Low-Level Metal Contamination and Chelation in Cardiovascular Disease-A Ripe Area for Toxicology Research

Cardiovascular disease remains the leading cause of death worldwide. In spite of cardiovascular prevention, there is residual risk not explicable by traditional risk factors. Metal contamination even at levels previously considered safe in humans may be a potential risk factor for atherosclerosis. This review examines evidence that 2 metals, lead, and cadmium, demonstrate sufficient toxicological and epidemiologic evidence to attribute causality for atherosclerotic disease. Basic science suggests that both metals have profound adverse effects on the human cardiovascular system, resulting in endothelial dysfunction, an increase in inflammatory markers, and reactive oxygen species, all of which are proatherosclerotic. Epidemiological studies have shown both metals to have an association with cardiovascular disease, such as peripheral arterial disease, ischemic heart disease, and cardiovascular mortality. This review also examines edetate disodium-based chelation as a possible pharmacotherapy to reduce metal burden in patients with a history of cardiovascular disease and thus potentially reduce cardiovascular events.

Dynamics of Cadmium Acclimation in Daphnia pulex: Linking Fitness Costs, Cross-Tolerance, and Hyper-Induction of Metallothionein

Acclimation increases tolerance to stress in individuals but is assumed to contribute fitness costs when the stressor is absent, though data supporting this widely held claim are sparse. Therefore, using clonal (i.e., genetically identical) cultures of Daphnia pulex, we isolated the contributions of acclimation to the regulation of the metal response gene, metallothionein 1 (MT1), and defined the reproductive benefits and costs of cadmium (Cd)-acclimation. Daphnia pulex were exposed for 50 parthenogenetic generations to environmentally realistic levels (1 μg Cd/L), and tolerance to Cd and other metals assessed during this period via standard toxicity tests. These tests revealed (1) increased tolerance to Cd compared to genetically identical nonacclimated cultures, (2) fitness costs in Cd-acclimated Daphnia when Cd was removed, and (3) cross-tolerance of Cd-acclimated Daphnia to zinc and silver, but not arsenic, thereby defining a functional role for metallothionein. Indeed, Cd-acclimated clones had significantly higher expression of MT1 mRNA than nonacclimated clones, when Cd exposed. Both the enhanced induction of MT1 and tolerant phenotype were rapidly lost when Cd was removed (1-2 generations), which is further evidence of acclimation costs. These findings provide evidence for the widely held view that acclimation is costly and are important for investigating evolutionary principles of genetic assimilation and the survival mechanisms of natural populations that face changing environments.

Consequences of trace metal variability and supplementation on Chinese hamster ovary (CHO) cell culture performance: A review of key mechanisms and considerations

Trace metals are supplied to chemically-defined media (CDM) for optimal Chinese hamster ovary (CHO) cell culture performance during the production of monoclonal antibodies and other therapeutic proteins. However, lot-to-lot and vendor-to-vendor variability in raw materials consequently leads to an imbalance of trace metals that are supplied to CDM. This imbalance can yield detrimental effects rooted in several primary mechanisms and pathways including oxidative stress, apoptosis, lactate accumulation, and unfavorable glycan synthesis. Recent research endeavors involve supplying zinc, copper, and manganese to CDM in excess to further maximize culture productivity and product quality. These treatments significantly impact critical quality attributes and furthermore highlight the degree to which trace metal availability can affect CHO cell culture performance. This review highlights the role of trace metal variability, supplementation, and interplay on key cellular mechanisms responsible for overall culture performance and the production and quality of therapeutic proteins.

Coordinative modulation of human zinc transporter 2 gene expression through active and suppressive regulators

Zinc transporter 2 (ZnT2) is one of the cellular factors responsible for Zn homeostasis. Upon Zn overload, ZnT2 reduces cellular Zn by transporting it into excretory vesicles. We investigated the molecular mechanism that regulates human ZnT2 (hZnT2) gene expression. Zn induces hZnT2 expression in dose- and time-dependent manners. Overexpression of metal-responsive transcription factor 1 (MTF-1) increases hZnT2 transcription, whereas depletion of MTF-1 reduces hZnT2 expression. There are five putative metal response elements (MREs) within 1kb upstream of the hZnT2 gene. A serial deletion of the hZnT2 promoter region (from 5' to 3') shows that the two MREs proximal to the gene are essential for Zn-induced promoter activity. Further mutation analysis concludes that the penultimate MRE (MREb) supports the metal-induced promoter activity. The hZnT2 promoter has also a zinc finger E-box binding homeobox (ZEB) binding element. Mutation or deletion of this ZEB binding element elevates the basal and Zn-induced hZnT2 promoter activities. Knockdown of ZEB1 mRNA enhances the hZnT2 transcript level in HEK-293 cells. In MCF-7 (ZEB-deficient) cells, expression of ZEB proteins attenuates the Zn-induced hZnT2 expression. However, expressions of MTF-1 target genes such as human ZnT1 and metallothionein IIA were not affected. Our study shows the expression of the hZnT2 gene is coordinately regulated via active and suppressive modulators.

