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

Zinc promotes cell proliferation via regulating metal-regulatory transcription factor 1 expression and transcriptional activity in pulmonary arterial hypertension

Metal responsive transcription factor 1 (MTF-1) is a zinc-dependent transcription factor involved in the development of pulmonary arterial hypertension (PAH), which is a life-threatening disease characterized by elevated pulmonary artery pressure and pulmonary vascular remodeling. However, little is known about the role and regulatory signaling of MTF-1 in PAH. This study aimed to investigate the effect and mechanism of MTF-1 on the proliferation of pulmonary arterial smooth muscle cells (PASMCs). Several techniques including intracellular-free zinc detected by fluorescent indicator-fluozinc-3-AM, western blot, luciferase reporter, and cell proliferation assay were conducted to perform a comprehensive analysis of MTF-1 in proliferation of PASMCs in PAH. Increased cytosolic zinc was shown in monocrotaline (MCT)-PASMCs and ZnSO₄-treated PASMCs, which led to overexpression and overactivation of MTF-1, followed by the up-regulation of placental growth factor (PlGF). Elevated MTF-1 and PlGF were observed in western blot, and high transcriptional activity of MTF-1 was confirmed by luciferase reporter in ZnSO4-treated cells. Further investigation of cell proliferation revealed a favorable impact of zinc ions on PASMCs proliferation, with the deletion of Mtf-1/Plgf attenuating ZnSO4-induced proliferation. Flow cytometry analysis showed that blockade of PKC signaling inhibited the cell cycle of MCT-PASMCs and ZnSO4-treated PASMCs. The Zinc/PKC/MTF-1/PlGF pathway is involved in the up-regulatory effect on the PASMCs proliferation in the process of PAH. This study provided novel insight into zinc homeostasis in the pathogenesis of PAHs, and the regulation of MTF-1 might be a potential target for therapeutic intervention in PAH.

Gawky modulates MTF-1-mediated transcription activation and metal discrimination

Metal-induced genes are usually transcribed at relatively low levels under normal conditions and are rapidly activated by heavy metal stress. Many of these genes respond preferentially to specific metal-stressed conditions. However, the mechanism by which the general transcription machinery discriminates metal stress from normal conditions and the regulation of MTF-1-meditated metal discrimination are poorly characterized. Using a focused RNAi screening in Drosophila Schneider 2 (S2) cells, we identified a novel activator, the Drosophila gawky, of metal-responsive genes. Depletion of gawky has almost no effect on the basal transcription of the metallothionein (MT) genes, but impairs the metal-induced transcription by inducing the dissociation of MTF-1 from the MT promoters and the deficient nuclear import of MTF-1 under metal-stressed conditions. This suggests that gawky serves as a 'checkpoint' for metal stress and metal-induced transcription. In fact, regular mRNAs are converted into gawky-controlled transcripts if expressed under the control of a metal-responsive promoter, suggesting that whether transcription undergoes gawky-mediated regulation is encrypted therein. Additionally, lack of gawky eliminates the DNA binding bias of MTF-1 and the transcription preference of metal-specific genes. This suggests a combinatorial control of metal discrimination by gawky, MTF-1, and MTF-1 binding sites.

Metallothionein 3 Promotes Osteoblast Differentiation in C2C12 Cells via Reduction of Oxidative Stress

Metallothioneins (MTs) are intracellular cysteine-rich proteins, and their expressions are enhanced under stress conditions. MTs are recognized as having the ability to regulate redox balance in living organisms; however, their role in regulating osteoblast differentiation is still unclear. In this research, we found that the expression of MT3, one member of the MT protein family, was specifically upregulated in the differentiation process of C2C12 myoblasts treated with bone morphogenetic protein 4 (BMP4). Transfection with MT3-overexpressing plasmids in C2C12 cells enhanced their differentiation to osteoblasts, together with upregulating the protein expression of bone specific transcription factors runt-related gene 2 (Runx2), Osterix, and distal-less homeobox 5 (Dlx5). Additionally, MT3 knockdown performed the opposite. Further studies revealed that overexpression of MT3 decreased reactive oxygen species (ROS) production in C2C12 cells treated with BMP4, and MT3 silencing enhanced ROS production. Treating C2C12 cells with antioxidant N-acetylcysteine also promoted osteoblast differentiation, and upregulated Runx2/Osterix/Dlx5, while ROS generator antimycin A treatment performed the opposite. Finally, antimycin A treatment inhibited osteoblast differentiation and Runx2/Osterix/Dlx5 expression in MT3-overexpressing C2C12 cells. These findings identify the role of MT3 in osteoblast differentiation and indicate that MT3 may have interesting potential in the field of osteogenesis research.

[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.

Tau Protein as a New Regulator of Cellular Prion Protein Transcription

Cellular prion protein (PrP^C) is largely responsible for transmissible spongiform encephalopathies (TSEs) when it becomes the abnormally processed and protease resistant form PrP^SC. Physiological functions of PrP^C include protective roles against oxidative stress and excitotoxicity. Relevantly, PrP^C downregulates tau levels, whose accumulation and modification are a hallmark in the advance of Alzheimer's disease (AD). In addition to the accumulation of misfolded proteins, in the initial stages of AD-affected brains display both increased reactive oxygen species (ROS) markers and levels of PrP^C. However, the factors responsible for the upregulation of PrP^C are unknown. Thus, the aim of this study was to uncover the different molecular actors promoting PrP^C overexpression. In order to mimic early stages of AD, we used β-amyloid-derived diffusible ligands (ADDLs) and tau cellular treatments, as well as ROS generation, to elucidate their particular roles in human PRNP promoter activity. In addition, we used specific chemical inhibitors and site-specific mutations of the PRNP promoter sequence to analyze the contribution of the main transcription factors involved in PRNP transcription under the analyzed conditions. Our results revealed that tau is a new modulator of PrP^C expression independently of ADDL treatment and ROS levels. Lastly, we discovered that the JNK/c-jun-AP-1 pathway is involved in increased PRNP transcription activity by tau but not in the promoter response to ROS.

Functional analysis of MTF-1 and MT promoters and their transcriptional response to zinc (Zn) and copper (Cu) in yellow catfish Pelteobagrus fulvidraco

Metal-responsive transcription factor-1 (MTF-1) and metallothionein (MT) expression are involved in metal homeostasis and detoxification. Here, we characterized the structure and functions of mtf-1 and mt promoters in yellow catfish Pelteobagrus fulvidraco. Many important binding sites of transcriptional factors, such as heat shock promoter element (HSE) and metal responsive element (MRE), were predicted on their promoter regions. Cu did not significantly influence the activity of mtf-1 promoter, but Zn increased its promoter activity. Cu and Zn induced the increase of mt promoter activity. HSE site of mtf-1 promoter was the functional binding locus responsible for Zn-induced mtf-1 transcriptional activation. Zn and Cu induced transcriptional activation of mt gene through the MTF-1- and MRE-dependent pathway. Using primary hepatocytes of yellow catfish, we found that Cu and Zn induced the mt expression; Cu did not significantly influence the mRNA and total protein levels of MTF-1, but Zn up-regulated its mRNA and total protein expression. Both Zn and Cu treatment also up-regulated MTF-1 nuclear protein expression, which in turn increased the mt expression. Taken together, these findings delineated the transcriptional regulation of MT and MTF-1 under Zn or Cu treatments, and provided some mechanisms for the regulation of Cu and Zn homeostasis in vertebrates.

