The role of ZIP8 down-regulation in cadmium-resistant metallothionein-null cells

Pancreas-Liver-Adipose Axis: Target of Environmental Cadmium Exposure Linked to Metabolic Diseases

Cadmium has been well recognized as a critical toxic agent in acute and chronic poisoning cases in occupational and nonoccupational settings and environmental exposure situations. Cadmium is released into the environment after natural and anthropogenic activities, particularly in contaminated and industrial areas, causing food pollution. In the body, cadmium has no biological activity, but it accumulates primarily in the liver and kidney, which are considered the main targets of its toxicity, through oxidative stress and inflammation. However, in the last few years, this metal has been linked to metabolic diseases. The pancreas-liver-adipose axis is largely affected by cadmium accumulation. Therefore, this review aims to collect bibliographic information that establishes the basis for understanding the molecular and cellular mechanisms linked to cadmium with carbohydrate, lipids, and endocrine impairments that contribute to developing insulin resistance, metabolic syndrome, prediabetes, and diabetes.

The schizophrenia-associated missense variant rs13107325 regulates dendritic spine density

The missense variant rs13107325 (C/T, p.Ala391Thr) in SLC39A8 consistently showed robust association with schizophrenia in recent genome-wide association studies (GWASs), suggesting the potential pathogenicity of this non-synonymous risk variant. Nevertheless, how this missense variant confers schizophrenia risk remains unknown. Here we constructed a knock-in mouse model (by introducing a threonine at the 393th amino acid of mouse SLC39A8 (SLC39A8-p.393T), which corresponds to rs13107325 (p.Ala391Thr) of human SLC39A8) to explore the potential roles and biological effects of this missense variant in schizophrenia pathogenesis. We assessed multiple phenotypes and traits (associated with rs13107325) of the knock-in mice, including body and brain weight, concentrations of metal ions (including cadmium, zinc, manganese, and iron) transported by SLC39A8, blood lipids, proliferation and migration of neural stem cells (NSCs), cortical development, behaviors and cognition, transcriptome, dendritic spine density, and synaptic transmission. Many of the tested phenotypes did not show differences in SLC39A8-p.393T knock-in and wild-type mice. However, we found that zinc concentration in brain and blood of SLC39A8-p.393T knock-in mice was dysregulated compared with wild-types, validating the functionality of rs13107325. Further analysis indicated that cortical dendritic spine density of the SLC39A8-p.393T knock-in mice was significantly decreased compared with wild-types, indicating the important role of SLC39A8-p.393T in dendritic spine morphogenesis. These results indicated that SLC39A8-p.393T knock-in resulted in decreased dendritic spine density, thus mimicking the dendritic spine pathology observed in schizophrenia. Our study indicates that rs13107325 might confer schizophrenia risk by regulating zinc concentration and dendritic spine density, a featured characteristic that was frequently reported to be decreased in schizophrenia.

Back-translating GWAS findings to animal models reveals a role for Hgfac and Slc39a8 in alcohol and nicotine consumption

Alcohol and tobacco are the most commonly used addictive substances, with high comorbidity rates between alcohol use disorder and tobacco use disorder. Risk for alcohol and nicotine addiction is highly heritable, and they share common genetic factors. A GWAS in over 1 million individuals has revealed 566 genetic variants in 406 loci associated with multiple stages of alcohol and tobacco use. Three novel genes-SLC39A8, GRK4 and HGFAC-within loci associated with altered alcoholic drinks per week (ADW) or cigarettes per day (CPD) were selected to further study their role in alcohol and tobacco use disorder. The role of these genes was assessed using the two-bottle choice addiction paradigm in transgenic mice for each of the genes. We found significant decreases in chronic alcohol consumption and preference in female Hgfac knockout (KO) mice, and decreased nicotine preference in male Hgfac KO compared with wild-type (WT) mice. Additionally, male Slc39a8 hypomorph mice showed greater overall nicotine preference compared with WT mice, while no differences were detected for Grk4 KO mice in alcohol or nicotine consumption and preference in either sex. Thus, this study implicates Hgfac and Slc39a8 in alcohol and tobacco use in a sex-specific manner.

Cadmium Accumulation in the Goat Liver and Kidney Is Partially Promoted by the Upregulation of Metal Transporter Genes

Metal transporters, including divalent metal-ion transporter-1 (DMT1), Zrt-/Irt-like protein 8 and 14 (ZIP8 and ZIP14), and ferroportin-1 (FPN1), reportedly participate in cellular cadmium (Cd) uptake, but those in farm animals remain unclarified. This study aimed to examine the growth, plasma biochemical indices, Cd accumulation, and expression of metal transporter genes in the liver, kidney, and muscle of goats exposed to rice paddies contaminated with different levels of Cd. Twenty-four goats were randomly assigned across three dietary treatments: 0.23, 0.63, and 1.07 mg of Cd/kg of dry matter (DM) for 60 days. The results showed that dietary Cd exposure increased (p < 0.05) both Cd accumulation and the mRNA expressions of metal transporter genes (DMT1, ZIP, and FPN1) in the liver and kidney but not in the muscle, suggesting dietary Cd exhibited different deposition rates between goat liver, kidney, and muscle. These outcomes suggest that high levels of dietary Cd stimulated the expression of metal transporter genes and thereby enhanced the uptake and accumulation of Cd in the goat liver and kidney. As such, higher Cd concentrations in the liver and kidney observed with Cd diets could be partly explained by upregulation of metal transport genes expression.

