Immunohistochemical localization of metallothionein in cell nucleus and cytoplasm of fetal human liver and kidney and its changes during development

Primary and Immortalized Cultures of Human Proximal Tubule Cells Possess Both Progenitor and Non-Progenitor Cells That Can Impact Experimental Results

Studies have reported the presence of renal proximal tubule specific progenitor cells which co-express PROM1 and CD24 markers on the cell surface. The RPTEC/TERT cell line is a telomerase-immortalized proximal tubule cell line that expresses two populations of cells, one co-expressing PROM1 and CD24 and another expressing only CD24, identical to primary cultures of human proximal tubule cells (HPT). The RPTEC/TERT cell line was used by the authors to generate two new cell lines, HRTPT co-expressing PROM1 and CD24 and HREC24T expressing only CD24. The HRTPT cell line has been shown to express properties expected of renal progenitor cells while HREC24T expresses none of these properties. The HPT cells were used in a previous study to determine the effects of elevated glucose concentrations on global gene expression. This study showed the alteration of expression of lysosomal and mTOR associated genes. In the present study, this gene set was used to determine if pure populations of cells expressing both PROM1 and CD24 had different patterns of expression than those expressing only CD24 when exposed to elevated glucose concentrations. In addition, experiments were performed to determine whether cross-talk might occur between the two cell lines based on their expression of PROM1 and CD24. It was shown that the expression of the mTOR and lysosomal genes was altered in expression between the HRTPT and HREC24T cell lines based on their PROM1 and CD24 expression. Using metallothionein (MT) expression as a marker demonstrated that both cell lines produced condition media that could alter the expression of the MT genes. It was also determined that PROM1 and CD24 co-expression was limited in renal cell carcinoma (RCC) cell lines.

Heterologous Expression of Human Metallothionein Gene HsMT1L Can Enhance the Tolerance of Tobacco (Nicotiana nudicaulis Watson) to Zinc and Cadmium

Metallothionein (MT) is a multifunctional inducible protein in animals, plants, and microorganisms. MT is rich in cysteine residues (10-30%), can combine with metal ions, has a low molecular weight, and plays an essential biological role in various stages of the growth and development of organisms. Due to its strong ability to bind metal ions and scavenge free radicals, metallothionein has been used in medicine, health care, and other areas. Zinc is essential for plant growth, but excessive zinc (Zn) is bound to poison plants, and cadmium (Cd) is a significant environmental pollutant. A high concentration of cadmium can significantly affect the growth and development of plants and even lead to plant death. In this study, the human metallothionein gene HsMT1L under the control of the CaMV 35S constitutive promoter was transformed into tobacco, and the tolerance and accumulation capacity of transgenic tobacco plants to Zn and Cd were explored. The results showed that the high-level expression of HsMT1L in tobacco could significantly enhance the accumulation of Zn²⁺ and Cd²⁺ in both the aboveground parts and the roots compared to wild-type tobacco plants and conferred a greater tolerance to Zn and Cd in transgenic tobacco. Subcellular localization showed that HsMT1L was localized to the nucleus and cytoplasm in the tobacco. Our study suggests that HsMT1L can be used for the phytoremediation of soil for heavy metal removal.

Metallothionein1A Regulates Rhizobial Infection and Nodulation in Phaseolus vulgaris

Metallothioneins (MTs) constitute a heterogeneous family of ubiquitous metal ion-binding proteins. In plants, MTs participate in the regulation of cell growth and proliferation, protection against heavy metal stress, oxidative stress responses, and responses to pathogen attack. Despite their wide variety of functions, the role of MTs in symbiotic associations, specifically nodule-fabacean symbiosis, is poorly understood. Here, we analyzed the role of the PvMT1A gene in Phaseolus vulgaris-Rhizobium tropici symbiosis using bioinformatics and reverse genetics approaches. Using in silico analysis, we identified six genes encoding MTs in P. vulgaris, which were clustered into three of the four classes described in plants. PvMT1A transcript levels were significantly higher in roots inoculated with R. tropici at 7 and 30 days post inoculation (dpi) than in non-inoculated roots. Functional analysis showed that downregulating PvMT1A by RNA interference (RNAi) reduced the number of infection events at 7 and 10 dpi and the number of nodules at 14 and 21 dpi. In addition, nodule development was negatively affected in PvMT1A:RNAi transgenic roots, and these nodules displayed a reduced nitrogen fixation rate at 21 dpi. These results strongly suggest that PvMT1A plays an important role in the infection process and nodule development in P. vulgaris during rhizobial symbiosis.

Metallothionein 2A gene polymorphisms in relation to diseases and trace element levels in humans

Human metallothioneins are a superfamily of low molecular weight intracellular proteins, whose synthesis can be induced by essential elements (primarily Zn and Cu), toxic elements and chemical agents, and stress-producing conditions. Of the four known isoforms in the human body MT2 is the most common. The expression of metallothioneins is encoded by a multigene family of linked genes and can be influenced by single nucleotide polymorphisms (SNPs) in these genes. To date, 24 SNPs in the MT2A gene have been identified with the incidence of about 1 % in various population groups, and three of them were shown to affect physiological and pathophysiological processes. This review summarises current knowledge about these three SNPs in the MT2A gene and their associations with element concentrations in the body of healthy and diseased persons. The most investigated SNP is rs28366003 (MT2A -5 A/G). Reports associate it with longevity, cancer (breast, prostate, laryngeal, and in paranasal sinuses), and chronic renal disease. The second most investigated SNP, rs10636 (MT2A +838G/C), is associated with breast cancer, cardiovascular disease, and type 2 diabetes. Both are also associated with several metal/metalloid concentrations in the organism. The third SNP, rs1610216 (MT2A -209A/G), has been studied for association with type 2 diabetes, cardiomyopathy, hyperglycaemia, and Zn concentrations. Metallothionein concentrations and MT2A polymorphisms have a potential to be used as biomarkers of metal exposure and clinical markers of a number of chronic diseases. This potential needs to be studied and verified in a large number of well-defined groups of participants (several hundreds and thousands) with a focus on particular physiological or pathological condition and taking into consideration other contributing factors, such as environmental exposure and individual genetic and epigenetic makeup.

Combined effects of DDE and hyperlipidic diet on metallothionein expression and synthesis in rat tissues

Metallothionein is well known for its detoxificant and anti-oxidant properties and has been shown to be effective to prevent hydroxyl radical-generated DNA degradation. The purpose of this investigation was to analyze the combined effect of two factors promoting cellular oxidative-stress, that is, the administration of the pesticide dichloro-diphenyl-dichloroethylene (DDE) and a high fat diet, on metallothionein expression and synthesis in rat liver and kidney. DDE is the main metabolite of dichloro-diphenyl-trichloroethane (DDT), and is commonly found in the food chain and in all tissues of living organisms, carried by the fats. Male Wistar rats were fed with a standard (N) or a high fat (HF) diet and exposed to DDE (10 mg/kg body mass, N + DDE and HF + DDE groups) or vehicle (corn oil, N, and HF groups) via gavage every day for 28 days. Tissues histology was determined by light microscopy analysis; differences in metallothionein gene expression and synthesis by real-time PCR and western blot, respectively. Finally, protein cellular localization was established by immunocytochemistry. The results showed a different involvement of metallothionein in defending tissues from HF- and DDE-induced oxidative stress, suggesting that hepatic and renal cells use different strategies against pro-oxidant species. In both cell types a marked increase in the metallothionein content was observed in the nucleus, with a concomitant drop of the cytoplasmatic protein, either under HF- and DDE-stress conditions; however, no synergistic or additive effects were observed between the action of fats and pesticide. These findings reinforce the role of metallothionein in protecting DNA from oxidative damage.

