Induction of metallothionein synthesis in cultured cells derived from rabbit kidney

In vitro effects of cadmium on two different animal cell models

The aim of this study was to assess comparatively the effects of cadmium on two different in vitro cell models, a cell line derived from proximal tubule renal cells (LLC-PK1) and haemocytes or blood cells of mussels (Mytilus galloprovincialis). Cells were seeded in 96-well microplates and exposed in vitro to different concentrations of cadmium (CdCl(2)) ranging from 10 to 2000 microM for haemocytes and from 1 to 100 microM for LLC-PK1 cells, added to the culture medium. After 24 h of exposure, different assays were performed on haemocytes: neutral red uptake, phagocytosis of neutral red-stained zymosan, XTT assay, activity of lysosomal acid phosphatase and demonstration of the actin cytoskeleton using TRITC-labeled phalloidin. Cell viability expressed as LC50 was 750 microM when using the neutral red assay and 400 microM with the XTT assay. The phagocytic ability and the activity of acid phosphatase increased significantly in cells treated with Cd in a non dose-dependent manner. Doses of Cd above 100 microM caused disruption of the actin cytoskeleton. In LLC-PK1 cells, cell viability expressed as LC50 was found to be around 40 microM when using the neutral red assay and 50-60 microM with MTT and LDH assays, respectively. These results show that mussel haemocytes are in general more resistant to Cd exposure than LLC-PK1 cells. Furthermore, Cd appears to stimulate phagocytic and lysosomal activities in haemocytes in vitro.

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.

Zinc transport and metallothionein induction in primary cultures of rabbit kidney proximal cells

Primary cultures of isolated rabbit renal proximal cells were grown on collagen-coated permeable supports. The confluent epithelia were polarized, making possible the measurement of uptakes and effluxes across the apical and the basolateral membranes. Uptakes of 65Zn were assessed under initial rate conditions, after 0.5 min incubation. The kinetic parameters of apical uptake were a Jmax of 25.1 +/- 5.3 pmol min-1 (micrograms DNA)-1, a Km of 43.3 +/- 7.3 microM and an unsaturable constant of 0.105 +/- 0.029 (n = 7) at 37 degrees C. Cadmium competitively inhibited the zinc uptake, with a Ki value of 24.5 +/- 7.3 microM. Basolateral uptake was characterized by a high capacity (Jmax = 227.9 +/- 46.6 pmol min-1 (micrograms DNA)-1) and an affinity similar to that of the apical uptake (Km = 35.4 +/- 14.2 microM). Cadmium had no effect on the basolateral zinc uptake. Effluxes across the basolateral face of the epithelium always exceeded those across the apical face. Excess zinc in the culture medium induced the synthesis of metallothionein in the epithelia, as judged by the rate of [35S]cysteine incorporation into a fraction of cytosolic proteins. Metallothionein induction did not appear to modify the kinetic parameters of the apical zinc uptake. These data suggest that separate saturable transport systems are responsible for the apical and basolateral zinc uptakes in proximal renal cells. Induction of metallothionein had no apparent effect on apical zinc uptake in this system.

Induction of metallothionein synthesis by cadmium and zinc in cultured rabbit kidney cells (RK-13)

The effects of increasing concentrations of Zn(II) and Cd(II) on the expression of the four isometallothioneins (isoMTs), namely MT-1a, MT-2a, MT-2d and MT-2e, in rabbit kidney cells (RK-13) and the development of cellular tolerance to these metal ions were studied. The results showed that, whereas in parental cells MT concentration was low and composed nearly exclusively of MT-2a and MT-1a, all four isoMTs increased massively in abundance when the cells were exposed to toxic concentrations of Zn(II) or Cd(II), the relative increase being largest in the two minor isoforms MT-2d and MT-2e. While the response of the four isoMTs to the challenge by Zn(II) or Cd(II) was qualitatively comparable, there were differences in sensitivity and delay time, Cd(II) being the more efficient inducer and much faster in eliciting the onset of isoMT synthesis. An even larger production of isoMTs resulted when RK-13 cells were cultured in the presence of a series of metal concentrations yielding sub-lines of increased metal tolerance. In this instance too, there were marked differences in the response to Cd(II) and Zn(II). Thus, in cells of sub-lines selected for tolerance to moderate concentrations of Cd(II) the kinetic analysis of isoMT accretion gave indications of a saturable induction process while no such evidence was forthcoming for Zn(II). In sub-line cells selected for tolerance to the highest concentrations of Cd(II) or Zn(II) isoMT formation was increased by another order of magnitude, reaching for some isoforms a 100- to 1000-fold augmentation over the amounts measured in cells of the unexposed parental cells. A potentiation of this magnitude goes beyond the range of ordinary regulation of gene expression. It is to be viewed instead as an enlargement of the capacity of isoMT synthesis acquired by a variety of mechanisms in the surviving cells.

