Detection of circulating metallothionein in rats injected with zinc or cadmium

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.

Redox and trace metal regulation of ion channels in the pain pathway

Given the clinical significance of pain disorders and the relative ineffectiveness of current therapeutics, it is important to identify alternative means of modulating nociception. The most obvious pharmacological targets are the ion channels that facilitate nervous transmission from pain sensors in the periphery to the processing regions within the brain and spinal cord. In order to design effective pharmacological tools for this purpose, however, it is first necessary to understand how these channels are regulated. A growing area of research involves the investigation of the role that trace metals and endogenous redox agents play in modulating the activity of a diverse group of ion channels within the pain pathway. In the present review, the most recent literature concerning trace metal and redox regulation of T-type calcium channels, NMDA (N-methyl-D-aspartate) receptors, GABA_A (γ-aminobutyric acid A) receptors and TRP (transient receptor potential) channels are described to gain a comprehensive understanding of the current state of the field as well as to provide a basis for future thought and experimentation.

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.

Proteomic analyzes of copper metabolism in an in vitro model of Wilson disease using surface enhanced laser desorption/ionization-time of flight-mass spectrometry

In Wilson disease, mutations in the ATP7B-gene lead to hepatic accumulation of copper that becomes toxic when the hepatic binding capacity is exceeded, leading to oxidative stress and acute liver failure. Several proteins are probably involved in dealing with the excess copper and oxidative stress. As a first step towards biomarker discovery and analyzes of copper metabolism in Wilson disease patients we characterized copper-induced changes in protein expression in cell lysates and culture media from an in vitro copper-overload model using surface enhanced laser desorption/ionization (SELDI) proteomics technology. HepG2 cells were cultured for 48 h with a physiological (0.5 microM) or a pathological (100 microM) copper concentration. Samples were applied to weak cation exchange (WCX) proteinchip arrays and chips were analyzed by time of flight (TOF)-mass spectrometry. Copper-coated IMAC chips were used to detect copper-binding proteins in cell lysate of copper depleted cells using buffers with increasing imidazole concentrations. Data from the 2 to 50 kDa range indicate that high extra-cellular copper substantially altered both intra-cellular protein expression as well as the composition of the secretome. In the lysate 15 proteins were found up-regulated, while 6 proteins were down-regulated. In culture media 21 proteins were increased while 4 proteins were decreased in abundance. Copper-coated protein chips revealed the presence of 18 high-affinity copper-binding proteins. Further identification is necessary to determine the exact cellular roles of the discovered proteins.

Metallothionein 1+2 protect the CNS during neuroglial degeneration induced by 6-aminonicotinamide

6-Aminonicotinamide (6-AN) is a niacin antagonist, which leads to degeneration of gray matter astrocytes. Metallothionein 1+2 (MT-1+2) are neuroprotective factors in the central nervous system (CNS), and to determine the roles for MT after 6-AN, we have examined transgenic mice overexpressing MT-1 (TgMTI* mice) after an i.p. injection with 6-AN. In control mice injected with 6-AN, astrocytes in specific gray matter areas of the brainstem showed degeneration. Reactive astrocytes surrounded the degenerated areas, which were heavily infiltrated by macrophages and T lymphocytes. MT-1+2 expression was significantly decreased in the damaged brainstem areas, but it increased in reactive astrocytes surrounding these areas and also in infiltrating macrophages. The levels of oxidative stress, as determined by immunoreactivity for inducible nitric-oxide synthase (iNOS), malondialdehyde (MDA), and nitrotyrosine (NITT), and the number of terminal deoxynucleotidyl transferase [TdT]-mediated deoxyuridine triphosphate [dUTP]-digoxigenin nick end labeling-positive (TUNEL+), caspase-3+ apoptotic cells were significantly increased in the brainstem of normal mice after 6-AN. In the TgMTI* mice, the 6-AN-induced tissue damage was decreased in comparison to control mice, and they showed significantly reduced numbers of recruited macrophages and T lymphocytes, and a drastic reduction of oxidative stress and apoptotic cell death. In addition, the accompanying reactive astrogliosis was increased in the transgenic mice. To further study the potential protective role of MT, we administered intraperitoneally Zn-MT-2 to 6-AN-injected normal mice and found essentially the same results as those obtained in TgMTI* mice. Thus, we hereby report that endogenous MT-1 overexpression and exogenous MT-2 treatment have significant neuroprotective roles during CNS pathological conditions.

