trans repression of the human metallothionein IIA gene promoter by PZ120, a novel 120-kilodalton zinc finger protein

Late fetal hematopoietic failure results from ZBTB11 deficiency despite abundant HSC specification

Hematopoiesis produces diverse blood cell lineages to meet the basal needs and sudden demands of injury or infection. A rapid response to such challenges requires the expansion of specific lineages and a prompt return to balanced steady-state levels, necessitating tightly coordinated regulation. Previously we identified a requirement for the zinc finger and broad complex, tramtrak, bric-a-brac domain-containing 11 (ZBTB11) transcription factor in definitive hematopoiesis using a forward genetic screen for zebrafish myeloid mutants. To understand its relevance to mammalian systems, we extended these studies to mice. When Zbtb11 was deleted in the hematopoietic compartment, embryos died at embryonic day (E) 18.5 with hematopoietic failure. Zbtb11 hematopoietic knockout (Zbtb11hKO) hematopoietic stem cells (HSCs) were overabundantly specified from E14.5 to E17.5 compared with those in controls. Overspecification was accompanied by loss of stemness, inability to differentiate into committed progenitors and mature lineages in the fetal liver, failure to seed fetal bone marrow, and total hematopoietic failure. The Zbtb11hKO HSCs did not proliferate in vitro and were constrained in cell cycle progression, demonstrating the cell-intrinsic role of Zbtb11 in proliferation and cell cycle regulation in mammalian HSCs. Single-cell RNA sequencing analysis identified that Zbtb11-deficient HSCs were underrepresented in an erythroid-primed subpopulation and showed downregulation of oxidative phosphorylation pathways and dysregulation of genes associated with the hematopoietic niche. We identified a cell-intrinsic requirement for Zbtb11-mediated gene regulatory networks in sustaining a pool of maturation-capable HSCs and progenitor cells.

Genetics of metallothioneins in Drosophilamelanogaster

Metallothioneins (MTs) are ubiquitous metal-chelating proteins involved in cellular metal homeostasis. MTs were found to be related with almost all the biological processes and their malfunctioning is responsible for a lot of important human diseases. Invertebrate MTs were also used broadly as biomarkers of metal contamination due to their inducible expression by metal exposure. MT system plays a significant role in maintaining human health and ecological stability. Drosophila melanogaster, the vinegar fly, is a perfect model for studying insect MT systems. Six MTs were identified in D. melanogaster, and were designated MtnA to F. All the MTs are considered as Cu-thioneins except for MtnF, which is putatively a Zn-thionein. Expression of all the MTs are regulated by MTF-1/MRE system, thus being able to be induced by heavy metal exposure. The expression pattern and function of separated MTs are partially overlapped and partially distinct. In this work, we made a summary of all the studies on D. melanogaster MTs. From this review, we noted that, compared with studies on mammalian MTs, the understanding of the MT system of D. melanogaster and other invertebrates, especially the regulation mechanism for MT expression and protein-protein interaction with them, is still in a low level.

The Bioinorganic Chemistry of Mammalian Metallothioneins

The functions, purposes, and roles of metallothioneins have been the subject of speculations since the discovery of the protein over 60 years ago. This article guides through the history of investigations and resolves multiple contentions by providing new interpretations of the structure-stability-function relationship. It challenges the dogma that the biologically relevant structure of the mammalian proteins is only the one determined by X-ray diffraction and NMR spectroscopy. The terms metallothionein and thionein are ambiguous and insufficient to understand biological function. The proteins need to be seen in their biological context, which limits and defines the chemistry possible. They exist in multiple forms with different degrees of metalation and types of metal ions. The homoleptic thiolate coordination of mammalian metallothioneins is important for their molecular mechanism. It endows the proteins with redox activity and a specific pH dependence of their metal affinities. The proteins, therefore, also exist in different redox states of the sulfur donor ligands. Their coordination dynamics allows a vast conformational landscape for interactions with other proteins and ligands. Many fundamental signal transduction pathways regulate the expression of the dozen of human metallothionein genes. Recent advances in understanding the control of cellular zinc and copper homeostasis are the foundation for suggesting that mammalian metallothioneins provide a highly dynamic, regulated, and uniquely biological metal buffer to control the availability, fluctuations, and signaling transients of the most competitive Zn(II) and Cu(I) ions in cellular space and time.

