Metallothionein and apoptosis during differentiation of myoblasts to myotubes: protection against free radical toxicity

The connection of α- and β-domains in mammalian metallothionein-2 differentiates Zn(II) binding affinities, affects folding, and determines zinc buffering properties

Mammalian metallothioneins (MTs) are small Cys-rich proteins involved in Zn(II) and Cu(I) homeostasis. They bind seven Zn(II) ions in two distinct β- and α-domains, forming Zn3Cys9 and Zn4Cys11 clusters, respectively. After six decades of research, their role in cellular buffering of Zn(II) ions has begun to be understood recently. This is because of different affinities of bound ions and the proteins' coexistence in variously Zn(II)-loaded Zn4-7MT species in the cell. To date, it has remained unclear how these mechanisms of action occur and how the affinities are differentiated despite the Zn(S-Cys)4 coordination environment being the same. Here, we dissect the molecular basis of these phenomena by using several MT2 mutants, hybrid protein, and isolated domains. Through a combination of spectroscopic and stability studies, thiol(ate) reactivity, and steered molecular dynamics, we demonstrate that both protein folding and thermodynamics of Zn(II) ion (un)binding significantly differ between isolated domains and the whole protein. Close proximity reduces the degrees of freedom of separated domains, making them less dynamic. It is caused by the formation of intra- and interdomain electrostatic interactions. The energetic consequence of domains connection has a critical impact on the role of MTs in the cellular environment, where they function not only as a zinc sponge but also as a zinc buffering system keeping free Zn(II) in the right concentrations. Any change of that subtle system affects the folding mechanism, zinc site stabilities, and cellular zinc buffer components.

An Integrated Mass Spectrometry and Molecular Dynamics Simulations Approach Reveals the Spatial Organization Impact of Metal-Binding Sites on the Stability of Metal-Depleted Metallothionein-2 Species

Mammalian metallothioneins (MTs) are a group of cysteine-rich proteins that bind metal ions in two α- and β-domains and represent a major cellular Zn(II)/Cu(I) buffering system in the cell. At cellular free Zn(II) concentrations (10-11-10-9 M), MTs do not exist in fully loaded forms with seven Zn(II)-bound ions (Zn7MTs). Instead, MTs exist as partially metal-depleted species (Zn4-6MT) because their Zn(II) binding affinities are on the nano- to picomolar range comparable to the concentrations of cellular Zn(II). The mode of action of MTs remains poorly understood, and thus, the aim of this study is to characterize the mechanism of Zn(II) (un)binding to MTs, the thermodynamic properties of the Zn1-6MT2 species, and their mechanostability properties. To this end, native mass spectrometry (MS) and label-free quantitative bottom-up and top-down MS in combination with steered molecular dynamics simulations, well-tempered metadynamics (WT-MetaD), and parallel-bias WT-MetaD (amounting to 3.5 μs) were integrated to unravel the chemical coordination of Zn(II) in all Zn1-6MT2 species and to explain the differences in binding affinities of Zn(II) ions to MTs. Differences are found to be the result of the degree of water participation in MT (un)folding and the hyper-reactive character of Cys21 and Cys29 residues. The thermodynamics properties of Zn(II) (un)binding to MT2 are found to differ from those of Cd(II), justifying their distinctive roles. The potential of this integrated strategy in the investigation of numerous unexplored metalloproteins is attested by the results highlighted in the present study.

Metal- and Affinity-Specific Dual Labeling of Cysteine-Rich Proteins for Identification of Metal-Binding Sites

Here, using human metallothionein (MT2) as an example, we describe an improved strategy based on differential alkylation coupled to MS, assisted by zinc probe monitoring, for identification of cysteine-rich binding sites with nanomolar and picomolar metal affinity utilizing iodoacetamide (IAM) and N-ethylmaleimide reagents. We concluded that an SN2 reaction provided by IAM is more suitable to label free Cys residues, avoiding nonspecific metal dissociation. Afterward, metal-bound Cys can be easily labeled in a nucleophilic addition reaction after separation by reverse-phase C18 at acidic pH. Finally, we evaluated the efficiency of the method by mapping metal-binding sites of Zn7-xMT species using a bottom-up MS approach with respect to metal-to-protein affinity and element(al) resolution. The methodology presented might be applied not only for MT2 but to identify metal-binding sites in other Cys-containing proteins.

Mass Spectrometry-Based Structural Analysis of Cysteine-Rich Metal-Binding Sites in Proteins with MetaOdysseus R Software

Identification of metal-binding sites in proteins and understanding metal-coupled protein folding mechanisms are aspects of high importance for the structure-to-function relationship. Mass spectrometry (MS) has brought a powerful adjunct perspective to structural biology, obtaining from metal-to-protein stoichiometry to quaternary structure information. Currently, the different experimental and/or instrumental setups usually require the use of multiple data analysis software, and in some cases, they lack some of the main data analysis steps (MS processing, scoring, identification). Here, we present a comprehensive data analysis pipeline that addresses charge-state deconvolution, statistical scoring, and mass assignment for native MS, bottom-up, and native top-down with emphasis on metal–protein complexes. We have evaluated all of the approaches using assemblies of increasing complexity, including free and chemically labeled proteins, from low- to high-resolution MS. In all cases, the results have been compared with common software and proved how MetaOdysseus outperformed them.

