The zinc transporter Zip14 influences c-Met phosphorylation and hepatocyte proliferation during liver regeneration in mice

Marginal Maternal Zinc Deficiency Produces Liver Damage and Altered Zinc Transporter Expression in Offspring Male Rats

The aim of this study was to investigate how zinc deficiency and supplementation affect liver markers including autotaxin, kallistatin, endocan, and zinc carrier proteins ZIP14 and ZnT9 in rats exposed to maternal zinc deficiency. Additionally, the study aimed to assess liver tissue damage through histological examination. A total of forty male pups were included in the research, with thirty originating from mothers who were given a zinc-deficient diet (Groups 1, 2, and 3), and the remaining ten born to mothers fed a standard diet (Group 4). Subsequently, Group 1 was subjected to a zinc-deficient diet, Group 2 received a standard diet, Group 3 received zinc supplementation, and Group 4 served as the control group without any supplementation. Upon completion of the experimental phases of the study, all animals were sacrificed under general anesthesia, and samples of liver tissue were obtained. The levels of autotaxin, kallistatin, endocan, ZIP 14, and ZnT9 in these liver tissue samples were determined using the ELISA technique. In addition, histological examination was performed to evaluate tissue damage in the liver samples. In the group experiencing zinc deficiency, both endocan and autotaxin levels increased compared to the control group. With zinc supplementation, the levels of endocan and autotaxin returned to the values observed in the control group. Similarly, the suppressed levels of kallistatin, ZIP14, and ZnT9 observed in the zinc deficiency group were reversed with zinc supplementation. Likewise, the reduced levels of kallistatin, ZIP14, and ZnT9 seen in the zinc deficiency group were rectified with zinc supplementation. Moreover, the application of zinc partially ameliorated the heightened liver tissue damage triggered by zinc deficiency. This study is the pioneering one to demonstrate that liver tissue dysfunction induced by a marginal zinc-deficient diet in rats with marginal maternal zinc deficiency can be alleviated through zinc supplementation.

Absence of Slc39a14/Zip14 in mouse pancreatic beta cells results in hyperinsulinemia

Zinc is an essential component of the insulin protein complex synthesized in β cells. The intracellular compartmentalization and distribution of zinc are controlled by 24 transmembrane zinc transporters belonging to the ZnT or Zrt/Irt-like protein (ZIP) family. Downregulation of SLC39A14/ZIP14 has been reported in pancreatic islets of patients with type 2 diabetes (T2D) as well as mouse models of high-fat diet (HFD)- or db/db-induced obesity. Our previous studies observed mild hyperinsulinemia in mice with whole body knockout of Slc39a14 (Zip14 KO). Based on our current secondary data analysis from an integrative single-cell RNA-seq dataset of human whole pancreatic tissue, SLC39A14 (coding ZIP14) is the only other zinc transporter expressed abundantly in human β cells besides well-known zinc transporter SLC30A8 (coding ZnT8). In the present work, using pancreatic β cell-specific knockout of Slc39a14 (β-Zip14 KO), we investigated the role of SLC39A14/ZIP14-mediated intracellular zinc trafficking in glucose-stimulated insulin secretion and subsequent metabolic responses. Glucose-stimulated insulin secretion, zinc concentrations, and cellular localization of ZIP14 were assessed using in vivo, ex vivo, and in vitro assays using β-Zip14 KO, isolated islets, and murine cell line MIN6. Metabolic evaluations were done on both chow- and HFD-fed mice using time-domain nuclear magnetic resonance and a comprehensive laboratory animal monitoring system. ZIP14 localizes on the endoplasmic reticulum regulating intracellular zinc trafficking in β cells and serves as a negative regulator of glucose-stimulated insulin secretion. Deletion of Zip14 resulted in greater glucose-stimulated insulin secretion, increased energy expenditure, and shifted energy metabolism toward fatty acid utilization. HFD caused β-Zip14 KO mice to develop greater islet hyperplasia, compensatory hyperinsulinemia, and mild insulin resistance and hyperglycemia. This study provided new insights into the contribution of metal transporter ZIP14-mediated intracellular zinc trafficking in glucose-stimulated insulin secretion and subsequent metabolic responses.NEW & NOTEWORTHY Metal transporter SLC39A14/ZIP14 is downregulated in pancreatic islets of patients with T2D and mouse models of HFD- or db/db-induced obesity. However, the function of ZIP14-mediated intracellular zinc trafficking in β cells is unknown. Our analyses revealed that SLC39A14 is the only Zn transporter expressed abundantly in human β cells besides SLC30A8. Within the β cells, ZIP14 is localized on the endoplasmic reticulum and serves as a negative regulator of insulin secretion, providing a potential therapeutic target for T2D.

Metal transporter SLC39A14/ZIP14 modulates regulation between the gut microbiome and host metabolism

Metal transporter SLC39A14/ZIP14 is localized on the basolateral side of the intestine, functioning to transport metals from blood to intestine epithelial cells. Deletion of Slc39a14/Zip14 causes spontaneous intestinal permeability with low-grade chronic inflammation, mild hyperinsulinemia, and greater body fat with insulin resistance in adipose. Importantly, antibiotic treatment reverses the adipocyte phenotype of Slc39a14/Zip14 knockout (KO), suggesting a potential gut microbial role in the metabolic alterations in the Slc39a14/Zip14 KO mice. Here, we investigated the hypothesis that increased intestinal permeability and subsequent metabolic alterations in the absence of Zip14 could be in part due to alterations in gut microbial composition. Dietary metals have been shown to be involved in the regulation of gut microbial diversity and composition. However, studies linking the action of intestinal metal transporters to gut microbial regulation are lacking. We showed the influence of deletion of metal transporter Slc39a14/Zip14 on gut microbiome composition and how ZIP14-linked changes to gut microbiome community composition are correlated with changes in host metabolism. Deletion of Slc39a14/Zip14 generated Zn-deficient epithelial cells and luminal content in the entire intestinal tract, a shift in gut microbial composition that partially overlapped with changes previously associated with obesity and inflammatory bowel disease (IBD), increased the fungi/bacteria ratio in the gut microbiome, altered the host metabolome, and shifted host energy metabolism toward glucose utilization. Collectively, our data suggest a potential predisease microbial susceptibility state dependent on host gene Slc39a14/Zip14 that contributes to intestinal permeability, a common trait of IBD, and metabolic disorders such as obesity and type 2 diabetes.NEW & NOTEWORTHY Metal dyshomeostasis, intestinal permeability, and gut dysbiosis are emerging signatures of chronic disorders, including inflammatory bowel diseases, type-2 diabetes, and obesity. Studies in reciprocal regulations between host intestinal metal transporters genes and gut microbiome are scarce. Our research revealed a potential predisease microbial susceptibility state dependent on the host metal transporter gene, Slc39a14/Zip14, that contributes to intestinal permeability providing new insight into understanding host metal transporter gene-microbiome interactions in developing chronic disease.

Interplay between zinc and cell proliferation and implications for the growth of livestock

Zinc (Zn) plays a critical role in the growth of livestock, which depends on cell proliferation. In addition to modifying the growth associated with its effects on food intake, mitogenic hormones, signal transduction and gene transcription, Zn also regulates body weight gain through mediating cell proliferation. Zn deficiency in animals leads to growth inhibition, along with an arrest of cell cycle progression at G0/G1 and S phase due to depression in the expression of cyclin D/E and DNA synthesis. Therefore, in the present study, the interplay between Zn and cell proliferation and implications for the growth of livestock were reviewed, in which Zn regulates cell proliferation in several ways, especially cell cycle progression at the G0/G1 phase DNA synthesis and mitosis. During the cell cycle, the Zn transporters and major Zn binding proteins such as metallothioneins are altered with the requirements of cellular Zn level and nuclear translocation of Zn. In addition, calcium signaling, MAPK pathway and PI3K/Akt cascades are also involved in the process of Zn-interfering cell proliferation. The evidence collected over the last decade highlights the necessity of Zn for normal cell proliferation, which suggests Zn supplementation should be considered for the growth and health of poultry.

Sucrose-induced hyperglycemia dysregulates intestinal zinc metabolism and integrity: risk factors for chronic diseases

Objective

Zinc is an essential micronutrient that is critical for many physiological processes, including glucose metabolism, regulation of inflammation, and intestinal barrier function. Further, zinc dysregulation is associated with an increased risk of chronic inflammatory diseases such as type II diabetes, obesity, and inflammatory bowel disease. However, whether altered zinc status is a symptom or cause of disease onset remains unclear. Common symptoms of these three chronic diseases include the onset of increased intestinal permeability and zinc dyshomeostasis. The specific focus of this work is to investigate how dietary sources of intestinal permeability, such as high sucrose consumption, impact transporter-mediated zinc homeostasis and subsequent zinc-dependent physiology contributing to disease development.

