Development of a radioimmunoassay for rat liver metallothionein-I and its application to the analysis of rat plasma and kidneys

Renal effects of metallothionein induction by zinc in vitro and in vivo

BACKGROUND

Metallothionein (MTT) is an endogenous antioxidant that can be induced by both zinc (Zn) and ischemia. In kidneys, increased MTT expression exerts a putative protective role in diabetes and hypoxia. Our goal was to further investigate the behavior of MTT under the influence of Zn and hypoxia in vitro and in vivo.

METHODS

MTT expression was measured in vitro in cell cultures of proximal tubular cells (LCC-PK1) by immune-histochemistry and real-time PCR after incubation with increasing concentrations of Zn under hypoxic and non-hypoxic conditions. In addition, in vivo studies were carried out in 54 patients to study MTT induction through Zn. This is a sub-study of a prospective, randomized, double-blind trial on prevention of contrast-media-induced nephropathy using Placebo, Zn and N-Acetylcysteine. Blood samples were obtained before and after 2 days p.o. treatment with or without Zn (60 mg). ELISA-based MTT level measurements were done to evaluate the effects of Zn administration. For in vivo analysis, we considered the ratio of MTT to baseline MTT (MTT₁/MTT₀) and the ratio of eGFR (eGFR₁/eGFR₀), correspondingly.

RESULTS

In vitro quantitative immuno-histochemical analysis (IHC) and real-time PCR showed that at increasing levels of Zn (5, 10, and 15 μg/ml) led to a progressive increase of MTTs: Median (IQR) expression of IHC also increased progressively from 0.10 (0.09-0.12), 0.15 (0.12-0.18), 0.25 (0.25-0.27), 0.59 (0.48-0.70) (p < 0.0001). Median (IQR) expression of PCR: 0.59 (0.51-1.72), 1.62 (1.38-4.70), 3.58 (3.06-10.42) and 10.81 (9.24-31.47) (p < 0.0001). In contrast, hypoxia did not change MTT-levels in vitro (p > 0.05). In vivo no significant differences (p = 0.96) occurred in MTT-levels after 2 days of Zn administration compared with no Zn intake. Nevertheless, there was a significant correlation between MTT (MTT₁/MTT₀) and eGFR (eGFR₁/eGFR₀) in case of Zn administration (rho = -0.49; 95%-CI: -0.78 to -0.03; p = 0.04).

CONCLUSIONS

We found that Zn did induce MTTs in vitro, whereas hypoxia had no significant impact. In contrast, no significant increase of MTTs was detected after in vivo administration of Zn. However, there was a significant negative correlation between MTT and eGFR in vivo in case of Zn administration, this could indicate a protective role of MTTs in a setting of reduced kidney function, which is possibly influenced by Zn.

TRIAL REGISTRATION

ClinicalTrials.gov Identifier: NCT00399256 . Retrospectively registered 11/13/2006.

Copper binding components of blood plasma and organs, and their responses to influx of large doses of (65)Cu, in the mouse

To establish for the first time how mice might differ from rats and humans in terms of copper transport, excretion, and copper binding proteins, plasma and organ cytosols from adult female C57CL6 mice were fractionated and analyzed by directly coupled size exclusion HPLC-ICP-MS, before and after i.p. injection of large doses of (65)Cu. Plasma from untreated mice had different proportions of Cu associated with transcuprein/macroglobulin, ceruloplasmin and albumin than in humans and rats, and two previously undetected copper peaks (Mr 700 k and 15 k) were observed. Cytosols had Cu peaks seen previously in rat liver (Mr > 1,000 k, 45 k and 11 k) plus one of 110 kDa. (65)Cu (141 microg) administered over 14 h, initially loaded plasma albumin and mainly entered liver and kidney (especially 28 kDa and 11 kDa components). Components of other organs were less (but still significantly) enriched. (63)Cu/(65)Cu ratios returned almost to normal by 14 days, indicating a robust system for excreting excess copper. We conclude that there are significant differences but also strong similarities in copper metabolism between mice, rats and humans; that the liver is able to buffer enormous changes in copper status; and that a large number of mammalian copper proteins remain to be identified.

A sensitive time-resolved fluorescent immunoassay for metallothionein protein

Metallothioneins (MTs) are a family of low molecular weight metal-binding proteins induced by a broad range of stress conditions, including exposure to transition metal ions. Biochemical and immunological methods to measure MT protein levels in tissues and cultured cells have been reported, but accuracy and sensitivity is impeded by high background levels, low specificity of currently available reagents, and relatively laborious and time-consuming multistep procedures. To address these difficulties, a protocol has been developed to measure MT protein levels using a competitive solid phase assay based on dissociation enhanced lanthanide fluoroimmuno (DELFIA) detection of anti-MT monoclonal antibody bound to solid phase MT. This assay allows time-resolved detection of antibody binding, based on binding and exchange of different lanthanide chelates followed by fluorescent detection, designed to reduce background fluorescence and increase sensitivity. The method allows measurement of low MT levels that are undetectable using current radioimmunoassay (RIA) and enzyme-linked immunosorbent assay (ELISA) protocols, and yields reproducible results with low background over a wide range of MT concentrations. Improved sensitivity of MT protein detection is of value in toxicological measurement of stress responses and assessment of MT expression and function.

Metallothionein gene expression and secretion in white adipose tissue

White adipose tissue (WAT) has been examined to determine whether the gene encoding metallothionein (MT), a low-molecular-weight stress response protein, is expressed in the tissue and whether MT may be a secretory product of adipocytes. The MT-1 gene was expressed in epididymal WAT, with MT-1 mRNA levels being similar in lean and obese (ob/ob) mice. MT-1 mRNA was found in each of the main adipose tissue sites (epididymal, perirenal, omental, subcutaneous), and there was no major difference between depots. Separation of adipocytes from the stromal-vascular fraction of WAT indicated that the MT gene (MT-1 and MT-2) was expressed in adipocytes themselves. Treatment of mice with zinc had no effect on MT-1 mRNA levels in WAT, despite strong induction of MT-1 expression in the liver. MT-1 gene expression in WAT was also unaltered by fasting or norepinephrine. However, administration of a beta(3)-adrenoceptor agonist, BRL-35153A, led to a significant increase in MT-1 mRNA. On differentiation of fibroblastic preadipocytes to adipocytes in primary culture, MT was detected in the medium, suggesting that the protein may be secreted from WAT. It is concluded that WAT may be a significant site of MT production; within adipocytes, MT could play an antioxidant role in protecting fatty acids from damage.