Sensors for measuring subcellular zinc pools

Zinc homeostasis is essential for normal cellular function, and defects in this process are associated with a number of diseases including type 2 diabetes (T2D), neurological disorders and cardiovascular disease. Thus, variants in the SLC30A8 gene, encoding the vesicular/granular zinc transporter ZnT8, are associated with altered insulin release and increased T2D risk while the zinc importer ZIP12 is implicated in pulmonary hypertension. In light of these, and findings in other diseases, recent efforts have focused on the development of refined sensors for intracellular free zinc ions that can be targeted to subcellular regions including the cytosol, endoplasmic reticulum (ER), secretory granules, Golgi apparatus, nucleus and the mitochondria. Here, we discuss recent advances in Zn²⁺ probe engineering and their applications to the measurement of labile subcellular zinc pools in different cell types.

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 Gene Family in the Sea Urchin Paracentrotus lividus: Gene Structure, Differential Expression and Phylogenetic Analysis

Metallothioneins (MT) are small and cysteine-rich proteins that bind metal ions such as zinc, copper, cadmium, and nickel. In order to shed some light on MT gene structure and evolution, we cloned seven Paracentrotus lividus MT genes, comparing them to Echinodermata and Chordata genes. Moreover, we performed a phylogenetic analysis of 32 MTs from different classes of echinoderms and 13 MTs from the most ancient chordates, highlighting the relationships between them. Since MTs have multiple roles in the cells, we performed RT-qPCR and in situ hybridization experiments to understand better MT functions in sea urchin embryos. Results showed that the expression of MTs is regulated throughout development in a cell type-specific manner and in response to various metals. The MT7 transcript is expressed in all tissues, especially in the stomach and in the intestine of the larva, but it is less metal-responsive. In contrast, MT8 is ectodermic and rises only at relatively high metal doses. MT5 and MT6 expression is highly stimulated by metals in the mesenchyme cells. Our results suggest that the P. lividus MT family originated after the speciation events by gene duplications, evolving developmental and environmental sub-functionalization.

Metallothionein 1G promotes the differentiation of HT-29 human colorectal cancer cells

Metallothioneins (MTs) are a family of low- molecular-weight, cysteine-rich proteins involved in zinc and redox metabolism, that are epigenetically downregulated during colorectal cancer (CRC) progression, but may be re-induced with a variety of agents. Since loss of MT expression is associated with a worse prognosis, in the present study we investigated the effects of overexpression of the most significantly downregulated isoform in CRC, namely MT1G, on the HT-29 cell line. Overexpression of MT1G resulted in xenograft tumors with an aberrant morphology, characterized by an evident increase in mucin-containing cells that were identified as goblet cells under electron microscopy. Immunohistochemical detection of CDX2 and cytokeratin 20 was also increased, as were goblet-cell and enterocyte-specific genes by qRT-PCR. Microarray analysis of gene expression confirmed the alteration of several differentiation signaling pathways, including the Notch pathway. Using sodium butyrate and post-confluent growth as inducers of differentiation, we demonstrated that MT1G does indeed play a functional role in promoting goblet over enterocyte differentiation in vitro. Labile zinc is also induced upon differentiation of CRC cells, functionally contributing to enterocyte over goblet differentiation, as revealed using zinc-specific chelating agents. Overall, our results uncover a new tumor-suppressor activity of MT1G in promoting the differentiation of at least some CRC tumors, and implicate MTs and zinc signaling as new players in colorectal differentiation. This further contributes to the hypothesis that re-induction of MTs may have therapeutic value by diminishing the aggressiveness of CRC tumors.

Advanced age-related denervation and fiber-type grouping in skeletal muscle of SOD1 knockout mice

In this study skeletal muscles from 1.5- and 10-month-old Cu/Zn superoxide dismutase (SOD1) homozygous knockout (JLSod1(-/-)) mice obtained from The Jackson Laboratory (C57Bl6/129SvEv background) were compared with muscles from age- and sex-matched heterozygous (JLSod1(+/-)) littermates. The results of this study were compared with previously published data on two different strains of Sod1(-/-) mice: one from Dr. Epstein's laboratory (ELSod1(-/-); C57Bl6 background) and the other from Cephalon, Inc. (CSod1(-/-); 129/CD-1 background). Grouping of succinate dehydrogenase-positive fibers characterized muscles of Sod1(-/-) mice from all three strains. The 10-month-old Sod1(-/-)C and JL mice displayed pronounced denervation of the gastrocnemius muscle, whereas the ELSod1(-/-) mice displayed a small degree of denervation at this age, but developed accelerated age-related denervation later on. Denervation markers were up-regulated in skeletal muscle of 10-month-old JLSod1(-/-) mice. This study is the first to show that metallothionein mRNA and protein expression was up-regulated in the skeletal muscle of 10-month-old JLSod1(-/-) mice and was mostly localized to the small atrophic muscle fibers. In conclusion, all three strains of Sod1(-/-) mice develop accelerated age-related muscle denervation, but the genetic background has significant influence on the progress of denervation.

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.

