Copper-induced apical trafficking of ATP7B in polarized hepatoma cells provides a mechanism for biliary copper excretion

Background & Aims: Mutations in the ATP7B gene, encoding a copper-transport ing P-type adenosine triphosphatase, lead to excessive hepatic copper accum ulation because of impaired biliary copper excretion in Wilson's disease. I n human liver, ATP7B is predominantly localized to the trans-Golgi network, which appears incompatible with a role of ATP7B in biliary copper excretio n. The aim of this study was to elucidate this discrepancy. Methods: Immuno fluorescence and electron-microscopic methods were used to study the effect s of excess copper on ATP7B localization in polarized HepG2 hepatoma cells. Results: ATP7B is localized to the trans-Golgi network only when extracell ular copper concentration is low (<1 mu mol/L). At increased copper levels, ATP7B redistributes to vesicular structures and to apical vacuoles reminis cent of bile canaliculi. After copper depletion, ATP7B returns to the trans -Golgi network. Brefeldin A and nocodazole impair copper-induced apical tra fficking of ATP7B and cause accumulation of apically retrieved transporters in a subapical compartment, suggesting continuous recycling of ATP7B betwe en this vesicular compartment and the apical membrane when copper is increa sed, Conclusions: Copper induces trafficking of its own transporter from th e trans-Golgi network to the apical membrane, where it may facilitate bilia ry copper excretion. This system of ligand-induced apical sorting provides a novel mechanism to control copper homeostasis in hepatic cells.