Identification of Methionine-rich Clusters That Regulate Copper-stimulated Endocytosis of the Human Ctr1 Copper Transporter

EGCG Enhances Cisplatin Sensitivity by Regulating Expression of the Copper and Cisplatin Influx Transporter CTR1 in Ovary Cancer

Cisplatin is one of the first-line platinum-based chemotherapeutic agents for treatment of many types of cancer, including ovary cancer. CTR1 (copper transporter 1), a transmembrane solute carrier transporter, has previously been shown to increase the cellular uptake and sensitivity of cisplatin. It is hypothesized that increased CTR1 expression would enhance the sensitivity of cancer cells to cisplatin (cDDP). The present study demonstrates for the first time that (-)-epigallocatechin-3-gallate (EGCG), a major polyphenol from green tea, can enhance CTR1 mRNA and protein expression in ovarian cancer cells and xenograft mice. EGCG inhibits the rapid degradation of CTR1 induced by cDDP. The combination of EGCG and cDDP increases the accumulation of cDDP and DNA-Pt adducts, and subsequently enhances the sensitivity of ovarian cancer SKOV3 and OVCAR3 cells to the chemotherapeutic agent. In the OVCAR3 ovarian cancer xenograft nude mice model, the combination of the lower concentration of cDDP and EGCG strongly repressed the tumor growth and exhibited protective effect on the nephrotoxicity induced by cisplatin. Overall, these findings uncover a novel chemotherapy mechanism of EGCG as an adjuvant for the treatment of ovarian cancer.

The Mitochondrial Metallochaperone SCO1 Is Required to Sustain Expression of the High-Affinity Copper Transporter CTR1 and Preserve Copper Homeostasis

Human SCO1 fulfills essential roles in cytochrome c oxidase (COX) assembly and the regulation of copper (Cu) homeostasis, yet it remains unclear why pathogenic mutations in this gene cause such clinically heterogeneous forms of disease. Here, we establish a Sco1 mouse model of human disease and show that ablation of Sco1 expression in the liver is lethal owing to severe COX and Cu deficiencies. We further demonstrate that the Cu deficiency is explained by a functional connection between SCO1 and CTR1, the high-affinity transporter that imports Cu into the cell. CTR1 is rapidly degraded in the absence of SCO1 protein, and we show that its levels are restored in Sco1^-/- mouse embryonic fibroblasts upon inhibition of the proteasome. These data suggest that mitochondrial signaling through SCO1 provides a post-translational mechanism to regulate CTR1-dependent Cu import into the cell, and they further underpin the importance of mitochondria in cellular Cu homeostasis.

Molecular mechanisms underlying copper homeostasis in Mammalian cells

Copper (Cu) is an essential metal for living organisms that utilize oxygen for respiration and is required as a cofactor of redox-regulating enzymes, such as superoxide dismutase, ceruloplasmin, lysyl oxidase, tyrosinase, and dopamine β-hydroxylase. However, the redox-active property of this metal may have toxic effects on cells due to the generation of harmful reactive oxygen species. Given these circumstances, it is said that cells have a dependable system for Cu homeostasis that efficiently distributes this essential metal to cuproenzymes, thereby preventing damage to proteins, nucleic acids, sugars, and lipids. In particular, influx, efflux, and intracellular distribution with maintenance of the oxidation state of Cu are strictly regulated. Several groups of Cu-regulating factors have been identified in mammalian cells, i.e., Cu transporters, Cu chaperones, Cu-binding proteins/peptides, and others. In this review, the features of the Cu-regulating factors are concisely examined in terms of molecular mechanisms underlying Cu homeostasis in cells.

The role of copper ions in pathophysiology and fluorescent sensors for the detection thereof

Copper ions are crucial to life, and some fundamental roles of copper in pathophysiology have been elucidated using fluorescent sensors.

Copper in Eukaryotes

Copper is essential for normal growth and development of eukaryotic organisms. Numerous physiological processes rely on sufficient availability of copper: from indispensable reactions such as mitochondrial respiration to more highly specialized processes such as pigment development in a skin. Copper misbalance has been linked to a variety of metabolic and neurodegenerative disorders in humans. Complex cellular machinery has evolved to mediate copper uptake, compartmentalization and incorporation into target proteins. Extensive studies revealed a predominant utilization of methionines and histidines by copper handling molecules for copper capture at the extracellular surface and delivery to cuproenzymes in the lumen of cellular compartments, respectively. Cu(I) is a predominant form within the cell, and copper binding and distribution inside the cell at the cytosolic sites relies heavily on cysteines. The selectivity and directionality of copper transfer reactions is determined by thermodynamic and kinetic factors as well as spatial distribution of copper donors and acceptors. In this chapter, we review current structural and mechanistic data on copper transport and distribution in yeast and mammalian cells and highlight important issues and questions for future studies.

Salinity-dependent copper accumulation in the guppy Poecilia vivipara is associated with CTR1 and ATP7B transcriptional regulation

