Breast Cancer-Derived Extracellular Vesicle miR-425-5p (miR-425) Promotes Brain Metastasis via Activating Astrocytes Through the Novel miR-425-ZNF24-CCL8 Signaling Axis.

Mechanisms underlying breast cancer brain metastasis (BCBM) are still not well understood. Here, we identified that BCBM patient serum contained extracellular vesicles (EVs) with high levels of microRNAs (miRNAs)-107 and -425. Levels of miR-107 and miR-425 were elevated in brain metastases, and the elevation was associated with poor patient prognoses. Ectopic expression of miR-107 and miR-425 promoted mammospheres; however, the inhibition of miR-425, but not miR-107, suppressed breast cancer mammosphere formation. We further observed that EVs from miR-425-overexpressing breast cancer cells strongly activated astrocytes whereas their inhibitors abrogated the effect. Conditioned media from miR-425-activated astrocytes promoted mammospheres. To elucidate how miR-425 activates astrocytes, we found that, within astrocytes, miR-425 suppressed the expression of transcription factor ZNF24, which downregulated CCL8 cytokine expression/secretion, leading to the subsequent activation of astrocytes. Using a mouse study, we further determined the role of miR-425 in brain metastasis formation and observed that miR-425-overexpressing breast cancer cells exhibited significantly more aggressive growth in mouse brains compared to control cells. Immunohistochemistry and immunofluorescence analyses of mouse brain metastases revealed that miR-425-overexpressing tumors exhibited significantly increased activation, intratumoral accumulation, and proliferation of astrocytes, and a decrease in ZNF24 expression compared to control tumors. Together, our findings demonstrate, for the first time, that breast cancer EV-derived miR-425 promotes BCBM via activating astrocytes in the brain microenvironment through the novel EV-miR-425-ZNF24-CCL8 signaling axis.