Abstract
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 P-glycoprotein (mMdr1a) mediated transport of a tetrahydroxy bile acid, 6α-OH-taurocholic acid (6α-OH-TC), and its biliary excretion in wild-type and Mrp2-/- mice in the presence or absence of GF120918, a P-gp and Bcrp inhibitor. 6α-OH-TC was rapidly excreted into bile of wild-type mice (78% recovery); co-infusion of GF120918 had no significant effect. In Mrp2-/- mice, biliary excretion was decreased (52% recovery) and co-infusion of GF120918 further decreased these values (34% recovery). In wild-type, but not Mrp2-/- mice, 6α-OH-TC increased bile flow 2.5-fold. Membrane vesicle transport studies of 6α-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 6α-OH-TC with positive cooperativity (Hill slope = 2.1). Human MRP2 and P-glycoprotein also transported 6α-OH-TC, but with positive cooperativity (Hill slope = 3.6 and 1.6, respectively). Following intra-ileal administration in wild-type mice, the time course of 6α-OH-TC biliary recovery was similar to that of co-infused taurocholate, implying that 6α-OH-TC can undergo enterohepatic cycling. Thus, Mrp2 plays a key role in 6α-OH-TC biliary excretion, while P-glycoprotein plays a secondary role; Bsep likely mediates excretion of 6α-OH-TC in the absence of Mrp2 and P-gp. In the Bsep-/- mouse, efficient synthesis of tetrahydroxy bile acids that are Mrp2 and P-glycoprotein substrates can explain the non-cholestatic phenotype.
- ABC transporters
- bile acid transport
- biliary excretion
- hepatobiliary disposition
- membrane transport
- multi-drug resistance
- p-glycoprotein
Footnotes
- Received March 25, 2010.
- Accepted July 19, 2010.
- The American Society for Pharmacology and Experimental Therapeutics