Characterization of mouse embryonic cells deficient in the ctr1 high affinity copper transporter. Identification of a Ctr1-independent copper transport system

Genetic polymorphism of copper transporter protein 1 is related to platinum resistance in Chinese non-small cell lung carcinoma patients

1. Chemotherapeutic resistance to platinum-based anticancer drugs is a major obstacle in the successful treatment of lung cancer. Cellular uptake and platinum accumulation are considered the most important factors contributing to platinum resistance. The copper transporter family is the major plasma membrane transporter for platinum uptake. Copper transporter protein 1 (CTR1) plays an essential role in cisplatin influx and is closely related to platinum resistance by influencing platinum uptake and accumulation. The aim of the present study was to determine whether CTR1 polymorphisms are associated with platinum resistance in non-small cell lung carcinoma (NSCLC) patients. 2. A total of 282 incident Chinese Han NSCLC patients were enrolled in the study and followed up at three different institutions. All patients underwent at least two cycles of platinum-based chemotherapy. Twenty single-nucleotide polymorphisms of CTR1 were detected from genomic DNA samples. 3. Genetic polymorphisms of CTR1 at rs7851395 and rs12686377 were associated with platinum resistance in NSCLC patients. Patients with a GT haplotype presented with increased susceptibility to platinum resistance (P < 0.05), whereas an AG haplotype contributed to longer survival (P < 0.05). 4. In conclusion, a significant relationship was found between rs7851395 and rs12686377 polymorphisms and platinum resistance, as well as clinical outcomes, in Chinese NSCLC patients. Thus, CTR1 plays an essential role in platinum resistance and could be considered a predictive marker for the pretreatment evaluation of NSCLC patients.

Mechanism of silver nanoparticles action on insect pigmentation reveals intervention of copper homeostasis

Silver nanoparticles (AgNPs), like almost all nanoparticles, are potentially toxic beyond a certain concentration because the survival of the organism is compromised due to scores of pathophysiological abnormalities past that concentration. However, the mechanism of AgNP toxicity remains undetermined. Instead of applying a toxic dose, we attempted to monitor the effects of AgNPs at a nonlethal concentration on wild type Drosophila melanogaster by exposing them throughout their development. All adult flies raised in AgNP doped food showed that up to 50 mg/L concentration AgNP has no negative influence on median survival; however, these flies appeared uniformly lighter in body color due to the loss of melanin pigments in their cuticle. Additionally, fertility and vertical movement ability were compromised due to AgNP feeding. Determination of the amount of free ionic silver (Ag⁺) led us to claim that the observed biological effects have resulted from the AgNPs and not from Ag⁺. Biochemical analysis suggests that the activity of copper dependent enzymes, namely tyrosinase and Cu-Zn superoxide dismutase, are decreased significantly following the consumption of AgNPs, despite the constant level of copper present in the tissue. Consequently, we propose a mechanism whereby consumption of excess AgNPs in association with membrane bound copper transporter proteins cause sequestration of copper, thus creating a condition that resembles copper starvation. This model also explains the cuticular demelanization effect resulting from AgNP since tyrosinase activity is essential for melanin biosynthesis. Finally, we claim that Drosophila, an established genetic model system, can be well utilized for further understanding of the biological effects of nanoparticles.

F-box and leucine-rich repeat protein 5 (FBXL5) is required for maintenance of cellular and systemic iron homeostasis

Maintenance of cellular iron homeostasis requires post-transcriptional regulation of iron metabolism genes by iron regulatory protein 2 (IRP2). The hemerythrin-like domain of F-box and leucine-rich repeat protein 5 (FBXL5), an E3 ubiquitin ligase subunit, senses iron and oxygen availability and facilitates IRP2 degradation in iron replete cells. Disruption of the ubiquitously expressed murine Fbxl5 gene results in a failure to sense increased cellular iron availability, accompanied by constitutive IRP2 accumulation and misexpression of IRP2 target genes. FBXL5-null mice die during embryogenesis, although viability is restored by simultaneous deletion of the IRP2, but not IRP1, gene. Mice containing a single functional Fbxl5 allele behave like their wild type littermates when fed an iron-sufficient diet. However, unlike wild type mice that manifest decreased hematocrit and hemoglobin levels when fed a low-iron diet, Fbxl5 heterozygotes maintain normal hematologic values due to increased iron absorption. The responsiveness of IRP2 to low iron is specifically enhanced in the duodena of the heterozygotes and is accompanied by increased expression of the divalent metal transporter-1. These results confirm the role of FBXL5 in the in vivo maintenance of cellular and systemic iron homeostasis and reveal a privileged role for the intestine in their regulation by virtue of its unique FBXL5 iron sensitivity.

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.

Copper transport and compartmentation in grape cells

Copper-based fungicides have been widely used against several grapevine (Vitis vinifera L.) diseases since the late 1800s when the Bordeaux mixture was developed, but their intensive use has raised phytotoxicity concerns. In this study, physiological, biochemical and molecular approaches were combined to investigate the impacts of copper in grape cells and how it is transported and compartmented intracellularly. Copper reduced the growth and viability of grape cells (CSB, Cabernet Sauvignon Berry) in a dose-dependent manner above 100 µM and was accumulated in specific metal ion sinks. The copper-sensitive probe Phen Green SK was used to characterize copper transport across the plasma membrane of CSB cells. The transport system (K(m) = 583 µM; V(max) = 177 × 10(-6) %ΔF min(-1) protoplast(-1)) was regulated by copper availability in the culture medium, stimulated by Ca(2+) and inhibited by Zn(2+). The pH-sensitive fluorescent probe ACMA (9-amino-6-chloro-2-methoxyacridine) was used to evaluate the involvement of proton-dependent copper transport across the tonoplast. Cu(2+) compartmentation in the vacuole was dependent on the transmembrane pH gradient generated by both V-H(+)-ATPase and V-H(+)-pyrophosphatase (PPase). High copper levels in the growth medium did not affect the activity of V-H(+)-PPase but decreased the magnitude of the H(+) gradient generated by V-H(+)-ATPase. Expression studies of VvCTr genes showed that VvCTr1 and VvCTr8 were distinctly affected by CuSO(4) availability in grape cell cultures and that both genes were highly expressed in the green stage of grape berries.