Zinc and Skin Disorders

The skin is the third most zinc (Zn)-abundant tissue in the body. The skin consists of the epidermis, dermis, and subcutaneous tissue, and each fraction is composed of various types of cells. Firstly, we review the physiological functions of Zn and Zn transporters in these cells. Several human disorders accompanied with skin manifestations are caused by mutations or dysregulation in Zn transporters; acrodermatitis enteropathica (Zrt-, Irt-like protein (ZIP)4 in the intestinal epithelium and possibly epidermal basal keratinocytes), the spondylocheiro dysplastic form of Ehlers-Danlos syndrome (ZIP13 in the dermal fibroblasts), transient neonatal Zn deficiency (Zn transporter (ZnT)2 in the secretory vesicles of mammary glands), and epidermodysplasia verruciformis (ZnT1 in the epidermal keratinocytes). Additionally, acquired Zn deficiency is deeply involved in the development of some diseases related to nutritional deficiencies (acquired acrodermatitis enteropathica, necrolytic migratory erythema, pellagra, and biotin deficiency), alopecia, and delayed wound healing. Therefore, it is important to associate the existence of mutations or dysregulation in Zn transporters and Zn deficiency with skin manifestations.

Accumulation of copper in the kidney of pigs fed high dietary zinc is due to metallothionein expression with minor effects on genes involved in copper metabolism

A study was conducted to determine the effect of high dietary zinc (Zn) oxide on trace element accumulation in various organs with special emphasis on the kidney. A total of 40 weaned piglets were allocated into two groups with 16 and 24 piglets each receiving a diet containing normal (NZn; 100mg Zn/kg) or high (HZn; 2,100mg Zn/kg) Zn concentration, respectively. After two weeks, eight piglets from each treatment were killed and organ samples were taken. Eight piglets from the remaining 16 pigs fed HZn diets were changed to NZn diets (CZn). All remaining piglets were killed after another two weeks for organ sampling. Trace element concentration was determined in the jejunum, liver, kidney, pancreas, bone (metacarpal IV), spleen, lung, thymus, tonsils and lymph nodes of jejunum, ileum and colon. Kidney mRNA expression of Zn transporter ZnT1 and ZIP4, genes involved in Cu metabolism (Ctr1, Atox1, SOD1, ATP7A, CCS, CP) and divalent metal ion transport (DMT1) and binding (MT-1a, MT-2b, MT-3) were determined. The Zn concentration in jejunum, liver, pancreas tissue and metacarpal IV was higher (P<0.05) in HZn group compared with NZn and CZn groups. Trace element concentration in organs of CZn pigs was similar to those fed NZn diets. Zn concentration in muscle, lung and lymphatic organs as thymus, tonsils, spleen and lymph nodes of jejunum, ileum and colon did not differ between the groups. Zn and Cu were positively correlated (R=0.67; P<0.05) in the kidney. No significant differences for Cu chaperones, Cu transporters and Cu-dependent factors were determined despite decreased expression of Atox1 after two weeks and increased Ctr1 expression over time in the HZn group. Expression of MT-1a, MT-2b and MT-3 were significantly higher in HZn fed pigs with most pronounced effects for MT-1a > MT-2b > MT-3. Gene expression of MTs in pigs fed CZn diets did not differ from pigs fed NZn diets. The data suggest that high dietary Zn feeding in pigs leads to Cu co-accumulation in the kidney of pigs with minor effect on genes relevant for Cu metabolism. In addition, the organ Zn and Cu accumulation is reversible after two weeks of withdrawal of high dietary Zn.

Identification of hepatocellular carcinoma-related genes with a machine learning and network analysis

Liver cancer is one of the leading causes of cancer mortality worldwide. Hepatocellular carcinoma (HCC) is the main type of liver cancer. We applied a machine learning approach with maximum-relevance-minimum-redundancy (mRMR) algorithm followed by incremental feature selection (IFS) to a set of microarray data generated from 43 tumor and 52 nontumor samples. With the machine learning approach, we identified 117 gene probes that could optimally separate tumor and nontumor samples. These genes not only include known HCC-relevant genes such as MT1X, BMI1, and CAP2, but also include cancer genes that were not found previously to be closely related to HCC, such as TACSTD2. Then, we constructed a molecular interaction network based on the protein-protein interaction (PPI) data from the STRING database and identified 187 genes on the shortest paths among the genes identified with the machine learning approach. Network analysis reveals new potential roles of ubiquitin C in the pathogenesis of HCC. Based on gene ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis, we showed that the identified subnetwork is significantly enriched in biological processes related to cell death. These results bring new insights of understanding the process of HCC.

Mapping the global mRNA transcriptome during development of the murine first molar

The main objective of this study was to map global gene expression in order to provide information about the populations of mRNA species participating in murine tooth development at 24 h intervals, starting at the 11th embryonic day (E11.5) up to the 7th post-natal day (P7). The levels of RNA species expressed during murine tooth development were mesured using a total of 58 deoxyoligonucleotide microarrays. Microarray data was validated using real-time RT-PCR. Differentially expressed genes (p < 0.05) were subjected to bioinformatic analysis to identify cellular activities significantly associated with these genes. Using ANOVA the microarray data yielded 4362 genes as being differentially expressed from the 11th embryonic day (E11.5) up to 7 days post-natal (P7), 1921 of these being genes without known functions. The remaining 2441 genes were subjected to further statistical analysis using a supervised procedure. Bioinformatic analysis results for each time-point studied suggests that the main molecular functions associated with genes expressed at the early pre-natal stages (E12.5–E18.5) were cell cycle progression, cell morphology, lipid metabolism, cellular growth, proliferation, senescence and apoptosis, whereas most genes expressed at post-natal and secretory stages (P0–P7) were significantly associated with regulation of cell migration, biosynthesis, differentiation, oxidative stress, polarization and cell death. Differentially expressed genes (DE) not described earlier during murine tooth development; Inositol 1, 4, 5-triphosphate receptor 3 (Itpr3), metallothionein 1(Mt1), cyclin-dependent kinase 4 (Cdk4), cathepsin D (Ctsd), keratin complex 2, basic, gene 6a (Krt2-6a), cofilin 1, non-muscle (Cfl1), cyclin 2 (Ccnd2), were verified by real-time RT-PCR.

Genistein versus ICI 182, 780: an ally or enemy in metastatic progression of prostate cancer

BACKGROUND

Androgen signalling through the androgen receptor (AR) plays a critical role in prostate cancer (PCa) initiation and progression. Estrogen in synergy with androgen is essential for cell growth of the normal and malignant prostate. However, the exact role that estrogen and the estrogen receptor play in prostate carcinogenesis remains unclear. We have previously demonstrated the metastasis-promoting effect of an estrogen receptor beta (ERβ) agonist (genistein) in a patient-derived PCa xenograft model mimicking localized and metastatic disease.

METHODS

To test the hypothesis that the tumor-promoting activity of genistein was due to its estrogenic properties, we treated the xenograft-bearing mice with genistein and an anti-estrogen compound (ICI 182, 780) and compared the differential gene expression using microarrays.

RESULTS

Using a second xenograft model which was derived from another patient, we showed that genistein promoted disease progression in vivo and ICI 182, 780 inhibited metastatic spread. The microarray analysis revealed that the metallothionein (MT) gene family was differentially expressed in tumors treated by these compounds. Using qRT-PCR, the differences in expression levels were validated in the metastatic and non-metastatic LTL313 PCa xenograft tumor lines, both of which were originally derived from the same PCa patient.

CONCLUSIONS

Together our data provide evidence that genistein stimulates and ICI 182, 780 inhibits metastatic progression, suggesting that these effects may be mediated by ERβ signalling.

