Activation of the complete mouse metallothionein gene locus in the maternal deciduum

Cadmium exposure during pregnancy and lactation: materno-fetal and newborn repercussions of Cd(ii), and Cd-metallothionein complexes

Cadmium (Cd) is a non-physiological heavy metal that can be harmful at low concentrations. Increasing anthropogenic activities are incrementing the risk of accumulation of this heavy metal in different organs and tissues of the body. In the case of pregnant women, the threat is more serious due to the implications affecting not only their own health but also fetal development as well. Metallothioneins (MTs), small cysteine-rich proteins, are involved in zinc (Zn) and copper homeostasis in mammals but can, however, also bind with Cd if present. The accumulation of Cd in maternal tissues (e.g. placenta, maternal blood, and mammary glands) induces the synthesis of MTs, preferably MT2, in an attempt to sequester the metal to avoid toxicity. The formed Cd-MT complexes will avoid the Cd transport from the placenta to the fetus and end up accumulating in the maternal kidneys. At the same time, high concentrations of MTs will increase the formation of Zn-MT complexes, therefore decreasing the amount of Zn ions available to be transported to the fetus by means of Zn transporters such as ZnT2, ZIP14 and DMT1. Although MTs cannot transport Cd from the mother to the fetus, the divalent DMT1 transporter is suggested to carry the metal to the fetus. As a consequence, the low levels of Zn(ii) in the fetus, together with the presence of Cd(ii) coming from the mother either via the placenta and cord blood or via breast milk induce changes in the fetal development including fetal growth retardation, and low weight or height of the newborn. Likewise, the concentrations of Cd(ii) in the newborn can cause alterations such as cognitive disabilities. In summary, the presence of Cd(ii) in the maternal tissues will induce MT synthesis in an attempt to detoxify these tissues and reduce the possible toxicity of Cd in fetal and newborn tissues.

Metallothioneins: Mercury Species-Specific Induction and Their Potential Role in Attenuating Neurotoxicity

Metallothionein (MT) proteins are widespread in bacteria, fungi, plants, and eukaryotic species. They are of low molecular weight (6–7 kDa) and of the 60+ amino acid residues, 20 are cysteines. Functions attributed to MTs include the sequestration and dispersal of metal ions, primarily in zinc and copper homeostasis; regulation of the biosynthesis and activity of zinc metalloproteins, most notably zinc-dependent transcription factors; and cellular cytoprotection from reactive oxygen species, ionizing radiation, electrophilic anticancer drugs and mutagens, and metals. Observations on the abundance of MTs within the central nervous system (CNS) and the identification of a brain-specific isoform, MT-III, suggest that it might have important neurophysiological and neuromodulatory functions. Reinforced by the potential Involvement of MT-III in a number of neurodegenerative disorders, the role of MTs in the CNS has become an intense focus of scientific pursuit. This manuscript represents a survey on the ability of MTs to modulate mercury neurotoxicity, a neurotoxin that has been implied to play an etiologic role in Minamata disease, erethism, and autism, just to name a few.

Ultrastructural Demonstration of Mercury Granules in the Placenta of Metallothionein-Null Pregnant Mice after Exposure to Mercury Vapor

The placenta plays an important role in the regulation of maternal to fetal transfer of toxic substances, including nonessential metals. Metallothioneins (MTs), which are known to have protective effects against heavy metal toxicity, exist in the placenta, but the exact localization of placental MTs (both MT-I and MT-III) and their physiological role in the placenta exposed to mercury are unclear. The present study was performed to examine the localization of MTs and mercury granules in the placenta of mice exposed to mercury vapor. On gestational day 16, MT-I & II-null and wild-type mice were exposed to mercury vapor at 4.9 to 5.9 mg/m3 for 2 hours. At 24 and 48 hours after exposure, the placentas were examined for mercury distribution (autometallography), MT immunoreactivity, and MT mRNA expression (in situ hybridization). No histological changes were observed in the placentas of either MT-null or wild-type mice. Mercury deposition was demonstrated along the boundary between the junctional zone and the labyrinth zone, as well as in the yolk sac, maternal decidual cells, and labyrinth trophoblasts of both MT-null and wild-type mice. MT-I & -II immunoreactivity, which was confined to wild-type mice, was demonstrated in the yolk sac and decidual cells; mercury was also shown in both structures, suggesting that mercury granules were bound to MTs. MT-III mRNA expression was observed in the yolk sac, decidual cells, and spongiotrophoblasts in both MT-null and wild-type mice. There was, however, no evidence of MT at the boundary between the junctional and labyrinth zones, where substantial mercury deposits were demonstrated. These results suggest that placental MTs and the other unknown molecules may be related to the barrier to the placental transfer of mercury.

Chemistry and biology of mammalian metallothioneins

Metallothioneins (MTs) are a class of ubiquitously occurring low molecular mass, cysteine- and metal-rich proteins containing sulfur-based metal clusters formed with Zn(II), Cd(II), and Cu(I) ions. In mammals, four distinct MT isoforms designated MT-1 through MT-4 exist. The first discovered MT-1/MT-2 are widely expressed isoforms, whose biosynthesis is inducible by a wide range of stimuli, including metals, drugs, and inflammatory mediators. In contrast, MT-3 and MT-4 are noninducible proteins, with their expression primarily confined to the central nervous system and certain squamous epithelia, respectively. MT-1 through MT-3 have been reported to be secreted, suggesting that they may play different biological roles in the intracellular and extracellular space. Recent reports established that these isoforms play an important protective role in brain injury and metal-linked neurodegenerative diseases. In the postgenomic era, it is becoming increasingly clear that MTs fulfill multiple functions, including the involvement in zinc and copper homeostasis, protection against heavy metal toxicity, and oxidative damage. All mammalian MTs are monomeric proteins, containing two metal-thiolate clusters. In this review, after a brief summary of the historical milestones of the MT-1/MT-2 research, the recent advances in the structure, chemistry, and biological function of MT-3 and MT-4 are discussed.