Separation and quantitation of metallothionein isoforms from liver of untreated rats by ion-exchange high-performance liquid chromatography and atomic absorption spectrometry

A sensitive method for determination of metallothionein (MT) isoform levels in rat liver by ion-exchange high-performance liquid chromatography and atomic absorption spectrometry was developed. Critical steps in sample preparation, like MT extraction, MT saturation with Cd and protein separation, were optimized. This method is capable of measuring levels of 2.0 microg/g liver for metallothionein-1 (MT-1) and 1.3 microg/g liver for metallothionein-2 (MT-2), respectively, with a high recovery of 103% on average. The method described, thus, proved suitable for analyzing metallothionein isoform concentrations even in untreated animals. The ratio of MT-1 to MT-2 was found to be 1:1 on average. MT decomposition during storage was very high in whole livers, but could be reduced by about 80% when extracted liver samples were used.

Identification and expression analysis of leptin-regulated immediate early response and late target genes

Using PC12 cells as an in vitro model system, we have identified a series of transcripts induced through activation of the leptin receptor. On the basis of kinetic studies, two distinct gene sets could be discerned: signal transducer and activator of transciption-3 (STAT-3), suppressor of cytokine signalling-3 (SOCS-3), MT-II (metallothionein-II), the serine/threonine kinase fibroblast-growth-factor-inducible kinase (Fnk) and modulator recognition factor (MRF-1), which are immediate early response genes, and pancreatitis-associated protein I (PAP I), squalene epoxidase, uridine diphosphate glucuronosyltransferase and annexin VIII, which are late induced target genes. At late time points a strong co-stimulation with beta-nerve growth factor or with the adenylate cyclase activator forskolin was observed. To assess the validity of the PC12-cell model system, we examined the effect of leptin administration on the gene transcription of STAT-3, MT-II, Fnk and PAP I in vivo. Leptin treatment of leptin-deficient ob/ob mice increased the STAT-3, SOCS-3, MT-II and Fnk mRNA, and MT-I protein levels in liver, whereas, in jejunum, expression of PAP I mRNA was down-regulated. Furthermore, administration of leptin to starved wild-type mice enhanced the expression of MT-II and Fnk mRNA in liver, but decreased MT-II and PAP I mRNA expression in jejunum. These findings may help to explain the obese phenotype observed in some colonies of MT-I- and MT-II-null mice and/or the observation that leptin protects against tumour-necrosis-factor toxicity in vivo.

Metallothionein is expressed in adipocytes of brown fat and is induced by catecholamines and zinc

Metallothionein (MT) is thought to have an antioxidant function and is strongly expressed during activation of thermogenesis and increased oxidative stress in brown adipose tissue (BAT). Localization and regulation of MT expression in BAT was therefore investigated in rats and mice. Immunohistochemical analysis of BAT from rats exposed to 4 degrees C for 24 h showed that MT and uncoupling protein 1 (UCP1) were coexpressed in differentiated adipocytes, and both cytoplasmic and nuclear localization of MT was observed. Cold induction of MT-1 expression in BAT was also observed in mice. Administration of norepinephrine to rats and isoproterenol to mice stimulated MT and UCP1 expression in BAT, implying a sympathetically mediated pathway for MT induction. In mice, zinc, and particularly dexamethasone, induced MT-2 expression in BAT and liver. Surprisingly, zinc also induced UCP1 in BAT, suggesting that elevated zinc may induce thermogenesis. We conclude that expression of MT in mature brown adipocytes upon beta-adrenoceptor activation is consistent with a role in protecting against physiological oxidative stress or in facilitating the mobilization or utilization of energy reserves.

Nuclear import of metallothionein requires its mRNA to be associated with the perinuclear cytoskeleton

The influence of mRNA localization on metallothionein-1 protein distribution was studied by immunocytochemistry. We used Chinese hamster ovary cells that had been transfected with either a native metallothionein-1 gene construct or metallothionein-1 5'-untranslated region and coding sequences linked to the 3'-untranslated region from glutathione peroxidase. The change in the 3'-untranslated region caused the delocalization of the mRNA with a loss of the perinuclear localization and association with the cytoskeleton. Clones were selected which expressed similar levels of metallothionein-1 protein, as assessed by radioimmunoassay. The results showed that loss of metallothionein-1 mRNA localization was associated with a loss of metallothionein-1 protein localization, most notably with a lack of metallothionein-1 protein in the nucleus of synchronized cells which were beginning to synthesize DNA. This indicates that the association of metallothionein-1 mRNA with the cytoskeleton around the nucleus is essential for efficient shuttling of the protein into the nucleus during the G(1) to S phase transition. This is the first demonstration of a physiological role for perinuclear mRNA localization and we propose that such localization may be important for a wide range of nuclear proteins, including those that shuttle between nucleus and cytoplasm in a cell cycle dependent manner.

Biological significance of non-acetylated metallothionein

The biological significance of non-acetylated metallothionein (MT) was investigated from the viewpoint of N(alpha)-acetylation after induction of MT synthesis by metallic and non-metallic inducers, by partial hepatectomy and under physiological conditions. N(alpha)-Acetylated and non-acetylated forms of MT-2 in liver supernatants and plasma were detected by the tandem size-exclusion and anion-exchange HPLC columns with in-line detection by mass spectrometry. The non-acetylated isoform of MT-2 (MT-2') was present at a comparable level to the N(alpha)-acetylated form of MT-2 (MT-2) at an early stage after induction by not only zinc but also cadmium, and by partial hepatectomy in the livers of rats. Plasma MT-2 in neonatal rats was similar to liver MT-2 in the composition of N(alpha)-acetylated and non-acetylated forms, suggesting that there are no differences in the roles of N(alpha)-acetylation of MT in the extracellular trafficking of MT. The column switching HPLC method with in-line detection by inductively coupled argon plasma mass spectrometry (ICP-MS) was shown to be a sensitive and powerful method to detect MT proteins at not only isoform level but also at acetylated and non-acetylated form levels.

