Degradation of 35S-labeled metallothionein in the liver and the kidney of brindled mice: model for Menkes' disease

Copper-metallothionein in the kidney of macular mice: a model for Menkes disease

Menkes disease is an X-linked disorder of copper metabolism. Excess amounts of copper in the kidney of Macular mice, a model for this disease, were found as copper-metallothionein (Cu-MT) from kidney of the mice. Histochemical studies of Cu-MT based on its autofluorescent emission properties showed that the protein was predominant in the proximal convoluted tubule (PCT) cells of the cortex. PCT cells are known to be the primary site of the nephrotoxicity caused by heavy metals. MT mRNA was also observed in the cortex, indicating that the protein was biosynthesized in this region. On the basis of these results, we suggest that biosynthesis and degradation of Cu-MT occur repeatedly in the PCT cells of the cortex. We also compared the histochemical localization of Cu-MT in Macular mice and Long-Evans cinnamon rats, a model for Wilson's disease. The significance of this comparison is discussed.

Developmental variation in copper, zinc and metallothionein mRNA in brindled mutant and nutritionally copper deficient mice

The concentrations of copper, zinc and metallothionein-I (MT-I) mRNA were determined in the liver, kidney and brain of the brindled mutant mouse from birth until the time of death. Despite accumulation of copper in the kidney of the mutant, MT-I mRNA concentrations were normal. There was no difference between the MT-I mRNA in the brain of mutant and normal in the first 10 days of life, but after day 10 metallothionein mRNA levels were increased in the mutant. The concentration of copper was very low in the liver of the mutant, and on day 6 after birth the metallothionein mRNA was also reduced by about 50%. This reduction was not seen in copper-deficient 6-day-old pups, despite very low hepatic copper levels. This suggests that the lower hepatic MT-I mRNA in the day 6 brindled mouse was not simply due to the reduction in hepatic copper and also that hepatic copper is not regulating metallothionein gene expression the liver of neonatal mice. After day 12 hepatic MT-I mRNA levels were elevated in mutant and in copper deficient mice, both of which die at 14 to 16 days. These increases and the increase in brain MT-I mRNA in older mutant mice are likely to be caused by stress. Overall the results support the conclusions that the brindled mutation does not cause a constitutive activation of the metallothionein genes, and that the differences in metallothionein mRNA between mutant and normal are most probably secondary consequences of the mutation.

Copper-induced toxicity in macular mutant mouse: an animal model for Menkes' kinky-hair disease

These studies were designed to determine if macular mutant mouse, which is a proposed animal model of Menkes' kinky-hair disease, is sensitive to the acute toxic effect of Cu as compared to normal and heterozygote mice. Single sc injection of Cu were administered to 6- to 8-day-old mice, and mortalities were recorded for 30 days. The copper treatment at high doses (12 to 25 mg Cu/kg) was very toxic to mutant mice as compared to normal mice, and almost all mutant mice died within 10 days after injection. The effect of Cu toxicity on heterozygote mice was intermediate. The LD50 values 3 days after injection of Cu were 29.5 mg Cu/kg for normal mice, 23.5 mg Cu/kg for heterozygote mice, and 15.5 mg Cu/kg for mutant mice. In Cu-injected mutant mice (11 and 18 mg Cu/kg), significant elevations in serum aspartate aminotransferase and lactate dehydrogenase activity occurred as compared to Cu-injected normal and heterozygote mice. However, no significant elevations in serum creatinine and urea nitrogen contents in Cu-injected mutant were observed as compared to normal and heterozygote mouse. No significant differences in hepatic metallothionein(MT) and MT-1 mRNA, and serum ceruloplasmin oxidase activity levels were observed between Cu-injected normal and mutant mouse. These results indicated that macular mutant mice was sensitive to the acute toxic or hepatotoxic effects of Cu as compared to normal and heterozygote mice.