The association of metallothionein-4 gene polymorphism and renal function in long-term lead-exposed workers

The goal of this study is to investigate if metallothionein (MT) gene polymorphism affects the susceptibility to lead as well as renal function parameters and blood pressures (BP) in workers exposed to lead for extended period of time. By means of real-time polymerase chain reaction, the MT4-216 A/G genotypes classified as rs396230 in the single nucleotide polymorphism database of the National Center for Biotechnology Information (database) were analyzed on 113 workers of a lead battery-recycling factory. Workers with G (mutant) allele were more susceptible to the toxic effects of lead on their systolic BP and serum renal function parameters. Their BP was 10 mmHg higher than those with wild-type (AA type) allele. Among subjects with the 3-genome, the GG mutant type subjects appear to be more susceptible to lead. Regression models of serum creatinine and BUN showed significant differences between the GG and GA types compared to AA type subjects. This cross-sectional study shows that workers with different MT-4 genotypes have different lead-induced adverse health effects. Those with the G allele have the greater susceptibility to lead so their exposure should be reduced.

Metallothionein as an anti-inflammatory mediator

The integration of knowledge concerning the regulation of MT, a highly conserved, low molecular weight, cystein-rich metalloprotein, on its proposed functions is necessary to clarify how MT affects cellular processes. MT expression is induced/enhanced in various tissues by a number of physiological mediators. The cellular accumulation of MT depends on the availability of cellular zinc derived from the diet. MT modulates the binding and exchange/transport of heavy metals such as zinc, cadmium, or copper under physiological conditions and cytoprotection from their toxicities, and the release of gaseous mediators such as hydroxyl radicals or nitric oxide. In addition, MT reportedly affects a number of cellular processes, such as gene expression, apoptosis, proliferation, and differentiation. Given the genetic approach, the apparently healthy status of MT-deficient mice argues against an essential biological role for MT; however, this molecule may be critical in cells/tissues/organs in times of stress, since MT expression is also evoked/enhanced by various stresses. In particular, because metallothionein (MT) is induced by inflammatory stress, its roles in inflammation are implied. Also, MT expression in various organs/tissues can be enhanced by inflammatory stimuli, implicating in inflammatory diseases. In this paper, we review the role of MT of various inflammatory conditions.

Impact of overexpression of metallothionein-1 on cell cycle progression and zinc toxicity

Metallothioneins (MTs) have an important role in zinc homeostasis and may counteract the impact of oversupply. Both intracellular zinc and MT expression have been implicated in proliferation control and resistance to cellular stress, although the interdependency is unclear. The study addresses the consequences of a steady-state overexpression of MT-1 for intracellular zinc levels, cell cycle progression, and protection from zinc toxicity using a panel of cell lines with differential expression of MT-1. The panel comprised parental Chinese hamster ovary-K1 cells with low endogenous expression of MT and transfectants with enhanced expression of mouse MT-1 on an autonomously replicating expression vector with a noninducible promoter. Cell cycle progression, determined by flow cytometry and time-lapse microscopy, revealed that enhanced cytoplasmic expression of MT-1 does not impact on normal cell cycle operation, suggesting that basal levels of MT-1 expression are not limiting for background levels of oxidative stress. MT-1 overexpression correlated with a steady-state increase in cytoplasmic free Zn(2+), assessed using the fluorescent zinc-sensor Zinquin, particularly at high levels of overexpression, further suggesting that zinc availability is normally not limiting for cell cycle progression. Enhanced MT-1 expression, over a 10-fold range, had a clear impact on resistance to Cd(2+) and Zn(2+) toxicity. In the case of Zn(2+), the degree of protection afforded was less, indicating that MT-1 has a limited range and saturable capacity for effecting resistance. The results have implications for the use of cellular stress responses to exogenously supplied zinc and zinc-based systemic therapies.

Use of stable isotopically enriched proteins and directly coupled high-performance liquid chromatography inductively coupled plasma mass spectrometry for quantitatively monitoring the transfer of metals between proteins

Studies have shown that metallothionein (MT) may play an important role in modulating the activity of certain Zn-regulated enzymes under various oxidoreductive conditions by either donating or removing Zn. To better determine the role of MT in interprotein metal transfer, we describe a procedure that uses stable isotopically enriched (67)Zn(7) metallothionein 2 ((67)Zn(7)-MT-2) to quantitatively determine the stoichiometry of transfer of Zn from the protein to a recipient apo-metalloenzyme, apo-carbonic anhydrase (apo-CA) by directly coupled ion exchange high-performance liquid chromatography inductively coupled plasma mass spectrometry. Quantitatively, the transfer of (67)Zn was consistent with the enzymatic activation of the apo-enzyme as judged by its esterase activity and ability to cleave p-nitrophenyl acetate. Maximum enzyme activation occurred at an MT-2:apo-CA molar ratio of 1, implying the release of a single atom of Zn from MT-2. Preincubation of (67)Zn(7)-MT-2 with an excess of oxidized glutathione (GSSG) increased metal donation fourfold, whereas reduced glutathione (GSH) inhibited donation by approximately 50%. By using multiple recipient and donor proteins having different stable isotopic signatures, the technique has the potential for quantitatively studying the kinetic and thermodynamic aspects of Zn transfer between numerous competing ligands in vitro, an important first step toward understanding the regulatory role of this metal in protein functioning and cellular metabolism in vivo.

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.

