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

The Regulation of ZIP8 by Dietary Manganese in Mice

ZIP8 is a newly identified manganese transporter. A lack of functional ZIP8 results in severe manganese deficiency in both humans and mice, indicating that ZIP8 plays a crucial role in maintaining body manganese homeostasis. Despite a well-acknowledged connection between ZIP8 and manganese metabolism, how ZIP8 is regulated under high-manganese conditions remains unclear. The primary goal of this study was to examine the regulation of ZIP8 by high-manganese intake. We used both neonatal and adult mouse models in which mice were supplied with dietary sources containing either a normal or a high level of manganese. We discovered that high-manganese intake caused a reduction in liver ZIP8 protein in young mice. Since a decrease in hepatic ZIP8 leads to reduced manganese reabsorption from the bile, our study identified a novel mechanism for the regulation of manganese homeostasis under high-manganese conditions: high dietary manganese intake results in a decrease in ZIP8 in the liver, which in turn decreases the reabsorption of manganese from the bile to prevent manganese overload in the liver. Interestingly, we found that a high-manganese diet did not cause a decrease in hepatic ZIP8 in adult animals. To determine the potential reason for this age-dependent variation, we compared the expressions of liver ZIP8 in 3-week-old and 12-week-old mice. We found that liver ZIP8 protein content in 12-week-old mice decreases when compared with that of 3-week-old mice under normal conditions. Overall, results from this study provide novel insights to facilitate the understanding of ZIP8's function in regulating manganese metabolism.

Iron Deficiency and Nephrotoxic Heavy Metals: A Dangerous Interplay?

Heavy metals are common in our environment, and all individuals are exposed to them to some extent. These toxic metals have several harmful effects on the body, including the kidney, which is a very sensitive organ. Indeed, heavy metal exposure has been linked to an increased risk of chronic kidney disease (CKD) and its progression, which may be explained by the well-established nephrotoxic effects of these metals. In this hypothesis and narrative literature review, we will shed light on the potential role that another highly common problem in patients with CKD, iron deficiency, may play in the damaging effects of heavy metal exposure in this patient group. Iron deficiency has previously been linked with an increased uptake of heavy metals in the intestine due to the upregulation of iron receptors that also take up other metals. Furthermore, recent research suggests a role of iron deficiency in the retention of heavy metals in the kidney. Therefore, we hypothesize that iron deficiency plays a crucial role in the damaging effects of heavy metal exposure in patients with CKD and that iron supplementation might be a strategy to combat these detrimental processes.

Impact of Zinc Transport Mechanisms on Embryonic and Brain Development

The trace element zinc (Zn) binds to over ten percent of proteins in eukaryotic cells. Zn flexible chemistry allows it to regulate the activity of hundreds of enzymes and influence scores of metabolic processes in cells throughout the body. Deficiency of Zn in humans has a profound effect on development and in adults later in life, particularly in the brain, where Zn deficiency is linked to several neurological disorders. In this review, we will summarize the importance of Zn during development through a description of the outcomes of both genetic and early dietary Zn deficiency, focusing on the pathological consequences on the whole body and brain. The epidemiology and the symptomology of Zn deficiency in humans will be described, including the most studied inherited Zn deficiency disease, Acrodermatitis enteropathica. In addition, we will give an overview of the different forms and animal models of Zn deficiency, as well as the 24 Zn transporters, distributed into two families: the ZIPs and the ZnTs, which control the balance of Zn throughout the body. Lastly, we will describe the TRPM7 ion channel, which was recently shown to contribute to intestinal Zn absorption and has its own significant impact on early embryonic development.

Manganese transport in mammals by zinc transporter family proteins, ZNT and ZIP

Manganese (Mn) is an essential trace element required for various biological processes. However, excess Mn causes serious side effects in humans, including parkinsonism. Thus, elucidation of Mn homeostasis at the systemic, cellular, and molecular levels is important. Many metal transporters and channels can be involved in the transport and homeostasis of Mn, and an increasing body of evidence shows that several zinc (Zn) transporters belonging to the ZIP and ZNT families, specifically, ZNT10, ZIP8, and ZIP14, play pivotal roles in Mn metabolism. Mutations in the genes encoding these transporter proteins are associated with congenital disorders related to dysregulated Mn homeostasis in humans. Moreover, single nucleotide polymorphisms of ZIP8 are associated with multiple clinical phenotypes. In this review, we discuss the recent literature on the structural and biochemical features of ZNT10, ZIP8, and ZIP14, including transport mechanisms, regulation of expression, and pathophysiological functions. Because a disturbance in Mn homeostasis is closely associated with a variety of phenotypes and risk of human diseases, these transporters constitute a significant target for drug development. An understanding of the roles of these key transporters in Mn metabolism should provide new insights into pharmacological applications of their inhibitors and enhancers in human diseases.