Role of metallothionein on Ag accumulation in hepatic and renal cytosol after Ag injection to rats

To examine the role of metallothionein on Ag accumulation in liver or kidney of rat after Ag injection, the relative Ag-binding capacity of Ag-induced metallothionein in hepatic or renal cytosol of rat after Ag injection was determined. The greater part of Ag increment in hepatic cytosol was attributable to a low molecular weight protein, while the main part of Ag increment in renal cytosol was ascribed to high molecular weight proteins. The low molecular weight, metal-binding protein was identified as metallothionein using ELISA. The maximal levels of hepatic and renal metallothionein mRNA induced by Ag occurred at 7 hr after Ag injection. There was a close relationship between Ag contents in the hepatic or renal cytosol and metallothionein after Ag injection. In dose-response and time-course studies, approximately 60-70% of the Ag increments in hepatic cytosol and approximately 30% of the Ag increments in renal cytosol were bound to metallothionein. These results suggest that the role of metallothionein on Ag accumulation in the liver after Ag injection is different from that of metallothionein on Ag accumulation in the kidney.

Obesity and hyperleptinemia in metallothionein (-I and -II) null mice

Metallothionein (MT) has several putative roles in metal detoxification, in Zn and Cu homeostasis, in scavenging free radicals, and in the acute phase response. Mice of mixed 129/Ola and C57BL/6J background with targeted disruption of MT-I and MT-II genes are more sensitive to toxic metals and oxidative stress. We noted that these animals were larger than most strains of mice, and we systematically studied aspects of their physiology and biochemistry relating to energy metabolism. During the first 2 weeks after weaning, the growth rates of MT-null and C57BL/6J mice were similar, but the transgenic mice became significantly heavier at age 5-6 weeks. At age 14 weeks, the body weight and food intake of MT-null mice was 16 and 30% higher, respectively, compared with C57BL/6J mice. Most 22- to 39-week-old male MT-null mice were obese, as shown by increased fat accretion, elevated obese (ob) gene expression, and high plasma leptin levels, similar to those recorded in Zucker fatty (fa/fa) rats. Seven-week-old MT-null mice also had significantly higher levels of plasma leptin and elevated expression of ob, lipoprotein lipase, and CCAAT enhancer binding protein alpha genes as compared with age-matched C57BL/6J mice. These observations indicate that abnormal accretion of body fat and adipocyte maturation is initiated at 5-7 weeks of age, possibly coincident with sexual maturation. Targeted disruption of MT-I and MT-II genes seems to induce moderate obesity, providing a new obese animal model. A link between MT and the regulation of energy balance is implied.

On-line solid-phase extraction-capillary electrophoresis for enhanced detection sensitivity and selectivity: application to the analysis of metallothionein isoforms in sheep fetal liver

A simple and rapid capillary electrophoresis (CE) method for analysis of metallothionein (MT) isoforms is described. A modified two-step solvent extraction procedure was used in combination with CE and an on-line solid-phase preconcentration device for sensitive and reproducible detection of MT isoforms in sheep fetal liver. Preparation of twenty samples was practicable within a working day with subsequent automated overnight analysis by solid-phase extraction (SPE)-CE. A commercially available divinylbenzene-based reversed-phase resin was found to be most suitable for the SPE device because it is resistant to extremes of pH and can be adequately regenerated between analyses. Each SPE device was readily constructed from commonly available materials and was used for the reproducible separation of over 100 biological samples before replacement. Precision of analysis within or between sample batches was < 10% and usually < 5% while detection limits were at least 28 ng/ml for standards and 272 ng/ml for biological samples. This would indicate a detection limit of about 0.5 microgram/g wet weight of tissue. Recovery of MT from tissue cytosols by solvent extraction was measured using radiolabeled MT and was found to be just over 50% increasing to almost 70% by addition of NaCl to the homogenisation buffer. The combined solvent extraction and SPE-CE methodology was applied to the analysis of MT in sheep fetal liver and the results compared favorably with those obtained by high-resolution chromatography. MT-1 levels were 2 to 4-times higher than those of MT-2 and both isoforms decreased from day 89 to day 136 of gestation. These results were compared with MT levels in fetal liver from sheep embryos that had been perturbed by temporary transfer to an advanced uterine environment. Hepatic MT levels at day 136 of gestation were 3 to 8-times higher than in normal fetal liver and significant differences were observed with both isoforms.

Evidence for differences in the post-transcriptional regulation of rat metallothionein isoforms

The expression of metallothionein (MT)-1 and -2 mRNAs in rat liver following administration of Cd or Cu was investigated using specific oligonucleotides. The specificity was confirmed using a competitive prehybridization assay. Cd injection caused a biphasic induction of both isoform mRNAs, whereas Cu induced a sustained, monophasic response. Analysis of polyribosomal RNA showed that, after both Cd and Cu treatments, the recruitment of MT-1 mRNA into polyribosomes paralleled the increase in transcription, but the increase of polyribosomal MT-2 mRNA was less than that of total MT-2 mRNA. This indicates that not all the MT-2 mRNA induced was translated, suggesting that there is translational control of MT-2 mRNA expression, but not of MT-1 mRNA. This hypothesis was supported by the observation that, after Cu treatment, the induction of MT-1 protein was induced to the same extent as MT-1 mRNA, whereas the total MT protein (MT-1 + MT-2) was increased far less (7-fold) than MT-2 mRNA (30-fold).

Metallothionein synthesis induced by interferon alpha/beta in mice of various zinc status

We studied the ability of interferon alpha/beta (IFN) to induce metallothionein (MT) synthesis in mice. Male mice were injected intraperitoneally with mouse IFN (5 x 10(5) IU/mouse). Plasma Zn levels were reduced at 4 hr after injection, reached a minimum value at 6 hr, and then returned to the control level at 8 hr. Hepatic MT concentrations began to increase at 4 hr and reached maximum values at 6 hr. Induction of MT gene expression and protein synthesis was confirmed by Northern blot analysis and radioimmunoassay, respectively. The induction of MT synthesis in the liver by IFN was dose-dependent. The data suggest that induction of MT-mRNA and the protein in the liver by IFN occurs rapidly but is rather transient. Furthermore, MT synthesis was not induced by IFN in the liver of mice given a Zn-deficient diet, whereas IFN induced increases in the activity of 2',5'-oligoadenylate synthetase in the spleen were unaffected by Zn status. Thus, induction of hepatic MT synthesis by IFN is influenced by Zn status.

