Reversible zinc exchange between metallothionein and the estrogen receptor zinc finger

Interaction of metallothionein with tumor suppressor p53 protein

Previous reports have shown that metallothionein (MT) may modulate p53 activity through zinc exchange. However, little is known on a direct interaction between MT and p53 in cells. The results demonstrate an interaction between MT and p53 can occur in vitro. The complex between MT and p53 was observed in breast cancer epithelial cells with both wild and inactive type of p53. Furthermore, it was shown that wt-p53 was preferentially associated with Apo-MT. Our data suggest that co-expression of MT and p53 and their complex formation in tumor cells may be involved in regulation of apoptosis in these cells.

Determination of Cd, Cu, Pb and Zn in neoplastic kidneys and in renal tissue of fetuses, newborns and corpses

The incidence of kidney tumors in USA and Europe (in particular, Central Europe and Italy) has been dramatically increasing since the 1970s, possibly as a consequence of ongoing environmental pollution. Environmental factors have been considered responsible for at least 80% of the incidence of neoplastic diseases. To shed some light on this issue, the amounts of Cd and Pb were measured in neoplastic tissue and adjacent normal part of kidney excised for carcinoma and compared with those in renal tissues of fetuses, newborns and subjects that died of non-neoplastic diseases. Cd and Pb were determined by Inductively Coupled Plasma Atomic Emission Spectrometry and Atomic Absorption Spectrometry with Electrothermal Atomization. Metallothionein immunoperoxidase staining technique was used to localize the accumulation of Cd and Zn in the nephrons. Content of Cd and Pb in kidneys of fetuses and newborns was extremely low. However, it was significantly increased in adjacent-normal tissues of kidneys with carcinomas, and significantly higher compared to kidneys of individuals that died of non-neoplastic diseases. In tumoral tissues of the excised kidneys, Cd content was very low, while that of Pb significantly elevated. High amounts of Cd and Pb in the adjacent-normal parts of kidneys with carcinomas are suggestive of possible, individual or synergistic, effects of these pollutants on enzymatic systems, priming an oncogenic pathway. Detection of metallothioneins, primary ligands of Cd, exclusively in the cells of proximal tubuli, i.e. wherein renal carcinoma develops in over 80% of cases, strongly supports the assumption that Cd exerts a carcinogenic effect.

Zinc prevention and treatment of alcoholic liver disease

Alcoholic liver disease (ALD) is associated with decreases in zinc (Zn) and its major binding protein, metallothionein (MT), in the liver. Studies using animal models have shown that Zn supplementation prevents alcohol-induced liver injury under both acute and chronic alcohol exposure conditions. There are hepatic and extrahepatic actions of Zn in the prevention of alcoholic liver injury. Zn supplementation attenuates ethanol-induced hepatic Zn depletion and suppresses ethanol-elevated cytochrome P450 2E1 (CYP2E1) activity, but increases the activity of alcohol dehydrogenase in the liver; an action that is likely responsible for Zn suppression of alcohol-induced oxidative stress. Zn also enhances glutathione-related antioxidant capacity in the liver. At the cellular level, Zn inhibits alcohol-induced hepatic apoptosis partially through suppression of the Fas/FasL-mediated pathway. Zn supplementation preserves intestinal integrity and prevents endotoxemia, leading to inhibition of endotoxin-induced tumor necrosis factor-alpha (TNF-alpha) production in the liver. Zn also directly inhibits the signaling pathway involved in endotoxin-induced TNF-alpha production. These hepatic and extrahepatic effects of Zn are independent of MT. However, low levels of MT in the liver sensitize the organ to alcohol-induced injury, and elevation of MT enhances the endogenous Zn reservoir and makes Zn available when oxidative stress is imposed. Zn has a high potential to be developed as an effective agent in the prevention and treatment of ALD.

Metallothionein transfers zinc to mitochondrial aconitase through a direct interaction in mouse hearts

Previous studies have shown that in a cell-free system, metallothionein (MT) releases zinc when the environment becomes oxidized and the released zinc is transferred to a zinc-binding protein if such a protein is present. However, it is unknown whether and how zinc transfers from MT to other proteins in vivo. The present study was undertaken to test the hypothesis that if zinc transfer from MT to other proteins occurs in vivo, the transfer would proceed through a direct interaction between MT and a specific group of proteins. The heart extract obtained from MT-null mice was incubated with 65Zn-MT or 65ZnCl2 and the proteins receiving 65Zn were separated by blue-native PAGE (BN-PAGE) or sodium dodecyl sulfate-PAGE (SDS-PAGE), and detected by autoradiography. A unique 65Zn-binding band was observed from the 65Zn-MT-incubated, but not the 65ZnCl2-incubated preparation. The analysis using matrix assisted laser desorption/ionization-time-of-flight (MALDI-TOF) mass spectrometry revealed that mitochondrial aconitase (m-aconitase) was among the proteins accepting Zn directly from Zn-MT. The m-aconitase, not the cytosolic aconitase (c-aconitase), was co-immunoprecipitated with MT. This study demonstrates that MT transfers zinc to m-aconitase through a direct interaction.

Sex-specific effects of neonatal exposures to low levels of cadmium through maternal milk on development and immune functions of juvenile and adult rats

Cadmium (Cd) is a major environmental contaminant. Although immunotoxic effects have been associated with Cd exposure, the inconsistency of experimental results underlines the need of an experimental approach more closely related to environmental conditions. We investigated the effects of exposing neonatal Sprague-Dawley rats to environmentally relevant doses of Cd through maternal milk. Dams received 10 parts per billion (ppb) or 5 parts per million (ppm) Cd chloride (CdCl2) in drinking water from parturition until the weaning of the pups. Half of the offspring was sampled at weaning time. The remaining juvenile rats received water without addition of Cd until adulthood. Cd accumulation in kidneys of juvenile rats fed from dams exposed to Cd indicated the transfer of the metal from mother to pups through maternal milk. This neonatal exposure resulted in decreased body, kidney and spleen weights of just weaned females but not of males. This effect was more pronounced in the less exposed females fed from dams exposed to 10 ppb Cd, which also displayed lower hepatic metallothionein-1 (MT-1) mRNA levels. The effect of Cd exposure on body and organ weights did not persist to adulthood. In contrast, we observed gender-specific effects of neonatal Cd exposure on the cytotoxic activity of splenic NK-cells of both juvenile and adult rats. Cd also strongly inhibited the proliferative response of Con A-stimulated thymocytes in both male and female adult rats 5 weeks after the cessation of Cd exposure. These immunotoxic effects were observed at doses much lower than those reported to produce similar effects when exposure occurred during adulthood. In conclusion, neonatal exposures to environmentally relevant levels of Cd through maternal milk represent a critical hazard liable to lead to both transitory and persistent immunotoxic effects.

