The MDM2–p53 feedback loop is tightly regulated to restrain inappropriate p53 activation while allowing rapid cellular stress responses. This dissertation investigates regulation of this pathway by the nucleolar protein nucleostemin (NS) and ribosomal proteins L5, L11, and L23. NS was found to bind the acidic domain of MDM2, inhibit its E3 ubiquitin ligase activity, and stabilize p53, leading to p21 induction and cell‑cycle arrest. Both overexpression and depletion of NS activated p53, indicating that aberrant NS levels disrupt cellular homeostasis. Nucleolar stress induced by mycophenolic acid or 5‑fluorouracil promoted release of NS and ribosomal proteins from the nucleolus, enhancing their interaction with MDM2 and activation of p53. These findings identify the nucleolus as a key stress sensor and reveal cooperative roles for NS and ribosomal proteins in modulating the MDM2–p53 pathway.
