FBXW7 Directly Ubiquitinates and Degrades CTNNB1 Mediating the Suppression of ENKUR in Endometrial Cancer

Enkurin (ENKUR) is a tumor suppressor in some malignancies. However, its role in endometrial cancer (EC) remains unknown. Here, we firstly observed that reduced ENKUR expression promotes progression and poor prognosis in EC. Moreover,the overexpression of ENKUR suppressed the proliferation, migration, invasion, and intrahepatic dissemination of EC in vitro and in vivo. Repressing ENKUR expression by small-interfering RNA significantly reversed the inhibition of cell proliferation and invasion in vitro. We used co-immunoprecipitation combined with mass spectral analysis to identify the potential interactive proteins of ENKUR. Based on Gene Ontology analysis, we discovered that Wnt/β-catenin (Wnt/CTNNB1) signaling is a ENKUR-modulated key pathway. ENKUR binds to CTNNB1, significantly repressing its protein expression. Furthermore, ENKUR also binds to E3 ligase F-box and WD repeat domain containing 7 (FBXW7), a critical tumor suppressor. Interestingly, the latter binds to CTNNB1 and S502 of CTNNB1 is the key binding site, thereby increasing its protein ubiquitination and degradation. Finally, we confirmed that the predominant ubiquitination sites of CTNNB1 are located at K281 and K394. Transfection of ENKUR-overexpressing EC cells with CTNNB1 reversed the suppressive effects on tumor growth and invasion. ENKUR may be a tumor suppressor via recruiting FBXW7 to directly ubiquitinate and degrade CTNNB1 in EC.