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

Function and regulation of human copper-transporting ATPases

Copper-transporting ATPases (Cu-ATPases) ATP7A and ATP7B are evolutionarily conserved polytopic membrane proteins with essential roles in human physiology. The Cu-ATPases are expressed in most tissues, and their transport activity is crucial for central nervous system development, liver function, connective tissue formation, and many other physiological processes. The loss of ATP7A or ATP7B function is associated with severe metabolic disorders, Menkes disease, and Wilson disease. In cells, the Cu-ATPases maintain intracellular copper concentration by transporting copper from the cytosol across cellular membranes. They also contribute to protein biosynthesis by delivering copper into the lumen of the secretory pathway where metal ion is incorporated into copper-dependent enzymes. The biosynthetic and homeostatic functions of Cu-ATPases are performed in different cell compartments; targeting to these compartments and the functional activity of Cu-ATPase are both regulated by copper. In recent years, significant progress has been made in understanding the structure, function, and regulation of these essential transporters. These studies raised many new questions related to specific physiological roles of Cu-ATPases in various tissues and complex mechanisms that control the Cu-ATPase function. This review summarizes current data on the structural organization and functional properties of ATP7A and ATP7B as well as their localization and functions in various tissues, and discusses the current models of regulated trafficking of human Cu-ATPases.

Human Copper Transporter hCTR1 Mediates Basolateral Uptake of Copper into Enterocytes

Copper is essential for human growth and survival. Enterocytes mediate the absorption of dietary copper from the intestinal lumen into blood as well as utilizing copper for their biosynthetic needs. Currently, the pathways for copper entry into enterocytes remain poorly understood. We demonstrate that the basolateral copper uptake into intestinal cells greatly exceeds the apical uptake. The basolateral but not apical transport is mediated by the high affinity copper transporter hCTR1. This unanticipated conclusion is supported by cell surface biotinylation and confocal microscopy of endogenous hCTR1 in Caco2 cells as well as copper influx measurements that show saturable high affinity uptake at the basolateral but not the apical membrane. Basolateral localization of hCTR1 and polarized copper uptake are also conserved in T84 cells, models for intestinal crypt cells. The lateral localization of hCTR1 seen in intestinal cell lines is recapitulated in immunohistochemical staining of mouse intestinal sections. Biochemical and functional assays reveal the basolateral localization of hCTR1 also in renal Madin-Darby canine kidney cells and opossum kidney cells. Overexpression of hCTR1 in Madin-Darby canine kidney cells results in both apical and basolateral delivery of the overexpressed protein and greatly enhanced copper uptake at both cell surfaces. We propose a model of intestinal copper uptake in which basolateral hCTR1 plays a key role in the physiologically important delivery of copper from blood to intracellular proteins, whereas its role in the initial apical uptake of dietary copper is indirect.

Distinct mechanisms for Ctr1-mediated copper and cisplatin transport

The Ctr1 family of integral membrane proteins is necessary for high affinity copper uptake in eukaryotes. Ctr1 is also involved in cellular accumulation of cisplatin, a platinum-based anticancer drug. Although the physiological role of Ctr1 has been revealed, the mechanism of action of Ctr1 remains to be elucidated. To gain a better understanding of Ctr1-mediated copper and cisplatin transport, we have monitored molecular dynamics and transport activities of yeast Saccharomyces cerevisiae Ctr1 and its mutant alleles. Co-expression of functional Ctr1 monomers fused with either cyan or yellow fluorescent protein resulted in fluorescence resonance energy transfer (FRET), which is consistent with multimer assembly of Ctr1. Copper near the K(m) value of Ctr1 enhanced FRET in a manner that correlated with cellular copper transport. In vitro cross-linking of Ctr1 confirmed that copper-induced FRET reflects conformational changes within pre-existing Ctr1 complexes. FRET assays in membrane-disrupted cells and protein extracts showed that intact cell structure is necessary for Ctr1 activity. Despite Ctr1-dependent cellular accumulation, cisplatin did not change Ctr1 FRET nor did it attenuate copper-induced FRET. A Ctr1 allele defective in copper transport enhanced cellular cisplatin accumulation. N-terminal methionine-rich motifs that are dispensable for copper transport play a critical role for cisplatin uptake. Taken together, our data reveal functional roles for structural remodeling of the Ctr1 multimeric complex in copper transport and suggest distinct mechanisms employed by Ctr1 for copper and cisplatin transport.

Vertebrate Ctr1 coordinates morphogenesis and progenitor cell fate and regulates embryonic stem cell differentiation

Embryogenesis involves two distinct processes. On the one hand, cells must specialize, acquiring fates appropriate to their positions (differentiation); on the other hand, they must physically construct the embryo through coordinated mechanical activity (morphogenesis). In early vertebrate development, fibroblast growth factor (FGF) regulates multiple embryonic events, including germ layer differentiation and morphogenesis; the cellular components that direct FGF signaling to evoke these different responses remain largely unknown. We show here that the copper transporter 1 (Ctr1) protein is a critical router of FGF signals during early embryogenesis. Ctr1 both promotes the differentiation and inhibits the morphogenesis of mesoderm and neurectoderm in embryos of the frog Xenopus laevis, thereby coordinating normal development. Signal sorting by Ctr1 involves the activation of the Ras-MAP kinase cascade and appears to be independent of its role in copper transport. Mouse embryonic stem (ES) cells deficient for Ctr1 (Ctr1(-/-)) retain characteristics of pluripotency under conditions that favor differentiation in wild-type ES cells, indicating a conserved role for Ctr1 during amphibian and mammalian cell fate determination. Our studies support a model in which vertebrate Ctr1 functions as a key regulator of the differentiation capacity of both stem and progenitor cell populations.

