Nitric oxide selectively releases metals from the amino-terminal domain of metallothioneins: potential role at inflammatory sites

Contribution of zinc accumulation to ischemic brain injury and its mechanisms about oxidative stress, inflammation, and autophagy: an update

Ischemic stroke is a leading cause of death and disability worldwide, and presently, there is no effective neuroprotective therapy. Zinc is an essential trace element that plays important physiological roles in the central nervous system. Free zinc concentration is tightly regulated by zinc-related proteins in the brain under normal conditions. Disruption of zinc homeostasis, however, has been found to play an important role in the mechanism of brain injury following ischemic stroke. A large of free zinc releases from storage sites after cerebral ischemia, which affects the functions and survival of nerve cells, including neurons, astrocytes, and microglia, resulting in cell death. Ischemia-triggered intracellular zinc accumulation also disrupts the function of blood-brain barrier via increasing its permeability, impairing endothelial cell function, and altering tight junction levels. Oxidative stress and neuroinflammation have been reported to be as major pathological mechanisms in cerebral ischemia/reperfusion injury. Studies have showed that the accumulation of intracellular free zinc could impair mitochondrial function to result in oxidative stress, and form a positive feedback loop between zinc accumulation and reactive oxygen species production, which leads to a series of harmful reactions. Meanwhile, elevated intracellular zinc leads to neuroinflammation. Recent studies also showed that autophagy is one of the important mechanisms of zinc toxicity after ischemic injury. Interrupting the accumulation of zinc will reduce cerebral ischemia injury and improve neurological outcomes. This review summarizes the role of zinc toxicity in cellular and tissue damage following cerebral ischemia, focusing on the mechanisms about oxidative stress, inflammation, and autophagy.

Metallothioneins, a Part of the Retinal Endogenous Protective System in Various Ocular Diseases

Metallothioneins are the metal-rich proteins that play important roles in metal homeostasis and detoxification. Moreover, these proteins protect cells against oxidative stress, inhibit proapoptotic mechanisms and enhance cell differentiation and survival. Furthermore, MTs, mainly MT-1/2 and MT-3, play a vital role in protecting the neuronal retinal cells in the eye. Expression disorders of these proteins may be responsible for the development of various age-related eye diseases, including glaucoma, age-related macular degeneration, diabetic retinopathy and retinitis pigmentosa. In this review, we focused on the literature reports suggesting that these proteins may be a key component of the endogenous protection system of the retinal neurons, and, when the expression of MTs is disrupted, this system becomes inefficient. Moreover, we described the location of different MT isoforms in ocular tissues. Then we discussed the changes in MT subtypes' expression in the context of the common eye diseases. Finally, we highlighted the possibility of the use of MTs as biomarkers for cancer diagnosis.

Structural Role of Cadmium and Zinc in Metallothionein Oxidation by Hydrogen Peroxide: The Resilience of Metal-Thiolate Clusters

Oxidative stress is a state involving an imbalance of reactive oxygen species in a cell and is linked to a variety of diseases. The metal-binding protein metallothionein (MT) may play a role in protection due to its high cysteine content. Many studies have shown that oxidative stress will cause MT to both form disulfide bonds and release bound metals. However, studies on the more biologically relevant partially metalated MTs have been largely neglected. Additionally, most studies to date have used spectroscopic methods that cannot detect specific intermediate species. In this paper, we describe the oxidation and the subsequent metal displacement pathway of fully and partially metalated MTs with hydrogen peroxide. The rates of the reactions were monitored using electrospray ionization mass spectrometry (ESI-MS) techniques, which resolved and characterized the individual intermediate M_x(SH)_yMT species. The rate constants were calculated for each species formation. Through ESI-MS and circular dichroism spectroscopy, it was found that the three metals in the β-domain were the first to be released from the fully metalated MTs. The Cd(II) in the partially metalated Cd(II)-bound MTs rearranged to form a protective Cd₄MT cluster structure upon exposure to oxidation. The partially metalated Zn(II)-bound MTs oxidized at a faster rate as the Zn(II) did not rearrange in response to oxidation. Additionally, density functional theory calculations showed that the terminally bound cysteines were more negative and thus more susceptible to oxidation than the bridging cysteines. The results of this study highlight the importance of metal-thiolate structures and metal identity in MT's response to oxidation.

Calcium complexes of oxicams: new dimensions in rheumatoid arthritis treatment

Various metals have been complexed with drugs to improve their cellular impact. Inflammatory diseases like rheumatoid arthritis (RA) are characterized by unbalanced production of proinflammatory cytokines (PICs) and prostaglandins with decreased levels of vitamin D and calcium. The inflammation can be suppressed through targeting the formation of PICs or related enzymes by various treatment strategies that involve the use of corticosteroids, disease-modifying antirheumatic drugs and NSAIDs. We present a detailed review on the impact of calcium complexes of oxicams as an advanced treatment strategy for RA. The calcium complexes demonstrate promising capabilities to cure the disease, improve the strength of bones and suppress PICs in RA.

The Bioinorganic Chemistry of Mammalian Metallothioneins

The functions, purposes, and roles of metallothioneins have been the subject of speculations since the discovery of the protein over 60 years ago. This article guides through the history of investigations and resolves multiple contentions by providing new interpretations of the structure-stability-function relationship. It challenges the dogma that the biologically relevant structure of the mammalian proteins is only the one determined by X-ray diffraction and NMR spectroscopy. The terms metallothionein and thionein are ambiguous and insufficient to understand biological function. The proteins need to be seen in their biological context, which limits and defines the chemistry possible. They exist in multiple forms with different degrees of metalation and types of metal ions. The homoleptic thiolate coordination of mammalian metallothioneins is important for their molecular mechanism. It endows the proteins with redox activity and a specific pH dependence of their metal affinities. The proteins, therefore, also exist in different redox states of the sulfur donor ligands. Their coordination dynamics allows a vast conformational landscape for interactions with other proteins and ligands. Many fundamental signal transduction pathways regulate the expression of the dozen of human metallothionein genes. Recent advances in understanding the control of cellular zinc and copper homeostasis are the foundation for suggesting that mammalian metallothioneins provide a highly dynamic, regulated, and uniquely biological metal buffer to control the availability, fluctuations, and signaling transients of the most competitive Zn(II) and Cu(I) ions in cellular space and time.