Copper (Cu) accumulation and regulation of key-genes involved in Cu homeostasis were evaluated in freshwater- and saltwater-acclimated guppies Poecilia vivipara. Fish were exposed (96h) to environmentally relevant concentrations of dissolved Cu (0, 5.0, 9.0 and 20.0μg/L). In freshwater guppies, gill and liver Cu accumulation was dependent on Cu concentration in the exposure medium. In saltwater guppies, this dependence was observed only in the gut. These findings indicate that Cu accumulation was salinity- and tissue-dependent. Key genes involved in Cu metabolism were sequenced for the first time in P. vivipara. Transcripts coding for the high-affinity copper transporter (CTR1) and copper-transporting ATPase (ATP7B) were identified using polymerase chain reaction (PCR) and gene sequencing. The full-length CTR1 open reading frame (1560bp) and a partial ATP7B (690bp) were discovered. Predicted amino acid sequences shared high identities with the CTR1 of Fundulus heteroclitus (81%) and the ATP7B of Sparus aurata (87%). Basal transcriptional levels addressed by RT-qPCR in control fish indicate that CTR1 and ATP7B was highly transcribed in liver of freshwater guppies while CTR1 was highly transcribed in gut of saltwater guppies. This could explain the higher Cu accumulation observed in liver of freshwater guppies and in gut of saltwater guppies, because CTR1 is involved in Cu uptake. Reduced gill mRNA expression of CTR1 was observed in freshwater guppies exposed to 20.0μg/L Cu and in saltwater guppies exposed to 5.0μg/L Cu. In turn, reduced mRNA expression of gut ATP7B was observed in freshwater and salt water guppies exposed to 9.0 and 20.0μg/L Cu. Liver CTR1 and ATP7B transcription were not affected by Cu exposure. These findings suggest that gill CTR1 and gut ATP7B are down-regulated to limit Cu absorption after exposure to dissolved Cu, while liver CTR1 and ATP7B levels are maintained to allow Cu storage and detoxification. In conclusion, findings reported here indicate that Cu accumulation in the euryhaline guppy P. vivipara is tissue specific and dependent on water salinity. They also suggest that Cu homeostasis involves a differential transcriptional regulation of the newly identified Cu transporters, CTR1 and ATP7B.

Molecular modulation of the copper and cisplatin transport function of CTR1 and its interaction with IRS-4

The copper influx transporter CTR1 is also a major influx transporter for cisplatin (cDDP) in tumor cells. It influences the cytotoxicity of cDDP both in vivo and in vitro. Whereas Cu triggers internalization of CTR1 from the plasma membrane, cDDP does not. To investigate the mechanisms of these effects, myc-tagged forms of wild type hCTR1 and variants in which Y103 was converted to alanine, C189 was converted to serine, or the K178/K179 dilysine motif was converted to alanines were re-expressed in mouse embryo cells in which both alleles of CTR1 had been knocked out and also in HEK293T cells. The Y103A mutation and to a lesser extent the C189S mutation reduced internalization of CTR1 induced by Cu while the K178A/K179A had little effect. Both Y103 and C189 were required for Cu and cDDP transport whereas the K178/K179 motif was not. While Y103 lies in an YXXM motif that, when phosphorylated, is a potential docking site for phosphatidylinositol 3-kinase and other proteins involved in endocytosis, Western blot analysis of immunoprecipitated myc-CTR1, and proteomic analysis of peptides derived from CTR1, failed to identify any basal or Cu-induced phosphorylation. However, proteomic analysis did identify an interaction of CTR1 with IRS-4 and this was confirmed by co-immunoprecipitation from HEK cells expressing either FLAG-CTR1 or myc-CTR1. The interaction was greater in the Y103A-expressing cells. We conclude that Y103 is required for the internalization of hCTR1 in response to Cu, that this occurs by a mechanism other than phosphorylation and that mutation of Y103 modulates the interaction with IRS-4.

What can flies tell us about copper homeostasis?

Copper (Cu) is an essential redox active metal that is potentially toxic in excess. Multicellular organisms acquire Cu from the diet and must regulate uptake, storage, distribution and export of Cu at both the cellular and organismal levels. Systemic Cu deficiency can be fatal, as seen in Menkes disease patients. Conversely Cu toxicity occurs in patients with Wilson disease. Cu dyshomeostasis has also been implicated in neurodegenerative disorders such as Alzheimer's disease. Over the last decade, the fly Drosophila melanogaster has become an important model organism for the elucidation of eukaryotic Cu regulatory mechanisms. Gene discovery approaches with Drosophila have identified novel genes with conserved protein functions relevant to Cu homeostasis in humans. This review focuses on our current understanding of Cu uptake, distribution and export in Drosophila and the implications for mammals.

How copper traverses cellular membranes through the mammalian copper transporter 1, Ctr1

The copper transporter 1, Ctr1, is part of a major pathway for cellular copper (Cu) uptake in the intestinal epithelium, in hepatic and cardiac tissue, and likely in many other mammalian cells and tissues. Here, we summarize what is currently known about how extracellular Cu travels across the plasma membrane to enter the cytoplasm for intracellular distribution and for use by proteins and enzymes, the physiological roles of Ctr1, and its regulation. As a critical Cu importer, Ctr1 occupies a strategic position to exert a strong modifying influence on diseases and pathophysiological states caused by imbalances in Cu homeostasis. A more thorough understanding of the mechanisms that regulate Ctr1 abundance, trafficking, and function will provide new insights and opportunities for disease therapies.

The role of Ctr1 and Ctr2 in mammalian copper homeostasis and platinum-based chemotherapy

Copper (Cu) is an essential metal for growth and development that has the potential to be toxic if levels accumulate beyond the ability of cells to homeostatically balance uptake with detoxification. One system for Cu acquisition is the integral membrane Cu(+) transporter, Ctr1, which has been quite well characterized in terms of its function and physiology. The mammalian Ctr2 protein has been a conundrum for the copper field, as it is structurally closely related to the high affinity Cu transporter Ctr1, sharing important motifs for Cu transport activity. However, in contrast to mammalian Ctr1, Ctr2 fails to suppress the Cu-dependent growth phenotype of yeast cells defective in Cu(+) import, nor does it appreciably stimulate Cu acquisition when over-expressed in mammalian cells, underscoring important functional dissimilarities between the two proteins. Several roles for the mammalian Ctr2 have been suggested both in vitro and in vivo. Here, we summarize and discuss current insights into the Ctr2 protein and its interaction with Ctr1, its functions in mammalian Cu homeostasis and platinum-based chemotherapy.

