P-glycoprotein (ABCB1) modulates collateral sensitivity of a multidrug resistant cell line to verapamil

P-glycoprotein (or P-gp1, ABCB1) expression in tumor cells is causative of multidrug resistance through the active efflux of drugs across the cell membrane. However, the over-expression of P-glycoprotein in some tumor cells has been associated with increased sensitivity, or "collateral sensitivity", of multidrug resistant cells to specific drugs, including the calcium channel blocker verapamil. We previously demonstrated that collateral sensitivity to verapamil correlates with the effect of this drug on P-gp1 ATPase, and is reversed by inhibitors of P-gp1 ATPase (e.g., PSC 833 and Ivermectin). In this report, we expand on our earlier study and demonstrate that P-gp1 expression in drug-resistant cells modulates collateral sensitivity. Using P-gp1-specific siRNA, P-gp1 expression in the multidrug resistant CH(R)C5 cells was significantly down-regulated beginning on day 2 post-transfection of siRNA. Furthermore, down-regulation of P-gp1 led to increased sensitivity of CH(R)C5 cells to paclitaxel and doxorubicin, but not to cis-platinum, due to inhibition of P-gp1 drug efflux pump. Down-regulation of P-gp1 expression completely reversed collateral sensitivity to verapamil. Moreover, known inhibitors of ETC, rotenone and antimycin A which cause an increase in reactive oxygen species, synergized with verapamil-induced collateral sensitivity leading to increased cell death as determined by MTT cell survival assay. Similarly, the addition of hydrogen peroxide also synergized with verapamil. Taken together, the results of this study demonstrate a direct link between P-gp1 expression and collateral sensitivity of drug-resistant cells, possibly due to an increase in reactive oxygen species.