Expression profiles of metallothionein-I/II and megalin/LRP-2 in uterine cervical squamous lesions

Metallothioneins (MTs) are phylogenetically old cysteine-rich proteins, which are implicated in a variety of physiological and pathological processes. Their growth-regulating, anti-apoptotic and anti-inflammatory functions have been attributed not only to intracellular free radical scavenging and to zinc and copper regulation but also to the ability of secreted MT to bind on surface lipoprotein receptor-megalin/LRP2, which enables the endocytosis of MT-I/II and a wide range of other functionally distinct ligands. In the present study, we analysed the expression pattern of both proteins in 55 cases of premalignant transformation of cervical squamous cells, i.e. in low- and high-grade squamous intraepithelial lesion (LSIL and HSIL). The data showed that in LSIL (cervical intraepithelial neoplasia CIN1; N = 25) MTs were present only in basal and parabasal cells and that megalin was only weakly expressed. In HSIL (CIN2; N = 15 and CIN 3/carcinoma in situ; N = 15), however, overexpression and co-localization of MT with megalin were found in the entire hyperplastic epithelium. Moreover, megalin immunoreactivity appeared on the glandular epithelium and vascular endothelium, as well as on lymphatic cells in stroma. Besides, multiple megalin-positive cells expressed phosphorylated Akt1, implying that MT- and/or megalin-dependent prosurvival signal transduction pathways might contribute to the development of severe cervical dysplasia. The data emphasize the diagnostic power of combined MT/megalin analysis in pre-cancer screening.

Metallothionein 2A gene polymorphisms in relation to diseases and trace element levels in humans

Human metallothioneins are a superfamily of low molecular weight intracellular proteins, whose synthesis can be induced by essential elements (primarily Zn and Cu), toxic elements and chemical agents, and stress-producing conditions. Of the four known isoforms in the human body MT2 is the most common. The expression of metallothioneins is encoded by a multigene family of linked genes and can be influenced by single nucleotide polymorphisms (SNPs) in these genes. To date, 24 SNPs in the MT2A gene have been identified with the incidence of about 1 % in various population groups, and three of them were shown to affect physiological and pathophysiological processes. This review summarises current knowledge about these three SNPs in the MT2A gene and their associations with element concentrations in the body of healthy and diseased persons. The most investigated SNP is rs28366003 (MT2A -5 A/G). Reports associate it with longevity, cancer (breast, prostate, laryngeal, and in paranasal sinuses), and chronic renal disease. The second most investigated SNP, rs10636 (MT2A +838G/C), is associated with breast cancer, cardiovascular disease, and type 2 diabetes. Both are also associated with several metal/metalloid concentrations in the organism. The third SNP, rs1610216 (MT2A -209A/G), has been studied for association with type 2 diabetes, cardiomyopathy, hyperglycaemia, and Zn concentrations. Metallothionein concentrations and MT2A polymorphisms have a potential to be used as biomarkers of metal exposure and clinical markers of a number of chronic diseases. This potential needs to be studied and verified in a large number of well-defined groups of participants (several hundreds and thousands) with a focus on particular physiological or pathological condition and taking into consideration other contributing factors, such as environmental exposure and individual genetic and epigenetic makeup.

Role of Zinc and Selenium in Oxidative Stress and Immunosenescence: Implications for Healthy Aging and Longevity

Aging is a complex process that includes gradual and spontaneous biochemical and physiological changes which contributes to a decline in performance and increased susceptibility to diseases. Zn and Se are essential trace elements that play a pivotal role in immune functions and antioxidant defense and, consequently, are claimed to play also a role in successful aging trajectories. Consistently with their nature of essential trace elements, a plethora of data obtained “in vitro” and “in vivo” (in humans and animal models) support the relevance of Zn and Se for both the innate and adoptive immune response. Moreover, Zn and Se are strictly involved in the synthesis and regulation of activity of proteins and enzymes, e.g., metallothioneins (MT) and glutathione peroxidase (GPX), that are necessary for our endogenous antioxidant response. This is clearly important to protect our cells from oxidative damage and to slow the decline of our immune system with aging. Age-related changes affecting tissue levels of Zn and Se may indicate that the risk of Zn and Se deficiency increases with aging. However, it is still unclear which of these changes can be the consequence of a “real deficiency” and which can be part of our physiological compensatory response to the accumulating damage occurring in aging. Furthermore, the upregulation of antioxidant proteins (Zn and Se dependent) may be a manifestation of self-induced oxidative stress. By the way, Zn and Se dependent proteins are modulated not only by nutritional status, but also by well-known hallmarks of aging that play antagonistic functions, such as the deregulated nutrient sensing pathways and cellular senescence. Thus, it is not an easy task to conduct Zn or Se supplementation in elderly and it is emerging consistent that these kind of supplementation requires an individualized approach. Anyway, there is consistent support that supplementation with Zn using doses around 10 mg/day is generally safe in elderly and may even improve part of immune performances in those subjects with a baseline deficiency. Regarding Se supplementation, it may induce both beneficial and detrimental effects on cellular immunity depending on the form of Se, supplemental dose, and delivery matrix. The nutritional association of supplements based on “Zn plus Se” is hypothesized to provide additional benefits, but this will likely need a more complex individualized approach. The improvement of our knowledge around screening and detection of Zn and Se deficiency in aging could lead to substantial benefits in terms of efficacy of nutritional supplements aimed at ameliorate performance and health in aging.

Genetic biosensors for imaging nitric oxide in single cells

Over the last decades a broad collection of sophisticated fluorescent protein-based probes was engineered with the aim to specifically monitor nitric oxide (NO), one of the most important signaling molecules in biology. Here we report and discuss the characteristics and fields of applications of currently available genetically encoded fluorescent sensors for the detection of NO and its metabolites in different cell types.

LONG ABSTRACT

Because of its radical nature and short half-life, real-time imaging of NO on the level of single cells is challenging. Herein we review state-of-the-art genetically encoded fluorescent sensors for NO and its byproducts such as peroxynitrite, nitrite and nitrate. Such probes enable the real-time visualization of NO signals directly or indirectly on the level of single cells and cellular organelles and, hence, extend our understanding of the spatiotemporal dynamics of NO formation, diffusion and degradation. Here, we discuss the significance of NO detection in individual cells and on subcellular level with genetic biosensors. Currently available genetically encoded fluorescent probes for NO and nitrogen species are critically discussed in order to provide insights in the functionality and applicability of these promising tools. As an outlook we provide ideas for novel approaches for the design and application of improved NO probes and fluorescence imaging protocols.

Metallothionein: A Multifunctional Protein from Toxicity to Cancer

The metallothionein (MT) family is a class of low molecular weight, intracellular and cysteine-rich proteins presenting high affinity for metal ions. Although the members of this family were discovered nearly 40 years ago, their functional significance remains obscure. Four major MT isoforms, MT-1, MT-2, MT-3 and MT-4, have been identified in mammals. MTs are involved in many pathophysiological processes such as metal ion homeostasis and detoxification, protection against oxidative damage, cell proliferation and apoptosis, chemoresistance and radiotherapy resistance. MT isoforms have been shown to be involved in several aspects of the carcinogenic process, cancer development and progression. MT expression has been implicated as a transient response to any form of stress or injury providing cytoprotective action. Although MT participates in the carcinogenic process, its use as a potential marker of tumor differentiation or cell proliferation, or as a predictor of poor prognosis remains unclear. In the present review the involvement of MT in defense mechanisms to toxicity and in carcinogenicity is discussed.

