[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.

Characterisation of the metallothionein gene in the Sydney rock oyster and its expression upon metal exposure in oysters with different prior metal exposure histories

The metal-binding protein metallothionein (MT) is widely used as a biomarker of metal contamination. In this study, we cloned a MT gene (sgMT) from the Sydney rock oyster Saccostrea glomerata. The gene encodes a MT-I protein with a classical αβ domain structure and is expressed as two transcripts resulting from alternative polyadenylation. The gene promoter contains two putative metal-responsive elements (MREs) which are known to be required for metal-inducible transcription. A specific and efficient qPCR assay was developed to quantify sgMT mRNA expression. Further, we assessed whether prior metal exposure history influences sgMT mRNA expression upon subsequent metal exposure. Oysters with varying prior metal exposure histories (contaminated and reference) were exposed to Cu, Cd and Zn. Expression of sgMT generally increased with metal dose, and oysters with an elevated past metal exposure history exhibited higher sgMT expression under Cd and Zn stress, representing a potential acclimatory response to prior metal exposure.

Specificity of the Metallothionein-1 Response by Cadmium-Exposed Normal Human Urothelial Cells

Occupational and environmental exposure to cadmium is associated with the development of urothelial cancer. The metallothionein (MT) family of genes encodes proteins that sequester metal ions and modulate physiological processes, including zinc homeostasis. Little is known about the selectivity of expression of the different MT isoforms. Here, we examined the effect of cadmium exposure on MT gene and isoform expression by normal human urothelial (NHU) cell cultures. Baseline and cadmium-induced MT gene expression was characterized by next-generation sequencing and RT-PCR; protein expression was assessed by Western blotting using isoform-specific antibodies. Expression of the zinc transporter-1 (SLC30A1) gene was also assessed. NHU cells displayed transcription of MT-2A, but neither MT-3 nor MT-4 genes. Most striking was a highly inducer-specific expression of MT-1 genes, with cadmium inducing transcription of MT-1A, MT-1G, MT-1H, and MT-1M. Whereas MT-1G was also induced by zinc and nickel ions and MT-1H by iron, both MT-1A and MT-1M were highly cadmium-specific, which was confirmed for protein using isoform-specific antibodies. Protein but not transcript endured post-exposure, probably reflecting sequestration. SLC30A1 transcription was also affected by cadmium ion exposure, potentially reflecting perturbation of intracellular zinc homeostasis. We conclude that human urothelium displays a highly inductive profile of MT-1 gene expression, with two isoforms identified as highly specific to cadmium, providing candidate transcript and long-lived protein biomarkers of cadmium exposure.

Expression of genes related to metal metabolism in the freshwater fish Hyphessobrycon luetkenii living in a historically contaminated area associated with copper mining

Copper (Cu) mining in Minas do Camaquã-Brazil, released significant amounts of metals into the João Dias creek, where Hyphessobrycon luetkenii inhabit. Because the involvement of Cu in biological processes its concentration and availability is regulated by molecules as the metal regulatory transcription factor (MTF-1), metallothionein (MT) and transporters (ATP7A and CTR1). These genes were whole sequenced and their expression (GE) evaluated in gills, liver and intestine. Were collected fish in non-contaminated and contaminated (Cu 3.4-fold higher) sites of the creek (CC and PP) and respectively translocated (CP and PC) for 96 h. The GE of the non-translocated groups evidenced that MT, MTF-1 and CTR1 have organ specific differences between both communities. Additionally the translocation allowed to identify organ specific changes associated with the activation/inactivation of protective mechanisms. These findings indicate that MTF-1, MT and CTR-1 GE play an important role in the tolerance of H. luetkenii to Cu contamination.

Optimization of ZnO-NPs to Investigate Their Safe Application by Assessing Their Effect on Soil Nematode Caenorhabditis elegans

Zinc oxide nanoparticles (ZnO-NPs) are increasingly receiving attention due to their widespread application in cosmetics, pigments and coatings. This has raised concerns in the public and scientific communities regarding their unexpected health effects. Toxicity effect of ZnO-NPs on the environment was assessed in the present study using Caenorhabditis elegans. Multiple toxicity end points including their mortality, behaviour, reproduction, in vitro distribution and expression of stress response mtl-1 and sod-1 genes were observed to evaluate safe application of ZnO-NPs. C. elegans were exposed to 10, 50, and 100 nm ZnO-NPs (0.1 to 2.0 g/l). Application of 10 nm ≥0.7g/l adversely affects the survivability of worms and was significantly not affected with exposure of 50 and 100 nm ≤1.0 g/l. However, reproduction was affected at much low concentration as compared to their survivability. LC50 was recorded 1.0 ± 0.06 (g/l) for 100 nm, 0.90 ± 0.60 for 50 nm and 0.620 ± 0.08 for 10 nm. Expression of mtl-1 and sod-1 was significantly increased with application of 10 nm ≥0.7g/l and significantly unaffected with exposure of 50 and 100 nm at the same concentration. ZnO-NPs (10 nm) had shown even distribution extended nearly the entire length of the body. The distribution pattern of ZnO-NPs indicates that the intestine is the major target tissues for NP toxicity. Study demonstrates that small-sized (10 nm) ZnO-NPs ≥0.7g/l is more toxic than larger-sized particles. This may be suggested on the basis of available data; application of 50 and 100 nm ≤1.0 g/l ZnO-NPs may be used to the environment as this shows no significant toxicity. However, further calibration is warranted to explore safe dose on soil compartments prior to their field application.

