Amphiregulin retains ERα expression in acquired aromatase inhibitor resistant breast cancer cells

Acquired resistance to aromatase inhibitors (AIs) is a significant clinical issue in endocrine therapy for estrogen receptor (ER) positive breast cancer which accounts for the majority of breast cancer. Despite estrogen production being suppressed, ERα signaling remains active and plays a key role in most AI-resistant breast tumors. Here, we found that amphiregulin (AREG), an ERα transcriptional target and epidermal growth factor receptor (EGFR) ligand, is crucial for maintaining ERα expression and signaling in acquired AI-resistant breast cancer cells. AREG was deregulated and critical for cell viability in ER+ AI-resistant breast cancer cells, and ectopic expression of AREG in hormone responsive breast cancer cells promoted endocrine resistance. RNA-sequencing and reverse phase protein array analyses revealed that AREG maintains ERα expression and signaling by activation of PI3K/Akt/mTOR signaling and upregulation of Forkhead Box M1 (FoxM1) and Serum- and glucocorticoid-inducible kinase 3 (SGK3) expression. Our study uncovers a previously unappreciated role of AREG in maintaining ERα expression and signaling, and establishes the AREG-ERα crosstalk as a driver of acquired AI resistance in breast cancer.