Relevance of infection with human papillomavirus: the role of the p53 tumor suppressor protein and E6/E7 zinc finger proteins (Review)

Human papillomaviruses (HPV) are small circular, double-stranded DNA viruses infecting epithelial tissues. HPV types can be classified both as high-risk or low-risk. Of the more than 120 different identified types of HPV, the majority are involved in infections of the genital tract, cancer of the cervix, vulva, vagina and penis, and of non-anogenital localizations, such as the head and neck areas. From the point of view of the infection, human papillomaviruses have developed several molecular mechanisms to enable infected cells to suppress apoptosis. This review provides a comprehensive and critical summary of the current literature that focuses on cervical carcinoma and cancer of the head and neck caused by HPV. In particular, we discuss HPV virology, the molecular mechanisms of carcinogenesis, the role of the tumor suppressor protein p53 and the E6/E7 zinc finger proteins. Classification of HPV according to diagnosis is also described.

Role of metallothionein 1E in the migration and invasion of human glioma cell lines

Metallothionein 1E (MT1E) has been found to be highly expressed in motile cell lines. We investigated whether MT1E actually modulates the migration and invasion of human glioma cell lines and the types of factors that have an effect on MT1E. RNA differential display was performed using Genefishing™ technology in the human glioma cell lines U343MG-A, U87MG and U87MG-10'; the results were validated by RT-PCR and northern blot analysis, in order to detect possible genetic changes as the determining factors for migration ability in malignant glioma. MT1E was identified in U87MG, a highly motile cell line. The migration and invasion abilities of human glioma cell lines, and MT1E transfectants were investigated using simple scratch testing and Matrigel invasion assays. Morphological and cytoskeletal (actin, vimentin) changes were documented by light and confocal microscopy. The expression of MT1E in four glioma cell lines was assessed by RT-PCR and western blotting. In addition, the effects of MT1E on the activity of the NF-κB p50/p65 transcription factor, MMP-2 and -9 were examined by western blotting and zymography. The endogenous MT1E expression in the human glioma cell lines was statistically correlated with their migratory abilities and invasion. The U87-MT-AS cells became more round and had decreased stress fibers, compared with the U87MG cells. Endogenous MT1E expression in the four human glioma cell lines was directly correlated with migration. Two antisense MT1E-transfected cell lines showed decreased NF-κB p50 translocation into the nucleus, which led to decreased activity of MMP-9 in conditioned media. It may be postulated that MT1E can enhance the migration and invasion of human glioma cells by inducing MMP-9 inactivation via the upregulation of NF-κB p50.

Lack of ceruloplasmin expression alters aspects of copper transport to the fetus and newborn, as determined in mice

Copper transport and accumulation were studied in virgin and lactating C57BL/6 mice, with and without expression of ceruloplasmin (Cp), to assess the importance of Cp to these processes. One hour after i.p. injection of tracer (64)Cu, liver and kidney accounted for 80% of the radioactivity, and mammary gland 1%, while in lactating Cp+/+ mice 2-4 days post partum, uptake by mammary gland was 9-fold higher and that of liver and other organs was decreased, with (64)Cu rapidly appearing in milk. Parallel studies in Cp-/- mice (siblings from same colony) gave virtually identical results. However, their milk contained less (64)Cu, and actual copper contents determined by furnace atomic absorption were less than half those for milk from normal dams. Liver copper concentrations of pups born to Cp-/- dams also were half those of pups from wild type dams. Copper in pup brains was unaffected; but iron concentrations were reduced. We conclude that absence of Cp, while not affecting entry of exchangeable copper from the blood into the mammary gland, does have a significant effect on the availability of this metal to the newborn through the milk and in the form of stores accumulating in gestation.

The potential effect of metallothionein 2A -5A/G single nucleotide polymorphism on blood cadmium, lead, zinc and copper levels

Metallothioneins (MTs) are low molecular weight, cysteine-rich, metal-binding proteins. Because of their rich thiol groups, MTs bind to the biologically essential metals and perform these metals' homeostatic regulations; absorb the heavy metals and assist with their transportation and extraction. The aim of this study was to investigate the association between the metallothionein 2A (MT2A) core promoter region -5 A/G single nucleotide polymorphism (SNP) and Cd, Pb, Zn and Cu levels in the blood samples. MT2A polymorphism was determined by the standard polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) technique using the 616 blood samples and the genotype frequencies were found as 86.6% homozygote typical (AA), 12.8% heterozygote (AG) and 0.6% homozygote atypical (GG). Metal levels were analyzed by dual atomic absorption spectrophotometer system and the average levels of Cd, Pb, Zn and Cu in the blood samples were 1.69±1.57 ppb, 30.62±14.13 ppb, 0.98±0.49 ppm and 1.04±0.45 ppm, respectively. As a result; highly statistically significant associations were detected between the -5 A/G core promoter region SNP in the MT2A gene and Cd, Pb and Zn levels (p=0.004, p=0.012 and p=0.002, respectively), but no association was found with Cu level (p=0.595). Individuals with the GG genotype had statistically lower Zn level and higher Cd and Pb levels in the blood samples than individuals with AA and AG genotypes. This study suggests that having the GG genotype individuals may be more sensitive for the metal toxicity and they should be more careful about protecting their health against the toxic effects of the heavy metals.

Redox biochemistry of mammalian metallothioneins

Metallothionein (MT) is a generic name for certain families of structurally rather variable metal-binding proteins. While purely chemical or biological approaches failed to establish a single physiologic function for MTs in any species, a combination of chemical and biological approaches and recent progress in defining the low but significant concentrations of cytosolic free zinc(II) ions have demonstrated that mammalian MTs function in cellular zinc metabolism in specific ways that differ from conventional knowledge about any other metalloprotein. Their thiolate coordination environments make MTs redox-active zinc proteins that exist in different molecular states depending on the availability of cellular zinc and the redox poise. The zinc affinities of MTs cover a range of physiologic zinc(II) ion concentrations and are modulated. Oxidative conditions make more zinc available, while reductive conditions make less zinc available. MTs move from the cytosol to cellular compartments, are secreted from cells, and are taken up by cells. They provide cellular zinc ions in a chemically available form and participate in cellular metal muffling: the combination of physiologic buffering in the steady state and the cellular redistribution and compartmentalization of transiently elevated zinc(II) ion concentrations in the pre-steady state. Cumulative evidence indicates that MTs primarily have a redox-dependent function in zinc metabolism, rather than a zinc-dependent function in redox metabolism.

Over-expression of metallothionein predicts chemoresistance in breast cancer

Metallothionein (MT) plays a role in fundamental cellular processes such as proliferation, apoptosis and differentiation. We examined MT expression in women with invasive breast ductal carcinoma who underwent mastectomy/lumpectomy without neo-adjuvant treatment. We showed that MT was over-expressed in 87.9% of breast cancer tissues examined, with the mean percentage of positive cells at 30%. There were two patterns of MT expression: predominantly cytoplasmic in 75.9% and nuclear in 24.1% of MT-positive cases. Higher MT scores were associated with poorer histological grade (p = 0.009) but were independent of age, tumour size and oestrogen receptor status. For patients who were treated with adjuvant chemotherapy (cyclophosphamide/methotrexate/5 fluorouracil- or doxorubicin-based regimes), those with high MT expression had a significantly lower recurrence-free survival (p = 0.048), suggesting a role of MT in predicting disease recurrence. Down-regulation of MT in MCF-7 cells by silencing the MT-2A gene (the most abundantly expressed of the 10 known functional MT isoforms) increased chemosensitivity of the cells to doxorubicin. To examine the mechanisms underlying these clinical data, we used siRNAs to decrease MT-2A mRNA expression and protein expression. In MT down-regulated cells challenged with the IC(50) concentration of doxorubicin, we observed a significant reduction in cell viability. Cell cycle analysis also revealed a corresponding increase in apoptosis in the MT down-regulated cells following doxorubicin exposure, showing that down-regulation of MT increased susceptibility to doxorubicin cytotoxicity. The data suggest that MT could be a potential marker of chemoresistance and a molecular therapeutic target.

