Chemical background of silver nanoparticles interfering with mammalian copper metabolism

The rapidly increasing application of silver nanoparticles (AgNPs) boosts their release into the environment, which raises a reasonable alarm for ecologists and health specialists. This is manifested as increased research devoted to the influence of AgNPs on physiological and cellular processes in various model systems, including mammals. The topic of the present paper is the ability of silver to interfere with copper metabolism, the potential health effects of this interference, and the danger of low silver concentrations to humans. The chemical properties of ionic and nanoparticle silver, supporting the possibility of silver release by AgNPs in extracellular and intracellular compartments of mammals, are discussed. The possibility of justified use of silver for the treatment of some severe diseases, including tumors and viral infections, based on the specific molecular mechanisms of the decrease in copper status by silver ions released from AgNPs is also discussed.

Properties of recombinant extracellular N-terminal domain of human high-affinity copper transporter 1 (hNdCTR1) and its interactions with Cu(II) and Ag(I) ions

High-affinity copper transporter 1 (CTR1) is a key link in the transfer of copper (Cu) from the extracellular environment to the cell. Violation in the control system of its expression, or mutations in this gene, cause a global copper imbalance. However, the mechanism of copper transfer via CTR1 remains unclear. It has been shown that transformed bacteria synthesizing the fused GB1-NdCTR become resistant to toxic silver ions. According to UV-Vis spectrophotometry and isothermal titration calorimetry, electrophoretically pure GB1-NdCTR specifically and reversibly binds copper and silver ions, and binding is associated with aggregation. Purified NdCTR1 forms SDS-resistant oligomers. The link between nontrivial properties of NdCTR1 and copper import mechanism from extracellular space, as well as potential chelating properties of NdCTR1, are discussed.

Aggregation Properties of Albumin in Interacting with Magnetic Fluids

In this study, interactions of Fe3O4 magnetic nanoparticles with serum albumin biomolecules in aqueous solutions were considered. The studies were conducted with the laser correlation spectroscopy and optical analysis of dehydrated films. It was shown that the addition of magnetite to an albumin solution at low concentrations of up to 10-6 g/L led to the formation of aggregates with sizes of up to 300 nm in the liquid phase and an increase in the number of spiral structures in the dehydrated films, which indicated an increase in their stability. With a further increase in the magnetite concentration in the solution (from 10-4 g/L), the magnetic particles stuck together and to albumin, thus forming aggregates with sizes larger than 1000 nm. At the same time, the formation of morphological structures in molecular films was disturbed, and a characteristic decrease in their stability occurred. Most stable films were formed at low concentrations of magnetic nanoparticles (less than 10-4 g/L) when small albumin-magnetic nanoparticle aggregates were formed. These results are important for characterizing the interaction processes of biomolecules with magnetic nanoparticles and can be useful for predicting the stability of biomolecular films with the inclusion of magnetite particles.

Size-Dependent Bioactivity of Silver Nanoparticles: Antibacterial Properties, Influence on Copper Status in Mice, and Whole-Body Turnover

Purpose

The ability of silver nanoparticles (AgNPs) of different sizes to influence copper metabolism in mice is assessed.

Materials and Methods

AgNPs with diameters of 10, 20, and 75 nm were fabricated through a chemical reduction of silver nitrate and characterized by UV/Vis spectrometry, transmission and scanning electronic microscopy, and laser diffractometry. To test their bioactivity, Escherichia coli cells, cultured A549 cells, and C57Bl/6 mice were used. The antibacterial activity of AgNPs was determined by inhibition of colony-forming ability, and cytotoxicity was tested using the MTT test (viability, %). Ceruloplasmin (Cp, the major mammalian extracellular copper-containing protein) concentration and enzymatic activity were measured using gel-assay analyses and WB, respectively. In vitro binding of AgNPs with serum proteins was monitored with UV/Vis spectroscopy. Metal concentrations were measured using atomic absorption spectrometry.

Results

The smallest AgNPs displayed the largest dose- and time-dependent antibacterial activity. All nanoparticles inhibited the metabolic activity of A549 cells in accordance with dose and time, but no correlation between cytotoxicity and nanoparticle size was found. Nanosilver was not uniformly distributed through the body of mice intraperitoneally treated with low AgNP concentrations. It was predominantly accumulated in liver. There, nanosilver was included in ceruloplasmin, and Ag-ceruloplasmin with low oxidase activity level was formed. Larger nanoparticles more effectively interfered with the copper metabolism of mice. Large AgNPs quickly induced a drop of blood serum oxidase activity to practically zero, but after cancellation of AgNP treatment, the activity was rapidly restored. A major fraction of the nanosilver was excreted in the bile with Cp. Nanosilver was bound by alpha-2-macroglobulin in vitro and in vivo, but silver did not substitute for the copper atoms of Cp in vitro.

Conclusion

The data showed that even at low concentrations, AgNPs influence murine copper metabolism in size-dependent manner. This property negatively correlated with the antibacterial activity of AgNPs.

