Understanding the mechanisms of zinc-sensing by metal-response element binding transcription factor-1 (MTF-1)

Ammonia increases nitric oxide, free Zn(2+), and metallothionein mRNA expression in cultured rat astrocytes

Ammonia is a major player in the pathogenesis of hepatic encephalopathy (HE) and affects astrocyte function by triggering a self-amplifying cycle between osmotic and oxidative stress. We recently demonstrated that hypoosmotic astrocyte swelling rapidly stimulates nitric oxide (NO) production and increases intracellular free Zn(2+) concentration ([Zn(2+)](i)). Here we report effects of ammonia on [Zn(2+)](i) homeostasis and NO synthesis. In cultured rat astrocytes, NH(4)Cl (5 mm) increased within 6 h both cytosolic and mitochondrial [Zn(2+)]. The [Zn(2+)](i) increase was transient and was mimicked by the nonmetabolizable CH(3)NH(3)Cl, and it was dependent on NO formation, as evidenced by the sensitivity toward the nitric oxide synthase inhibitor N(G)-monomethyl-l-arginine. The NH(4)Cl-induced NO formation was sensitive to the Ca(2+) chelator 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetra(acetoxymethyl) ester and increases in both NO and [Zn(2+)](i) were blocked by the N-methyl-d-aspartate receptor antagonist MK-801. The NH(4)Cl-triggered increase in [Zn(2+)](i) was followed by a Zn(2+)-dependent nuclear appearance of the metal response element-binding transcription factor and metallothionein messenger RNA (mRNA) induction. Metallothionein mRNA was also increased in vivo in rat cerebral cortex 6 h after an NH(4)Ac challenge. NH(4)Cl increased peripheral-type benzodiazepine receptor (PBR) protein expression, whereas PBR mRNA levels were decreased in a Zn(2+)-independent manner. The Zn(2+)-dependent upregulation of metallothionein following ammonia intoxication may reflect a cytoprotective response, whereas the increase in PBR expression may augment HE development.

Protein kinase CK2 triggers cytosolic zinc signaling pathways by phosphorylation of zinc channel ZIP7

The transition element zinc, which has recently been identified as an intracellular second messenger, has been implicated in various signaling pathways, including those leading to cell proliferation. Zinc channels of the ZIP (ZRT1- and IRT1-like protein) family [also known as solute carrier family 39A (SLC39A)] transiently increase the cytosolic free zinc (Zn(2+)) concentration in response to extracellular signals. We show that phosphorylation of evolutionarily conserved residues in endoplasmic reticulum zinc channel ZIP7 is associated with the gated release of Zn(2+) from intracellular stores, leading to activation of tyrosine kinases and the phosphorylation of AKT and extracellular signal-regulated kinases 1 and 2. Through pharmacological manipulation, proximity ligation assay, and mutagenesis, we identified protein kinase CK2 as the kinase responsible for ZIP7 activation. Together, the present results show that transition element channels in eukaryotes can be activated posttranslationally by phosphorylation, as part of a cell signaling cascade. Our study links the regulated release of zinc from intracellular stores to phosphorylation of kinases involved in proliferative responses and cell migration, suggesting a functional role for ZIP7 and zinc signals in these events. The connection with proliferation and migration, as well as the activation of ZIP7 by CK2, a kinase that is antiapoptotic and promotes cell division, suggests that ZIP7 may provide a target for anticancer drug development.

Metal ion-mediated DNA-protein interactions

The dramatic changes in the environmental conditions that organisms encountered during evolution and adaptation to life in specific niches, have influenced intracellular and extracellular metal ion contents and, as a consequence, the cellular ability to sense and utilize different metal ions. This metal-driven differentiation is reflected in the specific panels of metal-responsive transcriptional regulators found in different organisms, which finely tune the intracellular metal ion content and all metal-dependent processes. In order to understand the processes underlying this complex metal homeostasis network, the study of the molecular processes that determine the protein-metal ion recognition, as well as how this event is transduced into a transcriptional output, is necessary. This chapter describes how metal ion binding to specific proteins influences protein interaction with DNA and how this event can influence the fate of genetic expression, leading to specific transcriptional outputs. The features of representative metal-responsive transcriptional regulators, as well as the molecular basis of metal-protein and protein-DNA interactions, are discussed on the basis of the structural information available. An overview of the recent advances in the understanding of how these proteins choose specific metal ions among the intracellular metal ion pool, as well as how they allosterically respond to their effector binding, is given.

Coordinate expression and localization of iron and zinc transporters explain iron-zinc interactions during uptake in Caco-2 cells: implications for iron uptake at the enterocyte

Iron and zinc have diverse and important physiological functions. Yet, the mechanism of their absorption at the intestine remains controversial and is confounded by the fact that many studies have shown, to varying extents, that they inhibit the absorption of each other. We have studied the expression of iron and zinc transporters and storage proteins, and their regulation, in Caco-2 cells, an established enterocyte model, under normal culture conditions and under conditions of iron and zinc depletion and supplementation using a combination of immunoblotting, confocal microscopy and reverse transcriptase polymerase chain reaction. We show that divalent metal transporter-1 (DMT-1) delocalizes from the plasma membrane upon iron or zinc depletion, but its apical abundance increases with zinc supplementation. This translocation of DMT-1 coincides with an increase in iron uptake upon zinc supplementation, as previously reported by us. FPN-1 expression increases upon zinc supplementation and decreases with iron or zinc depletion, effluxing the excess sequestered iron and thus maintaining cellular iron homeostasis. Zinc influx transporters Zip-1 and Zip-14 and efflux transporters ZnT-1 and ZnT-4 are coordinately regulated under conditions of zinc supplementation and depletion to ensure cellular zinc homeostasis. We have previously reported that iron uptake can entail two transporters and that zinc noncompetitively inhibits iron uptake in Caco-2 cells. We now provide evidence that this inhibition is independent of DMT-1 and that Zip-14 may be a relevant iron transporter. These new observations provide experimental support to this two-transporter model of iron uptake and give mechanistic insight to iron-zinc interactions during uptake at the enterocyte.

