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

Zinc finger proteins and ATP-binding cassette transporter-dependent multidrug resistance

BACKGROUND

Multidrug resistance (MDR) remains a significant challenge in cancer treatment, leading to poor clinical outcomes. Dysregulation of ATP-binding cassette (ABC) transporters has been identified as a key contributor to MDR. Zinc finger proteins (ZNPs) are key regulators of transcription and have emerged as potential contributors to cancer drug resistance. Bridging the knowledge gap between ZNPs and MDR is essential to understand a source of heterogeneity in cancer treatment. This review sought to elucidate how different ZNPs modulate the transcriptional regulation of ABC genes, contributing to resistance to cancer therapies.

METHODS

The search was conducted using PubMed, Google Scholar, EMBASE and Web of Science.

RESULTS

In addition to ABC-blockers, the transcriptional features regulated by ZNP are expected to play a role in reversing ABC-mediated MDR and predicting the efficacy of anticancer treatments. Among the ZNP-induced epithelial to mesenchymal transition, SNAIL, SLUG and Zebs have been identified as important factors in promoting MDR through activation of ATM, NFκB and PI3K/Akt pathways, exposing the metabolism to potential ZNP-MDR interactions. Additionally, nuclear receptors, such as VDR, ER and PXR have been found to modulate certain ABC regulations. Other C2H2-type zinc fingers, including Kruppel-like factors, Gli and Sp also have the potential to contribute to MDR.

CONCLUSION

Besides reviewing evidence on the effects of ZNP dysregulation on ABC-related chemoresistance in malignancies, significant markers of ZNP functions are discussed to highlight the clinical implications of gene-to-gene and microenvironment-to-gene interactions on MDR prospects. Future research on ZNP-derived biomarkers is crucial for addressing heterogeneity in cancer therapy.

Regulation of zinc-dependent enzymes by metal carrier proteins

Zn²⁺ ions are essential in many physiological processes, including enzyme catalysis, protein structural stabilization, and the regulation of many proteins. The affinities of proteins for Zn²⁺ ions span several orders of magnitude, with catalytic Zn²⁺ ions generally held more tightly than structural or regulatory ones. Metal carrier proteins, most of which are not specific for Zn²⁺, bind these ions with a broad range of affinities that overlap those of catalytic, structural, and regulatory Zn²⁺ ions and are thought to be responsible for distributing the metal through most cells, tissues, and fluid compartments. While little is known about how many proteins obtain or release these ions, there is now considerable experimental evidence suggesting that metal carrier proteins may be responsible for transferring metals to and from some Zn²⁺-dependent proteins, thus serving as a major regulatory factor for them. In this review, the biological roles of Zn²⁺ and structures of Zn²⁺ binding sites are examined, and experimental evidence demonstrating the direct participation of metal carrier proteins in enzyme regulation is discussed. Mechanisms of metal ion transfer are also offered, and the potential physiological significance of this phenomenon is explored.

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.

Meta-analysis of gene expression profiling reveals novel basal gene signatures in MCF-10A cells transformed with cadmium

Cadmium (Cd²⁺) is an environmental toxicant and a human carcinogen. Several studies show an association of Cd²⁺ exposure to the development of breast cancer. Previously, we have transformed the immortalized non-tumorigenic cell line MCF-10A with Cd²⁺ and have demonstrated that the transformed cells have anchorage independent growth. In a separate study, we showed that transformation of the immortalized urothelial cells with the environmental carcinogen arsenite (As³⁺) results in an increase in expression of genes associated with the basal subtype of bladder cancer. In this study, we determined if transformation of the MCF-10A cells with Cd²⁺ would have a similar effect on the expression of basal genes. The results of our study indicate that there is a decrease in expression of genes associated with keratinization and cornification and this gene signature includes the genes associated with the basal subtype of breast cancer. An analysis of human breast cancer databases indicates an increased expression of this gene signature is associated with a positive correlation to patient survival whereas a reduced expression/absence of this gene signature is associated with poor patient survival. Thus, our study suggests that transformation of the MCF-10A cells with Cd²⁺ produces a decreased basal gene expression profile that correlates to patient outcome.