The effects of silver ions on copper metabolism in rats

The influence of short and prolonged diet containing silver ions (Ag-diet) on copper metabolism was studied. Two groups of animals were used: one group of adult rats received a Ag-diet for one month (Ag-A1) and another group received a Ag-diet for 6 months from birth (Ag-N6). In Ag-A1 rats, the Ag-diet caused a dramatic decrease of copper status indexes that was manifested as ceruloplasmin-associated copper deficiency. In Ag-N6 rats, copper status indexes decreased only 2-fold as compared to control rats. In rats of both groups, silver entered the bloodstream and accumulated in the liver. Silver was incorporated into ceruloplasmin (Cp), but not SOD1. In the liver, a prolonged Ag-diet caused a decrease of the expression level of genes, associated with copper metabolism. Comparative spectrophotometric analysis of partially purified Cp fractions has shown that Cp from Ag-N6 rats was closer to holo-Cp by specific enzymatic activities and tertiary structure than Cp from Ag-A1 rats. However, Cp of Ag-N6 differs from control holo-Cp and Cp of Ag-A1 in its affinity to DEAE-Sepharose and in its binding properties to lectins. In the bloodstream of Ag-N6, two Cp forms are present as shown in pulse-experiments on rats with the liver isolated from circulation. One of the Cp isoforms is of hepatic origin, and the other is of extrahepatic origin; the latter is characterized by a faster rate of secretion than hepatic Cp. These data allowed us to suggest that the disturbance of holo-Cp formation in the liver was compensated by induction of extrahepatic Cp synthesis. The possible biological importance of these effects is discussed.

Non-hepatic tumors change the activity of genes encoding copper trafficking proteins in the liver

To assess the statistical relationship between tumor growth and copper metabolism, we performed a metaanalysis of studies in which patients with neoplasms were characterized according to any of the copper status indexes (atomic copper serum concentration, serum oxidase activity, ceruloplasmin protein content). Our metaanalysis shows that in the majority of cases (more than 3100 patients), tumor growth positively correlates with the copper status indexes. Nude athymic CD-1 nu/nu mice with subcutaneous tumors of human origin, C57Bl/6J mice with murine melanoma and Apc(Min) mice with spontaneously developing adenomas throughout the intestinal tract were studied to experimentally determine the relationship between tumor progression, liver copper metabolism, and copper status indexes. We showed that the copper status indexes increased significantly during tumor growth. In the liver tissue of tumor-bearing mice, ceruloplasmin gene expression, as well as the expression of genes related to ceruloplasmin metallation (CTR1 and ATP7B), increased significantly. Moreover, the presence of an mRNA splice variant encoding a form of ceruloplasmin anchored to the plasma membrane by glycosylphosphatidyl inositol, which is atypical for hepatocytes, was also detected. The ATP7A copper transporter gene, which is normally expressed in the liver only during embryonic copper metabolism, was also activated. Depletion of holo-ceruloplasmin resulted in retardation of human HCT116 colon carcinoma cell growth in nude mice and induced DNA fragmentation in tumor cells. In addition, the concentration of cytochrome c increased significantly in the cytosol, while decreasing in the mitochondria. We discuss a possible trans-effect of developing tumors on copper metabolism in the liver.

Serum depletion of holo-ceruloplasmin induced by silver ions in vivo reduces uptake of cisplatin

There is an emerging link between extracellular copper concentration and the uptake of cisplatin mediated by copper transporter CTR1 in cell cultures and unicellular eukaryotes. To test the link between extracellular copper level and cisplatin uptake by organs in vivo we used mice with low copper status parameters induced by AgCl-containing diet (Ag-mice). In Ag-mice, serum copper status and liver copper metabolism were characterized. It was shown that the expression level of copper transporter genes and activity of ubiquitous intracellular cuproenzymes were not affected but the level of serum holo-ceruloplasmin was not detectable. Silver was selectively absorbed by liver and accumulated in the mitochondrial matrix. Silver was present in an exchangeable form and was excreted through bile. Ag-mice model is characterized by high reproducibility, reversibility, synchronicity, and definiteness of ceruloplasmin-associated copper deficiency. After cisplatin treatment Ag-mice, as compared to control mice, demonstrated the delay in platinum uptake by organs during first 30 min. This effect was not observed at later time points probably due to cisplatin induced copper release to blood, which resulted in the recovery of copper status. These data allowed us to conclude that cisplatin uptake was coupled to copper transport in vivo.

[Structure-functional organization of eukaryotic high-affinity copper importer CTR1 determines its ability to transport copper, silver and cisplatin]

It was shown recently, that high affinity Cu(I) importer eukaryotic protein CTR1 can also transport in vitro abiogenic Ag(I) ions and anticancer drug cisplatin. At present there is no rational explanation how CTR1 can transfer platinum group, which is different by coordination properties from highly similar Cu(I) and Ag(I). To understand this phenomenon we analyzed 25 sequences of chordate CTR1 proteins, and found out conserved patterns of organization of N-terminal extracellular part of CTR1 which correspond to initial metal binding. Extracellular copper-binding motifs were qualified by their coordination properties. It was shown that relative position of Met- and His-rich copper-binding motifs in CTR1 predisposes the extracellular CTR1 part to binding of copper, silver and cisplatin. Relation between tissue-specific expression of CTR1 gene, steady-state copper concentration, and silver and platinum accumulation in organs of mice in vivo was analyzed. Significant positive but incomplete correlation exists between these variables. Basing on structural and functional peculiarities of N-terminal part of CTR1 a hypothesis of coupled transport of copper and cisplatin has been suggested, which avoids the disagreement between CTR1-mediated cisplatin transport in vitro, and irreversible binding of platinum to Met-rich peptides.

