Targeting and germ-line transmission of a null mutation at the metallothionein I and II loci in mouse

Protection against carbon tetrachloride hepatotoxicity by oleanolic acid is not mediated through metallothionein

Oleanolic acid is a triterpenoid compound that has been shown to protect against liver injury produced by some hepatotoxicants. This study was designed to characterize the protective effects of oleanolic acid on carbon tetrachloride-induced hepatotoxicity, and the role of metallothionein in the protection. Oleanolic acid pretreatment (100-400 micromol/kg, s.c.) protected Sprague-Dawley rats and mice from carbon tetrachloride-induced liver injury in a dose- and time-dependent manner, as evidenced by serum alanine aminotransferase and sorbitol dehydrogenase activities, as well as by histopathology. The protection against carbon tetrachloride hepatotoxicity was not evident until animals were pretreated with oleanolic acid 12 h, and lasted for 72 h after a single injection. This suggests that the protection might be due to induction of some adaptive mechanisms. Metallothionein (MT), an acute-phase protein proposed to decrease carbon tetrachloride-induced liver injury, was dramatically induced following oleanolic acid treatment. To examine whether oleanolic acid protection is mediated through MT, MT-I and II knock-out (MT-null) mice were utilized. Oleanolic acid pretreatment increased MT levels in control mice (20-fold), but not in MT-null mice, however, it protected equally against carbon tetrachloride-induced hepatotoxicity in both control and MT-null mice. These data indicate that oleanolic acid is effective in protecting rats and mice from the hepatotoxicity produced by carbon tetrachloride, and the protection is not mediated through induction of MT.

The DNA binding activity of metal response element-binding transcription factor-1 is activated in vivo and in vitro by zinc, but not by other transition metals

We examined the DNA binding activity of mouse and human MTF-1 in whole cell extracts from cells cultured in medium containing zinc or cadmium and from untreated cells after the in vitro addition of zinc or cadmium, as well as using recombinant MTF-1 transcribed and translated in vitro and treated with various transition metals. Incubation of human (HeLa) or mouse (Hepa) cells in medium containing cadmium (5-15 microM) did not lead to a significant increase (<2-fold) in the amount of MTF-1 DNA binding activity, whereas zinc (100 microM) led to a 6-15-fold increase within 1 h. MTF-1 binding activity was low, but detectable, in control whole cell extracts and was increased (>10-fold) after the in vitro addition of zinc (30 microM) and incubation at 37 degrees C for 15 min. In contrast, addition of cadmium (6 or 60 microM) did not activate MTF-1 binding activity. Recombinant mouse and human MTF-1 were also dependent on exogenous zinc for DNA binding activity. Cadmium did not facilitate activation of recombinant MTF-1, but instead inhibited the activation of the recombinant protein by zinc. Interestingly, glutathione (1 mM) protected recombinant MTF-1 from inactivation by cadmium, and allowed for activation by zinc. It was also noted that zinc-activated recombinant MTF-1 was protected from cadmium only when bound to DNA. These results suggest that cadmium interacts with the zinc fingers of MTF-1 and forms an inactive complex. Of the several transition metals (zinc, cadmium, nickel, silver, copper, and cobalt) examined, only zinc facilitated activation of the DNA binding activity of recombinant MTF-1. These data suggest that transition metals, other than zinc, that activate MT gene expression may do so by mechanisms independent of an increase in the DNA binding activity of MTF-1.

Epidermal proliferation of the skin in metallothionein-null mice

Metallothionein (MT) is a low-molecular weight metal-binding protein. Although the physiologic function of MT is not fully known, it is present in various species and various organs including the skin. MT is strongly stained in hyperplastic epidermal tissues in normal skin and in hyperplastic skin lesions, and increased expression of mRNA of the MT gene has been demonstrated in skin stimulated by proliferative agents, suggesting that MT is involved in the proliferation of epidermal keratinocytes. To improve our understanding of the role of MT in epidermal hyperplasia, mice with null mutations in their MT-1 and MT-2 genes were used in this study. We compared the epidermal hyperplasia in MT-null mice and in normal C57BL/6 J mice after treatments with cholera toxin, 12-0-tetradecanoylphorbol-13-acetate, and ultraviolet B irradiation, which stimulate epidermal proliferation. Immunostaining of MT was not detected in the skin of MT-null mice, and these mice developed significantly less epidermal hyperplasia than the normal mice after exposure to each stimulator. We determined the metal contents of skin samples by the proton-induced x-ray emission method. The zinc content of the skin of the MT-null mice was lower than that of the control mice before stimulation. After stimulation of epidermal hyperplasia, MT-null and normal mice showed significantly reduced levels of zinc. These findings indicate that cellular MT is involved in the proliferative process of the epidermis induced by cholera toxin, 12-0-tetradecanoylphorbol-13-acetate, and ultraviolet B light through its regulatory action on the metal metabolism required for cell growth.

Metallothionein (MT)-null mice are sensitive to cisplatin-induced hepatotoxicity

cis-Diamminedichloroplatinum (cisplatin) is an important anticancer drug used to treat solid tumors. The nephrotoxicity of cisplatin is recognized as the most important dose-limiting factor, but high doses of cisplatin also produce hepatotoxicity. However, little is known about cisplatin-induced liver injury and the role of metallothionein, a cysteine-rich, metal-binding protein, in modulating its hepatotoxicity. This study was designed to examine cisplatin hepatotoxicity in control and metallothionein-I/II knockout (MT-null) mice. Animals were given a single injection of cisplatin (50-200 mumol/kg i.p.), and liver injury was evaluated 3-16 h later. Cisplatin produced dose- and time-dependent liver injury, as evidenced by increased serum activity of alanine aminotransferase (ALT), as well as by histopathology. Apoptosis, rather than necrosis, predominates in cisplatin-induced liver injury, as indicated by increased numbers of apoptotic cells (hematoxylin and eosin staining), in situ apoptotic DNA detection, and DNA fragmentation on agarose gel electrophoresis. MT-null mice were more sensitive than controls to cisplatin-induced hepatotoxicity. Cisplatin (200 mumol/kg) was lethal to 12% of control mice, but 60% of MT-null mice died within 16 h. At the dose of 150 mumol/kg, serum ALT activities were increased 2-fold in control mice compared to 6.5-fold in MT-null mice. Apoptotic lesions were more pronounced in MT-null than in control mice. MT-null mice were also more susceptible than controls to cisplatin-induced nephrotoxicity, as evidenced by having higher blood urea nitrogen concentrations. Furthermore, cultured MT-null hepatocytes were more sensitive than control cells to the cytotoxicity of cisplatin (50-200 microM), as indicated by lactate dehydrogenase leakage into the medium. These results demonstrate that (1) high doses of cisplatin produce hepatotoxicity, with apoptosis as the major lesion, and (2) MT protects against cisplatin-induced liver injury.

