Nuclear Beclin 1 Destabilizes Retinoblastoma Protein to Promote Cell Cycle Progression and Colorectal Cancer Growth

SIMPLE SUMMARY: The role of autophagy core-protein Beclin 1 in colorectal cancer (CRC) development remains controversial. Here, we show that nuclear Beclin 1 is up-regulated in CRC with a negative correlation to RB protein expression. Silencing of BECN1 up-regulates RB expression resulting in cell cycle G1 arrest and inhibition of xenograft tumor growth independent of p53. Ablation of BECN1 facilitates MDM2–MDMX complex formation to promote MDMX polyubiquitination and degradation, consequently leading to RB protein stabilization. These results reveal that nuclear Beclin 1 can promote CRC growth through modulation of RB protein stability and imply that nuclear Beclin 1 may be a prognostic indicator in human colorectal cancer. ABSTRACT: Autophagy is elevated in colorectal cancer (CRC) and is generally associated with poor prognosis. However, the role of autophagy core-protein Beclin 1 remains controversial in CRC development. Here, we show that the expression of nuclear Beclin 1 protein is upregulated in CRC with a negative correlation to retinoblastoma (RB) protein expression. Silencing of BECN1 upregulates RB resulting in cell cycle G1 arrest and growth inhibition of CRC cells independent of p53. Furthermore, ablation of BECN1 inhibits xenograft tumor growth through elevated RB expression and reduced autophagy, while simultaneous silencing of RB1 restores tumor growth but has little effect on autophagy. Mechanistically, knockdown of BECN1 promotes the complex formation of MDM2 and MDMX, resulting in MDM2-dependent MDMX instability and RB stabilization. Our results demonstrate that nuclear Beclin 1 can promote cell cycle progression through modulation of the MDM2/X-RB pathway and suggest that Beclin 1 promotes CRC development by facilitating both cell cycle progression and autophagy.