Metallothioneins (MTs) are small cysteine-rich proteins whose structure is conserved from fungi to man. MTs strongly bind heavy metals, notably zinc, copper and cadmium. Upon exposure of cells to heavy metal and other adverse treatments, MT gene transcription is strongly enhanced. Metal induction is mediated by several copies of a 15 bp consensus sequence (metal-responsive element, MRE) present in the promoter region of MT genes. The presence of an MRE-binding factor has been demonstrated in HeLa cell nuclear extracts. This factor, termed MTF-1 (MRE-binding transcription factor) is inactivated/reactivated in vitro by zinc withdrawal/addition. The amounts of MTF-1-DNA complexes are elevated several-fold in zinc-treated cells, as measured by bandshift assay. The cDNA of mouse MTF-1, a 72.5 kDa protein, was cloned. MTF-1 contains six zinc fingers and separate transcriptional activation domains with high contents of acidic and proline residues. Ectopic expression of MTF-1 in primate or rodent cells strongly enhances transcription of a reporter gene that is driven by four consensus MREd sites, or by the complete mouse MT-I promoter, even at normal zinc levels (Radtke, 1993).
Metallothioneins (MTs) are small cysteine-rich proteins that bind heavy metal ions such as zinc, cadmium and copper with high affinity, and have been functionally implicated in heavy metal detoxification and radical scavenging. Transcription of metallothioneins genes is induced by exposure of cells to heavy metals. This induction is mediated by metal-responsive promoter elements (MREs). The cDNA of an MRE-binding transcription factor (MTF-1) from the mouse has been cloned. This study presents the human cDNA equivalent of this metal-regulatory factor. Human MTF-1 is a protein of 753 amino acids with 93% amino acid sequence identity to mouse MTF-1 and has an extension of 78 amino acids at the C-terminus without counterpart in the mouse. The factors of both species have the same overall structure including six zinc fingers in the DNA binding domain. The human MTF-1 gene has been mapped to human chromosome 1 where it localizes to the short arm in the region 1p32-34, most likely 1p33. Both human and mouse MTF-1 when produced in transfected mammalian cells strongly bind to a consensus MRE of metallothionein promoters. However, human MTF-1 is more effective than the mouse MTF-1 clone in mediating zinc-induced transcription (Brugnera, 1994).
The pufferfish Fugu rubripes 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. 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, was isolated. 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 (Auf der Maur, 1999).
The metal response element (MRE)-binding transcription factor-1, MTF-1, is a zinc-responsive protein that controls transcription of metallothionein (MT) genes in many cell types. In addition, MTF-1 is also hypothesized to regulate transcription of a battery of genes involved in the defense against oxidative stress. Manipulating the Zn concentration in the low microM range reversibly modulates the DNA-binding activity of the mammalian MTF-1; this effect is inhibited at low temperature. This report examines the presence and binding properties of MTF-1 in cell lines derived from warm- and cold-water fishes (zebrafish and trout, respectively). Both species of fish express MRE-specific binding activities that are immunologically similar to mouse MTF-1. MTF-1-binding from the cells of both species of fish was activated when cells were treated with Zn but not with Cd. Zebrafish cells contained a single isoform of MTF-1 with binding properties similar to mammalian MTF-1. Trout cells, in contrast, contained two isoforms of MTF-1: MTF-1H and MTF-1L. Zn reversibly modulates MTF-1H binding in a temperature-dependent manner. Similarly, Zn reversibly modulates MTF-1L binding, but, in contrast, such modulation occurs readily at 4 degrees C. This data demonstrates the conservation of binding specificity, binding properties, and regulation of MTF-1 in fishes (Dalton, 2000).
