A nuclear factor that recognizes the metal-responsive elements of human metallothionein IIA gene

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

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

Co-treatment with sulforaphane–zein microparticles enhances the chemopreventive potential of zinc in a 1,2-dimethylhydrazine induced colon carcinogenesis rat model

Hypothesis of the study was that zinc and sulforaphane will act on common targets MT and Nrf2 in colon, increasing their intracellular levels leading to improvement in DMH induced oxidative stress thereby hindering the process of colon carcinogenesis in rat.

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

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

Induction, regulation, degradation, and biological significance of mammalian metallothioneins

MTs are small cysteine-rich metal-binding proteins found in many species and, although there are differences between them, it is of note that they have a great deal of sequence and structural homology. Mammalian MTs are 61 or 62 amino acid polypeptides containing 20 conserved cysteine residues that underpin the binding of metals. The existence of MT across species is indicative of its biological demand, while the conservation of cysteines indicates that these are undoubtedly central to the function of this protein. Four MT isoforms have been found so far, MT-1, MT-2, MT-3, and MT-4, but these also have subtypes with 17 MT genes identified in man, of which 10 are known to be functional. Different cells express different MT isoforms with varying levels of expression perhaps as a result of the different function of each isoform. Even different metals induce and bind to MTs to different extents. Over 40 years of research into MT have yielded much information on this protein, but have failed to assign to it a definitive biological role. The fact that multiple MT isoforms exist, and the great variety of substances and agents that act as inducers, further complicates the search for the biological role of MTs. This article reviews the current knowledge on the biochemistry, induction, regulation, and degradation of this protein in mammals, with a particular emphasis on human MTs. It also considers the possible biological roles of this protein, which include participation in cell proliferation and apoptosis, homeostasis of essential metals, cellular free radical scavenging, and metal detoxification.

Molecular analyses of metallothionein gene regulation

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

Regulation of ZiRF1 and basal SP1 transcription factor MRE-binding activity by transition metals

The metal-dependent activation of metallothionein (MT) genes requires the interaction of positive trans-activators (MRFs) with metal-regulatory (MRE) regions of MT promoters. In this report, we examined the role of transition metals in modulating the MRE-binding activities of two different MRE-binding proteins: the metal-regulated factor ZiRF1 and the basal factor SP1. We showed the ability of both proteins to interact with a similar sequence specificity with the cognate target site (MRE-S) of another known MRE-binding protein, mMTF1. We next evaluated the role of metal ions in modulating the MRE-binding activity of recombinant ZiRF1 and basal SP1 proteins by measuring the effect of different metal chelators on DNA interaction. We observed a dose-dependent inhibition of the GST-ZiRF1/MRE-binding activity using three different metal chelators: EDTA, 1,10 PHE and TPEN. Interestingly, EDTA treatment failed to inhibit the recombinant SP1 MRE-binding activity while the effect of 1,10 PHE was comparable to that obtained analyzing 1,10 PHE-treated GST-ZiRF1. The MRE-binding complexes detected in cell extracts showed a response to metal chelator treatment very similar to that displayed by the recombinant ZiRF1 and SP1 proteins. The hypothesis of mutual interactions of both basal and metal-regulated transcription factors with the same metal-regulatory regions is discussed.

Purification and characterization of a rat liver protein that recognizes CCAAT-homologous sequences of the metallothionein promoter and trans-activates this promoter

C'BP-1, a protein that binds to the MRE-c' region (-135 to -110) of the mouse metallothionein-I (MT-I) gene in metal-independent manner, was purified from rat liver nuclear extract by ion exchange and affinity chromatography. Analysis by SDS-PAGE, UV cross-linking, and glycerol gradient sedimentation, taken together, showed that C'BP-1 is a dimer of the 34-kDa polypeptides. Affinity-purified C'BP-1 could significantly stimulate transcription from mouse MT-I gene promoter. DNase I footprinting with the purified protein identified two binding sites for C'BP-1 located at positions -135 to -100 and -210 to -175 with respect to the start site of MT-I gene transcription. Both C'BP-1 binding sequences were found to contain imperfect dyad of the CCAAT homology. C'BP-1 was shown to make critical contacts with the CCAAT homology by methylation interference analysis and competition electrophoretic mobility shift assay with mutants harboring alterations in the CCAAT homology. An antibody that specifically recognizes C/EBP delta partially supershifted C'BP-1/MRE-c' complex, suggesting that C'BP-1 is identical to C/EBP delta or is closely related to C/EBP delta.