Method

We used in vivo subchronic sucrose treatment, ex vivo intestinal organoid culture, and in vitro cell systems. We analyze the alterations in zinc metabolism and intestinal permeability and metabolic outcomes.

Results

We found that subchronic sucrose treatment resulted in systemic changes in steady-state zinc distribution and increased 65Zn transport (blood-to-intestine) along with greater ZIP14 expression at the basolateral membrane of the intestine. Further, sucrose treatment enhanced cell survival of intestinal epithelial cells, activation of the EGFR-AKT-STAT3 pathway, and intestinal permeability.

Conclusion

Our work suggests that subchronic high sucrose consumption alters systemic and intestinal zinc homeostasis linking diet-induced changes in zinc homeostasis to the intestinal permeability and onset of precursors for chronic disease.

Structural Characterization of Zinc-Sucrose Complex and Its Ability to Promote Zinc Absorption in Caco-2 Monolayer Cells and Mice

Sucrose emerges as a metal-ion chelating agent with excellent stability that may increase metal-ion absorption. This study aimed to characterize the structure of zinc-sucrose complex and investigate its ability to promote zinc absorption in Caco-2 monolayer cells and mice. Based on the results of the inductively coupled plasma emission spectrometer (ICP-ES), scanning electron microscopy-energy-dispersive X-ray spectroscopy (SEM-EDX), and Fourier transform infrared spectroscopy (FT-IR), it can be inferred that zinc and sucrose were chelated at a 1:1 ratio, with the hydroxyl groups playing a significant role. The Caco-2 monolayer cell model revealed that zinc-sucrose complex increased the amount of zinc uptake, retention, and transport in a dose- and time-dependent manner. Through the upregulation of genes and proteins for ZIP4, MT1, and DMT1, treatment with zinc-sucrose complex improved the proportion of absorbed zinc utilized for transport compared to ZnCl2 (26.21 ± 4.96 versus 8.50 ± 1.51%). Pharmacokinetic analysis of mice confirmed the zinc absorption-promoting effect of zinc-sucrose complex, as indicated by the considerably higher serum zinc level (4.16 ± 0.53 versus 2.56 ± 0.45 mg/L) and intestinal ZIP4, MT1, and DMT1 gene expression than ZnCl2. Further treatment of different zinc channel inhibitors and CETSA demonstrated the direct interaction of zinc-sucrose complex with ZIP4 protein and ZIP4-mediated cellular transport of zinc-sucrose complex. These findings provide a novel insight into the zinc absorption-promoting mechanism of zinc-sucrose complex, which could be used as an ingredient in functional foods to treat zinc deficiency.

Pathophysiological studies of aging Slc39a14 knockout mice to assess the progression of manganese-induced dystonia-parkinsonism

Over the last decade, several clinical reports have outlined cases of early-onset manganese (Mn)-induced dystonia-parkinsonism, resulting from loss of function mutations of the Mn transporter gene SLC39A14. Previously, we have performed characterization of the behavioral, neurochemical, and neuropathological changes in 60-day old (PN60) Slc39a14-knockout (KO) murine model of the human disease. Here, we extend our studies to aging Slc39a14-KO mice to assess the progression of the disease. Our results indicate that 365-day old (PN365) Slc39a14-KO mice present with markedly elevated blood and brain Mn levels, similar to those found in the PN60 mice and representative of the human cases of the disease. Furthermore, aging Slc39a14-KO mice consistently manifest a hypoactive and dystonic behavioral deficits, similar to the PN60 animals, suggesting that the behavioral changes are established early in life without further age-associated deterioration. Neurochemical, neuropathological, and functional assessment of the dopaminergic system of the basal ganglia revealed absence of neurodegenerative changes of dopamine (DA) neurons in the substantia nigra pars compacta (SNc), with no changes in DA or metabolite concentrations in the striatum of Slc39a14-KO mice relative to wildtype (WT). Similar to the PN60 animals, aging Slc39a14-KO mice expressed a marked inhibition of potassium-stimulated DA release in the striatum. Together our findings indicate that the pathophysiological changes observed in the basal ganglia of aging Slc39a14-KO animals are similar to those at PN60 and aging does not have a significant effect on these parameters.

Mineral metabolism and ferroptosis in non-alcoholic fatty liver diseases

Nonalcoholic fatty liver disease (NAFLD) has become the most prevalent chronic liver disease worldwide. Minerals including iron, copper, zinc, and selenium, fulfil an essential role in various biochemical processes. Moreover, the identification of ferroptosis and cuproptosis further underscores the importance of intracellular mineral homeostasis. However, perturbation of minerals has been frequently reported in patients with NAFLD and related diseases. Interestingly, studies have attempted to establish an association between mineral disorders and NAFLD pathological features, including oxidative stress, mitochondrial dysfunction, inflammatory response, and fibrogenesis. In this review, we aim to provide an overview of the current understanding of mineral metabolism (i.e., absorption, utilization, and transport) and mineral interactions in the pathogenesis of NAFLD. More importantly, this review highlights potential therapeutic strategies, challenges, future directions for targeting mineral metabolism in the treatment of NAFLD.

Zinc deficiency as a possible risk factor for increased susceptibility and severe progression of Corona Virus Disease 19

The importance of Zn for human health becomes obvious during Zn deficiency. Even mild insufficiencies of Zn cause alterations in haematopoiesis and immune functions, resulting in a proinflammatory phenotype and a disturbed redox metabolism. Although immune system malfunction has the most obvious effect, the functions of several tissue cell types are disturbed if Zn supply is limiting. Adhesion molecules and tight junction proteins decrease, while cell death increases, generating barrier dysfunction and possibly organ failure. Taken together, Zn deficiency both weakens the resistance of the human body towards pathogens and at the same time increases the danger of an overactive immune response that may cause tissue damage. The case numbers of Corona Virus Disease 19 (COVID-19) are still increasing, which is causing enormous problems for health systems and economies. There is an urgent need to reduce both the number of severe cases and the resulting deaths. While therapeutic options are still under investigation, and first vaccines have been approved, cost-effective ways to reduce the likelihood of or even prevent infection, and the transition from mild symptoms to more serious detrimental disease, are highly desirable. Nutritional supplementation might be an effective option to achieve these aims. In this review, we discuss known Zn deficiency effects in the context of an infection with Severe Acute Respiratory Syndrome-Coronavirus-2 and its currently known pathogenic mechanisms and elaborate on how severe pre-existing Zn deficiency may pre-dispose patients to a severe progression of COVID-19. First published clinical data on the association of Zn homoeostasis with COVID-19 and registered studies in progress are listed.

Expression profiles of SLC39A/ZIP7, ZIP8 and ZIP14 in response to exercise-induced skeletal muscle damage

BACKGROUND

Zinc transporters are thought to facilitate the mobilization of zinc (Zn) and the role of Zn as a signaling mediator during cellular events. Little is known about the response of Zn movement and zinc transporters during muscle proliferation and differentiation processes after damage.

METHODS

After rats were subjected to one 90-min session of downhill running to cause muscle damage, the gastrocnemius muscles were harvested to assess the expression of zinc transporters SLC39A/ZIP7, ZIP8, ZIP14 and myogenic regulatory factors at the 0 h, 6 h, 12 h, 1 d, 2 d, 3 d, 1 w and 2 w time points after exercise.

RESULTS

SLC39A/ZIP7, ZIP8 and ZIP14 had translocated to different compartments of the cell following damage, and they exhibited differential expression profiles after eccentric exercise. The results regarding the myogenetic regulators showed that nf-κb was upregulated 2 d after exercise, and STAT3 and Akt1 mRNA levels were mostly expressed 2 w after exercise. The upregulation of phosphatidylinositol 3-kinase, catalytic subunit gamma (pik3cg), erk1 and erk2 mostly occurred at the early stage (6 h or 12 h) after exercise. In addition, we found that zip7, zip8 and zip14 expression was moderately correlated with certain markers of muscle regeneration.

CONCLUSION

The zinc transporters SLC39A/ZIP7, ZIP8 and ZIP14 have differential expression profiles upon eccentric exercise, and they might regulate muscle proliferation or differentiation processes through different cellular pathways after exercise-induced muscle damage.