Paracetamol hepatotoxicity in metallothionein-null mice

The role of metallothionein (MT) in protecting the liver against paracetamol (PCT) toxicity was investigated in vivo and in vitro in mice lacking expression of MT-1 and MT-2 genes (MT -/-). In the fed, glycogen replete state, hepatotoxicity (PCT 300 mg/kg i.p.) at 6 h was significantly greater in MT -/- than MT +/+ mice. Plasma lactate dehydrogenase (LD) and alanine aminotransferase (ALT) were 5- and 13-fold greater respectively than in MT +/+ mice. Liver glycogen, glucose and zinc levels were significantly lower in MT -/- mice at this time. In contrast, hepatotoxicity (PCT 135 mg/kg i.p.) at 6 h was similar in both MT +/+ and MT -/- mice fasted 24 h, despite a doubling in liver MT in MT +/+ mice. No differences were found between MT -/- and MT +/+ mice in cytochrome P450 activity. Liver glutathione levels were the same in both groups of mice prior to fasting and were decreased to a similar extent (55-65%) following PCT treatment. Investigation of lower PCT doses (< or = 120 mg/kg) in fasted mice over 24 h demonstrated a greater susceptibility in female MT -/- mice with plasma LD, 2.4-fold and ALT, 7.5-fold greater than in MT +/+ mice at 120 mg/kg PCT. In male MT -/- mice, there was only a trend towards greater susceptibility at 110 mg/kg PCT compared to male MT +/+ mice, and at 120 mg/kg, both male genotypes were equally affected. Investigations with cultured hepatocytes supported the in vivo findings in that there was a trend towards greater toxicity (PCT at 1 and 5 mM for 24 h) in hepatocytes from fed MT -/- mice, with the difference diminished in association with greater hepatotoxicity in hepatocytes from fasted mice. Use of dexamethasone (Dex) to increase MT in the MT +/+ mouse hepatocytes protected from PCT toxicity. Zn alone was not protective. Zn plus Dex offered no protection despite higher MT levels. Generation of apo-MT with Dex may offer more protection than Zn-MT. In conclusion, MT -/- mice were more susceptible than MT +/+ mice to PCT toxicity in the fed state, but the increased susceptibility was much smaller, but still significant, when the effects of glycogen were minimised by fasting.

Induction of metallothionein as an adaptive mechanism affecting the magnitude and progression of toxicological injury

Pretreatment of rats with low doses of Cd produces adaptive tolerance to a subsequent high dose of Cd-induced lethality, thus shifting the dose-response curve to the right. Cd pretreatment of animals also protects against the hepatotoxicity produced by high doses of Cd. This protection is attributable to the 10- to 50-fold induction of hepatic metallothionein (MT) by Cd pretreatment. As a result hepatic subcellular distribution of Cd is significantly altered, with more Cd bound to MT in the cytosol and a concomitant reduction of Cd in other critical organelles. In addition MT-transgenic animals are more resistant, whereas MT-null mice are more sensitive than controls to Cd-induced lethality and hepatotoxicity. This further demonstrates that MT is important in Cd detoxication. Induction of hepatic MT by zinc also protects mice from carbon tetrachloride (CCl4)-induced liver injury, with more 14C-CCl4 bound to MT in the cytosol. MT-null mice are more sensitive to CCl4-induced hepatotoxicity, which supports the hypothesis that induction of MT also plays a protective role for nonmetallic chemicals. These results indicate that MT is a part of cellular adaptive mechanisms affecting the magnitude and progression of toxic insults from metals such as Cd as well as from organic chemicals such as CCl4.

Obesity and hyperleptinemia in metallothionein (-I and -II) null mice

Metallothionein (MT) has several putative roles in metal detoxification, in Zn and Cu homeostasis, in scavenging free radicals, and in the acute phase response. Mice of mixed 129/Ola and C57BL/6J background with targeted disruption of MT-I and MT-II genes are more sensitive to toxic metals and oxidative stress. We noted that these animals were larger than most strains of mice, and we systematically studied aspects of their physiology and biochemistry relating to energy metabolism. During the first 2 weeks after weaning, the growth rates of MT-null and C57BL/6J mice were similar, but the transgenic mice became significantly heavier at age 5-6 weeks. At age 14 weeks, the body weight and food intake of MT-null mice was 16 and 30% higher, respectively, compared with C57BL/6J mice. Most 22- to 39-week-old male MT-null mice were obese, as shown by increased fat accretion, elevated obese (ob) gene expression, and high plasma leptin levels, similar to those recorded in Zucker fatty (fa/fa) rats. Seven-week-old MT-null mice also had significantly higher levels of plasma leptin and elevated expression of ob, lipoprotein lipase, and CCAAT enhancer binding protein alpha genes as compared with age-matched C57BL/6J mice. These observations indicate that abnormal accretion of body fat and adipocyte maturation is initiated at 5-7 weeks of age, possibly coincident with sexual maturation. Targeted disruption of MT-I and MT-II genes seems to induce moderate obesity, providing a new obese animal model. A link between MT and the regulation of energy balance is implied.

Cloning of a complementary DNA encoding an Ambystoma mexicanum metallothionein, AmMT, and expression of the gene during early development

We have used a polymerase chain reaction strategy to isolate a metallothionein (MT) cDNA from the amphibian Ambystoma mexicanum (axolotl). This cDNA is 875-bp long and encodes a 60 amino acid protein, AmMT, typical for family 1 MTs. It contains 20 cysteine (Cys) residues that can be aligned with those of other vertebrate MTs. The overall structure of the protein is unique among vertebrates in having only two amino acid residues before the first Cys at the amino-terminal end. Northern analyses showed that AmMT is expressed throughout embryogenesis, giving rise to three mRNA species of 650, 750, and 1,600 nucleotides (nt). The 750 and 1,600 nt transcripts appear to result from differential use of polyadenylation signals, whereas the 650 nt RNA could arise from deadenylation of the 750-nt transcript. Both the 750- and 1,600-nt RNAs were presented in embryos before the mid-blastula transition (MBT). After the MBT, the 750-nt RNA was replaced by the 650-nt RNA which was gradually degraded to undetectable levels in post-neurulation embryos. Levels of the 1,600-nt transcript increased at gastrulation and reach a maximum in Stage 30 embryos. In adult animals, levels of the 750-nt RNA were high in liver and testes, and very low in lung, gut, skin, and oviducts, whereas levels of the 1,600-nt transcript were similar and moderately elevated in all tissues examined. In contrast, in Xenopus laevis, Northern analysis did not detect XIMT-A mRNA in embryos before late neurulation (Stage 24). XIMT-A mRNA levels then increased sharply in Stage 36 hatched embryos at levels similar to those found in adult livers. These results show that AmMT presents a unique expression pattern among metazoans being transcribed as two transcripts differing in the length of their 3' untranslated regions, the levels of which vary during embryogenesis and in adult tissues.

Ectopic expression of metallothionein-III causes pancreatic acinar cell necrosis in transgenic mice

Mice express four distinct metallothioneins (MTs) that have similar metal-binding properties. MT-I and MT-II are expressed coordinately in most organs, whereas MT-III is expressed predominantly in a subset of neurons and MT-IV is expressed in certain stratified epithelia. The restricted expression of MT-III suggests that it may severe a specialized function. To test this hypothesis, transgenic mice were generated that express MT-III in the wider expression domain of MT-I. Similar transgenic lines expressing extra MT-I under the same regulation were generated as controls for the effect of over-expression of MT. Transgenic mice that express MT-III ectopically frequently die at 2-3 months of age. The pancreata of moribund mice were abnormally small and histological examination, at various ages, revealed a progressive degeneration of the acinar cells. At early stages multifocal acinar cell eosinophilia and swollen nuclei were seen and this pathology progressed to multifocal acinar cell necrosis and fibrosis. The terminal stages were characterized by a loss of the acinar compartment, leaving the islets embedded in a fibrotic remnant. Other organs of these mice were grossly and histologically normal. All organs examined from mice expressing excess MT-I were unremarkable even though expression of either MT-I or MT-III transgenes resulted in similar accumulations of zinc and copper in the pancreata. This study indicates that pancreatic acinar cells are unusually sensitive to chronic expression of MT-III. The mechanism by which MT-III disrupts pancreatic function is unclear, but the results provide further evidence that MT isoforms exhibit distinct properties and probably serve distinct biological functions.