Dynamics of endogenous ATP7A (Menkes protein) in intestinal epithelial cells: copper-dependent redistribution between two intracellular sites

We report for the first time on the copper-dependent behavior of endogenous ATP7A in two types of polarized intestinal epithelia, rat enterocytes in vivo and filter-grown Caco-2 cells, an accepted in vitro model of human small intestine. We used high-resolution, confocal immunofluorescence combined with quantitative cell surface biotinylation and found that the vast majority of endogenous ATP7A was localized intracellularly under all copper conditions. In copper-depleted cells, virtually all of the ATP7A localized to a post-TGN compartment, with <3% of the total protein detectable at the basolateral cell surface. When copper levels were elevated, ATP7A dispersed to the cell periphery in punctae whose pattern did not overlap with the steady-state distributions of post-Golgi, endosomal, or basolateral membrane markers; only approximately 8-10% of the recovered ATP7A was detected at the basolateral cell surface. These results raise several questions regarding prevailing models of ATP7A dynamics and the mechanism of copper efflux.

Copper and human health: biochemistry, genetics, and strategies for modeling dose-response relationships

Copper (Cu) and its alloys are used extensively in domestic and industrial applications. Cu is also an essential element in mammalian nutrition. Since both copper deficiency and copper excess produce adverse health effects, the dose-response curve is U-shaped, although the precise form has not yet been well characterized. Many animal and human studies were conducted on copper to provide a rich database from which data suitable for modeling the dose-response relationship for copper may be extracted. Possible dose-response modeling strategies are considered in this review, including those based on the benchmark dose and categorical regression. The usefulness of biologically based dose-response modeling techniques in understanding copper toxicity was difficult to assess at this time since the mechanisms underlying copper-induced toxicity have yet to be fully elucidated. A dose-response modeling strategy for copper toxicity was proposed associated with both deficiency and excess. This modeling strategy was applied to multiple studies of copper-induced toxicity, standardized with respect to severity of adverse health outcomes and selected on the basis of criteria reflecting the quality and relevance of individual studies. The use of a comprehensive database on copper-induced toxicity is essential for dose-response modeling since there is insufficient information in any single study to adequately characterize copper dose-response relationships. The dose-response modeling strategy envisioned here is designed to determine whether the existing toxicity data for copper excess or deficiency may be effectively utilized in defining the limits of the homeostatic range in humans and other species. By considering alternative techniques for determining a point of departure and low-dose extrapolation (including categorical regression, the benchmark dose, and identification of observed no-effect levels) this strategy will identify which techniques are most suitable for this purpose. This analysis also serves to identify areas in which additional data are needed to better define the characteristics of dose-response relationships for copper-induced toxicity in relation to excess or deficiency.

Mechanisms of dietary Cu uptake in freshwater rainbow trout: evidence for Na-assisted Cu transport and a specific metal carrier in the intestine

Copper (Cu) is both a vital nutrient and a potent toxicant. The objective of this study was to analyze the mechanistic nature of intestinal Cu transport in rainbow trout using radiolabeled Cu (64Cu) and an in vitro gut sac technique. Reduction of mucosal NaCl levels inhibited Cu transport while increase caused stimulation; Na(2)SO(4) had an identical effect, implicating Na(+) rather than the anion. These responses were unrelated to solvent drag, osmotic pressure or changes in transepithelial potential. The presence of elevated luminal Ag stimulated Cu and Na(+) uptake. Phenamil caused a partial inhibition of both Cu and Na(+) uptake while hypercapnia stimulated Na(+) and Cu transport. Cu uptake was sensitive to luminal pH and inhibited by a tenfold excess of Fe and Zn. These factors had no effect on Na(+ )uptake. On the basis of these results we propose a novel Na(+)-assisted mechanism of Cu uptake wherein the Na(+) gradient stimulates an increase in the H(+) concentration of the brushborder creating a suitable microenvironment for the effective transport of Cu via either DMT1 or Ctr1.

Copper entry into human cells: progress and unanswered questions

In this brief review we summarize what is known about the role of hCTR1 in mediating the entry of copper into human cells. There is a body of information that clearly identifies this protein as being a major source (though not the only source) of copper entry into human cells, and thus a crucial element of copper homeostasis. However, much remains that is poorly understood and key aspects of the physiological roles of hCTR1 and its regulation are only superficially appreciated. The particular characteristics of a transport process that in vivo involves the binding, transmembrane transport and release of a substrate that is not present in a free form in the intracellular or extracellular compartments poses particular challenges that are not encountered in the transport of more familiar physiologically important metal cations. Thus much of what we have learned about the more commonly encountered transported ions provides an inadequate model for studies of copper homeostasis. In this article we review progress made and identify the major questions that need to be resolved before an adequate description is attained of how copper entry into human cells is mediated and regulated by hCTR1.

A cadmium-transporting P1B-type ATPase in yeast Saccharomyces cerevisiae

Detoxification and homeostatic acquisition of metal ions are vital for all living organisms. We have identified PCA1 in yeast Saccharomyces cerevisiae as an overexpression suppressor of copper toxicity. PCA1 possesses signatures of a P1B-type heavy metal-transporting ATPase that is widely distributed from bacteria to humans. Copper resistance conferred by PCA1 is not dependent on catalytic activity, but it appears that a cysteine-rich region located in the N terminus sequesters copper. Unexpectedly, when compared with two independent natural isolates and an industrial S. cerevisiae strain, the PCA1 allele of the common laboratory strains we have examined possesses a missense mutation in a predicted ATP-binding residue conserved in P1B-type ATPases. Consistent with a previous report that identifies an equivalent mutation in a copper-transporting P1B-type ATPase of a Wilson disease patient, the PCA1 allele found in laboratory yeast strains is nonfunctional. Overexpression or deletion of the functional allele in yeast demonstrates that PCA1 is a cadmium efflux pump. Cadmium as well as copper and silver, but not other metals examined, dramatically increase PCA1 protein expression through post-transcriptional regulation and promote subcellular localization to the plasma membrane. Our study has revealed a novel metal detoxification mechanism in yeast mediated by a P1B-type ATPase that is unique in structure, substrate specificity, and mode of regulation.

