Abnormal MDMX Degradation In Tumor Cells Due To ARF Deficiency

MDMX is a heterodimeric partner of MDM2and a critical regulator of p53. The MDMX level is generally elevated in tumors with wild-type p53and contributes to p53inactivation. MDMX degradation is controlled in part by MDM2-mediated ubiquitination. Here, we show that MDMX turnover is highly responsive to changes in MDM2level in non-transformed cells, but not in tumor cells. We found that loss of alternate reading frame (ARF) expression, which occurs in most tumors with wild-type p53, significantly reduces MDMX sensitivity to MDM2. Restoration of ARF expression in tumor cells enables MDM2to degrade MDMX in a dose-dependent manner. ARF binds to MDM2and stimulates a second-site interaction between the central region of MDM2and MDMX, and thus increases MDMX-MDM2binding and MDMX ubiquitination. These results reveal an important abnormality in the p53-regulatory pathway as a consequence of ARF deficiency. Loss of ARF during tumor development not only prevents p53stabilization by proliferative stress but also causes accumulation of MDMX that compromises p53activity. This phenomenon may reduce the clinical efficacy of MDM2-specific inhibitors by preventing MDMX downregulation.