Cadmium toxicity and distribution in metallothionein-I and -II deficient transgenic mice

To date, numerous correlative studies have implicated metallothionein in the detoxification of heavy metals and in the regulation of metal distribution within an organism. In the present study cadmium-binding proteins (metallothionein equivalents), cadmium acute toxicity, and cadmium distribution in tissues and subcellular fractions were compared in metallothionein-I and -II deficient (MT-/-) mice and the parental strain carrying intact metallothionein genes (MT+/+) to determine if the absence of metallothionein altered any of these parameters. In an uninduced state, MT-/- mice expressed lower levels of cadmium-binding proteins relative to MT+/+ mice in several tissues. Administration of zinc enhanced the levels of cadmium-binding proteins in liver, small intestine, kidney, pancreas, and male sex organs, but not in cecum or brain of MT+/+ mice compared to zinc pretreated MT-/- mice. The cadmium LD50 was similar for MT-/-, MT+/+, and zinc-pretreated MT-/- mice (15-17 mumol CdCl2/kg body weight delivered i.p.). However, zinc-pretreated MT+/+ mice had a cadmium LD50 of 58-63 mumol CdCl2/kg body weight. Over two-thirds of cadmium was found in liver, cecum, small intestine, and kidney in both MT+/+ and MT-/- mice; therefore, metallothionein levels do not appear to play a major role in the tissue distribution of cadmium. However, after zinc pretreatment, MT+/+ mice accumulated more cadmium in the liver and less in other tissues, whereas the amount of cadmium in the liver was not altered by zinc pretreatment in MT-/- mice. In general, the cytosolic/particulate ratio of cadmium was significantly higher in tissues of noninduced MT+/+ mice relative to MT-/- mice. This difference was accentuated after zinc pretreatment. Together these results indicate that basal levels of metallothionein do not protect from the acute toxicity of a single i.p. cadmium challenge. Furthermore, it does not appear that the cytosolic compartmentalization of cadmium is correlated with reduced toxicity.

Metal-induced metallothionein gene expression can be inactivated by protein kinase C inhibitor

The effects of protein kinase C (PKC) inhibitors on the metallothionein (MT) gene expression induced by metals were investigated. When PKC inhibitor (H7 or chelerythrine) was administered to Cd resistant, MT gene-amplified Chinese hamster ovary (CdR) cells, the induction of MT mRNA by Cd or Zn was blocked. Treating the CdR cells with a PKA-specific inhibitor, HA1004, did not cause an inhibition of metal-induced MT gene transcription. The inhibitory effect was effectuated by adding inhibitors within 40 min of exposing the cells to Cd. Apparently, AP1 was not involved in this down-regulatory effect of PKC inhibitor on MT gene expression since the inducibility of MT promoter was blocked by H7 even in the absence of the AP1-binding sequence. For Cd-treated cells, Cd accumulation in the cell was similar with or without H7 treatment. However, H7 markedly reduced cellular Zn accumulation when the cells were treated with Zn. Cycloheximide treatment increased the level of MT mRNA. This elevation can also be blocked by treating the cell with PKC inhibitor. Results in this study suggest that PKC participates in the process of metal-induced MT gene expression.

Metallothionein-independent hepatoprotection by zinc and sakuraso-saponin

Hepatoprotective activities of zinc and sakuraso-saponin against toxicity of carbon tetrachloride were investigated in metallothionein (MT)-deficient mice. Pretreatment of control 129/Sv mice with zinc or sakuraso-saponin blocked carbon tetrachloride-induced elevation of plasma transaminase activities. Quantitatively equivalent protection against carbon tetrachloride-induced hepatic damage was also observed in MT-deficient mice. Zinc and sakuraso-saponin caused elevation of hepatic MT levels in control 129/Sv mice, whereas hepatic MT was undetectable in MT-deficient mice. To examine the possibility that sakuraso-saponin-induced hepatoprotection is mediated by endogenous zinc, the hepatic concentration of zinc was analyzed. Hepatic zinc concentration in MT-deficient mice was not changed by the treatment of sakuraso-saponin. Injection of sakuraso-saponin caused a decrease of activity of aniline hydroxylation. The suppression of cytochrome P450 appears to be a mechanism by which sakuraso-saponin protects mice from the hepatotoxic effects of carbon tetrachloride. These findings indicate that the hepatoprotective activity of zinc or sakuraso-saponin is not dependent on their MT-inducing activity.

Enhanced apoptosis in metallothionein null cells

Metallothioneins (MTs) are major intracellular, zinc-binding proteins with antioxidant properties. Mouse embryonic cells null for MT due to loss of functional MT I and II genes (MT-/-) were more susceptible to apoptotic death after exposure to tert-butyl hydroperoxide or the anti-cancer agents cytosine arabinoside, bleomycin, melphalan, and cis-dichlorodiammineplatinum(II) compared with wild-type mouse embryonic cells (MT+/+). We measured basal levels of the tumor suppressor protein p53 and the death effector protein Bax and found the basal levels of both proteins were higher in MT null cells compared with MT+/+ cells. After treatment with the DNA-damaging agent cis-dichlorodiammineplatinum(II), p53 protein levels were induced in both MT+/+ and MT-/- cells with MT null cells always maintaining the highest p53 levels. The elevated sensitivity to apoptosis was not restricted to embryonic cells. Primary pulmonary fibroblasts were isolated from distinct litters of MT null, heterozygous, and wild-type mice, and all had undetectable basal MT levels. Zinc exposure increased MT levels in the wild-type and heterozygous fibroblasts but not in the MT null fibroblasts. Consistent with the induced MT levels, we found MT+/+ and MT+/- embryonic cells were less sensitive to cis-dichlorodiammineplatinum(II)-induced apoptosis compared with MT-/- cells. Our results implicate MT as a stress-responsive factor that can regulate apoptotic engagement.

Expression of MT-3 mRNA in human kidney, proximal tubule cell cultures, and renal cell carcinoma

The human metallothionein 3 (MT-3) gene has recently been identified and characterized as a brain-specific MT having growth inhibitory activity for neuronal cells. One objective of the present study was to determine if MT-3 is brain-specific or also present in the renal system, a site for chronic toxicity due to heavy metal exposure. Using RT-PCR methodology, MT-3 mRNA was shown to be expressed in the human renal system at levels below mRNA for the beta-actin gene. MT-3 mRNA was shown to be expressed in all samples obtained from both the developing and adult renal systems, from 20 weeks of fetal age to 72 years. Cultures of human proximal tubule (HPT) cells were used to determine if MT-3 mRNA expression is influenced by metal exposure. Exposure of HPT cells to either Zn2+ or Cd2+ resulted in an early (within 24 h), but unsustained increase in MT-3 mRNA. The demonstration of MT-3 mRNA expression in the kidney indicates that MT-3 may play an important early role in the response of the cell to metal exposure. MT-3 mRNA expression was also examined in tissues and cells from three cases of renal cell carcinoma. MT-3 was found to be expressed in all three cases at levels similar to those found for normal kidney, providing evidence that MT-3 mRNA expression is not altered in this cancer.

Introduction of precise alterations into the mouse genome with high efficiency by stable tag-exchange gene targeting: implications for gene targeting in ES cells

The efficiency of tag-and-exchange gene targeting approaches for the introduction of precise genomic modifications is compromised by high levels of non-homologous recombinants which survive selection due to loss of tag gene expression, often by physical loss of the tag gene. We describe a modified approach, termed stable tag-exchange, which incorporates an additional positive selection (stability) cassette to circumvent this limitation. HPRT (tag) and neo (stability) cassettes, separated by 4.9 kb of homologous DNA, were introduced efficiently into the LIF locus of ES cells. The tag cassette was substituted for abeta-galactosidase gene in exchange step targeting. Direct comparison of the tag-and-exchange and stable tag-exchange approaches indicated respective targeting efficiencies of 21% and 88%. The increased stable tag-exchange targeting efficiency resulted from elimination of >75% of background lines which survived tag-and-exchange selection due to physical loss of the tag gene. These resulted from reversion of the tagged allele to wild-type which is therefore a major contributor to tag-and-exchange targeting background. Our results extend the application of gene targeting by demonstrating a rationale for single-step integration of multiple regions of extended non-homology, and providing an efficient system for the repeated introduction of precise alterations into the mammalian genome.

