Abstract

Medroxyprogesterone acetate (MPA) is a major fertility-preserving therapeutic option for patients with endometrial cancer (EC); however, MPA resistance markedly restricts the efficacy of conservative treatments. Downregulation of the progesterone receptor (PR) expression is a key determinant of MPA resistance; however, the underlying molecular mechanisms have not been elucidated comprehensively. In this study, we identified AMBRA1, a substrate-specific adaptor of CUL4-RING E3 ubiquitin ligase (CRL4), as a critical regulator of PR stability and the MPA response. AMBRA1 expression was significantly increased in MPA-resistant EC tissues, and its overexpression induced an MPA-resistant phenotype. Mechanistically, AMBRA1 promotes ubiquitin-mediated degradation of PR by targeting lysine 388 in a CRL4 AMBRA1 complex-dependent manner. Moreover, pharmacological inhibition of the CRL4 AMBRA1 complex with MLN4924, an FDA-approved antitumor drug that blocks NEDD8-dependent Cullin-RING ligase activation, stabilizes PR and markedly restores MPA sensitivity in MPA-resistant EC cell lines and patient-derived organoid models. Collectively, these findings suggest that the CRL4 AMBRA1 ubiquitin ligase facilitates PR degradation, inducing resistance to MPA. Furthermore, this study identified the CRL4 AMBRA1 complex as a potential therapeutic target for overcoming MPA resistance in EC.