Thionein (apometallothionein) can modulate DNA binding and transcription activation by zinc finger containing factor Sp1

Increased abundance of labile intracellular zinc during cell proliferation was due to increased retention of extracellular zinc in 3T3 cells

Platelet-derived growth factor (PDGF)-, epidermal growth factor (EGF)- and insulin-like growth factor I (IGF-I)-stimulated cell proliferation in 3T3 cells was accompanied by increased abundance of labile intracellular pool of zinc (LIPZ). However, the origin and regulation of this cell proliferation-associated increase in the abundance of LIPZ are unknown. Cellular zinc homeostasis involves zinc transporters and metallothionein. The objectives of this study were to determine whether cell proliferation-associated increase in the abundance of LIPZ was a result of an increased zinc uptake and to assess the involvement of zinc transporters and metallothionein in this cell proliferation-associated increase in the abundance of LIPZ in 3T3 fibroblasts. Zinc transporters assessed included both zinc importer (Zip1) and zinc exporters (ZnT1, ZnT2 and ZnT4). Growth factors increased the abundance of LIPZ while total cellular zinc concentration remained unaffected, demonstrating that LIPZ was responsive to the increased needs for zinc during growth factor-stimulated cell proliferation. Growth factors also increased net zinc retention as indicated by higher 65zinc radioactivity and elevated mRNA levels of Zip1, ZnT1 and ZnT4. Although zinc is essential to cell proliferation, excessive cellular zinc accumulation causes cytotoxicity. Collectively, these observations suggest that increase in the abundance of LIPZ during growth factor-stimulated cell proliferation was due to increased net retention of extracellular zinc, which was apparently achieved through a coordinated up-regulation of the expression of transporters involved in both zinc influx and efflux to ensure adequate supply of zinc to sustain cell proliferation, yet to prevent potential zinc cytotoxicity in 3T3 cells.

Interferonbeta-induced changes in metallothionein expression and subcellular distribution of zinc in HepG2 cells

We evaluated the changes of metallothionein induction and cellular zinc distribution in HepG2 cells by interferonbeta treatment. Immunohistochemical staining of metallothionein was observed in the cytoplasm and nuclei of hepatocytes; which was observed predominantly in the cells treated with interferon and zinc compared to those with zinc alone, interferon alone or the no-treated control. The cellular zinc level was higher in order of the interferon- and zinc-treated cells, the zinc-alone-treated cells, and the interferon-alone-treated cells. Flow cytometry showed that S-phase population increased in interferon-alone-treated cells and interferon- and zinc-treated cells, but not in zinc-alone-treated ones. Cellular elemental distribution was analyzed using in-air micro-particle induced X-ray emission. In zinc-alone-treated sample, X-ray spectra showed good consistency between the enhanced cellular zinc distribution and the phosphorous map. Localizations of bromine followed by interferon treatment were found accompanying a spatial correlation with the phosphorous map. The samples treated with interferon and zinc showed the marked accumulation of zinc and bromine. Discrete bromine accumulation sites were clearly visible with a strong spatial correlation followed by zinc accumulation. These findings suggest that interferonbeta in combination with zinc predominantly induces metallothionein expression in HepG2 cells. In addition, interferonbeta may promote the translocation of metallothionein-bound zinc from cytoplasm to S-phase nuclei.

Metallothionein and liver cell regeneration

Hepatocytes in adults are in a nonproliferative state but they have high capacity to regenerate within few hours after an injury. After partial hepatectomy or chemical injury, hepatocytes undergo a synchronized multistep process consisting of priming/initiation, proliferation, and termination. These distinct steps are essential for restoring the structure and functions of liver. The mechanisms involved in each of these steps of regeneration are well documented from various laboratories and are described in several reviews. We briefly describe these steps and the involvement of various cytokines and growth factors for cell regeneration in this short review. Liver cell regeneration may also involve stem cell proliferation. The regenerating cells require large amounts of zinc within a short time, and this requirement is met by induction of a zinc and copper binding protein, metallothionein (MT), during the priming step, soon after an injury. There are several reports on the transfer of zinc from MT to various metalloenzymes and transcription factors. Genetically modified mouse models have been used to study the involvement of interleukin (IL)-6 and tumor necrosis factor (TNF)-alpha in cell regeneration. The use of an MT-knockout mouse has enabled us to investigate the specific role of MT in liver regeneration after partial hepatectomy, chemical injury, and fibrosis. Several studies have suggested a defective liver regeneration after an injury in MT-knockout mice. There is cumulative evidence that indicates an essential role for MT in liver cell regeneration.

Interaction of metallothionein with tumor suppressor p53 protein

Previous reports have shown that metallothionein (MT) may modulate p53 activity through zinc exchange. However, little is known on a direct interaction between MT and p53 in cells. The results demonstrate an interaction between MT and p53 can occur in vitro. The complex between MT and p53 was observed in breast cancer epithelial cells with both wild and inactive type of p53. Furthermore, it was shown that wt-p53 was preferentially associated with Apo-MT. Our data suggest that co-expression of MT and p53 and their complex formation in tumor cells may be involved in regulation of apoptosis in these cells.

Immune-enhancing role of vitamin C and zinc and effect on clinical conditions

Vitamin C concentrations in the plasma and leukocytes rapidly decline during infections and stress. Supplementation of vitamin C was found to improve components of the human immune system such as antimicrobial and natural killer cell activities, lymphocyte proliferation, chemotaxis, and delayed-type hypersensitivity. Vitamin C contributes to maintaining the redox integrity of cells and thereby protects them against reactive oxygen species generated during the respiratory burst and in the inflammatory response. Likewise, zinc undernutrition or deficiency was shown to impair cellular mediators of innate immunity such as phagocytosis, natural killer cell activity, and the generation of oxidative burst. Therefore, both nutrients play important roles in immune function and the modulation of host resistance to infectious agents, reducing the risk, severity, and duration of infectious diseases. This is of special importance in populations in which insufficient intake of these nutrients is prevalent. In the developing world, this is the case in low- and middle-income countries, but also in subpopulations in industrialized countries, e.g. in the elderly. A large number of randomized controlled intervention trials with intakes of up to 1 g of vitamin C and up to 30 mg of zinc are available. These trials document that adequate intakes of vitamin C and zinc ameliorate symptoms and shorten the duration of respiratory tract infections including the common cold. Furthermore, vitamin C and zinc reduce the incidence and improve the outcome of pneumonia, malaria, and diarrhea infections, especially in children in developing countries.

Metallothionein expression in benign and malignant canine mammary gland tumours

The presence of metallothioneins (MTs) were demonstrated immunohistochemically using a monoclonal antibody (E9) against a conserved epitope of I and II isoforms in canine mammary tumours. In a semiquantitative analysis MT expression in the tumour cells was observed in 54/54 cases of benign and 32/40 malignant mammary neoplasms. A statistically significant difference at the level of P<0.01 was observed for MT expression between benign and malign mammary tumours in terms of immunoreactivity score. It is concluded that immunohistochemically demonstrated MT expression is significantly associated with benign canine mammary tumours.

Metallothionein mediates leukocyte chemotaxis

BACKGROUND

Metallothionein (MT) is a cysteine-rich, metal-binding protein that can be induced by a variety of agents. Modulation of MT levels has also been shown to alter specific immune functions. We have noticed that the MT genes map close to the chemokines Ccl17 and Cx3cl1. Cysteine motifs that characterize these chemokines are also found in the MT sequence suggesting that MT might also act as a chemotactic factor.