Impaired hepatic regeneration in metallothionein-I/II knockout mice after partial hepatectomy

Although the translocation of metallothionein (MT) from cytoplasm to nucleus has been demonstrated in liver during times of high requirement for zinc (fetal development and the neonatal period), the role of MT in cellular growth is not well understood. In this study, a potential role of MT in liver regeneration was investigated in wild type (WT) and MT-I and MT-II gene knockout (MT-null) mice after 35% partial hepatectomy (PH) or sham laparotomy. Hepatic MT levels and proliferation index were measured at 0, 5, 15, 24, 36, 48, and 60 hrs after PH and 48 hrs after sham laparotomy (control). MT levels were increased in WT mice (peak at 24 hrs after PH) and declined to normal levels by 60 hrs after PH. Immunohistochemical staining for MT in WT mice indicated the presence of MT in both nucleus and cytoplasm of hepatocytes at 24 hrs after PH, whereas MT was present mainly in the cytoplasm at 36-60 hrs after PH and 48 hrs after sham laparotomy. Hepatic proliferation index in both WT and MT-null mice, as determined by argyrophilic nucleolar organizing region staining and proliferating cell nuclear antigen immunohistochemical staining, reached a peak at 48 hrs and declined by 60 hrs after PH. Cell proliferation was significantly less in MT-null mice as compared to WT mice during liver regeneration after PH. These results suggest that MT may play a positive role in hepatic regeneration after PH.

Metallothionein expression in invasive and in situ breast carcinomas

BACKGROUND/METHODS

The primary aims of this study were to examine the expression of metallothionein (MT) in 123 primary invasive breast carcinomas and the in situ components of these carcinomas and to assess the association between MT expression and certain socio-demographic and clinico-pathologic characteristics. MT expression was assessed using immunohistochemical procedures and semi-quantified using an immunoreactivity score.

RESULTS

Results showed that 57.7% of the invasive tumors and 43.3% of the in situ carcinomas in the study were MT-positive. Chi-squared analyses showed that MT expression was significantly higher in the tumors of women categorized as being of 'other' race and of women with tumors of high histological grade.

CONCLUSIONS

The results of this study suggest that MT is a biomarker of tumor differentiation and aggressiveness and that MT expression may differ by race.

Metal binding of metallothionein-3 versus metallothionein-2: lower affinity and higher plasticity

Mammalian metallothioneins (MTs) are involved in cellular metabolism of zinc and copper and in cytoprotection against toxic metals and reactive oxygen species. MT-3 plays a specific role in the brain and is down-regulated in Alzheimer's disease. To evaluate differences in metal binding, we conducted direct metal competition experiments with MT-3 and MT-2 using electrospray ionization mass spectroscopy (ESI-MS). Results demonstrate that MT-3 binds Zn2+ and Cd2+ ions more weakly than MT-2 but exposes higher metal-binding capacity and plasticity. Titration with Cd2+ ions demonstrates that metal-binding affinities of individual clusters of MT-2 and MT-3 are decreasing in the following order: four-metal cluster of MT-2>three-metal cluster of MT-2 approximately four-metal cluster of MT-3>three-metal cluster of MT-3>extra metal-binding sites of MT-3. To evaluate the reasons for weaker metal-binding affinity of MT-3 and the enhanced resistance of MT-3 towards proteolysis under zinc-depleted cellular conditions, we studied the secondary structures of apo-MT-3 and apo-MT-2 by CD spectroscopy. Results showed that apo-MT-3 and apo-MT-2 have almost equal helical content (approximately 10%) in aqueous buffer, but that MT-3 had slightly higher tendency to form alpha-helical secondary structure in TFE-water mixtures. Secondary structure predictions also indicated some differences between MT-3 and MT-2, by predicting random coil for common MTs, but 22% alpha-helical structure for MT-3. Combined, all results highlight further differences between MT-3 and common MTs, which may be related with their functional specificities.

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.

Interprotein metal exchange between transcription factor IIIa and apo-metallothionein

Zn(2+) and Cd(2+) ion exchange between transcription factor IIIA (TFIIIA) and apo-metallothionein (MT) were studied using a combination of methods including chromatography, ultrafiltration and UV spectroscopy. Under near stoichiometric conditions, apoMT was able to remove most if not all of the zinc ions from TFIIIA, whether or not the TFIIIA was bound to the 5S DNA internal control region (ICR), and concomitantly inhibit its DNA-binding activity as indicated by an electrophoretic mobility shift assay. The kinetics of the two processes were similar. The rate of the metal exchange reaction increased with the concentrations of both reactants. A second-order rate constant of 30+/-10 M(-1)s(-1) was calculated. Similar observations were made for the reaction between apoMT and Cd-substituted TFIIIA, which proceeded without observable intermediates according to a spectrophotometric analysis. A very slow metal ion exchange occurred between Cd-TFIIIA and Zn-MT, but not between Cd-MT and Zn-TFIIIA. Comparative studies on the reaction of TFIIIA with a small competing ligand, ethylenedinitrilo-tetraacetic acid (EDTA), were also conducted. Although EDTA reacts with free Zn-TFIIIA, under similar conditions it failed to compete for Zn(2+) bound as Zn-TFIIIA-ICR.

Zinc-metallothionein genoprotective effect is independent of the glutathione depletion in HaCaT keratinocytes after solar light irradiation

UV radiations are the major environmental factors that induce DNA damage of skin cells either by direct absorption (UVB), or after inducing an oxidative stress (UVA and UVB). Cells maintain a reducing intracellular environment to avoid genomic damage. MTs have been expected not only to control metal homeostasis but also counteract the glutathione (GSH) depletion induced by oxidative stress because of their high thiol content. Induction and redistribution of MTs in cultured human keratinocytes (HaCaT) in response to SSL, is an important cellular defense mechanism against DNA damage. Reduced glutathione (GSH) is another way of cellular protection against UV-induced oxidative stress. This study which extend our previous finding focused on the relation between intracellular GSH and Zn genoprotective effects after solar irradiation. HaCaT cells, depleted or not in GSH by a chemical treatment were used to compare MTs induction by Northern blot, expression by Western blot and localization using immunocytochemistry. Zn genoprotection experiments after SSL irradiation was carried out by the comet assay. We demonstrated that in absence of GSH, Zn-MTs could protect DNA after SSL irradiation and that GSH depletion has no effect on MTs induction and localization. Nuclear Zn-MTs could be responsible for this observed genoprotection in GSH depleted cells. So the GSH/Zn and the MT/Zn systems could be two independent but interacting mechanisms of cellular protection against SSL injury.

Induction of hepatic metallothionein synthesis by endoplasmic reticulum stress in mice

Metallothionein (MT) is a small sulfhydryl-rich protein whose levels are elevated by various inducers of organelle stresses, such as nuclear stress (cisplatin), mitochondrial stress (antimycin A, 2,4-dinitrophenol) and lysosomal stress (paraquat). Although abnormal folding of protein in the endoplasmic reticulum (ER) causes ER stress, induction of MT synthesis by ER stress has never been investigated. In this study, we examined the induction of MT by an inducer of ER stress, tunicamycin (Tun), which induces ER stress by inhibiting N-linked glycosylation of protein in the ER. Administration of Tun (0.5-1.5 mg/kg, sc) increased hepatic MT levels in C57BL/6J mice (3.1-fold). The maximal increase in hepatic MT was observed 48-96 h after the administration of Tun (1.0 mg/kg). Expressions of MT-I, II and glucose-regulated protein 78 (Bip/GRP78), which is a molecular chaperone induced by ER stress, mRNA were also detected by administration of Tun. Thapsigargin (Thap), a generator of ER stress by inhibiting ER Ca(2+)-ATPase, also increased both hepatic MT levels and expression of MT-I and -II mRNA. The level of expression of Bip/GRP78 mRNA induced by Tun administration in MT-null mice was greater than that in wild-type mice. Taken together, these findings suggest that inhibitors of ER are potent inducers of MT.