[Characteristics of rat ceruloplasmin from the serum of animals, which received salts of silver with food]

Abiogenic Ag(I) ions have electronic structure, similar to Cu(I) ions and can compete with Cu(I) for binding sites of proteins which transport copper from extracellular media to sites of cuproenzyme formation in the cell. Rodents receiving Ag-salts with food develop extracellular deficiency of copper associated with ceruloplasmin (Cp, the major copper-transporting protein in blood serum of vertebrates). The present work focuses on the studies of biochemical and physicochemical properties of Cp, obtained from blood serum of rats, which received AgCl with food for 4 weeks (Ag-rats). Cp-fractions from blood serum of Ag-rats (Ag-Cp) were obtained by ion-exchange chromatography with stepped gradient of NaCl. Each fraction was tested for oxidase and ferroxidase activities by direct measurement of catalytic activity in the gel, and for specific activity in holo-Cp in oxidation of chromogenic substrate. Molecular mass, electrophoretic mobility and ratio of apo- and holo-forms in Ag-Cp fractions were evaluated by immunoblotting. Ag-Cp samples did not contain products of spontaneous partial proteolytic degradation, characteristic of holo-Cp samples. Fractions of Ag-Cp and holo-Cp (from blood serum of control rats) were compared by optical spectra, tertiary structure, susceptibility to thermal denaturation, and by atomic Cu and Ag content. Ag-Cp contained 1-2% Cp, which is similar by spectral and catalytic properties with holo-Cp. [Ag]:[Cu] ration in Ag-Cp samples was about 4:1. As evidenced by circular dichroism and differential scanning calorimetric studies, the major apo-fraction of Ag-Cp lacked tertiary structure of native Cp and was significantly misfolded, which might explain its resistance to spontaneous partial proteolytic degradation.

[Efficient method of analysis of optical spectra from kinetic studies]

The application of principal components for the analysis of kinetic data obtained by optical spectroscopy is described. The use of singular value decomposition (SVD) for stable and reproducible generation of principal components, details of realization, advantages and drawbacks of the method are discussed. The described method with minor modifications may be used in a wide variety of UV-spectroscopy applications in molecular biology and biophysics. The developed method was applied to study the reaction of platinum anticancer drug, cisplatin, with DNA and methionine. Use of sensitive UV-spectroscopy allowed to study low platinum concentrations, typical for biological systems. It has been shown, that reactions of cisplatin with DNA and L-methionine generally follow the same pathway both at high and low concentrations.

[Relations between CTR1 gene activity and copper status in different rat organs]

CTR1 gene (SLC31A1 according to Entrez data base) product is the main candidate for the role of eukaryotic copper importer, whose tissue-specific function is still unclear. In this research steady state CTR1-mRNA level was measured with semiquantitative RT-PCR analysis and compared with copper status in rat organs, in which copper metabolism is changed during development (liver, cerebellum, choroid plexus and mammary gland). It has been shown that CTR1 gene activity correlates with the rate of both intracellular and extracellular copper-containing enzymes formation. In mesenchymal origin cells of newborns the CTR1 gene activity decreases when high copper concentrations in cell nucleus is reached. According to phylogenetic analysis CTR1 has the most conservative transmembrane domains 2 and 3 (TMD), containing 7 amino acid residues able to coordinate copper atom. A model of cuprophylic channel has been proposed, which is formed by TMD2 and TMD3 in homotrimeric CTR1 complex. In this model copper is transported through the channel to cytosolic C-terminal motif His-Cys-His, which ability to coordinate Cu(I) was assessed by molecular modeling (MM+, ZINDO/1). Theoretical possibility of copper transfer from His-Cys-His CTR1 C-terminal motif to cytosolic Cys-X-X-Cys Cu(I) chaperon sites has been shown. The role of CTR1 in copper metabolism as copper donor and acceptor is discussed.

[HMG1 domains: the victims of circumstance]

The method of circular dichroism (CD) was used to compare DNA behavior during its interaction with linker histone H1 and with non-histone chromosomal protein HMG1 at different ionic strength and at different protein content in the system. The role of negatively charged C-terminal fragment of HMG1 was analyzed using recombinant protein HMG1-(A + B), which lacks the C terminal amino acid sequence. The psi-type CD spectra were common for DNA interaction with histone H1, but no spectra of this type were observed in HMG1-DNA systems even at high ionic strength. The CD spectrum of the truncated recombinant protein at high salt concentration somewhat resembled the psi-type spectrum. Two very intense positive bands were located near 215 nm and near 273 nm, and the whole CD spectrum was positive. The role of C-terminal tail of HMG1 in formation of the ordered DNA-protein complexes is discussed.