Brain FGF2 and NCAM1 contribute to FGFR1-dependent progression of ER+ breast cancer brain metastases in young and aged hosts

Estrogen receptor positive (ER+) breast cancer represents a significant proportion of breast cancer brain metastasis but remains understudied. Here, we report that FGFR1-amplification, a well-established driver of ER+ breast cancer endocrine resistance, promotes ER+ breast cancer brain metastatic colonization in young and aged female mice, through both canonical FGF2/FGFR1 signaling and non-canonical NCAM1/FGFR1 interactions. Astrocytic FGF2-mediated paracrine activation of FGFR1 promoted breast cancer brain metastasis in estrogen-treated young mice, but FGF2 levels and signaling decreased in the brain with aging and estrogen-depletion. Neuronal and astrocytic NCAM1, which remain unchanged in young and aged brains, promoted adhesion to neurons, migration, and growth of ER+ cells, suggesting that interactions with astrocytes and neurons facilitate early ER+ breast cancer brain metastasis colonization through FGFR1. Importantly, FDA-approved FGFR inhibitors effectively blocked early colonization but not late-stage brain metastases, suggesting prevention of FGFR1+ brain metastases as a window of opportunity for FGFR1 inhibitors.