AMP-Activated Protein Kinase Induces p53 by Phosphorylating MDMX and Inhibiting Its Activity

Abstract

h AMP-activated protein kinase (AMPK) has been shown to activate p53 in response to metabolic stress. However, the underlying mechanisms remain unclear. Here we show that metabolic stresses induce AMPK-mediated phosphorylation of human MDMX on Ser342 in vitro and in cells, leading to enhanced association between MDMX and 14-3-3. This markedly inhibits p53 ubiquitylation and significantly stabilizes and activates p53. By striking contrast, no phosphorylation of MDM2 by AMPK was noted. AMPK-mediated MDMX phosphorylation, MDMX-14-3-3 binding, and p53 activation were drastically reduced in mouse embryo fibroblasts harboring endogenous MDMX with S341A (mouse homologue of human serine 342), S367A, and S402A (mouse homologue of human serine 403) mutations. Moreover, deficiency of AMPK prevented MDMX-14-3-3 interaction and p53 activation. The activation of p53 through AMPK-mediated MDMX phosphorylation and inactivation was further confirmed by using cell and animal model systems with two AMPK activators, metformin and salicylate (the active form of aspirin). Together, the results unveil a mechanism by which metabolic stresses activate AMPK, which, in turn, phosphorylates and inactivates MDMX, resulting in p53 stabilization and activation.T he p53 tumor suppressor executes its antitumor functions primarily via its transcriptional activity to induce the expression of protein-encoding genes responsible for p53-dependent apoptosis, cell growth arrest, differentiation, and senescence (1) as well as its ability to induce apoptosis and autophagy by transcriptionindependent mechanisms (2). Since these cellular functions are detrimental to cells, p53 is often tightly monitored by a pair of partner proteins, MDM2 (called HDM2 in humans) and MDMX (also called MDM4), in normally growing cells (3-5). MDM2 and MDMX act as a complex during early embryogenesis (6-10) to ubiquitylate p53 and mediate its proteosomal turnover as well as inactivate its activity in a negative-feedback fashion (10-12), and cooperatively or individually restrain the p53 level to maintain the normal development and function of different tissues (13-16), by binding to p53, inhibiting its transcriptional activity and/or enhancing its ubiquitination. Hence, to activate p53, cells need to trigger different cellular mechanisms or pathways that block the MDM2-MDMX-p53 feedback loop through modifications of one of these proteins in response to a variety of stresses (17, 18). For instance, DNA damage signals can induce p53 by activating the ATM-Chk2 or ATR-Chk1 pathway that leads to phosphorylation of p53, MDMX, and MDM2 (19-21). Of relevance to MDMX, Ser367 phosphorylation by Chk2 or Chk1 triggers interaction between MDMX and 14-3-3, leading to MDMX inactivation and p53 activation (19,21,22). The importance of 14-3-3 binding to Ser367-phosphorylated MDMX for p53 activation by DNA damage was further emphasized in an animal knock-in study in which three serines, including Ser341, Ser367, and Ser402 (23), were mutated into alanines. This mutant MDMX exhibits substantial...

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