Metallothionein-1+2 protect the CNS after a focal brain injury

We have evaluated the physiological relevance of metallothionein-1+2 (MT-1+2) in the CNS following damage caused by a focal cryolesion onto the cortex. In comparison to normal mice, transgenic mice overexpressing the MT-1 isoform (TgMTI* mice) showed a significant decrease of the number of activated microglia/macrophage and of CD3+ T lymphocytes in the area surrounding the lesion, while astrocytosis was increased. The TgMTI* mice showed a diminished peripheral macrophage but not CD3 T cell response to the cryolesion. This altered inflammatory response produced a decreased expression of the proinflammatory cytokines IL-1beta, IL-6, and TNF-alpha and an increased expression of the growth factors bFGF, TGFbeta1, and VEGF in the TgMTI* mice relative to control mice, which might be related to the increased angiogenesis and regeneration of the parenchyma of the former mice. The overexpression of MT-1 dramatically reduced the cryolesion-induced oxidative stress and neuronal apoptosis. Remarkably, these effects were also obtained by the intraperitoneal administration of MT-2 to both normal and MT-1+2 knock-out mice. These results fully support the notion that MT-1+2 are essential in the CNS for coping with focal brain injury and suggest a potential therapeutic use of these proteins.

Cadmium-binding to transferrin in the plasma of the common carp Cyprinus carpio

In this study, it was investigated by autoradiography with radioactive cadmium after Western blotting of two-dimensional electrophoresis gels, to which proteins cadmium is mainly bound in plasma of common carp Cyprinus carpio. The obtained results demonstrate that in carp plasma, cadmium is primarily bound to two high molecular weight proteins. Relative small amounts are bound to a protein with M(r) approximately 60000. The other metal-binding protein, with M(r) approximately 70000 and pI approximately 6.7 was identified as transferrin. The conditional equilibrium constants for the binding of cadmium ions to the two metal-binding sites of this protein were calculated as logK(1)=5.40+/-0.12 and logK(2)=4.66+/-0.21, which are comparable to those of human transferrin under the same experimental conditions. Transport of cadmium in plasma of carp was found to be different from that of brown trout Salmo trutta and man, where cadmium is mainly bound to albumin and transferrin. The prominent binding of cadmium to transferrin can be explained by the absence or at least the very low concentrations in which albumin is present in carp plasma.

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.

Repeated administration of alpha-hederin results in alterations in maternal zinc status and adverse developmental outcome in the rat

The administration of alpha-hederin, an inducer of metallothionein, results in a secondary zinc deficiency that may be an important maternally mediated mechanism of developmental toxicity. Previous studies have shown adverse developmental outcome with a single administration of alpha-hederin to rats on gestation day (GD) 8 or 11. The objective of this study was to determine whether dosing of alpha-hederin throughout organogenesis would result in a sustained elevation of maternal hepatic metallothionein and subsequent developmental abnormalities. Rats were administered dosage levels of 0 (vehicle only), 20, or 30 mumol/kg from GD 6-15. Maternal hepatic metallothionein levels were 10-fold higher on GD 16 in the treatment groups than the controls. Consequently, liver zinc concentrations increased 60% and 54%, whereas plasma levels decreased 23% and 33% in the 20 and 30 mumol/kg treatment groups, respectively. At GD 20, mean fetal weights of the treatment litters were 11% less than control litters. The administration of alpha-hederin resulted in a threefold increase in the number of offspring that exhibited developmental abnormalities, including visceral and skeletal malformations. Following an oral pulse of 65Zn subsequent to treatment with 0 or 20 mumol/kg of alpha-hederin, the distribution of 65Zn to the liver of treated dams was twice that of controls, whereas the radiolabeled zinc apportioned to the decidua and uterus decreased by 44%. Furthermore, the 65Zn detected in the embryos from treated dams was 70% lower than in embryos from control dams. In conclusion, low doses of a metallothionein inducer administered to the dam from GD 6-15 resulted in a sustained elevation of hepatic metallothionein and a subsequent redistribution of zinc leading to a decrease in the zinc available to the embryo and ultimately to adverse development of the offspring. Repeated dosing throughout organogenesis, as required in regulated safety assessment testing, increased the severity of the effects previously observed with single large dosages of the toxicant administered during midgestation.

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.

Apparent deficiency of metallothionein in the liver of the Antarctic icefish Chionodraco hamatus. Identification and isolation of a zinc-containing protein unlike metallothionein

1. A zinc-binding protein has been isolated and purified from the liver of the icefish Chionodraco hamatus. 2. The icefish Zn-protein has characteristics distinct from those of metallothionein. 3. The amino acid composition shows a low content of cysteine and a high content of glutamate and aspartate. 4. No metallothionein has been detected in the extracts from icefish liver.

Dietary calcium restriction enhances cadmium-induced metallothionein synthesis in rats

Experiments were conducted with adult male rats to investigate the effects of dietary calcium (Ca) restriction upon intake and tissue distribution of cadmium (Cd), and Cd-metallothionein (Mt) synthesis. Four groups of animals were fed either a low-Ca, semisynthetic diet (0.1% Ca) or the same diet supplemented with 0.8% Ca (normal diet). The caloric intake was similar in all groups. Two groups (low-Ca and normal diet) were used as controls, and two groups (low-Ca and normal diet) received 100 mg/l Cd (as CdCl2) in drinking water. Cd levels in liver, kidney, spleen and red cells were measured in all animals after 8 weeks of treatment. Concomitantly, Mt levels in plasma, liver and kidney were evaluated by radioimmunoassay. Ca deficiency entailed marked and significant increases in accumulation of Cd and synthesis of Mt in all assayed tissues. It is concluded that dietary Ca restriction, independent of caloric intake, enhances Cd intestinal absorption and tissue accumulation, which is followed by increased tissue Mt synthesis.

