Effect of multidrug resistance-reversing agents on transporting activity of human canalicular multispecific organic anion transporter

The canalicular multispecific organic anion transporter (cMOAT), also termed MRP2, is a recently identified ATP-binding cassette transporter. We previously established stable human cMOAT cDNA-transfected cells, LLC/cMOAT-1 from LLC-PK1 cells, and LLC/CMV cells that were transfected with an empty vector. We found that LLC/cMOAT-1 cells have increased resistance to vincristine (VCR), 7-ethyl-10-hydroxycamptothecin, and cisplatin but not to etoposide. The multidrug resistance-reversing agents cyclosporin A (CsA) and 2-[4(diphenylmethyl)-1-piperazinyl]-5-(trans-4,6-dimethyl-1,3,2- dioxaphosphorinan-2-yl)-2,6-dimethyl-4-(3-nitrophenyl)-3-pyridinecarboxylate P-oxide (PAK-104P) almost completely reversed the resistance to VCR, 7-ethyl- 10-hydroxy-camptothecin, and cisplatin of LLC/cMOAT-1 cells; and DL- buthionine(S,R)-sulfoximine, (3'-oxo-4-butenyl-4-methyl-threonine, (valine) cyclosporin (PSC833), and 3-([{3-(2-[7-chloro-2-quinolinyl]ethenyl)phenyl}- {(3-dimethylamino-3-oxopropyl)thio}-methyl]thio)propanoic acid (MK571) partially reversed the resistance to these drugs. CsA and PAK-104P at 10 μM enhanced the accumulation of VCR in LLC/cMOAT-1 cells almost to the level in LLC/CMV cells without the agents. The efflux of VCR from LLC/cMOAT-1 cells was enhanced compared with LLC/CMV cells and inhibited by CsA and PAK-104P. Transport of leukotriene C₄ (LTC₄) and S-(2, 4-dinitrophenyl)glutathione also was studied with membrane vesicles prepared from these cells. LTC₄ and S-(2, 4-dinitrophenyl)glutathione were actively transported into membrane vesicles prepared from LLC/cMOAT-1 cells. The K(m) and V(max) values for the uptake of LTC₄ by the LLC/cMOAT-1 membrane vesicles were 0.26 ± 0.05 μM and 7.48 ± 0.67 pmol/min/mg protein, respectively. LTC₄ transport was competitively inhibited by PAK-104P, CsA, MK571, and PSC833, with K(i) values of 3.7, 4.7, 13.1, and 28.9 μM, respectively. These findings demonstrate that cMOAT confers a novel drug-resistance phenotype. CsA and PAK-104P may be useful for reversing cMOAT-mediated drug resistance in tumors.