MTF-1-mediated repression of the zinc transporter Zip10 is alleviated by zinc restriction

Role of zinc in health and disease

This review provides a concise overview of the cellular and clinical aspects of the role of zinc, an essential micronutrient, in human physiology and discusses zinc-related pathological states. Zinc cannot be stored in significant amounts, so regular dietary intake is essential. ZIP4 and/or ZnT5B transport dietary zinc ions from the duodenum into the enterocyte, ZnT1 transports zinc ions from the enterocyte into the circulation, and ZnT5B (bidirectional zinc transporter) facilitates endogenous zinc secretion into the intestinal lumen. Putative promoters of zinc absorption that increase its bioavailability include amino acids released from protein digestion and citrate, whereas dietary phytates, casein and calcium can reduce zinc bioavailability. In circulation, 70% of zinc is bound to albumin, and the majority in the body is found in skeletal muscle and bone. Zinc excretion is via faeces (predominantly), urine, sweat, menstrual flow and semen. Excessive zinc intake can inhibit the absorption of copper and iron, leading to copper deficiency and anaemia, respectively. Zinc toxicity can adversely affect the lipid profile and immune system, and its treatment depends on the mode of zinc acquisition. Acquired zinc deficiency usually presents later in life alongside risk factors like malabsorption syndromes, but medications like diuretics and angiotensin-receptor blockers can also cause zinc deficiency. Inherited zinc deficiency condition acrodermatitis enteropathica, which occurs due to mutation in the SLC39A4 gene (encoding ZIP4), presents from birth. Treatment involves zinc supplementation via zinc gluconate, zinc sulphate or zinc chloride. Notably, oral zinc supplementation may decrease the absorption of drugs like ciprofloxacin, doxycycline and risedronate.

Upregulation of Intracellular Zinc Ion Level after Differentiation of the Neural Stem/Progenitor Cells In Vitro with the Changes in Gene Expression of Zinc Transporters

We measured the intracellular zinc ion concentration of murine fetal neural stem/progenitor cells (NSPCs) and that in the differentiated cells. The NSPCs cultured with 1.5 μM Zn2+ proliferated slightly faster than that in the zinc-deficient medium and the intracellular zinc concentration of the NSPCs and that of their differentiated cells (DCs) cultured with 1.5 μM Zn2+ was 1.34-fold and 2.00-fold higher than those in the zinc-deficient medium, respectively. The zinc transporter genes upregulated over the 3.5-fold change were Zip1, Zip4, Zip12, Zip13, ZnT1, ZnT8, and ZnT10 whereas the only downregulated one was Zip8 during the differentiation of NSPCs to DCs. The cell morphologies of both NSPCs and DCs in the low oxygen culture condition consisting of 2%O2 and 5%CO2, the high carbon dioxide condition consisting of 21%O2 and 10%CO2, and the normal condition consisting of 21%O2 and 5%CO2 were essentially the same each other. The expression of Zip4, Zip8, Zip12, and Zip14 was not drastically changed depending on the O2 and CO2 concentrations.

Cellular zinc metabolism and zinc signaling: from biological functions to diseases and therapeutic targets

Zinc metabolism at the cellular level is critical for many biological processes in the body. A key observation is the disruption of cellular homeostasis, often coinciding with disease progression. As an essential factor in maintaining cellular equilibrium, cellular zinc has been increasingly spotlighted in the context of disease development. Extensive research suggests zinc's involvement in promoting malignancy and invasion in cancer cells, despite its low tissue concentration. This has led to a growing body of literature investigating zinc's cellular metabolism, particularly the functions of zinc transporters and storage mechanisms during cancer progression. Zinc transportation is under the control of two major transporter families: SLC30 (ZnT) for the excretion of zinc and SLC39 (ZIP) for the zinc intake. Additionally, the storage of this essential element is predominantly mediated by metallothioneins (MTs). This review consolidates knowledge on the critical functions of cellular zinc signaling and underscores potential molecular pathways linking zinc metabolism to disease progression, with a special focus on cancer. We also compile a summary of clinical trials involving zinc ions. Given the main localization of zinc transporters at the cell membrane, the potential for targeted therapies, including small molecules and monoclonal antibodies, offers promising avenues for future exploration.

Macrophage metallothioneins participate in the antileishmanial activity of antimonials

Host cell functions that participate in the pharmacokinetics and pharmacodynamics (PK/PD) of drugs against intracellular pathogen infections are critical for drug efficacy. In this study, we investigated whether macrophage mechanisms of xenobiotic detoxification contribute to the elimination of intracellular Leishmania upon exposure to pentavalent antimonials (SbV). Primary macrophages from patients with cutaneous leishmaniasis (CL) (n=6) were exposed ex vivo to L. V. panamensis infection and SbV, and transcriptomes were generated. Seven metallothionein (MT) genes, potent scavengers of heavy metals and central elements of the mammalian cell machinery for xenobiotic detoxification, were within the top 20 up-regulated genes. To functionally validate the participation of MTs in drug-mediated killing of intracellular Leishmania, tandem knockdown (KD) of MT2-A and MT1-E, MT1-F, and MT1-X was performed using a pan-MT shRNA approach in THP-1 cells. Parasite survival was unaffected in tandem-KD cells, as a consequence of strong transcriptional upregulation of MTs by infection and SbV, overcoming the KD effect. Gene silencing of the metal transcription factor-1 (MTF-1) abrogated expression of MT1 and MT2-A genes, but not ZnT-1. Upon exposure to SbV, intracellular survival of Leishmania in MTF-1KD cells was significantly enhanced. Results from this study highlight the participation of macrophage MTs in Sb-dependent parasite killing.

The role of Zn2+ in shaping intracellular Ca2+ dynamics in the heart

Increasing evidence suggests that Zn2+ acts as a second messenger capable of transducing extracellular stimuli into intracellular signaling events. The importance of Zn2+ as a signaling molecule in cardiovascular functioning is gaining traction. In the heart, Zn2+ plays important roles in excitation-contraction (EC) coupling, excitation-transcription coupling, and cardiac ventricular morphogenesis. Zn2+ homeostasis in cardiac tissue is tightly regulated through the action of a combination of transporters, buffers, and sensors. Zn2+ mishandling is a common feature of various cardiovascular diseases. However, the precise mechanisms controlling the intracellular distribution of Zn2+ and its variations during normal cardiac function and during pathological conditions are not fully understood. In this review, we consider the major pathways by which the concentration of intracellular Zn2+ is regulated in the heart, the role of Zn2+ in EC coupling, and discuss how Zn2+ dyshomeostasis resulting from altered expression levels and efficacy of Zn2+ regulatory proteins are key drivers in the progression of cardiac dysfunction.

Dietary zinc restriction affects the expression of genes related to immunity and stress response in the small intestine of pigs

Zinc (Zn) is an essential mineral and its deficiency manifests in non-specific clinical signs that require long time to develop. The response of swine intestine to Zn restriction was evaluated to identify early changes that can be indicative of Zn deficiency. Twenty-seven pigs (body weight = 77⋅5 ± 2⋅5 kg) were assigned to one of three diets: diet without added Zn (Zn-restricted diet, ZnR), and ZnR-supplemented with either 50 (Zn50) or 100 mg of Zn/kg of diet (Zn100) of Zn supplied by ZnCl₂. After 32 d consuming the diets, serum Zn concentration in ZnR pigs was below the range of 0⋅59-1⋅37 μg/ml considered sufficient, thereby confirming subclinical Zn deficiency. Pigs showed no obvious health or growth changes. RNA-seq analysis followed by qPCR showed decreased expression of metallothionein-1 (MT1) (P < 0⋅05) and increased expression of Zn transporter ZIP4 (P < 0⋅05) in jejunum and ileum of ZnR pigs compared with Zn-supplemented pigs. Ingenuity pathway analysis revealed that Zn50 and Zn100 induced changes in genes related to nucleotide excision repair and integrin signalling pathways. The top gene network in the ZnR group compared with Zn100 was related to lipid and drug metabolism; and compared with Zn50, was related to cellular proliferation, assembly and organisation. Dietary Zn concentrations resulted in differences in genes related to immune pathways. Our analysis showed that small intestine presents changes associated with Zn deficiency after 32 d of Zn restriction, suggesting that the intestine could be a sentinel organ for Zn deficiency.