Ammonia increases nitric oxide, free Zn(2+), and metallothionein mRNA expression in cultured rat astrocytes

Ammonia is a major player in the pathogenesis of hepatic encephalopathy (HE) and affects astrocyte function by triggering a self-amplifying cycle between osmotic and oxidative stress. We recently demonstrated that hypoosmotic astrocyte swelling rapidly stimulates nitric oxide (NO) production and increases intracellular free Zn(2+) concentration ([Zn(2+)](i)). Here we report effects of ammonia on [Zn(2+)](i) homeostasis and NO synthesis. In cultured rat astrocytes, NH(4)Cl (5 mm) increased within 6 h both cytosolic and mitochondrial [Zn(2+)]. The [Zn(2+)](i) increase was transient and was mimicked by the nonmetabolizable CH(3)NH(3)Cl, and it was dependent on NO formation, as evidenced by the sensitivity toward the nitric oxide synthase inhibitor N(G)-monomethyl-l-arginine. The NH(4)Cl-induced NO formation was sensitive to the Ca(2+) chelator 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetra(acetoxymethyl) ester and increases in both NO and [Zn(2+)](i) were blocked by the N-methyl-d-aspartate receptor antagonist MK-801. The NH(4)Cl-triggered increase in [Zn(2+)](i) was followed by a Zn(2+)-dependent nuclear appearance of the metal response element-binding transcription factor and metallothionein messenger RNA (mRNA) induction. Metallothionein mRNA was also increased in vivo in rat cerebral cortex 6 h after an NH(4)Ac challenge. NH(4)Cl increased peripheral-type benzodiazepine receptor (PBR) protein expression, whereas PBR mRNA levels were decreased in a Zn(2+)-independent manner. The Zn(2+)-dependent upregulation of metallothionein following ammonia intoxication may reflect a cytoprotective response, whereas the increase in PBR expression may augment HE development.

Transcriptomic analyses in a benthic fish exposed to contaminated estuarine sediments through laboratory and in situ bioassays

The transcription of contaminant response-related genes was investigated in juvenile Senegalese soles exposed to sediments from three distinct sites (a reference plus two contaminated) of a Portuguese estuary (the Sado, W Portugal) through simultaneous 28-day laboratory and in situ bioassays. Transcription of cytochrome P450 1A (CYP1A), metallothionein 1 (MT1), glutathione peroxidase (GPx), catalase (CAT), caspase 3 (CASP3) and 90 kDa heat-shock protein alpha (HSP90AA) was surveyed in the liver by real-time PCR. CASP3 transcription analysis was complemented by surveying apoptosis through the TUNEL reaction. After 14 days of exposure, relative transcription was either reduced or decreased in fish exposed to the contaminated sediments, revealing a disturbance stress phase during which animals failed to respond to insult. After 28 days of exposure all genes' transcription responded to contamination but laboratory and in situ assays depicted distinct patterns of regulation. Although sediments revealed a combination of organic and inorganic toxicants, transcription of the CYP1A gene was consistently correlated to organic contaminants. Metallothionein regulation was found correlated to metallic and organic xenobiotic contamination in the laboratory and in situ, respectively. The transcription of oxidative stress-related genes can be a good indicator of general stress but caution is mandatory when interpreting the results since regulation may be influenced by multiple factors. As for MT1, HSP90 up-regulation has potential to be a good indicator for total contamination, as well as the CASP3 gene, even though hepatocyte apoptosis depicted values inconsistent with sediment contamination, showing that programmed cell death did not directly depend on caspase transcription alone.

Role of the Nrf2-heme oxygenase-1 pathway in silver nanoparticle-mediated cytotoxicity

Silver nanoparticles (nano-Ag) have been widely used in various commercial products including textiles, electronic appliances and biomedical products. However, there remains insufficient information on the potential risk of nano-Ag to human health and environment. In the current study, we have investigated the role of NF-E2-related factor 2 (Nrf2) transcription factor in nano-Ag-induced cytotoxicity. When Nrf2 expression was blocked using interring RNA expression in ovarian carcinoma cell line, nano-Ag treatment showed a substantial decrease in cell viability with concomitant increases in apoptosis and DNA damage compared to the control cells. Target gene analysis revealed that the expression of heme oxygenase-1 (HO-1) was highly elevated by nano-Ag in nonspecific shRNA expressing cells, while Nrf2 knockdown cells (NRF2i) did not increase HO-1 expression. The role of HO-1 in cytoprotection against nano-Ag was reinforced by results using pharmacological inducer of HO-1: cobalt protoporphyrin-mediated HO-1 activation in the NRF2i cells prevented nano-Ag-mediated cell death. Similarly, pharmacological or genetic inhibition of HO-1 in nonspecific control cells exacerbated nano-Ag toxicity. As the upstream signaling mechanism, nano-Ag required the phosphoinositide 3-kinase (PI3K) and p38MAPK signaling cascades for HO-1 induction. The treatment with either PI3K inhibitor or p38MAPK inhibitor suppressed HO-1 induction and intensified nano-Ag-induced cell death. Taken together, these results suggest that Nrf2-dependent HO-1 up-regulation plays a protective role in nano-Ag-induced DNA damage and consequent cell death. In addition, nano-Ag-mediated HO-1 induction is associated with the PI3K and p38MAPK signaling pathways.

Regulatory responses to excess zinc ingestion in growing rats

The growth of weaning piglets is effectively improved by feeding a high-Zn diet (3000 mg Zn/kg of diet). The present study examined whether feeding a diet supplemented with Zn (1016-3000 mg/kg) for 10 d induces growth benefits in rats. In addition, tissue weight, Zn content of tissues and expression of Zn transporters were examined in these rats. Zn supplementation did not significantly increase body weight. Breaking line model analyses indicated that the weight of the pancreas, the organ most sensitive to excess Zn, significantly decreased with increasing Zn intake beyond 15·2 mg/d. Excess Zn has been suggested to accumulate in the liver, kidney and bone in order to protect the pancreas. Zn concentrations in the plasma, liver, kidney and femur increased with increasing Zn intake up to approximately 30 mg/d, whereas those in the pancreas increased up to 8·4 mg/d and decreased by Zn intake beyond 8·4 mg/d. The expression levels of the Zn transporters Zip4 and ZnT1 in the intestinal epithelium were significantly lower in rats fed a diet supplemented with 1016 mg/kg Zn compared to those fed the basal diet. The present study reveals that (1) excess Zn intake does not accelerate growth in rats, but is detrimental to the pancreas, (2) the excess Zn is effectively accumulated in the liver, kidney and bone, without sufficient protection of the pancreas and (3) expression of Zn transporters is down-regulated in response to excess Zn intake.