Synergistic induction of metallothionein synthesis by interleukin-6, dexamethasone and zinc in the rat

We investigated the reciprocal effects of interleukin-6 (IL-6), glucocorticoid and zinc (Zn) on metallothionein (MT) synthesis in rats. MT synthesis in the liver, which is a key responsible organ in acute phase responses, was induced by IL-6 or dexamethasone (Dex), and in an additive manner by a combination of IL-6 and Dex 18 h after injection. MT synthesis in the lung and heart was evaluated by immunoassay using a specific antibody to MT-1, because of its low concentration in these tissues. Heart concentrations of MT-1 were significantly increased by IL-6, and were further increased by the combination of IL-6 and Dex, although Dex by itself had no effect. This suggests a synergistic effect of IL-6 and Dex on MT-I synthesis in the heart. A similar synergism was observed in the lung. To study the effect of Zn on the induction of MT and acute phase proteins, Zn, IL-6 and Dex were administered in various concentrations. The increase in liver MT induced by the combination of IL-6 and Dex with Zn (130 micrograms MT/g of liver) was greater than the sum of the increases induced by (IL-6 + Zn) and by (Dex + Zn) (103 micrograms MT/g), suggesting a synergistic increase. The data indicate that the maximal increase in the induction of MT by a combination of IL-6 and Dex depends on an adequate liver Zn content. Thus, the in vivo synergistic induction of acute phase proteins by IL-6, glucocorticoid and Zn may be required for the maximal and rapid response, not only in liver but also in other tissues including heart and lung. This suggests that the synergistic reaction may be important for an enhancement of the radical scavenging ability of tissues in acute phase responses.

Induction of metallothionein synthesis by glutathione depletion after trans- and cis-stilbene oxide administration in rats

To investigate the relationship between glutathione (GSH) depletion and metallothionein (MT) synthesis, the effects of substrates and an inhibitor of GSH S-transferases on concentrations of hepatic GSH, zinc (Zn) and MT were studied in rats. Trans-stilbene oxide (TSO) is an inducer of drug metabolizing enzymes and also a substrate of GSH S-transferase, whereby it covalently reacts with and depletes GSH. The hepatic GSH level was decreased to 25% of the control 2 h after injection of TSO, and returned to the control level by 24 h. TSO significantly increased hepatic concentrations of Zn and MT in a dose-dependent manner. Two isoforms of MT (MT-I and MT-II) were increased by TSO; MT-II was the dominant form. Pretreatment with buthionine sulfoximine (BSO), an inhibitor of GSH synthesis, enhanced MT synthesis itself as well as that induced by TSO and cis-stilbene oxide (CSO). On the contrary, infection into rats of perfluorodecanoic acid (PFDA), an inhibitor of GSH S-transferase, resulted in a decrease in basal levels of Zn, and prevented the increase in MT synthesis by TSO and CSO. These results suggest that the decrease of GSH concentration in the liver which causes oxidative stress conditions may be related to MT induction.

Comparison of different techniques for the analysis of metallothionein isoforms by capillary electrophoresis

We have investigated free-solution capillary electrophoresis (FSCE) and micellar electrokinetic capillary chromatography (MECC) separations of metallothionein (MT) isoforms conducted in uncoated and surface-modified fused-silica capillaries. At alkaline pH, FSCE rapidly resolves isoforms belonging to the MT-1 and MT-2 charge classes. At acidic pH, additional resolution of MT isoforms is achieved. The use of high-ionic-strength (0.5 M) phosphate buffers can result in high peak efficiencies and increased resolution for some MT isoforms. Interior capillary surface coatings such as polyamine and linear polyacrylamide polymers permit separation of MT isoforms with enhanced resolution through their effects on electroosmotic flow (EOF) and protein-wall interactions. Improvements in MT isoform resolution can also be achieved by MECC using 100 mM borate buffer pH 8.4 containing 75 mM SDS. Deproteinization of tissue cytosol samples with acetonitrile (60-80%) or perchloric acid (7%) produces extracts that can be subjected to direct analysis of MT by FSCE or MECC. We conclude that optimal separation of MT isoforms by capillary electrophoresis (CE) can be achieved with the appropriate combination of different capillaries, buffers and sample preparation techniques.

Immunochemical detection of metallothionein in brain

Polyclonal and monoclonal antibodies have been used to determine the levels of metallothionein (MT) isoforms in the brain, its regions, and its cellular elements. MT is found abundantly in the astrocytes but not in the oligodendroglia or microglia. The induction of astrocytic metallothionein by cytokines suggests an important role for this protein in providing long-term protection against oxidative damage in tissue injury and inflammation.

Effect of zinc, copper and glucocorticoids on metallothionein levels of cultured neurons and astrocytes from rat brain