Cellular zinc homeostasis is a regulator in monocyte differentiation of HL-60 cells by 1 alpha,25-dihydroxyvitamin D3

It was reported previously that zinc-deficient mice show impaired lymphopoiesis. At the same time, monocyte numbers in these animals are increased, indicating a negative impact of zinc on monocyte development. Here, we investigate the role of zinc homeostasis in the differentiation of myeloid precursors into monocytes. Reduced gene expression of several zinc transporters, predominantly from the Zip family, was observed during 1 alpha, 25-dihydroxyvitamin D(3) (1,25D(3))-induced differentiation of HL-60 cells. This was accompanied by a reduction of intracellular-free zinc, measured by FluoZin-3. Amplifying this reduction with the zinc chelator TPEN or zinc-depleted cell-culture medium enhanced 1,25D(3)-induced expression of monocytic surface markers CD11b and CD14 on HL-60, THP-1, and NB4 cells. In contrast, differentiation of NB4 cells to granulocytes was not zinc-sensitive, pointing toward a specific effect of zinc on monocyte differentiation. Further, monocyte functions, such as TNF-alpha secretion, phagocytosis, and oxidative burst, were also augmented by differentiation in the presence of TPEN. The second messenger cAMP promotes monocyte differentiation. We could show that zinc inhibits the cAMP-synthesizing enzyme adenylate cyclase, and chelation of zinc by TPEN increases cAMP generation after stimulation with the adenylate cyclase activator forskolin. Based on our in vitro results and the in vivo observations from the literature, we suggest a model in which the intracellular-free zinc concentration limits AC activity, and the decrease of zinc after 1,25D(3) treatment promotes differentiation by relieving AC inhibition. Thus, cellular zinc homeostasis acts as an endogenous modulator of monocyte differentiation.

Alterations in mRNA expression and protein products following spinal cord injury in humans

We examined the effects of spinal cord injury (SCI) on alterations in gene expression and respective protein products in human skeletal muscle 2 days and 5 days post-SCI. Biopsies were taken from skeletal muscle of 9 men and 1 woman (n = 10) (43.9 +/- 6.7 years) 2 days and 5 days post-SCI and from 5 healthy young men who served as controls (20.4 +/- 0.5 years). Global changes in gene expression were analysed using Affymetrix GeneChips on a subsample of subjects (n = 3). Candidate genes were then pursued via qRT-PCR. Western blotting (WB) was used to quantify protein products of candidate genes. Immunohistochemistry (IHC) was used to localize proteins. Groups of transcripts showing the greatest percentage of altered expression, the most robust fold-changes, and indicative of involvement of an entire pathway using the GeneChip included genes involved in the ubiquitin proteasome pathway (UPP), metallothionein function, and protease inhibition. qRT-PCR analysis confirmed increases in gene expression for UPP components (UBE3C, Atrogin-1, MURF1, and PSMD11), the metallothioneins (MT1A, MT1F, MT1H), and the protease inhibitor, SLPI (P < 0.05) at 2 days and 5 days post-SCI. Protein levels of the proteasome subunit (PSMD11) and the metallothioneins were increased 5 days post-SCI. Protein levels of UBE3C, Atrogin-1, MURF1 and SLPI were unchanged (P > 0.05). IHC showed increased staining for PSMD11 and the metallothioneins 5 days post-SCI, along the peripheral region of the cells. IHC also showed altered staining for Atrogin-1 at 5 days post-SCI along the membrane region. Thus, there was a profound increase in gene expression of UPP components, the metallothioneins, and the protease inhibitor, SLPI, within 5 days of SCI. Increased protein levels for PSMD11 and the metallothioneins 5 days post-SCI, specifically along the cell periphery, indicate that proteins in this region may be early targets for degradation post-SCI.

Zinc dynamics in the myocardial redox signaling network

Zinc plays a vital role in various cellular functions. Zinc deprivation is associated with severe disorders related to growth, maturation, and stress responses. In the heart, zinc affects differentiation and regeneration of cardiac muscle, cardiac conductance, acute stress responses, and recovery of heart transplants. Recent discoveries of the molecular players in zinc homeostasis revealed that the amount of intracellular free zinc is tightly controlled on the level of uptake, intracellular sequestration, redistribution, storage, and elimination, consequently creating a narrow window of optimal zinc concentration in the cells. Most of intracellular zinc is bound to numerous structural and regulatory proteins, with metabolically active, labile zinc present in picoto nanomolar concentrations. The central position of zinc in the redox signaling network is built on its unique chemical nature. The redox inert zinc creates a redox active environment when it binds to a sulfur ligand. The reversible oxidation of the sulfur ligand is coupled to the reversible zinc release from the protein, thereby executing the task of so-called protein "redox zinc switch." Clearly, the impairment of zinc homeostasis will have far reaching physiological consequences.

Decreased neurofilament density in large myelinated axons of metallothionein-I, II knockout mice

Metallothioneins (MTs) are small proteins, two isoforms (I, II) of which bind metals. Their physiological role has been difficult to establish, but recent reports suggested that they serve an important function in nerve repair and in the protection against oxidative stress in the peripheral nervous system. We previously reported a decreased axon calibre in the large myelinated fibres of the phrenic nerve in the MT-I, II double knock out (MT-I, II KO) mouse model. We propose that this could be due to the effects of oxidative stress on neurofilaments (NFs). In this study, we examined the same subset of large myelinated axons using transmission electron microscopy (TEM). There was a decreased NF density in the axons of MT-I, II KO phrenic nerve (P<0.005). This observation may have novel therapeutic implications in the treatment of amyotrophic lateral sclerosis (ALS), particularly as the terminal phases of the disease involve respiratory insufficiency.

