Estrogen receptor-alpha (ER) is a DNA transcription factor that drives the majority of breast cancers, which are treated with endocrine therapy. ER also remains active in endocrine-resistant disease, a major contributor of breast cancer mortality. Many co-regulatory proteins bind ER to orchestrate changes in chromatin structure and gene regulation. We interrogated ER qPLEX-RIME proteomic datasets to identify IRF2BP2 as a high confidence ER interacting protein in breast cancer cell lines and clinical breast tumours. IRF2BP2 was originally described as a co-repressor of interferon regulatory factor 2 (IRF2) and was recently identified as a co-activator of the glucocorticoid receptor in human embryonic kidney (HEK) cells. The role of IRF2BP2 in ER signalling and in breast cancer is unknown.
We used live-cell imaging to show that silencing IRF2BP2 abrogated estrogen (E2)-stimulated proliferation of ER+ breast cancer cells (T-47D, ZR-75-1). RNA-sequencing identified ~140 E2-induced genes that became unresponsive to E2 treatment upon IRF2BP2 knockdown (FDR < 0.05, logFC > 1.0 or < -1.0). Whereas canonical ER target genes (e.g. GREB1, RARA) remained responsive to E2 in siIRF2BP2 cells, E2-induced expression of other known ER regulated genes (MYBL1, IL20, and LEF1) required IRF2BP2. Pathway analysis implicated IRF2BP2-dependent E2 regulated genes in apoptosis, proliferation, and interferon response. Importantly, we found that the interaction between ER and IRF2BP2 was unchanged by treatment with tamoxifen, indicating that targeting ER with conventional therapies may not abrogate IRF2BP2-regulated ER activity, which has implications for endocrine resistant disease. Collectively, our data identify a new ER co-factor in breast cancer that selectively activates genes contributing to ER oncogenic activity.