Folates provoke cellular efflux and drug resistance of substrates of the multidrug resistance protein 1 (MRP1)

Cellular folate concentration was earlier reported to be a critical factor in the activity and expression of the multidrug resistance protein MRP1 (ABCC1). Since MRP1 mediates resistance to a variety of therapeutic drugs, we investigated whether the cellular folate concentration influences the MRP1-mediated cellular resistance against drugs. As a model system, we used the human ovarian carcinoma cell line 2008wt, and its stably MRP1/ABCC1-transfected subline 2008/MRP1. These cell types have a moderate and high expression of MRP1, respectively. In folate-deprived 2008/MRP1 cells, the MRP1-mediated efflux of its model substrate calcein decreased to ∼55 % of the initial efflux rate under folate-rich conditions. In 2008wt cells, only a small decrease in efflux was observed. Folate depletion for 5-10 days markedly increased (∼500 %) cellular steady-state accumulation of calcein in 2008/MRP1 cells and moderately in 2008wt cells. A subsequent short (24 h) exposure to 2.3 μM l-leucovorin decreased calcein levels again in MRP1-overexpressing cells. Folate deprivation markedly increased growth inhibitory effects of the established MRP1 substrates daunorubicin (∼twofold), doxorubicin (∼fivefold), and methotrexate (∼83-fold) in MRP1-overexpressing cells, proportional to MRP1 expression. In conclusion, this study demonstrates that increased cellular folate concentrations induce MRP1/ABCC1-related drug efflux and drug resistance. These results have important implications in the understanding of the role of MRP1 and its homologs in clinical drug resistance.