Zinc Favors Triple-Negative Breast Cancer's Microenvironment Modulation and Cell Plasticity

Triple-negative breast cancer (TNBC) tends to metastasize to the brain, a step that worsens the patient’s prognosis. The specific hallmarks that determine successful metastasis are motility and invasion, microenvironment modulation, plasticity, and colonization. Zinc, an essential trace element, has been shown to be involved in all of these processes. In this work, we focus our attention on the potential role of zinc during TNBC metastasis. We used MDA-MB-BrM2 (BrM2) cells, a brain metastasis model derived from the parental TNBC cell line MDA-MB-231. Our studies show that BrM2 cells had double the zinc content of MDA-MB-231 cells. Moreover, exploring different metastatic hallmarks, we found that the zinc concentration is especially important in the microenvironment modulation of brain metastatic cells, enhancing the expression of SerpinB2. Furthermore, we show that zinc promotes the tumorigenic capacity of breast cancer stem cells. In addition, by causing a disturbance in MDA-MB-231 zinc homeostasis by overexpressing the Zip4 transporter, we were able to increase tumorigenicity. Nevertheless, this strategy did not completely recapitulate the BrM2 metastatic phenotype. Altogether, our work suggests that zinc plays an important role in the transformative steps that tumoral cells take to acquire tumorigenic potential and niche specificity.

Behavioral and neurochemical studies of inherited manganese-induced dystonia-parkinsonism in Slc39a14-knockout mice

Inherited autosomal recessive mutations of the manganese (Mn) transporter gene SLC39A14 in humans, results in elevated blood and brain Mn concentrations and childhood-onset dystonia-parkinsonism. The pathophysiology of this disease is unknown, but the nigrostriatal dopaminergic system of the basal ganglia has been implicated. Here, we describe pathophysiological studies in Slc39a14-knockout (KO) mice as a preclinical model of dystonia-parkinsonism in SLC39A14 mutation carriers. Blood and brain metal concentrations in Slc39a14-KO mice exhibited a pattern similar to the human disease with highly elevated Mn concentrations. We observed an early-onset backward-walking behavior at postnatal day (PN) 21 which was also noted in PN60 Slc39a14-KO mice as well as dystonia-like movements. Locomotor activity and motor coordination were also impaired in Slc39a14-KO relative to wildtype (WT) mice. From a neurochemical perspective, striatal dopamine (DA) and metabolite concentrations and their ratio in Slc39a14-KO mice did not differ from WT. Striatal tyrosine hydroxylase (TH) immunohistochemistry did not change in Slc39a14-KO mice relative to WT. Unbiased stereological cell quantification of TH-positive and Nissl-stained estimated neuron number, neuron density, and soma volume in the substantia nigra pars compacta (SNc) was the same in Slc39a14-KO mice as in WT. However, we measured a marked inhibition (85-90%) of potassium-stimulated DA release in the striatum of Slc39a14-KO mice relative to WT. Our findings indicate that the dystonia-parkinsonism observed in this genetic animal model of the human disease is associated with a dysfunctional but structurally intact nigrostriatal dopaminergic system. The presynaptic deficit in DA release is unlikely to explain the totality of the behavioral phenotype and points to the involvement of other neuronal systems and brain regions in the pathophysiology of the disease.

Dietary and Physiological Effects of Zinc on the Immune System

Evidence for the importance of zinc for all immune cells and for mounting an efficient and balanced immune response to various environmental stressors has been accumulating in recent years. This article describes the role of zinc in fundamental biological processes and summarizes our current knowledge of zinc's effect on hematopoiesis, including differentiation into immune cell subtypes. In addition, the important role of zinc during activation and function of immune cells is detailed and associated with the specific immune responses to bacteria, parasites, and viruses. The association of zinc with autoimmune reactions and cancers as diseases with increased or decreased immune responses is also discussed. This article provides a broad overview of the manifold roles that zinc, or its deficiency, plays in physiology and during various diseases. Consequently, we discuss why zinc supplementation should be considered, especially for people at risk of deficiency. Expected final online publication date for the Annual Review of Nutrition, Volume 41 is September 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

The Potential Impact of Zinc Supplementation on COVID-19 Pathogenesis

During the current corona pandemic, new therapeutic options against this viral disease are urgently desired. Due to the rapid spread and immense number of affected individuals worldwide, cost-effective, globally available, and safe options with minimal side effects and simple application are extremely warranted. This review will therefore discuss the potential of zinc as preventive and therapeutic agent alone or in combination with other strategies, as zinc meets all the above described criteria. While a variety of data on the association of the individual zinc status with viral and respiratory tract infections are available, study evidence regarding COVID-19 is so far missing but can be assumed as was indicated by others and is detailed in this perspective, focusing on re-balancing of the immune response by zinc supplementation. Especially, the role of zinc in viral-induced vascular complications has barely been discussed, so far. Interestingly, most of the risk groups described for COVID-19 are at the same time groups that were associated with zinc deficiency. As zinc is essential to preserve natural tissue barriers such as the respiratory epithelium, preventing pathogen entry, for a balanced function of the immune system and the redox system, zinc deficiency can probably be added to the factors predisposing individuals to infection and detrimental progression of COVID-19. Finally, due to its direct antiviral properties, it can be assumed that zinc administration is beneficial for most of the population, especially those with suboptimal zinc status.

Intestine-specific deletion of metal transporter Zip14 (Slc39a14) causes brain manganese overload and locomotor defects of manganism

Impaired manganese (Mn) homeostasis can result in excess Mn accumulation in specific brain regions and neuropathology. Maintaining Mn homeostasis and detoxification is dependent on effective Mn elimination. Specific metal transporters control Mn homeostasis. Human carriers of mutations in the metal transporter ZIP14 and whole body Zip14-knockout (WB-KO) mice display similar phenotypes, including spontaneous systemic and brain Mn overload and motor dysfunction. Initially, it was believed that Mn accumulation due to ZIP14 mutations was caused by impaired hepatobiliary Mn elimination. However, liver-specific Zip14-KO mice did not show systemic Mn accumulation or motor deficits. ZIP14 is highly expressed in the small intestine and is localized to the basolateral surface of enterocytes. Thus, we hypothesized that basolaterally localized ZIP14 in enterocytes provides another route for the elimination of Mn. Using wild-type and intestine-specific Zip14-KO (I-KO) mice, we have shown that ablation of intestinal Zip14 is sufficient to cause systemic and brain Mn accumulation. The lack of intestinal ZIP14-mediated Mn excretion was compensated for by the hepatobiliary system; however, it was not sufficient to maintain Mn homeostasis. When supplemented with extra dietary Mn, I-KO mice displayed some motor dysfunctions and brain Mn accumulation based on both MRI imaging and chemical analysis, thus demonstrating the importance of intestinal ZIP14 as a route of Mn excretion. A defect in intestinal Zip14 expresssion likely could contribute to the Parkinson-like Mn accumulation of manganism.NEW & NOTEWORTHY Mn-induced parkinsonism is recognized as rising in frequency because of both environmental factors and genetic vulnerability; yet currently, there is no cure. We provide evidence in an integrative animal model that basolaterally localized ZIP14 regulates Mn excretion and detoxification and that deletion of intestinal ZIP14 leads to systemic and brain Mn accumulation, providing robust evidence for the indispensable role of intestinal ZIP14 in Mn excretion.

Deletion of metal transporter Zip14 (Slc39a14) produces skeletal muscle wasting, endotoxemia, Mef2c activation and induction of miR-675 and Hspb7

Skeletal muscle represents the largest pool of body zinc, however, little is known about muscle zinc homeostasis or muscle-specific zinc functions. Zip14 (Slc39a14) was the most highly expressed zinc transporter in skeletal muscle of mice in response to LPS-induced inflammation. We compared metabolic parameters of skeletal muscle from global Zip14 knockout (KO) and wild-type mice (WT). At basal steady state Zip14 KO mice exhibited a phenotype that included muscle wasting and metabolic endotoxemia. Microarray and qPCR analysis of gastrocnemius muscle RNA revealed that ablation of Zip14 produced increased muscle p-Mef2c, Hspb7 and miR-675-5p expression and increased p38 activation. ChIP assays showed enhanced binding of NF-[Formula: see text] to the Mef2c promoter. In contrast, LPS-induced systemic inflammation enhanced Zip14-dependent zinc uptake by muscle, increased expression of Atrogin1 and MuRF1 and markedly reduced MyoD. These signatures of muscle atrophy and cachexia were not influenced by Zip14 ablation, however. LPS-induced miR-675-3p and -5p expression was Zip14-dependent. Collectively, these results with an integrative model are consistent with a Zip14 function in skeletal muscle at steady state that supports myogenesis through suppression of metabolic endotoxemia and that Zip14 ablation coincides with sustained activity of phosphorylated components of signaling pathways including p-Mef2c, which causes Hspb7-dependent muscle wasting.