Expression of metallothionein in oral squamous cell carcinoma: A systematic review

Free radicals are chemical particles containing one or more unpaired electrons, which may be part of the molecule making them highly reactive species. The free radicals are also known to play a dual role in biological systems, as they can be either beneficial or harmful. It has been proven that there are numerous mechanisms participating in the protection of a cell against free radicals. In this systematic review, we have reviewed metallothioneins (MTs) which are a small, cysteinerich and heavy metalbinding protein, that participates in an array of protective stress responses. The aim of this study was to systematically evaluate the role of MT in oral squamous cell carcinoma (OSCC). In this systematic review, we have found that in 9 studies involving 1340 cases and 542 controls concluded that MT was found to be present in the cytoplasm as well as the nucleus of the tumor tissue in 66.6% of the articles using immunohistochemistry and 11.1% of the articles reported the mosaic pattern of expression of MT in OSCC.

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.

Detection and Manipulation of the Stress Response Protein Metallothionein

Metallothioneins (MTs) are small molecular weight stress response proteins that play a central role as reservoir of essential divalent heavy metal cations such as zinc and copper, and also can diminish the effects of toxic heavy metals such as mercury and cadmium. Historically, MT has been considered to be an intracellular protein with roles to play in the management of heavy metals, as a regulator of cellular redox potential, and as a buffer of free radicals. Our recent studies have highlighted immunomodulatory role of MT in inflammatory diseases and also in the progression of metastatic cell movement. Hence, manipulation and detection of MT is essential for its possible use as a diagnostic and in therapeutic interventions of chronic inflammation. This review describes procedures used to detect MT using techniques such as western immunoblot, competition ELISA, flow cytometry and immunohistochemistry. Additionally, it also describes the use of a colorimetric cell proliferation assay (CellTiter 96 AQ_ueous One Solution/MTS) to study the proliferative effect of MT. © 2017 by John Wiley & Sons, Inc.

The Physiological Roles of Extracellular Metallothionein

Metallothionein (MT) is a low-molecular-weight protein with a number of roles to play in cellular homeostasis. MT is synthesized as a consequence of a variety of cellular stressors, and has been found in both intracellular compartments and in extracellular spaces. The intracellular pool of this cysteine-rich protein can act as a reservoir of essential heavy metals, as a scavenger of reactive oxygen and nitrogen species, as an antagonist of toxic metals and organic molecules, and as a regulator of transcription factor activity. The presence of MT outside of cells due to the Influence of stressors suggests that this protein may make important contributions as a “danger signal” that influences the management of responses to cellular damage. While conventional wisdom has held that extracellular MT is the result of cell death or leakage from stressed cells, there are numerous examples of selective release of proteins by nontraditional mechanisms, including stress response proteins. This suggests that MT may similarly be selectively released, and that the pool of extracellular MT represents an important regulator of various cellular functions. For example, extracellular MT has effects both on the severity of autoimmune disease, and on the development of adaptive immune functions. Extracellular MT may operate as a chemotactic factor that governs the trafficking of inflammatory cells that move to resolve damaged tissues, as a counter to extracellular oxidant-mediated damage, and as a signal that influences the functional behavior of wounded cells. A thorough understanding of the mechanisms of MT release from cells, the conditions under which MT is released to the extracellular environment, and the ways in which MT Interacts with sensitive cells may both illuminate our understanding of an important control mechanism that operates in stressful conditions, and should indicate new opportunities for therapeutic management via the manipulation of this pool of extracellular MT.

The Functions of Metallothionein and ZIP and ZnT Transporters: An Overview and Perspective

Around 3000 proteins are thought to bind zinc in vivo, which corresponds to ~10% of the human proteome. Zinc plays a pivotal role as a structural, catalytic, and signaling component that functions in numerous physiological processes. It is more widely used as a structural element in proteins than any other transition metal ion, is a catalytic component of many enzymes, and acts as a cellular signaling mediator. Thus, it is expected that zinc metabolism and homeostasis have sophisticated regulation, and elucidating the underlying molecular basis of this is essential to understanding zinc functions in cellular physiology and pathogenesis. In recent decades, an increasing amount of evidence has uncovered critical roles of a number of proteins in zinc metabolism and homeostasis through influxing, chelating, sequestrating, coordinating, releasing, and effluxing zinc. Metallothioneins (MT) and Zrt- and Irt-like proteins (ZIP) and Zn transporters (ZnT) are the proteins primarily involved in these processes, and their malfunction has been implicated in a number of inherited diseases such as acrodermatitis enteropathica. The present review updates our current understanding of the biological functions of MTs and ZIP and ZnT transporters from several new perspectives.