Hammering out details: regulating metal levels in eukaryotes

The transition metals zinc, iron and copper are common constituents in a wide range of proteins. Although these metals are all essential for life, when present in excess, they are frequently toxic to cell growth and viability. Therefore, all organisms rely on sophisticated mechanisms to maintain optimal levels of each metal. Genes that encode metal transport or storage proteins are often regulated at the transcriptional level in response to changes in metal status. In this review, we focus on what is known about the transcription factors that mediate these metal-dependent changes. Specifically, we highlight recent advances in our understanding of the mechanisms by which these factors sense metal ions.

Differential glucocorticoid receptor-mediated transcription mechanisms

Nuclear receptors such as the glucocorticoid receptor (GR) are ligand-dependent transcription factors that mediate transcription of target genes by recruiting factors that modulate chromatin structure. In this study, curcumin, a compound known to inhibit GR-mediated transcription, was used to examine the different mechanisms by which GR regulates transcription. The mechanisms of transcription regulation of metallothioneine-2A (MT2A) and solute carrier family 19 member 2 (SLC19A2), two GR target genes where the hormone-dependent gene activation is inhibited or unaffected by curcumin treatment, respectively, were analyzed by chromatin immunoprecipitation and RT-PCR experiments. The data suggest that the loss of hormone-dependent MT2A gene expression is due to the inhibition of continued transcription activity after initial assembly of the transcription machinery. In contrast, the hormone-dependent SLC19A2 gene expression is maintained because the continued transcription output after assembly of transcription machinery is unaffected by curcumin. These results suggest that the two GR target genes use alternate mechanisms to regulate expression levels at the level of continued transcription output after transcription machinery assembly.

Regulation of the adaptation to zinc deficiency in plants

The molecular mechanisms by which plants sense their micronutrient status, and adapt to their environment in order to ensure a sufficient micronutrient supply, are poorly understood. Zinc is an essential micronutrient for all living organisms. When facing a shortage in zinc supply, plants adapt by enhancing the zinc uptake capacity. The molecular regulators controlling this adaptation were recently identified. In this mini-review, we highlight recent progress in understanding the adaptation to zinc deficiency in plants and discuss the future challenges to fully unravel its molecular basis.

Induction of FPN1 transcription by MTF-1 reveals a role for ferroportin in transition metal efflux

Ferroportin (Fpn) is the only known iron exporter in vertebrate cells and plays a critical role in iron homeostasis regulating cytosolic iron levels and exporting iron to plasma. Ferroportin1 (FPN1) expression can be transcriptionally regulated by iron as well as other transition metals. Fpn can also be posttranslationally regulated by hepcidin-mediated internalization and degradation. We demonstrate that zinc and cadmium induce FPN1 transcription through the action of Metal Transcription Factor-1 (MTF-1). These transition metals induce MTF-1 translocation into the nucleus. Zinc leads to MTF-1 binding to the FPN1 promoter, while iron does not. Silencing of MTF-1 reduces FPN1 transcription in response to zinc but not in response to iron. The mouse FPN1 promoter contains 2 MTF-1 binding sites and mutation of those sites affects the zinc and cadmium-dependent expression of a FPN1 promoter reporter construct. We demonstrate that Fpn can transport zinc and can protect zinc sensitive cells from high zinc toxicity.

Cloning, characterization, expression, and copper sensitivity of the metallothionein-1 gene in the Chinese mitten crab, Eriocheir sinensis

A full-length metallothionein-1(MT-1) cDNA was cloned from the Chinese mitten crab, Eriocheir sinensis, based upon the hepatopancreas cDNA library. The full-length cDNA contained a single 180 bp open reading frame that encoded a 59 amino acid protein. The deduced amino acid sequence was cysteine (Cys)-rich, with residues observed in patterns characteristic of other reported MTs: Cys-X-Cys, Cys-X-X-Cys, or Cys-X-X-X-Cys. Gene structure obtained via PCR yielded a 3816 bp gene, which was comprised of three exons and two introns arranged in a "3 + 2" pattern. The cloned 5'flanking region (1,735 bp) contained several predicted binding sites, which included MREs, AP-1, SP1, USF, GATA, HNF-1, and HSF. MT-1 mRNA expression analysis revealed that while levels were highest in the hepatopancreas, expression was abundant in testis and thoracic ganglia, moderate in intestine (P<0.05), and weak in other tissues (P<0.05). MT-1 mRNA expression exhibited reproductive variation in the male, with levels approximately tenfold greater in August, during seasonal gonadal maturation, compared to other times of the year. Cu2+ exposure via tank water (0-1 mg/l for 7 days) resulted in a dose-dependent bell curve response in MT-1 mRNA expression, with peak expression observed after exposure to 0.1 mg/l Cu2+. A time course experiment (0.1 mg/l Cu2+ over 9 days) revealed MT-1 mRNA expression peaked sharply on day 5 before gradually decreasing with prolonged exposure. In the present report, we provide sequence analysis of the first MT-1 gene cloned in E. sinensis, and evidence that its physiological and toxicological regulation is evolutionary conserved.

Dynamic transcriptomic profiles of zebrafish gills in response to zinc supplementation

Background

Dietary zinc supplementation may help to promote growth, boost the immune system, protect against diabetes, and aid recovery from diarrhoea. We exploited the zebrafish (Danio rerio) gill as a unique vertebrate ion transporting epithelium model to study the time-dependent regulatory networks of gene-expression leading to homeostatic control during zinc supplementation. This organ forms a conduit for zinc uptake whilst exhibiting conservation of zinc trafficking components.