RESULTS

In the experiments reported here, we show that immune cells migrate chemotactically in the presence of a gradient of MT. This response can be specifically blocked by two different monoclonal anti-MT antibodies. Exposure of cells to MT also leads to a rapid increase in F-actin content. Incubation of Jurkat T cells with cholera toxin or pertussis toxin completely abrogates the chemotactic response to MT. Thus MT may act via G-protein coupled receptors and through the cyclic AMP signaling pathway to initiate chemotaxis.

CONCLUSION

These results suggest that, under inflammatory conditions, metallothionein in the extracellular environment may support the beneficial movement of leukocytes to the site of inflammation. MT may therefore represent a "danger signal"; modifying the character of the immune response when cells sense cellular stress. Elevated metallothionein produced in the context of exposure to environmental toxicants, or as a result of chronic inflammatory disease, may alter the normal chemotactic responses that regulate leukocyte trafficking. Thus, MT synthesis may represent an important factor in immunomodulation that is associated with autoimmune disease and toxicant exposure.

Brain, aging and neurodegeneration: role of zinc ion availability

Actual fields of research in neurobiology are not only aimed at understanding the different aspects of brain aging but also at developing strategies useful to preserve brain compensatory capacity and to prevent the onset of neurodegenerative diseases. Consistent with this trend much attention has been addressed to zinc metabolism. In fact, zinc acts as a neuromodulator at excitatory synapses and has a considerable role in the stress response and in the functionality of zinc-dependent enzymes contributing to maintaining brain compensatory capacity. In particular, the mechanisms that modulate the free zinc pool are pivotal for safeguarding brain health and performance. Alterations in zinc homeostasis have been reported in Parkinson's and Alzheimer's disease as well as in transient forebrain ischemia, seizures and traumatic brain injury, but little is known regarding aged brain. There is much evidence that that age-related changes, frequently associated to a decline in brain functions and impaired cognitive performances, could be related to dysfunctions affecting the intracellular zinc ion availability. A general agreement emerges from studies of humans' and rodents' old brains about an increased expression of metallothionein (MT) isoforms I and II, but dyshomogenous results are reported for MT-III, and it is still uncertain whether these proteins maintain in aging the protective role, as it occurs in adult/young age. At the same time, there is considerable evidence that amyloid-beta deposition in Alzheimer's disease is induced by zinc, but the pathological significance and the causes of this phenomenon are still an open question. The scientific debate on the role of zinc and of some zinc-binding proteins in aging and neurodegenerative disorders, as well as on the beneficial effect of zinc supplementation in aged brain and neurodegeneration, is extensively discussed in this review.

Metallothionein transfers zinc to mitochondrial aconitase through a direct interaction in mouse hearts

Previous studies have shown that in a cell-free system, metallothionein (MT) releases zinc when the environment becomes oxidized and the released zinc is transferred to a zinc-binding protein if such a protein is present. However, it is unknown whether and how zinc transfers from MT to other proteins in vivo. The present study was undertaken to test the hypothesis that if zinc transfer from MT to other proteins occurs in vivo, the transfer would proceed through a direct interaction between MT and a specific group of proteins. The heart extract obtained from MT-null mice was incubated with 65Zn-MT or 65ZnCl2 and the proteins receiving 65Zn were separated by blue-native PAGE (BN-PAGE) or sodium dodecyl sulfate-PAGE (SDS-PAGE), and detected by autoradiography. A unique 65Zn-binding band was observed from the 65Zn-MT-incubated, but not the 65ZnCl2-incubated preparation. The analysis using matrix assisted laser desorption/ionization-time-of-flight (MALDI-TOF) mass spectrometry revealed that mitochondrial aconitase (m-aconitase) was among the proteins accepting Zn directly from Zn-MT. The m-aconitase, not the cytosolic aconitase (c-aconitase), was co-immunoprecipitated with MT. This study demonstrates that MT transfers zinc to m-aconitase through a direct interaction.

Impaired hepatic regeneration in metallothionein-I/II knockout mice after partial hepatectomy

Although the translocation of metallothionein (MT) from cytoplasm to nucleus has been demonstrated in liver during times of high requirement for zinc (fetal development and the neonatal period), the role of MT in cellular growth is not well understood. In this study, a potential role of MT in liver regeneration was investigated in wild type (WT) and MT-I and MT-II gene knockout (MT-null) mice after 35% partial hepatectomy (PH) or sham laparotomy. Hepatic MT levels and proliferation index were measured at 0, 5, 15, 24, 36, 48, and 60 hrs after PH and 48 hrs after sham laparotomy (control). MT levels were increased in WT mice (peak at 24 hrs after PH) and declined to normal levels by 60 hrs after PH. Immunohistochemical staining for MT in WT mice indicated the presence of MT in both nucleus and cytoplasm of hepatocytes at 24 hrs after PH, whereas MT was present mainly in the cytoplasm at 36-60 hrs after PH and 48 hrs after sham laparotomy. Hepatic proliferation index in both WT and MT-null mice, as determined by argyrophilic nucleolar organizing region staining and proliferating cell nuclear antigen immunohistochemical staining, reached a peak at 48 hrs and declined by 60 hrs after PH. Cell proliferation was significantly less in MT-null mice as compared to WT mice during liver regeneration after PH. These results suggest that MT may play a positive role in hepatic regeneration after PH.

Metallothionein expression in human neoplasia

The metallothionein family is a class of low-molecular-weight, cysteine-rich proteins with high affinity for metal ions. Four major isoforms (metallothionein-1, -2, -3, and -4) have been identified in mammals, involved in many pathophysiological processes, including metal ion homeostasis and detoxification, protection against oxidative damage, cell proliferation and apoptosis, drug and radiotherapy resistance and several aspects of the carcinogenic process. In the present review we examine the expression of metallothionein in different human tumours and its correlation with histopathological variables, tumour cell proliferation or apoptosis, resistance to radiation or chemotherapy, patient survival and prognosis. A variable profile of metallothionein and its isoforms' expression has been observed in different cancer types. Although metallothionein expression has been implicated in carcinogenic evolution, its use as a marker of tumour differentiation, cell proliferation and prognosis predictor remains unclear. Detailed studies focused on the expression of metallothionein isoforms and isotypes in different tumour types could elucidate the role of this group of proteins in the carcinogenic process, delineating its possible clinical significance for the management of patients.

Metal binding of metallothionein-3 versus metallothionein-2: lower affinity and higher plasticity

Mammalian metallothioneins (MTs) are involved in cellular metabolism of zinc and copper and in cytoprotection against toxic metals and reactive oxygen species. MT-3 plays a specific role in the brain and is down-regulated in Alzheimer's disease. To evaluate differences in metal binding, we conducted direct metal competition experiments with MT-3 and MT-2 using electrospray ionization mass spectroscopy (ESI-MS). Results demonstrate that MT-3 binds Zn2+ and Cd2+ ions more weakly than MT-2 but exposes higher metal-binding capacity and plasticity. Titration with Cd2+ ions demonstrates that metal-binding affinities of individual clusters of MT-2 and MT-3 are decreasing in the following order: four-metal cluster of MT-2>three-metal cluster of MT-2 approximately four-metal cluster of MT-3>three-metal cluster of MT-3>extra metal-binding sites of MT-3. To evaluate the reasons for weaker metal-binding affinity of MT-3 and the enhanced resistance of MT-3 towards proteolysis under zinc-depleted cellular conditions, we studied the secondary structures of apo-MT-3 and apo-MT-2 by CD spectroscopy. Results showed that apo-MT-3 and apo-MT-2 have almost equal helical content (approximately 10%) in aqueous buffer, but that MT-3 had slightly higher tendency to form alpha-helical secondary structure in TFE-water mixtures. Secondary structure predictions also indicated some differences between MT-3 and MT-2, by predicting random coil for common MTs, but 22% alpha-helical structure for MT-3. Combined, all results highlight further differences between MT-3 and common MTs, which may be related with their functional specificities.