Pathogenic adaptations to host-derived antibacterial copper

Recent findings suggest that both host and pathogen manipulate copper content in infected host niches during infections. In this review, we summarize recent developments that implicate copper resistance as an important determinant of bacterial fitness at the host-pathogen interface. An essential mammalian nutrient, copper cycles between copper (I) (Cu(+)) in its reduced form and copper (II) (Cu(2+)) in its oxidized form under physiologic conditions. Cu(+) is significantly more bactericidal than Cu(2+) due to its ability to freely penetrate bacterial membranes and inactivate intracellular iron-sulfur clusters. Copper ions can also catalyze reactive oxygen species (ROS) generation, which may further contribute to their toxicity. Transporters, chaperones, redox proteins, receptors and transcription factors and even siderophores affect copper accumulation and distribution in both pathogenic microbes and their human hosts. This review will briefly cover evidence for copper as a mammalian antibacterial effector, the possible reasons for this toxicity, and pathogenic resistance mechanisms directed against it.

The families of zinc (SLC30 and SLC39) and copper (SLC31) transporters

The solute carriers families 30 (SLC30; ZnT), 39 (SLC39; ZIP), and 31 (SLC31; CTR) are involved in the essential maintenance of cellular zinc (Zn²⁺) and copper (Cu²⁺) homeostasis, respectively. ZnTs mediate Zn²⁺ extrusion from cells (SLC30A1) or transport Zn²⁺ into organelles and secretory vesicles/granules (SLC30A2-SLC30A8). SLC39 family members are predominantly localized to the cell membrane where they perform Zn²⁺ uptake and increase the availability of cytosolic Zn²⁺. SLC39A1 is ubiquitously expressed, whereas other ZIP transporters (e.g., SLC39A2 and SLC39A3) show a more tissue-restricted expression consistent with organ-specific functions of these proteins. The members A1 (CTR1) and A2 (CTR2) of the SLC31 family of solute carriers belong to a network of proteins that acts to regulate the intracellular Cu²⁺ concentration within a certain range. SLC31A1 is predominantly localized to the plasma membrane, whereas SLC31A2 is mainly found in intracellular membranes of the late endosome and lysosome. The specific function of SLC31A2 is not known. SLC31A1 is ubiquitously expressed and has been characterized as a high-affinity importer of reduced copper (Cu⁺). Cu²⁺ transport function of CTR proteins is associated with oligomerization; SLC31A1 trimerizes and thereby forms a channel-like structure enabling Cu²⁺ translocation across the cell membrane. The molecular characteristics and structural details (e.g., membrane topology, conserved Zn²⁺, and Cu²⁺ binding sites) and mechanisms of translational and posttranslational regulation of expression and/or activity have been described for SLC30 and SLC39 family members, and for SLC31A1. For SLC31A1, data on tissue-specific functions (e.g., in the intestine, heart, and liver) are also available. A link between SLC31A1, immune function, and disorders such as Alzheimer's disease or cancer makes the protein a candidate therapeutic target. In secretory tissues (e.g., the mammary gland and pancreas), Zn²⁺ transporters of SLC families 30 and 39 are involved in specific functions such as insulin synthesis and secretion, metallation of digestive proenzymes, and transfer of nutrients into milk. Defective or dysregulated Zn²⁺ metabolism in these organs is associated with disorders such as diabetes and cancer, and impaired Zn²⁺ secretion into milk.

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.

SLC31 (CTR) family of copper transporters in health and disease

Copper is a vital mineral for many organisms, yet it is highly toxic as demonstrated by serious health concerns associated with its deficiency or excess accumulation. The SLC31 (CTR) family of copper transporters is a major gateway of copper acquisition in eukaryotes, ranging from yeast to humans. Characterization of the function, modes of action, and regulation of CTR and other molecular factors that functionally cooperate with CTR for copper transport, compartmentalization, incorporation into cuproproteins, and detoxification has revealed that organisms have evolved fascinating mechanisms for tight control of copper metabolism. This research progress further indicates the significance of copper in health and disease and opens avenues for therapeutic control of copper bioavailability and its metabolic pathways.

Rate and regulation of copper transport by human copper transporter 1 (hCTR1)

Human copper transporter 1 (hCTR1) is a homotrimer of a 190-amino acid monomer having three transmembrane domains believed to form a pore for copper permeation through the plasma membrane. The hCTR1-mediated copper transport mechanism is not well understood, nor has any measurement been made of the rate at which copper ions are transported by hCTR1. In this study, we estimated the rate of copper transport by the hCTR1 trimer in cultured cells using (64)Cu uptake assays and quantification of plasma membrane hCTR1. For endogenous hCTR1, we estimated a turnover number of about 10 ions/trimer/s. When overexpressed in HEK293 cells, a second transmembrane domain mutant of hCTR1 (H139R) had a 3-fold higher Km value and a 4-fold higher turnover number than WT. Truncations of the intracellular C-terminal tail and an AAA substitution of the putative metal-binding HCH C-terminal tripeptide (thought to be required for transport) also exhibited elevated transport rates and Km values when compared with WT hCTR1. Unlike WT hCTR1, H139R and the C-terminal mutants did not undergo regulatory endocytosis in elevated copper. hCTR1 mutants combining methionine substitutions that block transport (M150L,M154L) on the extracellular side of the pore and the high transport H139R or AAA intracellular side mutations exhibited the blocked transport of M150L,M154L, confirming that Cu(+) first interacts with the methionines during permeation. Our results show that hCTR1 elements on the intracellular side of the hCTR1 pore, including the carboxyl tail, are not essential for permeation, but serve to regulate the rate of copper entry.

