1
Q
What is senescence?
A
- It is the irreversible cell cycle arrest in G1.
- It’s a process that occurs in mitotically competent cells, causing them to enter state of post-mitosis.
- Cells do not die – they remain viable and metabolically active.
2
Q
Define the two main biomarkers of senescence.
A
- SA-β-Gal (senescence-associated beta-galactosidase) –> is a hydrolase enzyme that catalyzes the hydrolysis of β-galactosides into monosaccharides found only in senescent cells. However it’s not completely definitive.
- SAHFs (senescence-associated heterochromatic foci) —> they are covalent alterations of histones forming chromatin are also seen in senescence.
- - Found in nuclei of senescent cells, reflecting the widespread epigenetic changes in gene expression and silencing of cell cycle genes.
- - They can be detected by labelled antibodies that react with the covalently modified histones.
3
Q
What are the different ways in which senescence can arise?
A
- Replicative senescence – induced by the erosion of telomeres.
- Oncogene-induced senescence – aberrant signalling of oncogenes causes the cells to enter a state the same as replicative senescence. Ras has been shown to induce it.
- DNA damage caused by ROS
- X-rays
- Chemotherapeutic drugs can also trigger senescence.
4
Q
Explain the mechanism of replicative senescence.
A
- Telomeres lie on ends of chromosome to prevent fusion.
- Replication of DNA strands is ‘problematic’ at the ends of the strands, so a region of strand is not covered by the primer and not replicated – gradual shortening.
- Termed ‘end replication problem’ – telomeres shorten after each division, eventually leading to ‘crisis’ where telomeres are too short to prevent fusion from occurring.
- Once telomeres are too short, they will be sensed by the cells as a DSB and elicit a DNA damage resonse which will activate ATM/ATR kinases which will signal downstream to p53.
- Or the telomere erosion will activate INK4A (also known as p16) and results in p53 activation. it sequesters MDM2 in the nucleolus which has ubiquinating activity of p53 – negative regulator of p53.
- The same mechanism is induced by DNA damage.
5
Q
Explain the mechanism of oncogene-induced senescence.
A
- It is triggered by cells to prevent oncogenic transformation.
- Overexpression of oncogenes, such as Ras will lead to induction of response similar to the DNA damage response.
- These strong mitogenic signals will induce DNA damage and activate the DDR.
6
Q
How can stress conditions induce senescence?
A
- Inadequate growth conditions and stress can initiate senescence via p16 in a telomere-independent mechanism.
- P16 will sequester MDM-2 at the nucleolus to cause p53 levels to rise.
7
Q
How can senescence promote tumour progression?
A
- Senescence is antagonistically pleiotropic- it can inhibit and promote tumour progression by different mechanisms.
- Senescent cells have been shown to promote malignant progression of cells when injected in vivo.
- The senescence associated secretory phenotype (SASP) includes pro-inflammatory cytokines which can promote tumour progression via inflammation.
- Also includes growth stimulating factors, cytokines that stimulate cells to invade the basement membrane, VEGF and MMPs.
- They also trigger regeneration of tumour stem cells.
8
Q
How do cancer cells avoid senescence?
A
- Telomerase – an enzyme that is expressed in germ and ES cells to lengthen telomeres.
- Telomerase is an enzyme capable of regenerating telomeres. It is detected in around 85% of tumours by using the TRAP assay.
- Several oncogenes (c-myc, ras, E6 from HPV) have been found to drive telomerase expression.
- When telomerase is expressed at significant levels, the telomeres are maintained at a length that allows unlimited replication – the cells are immortalised.
- The remaining 15% of tumour cells have to use another mechanism to maintain their telomere length not dependent on telomerase —> termed the alternative lengthening of telomeres (ALT), which is dependent on homologous recombination.