Alterations in the Levels of Metallothionein and Metals in the Liver, and Unique Serum Liver Enzyme Response in Metallothionein Knock-Out Mice after Burn Injury

OBJECTIVES

Metallothionein (MT) is a small cysteine-rich protein that sequesters and distributes metal ions. Its overexpression stimulates cell proliferation and inhibits apoptosis. We investigated the effects of burn injury on MT expression and metal localization. We also sought to determine roles of MT in the pathophysiologic alterations in the liver after injury.

METHODS

Mice (C57BLKS/J, MT-I/II knock-out, KO, and wild-type control mice) were subjected to an 18% burn. Liver tissues harvested after injury were analyzed for the MT expression and the levels of zinc, copper, manganese, and iron. Levels of alanine aminotransferase, aspartate aminotransferase, and alkaline phosphatase were measured in serum samples from MT-I/MT-II KO mice and controls after injury.

RESULTS

Transient induction of MT-I and MT-II mRNAs was observed 3-6 h after injury, while MT-I/MT-II protein peaked on day 1. The induction was localized to hepatocytic nuclei. The intrahepatic levels of zinc, copper, and iron were transiently elevated on day 1, when a downregulation of manganese was evident. Interestingly, only the serum levels of aspartate aminotransferase were significantly augmented in MT-I/MT-II KO mice compared to controls after injury.

CONCLUSIONS

These data suggest that MT and metals may participate in the pathogenesis of the liver after burn injury.

Interprotein metal exchange between transcription factor IIIa and apo-metallothionein

Zn(2+) and Cd(2+) ion exchange between transcription factor IIIA (TFIIIA) and apo-metallothionein (MT) were studied using a combination of methods including chromatography, ultrafiltration and UV spectroscopy. Under near stoichiometric conditions, apoMT was able to remove most if not all of the zinc ions from TFIIIA, whether or not the TFIIIA was bound to the 5S DNA internal control region (ICR), and concomitantly inhibit its DNA-binding activity as indicated by an electrophoretic mobility shift assay. The kinetics of the two processes were similar. The rate of the metal exchange reaction increased with the concentrations of both reactants. A second-order rate constant of 30+/-10 M(-1)s(-1) was calculated. Similar observations were made for the reaction between apoMT and Cd-substituted TFIIIA, which proceeded without observable intermediates according to a spectrophotometric analysis. A very slow metal ion exchange occurred between Cd-TFIIIA and Zn-MT, but not between Cd-MT and Zn-TFIIIA. Comparative studies on the reaction of TFIIIA with a small competing ligand, ethylenedinitrilo-tetraacetic acid (EDTA), were also conducted. Although EDTA reacts with free Zn-TFIIIA, under similar conditions it failed to compete for Zn(2+) bound as Zn-TFIIIA-ICR.

Zinc-metallothionein genoprotective effect is independent of the glutathione depletion in HaCaT keratinocytes after solar light irradiation

UV radiations are the major environmental factors that induce DNA damage of skin cells either by direct absorption (UVB), or after inducing an oxidative stress (UVA and UVB). Cells maintain a reducing intracellular environment to avoid genomic damage. MTs have been expected not only to control metal homeostasis but also counteract the glutathione (GSH) depletion induced by oxidative stress because of their high thiol content. Induction and redistribution of MTs in cultured human keratinocytes (HaCaT) in response to SSL, is an important cellular defense mechanism against DNA damage. Reduced glutathione (GSH) is another way of cellular protection against UV-induced oxidative stress. This study which extend our previous finding focused on the relation between intracellular GSH and Zn genoprotective effects after solar irradiation. HaCaT cells, depleted or not in GSH by a chemical treatment were used to compare MTs induction by Northern blot, expression by Western blot and localization using immunocytochemistry. Zn genoprotection experiments after SSL irradiation was carried out by the comet assay. We demonstrated that in absence of GSH, Zn-MTs could protect DNA after SSL irradiation and that GSH depletion has no effect on MTs induction and localization. Nuclear Zn-MTs could be responsible for this observed genoprotection in GSH depleted cells. So the GSH/Zn and the MT/Zn systems could be two independent but interacting mechanisms of cellular protection against SSL injury.

Metallothionein mediates the level and activity of nuclear factor kappa B in murine fibroblasts

The zinc-binding protein metallothionein (MT) is associated with resistance to apoptosis. We examined whether MT regulates the zinc-dependent antiapoptotic transcription factor nuclear factor KappaB (NF-KappaB), which is up-regulated under many conditions that lead to elevated MT expression. NF-KappaB protein levels and NF-KappaB-dependent reporter gene activity were examined in clonal MT(+) (MT-WT) and MT(-) (MT-KO) fibroblastic cell lines. The amount of cellular NF-KappaB p65 protein in MT-KO was less than 20% of the amount in MT-WT cells, in accord with increased sensitivity of MT-KO cells to apoptosis. NF-KappaB p65 mRNA levels, and NF-KappaB p50 subunit and IKappaBalpha protein levels, were unchanged. NF-KappaB activity assessed by expression of a transfected NF-KappaB reporter construct was less than half that observed in MT-KO cells. Decreased nuclear localization of NF-KappaB p65 in MT-KO clones was not responsible for differences in activity. In fact, MT-KO cells had higher nuclear levels of NF-KappaB p65 than did MT-WT cells, despite a lower cellular NF-KappaB level and function, suggesting that metallothionein mediated the specific activity of NF-KappaB. Reconstitution of MT by stable incorporation of an MT-1 expression vector in MT-KO cells resulted in increased NF-KappaB p65 (but not IKappaBalpha or NF-KappaB p50), increased NF-KappaB-dependent reporter activity, and increased resistance to apoptosis. These data support the hypothesis that metallothionein positively regulates the cellular level and activity of NF-KappaB.

Differentially expressed genes in HIV-1-infected macrophages following treatment with the virus-suppressive immunomodulator murabutide

The synthetic immunomodulator murabutide has been found to suppress human immunodeficiency virus type-1 (HIV-1) replication, in macrophages, through a regulated expression of cellular factors needed at different steps in the virus replication cycle. To identify cellular genes implicated in the murabutide-induced virus inhibition, we have carried out a differential display analysis on HIV-1-infected macrophages that were treated, or not, with murabutide. Sequencing of the differentially regulated cDNA bands and verification of the reproducibility of the murabutide effects, by reverse transcription-polymerase chain reaction or by Northern blotting, revealed an up-regulated expression of 21 genes and a down-regulation of seven others. The murabutide-regulated genes encoded proteins implicated in DNA binding, regulation of transcription, oxidative stress, metal binding, and other physiological functions. Six of the genes corresponded to unassigned/expressed sequence tags with yet unknown function. Among the genes which were up-regulated by murabutide and with established effects on inhibiting virus transcription, was the octamer binding factor 1 (Oct-1). We demonstrate the ability of murabutide to induce enhanced Oct-1 protein expression and DNA-binding activity in macrophages. Furthermore, our findings suggest the potential implication of additional transcription factors and metal-binding proteins in mediating the inhibitory effect of murabutide on virus transcription.