The knowledge of brain metallothionein (MT) regulation and especially of MT presence in specific cell types is scarce. Therefore, the effect of several well-known MT inducers, measured by radioimmunoassays using antibodies that cross-react with MT-I and MT-II or specific for MT-I and which do not cross-react with human growth inhibitory factor (GIF or MT-III), has been studied in primary cultures of neurons or astrocytes obtained from rat cerebrum. MT-I levels in glial cells were about ten times higher than those in neuronal cells (538 +/- 194 vs. 49 +/- 16 pg MT-I/micrograms protein, mean +/- S.D. from three separate cell preparations). Increasing the concentration of Zn in the bovine serum albumin (BSA)-containing culture medium up to 50 microM significantly increased MT-I levels by up to 3.5-fold in neurons and 2.5-fold in astrocytes. In contrast, Cu up to 50 microM increased MT-I levels in a saturable manner in both neurons (up to 5-fold) and astrocytes (up to 1.5-fold), the maximum effect occurring at 5 microM Cu. In general, the combination of Zn and Cu further increased MT-I levels. The effect of the metals on MT-I appeared to reflect metal uptake, since MT-I induction was less marked when the BSA concentration in the medium was increased from 2 to 10 mg/ml. Dexamethasone increased MT-I levels in both neurons and astrocytes in vitro in a concentration-dependent manner. Endotoxin, IL-1 and IL-6 did not have a significant effect on glial MT levels at the concentrations studied. The administration of dexamethasone to rats increased MT-I levels in non-frontal cortex, cerebellum, pons+medulla, midbrain and hippocampus, but not in hypothalamus, frontal cortex and striatum. Endotoxin increased liver but not brain MT-I levels. Immunocytochemical studies in adult rat brain preparations with a polyclonal antibody that cross-reacts with MT-I and MT-II indicated that immunostaining was always nuclear in glial cells, whereas in neurons it was nuclear in the cerebral cortex, hippocampus and the granular layer of the cerebellum, and nuclear plus cytoplasmic in Purkinje cells in the cerebellum, hypothalamic nuclei and gigantocellular reticular nucleus in the brain stem. Meninges, choroidal plexus, ependymal and endothelial cells were also MT-immunoreactive.

Regulation of metallothionein-I+II levels in specific brain areas and liver in the rat: role of catecholamines

The role of the catecholamines noradrenaline, adrenaline and dopamine on metallothionein (MT) levels of specific areas of the rat brain has been studied. MT-I or MT-I + II levels were measured by radioimmunoassay using specific antibodies that cross-react only slightly with human MT-III (growth inhibitory factor, GIF). The inhibition of tyrosine hydroxylase with alpha-methyl-p-tyrosine (MPT), which depletes brain dopamine, noradrenaline, and adrenaline, increased MT levels in all brain areas studied (frontal cortex, cortex, medulla oblongata plus pons, midbrain, striatum, hippocampus, hypothalamus, and cerebellum) when considering the results of two separate experiments. The alpha- and beta-receptor blockers, phentolamine, and propranolol, alone or together, did not increase brain MT levels in any area of the brain, suggesting that the effect of MPT in vivo is related to inhibition of the synthesis of dopamine rather than of noradrenaline and adrenaline. Dopamine, noradrenaline, and serotonin increased MT-I levels in primary cultures of neurons, whereas decreased them in astrocyte-enriched primary cultures. Since MT-I levels are about ten times higher in astrocytes than in neurons, the increased brain MT levels induced by MPT may reflect the suppression of the normal inhibitory effect of dopamine on astrocyte MT levels. The increase in MT concentrations induced in most parts of the brain by immobilization stress was not prevented by MPT, phentolamine, or propranolol, suggesting that it was not mediated by the central monoamines.

Regulation of metallothionein concentrations in rat brain: effect of glucocorticoids, zinc, copper, and endotoxin

The effects of known inducers of liver metallothionein (MT) synthesis on MT concentrations in the rat brain have been determined using antibodies that are specific for MT I and II and do not cross-react with MT III. There were substantial differences in the MT concentrations in different areas of the brain. Dexamethasone increased MT levels after 24 h in the frontal cortex, cortex, medulla oblongata plus pons, midbrain, striatum, hippocampus, and cerebellum but not in the hypothalamus. Corticosterone produced similar results except in the hippocampus. Long-lasting adrenocorticotropic hormone increased MT concentrations after 12 h in midbrain and striatum but not in the liver. Adrenalectomy decreased MT concentrations after 6 days in the medulla oblongata plus pons, striatum, hippocampus, and hypothalamus but increased concentrations in the liver and kidneys; these effects were reversed by corticosterone. The role of glucocorticoids in the regulation of MT levels therefore differs between tissues and within specific areas of the brain. Injection of zinc or copper intracerebroventricularly and the use of a zinc-deficient diet increased and decreased MT levels, respectively, in some but not all brain areas. Endotoxin increased liver MT but not brain MT I levels after 8 h.

The longitudinal distribution of cadmium, zinc, copper, iron, and metallothionein in the small-intestinal mucosa of rats after administration of cadmium chloride

Different routes of Cd intake may influence the intestinal distribution of Cd, metallothionein (MT), and trace metals differently. Therefore, we compared the effects of parenteral and enteral administration of Cd on the distribution of trace metals and MT along the small intestine. In a first experiment three groups of rats were employed: a control, one receiving CdCl2 within the drinking water, and another receiving sc injections of CdCl2. In a second experiment, rats were fed three different diets with either 0, 0.3, or 1 mmol CdCl2/kg for one and two weeks to study the time- and dose-dependent effects of orally administered Cd. Metal concentrations (Cd, Zn, Cu, Fe) were measured by atomic emission spectrometry and MT was determined by radioimmunoassay. Intestinal MT levels did not show proximodistal gradients in controls or after sc administration of Cd, but orally administered Cd increased mucosal MT levels longitudinally from the duodenum to the ileum. Cd levels paralleled those of MT. Compared with the metal concentrations in the controls, sc administration of Cd did not change intestinal Zn, Cu, and Fe levels. Oral administration of Cd, however, increased Cu and decreased Fe levels in the intestinal mucosa significantly. The second experiment revealed that only high dietary concentrations of Cd increase intestinal Cd and MT levels longitudinally toward the distal parts, whereas at lower dietary concentration the longitudinal distribution was reversed. This shows that different routes and doses of Cd intake lead to a different trace metal and MT distribution and emphasizes the role of dietary Cd in the local induction of small-intestinal MT.

Separation of metallothionein isoforms by micellar electrokinetic capillary chromatography

Current techniques for the separation and quantification of metallothionein isoforms have limited value for routine analysis. Isoforms having a similar charge have been separated successfully using reversed-phase HPLC but this technique suffers from a slow sample turnover time. The use of the surfactant sodium dodecyl sulphate for the separation of metallothionein isoforms by micellar electrokinetic capillary chromatography (MECC) is described. The charge-different isoforms MT-1 and MT-2 from rats, rabbits and sheep were separated within 9-12 min. In addition, a varying degree of heterogeneity was observed in purified samples of human MT-1, rat MT-2, rabbit MT-1, rabbit MT-2 and sheep MT-1. The behaviour of chicken MT was different from that of any other species. The separation of sheep liver extracts indicated the potential of MECC as the basis for a quantitative assay for both charge-different and charge-similar metallothionein isoforms.