A review of molecular mechanisms linked to potential renal injury agents in tropical rural farming communities

The chronic kidney disease of unknown etiology (CKDu) is a global health concern primarily impacting tropical farming communities. Although the precise etiology is debated, CKDu is associated with environmental exposures including heat stress and chemical contaminants such as fluoride, heavy metals, and herbicide glyphosate. However, a comprehensive synthesis is lacking on molecular networks underpinning renal damage induced by these factors. Addressing this gap, here we present key molecular events associated with heat and chemical exposures. We identified that caspase activation and lipid peroxidation are common endpoints of glyphosate exposure, while vasopressin and polyol pathways are associated with heat stress and dehydration. Heavy metal exposure is shown to induce lipid peroxidation and endoplasmic reticulum stress from ROS activated MAPK, NFĸB, and caspase. Collectively, we identify that environmental exposure induced increased cellular oxidative stress as a common mechanism mediating renal cell inflammation, apoptosis, and necrosis, likely contributing to CKDu initiation and progression.

Metallothionein and Cadmium Toxicology-Historical Review and Commentary

More than one and a half centuries ago, adverse human health effects were reported after use of a cadmium-containing silver polishing agent. Long-term cadmium exposure gives rise to kidney or bone disease, reproductive toxicity and cancer in animals and humans. At present, high human exposures to cadmium occur in small-scale mining, underlining the need for preventive measures. This is particularly urgent in view of the growing demand for minerals and metals in global climate change mitigation. This review deals with a specific part of cadmium toxicology that is important for understanding when toxic effects appear and, thus, is crucial for risk assessment. The discovery of the low-molecular-weight protein metallothionein (MT) in 1957 was an important milestone because, when this protein binds cadmium, it modifies cellular cadmium toxicity. The present authors contributed evidence in the 1970s concerning cadmium binding to MT and synthesis of the protein in tissues. We showed that binding of cadmium to metallothionein in tissues prevented some toxic effects, but that metallothionein can increase the transport of cadmium to the kidneys. Special studies showed the importance of the Cd/Zn ratio in MT for expression of toxicity in the kidneys. We also developed models of cadmium toxicokinetics based on our MT-related findings. This model combined with estimates of tissue levels giving rise to toxicity, made it possible to calculate expected risks in relation to exposure. Other scientists developed these models further and international organizations have successfully used these amended models in recent publications. Our contributions in recent decades included studies in humans of MT-related biomarkers showing the importance of MT gene expression in lymphocytes and MT autoantibodies for risks of Cd-related adverse effects in cadmium-exposed population groups. In a study of the impact of zinc status on the risk of kidney dysfunction in a cadmium-exposed group, the risks were low when zinc status was good and high when zinc status was poor. The present review summarizes this evidence in a risk assessment context and calls for its application in order to improve preventive measures against adverse effects of cadmium exposures in humans and animals.

Parkinson's Disease and the Metal-Microbiome-Gut-Brain Axis: A Systems Toxicology Approach

Parkinson's Disease (PD) is a neurodegenerative disease, leading to motor and non-motor complications. Autonomic alterations, including gastrointestinal symptoms, precede motor defects and act as early warning signs. Chronic exposure to dietary, environmental heavy metals impacts the gastrointestinal system and host-associated microbiome, eventually affecting the central nervous system. The correlation between dysbiosis and PD suggests a functional and bidirectional communication between the gut and the brain. The bioaccumulation of metals promotes stress mechanisms by increasing reactive oxygen species, likely altering the bidirectional gut-brain link. To better understand the differing molecular mechanisms underlying PD, integrative modeling approaches are necessary to connect multifactorial perturbations in this heterogeneous disorder. By exploring the effects of gut microbiota modulation on dietary heavy metal exposure in relation to PD onset, the modification of the host-associated microbiome to mitigate neurological stress may be a future treatment option against neurodegeneration through bioremediation. The progressive movement towards a systems toxicology framework for precision medicine can uncover molecular mechanisms underlying PD onset such as metal regulation and microbial community interactions by developing predictive models to better understand PD etiology to identify options for novel treatments and beyond. Several methodologies recently addressed the complexity of this interaction from different perspectives; however, to date, a comprehensive review of these approaches is still lacking. Therefore, our main aim through this manuscript is to fill this gap in the scientific literature by reviewing recently published papers to address the surrounding questions regarding the underlying molecular mechanisms between metals, microbiota, and the gut-brain-axis, as well as the regulation of this system to prevent neurodegeneration.

Pathophysiological effects of cadmium(II) on human health-a critical review

Cadmium(II) is an omnipresent environmental toxicant emitted from various industrial sources and by anthropogenic sources such as smoking. Cadmium(II) enters our body through various sources including contaminated food and drinks and from active or passive smoking. It spares no organs in our body and the calamities it invites include primarily nephrotoxicity, osteotoxicity, teratogenicity, endocrine disruption, hepatotoxicity and carcinogenicity above all. It brings about a bolt from the blue in the cellular biochemistry by generating reactive oxygen species (ROS), disrupting the factors involved in the repair of DNA lesions and many other toxic nuisances otherwise by modulating the cell signalling machinery and acting as a potent carcinogen above all. In this review, we have tried to decipher some of the mechanisms played by cadmium(II) in exhibiting its toxic effects on various system of our body.

