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• 描述MDM2 and XIAP are mutually regulated: Binding of MDM2 RING protein to the IRES region on XIAP mRNA results in MDM2 protein stabilization and enhanced XIAP translation. MX69, used for cancer treatment, is an inhibitor of MDM2/XIAP and able to bind to MDM2 RING protein with a binding K_d value of 2.34 μM. MX69 inhibited expression of both MDM2 and XIAP in a time- and dose-dependent manner. IP-Western blot assay showed that MX69 (10 μM) induced ubiquitination of endogenous MDM2 in cancer cells. Further, MX69-mediated downregulation of MDM2 protein was inhibited by protein-degradation inhibitor MG132 suggesting that downregulation of MDM2 by MX69 is through a protein-degradation mechanism. The half-life of MDM2 in control-treated EU-1 cells was greater than 90 min, whereas MX69 treatment decreased MDM2 half-life to < 30 min, further suggesting that MX69 induces MDM2 degradation. The p53 half-life in control-treated cells was less than 30 min, while it was remarkably increased by MX69 suggesting that p53 is stabilized and accumulates in MX69-treated cells. MX69 inhibited XIAP in control SH-EP1 cells but not in SH-EP1 with MDM2 KO suggesting that MX69-mediated inhibition of XIAP is MDM2-dependent. MX69 induced apoptosis and cell death in both a time- and dose-dependent manner. In vivo, MX69 was administered to SCID mice previously inoculated with EU-1 leukemia cells. All untreated control SCID mice died within 45 days after inoculation, while all mice treated with 100 mg/kg MX69 still survived after 150 days^[2].