The human metalloregulatory transcription factor, metal-response element (MRE)-binding transcription factor-1 (MTF-1), contains six TFIIIA-type Cys(2)-His(2) motifs, each of which was projected to form well-structured betabetaalpha domains upon Zn(II) binding. This study investigated the structure and backbone dynamics of a fragment containing the unusual C-terminal fingers F4-F6. (15)N heteronuclear single quantum coherence (HSQC) spectra of uniformly (15)N-labeled hMTF-zf46 show that Zn(II) induces the folding of hMTF-zf46. Analysis of the secondary structure of Zn(3) hMTF-zf46 determined by (13)Calpha chemical shift indexing and the magnitude of (3)J(Halpha-HN) clearly reveal that zinc fingers F4 and F6 adopt typical betabetaalpha structures. An analysis of the heteronuclear backbone (15)N relaxation dynamics behavior is consistent with this picture and further reveals independent tumbling of the finger domains in solution. Titration of apo-MTF-zf46 with Zn(II) reveals that the F4 domain binds Zn(II) significantly more tightly than do the other two finger domains. In contrast to fingers F4 and F6, the ßßalpha fold of finger F5 is unstable and only partially populated at substoichiometric Zn(II); a slight molar excess of zinc results in severe conformational exchange broadening of all F5 NH cross-peaks. Finally, although Cd(II) binds to apo-hMTF-zf46 as revealed by intense S(-)-->Cd(II) absorption, a non-native structure results; addition of stoichiometric Zn(II) to the Cd(II) complex results in quantitative refolding of the betabetaalpha structure in F4 and F6. The functional implications of these results are discussed (Giedrock 2001a).
Metal-response element (MRE)-binding transcription factor-1 (MTF-1) is a zinc-regulated transcriptional activator of metallothionein (MT) genes in mammalian cells. The MRE-binding domain of MTF-1 (MTF-zf) has six canonical Cys(2)-His(2) zinc finger domains that are distinguished on the basis of their apparent affinities for zinc and their specific roles in MRE-binding. In this paper, pulsed alkylation of the zinc-liganding cysteine thiolate pairs with the sulfhydryl-specific alkylating reagent d(5)-N-ethylmaleimide (d(5)-NEM) is used as a residue-specific probe of the relative stabilities of the individual zinc finger coordination complexes in Zn(6) MTF-zf. A chase with excess H(5)-N-ethylmaleimide (H(5)-NEM) to fully derivatize MTF-zf concomitant with complete proteolysis, followed by MALDI-TOF mass spectrometry allows quantitation of the mole fraction of d(5),d(5)-, d(5),H(5)-, and H(5),H(5)-NEM derivatized peptides corresponding to each individual zinc finger domain as a function of d(5)-NEM pulse time. This experiment establishes the hierarchy of cysteine thiolate reactivity in MTF-zf as F5 > F6 >> F1 > F2 approximately F3 approximately F4. The apparent second-order rate of reaction of F1 thiolates is comparable to that determined for the DNA binding domain of Sp1, Zn(3) Sp1-zf, under identical solution conditions. The reactivities of all Cys residues in MTF-zf are significantly reduced when bound to an MREd-containing oligonucleotide. An identical experiment carried out with Zn(5) MTF-zf26, an MTF-zf domain lacking the N-terminal F1 zinc finger, reveals that MTF-zf26 binds to the MREd very weakly, and is characterized by strongly increased reactivity of nonadjacent F4 thiolates. These findings are discussed in the context of existing models for metalloregulation by MTF-1 (Apuy, 2001).
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 (Chen, 2004).
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 ßßalpha 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 (Potter, 2005).
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, the potential role of the peptide linkers connecting the four N-terminal zinc fingers of MTF-1 were investigated 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 abolishes 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 has 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 renders 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 (Li, 2006).
