Effects of copper on rat hepatoma HTC cells and primary cultured rat hepatocytes

Effects of copper and zinc on proteoglycan metabolism in articular cartilage

Co-Cultures of porcine articular cartilage and synovium or synovial conditioned medium were used as an in vitro model to mimic inflammatory events at the cartilage/synovial junction in degenerative joint disease. This model provides a useful tool to assess the anti-inflammatory and antiarthritic properties of pharmacological agents. In this study the effects of copper and zinc on (i) PG synthesis by cartilage and (ii) synovial-induced PG depletion have been investigated. Copper sulphate at a concentration of 0.01 mM did not stimulate PG synthesis significantly in cultured cartilage explants but completely abrogated the inhibitory effects of synovial tissue in co-culture experiments. This finding was supported by the histological demonstration of copper-dependent reversal of the PG depletion in cartilage exposed to synovial conditioned medium. Zinc sulphate at 0.01 mM had no effect on PG synthesis and was unable to protect cartilage against synovialinduced PG depletion. These results reveal possible mechanisms by which copper exerts its anti-inflammatory and anti-arthritic actions.

Effects of copper nanoparticles on rat cerebral microvessel endothelial cells

AIM

The purpose of the current study was to determine whether copper nanoparticles (Cu-NPs) can induce the release of proinflammatory mediators that influence the restrictive characteristics of the blood-brain barrier.

MATERIAL & METHODS

Confluent rat brain microvessel endothelial cells (rBMECs) were treated with well-characterized Cu-NPs (40 or 60 nm). Cytotoxicity of the Cu-NPs was evaluated by cell proliferation assay (1.5-50 µg/ml). The extracellular concentrations of proinflammatory mediators (IL-1β, IL-2, TNF-α and prostaglandin E(2)) were evaluated by ELISA.

RESULTS

The exposure of Cu-NPs at low concentrations increases cellular proliferation of rBMECs, by contrast, high concentrations induce toxicity. Prostaglandin E(2) release was significantly increased (threefold; 8 h) for Cu-NPs (40 and 60 nm). The extracellular levels of both TNF-α and IL-1β were significantly elevated following exposure to Cu-NPs. The P-apparent ratio, as an indicator of increased permeability of rBMEC was approximately twofold for Cu-NPs (40 and 60 nm).

CONCLUSION

These data suggest that Cu-NPs can induce rBMEC, proliferation at low concentrations and/or induce blood-brain barrier toxicity and potential neurotoxicity at high concentrations.

Copper inhibits P2Y(2)-dependent Ca(2+) signaling through the effects on thapsigargin-sensitive Ca(2+) stores in HTC hepatoma cells

Purinergic P2Y(2) G-protein coupled receptors play a key role in the regulation of hepatic Ca(2+) signaling by extracellular ATP. The concentration of copper in serum is about 20muM. Since copper accumulates in the liver in certain disease states, the purpose of these studies was to assess the effects of copper on P2Y(2) receptors in a model liver cell line. Exposure to a P2Y(2) agonist UTP increased [Ca(2+)](i) by stimulating Ca(2+) release from thapsigargin-sensitive Ca(2+) stores. Pretreatment of HTC cells for several minutes with copper did not affect cell viability, but potently inhibited increases in [Ca(2+)](i) evoked by UTP and thapsigargin. During this pretreatment, copper was not transported into the cytosol, and inhibited P2Y(2) receptors in a concentration-dependent manner with the IC(50) of about 15muM. These results suggest that copper inhibits P2Y(2) receptors through the effects on thapsigargin-sensitive Ca(2+) stores by acting from an extracellular side. Further experiments indicated that these effect of copper may lead to inhibition of regulatory volume decrease (RVD) evoked by hypotonic solution. Thus, copper may contribute to defective regulation of purinergic signaling and liver cell volume in diseases associated with the increased serum copper concentration.

