Copper Ionophores as Novel Antiobesity Therapeutics

The therapeutic utility of the copper ionophore disulfiram was investigated in a diet-induced obesity mouse model (C57BL/6J background), both through administration in feed (0.05 to 1% (w/w)) and via oral gavage (150 mg/kg) for up to eight weeks. Mice were monitored for body weight, fat deposition (perigonadal fat pads), metabolic changes (e.g., glucose dyshomeostasis) and pathologies (e.g., hepatic steatosis, hyperglycaemia and hypertriglyceridemia) associated with a high-fat diet. Metal-related pharmacological effects across major organs and serums were investigated using inductively coupled plasma mass spectrometry (ICP-MS). Disulfiram treatments (all modes) augmented hepatic copper in mice, markedly moderated body weight and abolished the deleterious systemic changes associated with a high-fat diet. Likewise, another chemically distinct copper ionophore H2(gtsm), administered daily (oral gavage), also augmented hepatic copper and moderated mouse body weight. Postmortem histological examinations of the liver and other major organs, together with serum aminotransferases, supported the reported therapeutic safety of disulfiram. Disulfiram specifically altered systemic copper in mice and altered hepatic copper metabolism, perturbing the incorporation of copper into ceruloplasmin (holo-ceruloplasmin biosynthesis) and subsequently reducing serum copper concentrations. Serum ceruloplasmin represents a biomarker for disulfiram activity. Our results establish copper ionophores as a potential class of antiobesity agents.

Copper Complexes in Cancer Therapy

Copper homeostasis is tightly regulated in both prokaryotic and eukaryotic cells to ensure sufficient amounts for cuproprotein biosynthesis, while limiting oxidative stress production and toxicity. Over the last century, copper complexes have been developed as antimicrobials and for treating diseases involving copper dyshomeostasis (e.g., Wilson's disease). There now exists a repertoire of copper complexes that can regulate bodily copper through a myriad of mechanisms. Furthermore, many copper complexes are now being appraised for a variety of therapeutic indications (e.g., Alzheimer's disease and amyotrophic lateral sclerosis) that require a range of copper-related pharmacological affects. Cancer therapy is also drawing considerable attention since copper has been recognized as a limiting factor for multiple aspects of cancer progression including growth, angiogenesis, and metastasis. Consequently, 'old copper complexes' (e.g., tetrathiomolybdate and clioquinol) have been repurposed for cancer therapy and have demonstrated anticancer activity in vitro and in preclinical models. Likewise, new tailor-made copper complexes have been designed based on structural and biological features ideal for their anticancer activity. Human clinical trials continue to evaluate the therapeutic efficacy of copper complexes as anticancer agents and considerable progress has been made in understanding their pharmacological requirements. In this chapter, we present a historical perspective on the main copper complexes that are currently being repurposed for cancer therapy and detail several of the more recently developed compounds that have emerged as promising anticancer agents. We further provide an overview of the known mechanisms of action, including molecular targets and we discuss associated clinical trials.

Copper Complexes in Cancer Therapy

Copper homeostasis is tightly regulated in both prokaryotic and eukaryotic cells to ensure sufficient amounts for cuproprotein biosynthesis, while limiting oxidative stress production and toxicity. Over the last century, copper complexes have been developed as antimicrobials and for treating diseases involving copper dyshomeostasis (e.g., Wilson's disease). There now exists a repertoire of copper complexes that can regulate bodily copper through a myriad of mechanisms. Furthermore, many copper complexes are now being appraised for a variety of therapeutic indications (e.g., Alzheimer's disease and amyotrophic lateral sclerosis) that require a range of copper-related pharmacological affects. Cancer therapy is also drawing considerable attention since copper has been recognized as a limiting factor for multiple aspects of cancer progression including growth, angiogenesis, and metastasis. Consequently, 'old copper complexes' (e.g., tetrathiomolybdate and clioquinol) have been repurposed for cancer therapy and have demonstrated anticancer activity in vitro and in preclinical models. Likewise, new tailor-made copper complexes have been designed based on structural and biological features ideal for their anticancer activity. Human clinical trials continue to evaluate the therapeutic efficacy of copper complexes as anticancer agents and considerable progress has been made in understanding their pharmacological requirements. In this chapter, we present a historical perspective on the main copper complexes that are currently being repurposed for cancer therapy and detail several of the more recently developed compounds that have emerged as promising anticancer agents. We further provide an overview of the known mechanisms of action, including molecular targets and we discuss associated clinical trials.