MTF-1 binds specifically to heavy metal-responsive DNA sequence elements in the enhancer/promoter region of metallothionein genes. MTF-1 is a protein of 72.5 kDa that contains six zinc fingers and multiple domains for transcriptional activation. This study reports the disruption of both alleles of the MTF-1 gene in mouse embryonic stem cells by homologous recombination. The resulting null mutant cell line fails to produce detectable amounts of MTF-1. Moreover, due to the loss of MTF-1, the endogenous metallothionein I and II genes are silent, indicating that MTF-1 is required for both their basal and zinc-induced transcription. In addition to zinc, other heavy metals, including cadmium, copper, nickel and lead, also fail to activate metal-responsive promoters in null mutant cells. However, cotransfection of an MTF-1 expression vector and metal-responsive reporter genes yields strong basal transcription that can be further boosted by zinc treatment of cells. These results demonstrate that MTF-1 is essential for metallothionein gene regulation. Finally, evidence is presented that MTF-1 itself is a zinc sensor, which exhibits increased DNA binding activity upon zinc treatment (Heuchel, 1994).
The heavy metal-responsive transcriptional activator MTF-1 regulates the basal and heavy metal-induced expression of metallothioneins. To investigate the physiological function of MTF-1, null mutant mice were generated by targeted gene disruption. Embryos lacking MTF-1 die in utero at approximately day 14 of gestation. They show impaired development of hepatocytes and, at later stages, liver decay and generalized edema. MTF-1(-/-) embryos fail to transcribe metallothionein I and II genes, and also show diminished transcripts of the gene which encodes the heavy-chain subunit of the gamma-glutamylcysteine synthetase, a key enzyme for glutathione biosynthesis. Metallothionein and glutathione are involved in heavy metal homeostasis and detoxification processes, such as scavenging reactive oxygen intermediates. Accordingly, primary mouse embryo fibroblasts lacking MTF-1 show increased susceptibility to the cytotoxic effects of cadmium or hydrogen peroxide. Thus, MTF-1 may help to control metal homeostasis and probably cellular redox state, especially during liver development. It is also noted that the MTF-1 null mutant phenotype bears some similarity to those of two other regulators of cellular stress response, namely c-Jun and NF-kappaB (p65/RelA) (Günes, 1998).
Metal-responsive transcription factor-1 (MTF-1) activates the transcription of metallothionein genes and other target genes in response to heavy metal load and other stresses such as hypoxia and oxidative stress. It also has an essential function during embryogenesis: targeted disruption of Mtf1 in the mouse results in lethal liver degeneration on day 14 of gestation. Mtf1 knockout mice were studied at embryonic and adult stages, the latter by means of conditional knockout. Hepatocytes from Mtf1 null mutant and wild-type embryos were taken into culture on day 12.5 of gestation. Both initially appeared normal, but mutant cells were lost within a few days. Furthermore, Mtf1 null hepatocytes were poorly, if at all, rescued by cocultivation with wild-type rat embryo hepatocytes, indicating a cell-autonomous defect. When the Mtf1 gene was excised by Cre recombinase after birth in liver and bone marrow and to a lesser extent in other organs, mice were viable under non-stress conditions but highly susceptible to cadmium toxicity, in support of a role of MTF-1 in coping with heavy metal stress. An additional MTF-1 function was revealed upon analysis of the hematopoietic system in conditional knockout mice where leukocytes, especially lymphocytes, were found to be severely underrepresented. Together, these findings point to a critical role of MTF-1 in embryonic liver formation, heavy metal toxicity, and hematopoiesis (Wang, 2004).
Genetic predisposition to cancers is significant to public health because a high proportion of cancers probably arise in a susceptible human subpopulation. Using a mouse model of gamma-ray-induced thymic lymphomas, linkage analysis and haplotype mapping were performed that suggested Mtf-1, metal-responsive transcription factor-1 (Mtf-1), as a candidate lymphoma susceptibility gene. Sequence analysis revealed a polymorphism of Mtf-1 that alters the corresponding amino acid at position 424 in the proline-rich domain from a serine in susceptibility strains to proline in resistant strains. The transcriptional activity of Mtf-1 encoding serine and proline was compared by transfecting the DNA to Mtf-1-null cells, and the change to proline conferred a higher metal responsiveness in transfections. Furthermore, the resistant congenic strains possessing the Mtf-1 allele of proline type exhibited higher radiation inducibility of target genes than susceptible background strains having the Mtf-1 allele of serine type. Since products of the targets such as metallothionein are able to suppress cellular stresses generated by irradiation, these results suggest that highly inducible strains having Mtf-1 of proline type are refractory to radiation effects and hence are resistant to lymphoma development (Tamura, 2005).