Response of Xenopus laevis to cadmium administration

1. Levels of copper, zinc and cadmium have been determined in the plasma, liver and kidneys of Xenopus laevis. 2. Administration of cadmium by subcutaneous injection induces metallothionein synthesis in the liver of Xenopus laevis. Cadmium is found to accumulate in both liver and kidneys following injection. 3. Cadmium uptake, following injection, is biphasic, suggesting a saturable first-phase uptake mechanism, followed by a linear second phase. 4. Zinc is displaced from the liver by administration of cadmium in a dose-dependent manner.

The growth inhibitory factor that is deficient in the Alzheimer's disease brain is a 68 amino acid metallothionein-like protein

We have purified and characterized the growth inhibitory factor (GIF) that is abundant in the normal human brain, but greatly reduced in the Alzheimer's disease (AD) brain. GIF inhibited survival and neurite formation of cortical neurons in vitro. Purified GIF is a 68 amino acid small protein, and its amino acid sequence is 70% identical to that of human metallothionein II with a 1 amino acid insert and a unique 6 amino acid insert in the NH2-terminal and the COOH-terminal portions, respectively. The antibodies to the unique sequence of GIF revealed a distinct subset of astrocytes in the gray matter that appears to be closely associated with neuronal perikarya and dendrites. In the AD cortex, the number of GIF-positive astrocytes was drastically reduced, suggesting that GIF is down-regulated in the subset of astrocytes during AD.

Histochemical and immunocytochemical evaluation of copper and metallothionein in the liver and kidney of copper-loaded rats

Histochemical methods do not always show a good correlation with analytical measurement of copper content and consequently immunoreactive staining techniques for metallothionein (MT) have recently been employed for the differential diagnosis of copper-associated diseases. This study compares histochemical with immunocytochemical methods for the assessment of copper status. Male rats were fed a high copper (1 g/kg) diet for 16 weeks and killed sequentially during this period. The livers and kidneys were analysed for copper and zinc (atomic absorption spectrophotometry), and sections were stained with rubeanic acid and rhodanine for copper and for immunoreactive MT using the DNP localization system. Immunoreactive stains for MT corresponded better with copper content than histochemical stains and were more sensitive, albeit less selective, indicators of copper accumulation. Moreover, major differences in intracellular staining were apparent between the two methods, attributed to differences in copper binding and microcompartmentalization of metal.

Modulation of metal toxicity by metallothionein

Toxic properties of several metals may be modified, since they are bound to metallothionein in vivo. Such modulation is particularly well known for cadmium (Cd), whose acute effects are prevented by metallothionein induction, whereas chronic effects on the kidney are partly explained on the basis of transport of cadmium-metallothionein (CdMt) into the kidney. Although intracellular Mt synthesis is induced by Cd, offering partial protection, nephrotoxicity may occur at times when such protection is insufficient. Perturbations in renal calcium metabolism may be an important basis for membrane dysfunction leading to proteinuria.

Metallothionein immunoreactivity in the liver and kidney of copper injected rats

Adult male rats were injected intraperitoneally with copper sulphate in physiological saline (3 mg copper/kg body wt). Metallothionein-I (MT-I) levels in liver, kidney, plasma and red blood cells were measured by radioimmunoassay (RIA), prior to the injection and after 7, 16 and 24 h. Copper and zinc levels in liver and kidneys were also monitored. Concentrations of MT-I in liver and kidneys showed a rapid increase and remained elevated for 24 h. Copper concentrations also increased in both tissues but zinc levels remained constant in the kidney and rose only slightly in the liver. MT-I levels increased gradually in plasma but decreased in the red blood cells. Immunochemistry of liver and kidney, using the direct peroxidase technique with antiserum to rat MT-I, revealed an increase in staining in both tissues after copper administration, consistent with the RIA results. The change in distribution of immunoreactive material with time after copper injection indicates a role for MT in the sequestration and excretion of copper in acutely loaded animals.

Fate of cadmium in rat renal tubules: a micropuncture study

Free-flow micropuncture was carried out in superficial nephrons of Munich-Wistar type rats infused acutely with Cd acetate (CdA) or Cd-DTPA (141 microM Cd). Fluid obtained from Bowman's space (BS) or end-proximal tubule sites was analyzed for Cd and inulin. The fluid/plasma Cd concentration ratio in BS averaged 0.2 and 1.0 during CdA and Cd-DTPA infusions, respectively. End-proximal tubule fractional excretion of Cd during CdA infusion averaged 0.34. Previous administration of CdA (1.0 mg/kg, 48 hr before micropuncture) increased the level of circulating Cd-metallothioneins, as measured by radioimmunoassay, but did not affect the luminal tubular uptake of Cd during CdA infusion. No net transepithelial movement of Cd-DTPA was measured. It is concluded that Cd ultrafiltered during inorganic Cd administration is taken up to a large extent by the convoluted part of proximal tubules.