Roles of synthesis and degradation in the regulation of metallothionein accretion in a chicken macrophage-cell line

Metallothionein (MT) is a metal-binding protein rapidly accreted in many tissues in response to trace elements or hormones. To gain an understanding of the regulation of MT accretion, rates of MT synthesis and degradation were determined by using a decay-kinetics technique. A chicken macrophage-cell line (HD11) that rapidly accretes incremental amounts of MT when stimulated with increasing concentrations of Zn2+ or Cd2+ was studied. The maximum rate of MT accretion occurred at 50 microM-Zn2+ or 20 microM-Cd2+. The absolute rate of MT accretion was less in macrophages incubated with 25 microM- as compared with 50 microM-Zn2+, owing to decreased and increased rates of MT synthesis and degradation respectively. The absolute rate of MT accretion was less in macrophages incubated with 10 microM- as compared with 20 microM-Cd2+, owing to a decreased rate of MT synthesis with no change in degradation. Compared with macrophages continually incubated with 50 microM-Zn2+, removal of Zn2+ from medium previously containing 50 microM-Zn2+ decreased the absolute rate of MT accretion, owing to decreased and increased rates of MT synthesis and degradation respectively. Removal of Cd2+ from medium previously containing 20 microM-Cd2+ also decreased the absolute rate of MT accretion in macrophages. Unlike Zn2+ removal, the decrease in MT accretion was due to a decreased rate of MT synthesis with no change in degradation. When macrophages incubated with 50 microM-Zn2+ were subsequently incubated with 20 microM-Cd2+, rates of MT synthesis and accretion were decreased as compared with cells continually incubated with 50 microM-Zn2+ or 20 microM-Cd2+. When macrophages incubated with 20 microM-Cd2+ were subsequently incubated with 50 microM-Zn2+, rates of MT synthesis and accretion were increased as compared with cells continually incubated with 50 microM-Zn2+ or 20 microM-Cd2+. Switching the metal in the incubation medium did not influence the rate of MT degradation. Our results indicate that the rate of MT accretion is determined by variations in the rates of MT synthesis and degradation, depending upon the inducing metal and the concentration of the metal.

Molecular aspects, physiological function, and clinical significance of metallothioneins

Metallothioneins (MTs) are well-characterized low molecular weight, heat-stable cytosolic proteins with exceptional high content of cysteinyl sulfur and are known to bind heavy metals like cadmium (Cd), zinc (Zn), and copper (Cu). Since these proteins are induced on exposure to heavy metals, it is now accepted that they have a detoxifying role during heavy metal toxicity. It has also been suggested that the primary function of Mt is in the homeostasis of the essential metals Zn and Cu. Recently, a role MT in selenium metabolism in primates has been established. Further, MT has gained considerable importance in the clinical disorders related to trace metal metabolism.

Induction and degradation of Zn-, Cu- and Cd-thionein in Chang liver cells

Human liver cells (Chang liver) were exposed to 5 micrograms Zn, 2.5 micrograms Cu or 1 microgram Cd/ml in cultured medium. These exogeneous heavy metals were accumulated by the cells and induced de novo synthesis of metallothionein after a 3-h incubation period. The production of Zn-, Cu- or Cd-thionein started in the cells with accumulation of 1 nmol Zn, 0.3 nmol Cu and 0.1 nmol Cd/mg cytosol protein and subsequently the amounts of metal-binding thioneins increased in agreement with the relative amount of metal accumulated in the cytosol over a 24-h period. When cells containing Zn- or Cu-thionein were placed in metal free medium, 70% or 25% of the zinc or copper bound to each original metallothionein was released after 3 h; bound metals decreased to 85% and 65% respectively after 24 h. The disappearance of metal from metallothionein correlated with increases of metal in the medium. On the other hand, 35S-counts incorporated into Zn- and Cu-thionein decreased only to 40% and 15% of the levels in the original metallothionein after 3 h; 35S-counts decreased to 65% and 45%, respectively, after 24 h, indicating that metals bound to metallothionein decreased more quickly than 35S-counts. These results suggest that metals were released from metallothionein and were excreted into the medium. However, 35S- and 109Cd-counts in Cd-thionein changed very little, if at all, in the cells even after a 24-h incubation period. Our data strongly suggest that Zn- and Cu-thionein are degraded in the cells, but that Cd-thionein remains longer than either Zn- or Cu-thionein. When cells containing Zn-thionein were incubated in metal-free medium, Zn-thionein was digested in the cells and peptide fragments ranging about 200-400 daltons were excreted from the cells.