Structure of the Ctr1 copper trans'PORE'ter reveals novel architecture

Copper is essential for biological processes such as free radical detoxification, mitochondrial respiration and iron metabolism. A central player in copper homeostasis is the high-affinity integral plasma membrane copper transporter Ctr1. However, the precise mechanisms by which Ctr1 functions are not known. Here, we highlight an important breakthrough in our understanding of how Ctr1 facilitates Cu(I) movement across membranes: the publication of structural details for human Ctr1 obtained from 2D crystallography and electron microscopy.

Ctr1 drives intestinal copper absorption and is essential for growth, iron metabolism, and neonatal cardiac function

The trace element copper (Cu) is a cofactor for biochemical functions ranging from energy generation to iron (Fe) acquisition, angiogenesis, and free radical detoxification. While Cu is essential for life, the molecules that mediate dietary Cu uptake have not been identified. Ctr1 is a homotrimeric protein, conserved from yeast to humans, that transports Cu across the plasma membrane with high affinity and specificity. Here we describe the generation of intestinal epithelial cell-specific Ctr1 knockout mice. These mice exhibit striking neonatal defects in Cu accumulation in peripheral tissues, hepatic Fe overload, cardiac hypertrophy, and severe growth and viability defects. Consistent with an intestinal Cu absorption block, the growth and viability defects can be partially rescued by a single postnatal Cu administration, indicative of a critical neonatal metabolic requirement for Cu that is provided by intestinal Ctr1. These studies identify Ctr1 as the major factor driving intestinal Cu absorption in mammals.

Contribution of the major copper influx transporter CTR1 to the cellular accumulation of cisplatin, carboplatin, and oxaliplatin

The goal of this study was to determine the ability of the major copper influx transporter CTR1 to mediate the cellular accumulation of cisplatin (DDP), carboplatin (CBDCA), and oxaliplatin (L-OHP). Wild-type murine embryonic fibroblasts (CTR1+/+) and a subline in which both alleles of CTR1 were deleted (CTR1-/-) were tested for their ability to accumulate platinum when exposed to increasing concentrations of DDP, CBDCA, or L-OHP for 1 h. They were also tested for their sensitivity to the growth-inhibitory effect of each drug. Platinum content was measured by ion-coupled plasmon mass spectroscopy. The experimental model was validated by measuring copper accumulation and cytotoxicity. CTR1-/- cells accumulated only 5.7% as much copper as CTR1+/+ cells during a 1-h exposure to 2 microM copper. When exposed to DDP, CBDCA, or L-OHP at 2 microM, accumulation in the CTR1-/- cells was only 35 to 36% of that in the CTR1+/+ cells. When tested at a 5-fold higher concentration, this deficit remained for DDP and CBDCA, but accumulation of L-OHP was no longer CTR1-dependent. There was an association between the effect of loss of CTR1 function on uptake of the platinum drugs and their cytotoxicity. The CTR1-/- cells were 3.2-fold resistant to DDP, 2.0-fold resistant to CBDCA, but only 1.7-fold resistant to L-OHP. Thus, whereas CTR1 controls the cellular accumulation of all three drugs at low concentrations, accumulation of L-OHP is not dependent on CTR1 at higher concentrations. We conclude that L-OHP is a substrate for some other cellular entry mechanism, a feature consistent with its different clinical spectrum of activity.

Expression of the human copper influx transporter 1 in normal and malignant human tissues

The major copper influx transporter, copper transporter 1 (hCTR1), controls the cellular accumulation of cisplatin in mammalian cells. The goal of this study was to determine the pattern of hCTR1 expression in normal and malignant human tissues. Tissue arrays were stained with an antibody specific for hCTR1 using standard immunohistochemical techniques. Particularly strong staining was noted in the alpha cells of the pancreatic islets, enteroendocrine cells of the gastric mucosa and bronchioles, C cells of the thyroid, and a subset of cells in the anterior pituitary. Frequency and intensity of hCTR1 staining in malignant tissues reflected the levels found in their normal tissue counterparts. For example, neither normal prostate nor prostate cancers expressed hCTR1, whereas it was commonly expressed in both normal colonic epithelium and in colon carcinomas. Strong staining was observed in a limited number of cases of carcinoid tumors, Ewing's sarcoma, and undifferentiated carcinomas. Although all tissues require copper, expression of hCTR1 was highly variable among normal tissues and among the major human malignancies, with the highest levels found in enteroendocrine cells. No hCTR1 expression was found in several common types of cancer, suggesting that hCTR1 expression is not commonly enhanced by transformation.

Mechanisms of the Copper-dependent Turnover of the Copper Chaperone for Superoxide Dismutase

The copper chaperone for superoxide dismutase (CCS) is an intracellular metallochaperone required for incorporation of copper into the essential antioxidant enzyme copper/zinc superoxide dismutase (SOD1). Nutritional studies have revealed that the abundance of CCS is inversely proportional to the dietary and tissue copper content. To determine the mechanisms of copper-dependent regulation of CCS, copper incorporation into SOD1 and SOD1 enzymatic activity as well as CCS abundance and half-life were determined after metabolic labeling of CCS-/- fibroblasts transfected with wild-type or mutant CCS. Wild-type CCS restored SOD1 activity in CCS-/- fibroblasts, and the abundance of this chaperone in these cells was inversely proportional to the copper content of the media, indicating that copper-dependent regulation of CCS is entirely post-translational. Although mutational studies demonstrated no role for CCS Domain I in this copper-dependent regulation, similar analysis of the CXC motif in Domain III revealed a critical role for these cysteine residues in mediating copper-dependent turnover of CCS. Further mutational studies revealed that this CXC-dependent copper-mediated turnover of CCS is independent of the mechanisms of delivery of copper to SOD1 including CCS-SOD1 interaction. Taken together these data demonstrate a mechanism determining the abundance of CCS that is competitive with the process of copper delivery to SOD1, revealing a unique post-translational component of intracellular copper homeostasis.