Antioxidant Defenses in the Human Eye: A Focus on Metallothioneins

The human eye, the highly specialized organ of vision, is greatly influenced by oxidants of endogenous and exogenous origin. Oxidative stress affects all structures of the human eye with special emphasis on the ocular surface, the lens, the retina and its retinal pigment epithelium, which are considered natural barriers of antioxidant protection, contributing to the onset and/or progression of eye diseases. These ocular structures contain a complex antioxidant defense system slightly different along the eye depending on cell tissue. In addition to widely studied enzymatic antioxidants, including superoxide dismutase, glutathione peroxidase, catalase, peroxiredoxins and selenoproteins, inter alia, metallothioneins (MTs) are considered antioxidant proteins of growing interest with further cell-mediated functions. This family of cysteine rich and low molecular mass proteins captures and neutralizes free radicals in a redox-dependent mechanism involving zinc binding and release. The state of the art of MTs, including the isoforms classification, the main functions described to date, the Zn-MT redox cycle as antioxidant defense system, and the antioxidant activity of Zn-MTs in the ocular surface, lens, retina and its retinal pigment epithelium, dependent on the number of occupied zinc-binding sites, will be comprehensively reviewed.

Metallothionein-2 is associated with the amelioration of asthmatic pulmonary function by acupuncture through protein phosphorylation

BACKGROUND

Acupuncture has long been used for asthma treatment but the underlying mechanism remains unclear. Previous study showed that metallothionein-2 (MT-2) was significantly decreased in asthmatic lung tissue. However, the relationship between acupuncture treatment and MT-2 expression during asthma is still unknown, and the detailed effect analysis of MT-2 on phosphorylation in airway smooth muscle cells (ASMCs) is also unclear.

METHODS

The acupuncture effect on pulmonary resistance (RL) was investigated in a rat model of asthma, and the mRNA and protein levels of MT-2 in lung tissue were detected. Primary ASMCs were isolated and treated with MT-2 recombinant protein to study the MT-2 effects on ASMC relaxation. A Phospho Explorer antibody microarray was applied to detect protein phosphorylation changes associated with MT-2-induced ASMC relaxation. Bioinformatic analysis were performed with PANTHER database, DAVID and STRING. Phosphorylation changes in key proteins were confirmed by Western blot.

RESULTS

Acupuncture significantly reduced RL at 2-5 min (P < 0.05 vs asthma) in asthmatic rats. Acupuncture continued to increase MT-2 mRNA expression in lung tissue for up to 14 days (P < 0.05 vs asthma). The MT-2 protein expression was significantly decreased in the asthmatic rats (P < 0.05 vs control), while MT-2 protein expression was significantly increased in the asthmatic model group treated with acupuncture (P < 0.05 vs asthma). Primary ASMCs were successfully isolated and recombinant MT-2 protein (100, 200, 400 ng/ml) significantly relaxed ASMCs (P < 0.05 vs control). MT-2 induced phosphorylation changes in 51 proteins. Phosphorylation of 14 proteins were upregulated while 37 proteins were downregulated. PANTHER classification revealed eleven functional groups, and the phosphorylated proteins were identified as transferases (27.8 %), calcium-binding proteins (11.1 %), etc. DAVID functional classification showed that the phosphorylated proteins could be attributed to eight functions, including protein phosphorylation and regulation of GTPase activity. STRING protein-protein interaction network analysis showed that Akt1 was one of the most important hubs for the phosphorylated proteins. The phosphorylation changes of Akt1 and CaMK2β were consistent in both the Phospho Explorer antibody microarray and Western blot.

CONCLUSION

Acupuncture can significantly ameliorate RL, and the MT-2 mRNA and protein levels in lung tissue are increased during treatment. MT-2 significantly relaxes ASMCs and induces a series of protein phosphorylation. These phosphorylation changes, including Akt1 and CaMK2β, may play important roles in the therapeutic effects of acupuncture on asthma.