Suppression of ZIP8 expression is a common feature of cadmium-resistant and manganese-resistant RBL-2H3 cells

Rat basophilic leukemia RBL-2H3 cells show markedly high sensitivity to both CdCl2 and MnCl2 compared with other rat cell lines, due to efficient accumulation of cadmium and manganese. To clarify the roles of metal transporters in hyperaccumulation of cadmium and manganese in RBL-2H3 cells, Cd-resistant and Mn-resistant cells were developed from RBL-2H3 cells by continuous exposure to CdCl2 and MnCl2, respectively. The established Cd-resistant (RBL-Cdr) and Mn-resistant (RBL-Mnr) cells exhibited about 20 times higher LC50 values of CdCl2 and MnCl2, respectively, than parental RBL-2H3 cells, and showed cross-resistance to each metal. The resistance to cadmium and manganese was primarily conferred by a marked decrease in the uptake of both metals. RBL-Cdr cells also showed cross-resistance to HgCl2 and AgNO3 probably due to enhanced expression of metallothionein. Among the possible transporters involved in the uptake of Cd(2+) and Mn(2+), the expression of ZIP8 (Zrt-, Irt-related protein 8), encoded by Slc39a8, showed a marked suppression in both RBL-Cdr and RBL-Mnr cells. These results suggest that ZIP8 plays a pivotal role in the transport and toxicity of Cd(2+) and Mn(2+) in RBL-2H3 cells.

High sensitivity of RBL-2H3 cells to cadmium and manganese: an implication of the role of ZIP8

Cellular incorporation of Cd involves multiple transport systems for other metals such as Fe, Zn, Mn, and Ca. Metal transporters including divalent metal transporter 1, Zrt/Irt-related protein (ZIP) 8, and ZIP14, and certain types of voltage-dependent Ca channels have been shown to be involved in cellular Cd uptake. However, tissue- or cell-specific roles of these metal transporters in the accumulation and toxicity of Cd remains unclear. In the present study, we compared the sensitivity to and accumulation of Cd, Mn, and Zn among four types of rat cell lines. Rat basophilic leukemia RBL-2H3 cells showed the highest sensitivity to Cd and Mn due to the highest accumulation of Cd and Mn among the four cell lines. The high accumulation of Cd and Mn was caused by high uptake rates of Cd and Mn. Since relatively high expression of ZIP8 and ZIP14 was found in RBL-2H3 cells, siRNAs of ZIP8 and ZIP14 were transfected into RBL-2H3 cells. The knockdown of ZIP8, but not of ZIP14, significantly reduced the uptake rates of Cd and Mn in RBL-2H3 cells, especially in the presence of bicarbonate. These results suggest that the high expression of ZIP8, which is known to have affinities for both Cd and Mn, resulted in high accumulation of Cd and Mn, leading to high sensitivity to these metals in RBL-2H3 cells. Thus, RBL-2H3 cells may serve as a good model for clarifying the mechanisms of Cd and Mn transport via ZIP8.

Alkaline induces metallothionein gene expression and potentiates cell proliferation in Chinese hamster ovary cells

Metallothionein (MT) gene expression is increased in cadmium resistant Chinese hamster ovary cells (CHO Cd(R)) upon medium (regular or serum-free) change during culturing. Among the major components of the medium, NaHCO3 was found to be able to induce MT gene expression in a dose- and time-dependent manner. The same effect was observed with other alkaline solutions, such as HEPES and NaOH. Using MT promoter-luciferase reporter gene constructs, we found that the presence of metal response elements (MREs) in the promoter region is necessary for NaHCO3-induced MT gene transcription. This finding is further supported by the observation that the binding activity between the metal-responsive transcription factor 1 (MTF-1) and the MRE were increased after NaHCO3 treatment. Following NaHCO3 treatment, an increase in cell proliferation was observed in CdR cells but not in the parental CHO K1 cells that do not express MT transcripts due to MT gene methylation. Using synchronized cells, an increase in cell proliferation was observed 9 h after NaHCO3 addition. Notably, proliferation of CHO K1 cells was increased when transfected with an MT gene. The effect of MT on cell growth was affirmed by treating CHO K1 cells with 5-azacytidine (Aza) to demethylate the MT gene. Proliferation increased in Aza-treated CHO K1 cells after NaHCO3 treatment. These results demonstrate that NaHCO3 stimulates MT gene expression and causes an enhancement of cell proliferation in CHO cells.

