Metallothionein mRNA and protein induction by cadmium in peripheral-blood leucocytes

Expression of metallothionein isoforms in peripheral blood leukocytes from Thai population residing in cadmium-contaminated areas

Metallothionein (MT) is a group of proteins with high cadmium (Cd) affinity and with a potential role in Cd transportation and detoxification. The aim of the present study was to investigate the relationship between MT (MT-1A, MT-2A, and MT-3 isoforms) gene expression level in peripheral blood leukocytes and Cd-associated renal injury in non-occupational exposed Thai population. The study was conducted in adult subjects residing in Cd-contaminated areas of Mae Sot District, Thailand. The basal levels of MT-1A, MT-2A, and MT-3 mRNA expression were determined in leukocytes by quantitative RT-PCR. MT-1A and MT-2A expressions, particularly MT-1A, were found to be significantly increased with elevated levels of blood and urinary Cd levels. In subjects with high urinary Cd levels, negative correlations between MT-1A and microalbumin, and between MT-2A and β(2)-MG, were observed. These results suggest that MT gene expression may reflect susceptibility of the exposed population to Cd-induced renal dysfunction. MT-1A mRNA expression in leukocytes might be developed as a potential biomarker of Cd exposure and Cd-induced renal dysfunction.

Kidney dysfunction and cadmium exposure--factors influencing dose-response relationships

Our early toxicological studies showed that metallothionein (MT) is a protein that carries cadmium (Cd) to the kidney, explaining why Cd exposures during long time periods may give rise to kidney dysfunction. This dysfunction is usually considered to be the critical effect, i.e. the adverse effect that occurs at the lowest exposure level. MT also provides intracellular protection against cadmium toxicity. In studies of population groups in cadmium contaminated areas in China, we investigated factors that affected the relationship between internal dose of Cd, as indicated by blood Cd (BCd) or urinary Cd (UCd), and the prevalence of kidney dysfunction. We found dose-response relationships between UCd and the prevalence of increased levels of biomarkers of renal tubular dysfunction (urinary beta-2-microglobulin, B2M, or N-acetyl-beta-d-glucosaminidase - NAG) or urinary albumin (UAlb), a biomarker of glomerular kidney dysfunction. Two years after Cd intake from contaminated rice was diminished, renal tubular dysfunction appeared unchanged or aggravated among those with higher UCd; Another 8 years later, i.e. 10 years after Cd intake was decreased, the prevalence of renal tubular dysfunction was still increased but UAlb had returned to normal. Factors that influenced the dose-response relationships were: (1) time after maximum exposure. (2) Concomitant exposure to other nephrotoxic agents such as inorganic arsenic. (3) Cd induced metallothionein mRNA levels in peripheral blood lymphocytes, used as a biomarker of the ability of each person, to synthesize MT. (4) The occurrence of increased levels in blood plasma of autoantibodies against MT. The two last points further support a role in humans of MT as a protective protein against tissue damage from cadmium and gives support to previous ideas developed partly in experimental systems.

Increased metallothionein gene expression, zinc, and zinc-dependent resistance to apoptosis in circulating monocytes during HIV viremia

Circulating monocytes exhibit an apoptotic resistance phenotype during HIV viremia in association with increased MT expression. MTs are known to play an important role in zinc metabolism and immune function. We now show, in a cross-sectional study using peripheral monocytes, that expression of MT1 isoforms E, G, H, and X is increased significantly in circulating monocyte cells from HIV+ subjects during chronic viremic episodes as compared with uninfected subjects. This increase in expression is also observed during acute viremia following interruption of suppressive ART. Circulating monocytes from HIV+ donors were also found to have elevated zinc importer gene Zip8 expression in conjunction with elevated intracellular zinc levels in contrast to CD4(+)T-lymphocytes. In vitro HIV-1 infection studies with elutriated MDM confirm a direct relation between HIV-1 infection and increased MDM MT1 (isoform G) gene expression and increased intracellular zinc levels. A direct link between elevated zinc levels and apoptosis resistance was established using a cell-permeable zinc chelator TPEN, which reversed apoptosis resistance effectively in monocytes from HIV-infected to levels comparable with uninfected controls. Taken together, increases in MT gene expression and intracellular zinc levels may contribute directly to maintenance of an immune-activated monocyte by mediating an increased resistance to apoptosis during active HIV-1 viremia.