Variable response of selected cuproproteins in rat choroid plexus and cerebellum following perinatal copper deficiency

Recent immunohistochemical characterization of the copper transport protein, Ctr1, reported enriched levels in mouse choroid plexus, and enhancement by copper deficiency. To extend and confirm this, experiments were conducted with Holtzman rats. Following perinatal copper deficiency there was an 80% reduction in brain copper of 24-27 day old copper-deficient (Cu-) rat pups compared to copper-adequate (Cu+) controls. Choroid plexus immunoblot analysis with rabbit anti-hCtr1 demonstrated a 50% higher Ctr1 protein expression in Cu-samples. However, levels of copper chaperone for superoxide dismutase (CCS) were unchanged, suggesting that Ctr1 buffers the choroid plexus against copper deficiency, since CCS normally is much higher in Cu-tissues. There were 13% lower levels of cytochrome c oxidase subunit IV (COX IV) detected in Cuchoroid plexus. In contrast, in cerebellum of Cu-rats CCS was 2-fold higher and COXIV 1.7-fold lower than Cu+ rats consistent with severe copper deficiency. Brain mitochondria from Cu-rats had severe reductions in COXIV content and CCO activity and modest but significant elevations in CCS and reductions in Cu, Zn-superoxide dismutase. COXIV may be a more sensitive marker for copper deficiency than CCS and may prove useful to assess copper status.

Astrocyte functions in the copper homeostasis of the brain

Copper is an essential element that is required for a variety of important cellular functions. Since not only copper deficiency but also excess of copper can seriously affect cellular functions, the cellular copper metabolism is tightly regulated. In brain, astrocytes appear to play a pivotal role in the copper metabolism. With their strategically important localization between capillary endothelial cells and neuronal structures they are ideally positioned to transport copper from the blood-brain barrier to parenchymal brain cells. Accordingly, astrocytes have the capacity to efficiently take up, store and to export copper. Cultured astrocytes appear to be remarkably resistant against copper-induced toxicity. However, copper exposure can lead to profound alterations in the metabolism of these cells. This article will summarize the current knowledge on the copper metabolism of astrocytes, will describe copper-induced alterations in the glucose and glutathione metabolism of astrocytes and will address the potential role of astrocytes in the copper metabolism of the brain in diseases that have been connected with disturbances in brain copper homeostasis.

Copper influx transporter 1 is required for FGF, PDGF and EGF-induced MAPK signaling

Copper transporter 1 (CTR1) is the major copper (Cu) influx transporter in mammalian cells. We report here that CTR1 is required for the activation of signaling to the MAPK pathway by the ligands of three major receptor tyrosine kinases (RTK) including FGF, PDGF and EGF. Induction of Erk1/2 phosphorylation was compared in isogenic wild type CTR1(+/+) and CTR1(-/-) cells. Whereas all three ligands increased pErk1/2 in the CTR1(+/+) cells, they failed to do this in CTR1(-/-) cells. While FGF did not enhance the phosphorylation of AKT in the CTR1(+/+) cells, both PDGF and EGF increased pAKT in the CTR1(+/+) but not CTR1(-/-) cells. The deficit in Erk1/2 phosphorylation in the CTR1(-/-) cells was rescued by adding Cu to the medium, and it was induced in CTR1(+/+) cells by treatment with a Cu chelator. Intracellular Cu availability was reduced in the CTR1(-/-) cells as reflected by increased expression of the Cu chaperone CCS. The failure of RTK-induced signaling to both Erk1/2 and AKT suggested the presence of a Cu-dependent step upstream of Ras. The Cu-dependent enzyme SOD1 is responsible for generating the hydrogen peroxide in response to RTK activation that serves to inhibit phosphatases that normally limit RTK signaling. SOD1 activity was reduced by a factor of 17-fold in the CTR1(-/-) cells, and addition of hydrogen peroxide restored signaling. We conclude that Cu acquired from CTR1 is required for signaling in pathways regulated by RTKs that play major roles in development and cancer.

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.

Mfc1 is a novel copper transporter during meiosis

Meiosis is a specialized cell division process by which diploid germ line cells generate haploid gametes, which are required for sexual reproduction. During this process, several micronutrients are required, including copper ions. Despite important roles for copper-dependent proteins during meiosis, their mechanisms of action remain poorly understood. In a recently publication, we reported the discovery of Mfc1, the first example ever reported of a meiosis-specific copper transporter. Although Mfc1 did not exhibit any significant amino acid sequence similarities with members of the Ctr family of copper transporters, it harbored putative copper coordination motifs. Microarray data showed that mfc1⁺ was the most highly induced of all meiotic genes detected under copper-limiting conditions. Analysis of Mfc1 localization during meiosis revealed that it localized at the forespore membrane during middle and late phases of the meiotic program. Interestingly, live-cell copper imaging using a copper-binding tracker revealed accumulation of copper ions into the forespore in wild-type cells. In contrast, mutant cells lacking Mfc1 displayed an intracellular distribution of copper ions that was dispersed throughout the ascospores without any marked preference for the forespore. We propose that Mfc1 is required to mobilize copper into the forespore, thereby providing copper to copper-requiring enzymes of the developing spores.

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.