Serum-induced expression of metallothionein isoforms in K-562 cells

The metallothionein (MT) expression was studied in the hematopoietic precursor cell line K-562, after serum deprivation and reconstitution of the cells in medium with 10% (v/v) FCS. Serum deprivation for 72 h markedly downregulated the MT mRNA expression, only the isoforms most abundant in normal K-562 cells were clearly detectable. Within 1-1.5 h after serum supplementation however, a definite induction of MT mRNA was noticed, and all isoforms were induced. Forty-eight hours after serum stimulation, the MT mRNA expression of all isoforms decreased again. Also MT protein levels increased twofold 24 h after serum stimulation. These results suggest that MT has a function in the re-entry of resting cells into the cell cycle, this function however could not be assigned to a specific MT isoform. The induction of MT after serum stimulation was independent of protein synthesis, but dependent on phosphorylation.

Involvement of Oxidative Stress in the Synthesis of Metallothionein Induced by Mitochondrial Inhibitors

We previously reported that synthesis of metallothionein (MT) was induced by mitochondrial inhibitors such as 2,4-dinitrophenol (DNP) or antimycin A (Kondoh et al., 2001), which are potent inhibitors of mitochondrial respiration. Although the inhibitors are known to be radical generators in mitochondria, the involvement of oxidative stress in the synthesis of MT induced by mitochondrial inhibitors and the biological functions of MT remain obscure. In this study, therefore, we examined the involvement of oxidative stress in MT synthesis induced by mitochondrial inhibitors and the biological functions of MT. In cultured mouse fibroblast cells, the addition of DNP increased both MT concentration and MT mRNA level. Administration of DNP to L-buthionine-SR-sulfoximine (BSO)-pretreated mice increased hepatic lipid peroxidation and induction of MT synthesis. In addition, vitamin E prevented induction of MT synthesis as well as lipid peroxidation in the liver of mice caused by administration of DNP. Administration of mitochondrial inhibitor to mice elevated the levels of lipid peroxidation in the liver and mitochondria, and MT in the liver, indicating the generation of mitochondrial oxidative stress. These data suggest that the induction of MT synthesis by mitochondrial inhibitors is correlated with generation of oxidative stress in mitochondria. Furthermore, the level of DNP-induced alanine aminotransferase (ALT) activity, reflecting hepatic damage, was greater in MT-null mice than in wild-type mice, and intracellular accumulation of reactive oxygen species (ROS) caused by the action of mitochondrial inhibitors was greater in MT-null fibloblast cells than in wild-type cells. The results suggest that MT plays a role as a radical scavenger of intracellular ROS produced in mitochondria. Taken together, the results suggest that mitochondrial oxidative stress induces the synthesis of MT, which may contribute to regulation of mitochondrial ROS production.

Loss of PNA staining in mouse aortico-pulmonary septum is associated with mesenchymal cell apoptosis

Massive apoptosis of mesenchymal cells in the septum of the aortico-pulmonary trunk was found in mouse fetuses at stage 14.5dpc. It was associated with the appearance of cavities in the mesenchymal tissue, presumably due to cell loss, a strong reduction in the extent of lectin PNA staining, and the induction of metallothioneins in specialized mesenchymal cells. Cell loss was spatially restricted to an inner area of the septum and was due to a distinct apoptotic pattern of cells, different from that in the heart wall. These events led to a rapid reduction of the aortico-pulmonary septum as occurs during the late stages of heart morphogenesis. It coincided with the migration of other cell types that invaded the cell-depleted septum, and contributed to the histiogenesis of the mature heart.

Prognostic significance of metallothionein expression in renal cell carcinoma

Background

Metallothionein (MT) protein expression deficiency has been implicated in carcinogenesis while MT over expression in tumors is indicative of tumor resistance to anti-cancer treatment. The purpose of the study was to examine the expression of MT expression in human renal cell carcinoma (RCC) and to correlate MT positivity, the pattern and extent of MT expression with tumor histologic cell type and nuclear grade, pathologic stage and patients' survival.

Patients and methods

The immunohistochemical expression of MT was determined in 43 formalin-fixed and paraffin-embedded RCC specimens, using a mouse monoclonal antibody that reacts with both human MT-I and MT-II. Correlation was sought between immunohistochemical (MT positivity, intensity and extension of staining) and clinico-pathological data (histological cell type, tumor nuclear grade, pathologic stage and patients' survival).

Results

Positive MT staining was present in 21 cases (49%), being mild/moderate and intense in 8 and 13 cases, respectively. The pattern was cytoplasmic in 7 cases and was both cytoplasmic and nuclear in 14 cases. MT expression in a percentage of up to 25% of tumor cells (negative MT staining included) was observed in 31 cases, in a percentage 25–50% of tumor cells in 7 cases, and in a percentage of 50–75% of tumor cells in 5 cases. There was no significant correlation of MT intensity of staining to histological type, stage and patients' survival, while it was inversely correlated to higher tumor nuclear grade. MT extent of staining did not correlate with histological type, nuclear grade, and pathologic stage while a statistically significant association was found with patients' survival.