Iron accumulation in senescent cells is coupled with impaired ferritinophagy and inhibition of ferroptosis

Cellular senescence is characterised by the irreversible arrest of proliferation, a pro-inflammatory secretory phenotype and evasion of programmed cell death mechanisms. We report that senescence alters cellular iron acquisition and storage and also impedes iron-mediated cell death pathways. Senescent cells, regardless of stimuli (irradiation, replicative or oncogenic), accumulate vast amounts of intracellular iron (up to 30-fold) with concomitant changes in the levels of iron homeostasis proteins. For instance, ferritin (iron storage) levels provided a robust biomarker of cellular senescence, for associated iron accumulation and for resistance to iron-induced toxicity. Cellular senescence preceded iron accumulation and was not perturbed by sustained iron chelation (deferiprone). Iron accumulation in senescent cells was driven by impaired ferritinophagy, a lysosomal process that promotes ferritin degradation and ferroptosis. Lysosomal dysfunction in senescent cells was confirmed through several markers, including the build-up of microtubule-associated protein light chain 3 (LC3-II) in autophagosomes. Impaired ferritin degradation explains the iron accumulation phenotype of senescent cells, whereby iron is effectively trapped in ferritin creating a perceived cellular deficiency. Accordingly, senescent cells were highly resistant to ferroptosis. Promoting ferritin degradation by using the autophagy activator rapamycin averted the iron accumulation phenotype of senescent cells, preventing the increase of TfR1, ferritin and intracellular iron, but failed to re-sensitize these cells to ferroptosis. Finally, the enrichment of senescent cells in mouse ageing hepatic tissue was found to accompany iron accumulation, an elevation in ferritin and mirrored our observations using cultured senescent cells.

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Altered iron homeostasis in senescent cells is driven by impaired ferritinophagy.

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Impaired ferritinophagy causes functional cellular iron deficiency.

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senescent cells are resistant to iron mediated cell death including ferroptosis.

Ceruloplasmin is regulated by copper and lactational hormones in PMC42-LA mammary epithelial cell culture models

Ceruloplasmin (Cp) is a multicopper ferroxidase that is considered to be an important source of copper in milk for normal neonatal development. We investigated the expression, subcellular localization and secretion of Cp in PMC42-LA cell culture models representative of resting, lactating and suckled human mammary epithelia. Both secreted Cp (sCp) and plasma membrane associated glycosylphosphatidylinositol-linked Cp (GPI-Cp) were expressed in PMC42-LA cells. In all three epithelial models (resting, lactating and suckled), the expression and secretion of copper-bound, ferroxidase active, Cp (holo-Cp) was dependent on media copper concentration. In low copper (bathocuproinedisulphonic acid/d-penicillamine treated models) there was greater than a 2-fold decrease in holo-Cp expression and secretion, which was mirrored by a 2-fold increase in the expression and secretion of copper-free Cp protein (apo-Cp). Cell surface biotinylation studies revealed that the state of PMC42-LA cell differentiation (functionality), and the level of extracellular copper, had no significant effect on the level of plasma membrane bound GPI-Cp. Quantitative real time PCR analyses determined that there was no significant (P > 0.05) difference in Cp mRNA levels across all copper conditions investigated (0, 5, 50 μM). However, there was a significant (P < 0.05) increase (∼2-fold) in Cp mRNA in both the lactating and suckled models in comparison to the resting model. Furthermore, the Cp mRNA increase in response to PMC42-LA differentiation corresponded with more secreted Cp protein, both apo and holo forms, indicating a link between function and Cp requirement. Our results provide significant insight on the regulation of Cp expression and secretion in lactation and copper incorporation into milk.

Heterogeneous copper concentrations in cancerous human prostate tissues

BACKGROUND

Therapeutics that target copper for the treatment of prostate cancer are being evaluated in human clinical trials. Elevated intracellular copper is considered to sensitize prostate cancer cells to certain copper-coordination compounds, especially those with ionophoric properties. While there is compelling in vitro evidence that prostate cancer cells accumulate intracellular copper, a corresponding status for copper in patient tissues has not been corroborated. We therefore established whether copper concentrations increase in cancerous prostate tissues, and in sera, in patients throughout disease progression.