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

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

Biological chemistry of reactive oxygen and nitrogen and radiation-induced signal transduction mechanisms

In the past few years, nuclear DNA damage-sensing mechanisms activated by ionizing radiation have been identified, including ATM/ATR and the DNA-dependent protein kinase. Less is known about sensing mechanisms for cytoplasmic ionization events and how these events influence nuclear processes. Several studies have demonstrated the importance of cytoplasmic signaling pathways in cytoprotection and mutagenesis. For cytoplasmic signaling, radiation-stimulated reactive oxygen species (ROS) and reactive nitrogen species (RNS) are essential activators of these pathways. This review summarizes recent studies on the chemistry of radiation-induced ROS/RNS generation and emphasizes interactions between ROS and RNS and the relative roles of cellular ROS/RNS generators as amplifiers of the initial ionization events. Cellular mechanisms for regulating ROS/RNS levels are discussed. The mechanisms by which cells sense ROS/RNS are examined in terms of how ROS/RNS modify protein structure and function, for example, interactions with metal-thiol clusters, protein tyrosine nitration, protein cysteine oxidation, S-thiolation and S-nitrosylation. We propose that radiation-induced ROS are the initiators and that nitric oxide (NO*) or derivatives are the effectors activating these signal transduction pathways. In responding to cellular ionization events, the cell converts an oxidative signal to a nitrosative one because ROS are too reactive and unspecific in their reactions for regulatory purposes and the cell is equipped to precisely modulate NO* levels.

Role of metallothionein in regulating the abundance of histochemically reactive zinc in rat tissues

The objectives of this study were (i) to investigate the modulating effects of zinc nutrition on histochemically reactive zinc in the rat intestine and liver and (ii) to assess the relationship between histochemically reactive zinc and metallothionein-bound zinc in these tissues under varying zinc nutrition. Male Wistar rats were fed a zinc-deficient (3 mg zinc/kg), adequate-zinc (30 mg zinc/kg, ad libitum or pair-fed), or zinc-supplemented (155 mg zinc/kg) diet for 2 or 6 weeks. Plasma N-(6-methoxy-8-quinolyl)-para-toluenesulfonamide-reactive zinc reflected dietary zinc intake. Abundance of the intestine histochemically reactive zinc was correlated with dietary zinc intake after 2 weeks of dietary treatment. Dietary zinc intake had no effect on the abundance of the intestine histochemically reactive zinc after 6 weeks of dietary treatment and the hepatic histochemically reactive zinc after both 2 and 6 weeks of dietary treatment. This lack of effect of dietary zinc intake on the abundance of histochemically reactive zinc was associated with a higher level of metallothionein. The molecular-mass distribution profile revealed that N-(6-methoxy-8-quinolyl)-para-toluenesulfonamide-reactive zinc and metallothionein-bound zinc represented two different, but interrelated, pools of zinc. Overall, these results suggested that the abundance of histochemically reactive zinc was homeostatically regulated, which was partially achieved through the regulation of metallothionein levels in rats.

Zinc-induced NF-kappaB inhibition can be modulated by changes in the intracellular metallothionein level

Metallothionein (MT), a small metal-binding protein, is involved in the regulation of cellular metal homeostasis. Sequestration and the release of metals to and from MT plays an important role in the attenuation or amplification of signal transduction. Zinc has been suggested to be an important regulator of nuclear factor kappaB (NF-kappaB). In this study, the effect of MT expression on the zinc-induced inhibition of NF-kappaB activity was examined. In HeLa cells, pyrrolidine dithiocarbamate (PDTC), a zinc ionophore, and zinc itself inhibited NF-kappaB activity. When the cells were pretreated with MT-inducers, cadmium, or dexamethasone, PDTC did not inhibit NF-kappaB activity. We transfected HeLa cells with a DNA construct in which expression of MT-IIA is controlled by tet operator protein. Treatment of HeLa cells with doxycycline, a tetracycline analogue, induced the expression of MT-IIA, which attenuated the effect of PDTC on NF-kappaB activity. These results implicate MT in the zinc regulation of NF-kappaB and identify MT as one of the potential intracellular modulators of NF-kappaB activation.

Zn(II) metabolism in prokaryotes

It is difficult to over-state the importance of Zn(II) in biology. It is a ubiquitous essential metal ion and plays a role in catalysis, protein structure and perhaps as a signal molecule, in organisms from all three kingdoms. Of necessity, organisms have evolved to optimise the intracellular availability of Zn(II) despite the extracellular milieu. To this end, prokaryotes contain a range of Zn(II) import, Zn(II) export and/or binding proteins, some of which utilise either ATP or the chemiosmotic potential to drive the movement of Zn(II) across the cytosolic membrane, together with proteins that facilitate the diffusion of this ion across either the outer or inner membranes of prokaryotes. This review seeks to give an overview of the systems currently classified as altering Zn(II) availability in prokaryotes.

Astrocyte-targeted expression of IL-6 protects the CNS against a focal brain injury

The effect of CNS-targeted IL-6 gene expression has been thoroughly investigated in the otherwise nonperturbed brain but not following brain injury. Here we examined the impact of astrocyte-targeted IL-6 production in a traumatic brain injury (cryolesion) model using GFAP-IL6 transgenic mice. This study demonstrated that transgenic IL-6 production significantly increased wound healing following the cryolesion. Thus, at 20 days postlesion (dpl) the GFAP-IL6 mice showed almost complete wound healing compared to litter mate nontransgenic controls. It seems likely that a reduced inflammatory response in the long term could be responsible for this IL-6-related effect. Thus, while in the acute phase following cryolesion (1-6 dpl) the recruitment of macrophages and T lymphocytes was higher in GFAP-IL6 mice, at 10-20 dpl it was significantly reduced compared to controls. Reactive astrogliosis was also significantly increased up to but not including 20 dpl in the GFAP-IL6 mice. Oxidative stress as well as apoptotic cell death was significantly decreased throughout the time period studied in the GFAP-IL6 mice compared to controls. This could be linked to the altered inflammatory response as well as to the transgenic IL-6-induced increase of the antioxidant, neuroprotective proteins metallothionein-I + II. These results indicate that although in the brain the chronic astrocyte-targeted expression of IL-6 spontaneously induces an inflammatory response causing significant damage, during an acute neuropathological insult such as following traumatic injury, a clear neuroprotective role is evident.

Isolation and characterisation of metallothionein from the clam Ruditapes decussatus

Metallothioneins (MT) were obtained after purification from metal-exposed clams (Ruditapes decussatus) using gel-permeation and ion-exchange chromatography. Four cadmium-metallothioneins (CdMTs) were resolved by ion-exchange chromatography and they all had similar molecular weights, high cadmium content and an absorption spectra indicative of the presence of characteristic Cd-S aggregates. The NH(2)-terminal sequence suggests the presence of at least two class I clam MT isoforms. For the other two putative clam CdMTs isolated, the results of the amino acid determination were inconclusive. One was slightly contaminated and the other one had a blocked NH(2)-terminal. These clam metalothioneins contain glycine, which seems to be a common feature of molluscan MT family and exhibited more similarity to oysters than to mussels. Further investigation on the inducibility of these isoforms will be necessary if clams are to be used as biomarkers of metal exposure.