Contributions of rat Ctr1 to the uptake and toxicity of copper and platinum anticancer drugs in dorsal root ganglion neurons

Dorsal root ganglion (DRG) neurons are affected by platinum-induced neurotoxicity and neurodegenerative processes associated with disturbed copper homeostasis and transport. This study aimed to understand the role of copper transporter 1 (Ctr1) in the uptake and toxicity of copper and platinum drugs in cultured rat DRG neurons, and the functional activities of rat Ctr1 (rCtr1) as a membrane transporter of copper and platinum drugs. Heterologous expression of rCtr1 in HEK293 cells (HEK/rCtr1 cells) increased the uptake and cytotoxicity of copper, oxaliplatin, cisplatin and carboplatin, in comparison to isogenic vector-transfected control cells. Cultured rat DRG neurons endogenously expressed rCtr1 protein on their neuronal cell body plasma membranes and cytoplasm, and displayed substantial capacity for taking up copper, but were resistant to copper toxicity. The uptake of copper by both cultured rat DRG neurons and HEK/rCtr1 cells was saturable and inhibited by cold temperature, silver and zinc, consistent with it being mediated by rCtr1. Cultured rat DRG neurons accumulated platinum during their exposure to oxaliplatin and were sensitive to oxaliplatin cytotoxicity. The accumulation of platinum by both cultured rat DRG neurons and HEK/rCtr1 cells, during oxaliplatin exposure, was saturable and temperature dependent, but was inhibited by copper only in HEK/rCtr1 cells. In conclusion, rCtr1 can transport copper and platinum drugs, and sensitizes cells to their cytotoxicities. DRG neurons display substantial capacity for accumulating copper via a transport process mediated by rCtr1, but appear able to resist copper toxicity and use alternative mechanisms to take up oxaliplatin.

Role of the human high-affinity copper transporter in copper homeostasis regulation and cisplatin sensitivity in cancer chemotherapy

The high-affinity copper transporter (Ctr1; SCLC31A1) plays an important role in regulating copper homeostasis because copper is an essential micronutrient and copper deficiency is detrimental to many important cellular functions, but excess copper is toxic. Recent research has revealed that human copper homeostasis is tightly controlled by interregulatory circuitry involving copper, Sp1, and human (hCtr1). This circuitry uses Sp1 transcription factor as a copper sensor in modulating hCtr1 expression, which in turn controls cellular copper and Sp1 levels in a 3-way mutual regulatory loop. Posttranslational regulation of hCtr1 expression by copper stresses has also been described in the literature. Because hCtr1 can also transport platinum drugs, this finding underscores the important role of hCtr1 in platinum-drug sensitivity in cancer chemotherapy. Consistent with this notion is the finding that elevated hCtr1 expression was associated with favorable treatment outcomes in cisplatin-based cancer chemotherapy. Moreover, cultured cell studies showed that elevated hCtr1 expression can be induced by depleting cellular copper levels, resulting in enhanced cisplatin uptake and its cell-killing activity. A phase I clinical trial using a combination of trientine (a copper chelator) and carboplatin has been carried out with encouraging results. This review discusses new insights into the role of hCtr1 in regulating copper homeostasis and explains how modulating cellular copper availability could influence treatment efficacy in platinum-based cancer chemotherapy through hCtr1 regulation.

An all-atom model of the structure of human copper transporter 1

Human copper transporter 1 (hCTR1) is the major high affinity copper influx transporter in mammalian cells that also mediates uptake of the cancer chemotherapeutic agent cisplatin. A low resolution structure of hCTR1 determined by cryoelectron microscopy was recently published. Several protein structure simulation techniques were used to create an all-atom model of this important transporter using the low resolution structure as a starting point. The all-atom model provides new insights into the roles of specific residues of the N-terminal extracellular domain, the intracellular loop, and C-terminal region in metal ion transport. In particular, the model demonstrates that the central region of the pore contains four sets of methionine triads in the intramembranous region. The structure confirms that two triads of methionine residues delineate the intramembranous region of the transporter, and further identifies two additional methionine triads that are located in the extracellular N-terminal part of the transporter. Together, the four triads create a structure that promotes stepwise transport of metal ions into and then through the intramembranous channel of the transporter via transient thioether bonds to methionine residues. Putative copper-binding sites in the hCTR1 trimer were identified by a program developed by us for prediction of metal-binding sites. These sites correspond well with the known effects of mutations on the ability of the protein to transport copper and cisplatin.

Sec61β controls sensitivity to platinum-containing chemotherapeutic agents through modulation of the copper-transporting ATPase ATP7A

The Sec61 protein translocon is a multimeric complex that transports proteins across lipid bilayers. We discovered that the Sec61β subunit modulates cellular sensitivity to chemotherapeutic agents, particularly the platinum drugs. To investigate the mechanism, expression of Sec61β was constitutively knocked down in 2008 ovarian cancer cells. Sec61β knockdown (KD) resulted in 8-, 16.8-, and 9-fold resistance to cisplatin (cDDP), carboplatin, and oxaliplatin, respectively. Sec61β KD reduced the cellular accumulation of cDDP to 67% of that in parental cells. Baseline copper levels, copper uptake, and copper cytotoxicity were also reduced. Because copper transporters and chaperones regulate platinum drug accumulation and efflux, their expression in 2008 Sec61β-KD cells was analyzed; ATP7A was found to be 2- to 3-fold overexpressed, whereas there was no change in ATP7B, ATOX1, CTR1, or CTR2 levels. Cells lacking ATP7A did not exhibit increased cDDP resistance upon knockdown of Sec61β. Sec61β-KD cells also exhibited altered ATP7A cellular distribution. We conclude that Sec61β modulates the cytotoxicity of many chemotherapeutic agents, with the largest effect being on the platinum drugs. This modulation occurs through effects of Sec61β on the expression and distribution of ATP7A, which was shown previously to control platinum drug sequestration and cytotoxicity.

Charting the travels of copper in eukaryotes from yeast to mammals

Throughout evolution, all organisms have harnessed the redox properties of copper (Cu) and iron (Fe) as a cofactor or structural determinant of proteins that perform critical functions in biology. At its most sobering stance to Earth's biome, Cu biochemistry allows photosynthetic organisms to harness solar energy and convert it into the organic energy that sustains the existence of all nonphotosynthetic life forms. The conversion of organic energy, in the form of nutrients that include carbohydrates, amino acids and fatty acids, is subsequently released during cellular respiration, itself a Cu-dependent process, and stored as ATP that is used to drive a myriad of critical biological processes such as enzyme-catalyzed biosynthetic processes, transport of cargo around cells and across membranes, and protein degradation. The life-supporting properties of Cu incur a significant challenge to cells that must not only exquisitely balance intracellular Cu concentrations, but also chaperone this redox-active metal from its point of cellular entry to its ultimate destination so as to avert the potential for inappropriate biochemical interactions or generation of damaging reactive oxidative species (ROS). In this review we chart the travels of Cu from the extracellular milieu of fungal and mammalian cells, its path within the cytosol as inferred by the proteins and ligands that escort and deliver Cu to intracellular organelles and protein targets, and its journey throughout the body of mammals. This article is part of a Special Issue entitled: Cell Biology of Metals.