The transfer of trace metals in the soil-plant-arthropod system

Essential and non-essential trace metals are capable of causing toxicity to organisms above a threshold concentration. Extensive research has assessed the behaviour of trace metals in biological and ecological systems, but has typically focused on single organisms within a trophic level and not on multi-trophic transfer through terrestrial food chains. This reinforces the notion of metal toxicity as a closed system, failing to consider one trophic level as a pollution source to another; therefore, obscuring the full extent of ecosystem effects. Given the relatively few studies on trophic transfer of metals, this review has taken a compartment-based approach, where transfer of metals through trophic pathways is considered as a series of linked compartments (soil-plant-arthropod herbivore-arthropod predator). In particular, we consider the mechanisms by which trace metals are taken up by organisms, the forms and transformations that can occur within the organism and the consequences for trace metal availability to the next trophic level. The review focuses on four of the most prevalent metal cations in soil which are labile in terrestrial food chains: Cd, Cu, Zn and Ni. Current knowledge of the processes and mechanisms by which these metals are transformed and moved within and between trophic levels in the soil-plant-arthropod system are evaluated. We demonstrate that the key factors controlling the transfer of trace metals through the soil-plant-arthropod system are the form and location in which the metal occurs in the lower trophic level and the physiological mechanisms of each organism in regulating uptake, transformation, detoxification and transfer. The magnitude of transfer varies considerably depending on the trace metal concerned, as does its toxicity, and we conclude that biomagnification is not a general property of plant-arthropod and arthropod-arthropod systems. To deliver a more holistic assessment of ecosystem toxicity, integrated studies across ecosystem compartments are needed to identify critical pathways that can result in secondary toxicity across terrestrial food-chains.

[Roles of Zinc Transporters That Control the Essentiality and Toxicity of Manganese and Cadmium]

Cellular transport systems for both essential and toxic trace elements remain elusive. In our studies on the transport systems for cadmium (Cd), we found that the cellular uptake of Cd is mediated by the transporter for manganese (Mn). We identified ZIP8 and ZIP14, members of the ZIP zinc (Zn) transporter family, as transporters having high affinities for both Cd and Mn. Notably, the uptake of Cd into rice root from soil is mediated by a transporter for Mn as well. We found that ZIP8 is highly expressed at the S3 segment of the kidney proximal tubule and can transport glomerulus-filtered Cd and Mn ions in the lumen into epithelial cells of the proximal tubule, suggesting that ZIP8 has an important role in the renal reabsorption of both toxic Cd and essential Mn. Mutations in ZIP8 and ZIP14 genes were found in humans having congenital disorders associated with the disturbed transport of Mn, although ZIP8 mutation causes whole-body Mn deficiency while ZIP14 mutation causes Mn accumulation in the brain. Mutations in ZnT10, a Zn transporter responsible for Mn excretion, also cause hyperaccumulation of Mn in the brain. Results of genome-wide association studies have indicated that ZIP8 SNPs are involved in a variety of common diseases. Thus, ZIP8, ZIP14, and ZnT10 play crucial roles in the transport of Mn and thereby control Mn- and Cd-related biological events in the body.

Zinc as a countermeasure for cadmium toxicity

Cadmium (Cd) is an important environmental pollutant and long-term Cd exposure is closely related to autoimmune diseases, cancer, cardiovascular diseases (CVD), and hepatic dysfunction. Zinc (Zn) is an essential metal that plays key roles in protein structure, catalysis, and regulation of their function. Numerous studies have shown that Zn can reduce Cd toxicity; however, the underlying mechanisms have not been extensively explored. Preclinical studies have revealed direct competition for sarcolemmal uptake between these two metals. Multiple sarcolemmal transporters participate in Cd uptake, including Zn transporters, calcium channels, and DMT1 (divalent metal transporter 1). Zn also induces several protective mechanisms, including MT (metallothionein) induction and favorable redox homeostasis. This review summarizes current knowledge related to the role of Zn and metal transporters in reducing Cd toxicity and discusses potential future directions of related research.

Involvement of metal transporters in the intestinal uptake of cadmium

We investigated the mechanism underlying intestinal cadmium (Cd) uptake based on the mediators (metal transporters) of essential elements, such as Fe, Zn, Cu, and Ca, under normal conditions in female rats. These elements interact with Cd uptake from the intestinal tract. Cd concentration at each site of the small intestine (duodenum, jejunum, and ileum) increased as Cd exposure increased. However, Cd concentration was the highest in the duodenum. The gene expression of ZIP14, DMT1, and ATP7A increased with increase in Cd concentration. Further, Cu concentration decreased as Cd concentration increased. In contrast, Fe concentration displayed a decreasing tendency with the increase in Cd concentration. The gene expression levels of ZIP14, DMT1, and ATP7A were positively correlated with Cd concentration. Immunohistochemical staining revealed the positive sites of ZIP14 and DMT1 scattered in the area adjacent to the goblet cells, resorbable epithelial cells, and lamina propria in the duodenum tissue, according to the increase in Cd concentration. Cd is induced to synthesize and bind to metallothionein (MT-I and -II) and accumulate in the intestinal tissues, mainly in the duodenum. Such findings suggest that Cd, a contaminant element, is taken up from the intestinal tract by multiple metal transporters such as Cu, Fe, and Zn, thereby involving in the intestinal Cd absorption.