Effect of Copper Sulfate on Performance of a Serum-Free CHO Cell Culture Process and the Level of Free Thiol in the Recombinant Antibody Expressed

A recombinant Chinese hamster ovary (CHO) cell line was used to express a humanized antibody. Product quality analysis of this humanized antibody showed the presence of free thiol, due to unpaired cysteine residues in the Fab region. Decreased potency of this thiol Fab made it critical to minimize the levels of free thiol. In an effort to do this, we evaluated the effect of copper sulfate addition to the cell culture production medium. As a component of the production medium, copper sulfate can act as an oxidizing agent, thereby facilitating disulfide bond formation. Four concentrations of copper sulfate were added at the beginning of 2-L benchtop production cultures of the recombinant CHO cell line: 0, 5, 50, and 100 microM. We found that these copper sulfate additions had no effect on cell growth or antibody production. However, a slight dose-dependent depression in culture viability was observed. Analysis of the purified antibody showed that either the 50 or 100 microM copper sulfate additions reduced the level of free thiol by more than 10-fold.

In vivo cytokine secretion and NF-kappaB activation around titanium and copper implants

The early biological response at titanium (Ti), copper (Cu)-coated Ti and sham sites was evaluated in an in vivo rat model. Material surface chemical and topographical properties were characterized using Auger electron spectroscopy, energy dispersive X-ray spectroscopy and interferometry, respectively. The number of leukocytes, cell types and cell viability (release of lactate dehydrogenase) were determined in the implant-interface exudate. The contents of activated nuclear transcription factor NF-kappaB, interleukin-6 (IL-6) and interleukin-10 (IL-10) were determined by enzyme linked immunosorbent assay. An increase in the number of leukocytes, in particular, polymorphonuclear leukocytes, was observed between 12 and 48 h around Cu. A marked decrease of exudate cell viability was found around Cu after 48 h. The total amounts of activated NF-kappaB after 12 h was highest in Ti exudates whereas after 48 h the highest amount of NF-kappaB was detected around Cu. The levels of cytokine IL-6 were consistently high around Cu at both time periods. No differences in IL-10 contents were detected, irrespective of material/sham and time. The results show that materials with different toxicity grades (titanium with low and copper with high toxicity) exhibit early differences in the activation of NF-kappaB, extracellular expression and secretion of mediators, causing major differences in inflammatory cell accumulation and death in vivo.

Metallothionein is crucial for safe intracellular copper storage and cell survival at normal and supra-physiological exposure levels

MTs (metallothioneins) increase the resistance of cells to exposure to high Cu (copper) levels. Characterization of the MT-Cu complex suggests that MT has an important role in the cellular storage and/or delivery of Cu ions to cuproenzymes. In this work we investigate how these properties contribute to Cu homoeostasis by evaluating the uptake, accumulation and efflux of Cu in wild-type and MT I/II null rat fibroblast cell lines. We also assessed changes in the expression of Cu metabolism-related genes in response to Cu exposure. At sub-physiological Cu levels (0.4 microM), the metal content was not dependent on MT; however, when extracellular Cu was increased to physiological levels (10 microM), MTs were required for the cell's ability to accumulate the metal. The subcellular localization of the accumulated metal in the cytoplasm was MT-dependent. Following supra-physiological Cu exposure (>50 microM), MT null cells had a decreased capacity for Cu storage and an elevated sensitivity to a minor increment in intracellular metal levels, suggesting that intracellular Cu toxicity is due not to the metal content but to the interactions of the metal with cellular components. Moreover, MT null cells failed to show increased levels of mRNAs encoding MT I, SOD1 (superoxide dismutase 1) and Ccs1 (Cu chaperone for SOD) in response to Cu exposure. These results support a role for MT in the storage of Cu in a safe compartment and in sequestering an intracellular excess of Cu in response to supra-physiological Cu exposure. Gene expression analysis suggests the necessity of having MT as part of the signalling pathway that induces gene expression in response to Cu.