Translationally controlled tumor protein promotes liver regeneration by activating mTORC2/AKT signaling

Translationally controlled tumor protein (TCTP), which is a protein characterized by its potent proliferation promoting activity, has been well studied in the area of growth and tumorigenesis. However, the specific role of TCTP in liver regeneration (LR) and its underlying mechanism remains unclear. In order to investigate the contribution of TCTP during LR, heterozygous TCTP mice were generated, and a mimic LR model was applied to TCTP-knockdown (KD) hepatic cell lines. The results revealed that TCTP-KD impaired LR in mice, and manifested as the following aspects: delayed proliferation of hepatocytes, accompanied by disruption of the mRNA expression of markers of the cell cycle, degenerated lipid metabolism, and abnormal immune response. Furthermore, it was found out that TCTP activated PI3K/AKT signaling by regulating mTORC2. Lastly, the increasing rate of serum TCTP positively correlated to the recovery of alanine aminotransferase (ALT) and aspartate aminotransferase (AST) after liver resection in humans. In summary, the present study is the first to reveal the crucial role of intracellular TCTP in LR.

Manganese Uptake by A549 Cells is Mediated by Both ZIP8 and ZIP14

The alveolar epithelia of the lungs require manganese (Mn) as an essential nutrient, but also provide an entry route for airborne Mn that can cause neurotoxicity. Transporters involved in Mn uptake by alveolar epithelial cells are unknown. Recently, two members of the Zrt- and Irt-like protein (ZIP) family of metal transporters, ZIP8 and ZIP14, have been identified as crucial Mn importers in vivo. ZIP8 is by far most abundantly expressed in the lungs, whereas ZIP14 expression in the lungs is low compared to other tissues. We hypothesized that Mn uptake by alveolar epithelial cells is primarily mediated by ZIP8. To test our hypothesis, we used A549 cells, a type II alveolar cell line. Mirroring the in vivo situation, A549 cells expressed higher levels of ZIP8 than cell models for the liver, intestines, and kidney. Quantification of ZIP8 and ZIP14 revealed a strong enrichment of ZIP8 over ZIP14 in A549 cells. Using siRNA technology, we identified ZIP8 and ZIP14 as the major transporters mediating Mn uptake by A549 cells. To our surprise, knockdown of either ZIP8 or ZIP14 impaired Mn accumulation to a similar extent, which we traced back to similar amounts of ZIP8 and ZIP14 at the plasma membrane. Our study highlights the importance of both ZIP8 and ZIP14 in Mn metabolism of alveolar epithelial cells.

The zinc transporter Zip14 (SLC39a14) affects Beta-cell Function: Proteomics, Gene expression, and Insulin secretion studies in INS-1E cells

Insulin secretion from pancreatic beta-cells is dependent on zinc ions as essential components of insulin crystals, zinc transporters are thus involved in the insulin secretory process. Zip14 (SLC39a14) is a zinc importing protein that has an important role in glucose homeostasis. Zip14 knockout mice display hyperinsulinemia and impaired insulin secretion in high glucose conditions. Endocrine roles for Zip14 have been established in adipocytes and hepatocytes, but not yet confirmed in beta-cells. In this study, we investigated the role of Zip14 in the INS-1E beta-cell line. Zip14 mRNA was upregulated during high glucose stimulation and Zip14 silencing led to increased intracellular insulin content. Large-scale proteomics showed that Zip14 silencing down-regulated ribosomal mitochondrial proteins, many metal-binding proteins, and others involved in oxidative phosphorylation and insulin secretion. Furthermore, proliferation marker Mki67 was down-regulated in Zip14 siRNA-treated cells. In conclusion, Zip14 gene expression is glucose sensitive and silencing of Zip14 directly affects insulin processing in INS-1E beta-cells. A link between Zip14 and ribosomal mitochondrial proteins suggests altered mitochondrial RNA translation, which could disturb mitochondrial function and thereby insulin secretion. This highlights a role for Zip14 in beta-cell functioning and suggests Zip14 as a future pharmacological target in the treatment of beta-cell dysfunction.

The pro-inflammatory stimulus of zinc- and copper-containing welding fumes in whole blood assay via protein tyrosine phosphatase 1B inhibition

An asymptomatic systemic inflammation after exposure to zinc- and copper-containing welding fumes has been described as mild form of metal fume fever in recent studies. Since chronic systemic inflammation leads to a higher cardiovascular risk, examining the inflammation with the underlying pathomechanism is necessary to estimate and hopefully prevent long-term effects of welding. We established a whole blood assay to investigate the effects of zinc- and copper-containing welding fume particles on the blood immune response. Increased levels of IL-6, IL-8, TNFα and IL-1β determined after 24 hours of exposure indicated an acute systemic inflammatory reaction. In vitro increases of IL-6 were comparable to in vivo increases of serum IL-6 levels in a study with welding fume exposure of human subjects. Inhibition of PTP1B was identified as one pathway responsible for the effects of zinc- and copper-containing welding fumes and therefore welding fume fever. In conclusion, the whole blood assay is a reliable and feasible method to investigate effects of zinc- and copper-containing welding fumes on the immune system and as a surrogate for systemic inflammation and welding fume fever. Future research can utilize whole blood assays to reduce and partially replace human exposure studies for further investigations of welding fume fever.

B cell activation and proliferation increase intracellular zinc levels

Zinc ions serve as second messengers in major cellular pathways, including the regulation pathways of proliferation and their proper regulation is necessary for homeostasis and a healthy organism. Accordingly, expression of zinc transporters can be altered in various cancer cell lines and is often involved in producing elevated intracellular zinc levels. In this study, human B cells were infected with Epstein–Barr virus (EBV) to generate immortalized cells, which revealed traits of tumor cells, such as high proliferation rates and an extended lifespan. These cells showed differentially altered zinc transporter expression with ZIP7 RNA and protein expression being especially increased as well as a corresponding increased phosphorylation of ZIP7 in EBV-transformed B cells. Accordingly, free zinc levels were elevated within these cells. To prove whether the observed changes resulted from immortalization or rather high proliferation, free zinc levels in in vitro activated B cells and in freshly isolated B cells expressing the activation marker CD69 were determined. Here, comparatively increased zinc levels were found, suggesting that activation and proliferation, but not immortalization, act as crucial factors for the elevation of intracellular free zinc.

Differential expression of zinc transporters accompanies the differentiation of C2C12 myoblasts

Zinc transporters facilitate metal mobilization and compartmentalization, playing a key role in cellular development. Little is known about the mechanisms and pathways of Zn movement between Zn transporters and metalloproteins during myoblast differentiation. We analyzed the differential expression of ZIP and ZnT transporters during C2C12 myoblast differentiation. Zn transporters account for a transient decrease of intracellular Zn upon myogenesis induction followed by a gradual increase of Zn in myotubes. Considering the subcellular localization and function of each of the Zn transporters, our findings indicate that a fine regulation is necessary to maintain correct metal concentrations in the cytosol and subcellular compartments to avoid toxicity, maintain homeostasis, and for loading metalloproteins needed during myogenesis. This study advances our basic understanding of the complex Zn transport network during muscle differentiation.

Prolonged stimulation of insulin release from MIN6 cells causes zinc depletion and loss of β-cell markers

Zinc is integral for the normal function of pancreatic β-cells in glycaemic control. Large amounts of zinc are secreted from β-cells following insulin exocytosis and regulated replenishment is required, which is thought to be mediated by the ZIP family of zinc importer proteins. Within Type 2 Diabetic patients, β-cells are stressed through prolonged stimulation by hyperglycaemia and this is thought to be a major factor contributing to loss of β-cell identity and mass. However, the consequences for the β-cell zinc status remain largely unexplored. We used inductively coupled plasma mass spectrometry (ICP-MS) to show that 24 h treatment of MIN6 cells with potassium chloride, mimicking hyperglycaemic stimulation, reduces the total cellular zinc content 2.8-fold, and qPCR to show an increase in mRNA expression for metallothioneins (Mt1 and Mt2) following 4 and 24 h of stimulation, suggestive of an early rise in cytosolic zinc. To determine which ZIP paralogues may be responsible for zinc replenishment, we used immunocytochemistry, Western blot and qPCR to demonstrate initial ZIP1 protein upregulation proceeded by downregulation of mRNA coding for ZIP1, ZIP6, ZIP7 and ZIP14. To assign a biological significance to the decreased total cellular zinc content, we assessed expression of key β-cell markers to show downregulation of mRNA for MafA, Mnx-1, Nkx2.2 and Pax6. Our data suggest hyperglycaemia-induced zinc depletion may contribute to loss of β-cell markers and promote β-cell dedifferentiation through disrupting expression of key transcription factors.

Expression of zinc transporters ZIP4, ZIP14 and ZnT9 in hepatic carcinogenesis-An immunohistochemical study

INTRODUCTION

Dysregulation of both, systemic zinc levels and tissue-specific zinc transporters, is reported in chronic inflammatory and malignant liver disease (hepatocellular carcinoma, HCC). Aim of this study is to assess the expression level of three zinc transporters in liver tissue and HCC: ZIP4, ZIP14 and ZnT9.