Chelation therapy in Wilson's disease: from D-penicillamine to the design of selective bioinspired intracellular Cu(I) chelators

Wilson's disease is an orphan disease due to copper homeostasis dysfunction. Mutations of the ATP7B gene induces an impaired functioning of a Cu-ATPase, impaired Cu detoxification in the liver and copper overload in the body. Indeed, even though copper is an essential element, which is used as cofactor by many enzymes playing vital roles, it becomes toxic when in excess as it promotes cytotoxic reactions leading to oxidative stress. In this perspective, human copper homeostasis is first described in order to explain the mechanisms promoting copper overload in Wilson's disease. We will see that the liver is the main organ for copper distribution and detoxification in the body. Nowadays this disease is treated life-long by systemic chelation therapy, which is not satisfactory in many cases. Therefore the design of more selective and efficient drugs is of great interest. A strategy to design more specific chelators to treat localized copper accumulation in the liver will then be presented. In particular we will show how bioinorganic chemistry may help in the design of such novel chelators by taking inspiration from the biological copper cell transporters.

Regulation of copper transporters in human cells

Copper is essential for normal growth and development of human organisms. The role of copper as a cofactor of important metabolic enzymes, such as cytochrome c oxidase, superoxide dismutase, lysyl oxidase, dopamine-β-hydroxylase, and many others, has been well established. In recent years, new regulatory roles of copper have emerged. Accumulating evidence points to the involvement of copper in lipid metabolism, antimicrobial defense, neuronal activity, resistance of tumor cells to platinum-based chemotherapeutic drugs, kinase-mediated signal transduction, and other essential cellular processes. For many of these processes, the precise mechanism of copper action remains to be established. Nevertheless, it is increasingly clear that many regulatory and signaling events are associated with changes in the intracellular localization and abundance of copper transporters, as well as distinct compartmentalization of copper itself. In this review, we discuss current data on regulation of the localization and abundance of copper transporters in response to metabolic and signaling events in human cells. Regulation by kinase-mediated phosphorylation will be addressed along with the emerging area of the redox-driven control of copper transport. We highlight mechanistic questions that await further testing.

Molecular basis of the cooperative binding of Cu(I) and Cu(II) to the CopK protein from Cupriavidus metallidurans CH34

The bacterium Cupriavidus metallidurans CH34 is resistant to high environmental concentrations of many metal ions. Upon copper challenge, it upregulates the periplasmic protein CopK (8.3 kDa). The function of CopK in the copper resistance response is ill-defined, but CopK demonstrates an intriguing cooperativity: occupation of a high-affinity Cu(I) binding site generates a high-affinity Cu(II) binding site, and the high-affinity Cu(II) binding enhances Cu(I) binding. Native CopK and targeted variants were examined by chromatographic, spectroscopic, and X-ray crystallographic probes. Structures of two distinct forms of Cu(I)Cu(II)-CopK were defined, and structural changes associated with occupation of the Cu(II) site were demonstrated. In solution, monomeric Cu(I)Cu(II)-CopK features the previously elucidated Cu(I) site in Cu(I)-CopK, formed from four S(δ) atoms of Met28, -38, -44, and -54 (site 4S). Binding of Cu(I) to apo-CopK induces a conformational change that releases the C-terminal β-strand from the β-sandwich structure. In turn, this allows His70 and N-terminal residues to form a large loop that includes the Cu(II) binding site. In crystals, a polymeric form of Cu(I)Cu(II)-CopK displays a Cu(I) site defined by the S(δ) atoms of Met26, -38, and -54 (site 3S) and an exogenous ligand (modeled as H(2)O) and a Cu(II) site that bridges dimeric CopK molecules. The 3S Cu(I) binding mode observed in crystals was demonstrated in solution in protein variant M44L where site 4S is disabled. The intriguing copper binding chemistry of CopK provides molecular insight into Cu(I) transfer processes. The adaptable nature of the Cu(I) coordination sphere in methionine-rich clusters allows copper to be relayed between clusters during transport across membranes in molecular pumps such as CusA and Ctr1.

Human copper transporters: mechanism, role in human diseases and therapeutic potential

Normal copper homeostasis is essential for human growth and development. Copper deficiency, caused by genetic mutations, inadequate diet or surgical interventions, may lead to cardiac hypertrophy, poor neuronal myelination, blood vessel abnormalities and impaired immune response. Copper overload is associated with morphological and metabolic changes in tissues and, if untreated, eventual death. Recent reports also indicate that changes in the expression of copper transporters alter the sensitivity of cancer cells to major chemotherapeutic drugs, such as cisplatin, although the mechanism behind this important phenomenon remains unclear. This review summarizes current information on the molecular characteristics of copper transporters CTR1, CTR2, ATP7A and ATP7B, their roles in mammalian copper homeostasis and the physiological consequences of their inactivation. The mechanisms through which copper transporters may influence cell sensitivity to cisplatin are discussed. Regulation of human copper homeostasis has significant therapeutic potential and requires the detailed understanding of copper transport mechanisms.

Advances in the understanding of mammalian copper transporters

Copper (Cu) is an essential micronutrient. Its ability to exist in 2 oxidation states (Cu(1+) and Cu(2+)) allows it to function as an enzymatic cofactor in hydrolytic, electron transfer, and oxygen utilization reactions. Cu transporters CTR1, ATP7A, and ATP7B play key roles in ensuring that adequate Cu is available for Cu-requiring processes and the prevention of excess Cu accumulation within cells. Two diseases of Cu metabolism, Menkes disease and Wilson disease, which are caused by mutations in ATP7A and ATP7B, respectively, exemplify the critical importance of regulating Cu balance in humans. Herein, we review recent studies of the biochemical and cell biological characteristics of CTR1, ATP7A, and ATP7B, as well as emerging roles for Cu in new areas of physiology.

