Ionizing radiation induces ATM-independent degradation of p21Cip1 in transformed cells

J Biol Chem. 2009 May 29;284(22):15061-70. doi: 10.1074/jbc.M808810200. Epub 2009 Mar 30.


The cyclin-dependent kinase inhibitor p21(Cip1) plays an important role in the cellular response to DNA damage. In normal cells, genotoxic stress activates the ATM-p53 pathway that up-regulates the expression of p21(Cip1) leading to cell cycle arrest. However, we have found that in several neoplastic cell lines, ionizing radiation (IR) induces ubiquitin-dependent degradation of p21(Cip1). This process is independent of the ATM pathway as it occurs in immortalized A-T fibroblasts. Knockdown of Skp2, an F-box protein capable of regulating the normal turnover of p21(Cip1), does not prevent the IR-induced degradation. Instead, this process requires the Cul4-DDB1(Cdt2) E3 ligase as knockdown of either DDB1 or Cdt2 rescues p21(Cip1) degradation after IR. Mutating the proliferating cell nuclear antigen-binding site of p21(Cip1) also prevents its IR-induced degradation suggesting that the p21(Cip1)-proliferating cell nuclear antigen interaction is critical for this event. Although ectopic expression of a nondegradable p21(Cip1) did not by itself affect the clonogenic survival of HEK293 cells after IR, the degradation of p21(Cip1) and other targets of the Cul4-DDB1(Cdt2) E3 ligase may collectively contribute to the survival of neoplastic cells after ionizing radiation.

Publication types

  • Research Support, N.I.H., Extramural

MeSH terms

  • Animals
  • Ataxia Telangiectasia Mutated Proteins
  • Cell Cycle Proteins / metabolism
  • Cell Line, Transformed
  • Cyclin-Dependent Kinase Inhibitor p21 / metabolism*
  • DNA-Binding Proteins / metabolism
  • Humans
  • Proliferating Cell Nuclear Antigen / metabolism
  • Proteasome Endopeptidase Complex / metabolism
  • Protein Binding / radiation effects
  • Protein Processing, Post-Translational / radiation effects*
  • Protein Serine-Threonine Kinases / metabolism
  • Radiation, Ionizing
  • S-Phase Kinase-Associated Proteins / metabolism
  • Tumor Suppressor Proteins / metabolism
  • Ubiquitin / metabolism


  • Cell Cycle Proteins
  • Cyclin-Dependent Kinase Inhibitor p21
  • DDB1 protein, human
  • DNA-Binding Proteins
  • Proliferating Cell Nuclear Antigen
  • S-Phase Kinase-Associated Proteins
  • Tumor Suppressor Proteins
  • Ubiquitin
  • ATM protein, human
  • Ataxia Telangiectasia Mutated Proteins
  • Protein Serine-Threonine Kinases
  • Proteasome Endopeptidase Complex