Functional expression of a low-affinity zinc uptake transporter (FrZIP2) from pufferfish (Takifugu rubripes) in MDCK cells

Zinc is a vital micronutrient to all organisms and it is therefore very important to determine the mechanisms that regulate cellular zinc uptake. Previously, we reported on zinc uptake transporters from zebrafish (Danio rerio; DrZIP1) and Fugu pufferfish (Takifugu rubripes; FrZIP1) that facilitated cellular zinc uptake of high affinity (K(m)<0.5 microM) in both CHSE214 [chinook salmon (Oncorhynchus tshawytscha) embryonic 214] cells and Xenopus laevis oocytes. To investigate additional biochemical pathways of zinc uptake in fish, we molecularly cloned the second fish member (FrZIP2) of the SLC39 subfamily II from Fugu pufferfish gill. Functional characterization suggests that FrZIP2 stimulated zinc uptake in a temperature-, time-, concentration- and pH-dependent manner when overexpressed in MDCK cells (Madin-Darby canine kidney cells). In comparison with FrZIP1 and DrZIP1 (<0.5 microM), FrZIP2 appears to represent a low-affinity zinc uptake transporter (K(m)=13.6 microM) in pufferfish. FrZIP2 protein was selective for zinc, but it might also transport Cu2+, since 20 times excess of Cu2+ completely abolished its zinc uptake activity. The zinc uptake by FrZIP2 was stimulated in a slightly acidic medium (pH 5.5-6.5) and was completely blocked at pH 7.5 and above, suggesting that an inward H+ gradient might provide a driving force for zinc transport by FrZIP2. Furthermore, FrZIP2-mediated zinc uptake activity was slightly inhibited by 0.5 mM HCO3-, indicating that FrZIP2 may employ a different mechanism of zinc translocation from the assumed HCO3--coupled zinc transport used by human SLC39A2. The FrZIP2 gene was expressed in all the tissues studied herein, with especially high levels in the ovary and intestines. Thus FrZIP2 may be a prominent zinc uptake transporter of low affinity in many cell types of Fugu pufferfish.

Role of copper transporters in the uptake and efflux of platinum containing drugs

Cellular mechanisms for the uptake, intracellular distribution and efflux of the platinum (Pt) containing compounds cisplatin (DDP), carboplatin (CBDCA) and oxaliplatin (LOHP) are unknown. Current data suggest that specialized transporters/carriers mediate the transport of Pt drugs across the cellular membranes. Specific roles for the copper (Cu) transporters CTR1, ATP7A and ATP7B have been demonstrated during recent years. The finding that in cultured cells and tumor samples a correlation can be found between the expression of Cu transporters and the degree of the acquired resistance to Pt drug suggests that the Cu transporters are important constituents of the program that regulates sensitivity to Pt drugs. A model is presented that describes the function of Cu transporters in the regulation of Pt drug uptake and efflux.

Modulation of the cellular pharmacology of JM118, the major metabolite of satraplatin, by copper influx and efflux transporters

Satraplatin is an orally bioavailable platinum analog that has activity in prostate cancer. JM118 is the most abundant species found in the plasma following the oral ingestion of satraplatin and has anti-tumor activity in vitro against cell lines that are resistant to cisplatin (DDP). The goal of the current study was to determine whether the activity of JM118 in some DDP-resistant cells can be explained by differences in the cellular pharmacology of the two drugs. The effect of each of the Cu transporters CTR1, ATP7A and ATP7B on sensitivity to the growth inhibitory effect of JM118 and its cellular pharmacology was examined to identify the characteristics of JM118 that distinguish it from DDP. These studies were performed using wild type and CTR1-/- homozygous knockout mouse embryo cells, and human Me32a Menkes disease fibroblasts that do not express either ATP7A or ATP7B plus sublines molecularly engineered to express either ATP7A (MeMNK cells) or ATP7B (MeWND cells). Knockout of the Cu influx transporter CTR1 in murine embryo cells increased their resistance to DDP and reduced its cellular accumulation but had no effect on sensitivity to JM118 or its uptake. In the case of DDP, forced expression of either of the two Cu efflux transporters, ATP7A or ATP7B, in Me32a cells rendered them resistant to DDP, increased whole cell accumulation of Pt but reduced the amount of Pt in DNA. In the case of JM118, forced expression of either ATP7A or ATP7B rendered Me32a cells resistant, increased not only whole cell Pt accumulation but also increased rather than decreased the amount of Pt in DNA. These results demonstrate that both ATP7A and ATP7B mediate resistance to JM118 as well as DDP and suggest that they sequester both DDP and JM118 into vesicular compartments within the cell resulting in enhanced whole cell accumulation and reduced cytotoxicity. We conclude that there are two important differences between DDP and JM118 with respect to the effect of Cu transporters on their cellular pharmacology. First, whereas CTR1 is involved in DDP accumulation it does not play a role in the uptake of JM118. Second, ATP7A and ATP7B, while they both mediate resistance, have opposite effects on the accumulation of Pt in DNA following exposure to the two drugs. ATP7A and ATP7B appear to be able to modulate the toxicity of the Pt that accumulates in DNA following exposure to JM118. These results suggest that JM118 will retain activity in cells in which DDP resistance is due to the loss of CTR1, but not in cells in which resistance is due to enhanced expression of ATP7A or ATP7B.