IL-1alpha, IL-1beta and TNF-alpha secretion during in vivo/ex vivo cellular interactions with titanium and copper

Titanium (Ti) and copper (Cu) were used to evaluate cytokine secretion around materials with different chemical properties. Ti disks were coated with Cu or left uncoated. The disks were inserted subcutaneously in rats for 1, 3, 12, 18, 24 and 48 h. Interleukin-1alpha (IL-1alpha), IL-1beta and tumor necrosis factor-alpha (TNF-alpha) concentrations were measured in vivo around the materials, in sham operated sites, and after ex vivo incubation of surface adherent cells. Ti and Cu revealed distinct cytokine expression patterns. Cu recruited cells showed higher and prolonged release of IL-1alpha than Ti at longer times (>24 h), whereas Ti exhibited a transient IL-1alpha response at earlier periods (<24 h). An early enhanced secretion of TNF-alpha characterized Ti. Low amounts of IL-1beta were found around both materials. Sham site recruited cells produced lower levels of cytokines. The results after ex vivo incubations were similar to those in vivo. This study shows that material chemical properties influence early cytokine production. The Ti-associated transient rise of IL-1alpha and TNF-alpha may be of importance for the early tissue response around biocompatible materials, while a delayed high IL-1alpha expression could be a marker of inflammation induced by toxic materials.

Contribution of glutathione and metallothioneins to protection against copper toxicity and redox cycling: quantitative analysis using MT+/+ and MT-/- mouse lung fibroblast cells

Glutathione (GSH) and metallothioneins (MT) are believed to play important roles in protecting cells against high copper (Cu) concentrations. Little is known, however, about their specific intracellular interactions and the coordination of protective functions. We investigated contributions of GSH and MT to protection against Cu toxicity in fibroblasts derived from wild-type (MT+/+) and knockout (MT-/-) mice that were challenged with cupric nitrilotriacetate (Cu-NTA). Endogenous levels of GSH and MT were manipulated using an inhibitor of gamma-glutamylcysteine synthetase, buthionine sulfoximine (BSO, 5 microM), as GSH depletor and ZnCl(2) (100 microM) as inducer of MT expression. BSO pretreatment markedly decreased cellular GSH levels in MT+/+ and MT-/- cells, by 65% and 70%, respectively, which resulted in Cu cytotoxicity accompanied by its elevated redox-cycling activity and enhanced Cu-induced membrane phospholipid peroxidation. BSO-pretreated MT-/- cells were markedly more sensitive to Cu despite the fact that the residual levels of GSH were similar in both BSO-pretreated MT+/+ and MT-/- cells. Zn pretreatment resulted in more than 10-fold induction of MT in MT+/+ cells but not in MT-/- cells. Accordingly, Zn pretreatment afforded significant protection of MT+/+ cells against Cu cytotoxicity, likely associated with MT-dependent suppression of Cu redox-cycling activity and phospholipid peroxidation, but it exerted no protection in MT-/- cells (as compared to naive cells). To determine whether MT functions specifically in Cu regulation or rather acts as a nonspecific Cu-binding cysteine-rich nucleophile, experiments were performed using MT+/+ and MT-/- cells pretreated with both BSO and Zn. BSO pretreatment did not affect Zn-induced MT expression in MT+/+ cells. As compared with BSO pretreatment alone, exposure to Cu of MT+/+ cells after Zn/BSO pretreatment resulted in the following: (i) a significantly higher viability; (ii) attenuated Cu-dependent redox-cycling activity; and (iii) a lower level of phospholipid peroxidation. In BSO/Zn-pretreated MT-/- cells, the redox-cycling activity of Cu and the level of phospholipid peroxidation remained remarkably higher than in naive cells and were not significantly different from those in cells pretreated with BSO alone. Cu-induced toxicity was remarkably higher in BSO/Zn-pretreated MT-/- cells than in naive or Zn-pretreated cells, although slightly lower than in the MT-/- cells pretreated with BSO alone.

Impact of copper on biomonitoring enzyme ethoxyresorufin-o-deethylase in cultured catfish hepatocytes

The enzyme ethoxyresorufin-o-deethylase (EROD) of the cytochrome P4501A family (CYP1A) in fish liver is increasingly being used as a molecular marker for qualitative and quantitative estimation of aquatic pollution throughout the world. The regulation and expression of this enzyme protein is very important from the toxicological point of view. The regulation of gene expression for this enzyme is mediated by the aromatic hydrocarbon receptor. In addition, cellular glutathione status influences expression of CYP1A. In this study, we explored the relationships among glutathione, EROD, and copper in cultured hepatocytes from Indian catfish. EROD activity in cultured hepatocytes was induced by carbofuran (CF), a widely used agricultural pesticide, and by beta-napthoflavone (BNF), a known inducer of CYP1A. Addition of copper into the culture media of hepatocytes inhibited EROD activity significantly. The activity of EROD elevated by CF and BNF was inhibited in hepatocytes pretreated with CF and BNF exposed to CuSO4. This effect was reflected in the glutathione status of the cells. The level of glutathione was increased by 3.4 and 3.0 times in hepatocytes treated with CF and BNF, respectively. These levels were inhibited in hepatocytes exposed to CuSO(4). Thus, copper interactions with glutathione may play a role in regulating EROD in hepatocytes.

