Purification and characterization of a protein that binds to metal responsive elements of the human metallothionein IIA gene

[Transcription Factor MTF-1 Involved in the Cellular Response to Zinc]

Heavy metals, both toxic and essential, have long been an important research focus in life science. To investigate the intracellular actions of heavy metals at the molecular level, I have been exploring protein factors involved in induction of metallothionein (MT) genes by heavy metals that specifically bind to a metal responsive element (MRE) in the region upstream of the human MT-IIA gene. Purification of a zinc-dependent MRE-binding factor, and cloning of its cDNA identified a sequence identical to that of metal-responsive transcription factor-1 (MTF-1). MTF-1, which is characterized by six tandem repeats of the C2H2 type zinc finger motif, is indispensable for induction of MT gene expression by multiple types of heavy metal, but zinc is the only metal that can directly activate MTF-1 binding to the MRE, indicating that other heavy metal signals act through zinc as a second messenger. Functional analysis of various MTF-1 point mutants revealed several cysteine (Cys) residues critical for DNA binding and/or transactivation activity. Interestingly, six finger motifs seem to mediate several MTF-1 functions other than DNA binding. Immunohistochemical analyses of various mouse tissues revealed selective expression of MTF-1 in spermatocytes among the testicular cells, suggesting roles relevant to spermatogenesis. The zinc regulon, under the control of MTF-1, will likely provide good clues to aid in unraveling novel functions of intracellular zinc ions.

Metallothionein 2A gene polymorphisms in relation to diseases and trace element levels in humans

Human metallothioneins are a superfamily of low molecular weight intracellular proteins, whose synthesis can be induced by essential elements (primarily Zn and Cu), toxic elements and chemical agents, and stress-producing conditions. Of the four known isoforms in the human body MT2 is the most common. The expression of metallothioneins is encoded by a multigene family of linked genes and can be influenced by single nucleotide polymorphisms (SNPs) in these genes. To date, 24 SNPs in the MT2A gene have been identified with the incidence of about 1 % in various population groups, and three of them were shown to affect physiological and pathophysiological processes. This review summarises current knowledge about these three SNPs in the MT2A gene and their associations with element concentrations in the body of healthy and diseased persons. The most investigated SNP is rs28366003 (MT2A -5 A/G). Reports associate it with longevity, cancer (breast, prostate, laryngeal, and in paranasal sinuses), and chronic renal disease. The second most investigated SNP, rs10636 (MT2A +838G/C), is associated with breast cancer, cardiovascular disease, and type 2 diabetes. Both are also associated with several metal/metalloid concentrations in the organism. The third SNP, rs1610216 (MT2A -209A/G), has been studied for association with type 2 diabetes, cardiomyopathy, hyperglycaemia, and Zn concentrations. Metallothionein concentrations and MT2A polymorphisms have a potential to be used as biomarkers of metal exposure and clinical markers of a number of chronic diseases. This potential needs to be studied and verified in a large number of well-defined groups of participants (several hundreds and thousands) with a focus on particular physiological or pathological condition and taking into consideration other contributing factors, such as environmental exposure and individual genetic and epigenetic makeup.

Chromium (VI)-induced transformation is enhanced by Zn deficiency in BALB/c 3T3 cells

Hexavalent chromium [Cr(VI)] is a carcinogenic heavy metal that is reduced to intermediate oxidation states, such as Cr(V) and Cr(IV), in the process of forming stable Cr(III) forms; it is these intermediate forms that are thought to be responsible for much of the DNA damage and mutations that are induced by Cr(VI). Metallothionein (MT), a heavy metal-binding protein, is induced by zinc and other heavy metals and protects cells from the toxic effects of these metals by sequestering them. MT cannot bind Cr, but by scavenging reactive oxygen species through its cysteine residues, it may act as a protective factor against Cr(VI)-induced DNA lesions by reducing Cr(VI) directly to Cr(III), thereby avoiding the creation of the toxic intermediates. Here, we showed that Zn deficiency decreased MT expression in BALB/3T3 clone A31-1-1 cells and caused them to become highly susceptible to Cr(VI)-induced transformation. To obtain Zn-deficient cultures, cells were cultured in medium supplemented with 10% Chelex(®)-100 chelating resin-treated FBS. The increase in susceptibility to transformation was abolished by culturing the cells with supplemental Zn (50 µM). Previously, we reported that Cr(VI) inhibits MT transcription by preventing the zinc-dependent formation of a complex of metal response element-binding transcription factor-1 (MTF-1) and the co-activator p300. Our results suggest that the carcinogenicity of Cr(VI) is enhanced by MTF-1 dysfunction.

Molecular response of 4T1-induced mouse mammary tumours and healthy tissues to zinc treatment

Breast cancer patients negative for the nuclear oestrogen receptor α have a particularly poor prognosis. Therefore, the 4T1 cell line (considered as a triple-negative model) was chosen to induce malignancy in mice. The aim of the present study was to assess if zinc ions, provided in excess, may significantly modify the process of mammary oncogenesis. Zn(II) ions were chosen because of their documented antitumour effects. Zn(II) is also known to induce the expression of metallothioneins (MT) and glutathion (GSH). A total dose of zinc sulphate per one gram of mouse weight used in the experiment was 0.15 mg. We studied the expression of MT1, MT2, TP53 and MTF-1 genes and also examined the effect of the tumour on antioxidant capacity. Tumour-free mice had significantly higher expression levels of the studied genes (p<0.003). Significant differences were also revealed in the gene expression between the tissues (p<0.001). The highest expression levels were observed in the liver. As compared to brain, lung and liver, significantly lower concentrations of MT protein were found in the primary tumour; an inverse trend was observed in the concentration of Zinc(II). In non-tumour mice, the amount of hepatic hydrosulphuryl groups significantly increased by the exposure to Zn(II), but the animals with tumour induction showed no similar trend. The primary tumour size of zinc-treated animals was 20% smaller (p=0.002); however, no significant effect on metastasis progression due to the zinc treatment was discovered. In conclusion, Zn(II) itself may mute the growth of primary breast tumours especially at their early stages.

Metallothionein polymorphisms in pathological processes

Metallothioneins (MTs) are a class of metal-binding proteins characterized by a high cysteine content and low molecular weight. MTs play an important role in metal metabolism and protect cells against the toxic effects of radiation, alkylating agents and oxygen free radicals. The evidence that individual genetic characteristics of MTs play an important role in physiological and pathological processes associated with antioxidant defense and detoxification inspired targeted studies of genetic polymorphisms in a clinical context. In recent years, common MT polymorphisms were identified and associated with, particularly, western lifestyle diseases such as cancer, complications of atherosclerosis, and type 2 diabetes mellitus along with related complications. This review summarizes all evidence regarding MT polymorphisms of major human MTs (MT1, MT2, MT3 and MT4), their relation to pathological processes, and outlines specific applications of MTs as a set of genetic markers for certain pathologies.

