Metallothionein: an intracellular protein to protect against cadmium toxicity

Different induction of metallothioneins and Hsp70 and presence of the membrane transporter ZnT-1 in HepG2 cells exposed to copper and zinc

Eukaryotic cells respond to stressful environmental stimuli, such as toxic concentrations of heavy metals, by rapidly synthesising defence proteins: the metallothioneins (MT) and the heat shock protein 70 (Hsp70). In this study we have analysed how the human hepatoblastoma cell line HepG2 responds to exposure to excess copper (30 microg/ml) and zinc (50 microg/ml) for long exposure times (48 and 72 h). Accumulation of the two metals, as measured by ICP-AES, was time-dependent reaching a plateau after 72 h. HepG2 cells responded by dramatically increasing levels of MT during stress, mostly during zinc exposure. A time lag in Hsp70 induction was observed as the levels of this protein increased only after removal of the stress from culture medium (recovery) for 24 h, thus suggesting that the two defence mechanisms are not coordinated in a metal-induced stress response. Moreover in HepG2 cells, immunochemical and fluorescence techniques showed the presence and the localisation of the zinc membrane exporter ZnT-1 as a further mechanism of defence/homeostasis against zinc toxicity.

Tissue-dependent induction of heme oxygenase-1 and metallothionein-1/2 by methyl methanesulfonate

Methyl methanesulfonate (MMS), a methylating agent, is known to be a genotoxicant in testis. The purpose of this study was to investigate roles of oxidative stress-responsive proteins, heme oxygenase-1 (HO-1) and metallothionein-1/2 (MT-1/2), in genotoxicity of MMS. Cadmium, a potent genotoxicity inducer, induced HO-1 and MT-1/2 in rat livers and kidneys. Then we comparatively investigated MMS-induced HO-1 and MT-1/2 in rat livers, kidneys and testes. We found that a single administration of MMS (40 mg/kg) resulted in the induction of MT-1/2 mRNA in the liver, but not HO-1 mRNA, reaching maximum level at 6 hr and returning to the control levels by 24 hr. Interestingly, MMS induced both HO-1 and MT-1/2 mRNAs in the kidney. In contrast, MMS induced HO-1 mRNA, but not MT-1/2 mRNA in the testis. Since HO-1 and MT-1/2 have been recognized to respond to various oxidative stimuli, we further examined the inducing effect of MMS on these two proteins. MMS at dosages of 20 to 40 mg/kg for 2 consecutive weeks induced HO-1 mRNA (123 to 187% of the control) and protein (274 to 404% of the control) in rat testes. However, MT-1/2 mRNA was not induced by MMS administration, although a high level of expression was observed in comparison with the liver and kidney. These findings suggest that MMS induces HO-1 and/or MT-1/2 mRNA and its protein tissue-dependently, and the heme catabolites by HO-1 in the testis may contribute in some manner to its genotoxicity.

Characterization of the Arabidopsis metallothionein gene family: tissue-specific expression and induction during senescence and in response to copper

• Expression and regulation of Arabidopsis metallothionein (MT) genes were investigated to examine the functions of MTs in plants. • To examine the tissue-specific expression of MT genes, GUS reporter gene activity driven by promoters of MT1a, MT2a, MT2b and MT3 was analysed in transgenic plants. • MT1a and MT2b are expressed in the phloem of all organs and are copper (Cu)-inducible; MT2a and MT3, by contrast, are expressed predominantly in mesophyll cells and are also induced by Cu in young leaves and at root tips. Expression of MT genes is highly induced by Cu in trichomes and increases during senescence. Expression of MT4 genes is restricted to seeds. • We propose that plant MTs have distinct functions in heavy metal homeostasis, especially for Cu: MT1a and MT2b are involved in the distribution of Cu via the phloem, while MT2a and MT3 chaperone excess metals in mesophyll cells and root tips. These functional capabilities may allow MTs to play a role in mobilization of metal ions from senescing leaves and the sequestration of excess metal ions in trichomes.

Structural consequences of metallothionein dimerization: solution structure of the isolated Cd4-alpha-domain and comparison with the holoprotein dimer

The NMR determination of the structure of Cd(7)-metallothionein was done previously using a relatively large protein concentration that favors dimer formation. The reactivity of the protein is also affected under this condition. To examine the influence of protein concentration on metallothionein conformation, the isolated Cd(4)-alpha-domain was prepared from rabbit metallothionein-2 (MT 2), and its three-dimensional structure was determined by heteronuclear, (1)H-(111)Cd, and homonuclear, (1)H-(1)H NMR, correlation experiments. The three-dimensional structure was refined using distance and angle constraints derived from these two-dimensional NMR data sets and a distance geometry/simulated annealing protocol. The backbone superposition of the alpha-domain from rabbit holoprotein Cd(7)-MT 2 and the isolated rabbit Cd(4)-alpha was measured at a RMSD of 2.0 A. Nevertheless, the conformations of the two Cd-thiolate clusters were distinctly different at two of the cadmium centers. In addition, solvent access to the sulfhydryl ligands of the isolated Cd(4)-alpha cluster was 130% larger due to this small change in cluster geometry. To probe whether these differences were an artifact of the structure calculation, the Cd(4)-alpha-domain structure in rabbit Cd(7)-MT 2 was redetermined, using the previously defined set of NOEs and the present calculation protocol. All calculations employed the same ionic radius for Cd(2+) and same cadmium-thiolate bond distance. The newly calculated structure matched the original with an RMSD of 1.24 A. It is hypothesized that differences in the two alpha-domain structures result from a perturbation of the holoprotein structure because of head-to-tail dimerization under the conditions of the NMR experiments.

Hepatic metallothioneins in two neotenic salamanders, Proteus anguinus and Necturus maculosus (Amphibia, Caudata)

The presence of metallothionein (MT) and the subcellular distribution of copper, zinc and cadmium were investigated in livers of two neotenic salamanders, Proteus anguinus and Necturus maculosus. In P. anguinus, caught in the wild, hepatic MTs were present as a single isoform of (Zn, Cu, Cd)-thioneins, whose molecular weight was estimated to be approximately 12000 by size exclusion chromatography. The percentage of zinc and cadmium was higher in the cytosol and of copper in the pellet. Cytosolic cadmium was almost exclusively associated with MTs (80%), while zinc and copper were also present in the regions of higher-molecular weight proteins. In laboratory bred N. maculosus, MTs were isolated from the liver cytosol and extract of the pellet as (Cu, Zn)- and (Zn, Cu)-thioneins, respectively. According to the low amount of copper extracting from liver pellets of N. maculosus, the presence of water insoluble aggregated forms of Cu-thioneins should be checked in further investigations.

Interaction of trivalent arsenicals with metallothionein

Arsenic is a human carcinogen, causing skin, bladder, and lung cancers. Although arsenic in drinking water affects millions of people worldwide, the mechanism(s) of action by which arsenic causes cancers is not known. Arsenic probably exerts some toxic effects by binding with proteins. However, few experimental data are available on arsenic-containing proteins in biological systems. This study reports on arsenic interaction with metallothionein and established binding stoichiometries between metallothionein and the recently discovered trivalent metabolites of arsenic metabolism. Size exclusion chromatography with inductively coupled plasma mass spectrometry analysis of reaction mixtures between trivalent arsenicals and metallothionein clearly demonstrated the formation of complexes of arsenic with metallothionein. Analysis of the complexes using electrospray quadrupole time-of-flight tandem mass spectrometry revealed the detailed binding stoichiometry between arsenic and the 20 Cys residues in the metallothionein molecule. Inorganic arsenite (As(III)) and its two trivalent methylation metabolites, monomethylarsonous acid (MMA(III)) and dimethylarsinous acid (DMA(III)), readily bind with metallothionein. Each metallothionein molecule could bind with up to six As(III), 10 MMA(III), and 20 DMA(III) molecules, consistent with the coordination chemistry of these arsenicals. The findings on arsenic interaction with proteins are useful for a better understanding of arsenic health effects.

