IL6/CCL2 from M2-polarized microglia promotes breast cancer brain metastasis and the reversal effect of β-elemene

Introduction: Brain metastasis (BM) is the most common and serious complication of breast cancer (BC). There is significant interest in investigating the crosstalk between BC cells and immune cells. β-elemene is the main pharmacodynamic component of Curcumae Rhizoma, a traditional Chinese medicine that is commonly used for the clinical treatment and prevention of various tumors. However, the specific underlying mechanism of β-elemene in BC-BM is still unclear.Methods: An intracardiac (ICT) injection model was used to establish specific BC-BM cells, then an intracarotid (ICD) injection model was used to verify the inhibitory effect of β-elemene in BC-BM. Tumor-cell-conditioned media, a primary microglia co-culture model, and an in vitro recruitment experiment were used to explore crosstalk between BC cells and immune cells. TMT-based quantitative proteomic, ELISA, IF, and other molecular biotechnologies were used to investigate the mechanisms.Results: The BC-BM cells established in our study not only increased BM rates but also exhibit mesenchymal phenotype and activated the JAK–STAT signaling pathway. Microglia, particularly M2 microglia, were enriched in BM lesions and secreted high levels of both IL6 and CCL2. Hypersecretory IL6 reversed the MET process of BC cells by regulating the JAK2/STAT3 signaling pathway to promote colonization in the brain. Increased levels of CCL2 significantly recruited monocytic myeloid-derived suppressor cells (M-MDSCs) to induce an immunosuppressive brain microenvironment. β-elemene could significantly inhibit BC-BM in mice by regulating the IL6/STAT3 signaling pathway and suppressing the M-MDSC recruitment.Conclusion: Our work first demonstrated that β-elemene regulated the IL6/STAT3 axis and M-MDSC recruitment to reconstruct immunosuppressive brain microenvironment to suppress BC-BM.