Heavy metal response of the heat shock protein 70 gene is mediated by duplicated heat shock elements and heat shock factor 1

Heavy metals induce transcription of a number of mammalian genes, but in most cases the mechanism of induction has not been well characterized. The human heat shock protein 70 gene (hsp70) is activated by several heavy metals such as Cd and Zn, and the heat shock element (HSE) has been proposed to mediate metal response by previous studies. However, it was observed that the lack of further upstream sequences rendered the hsp70 promoter unresponsive to metals. A detailed deletion analysis of the promoter revealed that the distal HSE-like sequence (dHSE) is required for heavy metal response. A supershift assay showed that the heat shock factor 1 (HSF1) recognized dHSE, as well as the proximal HSE (pHSE). An hsp70 promoter variant with a dHSE mutation that blocks HSF1 binding completely lost metal response, demonstrating that the HSF1/dHSE interaction is crucial. Another promoter variant with a similar pHSE mutation partially lost metal response, indicating that both HSEs are required for full activity. Knockdown of HSF1 dramatically reduced the metal response of hsp70, demonstrating its essential role. Furthermore, a reporter gene regulated by the human hsp70 promoter, which lacked metal response in HSF1-null mouse cells, acquired the response upon over-expression of a recombinant human HSF1. These results demonstrate that the duplicated HSEs and HSF1 constitute the mechanism for the heavy metal response of hsp70, which is distinct from the known metal regulatory system for the metallothionein genes.

Dissection of Drosophila MTF-1 reveals a domain for differential target gene activation upon copper overload vs. copper starvation

Metal-responsive transcription factor-1 (MTF-1) is a zinc finger protein conserved from mammals to insects. It mediates protection against heavy metal load by activating the expression of metallothionein and other genes. In Drosophila, MTF-1 serves a dual function in that it not only helps to protect against heavy metal load but also induces the expression of Ctr1B, the gene for an intestinal copper importer, upon copper starvation. By dissecting Drosophila MTF-1 function, we have identified determinants for nuclear import and export, and characterized a phosphorylation site mutant (T127A) that differentially affects MTF-1 target genes. Further, by generating a series of fusion proteins with the heterologous DNA binding domain of Gal4 we identified a strong, constitutive activation domain in the central region of MTF-1 (aa 352-540). By contrast, an extended fusion protein that includes MTF-1's C-terminus (aa 352-791) is not active in standard conditions but induced by copper load. The paramount regulatory importance of the C-terminal part, that harbors a cysteine-rich "metallothionein-like" domain, was corroborated by different experiments. Transgenic flies expressing C-terminally truncated MTF-1 variants displayed high constitutive transcription of both, the genes for metallothioneins and the copper importer Ctr1B. The indiscriminate activation of these genes that are normally induced under opposite conditions of copper load and copper starvation manifested itself in a shortened lifespan, crippled wings, and female sterility.

Functional genomics in aquatic toxicology-do not forget the function

Toxicological responses of an organism are disturbances of function. This as a starting point we review and discuss issues that we consider important in applying functional genomics to aquatic toxicology. Functional genomics includes all the steps in gene expression pathway. Thus, ultimately the goal is to relate genome information to protein activity. In ecotoxicogenomics the toxicological responses must further be combined with responses to natural environmental changes. We focus on fish, but also consider commonly used invertebrates, mainly Daphnia. We first go through the toxicologically important features of genomes of aquatic animals, and then review the reference gene approach to quantify transcript amount. Thereafter we emphasize the need to relate the mRNA and protein levels, and protein activity of individual genes. Finally we discuss how functional genomic investigations may be important in resolving current environmental problems and give our views of valuable future research topics.

Chromium (VI) inhibits mouse metallothionein-I gene transcription by modifying the transcription potential of the co-activator p300

The production of the heavy metal-binding proteins, the metallothioneins (MTs), is induced by heavy metals such as Zn, Cd, and Hg. MTs maintain Zn homeostasis and attenuate heavy metal-induced cytotoxicity by sequestering these metals and lowering their intracellular concentrations. Previously, we had reported that Zn induced the formation of a co-activator complex containing metal response element-binding transcription factor-1 (MTF-1) and the histone acetyltransferase (HAT), p300, which plays an essential role in the activation of MT-1 transcription. In addition, we had shown that Cr(VI) inhibits Zn-induced MT-1 transcription by preventing the Zn-dependent formation of the MTF-1-p300 complex. In the current study, we have shown that the inhibition by Cr(VI) was partially overcome by the overexpression of p300 or MTF-1 in an MT-I promoter-driven luciferase reporter assay system and have used real-time RT-PCR to determine MT-I mRNA levels. It has been reported that Cr(VI) inhibits CYP1A1 transcription by crosslinking histone deacetylase (HDAC) to the promoter. The crosslink inhibits the recruitment of p300 to the MT-1 promoter and blocks HAT-dependent transactivation by p300. However, our results demonstrate that trichostatin A, an HDAC inhibitor, could not block the inhibitory effects of Cr(VI) on MT-1 transcription and that there were no significant differences in the in vitro inhibitory effects of Cr(VI), Cr(III), and Zn on p300 HAT activity. This suggests that the inhibitory effects of Cr(VI) on MT-I transcription may be due to its effects on the HAT-independent transactivation ability rather than the HAT-dependent, HDAC release-related transactivation ability of p300.

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.

Identification of a splice variant of the metal-responsive transcription factor MTF-1 in common carp

Transactivation of the expression of metallothionein genes involves the Metal-responsive Transcription Factor (MTF-1). We report here the identification of mtf-1.1a, the first known splice variant of mtf-1.1 mRNA, in common carp (Cyprinus carpio). The lack of a 103 nt internal segment results in a frame shift, causing the early termination of translation. mtf-1.1a mRNA encodes a protein consisting of the first 349 amino acids of MTF-1.1 plus an additional 64 amino acids, with no significant similarity to any of the proteins in the databases. The predicted MTF-1.1a protein carries the Zn-finger domain and the nuclear exporting and nuclear localization signals, and lacks the transcription activation domains. mtf-1.1a was detected in all tissues examined but the liver, with the highest level in the brain. Arsenic alters the levels of both mtf-1.1 and mtf-1.1a transcripts, in an isoform- and tissue-specific manner.