Differentially expressed genes in HIV-1-infected macrophages following treatment with the virus-suppressive immunomodulator murabutide

The synthetic immunomodulator murabutide has been found to suppress human immunodeficiency virus type-1 (HIV-1) replication, in macrophages, through a regulated expression of cellular factors needed at different steps in the virus replication cycle. To identify cellular genes implicated in the murabutide-induced virus inhibition, we have carried out a differential display analysis on HIV-1-infected macrophages that were treated, or not, with murabutide. Sequencing of the differentially regulated cDNA bands and verification of the reproducibility of the murabutide effects, by reverse transcription-polymerase chain reaction or by Northern blotting, revealed an up-regulated expression of 21 genes and a down-regulation of seven others. The murabutide-regulated genes encoded proteins implicated in DNA binding, regulation of transcription, oxidative stress, metal binding, and other physiological functions. Six of the genes corresponded to unassigned/expressed sequence tags with yet unknown function. Among the genes which were up-regulated by murabutide and with established effects on inhibiting virus transcription, was the octamer binding factor 1 (Oct-1). We demonstrate the ability of murabutide to induce enhanced Oct-1 protein expression and DNA-binding activity in macrophages. Furthermore, our findings suggest the potential implication of additional transcription factors and metal-binding proteins in mediating the inhibitory effect of murabutide on virus transcription.

Suppression of breast cancer by chemical modulation of vulnerable zinc fingers in estrogen receptor

Current antiestrogen therapy for breast cancer is limited by the mixed estrogenic and antiestrogenic activity of selective estrogen receptor modulators. Here we show that the function of zinc fingers in the estrogen receptor DNA-binding domain (DBD) is susceptible to chemical inhibition by electrophilic disulfide benzamide and benzisothiazolone derivatives, which selectively block binding of the estrogen receptor to its responsive element and subsequent transcription. These compounds also significantly inhibit estrogen-stimulated cell proliferation, markedly reduce tumor mass in nude mice bearing human MCF-7 breast cancer xenografts, and interfere with cell-cycle and apoptosis regulatory gene expression. Functional assays and computational analysis support a molecular mechanism whereby electrophilic agents preferentially disrupt the vulnerable C-terminal zinc finger, thus suppressing estrogen receptor-mediated breast carcinoma progression. Our results provide the proof of principle for a new strategy to inhibit breast cancer at the level of DNA binding, rather than the classical antagonism of estrogen binding.

Activity of metal-responsive transcription factor 1 by toxic heavy metals and H2O2 in vitro is modulated by metallothionein

Metallothioneins are small, cysteine-rich proteins that avidly bind heavy metals such as zinc, copper, and cadmium to reduce their concentration to a physiological or nontoxic level. Metallothionein gene transcription is induced by several stimuli, notably heavy metal load and oxidative stress. Transcriptional induction of metallothionein genes is mediated by the metal-responsive transcription factor 1 (MTF-1), an essential zinc finger protein that binds to specific DNA motifs termed metal-response elements. In cell-free DNA binding reactions with nuclear extracts, MTF-1 requires elevated zinc concentrations for efficient DNA binding but paradoxically is inactivated by other in vivo inducers such as cadmium, copper, and hydrogen peroxide. Here we have developed a cell-free, MTF-1-dependent transcription system which accurately reproduces the activation of metallothionein gene promoters not only by zinc but also by these other inducers. We found that while transcriptional induction by zinc can be achieved by elevated zinc concentration alone, induction by cadmium, copper, or H2O2 additionally requires the presence of zinc-saturated metallothionein. This is explained by the preferential binding of cadmium or copper to metallothionein or its oxidation by H2O2; the concomitant release of zinc in turn leads to the activation of transcription factor MTF-1. Conversely, thionein, the metal-free form of metallothionein, inhibits activation of MTF-1. The release of zinc from cellular components, including metallothioneins, and the sequestration of zinc by newly produced apometallothionein might be a basic mechanism to regulate MTF-1 activity upon cellular stress.

Zinc-induced NF-kappaB inhibition can be modulated by changes in the intracellular metallothionein level

Metallothionein (MT), a small metal-binding protein, is involved in the regulation of cellular metal homeostasis. Sequestration and the release of metals to and from MT plays an important role in the attenuation or amplification of signal transduction. Zinc has been suggested to be an important regulator of nuclear factor kappaB (NF-kappaB). In this study, the effect of MT expression on the zinc-induced inhibition of NF-kappaB activity was examined. In HeLa cells, pyrrolidine dithiocarbamate (PDTC), a zinc ionophore, and zinc itself inhibited NF-kappaB activity. When the cells were pretreated with MT-inducers, cadmium, or dexamethasone, PDTC did not inhibit NF-kappaB activity. We transfected HeLa cells with a DNA construct in which expression of MT-IIA is controlled by tet operator protein. Treatment of HeLa cells with doxycycline, a tetracycline analogue, induced the expression of MT-IIA, which attenuated the effect of PDTC on NF-kappaB activity. These results implicate MT in the zinc regulation of NF-kappaB and identify MT as one of the potential intracellular modulators of NF-kappaB activation.

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.

Effects of cadmium and zinc on solar-simulated light-irradiated cells: potential role of zinc-metallothionein in zinc-induced genoprotection

Zinc is an essential oligoelement for cell growth and cell survival and has been demonstrated to protect cells from oxidative stress induced by UVA or from genotoxic stress due to UVB. In a recent work we demonstrated that the antioxidant role of zinc could be related to its ability to induce metallothioneins (MTs). In this study we identified the mechanism of zinc protection against solar-simulated light (SSL) injury. Cultured human keratinocytes (HaCaT) were used to examine MTs expression and localization in response to solar-simulated radiation. We found translocation to the nucleus, with overexpression of MTs in irradiated cells, a novel observation. The genoprotective effect of zinc was dependent on time and protein synthesis. DNA damage was significantly decreased after 48 h of ZnCl(2) (100 microM) treatment and is inhibited by actinomycin D. ZnCl(2) treatment (100 microM) led to an intense induction, redistribution, and accumulation of MT in the nucleus of irradiated cells. MT expression correlated with the time period of ZnCl(2) treatment. CdCl(2), a potent MT inducer, did not show any genoprotection, although the MTs were expressed in the nucleus. Overall our findings demonstrate that MTs could be a good candidate for explaining the genoprotection mediated by zinc on irradiated cells.