Cellular copper transport and metabolism

The transport and cellular metabolism of Cu depends on a series of membrane proteins and smaller soluble peptides that comprise a functionally integrated system for maintaining cellular Cu homeostasis. Inward transport across the plasma membrane appears to be a function of integral membrane proteins that form the channels that select Cu ions for passage. Two membrane-bound Cu-transporting ATPase enzymes, ATP7A and ATP7B, the products of the Menkes and Wilson disease genes, respectively, catalyze an ATP-dependent transfer of Cu to intracellular compartments or expel Cu from the cell. ATP7A and ATP7B work in concert with a series of smaller peptides, the copper chaperones, that exchange Cu at the ATPase sites or incorporate the Cu directly into the structure of Cu-dependent enzymes such as cytochrome c oxidase and Cu, Zn superoxide dismutase. These mechanisms come into play in response to a high influx of Cu or during the course of normal Cu metabolism.

Hormesis: a stress response in cells exposed to low levels of heavy metals

Cytotoxicity studies using a 3-(4,5 dimethylthiazol-2-yl)-2,5 diphenyl tetrazolium bromide (MTT)-based in vitro toxicity assay revealed that McCoy cells exposed to low concentrations of mercuric (0.7 microM), cadmium (1 microM) and cupric chloride (3 microM) exhibited significant increases in cellular activity. This increased activity, previously termed hormesis, coincided with the production of high levels of the stress proteins, heat shock protein 70 (Hsp 70) and metallothionein, while the high constitutive expression of these proteins in cadmium-resistant mutant (CRM) cells corresponded to constitutive hormetic activity. Hormesis was found to obey uniform kinetics allowing for a mathematical description of this increased activity. These results suggest that hormetic activity is a specific cellular response, and most likely, a stress response to low but harmful levels of toxic agents and may therefore provide a rapid test for the presence of toxicants at concentrations associated with chronic toxicity.

Regulation of mitochondrial cytochrome b mRNA by copper in cultured human hepatoma cells and rat liver

Copper overload and deficiency are known to cause morphological and functional mitochondrial abnormalities. The reverse transcriptase-polymerase chain reaction (RT-PCR)-based method of differential display of mRNA was used to identify genes with altered expression in cultured human hepatoma cells (Hep G2) exposed to increasing concentrations of copper (0-100 microM, 24 h). Copper regulation of a cloned PCR product, identified as the gene for the mitochondrially encoded cytochrome b, was confirmed by Northern analysis and in situ hybridization. Copper toxicity increased cytochrome b mRNA abundance up to 3.6-fold, and copper chelation reduced it by 50%. Hepatic cytochrome b mRNA was also increased in rats fed a high-copper diet. Thapsigargin treatment resulted in a significant increase in cytochrome b mRNA, suggesting that an increase in intracellular calcium may be involved in the mechanism of copper action. Furthermore, although cyclohexamide (CHX) alone did not increase cytochrome b mRNA, the addition of CHX and copper resulted in a sixfold increase. These data suggest a role for cytochrome b in the response to increases or decreases in hepatic copper.