Metallothionein and the development of the mottled disorder in the mouse

Copper accumulates in kidney tissue of mottled (Mo) mice largely in association with a low MW cytosol protein, and the reduced copper levels in neonatal mutant liver are largely the result of a reduction in the amount of copper associated with this same protein. On the basis of ion-exchange chromatographic profile, heat stability, absence of a 280nm absorption peak, and the binding of Cd109 and Zn65 the protein mutants in the kidney is identified as metallothionein (MT). Amino acid analysis, however, failed to confirm this, and it is suggested that the high copper content of the mutant protein results in its oxidative degradation during purification, even when normal anaerobic precautions are taken. Estimates of thionein protein content of tissues from mutant and normal mice demonstrated that the levels are significantly elevated in both young and adult mutant kidney and depressed in young mutant liver, in parallel therefore with the changes in tissue copper levels. In adult mutant liver tissue, however, thionein levels are significantly raised, even though tissue copper content is normal. The synthesis and degradation of MT was examined in some detail. Incorporation of S35-cysteine in kidney MT was significantly raised in both young and adult mutant mice, while in adult tissue the rate of degradation of MT was significantly depressed. The elevated kidney MT levels arise therefore in young mutant mice from an increased rate of synthesis and in adult mice from the combined effects of increased synthesis and reduced degradation.

Hepatic metallothionein synthesis in neonatal Mottled-Brindled mutant mice

Mottled-brindled mutant mice did not display the elevated hepatic metallothionein synthesis normally observed in 2- to 6-day-old wild-type mice. This difference between normal and mutant mice was not due to a decreased ability to synthesize metallothionein in the liver, since hepatic metallothionein synthesis was inducible in response to copper, cadmium, zinc, or hydrocortisone administration to neonatal mutant mice. Hydrocortisone treatment resulted in increased metallothionein synthesis in liver of mutant mice but had no ameliorative effect on the mottled-brindled disease.

Synthesis and degradation of hepatic metallothionein in mice differing in susceptibility to cadmium mortality

We have confirmed the observations of Tsunoo et al. (Toxicol. Lett. 4:253, 1979) that (a) DBA/2 mice are resistant to cadmium mortality than C3H mice and (b) DBA/2 mice accumulate more 109Cd into hepatic metallothionein than do C3H mice in response to an injection of 30 mumol CdCl2/kg, a dose of CdCl2 which is lethal to C3H mice. We now report, using a nonlethal dose of 8 mumol CdCl2/kg, that the rates of both the synthesis and the degradation of cadmium-induced hepatic metallothionein are increased in C3H mice. The rate of metallothionein synthesis, measured 6 hr after cadmium administration and expressed as the percentage of injected [35S]cysteine incorporated into metallothionein/g liver, was 0.33 +/- 0.04% (SD) in C3H mice, compared to 0.19 +/- 0.06% in DBA/2 mice (significantly different rates by Students' t test. P less than 0.01). Also, at this dose, hepatic 35S-labeled metallothionein was degraded with a half-life of 22.5 +/- 0.7 hr in C3H mice, compared to 30.1 +/- 2.5 hr in DBA/2 mice (significantly different half-lives by F test, within 95% confidence limits). The increased accumulation of metallothionein in resistant DBA/2 mice compared to sensitive C3H mice after cadmium exposure appears to be due primarily to a difference in metallothionein degradation, rather than metallothionein synthesis.

Mutations in humans and animals which affect copper metabolism

Various inherited disorders of copper metabolism in man and animals are reviewed. Emphasis is placed on the use of cultured cells from mutants to determine the primary molecular defects and to acquire basic knowledge of normal copper metabolism. This allows better diagnostic tests and possible treatment of the disorders. Menkes' disease in humans and mottled mouse mutants are discussed in detail, as they illustrate these approaches.