Stromal MTA1 silencing reprograms mesenchymal stem cells to suppress EMT and distant metastasis in triple-negative breast cancer

Background: Metastasis-associated protein-1 (MTA1) is a chromatin regulator that drives epithelial-to-mesenchymal transition (EMT), invasion, and metastatic competence in aggressive cancers including triple-negative breast cancer (TNBC). While tumor-cell MTA1 roles are well documented, its stromal contribution within mesenchymal stem cells (MSCs)-key regulators of the tumor microenvironment-remains largely unexplored. We investigated whether genetic silencing of MTA1 in human MSCs reprograms the tumor microenvironment to restrain TNBC progression and distant metastasis. Methods: Human MSCs were stably transfected with MTA1-shRNA or scrambled control. Knockdown was confirmed by qPCR and immunoblotting. Functional assays included MMP-2 activity, osteogenic differentiation markers (RUNX2, DMP1), endothelial tube formation, and TNBC (MDA-MB-231) 3D spheroid migration, invasion, and EMT profiling. Orthotopic MSC-TNBC spheroid co-implants were established in NSG mice (n=5-8/group) to assess tumor growth, lung and brain metastases, and histologic EMT and proliferation markers. Metastasis was quantified using macrometastatic scoring, H&E staining, IHC, and micro-CT of the lungs and whole body. Results: MTA1 knockdown reduced MSC-MTA1 expression by >80% and suppressed MMP-2 activity by 66%. Osteogenic differentiation was markedly reduced (RUNX2 and DMP1 each ↓>60%), and angiogenic tube formation decreased by 61%. In 3D co-culture, MTA1-KD MSCs reduced TNBC migration (↓54%) and invasion (↓57%) and induced EMT reversal with increased E-cadherin and decreased Vimentin and Snail. In vivo, MTA1-KD MSCs significantly inhibited tumor growth (~48% reduction at day 56). Lung and brain metastases were profoundly reduced, with a 93% decrease in lung micrometastatic foci validated by macrometastasis scoring, H&E, IHC, and micro-CT imaging. Tumors and metastatic lesions showed reduced Ki-67, N-cadherin, and Vimentin, confirming impaired proliferation and EMT signaling. Conclusion: Our data reveal that MTA1 acts as a master stromal determinant of metastatic competence and that its suppression in MSCs yields marked inhibition of tumor-stroma crosstalk driving EMT, invasion, and organotropic colonization. The profound reduction of lung and brain metastases-validated across macroscopic, histologic, immunophenotypic, and imaging platforms-highlights the therapeutic leverage gained by targeting stromal epigenetic programs rather than tumor cells alone. These findings nominate stromal MTA1 inhibition as a powerful, mechanistically grounded strategy to remodel the metastatic niche and provide a strong translational strategy for engineering MSC-based interventions for high-risk TNBC.