Metallothionein-3 Is a Component of a Multiprotein Complex in the Mouse Brain

Metallothlonein (MT)-3, originally called growth inhibitory factor (GIF), was initially identified through its ability to Inhibit the growth of neuronal cells in the presence of brain extract. MT-3 is the brain specific isoform of the MT family whose specific biological activity associates it with neurological disorders. Indeed, studies report that MT-3 is decreased by ~30% in brains of patients with Alzheimer disease (AD). Furthermore, many lines of evidence suggest that MT-3 engages in specific protein interactions. To address this, we conducted Immunoaffinity chromatography experiments using an immobilized anti-mouse MT-3 antibody. We identified five associated proteins from the pool of sixteen recovered using mass spectrometry and tandem mass spectrometry after in-gel trypsin digestion of bands from the affinity chromatography. The proteins identified were: heat shock protein 84 (HSP84), heat shock protein 70 (HSP70), dihydropyrimidinase-like protein-2 (DRP-2), creatine kinase (CK) and β-actin. Coimmunoprecipitation experiments, also conducted on whole mouse brain extract using the anti-mouse MT-3 antibody along with commercially available antibodies against HSP84 and CK, confirmed that these three proteins were in a single protein complex. Immunohistochemical experiments were then conducted on the perfused mouse brain that confirmed the in situ colocallzation of CK and MT-3 in the hippocampus region. These data provide new Insights into the involvement of MT-3 in a multiprotein complex, which will be used to understand the biological activity of MT-3 and its role in neurological disease.

Oxidation of the N-terminal domain of the wheat metallothionein Ec -1 leads to the formation of three distinct disulfide bridges

Metallothioneins (MTs) are low molecular weight proteins, characterized by a high cysteine content and the ability to coordinate large amounts of d(10) metal ions, for example, Zn(II), Cd(II), and Cu(I), in form of metal-thiolate clusters. Depending on intracellular conditions such as redox potential or metal ion concentrations, MTs can occur in various states ranging from the fully metal-loaded holo- to the metal-free apo-form. The Cys thiolate groups in the apo-form can be either reduced or be involved in disulfide bridges. Although oxidation-mediated Zn(II) release might be a possible mechanism for the regulation of Zn(II) availability by MTs, no concise information regarding the associated pathways and the structure of oxidized apo-MT forms is available. Using the well-studied Zn2 γ-Ec -1 domain of the wheat Zn6 Ec -1 MT we attempt here to answer several question regarding the structure and biophysical properties of oxidized MT forms, such as: (1) does disulfide bond formation increase the stability against proteolysis, (2) is the overall peptide backbone fold similar for the holo- and the oxidized apo-MT form, and (3) are disulfide bridges specifically or randomly formed? Our investigations show that oxidation leads to three distinct disulfide bridges independently of the applied oxidation conditions and of the initial species used for oxidation, that is, the apo- or the holo-form. In addition, the oxidized apo-form is as stable against proteolysis as Zn2 γ-Ec -1, rendering the currently assumed degradation of oxidized MTs unlikely and suggesting a role of the oxidation process for the extension of protein lifetime in absence of sufficient amounts of metal ions. © 2016 Wiley Periodicals, Inc. Biopolymers (Pept Sci) 106: 295-308, 2016.

Redox Reactivity of Cerium Oxide Nanoparticles Induces the Formation of Disulfide Bridges in Thiol-Containing Biomolecules

The redox state of disulfide bonds is implicated in many redox control systems, such as the cysteine-cystine couple. Among proteins, ubiquitous cysteine-rich metallothioneins possess thiolate metal binding groups susceptible to metal exchange in detoxification processes. CeO2 NPs are commonly used in various industrial applications due to their redox properties. These redox properties that enable dual oxidation states (Ce(IV)/Ce(III)) to exist at their surface may act as oxidants for biomolecules. The interaction among metallothioneins, cysteine, and CeO2 NPs was investigated through various biophysical approaches to shed light on the potential effects of the Ce(4+)/Ce(3+) redox system on the thiol groups of these biomolecules. The possible reaction mechanisms include the formation of a disulfide bridge/Ce(III) complex resulting from the interaction between Ce(IV) and the thiol groups, leading to metal unloading from the MTs, depending on their metal content and cluster type. The formation of stable Ce(3+) disulfide complexes has been demonstrated via their fluorescence properties. This work provides the first evidence of thiol concentration-dependent catalytic oxidation mechanisms between pristine CeO2 NPs and thiol-containing biomolecules.

The associations between serum zinc levels and metabolic syndrome in the Korean population: findings from the 2010 Korean National Health and Nutrition Examination Survey

The prevalence of metabolic syndrome has been increasing rapidly worldwide. The functions of zinc may have a potential association with metabolic syndrome, but such associations have not been investigated extensively. Therefore, we examined the relationship between serum zinc levels and metabolic syndrome or metabolic risk factors among South Korean adults ≥20 years of age. The analysis used data from the Korean National Health and Nutrition Examination Survey, a cross-sectional survey of Korean civilians, conducted from January to December 2010. A total of 1,926 participants were analyzed in this study. Serum zinc levels in men were negatively associated with elevated fasting glucose (adjusted odds ratio [aOR], 0.58; 95% confidence interval [CI], 0.36–0.93) and positively associated with elevated triglycerides (aOR, 1.47; 95% CI, 1.01–2.13). A difference in serum zinc levels was detected in women, depending on the number of metabolic syndrome components (p = 0.002). Furthermore, serum zinc levels showed a decreasing trend with increasing numbers of metabolic syndrome components in women with metabolic syndrome. These findings suggest that serum zinc levels might be associated with metabolic syndrome or metabolic risk factors. Further gender-specific studies are needed to evaluate the effect of dietary or supplemental zinc intake on metabolic syndrome.

Functional role of intracellular labile zinc in pulmonary endothelium

After iron, zinc is the most abundant essential trace metal. Intracellular zinc ([Zn]_i) is maintained across a wide range of cells and species in a tight quota (100 to 500 μM) by a dynamic process of transport, intracellular vesicular storage, and binding to a large number of proteins (estimated at 3-10% of human proteome). As such, zinc is an integral component of numerous metalloenzymes, structural proteins, and transcription factors. It is generally assumed that a vanishingly small component of [Zn]_i, referred to as free or labile zinc, and operationally defined as the pool sensitive to chelation (by agents such as N, N, N’, N’-tetrakis [2-pyridylmethyl] ethylenediamine [TPEN]) and capable of detection by a variety of chemical and genetic sensors, participates in signal transduction pathways. Zinc deficiencies, per se, can arise from acquired (malnutrition, alcoholism) or genetic (mutations in molecules affecting zinc homeostasis, the informative and first example being acrodermatitis enteropathica) factors or as a component of various diseases (e.g., sickle cell disease, cystic fibrosis, sepsis). Hypozincemia has profound effects on developing humans, and all facets of physiological function (neuronal, endocrine, immunological) are affected, although considerably less is known regarding cardiovascular pathophysiology. In this review, we provide an update on current knowledge of molecular and cellular aspects of zinc homeostasis and then focus on implications of zinc signaling in pulmonary endothelium as it relates to programmed cell death, altered contractility, and septic and aseptic injury to this segment of the lung.