A new metallothionein gene from the giant keyhole limpet Megathura crenulata

Metallothioneins (MTs) are small soluble proteins ubiquitously expressed in animals and plants. Different isoforms are present in deuterostomes and protostomes. They do not differ greatly in primary structure, but are clearly distinguishable. Here, I present the gene and the complete cDNA of a novel MT from the mollusk Megathura crenulata. This protein is closely related to the Cu-inducible MTs of the vineyard snail Helix pomatia, but has also some minor sequence features typical of Cd-inducible isoforms of H. pomatia and other molluscs. Overall, the deduced primary structure is similar to the known molluscan MTs, but in addition possesses an insertion of 5 amino acids not found in any other molluscan MTs, protostomic or deuterostomic MTs. In addition, a pentapeptide insertion, characteristic of mammalian MT-3 is present but it lacks the functional tetrapeptide CPCP within the beta-region of those MT-3 proteins that are known to suppress neuronal growth processes. The M. crenulata MT is a novel form of MT in comparison to all other known MTs. Possible functional aspects for this new MT are discussed.

Metallothionein is required for zinc-induced expression of the macrophage colony stimulating factor gene

Macrophage colony stimulating factor (M-CSF) plays an important role in the proliferation and differentiation of mononuclear phagocytes. The present study investigates the effect of zinc on M-CSF expression in MC3T3-E1 and L929 cells. Zinc dose-dependently increased M-CSF mRNA levels. The time-course of zinc-induced M-CSF mRNA expression peaked at 6 h. Stability studies of mRNA using actinomycin D revealed that zinc does not affect M-CSF mRNA stability. We examined the function of the M-CSF gene promoter using a luciferase reporter assay. A construct containing the -467/+39 region of the promoter was upregulated by zinc. In the presence of cycloheximide, zinc did not induce a greater increase in the M-CSF mRNA than cycloheximide alone. To confirm the effect of MT on M-CSF mRNA expression, mouse lung fibroblasts (MLFs) were prepared from MT+/+ and MT-/- mice. Zinc induced an increase in the expression of M-CSF in MT+/+ MLFs, but this response was not evident in MT-/- MLFs. Moreover, overexpression of MT upregulated M-CSF mRNA expression as well as M-CSF secretion. Our findings suggest that MT expression mediates zinc regulation of M-CSF gene expression at the transcriptional level.

Effects of cadmium and zinc on solar-simulated light-irradiated cells: potential role of zinc-metallothionein in zinc-induced genoprotection

Zinc is an essential oligoelement for cell growth and cell survival and has been demonstrated to protect cells from oxidative stress induced by UVA or from genotoxic stress due to UVB. In a recent work we demonstrated that the antioxidant role of zinc could be related to its ability to induce metallothioneins (MTs). In this study we identified the mechanism of zinc protection against solar-simulated light (SSL) injury. Cultured human keratinocytes (HaCaT) were used to examine MTs expression and localization in response to solar-simulated radiation. We found translocation to the nucleus, with overexpression of MTs in irradiated cells, a novel observation. The genoprotective effect of zinc was dependent on time and protein synthesis. DNA damage was significantly decreased after 48 h of ZnCl(2) (100 microM) treatment and is inhibited by actinomycin D. ZnCl(2) treatment (100 microM) led to an intense induction, redistribution, and accumulation of MT in the nucleus of irradiated cells. MT expression correlated with the time period of ZnCl(2) treatment. CdCl(2), a potent MT inducer, did not show any genoprotection, although the MTs were expressed in the nucleus. Overall our findings demonstrate that MTs could be a good candidate for explaining the genoprotection mediated by zinc on irradiated cells.

Prior increase in metallothionein levels is required to prevent doxorubicin cardiotoxicity

Many studies have shown that metallothionein (MT) can be increased significantly by different oxidative insults in multiple organ systems. However, the increase in MT production often fails to protect against oxidative tissue injury. On the other hand, recent studies using a cardiac-specific, MT-overexpressing, transgenic mouse model have shown that MT protects against oxidative heart injury. Thus, the present study was undertaken to test the hypothesis that prior increase in MT levels is required to prevent oxidative injury. Oxidative heart injury was induced by doxorubicin (DOX), an important anticancer drug that causes severe cardiotoxicity through oxidative stress. Cardiac-specific, MT-overexpressing, transgenic mice and wild-type (WT) FVB mice were treated with DOX at 20 mg/kg. Four days after the treatment, MT concentrations were markedly elevated in the WT mouse heart. The elevated MT concentrations were comparable with those found in the transgenic mouse heart, which did not show further MT elevation in response to DOX challenge. Severe oxidative injury occurred in the heart of WT mice, including myocardial lipid peroxidation, morphological changes as examined by electron microscopy, high levels of serum creatine kinase activity, and decreased total glutathione concentrations in the heart. However, all of these pathological changes were significantly inhibited in the MT-transgenic mice. Therefore, this study demonstrates that there is a correlation between MT induction and oxidative stress in the DOX-treated mouse heart. However, MT can protect the heart from oxidative injury only if it is present prior to induction of oxidative stress.

Metallothionein: early marker in the carcinogenesis of ulcerative colitis-associated colorectal carcinoma

Metallothioneins (MTs) are zinc-binding proteins whose overexpression may lead to sequestration of zinc ions and consequently to functional inactivation of the p53 tumor suppressor gene. The aim of the study was to investigate the potential role of MTs in the carcinogenesis of ulcerative colitis (UC) as well as possible effects on p53 function. The monoclonal antibodies E9 (anti-MT), DO-7, and 1801 (anti-p53) and the polyclonal antibody CM-1 (anti-p53) were used to stain formalin-fixed, paraffin-embedded colon specimens obtained from 14 patients with UC-associated colorectal carcinoma (CAC), 13 with high-grade dysplasia (HGD), 10 with low-grade dysplasia (LGD), and 30 with UC without dysplasia or carcinoma. Statistical significance (p <0.05) was assessed using Fisher's exact test. Positive MT staining (> 20% of tumor, dysplastic, or epithelial cells) was found in most UC and LGD but in only a small percentage of HGD and CAC (p <0.01 for CAC vs. UC and LGD vs. HGD). Positive p53 immunoreactivity was observed predominantly in HGD and CAC but not in LGD and UC (p <0.01 for CAC vs. UC and HGD vs. LGD). In histologically normal tissue neighboring CAC, significant MT expression was found in six of seven specimens with simultaneous lack of p53 expression. MT overexpression may represent an important early step in the development of CAC independent of p53 expression and should be investigated in the long term as an independent cancer risk factor in UC.

Role of de novo DNA methyltransferases and methyl CpG-binding proteins in gene silencing in a rat hepatoma

The expression of metallothionein-I (MT-I), a known antioxidant, was suppressed in a transplanted rat hepatoma because of promoter methylation and was induced by heavy metals only after demethylation by 5-azacytidine (5-AzaC). Treatment of the tumor-bearing rats with 5-AzaC resulted in significant regression of the hepatoma. When the inhibitor-treated tumor was allowed to grow in a new host, MT-I promoter was remethylated, which suggested de novo methylation. The activities of both de novo (3-fold) and maintenance DNA methyltransferases (DNMT) (5-fold) were higher in the hepatoma than in the host liver. The mRNA levels of the de novo methyltransferases DNMT3a and DNMT3b were 3- and 6-fold higher, respectively, in the tumor implicating transcriptional up-regulation of these two genes in this tissue. Immunohistochemical analysis showed exclusive localization of DNMT3a in the nuclei of both the liver and hepatoma, whereas DNMT3b was detected in the nuclei as well as the cytoplasm. Immunoblot assay showed that the levels of DNMT1, DNMT3a, and DNMT3b proteins in the hepatoma were 5-, 10-, and 4-fold higher, respectively, than in the liver. The mRNA level of the major methyl CpG-binding protein (MeCP2) was 8-fold higher in the tumor compared with the liver. Immunohistochemical studies showed that MeCP2 is localized exclusively in the nuclei of both tissues. A chromatin immunoprecipitation assay demonstrated that MeCP2 was associated with the MT-I promoter in the hepatoma implicating its involvement in repressing the methylated promoter. Analysis of the DNA isolated from the liver and hepatoma by RLGS-M (restriction landmark genomic scanning with methylation-sensitive enzyme) (NotI) showed that many genes in addition to MT-I were methylated in the hepatoma. These data demonstrate suppression of the MT-I gene and probably other genes in a solid tumor by promoter methylation and have provided potential molecular mechanisms for the altered methylation profile of the genes in this tumor.