Conclusions

The inverse correlation between MT staining intensity and tumor nuclear grade in RCC suggests a role of MT in tumor differentiation process. Since extent of MT expression is inversely correlated with survival it may be possibly used as a clinical prognostic parameter.

Identification and characterization of a novel gene EOLA1 stimulating ECV304 cell proliferation

To study the changes in gene expression in endothelial cells stimulated by lipopolysaccharide (LPS) we performed subtraction hybridization on control human umbilical vein endothelial cells (HUVEC) versus HUVEC stimulated by LPS. A novel cDNA, named endothelial-overexpressed lipopolysaccharide-associated factor 1 (EOLA1), was cloned from our differentially expressed EST database of HUVEC cDNA library (GenBank Accession No. ). Computational analysis showed that EOLA1 is 1404bp long, encoding a 158aa, 17.8kDa protein, mapped to chromosome Xq27.4 with 5 exons, expressed in different human normal tissues and cancer cell lines. Using the EOLA1 cDNA as bait, we performed a yeast two-hybrid screening of a human liver cDNA library and identified metallothionein 2A (MT2A) as associated protein. Stable transfection of EOLA1 stimulates ECV304 cell proliferation. Our data suggest that the physical interaction of EOLA1 and MT2A may have an important role of cell protection in inflammation reaction.

Cellular responses to H(2)O(2) and bleomycin-induced oxidative stress in L6C5 rat myoblasts

In muscle cells, reactive oxygen species (ROS) are continually generated. It is believed that these molecules have a well-established role as physiological modulators of skeletal muscle functions, ranging from development to metabolism and from blood flow to contractile functions. Moreover, ROS may contribute to the development of muscle fatigue, inflammation, and degeneration, and may be implicated in many muscle diseases. The aim of the present study was to verify the role of short or prolonged exposure to oxidative stress, generated by different concentrations of H(2)O(2), on growth, chromosomal aberrations, and apoptosis induced in cultured L6C5 rat muscle cells used as model for myoblasts. Our results indicate that, in L6C5 cells, reactive oxygen intermediates (ROI) can activate distinct cell pathways leading to cell growth induction and development of resistant phenotype, or to chromosomal aberrations, cell cycle arrest, or cell death. The positive vs. negative effects of H(2)O(2)-altered redox potential in myoblasts are strictly related to the intensity of oxidative stress, likely depending on the types and number of cellular targets involved. Among these, DNA molecules appear to be very sensitive to breakage by H(2)O(2), although DNA damage is not directly responsible for ROI-induced apoptosis in L6C5 rat myoblasts.

Intracellular trafficking of micronutrients: from gene regulation to nutrient requirements

The intracellular distribution of micronutrients, as well as their uptake, is important for cell function. In some cases the distribution of micronutrients or their related proteins is determined by gene expression mechanisms. The 3' untranslated region (3'UTR) of metallothionein-1 mRNA determines localisation of the mRNA, and in turn intracellular trafficking of the protein product. Using transfected cells we have evidence for the trafficking of metallothionein-1 into the nucleus and for its involvement in protection from oxidative stress and DNA damage. When nutritional supply of Se is limited, selenoprotein expression is altered, but not all selenoproteins are affected equally; the available Se is prioritised for synthesis of particular selenoproteins. The prioritisation involves differences in mRNA translation and stability due to 3'UTR sequences. Potentially, genetic variation in these regulatory mechanisms may affect nutrient requirements. Genetic polymorphisms in the 3'UTR from two selenoprotein genes have been observed; one polymorphism affects selenoprotein synthesis. These examples illustrate how molecular approaches can contribute at several levels to an increased understanding of nutrient metabolism and requirements. First, they provide the tools to investigate regulatory features in genes and their products. Second, understanding these processes can provide model systems to investigate nutrient metabolism at the cellular level. Third, once key features have been identified, the availability of human genome sequence information and single nucleotide polymorphism databases present possibilities to define the extent of genetic variation in genes of nutritional relevance. Ultimately, the functionality of any variations can be defined and subgroups of the population with subtly different nutrient requirements identified.

Preconditioning with millimolar concentrations of vitamin C or N-acetylcysteine protects L6 muscle cells insulin-stimulated viability and DNA synthesis under oxidative stress

The effect of reactive oxygen/nitrogen species (ROS/RNS)(hydrogen peroxide -- H(2)O(2), superoxide anion radical O(2)*- and hydroxyl radical *OH -- the reaction products of hypoxanthine/xanthine oxidase system), nitric oxide (NO* from sodium nitroprusside -- SNP), and peroxynitrite (ONOO(-) from 3-morpholinosydnonimine -- SIN-1) on insulin mitogenic effect was studied in L6 muscle cells after one day pretreatment with/or without antioxidants. ROS/RNS inhibited insulin-induced mitogenicity (DNA synthesis). Insulin (0.1 microM), however, markedly improved mitogenicity in the muscle cells treated with increased concentrations (0.1, 0.5, 1 mM) of donors of H(2)O(2), O(2)*-, *OH, ONOO(-) and NO*. Cell viability assessed by morphological criteria was also monitored. Massive apoptosis was induced by 1 mM of donors of H(2)O(2) and ONOO(-), while NO* additionally induced necrotic cell death. Taken together, these results have shown that ROS/RNS provide a good explanation for the developing resistance to the growth promoting activity of insulin in myoblasts under conditions of oxidative or nitrosative stress. Cell viability showed that neither donor induced cell death when given below 0.5 mM. In order to confirm the deleterious effects of ROS/RNS prior to the subsequent treatment with ROS/RNS plus insulin one day pretreatment with selected antioxidants (sodium ascorbate - ASC (0.01, 0.1, 1 mM), or N-acetylcysteine - NAC (0.1, 1, 10 mM) was carried out. Surprisingly, at a low dose (micromolar) antioxidants did not abrogate and even worsened the concentration-dependent effects of ROS/RNS. In contrast, pretreatment with millimolar dose of ASC or NAC maintained an elevated mitogenicity in response to insulin irrespective of the ROS/RNS donor type used.

