Lecture 12 - p53 and Apoptosis Flashcards
What is p53 and what can it trigger? What does the loss of apoptosis allow [in relation to cancer]?
Why is it interesting?
p53 is a transcription factor that can trigger cell cycle arrest and apoptosis.
The loss of apoptosis allows cancer cells to survive under normally lethal pathological stresses. Cancer cells have learned how to avoid apoptosis.
p53 is interesting because it is the most common mutated gene found in all forms of cancer. It even has a higher frequency of mutation than Rb. It is almost always lost eventually (at some point) in every form of cancer.
Why was p53 originally thought to be an oncogene? Why was this false?
p53 was originally thought to be an oncogene as p53 cDNA transformed rodent cells. However, it was found that the p52 used in that experiment was a mutated form and therefore cause injected rodents to develop cancer. (promotes proliferation).
Regular Wild Type p53 suppresses proliferation and is a tumor suppressor gene.
What is the relationship between p53 and many tumor viruses?
p53 is a target for many tumor viruses.
How is p53 different from other tumor suppressor genes?
p53’s function is different from other tumor suppressor genes as it specifically prevents the proliferation of abnormal cells.
What is the most common form of a p53 mutation? Why is this form the most detrimental form of p53 mutation?
The most common p53 mutation is a “missense mutation” where one Amino Acid has been switched for another.
This is the most detrimental mutation as it is similar enough in shape to allow p53 to stay in the cell and in the game, so to speak, but powerful enough to prevent the protein from performing its proper function.
Explain the statement: “p53 is a dominant negative mutant”.
p53 is a protein that functions as a homotetramer. This means it is made up of four smaller subunits that all need to be functional in order for the overall protein to function.
Therefore, only 1 out of every 16 p53 proteins is truly +/+/+/+ or simply +/+ which means that the mutated version are more common, favoured, and will naturally out-compete the small amount of +/+ in the cell.
What is so significant about p53 levels? Explain the futile cycle.
p53 shows nuclear localization and levels differ between cells. Furthermore, levels of p53 increase in response to stress and p53 has a ~20 minute half life.
The cycle of p53 production is therefore futile because it will continously degrade/die-off.
What are the 6 most common activating signals of p53 release? What are the 4 most common response effects by p53 to these signals?
Activating signals of p53: 1 - lack of nucleotides 2 - UV radiation 3 - Ionizing radiation 4 - Oncogene signalling 5 - Hypoxia 6 - Blockage of transcription
Effects of p53: 1 - Cell cycle arrest 2 - DNA repair 3 - Block of angiogenesis 4 - Apoptosis
What are the two subcategories to the cell cycle arrest response of p53? What do they mean?
What role do activating signals play over a prolonged period of time? What is the main role of p53?
1 - Senescence: cell goes into a Go style stage where it cannot proliferate and remains static. It is still functioning, but stagnant. Cell may or may not go back into the cell cycle down the road (pretty doubtful though)
2 - Return to proliferation: basically, instead of going into the Go stage, you just let the cell go to do its thing.
Over an extended period of time, activating signals usually prevent p53 breakdown: the transcription levels will remain relatively constant.
What is Mdm2 and what is its relationship to p53?
Mdm2 is an oncoprotein that binds to all four domains of the p53 tetramer (1 Mdm2 per subunit). It then initiates the ubiquitylation (3 Ubi per subunit) of p53 and its degradation bia proteasomes.
Essentially, Mdm2 acts to inhibit p53 and thereby inhibit apoptosis and/or senescence.
How is the relationship between p53 and Mdm2 transcription levels a negative feedback loop? Describe this process.
p53 transcribes certain genes to perform its various functions. At increased levels, it will transcribe Mdm2 which then acts to reduce the levels of p53 and subsequently reduce its own levels.
Can p53 be protected from Mdm2? How does this occur?
p53 can be phosphorylated in response to DNA damage. This prevents Mdm2 from binding and furhter prevents ubiquitylation and degradation via proteasomes.
Aside from phosphorylation, what is the other way p53 can be protected from Mdm2?
Mdm2 can be phosphorylated or broken down/degraded itself by Chk1 and/or Chk2 kinases.
What is the relationship between ARF and Mdm2? What does the ARF molecule look like?
ARF bind to Mdm2 NOT for breakdown, but rather for sequesteration. It mobes Mdm2 into the nucleus for storage until it is needed.
It looks like a circle with a rectangular slot for the square end of the Mdm2 rectangle.
What can p53 do in emergency situations.
Ex: when there is massive genomic damage
Where is the p53 apoptotic pathway commonly seen in development?
In emergency situations, p53 can trigger apoptosis, but Cell Cycle arrest is still the primary response.
p53 apoptotic activity is commonly observed in the separation of digits, etc…