Cadmium reduces adipocyte size and expression levels of adiponectin and Peg1/Mest in adipose tissue

Adipose tissue dysfunction has been associated with diabetogenic effects. The effects of repeated Cd exposure on adipocytes remain largely unknown. We administered Cd at doses of 0, 5, 10, and 20 micromol/kgbw sc for 2 weeks (3.5 times/week) to mice and assessed the possible alteration of epididymal white adipose tissue (WAT), including histological difference, adipocyte differentiation and functional capacity. Whereas hepatic weight did not differ between the control and Cd-exposed groups, WAT weight, as well as adipose cell mass, significantly decreased in a dose-dependent manner in Cd-treated mice. The Cd concentration in WAT significantly increased in Cd-treated groups after 2 weeks of exposure. Next, we examined the effects of Cd on adipocyte differentiation and hypertrophy. Cd exposure significantly decreased the paternally expressed gene 1/Mesoderm-specific transcript mRNA expression levels. Both peroxisome proliferator-activated receptor gamma2 and CCAAT/enhancer-binding protein alpha mRNA expression levels in WAT tended to decrease in the Cd-treated groups. Next, we determined the effects of Cd exposure on the mRNA expression levels of adipose-derived hormones, such as adiponectin and resistin. The adiponectin mRNA expression level in WAT decreased after both 6h and 2 weeks of exposure to a high dose of Cd, and the reduction in resistin mRNA expression levels was observed after 2 weeks of exposure. These results suggest that Cd exposure causes abnormal adipocyte differentiation, expansion, and function, which might lead to development of insulin resistance, hypertension, and cardiovascular disease.

Metallothionein in human thrombocyte precursors, CD61+ megakaryocytes

The in vitro biosynthesis of metallothionein (MT) was investigated in thrombocyte precursors (megakaryocytes) isolated from human cord blood. Biosynthesis and induction of MT in magnetic cell sorting-separated CD61(+) megakaryocytes was confirmed by immunohistochemical staining using monoclonal mouse anti-MT. The presence of MT was detected both in the nuclear and in the cytoplasmic area. Using RT-PCR, in vitro upregulation/induction of total MT transcripts was observed in CD61(+) cells at 48 h post-treatment with 100 micromol/L of zinc supplement. Seven isoform-specific mRNAs namely, MT-1A, MT-1B, MT-1E, MT-1G, MT-1H, MT-1X, and MT-2A were detected in the similar cell populations left untreated with zinc.

Lymphocyte metallothionein mRNA responds to marginal zinc intake in human volunteers

Marginal Zn deficiency is thought to be prevalent in both developed and developing countries. However, the extent of Zn deficiency is not known, due to the lack of a reliable diagnostic indicator. Blood plasma and erythrocyte concentrations of metallothionein (MT) reflect Zn status, but measurement of MT is dependent on the availability of sensitive immunoassays. Our aim was to show whether measurement of T lymphocyte MT-2A mRNA, using a competitive reverse transcriptase (RT)–polymerase chain reaction (PCR) assay, could indicate Zn status in human subjects in a residential Zn-depletion study. In the study, the Zn intake of seven volunteers was maintained at 13·7 mg/d for 5 weeks (baseline) followed by 4·6 mg/d for 10 weeks (marginal intake) and then 13·7 mg/d (repletion) for 5 weeks. The quantitative assay was developed using standard techniques and concentrations of MT-2A mRNA were normalized by reference to β-actin mRNA which was also measured by competitive RT–PCR assay. An alternative method of measuring the PCR product using capillary electrophoresis with laser-induced fluorescence detection was also evaluated. There was considerable inter-individual variation in MT-2A mRNA concentration and the mean level at the end of the baseline period was 10·3 (SE 3·7) fg MT-2A mRNA/pg β-actin mRNA, which then decreased by 64 % during the low Zn intake period. After repletion, MT-2A mRNA returned to baseline concentrations. In contrast, plasma Zn was unchanged by marginal Zn intake or repletion. The effect of low Zn in all individuals was consistent. We conclude that this assay is a sensitive method of evaluating marginal changes in dietary Zn intake.

Cultured lymphocytes from autistic children and non-autistic siblings up-regulate heat shock protein RNA in response to thimerosal challenge

There are reports suggesting that some autistic children are unable to mount an adequate response following exposure to environmental toxins. This potential deficit, coupled with the similarity in clinical presentations of autism and some heavy metal toxicities, has led to the suggestion that heavy metal poisoning might play a role in the etiology of autism in uniquely susceptible individuals. Thimerosal, an anti-microbial preservative previously added routinely to childhood multi-dose vaccines, is composed of 49.6% ethyl mercury. Based on the levels of this toxin that children receive through routine immunization schedules in the first years of life, it has been postulated that thimerosal may be a potential triggering mechanism contributing to autism in susceptible individuals. One potential risk factor in these individuals may be an inability to adequately up-regulate metallothionein (MT) biosynthesis in response to presentation of a heavy metal challenge. To investigate this hypothesis, cultured lymphocytes (obtained from the Autism Genetic Resource Exchange, AGRE) from autistic children and non-autistic siblings were challenged with either 10 microM ethyl mercury, 150 microM zinc, or fresh media (control). Following the challenge, total RNA was extracted and used to query "whole genome" DNA microarrays. Cultured lymphocytes challenged with zinc responded with an impressive up-regulation of MT transcripts (at least nine different MTs were over-expressed) while cells challenged with thimerosal responded by up-regulating numerous heat shock protein transcripts, but not MTs. Although there were no apparent differences between autistic and non-autistic sibling responses in this very small sampling group, the differences in expression profiles between those cells treated with zinc versus thimerosal were dramatic. Determining cellular response, at the level of gene expression, has important implications for the understanding and treatment of conditions that result from exposure to neurotoxic compounds.