Biallelic missense variants in ZBTB11 can cause intellectual disability in humans

Exploring genes and pathways underlying intellectual disability (ID) provides insight into brain development and function, clarifying the complex puzzle of how cognition develops. As part of ongoing systematic studies to identify candidate ID genes, linkage analysis and next-generation sequencing revealed Zinc Finger and BTB Domain Containing 11 (ZBTB11) as a novel candidate ID gene. ZBTB11 encodes a little-studied transcription regulator, and the two identified missense variants in this study are predicted to disrupt canonical Zn2+-binding residues of its C2H2 zinc finger domain, leading to possible altered DNA binding. Using HEK293T cells transfected with wild-type and mutant GFP-ZBTB11 constructs, we found the ZBTB11 mutants being excluded from the nucleolus, where the wild-type recombinant protein is predominantly localized. Pathway analysis applied to ChIP-seq data deposited in the ENCODE database supports the localization of ZBTB11 in nucleoli, highlighting associated pathways such as ribosomal RNA synthesis, ribosomal assembly, RNA modification and stress sensing, and provides a direct link between subcellular ZBTB11 location and its function. Furthermore, given the report of prominent brain and spinal cord degeneration in a zebrafish Zbtb11 mutant, we investigated ZBTB11-ortholog knockdown in Drosophila melanogaster brain by targeting RNAi using the UAS/Gal4 system. The observed approximate reduction to a third of the mushroom body size-possibly through neuronal reduction or degeneration-may affect neuronal circuits in the brain that are required for adaptive behavior, specifying the role of this gene in the nervous system. In conclusion, we report two ID families segregating ZBTB11 biallelic mutations disrupting Zn2+-binding motifs and provide functional evidence linking ZBTB11 dysfunction to this phenotype.

The Functions of Metamorphic Metallothioneins in Zinc and Copper Metabolism

Recent discoveries in zinc biology provide a new platform for discussing the primary physiological functions of mammalian metallothioneins (MTs) and their exquisite zinc-dependent regulation. It is now understood that the control of cellular zinc homeostasis includes buffering of Zn²⁺ ions at picomolar concentrations, extensive subcellular re-distribution of Zn²⁺, the loading of exocytotic vesicles with zinc species, and the control of Zn²⁺ ion signalling. In parallel, characteristic features of human MTs became known: their graded affinities for Zn²⁺ and the redox activity of their thiolate coordination environments. Unlike the single species that structural models of mammalian MTs describe with a set of seven divalent or eight to twelve monovalent metal ions, MTs are metamorphic. In vivo, they exist as many species differing in redox state and load with different metal ions. The functions of mammalian MTs should no longer be considered elusive or enigmatic because it is now evident that the reactivity and coordination dynamics of MTs with Zn²⁺ and Cu⁺ match the biological requirements for controlling—binding and delivering—these cellular metal ions, thus completing a 60-year search for their functions. MT represents a unique biological principle for buffering the most competitive essential metal ions Zn²⁺ and Cu⁺. How this knowledge translates to the function of other families of MTs awaits further insights into the specifics of how their properties relate to zinc and copper metabolism in other organisms.

The Pu.1 target gene Zbtb11 regulates neutrophil development through its integrase-like HHCC zinc finger

In response to infection and injury, the neutrophil population rapidly expands and then quickly re-establishes the basal state when inflammation resolves. The exact pathways governing neutrophil/macrophage lineage outputs from a common granulocyte-macrophage progenitor are still not completely understood. From a forward genetic screen in zebrafish, we identify the transcriptional repressor, ZBTB11, as critical for basal and emergency granulopoiesis. ZBTB11 sits in a pathway directly downstream of master myeloid regulators including PU.1, and TP53 is one direct ZBTB11 transcriptional target. TP53 repression is dependent on ZBTB11 cys116, which is a functionally critical, metal ion-coordinating residue within a novel viral integrase-like zinc finger domain. To our knowledge, this is the first description of a function for this domain in a cellular protein. We demonstrate that the PU.1–ZBTB11–TP53 pathway is conserved from fish to mammals. Finally, Zbtb11 mutant rescue experiments point to a ZBTB11-regulated TP53 requirement in development of other organs.

Neutrophils are increased in response to injury and infection but how they form from a common granulocyte-macrophage progenitor is unclear. Here, the authors identify a role for the transcriptional repressor ZBTB11 in zebrafish, which is regulated by master myeloid regulators and represses TP53.

