Interleukin-6 regulates the zinc transporter Zip14 in liver and contributes to the hypozincemia of the acute-phase response

Zinc-transporter genes in human visceral and subcutaneous adipocytes: lean versus obese

Zinc ions influence adipose tissue metabolism by regulating leptin secretion and by promoting free fatty acid release and glucose uptake. The mechanisms controlling zinc metabolism in adipose tissue are unknown. We therefore examined the gene-expression levels of a number of zinc-transporting proteins in adipose tissue, comparing subcutaneous fat with visceral fat from lean and obese humans. Both ZnT-proteins responsible for zinc transport from cytosol to extracellular compartments and intracellular vesicles and Zip-proteins responsible for zinc transport to the cytoplasm were expressed in all samples. This suggests that zinc metabolism in adipocytes is actively controlled by zinc-transporters. The expression levels were different in lean and obese subjects suggesting a role for these proteins in obesity. Furthermore, the expression levels were different from subcutaneous fat to intra-abdominal fat suggesting that the metabolic activity in adipocytes is to some extent dependent upon zinc and the activity of zinc-transporting proteins or vice versa.

Upregulation of Slc39a10 gene expression in response to thyroid hormones in intestine and kidney

A novel zinc transporter has been purified and cloned from rat renal brush border membrane. This transporter was designated as Zip10 encoded by Slc39a10 gene and characterized as zinc importer. Present study documents the impact of thyroid hormones on the expression of Zip10 encoded by Slc39a10 gene in rat model of hypo and hyperthyroidism. Serum T(3) and T(4) levels were reduced significantly in hypothyroid rats whereas these levels were significantly elevated in hyperthyroid rats as compared to euthyroid rats thereby confirming the validity of the model. Kinetic studies revealed a significant increase in the initial and equilibrium uptake of Zn(++) in both intestinal and renal BBMV of hyperthyroid rats in comparison to hypothyroid and euthyroid rats. By RT-PCR, Slc39a10 mRNA expression was found to be significantly decreased in hypothyroid and increased in hyperthyroid as compared to euthyroid rats. These findings are in conformity with the immunofluorescence studies that revealed markedly higher fluorescence intensity at periphery of both intestinal and renal cells isolated from hyperthyroid rats as compared to hypothyroid and euthyroid rats. Higher expression of Zip10 protein in hyperthyroid group was also confirmed by western blot. These findings suggest that expression of zinc transporter protein Zip10 (Slc39a10) in intestine and kidney is positively regulated by thyroid hormones.

Acrodermatitis enteropathica and an overview of zinc metabolism

Acrodermatitis enteropathica is a rare autosomal recessive disorder of zinc deficiency. The genetic defect has been mapped to 8q24 and the defective gene identified as SLC39A4, which encodes the zinc transporter Zip4. The diagnosis is made by way of clinical presentation together with histopathology and laboratory tests. Here we provide an overview of zinc metabolism and a description of inherited and acquired zinc deficiency.

Zinc homeostasis and immunity

Zinc is an essential trace element for the immune system, and zinc deficiency compromises the function of primarily T cells but also of several other immune cells. Recently, zinc homeostasis has been demonstrated to affect dendritic cells, in particular the involvement of zinc transport proteins during lipopolysaccharide-induced upregulation of major histocompatibility complex proteins and co-stimulatory molecules. This adds to our understanding of the immunomodulatory potential of zinc and highlights its significance for immune function.

Anti-inflammatory effects of zinc and alterations in zinc transporter mRNA in mouse models of allergic inflammation

There is clinical evidence linking asthma with the trace element, zinc (Zn). Using a mouse model of allergic inflammation, we have previously shown that labile Zn decreases in inflamed airway epithelium (Truong-Tran AQ, Ruffin RE, Foster PS, Koskinen AM, Coyle P, Philcox JC, Rofe AM, Zalewski PD. Am J Respir Cell Mol Biol 27: 286-296, 2002). Moreover, mild nutritional Zn deficiency worsens lung function. Recently, a number of proteins belonging to the Solute Carrier Family 39 (ZIP) and Solute Carrier Family 30 (ZnT) have been identified that bind Zn and regulate Zn homeostasis. Mice were sensitized, and subsequently aerochallenged, with ovalbumin to induce acute and chronic airway inflammation. Mice received 0, 54, or 100 microg of Zn intraperitoneally. Tissues were analyzed for Zn content and histopathology. Inflammatory cells were counted in bronchoalveolar lavage fluid. Cytokine and Zn transporter mRNA levels were determined by cDNA gene array and/or real-time PCR. Zn supplementation decreased bronchoalveolar lavage fluid eosinophils by 40 and 80%, and lymphocytes by 55 and 66%, in the acute and chronic models, respectively. Alterations in Zn transporter expression were observed during acute inflammation, including increases in ZIP1 and ZIP14 and decreases in ZIP4 and ZnT4. Zn supplementation normalized ZIP1 and ZIP14, but it did not affect mRNA levels of cytokines or their receptors. Our results indicate that inflammation-induced alterations in Zn transporter gene expression are directed toward increasing Zn uptake. Increases in Zn uptake may be needed to counteract the local loss of Zn in the airway and to meet an increased demand for Zn-dependent proteins. The reduction of inflammatory cells by Zn in the airways provides support for Zn supplementation trials in human asthmatic individuals.