Transcriptional Regulation and Protein Localization of Zip10, Zip13 and Zip14 Transporters of Freshwater Teleost Yellow Catfish Pelteobagrus fulvidraco Following Zn Exposure in a Heterologous HEK293T Model

Zip family proteins are involved in the control of zinc (Zn) ion homeostasis. The present study cloned the promoters and investigated the transcription responses and protein subcellular localizations of three LIV-1 subfamily members (zip10, zip13, and zip14) from common freshwater teleost yellow catfish, Pelteobagrus fulvidraco, using in vitro cultured HEK293T model cells. The 2278 bp, 1917 bp, and 1989 bp sequences of zip10, zip13, and zip14 promoters, respectively, were subcloned into pGL3-Basic plasmid for promoter activity analysis. The pcDNA3.1 plasmid coding EGFP tagged pfZip10, pfZip13, and pfZip14 were generated for subsequent confocal microscope analysis. Several potential transcription factors’ binding sites were predicted within the promoters. In vitro promoter analysis in the HEK293T cells showed that high Zn administration significantly reduced the transcriptional activities of the zip10, zip13, and zip14 promoters. The −2017 bp/−2004 bp MRE in the zip10 promoter, the −360 bp/−345 bp MRE in the zip13 promoter, and the −1457 bp/−1442 bp MRE in the zip14 promoter were functional loci that were involved in the regulation of the three zips. The −606 bp/−594 bp KLF4 binding site in the zip13 promoter was a functional locus responsible for zinc-responsive regulation of zip13. The −1383 bp/−1375 bp STAT3 binding site in the zip14 promoter was a functional locus responsible for zinc-responsive regulation of zip14. Moreover, confocal microscope analysis indicated that zinc incubation significantly reduced the fluorescence intensity of pfZip10-EGFP and pfZip14-EGFP but had no significant influence on pfZip13-EGFP fluorescence intensity. Further investigation found that pfZip10 localizes on cell membranes, pfZip14 colocalized with both cell membranes and lysosome, and pfZip13 colocalized with intracellular ER and Golgi. Our research illustrated the transcription regulation of zip10, zip13, and zip14 from P. fulvidraco under zinc administration, which provided a reference value for the mechanisms involved in Zip-family-mediated control of zinc homeostasis in vertebrates.

ZIP11 Regulates Nuclear Zinc Homeostasis in HeLa Cells and Is Required for Proliferation and Establishment of the Carcinogenic Phenotype

Zinc (Zn) is an essential trace element that plays a key role in several biological processes, including transcription, signaling, and catalysis. A subcellular network of transporters ensures adequate distribution of Zn to facilitate homeostasis. Among these are a family of importers, the Zrt/Irt-like proteins (ZIP), which consists of 14 members (ZIP1-ZIP14) that mobilize Zn from the extracellular domain and organelles into the cytosol. Expression of these transporters varies among tissues and during developmental stages, and their distribution at various cellular locations is essential for defining the net cellular Zn transport. Normally, the ion is bound to proteins or sequestered in organelles and vesicles. However, though research has focused on Zn internalization in mammalian cells, little is known about Zn mobilization within organelles, including within the nuclei under both normal and pathological conditions. Analyses from stomach and colon tissues isolated from mouse suggested that ZIP11 is the only ZIP transporter localized to the nucleus of mammalian cells, yet no clear cellular role has been attributed to this protein. We hypothesized that ZIP11 is essential to maintaining nuclear Zn homeostasis in mammalian cells. To test this, we utilized HeLa cells, as research in humans correlated elevated expression of ZIP11 with poor prognosis in cervical cancer patients. We stably knocked down ZIP11 in HeLa cancer cells and investigated the effect of Zn dysregulation in vitro. Our data show that ZIP11 knockdown (KD) reduced HeLa cells proliferation due to nuclear accumulation of Zn. RNA-seq analyses revealed that genes related to angiogenesis, apoptosis, mRNA metabolism, and signaling pathways are dysregulated. Although the KD cells undergoing nuclear Zn stress can activate the homeostasis response by MTF1 and MT1, the RNA-seq analyses showed that only ZIP14 (an importer expressed on the plasma membrane and endocytic vesicles) is mildly induced, which may explain the sensitivity to elevated levels of extracellular Zn. Consequently, ZIP11 KD HeLa cells have impaired migration, invasive properties and decreased mitochondrial potential. Furthermore, KD of ZIP11 delayed cell cycle progression and rendered an enhanced senescent state in HeLa cells, pointing to a novel mechanism whereby maintenance of nuclear Zn homeostasis is essential for cancer progression.

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.

miRNA-22 Upregulates Mtf1 in Dorsal Horn Neurons and Is Essential for Inflammatory Pain

Chronic inflammatory pain seriously affects patients' quality of life because of a paucity of effective clinical treatments caused, at least in part, by lack of full understanding of the underlying mechanisms. miRNAs are known to be involved in inflammatory pain via silencing or degrading of target mRNA in the cytoplasm. The present study provides a novel mechanism by which miRNA-22 positively regulates metal-regulatory transcription factor 1 (Mtf1) in the nuclei of neurons in the dorsal horn of the spinal cord. We found that miRNA-22 was significantly increased in the dorsal horn of mice with either inflammatory pain induced by plantar injection of complete Freund's adjuvant (CFA) or neuropathic pain induced by unilateral sciatic nerve chronic constrictive injury (CCI). Knocking down or blocking miRNA-22 alleviated CFA-induced mechanical allodynia and heat hyperalgesia, whereas overexpressing miRNA-22 produced pain-like behaviors. Mechanistically, the increased miRNA-22 binds directly to the Mtf1 promoter to recruit RNA polymerase II and elevate Mtf1 expression. The increased Mtf1 subsequently enhances spinal central sensitization, as evidenced by increased expression of p-ERK1/2, GFAP, and c-Fos in the dorsal horn. Our findings suggest that the miRNA-22-Mtf1 signaling axis in the dorsal horn plays a critical role in the induction and maintenance of inflammatory pain. This signaling pathway may be a promising therapeutic target in inflammatory pain.

Genetics of metallothioneins in Drosophilamelanogaster

Metallothioneins (MTs) are ubiquitous metal-chelating proteins involved in cellular metal homeostasis. MTs were found to be related with almost all the biological processes and their malfunctioning is responsible for a lot of important human diseases. Invertebrate MTs were also used broadly as biomarkers of metal contamination due to their inducible expression by metal exposure. MT system plays a significant role in maintaining human health and ecological stability. Drosophila melanogaster, the vinegar fly, is a perfect model for studying insect MT systems. Six MTs were identified in D. melanogaster, and were designated MtnA to F. All the MTs are considered as Cu-thioneins except for MtnF, which is putatively a Zn-thionein. Expression of all the MTs are regulated by MTF-1/MRE system, thus being able to be induced by heavy metal exposure. The expression pattern and function of separated MTs are partially overlapped and partially distinct. In this work, we made a summary of all the studies on D. melanogaster MTs. From this review, we noted that, compared with studies on mammalian MTs, the understanding of the MT system of D. melanogaster and other invertebrates, especially the regulation mechanism for MT expression and protein-protein interaction with them, is still in a low level.