Factors influencing the separation of metallothioneins by capillary zone electrophoresis

The factors influencing the separation of metallothioneins (MTs) by capillary zone electrophoresis (CZE) were studied using untreated fused-silica capillaries. A comparison was made between the various buffers of 20 mM Na2B4O7, 10 mM Na2B4O7-10 mM Na2HPO4 and 20 mM Na2B4O7-10 mM Tris for the separation of MTs in terms of migration time and UV-absorbance under the various applied voltages. The migration times for MT-I and MT-II decreased with increase in the applied voltage and column temperature. However, no significant change in UV absorbance was observed. When Na2B4O7-Tris buffer was chosen for the separation of MTs, the migration time, UV absorbance, theoretical plates and resolutions also increased with increasing buffer concentrations. The effect of buffer pH on migration time was relatively complicated. Under the optimal operating conditions the standard MTs were well separated. Determination of MTs in the real samples failed due to the adsorption of other proteins onto the capillary walls.

Differential induction of metallothionein synthesis by interleukin-6 and tumor necrosis factor-alpha in rat tissues

Metallothionein (MT) synthesis induced by the inflammatory cytokines, interleukin-6 (IL-6) and tumor necrosis factor-alpha (TNF), was studied in vivo. Administration of recombinant human IL-6 or TNF to rats caused the acute phase responses including rapid decreases in plasma zinc (Zn), and increases in plasma copper (Cu) and ceruloplasmin. Hepatic concentration of MT-I, one of MT isoforms, began to increase within 3 h after the injection of IL-6 or TNF. In IL-6-treated rats, MT-I concentration in liver reached a maximum level at 12 h and decreased with a transient rebound, whereas, in TNF-treated rats, a high level of MT-I lasted for about 48 h. MT-II, the other MT isoform, was induced more than MT-I in liver by both cytokines. MT-I was also induced in lung and heart by TNF, but little by IL-6. The data suggest that IL-6 may be responsible for MT synthesis in liver, whereas TNF may be responsible not only in liver but also in lung and heart. Furthermore plasma concentration of MT did not always reflect the enhanced concentration of MT by TNF and IL-6 in liver, suggesting involvement of many factors influencing plasma MT levels. The interrelation between IL-6 and TNF for MT synthesis has also been discussed.

Development of a competitive double antibody radioimmunoassay for rat metallothionein

A competitive double antibody radioimmunoassay (RIA) for rat metallothionein (MT) has been developed that has a detection limit of 100 pg and a range of 100 to 100000 pg. The antibody was raised in rabbits against rat MT-2 but it crossreacts equally with MT-1 and MT-2. However, when the assay is done in the presence of 2-mercaptoethanol the antibody is more specific for MT-2. Zn- and Cd- saturated MTs have similar responses in the assay. Addition of Cu(II) to Zn-MT (more than 6 mol Cu/mol MT) in non-reducing conditions modifies the response of the antibody, probably because of Cu(II) oxidation and later MT polymerization. Standard curves developed in the presence of cytosols from brain cortex, hypothalamus or liver did not differ from the standard curve, indicating the absence of interfering substances in the assay. Furthermore, serial dilutions of those cytosols paralleled the response of the standard curve, indicating that the response of the antibody was specific. For comparison, MT levels in some brain areas measured with the present RIA were compared with those measured with an established RIA. In addition, the expected effect of dexamethasone and stress on liver MT levels was clearly identified by this RIA. The results suggest that the present RIA can be used for quantitation of metallothionein.

Characterization of metallothionein isoforms. Comparison of capillary zone electrophoresis with reversed-phase high-performance liquid chromatography

The purpose of this study was to compare and contrast the separation of metallothionein (MT) isoforms by reversed-phase high-performance liquid chromatography (RP-HPLC) with capillary zone electrophoresis (CZE). RP-HPLC was performed on a Vydac C8 column eluted with a linear acetonitrile gradient. CZE was performed in a 57 cm x 75 microns I.D. fused-silica tube at an operating voltage of 30 kV. Phosphate buffer (10 mM) at pH 2.5, 7.0 and 11.0 was used for both separations. CZE at pH 2.5 resolved three distinct peaks of rabbit liver MT which were incompletely resolved at pH 7.0 or 11.0. RP-HPLC at pH 2.5 gave two peaks and the resolution was not as good as with CZE at the same pH. At pH 7.0 or 11.0, RP-HPLC of rabbit liver MT gave a single predominant peak of unresolved MT-1 and MT-2. Purified rabbit liver MT-1 and MT-2 were used to verify the identity of these peaks. In contrast, MT from horse kidney demonstrated three predominant peaks which were best resolved by CZE at pH 11.0, whereas RP-HPLC resolved only two peaks at pH 11.0 and 7.0. At pH 2.5, RP-HPLC of horse kidney MT gave three peaks, though two of the peaks were incompletely separated. We conclude that pH has a considerable impact on the resolution of MT isoforms by CZE and RP-HPLC and that it is possible to exploit changes in pH to optimize the separation of isoforms for a particular species of MT. When samples of human and sheep liver MT-1, both of which exhibit microheterogeneity, were subjected to CZE, a single predominant peak was observed at each pH value. RP-HPLC of human liver MT-1 at pH 2.5 yielded two peaks that were incompletely resolved. Purified chick liver MT and rat liver MT-1 and MT-2 gave a single predominant peak at all pH values on CZE. In contrast, pig liver MT-1 and MT-2 each exhibited multiple peaks when subjected to CZE, the number of which depended on the pH used to separate the MT. In conclusion, CZE, with its orthogonal selectivity, and RP-HPLC make an excellent combination for the separation and characterization of MT isoforms. Because CZE is rapid (run times typically < 10 min) and requires little sample (< 100 nl), MT samples can readily be analyzed by CZE in conjunction with RP-HPLC or other techniques in order to maximize the information obtained about the individual isoforms.