Chromium(VI) inhibits mouse metallothionein-I gene transcription by preventing the zinc-dependent formation of an MTF-1-p300 complex

Mouse MT-I (metallothionein-I) transcription is regulated by MTF-1 (metal-response-element-binding transcription factor-1) which is recruited to the promoter in response to zinc. Cr(VI) [chromium(VI)] pretreatment blocks zinc-activation of the endogenous MT-I gene and attenuates zinc-activation of MT-I-promoter-driven luciferase reporter genes in transient transfection assays. Chromatin immunoprecipitation assays revealed that Cr(VI) only modestly reduces recruitment of MTF-1 to the MT-I promoter in response to zinc, but drastically reduces the recruitment of RNA polymerase II. These results suggest that Cr(VI) inhibits the ability of MTF-1 to transactivate this gene in response to zinc. Zinc has recently been shown to induce the formation of a co-activator complex containing MTF-1 and the histone acetyltransferase p300 which plays an essential role in the activation of MT-I transcription. In the present study, co-immunoprecipitation assays demonstrated that Cr(VI) pretreatment blocks the zinc-induced formation of this co-activator complex. Thus Cr(VI) inhibits mouse MT-I gene expression in response to zinc by interfering with the ability of MTF-1 to form a co-activator complex containing p300 and recruiting RNA polymerase II to the promoter.

[Molecular mechanism involved in chromium(VI) toxicity]

Chromium exists in many different oxidation states in the environment, Cr(VI) and Cr(III) being the most stable forms. Chromium has been known for over 100 years to be a human carcinogen. The greatest risk of cancer from chromium exposure is associated with Cr(VI). Cr(VI) enters cells via the sulfate anion transporter system and is reduced to intermediate oxidation states, such as Cr(V) and Cr(IV), in the process of forming stable Cr(III) forms. It is known that Cr(VI) affects expression of various genes. Metal responsive element-binding transcription factor-1 (MTF-1) is involved in sensing heavy metal load and the induced transcription of several protective genes, including metallothionein (MT)-I, MT-II, zinc transporter-1, and gamma-glutamylcysteine synthetase. Cr(VI) inhibits zinc-induced MT transcription via modifying transactivation potential of MTF-1. However, the molecular mechanism for the Cr(VI)-mediated inhibition of MTF-1 has not been fully elucidated. In this review, I briefly summarize the previous studies and discuss the current status of research on Cr(VI) toxicity and Cr(VI)-mediated inhibition against transcription.

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

The regulation of divalent zinc has been observed in a wide range of organisms. Since this metal is an essential nutrient, but also toxic in excess, zinc homeostasis is crucial for normal cellular functioning. The metal-responsive-element-binding transcription factor-1 (MTF-1) is a key regulator of zinc in higher eukaryotes ranging from insects to mammals. MTF-1 controls the expression of metallothioneins (MTs) and a number of other genes directly involved in the intracellular sequestration and transport of zinc. Although the diverse functions of MTF-1 extend well beyond zinc homeostasis to include stress-responses to heavy metal toxicity, oxidative stress, and selected chemical agents, in this review we focus on the recent advances in understanding the mechanisms whereby MTF-1 regulates MT gene expression to protect the cell from fluctuations in environmental zinc. Particular emphasis is devoted to recent studies involving the Cys2His2 zinc finger DNA-binding domain of MTF-1, which is an important contributor to the zinc-sensing and metal-dependent transcriptional activation functions of this protein.

Analysis of human proteins that have an affinity to heavy metals by metal-chelating column chromatography

To clarify the molecular basis of toxicities of industrial chemicals, it is demanded to develop appropriate methods whereby their cellular target molecules can be directly identified. In the present study, we focused on target proteins of heavy metals and established the method to detect them using a combination of metal-chelating column chromatography and a subsequent analysis by electrophoresis. Protein samples prepared from HeLa cells were applied to the Zn- or Cd-chelating column, and the bound proteins were analyzed by SDS-polyacrylamide gel electrophoresis followed by either silver staining, or fluorography when using radiolabel protein samples. Among several protein species trapped in the columns, a 36-kDa protein apparently has an affinity to both Zn and Cd, indicating the possibility that Cd can exchange essential Zn on this protein. These results suggest that the established method is useful for the target protein screening and further analyses of separated proteins.

Role of metal-responsive transcription factor-1 (MTF-1) in EGF-dependent DNA synthesis in primary hepatocytes

Metal-responsive transcription factor-1 (MTF-1), which is involved in sensing heavy metal load, induces the transcription of several protective genes. The mouse Mtf-1 gene is essential, and Mtf-1(-/-) embryos die from liver degeneration. We showed that DNA synthesis induced in hepatocytes by epidermal growth factor (EGF) was delayed by inhibition of MTF-1. To inhibit MTF-1 activity, MTFDeltaC, a C-terminal deletion mutant of MTF-1, was expressed by infection with the virus Ad5MTFDeltaC. Lactate dehydrogenase (LDH) release and/or caspase-3/7 activation was not observed under our experimental conditions. The inhibitory effect of MTFDeltaC on EGF-dependent DNA synthesis in hepatocytes was not eliminated by zinc addition. EGF-dependent extracellular signal-related kinase (ERK) phosphorylation, an essential reaction for EGF-dependent DNA synthesis, was decreased in MTF-1-inhibited hepatocytes. Moreover, decrease of ERK phosphorylation was observed by using siRNA in MTF-1-downregulated hepatocytes. These results indicate that MTF-1 is particularly important for proper hepatocyte proliferation. This is the first report to suggest the function of MTF-1 in the ERK pathway.

Potential effect on cellular response to cadmium of a single-nucleotide A --> G polymorphism in the promoter of the human gene for metallothionein IIA

Most people generally ingest cadmium in their food. Cadmium that has accumulated in tissues induces the synthesis of metallothioneins (MTs) which are metal-binding proteins that bind tightly to cadmium to inhibit its renal toxicity. Individuals whose ability to induce the synthesis of MTs is low seem likely to be particularly susceptible to the toxic effects of cadmium. In this study, we analyzed the polymorphism of the promoter region of the gene for MT-IIA, the major species of MT in humans, in 119 adult Japanese subjects. We found that about 18% of the subjects had an A --> G single-nucleotide polymorphism in the core region of the promoter near the TATA box. A reporter-gene assay using HEK293 cells showed that replacement of A by G at position -5 reduced the efficiency of the cadmium-induced transcription of the gene for MT-IIA. This single-nucleotide polymorphism inhibited the binding of nuclear proteins to the core promoter region of the gene for MT-IIA. When the promoter region upstream of the TATA box was replaced by a sequence that contained three dioxin-responsive elements, the reporter-gene assay demonstrated that the A --> G single-nucleotide polymorphism resulted in a marked reduction in the rate of dioxin-induced transcription. These results suggest that the A --> G single-nucleotide polymorphism reduces the efficiency of those aspects of the transcription of the gene for MT-IIA that are controlled by general transcription factors.