Chronic Running Exercise Regulates Cytotoxic Cell Functions and Zinc Transporter SLC39A10/ZIP10 Levels in Diabetic Rats

The aim of this study is to investigate how chronic running exercise affects ZIP10 levels in thymus and spleen tissue as well as immune parameters in diabetic rats. A total of 40 adult male Wistar rats were divided into 4 equal groups: group 1, control; group 2, exercise control; group 3, diabetes; group 4, diabetes + exercise. Diabetes was induced by injecting intraperitoneal streptozotocin (STZ) at a dose of 40 mg/kg twice with 24-h intervals to the animals in groups 3 and 4. The animals in group 2 and group 4 underwent exercise for 45 min on the rat treadmill for 4 weeks at 20 m/min. Twenty-four hours after the last running exercise, the animals were sacrificed under general anesthesia. Immunological parameters were determined by flow cytometric method; tissue ZIP 10 levels were determined by ELISA method. The diabetic group had the lowest natural killer (NK) and natural killer T (NKT) cells percentages. Chronic exercise partially improved NK and NKT cell percentages in diabetic rats. The diabetic group had the lowest ZIP10 levels in spleen and thymus tissue. ZIP10 values in spleen and thymus tissue of diabetes exercise group were significantly higher than diabetes group. The results of our study show that the impaired cytotoxic cell functions in diabetes are partially corrected with 4 weeks of chronic exercise, and that the suppressed ZIP 10 levels in diabetic rats are reversed by 4 weeks of chronic exercise.

The Bioinorganic Chemistry of Mammalian Metallothioneins

The functions, purposes, and roles of metallothioneins have been the subject of speculations since the discovery of the protein over 60 years ago. This article guides through the history of investigations and resolves multiple contentions by providing new interpretations of the structure-stability-function relationship. It challenges the dogma that the biologically relevant structure of the mammalian proteins is only the one determined by X-ray diffraction and NMR spectroscopy. The terms metallothionein and thionein are ambiguous and insufficient to understand biological function. The proteins need to be seen in their biological context, which limits and defines the chemistry possible. They exist in multiple forms with different degrees of metalation and types of metal ions. The homoleptic thiolate coordination of mammalian metallothioneins is important for their molecular mechanism. It endows the proteins with redox activity and a specific pH dependence of their metal affinities. The proteins, therefore, also exist in different redox states of the sulfur donor ligands. Their coordination dynamics allows a vast conformational landscape for interactions with other proteins and ligands. Many fundamental signal transduction pathways regulate the expression of the dozen of human metallothionein genes. Recent advances in understanding the control of cellular zinc and copper homeostasis are the foundation for suggesting that mammalian metallothioneins provide a highly dynamic, regulated, and uniquely biological metal buffer to control the availability, fluctuations, and signaling transients of the most competitive Zn(II) and Cu(I) ions in cellular space and time.

Physiological role of CYP17A1-like in cadmium detoxification and its transcriptional regulation in the Pacific oyster, Crassostrea gigas

Cadmium (Cd) is one of the most harmful heavy metals due to its persistence and bioaccumulation through the food chains, posing health risks to human. Oysters can bioaccumulate and tolerate high concentrations of Cd, providing a great model for studying molecular mechanism of Cd detoxification. In a previous study, we identified two CYP genes, CYP17A1-like and CYP2C50, that were potentially involved in Cd detoxification in the Pacific oyster, Crassostrea gigas. In this work, we performed further investigations on their physiological roles in Cd detoxification through RNA interference (RNAi). After injection of double-stranded RNA (dsRNA) into the adductor muscle of oysters followed by Cd exposure for 7 days, we observed that the expressions of CYP17A1-like and CYP2C50 in interference group were significantly suppressed on day 3 compared with control group injected with PBS. Moreover, the mortality rate and Cd content in the CYP17A1-like dsRNA interference group (dsCYP17A1-like) was significantly higher than those of the control on day 3. Furthermore, the activities of antioxidant enzymes, including SOD, CAT, GST, were significantly increased in dsCYP17A1-like group, while were not changed in dsCYP2C50 group. More significant tissue damage was observed in gill and digestive gland of oysters in RNAi group than control group, demonstrating the critical role of CYP17A1-like in Cd detoxification. Dual luciferase reporter assay revealed three core regulatory elements of MTF-1 within promoter region of CYP17A1-like, suggesting the potential transcriptional regulation of CYP17A1-like by MTF-1 in oysters. This work demonstrated a critical role of CYP17A1-like in Cd detoxification in C. gigas and provided a new perspective toward unravelling detoxification mechanisms of bivalves under heavy metal stress.

Strategies for Zinc Uptake in Pseudomonas aeruginosa at the Host-Pathogen Interface

As a structural, catalytic, and signaling component, zinc is necessary for the growth and development of plants, animals, and microorganisms. Zinc is also essential for the growth of pathogenic microorganisms and is involved in their metabolism as well as the regulation of various virulence factors. Additionally, zinc is necessary for infection and colonization of pathogenic microorganisms in the host. Upon infection in healthy organisms, the host sequesters zinc both intracellularly and extracellularly to enhance the immune response and prevent the proliferation and infection of the pathogen. Intracellularly, the host manipulates zinc levels through Zrt/Irt-like protein (ZIP)/ZnT family proteins and various zinc storage proteins. Extracellularly, members of the S100 protein family, such as calgranulin C, sequester zinc to inhibit microbial growth. In the face of these nutritional limitations, bacteria rely on an efficient zinc transport system to maintain zinc supplementation for proliferation and disruption of the host defense system to establish infection. Here, we summarize the strategies for zinc uptake in conditional pathogenic Pseudomonas aeruginosa, including known zinc uptake systems (ZnuABC, HmtA, and ZrmABCD) and the zinc uptake regulator (Zur). In addition, other potential zinc uptake pathways were analyzed. This review systematically summarizes the process of zinc uptake by P. aeruginosa to provide guidance for the development of new drug targets.

Cadmium induced cerebral toxicity via modulating MTF1-MTs regulatory axis

Metal-responsive transcription factor 1 (MTF1) participates in redox homeostasis and heavy metals detoxification via regulating the expression of metal responsive genes. However, the exact role of MTF1 in Cd-induced cerebral toxicity remains unclear. Herein, we explored the mechanism of Cd-elicited cerebral toxicity through modulating MTF1/MTs pathway in chicken cerebrum exposed to different concentrations of Cd (35 mg, 70 mg, and 140 mg/kg CdCl₂) via diet. Notably, cerebral tissues showed varying degrees of microstructural changes under Cd exposure. Cd exposure significantly up-regulated the expression of metal transporters (DMT1, ZIP8, and ZIP10) with concomitant elevated Cd level, as determined by ICP-MS. Cd significantly altered other cerebral biometals concentrations (particularly, Zn, Fe, Se, Cr, Mo, and Pb) and redox balance, resulting in increased cerebral oxidative stress. More importantly, Cd exposure suppressed MTF1 mRNA and nuclear protein levels and its target metal-responsive genes, notably metallothioneins (MT1 and MT2), and Fe and Cu transporter genes (FPN1, ATOX1, and XIAP). Moreover, Cd disrupted the regulation of expression of selenoproteome (particularly, GPxs and SelW), and cerebral Se level. Overall, our data revealed that molecular mechanisms associated with Cd-induced cerebral damage might include over-expression of DMT1, ZIP8 and ZIP10, and suppression of MTF1 and its main target metal-responsive genes as well as several selenoproteins.

