The SLC39 family of metal ion transporters

Zinc homeostasis and signaling in health and diseases: Zinc signaling

The essential trace element zinc (Zn) is widely required in cellular functions, and abnormal Zn homeostasis causes a variety of health problems that include growth retardation, immunodeficiency, hypogonadism, and neuronal and sensory dysfunctions. Zn homeostasis is regulated through Zn transporters, permeable channels, and metallothioneins. Recent studies highlight Zn's dynamic activity and its role as a signaling mediator. Zn acts as an intracellular signaling molecule, capable of communicating between cells, converting extracellular stimuli to intracellular signals, and controlling intracellular events. We have proposed that intracellular Zn signaling falls into two classes, early and late Zn signaling. This review addresses recent findings regarding Zn signaling and its role in physiological processes and pathogenesis.

Zip14 is a complex broad-scope metal-ion transporter whose functional properties support roles in the cellular uptake of zinc and nontransferrin-bound iron

Recent studies have shown that overexpression of the transmembrane protein Zrt- and Irt-like protein 14 (Zip14) stimulates the cellular uptake of zinc and nontransferrin-bound iron (NTBI). Here, we directly tested the hypothesis that Zip14 transports free zinc, iron, and other metal ions by using the Xenopus laevis oocyte heterologous expression system, and use of this approach also allowed us to characterize the functional properties of Zip14. Expression of mouse Zip14 in RNA-injected oocytes stimulated the uptake of (55)Fe in the presence of l-ascorbate but not nitrilotriacetic acid, indicating that Zip14 is an iron transporter specific for ferrous ion (Fe(2+)) over ferric ion (Fe(3+)). Zip14-mediated (55)Fe(2+) uptake was saturable (K(0.5) ≈ 2 μM), temperature-dependent (apparent activation energy, E(a) = 15 kcal/mol), pH-sensitive, Ca(2+)-dependent, and inhibited by Co(2+), Mn(2+), and Zn(2+). HCO(3)(-) stimulated (55)Fe(2+) transport. These properties are in close agreement with those of NTBI uptake in the perfused rat liver and in isolated hepatocytes reported in the literature. Zip14 also mediated the uptake of (109)Cd(2+), (54)Mn(2+), and (65)Zn(2+) but not (64)Cu (I or II). (65)Zn(2+) uptake also was saturable (K(0.5) ≈ 2 μM) but, notably, the metal-ion inhibition profile and Ca(2+) dependence of Zn(2+) transport differed from those of Fe(2+) transport, and we propose a model to account for these observations. Our data reveal that Zip14 is a complex, broad-scope metal-ion transporter. Whereas zinc appears to be a preferred substrate under normal conditions, we found that Zip14 is capable of mediating cellular uptake of NTBI characteristic of iron-overload conditions.

Metal binding ability of cysteine-rich peptide domain of ZIP13 Zn2+ ions transporter

The coordination modes and thermodynamic stabilities of the complexes of the cysteine-rich N-terminal domain fragment of the ZIP13 zinc transporter (MPGCPCPGCG-NH(2)) with Zn(2+), Cd(2+), Bi(3+), and Ni(2+) have been studied by potentiometric, mass spectrometric, NMR, CD, and UV-vis spectroscopic methods. All of the studied metals had similar binding modes, with the three thiol sulfurs of cysteine residues involved in metal ion coordination. The stability of the complexes formed in solution changes in the series Bi(3+) ≫ Cd(2+) > Zn(2+) > Ni(2+), the strongest being for bismuth and the weakest for nickel. The N-terminal fragment of the human metalothionein-3 (MDPETCPCP-NH(2)) and unique histidine- and cysteine-rich domain of the C-terminus of Helicobacter pyroli HspA protein (Ac-ACCHDHKKH-NH(2)) have been chosen for the comparison studies. It confirmed indirectly which groups were the anchoring ones of ZIP13 domain. Experimental data from all of the used techniques and comparisons allowed us to propose possible coordination modes for all of the studied ZIP13 complexes.

Molecular and genetic features of zinc transporters in physiology and pathogenesis

Zinc (Zn) is a vital element. It plays indispensable roles in multifarious cellular processes, affecting the expression and activity of a variety of molecules, including transcription factors, enzymes, adapters, channels, growth factors, and their receptors. A disturbance in Zn homeostasis due to Zn deficiency or an excess of Zn absorption can therefore impair the cellular machinery and exert various influences on physiological programs, such as systemic growth, morphogenetic processes, and immune responses, as well as neuro-sensory and endocrine functions. Thus, Zn imbalance becomes pathogenic in humans. Zn homeostasis is controlled by the coordinated actions of Zn transporters, which are responsible for Zn influx and efflux, and intricately regulate the intracellular and extracellular Zn concentration and distribution. In this review, we describe crucial roles of Zn transporters in biological phenomena, focusing in particular on how Zn transporters contribute to cellular events at the molecular, biochemical, and genetic level, with recent progress uncovering the roles of Zn transporters in physiology and pathogenesis.

Functional specialization in nucleotide sugar transporters occurred through differentiation of the gene cluster EamA (DUF6) before the radiation of Viridiplantae

Background

The drug/metabolite transporter superfamily comprises a diversity of protein domain families with multiple functions including transport of nucleotide sugars. Drug/metabolite transporter domains are contained in both solute carrier families 30, 35 and 39 proteins as well as in acyl-malonyl condensing enzyme proteins. In this paper, we present an evolutionary analysis of nucleotide sugar transporters in relation to the entire superfamily of drug/metabolite transporters that considers crucial intra-protein duplication events that have shaped the transporters. We use a method that combines the strengths of hidden Markov models and maximum likelihood to find relationships between drug/metabolite transporter families, and branches within families.

Results

We present evidence that the triose-phosphate transporters, domain unknown function 914, uracil-diphosphate glucose-N-acetylglucosamine, and nucleotide sugar transporter families have evolved from a domain duplication event before the radiation of Viridiplantae in the EamA family (previously called domain unknown function 6). We identify previously unknown branches in the solute carrier 30, 35 and 39 protein families that emerged simultaneously as key physiological developments after the radiation of Viridiplantae, including the "35C/E" branch of EamA, which formed in the lineage of T. adhaerens (Animalia). We identify a second cluster of DMTs, called the domain unknown function 1632 cluster, which has non-cytosolic N- and C-termini, and thus appears to have been formed from a different domain duplication event. We identify a previously uncharacterized motif, G-X(6)-G, which is overrepresented in the fifth transmembrane helix of C-terminal domains. We present evidence that the family called fatty acid elongases are homologous to transporters, not enzymes as had previously been thought.

Conclusions

The nucleotide sugar transporters families were formed through differentiation of the gene cluster EamA (domain unknown function 6) before Viridiplantae, showing for the first time the significance of EamA.

