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J. Biol. Chem., Vol. 281, Issue 34, 24085-24089, August 25, 2006
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Mammalian Zinc Transport, Trafficking, and Signals*
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| INTRODUCTION |
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Loss-of-function studies have not provided a delineation of how zinc participates in physiologic activities. Nevertheless, immune function and resistance to infection and control of nitrosative and/or oxidative stress of inflammation are two areas of particular contemporary interest. Unfolding knowledge of cell type-specific expression of zinc transporter genes that respond differentially to hormonal and cytokine stimulation should aid in understanding the role of zinc in these physiologic systems. Coordination of intracellular zinc trafficking has focused on the cysteine-rich protein metallothionein (MT).2 Zinc-dependent expression of Mt and its well documented response to multiple mediators, including oxidants, cytokines, hormones, and nitric oxide, have been extensively investigated. Here we review recent findings of how zinc transporters and MT influence mammalian cellular zinc metabolism and signaling pathways.
| Zinc Transporters |
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The mammalian ZnT family consists of 10 members (ZnT110). Zinc transporter activity for most ZnT proteins (ZnT1,2,48) has been confirmed indirectly by survival of cells in medium of high zinc content or directly through measuring zinc uptake/efflux or zinc accumulation in transfected or mutated mammalian cells, zinc-sensitive yeast strains, and Xenopus oocytes (4, 1016). The ZnT-mediated transport mechanism is unknown. Cellular extrusion of zinc and zinc vesicular deposition occur against a zinc concentration gradient; therefore, it is likely that ZnT zinc transporters function as secondary active transporters or perhaps as antiporters. Homologous proteins function as antiporters exchanging Zn2+ for H+ or K+ (17, 18) or are able to transport zinc without an energy source or proton gradient when reconstituted in proteo-liposomes (19).
There is considerable sequence homology among human ZnT proteins (20). These sequences vary in size, and most of them are predicted to have 6 transmembrane domains (TMD) with the exception of ZnT5, which has 12. These proteins have both N and C termini on the cytoplasmic side of the membrane. In addition, most ZnT proteins have a long intracellular loop with a variable number of histidine residues (20). These loops should bind zinc and have been implicated as ion-binding domains (21). Hydropathy plots of SLC30 proteins from different species show that four amphipathic TMDs likely form the channel through which zinc is translocated (22, 23). Some ZnT proteins function as homoor hetero-oligomers (24). These proteins are purportedly involved in the incorporation of zinc into enzymes, e.g. alkaline phosphatase, by mediating Zn2+ transport into the lumen of the Golgi apparatus (24). Motifs have been found in ZnT sequences that may allow for protein-protein interactions (21, 25).
The mammalian Zip family (SLC39) consists of 14 members. Zinc transport activity for Zip18 and 14 has been confirmed (2634) by using transfection of DNA into mammalian (human embryonic kidney, K562, and Chinese hamster ovary) cells. Transport activity has been measured by 65Zn uptake or by using probes that produce emission fluorescence upon binding intracellular labile zinc. Cell-permeable fluorophores include Zinquin, Zin-naphthopyr 1, Newport Green, and FluoZin-3AM. Ion specificity and advances in fluorescence output and photostability of these fluorophores have improved their use for zinc trafficking and imaging studies. The mechanism of Zip-mediated transport is not well understood. Zinc uptake could be a facilitated process driven by a concentration gradient. hZip1 and hZip2 transporter activities do not require ATP (26, 27). Zip-dependent transport activity is induced by HCO3, which suggests a symport mechanism (27). Metal competition studies indicate some mammalian Zip proteins are specific for zinc (2830); however, zinc uptake activity for h/mZip1, mZip3, and hZip2 was found to be inhibited by multiple cations (2628). Recently, the transport activity of Zip14 for both zinc and non-transferrin-bound iron, when overexpressed in HEK cells and Sf9 insect cells, has been demonstrated.3
A dendogram shows sequence similarity among mouse Zip proteins (supplemental Fig. 1). The Zip family can be divided into four subfamilies: Zip I, Zip II, gufA, and LZT (35). Most mammalian Zip proteins, including Zip48, Zip10, and Zip1214, belong to the LZT subfamily. Zip13 are from the Zip II subfamily; Zip9 is from the Zip I subfamily; and Zip11 clusters within the gufA subfamily. The LZT transporter family was named after LIV-1 (Zip6), the first Zip member (36). Most Zip proteins are predicted to have eight TMDs, but LIV-1 has only six. TMDs IV and V are highly conserved and could form the pore through which metals pass (37, 38). Zip proteins are predicted to have extracellular N and C termini and a long intracellular loop with a histidine-rich repeat (35). In contrast, immunolocalization studies suggest mouse Zip14 has an extracellular histidine-rich loop (34). The presence of specific motifs may confer to Zip proteins the option of other functions separate from zinc transport or for protein-protein interactions involving zinc. For example, the novel metalloprotease motif (HEXPHEXGD) of LZT proteins may allow them to function as matrix metalloproteases or participate in the catalytic properties of these enzymes. Zip10 has putative C2H2 zinc finger and cytochrome c motifs in its first TMD, suggesting novel roles for targeted metal transport.