Treatment with metallothionein prevents demyelination and axonal damage and increases oligodendrocyte precursors and tissue repair during experimental autoimmune encephalomyelitis

Experimental autoimmune encephalomyelitis (EAE) is an animal model for the human demyelinating disease multiple sclerosis (MS). EAE and MS are characterized by significant inflammation, demyelination, neuroglial damage, and cell death. Metallothionein-I and -II (MT-I + II) are antiinflammatory and neuroprotective proteins that are expressed during EAE and MS. We have shown recently that exogenous administration of Zn-MT-II to Lewis rats with EAE significantly reduced clinical symptoms and the inflammatory response, oxidative stress, and apoptosis of the infiltrated central nervous system areas. We show for the first time that Zn-MT-II treatment during EAE significantly prevents demyelination and axonal damage and transection, and stimulates oligodendroglial regeneration from precursor cells, as well as the expression of the growth factors basic fibroblast growth factor (bFGF), transforming growth factor (TGF)beta, neurotrophin-3 (NT-3), NT-4/5, and nerve growth factor (NGF). These beneficial effects of Zn-MT-II treatment could not be attributable to its zinc content per se. The present results support further the use of Zn-MT-II as a safe and successful therapy for multiple sclerosis.

Zn(II) metabolism in prokaryotes

It is difficult to over-state the importance of Zn(II) in biology. It is a ubiquitous essential metal ion and plays a role in catalysis, protein structure and perhaps as a signal molecule, in organisms from all three kingdoms. Of necessity, organisms have evolved to optimise the intracellular availability of Zn(II) despite the extracellular milieu. To this end, prokaryotes contain a range of Zn(II) import, Zn(II) export and/or binding proteins, some of which utilise either ATP or the chemiosmotic potential to drive the movement of Zn(II) across the cytosolic membrane, together with proteins that facilitate the diffusion of this ion across either the outer or inner membranes of prokaryotes. This review seeks to give an overview of the systems currently classified as altering Zn(II) availability in prokaryotes.

Nephrotoxicity and the proximal tubule. Insights from cadmium

Cadmium (Cd(2+)) is a non-essential heavy metal, which is taken up from the environment into the body through pulmonary and enteral pathways. The S1 segment of the kidney proximal tubule (PT) is a major target of chronic Cd(2+) toxicity. Renal dysfunction develops in up to 7% of the general population and in its most severe form displays major features of Fanconi syndrome, such as a defective protein, amino acid, glucose, bicarbonate and phosphate reabsorption. The major pathway for Cd(2+) uptake by PT cells (PTCs) in vivo is apical endocytosis of Cd(2+) complexed to the high-affinity metal-binding protein metallothionein (MT), which may be receptor-mediated. MT is subsequently degraded in endo-lysosomes, and Cd(2+) is liberated for translocation into the cytosolic compartment, possibly using transporters for Fe(2+), Zn(2+) or Cu(2+), such as the divalent metal transporter DMT1. Free Cd(2+) ions in the extracellular space are translocated across apical and/or basolateral PTC membranes into the cytosol via transporters, whose identity remains unknown. Cytosolic Cd(2+) generates reactive oxygen species (ROS), which deplete endogenous radical scavengers. ROS also damage a variety of transport proteins, including the Na(+)/K(+)-ATPase, which are subsequently degraded by the proteasome and endo-lysosomal proteases. Cd(2+) causes mitochondrial swelling and release of cytochrome C. If these ROS-mediated stress events are not balanced by repair processes, affected cells undergo apoptosis. But Cd(2+) also induces the upregulation of cytoprotective stress and metal-scavenging proteins, such as MT. In addition, Cd(2+) upregulates the detoxifying pump multidrug resistance P-glycoprotein, which appears to protect PTCs against Cd(2+)-induced apoptosis. Thus, Cd(2+) interferes with various cellular events ranging from mechanisms of induction of programmed cell death to activation of cell survival genes. A better understanding of the cellular mechanisms involved in Cd(2+) nephrotoxicity should provide insights into other heavy metal (e.g. Pb(2+), Hg(2+)) nephropathies and various forms of acquired Fanconi syndrome.

Metallothionein-1 messenger RNA transcription in steroid-secreting cells of the rat ovary during the periovulatory period

An increase in metallothionein 1 (MT-1) mRNA was detected in the ovaries of immature Wistar rats that were primed with s.c. injection of 10 IU eCG followed 48 h later by 10 IU hCG s.c. to initiate the ovulatory process. Ovarian RNA was extracted at 0, 2, 4, 8, 12, 24, 72, 144, and 288 h after the primed animals were injected with hCG. These extracts were used for reverse transcription polymerase chain reaction (RT-PCR) differential display and Northern analyses that yielded complementary gene fragments for MT-1. Expression of MT-1 mRNA increased significantly by 24 h after hCG treatment and reached a peak at 144 h after hCG. In contrast, a disintegrin and metalloproteinase with thrombospondin motifs and a tissue inhibitor of metalloproteinase 1, which were also detected by the RT-PCR differential display procedure, reached a peak at 12 h after hCG and returned to control levels in the ovaries by 72 h after hCG. In situ hybridization indicated that most of the MT-1 mRNA was expressed in the vicinity of the theca interna of preovulatory follicles and in the lutein granulosa of postovulatory follicles. Thus, MT-1 mRNA expression is primarily in the vicinity of steroid-secreting areas of the ovary. The substantial increase in MT-1 mRNA expression might be important in protecting the ovarian tissues from oxidative stress generated by ovarian inflammatory events during the ovulatory process and luteinization.

Cadmium intake of moose hunters in Finland from consumption of moose meat, liver and kidney

Although the average cadmium intake in Finland is about 10 microg day(-1), some risk groups can be identified. This study assessed cadmium intake from the consumption of moose meat, liver and kidneys by moose hunters. Consumption data from a postal questionnaire were combined with a representative database on moose cadmium concentrations. Cadmium intakes were calculated as point estimates for all respondents (n = 711), for those consuming moose meat, liver and/or kidneys, and for the highest decile of those. Probabilistic modelling using the Monte Carlo technique was used to simulate the distribution of dietary cadmium exposure. Of the respondents, 69% consumed moose liver and only 23% moose kidneys. The consumption of moose liver or kidneys significantly increased cadmium intake, whereas moose meat (median consumption 17 kg year(-1) person(-1)) contributed only slightly (0.16 microg day(-1) person(-1)) to the daily total cadmium intake. In the simulation, 10% of the moose hunters had an intake of > 8.76 microg day(-1) (14.6% of PTWI for a 60-kg person) from moose. Point estimates provided only a partial understanding of the potential exposure. Simulated distributions of intake were more useful in characterizing exposure. The study revealed that heavy users of moose organs have a relatively narrow safety margin from the levels of cadmium probably causing adverse health effects.

Low-Dose Cadmium Exposure Reduces Human Prostate Cell Transformation in Culture and Up-Regulates Metallothionein and MT-1G mRNA

Chronic low-level exposure to environmental toxins, including cadmium (Cd), is a growing problem in the industrialized world. One promising strategy for protection from these toxins is the use of low-dose exposure of environmental chemicals to induce cell tolerance and recovery, a phenomenon known as "protective hormesis". Hormetic [low-dose stimulatory] effects occur in a variety of systems and with a number of chemicals. Cd is a potent carcinogen in rodents and has also been linked to human lung and prostate cancers. In the present study, we have evaluated the protective effects of low and ultra-low dose, long-term Cd exposure in the normal human prostate cells, RWPE-1. Cells were exposed to low and ultra-low doses (0, 0 (S(-36)), 10(-6), 10(-7), 10(-18), 10(-21), 10(-32), or 10(-36)M) of Cd for 20 weeks followed by treatment with 10(-5)M Cd for another 8 weeks. Continuous exposure of RWPE-1 cells to 10(-5)M Cd results in malignant transformation. However, cells pretreated with low and ultra-low doses of Cd had delayed transformation compared with controls. In addition, the number of transformed cell mounds was lower in pretreated cells indicating that low and ultra-low dose exposure had protective effects against high-dose Cd induced carcinogenesis. The expression of metallothionein (MT), the primary Cd detoxification protein, was induced by low-dose exposure to Cd and maintained during the 20 weeks. In addition, MT-1G mRNA was up-regulated 2- to 3-fold by low-dose and ultralow-dose Cd exposures and may be the mechanism of protective hormesis in this model. MT-1G mRNA might also serve as a biological indicator of very low-dose environmental Cd exposure.