Mechanotransduction Activity Facilitates Hair Cell Toxicity Caused by the Heavy Metal Cadmium

Hair cells are sensitive to many insults including environmental toxins such as heavy metals. We show here that cadmium can consistently kill hair cells of the zebrafish lateral line. Disrupting hair cell mechanotransduction genetically or pharmacologically significantly reduces the amount of hair cell death seen in response to cadmium, suggesting a role for mechanotransduction in this cell death process, possibly as a means for cadmium uptake into the cells. Likewise, when looking at multiple cilia-associated gene mutants that have previously been shown to be resistant to aminoglycoside-induced hair cell death, resistance to cadmium-induced hair cell death is only seen in those with mechanotransduction defects. In contrast to what was seen with mechanotransduction, significant protection was not consistently seen from other ions previously shown to compete for cadmium uptake into cells or tissue including zinc and copper. These results show that functional mechanotransduction activity is playing a significant role in cadmium-induced hair cell death.

Segment-specific and direction-dependent transport of cadmium and manganese in immortalized S1, S2, and S3 cells derived from mouse kidney proximal tubules

The kidney proximal tubule is a target of many renal toxicants, including cadmium (Cd), and also a place of reabsorption of essential metals in glomerular filtrate to systemic circulation. Although the mechanisms of metal transport in the convoluted proximal tubule (S1 and S2 segments) and the straight proximal tubule (S3 segment) may differ, little is known about the segment-specific modes of metal transport. Here, we utilized immortalized cell lines derived from the S1, S2, and S3 segments of mouse kidney proximal tubules, and examined the segment-specific and direction-dependent transport of Cd and manganese (Mn) using a trans-well culture system. The results showed that the uptakes of Cd²⁺ and Mn²⁺ from apical sides were the highest in S3 cells, and Cd²⁺, Mn²⁺, and Zn²⁺ mutually inhibited the apical uptake of each metal. As the expression of ZIP8, a zinc transporter having affinities for Cd²⁺ and Mn²⁺, was the highest in S3 cells, ZIP8 may contribute largely to the apical uptakes of these metals. The efficient uptake of Mn²⁺ from apical side of S3 cells may suggest an important role of ZIP8 in proximal tubule in reabsorption of Mn, an essential metal. Our study demonstrated that S1, S2, and S3 cells provide a useful tool for studying the segment-specific and direction-dependent transport of both toxic and essential metals in the kidney's proximal tubules.

New Insights into the Roles of ZIP8, a Cadmium and Manganese Transporter, and Its Relation to Human Diseases

ZIP8, a Zrt-/Irt-related protein encoded by Slc39A8, was originally discovered as a zinc transporter, but since then its roles as a transporter for cadmium (Cd) and manganese (Mn) have also been well characterized. ZIP8 is highly expressed in the S3 segment of the proximal tubules of the mouse kidney and may play a significant role in reabsorption of both toxic Cd and essential Mn from the lumen to the epithelial cells of the proximal tubule. In recent years, associations between various human diseases and genetic variations of ZIP8 have been reported. Missense mutations in the human SLC39A8 gene are associated with serious disorders of Mn metabolism, showing symptoms similar to congenital glycosylation deficiency. Enhanced excretion of Mn via bile or urine might be the cause of extremely low blood Mn levels in ZIP8-mutated patients, leading to the defects in Mn-dependent glycosylation. Several genome-wide association studies have demonstrated the associations of multiple diseases and ZIP8 SNPs constituting missense mutations. These findings suggest that ZIP8 plays more important roles than previously expected as a modulator of Mn homeostasis in the body. Elucidation of biochemical mechanisms regarding the metal-transporting ability of ZIP8 and its alteration by mutation is required for better understanding of the role of ZIP8 in human diseases.

Toxicometallomics of Cadmium, Manganese and Arsenic with Special Reference to the Roles of Metal Transporters

The transport systems for metals play crucial roles in both the physiological functions of essential metals and the toxic effects of hazardous metals in mammals and plants. In mammalian cells, Zn transporters such as ZIP8 and ZIP14 have been found to function as the transporters for Mn(II) and Cd(II), contributing to the maintenance of Mn homeostasis and metallothionein-independent transports of Cd, respectively. In rice, the Mn transporter OsNramp5 expressed in the root is used for the uptake of Cd from the soil. Japan began to cultivate OsNramp5 mutant rice, which was found to accumulate little Cd, to prevent Cd accumulation. Inorganic trivalent arsenic (As(III)) is absorbed into mammalian cells via aquaglyceroporin, a water and glycerol channel. The ortholog of aquaporin in rice, OsLsi1, was found to be an Si transporter expressed in rice root, and is responsible for the absorption of soil As(III) into the root. Since rice is a hyperaccumulator of Si, higher amounts of As(III) are incorporated into rice compared to other plants. Thus, the transporters of essential metals are also utilized to incorporate toxic metals in both mammals and plants, and understanding the mechanisms of metal transports is important for the development of mitigation strategies against food contamination.

Long-term cadmium exposure enhances metallothionein-1 induction after subsequent exposure to high concentrations of cadmium in P1798 mouse lymphosarcoma cells

Cadmium, a ubiquitous heavy metal, is a toxic industrial and environmental pollutant. The initial biological response to cadmium exposure is induction of metallothioneins (MTs), a family of cysteine-rich, low-molecular-weight proteins that bind primarily zinc, cadmium, or both. This MT induction protects against cadmium toxicity by quenching cadmium. However, the effects of long-term cadmium exposure on MT1 gene expression are largely unknown. To investigate these effects, we used P1798 mouse lymphosarcoma cells, in which the MT1 gene is suppressed. As previously reported, MT1 expression remained unchanged after cadmium treatment. However, MT1 induction was observed in cells treated with 0.1 µM cadmium for 7 days, then exposed to 10 µM cadmium for 3 hr. In cells treated with 0.1 µM cadmium for 7 days, the transfected MT1 promoter reporter gene transcription and the cadmium incorporation in response to 10 µM cadmium induction were similar to those in untreated P1798 cells. Bisulfite genomic sequencing revealed that 7 day treatment with 0.1 µM cadmium slightly decreased CpG methylation in the 5´ flanking region of the MT1 gene. Our results together show that cadmium treatment results in MT1 induction and epigenetic modification of the MT1 gene.