MRE-binding transcription factor-1 is activated during endotoxemia: a central role for metallothionein

Endotoxin (LPS) has been established to induce hepatic metallothionein (MT), but the specific role of MT remains unknown. In this study, we examined whether MT can modulate MTF-1 activity during endotoxemia. Treatment with IL-6, the main mediator of MT induction during endotoxemia, enhanced the expression of the MRE(d)-driven reporter gene. MTF-1 DNA-binding activity was increased 16-24 h after LPS administration in wild-type mice, while no such activation was observed in MT-null mice during the same period. The expression of alpha(1)-acid glycoprotein (AGP) mRNA, an RNA regulated by MTF-1, was lower in MT-null than in wild-type mice. Our results suggested that MTF-1 was activated during endotoxemia. MT can act as an activator of MTF-1, and MT can induce MTF-1 targeted gene expression during endotoxemia.

Zinc-metallothionein levels are correlated with enhanced glucocorticoid responsiveness in mouse cells exposed to ZnCl(2), HgCl(2), and heat shock

Metallothioneins (MTs) are the major low molecular weight, zinc-binding proteins in mammalian cells. It has been hypothesized that they play a role in the function of zinc-dependent signal transduction proteins and transcription factors. We investigated the capacity of zinc and other metal ions and conditions to increase both Zn-associated MT levels and the receptiveness of cells to transcriptional activation mediated by the zinc-dependent glucocorticoid receptor (GR). We studied, in a GR-responsive mouse mammary-tumor cell line, the ability of dexamethasone (DEX) to stimulate transcription of a chloramphenicol acetyltransferase (CAT) gene controlled by a mouse mammary-tumor virus promoter. In cells pretreated with 20 to 100 microM ZnCl(2), DEX-induced CAT activity correlated with zinc-induced MT levels. However, 0.05 to 0.5 microM CdCl(2) had no effect on CAT activity, despite an increase in Cd-associated MT. Copper-associated MT was detected in cells treated with 20 microM CuCl(2,) but there was no change in the level of Zn-MT, nor was CAT activity altered in cells exposed to 5 to 20 microM CuCl(2). These results may reflect a functional difference between zinc-associated MT, and MT associated with other metals. Significantly more CAT activity was observed in both heat-shocked cells and in cells exposed to 40 or 50 nM HgCl(2). Although absolute amounts of MT were unchanged by these two treatments, a higher percentage of total cellular zinc was associated with the MT protein fractions after treatment. Changes in GR levels could not account for variations in CAT activity. These data indicate that hormonal signalling can be altered by exposure to metal salts and heat shock, and the effect is correlated with the level of Zn-MT.

The effects of physiologically important nonmetallic ligands in the reactivity of metallothionein towards 5,5'-dithiobis(2-nitrobenzoic acid). A new method for the determination of ligand interactions with metallothionein

The reaction of Cd5Zn2-metallothionein (MT) with 5,5'-dithiobis(2-nitrobenzoic acid) (Nbs2) has been studied at different reagent stoichiometries, pH and temperature conditions and in the presence of several ligands. At stoichiometries of Nbs2 to MT from 0.5 to 5, the reaction followed first order kinetics. The first order rate constants obtained were independent from the concentration of Nbs2 but were linearly dependent on the concentration of MT. At higher Nbs2/MT stoichiometries, the reaction deviates from first order kinetics and the observed rate constant increases. The reactivity of MT towards Nbs2 has been probed at 4 microM concentration of both reagents where the reaction is monophasic and is characterized by a linear Arrhenius plot (Ea = 45.8 +/- 2.7 kJ.mol-1). It has been demonstrated that metal release at low pH or subtraction from MT by EDTA substantially increases the reactivity of MT towards Nbs2. At the same time, a number of nonmetallic ligands moderately accelerate the reaction of MT with Nbs2 and hyperbolic dose-response curves were obtained. The data have been interpreted with the binding of ligands to MT and following MT. Ligand binding constants were calculated as follows: ATP, K = 0.31 +/- 0.06 mM; ADP, K = 0.26 +/- 0.07 mM. Several compounds such as AMP, S-methylglutathione, and phosphate had no effect on the reaction, but Zn2+ ions showed an inhibitory effect at micromolar concentrations.

A metallothionein containing a zinc finger within a four-metal cluster protects a bacterium from zinc toxicity

Zinc is essential for many cellular processes, including DNA synthesis, transcription, and translation, but excess can be toxic. A zinc-induced gene, smtA, is required for normal zinc-tolerance in the cyanobacterium Synechococcus PCC 7942. Here we report that the protein SmtA contains a cleft lined with Cys-sulfur and His-imidazole ligands that binds four zinc ions in a Zn(4)Cys(9)His(2) cluster. The thiolate sulfurs of five Cys ligands provide bridges between the two ZnCys(4) and two ZnCys(3)His sites, giving two fused six-membered rings with distorted boat conformations. The inorganic core strongly resembles the Zn(4)Cys(11) cluster of mammalian metallothionein, despite different amino acid sequences, a different linear order of the ligands, and presence of histidine ligands. Also, SmtA contains elements of secondary structure not found in metallothioneins. One of the two Cys(4)-coordinated zinc ions in SmtA readily exchanges with exogenous metal ((111)Cd), whereas the other is inert. The thiolate sulfur ligands bound to zinc in this site are buried within the protein. Regions of beta-strand and alpha-helix surround the inert site to form a zinc finger resembling the zinc fingers in GATA and LIM-domain proteins. Eukaryotic zinc fingers interact specifically with other proteins or DNA and an analogous interaction can therefore be anticipated for prokaryotic zinc fingers. SmtA now provides structural proof for the existence of zinc fingers in prokaryotes, and sequences related to the zinc finger motif can be identified in several bacterial genomes.

Oxidation of zinc-binding cysteine residues in transcription factor proteins

Recent results on the oxidation of cysteine residues that bind zinc in transcription factors and their analogous peptides and in related proteins and model systems are reviewed. Two classes of oxidants, the transition metals and dioxygen, hydrogen peroxide, and related species, are considered, and the role of metal ions in suppressing or enhancing Cys oxidation is a major focus. Cysteines in the zinc-bound structures of transcription factors are less susceptible to oxidation than in the metal-free form, and this appears to correlate with reduced accessibility of the thiolates to oxidants. Substitution of other metal ions for Zn(II) increases the rate of Cys oxidation, apparently through increased oxidant accessibility. Reactions that result in reversible or irreversible oxidation of these zinc-binding cysteines under biological conditions are identified in the context of deleterious implications for gene expression.

Regulation of metallothionein II messenger ribonucleic acid measures exogenous estrogen receptor-beta activity in SAOS-2 and LNCaPLN3 cells

Estrogen receptor-beta (ERbeta) is a recently discovered member of the steroid hormone superfamily. Because its distribution is distinct from that of the classical estrogen receptor, and it is expressed in several nonclassical estrogen target tissues (e.g. prostate and bladder), its role in mediating the action of estrogen is unclear. One approach to elucidating the function of this receptor is to identify genes that it regulates. Using differential display, we profiled the messenger RNAs regulated by 17beta-estradiol in SAOS-2 and LNCaPLN3 cells overexpressing ERbeta. Follow-up studies used cells expressing either ERalpha or ERbeta. One gene, metallothionein II, was regulated by both receptor subtypes in LNCaPLN3 cells, but only by ERbeta in SAOS-2 cells. Because cycloheximide blocks this response, induction is probably mediated through regulation of at least one other protein. Identification of endogenous genes that are regulated differentially by ERalpha and ERbeta will be valuable tools in elucidating the function of ERbeta and the mechanisms by which these two receptors regulate transcription.