Stressor-dependent regulation of the heat shock response in zebrafish, Danio rerio

Heat shock transcription factors (HSFs) regulate expression of heat shock proteins (Hsps). We have previously shown that in zebrafish a unique isoform, zHSF1b, disappears concomitant with heat shock-induced Hsp70 expression. To characterize the role of zHSF1a and zHSF1b isoforms in the regulation of the stress response in vivo, we have carried out cadmium (10-100 microM) and copper (10-30 microM) exposures in order to specify whether the disappearance of HSF1b is specific for heat stress. After 4-h metal exposures we analyzed the expression of hsp70, zHSF1a, zHSF1b and metallothionein (MT) by reverse transcriptase polymerase chain reaction in zebrafish liver, gonads and gills. Although cadmium is a known inducer of Hsps, it did not affect hsp70 expression significantly in the studied tissues. Induction of hsp70 was observed upon copper exposure in liver and gonads, but not in gills. Neither metal affected the zHSF1a/b ratio. Both cadmium and copper exposure caused upregulation of MT, regulator of metal homeostasis and detoxification, confirming that the tissues were subjected to metal loads. Thus, hsp70 appears to be more weakly induced upon metal exposure than in response to heat shock and HSF1 isoforms may participate in stressor-specific regulation of hsp70.

An iron-rich diet protects the liver and kidneys against cadmium-induced injury in the bank vole (Clethrionomys glareolus)

The objective of this study was to determine whether supplemental dietary iron (Fe) would protect against cadmium (Cd)-induced injury in the liver and kidneys of bank voles. The rodents were provided, for 6 weeks, Fe-adequate (60-80 microg/g) and Fe-enriched (250-270 microg/g) diets containing 0.05 (control), 40, and 80 microg Cd/g. Histological examinations and analyses of Cd, Cd bound and not bound to metallothionein (MT), Fe, and lipid peroxidation in liver and kidneys were carried out. The Fe-enriched diet prevented Cd-induced histopathological changes as well as deprivation of tissue Fe and lipid peroxidation. Also, supplemental Fe significantly decreased hepatic and renal Cd burden. However, in the Cd-80 bank voles fed the Fe-enriched diet, the non-MT-bound Cd, considered a toxic species, reached 4.7 microg/g liver and 13.7 microg/g kidney, these values being similar to those at which histopathological changes occurred in the voles fed Cd diets not supplemented with Fe. The data indicate that the protective effect of supplemental Fe in the bank vole may be due to the prevention of Cd-induced deprivation of tissue Fe and Fe-dependent oxidative processes rather than to reduction of cadmium accumulation.

Rat liver tyrosine aminotransferase activity and induction by dexamethasone upon cadmium intoxication

This study was focused on Cd effects on basal and dexamethasone-induced tyrosine aminotransferase (TAT) activity in the rat liver cytosol. Cadmium (Cd), applied in the dose of 2 mg/kg b.w., stimulated both TAT activity and its induction by dexamethasone, inducing the most prominent alterations 24 h after administration. Doses lower than 2 mg Cd/kg b.w. were ineffective while the higher ones (3 and 4 mg Cd/kg b.w) led to the changes similar to those reached by 2 mg Cd/kg. The in vitro application of different Cd concentrations to the liver cytosol rendered the enzyme activity unchanged suggesting that the metal acted at the level of TAT gene transcription.

Activities of glutamate dehydrogenase and aspartate and alanine aminotransferases in freshwater snails Helisoma duryi and Lymnaea natalensis exposed to copper

In this paper we investigate the potential of glutamate dehydrogenase (GDH) and aspartate and alanine aminotransferases (AST and ALT) as biomarkers of water pollution due to copper in the freshwater snails Helisoma duryi and Lymnaea natalensis. Snails were dosed with copper(II) ion concentrations of 0.01, 0.1 and 1 mg kg(-1) breeding water for a period of 96 h, after which those surviving were shelled. The copper content in the breeding water, in whole snail tissue and in the snail shells was determined at the end of the period of exposure. For enzyme determinations, whole snail tissue was first homogenized and fractionated by centrifugation at 500 g to remove the nuclei. The resulting supernatant was then centrifuged at 10,000 g to give a pellet fraction representing the mitochondrial fraction and a supernatant representing the cytosolic fraction. Copper was very toxic to both snail species at concentrations above 0.2 mg l(-1), with only 3% of the Helisoma and 12% of the Lymnaea surviving at concentrations of approximately 1 mg l(-1). The copper content in the shells and tissues of snails rose with increasing copper concentration in the breeding water, and was 2.1- to 4.9-fold in snails exposed to copper ion at a dose of 1 mg kg(-1) water compared with undosed snails. Similarly, the activities of GDH and AST rose by up to 4.7-fold in the homogenate and the mitochondrial and cytosolic fractions with increasing concentrations of copper. These activities, however, fell at copper concentrations of approximately 1 mg l(-1), which coincided with massive death of snails. Mitochondrial ALT disappeared at copper ion concentrations of approximately 0.2 mg l(-1) for Lymnaea and 1 mg l(-1) for Helisoma, possibly indicating mitochondrial degeneration. These results show that GDH, AST and ALT have the potential to be biomarkers of sublethal copper pollution in these two snail species, since their activities were significantly altered by low copper concentrations.

Engineering of technetium-99m-binding artificial receptors for imaging gene expression

BACKGROUND

Optimization of gene therapy protocols requires accurate and non-invasive quantification of vector delivery and gene expression. To facilitate non-invasive imaging of gene expression, we have genetically engineered 'artificial receptors', i.e. membrane proteins that bind (99m)Tc-oxotechnetate ((99m)TcOT) via transchelation from a complex with glucoheptonate. The latter is a component of a widely used clinical imaging kit.

METHODS

The engineered marker proteins were designed as type I and II membrane proteins and consisted of (1) an (99m)TcOT-binding domain, metallothionein (MT), and (2) a membrane-anchoring domain. Engineered constructs were used for transfection of COS-1 and 293 cells; the expression of mRNA was verified by RT-PCR.

RESULTS

Immunofluorescent analysis, cell fractionation and immunoblotting revealed expression of marker proteins on plasma membrane. Transfection of cells resulted in strong positive staining of plasma membrane with anti-His-tag antibodies. Scintigraphic imaging in vitro confirmed the ability of transfected cells to bind (99m)TcOT. The fraction of bound radioactivity reached a peak (3.53%) when 0.93 MBq (99m)TcOT was added to transfected COS-1 cells. The experiment-to-control signal ratio was equal to 32 at the same added dose.

CONCLUSIONS

(1) Both types of engineered 'artificial receptors' were expressed on the surface of eukaryotic cells; (2) marker proteins were functional in binding (99m)TcOT; and (3) type II membrane proteins were more efficient in binding (99m)TcOT than type I proteins. We anticipate that the developed approach could be useful for 'tagging' transfected cells with (99m)TcOT enabling imaging of tracking in vivo transduced cells or cell therapies.

Evidence for oligomerization of metallothioneins in their functional state

Capillary zone electrophoresis in the polyacrylamide-coated capillary was used to study metallothionein (MT) isoforms at physiological pH in horse kidney and rabbit liver MT preparations produced commercially by Sigma. Evidence is put forward that MT develops oligomers or aggregates in its metal binding situation at these pH values in both species. For the horse kidney preparation two forms were found for both the MT-IA and the MT-IB forms, for the rabbit liver three forms could be seen for the MT-I form and two for the MT-II form. At pH values above the physiological range (pH 8-10) up to four forms could be seen for the MT-I form (MT-IA in the horse) in both preparations. Compared to the MTCd-II form, the rabbit liver MTZn-II form not only behaved electrophoretically identical, but also showed a corresponding oligomerization behaviour. Our results indicate that the oligomerized MT-I form in the rabbit liver and the MT-IA form in the horse kidney bind more Cd atoms than the expected number of 7 per monomer.

Metallothionein in bovine spongiform encephalopathy

An increase in metallothionein I and II (MT I/II) mRNA concentrations has been reported in the central nervous system of scrapie-infected rodents. In this study we compared cattle with bovine spongiform encephalopathy (BSE), cattle affected by neurological disease other than BSE, and clinically healthy cattle in respect of MT I/II immunoreactivity in brainstem medullary tissue. Marked astrocytic MT I/II immunolabelling was seen in all BSE-affected animals, in contrast to clinically healthy cases, in which no such labelling was detected. In BSE, MT I/II immunoreactive astrocytes were confined specifically to areas of vacuolation or abnormal prion protein (PrP(BSE)) deposition, or both. MT I/II immunolabelling was also seen in a small number of animals with a neurological disease other than BSE. These findings complement previous studies by demonstrating increased levels of MT I/II in transmissible spongiform encephalopathy (TSE)-infected brain tissue, indicating that MT I/II may play some as yet unidentified role in the response to TSE infection.