Physiological role of glucocorticoids on rat serum and liver metallothionein in basal and stress conditions

Serious contradictions exist at present in our understanding of the physiological role of glucocorticoids on the synthesis of the metal-binding protein, metallothionein (MT). In addressing this problem, we have examined in vivo the role of glucocorticoids on liver and serum MT levels in the rat under a spectrum of experimental conditions. The experiments confirm that stress has a major positive effect on hepatic MT levels. It was found that adrenocorticotropic hormone (ACTH) administration has an inhibitory effect on hepatic MT levels in response to restraint stress and that adrenalectomy (ADX) leads to an increase in basal MT levels and in MT levels in response to acute and chronic immobilization stress. Similar results followed treatment with the glucocorticoid receptor blocker, RU 486. The effect of ADX was abolished by corticosterone replacement. The relations found among hepatic MT, serum MT, and glucocorticoid concentrations indicate that in some circumstances glucocorticoids have a permissive role in mobilizing MT from tissues to serum and that in physiological conditions corticosterone has an inhibitory role in the maintenance of hepatic MT levels.

Ferritin: an expanded role in metabolic regulation

Synthesis of ferritin, a constitutive protein, is increased by iron. This protein is well recognized as a protein which detoxifies, stores and transports iron. The 24 subunits of ferritin assemble to form a protomer of Mr 480,000. This protein shell can sequester up to 4500 g atoms of iron as ferrichydroxyphosphate. Ferritin in vitro and in vivo binds other metal ions such as Cu, Zn, Cd, Pb, Be and Al. Next to Fe it binds large quantities of Be. Therefore, in vitro ferritin protects against and reverses the inhibition by Be of enzymes susceptible to this metal ion. Also, rats pretreated with Fe survive otherwise toxic levels of either pulmonary or intravenous exposure of Be. Liver ferritin from rats injected with Zn contains some of the injected metal ion. Incubation of such ferritin-zinc complex with zinc-requiring apoenzymes restores their activity. Fe(III) of ferritin is released only after its reduction to Fe(II) by a reductant. Incubation of phosphoglucomutase, a phosphoserine containing enzyme with ferritin and a reductant causes irreversible inactivation of the enzyme and removes 70% of its phosphate. Some other phosphoproteins are similarly inactivated but without the loss of the bound phosphate. Thus, uncontrolled release of iron from ferritin, in the presence of a reductant and oxygen can modify several biomolecules and can affect metabolic processes. A subclass of ferritin, acidic isoferritins, have been implicated in leukemia-associated inhibitory activity and has been suggested to inhibit production of Ia+ macrophage progenitors.

Influence of chemical and environmental stressors on acute cadmium toxicity

Previous investigations have demonstrated that the cytosolic protein metallothionein (MT) is induced not only by exposure to certain heavy metals but also by a variety of other factors, including environmental stress. While MT synthesis has been observed with exposure to cold temperatures, there is a paucity of data concerning the influence of cold on heavy-metal toxicity. The present investigation focused on the influence of metal and cold pretreatments on the acute toxicity of cadmium. Mortalities of 80% and 100% were observed for mice orally administered challenge doses of 100 mg Cd/kg and 150 mg Cd/kg, respectively. To determine a protective cadmium pretreatment dose, animals were administered 2.5, 5, 10, 20, 25, and 50 mg Cd/kg 24 h prior to cadmium challenge. In animals pretreated with 10 mg Cd/kg, mortalities of 20% and 70% were observed with the respective challenge doses. Immediately following cold stress (4 degrees C, 12 h), mortalities of 30% and 90% were observed with cadmium challenge doses of 100 and 150 mg Cd/kg, respectively. Significant correlations were demonstrated between induced hepatic MT concentrations and cadmium pretreatment (r = 0.99), as well as cold pretreatment (r = 0.87). Results of this investigation indicate that stressors, such as cold, influence the acute toxicity of cadmium to the same magnitude as metal pretreatment. This induced tolerance to cadmium was attributed, in part, to the induction of MT synthesis. Furthermore, the induced levels of MT resulting from cold stress may confound the simplistic approach of using MT as a biological monitor of occupational exposure to cadmium.