Oxygen free radicals and metallothionein

It is generally accepted that the principal roles of metallothionein lie in the detoxification of heavy metals and regulation of the metabolism of essential trace metals. However, there is increasing evidence that it can act as a free radical scavenger. This article reviews the evidence supporting such a physiological role and describes induction of metallothionein synthesis by oxidative stress, possible mediators for this induction, and the radical scavenging capability of metallothionein in tissues and cells. The relationship between metallothionein and other antioxidant defense systems and the medical implications of the free radical scavenging properties of metallothionein are also discussed.

Separation of metallothionein isoforms by capillary zone electrophoresis

The potential of capillary zone electrophoresis (CZE) for the analysis of metallothionein (MT) isoforms was investigated. CZE was performed using two different systems, (1) a laboratory-constructed instrument with an ISCO UV detector and (2) a Waters Quanta 4000 system. Capillaries were of 75 microns I.D. x ca. 1 m in length and loading times were up to 40 s by gravity or 4 s by electrokinetic migration at 30 kV. Samples were dissolved in 10 mM Tris-HCl buffer, pH 9.1, and electrophoresis was performed at 30 kV using a 50 mM Tris-HCl, pH 9.1 running buffer. Detection was by UV absorbance at 185 or 214 nm. Purified and semipurified MT samples were analysed for qualitative assessment of purity, relative isoform abundance and separation characteristics of MT from different species. As progress towards the development of a quantitative assay, the linearity of calibration curves and simple methods of sample preparation for analysis by CZE were investigated. Complete separation of a mixture of the two major MT isoforms was achieved in less than 5 min and the technique was found to be very useful for qualitative analysis of MT. Using a rabbit liver MT standard (500 micrograms/ml-1), a linear relationship was found between the gravity load time and the integrated peak area. Standard calibration curves were also linear and the detection limit for both CZE instruments under our separation conditions was 1-10 micrograms MT ml-1. The successful use of two solvent extraction procedures for tissue samples demonstrated the potential of CZE for routine quantitative analysis of MT.

A gel filtration high-performance liquid chromatographic method for determination of hepatic and renal metallothionein of rat and in comparison with the cadmium-saturation method

Metallothionein (MT) is a low-mol-wt protein. The essential trace elements copper and zinc and the potentially toxic elements, such as cadmium, can induce the synthesis of and bind to MT. The major functions of MT are related to metal metabolism. This paper reported and evaluated a new method for determination of hepatic and renal MT of rat by using high performance liquid chromatography (HPLC) with a Superdex 75 gel filtration column. The tissue was homogenized and centrifuged, then the supernatant was pretreated simply by cadmium saturation and heating before HPLC determination. The MT was completely and clearly separated from other proteins in the rat tissues in a short time, and was quantitated directly as a function of UV absorbance at 250 nm. The recovery both for hepatic and renal MT of rat were exceed 90%. The coefficient of variation was 1.3% for hepatic MT of rat and 1.7% for renal MT of rat. The detection limit was 0.265 microgram for hepatic MT and 0.095 microgram for renal MT of rat. The present method was compared with the traditional cadmium-saturation method for determination of hepatic and renal MT of rat. A good correlation was found in these two methods.

Exogenous metallothionein and renal toxicity of cadmium and mercury in rats

The relative tissue distribution and toxicity of cadmium (Cd) and mercury (Hg) in the liver and kidneys of rats when the metals are administered as either inorganic salts or complexed with MT were studied. Male Sprague-Dawley rats were injected (i.v.) with Cd or Hg inorganic salt of chloride or in a complex of MT at a dose of 0.3 mg/kg body weight. The concentration of MT and metals in plasma and urine was monitored for 7 days, at the end of which the rats were killed. Injection of both HgCl2 and Hg-MT induced the synthesis of MT only in the kidney but not in the liver, whereas CdCl2 and Cd-MT injections induced MT synthesis in both liver and kidney, respectively. Plasma MT levels increased 3 days after CdCl2 but not after HgCl2 injection, suggesting that hepatic MT may be an important source of plasma MT under our experimental conditions. Renal toxicity was observed morphologically and by an increase in blood urea nitrogen, plasma creatinine, proteinuria in rats injected with Cd-MT and both forms of Hg. Urinary MT excretion was significantly elevated in Cd-MT injected rats compared with those injected with CdCl2. However, HgCl2 and Hg-MT injected rats showed no significant difference in urinary MT excretion. The magnitude in the renal accumulation of Hg is similar after the administration of Hg-MT or HgCl2, but our findings suggest that the site of epithelial injury may be different. Injury effects of Hg-MT localized mainly in the terminal portions of the proximal convoluted tubule and the initial portions of the proximal straight tubule whereas inorganic Hg caused necrosis in pars recta segments of the proximal tubule.

Quantification of metallothionein isoforms using an enzyme-linked immunosorbent assay (ELISA) with two specific antisera

The specificity of three populations of antibodies to the two metallothionein (MT) isoforms has been characterized using a competitive enzyme-linked immunosorbent assay (ELISA). A polyclonal sheep anti-MT-1 antibody (PAb 1) showed higher affinity to MT-1 than to MT-2 with I50 (concentrations required to achieve 50% inhibition) being 18 and 480 ng for MT-1 and MT-2, respectively. Conversely, a polyclonal rabbit anti-MT-2 antibody (PAb 2) showed higher affinity to MT-2 than to MT-1 (I50 being 250 and 15 ng for MT-1 and MT-2, respectively). A third antibody (PAb 3), isolated by removal of anti-MT-1 antibodies in PAB 2 by immunoaffinity purification, showed no cross-reactivity with MT-1. This result suggests that there are two populations of antibodies which cross-react specifically with the MT isoforms. Rat tissue MT concentrations were quantified by ELISA using the three different antibodies and silver-saturation method. The sum of MT isoform concentrations (ELISA estimation using antibodies PAb 1 and PAb 3) were not significantly different (p < 0.05) from the total MT concentrations obtained by silver-saturation method. Therefore, the ELISA can be used to estimate the relative quantities of the two MT isoforms in rat tissues. The predominant isoform in rat tissues was MT-2 and the proportion of MT-1 increased with induced synthesis of MT by metal injection.