METHODS

Human prostate tissue samples were obtained from patient prostatectomies (n = 28), and together with patient-matched sera, were analyzed for copper content by inductively coupled plasma mass spectrometry.

RESULTS

When grouped together, cancerous prostate tissues exhibiting moderate disease severity (Gleason Score 7) (n = 10) had 1.6-fold more copper than age-matched normal tissues (n = 10) (P < 0.05). Those with more aggressive disease (Gleason Score 9) (n = 8) had 1.8-fold more copper (P < 0.05). In both disease stages however, the copper concentrations between individual samples were rather variable (0.55-3.02 μg/g), with many clearly within the normal range (0.52-1.28 μg/g). Additionally, we found that there was no change in serum copper concentrations in patients with either moderate or aggressive prostate cancer (Gleason Score 7 or 9), compared with reference intervals and to age-matched controls.

CONCLUSIONS

The heterogeneous nature of copper concentrations in cancerous prostate tissues, suggest that a small subset of patients may respond to treatments that target elevated intratumoral copper. Therefore, such approaches would likely require personalized treatment strategies.

Comparative Microarray Analysis Identifies Commonalities in Neuronal Injury: Evidence for Oxidative Stress, Dysfunction of Calcium Signalling, and Inhibition of Autophagy-Lysosomal Pathway

Mitochondrial dysfunction, ubiquitin-proteasomal system impairment and excitotoxicity occur during the injury and death of neurons in neurodegenerative conditions. The aim of this work was to elucidate the cellular mechanisms that are universally altered by these conditions. Through overlapping expression profiles of rotenone-, lactacystin- and N-methyl-D-aspartate-treated cortical neurons, we have identified three affected biological processes that are commonly affected; oxidative stress, dysfunction of calcium signalling and inhibition of the autophagic-lysosomal pathway. These data provides many opportunities for therapeutic intervention in neurodegenerative conditions, where mitochondrial dysfunction, proteasomal inhibition and excitotoxicity are evident.

Targeting copper in cancer therapy: 'Copper That Cancer'

Copper is an essential micronutrient involved in fundamental life processes that are conserved throughout all forms of life. The ability of copper to catalyze oxidation-reduction (redox) reactions, which can inadvertently lead to the production of reactive oxygen species (ROS), necessitates the tight homeostatic regulation of copper within the body. Many cancer types exhibit increased intratumoral copper and/or altered systemic copper distribution. The realization that copper serves as a limiting factor for multiple aspects of tumor progression, including growth, angiogenesis and metastasis, has prompted the development of copper-specific chelators as therapies to inhibit these processes. Another therapeutic approach utilizes specific ionophores that deliver copper to cells to increase intracellular copper levels. The therapeutic window between normal and cancerous cells when intracellular copper is forcibly increased, is the premise for the development of copper-ionophores endowed with anticancer properties. Also under investigation is the use of copper to replace platinum in coordination complexes currently used as mainstream chemotherapies. In comparison to platinum-based drugs, these promising copper coordination complexes may be more potent anticancer agents, with reduced toxicity toward normal cells and they may potentially circumvent the chemoresistance associated with recurrent platinum treatment. In addition, cancerous cells can adapt their copper homeostatic mechanisms to acquire resistance to conventional platinum-based drugs and certain copper coordination complexes can re-sensitize cancer cells to these drugs. This review will outline the biological importance of copper and copper homeostasis in mammalian cells, followed by a discussion of our current understanding of copper dysregulation in cancer, and the recent therapeutic advances using copper coordination complexes as anticancer agents.

Copper and lactational hormones influence the CTR1 copper transporter in PMC42-LA mammary epithelial cell culture models

Adequate amounts of copper in milk are critical for normal neonatal development, however the mechanisms regulating copper supply to milk have not been clearly defined. PMC42-LA cell cultures representative of resting, lactating and suckled mammary epithelia were used to investigate the regulation of the copper uptake protein, CTR1. Both the degree of mammary epithelial differentiation (functionality) and extracellular copper concentration greatly impacted upon CTR1 expression and its plasma membrane association. In all three models (resting, lactating and suckling) there was an inverse correlation between extracellular copper concentration and the level of CTR1. Cell surface biotinylation studies demonstrated that as extracellular copper concentration increased membrane associated CTR1 was reduced. There was a significant increase in CTR1 expression (total and membrane associated) in the suckled gland model in comparison to the resting gland model, across all copper concentrations investigated (0-50 μM). Regulation of CTR1 expression was entirely post-translational, as quantitative real-time PCR analyses showed no change to CTR1 mRNA between all models and culture conditions. X-ray fluorescence microscopy on the differentiated PMC42-LA models revealed that organoid structures distinctively accumulated copper. Furthermore, as PMC42-LA cell cultures became progressively more specialised, successively more copper accumulated in organoids (resting<lactating<suckling), indicating a link between function and copper requirement. Based on previous data showing a function for CTR1 in copper uptake, we have concluded that under the influence of hormones and increased extracellular copper levels, CTR1 participates in uptake of copper by mammary epithelial cells, as a prerequisite for secretion of copper into milk.

