Reversible induction of translational isoforms of p53 in glucose deprivation

Abstract

Tumor suppressor protein p53 is a master transcription regulator, indispensable for controlling several cellular pathways. Earlier work in our laboratory led to the identification of dual internal ribosome entry site (IRES) structure of p53 mRNA that regulates translation of full-length p53 and Δ40p53. IRES-mediated translation of both isoforms is enhanced under different stress conditions that induce DNA damage, ionizing radiation and endoplasmic reticulum stress, oncogene-induced senescence and cancer. In this study, we addressed nutrient-mediated translational regulation of p53 mRNA using glucose depletion. In cell lines, this nutrient-depletion stress relatively induced p53 IRES activities from bicistronic reporter constructs with concomitant increase in levels of p53 isoforms. Surprisingly, we found scaffold/matrix attachment region-binding protein 1 (SMAR1), a predominantly nuclear protein is abundant in the cytoplasm under glucose deprivation. Importantly under these conditions polypyrimidine-tractbinding protein, an established p53 ITAF did not show nuclear-cytoplasmic relocalization highlighting the novelty of SMAR1-mediated control in stress. In vivo studies in mice revealed starvation-induced increase in SMAR1, p53 and Δ40p53 levels that was reversible on dietary replenishment. SMAR1 associated with p53 IRES sequences ex vivo, with an increase in interaction on glucose starvation. RNAi-mediated-transient SMAR1 knockdown decreased p53 IRES activities in normal conditions and under glucose deprivation, this being reflected in changes in mRNAs in the p53 and Δ40p53 target genes involved in cell-cycle arrest, metabolism and apoptosis such as p21, TIGAR and Bax. This study provides a new physiological insight into the regulation of this critical tumor suppressor in nutrient starvation, also suggesting important functions of the p53 isoforms in these conditions as evident from the downstream transcriptional target activation. Cell Death and Differentiation (2015) 22, 1203-1218; doi:10.1038/cdd.2014.220; published online 27 February 2015 p53 is a master transcription factor and tumor suppressor. Apart from post-translational modifications of p53 and concomitant protein-protein interactions of p53 with diverse factors, translational control of p53 mRNA also plays an important role under stress conditions. 1 p53 and its N-terminally truncated isoform Δ40p53 (also known as ΔN-p53 or p53/47) are translated by internal ribosome entry site (IRES)-mediated translation initiation from the same mRNA under different stress conditions that induce DNA damage, ionizing radiation and endoplasmic reticulum (ER) stress, oncogene-induced senescence and cancer. [2][3][4][5][6][7] Thus, p53 mRNA has a dual IRES structure. 8 For their function, these IRESs rely on IRES trans-acting factors (ITAFs). Polypyrimidine-tract-binding protein (PTB) was shown to have differential affinity for the two IRESs of p53 and regulated p53 IRES functions by translocating from nucleus to cytoplasm on doxorubicin-induced DNA damage. 3 This PTB-med...

Deregulation of Internal Ribosome Entry Site-Mediated p53 Translation in Cancer Cells with Defective p53 Response to DNA Damage

“…It was recently found that mouse p53 mRNA also contains an IRES sequence (14), which is consistent with a previous finding that mouse p53 synthesis is upregulated by DNA damage (5). The increase in p53 IRES activity following different cellular stress has been observed by a number of recent reports (15)(16)(17)(18)(19)(20). For instance, it was found that during DNA damage or oncogene-induced senescence (OIS), the p53 IRES exhibits enhanced activity to facilitate p53 translation (17), which provides further evidence that the p53 IRES plays a key role in regulation of p53 synthesis following geno-or cytotoxic stress.…”

“…Recently, multiple new ITAFs of the p53 IRES have been identified and have been shown to affect the translation of p53 and its isoform p47 under different stressful conditions (15,(18)(19)(20). However, it is not known how altered expression/localization and/or function of these ITAFs link to defective p53 function and cancer development.…”

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“…Its activation can be directly caused by the DNA damage that we and others previously reported in cells exposed to TiO 2 -NPs either acutely [4,21,48] or chronically [13] and which is characterized by oxidative lesions and DNA strand breaks, including double-strand breaks that may result from the duplication of cells with replication fork blockade [48]. It may also be linked to the impaired glucose metabolism that our proteomics results reveal; indeed p53 is also activated upon cell starvation and metabolic stress [49]. Its activation can lead to cell cycle arrest, senescence or apoptosis, depending on the mode of activation and DNA damage duration [50].…”

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“…Animal models for the study of p53 family members p53 +/M , TAp63 −/− and TAp73 −/− display similar premature ageing phenotype (Fitzgerald et al, 2015;Khan et al, 2015; Fig. 3.…”