Challenging conventional wisdom: single domain metallothioneins

Metallation studies of human metallothioneins support the role of single metal-binding-domains as commonplace with the typical two-domain-cluster structure as exceptional.

Rare-earth-incorporated polymeric vector for enhanced gene delivery

Cationic polymer PEI-CyD is doped with Nd by plasma technology to produce the gene vector: Nd@PEI-CyD. Luciferase expression and EGFP transfection experiments performed in vitro reveal that Nd@PEI-CyD has significantly higher transfection efficiency than lipofectamine 2000 and PEI-CyD and the mechanism is studied and proposed. The rare-earth element, Nd, stimulates the energy metabolism of cells, enhances cell uptake of complexes/pDNA, and regulates the cellular pathways. These special features suggest a new strategy involving metal-incorporated non-viral gene vectors.

Increased metallothionein expression reflects steroid resistance in renal allograft recipients

Steroid-refractory acute rejection is a risk factor for inferior renal allograft outcome. We aimed to gain insight into the mechanisms underlying steroid resistance by identifying novel molecular markers of steroid-refractory acute rejection. Eighty-three kidney transplant recipients (1995-2005), who were treated with methylprednisolone during a first acute rejection episode, were included in this study. Gene expression patterns were investigated in a discovery cohort of 36 acute rejection biopsies, and verified in a validation cohort of 47 acute rejection biopsies. In the discovery set, expression of metallothioneins (MT) was significantly (p < 0.000001) associated with decreased response to steroid treatment. Multivariate analysis resulted in a predictive model containing MT-1 as an independent covariate (AUC = 0.88, p < 0.0000001). In the validation set, MT-1 expression was also significantly associated with steroid resistance (p = 0.029). Metallothionein expression was detected in macrophages and tubular epithelial cells. Parallel to the findings in patients, in vitro experiments of peripheral blood mononuclear cells from 11 donors showed that nonresponse to methylprednisolone treatment is related to highly elevated MT levels. High expression of metallothioneins in renal allografts is associated with resistance to steroid treatment. Metallothioneins regulate intracellular concentrations of zinc, through which they may diminish the zinc-requiring anti-inflammatory effect of the glucocorticoid receptor.

Single-domain metallothioneins: evidence of the onset of clustered metal binding domains in Zn-rhMT 1a

Mammalian metallothioneins bind up to seven Zn(2+) ions in two distinct domains: an N-terminal β-domain that binds three Zn(2+) ions and a C-terminal α-domain that binds four Zn(2+) ions. Domain specificity has been invoked in the metalation mechanism with cluster formation and bridging of the 20 Cys residues taking place prior to saturation with seven Zn(2+) ions. We report a novel experiment that examines Zn(2+) metalation by exploiting the expected decrease in K(F) at the onset of clustering using electrospray ionization mass spectrometry (ESI-MS). During the titration with Zn(2+), the ESI-MS data show that several metalated species coexist until the fully saturated proteins are formed. The relative Zn binding affinities of the seven total sites in the α- and β-fragments were determined through direct competition for added Zn(2+). The K(F) values for each Zn(2+) are expected to decrease as a function of the remaining available sites and the onset of clustering. Analysis shows that Zn(2+) binds to β-rhMT with a greater affinity than α-rhMT. The incremental distribution of Zn(2+) between the competing fragments and apo-βα-rhMT (essentially three and four sites competing with seven sites) identifies the exact point at which clustering begins in the full protein. Analysis of the speciation data shows that Zn(5)-MT forms before clustering begins. This means that all 20 Cys residues of apo-βα-rhMT are bound terminally to Zn(2+) as [Zn(Cys)(4)](2-) units before clustering begins; there is no domain preference in this first metalation stage. Preferential binding of Zn(2+) to β- and α-rhMT at the point where βα-rhMT must form clusters is caused by a significant decrease in the affinity of βα-rhMT for further Zn(2+). The single-domain Zn(5)-rhMT, in which there are no exposed cysteine sulfurs, is a key component of the metalation pathway because the lower affinities of the two clustered Zn(2+) ions allow donation to apoenzymes.