Results

Fish were maintained with either zinc supplemented water (4.0 μM) and diet (2023 mg zinc kg^-1) or water and diet containing Zn²⁺ at 0.25 μM and 233 mg zinc kg^-1, respectively. Gill tissues were harvested at five time points (8 hours to 14 days) and transcriptome changes analysed in quintuplicate using a 16 K microarray with results anchored to gill Zn²⁺ influx and whole body nutrient composition (protein, carbohydrate, lipid, elements). The number of regulated genes increased up to day 7 but declined as the fish acclimated. In total 525 genes were regulated (having a fold-change more than 1.8 fold change and an adjusted P-value less than 0.1 which is controlling a 10% False discovery rate, FDR) by zinc supplementation, but little overlap was observed between genes regulated at successive time-points. Many genes displayed cyclic expression, typical for homeostatic control mechanisms. Annotation enrichment analysis revealed strong overrepresentation of "transcription factors", with specific association evident with "steroid hormone receptors". A suite of genes linked to "development" were also statistically overrepresented. More specifically, early regulation of genes was linked to a few key transcription factors (e.g. Mtf1, Jun, Stat1, Ppara, Gata3) and was followed by hedgehog and bone morphogenic protein signalling.

Conclusions

The results suggest that zinc supplementation reactivated developmental pathways in the gill and stimulated stem cell differentiation, a response likely reflecting gill remodelling in response to its altered environment. This provides insight to the role of zinc during cell differentiation and illustrates the critical nature of maintaining zinc status. The study also highlights the importance of temporal transcriptomics analysis in order resolve the discrete elements of biological processes, such as zinc acclimation.

Activation of metallothionein transcription by 4-hydroxynonenal

Metallothioneins (MTs) protect cells from oxidative damage by scavenging reactive oxygen species (ROS). Concurrent with protecting cells from ROS-mediated damage, MT transcription is induced by ROS. ROS activate transcription by affecting several signal transduction pathways, many of which have been implicated in regulating MT transcription. ROS-activated intracellular signaling is mediated by the stable lipid peroxide 4-hydroxynonenal (HNE). After determining the level of sensitivity of Hepa 1-6 cells to HNE, MT-1 mRNA expression was shown to be induced in a concentration and time-dependent manner after HNE exposure. Finally, using MT-based reporters, HNE was found to induce MT transcription via both antioxidant response and metal response elements. Thus, ROS may activate MT transcription by generating HNE that in turn affects signaling pathways that regulate MT transcription via the transcription factors AP-1 and MTF-1.

Characterization and localization of metal-responsive-element-binding transcription factors from tilapia

Two isoforms of MTF-1, MTF-1L (long form) and MTF-1S (short form), were cloned in tilapia (Ti) and characterized in a tilapia liver cell line, Hepa-T1. The cloned tiMTF-1L has the characteristics of all of the tiMTF-1S identified so far with the zinc finger domain having six fingers, the acidic-rich, proline-rich, and serine/threonine-rich domains; however, the short form encodes for the zinc finger domain with five zinc fingers only and no other domains. The transient transfection of tiMTF-1L into human HepG2 cells showed both constitutive and zinc-induced metal-responsive-element (MRE)-driven reporter gene expression. However, the transfection of tiMTF-1S (which lacks all three transactivation domains) into a human cell line showed reduced transcriptional activities compared with an endogenous control in both basal- and Zn(2+)-induced conditions. The tiMTF-1 isoforms were tagged with GFP and transfected into Hepa-T1 cells (tilapia hepatocytes). The nuclear translocation of tiMTF-1L was observed when the cells were exposed to a sufficient concentration of metals for 6h. However, tiMTF-1S, was localized in the nucleus with or without metal treatment. Electrophoretic mobility shift assay (EMSA) confirmed that both of the isoforms were able to bind to the MRE specifically in vitro. Tissue distribution studies showed that tiMTF-1L was more abundant than tiMTF-1S in all of the tissues tested.

Cytosolic zinc buffering and muffling: their role in intracellular zinc homeostasis

Our knowledge of the molecular mechanisms of intracellular homeostatic control of zinc ions is now firmly grounded on experimental findings gleaned from the study of zinc proteomes and metallomes, zinc transporters, and insights from the use of computational approaches. A cell's repertoire of zinc homeostatic molecules includes cytosolic zinc-binding proteins, transporters localized to cytoplasmic and organellar membranes, and sensors of cytoplasmic free zinc ions. Under steady state conditions, a primary function of cytosolic zinc-binding proteins is to buffer the relatively large zinc content found in most cells to a cytosolic zinc(ii) ion concentration in the picomolar range. Under non-steady state conditions, zinc-binding proteins and transporters act in concert to modulate transient changes in cytosolic zinc ion concentration in a process that is called zinc muffling. For example, if a cell is challenged by an influx of zinc ions, muffling reactions will dampen the resulting rise in cytosolic zinc ion concentration and eventually restore the cytosolic zinc ion concentration to its original value by shuttling zinc ions into subcellular stores or by removing zinc ions from the cell. In addition, muffling reactions provide a potential means to control changes in cytosolic zinc ion concentrations for purposes of cell signalling in what would otherwise be considered a buffered environment not conducive for signalling. Such intracellular zinc ion signals are known to derive from redox modifications of zinc-thiolate coordination environments, release from subcellular zinc stores, and zinc ion influx via channels. Recently, it has been discovered that metallothionein binds its seven zinc ions with different affinities. This property makes metallothionein particularly well positioned to participate in zinc buffering and muffling reactions. In addition, it is well established that metallothionein is a source of zinc ions under conditions of redox signalling. We suggest that the biological functions of transient changes in cytosolic zinc ion concentrations (presumptive zinc signals) complement those of calcium ions in both spatial and temporal dimensions.