Breaking-off tissue specific activity of the oil palm metallothionein-like gene promoter in T(1) seedlings of tomato exposed to metal ions

Metallothioneins (MTs) are cysteine-rich metal-binding proteins that are involved in cell growth regulation, transportation of metal ions and detoxification of heavy metals. A mesocarp-specific metallothionein-like gene (MT3-A) promoter was isolated from the oil palm (Elaeis guineensis Jacq). A vector construct containing the MT3-A promoter fused to the β-glucuronidase (GUS) gene in the pCAMBIA 1304 vector was produced and used in Agrobacterium-mediated transformation of tomato. Histochemical GUS assay of different tissues of transgenic tomato showed that the MT3-A promoter only drove GUS expression in the reproductive tissues and organs, including the anther, fruit and seed coat. Competitive RT-PCR and GUS fluorometric assay showed changes in the level of GUS mRNA and enzyme activity in the transgenic tomato (T(0)). No GUS mRNA was found in roots and leaves of transgenic tomato. In contrast, the leaves of transgenic tomato seedlings (T(1)) produced the highest GUS activity when treated with 150 μM Cu(2+) compared to the control (without Cu(2+)). However, Zn(2+) and Fe(2+) treatments did not show GUS expression in the leaves of the transgenic tomato seedlings. Interestingly, the results showed a breaking-off tissue-specific activity of the oil palm MT3-A promoter in T(1) seedlings of tomato when subjected to Cu(2+) ions.

The toxicity redox mechanisms of cadmium alone or together with copper and zinc homeostasis alteration: its redox biomarkers

Cadmium (Cd) is a toxic metal and can induce and/or promote diseases in humans (cancer, aging diseases, kidney and bone diseases, etc.). Its toxicity involves many mechanisms including the alteration of copper (Cu) and zinc (Zn) homeostasis leading to reactive oxygen species (ROS) production, either directly or through the inhibition of antioxidant activities. Importantly, ROS can induce oxidative damages in cells. Cadmium, Cu and Zn are also able to induce glutathione (GSH) and metallothioneins (MT) synthesis in a cell-type-dependent manner. As a consequence, the effects induced by these three metals result simultaneously from the inhibition of antioxidant activities and the induction of other factors such as GSH and MT synthesis. MT levels are regulated not only by the p53 protein in a cell-type-dependent manner, or by transcription factors such as metal-responsive transcription factor 1 (MTF-1) and cellular Zn levels but also by cellular GSH level. As described in the literature, DNA damage, GSH and MT levels are sensitive biomarkers used to identify Cd-induced toxicity alone or together with Cu and Zn homeostasis alteration.

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.

Metallothionein expression in HaCaT and C6 cell lines exposed to cadmium

Metallothioneins (MT) are low-molecular weight, cysteine-rich metal-binding proteins. MT play a role in the homeostasis of essential metals such as zinc (Zn) and copper (Cu), detoxification of toxic metals such as cadmium (Cd) and protection against oxidative stress. In this study, we examined the expression of MT in HaCaT and C6 cells as a strategy to enhance protection against Cd-mediated toxicity. At basal level, HaCaT cells showed higher MT level than C6 cells which could explain the resistance of HaCaT cells. Western blot showed that C6 cells treated with 20micromol/L Cd for 24h did not express any MT. MT were initially expressed in the cytoplasmic or periplasmic compartment and were then translocated in the nucleus after 24h treatment by Cd both in HaCaT and C6 cells. In addition, the cell treatment with Cd was followed by an increase in the cellular zinc level but the electrophoretic mobility shift assay (EMSA) experiment did not show any translocation of metal-responsive transcription factor-1 (MTF-1) to the nucleus of HaCaT cells. These absence of translocation could be due to the presence of MT in these cells at the basal state. The translocation study in HaCaT cells suggested that the MT translocation in the nucleus was greater than observed in C6 cells. The latter observation could explain HaCaT cells resistance to Cd concentrations up to 50micromol/L. Our results suggested that the C6 cell sensitivity was correlated with the decrease in MT level at 20micromol/L Cd occurring after the transcription of MT gene.