METHODS

The study is based on tissue samples obtained from 138 patients with histologically proven HCC. Tissue specimens from tumor (n = 138) and extra-lesional specimens (n = 72) were assessed immunohistochemically for the expression of the three zinc transporters. Expression levels were semi-quantitatively scored and statistically analyzed with respect to the etiology of HCC (alcohol, AFLD; non-alcoholic fatty liver disease, NAFLD; virus-hepatitis, VH) and survival.

RESULTS

Overall, expression levels of ZIP4, ZIP14 and ZnT9 were significantly higher in HCC tissue than in adjacent extra-lesional liver tissue. Expression levels in tumor tissue and survival time revealed a negative correlation for ZIP4 and ZIP14, and in part for ZnT9 (nuclear staining) (p < 0.05), whereas cytoplasmic staining of ZnT9 did not correlate with survival. Furthermore, the expression level of ZIP4 in extra-lesional tissue showed inverse correlation with survival time.

CONCLUSION

The upregulation of zinc transporters in hepatic carcinogenesis and its negative correlation with survival time implies a regulatory/functional link between zinc-homeostasis and development/progression of HCC that deserves to be further explored.

Conditional mouse models support the role of SLC39A14 (ZIP14) in Hyperostosis Cranialis Interna and in bone homeostasis

Hyperostosis Cranialis Interna (HCI) is a rare bone disorder characterized by progressive intracranial bone overgrowth at the skull. Here we identified by whole-exome sequencing a dominant mutation (L441R) in SLC39A14 (ZIP14). We show that L441R ZIP14 is no longer trafficked towards the plasma membrane and excessively accumulates intracellular zinc, resulting in hyper-activation of cAMP-CREB and NFAT signaling. Conditional knock-in mice overexpressing L438R Zip14 in osteoblasts have a severe skeletal phenotype marked by a drastic increase in cortical thickness due to an enhanced endosteal bone formation, resembling the underlying pathology in HCI patients. Remarkably, L438R Zip14 also generates an osteoporotic trabecular bone phenotype. The effects of osteoblastic overexpression of L438R Zip14 therefore mimic the disparate actions of estrogen on cortical and trabecular bone through osteoblasts. Collectively, we reveal ZIP14 as a novel regulator of bone homeostasis, and that manipulating ZIP14 might be a therapeutic strategy for bone diseases.

SLC39A14 deficiency alters manganese homeostasis and excretion resulting in brain manganese accumulation and motor deficits in mice

Solute carrier family 39, member 14 (SLC39A14) is a transmembrane transporter that can mediate the cellular uptake of zinc, iron, and manganese (Mn). Studies of Slc39a14 knockout (Slc39a14^-/-) mice have documented that SLC39A14 is required for systemic growth, hepatic zinc uptake during inflammation, and iron loading of the liver in iron overload. The normal physiological roles of SLC39A14, however, remain incompletely characterized. Here, we report that Slc39a14^-/- mice spontaneously display dramatic alterations in tissue Mn concentrations, suggesting that Mn is a main physiological substrate for SLC39A14. Specifically, Slc39a14^-/- mice have abnormally low Mn levels in the liver coupled with markedly elevated Mn concentrations in blood and most other organs, especially the brain and bone. Radiotracer studies using ⁵⁴Mn reveal that Slc39a14^-/- mice have impaired Mn uptake by the liver and pancreas and reduced gastrointestinal Mn excretion. In the brain of Slc39a14^-/- mice, Mn accumulated in the pons and basal ganglia, including the globus pallidus, a region susceptible to Mn-related neurotoxicity. Brain Mn accumulation in Slc39a14^-/- mice was associated with locomotor impairments, as assessed by various behavioral tests. Although a low-Mn diet started at weaning was able to reverse brain Mn accumulation in Slc39a14^-/- mice, it did not correct their motor deficits. We conclude that SLC39A14 is essential for efficient Mn uptake by the liver and pancreas, and its deficiency results in impaired Mn excretion and accumulation of the metal in other tissues. The inability of Mn depletion to correct the motor deficits in Slc39a14^-/- mice suggests that the motor impairments represent lasting effects of early-life Mn exposure.

The Multiple Faces of the Metal Transporter ZIP14 (SLC39A14)

The SLC39A family of metal transporters was identified through homologies with the Zrt- and Irt-like (ZIP) proteins from yeast and plants. Of all the ZIP transporters, ZIP14 is arguably the most robustly characterized in terms of function at the integrative level. Mice with a global knockout of Zip14 are viable, thus providing the opportunity to conduct physiologic experiments. In mice, Zip14 expression is highly tissue specific, with the greatest abundance in the jejunum > liver > heart > kidney > white adipose tissue > skeletal muscle > spleen > pancreas. A unique feature of Zip14 is its upregulation by proinflammatory conditions, particularly increased interleukin 6 (IL-6) and nitric oxide. The transcription factors AP-1, ATF4, and ATF6α are involved in Zip14 regulation. ZIP14 does not appear to be zinc-regulated. The Zip14 knockout phenotype shows multiple sites of ZIP14 function, including the liver, adipose tissue, brain, pancreas, and bone. A prominent feature of the Zip14 ablation is a reduction in intestinal barrier function and onset of metabolic endotoxemia. Many aspects of the phenotype are accentuated with age and accompany increased circulating IL-6. Studies with 65Zn, 59Fe [nontransferrin-bound iron (NTBI)] and 54Mn show that ZIP14 transports these metals. At a steady state, the plasma concentrations of zinc, NTBI, and manganese are such that zinc ions are the major substrate available for ZIP14 at the cell surface. Upregulation of ZIP14 accounts for the hypozincemia and hepatic zinc accumulation associated with acute inflammation and sepsis and is required for liver regeneration and resistance to endoplasmic reticulum (ER) stress. Zip14 ablation in mice produces a defect in manganese excretion that leads to excess manganese accumulation in the brain that produces characteristics of Parkinsonism.

The Zinc Sensing Receptor, ZnR/GPR39, in Health and Disease

A distinct G-protein coupled receptor that senses changes in extracellular Zn²⁺, ZnR/GPR39, was found in cells from tissues in which Zn²⁺ plays a physiological role. Most prominently, ZnR/GPR39 activity was described in prostate cancer, skin keratinocytes, and colon epithelial cells, where zinc is essential for cell growth, wound closure, and barrier formation. ZnR/GPR39 activity was also described in neurons that are postsynaptic to vesicular Zn²⁺ release. Activation of ZnR/GPR39 triggers Gαq-dependent signaling and subsequent cellular pathways associated with cell growth and survival. Furthermore, ZnR/GPR39 was shown to regulate the activity of ion transport mechanisms that are essential for the physiological function of epithelial and neuronal cells. Thus, ZnR/GPR39 provides a unique target for therapeutically modifying the actions of zinc in a specific and selective manner.

Expression of the ZIP/SLC39A transporters in β-cells: a systematic review and integration of multiple datasets

BACKGROUND

Pancreatic β-cells require a constant supply of zinc to maintain normal insulin secretory function. Following co-exocytosis with insulin, zinc is replenished via the Zrt- and Irt-like (ZIP; SLC39A) family of transporters. However the ZIP paralogues of particular importance for zinc uptake, and associations with β-cell function and Type 2 Diabetes remain largely unexplored. We retrieved and statistically analysed publically available microarray and RNA-seq datasets to perform a systematic review on the expression of β-cell SLC39A paralogues. We complemented results with experimental data on expression profiling of human islets and mouse β-cell derived MIN6 cells, and compared transcriptomic and proteomic sequence conservation between human, mouse and rat.

RESULTS

The 14 ZIP paralogues have 73-98% amino sequence conservation between human and rodents. We identified 18 datasets for β-cell SLC39A analysis, which compared relative expression to non-β-cells, and expression in response to PDX-1 activity, cytokines, glucose and type 2 diabetic status. Published expression data demonstrate enrichment of transcripts for ZIP7 and ZIP9 transporters within rodent β-cells and of ZIP6, ZIP7 and ZIP14 within human β-cells, with ZIP1 most differentially expressed in response to cytokines and PDX-1 within rodent, and ZIP6 in response to diabetic status in human and glucose in rat. Our qPCR expression profiling data indicate that SLC39A6, -9, -13, and - 14 are the highest expressed paralogues in human β-cells and Slc39a6 and -7 in MIN6 cells.

CONCLUSIONS

Our systematic review, expression profiling and sequence alignment reveal similarities and potentially important differences in ZIP complements between human and rodent β-cells. We identify ZIP6, ZIP7, ZIP9, ZIP13 and ZIP14 in human and rodent and ZIP1 in rodent as potentially biologically important for β-cell zinc trafficking. We propose ZIP6 and ZIP7 are key functional orthologues in human and rodent β-cells and highlight these zinc importers as important targets for exploring associations between zinc status and normal physiology of β-cells and their decline in Type 2 Diabetes.