Reversible activation of mouse metal response element-binding transcription factor 1 DNA binding involves zinc interaction with the zinc finger domain

The DNA-binding activity of the Zn finger protein metal response element-binding transcription factor 1 (MTF-1) was rapidly induced both in vivo in mouse Hepa cells, canine MDCK, and human HeLa cells after incubation in medium containing zinc and in vitro in whole-cell extracts to which zinc was added. Acquisition of DNA-binding capacity in the presence of free zinc was temperature and time dependent and did not occur at 4 degrees C. In contrast, activated MTF-1 binding to the metal response element occurred at 4 degrees C. After Zn activation, mouse MTF-1 binding activity was more sensitive to EDTA and was stabilized by DNA binding relative to the Zn finger transcription factor Sp1. After dilution of nuclear or whole-cell extracts from Zn-treated cells and incubation at 37 degrees C, mouse MTF-1 DNA-binding activity was no longer detected but could be completely reconstituted by the subsequent readdition of zinc. In vitro-synthesized, recombinant mouse MTF-1 displayed a similar, reversible temperature- and Zn-dependent activation of DNA-binding activity. Analysis of deletion mutants of recombinant MTF-1 suggests that the Zn finger domain is important for the Zn-dependent activation of DNA-binding capacity. Thus, mouse MTF-1 functions as a reversibly activated sensor of free zinc pools in the cell.

Identification and functional expression of HAH1, a novel human gene involved in copper homeostasis

To search for a mammalian homologue of ATX1, a human liver cDNA library was screened and a cDNA clone was isolated, which encodes a protein with 47% amino acid identity to Atx1p including conservation of the MTCXGC copper-binding domain. RNA blot analysis using this cDNA identified an abundant 0.5-kilobase mRNA in all human tissues and cell lines examined. Southern blot analysis using this same clone indicated that the corresponding gene exists as a single copy in the haploid genome, and chromosomal localization by fluorescence in situ hybridization detected this locus at the interface between bands 5q32 and 5q33. Yeast strains lacking copper/zinc superoxide dismutase (SOD1) are sensitive to redox cycling agents and dioxygen and are auxotrophic for lysine when grown in air, and expression of this human ATX1 homologue (HAH1) in these strains restored growth on lysine-deficient media. Yeast strains lacking ATX1 are deficient in high affinity iron uptake and expression of HAH1 in these strains permits growth on iron-depleted media and results in restoration of copper incorporation into newly synthesized Fet3p. These results identify HAH1 as a novel ubiquitously expressed protein, which may play an essential role in antioxidant defense and copper homeostasis in humans.

Analysis of the possible protective role of metallothionein in streptozotocin-induced diabetes using metallothionein-null mice

In order to clarify a possible protective role of metallothionein (MT) in the development of streptozotocin (STZ)-caused insulin-dependent diabetes mellitus (IDDM) and its mechanisms, we studied whether MT is effective for protection against STZ-caused IDDM by utilizing MT-null (isoforms MT-I and II) transgenic mice. It was found that Zn pretreatment (I mg/kg body weight as ZnSO4) has a unique inhibitory effect on IDDM development in MT-null mice in contrast to no marked effect in control (C57BL/6J) mice, suggesting that Zn ions free from MT molecules exerted this protective effect. The highest Zn dose (10 mg/kg body weight) fully suppressed development of hyperglycaemia in both types of mice. Pretreatment with Zn partially led to recovery of superoxide dismutase activities in the liver and pancreas in which STZ administration suppressed superoxide dismutase activity in both types of mice. The present study suggests that Zn plays an important role in the pathogenesis of IDDM, although a possible involvement of MT in the protection of STZ-caused IDDM cannot be completely negated.

Disruption of the metallothionein-III gene in mice: analysis of brain zinc, behavior, and neuron vulnerability to metals, aging, and seizures

Metallothionein-III (MT-III), a brain-specific member of the metallothionein family of metal-binding proteins, is abundant in glutamatergic neurons that release zinc from their synaptic terminals, such as hippocampal pyramidal neurons and dentate granule cells. MT-III may be an important regulator of zinc in the nervous system, and its absence has been implicated in the development of Alzheimer's disease. However, the roles of MT-III in brain physiology and pathophysiology have not been elucidated. Mice lacking MT-III because of targeted gene inactivation were generated to evaluate the neurobiological significance of MT-III. MT-III-deficient mice had decreased concentrations of zinc in several brain regions, including hippocampus, but the pool of histochemically reactive zinc was not disturbed. Mutant mice exhibited normal spatial learning in the Morris water maze and were not sensitive to systemic zinc or cadmium exposure. No neuropathology or behavioral deficits were detected in 2-year-old MT-III-deficient mice, but the age-related increase in glial fibrillary acidic protein expression was more pronounced in mutant brain. MT-III-deficient mice were more susceptible to seizures induced by kainic acid and subsequently exhibited greater neuron injury in the CA3 field of hippocampus. Conversely, transgenic mice containing elevated levels of MT-III were more resistant to CA3 neuron injury induced by seizures. These observations suggest a potential role for MT-III in zinc regulation during neural stimulation.

Metallothioneins 1 and 2 are expressed in the olfactory mucosa of mice in untreated animals and during the regeneration of the epithelial layer

We have examined the expression of the MT1 and MT2 isoforms of metallothionein in the mouse olfactory mucosa. In untreated mice, metallothionein was strongly expressed in supporting cells, acinar cells of the Bowman's glands, and olfactory neurons. Expression was however restricted to a subset of cells within each type, and to zones within the olfactory system. Irrigation with ZnSO4 solution caused exfoliation of the olfactory epithelium and during the resultant regeneration, metallothionein immunoreactivity was associated with the proliferating basal cells. The ability to express MTs 1 and 2 did not appear to be obligatory for the early stages of regeneration since mice which do not express these isoforms responded similarly to wild type mice. Strong nuclear expression of metallothionein was noted in the untreated olfactory chamber following unilateral irrigation.

Disruption of the metallothionein-III gene in mice: analysis of brain zinc, behavior, and neuron vulnerability to metals, aging, and seizures

Metallothionein-III (MT-III), a brain-specific member of the metallothionein family of metal-binding proteins, is abundant in glutamatergic neurons that release zinc from their synaptic terminals, such as hippocampal pyramidal neurons and dentate granule cells. MT-III may be an important regulator of zinc in the nervous system, and its absence has been implicated in the development of Alzheimer's disease. However, the roles of MT-III in brain physiology and pathophysiology have not been elucidated. Mice lacking MT-III because of targeted gene inactivation were generated to evaluate the neurobiological significance of MT-III. MT-III-deficient mice had decreased concentrations of zinc in several brain regions, including hippocampus, but the pool of histochemically reactive zinc was not disturbed. Mutant mice exhibited normal spatial learning in the Morris water maze and were not sensitive to systemic zinc or cadmium exposure. No neuropathology or behavioral deficits were detected in 2-year-old MT-III-deficient mice, but the age-related increase in glial fibrillary acidic protein expression was more pronounced in mutant brain. MT-III-deficient mice were more susceptible to seizures induced by kainic acid and subsequently exhibited greater neuron injury in the CA3 field of hippocampus. Conversely, transgenic mice containing elevated levels of MT-III were more resistant to CA3 neuron injury induced by seizures. These observations suggest a potential role for MT-III in zinc regulation during neural stimulation.