Analysis of mammalian metallothionein isoforms by high-resolution SDS-gel electrophoresis

Metallothioneins (MTs) are cysteine-rich heavy metal-binding proteins, whose possible functions are thought to be the protection against toxic metals as well as the regulation of essential metals. It is known that there are several MT isoforms, but the biological roles of the individual isoforms have not been elucidated. To facilitate the functional analysis of these isoforms, we improved an analytical method of MTs developed previously, which is based on a denaturing gel electrophoresis of chemically modified MTs. The established technique makes it possible not only to separate MT isoforms with a high resolution, but to estimate the levels of the individual isoforms by analyzing directly crude cell extracts. By this method, six MT isoforms were identified in the extracts of Cd-exposed human cells. It was also revealed that there is an apparent heterogeneity of the rat liver MT; five isoforms were identified in the liver extracts of Cd-injected rats. The present method will be useful in the functional analysis of the MT isoforms, as well as in a variety of aspects of the MT studies.

Zinc-specific activation of a HeLa cell nuclear protein which interacts with a metal responsive element of the human metallothionein-IIA gene

Transcription of metallothionein genes is activated by heavy metals such as zinc and cadmium, and a DNA element called metal responsive element (MRE) is essential for this process. By mobility-shift assay, we identified a HeLa-cell nuclear protein which specifically binds to MREa of human metallothionein-IIA gene. This protein, named ZRF (zinc-regulatory factor), is present in the cells untreated with heavy metals. Zinc is essential for, and increases in a dose-dependent manner, the binding of ZRF to MREa. Other heavy metals which can also induce metallothioneins, including cadmium, copper and mercury, do not activate ZRF. A MREa-containing oligonucleotide that can bind ZRF confers heavy metal-inducibility to a heterologous promoter, suggesting that ZRF is a zinc-dependent transcriptional activator. In addition to the MRE core sequence, the surrounding sequences are also important for both ZRF binding in vitro, and zinc-dependent transcriptional activation in vivo. MREa by itself responds not only to zinc but also to other metallothionein-inducing heavy metals, indicating that the ZRF protein, not the MREa sequence, is responsible for the zinc specificity.

Regulation of metallothionein production in HeLa cells

Metallothioneins are cysteine-rich, heavy-metal-binding proteins which have been assumed to participate in the detoxification of toxic metals. The mechanism of thionein (apoprotein of metallothionein) induction by cadmium was studied using cultured human cells. It was found that when thionein synthesis reaches a maximum (6-8 h after induction), it no longer responds to additional cadmium. Changes in cadmium uptake or induction of inhibitory proteins were not responsible. Together with our previous findings, a possible mechanism is proposed: loss of the secondary induction response might be due to increased intracellular levels of thionein, which has been overproduced by the initial induction.

Cadmium uptake and induction of metallothionein synthesis in a renal epithelial cell line (LLC-PK1)

LLC-PK1 cells, an established cell line from pig kidney with proximal tubule properties, were cultivated in vitro at confluence on plastic dishes. They were then exposed (apical side) to inorganic cadmium (CdCl2, 5 microM) for periods ranging between 1 to 24 h. Analysis of the cell supernatant after homogenisation and ultracentrifugation indicated that Cd taken up in the first 3 h was bound to cytosolic high molecular weight proteins, but was redistributed to low molecular weight proteins at later stages. Induction of Cd-metallothionein (Cd-Mt) synthesis, as judged from Cd-Mt binding to a specific anti-Cd-Mt antibody and from the rate of 35S-cys incorporation into a specific protein fraction, was apparent 3-6 h after the addition of Cd to the incubation medium.

Zinc-induced metallothionein synthesis by Caco-2 cells

Caco-2 cells possess many morphological and biochemical characteristics of intestinal absorptive cells, including the ability to transport zinc. In the present study, metallothionein (MT) synthesis in response to increased levels of zinc was examined. Increased incorporation of [35S]cysteine into MTs was observed when excess ZnCl2 was added to the medium. The rate of MT synthesis was found to be concentration dependent. Also, induction of MT synthesis was greater early in the culture, before the cells were fully differentiated. Incubation of the monolayers with 65Zn and 200 microM zinc revealed that approximately 50% of the zinc incorporated into the cells was associated with MTs. The remainder was associated with large proteins as well as amino acids and small peptides. Actinomycin D and cycloheximide both inhibited the induction of MT synthesis, suggesting that the newly synthesized MTs are a result of expression of MT genes. Hence, Caco-2 cells, a model of intestinal absorptive cells, may be used to examine the role of MTs in zinc absorption.

