NorUDCA (24-norursodeoxycholic acid), the C23-homolog of ursodeoxycholic acid (UDCA), showed remarkable therapeutic effects in cholestatic Mdr2 (Abcb4) (multidrug resistance protein 2/ATP-binding cassette b4) knockout mice with sclerosing/fibrosing cholangitis. In contrast to UDCA, norUDCA is inefficiently conjugated in human and rodent liver, and conjugation has been discussed as a key step for the anticholestatic action of UDCA in cholestasis. We compared the choleretic, anticholestatic, and antiapoptotic properties of unconjugated and taurine-conjugated UDCA (C24) and norUDCA (C23) in isolated perfused rat liver (IPRL) and in natrium/taurocholate cotransporting polypeptide (Ntcp)-transfected human hepatoma (HepG2) cells. Taurolithocholic acid (TLCA) was used to induce a predominantly hepatocellular cholestasis in IPRL. Bile flow was determined gravimetrically; bile acids determined by gas chromatography and liquid chromatography/tandem mass spectrometry; the Mrp2 model substrate, 2,4-dinitrophenyl-S-glutathione (GS-DNP) was determined spectrophotometrically; and apoptosis was determined immunocytochemically. The choleretic effect of C23-bile acids was comparable to their C24-homologs in IPRL. In contrast, TnorUDCA, but not norUDCA antagonized the cholestatic effect of TLCA. Bile flow (percent of controls) was 8% with TLCA-induced cholestasis, and unchanged by coinfusion of norUDCA (14%). However, it was increased by TnorUDCA (83%), UDCA (73%) and TUDCA (136%). Secretion of GS-DNP was markedly reduced by TLCA (5%), unimproved by norUDCA (4%) or UDCA (17%), but was improved modestly by TnorUDCA (26%) or TUDCA (58%). No apoptosis was observed in IPRL exposed to low micromolar TLCA, but equivalent antiapoptotic effects of TUDCA and TnorUDCA were observed in Ntcp-HepG2 cells exposed to TLCA. Conclusion: Conjugation is essential for the anticholestatic effect of norUDCA in a model of hepatocellular cholestasis. Combined therapy with UDCA and norUDCA may be superior to UDCA or norUDCA monotherapy in biliary disorders in which hepatocyte as well as cholangiocyte dysfunction contribute to disease progression.
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