Role of metallothionein in cadmium-induced hepatotoxicity and nephrotoxicity

MT, a cysteine-rich, metal-binding protein, exists in most tissues and is easily induced by many stimuli. There are four major MT isoforms in mammalian tissues, with MT-I and -II present in all tissues, MT-III only in brain, and MT-IV located in epithelium. Many factors regulate MT synthesis, such as age, species, hormones, inflammation, and various chemical treatments. Not only is MT synthesis important, but degradation of MT is also an important mechanism of MT regulation. The importance of MT in Cd toxicology has been extensively investigated. MT does not have a major effect on absorption and tissue distribution of Cd, but it does play a major role in binding Cd in the cell, thus decreasing its elimination from the body, especially into the bile. MT is at least partially responsible for the retention of Cd in tissues and the long biological half-life of the metal. MT plays an important role in Cd tolerance and Cd-induced hepatotoxicity. MT binds Cd in the hepatic cytosol and renders it "inert." Therefore, MT is beneficial to the liver. However, the Cd-MT complex is nephrotoxic and is proposed to be responsible for chronic Cd poisoning. MT appears to play less of a protective role in Cd-MT-induced acute nephrotoxicity, and Zn-induced protection against CdMT acute renal injury is not mediated by MT. The role of MT in chronic Cd nephrotoxicity needs to be further clarified.

Metallothioneins in brain--the role in physiology and pathology

A symposium on the role of brain metallothioneins (MTs) in physiology and pathology was held at the 1996 Annual Society of Toxicology Meeting in Anaheim, California. The objectives of this symposium were to: (1) review the physiologic function of MTs, (2) examine the distribution of brain MTs with particular emphasis on cell-specific localization (neurons vs neuroglia), (3) discuss MT gene responsiveness upon toxic insult with metals, and (4) discuss the potential role of MTs in the etiology of neurodegenerative disorders. Dr. Cherian discussed the biochemical properties of the MTs, emphasizing structural similarities and differences between the MTs. Dr. Klaassen addressed the expression and distribution of the MTs in brains with special reference to the cell-specific localization of MTs. Dr. Aschner provided data illustrating a potential role for MTs in attenuating the cytotoxicity caused by methylmercury (MeHg) in cultured neonatal astrocytes. Dr. Palmiter discussed the properties of MT-III and the increased sensitivity of MT-III knockout mice to kainate-induced seizures. Cerebral zinc metabolism, its relationship to MT homeostasis, and its pathogenic potential in Alzheimer's disease was addressed by Dr. Bush.

Metallothionein in physiological and physiopathological processes

The multipurpose nature of MT that we have presented in this review has drawn attention from many different fields of research: biochemistry, molecular biology, toxicology, pharmacology, etc. In recent years, considerable advances have been made concerning the regulation of MT genes by metals. Little, however, is known at the molecular level about the mechanisms of MT induction by nonmetallic inducers such as growth factors. This is of particular interest since MT is highly expressed during liver regeneration, an event orchestrated by a series of growth stimulators and inhibitors. The significance of the nuclear distribution of MT in growing cells and what controls its translocation are questions that remain unanswered at the present time. The possibility that MT could participate in a DNA synthesis-related process through donation or abstraction of Zn to and from transcription factors has been inferred from in vitro studies. Such transfer mechanisms, however, have yet to be confirmed in vivo. Overexpression of MT is often accompanied by increased resistance towards a variety of alkylating agents and chemotherapeutic drugs. The mechanisms by which MT protects cells against these agents may depend on their distinct mode of toxic action. For some, MT cysteines can be the target of the direct attack from the parent compound. For others such as N-methyl-N-nitroso compounds, MT cysteines may serve as a sink for the reactive oxygen species now known to be derived from their metabolism. In either case, a primary consequence of such interactions is the release of the metals initially bound to MT. Therefore, the metal composition of MT appears to be an important factor to consider in determining the overall effect of MT in the resistance process.

Enhanced effectiveness of copper ion buffering by CUP1 metallothionein compared with CRS5 metallothionein in Saccharomyces cerevisiae

The bakers' yeast Saccharomyces cerevisiae contains a metallothionein (MT) gene family comprised of the amplified CUP1 locus and the single copy CRS5 gene. We demonstrate that CUP1 plays the dominant role in copper detoxification. A single copy of CUP1 was far more effective in conferring copper resistance than was CRS5. The CUP1 promoter contributes to this resistance; in a promoter exchange experiment, the Crs5 MT conferred strong copper resistance when its expression was driven by the CUP1 promoter, and conversely, the CRS5 promoter reduced the effectiveness of Cup1 MT. Unlike CUP1, the CRS5 promoter appears to be refractory to high concentrations of copper. The CUP1 coding sequences also contribute to copper tolerance, presumably reflecting the enhanced binding avidity of Cup1 MT for Cu(I) ions. In studies with the bathocuproine Cu(I) chelator, the Cu(I) ions bound to Crs5 were kinetically more labile than the Cu(I) binding to Cup1. Our findings are consistent with the assembly of Crs5 into two metal-binding clusters, similar to mammalian MTs, but unlike Cup1. Overall, the striking differences in gene structure, regulation, and function of CUP1 and CRS5 are remarkably reminiscent of the MTI and MTII genes of the pathogenic yeast Candida glabrata.

A murine model of Menkes disease reveals a physiological function of metallothionein

Human Menkes disease and the murine Mottled phenotype are X-linked diseases that result from copper deficiency due to mutations in a copper-effluxing ATPase, designated ATP7A. Male mice with the Mottled-Brindled allele (Mo-brJ) accumulate copper in the intestine, fail to export copper to peripheral organs and die a few weeks after birth. Much of the intestinal copper is bound by metallothionein (MT). To determine the function of MT in the presence of Atp7a deficiency, we crossed Mo-brJ females with males that bear a targeted disruption of the Mt1 and Mt2 genes (Mt-/-). On an Mt -/- background, most Mo-brJ males as well as heterozygous Mo-brJ females die before embryonic day 11. The lethality in Mo-brJ females can be explained by preferential inactivation of the paternal X chromosome in extraembryonic tissues and resultant copper toxicity in the absence of MT. In support of this hypothesis, cell lines derived from Mt -/-, Mo-brJ embryos are very sensitive to copper toxicity.

Trace metal, acute phase and metabolic response to endotoxin in metallothionein-null mice

Accumulation of hepatic zinc via metallothionein (MT) induction during infection/inflammation is postulated to benefit a range of metabolic processes. The metabolic consequences of two doses of endotoxin (LPS) (1 and 5 mg/kg, intraperitoneally) were examined in normal (MT+/+) and MT-null (MT-/-) mice (all results means =/- S.E.M., n=6). At 16 h after 1 mg/kg LPS, hypozincaemia was pronounced in the MT+/+ mice (4.4+/-0.2 microM), concomitant with a 36% increase in hepatic Zn and a > 10-fold increase in hepatic MT. Plasma Zn (16.6+/-0.7 microM) and total hepatic Zn were unchanged in MT -/- mice, confirming the importance of MT in altering plasma and hepatic Zn during inflammation. Plasma iron was lower in LPS-treated MT-/- mice, whereas plasma copper increased to a similar extent in both groups of mice. Plasma fibrinogen more than doubled, and was similar in both groups of mice, which questions the importance of MT in acute-phase protein synthesis. Blood and liver glucose concentrations were not significantly different between groups before or after LPS, whereas blood and liver lactate concentrations were significantly lower (31% and 24% respectively) in MT-/- mice after LPS. At 16 h after 5 mg/kg LPS, plasma Zn was decreased even further in MT+/+ mice (2.6+/-0.3 microM), but remained unchanged in MT-/- mice at concentrations significantly above those in 16-h fasted MT-/- mice (15.8+/-0.5 versus 11.3+/-0.3 microM). Total liver Zn was 17% lower than fasting values in MT-/- mice, in contrast with 32% higher in MT+/+ mice. Synthesis of MT (in MT+/+ mice) and fibrinogen in all mice was not further enhanced by the higher LPS dose. Blood glucose was significantly decreased by 18% in MT+/+ mice and by 38% in MT-/- mice after 5 mg/kg LPS. There was a marked 44% decrease in liver glucose in MT-/- mice; that in MT+/+ mice was unchanged from fasting levels, implying a deficit in hepatic gluconeogenesis in LPS-treated MT-/- mice. In the absence of any indication of major hepatotoxicity, the results of this study indicate that energy production, and not acute-phase protein synthesis, may be most influenced by Zn supply during endotoxaemia, suggesting that MT has a role in maintaining hepatic and blood glucose in this metabolic setting.