Genetic and transcriptional variations in NRAMP-2 and OPAQUE1 genes are associated with salt stress response in wheat

SNP alleles on chromosomes 4BL and 6AL are associated with sensitivity to salt tolerance in wheat and upon validation can be exploited in the development of salt-tolerant wheat varieties. The dissection of the genetic and molecular components of salt stress response offers strong opportunities toward understanding and improving salt tolerance in crops. In this study, GWAS was employed to identify a total of 106 SNP loci (R² = 0.12-63.44%) linked to salt stress response in wheat using leaf chlorophyll fluorescence, grain quality and shoot ionic (Na⁺ and K⁺ ions) attributes. Among them, 14 SNP loci individually conferred pleiotropic effects on multiple independent salinity tolerance traits including loci at 99.04 cM (R² ≥ 14.7%) and 68.45 cM (R² ≥ 4.10%) on chromosomes 6AL and 4BL, respectively, that influenced shoot Na⁺-uptake, shoot K⁺/Na⁺ ratio, and specific energy fluxes for absorption (ABS/RC) and dissipation (DIo/RC). Analysis of the open reading frame (ORF) containing the SNP markers revealed that they are orthologous to genes involved in photosynthesis and plant stress (salt) response. Further transcript abundance and qRT-PCR analyses indicated that the genes are mostly up-regulated in salt-tolerant and down-regulated in salt-sensitive wheat genotypes including NRAMP-2 and OPAQUE1 genes on 4BL and 6AL, respectively. Both genes showed highest differential expression between contrasting genotypes when expressions of all the genes within their genetic intervals were analyzed. Possible cis-acting regulatory elements and coding sequence variation that may be involved in salt stress response were also identified in both genes. This study identified genetic and molecular components of salt stress response that are associated with Na⁺-uptake, shoot Na⁺/K⁺ ratio, ABS/RC, DIo/RC, and grain quality traits and upon functional validation would facilitate the development of gene-specific markers that could be deployed to improve salinity tolerance in wheat.

Bioinformatic analysis of the metal response element and zinc-dependent gene regulation via the metal response element-binding transcription factor 1 in Caco-2 cells

The purpose of this study was to determine the correlation between the position or number of metal regulatory elements (MREs) near gene transcriptional or translational start sites, and the strength of metal response element-binding transcription factor 1 (MTF-1) regulation. A secondary analysis was performed in silico on published results measuring the effects of Zn and MTF-1 on transcriptional regulation of genes (n = 120) in the Caco-2 cell line. MRE sequence variations throughout the human genome were sorted using a position weight matrix. Three null hypotheses (H0) were tested: (1) there is no correlation between the number of MREs and MTF-1 transcriptional strength, (2) there is no correlation between the distance of the MRE upstream from the transcriptional start site (TSS) and MTF-1 transcriptional strength, and (3) there is no correlation between the distance of the MRE downstream from the translational start site (TrSS) and MTF-1 transcriptional strength. Spearman correlation was used to test for significance (p < 0.05). From our results we rejected the first H0; we observed a significant correlation between the total number of MRE sequences - 7Kbp upstream from the TSS, within the 5' untranslated region, and + 1Kbp downstream from the TrSS, versus the strength of MTF-1 regulation (r = 0.202; p = 0.027). The second and third H0 were accepted. These results expand our understanding of the role of the MRE in Zn-dependent gene regulation. The data indicate that Zn influences the transcriptional control of gene expression beyond maintaining intracellular Zn homeostasis.

Female Hormone 17β-Estradiol Downregulated MMP-2 Expression and Upregulated A1PI Expression in Human Corneal Stromal Cells

Collagens are essential for cornea functions. In non-ocular tissues, it has been demonstrated that sex hormones modulate the collagen remodeling. In this study, we investigated whether the primary female hormone 17β-estradiol plays a role in the expressions of matrix metalloproteinases and proteinase inhibitors in cultured human corneal stromal cells. We found that 17β-estradiol treatment significantly reduced the matrix metalloproteinase-2 mRNA in human corneal stromal cells as well as the matrix metalloproteinase-2 proteins, while the matrix metalloproteinase-9 mRNA level was not significantly altered. 17β-estradiol also upregulated the expression of proteinase inhibitor, alpha1-proteinase inhibitor. The expression of transcription factor specificity protein 1 was reduced by 17β-estradiol. Furthermore, 17β-estradiol did not change the viability and apoptosis of the corneal stromal cells. The downregulation of matrix metalloproteinase-2 and upregulation of alpha1-proteinase inhibitor by 17β-estradiol possibly serve as protective factor for the normal tomography in antagonizing the extracellular matrix degeneration in many cornea diseases.

Cu(I) Disrupts the Structure and Function of the Nonclassical Zinc Finger Protein Tristetraprolin (TTP)

Tristetraprolin (TTP) is a nonclassical zinc finger (ZF) protein that plays a key role in regulating inflammatory response. TTP regulates cytokines at the mRNA level by binding to AU-rich sequences present at the 3'-untranslated region, forming a complex that is then degraded. TTP contains two conserved CCCH domains with the sequence CysX₈CysX₅CysX₃His that are activated to bind RNA when zinc is coordinated. During inflammation, copper levels are elevated, which is associated with increased inflammatory response. A potential target for Cu(I) during inflammation is TTP. To determine whether Cu(I) binds to TTP and how Cu(I) can affect TTP/RNA binding, two TTP constructs were prepared. One construct contained just the first CCCH domain (TTP-1D) and serves as a peptide model for a CCCH domain; the second construct contains both CCCH domains (TTP-2D) and is functional (binds RNA) when Zn(II) is coordinated. Cu(I) binding to TTP-1D was assessed via electronic absorption spectroscopy titrations, and Cu(I) binding to TTP-2D was assessed via both absorption spectroscopy and a spin filter/inductively coupled plasma mass spectrometry (ICP-MS) assay. Cu(I) binds to TTP-1D with a 1:1 stoichiometry and to TTP-2D with a 3:1 stoichiometry. The CD spectrum of Cu(I)-TTP-2D did not exhibit any secondary structure, matching that of apo-TTP-2D, while Zn(II)-TTP-2D exhibited a secondary structure. Measurement of RNA binding via fluorescence anisotropy revealed that Cu(I)-TTP-2D does not bind to the TTP-2D RNA target sequence UUUAUUUAUUU with any measurable affinity, while Zn(II)-TTP-2D binds to this site with nanomolar affinity. Similarly, addition of Cu(I) to the Zn(II)-TTP-2D/RNA complex resulted in inhibition of RNA binding. Together, these data indicate that, while Cu(I) binds to TTP-2D, it does not result in a folded or functional protein and that Cu(I) inhibits Zn(II)-TTP-2D/RNA binding.