Novel gene functions required for melanization of the human pathogenCryptococcus neoformans

The ability to produce melanin is a key virulence factor in many fungal pathogens including the human basidiomycete pathogen Cryptococcus neoformans, a major cause of life-threatening infections among immunocompromised persons. Despite the significance of melanin biosynthesis in virulence of C. neoformans, the cellular and molecular processes involved in this pathway have not yet been fully elucidated. Here, we used Agrobacterium to isolate insertional mutants and screened 12 000 mutants to uncover genes involved in melanin production in C. neoformans. Four new mutant alleles of the well-known melanin biosynthesis gene, LAC1, which encodes laccase were identified, and the T-DNA was shown to have a possible predisposition for insertion into the promoters of genes, in particular LAC1. Melanization in C. neoformans is dependent on five additional genes identified in this screen encoding homologues of the copper transporter Ccc2, the copper chaperone Atx1, the chitin synthase Chs3, the transcriptional coactivator Mbf1 and the chromatin-remodelling enzyme Snf5. Illumination of the molecular and genetic components of this virulence pathway reveals potential novel targets for drug development against C. neoformans and provides further insight into the intimate relationship between metal ion homeostasis and melanin biosynthesis.

The effects of tetrathiomolybdate (TTM, NSC-714598) and copper supplementation on fertility and early embryonic development in rats

Based on its ability to chelate copper, TTM is being studied as an antiangiogenic agent for cancer therapy. The purpose of this study was to evaluate the toxicity of TTM and the protection of copper supplementation on the reproductive capability of male and female CD rats. Doses of 0, 1, 4, and 12 mg/kg/day with copper supplementation (110 mg/kg of diet) were given by gavage. There were no effects on the estrous cycle or reproductive indices, or maternal toxicity in any female dose group. Male rats given 12 mg/kg/day showed significant decreases in body weight gains and food consumption, and anemia. Serum ceruloplasmin levels were dose-dependently decreased in all male dose groups. Reduced epididymal weights, sperm counts, and sperm motility, sperm morphologic abnormalities and histopathologic changes in testis and epididymis occurred only at 12 mg/kg/day. Dietary copper supplementation prevented the adverse sperm effects produced by 12 mg/kg/day of TTM.

Copper transporters regulate the cellular pharmacology and sensitivity to Pt drugs

Recent studies have demonstrated that the major Cu influx transporter CTR1 regulates tumor cell uptake of cisplatin (DDP), carboplatin (CBDCA) and oxaliplatin (L-OHP), and that the two Cu efflux transporters ATP7A and ATP7B regulate the efflux of these drugs. Evidence for the concept that these platinum (Pt) drugs enter cells and are distributed to various subcellular compartments via transporters that have evolved to manage Cu homeostasis includes the demonstration of: (1) bidirectional cross-resistance between cells selected for resistance to either the Pt drugs or Cu; (2) parallel changes in the transport of Pt and Cu drugs in resistant cells; (3) altered cytotoxic sensitivity and Pt drug accumulation in cells transfected with Cu transporters; and (4) altered expression of Cu transporters in Pt drug-resistant tumors. Appreciation of the role of the Cu transporters in the development of resistance to DDP, CBDCA, and L-OHP offers novel insights into strategies for preventing or reversing resistance to this very important family of anticancer drugs.

Metal-responsive transcription factor (MTF-1) handles both extremes, copper load and copper starvation, by activating different genes

From insects to mammals, metallothionein genes are induced in response to heavy metal load by the transcription factor MTF-1, which binds to short DNA sequence motifs, termed metal response elements (MREs). Here we describe a novel and seemingly paradoxical role for MTF-1 in Drosophila in that it also mediates transcriptional activation of Ctr1B, a copper importer, upon copper depletion. Activation depends on the same type of MRE motifs in the upstream region of the Ctr1B gene as are normally required for metal induction. Thus, a single transcription factor, MTF-1, plays a direct role in both copper detoxification and acquisition by inducing the expression of metallothioneins and of a copper importer, respectively.

Cisplatin rapidly down-regulates its own influx transporter hCTR1 in cultured human ovarian carcinoma cells

PURPOSE

Cisplatin (DDP)-resistant cells commonly exhibit reduced drug accumulation. Previous studies have shown that the major copper (Cu) influx transporter CTR1 controls the uptake of DDP in yeast and mammalian cells. The goal of this study was to examine the effect of Cu and DDP on the level and subcellular localization of hCTR1 protein in human ovarian carcinoma cells.

EXPERIMENTAL DESIGN

Cultured human ovarian carcinoma A2780 cells were exposed to DDP and Cu, and the effect on hCTR1 was determined using Western blot analysis and confocal digital deconvolution microscopy.

RESULTS

Loss of hCTR1 was triggered by DDP exposure in a concentration and time-dependent manner. Exposure to 0.5 micromol/L DDP for 5 minutes reduced hCTR1 levels and exposure to DDP concentrations > or =2 micromol/L caused almost complete disappearance. The loss of hCTR1 was observed within 1 minute of the start of exposure to 2 micromol/L DDP. Treatment of cells with 100 micromol/L Cu for 5 minutes produced a smaller effect. Pretreatment of cells with 2 micromol/L DDP for 5 minutes resulted in a 50% decrease in 64Cu uptake, demonstrating that the DDP-induced loss of hCTR1 detected by Western blot analysis and imaging was functionally significant.

CONCLUSIONS

DDP down-regulated the amount of its major influx transporter in cultured human ovarian carcinoma cells in a concentration- and time-dependent manner. The effect was observed at DDP concentrations within the range found in the plasma of patients being treated with DDP, and it occurred very quickly relative to the half-life of the drug.

