Abstract
The major event that leads to death from breast cancer (BrCa) is the emergence of micrometastases into lethal growing metastases. While it is still uncertain what regulates the cell fate decision between remaining in dormancy and aggressive proliferative progression, accumulating evidence demonstrates a major role for the metastatic microenvironment. One area of interest is that of tissue and circulating mesenchymal stem cells (MSCs), which have been shown to alter the proliferative and metastatic potential of BrCa. Herein, we investigate how these cells impact the phenotype of metastatic BrCa. As the disseminated BrCa cells initially adopt an epithelial phenotype in ectopic organs, one that is dormant in having limited proliferation and being immune-silent, interactions that revert the disseminated metastatic BrCa to aggressive mesenchymal phenotypes, would be a driver of metastatic progression. BrCa cells exhibited phenotypic changes including increased E-cadherin expression, altered proliferation, and differential sensitivity to TRAIL-induced apoptosis when directly co-cultured with immortalized human MSCs, compared to the BrCa cells not co-cultured. These regulatory effects were dependent upon the BrCa cell's epithelial-mesenchymal status and involved distinct juxtacrine and paracrine signaling mechanisms, as evidenced by differing responses in direct co-culture, conditioned medium, and Transwell systems. Our findings highlight the complex and context-dependent roles of MSCs in BrCa progression, improving our understanding of tumor-stroma interactions and laying groundwork for future therapeutic exploration.