Human multidrug resistance protein 2 transports the therapeutic bile salt tauroursodeoxycholate

The multidrug resistance protein 2 (MRP2/ABCC2) mediates the biliary excretion of glucuronide and glutathione conjugates of endogenous and exogenous compounds. We examined the activation of human MRP2-mediated ATP-dependent transport by the choleretic bile salt ursodeoxycholic acid (UDC) and its taurine and glycine amidates in Sf9 cell membranes expressing MRP2 using beta-estradiol 17-(beta-D-glucuronide) (E(2)17G) and beta-estradiol 3-(beta-D-glucuronide) (E(2)3G) as substrates. MRP2 transported E(2)3G via classic Michaelis-Menten kinetics (K-m = 122 mu M; V-max = 3.0 nmol/mg/min), whereas E(2)17G transport showed positive cooperativity ( Hill slope, 2.15; K-m = 75 mu M; V-max = 3.8 nmol/mg/min). UDC, tauroursodeoxycholate, and glycoursodeoxycholate (80-100 mu M) maximally stimulated E(2)3G transport 9-, 7.9-, and 3.6-fold, respectively, whereas higher concentrations (1-2 mM) inhibited transport. At low ( 0.3 mu M) concentrations, tauroursodeoxycholate was transported only in the presence of E(2)17G or E(2)3G, but not other MRP2 substrates such as methotrexate, leukotriene C-4, or S-methylglutathione. Kinetic analysis of higher concentrations of tauroursodeoxycholate transport by MRP2 showed positive cooperativity ( Hill slope, 1.84; K-m = 127 mu M; V-max = 779 pmol/mg/ min). Taurocholate (2 - 100 mu M) was not detectably transported by MRP2 either alone or in the presence of E(2)17G but was transported in the presence of E(2)3G. Thus, UDC, tauroursodeoxycholate, and glycoursodeoxycholate activated MRP2 transport. Tauroursodeoxycholate was transported by MRP2 and demonstrated positive cooperativity, identifying it as the second MRP2 substrate able to stimulate its own transport. The data suggest MRP2 binding sites that can require specific complementarities between substrates and modulators of MRP2-mediated transport.