TAUROURSODEOXYCHOLIC ACID ACTIVATES PROTEIN-KINASE-C IN ISOLATED RAT HEPATOCYTES

Background & Aims: Ursodeoxycholic acid (UDCA) improves liver function in patients with chronic cholestatic liver diseases by an unknown mec hanism. UDCA is conjugated to taurine in vivo, and tauroursodeoxycholi c acid (TUDCA) is a potent hepatocellular Ca2+ agonist and stimulates biliary exocytosis and hepatocellular Ca2+ influx, both of which are d efective in experimental cholestasis. Protein kinase C (PKC) mediates stimulation of exocytosis in the liver. The aim of this study was to d etermine the effects of TUDCA on PKC in isolated hepatocytes. Methods: The effect of TUDCA on the distribution of PKC isoenzymes within the hepatocyte was studied using immunoblotting and immunofluorescence tec hniques. In addition, the effect of TUDCA on the accumulation of sn-1, 2-diacylglycerol (DAG), the intracellular activator of PKC, and hepato cellular PKC activity was studied using radioenzymatic techniques. Res ults: Immunoblotting studies showed the presence of four isoenzymes (a lpha, delta, epsilon, and zeta). The phorbol ester phorbol 12-myristat e 13-acetate (1 mu mol/L) induced translocation of alpha-PKC, delta-PK C, and epsilon-PKC from cytosol to a particulate membrane fraction, a key step for activation of PKC. TUDCA, but not taurocholic acid, selec tively induced translocation of the alpha-PKC isoenzyme from cytosol t o the membranes. In addition, TUDCA induced a significant increase in hepatocellular DAG mass and stimulated membrane-associated PKC activit y. Conclusions: TUDCA might stimulate Ca2+-dependent hepatocellular ex ocytosis into bile in part by activation of alpha-PKC.