Structure and Function of ABCG2-Rich Extracellular Vesicles Mediating Multidrug Resistance

Abstract

Multidrug resistance (MDR) is a major impediment to curative cancer chemotherapy. The ATP-Binding Cassette transporters ABCG2, ABCB1 and ABCC2 form a unique defense network against multiple structurally and functionally distinct chemotherapeutics, thereby resulting in MDR. Thus, deciphering novel mechanisms of MDR and their overcoming is a major goal of cancer research. Recently we have shown that overexpression of ABCG2 in the membrane of novel extracellular vesicles (EVs) in breast cancer cells results in mitoxantrone resistance due to its dramatic sequestration in EVs. However, nothing is known about EVs structure, biogenesis and their ability to concentrate multiple antitumor agents. To this end, we here found that EVs are structural and functional homologues of bile canaliculi, are apically localized, sealed structures reinforced by an actin-based cytoskeleton and secluded from the extracellular milieu by the tight junction proteins occludin and ZO-1. Apart from ABCG2, ABCB1 and ABCC2 were also selectively targeted to the membrane of EVs. Moreover, Ezrin-Radixin-Moesin protein complex selectively localized to the border of the EVs membrane, suggesting a key role for the tethering of MDR pumps to the actin cytoskeleton. The ability of EVs to concentrate and sequester different antitumor drugs was also explored. Taking advantage of the endogenous fluorescence of anticancer drugs, we found that EVs-forming breast cancer cells display high level resistance to topotecan, imidazoacridinones and methotrexate via efficient intravesicular drug concentration hence sequestering them away from their cellular targets. Thus, we identified a new modality of anticancer drug compartmentalization and resistance in which multiple chemotherapeutics are actively pumped from the cytoplasm and highly concentrated within the lumen of EVs via a network of MDR transporters differentially targeted to the EVs membrane. We propose a composite model for the structure and function of MDR pump-rich EVs in cancer cells and their ability to confer multiple anticancer drug resistance.

Nanoparticle delivery of anticancer drugs overcomes multidrug resistance in breast cancer

“…In breast cancer cells, BCRP is not only expressed at the cell membrane, but also in intracellular vesicles that segregate and retain drugs, leading to drug compartmentalization. This is another reason for increased drug resistance of cells (Ifergan et al, 2005;Goler-Baron & Assaraf, 2011). Resistance of BCRP extends to the class of anthracyclines, but also to antifolatessuch as methotrexate (Doyle et al, 1998;Saraswathy & Gong, 2013).…”

Implications of Akt2/Twist crosstalk on breast cancer metastatic outcome

“…More recently, the possible role of the PI3K/Akt pathway in the subcellular localization of BCRP was suggested. Activation of this pathway was required for BCRP targeting the membrane of BCRP-rich extracellular vesicles, which actively sequester drugs from the cytosol and concentrate them in their lumen, contributing to multidrug resistance [48,49].…”

Intercellular Transfer of Cancer Drug Resistance Traits by Extracellular Vesicles

“…In addition, ABCG2-rich EVs were also shown to sequester riboflavin, topotecan, imidazoacridinones, and methotrexate [34]. This ability of ABCG2-rich EVs to sequester chemotherapeutic drugs could be a cellular mechanism used to overcome MDR.…”