p21(Waf1/Cip1/Sdi1) mediates retinoblastoma protein degradation

Oncogene. 2007 Oct 18;26(48):6954-8. doi: 10.1038/sj.onc.1210516. Epub 2007 May 7.


Damage-induced G1 checkpoint in mammalian cells involves upregulation of p53, which activates transcription of p21(Waf1) (CDKN1A). Inhibition of cyclin-dependent kinase (CDK)2 and CDK4/6 by p21 leads to dephosphorylation and activation of Rb. We now show that ectopic p21 expression in human HT1080 fibrosarcoma cells causes not only dephosphorylation but also depletion of Rb; this effect was p53-independent and susceptible to a proteasome inhibitor. CDK inhibitor p27 (CDKN1B) also caused Rb dephosphorylation and depletion, but another CDK inhibitor p16 (CDKN2A) induced only dephosphorylation but not depletion of Rb. Rb depletion was observed in both HT1080 and HCT116 colon carcinoma cells, where p21 was induced by DNA-damaging agents. Rb depletion after DNA damage did not occur in the absence of p21, and it was reduced when p21 induction was inhibited by p21-targeting short hairpin RNA or by a transdominant inhibitor of p53. These results indicate that p21 both activates Rb through dephosphorylation and inactivates it through degradation, suggesting negative feedback regulation of damage-induced cell-cycle checkpoint arrest.

Publication types

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

MeSH terms

  • Antibiotics, Antineoplastic / pharmacology
  • Colonic Neoplasms / metabolism*
  • Colonic Neoplasms / pathology
  • Cyclin-Dependent Kinase Inhibitor p16 / metabolism
  • Cyclin-Dependent Kinase Inhibitor p21 / metabolism*
  • Cyclin-Dependent Kinase Inhibitor p27 / metabolism*
  • DNA Damage / drug effects
  • Doxorubicin / pharmacology
  • Fibrosarcoma / metabolism*
  • Fibrosarcoma / pathology
  • Humans
  • Immunoblotting
  • Phosphorylation / drug effects
  • Retinoblastoma Protein / metabolism*
  • Tumor Cells, Cultured / drug effects
  • Tumor Cells, Cultured / metabolism
  • Tumor Suppressor Protein p53 / metabolism


  • Antibiotics, Antineoplastic
  • CDKN1A protein, human
  • Cyclin-Dependent Kinase Inhibitor p16
  • Cyclin-Dependent Kinase Inhibitor p21
  • Retinoblastoma Protein
  • Tumor Suppressor Protein p53
  • Cyclin-Dependent Kinase Inhibitor p27
  • Doxorubicin