Metalloregulation of the tumor suppressor protein p53: zinc mediates the renaturation of p53 after exposure to metal chelators in vitro and in intact cells

The tumor suppressor p53 is a transcription factor which binds DNA through a structurally complex domain stabilized by a zinc atom. Zinc chelation disrupts the architecture of this domain, inducing the protein to adopt an immunological phenotype identical to that of many mutant forms of p53. In this report, we used 65Zn to show that incorporation of zinc within the protein was required for folding in the 'wild-type' conformation capable of specific DNA-binding. Using a cellular assay, we show that addition of extracellular zinc at concentrations within the physiological range (5 microM) was required for renaturation and reactivation of wild-type p53. Among other divalent metals tested (Cd2+, Cu2+, Co2+, Fe2+ and Ni2+), only Co2+ at 125 microM had a similar effect. Recombinant metallothionein (MT), a metal chelator protein, was found to modulate p53 conformation in vitro. In cultured cells, overexpression of MT by transfection could modulate p53 transcriptional activity. Taken together, these results suggest that zinc binding plays a regulatory role in the control of p53 folding and DNA-binding activity.

Superoxide-induced stimulation of protein kinase C via thiol modification and modulation of zinc content

We investigated the effects of mild oxidation on protein kinase C (PKC) using the xanthine/xanthine oxidase system of generating superoxide. Exposure of various PKC preparations to superoxide stimulated the autonomous activity of PKC. Similarly, thiol oxidation increased autonomous PKC activity, consistent with the notion that superoxide stimulates PKC via thiol oxidation. The superoxide-induced stimulation of PKC activity was partially reversed by reducing agents, suggesting that disulfide bond formation contributed to the oxidative stimulation of PKC. In addition, superoxide increased the autonomous activity of the alpha, beta(II), epsilon, and zeta PKC isoforms, all of which contain at least one cysteine-rich region. Taken together, our observations suggested that superoxide interacts with PKC at the cysteine-rich region, zinc finger motif of the enzyme. Therefore, we examined the effects of superoxide on this region by testing the hypothesis that superoxide stimulates PKC by promoting the release of zinc from PKC. We found that a zinc chelator stimulated the autonomous activity of PKC and that superoxide induced zinc release from an PKC-enriched enzyme preparation. In addition, oxidized PKC contained significantly less zinc than reduced PKC. Finally, we have isolated a persistent, autonomously active PKC by DEAE-cellulose column chromatography from hippocampal slices incubated with superoxide. Taken together, these data suggest that superoxide stimulates autonomous PKC activity via thiol oxidation and release of zinc from cysteine-rich region of PKC.

Augmented humoral immune function in metallothionein-null mice

Stress response proteins can play integral roles as modulators of cellular function and can be involved in mechanisms that are important to immune function. Metallothionein (MT), a cysteine-rich stress response protein, has been shown to play numerous roles in the cell: it serves as a reservoir of essential heavy metals, it scavenges free radicals, and it can sequester heavy metals. These various functions suggest that MT may also participate in modulating immune responses. In previous work, we have shown that exogenous metallothionein can suppress the developing humoral immune response when coinjected with antigen. The present study was designed to evaluate the effects of endogenous MT on the development of humoral immunity. We compared the humoral immune function of animals with a targeted disruption of Mt-1 and -2 genes (MTKO) and their wild-type counterparts. MTKO mice displayed a significantly higher humoral response to challenge with ovalbumin (OVA) compared to wild-type controls. The secondary anti-OVA response in MTKO mice is as much as 58% higher than the response in control mice injected at the same time. Overall circulatory immunoglobulin levels are also substantially higher in MTKO mice (0.039 mg/ml IgM and 0.42 mg/ml IgG) than wild-type controls. MTKO mice displayed increased B cell differentiation following OVA challenge and an enhanced lymphoproliferative response to mitogenic stimulation. These changes in immune functional capacity occur in the context of changes in the makeup of the lymphoid compartments of the blood and spleen. There are substantially fewer T and B cells in the circulation of MTKO mice, but more T cells in the spleen of these mice than in control animals. Finally, we have found that splenocytes from MTKO animals displayed significantly elevated levels of NF-kappaB activity compared to wild-type controls. In conclusion, we have provided evidence that endogenous metallothionein can modulate the immune response in vivo and that intracellular MT may modulate immune function by regulation of transcription factor activity.

Induction, regulation, degradation, and biological significance of mammalian metallothioneins

MTs are small cysteine-rich metal-binding proteins found in many species and, although there are differences between them, it is of note that they have a great deal of sequence and structural homology. Mammalian MTs are 61 or 62 amino acid polypeptides containing 20 conserved cysteine residues that underpin the binding of metals. The existence of MT across species is indicative of its biological demand, while the conservation of cysteines indicates that these are undoubtedly central to the function of this protein. Four MT isoforms have been found so far, MT-1, MT-2, MT-3, and MT-4, but these also have subtypes with 17 MT genes identified in man, of which 10 are known to be functional. Different cells express different MT isoforms with varying levels of expression perhaps as a result of the different function of each isoform. Even different metals induce and bind to MTs to different extents. Over 40 years of research into MT have yielded much information on this protein, but have failed to assign to it a definitive biological role. The fact that multiple MT isoforms exist, and the great variety of substances and agents that act as inducers, further complicates the search for the biological role of MTs. This article reviews the current knowledge on the biochemistry, induction, regulation, and degradation of this protein in mammals, with a particular emphasis on human MTs. It also considers the possible biological roles of this protein, which include participation in cell proliferation and apoptosis, homeostasis of essential metals, cellular free radical scavenging, and metal detoxification.

Delay of M-phase onset by aphidicolin can retain the nuclear localization of zinc and metallothionein in 3T3-L1 fibroblasts

The transient nuclear localization of metallothionein during cell growth and differentiation may be related to the increased requirement of zinc for DNA synthesis, activation of metalloenzymes, and transcription factors. Treatment of 3T3-L1 fibroblasts with aphidicolin, an inhibitor of nuclear DNA synthesis, caused a cell-cycle block at G1/S phase and a delay in the onset of M phase. This also resulted in the accumulation of both zinc and metallothionein in the nucleus. After removal of aphidicolin, the cells rapidly reentered S phase, and during the G2/M phase of cell cycle both zinc and metallothionein began to relocate to the cytoplasm. Delaying the onset of M phase in 3T3-L1 cells could prevent the cytoplasmic relocation of metallothionein. The nuclear translocation of both zinc and metallothionein during the cell cycle can be considered as a normal process and this may be a general mechanism in response to mitogenic signals.