Expression of metallothionein in lung carcinoma: correlation with histological type and grade

AIMS

Over-expression of cellular metallothionein occurs frequently in human tumours but the underlying mechanism remains unknown. The aim of this study was to assess metallothionein expression in cases of lung carcinoma and to correlate it with histopathological parameters.

METHODS AND RESULTS

Tumour tissue samples from 89 patients with lung carcinoma were immunostained by the streptavidin-biotin-peroxidase technique, using a monoclonal antibody against both metallothionein-1 and -2 isoforms. Positive matallothionein immunostaining was prominent in 44 out of 89 (49%) and negative in 45 out of 89 (51%) cases of lung carcinoma examined. Metallothionein positivity was prominent in 32 out of 43 (74%) cases of squamous cell lung carcinoma, and in 12 out of 35 (34%) cases of adenocarcinoma, while it was negative in all 11 cases of small-cell lung carcinoma examined, presenting a statistically significant difference between the different histological types. The intensity of metallothionein staining revealed a statistically significant difference between the squamous cell and adenocarcinoma cases examined. The pattern and extent of metallothionein staining in tumour cells and the expression of metallothionein in stromal cells were not correlated with histopathological parameters (type and grade) in metallothionein-positive cases of lung carcinoma examined. No association was found between metallothionein expression and lymph node status in the examined cases of lung carcinoma.

CONCLUSIONS

Our findings indicate that expression of metallothionein was evident in squamous cell lung carcinoma and adenocarcinoma, but absent in small-cell lung carcinoma, supporting evidence for participation of this protein in the biological mechanisms underlying the carcinogenic evolution in the lung.

High-glucose-induced metallothionein expression in endothelial cells: an endothelin-mediated mechanism

Vascular endothelial cells are constantly exposed to oxidative stress and must be protected by physiological responses. In diabetes mellitus, endothelial cell permeability is impaired and may be increased by high extracellular glucose concentrations. It has been postulated that metallothionein (MT) can protect endothelial cells from oxidative stress with its increased expression by cytokines, thrombin, and endothelin (ET)-1. In this study, we demonstrate that high glucose concentration can induce MT expression in endothelial cells through a distinct ET-dependent pathway. Exposure of human umbilical vein endothelial cells (HUVEC) to increasing concentrations of glucose resulted in a rapid dose-dependent increase in MT-2 and ET-1 mRNA expression. MT expression may be further augmented with addition of ET-1. Preincubation of the cells with the specific ET(B) antagonist BQ-788 blocked MT-2 mRNA expression more effectively than the ET(A) inhibitor TBC-11251. High glucose also increased immunoreactive MT protein expression and induced translocation of MT into the perinuclear area. Perinuclear localization of MT was related to high-glucose-induced reorganization of F-actin filaments. These results demonstrate that an increase in extracellular glucose in HUVEC can lead to a rapid dose-dependent increase in MT-2 mRNA expression and to perinuclear localization of MT protein with changes to the cytoskeleton. These effects are mediated via the ET receptor-dependent pathway.

Metallothionein treatment reduces proinflammatory cytokines IL-6 and TNF-alpha and apoptotic cell death during experimental autoimmune encephalomyelitis (EAE)

Experimental autoimmune encephalomyelitis (EAE) is an animal model for the human autoimmune disease multiple sclerosis (MS). Proinflammatory cytokines such as interleukin-6 (IL-6) and tumor necrosis factor-alpha (TNF-alpha) are considered important for induction and pathogenesis of EAE/MS disease, which is characterized by significant inflammation and neuroglial damage. We have recently shown that the exogenous administration of the antioxidant protein zinc-metallothionein-II (Zn-MT-II) significantly decreased the clinical symptoms, mortality, and leukocyte infiltration of the CNS during EAE. However, it is not known how EAE progression is regulated nor how cytokine production and cell death can be reduced. We herewith demonstrate that treatment with Zn-MT-II significantly decreased the CNS expression of IL-6 and TNF-alpha during EAE. Zn-MT-II treatment could also significantly reduce apoptotic cell death of neurons and oligodendrocytes during EAE, as judged by using TUNEL and immunoreactivity for cytochrome c and caspases 1 and 3. In contrast, the number of apoptotic lymphocytes and macrophages was less affected by Zn-MT-II treatment. The Zn-MT-II-induced decrease in proinflammatory cytokines and apoptosis during EAE could contribute to the reported diminution of clinical symptoms and mortality in EAE-immunized rats receiving Zn-MT-II treatment. Our results demonstrate that MT-II reduces the CNS expression of proinflammatory cytokines and the number of apoptotic neurons during EAE in vivo and that MT-II might be a potentially useful factor for treatment of EAE/MS.