Confocal microscopic analysis of the interaction between cisplatin and the copper transporter ATP7B in human ovarian carcinoma cells

Some cisplatin (DDP)-resistant cells overexpress the copper export transporter ATP7B, and cells molecularly engineered to overexpress ATP7B are resistant to DDP. The interaction of Cu with ATP7B normally triggers its relocalization from the perinuclear region to more peripheral vesicles. To investigate the interaction of DDP with ATP7B, we examined the effect of DDP on the subcellular localization of ATP7B using human ovarian carcinoma cells expressing a cyan fluorescent protein (ECFP)-tagged ATP7B (2008/ECFP-ATP7B). ATP7B expression was confirmed in 2008/ECFP-ATP7B cells by Western blotting, and its functionality was documented by showing that it rendered the cells 1.9-fold resistant to CuSO(4) and 4.1-fold resistant to DDP and also reduced the accumulation of both drugs. There was greater sequestration of Pt into intracellular vesicles in the 2008/ECFP-ATP7B cells than in the 2008/ECFP cells. Confocal digital microscopy revealed that ECFP-ATP7B localized in the perinuclear region in absence of drug exposure and that both Cu and DDP triggered relocalization to more peripheral vesicular structures. A fluorescein-labeled form of DDP that retained cytotoxicity and was subject to the same mechanisms of resistance as DDP colocalized with ECFP-ATP7B in the 2008/ECFP-ATP7B cells, whereas the same fluorochrome lacking the DDP moiety did not. These results provide evidence that DDP directly interacts with ATP7B to trigger its relocalization and that ATP7B mediates resistance to DDP by sequestering it into vesicles of the secretory pathway for export from the cell.

Sodium-sensitive and -insensitive copper accumulation by isolated intestinal cells of rainbow troutOncorhynchus mykiss

The pathway for copper (Cu) uptake across the mucosal membrane into intestinal cells has not been elucidated in fish. Copper accumulation in freshly isolated intestinal cells from rainbow trout Oncorhynchus mykiss was measured after exposure to 0–800 μmol l–1 CuSO4 for 15 min. With external Cu concentration (Cuo) of 800 μmol l–1, the rate of Cu accumulation by cells was 1.88±0.52 nmol Cu mg–1cell protein h–1 compared to 0.05±0.01 nmol Cu mg–1 cell protein h–1 with no added Cuo (means ± s.e.m., N=6). Deduction of a rapid Cu accumulation measured on/in cells at time zero (about 12% of the total Cu uptake when Cuo was 800 μmol l–1)revealed a saturable uptake curve, which reached a plateau at 400 μmol l–1 Cuo (Km=216 μmol l–1 Cuo; Vmax=1.09 nmol Cu mg–1 cell protein h–1; 140 mmol l–1 NaCl throughout). Incubation of cells at 4°C did not prevent Cu accumulation. Lowering external [Na+] to 11 mmol l–1 (low Na+o) generally did not alter the rate of Cu accumulation into the cells over a 15 min period. Under low Na+o conditions Cu accumulation was exponential(non-saturable). Na+-insensitive Cu accumulation dominated (59% of total Cu accumulation) when Cuo was 400 μmol l–1 or less. At high Cuo (800 μmol l–1), removal of Na+ caused a 45% increase in Cu accumulation. Pre-incubation of cells with blocking agents of epithelial Na+ channel (ENaC) for 15 min (normal [NaCl] throughout) caused Cu accumulation rates to increase by 40-fold (100 μmol l–1phenamil), 21-fold (10 μmol l–1 CDPC) or 12-fold (2 mmol l–1 amiloride) when Cuo was 800 μmol l–1 compared to those in drug-free controls. Lowering the external chloride concentration [Cl–]o from 131.6 to 6.6 mmol l–1 (replaced by sodium gluconate) caused the rate of Cu accumulation to increase 11-fold when Cuo was 800μmol l–1. Application of 0.1 mmol l–1DIDS (normal Cl–o) caused a similar effect. Lowering external pH from 7.4 to pH 5.5 produced a 17-fold, saturable,increase in Cu accumulation rate, which was not explained by increased instantaneous Cu accumulation on/in cells at low pH. We conclude that Cu accumulation by intestinal cells is mainly Na+-insensitive and more characteristic of a pH- and K+-sensitive Ctr1-like pathway than Cu uptake through ENaCs.

Copper homoeostasis in Drosophila melanogaster S2 cells

Copper homoeostasis was investigated in the Drosophila melanogaster S2 cell line to develop an insect model for the study of copper regulation. Real-time PCR studies have demonstrated expression in S2 cells of putative orthologues of human Cu regulatory genes involved in the uptake, transport, sequestration and efflux of Cu. Drosophila orthologues of the mammalian Cu chaperones, ATOX1 (a human orthologue of yeast ATX1), CCS (copper chaperone for superoxide dismutase), COX17 (a human orthologue of yeast COX17), and SCO1 and SCO2, did not significantly respond transcriptionally to increased Cu levels, whereas MtnA, MtnB and MtnD (Drosophila orthologues of human metallothioneins) were up-regulated by Cu in a time- and dose-dependent manner. To examine the effect on Cu homoeostasis, expression of several key copper homoeostasis genes was suppressed using double-stranded RNA interference. Suppression of the MTF-1 (metal-regulatory transcription factor 1), reduced both basal and Cu-induced gene expressions of MtnA, MtnB and MtnD, significantly reducing the tolerance of these cells to increased Cu. Suppression of either Ctr1A (a Drosophila orthologue of yeast CTR1) or Ctr1B significantly reduced Cu uptake from media, demonstrating that both these proteins function to transport Cu into S2 cells. Significantly, Cu induced Ctr1B gene expression, and this could be prevented by suppressing MTF-1, suggesting that Ctr1B might be involved in Cu detoxification. Suppression of DmATP7, the putative homologue of human Cu transporter genes ATP7A and ATP7B, significantly increased Cu accumulation, demonstrating that DmATP7 is essential for efflux of excess Cu. This work is consistent with previous studies in mammalian cells, validating S2 cells as a model system for studying Cu transport and identifying novel Cu regulatory mechanisms.