Most ZnT proteins have been found in intracellular compartments, usually associated with endosomes, Golgi, or endoplasmic reticulum. ZnT1 appears to be the only ZnT transporter located at the plasma membrane, congruent with its role as the primary regulator of cellular zinc efflux (4). ZnT2 has a vesicular localization in pancreatic acinar cells, whereas ZnT1 has a vesicular localization but is also at the plasma membrane (39). ZnT9 is located in the nucleus during mitosis (25). ZnT5 localizes with secretory vesicles of pancreatic
cells and at the apical membrane of enterocytes. ZnT10 could be located at the plasma membrane according to software calculations. Most Zip proteins have been observed at the plasma membrane; however, Zip7 was located at the Golgi apparatus (32). The localization of some Zip transporters may change according to zinc availability or physiologic conditions. Zip5 has a basolateral plasma membrane orientation in polarized cells during dietary zinc sufficiency, but its regulation by zinc is not defined (30, 40). Similarly, Zip14 is mobilized to the sinusoidal membrane of the mouse hepatocyte during acute inflammation and, therefore, increases zinc uptake as a component of the acute phase response.
| Transporter Regulation |
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Geo reporter gene to zinc concentrations was similar. Homozygous knock-outs of either ZnT1 or MTF1 in mice are lethal to the embryo (41). Homozygous MTF1/ embryos do not express any ZnT1 mRNA compared with those from wild-type or heterozygous mice, suggesting ZnT1 is MTF1-regulated. In rodents, ZnT1 is ubiquitously expressed, but mRNA abundance exhibits wide differences among tissues (7, 9). The response of ZnT1 to dietary zinc restriction or supplementation is also variable among different tissues (4143). ZnT1 transcript levels are markedly reduced in peripheral blood mononuclear cells of zinc-deficient mice (39). In humans, ZnT1 mRNA levels in leukocytes increase markedly upon dietary zinc supplementation (44). Among human leukocyte subsets, ZnT1 transcripts are more abundant in monocytes than T-lymphocytes or neutrophils. Zinc responsiveness of ZnT1 is not dependent upon MT expression as it is normal in MT/ mice (45). Expression of neuronal ZnT1 is induced by forebrain ischemia (46), and considerable evidence shows ZnT1 is protective against Zn2+-induced neuronal cell damage by promoting efflux (47, 48).
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and endotoxin-induced gene expressed in monocytes (33). Expression is very low in unstimulated monocytes. DNA microarray analysis of the response of humans to acute systemic inflammation induced by endotoxin administration revealed a marked transient increase in Zip8 expression in total leukocytic RNA.4 Among human leukocytes, Zip8 exhibits extremely high transcript abundance in T-lymphocytes (44).