The Zinc-Schiff Base-Novicidin Complex as a Potential Prostate Cancer Therapy

Prostate cancer cells control energy metabolism by chelating intracellular zinc. Thus, zinc delivery has been a popular therapeutic approach for prostate cancer. Here, we propose the use of the membrane-penetrating peptide Novicidin connected to zinc-Schiff base as a carrier vehicle for the delivery of zinc to prostate cells. Mass spectrometry, electrochemistry and spectrophotometry confirmed the formation/stability of this complex and provided insight regarding the availability of zinc for complex interactions. This delivery system showed minor toxicity in normal PNT1A cells and high potency towards PC3 tumor cells. The complex preferentially penetrated PC3 tumor cells in contrast to confinement to the membranes of PNT1A. Furthermore, zinc uptake was confirmed in both cell lines. Molecular analysis was used to confirm the activation of zinc stress (e.g., ZnT-1) and apoptosis (e.g., CASP-1). Our results strongly suggest that the zinc-Schiff base-Novicidin complex has great potential as a novel anticancer drug.

Analysis of cysteine and histidine residues required for zinc response of the transcription factor human MTF-1

Metal responsive element (MRE)-binding transcription factor-1 (MTF-1) is a zinc finger (ZF) transcription factor that plays a key role in heavy metal homeostasis by regulating relevant genes in response to metals. MTF-1 is known to be activated by heavy metals such as Zn and Cd, but the mechanism of activation remains unclear. In the present study, Cys and His residues of human MTF-1 (hMTF-1), some of which may be involved in interaction with metals or with each other, were screened for their contribution to Zn-dependent transcription. To avoid poor induction ratios of previous transfection assays, we re-examined experimental conditions to establish an assay able to correctly detect Zn-responsive transcription. Using this assay, a series of Cys and/or His substitution mutants were analyzed over the entire hMTF-1 molecule. In five out of the six ZFs (ZF1 to ZF5), Cys mutations that disrupt the ZF structure abolished response to Zn. Of these, ZF5 was shown for the first time to be essential for Zn-responsive transcription, despite it being unnecessary for Zn-induced DNA binding. These results indicate that Zn activation of hMTF-1 involves an additional process besides induction of DNA binding activity. Our assay also confirmed the importance of Cys in the acidic activation domain, as well as those in the C-terminal Cys cluster, implicated in transcription in other studies. The identified Cys residues might contribute to metal response of hMTF-1 through direct metal binding and/or intramolecular interactions, analysis of which will be helpful in understanding the mechanism of metal response.

The taste of heavy metals: gene regulation by MTF-1

The metal-responsive transcription factor-1 (MTF-1, also termed MRE-binding transcription factor-1 or metal regulatory transcription factor-1) is a pluripotent transcriptional regulator involved in cellular adaptation to various stress conditions, primarily exposure to heavy metals but also to hypoxia or oxidative stress. MTF-1 is evolutionarily conserved from insects to humans and is the main activator of metallothionein genes, which encode small cysteine-rich proteins that can scavenge toxic heavy metals and free radicals. MTF-1 has been suggested to act as an intracellular metal sensor but evidence for direct metal sensing was scarce. Here we review recent advances in our understanding of MTF-1 regulation with a focus on the mechanism underlying heavy metal responsiveness and transcriptional activation mediated by mammalian or Drosophila MTF-1. This article is part of a Special Issue entitled: Cell Biology of Metals.

Induction and transcriptional regulation of laccases in fungi

Fungal laccases are phenol oxidases widely studied for their use in several industrial applications, including pulp bleaching in paper industry, dye decolourisation, detoxification of environmental pollutants and revalorization of wastes and wastewaters. The main difficulty in using these enzymes at industrial scale ensues from their production costs. Elucidation of the components and the mechanisms involved in regulation of laccase gene expression is crucial for increasing the productivity of native laccases in fungi. Laccase gene transcription is regulated by metal ions, various aromatic compounds related to lignin or lignin derivatives, nitrogen and carbon sources. In this manuscript, most of the published results on fungal laccase induction, as well as analyses of both the sequences and putative functions of laccase gene promoters are reviewed. Analyses of promoter sequences allow defining a correlation between the observed regulatory effects on laccase gene transcription and the presence of specific responsive elements, and postulating, in some cases, a mechanism for their functioning. Only few reports have investigated the molecular mechanisms underlying laccase regulation by different stimuli. The reported analyses suggest the existence of a complex picture of laccase regulation phenomena acting through a variety of cis acting elements. However, the general mechanisms for laccase transcriptional regulation are far from being unravelled yet.

Coping with cadmium exposure in various ways: the two helicid snails Helix pomatia and Cantareus aspersus share the metal transcription factor-2, but differ in promoter organization and transcription of their Cd-metallothionein genes

Gastropods are able to withstand fluctuating availabilities of nonessential trace elements such as cadmium by induction of Cd-specific metallothionein isoform (Cd-MT) expression. As in other species, the induction mechanism involves the binding of metal-regulatory transcription factors (MTF-1 or MTF-2) to metal responsive elements (MREs) in the MT promoter regions. Cd-dependent transcription of Cd-MT genes was assessed by quantitative real time PCR in two helicid gastropods, Helix pomatia and Cantareus aspersus, over a period of eight days. The promoter regions of the Cd-MT genes of the two species were sequenced and compared regarding the position of MREs and other relevant potential transcription factor binding sites (TFBs). Cd-MT gene transcription is induced after Cd exposure in Helix pomatia and Cantareus aspersus, showing a transient peak in Helix pomatia, contrasting with a persistent induction rate in Cantareus aspersus. Since the existence of MTF-2 was verified in both species, differing transcription patterns of Cd-MT genes must be due to functional differences in their metal-responsive promoter regions. Both promoters contain a proximal cluster of three MREs overlapping with TFBs for the transcriptional regulator Sp1. In contrast to Cantareus aspersus, however, the Cd-MT gene of Helix pomatia hosts an additional distal MRE overlapping with a Sp1 binding site and a CACCC box. Inhibitory effects of MRE overlapping Sp1 binding sites were observed in other MT genes. We therefore suggest that transient Cd-MT transcription upon Cd(2+) exposure in Helix pomatia may be the result of an inhibitory action of the distal MRE cluster.