Thiol and redox reactive agents exert different effects on glutathione metabolism in HeLa cell cultures

Glutathione protects cells against oxidative damage, free radical damage and other types of toxicity. The aim of the present study was to investigate the impact on glutathione metabolism exerted by different thiol or redox reactive agents. Intracellular concentrations of glutathione in HeLa cell cultures were lowered after addition of agents mainly exerting oxidative stress (homocysteine and hydrogen peroxide), whereas thiol reactive oxidative agents (mercury ions, copper ions and hydroquinone) in concentrations not affecting cell growth seemed to stimulate the production of glutathione. Possibly, the thiol reactive agents decrease the concentration of glutathione, thereby stimulating further synthesis of glutathione, since glutathione synthesis is subject to feedback regulation by glutathione on gamma-glutamylcysteine synthase. Redox changes after addition of thiol and redox reactive agents were also investigated. Copper ions lowered the concentrations of reduced forms of all extracellular thiols and of intracellular reduced cysteine in HeLa cell cultures. The addition of mercury ions, hydroquinone, homocysteine or hydrogen peroxide did not change the proportions between reduced and total thiol concentrations. After addition of buthionine sulfoxime, the total concentrations of intra- and extracellular glutathione were markedly decreased and the ratio between reduced and total glutathione concentrations was lowered. However, both cysteine and homocysteine exhibited normal ratios between the concentrations of reduced and total thiols in the presence of buthionine sulfoxime. This finding could be due to other cellular antioxidants, such as thioredoxin, ascorbic acid or tocopherols, maintaining redox status of these thiols.

Exacerbation of copper toxicity in primary neuronal cultures depleted of cellular glutathione

Perturbations to glutathione (GSH) metabolism may play an important role in neurodegenerative disorders such as Alzheimer's, Parkinson's, and prion diseases. A primary function of GSH is to prevent the toxic interaction between free radicals and reactive transition metals such as copper (Cu). Due to the potential role of Cu in neurodegeneration, we examined the effect of GSH depletion on Cu toxicity in murine primary neuronal cultures. Depletion of cellular GSH with L-buthionine-[S,R]-sulfoximine resulted in a dramatic potentiation of Cu toxicity in neurons without effect on iron (Fe) toxicity. Similarly, inhibition of glutathione reductase (GR) activity with 1,3-bis(2-chloroethyl)-1-nitrosurea also increased Cu toxicity in neurons. To determine if the Alzheimer's amyloid-beta (Abeta) peptide can affect neuronal resistance to transition metal toxicity, we exposed cultures to nontoxic concentrations of Abeta25-35 in the presence or absence of Cu or Fe. Abeta25-35 pretreatment was found to deplete neuronal GSH and increase GR activity, confirming the ability of Abeta to perturb neuronal GSH homeostasis. Abeta25-35 pretreatment potently increased Cu toxicity but had no effect on Fe toxicity. These studies demonstrate an important role for neuronal GSH homeostasis in selective protection against Cu toxicity, a finding with widespread implications for neurodegenerative disorders.

Alterations of thiol metabolism in human cell lines induced by low amounts of copper, mercury or cadmium ions

Ions of metals such as mercury, cadmium and copper are known to exhibit a high affinity for thiol groups and may therefore severely disturb many metabolic functions in the cell. The aim of the present study was to identify the most sensitive changes of thiol metabolism induced by the addition of low concentrations of metal ions in order to elucidate the mechanisms of metal-toxicity. The effects on thiol metabolism by copper ions seemed to differ from that of mercury and cadmium ions. Copper ions exhibited mainly two effects that were different from those of mercury and cadmium ions. They lowered the reduced fractions of thiols and increased the release of homocysteine into the medium, whereas mercury and cadmium ions mainly influenced the metabolism of glutathione by increasing its synthesis. Even 0.1 micromol/l of copper ions increased the release of homocysteine in HeLa cell lines. An increased cellular concentration of glutathione and an increased release of glutathione into the medium were observed after addition of mercury and cadmium ions at a concentration of 1 micromol/l, which is just above the toxicity limit in human blood. The different cell lines varied in some respects in their response to the addition of metal ions. Cadmium ions had no effect on thiol metabolism in endothelial cell lines, and copper ions did not significantly increase the release of homocysteine into the medium in hepatoma cell lines. Furthermore, the metabolism of thiols during basal conditions (without the addition of metal ions) differed somewhat in the three cell lines investigated. One example is the low amount of extracellular glutathione in hepatoma cell lines, which probably was due to its rapid degradation to cysteinylglycine by gamma-glutamyl-transpeptidase.