A novel role for copper in Ras/mitogen-activated protein kinase signaling

Copper (Cu) is essential for development and proliferation, yet the cellular requirements for Cu in these processes are not well defined. We report that Cu plays an unanticipated role in the mitogen-activated protein (MAP) kinase pathway. Ablation of the Ctr1 high-affinity Cu transporter in flies and mouse cells, mutation of Ctr1, and Cu chelators all reduce the ability of the MAP kinase kinase Mek1 to phosphorylate the MAP kinase Erk. Moreover, mice bearing a cardiac-tissue-specific knockout of Ctr1 are deficient in Erk phosphorylation in cardiac tissue. in vitro investigations reveal that recombinant Mek1 binds two Cu atoms with high affinity and that Cu enhances Mek1 phosphorylation of Erk in a dose-dependent fashion. Coimmunoprecipitation experiments suggest that Cu is important for promoting the Mek1-Erk physical interaction that precedes the phosphorylation of Erk by Mek1. These results demonstrate a role for Ctr1 and Cu in activating a pathway well known to play a key role in normal physiology and in cancer.

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.

Detection of genetically altered copper levels in Drosophila tissues by synchrotron x-ray fluorescence microscopy

Tissue-specific manipulation of known copper transport genes in Drosophila tissues results in phenotypes that are presumably due to an alteration in copper levels in the targeted cells. However direct confirmation of this has to date been technically challenging. Measures of cellular copper content such as expression levels of copper-responsive genes or cuproenzyme activity levels, while useful, are indirect. First-generation copper-sensitive fluorophores show promise but currently lack the sensitivity required to detect subtle changes in copper levels. Moreover such techniques do not provide information regarding other relevant biometals such as zinc or iron. Traditional techniques for measuring elemental composition such as inductively coupled plasma mass spectroscopy are not sensitive enough for use with the small tissue amounts available in Drosophila research. Here we present synchrotron x-ray fluorescence microscopy analysis of two different Drosophila tissues, the larval wing imaginal disc, and sectioned adult fly heads and show that this technique can be used to detect changes in tissue copper levels caused by targeted manipulation of known copper homeostasis genes.

Mfc1 is a novel forespore membrane copper transporter in meiotic and sporulating cells

To gain insight in the molecular basis of copper homeostasis during meiosis, we have used DNA microarrays to analyze meiotic gene expression in the model yeast Schizosaccharomyces pombe. Profiling data identified a novel meiosis-specific gene, termed mfc1(+), that encodes a putative major facilitator superfamily-type transporter. Although Mfc1 does not exhibit any significant sequence homology with the copper permease Ctr4, it contains four putative copper-binding motifs that are typically found in members of the copper transporter family of copper transporters. Similarly to the ctr4(+) gene, the transcription of mfc1(+) was induced by low concentrations of copper. However, its temporal expression profile during meiosis was distinct to ctr4(+). Whereas Ctr4 was observed at the plasma membrane shortly after induction of meiosis, Mfc1 appeared later in precursor vesicles and, subsequently, at the forespore membrane of ascospores. Using the fluorescent copper-binding tracker Coppersensor-1 (CS1), labile cellular copper was primarily detected in the forespores in an mfc1(+)/mfc1(+) strain, whereas an mfc1Δ/mfc1Δ mutant exhibited an intracellular dispersed punctate distribution of labile copper ions. In addition, the copper amine oxidase Cao1, which localized primarily in the forespores of asci, was fully active in mfc1(+)/mfc1(+) cells, but its activity was drastically reduced in an mfc1Δ/mfc1Δ strain. Furthermore, our data showed that meiotic cells that express the mfc1(+) gene have a distinct developmental advantage over mfc1Δ/mfc1Δ mutant cells when copper is limiting. Taken together, the data reveal that Mfc1 serves to transport copper for accurate and timely meiotic differentiation under copper-limiting conditions.

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.

Model peptides provide new insights into the role of histidine residues as potential ligands in human cellular copper acquisition via Ctr1

Cellular acquisition of copper in eukaryotes is primarily accomplished through the Ctr family of copper transport proteins. In both humans and yeast, methionine-rich "Mets" motifs in the amino-terminal extracellular domain of Ctr1 are thought to be responsible for recruitment of copper at the cell surface. Unlike yeast, mammalian Ctr1 also contains extracellular histidine-rich motifs, although a role for these regions in copper uptake has not been explored in detail. Herein, synthetic model peptides containing the first 14 residues of the extracellular domain of human Ctr1 (MDHSHHMGMSYMDS) have been prepared and evaluated for their apparent binding affinity to both Cu(I) and Cu(II). These studies reveal a high affinity Cu(II) binding site (log K = 11.0 ± 0.3 at pH 7.4) at the amino-terminus of the peptide as well as a high affinity Cu(I) site (log K = 10.2 ± 0.2 at pH 7.4) that utilizes adjacent HH residues along with an additional His or Met ligand. These model studies suggest that the histidine domains may play a direct role in copper acquisition from serum copper-binding proteins and in facilitating the reduction of Cu(II) to the active Ctr1 substrate, Cu(I). We tested this hypothesis by expressing a Ctr1 mutant lacking only extracellular histidine residues in Ctr1-knockout mouse embryonic fibroblasts. Results from live cell studies support the hypothesis that extracellular amino-terminal His residues directly participate in the copper transport function of Ctr1.