Phosphorylation of eukaryotic initiation factor 2 by heme-regulated inhibitor kinase-related protein kinases in Schizosaccharomyces pombe is important for fesistance to environmental stresses

Protein synthesis is regulated by the phosphorylation of the alpha subunit of eukaryotic initiation factor 2 (eIF2alpha) in response to different environmental stresses. One member of the eIF2alpha kinase family, heme-regulated inhibitor kinase (HRI), is activated under heme-deficient conditions and blocks protein synthesis, principally globin, in mammalian erythroid cells. We identified two HRI-related kinases from Schizosaccharomyces pombe which have full-length homology with mammalian HRI. The two HRI-related kinases, named Hri1p and Hri2p, exhibit autokinase and kinase activity specific for Ser-51 of eIF2alpha, and both activities were inhibited in vitro by hemin, as previously described for mammalian HRI. Overexpression of Hri1p, Hri2p, or the human eIF2alpha kinase, double-stranded-RNA-dependent protein kinase (PKR), impeded growth of S. pombe due to elevated phosphorylation of eIF2alpha. Cells from strains with deletions of the hri1(+) and hri2(+) genes, individually or in combination, exhibited a reduced growth rate when exposed to heat shock or to arsenic compounds. Measurements of in vivo phosphorylation of eIF2alpha suggest that Hri1p and Hri2p differentially phosphorylate eIF2alpha in response to these stress conditions. These results demonstrate that HRI-related enzymes are not unique to vertebrates and suggest that these eIF2alpha kinases are important participants in diverse stress response pathways in some lower eukaryotes.

Potential role of p53 on metallothionein induction in human epithelial breast cancer cells

The expression and induction of metallothionein has been associated with protection against oxidative stress and apoptosis. This study examines the effect of tumour suppressor protein p53 on metallothionein expression following CdCl2 treatment in eight human epithelial breast cancer cell lines differing in p53 and oestrogen-receptor status. Cells were treated with 10 microM CdCl2 for 24 h and metallothionein protein levels were measured by cadmium binding assay. MCF7 cells which are p53-positive (p53+) and oestrogen-receptor-positive showed a large induction in metallothionein synthesis by 10.79+/-1.36-fold. Other breast cancer cell lines which are p53-negative (p53-) and oestrogen-receptor-negative or weakly oestrogen-receptor-positive showed a small induction ranging from 1.40+/-0.10 to 3.65+/-0.30-fold. RT-PCR analysis showed an induction of metallothionein mRNA in MCF7 cells by about 1.61+/-0.08-fold, while in HCC1806 cells (p53-, oestrogen-receptor-negative) by 1.11+/-0.13-fold, and in MDA-MB-231 (p53-, oestrogen-receptor-negative) by 1.25+/-0.06-fold. Metallothionein localisation was determined by immunohistochemical staining. Prior to metal treatment, metallothionein was localised mainly in the cytoplasm of MCF7 and MDA-MB-231 cells. After treatment with 10 microM CdCl2 for 24 h, MCF7 cells showed intense nuclear and cytoplasmic staining for metallothionein, while MDA-MB-231 cells showed staining in the cytoplasm with weak nuclear staining. Apoptosis induced by 10-40 microM CdCl2 at time points between 4 and 48 h was examined with TUNEL assay. In MCF7 cells, apoptosis increased with higher concentrations of CdCl2, it peaked at 6-8 h and appeared again at 48 h for all concentrations of CdCl2 tested. In MDA-MB-231 cells, apoptosis remained at low levels for 10-40 microM CdCl2 at all time points. Studies on cadmium uptake showed similar uptake and accumulation of cadmium at 8 and 24 h in all the cell lines. The data demonstrate that treatment of epithelial breast cancer cells with 10 microM CdCl2 for 24 h caused a greater induction of metallothionein protein and mRNA expression in p53+ and oestrogen-receptor-positive cells as compared to p53- and oestrogen-receptor-negative or weakly oestrogen-receptor-positive cells. This effect may be associated with the occurrence of apoptosis and suggests a role for p53 and oestrogen-receptor on the expression and induction of metallothionein in epithelial cells.

Identification of cadmium-sensitive genes in the Antarctic fish Chionodraco hamatus by messenger RNA differential display

To investigate the ability of cadmium to affect gene transcription in fish, the messenger RNA (mRNA) differential display technique was used to analyze gene expression in the Antarctic icefish Chionodraco hamatus exposed to sublethal doses of cadmium salt. Seven DNA complementary to RNA (cDNA) bands whose steady-state levels of expression significantly changed in response to cadmium exposure were identified. The results obtained show that two groups of genes are affected by cadmium in icefish liver. The first group comprises genes that are up-regulated by the metal: in particular, a gene encoding the heat-shock protein HSP70 and another encoding a protein homologous to GP49 of Sparus aurata egg envelope. The other group comprises genes down-regulated by cadmium. These are the transferrin gene and a gene encoding a protein presenting homology to mouse T2K, a kinase having a role in the prevention of apoptosis. Three cDNAs had no homology to known gene sequences, thus suggesting that may either encode not yet identified proteins, or correspond to untranslated regions of mRNA molecules.

Cellular stress induced in cultured human cells by exposure to sludge extracts from water treatment plants

Sludge extracts from three water treatment plants, corresponding either to the readily water-soluble fraction or to the heavy metal content found in the solid fraction were tested separately or in combination on human cultured cells for their ability to affect the growth rate and/or to trigger a synthesis of the stress-related hsp72 and metallothionein proteins. When given separately, the soluble extract or the metal mixture corresponding to the solid fraction of sludge failed to exert significant effects on cell growth rate and expression level of the stress proteins. However, when given in combination, they were found to exert a strong synergistic effect, as they impaired cell growth and induced a significant overexpression of both hsp72 and metallothionein. This result points out the complex molecular interactions in actual environmental samples when acting on biological structures. It underscores the need for biological tests to complement chemical analyses in environment monitoring.

Prior increase in metallothionein levels is required to prevent doxorubicin cardiotoxicity

Many studies have shown that metallothionein (MT) can be increased significantly by different oxidative insults in multiple organ systems. However, the increase in MT production often fails to protect against oxidative tissue injury. On the other hand, recent studies using a cardiac-specific, MT-overexpressing, transgenic mouse model have shown that MT protects against oxidative heart injury. Thus, the present study was undertaken to test the hypothesis that prior increase in MT levels is required to prevent oxidative injury. Oxidative heart injury was induced by doxorubicin (DOX), an important anticancer drug that causes severe cardiotoxicity through oxidative stress. Cardiac-specific, MT-overexpressing, transgenic mice and wild-type (WT) FVB mice were treated with DOX at 20 mg/kg. Four days after the treatment, MT concentrations were markedly elevated in the WT mouse heart. The elevated MT concentrations were comparable with those found in the transgenic mouse heart, which did not show further MT elevation in response to DOX challenge. Severe oxidative injury occurred in the heart of WT mice, including myocardial lipid peroxidation, morphological changes as examined by electron microscopy, high levels of serum creatine kinase activity, and decreased total glutathione concentrations in the heart. However, all of these pathological changes were significantly inhibited in the MT-transgenic mice. Therefore, this study demonstrates that there is a correlation between MT induction and oxidative stress in the DOX-treated mouse heart. However, MT can protect the heart from oxidative injury only if it is present prior to induction of oxidative stress.