Transcriptional responses of four slc30a/znt family members and their roles in Zn homeostatic modulation in yellow catfish Pelteobagrus fulvidraco

The study characterized their regulatory functions of four znt members (znt1, znt2, znt6 and znt8) in Zn homeostasis in vertebrates. We found that the -1281/-1296 bp locus on the znt1 promoter, the -1/-16 bp locus on the znt2 promoter, the -825/-839 bp locus on the znt6 promoter, the -165/-180 bp locus and the -274/-292 bp STAT3 locus on the znt8 promoter were functional MTF-1 binding sites and had metal responsive element (MRE). Zn incubation increased activities of four znt promoters, which was mediated by MRE sites on znt1, znt2, znt6 and znt8 promoters and by STAT3 binding site on znt8 promoter. Moreover, Zn activated the transcription of these znts genes through MTF-1-MRE-dependent pathway. Zn incubation up-regulated the mRNA and total protein expression of ZnT1, ZnT2 and ZnT8 at both 24 h and 48 h. Overall, for the first time, this study offered novel insights for regulatory mechanism of Zn homeostasis in vertebrates.

Levels of Zinc Transporters mRNA Depending on Zinc Status and HIV-1 Tat Induced Inflammation in Muscle (Rhabdomyosarcoma) and Monocyte (THP-1) Cell Lines

Monocytes and muscles demonstrate functionally contrasting behavior under conditions of zinc deficiency with relation to zinc storage system (muscle retain zinc in contrast to monocytes). We aimed to understand the effects of zinc status and HIV-1 Tat mediated inflammation on expression of zinc transporters in these types of cells. Expression of zinc transporters [ZnTs, ZIPs, and metallothionein (MT)] was quantified by qRT-PCR in RD, THP-1 cells separately and in co-cultured THP-1-RD cells. ZnT1 protein expression levels were confirmed by Western blot. Significant increase of MT and ZnT1 mRNA in response to zinc supplementation and decrease during zinc deficiency indicates significance of the genes encoding transporters in maintaining zinc homeostasis in these tissues. In the RD cells ZIP10 exhibited inverse relation to zinc status whereas no correlation was found in the THP-1 cells. Tat-induced inflammation resulted in the significant elevation of MT, IL6, ZIP7, ZIP8, ZIP9 transcripts in the co-cultured RD cells, whereas THP-1 cells demonstrated increased IL-1β levels and reduced levels of ZIP7 and ZIP14. Zinc status and HIV-1Tat induced inflammation appear to influence differential expression of MT, ZnTs, and ZIPs in the muscle and monocyte cells.

Gawky modulates MTF-1-mediated transcription activation and metal discrimination

Metal-induced genes are usually transcribed at relatively low levels under normal conditions and are rapidly activated by heavy metal stress. Many of these genes respond preferentially to specific metal-stressed conditions. However, the mechanism by which the general transcription machinery discriminates metal stress from normal conditions and the regulation of MTF-1-meditated metal discrimination are poorly characterized. Using a focused RNAi screening in Drosophila Schneider 2 (S2) cells, we identified a novel activator, the Drosophila gawky, of metal-responsive genes. Depletion of gawky has almost no effect on the basal transcription of the metallothionein (MT) genes, but impairs the metal-induced transcription by inducing the dissociation of MTF-1 from the MT promoters and the deficient nuclear import of MTF-1 under metal-stressed conditions. This suggests that gawky serves as a 'checkpoint' for metal stress and metal-induced transcription. In fact, regular mRNAs are converted into gawky-controlled transcripts if expressed under the control of a metal-responsive promoter, suggesting that whether transcription undergoes gawky-mediated regulation is encrypted therein. Additionally, lack of gawky eliminates the DNA binding bias of MTF-1 and the transcription preference of metal-specific genes. This suggests a combinatorial control of metal discrimination by gawky, MTF-1, and MTF-1 binding sites.

Transcriptomic insights into the zinc homeostasis of MCF-7 breast cancer cells via next-generation RNA sequencing

A significant gap in the knowledge of zinc homeostasis exists for breast cancer cells. In this study, we investigated the transcriptomic response of the luminal breast cancer cells (MCF-7) to the exposure of extracellular zinc using next-generation RNA sequencing. The dataset was collected for three time points (T0, T30, and T120) in the time course of zinc treatment, which revealed the dramatic increase, up to 869-fold, of the gene expression for metallothioneins (MT1B, MT1F, MT1X, and MT2A) and the zinc exporter ZnT1 (SLC30A1) at T30, continuingly through to T120. The similar dynamic expression pattern was found for the autophagy-related gene (VMP1) and numerous genes for zinc finger proteins (e.g. RNF165, ZNF365, ZBTB2, SNAI1, ZNF442, ZNF547, ZNF563, and ZNF296). These findings point to the all-hands-on-deck strategy adopted by the cancer cells for maintaining zinc homeostasis. The stress responsive genes encoding heat shock proteins (HSPA1A, HSPA1B, HSPA1L, HSPA4L, HSPA6, HSPA8, HSPH1, HSP90AA1, and HSP90AB1) and the MTF-1 biomarker genes (AKR1C2, CLU, ATF3, GDF15, HMOX1, MAP1A, MAFG, SESN2, and UBC) were also differentially up-regulated at T120, suggesting a role of heat shock proteins and the MTF-1 related stress proteins in dealing with zinc exposure. It is for the first time that the gene encoding Polo-like kinase 2 (PLK2) was found to be involved in zinc-related response. The top differentially expressed genes were validated by qRT-PCR and further extended to the basal type breast cancer cells (MDA-MB-231). It was found that the expression level of SLC30A1 in MDA-MB-231 was higher than MCF-7 in response to zinc exposure. Taken together, the findings contribute to our knowledge and understanding of zinc homeostasis in breast cancer cells.

The Function and Regulation of Zinc in the Brain

Nearly sixty years ago Fredrich Timm developed a histochemical technique that revealed a rich reserve of free zinc in distinct regions of the brain. Subsequent electron microscopy studies in Timm- stained brain tissue found that this "labile" pool of cellular zinc was highly concentrated at synaptic boutons, hinting a possible role for the metal in synaptic transmission. Although evidence for activity-dependent synaptic release of zinc would not be reported for another twenty years, these initial findings spurred decades of research into zinc's role in neuronal function and revealed a diverse array of signaling cascades triggered or regulated by the metal. Here, we delve into our current understanding of the many roles zinc plays in the brain, from influencing neurotransmission and sensory processing, to activating both pro-survival and pro-death neuronal signaling pathways. Moreover, we detail the many mechanisms that tightly regulate cellular zinc levels, including metal binding proteins and a large array of zinc transporters.

In silico analysis of putative metal response elements (MREs) in the zinc-responsive genes from Trichomonas vaginalis and the identification of novel palindromic MRE-like motif

Zinc is an essential micronutrient that plays an important role as a co-factor to several proteins, including zinc-responsive transcription factors. Trichomonas vaginalis is able to survive in the presence of high zinc concentrations in the male urogenital tract. Several genes in T. vaginalis have been shown to respond to changes in zinc concentrations, however, the zinc-dependent mechanism remains undetermined. Recently, we identified in T. vaginalis the zinc finger protein, TvZNF1, which is an ortholog of the mammal metal transcription factor (MTF1). We searched for several of the zinc-responsive genes in T. vaginalis to determine whether if they contain metal response elements (MRE), cis-acting DNA elements that specifically bind MTF1. Six highly conserved over-represented sequence motifs (TvMREs), which share similarity with other eukaryotic MREs, were identified in the zinc-responsive genes in T. vaginalis. We also demonstrated that some of the TvMREs assemble as divalent complexes either as two closely spaced TvMREs or as two overlapping TvMREs forming a palindromic-like sequence: TGCC(N3)GGCA. Electrophoretic mobility shift assays were used to detect the zinc-dependent binding of TvZNF1 and nuclear proteins from T. vaginalis to this specific palindromic motif. Our results support a novel mechanism used by T. vaginalis for the transcriptional regulation of associated zinc-responsive genes through a MTF1/MRE-like system.