Presenilins promote the cellular uptake of copper and zinc and maintain copper chaperone of SOD1-dependent copper/zinc superoxide dismutase activity

Dyshomeostasis of extracellular zinc and copper has been implicated in β-amyloid aggregation, the major pathology associated with Alzheimer disease. Presenilin mediates the proteolytic cleavage of the β-amyloid precursor protein to release β-amyloid, and mutations in presenilin can cause familial Alzheimer disease. We tested whether presenilin expression affects copper and zinc transport. Studying murine embryonic fibroblasts (MEFs) from presenilin knock-out mice or RNA interference of presenilin expression in HEK293T cells, we observed a marked decrease in saturable uptake of radiolabeled copper and zinc. Measurement of basal metal levels in 6-month-old presenilin 1 heterozygous knock-out (PS1(+/-)) mice revealed significant deficiencies of copper and zinc in several tissues, including brain. Copper/zinc superoxide dismutase (SOD1) activity was significantly decreased in both presenilin knock-out MEFs and brain tissue of presenilin 1 heterozygous knock-out mice. In the MEFs and PS1(+/-) brains, copper chaperone of SOD1 (CCS) levels were decreased. Zinc-dependent alkaline phosphatase activity was not decreased in the PS null MEFs. These data indicate that presenilins are important for cellular copper and zinc turnover, influencing SOD1 activity, and having the potential to indirectly impact β-amyloid aggregation through metal ion clearance.

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.

Acquisition of dietary copper: a role for anion transporters in intestinal apical copper uptake

Copper is an essential micronutrient in humans and is required for a wide range of physiological processes, including neurotransmitter biosynthesis, oxidative metabolism, protection against reactive oxygen species, and angiogenesis. The first step in the acquisition of dietary copper is absorption from the intestinal lumen. The major human high-affinity copper uptake protein, human copper transporter hCTR1, was recently shown to be at the basolateral or blood side of both intestinal and renal epithelial cell lines and thus does not play a direct role in this initial step. We sought to functionally identify the major transport pathways available for the absorption of dietary copper across the apical intestinal membrane using Caco2 cells, a well-established model for human enterocytes. The initial rate of apical copper uptake into confluent monolayers of Caco2 cells is greatly elevated if amino acids and serum proteins are removed from the growth media. Uptake from buffered saline solutions at neutral pH (but not at lower pH) is inhibited by either d- or l-histidine, unaltered by the removal of sodium ions, and inhibited by ∼90% when chloride ions are replaced by gluconate or sulfate. Chloride-dependent copper uptake occurs with Cu(II) or Cu(I), although Cu(I) uptake is not inhibited by histidine, nor by silver ions. A well-characterized inhibitor of anion exchange systems, DIDS, inhibited apical copper uptake by 60-70%, while the addition of Mn(II) or Fe(II), competitive substrates for the divalent metal transporter DMT1, had no effect on copper uptake. We propose that anion exchangers play an unexpected role in copper absorption, utilizing copper-chloride complexes as pseudo-substrates. This pathway is also observed in mouse embryonic fibroblasts, human embryonic kidney cells, and Cos-7 cells. The special environment of low pH, low concentration of protein, and protonation of amino acids in the early intestinal lumen make this pathway especially important in dietary copper acquisition.

Copper transporter 2 regulates endocytosis and controls tumor growth and sensitivity to cisplatin in vivo

Copper transporter 2 (CTR2) is one of the four copper transporters in mammalian cells that influence the cellular pharmacology of cisplatin and carboplatin. CTR2 was knocked down using a short hairpin RNA interference. Robust expression of CTR2 was observed in parental tumors grown in vivo, whereas no staining was found in the tumors formed from cells in which CTR2 had been knocked down. Knockdown of CTR2 reduced growth rate by 5.8-fold, increased the frequency of apoptotic cells, and decreased the vascular density, but it did not change copper content. Knockdown of CTR2 increased the tumor accumulation of cis-diamminedichloroplatinum(II) [cisplatin (cDDP)] by 9.1-fold and greatly increased its therapeutic efficacy. Because altered endocytosis has been implicated in cDDP resistance, uptake of dextran was used to quantify the rate of macropinocytosis. Knockdown of CTR2 increased dextran uptake 2.5-fold without reducing exocytosis. Inhibition of macropinocytosis with either amiloride or wortmannin blocked the increase in macropinocytosis mediated by CTR2 knockdown. Stimulation of macropinocytosis by platelet-derived growth factor coordinately increased dextran and cDDP uptake. Knockdown of CTR2 was associated with activation of the Rac1 and cdc42 GTPases that control macropinocytosis but not activation of the phosphoinositide-3 kinase pathway. We conclude that CTR2 is required for optimal tumor growth and that it is an unusually strong regulator of cisplatin accumulation and cytotoxicity. CTR2 regulates the transport of cDDP in part through control of the rate of macropinocytosis via activation of Rac1 and cdc42. Selective knockdown of CTR2 in tumors offers a strategy for enhancing the efficacy of cDDP.

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.

Unexpected role of the copper transporter ATP7A in PDGF-induced vascular smooth muscle cell migration

RATIONALE

Copper, an essential nutrient, has been implicated in vascular remodeling and atherosclerosis with unknown mechanism. Bioavailability of intracellular copper is regulated not only by the copper importer CTR1 (copper transporter 1) but also by the copper exporter ATP7A (Menkes ATPase), whose function is achieved through copper-dependent translocation from trans-Golgi network (TGN). Platelet-derived growth factor (PDGF) promotes vascular smooth muscle cell (VSMC) migration, a key component of neointimal formation.

OBJECTIVE

To determine the role of copper transporter ATP7A in PDGF-induced VSMC migration.

METHODS AND RESULTS

Depletion of ATP7A inhibited VSMC migration in response to PDGF or wound scratch in a CTR1/copper-dependent manner. PDGF stimulation promoted ATP7A translocation from the TGN to lipid rafts, which localized at the leading edge, where it colocalized with PDGF receptor and Rac1, in migrating VSMCs. Mechanistically, ATP7A small interfering RNA or CTR small interfering RNA prevented PDGF-induced Rac1 translocation to the leading edge, thereby inhibiting lamellipodia formation. In addition, ATP7A depletion prevented a PDGF-induced decrease in copper level and secretory copper enzyme precursor prolysyl oxidase (Pro-LOX) in lipid raft fraction, as well as PDGF-induced increase in LOX activity. In vivo, ATP7A expression was markedly increased and copper accumulation was observed by synchrotron-based x-ray fluorescence microscopy at neointimal VSMCs in wire injury model.

