Exchange of microRNAs between Breast Cancer Cells and Bone Marrow Stroma: Potential for Avenues To Revisit Hematopoietic Stem Cell Transplantation for Breast and Other Cancers That Show Preference for Bone Marrow.

Previous failure of autologous bone marrow transplantation as a viable treatment of breast cancer opens the field for investigational studies since the bone marrow, as a source of cancer metastasis, remains a clinical dilemma. This study hypothesizes that a subset of breast cancer cells enter the bone marrow at an early period of disease development, perhaps prior to clinical detection, where they evade chemotherapeutic damage even at high doses. An understanding of this cell population might be crucial for the future success of bone marrow transplants in patients suffering from breast cancer metastasis to the bone marrow. In vivo and in vitro studies have demonstrated gap junctions between bone marrow stroma and breast cancer cells. The gap junctions can passage CFDA cell tracer between stroma and cancer cells. This exchange can be blocked by the gap junctional intercellular communication inhibitor, 1-Octanol, suggesting functional gap junctions. We have now implemented quantitative studies to determine the role of specific microRNAs (miRNAs) in blunting of mRNA translation of the Tac1 gene in cancer cells. The first set of studies optimized the effects of miRNAs, 130a and 206 in the suppression of Tac1 mRNA in the low invasive T47D and the aggressive MDA-MB-231 breast cancer cell lines. The miRNAs were transfected into cells containing the pMIR-R/Tac1/SG vector, which includes the 3′ UTR of Tac1 mRNA. Luciferase activity in cell lysates showed synergism between the two miRNAs, indicating that the predicted miRNAs can inhibit Tac1 translation. The next set of studies determined whether miRNAs 130a and 206 can cross gap junctions between stroma and cancer cells to suppress Tac1 mRNA. The miRNAs were transfected in one cell type and pMIR-R/Tac1/SG in the other cell type. Luciferase activity and parallel studies with 1-Octanol verified movement of miRNAs via the gap junction. These studies recapitulate the downregulation of endogenous Tac1 and other cytokines in the cancer cells. In summary, this study shows a role for miRNA exchange, between bone marrow stroma and breast cancer cells, as a mechanism by which the cancer cells might evade detection and current methods of treatment. An understanding of these mechanisms might lead to new methods to remove cancer from bone marrow in future therapies.