Monitoring micronutrients in cigarette smokers

Smoking is associated with oxidative stress and increased risks of many chronic diseases that both shorten life and impair its quality. Low concentrations of several micronutrients, especially the antioxidants vitamin C and beta-carotene, are also associated with smoking, and there has been much interest in determining whether deficiencies in micronutrients are involved etiologically in smoking-related diseases. The objective of this review was to bring together reports on dietary intakes, biochemical indicators of micronutrient status, and results of some intervention studies on micronutrients where authors had compared outcomes in smokers and non-smokers. The micronutrients discussed are vitamins A, E, and C; the carotenoids; some of the B-vitamin group; and the minerals selenium, zinc, copper, and iron. The data were then examined to determine whether effects on the biochemical markers of micronutrient status were due to differences in dietary intakes between smokers and non-smokers or to the consequences of inflammatory changes caused by the oxidative stress of smoking. It was concluded that although smoking is associated with reduced dietary intake of vitamin C and carotenoid-containing foods, inflammatory changes increase turnover of these micronutrients so that blood concentrations are still lower in smokers than non-smokers even when there is control for dietary differences. In the case of vitamin E, there is some evidence for increased turnover of this nutrient in smokers, but this has little to no influence on blood concentrations, and there are no differences in dietary intake of vitamin E between smokers and non-smokers. Serum concentrations of vitamin A, folate, and vitamin B12 and B6 markers do not appear to be influenced by smoking, although there is some influence of dietary intake on concentrations of these nutrients in the body. In the case of the minerals examined, the main effects on biochemical markers of mineral status were attributed to inflammation and were therefore greater in heavy or long-term smokers. Serum concentrations of selenium and erythrocyte GPx activity were lower in smokers. Erythrocyte CuZn-SOD activity and serum ceruloplasmin concentrations were elevated, while serum zinc concentrations were depressed only in heavy smokers. Lastly, smoking appears to affect iron homeostasis mainly by changing hemoglobin concentrations, which were in general increased. Serum iron, TfR, and ferritin were mostly unaffected by smoking, except in pregnancy where there is evidence of increased erythropoiesis causing lower saturation of plasma transferrin and some evidence of lowering of iron stores.

Zip14 (Slc39a14) mediates non-transferrin-bound iron uptake into cells

Zip14 is a member of the SLC39A zinc transporter family, which is involved in zinc uptake by cells. Up-regulation of Zip14 by IL-6 appears to contribute to the hepatic zinc accumulation and hypozincemia of inflammation. At least three members of the SLC39A family transport other trace elements, such as iron and manganese, in addition to zinc. We analyzed the capability of Zip14 to mediate non-transferrin-bound iron (NTBI) uptake by overexpressing mouse Zip14 in HEK 293H cells and Sf9 insect cells. Zip14 was found to localize to the plasma membrane, and its overexpression increased the uptake of both (65)Zn and (59)Fe. Addition of bathophenanthroline sulfonate, a cell-impermeant ferrous iron chelator, inhibited Zip14-mediated iron uptake from ferric citrate, suggesting that iron is taken up by HEK cells as Fe(2+). Iron uptake by HEK and Sf9 cells expressing Zip14 was inhibited by zinc. Suppression of endogenous Zip14 expression by using Zip14 siRNA reduced the uptake of both iron and zinc by AML12 mouse hepatocytes. Zip14 siRNA treatment also decreased metallothionein mRNA levels, suggesting that compensatory mechanisms were not sufficient to restore intracellular zinc. Collectively, these results indicate that Zip14 can mediate the uptake of zinc and NTBI into cells and that it may play a role in zinc and iron metabolism in hepatocytes, where this transporter is abundantly expressed. Because NTBI is commonly found in plasma of patients with hemochromatosis and transfusional iron overload, Zip14-mediated NTBI uptake may contribute to the hepatic iron loading that characterizes these diseases.

