Apoptosis and Replicative Immortality - L4 Flashcards
What is apoptosis?
PROGRAMMED CELL DEATH
it is a normal physiological process in development/health of multicellular organisms involving active cell suicide
Why is apoptosis important during embryogenesis?
it is important because it sculpts the organism
- about 50% of neurons die by apoptosis during brain development
What is the link between apoptosis and homeostasis ?
apoptosis is a means of cells talking to each other to ensure the correct number of cells within a tissue
- regulates the total number of cells
- removes unwanted cells like potential tumour cells, virally infected cells and damaged cells
What do cancer cells do to apoptotic processes?
it is essential for cancer cells to deactivate apoptosis
What are the stages of apoptosis ?
1) cellular membrane disruption
2) cytosol extruded causing BLEBBING
3) cytoplasmic and nuclear skeletons are broken down
4) chromosomes are degraded
5) nuclear condensation and fragmentation
What are the 2 overlapping pathways controlling apoptosis ?
INTRINSIC PATHWAY= responds to internal stress/damage within the cell- cell knows something is wrong with it
EXTRINISIC PATHWAY= responds to signals from other cells- another cell is telling its neighbour that its not needed anymore
What are the 3 classes of molecular components ?
Sensors
Signal transduction
Effectors
What are the sensors purpose in apoptosis ?
intrinsic= molecules induced by cellular stress- p53, HIF-alpha and E2F1 extrinsic= the "death" receptors pairing of extracellular ligands with cells surface death receptors - FAS-R, DR3-5, TNF-alpha-R1
What stress signals activate p53, HIF-alpha and E2F1 ?
p53- DNA damage - too much damage to repair then apoptosis is induced
HIF-alpha- hypoxia
E2F1- oncogene activation - activates apoptotic pathways
What are the signal transduction molecules purpose in apoptosis ?
intrinisic= bcl2 and family members, cytochrome c(released from mitochondria) and apaf-1 extrinsic= FADD and TRADD - death inducing signalling complex= DISC
What is bcl2?
it is an oncogene that maintains the integrity of mitochondria, prevents them releasing cytochrome c
apoptotic signals want to release cytochrome c
overexpressed a lot in cancers
What are the effector molecules purpose in apoptosis ?
caspases= cysteine-aspartic acid proteases - split into initiator and executioner - intrinsic and extrinsic pathways converge onto caspases
caspase activated DNase (CAD)- actually degrades DNA
What is the main decision of the intrinsic apoptotic pathway ?
key decision is at the mitochondrion, deciding whether or not to release cytochrome c - this is influence by the bcl2 family
What happens when cytochrome c is released ?
1) it forms an apoptosome with apaf-1 and the initiator caspase pro-caspase 9
2) pro-caspase 9 is activated to form capsase 9
3) caspase 9 then activates the executioner capsases
4) DIABLO is another mechanism coming out of the mitochondria which increases apoptosis by inhibiting IAPs which inhibit apoptosis
5) executioner caspases cleave essential cellular proteins
What are examples of essential cellular proteins cleaved by executioner capsases?
ICAD, iamin, vimentin and actin etc
What are BAK/BAX?
they are tumour suppressors which respond to apoptotic signals and try to degrade mitochondrial membranes to release cytochrome c
How many cancers have overexpression of bcl2 ?
over half of all cancers
What are examples of proteins that try to prevent bcl2 taking control ?
NOXA, Poma, Bad, HrK, Bim, Bmf and Bik
What ratio is critical in apoptosis ?
the balance between bax and bcl2
What are examples of ligands that bind to death receptors in the extrinsic pathway ?
Fas, TRAIL, TNF-alpha and TL1A
When death receptors are activated what happens?
1) they recruit FADD (or TRADD and FADD) and pro-caspase 8 or 10
2) this forms a death inducing signalling complex DISC
3) pro-caspase 8/10 is activated to form caspase 8/10
4) this leads to activation of executioner capsases
Why is evasion of apoptosis essential for tumour growth ?
because. ..
- excessive DNA damage and mutations occur
- loss of growth factor/survival signalling
- resistance to detachment induced cell death- anoikis
- dysregulation of cell cycle
What is anoikis?
it is a particular type of cell death when cells lose adherence to their neighbours
its a way of preventing cells floating off around the body
What are different ways in which cancer cells evade apoptosis ?
- overepression of bcl2
- mutated bax
- apaf-1 methylated in melanoma
- IPA-1 is amplified and overexpressed
- methylated capsase 8
- caspase 3 down regulated
What is a key research area in cancer?
reactivating apoptotic pathways
What is replicative immortality ?
it is the limitless potential to grow/divide in appropriate conditions
NO INDIVIDUAL CELL is immortal- but some give an immortal lineage of parent/daughter cells
What types of cells are immortal?
germ cells= subset of stem cells
stem cells= divide very slowly, asymmetrically and continuously
cell lines= grown in incubators
cancer cells= probably immortal
What are the processes that occur when a cell is not immortal ?
Terminal differentiation - they acquire an end functional purpose = highly specialised
Apoptosis/Phagocytosis/Necrosis
Divide infrequently until death of the organism
Senescence
What do normal cells do ?
they divide finite number of times known as the Hayflick limit - this describes a molecular counter
cells can be frozen for decades and still remember their replicative lifespan
What is senescence ?
permanent cell cycle arrest accompanied by whole scale changes in gene expression
Why does timing of senesence vary with conditions ?
the number of divisions depends on growth conditions- too much oxygen or wrong growth medium reduces number of divisions
What do senescent cells look like ?
their cell shape changes -flatter and more squashed out
express beta galactosidase
changes in nuclear structure- heterochromatin foci are present in senescent cells
Why is the activity of p53 important ?
if p53 is active then cells enter senescence
if p53 is inactivated the cells continue to divide leading to crisis
What happens in crisis?
crisis- cells continue to divide inducing numerous mutations , rapid proliferation and death
chromosome fusion can occur, massive cell death can occur and some cells emerge with ability to fix themselves and it is these cells that are immortal
What does it mean by senescence being double back up plan ?
- natural halt on division after enough have occurred (telomeres signal to p53/pRb)
- induced permanent halt after oncogenic/physical distress
Why are senescent cells rare in humans ?
because cells never reach the hay flick replicative limit, apoptosis occurs instead
macrophages may scavenge them
may get odd ones in old persons skin
What are telomeres ?
1000s of repairs of DNA sequence TTAGGG doesn’t code for anything
they protect chromosome ends from recognition by DNA repair machinery
What happens to telomeres?
They get shorter after every division due to end replication problem
What happens if telomere erosion occurs ?
the ends of chromatids are unprotected and this can cause the ends of the 2 chromatids to fuse together and this prevents the cell functioning
Why are telomeres not recognised as double strand breaks of DNA?
because there are lots of telomere binding proteins that recognise the sequence and form a protective cap
When does alternative lengthening of telomeres (ALT) occur?
found in tumours where hTERT is not active
What does the end replication problem cause?
causes DNA damage and activates p53
how many cancers is hTERT activated ?
> 90%, others utilise ALT