From aging to virulence: forging connections through the study of copper homeostasis in eukaryotic microorganisms

Recent years have witnessed an explosion in the breadth of investigations on transition metal homeostasis and the subsequent depth of our understanding of metals in biology. Many genes and proteins that serve in the uptake, distribution, sensing and detoxification of one such transition metal, copper, have been identified. Through genetic and biochemical studies, the molecular details of copper uptake are being elucidated, and evidence suggests a largely conserved mechanism for copper acquisition and distribution from yeast to humans. Investigations of the mitochondrial copper pathway reveal the complexity surrounding copper delivery to cytochrome oxidase and highlight additional roles for some of the participants in copper homeostasis, such as a copper chaperone that influences the subcellular distribution of its target for copper incorporation. Furthermore, our understanding of the structure and function of copper transporters, chaperones and cupro-proteins, coupled with the emergence of additional model systems, is providing surprising examples of the integration of copper homeostasis with other physiological and pathophysiological processes and states, such as cancer, aging and virulence.

Cloning, expression pattern and essentiality of the high-affinity copper transporter 1 (ctr1) gene in zebrafish

The high-affinity copper transporter 1 (Ctr1) is a highly conserved transmembrane protein that mediates the internalization of copper ions from the extracellular medium. In this study, we have isolated the zebrafish ctr1 gene. The zebrafish ctr1 cDNA encodes a protein with 69% identity to the human orthologue and shows conservation of specific amino acid residues involved in copper transport. We find only a single ctr1 gene in the zebrafish genome which maps to linkage group 5. The genomic structure of the zebrafish gene shows that it consists of five exons and that exon-intron boundaries are absolutely conserved with the mammalian ctr1 genes. Expression in embryos was analyzed by reverse transcription-polymerase chain reaction (RT-PCR) and by in situ hybridization. Zebrafish ctr1 is maternally loaded, and transcripts can be detected throughout development and in adult fish. Distribution of ctr1 message appears ubiquitous during early stages becoming restricted to the brain and ventral tissues by 24 h post fertilization (hpf). Beginning at 3 days post fertilization (dpf), expression is found mainly in the developing intestine. Specific knockdown of ctr1 by antisense morpholino oligonucleotides (MOs) causes early larval lethality. Defects include cell death in tissues where ctr1 is most heavily expressed, a finding similar to that described for a mouse knockout of mCtr1. Despite the existence of at least one other copper transport mechanism in the fish, our studies show that zebrafish ctr1 is an essential gene for development.

Maintaining copper homeostasis: regulation of copper-trafficking proteins in response to copper deficiency or overload

Copper is an essential micronutrient that plays a vital role as a catalytic co-factor for a variety of metalloenzymes. The redox chemistry of copper also makes it a potentially toxic metal if not properly used. Therefore, elaborate mechanisms have evolved for controlling its cellular uptake, elimination, and distribution. In the last decade, our understanding of the systems involved in maintaining copper homeostasis has improved considerably with the characterization of copper transporters that mediate cellular copper uptake or efflux and with the identification of copper chaperones, a family of proteins required for delivering copper to specific targets in the cell. Despite the distinct roles of these proteins in copper trafficking, all seem able to respond to changes in copper status. Here, we describe recent advances in our knowledge of how copper-trafficking proteins respond to copper deficiency or overload in mammalian cells in order to maintain copper balance.

A copper-regulated transporter required for copper acquisition, pigmentation, and specific stages of development in Drosophila melanogaster

The trace element copper is required for normal growth and development, serving as an essential catalytic co-factor for enzymes involved in energy generation, oxidative stress protection, neuropeptide maturation, and other fundamental processes. In yeast and mammals copper acquisition occurs through the action of the Ctr1 family of high affinity copper transporters. Here we describe studies using Drosophila melanogaster to investigate the role of copper acquisition through Ctr1 in normal growth and development. Three distinct Drosophila Ctr1 genes (Ctr1A, Ctr1B, and Ctr1C) have been identified, which have unique expression patterns over the course of development. Interestingly, Ctr1B, which is expressed exclusively during the late embryonic and larval stages of development, is transcriptionally activated in response to nutritionally induced copper deprivation and down-regulated in response to copper adequacy. The generation of Ctr1B mutant flies results in decreased larval copper accumulation, marked body pigmentation defects that parallel defects in tyrosinase activity, and specific developmental arrest under conditions of both nutritional copper limitation and excess. These studies establish that copper acquisition through the Drosophila Ctr1B transporter is crucial for normal growth and in early and specific stages of metazoan development.

The many highways for intracellular trafficking of metals

Metal ions such as copper and manganese represent a unique problem to living cells in that these ions are not only essential co-factors for metalloproteins, but are also potentially toxic. To aid in the homeostatic balance of essential but toxic metals, cells have evolved with a complex network of metal trafficking pathways. The object of such pathways is two-fold: to prevent accumulation of the metal in the freely reactive form (metal detoxification pathways) and to ensure proper delivery of the ion to target metalloproteins (metal utilization pathways). Much of what we currently know regarding these complex pathways of metal trafficking has emerged from molecular genetic studies in baker's yeast, Saccharomyces cerevisiae. In this review, we shall briefly highlight the current understanding of factors that function in the trafficking and handling of copper, including copper detoxification factors, copper transporters and copper chaperones. In addition, very recent findings on the players involved in manganese trafficking will be presented. The goal is to provide a paradigm for the intracellular handling of metals that may be applied in a more general sense to metals that serve essential functions in biology.

