Hdm2-and proteasome-dependent turnover limits p21 accumulation during S phase

Program in Molecular Biology, Sloan-Kettering Institute, Memorial Sloan-Kettering Cancer Center, New York, NY, USA.
Cell cycle (Georgetown, Tex.) (Impact Factor: 4.57). 08/2011; 10(16):2714-23. DOI: 10.4161/cc.10.16.16725
Source: PubMed


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.

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Available from: Daniel Ciznadija, May 26, 2015
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    ABSTRACT: Comment on: Ciznadija D, et al. Cell Cycle 2011; 10:2714-23.
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    ABSTRACT: Comment on: Ciznadija D, et al. Cell Cycle 2011; 10:2714-23.
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    ABSTRACT: The cyclin-dependent kinase inhibitor p21(CIP1/WAF1) is a regulatory factor of the cell cycle. Its transcriptional activation and its protein stability are tightly controlled by several distinct mechanisms. S100A11 is a member of the S100 family of Ca(2+) -binding proteins involved in several biological processes including cell cycle progression and signal transduction. Here, we show that down-regulation of S100A11 results in the reduction of p21 protein in human HaCaT keratinocytes. It seems that a ubiquitin-independent proteasomal degradation process is involved in p21 degradation in S100A11 down-regulated cells. The application of a proteasome inhibitor stabilized p21 protein in these cells. Analyses of distinct signal transduction pathways revealed a disturbed phosphatidylinositol-3-kinase/Akt pathway after S100A11 knock-down. We determined that the glycogen synthase kinase-3, which is negatively regulated by PI3K/Akt, was activated in cells possessing knocked-down S100A11 and seems to be involved in p21 protein destabilization. The application of a specific inhibitor of GSK3 resulted in an increase of the p21 protein level in S100A11 down-regulated HaCaT cells. GSK3 is able to phosphorylate p21 at T57 that induces p21 proteasomal turnover. Mutation of the GSK3 site threonine-57 into alanine (T57A) stabilizes p21 in HaCaT cells lacking S100A11. Beside decreased p21 protein, down-regulation of S100A11 triggered the induction of apoptosis in HaCaT cells. These observations suggest that S100A11 is involved in maintenance of p21 protein stability and seems to function as an inhibitor of apoptosis in human HaCaT keratinocyte cells. Thus, the data shed light on a novel pathway regulating p21 protein stability. This article is protected by copyright. All rights reserved.
    No preview · Article · Jun 2013 · FEBS Journal
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