Ubr5 Over-Expression Contributes To Poor Prognosis And Tamoxifen Resistance Of Era Plus Breast Cancer By Stabilizing Beta-Catenin

Background Tamoxifen (TAM) resistance is a critical clinical challenge in the treatment of ERa+ breast cancer. However, the underlying mechanisms involved in TAM-resistance are not fully understood. Here we study the efficacy of UBR5 in predicting TAM-resistance in ERa+ breast cancer.Methods Western blot RT-PCR and IHC staining were used to evaluate UBR5 protein and mRNA levels in ERa+ breast cancer cell and tissues. MTT assays and colony formation assays were used to measure cell proliferation. The xeno-graft tumor model was used for in vivo study. We performed protein stability assay and ubiquitin assay to detect beta-catenin protein degradation. Immuno-precipitation assay was used to detect the interaction between UBR5 and beta-catenin. The ubiquitin-based immuno-precipitation based assay was used to detect the ubiquitination of beta-catenin.Results High UBR5 expression was correlated with poor prognosis in ER+ breast cancer. Importantly, UBR5 expression was remarkably upregulated in TAM-refractory breast cancer tissues compared with their primary paired TAM-untreated tissues. Additionally, UBR5 overexpression caused tamoxifen-resistance in vitro, whereas UBR5 knockdown increased TAM sensitivity. Mechanistic investigations revealed that UBR5 overexpression, through its ubiquitin ligase catalyzing activity, led to up-regulation of beta-catenin expression and activity. Finally, our results confirmed that TAM-resistance promoting effects by UBR5 in ERa+ breast cancer cells was at least partly due to beta-catenin stabilization, and inhibition of the UBR5/beta-catenin signaling re-sensitizing the resistant breast cancer cells to tamoxifen in vivo.Conclusions These findings suggested that UBR5/beta-catenin signaling might be a potential therapeutic target for TAM-resistant ERa+ breast cancer.