Transgenic mouse blastocysts that overexpress metallothionein-I resist cadmium toxicity in vitro

The role of metallothionein with regard to cadmium toxicity in vitro was investigated using preimplantation mouse blastocysts derived from a transgenic strain that constitutively overexpresses metallothionein-I transgenes (MT-I*). Northern blot and in situ hybridization revealed high levels of MT-I mRNA in transgenic blastocysts when compared with control blastocysts, and reverse transcriptase-polymerase chain reaction-amplified MT-I mRNA was almost exclusively MT-I*. Moreover, pulse-labeling experiments showed that the relative rate of synthesis of MT was 9-fold higher in transgenic blastocysts. Cadmium (Cd2+) toxicity was assessed after incubating blastocysts for 4 hr in Whitten's medium containing 50 microM Cd2+. Embryos that displayed abnormal morphology were judged "sensitive". Transgenic blastocysts were more resistant to cadmium-induced morphological changes than were control blastocysts. "Sensitive" and "resistant" blastocysts were individually genotyped by polymerase chain reaction, or they were transferred to foster mothers, and embryonic development to midterm was monitored. Of the blastocysts derived from mating heterozygous transgenic males with control females, 56% were transgenic before incubation with Cd2+, whereas 95% of the blastocysts that retained normal morphology after incubation were transgenic. Moreover, after Cd2+ exposure, transgenic blastocysts with normal morphology were nine times more likely to develop to midterm than were control blastocysts with normal morphology. Blastocysts with abnormal morphology failed to develop to midterm. These studies indicate that MT plays a central role in protection from Cd2+ toxicity within the physiological context of the developing mouse embryo.

Analysis of the effects of overexpression of metallothionein-I in transgenic mice on the reproductive toxicology of cadmium

Exposure to low levels of cadmium reduces fertility. In male mice spermatogenesis is highly sensitive to cadmium, whereas in females the peri-implantation period of pregnancy is sensitive. To examine the potential roles of the cadmium-binding protein, metallothionein (MT), in the reproductive toxicology of cadmium, we examined a transgenic mouse strain that overexpresses metallothionein-I (MT-I). These mice had dramatically increased steady-state levels of MT-I mRNA and MT in the testes and in the female reproductive tract during the peri-implantation period of pregnancy, and this overexpression occurred in a cell-specific and temporally regulated manner similar to that of the endogenous MT-I gene. Transgenic and control males were injected with cadmium, and the histology of the testes was examined. An injection of 7.5 mumol Cd/kg had no effect on histology of the testes in either transgenic or control mice. In contrast, an injection of 10 mumol Cd/kg caused rapid changes in the histology of the testes and resulted in pronounced testicular necrosis in both control and transgenic mice. Female transgenic and control mice were mated and then injected with cadmium (30-45 mumol Cd/kg) on the day of blastocyst implantation (day 4). In both of these groups, injection of cadmium reduced pregnancy rate, and no dramatic protection was afforded by maternal and/or embryonic overexpression of MT. Thus, overexpression of MT-I does not significantly protect against either of these cadmium-induced effects on fertility.

Activation of the complete mouse metallothionein gene locus in the maternal deciduum

The mouse metallothionein (MT) gene family consists of four known members (MT-I through IV) clustered on chromosome 8. Studies reported herein examine the expression and regulation of the MT-III and MT-IV genes in specific cell types in the maternal reproductive tract, developing embryo, and fetus known to express the MT-I and -II genes. MT-III and MT-IV mRNAs were absent from the visceral yolk sac, placenta, and fetal liver, tissues with high levels of MT-I and MT-II mRNAs. In contrast, MT-III and MT-IV mRNAs were both abundant in the maternal deciduum, and in experimentally induced deciduoma on 7 and 8 days postcoitum (1 dpc = vaginal plug), as are MT-I and -II mRNAs. The abundance of each of these MT mRNAs increased coordinately during development of the deciduum (6-8 dpc), and in situ hybridization localized MT-I, MT-III, and MT-IV mRNAs to the secondary decidual zone of the antimesometrial region on 8 dpc, where in some regions all of the cells were apparently positive. Thus, all of the known mouse MT genes are co-expressed in at least some of the cells in the secondary decidual zone. Electrophoretic analysis of decidual MT suggested that the MT-I, -II, and -III isoforms are abundant proteins in the secondary deciduum. Bacterial endotoxin-lipopolysaccharide (LPS) and Zn are powerful inducers of MT-I and MT-II gene expression in many adult organs, whereas these agents apparently have little effect on MT-III and MT-IV gene expression. Neither of these agents significantly effected levels of decidual MT-III or MT-IV mRNAs in vivo or in primary cultures of decidual cells in vitro, and only modest effects of Zn on MT-I mRNA levels were noted. During 2 days of in vitro culture, decidual cell MT-I and MT-III mRNA levels remained elevated while MT-IV mRNA levels decreased. Thus, expression of the mouse MT gene locus in the deciduum appears to be developmentally regulated, and in this tissue, the MT genes are refractory to induction by Zn or inflammation.

Toxic metal-responsive gene transcription

Metals play a dual role in biological systems, serving as essential co-factors for a wide range of biochemical reactions yet these same metals may be extremely toxic to cells. To cope with the stress of increases in environmental metal concentrations, eukaryotic cells have developed sophisticated toxic metal sensing proteins which respond to elevations in metal concentrations. This signal is transmitted to stimulate the cellular transcriptional machinery to activate expression of metal detoxification and homeostasis genes. This review summarizes our current understanding of the biochemical and genetic mechanisms which underlie cellular responses to toxic metals via metalloregulatory transcription factors.

Expression of metallothionein genes during the post-embryonic development of Drosophila melanogaster

Expression of the two Drosophila melanogaster metallothionein genes, Mtn and Mto, has been analyzed by in situ hybridization during post-embryonic development. Mtn and Mto transcripts were detected exclusively in the digestive tract of larvae, pupae and adults reared on standard medium. Mtn and Mto expression domains overlap, but each gene is also expressed at unique sites. Mtn mRNA levels are approximately 10 and 20 times higher than those of Mto in larvae and adults, respectively. Copper and cadmium ions strongly induce Mtn and Mto mRNA accumulation in the midgut. Zinc is a weaker inducer, acting only at high concentrations. Mtn gene expression is induced by these three metals in Malpighian tubules, while Mto gene expression in this organ is induced only by zinc. Iron is a poor inducer of metallothionein mRNA accumulation. Functions of MTN and MTO proteins in metal homeostasis and detoxification are considered.