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

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

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

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

Regulation of prion gene expression by transcription factors SP1 and metal transcription factor-1

Prion diseases are associated with the conformational conversion of the host-encoded cellular prion protein into an abnormal pathogenic isoform. Reduction in prion protein levels has potential as a therapeutic approach in treating these diseases. Key targets for this goal are factors that affect the regulation of the prion protein gene. Recent in vivo and in vitro studies have suggested a role for prion protein in copper homeostasis. Copper can also induce prion gene expression in rat neurons. However, the mechanism involved in this regulation remains to be determined. We hypothesized that transcription factors SP1 and metal transcription factor-1 (MTF-1) may be involved in copper-mediated regulation of human prion gene. To test the hypothesis, we utilized human fibroblasts that are deleted or overexpressing the Menkes protein (MNK), a major mammalian copper efflux protein. Menkes deletion fibroblasts have high intracellular copper, whereas Menkes overexpressed fibroblasts have severely depleted intracellular copper. We have utilized this system previously to demonstrate copper-dependent regulation of the Alzheimer amyloid precursor protein. Here we demonstrate that copper depletion in MNK overexpressed fibroblasts decreases cellular prion protein and PRNP gene levels. Conversely, expression of transcription factors SP1 and/or MTF-1 significantly increases prion protein levels and up-regulates prion gene expression in copper-replete MNK deletion cells. Furthermore, siRNA "knockdown" of SP1 or MTF-1 in MNK deletion cells decreases prion protein levels and down-regulates prion gene expression. These data support a novel mechanism whereby SP1 and MTF-1 act as copper-sensing transcriptional activators to regulate human prion gene expression and further support a role for the prion protein to function in copper homeostasis. Expression of the prion protein is a vital component for the propagation of prion diseases; thus SP1 and MTF-1 represent new targets in the development of key therapeutics toward modulating the expression of the cellular prion protein and ultimately the prevention of prion disease.

Zinc-induced formation of a coactivator complex containing the zinc-sensing transcription factor MTF-1, p300/CBP, and Sp1

Herein, the mechanisms of transactivation of gene expression by mouse metal response element-binding transcription factor 1 (MTF-1) were investigated. Evidence obtained from coimmunoprecipitation assays revealed that exposure of the cells to zinc resulted in the rapid formation of a multiprotein complex containing MTF-1, the histone acetyltransferase p300/CBP, and the transcription factor Sp1. Down-regulation of endogenous p300 expression by small interfering RNA transfection significantly decreased zinc-dependent metallothionein I (MT-I) gene transcription without altering induction of zinc transporter 1 (ZnT1). MTF-1 independently facilitated the recruitment of Sp1 and p300 to the protein complex in response to zinc. Mutagenesis demonstrated that the acidic domain, one of three transactivation domains of MTF-1, is required for recruitment of p300 but not Sp1 as well as for zinc-dependent activation of MT-I gene transcription. Furthermore, mutation of leucine residues (L-->A) within a nuclear exclusion signal in the MTF-1 acidic domain impaired recruitment of p300 and zinc-dependent activation of the MT-I gene. Nuclear magnetic resonance characterization of an isolated protein fragment corresponding to the MTF-1 acidic region demonstrated that this region is largely unstructured in the presence and absence of excess stoichiometric amounts of zinc. This suggests that the mechanism by which MTF-1 recruits p300 to this complex involves extrinsic-zinc-dependent steps. These studies reveal a novel zinc-responsive mechanism requiring an acidic region of MTF-1 that functions as a nuclear exclusion signal as well as participating in formation of a coactivator complex essential for transactivation of MT-I gene expression.

Dumpy-30 family members as determinants of male fertility and interaction partners of metal-responsive transcription factor 1 (MTF-1) in Drosophila

BACKGROUND

Metal-responsive transcription factor 1 (MTF-1), which binds to metal response elements (MREs), plays a central role in transition metal detoxification and homeostasis. A Drosophila interactome analysis revealed two candidate dMTF-1 interactors, both of which are related to the small regulatory protein Dumpy-30 (Dpy-30) of the worm C. elegans. Dpy-30 is the founding member of a protein family involved in chromatin modifications, notably histone methylation. Mutants affect mating type in yeast and male mating in C. elegans.

RESULTS

Constitutive expression of the stronger interactor, Dpy-30L1 (CG6444), in transgenic flies inhibits MTF-1 activity and results in elevated sensitivity to Cd(II) and Zn(II), an effect that could be rescued by co-overexpression of dMTF-1. Electrophoretic mobility shift assays (EMSA) suggest that Dpy-30L1 interferes with the binding of MTF-1 to its cognate MRE binding site. Dpy-30L1 is expressed in the larval brain, gonads, imaginal discs, salivary glands and in the brain, testes, ovaries and salivary glands of adult flies. Expression of the second interactor, Dpy-30L2 (CG11591), is restricted to larval male gonads, and to the testes of adult males. Consistent with these findings, dpy-30-like transcripts are also prominently expressed in mouse testes. Targeted gene disruption by homologous recombination revealed that dpy-30L1 knockout flies are viable and show no overt disruption of metal homeostasis. In contrast, the knockout of the male-specific dpy-30L2 gene results in male sterility, as does the double knockout of dpy-30L1 and dpy-30L2. A closer inspection showed that Dpy-30L2 is expressed in elongated spermatids but not in early or mature sperm. Mutant sperm had impaired motility and failed to accumulate in sperm storage organs of females.

CONCLUSION

Our studies help to elucidate the physiological roles of the Dumpy-30 proteins, which are conserved from yeast to humans and typically act in concert with other nuclear proteins to modify chromatin structure and gene expression. The results from these studies reveal an inhibitory effect of Dpy-30L1 on MTF-1 and an essential role for Dpy-30L2 in male fertility.

Keep your fingers off my DNA: protein-protein interactions mediated by C2H2 zinc finger domains

Cys2-His2 (C2H2) zinc finger domains (ZFs) were originally identified as DNA-binding domains, and uncharacterized domains are typically assumed to function in DNA binding. However, a growing body of evidence suggests an important and widespread role for these domains in protein binding. There are even examples of zinc fingers that support both DNA and protein interactions, which can be found in well-known DNA-binding proteins such as Sp1, Zif268, and Ying Yang 1 (YY1). C2H2 protein-protein interactions (PPIs) are proving to be more abundant than previously appreciated, more plastic than their DNA-binding counterparts, and more variable and complex in their interactions surfaces. Here we review the current knowledge of over 100 C2H2 zinc finger-mediated PPIs, focusing on what is known about the binding surface, contributions of individual fingers to the interaction, and function. An accurate understanding of zinc finger biology will likely require greater insights into the potential protein interaction capabilities of C2H2 ZFs.

