Metallothionein protects DNA from oxidative damage

Metallothionein protects against the cytotoxic and DNA-damaging effects of nitric oxide

In inflammatory states, nitric oxide (.NO) may be synthesized from precursor L-arginine via inducible .NO synthase (iNOS) in large amounts for prolonged periods of time. When .NO acts as an effector molecule under these conditions, it may be toxic to cells by inhibition of iron-containing enzymes or initiation of DNA single-strand breaks. In contrast to molecular targets of .NO, considerably less is known regarding mechanisms by which cells become resistant to .NO. Metallothionein (MT), the major protein thiol induced in cells exposed to cytokines and bacterial products, is capable of forming iron-dinitrosyl thiolates in vitro. Therefore, we tested the hypothesis that overexpression of MT reduces the sensitivity of NIH 3T3 cells to the .NO donor, S-nitrosoacetylpenicillamine (SNAP), and to .NO released from cells (NIH 3T3-DFG-iNOS) after infection with a retroviral vector expressing human iNOS gene. There was a 4-fold increase in MT in cells transfected with the mouse MT-1 gene (NIH 3T3/MT) compared to cells transfected with the promoter-free inverted gene (NIH 3T3/TM). NIH 3T3/MT cells were more resistant than NIH 3T3/TM cells to the cytotoxic effects of SNAP (0.1-1.0 mM) or .NO released from NIH 3T3-DFG-iNOS cells. A brief (1 h) exposure to 10 mM SNAP caused DNA single-strand breaks that were 9-fold greater in NIH 3T3/TM compared to NIH 3T3/MT cells. Electron paramagnetic resonance spectroscopy of NIH 3T3 cells revealed a greater peak at g = 2.04 (e.g., iron-dinitrosyl complex) in NIH 3T3/MT than NIH 3T3/TM cells. These data are consistent with a role for cytoplasmic MT in interacting with .NO and reducing .NO-induced cyto- and nuclear toxicity.

Cellular adaptation to chronic cadmium exposure: intracellular localization of metallothionein protein in human trophoblast cells (JAr)

Trophoblast cells are the first embryonic cells that modulate the transfer of a variety of compounds (oxygen, amino acids, xenobiotics, metals) from the maternal to the fetal circulation in the human placenta. Human placental exposure to the toxic metal, cadmium (Cd) results in a decrease in the production of human chorionic gonadotropin (hCG), a decrease in the maternal to fetal transport of zinc (Zn), and trophoblastic necrosis. Thus, the ability of trophoblast cells to adapt to exposure to the toxic metal Cd has been considered crucial. In this study, the expression and intracellular localization of metallothionein (MT), a small molecular weight, metal binding protein, was examined in trophoblast cells (JAr) grown in normal media and in cells exposed chronically (6 months) to 2 microM CdCl2. Conventional and confocal fluorescence microscopy were used to examine the intracellular localization of MT protein in control cells and cells grown chronically in Cd. In unexposed trophoblast cells, MT protein was primarily perinuclear with low level, punctate expression in the cytosol. Following both chronic and 24 hour exposure to Cd, MT protein levels were increased (at least 3-fold in both chronic and acute exposures) and the protein was now concentrated inside the nucleus with a lacy, cytoskeletal pattern of expression in the cytosol. To determine if the nuclear accumulation of MT protein was dependent on new protein synthesis, control cells were exposed to CdCl2 (2 microM) and cycloheximide (2 micrograms/. ml) for 24 hours.(ABSTRACT TRUNCATED AT 250 WORDS)

The prognostic significance of immunohistochemically detectable metallothionein in primary breast carcinomas

Metallothionein (MT) is a low-molecular-weight protein with a high affinity for group II metal ions, especially zinc and copper. MT serves as an intracellular reservoir of these ions, but may also be involved in the detoxification of certain toxic metal ions such as cadmium. In addition, high MT contents might protect tumour cells from alkylating agents and irradiation. The aim of this study was to evaluate the prognostic significance of immunohistochemically detected MT overexpression in patients with primary breast carcinoma: 478 patients with primary breast carcinoma diagnosed during the period 1980-1985 were included. Formalin-fixed and paraffin-embedded tissue was used. Immunoreactivity for MT was found to be independent of the length of formalin fixation if the sections were microwave processed before incubation with the primary antibody. Patients were divided into two groups: those with MT overexpression (more than 10% positive tumour cells) and those with low expression (less than 10% positive tumour cells). MT overexpression was correlated with postmenopausal status, large tumour size, presence of lymph node metastases, high number of mitoses, severe nuclear pleomorphism, high histological grade (poor differentiation), and absence of PgR. In univariate analysis of survival data, MT overexpression was a predictor of poor overall survival in the entire group of patients. In multivariate analysis, MT overexpression failed to be of prognostic significance, whereas classical histopathological parameters such as tumour size, histological grade, and PgR were of independent prognostic significance.

