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Folate homeostasis of cancer cells affects sensitivity to not only antifolates but also other non-folate drugs: effect of MRP expression

Abstract Sensitivity to antifolates can be decreased by endogenous or exogenous folates. Leucovorin protects cancer patients against toxicity of the dihydrofolate reductase inhibitor methotrexate (MTX), while folic acid is used to protect rheumatoid arthritis patients against MTX. Folates and antifolates can be effluxed from the cell by ABC transporters multidrug resistance protein 1 (MRP1), 2 and 3. We previously demonstrated in 2008 ovarian cancer cells that MRP overexpression reduced cellular folate content by 40%, while folate depletion increased expression of MRP1. As MRPs mediate resistance to several unrelated drugs, we investigated whether folate status would affect sensitivity to doxorubicin, daunorubicin, etoposide and vincristine. Ovarian cancer 2008 cells and its MRP1 transfected variant (2008/MRP1) were adapted from normal folate medium [2.3 μM; high folate (HF) cells] to short-term folate depletion (up to 7 days) (low folate cells); drugs were added after 2 days and sensitivity was tested by the MTT test after 3 additional days. The effect on folate homeostasis was evaluated by measurement of intracellular homocysteine using high-performance liquid chromatography and glutathione using a kit. MRP expression of wild-type (WT) 2008 cells did not increase homocysteine pools in 2008/MRP1 cells. Three day folate depletion increased homocysteine pools 23-fold in 2008 cells and 8.6-fold in the MRP variant. Folate depletion increased glutathione 20%–40% in 2008/WT and 2008/MRP1. In 2008 HF cells MRP1 expression did not affect sensitivity to MTX, but induced 4- to 10-fold resistance to doxorubicin, daunorubicin, etoposide and vincristine. Folate depletion decreased 50% growth inhibition (IC50) for MTX in both 2008 variants 25- to 4-fold, but that to doxorubicin and daunorubicin approximately 2-fold. Sensitivity to etoposide and vincristine was not affected. In conclusion, folate depletion markedly increased homocysteine, but moderately increased glutathione. Folate depletion increased MTX sensitivity, but effects on other drugs were most pronounced in WT cells, probably because MRP expression is already high in transfected variants.