Inactivation of mammalian 8-oxoguanine-DNA glycosylase by cadmium(II): implications for cadmium genotoxicity

Cadmium(II) is a toxic, mutagenic and carcinogenic metal (IARC Class 1 human carcinogen). It causes damage to eukaryotic cells both in acute and chronic modes of exposure via multiple biochemical mechanisms. In particular, Cd diminishes the capacity of cells to repair oxidative DNA damage. Oxidative DNA lesions are important precursors to mutations and ultimately may lead to neoplastic transformation of human cells. We investigated interactions of Cd with murine Ogg1 (mOgg1), an enzyme that removes 8-oxoguanine (8-oxoG), an abundant oxidative lesion, from DNA. Cd(2+) and Zn(2+), but not other divalent cations tested, suppressed mOgg1-catalyzed reactions. The apparent inhibition by Cd consisted of at least two independent processes: irreversible, DNA-independent first-order inactivation of mOgg1 and DNA-dependent inhibition. Irreversibly inactivated mOgg1 has nearly normal affinity for damaged DNA and a normal catalytic rate constant but is defective in formation of the covalent reaction intermediate. When both modes of inhibition are in effect, the catalytic rate constant is dramatically lowered, while affinity to damaged DNA is decreased moderately. Potential sites for Cd binding in mOgg1 and mOgg1-DNA complex are identified. Inactivation of Ogg1 may play a role in the mutagenic and carcinogenic action of Cd.

Relationships between trace element concentrations in human blood and serum

Trace element interactions can affect the absorption, metabolism, or effects of elements. Also, different elements may derive from the same source. Associations in biological media between element concentrations may indicate such phenomena. A large number of correlations were found between 13 trace elements (Co, Cu, Zn, Se, Rb, Rh, Pd, Cd, W, Pt, Hg, Tl, and Pb) in human blood and/or serum, as investigated in 372 Swedish adolescents. Notably, serum Se correlated with blood Pb and blood Hg and Cu and Zn were correlated to each other in both blood and serum. The elements Pt, Pd and Rh, spread in the environment through use of catalytic converters in cars, were closely correlated in both blood and serum. Apart from the correlations with a probable biological or exposure-related explanation, several other correlations, of yet unknown importance and origin, were found.

DNA damage and metallothionein synthesis in human hepatoma cells (HepG2) exposed to cadmium

Cadmium is an important heavy metal environmental toxicant, which is classified as a human carcinogen. The comet assay was used to evaluate the levels of DNA damage in a metabolically competent HepG2 cell line after treatment with low, non-cytotoxic and physiologically relevant concentrations of cadmium, alone and in combination with the dietary mutagen 2-amino-3-methyl-imidazo[4,5-f]quinoline (IQ) and with the environmental mutagen benzo[a]pyrene (B(a)P). After exposure of the cells to 10, 100 and 1000 nM CdCl(2), a dose- and time-dependent increase of DNA damage was detected. Maximal damage was found after 12 h of treatment, but declined with further incubation with CdCl(2). The increased synthesis of metallothioneins on exposure to CdCl(2) up to 12 h suggests that they are responsible for the adaptation of HepG2 cells to the DNA damaging effects of CdCl(2). Co-treatment of the cells with CdCl(2) (10-1000 nM) and IQ (300 microM) induced a dose-dependent increase of DNA damage compared to cells treated with IQ alone. Co-genotoxic activity was also observed by increased formation of micronuclei in cells exposed to IQ and 1000 nM CdCl(2); at this concentration, CdCl(2) alone also induced micronuclei in HepG2 cells. Our results support the hypothesis that direct and indirect mechanisms are involved in cadmium-induced DNA damage.

Interference by toxic metal ions with zinc-dependent proteins involved in maintaining genomic stability

Metal ions are essential components of biological systems; nevertheless, even essential elements may have toxic or carcinogenic properties. Thus, besides As(III) and Cd(II), also Ni(II) and Co(II) have been shown previously to disturb different types of DNA repair systems at low, non-cytotoxic concentrations. Since some metals exert high affinities for SH groups, we investigated whether zinc finger structures in DNA-binding motifs of DNA repair proteins are potential targets for toxic metal ions. The bacterial formamidopyrimidine-DNA glycosylase (Fpg protein) involved in base excision repair was inhibited by Cd(II), Cu(II) and Hg(II) with increasing efficiencies, whereas Co(II), As(III), Pb(II) and Ni(II) had no effect. Furthermore, Cd(II) still disturbed enzyme function when bound to metallothionein. Strong inhibition was also observed in the presence of phenylselenyl chloride, followed by selenocystine, while selenomethionine was not inhibitory. Regarding the mammalian XPA protein involved in the recognition of DNA lesions during nucleotide excision repair, its DNA-binding capacity was diminished by Cd(II), Cu(II), Ni(II) and Co(II), while Hg(II), Pb(II) and As(III) were ineffective. Finally, the H(2)O(2)-induced activation of the poly(ADP-ribose)polymerase (PARP) involved in DNA strand break detection and apoptosis was greatly reduced by Cd(II), Co(II), Ni(II) and As(III). Similarly, the disruption of correct p53 folding and DNA binding by Cd(II), Ni(II) and Co(II) has been shown by other authors. Therefore, zinc-dependent proteins involved in DNA repair and cell-cycle control may represent sensitive targets for some toxic metals such as Cd(II), Ni(II), Co(II) and Cu(II), as well as for some selenium compounds. Relevant mechanisms of inhibition appear to be the displacement of zinc by other transition metals as well as redox reactions leading to thiol/disulfide interchange.

iNOS-null mice are not resistant to cadmium chloride-induced hepatotoxicity

Acute administration of cadmium (Cd) to rats results in hepatotoxicity. Recent reports indicate that Kupffer cells, the resident macrophages of the liver, participate in the manifestation of Cd-induced hepatotoxicity. Nitric oxide (NO) is a reactive nitrogen radical produced by activated Kupffer cells via the induction of inducible nitric oxide synthase (iNOS). Nitric oxide can combine with superoxide to form peroxynitrite, a molecule that may participate in the toxic mechanisms of hepatotoxins, such as acetaminophen and bacterial endotoxin. It has been speculated that Cd also may exert its hepatotoxicity, in part, via the production of NO by iNOS. Therefore, this study was undertaken to determine whether iNOS contributes to Cd-induced hepatotoxicity. Wild-type (WT) mice were administered selective iNOS inhibitors (AMT and 1400W) concurrently and 3 h after administration of a hepatotoxic dose of Cd (4.0 mg Cd/mg). Additionally, WT and iNOS-null (iNOS-KO) mice were dosed iv with saline or 2.0, 2.5, 3.0, 3.5 or 4.0 mg Cd/kg. Serum alanine aminotransferase (ALT) and sorbitol dehydrogenase (SDH) activities were quantified to assess liver injury. Administration of iNOS inhibitors failed to prevent Cd-induced hepatotoxicity. Also, Cd caused a dose-dependent increase in liver injury in both WT and iNOS-KO mice. The liver injury produced by Cd in the iNOS-KO mice was not different from that in WT at any dose. These data indicate that iNOS does not appear to mediate Cd-induced hepatotoxicity.

Metallothionein-independent zinc protection from alcoholic liver injury

Previous studies using metallothionein (MT)-overexpressing transgenic mice have demonstrated that MT protects the liver from oxidative injury induced by alcohol. The mechanism of action of MT is unknown. Because MT primarily binds to zinc under physiological conditions and releases zinc under oxidative stress and zinc is an antioxidant element, it is likely that zinc mediates the protective action of MT. The present study was undertaken to determine the distinct role of zinc in hepatic protection from alcoholic injury. MT I/II-knockout (MT-KO) mice along with their wild-type controls were treated with three gastric doses of ethanol at 5 g/kg at 12-hour intervals. Zinc sulfate was injected intraperitoneally in a dosage of 5 mg/kg/day for 3 days before ethanol treatment. MT concentrations in MT-KO mice were very low and zinc concentrations in MT-KO mice were lower than in wild-type mice. Zinc treatment significantly elevated hepatic MT concentrations only in wild-type mice and increased zinc concentrations in both MT-KO and wild-type mice. Ethanol treatment caused degenerative morphological changes and necrotic appearance in the livers of MT-KO mice. Microvesicular steatosis was the only ethanol-induced change in the liver of wild-type mice. Ethanol treatment decreased hepatic glutathione concentrations and increased hepatic lipid peroxidation, and the concentrations of lipid peroxide products in the wild-type mice were lower than in the MT-KO mice. All of these alcohol-induced toxic responses were significantly suppressed by zinc treatment in both MT-KO and wild-type mouse livers. These results demonstrate that zinc, independent of MT, plays an important role in protection from alcoholic liver injury. However, MT is required to maintain high levels of zinc in the liver, suggesting that the protective action of MT in the liver is likely mediated by zinc.