Regulation of Cisplatin cytotoxicity by cu influx transporters

Platinum drugs are an important class of cancer chemotherapeutics. However, the use of these drugs is limited by the development of resistance during treatment with decreased accumulation being a common mechanism. Both Cu transporters CTR1 and CTR2 influence the uptake and cytotoxicity of cisplatin. Although it is structurally similar to CTR1, CTR2 functions in a manner opposite to that of CTR1 with respect to Pt drug uptake. Whereas knockout of CTR1 reduces Pt drug uptake, knockdown of CTR2 enhances cisplatin uptake and cytotoxicity. CTR2 is subject to transcriptional and posttranscriptional regulation by both Cu and cisplatin; this regulation is partly dependent on the Cu chaperone ATOX1. Insight into the mechanisms by which CTR1 and CTR2 regulate sensitivity to the Pt-containing drugs has served as the basis for novel pharmacologic strategies for improving their efficacy.

Transmembrane nine proteins in yeast and Arabidopsis affect cellular metal contents without changing vacuolar morphology

Transmembrane nine (TM9) proteins are localized in the secretory pathway of eukaryotic cells and are involved in cell adhesion and phagocytosis. The mechanism by which TM9 proteins operate is, however, not well understood. Here we have utilized elemental profiling by inductively coupled plasma mass spectrometry (ICP-MS) to further investigate the physiological function of TM9 proteins. Cellular copper contents in Saccharomyces cerevisiae varied depending on the presence of TM9 homologues from both yeast and Arabidopsis thaliana. A yeast tmn1-3 triple mutant lacking all three yeast endogenous TMNs showed altered metal homeostasis with a reduction in the cellular Cu contents to 25% of that in the wild-type. Conversely, when TMN1 was overexpressed in yeast, cellular Cu concentrations were more than doubled. Both Tmn1p-GFP and Tmn2p-GFP fusion proteins localized to the tonoplast. Yeast vacuolar biogenesis was not affected by the lack or presence of TM9 proteins neither in the tmn1-3 triple mutant nor in TM9 overexpressing strains. Heterologous expression in yeast of AtTMN7, a TM9 homologue from Arabidopsis, affected Cu homeostasis similar to the overexpression of TMN1. In Arabidopsis, the two TM9 homologues AtTMN1 and AtTMN7 were ubiquitously expressed. AtTMN7 promoter constructs driving the expression of GFP showed elevated expression of AtTMN7 in the root elongation zone. It is concluded that TM9 homologues from S. cerevisiae and A. thaliana have the ability to affect the intracellular Cu balance. Tmn1p and Tmn2p operate from the yeast vacuolar membrane without influencing vacuolar biogenesis. A new physiological function of the TM9 family coupled to vacuolar Cu homeostasis is proposed.

Ctr1 is an apical copper transporter in mammalian intestinal epithelial cells in vivo that is controlled at the level of protein stability

Copper is an essential trace element that functions in a diverse array of biochemical processes that include mitochondrial respiration, neurotransmitter biogenesis, connective tissue maturation, and reactive oxygen chemistry. The Ctr1 protein is a high-affinity Cu(+) importer that is structurally and functionally conserved in yeast, plants, fruit flies, and humans and that, in all of these organisms, is localized to the plasma membrane and intracellular vesicles. Although intestinal epithelial cell-specific deletion of Ctr1 in mice demonstrated a critical role for Ctr1 in dietary copper absorption, some controversy exists over the localization of Ctr1 in intestinal epithelial cells in vivo. In this work, we assess the localization of Ctr1 in intestinal epithelial cells through two independent mechanisms. Using immunohistochemistry, we demonstrate that Ctr1 localizes to the apical membrane in intestinal epithelial cells of the mouse, rat, and pig. Moreover, biotinylation of intestinal luminal proteins from mice fed a control or a copper-deficient diet showed elevated levels of both total and apical membrane Ctr1 protein in response to transient dietary copper limitation. Experiments in cultured HEK293T cells demonstrated that alterations in the levels of the glycosylated form of Ctr1 in response to copper availability were a time-dependent, copper-specific posttranslational response. Taken together, these results demonstrate apical localization of Ctr1 in intestinal epithelia across three mammalian species and suggest that increased Ctr1 apical localization in response to dietary copper limitation may represent an adaptive response to homeostatically modulate Ctr1 availability at the site of intestinal copper absorption.

Copper-dependent trafficking of the Ctr4-Ctr5 copper transporting complex

BACKGROUND

In Schizosaccharomyces pombe, copper uptake is carried out by a heteromeric complex formed by the Ctr4 and Ctr5 proteins. Copper-induced differential subcellular localization may play a critical role with respect to fine tuning the number of Ctr4 and Ctr5 molecules at the cell surface.

METHODOLOGY/PRINCIPAL FINDINGS

We have developed a bimolecular fluorescence complementation (BiFC) assay to analyze protein-protein interactions in vivo in S. pombe. The assay is based on the observation that N- and C-terminal subfragments of the Venus fluorescent protein can reconstitute a functional fluorophore only when they are brought into tight contact. Wild-type copies of the ctr4(+) and ctr5(+) genes were inserted downstream of and in-frame with the nonfluorescent C-terminal (VC) and N-terminal (VN) coding fragments of Venus, respectively. Co-expression of Ctr4-VC and Ctr5-VN fusion proteins allowed their detection at the plasma membrane of copper-limited cells. Similarly, cells co-expressing Ctr4-VN and Ctr4-VC in the presence of Ctr5-Myc(12) displayed a fluorescence signal at the plasma membrane. In contrast, Ctr5-VN and Ctr5-VC co-expressed in the presence of Ctr4-Flag(2) failed to be visualized at the plasma membrane, suggesting a requirement for a combination of two Ctr4 molecules with one Ctr5 molecule. We found that plasma membrane-located Ctr4-VC-Ctr5-VN fluorescent complexes were internalized when the cells were exposed to high levels of copper. The copper-induced internalization of Ctr4-VC-Ctr5-VN complexes was not dependent on de novo protein synthesis. When cells were transferred back from high to low copper levels, there was reappearance of the BiFC fluorescent signal at the plasma membrane.