Towards a multidisciplinary and integrated strategy in the assessment of adverse health effects related to air pollution: the case study of Cracow (Poland) and asthma

Complex interaction between anthropogenic activities, air quality and human health in urban areas, such as in Cracow sustains the need for the development of an interdisciplinary and integrated risk-assessment methodology. In such purpose, we propose a pilot study performed on asthmatics and based on a combined use of a biomarker, such as metallothionein 2A (MT-2A) in the characterization of human exposure to one or a mixture of pollutants and of Geographical Information Systems (G.I.S.) which integrates climatic and urban anthropogenic parameters in the assessment of spatio-temporal dispersion of air pollutants. Considering global incidence of air pollution on asthma and on peripheral blood lymphocytes MT-2A expression should provide a complementary information on biological risks linked to urban anthropogenic activities. Such study would help for the establishment of a sustainable development in urban areas that can maintain the integrity of air quality and preserve human health.

Metallothionein gene expression in peripheral lymphocytes and renal dysfunction in a population environmentally exposed to cadmium

In order to study the validity of metallothionein (MT) gene expression in peripheral blood lymphocytes (PBLs) as a biomarker of cadmium exposure and susceptibility to renal dysfunction, MT mRNA levels were measured using reverse transcription polymerase chain reaction (RT-PCR) in PBLs from residents living in a cadmium-contaminated area. MT mRNA levels were found to increase with the increase of blood cadmium (BCd) and urinary cadmium (UCd) levels. Basal MT mRNA levels were significantly correlated with the logarithm of BCd levels and the logarithm of UCd levels confirming that MT expression in PBLs is a biomarker of cadmium exposure and internal dose. An inverse relationship was observed between in vitro induced MT-mRNA level in PBLs and urinary N-acetyl-beta-d-glucosaminidase (UNAG) suggesting that MT gene expression in PBLs may be used as a biomarker of susceptibility to renal toxicity of cadmium.

Differential effects of mercury, lead, and cadmium on IL-2 production by Jurkat T cells

Mercury, lead, and cadmium are widespread and highly toxic pollutants. The aim of this study was to determine the effects of sublethal doses of CH(3)HgCl, CdCl(2), and PbCl(2) on IL-2 production by T lymphocytes. Jurkat T cells were stimulated by triggering CD3 and CD28 molecules before, in conjunction with, or following heavy metal exposure. Heavy metals, individually or mixed together at equimolar concentrations, were used. Results demonstrated that low, noncytotoxic doses of metals induce tyrosine phosphorylation. Mercury and lead (1 microM) inhibit IL-2 production regardless of the state of T cell activation. Cadmium stimulated IL-2 production only in preactivated T cells. Surprisingly, a mixture of these three metals had no effect. We subsequently determined the effects of heavy metals on NFAT (nuclear factors of activated T cells) activity. When cells were stimulated by potent stimulation involving the CD3 and CD28 molecules, an increased NFAT activation was noted when the cells were exposed to mercury and to the metal mixture. Activation with PMA/calcium ionophores indicated that the target of heavy metals is located downstream from PKC and calcium mobilization. These results suggest that the state and mode of T cell activation are important parameters to consider in heavy metal toxicity.