Renal metallothionein responds rapidly and site specifically to zinc repletion in growing rats

Metallothionein (MT) is important for heavy metals and free radical protection in the kidney. MT is responsive to zinc and primarily localized within the renal cortex. However, site-specific renal responses to dietary zinc repletion are understudied. The objective of this study was to examine the effects of dietary zinc deficiency and repletion on renal MT concentration and immunolocalization in rats. Weanling male Sprague Dawley rats were randomly assigned to either a zinc-deficient, zinc control, or pair-fed to zinc-deficient group. Half of the zinc-deficient and pair-fed rats were repleted with the control diet ad libitum for an additional 24h. Renal tissue samples were assessed for total zinc, MT concentrations and MT immunostaining. Dietary zinc deficiency reduced renal zinc and MT concentrations, and attenuated intensity and localization of MT. Dietary zinc repletion for 24h restored renal zinc and MT concentrations, the latter primarily in the proximal convoluted tubules of the cortex. Concentrations of renal MT, but not zinc, were elevated by diet restriction and MT (microg/mg protein) and partially normalized by 24h diet repletion. In conclusion, renal MT modification due to zinc deficiency or diet restriction can be rapidly normalized in a site-specific manner with normal dietary zinc intake. The results support a role for MT in kidney homeostasis, in particular at the level of the proximal tubules in the cortex. The speed of MT repletion may have clinical implications for dietary zinc in the treatment of acute and chronic renal pathology due to toxins and free radicals.

The renal metallothionein expression profile is altered in human lupus nephritis

Introduction

Metallothionein (MT) isoforms I + II are polypeptides with potent antioxidative and anti-inflammatory properties. In healthy kidneys, MT-I+II have been described as intracellular proteins of proximal tubular cells. The aim of the present study was to investigate whether the renal MT-I+II expression profile is altered during lupus nephritis.

Methods

Immunohistochemistry was performed on renal biopsies from 37 patients with lupus nephritis. Four specimens of healthy renal tissue served as controls. Clinicopathological correlation studies and renal survival analyses were performed by means of standard statistical methods.

Results

Proximal tubules displaying epithelial cell MT-I+II depletion in combination with luminal MT-I+II expression were observed in 31 out of 37 of the lupus nephritis specimens, but not in any of the control sections (P = 0.006). The tubular MT score, defined as the median number of proximal tubules displaying this MT expression pattern per high-power microscope field (40× magnification), was positively correlated to the creatinine clearance in the lupus nephritis cohort (P = 0.01). Furthermore, a tubular MT score below the median value of the cohort emerged as a significant predictor of a poor renal outcome in renal survival analyses. Thus, patients with a tubular MT score < 1.0 had a 6.2-times higher risk of developing end-stage renal disease than patients with a tubular MT score ≥ 1.0 (P = 0.03).

Conclusion

Lupus nephritis is associated with significant alterations in renal MT-I+II expression. Our data indicate that important prognostic information can be deduced from the renal MT-I+II expression profile in systemic lupus erythematosus patients with nephritis.

Transmission electron microscopy study of the effects of cadmium and copper on fetal rat liver tissue

During the entire period of their pregnancies, three groups of adult pregnant Wistar albino rats were provided with tap water (control; group I) or with tap water containing 10 mg/kg CdCl2 (group II) or 10 mg/kg CdCl2 plus 10 mg/kg CuSO4 (group III). At term, the animals were sacrificed and the fetal livers were removed and examined under electron microscopy. The liver tissue of the fetuses in maternal groups II and III showed degenerative changes to their hepatocytes. In group II, the smooth endoplasmic reticulum tubules showed dilatation, and the mitochondria showed a dense matrix. In group III, some mitochondrial degeneration was also seen, with a diluted matrix and mitochondrial dilatation. There were also more heterochromatic nuclei and an increased number of ribosomes. None of these histopathological changes were present in the fetal liver samples from the maternal group I control animals.

Increased abundance of labile intracellular zinc during cell proliferation was due to increased retention of extracellular zinc in 3T3 cells

Platelet-derived growth factor (PDGF)-, epidermal growth factor (EGF)- and insulin-like growth factor I (IGF-I)-stimulated cell proliferation in 3T3 cells was accompanied by increased abundance of labile intracellular pool of zinc (LIPZ). However, the origin and regulation of this cell proliferation-associated increase in the abundance of LIPZ are unknown. Cellular zinc homeostasis involves zinc transporters and metallothionein. The objectives of this study were to determine whether cell proliferation-associated increase in the abundance of LIPZ was a result of an increased zinc uptake and to assess the involvement of zinc transporters and metallothionein in this cell proliferation-associated increase in the abundance of LIPZ in 3T3 fibroblasts. Zinc transporters assessed included both zinc importer (Zip1) and zinc exporters (ZnT1, ZnT2 and ZnT4). Growth factors increased the abundance of LIPZ while total cellular zinc concentration remained unaffected, demonstrating that LIPZ was responsive to the increased needs for zinc during growth factor-stimulated cell proliferation. Growth factors also increased net zinc retention as indicated by higher 65zinc radioactivity and elevated mRNA levels of Zip1, ZnT1 and ZnT4. Although zinc is essential to cell proliferation, excessive cellular zinc accumulation causes cytotoxicity. Collectively, these observations suggest that increase in the abundance of LIPZ during growth factor-stimulated cell proliferation was due to increased net retention of extracellular zinc, which was apparently achieved through a coordinated up-regulation of the expression of transporters involved in both zinc influx and efflux to ensure adequate supply of zinc to sustain cell proliferation, yet to prevent potential zinc cytotoxicity in 3T3 cells.

Interferonbeta-induced changes in metallothionein expression and subcellular distribution of zinc in HepG2 cells

We evaluated the changes of metallothionein induction and cellular zinc distribution in HepG2 cells by interferonbeta treatment. Immunohistochemical staining of metallothionein was observed in the cytoplasm and nuclei of hepatocytes; which was observed predominantly in the cells treated with interferon and zinc compared to those with zinc alone, interferon alone or the no-treated control. The cellular zinc level was higher in order of the interferon- and zinc-treated cells, the zinc-alone-treated cells, and the interferon-alone-treated cells. Flow cytometry showed that S-phase population increased in interferon-alone-treated cells and interferon- and zinc-treated cells, but not in zinc-alone-treated ones. Cellular elemental distribution was analyzed using in-air micro-particle induced X-ray emission. In zinc-alone-treated sample, X-ray spectra showed good consistency between the enhanced cellular zinc distribution and the phosphorous map. Localizations of bromine followed by interferon treatment were found accompanying a spatial correlation with the phosphorous map. The samples treated with interferon and zinc showed the marked accumulation of zinc and bromine. Discrete bromine accumulation sites were clearly visible with a strong spatial correlation followed by zinc accumulation. These findings suggest that interferonbeta in combination with zinc predominantly induces metallothionein expression in HepG2 cells. In addition, interferonbeta may promote the translocation of metallothionein-bound zinc from cytoplasm to S-phase nuclei.

