Role of protein kinase C isoforms in bile formation and cholestasis.

Transhepatic solute transport provides the osmotic driving force for canalicular bile formation. Choleretic and cholestatic agents affect bile formation, in part, by altering plasma membrane localizations of transporters involved in bile formation. These short-term dynamic changes in transporter location are highly regulated posttranslational events requiring various cellular signaling pathways. Interestingly, both choleretic and cholestatic agents activate the same intracellular signaling kinases, such as phosphoinositide-3-kinase (PI3K), protein kinase C (PKC), and mitogen-activated protein kinase (MAPK). An emerging theme is that choleretic and cholestatic effects may be mediated by different isoforms of these kinases. This is most evident for PKC-mediated regulation of plasma membrane localization of Na+-taurocholate cotransporting polypeptide (NTCP) and multidrug resistance-associated protein 2 (MRP2) by conventional PKCα (cPKCα), novel PKCδ (nPKCδ), nPKCε, and atypical PKCζ (aPKCζ). aPKCζ may mediate choleretic effects by inserting NTCP into the plasma membrane, and nPKCε may mediate cholestatic effects by retrieving MRP2 from the plasma membrane. On the other hand, cPKCα and nPKCδ may be involved in choleretic, cholestatic, and anticholestatic effects by inserting, retrieving, and inhibiting retrieval of transporters, respectively. The effects of PKC isoforms may be mediated by phosphorylation of the transporters, actin binding proteins (radixin and myristoylated alanine-rich C kinase substrate), and Rab proteins. Human NTCP plays an important role in the entry of hepatitis B and D viruses into hepatocytes and consequent infection. Thus, PKCs, by regulating NTCP trafficking, may also play an important role in hepatic viral infections.