Mechanism of cadmium-metallothionein-induced nephrotoxicity: relationship to altered renal calcium metabolism

Prolonged cadmium exposure has been associated with proteinuria, calcuria and loss of calcium from bones in humans. Previous studies have shown that kidney uptake of cadmium in vivo results from proximal tubule absorption of the circulating cadmium metallothionein complex (CdMT), and intracellular release of the Cd2+ ion prior to induction of renal metallothionein. Parenteral administration of CdMT has been found to selectively damage the proximal tubule cell lysosome system with development of a tubular proteinuria pattern similar to that observed under chronic exposure conditions. The present studies also demonstrate a concomitant calcuria but no changes in the excretion of other electrolytes or glucose using this model. These marked changes in renal calcium metabolism occurred in the absence of mitochondrial damage, changes in total, Na/K or Mg-stimulated ATPase activities, renal ATP levels, membrane 45Ca2+ transport or overt tubule cell necrosis during an 8 hour period following CdMT injection. Proteinuria and calcuria were prevented by prior zinc induction of the renal MT pool. Data from these studies indicate that renal proximal tubule cell uptake and degradation of the circulating CdMT complex produces both a marked proteinuria and calcuria. The calcuria does not appear to stem from changes in renal energy metabolism or membrane transport of this element but is probably a secondary result of calcium binding to excreted proteins which are increased in urine to a similar extent. The studies also suggest that zinc status and maintenance of the renal ZnMT pool may play an important role in regulating cadmium-induced renal proteinuria and calcuria by preventing Cd2+ perturbation of the proximal tubule cell lysosome system.

Metallothionein in blood, bile and urine

The development of radioimmunoassays for rat metallothionein-I (MT-I) has revealed that this protein is present in many extracellular fluids. Plasma concentrations are low in normal animals but may be substantially increased in animals with elevated tissue MT levels. MT-I is also excreted in varying amounts into urine and bile, and there is evidence for the presence of degradation products of MT-I, detectable by radioimmunoassay, in these fluids. Investigation of the secretion of MT can provide valuable information on the turnover of the protein in tissues and lead to the development of novel diagnostic tests for the assessment of trace metal status.

Enzyme-linked immunosorbent assay (ELISA) for metallothionein

A heterogeneous, double-antibody, fluorometric enzyme-linked immunosorbent assay (ELISA) is described for the detection and quantitation of metallothionein(MT) The protocol uses the same immunological reagents as the radioimmunoassay(RIA) developed previously in this laboratory; fluorescence replaces radioactivity for detection of the reference antigen in the specific binding reaction. Present results indicate that the developed ELISA has approximately the same range of capability in detecting and quantitating MT as is characteristic of the RIA. The ELISA has the advantage that the time required to perform a typical assay is significantly less than that required for the RIA.

Restraint stress induced changes in rat liver and serum metallothionein and in Zn metabolism

24 h of a psychogenic stress (restraint) caused a strong increase of liver metallothionein (MT) levels. 3 h of stress were sufficient to induce an increase in liver MT, measured 21 h later, but the increase was much lower than in continuously restrained rats. Stress induction of liver MT was not due to food deprivation, since rats deprived for 24 h showed lower MT levels than stressed ones. Zn on MT presented the same qualitative but not quantitative pattern of response as MT protein. Liver cytosolic Zn was increased by restraint in spite of their being no decrease in serum Zn. Any treatment altered serum MT. Liver and serum MT were not correlated. The present results demonstrate that basically psychogenic stresses increased liver but not serum MT levels. No positive evidence for a relationship between corticosterone secretion and MT induction was found.

Fluorometric ELISA for the detection and quantitation of metallothionein

The development of a heterogeneous fluorometric ELISA for the detection and quantitation of metallothionein (MT) is described. A radioimmunoassay (RIA) previously developed in our laboratory is used as a reference assay to characterize the performance of the ELISA. The standard curves (logit-log regressions) that are typical of either assay have similar ranges (customarily from 20 000 to 100 pg of competing antigen); both assays are capable of quantitating MT in unknowns with 5-10% accuracy. Aspects of MT measurement in cytosols and physiological fluids are discussed.

Relative zinc-binding activities of ligands in the cytosol of rat small intestine

Relative zinc-binding activities of high-molecular-weight zinc-binding ligand (HMW-ZBL), metallothionein (MT) and low-molecular-weight zinc-binding ligand (LMW-ZBL) in the cytosols of rat small intestines under various experimental conditions were examined. Zinc-binding activities of MT decreased and those of LMW-ZBL increased in the intestinal cytosols from most of the experimental rat groups after incubating at 37 degrees C for 2 hr. The relative zinc-binding activity of MT increased with increasing doses of injected zinc and decreased with orally-administered zinc. Isolated MT did not lose zinc-binding activity during incubation at 37 degrees C for 48 hr, but moved from the MT eluting peak to the LMW-ZBL eluting peak after 1 week.

Comparison of methods of metallothionein quantification: cadmium radioassay, mercury radioassay, and radioimmunoassay

A comparison of the Hg radioassay (i.e. "Piotrowski" assay), the Cd radioassay (i.e. "cadmium-hemolysate" assay), and the radioimmunoassay (RIA) for metallothionein (MT) was performed for livers from control, Cd-, and Zn-pretreated rats. Results indicate that the Cd and Hg radioassays gave similar results in all cases. The RIA values compared very favorably to those obtained by the metal saturation assays for livers from control rats. The values obtained for livers from Cd- or Zn-treated animals were generally lower in the RIA than in the Hg or Cd radioassay, ranging from 36 to 75% of the metal saturation assays. The source of this discrepancy cannot be discerned from the present data. However, the various assay systems employed did yield MT concentrations that followed similar intraassay patterns, being lowest in control, elevated to an intermediate level with Cd treatment, and highest following Zn treatment.