The protective role of ascorbic acid in the hepatotoxicity of cadmium and mercury in rabbits

The liver is one of the vital and sensitive organs which are usually exposed against the toxicity of mercury (Hg) and cadmium (Cd). The main objective of the current study was to evaluate the potential toxicological effects of both Cd and Hg as individual and combined. Hepatotoxicity was evaluated by monitoring the biochemical parameters of the liver and their accumulation in the liver as well as therapeutic role of vitamin C in said toxicity in rabbits (Oryctolagus cuniculus). In this research, cadmium chloride (1.5 mg/kg), mercuric chloride (1.2 mg/kg), and vitamin C (150 mg/kg of body weight) were orally administered to treatment groups of the rabbits for 28 alternative days. Various biochemical parameters of the liver such as lactate dehydrogenase (LDH), aspartate aminotransferase (ASAT), bilirubin, alanine aminotransferase (ALAT), total protein, and gamma glutamyl transferase (GGT) were estimated using blood samples. Some biochemical parameters like ASAT, ALAT, LDH, GGT, and bilirubin were significantly elevated (P ≤ 0.001) in individual Cd and Hg treatment groups, while the level of total protein was found to be significantly declined. The effects of Cd and Hg in the presence of vitamin C on these biochemical parameters were low as compared to metals-treated groups. Similar results were found when rabbits were treated with co-administration of both metals and vitamin C. Accumulation of Cd and Hg found to be higher in the liver. However, chemoprevention and chemotreatment with vitamin C significantly (P ≤ 0.01) minimized the toxicological effects of both metals but not regained the accumulation similar to that of the control group. The findings of this study provide awareness on accumulation of metals in the liver in rabbits and their toxicity tested through biochemical parameters as well as the therapeutic role of vitamin C in such alterations.

Zinc supplementation can reduce accumulation of cadmium in aged metallothionein transgenic mice

Epidemiologic studies suggest that exposure to Cd is related to a multitude of age-related diseases. There is evidence that Cd toxicity emerges from an interference with Zn metabolism as they compete for the same binding sites of ligands. The most responsive proteins to Cd exposure are the metal-binding proteins termed metallothioneins (MTs), which display a much greater affinity for Cd than for Zn. Most studies have considered the effect of Zn on the accumulation of exogenous Cd and tissue damage, whereas observational studies have addressed the association between Zn intake and Cd levels in body fluids. However, it has not been addressed whether supplemental Zn can lower Cd levels in organs of healthy aged animals without affecting Cu stores, a question more pertinent to human aging. We therefore aimed to investigate the effect of Zn supplementation on Cd levels in liver and kidney of aged MT transgenic mice (MT1-tg) overexpressing MT1 at levels more comparable to those observed in humans than non-transgenic mice. We found a >30% reduction of kidney and liver Cd levels in Zn supplemented MT1-tg mice compared to non-supplemented controls, independently of the dose of Zn, without a significant reduction of Cu. Our data support the idea of a causal and inverse relationship between Zn intake and Cd content in organs of aged MT1-tg mice as suggested by observational studies in humans. Our work provides the rationale for interventional studies to address the effects of Zn supplementation on Cd burden in elderly people.

[Transport and Toxicity of Cadmium]

Cadmium is a nonessential heavy metal and an industrial and environmental pollutant. It has been known that cadmium must enter cells to cause damage. To understand the transport systems responsible for cadmium entry into cells, it is important to determine the precise mechanisms underlying cadmium toxicity. Numerous studies have sought to unravel the exact pathways by which cadmium enters various cells and the mechanisms by which it causes toxicity in the organs of human and animals. The purpose of this review is to present the progress made regarding the mechanisms of cadmium transport in various cells and the mechanisms underlying cadmium toxicity in organs.

Ameliorative effect of the anticancer agent salinomycin on cadmium-induced hepatotoxicity and renal dysfunction in mice

This study presents experimental data on the effects of the tetraethylammonium salt of salinomycinic acid (Sal) on Cd-induced hepatotoxicity and renal dysfunction in Cd-treated mice compared to those of meso-2,3-dimercaptosuccinic acid (DMSA). Forty 60-day-old male ICR mice were randomized into five groups: control group (untreated mice), Cd group (Cd(II) acetate 20 mg/kg body weight provided orally once per day for 14 days), Cd + DMSA group (exposed to Cd(II) acetate as the Cd-exposed group followed by DMSA 20 mg/kg body weight provided orally once per day for 14 days), and Cd + Sal group (exposed to Cd(II) acetate as the Cd-exposed group followed by Sal 20 mg/kg body weight once per day for 14 days). Cd intoxication of mice induced significant liver and kidney injury and a significant elevation of the concentration of Cd in both organs. Treatment of Cd-exposed mice with DMSA or Sal restored the levels of the renal and hepatic functional markers and significantly decreased the concentration of the toxic metal ion in both organs. Administration of Sal improved Cd-induced alterations of the endogenous levels of the essential metal ions. Histological studies revealed that the antibiotic more effectively ameliorated the Cd effect on the liver morphology compared to DMSA. Taken together, the results confirm that the anticancer agent salinomycin is a promising antidote to Cd poisoning.

