Editors' ChoiceDNA damage

Damage Switch

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Science Signaling  31 Jan 2012:
Vol. 5, Issue 209, pp. ec38
DOI: 10.1126/scisignal.2002896

The ubiquitin ligase Mdm2 can inhibit the activity of the transcription factor p53 by targeting p53 for proteasomal degradation. Mdm2 can also increase p53 synthesis by binding to and promoting the translation of p53 mRNA (see commentary by Hamard and Manfredi). Gajjar et al. noted that, in H1299 cells expressing p53 (these cells normally lack p53), siRNA directed against Mdm2 increased apoptosis under basal conditions, whereas overexpression of Mdm2 increased apoptosis after DNA damage induced by doxorubicin, effects partially reversed by siRNA directed against the kinase ATM (ataxia telangiectasia mutated). These results suggested that Mdm2 inhibits p53 activity in the absence of genotoxic stress but increases p53 activity after DNA damage. ATM phosphorylates Mdm2 at Ser395, and apoptosis was reduced after doxorubicin treatment in various cell lines expressing a form of Mdm2 that could not be phosphorylated at this site (Ser395→Ala; S395A), ATM, and p53. Expression of the p76 Mdm2 isoform, which lacks the p53 binding site, increased apoptosis after DNA damage in H1299 cells expressing p53. In contrast, expression of forms of p53 mRNA or Mdm2 with mutations that disrupted the Mdm2–p53 mRNA interaction failed to increase apoptosis after doxorubicin treatment. These results suggested that the ability of Mdm2 to increase p53 activity after DNA damage required binding of Mdm2 to p53 mRNA, rather than p53 protein. More p53 mRNA associated with Mdm2 in cells treated with doxorubicin or cells expressing a form of Mdm2 with a phosphomimetic mutation (Ser395→Asp; S395D), whereas less p53 mRNA associated with Mdm2 in AT5-BIVA cells, which lack ATM. Mdm2 translocates to nucleoli after DNA damage, a localization that was attenuated by expressing the form of Mdm2 that could not be phosphorylated by ATM at Ser395 (S395A) or the mutant p53 mRNA that does not bind to Mdm2. In addition, a p53 construct (Sp53) that lacks initiation codons and thus does not generate p53 protein promoted the genotoxic stress-induced translocation of Mdm2 to nucleoli, and Mdm2 with the phosphomimetic mutation (S395D) constitutively localized to nucleoli when cotransfected with Sp53. Coexpression of Mdm2 increased p53 abundance in H1299 cells treated with doxorubicin, an increase that was partially attenuated if cells were also treated with an inhibitor of ATM. Mdm2-mediated ubiquitination of p53 was reduced in doxorubicin-treated cells, and Mdm2 with the S395D phosphomimetic mutation showed decreased ubiquitination of p53 compared with wild-type Mdm2. In cells expressing forms of Mdm2 with impaired binding to p53 mRNA, the turnover of p53 was higher than in cells expressing wild-type Mdm2. Thus, ATM-mediated phosphorylation of Mdm2 after DNA damage switches Mdm2 from an inhibitor of p53 to a factor that stimulates p53 activity, an effect that involves the Mdm2–p53 mRNA interaction.

M. Gajjar, M. M. Candeias, L. Malbert-Colas, A. Mazars, J. Fujita, V. Olivares-Illana, R. Fåhraeus, The p53 mRNA-Mdm2 interaction controls Mdm2 nuclear trafficking and is required for p53 activation following DNA damage. Cancer Cell 21, 25–35 (2012). [PubMed]

P.-J. Hamard, J. J. Manfredi, Mdm2’s dilemma: To degrade or to translate p53? Cancer Cell 21, 3–5 (2012). [PubMed]

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