CONCLUSIONS

These findings suggest that ATP7A plays an important role in copper-dependent PDGF-stimulated VSMC migration via recruiting Rac1 to lipid rafts at the leading edge, as well as regulating LOX activity. This may contribute to neointimal formation after vascular injury. Our findings provide insight into ATP7A as a novel therapeutic target for vascular remodeling and atherosclerosis.

Ctr1 transports silver into mammalian cells

Silver is a non-essential, toxic metal. The use of silver as an antimicrobial agent in many applications and its presence as a contaminant in foods and air can lead to accumulation in tissues. Despite its widespread use, the systems involved in the uptake of silver into mammalian cells are presently unknown. Previous studies have shown that copper uptake at the plasma membrane by copper transporter 1 (Ctr1) is inhibited by an excess of silver, suggesting that Ctr1 may function in importing silver into cells. In this study we examined directly the role of Ctr1 in the accumulation of silver in mammalian cells using over-expression experiments and mouse embryonic fibroblast cells lacking Ctr1. COS-7 cells transfected to express a human Ctr1-green fluorescent protein (hCtr1-GFP) fusion protein hyper-accumulated silver when incubated in medium supplemented with low micromolar concentrations (2.5-10 micromol/L) of AgNO(3). An hCtr1-GFPM150L,M154L variant deficient for copper transport failed to stimulate accumulation of silver. Mouse embryonic fibroblast cells lacking Ctr1 showed approximately a 50% reduction in silver content when incubated in silver-supplemented medium compared to a wild-type isogenic cell line. Collectively, these data demonstrate that Ctr1 transports both copper and silver and suggest that Ctr1 is an important transport protein in the accumulation of silver in mammalian cells.

Roles of COMM-domain-containing 1 in stability and recruitment of the copper-transporting ATPase in a mouse hepatoma cell line

A novel function of COMMD1 {COMM [copper metabolism MURR1 (mouse U2af1-rs1 region 1)]-domain-containing 1}, a protein relevant to canine copper toxicosis, was examined in the mouse hepatoma cell line Hepa 1-6 with multi-disciplinary techniques consisting of molecular and cellular biological techniques, speciation and elemental imaging. To clarify the function of COMMD1, COMMD1-knockdown was accomplished by introducing siRNA (small interfering RNA) into the cells. Although COMMD1-knockdown did not affect copper incorporation, it inhibited copper excretion, resulting in copper accumulation, which predominantly existed in the form bound to MT (metallothionein). It is known that the liver copper transporter Atp7b (ATP-dependent copper transporter 7beta), localizes on the trans-Golgi network membrane under basal copper conditions and translocates to cytoplasmic vesicles to excrete copper when its concentration exceeds a certain threshold, with the vesicles dispersing in the periphery of the cell. COMMD1-knockdown reduced the expression of Atp7b, and abolished the relocation of Atp7b back from the periphery to the trans-Golgi network membrane when the copper concentration was reduced by treatment with a Cu(I) chelator. The same phenomena were observed during COMMD1-knockdown when another Atp7b substrate, cis-diamminedichloroplatinum, and its sequestrator, glutathione ethylester, were applied. These results suggest that COMMD1 maintains the amount of Atp7b and facilitates recruitment of Atp7b from cytoplasmic vesicles to the trans-Golgi network membrane, i.e. COMMD1 is required to shuttle Atp7b when the intracellular copper level returns below the threshold.

Enhancing tumor-specific uptake of the anticancer drug cisplatin with a copper chelator

Uptake of the anticancer drug cisplatin is mediated by the copper transporter CTR1 in cultured cells. Here we show in human ovarian tumors that low levels of Ctr1 mRNA are associated with poor clinical response to platinum-based therapy. Using a mouse model of human cervical cancer, we demonstrate that combined treatment with a copper chelator and cisplatin increases cisplatin-DNA adduct levels in cancerous but not in normal tissues, impairs angiogenesis, and improves therapeutic efficacy. The copper chelator also enhances the killing of cultured human cervical and ovarian cancer cells with cisplatin. Our results identify the copper transporter as a therapeutic target, which can be manipulated with copper chelating drugs to selectively enhance the benefits of platinum-containing chemotherapeutic agents.

The role of the methionines and histidines in the transmembrane domain of mammalian copper transporter 1 in the cellular accumulation of cisplatin

Mammalian copper transporter 1 (CTR1) is a high-affinity copper influx transporter that also mediates the uptake of platinum-containing chemotherapeutic agents including cisplatin (cDDP). Methionines 150, 154, and histidine 139 have been proposed to form a series of stacked rings in the pore formed by the CTR1 homotrimer, each of which is required for maximal copper transport. To examine the mechanism by which hCTR1 also transports cDDP, variant forms of hCTR1 in which methionines 150 and 154 were converted to isoleucines or in which histidine 139 was converted to alanine were re-expressed in cells in which both alleles of CTR1 had been knocked out. Each of these conversions disabled copper transport and increased cellular resistance to the cytotoxic effect of copper. In contrast, conversion of the methionines increased the uptake and cytotoxicity of cDDP well above that attained with wild-type hCTR1. Conversion of His139 to alanine did not impair cDDP uptake and actually enhanced cytotoxicity. Thus, although Met150 and Met154 facilitate the movement of copper through the pore, they serve to obstruct the passage of cDDP. None of the modifications altered the ability of cDDP to trigger the degradation of hCTR1, indicating that cDDP must interact with hCTR1 at other sites as well. Although both copper and cDDP may rely on a series of transchelation reactions to pass through the hCTR1 trimeric complex, the details of the molecular interactions must be different, which provides a potential basis for selective pharmacological modulation of copper versus cDDP cytotoxicity.

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.