[Mechanism of metallothionein gene activation mediated by heavy-metal dependent transcription factor MTF-1]

Transcriptional activation of metallothionein (MT) genes by heavy metals is a valuable system for understanding the functions of MT as well as the cellular response against heavy metals. Although it is now known that heavy metal signals culminating in MT induction converge upon a transcription factor MTF-1, the mechanism underlying the MTF-1 response to heavy metals has not been elucidated. To address this issue, we investigated various aspects of the in vivo response of MTF-1 against heavy metals. Chromatin immunoprecipitation assay showed that heavy metal-dependent DNA binding of MTF-1 is the critical step in vivo. MTF-1 is primarily localized in the nucleus so that heavy metal-dependent nuclear translocation demonstrated by other groups does not seem to be universal and hence may not be critical for activation of MTF-1. In the six Zn finger motifs, the hallmark of MTF-1, the third and the fourth fingers are essential for the nuclear localization of MTF-1. Furthermore, all fingers except the last are important for transcriptional activation function of MTF-1, suggesting their key role for MTF-1 function. Also, a cysteine cluster structure located in the C-terminal region of MTF-1 is critical for transactivating function of MTF-1. These results suggest a central role of the Zn-finger domain and intramolecular cooperation through a structural change of MTF-1 for its response to heavy metal challenge.

Transcriptional regulation of thioredoxin reductase 1 expression by cadmium in vascular endothelial cells: role of NF-E2-related factor-2

Thioredoxin reductase (TrxR) is a selenoprotein that catalyzes the reduction of the active site disulfide of thioredoxin (Trx), which regulates the redox status of the cells. In the present study, we found that TrxR1, one of the three TrxR isozymes, was induced by cadmium as well as tumor necrosis factor alpha (TNFalpha) in bovine arterial endothelial cells (BAEC), and investigated the mechanism of cadmium-induced TrxR1 expression. We here showed that cadmium, differently from TNFalpha, enhanced the promoter activity of the 5'-flanking region of human TrxR1 gene (nucleotides -1692 to +49). Deletion and site-directed mutation of antioxidant responsive element (ARE) (nucleotides -62 to -48) in this region abolished the response to cadmium. Overexpression of NF-E2-related factor-2 (Nrf2) augmented the TrxR1 promoter activity. In contrast, overexpression of the dominant negative mutant of Nrf2 suppressed cadmium-induced activation of TrxR1 promoter through the ARE. Chromatin immunoprecipitation (ChIP) assays showed that anti-Nrf2 antibody precipitated ARE from the chromatin of the cadmium-treated cells. These results indicated that cadmium-induced TrxR1 gene expression is mediated by the activation of Nrf2 transcription factor and its binding to ARE in the TrxR1 gene promoter. We further found that in addition to cadmium, the activators of Nrf2, such as diethyl maleate (DEM) and arsenite, induced both TrxR1 and Trx gene expression in BAEC. Nrf2 might play an important role in the regulation of the cellular Trx system consisting of Trx and TrxR.

Investigation of lymphocyte gene expression for use as biomarkers for zinc status in humans

A bioassay for zinc status in humans has been sought due to the importance of zinc, an essential trace metal, for many divergent functions in the human body; however, a sensitive bioassay for zinc status in humans is lacking. To address this issue, we established gene expression profiles of human lymphoblastoid cells treated with 0 or 30 micro mol/L ZnSO(4) using microarray technology. A limited number of genes were responsive to 30 micro mol/L zinc based on the analysis of Affymetrix human genome U133A GeneChips. We also examined the gene expression patterns of zinc transporters in human lymphoblastoid cells using quantitative RT-PCR analysis. ZNT1 was upregulated in lymphoblastoid cells, whereas ZIP1 was downregulated in response to the increased zinc concentrations in the culture media. To evaluate the potential applications of using both zinc transporter genes as biomarkers of zinc status, we measured the expression levels of ZIP1 and ZNT1 in the peripheral leukocytes collected from 2 different age groups of Korean women. After administration of a zinc supplement (22 mg zinc gluconate/d for 27 d), ZIP1 expression decreased by 17% (P < 0.01) and 21% (P < 0.05) in the peripheral leukocytes collected from 15 young (20-25 y) and 10 elderly (64-75 y) subjects, respectively. ZNT1 expression was not affected by taking the zinc supplement. These data suggest a potential application of ZIP1 as a biomarker of zinc status in humans.