Cross talk of heat shock and heavy metal regulatory pathways

Heavy metals induce transcription of human genes including those coding for metallothionein and heat shock protein 70 (HSP70). It has been suggested that these processes are mediated by metal-activated transcription factors, MTF-1 and HSF1, respectively, and are independent of each other. We raised an antibody against human MTF-1 which efficiently supershifts the protein-DNA complex formed by MTF-1 and its cognate binding sequence, MRE. We discovered that this antibody could also supershift complexes formed by HSF1 and its recognition sequence HSE, which suggested the involvement of MTF-1 in these complexes. This supershift was observed for HSF1/HSE complexes induced by Zn, Cd, Ag, and heat shock. Furthermore, overexpression of MTF-1 in HeLa cells markedly reduced metal-induced transcription from the hsp70-1 gene promoter which depends on HSF1. These data indicate that MTF-1 represses HSF1-mediated transcription probably through a direct protein-protein interaction, suggesting a cross talk of two lines of stress-responsive regulatory pathways.

The ubiquitin-specific protease USP10 modulates androgen receptor function

The role of the ubiquitin/proteasome system in degrading nuclear hormone receptors and regulating their transcriptional function has emerged in the last few years. We identified the ubiquitin-specific protease USP10 as part of DNA-bound androgen receptor (AR) complexes purified from nuclear extracts of PC-3 cells stably expressing the AR. The interaction between USP10 and the AR was confirmed by GST pull-down assays. Fluorescence microscopy documented that USP10 was localised in the nucleus and the cytoplasm. Cell-based transactivation assays in PC-3/AR cells revealed that overexpression of wild-type USP10, but not of an enzymatically inactive form, stimulated AR activity mediated by reporter constructs harbouring selective androgen response elements (AREs), non-selective steroid response elements (SREs) or the mouse mammary tumour virus (MMTV) promoter. Conversely, USP10 expression knock-down by siRNAs impaired the MMTV response to androgen. In summary, the data indicate that USP10 is a new cofactor that binds to the AR and stimulates the androgen response of target promoters. This finding underlines the role of the ubiquitin/proteasome system in modulating the AR function.

The six zinc fingers of metal-responsive element binding transcription factor-1 form stable and quasi-ordered structures with relatively small differences in zinc affinities

Six Cys(2)His(2) zinc fingers (F1-6) comprise the DNA binding domain of metal-responsive element binding transcription factor-1 (MTF-1). F1-6 is necessary for basal and zinc-induced expression of metallothionein genes. Analysis of NMR structural and dynamic data for an F1-6 protein construct demonstrates that each zinc finger adopts a stable betabetaalpha fold in the presence of stoichiometric Zn(II), provided that all cysteine ligands are in a reduced state. Parallel studies of protein constructs spanning the four N-terminal core DNA binding fingers (F1-4) and two C-terminal low DNA affinity fingers (F5-6) reveal similar stable zinc finger structures. In both the F1-6 and F5-6 proteins, the finger 5 cysteines were found to readily oxidize at neutral pH. Detailed spectral density and hydrodynamic analysis of (15)N relaxation data revealed quasi-ordered anisotropic rotational diffusion properties of the six F1-6 zinc fingers that could influence MTF-1 DNA binding function. A more general effect on the rotational diffusion properties of Cys(2)His(2) zinc fingers was also uncovered that is dependent upon the position of each finger within multifinger domains. Analysis of NMR (1)H-(15)N-heteronuclear single quantum coherence spectral peak intensities measured as a function of added Zn(II) in conjunction with Zn(II) binding modeling studies indicated that the Zn(II) affinities of all MTF-1 zinc fingers are within approximately 10-50-fold. These analyses further suggested that metal sensing by MTF-1 in eukaryotic cells involves multiple zinc fingers and occurs over a 100-fold or less range of accessible Zn(II) concentration.

Aly/ REF, a factor for mRNA transport, activates RH gene promoter function

The rhesus (Rh) blood group antigens are of considerable importance in transfusion medicine as well as in newborn or autoimmune hemolytic diseases due to their high antigenicity. We identified a major DNaseI hypersensitive site at the 5' flanking regions of both RHD and RHCE exon 1. A 34 bp fragment located at -191 to -158 from a translation start position, and containing the TCCCCTCCC sequence, was involved in enhancing promoter activity, which was assessed by luciferase reporter gene assay. A biotin-labelled 34 bp probe isolated an mRNA transporter protein, Aly/REF. The specific binding of Aly/REF to RH promoter in erythroid was confirmed by chromatin immunoprecipitation assay. The silencing of Aly/REF by siRNA reduced not only the RH promoter activity of the reporter gene but also transcription from the native genome. These facts provide second proof of Aly/REF as a transcription coactivator, initially identified as a coactivator for the TCRalpha enhancer function. Aly/REF might be a novel transcription cofactor for erythroid-specific genes.

C-terminal deletion mutant of MRE-binding transcription factor-1 inhibits MRE-driven gene expression

Heavy metal-induced transcriptional activation of the genes coding for metallothionein (MT) is mediated by a cis-acting DNA element, the metal-responsive element (MRE). MRE-binding transcription factor-1 (MTF-1) is a highly conserved heavy metal-induced transcriptional activator. MTF-1 also activates transcription in response to oxidative stress and regulates the expression of several cytoprotective factor genes, including MT, gamma-glutamylcysteine synthetase, and Cu/Zn-superoxide dismutase. It is thus thought that MTF-1 plays a role in cellular stress response. The physiological role of MTF-1 remains unclear because of the lack of MTF-1-specific activators and/or inhibitors. To obtain an MTF-1-specific inhibitor, we constructed an MTFDeltaC (amino acids 1-317), a C-terminal deletion mutant of MTF-1. MTFDeltaC could bind MRE and competed with MTF-1 for MTF-MRE complex formation. Transient expression of MTFDeltaC in HepG2 cells reduced MRE-driven gene expression, demonstrating that MTFDeltaC is dominant to MTF-1. HepG2 cells stably expressing MTFDeltaC showed increased susceptibility to the cytotoxic effects of tert-butyl hydroperoxide (tBH). Furthermore, we constructed Ad5MTFDeltaC, a recombinant adenovirus that expresses MTFDeltaC. Infection with the virus induced MTFDeltaC expression and increased susceptibility to the cytotoxic effects of tBH. These results indicate that MTF-1 participates in controlling the cellular redox state.

