Background <p>Tamoxifen (TAM) resistance remains a challenge in estrogen receptor-positive breast cancer treatment. Current research suggested that mesenchymal stem cells (MSCs) derived exosomes may mediate chemoresistance, but the underlying mechanisms are unclear. This study investigates how exosomes from tamoxifen-pretreated MSCs (Tt-MSC-exos) promote tamoxifen resistance through the miR-137/SERPINA3 axis.</p> Methods <p>We isolated exosomes using ultracentrifugation. To investigate the effect of Tt-MSC-exos on TAM resistance, we co-cultured Tt-MSC-exos with ER-positive breast cancer cells and evaluated the cellular functional changes and apoptosis levels. Second-generation transcriptome sequencing was used to analyze the key genes for TAM resistance, and a dual luciferase reporter assay was used to detect the upstream factors of the key genes. The results were further validated by cellular function assays, xenograft modeling studies, and database analysis.</p> Results <p>We demonstrated that Tt-MSC-exos promote TAM resistance in ER-positive breast cancer cells. Next-generation sequencing revealed that the key gene for Tt-MSC-exos promoting resistance was SERPINA3. Downregulation of SERPINA3 expression enhances TAM resistance in breast cancer cells. In addition, we found that miR-137, which was highly expressed in Tt-MSC-exos, could bind to the 3’ UTR region of SERPINA3 in breast cancer cells and thus inhibit the expression of SERPINA3.</p> Conclusions <p>In this study, we clarified that Tt-MSC-exos inhibit the expression level of SERPINA3 in cancer cells by delivering miR-137 to ER-positive breast cancer cells, which in turn leads to TAM resistance in breast cancer. This study provides a new idea to overcome endocrine therapy resistance in ER-positive breast cancer.</p>