Hepatobiliary disposition of 3α,6α,7α,12α-tetrahydroxy-cholanoyl taurine: A substrate for multiple canalicular transporters

Tetrahydroxy bile acids become major biliary bile acids in Bsep( / ) mice and Fxr( / ) mice fed cholic acid; we characterized disposition of these novel bile acids that also occur in patients with cholestasis. We investigated mouse Mrp2 (mMrp2) and Pglycoprotein [(P-gp) mMdr1a]-mediated transport of a tetrahydroxy bile acid, 6 -OH-taurocholic acid (6 -OH-TC), and its biliary excretion in wild-type and Mrp2( / ) mice in the presence or absence of N-(4-[2-(1,2,3,4-tetrahydro-6,7-dimethoxy-2-isoquinolinyl)ethyl]-phenyl)-9,10-dihydro-5-methoxy-9-oxo-4-acridine carboxamide (GF120918), a P-gp and breast cancer resistance protein inhibitor. 6 -OH-TC was rapidly excreted into bile of wild-type mice (78% recovery); coinfusion of GF120918 had no significant effect. In Mrp2( / ) mice, biliary excretion was decreased (52% recovery) and coinfusion of GF120918 further decreased these values (34% recovery). In wild-type, but not Mrp2( / ), mice, 6 OH-TC increased bile flow 2.5-fold. Membrane vesicle transport studies of 6 -OH-TC (0.05–0.75 mM) yielded saturation kinetics with a higher apparent affinity for mMrp2 (Km 0.13 mM) than for mMdr1a (Km 0.33 mM); mBsep transported 6 -OH-TC with positive cooperativity (Hill slope 2.1). Human multidrug resistanceassociated protein (MRP) 2 and P-gp also transported 6 -OH-TC but with positive cooperativity (Hill slope 3.6 and 1.6, respectively). After intraileal administration, the time course of 6 -OH-TC biliary recovery was similar to that of coinfused taurocholate, implying that 6 -OH-TC can undergo enterohepatic cycling. Thus, Mrp2 plays a key role in 6 -OH-TC biliary excretion, whereas P-glycoprotein plays a secondary role; Bsep likely mediates excretion of 6 -OH-TC in the absence of Mrp2 and P-gp. In Bsep( / ) mice, efficient synthesis of tetrahydroxy bile acids that are Mrp2 and P-gp substrates can explain the noncholestatic phenotype.