Hepatic ZIP14-mediated zinc transport is required for adaptation to endoplasmic reticulum stress

Extensive endoplasmic reticulum (ER) stress damages the liver, causing apoptosis and steatosis despite the activation of the unfolded protein response (UPR). Restriction of zinc from cells can induce ER stress, indicating that zinc is essential to maintain normal ER function. However, a role for zinc during hepatic ER stress is largely unknown despite important roles in metabolic disorders, including obesity and nonalcoholic liver disease. We have explored a role for the metal transporter ZIP14 during pharmacologically and high-fat diet-induced ER stress using Zip14^-/- (KO) mice, which exhibit impaired hepatic zinc uptake. Here, we report that ZIP14-mediated hepatic zinc uptake is critical for adaptation to ER stress, preventing sustained apoptosis and steatosis. Impaired hepatic zinc uptake in Zip14 KO mice during ER stress coincides with greater expression of proapoptotic proteins. ER stress-induced Zip14 KO mice show greater levels of hepatic steatosis due to higher expression of genes involved in de novo fatty acid synthesis, which are suppressed in ER stress-induced WT mice. During ER stress, the UPR-activated transcription factors ATF4 and ATF6α transcriptionally up-regulate Zip14 expression. We propose ZIP14 mediates zinc transport into hepatocytes to inhibit protein-tyrosine phosphatase 1B (PTP1B) activity, which acts to suppress apoptosis and steatosis associated with hepatic ER stress. Zip14 KO mice showed greater hepatic PTP1B activity during ER stress. These results show the importance of zinc trafficking and functional ZIP14 transporter activity for adaptation to ER stress associated with chronic metabolic disorders.

Metal Transporter Zip14 (Slc39a14) Deletion in Mice Increases Manganese Deposition and Produces Neurotoxic Signatures and Diminished Motor Activity

Mutations in human ZIP14 have been linked to symptoms of the early onset of Parkinsonism and Dystonia. This phenotype is likely related to excess manganese accumulation in the CNS. The metal transporter ZIP14 (SLC39A14) is viewed primarily as a zinc transporter that is inducible via proinflammatory stimuli. In vitro evidence shows that ZIP14 can also transport manganese. To examine a role for ZIP14 in manganese homeostasis, we used Zip14 knock-out (KO) male and female mice to conduct comparative metabolic, imaging, and functional studies. Manganese accumulation was fourfold to fivefold higher in brains of Zip14 KO mice compared with young adult wild-type mice. There was less accumulation of subcutaneously administered ⁵⁴Mn in the liver, gallbladder, and gastrointestinal tract of the KO mice, suggesting that manganese elimination is impaired with Zip14 ablation. Impaired elimination creates the opportunity for atypical manganese accumulation in tissues, including the brain. The intensity of MR images from brains of the Zip14 KO mice is indicative of major manganese accumulation. In agreement with excessive manganese accumulation was the impaired motor function observed in the Zip14 KO mice. These results also demonstrate that ZIP14 is not essential for manganese uptake by the brain. Nevertheless, the upregulation of signatures of brain injury observed in the Zip14 KO mice demonstrates that normal ZIP14 function is an essential factor required to prevent manganese-linked neurodegeneration.SIGNIFICANCE STATEMENT Manganese is an essential micronutrient. When acquired in excess, manganese accumulates in tissues of the CNS and is associated with neurodegenerative disease, particularly Parkinson-like syndrome and dystonia. Some members of the ZIP metal transporter family transport manganese. Using mutant mice deficient in the ZIP14 metal transporter, we have discovered that ZIP14 is essential for manganese elimination via the gastrointestinal tract, and a lack of ZIP14 results in manganese accumulation in critical tissues such as the brain, as measured by MRI, and produces signatures of brain injury and impaired motor function. Humans with altered ZIP14 function would lack this gatekeeper function of ZIP14 and therefore would be prone to manganese-related neurological diseases.

Physiological roles of zinc transporters: molecular and genetic importance in zinc homeostasis

Zinc (Zn) is an essential trace mineral that regulates the expression and activation of biological molecules such as transcription factors, enzymes, adapters, channels, and growth factors, along with their receptors. Zn deficiency or excessive Zn absorption disrupts Zn homeostasis and affects growth, morphogenesis, and immune response, as well as neurosensory and endocrine functions. Zn levels must be adjusted properly to maintain the cellular processes and biological responses necessary for life. Zn transporters regulate Zn levels by controlling Zn influx and efflux between extracellular and intracellular compartments, thus, modulating the Zn concentration and distribution. Although the physiological functions of the Zn transporters remain to be clarified, there is growing evidence that Zn transporters are related to human diseases, and that Zn transporter-mediated Zn ion acts as a signaling factor, called "Zinc signal". Here we describe critical roles of Zn transporters in the body and their contribution at the molecular, biochemical, and genetic levels, and review recently reported disease-related mutations in the Zn transporter genes.

Inherited Disorders of Manganese Metabolism

While the neurotoxic effects of manganese were recognized in 1837, the first genetic disorder of manganese metabolism was described only in 2012 when homozygous mutations in SLC30A10 were reported to cause manganese-induced neurotoxicity. Two other genetic disorders of manganese metabolism have now been described - mutations in SLC39A14 cause manganese toxicity, while mutations in SLC39A8 cause manganese and zinc deficiency. Study of rare genetic disorders often provides unique insights into disease pathobiology, and the discoveries of these three inherited disorders of manganese metabolism are already transforming our understanding of manganese homeostasis, detoxification, and neurotoxicity. Here, we review the mechanisms by which mutations in SLC30A10, SLC39A14, and SLC39A8 impact manganese homeostasis to cause human disease.

Dietary Zinc Regulates Apoptosis through the Phosphorylated Eukaryotic Initiation Factor 2α/Activating Transcription Factor-4/C/EBP-Homologous Protein Pathway during Pharmacologically Induced Endoplasmic Reticulum Stress in Livers of Mice

BACKGROUND

Several in vitro studies have shown that zinc deficiency could induce endoplasmic reticulum (ER) stress, resulting in activation of the unfolded protein response.

OBJECTIVE

We aimed to determine whether consumption of a zinc-deficient diet (ZnD) triggers ER stress and to understand the impact of dietary zinc intake on ER stress-induced apoptosis using a mouse model.

METHODS

Young adult (8-16 wk of age) male mice of strain C57BL/6 were fed either a ZnD (<1 mg/kg diet), or a zinc-adequate diet (ZnA; 30 mg/kg diet). After 2 wk, liver, pancreas, and serum samples were collected and analyzed for indexes of ER stress. In another experiment, mice were fed either a ZnD, a ZnA, or a zinc-supplementation diet (ZnS; 180 mg/kg diet). After 2 wk, vehicle or tunicamycin (TM; 2 mg/kg body weight) was administered to mice to model ER stress. Liver and serum were analyzed for indexes of ER stress to evaluate the effects of zinc status.

RESULTS

Mice fed a ZnD did not activate the apoptotic and ER stress markers in the liver or pancreas. During the TM challenge, mice fed a ZnD showed greater C/EBP-homologous protein expression in the liver (3.8-fold, P < 0.01) than did ZnA-fed mice. TM-treated mice fed a ZnD also had greater terminal deoxynucleotidyl transferase deoxyuridine triphosphate nick-end labeling-positive cells in the liver (2.2-fold, P < 0.05), greater hepatic triglyceride accumulation (1.5-fold, P < 0.05), greater serum alanine aminotransferase activity (1.6-fold, P < 0.05), and greater protein-tyrosine phosphatase 1B activity (1.5-fold, P < 0.05), respectively, than did those fed a ZnA. No significant differences were observed in these parameters between mice fed ZnAs and ZnSs.

CONCLUSIONS

Consumption of a ZnD per se is not a critical factor for induction of ER stress in mice; however, once ER stress is triggered, adequate dietary zinc intake is required for suppressing apoptotic cell death and further insults in the liver of mice.

Roles of Zinc Signaling in the Immune System

Zinc (Zn) is an essential micronutrient for basic cell activities such as cell growth, differentiation, and survival. Zn deficiency depresses both innate and adaptive immune responses. However, the precise physiological mechanisms of the Zn-mediated regulation of the immune system have been largely unclear. Zn homeostasis is tightly controlled by the coordinated activity of Zn transporters and metallothioneins, which regulate the transport, distribution, and storage of Zn. There is growing evidence that Zn behaves like a signaling molecule, facilitating the transduction of a variety of signaling cascades in response to extracellular stimuli. In this review, we highlight the emerging functional roles of Zn and Zn transporters in immunity, focusing on how crosstalk between Zn and immune-related signaling guides the normal development and function of immune cells.