A uterine decidual cell cytokine ensures pregnancy-dependent adaptations to a physiological stressor

In the mouse, decidual cells differentiate from uterine stromal cells in response to steroid hormones and signals arising from the embryo. Decidual cells are crucially involved in creating the intrauterine environment conducive to embryonic development. Among their many functions is the production of cytokines related to prolactin (PRL), including decidual prolactin-related protein (DPRP). DPRP is a heparin-binding cytokine, which is abundantly expressed in uterine decidua. In this investigation, we have isolated the mouse Dprp gene, characterized its structure and evaluated its biological role. Dprp-null mice were made by replacing exons 2 to 6 of the Dprp gene with an in-frame enhanced green fluorescent protein (EGFP) gene and a neomycin (neo) resistance cassette. Heterozygous intercross breeding of the mutant mice yielded the expected mendelian ratio. Pregnant heterozygote females expressed EGFP within decidual tissue in locations identical to endogenous Dprp mRNA and protein expression. Homozygous Dprp-null mutant male and female mice were viable, exhibited normal postnatal growth rates, were fertile and produced normal litter sizes. A prominent phenotype was observed when pregnant Dprp-null mice were exposed to a physiological stressor. DPRP deficiency interfered with pregnancy-dependent adaptations to hypoxia resulting in pregnancy failure. Termination of pregnancy was associated with aberrations in mesometrial decidual cells, mesometrial vascular integrity, and disruptions in chorioallantoic placenta morphogenesis. The observations suggest that DPRP participates in pregnancy-dependent adaptations to a physiological stressor.

The properties of the metal-thiolate clusters in recombinant mouse metallothionein-4

Metallothioneins (MTs) are metal-binding proteins with low molecular weight and conservative cysteine residues. Metallothionein-4 (MT-4), one of MT isoforms, is first reported to be distributed in a tissue-specific manner, mainly in stratified squamous epithelia. Here, we compare the properties of metal-thiolate clusters in MT-4 to those in MT-1 and MT-3, including the stabilities toward both pH change and EDTA, as well as the exposure of thiolates to solvent. The metal-thiolate clusters in MT-3 show different property and activity to the reactions compared with MT-4 and MT-1. The structure of metal-thiolate clusters in MT-4 is similar to that of MT-1 from the UV and CD spectra. During pH titration and DTNB reaction, MT-4 and MT-1 exhibit comparable behavior. But while reacting with EDTA, the metal-thiolate clusters in MT-4 are more stable than those of MT-1. We suppose the negative charge of the beta-domain of MT-4 prevents the EDTA attack to MT-4.

Smoking specifically induces metallothionein-2 isoform in human placenta at term

Recently, we reported the presence of higher levels of metallothionein (MT) in placentas of smokers compared to non-smokers. In the present study, we designed experiments to separate and evaluate two isoforms of MT (MT-1 and MT-2) in placentas of smokers and non-smokers. Metallothionein was extracted and separated by ion-exchange high performance liquid chromatography (HPLC), previous saturation with cadmium chloride. Two peaks eluting at 6 and 12.5 min, corresponding to MT-1 and MT-2, respectively, were obtained. Metallothionein present in both peaks was identified by Western blot analysis using a monoclonal antibody directed against MT-1 and MT-2. Each isoform concentration was calculated after measuring its cadmium content by atomic absorption spectrometry with inductively coupled-plasma. In placentas of smokers, MT-2 levels increased by seven-fold compared to non-smokers, whereas MT-1 was not changed. Total placental cadmium and zinc concentrations, determined by atomic absorption spectrometry and neutron activation analysis, respectively, were higher in smokers. Metallothioneins levels were clearly in excess to bind all cadmium ions present in placentas. However, most of placental zinc remains unbound to MTs, although as much as twice zinc ions could be bound to MT in smokers. In conclusion, MT-2 is the main isoform induced by smoking, suggesting that this isoform could be involved in placental cadmium and zinc retention. This fact, which could contribute to reduce the transference of zinc to the fetus, may be associated to detrimental effects on fetal growth and development.

Organization and assembly of metal-thiolate clusters in epithelium-specific metallothionein-4

Mammalian metallothionein-4 (MT-4) was found to be specifically expressed in stratified squamous epithelia where it plays an essential but poorly defined role in regulating zinc or copper metabolism. Here we report on the organization, stability, and the pathway of metal-thiolate cluster assembly in MT-4 reconstituted with Cd(2+) and Co(2+) ions. Both the (113)Cd NMR studies of (113)Cd(7)MT-4 and the spectroscopic characterization of Co(7)MT-4 showed that, similar to the classical MT-1 and MT-2 proteins, metal ions are organized in two independent Cd(4)Cys(11) and Cd(3)Cys(9) clusters with each metal ion tetrahedrally coordinated by terminal and bridging cysteine ligands. Moreover, we have demonstrated that the cluster formation in Cd(7)MT-4 is cooperative and sequential, with the Cd(4)Cys(11) cluster being formed first, and that a distinct single-metal nucleation intermediate Cd(1)MT-4 is required in the cluster formation process. Conversely, the absorption and circular dichroism features of metal-thiolate clusters in Cd(7)MT-4 indicate that marked differences in the cluster geometry exist when compared with those in Cd(7)MT-1/2. The biological implication of our studies as to the role of MT-4 in zinc metabolism of stratified epithelia is discussed.