Construction of Mode of Action for Cadmium-Induced Renal Tubular Dysfunction Based on a Toxicity Pathway-Oriented Approach

Although it is recognized that cadmium (Cd) causes renal tubular dysfunction, the mechanism of Cd-induced nephrotoxicity is not yet fully understood. Mode of action (MOA) is a developing tool for chemical risk assessment. To establish the mechanistic MOA of Cd-induced renal tubular dysfunction, the Comparative Toxicogenomics Database (CTD) was used to obtain genomics data of Cd-induced nephrotoxicity, and Ingenuity^® Pathway Analysis (IPA) software was applied for bioinformatics analysis. Based on the perturbed toxicity pathways during the process of Cd-induced nephrotoxicity, we established the MOA of Cd-induced renal tubular dysfunction and assessed its confidence with the tailored Bradford Hill criteria. Bioinformatics analysis showed that oxidative stress, DNA damage, cell cycle arrest, and cell death were the probable key events (KEs). Assessment of the overall MOA of Cd-induced renal tubular dysfunction indicated a moderate confidence, and there are still some evidence gaps to be filled by rational experimental designs.

[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.

Comparisons of segment-specific toxicity of platinum-based agents and cadmium using S1, S2, and S3 cells derived from mouse kidney proximal tubules

Renal toxicants such as cisplatin and cadmium cause segment-specific damages in kidney proximal tubules. Recently, we established an in vitro experimental system for evaluating segment-specific toxicity and transport of chemicals using immortalized S1, S2, and S3 cells derived from the S1, S2, and S3 regions of mouse kidney proximal tubules. In the present study, we examined the toxicity and accumulation of cisplatin, carboplatin, oxaliplatin, and cadmium in S1, S2, and S3 cells. We found that not only cisplatin but also carboplatin and oxaliplatin exhibited higher lethal toxicity in S3 cells than in S1 and S2 cells. At sublethal doses, cisplatin showed delayed induction of Kim-1 and clusterin on days 3 and 6, which may reflect the latent renal toxicity of cisplatin in vivo. The high sensitivities of S3 cells to the platinum-based agents were not due to the high accumulation of Pt in S3 cells. Exposure to cadmium resulted in similar toxicity among these cells, suggesting that S3 cells were not sensitive to any renal toxicants. Thus, the utilization of S1, S2, and S3 cells may provide a useful tool for the in vitro evaluation of the proximal tubule segment-specific toxicity of chemicals.

Manganese exposure induces permeability in renal glomerular endothelial cells via the Smad2/3-Snail-VE-cadherin axis

Manganese (Mn) is an essential micronutrient. However, it is well established that Mn overexposure causes nervous system diseases. In contrast, there are few reports on the effects of Mn exposure on glomerular endothelium. In the present study, the potential effects of Mn exposure on glomerular endothelium were evaluated. Sprague Dawley rats were used as a model of Mn overexposure by intraperitoneal injection of MnCl₂·H₂O at 25 mg/kg body weight. Mn exposure decreased expression of vascular endothelial-cadherin, a key component of adherens junctions, and increased exudate from glomeruli in Sprague Dawley rats. Human renal glomerular endothelial cells were cultured with different concentration of Mn. Exposure to 0.2 mM Mn increased permeability of human renal glomerular endothelial cell monolayers and decreased vascular endothelial-cadherin expression without inducing cytotoxicity. In addition, Mn exposure increased phosphorylation of mothers against decapentaplegic homolog 2/3 and upregulated expression of zinc finger protein SNAI1, a negative transcriptional regulator of vascular endothelial-cadherin. Our data suggest Mn exposure may contribute to development of glomerular diseases by inducing permeability of glomerular endothelium.

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.

In vitro Evaluation of The Effects of Cadmium on Endocytic Uptakes of Proteins into Cultured Proximal Tubule Epithelial Cells

Cadmium (Cd) is an environmental pollutant known to cause dysfunctions of the tubular reabsorption of biomolecules in the kidney. Elevated levels of urinary excretion of low-molecular-weight proteins such as β₂-microglobulin (β₂-MG) have been used as an indicator of Cd-induced renal tubular dysfunctions. However, very few studies have examined the direct effects of Cd on the reabsorption efficiency of proteins using cultured renal cells. Here, we developed an in vitro assay system for quantifying the endocytic uptakes of fluorescent-labeled proteins by flow cytometry in S1 and S2 cells derived from mouse kidney proximal tubules. Endocytic uptakes of fluorescent-labeled albumin, transferrin, β₂-MG, and metallothionein into S1 cells were confirmed by fluorescence imaging and flow cytometry. The exposure of S1 and S2 cells to Cd at 1 and 3 µM for 3 days resulted in significant decreases in the uptakes of β₂-MG and metallothionein but not in those of albumin or transferrin. These results suggest that Cd affects the tubular reabsorption of low-molecular-weight proteins even at nonlethal concentrations. The in vitro assay system developed in this study to evaluate the endocytic uptakes of proteins may serve as a useful tool for detecting toxicants that cause renal tubular dysfunctions.

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.