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

A novel report of metallothioneins in fish bile: SDS-PAGE analysis, spectrophotometry quantification and metal speciation characterization by liquid chromatography coupled to ICP-MS

Metallothioneins (MT) are a validated biomarker for metal exposure. In fish they are usually determined in the liver, while some studies have observed MT in kidney, muscle and gills. There are, however, no reports on MT in fish bile. This study aimed to verify if these proteins are present in fish bile, if they can be used as biomarkers for metal exposure and to characterize the metal speciation present in the different species. Three independent methods to verify MT presence in fish bile were conducted: 1D/2D gel electrophoresis, spectrophotometric analyses and SEC-HPLC-ICP-MS. Results show that all methods verified the presence of these proteins in fish bile, while SEC-HPLC-ICP-MS also characterized also characterized metals responsible for the contamination of the studied sites. Quantification was successfully conducted by two methods, densitometry (1D electrophoresis) and spectrophotometry, and the data indicates that bile MTs follow the same trend as in liver. Therefore, Biliary MT show the potential to be used the same way as MT from other matrices in metal exposure studies, with the advantage of being a simpler matrix and showing the possibility of not sacrificing the animals, as chronic duct cannulation is available.

Metallothionein-I overexpression decreases brain pathology in transgenic mice with astrocyte-targeted expression of interleukin-6

Transgenic expression of interleukin-6 (IL-6) in the CNS under the control of the glial fibrillary acidic protein (GFAP) gene promoter (GFAP-IL6 mice) causes significant damage and alters the expression of many genes, including a dramatic upregulation of metallothionein-I (MT-I). The findings in this report support the idea that the upregulation of MT-I observed in GFAP-IL6 mice is an important mechanism for coping with brain damage. Thus, GFAP-IL6 mice that were crossed with TgMTI transgenic mice (GFAP-IL6xTgMTI) and overexpressed MT-I in the brain showed a decreased upregulation of cytokines such as IL-6 and a diminished recruitment and activation of macrophages and T cells throughout the CNS but mainly in the cerebellum. The GFAP-IL6 mice showed clear evidence of increased oxidative stress, which was significantly decreased by MT-I overexpression. Interestingly, MT-I overexpression increased angiogenesis in GFAP-IL6 mice but not in control littermates. Overall, the results strongly suggest that MT-I+II proteins are valuable factors that protect against cytokine-induced CNS injury.

Two forms of Wilson disease protein produced by alternative splicing are localized in distinct cellular compartments

Copper is an essential trace element in prokaryotes and eukaryotes and is strictly regulated by biological mechanisms. Menkes and Wilson diseases are human disorders that arise from disruption of the normal process of copper export from the cytosol to the extracellular environment. Recently a gene for Wilson disease (WD)(also named the ATP7B gene) was cloned. This gene encodes a copper transporter of the P-type ATPase. We prepared monoclonal and polyclonal anti-(WD protein) antibodies and characterized the full-length WD protein as well as a shorter form that is produced by alternative splicing in the human brain. We found that the WD protein is localized mainly in the Golgi apparatus, whereas the shorter form is present in the cytosol. These results suggest that the alternative WD proteins act as key regulators of copper metabolism, perhaps by performing distinct roles in the intracellular transport and export of copper.

Outputs of hepatic copper and cadmium stimulated by tetrathiomolybdate (TTM) injection in Long-Evans Cinnamon (LEC) rats pretreated with cadmium, and in Fischer rats pretreated with copper and cadmium

