Age-dependent variation for inducibility of metallothionein genes in mouse liver by cadmium

The Effect of Smoking Habits on Blood Cadmium and Lead Levels in Residents Living Near a Mining and Smelting Area in Northwest China: a Cross-Sectional Study

Few studies have focused on environmental cadmium (Cd) and lead (Pb) exposure while exploring the effect of smoking on blood Cd (BCd) and blood Pb (BPb) levels. Moreover, essential trace elements affect the absorption, accumulation, and toxicity of Cd and Pb. To investigate the effect of smoking on BCd and BPb levels under high Cd and Pb exposure and the influence of essential trace elements on the effect, 301 residents living near a mining and smelting area in Northwest China were included in our study. After collecting health information and measuring BCd, BPb, serum iron, magnesium, and total calcium levels, we analyzed the association between smoking and BCd and BPb levels and the influence of the essential trace elements on the association. The results showed that BCd and BPb levels in smokers were significantly higher than those in non-smokers. There was a dose-response association between pack-years and the odds ratios (ORs) of high BCd and BPb levels in all participants compared with non-smokers. Serum iron, magnesium, and calcium had a negative effect on the elevations of the ORs of high BCd and BPb levels. In addition, smoking-related elevations of BCd and BPb levels vary by sex, age, BMI, and age of smoking initiation. Our findings present evidence for the effect of smoking on BCd and BPb levels under high Cd and Pb exposure and may provide guidance for the prevention and control of BCd and BPb elevations in residents living in Cd- and Pb-polluted areas.

Effect of size on concentrations and cadmium inducibility of metallothionein in the shore crab Carcinus maenas

Metallothionein (MT) plays an important role in protecting organisms from the adverse effects of Cd, Hg, Zn and Cu. Investigations on mammals show variations in metallothionein concentrations and inducibility with age. This has never been investigated in invertebrates, and we determined the concentrations and inducibility of metallothionein in gills and midgut gland of different size classes of shore crabs from uncontaminated areas. Metallothionein concentrations in gills and midgut gland ranged between 30 and 40 μg g^-1 dry weight with no differences among the different size classes. Concentrations of cadmium, copper and zinc in the midgut gland increased with increasing size of the crabs when the concentrations were expressed on a dry weight basis; water content in the midgut gland increased with the size and only the cadmium concentration increased with size when concentrations were expressed on wet weight basis. There was an inverse relationship between metallothionein and both copper and cadmium concentrations. Smaller crabs exposed to 1 mg Cd L^-1 accumulated higher concentrations of cadmium in midgut gland and gills than larger ones and metallothionein concentrations in the midgut gland were higher in the smaller crabs. However, the increase in metallothionein concentration per accumulated unit of cadmium showed a linear increase with the size of the crabs. The ratio [Cd]_midgut/[Cd]_gills decreased with the size of the crabs. The overall conclusion is that baseline metallothionein concentrations do not change with age in shore crabs, but that the inducibility of metallothionein upon cadmium challenge does.

Perinatal and early-life cobalt exposure impairs essential metal metabolism in immature ICR mice

The objective of the present study was to assess the impact of cobalt (Co) exposure on tissue distribution of iron (Fe), copper (Cu), manganese (Mn), and zinc (Zn), as well as serum hepcidin levels in immature mice (18, 25, 30 days). Pregnant mice were exposed to 75 mg/kg b.w. cobalt chloride (CoCl₂ × 6H2O) with drinking water starting from 3 days before delivery and during lactation. At weaning (day 25) the offspring were separated and housed in individual cages with subsequent exposure to 75 mg/kg b.w. CoCl₂ until 30 days postnatally. Evaluation of tissue metal levels was performed by an inductively coupled plasma-mass spectrometry (ICP-MS). Serum hepcidin level was assayed by enzyme linked immunosorbent assay (ELISA). Cobalt exposure resulted in a time- and tissue-dependent increase in Co levels in kidney, spleen, liver, muscle, erythrocytes, and serum on days 18, 25, and 30. In parallel with increasing Co levels, CoCl₂ exposure resulted in a significant accumulation of Cu, Fe, Mn, and Zn in the studied tissues, with the effect being most pronounced in 25-day-old mice. Cobalt exposure significantly increased serum hepcidin levels only in day18 mice. The obtained data demonstrate that Co exposure may alter essential metal metabolism in vivo.

