Migrasome-propagated ITGβ3-TGFβ1 circuit orchestrates breast cancer brain metastasis by reprogramming the microglial niche.

OBJECTIVE: The dismal prognosis of breast cancer brain metastasis (BCBM) is attributed to a maladapted tumor microenvironment. This study was aimed at deciphering the role of novel intercellular communicators, tumor-derived migrasomes, in driving BCBM progression, with a focus on their ability to co-opt brain-resident microglia. METHODS: Migrasomes were isolated and characterized from brain-tropic (231-BR; Mig-BCBM) and parental breast cancer cells, patient-derived metastatic breast cancer tissues, and mice BCBM tissues. The functional axis was delineated through an integrative approach encompassing proteomics, in vitro co-culture systems, in vivo brain colonization assays, genetic perturbations, and a pharmacological intervention with the clinical-stage TGFβR1 inhibitor galunisertib. RESULTS: Mig-BCBM were internalized by microglia, thus triggering their repolarization to a pro-tumorigenic, M2-like state, both in vitro and in vivo. Proteomic profiling identified integrin β3 (ITGβ3) as a key regulator selectively packaged into Mig-BCBM. Mechanistically, migrasomal ITGβ3 activated the PI3K/AKT pathway in microglia, thus driving M2 polarization. These reprogrammed microglia secreted high levels of TGFβ1, which in turn fostered a metastatic niche by inducing Smad2/3-dependent epithelial-mesenchymal transition (EMT) in tumor cells. Crucially, we uncovered a self-amplifying feedforward loop in which microglial-derived TGFβ1 transcriptionally upregulates TSPAN4 via EGR1 in cancer cells, thus enhancing migrasome biogenesis and subsequent ITGβ3 loading. Disrupting this loop with galunisertib potently inhibited BCBM outgrowth and colonization in vivo, without systemic toxicity. CONCLUSIONS: This study identified a targetable circuit in BCBM wherein tumor-derived migrasomes, via ITGβ3, engage microglia in a TGFβ1-centered crosstalk. Because this self-reinforcing ITGβ3-TGFβ1 axis sustains the pro-metastatic niche, its disruption might provide a rational therapeutic strategy.