Why and how to investigate the role of protein phosphorylation in ZIP and ZnT zinc transporter activity and regulation

Zinc is required for the regulation of proliferation, metabolism, and cell signaling. It is an intracellular second messenger, and the cellular level of ionic, mobile zinc is strictly controlled by zinc transporters. In mammals, zinc homeostasis is primarily regulated by ZIP and ZnT zinc transporters. The importance of these transporters is underscored by the list of diseases resulting from changes in transporter expression and activity. However, despite numerous structural studies of the transporters revealing both zinc binding sites and motifs important for transporter function, the exact molecular mechanisms regulating ZIP and ZnT activities are still not clear. For example, protein phosphorylation was found to regulate ZIP7 activity resulting in the release of Zn2+ from intracellular stores leading to phosphorylation of tyrosine kinases and activation of signaling pathways. In addition, sequence analyses predict all 24 human zinc transporters to be phosphorylated suggesting that protein phosphorylation is important for regulation of transporter function. This review describes how zinc transporters are implicated in a number of important human diseases. It summarizes the current knowledge regarding ZIP and ZnT transporter structures and points to how protein phosphorylation seems to be important for the regulation of zinc transporter activity. The review addresses the need to investigate the role of protein phosphorylation in zinc transporter function and regulation, and argues for a pressing need to introduce quantitative phosphoproteomics to specifically target zinc transporters and proteins involved in zinc signaling. Finally, different quantitative phosphoproteomic strategies are suggested.

Sensitivity and reliability of zinc transporter and metallothionein gene expression in peripheral blood mononuclear cells as indicators of zinc status: responses to ex vivo zinc exposure and habitual zinc intake in humans

Zn is an essential nutrient for humans; however, a sensitive biomarker to assess Zn status has not been identified. The objective of this study was to determine the reliability and sensitivity of Zn transporter and metallothionein (MT) genes in peripheral blood mononuclear cells (PBMCs) to Zn exposure ex vivo and to habitual Zn intake in human subjects. In study 1, human PBMCs were cultured for 24 h with 0-50 µm ZnSO4 with or without 5 µm N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine (TPEN), and mRNA expression of SLC30A1-10, SLC39A1-14, MT1 subtypes (A, B, E, F, G, H, L, M and X), MT2A, MT3 and MT4 mRNA was determined. In study 2, fifty-four healthy male and female volunteers (31·9 (sd 13·8) years, BMI 25·7 (sd 2·9) kg/m2) completed a FFQ, blood was collected, PBMCs were isolated and mRNA expression of selected Zn transporters and MT isoforms was determined. Study 1: MT1E, MT1F, MT1G, MT1H, MT1L, MT1M, MT1X, MT2A and SLC30A1 increased with increasing concentrations of Zn and declined with the addition of TPEN. Study 2: Average daily Zn intake was 16·0 (sd 5·3) mg/d (range: 9-31 mg/d), and plasma Zn concentrations were 15·5 (SD 2·8) μmol/l (range 11-23 μmol/l). PBMC MT2A was positively correlated with dietary Zn intake (r 0·306, P = 0·03) and total Zn intake (r 0·382, P < 0·01), whereas plasma Zn was not (P > 0·05 for both). Findings suggest that MT2A mRNA in PBMCs reflects dietary Zn intake in healthy adults and may be a component in determining Zn status.

Transcriptional Regulation of Zinc Transporters in Human Osteogenic Sarcoma (Saos-2) Cells to Zinc Supplementation and Zinc Depletion

Bone is a passive storage organ for zinc, which contains about 30% of the total body zinc. However, during extreme zinc deficiency, only a small fraction of zinc is released in contrast to other tissues where zinc is released like monocytes or conserved, e.g., skeletal muscle. Zinc plays an important role in bone tissue remodeling. Zinc homeostasis is regulated by several zinc transporters (ZnTs) and importers (ZIPs), but their expression dynamics concerning zinc status of bone cells is not well understood. The study aimed to elucidate the effects of zinc supplementation and depletion on the transcript levels of various zinc transporters. Saos-2, a human osteoblastic cell line, was used as representative bone tissue. Zinc sulfate was used for simulating sufficient zinc status whereas TPEN, a zinc chelator, was used to simulate zinc-deficient state. Expression of various transcripts was measured by qRT-PCR. Subcellular localization of ZnT-1 was carried out by immunofluorescent microscopy, and Western Blotting was carried out to measure the expression of ZnT-1 at the protein level. Among the export transporters the transcript levels of MT, ZnT-1 showed higher levels in zinc sufficient and lower levels in TPEN treated cells. Expression of ZnT-4 was decreased under both the conditions. ZIP-6 and ZIP-13 were downregulated in zinc sufficiency, and ZIP-10 upregulated probably to prevent an excess zinc accumulation in bone cells. Further, ZnT-1 was found to be localized in the nuclear region of SaOS-2 cells. ZnT-1, ZnT-4, ZIP-6, ZIP-11, ZIP-10, and ZIP-13 along with MT may be responsible for maintaining bone zinc homeostasis.

The Interaction of Sodium and Zinc in the Priming of T Cell Subpopulations Regarding Th17 and Treg Cells

SCOPE

Nutrition is a critical determinant of a functional immune system. The aim of this study is to investigate the molecular mechanisms by which immune cells are influenced by zinc and sodium.

METHODS AND RESULTS

Mixed lymphocyte cultures and Jurkat cells are generated and incubated with zinc, sodium, or a combination of both for further tests. Zinc induces the number of regulatory T cells (Treg) and decreases T helper 17 cells (Th17), and sodium has the opposite effect. The transforming growth factor beta receptor signaling pathway is also enhanced by zinc and reduced by sodium as indicated by contrary phosphoSmad 2/3 induction. Antagonistic effects can also be seen on zinc transporter and metallothionein-1 (MT-1) mRNA expression: zinc declines Zip10 mRNA expression while sodium induces it, whereas MT-1 mRNA expression is induced by zinc while it is reduced by sodium.

CONCLUSION

This data indicate that zinc and sodium display opposite effects regarding Treg and Th17 induction in MLC, respectively, resulting in a contrary effect on the immune system. Additionally, it reveals a direct interaction of zinc and sodium in the priming of T cell subpopulations and shows that Zip10 and MT-1 play a significant role in those differentiation pathways.

Metalloimmunology: The metal ion-controlled immunity

Metals are essential components in all forms of life required for the function of nearly half of all enzymes and are critically involved in virtually all fundamental biological processes. Especially, the transition metals iron (Fe), zinc (Zn), manganese (Mn), nickel (Ni), copper (Cu) and cobalt (Co) are crucial micronutrients known to play vital roles in metabolism as well due to their unique redox properties. Metals carry out three major functions within metalloproteins: to provide structural support, to serve as enzymatic cofactors, and to mediate electron transportation. Metal ions are also involved in the immune system from metal allergies to nutritional immunity. Within the past decade, much attention has been drawn to the roles of metal ions in the immune system, since increasing evidence has mounted to suggest that metals are critically implicated in regulating both the innate immune sensing of and the host defense against invading pathogens. The importance of ions in immunity is also evidenced by the identification of various immunodeficiencies in patients with mutations in ion channels and transporters. In addition, cancer immunotherapy has recently been conclusively demonstrated to be effective and important for future tumor treatment, although only a small percentage of cancer patients respond to immunotherapy because of inadequate immune activation. Importantly, metal ion-activated immunotherapy is becoming an effective and potential way in tumor therapy for better clinical application. Nevertheless, we are still in a primary stage of discovering the diverse immunological functions of ions and mechanistically understanding the roles of these ions in immune regulation. This review summarizes recent advances in the understanding of metal-controlled immunity. Particular emphasis is put on the mechanisms of innate immune stimulation and T cell activation by the essential metal ions like calcium (Ca²⁺), zinc (Zn²⁺), manganese (Mn²⁺), iron (Fe²⁺/Fe³⁺), and potassium (K⁺), followed by a few unessential metals, in order to draw a general diagram of metalloimmunology.