Interleukin-6 deficiency reduces the brain inflammatory response and increases oxidative stress and neurodegeneration after kainic acid-induced seizures

The role of interleukin-6 in hippocampal tissue damage after injection with kainic acid, a rigid glutamate analogue inducing epileptic seizures, has been studied by means of interleukin-6 null mice. At 35mg/kg, kainic acid induced convulsions in both control (75%) and interleukin-6 null (100%) mice, and caused a significant mortality (62%) only in the latter mice, indicating that interleukin-6 deficiency increased the susceptibility to kainic acid-induced brain damage. To compare the histopathological damage caused to the brain, control and interleukin-6 null mice were administered 8.75mg/kg kainic acid and were killed six days later. Morphological damage to the hippocampal field CA1-CA3 was seen after kainic acid treatment. Reactive astrogliosis and microgliosis were prominent in kainic acid-injected normal mice hippocampus, and clear signs of increased oxidative stress were evident. Thus, the immunoreactivity for inducible nitric oxide synthase, peroxynitrite-induced nitration of proteins and byproducts of fatty acid peroxidation were dramatically increased, as was that for metallothionein I+II, Mn-superoxide dismutase and Cu/Zn-superoxide dismutase. In accordance, a significant neuronal apoptosis was caused by kainic acid, as revealed by terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate-biotin nick end labeling and interleukin-1beta converting enzyme/Caspase-1 stainings. In kainic acid-injected interleukin-6 null mice, reactive astrogliosis and microgliosis were reduced, while morphological hippocampal damage, oxidative stress and apoptotic neuronal death were increased. Since metallothionein-I+II levels were lower, and those of inducible nitric oxide synthase higher, these concomitant changes are likely to contribute to the observed increased oxidative stress and neuronal death in the interleukin-6 null mice. The present results demonstrate that interleukin-6 deficiency increases neuronal injury and impairs the inflammatory response after kainic acid-induced seizures.

Zinc and the gene

A significant portion of cellular zinc is found in the nucleus where it appears to be critically involved in maintaining genetic stability and in the process of gene expression. With regard to gene expression zinc functions mechanistically at several levels but recent interest has focussed especially on the involvement of zinc in DNA transcription through the activity of transcription factors which contain specific zinc-finger regions which bind to DNA and, in conjunction with other families of transcription factors, control cell proliferation, differentiation and cell death. Because of the central importance of zinc in cell division and growth, considerable attention is paid to zinc as an essential trace element and much has been written concerning dietary sources of zinc and recommended dietary intakes of the metal.

Influence of metallothionein-1 localization on its function

Metallothioneins (MTs) have a major role to play in metal metabolism, and may also protect DNA against oxidative damage. MT protein has been found localized in the nucleus during S-phase. The mRNA encoding the MT-1 isoform has a perinuclear localization, and is associated with the cytoskeleton; this targeting, due to signals within the 3'-untranslated region (3'-UTR), facilitates nuclear localization of MT-1 during S-phase [Levadoux, Mahon, Beattie, Wallace and Hesketh (1999) J. Biol. Chem. 274, 34961-34966]. Using cells transfected with MT gene constructs differing in their 3'-UTRs, the role of MT protein in the nucleus has been studied. Chinese hamster ovary cells were transfected with either the full MT gene (MTMT cells) or with the MT 5'-UTR and coding region linked to the 3'-UTR of glutathione peroxidase (MTGSH cells). Cell survival following exposure to oxidative stress and chemical agents was higher in cells expressing the native MT gene than in cells where MT localization was disrupted, or in untransfected cells. Also, MTMT cells showed less DNA damage than MTGSH cells in response to either hydrogen peroxide or mutagen. After exposure to UV light or mutagen, MTMT cells showed less apoptosis than MTGSH cells, as assessed by DNA fragmentation and flow cytometry. The data indicate that the perinuclear localization of MT mRNA is important for the function of MT in a protective role against DNA damage and apoptosis induced by external stress.

Induction of metallothionein in the liver of carbon tetrachloride intoxicated rats: an immunohistochemical study

Metallothioneins (MTs), are low molecular weight proteins, mainly implicated in metal ion detoxification. In the present study, we investigated the expression of hepatic MT in a rat model of injury and regeneration, induced by carbon tetrachloride (CCl(4)) administration. A single intraperitoneal injection of 1 ml CCl(4)/kg body weight was performed in male Wistar rats, killed at different time points post-administration. The enzymatic activities of aspartate and alanine aminotransferases in serum were determined, in addition to the liver histological findings, to estimate hepatotoxicity. The rate of tritiated thymidine incorporation into hepatic DNA, the enzymatic activity of thymidine kinase in liver tissue and the assessment of the mitotic index in hepatocytes, were used as indices of regeneration. MT was detected immunohistochemically in liver tissue sections. CCl(4) administration caused severe hepatic injury, followed by regeneration. MT expression became prominent as early as 12 h after the administration of CCl(4), in the nuclei of hepatocytes, while at 24 and 36 h intense cytoplasmic staining for MT appeared in the hepatocytes in the vicinity of necrotic areas. The peak of hepatocyte proliferative capacity, occurring at 48 h post-CCl(4) administration coincides with the maximum nuclear and cytoplasmic MT expression. At further time points MT expression presented a decreasing trend. Induction of MT expression was observed in the liver after a single administration of CCl(4), being more prominent at the time of maximum hepatocellular proliferation, participating actively in the replication of hepatocytes.