The Long-Evans Cinnamon (LEC) rat, an inbred mutant rat derived from the Long-Evans strain, is characterized by spontaneous hepatitis due to gross accumulation of hepatic Cu. The accumulation, accompanied by marked induction of metallothionein (MT), is believed to be due to the inherent lack of output of Cu into the bile duct and blood vessels. In this study, the acute effect of tetrathiomolybdate (TTM), a chelator for output of hepatic Cu and Cd in LEC rats treated with Cd, was investigated. Female LEC rats were injected subcutaneously with Cd (Cd; 1.0 mg/kg) to induce Cd, Cu-MT. Fischer rats were treated with Cd (Cd; 1.0 mg/kg) and Cu (Cu; 3.0 mg/kg). Forty-eight hours after the injections of metals, TTM (5 mg/kg bw) was injected intravenously under anesthesia. The TTM injection rapidly stimulated biliary excretions of Cu (at a microgram/ml level) and Cd (at a ng/ml level). Furthermore, Cu and Cd concentrations were increased in serum sampled 60 min after the TTM injection. The increase of biliary Cu excretion was not accompanied by increased biliary excretion of MT. The TTM injection caused the hepatic Cu concentrations to decrease from 306 +/- 2 to 262 +/- 12 and from 43 +/- 6 to 20 +/- 5 micrograms/g in LEC and Fischer rats, respectively. The hepatic Cd concentration was not decreased by TTM treatment. Hepatic MT and Cu, but not Cd, concentrations in the MT fraction were also reduced by TTM injection. Our results showed that TTM can rapidly remove Cu from MT to increase bile and blood Cu levels. The output of Cd stimulated by TTM injection may be related to MT reduction resulting from removal of MT-bound Cu. Our results indicate that to avoid the toxic effect of Cu, TTM injection is an effective initial treatment, although it remains to be established how metals, including Cu, are finally metabolized.

Biliary excretion of copper, manganese, and horseradish peroxidase in Eisai hyperbilirubinemic mutant rats (EHBRs) with defective biliary excretion of glutathione

A mixture of copper (Cu) (0.38 mg/kg), manganese (Mn (0.038 mg/kg), and horseradish peroxidase (HRP) (5.0 mg/kg) was injected intravenously (i.v.) into mature Eisai hyperbilirubinemic rats (EHBRs) and Sprague-Dawley rats (SDRs). Bile was collected at 10-min intervals before and after the injection, under anesthesia. The liver, kidneys, and blood were removed 40 min after the injection. The serum conjugated bilirubin concentration was 0.85 mg/dL in the EHBRs, but was below detection limits in the SDRs. The bile-reduced glutathione (GSH) concentration was much lower in the EHBRs (0.04 mg/mL) than in the SDRs (1.30 mg/mL). However, the hepatic GSH concentration was about 1.6 times higher in EHBRs (2.26 mg/g liver) than in SDRs (1.43 mg/g liver). The low excretion of biliary GSH was not caused by the activity of GGT in the liver, since there was no significant difference in the activity between the two groups (5.8 +/- 3.4 and 4.6 +/- 2.4 mumol p-nitroaniline/g protein/30 min in SDR and EHBR groups, respectively). There was a delay of initial biliary excretion of Cu in EHBRs compared to SDRs. The biliary concentration of Mn was slightly lower in EHBRs than in SDRs. Forty min after the injection of metals, however, there was no difference between hepatic concentrations of the two metals in the two groups. Our results suggest that abnormal deposition of the two metals is not observed naturally in EHBRs. Injected HRP was excreted rapidly and notably in the EHBRs compared to SDRs. Furthermore, the biliary concentration of beta-N-acetyl-D-glucosaminidase (beta-NAG) was significantly higher in EHBRs than in SDRs, Rapid biliary excretion of Cu, but not of Mn, may be related to the hepatobiliary transport of GSH, but the transport and lysosomal function do not originally regulate the biliary excretion of Cu.

Evidence for a high molecular weight cytosolic factor that binds brain and liver metallothionein

When brain extracts were fractionated in a Sephadex G-75 chromatography and MT levels were assayed by RIA or ELISA using polyclonal antibodies specific for the MT-I and MT-II isoforms, it was found that MT mostly eluted in the high molecular weight (HMW) peak even in reducing or anaerobic conditions. This was also the case for the liver extracts of control rats; in stressed animals MT immunoreactivity in the HMW peak (> 80 Kd) was increased compared with undisturbed animals, but the major amount of the newly induced MT eluted, as expected from the current literature, in the low molecular weight (LMW) peak, around 10 Kd. The addition of purified MT to brain extracts precluded its binding to a DEAE-Sephadex column. Furthermore, immunoblot results of native PAGE showed that MT changed its electrophoretic mobility in the presence of HMW proteins from brain cytosol. Altogether, these results suggest that a cytosolic factor binds MT in a saturable manner, which may have strong physiological implications.

