Phosphorylated NCOA6 promotes aggressiveness and contributes patient outcomes in estrogen receptor-positive breast cancer

BACKGROUND: Breast cancer heterogeneity is not only involved by genetic and epigenetic alterations, but may also associated with the complexities at levels of protein modifications. Estrogen receptor (ER) positive breast cancer constitutes approximately 70% of all breast cancer cases, significantly impacting global female morbidity and mortality. Nuclear receptor co-activator 6 (NCOA6, also referred as AIB3) is a coregulator of sex hormone receptors as well as other transcription factors. Although the expression and functions of NCOA6 have been studied in breast, colon, lung, pancreatic cancers, and hepatocellular carcinoma, post-tranlational modification of NCOA6 is barely known. This study aims to study the NCOA6 phosphorylation, in particular at Serine 884, and its relevance to breast tumor aggressiveness and the prognostic significance. METHODS: We used bioinformatics and immunohistochemistry to detect NCOA6 expression and Ser884 phosphorylation in ER-positive breast cancer tissues. Cell proliferation, migration, and invasion were measured using CCK-8, transwell, and wound healing assays. Immunoblotting was used to detect protein levels and phosphorylation. Mouse xenograft models were established to evaluate tumor growth in vivo. Transcriptome and proteome analyses were performed to identify related signaling pathways. RESULTS: Total NCOA6 expression was not linked to cancer stage, subtype, or prognosis. However, phosphorylation at Ser884 was significantly increased in ER-positive breast cancer and associated with poorer outcomes, indicating its potential as an independent prognostic marker. Functional studies demonstrated that Ser884 phosphorylation promoted tumor cell proliferation, migration, and invasion in vitro and enhanced tumor growth in vivo. Mechanistically, ERK2 activation induced NCOA6 phosphorylation at Ser884, promoting epithelial-mesenchymal transition (EMT) and metastatic behavior. Only the phosphomimetic S884E mutant, not the non-phosphorylatable S884F mutant, restored migration and invasion in NCOA6-deficient cells. Transcriptomic and proteomic analyses further suggested that NCOA6 influences cell cycle, PI3K/AKT/mTOR signaling, and protein synthesis. CONCLUSION: Phosphorylation of NCOA6 at Ser884 is markedly increased in ER-positive breast cancer, associates with aggressive clinicopathologic features, and portends poor prognosis. Functionally, enhanced NCOA6 phosphorylation promotes invasion and metastasis of ER-positive breast cancer cells. Collectively, these findings identify phospho-NCOA6 (Ser884) as a promising prognostic biomarker and potential therapeutic target, supporting more personalized management of ER-positive disease.