The role of metallothionein in oxidative stress

Free radicals are chemical particles containing one or more unpaired electrons, which may be part of the molecule. They cause the molecule to become highly reactive. The free radicals are also known to play a dual role in biological systems, as they can be either beneficial or harmful for living systems. It is clear that there are numerous mechanisms participating on the protection of a cell against free radicals. In this review, our attention is paid to metallothioneins (MTs) as small, cysteine-rich and heavy metal-binding proteins, which participate in an array of protective stress responses. The mechanism of the reaction of metallothioneins with oxidants and electrophilic compounds is discussed. Numerous reports indicate that MT protects cells from exposure to oxidants and electrophiles, which react readily with sulfhydryl groups. Moreover, MT plays a key role in regulation of zinc levels and distribution in the intracellular space. The connections between zinc, MT and cancer are highlighted.

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.

Molecular functions of metallothionein and its role in hematological malignancies

Metallothionein (MT) was reported to be a potential negative regulator of apoptosis, and various reports have suggested that it may play roles in carcinogenesis and drug resistance, in at least a portion of cancer cells. The author summarizes the current understanding of the molecular functions of MT for tumor cell growth and drug resistance. These activities are regulated through intracellular metal ion modulation and free radical scavenging. Compared with analyses of solid tumors, few studies have analyzed the roles of MT in hematological malignancies. This review mainly describes the functions of MT in hematopoietic cells. Furthermore, through expression analyses of leukemias and lymphomas, the roles of MT in the biology of these diseases are particularly focused upon.

Mammalian metallothioneins: properties and functions

Metallothioneins (MT) are a family of ubiquitous proteins, whose role is still discussed in numerous papers, but their affinity to some metal ions is undisputable. These cysteine-rich proteins are connected with antioxidant activity and protective effects on biomolecules against free radicals, especially reactive oxygen species. In this review, the connection between zinc(II) ions, reactive oxygen species, heavy metal ions and metallothioneins is demonstrated with respect to effect of these proteins on cell proliferation and a possible negative role in resistance to heavy metal-based and non-heavy metal-based drugs.

Thermodynamics of Zn2+ binding to Cys2His2 and Cys2HisCys zinc fingers and a Cys4 transcription factor site

The thermodynamics of Zn(2+) binding to three peptides corresponding to naturally occurring Zn-binding sequences in transcription factors have been quantified with isothermal titration calorimetry (ITC). These peptides, the third zinc finger of Sp1 (Sp1-3), the second zinc finger of myelin transcription factor 1 (MyT1-2), and the second Zn-binding sequence of the DNA-binding domain of glucocorticoid receptor (GR-2), bind Zn(2+) with Cys(2)His(2), Cys(2)HisCys, and Cys(4) coordination, respectively. Circular dichroism confirms that Sp1-3 and MyT1-2 have considerable and negligible Zn-stabilized secondary structure, respectively, and indicate only a small amount for GR-2. The pK(a)'s of the Sp1-3 cysteines and histidines were determined by NMR and used to estimate the number of protons displaced by Zn(2+) at pH 7.4. ITC was also used to determine this number, and the two methods agree. Subtraction of buffer contributions to the calorimetric data reveals that all three peptides have a similar affinity for Zn(2+), which has equal enthalpy and entropy components for Sp1-3 but is more enthalpically disfavored and entropically favored with increasing Cys ligands. The resulting enthalpy-entropy compensation originates from the Zn-Cys coordination, as subtraction of the cysteine deprotonation enthalpy results in a similar Zn(2+)-binding enthalpy for all three peptides, and the binding entropy tracks with the number of displaced protons. Metal and protein components of the binding enthalpy and entropy have been estimated. While dominated by Zn(2+) coordination to the cysteines and histidines, other residues in the sequence affect the protein contributions that modulate the stability of these motifs.