Gadolinium chloride pretreatment prevents cadmium chloride-induced liver damage in both wild-type and MT-null mice

The heavy metal cadmium (Cd) causes hepatotoxicity upon acute administration. Kupffer cells, the resident macrophages of the liver, have been suggested to play a role in Cd-induced hepatotoxicity. Gadolinium chloride (GdCl3) may prevent Cd-induced hepatotoxicity by suppressing Kupffer cells. However, GdCl3 also induces the Cd-binding protein, metallothionein (MT). Therefore, this study was conducted to determine whether GdCl3 prevents Cd-induced hepatotoxicity via the induction of MT. Hepatic MT and Kupffer cell counts were analyzed 24 h after wild-type (WT) mice were administered saline or 10, 30, or 60 mg GdCl3/kg. GdCl3 induced MT in a dose-dependent manner without affecting nonprotein sulfhydryl content. All examined doses of GdCl3 were effective at eliminating Kupffer cells from the liver. To examine the hepatoprotective effects of GdCl3, WT and MT-null mice were pretreated with saline or 10, 30, or 60 mg GdCl3 24 h prior to a hepatotoxic dose of Cd (2.5 mg Cd/kg). Blood and livers were removed 16 h later and analyzed for hepatotoxicity as well as MT, Cd, and Kupffer cell content. Hepatotoxicity was alleviated in both WT and MT-null mice that were pretreated with 30 or 60 mg GdCl3/kg, indicating that MT induction is not required for the hepatoprotective effects of GdCl3. Hepatic Cd content was not decreased by GdCl3, demonstrating that GdCl3 does not negatively affect Cd distribution to the liver. Kupffer cells were depleted at all three doses of GdCl3, whereas hepatoprotection was only observed at doses of 30 and 60 mg GdCl3/kg. This does not rule out Kupffer cells in the mechanism of Cd-induced hepatotoxicity, but it does suggest that GdCl3 exerts hepatoprotective effects on the liver aside from depleting Kupffer cells. In summary, these data substantially rule out MT induction and decrease the importance of Kupffer cells as mechanisms of GdCl3-induced protection from Cd-induced hepatotoxicity.

Acute cadmium exposure induces stress-related gene expression in wild-type and metallothionein-I/II-null mice

This study examined the effect of acute cadmium on stress-related gene expression and free radical production in wild-type and metallothionein-I/II-null (MT-null) mice. Atlas Toxicology arrays showed that acute cadmium (40 micromol/kg as CdCl(2), ip for 3 h) markedly increased the expression of genes encoding heat-shock proteins, heme oxygenase-1, and genes in response to DNA damage/repair. The expression of genes encoding cytochrome P450 enzymes, UDP-glucuronosyltransferases, Mn-superoxide dismutase, and catalase was suppressed by cadmium. MT-null mice were more sensitive than wild-type mice to cadmium-induced, stress-related gene expression, in accord with greater activation of transcription factor AP-1 and phosphorylated JNK and ERK. To evaluate free radical production, mice were simultaneously given the spin trap agent, N-tert-butyl-alpha-phenylnitrone (PBN, 250 mg in DMSO/kg, ip) with cadmium, and livers were removed 30 min later for PBN-trapped radical extraction with chloroform:methanol (2:1), and detected with electron spin resonance (ESR). Cadmium treatment caused detectable ESR signals for PBN adducts as well as lipid peroxidation in the liver similarly in both wild-type and MT-null mice. Thus, the mechanism of acute cadmium toxicity involves multiple facets including oxidative damage and aberrant gene expression, and absence of MT exacerbates Cd-induced aberrant gene expression.

White-sided dolphin metallothioneins: purification, characterisation and potential role

Metallothioneins (MTs) were characterised in the kidneys of a white-sided dolphin Lagenorhynchus acutus stranded along the Belgian coast, displaying high levels of cadmium (Cd) and mercury (Hg) in liver and kidney. The protein has two isoforms: MT-1 and MT-2. MT-1 binds Cu, Zn, Hg and Cd, while MT-2 only binds Zn, Hg and Cd. This suggests different metabolic functions for the two isoforms: MT-1 is mainly involved in Cu homeostasis; MT-2, which was four-fold more abundant than MT-1, detoxifies most of the accumulated cadmium.

Stability and conformational dynamics of metallothioneins from the antarctic fish Notothenia coriiceps and mouse

The structural properties and the conformational dynamics of antarctic fish Notothenia coriiceps and mouse metallothioneins were studied by Fourier-transform infrared and fluorescence spectroscopy. Infrared data revealed that the secondary structure of the two metallothioneins is similar to that of other metallothioneins, most of which lack periodical secondary structure elements such as alpha-helices and beta-sheets. However, the infrared spectra of the N. coriiceps metallothionein indicated the presence of a band, which for its typical position in the spectrum and for its sensitivity to temperature was assigned to alpha-helices whose content resulted in 5% of the total secondary structure of the protein. The short alpha-helix found in N. coriiceps metallothionein showed an onset of denaturation at 30 degrees C and a T(m) at 48 degrees C. The data suggest that in N. coriiceps metallothionein a particular cysteine is involved in the alpha-helix and in the metal-thiolate complex. Moreover, infrared spectra revealed that both proteins investigated possess a structure largely accessible to the solvent. The time-resolved fluorescence data show that N. coriiceps metallothionein possesses a more flexible structure than mouse metallothionein. The spectroscopic data are discussed in terms of the biological function of the metallothioneins.

Cadmium resistance conferred to yeast by a non-metallothionein-encoding gene of the earthworm Enchytraeus

The earthworm Enchytraeus is able to survive in cadmium (Cd)-polluted environments. Upon Cd exposure, the worms express a gene encoding the putative non-metallothionein 25-kDa cysteine-rich protein (CRP), which contains eight repeats with highly conserved cysteines in Cys-X-Cys and Cys-Cys arrangements exhibiting 36-53% identities to the 6-7-kDa metallothioneins of different organisms. Here, we demonstrate that the CRP protein confers a highly Cd-resistant phenotype to a Cd-hypersensitive yeast strain. Cd resistance increases with increasing numbers of expressed CRP repeats, but even one 3-kDa CRP repeat still mediates Cd resistance. Site-directed mutagenesis reveals that each single cysteine within a given repeat is important for Cd resistance, though to a different extent. However, replacement of other conserved amino acids such as Pro(136) and Asp(196) at the CRP repeat junctions does not affect Cd resistance. Our data indicate (i) that the non-metallothionein CRP protein is able to detoxify Cd and (ii) that this is dependent on the availability of sulfhydryl groups of the conserved cysteines.

Overexpression of metallothionein-II sensitizes rodent cells to apoptosis induced by DNA cross-linking agent through inhibition of NF-kappa B activation

DNA cross-linking agents such as mitomycin C (MMC) and cisplatin are used as chemotherapeutic agents in cancer treatment. However, the molecular mechanism underlying their antitumor activity is not entirely clear. Critical steps in cytotoxicity toward cross-linking agents can involve DNA repair efficiency, inhibition of replication, cell-cycle checkpoints, regulation, and induction of apoptosis. The complexity of the mechanisms of the mammalian cell defense against cross-linking agents is reflected by the existence of many complementation groups identified in rodent cells that are specifically sensitive to MMC. We recently showed that increased induction of apoptosis contributes to the MMC sensitivity of the group represented by the V-H4 hamster mutant cell line. In this study, through the analyses of a substractive library, we discovered that sensitive V-H4 cells display a 40-fold increase of steady-state expression of metallothionein II (MT-II) mRNA compared with resistant parental V79 cells. Down-regulation of MT-II by antisense oligonucleotides partially restores MMC resistance in V-H4 cells, indicating that MT-II overexpression is directly involved in MMC hypersensitivity of these cells. MTs have been reported to regulate the activation of NF-kappaB, one of the key proteins that modulates the apoptotic response. Here we found that NF-kappaB activation by MMC is impaired in V-H4 cells and is partially restored following down-regulation of MT-II by antisense oligonucleotides. All these data suggest that the overexpression of MT-II in V-H4 cells impairs NF-kappaB activation by MMC, resulting in decreased cell survival and enhanced induction of apoptosis.