The essential toxin: impact of zinc on human health

Compared to several other metal ions with similar chemical properties, zinc is relatively harmless. Only exposure to high doses has toxic effects, making acute zinc intoxication a rare event. In addition to acute intoxication, long-term, high-dose zinc supplementation interferes with the uptake of copper. Hence, many of its toxic effects are in fact due to copper deficiency. While systemic homeostasis and efficient regulatory mechanisms on the cellular level generally prevent the uptake of cytotoxic doses of exogenous zinc, endogenous zinc plays a significant role in cytotoxic events in single cells. Here, zinc influences apoptosis by acting on several molecular regulators of programmed cell death, including caspases and proteins from the Bcl and Bax families. One organ where zinc is prominently involved in cell death is the brain, and cytotoxicity in consequence of ischemia or trauma involves the accumulation of free zinc. Rather than being a toxic metal ion, zinc is an essential trace element. Whereas intoxication by excessive exposure is rare, zinc deficiency is widespread and has a detrimental impact on growth, neuronal development, and immunity, and in severe cases its consequences are lethal. Zinc deficiency caused by malnutrition and foods with low bioavailability, aging, certain diseases, or deregulated homeostasis is a far more common risk to human health than intoxication.

The metal-responsive transcription factor-1 protein is elevated in human tumors

We previously identified metal-responsive transcription factor-1 (MTF-1) as a positive contributor to mouse fibrosarcoma growth through effects on cell survival, proliferation, tumor angiogenesis and extracellular matrix remodeling. In the present study, we investigated MTF-1 protein expression in human tissues by specific immunostaining of both normal and tumor tissue samples. Immunohistochemical (IHC) staining of a human tissue microarray (TMA), using a unique anti-human MTF-1 antibody, indicated constitutive MTF-1 expression in most normal tissues, with liver and testis displaying comparatively high levels of expression. Nevertheless, MTF-1 protein levels were found to be significantly elevated in diverse human tumor types, including breast, lung and cervical carcinomas. IHC analysis of a separate panel of full-size tissue sections of human breast cancers, including tumor and normal adjacent, surrounding tissue, confirmed and extended the results of the TMA analysis. Taken with our previous findings, this new study suggests a role for MTF-1 in human tumor development, growth or spread. Moreover, the study suggests that MTF-1 could be a novel therapeutic target that offers the opportunity to manipulate metal or redox homeostasis in tumor cells.

Metal transcription factor-1 regulation via MREs in the transcribed regions of selenoprotein H and other metal-responsive genes

Selenoprotein H is a redox-sensing DNA binding protein that upregulates genes involved in antioxidant responses. Given the known links between oxidative stress and heavy metals, we investigated the potential for regulation of selenoprotein H by metals. In silico analysis of the selenoprotein H genes from nine species reveals multiple predicted metal response elements (MREs). To validate MRE function, we investigated the effects of zinc or cadmium addition and metal-responsive transcription factor 1 (MTF-1) knockout on selenoprotein H mRNA levels. Chromatin immunoprecipitation was used to directly assess physical binding of the transcription factor to MREs in the human and mouse selenoprotein H genes. The results reported herein show that selenoprotein H is a newly identified target for MTF-1. Further, whereas nearly all prior studies of MREs focused on those located in promoters, we demonstrate binding of MTF-1 to MREs located downstream of the transcription start sites in the human and murine selenoprotein H genes. Finally, we identified MREs in downstream sequences in 15 additional MTF-1 regulated genes lacking promoter MREs, and demonstrated MTF-1 binding in three of these genes. This regulation via sequences downstream of promoters highlights a new direction for identifying previously unrecognized target genes for MTF-1.

Combined analysis of hypoxia-inducible factor 1 alpha and metallothionein indicates an aggressive subtype of colorectal carcinoma

PURPOSE

Hypoxia-inducible factor 1 (HIF-1) is a hypoxia-induced transcription factor that regulates gene expression in critical pathways involved in tumour growth and metastasis. Metallothionein (MT) is a group of small molecular weight cysteine-rich proteins with a broad variety of functions. The present study aimed to analyse the prognostic impact of HIF-1alpha and MT expression in colorectal cancer and to evaluate a possible link of combined HIF-1alpha and MT expression with colorectal cancer progression.

MATERIALS AND METHODS

We investigated the relationship of HIF-1alpha and MT with each other and clinicopathological parameters including proliferative activity (Ki67) and apoptosis (terminal desoxyribonucleotide transferase-mediated dUTP nick-end labelling) using immunohistochemistry.

RESULTS

HIF-1alpha expression was identified as an independent prognostic parameter in multivariate survival analysis and characterised an aggressive cancer phenotype. In addition, HIF-1alpha was significantly linked to an increased expression of MT.

CONCLUSIONS

HIF-1alpha expression qualified as an independent prognostic and characterised an aggressive cancer phenotype associated with an increased expression of MT. Our study suggests that MT can be added to the complex biological pathways induced by hypoxia in human cancer tissue.

Functional significance of zinc-related signaling pathways in immune cells

Recent years have brought a paradigm shift for the role of the essential trace element zinc in immunity. Although its function as a structural component of many enzymes has been known for decades, current experimental evidence points to an additional function of the concentration of free or loosely bound zinc ions as an intracellular signal. The activity of virtually all immune cells is modulated by zinc in vitro and in vivo. In this review, we discuss the interactions of zinc with major signaling pathways that regulate immune cell activity, and the implications of zinc deficiency or supplementation on zinc signaling as the molecular basis for an effect of zinc on immune cell function.

Metalloproteins and metal sensing

Almost half of all enzymes must associate with a particular metal to function. An ambition is to understand why each metal-protein partnership arose and how it is maintained. Metal availability provides part of the explanation, and has changed over geological time and varies between habitats but is held within vital limits in cells. Such homeostasis needs metal sensors, and there is an ongoing search to discover the metal-sensing mechanisms. For metalloproteins to acquire the right metals, metal sensors must correctly distinguish between the inorganic elements.