Metallothionein Zn2+- and Cu2+-clusters from first-principles calculations

Detailed electronic structures of Zn(II) and Cu(II) clusters from metallothioneins (MT) have been obtained using density functional theory (DFT), in order to investigate how oxidative stress-caused Cu(II) intermediates affect Zn-binding to MT and cooperatively lead to Cu(I)MT. The inferred accuracy is ∼0.02-0.03 Å for metal-thiolate bond lengths for the models that are the most realistic MT models so far studied by DFT. We find terminal Zn-S and Cu-S bond lengths of 2.35-2.38 Å and 2.30-2.34 Å, whereas bridging M-S bonds are 0.05-0.11 Å longer. This electronic effect is also reflected in changes in electron density on bridging sulfurs. Various imposed backbone constraints quantify the sensitivity of cluster electronic structure towards protein conformational changes. The large negative charge densities of the clusters are central to MT function, and the smaller β-clusters are more prone to modification. Oxidative stress-associated Cu(II) binding weakens the Zn-S bonds and is thus likely to impair the Zn(II) transfer function of MTs, providing a mechanism for cooperative Cu(II) binding leading to loss of Zn(II) and dysfunctional Cu(I)MT clusters.

Full quantum-mechanical structure of the human protein Metallothionein-2

Metallothioneins (MT) are small, metal-binding proteins with diverse functions related to metal ion homeostasis. This paper presents the full 384-388-atom structures of the two native Zn(II)- and the Cd(II)-containing domains of human MT2, optimized with density functional theory. The presented structures are accurate to ~0.03 Å for bond lengths and thus provide new physical insight into the detailed electronic structures of MTs, in particular with accurate accounts of bridging vs. terminal bonds not available from NMR or EXAFS. The MT protein enhances the asymmetry, as compared to the protein-free clusters, causing a hierarchy in binding that most likely allows MTs to transfer ions to multiple targets in vivo. The protein polarization is substantial and occurs primarily via the terminal sulfurs, a key mechanism in providing domain-specific electronic structures. The β-domain polarizes its smaller cluster less on average, due to its less polarizable, higher negative charge density, as reflected in longer MS bond lengths and smaller bond orders. This may explain why MT2β is more reactive and dynamic and why MTs have evolved two different-size, asymmetric domains with different metal binding affinities fit for different molecular targets of metal ion transfer.

Metallothionein expression in colorectal cancer: relevance of different isoforms for tumor progression and patient survival

Metallothioneins are a family of small, cysteine-rich proteins with many functions. Immunohistochemical evaluation of all metallothionein 1 + 2 isoforms in colorectal tumors has demonstrated an important down-regulation compared with normal tissue, although its prognostic significance is unclear. Moreover, the contribution of individual isoforms to overall metallothionein down-regulation is not known. To address these important issues, we analyzed the messenger RNA expression levels of all functional metallothionein 1 + 2 isoforms by quantitative reverse transcription polymerase chain reaction in 22 pairs of normal and tumor-microdissected epithelia and correlated these to the overall immunohistochemical protein expression. Our results showed that 5 isoforms (MT1G, 1E, 1F, 1H, and 1M) were lost during the transition from normal mucosa to tumor, whereas MT1X and MT2A were less down-regulated, and their expression was correlated with overall protein positivity. Second, we showed that MT1G hypermethylation occurred in cell lines and in 29% of tumor samples, whereas histone deacetylase inhibitors are able to induce most isoforms. Furthermore, we analyzed by immunohistochemistry 107 normal mucosae, 25 adenomas, 81 carcinomas, and 19 lymph node metastases to evaluate metallothionein expression during different stages of cancer development and to assess its relationship to patient survival. A lower immunohistochemical expression was associated with poorer survival, although it was not an independent predictor. Overall, this study identifies for the first time the relevant metallothionein isoforms for colorectal cancer progression, supports the concept that their loss is associated with worse prognosis, and suggests 2 mechanisms for epigenetic repression of metallothionein expression in colorectal tumors.