Concentration-dependent roles of DMT1 and ZIP14 in cadmium absorption in Caco-2 cells

Intestinal absorption of cadmium (Cd) is considered to be mediated mainly by the ferrous iron transporter DMT1, or the calcium transporter CaT1. The roles of zinc transporters such as ZIP8 and ZIP14 remain unclear, and the roles of these four transporters in the intestinal uptake of Cd under physiological conditions have not been compared. Here, we used a trans-well cell culture system to investigate the effects of the down-regulation of these four transporters on the uptake of Cd from the apical side of enterocytes. We used a Caco-2-kh cell line that can form tight junctions within a few days. The transfection of DMT1 siRNA significantly decreased the Cd uptake from the apical side at 5 μM, but not at 0.1 or 1 μM. The transfection of ZIP14 siRNA markedly decreased the Cd uptake at 0.1 and 1 μM, but not at 5 μM. The transfection of siRNA of CaT1 or ZIP8 did not alter the Cd uptake at any concentrations of Cd examined. These results suggest that DMT1 and ZIP14 play different roles in intestinal Cd absorption depending on the concentration of Cd.

Zinc Transporter Proteins

Zinc, which is involved in the structure of all enzyme classes, is a micro nutrient element and necessary for growth and development. The ability of zinc to function without causing toxic effects is depends on the protection of its homeostasis. Zinc transporter proteins are responsible for keeping zinc at certain concentrations. Based on their predicted membrane topology, Zn transporters are divided into two major families, SLC39s/ZIPs and SLC30s/ZnTs, which transport Zn in opposite directions through cellular and intracellular membranes. ZIPs increases the zinc concentration in the cytosol. For this, the ZIPs carries the zinc from extracellular and intracellular compartments to the cytosol. ZnTs, reduces the concentration of zinc in the cytosol. For this, ZnTs carries the zinc from the cytosol to extracellular and intracellular compartments. After being transported to the cell, 50% of the zinc is found in the cytoplasm, 30-40% in the nucleus, and 10% in the plasma and organelle membranes. The expression of many zinc transporter proteins in the cell is depending on the concentration of zinc and the physiological problems. The aim of this study is to give information about association of zinc transporter proteins with physiological events and health problems.

Chronic Kidney Disease and Exposure to Nephrotoxic Metals

Chronic kidney disease (CKD) is a common progressive disease that is typically characterized by the permanent loss of functional nephrons. As injured nephrons become sclerotic and die, the remaining healthy nephrons undergo numerous structural, molecular, and functional changes in an attempt to compensate for the loss of diseased nephrons. These compensatory changes enable the kidney to maintain fluid and solute homeostasis until approximately 75% of nephrons are lost. As CKD continues to progress, glomerular filtration rate decreases, and remaining nephrons are unable to effectively eliminate metabolic wastes and environmental toxicants from the body. This inability may enhance mortality and/or morbidity of an individual. Environmental toxicants of particular concern are arsenic, cadmium, lead, and mercury. Since these metals are present throughout the environment and exposure to one or more of these metals is unavoidable, it is important that the way in which these metals are handled by target organs in normal and disease states is understood completely.

Progressive silencing of the zinc transporter Zip8 (Slc39a8) in chronic cadmium-exposed lung epithelial cells

Cadmium (Cd), a non-essential metal, stealthily enters the cells by utilizing the essential metal importing pathways. The zinc transporters Zip8, Zip14, and divalent metal transporter 1 (Dmt1) are now emerging as several important metal transporters involved in cellular Cd incorporation and their expressions have been shown to be down-regulated in several Cd-resistant (CdR) cell lines, however, the involvement of these transporters during the development of Cd-resistance in lung cells is unclear. In this study, we therefore check the expression of these metal transporters in our previously established rat lung epithelial cells (LECs) and show that the level of Zip8 is progressively silenced when LECs are adapted to increasing concentrations of CdCl2 (from 1 to 20 μM). Subsequent measurement of the cellular Cd content indicated that CdR LECs exhibit a marked decrease of Cd accumulation, possibly due to the loss of Zip8 expression. We investigate the possibility that epigenetic silencing of the Zip8 gene by DNA hypermethylation is involved in the down-regulation of Zip8 expression. CdR LECs show a higher mRNA level of DNA methyltransferase 3b (Dnmt3b) than parental cells. Treatment of CdR LECs with 5-aza-2'-deoxycytidine, an inhibitor of DNA methyltransferases, reverted the expression of Zip8 and sensitivity to Cd in these cells, indicating the critical role of Zip8 for Cd import. Taken together, our results demonstrate that the progressive silencing of Zip8 expression is involved in the acquisition of resistance against Cd in lung cells, representing an adaptive survival mechanism that resists Cd-induced cytotoxicity.

Aberrant cytokine secretion and zinc uptake in chronic cadmium-exposed lung epithelial cells

PURPOSE

Our previous results showed that cadmium (Cd)-adapted lung epithelial cells (LECs) developed resistance to apoptosis due to non-responsiveness of the c-Jun N-terminal kinase pathway and augmented expression of cytokeratin 8. Since cellular Cd entry is a prerequisite in order for Cd to elicit its cytotoxicity, therefore, we wonder if there are differential metal ion transport ability and also other phenotypic changes that occurred in these Cd-resistant LECs.

EXPERIMENTAL DESIGN AND RESULTS

Here, we explored further and found that the zinc (Zn) importer Zip8 was stably abolished in these cells along with a marked decrease of Cd and Zn accumulation. Moreover, by cell migration assays and cytokine antibody array analysis, we found that Cd-adapted cells exhibit enhanced migratory ability possibly due to elevated secretions of vascular endothelial growth factor and macrophage inflammatory protein-3 alpha (MIP-3α).