Cadmium induces apoptosis partly via caspase-9 activation in HL-60 cells

Cadmium (Cd), a potent immunotoxic metal, induces apoptosis both in vitro and in vivo. However, the mode of action remains unclear. We previously reported that Cd-induced apoptosis was partly dependent on mitochondria. In the present study, we investigated the involvement of caspase-9, which is the apex caspase in the mitochondoria-dependent apoptosis pathway, in Cd-induced apoptosis in human promyelocytic leukemia HL-60 cells. A specific inhibitor of caspase-9, Z-LEHD-FMK, partly inhibited DNA fragmentation induced by Cd treatment in HL-60 cells. Moreover, treatment of HL-60 cells with Cd resulted in the appearance of Cytochrome c (Cyt c), a potent activator of caspase-9, in the cytosol at 3 h, which closely paralleled the activation of caspase-9. Caspase-9 is an initiator caspase that is a potent activator of downstream effector caspases such as caspase-3. Caspase-3 activation was subsequent to the Cyt c release at 6 h. DNA fragmentation, an index of induction of apoptosis, also appeared 6 h after Cd treatment. The effects were more pronounced at 9 h after Cd addition. A broad-specificity inhibitor of caspases, Z-Asp-CH(2)-DCB, inhibited caspase-3 activation and DNA fragmentation induced by Cd in a dose-dependent fashion. The results suggest that Cd-induced apoptosis is partly caused by caspase-9 activation triggered by Cyt c.

Metallothionein-1+2 protect the CNS after a focal brain injury

We have evaluated the physiological relevance of metallothionein-1+2 (MT-1+2) in the CNS following damage caused by a focal cryolesion onto the cortex. In comparison to normal mice, transgenic mice overexpressing the MT-1 isoform (TgMTI* mice) showed a significant decrease of the number of activated microglia/macrophage and of CD3+ T lymphocytes in the area surrounding the lesion, while astrocytosis was increased. The TgMTI* mice showed a diminished peripheral macrophage but not CD3 T cell response to the cryolesion. This altered inflammatory response produced a decreased expression of the proinflammatory cytokines IL-1beta, IL-6, and TNF-alpha and an increased expression of the growth factors bFGF, TGFbeta1, and VEGF in the TgMTI* mice relative to control mice, which might be related to the increased angiogenesis and regeneration of the parenchyma of the former mice. The overexpression of MT-1 dramatically reduced the cryolesion-induced oxidative stress and neuronal apoptosis. Remarkably, these effects were also obtained by the intraperitoneal administration of MT-2 to both normal and MT-1+2 knock-out mice. These results fully support the notion that MT-1+2 are essential in the CNS for coping with focal brain injury and suggest a potential therapeutic use of these proteins.

Roles of the metallothionein family of proteins in the central nervous system

Metallothioneins (MTs) constitute a family of proteins characterized by a high heavy metal [Zn(II), Cu(I)] content and also by an unusual cysteine abundance. Mammalian MTs are comprised of four major isoforms designated MT-1 trough MT-4. MT-1 and MT-2 are expressed in most tissues including the brain, whereas MT-3 (also called growth inhibitory factor) and MT-4 are expressed predominantly in the central nervous system and in keratinizing epithelia, respectively. All MT isoforms have been implicated in disparate physiological functions, such as zinc and copper metabolism, protection against reactive oxygen species, or adaptation to stress. In the case of MT-3, an additional involvement of this isoform in neuromodulatory events and in the pathogenesis of Alzheimer's disease has also been suggested. It is essential to gain insight into how MTs are regulated in the brain in order to characterize MT functions, both in normal brain physiology, as well as in pathophysiological states. The focus of this review concerns the biology of the MT family in the context of their expression and functional roles in the central nervous system.

Metallothionein gene expression in peripheral lymphocytes from cadmium-exposed workers

Metallothionein (MT) plays an important role in the detoxification of cadmium. To investigate the usefulness of MT gene expression in peripheral blood lymphocytes (PBLs) as a biomarker of cadmium exposure and susceptibility, reverse transcriptase-polymerase chain reaction was used to measure the MT gene expression in PBLs from cadmium-exposed workers. Both basal and induced MT expressions were found to increase with increased blood cadmium (BCd) and urinary cadmium (UCd) levels. Both basal and induced MT expression levels were significantly correlated with the logarithm of BCd and the logarithm of UCd levels. The dose-response relationship between internal dose of cadmium and MT expression suggested the validity of MT expression in PBLs as a biomarker of cadmium exposure. In vitro induced MT expression level in PBLs was found to be inversely related to the level of renal dysfunction indicator, urinary N-acetyl-beta-D-glucosaminidase (UNAG). The latter finding indicates that MT expression in PBLs may be a useful biomarker of susceptibility to renal toxicity of cadmium.