Metallothionein-induced zinc partitioning exacerbates hyperoxic acute lung injury

Hypozincemia, with hepatic zinc accumulation at the expense of other organs, occurs in infection, inflammation, and aseptic lung injury. Mechanisms underlying zinc partitioning or its impact on extrahepatic organs are unclear. Here we show that the major zinc-binding protein, metallothionein (MT), is critical for zinc transmigration from lung to liver during hyperoxia and preservation of intrapulmonary zinc during hyperoxia is associated with an injury-resistant phenotype in MT-null mice. Particularly, lung-to-liver zinc ratios decreased in wild-type (WT) and increased significantly in MT-null mice breathing 95% oxygen for 72 h. Compared with female adult WT mice, MT-null mice were significantly protected against hyperoxic lung injury indicated by reduced inflammation and interstitial edema, fewer necrotic changes to distal airway epithelium, and sustained lung function at 72 h hyperoxia. Lungs of MT-null mice showed decreased levels of immunoreactive LC3, an autophagy marker, compared with WT mice. Analysis of superoxide dismutase (SOD) activity in the lungs revealed similar levels of manganese-SOD activity between strains under normoxia and hyperoxia. Lung extracellular SOD activity decreased significantly in both strains at 72 h of hyperoxia, although there was no difference between strains. Copper-zinc-SOD activity was ~4× higher under normoxic conditions in MT-null compared with WT mice but was not affected in either group by hyperoxia. Collectively the data suggest that genetic deletion of MT-I/II in mice is associated with compensatory increase in copper-zinc-SOD activity, prevention of hyperoxia-induced zinc transmigration from lung to liver, and hyperoxia-resistant phenotype strongly associated with differences in zinc homeostasis during hyperoxic acute lung injury.

Zinc transporter ZIP14 functions in hepatic zinc, iron and glucose homeostasis during the innate immune response (endotoxemia)

ZIP14 (slc39A14) is a zinc transporter induced in response to pro-inflammatory stimuli. ZIP14 induction accompanies the reduction in serum zinc (hypozincemia) of acute inflammation. ZIP14 can transport Zn²⁺ and non-transferrin-bound Fe²⁺ in vitro. Using a Zip14^−/− mouse model we demonstrated that ZIP14 was essential for control of phosphatase PTP1B activity and phosphorylation of c-Met during liver regeneration. In the current studies, a global screening of ZIP transporter gene expression in response to LPS-induced endotoxemia was conducted. Following LPS, Zip14 was the most highly up-regulated Zip transcript in liver, but also in white adipose tissue and muscle. Using ZIP14^−/− mice we show that ZIP14 contributes to zinc absorption from the gastrointestinal tract directly or indirectly as zinc absorption was decreased in the KOs. In contrast, Zip14^−/− mice absorbed more iron. The Zip14 KO mice did not exhibit hypozincemia following LPS, but do have hypoferremia. Livers of Zip14−/− mice had increased transcript abundance for hepcidin, divalent metal transporter-1, ferritin and transferrin receptor-1 and greater accumulation of iron. The Zip14^−/− phenotype included greater body fat, hypoglycemia and higher insulin levels, as well as increased liver glucose and greater phosphorylation of the insulin receptor and increased GLUT2, SREBP-1c and FASN expression. The Zip14 KO mice exhibited decreased circulating IL-6 with increased hepatic SOCS-3 following LPS, suggesting SOCS-3 inhibited insulin signaling which produced the hypoglycemia in this genotype. The results are consistent with ZIP14 ablation yielding abnormal labile zinc pools which lead to increased SOCS-3 production through G-coupled receptor activation and increased cAMP production as well as signaled by increased pSTAT3 via the IL-6 receptor, which inhibits IRS 1/2 phosphorylation. Our data show the role of ZIP14 in the hepatocyte is multi-functional since zinc and iron trafficking are altered in the Zip14^−/− mice and their phenotype shows defects in glucose homeostasis.

Survival study of metallothionein-1 transgenic mice and respective controls (C57BL/6J): influence of a zinc-enriched environment

The role of metallothioneins (MTs) in aging is not completely understood. Several studies have shown evidence that these proteins could represent a defense system against oxidative damage, but survival studies on mice overexpressing MTs are poor. Here we describe a survival study performed on old MT-1-overexpressing mice (MT-TG) and their respective controls (C57BL/6J) fed a standard or zinc (Zn)-supplemented diet. MT-TG mice had significantly increased survival compared with control. Zn supplementation affects the survival curves of MT-TG and C57BL/6J mice differently. This study poses the basis for intervention based on gene therapy with MTs to enhance the health span of laboratory mice.

Cellular mechanisms of zinc dysregulation: a perspective on zinc homeostasis as an etiological factor in the development and progression of breast cancer

Worldwide, breast cancer is the most commonly diagnosed cancer among women and is the leading cause of female cancer deaths. Zinc (Zn) functions as an antioxidant and plays a role in maintaining genomic stability. Zn deficiency results in oxidative DNA damage and increased cancer risk. Studies suggest an inverse association between dietary and plasma Zn levels and the risk for developing breast cancer. In contrast, breast tumor biopsies display significantly higher Zn levels compared with normal tissue. Zn accumulation in tumor tissue also correlates with increased levels of Zn importing proteins. Further, aberrant expression of Zn transporters in tumors correlates with malignancy, suggesting that altered metal homeostasis in the breast could contribute to malignant transformation and the severity of cancer. However, studies have yet to link dysregulated Zn transport and abnormal Zn-dependent functions in breast cancer development. Herein, we summarize studies that address the multi-modal role of Zn dyshomeostasis in breast cancer with respect to the role of Zn in modulating oxidative stress, DNA damage response/repair pathways and cell proliferation/apoptosis, and the relationship to aberrant regulation of Zn transporters. We also compare Zn dysregulation in breast tissue to that of prostate, pancreatic and ovarian cancer where possible.

Role of metallothionein 1E in the migration and invasion of human glioma cell lines

Metallothionein 1E (MT1E) has been found to be highly expressed in motile cell lines. We investigated whether MT1E actually modulates the migration and invasion of human glioma cell lines and the types of factors that have an effect on MT1E. RNA differential display was performed using Genefishing™ technology in the human glioma cell lines U343MG-A, U87MG and U87MG-10'; the results were validated by RT-PCR and northern blot analysis, in order to detect possible genetic changes as the determining factors for migration ability in malignant glioma. MT1E was identified in U87MG, a highly motile cell line. The migration and invasion abilities of human glioma cell lines, and MT1E transfectants were investigated using simple scratch testing and Matrigel invasion assays. Morphological and cytoskeletal (actin, vimentin) changes were documented by light and confocal microscopy. The expression of MT1E in four glioma cell lines was assessed by RT-PCR and western blotting. In addition, the effects of MT1E on the activity of the NF-κB p50/p65 transcription factor, MMP-2 and -9 were examined by western blotting and zymography. The endogenous MT1E expression in the human glioma cell lines was statistically correlated with their migratory abilities and invasion. The U87-MT-AS cells became more round and had decreased stress fibers, compared with the U87MG cells. Endogenous MT1E expression in the four human glioma cell lines was directly correlated with migration. Two antisense MT1E-transfected cell lines showed decreased NF-κB p50 translocation into the nucleus, which led to decreased activity of MMP-9 in conditioned media. It may be postulated that MT1E can enhance the migration and invasion of human glioma cells by inducing MMP-9 inactivation via the upregulation of NF-κB p50.