Biphasic response of the metallothionein promoter to ultraviolet radiation in human melanoma cells

Because metallothionein (MT) is elevated and may be protective in UV-irradiated skin, we have studied the effects of UV and other agents on MT transcription using the sheep MT 1A promoter, linked to the beta-galactosidase gene and stably transfected into human cell lines. beta-galactosidase reporter activity was inducible by adding Zn2+ ions to the medium (100 microM for 2-4 h). Two differentiating agents, butyric acid and azelaic bishydroxamic acid (ABHA), significantly increased the response to Zn2+ in a melanoma cell line (MM96L-gal). UVB (280-315 nm) had two distinct, time-dependent effects. During the first 4 h after irradiation, high doses of UVB inhibited induction by Zn2+, an effect that was made more acute by simultaneous exposure to the differentiating agents. These changes in reporter activity were not due to alterations in Zn2+ transport into the cell. The UVB-depressed MT response subsequently recovered and by 24 h was double the control, yet remained sensitive to ABHA. Reporter activity in transfected HeLa cells differed from that in MM96L, being depressed 4 and 24 h after UVB and insensitive to ABHA at both times. Galactosidase reporter activity driven by non-MT promoters was not affected by these treatments. Dependence of MT transcriptional activity on UV-related DNA damage could be inferred because equitoxic UVC (254 nm) affected the response to Zn2+ in a similar fashion, whereas UVA, cisplatin and a methylating agent had no effect. The MT response was partly dependent on the PKC signal transduction pathway because it was inhibited by phorbol ester in HeLa, and by bisindolyl maleimide in HeLa and MM96L. The biphasic MT transcriptional response may model a signal transduction pathway that gives an early, depressed response to acute UV damage, with exacerbation by concurrent differentiation stimuli, but switches to a positive, cell-specific and potentially protective response at later times.

Metallothionein in physiological and physiopathological processes

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

Regulation of metallothionein gene expression. Studies in transfected primary cultures of chick embryo liver cells

To study the regulation of expression of the metallothionein gene in normal liver cells, we transfected chick embryo liver cells in primary cultures with constructs containing luciferase or chloramphenicol acetyl transferase (as reporter genes) under the control of differing lengths of the 5'-promoter region of the chick metallothionein gene (containing 30, 122, 190, or 623 base pairs upstream of the transcriptional start site). We controlled for efficiency of transfection by co-transfections with a plasmid containing a bacterial beta-galactosidase gene under the control of the SV 40 promoter and enhancer. Treatment of the transfected cells with transition metallic ions (cadmium, cobalt, and zinc) or sodium arsenite produced increases in activities of luciferase or chloramphenicol acetyl transferase, relative to beta-galactosidase, and this activity mapped to the first 122 base pairs of the promoter. Although heme has recently been reported to induce the endogenous metallothionein gene in chick embryo liver cells, 10-50 microM heme did not increase reporter gene activities in transfected cells. Nevertheless, the heme-dependent induction of endogenous heme oxygenase-1 in these cells was normal. We conclude that the heme-dependent induction of the liver metallothionein gene depends upon DNA region(s) outside the regulatory region of the chick metallothionein gene studied here and that elements within the first 122 base pairs of the metallothionein promoter are sufficient to confer responsiveness to transition metals or sodium arsenite.