Metal ion affinities of the zinc finger domains of the metal responsive element-binding transcription factor-1 (MTF1)

Metal response element (MRE) binding transcription factor-1 (MTF1) is a six Cys(2)His(2) zinc finger-containing transcription factor required for basal and zinc-induced transcription of metallothionein genes. The cobalt(II) and zinc(II) affinities of a protein fragment comprising the six zinc finger domains have been examined to reveal apparent dissociation constants (for the six domains collectively) of 0.5 +/- 0.2 microM for cobalt(II) and 31 +/- 14 pM for zinc(II). Two approaches have been used to determine the metal ion affinities of the individual domains. First, the six domains have been examined as single domain peptides revealing dissociation constants ranging from 0.3 to 1.7 microM for cobalt(II). The domains fall into two sets with peptides corresponding to domains 2, 3, and 4 showing relatively high affinity (K(d)(Co(II)) 0.3-0.5 microM) and peptides corresponding to domains 1, 5, and 6 showing lower affinity (K(d)(Co(II)) 1.6-1.7 microM). Second, we examined the affinity of each domain in the context of the six zinc finger domain protein by individually mutating one metal-binding His residue to Cys to allow independent monitoring of the cobalt(II) occupancy of each site. The affinity of each domain was higher in this context than as a single domain peptide with affinities (corrected for the effect of the mutation) ranging from 0.02 to 0.5 microM. The increase in affinity for the individual domains ranged from factors of 1.1 to 20. The order of affinities (from higher to lowest) was observed to be 4 > 2 approximately 5 > 6 approximately 3 approximately 1. These results reveal that none of the Cys(2)His(2) zinc finger domains of MTF1 have dramatically low metal ion affinities, certainly none low enough to respond to changes in free zinc ion concentrations in the micromolar range. Nonetheless, the metal ion affinities of some domains do differ by a factor of 25 with domains at both the amino- and carboxyl-termini showing lower intrinsic affinities for metal ions than the central domains.

A novel cysteine cluster in human metal-responsive transcription factor 1 is required for heavy metal-induced transcriptional activation in vivo

Metal-responsive transcription factor 1 (MTF-1) specifically binds to metal response elements (MREs) associated with a number of metal- and stress-responsive genes. Human MTF-1 contains a cysteine-rich cluster, -632Cys-Gln-Cys-Gln-Cys-Ala-Cys638-, conserved from pufferfish to humans far removed from the MRE-binding zinc finger domain and just C-terminal to a previously mapped serine/threonine-rich transcriptional activation domain. MTF-1 proteins containing two Cys-->Ala substitutions (C632A/C634A) or a deletion in this region altogether (Delta(632-644)) are significantly impaired in their ability to induce Zn(II)- and Cd(II)-responsive transcription of a MRE-linked reporter gene in transiently transfected mouse dko7 (MTF-1-/-) cells in culture under moderate metal stress but retain the ability to drive basal levels of transcription in a MRE-dependent manner in vivo and in vitro. In addition, the mutated proteins respond to induction by Zn(II) or Cd(II) with nuclear translocation and MRE binding activities comparable with wild-type MTF-1. Attempts to rescue the Delta(632-644) deletion mutant phenotype by inserting similar Cys-rich sequences from Drosophila MTF-1 were unsuccessful, suggesting that the structure of this motif within intact human MTF-1, rather than the simple presence of multiple closely spaced Cys residues, is required for function. This cysteine cluster therefore functions at a step subsequent to nuclear translocation and MRE-binding DNA to naked promoter-containing DNA and appears to be specifically required for MTF-1 to activate transcription in the presence of inducing heavy metal ions.

Association of transcription factor YY1 with the high molecular weight Notch complex suppresses the transactivation activity of Notch

Notch receptors are evolutionarily conserved from Drosophila to human and play important roles in cell fate decisions. After ligand binding, Notch receptors are cleaved to release their intracellular domains. The intracellular domains, the activated form of Notch receptors, are then translocated into the nucleus where they interact with other transcriptional machinery to regulate the expression of cellular genes. To dissect the molecular mechanisms of Notch signaling, the cellular targets that interact with Notch1 receptor intracellular domain (N1IC) were screened. In this study, we found that endogenous transcription factor Ying Yang 1 (YY1) was associated with exogenous N1IC in human K562 erythroleukemic cells. The ankyrin (ANK) domain of N1IC and zinc finger domains of YY1 were essential for the association of N1IC and YY1 according to the pull-down assay of glutathione S-transferase fusion proteins. Furthermore, both YY1 and N1IC were present in a large complex of the nucleus to suppress the luciferase reporter activity transactivated by Notch signaling. The transcription factor YY1 indirectly regulated the transcriptional activity of the wild-type CBF1-response elements via the direct interaction of N1IC and CBF1. We also demonstrated the association between endogenous N1IC and intrinsic YY1 in human acute T-cell lymphoblastic leukemia cell lines. Taken together, these results indicate that transcription factor YY1 may modulate Notch signaling via association with the high molecular weight Notch complex.

Nuclear proteins that bind to metal response element a (MREa) in the Wilson disease gene promoter are Ku autoantigens and the Ku-80 subunit is necessary for basal transcription of the WD gene

Wilson disease (WD), an inherited disorder affecting copper metabolism, is characterized by hepatic cirrhosis and neuronal degeneration, which result from toxic levels of copper that accumulate in the liver and brain, respectively. We reported previously that the approximately 1.3-kb promoter of the WD gene contains four metal response elements (MREs). Among the four MREs, MREa plays the most important role in the transcriptional activation of the WD promoter. Electrophoretic mobility shift assays (EMSAs) using synthetic MREa and an oligonucleotide containing the binding site for transcription factor Sp1 revealed the presence of nuclear factors that bind specifically to MREa. Two MREa-binding proteins of 70 and 82 kDa were purified using avidin-biotin affinity chromatography. Amino acid sequences of peptides from each protein were found to be highly homologous to the Ku proteins. Immunoblot analysis and EMSAs showed that the MREa-binding proteins are immunologically related to the Ku proteins. To study further the functional significance of these Ku-related proteins in transcriptional regulation of the WD gene, we performed RNA interference (RNAi) assays using a Ku-80 inverted-repeat gene to inhibit expression of the Ku-80 gene in vivo. Results of the RNAi assays showed that expression of the Ku-80 protein was suppressed in transfected cells, which in turn led to the suppression of the WD gene. In addition, a truncated Ku-80 (DeltaKu-80) mutant inhibited WD promoter activity in HepG2 cells in a dominant-negative manner. We also found that WD promoter activity was decreased in Xrs5 cells, which, unlike the CHO-K1 cells, are defective in the Ku-80 protein. When Ku-80 cDNA was transfected into Xrs5 and CHO cells, WD promoter activity was recovered only in Xrs5 cells. Taken together, our findings suggest that the Ku-80 subunit is required for constitutive expression of the WD gene.

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.

A new analytical scale DNA affinity binding assay for analyses of specific protein-DNA interactions

We describe a rapid analytical assay for identification of proteins binding to specific DNA sequences. The DAPSTER assay (DNA affinity preincubation specificity test of recognition assay) is a DNA affinity chromatography-based microassay that can discriminate between specific and nonspecific protein-DNA interactions. The assay is sensitive and can detect protein-DNA interactions and larger multicomponent complexes that can be missed by other analytical methods. Here we describe in detail the optimization and utilization of the DAPSTER assay to isolate AP-1 complexes and associated proteins in multimeric complexes bound to the AP-1 DNA element.