Metallothionein genes are transcriptionally regulated by a number of inducers including heavy metals. Previous mutational analyses of the mouse metallothionein-I gene (mMTI) promoter have delineated a heavy-metal regulatory region between -60 and -42 relative to the transcription start site. A synthetic copy of a 12-base-pair (bp) conserved sequence located within this region confers heavy-metal regulation on a heterologous gene. However, specific disruption of this metal regulatory element (MRE) within a wild-type mMTI promoter reduces but does not eliminate the heavy-metal response. The additional metal regulatory activity is localized to an upstream region containing four sequences homologous to the identified MRE. Similar sequences are also found in multiple copies in metallothionein genes from other species. Synthetic copies of all five mMTI MRE homologues were tested for metal regulatory activity. At least four of these sequences are able to confer heavy-metal regulation on a heterologous promoter (Stuart, 1985).
Heavy metal ions are effective inducers of metallothionein gene transcription. The metal response is dependent on short DNA motifs, so-called MREs (metal responsive elements) that occur in multiple copies in the promoter region of these genes. This study analysed an MRE of the mouse metallothionein-I gene (MREd); this can function over long distances as a bona fide metal ion-inducible enhancer. The transcription factor Sp1 and a zinc-inducible factor, designated MTF-1, bind to the MREd enhancer in vitro. The combined use of MREd mutants in a transient assay in HeLa cells and a competition band shift assay show that the zinc-inducible formation of the MTF-1/DNA complex in vitro correlates with zinc-inducible transcription in vivo. A chemical methylation interference assay revealed remarkably similar but non-identical guanine interference patterns for the MTF-1 and Sp1 complexes, which may mean that MTF-1 is related to the Sp1 factor (Westin, 1988).
Metallothioneins (MTs) are a family of stress-induced proteins with diverse physiological functions, including protection against metal toxicity and oxidants. They may also contribute to the regulation of cellular proliferation, apoptosis, and malignant progression. The human (h)MT-IIA isoform is induced in carcinoma cells (A431, SiHa, and HT29) exposed to low oxygen, conditions commonly found in solid tumors. The present study demonstrates that the genes for hMT-IIA and mouse (m)MT-I are transcriptionally activated by hypoxia through metal response elements (MREs) in their proximal promoter regions. These elements bind metal transcription factor-1 (MTF-1). Deletion and mutational analyses of the hMT-IIA promoter indicate that the hMRE-a element is essential for basal promoter activity and for induction by hypoxia, but that other elements contribute to the full transcriptional response. Functional studies of the mMT-I promoter demonstrate that at least two other MREs (mMRE-d and mMRE-c) are responsive to hypoxia. Multiple copies of either hMRE-a or mMRE-d confer hypoxia responsiveness to a minimal MT promoter. Mouse MT-I gene transcripts in fibroblasts with targeted deletions of both MTF-1 alleles (MTF-1-/-; dko7 cells) are not induced by zinc and show low responsiveness to hypoxia. A transiently transfected MT promoter is unresponsive to hypoxia or zinc in dko7 cells, but inductions are restored by cotransfecting a mouse MTF-1 expression vector. Electrophoretic mobility shift assays detect a specific protein-DNA complex containing MTF-1 in nuclear extracts from hypoxic cells. Together, these results demonstrate that hypoxia activates MT gene expression through MREs and that this activation involves MTF-1 (Murphy, 1999).
