The atypical ubiquitin ligase RNF31 stabilizes estrogen receptor α and modulates estrogen-stimulated breast cancer cell proliferation

Estrogen receptor alpha (ER alpha) is initially expressed in the majority of breast cancers and promotes estrogen-dependent cancer progression by regulating the transcription of genes linked to cell proliferation. ER alpha status is of clinical importance, as ER alpha-positive breast cancers can be successfully treated by adjuvant therapy with antiestrogens or aromatase inhibitors. Complications arise from the frequent development of drug resistance that might be caused by multiple alterations, including components of ER alpha signaling, during tumor progression and metastasis. Therefore, insights into the molecular mechanisms that control ER alpha expression and stability are of utmost importance to improve breast cancer diagnostics and therapeutics. Here we report that the atypical E3 ubiquitin ligase RNF31 stabilizes ER alpha and facilitates ER alpha-stimulated proliferation in breast cancer cell lines. We show that depletion of RNF31 decreases the number of cells in the S phase and reduces the levels of ER alpha and its downstream target genes, including cyclin D1 and c-myc. Analysis of data from clinical samples confirms correlation between RNF31 expression and the expression of ER alpha target genes. Immunoprecipitation indicates that RNF31 associates with ER alpha and increases its stability and mono-ubiquitination, dependent on the ubiquitin ligase activity of RNF31. Our data suggest that association of RNF31 and ER alpha occurs mainly in the cytosol, consistent with the lack of RNF31 recruitment to ER alpha-occupied promoters. In conclusion, our study establishes a non-genomic mechanism by which RNF31 via stabilizing ER alpha levels controls the transcription of estrogen-dependent genes linked to breast cancer cell proliferation.