Evidence for operation of the direct zinc ligand exchange mechanism for trafficking, transport, and reactivity of zinc in mammalian cells

In addition to its critical role in normal cell function, growth, and metabolism, zinc is implicated as a major factor in the development and progression of many pathological conditions and diseases. Despite this importance of zinc, many important factors, processes, and mechanisms of the physiology, biochemistry, and molecular biology of zinc remain unknown. Especially important is the unresolved issue regarding the mechanism and process of the trafficking, transport, and reactivity of zinc in cells; especially in mammalian cells. This presentation focuses on the concept that, due to the existence of a negligible pool of free Zn(2+) ions in the mammalian cell environment, the trafficking, transport and reactivity of zinc occurs via a direct exchange of zinc from donor Zn-ligands to acceptor ligands. This Zn exchange process occurs without the requirement for production of free Zn(2+) ions. The direct evidence from mammalian cell studies is presented in support of the operation of the direct Zn-ligand exchange mechanism. The paper also provides important information and conditions that should be considered and employed in the conduct of studies regarding the role and effects of zinc in biological/biomedical research; and in its clinical interpretation and application.

Transition metal homeostasis: from yeast to human disease

Transition metal ions are essential nutrients to all forms of life. Iron, copper, zinc, manganese, cobalt and nickel all have unique chemical and physical properties that make them attractive molecules for use in biological systems. Many of these same properties that allow these metals to provide essential biochemical activities and structural motifs to a multitude of proteins including enzymes and other cellular constituents also lead to a potential for cytotoxicity. Organisms have been required to evolve a number of systems for the efficient uptake, intracellular transport, protein loading and storage of metal ions to ensure that the needs of the cells can be met while minimizing the associated toxic effects. Disruptions in the cellular systems for handling transition metals are observed as a number of diseases ranging from hemochromatosis and anemias to neurodegenerative disorders including Alzheimer's and Parkinson's disease. The yeast Saccharomyces cerevisiae has proved useful as a model organism for the investigation of these processes and many of the genes and biological systems that function in yeast metal homeostasis are conserved throughout eukaryotes to humans. This review focuses on the biological roles of iron, copper, zinc, manganese, nickel and cobalt, the homeostatic mechanisms that function in S. cerevisiae and the human diseases in which these metals have been implicated.

Regulation of T cell receptor signaling by activation-induced zinc influx

Zinc is a trace element that is essential for innate and adaptive immune responses. In addition to being a structural element of many proteins, zinc also functions as a neurotransmitter and an intracellular messenger. Temporal or spatial changes in bioavailable zinc may influence the activity of several enzymes, including kinases and phosphatases. We provide evidence that zinc functions as an ionic signaling molecule after T cell activation. Cytoplasmic zinc concentrations increased within 1 min after T cell receptor (TCR) triggering, in particular in the subsynaptic compartment. The increase depended on the extracellular zinc concentrations and was inhibited by silencing zinc transporter Zip6. Increased zinc influx reduced the recruitment of SHP-1 to the TCR activation complex, augmented ZAP70 phosphorylation and sustained calcium influx. By calibrating TCR activation thresholds, increased extracellular zinc bioavailability facilitated the induction of T cell proliferative responses to suboptimal stimuli.

Molecular analysis of zinc transporters in Schistosoma japonicum

Members of the zinc-regulated transporter/iron-regulated transporter-like protein (ZIP) family of proteins transport metal ions across cell membranes. Genes encoding ZIPs are present in the genomes of schistosomes. Here, we describe molecular characterisation of six ZIPs (SjZIPA-F) from Schistosoma japonicum. Quantitative PCR analyses of these ZIPs through the lifecycle showed that each is expressed predominantly during the intramammalian stage and are particularly enriched in adult females. Using laser microdissected tissue as template, SjZIPA-D were transcriptionally enriched in female reproductive tissues, SjZIPE was not expressed in specific tissues and SjZIPF was expressed similarly in each tissue. Whole mount in situ hybridization revealed that SjZIPA and SjZIPB were localised to the oesophageal gland of adults and the vitellaria. We have shown that multiple ZIPs are expressed by schistosomes during the intramammalian parasitic phases and propose that the encoded products perform diverse cellular functions related to metal transport in different cells of S. japonicum.

LPS-induced decrease in intracellular labile zinc, [Zn]i, contributes to apoptosis in cultured sheep pulmonary artery endothelial cells

A role in signal transduction for a vanishingly small labile pool of intracellular zinc ([Zn](i)) has been inferred by the sensitivity of various physiological pathways to zinc chelators such as N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine (TPEN) and/or associations with changes in nonprotein-bound zinc-sensitive fluorophores. Although we (44) reported that LPS-induced apoptosis in cultured sheep pulmonary artery endothelial cells (SPAEC) was exacerbated by TPEN, 1) we did not detect acute (30 min) changes in [Zn](i), and 2) it is unclear from other reports whether LPS increases or decreases [Zn](i) and whether elevations or decreases in [Zn](i) are associated with cell death and/or apoptosis. In the present study, we used both chemical (FluoZin-3 via live cell epifluorescence microscopy and fluorescence-activated cell sorting) and genetic (luciferase activity of a chimeric reporter encoding zinc-sensitive metal-response element and changes in steady-state mRNA of zinc importer, SLC39A14 or ZIP14) techniques to show that LPS caused a delayed time-dependent (2-4 h) decrease in [Zn](i) in SPAEC. A contributory role of decreases in [Zn](i) in LPS-induced apoptosis (as determined by caspase-3/7 activation, annexin-V binding, and cytochrome c release) in SPAECs was revealed by mimicking the effect of LPS with the zinc chelator, TPEN, and inhibiting LPS- (or TPEN)-induced apoptosis with exogenous zinc. Collectively, these are the first data demonstrating a signaling role for decrease in [Zn](i) in pulmonary endothelial cells and suggest that endogenous levels of labile zinc may affect sensitivity of pulmonary endothelium to the important and complex proapoptotic stimulus of LPS.

[Role of zinc transporter in allergic reactions]

Zinc (Zn) is an essential nutrient and its deficiency causes growth retardation, immunodeficiency, and neuronal degeneration. Zn homeostasis is tightly controlled by transporting through Zn transporters and by buffering via metallothioneins, all of which are involved in the intricate regulation of Zn concentration and distribution in individual cells. Research in understanding of these molecules has progressed with application of genetic techniques, which allow us to clarify the diverse role of Zn in vivo and in vitro. However, the precise roles and molecular mechanism(s) of Zn's function in allergic response have not been clarified. Mast cells are granulated cells that play a pivotal role in allergic reactions. The granules of mast cells contain various chemical mediators and inflammatory cytokines that are released upon FcεRI crosslinking. In this article, I will describe a role of Zn/Zn transporter in FcεRI-mediated mast cell degranulation and cytokine production. Furthermore, Zn acts as an intracellular signaling molecule, that is, a molecule whose intracellular status is altered in response to an extracellular stimulus in mast cell, and that is capable of transducing the extracellular stimulus into an intracellular signaling event, like Ca(2+). I have proposed that there are two classes of Zn signaling: "Early" and "Late" Zn signaling. In this review, I discussed how Zn and its homeostasis affect biological events especially for mast cell-mediated allergy response.

