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Copper Does Not Alter the Intracellular Distribution of ATP7B, a Copper-Transporting ATPase

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Abstract

Wilson's disease is a genetic disorder characterized by the accumulation of copper in the body due to a defect of biliary copper excretion. However, the mechanism of biliary copper excretion has not been fully clarified. We examined the effect of copper on the intracellular localization of the Wilson disease gene product (ATP7B) and green fluorescent protein (GFP)-tagged ATP7B in a human hepatoma cell line (Huh7). The intracellular organelles were visualized by fluorescence microscopy. GFP-ATP7B colocalized with late endosome markers, but not with endoplasmic reticulum, Golgi, or lysosome markers in both the steady and copper-loaded states. ATP7B mainly localized at the perinuclear regions in both states. These results suggest that the main localization of ATP7B is in the late endosomes in both the steady and copper-loaded states. ATP7B seems to translocate copper from the cytosol to the late endosomal lumen, thus participating in biliary copper excretion via lysosomes.

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      Despite the fundamental role of ATP7B trafficking in Cu homeostasis, the intracellular itinerary of ATP7B transport remains poorly understood and controversial (La Fontaine and Mercer, 2007; Polishchuk and Lutsenko, 2013). First, in contrast to common view, several studies conducted in hepatic cells indicate that Cu does not alter the intracellular distribution of ATP7B (Harada et al., 2000, 2005). Second, the uncertainty in ATP7B trafficking concerns the identity of the peripheral vesicular structures, whose ability to receive ATP7B upon Cu overload was associated with a Cu excretion process (La Fontaine and Mercer, 2007; Polishchuk and Lutsenko, 2013).

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      2007, Archives of Biochemistry and Biophysics
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      Trafficking of exogenously expressed ATP7B also could not be detected in HeLa cells [49] and in Menkes patient fibroblasts [47], but in these cases this observation may be attributed to a lack of the appropriate machinery in these cell types, in which ATP7B is not normally expressed, to support trafficking. Contrary to all of the previous studies, two reports by Harada et al. (2000) found that ATP7B was localized to late endosomes rather than the TGN in a human hepatoma cell line (HuH-7) and in primary rat hepatocytes, and this localization was not affected by elevated copper levels [53,54]. The reason for the discrepancy between these and the previous reports is not clear but may involve differences in expression levels of ATP7B in these cells, as previously suggested [50,55].

    • The Wilson disease protein ATP7B resides in the late endosomes with Rab7 and the Niemann-Pick C1 protein

      2005, American Journal of Pathology
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      The reason for the different results among the previous studies has not been clear. The difference is not likely to attribute to the difference in cells used for analysis, because we obtained the same results regarding the localization of ATP7B in various cell types, including isolated rat hepatocytes, Huh7, HEK293, Hep3B, OUMS29, and MDCK cells.11–14 Endogeneous ATP7B was detected with the anti-ATP7B antibody, and it was distributed as a punctate vesicular pattern around the nucleus.

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    Abbreviations used: ER, endoplasmic reticulum; FCS, fetal calf serum; FITC, fluorescein isothiocyanate; GalT, β1,4-galactosyltransferase; GFP, green fluorescent protein; lamp, lysosome-associated membrane protein; PBS, phosphate-buffered saline; TGN, trans-Golgi network; TRITC, tetramethylrhodamine isothiocyanate.

    1

    To whom reprint requests should be addressed at Second Department of Medicine, Kurume University School of Medicine, 67 Asahi-Machi, Kurume 830-0011, Japan. Fax: 81.942.34.2623. E-mail: [email protected].

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