Comparative Response of Cardiomyocyte ZIPs and ZnTs to Extracellular Zinc and TPEN

Intracellular zinc concentrations are tightly regulated by the coordinated regulation of ZIPs and ZnTs. Very little is known about the regulation of these transporters in cardiomyocytes, in response to extracellular zinc. Adult rat cardiomyocytes express ZnTs 1, 2, 5, and 9, in addition to ZIPs 1, 2, 3, 6, 7, 9, 10, 11, 13, and 14. We have determined the intracellular free zinc levels using Zinpyr-1 fluorescence and studied response of ZIP and ZnT mRNA by real-time PCR to the changes in extracellular zinc and TPEN in adult rat ventricular myocytes. TPEN downregulated ZnT1, ZnT2, and ZIP11 mRNAs but upregulated ZnT5, ZIP2, ZIP7, ZIP10, ZIP13, and ZIP14 mRNAs. Zinc supplementation upregulated ZnT1, ZnT2 mRNA but downregulated ZnT5, ZIP1, ZIP2, ZIP3, ZIP7, ZIP9, and ZIP10 mRNA. The negative regulation of ZIPs by zinc excess can be explained in terms of zinc homeostasis as these transporters may act to protect cells from zinc over accumulation by reducing zinc influx when the extracellular concentration of zinc is high. Similarly, the ZnT expression appears to be regulated to avoid loss of zinc from the intracellular milieu, under zinc-deficient conditions.

A Sensitive, Nonradioactive Assay for Zn(II) Uptake into Metazoan Cells

Sensitive measurements of cellular Zn(II) uptake currently rely on quantitating radioactive emissions from cells treated with 65Zn(II). Here, we describe a straightforward and reliable method employing a stable isotope to sensitively measure Zn(II) uptake by metazoan cells. First, biological medium selectively depleted of natural abundance Zn(II) using A12-resin [Richardson, C. E. R., et al. (2018) J. Am. Chem. Soc. 140, 2413] is restored to physiological levels of Zn(II) by addition of a non-natural Zn(II) isotope distribution comprising 70% 70Zn(II). The resulting 70Zn(II)-enriched medium facilitates quantitation of Zn(II) uptake using inductively coupled plasma-mass spectrometry (ICP-MS). This sensitive and reliable assay assesses Zn(II)-uptake kinetics at early time points and can be used to delineate how chemical and genetic perturbations influence Zn(II) uptake. Further, the use of ICP-MS in a Zn(II)-uptake assay permits simultaneous measurement of multiple metal ion concentrations. We used this capability to show that, across three cell lines, Zn(II) deficiency enhances selectivity for Zn(II) over Cd(II) uptake.

Cloning, function, and localization of human, canine, and Drosophila ZIP10 (SLC39A10), a Zn2+ transporter

Zinc (Zn²⁺) is the second most abundant trace element, but is considered a micronutrient, as it is a cofactor for many enzymes and transcription factors. Whereas Zn²⁺ deficiency can cause cognitive immune or metabolic dysfunction and infertility, excess Zn²⁺ is nephrotoxic. As for other ions and solutes, Zn²⁺ is moved into and out of cells by specific membrane transporters: ZnT, Zip, and NRAMP/DMT proteins. ZIP10 is reported to be localized at the apical membrane of renal proximal tubules in rats, where it is believed to play a role in Zn²⁺ import. Renal regulation of Zn²⁺ is of particular interest in light of growing evidence that Zn²⁺ may play a role in kidney stone formation. The objective of this study was to show that ZIP10 homologs transport Zn²⁺, as well as ZIP10, kidney localization across species. We cloned ZIP10 from dog, human, and Drosophila ( CG10006), tested clones for Zn²⁺ uptake in Xenopus oocytes and localized the protein in renal structures. CG10006, rather than foi (fear-of-intimacy, CG6817) is the primary ZIP10 homolog found in Drosophila Malpighian tubules. The ZIP10 antibody recognizes recombinant dog, human, and Drosophila ZIP10 proteins. Immunohistochemistry reveals that ZIP10 in higher mammals is found not only in the proximal tubule, but also in the collecting duct system. These ZIP10 proteins show Zn²⁺ transport. Together, these studies reveal ZIP10 kidney localization, a role in renal Zn²⁺ transport, and indicates that CG10006 is a Drosophila homolog of ZIP10.

Zinc and Insulin Resistance: Biochemical and Molecular Aspects

Studies have shown the participation of minerals in mechanisms involved in the pathogenesis of insulin resistance. Zinc, in particular, seems to play an important role in the secretion and action of this hormone. Therefore, the aim of this review is to understand the role of zinc in increasing insulin sensitivity. We conducted a search of articles published in the PubMed and ScienceDirect database selected from March 2016 to February 2018, using the keywords "zinc," "insulin," "insulin resistance," "insulin sensitivity," and "supplementation." Following the eligibility criteria were selected 53 articles. The scientific evidences presented in this review show the importance of zinc and their carrier proteins in the synthesis and secretion of insulin, as well as in the signaling pathway of action of this hormone. Zinc deficiency is associated with glucose intolerance and insulin resistance; however, the effectiveness of the intervention with the zinc supplementation is still inconclusive.

Altered zinc balance in the Atp7b-/- mouse reveals a mechanism of copper toxicity in Wilson disease

Wilson disease (WD) is an autosomal recessive disorder caused by mutation in the ATP7B gene that affects copper transport in the body. ATP7B mutation damages copper transporter function, ultimately resulting in excessive copper accumulation and subsequent toxicity in both the liver and brain. Mechanisms of copper toxicity, however, are not well defined. The Atp7b-/- mouse model is well-characterized and presents a hepatic phenotype consistent with WD. In this study, we found that the untreated Atp7b-/- mice accumulate approximately 2-fold excess hepatic zinc compared to the wild type. We used targeted transcriptomics and proteomics to analyze the molecular events associated with zinc and copper accumulation in the Atp7b-/- mouse liver. Altered gene expression of Zip5 and ZnT1 zinc transporters indicated a transcriptional homeostatic response, while increased copper/zinc ratios associated with high levels of metallothioneins 1 and 2, indicated altered Zn availability in cells. These data suggest that copper toxicity in Wilson disease includes effects on zinc-dependent proteins. Transcriptional network analysis of RNA-seq data reveals an interconnected network of transcriptional activators with over-representation of zinc-dependent and zinc-responsive transcription factors. In the context of previous research, these observations support the hypothesis that mechanisms of copper toxicity include disruption of intracellular zinc distribution in liver cells. The translational significance of this work lies in oral zinc supplementation in treatment for WD, which is thought to mediate protective effects through the induction of metallothionein synthesis in the intestine. This work indicates broader impacts of altered zinc-copper balance in WD, including global transcriptional responses and altered zinc balance in the liver.

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.

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.