Armour GE, Thakkar E, Mackay TFC, Anholt RRH. A Drosophila model for toxicogenomics: Genetic variation in susceptibility to heavy metal exposure

The genetic factors that give rise to variation in susceptibility to environmental toxins remain largely unexplored. Studies on genetic variation in susceptibility to environmental toxins are challenging in human populations, due to the variety of clinical symptoms and difficulty in determining which symptoms causally result from toxic exposure; uncontrolled environments, often with exposure to multiple toxicants; and difficulty in relating phenotypic effect size to toxic dose, especially when symptoms become manifest with a substantial time lag. Drosophila melanogaster is a powerful model that enables genome-wide studies for the identification of allelic variants that contribute to variation in susceptibility to environmental toxins, since the genetic background, environmental rearing conditions and toxic exposure can be precisely controlled. Here, we used extreme QTL mapping in an outbred population derived from the D. melanogaster Genetic Reference Panel to identify alleles associated with resistance to lead and/or cadmium, two ubiquitous environmental toxins that present serious health risks. We identified single nucleotide polymorphisms (SNPs) associated with variation in resistance to both heavy metals as well as SNPs associated with resistance specific to each of them. The effects of these SNPs were largely sex-specific. We applied mutational and RNAi analyses to 33 candidate genes and functionally validated 28 of them. We constructed networks of candidate genes as blueprints for orthologous networks of human genes. The latter not only provided functional contexts for known human targets of heavy metal toxicity, but also implicated novel candidate susceptibility genes. These studies validate Drosophila as a translational toxicogenomics gene discovery system.

Although physiological effects of environmental toxins are well documented, we know little about the genetic factors that determine individual variation in susceptibility to toxins. Such information is difficult to obtain in human populations due to heterogeneity in genetic background and environmental exposure, and the diversity of symptoms and time lag with which they appear after toxic exposure. Here, we show that the fruit fly, Drosophila, can serve as a powerful genetic model system to elucidate the genetic underpinnings that contribute to individual variation in resistance to toxicity, using lead and cadmium exposure as an experimental paradigm. We identified genes that harbor genetic variants that contribute to individual variation in resistance to heavy metal exposure. Furthermore, we constructed genetic networks on which we could superimpose human counterparts of Drosophila genes. We were able to place human genes previously implicated in heavy metal toxicity in biological context and identify novel targets for heavy metal toxicity. Thus, we demonstrate that based on evolutionary conservation of fundamental biological processes, we can use Drosophila as a powerful translational model for toxicogenomics studies.

Melatonin prevents the estrogenic effects of sub-chronic administration of cadmium on mice mammary glands and uterus

Cadmium (Cd) is a heavy metal classified as a human carcinogen. Occupational exposure, dietary consumption and cigarette smoking are sources of Cd contamination. Cd-induced carcinogenicity depends on its oxidative and estrogenic actions. A possible role of Cd in breast cancer etiology has been recently suggested. Melatonin, because of its antioxidant and antiestrogenic properties could counteract the toxic effects of this metalloestrogen. Our aim was both to determine the effects of relevant doses of Cd on mice mammary glands and uterus and to test whether melatonin would counteract its effects. Female mice of different ages and estrogenic status (prepuberal, adult intact, adult ovariectomized) were treated with CdCl(2) (2-3 mg/kg, i.p.), melatonin (10 microg/mL in drinking water), CdCl(2) + melatonin, or diluents. Whereas in prepuberal animals Cd disturbs mammary ductal growth and reduces the number of terminal end buds, in adults, regardless of the steroidal milieu, Cd exerts estrogenic effects on mammary glands, increasing lobuloalveolar development and ductal branching. Uterine weight also increased as a result of Cd treatment. The effects of Cd are partially inhibited by melatonin. In adult ovariectomized mice, Cd concentration in blood of animals treated with CdCl(2) + melatonin was lower than in mice receiving only Cd; the opposite effects were found in non-castrated animals. As Cd mimics the effect of estrogens, the high incidence of breast cancer in tobacco smokers and women working in industries related with Cd could be explained because of the properties of this metal. The effects of melatonin point to a possible role of this indoleamine as a preventive agent for environmental or occupational Cd contamination.