Covalent sequestration of melphalan by metallothionein and selective alkylation of cysteines

Rabbit liver metallothionein-2 is shown to form covalent bonds with the anticancer agent melphalan, in support of the hypothesis that covalent sequestration by metallothionein constitutes one mechanism for the cross-resistance acquired by cancer patients to therapeutic alkylating agents. Among 20 cysteines in the 2-domain protein, 89% of the first alkylation reaction occurs with 2 that cochelate a zinc cation in the carboxy domain. Computer-supported docking studies indicate a favorable binding site for melphalan near these cysteine sulfhydryl groups. Although folded metallothionein-2 is resistant to trypsin cleavage, alkylation by 1 mol of melphalan allows cleavage by trypsin between the two globular domains.

Enhanced sensitivity to oxidative stress in cultured embryonic cells from transgenic mice deficient in metallothionein I and II genes

Embryonic cells from transgenic mice with targeted disruption of metallothionein I and II genes expressed no detectable metallothionein either constitutively or after treatment with cadmium, in contrast to cultured cells that were wild type or heterozygous for the loss of the metallothionein genes. Metallothionein null cells were most sensitive to the cytotoxic effects of cadmium, the membrane permeant oxidant tert-butylhydroperoxide, and the redox cycling toxin paraquat. No marked differences were seen among the wild type, heterozygous, or metallothionein null cells in glutathione levels or in the activity of CuZn-superoxide dismutase, glutathione peroxidase, or catalase. Nevertheless, metallothionein null cells were more sensitive to tert-butylhydroperoxide-induced oxidation as ascertained by confocal microscopic imaging of dichlorofluoroscein fluorescence. These results indicate basal metallothionein levels can function to regulate intracellular redox status in mammalian cells.

Cyanobacterial metallothioneins: biochemistry and molecular genetics

Metallothioneins have been extensively studied in many different eukaryotes where they sequester, and hence detoxify, excess amounts of certain metal ions. However, the precise functions of many of these molecules are not fully understood. This article reviews literature concerning their namesakes in prokaryotes.

Characterisation of an mRNA encoding a metallothionein-like protein that accumulates during ethylene-promoted abscission of Sambucus nigra L. leaflets

A cDNA encoding a metallothionein-like protein has been isolated from a cDNA library from the abscission zones of ethylene-treated Sambucus nigra leaflets. The precise function of this group of proteins in plants has yet to be confirmed but in animals there is convincing evidence that they bind heavy metals. Several of these proteins have recently been characterised from plants and it has been demonstrated that heavy metals have no stimulatory effect on their expression. In this paper we describe the isolation and characterisation of a metallothionein-like mRNA identified as a consequence of differentially screening a cDNA library for messages up-regulated during abscission. The accumulation of the mRNA occurred in the abscission zone tissue within 18 h of exposure to ethylene while, in contrast, no expression was detectable in adjacent non-abscission-zone tissue. The transcript size of the message was approximately 0.6 kb. Northern analysis revealed that the cDNA insert (JET12) did not hybridise to mRNA from either green or senescing leaflets but a signal was detectable with mRNA extracted from senescent tissue. The size of this hybridising transcript was approximately 0.5 kb. The predicted metallothionein-like protein encoded by JET12 was cysteine-rich (18.4%) and had a molecular weight of approximately 7.5 kDa. Southern analysis of S. nigra genomic DNA showed that the mRNA was encoded by a small gene family. The protein exhibited greatest homology to other metallothioneins belonging to the Type 2 family including those from Mimulus (62%) and Arabidopsis (57%). This homology was greatest around the cysteine-rich amino and carboxy termini.(ABSTRACT TRUNCATED AT 250 WORDS)

Immunohistochemical detection of metallothionein in primary breast carcinomas and their axillary lymph node metastases

Metallothioneins (MTs) are low-molecular-weight proteins with specific binding for group II metal ions. MTs are involved in the detoxification of metals, but can also play a role in protection of the cell against certain anticancer agents and from damage of irradiation. High expression of MTs in primary breast carcinomas has been found to be associated with poorer prognosis. Expression of MT (MT) was examined immunohistochemically in 160 breast carcinomas and their concomitant lymph node metastases. The immunoreactivity appeared to be independent of the length of fixation when the section was microwaved before incubation with the primary antibody, a monoclonal antibody E-9. The findings were correlated with various histopathological factors, disease-free survival and over-all survival. Patients were divided into two groups, those with MT over-expression (above 10% of positive tumour cells), and those with low MT expression (below 10% positive). MT over-expression was found to be correlated with postmenopausal status and inversely with positive progesterone receptor status (PgR). MT over-expression showed statistically significant correlation with poor over-all survival. No differences in survival were seen between pre- and postmenopausal patients. PgR was in univariate analysis a poor prognostic parameter. In one fourth of the patients, the lymph node metastases showed increased MT expression compared with the primary tumour. These patients had a poorer, but not statistically significant different survival. MT expression was not correlated to chemo- or radiation therapy.(ABSTRACT TRUNCATED AT 250 WORDS)

