ERα-regulated IRX3 controls the growth of ER-positive breast tumors

Estrogen receptor positive (ER+) breast cancer is primarily treated with endocrine therapies targeting ERα signaling. Although endocrine therapy has substantially improved survival in ER+ breast cancer, metastatic disease remains largely incurable, underscoring the need to elucidate additional mechanisms driving growth and proliferation. Here, we show that the homeobox protein IRX3 is selectively overexpressed in ER+ breast cancer and define the molecular function of IRX3 in ER+ breast cancer using an integrated combination of in vitro , in vivo and in silico approaches. We uncover a previously uncharacterized distal regulatory region that controls IRX3 transcription via ERα and associated steroid receptor coactivators. Consistent with this regulatory axis, anti-estrogen treatment resulted in marked downregulation of cellular IRX3 levels. Functionally, depletion of IRX3 suppresses proliferation of the human ER+ breast cancer cells in vitro, but paradoxically promotes tumor growth and metastatic dissemination in orthotopic xenografts in vivo by stimulating enhanced tumor vascularization. Finally, low tumor expression of IRX3 correlates with poorer survival outcomes in patients with ER+ breast cancer. Collectively, these findings establish IRX3 as an important regulator of ER+ breast tumor biology and reveal an ERα-dependent role for IRX3 in modulating proliferative and vascular programs in tumor progression.