Metallothionein mRNA in monocytes and peripheral blood mononuclear cells and in cells from dried blood spots increases after zinc supplementation of men

A specific, sensitive and reliable index for assessment of human zinc status has not been developed, and continues to present a considerable challenge for nutritionists in the trace element field. We have focused on metallothionein (MT) expression as a potential index. A protocol involving 16 men and a 10-d supplementation period plus a 4-d postsupplementation period was used to examine the relative response of MT expression in erythrocytes, monocytes, peripheral blood mononuclear cells (PBMC) and cells from a dried blood spot (DBS). Zinc was supplemented at the current adult male recommended dietary allowance (RDA) of 15 mg. Erythrocyte MT protein, as measured by ELISA, increased gradually to about twofold over the placebo group during zinc supplementation and remained elevated for 4 d postsupplementation. Competitive reverse transcriptase-polymerase chain reaction showed that MT mRNA levels in both monocytes and PBMC increased (up to 4.7- and 2.7-fold, respectively) after 2 d of supplementation, with greater expression in monocytes compared with PBMC. Total RNA extracted from dried blood spots, representing cells from 50 microL of blood, showed a comparable change in MT mRNA upon zinc supplementation. In each leukocyte population isolated, when zinc supplementation was withdrawn, MT mRNA levels decreased. Collectively, these experiments show that, in men, MT gene expression increases during supplementation at the RDA, and that the DBS sampling method will be of value in measuring MT expression in a variety of clinical and survey situations.

Metallothionein expression is increased in monocytes and erythrocytes of young men during zinc supplementation

The metallothionein gene is transcriptionally regulated by zinc. Consequently, metallothionein has potential for serving as an index of dietary zinc status in humans. To examine this possibility, an enzyme-linked immunoassay (ELISA) based on a sandwich approach that utilizes monoclonal and chicken egg yolk antibodies was used to compare the response of erythrocyte metallothionein protein levels with the response of monocyte metallothionein mRNA levels as measured by competitive reverse transcriptase-polymerase chain reaction (CRT-PCR) during zinc supplementation. Young male subjects participated in an 18-d supplementation study in which zinc was provided at 50 mg/d. Control subjects received a placebo. The zinc supplement resulted in significantly greater erythrocyte metallothionein levels by d 8 of supplementation compared with controls. Monocyte metallothionein mRNA levels were significantly greater than those of controls by d 2 of supplementation. Both remained elevated through d 18. They returned to base line by 8 and 4 d after supplementation, respectively. The plasma zinc concentration was significantly greater than in controls by d 6 and had returned to control levels by d 22 of supplementation. The results presented here show that both monocyte metallothionein mRNA and erythrocyte metallothionein protein concentrations change in human subjects in response to elevated dietary zinc intake and that monocyte metallothionein mRNA responds more rapidly to elevation of dietary zinc status than erythrocyte metallothionein protein. Consequently, both erythrocyte metallothionein and monocyte metallothionein mRNA may prove to be measures useful for assessment of either zinc depletion or the bioavailability of zinc supplements.

Competitive reverse transcriptase-polymerase chain reaction shows that dietary zinc supplementation in humans increases monocyte metallothionein mRNA levels

Zinc status is difficult to evaluate in humans. Metallothionein gene expression is transcriptionally regulated by dietary zinc and thus could serve as an assessment parameter based on zinc-dependent function. We used semiquantitative reverse transcriptase-polymerase chain reaction (RT-PCR) to establish that MT mRNA is increased in a human monocytic cell line by addition of zinc to the medium. To examine this response in human subjects, a dietary supplement of 50 mg zinc gluconate/d was given for 15 d. Monocytes were purified from venous blood using NycoPrep 1.068. Monocyte purity was determined by flow cytometry using fluorescent anti-human monocyte CD14 antibodies. Total monocyte RNA was extracted and converted to cDNA by reverse transcription. Competitive RT-PCR was used to analyze differences between cDNA levels that are proportional to MT mRNA levels in monocytes from zinc-supplemented and control subjects. RT-PCR oligonucleotide primers were designed to amplify both a 201 bp segment of the human MT cDNA and a 180 bp competitor cDNA template. The 180 bp competitor cDNA template was used for MT cDNA quantitation. The RT-PCR data show that there was a significant increase in monocyte MT mRNA in subjects within 6 d of zinc supplementation, which remained elevated at d 15 of supplementation. In contrast, plasma zinc was greater at d 6 of zinc supplementation, but by d 15 of supplementation, while still elevated, was close to control levels. These data suggest that monocyte MT mRNA levels respond to zinc supplementation and that the response could serve as a more useful assessment variable than plasma zinc for the measurement of zinc status in humans.

An immunohistochemical study on the tissue-specific localization of metallothionein in dogs

To study the tissue specificity of metallothionein (MT) expression, tissues of dogs ranging in age from 1 day to 18 years were examined immunohistochemically. Of the organs examined, liver and kidney showed the strongest immunoreactivity; a comparable intensity of MT immunolabelling was noted in the two organs in adult animals. In the central nervous system, astrocytes and ependymal cells showed MT immunoreactivity. MT labelling was shown in the sustentacular cells of the olfactory epithelium, but immunoreactivity was slight in the epithelium of the respiratory tract. Slight MT immunoreactivity was demonstrated in the epithelium of a variety of glands (sweat, uterine, mammary, olfactory, perianal and thyroid) and in parietal cells of the fundic glands of the stomach. Sporadic MT immunolabelling was demonstrated in the columnar and goblet cells of the small and large intestines, surface mucosal cells of the stomach and epithelial cells of the hair follicles. These findings may help in understanding important features of MT, such as its dynamic induction mechanism, its systemic degradation pathway and the possible biological functions.