Menkes disease

Menkes disease (MD) is a lethal multisystemic disorder of copper metabolism. Progressive neurodegeneration and connective tissue disturbances, together with the peculiar 'kinky' hair are the main manifestations. MD is inherited as an X-linked recessive trait, and as expected the vast majority of patients are males. MD occurs due to mutations in the ATP7A gene and the vast majority of ATP7A mutations are intragenic mutations or partial gene deletions. ATP7A is an energy dependent transmembrane protein, which is involved in the delivery of copper to the secreted copper enzymes and in the export of surplus copper from cells. Severely affected MD patients die usually before the third year of life. A cure for the disease does not exist, but very early copper-histidine treatment may correct some of the neurological symptoms.

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.

Copper-triggered aggregation of ubiquitin

Neurodegenerative disorders share common features comprising aggregation of misfolded proteins, failure of the ubiquitin-proteasome system, and increased levels of metal ions in the brain. Protein aggregates within affected cells often contain ubiquitin, however no report has focused on the aggregation propensity of this protein. Recently it was shown that copper, differently from zinc, nickel, aluminum, or cadmium, compromises ubiquitin stability and binds to the N-terminus with 0.1 micromolar affinity. This paper addresses the role of copper upon ubiquitin aggregation. In water, incubation with Cu(II) leads to formation of spherical particles that can progress from dimers to larger conglomerates. These spherical oligomers are SDS-resistant and are destroyed upon Cu(II) chelation or reduction to Cu(I). In water/trifluoroethanol (80∶20, v/v), a mimic of the local decrease in dielectric constant experienced in proximity to a membrane surface, ubiquitin incubation with Cu(II) causes time-dependent changes in circular dichroism and Fourier-transform infrared spectra, indicative of increasing β-sheet content. Analysis by atomic force and transmission electron microscopy reveals, in the given order, formation of spherical particles consistent with the size of early oligomers detected by gel electrophoresis, clustering of these particles in straight and curved chains, formation of ring structures, growth of trigonal branches from the rings, coalescence of the trigonal branched structures in a network. Notably, none of these ubiquitin aggregates was positive to tests for amyloid and Cu(II) chelation or reduction produced aggregate disassembly. The early formed Cu(II)-stabilized spherical oligomers, when reconstituted in 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) liposomes and in POPC planar bilayers, form annular and pore-like structures, respectively, which are common to several neurodegenerative disorders including Parkinson's, Alzheimer's, amyotrophic lateral sclerosis, and prion diseases, and have been proposed to be the primary toxic species. Susceptibility to aggregation of ubiquitin, as it emerges from the present study, may represent a potential risk factor for disease onset or progression while cells attempt to tag and process toxic substrates.

Generation of oxidant response to copper and iron nanoparticles and salts: Stimulation by ascorbate

The present work describes a two-stage approach to analyzing combustion-generated samples for their potential to produce oxidant stress. This approach is illustrated with the two commonly encountered transition metals, copper and iron. First, their abilities to generate hydroxyl radical were measured in a cell-free, phosphate-buffered saline solution containing ascorbate and/or citrate. Second, their abilities to induce heme oxygenase-1 in cultured human epidermal keratinocytes were assessed in cell culture. Combustion-generated copper oxide nanoparticles were active in both assays and were found to be soluble in culture medium. Depletion of glutathione in the cells or loading the cells with ascorbate greatly increased heme oxygenase-1 induction in the presence of copper. By contrast, iron oxide nanoparticles were active in the phosphate-buffered saline but not in cell culture, and they aggregated in culture medium. Soluble salts of copper and iron exhibited the same contrast in activities as the respective combustion-generated particles. The results suggest that the capability of combustion-generated environmental samples to produce oxidant stress can be screened effectively in a two step process, first in phosphate-buffered saline with ascorbate and subsequently in epithelial cell culture for those exhibiting activity initially. The results also point to an unanticipated interaction in cells of oxidant stress-generating metals with an antioxidant (ascorbate) that is usually missing in culture medium formulations. Thus, ascorbate supplementation of cultured human cells is likely to improve their ability to model the in vivo effects of particulate matter containing copper and other redox-active metals.

Copper transport in mammalian cells: special care for a metal with special needs

Copper plays an essential role in human physiology. It is required for respiration, radical defense, neuronal myelination, angiogenesis, and many other processes. Copper has distinct physicochemical properties that pose uncommon challenges for its transport across biological membranes. Only small amounts of copper are present in biological fluids, and essentially none of it exists in a free ion form. These properties and the low redox potential of copper dictate special structural and mechanistic features in copper transporters. This minireview discusses molecular mechanisms through which copper enters and exits human cells.

Copper transporter 2 regulates the cellular accumulation and cytotoxicity of Cisplatin and Carboplatin

PURPOSE

Copper transporter 2 (CTR2) is known to mediate the uptake of Cu(+1) by mammalian cells. Several other Cu transporters, including the influx transporter CTR1 and the two efflux transporters ATP7A and ATP7B, also regulate sensitivity to the platinum-containing drugs. We sought to determine the effect of CTR2 on influx, intracellular trafficking, and efflux of cisplatin and carboplatin.

EXPERIMENTAL DESIGN

The role of CTR2 was examined by knocking down CTR2 expression in an isogenic pair of mouse embryo fibroblasts consisting of a CTR1(+/+) line and a CTR1(-/-) line in which both CTR1 alleles had been deleted. CTR2 levels were determined by quantitative reverse transcription-PCR and Western blot analysis. Cisplatin (DDP) was quantified by inductively coupled plasma mass spectrometry and (64)Cu and [(14)C]carboplatin (CBDCA) accumulation by gamma and scintillation counting.

