Gap junction-mediated import of microRNA from bone marrow stromal cells can elicit cell cycle quiescence in breast cancer cells.

Bone marrow (BM) metastasis of breast cancer (BC) can recur even decades after initial diagnosis and treatment, implying the long-term survival of disseminated cancer cells in a dormant state. Here we investigated the role of microRNAs (miRNA) transmitted from BM stroma to BC cells via gap junctions and exosomes in tumor cell quiescence. MDA-MB-231 and T47D BC cells arrest in G(0) phase of the cell cycle when cocultured with BM stroma. Analyses of miRNA expression profiles identified numerous miRNAs implicated in cell proliferation including miR-127, -197, -222, and -223 targeting CXCL12. Subsequently, we showed that these CXCL12-specific miRNAs are transported from BM stroma to BC cells via gap junctions, leading to reduced CXCL12 levels and decreased proliferation. Stroma-derived exosomes containing miRNAs also contributed to BC cell quiescence, although to a lesser degree than miRNAs transmitted via gap junctions. This study shows that the transfer of miRNAs from BM stroma to BC cells might play a role in the dormancy of BM metastases.