Background Tauroursodeoxycholic acid (TUDCA) may exert anticholestatic effects via Ca++- and α-protein kinase C (α-PKC)-dependent apical vesicular insertion of canalicular transporters in cholestatic hepatocytes (Hepatology 2001; 33: 1206–16). Tauroursodeoxycholic acid is mainly taken up into liver cells by Na+-taurocholate cotransporting polypeptide (Ntcp). Tauroursodeoxycholic acid selectively translocates α-PKC, a key mediator of regulated exocytosis, to hepatocellular membranes. It is unclear whether TUDCA exerts its effects on α-PKC after carrier-mediated uptake into liver cells or by interaction with extracellular/membraneous structures.
Materials and methods Human hepatoblastoma HepG2 cells lacking Ntcp were stably transfected with pcDNA3·1/Ntcp or sham-transfected with pcDNA3·1 [+]. Distribution of α-PKC was studied using a Western blotting technique. Uptake of [3H]taurocholic acid (TCA) was determined radiochemically.
Results [3H]taurocholic acid uptake was approximately 180-fold higher in Ntcp-transfected than in sham-transfected cells. Phorbol 12-myristate 13-acetate (1 µmol L−1; positive control) increased membrane binding of α-PKC by 34% in Ntcp-transfected and by 37% in sham-transfected cells. Tauroursodeoxycholic acid (10 µmol L−1) increased membrane-associated α-PKC by 19% in Ntcp-transfected, but not in sham-transfected cells (−13%). Taurocholic acid (10 µmol L−1) did not affect the distribution of α-PKC.
Conclusion Carrier-mediated uptake is a prerequisite for TUDCA-induced translocation of α-PKC to hepatocellular membranes.