RESULTS

Deletion of CTR1 reduced the uptake of Cu, DDP, and CBDCA and increased resistance to their cytotoxic effects by 2- to 3-fold. Knockdown of CTR2 increased uptake of Cu only in the CTR1(+/+) cells. In contrast, knockdown of CTR2 increased whole-cell DDP uptake and DNA platination in both CTR1(+/+) and CTR1(-/-) cells and proportionately enhanced cytotoxicity while producing no effect on vesicular accumulation or efflux. A significant correlation was found between CTR2 mRNA and protein levels and sensitivity to DDP in a panel of six ovarian carcinoma cell lines.

CONCLUSIONS

CTR2 is a major determinant of sensitivity to the cytotoxic effects of DDP and CBDCA. CTR2 functions by limiting drug accumulation, and its expression correlates with the sensitivity of human ovarian carcinoma cell lines to DDP.

Neuronal expression of copper transporter 1 in rat dorsal root ganglia: association with platinum neurotoxicity

PURPOSE

We report the neuronal expression of copper transporter 1 (CTR1) in rat dorsal root ganglia (DRG) and its association with the neurotoxicity of platinum-based drugs.

METHODS

CTR1 expression was studied by immunohistochemistry and RT-PCR. The toxicity of platinum drugs to CTR1-positive and CTR1-negative neurons was compared in DRG from animals treated with maximum tolerated doses of oxaliplatin (1.85 mg/kg), cisplatin (1 mg/kg) or carboplatin (8 mg/kg) twice weekly for 8 weeks.

RESULTS

Abundant CTR1 mRNA was detected in DRG tissue. CTR1 immunoreactivity was associated with plasma membranes and cytoplasmic vesicular structures of a subpopulation (13.6 +/- 3.1%) of mainly large-sized (mean cell body area, 1,787 +/- 127 microm(2)) DRG neurons. After treatment with platinum drugs, the cell bodies of these CTR1-positive neurons became atrophied, with oxaliplatin causing the greatest percentage reduction in the mean cell body area relative to controls (42%; P < 0.05), followed by cisplatin (18%; P < 0.05) and carboplatin causing the least reduction (3.2%; P = NS). CTR1-negative neurons, with no immunoreactivity or only diffuse cytoplasmic staining, showed less treatment-induced cell body atrophy than CTR1-positive neurons.

CONCLUSIONS

CTR1 is preferentially expressed by a subset of DRG neurons that are particularly vulnerable to the toxicity of platinum drugs. These findings, together with its neuronal membrane localization, are suggestive of CTR1-related mechanisms of platinum drug neuronal uptake and neurotoxicity.

Deletion of hepatic Ctr1 reveals its function in copper acquisition and compensatory mechanisms for copper homeostasis

Copper is a vital trace element required for normal growth and development of many organisms. To determine the roles for copper transporter 1 (Ctr1) in hepatic copper metabolism and the contribution of the liver to systemic copper homeostasis, we have generated and characterized mice in which Ctr1 is deleted specifically in the liver. These mice express less than 10% residual Ctr1 protein in the liver and exhibit a small but significant growth retardation, which disappears with age. Hepatic copper concentrations and the activities of copper-requiring enzymes are reduced; however, mild copper deficiency relative to Ctr1 protein deficit indicates compensatory mechanisms for copper metabolism. Copper concentrations of other organs did not alter despite the defect in hepatic copper uptake. Whereas biliary copper excretion is reduced, urinary copper concentration in these mice is higher than that of control mice. Our data indicate that Ctr1 plays a critical role in copper acquisition in the liver, and, when Ctr1 expression is compromised, compensatory mechanisms facilitate copper uptake and/or retention in the liver and excretion of copper via urine.

The role of the mammalian copper transporter 1 in the cellular accumulation of platinum-based drugs

The mammalian copper transporter 1 (CTR1) is responsible for the uptake of copper from the extracellular space. In this study, we used an isogenic pair of CTR1(+/+) and CTR1(-/-) mouse embryo fibroblasts to examine the contribution of CTR1 to the influx of cisplatin (DDP), carboplatin (CBDCA), oxaliplatin (L-OHP), and transplatin. Exposure to DDP triggered the rapid degradation of CTR1, suggesting that its contribution to influx was likely to be on the initial phase of drug entry. Loss of CTR1 decreased the initial binding of DDP to cells and reduced influx measured over the first 5 min of drug exposure by 81%. Loss of CTR1 almost completely eliminated the initial influx of CBDCA and reduced the initial uptake of L-OHP by 68% but had no effect on the influx of transplatin. Loss of CTR1 rendered cells resistant to even high concentrations of DDP when measured in vitro, and re-expression of CTR1 in the CTR1(-/-) cells restored both DDP uptake and cytotoxicity. The growth of CTR1(-/-) tumor xenografts in which CTR1 levels were restored by infection with a lentivirus expressing wild-type CTR1 was reduced by a single maximum tolerated dose of DDP in vivo, whereas the CTR1(-/-) xenografts failed to respond at all. We conclude that CTR1 mediates the initial influx of DDP, CBDCA, and L-OHP and is a major determinant of responsiveness to DDP both in vitro and in vivo.

Copper is taken up efficiently from albumin and alpha2-macroglobulin by cultured human cells by more than one mechanism

Ionic copper entering blood plasma binds tightly to albumin and the macroglobulin transcuprein. It then goes primarily to the liver and kidney except in lactation, where a large portion goes directly to the mammary gland. Little is known about how this copper is taken up from these plasma proteins. To examine this, the kinetics of uptake from purified human albumin and alpha(2)-macroglobulin, and the effects of inhibitors, were measured using human hepatic (HepG2) and mammary epithelial (PMC42) cell lines. At physiological concentrations (3-6 muM), both cell types took up copper from these proteins independently and at rates similar to each other and to those for Cu-dihistidine or Cu-nitrilotriacetate (NTA). Uptakes from alpha(2)-macroglobulin indicated a single saturable system in each cell type, but with different kinetics, and 65-80% inhibition by Ag(I) in HepG2 cells but not PMC42 cells. Uptake kinetics for Cu-albumin were more complex and also differed with cell type (as was the case for Cu-histidine and NTA), and there was little or no inhibition by Ag(I). High Fe(II) concentrations (100-500 microM) inhibited copper uptake from albumin by 20-30% in both cell types and that from alpha(2)-macroglobulin by 0-30%, and there was no inhibition of the latter by Mn(II) or Zn(II). We conclude that the proteins mainly responsible for the plasma-exchangeable copper pool deliver the metal to mammalian cells efficiently and by several different mechanisms. alpha(2)-Macroglobulin delivers it primarily to copper transporter 1 in hepatic cells but not mammary epithelial cells, and additional as-yet-unidentified copper transporters or systems for uptake from these proteins remain to be identified.