Cloning and characterization of a tilapia (Oreochromis aureus) metallothionein gene promoter in Hepa-T1 cells following the administration of various heavy metal ions

Metallothioneins (MTs) are highly conserved intracellular metal-binding proteins that contribute to the homeostasis of essential metals and the detoxification of non-essential heavy metals. MT gene expression is induced by various heavy metal ions, and Zn(2+) is able to bind and activate a transcription factor associated with the MT gene that is known as the metal responsive element (MRE) binding transcription factor-1 (MTF-1). Heavy metals other than Zn(2+), such as Cd(2+) and Cu(2+), fail to activate the binding of MTF-1 to MREs despite their ability to induce the transcription of the MT gene. To study how different metal ions regulate MT gene expression, a tilapia (ti)-MT gene promoter was cloned and its responses to activation by various metal ions measured using a Hepa T1 cell culture model. The tiMT gene promoter contains six functional MREs within 2118bp 5' of the translational start site. A transient gene expression study showed the tiMT gene promoter fragment to be responsive to Cd(2+), Cu(2+), Hg(2+), Pb(2+), and Zn(2+). Deletions from the 5' end and the site-directed mutagenesis of individual MREs in the tiMT gene promoter confirmed that both proximal and distal clusters of MREs were required for the maximal metal induction of the tiMT gene. The distal cluster of MREs greatly enhanced the induction of tiMT gene expression by several of the heavy metal ions, and especially the non-Zn(2+) ions. Individual MREs showed a different responsiveness to metal ions, with MREe being the most potent, MREb being responsive to Zn(2+) but not to other metal ions, and MREa being mainly for the basal expression of the tiMT gene. Electrophoretic mobility shift assay (EMSA) identified a transcription factor that was able to bind most of the MREs, with the exception of MREd, but the binding was only activated by the in vivo administration of Zn(2+), not the administration of Cd(2+) or Cu(2+). In conclusion, the results of this study on a Hepa T1 cell model suggest that the mechanism of MT gene activation by non-Zn(2+) metal ions is different from that of activation by Zn(2+), and that different MREs may be involved in the activation of the tiMT gene by different metal ions without enhancing the binding of MTF-1 to MREs.

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.

Molecular characterization of two metallothionein isoforms in avian species: evolutionary history, tissue distribution profile, and expression associated with metal accumulation

To characterize avian MTs, MT cDNAs were cloned from liver of cormorant (Phalacrocorax carbo) and mallard (Anas platyrhynchos). Expression profiles of MT isoforms and relationships between metal accumulation and MT mRNA expression in tissues were also investigated. We succeeded in cDNA cloning of MT1/2 from cormorant and MT1 in mallard. DNA sequence of chicken MT1 was obtained from chicken (Gallus gallus) genomic database. Considering previous reports on avian MTs, birds possess at least two distinct MT isoforms. Comparison of genomic synteny among vertebrates and phylogenetic analysis of MT amino acid sequences revealed that avian MT1/2 are evolutionarily close to mammalian MT3. Messenger RNAs of both MT isoforms were detected in all the tissues/organs in cormorant and mallard. Liver was the primary organ for cormorant MT1/2, and mallard MT2, whereas MT1 was dominant in mallard heart. Interspecies comparison of tissue distribution of MT mRNA expression between cormorant and mallard indicated that MT2 profile was similar, but MT1 was not. Significant positive correlations of mRNA expression levels between MT1 and MT2 were observed in the liver and kidney of cormorants, whereas no correlation was found in mallards. Expression levels of cormorant MT1/2 showed significant positive correlations with hepatic Cu and Zn concentrations, suggesting that both MT isoforms were induced by Cu and Zn in livers. Cormorant MT2 expression level exhibited a significant positive correlation with hepatic Ag, and a negative correlation with Rb, indicating that Ag and Rb concentrations depend on the expression of MT2 by Cu and Zn. In mallard, MT1 had no correlation with any metal concentration, and MT2 expression was positively correlated only with Cu, even though hepatic Cu and Zn concentrations in mallard were much higher than in cormorant. This may indicate that cormorant is a more susceptible species than mallard in terms of MT induction. These findings suggest tissue-, species-, and MT isoform-specific responses to metal stresses in these aquatic birds.

The C/A(-18) polymorphism in the surfactant protein B gene influences transcription and protein levels of surfactant protein B

Surfactant protein B (SP-B) is an essential component of surfactant that promotes adsorption and spreading of surfactant phospholipids and stabilizes the phospholipid monolayer. SP-B is essential for respiratory function in newborn humans and mice; adult mice with levels of SP-B below 25% of wild-type develop fatal respiratory distress syndrome. A potential regulatory function of the C/A(-18) single nucleotide polymorphism (SNP) in the promoter of the SP-B gene was examined. Transcriptional analysis and ELISA on bronchoalveolar lavage fluid revealed that the presence of the C allele correlated with more SP-B promoter activity and protein. There was approximately threefold difference in amounts of SP-B in bronchoalveolar lavage fluid from CA(-18) and AA(-18) individuals. By EMSA, Sp1 bound more tightly to the C allele sequence than to the A allele sequence, perhaps accounting for the differences in transcription. Genotyping of a normal volunteer population showed approximately 31% of the population were AA homozygotes, suggesting that these individuals produce less SP-B. Differences in amounts of SP-B resulting from the promoter SNP could affect the clinical presentation of pulmonary disease.

The zinc-sensing mechanism of mouse MTF-1 involves linker peptides between the zinc fingers

Mouse metal response element-binding transcription factor-1 (MTF-1) regulates the transcription of genes in response to a variety of stimuli, including exposure to zinc or cadmium, hypoxia, and oxidative stress. Each of these stresses may increase labile cellular zinc, leading to nuclear translocation, DNA binding, and transcriptional activation of metallothionein genes (MT genes) by MTF-1. Several lines of evidence suggest that the highly conserved six-zinc finger DNA-binding domain of MTF-1 also functions as a zinc-sensing domain. In this study, we investigated the potential role of the peptide linkers connecting the four N-terminal zinc fingers of MTF-1 in their zinc-sensing function. Each of these three linkers is unique, completely conserved among all known vertebrate MTF-1 orthologs, and different from the canonical Cys2His2 zinc finger TGEKP linker sequence. Replacing the RGEYT linker between zinc fingers 1 and 2 with TGEKP abolished the zinc-sensing function of MTF-1, resulting in constitutive DNA binding, nuclear translocation, and transcriptional activation of the MT-I gene. In contrast, swapping the TKEKP linker between fingers 2 and 3 with TGEKP had little effect on the metal-sensing functions of MTF-1, whereas swapping the canonical linker for the shorter TGKT linker between fingers 3 and 4 rendered MTF-1 less sensitive to zinc-dependent activation both in vivo and in vitro. These observations suggest a mechanism by which physiological concentrations of accessible cellular zinc affect MTF-1 activity. Zinc may modulate highly specific, linker-mediated zinc finger interactions in MTF-1, thus affecting its zinc- and DNA-binding activities, resulting in translocation to the nucleus and binding to the MT-I gene promoter.

