MDMX is essential for the regulation of p53 protein levels in the absence of a functional MDM2 C-terminal tail.

Background: MDM2 is an E3 ubiquitin ligase that is able to ubiquitinate p53, targeting it for proteasomal degradation. Its homologue MDMX does not have innate E3 activity, but is able to dimerize with MDM2. Although mouse models have demonstrated both MDM2 and MDMX are individually essential for p53 regulation, the significance of MDM2-MDMX heterodimerization is only partially understood and sometimes controversial. MDM2^C462A mice, where the C462A mutation abolishes MDM2 E3 ligase activity as well as its ability to dimerize with MDMX, die during embryogenesis. In contrast, the MDM2^Y487A mice, where the Y487A mutation at MDM2 C-terminus significantly reduces its E3 ligase activity without disrupting MDM2-MDMX binding, survive normally even though p53 is expressed to high levels. This indicates that the MDM2-MDMX heterodimerization plays a critical role in the regulation of p53. However, it remains unclear whether MDMX is essential for the regulation of p53 protein levels in the context of an endogenous MDM2 C-terminal tail mutation.

Results: Here, we studied the significance of MDM2-MDMX binding in an MDM2 E3 ligase deficient context using the MDM2^Y487A mouse embryonic fibroblast (MEF) cells. Surprisingly, down-regulation of MDMX in MDM2^Y487A MEFs resulted in a significant increase of p53 protein levels. Conversely, ectopic overexpression of MDMX reduced p53 protein levels in MDM2^Y487A MEFs. Mutations of the RING domain of MDMX prevented MDMX-MDM2 binding, and ablated MDMX-mediated suppression of p53 protein expression. Additionally, DNA damage treatment and nuclear sequestration of MDMX inhibited MDMX activity to suppress p53 protein expression.

Conclusions: These results suggest that MDMX plays a key role in suppressing p53 protein expression in the absence of normal MDM2 E3 ligase activity. We found that the ability of MDMX to suppress p53 levels requires MDM2 binding and its cytoplasmic localization, and this ability is abrogated by DNA damage. Hence, MDMX is essential for the regulation of p53 protein levels in the context of an MDM2 C-terminal mutation that disrupts its E3 ligase activity but not MDMX binding. Our study is the first to examine the role of MDMX in the regulation of p53 in the context of endogenous MDM2 C-terminal mutant MEF cells.