Mammalian metallothionein in toxicology, cancer, and cancer chemotherapy

The present paper centers on mammalian metallothionein 1 and 2 in relationship to cell and tissue injury beginning with its reaction with Cd²⁺ and then considering its role in the toxicology and chemotherapy of both metals and non-metal electrophiles and oxidants. Intertwined is a consideration of MTs role in tumor cell Zn²⁺ metabolism. The paper updates and expands on our recent review by Petering et al. (Met Ions Life Sci 5:353-398, 2009).

Zinc, metallothioneins and immunosenescence: effect of zinc supply as nutrigenomic approach

Ageing is an inevitable biological process associated with gradual and spontaneous biochemical and physiological changes and increased susceptibility to diseases. Nutritional factor, zinc, known to be involved in improving immunity, may remodel some of the age-associated changes, leading to a healthy ageing. "In Vitro" studies involving human lymphocytes exposed to endotoxins, and "in vivo" studies comparing old and young mice fed with low dietary zinc suggest that zinc is important for both innate and adaptive immune efficiency, and more optimal inflammatory/immune response. The intracellular zinc homeostasis is mainly regulated by Metallothioneins (MT), via ion release through the reduction of thiol groups in MT molecule. These processes are crucial because mediating the zinc signalling within the immune cells assigning to zinc a role of "second messenger". Zinc homeostasis is altered in ageing partly due to higher expression levels of MT, leading to an increased sequestration of zinc, resulting in less availability of free intracellular zinc. Improvement of immune functions and stress response systems occurs in elderly after physiological zinc supplementation. The main reason behind these effects seems to be related to a like "hormetic" response induced by zinc. However, the choice of old subjects for zinc supplementation has to be performed in relationship to the specific genetic background of MT and pro-inflammatory cytokine (IL-6) because the latter is involved both in MT gene expression and in intracellular zinc homeostasis. Old subjects carrying GG genotypes (termed C- carriers) in IL-6--174G/C locus display increased IL-6 production, low intracellular zinc ion availability, impaired innate immune response and enhanced MT. By contrast, old subjects carrying GC and CC genotypes (termed C+ carriers) in the same IL-6--174 locus displayed satisfactory intracellular zinc and innate immune response. Moreover, male carriers of C+ allele are more prone to reach centenarian age than C- ones. Therefore, old C- subjects are likely to benefit more from zinc supplementation restoring NK cell cytotoxicity and improving the zinc status. Plasma zinc deficiency and the altered immune response is more evident when the genetic variations of IL-6 polymorphism are associated with the genetic variations of MT1A in position +647, suggesting that the genetic variations of IL-6 and MT1A are very useful tools for the identification of old people who effectively need zinc supplementation.

Unraveling the response of plant cells to cytotoxic saponins: role of metallothionein and nitric oxide

A wide range of pharmacological properties are ascribed to natural saponins, in addition to their biological activities against herbivores, plant soil-borne pathogens and pests. As for animal cells, the cytotoxicity and the chemopreventive role of saponins are mediated by a complex network of signal transduction pathways which include reactive oxygen species (ROS) and nitric oxide (NO). The involvement of other relevant components of the saponin-related signaling routes, such as the Tumor Necrosis Factor(TNF)-α, the interleukin(IL)-6 and the Nuclear Transcription Factor-kB (NF-κB), has been highlighted in animal cells. By contrast, information concerning the response of plant cells to saponins and the related signal transduction pathways is almost missing. To date, there are only a few common features which link plant and animal cells in their response to saponins, such as the early burst in ROS and NO production and the induction of metallothioneins (MTs), small cysteine-rich, metal-binding proteins. This aspect is discussed in the present paper in view of the recent hypothesis that MTs and NO are part of a novel signal transduction pathway participating in the cell response to oxidative stress.

Mobilizing Cu(I) for carbon-carbon bond forming catalysis in the presence of thiolate. Chemical mimicking of metallothioneins

Copper(I) is rendered catalytically viable in the presence of thiolate by the design of a small molecule chemical analogue of the metallothionein system in which an N-O reactant serves the same conceptual purpose of the S-S reactant of the biological system.

The "magic numbers" of metallothionein

Metallothioneins (MT) are a family of small cysteine rich proteins, which since their discovery in 1957, have been implicated in a range of roles including toxic metal detoxification, protection against oxidative stress, and as a metallochaperone involved in the homeostasis of both zinc and copper. The most well studied member of the family is the mammalian metallothionein, which consists of two domains: a β-domain with 9 cysteine residues, which sequesters 3 Cd(2+) or Zn(2+) or 6 Cu(+) ions, and an α-domain with 11 cysteine residues and, which sequesters 4 Cd(2+) or Zn(2+) or 6 Cu(+) ions. Despite over half a century of research, the exact functions of MT are still unknown. Much of current research aims to elucidate the mechanism of metal binding, as well as to isolate intermediates in metal exchange reactions; reactions necessary to maintain homeostatic equilibrium. These studies further our understanding of the role(s) of this remarkable and ubiquitous protein. Recently, supermetallated forms of the protein, where supermetallation describes metallation in excess of traditional levels, have been reported. These species may potentially be the metal exchange intermediates necessary to maintain homeostatic equilibrium. This review focuses on recent advances in the understanding of the mechanistic properties of metal binding, the implications for the metal induced protein folding reactions proposed for metallothionein metallation, the value of "magic numbers", which we informally define as the commonly determined metal-to-protein stoichiometric ratios and the significance of the new supermetallated states of the protein and the possible interpretation of the structural properties of this new metallation status. Together we provide a commentary on current experimental and theoretical advances and frame our consideration in terms of the possible functions of MT.

Nitric oxide and zinc homeostasis in pulmonary endothelium

We have shown that zinc-thiolate moieties of the metal binding protein metallothionein (MT) are critical targets for nitric oxide (NO) with resultant increases in intracellular labile zinc. Such an NO-MT-Zn signaling pathway appears to participate in important cardiovascular functions associated with biosynthesis of NO including hypoxic vasoconstriction in the lung. Although downstream effector signaling molecules and critical contractile targets remain unclear, current investigations reveal a contributory role for zinc dependent protein kinases and cytoskeletal proteins in mediating hypoxic induced constriction of pulmonary endothelial cells.