The origin of metallothionein in red blood cells

The origin of metallothionein (MT) in red blood cells (RBCs) from a mouse given cadmium was studied in connection with RBC kinetics. Plasma Cd concentration rapidly decreased 3 hr following 109CdCl2 (2 mg/kg, sc) administration, whereas RBC Cd increased from 2 to 4 days, followed by a gradual decrease. RBC Cd was found to be distributed more in the high-molecular-weight fraction than in the MT fraction 12 hr after administration. But, thereafter, Cd increased rapidly in the MT fraction to show changes with time similar to Cd level in RBCs. Hepatic damage induced in a mouse given 21 injections of Cd, with resultant marked elevation of plasma MT concentrations, did not cause any change in RBC Cd concentration. MT was hardly transferred to RBC when a mouse RBC suspension was incubated with mouse hepatic MT. To examine the relationship of Cd-MT and erythropoietic function, mice in the normal group, the phenylhydrazine-induced anemia group (PH), the transfusion-induced plethora group (TR), and the erythropoietin administered plethora group (TR + EP) were given 109CdCl2. Three days after administration, Cd concentration in its RBCs and its MT fraction remarkably increased in the PH group, and was greatly decreased in the TR group. A significant increase was noted in the TR + EP group as compared with the TR group. These results indicate that MT in the RBCs is formed in erythroblasts.

Cadmium-induced hepatic and renal injury in chronically exposed rats: likely role of hepatic cadmium-metallothionein in nephrotoxicity

Rats were injected sc with 0.5 mg Cd/kg, 6 days/week, for up to 26 weeks. Hepatic and renal function and tissue Cd and metallothionein (MT) content were determined in tissues and plasma at various times after Cd injection. Cd in liver and kidney increased linearly for the first 10 weeks of treatment, but thereafter hepatic concentrations of Cd decreased by 33% whereas the content of Cd in kidney remained constant. MT in liver and kidney increased linearly during the first 12 weeks of Cd treatment to 4400 and 2300 micrograms MT/g, respectively, but rose only slightly thereafter. Circulating concentrations of MT progressively increased beginning 2 weeks after Cd treatment and were approximately 10 times control values in rats dosed with Cd for 12 or more weeks. Plasma activities of alanine and aspartate aminotransferase exhibited a time course similar to that observed with MT, and were elevated as early as the sixth week of Cd exposure. Sharp increases in activities of these enzymes also occurred after 10 to 12 weeks of dosing. Hepatic microsomal metabolism of benzo[a]pyrene and ethylmorphine was severely attenuated beginning 4 weeks after Cd. Renal injury occurred after hepatic damage, as evidenced by decreased in vitro p-aminohippuric acid uptake beginning 8 weeks after exposure. Urine outflow increased threefold 11 weeks after Cd exposure began, while urinary protein and Cd excretion increased beginning at Week 9. These data indicate the liver is a major target organ of chronic Cd poisoning, and suggest that Cd-induced hepatic injury, via release of Cd-MT, may play an important role in the nephrotoxicity observed in response to long-term exposure to Cd.

Biliary excretion of metallothionein and a possible degradation product in rats injected with copper and zinc

The concentrations of metallothionein-I (MT-I) and related immunoreactive products in bile from adult female rats were measured by radioimmunoassay. Concentrations in normal animals were 20-30 ng/ml, but increased to 600 and 75 ng/ml after injection of Cu2+ and Zn2+ respectively (3 mg of metal/kg body wt.). However, only 1-2% of the biliary Cu was bound to MT, and less than 1% of the total liver MT in control or Cu2+-injected rats appeared to be secreted in intact form into bile. Other major immunoreactive components in bile from Cu2+-injected rats included an aggregated form of MT-I and a possible degradation product of the isoprotein.

Metallothionein-like proteins in human placenta and fetal membranes

Metallothionein (MT) levels were measured in the term human placenta, chorion, and amnion by the Cd-radioassay method. MT concentrations in the amnion and chorion were 8 to 9 micrograms MT/g tissue while placental levels were approximately half this value. For individual subjects a significant correlation (r2 = 0.96) was found for placental and chorionic MT, while amnionic MT levels did not correlate with either placental or chorionic MT. Gel filtration and uv spectral analysis confirmed the presence of a MT-like protein in cytosol obtained from amnion, chorion, and placenta. MT levels in trophoblasts cultured from chorion were increased upon exposure in vitro to either Cd or Zn.