Worms take the 'phyto' out of 'phytochelatins'

Phytochelatin synthase is the enzyme responsible for the synthesis of heavy-metal-binding peptides (phytochelatins) from glutathione and related thiols. It has recently been determined that it is not only restricted to plants and some fungi, as was once thought, but also has an essential role in heavy-metal detoxification in the model nematode Caenorhabditis elegans. These findings and others that demonstrate phytochelatin synthase-coding sequences in the genomes of several other invertebrates, including pathogenic nematodes, schistosomes and roundworms, herald a new era in phytochelatin research, in which these novel post-translationally synthesized peptides will not only be investigated in the context of phytoremediation but also from a clinical parasitological standpoint.

Retrovirally expressed metal response element-binding transcription factor-1 normalizes metallothionein-1 gene expression and protects cells against zinc, but not cadmium, toxicity

Metal response element (MRE) transcription factor-1 (MTF1), a member of the Cys2-His2 class of zinc-finger transcription factors, is best known for its robust transcriptional regulation of mammalian metallothionein (MT) genes. MTF1 is also believed to play a generalized role in regulating genes involved in protection against heavy metals and oxidative stress. MTF1 binding to MRE motifs is regulated by changes in intracellular zinc (Zn(2+)) concentration. Molecular dissection of MTF1 has been hindered by its high constitutive trans-activity following transient transfection and the failure of these systems to examine genes packaged in native chromatin. In developing a system to avoid these problems, we employed a high-efficiency retroviral transduction system to reintroduce MTF1 into mouse Mtf1(-/-) knockout cells (dko7). Electrophoretic mobility shift assays demonstrated that MTF1 retrovirally transduced dko7 cells (MTF1dko7) possess levels of inducible MTF1-MRE binding activity similar to that seen in mouse hepatoma Hepa-1 cells, and MTF1 binding could be modulated over a 20-fold range by varying the concentration of Zn(2+) present in the culture medium. The dko7 cells exhibited no change in Mt1 gene expression upon Zn(2+) or cadmium (Cd(2+)) treatment; in contrast, in MTF1dko7 cells, Zn(2+) or Cd(2+) induced MT1 mRNA accumulation in a dose-dependent manner. Interestingly, MTF1dko7 cells showed resistance to Zn(2+) toxicity, but negligible resistance to Cd(2+). Concomitantly, MT1 protein levels in MTF1dko7 cells were inducible to the same degree as that in Hepa-1 cells when treated with Zn(2+), but not with Cd(2+). Together, our studies suggest that MTF1-mediated regulation of gene expression is sufficient to protect cells against Zn(2+) toxicity and may be necessary but not sufficient to protect cells against Cd(2+) toxicity.

Perspectives on cadmium toxicity research

Since there are a plethora of studies on cadmium toxicity and poisoning in laboratory animals and humans, we have limited this review to studies that are relevant to human health issues by focusing on carcinogenicity, genotoxicity, circulatory disease, nephrotoxicity and life expectancy. Cadmium exposure has been established to induce cancer in various tissues of laboratory animals. Contrary to early findings of the lack of genotoxicity by cadmium, recent findings of mammalian cell culture studies have revealed genotoxic effects. Furthermore, cadmium exposure at relatively low doses induces circulatory diseases in laboratory animals. Despite such results of various cadmium toxicities in animal studies, data from human studies are lacking and insufficient to support the cause-effect relationship. Although cadmium is currently considered to be a human carcinogen by the International Agency for Research and Cancer, it is inappropriate to conclude that sufficient evidence on the carcinogenicity of cadmium in humans exists. It is also thought that epidemiological studies so far reported do not support the occurrence of cadmium-induced circulatory disease in humans. Since there are inconsistent reports on the relationship of cadmium exposure with the life expectancy of people living in cadmium-polluted areas, further studies are needed for clarification. It is also necessary to examine apparent discrepancies in result between humans and experimental animals. It has been established that long-term exposure to cadmium causes renal dysfunction in both humans and experimental animals, and whether there are any differences in the inducibility of metallothionein in the kidney warrants further study.

Role of cadmium in the regulation of AR gene expression and activity

Treatment of human prostate cancer cells, LNCaP, with cadmium stimulated cell growth. There was a 2.4-fold increase in the population of cells in the S + G(2)M phase by d 4 and a 2.7-fold increase in cell number by d 8. The metal decreased the concentration of AR protein and mRNA (80 and 60%, respectively) and increased the expression of prostate-specific antigen and the homeobox gene, NKX 3.1 (6-fold) that was blocked by an antiandrogen. In addition, cadmium activated the AR in mouse L cells containing an MMTV-luciferase reporter gene (4-fold increase) and in COS-1 cells transfected with wild-type AR and an MMTV-CAT reporter gene (7-fold increase). Cadmium also activated a chimeric receptor (GAL-AR) containing the hormone-binding domain of AR. The metal bound to AR with an equilibrium dissociation constant of 1.19 x 10(-10) M. Cadmium blocked the binding of androgen to the receptor but did not alter its affinity (dissociation constant = 2.8 x 10(-10) M), suggesting that the metal is an inhibitor of hormone binding. In castrated animals, a single, low, environmentally relevant dose of cadmium (20 microg/kg body weight) increased the wet weight of the prostate (1.97- to 3-fold) and the seminal vesicle complex (approximately 1.5-fold) and increased the expression of the androgen-regulated gene, probasin (27-fold). The in vivo effects were also blocked by an antiandrogen.

Phytochelatins and metallothioneins: roles in heavy metal detoxification and homeostasis

Among the heavy metal-binding ligands in plant cells the phytochelatins (PCs) and metallothioneins (MTs) are the best characterized. PCs and MTs are different classes of cysteine-rich, heavy metal-binding protein molecules. PCs are enzymatically synthesized peptides, whereas MTs are gene-encoded polypeptides. Recently, genes encoding the enzyme PC synthase have been identified in plants and other species while the completion of the Arabidopsis genome sequence has allowed the identification of the entire suite of MT genes in a higher plant. Recent advances in understanding the regulation of PC biosynthesis and MT gene expression and the possible roles of PCs and MTs in heavy metal detoxification and homeostasis are reviewed.

Metallothionein protects retinal pigment epithelial cells against apoptosis and oxidative stress

The retina expresses metallothionein (MT) which has been reported to protect cells against oxidative stress and apoptosis. The types of MT expressed by human retinal cells were identified by laser capture microdissection and RT--PCR and it was found that MT-2a is expressed by retinal pigment epithelial (RPE) cells, photoreceptor cells, inner nuclear layer cells and ganglion cells while MT-1a is expressed by RPE cells and MT-3 by cells of the neural retina. MT is induced in cultured human RPE cells under stress conditions such as the presence of glucocorticoids, interleukin-1/TNF alpha, oxygen and TGF beta 1. Cultured human D407 RPE cells were transfected with plasmids that allowed the expression of MT to be controlled via the tet operator protein by the level of tetracycline in the medium. These experiments showed that elevation of MT levels by transfection of RPE cells protects them against toxic levels of cadmium, heme- and iron-induced oxidation and UV light-induced apoptosis.

Cadmium-containing granules in kidney tissue of the Atlantic white-sided dolphin (Lagenorhyncus acutus) off the Faroe Islands

Top predators from the northern sub-polar and polar areas exhibit high cadmium concentrations in their tissues. In the aim to reveal possible adverse effects, samples of five Atlantic white-sided dolphins Lagenorhyncus acutus have been collected on the occasion of the drive fishery in the Faroe Islands, for ultrastructural investigations and energy dispersive X-ray microanalyses. Cadmium concentrations were less than the limit of detection in both immature individuals and ranged from 22.7 to 31.1 microg x g(-1) wet weight in the mature individuals. Two individuals with the highest cadmium concentrations exhibited electron dense mineral concretions in the basal membranes of the proximal tubules. They are spherocrystals made up of numerous strata mineral deposit of calcium and phosphorus together with cadmium. Cadmium has been detected with a molar ratio of Ca:Cd of 10:1 in the middle of these concretions. To our knowledge, this is the first report of such granules in a wild vertebrate. The role of these granules in the detoxification of the metal and the possible pathological effects are considered.