CONCLUSION AND CLINICAL RELEVANCE

Taken together, our results show that during chronic Cd exposure, lung cells antagonize excessive cellular Cd-influx by abolishing Zip8 expression to reduce Cd-toxicity; however, this also renders cells with a diminished Zn uptake. The imbalance of Zn homeostasis and elevation of angiogenic and epithelial-mesenchymal transition-promoting cytokines in Cd-adapted cells might thus likely promote Zn deficiency, angiogenesis, and cell invasion.

Cellular mechanisms of cadmium-induced toxicity: a review

Cadmium is a widespread toxic pollutant of occupational and environmental concern because of its diverse toxic effects: extremely protracted biological half-life (approximately 20-30 years in humans), low rate of excretion from the body and storage predominantly in soft tissues (primarily, liver and kidneys). It is an extremely toxic element of continuing concern because environmental levels have risen steadily due to continued worldwide anthropogenic mobilization. Cadmium is absorbed in significant quantities from cigarette smoke, food, water and air contamination and is known to have numerous undesirable effects in both humans and animals. Cadmium has a diversity of toxic effects including nephrotoxicity, carcinogenicity, teratogenicity and endocrine and reproductive toxicities. At the cellular level, cadmium affects cell proliferation, differentiation, apoptosis and other cellular activities. Current evidence suggests that exposure to cadmium induces genomic instability through complex and multifactorial mechanisms. Most important seems to be cadmium interaction with DNA repair mechanism, generation of reactive oxygen species and induction of apoptosis. In this article, we have reviewed recent developments and findings on cadmium toxicology.

Fibroblasts from long-lived rodent species exclude cadmium

Resistance to the lethal effects of cellular stressors, including the toxic heavy metal cadmium (Cd), is characteristic of fibroblast cell lines derived from long-lived bird and rodent species, as well as cell lines from several varieties of long-lived mutant mice. To explore the mechanism of resistance to Cd, we used inductively coupled plasma mass spectroscopy to measure the rate of Cd uptake into primary fibroblasts of 15 rodent species. These data indicate that fibroblasts from long-lived rodent species have slower rates of Cd uptake from the extracellular medium than those from short-lived species. In addition, fibroblasts from short-lived species export more zinc after exposure to extracellular Cd than cells from long-lived species. Lastly, fibroblasts from long-lived rodent species have lower baseline concentrations of two redox-active metals, iron and copper. Our results suggest that evolution of longevity among rodents required adjustment of cellular properties to alter metal homeostasis and to reduce the toxic effects of heavy metals that accumulate over the course of a longer life span.

Interleukin-6 enhances manganese accumulation in SH-SY5Y cells: implications of the up-regulation of ZIP14 and the down-regulation of ZnT10

Manganese accumulation in neuronal cells may be enhanced by interleukin-6viathe up-regulation of ZIP14 and the down-regulation of ZnT10.

The SLC39 family of zinc transporters

Zinc is a trace element nutrient that is essential for life. This mineral serves as a cofactor for enzymes that are involved in critical biochemical processes and it plays many structural roles as well. At the cellular level, zinc is tightly regulated and disruption of zinc homeostasis results in serious physiological or pathological issues. Despite the high demand for zinc in cells, free or labile zinc must be kept at very low levels. In humans, two major zinc transporter families, the SLC30 (ZnT) family and SLC39 (ZIP) family control cellular zinc homeostasis. This review will focus on the SLC39 transporters. SLC39 transporters primarily serve to pass zinc into the cytoplasm, and play critical roles in maintaining cellular zinc homeostasis. These proteins are also significant at the organismal level, and studies are revealing their link to human diseases. Therefore, we will discuss the function, structure, physiology, and pathology of SLC39 transporters.

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.

Roles of ZIP8, ZIP14, and DMT1 in transport of cadmium and manganese in mouse kidney proximal tubule cells

Chronic exposure to cadmium causes preferential accumulation of cadmium in the kidney, leading to nephrotoxicity. In the process of renal cadmium accumulation, the cadmium bound to a low-molecular-weight metal-binding protein, metallothionein, has been considered to play an important role in reabsorption by epithelial cells of proximal tubules in the kidney. However, the role and mechanism of the transport of Cd(2+) ions in proximal tubule cells remain unclear. Zinc transporters such as Zrt, Irt-related protein 8 (ZIP8) and ZIP14, and divalent metal transporter 1 (DMT1) have been reported to have affinities for Cd(2+) and Mn(2+). To examine the roles of these metal transporters in the absorption of luminal Cd(2+) and Mn(2+) into proximal tubule cells, we utilized a cell culture system, in which apical and basolateral transport of metals can be separately examined. The uptake of Cd(2+) and Mn(2+) from the apical side of proximal tubule cells was inhibited by simultaneous addition of Mn(2+) and Cd(2+), respectively. The knockdown of ZIP8, ZIP14 or DMT1 by siRNA transfection significantly reduced the uptake of Cd(2+) and Mn(2+) from the apical membrane. The excretion of Cd(2+) and Mn(2+) was detected predominantly in the apical side of the proximal tubule cells. In situ hybridization of these transporters revealed that ZIP8 and ZIP14 are highly expressed in the proximal tubules of the outer stripe of the outer medulla. These results suggest that ZIP8 and ZIP14 expressed in the S3 segment of proximal tubules play significant roles in the absorption of Cd(2+) and Mn(2+) in the kidney.

