Copper-induced apical trafficking of ATP7B in polarized hepatoma cells provides a mechanism for biliary copper excretion.
OBJECTIVE
Mutations in the ATP7B gene, encoding a copper-transporting P-type adenosine triphosphatase, lead to excessive hepatic copper accumulation because of impaired biliary copper excretion in Wilson's disease. In human liver, ATP7B is predominantly localized to the trans-Golgi network, which appears incompatible with a role of ATP7B in biliary copper excretion. The aim of this study was to elucidate this discrepancy.


METHODS
Immunofluorescence and electron-microscopic methods were used to study the effects of excess copper on ATP7B localization in polarized HepG2 hepatoma cells.


RESULTS
ATP7B is localized to the trans-Golgi network only when extracellular copper concentration is low (<1 micromol/L). At increased copper levels, ATP7B redistributes to vesicular structures and to apical vacuoles reminiscent of bile canaliculi. After copper depletion, ATP7B returns to the trans-Golgi network. Brefeldin A and nocodazole impair copper-induced apical trafficking of ATP7B and cause accumulation of apically retrieved transporters in a subapical compartment, suggesting continuous recycling of ATP7B between this vesicular compartment and the apical membrane when copper is increased.


CONCLUSIONS
Copper induces trafficking of its own transporter from the trans-Golgi network to the apical membrane, where it may facilitate biliary copper excretion. This system of ligand-induced apical sorting provides a novel mechanism to control copper homeostasis in hepatic cells.
Roelofsen
H
H
Groningen University Institute for Drug Exploration (GUIDE), Center for Liver, Digestive and Metabolic Diseases, University Hospital Groningen, Groningen, The Netherlands. j.roelofsen@med.rug.nl
Wolters
H
H
Van Luyn
M J
MJ
Miura
N
N
Kuipers
F
F
Vonk
R J
RJ
D000251:Adenosine Triphosphatases / Q000378:metabolism*; D001646:Bile / Q000378:metabolism*; D001692:Biological Transport / Q000187:drug effects; D006528:Carcinoma, Hepatocellular / Q000378:metabolism* / Q000473:pathology; D002352:Carrier Proteins / Q000378:metabolism*; D027682:Cation Transport Proteins; D002462:Cell Membrane / Q000378:metabolism; D016764:Cell Polarity; D003300:Copper / Q000378:metabolism* / Q000494:pharmacology*; D000073840:Copper-Transporting ATPases; D005455:Fluorescent Antibody Technique; D006056:Golgi Apparatus / Q000378:metabolism; D006801:Humans; D008113:Liver Neoplasms / Q000378:metabolism* / Q000473:pathology; D008854:Microscopy, Electron; D014407:Tumor Cells, Cultured
D002352:Carrier Proteins; D027682:Cation Transport Proteins; D003300:Copper; D000251:Adenosine Triphosphatases; C000612464:ATP7B protein, human; D000073840:Copper-Transporting ATPases