Protein binding of homocysteine and other thiols in HeLa cell cultures after addition of homocysteine and copper ions

Homocysteine can be viewed as the first risk factor for atherosclerosis, believed to exert its effects through a mechanism involving oxidative damage. Oxygen radicals are known to interact with a variety of macromolecules leading to lipid peroxidation, DNA strand breakage and a variety of changes in proteins, including thiol oxidation. The present study deals with the protein-binding of homocysteine, cysteine and glutathione in a human cell line culture exposed to homocysteine and copper ions in order to elucidate the possible role of homocysteine in cell injury and atherogenesis. It is shown that homocysteine has the highest tendency of the thiols investigated to create disulfide bonds with proteins. The interaction with the protein cysteine thiol groups, which are involved in the function of many enzymes, structural proteins and receptors might disturb many metabolic functions in the cell. This finding might therefore be one reason for the cell-damaging effects of homocysteine. Addition of reduced homocysteine to cell cultures decreased the intra- and extracellular proportions of protein-bound thiols except that of intracellular glutathione. In agreement with the pro-oxidative effects of copper ions, the findings in the present study showed an increase of the protein-bound fractions of all thiols after the addition of copper ions. Another finding is that increased (in the presence of 100 mumol/l of copper ions) or decreased (in the presence of 100-2000 mumol/l of homocysteine) proportions of protein-bound fractions of the thiols in these short-term experiments did not seriously affect the cells since the cell growth was unchanged.

The cell-damaging effects of low amounts of homocysteine and copper ions in human cell line cultures are caused by oxidative stress

In the present study, we have investigated the increase of cell protein and the concentration of glutathione, cysteine and homocysteine in cell culture systems (HeLa cell line) after addition of low amounts (100-500 micromol/l) of homocysteine and/or copper. The thiols and cell protein were determined in cell cultures with daily additions of new medium with and without homocysteine and/or copper ions for 3 days. The present study shows that extracellularly added homocysteine (500 and 2000 micromol/l) resulted in signs of cell toxicity (decreased intracellular glutathione level and/or retarded cell growth). After the addition of copper ions (10, 50 or 100 micromol/l), complex changes in the concentrations of thiols in cell cultures occurred but cell growth was normal. After the addition of both homocysteine and copper ions, changes similar to those seen with the addition of copper ions and homocysteine alone were noted. However, synergistic features after addition of 500 micromol/l homocysteine and 10 or 50 micromol/l of copper ions were a significantly retarded cell growth and decreased concentration of cellular glutathione. In HeLa cell lines with initial low cell density and in an endothelial cell line (ECV 304), even the presence of 100 micromol/l of homocysteine and 10 micromol/l of copper ions inhibited cell growth and decreased the cellular level of glutathione. Whilst the level of homocysteine in our 3-day cell-culture experiments is higher than the mild hyperhomocysteinemia thought to be atherogenic in humans (20-30 micromol/l), it is conceivable that over a longer time course (several decades), this mild hyperhomocysteinemia could be sufficient to induce cellular effects similar to those found in the present study, eventually leading to atherosclerosis.

The effects of homocysteine and copper ions on the concentration and redox status of thiols in cell line cultures

Different fractions (reduced and total) of thiols (homocysteine, cysteine and glutathione) were determined in HeLa cell cultures with and without addition of copper ions and/or homocysteine. In cell cultures without any addition the concentration of all intracellular thiols increased between 1 and 24 h of culture. Glutathione had the highest, whereas homocysteine showed the lowest, proportion of the reduced form. In the medium, there was a decrease of total cysteine during the incubation, but the amount of extracellular reduced cysteine increased. Both homocysteine and glutathione were released into the medium. The amount of exported homocysteine during the incubation exceeded several-fold the intracellular amount. There were no signs of cell toxicity induced by the high amounts of extracellularly added homocysteine (2000 mumol/l) in HeLa cell cultures, except a slight decrease in the concentration of intracellular glutathione. After the addition of copper ions (500 mumol/l) there was a retarded cell growth, decreased intracellular concentration of glutathione, increased release of glutathione into the medium and a lower proportion of all intra- and extracellular reduced thiols. After the addition of both copper ions and homocysteine to HeLa cell cultures, similar changes as with the addition of copper ions were noted except that the cell growth was still more retarded and that a very high level of intracellular homocysteine was noted at 1 h of incubation. N-acetylcysteine lowered, in these experiments, the intracellular accumulation of homocysteine and restored, to some extent, the cell growth. In an endothelial cell line even the presence of 500 mumol/l of homocysteine and 50 mumol/l of copper ions inhibited the cell growth and decreased the cellular level of glutathione. Whilst the levels of homocysteine in our short-time cell culture experiments are higher than the mild hyperhomocysteinemia thought to be atherogenic in humans (20-30 mumol/l), it is conceivable that over a longer time-course (several decades) these lower levels of homocysteine in the presence of copper ions could be sufficient to induce cellular effects similar to those found in the present study, eventually leading to atherosclerosis.