Sp1 and NFkappaB pathways are regulated in brain in response to acute and chronic ethanol

DNA microarray analysis was used to identify candidate ethanol-regulated genes, as a first step towards exploring how transcriptional changes might lead to ethanol-induced changes in behaviour. Mice were treated with a single acute intraperitoneal ethanol dose and DNA microarray analysis performed on midbrain 2 h posttreatment. We predicted that if ethanol-regulated genes contribute towards behaviour, then constitutive variation in brain expression levels may also contribute to strain-specific differences in ethanol-related behaviour of inbred mouse strains. On the basis of this assumption, we interrogated the BXD inbred strain phenotype database and the U74Av2 MAS5 brain expression database using the WebQTL tool (http://www.genenetwork.org/) and correlated ethanol-related behaviours to expression levels. Constitutive expression levels of 70/90 candidate genes, identified from the DNA microarray analysis, varied significantly between inbred strains and correlated significantly with strain-specific differences in ethanol-related behaviours. These genes were then mapped onto biochemical pathways using Stratagene's PathwayAssist software. This analysis identified the transcription factor Sp1 and NFkappaB pathways in the acute response to ethanol. Ethanol regulation of Sp1 transcription was conserved between humans and mouse. As predicted, downstream targets of Sp1 were also ethanol regulated. NFkappaBia, an important regulator of NFkappaB function and Rela, an NFkappaB-binding partner, were both regulated by ethanol. Expression of both Sp1 and NFkappaBialpha were also downregulated following chronic ethanol treatment. As Sp1 and NFkappaB are implicated in plasticity and behaviour, our data suggest a role for these transcription factors in the long-term behavioural adaptations to ethanol.

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.

Tilapia metallothionein genes: PCR-cloning and gene expression studies

Genomic PCR reactions were performed to isolate gene sequences of tilapia metallothionein (tiMT) from Oreochromis mossambicus and Oreochromis aureus. Two AP1 binding sites, four metal responsive elements, and a TATA box are the major cis-acting elements identified in the 800-bp 5' flanking region of the tiMTs obtained in this study. The tiMT gene promoter cloned from O. aureus was characterized in vitro using PLHC-1 cell-line, a hepatocellular carcinoma of a desert topminnow (Poecciliopsis lucida), following the administrations of Cd2+, Co2+, Cu2+, Ni2+, Pb2+ and Zn2+. Only Cd2+, Pb2+ and Zn2+ were able to induce the transcription of tiMT gene promoter in PLHC-1 cells in a dose-dependent manner. Zn2+ had the highest fold induction of tiMT gene promoter activity. Deletion mutants were tested for their abilities to drive the transcription of reporter gene following Cd2+ and Zn2+ administrations. However, Cu2+ and Ni2+ also induced the production of hepatic MT mRNA in vivo. Northern blot analysis showed that liver gave the highest fold induction of MT gene expression following the administration of heavy metal ions. These data indicated that hepatic MT mRNA level in tilapia is a potential sensitive biomarker of exposure to various metal ions including Cu2+, Cd2+, Ni2+, Pb2+, Hg2+ and Zn2+ ions.

Transcriptional Activity of Sp1 Is Regulated by Molecular Interactions between the Zinc Finger DNA Binding Domain and the Inhibitory Domain with Corepressors, and This Interaction Is Modulated by MEK

Sp1 activates the transcription of many cellular and viral genes with the GC-box in either the proximal promoter or the enhancer. Sp1 is composed of several functional domains, such as the inhibitory domain (ID), two serine/threonine-rich domains, two glutamine-rich domains, three C2H2-type zinc finger DNA binding domains (ZFDBD), and a C-terminal D domain. The ZDDBD is the most highly conserved domain among the Sp-family transcription factors and plays a critical role in GC-box recognition. In this study, we investigated the protein-protein interactions occurring at the Sp1ZFDBD and the Sp1ID, and the molecular mechanisms controlling the interaction. Our results found that Sp1ZFDBD and Sp1ID repressed transcription once they were targeted to the proximal promoter of the pGal4 UAS reporter fusion gene system, suggesting molecular interaction with the repressor molecules. Indeed, mammalian two-hybrid assays, GST fusion protein pull-down assays, and co-immunoprecipitation assays showed that Sp1ZFDBD and Sp1ID are able to interact with corepressor proteins such as SMRT, NcoR, and BCoR. The molecular interactions appear to be regulated by MAP kinase/Erk kinase kinase (MEK). The molecular interactions between Sp1ID and the corepressor might explain the role of Sp1 as a repressor under certain circumstances. The siRNA-induced degradation of the corepressors resulted in an up-regulation of Sp1-dependent transcription. The cellular context of the corepressors and the regulation of molecular interaction between corepressors and Sp1ZFDBD or Sp1ID might be important in controlling Sp1 activity.

Arsenic suppresses gene expression in promyelocytic leukemia cells partly through Sp1 oxidation

The mechanism by which arsenic dramatically affects gene expression remains poorly understood. Here we report that prolonged exposure of acute promyelocytic leukemia NB4 cells to low levels of arsenic trioxide increased the expression of a set of genes responsible for reactive oxygen species (ROS) production. We hypothesize that arsenic-induced ROS in turn contribute partially to altered gene expression. To identify genes responsive to arsenic-induced ROS, we used microarray gene expression analysis and identified genes that responded to arsenic and hydrogen peroxide but whose response to arsenic was reversed by an ROS scavenger, N-acetyl-L-cysteine. We found that 26% of the genes significantly responsive to arsenic might have been directly altered by ROS. We further explored the mechanisms by which ROS affects gene regulation and found that the Sp1 transcription factor was oxidized by arsenic treatment, with a corresponding decrease in its in situ binding on the promoters of 3 genes, hTERT, C17, and c-Myc, whose expressions were significantly suppressed. We conclude that ROS contributed partly to arsenic-mediated gene regulation and that Sp1 oxidation contributed to gene suppression by arsenic-induced ROS.

Modulation of iron on mitochondrial aconitase expression in human prostatic carcinoma cells

The mitochondrial aconitase (mACON) containing a [4Fe-4S] cluster is regarded as the key enzyme for citrate oxidation in the epithelial cells of human prostate. In vitro studies using the human prostatic carcinoma cells, PC-3 cells, found that both hemin and ferric ammonium citrate (FAC) significantly increased mACON enzymatic activity and gene expression. The effect of FAC on mACON was enhanced 2-fold by co-treating with ascorbic acid but blocked by co-treating with iron chelator, deferoxamine mesylate. Hemin treatments blocked 30% of citrate secretion from PC-3 cells but upregualted 2-fold of intracellular ATP biosynthesis. Results from reporter assay by using a cytomegalovirus enhance/promoter driven luciferase mRNA ligated to the iron response element (IRE) of mACON as a reporter construct demonstrated that modulation of FAC on gene translation of mACON gene is dependent on the IRE. Transient gene expression assays indicated that upregulation of mACON gene transcription by FAC may through the putative antioxidant response element (ARE) signal pathway. This study provides the first evidence of the biologic mechanism of human mACON gene translation/transcription and suggests a regulatory link between the energy utilization and the iron metabolism in human prostatic carcinoma cells.