Comparison of differential gene expression of hot and cold thyroid nodules with primary epithelial cell culture models by investigation of co-regulated gene sets

Both autonomously functioning thyroid nodules (AFTNs) and cold thyroid nodules (CTNs) are characterized by an increased proliferation, however, they have opposite functional activities. Therefore, with the aim to further understand the distinct molecular pathology of each entity and to discover common mechanisms like those leading to increased proliferation in both, AFTNs and CTNs, we now compared gene expression of AFTNs and CTNs with in vitro model systems (TSH-stimulated and ras-transfected primary cultures (PC)) whose gene expression patterns can be attributed to specific molecular alterations. Since combinations of co-regulated genes are more likely to reveal molecular mechanisms, we used a procedure which groups co-regulated genes within "gene sets". We found a co-regulated gene set in the AFTNs that overlaps with differential expression in TSH-stimulated PCs but not in CTNs or ras-transfected PCs. In addition to thyroid peroxidase and sialyltransferase 1, this set of co-regulated genes comprises metallothioneins and the G-protein-coupled receptor 56. Although their role in the thyroid is unknown so far, their appearance in one group indicates a functional relevance in TSH-TSH receptor-stimulated mechanisms. Furthermore, we identified down-regulated gene sets with concordant expression patterns in AFTNs, CTNs and ras-transfected PCs. However, these expression patterns are not of relevance in the TSH-stimulated PCs. These findings suggest that TSH-stimulated PCs can be used as a model of increased thyroid function (AFTNs), whereas the ras-transfected PCs better reflect the increased proliferation of both AFTNs and CTNs.

Localization and role of metallothioneins in the olfactory pathway after exposure to mercury vapor

We have investigated the localization and physiological roles of metallothioneins (MTs) in the olfactory pathway after exposure to mercury (Hg0) vapor. Male MT-null and wild-type mice were examined for the distribution of mercury, MT immunoreactivity and MT-III mRNA expression. There were no signs of histological changes in MT-null or wild-type mice. Light and electron microscopy of the samples stained with autometallography demonstrated chronological transfer of exposed mercury granules to the olfactory bulb by way of the olfactory tract. Basal expression of MT-I and -II immunoreactivity was observed in supporting cells, basal cells and acinar cells of the Bowman's gland of the olfactory mucosa in wild-type mice even without mercury exposure. In situ hybridization showed that signals for MT-III mRNA dominated in the olfactory cells of the olfactory mucosa, neurons in the olfactory bulb and those of brain in MT-null and wild-type mice. No difference in these findings was observed between samples taken at any interval after mercury exposure.

Increased levels of metallothionein in placenta of smokers

Experiments were designed to evaluate and compare metallothionein (MT), zinc and cadmium levels in human placentas of smoking and non-smoking women. Smoking was assessed by self-reported cigarette consumption and urine cotinine levels before delivery. Smoking pregnant women with urine cotinine levels higher than 130 ng/ml were included in the smoking group. Determination of placental MT was performed by western blot analysis after tissue homogenization and saturation with cadmium chloride (1000 ppm). Metallothionein was analyzed with a monoclonal antibody raised against MT-1 and MT-2 and with a second anti mouse antibody conjugated to alkaline phosphatase. Zinc and cadmium were determined by neutron activation analysis and atomic absorption spectrometry respectively. Smokers showed higher placental MT and cadmium levels, together with decreased newborn birth weights, as compared to non-smokers. The semi-quantitative analysis of western blots by band densitometry indicated that darker bands corresponded to MT present in smokers' samples. This study confirms that cigarette smoking increases cadmium accumulation in placental tissue and suggests that this element has a stimulatory effect on placental MT production.

Nuclear trafficking of metallothionein requires oxidation of a cytosolic partner

The present study revealed the mechanism underlying the nuclear trafficking of metallothionein (MT). Nuclear localization of MT in digitonin-permeabilized BALB 3T3 cells was enhanced in the presence of a cytosolic factor added as a rat red blood cell lysate by oxidation with H2O2 in a dose-dependent manner, but inhibited with excess glutathione. A cytosolic partner was assumed to bind MT and retain it in the cytoplasm, and its oxidation can mobilize MT to the nuclei on cellular oxidation. Pre-treatment of nuclei with H2O2 did not enhance the localization, and MT that had been localized in the nuclei was washed out, indicating that MT is in the nuclei as a result of a higher rate of uptake by the nuclei than the rate of diffusion from the nuclei. Nuclear localization of lysozyme and nuclear localization signal (NLS)-bearing allophycocyanin were not enhanced by the oxidation in the presence of cytosolic factor, suggesting that the nuclear traffic occurring on oxidation is specific to MT. Moreover, when cells were arrested the cell cycle at the S phase, MT was localized in the nuclei in response to coincidental generation of a feeble reactive oxygen species (ROS). These observations suggest that MT comes localized in the nuclei on the sensing of intracellular oxidation, whereby a cytosolic partner specific to MT comes oxidized as a cargo system, MT being localized as a result of enhanced uptake in the nuclei and re-localized in the cytoplasm diffusely. Nuclear MT was proposed to protect the nuclei from the oxidation occurring with progression of the cell cycle.

Impaired hepatic regeneration in metallothionein-I/II knockout mice after partial hepatectomy

Although the translocation of metallothionein (MT) from cytoplasm to nucleus has been demonstrated in liver during times of high requirement for zinc (fetal development and the neonatal period), the role of MT in cellular growth is not well understood. In this study, a potential role of MT in liver regeneration was investigated in wild type (WT) and MT-I and MT-II gene knockout (MT-null) mice after 35% partial hepatectomy (PH) or sham laparotomy. Hepatic MT levels and proliferation index were measured at 0, 5, 15, 24, 36, 48, and 60 hrs after PH and 48 hrs after sham laparotomy (control). MT levels were increased in WT mice (peak at 24 hrs after PH) and declined to normal levels by 60 hrs after PH. Immunohistochemical staining for MT in WT mice indicated the presence of MT in both nucleus and cytoplasm of hepatocytes at 24 hrs after PH, whereas MT was present mainly in the cytoplasm at 36-60 hrs after PH and 48 hrs after sham laparotomy. Hepatic proliferation index in both WT and MT-null mice, as determined by argyrophilic nucleolar organizing region staining and proliferating cell nuclear antigen immunohistochemical staining, reached a peak at 48 hrs and declined by 60 hrs after PH. Cell proliferation was significantly less in MT-null mice as compared to WT mice during liver regeneration after PH. These results suggest that MT may play a positive role in hepatic regeneration after PH.

Trace elements in human physiology and pathology: zinc and metallothioneins

Zinc is one of the most abundant nutritionally essential elements in the human body. It is found in all body tissues with 85% of the whole body zinc in muscle and bone, 11% in the skin and the liver and the remaining in all the other tissues. In multicellular organisms, virtually all zinc is intracellular, 30-40% is located in the nucleus, 50% in the cytoplasm, organelles and specialized vesicles (for digestive enzymes or hormone storage) and the remainder in the cell membrane. Zinc intake ranges from 107 to 231 micromol/d depending on the source, and human zinc requirement is estimated at 15 mg/d. Zinc has been shown to be essential to the structure and function of a large number of macromolecules and for over 300 enzymic reactions. It has both catalytic and structural roles in enzymes, while in zinc finger motifs, it provides a scaffold that organizes protein sub-domains for the interaction with either DNA or other proteins. It is critical for the function of a number of metalloproteins, inducing members of oxido-reductase, hydrolase ligase, lyase family and has co-activating functions with copper in superoxide dismutase or phospholipase C. The zinc ion (Zn(++)) does not participate in redox reactions, which makes it a stable ion in a biological medium whose potential is in constant flux. Zinc ions are hydrophilic and do not cross cell membranes by passive diffusion. In general, transport has been described as having both saturable and non-saturable components, depending on the Zn(II) concentrations involved. Zinc ions exist primarily in the form of complexes with proteins and nucleic acids and participate in all aspects of intermediary metabolism, transmission and regulation of the expression of genetic information, storage, synthesis and action of peptide hormones and structural maintenance of chromatin and biomembranes.