Induction of a putative metallothionein gene in the blood cockle, Anadara granosa, exposed to cadmium

The relationship between a putative metallothionein gene (MT) and exposure to cadmium (Cd) in blood cockles (Anadara granosa) is reported. In a 96-h dose-response experiment, mortality of cockles was found to proportionately increase in the range of 0.2-5.0 mg/l Cd with a calculated LC(50) of 2.94 mg/l. Exposure to 0.25 mg/l Cd for 16 days caused significant increases (P<0.05) in Cd concentrations in whole tissues, gills and hepatopancreas, and the accumulation of Cd in these tissues increased with the duration of exposure. Two cDNA libraries constructed using the hepatopancreas from control and Cd-treated cockles gave titres of 5.62 x 10(5) and 1.94 x 10(5) pfu/microg vector, respectively. A putative MT gene, AnaMT, of 510 nucleotides in length, was isolated from the treated cDNA library using a heterologous probe MT20 from the blue mussel, Mytilus edulis. Northern analyses using AnaMT as a probe indicated low expression of the MT mRNA in control animals. In cockles treated with 0.25 mg/l Cd for 4 days, MT mRNA level increased to approximately 168%, but declined to 108% at day 8. After 12 and 16 days of Cd treatment, expression of the MT gene was 138% and 187%, respectively, compared to the controls. These observations suggest that induction of the MT gene by a sublethal dose of Cd is rapid, occurring within 4 days of treatment.

Nitric oxide-induced changes in intracellular zinc homeostasis are mediated by metallothionein/thionein

We hypothesized that metallothionein (MT), a cysteine-rich protein with a strong affinity for Zn(2+), plays a role in nitric oxide (NO) signaling events via sequestration or release of Zn(2+) by the unique thiolate clusters of the protein. Exposing mouse lung fibroblasts (MLF) to the NO donor S-nitrosocysteine resulted in 20-30% increases in fluorescence of the Zn(2+)-specific fluorophore Zinquin that were rapidly reversed by the Zn(2+) chelator N,N,N',N'-tetrakis-(2-pyridylmethyl)ethylenediamine. The absence of a NO-mediated increase in labile Zn(2+) in MLF from MT knockouts and its restoration after MT complementation by adenoviral gene transfer inferred a critical role for MT in the regulation of Zn(2+) homeostasis by NO. Additional data obtained in sheep pulmonary artery endothelial cells suggested a role for the apo form of MT, thionein (T), as a Zn(2+)-binding protein in intact cells, as overexpression of MT caused inhibition of NO-induced changes in labile Zn(2+) that were reversed by Zn(2+) supplementation. Furthermore, fluorescence-resonance energy-transfer data showed that overexpression of green fluorescent protein-modified MT prevented NO-induced conformational changes, which are indicative of Zn(2+) release from thiolate clusters. This effect was restored by Zn(2+) supplementation. Collectively, these data show that MT mediates NO-induced changes in intracellular Zn(2+) and suggest that the ratio of MT to T can regulate Zn(2+) homeostasis in response to nitrosative stress.

Metallothionein modulates lipopolysaccharide-stimulated tumour necrosis factor expression in mouse peritoneal macrophages

Metallothionein (MT) is a low-molecular-mass, cysteine-rich metal binding protein thought to be involved in the detoxification of heavy metals and scavenging of free radicals. MT is directly induced not only by heavy metals, but also by hormones and cytokines. The present study, which uses mice with genetic deletions of the MT proteins (MT(-/-) mice), was designed to evaluate the effects of MT on the expression of pro-inflammatory cytokines in macrophages. We found that the production of tumour necrosis factor (TNF) induced by lipopolysaccharide (LPS) in peritoneal macrophages is up-regulated by MT via the modulation of nuclear factor kappaB (NF-kappaB) activity. This conclusion is supported by the following observations: (1) LPS stimulated the secretion of less TNF activity from MT(-/-) peritoneal exudate macrophages (PEMs) than from wild-type controls (MT(+/+) mice) without a difference in the pattern of kinetics; (2) LPS-stimulated expression of TNF-alpha mRNA was decreased in MT(-/-) PEMs; (3) LPS-stimulated activation of NF-kappaB was decreased in MT(-/-) PEMs; and (4) production of TNF in PEMs of MT(-/-) mice after LPS treatment in vivo was decreased (compared with MT(+/+) PEMs). Expression of other inflammatory cytokines, interleukin (IL)-1alpha and IL-6 mRNA, which were modulated by NF-kappaB, were also down-regulated in MT(-/-) PEMs. Thus MT plays a key role in the LPS-induced activation of PEMs via the modulation of NF-kappaB activity.

Zinc-metallothionein levels are correlated with enhanced glucocorticoid responsiveness in mouse cells exposed to ZnCl(2), HgCl(2), and heat shock

Metallothioneins (MTs) are the major low molecular weight, zinc-binding proteins in mammalian cells. It has been hypothesized that they play a role in the function of zinc-dependent signal transduction proteins and transcription factors. We investigated the capacity of zinc and other metal ions and conditions to increase both Zn-associated MT levels and the receptiveness of cells to transcriptional activation mediated by the zinc-dependent glucocorticoid receptor (GR). We studied, in a GR-responsive mouse mammary-tumor cell line, the ability of dexamethasone (DEX) to stimulate transcription of a chloramphenicol acetyltransferase (CAT) gene controlled by a mouse mammary-tumor virus promoter. In cells pretreated with 20 to 100 microM ZnCl(2), DEX-induced CAT activity correlated with zinc-induced MT levels. However, 0.05 to 0.5 microM CdCl(2) had no effect on CAT activity, despite an increase in Cd-associated MT. Copper-associated MT was detected in cells treated with 20 microM CuCl(2,) but there was no change in the level of Zn-MT, nor was CAT activity altered in cells exposed to 5 to 20 microM CuCl(2). These results may reflect a functional difference between zinc-associated MT, and MT associated with other metals. Significantly more CAT activity was observed in both heat-shocked cells and in cells exposed to 40 or 50 nM HgCl(2). Although absolute amounts of MT were unchanged by these two treatments, a higher percentage of total cellular zinc was associated with the MT protein fractions after treatment. Changes in GR levels could not account for variations in CAT activity. These data indicate that hormonal signalling can be altered by exposure to metal salts and heat shock, and the effect is correlated with the level of Zn-MT.

Putting its fingers on stressful situations: the heavy metal-regulatory transcription factor MTF-1

It has been suggested that metallothioneins, discovered about 45 years ago, play a central role in heavy metal metabolism and detoxification, and in the management of various forms of stress. The metal-regulatory transcription factor-1 (MTF-1) was shown to be essential for basal and heavy metal-induced transcription of the stress-responsive metallothionein-I and metallothionein-II. Recently it has become obvious that MTF-1 has further roles in the transcriptional regulation of genes induced by various stressors and might even contribute to some aspects of malignant cell growth. Furthermore, MTF-1 is an essential gene, as mice null-mutant for MTF-1 die in utero due to liver degeneration. We describe here the state of knowledge on the complex activation of MTF-1, and propose a model with MTF-1 as an interconnected cellular stress-sensor protein involved in heavy metal metabolism, hepatocyte differentiation and detoxification of toxic agents.

