Mouse double minute 2 (MDM2) participates in protein synthesis, folding, and ubiquitin-mediated degradation and is therefore a proteostasic hub protein. The MDM2 interactome contains over 100 proteins, yet stratification of dominant MDM2-interacting proteins has not been achieved. 8-plex iTRAQ (nanoLC-MS/MS) of MCF7 cells treated with the MDM2-binding ligand Nutlin-3 identified the most abundant cellular protein changes over early time points; 1,323 unique proteins were identified including 35 with altered steady-state levels within 2 h of Nutlin-3 treatment, identifying a core group of MDM2 related proteins. Six of these proteins were previously identified MDM2 interactors, and the effects of Nutlin-3 on the MDM2-nucleophosmin interaction (NPM) was further validated. This revealed that Nutlin-3 mediates the in vivo conversion of NPM from an oligomer to a monomer as an MDM2-dependent phenomenon, with Nutlin-3 stimulating MDM2 binding to a peptide motif derived from the oligomerization interface of NPM. These data form the first proteomic screen of Nutlin-3 in cells whereby we (i) identify the most abundant MDM2-interacting proteins whose steady-state levels change early after Nutlin-3 treatment; (ii) identify the first protein apart from p53, nucleophosmin (NPM), whose interaction with MDM2 can be stimulated allosterically by Nutlin-3; and (iii) raise the possibility that Nutlin-3 might act as a general agonist of other MDM2 protein-protein interactions.