?
A
- cells continue dividing, but frequently die
- rarely, a mutation or gene rearrangement in a cell permits telomere extension (usually TERT promoter mutation)
- this leads to escape from crisis = immortalisation
- new variant clone can proliferate again= cancer progression
