AKT/PKB phosphorylation of p21Cip/WAF1 enhances protein stability of p21Cip/WAF1 and promotes cell survival

J Biol Chem. 2002 Mar 29;277(13):11352-61. doi: 10.1074/jbc.M109062200. Epub 2001 Dec 27.


p21(Cip1/WAF1) (p21), a p53-inducible protein, is a critical regulator of cell cycle and cell survival. p21 binds to and inhibits both the DNA synthesis regulator proliferating cell nuclear antigen and cyclin A/E-CDK2 complexes. Recently, p21 has also been shown to be a positive regulator of cell cycle progression as p21 is necessary for the assembly and activation of cyclin D1-CDK4/6 complexes. Furthermore, elevated p21 protein levels have been observed in various aggressive tumors as well as linked to chemoresistance. Here we demonstrate that p21 is directly phosphorylated by AKT/PKB, a survival kinase that is hyperactivated in many late stage tumors. Two sites (Thr(145) and Ser(146)) in the carboxyl terminus of p21 are phosphorylated by AKT/PKB in vitro and in vivo. Phosphorylation of Thr(145) inhibits PCNA binding, whereas phosphorylation of Ser(146) significantly increases p21 protein stability. Glioblastoma cell lines with activated AKT/PKB show enhanced p21 stability, and they are more resistant to taxol-mediated toxicity. Finally, AKT/PKB controls the assembly of cyclin D1-CDK4 complexes through modulation of p21 and cyclin D1 levels. These data imply that enhanced levels of p21 in tumors are due, in part, to phosphorylation by activated AKT/PKB. Furthermore, they suggest that one mechanism of AKT/PKB regulation of tumor cell survival and/or proliferation is to stabilize p21 protein.

MeSH terms

  • Amino Acid Sequence
  • Base Sequence
  • Cell Survival*
  • Cyclin D1 / metabolism
  • Cyclin-Dependent Kinase 4
  • Cyclin-Dependent Kinase Inhibitor p21
  • Cyclin-Dependent Kinases / metabolism
  • Cyclins / metabolism*
  • DNA Primers
  • Humans
  • Molecular Sequence Data
  • Phosphorylation
  • Protein-Serine-Threonine Kinases*
  • Protein-Tyrosine Kinases / chemistry
  • Protein-Tyrosine Kinases / metabolism*
  • Proto-Oncogene Proteins / chemistry
  • Proto-Oncogene Proteins / metabolism*
  • Proto-Oncogene Proteins c-akt
  • Tumor Cells, Cultured
  • Up-Regulation


  • CDKN1A protein, human
  • Cyclin-Dependent Kinase Inhibitor p21
  • Cyclins
  • DNA Primers
  • Proto-Oncogene Proteins
  • Cyclin D1
  • Protein-Tyrosine Kinases
  • AKT1 protein, human
  • Protein-Serine-Threonine Kinases
  • Proto-Oncogene Proteins c-akt
  • CDK4 protein, human
  • Cyclin-Dependent Kinase 4
  • Cyclin-Dependent Kinases