title: Rab4 facilitates cyclic adenosine monophosphate-stimulated bile acid uptake and Na+-taurocholate cotransporting polypeptide translocation.
abstract: Cyclic adenosine monophosphate (cAMP) stimulates hepatic bile acid uptake by translocating sodium-taurocholate (TC) cotransporting polypeptide (Ntcp) from an endosomal compartment to the plasma membrane. Rab4 is associated with early endosomes and involved in vesicular trafficking. This study was designed to determine the role of Rab4 in cAMP-induced TC uptake and Ntcp translocation. HuH-Ntcp cells transiently transfected with empty vector, guanosine triphosphate (GTP) locked dominant active Rab4 (Rab4(GTP)), or guanosine diphosphate (GDP) locked dominant inactive Rab4 (Rab4(GDP)) were used to study the role of Rab4. Neither Rab4(GTP) nor Rab4(GDP) affected either basal TC uptake or plasma membrane Ntcp level. However, cAMP-induced increases in TC uptake and Ntcp translocation were enhanced by Rab4(GTP) and inhibited by Rab4(GDP). In addition, cAMP increased GTP binding to endogenous Rab4 in a time-dependent, but phosphoinositide-3-kinase-independent manner.
  
  
  CONCLUSIONS
  Taken together, these results suggest that cAMP-mediated phosphoinositide-3-kinase-independent activation of Rab4 facilitates Ntcp translocation in HuH-Ntcp cells.
authors.affiliation: Department of Biomedical Sciences, Tufts Cummings School of Veterinary Medicine, North Grafton, MA 01536, USA.
authors.forename: Christopher M
authors.initials: CM
authors.lastname: Schonhoff
authors.forename: Krishna
authors.initials: K
authors.lastname: Thankey
authors.forename: Cynthia R L
authors.initials: CR
authors.lastname: Webster
authors.forename: Yoshiyuki
authors.initials: Y
authors.lastname: Wakabayashi
authors.forename: Allan W
authors.initials: AW
authors.lastname: Wolkoff
authors.forename: M Sawkat
authors.initials: MS
authors.lastname: Anwer
mesh_terms: D001647:Bile Acids and Salts / Q000378:metabolism*; D001692:Biological Transport / Q000187:drug effects; D006528:Carcinoma, Hepatocellular; D045744:Cell Line, Tumor; D002462:Cell Membrane / Q000378:metabolism; D000242:Cyclic AMP / Q000494:pharmacology*; D011992:Endosomes / Q000378:metabolism; D006801:Humans; D008113:Liver Neoplasms; D029363:Organic Anion Transporters, Sodium-Dependent / Q000378:metabolism*; D021381:Protein Transport; D027981:Symporters / Q000378:metabolism*; D014162:Transfection; D020695:rab4 GTP-Binding Proteins / Q000502:physiology*
chemical_list: D001647:Bile Acids and Salts; D029363:Organic Anion Transporters, Sodium-Dependent; D027981:Symporters; C071908:sodium-bile acid cotransporter; D000242:Cyclic AMP; D020695:rab4 GTP-Binding Proteins