8028 Targeted Therapy-Induced ER and HER2 Chromatin Binding: a Novel Driver of Poor Response to Treatment in Triple Positive Breast Cancer

Abstract Disclosure: S. Tam: None. M.D. McCoy: None. S. Bahnassy: None. R.B. Riggins: None. Triple positive breast cancer (TPBC) is a unique variant of breast cancer characterized by co-expression of two oncogenic drivers, estrogen receptor alpha (ER) and human epidermal growth factor receptor 2 (HER2). Both drivers promote tumor progression and serve as targets for effective small molecule and monoclonal antibody (mAb) therapies. Previous studies showed that the co-expression of ER reduces the efficacy of anti-HER2 targeted therapy in TPBC, and vice versa. ER is a well-established nuclear receptor and transcription factor. HER2 mainly functions as a transmembrane receptor, activating cytosolic pro-tumorigenic signaling pathways. However, less-studied nuclear HER2 is involved in the transcriptional regulation of several genes, leading to increased BC cell growth and survival. In this study, we investigate the subcellular localization and transcriptional regulatory functions of ER and HER2 in the unique context of TPBC, using cell fractionation, immunofluorescence, chromatin immunoprecipitation (ChIP)-qPCR, and integrated analysis of ER ChIP-seq/RNA-seq data. We hypothesize that crosstalk between ER and HER2 in the nucleus may, in part, underlie poor response to targeted therapy. At baseline, ER has strong nuclear localization in TPBC cells, BT474 and MDA-MB-361, but is predominantly cytosolic in ER+ MCF7 cells. Activated, phospho-HER2 is also localized to the nucleus of TPBC and HER2+ SkBr3 cells at baseline. Treatment with the anti-HER2 mAb combination of trastuzumab and pertuzumab (TP) induces ER binding to specific ER binding sites (ERBSs) in the enhancers or promoters of PGR, TFF1, and FOXC1 in BT474, but this is not observed in MCF7 or SkBr3 cells. Unlike TP, which increases HER2 nuclear localization but not ERBS recruitment, fulvestrant (a selective ER degrader) induces HER2 binding at the same ERBSs in BT474, but not MCF7 or SkBr3 cells. Collectively, these results suggest that the inhibition of either ER or HER2 can promote chromatin binding of the other at ERBSs. Integration of ER ChIP-seq peaks and differentially expressed genes from RNA-seq of BT474 cells treated with E2 vs control following short-term estrogen deprivation shows the ER cistrome is enriched for genes associated with RNA binding, RNA processing, and ribosome biogenesis. Interestingly, nuclear HER2 is reported to increase BC cell growth by activating transcription of ribosomal RNA genes. By contrast, the ER cistrome in MCF7 cells is enriched for genes associated with cell cycle regulation. In summary, anti-HER2 therapy induces recruitment of ER to well-established ERBSs, while anti-ER treatment causes recruitment of HER2 to these same sites in TPBC. Ongoing and future work leverages HER2 cistrome data to further define nuclear HER2/ER functional interactions, at the level of transcriptional regulation, that may contribute to poor treatment outcomes in TPBC. Presentation: 6/2/2024