Metallothionein expression in canine and feline mammary and melanotic tumours

Moderate to strong immunohistochemical metallothionein (MT) positivity (MT expression) is associated with a poor prognosis in some human tumours. The aim of this study was to determine MT expression in mammary tumours and cutaneous melanomas in dogs and cats. Canine (67) and feline (47) mammary tumours, and cutaneous melanomas (canine 40, feline 26) were immunolabelled with MT monoclonal antibody E9. The overall incidence of MT expression of these tumours was similar to that observed in various human neoplasms. However, a striking interspecies difference was detected. In dogs, MT expression occurred in 100% of benign and 57% of malignant mammary tumours. In cats, however, 30% of malignant mammary tumours expressed MT but benign mammary tumours and cases of fibroadenomatous hyperplasia did not. Moderate to strong MT immunoreactivity was detected in 30% of benign and 25% of malignant cutaneous melanomas in dogs, and in 6% of malignant melanomas in cats. The findings in feline mammary tumours resembled findings reported in human breast cancer, but the cause of tumour-associated MT expression is unknown. Studies are in progress to determine whether the MT state (apo [metal-free] or holo [metal-bound]) accounts for the paradoxical association of MT expression with individual types of tumours and the animal species in which they arise.

Oxidation of zinc finger transcription factors: physiological consequences

Redox-sensitive cysteine residues are present in the interaction domains of many protein complexes. There are examples in all of the major categories of transcription factors, including basic region, leucine zipper, helix-loop-helix, and zinc finger. Zinc finger structures require at least two zinc-coordinated cysteine sulfhydryl groups, and oxidation or alkylation of these can eliminate DNA-binding and transcriptional functions. We review here the evidence for oxidation of zinc finger cysteines, the pathways and reactive oxygen intermediates involved, and the functional and physiological consequences of these reactions. Despite skepticism that the strongly reducing intracellular environment would permit significant oxidation of cysteine residues within zinc finger transcription factors, there is compelling evidence that oxidation occurs both in vitro and in vivo. Early reports demonstrating reversible oxidation of zinc-coordinated cysteines with loss of binding function in vitro were shown to reflect accurately the changes in intact cells, and these in turn have been shown to correlate with physiological changes. In particular, the accumulation of oxidized Spl zinc fingers during aging, and estrogen receptors in tamoxifen-resistant breast cancers are dramatic examples of what may be a general sensitivity of zinc finger factors to changes in the redox state of the cell.

Oxidation of zinc-binding cysteine residues in transcription factor proteins

Recent results on the oxidation of cysteine residues that bind zinc in transcription factors and their analogous peptides and in related proteins and model systems are reviewed. Two classes of oxidants, the transition metals and dioxygen, hydrogen peroxide, and related species, are considered, and the role of metal ions in suppressing or enhancing Cys oxidation is a major focus. Cysteines in the zinc-bound structures of transcription factors are less susceptible to oxidation than in the metal-free form, and this appears to correlate with reduced accessibility of the thiolates to oxidants. Substitution of other metal ions for Zn(II) increases the rate of Cys oxidation, apparently through increased oxidant accessibility. Reactions that result in reversible or irreversible oxidation of these zinc-binding cysteines under biological conditions are identified in the context of deleterious implications for gene expression.

Metallothionein isoform 3 overexpression is associated with breast cancers having a poor prognosis

The third isoform (MT-3) of the metallothionein gene family is unique in that it has a limited tissue distribution, is not induced by metals, has a neuronal growth inhibitory activity, and sequesters zinc more effectively under zinc-depleted conditions. The goal of the present study was to determine whether MT-3 was absent in normal breast tissue, was overexpressed in breast cancers, and if MT-3 overexpression would be associated with disease outcome. A combination of immunohistochemistry and reverse-transcription polymerase chain reaction was used to demonstrate that the normal breast had no detectable expression of MT-3 mRNA or protein. Using immunohistochemistry, it was shown that MT-3 was overexpressed in 25 of 34 cases of breast cancer. In all cases of positive staining, MT-3 was diffusely localized to the cytoplasm. The tumors from these 34 cases were divided as to outcome based on known 5-year survival, with 20 patients being disease free at 5 years (good outcome) and the other 14 having recurring disease within 5 years (bad outcome). When analyzed for MT-3 staining, it was shown that there was a trend for increased MT-3 immunoreactivity in the group having bad outcomes. However, when the tumor subgrouping was further defined on the basis of carcinoma in situ (CIS), there was a marked significant difference in MT-3 staining between patients with good and bad outcomes. Limited to DCIS, MT-3 staining was significantly increased in patients with bad outcomes compared to those with good outcomes. Thus, these studies demonstrate that MT-3 is overexpressed in selected breast cancers and that overexpression is associated with tumors having a poor prognosis.

In vivo manipulation of endogenous metallothionein with a monoclonal antibody enhances a T-dependent humoral immune response

Metallothionein (MT) is a small stress response protein that can be induced by exposure to heavy metal cations, oxidative stressors, and acute phase cytokines that mediate inflammation. In previous experiments, we have shown that exogenous MT can affect cell proliferation, macrophage and cytotoxic T lymphocyte function, and humoral immunity to T-dependent antigens. In the studies described here, we have explored the effect of a monoclonal anti-MT antibody (clone UC1MT) on the role that endogenous MT plays in the humoral immune response. In vivo injection of UC1MT significantly increased the humoral response to simultaneous challenge with ovalbumin (OVA). In contrast, mice immunized with OVA in the presence of an isotype-matched antibody control (MOPC 21) showed no change in the anti-OVA humoral response. The predominant anti-OVA response that was enhanced by UC1MT treatment was the IgG(1) response; the IgG(2a) anti-OVA response was not altered by UC1MT treatment. UC1MT treatment increased the numbers of IgG anti-OVA secreting cells as measured by ELISPOT assay, suggesting that blocking the effects of MT synthesized during the immune response augments the differentiation of antigen-specific plasma cells. The percentages of T and B cells in the spleens of animals from each treatment group were not significantly different, suggesting that this regimen of UC1MT treatment does not significantly affect hematopoiesis, but rather alters antigen-induced differentiation of lymphocytes. These observations are compatible with previous results from our laboratory that suggest that endogenous MT synthesized during the normal immune response or as a consequence of toxicant exposure suppresses in vivo immune function. In light of the fact that significant amounts of MT can be synthesized during toxicant exposure, manipulation of MT levels with an anti-MT antibody may ultimately represent an important therapeutic approach to the treatment of immune dysfunctions that result from toxicant exposure.

Zinc-induced activation of the human cytomegalovirus major immediate-early promoter is mediated by metallothionein and nuclear factor-kappaB

We previously reported that major immediate-early promoter (MIEP) activity was regulated by intercellular zinc levels. In this report, we elucidate the mechanisms involved in this phenomenon. In luciferase reporter assays, zinc-induced activation of MIEP (-735/+62) was decreased with deletion of the promoter in stages, and MIEP (-117/+62) did not respond to zinc. The time course of the activity of MIEP responding to diethylenetriamine pentaacetic acid and zinc was not parallel with metallothionein (MT) promoter, which contains metal responsive elements. SV40 promoter that contains AP-1 binding sites, a candidate for the zinc-responsive motif in the MIEP, was not affected by zinc under our conditions. The activation of MIEP (-735/+62) by zinc was prevented with NF-kappaB decoy. When three kappaB motifs from the enhancer in the MIEP were inserted in the front of the zinc-nonresponsive MIEP (-117/+62), it became responsive to zinc. Moreover, overexpression of MT up-regulates the DNA binding of NF-kappaB and NF-kappaB-induced activation of transcription. These findings strongly suggest that MT and NF-kappaB act as mediator/regulator in zinc-induced activation of MIEP.

