Novel extracellular vesicles mediate an ABCG2-dependent anticancer drug sequestration and resistance

Overexpression of the multidrug efflux transporter ABCG2 in the plasma membrane of cancer cells confers resistance to various anticancer drugs, including mitoxantrone. Here, we explored the mechanism underlying drug resistance in the MCF-7 breast cancer sublines MCF-7/MR and MCF-7/FLV1000 cells in which wild-type (R482) ABCG2 overexpression is highly confined to cell-cell attachment zones. The latter comprised the membrane of novel extracellular vesicles in which mitoxantrone was rapidly and dramatically sequestered. After 12 hours of incubation with mitoxantrone, the estimated intravesicular drug concentration was approximately 1,000-fold higher than in the culture medium. This drug compartmentalization was prevented by the specific and potent ABCG2 transport inhibitors Ko143 and fumitremorgin C, thereby resulting in restoration of drug sensitivity. Consistently, this intravesicular drug concentration was abrogated by energy deprivation and was restored upon provision of energy substrates. Fine-structure studies corroborated the presence of numerous large extracellular vesicles that were highly confined to cell-cell attachment zones between neighbor cells. Furthermore, high-resolution electron microscopy revealed that the membrane of these extracellular vesicles contained microvilli-like invaginations protruding into the intravesicular lumen. It is likely that these microvilli-like projections increase the vesicular membrane surface, thereby allowing for a more efficient ABCG2-dependent intravesicular anticancer drug concentration. Hence, these novel extracellular vesicles mediate the ABCG2-dependent extraction of intracellular drug, thereby serving as cytotoxic drug disposal chambers shared by multiple neighbor cancer cells. This constitutes a novel modality of anticancer drug resistance.