Accumulation of metallothionein and its multiple forms by zinc, cadmium and dexamethasone in human peripheral T and B lymphocytes and monocytes

Metallothionein (MT) is produced at high rates in isolated monocytes, and T and B lymphocytes during induction in vitro. At optimal concentrations, 125 microM for Zn and 10 microM for Cd and dexamethasone (dex), MT was demonstrated after only 2 h in some cases, and in all cell types substantial levels were measured after 1 day of exposure to all three inductors. With Cd, lower amounts of MT were found, but maximum levels were reached faster than with Zn. The same result was found for dex compared to Zn. Zn and dex in combination showed the same accumulation rate as Zn alone. If the inductors were used in lower concentrations than optimal, reduced accumulation rates occurred, particularly during the first part of the exposure period. No MT was found for concentrations below 5 microM Zn, 1 microM Cd or 0.5 microM dex. The constitutive levels of MT (mean +/- S.E.M.) were 0.11 +/- 0.05, 0.54 +/- 0.3, 0.06 +/- 0.05 and 0.15 +/- 0.08 nmol Cd bound/5 x 10(6) unseparated mononuclear cells (MNC), monocytes, T lymphocytes and B lymphocytes, respectively. Monocytes accumulated 19 times and B lymphocytes 6 times more MT than T lymphocytes after 2 days of exposure to 125 microM Zn. Despite these differences in accumulated amounts of MT, the fold accumulation values were rather similar between the cell types, reflecting corresponding variations in background MT levels. After exposure of unseparated MNC to 125 microM Zn for 2 days, removal of the metal caused constitutive MT levels to be reestablished in 5 days. Five different MT forms, all capable of Cd complexation, were demonstrated in these cells. These forms had kinetically different behaviour during Zn exposure among the cell types, and the response to Cd was different from the Zn response. The results indicate metals to be closely controlled in MNC and emphasize a role for multiple MT forms in the process of regulation.

Effect of prior, low-level cadmium exposure in vivo on metallothionein expression in cultured lymphocytes

Exposure to cadmium (Cd) is currently monitored by measurement of the metal in blood or urine, or by observation of excreted compounds such as beta 2-microglobulin or N-acetyl-beta-D-glucose. Whilst these approaches are useful for the detection of acute exposure to Cd, their applicability in the management of long-term, low-level exposure is less clear. Metallothioneins are ubiquitous proteins that are synthesized in response to heavy metal ions and may offer themselves as being a biologically sensitive indicator of Cd exposure. We have examined both basal and Cd-induced metallothionein mRNA levels in cultured lymphocytes from groups with different exposures to Cd, attempting to assess their potential as an indicator of Cd exposure and the suitability of such an assay for routine analysis. We found that induced metallothionein mRNA levels, rather than basal mRNA levels, increased in groups known to have received elevated body burdens of Cd, although these increases were not significant between groups. There was, however, a significant correlation between induced metallothionein mRNA levels and urinary beta 2-microglobulin. These results suggest that further work on the in vitro lymphocyte response to Cd as a diagnostic tool is warranted.

Cytotoxicity and accumulation of Hg, Ag, Cd, Cu, Pb and Zn in human peripheral T and B lymphocytes and monocytes in vitro

1. The cytotoxic effects of various heavy metals were assayed by trypan blue exclusion in vitro in human peripheral immune cells separated to high purity. T and B lymphocytes and monocytes were equally sensitive to metals. The individual metals could be ranked in the following decreasing order of cytotoxic potency, Hg approximately Ag > Cd approximately Cu > Pb approximately Zn, based on exposure time and concentration needed to give a particular percentage of dead cells. 2. The cytotoxic effects became irreversible after about 13 hr of metal exposure. 3. Examination by scanning electron microscopy showed that the heavy metals caused serious destruction of the cell membranes. 4. Solubility and uptake of metals into the cells were studied and discussed in relation to the cytotoxic effects. It was concluded that metal binding to cell surfaces or precipitate formation could inhibit ordinary uptake, thereby affecting cytotoxicity. For Pb in monocytes this appeared to lead to uptake of non-toxic complexes, probably by phagocytosis.

