Abstract

Based upon several previous reports, no consistent relationship between multidrug resistance protein 1 (MRP1, ABCC1) expression and cellular sensitivity to mitoxantrone (MX) toxicity can be ascertained; thus, the role of MRP1 in MX resistance remains controversial. The present study, using paired parental, MRP1-poor, and transduced MRP1-overexpressing MCF7 cells, unequivocally demonstrates that MRP1 confers resistance to MX cytotoxicity and that resistance is associated with reduced cellular accumulation of MX. This MRP1-associated reduced accumulation of MX was partially reversed by treatment of cells with 50 μM MK571 [3-[[3-[2-(7-chloroquinolin-2-yl)vinyl]phenyl]-(2-dimethylcarbamoylethylsulfanyl)methylsulfanyl] propionic acid]—an MRP inhibitor that increased MX accumulation in MRP1-expressing MCF7 cells but had no effect on MRP-poor MCF7 cells. Moreover, in vitro experiments using inside-out membrane vesicles show that MRP1 supports ATP-dependent, osmotically sensitive uptake of MX. Unlike ABCG2 (breast cancer resistance protein, mitoxantrone-resistant protein), MRP1-mediated MX transport is dependent upon the presence of glutathione or its S-methyl analog. In addition, MX stimulates transport of [³H]glutathione. Together, these data are consistent with the interpretation that MX efflux by MRP1 involves cotransport of MX and glutathione. The results suggest that MRP1—like the alternative MX transporters ABCG2 and ABCB1 (MDR1, P-glycoprotein)—can significantly influence tumor cell sensitivity to and pharmacological disposition of MX.