It has been shown that MDMX inhibits the activity of the tumor suppressor p53 by primarily cooperating with the p53 feedback regulator MDM2. Here, our study shows that this inhibition can be overcome by 14-3-3c and Chk1. 14-3-3c was identified as an MDMX-associated protein via an immuno-affinity purification-coupled mass spectrometry. Consistently, 14-3-3c directly interacted with MDMX in vitro, and this interaction was stimulated by MDMX phosphorylation in vitro and in cells. Interestingly, in response to UV irradiation, the wild-type, but not the kinase-dead mutant, Chk1 phosphorylated MDMX at serine 367, enhanced the 14-3-3c-MDMX binding and the cytoplasmic retaining of MDMX. The Chk1 specific inhibitor UCN-01 repressed all of these effects. Moreover, overexpression of 14-3-3c, but not its mutant K50E, which did not bind to MDMX, suppressed MDMX-enhanced p53 ubiquitination, leading to p53 stabilization and activation. Finally, ablation of 14-3-3c by siRNA reduced UV-induced p53 level and G1 arrest. Thus, these results demonstrate 14-3-3c and Chk1 as two novel regulators of MDMX in response to UV irradiation.

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