Hepatic ZIP14-mediated Zinc Transport Contributes to Endosomal Insulin Receptor Trafficking and Glucose Metabolism

Zinc influences signaling pathways through controlled targeted zinc transport. Zinc transporter Zip14 KO mice display a phenotype that includes impaired intestinal barrier function with low grade chronic inflammation, hyperinsulinemia, and increased body fat, which are signatures of diet-induced diabetes (type 2 diabetes) and obesity in humans. Hyperglycemia in type 2 diabetes and obesity is caused by insulin resistance. Insulin resistance results in inhibition of glucose uptake by liver and other peripheral tissues, principally adipose and muscle and with concurrently higher hepatic glucose production. Therefore, modulation of hepatic glucose metabolism is an important target for antidiabetic treatment approaches. We demonstrate that during glucose uptake, cell surface abundance of zinc transporter ZIP14 and mediated zinc transport increases. Zinc is distributed to multiple sites in hepatocytes through sequential translocation of ZIP14 from plasma membrane to early and late endosomes. Endosomes from Zip14 KO mice were zinc-deficient because activities of the zinc-dependent insulin-degrading proteases insulin-degrading enzyme and cathepsin D were impaired; hence insulin receptor activity increased. Transient increases in cytosolic zinc levels are concurrent with glucose uptake and suppression of glycogen synthesis. In contrast, Zip14 KO mice exhibited greater hepatic glycogen synthesis and impaired gluconeogenesis and glycolysis related to low cytosolic zinc levels. We can conclude that ZIP14-mediated zinc transport contributes to regulation of endosomal insulin receptor activity and glucose homeostasis in hepatocytes. Therefore, modulation of ZIP14 transport activity presents a new target for management of diabetes and other glucose-related disorders.

The role of zinc in liver cirrhosis

Zinc is an essential trace element playing fundamental roles in cellular metabolism. It acts mostly by binding a wide range of proteins, thus affecting a broad spectrum of biological processes, which include cell division, growth and differentiation. Zinc is critical to a large number of structural proteins, enzymatic processes, and transcription factors. Zinc deficiency can result in a spectrum of clinical manifestations, such as poor of appetite, loss of body hair, altered taste and smell, testicular atrophy, cerebral and immune dysfunction, and diminished drug elimination capacity. These are common symptoms in patients with chronic liver diseases, especially liver cirrhosis. The liver is the main organ responsible for the zinc metabolism which can be affected by liver diseases. On the other hand, zinc deficiency may alter hepatocyte functions and also immune responses in inflammatory liver diseases. Liver cirrhosis represents the most advanced stage of chronic liver diseases and is the common outcome of chronic liver injury. It is associated with energy malnutrition, with numerous metabolic disorders, such as hypoalbuminemia, with imbalance between branched-chain amino acids and aromatic amino acids, and with reduced zinc serum concentrations. All these processes can influence the clinical outcome of patients, such ascites, hepatic encephalopathy and hepatocellular carcinoma. In the present review, we summarize the emerging evidence on the pitoval role of zinc in the pathogenesis of liver cirrhosis.

Driving Along the Zinc Road

After having written hundreds of research articles, reviews, and book chapters, I find it awkward to pen an autobiography. I still do use a pen. As stated by others in the nutrition field who have written of their own experiences in a perspective article for the Annual Review of Nutrition, my course through this field of science has been serendipitous. My interest in nutrition developed during my experiences with horses and then Angus cattle and entry into an animal science degree program. As the age of molecular biology was unfolding, I pursued a PhD in nutritional biochemistry with Hamilton Eaton at the University of Connecticut followed by postdoctoral work with Hector DeLuca at the University of Wisconsin, working on vitamins A and D, respectively. At Rutgers University, one of the two institutions where I have served on the faculty, I started my research program on trace elements with a focus on cadmium toxicity but soon thereafter began my research on zinc metabolism and function. I moved to the University of Florida in 1982 for an endowed position and have been a Florida Gator ever since. At the University of Florida, research expanded to include identification of zinc-responsive genes and physiological outcomes of zinc transport influencing health and disease, particularly as related to inflammation. I had the opportunity to contribute national science policy as president of both the Federation of American Societies for Experimental Biology and the American Society for Nutrition. As the time of this writing, I maintain an active laboratory.

The Functions of Metallothionein and ZIP and ZnT Transporters: An Overview and Perspective

Around 3000 proteins are thought to bind zinc in vivo, which corresponds to ~10% of the human proteome. Zinc plays a pivotal role as a structural, catalytic, and signaling component that functions in numerous physiological processes. It is more widely used as a structural element in proteins than any other transition metal ion, is a catalytic component of many enzymes, and acts as a cellular signaling mediator. Thus, it is expected that zinc metabolism and homeostasis have sophisticated regulation, and elucidating the underlying molecular basis of this is essential to understanding zinc functions in cellular physiology and pathogenesis. In recent decades, an increasing amount of evidence has uncovered critical roles of a number of proteins in zinc metabolism and homeostasis through influxing, chelating, sequestrating, coordinating, releasing, and effluxing zinc. Metallothioneins (MT) and Zrt- and Irt-like proteins (ZIP) and Zn transporters (ZnT) are the proteins primarily involved in these processes, and their malfunction has been implicated in a number of inherited diseases such as acrodermatitis enteropathica. The present review updates our current understanding of the biological functions of MTs and ZIP and ZnT transporters from several new perspectives.

Zinc transporter Slc39a14 regulates inflammatory signaling associated with hypertrophic adiposity

Zinc is a signaling molecule in numerous metabolic pathways, the coordination of which occurs through activity of zinc transporters. The expression of zinc transporter Zip14 (Slc39a14), a zinc importer of the solute carrier 39 family, is stimulated under proinflammatory conditions. Adipose tissue upregulates Zip14 during lipopolysaccharide-induced endotoxemia. A null mutation of Zip14 (KO) revealed that phenotypic changes in adipose include increased cytokine production, increased plasma leptin, hypertrophied adipocytes, and dampened insulin signaling. Adipose tissue from KO mice had increased levels of preadipocyte markers, lower expression of the differentiation marker (PPARγ), and activation of NF-κB and STAT3 pathways. Our overall hypothesis was that ZIP14 would play a role in adipocyte differentiation and inflammatory obesity. Global Zip14 KO causes systemic endotoxemia. The observed metabolic changes in adipose metabolism were reversed when oral antibiotics were administrated, indicating that circulating levels of endotoxin were in part responsible for the adipose phenotype. To evaluate a mechanism, 3T3-L1 cells were differentiated into adipocytes and treated with siRNA to knock down Zip14. These cells had an impaired ability to mobilize zinc, which caused dysregulation of inflammatory pathways (JAK2/STAT3 and NF-κB). The Zip14 deletion may limit the availability of intracellular zinc, yielding the unique phenotype of inflammation coupled with hypertrophy. Taken together, these results suggest that aberrant zinc distribution observed with Zip14 ablation impacts adipose cytokine production and metabolism, ultimately increasing fat deposition when exposed to endotoxin. To our knowledge, this is the first investigation into the mechanistic role of ZIP14 in adipose tissue regulation and metabolism.

Gene expression of the zinc transporter ZIP14 (SLC39a14) is affected by weight loss and metabolic status and associates with PPARγ in human adipose tissue and 3T3-L1 pre-adipocytes

Background

The expansion and function of adipose tissue are important during the development of insulin resistance and inflammation in obesity. Zinc dyshomeostasis is common in obese individuals. In the liver, zinc influx transporter ZIP14, affects proliferation and glucose metabolism but the role of ZIP14 in adipose tissue is still unknown. This study investigates ZIP14 gene expression in human adipose tissue before and after weight loss as well as the regulation of ZIP14 during early adipogenesis.

Methods

Fourteen obese individuals were investigated before and after a 10 week weight loss intervention and compared to 14 non-obese controls. Gene expressions of ZIP14 and peroxisome proliferator-activated receptor γ (PPARγ) were measured in subcutaneous adipose tissue and correlated with metabolic and inflammatory markers. Further, we investigated gene expression of ZIP14 and PPARγ during early adipogenesis of 3T3-L1 pre-adipocytes, together with an in silico analysis of PPARγ binding motifs in the promoter sequence of ZIP14.

Results

ZIP14 was down-regulated in obese individuals compared to non-obese controls (p = 0.0007) and was up-regulated after weight loss (p = 0.0005). Several metabolic markers of clinical importance, including body mass index, triglyceride, and insulin resistance, were inversely correlated with ZIP14. During early adipogensis an up-regulation of ZIP14 gene expression was found. PPARγ gene expression was positively correlated with the ZIP14 gene expression in both adipose tissue and during adipogenesis. However, in silico analysis revealed that the ZIP14 promoter does not contain PPARγ-binding motifs.