SIGNIFICANCE

These findings reveal a copper-dependent internalization and recycling of the heteromeric Ctr4-Ctr5 complex as a function of copper availability.

The role of the N-terminus of mammalian copper transporter 1 in the cellular accumulation of cisplatin

The mammalian copper transporter 1 (CTR1) is responsible for the uptake of copper (Cu) from the extracellular space, and has been shown to play a major role in the initial accumulation of platinum-based drugs. In this study we re-expressed wild type and structural variants of hCTR1 in mouse embryo fibroblasts in which both alleles of mCTR1 had been knocked out (CTR1(-/-)) to examine the role of the N-terminal extracellular domain of hCTR1 in the accumulation of cisplatin (cDDP). Deletion of either the first 45 amino acids or just the (40)MXXM(45) motif in the N-terminal domain did not alter subcellular distribution or the amount of protein in the plasma membrane but it eliminated the ability of hCTR1 to mediate the uptake of Cu. In contrast it only partially reduced cDDP transport capacity. Neither of these structural changes prevented cDDP from triggering the rapid degradation of hCTR1. However, they did alter the potency of the cDDP that achieved cell entry, possibly reflecting the fact that hCTR1 may mediate the transport of cDDP both through the pore it forms in the plasma membrane and via endocytosis. We conclude that cDDP interacts with hCTR1 both at (40)MXXM(45) and at sites outside the N-terminal domain that produce the conformational changes that trigger degradation.

Deregulated copper transport affects Arabidopsis development especially in the absence of environmental cycles

Copper is an essential cofactor for key processes in plants, but it exerts harmful effects when in excess. Previous work has shown that the Arabidopsis (Arabidopsis thaliana) COPT1 high-affinity copper transport protein participates in copper uptake through plant root tips. Here, we show that COPT1 protein localizes to the plasma membrane of Arabidopsis cells and the phenotypic effects of transgenic plants overexpressing either COPT1 or COPT3, the latter being another high-affinity copper transport protein family member. Both transgenic lines exhibit increased endogenous copper levels and are sensitive to the copper in the growth medium. Additional phenotypes include decreased hypocotyl growth in red light and differentially affected flowering times depending on the photoperiod. Furthermore, in the absence of environmental cycles, such as light and temperature, the survival of plants overexpressing COPT1 or COPT3 is compromised. Consistent with altered circadian rhythms, the expression of the nuclear circadian clock genes CIRCADIAN CLOCK-ASSOCIATED1 (CCA1) and LATE ELONGATED HYPOCOTYL (LHY) is substantially reduced in either COPT1- or COPT3-overexpressing plants. Copper affects the amplitude and the phase, but not the period, of the CCA1 and LHY oscillations in wild-type plants. Copper also drives a reduction in the expression of circadian clock output genes. These results reveal that the spatiotemporal control of copper transport is a key aspect of metal homeostasis that is required for Arabidopsis fitness, especially in the absence of environmental cues.

Tryptophan scanning analysis of the membrane domain of CTR-copper transporters

Membrane proteins of the CTR family mediate cellular copper uptake in all eukaryotic cells and have been shown to participate in uptake of platinum-based anticancer drugs. Despite their importance for life and the clinical treatment of malignancies, directed biochemical studies of CTR proteins have been difficult because high-resolution structural information is missing. Building on our recent 7A structure of the human copper transporter hCTR1, we present the results of an extensive tryptophan-scanning analysis of hCTR1 and its distant relative, yeast CTR3. The comparative analysis supports our previous assignment of the transmembrane helices and shows that most functionally and structurally important residues are clustered around the threefold axis of CTR trimers or engage in helix packing interactions. The scan also identified residues that may play roles in interactions between CTR trimers and suggested that the first transmembrane helix serves as an adaptor that allows evolutionarily diverse CTRs to adopt the same overall structure. Together with previous biochemical and biophysical data, the results of the tryptophan scan are consistent with a mechanistic model in which copper transport occurs along the center of the trimer.

Copper transporters and the cellular pharmacology of the platinum-containing cancer drugs

Multiple lines of evidence indicate that the platinum-containing cancer drugs enter cells, are distributed to various subcellular compartments, and are exported from cells via transporters that evolved to manage copper homeostasis. The cytotoxicity of the platinum drugs is directly related to how much drug enters the cell, and almost all cells that have acquired resistance to the platinum drugs exhibit reduced drug accumulation. The major copper influx transporter, copper transporter 1 (CTR1), has now been shown to control the tumor cell accumulation and cytotoxic effect of cisplatin, carboplatin, and oxaliplatin. There is a good correlation between change in CTR1 expression and acquired cisplatin resistance among ovarian cancer cell lines, and genetic knockout of CTR1 renders cells resistant to cisplatin in vivo. The expression of CTR1 is regulated at the transcriptional level by copper via Sp1 and at the post-translational level by the proteosome. Copper and cisplatin both trigger the down-regulation of CTR1 via a process that involves ubiquitination and proteosomal degradation and requires the copper chaperone antioxidant protein 1 (ATOX1). The cisplatin-induced degradation of CTR1 can be blocked with the proteosome inhibitor bortezomib, and this increases the cellular uptake and the cytotoxicity of cisplatin in a synergistic manner. Copper and platinum(II) have similar sulfur binding characteristics, and the presence of stacked rings of methionines and cysteines in the CTR1 trimer suggest a mechanism by which CTR1 selectively transports copper and the platinum-containing drugs via sequential transchelation reactions similar to the manner in which copper is passed from ATOX1 to the copper efflux transporters.