Zinc in innate and adaptive tumor immunity

Zinc is important. It is the second most abundant trace metal with 2-4 grams in humans. It is an essential trace element, critical for cell growth, development and differentiation, DNA synthesis, RNA transcription, cell division, and cell activation. Zinc deficiency has adverse consequences during embryogenesis and early childhood development, particularly on immune functioning. It is essential in members of all enzyme classes, including over 300 signaling molecules and transcription factors. Free zinc in immune and tumor cells is regulated by 14 distinct zinc importers (ZIP) and transporters (ZNT1-8). Zinc depletion induces cell death via apoptosis (or necrosis if apoptotic pathways are blocked) while sufficient zinc levels allows maintenance of autophagy. Cancer cells have upregulated zinc importers, and frequently increased zinc levels, which allow them to survive. Based on this novel synthesis, approaches which locally regulate zinc levels to promote survival of immune cells and/or induce tumor apoptosis are in order.

Intracellular free zinc during cardiac excitation-contraction cycle: calcium and redox dependencies

AIMS

Zinc exists in biological systems as bound and histochemically reactive free Zn(2+). It is an essential structural constituent of many proteins, including enzymes from cellular signalling pathways, in which it functions as a signalling molecule. In cardiomyocytes at rest, Zn(2+) concentration is in the nanomolar range. Very little is known about precise mechanisms controlling the intracellular distribution of Zn(2+) and its variations during cardiac function.

METHODS AND RESULTS

Live-cell detection of intracellular Zn(2+) has become feasible through the recent development of Zn(2+)-sensitive and -selective fluorophores able to distinguish Zn(2+) from Ca(2+). Here, in freshly isolated rat cardiomyocytes, we investigated the rapid changes in Zn(2+) homeostasis using the Zn(2+)-specific fluorescent dye, FluoZin-3, in comparison to Ca(2+)-dependent fluo-3 fluorescence. Zn(2+) sparks and Zn(2+) transients, in quiescent and electrically stimulated cardiomyocytes, respectively, were visualized in a similar manner to known rapid Ca(2+) changes. Both Zn(2+) sparks and Zn(2+) transients required Ca(2+) entry. Inhibiting the sarcoplasmic reticulum Ca(2+) release or increasing the Ca(2+) load in a low-Na(+) solution suppressed or increased Zn(2+) movements, respectively. Mitochondrial inhibitors slightly reduced both Zn(2+) sparks and Zn(2+) transients. Oxidation by H₂O₂ facilitated and acidic pH inhibited the Ca(2+)-dependent Zn(2+) release.

CONCLUSION

It is proposed that Zn(2+) release during the cardiac cycle results mostly from intracellular free Ca(2+) increase, triggering production of reactive oxygen species that induce changes in metal-binding properties of metallothioneins and other redox-active proteins, aside from ionic exchange on these proteins.

Selective electrodiffusion of zinc ions in a Zrt-, Irt-like protein, ZIPB

All living cells need zinc ions to support cell growth. Zrt-, Irt-like proteins (ZIPs) represent a major route for entry of zinc ions into cells, but how ZIPs promote zinc uptake has been unclear. Here we report the molecular characterization of ZIPB from Bordetella bronchiseptica, the first ZIP homolog to be purified and functionally reconstituted into proteoliposomes. Zinc flux through ZIPB was found to be nonsaturable and electrogenic, yielding membrane potentials as predicted by the Nernst equation. Conversely, membrane potentials drove zinc fluxes with a linear voltage-flux relationship. Direct measurements of metal uptake by inductively coupled plasma mass spectroscopy demonstrated that ZIPB is selective for two group 12 transition metal ions, Zn(2+) and Cd(2+), whereas rejecting transition metal ions in groups 7 through 11. Our results provide the molecular basis for cellular zinc acquisition by a zinc-selective channel that exploits in vivo zinc concentration gradients to move zinc ions into the cytoplasm.

Over-expression of Zip-13 mRNA in kidney and lung during dietary zinc deficiency in Wistar rats

Zinc is an essential nutrient for all organisms, which is involved in the function of numerous key enzymes in metabolism. Two gene families have been identified involved in zinc homeostasis. ZnT transporters reduce intracellular zinc while Zip transporters increase intracellular zinc. Previous studies in our laboratory have shown that Zip-1, ZnT-1, Zip-2 and LIV-1 mRNA are associated with zinc level in established human breast cancer in nude mice model. In this study, six zinc transporters: ZnT-1, ZnT-2, ZnT-4, Zip-1, Zip-8 and Zip-13 were chosen. We aim to determine the relation between zinc transporters and zinc level in kidney and lung of Wistar rats. Eighteen Wistar rats were randomly divided into three groups: normal group, zinc-deficiency group and pair-fed group. After 22 days, the rats were killed and organs samples were taken, then zinc transporters mRNA were detected by RT-PCR. Compared with the normal group, Zip-13 shows an up-regulation (P < 0.05) in zinc-deficiency group both in kidney and lung, and Zip-8 was significantly lower (P < 0.05) in zinc-deficiency group in kidney.

Effects of zinc exposure on zinc transporter expression in human intestinal cells of varying maturity

OBJECTIVES

Zinc (Zn) homeostasis in adults is achieved principally through a balance between intestinal absorption and excretion involving adaptive mechanisms programmed by levels of dietary Zn. Zn absorption in infants is not as tightly regulated as that in adults, which may induce potential toxicity in infants due to the relatively high capacity of Zn absorption. We hypothesized that intestinal Zn homeostasis is developmentally regulated and depends on intestinal maturation, which in turn affects Zn transporter regulation.

MATERIALS AND METHODS

Cultured human fetal (FHs 74 Int, F) and adult (Caco-2: undifferentiated, U; differentiated, D) intestinal cells were used to determine developmental differences in Zn uptake and effects of Zn exposure on Zn transporters.

RESULTS

Zn uptake rates in F and U cells were higher compared with D cells (F, 9-fold; U, 3-fold). F cells were more intolerant to Zn exposure than were U or D cells (LD50 = 67.9 +/- 5.3; 117.0 +/- 5.2; 224.4 +/- 3.7 micromol/L, respectively). Two mechanisms were involved in developmental regulation of Zn homeostasis: differential Zn transporter expression and differential response to Zn exposure. In F cells, zinc-regulated transporter (ZRT)/iron-regulated transporter (IRT)-like protein (Zip)4 expression was undetectable; Zn (50 micromol/L) increased levels of Zn transporter (ZnT)1, ZnT2, and metallothionein-1 mRNA and ZnT1 protein. U and D cells had higher mRNA expression of ZnT1 (U: 5-fold; D: 7-fold, respectively) and ZnT2 (U: 2-fold; D: 9-fold, respectively) than F cells, and D cells also had higher Zip4 expression (3-fold) than U cells. In U cells, Zn exposure increased Zip4 protein level, but not membrane-associated abundance. However, in D cells, Zn exposure decreased both the Zip4 protein level and membrane-associated abundance.