Increasing intracellular bioavailable copper selectively targets prostate cancer cells

The therapeutic efficacy of two bis(thiosemicarbazonato) copper complexes, glyoxalbis[N4-methylthiosemicarbazonato]Cu(II) [Cu(II)(gtsm)] and diacetylbis[N4-methylthiosemicarbazonato]Cu(II) [Cu(II)(atsm)], for the treatment of prostate cancer was assessed in cell culture and animal models. Distinctively, copper dissociates intracellularly from Cu(II)(gtsm) but is retained by Cu(II)(atsm). We further demonstrated that intracellular H2gtsm [reduced Cu(II)(gtsm)] continues to redistribute copper into a bioavailable (exchangeable) pool. Both Cu(II)(gtsm) and Cu(II)(atsm) selectively kill transformed (hyperplastic and carcinoma) prostate cell lines but, importantly, do not affect the viability of primary prostate epithelial cells. Increasing extracellular copper concentrations enhanced the therapeutic capacity of both Cu(II)(gtsm) and Cu(II)(atsm), and their ligands (H2gtsm and H2atsm) were toxic only toward cancerous prostate cells when combined with copper. Treatment of the Transgenic Adenocarcinoma of Mouse Prostate (TRAMP) model with Cu(II)(gtsm) (2.5 mg/kg) significantly reduced prostate cancer burden (∼70%) and severity (grade), while treatment with Cu(II)(atsm) (30 mg/kg) was ineffective at the given dose. However, Cu(II)(gtsm) caused mild kidney toxicity in the mice, associated primarily with interstitial nephritis and luminal distention. Mechanistically, we demonstrated that Cu(II)(gtsm) inhibits proteasomal chymotrypsin-like activity, a feature further established as being common to copper-ionophores that increase intracellular bioavailable copper. We have demonstrated that increasing intracellular bioavailable copper can selectively kill cancerous prostate cells in vitro and in vivo and have revealed the potential for bis(thiosemicarbazone) copper complexes to be developed as therapeutics for prostate cancer.

Clusterin and COMMD1 independently regulate degradation of the mammalian copper ATPases ATP7A and ATP7B

ATP7A and ATP7B are copper-transporting P(1B)-type ATPases (Cu-ATPases) that are critical for regulating intracellular copper homeostasis. Mutations in the genes encoding ATP7A and ATP7B lead to copper deficiency and copper toxicity disorders, Menkes and Wilson diseases, respectively. Clusterin and COMMD1 were previously identified as interacting partners of these Cu-ATPases. In this study, we confirmed that clusterin and COMMD1 interact to down-regulate both ATP7A and ATP7B. Overexpression and knockdown of clusterin/COMMD1 decreased and increased, respectively, endogenous levels of ATP7A and ATP7B, consistent with a role in facilitating Cu-ATPase degradation. We demonstrate that whereas the clusterin/ATP7B interaction was enhanced by oxidative stress or mutation of ATP7B, the COMMD1/ATP7B interaction did not change under oxidative stress conditions, and only increased with ATP7B mutations that led to its misfolding. Clusterin and COMMD1 facilitated the degradation of ATP7B containing the same Wilson disease-causing C-terminal mutations via different degradation pathways, clusterin via the lysosomal pathway and COMMD1 via the proteasomal pathway. Furthermore, endogenous ATP7B existed in a complex with clusterin and COMMD1, but these interactions were neither competitive nor cooperative and occurred independently of each other. Together these data indicate that clusterin and COMMD1 represent alternative and independent systems regulating Cu-ATPase quality control, and consequently contributing to the maintenance of copper homeostasis.