Biliary excretion of copper in Fischer rats treated with copper salt and in Long-Evans cinnamon (LEC) rats with an inherently abnormal copper metabolism

Increased biliary Cu excretion was found in Fischer rats injected with Cu. The biliary Cu was located at the void (large-molecule region) and total (small-molecule region) volume of a Sephadex G-75 column. The most Cu was found in the total volume. The two Cu peaks comigrated with absorbance at 280 nm. Although the bile from Cu-untreated Fischer rats did not show Cu absorbance in the total volume, absorbance at 280 nm was also found in this region. Even though Long-Evans Cinnamon (LEC) rats deposited a gross amount of Cu (194.0 +/- 27.8 micrograms/g liver) in the liver, they conversely showed reduced Cu excretion into the bile. LEC bile did not show Cu absorbance but rather absorbance at 280 nm in the total volume. Therefore, it seems unlikely that the small molecules found in the Sephadex G-75 regulate biliary Cu excretion in Cu-loaded rats, although the molecules bind to Cu. When the bile from Cu-untreated Fischer and LEC rats was incubated with CuCl2 solution, the most Cu was recovered in the total volume of this column. Our results suggest that reduced biliary Cu excretion in LEC rats is not related to the small molecules, and that Cu cannot be excreted in the form of macromolecules in rats to decrease Cu from the Cu-loaded liver.

Effects of metallothionein on the observed copper distribution in cell extracts

Systematic studies have been undertaken to compare the effects of cell lysis and chromatography conditions on the observed distribution of Cu amongst Cu-binding proteins in cultured cells. The variables included rate of centrifugation, presence or absence of non-ionic detergent, and presence or absence of dithiothreitol. The application of an improved FPLC gel filtration system has permitted us to examine the effects of the addition of exogenous metallothionein (MT) to cell extracts. When the cell extract contains low levels of endogenous MT, the addition of MT in the presence of dithiothreitol causes a shift of copper to the MT peak. High levels of MT can therefore remove copper from other Cu-binding ligands during cell homogenization, hence producing artifactual Cu distribution results. The use of an anaerobic buffer system has greatly reduced the observed level of Cu exchange, and has allowed comparison of Cu distribution in normal cells and cells from patients with Menkes' disease.

Metallothionein biodegradation in rat hepatoma cells: a compartmental analysis aided 35S-radiotracer study

Disappearance rates, synthesis and biodegradation of rat HTC cell metallothioneins (MT) were examined by measurements of [35S]MT, and expressed by their rate constants in experiments carried out under steady-state conditions, with physiological doses of zinc and copper. Overall rates of disappearance of [35S]MT did not obey first-order kinetics, apparently due to the presence of chemically and/or spatially distinguishable MT (sub)pools. Application of compartmental analysis yielded results indicating that: (a) MT in copper-treated cells is stabler than MT in zinc-treated cells, (b) apparent half-lives for MT disappearance are 22-25 h under the conditions as described, (c) addition of 0.1 mM chloroquine reduces the overall rate of MT disappearance by 25%, (d) total MT should be regarded as significantly consisting of at least two MT sub-forms, i.e., Apo-MT and M-MT and/or cytosolic MT and lysosomal MT, each being depleted and replenished at different rates, (e) mean half-lives for total MT, based on actual degradation rates, were 3-7 h, (f) addition of 0.1 mM chloroquine resulted in both increased synthesis and increased degradation of non-lysosomal MT, the latter probably due to free amino acid depletion in the cells, and (g) MT behaviour may be best examined by combination of tracer experiments (35S-labeled amino acids), simultaneous determination of MT levels in all (sub)MT pools, and application of compartmental analysis.

