Inactivation of the MDM2 RING domain enhances p53 transcriptional activity in mice

The MDM2 RING domain harbors E3 ubiquitin ligase activity critical for regulating the degradation of tumor suppressor p53, which controls many cellular pathways. The MDM2 RING domain also is required for an interaction with MDMX. Mice containing a substitution in the MDM2 RING domain, MDM2^C462A, disrupting MDM2 E3 function and the MDMX interaction, die during early embryogenesis that can be rescued by p53 deletion. To investigate whether MDM2^C462A, which retains p53 binding, has p53-suppressing activity, we generated Mdm2 ;p53^/- mice, in which we replaced the endogenous p53 alleles with an inducible p53^/- allele, and compared survival with that of similarly generated Mdm2^-/-;p53^/- mice. Adult Mdm2-null mice died ∼7 days after tamoxifen-induced p53 activation, indicating that in the absence of MDM2, MDMX cannot suppress p53. Surprisingly, Mdm2 ;p53^/- mice died ∼5 days after tamoxifen injection, suggesting that p53 activity is higher in the presence of MDM2^C462A than in the absence of MDM2. Indeed, in MDM2^C462A-expressing mouse tissues and embryonic fibroblasts, p53 exhibited higher transcriptional activity than in those expressing no MDM2 or no MDM2 and MDMX. This observation indicated that MDM2^C462A not only is unable to suppress p53 but may have gained the ability to enhance p53 activity. We also found that p53 acetylation, a measure of p53 transcriptional activity, was higher in the presence of MDM2^C462A than in the absence of MDM2. These results reveal an unexpected role of MDM2^C462A in enhancing p53 activity and suggest the possibility that compounds targeting MDM2 RING domain function could produce even more robust p53 activation.