Chelation of cadmium

The toxicity of cadmium is determined by chelation reactions: in vivo, Cd2+ exists exclusively in coordination complexes with biological ligands, or with administered chelating agents. The Cd2+ ion has some soft character, but it is not a typical soft ion. It has a high degree of polarizability, and its complexes with soft ligands have predominantly covalent bond characteristics. Cd2+ forms the most stable complexes with soft donor atoms (S much greater than N greater than 0). The coordination stereochemistry of Cd2+ is unusually varied, including coordination numbers from 2 to 8. Even though the Cd2+ ion is a d10 ion, disturbed coordination geometries are often seen. Generally, the stability of complexes increases with the number of coordination groups contributed by the ligand; consequently, complexes of Cd2+ with polydentate ligands containing SH groups are very stable. Cd2+ in metallothionein (MT) is coordinated with 4 thiolate groups, and the log stability constant is estimated to 25.5. Complexes between Cd2+ and low molecular weight monodentate or bidentate ligands, e.g., free amino acids (LMW-Cd), seem to exist very briefly, and Cd2+ is rapidly bound to high molecular weight proteins, mainly serum albumin. These complexes (HMW-Cd) are rapidly scavenged from blood, mainly by the liver, and Cd2+ is redistributed to MT. After about 1 day the Cd-MT complex (MT-Cd) almost exclusively accounts for the total retained dose of Cd2+, independent of the route of exposure. MT-Cd is slowly transferred to and accumulated in kidney cortex. The acute toxicity and interorgan distribution of parenterally administered Cd2+ are strongly influenced by preceding MT induction, or decreased capacity for MT synthesis; however, the gastrointestinal (GI) uptake of Cd2+ seems unaffected by preceding MT induction resulting in considerable capacity for Cd2+ chelation in intestinal mucosa, and this finding indicates that endogenous MT is not involved in Cd2+ absorption. The toxicity of parenterally administered Cd2+ is strongly enhanced when administered as complexes with NTA or STPP , but it is much decreased when administered as a complex with EDTA. In chronic oral exposure the toxicity and GI uptake of Cd2+ is not changed when Cd2+ is administered as a complex with the detergent formula chelating agents NTA, EDTA and STPP . The uptake of Cd2+ from ligated intestine in vivo was not affected by administration of Cd2+ as complexes with CYS or GSH, but significantly reduced by complexation with EDTA or BAL. The acute toxicity of orally administered Cd2+ is reduced when Cd2+ is administered as a complex with EDTA.(ABSTRACT TRUNCATED AT 400 WORDS)

Metallothionein-I in the plasma and liver of neonatal rats

The concentrations of metallothionein-I in the plasma and liver of neonatal rats were measured by radioimmunoassay. Plasma concentrations of the protein in male and female 4-day-old rats were 350 and 740 ng/ml respectively, and declined rapidly to only 3.5 ng/ml at 32 days of age. Concentrations in liver were also high in the newborn rats (200 micrograms/g), and declined from 12 days of age onwards.

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

A sensitive radioimmunoassay for rat liver metallothionein-I has been developed using avid and high-titre antibodies obtained from sheep that were immunized with a conjugate of metallothionein and rabbit immunoglobulin G. The assay was specific for metallothionein-I, and did not depend on the particular metal bound to the protein. There was no significant cross-reaction with rat liver metallothionein-II. The use of the assay to measure metallothionein concentrations in rat plasma and kidneys is described.

Urinary metallothionein and tissue metal levels of rats injected with cadmium, mercury, lead, copper or zinc

Since Cd exposure results in a dose dependent increase in metallothionein level in urine, the present investigation was conducted to examine whether exposure to other divalent cations would also cause an elevation in urinary metallothionein. Female Sprague-Dawley rats were injected subcutaneously with either saline, 5 mumol/kg/day of CdCl2, HgCl2, Pb(C2H3O2)2, CuSO4 or ZnCl2 for 5 days. Significant increases in hepatic Cu levels in rats treated with not only Cu, but also Zn, Cd, or Hg, and in hepatic Zn levels in rats treated with Zn or Cd were noted. Similarly, renal Cu and Zn levels were elevated significantly in all groups except the Pb-injected group. These increases in tissue metal levels were presumably due to induction of metallothionein. The urinary metallothionein level in control rats on day 0, determined by radioimmunoassay, was 0.85 +/- 0.17 mg/g creatinine. There was no significant change in urinary metallothionein level in rats given up to 5 injections of saline or Pb. Hg-injected rats showed 25-fold increase in urinary metallothionein after 5 injections, whereas Cd-injected rats had 9-fold increase. There were also 2- and 3-fold increases of urinary metallothionein by Cu and Zn treatments for 5 days, respectively. Thus, urinary metallothionein levels were elevated in response to Cd, Hg, Cu and Zn, but not Pb; Hg had the most profound effect at equimolar doses.