Clusterin (apolipoprotein J), a molecular chaperone that facilitates degradation of the copper-ATPases ATP7A and ATP7B

The copper-transporting P(1B)-type ATPases (Cu-ATPases) ATP7A and ATP7B are key regulators of physiological copper levels. They function to maintain intracellular copper homeostasis by delivering copper to secretory compartments and by trafficking toward the cell periphery to export excess copper. Mutations in the genes encoding ATP7A and ATP7B lead to copper deficiency and toxicity disorders, Menkes and Wilson diseases, respectively. This report describes the interaction between the Cu-ATPases and clusterin and demonstrates a chaperone-like role for clusterin in facilitating their degradation. Clusterin interacted with both ATP7A and ATP7B in mammalian cells. This interaction increased under conditions of oxidative stress and with mutations in ATP7B that led to its misfolding and mislocalization. A Wilson disease patient mutation (G85V) led to enhanced ATP7B turnover, which was further exacerbated when cells overexpressed clusterin. We demonstrated that clusterin-facilitated degradation of mutant ATP7B is likely to involve the lysosomal pathway. The knockdown and overexpression of clusterin increased and decreased, respectively, the Cu-ATPase-mediated copper export capacity of cells. These results highlight a new role for intracellular clusterin in mediating Cu-ATPase quality control and hence in the normal maintenance of copper homeostasis, and in promoting cell survival in the context of disease. Based on our findings, it is possible that variations in clusterin expression and function could contribute to the variable clinical expression of Menkes and Wilson diseases.

Role of glutaredoxin1 and glutathione in regulating the activity of the copper-transporting P-type ATPases, ATP7A and ATP7B

The copper-transporting P-type ATPases (Cu-ATPases), ATP7A and ATP7B, are essential for the regulation of intracellular copper homeostasis. In this report we describe new roles for glutathione (GSH) and glutaredoxin1 (GRX1) in Cu homeostasis through their regulation of Cu-ATPase activity. GRX1 is a thiol oxidoreductase that catalyzes the reversible reduction of GSH-mixed disulfides to their respective sulfhydryls (deglutathionylation). Here, we demonstrated that glutathionylation of the Cu-ATPases and their interaction with GRX1 were affected by alterations in Cu levels. The data support our hypothesis that the Cu-ATPases serve as substrates for Cu-dependent GRX1-mediated deglutathionylation. This in turn liberates the Cu-ATPase cysteinyl thiol groups for Cu binding and transport. GSH depletion experiments led to reversible inhibition of the Cu-ATPases that correlated with effects on intracellular Cu levels and GRX1 activity. Finally, knockdown of GRX1 expression resulted in an increase in intracellular Cu accumulation. Together, these data directly implicate GSH and GRX1 with important new roles in redox regulation of the Cu-ATPases, through modulation of Cu binding by the Cu-ATPase cysteine motifs.

Copper-dependent interaction of glutaredoxin with the N termini of the copper-ATPases (ATP7A and ATP7B) defective in Menkes and Wilson diseases

The P-type ATPases affected in Menkes and Wilson diseases, ATP7A and ATP7B, respectively, are key copper transporters that regulate copper homeostasis. The N termini of these proteins are critical in regulating their function and activity, and contain six copper-binding motifs MxCxxC. In this study, we describe the identification of glutaredoxin (GRX1) as an interacting partner of both ATP7A and ATP7B, confirmed by yeast two-hybrid technology and by co-immunoprecipitation from mammalian cells. The interaction required the presence of copper and intact metal-binding motifs. In addition, the interaction was related to the number of metal-binding domains available. GRX1 catalyses the reduction of disulphide bridges and reverses the glutathionylation of proteins to regulate and/or protect protein activity. We propose that GRX1 is essential for ATPase function and catalyses either the reduction of intramolecular disulphide bonds or the deglutathionylation of the cysteine residues within the CxxC motifs to facilitate copper-binding for subsequent transport.