New developments in the regulation of intestinal copper absorption

The transition metal copper is an essential trace element involved in many enzymatic processes that require redox-chemistry. The redox-activity of copper is potentially harmful. Severe imbalance of copper homeostasis can occur with some hereditary disorders of copper metabolism. Copper is acquired from the diet by intestinal absorption and is subsequently distributed throughout the body. The regulation of intestinal copper absorption to maintain whole-body copper homeostasis is currently poorly understood. This review evaluates novel findings regarding the molecular mechanism of intestinal copper uptake. The role of recently identified transporters in enterocyte copper uptake and excretion into the portal circulation is described, and the regulation of dietary copper uptake during physiological and pathophysiological conditions is discussed.

Copper accumulation by cultured astrocytes

To study copper transport in brain astrocytes, we have used astrocyte-rich primary cultures as model system. Cells in these cultures contained a basal copper content of 1.1+/-0.4 nmol per mg protein. The cellular copper content increased strongly after application of copper chloride in a time and concentration-dependent manner. Analysis of the linear copper accumulation during the first 5 min of copper exposure revealed that cultured astrocytes accumulated copper with saturable kinetics with apparent K(M)- and V(max)-values of 9.4+/-1.8 microM and 0.76+/-0.13 nmol/(min x mg protein), respectively. In contrast, incubation of astrocytes with copper in the presence of ascorbate caused a linear increase of the copper accumulation rates for copper concentrations of up to 30 microM. In addition, copper accumulation was strongly inhibited by the presence of an excess of zinc or of various other divalent metal ions. The presence of mRNA and of immunoreactivity of the copper transport protein Ctr1 in astrocyte cultures suggests that Ctr1 contributes to the observed copper accumulation. However, since some characteristics of the observed copper accumulation are not consistent with Ctr1-mediated copper transport, additional Ctr1-independent mechanism(s) are likely to be involved in astrocytic copper accumulation.

Endocytosis of FcalphaR is clathrin and dynamin dependent, but its cytoplasmic domain is not required

FcalphaR, the Fc receptor for IgA, is essential for IgA-mediated immune responses. Previous studies have shown that IgA and IgA immune complexes can be rapidly endocytosed by FcalphaR. However, the underlying mechanism remains unclear. Here, we investigated the endocytic pathway of FcalphaR in monocytic cell line, U937, that naturally express FcalphaR and in transfected Chinese hamster ovary (CHO), COS-7 and Hela cells. By using selective chemical inhibitors of different endocytic pathways, overexpression of dominant-negative mutants of Eps15 and knockdown of clathrin heavy chain (CHC) via RNA interference, we demonstrated that endocytosis of FcalphaR was through a clathrin-mediated pathway. The endocytosed FcalphaR went into Rab5- and Rab11-positive endosomes. However, endocytosis of FcalphaR could not be blocked by a dominant-negative mutant of Rab5. We also demonstrated that endocytosis of FcalphaR was dynamin-dependent by overexpressing a dominant-negative mutant of dynamin. The potential endocytic motif for FcalphaR was also examined. Unexpectedly, we found that the entire cytoplasmic domain of FcalphaR was not required for the endocytic process of FcalphaR. We conclude that endocytosis of FcalphaR is clathrin- and dynamin-dependent, but is not regulated by Rab5, and the endocytic motif is not located in the cytoplasmic domain of FcalphaR.

Copper-dependent recycling of hCTR1, the human high affinity copper transporter

Copper is an essential co-factor in many important physiological processes, but at elevated levels it is toxic to cells. Thus at both the organism and cellular level mechanisms have evolved to finely tune copper homeostasis. The protein responsible for copper entry from the circulation in most human cells is hCTR1, a small protein (190 amino acid residues) that functions as a trimer in the plasma membrane. In the present work we employ cell surface biotinylation and isotopic copper uptake studies of overexpressed hCTR1 in HEK293 cells to examine the acute (minutes) response of hCTR1 to changes in extracellular copper. We show that within 10 min of exposure to copper at 2.5 microM or higher, plasma membrane hCTR1 levels are reduced (by approximately 40%), with a concomitant reduction in copper uptake rates. We are unable to detect any degradation of internalized hCTR1 in the presence of cycloheximide after up to 2 h of exposure to 0-100 microM copper. Using a reversible biotinylation assay, we quantified internalized hCTR1, which increased upon the addition of copper and corresponded to the hCTR1 lost from the surface. In addition, when extracellular copper is then removed, internalized hCTR1 is promptly (within 30 min) recycled to the plasma membrane. We have shown that in the absence of added extracellular copper, there is a small but detectable amount of internalized hCTR1 that is increased in the presence of copper. Similar studies on endogenous hCTR1 show a cell-specific response to elevated extracellular copper. Copper-dependent internalization and recycling of hCTR1 provides an acute and reversible mechanism for the regulation of cellular copper entry.

Regulation of copper transport in Arabidopsis thaliana: a biochemical oscillator?

Plants are among the most versatile higher eukaryotes in accommodating environmental copper availability to largely variable demands. In particular, copper deficiency in soils is a threat for plant survival since it mostly affects reproductive structures. One of the strategies that plant cells use to overcome this situation is to increase copper levels by expressing high-affinity copper transporters delivering the metal to the cytosol. In this minireview, we discuss recent advances in the structure, function, and regulation of the CTR/COPT family of copper transporters, and pay special attention to the Arabidopsis thaliana counterparts. These are constituted by transmembrane polypeptides, containing several copper-binding sequences of functional and/or regulatory value, and assembling as trimers. Copper deficiency activates the expression of some members of the COPT family via the interaction of the SPL7 transcription factor with reiterative GTAC motifs present in their promoters. Interestingly, the regulation of the synthesis of these transporters by copper itself constitutes a negative-feedback loop that could cause a sustained oscillation in the cytosolic copper levels. We analyze the theoretical conditions required for this hypothetical copper oscillation and the potential advantages of synchronization with other cycles. Diverse data in other organisms point to the relationship between copper homeostasis and circadian cycles.