Expression of metallothionein in synovial sarcoma cells

Other authors have demonstrated earlier that cells of normal synovium contain metallothionein. The protein was also detected in several other normal cell types and in tumors derived from the cells. Metallothionein content is thought to reflect proliferative activity of neoplastic cells. Therefore, it was decided to demonstrate metallothionein expression in various types of synovial sarcoma. The present study aimed to determine metallothionein cellular expression by immunocytochemical techniques in nine cases of biphasic, six cases of monophasic (spindle cell), and five cases of poorly differentiated synovial sarcoma, and to compare the expression with those of vimentin and cytokeratin 19. Metallothionein expression was demonstrated in epithelioid cells in all cases of biphasic type sarcoma and in spindle cells in all cases of monophasic type tumors. In poorly differentiated tumors, metallothionein expression was detected in four of five cases (80%). Expression of cytokeratin 19 was typical for epithelioid cells and expression of vimentin for spindle cells of synovial sarcoma. A much less pronounced expression of the proteins was observed in poorly differentiated tumors. The results indicate that metallothionein expression may prove useful in differential diagnosis and for defining prognosis in cases of synovial sarcomas.

Metallothionein in liver-biopsies from patients with different diseases

Metallothioneins (MT) are ubiquitous found in eukaryotic organism. MT have a potential for metal-storage and protect the cells against stress. On the genomic level, proinflammatory cytokines like interleukin-6 and transition metals like copper cause induction of MT. Therefore, an estimation of MT in liver-biopsies from patients with different diseases probably could help in identifying acute-phase reactions and processes which lead to increased copper. We investigated paraffin embedded liver biopsies from 170 patients and 13 control biopsies from cases of sudden death. Tissue was stained with a primary antibody against MT and a peroxidase technique was used to make results visible. A grading was performed using an immunoreactive score (IRS from 0-24) and by computer-aided measurement of the optical density (OD) of the stained tissue slides. Patients with cholestasis (IRS: 12.1 +/- 2.8, n = 11), autoimmune (10.6 +/- 3.1, n = 7) or inflammatory bowel diseases (IBD) (13.3 +/- 5.1, n = 4) and lymphoma (9.8 +/- 5.8, n = 21) showed marked increases in MT compared to the controls (5.2 +/- 2.8, n = 13). Patients with chronic hepatitis B or C or chronic alcoholic abuse had no elevation of MT. Furthermore, no correlation was found between histological damage and amount of MT except in cases of cholestasis, in which increased MT was observed. Results by OD confirmed the findings. In summary, we were able to demonstrate a clear increase of MT content in liver-biopsies in proinflammatory and cholestatic conditions. Marked elevation in patients with systemic diseases (like autoimmune-, IBD and lymphoma) seems to be best explained by an acute-phase induction of MT by proinflammatory cytokines. This could help in identifying these conditions in liver biopsies.

Membrane phospholipid reorganization differentially regulates metallothionein and heme oxygenase by heme-hemopexin

Heme-hemopexin coordinately regulates genes encoding protective proteins including metallothionein-I (MT-I) and heme oxygenase 1 (HO-1). Hexamethylene-bisacetamide (HMBA), which induces differentiation and activates protein kinase C (PKC), synergistically augments the induction of both MT-I and MT-II mRNAs in response to heme-hemopexin, but attenuates the induction of HO-1. HMBA also augments the increase in MT mRNA in response to cobalt protoporphyrin-hemopexin, a hemopexin (HPX) receptor ligand that activates signaling cascades without tetrapyrrole uptake. Unlike the PKC-activating phorbol esters that induce MT-I and HO-1, HMBA has minimal effects on MT-I or HO-1. HMBA is an amphipathic molecule, and is shown here to interact physically with lipids in model membranes using differential scanning calorimetry (DSC). The data are consistent with a stabilization of the lipid bilayer and an HMBA-induced segregation of lipids into separate domains each relatively enriched in one of the lipids. HMBA also perturbs membrane-protein interactions, and causes a loss of PKC and G-protein subunits from plasma membranes in vitro. Taken together, these observations reveal an additional level of complexity in the regulation of protective proteins induced by HPX, and which may take place in vivo in response to natural compounds that reorganize membrane phospholipids. A model is proposed whereby a reorganization of lipids by HMBA alters signaling pathways and fusion events considered to be the etiology of the differential response of the MT-1 (and MT-II) and the HO-1 genes to HMBA and heme-HPX.

Reduction of copper and metallothionein in toxic milk mice by tetrathiomolybdate, but not deferiprone

Copper is both essential for life and toxic. Aberrant regulation of copper at the level of intracellular transport has been associated with inherited diseases, including Wilson's disease (WND) in humans. WND results in accumulation of copper and the copper and zinc-binding protein metallothionein (MT) in liver and other tissues, liver degeneration, and neurological dysfunction. The toxic milk (TX) mutation in mice results in a phenotype that mimics human WND, and TX has been proposed to be a model of the disease. We characterized TX mice as a model of altered metal ion and MT levels during development, and after treatment with the metal ion chelators tetrathiomolybdate (TTM) and deferiprone (L1). We report that hepatic, renal and brain copper and MT are elevated in TX mice at 3 and 12 months of age. Zinc was significantly higher in TX mouse liver, but not brain and kidney, at both time points. Nodules appeared spontaneously in TX mouse livers at 8-12 months that maintained high copper levels, but with more normal morphology and decreased MT levels. Treatment of TX mice with TTM significantly reduced elevated hepatic copper and MT. Transient increases in blood and kidney copper accompanied TTM treatment and indicated that renal excretion was a significant route of removal. Treatment with L1, on the other hand, had no effect on liver or kidney copper and MT, but resulted in increased brain copper and MT levels. These data indicate that TTM, but not L1, may be useful in treating diseases of copper overload including WND.

Metallothionein expression and localization in rat bone tissue after cadmium injection

We investigated the induction of metallothionein (MT) by cadmium (Cd) in the bone tissue of rats. To clarify the cell response to Cd in bone, the isoform-specific expression of MT mRNAs (MT-I and MT-II) was examined by reverse transcriptase-polymerase chain reaction (RT-PCR). Both MT-I and MT-II mRNA levels were increased within 3 h by Cd administration. MT (MT-I/MT-II) localization after single Cd injection were also confirmed by immunohistochemical studies. Notably, MT-positive cells were time-dependently increased, and the positive cells were mainly localized in osteocytes. The cell-specific induction of MT may be associated with Cd accumulation and Cd-induced bone injury in vivo. Furthermore, we also found that MT was consecutively expressed in some osteoclasts of control rats. This finding suggested a new role of osteoclasts in bone metabolism.

Dietary zinc deficiency and repletion modulate metallothionein immunolocalization and concentration in small intestine and liver of rats

Metallothionein (MT) functions in zinc (Zn) homeostasis and dietary Zn affects tissue MT concentration. The objective of this study was to investigate the effects of dietary Zn deficiency and 24-h Zn repletion on MT immunolocalization and concentration in the small intestine and liver of growing rats. Three-week-old rats fed Zn-deficient diet (< 1 mg Zn/kg) for 16 d had no MT staining in either small intestine or liver. After 24-h Zn repletion with control diet (30 mg Zn/kg), strong MT staining was observed in intestinal Paneth cells and surface epithelial cells in the proliferative regions of villi. Pair-fed control rats had strong MT staining in liver that was localized around central veins. After 24-h energy repletion, the hepatic MT staining diminished. Furthermore, Zn-deficient rats had significantly reduced intestinal (57%) and hepatic (61%) MT concentrations but unaffected Zn concentrations compared with controls that consumed food ad libitum. Zn repletion for 24 h restored intestinal and hepatic MT concentrations and reduced hepatic Zn concentration. Pair-fed control rats had elevated MT concentration in liver that was normalized by energy repletion. There was a significant positive correlation between tissue Zn and MT concentrations in liver (r = 0.60, P = 0.0001), but not in small intestine. In summary, MT immunolocalization and concentration in rat small intestine and liver were responsive to changes in Zn status, supporting the role of MT in Zn metabolism. Cell-type-specific localization of MT in small intestine after dietary Zn manipulations indicates a function of Zn and MT in gut immunity and intestinal mucosal turnover, and the pattern of hepatic MT distribution with energy restriction may be linked to detoxification processes.