Regulation of metallothionein gene expression

The rapid and robust induction of metallothioneins (MT)-I and II by a variety of inducers that include heavy toxic metals, reactive oxygen species, and different types of stress provide a useful system to study the molecular mechanisms of this unique induction process. The specific expression of MT-III in the brain and of MT-IV in the squamous epithelium of skin and tongue offers a unique opportunity to identify and characterize the tissue-specific factors involved in their expression. Studies using transgenic mice that overexpress MTs or MT null mice have revealed the role of MT in the protection of cells against numerous tissue-damaging agents such as reactive oxygen species. The primary physiological function of these proteins, however, remains an enigma. Considerable advances have been made in the identification of the cis-acting elements that are involved in the constitutive and induced expression of MT-I and MT-II. By contrast, only one key trans-activating factor, namely MTF-1, has been extensively characterized. Studies on the epigenetic silencing of MT-I and MT-II by promoter hypermethylation in some cancer cells have posed interesting questions concerning the functional relevance of MT gene silencing, the molecular mechanisms of MT suppression in these cells, particularly chromatin modifications, and the characteristics of the repressors.

Comparative effects of metal chelating agents on the neuronal cytotoxicity induced by copper (Cu+2), iron (Fe+3) and zinc in the hippocampus

The ability of metal chelating agents to prevent neuronal death caused by intra-hippocampal injections of cupric sulphate, ferric citrate and zinc chloride was investigated. Ammonium tetrathiomolybdate was itself toxic after injection into the hippocampus, but this toxicity was reduced by formation of a metal ion/tetrathiomolybdate complex with Cu+2. Disodium bathocuproine disulphonate (BCDS) prevented neuronal death caused by Cu+2, but not that induced by Fe+3 or Zn+2. Desferrioxamine prevented death caused by Fe+3, had no significant effect of the toxicity of Zn+2, and increased that caused by Cu+2. Even though N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine (TPEN) has a higher affinity for Cu+2 than for Zn+2, TPEN had no effect on the toxicity of Cu+2 while totally preventing damage caused by Fe+3 or Zn+2. Ethylenediaminetetra-acetic acid (EDTA) prevented the toxicity of all three metal ions. Motor seizure activity occurred in most rats after injections of Fe+3; or combinations of Cu+2 plus TPEN, or 4 nmol Fe+3 plus 0.1 nmol desferrioxamine. However, apart from the low dose desferrioxamine/Fe+3 combination, only the occasional brain contained seizure-induced neuronal loss in limbic regions outside the injected hippocampus, and these brains were not used for analysis. Seizure activity was found even with very low levels of Cu+2 with a fixed amount of TPEN (a ratio of Cu+2/TPEN of 1:100), but the extent of hippocampal damage in these brains was not significantly different to that caused by injections of saline. These studies demonstrate that idiosyncratic interactions can occur between metal ions and chelating agents. Thus further investigations are needed before chelating agents can be examined for their protective properties in various neurodegenerative diseases.

Reactivity of zinc finger cores: analysis of protein packing and electrostatic screening

The chemical stability of 207 zinc fingers, derived from 92 experimental protein structures, is evaluated according to the protein packing and electrostatic screening of their zinc cores. These properties are used as measures of the protein protection of zinc cores, to predictively rank relative zinc finger reactivities and assess differences in function. On average, there is a substantial and concomitant increase in the screening of increasingly anionic core motifs, suggesting zinc fingers have evolved in a manner that promotes shielding of their potentially reactive core thiolates. In contrast, enzymatic zinc cores are functionally differentiated by negative electrostatic screening. Zinc finger cores are predominantly screened by networks of backbone:core NH-S hydrogen bonds that electronically stabilize core thiolates and enhance backbone packing. Stabilizing protein:core interactions can be mapped to conserved residues, including [Arg,Lys]:core salt-bridges in some protein families. Labile zinc fingers are identified by poorly screened cores, possibly indicating redox or metallothionein (MT) regulated function. Consistent with experiment, the cores of the C-terminal finger of the human immunodeficiency virus type 1 (HIV-1) nucleocapsid protein p7 (NCp7) and Escherichia coli Ada protein (Ada) "finger" are identified as reactive. The C-terminal zinc fingers of nuclear receptors are predicted to be the most labile in this study, particularly the human estrogen receptor (hER), which contains a triad of reactive thiolates. We propose that hER DNA binding is redox and MT regulated through the C-terminal finger and that weak electrophilic agents may inhibit hER-mediated transcription, implicated in breast cancer progression.

High yield expression and single step purification of human thionein/metallothionein

Human metallothionein (MT), isoform 2, was expressed in Escherichia coli as an intein (protein splicing element) fusion protein in the absence of added metals and purified by intein-mediated purification with an affinity chitin-binding tag (IMPACT system). This procedure constitutes a novel and simple strategy to prepare thionein (T), the metal-free form, or MT when reconstituting T with metals in vitro. The yield was 8 mg of T or 6 mg of pure Cd(7)- or Zn(7)-MT from a 1-L culture, significantly higher than yields from any other expression system. Purified recombinant protein is indistinguishable from the native protein on the basis of its metal-binding ability, titration of its sulfhydryls, and UV and CD spectra. The MALDI-TOF mass spectrum is consistent with that of T with a free N-terminus.

Metalloregulation of the tumor suppressor protein p53: zinc mediates the renaturation of p53 after exposure to metal chelators in vitro and in intact cells

The tumor suppressor p53 is a transcription factor which binds DNA through a structurally complex domain stabilized by a zinc atom. Zinc chelation disrupts the architecture of this domain, inducing the protein to adopt an immunological phenotype identical to that of many mutant forms of p53. In this report, we used 65Zn to show that incorporation of zinc within the protein was required for folding in the 'wild-type' conformation capable of specific DNA-binding. Using a cellular assay, we show that addition of extracellular zinc at concentrations within the physiological range (5 microM) was required for renaturation and reactivation of wild-type p53. Among other divalent metals tested (Cd2+, Cu2+, Co2+, Fe2+ and Ni2+), only Co2+ at 125 microM had a similar effect. Recombinant metallothionein (MT), a metal chelator protein, was found to modulate p53 conformation in vitro. In cultured cells, overexpression of MT by transfection could modulate p53 transcriptional activity. Taken together, these results suggest that zinc binding plays a regulatory role in the control of p53 folding and DNA-binding activity.

Tau promoter confers neuronal specificity and binds Sp1 and AP-2

Tau, a microtubule-associated protein, is encoded by a single gene, whose expression is primarily neuronal. In this work, we defined an 80-bp region of the tau promoter that confers tau protein with neuronal expression. This fragment works in conjunction with an endogenous initiation region to activate neuronal precursor-specific transcription of the tau promoter and works independently of this initiation region to confer nerve growth factor inducibility. Furthermore, this 80-bp fragment binds both Sp1 and AP-2 proteins. DNase I foot-print analysis revealed a third protein binding region at the center of this 80-bp fragment in neuronal cells. Mutation within any of these three protein binding sites decreases transcriptional activation of the tau gene. Comprehension of the interactions that occur between cis- and trans-regulatory elements of the tau promoter is important to understand the regulation of tau expression during normal development and changes that may occur in many cases of dementia, including Alzheimer's disease.