Zinc increases the activity of vitamin D-dependent promoters in osteoblasts

Zinc modulates the structure and binding of the DNA binding domain of the 1alpha,25-dihydroxyvitamin D(3) receptor to specific vitamin D response element DNA (Nature Biotechnology 16, 262-266, 1998). To determine whether zinc alters 1alpha,25-dihydroxyvitamin D(3)-regulated genes in cells, we permanently transfected rat osteoblasts with two vitamin D-dependent promoter-reporter systems and examined their responses to 1alpha,25-dihydroxyvitamin D(3) in the presence of increasing amounts of extracellular zinc. When extracellular zinc concentrations were increased in the presence of 1alpha,25-dihydroxyvitamin D(3), there was an increase in the activity of 1alpha,25-dihydroxyvitamin D(3)-dependent promoters with increasing concentrations of zinc. The effect was specific for zinc since metals such as copper failed to increase the activity of 1alpha,25-dihydroxyvitamin D(3)-dependent promoters. The concentration of the vitamin D receptor within the cell and the affinity of 1alpha,25-dihydroxyvitamin D(3) for its receptor remained unchanged with added zinc. Our results show that zinc increases the activity of 1alpha,25-dihydroxyvitamin D(3)-dependent promoters in osteoblasts.

Metallothionein-null mice express altered genes during development

Metallothionein (MT) can modulate transcriptional activity in vitro. We examined whether the absence of MT affects gene expression in vivo. We compared the hepatic RNA profiles of wild-type and MT-null neonatal mice using improved differential display. The hepatic MT level was maximal during neonatal development. We identified five cDNA fragments that were expressed in MT-null mice at different levels from those in wild-type mice. Two were fragments of MT-I and mutant MT-I cDNA. The sequences of the other cDNA fragments were identical to those of contrapsin, transketolase, and vanin-3. The latter two were up-regulated, whereas contrapsin was down-regulated in neonatal MT-null mice. These mRNA levels were remarkably different between the two strains of neonatal mice. Further characterization of the regulated mRNA identified here will determine whether or not they are primary or secondary effects of an MT deficiency.

Metallothionein is expressed in adipocytes of brown fat and is induced by catecholamines and zinc

Metallothionein (MT) is thought to have an antioxidant function and is strongly expressed during activation of thermogenesis and increased oxidative stress in brown adipose tissue (BAT). Localization and regulation of MT expression in BAT was therefore investigated in rats and mice. Immunohistochemical analysis of BAT from rats exposed to 4 degrees C for 24 h showed that MT and uncoupling protein 1 (UCP1) were coexpressed in differentiated adipocytes, and both cytoplasmic and nuclear localization of MT was observed. Cold induction of MT-1 expression in BAT was also observed in mice. Administration of norepinephrine to rats and isoproterenol to mice stimulated MT and UCP1 expression in BAT, implying a sympathetically mediated pathway for MT induction. In mice, zinc, and particularly dexamethasone, induced MT-2 expression in BAT and liver. Surprisingly, zinc also induced UCP1 in BAT, suggesting that elevated zinc may induce thermogenesis. We conclude that expression of MT in mature brown adipocytes upon beta-adrenoceptor activation is consistent with a role in protecting against physiological oxidative stress or in facilitating the mobilization or utilization of energy reserves.

Signal transduction pathways, and nuclear translocation of zinc and metallothionein during differentiation of myoblasts

The changes in subcellular localization of metallothionein during differentiation were studied in two myoblast cell lines, L6 and H9C2. Addition of insulin like growth factor-I or lowering foetal bovine serum to 1% can induce differentiation of myoblasts to myotubes. Metallothionein and zinc were localized mainly in the cytoplasm in myoblasts but were translocated into the nucleus of newly formed myotubes during early differentiation. In fully differentiated myotubes, metallothionein content was decreased with a cytoplasmic localization. Addition of an inhibitor of mitogen-activated protein kinase, PD 98059, did not affect differentiation but blocked nuclear translocation of metallothionein. LY 294092, an inhibitor of PI3 kinase, and rapamycin, an inhibitor of p70S6 serine/threonine kinase, abolished insulin-like growth factor-I induced differentiation of myoblasts, retained metallothionein in the cytoplasm, and decreased metallothionein content. These results demonstrate that the cytoplasmic-nuclear translocation of metallothionein occurs during the early stage of differentiation of myoblasts to myotubes and can be blocked by inhibition of certain signal transduction pathways. The transient nuclear localization of metallothionein and zinc may be related to a high requirement for zinc for metabolic activities during the early stage of differentiation.

Cysteine, glutathione (GSH) and zinc and copper ions together are effective, natural, intracellular inhibitors of (AIDS) viruses

Sufficient essential nutrients such as methionine, cysteine, copper, selenium, zinc and vitamins C and E are indispensable for the maintenance of optimal (immune) cell functions. Parasitic organisms such as protozoa, fungi, bacteria and viruses also depend on these essential nutrients for their multiplication and functioning. An evolutionarily developed optimal distribution of available nutrients between host (cells) and parasitic organisms normally prevents diseases, the nature of which will depend on genetic and environmental factors. The way in which the right amount of cysteine, glutathione (GSH), and copper and zinc ions made available in the right place at the right time and in the right form can prevent an unchecked multiplication of (AIDS) viruses in a more passive or active way forms the basis for the AIDS zinc-deficiency hypothesis (A-Z hypothesis) presented in this article. Zinc and copper ions stimulate/inhibit/block in a concentration-dependent way the (intracellular) activation of essential protein-splitting enzymes such as HIV proteases. Zinc and copper ions as 'passive' virus inhibitors. Apart from this, zinc ions directly or indirectly regulate, via zinc finger protein molecular structures, the activities of virus-combating Th-1 cells such as cytotoxic T-cells (CTLs). Zinc ions as regulators of the active, virus-combating Th-1 cells. Zinc and copper ions that remain available in sufficient amounts via cysteine/GSH are effective natural inhibitors/combaters of (AIDS) viruses and thereby prevent the development of chronic virus diseases that can lead to AIDS, autoimmune diseases, (food) allergies and/or cancer. A safe, relatively inexpensive and extensively tested medicine such as N-acetylcysteine (NAC) can help in supplying extra cysteine. The anti-HIV peptide T22, synthesized on the basis of two natural peptides from the Tachypleus tridentatus and Limnus polyphemus crabs, appears to be able to serve as supplier/carrier molecule of cysteine and zinc and/or to hinder the entry of HIVs into cells by way of the CD4 receptor.

Transient peaks in zinc and metallothionein levels during differentiation of 3T3L1 cells

A novel role for zinc mediated by metallothionein (MT) is found in the process of differentiation of 3T3L1 mouse fibroblasts to adipocytes. Twenty-four hours after the stimulation of differentiation by hormones, the cells enter into a phase of synchronous proliferation. In this phase the cellular contents of zinc and metallothionein rise rapidly to fivefold and threefold levels, respectively. Simultaneously MT is translocated from the cytoplasm to the nucleus. The rise of intracellular zinc is essential for the transition from G0/G1- to S-phase of the cell cycle. Deprivation of zinc with N,N,N', N'-tetrakis[2-pyridyl]ethylenediamine, a membrane-permeable zinc chelator, inhibited hormonal induced proliferation. After the short phase of proliferation a slower stage of actual differentiation to adipocytes begins. The elevated levels of MT and zinc decline quickly to start levels, and a rapid redistribution of MT to the cytoplasm occurs. We propose that the nuclear translocation of MT mediates the transfer of zinc to nuclear factors in the mitogenic process. The redistribution of MT to the cytoplasm and the decrease of the zinc content are postulated to be required for the start of the actual differentiation.