A nuclear factor that interacts with metal responsive elements of a human metallothionein gene

Metallothioneins (MTs) are low molecular weight heavy metal-binding proteins which are known to play a major role in heavy metal detoxification and understanding of their regulatory mechanism is toxicologically important. Expression of MT genes is induced by heavy metals and metal responsive elements (MREs) upstream of MT genes are essential for the transcriptional activation. By several types of mobility shift assay with 32P-labeled oligonucleotide probes, we detected HeLa cell nuclear as well as cytoplasmic factors that bind to MRE sequences of human MTIIA (hMTIIA) gene. One of the nuclear factors, which gives stronger signal than others, was further characterized. Competition experiments showed that the nuclear factor (named MREBP) specifically recognizes MREs of hMTIIA gene. EDTA abolished the binding of MREBP to MRE, suggesting that a divalent cation(s) is required for the complex formation. Also in blotting experiments with HeLa nuclear extract and the [32P]MRE probes an EDTA-sensitive 95k protein band, which possibly represents MREBP, was detected.

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.

Isolation of mammalian cell variants with enhanced endogenous thiol content at low survival levels following irradiation

Approximately half of a group of Chinese hamster V79 cell clones isolated from radiation survivors at low surviving fractions had significantly higher endogenous levels of non-protein and protein thiols than unirradiated cells. A similar group of cell lines cloned from unirradiated cells had thiol levels in the same range as the original unirradiated population. In some cases, clones isolated following irradiation are also more resistant to misonidazole toxicity and to radiation. This phenotype can persist through many cell generations for weeks or months of continuous growth; however, in many clones with altered phenotypes isolated following irradiation, reversion of the population to the same phenotype as that of unirradiated populations has been observed. Induction of elevated thiol levels in tumours by radiotherapy could reduce both the efficacy of the radiation itself and of radiation-modifying or chemotherapeutic drugs given in combination with the radiation.

Cadmium-induced synthesis of metallothioneins in human T and B cell purified by a fluorescence activated cell sorter

Human peripheral blood lymphocytes were reacted with fluorescein-conjugated antibodies specific to T or B cell surface antigen and fractionated with a fluorescence activated cell sorter. The isolated T and B cells were examined for their capacity to synthesize metallothioneins (MTs). Analysis by gel electrophoresis indicated that both T and B cells were able to produce MTs in a Cd2+-inducible manner, suggesting that both cells types have a mechanism of protection against Cd2+ toxicity.

Cadmium-induced synthesis of metallothioneins in human lymphocytes and monocytes

Cd2+-binding proteins of peripheral blood lymphocytes and monocytes have not well been characterized so far, although they are expected to be a clue for understanding Cd2+ toxicity in those immune competent cells. We separated a family of Cd2+-binding proteins from Cd2+-exposed human peripheral blood lymphocytes by gel filtration chromatography, and characterized them by SDS-gel electrophoresis. The proteins showed electrophoretic behaviours closely similar to metallothioneins (MTs) of HeLa cells derived from human cervical carcinoma. The proteins were also found in Cd2+-exposed monocytes, and were inducible by Cd2+ in both lymphocytes and monocytes. Anti-MT serum specifically precipitated these proteins, which were thus identified as MTs. These results suggest that the two classes of the cells involved in the immune system possess a protective mechanism against Cd2+ through MTs. A variety of human lymphoid cell lines derived from both T and B cells were also found to have capacity to synthesize MTs in response to Cd2+.

Metallothionein metabolism in the liver and kidney of the streptozotocin-diabetic rat

1. Elevated levels of metallothionein (MT)-I and -II were identified in the liver and kidney of insulin-deficient diabetic rats. 2. The relative rate of MT synthesis and the turnover of cytoplasmic MT were both accelerated in the liver of diabetic rats. 3. The rate of synthesis of MT, but not its cytoplasmic turnover, was increased in diabetic kidney. 4. Maximal relative rates of MT synthesis in liver and kidney were first observed at 4 and 10 days, respectively, after inducing the diabetic condition. 5. The altered metabolism of hepatic MT in diabetic rats was attributed primarily to disturbances in endocrine status, while the altered metabolism of renal MT was largely due to accumulation of excessive dietary copper in the kidney.