title: AKT/PKB phosphorylation of p21Cip/WAF1 enhances protein stability of p21Cip/WAF1 and promotes cell survival.
abstract: 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.
authors.affiliation: Department of Molecular Oncology, Genentech, Inc., San Francisco, California 94080, USA.
authors.forename: Ying
authors.initials: Y
authors.lastname: Li
authors.forename: Donald
authors.initials: D
authors.lastname: Dowbenko
authors.forename: Laurence A
authors.initials: LA
authors.lastname: Lasky
mesh_terms: D000595:Amino Acid Sequence; D001483:Base Sequence; D002470:Cell Survival; D019938:Cyclin D1 / Q000378:metabolism; D051358:Cyclin-Dependent Kinase 4; D050759:Cyclin-Dependent Kinase Inhibitor p21; D018844:Cyclin-Dependent Kinases / Q000378:metabolism; D016213:Cyclins / Q000378:metabolism*; D017931:DNA Primers; D006801:Humans; D008969:Molecular Sequence Data; D010766:Phosphorylation; D017346:Protein Serine-Threonine Kinases; D011505:Protein-Tyrosine Kinases / Q000737:chemistry / Q000378:metabolism*; D011518:Proto-Oncogene Proteins / Q000737:chemistry / Q000378:metabolism*; D051057:Proto-Oncogene Proteins c-akt; D014407:Tumor Cells, Cultured; D015854:Up-Regulation
chemical_list: C493956:CDKN1A protein, human; D050759:Cyclin-Dependent Kinase Inhibitor p21; D016213:Cyclins; D017931:DNA Primers; D011518:Proto-Oncogene Proteins; D019938:Cyclin D1; D011505:Protein-Tyrosine Kinases; C494918:AKT1 protein, human; D017346:Protein Serine-Threonine Kinases; D051057:Proto-Oncogene Proteins c-akt; C495900:CDK4 protein, human; D051358:Cyclin-Dependent Kinase 4; D018844:Cyclin-Dependent Kinases