Stable expression of C/EBPalpha in prostate cancer cells down-regulates metallothionein and increases zinc-induced toxicity

BACKGROUND

The transcription factor C/EBPalpha regulates terminal differentiation of various cell types. C/EBPalpha is expressed in prostate epithelium but its role in prostate development and malignant transformation is unknown. In examining the effect of forced expression of C/EBPalpha on the global gene expression profile in prostate cancer cells, we found that C/EBPalpha significantly decreased the RNA level of metallothioneins (MTs).

METHODS

The prostate cancer cell lines DU145, LNCaP, and PC3 with stable overexpression of C/EBPalpha were established with a retroviral expression system. MT expression was assayed by Western blot analysis and with the MT promoter in a plasmid using luciferase as a reporter.

RESULTS

Under basal conditions and in response to zinc, forced overexpression of C/EBPalpha decreased expression of MT isoforms 1A, B, F, and H, IIA and III. Following zinc exposure C/EBPalpha inhibited MT promoter activity by 1.5-2.5-fold. Overexpression of C/EBPalpha led to increased cytotoxicity of zinc at concentration of 150 microM in DU145 and LNCaP cells.

CONCLUSIONS

Our data demonstrated that expression of MTs in prostate cancer cells is inhibited by C/EBPalpha and the effect may have functional significance in regulating the growth of prostate cancer cells and the response of these cells to environment stresses.

Cloning of zebrafish metallothionein gene and characterization of its gene promoter region in HepG2 cell line

The coding region of cDNA and genomic DNA, with its promoter region, of zebrafish metallothionein (zMT) gene homologous to the piscine MT-II was obtained. The A/T-rich promoter region contains four metal regulatory elements (MREs), three activator protein 1 (AP1) and one specific protein 1 (Sp1) binding sites. The four MREs are organized into two clusters, a distal cluster with one MRE lying around 740 bp upstream of the transcription start point and a proximal cluster with three MREs located close to the TATA box. The metal induction ability of the promoter was assessed by transient luciferase gene expression assays in HepG2 cells. The zMT promoter was inducible by Zn2+, Cd2+, Cu2+ and Hg2+ ions in decreasing inducibility, while inert to Ni2+, Pb2+ and Co2+ ions, and H2O2 treatment in vitro. Deletion of the putative cis-acting elements in the promoter region revealed that the distal MRE (MREd) was important in mediating metal inducibility. Despite the binding of HepG2 cell nuclear protein factors to all MREs as confirmed by electrophoretic mobility shift assay (EMSA), the proximal MREs did not provide significant contribution to metal induction of zMT gene in HepG2 cells. The metal inducibility of zMT promoter required the cooperative effect of at least three MRE sites.

Gelsolin gene silencing involving unusual hypersensitivities to dimethylsulfate and KMnO4 in vivo footprinting on its promoter region

We previously reported that gelsolin gene expression is reduced in various tumors. In an effort to gain further insights into the mechanism of gelsolin downregulation in tumors, we examined the in vivo properties of the gelsolin promoter in urinary bladder cancer cell lines. Neither mutation nor hypermethylation was responsible for gene silencing at the promoter. After exposure to trichostatin A (TSA), a histone deacetylase inhibitor, gelsolin promoter activity was markedly enhanced in the cancer cells, not in cells derived from normal tissue. Chromatin immunoprecipitation assays revealed that both histones H3 and H4 were hypoacetylated in the promoter region of the cancer cells, and the accumulation of acetylated histones was detected by TSA treatment. In vivo footprinting analysis revealed the presence of dimethylsulfate (DMS) hypersensitive site in the untranslated region around nucleotide--35 only in the cancer cells but not in cells derived from normal tissue, and analysis of KMnO4 reactive nucleotides showed that the stem loop structure could be formed in vivo of the cancer cells. This novel stem loop structure may play a part in regulating the transcription of the gelsolin gene in the cancer cells. These results suggest that nucleosome accessibility through histone deacetylation and structural changes (DMS hypersensitivity and stem loop structure) in the promoter region form the basis of the mechanism leading to the silencing of gelsolin gene in human bladder cancer.

Identification of putative transcription factor binding sites in rodent selenoprotein W promoter

To understand transcriptional regulation of the selenoprotein W (SeW) gene, we used in vitro binding assays to identify transcription factors that may be involved in the transcriptional regulation of the SeW gene. Using protein from rat C6 (glial) cell nuclear extracts, oligonucleotides containing putative regulatory elements in the SeW promoter and antibodies, we observed that specificity protein 1(Sp1) transcription factor binds to the Sp1 consensus sequence in the SeW promoter as well as to the metal response element (MRE). Although competition analysis showed specific binding at the TFII-1 site, super-shift analysis using anti-TFII-1 antibody did not yield any super-shifted band. Therefore, the SeW gene may be a target for Sp1 whose binding to various regulatory sequences of the SeW promoter may activate or repress the transcription of SeW. The MRE, GRE, AP-1 and LF-A1 sites were also tested but no evidence was obtained for specific binding as indicated by lack of competition with unlabeled probes.

Valproic acid teratogenicity: a toxicogenomics approach

Embryonic development is a highly coordinated set of processes that depend on hierarchies of signaling and gene regulatory networks, and the disruption of such networks may underlie many cases of chemically induced birth defects. The antiepileptic drug valproic acid (VPA) is a potent inducer of neural tube defects (NTDs) in human and mouse embryos. As with many other developmental toxicants however, the mechanism of VPA teratogenicity is unknown. Using microarray analysis, we compared the global gene expression responses to VPA in mouse embryos during the critical stages of teratogen action in vivo with those in cultured P19 embryocarcinoma cells in vitro. Among the identified VPA-responsive genes, some have been associated previously with NTDs or VPA effects [vinculin, metallothioneins 1 and 2 (Mt1, Mt2), keratin 1-18 (Krt1-18)], whereas others provide novel putative VPA targets, some of which are associated with processes relevant to neural tube formation and closure [transgelin 2 (Tagln2), thyroid hormone receptor interacting protein 6, galectin-1 (Lgals1), inhibitor of DNA binding 1 (Idb1), fatty acid synthase (Fasn), annexins A5 and A11 (Anxa5, Anxa11)], or with VPA effects or known molecular actions of VPA (Lgals1, Mt1, Mt2, Id1, Fasn, Anxa5, Anxa11, Krt1-18). A subset of genes with a transcriptional response to VPA that is similar in embryos and the cell model can be evaluated as potential biomarkers for VPA-induced teratogenicity that could be exploited directly in P19 cell-based in vitro assays. As several of the identified genes may be activated or repressed through a pathway of histone deacetylase (HDAC) inhibition and specificity protein 1 activation, our data support a role of HDAC as an important molecular target of VPA action in vivo.