Basal and copper-induced expression of metallothionein isoform 1,2 and 3 genes in epithelial cancer cells: The role of tumor suppressor p53

Metallothioneins (MTs) are a ubiquitous low-molecular weight, cysteine rich proteins with a high affinity for metal ions. The expression and induction of MTs have been associated with protection against DNA damage, oxidative stress, and apoptosis. Our past research had shown that p53 is an important factor in metal regulation of MTs. The present study was undertaken to explore further the interrelationship between p53 and MTs. We investigated whether silencing of p53 could affect expression pattern of basal and copper induced metallothioneins. The silencing of wild-type p53 (wt-p53) in epithelial breast cancer MCF7 cells affected the basal level of MT-2A RNA, whereas the levels of MT-1A and MT-1X RNA remained largely unchanged. The expression of MT-3 was undetectable in MCF7 with either functional or silenced p53. MCF7 cells with silenced wt-p53 failed to upregulate MT-2A in response to copper and showed a reduced sensitivity toward copper induced cell apoptotic death. Similarly in MCF7-E6 and MDA-MB-231 cells, the presence of inactive/mutated p53 halted MT-1A and MT-2A gene expression in response to copper. Constitutive expression of MT-3 RNA was detectable in the presence of mutated p53 (mtp53). Transient transfection of MDA-MB-231 cells with wt-p53 enabled copper induced upregulation of both MT-1A and MT-2A but not basal level of MT-2A, MT-1E, MT-1X and MT-3. Inactivation of p53 in HepG2 cells amplified the basal expression of studied MT isoforms, including MT-3, as well as copper-induced mRNA expression of MTs except MT-1H and MT-3. Presented data demonstrate a direct relation between p53 and MT-1A and MT-2A and they also indicate that wt-p53 might be a negative regulator of MT-3 in epithelial cancer cells.

Positive and negative regulators of the metallothionein gene (review)

Metallothioneins (MTs) are metal-binding proteins involved in diverse processes, including metal homeostasis and detoxification, the oxidative stress response and cell proliferation. Aberrant expression and silencing of these genes are important in a number of diseases. Several positive regulators of MT genes, including metal-responsive element-binding transcription factor (MTF)-1 and upstream stimulatory factor (USF)-1, have been identified and mechanisms of induction have been well described. However, the negative regulators of MT genes remain to be elucidated. Previous studies from the group of the present review have revealed that the hematopoietic master transcription factor, PU.1, directly represses the expression levels of MT genes through its epigenetic activities, and upregulation of MT results in the potent inhibition of myeloid differentiation. The present review focuses on PU.1 and several other negative regulators of this gene, including PZ120, DNA methyltransferase 3a with Mbd3 and Brg1 complex, CCAAT enhancer binding protein α and Ku protein, and describes the suppression of the MT genes through these transcription factors.

Deregulation of biometal homeostasis: the missing link for neuronal ceroid lipofuscinoses?

Neuronal ceroid lipofuscinoses (NCLs), a group of genetically distinct fatal neurodegenerative disorders with no treatment or cure, are clinically characterised by progressive motor and visual decline leading to premature death. While the underlying pathological mechanisms are yet to be precisely determined, the diseases share several common features including inflammation, lysosomal lipofuscin deposits and lipid abnormalities. An important hallmark of most common neurodegenerative disorders including Alzheimer's, Parkinson's and motor neuron diseases is deregulation of biologically active metal homeostasis. Metals such as zinc, copper and iron are critical enzyme cofactors and are important for synaptic transmission in the brain, but can mediate oxidative neurotoxicity when homeostatic regulatory mechanisms fail. We previously demonstrated biometal accumulation and altered biometal transporter expression in 3 animal models of CLN6 NCL disease. In this study we investigated the hypothesis that altered biometal homeostasis may be a feature of NCLs in general using 3 additional animal models of CLN1, CLN3 and CLN5 disease. We demonstrated significant accumulation of the biometals zinc, copper, manganese, iron and cobalt in these mice. Patterns of biometal accumulation in each model preceded significant neurodegeneration, and paralleled the relative severity of disease previously described for each model. Additionally, we observed deregulation of transcripts encoding the anti-oxidant protein, metallothionein (Mt), indicative of disruptions to biometal homeostasis. These results demonstrate that altered biometal homeostasis is a key feature of at least 4 genetically distinct forms of NCL disease.

Vitreous induces heme oxygenase-1 expression mediated by transforming growth factor-beta and reactive oxygen species generation in human retinal pigment epithelial cells

PURPOSE

When human retinal pigment epithelial (RPE) cells come in contact with vitreous, they undergo changes in gene expression that include inflammatory and anti-oxidant responses. The effects of vitreous on expression of heme oxygenase-1 (HO-1), metallothionein (MT) -1a and -2a, and c-fos were investigated. Activator protein-1 (AP-1) binding sites are located in the promoter region of HO-1 and MT genes and the effects of vitreous on c-fos activity were investigated.