Conclusions

We hypothesize that ZIP14-mediated zinc influx might directly influence PPARγ activity and that ZIP14 may regulate expansion and function of adipose tissue and serve as a potential biomarker for metabolic stress.

Electronic supplementary material

The online version of this article (doi:10.1186/s40608-015-0076-y) contains supplementary material, which is available to authorized users.

Zinc dyshomeostasis during polymicrobial sepsis in mice involves zinc transporter Zip14 and can be overcome by zinc supplementation

Integrity of the immune system is particularly dependent on the availability of zinc. Recent data suggest that zinc is involved in the development of sepsis, a life-threatening systemic inflammation with high death rates, but with limited therapeutic options. Altered cell zinc transport mechanisms could contribute to the inflammatory effects of sepsis. Zip14, a zinc importer induced by proinflammatory stimuli, could influence zinc metabolism during sepsis and serve as a target for therapy. Using cecal ligation-and-puncture (CLP) to model polymicrobial sepsis, we narrowed the function of ZIP14 to regulation of zinc homeostasis in hepatocytes, while hepatic leukocytes were mostly responsible for driving inflammation, as shown by higher expression of IL-1β, TNFα, S100A8, and matrix metalloproteinase-8. Using Zip14 knockout (KO) mice as a novel approach, we found that ablation of Zip14 produced a delay in development of leukocytosis, prevented zinc accumulation in the liver, altered the kinetics of hypozincemia, and drastically increased serum IL-6, TNFα, and IL-10 concentrations following CLP. Hence, this model revealed that the zinc transporter ZIP14 is a component of the pathway for zinc redistribution that contributes to zinc dyshomeostasis during polymicrobial sepsis. In contrast, using the identical CLP model, we found that supplemental dietary zinc reduced the severity of sepsis, as shown by amelioration of cytokines, calprotectins, and blood bacterial loads. We conclude that the zinc transporter ZIP14 influences aspects of the pathophysiology of nonlethal polymicrobial murine sepsis induced by CLP through zinc delivery. The results are promising for the use of zinc and its transporters as targets for future sepsis therapy.

Amiodarone as an autophagy promoter reduces liver injury and enhances liver regeneration and survival in mice after partial hepatectomy

The deregulation of autophagy is involved in liver regeneration. Here, we investigated the role of autophagy in the regulation of liver regeneration after partial hepatectomy (PHx) and the development of pharmacological interventions for improved liver regeneration after PHx. We show that autophagy was activated in the early stages of liver regeneration following 70% PHx in vivo. Moreover, amiodarone was associated with a significant enhancement of autophagy, liver growth, and hepatocyte proliferation, along with reduced liver injury and the termination of liver regeneration due to decreased transforming growth factor-β1 expression after 70% PHx. The promotion of autophagy appeared to selectively increase the removal of damaged mitochondria. We also found that Atg7 knockdown or pretreatment with chloroquine aggravated the liver injury associated with 70% PHx and reduced liver growth and hepatocyte proliferation. Finally, amiodarone improved liver regeneration, survival, and liver injury after 90% PHx. In conclusion, our results indicate that autophagy plays an important role in mouse liver regeneration and that modulating autophagy with amiodarone may be an effective method of improving liver regeneration, increasing survival, and ameliorating liver injury following PHx.

The Physiological, Biochemical, and Molecular Roles of Zinc Transporters in Zinc Homeostasis and Metabolism

Zinc is involved in a variety of biological processes, as a structural, catalytic, and intracellular and intercellular signaling component. Thus zinc homeostasis is tightly controlled at the whole body, tissue, cellular, and subcellular levels by a number of proteins, with zinc transporters being particularly important. In metazoan, two zinc transporter families, Zn transporters (ZnT) and Zrt-, Irt-related proteins (ZIP) function in zinc mobilization of influx, efflux, and compartmentalization/sequestration across biological membranes. During the last two decades, significant progress has been made in understanding the molecular properties, expression, regulation, and cellular and physiological roles of ZnT and ZIP transporters, which underpin the multifarious functions of zinc. Moreover, growing evidence indicates that malfunctioning zinc homeostasis due to zinc transporter dysfunction results in the onset and progression of a variety of diseases. This review summarizes current progress in our understanding of each ZnT and ZIP transporter from the perspective of zinc physiology and pathogenesis, discussing challenging issues in their structure and zinc transport mechanisms.

Influence of ZIP14 (slc39A14) on intestinal zinc processing and barrier function

ZIP14 is a zinc transport protein with high expression in the small intestine and liver. Zip14 is upregulated during endotoxemia and leads to increased liver zinc content and transient hypozinemia. Since body zinc status and inflammation are associated with changes in intestinal permeability, we hypothesized that ZIP14 may influence intestinal permeability. Wild-type (WT) and Zip14 knockout (KO) mice were used to determine ZIP14-associated intestinal zinc metabolism and effects on permeability. Fractionation of plasma membranes revealed that ZIP14 is localized to the basolateral membrane of enterocytes. Studies utilizing (65)Zn administered by subcutaneous injection revealed greater zinc accumulation in the SI of Zip14 KO mice compared with WT mice. Isolation of endosomes confirmed the presence of ZIP14. Quantification of endosomal zinc concentration by FluoZin-3AM fluorescence demonstrated that zinc is trapped in endosomes of Zip14 KO mice. Intestinal permeability assessed both by plasma FITC-dextran following gavage and by serum endotoxin content was greater in Zip14 KO mice. Threonine phosphorylation of the tight junction protein occludin, which is necessary for tight junction assembly, was reduced in KO mice. Claudin 1 and 2, known to have an inverse relationship in regards to tight junction integrity, reflected impaired barrier function in KO jejunum. These data suggest involvement of ZIP14 in providing zinc for a regulatory role needed for maintenance of the intestinal barrier. In conclusion, ZIP14 is a basolaterally localized protein in enterocytes and is involved in endosomal trafficking of zinc and is necessary for proper maintenance of intestinal tight junctions.

Zinc regulates the acute phase response and serum amyloid A production in response to sepsis through JAK-STAT3 signaling

Sepsis rapidly activates the host inflammatory response and acute phase response. Severe sepsis, complicated by multiple organ failure, is associated with overwhelming inflammation and high mortality. We previously observed that zinc (Zn) deficiency significantly increases mortality in a mouse model of polymicrobial sepsis due to over-activation of the inflammatory response. In order to identify potential mechanisms that account for Zn-responsive effects, we generated whole exome expression profiles from the lung tissue of septic mice that were maintained on Zn modified diets. Based on systems analysis, we observed that Zn deficiency enhances the acute phase response and particularly the JAK-STAT3 pathway, resulting in increased serum amyloid A production. In vitro studies of primary hepatocytes and HepG2 cells substantiated that Zn-deficiency augments serum amyloid A production through up-regulation of the JAK-STAT3 and NF-κB pathways. In contrast, Zn inhibited STAT3 activation through the up-regulation of SHP1 activity. Collectively, these findings demonstrate that Zn deficiency enhances the acute phase response through up-regulation of the JAK-STAT3 pathway, thereby perpetuating increased inflammation that may lead to increased morbidity and mortality in response to sepsis.

Expression analysis of zinc transporters in resting and stimulated human peripheral blood mononuclear cells

Intracellular zinc homeostasis is tightly regulated under physiological conditions; however, dysregulation of zinc levels has been reported in various chronic inflammatory and malignant diseases. In this study, we aimed to assess the expression pattern of the 24 currently known zinc transporters in resting and stimulated human peripheral blood mononuclear cells (PBMCs). The cells were isolated from healthy probands and subsequently stimulated with phytohaemagglutinin (PHA) for 3 days. The expression levels of zinc transporters [Zrt/IRT-like protein (ZIP) and cation diffusion facilitator/zinc transporter protein (CDF/ZnT) families] were analyzed by quantitative reverse transcription-polymerase chain reaction. Of the 24 genes encoding for zinc transporters, 19 were found to be ubiquitously expressed in PBMCs. ZIP5 and ZnT10 were not found in all 5 samples, whereas ZIP12, ZnT3 and ZIP2 were expressed in only 1-2 out of 5 PBMC samples. Of note, stimulation by PHA led to an overall downregulation of zinc transporters in the PBMCs of 4 out of the 5 subjects. Notably, the transcript levels of ZIP14 were consistently induced and those of ZIP3 and ZIP4 consistently downregulated in all 5 subjects, whereas the corresponding levels of the remaining 21 genes varied. Data from this study may facilitate a better understanding of the pathophysiological role of deregulated zinc transporters in chronic inflammatory diseases.