title: Hdm2- and proteasome-dependent turnover limits p21 accumulation during S phase.
abstract: Double-strand DNA breaks detected in different phases of the cell cycle induce molecularly distinct checkpoints downstream of the ATM kinase. p53 is known to induce arrest of cells in G 1 and occasionally G 2 phase but not S phase following ionizing radiation, a time at which the MRN complex and cdc25-dependent mechanisms induce arrest. Our understanding of how cell cycle phase modulates pathway choice and the reasons certain pathways might be favored at different times is limited. In this report, we examined how cell cycle phase affects the activation of the p53 checkpoint and its ability to induce accumulation of the cdk2 inhibitor p21. Using flow cytometric tools and centrifugal elutriation, we found that the p53 response to ionizing radiation is largely intact in all phases of the cell cycle; however, the accumulation of p21 protein is limited to the G 1 and G 2 phase of the cell cycle because of the activity of a proteasome-dependent p21 turnover pathway in S-phase cells. We found that the turnover of p21 was independent of the SCF (skp2) E3 ligase but could be inhibited, at least in part, by reducing hdm2, although this depended on the cell type studied. Our results suggest that there are several redundant pathways active in S-phase cells that can prevent the accumulation of p21.
authors.affiliation: Program in Molecular Biology, Sloan-Kettering Institute, Memorial Sloan-Kettering Cancer Center, New York, NY, USA.
authors.forename: Daniel
authors.initials: D
authors.lastname: Ciznadija
authors.forename: Xin-Hua
authors.initials: XH
authors.lastname: Zhu
authors.forename: Andrew
authors.initials: A
authors.lastname: Koff
mesh_terms: D059447:Cell Cycle Checkpoints; D002461:Cell Line, Transformed; D050759:Cyclin-Dependent Kinase Inhibitor p21 / Q000235:genetics / Q000378:metabolism*; D053903:DNA Breaks, Double-Stranded / Q000528:radiation effects; D006801:Humans; D046988:Proteasome Endopeptidase Complex / Q000235:genetics / Q000378:metabolism*; D051736:Proto-Oncogene Proteins c-mdm2 / Q000235:genetics / Q000378:metabolism*; D034622:RNA Interference; D034741:RNA, Small Interfering; D011839:Radiation, Ionizing; D016196:S Phase; D044786:S-Phase Kinase-Associated Proteins / Q000096:biosynthesis / Q000378:metabolism*; D016159:Tumor Suppressor Protein p53 / Q000235:genetics / Q000378:metabolism
chemical_list: D050759:Cyclin-Dependent Kinase Inhibitor p21; D034741:RNA, Small Interfering; D044786:S-Phase Kinase-Associated Proteins; C495901:TP53 protein, human; D016159:Tumor Suppressor Protein p53; C497206:MDM2 protein, human; D051736:Proto-Oncogene Proteins c-mdm2; D046988:Proteasome Endopeptidase Complex