Zinc, metallothioneins and immunosenescence

Ageing is an inevitable biological process with gradual and spontaneous biochemical and physiological changes and increased susceptibility to diseases. The nutritional factor, zinc, may remodel these changes with subsequent healthy ageing, because zinc improves the inflammatory/immune response as shown by in vitro and in vivo studies. The intracellular zinc homeostasis is regulated by buffering metallothioneins (MT) and zinc transporters (ZnT and ZIP families) that mediate the intracellular zinc signalling assigning to zinc a role of ‘second messenger’. In ageing, the intracellular zinc homeostasis is altered, because high MT are unable to release zinc and some zinc transporters deputed to zinc influx (ZIP family) are defective leading to low intracellular zinc content for the immune efficiency. Physiological zinc supplementation in the elderly improves these functions. However, the choice of old subjects for zinc supplementation has to be performed in relation to the specific genetic background of MT and IL-6, because the latter is involved both in MTmRNA and in intracellular zinc homeostasis. Old subjects carrying GG genotypes (C–carriers) in the IL-6–174G/C locus display high IL-6, low intracellular zinc content, impaired innate immunity and enhanced MT. Old subjects carrying GC and CC genotypes (C+carriers) display satisfactory intracellular zinc content, adequate innate immunity and are more prone to reach longevity. Zinc supplementation in old C–carriers restores natural killer cell cytotoxicity and zinc status. The genetic variations of the IL-6−174G/C locus when associated with those of the MT1A+647A/C locus are useful tools for the choice of old people for zinc supplementation.

Reactivity of Zn-, Cd-, and apo-metallothionein with nitric oxide compounds: in vitro and cellular comparison

The reactivity of Zn(7)- and Cd(7)-metallothionein (MT) with S-nitrosopenicillamine (SNAP), S-nitrosoglutathione (GSNO), and 2-(N,N-diethylamino)-diazenolate-2-oxide (DEA/NO) was investigated to explore the hypothesis that metallothionein is a signficant site of cellular reaction of nitric oxide or NO compounds. Zn(7)-MT reacted with SNAP or GSNO only under aerobic conditions and in the presence of light, which stimulates the decomposition of S-nitrosothiolates to NO. Zn(2+) is released, and protein thiols are modified. DEA/NO, which degrades spontaneously to release NO, also reacted with Zn(7)-MT only when oxygen was present. Anaerobically, DEA/NO reacted with Zn(7)-MT in the presence of 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide, which converts NO to NO(2). Glutathione competed effectively with Zn(7)-MT for reactive nitrogen oxide species in reaction mixtures. Reaction of Cd(7)-MT with SNAP also required oxygen and light to react. In this case, only a fraction of the Cd(2+) bound to Cd(7)-MT was displaced by SNAP. Apo-metallothionein was much more reactive with SNAP and DEA-NO than Zn(7)- or Cd(7)-MT. TE671 and LLC-PK(1) cell lines were incubated with DEA/NO to examine the role that MT might play in the cellular reactions of this NO donor compound. Incubation of cells with 0-80 microM Zn(2+) for 24 h resulted in progressively increasing concentrations of Zn-unsaturated MT. One hour of cellular exposure to a range of DEA/NO concentrations followed by 24 h of incubation caused no evident acute toxicity at less than 0.45 mM. Preinduction of MT did not alter this response. The effects of DEA/NO on proteomic, metallothionein, and low molecular weight (LMW) thiol pools, including glutathione (GSH), were measured. Substantial fractions of the proteomic and LMW thiol pools underwent reaction with little dislocation of Zn(2+). In addition, one-third of the MT thiol pool reacted without labilizing any of the bound Zn(2+). These results demonstrated that it was free thiols associated with MT that reacted with DEA/NO not those bound to Zn(2+). Moreover, under the conditions of the experiments, DEA/NO reacted with the spectrum of cellular thiols in proportion to their fraction in the cytosol and did not preferentially react with MT sulfhydryl groups.

Effects of cadmium exposure and intermittent anoxia on nitric oxide metabolism in eastern oysters, Crassostrea virginica

Nitric oxide (NO) is an intracellular signaling molecule synthesized by a group of enzymes called nitric oxide synthases (NOS) and involved in regulation of many cellular functions including mitochondrial metabolism and bioenergetics. In invertebrates, the involvement of NO in bioenergetics and metabolic responses to environmental stress is poorly understood. We determined sensitivity of mitochondrial and cellular respiration to NO and the effects of cadmium (Cd) and intermittent anoxia on NO metabolism in eastern oysters, Crassostrea virginica. NOS activity was strongly suppressed by exposure to 50 μg l–1 Cd for 30 days (4.76 vs 1.19 pmol NO min–1 mg–1 protein in control and Cd-exposed oysters, respectively) and further decreased during anoxic exposure in Cd-exposed oysters but not in their control counterparts. Nitrate/nitrite content (indicative of NO levels) decreased during anoxic exposure to less than 10% of the normoxic values and recovered within 1 h of re-oxygenation in control oysters. In Cd-exposed oysters, the recovery of the normoxic NO levels lagged behind, reflecting their lower NOS activity. Oyster mitochondrial respiration was inhibited by exogenous NO, with sensitivity on a par with that of mammalian mitochondria, and ADP-stimulated mitochondrial respiration was significantly more sensitive to NO than resting respiration. In isolated gill cells, manipulations of endogenous NOS activity either with a specific NOS inhibitor (aminoguanidine) or a NOS substrate (l-arginine) had no effect on respiration, likely due to the fact that mitochondria in the resting state are relatively NO insensitive. Likewise, Cd-induced stimulation of cellular respiration did not correlate with decreased NOS activity in isolated gill cells. High sensitivity of phosphorylating (ADP-stimulated) oyster mitochondria to NO suggests that regulation of bioenergetics is an evolutionarily conserved function of NO and that NO-dependent regulation of metabolism may be most prominent under the conditions of high metabolic flux when the ADP-to-ATP ratio is high.

Zinc transporter ZIP8 (SLC39A8) and zinc influence IFN-gamma expression in activated human T cells

The zinc transporter ZIP8 is highly expressed in T cells derived from human subjects. T cell ZIP8 expression was markedly up-regulated upon in vitro activation. T cells collected from human subjects who had received oral zinc supplementation (15 mg/day) had higher expression of the activation marker IFN-gamma upon in vitro activation, indicating a potentiating effect of zinc on T cell activation. Similarly, in vitro zinc treatment of T cells along with activation resulted in increased IFN-gamma expression with a maximum effect at 3.1 microM. Knockdown of ZIP8 in T cells by siRNA decreased ZIP8 levels in nonactivated and activated cells and concomitantly reduced secretion of IFN-gamma and perforin, both signatures of activation. Overexpression of ZIP8 by transient transfection caused T cells to exhibit enhanced activation. Confocal microscopy established that ZIP8 is localized to the lysosome where ZIP8 abundance is increased upon activation. Loss of lysosomal labile zinc in response to activation was measured by flow cytometry using a zinc fluorophore. Zinc between 0.8 and 3.1 microM reduced CN phosphatase activity. CN was also inhibited by the CN inhibitor FK506 and ZIP8 overexpression. The results suggest that zinc at low concentrations, through inhibition of CN, sustains phosphorylation of the transcription factor CREB, yielding greater IFN-gamma expression in T cells. ZIP8, through control of zinc transport from the lysosome, may provide a secondary level of IFN-gamma regulation in T cells.