Physiologically based models of metal kinetics

The issues confronting the modeler of metals kinetics are somewhat different from those with which the modeler of organic chemical behavior is faced. Particularly important features of metals kinetics include metal-protein binding and metal-metal interactions. Reduction, and for some metals oxidation, is frequently an intrinsic part of metal metabolism. Alkylation/dealkylation reactions may or may not render the metal less active, and the behavior of alkylated or dealkylated metabolites must often be included in a complete kinetic model. Despite these complexities, the kinetics of metals are as amenable to the techniques of physiologically based modeling as are the kinetics of organic chemicals. Like all models, those for metals kinetics have the potential to organize a variety of observations, sometimes including apparently inconsistent observations, into a coherent framework of behavior, to identify needs for more complete experimental information, and to assist the risk assessor in making judgments concerning dose-response relationships. Development of physiologically based models of the kinetic behavior of metals is in its very early stages. The kinetics of only four metals, arsenic, chromium, mercury, and lead, have been modeled with any degree of completeness. Of these, the lead model is the most fully realized at the present time. The chromium and mercury models are still in the process of development, and experimental data are being gathered to support further development and refinement of the arsenic model. We may expect to see continued progress made on these models and their practical applications, as well as the development of new models for other toxicologically significant metals such as cadmium, manganese, nickel, and aluminum.

Effect of sodium butyrate on metallothionein induction and cadmium cytotoxicity in ROS 17/2.8 cells

ROS 17/2.8 cells, a cloned rat osteosarcoma cell line, are exceptionally sensitive to the cytotoxic effects of cadmium. This sensitivity is associated with the inability of this metal to induce the synthesis of metallothionein, a transition metal-binding protein, which detoxifies this metal by its sequestration. Sodium butyrate induces the synthesis of metallothionein in these cells in a concentration-dependent manner. Treatment with this agent also significantly increases the resistance of these cells to the cytotoxic effects of cadmium and the protective effect of butyrate is reversed upon its removal from culture medium. Butyrate treatment did not significantly alter the accumulation of cadmium by these cells. Hence, the increased synthesis of metallothionein in butyrate-treated cells is not due to increased cellular uptake of cadmium. Inhibition of DNA synthesis due to butyrate was not a sufficient condition to alter metallothionein synthesis or to protect against Cd-induced cytotoxicity. Equivalent inhibition of DNA synthesis with hydroxyurea failed to increase metallothionein synthesis in cadmium-treated cells. These results indicate that modulation of metallothionein gene expression in this cell line is the critical factor in determining cellular sensitivity to the cytotoxic effects of cadmium.

Effect of 5-azacytidine on metallothionein inducibility and sensitivity to lethality of cadmium in rat osteosarcoma (ROS 17/2.8) cells

ROS 17/2.8 cells, a cloned rat osteoblastic osteosarcoma cell line, were found to be extremely sensitive to the lethal effects of cadmium and to synthesize little, if any, metallothionein in response to cadmium exposure. Culture of cells for 24 h in the presence of 1 microM 5-azacytidine, a cytidine analog, increased the inducibility of metallothionein by cadmium and significantly reduced (P less than 0.001) cytotoxicity. Anion exchange chromatographic analysis of cadmium binding to low molecular mass cytotoxicity. Anion exchange chromatographic analysis of cadmium binding to low molecular mass cytosolic proteins showed that cells treated with cadmium and 5-azacytidine expressed at least 2 isoforms of metallothionein. One isoform of metallothionein with a low affinity for cadmium was constitutively expressed by these cells. The association of poor inducibility of metallothionein by cadmium with extreme sensitivity of cells to cadmium emphasizes the role of this protein in the cellular response to this toxic metal. The modulation of metallothionein inducibility and sensitivity to cadmium by 5-azacytidine treatment suggest that metallothionein gene structure and regulation are altered in ROS 17/2.8 cells.