Metal regulation of the mouse zinc transporter (ZnT)-1 gene was examined in cultured cells and in the developing conceptus. Zinc or cadmium treatment of cell lines rapidly (3 h) and dramatically (about 12-fold) induced ZnT1 mRNA levels. In cells incubated in medium supplemented with Chelex-treated fetal bovine serum, to remove metal ions, levels of ZnT1 mRNA are reduced, and induction of this message in response to zinc or cadmium is accentuated (up to 31-fold induction). Changes in ZnT1 gene expression in these experiments parallel those of metallothionein I (MT-I). Inhibition of RNA synthesis blocks metal induction of ZnT1 and MT-I mRNAs, whereas inhibition of protein synthesis does not. Metal response element-binding transcription factor (MTF)-1 mediates metal regulation of the metallothionein I gene. In vitro DNA-binding assays demonstrate that mouse MTF-1 can bind avidly to the two metal-response element sequences found in the ZnT1 promoter. Using mouse embryo fibroblasts with homozygous deletions of the MTF-1 gene, it was shown that this transcription factor is essential for basal as well as metal (zinc and cadmium) regulation of the ZnT1 gene in these cells. In vivo, ZnT1 mRNA is abundant in the midgestation visceral yolk sac and placenta. Dietary zinc deficiency during pregnancy down-regulates ZnT1 and MT-I mRNA levels (4-5-fold and >20-fold, respectively) in the visceral yolk sac, but has little effect on these mRNAs in the placenta. Homozygous knockout of the MTF-1 gene in transgenic mice also leads to a 4-6-fold reduction in ZnT1 mRNA levels and a loss of MT-I mRNA in the visceral yolk sac. These results suggest that MTF-1 mediates the response to metal ions of both the ZnT1 and the MT-I genes the visceral yolk sac. Overall, these studies suggest that MTF-1 directly coordinates the regulation of genes involved in zinc homeostasis and protection against metal toxicity (Langmade, 2000).
The metallothioneins (MT) are small, cysteine-rich heavy metal-binding proteins that participate in an array of protective stress responses. Although a single essential function of MT has not been demonstrated, MT of higher eukaryotes evolved as a mechanism to regulate zinc levels and distribution within cells and organisms. These proteins can also protect against some toxic metals and oxidative stress-inducing agents. In mice, among the four known MT genes, the MT-I and -II genes are most widely expressed. Transcription of these genes is rapidly and dramatically up-regulated in response to zinc and cadmium, as well as in response to agents which cause oxidative stress and/or inflammation. The six zinc-finger metal-responsive transcription factor MTF-1 plays a central role in transcriptional activation of the MT-I gene in response to metals and oxidative stress. Mutation of the MTF-1 gene abolishes these responses, and MTF-1 is induced to bind to the metal response elements in proximal MT promoter in cells treated with zinc or during oxidative stress. The exact molecular mechanisms of action of MTF-1 are not fully understood. These studies suggest that the DNA-binding activity of MTF-1 in vivo and in vitro is reversibly activated by zinc interactions with the zinc-finger domain. This reflects heterogeneity in the structure and function of the six zinc fingers. It is hypothesized that MTF-1 functions as a sensor of free zinc pools in the cell. Changes in free zinc may occur in response to chemically diverse inducers. MTF-1 also exerts effects on MT-I gene transcription which are independent of a large increase in MTF-1 DNA-binding activity. For example, cadmium, which has little effect on the DNA-binding activity of MTF-1 in vivo or in vitro, is a more potent inducer of MT gene expression than is zinc. The basic helix-loop-helix-leucine zipper protein, USF (upstream stimulatory factor family), also plays a role in regulating transcription of the mouse MT-I gene in response to cadmium or H2O2. Expression of dominant negative USF-1 or deletion of its binding site from the proximal promoter attenuates induction of the mouse MT-I gene. USF apparently functions in this context by interacting with as yet unidentified proteins which bind to an antioxidant response element which overlaps the USF-binding site (USF/ARE). Interestingly, this composite element does not participate in the induction of MT-I gene transcription by zinc or redox-cycling quinones. Thus, regulation of the mouse MT-I gene by metals and oxidative stress involves multiple signaling pathways which depend on the species of metal ion and the nature of the oxidative stress (Andrews, 2001).