Chemistry and biology of mammalian metallothioneins

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

The "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.

Cardioprotective effect of zinc requires ErbB2 and Akt during hypoxia/reoxygenation

Recent literature suggests that exogenous zinc can prevent ischemia reperfusion injury by activating phosphoinositide-3 kinase (PI3K)/Akt and by targeting the mitochondrial permeability transition pore (mPTP). It is known that ErbB2 expression promotes association and activation of PI3-kinase/Akt, resulting in growth and survival of cardiac myocytes. In this study, we found that zinc-induced ErbB2 protein expression and Akt activation are required for preventing reperfusion injury. Neonatal rat cardiac myocytes subjected to 8 h of hypoxia, followed by 16 h of reoxygenation induced cardiomyocyte apoptosis, as assessed by increased caspase-3 activity, annexin V staining and lowered MTT reduction and ATP levels. However, addition of zinc-pyrithione (ZPT) before onset of reoxygenation effectively lowered the apoptotic indices and restored the ATP levels. ZPT induced a significant increase in ErbB2 protein expression and Akt activation. Pretreatment with Hsp 90 inhibitor, geldanamycin or PI3-kinase inhibitor, wortmannin prevented the increase in ATP levels and abrogated the protective effect of zinc-pyrithione. Taken together, these data suggest that zinc prevents reperfusion injury by modulating the ErbB2 protein expression and Akt activation.

Increased metallothionein gene expression, zinc, and zinc-dependent resistance to apoptosis in circulating monocytes during HIV viremia

Circulating monocytes exhibit an apoptotic resistance phenotype during HIV viremia in association with increased MT expression. MTs are known to play an important role in zinc metabolism and immune function. We now show, in a cross-sectional study using peripheral monocytes, that expression of MT1 isoforms E, G, H, and X is increased significantly in circulating monocyte cells from HIV+ subjects during chronic viremic episodes as compared with uninfected subjects. This increase in expression is also observed during acute viremia following interruption of suppressive ART. Circulating monocytes from HIV+ donors were also found to have elevated zinc importer gene Zip8 expression in conjunction with elevated intracellular zinc levels in contrast to CD4(+)T-lymphocytes. In vitro HIV-1 infection studies with elutriated MDM confirm a direct relation between HIV-1 infection and increased MDM MT1 (isoform G) gene expression and increased intracellular zinc levels. A direct link between elevated zinc levels and apoptosis resistance was established using a cell-permeable zinc chelator TPEN, which reversed apoptosis resistance effectively in monocytes from HIV-infected to levels comparable with uninfected controls. Taken together, increases in MT gene expression and intracellular zinc levels may contribute directly to maintenance of an immune-activated monocyte by mediating an increased resistance to apoptosis during active HIV-1 viremia.

Immunohistochemical localization of metallothionein and p53 protein in pancreatic serous cystadenomas

INTRODUCTION

The objective of this study was to determine the expression levels of metallothionein (MT) and p53 protein, recognized neoplastic transformation markers, in pancreatic serous cystadenomas (SCA) and adenomocarcinomas.

MATERIALS AND METHODS

Neoplastic pancreatic tissue was taken from 20 patients with diagnosed benign (SCA: 5 cases) or malignant tumors (adenomocarcinomas: 15 cases) and control pancreatic tissue from healthy persons who had died in car accidents. Sections were stained with hematoxylin-eosin. Immunohistochemical localization of MT and p53 protein was carried out by LSAB2-HRP using specific antibodies against MT and p53.

RESULTS

Metallothionein expression was observed only in the epithelial cells of the neoplastic tissue of SCAs. MT expression in the cystadenomas was weaker than in the healthy pancreatic tissue. No tissue was found with p53 protein expression. In the adenomocarcinomas, positive staining for MT was observed in 67% and p53 was positive in the carcinoma cells.

CONCLUSION

The weak MT expression and lack of p53 protein expression in pancreatic SCAs confirms the lack of local invasive potential of the neoplastic lesion. Increased expressions of MT and p53 were observed in the less differentiated tumors. Thus the expression of MT may be a potential prognostic marker for tumors.