Zinc modulates cytokine-induced lung epithelial cell barrier permeability

Apoptosis plays a causative role in acute lung injury in part due to epithelial cell loss. We recently reported that zinc protects the lung epithelium during inflammatory stress whereas depletion of intracellular zinc enhances extrinsic apoptosis. In this investigation, we evaluated the relationship between zinc, caspase-3, and cell-to-cell contact via proteins that form the adherens junction complex. Cell adhesion proteins are directly responsible for formation of the mechanical barrier of the lung epithelium. We hypothesized that exposure to inflammatory cytokines, in conjunction with zinc deprivation, would induce caspase-3, leading to degradation of junction proteins, loss of cell-to-cell contact, and compromised barrier function. Primary human upper airway and type I/II alveolar epithelial cultures were obtained from multiple donors and exposed to inflammatory stimuli that provoke extrinsic apoptosis in addition to depletion of intracellular zinc. We observed that zinc deprivation combined with tumor necrosis factor-alpha, interferon-gamma, and Fas receptor ligation accelerates caspase-3 activation, proteolysis of E-cadherin and beta-catenin, and cellular apoptosis, leading to increased paracellular leak across monolayers of both upper airway and alveolar lung epithelial cultures. Zinc supplementation inhibited apoptosis and paracellular leak, whereas caspase inhibition was less effective. We conclude that zinc is a vital factor in the lung epithelium that protects against death receptor-mediated apoptosis and barrier dysfunction. Furthermore, our findings suggest that although caspase-3 inhibition reduces lung epithelial apoptosis it does not prevent mechanical dysfunction. These findings facilitate future studies aimed at developing therapeutic strategies to prevent acute lung injury.

Zinc homeostasis in aging: two elusive faces of the same "metal"

Proteins involved in zinc homeostasis may be altered in aging. This phenomenon may lead to zinc deficiency in the peripheral blood and an accumulation of zinc bound to insoluble aggregates at the extracellular level in the brain. Therefore, it should be more correct to talk about aging as a condition associated with zinc dyshomeostasis rather than deficiency. Restoring functional zinc homeostasis in aging people is an attractive field for antiaging research, but requires further knowledge than the current state of the art.

Sequence similarity and functional relationship among eukaryotic ZIP and CDF transporters

ZIP (ZRT/IRT-like Protein) and CDF (Cation Diffusion Facilitator) are two large metal transporter families mainly transporting zinc into and out of the cytosol. Several ZIP and CDF transporters have been characterized in mammals and various model organisms, such as yeast, nematode, fruit fly, and zebrafish, and many candidate genes have been identified by genome projects. Unexpected functions of ZIP and CDF transporters have been recently reported in some model organisms, leading to major advances in our understanding of the functions of mammalian counterparts. Here, we review the recent information on the sequence similarity and functional relationship among eukaryotic ZIP and CDF transporters obtained from the representative model organisms.

Zinc transporters and the cellular trafficking of zinc

Zinc is an essential nutrient for all organisms because this metal serves as a catalytic or structural cofactor for many different proteins. Zinc-dependent proteins are found in the cytoplasm and within many organelles of the eukaryotic cell including the nucleus, the endoplasmic reticulum, Golgi, secretory vesicles, and mitochondria. Thus, cells require zinc transport mechanisms to allow cells to efficiently accumulate the metal ion and distribute it within the cell. Our current knowledge of these transport systems in eukaryotes is the focus of this review.

Modulation of Iron Availability at the Host-Pathogen Interface in Phagocytic Cells

This review summarizes recent data on iron metabolism in macrophages, with a special emphasis on possible bacteriostatic and bactericidal consequences for intracellular pathogens. It includes the role of biological chelators and transporters in normal macrophage physiology and antimicrobial defense. Iron is an essential metal cofactor for many biochemical pathways in mammals. However, excess iron promotes the formation of cytotoxic oxygen derivatives so that systemic iron levels must be tightly regulated. The mechanism of iron recycling by macrophages including iron efflux from erythrocyte-containing phagosomes, iron release from macrophages, and entry into the transferrin (Tf) cycle remain poorly understood. Ferroportin expression in the liver, spleen, and bone marrow cells appears to be restricted to macrophages. Mutant mice bearing a conditional deletion of the ferroportin gene in macrophages show retention of iron by hepatic Kupffer cells and splenic macrophages. Hepcidin is induced by lipopolysaccharide (LPS) in mouse spleens and splenic macrophage in vitro and appears to mediate the LPS-induced down-regulation of ferroportin in the intestine and in splenic macrophages, suggesting that inflammatory agents may regulate iron metabolism through modulation of ferroportin expression. The host transporter Nramp1 may compete directly with bacterial divalent-metal transport systems for the acquisition of divalent metals within the phagosomal space. The ultimate outcome of these competing interactions influences the ability of pathogens to survive and replicate intracellularly. This seems particularly relevant to the Salmonella, Leishmania, and Mycobacterium spp., in which inactivating mutations in Nramp1 abrogate the natural resistance of macrophages to these pathogens.