CONCLUSIONS

Zn absorption is developmentally regulated through intestinal Zn efflux and sequestration and import mechanisms, which may be responsible for differences in Zn absorption observed between infants and adults.

Cytosolic zinc buffering and muffling: their role in intracellular zinc homeostasis

Our knowledge of the molecular mechanisms of intracellular homeostatic control of zinc ions is now firmly grounded on experimental findings gleaned from the study of zinc proteomes and metallomes, zinc transporters, and insights from the use of computational approaches. A cell's repertoire of zinc homeostatic molecules includes cytosolic zinc-binding proteins, transporters localized to cytoplasmic and organellar membranes, and sensors of cytoplasmic free zinc ions. Under steady state conditions, a primary function of cytosolic zinc-binding proteins is to buffer the relatively large zinc content found in most cells to a cytosolic zinc(ii) ion concentration in the picomolar range. Under non-steady state conditions, zinc-binding proteins and transporters act in concert to modulate transient changes in cytosolic zinc ion concentration in a process that is called zinc muffling. For example, if a cell is challenged by an influx of zinc ions, muffling reactions will dampen the resulting rise in cytosolic zinc ion concentration and eventually restore the cytosolic zinc ion concentration to its original value by shuttling zinc ions into subcellular stores or by removing zinc ions from the cell. In addition, muffling reactions provide a potential means to control changes in cytosolic zinc ion concentrations for purposes of cell signalling in what would otherwise be considered a buffered environment not conducive for signalling. Such intracellular zinc ion signals are known to derive from redox modifications of zinc-thiolate coordination environments, release from subcellular zinc stores, and zinc ion influx via channels. Recently, it has been discovered that metallothionein binds its seven zinc ions with different affinities. This property makes metallothionein particularly well positioned to participate in zinc buffering and muffling reactions. In addition, it is well established that metallothionein is a source of zinc ions under conditions of redox signalling. We suggest that the biological functions of transient changes in cytosolic zinc ion concentrations (presumptive zinc signals) complement those of calcium ions in both spatial and temporal dimensions.

Anticancer activity and mode of action of titanocene C

Titanocenes constitute a class of metal-based anticancer agents that seem to display a mode of action distinct from that of platinum complexes and to be more tolerable with a differing spectrum of activity. In the present study, titanocene C (bis-(N,N-dimethylamino-2(N-methylpyrrolyl)-methyl-cyclopentadienyl) titanium(IV) dichloride) was shown to exhibit antiproliferative activity against human tumor cell lines with a mean IC₅₀ value of 48.3 ± 32.5 µM. In particular, high activity was found against small cell lung cancer (SCLC) cell lines with a profile different from cisplatin. Titanocene C induced cell cycle arrest at the G1/0-S interphase. Cross-resistance to either cisplatin or oxoplatin, respectively, was low for titanocene C and absent for titanocene Y in variant HL-60 cell lines. Alterations in gene expression of NCI-H526 SCLC cells induced by titanocene C were investigated using genome-wide expression arrays. Downregulation was found for genes coding for topoisomerases I and IIα, histones of the HIST1H4 cluster, enzymes involved in glycolysis, components of the cytoskeleton and vesicular transport, among others. In contrast, expression of genes involved in apoptosis, stress response, particularly members of the metallothionein gene cluster 1, DNA damage and growth factors was upregulated following exposure to titanocene C. Approximately 50% of those genes downregulated by titanocene C and cisplatin were concordant, including the previously identified markers of cisplatin-sensitivity, tubulin and stathmin, indicating partial overlap of the pathways affected by these metal complexes. The present findings point helicases/topoisomerases and HIST1H4 core histones out as targets of titanocene C and metallothioneins as putative main effectors of drug resistance.

GintABC1 encodes a putative ABC transporter of the MRP subfamily induced by Cu, Cd, and oxidative stress in Glomus intraradices

A full-length cDNA sequence putatively encoding an ATP-binding cassette (ABC) transporter (GintABC1) was isolated from the extraradical mycelia of the arbuscular mycorrhizal fungus Glomus intraradices. Bioinformatic analysis of the sequence indicated that GintABC1 encodes a 1513 amino acid polypeptide, containing two six-transmembrane clusters (TMD) intercalated with sequences characteristics of the nucleotide binding domains (NBD) and an extra N-terminus extension (TMD0). GintABC1 presents a predicted TMD0-(TMD-NBD)(2) topology, typical of the multidrug resistance-associated protein subfamily of ABC transporters. Gene expression analyses revealed no difference in the expression levels of GintABC1 in the extra- vs the intraradical mycelia. GintABC1 was up-regulated by Cd and Cu, but not by Zn, suggesting that this transporter might be involved in Cu and Cd detoxification. Paraquat, an oxidative agent, also induced the transcription of GintABC1. These data suggest that redox changes may be involved in the transcriptional regulation of GintABC1 by Cd and Cu.

High-dose dietary zinc promotes prostate intraepithelial neoplasia in a murine tumor induction model

To evaluate the role of high-dose dietary zinc in the process of prostate malignancy, 60 Sprague-Dawley rats were randomly divided into four groups: tumor induction with carcinogen and hormone (group 1), oral zinc administration without tumor induction (group 2), oral zinc administration with tumor induction (group 3) and a control without zinc administration or tumor induction (group 4). Zinc was supplied orally in the form of zinc sulfate heptahydrate dissolved in drinking water to groups 2 and 3 for 20 weeks. Although the serum level of zinc measured at 20 weeks was maintained similarly in each group (P = 0.082), intraprostatic zinc concentrations were statistically different. Group 1 prostates contained the least amount of zinc in both the dorsolateral and ventral lobes at levels of 36.3 and 4.8 microg g(-1), respectively. However, in group 3, zinc levels increased in both lobes to 59.3 and 12.1 microg g(-1), respectively, comparable with that of group 4 (54.5 +/- 14.6 and 14.1 +/- 2.4 microg g(-1)). In spite of these increases in zinc concentration, the prevalence of prostate intraepithelial neoplasm was rather increased in group 3 (53.3% and 46.7%) compared with group 1 (33.3% and 33.3%) in both dorsolateral and ventral prostate lobes. Although prostate intraepithelial neoplasm did not develop in any prostate in group 4, zinc administration did induce prostate intraepithelial neoplasm in group 2 (46.7% and 40.0%). Thus, although high dietary zinc increased intraprostatic zinc concentrations, it promoted, instead of preventing, prostate intraepithelial neoplasm in a murine prostate malignancy induction model.