Toxicity studies on depleted uranium in primary rat cortical neurons and in Caenorhabditis elegans: what have we learned?

Depleted uranium (DU) is the major by-product of the uranium enrichment process for its more radioactive isotopes, retaining approximately 60% of its natural radioactivity. Given its properties as a pyrophoric and dense metal, it has been extensively used in armor and ammunitions. Questions have been raised regarding the possible neurotoxic effects of DU in humans based on follow-up studies in Gulf War veterans, where a decrease in neurocognitive behavior in a small population was noted. Additional studies in rodents indicated that DU readily traverses the blood-brain barrier, accumulates in specific brain regions, and results in increased oxidative stress, altered electrophysiological profiles, and sensorimotor deficits. This review summarizes the toxic potential of DU with emphasis on studies on thiol metabolite levels, high-energy phosphate levels, and isoprostane levels in primary rat cortical neurons. Studies in Caenorhabditis elegans detail the role of metallothioneins, small thiol-rich proteins, in protecting against DU exposure. In addition, recent studies also demonstrate that only one of the two forms, metallothionein-1, is important in the accumulation of uranium in worms.

l-Arginine normalizes NOS activity and zinc-MT homeostasis in the kidney of mice chronically exposed to inorganic mercury

Inorganic mercury (HgCl2) exposure provokes damage in many organs, especially kidney. Inducible nitric oxide synthase (iNOS) expression, total NOS activity and the profiles of zinc (Zn), copper (Cu) and Hg as well as their distribution when bound to specific intracellular proteins, including metallothioneins (MT), were studied during HgCl2 exposure and after l-arginine treatment in C57BL/6 mouse kidney. HgCl2 exposure modulates differently iNOS expression and NOS activity, increasing iNOS expression but, conversely, decreasing total NOS activity in the mouse kidney. Moreover, during Hg exposure an increased MT production occurs. The kidney damage leads to a loss of urinary proteins, increased plasma creatinine and high Zn mobilization with consequent increased urinary Zn excretion. l-arginine treatment recovers NOS activity and induces a normalization of MT induction, plasma creatinine values and urinary proteins excretion, suggesting that l-arginine may limit kidney damages by Hg exposure.

L-arginine reduces mercury accumulation in thymus of mercury-exposed mice: role of nitric oxide synthase activity and metallothioneins

Mercury, an occupational and environmental contaminant, is a well-recognized health hazard. The thymus is a target for inorganic mercury (Hg2+); thymic function is impaired in Hg2+ intoxication and is partially restored by simultaneous L-arginine supplementation. The nitric oxide (NO)-nitric oxide synthase (NOS) pathway and metallothioneins (MTs) were studied to investigate the role of L-arginine in thymic function restoration after mercury exposure. Mice received a higher and a lower dose of inorganic mercury, with and without L-arginine supplementation. Saline-treated mice were used as controls. Thymus weight and thymulin were measured as indices of thymic function. Mice treated with Hg2+ alone displayed an accumulation of metal in the thymus, reduced NOS activity, a lower plasma nitrite plus nitrate concentration and an increased MTs expression compared with control mice. L-arginine supplementation was associated with lower Hg2+ concentrations in the organ and partial preservation of other measures. Reduced accumulation of Hg2+ in mice dosed with L-arginine was probably related to greater NO production and NO-MTs interactions.

Role of Zinc and Selenium in Oxidative Stress and Immunosenescence: Implications for Healthy Ageing and Longevity

Ageing is an inevitable biological process with gradual and spontaneous biochemical and physiological changes and increased susceptibility to diseases. Some nutritional factors (zinc and selenium) may remodel these changes leading to a possible escaping of diseases with subsequent healthy ageing, because they are especially involved in improving immune functions as well as antioxidant defense. Experiments performed “in vitro” (human lymphocytes exposed to endotoxins) and “in vivo” (old mice or young mice fed with low zinc dietary intake) show that zinc is important for immune response both innate and adoptive. Selenium provokes zinc release by Metallothioneins (MT), via reduction of glutathione peroxidase. This fact is crucial in ageing because high MT may be unable to release zinc with subsequent low intracellular free zinc ion availability for immune response. Taking into account the existence of zinc transporters (ZnT and ZIP family) for cellular zinc efflux and influx, respectively, the association between ZnT and MT is important in maintaining satisfactory intracellular zinc homeostasis in ageing. Improved immune performance occur in elderly after physiological zinc supplementation, which also induces prolonged survival in old, nude and neonatal thymectomized mice. The association “zinc plus selenium” improves humoral immunity in old subjects after influenza vaccination. Therefore, zinc and selenium are relevant for immunosenescence in order to achieve healthy ageing and longevity.

+647 A/C and +1245 MT1A polymorphisms in the susceptibility of diabetes mellitus and cardiovascular complications

Diabetes mellitus is a chronic disease characterized by an overproduction of reactive oxygen species, which perturbs zinc metabolism and promotes the onset of cardiovascular disease (CVD) in diabetic patients. Metallothioneins (MT) are cysteine-rich metal-binding proteins which, by means of their antioxidant and zinc-buffering properties, might prevent the development of diabetic cardiovascular complications. A recent investigation shows that a polymorphism (+647 A/C) in the human MT-1A gene, affects the intracellular zinc ion release (iZnR) from the proteins and is associated with longevity in Italian population. The aim of the present study is to assess the involvement of +647 A/C and +1245 A/G MT1A polymorphisms with the susceptibility to type 2 diabetes (DM2) and cardiovascular complications. The study included 694 old individuals: 242 old healthy controls, 217 DM2 patients without clinical evidence of CVD (DNC) and 235 diabetic patients with diagnosis of CVD (DCVD). +647 A/C MT1A polymorphism, but not the second SNP, was associated with DM2. C allele carriers were more prevalent in DNC and DCVD patients than in control group (OR=1.37, p=0.034; OR=1.54, p=0.002, respectively). C+ carriers was associated with higher glycemia and glycosylated hemoglobin in DCVD patients, but not in DNC or control subjects. No differences in plasma zinc, but a modulation of MT levels and iZnR in PBMCs were observed in DCVD cohort when related to +647 A/C MT1A polymorphism. In summary, this work provides novel evidence on the association of the +647 A/C MT1A polymorphism with DM2. Moreover, C+ carriers in DCVD patients presented a worse glycemic control, a reduced iZnR and a higher MT levels, suggesting a possible role of MT in diabetic cardiovascular complications.