Negative regulatory role of Sp1 in metal responsive element-mediated transcriptional activation

Transcription of mammalian metallothionein (MT) genes is activated by heavy metals via multiple copies of a cis-acting DNA element, the metal-responsive element (MRE). Our previous studies have shown that certain MREs of the human MT-IIA gene (MREb, MREc, MREd, and MREf) are less active than the others (MREa, MREe, and MREg). Gel shift analysis of HeLa cell nuclear proteins revealed that whereas the active MREs strongly bind the transcription factor MTF-1 essential for metal regulation, the less active MREs bind another distinct protein, MREb-BF. This protein recognizes the GC-rich region of MREb rather than the MRE core required for MTF-1 binding. All the MREs recognized by MREb-BF contain the CGCCC and/or CACCC motif, suggesting that the MREb-BF.MRE complex contains Sp1 or related proteins. Supershift analysis using antibodies against Sp1 family proteins as well as gel shift analysis using the recombinant Sp1 demonstrated that Sp1 represents the majority of MREb-BF activity. An MREb mutant with reduced affinity to Sp1 mediated zinc-inducible transcription much more actively than the wild-type MREb. Furthermore, when placed in the native promoter, this mutant MREb raised the overall promoter activity. These results strongly suggest that Sp1 acts as a negative regulator of transcription mediated by specific MREs.

Functional heterogeneity in the zinc fingers of metalloregulatory protein metal response element-binding transcription factor-1

Metal response element-binding transcription factor-1 (MTF-1) is a unique, zinc-inducible transcription factor that binds to metal response elements in the metallothionein promoter and activates transcription in response to metals and oxidative stress. MTF-1 contains six zinc fingers of the Cys(2)-His(2) type. It was previously shown that MTF-1 is reversibly activated to bind DNA in response to changes in zinc status, unlike other zinc finger transcription factors, which do not appear to be reversibly activated by zinc in the cellular environment. Here we show that zinc fingers 2-4 constitute the core DNA-binding domain, whereas fingers 5 and 6 appear to be unnecessary for DNA binding in vitro. Deletion of finger 1 resulted in a protein that bound DNA constitutively in vitro. Furthermore, transfer of MTF-1 finger 1 to a position immediately preceding the three zinc fingers of Sp1 resulted in a chimeric protein that required exogenous zinc to activate DNA binding in vitro, unlike native Sp1, which binds DNA constitutively. Transient transfection experiments demonstrated that intact MTF-1 activated a reporter 2.5-4-fold above basal levels after metal treatment in mouse MTF-1 knockout cells, Drosophila SL2 cells, and yeast. However, the metal response was lost in all three systems when finger 1 was deleted, but was unaffected by deletion of fingers 5 and 6. These data suggest that finger 1 of MTF-1 constitutes a unique metal-sensing domain that, in cooperation with the transactivation domains, produces a zinc-sensing metalloregulatory transcription factor.

Novel responses of ZRF, a variant of human MTF-1, to in vivo treatment with heavy metals

Heavy metal-dependent transcriptional activation of metallothionein (MT) genes is mediated by multiple enhancer sequences, metal responsive element (MRE), located in the upstream region of the genes. Previously, we have reported purification of a zinc-dependent MRE-binding protein, zinc regulatory factor (ZRF), from HeLa cells, and have pointed to the close relationship between ZRF and mouse MRE-binding transcription factor-1 (MTF-1) according to the analysis of partial amino acid sequences. By means of cDNA cloning and the product analyses, we show that ZRF is a variant of human MTF-1 (hMTF-1), which carries a single amino acid exchange in the zinc finger domain. Accordingly, ZRF is renamed hMTF-1b. Expression of hMTF-1b in HeLa cells is constitutive at both mRNA and protein levels, and is unaffected by treatment with cadmium (Cd). On the other hand, when cells were fractionated into nuclear extract and cytosol, a large part of the hMTF-1b was recovered in the cytosol fraction. A significant increase in the amount of nuclear hMTF-1b occurs when cells are treated with various heavy metals, including Cd, Zn, Cu and Ag, which is associated with concomitant decrease in the amount recovered in the cytosol fraction. Since immunocytochemical analysis revealed that intracellular distribution of hMTF-1b is restricted to the nucleus irrespective of the heavy metal treatment, such an increment in the nuclear extracts apparently results from promotion of nuclear retention of hMTF-1b by the heavy metal treatment. Analysis by native gel electrophoresis shows that the mobility of hMTF-1b significantly changes in association with Cd treatment, raising the possibility that a conformational change of hMTF-1b occurs in response to treatment with heavy metals in vivo.

Characterization of the mouse gene for the heavy metal-responsive transcription factor MTF-1

MTF-1 is a zinc finger transcription factor that mediates the cellular response to heavy metal stress; its targeted disruption in the mouse leads to liver decay and embryonic lethality at day E14. Recently, we have sequenced the entire MTF-1 gene in the compact genome of the pufferfish Fugu rubripes. Here we have defined the promoter sequences of human and mouse MTF-1 and the genomic structure of the mouse MTF-1 locus. The transcription unit of MTF-1 spans 42 kb (compared to 8.5 kb in Fugu) and is located downstream of the gene for a phosphatase (INPP5P) in mouse, human, and fish. In all of these species, the MTF promoter region has the features of a CpG island. In both mouse and human, the 5' untranslated region harbors conserved short reading frames of unknown function. RNA mapping experiments revealed that in these two species, MTF-1 mRNA is transcribed from a cluster of multiple initiation sites from a TATA-less promoter without metal-responsive elements. Transcription from endogenous and transfected MTF-1 promoters was not affected by heavy metal load or other stressors, in support of the notion that MTF-1 activity is regulated at the posttranscriptional level. Tissue Northern blots normalized for poly A+ RNA indicate that MTF-1 is expressed at similar levels in all tissues, except in the testes, that contain more than 10-fold higher mRNA levels.