Seasonal changes in subcellular distribution of zinc, copper, cadmium and metallothionein in the liver of bank vole (Clethrionomys glareolus): a possible essential role of cadmium and metallothionein in the hepatic metabolism of copper

1. An increase in total Zn concentration in the liver of sexually mature bank voles in spring and summer was primarily accompanied by an increase in Zn levels in the nuclear and mitochondrial-lysosomal fractions, while in immature voles in an analogous situation, Zn tended to concentrate mainly in the post-mitochondrial fraction. 2. Seasonal changes in Cu concentration in all subcellular fractions followed closely those of total hepatic Cu; however, the mitochondrial-lysosomal Cu underwent the most dramatic changes. 3. Multiple regression analysis between the concentration of metallothionein (MT) in the postmitochondrial fraction and the levels of Zn, Cu and Cd in the particular subcellular fractions showed that MT was principally induced by small amounts of Cd concentration of which in the liver ranged from 0.019 microgram/g in winter to 1.1 micrograms/g wet wt in spring. 4. The analysis revealed further that the level of MT in the post-mitochondrial fraction concurrently decreased as total hepatic Cu, as well as mitochondrial-lysosomal Cu, increased. 5. The data indicate that MT, first induced by small amounts of Cd, is involved in the hepatic metabolism of Cu in bank voles; most probably the protein sequesters free Cu ions in cytoplasm and then transfer them into the mitochondrial-lysosomal fraction.

Mechanisms of metal transport across liver cell plasma membranes

The liver's pivotal role in the homeostasis of essential trace metals and detoxification of exogenous metals is attributed to its ability to efficiently extract metals from plasma, metabolize, store, and redistribute them in various forms either into bile or back into the bloodstream. Bidirectional transport across the sinusoidal plasma membrane allows the liver to control plasma concentrations and therefore availability to other tissues. In contrast, transport across the canalicular membrane is largely, but not exclusively, unidirectional and is a major excretory pathway. Although each metal has relatively distinct hepatic transport characteristics, some generalizations can be made. First, movement of metals from plasma to bile follows primarily a transcellular route. The roles of the paracellular pathway and of ductular secretion appear minimal. Second, intracellular binding proteins and in particular metallothionein play only indirect roles in transmembrane flux. The amounts of metallothionein normally secreted into plasma and bile are quite small and cannot account for total metal efflux. Third, metals traverse liver cell plasma membranes largely by facilitated diffusion, and by fluid-phase, adsorptive, and receptor-mediated endocytosis/exocytosis. There is currently no evidence for primary active transport. Because of the high rate of hepatocellular membrane turnover, metal transport via endocytic vesicles probably makes a larger contribution than previously recognized. Finally, there is significant overlap in substrate specificity on the putative membrane carriers for the essential trace metals. For example, zinc and copper share many transport characteristics and apparently compete for at least one common transport pathway. Similarly, canalicular transport of five of the metals discussed in this overview (Cu, Zn, Cd, Hg, and Pb) is linked to biliary GSH excretion. These metals may be transported as GSH complexes by the canalicular glutathione transport system(s). Unfortunately, none of the putative membrane carrier proteins have been studied at the subcellular or molecular level. Our knowledge of their biochemical properties is rudimentary and rests almost entirely on indirect evidence obtained in vivo or in intact cell systems. The challenge for the future is to isolate and characterize these putative metal carriers, and to determine how they are functionally regulated.

The protective role of metallothionein in copper-overload: II. Transport and excretion of immunoreactive MT-1 in blood, bile and urine of copper-loaded rats