Localization of zip1 and zip4 mRNA in the adult rat brain

The localization of two members of the Slc39a (zip1 and zip4) family of zinc transporters was examined in the brains of adult mice. Zip1 was highly enriched in brain regions with high densities of neuronal cell bodies, including the hippocampus, thalamus, and perifontal cortex. Zip1 was also expressed in commissural fiber tracts such as the corpus callosum and anterior commissure, but little was found in the internal and external capsules. Also, very low amounts of zip1 mRNA were detected in resting astrocytes and reactive astrocytes that were examined at 14 days after inflicting a stab wound. Zip1 mRNA was detected in ependymal cells lining the third and lateral ventricles and epithelium cells in the choroid plexus. Interestingly, zip4 mRNA was detected in the choroid plexus but not in the ependymal cells or other neural elements. Zip4 mRNA was also detected in brain capillaries, but zip1 mRNA was not. In zip4 knockout heterozygotes that express green fluorescent protein regulated by the zip4 promoter, green fluorescent protein was detected in brain capillaries. Because zip4 levels are regulated by dietary Zn, our studies suggest that the brain has the potential of adapting to changes in Zn status.

Zinc: the brain's dark horse

Zinc is a life-sustaining trace element, serving structural, catalytic, and regulatory roles in cellular biology. It is required for normal mammalian brain development and physiology, such that deficiency or excess of zinc has been shown to contribute to alterations in behavior, abnormal central nervous system development, and neurological disease. In this light, it is not surprising that zinc ions have now been shown to play a role in the neuromodulation of synaptic transmission as well as in cortical plasticity. Zinc is stored in specific synaptic vesicles by a class of glutamatergic or "gluzinergic" neurons and is released in an activity-dependent manner. Because gluzinergic neurons are found almost exclusively in the cerebral cortex and limbic structures, zinc may be critical for normal cognitive and emotional functioning. Conversely, direct evidence shows that zinc might be a relatively potent neurotoxin. Neuronal injury secondary to in vivo zinc mobilization and release occurs in several neurological disorders such as Alzheimer's disease and amyotrophic lateral sclerosis, in addition to epilepsy and ischemia. Thus, zinc homeostasis is integral to normal central nervous system functioning, and in fact its role may be underappreciated. This article provides an overview of zinc neurobiology and reviews the experimental evidence that implicates zinc signals in the pathophysiology of neuropsychiatric diseases. A greater understanding of zinc's role in the central nervous system may therefore allow for the development of therapeutic approaches where aberrant metal homeostasis is implicated in disease pathogenesis.

Mammalian zinc transporters: nutritional and physiologic regulation

Research advances defining how zinc is transported into and out of cells and organelles have increased exponentially within the past five years. Research has progressed through application of molecular techniques including genomic analysis, cell transfection, RNA interference, kinetic analysis of ion transport, and application of cell and animal models including knockout mice. The knowledge base has increased for most of 10 members of the ZnT family and 14 members of the Zrt-, Irt-like protein (ZIP) family. Relative to the handling of dietary zinc is the involvement of ZnT1, ZIP4, and ZIP5 in intestinal zinc transport, involvement of ZIP10 and ZnT1 in renal zinc reabsorption, and the roles of ZIP5, ZnT2, and ZnT1 in pancreatic release of endogenous zinc. These events are major factors in regulation of zinc homeostasis. Other salient findings are the involvement of ZnT2 in lactation, ZIP14 in the hypozincemia of inflammation, ZIP6, ZIP7, and ZIP10 in metastatic breast cancer, and ZnT8 in insulin processing and as an autoantigen in diabetes.

Down-regulation of ZIP4 by RNA interference inhibits pancreatic cancer growth and increases the survival of nude mice with pancreatic cancer xenografts

PURPOSE

Zinc levels have been correlated with cancer risk, although the role of zinc and zinc transporters in cancer progression is largely unknown. We recently found that a zinc transporter, ZIP4, is overexpressed in pancreatic cancer. In this study, we further deciphered the role that ZIP4 plays in a pancreatic cancer mouse model by silencing ZIP4.

EXPERIMENTAL DESIGN

ZIP4 stable silencing was established in pancreatic cancer cell lines ASPC-1 (ASPC-shZIP4) and BxPC-3 (BxPC-shZIP4) by short hairpin RNA using retrovirus vectors. The stable cells were characterized in vitro and in vivo using a nude mouse xenograft model.

RESULTS

Silencing of ZIP4 was associated with decreased cell proliferation, migration, and invasion. Both ASPC-shZIP4 and BxPC-shZIP4 cells showed a significant reduction in tumor volume and weight in the s.c. model, and decreased primary tumor weight in the orthotopic model compared with the vector control cells (ASPC-shV and BxPC-shV). Silencing of ZIP4 also caused reduced incidence of tumor metastasis in the mice and downsized the tumor grade. More importantly, silencing of ZIP4 significantly increased the survival rate of nude mice with orthotopic xenografts (P = 0.005). All ASPC-shZIP4-injected mice (100%) remained alive up to 32 days after tumor implantation, whereas only 30% of the ASPC-shV mice were alive at the same time point. CyclinD1 expression was decreased in the ASPC-shZIP4 xenografts.

CONCLUSIONS

These results identify a previously uncharacterized role of ZIP4 in pancreatic cancer progression, and indicate that knocking down ZIP4 by short hairpin RNA might be a novel treatment strategy for pancreatic cancers with ZIP4 overexpression.

The cation diffusion facilitator gene cdf-2 mediates zinc metabolism in Caenorhabditis elegans

Zinc is essential for many cellular processes. To use Caenorhabditis elegans to study zinc metabolism, we developed culture conditions allowing full control of dietary zinc and methods to measure zinc content of animals. Dietary zinc dramatically affected growth and zinc content; wild-type worms survived from 7 microm to 1.3 mm dietary zinc, and zinc content varied 27-fold. We investigated cdf-2, which encodes a predicted zinc transporter in the cation diffusion facilitator family. cdf-2 mRNA levels were increased by high dietary zinc, suggesting cdf-2 promotes zinc homeostasis. CDF-2 protein was expressed in intestinal cells and localized to cytosolic vesicles. A cdf-2 loss-of-function mutant displayed impaired growth and reduced zinc content, indicating that CDF-2 stores zinc by transport into the lumen of vesicles. The relationships between three cdf genes, cdf-1, cdf-2, and sur-7, were analyzed in double and triple mutant animals. A cdf-1 mutant displayed increased zinc content, whereas a cdf-1 cdf-2 double mutant had intermediate zinc content, suggesting cdf-1 and cdf-2 have antagonistic functions. These studies advance C. elegans as a model of zinc metabolism and identify cdf-2 as a new gene that has a critical role in zinc storage.