In vitro exposure of male and female mice gametes to cadmium chloride during the fertilization process, and its effects on pregnancy outcome

The effect of cadmium chloride (Cd) on gamete fusion in vitro was evaluated, with further observations of the embryonic development and assessment of the pregnancy outcome of the in vitro fertilized mice. Oocytes were recovered from superovulated B6C3F1 females. Of 1210 control oocytes, 53.2% cleaved into two-cell stage embryos. Of these, 46.6% developed into blastocyst stage embryos which were then surgically transferred to pseudopregnant female CD-1 mice. Of a total of 63 implanted embryos, 8 (12.7%) developed in utero to live fetuses. Teratological examinations of these "test-tube" mice revealed no signs of abnormalities caused by in vitro culture. Male and female gametes were exposed to 0.4, 0.8, or 1.6 microM of Cd and a decrease in sperm motility was noted in the 1.6 microM group. Nevertheless, even in the highest concentration used, 56.4% of the ova cleaved into the two-cell stage, thus indicating no effect of Cd on initial gamete interaction. Gametes that had been treated with 0.4 and 0.8 microM Cd developed to blastocysts at rates comparable to that of the controls. In the 1.6 microM group, however, only one (3.2%) of the two-cell embryos developed to the blastocyst stage. Blastocysts from 0.4 microM Cd-treated gametes were then transferred to surrogate dams. Statistically significant blastocyst losses were recorded during the implantation period, whereas the pregnancy rate and the numbers of resorbed and live fetuses, were comparable to those of the controls. The offspring exhibited no malformations, and their body weights remained within the control values.

Metallothionein--aspects related to copper and zinc metabolism

Metallothionein is a cysteine-rich, low molecular weight protein that binds zinc, copper and cadmium. It is inducible in liver, kidney and intestine by glucocorticoids, changes in the dietary zinc supply, acute administration of various metals, food restriction, infection, stress and endotoxin treatment. Regulation of synthesis involves altered gene expression. The protein is fairly rapidly degraded when zinc is the primary metal species bound, but the degradation rate is diminished when cadmium or copper are bound as well. The net result of metallothionein production seems to be accumulation of bound metal and/or intracellular metal redistribution. The accumulation of copper in various tissues of individuals with Menkes' and Wilson's diseases may be related to altered metallothionein turnover. The physiological function is not clear, but the response of metallothionein to hormonal stimuli is suggestive of an important role in cellular metabolism.

Inducible protective proteins: a potentially novel approach to chemotherapy

A number of toxic chemical and physical agents elicit the induction of a series of protein species, some of which react with the agents and render them nontoxic. A few of the induced species (such as metallothionein) are rich in thiol groups that might be expected to react with alkylating agents and render them nontoxic. If a safe means could be found for selectively enhancing the synthesis of alkylating-agent-reactive species in normal but not tumor cells, such a procedure would have ramifications in the area of cancer chemotherapy. In this report, we have utilized a variety of trace elements (Zn, Se, Cu, As) as inducers of synthesis of protective species in line CHO Chinese hamster cells and in a number of derived variants to determine whether this type of approach can be utilized to increase resistance to alkylating-agent toxicity. Our results indicate that Zn, Se and Cu elicit a protective response (increased survival, monitored by colony-forming ability) against the toxic effects of iodoacetate or melphalan, and, at least in the case of zinc, at levels that are physiologically reasonable. Arsenite appears to be a marginally effective inducer in the CHO cell and an ineffective inducer in the Cdr20F4 variant cell. The increased survival is not attributable to metallothionein inducibility, decreased availability of the alkylating agent in the medium, or decreased uptake of the drug into the trace-element-pretreated cells. The protective responses induced by zinc or selenite alone are additive in cells receiving both trace elements prior to exposure to alkylating agent, which suggests that different domains of response are elicited by the two metals. In view of reported differences in inducibility of protective proteins between normal and tumor cells, a possibility is raised for a novel approach to alkylating-agent chemotherapy that is somewhat analogous to the protocol utilized in high-dose methotrexate therapy.

Urinary metallothionein as a new index of renal dysfunction in "Itai-Itai" disease patients and other Japanese women environmentally exposed to cadmium

The significance of elevated excretion of metallothionein in urine of women living in cadmium-polluted areas of Japan was studied with respect to renal dysfunction. The relationships between the concentrations of metallothionein in urine and those of other non-specific urinary indices of renal dysfunction, i.e., total protein, glucose, beta 2-microglobulin, retinol-binding protein, alpha-amino nitrogen and proline were examined. In addition, the relationships between urinary metallothionein and urinary cadmium and copper were also evaluated. It was found that the logarithm of the metallothionein concentration in urine was significantly correlated with the logarithm of the concentrations of each of the above parameters. When subjects with signs of renal dysfunction, including "itai'itai" disease patients and patients suspected of the disease, were compared with subjects with normal renal functions, as a group, the former excreted significantly higher concentrations of metallothionein in their urine than the latter. The results suggest that the elevated excretion of metallothionein is not only an index of excessive cadmium exposure, but also of renal dysfunction caused by chronic exposure to this metal.