Metal-responsive elements of the rainbow trout metallothionein-B gene function for basal and metal-induced activity

In this study, the contributions of the two metal-responsive elements (MREs) of the rainbow trout (Salmo gairdnerii) metallothionein (tMT)-B gene promoter (-137 to +5) were analyzed. The effect of MRE mutations on the basal and zinc-induced activities of tMT-B promoter-reporter gene fusions were determined by transfection of a rainbow trout hepatoma (RTH-149) cell line. Together, MREa and MREb cooperate to elicit a significant response to zinc but exhibit differential basal and metal-induced activity. The MREa sequence (-62 to -51) is important for basal promoter activity and can function independently, whereas the more distal MREb (-89 to -100) mainly contributes to metal induction through cooperative interactions with MREa. The degree of basal character of the MREs is partially determined by nucleotide differences at the flexible position N of the MRE consensus TGC(G/A)CNC. In mouse L and HepG2 cells, MREa activity is conserved, but the contributions of the MREb region differ, including reduced cooperativity with MREa. There are also differences in the apparent molecular masses of the rainbow trout and mammalian nuclear factors that bind to the tMT-B promoter and MREa sequence.

Interactions of nuclear proteins from uninduced, induced and superinduced HeLa cells with metal regulatory elements MRE3 and 4 of the human metallothionein IIa-encoding gene

Transcriptional activation of metallothionein (MT)-encoding genes(MT) is regulated during heavy metal induction by short non-identical repeats, termed 'metal regulatory elements' (MRE), present in multiple imperfect copies in MT promoter regions of eukaryotes. Using mobility shift assays, we have studied the interaction between the human MRE 3 and 4 regions (hMRE3/4) of the MTIIa promoter and nuclear proteins from uninduced and Cd(2+)-induced HeLa cells, and from Cd(2+)-superinduced H454 cells, a HeLa-derived Cd(2+)-resistant cell isolate which overexpresses hMTIIa after exposure to metal. A specific complex with a similar electrophoretic mobility was formed in all three extracts. Dialysis of the extracts using EDTA inhibited the formation of the complexes, which could be reconstituted only after the addition of Zn2+. UV cross-linking analyses of the specific complexes formed by the three nuclear extracts interacting with the hMRE3/4 region revealed that in all of them polypeptides were present having similar electrophoretic mobilities and different molecular masses. Mobility shift assays showed no major differences in the binding of nuclear proteins from induced or uninduced cells. Proposed models of activation of metal-induced MT transcription are discussed.

The mammalian UV response: mechanism of DNA damage induced gene expression

DNA damage inducing treatment of cultured mammalian cells triggers the activation of transcription factors and the prolongation of the half life of p53. As the earliest event detectable in the nucleus (5 min), AP-1 (c-Jun/c-Fos) is post-translationally modified. Triggering this early event and triggering subsequent transcription factor dependent processes requires extra-nuclear components of signal transduction such as Src, Ras, Raf-1 and MAP-2 kinase. Recent efforts have concentrated on examining whether DNA damage or other secondary effects of the damaging agent generate the signal then passed on to transcription factors. Further, it has been studied whether a pathway of reverse signalling exists that originates in the nucleus and reaches the cell surface. At the cell surface the UV induced signalling chain can be interrupted experimentally. Beyond this step DNA damage and signal transduction induced by phorbol esters and growth factors merge and reach the nuclear proteins through common components.

Involvement of metallothionein and copper in cell proliferation

Metallothionein is a low-molecular weight, cysteine-rich, metal-binding protein which has been implicated in the detoxification of toxic metals (cadmium, mercury), metabolism of zinc and copper, as well as in the scavenging of free radicals. Recent evidence suggests that the protein may also be involved in cell proliferation. Based on the experiments carried out so far, it is assumed that the fundamental role of metallothionein in cell proliferation may be to detoxify and/or transfer copper ions from the cytoplasm to the nucleus at the G1/S phase, which in turn participate in some way in nuclear DNA synthesis.