Zinc and DHA have opposing effects on the expression levels of histones H3 and H4 in human neuronal cells

Zn and DHA have putative neuroprotective effects and these two essential nutrients are known to interact biochemically. We aimed to identify novel protein candidates that are differentially expressed in human neuronal cell line M17 in response to Zn and DHA that would explain the molecular basis of this interaction. Two-dimensional gel electrophoresis and MS were applied to identify major protein expression changes in the protein lysates of human Ml7 neuronal cells that had been grown in the presence and absence of Zn and DHA. Proteomic findings were further investigated using Western immunoblot and real-time PCR analyses. Four protein spots, which had significant differential expression, were identified and selected for in-gel trypsin digestion followed by matrix-assisted laser desorption ionisation MS analysis. The resultant peptide mass fingerprint for each spot allowed their respective identities to be deduced. Two human histone variants H3 and H4 were identified. Both H3 and H4 were downregulated by Zn in the absence of DHA (Zn effect) and upregulated by DHA (DHA effect) in the presence of Zn (physiological condition). These proteomic findings were further supported by Western immunoblot and real-time PCR analyses using H3- and H4-specific monoclonal antibodies and oligonucleotide primers, respectively. We propose that dietary Zn and DHA cause a global effect on gene expression, which is mediated by histones. Such novel information provides possible clues to the molecular basis of neuroprotection by Zn and DHA that may contribute to the future treatment, prevention and management of neurodegenerative diseases such as Alzheimer's disease.

Nickel mobilizes intracellular zinc to induce metallothionein in human airway epithelial cells

We recently reported that induction of metallothionein (MT) was critical in limiting nickel (Ni)-induced lung injury in intact mice. Nonetheless, the mechanism by which Ni induces MT expression is unclear. We hypothesized that the ability of Ni to mobilize zinc (Zn) may contribute to such regulation and therefore, we examined the mechanism for Ni-induced MT2A expression in human airway epithelial (BEAS-2B) cells. Ni induced MT2A transcript levels and protein expression by 4 hours. Ni also increased the activity of a metal response element (MRE) promoter luciferase reporter construct, suggesting that Ni induces MRE binding of the metal transcription factor (MTF-1). Exposure to Ni resulted in the nuclear translocation of MTF-1, and Ni failed to induce MT in mouse embryonic fibroblasts lacking MTF-1. As Zn is the only metal known to directly bind MTF-1, we then showed that Ni increased a labile pool of intracellular Zn in cells as revealed by fluorescence-activated cell sorter using the Zn-sensitive fluorophore, FluoZin-3. Ni-induced increases in MT2A mRNA and MRE-luciferase activity were sensitive to the Zn chelator, TPEN, supporting an important role for Zn in mediating the effect of Ni. Although neither the source of labile Zn nor the mechanism by which Ni liberates labile Zn was apparent, it was noteworthy that Ni increased intracellular reactive oxygen species (ROS). Although both N-acetyl cysteine (NAC) and ascorbic acid (AA) decreased Ni-induced increases in ROS, only NAC prevented Ni-induced increases in MT2A mRNA, suggesting a special role for interactions of Ni, thiols, and Zn release.

Cellular stress response pathway system as a sentinel ensemble in toxicological screening

High costs, long test times, and societal concerns related to animal use have required the development of in vitro assays for the rapid and cost-effective toxicological evaluation and characterization of compounds in both the pharmaceutical and environmental arenas. Although the pharmaceutical industry has developed very effective, high-throughput in vitro assays for determining the therapeutic potential of compounds, the application of this approach to toxicological screening has been limited. A primary reason for this is that while drug candidate screens are directed to a specific target/mechanism, xenobiotics can cause toxicity through any of a myriad of undefined interactions with cellular components and processes. Given that it is not practical to design assays that can interrogate each potential toxicological target, an integrative approach is required if there is to be a rapid and low-cost toxicological evaluation of chemicals. Cellular stress response pathways offer a viable solution to the creation of a set of integrative assays as there is a limited and hence manageable set (a small ensemble of 10 or less) of major cellular stress response pathways through which cells mount a homoeostatic response to toxicants and which also participate in cell fate/death decisions. Further, over the past decades, these pathways have been well characterized at a molecular level thereby enabling the development of high-throughput cell-based assays using the components of the pathways. Utilization of the set of cellular stress response pathway-based assays as indicators of toxic interactions of chemicals with basic cellular machinery will potentially permit the clustering of chemicals based on biological response profiles of common mode of action (MOA) and also the inference of the specific MOA of a toxicant. This article reviews the biochemical characteristics of the stress response pathways, their common architecture that enables rapid activation during stress, their participation in cell fate decisions, the essential nature of these pathways to the organism, and the biochemical basis of their cross-talk that permits an assay ensemble screening approach. Subsequent sections describe how the stress pathway ensemble assay approach could be applied to screening potentially toxic compounds and discuss how this approach may be used to derive toxicant MOA from the biological activity profiles that the ensemble strategy provides. The article concludes with a review of the application of the stress assay concept to noninvasive in vivo assessments of chemical toxicants.

The immune system and the impact of zinc during aging

The trace element zinc is essential for the immune system, and zinc deficiency affects multiple aspects of innate and adaptive immunity. There are remarkable parallels in the immunological changes during aging and zinc deficiency, including a reduction in the activity of the thymus and thymic hormones, a shift of the T helper cell balance toward T helper type 2 cells, decreased response to vaccination, and impaired functions of innate immune cells. Many studies confirm a decline of zinc levels with age. Most of these studies do not classify the majority of elderly as zinc deficient, but even marginal zinc deprivation can affect immune function. Consequently, oral zinc supplementation demonstrates the potential to improve immunity and efficiently downregulates chronic inflammatory responses in the elderly. These data indicate that a wide prevalence of marginal zinc deficiency in elderly people may contribute to immunosenescence.

SLC39A9 (ZIP9) regulates zinc homeostasis in the secretory pathway: characterization of the ZIP subfamily I protein in vertebrate cells

The SLC39A family of zinc transporters can be divided into four subfamilies (I, II, LIV-1, and gufA) in vertebrates, but studies of their functions have been restricted exclusively to members of subfamilies II and LIV-1. In this study, we characterized SLC39A9 (ZIP9), the only member of subfamily I in vertebrates. Confocal microscopy demonstrated that transiently expressed, HA-tagged human ZIP9 (hZIP9-HA) was localized to the trans-Golgi network regardless of zinc status. Disruption of the ZIP9 gene in DT40 cells did not change the growth rate, sensitivity to high zinc and manganese concentrations during long-term culture, or cellular zinc status after short-term incubation with zinc. The alkaline phosphatase activity of ZIP9(-/-) cells did not change in cells cultured in medium containing normal zinc levels. In contrast, the activity of this enzyme decreased in wild-type cells cultured in zinc deficient medium but less so in ZIP9(-/-) cells under these conditions. Stable over-expression of hZIP9-HA moderately decreased alkaline phophatase activity. These results suggest that ZIP9 functions to regulate zinc homeostasis in the secretory pathway without significantly altering cytosolic zinc homeostasis.