Zinc transporter-2 (ZnT2) variants are localized to distinct subcellular compartments and functionally transport zinc

ZnT2 (zinc transporter-2) expression is restricted to tissues with unique zinc requirements such as mammary and prostate glands. We previously determined that ZnT2 plays a major role in zinc export from mammary glands, as women with a mutation in the gene encoding ZnT2 (SLC30A2) had an approximately 75% reduction in milk zinc concentration. Two distinct human ZnT2 isoforms (approximately 42 and 35 kDa) are predicted to result from alternative splicing of SLC30A2. We examined the localization and function of each ZnT2 isoform, in cells generated to express ZnT2-HA (haemagglutinin) fusion proteins. The 42 kDa isoform was localized primarily to the endosomal/secretory compartment and overexpression resulted in increased zinc vesicularization. In contrast, the 35 kDa isoform is associated with the plasma membrane. Importantly, zinc transport was higher in cells over-expressing each isoform, indicating that both proteins are functional. Endogenous expression of the secretory vesicle-associated ZnT2 isoform predominates in mammary cells and expression is higher in secreting cells, whereas the smaller isoform plays a minor role in zinc export, directly reflecting the secretory function of the mammary gland. Together our data shed further light on the complex integration of cellular zinc transport mechanisms, which may be facilitated by multiple isoforms of specific zinc transporters with unique cellular functions.

Recent advances in knowledge of zinc nutrition and human health

Zinc deficiency increases the risk and severity of a variety of infections, restricts physical growth, and affects specific outcomes of pregnancy. Global recognition of the importance of zinc nutrition in public health has expanded dramatically in recent years, and more experience has accumulated on the design and implementation of zinc intervention programs. Therefore, the Steering Committee of the International Zinc Nutrition Consultative Group (IZiNCG) completed a second IZiNCG technical document that reexamines the latest information on the intervention strategies that have been developed to enhance zinc nutrition and control zinc deficiency. In particular, the document reviews the current evidence regarding preventive zinc supplementation and the role of zinc as adjunctive therapy for selected infections, zinc fortification, and dietary diversification or modification strategies, including the promotion and protection of breastfeeding and biofortification. The purposes of this introductory paper are to summarize new guidelines on the assessment of population zinc status, as recommended by the World Health Organization (WHO), the United Nations Children's Fund (UNICEF), the International Atomic Energy Agency (IAEA), and IZiNCG, and to provide an overview on several new advances in zinc metabolism. The following papers will then review the intervention strategies individually.

Strain difference of cadmium-induced testicular toxicity in inbred Wistar-Imamichi and Fischer 344 rats

Previously, we reported that Wistar-Imamichi (WI) rats are highly resistant to cadmium (Cd)-induced lethality and hepatotoxicity compared to Fischer 344 (F344) rats. Since the testes are one of the most sensitive organs to acute Cd toxicity, we examined possible strain-related differences in Cd-induced testicular toxicity between inbred WI and F344 rats. Rats were treated with a single dose of 0.5, 1.0 or 2.0 mg Cd/kg, as CdCl(2), sc and killed 24 h later. Cd at doses of 1.0 and 2.0 mg/kg induced severe testicular hemorrhage, as assessed by pathological and testis hemoglobin content, in F344 rats, but not WI rats. After Cd treatment (2.0 mg/kg), the testicular Cd content was significantly lower in WI rats than in the F344 rats, indicating a toxiokinetic mechanism for the observed strain difference. Thus, the remarkable resistance to Cd-induced testicular toxicity in WI rats is associated, at least in part, with lower testicular accumulation of Cd. When zinc (Zn; 10 mg/kg, sc) was administered in combination with Cd (2.0 mg/kg) to F344 rats, the Cd-induced increase in testicular hemoglobin content, indicative of hemorrhage, was significantly reduced. Similarly, the testicular Cd content was significantly decreased with Zn co-treatment compared to Cd treatment alone. Thus, it can be concluded that the testicular Cd accumulation partly competes with Zn transport systems and that these systems may play an important role in the strain-related differences in Cd-induced testicular toxicity between WI and F344 rats.

Zinc transporter Znt5/Slc30a5 is required for the mast cell-mediated delayed-type allergic reaction but not the immediate-type reaction

Zinc (Zn) is an essential nutrient and its deficiency causes immunodeficiency. However, it remains unknown how Zn homeostasis is regulated in mast cells and if Zn transporters are involved in allergic reactions. We show that Znt5/Slc30a5 is required for contact hypersensitivity and mast cell-mediated delayed-type allergic response but not for immediate passive cutaneous anaphylaxis. In mast cells from Znt5(-/-) mice, Fc epsilon receptor I (Fc epsilonRI)-induced cytokine production was diminished, but degranulation was intact. Znt5 was involved in Fc epsilonRI-induced translocation of protein kinase C (PKC) to the plasma membrane and the nuclear translocation of nuclear factor kappaB. In addition, the Zn finger-like motif of PKC was required for its plasma membrane translocation and binding to diacylglycerol. Thus, Znt5 is selectively required for the mast cell-mediated delayed-type allergic response, and it is a novel player in mast cell activation.

SLC39A9 (ZIP9) regulates zinc homeostasis in the secretory pathway: characterization of the ZIP subfamily I protein in vertebrate cells

The SLC39A family of zinc transporters can be divided into four subfamilies (I, II, LIV-1, and gufA) in vertebrates, but studies of their functions have been restricted exclusively to members of subfamilies II and LIV-1. In this study, we characterized SLC39A9 (ZIP9), the only member of subfamily I in vertebrates. Confocal microscopy demonstrated that transiently expressed, HA-tagged human ZIP9 (hZIP9-HA) was localized to the trans-Golgi network regardless of zinc status. Disruption of the ZIP9 gene in DT40 cells did not change the growth rate, sensitivity to high zinc and manganese concentrations during long-term culture, or cellular zinc status after short-term incubation with zinc. The alkaline phosphatase activity of ZIP9(-/-) cells did not change in cells cultured in medium containing normal zinc levels. In contrast, the activity of this enzyme decreased in wild-type cells cultured in zinc deficient medium but less so in ZIP9(-/-) cells under these conditions. Stable over-expression of hZIP9-HA moderately decreased alkaline phophatase activity. These results suggest that ZIP9 functions to regulate zinc homeostasis in the secretory pathway without significantly altering cytosolic zinc homeostasis.

