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Decreased p21 levels are required for efficient restart of DNA synthesis after S phase block

Vanesa Gottifredi1, Kristine McKinney, Masha V Poyurovsky

  • 1Department of Biological Sciences, Columbia University, New York, New York 10027, USA.

The Journal of Biological Chemistry
|November 5, 2003
PubMed

Insights

The cyclin-dependent kinase inhibitor p21 is degraded during S phase arrest, independent of p53. This down-regulation is crucial for restarting DNA synthesis and reversibility of cell cycle arrest.

Area of Science:

  • Molecular Biology
  • Cell Biology
  • Cancer Research

Background:

  • p21 is a key inhibitor of cyclin-dependent kinases, mediating cell cycle arrest after DNA damage.
  • p53, a tumor suppressor, transcriptionally regulates p21.
  • Previous studies indicated impaired p53 activity and weak p21 induction during S phase arrest.

Purpose of the Study:

  • To investigate the regulation of p21 during S phase arrest.
  • To determine the role of p21 down-regulation in the reversibility of S phase arrest.

Main Methods:

  • Assessing proteasome-mediated turnover of p21 during S phase arrest.
  • Analyzing p21 interactions with cyclin-dependent kinase complexes and PCNA.
  • Performing DNA elongation assays in isolated nuclei.
  • Observing cell cycle progression after release from S phase arrest based on p21 levels.

Main Results:

  • Proteasome-mediated turnover of p21 increases significantly during S phase arrest, independently of p53.
  • Low p21 levels during S phase arrest prevent full saturation of cyclin A-CDK2 complexes and interaction with PCNA.
  • The C-terminus of p21 inhibits DNA elongation, suggesting its down-regulation is necessary for DNA synthesis restart.
  • Cells with low p21 levels resume cell cycle progression faster upon release from S phase arrest.

Conclusions:

  • p21 down-regulation during S phase arrest is essential for the efficient restart of DNA synthesis.
  • p21 levels are multiply determined and its reduction is required for the reversibility of S phase arrest.
  • This suggests a novel mechanism for cell cycle control beyond p53-dependent transcriptional regulation.

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