Redox regulation of copper-metallothionein

Copper (Cu) is an essential element whose localization within cells must be carefully controlled to avoid Cu-dependent redox cycling. Metallothioneins (MTs) are cysteine-rich metal-binding proteins that exert cytoprotective effects during metal exposure and oxidative stress. The specific role of MTs, however, in modulating Cu-dependent redox cycling remains unresolved. Our studies utilized a chemically defined model system to study MT modulation of Cu-dependent redox cycling under reducing (Cu/ascorbate) and mild oxidizing (Cu/ascorbate + H2O2) conditions. In the presence of Cu and ascorbate, MT blocked Cu-dependent lipid oxidation and ascorbyl radical formation with a stoichiometry corresponding to Cu/MT ratios </=12. In the presence of H2O2 the degree of protection by MT was less and biological oxidations and radical formation were inhibited only up to Cu/MT ratios of 6. Physical interaction of MT and Cu was measured by using low-temperature EPR of free Cu2+ in solution. The maximal amount of EPR-silent Cu1+ (presumably in complex with MT) corresponded to 12 molar equivalents of Cu/MT under reducing conditions, but only 9 in the presence of H2O2. H2O2 modulated the ability of MT to protect HL-60 cells from Cu-induced cell death in a manner that correlated with the ability of MT to mitigate Cu-redox cycling in cell-free systems. Thus, optimal binding of Cu to MT is achieved under reducing conditions; however, a portion of this Cu appears releasable under oxidizing conditions. Release of free Cu from MT during oxidative stress could enhance the formation of reactive oxygen species and potentiate cellular damage.

Monitoring metal ion flux in reactions of metallothionein and drug-modified metallothionein by electrospray mass spectrometry

The capabilities of electrospray ionization mass spectrometry are demonstrated for monitoring the flux of metal ions out of and into the metalloprotein rabbit liver metallothionein and, in one example, chlorambucil-alkylated metallothionein. Metal ion transfers may be followed as the reactions proceed in situ to provide kinetic information. More uniquely to this technique, metal ion stoichiometries may be determined for reaction intermediates and products. Partners used in these studies include EDTA, carbonic anhydrase, a zinc-bound hexamer of insulin, and the core domain of bacteriophage T4 gene 32 protein, a binding protein for single-stranded DNA.

Coordination of Zn2+ (and Cd2+) by prokaryotic metallothionein. Involvement of his-imidazole

In mammalian metallothionein Zn2+ is exclusively coordinated to Cys-thiolate to form clusters in which the metal is thermodynamically stable but also kinetically labile. By contrast, little is known about coordination to prokaryotic metallothionein, SmtA. 3 nmol of Zn2+ nmol-1 SmtA were displaced by 8 nmol of p-(hydroxymercuri)phenylsulfonate implicating eight of the nine Cys in the coordination of three metal ions. None of the Zn2+ associated with SmtA was accessible to 4-(2-pyridylazo)resorcinol prior to the addition of p-(hydroxymercuri)phenylsulfonate. An unusual feature of SmtA is the presence of three His residues, and we have investigated whether these contribute to metal coordination. Less Zn2+ was associated with purified SmtA(H40R/H49R/H55R), in which all three His residues were substituted with Arg, and approximately one equivalent of Zn2+ was immediately accessible to 4-(2-pyridylazo)resorcinol. Following incubation of SmtA with 111Cd, three 111Cd resonances were detected, two in a range expected for CdS4 and the third indicative of either CdNS3 or CdN2S2 coordination. Two-dimensional TOCSY 1H NMR and 111Cd-edited 1H NMR showed two His residues bound to 111Cd, confirming CdN2S2 coordination. The pH of half-dissociation of Zn2+ increased from 4.05 for SmtA to 5.37 for SmtA(H40R/H49R/H55R). Equivalent values for single His mutants SmtA(H40R), SmtA(H49R), and SmtA(H55R) were 4.62, 4.48, and 3.81, respectively, revealing that conversion of His40 or His49 to Arg impairs Zn2+ binding at the CdN2S2 and CdS4 sites. Only approximately two equivalents of Zn2+ were associated with purified SmtA(H49R). The appearance of a fourth 111Cd resonance at lower pH suggests that an alternative CdN2S2 site also exists.

Effect of redox conditions on the DNA-binding efficiency of the retinoic acid receptor zinc-finger

Retinoic acid and its derivatives are involved in many important biological processes. In the present study, we have shown that the DNA binding domain of the retinoic acid receptor, which contains two zinc fingers with the Zn(II) tetrahedrally coordinated by four Cys, is susceptible to intracellularly relevant oxidizing agents. In the presence of hydrogen peroxide or hypochlorite, the zinc-finger DNA binding activity was abolished in a concentration dependent manner. The loss of DNA binding activity was correlated with the release of Zn(II) from the zinc-finger motif as a consequence of Zn(II)-thiolate bond oxidation. A combination of glutathione and Zn(II) was able to restore the activity, suggesting that oxidation of the zinc-finger by hydrogen peroxide or hypochlorite resulted in the formation of disulfide bonds between the Cys present in the Zn(II)-binding motif. Our results indicate that in situations of oxidative-stress zinc-finger containing transcription factors may be particularly susceptible to oxidation, resulting in the disruption of control and regulation of gene expression.

A carboxyl-terminal Cys2/His2-type zinc-finger motif in DNA primase influences DNA content in Synechococcus PCC 7942

The DNA primase gene, dnaG, has been isolated from the cyanobacterium Synechococcus PCC 7942. It is not part of a macromolecular synthesis operon but is co-transcribed with pheT and located adjacent to the metallothionein divergon, smt. At the carboxyl terminus of this DnaG is a Cys2/His2 zinc-finger motif. The carboxyl-terminal 91 residues bound 65Zn and 0.95 g atom of Zn2+ mol-1 were detected with 4-(2-pyridylazo)resorcinol. Following exposure to Cd2+, 0.95 g atom of Cd2+ was displaced by 2 equivalents of p-(hydroxymercuri) phenylsulfonate mol-1, while only 0.03 g atom of Cd2+ was displaced mol-1 polypeptide missing the carboxyl-terminal (residue 592 onward) zinc-finger motif. Zn2+ caused an increase in intensity, and a reduction in wavelength, of Trp fluorescence at the tip of the predicted zinc-finger, while EDTA caused the converse. Cells containing a single chromosomal codon substitution (C597S), altering the zinc-finger, were generated by exploiting Zn2+-sensitive smt mutants and the proximity of dnaG to smt. Cells in which smt and dnaG(C597S) had integrated into the chromosome were selected via restored Zn2+ tolerance. Synechococcus PCC 7942 and its dnaG(C597S) mutant grew at equivalent rates, but the latter had a reduced number of chromosomes.