Coordinate regulation of the ribosomal protein genes is entrusted to a number of signal transduction pathways that can abruptly induce or silence the ribosomal genes. A cellular model system has been uncovered that selectively induces the ribosomal protein S25 gene in hepatoma cells that are stressed by nutrient deprivation. The results indicate that p53 along with two other identified proteins, MTF-1 and La, post-transcriptionally regulate the synthesis of the S25 protein by controlling the nuclear export of the stress-induced S25 mRNA. This system is unique in that the nuclear-retained S25 mRNA is exported to the cytosol only upon replenishment or alternatively after prolonged starvation to participate in a p53-mediated apoptotic sequence of events. This p53-dependent survival or death pathway involves a previously unreported protein relationship among these three actors, one of which, MTF-1, has not yet been shown to have RNA-binding characteristics (Adilakshmi, 2002).
Metal-responsive transcription factor 1 (MTF-1) regulates expression of its target genes in response to various stress conditions, notably heavy metal load, via binding to metal response elements (MREs) in the respective enhancer/promoter regions. Furthermore, it serves a vital function in embryonic liver development. However, targeted deletion of Mtf1 in the liver after birth is no longer lethal. For this study, Mtf1 conditional knockout mice and control littermates were both mock- or cadmium-treated and liver-specific transcription was analyzed. Besides the well-characterized metallothionein genes, several new MTF-1 target genes with MRE motifs in the promoter region emerged. MTF-1 is required for the basal expression of selenoprotein W, muscle 1 gene (Sepw1) that encodes a glutathione-binding and putative antioxidant protein, supporting a role of MTF-1 in the oxidative stress response. Furthermore, MTF-1 mediates the cadmium-induced expression of N-myc downstream regulated gene 1 (Ndrg1), which is induced by several stress conditions and is overexpressed in many cancers. MTF-1 is also involved in the cadmium response of cysteine- and glycine-rich protein 1 gene (Csrp1), which is implicated in cytoskeletal organization. In contrast, MTF-1 represses the basal expression of Slc39a10, a putative zinc transporter. In a pathway independent of MTF-1, cadmium also induced the transcription of genes involved in the synthesis and regeneration of glutathione, a cadmium-binding antioxidant. These data provide strong evidence for two major branches of cellular anti-cadmium defense, one via MTF-1 and its target genes, notably metallothioneins, the other via glutathione, with an apparent overlap in selenoprotein W (Wimmer, 2005).
Placenta growth factor (PlGF) is a member of the vascular endothelial growth factor family of cytokines that control vascular and lymphatic endothelium development. It has been implicated in promoting angiogenesis in pathological conditions via signaling to vascular endothelial growth factor receptor-1. PlGF expression is induced by hypoxia and proinflammatory stimuli. Metal responsive transcription factor 1 (MTF-1) was shown to take part in the hypoxic induction of PlGF in Ras-transformed mouse embryonic fibroblasts. PlGF expression is also controlled by NF-kappaB. Several putative binding sites for NF-kappaB were identifed in the PlGF promoter/enhancer region by sequence analyses, and binding and transcriptional activity of NF-kappaB p65 was demonstrated at these sites. Expression of NF-kappaB p65 from a plasmid vector in HEK293 cells causes a substantial increase of PlGF transcript levels. Furthermore, hypoxic conditions induce nuclear translocation and interaction of MTF-1 and NF-kappaB p65 proteins, suggesting a role for this complex in hypoxia-induced transcription of PlGF (Cramer, 2006).
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