Functional differentiation in the mammalian metallothionein gene family: metal binding features of mouse MT4 and comparison with its paralog MT1

This paper reports on the characterization of the metal binding abilities of mammalian MT4 and their comparison with those of the well known MT1. Heterologous Escherichia coli expression in cultures supplemented with zinc, cadmium, or copper was achieved for MT4 and for its separate alphaMT4 and betaMT4 domains as well as for MT1 and its alphaMT1 domain in cadmium-enriched medium. The in vivo conformed metal complexes and the in vitro substituted zinc/cadmium and zinc/copper MT4 aggregates were characterized. Biosynthesis of MT4 and betaMT4 in Cd(II)-supplemented medium revealed that these peptides failed to form the same homometallic species as MT1, thus appearing less effective for cadmium coordination. Conversely, the entire MT4 and both of its domains showed better Cu(I) binding properties than MT1, affording Cu(10)-MT4, Cu(5)-alphaMT4 and Cu(7)-betaMT4, stoichiometries that make the domain dependence toward Cu(I) clear. Overall results allow consideration of MT4 as a novel copper-thionein, made up of two copper-thionein domains, the first of this class reported in mammals, and by extension in vertebrates. Furthermore, the in silico protein sequence analyses corroborated the copper-thionein nature of the MT4 peptides. As a consequence, there is the suggestion of a possible physiological role played by MT4 related with copper requirements in epithelial differentiating tissues, where MT4 is expressed.

Mousezinc transporter 1 gene provides an essential function during early embryonic development

The SLC30 family of cation diffusion transporters includes at least nine members in mammals, most of which have been documented to play a role in zinc transport. The founding member of this family, Znt1, was discovered by virtue of its ability to efflux zinc from cells and to protect them from zinc toxicity. However, its physiological functions remain unknown. To address this issue, mice with targeted knockout of the Znt1 gene were generated by homologous recombination in embryonic stem cells. Heterozygous Znt1 mice were viable. In contrast, homozygous Znt1 mice died in utero soon after implantation due to a catastrophic failure of embryonic development. Although extraembryonic membranes formed around these embryos, the embryo proper failed to undergo morphogenesis past the egg cylinder stage and was amorphous by d9 of pregnancy. Expression of the Znt1 gene was detected predominantly in trophoblasts and in the maternal deciduum during the postimplantation period (d5 to d8). The failure of homozygous Znt1 embryos to develop could not be rescued by manipulating maternal dietary zinc (either excess or deficiency) during pregnancy. However, embryos in Znt1 heterozygous females were approximately 3 times more likely to develop abnormally when exposed to maternal dietary zinc deficiency during later pregnancy than were those in wildtype females. These studies suggest that Znt1 serves an essential function of transporting maternal zinc into the embryonic environment during the egg cylinder stage of development, and further suggest that Znt1 plays a role in zinc homeostasis in adult mice.

Transcriptional activation of the metallothionein I gene by electric pulses in vivo: basis for the development of a new gene switch system

BACKGROUND

In vivo gene transfer to skeletal muscle is a promising strategy for the treatment of muscular disorders and for the systemic delivery of therapeutic proteins. Nevertheless, for a safe and effective protein production, the spatial and temporal control of gene expression is critical. The existing regulating systems rely on the use of an exogenously regulatory protein and/or an inducer drug whose pharmacological properties are of major concerns for therapeutic applications in humans. Therefore, new strategies based on endogenous regulatable elements have been explored.

METHODS

Gene expression profiles of skeletal muscle submitted or not to electrical pulses and harvested at different times were compared using the Affymetrix GeneChip technology. The endogenous metallothionein promoter was studied by Northern blot and semiquantitative and quantitative RT-PCR. The inducibility of the metallothionein I promoter placed in a plasmid exogenous context was studied using the murine SEAP reporter gene.

RESULTS

The expression of metallothionein I mRNA is significantly increased 6 h after electric pulses delivery. This induction is transient. Identical MT-I expression level is observed after several sequential series of pulses delivery. We demonstrated as well that the MT-II promoter was sensitive to electric pulses delivery. Moreover, the metallothionein I promoter, placed in a plasmid context in front of a reporter gene, was also activated by the application of transient electric field.

CONCLUSIONS

We identified a promoter highly inducible by the controlled electric stimuli applied for electrotransfer experiments. The use of the metallothionein promoter is promising for the time-control by physical stimuli of the expression of a therapeutic gene.

Expression, purification, and characterization of metallothionein-A from rainbow trout

Recombinant metallothionein A (MT-A) from rainbow trout has been successfully produced in milligram quantities in Escherichia coli. cDNA has been subcloned into pGEX-6P.1 vector, in-frame with a sequence encoding an N-terminal glutathione-S-transferase (GST) tail. Purification to electrophoretic homogeneity has been obtained by affinity chromatography using GSH-Sepharose. After enzymatic cleavage of GST tail, the MT-A moiety shows a molecular weight, corresponding to the expected one (6630 Da). The final yield of the entire expression and purification process was about 5 mg of pure metallothionein per liter of bacterial culture. The effects of different reducing and alkylating agents have been evaluated at the level of the formation of higher molecular weight aggregates. To investigate the metal-binding ability of the recombinant MT-A, we carried out a spectrophotometrical titration with cadmium ions. Finally, we checked the metal dissociation by recording the UV absorbance of the protein as a function of the environmental pH.

Expression of metallothionein isoforms in human chorioretinal complex

PURPOSE

To determine the relative expression of metallothionein isoforms and their differential induction by oxidative stress in cultured RPE cells and to localize the isoforms in the human chorioretinal complex.

METHODS

Total RNA was isolated from cultured human retinal pigment epithelial cells using TRI-Reagent. An "anchor-oligo-dT primer" was used for the synthesis of cDNA, reverse transcribed using avian reverse transcriptase and subsequently subjected to PCR analysis using oligonucleotides specific for metallothionein (MT) I, MT II, and MT III. The selected transcripts were then used to assess the expression of the above elements in fixed tissue sections by in situ hybridization. Cultured RPE cells were allowed to phagocytose bovine photoreceptor outer segments (ROS) or were treated with H(2)O(2) for 6 hours and then analyzed by RT-PCR or in situ hybridization to ascertain the effect of oxidative stress on metallothionein mRNA isoform expression.