Modulation of stress proteins by Cd2+ in a human T cell line

We previously showed in a human T cell line (CEM-C12 cells) that Cd2+ induced gene expression of stress proteins, metallothionein-IIA and heat shock protein 70 in a time- and dose-dependent manner. In the present study, CEM-C12 cells were pretreated for 24 h with 1 microM Cd2+ and then challenged with toxic concentrations of this metal. We found that maximal expression of the metallothionein-IIA and heat shock protein 70 genes was increased and this maximal level occurred at higher Cd2+ toxic concentrations. Actinomycin D chase experiments indicated that Cd2+ pretreatment did not modify metallothionein-IIA mRNA stability. The modulatory effect of Cd2+ pretreatment was dose-dependent from 100 pM to 1 microM. Such pretreatment also enhanced resistance to Cd2+ toxicity. Finally, verapamil, a calcium/potassium channel blocker displaced the dose-response curve for Cd2+ toxicity as well as metallothionein-IIA and heat shock protein 70 gene expression to higher Cd2+ concentrations.

Cadmium induces apoptosis in a human T cell line

Cadmium, a potent toxic metal, poses a serious environmental threat but the mechanisms of its toxicity remain unclear. In the present study, we investigated the nature of cadmium-induced cell death in the human T cell line CEM-C12. Cadmium was time- and dose-dependently toxic for CEM-C12 cells, cell death being preceded by chromatin condensation and DNA fragmentation. Quantification of the latter indicated an increase above 4 microM cadmium, with maximal fragmentation at 8 to 10 microM. By contrast, when CEM-C12 cells were exposed to higher cadmium concentrations (50 microM), cell death increased without concomitant chromatin condensation or DNA fragmentation. Thus, cadmium at low and high concentration kills CEM-C12 cells by apoptosis and necrosis, respectively. Addition of cycloheximide reduced the apoptotic effect of cadmium, suggesting that cadmium-induced apoptosis is an process depending on protein synthesis. Verapamil, a calcium/potassium channel blocker, markedly increased the viability of CEM-C12 cells treated by low cadmium concentrations and prevented DNA fragmentation. The apoptotic effect of cadmium suggests a possible mechanism for lymphocyte damage occurring after in vivo exposure to cadmium.

Induction by zinc of specific metallothionein isoforms in human monocytes

A low-molecular-mass zinc-containing protein was isolated by gel permeation and anion-exchange chromatography of lysates of human monocytes induced with zinc acetate. Characterization by sodium dodecyl sulphate/polyacrylamide gel electrophoresis and amino acid sequencing identified the two major charge-separable fractions and an occasionally occurring third fraction as metallothionein-1, metallothionein-2 and metallothionein-0, respectively. Metallothionein-1 was shown to consist of a mixture of isoforms, confirmed as metallothionein-1e, metallothionein-1g and metallothionein-1l by comparison with cDNA sequences obtained by screening a human monocyte cDNA library. We can find no previous observation in the literature of metallothionein-1g at both the protein and RNA level in a non-tumour cell, and of metallothionein-0 in a non-fetal cell or tissue. Since isoform-specific polymerase-chain-reaction amplification showed the presence of metallothionein-0 mRNA in zinc-induced but not in untreated monocytes, these cells can be used as an in vitro system to investigate the expression of this previously considered fetal isoform.

Metallothionein and anticancer agents: the role of metallothionein in cancer chemotherapy

Metallothioneins (MTs) are intracellular proteins containing the highest amount of thiol groups within the cytoplasm. These thiol groups are able to bind several cytotoxic agents, such as platinum compounds and alkylating agents. Increased levels of MT are one mechanism of resistance to these anticancer drugs, as intracytoplasmic binding of MT prevents the active molecules from reaching their target, the intranuclear DNA of tumor cells. MT synthesis can easily be induced by physiologic heavy metals such as zinc and copper. Pharmacological modulation of MT levels has been used to increase the MT pool in normal tissues and decrease their susceptibility to the toxicity of anticancer drugs. In the case of tumors arising in the brain, where the inducibility of MT synthesis is low, this approach would allow protection of normal tissues without decreasing the antitumor activity of the cytotoxic agents. The interaction of MT with cytotoxic agents is not limited to covalent binding. A correlation between MT synthesis and amplification of oncogenes such as ras has been reported. Furthermore, the cytotoxic drugs are bound by MT after competition with zinc and copper; these metals are cofactors of numerous metalloenzymes, some of which are involved in the metabolism of nucleic acids. Competitive displacement of these metals might modify nucleic acid metabolism and influence cellular proliferation. On the other hand, increased MT levels could provide a zinc cofactor reserve that increases the cell's reparative potential when faced by DNA damage by cytotoxic agents.(ABSTRACT TRUNCATED AT 250 WORDS)

