Effect of maternal zinc supply on blood and tissue metallothionein I concentrations in suckling rats

The zinc/thiolate redox biochemistry of metallothionein and the control of zinc ion fluctuations in cell signaling

Free zinc ions are potent effectors of proteins. Their tightly controlled fluctuations ("zinc signals") in the picomolar range of concentrations modulate cellular signaling pathways. Sulfur (cysteine) donors generate redox-active coordination environments in proteins for the redox-inert zinc ion and make it possible for redox signals to induce zinc signals. Amplitudes of zinc signals are determined by the cellular zinc buffering capacity, which itself is redox-sensitive. In part by interfering with zinc and redox buffering, reactive species, drugs, toxins, and metal ions can elicit zinc signals that initiate physiological and pathobiochemical changes or lead to cellular injury when free zinc ions are sustained at higher concentrations. These interactions establish redox-inert zinc as an important factor in redox signaling. At the center of zinc/redox signaling are the zinc/thiolate clusters of metallothionein. They can transduce zinc and redox signals and thereby attenuate or amplify these signals.

Lipid raft-dependent endocytosis of metallothionein in HepG2 cells

Human hepatocellular carcinoma (HepG2) cells take up metallothionein (MT) by endocytosis. MT co-localizes with albumin but not with transferrin, indicating uptake via a non-classical pathway rather than via clathrin-mediated endocytosis. A lipid raft-dependent uptake is indicated by pravastatin inhibition of cholesterol synthesis and methyl-beta-cyclodextrin inhibition of cholesterol translocation to the plasma membrane, reducing MT uptake by 29% and 69%, respectively. Subcellular fractionation after MT uptake reveals significant amounts of MT in vesicular fractions including lysosomes but virtually no MT in the cytosol. Metals bound to MT are released into the cytosol, however. The findings define a pathway for cellular metal acquisition. Together with results from other studies demonstrating secretion of MT from different cells and the presence of MT in extracellular fluids, the results suggest a function of MT in intercellular communication.

Metallothionein is part of a zinc-scavenging mechanism for cell survival under conditions of extreme zinc deprivation

Metallothionein (MT) is a small cysteine-rich protein thought to play a critical role in cellular detoxification of inorganic species by sequestering metal ions that are present in elevated concentrations. We demonstrate here that metallothionein can play an important role at the other end of the homeostatic spectrum by scavenging an essential metal in a mouse fibroblast cell line that has been cultured under conditions of extreme zinc deprivation (LZA-LTK-). These cells unexpectedly produce constitutively high levels of metallothionein mRNA; however, the MT protein accumulates only when high concentrations of zinc are provided in the media. Until this MT pool is saturated, no measurable zinc remains in the external media. In this case, zinc deprivation leads to amplification of the MT gene locus in the LZA-LTK- cell line. Furthermore, the intracellular zinc levels in the fully adapted cells remain at the normal level of 0.4 fmol zinc/cell, even when extracellular zinc concentration is decreased by 2 orders of magnitude relative to normal media.

Zinc intake, zinc status and growth in a longitudinal study of healthy Danish infants

Mild, growth-limiting zinc deficiency might be prevalent in otherwise healthy infants according to recent studies. We examined zinc intake and status in 91 healthy term infants from birth to 12 months, as part of the Copenhagen Cohort Study on Infant Nutrition and Growth. Zinc intake was recorded monthly and the amount of zinc absorbed was estimated. These estimates were below recently published FAO/WHO/IAEA values for basal requirements in 68%, 62% and 14% of the infants at 2, 4 and 9 months of age, respectively. Serum zinc decreased significantly (p < 0.01) from 10.6 mumol/l at 6 months to 8.4 mumol/l at 9 months of age (normal range 10-18 mumol/l). Erythrocyte metallothionein values, a tentative indicator of long-term zinc status, decreased significantly from 2 to 6 months (p < 0.001) and from 6 to 9 months (p < 0.01). Serum zinc at 9 months was positively associated with growth velocity during the period from 6 to 9 months (weight: p = 0.05; knee-heel length: p = 0.002). The results provide descriptive data on zinc intake and zinc status in healthy Danish infants. Although some of our data suggest suboptimal zinc status during late infancy, evidence for this can only be obtained through a randomized intervention study.

Chronic stress reduces serum but not liver metallothionein response to acute stress

Rats subjected to chronic immobilization stress showed a reduced serum metallothionein (MT) response to acute immobilization stress compared to nonchronically stressed rats. In contrast, liver MT response to acute immobilization stress was not influenced by previous chronic immobilization stress. These results suggest that serum MT levels are likely under endocrine regulation and that they do not reflect directly liver MT levels. Instead it appears that both MT pools are regulated differently. The fact that liver MT is resistant to adaptation to chronic stress may be related to its physiological function.

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 response to stress in rats: role in free radical scavenging

The possibility that liver metallothionein (MT) can function as an antioxidant in vivo has been studied in the rat. It was found that the stress of food and water deprivation with or without physical immobilization consistently increased liver lipid peroxidation (LLP), suggesting that liver MT induction by stress might be related to the stress-induced LLP. This was supported by results with the lipid peroxidation promoter dimethyl sulfoxide (DMSO) and the natural antioxidant vitamin E. Whereas DMSO administration increased LLP levels in basal and stress situations, vitamin E decreased them. Liver MT levels were increased by DMSO in basal and stress situations, whereas they were decreased by vitamin E during stress. These in vivo results are consistent with an antioxidant role of liver MT suggested by previous in vitro results. However, liver MT preinduction by Zn treatment did not result in a lower MT response to stress. Instead a positive synergistic effect between Zn and stress appeared to be present. This result indicates that the mechanism of action of MT as antioxidant remains unclear.