"Pulling the plug" on cellular copper: the role of mitochondria in copper export

Mitochondria contain two enzymes, Cu,Zn superoxide dismutase (Sod1) and cytochrome c oxidase (CcO), that require copper as a cofactor for their biological activity. The copper used for their metallation originates from a conserved, bioactive pool contained within the mitochondrial matrix, the size of which changes in response to either genetic or pharmacological manipulation of cellular copper status. Its dynamic nature implies molecular mechanisms exist that functionally couple mitochondrial copper handling with other, extramitochondrial copper trafficking pathways. The recent finding that mitochondrial proteins with established roles in CcO assembly can also effect changes in cellular copper levels by modulating copper efflux from the cell supports a mechanistic link between organellar and cellular copper metabolism. However, the proteins and molecular mechanisms that link trafficking of copper to and from the organelle with other cellular copper trafficking pathways are unknown. This review documents our current understanding of copper trafficking to, and within, the mitochondrion for metallation of CcO and Sod1; the pathways by which the two copper centers in CcO are formed; and, the interconnections between mitochondrial function and the regulation of cellular copper homeostasis.

Ctr2 is partially localized to the plasma membrane and stimulates copper uptake in COS-7 cells

Ctr1 (copper transporter 1) mediates high-affinity copper uptake. Ctr2 (copper transporter 2) shares sequence similarity with Ctr1, yet its function in mammalian cells is poorly understood. In African green monkey kidney COS-7 cells and rat tissues, Ctr2 migrated as a predominant band of approximately 70 kDa and was most abundantly expressed in placenta and heart. A transiently expressed hCtr2-GFP (human Ctr2-green fluorescent protein) fusion protein and the endogenous Ctr2 in COS-7 cells were mainly localized to the outer membrane of cytoplasmic vesicles, but were also detected at the plasma membrane. Biotinylation of Ctr2 with the membrane-impermeant reagent sulfo-NHS-SS-biotin [sulfosuccinimidyl-2-(biotinamido)ethyl-1,3-dithiopropionate] confirmed localization at the cell surface. Cells expressing hCtr2-GFP hyperaccumulated copper when incubated in medium supplemented with 10 microM CuSO(4), whereas cells depleted of endogenous Ctr2 by siRNAs (small interfering RNAs) accumulated lower levels of copper. hCtr2-GFP expression did not affect copper efflux, suggesting that hCtr2-GFP increased cellular copper concentrations by promoting uptake at the cell surface. Kinetic analyses showed that hCtr2-GFP stimulated saturable copper uptake with a K(m) of 11.0+/-2.5 microM and a K(0.5) of 6.9+/-0.7 microM when data were fitted to a rectangular hyperbola or Hill equation respectively. Competition experiments revealed that silver completely inhibited hCtr2-GFP-dependent copper uptake, whereas zinc, iron and manganese had no effect on uptake. Furthermore, increased copper concentrations in hCtr2-GFP-expressing cells were inversely correlated with copper chaperone for Cu/Zn superoxide dismutase protein expression. Collectively, these results suggest that Ctr2 promotes copper uptake at the plasma membrane and plays a role in regulating copper levels in COS-7 cells.

Human copper transporter 2 is localized in late endosomes and lysosomes and facilitates cellular copper uptake

High-affinity cellular copper uptake is mediated by the CTR (copper transporter) 1 family of proteins. The highly homologous hCTR (human CTR) 2 protein has been identified, but its function in copper uptake is currently unknown. To characterize the role of hCTR2 in copper homoeostasis, epitope-tagged hCTR2 was transiently expressed in different cell lines. hCTR2-vsvG (vesicular-stomatitis-virus glycoprotein) predominantly migrated as a 17 kDa protein after imunoblot analysis, consistent with its predicted molecular mass. Chemical cross-linking resulted in the detection of higher-molecular-mass complexes containing hCTR2-vsvG. Furthermore, hCTR2-vsvG was co-immunoprecipitated with hCTR2-FLAG, suggesting that hCTR2 can form multimers, like hCTR1. Transiently transfected hCTR2-eGFP (enhanced green fluorescent protein) was localized exclusively to late endosomes and lysosomes, and was not detected at the plasma membrane. To functionally address the role of hCTR2 in copper metabolism, a novel transcription-based copper sensor was developed. This MRE (metal-responsive element)-luciferase reporter contained four MREs from the mouse metallothionein 1A promoter upstream of the firefly luciferase open reading frame. Thus the MRE-luciferase reporter measured bioavailable cytosolic copper. Expression of hCTR1 resulted in strong activation of the reporter, with maximal induction at 1 muM CuCl2, consistent with the K(m) of hCTR1. Interestingly, expression of hCTR2 significantly induced MRE-luciferase reporter activation in a copper-dependent manner at 40 and 100 microM CuCl2. Taken together, these results identify hCTR2 as an oligomeric membrane protein localized in lysosomes, which stimulates copper delivery to the cytosol of human cells at relatively high copper concentrations. This work suggests a role for endosomal and lysosomal copper pools in the maintenance of cellular copper homoeostasis.