Basal metallothionein in tumors: widespread presence of apoprotein

Metallothionein isoform 3 overexpression is associated with breast cancers having a poor prognosis

The third isoform (MT-3) of the metallothionein gene family is unique in that it has a limited tissue distribution, is not induced by metals, has a neuronal growth inhibitory activity, and sequesters zinc more effectively under zinc-depleted conditions. The goal of the present study was to determine whether MT-3 was absent in normal breast tissue, was overexpressed in breast cancers, and if MT-3 overexpression would be associated with disease outcome. A combination of immunohistochemistry and reverse-transcription polymerase chain reaction was used to demonstrate that the normal breast had no detectable expression of MT-3 mRNA or protein. Using immunohistochemistry, it was shown that MT-3 was overexpressed in 25 of 34 cases of breast cancer. In all cases of positive staining, MT-3 was diffusely localized to the cytoplasm. The tumors from these 34 cases were divided as to outcome based on known 5-year survival, with 20 patients being disease free at 5 years (good outcome) and the other 14 having recurring disease within 5 years (bad outcome). When analyzed for MT-3 staining, it was shown that there was a trend for increased MT-3 immunoreactivity in the group having bad outcomes. However, when the tumor subgrouping was further defined on the basis of carcinoma in situ (CIS), there was a marked significant difference in MT-3 staining between patients with good and bad outcomes. Limited to DCIS, MT-3 staining was significantly increased in patients with bad outcomes compared to those with good outcomes. Thus, these studies demonstrate that MT-3 is overexpressed in selected breast cancers and that overexpression is associated with tumors having a poor prognosis.

Differential fluorescence labeling of cysteinyl clusters uncovers high tissue levels of thionein

The isolation of thionein (T) from tissues has not been reported heretofore. T contains 20 cysteinyl residues that react with 7-fluorobenz-2-oxa-1,3-diazole-4-sulfonamide to form fluorescent adducts. In metallothionein (MT) the cysteinyl residues, which are bound to zinc, do not react. However, they do react in the presence of a chelating agent such as EDTA. The resultant difference in chemical reactivity provides a means to measure T in the absence of EDTA, (MT + T) in its presence, and, of course, MT by difference. The 7-fluorobenz-2-oxa-1,3-diazole-4-sulfonamide derivative of T can be isolated from tissue homogenates by HPLC and quantified fluorimetrically with a detection limit in the femtomolar range and a linear response over 3 orders of magnitude. Analysis of liver, kidney, and brain of rats reveals almost as much T as MT. Moreover, in contrast to earlier views, MT in tissue extracts appears to be less stable than T. The existence of T in tissues under normal physiological conditions has important implications for its function both in zinc metabolism and the redox balance of the cell.

Tolerance to cadmium hepatotoxicity by metallothionein and zinc: in vivo and in vitro studies with MT-null mice

The protective role of metallothionein (MT) in Cd-mediated hepatotoxicity was investigated in vivo and in vitro. Following injection of Cd (2 mg/kg, intraperitoneal or subcutaneous) hepatoxicity was significantly greater at 20 h in metallothionein-null (MT-/-) mice, compared with controls (MT+/+). The decrease in the blood and liver glucose concentrations correlated with the extent of hepatotoxicity, with blood glucose 43% lower in MT-/- mice. Zinc (50 microM) and/or Dex (1 microM) were used in hepatocyte cultures to raise MT 2-5-fold. When Cd at 10 microM was co-treated with Zn and/or Dex, lactate dehydrogenase (LD) leakage in the MT+/+ and MT-/- hepatocytes was reduced only when Zn was present. Cellular glutathione (GSH) was the same in control MT+/+ and MT-/- cultures and was uninfluenced by Zn and Dex. After treatment with 5 and 10 microM Cd, GSH levels were lower in MT-/- than MT+/+ hepatocytes in the control and Dex groups. Higher GSH concentrations were maintained in Zn co-treated cultures from both genotypes, indicating that the superior protective effect of Zn may in part derive from its influence on cellular GSH. Pre-treatment with Zn and/or Dex provided no further protection than co-treatment. Tolerance to brief (15 min) Cd exposure was also investigated in the presence of MT inducers including progesterone (100 microM). Zn, Dex and progesterone treated hepatocytes had less LD leakage than controls with Zn giving the greatest protection (LD leakage 18% of controls at 100 microM Cd). Zn pre-treated cells had higher cytosolic/particulate ratios of Cd. These findings demonstrate that MT protects primary cultures of mouse hepatocytes from short-term exposure to Cd. Zn enhances the protection through MT and non-MT mechanisms.

