MDM2 and promyelocytic leukemia antagonize each other through their direct interaction with p53.

p53 can be regulated through post-translational modifications and through interactions with positive and negative regulatory factors. MDM2 binding inhibits p53 and promotes its degradation by the proteasome, whereas promyelocytic leukemia (PML) activates p53 by recruiting it to multiprotein complexes termed PML-nuclear bodies. We reported previously an in vivo and in vitro interaction between PML and MDM2 that is independent of p53. In the current study, we investigated whether interaction between MDM2 and PML can indirectly affect p53 activity. Increasing amounts of MDM2 inhibited p53 activation by PML but could not inhibit PML-mediated activation of a p53 fusion protein that lacked the MDM2-binding domain. Conversely, increasing amounts of PML could overcome p53 inhibition by MDM2 but could not overcome MDM2-mediated inhibition of a p53 fusion protein that lacked the PML-binding domain. These results demonstrate that MDM2 and PML can antagonize each other through their direct interaction with p53 and suggest the combined effects of MDM2 and PML on p53 function are determined by the relative level of each protein. Furthermore, these results imply that interactions between MDM2 and PML by themselves have little or no effect on p53 activity.