Zinc metallothionein imported into liver mitochondria modulates respiration

Metallothionein (MT) localizes in the intermembrane space of liver mitochondria as well as in the cytosol and nucleus. Incubation of intact liver mitochondria with physiological, micromolar concentrations of MT leads to the import of MT into the mitochondria where it inhibits respiration. This activity is caused by the N-terminal beta-domain of MT; in this system, the isolated C-terminal alpha-domain is inactive. Free zinc inhibits respiration at concentrations commensurate with the zinc content of either MT or the isolated beta-domain, indicating that MT inhibition involves zinc delivery to mitochondria. Respiratory inhibition of uncoupled mitochondria identifies the electron transfer chain as the primary site of inhibition. The apoform of MT, thionein, is an endogenous chelating agent and activates zinc-inhibited respiration with a 1:1 stoichiometry ([zinc binding sites]/[zinc]). Carbamoylation of the lysines of MT significantly attenuates the inhibitory effect, suggesting that these residues are critical for the passage of MT through the outer mitochondrial membrane. Such an import pathway has been proposed for other proteins that also lack a mitochondrial targeting sequence, e.g., apocytochrome c, and possibly Cox17, a mitochondrial copper chaperone that is the only protein known so far to exhibit significant primary sequence homology to MT. The presence and respiratory inhibition of MT in liver, but not heart, mitochondria suggest a hitherto unknown biological modulating activity of MT in cellular respiration and energy metabolism in a tissue-specific manner.

The transport mechanism of metallothionein is different from that of classical NLS-bearing protein

A nuclear localization signal (NLS) has been detected in several nuclear proteins. Classical NLS-mediated nuclear pore targeting is performed by using the cytosolic factors, importin alpha and importin beta, whereas nuclear translocation requires the small GTPase, Ran. In the present study, we demonstrated that nuclear localization of metallothionein (MT) differs from that of classical NLS-mediated substrates. In digitonin-permeabilized BALB/c3T3 cells, biotinylated MT was localized in the nucleus in the presence of ATP and erythrocyte cytosol in the same manner as for SV40 large T NLS-conjugated allophycocyanin (APC-NLS). Under ATP-free conditions, nuclear rim-binding was observed in both transport substrates. Rim-binding of labeled MT was competitively inhibited by the addition of an excess amount of unlabeled MT. Different elution profiles were observed for the localization-promoting activities of MT in the cytosol compared to those of APC-NLS. Furthermore, nuclear localization of MT was determined to be a wheat germ agglutinin-insensitive, GTPgammaS-sensitive, and anti-Ran antibody-sensitive process. Green fluorescent protein-metallothionein (GFP-MT) fusion protein was also localized in the nucleus in the stable transformant of CHL-IU cells. These results strongly suggest that the targeting by MT of the nuclear pore is mediated by cytosolic factor(s) other than importins and that MT requires Ran for its nuclear localization.

Delay of M-phase onset by aphidicolin can retain the nuclear localization of zinc and metallothionein in 3T3-L1 fibroblasts

The transient nuclear localization of metallothionein during cell growth and differentiation may be related to the increased requirement of zinc for DNA synthesis, activation of metalloenzymes, and transcription factors. Treatment of 3T3-L1 fibroblasts with aphidicolin, an inhibitor of nuclear DNA synthesis, caused a cell-cycle block at G1/S phase and a delay in the onset of M phase. This also resulted in the accumulation of both zinc and metallothionein in the nucleus. After removal of aphidicolin, the cells rapidly reentered S phase, and during the G2/M phase of cell cycle both zinc and metallothionein began to relocate to the cytoplasm. Delaying the onset of M phase in 3T3-L1 cells could prevent the cytoplasmic relocation of metallothionein. The nuclear translocation of both zinc and metallothionein during the cell cycle can be considered as a normal process and this may be a general mechanism in response to mitogenic signals.

Signal transduction pathways, and nuclear translocation of zinc and metallothionein during differentiation of myoblasts

The changes in subcellular localization of metallothionein during differentiation were studied in two myoblast cell lines, L6 and H9C2. Addition of insulin like growth factor-I or lowering foetal bovine serum to 1% can induce differentiation of myoblasts to myotubes. Metallothionein and zinc were localized mainly in the cytoplasm in myoblasts but were translocated into the nucleus of newly formed myotubes during early differentiation. In fully differentiated myotubes, metallothionein content was decreased with a cytoplasmic localization. Addition of an inhibitor of mitogen-activated protein kinase, PD 98059, did not affect differentiation but blocked nuclear translocation of metallothionein. LY 294092, an inhibitor of PI3 kinase, and rapamycin, an inhibitor of p70S6 serine/threonine kinase, abolished insulin-like growth factor-I induced differentiation of myoblasts, retained metallothionein in the cytoplasm, and decreased metallothionein content. These results demonstrate that the cytoplasmic-nuclear translocation of metallothionein occurs during the early stage of differentiation of myoblasts to myotubes and can be blocked by inhibition of certain signal transduction pathways. The transient nuclear localization of metallothionein and zinc may be related to a high requirement for zinc for metabolic activities during the early stage of differentiation.