Metallothioneins in human kidneys and associated tumors

Human kidneys and their associated tumors (nonneoplastic kidney tissues from patients with a transitional cell carcinoma or an adenocarcinoma and the adenocarcinomas themselves) were evaluated for their Zn, Cd, and Cu contents as well as for their metallothionein (MT) level. The total Cd content was correlated with the MT content, and both values were significantly decreased in the adenocarcinomas in comparison with the other tissues. After extraction and separation by anion-exchange chromatography, MT-0 was identified in the nonneoplastic tissues from both the adenocarcinomas as well as the transitional cell carcinomas. Since until now MT-0 protein was only found in human fetal liver and in Zn-stimulated human monocytes, a possible role for this isoform as an oncofetal marker is hypothesized. Separation of the isoforms of MT by reversed-phase high-performance liquid chromatography and sequence analysis showed besides MT-1e and MT-1l the isoform-MT-1g, which is not expressed in the healthy kidney, and MT-1k, an isoform which is not yet demonstrated in renal tissues. We conclude that the expression profile of the MT isoforms in the kidney changes due to the presence of a tumor.

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.

Modulation of DNA binding of a tramtrack zinc finger peptide by the metallothionein-thionein conjugate pair

The ability of metallothionein (MT) to modulate DNA binding by a two-finger peptide of Tramtrack (TTK), a CCHH zinc transcription factor, was investigated using metal-bound and metal-deficient forms of rabbit MT-2 and the TTK peptide. Thionein inhibited DNA binding by zinc-bound TTK, and Zn-MT restored DNA-binding by zinc-deficient apo-TTK. "Free" zinc at low concentrations was as effective as Zn-MT in restoring DNA binding by apopeptide but was inhibitory at concentrations equal to zinc bound to 2 mol eq and higher of Zn-MT. Substitution of cadmium for zinc reduced the affinity of the peptide for its DNA binding site. This effect was reversed by incubation with Zn-MT. The circular dichroic spectra of the TTK peptide indicated that zinc removal resulted in loss of alpha-helical structures, which are sites of DNA contact points. Reconstitution with cadmium resulted in stoichiometric substitution of 2 mol of Cd/mol of peptide but not recovery of alpha-helical structures. Incubation of Cd-TTK with Zn-MT restored the secondary structure expected for zinc-bound TTK. The ability of Zn-MT and thionein to restore or inhibit DNA-binding by TTK was associated with effects on the metallation status of the peptide and related alterations in its secondary structure.

Control of zinc transfer between thionein, metallothionein, and zinc proteins

Metallothionein (MT), despite its high metal binding constant (KZn = 3.2 x 10(13) M-1 at pH 7.4), can transfer zinc to the apoforms of zinc enzymes that have inherently lower stability constants. To gain insight into this paradox, we have studied zinc transfer between zinc enzymes and MT. Zinc can be transferred in both directions-i.e., from the enzymes to thionein (the apoform of MT) and from MT to the apoenzymes. Agents that mediate or enhance zinc transfer have been identified that provide kinetic pathways in either direction. MT does not transfer all of its seven zinc atoms to an apoenzyme, but apparently contains at least one that is more prone to transfer than the others. Modification of thiol ligands in MT zinc clusters increases the total number of zinc ions released and, hence, the extent of transfer. Aside from disulfide reagents, we show that selenium compounds are potential cellular enhancers of zinc transfer from MT to apoenzymes. Zinc transfer from zinc enzymes to thionein, on the other hand, is mediated by zinc-chelating agents such as Tris buffer, citrate, or glutathione. Redox agents are asymmetrically involved in both directions of zinc transfer. For example, reduced glutathione mediates zinc transfer from enzymes to thionein, whereas glutathione disulfide oxidizes MT with enhanced release of zinc and transfer of zinc to apoenzymes. Therefore, the cellular redox state as well as the concentration of other biological chelating agents might well determine the direction of zinc transfer and ultimately affect zinc distribution.

