Modulation of protein kinase C by taurolithocholic acid in isolated rat hepatocytes

The protein kinase C (PKC) family of isoenzymes plays a key role in the reg ulation of hepatocellular secretion, The hydrophobic and cholestatic bile a cid, taurolithocholic acid (TLCA), acts as a potent Ca++ agonist in isolate d hepatocytes. However, its effect on PKC isoforms has not been elucidated. Here we investigate the effects of TLCA at low micromolar concentrations o n the distribution of PKC isoforms and on membrane-associated PKC activity. The distribution of PKC isoforms was determined in isolated rat hepatocyte s in short-term culture using Western blotting and immunofluorescence techn iques. PKC activity was measured radiochemically, TLCA (10 mu mol/L) induce d selective translocation of epsilon-PKC by 47.9% +/- 20.5% (P < .02 vs. co ntrols; n = 7), but not of alpha-, delta-, and zeta-PKC to the hepatocellul ar membranes, whereas the phorbol ester, phorbol It-myristate 13-acetate (P MA) (1 mu mol/L) caused translocation of all mobile isoforms, alpha-, delta -, and epsilon-PKC, as shown by immunoblotting. Immunofluorescence studies demonstrated selective translocation of epsilon-PKC to the canalicular memb ranes of isolated rat hepatocyte couplers by TLCA (10 mu mol/L), but predom inant translocation to intracellular and basolateral membranes by PMA (1 mu mol/L), Both TLCA (10 mu mol/L) and PMA (1 mu mol/L) stimulated membrane-b ound PKC activity by 60.5% +/- 45.8% (P < .05 vs. controls; n = 5) and 72.4 % +/- 37.2% (P < .05; n = 5), respectively. TLCA at lower concentrations (5 mu mol/L) was less effective. Because activation of epsilon-PKC has been a ssociated with impairment of vesicle-mediated targeting and insertion of me mbrane proteins in secretory cells, it is attractive to speculate that TLCA reduces bile secretory capacity of the liver cell by activation of epsilon -PKC at the canalicular membrane.