Induction of metallothionein and stomatin by interleukin-6 and glucocorticoids in a human amniotic cell line

Interleukin 6 (IL-6) is an important mediator of various kinds of inflammatory and immune responses. The human amniotic cell line UAC has an increased number of IL-6 receptors after treatment by glucocorticoids. To find a possible activity of IL-6 on these cells, a cDNA library of IL-6- and dexamethasone-treated cells was screened with cDNA probes from both induced and non-induced cells. Two cDNAs showed a differential hybridization signal. The first one corresponds to metallothionein, a group of small cysteine-rich proteins thought to participate in the metabolism and storage of zinc and to protect cells against oxidative damage. A second cDNA corresponds to the recently cloned cDNA of band 7 integral membrane protein also called stomatin. In hereditary stomatocytosis, absence of this protein in erythrocyte membranes is associated with high Na+ and low K+ intracellular concentrations [Stewart, G. W., Hepworth-Jones, B. E., Keen, J. N., Dash, B. C. J., Argent, A. C. & Casimir, C. M. (1992) Blood 79, 1593-1601]. In UAC cells both metallothionein and stomatin are induced by dexamethasone and IL-6 in a more than additive manner. Western blot analysis shows that stomatin protein is induced in a similar way as its mRNA. IL-6 and dexamethasone induce a state of resistance against hydrogen peroxide toxicity in UAC cells. Metallothionein induction might be partly responsible for this cytoprotection against oxidative stress.

Isolation of cDNA clones for genes that are expressed during leaf senescence in Brassica napus. Identification of a gene encoding a senescence-specific metallothionein-like protein

cDNA clones representing genes that are expressed during leaf senescence in Brassica napus were identified by differential screening of a cDNA library made from RNA isolated from leaves at different stages of senescence. The expression of these genes at different stages of leaf development was examined by northern blot analysis, and several different patterns of expression were observed. One of the clones, LSC54, represented a gene that is expressed at high levels during leaf senescence. Analysis of this gene indicated strong expression in flowers as well as in senescing leaves. DNA sequence analysis of the LSC54 cDNA indicated a similarity between the deduced amino acid sequence and several metallothionein-like proteins previously identified in plants.

Protective effects of metallothionein on streptozotocin-induced diabetes in rats

The protective roles of metallothionein (MT) in streptozotocin (STZ)-induced pancreatic damage were investigated using zinc (Zn) as the inducer of MT synthesis in rats. Diabetes was produced in a group of Sprague Dawley rats by a single injection of STZ. In another group of rats, to induce the synthesis of MT, Zn was injected subcutaneously about 12 h before injection of STZ. Rats were sacrificed at about 30 h, 3 and 6 weeks after administration of STZ. The plasma glucose levels in STZ and Zn+STZ group increased significantly in 30 h, 3 and 6 weeks compared with the control group, but the level of glucose in Zn+STZ group was lower than that in the STZ group. Both Zn and MT levels increased in pancreas, liver, and kidney of rats injected with Zn or Zn+STZ. The induction of MT synthesis in pancreas of Zn-injected rats was also demonstrated by Western Blot analysis. Lipid peroxidation induced by STZ was inhibited significantly by pretreatment with Zn. Injection of STZ decreased the activities of superoxide dismutase (SOD) in plasma, liver and pancreas, but injection of Zn had no effect on the activities of SOD. The results support that pretreatment with Zn can partially prevent the development of diabetes induced by STZ injection and it may be related to the effects of MT as a scavenger for the oxygen free radicals.

The effects of 6-hydroxydopamine and oxidative stress on the level of brain metallothionein

Oxidative stress, resulting either from excess generation or reduced scavenging of free radicals, has been proposed to play a role in damaging striatal neurons in Parkinson's disease. Since metallothionein is able to regulate the intracellular redox potential, we have undertaken a group of experiments to see whether or not 6-hydroxydopamine, which generates free radicals and is toxic to dopaminergic neurons, could alter the level of zinc and metallothionein. 6-Hydroxydopamine (8 micrograms in 4 microliters 0.02% ascorbic acid) reduced the level of zinc and metallothionein in the striatum but not other brain regions tested. Dopamine plus selegiline increased the synthesis of metallothionein in Chang cells as judged by enhanced incorporation of [35S]cysteine into metallothionein. The effect of dopamine was selective, in that dopamine could not stimulate the synthesis of metallothionein in neuroblastoma IMR-32 cells, which are devoid of dopaminergic receptors. The effect of dopamine in stimulating the synthesis of metallothionein was similar to that of zinc, known to generate the synthesis of metallothionein, and to that of H2O2 and FeS04, known to generate free radicals. The results of these experiments provide additional evidence that zinc or zinc metallothionein are altered in conditions where oxidative stress has taken place.

Plant metallothioneins

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