Estradiol-associated extracellular signaling in the human breast in vivo identifies mast cells as tumor-supportive stromal mediators

Sex steroid exposure is a major risk factor for breast cancer initiation and progression through incompletely understood mechanisms. Novel therapeutics for prevention and treatment of estrogen-dependent breast cancer are needed. Cell-cell interactions within the microenvironment contribute to breast cancer progression through extracellular signaling. Here, extracellular proteins were sampled directly from live normal human breast tissue and breast cancers using microdialysis, and 1,101 proteins were quantified. Mechanistic studies were performed in cultured intact normal human breast tissue and mast cells. Findings were validated in vivo using ER+ breast cancer models in immunocompetent and immunocompromised mice and in a zebrafish xenograft model. We identified a breast tissue-specific extracellular protein signature comprising 150 proteins associated with local estradiol (E2) levels, of which several were linked to mast cell biology, including stem cell factor (SCF). Human ER+ breast tumors exhibited increased extracellular SCF levels and higher mast cell density, which was associated with poorer overall survival. In normal human breast tissue, E2 increased mast cell abundance and activity. In vitro, E2 rapidly activated mast cells, an effect blocked by desloratadine. In mouse breast cancer models, E2 increased and fulvestrant reduced mast cell density. In the zebrafish, mast cells promoted ER+ breast cancer growth, which was further enhanced by E2 and abrogated by desloratadine. Overall, our findings identify mast cells as estrogen-responsive stromal mediators and support targeting mast cell-dependent pathways as a potential strategy for the prevention and treatment of estrogen-dependent breast cancer.