Analysis of the heavy metal-responsive transcription factor MTF-1 from human and mouse

Heavy metal-induced transcription in mammalian cells is conferred by the metal-responsive 70 kDa transcription factor MTF-1 which contains six zinc fingers and at least three activation domains. In previous cell transfection experiments we have shown that the zinc finger region confers an about 3 fold metal inducibility of transcription, due to its differential zinc binding. However, we also noted that human MTF-1 was more metal-responsive than the mouse factor (about 10 fold versus 3 fold, respectively). Here we analyze this difference in more detail by using chimeric human-mouse factors and narrow the critical region to a 64 amino acid stretch immediately downstream of the zinc fingers, overlapping with the acidic activation domain. A short human segment of this region (aa 313-377) confers efficient metal induction to the mouse MTF-1 when replacing the corresponding mouse region. However, high metal inducibility requires an unaltered MTF-1 and is lost when human MTF-1 is fused to the general activation domain of herpesvirus VP16. Wild type and truncation mutants of MTF-1 fused to VP16 yield chimeras of high transcriptional activity, some exceeding the wildtype regulator, but only limited (about 3 fold) heavy metal inducibility.

Structure, organization and expression of the metallothionein gene family in Arabidopsis

Metallothioneins (MTs) are cysteine-rich proteins required for heavy metal tolerance in animals and fungi. Recent results indicate that plants also possess functional metallothionein genes. Here we report the cloning and characterization of five metallothionein genes from Arabidopsis thaliana. The position of the single intron in each gene is conserved. The proteins encoded by these genes can be divided into two groups (MT1 and MT2) based on the presence or absence of a central domain separating two cysteine-rich domains. Four of the MT genes (MT1a, MT1c, MT2a and MT2b) are transcribed in Arabidopsis. Several lines of evidence suggest that the fifth gene, MT1b, is inactive. There is differential regulation of the MT gene family. MT1 mRNA is expressed highly in roots, moderately in leaves and is barely detected in inflorescences and siliques. MT2a and MT2b mRNAs are more abundant in leaves, inflorescences and in roots from mature plants, but are also detected in roots of young plants, and in siliques. MT2a mRNA is strongly induced in seedlings by CuSO4, whereas MT2b mRNA is relatively abundant in this tissue and levels increase only slightly upon exposure to copper. MT1a and MT1c are located within 2 kb of each other and have been mapped to chromosome I. MT1b and MT2b map to separate loci on chromosome V, and MT2a is located on chromosome III. The locations of these MT genes are different from that of CAD1, a gene involved in cadmium tolerance in Arabidopsis.

Hepatic zinc in metallothionein-null mice following zinc challenge: in vivo and in vitro studies

Hepatic zinc uptake and accumulation were compared in freshly isolated and cultured hepatocytes prepared from control (MT+/+) and metallothionein (MT)-null (MT-/-) mice. In freshly isolated hepatocytes, rapid (10-15 min) exchange of 65Zn was proportional to the Zn concentration in the medium and occurred to the same extent in hepatocytes from MT+/+ and MT-/- mice. In 24 h culture experiments with MT+/+ and MT-/- hepatocytes it was shown that approx. 40% of newly acquired cell-associated Zn was attached to the cell surface and not internalized. In MT+/+ and MT-/- hepatocyte cultures, internalized Zn (intZn) increased in proportion to extracellular Zn. Zn accumulation in MT-/- hepatocytes was only 60% that of MT+/+ cells. Addition of 1 microM dexamethasone (Dex) and recombinant mouse interleukin-6 (IL-6; 100 units/ml) increased MT accumulation by 8.6-fold in MT+/+ hepatocytes (at 50 microM Zn) and there was an associated parallel increase in intZn. Dex and IL-6 did not increase intZn in the MT-/- hepatocytes. At 16 h after an intraperitoneal injection of 5 micrograms/g Zn, plasma and urine Zn concentrations were 69 +/- 10 microM and 86 +/- 25 microM respectively in MT-/- mice (n = 10) and 27 +/- 1 microM and 23 +/- 4 microM respectively in MT+/+ controls (n = 9) (P < 0.001, plasma; P < 0.05, urine). Hepatic cytosolic Zn concentrations doubled in MT+/+ mice and increased by a significant 15% in MT-/- mice. There was no increase in hepatic Zn (dry wt.) concentrations or in total hepatic Zn, demonstrating that the increase in cytosolic Zn in MT-/- mice was due to hepatic water loss rather than net Zn uptake. It appears that even at extreme plasma concentrations of Zn, little if any accumulates within the liver when there is no MT available for its sequestration. That this is not fully demonstrated in vitro is probably due to nature of cell culture, where organ architecture is lost and the external protein binding milieu is less complex.

Functional domains of the heavy metal-responsive transcription regulator MTF-1

Metallothioneins (MTs) constitute a class of low molecular weight, cysteine-rich, metal binding proteins which are regulated at the level of gene transcription in response to heavy metals and other adverse treatments. We have previously cloned a zinc finger factor (MTF-1) that binds specifically to heavy metal-responsive DNA sequence elements in metallothionein promoters and shown that this factor is essential for basal and heavy metal-induced transcription. Here we report that the C-terminal part of MTF-1 downstream of the DNA binding zinc fingers harbours three different transactivation domains, namely an acidic domain, a proline-rich domain and a domain rich in serine and threonine. When fused to the heterologous DNA binding domain of the yeast factor GAL4 these activation domains function constitutively, i.e. transcription of a GAL4-driven reporter gene is not induced by heavy metals. In search of the region(s) responsible for metal induction, external and internal deletion mutations of mouse and human MTF-1 and chimeric variants thereof were tested with a reporter gene driven by a metal-responsive promoter. The N-terminal part of MTF-1 containing the zinc fingers, which are dependent on zinc for efficient DNA binding, can indeed confer a limited (3- to 4-fold) zinc-responsive transcription when fused to the heterologous activation domain of the viral VP16 protein. Another region containing the acidic and proline-rich activation domains also contributes to metal inducibility, but only in the context of intact MTF-1. This indicates that the activity of MTF-1 results from a complex interplay of different functional domains.

Endotoxin-induced inflammation does not cause hepatic zinc accumulation in mice lacking metallothionein gene expression

The action of endotoxin lipopolysaccharide (LPS) on hepatic Zn uptake was examined in mice lacking expression of metallothionein (MT)-1 and MT-II genes. Hepatic Zn concentrations, which in normal control mice increased by a mean 29% (MT elevated 20-fold) 16 h post-LPS exposure, did not increase in MT-null mice. Plasma Zn fell by 68% in controls and 32% in MT-null mice. The time course of LPS action in normal mice was characterized by a rapid reduction (-74% at 4 h, -81% at 8 h) and partial recovery (-39% at 24 h) in plasma Zn, with a progressive increase over 24 h in hepatic concentrations of MT (by 36-fold) and Zn (by 40%). In contrast, the MT-null mice had a linear decrease in plasma Zn (-15% at 8 h, -41% at 24 h) and early loss of Zn from the liver. The Zn changes seen in MT-null mice were largely attributable to LPS-associated anorexia. Food deprivation (20 h) alone caused respective 14% and 30% decreases in hepatic and plasma Zn concentrations and a 27% reduction in total liver Zn reserves, whereas fasted normal mice conserved Zn with a 4-fold increase in hepatic MT. This study confirms that MT synthesis is essential for endotoxin-induced liver Zn accumulation.