Effect of cadmium on cell cycle progression in chinese hamster ovary cells

Chinese hamster ovary K1 (CHO K1) cells are very sensitive to cadmium (Cd) toxicity. They were used to investigate the effect of Cd on cell cycle progression. Cells were cultured with 0.1, 0.4, 1 or 4 microM Cd for various time intervals. There was no difference in growth rate when less than 0.4 microM Cd was given within 24 h. A dose-dependent reduction of cell proliferation was observed when more than 0.4 microM of Cd was given. The cells were pulse-labeled with 5-bromodeoxyuridine (BrdU), and the labeled cells were cultured in the presence of increasing concentrations of Cd. Cell cycle progression was retarded as a function of Cd concentration. G2/M arrest was observed when the BrdU-labeled cells were treated with 1 microM Cd for 8h, whereas cells receiving 4 microM Cd stopped at the S phase within 4 h. Cell cycle analysis of cells treated with Cd for 24 h showed that G2/M arrest occurred only when cells received 0.8 to 2 microM Cd. Despite the occurrence of G2/M arrest in the Cd treatment, only a limited proportion of the cells were blocked in the M phase. However, the increase in M phase cells coincided with an elevation in the cyclin-dependent kinase 1 activity. To examine whether Cd acts on cells at a specific cell stage, they were synchronized at the G1 or G2/M phase then treated with 1 microM Cd for 12 h. The cells were blocked at the G2/M and G1/S phase, respectively. This finding indicates that Cd toxicity is global and not cell phase specific. We also investigated the involvement of Cd-induced reactive oxygen species (ROS) with the occurrence of G2/M block and found a lack of correlation between cell cycle arrest and ROS production. We measured the Cd content that caused G2/M arrest from a series of Cd treatments and determined the ranges of cumulative Cd concentrations that could result in cell cycle arrest.

Photoperiod affects hepatic and renal cadmium accumulation, metallothionein induction, and cadmium toxicity in the wild bank vole (Clethrionomys glareolus)

The objective of this study was to examine the toxic effects of dietary cadmium (Cd) on bank voles, being the F1 offspring of a wild-caught population. For 6 weeks, the rodents were provided with diets containing 0.05 (control), 40, 80, and 120 microg Cd/g dry wt of diet under moderate (12 h) and long (16 h) photoperiods. Histological examinations and analyses of metallothionein (MT), Cd, Cd bound and not bound to MT, iron and lipid peroxidation in the liver and kidneys were carried out. Histopathological changes occurred in the liver (infiltrations of leukocytes) and kidneys (hemorrhage, glomerular injury, tubular cell degeneration) of bank voles fed the highest dose of dietary Cd only under the moderate photoperiod. The same voles also exhibited the highest values of hepatic and renal Cd, Cd not bound to MT, and renal lipid peroxidation. It seems that under the long photoperiod the liver and kidneys of bank voles were protected against Cd-induced injury through decreasing Cd accumulation and increasing synthesis of MT.

Rottlerin stimulates metallothionein gene expression but inhibits metal transport in Chinese hamster ovary cells

Metallothionein (MT) can be induced by various metals. We have shown previously that H7, a protein kinase C (PKC) inhibitor, inactivates metal-induced MT gene expression. To investigate whether a specific PKC isoform is involved in the induction process, inhibitors for various PKC isoforms were administered to cadmium-resistant Chinese hamster ovary (Cd(R)) cells. None of the inhibitors used can reduce metal-induced MT gene expression. However, a PKCdelta inhibitor, rottlerin, induced MT mRNA expression in Cd(R) cells in the presence or absence of Cd. Notably, the induction occurs through the activation of the MT transcriptional factor (MTF-1) and is not related to an increase of metal influx. Furthermore, metal accumulation is reduced in the presence of rottlerin. Pulse-labeling analysis indicated that MT protein synthesis increased in Cd(R) cells upon rottlerin treatment. These results suggest that rottlerin blocks metal transport but stimulates MT synthesis in Cd(R) cells. Since rottlerin is capable of reducing the cellular accumulation of Cd, it was expected that the cytotoxic effect of Cd would decrease in the presence of rottlerin. Treating the parental cell of Cd(R) with Cd and rottlerin together indeed showed a decline of cytotoxicity compared to cells treated with Cd alone. We further examined how MTF-1 was activated by rottlerin. Rottlerin-induced MTF-1 activity was not affected in Cd(R) cells by the addition of EDTA. It was, however, diminished by administering an intracellular Zn chelator, TPEN. The result implies a mobilization of intracellular Zn ions after rottlerin treatment in Cd(R) cells. To investigate whether the described results occur in all types of cells, another cell line (GH(3)) was used to study the effect of rottlerin on MT gene expression. The result revealed that rottlerin did not increase the amount of MT mRNA in GH(3) cells. This differential effect between cell lines may be useful for investigating the regulatory mechanism of MT gene expression.