Metallothionein-induced increase in mitochondrial inner membrane permeability

OBJECTIVE

To elucidate the effect of metallothionein on the permeability of the mitochondrial inner membrane.

BACKGROUND

Metallothionein (MT) is a 6-7-kDa protein which is rapidly produced by stressed cells. MT is induced by cytokines and other factors thought to mediate the stress response. The organelle with the highest concentration of MT is the mitochondrion. Therefore we studied the effect of MT on mitochondrial function. We chose to study the effect of MT on mitochondrial inner membrane permeability because of the role of this function in numerous cellular processes.

METHODS

Mitochondria were isolated from livers of Sprague-Dawley rats by differential centrifugation and suspended in sucrose-containing buffer. Changes in mitochondrial inner membrane permeability were monitored by following the change in absorbance at 540 nm. All experiments were of a paired design.

RESULTS

We found that an increase in inner membrane permeability was induced by physiological metallothionein 1 (MT1) concentrations between 6 and 50 microM. There was no increase in the effect beyond 50 microM. The metals of MT1, zinc, and cadmium alone had no effect at physiological concentrations. The action of MT1 was inhibited by the aliphatic polyamine, spermine, as well as magnesium both at physiological concentrations. Spermine was effective whether added before or after MT1. Metallothionein 2 of different Zn2+ and Cd2+ compositions induced different kinetics of pore opening.

CONCLUSIONS

These experiments reveal the possibility that the permeability of the mitochondrial inner membrane is regulated by relative concentrations of MT, spermine, and magnesium. The metal composition of MT could also play a role in this regulation.

Clonal overexpression of metallothionein is induced by somatic mutation in morphologically normal colonic mucosa

Metallothionein (MT) overexpression occurs frequently in human tumours but the underlying mechanism is unknown. Morphologically normal-appearing mucosa from human colorectal carcinoma resection specimens and of the colons of ageing laboratory mice contains scattered single crypts whose cells show uniformly increased MT immunostaining, suggesting that MT overexpression arises directly from random crypt stem cell somatic mutation, followed by colonization of the clonal unit by the mutated progeny. This hypothesis has now been tested by quantifying the frequency of immunocytochemically detectable monocryptal colorectal MT overexpression, 5 and 30 days after injection of 8-week-old mice with a single dose of the mutagen dimethylhydrazine (DMH, 30 mg/kg subcutaneous). Otherwise normal-appearing MT-positive crypts were recorded as either wholly or partially involved by the overexpressing phenotype. Five days after DMH injection, the median frequency of partially involved MT-positive crypts was 11.7 x 10(-4), declining significantly to 1.8 x 10(-4) at 30 days (Mann-Whitney U, P < 0.01). In contrast, the median frequency of wholly involved crypts was 0.2 x 10(-4) at 5 days, increasing significantly (P < 0.005) to 12.9 x 10(-4) at 30 days. The frequency of MT-positive crypts and the time course of evolution of partially involved to wholly involved forms were similar to those described for mutation-induced crypt-restricted loss of glucose-6-phosphate dehydrogenase activity in mice treated with an identical DMH regimen. The findings indicate that cellular MT overexpression can occur as a direct consequence of somatic mutation, either cis-activating mutation(s) of the MT gene itself, or trans-activating mutation(s) of other genes involved in controlling MT expression. This is likely to be an important mechanism underlying MT overexpression in neoplasia. Such mutation-induced aberrant MT expression may be involved in the acquisition of selective cellular growth of survival advantage during tumour progression.

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

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