Zinc fingers as biologic redox switches?

Despite zinc ions being redox inert in biologic systems, zinc-finger structures act as redox-sensitive molecular switches controlling several crucial cellular processes. Oxidative or nitrosative stress, via modification of zinc finger cysteine thiols, leads to a release of Zn(2+) from these structures, causing not only a loss of zinc-finger function but also an increase of cytoplasmic or nuclear free Zn(2+) that may, in turn, stimulate and interfere with cellular signaling cascades. A signaling cascade stimulated by exposure of cells to zinc ions or to stressful stimuli that are reported to cause an intracellular release of zinc ions involves phosphoinositide 3'-kinases and the Ser/Thr protein kinase Akt, resulting in an inactivation of transcriptional regulators of the FoxO family. Possible modes of action of zinc ions to stimulate this signaling cascade and consequences of stimulation are discussed. Moreover, we present an overview on human diseases or disorders characterized by an intracellular Zn(2+) dyshomeostasis.

Two metallothionein genes from mud loach Misgurnus mizolepis (Teleostei; Cypriniformes): gene structure, genomic organization, and mRNA expression analysis

Two metallothionein genes, MLMT-IA and MLMT-IB, were isolated and characterized from the mud loach Misgurnus mizolepis (Teleostei; Cypriniformes). For these MTs, we determined a tandem "tail-to-head" genomic organizational pattern, identified conserved genomic features, showed high sequence identities in the coding regions, and examined the closest phylogenetic affiliation, suggesting their divergence by a recent gene duplication event. However, the 5'-flanking upstream regions in MLMT-IA and MLMT-IB exposed large differences in the composition and distribution patterns of various transcription factor binding motifs, especially regarding the organization of the metal response element clusters. Real-time RT-PCR assays showed that mRNA levels of both MLMT-IA and MLMT-IB isoforms were variable among tissues and the ratios between them were also variable across tissues, although the MLMT-IA was always predominant in every adult tissue tested. We also found that the MLMT-IA and MLMT-IB mRNA expression levels were regulated dynamically during embryonic and larval development stages, in which the basal expression level of MLMT-IA was also consistently higher than that of MLMT-IB. Upon acute in vivo metal exposure to cadmium, chromium, copper, iron, manganese, nickel, or zinc at 5 microM for 48 h, the transcriptional modulations of MLMT-IA and MLMT-IB were quite different from each other and the type of response was affected significantly by the kind of metals and tissues.

Expression of metallothionein mRNAs on mouse cerebellum microglia cells by thimerosal and its metabolites

Effects of thimerosal and its metabolites, ethyl mercury and thiosalicylate, on the expression of metallothionein (MT) mRNAs in mouse cerebellum microglia cell line, C8-B4 cells, were studied. The level of MT-1 mRNA significantly decreased at early hours and recovered time-dependently 24h after thimerosal was added to the C8-B4 cells. However, MT-2 and MT-3 mRNA expressions did not change from the control group. In contrast, the expression of MT-1 mRNA increased in a mouse neuroblastoma cell line 6h after incubation with thimerosal. In addition, the level of MT-1 mRNA decreased in C8-B4 cells 6h after the addition of thiosalicylate, but ethyl mercury induced MT-1 mRNA expression. When cell viability was compared with thimerosal, thiosalicylate, and ethyl mercury, the viability of C8-B4 cells decreased dose-dependently 24h after either thimerosal or ethyl mercury was added; however, the viability increased dose-dependently until 15 microM thiosalicylate was added. From the present results, it is concluded that the expression of MT-1 mRNA may be mediated by different factors than the expression of MT-2 mRNA in C8-B4 cells. The reduction of MT-1 mRNA level by thiosalicylate may affect the proliferation of C8-B4 cells.

RNA oxidation and zinc in hepatic encephalopathy and hyperammonemia

Hepatic encephalopathy is a neuropsychiatric manifestation of acute and chronic liver failure. Ammonia plays a key role in the pathogenesis of hepatic encephalopathy by inducing astrocyte swelling and/or sensitizing astrocytes to swelling by a heterogeneous panel of precipitating factors and conditions. Whereas astrocyte swelling in acute liver failure contributes to a clinically overt brain edema, a low grade glial edema without clinically overt brain edema is observed in hepatic encephalopathy in liver cirrhosis. Astrocyte swelling produces reactive oxygen and nitrogen oxide species (ROS/RNOS), which again increase astrocyte swelling, thereby creating a self-amplifying signaling loop. Astroglial swelling and ROS/RNOS increase protein tyrosine nitration and may account for neurotoxic effects of ammonia and other precipitants of hepatic encephalopathy. Recently, RNA oxidation and an increase of free intracellular zinc ([Zn(2+)](i)) were identified as further consequences of astrocyte swelling and ROS/RNOS production. An elevation of [Zn(2+)](i) mediates mRNA expression of metallothionein and the peripheral benzodiazepine receptor (PBR) induced by hypoosmotic astrocyte swelling. Further, Zn(2+) mediates RNA oxidation in ammonia-treated astrocytes. In the brain of hyperammonemic rats oxidized RNA localizes in part to perivascular astrocyte processes and to postsynaptic dendritic spines. RNA oxidation may impair postsynaptic protein synthesis, which is critically involved in learning and memory consolidation. RNA oxidation offers a novel explanation for multiple disturbances of neurotransmitter systems and gene expression and the cognitive deficits observed in hepatic encephalopathy.