The regulation of copper homeostasis in copper overloaded animals occurs by excretion of excess of the metal in bile and urine, which may be facilitated by metallothionein (MT) a copper binding protein. The role of MT in the mobilisation and excretion of copper excess has been studied in copper-loaded rats during the development of tolerance. Young male Wistar rats were fed a high copper (1 g/kg) diet for 16 weeks during which period they were killed after prior collection of bile, blood and urine for analysis for copper and immunoreactive MT-1. In addition bile was separated chromatographically and the eluant fractions were assessed likewise for copper and MT-1. Biliary excretion of copper and MT-1 rose to a maximum after 6 weeks, falling subsequently as the rats became copper tolerant. Early increases in circulating copper and MT-1 occurred likewise but whereas MT-1 fell subsequently during the recovery period, serum copper remained elevated. By contrast, urinary copper and MT-1 maintained an increased output throughout. Chromatographic separation of bile revealed the presence of a range of immunoreactive MT-1 degradation products. It was concluded that the close correspondence between bile and serum MT reflected their hepatic derivation and implicated liver MT as an export protein in the early stages of copper overload. By contrast, urine MT, maintained independently of circulating MT levels, established the active secretory participation of the kidney in promoting the continued depletion of excess copper.

Extracellular metallothionein effects on lymphocyte activities

Metal cation influences on the immune response have been reported in a wide variety of experimental systems. These influences can either result in the augmentation or suppression of immunological activities. In order to investigate possible mechanisms of these influences, we examined the role that a metal cation-induced protein, metallothionein (MT), might play. Our findings suggest that thioneins, either as apoproteins or when complexed as Cd,Zn-MT, Zn-MT, or Cd-MT, are capable of inducing lymphocyte proliferation. This level of induction is substantially reduced when Zn,Cd-MT is added to lymphocyte cultures in the presence of 50 microM 2-mercaptoethanol. Apoprotein, Zn,Cd-MT, Zn-MT and Cd-MT also augment LPS-induced proliferation of splenic lymphocytes. Only the Zn,Cd-MT preparation significantly augmented ConA-induced proliferation. Hg-MT and Cu-MT were inhibitory as additions in either LPS or ConA mitogen proliferation assays, and did not stimulate proliferation when added alone to lymphocyte cultures. The capacity to induce proliferation correlates with the measurable thiol level of the particular thionein. Interestingly, Zn,Cd-MT and apothionein had an equivalent number of accessible thiols. Although Zn, Pb, Hg and Cu lowered the number of these sites, the immunoreactivity of these MTs was not altered substantially except by Pb. These results suggest that some metal influences on lymphocytes might be through a thionein intermediary. Our results also demonstrate that thioneins complexed with certain metal cations are detrimental to the normal cellular activities of lymphocytes. At least in these circumstances, MT does not play a role as a protective agent.

Urinary excretion of metallothionein-I and its degradation product in rats treated with cadmium, copper, zinc or mercury

The metallothionein-I (MT-I) content of urine following administration of cadmium (Cd), copper (Cu), mercury (Hg) or zinc (Zn) to rats was determined by radioimmunoassay. Urinary excretion of MT-I was increased significantly after injection of each of these metals. Fractionation of urine from Cd-treated rats on Sephadex G-50 showed a single immunoreactive component corresponding to native MT-I, whereas in urine from Cu, Zn or Hg-treated rats 2 immunoreactive components corresponding to MT-I and a possible degradation production were observed. Since a comparable low molecular weight component corresponding to this degradation product was not detected to the same extent on fractionation of plasma from Cu-exposed rat, it seemed to be derived from degradation of MT in the kidney.

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.

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.

Intestinal uptake and retention of copper in the suckling rat, Rattus rattus--IV. Mechanisms of intestinal copper accumulation

Copper-67, administered either parenterally or via the maternal milk, accumulates principally in the intestine and liver of the 6-day-old pup. Most of the 67Cu in the soluble fraction of the intestine is associated with the heterogeneous Cu-complex, which is located predominantly in the ileum. The rates of uptake and loss of 67Cu in the liver and intestine indicate that enterohepatic circulation of Cu in the neonate is appreciable. Whilst the concentration of Cu in the bile of the 13-day-old pup is high (16-fold greater than that in the adult male rat), translocation of Cu from both the liver and duodenum to the ileum probably occurs via the blood, rather than by the reabsorption of biliary Cu. Although the Cu-complex normally seems to be retained within the distal intestine until the enterocytes are desquamated, Cu in this form is utilized when the Cu-intake of the neonate is restricted.