The elusive function of metallothioneins

Biochemistry and genetics are both required to elucidate the function of macromolecules. There is no question that metallothioneins (MTs) have unique biochemical properties, but genetic experiments have not substantiated the importance of MTs under physiological conditions. Even after thousands of studies describing the structure, biochemical characteristics, tissue distribution, induction, and consequences of genetic disruption and deliberate overexpression, the evolutionary forces that led to the initial appearance, gene duplications, and nearly ubiquitous expression of MTs remain enigmatic.

Modulation of DNA binding of a tramtrack zinc finger peptide by the metallothionein-thionein conjugate pair

The ability of metallothionein (MT) to modulate DNA binding by a two-finger peptide of Tramtrack (TTK), a CCHH zinc transcription factor, was investigated using metal-bound and metal-deficient forms of rabbit MT-2 and the TTK peptide. Thionein inhibited DNA binding by zinc-bound TTK, and Zn-MT restored DNA-binding by zinc-deficient apo-TTK. "Free" zinc at low concentrations was as effective as Zn-MT in restoring DNA binding by apopeptide but was inhibitory at concentrations equal to zinc bound to 2 mol eq and higher of Zn-MT. Substitution of cadmium for zinc reduced the affinity of the peptide for its DNA binding site. This effect was reversed by incubation with Zn-MT. The circular dichroic spectra of the TTK peptide indicated that zinc removal resulted in loss of alpha-helical structures, which are sites of DNA contact points. Reconstitution with cadmium resulted in stoichiometric substitution of 2 mol of Cd/mol of peptide but not recovery of alpha-helical structures. Incubation of Cd-TTK with Zn-MT restored the secondary structure expected for zinc-bound TTK. The ability of Zn-MT and thionein to restore or inhibit DNA-binding by TTK was associated with effects on the metallation status of the peptide and related alterations in its secondary structure.

Zinc metabolism in the brain: relevance to human neurodegenerative disorders

Zinc is an important trace element in biology. An important pool of zinc in the brain is the one present in synaptic vesicles in a subgroup of glutamatergic neurons. In this form it can be released by electrical stimulation and may serve to modulate responses at receptors for a number of different neurotransmitters. These include both excitatory and inhibitory receptors, particularly the NMDA and GABA(A) receptors. This pool of zinc is the only form of zinc readily stained histochemically (the chelatable zinc pool), but constitutes only about 8% of the total zinc content in the brain. The remainder of the zinc is more or less tightly bound to proteins where it acts either as a component of the catalytic site of enzymes or in a structural capacity. The metabolism of zinc in the brain is regulated by a number of transport proteins, some of which have been recently characterized by gene cloning techniques. The intracellular concentration may be mediated both by efflux from the cell by the zinc transporter ZrT1 and by complexing with apothionein to form metallothlonein. Metallothionein may serve as the source of zinc for incorporation into proteins, including a number of DNA transcription factors. However, zinc is readily released from metallothionein by disulfides, increasing concentrations of which are formed under oxidative stress. Metallothionein is a very good scavenger of free radicals, and zinc itself can also reduce oxidative stress by binding to thiol groups, decreasing their oxidation. Zinc is also a very potent inhibitor of nitric oxide synthase. Increased levels of chelatable zinc have been shown to be present in cell cultures of immune cells undergoing apoptosis. This is very reminiscent of the zinc staining of neuronal perikarya dying after an episode of ischemia or seizure activity. Thus a possible role of zinc in causing neuronal death in the brain needs to be fully investigated. intraventricular injections of calcium EDTA have already been shown to reduce neuronal death after a period of ischemia. Pharmacological doses of zinc cause neuronal death, and some estimates indicate that extracellular concentrations of zinc could reach neurotoxic levels under pathological conditions. Zinc is released in high concentrations from the hippocampus during seizures. Unfortunately, there are contrasting observations as to whether this zinc serves to potentiate or decrease seizure activity. Zinc may have an additional role in causing death in at least some neurons damaged by seizure activity and be involved in the sprouting phenomenon which may give rise to recurrent seizure propagation in the hippocampus. In Alzheimer's disease, zinc has been shown to aggregate beta-amyloid, a form which is potentially neurotoxic. The zinc-dependent transcription factors NF-kappa B and Sp1 bind to the promoter region of the amyloid precursor protein (APP) gene. Zinc also inhibits enzymes which degrade APP to nonamyloidogenic peptides and which degrade the soluble form of beta-amyloid. The changes in zinc metabolism which occur during oxidative stress may be important in neurological diseases where oxidative stress is implicated, such as Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis (ALS). Zinc is a structural component of superoxide dismutase 1, mutations in which give rise to one form of familiar ALS. After HIV infection, zinc deficiency is found which may be secondary to immune-induced cytokine synthesis. Zinc is involved in the replication of the HIV virus at a number of sites. These observations should stimulate further research into the role of zinc in neuropathology.

Metallothionein accretion in human hepatic cells is linked to cellular proliferation

The basal amounts of metallothionein (MT) and its rates of biosynthesis were compared in resting and proliferating Chang liver (CCl-13) cells. In resting cells the total amounts of the detectable isoforms MT-2 and MT-1e were approx. 1.6x10(6) and 4x10(5) molecules per cell respectively. In exponentially growing cultures the cellular contents of both isoforms increased co-ordinately approx. 4-fold and decreased again to the initial values within 48 h after entering density-mediated growth arrest. As documented for MT-2 its transient accretion was attributable to a 10-fold rise in the rate of biosynthesis of this protein during the growth phase. Measurements of the relative amounts of MT-2 mRNA indicated the occurrence of a more than 50% increase within the first 12 h after subculturing of the cells, followed by a return to basal levels thereafter. These results denote a direct link between the programming of MT synthesis and proliferation and thus attest to a central housekeeping function of the MTs.

Zinc and Alzheimer's disease: is there a direct link?

Zinc is an essential trace element in human biology, but is neurotoxic at high concentrations. Several studies show that zinc promotes aggregations of beta-amyloid protein, the main component of the senile plaques typically found in Alzheimer's disease brains. In other neurological disorders where neurons appear to be dying by apoptosis (gene-directed cell death), chelatable zinc accumulates in the perikarya of neurons before, or during degeneration. As there is evidence for apoptotic death of neurons in Alzheimer's disease, an involvement of zinc in this process needs to be investigated. Zinc interacts with enzymes and proteins, including transcription factors, which are critical for cell survival and could be linked to apoptotic processes. While controversial, some studies indicate that total tissue zinc is markedly reduced in several brain regions of Alzheimer's patients. At face value, it seems that a paradox exists between reports of a decrease in zinc in the Alzheimer's brain and the putative link to aberrant high zinc levels promoting plaque formation. An hypothesis to explain this inconsistency is presented. Neuropathological changes mediated by endogenous or exogenous stressors may be relevant factors affecting abnormal zinc metabolism. This paper reviews current investigations that suggest a role of zinc in the etiology of Alzheimer's disease.