Redox silencing of copper in metal-linked neurodegenerative disorders: reaction of Zn7metallothionein-3 with Cu2+ ions

Dysregulation of copper and zinc homeostasis in the brain plays a critical role in Alzheimer disease (AD). Copper binding to amyloid-beta peptide (Abeta) is linked with the neurotoxicity of Abeta and free radical damage. Metallothionein-3 (MT-3) is a small cysteine- and metal-rich protein expressed in the brain and found down-regulated in AD. This protein occurs intra- and extracellularly, and it plays an important role in the metabolism of zinc and copper. In cell cultures Zn7MT-3, by an unknown mechanism, protects neurons from the toxicity of Abeta. We have, therefore, used a range of complementary spectroscopic and biochemical methods to characterize the interaction of Zn7MT-3 with free Cu2+ ions. We show that Zn7MT-3 scavenges free Cu2+ ions through their reduction to Cu+ and binding to the protein. In this reaction thiolate ligands are oxidized to disulfides concomitant with Zn2+ release. The binding of the first four Cu2+ is cooperative forming a Cu(I)4-thiolate cluster in the N-terminal domain of Cu4,Zn4MT-3 together with two disulfides bonds. The Cu4-thiolate cluster exhibits an unusual stability toward air oxygen. The results of UV-visible, CD, and Cu(I) phosphorescence at 77 K suggest the existence of metal-metal interactions in this cluster. We have demonstrated that Zn7MT-3 in the presence of ascorbate completely quenches the copper-catalyzed hydroxyl radical (OH.) production. Thus, zinc-thiolate clusters in Zn7MT-3 can efficiently silence the redox-active free Cu2+ ions. The biological implication of our studies as to the protective role of Zn7MT-3 from the Cu2+ toxicity in AD and other neurodegenerative disorders is discussed.

Metallothionein functions and structural characteristics

Metallothioneins (MTs) are low molecular weight proteins characterized by a high cysteine content and give rise to metal-thiolate clusters. Most MTs have two metal clusters containing three and four bivalent metal ions, respectively. The MT gene family in mammals consists of four subfamilies designated MT-1 through MT-4. MT-3 is expressed predominantly in brain and MT-4 in differentiating stratified squamous epithelial cells. Many reports have addressed MT structure and function, but despite the increasing experimental data several topics remain to be clarified, and the true function of this elusive protein has yet to be disclosed. Owing to their induction by a variety of stimuli, MTs are considered valid biomarkers in medicine and environmental studies. Here, we will discuss only a few topics taken from the latest literature. Special emphasis will be placed on MT antioxidant functions, the related oxidation of cysteines, which can give rise to intra/intermolecular bridges, and the relations between MTs and diseases which could be originated by metal dysregulation.

Dietary phenolic antioxidants, caffeic acid and Trolox, protect rainbow trout gill cells from nitric oxide-induced apoptosis

Caffeic acid (CA) and Trolox are phenolic acids that have beneficial antioxidant effect, but the underlying mechanisms involved are not fully understood. The extent to which CA and Trolox protect against sodium nitroprusside (SNP)-induced oxidative cell injury was investigated in cultured rainbow trout gill cells. The cells exposed to SNP for 24 h displayed a dose-dependent leakage of lactate dehydrogenase (LDH) and decreased cell viability as indicated by the MTT assay (mitochondrial dehydrogenase activity). Both effects were prevented by treatment with 50 microM CA or Trolox. CA or Trolox, protected against SNP-induced caspase-3 activation and DNA fragmentation, indicating a reduction of apoptosis. Thus, the results indicate that SNP induced cell death is caspase-3 related apoptosis and the treatment with CA inhibited the apoptotic pathway. In addition, we studied the effect of CA and Trolox on expression of zinc-responsive antioxidant genes such as metallothioneins (MT), glutathione-S-transferase (GST Class pi) and glucose-6-phosphate dehydrogenase (G6PD) in cultured gill cells. CA, 100 microM, increased accumulation of mRNA for MTA, MTB, GST and G6PD in cells. Thus, in addition to its ability to sequester free radicals, CA may protect against oxidative stress through expression of zinc-induced antioxidant proteins. Because of these properties we suggest that CA could be a beneficial additive to fish feeds in aquaculture.

Metallothionein redox cycle and function

The biologic function of metallothionein (MT) has been a perplexing topic ever since the discovery of this protein. Many studies have suggested that MT plays a role in the homeostasis of essential metals such as zinc and copper, detoxification of toxic metals such as cadmium, and protection against oxidative stress. However, mechanistic insights into the actions of MT have not been adequately achieved. MT contains high levels of sulfur. The mutual affinity of sulfur and transition metals makes the binding of these metals to MT thermodynamically stable. Under physiologic conditions, zinc-MT is the predominant form of the metal-binding protein. The recognition of the redox regulation of zinc release from or binding to MT provides an alternate perspective on biologic function of MT. Oxidation of the thiolate cluster by a number of mild cellular oxidants causes zinc release and formation of MT-disulfide (or thionin if all metals are released from MT, but this is unlikely to occur in vivo), which have been demonstrated in vivo. Therefore, the thermodynamic stability of zinc binding makes MT an ideal zinc reservoir in vivo, and the redox regulation of zinc mobilization enables MT function in zinc homeostasis. MT-disulfide can be reduced by glutathione in the presence of selenium catalyst, restoring the capacity of the protein to bind zinc. This MT redox cycle may play a crucial role in MT biologic function. It may link to the homeostasis of essential metals, detoxification of toxic metals and protection against oxidative stress.

Cytotoxicity of nitric oxide is alleviated by zinc-mediated expression of antioxidant genes

Metallothioneins (MTs) are small, cysteine-rich zinc binding proteins that are powerful antioxidants. In this study, we investigated the interaction between zinc, MTs, and other components of the antioxidant defense system in HepG2 cells. Cells were preincubated with zinc and then exposed to sodium nitroprusside (SNP), a nitric oxide (NO) donor. Both zinc pretreatment and SNP exposure separately induced transcription of MT genes (MT1A, MT2A, MT1E, MT1X), as measured using real time-polymerase chain reaction (PCR) after reverse transcription (RT). Pretreatment of HepG2 cells with zinc sulfate (ZnSO4) followed by SNP exposure caused MT and glucose-6-phosphate dehydrogenase (G6PD) mRNA levels to increase more than in cells only exposed to SNP. However, when cells were incubated with N,N,N',N'-tetrakis(2-pyridylmethyl)ethyl-enediamine (TPEN), a membrane-permeant Zn2+ chelator, the stimulation of MT transcription by SNP was blocked, suggesting that SNP-induced upregulation of these genes is zinc-dependent. Human glutathione-S-transferase (hGSTA1) and G6PD mRNA levels in the cells treated with 5 microM TPEN decreased. Additionally, the induction of MT by SNP after zinc pretreatment appears to be mediated by metal-activated transcription factor-1 (MTF-1), which is induced by labile zinc in the cytosol. SNP cytotoxicity was inhibited by preincubation with zinc. Taken together, these results suggest that NO plays an important role in regulation of cellular zinc homeostasis and that NO-mediated release of protein-bound Zn2+ may be an important signal in antioxidant defense.