A novel mechanism governing the transcriptional regulation of ABC transporters in MDR cancer cells.

P-glycoprotein (P-gp/ABCB1) and multidrug resistance-associated protein 1 (MRP1/ABCC1) are the main drug efflux transporters associated with treatment failure in cancer. Much attention has been focused on the molecular mechanisms regulating the expression of these transporters as a viable approach for identifying novel drug targets in circumventing cancer multidrug resistance (MDR) clinically. In this paper, we examine the role of miR-326 in the context of its intercellular transfer between cancer cells by extracellular membrane vesicles called microparticles (MPs). We observe that cellular suppression of ABCC1 by miR-326 is modulated by the presence of ABCB1 transcript. Specifically, we show that siRNA silencing of MP-transferred ABCB1 transcript reverses the knockdown effects of miRNA-326 on target MRP1/ABCC1 transcripts. We also demonstrate a dominance of ABCB1 transcripts when co-localized with ABCC1 transcripts, which is consistent with the facilitation of miR-326 function by ABCB1. This study identifies a novel pathway regulating the expression of ABC transporters and positions ABCB1 mRNA as a transcriptional regulator of other members of this superfamily in multidrug resistant cells through its actions on miRNAs.