MRE-Binding transcription factor-1: weak zinc-binding finger domains 5 and 6 modulate the structure, affinity, and specificity of the metal-response element complex

MRE-binding transcription factor-1 (MTF-1) contains six Cys(2)-His(2) zinc finger sequences, and it has been suggested that the zinc finger domain itself may function as a zinc sensor in zinc-activated expression of metallothioneins (MTs). Previous work has shown that a subset ( approximately 3-4) of the zinc fingers in MTF-zf play a structural role in folding and high-affinity metal-response element (MREd) binding, while one or more other fingers have properties consistent with a metalloregulatory role (weak zinc binding affinity in the absence of DNA). We show here that zinc fingers 5 and 6 correspond to the weak zinc-binding fingers in MTF-zf. Limited trypsinolysis of a Zn(6)-MTF-zf:MREd complex gives rise to a highly protease-resistant core fragment corresponding to amino acids 137-260 or N-terminal zinc fingers 1-4 of MTF-zf. Characterization of a collection of broken-finger (His --> Asn) and missing-finger mutants of MTF-zf reveals that deletion of zinc fingers 5 and 6 to create MTF-zf14 attenuates MREd binding affinity ( approximately 20-fold), while deletion of fingers 4-6 (MTF-zf13) results in a further 20-fold reduction of binding affinity with a nearly complete loss of specificity. Circular dichroism studies reveal that the binding of MTF-zf to the MREd induces a dramatic alteration of the structure of the MREd from a B-form to a double-helical conformation with A-like features. Formation of stoichiometric complexes with MTF-zf14, H279N (Deltazf5) MTF-zf, and MTF-zf13 induces comparatively less A-like structure. Steady-state fluorescence resonance energy transfer (FRET) spectroscopy has been used to globally define the orientation of the multifinger MTF-zf on the MREd. These experiments suggest that fingers 1-4 are oriented on the highly conserved TGCRCnC side of the MREd with fingers 5-6 bound at or near the gGCCc sequence. These findings are consistent with a model in which the N-terminal zinc fingers in MTF-zf are required for high affinity and specific binding to the consensus TGCRCnC core in a way which is subjected to structural and allosteric modulation by the weak zinc-binding C-terminal zinc fingers.

Differential metal response and regulation of human heavy metal-inducible genes

A number of heavy metal-inducible genes have been reported, but their ranges of response to various metal species are not well known. It is also unclear if these genes are regulated through common mechanisms. To answer these questions, we compared induction kinetics of human metal-inducible genes including the MT-IIA (coding for a metallothionein isoform), hsp70 (coding for the 70-kDa heat-shock protein), and c-fos genes in HeLa cells exposed to Zn, Cd, Ag, Hg, Cu(II), Co, or Ni ions. Transcripts from these three genes were increased after exposure to wide ranges of metals, but each gene was unique in its induction kinetics. Generally, induction was observed at lower metal concentrations in the order of MT-IIA, hsp70, and c-fos. These genes also showed differential responses in time course: more rapid induction was observed in the order of c-fos, hsp70, and MT-IIA after exposure to Zn or Cd. Since the metal-responsive element (MRE) and heat shock element (HSE) of the MT-IIA and hsp70 genes, respectively, are thought to be the cis-acting DNA elements that mediate metal response, we compared the properties of proteins that specifically bind to these elements. MRE-binding activity was detected only in the extract from cells exposed to Zn. By contrast, HSE-binding activity was detected in extracts from cells treated with Zn, Cd, Ag, and Cu. The former was also activated by Zn in vitro, while the latter was not. Each of these DNA-binding activities showed no affinity to the recognition sequence of the other. These results demonstrate that the human metal-inducible genes have broad ranges of response to a variety of heavy metals, but suggest that they are probably regulated through independent mechanisms.

The heavy metal-responsive transcription factor-1 (MTF-1) is not required for neural differentiation

The zinc finger transcription factor MTF-1 is essential for proper response to heavy metal load and other stress conditions in vertebrates, and also contributes to the maintenance of the cellular redox state. Target genes include metallothioneins (MT-I and MT-II) and gamma-glutamylcysteine synthetase (gamma-GCS), an enzyme involved in glutathione biosynthesis. Although MTF-1 is expressed ubiquitously, the primary defect in null mutant mice is hepatocyte necrosis, which results in embryonic lethality around day E14 and prevents the analysis of delayed effects on other organs. To assess the impact of MTF-1 deficiency on the function of the mature central nervous system, we employed the neural grafting strategy. Neuroectodermal brain tissue obtained from transgenic mouse embryos at gestational day 12.5 was transplanted into the caudoputamen of adult wild-type mice. 33 days later, grafts derived from MTF-1 deficient mice consisted of fully differentiated neuroectodermal tissue and showed no differences to heterozygous control grafts. This indicates that MTF-1 is dispensable for the development and differentiation of the nervous system. Such transplants devoid of MTF-1 may provide a useful tool for the further investigation of the effect of cell stress, including oxidative stress.

Regulation of gene expression by reactive oxygen

Reactive oxygen intermediates are produced in all aerobic organisms during respiration and exist in the cell in a balance with biochemical antioxidants. Excess reactive oxygen resulting from exposure to environmental oxidants, toxicants, and heavy metals perturbs cellular redox balance and disrupts normal biological functions. The resulting imbalance may be detrimental to the organism and contribute to the pathogenesis of disease and aging. To counteract the oxidant effects and to restore a state of redox balance, cells must reset critical homeostatic parameters. Changes associated with oxidative damage and with restoration of cellular homeostasis often lead to activation or silencing of genes encoding regulatory transcription factors, antioxidant defense enzymes, and structural proteins. In this review, we examine the sources and generation of free radicals and oxidative stress in biological systems and the mechanisms used by reactive oxygen to modulate signal transduction cascades and redirect gene expression.

Transcriptional activity and regulatory protein binding of metal-responsive elements of the human metallothionein-IIA gene

Multiple copies of a cis-acting DNA element, metal-responsive element (MRE) are required for heavy metal-induced transcriptional activation of mammalian metallothionein genes. To approach the regulatory mechanism mediated by these multiple elements, we studied the properties of seven MREs located upstream of the human metallothionein-IIA (hMT-IIA) gene in detail. Transfection assays of reporter gene constructs each containing one of these MREs as the promoter element revealed that only four MREs can mediate zinc response. With respect to the distribution of active MREs over the promoter region, the hMT-IIA gene is largely different from the mouse metallothionein-I gene, suggesting that MRE arrangement is not an important factor for metal regulation. Experiments using various model promoters showed that multiple MRE copies act highly synergistically, supporting the biological significance of the multiplicity. Only the four active MREs efficiently bound the purified transcription factor human MTF-1, and MRE mutants defective in binding this protein lost the ability to support zinc-induced reporter gene expression, strongly suggesting that the direct interaction between human MTF-1 and a set of the selected MREs plays the major role in heavy metal regulation. In protein/DNA binding reactions in vitro, the purified human MTF-1 was activated by zinc but not by other metallothionein-inducing heavy metals, supporting the idea that zinc is the direct modulator of human MTF-1.