Killifish metallothionein messenger RNA expression following temperature perturbation and cadmium exposure

Metallothionein (MT), a cysteine-rich metal binding protein, is considered to play an essential role in the regulation of intracellular metals. Induction of MT in mammalian and nonmammalian tissues following heavy metal exposure may serve as a defense mechanism and a biomarker of environmental exposure to chemical stressors such as toxic metals. In this study, MT messenger RNA (mRNA) expression was characterized in male and female nonspawning and spawning killifish (Fundulus heteroclitus) following an 8-day exposure to specific sublethal stressors, which included temperature perturbation (26 degrees C or 10 degrees C) and/or 6 ppb of waterborne cadmium chloride (CdCl2). Hepatic, gill, and intestinal MT mRNA, expressed as copy number per microgram of total RNA, was assessed by reverse transcriptase-polymerase chain reaction and electrochemiluminescence using winter flounder (Pleuronectes americanus) MT complementary DNA primers. Liver, gill, and intestine MT mRNA expression was significantly (P < 0.05) increased in nonspawning killifish exposed to 26 degrees C compared with those exposed to 19 degrees C (control). In addition, a significant (P < 0.05) increase in gill MT mRNA induction was observed in nonspawning killifish exposed to 6 ppb of waterborne CdCl2 compared with controls. The results of this study demonstrate significant MT mRNA induction in nonspawning killifish following short-term exposure to physiological and chemical stressors. Thus, further research may be necessary before the use of killifish MT mRNA induction as a biomarker of environmental chemical stress exposure alone.

Interactions between cadmium and zinc in the organism

It is well known that many toxic effects of cadmium (Cd) action result from interactions with essential elements, including zinc (Zn). These interactions can take place at different stages of absorption, distribution in the organism and excretion of both metals and at the stage of Zn biological functions. Exposure to Cd leads to disturbance in Zn in the organism on the one hand, while dietary Zn intake has an important effect on Cd absorption, accumulation and toxicity on the other. The Zn status of the body is important in relation to development of Cd toxicity. Numerous data show that increased Zn supply may reduce Cd absorption and accumulation and prevent or reduce the adverse actions of Cd, whereas Zn deficiency can intensify Cd accumulation and toxicity. In this review, the interactions between these two trace elements in humans and animals are discussed on the basis of the available literature and our own results, against the background of general population exposure to Cd and common nutritional deficiency of Zn.

Metallothionein expression and localization in rat bone tissue after cadmium injection

We investigated the induction of metallothionein (MT) by cadmium (Cd) in the bone tissue of rats. To clarify the cell response to Cd in bone, the isoform-specific expression of MT mRNAs (MT-I and MT-II) was examined by reverse transcriptase-polymerase chain reaction (RT-PCR). Both MT-I and MT-II mRNA levels were increased within 3 h by Cd administration. MT (MT-I/MT-II) localization after single Cd injection were also confirmed by immunohistochemical studies. Notably, MT-positive cells were time-dependently increased, and the positive cells were mainly localized in osteocytes. The cell-specific induction of MT may be associated with Cd accumulation and Cd-induced bone injury in vivo. Furthermore, we also found that MT was consecutively expressed in some osteoclasts of control rats. This finding suggested a new role of osteoclasts in bone metabolism.

Metallothionein-null and wild-type mice show similar cadmium absorption and tissue distribution following oral cadmium administration

Cadmium (Cd) is an environmental pollutant and is toxic to many tissues. Food is the primary source of Cd exposure for the general population. Metallothionein (MT), a cysteine-rich, Cd-binding protein, plays an important role in Cd detoxication. However, the role of MT in Cd absorption and distribution is still controversial. For example, some reports assert that MT in the intestine decreases Cd absorption and increases its distribution to the kidney, relative to the liver. Therefore, to further clarify the role of MT in Cd absorption and tissue distribution, MT-I/II knockout (MT-null) mice and their parental background wild-type mice were given a single dose of (109)Cd (1-300 micromol/kg po or 0.1-30 micromol/kg iv). Cd content in 15 organs was determined 4 h after Cd administration by gamma scintillation spectrometry. Approximately 60% of the Cd administered iv was retained in liver, and about 5% was retained in kidney in both MT-null and wild-type mice. The distribution of iv administered Cd was independent of dose. In contrast, when administered po, approximately 0.15% of the lowest dose (1 micromol/kg) and 0.75% of the highest dose (300 micromol/kg) was detected in the liver of both MT-null and wild-type mice. Similarly in kidney, approximately 0.05% of the dose was detected after the lowest dose and about 0.15% after the higher doses in both MT-null and wild-type mice. In summary, this study demonstrates that the absorption and initial distribution of orally administered Cd is dose dependent but is not influenced by MT.

Overexpression of glutathione S-transferase II and multidrug resistance transport proteins is associated with acquired tolerance to inorganic arsenic

Recent work shows that long-term exposure to low levels of arsenite induces malignant transformation in a rat liver epithelial cell line. Importantly, these chronic arsenic-exposed (CAsE) cells also develop self-tolerance to acute arsenic exposure. Tolerance is accompanied by reduced cellular arsenic accumulation, suggesting a mechanistic basis for reduced arsenic sensitivity. The present study examined the role of xenobiotic export pumps in acquired arsenic tolerance. Microarray analysis of CAsE cells showed increased expression of the genes encoding for glutathione S-transferase Pi (GST-Pi), multidrug resistance-associated protein genes (MRP1/MRP2, which encode for the efflux transporter Mrp1/Mrp2) and the multidrug resistance gene (MDR1, which encodes for the efflux transporter P-glycoprotein). These findings were confirmed at the transcription level by reverse transcription-polymerase chain reaction and at the translation level by Western-blot analysis. Acquired arsenic tolerance was abolished when cells were exposed to ethacrynic acid (an inhibitor of GST-Pi), buthionine sulfoximine (a glutathione synthesis inhibitor), MK571 (a specific inhibitor for Mrps), and PSC833 (a specific inhibitor for P-glycoprotein) in dose-dependent fashions. MK571, PSC833, and buthionine sulfoximine markedly increased cellular arsenic accumulation. Consistent with a role for multidrug resistance efflux pumps in arsenic resistance, CAsE cells were found to be cross-resistant to cytotoxicity of several anticancer drugs, such as vinblastine, doxorubicin, actinomycin-D, and cisplatin, that are also substrates for Mrps and P-glycoprotein. Thus, acquired tolerance to arsenic is associated with increased expression GST-Pi, Mrp1/Mrp2 and P-glycoprotein, which function together to reduce cellular arsenic accumulation.

Mercury vapor uptake into the nervous system of developing mice

The localisation of mercury in the developing nervous system following mercury vapor (Hg(0)) exposure is not clear. We therefore looked for mercury in the mouse nervous system following fetal or neonatal exposure to Hg(0). Mice were exposed to 50 or 500 microg/m(3) Hg(0) for 4 h a day for 5 days in late pregnancy, and pups sacrificed on postnatal day (P)1 or P40. Neonatal mice were exposed to 500 microg/m(3) Hg(0) for 2 h between P1 and P23, and were sacrificed 2 days later or at P40. Paraffin sections of the nervous system were stained with autometallography to detect inorganic mercury. No mercury was seen in the nervous system of pups after fetal exposure to the 50 microg/m(3) Hg(0) dose rate. After fetal exposure to the 500 microg/m(3) Hg(0) dose rate, mercury was seen in nervous system blood vessels and sensory ganglia. No mercury was seen in the nervous system after neonatal exposure to 500 microg/m(3) Hg(0) for 2 h between P1 and P10. From this exposure at P11 onwards, mercury was detected in motor neurons. The lack of stainable mercury in early developing central neurons suggests that the fetal and neonatal nervous systems are somehow protected from Hg(0) uptake.

Identification of a cadmium-induced gene in Mycobacterium bovis and Mycobacterium tuberculosis

A 17-kDa protein (CadI) was induced by cadmium in Mycobacterium bovis and Mycobacterium tuberculosis. Comparison of the N-terminal sequence from M. bovis CadI with the annotated M. tuberculosis genome database identified Rv2641 as the encoding gene. Long and short promoter fragments from M. bovis cadI were fused to the lacZ reporter gene in pYUB76. Only the long fragment directed cadmium-inducible activity when electroporated into M. bovis. The cadI promoter has potential for both constitutive and inducible expression studies in M. bovis and M. tuberculosis.