Tissue-specific alterations in zinc transporter expression in intestine and liver reflect a threshold for homeostatic compensation during dietary zinc deficiency in weanling rats

Intestinal zinc (Zn) absorption and liver Zn mobilization are presumed to regulate Zn homeostasis. Several Zn transporters have been identified; however, their contribution to Zn homeostasis is poorly understood. Moreover, their regulation during periods of growth is unknown. To characterize the mechanisms that maintain Zn status, weanling rats were fed control (25 mg/kg), marginally low (MLZ; 15 mg/kg), low (LZ; 7 mg/kg), or very low (VLZ; <1 mg/kg) Zn diets for 3 wk and effects on jejunum Zip4 and ZnT1 and hepatic Zip1 and ZnT1 were assessed. Another control group was pair-fed (PF) to VLZ. The MLZ rats had lower jejunum ZnT1 protein abundance than the control. In the LZ group, we detected increased jejunum Zip4 mRNA expression and hepatic ZnT1 protein abundance and reduced jejunum Zip4 and ZnT1 and hepatic Zip1 protein abundance. VLZ had lower jejunum ZnT1 mRNA and protein abundance and hepatic Zip1 and ZnT1 protein abundance compared with the PF group. Zip4 protein was present at the intestinal villus tip in controls but was detected on the apical membrane throughout the entire villus in LZ rats. ZnT5 protein in jejunum was always detected at the apical membrane and also at the basolateral membrane of VLZ rats. In contrast, ZnT7 was found intracellularly in jejunum. Our data suggest that effects of Zn deficiency on Zn homeostasis occurs biphasically during marginal Zn deficiency through increased intestinal Zn uptake capacity and reduced intestinal Zn efflux, then during more pronounced degrees of Zn deficiency through decreased liver Zn accretion and increased hepatic Zn efflux back into circulation. These results assist in our understanding of how mammals regulate Zn homeostasis.

Analysis and update of the human solute carrier (SLC) gene superfamily

The solute-carrier gene (SLC) superfamily encodes membrane-bound transporters. The SLC superfamily comprises 55 gene families having at least 362 putatively functional protein-coding genes. The gene products include passive transporters, symporters and antiporters, located in all cellular and organelle membranes, except, perhaps, the nuclear membrane. Transport substrates include amino acids and oligopeptides, glucose and other sugars, inorganic cations and anions (H⁺, HCO₃^-, Cl^-, Na⁺, K⁺, Ca²⁺, Mg²⁺, PO₄^3-, HPO₄^2-, H₂PO₄^-, SO₄^2-, C₂O₄^2-, OH^-,CO₃^2-), bile salts, carboxylate and other organic anions, acetyl coenzyme A, essential metals, biogenic amines, neurotransmitters, vitamins, fatty acids and lipids, nucleosides, ammonium, choline, thyroid hormone and urea. Contrary to gene nomenclature commonly assigned on the basis of evolutionary divergence http://www.genenames.org/, the SLC gene superfamily has been named based largely on transporter function by proteins having multiple transmembrane domains. Whereas all the transporters exist for endogenous substrates, it is likely that drugs, non-essential metals and many other environmental toxicants are able to 'hitch-hike' on one or another of these transporters, thereby enabling these moieties to enter (or leave) the cell. Understanding and characterising the functions of these transporters is relevant to medicine, genetics, developmental biology, pharmacology and cancer chemotherapy.

Age-dependent signature of metallothionein expression in primary CD4 T cell responses is due to sustained zinc signaling

The ability to mount adaptive immune responses to vaccinations and viral infections declines with increasing age. To identify mechanisms leading to immunosenescence, primary CD4 T cell responses were examined in 60- to 75-year-old individuals lacking overt functional defects. Transcriptome analysis indicated a selective defect in zinc homeostasis. CD4 T cell activation was associated with zinc influx via the zinc transporter Zip6, leading to increased free cytoplasmic zinc and activation of negative feedback loops, including the induction of zinc-binding metallothioneins. In young adults, activation-induced cytoplasmic zinc concentrations declined after 2 days to below prestimulation levels. In contrast, activated naïve CD4 T cells from older individuals failed to downregulate cytoplasmic zinc, resulting in excessive induction of metallothioneins. Activation-induced metallothioneins regulated the redox state in activated T cells and accounted for an increased proliferation of old CD4 T cells, suggesting that regulation of T cell zinc homeostasis functions as a compensatory mechanism to preserve the replicative potential of naïve CD4 T cells with age.

Cooperation of metallothionein and zinc transporters for regulating zinc homeostasis in human intestinal Caco-2 cells

This investigation examined the effects of zinc status on cell proliferation and the synergic roles of the metallothionein (MT) and zinc transporter (ZnT) in the human colon adenocarcinoma cell line Caco-2. Cells were treated with 0 to 300 micromol/L ZnSO(4) or 0 to 10 micromol/L N,N,N',N'-tetrakis(2-phridylmethyl) ethylenediamine (TPEN). Cell proliferation was determined by MTT assay and apoptotic cells detected by flow cytometry (Hoechst 33258 dye). mRNA expression of MT1; ZnT1; zrt, irt-like protein 1, 4 (ZIP1, 4); and divalent metal transporter (DMT1) were determined by the reverse transcription polymerase chain reaction or real-time polymerase chain reaction. The results showed that either high or low zinc could inhibit the cell proliferation. The number of apoptotic cells increased with incremental increases in the concentrations of ZnSO(4) and TPEN. The mRNA expression of ZnT1 and MT1 responded significantly after 6 and 12 hours with 200 micromol/L zinc treatment, respectively, and increased gradually with zinc levels from 0 to 200 micromol/L. Compared with the unchanged ZIP1 mRNA expression, ZIP4 was closely dependent on TPEN treatment duration and concentration. The DMT1 mRNA expression was upregulated time-dependently but not concentration-dependently in the late TPEN treatment duration. The results suggest that ZIP4 and DMT1 mRNA expressions are susceptible to low extracellular zinc concentration and upregulated to enhance zinc absorption, whereas the ZnT1 and MT1 act as the key regulators under high zinc conditions to enhance the intracellular zinc efflux to maintain zinc homeostasis. We propose that in response to variations in zinc concentration, the cooperated regulative roles of ZnT1, MT1, DMT1, and ZIP4 contribute to zinc homeostasis.

A distinct role in breast cancer for two LIV-1 family zinc transporters

Zinc, essential for normal cell growth, is tightly controlled in cells by two families of zinc transporters. The aberrant expression of zinc transporters from the LIV-1 family of ZIP (Zrt/Irt-like protein) transporters is increasingly being implicated in a variety of disease states. In the present paper, I describe a mechanism for the role of ZIP7 in the progression of breast cancer, identifying it as a new target in breast cancer. Furthermore, I document a link between another zinc transporter, LIV-1, and breast cancer metastasis, identifying it as a potential new prognostic indicator of breast cancer spread.