Metallothionein expression in renal cell carcinoma: subcellular localization and prognostic significance

PURPOSE

We investigated the immunohistochemical localization of metallothionein (MT) in renal cell carcinoma and determined the potential role of MT expression as a possible prognostic variable for tumor proliferation and progression.

MATERIALS AND METHODS

Tumor tissue blocks from 70 patients with renal cell carcinoma who underwent radical or partial nephrectomy were investigated. Mean followup plus or minus standard error was 36 +/- 3 months. Immunohistochemical testing was performed by the avidin-streptavidin method using a monoclonal mouse antiMT antibody. MT staining intensity in samples was evaluated semiquantitatively. The subcellular distribution of MT was also determined. Staining characteristics were compared with the clinicopathological results.

RESULTS

MT immunostaining was found in 39 of 70 tumors (55.7%) and subcellulary MT was localized in the cytoplasm, nucleus and cell membrane. The survival of patients with MT immunostaining was significantly worse than that of those with MT negative results (p = 0.02). A significant relationship of higher tumor grade and MT staining intensity was observed in grades I and III (p = 0.01), and grades II and III (p = 0.02) tumors. No association was found of MT expression and pathological stage. Sarcomatoid tumors showed significantly higher MT expression than clear cell, papillary, granular or chromophobe tumors (p = 0.02, 0.001, 0.01 and 0.01, respectively). MT expression was not an independent prognostic variable.

CONCLUSIONS

MT over expression seems to be associated with malignant behavior and poor prognosis in renal cell carcinoma. Therefore, MT expression may be considered a useful marker of less differentiated and more aggressive renal cell carcinoma.

Localization of metallothionein (MT) and expression of MT isoforms induced by cadmium in rat dental pulp

We investigated the induction of metallothionein (MT) by cadmium (Cd) in the dental pulp of rat incisors. Time-course studies of MT mRNA expression after single Cd injection were observed by Northern-blot analysis. The isoform-specific expressions of MT mRNAs (MT-I, MT-II and MT-III) were observed using the reverse transcriptase-polymerase chain reaction (RT-PCR) method. Both MT-I and MT-II mRNA levels increased within 3 h, peaked at 3 h and then decreased. These findings demonstrated that MT-I and MT-II mRNA were rapidly induced by Cd in dental pulp. MT-III mRNA was constitutively expressed in rat dental pulp, but the expression level did not change by Cd treatment. The localization of MT protein in Cd-treated rat dental pulp was determined by immunohistochemical staining using anti-MT antibody against MT-I and MT-II. MT protein was localized in the specific cell type of odontoblasts (secretory odontoblasts and resting odontoblasts). In conclusion, it is likely that stained MT in the immunohistochemical study should be MT-I and/or MT-II. Furthermore, MT-I and/or MT-II in Cd-treated rat dental pulp was localized in odontoblasts, in which accumulation of Cd were reported. The cell-specific synthesis of MT may be associated with its metal storage and detoxification role in dental tissues.

Copper and genomic stability in mammals

As the free ion and in the form of some complexes, there is no doubt that copper can promote damage to cellular molecules and structures through radical formation. At the same time, and perhaps as a consequence, mammals have evolved means of minimizing levels of free copper ions and destructive copper complexes that enter the organism and its cells. These means include tight binding of copper ions to protein carriers and transporters; direct exchange of copper between protein carriers, transporters, and cuproenzymes; and mobilization of secretory mechanisms and excretory pathways, as needed. As a consequence, normally, and except under certain genetic conditions, copper is likely to be benign to most mammals and not responsible for genomic instability, including fragmentation of and/or alterations to DNA, induction of mutations or apoptosis, or other toxic events. Indeed, cuproenzymes are important members of the antioxidant system of the organism.

The transport mechanism of metallothionein is different from that of classical NLS-bearing protein

A nuclear localization signal (NLS) has been detected in several nuclear proteins. Classical NLS-mediated nuclear pore targeting is performed by using the cytosolic factors, importin alpha and importin beta, whereas nuclear translocation requires the small GTPase, Ran. In the present study, we demonstrated that nuclear localization of metallothionein (MT) differs from that of classical NLS-mediated substrates. In digitonin-permeabilized BALB/c3T3 cells, biotinylated MT was localized in the nucleus in the presence of ATP and erythrocyte cytosol in the same manner as for SV40 large T NLS-conjugated allophycocyanin (APC-NLS). Under ATP-free conditions, nuclear rim-binding was observed in both transport substrates. Rim-binding of labeled MT was competitively inhibited by the addition of an excess amount of unlabeled MT. Different elution profiles were observed for the localization-promoting activities of MT in the cytosol compared to those of APC-NLS. Furthermore, nuclear localization of MT was determined to be a wheat germ agglutinin-insensitive, GTPgammaS-sensitive, and anti-Ran antibody-sensitive process. Green fluorescent protein-metallothionein (GFP-MT) fusion protein was also localized in the nucleus in the stable transformant of CHL-IU cells. These results strongly suggest that the targeting by MT of the nuclear pore is mediated by cytosolic factor(s) other than importins and that MT requires Ran for its nuclear localization.

Induction, regulation, degradation, and biological significance of mammalian metallothioneins

MTs are small cysteine-rich metal-binding proteins found in many species and, although there are differences between them, it is of note that they have a great deal of sequence and structural homology. Mammalian MTs are 61 or 62 amino acid polypeptides containing 20 conserved cysteine residues that underpin the binding of metals. The existence of MT across species is indicative of its biological demand, while the conservation of cysteines indicates that these are undoubtedly central to the function of this protein. Four MT isoforms have been found so far, MT-1, MT-2, MT-3, and MT-4, but these also have subtypes with 17 MT genes identified in man, of which 10 are known to be functional. Different cells express different MT isoforms with varying levels of expression perhaps as a result of the different function of each isoform. Even different metals induce and bind to MTs to different extents. Over 40 years of research into MT have yielded much information on this protein, but have failed to assign to it a definitive biological role. The fact that multiple MT isoforms exist, and the great variety of substances and agents that act as inducers, further complicates the search for the biological role of MTs. This article reviews the current knowledge on the biochemistry, induction, regulation, and degradation of this protein in mammals, with a particular emphasis on human MTs. It also considers the possible biological roles of this protein, which include participation in cell proliferation and apoptosis, homeostasis of essential metals, cellular free radical scavenging, and metal detoxification.

Delay of M-phase onset by aphidicolin can retain the nuclear localization of zinc and metallothionein in 3T3-L1 fibroblasts

The transient nuclear localization of metallothionein during cell growth and differentiation may be related to the increased requirement of zinc for DNA synthesis, activation of metalloenzymes, and transcription factors. Treatment of 3T3-L1 fibroblasts with aphidicolin, an inhibitor of nuclear DNA synthesis, caused a cell-cycle block at G1/S phase and a delay in the onset of M phase. This also resulted in the accumulation of both zinc and metallothionein in the nucleus. After removal of aphidicolin, the cells rapidly reentered S phase, and during the G2/M phase of cell cycle both zinc and metallothionein began to relocate to the cytoplasm. Delaying the onset of M phase in 3T3-L1 cells could prevent the cytoplasmic relocation of metallothionein. The nuclear translocation of both zinc and metallothionein during the cell cycle can be considered as a normal process and this may be a general mechanism in response to mitogenic signals.