A potential role for alterations of zinc and zinc transport proteins in the progression of Alzheimer's disease

Although multiple studies have suggested a role for alterations of zinc (Zn) and zinc transport (ZnT) proteins in the pathogenesis of Alzheimer's disease, the exact role of this essential trace element in the progression of the disease remains unclear. The following review discusses the normal role of Zn and ZnT proteins in brain and the potential effects of their alteration in the pathogenesis of Alzheimer's disease, particularly in the processing of the amyloid-beta protein precursor and amyloid-beta peptide generation and aggregation.

The role of metallothionein in oncogenesis and cancer prognosis

The antiapoptotic, antioxidant, proliferative, and angiogenic effects of metallothionein (MT)-I+II has resulted in increased focus on their role in oncogenesis, tumor progression, therapy response, and patient prognosis. Studies have reported increased expression of MT-I+II mRNA and protein in various human cancers; such as breast, kidney, lung, nasopharynx, ovary, prostate, salivary gland, testes, urinary bladder, cervical, endometrial, skin carcinoma, melanoma, acute lymphoblastic leukemia (ALL), and pancreatic cancers, where MT-I+II expression is sometimes correlated to higher tumor grade/stage, chemotherapy/radiation resistance, and poor prognosis. However, MT-I+II are downregulated in other types of tumors (e.g. hepatocellular, gastric, colorectal, central nervous system (CNS), and thyroid cancers) where MT-I+II is either inversely correlated or unrelated to mortality. Large discrepancies exist between different tumor types, and no distinct and reliable association exists between MT-I+II expression in tumor tissues and prognosis and therapy resistance. Furthermore, a parallel has been drawn between MT-I+II expression as a potential marker for prognosis, and MT-I+II's role as oncogenic factors, without any direct evidence supporting such a parallel. This review aims at discussing the role of MT-I+II both as a prognostic marker for survival and therapy response, as well as for the hypothesized role of MT-I+II as causal oncogenes.

Zinc binding ligands and cellular zinc trafficking: apo-metallothionein, glutathione, TPEN, proteomic zinc, and Zn-Sp1

Many cell types contain metal-ion unsaturated metallothionein (MT). Considering the Zn(2+) binding affinity of metallothionein, the existence of this species in the intracellular environment constitutes a substantial "thermodynamic sink". Indeed, the mM concentration of glutathione may be thought of in the same way. In order to understand how apo-MT and the rest of the Zn-proteome manage to co-exist, experiments examined the in vitro reactivity of Zn-proteome with apo-MT, glutathione (GSH), and a series of common Zn(2+) chelating agents including N,N,N',N'-(2-pyridylethyl)ethylenediammine (TPEN), EDTA, and [(2,2'-oxyproplylene-dinitrilo]tetraacetic acid (EGTA). Less than 10% of Zn-proteome from U87mg cells reacted with apo-MT or GSH. In contrast, each of the synthetic chelators was 2-3 times more reactive. TPEN, a cell permeant reagent, also reacted rapidly with both Zn-proteome and Zn-MT in LLC-PK(1) cells. Taking a specific zinc finger protein for further study, apo-MT, GSH, and TPEN inhibited the binding of Zn(3)-Sp1 with its cognate DNA site (GC-1) in the sodium-glucose co-transporter promoter of mouse kidney. In contrast, preformation of Zn(3)-Sp1-(GC-1) prevented reaction with apo-MT and GSH; TPEN remained active but at a higher concentration. Whereas, Zn(3)-Sp1 is active in cells containing apo-MT and GSH, exposure of LLC-PK(1) cells to TPEN for 24h largely inactivated its DNA binding activity. The results help to rationalize the steady state presence of cellular apo-MT in the midst of the many, diverse members of the Zn-proteome. They also show that TPEN is a robust intracellular chelator of proteomic Zn(2+).

Screening of genes associated with termination of the critical period of visual cortex plasticity in rats

PURPOSE

To investigate the molecular mechanism involved in the termination of the critical period of visual cortex plasticity in the rat.