RESULTS

Relative density analysis of amplified products demonstrate the presence of MT I, MT II and MT III in RPE cells, with an apparent relative expression MT II > MT I > MT III [corrected]. Expression of MT I and MT II mRNA was increased by both phagocytosis and hydrogen peroxide, however MT III was not induced by either stress. In situ hybridization corroborated the findings of the RT-PCR analysis and showed that MTs were mainly localized in the RPE and the photoreceptor layer of the retina.

CONCLUSIONS

The localization of MT and the response of MT to oxidative stress are consistent with a role for MT as an antioxidant in the RPE and retina. Studies are ongoing to determine the specific mechanisms of action of these antioxidants in RPE cells.

Roles of the metallothionein family of proteins in the central nervous system

Metallothioneins (MTs) constitute a family of proteins characterized by a high heavy metal [Zn(II), Cu(I)] content and also by an unusual cysteine abundance. Mammalian MTs are comprised of four major isoforms designated MT-1 trough MT-4. MT-1 and MT-2 are expressed in most tissues including the brain, whereas MT-3 (also called growth inhibitory factor) and MT-4 are expressed predominantly in the central nervous system and in keratinizing epithelia, respectively. All MT isoforms have been implicated in disparate physiological functions, such as zinc and copper metabolism, protection against reactive oxygen species, or adaptation to stress. In the case of MT-3, an additional involvement of this isoform in neuromodulatory events and in the pathogenesis of Alzheimer's disease has also been suggested. It is essential to gain insight into how MTs are regulated in the brain in order to characterize MT functions, both in normal brain physiology, as well as in pathophysiological states. The focus of this review concerns the biology of the MT family in the context of their expression and functional roles in the central nervous system.

Astrocyte-targeted expression of interleukin-3 and interferon-alpha causes region-specific changes in metallothionein expression in the brain

Transgenic mice expressing IL-3 and IFN-alpha under the regulatory control of the GFAP gene promoter (GFAP-IL3 and GFAP-IFNalpha mice) exhibit a cytokine-specific, late-onset chronic-progressive neurological disorder which resemble many of the features of human diseases such as multiple sclerosis, Aicardi-Goutières syndrome, and some viral encephalopathies including HIV leukoencephalopathy. In this report we show that the metallothionein-I+II (MT-I+II) isoforms were upregulated in the brain of both GFAP-IL3 and GFAP-IFNalpha mice in accordance with the site and amount of expression of the cytokines. In the GFAP-IL3 mice, in situ hybridization analysis for MT-I RNA and radioimmunoassay results for MT-I+II protein revealed that a significant upregulation was observed in the cerebellum and medulla plus pons at the two ages studied, 1-3 and 6-10 months. Increased MT-I RNA levels occurred in the Purkinje and granular layers of the cerebellum, as well as in its white matter tracts. In contrast to the cerebellum and brain stem, MT-I+II were downregulated by IL-3 in the hippocampus and the remaining brain in the older mice. In situ hybridization for MT-III RNA revealed a modest increase in the cerebellum, which was confirmed by immunohistochemistry. MT-III immunoreactivity was present in cells that were mainly round or amoeboid monocytes/macrophages and in astrocytes. MT-I+II induction was more generalized in the GFAP-IFNalpha (GIFN12 and GIFN39 lines) mice, with significant increases in the cerebellum, thalamus, hippocampus, and cortex. In the high expressor line GIFN39, MT-III RNA levels were significantly increased in the cerebellum (Purkinje, granular, and molecular layers), thalamus, and hippocampus (CA2/CA3 and especially lacunosum molecular layers). Reactive astrocytes, activated rod-like microglia, and macrophages, but not the perivenular infiltrating cells, were identified as the cellular sources of the MT-I+II and MT-III proteins. The pattern of expression of the different MT isoforms in these transgenic mice differed substantially, demonstrating unique effects associated with the expression of each cytokine. The results indicate that the MT expression in the CNS is significantly affected by the cytokine-induced inflammatory response and support a major role of these proteins during CNS injury.

The transcription factors MTF-1 and USF1 cooperate to regulate mouse metallothionein-I expression in response to the essential metal zinc in visceral endoderm cells during early development

During early development of the mouse embryo, expression of the metallothionein-I (MT-I) gene is heightened specifically in the endoderm cells of the visceral yolk sac. The mechanisms of regulation of this cell-specific pattern of expression of metallothionein-I are unknown. However, it has recently been shown that MTF-1, functioning as a metalloregulatory transcription factor, activates metallothionein genes in response to the essential metal zinc. In contrast with the metallothionein genes, MTF-1 is essential for development; null mutant embryos die due to liver degeneration. We report here that MTF-1 is absolutely essential for upregulation of MT-I gene expression in visceral endoderm cells and that optimal expression also involves interactions of the basic helix-loop-helix upstream stimulatory factor-1 (USF1) with an E-box1-containing sequence at -223 bp in the MT-I promoter. Expression of MT-I in visceral endoderm cells was dependent on maternal dietary zinc. Thus, the essential metal, zinc, apparently provides the signaling ligand that activates cell-specific MT-I expression in visceral endoderm cells.

