16th Feb - p53 Flashcards
When was p53 discovered?
1979
How was p53 identified?
Interacted with viral proteins:
- SV40 large T antigen
- E1B of Adenovirus
- E6 of papilloma virus
How was it identified that p53 was a tumour suppressor?
Originally thought to be an oncogene. Performed genetic studies to identify as TS
ras + p53 deletion mutant –> cell growth
ras + p53 val-135 point mutant –> increased proliferation
ras + WT p 53 –> no cell growth
What is Li-Fraumeni syndrome?
A rare dominant inherited cancer syndrome where patients have mutations in TP53 gene
How was it demonstrated that p53 protects against cancer?
Genetic studies in mice
p53-/- die very fast (0% survival at 250 days), as they get tumours very fast
p53+/- start to die at about 300 days
p53+/+ have 100% survival at 500 days
What is the main form of regulation of p53?
Phosphorylation
Does p53 function as an oligomer?
Yes it functions as a homotetramer
How does p53 recognise target genes?
It recognises a 10 base pair consensus sequence
In which exons of p53 are the majority of mutations located?
5-8
Which amino acids of p53 are hotspots for mutation?
175
248
273
How is the haploid insufficiency of p53 explained?
Mutant subunits from the mutated chromosome, form part of the tetramer with unmutated p53 subunits, meaning that only 1/16th of the tetramers produced are wholly WT
What is the half life of p53?
20 minutes
What is the main role of p53?
To define the cellular responses to different kinds of damage
Do all of p53s functions work harmoniously?
Some are antagonistic and can occur simultaneously
Where does it carry out its function?
Mainly in the nucleus but also in the cytoplasm
How can p53 by activated?
The MDM2 binding region can be phosphorylated in a variety of positions to prevent binding to MDM2
How does MDM2 inhibit p53?
MDM2 binds to p53 in a normal state and inhibits it by ubiquitination
This is reinforced by a negative feedback loop, as p53 targets the MDM2 gene, so if there is too much p53 MDM2 transcription will increase correspondingly
Disrupting this negative feedback loop, leads to a massive increase in p53
What are the different functions of p53?
Cell cycle arrest
DNA repair
Angiogenesis Block
Apoptosis
What are the downstream effectors of p53?
p53 antagonist - MDM2 Growth arrest - p21 cip1 and reprimo DNA repair - p53R2, XPC, DNAPk Regulators of apoptosis - BAX, PUMA, NFkB, APAF-1, Fas/APO1, Bcl-2 Anti-angiogenic - TSP-1
What are the two main categories of p53s pro-survival functions?
Prevent apoptotic signaling
Anti-oxidant function at basal levels
Outline the mechanism of p53 blocking apoptotic signaling
p53 activates:
p21 which inhibits CDK, E2F, ASK1, and Caspase 3
BTC inhibits CDK, thus allowing Akt to inhibit apoptosis
14-3-3-delta inhibits Bax, Foxo 3, YAP and ASK1 preventing apoptotic signaling
What genes does p53 target to inhibit ROS production?
SESN-1 SESN-2 MnSOD ALPHA GPX TIGAR
How is the response of p53 determined?
Believed to be based on the post-translational modification of p53 e.g. Phosphorylation of ser 56 –> apoptosis and acetylation on lys 320 –> survival
What is the barcode model?
Proposed by Zmijewski (2008) the cell response of p53 is determined by a series of factors dependent on PTM. The amount/type of stress will partially determine the PTM.
Along with the level of p53, its localisation and co-factors/modifiers present
What protein acts in opposition to MDM2, stabilising p53?
HAUSP which deubiquitinylates p53 increasing its stability and causing the induction of cell cycle arrest
What are the different isoforms of p53?
p53beta
p53 gamma
delta133p53
delta40p53
Some of the isoforms of p53 are overexpressed in cancer, what is the possible function of this?
Perhaps block p53 or increase survival function of p53
e.g. in breast cancer delta133p53alpha is OE
What are the other family members of p53?
p63
p73
What is the function of p63 and p73?
They help p53 to create a different combination of responses
Outline the experiment to elucidate p63/p73 function and the conclusion of the experiment
KO studies in mice
p63 -/- –> developmental defects –> death
p73 -/- –> developmental defects
p63 +/- and p73 +/- –> increased predisposition to cancer
Therefore p53 needs both p63 and p73 to induce apoptosis
List the other physiological roles of p53
Ageing Development Stem cell regulation Endurance Fecundity Sun Tanning Neurodegeneration Ischaemia
What is the role of p53 in ageing?
Contributes to both ageing and longevity have been reported, this may reflect its regulation of ROS
Give evidence for the role of p53 in development
Encephaly in Trp53-deficient mouse models
What is the role of p53 in stem cell regulation?
Involved in controlling self renewal and quiescence in adult stem cells
What is the role of p53 in fecundity?
p53 is an important activator of leukaemia inhibitory factor, which is required for implantation of blastocytes
What is fecundity?
the ability to produce an abundance of offspring or new growth
What is the role of p53 in sun tanning?
p53 induces the expression of pro-opiomelanocortin
What is the role of p53 in ischaemia?
It can have a protective function in models of stroke, MI, kidney ischaemia and reperfusion injury
Outline the role of p53 in metabolism
It enhances OXPHOS through SCO2 and fatty acid oxidation through GAMT
It inhibits GLUT, GLUT and TIGAR therefore inhibiting the Warburg effect
