Down-regulation of ABCB1 by collateral sensitivity drugs reverses multidrug resistance and up-regulates enolase I

The emergence of drug resistance remains an obstacle in the clinical treatment of cancer. Recent developments in the studies of drug resistance have identified compounds such as verapamil and tamoxifen that specifically target ABCB1-expressing multidrug-resistant (MDR) cells, through an ATP-dependent ROS-generating mechanism. In this report, we demonstrate that treatment of ABCB1-expressing MDR cells (CHO(R)C5 or MDA-Doxo(400)) or individual clones of the latter with sub-lethal concentrations of tamoxifen or verapamil down-regulates ABCB1 protein and mRNA expression in surviving clones. Consequently, tamoxifen- and verapamil-treated cells show increased sensitivity to chemotherapeutic drugs (e.g., colchicine and doxorubicin) and decreased sensitivity to collateral sensitivity drugs (e.g., verapamil and tamoxifen). Importantly, we show for the first time that downregulation of ABCB1 expression resulting from tamoxifen treatment and CRISPR-knockout of ABCB1 expression up-regulate a -enolase (enolase I) protein levels and activity. These findings demonstrate a possible effect of ABCB1 expression on the metabolic homeostasis of MDR cells. Moreover, given the use of tamoxifen to prevent the recurrence of oestrogen receptor-positive breast cancer, the findings of this study may be clinically important in modulating activity of other drugs. [GRAPHICS] .