Strain-related patterns of biliary excretion and hepatic distribution of copper in the rat

Biliary copper excretion was studied in male, bile-cannulated rats of the inbred strains Fischer, Brown Norway, WAG/Rij and Lewis. After intravenous injection of 10, 30 and 50 micrograms copper per 100 gm body weight, two patterns of copper excretion were observed; their profiles varied with the copper dose and the strain of the rats used. The lowest amounts of copper were excreted by Fischer rats, the highest by WAG/Rij rats; this was related to the effect of the copper dose on both patterns. The subcellular distribution of copper in the liver was studied in Fischer and Brown Norway rats after doses of 50, 100, and 200 micrograms per 100 gm body weight. Brown Norway rats accumulated more copper in the liver, although the copper concentration was the same in both strains 1 hr after injection of all doses. Fischer rats accumulated proportionally more copper in lysosomal and nuclear mitochondrial fractions whereas Brown Norway rats accumulated proportionally more copper in the cytosol. Gel filtration of liver supernatants revealed that the amount of copper accumulating in the protein presumed to be metallothionein was 2 to 3 times higher in Brown Norway rats, whereas in the Fischer rats more copper eluted in the void volume fraction. We conclude that both biliary copper excretion and copper distribution in the liver are under genetic control. Because of its low copper excretion and reduced binding of copper to metallothionein the Fischer rat, compared to other strains, may be a suitable model for studying the involvement of the liver in copper intoxication.

Effects of dietary copper supplementation of rats on the occurrence of metallothionein-I in liver and its secretion into blood, bile and urine

The appearance and excretion of metallothionein-I (MT-I) was studied in rats given a diet containing 1000 mg of Cu/kg for several weeks. No significant increase in MT-I concentrations in liver, plasma or bile was detected in rats with liver copper concentrations less than 600 micrograms of Cu/g fresh wt. Above this concentration, liver MT-I concentrations increased in proportion to the increase in hepatic copper content. Plasma and bile MT-I concentrations were directly related to those in the liver and were about 10 times those in normal rats. Urinary MT-I concentration also increased 10-fold within 1 week. Fractionation of bile and urine on Sephadex G-50 revealed the presence of monomeric MT-I and a range of possible degradation products of the isoprotein.

Biliary copper excretion in the chicken

A series of experiments were performed to examine the nature of biliary copper excretion in the chicken. Gallbladder and hepatic bile were collected from chickens fed diets that altered copper excretion. Bile was fractionated using gel filtration chromatography and SDS-PAGE. Chicks fed the control diet excreted copper that was bound primarily to a protein aggregate of greater than 600,000 daltons and secondarily, to a 7400 dalton compound. When biliary copper levels were elevated, the distribution of copper associated with the binding compounds was changed. Both the proportion and the absolute amounts of copper in the secondary pool increased dramatically when biliary copper increased. The excretion patterns observed in the control animals are believed to represent the steady-state distribution of copper in bile. A similar distribution was observed with rat bile that was obtained under steady-state conditions. These distribution patterns differ from those reported by other investigators who examined biliary copper excretion in the rat using different experimental conditions.

Studies on the metabolism of rat liver copper-metallothionein

The degradation of purified 35S-labelled rat liver isometallothioneins (MT) by lysosomal extracts was studied. Zn-MT-I was more readily hydrolysed than Zn-MT-II, but no significant degradation of the Cu-containing metallothioneins could be detected, even after 24 h incubation. The susceptibility of MT to degradation in vitro may be related to the strength of the metal-thiolate bonds. However, the turnover rates of cytosolic MT in vivo, as established by pulse-labelling techniques, are apparently subject to different controls. The half-lives of MT-I and -II in the liver cytosol of Cu2+-injected rats were only 15.4 +/- 1.5 and 18.2 +/- 1.1 h respectively. Approx. 25% of the total liver MT was present in particulate fractions (probably in lysosomes) of the liver and had a half-life of 25.1 +/- 4.1 h.