DNA methylation-mediated silencing of STAT5A drives breast cancer metastasis via dual regulation of EMT and immunosuppressive microenvironment

Signal transducer and activator of transcription 5 A (STAT5A) modulates breast cancer cell proliferation and metastasis, yet its epigenetic regulatory mechanisms and impacts on the tumor immune microenvironment (TIME) remain unclear. Here, we explored STAT5A’s epigenetic traits, metastatic roles, and effects on tumor-associated immune cells via two mouse models (immunodeficient vs. immunocompetent) with metastasis established by tail vein injection or orthotopic tumorigenesis, flow cytometry to characterize immune cell subsets, cell co-cultures, clinical tissue analyses, and dCas9-TET1CD-mediated STAT5A promoter methylation intervention. Low STAT5A expression reduced tumor cell proliferation but enhanced migration/invasion, with in vivo metastatic capacity dependent on TIME. Tumors with low STAT5A expression grew slower and metastasized less in immunodeficient mice, but exhibited opposite phenotypes in immunocompetent cohorts. Mechanistically, STAT5A downregulation was associated with epithelial-mesenchymal transition (EMT), cytokine pathway remodeling, impaired T-cell cytotoxicity, and macrophage-associated immunosuppressive remodeling. Macrophage-derived LIF activated STAT5A phosphorylation via LIFR. Clinical samples showed sequentially increasing STAT5A promoter methylation and decreasing protein expression from normal tissues to primary/metastatic tumors. dCas9-TET1CD-mediated targeted demethylation restored STAT5A expression and reduced cell migration. These findings suggest that epigenetic silencing of STAT5A contributes to breast cancer metastasis through coordinated regulation of EMT-like plasticity and the TIME, and support further investigation of STAT5A reactivation or LIF-LIFR modulation as potential anti-metastatic strategies. Breast cancer becomes much harder to treat once it spreads to other parts of the body. In this study, we examined a gene called STAT5A, which helps control how breast cancer cells grow and spread. We wanted to know whether DNA methylation, a change that can switch genes off, reduces STAT5A activity during breast cancer progression. To answer this, we analyzed patient samples, breast cancer cells, and mouse models. We found that STAT5A was gradually silenced as breast cancer progressed. Low STAT5A was linked to more invasive cancer cells and a tumor environment that weakened immune attack. Restoring STAT5A partly reversed these changes. These findings suggest that reactivating STAT5A may be a promising future strategy for limiting breast cancer metastasis.