Decreased translation of p21 waf1 mRNA causes attenuated p53 signaling in some p53 wild-type tumors

Department of Pharmacology and Toxicology, Dartmouth Medical School and Norris Cotton Cancer Center, Lebanon, NH, USA.
Cell cycle (Georgetown, Tex.) (Impact Factor: 4.57). 05/2012; 11(9):1818-26. DOI: 10.4161/cc.20208
Source: PubMed


DNA damage induces cell cycle arrest through both Chk1 and the p53 tumor suppressor protein, the latter arresting cells through induction of p21(waf1) protein. Arrest permits cells to repair the damage and recover. The frequent loss of p53 in tumor cells makes them more dependent on Chk1 for arrest and survival. However, some p53 wild type tumor cell lines, such as HCT116 and U2OS, are also sensitive to inhibition of Chk1 due to attenuated p21(waf1) induction upon DNA damage. The purpose of this study is to determine the cause of this attenuated p21(waf1) protein induction. We find that neither the induction of p21(waf1) mRNA nor protein half-life is sufficient to explain the low p21(waf1) protein levels in HCT116 and U2OS cells. The induced mRNA associates with polysomes but little protein is made suggesting these two cell lines have a reduced rate of p21(waf1) mRNA translation. This represents a novel mechanism for disruption of the p53-p21(waf1) pathway as currently known mechanisms involve either mutation of p53 or reduction of p53 protein levels. As a consequence, this attenuated p21(waf1) expression may render some p53 wild type tumors sensitive to a combination of DNA damage plus checkpoint inhibition.

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    ABSTRACT: DNA damage induces the canonical p53 pathway including elevation of p21 (waf1) resulting in arrest of cell cycle progression. This can protect cells from subsequent Chk1 inhibition. Some p53 wild-type cancer cells such as HCT116 and U2OS exhibit attenuated p21 (waf1) induction upon DNA damage due to translational inhibition, and are incapable of maintaining arrest upon Chk1 inhibition. The purpose of this study was to determine whether this attenuated p21 (waf1) induction also occurred with the non-DNA damaging agent Nutlin-3 which induces p53 by disrupting binding to its negative regulator MDM2. We find that Nutlin-3 circumvented the attenuated induction of p21 (waf1) protein by increasing its half-life which led to G 1 and G 2 arrest in both cell lines. Interestingly, the p21 (waf1) protein half-life remained short on Nutlin-3 in p53 wild-type MCF10A cells; these cells achieve high p21 (waf1) levels through transcriptional upregulation. Consequently, all three p53 wild-type cells but not p53 mutant MDA-MB-231 cancer cells were protected from subsequent incubation with a combination of DNA damage plus a checkpoint inhibitor.
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    ABSTRACT: It is now largely accepted that ribosomal proteins may be implicated in a variety of biological functions besides that of components of the translation machinery. Many evidences show that a subset of ribosomal proteins are involved in the regulation of the cell cycle and apoptosis through modulation of p53 activity. In addition, p53-independent mechanisms of cell cycle arrest in response to alterations of ribosomal proteins availability have been described. Here, we identify human rpL3 as a new regulator of cell cycle and apoptosis through positive regulation of p21 expression in a p53-independent system. We demonstrate that the rpL3-mediated p21 upregulation requires the specific interaction between rpL3 and Sp1. Furthermore, in our experimental system, p21 overexpression leads to a dual outcome, activating the G 1/S arrest of the cell cycle or the apoptotic pathway through mitochondria, depending on its intracellular levels. It is noteworthy that depletion of p21 abrogates both effects. Taken together, our findings unravel a novel extraribosomal function of rpL3 and reinforce the proapoptotic role of p21 in addition to its widely reported ability as an inhibitor of cell proliferation.
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