Physiologic implications of metal-ion transport by ZIP14 and ZIP8

Zinc, iron, and manganese are essential trace elements that serve as catalytic or structural components of larger molecules that are indispensable for life. The three metal ions possess similar chemical properties and have been shown to compete for uptake in a variety of tissues, suggesting that they share common transport proteins. Two likely candidates are the recently identified transmembrane proteins ZIP14 and ZIP8, which have been shown to mediate the cellular uptake of a number of divalent metal ions including zinc, iron, manganese, and cadmium. Although knockout and transgenic mouse models are beginning to define the physiologic roles of ZIP14 and ZIP8 in the handling of zinc and cadmium, their roles in the metabolism of iron and manganese remain to be defined. Here we review similarities and differences in ZIP14 and ZIP8 in terms of structure, metal transport, tissue distribution, subcellular localization, and regulation. We also discuss potential roles of these proteins in the metabolism of zinc, iron, manganese, and cadmium as well as recent associations with human diseases.

Environmental toxicants--induced epigenetic alterations and their reversers

Epigenetics has been emphasized in the postgenome era to clarify obscure health risks of environmental toxicants including endocrine disrupting chemicals (EDCs). In addition, mixed exposure in real life can modify health consequences of the toxicants. Particularly, some nutritional and dietary materials modify individual susceptibility through changes in the epigenome. Therefore, we focused on some environmental toxicants that induce epigenetic alterations, and introduced chemopreventive materials to reverse the toxicants-induced epigenetic alterations. Methodologically, we used global and specific DNA methylation as epigenetic end points and searched epigenetic modulators in food. We reviewed various epigenetic end points induced by environmental toxicants including alcohol, asbestos, nanomaterials, benzene, EDCs, metals, and ionizing radiation. The epigenetic end points can be summarized into global hypomethylation and specific hypermethylation at diverse tumor suppress genes. Exposure timing, dose, sex, or organ specificity should be considered to use the epigenetic end points as biomarkers for exposure to the epimutagenic toxicants. Particularly, neonatal exposure to the epimutagens can influence their future adult health because of characteristics of the epimutagens, which disrupt epigenetic regulation in imprinting, organogenesis, development, etc. Considering interaction between epimutagenic toxicants and their reversers in food, we suggest that multiple exposures to them can alleviate or mask epigenetic toxicity in real life. Our present review provides useful information to find new end points of environmental toxicants and to prevention from environment-related diseases.

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.

Role of metallothionein in cadmium traffic and toxicity in kidneys and other mammalian organs

Metallothioneins are cysteine-rich, small metal-binding proteins present in various mammalian tissues. Of the four common metallothioneins, MT-1 and MT-2 (MTs) are expressed in most tissues, MT-3 is predominantly present in brain, whereas MT-4 is restricted to the squamous epithelia. The expression of MT-1 and MT-2 in some organs exhibits sex, age, and strain differences, and inducibility with a variety of stimuli. In adult mammals, MTs have been localized largely in the cell cytoplasm, but also in lysosomes, mitochondria and nuclei. The major physiological functions of MTs include homeostasis of essential metals Zn and Cu, protection against cytotoxicity of Cd and other toxic metals, and scavenging free radicals generated in oxidative stress. The role of MTs in Cd-induced acute and chronic toxicity, particularly in liver and kidneys, is reviewed in more details. In acute toxicity, liver is the primary target, whereas in chronic toxicity, kidneys are major targets of Cd. The intracellular MTs bind Cd ions and form CdMT. In chronic intoxication, Cd stimulates de novo synthesis of MTs; it is assumed that toxicity in the cells starts when loading with Cd ions exceeds the buffering capacity of intracellular MTs. CdMT, released from the Cd-injured organs, or when applied parenterally for experimental purposes, reaches the kidneys via circulation, where it is filtered, endocytosed in the proximal tubule cells, and degraded in lysosomes. Liberated Cd can immediately affect the cell structures and functions. The resulting proteinuria and CdMT in the urine can be used as biomarkers of tubular injury.

Involvement of DNA hypermethylation in down-regulation of the zinc transporter ZIP8 in cadmium-resistant metallothionein-null cells

The Zrt/Irt-related protein 8 (ZIP8) encoded by slc39a8 is now emerging as an important zinc transporter involved in cellular cadmium incorporation. We have previously shown that mRNA and protein levels of ZIP8 were decreased in cadmium-resistant metallothionein-null (A7) cells, leading to a decrease in cadmium accumulation. However, the mechanism by which ZIP8 expression is suppressed in these cells remains to be elucidated. In the present study, we investigated the possibility that epigenetic silencing of the slc39a8 gene by DNA hypermethylation is involved in the down-regulation of ZIP8 expression. A7 cells showed a higher mRNA level of DNA methyltransferase 3b than parental cells. Hypermethylation of the CpG island of the slc39a8 gene was detected in A7 cells. Treatment of A7 cells with 5-aza-deoxycytidine, an inhibitor of DNA methyltransferase, caused demethylation of the CpG island of the slc39a8 gene and enhancement of mRNA and protein levels of ZIP8. In response to the recovery of ZIP8 expression, A7 cells treated with 5-aza-deoxycytidine showed an increase in cadmium accumulation and consequently an increase in sensitivity to cadmium. These results suggest that epigenetic silencing of the slc39a8 gene by DNA hypermethylation plays an important role in the down-regulation of ZIP8 in cadmium-resistant metallothionein-null cells.