Signaling mechanisms that regulate mammary stem/progenitor cell (MaSC) self-renewal are essential for developmental changes that occur in the mammary gland during pregnancy, lactation, and involution. We observed that equine MaSCs (eMaSCs) maintain their growth potential in culture for an indefinite period, whereas canine MaSCs (cMaSCs) lose their growth potential in long term cultures. We then used this system to investigate the role of microvesicles (MVs) in promoting self-renewal properties. We found that Wnt3a and Wnt1 were expressed at higher levels in MVs isolated from eMaSCs compared with those from cMaSCs. Furthermore, eMaSC-MVs were able to induce Wnt/β-catenin signaling in different target cells, including cMaSCs. Interestingly, the induction of Wnt/β-catenin signaling in cMaSCs was prolonged when using eMaSC-MVs compared with recombinant Wnt proteins, indicating that MVs are not only important for transport of Wnt proteins, but they also enhance their signaling activity. Finally, we demonstrate that the eMaSC-MVs-mediated activation of the Wnt/β-catenin signaling pathway in cMaSCs significantly improves the ability of cMaSCs to grow as mammospheres and, importantly, that this effect is abolished when eMaSC-MVs are treated with Wnt ligand inhibitors. This suggests that this novel form of intercellular communication plays an important role in self-renewal.