Recent Advances in the Role of SLC39A/ZIP Zinc Transporters In Vivo

Zinc (Zn), which is an essential trace element, is involved in numerous mammalian physiological events; therefore, either a deficiency or excess of Zn impairs cellular machineries and influences physiological events, such as systemic growth, bone homeostasis, skin formation, immune responses, endocrine function, and neuronal function. Zn transporters are thought to mainly contribute to Zn homeostasis within cells and in the whole body. Recent genetic, cellular, and molecular studies of Zn transporters highlight the dynamic role of Zn as a signaling mediator linking several cellular events and signaling pathways. Dysfunction in Zn transporters causes various diseases. This review aims to provide an update of Zn transporters and Zn signaling studies and discusses the remaining questions and future directions by focusing on recent progress in determining the roles of SLC39A/ZIP family members in vivo.

A pathway for low zinc homeostasis that is conserved in animals and acts in parallel to the pathway for high zinc homeostasis

The essential element zinc plays critical roles in biology. High zinc homeostasis mechanisms are beginning to be defined in animals, but low zinc homeostasis is poorly characterized. We investigated low zinc homeostasis in Caenorhabditis elegans because the genome encodes 14 evolutionarily conserved Zrt, Irt-like protein (ZIP) zinc transporter family members. Three C. elegans zipt genes were regulated in zinc-deficient conditions; these promoters contained an evolutionarily conserved motif that we named the low zinc activation (LZA) element that was both necessary and sufficient for activation of transcription in response to zinc deficiency. These results demonstrated that the LZA element is a critical part of the low zinc homeostasis pathway. Transcriptional regulation of the LZA element required the transcription factor ELT-2 and mediator complex member MDT-15. We investigated conservation in mammals by analyzing LZA element function in human cultured cells; the LZA element-mediated transcriptional activation in response to zinc deficiency in cells, suggesting a conserved pathway of low zinc homeostasis. We propose that the pathway for low zinc homeostasis, which includes the LZA element and ZIP transporters, acts in parallel to the pathway for high zinc homeostasis, which includes the HZA element, HIZR-1 transcription factor and cation diffusion facilitator transporters.

Metal transporter Slc39a10 regulates susceptibility to inflammatory stimuli by controlling macrophage survival

Zn is essential for maintaining the integrity of the immune system, and Zn homeostasis is tightly regulated by two families of ion transporters, SLC39A and SLC30A. Worldwide, an estimated two billion people have Zn deficiency, a condition that can impair immune function and increase susceptibility to a variety of infections. Despite their important roles in health and disease, the molecular mechanisms that underlie Zn transport and Zn homeostasis in macrophages are poorly understood. Here, we report that SLC39A10 plays an essential role in Zn homeostasis in macrophages, regulating the immune response following inflammatory stimuli. Specifically, we identified a role for SLC39A10 in regulating the survival of macrophages via a Zn/p53-dependent axis during the inflammatory response.

Zn plays a key role in controlling macrophage function during an inflammatory event. Cellular Zn homeostasis is regulated by two families of metal transporters, the SLC39A family of importers and the SLC30A family of exporters; however, the precise role of these transporters in maintaining macrophage function is poorly understood. Using macrophage-specific Slc39a10-knockout (Slc39a10^fl/fl;LysM-Cre⁺) mice, we found that Slc39a10 plays an essential role in macrophage survival by mediating Zn homeostasis in response to LPS stimulation. Compared with Slc39a10^fl/fl mice, Slc39a10^fl/fl;LysM-Cre⁺ mice had significantly lower mortality following LPS stimulation as well as reduced liver damage and lower levels of circulating inflammatory cytokines. Moreover, reduced intracellular Zn concentration in Slc39a10^fl/fl;LysM-Cre⁺ macrophages led to the stabilization of p53, which increased apoptosis upon LPS stimulation. Concomitant knockout of p53 largely rescued the phenotype of Slc39a10^fl/fl;LysM-Cre⁺ mice. Finally, the phenotype in Slc39a10^fl/fl;LysM-Cre⁺ mice was mimicked in wild-type mice using the Zn chelator TPEN and was reversed with Zn supplementation. Taken together, these results suggest that Slc39a10 plays a role in promoting the survival of macrophages through a Zn/p53-dependent axis in response to inflammatory stimuli.

Elemental Ingredients in the Macrophage Cocktail: Role of ZIP8 in Host Response to Mycobacterium tuberculosis

Tuberculosis (TB) is a global epidemic caused by the infection of human macrophages with the world’s most deadly single bacterial pathogen, Mycobacterium tuberculosis (M.tb). M.tb resides in a phagosomal niche within macrophages, where trace element concentrations impact the immune response, bacterial metal metabolism, and bacterial survival. The manipulation of micronutrients is a critical mechanism of host defense against infection. In particular, the human zinc transporter Zrt-/Irt-like protein 8 (ZIP8), one of 14 ZIP family members, is important in the flux of divalent cations, including zinc, into the cytoplasm of macrophages. It also has been observed to exist on the membrane of cellular organelles, where it can serve as an efflux pump that transports zinc into the cytosol. ZIP8 is highly inducible in response to M.tb infection of macrophages, and we have observed its localization to the M.tb phagosome. The expression, localization, and function of ZIP8 and other divalent cation transporters within macrophages have important implications for TB prevention and dissemination and warrant further study. In particular, given the importance of zinc as an essential nutrient required for humans and M.tb, it is not yet clear whether ZIP-guided zinc transport serves as a host protective factor or, rather, is targeted by M.tb to enable its phagosomal survival.

Zinc Dyshomeostasis in Cardiomyocytes after Acute Hypoxia/Reoxygenation

Zinc dyshomeostasis may play a role in the pathogenesis of myocardial ischemia/reperfusion injury. The objective of this study was to investigate the expression profile of zinc regulated transporter like- and iron-regulated transporter-like proteins (ZIPs) and zinc transporter proteins (ZnTs) in cardiomyocytes and their modulation in response to hypoxia and reoxygenation. Adult rat ventricular myocytes (ARVMs) were subjected to 6 h of hypoxia, followed by 18 h of reoxygenation. Intracellular and extracellular zinc concentrations were determined using Fluozin-3 and Newport Green fluorescence, respectively. Expression of ZnTs 1, 2, 5, and 9 along with ZIPs 1, 2, 3, 6, 7, 9, 10, 11, 13, and 14 was detectable in the cardiomyocytes by real-time reverse transcriptase polymerase chain reaction. Hypoxia elicited accumulation of intracellular free zinc, but subsequent reoxygenation resulted in striking loss of intracellular free zinc and decreased the cardiomyocyte viability. Concomitantly, extracellular zinc levels dropped rapidly during hypoxia, but increased after reoxygenation. Immunoblotting analysis revealed that hypoxia increased the expression of ZnT1, but reoxygenation significantly increased the expression of ZnTs 2 and 5. Neither hypoxia nor reoxygenation altered the levels of ZnT9. Increased intracellular zinc at the end of hypoxia is related to enhanced expression of ZIPs, whereas decreased intracellular zinc during reoxygenation appears to be due to lowered expression of all ZIPs, in addition to elevated levels of ZnTs 2 and 5. These results thus suggest that there is impaired accumulation of intracellular zinc during reoxygenation, due to overexpression of specific ZnTs and downregulation of ZIP expression.

