Characterization of the transport properties of cloned rat multidrug resistance-associated protein 3 (MRP3)

We have previously cloned rat MRP3 as an inducible transporter in the liver (Hirohashi, T,, Suzuki, H., Ito, K,, Ogawa, K,, Kume, K,, Shimizu, T,, and Sugiyama, Y, (1998) Mol. Pharmacol, 53, 1068-1075), In the present study, the function of rat MRP3 was investigated using membrane vesicles isolated from LLC-PK1 and HeLa cell population transfected with corresponding cDNA, The ATP-dependent uptake of both 17 beta estradiol 17-beta-D-glucuronide ([H-3]E(2)17 beta G) and glucuronide of [C-14] 6-hydroxy-5,7- dimethyl-2-methylamino-4- (3-pyridylmethyl) benzothiazole (E3040), but not that of [H-3]leukotriene C-4 and [H-3]2,4-dinitrophenyl-S-glutathione, was markedly stimulated by MRP3 transfection in both cell lines. The K-m and V-max values for the uptake of [H-3]E(2)17 beta G were 67 +/- 14 mu M and 415 +/- 73 pmol/min/mg of protein, respectively, for MRP3-expressing membrane vesicles and 3.0 +/- 0.7 mu M and 3.4 +/- 0.4 pmol/min/mg of protein, respectively, for the endogenous transporter expressed on HeLa cells. [H-3]E(2)17 beta G had also a similar K-m value for MRP3 when LLC-PK1 cells were used as the host. All glucuronide conjugates examined (E3040 glucuronide, 4-methylumbelliferone glucuronide, and naphthyl glucuronide) and methotrexate inhibited MRP3-mediated [H-3]E(2)17 beta G transport in LLC-PK1 cells, Moreover, [H-3]methotrexate was transported via MRP3, The inhibitory effect of estrone sulfate, [H-3]2,4-dinitrophenyl-S-glutathione, and [H-3]leukotriene C-4 was moderate or minimal, whereas N-acetyl-2,4-dinitrophenylcysteine had no effect on the uptake of [H-3]E(2)17 beta G. The uptake of [H-3]E(2)17 beta G was enhanced by E3040 sulfate and 4-methylumbelliferone sulfate. Thus we were able to demonstrate that several kinds of organic anions are transported via MRP3, although the substrate specificity of MRP3 differs from that of MRP1 and cMOAT/MRP2 in that glutathione conjugates are poor substrates for MRP3.