Fig. 5. The efflux of the MRP1 substrate, 3H -vincristine (3H-VCR), is greater from the MRPl-hyper-expressing cells, MCF7-VP, compared to control cells (MCF7-WT). (B,C) In contrast to other ABC transporters, only MRP1 is hyper-expressed in MCF7-VP cells relative to MCF7-WT. (A) 3H-VCR efflux from MCF7-WT and MCF7-VP cells. Cells were prelabelled with 3H-VCR (20 mM) for 30 min at 37°C, washed, and reincubated in the presence of non- radioactive VCR (20 mM). Released 3H-VCR was expressed as a percentage of the total. Results are mean ± SD (4 determinations) in a typical experiment from 4. (B) The expression of MRP1 mRNA and MRP1 protein in total cell lysates of MCF7-WT and MCF7-VP cells and MRP1 protein in the membrane fraction. (C) MRP1 mRNA expression compared to MRP2, MRP3, MRP4, CFTR and P-glycoprotein (MDR1) in MCF7-WT, MCF7-VP and SK-N-SH (childhood neuroblastoma) cells. The results shown in (B) and (C) are representative from 3 experiments.

these later species correlates with the ability of these inhibitors to prevent NO-mediated 59Fe efflux from MRPl-hyper-expressing cells (Fig. 8B) [89].

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