Copper-dependent interaction of dynactin subunit p62 with the N terminus of ATP7B but not ATP7A

The P-type ATPase affected in Wilson disease, ATP7B, is a key liver protein required to regulate and maintain copper homeostasis. When hepatocytes are exposed to elevated copper levels, ATP7B traffics from the trans-Golgi network toward the biliary canalicular membrane to excrete excess copper into bile. The N-terminal region of ATP7B contains six metal-binding sites (MBS), each with the copper-binding motif MXCXXC. These sites are required for the activity and copper-regulated intracellular redistribution of ATP7B. Two proteins are known to interact with the ATP7B N-terminal region: the copper chaperone ATOX1 that delivers copper to ATP7B, and COMMD1 (MURR1) that is potentially involved in vesicular copper sequestration. To identify additional proteins that interact with ATP7B and hence are involved in copper homeostasis, a yeast two-hybrid approach was employed to screen a human liver cDNA library. The dynactin subunit p62 (dynactin 4; DCTN4) was identified as an interacting partner, and this interaction was confirmed by co-immunoprecipitation from mammalian cells. The dynactin complex binds cargo, such as vesicles and organelles, to cytoplasmic dynein for retrograde microtubule-mediated trafficking and could feasibly be involved in the copper-regulated trafficking of ATP7B. The ATP7B/p62 interaction required copper, the metal-binding CXXC motifs, and the region between MBS 4 and MBS 6 of ATP7B. The p62 subunit did not interact with the related copper ATPase, ATP7A. We propose that the ATP7B interaction with p62 is a key component of the copper-induced trafficking pathway that delivers ATP7B to subapical vesicles of hepatocytes for the removal of excess copper into bile.

ATP7B mediates vesicular sequestration of copper: insight into biliary copper excretion

BACKGROUND & AIMS

The Wilson protein (ATP7B) regulates levels of systemic copper by excreting excess copper into bile. It is not clear whether ATP7B translocates excess intrahepatic copper directly across the canalicular membrane or sequesters this copper into exocytic vesicles, which subsequently fuse with canalicular membrane to expel their contents into bile. The aim of this study was to clarify the mechanism underlying ATP7B-mediated copper detoxification by investigating endogenous ATP7B localization in the HepG2 hepatoma cell line and its ability to mediate vesicular sequestration of excess intracellular copper.

METHODS

Immunofluorescence microscopy was used to investigate the effect of copper concentration on the localization of endogenous ATP7B in HepG2 cells. Copper accumulation studies to determine whether ATP7B can mediate vesicular sequestration of excess intracellular copper were performed using Chinese hamster ovary cells that exogenously expressed wild-type and mutant ATP7B proteins.

RESULTS

In HepG2 cells, elevated copper levels stimulated trafficking of ATP7B to pericanalicular vesicles and not to the canalicular membrane as previously reported. Mutation of an endocytic retrieval signal in ATP7B caused the protein to constitutively localize to vesicles and not to the plasma membrane, suggesting that a vesicular compartment(s) is the final trafficking destination for ATP7B. Expression of wild-type and mutant ATP7B caused Chinese hamster ovary cells to accumulate copper in vesicles, which subsequently undergo exocytosis, releasing copper across the plasma membrane.

CONCLUSIONS

This report provides compelling evidence that the primary mechanism of biliary copper excretion involves ATP7B-mediated vesicular sequestration of copper rather than direct copper translocation across the canalicular membrane.

Intracellular trafficking of the human Wilson protein: the role of the six N-terminal metal-binding sites

The Wilson protein (ATP7B) is a copper-transporting CPx-type ATPase defective in the copper toxicity disorder Wilson disease. In hepatocytes, ATP7B delivers copper to apo-ceruloplasmin and mediates the excretion of excess copper into bile. These distinct functions require the protein to localize at two different subcellular compartments. At the trans-Golgi network, ATP7B transports copper for incorporation into apo-ceruloplasmin. When intracellular copper levels are increased, ATP7B traffics to post-Golgi vesicles in close proximity to the canalicular membrane to facilitate biliary copper excretion. In the present study, we investigated the role of the six N-terminal MBSs (metal-binding sites) in the trafficking process. Using site-directed mutagenesis, we mutated or deleted various combinations of the MBSs and assessed the effect of these changes on the localization and trafficking of ATP7B. Results show that the MBSs required for trafficking are the same as those previously found essential for the copper transport function. Either MBS 5 or MBS 6 alone was sufficient to support the redistribution of ATP7B to vesicular compartments. The first three N-terminal motifs were not required for copper-dependent intracellular trafficking and could not functionally replace sites 4-6 when placed in the same sequence position. Furthermore, the N-terminal region encompassing MBSs 1-5 (amino acids 64-540) was not essential for trafficking, with only one MBS close to the membrane channel, necessary and sufficient to support trafficking. Our findings were similar to those obtained for the closely related ATP7A protein, suggesting similar mechanisms for trafficking between copper-transporting CPx-type ATPases.