Evaluation of differential pulse polarography for the quantification of metallothionein--a comparison with RIA

Two methods to quantify metallothionein (MT), differential pulse polarography (DPP) and radioimmunoassay (RIA), were compared for MT analysis of liver from Zn- and Cd-injected perch (Perca fluviatilis). Nine perch were intraperitoneally injected, twice a week during 2 weeks with ZnSO4 and CdCl2 to yield a total dose of 30 mg Zn and 3 mg Cd per kilogram body weight. Two samples, 100 and 200 mg from each liver, were homogenized separately and further prepared for DPP, RIA, and atomic absorption spectroscopy. MT values obtained by DPP were in good agreement with the MT values determined by RIA (r = 0.92). The relationship between the MT values analyzed with the two methods is described by the formula MTRIA = MTDPP x 0.99-0.048. Analysis of MT was not affected by sample size. MT values from individual liver samples plotted against the Cd and Zn content of the corresponding samples provided a high correlation. The correlation coefficient was 0.86 for MT values obtained by DPP and 0.92 for MT measured by RIA. It is concluded that DPP is a reliable method for analyzing MT in liver.

Heterogeneity of antibodies to metallothionein isomers and development of a simple enzyme-linked immunosorbent assay

A competitive enzyme-linked immunosorbent assay (ELISA) for the measurement of metallothionein (MT) in tissues and body fluids has been developed. The ELISA employs the IgG fraction of a rabbit antiserum to rat liver Cd-MT-2 polymer, a biotinylated secondary antibody, and peroxidase conjugated avidin. With a 1:4000 dilution of the immunoglobulins, typical standard curves (logit-log regression) provide a linear range of 0.1-100 ng for MT-2 and 10-1000 ng for MT-1. Fifty percent inhibition is accomplished with 15 ng and 250 ng for MT-2 and MT-1, respectively. Rat liver MT-1 and MT-2 containing different metals (Ag, Cu, and Zn) inhibited the antibodies as effectively as CdMT. However, the antibodies exhibited greater affinity for both Apo-MT isoforms. Previously reported discrepancies between results obtained by metal binding assays (e.g., Ag-hem binding) and radioimmunoassay for MT levels in tissues have been largely resolved. By addition of 1% Tween 20 to samples, the ELISA routinely estimated the total MT in samples of rat, mouse, and human liver and kidney at 88% of the value obtained by the silver-hem binding assay. Specific antibodies to MT-2 were purified from our antiserum by affinity purification using CH-Sepharose 4B coupled with rat liver MT-1. Estimation of MT in samples using purified MT-2 antibodies provided slightly lower values (72%) for MT in tissues as compared to the Ag-hem method. The predominant form of MT in tissues of control animals was found to be MT-2.(ABSTRACT TRUNCATED AT 250 WORDS)

Metallothionein-I induction by stress in specific brain areas

The distribution of metallothionein-I (MT) in several areas of the brain and its induction by immobilization stress has been studied in the rat. MT content was highest in hippocampus and midbrain and lowest in frontal cortex and pons plus medulla oblongata. Immobilization stress for 18 hours (which was accompanied by food and water deprivation) significantly increased MT levels in the frontal cortex, pons plus medulla oblongata and hypothalamus, but not in midbrain and hippocampus. The effect of stress on MT levels was specific as food and water deprivation along had no significant effect on MT levels in any of the brain areas studied. The effect of stress on MT levels was independent of changes in cytosolic Zn content; this was generally unaffected by stress or food and water deprivation but decreased in pons plus medulla oblongata from stressed rats. The results suggest that MT is induced more significantly in the brain areas that are usually involved in the response of animals to stress.

Determination and purification of metallothioneins by high performance liquid chromatography

A rapid, reproducible and sensitive high performance liquid chromatography (HPLC) method for the determination and purification of metallothionein-I (MT-I) and metallothionein-II (MT-II) in mouse and rabbit livers has been developed. Methallothioneins (MTs) were separated by an HPLC anion exchange column, eluted through a linear gradient of Tris buffer and the peak containing MTs was determined by atomic absorption spectrophotometry. Furthermore, the content of MT-I or MT-II was calculated by protein peak area in a short time (about 20 min). The sample to be tested was homogenized, centrifuged and saturated by cadmium. MT-I and MT-II were eluted at 15.9 and 19.3 min, respectively. The following mouse liver cytosols were tested: controls, Cd-injected samples and 60Co-irradiated samples. A detection limit of 5 micrograms/g liver was established for this method. We have analysed more than 100 biological samples and obtained satisfactory results.

Radioimmunoassay of metallothionein in rabbit, rat, mouse, Chinese hamster, and human cells

We describe a competitive, solid-phase radioimmunoassay for metallothionein, which employs a rabbit antiserum directed against rat MT-2 to detect metallothionein (MT) from several different species (rabbit, mouse, rat, Chinese hamster, and human). The lower limit of detection of the assay for rat MT-2 was 0.7 ng; for rabbit MT-2 it was 2 ng. The method is capable of measuring both isoforms of MT (MT-1 and MT-2). When MT levels in rat and mouse tissues were estimated with this RIA and the silver-saturation method, both assays gave the same pattern of MT induction in control and cadmium-treated animals. Both methods measured high levels of MT in human liver samples. Chinese hamster ovary cells induced with cadmium also showed elevated MT expression. The detectability of MTs from a broad range of species is facilitated by the use of solid-phase MT, which has an avidity for the antiserum similar to that of the MT in the tested sample.

Plasma 65Zn kinetics in the rat

The amount of 65Zn in the plasma of rats after intravenous injection was found to decline following closely two-compartment kinetics over a period of 90 min. Comparative analysis of the amount of 65Zn present in the two kinetic pools at various time-intervals post-injection with the actual physiological location of the 65Zn, revealed that the initial pool (Qa) is primarily the blood plasma, while the second pool (Qb) is primarily within the liver. The plasma Zn concentration and Qa were both found to fall reproducibly during Zn depletion, whereas Qa and Qb increased following injection of Escherichia coli endotoxin in contrast to the decline in plasma Zn concentration. Further investigation of the nature of Qb indicates that it represents in part a metabolic pool within the liver which varies substantially in response to Zn status in a manner similar to metallothionein.