METHODS

The rats were divided into two groups, one group was dark-reared for 67 days after birth, while the other group was dark-reared for 60 days and then put under a normal light/dark cycle for 7 days. A subtracted cDNA library was constructed from the visual cortex, and differentially expressed genes were screened by nested PCR, reverse Northern hybridization, sequencing, and homology analysis.

RESULTS

Fourteen genes were found to be up-regulated in the visual cortex. These included 13 known genes and a novel fragment (Genbank submission EB174193). Of the known genes, three genes encoding beta -tubulin, myelin basic protein, and cyclophilin were previously reported to be associated with visual cortex plasticity.

CONCLUSION

A set of candidate genes related to the termination of the critical period was identified using the subtracted cDNA library. This work provides an important basis for understanding the molecular mechanisms involved in the termination of plasticity in the visual cortex.

The use of biomarkers in biomonitoring: a 2-tier approach assessing the level of pollutant-induced stress syndrome in sentinel organisms

The paper outlines a 2-tier approach for wide-scale biomonitoring programmes. To obtain a high level of standardization, we suggest the use of caged organisms (mussels or fish). An "early warning", highly sensitive, low-cost biomarker is employed in tier 1 (i.e. lysosomal membrane stability (LMS) and survival rate, a marker for highly polluted sites). Tier 2 is used only for animals sampled at sites in which LMS changes are evident and there is no mortality, with a complete battery of biomarkers assessing the levels of pollutant-induced stress syndrome. Possible approaches for integrating biomarker data in a synthetic index are discussed, along with our proposal to use a recently developed Expert System. The latter system allows a correct selection of biomarkers at different levels of biological organisation (molecular/cellular/tissue/organism) taking into account trends in pollutant-induced biomarker changes (increasing, decreasing, bell-shape). A selection of biomarkers of stress, genotoxicity and exposure usually employed in biomonitoring programmes is presented, together with a brief overview of new biomolecular approaches.

Cytokine-induced Metallothionein Expression and Modulation of Cytokine Expression by Metallothionein

A multifunctional protein metallothionein (MT) is induced by various chemicals and cytokines. We have found novel functions of MT as follows: 1) Cytokine expression such as IL-1alpha, IL-6, and TNFalpha responding to lipopolysaccharide is reduced in MT-deficient macrophages compared with in wild-type cells. 2) Nitric oxide production responding to TNFalpha and LPS is reduced in MT-deficient macrophages compared with in wild-type cells. 3) M-CSF expression responding to zinc is reduced in MT-deficient fibroblasts compared with in wild-type cells, and increased in MT-overexpressed fibroblasts compared with in control cells. 4) LIF, a STAT3 activating cytokine, protects the heart from ischemia/reperfusion injury. Transgenic mice overexpressing STAT3 have tolerance to ischemia/reperfusion-induced damage, whereas MT-null mutation cancels the myocardial protection. In this review, we discuss the relation of MT and stress responses from the point of view of cytokine-induced expression of MT and modulation of cytokine expression by MT.

Challenging the model for induction of metallothionein gene expression

Metallothioneins (MTs) are low-molecular-weight, cysteine-rich metal-binding proteins found in a wide variety of organisms including bacteria, fungi and all eukaryotic plant and animal species. MTs bind essential and non-essential heavy metals. In mammalian cells MT genes are highly inducible by many heavy metals including Zn, Cd, Hg, and Cu. Aquatic systems are contaminated by different pollutants, including metals, as a result of man's activities. Bivalve molluscs are known to accumulate high concentrations of heavy metals in their tissue and are widely used as bioindicators for pollution in marine and freshwater environments, with MTs frequently used as a valuable marker of metal contamination. We here describe the MT isoform gene expression patterns of marine and freshwater molluscs and fish species after Cd or Zn contamination. Contamination was carried out at a river site polluted by a zinc ore extraction plant or in the laboratory at low, environmentally relevant metal concentrations. A comparison for each species based on the accumulated MT protein levels often shows discrepancies between gene expression and protein level. In addition, several differences observed in the pattern of MT gene expression between mollusc and mammalian species enable us to discuss and challenge a model for the induction of MT gene expression.

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.