Drosophila melanogaster Models of Metal-Related Human Diseases and Metal Toxicity

Iron, copper and zinc are transition metals essential for life because they are required in a multitude of biological processes. Organisms have evolved to acquire metals from nutrition and to maintain adequate levels of each metal to avoid damaging effects associated with its deficiency, excess or misplacement. Interestingly, the main components of metal homeostatic pathways are conserved, with many orthologues of the human metal-related genes having been identified and characterized in Drosophila melanogaster. Drosophila has gained appreciation as a useful model for studying human diseases, including those caused by mutations in pathways controlling cellular metal homeostasis. Flies have many advantages in the laboratory, such as a short life cycle, easy handling and inexpensive maintenance. Furthermore, they can be raised in a large number. In addition, flies are greatly appreciated because they offer a considerable number of genetic tools to address some of the unresolved questions concerning disease pathology, which in turn could contribute to our understanding of the metal metabolism and homeostasis. This review recapitulates the metabolism of the principal transition metals, namely iron, zinc and copper, in Drosophila and the utility of this organism as an experimental model to explore the role of metal dyshomeostasis in different human diseases. Finally, a summary of the contribution of Drosophila as a model for testing metal toxicity is provided.

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.

Metallothionein and Zinc Transporter Expression in Circulating Human Blood Cells as Biomarkers of Zinc Status: a Systematic Review

Zinc is an essential nutrient for humans; however, a sensitive biomarker to assess zinc status has not been identified. The objective of this systematic review was to compile and assess studies that determined zinc transporter and/or metallothionein expression in various blood cell types and to determine their reliability and sensitivity to changes in dietary zinc. Sixteen studies were identified that determined the expression of zrt-, irt-like protein (ZIP) 1 [solute carrier family (SLC) 39A1], ZIP3 (SLC39A3), ZIP5 (SLC39A5), ZIP6 (SLC39A6), ZIP7 (SLC39A7), ZIP8 (SLC39A8), ZIP10 (SLC39A10), ZIP14 (SLC39A14), zinc transporter (ZnT)1 (SLC30A1), ZnT2 (SLC30A2), ZnT4 (SLC30A4), ZnT5 (SLC30A5), ZnT6 (SLC30A6), ZnT7 (SLC30A7), ZnT9 (SLC30A9), and/or metallothionein in various blood cells isolated from healthy adult men and women in response to zinc supplementation or depletion. Cell types included leukocytes, peripheral blood mononuclear cells, T lymphocytes, monocytes, and erythrocytes. ZIP1, ZnT1, and metallothionein were the most commonly measured proteins. Changes in ZIP1 and ZnT1 in response to zinc supplementation or depletion were not consistent across studies. Leukocyte metallothionein decreased with zinc depletion (-39% change from baseline, <5 mg Zn/d, n = 2 studies) and increased with zinc supplementation in a dose-dependent manner (35%, 15-22 mg Zn/d, n = 7 studies; 267%, 50 mg Zn/d, n = 2 studies) and at the earliest time points measured; however, no change or delayed response was observed in metallothionein in erythrocytes. A greater percentage of studies demonstrated that metallothionein in leukocyte subtypes was a more reliable (100%, n = 12; 69%, n = 16) and responsive (92%, n = 12; 82%, n = 11) indicator of zinc exposure than was plasma zinc, respectively. In conclusion, current evidence indicates that metallothionein in leukocyte subtypes may be a component in determining zinc status.

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 sensing by metal-responsive transcription factor 1 (MTF1) controls metallothionein and ZnT1 expression to buffer the sensitivity of the transcriptome response to zinc

Only a small number of genes are known direct targets of the zinc-responsive transcription factor MTF1; therefore, the aim of this study was to gain a more complete understanding of the MTF-1 regulated zinc-responsive component of the transcriptome. A targeted siRNA was used to deplete MTF1 expression in the human intestinal cell line Caco-2. We predicted that the response to zinc of direct MTF1 target genes would be abrogated by MTF1 knockdown. Surprisingly, a greater number of genes were regulated by zinc following MFT1 knockdown, and most genes that responded to zinc under both control and MTF1-depleted conditions had an augmented response in the latter condition. Exceptions were the zinc effluxer ZnT1 and a suite of metallothionein genes, suggesting that responses of other genes to zinc are usually buffered by increases in these proteins. We propose that MTF1 heads a hierarchy of zinc sensors, and through controlling the expression of a raft of metallothioneins and other key proteins involved in controlling intracellular zinc levels (e.g. ZnT1) alters zinc buffering capacity and total cellular zinc content. We tested and validated this model by overexpressing metallothionein and observing the predicted curtailment in response of the zinc-repressed SLC30A5 (ZnT5) promoter. The model provides the framework for an integrated understanding of cellular zinc homeostasis. Because MTs can bind metals other than zinc, this framework links with overall cellular metal homeostasis.

Imaging trace element distributions in single organelles and subcellular features

The distributions of chemical elements within cells are of prime importance in a wide range of basic and applied biochemical research. An example is the role of the subcellular Zn distribution in Zn homeostasis in insulin producing pancreatic beta cells and the development of type 2 diabetes mellitus. We combined transmission electron microscopy with micro- and nano-synchrotron X-ray fluorescence to image unequivocally for the first time, to the best of our knowledge, the natural elemental distributions, including those of trace elements, in single organelles and other subcellular features. Detected elements include Cl, K, Ca, Co, Ni, Cu, Zn and Cd (which some cells were supplemented with). Cell samples were prepared by a technique that minimally affects the natural elemental concentrations and distributions, and without using fluorescent indicators. It could likely be applied to all cell types and provide new biochemical insights at the single organelle level not available from organelle population level studies.

Subtype-specific accumulation of intracellular zinc pools is associated with the malignant phenotype in breast cancer

BACKGROUND

Zinc (Zn) hyper-accumulates in breast tumors and malignant cell lines compared to normal mammary epithelium. The mechanisms responsible for Zn accumulation and the consequence of Zn dysregulation are poorly understood.

METHODS

Microarrays were performed to assess differences in the expression of Zn transporters and metallothioneins (MTs) in human breast tumors and breast cancer cell lines. Real-time PCR and immunoblotting were employed to profile Zn transporter expression in representative luminal (T47D), basal (MDA-MB-231), and non-malignant (MCF10A) cell lines. Zn distribution in human tumors was assessed by X-ray fluorescence imaging. Zn distribution and content in cell lines was measured using FluoZin-3 imaging, and quantification and atomic absorption spectroscopy. Functional consequences of ZnT2 over-expression in MDA-MB-231 cells including invasion, proliferation, and cell cycle were measured using Boyden chambers, MTT assays, and flow cytometry, respectively.

RESULTS

Gene expression profiling of human breast tumors and breast cancer cell lines identified subtype-specific dysregulation in the Zn transporting network. X-ray fluorescence imaging of breast tumor tissues revealed Zn hyper-accumulation at the margins of Luminal breast tumors while Zn was more evenly distributed within Basal tumors. While both T47D and MDA-MB-231 cells hyper-accumulated Zn relative to MCF10A cells, T47D cells accumulated 2.5-fold more Zn compared to MDA-MB-231 cells. FluoZin-3 imaging indicated that Zn was sequestered into numerous large vesicles in T47D cells, but was retained in the cytoplasm and found in fewer and larger, amorphous sub-cellular compartments in MDA-MB-231 cells. The differences in Zn localization mirrored the relative abundance of the Zn transporter ZnT2; T47D cells over-expressed ZnT2, whereas MDA-MB-231 cells did not express ZnT2 protein due to proteasomal degradation. To determine the functional relevance of the lack of ZnT2 in MDA-MB-231cells, cells were transfected to express ZnT2. ZnT2 over-expression led to Zn vesicularization, shifts in cell cycle, enhanced apoptosis, and reduced proliferation and invasion.

CONCLUSIONS

This comprehensive analysis of the Zn transporting network in malignant breast tumors and cell lines illustrates that distinct subtype-specific dysregulation of Zn management may underlie phenotypic characteristics of breast cancers such as grade, invasiveness, metastatic potential, and response to therapy.

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.