Characterization of the transcription factor MTF-1 from the Japanese pufferfish (Fugu rubripes) reveals evolutionary conservation of heavy metal stress response

The pufferfish Fugu rubripes was recently introduced as a new model organism for genomic studies, since it contains a full set of vertebrate genes but only 13% as much DNA as a mammal. Fugu genes tend to be smaller and densely spaced due to shortening of introns and intergenic spacers. We isolated the Fugu gene for the metal-responsive transcription factor MTF-1 (MTF1), a mediator of heavy metal regulation and oxidative stress response previously characterized in mammals. In addition, most of the cDNA sequence was also determined. The 780 amino acid MTF-1 protein of Fugu is very similar to that of mouse and human, with 90% amino acid identity in the DNA binding zinc finger domain and 57% overall identity. Expression of the pufferfish cDNA in mammalian cells shows that Fugu MTF-1 has the same DNA binding specificity as its mammalian counterpart and also induces transcription in response to zinc and cadmium. The protein-coding part of the Fugu MTF-1 gene spans 6.4 kb and consists of 11 exons. Upstream region and first exon constitute a CpG island. The distance between stop codon and polyadenylation motifs is >2 kb, suggesting a very long 3' untranslated mRNA region, followed by another CpG island which may represent the promoter of the next gene downstream. Part of the MTF-1 genomic structure was also determined in the mouse, and some striking similarities were found: for example, the upstream adjacent gene in both species is INPP5P, encoding a phosphatase. The mouse MTF-1 promoter is also embedded in a CpG island, which however shares no sequence similarity to the one of Fugu. The Fugu CpG island is shorter than the one of the mouse and has no elevated [G+C] content; these and other data indicate that CpG islands of fish may represent a primordial stage of CpG island evolution.

Molecular analyses of metallothionein gene regulation

Metallothionein (MT) genes encode small proteins that chelate metal ions through metal-thiolate bonds with cysteine residues. MTs may have a role in cellular zinc homeostasis and metal detoxification. Congruent with these putative functions, MT gene transcription is induced by metals via multiple metal-responsive elements (MREs) present in the MT gene 5'-regulatory regions. This chapter mainly is focused on studies of the functional and physical interactions of MRE binding proteins with MT promoters from human and rainbow trout. In addition to mediating zinc induction, MREs may make important contributions to nonmetal induced promoter activity. In part, differential basal activity of MREs appears to be determined by sequence and position in the promoter. During zinc induction, increased functional MRE activity correlates with increased activity of mammalian MRE binding proteins by zinc treatment in vivo or in vitro, as detected by electrophoretic mobility shift assays. Interestingly, the addition of cadmium in vitro or in vivo has no detectable effect even though it strongly induces MT gene expression in the same time course. This raises questions about how the effects of cadmium are mediated by MREs. The molecular masses and MRE complex migration of the zinc-responsive factors we detect are consistent with mouse and human metal-responsive transcription factor (MTF) and expression of the MTF cDNAs increases co-transfected MT promoter activity in both mammalian and trout cell lines underlining the conservation of MRE binding factor function among diverse species.

Metallothionein in physiological and physiopathological processes

The multipurpose nature of MT that we have presented in this review has drawn attention from many different fields of research: biochemistry, molecular biology, toxicology, pharmacology, etc. In recent years, considerable advances have been made concerning the regulation of MT genes by metals. Little, however, is known at the molecular level about the mechanisms of MT induction by nonmetallic inducers such as growth factors. This is of particular interest since MT is highly expressed during liver regeneration, an event orchestrated by a series of growth stimulators and inhibitors. The significance of the nuclear distribution of MT in growing cells and what controls its translocation are questions that remain unanswered at the present time. The possibility that MT could participate in a DNA synthesis-related process through donation or abstraction of Zn to and from transcription factors has been inferred from in vitro studies. Such transfer mechanisms, however, have yet to be confirmed in vivo. Overexpression of MT is often accompanied by increased resistance towards a variety of alkylating agents and chemotherapeutic drugs. The mechanisms by which MT protects cells against these agents may depend on their distinct mode of toxic action. For some, MT cysteines can be the target of the direct attack from the parent compound. For others such as N-methyl-N-nitroso compounds, MT cysteines may serve as a sink for the reactive oxygen species now known to be derived from their metabolism. In either case, a primary consequence of such interactions is the release of the metals initially bound to MT. Therefore, the metal composition of MT appears to be an important factor to consider in determining the overall effect of MT in the resistance process.

A binding site for chlorambucil on metallothionein

It is of interest to test the hypothesis that induced metallothionein (MT) acts in acquired drug resistance by covalent sequestration. In this study MT was incubated in vitro with chlorambucil (CHB) under conditions where only 1:1 covalent adducts were formed. The proteolytic products of these adducts were analyzed by HPLC and mass spectrometry to reveal two major sites of modification. These were the sulfur atoms of cysteines 33 and 48, which cochelate the same metal atom in native MT. The time course of the reaction was followed using on-line electrospray ionization with a double-focusing mass spectrometer. These experiments showed that drug-modified MT binds seven metal ions, as does the unmodified protein. Molecular docking experiments showed that the selectively of drug binding is influenced by the presence of the aziridinium ion in the drug structure and complementary charge densities in the protein structure.

Expression of the type 2 metallothionein-like gene MT2 from Arabidopsis thaliana in Zn(2+)-metallothionein-deficient Synechococcus PCC 7942: putative role for MT2 in Zn2+ metabolism

Zn2+ proteins pervade metabolism and are essential for gene expression. However, no proteins have been ascribed the central roles of Zn2+ donation to, or removal from, metalloproteins, or Zn2+ storage in vegetative plant tissue. In animals, such functions have been proposed for metallothioneins. Plants contain multiple metallothionein-like genes but their predicted products, which differ significantly from animal metallothioneins, remain to be isolated from vegetative tissue and their roles are uncertain. The type 2 metallothionein-like gene from Arabidopsis, MT2, was expressed under the control of Zn2+-responsive elements derived from the cyanobacterial metallothionein divergon, smt. Zn2+-dependent expression of MT2 transcripts in Synechococcus PCC 7942 was confirmed by northern analysis. The Arabidopsis MT2 gene partly complemented Zn2+ hypersensitivity in mutants of Synechococcus PCC 7942 which are functionally deficient in an endogenous Zn2+-metallothionein gene, smtA. MT2 was also expressed as a recombinant fusion protein in Escherichia coli, purified and shown to bind Zn2+ in vitro. The mean pH of half displacement of Zn2+ from MT2 was estimated to be 5.05. This suggests that MT2 has a greater affinity for Zn2+ than phytochelatins. The results presented here reveal that MT2 is capable of binding Zn2+ in vitro, conferring tolerance to elevated [Zn2+] in vivo within cyanobacteria and is likely to compete with other polypeptides for cellular Zn2+ in planta.

Toxic metal-responsive gene transcription

Metals play a dual role in biological systems, serving as essential co-factors for a wide range of biochemical reactions yet these same metals may be extremely toxic to cells. To cope with the stress of increases in environmental metal concentrations, eukaryotic cells have developed sophisticated toxic metal sensing proteins which respond to elevations in metal concentrations. This signal is transmitted to stimulate the cellular transcriptional machinery to activate expression of metal detoxification and homeostasis genes. This review summarizes our current understanding of the biochemical and genetic mechanisms which underlie cellular responses to toxic metals via metalloregulatory transcription factors.