Manganese and its role in Parkinson's disease: from transport to neuropathology

The purpose of this review is to highlight recent advances in the neuropathology associated with Mn exposures. We commence with a discussion on occupational manganism and clinical aspects of the disorder. This is followed by novel considerations on Mn transport (see also chapter by Yokel, this volume), advancing new hypotheses on the involvement of several transporters in Mn entry into the brain. This is followed by a brief description of the effects of Mn on neurotransmitter systems that are putative modulators of dopamine (DA) biology (the primary target of Mn neurotoxicity), as well as its effects on mitochondrial dysfunction and disruption of cellular energy metabolism. Next, we discuss inflammatory activation of glia in neuronal injury and how disruption of synaptic transmission and glial-neuronal communication may serve as underlying mechanisms of Mn-induced neurodegeneration commensurate with the cross-talk between glia and neurons. We conclude with a discussion on therapeutic aspects of Mn exposure. Emphasis is directed at treatment modalities and the utility of chelators in attenuating the neurodegenerative sequelae of exposure to Mn. For additional reading on several topics inherent to this review as well as others, the reader may wish to consult Aschner and Dorman (Toxicological Review 25:147-154, 2007) and Bowman et al. (Metals and neurodegeneration, 2009).

Functional characterization of tzn1 and tzn2-zinc transporter genes in Neurospora crassa

Previous work from our laboratory involved the description of the Neurospora metal transportome, which included seven hypothetical zinc transporters belonging to the ZIP family. The aim of the present study was to make a comparative functional evaluation of two hypothetical zinc transporters named tzn1 (NCU07621.3) and tzn2 (NCU11414.3). Phenotypic analysis of tzn1 and tzn2 mutants and a double mutant (tzn1tzn2) revealed that the deletion of tzn1 causes aconidiation and a greater defect in growth than the single deletion of tzn2. Supplementation with zinc restores growth but not conidiation in tzn1 and tzn1tzn2. TZN1 complemented a zinc-uptake-deficient Saccharomyces cerevisiae mutant (zrt1zrt2) in zinc-deficient conditions, while tzn2 restored growth upon supplementation with zinc (0.05 mM). Furthermore, the Deltatzn1 mutant was found to have severely reduced zinc content indicating that tzn1 functions as a key regulator of intracellular zinc levels in Neurospora crassa. Zinc uptake studies indicate tzn1 is a specific transporter of zinc, while tzn2 transports both zinc and cadmium. Quantitative RT-PCR showed up-regulation of tzn1 (128-fold) under zinc-depleted conditions and down-regulation (>1,000-fold) in zinc-replete conditions. The present study indicates that the zinc transport proteins encoded by tzn1 and tzn2 are members of the zinc uptake system regulated by zinc status in N. crassa.

The zinc transporter SLC39A13/ZIP13 is required for connective tissue development; its involvement in BMP/TGF-beta signaling pathways

BACKGROUND

Zinc (Zn) is an essential trace element and it is abundant in connective tissues, however biological roles of Zn and its transporters in those tissues and cells remain unknown.

METHODOLOGY/PRINCIPAL FINDINGS

Here we report that mice deficient in Zn transporter Slc39a13/Zip13 show changes in bone, teeth and connective tissue reminiscent of the clinical spectrum of human Ehlers-Danlos syndrome (EDS). The Slc39a13 knockout (Slc39a13-KO) mice show defects in the maturation of osteoblasts, chondrocytes, odontoblasts, and fibroblasts. In the corresponding tissues and cells, impairment in bone morphogenic protein (BMP) and TGF-beta signaling were observed. Homozygosity for a SLC39A13 loss of function mutation was detected in sibs affected by a unique variant of EDS that recapitulates the phenotype observed in Slc39a13-KO mice.

CONCLUSIONS/SIGNIFICANCE

Hence, our results reveal a crucial role of SLC39A13/ZIP13 in connective tissue development at least in part due to its involvement in the BMP/TGF-beta signaling pathways. The Slc39a13-KO mouse represents a novel animal model linking zinc metabolism, BMP/TGF-beta signaling and connective tissue dysfunction.

Overexpression of the zinc uptake transporter hZIP1 inhibits nuclear factor-kappaB and reduces the malignant potential of prostate cancer cells in vitro and in vivo

PURPOSE

Intracellular zinc levels and expression of the zinc uptake transporter, hZIP1, are markedly down-regulated in prostate adenocarcinomatous tissue compared with normal prostate tissue. Our previous studies have shown that zinc inhibits nuclear factor-kappaB (NF-kappaB) activity and reduces the malignant potential of prostate cancer cells in vitro. In this study, we investigate the functional effect of hZIP1 overexpression on NF-kappaB activity and tumorigenic potential in human prostate cancer cells in vitro and in vivo.

EXPERIMENTAL DESIGN

NF-kappaB activity in PC-3 prostate cancer cells was examined by Western blotting and luciferase assay. ELISA was used to examine the expression of tumorigenic cytokines. Terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling, adhesion, and invasiveness assays were used to assess the malignant potential of tumor cells. The effect of hZIP1 overexpression on prostate tumor progression in vivo was assessed using a xenograft model.

RESULTS

Overexpression of the hZIP1 transporter in PC-3 cells results in significant inhibition of NF-kappaB activity in the presence of physiologic levels of zinc. NF-kappaB inhibition coincides with a reduction in expression of several NF-kappaB controlled prometastatic and antiapoptotic factors as well as sensitization of the cells to etoposide and tumor necrosis factor-mediated apoptosis-inducing ligand-mediated cell death. Moreover, overexpression of the hZIP1 transporter induces regression of prostate tumor growth in a xenograft model.

CONCLUSIONS

Our results show that hZIP1 overexpression has a functional effect on the malignant potential of prostate cancer cells via inhibition of NF-kappaB-dependent pathways and support the concept that hZIP1 may function as a tumor suppressor gene in prostate cancer.

Intracellular zinc homeostasis and zinc signaling

Zinc (Zn) is an essential heavy metal that is incorporated into a number of human Zn metalloproteins. Zn plays important roles in nucleic acid metabolism, cell replication, and tissue repair and growth. Zn deficiency is associated with a range of pathological conditions, including impaired immunity, retarded growth, brain development disorders and delayed wound healing. Moreover, many reports have suggested that Zn is involved in cancer development and levels of Zn in serum and malignant tissues of patients with various types of cancer are abnormal. Zn may directly affect tumor cells by regulating gene expression profiles and/or cell viability, both of which are mediated in part by tumor-induced changes in Zn transporter expression. On the other hand, Zn may indirectly influence tumor cells by affecting processes within the cancer microenvironment, including immune responses; the functions and/or activity levels of immune cells that attack tumor cells are influenced by the intracellular Zn concentrations within those cells. In both cases, Zn contributes to intracellular metal homeostasis and/or signal transduction in tumor and immune cells. In this review article, we will summarize the current understanding of the roles of Zn homeostasis and signaling primarily in immune cells, with a discussion of the contributions of these processes to oncogenesis.