STAT5-glucocorticoid receptor interaction and MTF-1 regulate the expression of ZnT2 (Slc30a2) in pancreatic acinar cells

The exocrine pancreas plays an important role in endogenous zinc loss by regulating excretion into the intestinal tract and hence influences the dietary zinc requirement. The present experiments show that the zinc transporter ZnT2 (Slc30a2) is localized to the zymogen granules and that dietary zinc restriction in mice decreased the zinc concentration of zymogen granules and ZnT2 expression. Excess zinc given orally increased ZnT2 expression and was associated with increased pancreatic zinc accumulation. Rat AR42J acinar cells when induced into a secretory phenotype, using the glucocorticoid analog dexamethasone (DEX), exhibited increased ZnT2 expression and labile zinc as measured with a fluorophore. DEX administrated to mice also induced ZnT2 expression that accompanied a reduction of the pancreatic zinc content. ZnT2 promoter analyses identified elements required for responsiveness to zinc and DEX. Zinc regulation was traced to a MRE located downstream from the ZnT2 transcription start site. Responsiveness to DEX is produced by two upstream STAT5 binding sites that require the glucocorticoid receptor for activation. ZnT2 knockdown in the AR42J cells using siRNA resulted in increased cytoplasmic zinc and decreased zymogen granule zinc that further demonstrated that ZnT2 may mediate the sequestration of zinc into zymogen granules. We conclude, based upon experiments with intact mice and pancreatic acinar cells in culture, that ZnT2 participates in zinc transport into pancreatic zymogen granules through a glucocorticoid pathway requiring glucocorticoid receptor and STAT5, and zinc-regulated signaling pathways requiring MTF-1. The ZnT2 transporter appears to function in a physiologically responsive manner involving entero-pancreatic zinc trafficking.

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.

Zinc transporter ZIP8 (SLC39A8) and zinc influence IFN-gamma expression in activated human T cells

The zinc transporter ZIP8 is highly expressed in T cells derived from human subjects. T cell ZIP8 expression was markedly up-regulated upon in vitro activation. T cells collected from human subjects who had received oral zinc supplementation (15 mg/day) had higher expression of the activation marker IFN-gamma upon in vitro activation, indicating a potentiating effect of zinc on T cell activation. Similarly, in vitro zinc treatment of T cells along with activation resulted in increased IFN-gamma expression with a maximum effect at 3.1 microM. Knockdown of ZIP8 in T cells by siRNA decreased ZIP8 levels in nonactivated and activated cells and concomitantly reduced secretion of IFN-gamma and perforin, both signatures of activation. Overexpression of ZIP8 by transient transfection caused T cells to exhibit enhanced activation. Confocal microscopy established that ZIP8 is localized to the lysosome where ZIP8 abundance is increased upon activation. Loss of lysosomal labile zinc in response to activation was measured by flow cytometry using a zinc fluorophore. Zinc between 0.8 and 3.1 microM reduced CN phosphatase activity. CN was also inhibited by the CN inhibitor FK506 and ZIP8 overexpression. The results suggest that zinc at low concentrations, through inhibition of CN, sustains phosphorylation of the transcription factor CREB, yielding greater IFN-gamma expression in T cells. ZIP8, through control of zinc transport from the lysosome, may provide a secondary level of IFN-gamma regulation in T cells.

Interleukin-1beta contributes via nitric oxide to the upregulation and functional activity of the zinc transporter Zip14 (Slc39a14) in murine hepatocytes

Zinc metabolism during chronic disease is dysregulated by inflammatory cytokines. Experiments with IL-6 knockout mice show that LPS regulates expression of the zinc transporter, Zip14, by a mechanism that is partially independent of IL-6. The LPS-induced model of sepsis may occur by a mechanism signaled by nitric oxide (NO) as a secondary messenger. To address the hypothesis that NO can modulate Zip14 expression, we treated primary hepatocytes from wild-type mice with the NO donor S-nitroso N-acetyl penicillamine (SNAP). After treatment with SNAP, steady-state Zip14 mRNA levels displayed a maximal increase after 8 h and a concomitant increase in the transcriptional activity of the gene. Chromatin immunoprecipitation documented the kinetics of activator protein (AP)-1 and RNA polymerase II association with the Zip14 promoter after NO exposure, indicating a role of AP-1 in transcription of Zip14. We then stimulated the primary murine hepatocytes with IL-1beta, an LPS-induced proinflammatory cytokine and a potent activator of inducible NO synthase (iNOS) and NO production. In support of our hypothesis, IL-1beta treatment led to a threefold increase in Zip14 mRNA and enhanced zinc transport, as measured with a zinc fluorophore, in wild-type but not iNOS-/- hepatocytes. These data suggest that signaling pathways activated by NO are factors in the upregulation of Zip14, which in turn mediates hepatic zinc accumulation and hypozincemia during inflammation and sepsis.