The transcription factor MTF-1 mediates metal regulation of the mouse ZnT1 gene

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 were reduced, and induction of this message in response to zinc or cadmium was accentuated (up to 31-fold induction). Changes in ZnT1 gene expression in these experiments paralleled those of metallothionein I (MT-I). Inhibition of RNA synthesis blocked metal induction of ZnT1 and MT-I mRNAs, whereas inhibition of protein synthesis did not. Metal response element-binding transcription factor (MTF)-1 mediates metal regulation of the metallothionein I gene. In vitro DNA-binding assays demonstrated 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 was abundant in the midgestation visceral yolk sac and placenta. Dietary zinc deficiency during pregnancy down-regulated ZnT1 and MT-I mRNA levels (4-5-fold and >20-fold, respectively) in the visceral yolk sac, but had little effect on these mRNAs in the placenta. Homozygous knockout of the MTF-1 gene in transgenic mice also led 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.

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.

Metallothioneins are upregulated in symptomatic mice with astrocyte-targeted expression of tumor necrosis factor-alpha

Transgenic mice expressing TNF-alpha under the regulatory control of the GFAP gene promoter (GFAP-TNFalpha mice) exhibit a unique, late-onset chronic-progressive neurological disorder with meningoencephalomyelitis, neurodegeneration, and demyelination with paralysis. Here we show that the metallothionein-I + II (MT-I + II) isoforms were dramatically upregulated in the brain of symptomatic but not presymptomatic GFAP-TNFalpha mice despite TNF-alpha expression being present in both cases. In situ hybridization analysis for MT-I RNA and radioimmunoassay results for MT-I + II protein revealed that the induction was observed in the cerebellum but not in other brain areas. Increased MT-I RNA levels occurred in the Purkinje and granular neuronal layers of the cerebellum but also in the molecular layer. Reactive astrocytes, activated rod-like microglia, and macrophages, but not the infiltrating lymphocytes, were identified as the cellular sources of the MT-I + II proteins. In situ hybridization for MT-III RNA revealed a modest increase in the white matter of the cerebellum, which was confirmed by immunocytochemistry. MT-III immunoreactivity was present in cells which were mainly round or amoeboid monocytes/macrophages. The pattern of expression of the different MT isoforms in the GFAP-TNFalpha mice differed substantially from that described previously in GFAP-IL6 mice, demonstrating unique effects associated with the expression of each cytokine. The results suggest that the MT expression in the CNS reflects the inflammatory response and associated damage rather than a direct role of the TNF-alpha in their regulation and support a major role of these proteins during CNS injury.

Regulation of metallothionein gene expression by oxidative stress and metal ions

The metallothioneins (MT) are small, cysteine-rich heavy metal-binding proteins which 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. Our 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. We hypothesize 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.

Regulation of metallothionein gene transcription. Identification of upstream regulatory elements and transcription factors responsible for cell-specific expression of the metallothionein genes from Caenorhabditis elegans

Metallothioneins are small, cysteine-rich proteins that function in metal detoxification and homeostasis. Metallothionein transcription is controlled by cell-specific factors, as well as developmentally modulated and metal-responsive pathways. By using the nematode Caenorhabditis elegans as a model system, the mechanism that controls cell-specific metallothionein transcription in vivo was investigated. The inducible expression of the C. elegans metallothionein genes, mtl-1 and mtl-2, occurs exclusively in intestinal cells. Sequence comparisons of these genes with other C. elegans intestinal cell-specific genes identified multiple repeats of GATA transcription factor-binding sites (i.e. GATA elements). In vivo deletion and site-directed mutation analyses confirm that one GATA element in mtl-1 and two in mtl-2 are required for transcription. Electrophoretic mobility shift assays show that the C. elegans GATA transcription factor ELT-2 specifically binds to these elements. Ectopic expression of ELT-2 in non-intestinal cells of C. elegans activates mtl-2 transcription in these cells. Likewise, mtl-2 is not expressed in nematodes in which elt-2 has been disrupted. These results indicate that cell-specific transcription of the C. elegans metallothionein genes is regulated by the binding of ELT-2 to GATA elements in these promoters. Furthermore, a model is proposed where ELT-2 constitutively activates metallothionein expression; however, a second metal-responsive factor prevents transcription in the absence of metals.

Expression of the mouse metallothionein-I and -II genes provides a reproductive advantage during maternal dietary zinc deficiency

The function of metallothionein in zinc homeostasis was examined by using mice homozygous for knockout (KO) of the metallothionein-I or -II (MT-I and MT-II) genes. Pregnant MT-I/II KO mice or control mice were fed a zinc-deficient (1 microg/g or 5 microg/g) diet or a zinc-adequate (50 microg/g) diet during specific periods of pregnancy, and the effects on morphogenesis of the embryos were determined at day 14 of pregnancy (day 1 = vaginal plug). In the homozygous MT-I/II KO, as well as in the nontransgenic control mice, severe dietary zinc deficiency (1 microg/g) beginning on day 1 of pregnancy was embryotoxic and teratogenic, and the majority of the embryos in both strains were dead by mid-gestation. However, 53% of the surviving embryos in the MT-I/II KO mice were morphologically abnormal compared to only 32% of the embryos in the control mice. In subsequent experiments, moderate dietary zinc deficiency (5 microg/g beginning on day 1 of pregnancy or 1 microg/g dietary zinc beginning on day 8 of pregnancy) exerted teratogenic, but not embryotoxic effects. Embryos in the MT-I/II KO mice were 260 to 290% as likely to develop abnormally than were embryos in the control mice fed these same diets. These results demonstrate that the expression of the MT-I and -II genes in pregnant females improves reproductive success during maternal dietary zinc deficiency.