METHODS

Low passage cultures of human RPE cells were grown in the presence or absence of vitreous or transforming growth factor-beta (TGF-beta). The expression of HO-1 and MTs was measured by real time PCR and, in the case of HO-1, by immunoblotting and immunofluorescence microscopy. Specific inhibitors were used to investigate possible signaling pathways. The effect of vitreous on activation of AP-1 transcription factor was determined by immunoblotting, electrophoretic mobility shift assays, or immunofluorescence microscopy.

RESULTS

Incubation of RPE cells with vitreous resulted in increased expression of HO-1, MT-1a and MT-2a. TGF-beta caused an increase in HO-1 expression, although not to the extent mediated by vitreous, but had little effect on MT expression. Addition of inhibitors of TGF-beta signaling (SB431542 or TGF-beta-neutralizing antibodies) decreased the vitreous induction of HO-1. Several reactive oxygen species (ROS) quenchers inhibited the TGF-beta-induced or vitreous-induced elevation of HO-1 mRNA but had no effect on vitreous-mediated induction of MT expression. Inhibitors of the mitogen-activated protein kinase (p38MAPK; SB203580) and Jun N-terminal kinase (JNK; SP600125) pathways inhibited vitreous-induction of HO-1. C-fos, a component of AP-1 transcription factor complexes, exhibited increased expression and activation in the presence of vitreous.

CONCLUSIONS

TGF-beta, a known component of vitreous, can account for some but not all of the regulation of the anti-oxidant, anti-inflammatory HO-1 gene in human RPE cells, but it does not participate in the vitreous-mediated upregulation of MTs. Both vitreous and TGF-beta signals increased HO-1 expression via ROS but the latter were not involved in vitreous-mediated MT expression. Increased p38, JNK, and c-fos activation may be implicated in vitreous modulation of HO-1.

Zinc coordination environments in proteins as redox sensors and signal transducers

Zinc/cysteine coordination environments in proteins are redox-active. Oxidation of the sulfur ligands mobilizes zinc, while reduction of the oxidized ligands enhances zinc binding, providing redox control over the availability of zinc ions. Some zinc proteins are redox sensors, in which zinc release is coupled to conformational changes that control varied functions such as enzymatic activity, binding interactions, and molecular chaperone activity. Whereas the released zinc ion in redox sensors has no known function, the redox signal is transduced to specific and sensitive zinc signals in redox transducers. Released zinc can bind to sites on other proteins and modulate signal transduction, generation of metabolic energy, mitochondrial function, and gene expression. The paradigm of such redox transducers is the zinc protein metallothionein, which, together with its apoprotein, thionein, functions at a central node in cellular signaling by redistributing cellular zinc, presiding over the availability of zinc, and interconverting redox and zinc signals. In this regard, the transduction of nitric oxide (NO) signals into zinc signals by metallothionein has received particular attention. It appears that redox-inert zinc has been chosen to control some aspects of cellular thiol/disulfide redox metabolism. Tight control of zinc is essential for redox homeostasis because both increases and decreases of cellular zinc elicit oxidative stress. Depending on its availability, zinc can be cytoprotective as a pro-antioxidant or cytotoxic as a pro-oxidant. Any condition with acute or chronic oxidative stress is expected to perturb zinc homeostasis.

Metallothioneins are multipurpose neuroprotectants during brain pathology

Metallothioneins (MTs) constitute a family of cysteine-rich metalloproteins involved in cytoprotection during pathology. In mammals there are four isoforms (MT-I - IV), of which MT-I and -II (MT-I + II) are the best characterized MT proteins in the brain. Accumulating studies have demonstrated MT-I + II as multipurpose factors important for host defense responses, immunoregulation, cell survival and brain repair. This review will focus on expression and roles of MT-I + II in the disordered brain. Initially, studies of genetically modified mice with MT-I + II deficiency or endogenous MT-I overexpression demonstrated the importance of MT-I + II for coping with brain pathology. In addition, exogenous MT-I or MT-II injected intraperitoneally is able to promote similar effects as those of endogenous MT-I + II, which indicates that MT-I + II have both extra- and intracellular actions. In injured brain, MT-I + II inhibit macrophages, T lymphocytes and their formation of interleukins, tumor necrosis factor-alpha, matrix metalloproteinases, and reactive oxygen species. In addition, MT-I + II enhance cell cycle progression, mitosis and cell survival, while neuronal apoptosis is inhibited. The precise mechanisms downstream of MT-I + II have not been fully established, but convincing data show that MT-I + II are essential for coping with neuropathology and for brain recovery. As MT-I and/or MT-II compounds are well tolerated, they may provide a potential therapy for a range of brain disorders.

