Stromal MTA1 Silencing Reshapes Mesenchymal Stem Cells to Inhibit EMT in 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. Citation Format: Adel Zaid I Mutahar, Bharathi P Salimath. Stromal MTA1 silencing reprograms mesenchymal stem cells to suppress EMT and distant metastasis in triple-negative breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 4846.