Metallothionein: an intracellular protein to protect against cadmium toxicity

Metallothioneins (MT) are low-molecular-weight, cysteine-rich, metal-binding proteins. MT genes are readily induced by various physiologic and toxicologic stimuli. Because the cysteines in MT are absolutely conserved across species, it was suspected that the cysteines are necessary for function and MT is essential for life. In attempts to determine the function(s) of MT, studies have been performed using four different experimental paradigms: (a) animals injected with chemicals known to induce MT; (b) cells adapted to survive and grow in high concentrations of MT-inducing toxicants; (c) cells transfected with the MT gene; and (d) MT-transgenic and MT-null mice. Most often, results from studies using the first three approaches have indicated multiple functions of MT in cell biology: MT (a) is a "storehouse" for zinc, (b) is a free-radical scavenger, and (c) protects against cadmium (Cd) toxicity. However, studies using MT-transgenic and null mice have not strongly supported the first two proposed functions but strongly support its function in protecting against Cd toxicity. Repeated administration of Cd to MT-null mice results in nephrotoxicity at one tenth the dose that produces nephrotoxicity in control mice. Human studies indicate that 7% of the general population have renal dysfunction from Cd exposure. Therefore, if humans did not have MT, "normal" Cd exposure would be nephrotoxic to humans. Thus, it appears that during evolution, the ability of MT to protect against Cd toxicity might have taken a more pivotal role in the maintenance of life processes, as compared with its other proposed functions (i.e. storehouse for zinc and free radical scavenger).

Regulation of gene expression by reactive oxygen

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

Effects of cadmium on metallothionein-I and metallothionein-II mRNA expression in rat ventral, lateral, and dorsal prostatic lobes: quantification by competitive RT-PCR

Highly sensitive, sequence-specific competitive reverse transcriptase-polymerase chain reaction (RT-PCR) protocols were established for the detection and quantification of metallothionein (MT)-I and MT-II messages, in absolute values, in rat tissues. Detection limits for these protocols were in the range of 5 to 10 amol per microgram total RNA. Levels of MT-I and MT-II transcripts in the three major prostatic lobes, kidney, and testis were measured in untreated and cadmium (Cd)-treated rats. The dorsal prostate (DP), lateral prostate (LP), kidney, and testis expressed substantial levels of MT-I and MT-II mRNA while the ventral prostate (VP) had extremely low levels of the transcripts. Cd treatment induced higher levels of MT-I and/or MT-II mRNA expression in all tissues studied with the exception of LP. In the LP, Cd treatment caused reductions of MT-I and MT-II mRNA levels. The Cd-induced levels attained in the VP following Cd exposure were still markedly lower than those found in the kidney, testis, LP, and DP of untreated animals. These findings contradict previous claims that the MT genes in rat VP are unresponsive to Cd activation. The susceptibility of VP to Cd toxicity/carcinogenicity may therefore be explained by low levels of Cd-induced expression rather than lack of induction of MTs.

Participation of upstream stimulator factor (USF) in cadmium-induction of the mouse metallothionein-I gene

The roles of the bHLH-Zip protein, upstream stimulatory factor (USF), in mouse metallothionein-I (MT-I) gene expression were examined. The promoter contains a putative USF binding site which overlaps an antioxidant response element (ARE) located at -101 bp relative to the transcription start point. The USF/ARE composite element increases basal expression of the mouse MT-I gene, and partly mediates response to oxidative stress. However, other functions of this composite element and the in vivo roles for USF in MT-I promoter functions have not been examined. We report studies which indicate that USF participates via the USF/ARE element in cadmium responsiveness of the mouse MT-I promoter. During the course of these studies a second, higher affinity USF binding site at -223 bp was identified. Stable and transient transfection assays in mouse hepatoma cells, using the USF/ARE in the context of a minimal promoter and site-directed and truncation mutants of the MT-I promoter, revealed that the USF and the ARE sites contribute to cadmium (2-30 microM) but not zinc responsiveness, and to basal promoter activity. Overexpression of dominant-negative (dn)USF in co-transfection assays significantly attenuated cadmium induction of the USF/ARE in the context of a minimal promoter, and attenuated cadmium, but not zinc, induction of the intact MT-I promoter. A consensus E-box (CACATG) at -223 bp in the MT-I promoter was also found to bind USF in vitro , and to be constitutively footprinted in vivo . The interaction of USF with E-box1 was apparently 10-fold stronger than that with the USF/ARE. However, in contrast, E-box1 was not a strong basal promoter element nor was it metal ions responsive in mouse Hepa cells. In conclusion, these studies demonstrate a role for USF in cadmium-specific induction of the mouse MT-I gene, but bring into question an obligate role for USF in regulating basal activity of this gene. The data further suggest that USF interacts with ARE-binding proteins to influence MT-I gene expression.

The elusive function of metallothioneins

Biochemistry and genetics are both required to elucidate the function of macromolecules. There is no question that metallothioneins (MTs) have unique biochemical properties, but genetic experiments have not substantiated the importance of MTs under physiological conditions. Even after thousands of studies describing the structure, biochemical characteristics, tissue distribution, induction, and consequences of genetic disruption and deliberate overexpression, the evolutionary forces that led to the initial appearance, gene duplications, and nearly ubiquitous expression of MTs remain enigmatic.