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

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

Clofibric acid down-regulation of metallothionein IIA in HepG2 human hepatoma cells

Among the different hypotheses advanced to explain the peroxisome proliferator (PP)-induced hepatocarcinogenicity in rodents, one is based on the development of an oxidative stress due to an imbalance in the production of reactive oxygen species that leads to DNA damages and lipid peroxidation. On the other hand, human cells appear to be nonresponsive to PPs. As metallothionein proteins play an important antioxidant role, the aim of the present study was to investigate the expression of metallothionein IA (MTIA) and IIA (MTIIA) in HepG2 human hepatoma cells exposed to clofibric acid. When HepG2 cells were treated for 24 hr with 0.50 or 0.75 mM CA, a significant decrease was observed in MT protein-level determined by Western blotting and in the MTIIA mRNA content analyzed by RT-PCR and Northern blotting. No significant change was observed in the MTIA mRNA amount whatever the CA concentration and the duration of treatment. The decrease in MTIIA mRNA-level was not mediated via peroxisome proliferator-activated receptor alpha as attested by our data from gel mobility shift DNA binding assays, Dot blotting and cotransfection experiments with MTIIA promoter-driven luciferase reporter gene and PPARalpha expression vector. These results provide new insights about the pleiotropic effects of PPs on human cells.

Regulation of metallothionein gene expression

The rapid and robust induction of metallothioneins (MT)-I and II by a variety of inducers that include heavy toxic metals, reactive oxygen species, and different types of stress provide a useful system to study the molecular mechanisms of this unique induction process. The specific expression of MT-III in the brain and of MT-IV in the squamous epithelium of skin and tongue offers a unique opportunity to identify and characterize the tissue-specific factors involved in their expression. Studies using transgenic mice that overexpress MTs or MT null mice have revealed the role of MT in the protection of cells against numerous tissue-damaging agents such as reactive oxygen species. The primary physiological function of these proteins, however, remains an enigma. Considerable advances have been made in the identification of the cis-acting elements that are involved in the constitutive and induced expression of MT-I and MT-II. By contrast, only one key trans-activating factor, namely MTF-1, has been extensively characterized. Studies on the epigenetic silencing of MT-I and MT-II by promoter hypermethylation in some cancer cells have posed interesting questions concerning the functional relevance of MT gene silencing, the molecular mechanisms of MT suppression in these cells, particularly chromatin modifications, and the characteristics of the repressors.

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.

Characterization of cis-acting elements in the promoter of the mouse metallothionein-3 gene. Activation of gene expression during neuronal differentiation of P19 embryonal carcinoma cells

The metallothionein (MT)3 gene is expressed predominantly in the brain and the organs of the reproductive system, and fails to respond to metal ions in vivo. A CTG repeat was proposed to function as a potential repressor element in nonpermissive cells, and a sequence similar to the JC virus silencer element was found to function as a negative element in permissive primary astrocytes. The objective of this study was to characterize further the mechanisms governing cell-type specific MT-3 gene transcription. We searched for a suitable cell line expressing the MT-3 gene to be used for determination of MT-3 promoter tissue specificity, and showed that MT-3 expression is activated during neuroectodermal differentiation of P19 cells induced by retinoic acid to levels similar to those found in whole brain. Deletion of the CTG repeat or of the JC virus silencer did not promote MT-3 promoter activity in nonpermissive cells, or enhance expression in permissive cells. We identified MT-3 promoter sequences interacting with liver and brain nuclear proteins, as assayed by DNase I footprinting analyses and electrophoretic mobility shift assay, and assessed the role of these sequences in the regulation of MT-3 expression by cotransfection experiments. We generated stable transfectants in permissive C6 and nonpermissive NIH-3T3 cells, and analysed the methylation status of the MT-3 gene. These studies show that regulation of tissue-specific MT-3 gene expression does not appear to involve a repressor, and suggest that other mechanisms such as chromatin organization and epigenetic modifications could account for the absence of MT-3 gene transcription in nonpermissive cells.