Copper modulates the degradation of copper chaperone for Cu,Zn superoxide dismutase by the 26 S proteosome

Copper chaperones are copper-binding proteins that directly insert copper into specific targets, preventing the accumulation of free copper ions that can be toxic to the cell. Despite considerable advances in the understanding of copper transfer from copper chaperones to their target, to date, there is no information regarding how the activity of these proteins is regulated in higher eukaryotes. The insertion of copper into the antioxidant enzyme Cu,Zn superoxide dismutase (SOD1) depends on the copper chaperone for SOD1 (CCS). We have recently reported that CCS protein is increased in tissues of rats fed copper-deficient diets suggesting that copper may regulate CCS expression. Here we show that whereas copper deficiency increased CCS protein in rats, mRNA level was unaffected. Rodent and human cell lines cultured in the presence of the specific copper chelator 2,3,2-tetraamine displayed a dose-dependent increase in CCS protein that could be reversed with the addition of copper but not iron or zinc to the cells. Switching cells from copper-deficient to copper-rich medium promoted the rapid degradation of CCS, which could be blocked by the proteosome inhibitors MG132 and lactacystin but not a cysteine protease inhibitor or inhibitors of the lysosomal degradation pathway. In addition, CCS degradation was slower in copper-deficient cells than in cells cultured in copper-rich medium. Together, these data show that copper regulates CCS expression by modulating its degradation by the 26 S proteosome and suggest a novel role for CCS in prioritizing the utilization of copper when it is scarce.

The copper export pump ATP7B modulates the cellular pharmacology of carboplatin in ovarian carcinoma cells

Human tumor cells lines with acquired resistance to cisplatin (DDP) and carboplatin (CBDCA) are often cross-resistant to copper and vice versa, and some DDP-resistant cells overexpress the copper export pump ATP7B. We sought to demonstrate that ATP7B directly mediates resistance to DDP and CBDCA by stably transfecting human carcinoma cells with a vector designed to express ATP7B. Increased expression of ATP7B rendered all three cell lines tested more resistant to a 1-h exposure to DDP (1.6-2.6-fold), CBDCA (1.5-1.6-fold), and copper (1.2-1.4-fold). The effect of ATP7B on the cellular pharmacology of 64Cu and [14C]CBDCA was investigated in more detail using one cell pair (2008 cells transfected with an empty vector or an ATP7B-expressing vector). In the 2008/ATP7B subline, steady-state copper levels were decreased under both basal and copper-supplemented conditions, as was steady-state CBDCA content upon exposure to 50 microM [14C]CBDCA. Over the first 5 min, the average rate of accumulation of copper and CBCDA in the 2008/ATP7B cells was reduced by 37 and 61%, respectively. Efflux was more rapid from 2008/ATP7B cells for both copper and CBDCA. Two-compartment modeling indicated that the second phase of efflux was increased by a factor of 3.9-fold for CBCDA and to an even greater extent for copper. We conclude that expression of ATP7B regulates sensitivity to CBDCA as well as to DDP and copper and that a transporter that normally mediates copper homeostasis modulates the cellular pharmacology of CBDCA.

Regulation of copper absorption by copper availability in the Caco-2 cell intestinal model

Relatively little is known about the individual steps in intestinal copper absorption and whether or how they may be regulated. Polarized Caco-2 cell monolayers with tight junctions offer an already tested model in which to study intestinal metal transport. This model was used to examine potential effects of cellular copper availability on copper absorption. Uptake and transport were determined on application of (64)Cu(II) to the brush border. In the range of 0.2-2 micro M, uptake was dose dependent and was approximately 20% of dose/90 min. Overall transport of (64)Cu across the basolateral surface was approximately 0.3%. When cellular copper levels were depleted 40% by 18-h pretreatment with the specific copper chelator triethylenetetraamine, uptake and overall transport were markedly increased, going to 80 and 65% of dose, respectively. Cellular retention of (64)Cu fell fourfold, from 6 to 1.5%. Depletion of copper with the chelator was rapid and preceded initial changes in uptake and overall transport by 4 h. A lesser depletion of cellular copper (13%) failed to enhance copper uptake but doubled the rate of overall transport, as measured with (64)Cu and by atomic absorption. As previously reported, preexposure of the cells to excess copper (10 micro M, 18 h) also enhanced copper uptake ( approximately 3-fold). In contrast, ascorbate (10-1,000 micro M) failed to significantly alter uptake and transport of 1 micro M (64)Cu. Our findings are consistent with the concepts that, in the low physiological range, copper availability alters the absorption capacity of the intestine to support whole body homeostasis and that basolateral transport is more sensitively regulated than uptake.

Copper-stimulated endocytosis and degradation of the human copper transporter, hCtr1

Copper uptake at the plasma membrane and subsequent delivery to copper-dependent enzymes is essential for many cellular processes, including mitochondrial oxidative phosphorylation, free radical detoxification, pigmentation, neurotransmitter synthesis, and iron metabolism. However, intracellular levels of this nutrient must be controlled because it is potentially toxic in excess concentrations. The hCtr1 protein functions in high affinity copper uptake at the plasma membrane of human cells. In this study, we demonstrate that levels of the hCtr1 protein at the plasma membrane of HEK293 cells were reduced when cells were exposed to elevated copper. This decrease in surface hCtr1 levels was associated with an increased rate of endocytosis, and low micromolar concentrations of copper were sufficient to stimulate this process. Inhibitors of clathrin-dependent endocytosis prevented the trafficking of hCtr1 from the plasma membrane, and newly internalized hCtr1 and transferrin were co-localized. Significantly, elevated copper concentrations also resulted in the degradation of the hCtr1 protein. Our findings suggest that hCtr1-mediated copper uptake into mammalian cells is regulated by a post-translational mechanism involving copper-stimulated endocytosis and degradation of the transporter.