Protective effect of zinc on cadmium-induced micronuclei in V79 cells

The induction of micronuclei (MN) in mitotically active cells has been widely used and promoted as a biological marker of exposure to environmental toxins. In our study the effect of zinc on cadmium genotoxicity was investigated in V 79 cells. The results indicate that cadmium chloride exposure for 24 h increased micronucleus frequency and the percentage of binucleated cells in dose-dependent manner. At the highest concentration of cadmium (50 microM Cd) 23 MN were found in 1000 cells. The protective effect of zinc on cadmium genotoxicity was investigated at lower concentrations (5-25 microM CdCl2). At 50 microM Cd, the number of MN increased significantly (16 MN).

Signal transduction pathways, and nuclear translocation of zinc and metallothionein during differentiation of myoblasts

The changes in subcellular localization of metallothionein during differentiation were studied in two myoblast cell lines, L6 and H9C2. Addition of insulin like growth factor-I or lowering foetal bovine serum to 1% can induce differentiation of myoblasts to myotubes. Metallothionein and zinc were localized mainly in the cytoplasm in myoblasts but were translocated into the nucleus of newly formed myotubes during early differentiation. In fully differentiated myotubes, metallothionein content was decreased with a cytoplasmic localization. Addition of an inhibitor of mitogen-activated protein kinase, PD 98059, did not affect differentiation but blocked nuclear translocation of metallothionein. LY 294092, an inhibitor of PI3 kinase, and rapamycin, an inhibitor of p70S6 serine/threonine kinase, abolished insulin-like growth factor-I induced differentiation of myoblasts, retained metallothionein in the cytoplasm, and decreased metallothionein content. These results demonstrate that the cytoplasmic-nuclear translocation of metallothionein occurs during the early stage of differentiation of myoblasts to myotubes and can be blocked by inhibition of certain signal transduction pathways. The transient nuclear localization of metallothionein and zinc may be related to a high requirement for zinc for metabolic activities during the early stage of differentiation.

Biological significance of non-acetylated metallothionein

The biological significance of non-acetylated metallothionein (MT) was investigated from the viewpoint of N(alpha)-acetylation after induction of MT synthesis by metallic and non-metallic inducers, by partial hepatectomy and under physiological conditions. N(alpha)-Acetylated and non-acetylated forms of MT-2 in liver supernatants and plasma were detected by the tandem size-exclusion and anion-exchange HPLC columns with in-line detection by mass spectrometry. The non-acetylated isoform of MT-2 (MT-2') was present at a comparable level to the N(alpha)-acetylated form of MT-2 (MT-2) at an early stage after induction by not only zinc but also cadmium, and by partial hepatectomy in the livers of rats. Plasma MT-2 in neonatal rats was similar to liver MT-2 in the composition of N(alpha)-acetylated and non-acetylated forms, suggesting that there are no differences in the roles of N(alpha)-acetylation of MT in the extracellular trafficking of MT. The column switching HPLC method with in-line detection by inductively coupled argon plasma mass spectrometry (ICP-MS) was shown to be a sensitive and powerful method to detect MT proteins at not only isoform level but also at acetylated and non-acetylated form levels.

Heavy metal content in liver and kidneys of grey seals (Halichoerus grypus) in various life stages correlated with metallothionein levels: Some metal-binding characteristics of this protein

Liver and kidneys from grey seals (Halichoerus grypus) in their natural habitats in Norwegian waters contain high levels of Cd and Hg as well as metallothionein (MT). The data show significant placental transfer of heavy metals in seals. The MT concentrations were higher in fetus liver and kidneys than in the liver and kidneys of juveniles and adults. A significantly higher proportion of the total Cd and Cu, as compared to Zn and Hg, in fetal liver was associated with MT. Only low amounts of metals and MT were found in juveniles. In kidneys, little metal was associated with MT, although high amounts of Cd were bound in fetal stages. Positive correlations were found between total metal, MT content, and MT-bound metals, particularly for Cd and Zn. Grey seal MT may have higher affinity to Zn than to Cd. The use of seal MT as a biomarker for environmental heavy metal exposure seems promising.

Purification and characterization of a major zinc binding protein from renal brush border membrane of rat

In spite of the fact that zinc is an essential trace element, mechanisms that contribute to zinc homeostasis in mammals are poorly understood. An attempt has been made to identify and purify zinc binding components from renal brush border membrane (BBM), which could be involved in the binding of zinc and the subsequent translocation across the BBM. A 40 kDa major zinc binding protein has been identified and purified from renal BBM, which showed a dissociation constant (Kd) of 211 microM and maximal binding (Bmax) of 207 nmol/mg protein. 8 g zinc atoms could interact with 1 mol of protein. Specificity of the protein for zinc was checked by metal displacement and UV-absorption assay. It was found that only Cd2+ could displace the zinc bound to the protein. Other metals tested (Cu2+, Mg2+, Ca2+) did not show any interaction with the protein. These data indicated that purified protein is highly specific and has a high affinity for zinc. The carbohydrate content was found to be 7.85 mg% in the purified protein. Immunofluorescence localization of this protein in kidney sections revealed that this major zinc binding protein is exclusively localized in the proximal convoluted tubules. These results suggested that the 40 kDa major zinc binding transmembrane glycoprotein is highly specific for zinc and has a high affinity for zinc.

Increased radiation-induced apoptosis in mouse thymus in the absence of metallothionein

Metallothionein (MT) has been shown to protect cells from free radical induced DNA damage after exposure to copper, hydrogen peroxide and also radiation. In order to study the role of MT in radiation induced apoptosis, age-matched male control mice (C57BL/6J), MT-I overexpressing (MT-I*) and MT-null transgenic mice were exposed to whole body cobalt 60 gamma-irradiation at 0, 5, or 10 Gy, and their thymus were removed 24 h later. The basal levels of MT and zinc concentrations in the thymus were measured by 109Cadmium-heme assay and atomic absorption spectrophotometry, respectively. The MT expression after radiation was determined by immunohistochemical staining using a polyclonal antibody to MT. The extent of apoptosis in thymocytes was determined by histology (H&E stain). DNA was isolated from the thymus, and DNA fragmentation was determined by agarose gel electrophoresis. The results showed that the basal level of MT protein in MT-I* thymus was 2.4-fold higher than control mice, and that MT was inducible in both MT-I* and control C57BL6 thymus after radiation exposure. Minimal MT protein was detected in MT-null mice thymus before or after radiation, while, a significantly higher number of apoptotic cells and DNA fragmentation were found in MT-null thymus after whole body irradiation. These data demonstrated a protective role for MT in radiation-induced apoptosis in mouse thymus.

Expression of MT-3 protein in the human kidney

The objective of the present study was to determine the expression of MT-3 in the human kidney. To accomplish this, an antibody was generated against a unique 8 amino acid sequence present in MT-3 that is not shared by any other MT family member. Western analysis demonstrated that the resulting antibody reacted with a protein band of approximately 6 kDa, corresponding to the known molecular weight of MT-3. Immunohistochemical staining using this antibody demonstrated reactivity with several epithelial components of the nephron. In the glomerulus, moderate intensity was demonstrated in parietal epithelial cells of Bowman's capsule and in visceral epithelial cells of the glomerular tuft. Proximal convoluted tubule cells exhibited moderate cytoplasmic MT-3 reactivity. Distal tubules showed strong cytoplasmic staining for MT-3, particularly in the medullary rays. In the medulla, MT-3 staining was the most variable, with weak to moderate staining in the medullary collecting ducts and a general absence of staining in the thin loops of Henle and in the transitional epithelium of the renal pelvis. The finding that MT-3 is constitutively expressed in several glomerular and tubular epithelial elements of the human kidney warrants consideration of an expanded role for this protein family in maintaining renal homeostasis.