Krüppel-like factor 4 regulates adaptive expression of the zinc transporter Zip4 in mouse small intestine

Epithelial cells of the small intestine are the site of zinc absorption. Intestinal uptake of zinc is inversely proportional to the dietary supply of this essential micronutrient. The mechanism responsible for this adaptive differential in apical zinc transport is not known. The zinc transporter Zip4 (Slc39a4) is essential for adequate enteric zinc uptake. In mice, Zip4 expression is upregulated at low zinc intakes with a concomitant ZIP4 localization to the apical enterocyte plasma membrane. With the present experiments, we show that the zinc finger transcription factor Krüppel-like factor 4 (KLF4), produced in high abundance in the intestine, is expressed at elevated levels in mice fed a low-zinc diet. In the murine intestinal epithelial cell (IEC) line MODE-K, zinc depletion of culture medium with cell-permeant and cell-impermeant chelators increased Zip4 and Klf4 mRNA and Zip4 heterogeneous nuclear RNA expression. Zinc depletion led to increased KLF4 in nuclear extracts. Knockdown of KLF4 using small interfering RNA transfection drastically limited ZIP4 induction upon zinc depletion and reduced 65Zn uptake by depleted IECs. EMSAs with nuclear extracts of IECs showed KLF4 binding to cis elements of the mouse Zip4 promoter, with increased binding under zinc-limited conditions. Reporter constructs with the Zip4 promoter and mutation studies further demonstrated that Zip4 is regulated through a KLF4 response element. These data from experiments with mice and murine IECs demonstrate that KLF4 is induced during zinc restriction and is a transcription factor involved in adaptive regulation of the zinc transporter ZIP4.

Zinc transporters ZnT1 (Slc30a1), Zip8 (Slc39a8), and Zip10 (Slc39a10) in mouse red blood cells are differentially regulated during erythroid development and by dietary zinc deficiency

Zinc is essential for normal erythroid cell functions and therefore intracellular zinc homeostasis during erythroid differentiation is tightly regulated. However, a characterization of zinc transporters in erythrocytes has not been conducted. The membrane fraction of mature mouse RBC was screened for zinc transporter expression using western analysis as a first step in the characterization process. ZnT1, Zip8, and Zip10 were detected among the 12 transporter proteins tested. We examined expression of these zinc transporters during erythropoietin (EPO)-induced differentiation of splenic erythroid progenitor cells into reticulocytes. Both Zip8 and Zip10 mRNA increased by 2-6 h after addition of EPO to the cells. In contrast, maximal RNA levels for the zinc transporter ZnT1 and erythroid delta-aminolevulinic acid synthase were only produced by 24 h after EPO. We confirmed these changes in transcript abundance by western analysis. Dietary zinc status influences zinc-dependent functions of RBC. To determine whether the identified zinc transporters respond to dietary zinc status, mice were fed a zinc-deficient or control diet. Incorporation of (65)Zn into erythrocytes in vitro was significantly increased in cells from the zinc-deficient mice. Western analysis and densitometry revealed that erythrocyte Zip10 was upregulated and ZnT1 was downregulated in the zinc-depleted mice. Zip8 was not affected by restricted zinc intake. Collectively, these data suggest that the zinc transporters ZnT1, Zip8, and Zip10 are important for zinc homeostasis in erythrocytes and that ZnT1 and Zip10 respond to the dietary zinc supply.

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 supplementation of young men alters metallothionein, zinc transporter, and cytokine gene expression in leukocyte populations

An effective measure to assess zinc status of humans has remained elusive, in contrast to iron, where a number of indicators of metabolism/function are available. Using monocytes, T lymphocytes, and granulocytes isolated by magnetic sorting and dried blood spots (DBS) derived from 50 mul of peripheral blood, we evaluated the response of metallothionein (MT), zinc transporter, and cytokine genes to a modest (15 mg of Zn per day) dietary zinc supplement in human subjects. Transcript abundance was measured by quantitative real-time RT-PCR (QRT-PCR). Zinc supplementation increased MT mRNA abundance by up to 2-fold in RNA from leukocyte subsets, and 4-fold in RNA from DBS. Transcript levels for the zinc transporter genes ZnT1 and Zip3 were increased and decreased, respectively, by zinc supplementation. Expression of the ZnT and Zip genes among leukocyte subsets differ by up to 270-fold. Monocytes and granulocytes from supplemented subjects were activated by LPS, whereas T lymphocytes were activated by mimicking antigen presentation. With zinc consumption, TNF-alpha and IL-1beta expression was greater in activated monocytes and granulocytes, and IFN-gamma mRNA levels were higher in activated T lymphocytes. These studies show that QRT-PCR is a tool to reliably measure transcript abundance for nutritionally responsive genes in human subjects, and that a small sample of whole dried blood, when appropriately collected, can be used as the source of total RNA for QRT-PCR analysis. The results obtained also show that zinc supplementation of human subjects programs specific leukocytic subsets to show enhanced cytokine expression upon activation by stimulators of immunity.