PKCs mediate effects by phosphorylating their substrates. Myristoylated alanine-rich C kinase substrate (MARCKS) is one such substrate and plays a key role in cytoskeletal dynamics.11, 12 MARCKS Transmembrane Transporters modulator is an F-actin crosslinking protein and is phosphorylated by cPKCα, PKCδ, and PKCϵ in vitro.13, 14 Phosphorylation of MARCKS by PKCδ and PKCϵ has been shown to be involved in exocytosis and endocytosis
in nonhepatic cells. Thus, MARCKS phosphorylation by PKCδ is involved in airway mucin secretion15, 16 and gut peptide secretion.17 MARCKS phosphorylation by PKCϵ has been shown to stimulate vesicle translocation in chromaffin cells18 and basolateral fluid-phase endocytosis in T84 cells.19 Phosphorylation of MARCKS by PKCs results
in the retrieval of MARCKS from the plasma membrane (PM) to the cytosol and in F-actin disassembly.18 It may be noted that actin plays an important role in hepatobiliary transporter translocation20-22 and that TLC induces F-actin accumulation around bile canaliculi.23 Phosphorylation of MARCKS by PKCs requires the translocation of PKCs to MARCKS located in the PM, and as a result, MARCKS phosphorylation and the consequent effect are dependent on subcellular targeting of PKC.24, 25 These studies raise the possibility that TLC-induced endocytic retrieval of Mrp2 may result from PKCϵ-dependent MARCKS phosphorylation. In the present study, we determined whether TLC-induced MRP2 retrieval is mediated via PKCϵ and whether the effect of PKCϵ is mediated Selleck ABT 888 via MARCKS phosphorylation. The results of our studies with dominant-negative (DN)–PKCϵ and phosphorylation-deficient (PD)–MARCKS in HuH7 cells stably transfected with sodium taurocholate cotransporting polypeptide (HuH-NTCP cells) are consistent with the following signaling pathway: TLC PKCϵ MARCKS phosphorylation MRP2 retrieval. 8-(4-Chlorophenylthio)–cyclic
adenosine monophosphate (CPT-cAMP), wortmannin, and the antibody for human MRP2 were purchased from Sigma-Aldrich (St. Louis, MO). The commercial sources of other antibodies were Cell Signaling [phosphorylated 上海皓元 myristoylated alanine-rich C kinase substrate (pMARCKS) and hemagglutinin (HA)], Calbiochem (actin), Clontech [green fluorescent protein (GFP)], Upstate (PKCϵ), and BD Transduction Laboratories (E-cadherin). Sulfosuccinimidyl-6-(biotin-amido)hexanoate was purchased from Pierce (Rockford, IL). Streptavidin beads were purchased from Novagen (Madison, WI). Lipofectamine 2000 was obtained from Invitrogen (Carlsbad, CA). Plasmid constructs for wild-type (WT)–MARCKS and PD-MARCKS (with the effector domain phosphorylation sites at S152, S156, and S163 replaced by alanine) were kind gifts from Dr. Saito.26 Kinase-dead DN-PKCϵ plasmids were purchased from Addgene (Cambridge, MA).