SignificanceIncreasing evidence suggests that extracellular vesicles (EVs) can transfer genetic material to recipient cells. However, the mechanism and role of this phenomenon are largely unknown. Here we have made a remarkable discovery: EVs can harbor the full mitochondrial genome. These extracellular vesicles can in turn transfer their mtDNA to cells with impaired metabolism, leading to restoration of metabolic activity. We determined that hormonal therapy induces oxidative phosphorylation-deficient breast cancer cells, which can be rescued via the transfer of mtDNA-laden extracellular vesicles. Horizontal transfer of mtDNA occurred in cancer stem-like cells and was associated with increased self-renewal potential of these cells, leading to resistance to hormonal therapy. We propose that mtDNA transfer occurs in human cancer via EVs.

The hypothesis that microvesicle (MV)-mediated microRNA transfer converts non-cancer stem cells into cancer stem cells (CSCs) leading to therapy resistance remains poorly investigated. Here we provide direct evidence supporting this hypothesis, by demonstrating how MV derived from cancer associated fibroblasts (CAF) transfer miR-221 to promote hormonal therapy resistance (HTR) in models of luminal breast cancer. We determined that CAF-derived MV horizontally transferred miR221 to tumor cells and, in combination with hormone therapy activated an ERlo/Notchhi feed-forward loop responsible for the generation of CD133hi CSC. Importantly, MV from patients with HTR metastatic disease expressed high levels of miR221. We further determined that the IL6-pStat3 pathway in promoted the biogenesis of onco-miR-221hi CAF MV and established stromal CSC niches in experimental and patient-derived breast cancer models. Co-injection of patient-derived CAF from bone metastases led to de novo HTR tumors, which was reversed with IL6R blockade. Finally, we generated PDX models from patient-derived HTR bone metastases and analyzed tumor cells, stroma, and MV. Murine and human CAF were enriched in HTR tumors expressing high levels of CD133hi cells. Depletion of murine CAF from PDX restored sensitivity to HT, with a concurrent reduction of CD133hi CSC. Conversely, in models of CD133neg, HT-sensitive cancer cells, both murine and human CAF promoted de novo HT resistance via the generation of CD133hi CSC that expressed low levels of estrogen receptor alpha (ER). Overall, our results illuminate how MV-mediated horizontal transfer of genetic material from host stromal cells to cancer cells trigger the evolution of therapy-resistant metastases, with potentially broad implications for their control.