Cancer Bio (Dan Lecture 4 - p53) Flashcards
p53 mutagenesis is common in cancers. True or false?
True
p53 mutation levels are the same in all cancer types. True or false?
False
p53 knockout mice showed what 2 main differences?
- Reduced survival
- Enhanced tumourigenesis
What is the structure of native p53?
- A tetramer in its native state
- Expressed as a monomer (single gene)
- 4 copies produce native state
Name some factors that can induce p53 formation
- lack of nucleotides
- UV radiation
- Ionizing radiation
- Oncogene signalling
- Hypoxia
- Blockage of transcription
Name some processes that p53 regulates
- Cell cycle arrest
- DNA repair
- Blockage of angiogenesis
- Apoptosis
What is p53’s normal function?
To upregulate expression of target genes e.g p21 gene
What is the Mdm2 gene and how does it function to regulate p53 levels
- Gene products acts as negative feedback mechanism:
- p53 upregulates Mdm2 gene
- Mdm2 protein moves back to nucleus and binds p53
- Prevents its activation by preventing phosphorylation
- Complex exported from nucleus to cytoplasm, ubiquitinated and degraded by proteasome
LOOK AT DIAGRAM ON SLIDE TO CONFIRM
Is p53 a stable protein? What is its half life?
Very unstable protein
- 20 min half life
How can p53 prevent degradation by mdm2 and therefore increase its half life?
- Phosphorylation of p53 at the N terminus in the mdm2 binding domain
- Prevents mdm2 binding
- Increases half life
What affect will p53 knockout have on cell viability?
- loss of p53 HUGELY increases cell survival after DNA damage
- Opposite to what was observed in the mice
- No p53 in cells = loss of apoptosis = cell survival
- In mice: No p53 = no apoptosis = damaged DNA allowed to replicate leading to tumour formation therefore no p53 = mice death
In addition to p21, what other important protein is is upregulated by p53 as a result of UV induced DNA damage?
p48
What is p48?
- A protein subunit of the UV-DNA damage-binding protein (UV-DDB)
- Otherwise known as DDB2
What do intrinsic and extrinsic apoptotic pathways mean?
Intrinsic - apoptosis induces by intracellular components
Extrinsic - apoptosis induced by extracellular signals
What is the main intrinsic pathway of apoptosis?
BAX signalling:
1) p53 transcriptionally upregulates BAX
2) Bax increases permeability of mitochondrial membrane by opening channels
3) Release of pro-apoptotic cytochrome c from mitochondria
4) Cyt c associates with Apaf-1
5) Procaspase 9 binds to complex forming apoptosome
6) Apoptosome causes cleavage and acitvation of
executioner caspases
7) Executioner caspases cleave death substrates resulting in apoptosis
What is the main extrinsic pathway of apoptosis?
Apoptosis can be activated by transmembrane death receptors e.g FAS:
1) p53 transcriptionally upregulates FAS
2) Extracellular ligands part of the TNF family e.g. TNF-alpha, TRAIL
3) Receptor activation leads to activation of the cytoplasmic Fas-associated death domain protein (FADD)
4) Initiates caspase cascade via caspase 8 activation
Describe another extrinsic p53 regulated pathway of apoptosis
Insulin-like growth factor binding protein-3 (IGFBP-3):
1) p53 transcriptionally upregulates IGFBP-3 gene
2) Binds to extracellular IGF-1/2
3) Normally, IGF-1/2 bind to IGF-1/2 receptor on cell membrane and give pro-survival signals via the PI3 kinase pathway
4) Binding of IGFBP-3 to IGF-1/2 prevents binding of IGF-1/2 to receptor - induced apoptosis
How do Nutlins target the p53 pathway as a cancer therapy?
- Bind to mdm2 and prevent p53 interaction
- Leads to stabilisation and activation of p53
- Cancer cells arrest in G1 or G2 and undergo apoptosis
- In phase 1 clinical trials
- Requires non mutant p53
How do RITAs target the p53 pathway as a cancer therapy?
- Inhibit same interaction as Nutlins
- Binds to p53 instead of mdm2
What treatments are available in patients with mutant p53?
Tumour-associated mutations destabalise p53 tetramer
Therapies aim to stabilise tetramer
PRIMA-1
- Prevents unfolding of mutant protein
- Maintains stability and activity of p53