Enhanced sensitivity to oxidative stress in cultured embryonic cells from transgenic mice deficient in metallothionein I and II genes

Embryonic cells from transgenic mice with targeted disruption of metallothionein I and II genes expressed no detectable metallothionein either constitutively or after treatment with cadmium, in contrast to cultured cells that were wild type or heterozygous for the loss of the metallothionein genes. Metallothionein null cells were most sensitive to the cytotoxic effects of cadmium, the membrane permeant oxidant tert-butylhydroperoxide, and the redox cycling toxin paraquat. No marked differences were seen among the wild type, heterozygous, or metallothionein null cells in glutathione levels or in the activity of CuZn-superoxide dismutase, glutathione peroxidase, or catalase. Nevertheless, metallothionein null cells were more sensitive to tert-butylhydroperoxide-induced oxidation as ascertained by confocal microscopic imaging of dichlorofluoroscein fluorescence. These results indicate basal metallothionein levels can function to regulate intracellular redox status in mammalian cells.

Early mouse preimplantation development is unaffected by microinjection of metallothionein antibodies

Polyclonal antibodies that cross-react with rodent metallothionein I (MT I) and metallothionein II (MT II) were microinjected in 1-cell and 2-cell mouse embryos, into either the cytoplasm or the nucleus. Regardless of the experimental treatment, mouse embryo development in vitro was not affected and most of the embryos cleaved normally until the morula stage. The results suggest that metallothionein is not essential for normal mouse early preimplantational development, in agreement with recent studies in mice with inactivated MT I and MT II genes.

Cell cycle regulation of metallothionein in human colonic cancer cells

Elevated levels of metallothionein (MT) found in rapidly growing tissues such as neonatal liver and various types of human tumors have suggested a role for MT in cell proliferation. To further explore this possibility we investigated the concentration of MT in human colonic cancer (HT-29) cells at different stages of proliferation by means of immunocytochemistry and competitive binding. MT is increased in subconfluent proliferating cells relative to growth-inhibited confluent cells, much as it is in growing tissues. Cycling cells synchronized with compactin, an inhibitor of 3-hydroxy-3-methylglutaryl-coenzyme A reductase, revealed an oscillation of cytoplasmic MT that reached a maximum in successive late G1 phases and at the G1/S transition. Individual phase of the cell cycle were assessed by [3H]thymidine incorporation and by immunofluorescence employing an antibody that detects a nuclear antigen associated with proliferation. An enzyme-linked immunosorbent assay was used to quantify the relative amounts of MT in homogenate supernatants of HT-29 cells. A 2- to 3-fold increase in MT in actively proliferating cells and the regulation of the protein during the mitotic cell cycle point to a physiological role for MT in cellular proliferation and suggest that it may also serve as a proliferation marker.

Developmental variability of metallothionein Mtn gene expression in the species of the Drosophila melanogaster subgroup

Developmental expression of the Drosophila melanogaster metallothionein Mtn gene has been analysed. Transcripts of this gene accumulate during the vitellogenic phase of oogenesis in a ring of follicular cells at the oocyte-nurse cell margin and in the follicular cells surrounding the oocyte. There is also strong expression of the Mtn gene during the second half of embryogenesis in hemocytes, the endoderm midgut, and Malpighian tubules. A banded expression pattern is observed transiently in the midgut at stage 13. The two Mtn alleles, Mtn and Mtn, show quantitative differences in their expression patterns. Copper intoxication of flies does not induce ectopic expression of the Mtn gene, but rather leads to over-expression of the gene in the structures where it is normally transcribed. Mtn transcription is not altered in homozygous mutants of four genes (lab, wg, dpp, bap) known to be involved in midgut morphogenesis. Expression of Mtn has been also studied in six other species of the melanogaster subgroup. This analysis demonstrates that regulation of Mtn gene transcription has changed during evolution of the Drosophila lineage. For example, Mtn is expressed specifically in the Malpighian tubules of D. melanogaster, while in D. mauritiana and D. sechellia the amnioserosa is a specific location of expression. Nonetheless, expression of Mtn in the midgut is common to the seven species, suggesting a basic role for the MTN protein during embryogenesis in this organ, possibly in the release of metallic ions from vitellogenins. In contrast, two genes also expressed in the embryonic midgut, lab and dFRA, display identical patterns in all species of the melanogaster subgroup. The diversity of Mtn patterns in closely related Drosophila species exemplifies the rapid evolution of a gene regulatory system.

Cloning, chromosomal mapping and characterization of the human metal-regulatory transcription factor MTF-1

Metallothioneins (MTs) are small cysteine-rich proteins that bind heavy metal ions such as zinc, cadmium and copper with high affinity, and have been functionally implicated in heavy metal detoxification and radical scavenging. Transcription of metallothioneins genes is induced by exposure of cells to heavy metals. This induction is mediated by metal-responsive promoter elements (MREs). We have previously cloned the cDNA of an MRE-binding transcription factor (MTF-1) from the mouse. Here we present the human cDNA equivalent of this metal-regulatory factor. Human MTF-1 is a protein of 753 amino acids with 93% amino acid sequence identity to mouse MTF-1 and has an extension of 78 amino acids at the C-terminus without counterpart in the mouse. The factors of both species have the same overall structure including six zinc fingers in the DNA binding domain. We have physically mapped the human MTF-1 gene to human chromosome 1 where it localizes to the short arm in the region 1p32-34, most likely 1p33. Both human and mouse MTF-1 when produced in transfected mammalian cells strongly bind to a consensus MRE of metallothionein promoters. However, human MTF-1 is more effective than the mouse MTF-1 clone in mediating zinc-induced transcription.

More to learn from gene knockouts

Gene targeting by homologous recombination in mouse embryonic stem cells is a powerful technique to determine the physiological function of any gene product in embryonic and postnatal development and in molecular pathogenesis. Although the technique is very demanding and still in its developing stage several knockout mice carrying disrupted genes, which were once thought important for the development or molecular pathogenesis of certain tissues, have given unexpected results. A gene/function redundancy or superfluous and on-functional theory has been advanced by many investigators to explain the unexpected results. These surprising results may teach us a new lesson and lead to a revision of the strongly held view that highly conserved and abundantly expressed genes have a prominent role and function in cell physiology and development. Additional, they may also support the notion that molecular cross-talk among the genes may play an important role in determining the minimal phenotype.

Functional homologs of fungal metallothionein genes from Arabidopsis

Metallothioneins (MTs) are cysteine-rich proteins required for heavy metal tolerance in animals and fungi. Two cDNAs encoding proteins with homology to animal and fungal MTs have been isolated from Arabidopsis. The genes represented by these cDNAs are referred to as MT1 and MT2. When expressed in an MT-deficient (cup1 delta) mutant of yeast, both MT1 and MT2 complemented the cup1 delta mutation, providing a high level of resistance to CuSO4 and moderate resistance to CdSO4. Although the MT-deficient yeast was not viable in the presence of either 300 microM CuSO4 or 5 microM CdSO4, cells expressing MT1 were able to grow in medium supplemented with 3 mM CuSO4 and 10 microM CdSO4, and those expressing MT2 grew in the presence of 3 mM CuSO4 and 100 microM CdSO4. In plants, MT1 mRNA was more abundant in roots and dark-grown seedlings than in leaves. In contrast, MT2 mRNA accumulated more in leaves than in either roots or darkgrown seedlings. MT2 mRNA was strongly induced in seedlings by CuSO4, but only slightly by CdSO4 or ZnSO4. However, MT1 mRNA was induced by CuSO4 in excised leaves that were submerged in medium. These results indicated that Arabidopsis MT genes are involved in copper tolerance. Plants also synthesized metal binding phytochelatins (poly[gamma-glutamylcysteine]glycine) when exposed to heavy metals. The results presented here argue against the hypothesis that phytochelatins are the sole molecules involved in heavy metal tolerance in plants. We conclude that Arabidopsis MT1 and MT2 are functional homologs of yeast MT.