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

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

Adjacent sequence controls the response polarity of nitric oxide-sensitive Sp factor binding sites

Nitric oxide (NO*) and cAMP-dependent protein kinase (PKA) inhibitors up-regulate tumor necrosis factor alpha (TNFalpha) by decreasing Sp1 binding to a proximal GC box element. Here, elements flanking GC boxes were tested for their role in determining whether Sp sites act as activators or repressors. Promoter studies in receptive human cell lines demonstrated that NO* down-regulated endothelial NO* synthase (eNOS) but up-regulated TNFalpha. Like TNFalpha, Sp1 binding to the eNOS promoter was decreased by NO* and a PKA inhibitor, H89, and increased by a PKA activator, dibutyryl cAMP (Bt2cAMP). For either promoter, mutation of Sp sites abolished NO* responses. In contrast, mutation of an upstream AP1 site in the TNFalpha promoter (not present in eNOS) maintained NO* responsiveness, but reversed the direction of NO* and cAMP effects. Using artificial constructs, NO* increased transcription when Sp and AP1 sites were both present (TNFalpha-like response), but decreased it when the adjacent AP1 site was disrupted (eNOS-like response). NO*, H89, and Bt2cAMP were found to produce reciprocal protein binding changes at contiguous AP1 and Sp sites (p < 0.0001 for an interaction). Chromatin immunoprecipitation assays demonstrated that Sp1 and to a lesser extent Sp3 bound to the GC box regions of eNOS and TNFalpha in intact cells. Thus, this NO*- and cAMP-responsive regulatory module has a Sp site sensor variably coupled to an adjacent element that determines response polarity. These results define a composite element that can utilize secondary inputs to convert off signals to on, thereby conferring complex functionalities to the same DNA binding motif.

Metal-responsive elements in Pleurotus ostreatus laccase gene promoters

Fungal laccase gene transcription is strongly induced by copper ions; notably, some laccase promoters contain multiple putative metal-responsive elements (MREs). Previously, it has been demonstrated that the Pleurotus ostreatus laccase genes poxc and poxa1b are transcriptionally induced by copper, and several putative MREs were found in the promoter regions of these genes, which extend for about 400 nt upstream of the start codon (ATG). Identification of MRE sequences, which are protected by protein binding in the poxc and poxa1b promoter regions, has been achieved by footprinting analyses. Electromobility shift assays led to the evaluation of the ability of the identified MREs to bind protein(s), and the role of specific nucleotides of these elements in complex formation has also been analysed. The formation of complexes between analysed MREs and fungal proteins requires the absence of metal ions. Proteins extracted from fungus grown in copper-depleted medium are able to form complexes with MREs, whilst proteins extracted from fungus grown in copper-containing medium are able to form complexes only in the presence of a metal chelator. Moreover, copper-depleted proteins are unable to form complexes when copper or zinc ions are added. UV-cross-linking analyses led to the determination of the molecular masses of the MRE-binding proteins. In the poxa1b promoter, a GC-rich region, homologous to the core binding site for transcription factor Sp1, decreases the binding affinity of the adjacent MRE, affecting its interactions with fungal protein factors.

Mapping of Sp1 regulation sites in the promoter of the human alpha1-proteinase inhibitor gene

Keratoconus is a potentially blinding disease that thins the central cornea. In afflicted corneas, the level of an inhibitor, alpha1-proteinase inhibitor (alpha1-PI), is found reduced. An increased expression of transcription factor Sp1 is also demonstrated. To examine the role of Sp1 in regulation of the human alpha1-PI gene, a 1.4-kb (-1397/+9) 5'-flanking promoter sequence that contains 10 Sp1 sites was cloned. Previous transient transfection experiments showed that Sp1 expression indeed repressed the alpha1-PI promoter activity. In this study, 12 DNA segments, a series of 5', 3', and internal deletions of the 1.4-kb alpha1-PI promoter sequence, were ligated into the SEAP (secreted alkaline phosphatase) reporter gene vector and transfected into human corneal stromal cells. Co-transfection with a Sp1 expression vector pPacSp1 was also performed in parallel. The SEAP enzyme activity was assayed. A fragment with 489 bp (-480/+9) of the 3' sequence, and three fragments with internal deletions, were found to confer a majority of the full promoter activity. Other deletions significantly abolished the promoter activity. Site-directed mutagenesis experiments further revealed that the most proximal Sp1 site (-100/-87) may be an essential element involved in the negative regulation of alpha1-PI promoter activity by Sp1. Interaction between the proximal and distal Sp1 sites also seemed to be important. These results provide the first in-depth characterization of the transcription mechanisms regulating the expression of alpha1-PI. Mapping of the Sp1 sites may help elucidate the molecular pathway leading to the alterations observed in keratoconus.

Characterization of zebrafish metallothionein gene promoter in a zebrafish caudal fin cell-line, SJD. 1

Metallothioneins (MTs) are low molecular weight metal binding proteins that are implicated in heavy metal detoxification processes, and their quantification are commonly used as biomarkers of exposure to heavy metal ions. To verify the use of zebrafish MT gene promoter for aquatic toxicology studies, an 835 bp zebrafish MT (zMT) gene promoter with four putative metal regulatory elements (MREs) was isolated by DNA walking method and characterized for metal induction of gene transcription using transient expression assays in a zebrafish caudal fin cell line, SJD.1. The cloned promoter was linked upstream to a luciferase reporter gene and found to be responsive to administrations of Zn2+, Cd2+, and Cu2+ in SID.1 cell-line, but Hg2+, Ni2+, Pb2+, and Co2+ showed no significant induction of zMT gene transcription.

Activation of the metallothionein IIA promoter and other key stress response elements by ursodeoxycholate in HepG2 cells: relevance to the cytoprotective function of ursodeoxycholate

Ursodeoxycholate, used to treat a variety of pathologies, has the ability to reverse cytotoxic and hepatotoxic conditions. We examined HepG2, a hepatic cell line, treated with increasing levels of ursodeoxycholate, for responses of a range of promoters/response elements responsive to DNA damage, heavy metal ions, protein denaturants, aromatic hydrocarbons, retinoids, changes in intracellular AMP levels, end endoplasmic reticulum stress. The metallothionein IIA promoter was the most highly activated by ursodeoxycholate. Since ursodeoxycholate protects against the cytotoxic effects of deoxycholate, our data, combined with observations made by others, implicate metallothionein IIA as being important in this protective pathway.

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.

Metal response element (MRE)-binding transcription factor-1 (MTF-1): structure, function, and regulation

Metal-responsive control of the expression of genes involved in metal metabolism and metal homeostasis allows an organism to tightly regulate the free or bioavailable concentration of beneficial metal ions, such as zinc, copper, and iron, within an acceptable range, while efficiently removing nonbeneficial or toxic metals. Emerging evidence also suggests that metal homeostasis is intimately coupled to the oxidative stress response in many cell types. The expression of genes that encode metallothioneins in all vertebrate cells is strongly induced by potentially toxic concentrations of zinc and cadmium, as well as in response to strong oxidizing agents, including hydrogen peroxide. This induction requires a cis-acting DNA element, termed a metal response element (MRE), and MRE-binding transcription factor-1 (MTF-1), a Cys2-His2 zinc finger protein. This review summarizes recent progress that has been made toward understanding the structure, function, and metalloregulation of mammalian MTF-1.