Metabolism of homocysteine, its relation to the other cellular thiols and its mechanism of cell damage in a cell culture line (human histiocytic cell line U-937)

This study shows that the intracellular concentration of homocysteine in cultured cells is kept low due to an accumulation in the medium. The intracellular level of homocysteine was decreased when its precursor, methionine, was omitted from the culture medium. Intracellular glutathione and cysteine were lowered in cystine-deficient medium. Intracellular glutathione was also lowered when copper ions were added to the culture medium. It is evident from this study that the intracellular concentration of homocysteine was not influenced by the lowered level of glutathione and/or cysteine. High amounts of homocysteine added to the medium give rise to an increase of intracellular reduced homocysteine, which participates in the transsulfuration pathway and can replace cysteine in the synthesis of glutathione. The addition of relatively high amounts of reduced homocysteine (500 mumol/l) in the presence of copper ions (100 mumol/l) to the culture medium can be directly toxic to the cells, possibly due to oxygen radicals formed by thiol auto-oxidation. Whilst the level of homocysteine in this study using short-time cell culture experiment is much higher than the mild hyperhomocysteinemia thought to be atherogenic in humans, it is conceivable that over a longer time course these levels of homocysteine could be sufficient to induce endothelial dysfunction, eventually leading to atherosclerosis.

Role of cytosolic copper, metallothionein and glutathione in copper toxicity in rat hepatoma tissue culture cells

Effects of metallothionein (MT) synthesis inhibiting compounds (actinomycin D, cycloheximide), MT synthesis stimulating compounds (dexamethasone, dibu-cAMP) and interfering metals (Cd, Zn) on copper accumulation were investigated in rat hepatoma tissue culture cells. Copper-metallothionein (Cu-MT) and MT-associated copper levels were determined to find a possible correlation between cytosolic copper concentrations and MT as a Cu-detoxifying protein. Further, intracellular non-MT associated copper levels and levels of GSH and SOD were determined. Cell viability was tested under all experimental conditions by measuring LDH-release, K+ uptake and total cell protein. Administration of dexamethasone and dibu-cAMP showed no effect on MT levels (compared with controls), and only a marginal effect on 64Cu and total Cu accumulation. Administration of actinomycin D resulted in increased copper accumulation in the particulate fraction, possibly due to inhibition of copper secretion processes and/or protein synthesis. Presence of zinc had no effect on MT levels nor on total Cu and 64Cu levels, in contrast with cadmium which drastically enhanced copper accumulation and MT levels in the cells. Cu/MT ratios varied from 1.0 +/- 0.3 to 3.3 +/- 1.2, which is far below the assumed maximum molar ratio of 8-12 mol Cu per mol MT. SOD levels appeared to be enhanced up to 2- or 3-fold in the presence of Cd2+, relative to control values. The role of GSH as Cu-intermediate in intracellular Cu distribution plus its role in copper defence mechanism(s) was tested by application of BSO, an inhibitor of GSH synthesis. It was found that BSO had no effect on intracellular MT level; it was found however that MT-bound copper levels were markedly decreased. The results presented support a model for copper metabolism in hepatoma tissue culture (HTC) cells, where Cu(I) is complexed by GSH immediately after entering the cell. GSH is capable of transferring copper to MT where it is stored. Depletion of GSH (by administration of Cd2+, actinomycin D, cycloheximide) almost instantaneously results in enhanced cellular toxicity. When also MT is depleted (by actinomycin D) non-MT associated, 'free' cytosolic Cu2+ is elevated, and HTC cells rapidly loose their resistance to copper toxicity, as also reflected in loss of cell viability (LDH, K+ and total cell protein).