Interleukin-6 and tumor necrosis factor-alpha type 1 receptor deficient mice reveal a role of IL-6 and TNF-alpha on brain metallothionein-I and -III regulation

Metallothioneins (MTs) are a family of low molecular weight proteins which in rodents is comprised of several isoforms (MT-I to MT-IV). MT-I and MT-II are widely expressed isoforms, whereas MT-III is mainly expressed in the central nervous system and is the only isoform that inhibits survival and neurite formation of rat cortical neurons in vitro. However, the physiological roles and regulation of these proteins in the brain are poorly characterized. In this report we have studied the putative role of IL-6 and TNF-alpha on the regulation of brain MT-I and MT-III, by using mice carrying a null mutation in the IL-6 or the TNF-alpha type 1 receptor genes or both. In situ hybridization analysis revealed that brain MT-I induction by bacterial lipopolysaccharide (LPS) was significantly lower in IL-6- and TNFR1-deficient mice, and to a greater extent in the double mutant mice, in most brain areas studied. These results suggest that the MT-I isoform could be considered an acute-phase protein in the brain, which is consistent with previous studies in transgenic mice overexpressing IL-6 in astrocytes. In contrast to LPS, brain MT-I induction by restraint stress was not affected significantly by IL-6 or TNFR1 deficiencies, suggesting that these cytokines are not important during the stress response in the brain. In basal conditions, it was also observed that the double mutant mice had diminished MT-I mRNA levels in several brain areas. In contrast to MT-I, MT-III mRNA levels were minimally affected by either LPS or stress. Yet, significant decreasing effects of IL-6 and TNFR1 deficiencies were observed in the Purkinje neuronal layer of the cerebellum (after LPS) and ependymal cells (after LPS and stress). In contrast, significant increasing effects, especially of TNFR1 deficiency, were observed in CA1 hippocampal area, retrosplenial and parietal cortex, and in thalamic nuclei (after LPS). These results demonstrate that IL-6 and TNF-alpha are involved in brain MTs regulation during LPS-elicited inflammatory response but not during the stress response.

Expression of the gene encoding metallothionein-3 in organs of the reproductive system

Metallothionein-3 (MT-3) is a new MT gene-family member that inhibits survival of rat neurons cultured in presence of brain extracts. Contrary to other MT genes, which are expressed in most tissues and which are highly inducible by metals, MT-3 expression was reported to be mainly in the brain, and it failed to respond to metals in vivo. We show here that MT-3 mRNA is present in several organs other than the brain, as assayed by Northern analyses. In the rat, MT-3 mRNA was detected in the testis, prostate, epididymis, tongue, ovary, uterus, stomach, heart, and seminal vesicles. The MT-3 mRNA levels in the testis, epididymis, prostate, and tongue were 22% of those in brain, while in ovary, uterus, and stomach, they were 4% of the brain level, and they were lower still in the other organs. The MT-3 gene was not inducible by CdCl2 or lipopolysaccharide in rat testis and prostate. In the mouse and the human, relative MT-3 mRNA levels were lower than those found in the rat when compared with those present in brain. Testicular MT-3 transcript levels remained quite constant during rat postnatal development in animals aged from 6 to 43 days. In situ hybridization analyses on human testis sections showed that MT-3 mRNA was present at different levels in both the Leydig cells and the seminiferous tubules. In orchiectomized rats, prostatic MT-3 mRNA was decreased by 75%, and injections of dihydrotestosterone restored MT-3 mRNA levels to control values. Overall, these results show that MT-3 tissue-specific gene expression is broader than previously reported and provide new experimental systems to study the function and mechanism of action of the MT-3 protein.

Role of Glucocorticoids on Rat Brain Metallothionein-I and -III Response to Stress

The metallothionein (MT) gene family consists of four members (MT-I through -IV) that are tightly regulated during development. Whereas MT-I and MT-II are widely expressed isoforms, MT-III has been found to be mainly expressed in the central nervous system in adult animals, and is the only isoform that inhibits survival and neurite formation of cortical neurons in vitro. A number of models of brain injury have been shown to affect MT-III mRNA levels, which has been suggested to be related to the putative neurotrophic role of this protein. However, a stress response will presumably be associated to the brain injury which could, in turn, drive MT-III regulation. In the present report the effect of a classical stress model, immobilization stress, on brain MT regulation has been studied in rats. MT-I+II protein levels were measured by radioimmunoassay in up to eight brain areas and, as expected, it was found that stress increased selectively MT-I+II levels. Adrenalectomy (ADX) had a general decreasing effect on basal MT-I+II levels; however, ADX blunted the MT-I+II response to stress in cerebellum and presumably in frontal cortex and medulla plus pons but not in the hypothalamus. MT-I mRNA measurements were in accordance with the MT-I+II protein levels in the brain areas studied. In contrast to MT-I mRNA, MT-III mRNA levels of brain cortex tended to decrease during stress, although this effect was not statistically significant. ADX also tended to decrease basal MT-III mRNA levels. Northern blot assays of pooled mRNAs suggested similar differential regulation of these two brain MT isoforms in the cerebellum. These results indicate that glucocorticoids mediate brain MT-I+II response to stress in some but not all brain areas, that a role of these hormones is likely also for MT-III, and that the regulation of MT isoforms differs substantially in the brain.

Expression of MT-3 mRNA in human kidney, proximal tubule cell cultures, and renal cell carcinoma

The human metallothionein 3 (MT-3) gene has recently been identified and characterized as a brain-specific MT having growth inhibitory activity for neuronal cells. One objective of the present study was to determine if MT-3 is brain-specific or also present in the renal system, a site for chronic toxicity due to heavy metal exposure. Using RT-PCR methodology, MT-3 mRNA was shown to be expressed in the human renal system at levels below mRNA for the beta-actin gene. MT-3 mRNA was shown to be expressed in all samples obtained from both the developing and adult renal systems, from 20 weeks of fetal age to 72 years. Cultures of human proximal tubule (HPT) cells were used to determine if MT-3 mRNA expression is influenced by metal exposure. Exposure of HPT cells to either Zn2+ or Cd2+ resulted in an early (within 24 h), but unsustained increase in MT-3 mRNA. The demonstration of MT-3 mRNA expression in the kidney indicates that MT-3 may play an important early role in the response of the cell to metal exposure. MT-3 mRNA expression was also examined in tissues and cells from three cases of renal cell carcinoma. MT-3 was found to be expressed in all three cases at levels similar to those found for normal kidney, providing evidence that MT-3 mRNA expression is not altered in this cancer.