Molecular biology approaches to biological monitoring of genotoxic substances

Genetic testing is subdivided into genetic monitoring (evaluation over time of induced genetic changes) and genetic screening (detection of inherited traits). Genetic factors in relation to susceptibility to environmental agents are briefly examined, as well as mutation assays suitable for use in genetic monitoring, techniques for identifying specific DNA lesions, and oncogene products as biomarkers. In vitro studies with AS52 Chinese hamster ovary cells indicate that the distribution of lesions (e.g., point mutations or segment deletions) at the xanthine-guanine phosphoribosyl transferase (gpt) gene in mutants generated by exposure to nickel compounds show some substance specificity. This ability is viewed as a promising development for the molecular epidemiology of occupational and environmental cancers. It is concluded that technical limitations pertaining to specificity and sensitivity, as well as ethical and legal implications, need to be resolved before routine application of genetic monitoring and screening is feasible.

Experimental basis for increasing the therapeutic index of cis-diamminedicarboxylatocyclobutaneplatinum(II) in brain tumor therapy by a high-zinc diet

Metallothionein (MT), a ubiquitous intracellular protein, confers resistance to the toxic effects of platinum compounds. Since a high-zinc diet has been shown to induce MT synthesis in extracerebral tissues but not in brain, we investigated whether it could provide an experimental basis for decreasing the hematotoxicity of carboplatin without impairing its activity against brain tumors. After 2 weeks on either a high-zinc diet or a control diet (zinc content, 180 vs 10 ppm), mice and rats received various doses of carboplatin or Hanks' balanced salt solution by i.p. injection. The hematotoxicity of carboplatin was evaluated with an assay of colony-forming units of granulocytes and mononuclear cells in mice. The high-zinc diet enabled a 50% increase in the carboplatin dose without increasing hematotoxicity. The antitumor activity was evaluated with an assay of the colony-forming efficiency of gliosarcoma cells from 9L brain tumors in rats. The high-zinc diet did not alter the efficacy of carboplatin against this brain tumor. Northern blot analysis confirmed that the high-zinc diet induced MT mRNA in the kidney but not in the brain of mice and rats; it also showed MT mRNA induction in bone marrow cells of mice but not in rat 9L brain tumors. These results suggest that increasing the dietary intake of zinc might increase the therapeutic index of carboplatin in the treatment of brain tumors.

Detection of cadmium exposure in rats by induction of lymphocyte metallothionein gene expression

The induction of metallothionein (MT) gene expression in lymphocytes of rats was determined in order to detect exposure in vivo to cadmium. Both acute and chronic CdCl2 exposures resulted in the induction of the MT-1 gene in lymphocytes as measured by standard RNA Northern blot analysis. Twenty-four hours following an ip injection of 3.4 mg/kg CdCl2, a ninefold increase in MT gene expression was observed in lymphocytes, as well as five- and sevenfold increases in liver and kidney, respectively. Oral exposure of rats to 1-100 ppm CdCl2 via drinking water resulted in an approximate twofold enhanced MT signal in lymphocytes after 6 wk, and a threefold increase after 13 wk of exposure to 100 ppm Cd. No increases in lymphocyte MT gene expression were observed after 3 wk of Cd exposure. Liver MT gene expression was substantially induced following chronic Cd exposure, while kidney was not, although this organ had a higher basal expression of the MT-1 gene. Analysis of tissue Cd burdens demonstrated a dose-response Cd accumulation in liver and kidney, but only kidney burdens increased substantially with prolonged Cd exposure. These results demonstrate the utility of lymphocyte gene expression assays to detect in vivo toxicant exposure, and thus their applicability as molecular biomarker assays for human exposure assessment.

Metallothionein induction in human peripheral blood lymphocytes by heavy metals

Human peripheral blood lymphocytes have the capacity to produce metallothioneins (MTs) as a protective response to cadmium exposure. To define the range of metal species inducing lymphocyte MTs, cellular proteins synthesized after exposure to each of 11 heavy metals were analyzed by gel electrophoresis. Toxic metals such as cadmium, mercury and silver were found to induce thioneins (apoproteins of MTs) at relatively low concentrations (maximum at approximately 10 microM), whereas less toxic metals such as zinc, copper and nickel were inductive at relatively high concentrations (maximum at approximately 200 microM). Tin, lead, iron, cobalt, and manganese did not induce thioneins. The heavy metal specificity of MT induction in the lymphocyte resembles that in the liver, and the regulatory mechanism of MT production seems to be similar in both of these tissues. In the cells exposed to highly toxic metals such as cadmium and mercury, expression of cytotoxicity (represented by decline of cysteine uptake) was remarkable at the metal concentrations higher than those saturating thionein induction, supporting the protective role of MTs against heavy metals.