Biology of P53

• p53


  Tumour suppressor gene that conserves the integrity of the genome
• Hallmarks of cancer


  • Evading apoptosis
  • Insensitivity to antigrowth signals
  • Self sufficiency in growth signals
  • Sustained angiogenesis
  • Limitless replicative potential
  • Tissue invasion and metastasis
  • Deregulating cellular energetics
  • Avoiding immune destruction
  • Genome instability and mutation
  • Tumour promoting inflammation
• p53 is the most researched gene
• p53 identification


  Immunoprecipitation experiment found a 53 kDa protein that was p53
• p53 gene


  Tumour suppressor
• ras oncogene + mutated p53

  Study Shows bacteria Colonies growing
• ras oncogene + wild type p53

  Cells react to ras and die or undergo senescence
• Tumour suppressor genes


  Needed to keep cells under control, act as brakes to the cell cycle and DNA replication
• Structure of TP53 gene and p53 protein

  • 11 exons
  • Central domain binds DNA and acts as transcription factor
  • C-terminal domain for attachments, tetramerisation, and negative regulation
  • N-terminal domain is transcriptional activation domain
• Control of p53 levels

  p53 is continuously expressed but continuously degraded with short half-life, controlled by Mdm-2 protein
• When p53 is activated

  Degradation of p53 stops, allowing rapid accumulation of the protein
• Role of Mdm2


  Mdm2 is an oncoprotein that binds to p53 and targets it for degradation by ubiquitination
• Stimulus: DNA damage or mitogen cell survival


  1. DNA damage signals through other proteins to phosphorylate p53, making it active

  2. DNA damage can cause phosphorylation of Mdm2, inactivating it

  3. p53 stops cells from dividing, but there are also cell survival signals that activate Mdm2 to degrade p53
• Role of E1a, c-myc and ras in p53 activation

  They activate ARF, which targets Mdm2 and hides it in the nucleolus so it cannot degrade p53
• Signals that cause p53 induction and stabilisation

  • DNA damage, oncogene activation, hypoxia, ribonucleotide depletion, telomere erosion
• Key effects of p53 activation

  Cell cycle arrest to allow DNA repair, and apoptosis of cells
• UV radiation


  Increases DNA damage and apoptotic cells, requiring p53
• Role of p21


  Needed for cells to exit the cell cycle, helps G1 to S transition for DNA synthesis, activated by p53
• How p53 is activated

  Mainly by phosphorylation, with different phosphorylation patterns depending on the stress stimulus
• P53 role as a transcription factor


  • Activates genes in response to the nature of the insult, e.g. low DNA damage leads to cell cycle inhibition, high DNA damage leads to apoptosis
• How p53 causes cell cycle arrest through p21

  p53 activates p21, a CDK inhibitor, which inhibits cyclin-CDK complexes, preventing phosphorylation of pRB and allowing it to remain active to cause cell cycle inhibition
• Apoptosis


  Programmed cell death required during development, immune cell control, and to remove cells before they cause damage, occurring in response to overwhelming DNA damage, anoxia, or severe imbalances in cell signalling pathways
• p53-dependent apoptosis


  p53 initiates a cascade of proteases and checkpoints of caspases to induce apoptosis, with cytochrome c release from mitochondria as a key step
• Apoptosis


  • Removes cells before they cause damage
  • Occurs in response to: overwhelming DNA damage, anoxia, severe imbalances in cell signalling pathways
• Apoptotic characteristics


  • Cell blebbing (left) and nuclear fragmentation (right)
• P53 dependent apoptosis


  Role of p53 in apoptosis
• Apoptosis has to be tightly controlled - so has a cascade of proteases and checkpoints of caspases
• Initiation of caspase response

  1. Cytochrome c's release from the mitochondrial membrane

  2. Cytochrome c in cytosol associated with protein Apaf that initiates the caspase cascade

  3. Cytochrome C release is controlled by Bcl-2 and Bax
• Low level cellular stress

  p53 activated -> p21 activated -> cell cycle arrest
• High level cellular stress

  p53 activated -> puma activated -> bcl2 inhibited and activates Bax -> cytochrome C release -> apaf -> caspases cascade -> apoptosis
• Bcl2


  • Large family of proteins that control flow of cytochrome C via channels in outer mitochondrial membrane
  • Active bcl2 would inhibit cytochrome C release preventing apoptosis
  • Bcl2 is inhibited by PUMA = cell apoptosis
  • PUMA expression is controlled by p53
  • Bcl2 also inhibited by NOXA, NOXA expression is controlled by p53
• Bax, Bad, Bak


  • Function to open channels to release cytochrome C
  • Bax expression is controlled by p53
• Cytochrome C


  • Associates with Apaf-1 to form the apoptosome
  • Activates a protease (caspase 9) initiating a cascade of protease activation = apoptosis
• Induction of apoptosis depends on the balance of promoting and inhibiting factors
• Activated p53


  Induces Bax and inhibits bcl2, tipping the balance in favour of apoptosis
• The mechanism of how p53 decides between cell cycle arrest or apoptosis is not fully understood
• Myc (oncogene) binds p21 promoter preventing p53 from activating p21, changing the balance of p53 response from cell cycle arrest to apoptosis
• Loss of p53 function can lead to cancers
• Inactivating p53 mutations are the most common genetic alteration found in human cells
• Even in cancer types in which p53 mutations are rare, p53 function is indirectly abolished