Zinc transporters and cancer: a potential role for ZIP7 as a hub for tyrosine kinase activation

Zinc, which is essential for many cellular processes, is controlled by zinc transporters and through buffering by metallothioneins and glutathione. Although zinc is increasingly implicated in disease states, little is known about how zinc regulates cellular biochemical pathways. Recent seminal articles have revealed discrete zinc-trafficking pathways that are linked to signalling cascades, particularly those involving protein phosphatase inhibition and downstream activation of mitogen-activated protein kinases and tyrosine kinases. Here, we discuss the mechanisms of cellular zinc homeostasis, and we propose an important role for the zinc transporter solute carrier family 39, member 7 (SLC39A7; commonly referred to as ZIP7). ZIP7 releases zinc from the endoplasmic reticulum and might be required for tyrosine kinase activation. These observations position ZIP7 at a critical node in zinc-mediated tyrosine kinase signalling and suggest that this protein might form a novel target for diseases such as cancer where prevention of tyrosine kinase activation would be therapeutically advantageous.

Mechanisms of mammalian zinc-regulated gene expression

Mechanisms through which gene expression is regulated by zinc are central to cellular zinc homoeostasis. In this context, evidence for the involvement of zinc dyshomoeostasis in the aetiology of diseases, including Type 2 diabetes, Alzheimer's disease and cancer, highlights the importance of zinc-regulated gene expression. Mechanisms elucidated in bacteria and yeast provide examples of different possible modes of zinc-sensitive gene regulation, involving the zinc-regulated binding of transcriptional activators and repressors to gene promoter regions. A mammalian transcriptional regulatory mechanism that mediates zinc-induced transcriptional up-regulation, involving the transcription factor MTF1 (metal-response element-binding transcription factor 1), has been studied extensively. Gene responses in the opposite direction (reduced mRNA levels in response to increased zinc availability) have been observed in mammalian cells, but a specific transcriptional regulatory process responsible for such a response has yet to be identified. Examples of single zinc-sensitive transcription factors regulating gene expression in opposite directions are emerging. Although zinc-induced transcriptional repression by MTF1 is a possible explanation in some specific instances, such a mechanism cannot account for repression by zinc of all mammalian genes that show this mode of regulation, indicating the existence of as yet uncharacterized mechanisms of zinc-regulated transcription in mammalian cells. In addition, recent findings reveal a role for effects of zinc on mRNA stability in the regulation of specific zinc transporters. Our studies on the regulation of the human gene SLC30A5 (solute carrier 30A5), which codes for the zinc transporter ZnT5, have revealed that this gene provides a model system by which to study both zinc-induced transcriptional down-regulation and zinc-regulated mRNA stabilization.

The ligand environment of zinc stored in vesicles

Zinc serves regulatory functions in cells and thus, several mechanisms exist for tight control of its homeostasis. One mechanism is storage in and retrieval from vesicles, so-called zincosomes, but the chemical speciation of zincosomal zinc has remained enigmatic. Here, we determine the intravesicular zinc-coordination in isolated zincosomes in comparison to intact RAW264.7 murine macrophage cells. In elemental maps of a cell monolayer, generated by microbeam X-ray fluorescence, zincosomes were identified as spots of high zinc accumulation. A fingerprint for the binding motif obtained by muXANES (X-ray absorption near edge structure) matches the XANES from isolated vesicles; zinc is not free, but present as a complexed form (average coordination; 1.0 sulfur, 2,5 histidines 30 and 1.0 oxygen), resembling regulatory or catalytic zinc sites in proteins. Such coordination enables reversible binding, acting as a 'zinc sink', facilitating the accumulation of high amounts of zinc against a concentration gradient.

Regulation of Metallothionein Gene Expression

Organisms from bacteria to humans use elaborate systems to regulate levels of bioavailable zinc, copper, and other essential metals. An excess of them, or even traces of non-essential metals such as cadmium and mercury, can be highly toxic. Metallothioneins (MTs), short, cysteine-rich proteins, play pivotal roles in metal homeostasis and detoxification. With their sulfhydryl groups they avidly bind toxic metals and also play a role in cellular redox balance and radical scavenging. The intracellular concentration of MTs is adjusted to cellular demand primarily via regulated transcription. Especially upon heavy metal load, metallothionein gene transcription is strongly induced. From insects to mammals, the major regulator of MT transcription is MTF-1 (metal-responsive transcription factor 1), a zinc finger protein that binds to specific DNA sequence motifs (MREs) in the promoters of MT genes and other metal-regulated genes. This chapter provides an overview of our current knowledge on the expression and regulation of MT genes in higher eukaryotes, with some reference also to fungi which apparently have independently evolved their own regulatory systems.

Differential regulation of zebrafish metallothionein-II (zMT-II) gene transcription in ZFL and SJD cell lines by metal ions

Two alleles of a zebrafish metallothionein II gene (zMT-II) promoter (zMT-IIA and zMT-IIB) containing 10 MREs in the 5'-flanking region (1514bp) were identified in zebrafish. These putative MREs were confirmed via electrophoretic mobility shift assay (EMSA) to have binding activities from the cellular and nuclear extracts of a zebrafish cell line, ZFL. Transient gene expression studies using zebrafish liver (ZFL) and caudal fin (SJD) cell lines also confirmed that the most distal cluster of MREs contributed to the maximal induction of zMT-IIA activity by Zn(2+) and that this Zn(2+) induction was dose-dependent. Further transient gene expression assay of the zMT-IIA gene promoter was carried out to study the effects of various metal ions (Zn(2+), Cd(2+), Cu(2+), Hg(+), As(3+), As(5+), Cr(3+) and Cr(6+)), and Zn(2+) and Cd(2+) were found to be the most efficient MT gene inducers of zMT-II. As(3+) was a weak inducer of zMT-II in the two cell lines, and Hg(+) caused significant induction only in the SJD cells. No significant induction was found in the other metal ion exposures. EMSA also identified transcription factor(s) of two different sizes from the cytoplasmic and nuclear extracts of the ZFL cells that were able to bind with the MREs, but no increase in MRE binding was detected in the extracts of these cells after Zn(2+) or Cd(2+) treatment, compared with untreated control cells. The mechanisms of MT gene transcription induction via metal ions are discussed herein.