Systemic responses to sepsis and inflammation include hepatic zinc uptake, which produces a transient hypozincemia (3). Zip14 was identified as highly induced in hepatocytes of mice during the acute phase response (34). This novel Zip protein was demonstrated to transport zinc (31, 34). Wild-type mice produce robust amounts of Zip14 mRNA in response to acute inflammation and exhibit hypozincemia, whereas IL6/ mice produce no Zip14 and do not experience hypozincemia. Inflammation and IL6 in vitro increase Zip14 at the plasma membrane of hepatocytes. A probable signal pathway for this IL6-mediated response is via STAT regulation. Zip14 regulation by lipopolysaccharide during the acute phase is more complex, including NO-induced activation of AP-1.5 Zip14 regulation by IL6 and NO has implications for a role of zinc in resistance to toxin-induced liver injury and cancer progression.
| Integrative Interactions of Metallothionein-bound Zinc |
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domain binds three Zn2+ atoms via thiolate ligands from nine cysteines. The
domain binds four Zn2+ atoms via eleven cysteines. The metabolic pool from which the apo-MT (thionein) molecule acquires these seven Zn2+ atoms has not been identified. Structural studies using 1Hand 113Cd-NMR spectroscopy reveal that Zn2+ atoms bound to thiolates of the
domain are more labile than those of the
domain (54). This suggests the
domain is physiologically relevant, whereas the
domain may be related to metal detoxification. Such dual properties for the same molecule complicate interpretation of studies aimed at function. MT binds Zn2+ strongly (up to 1013 M1). However, experiments using model systems suggest MT may assume a donor/acceptor role for zinc-binding motifs and could activate or deactivate apozinc finger proteins or other zinc metalloproteins (55). Viability of MT/ mice suggests essential zinc-dependent functions, such as formation of zinc finger proteins, acquire zinc from sources other than MT. A widely studied role of MT is its ability to protect in vivo and in vitro against cellular stressors such as carbon-centered radicals and reactive oxygen and nitrogen species. The mechanism of cytoprotection remains unclear. In vitro studies have shown that the metal thiolate clusters of MT possess the unique ability to function as a redox unit; therefore, the protein has the potential to be involved in redox-sensitive signaling pathways (56). Comparative studies with nitric oxide (NO), H2O2, singlet oxygen, peroxyl radicals, and peroxynitrite suggest only stress from NO causing S-nitrosylation of MT cysteines and Zn2+ release is sufficiently mild to allow reconstitution of MT through Zn2+ rebinding (57). Fluorescence resonance energy transfer experiments have confirmed that in cultured endothelial cells MT undergoes conformational changes in the presence of NO donors, e.g. S-nitrocysteine (58). These alterations in cellular zinc homeostasis suggest protective effects of MT against nitric oxide toxicity (54). During inflammation or endotoxemia, hepatocytes respond to cytokines by up-regulating inducible nitric oxide synthase (iNOS), which generates large amounts of NO from arginine. Increased MT expression parallels the rise in cellular NO (59). NO promotes zinc release from MT, which in turn may repress iNOS (54). In zinc-deficient rodents, iNOS expression increases (60), perhaps because MT-bound zinc is not available. Another link between NO and MT may be through MTF1 activation. MTF1 nuclear translocation occurs under a variety of stress conditions. Hypoxia and oxidants such as H2O2 and tert-butylhydroquinone have been shown to increase MTF1 binding to the metal-responsive elements of the MT promoter. Release of labile zinc from MT or other thiolate ligands by NO may provide the means, via zinc binding to MTF1 and other zinc finger proteins, for regulation of other metal-responsive genes that are involved in cellular protection, including those of the ZnT and Zip families.
| Non-mammalian Zinc Transporters and Signaling |
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Zip6 (LIV1) may be involved in control of epithelial-mesenchymal transition (EMT). EMT is essential for embryonic development, tissue regeneration, and metastasis of neoplastic cells. During gastrulation in zebrafish, STAT3-dependent zLIV-1 (Zip6) expression regulates the nuclear translocation of the zinc finger protein Snail, which regulates EMT in organizer cells, and their invasive behavior (63). Through comparable mechanisms, Zip6 and Snail may be involved in metastasis of breast cancer cells, possibly through constitutive activation by STAT3 (64). Similarly, the LIV1 (Zip6) homologue FOI, the fear of intimacy gene, is required for proper gonad formation in Drosophila (65). A common downstream target for LIV1 (Zip6) and its homologues could be to inactivate cadherin expression, which has important roles in development, tissue remodeling, wound healing, and tumor promotion and metastasis.
| Conclusions |
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| FOOTNOTES |
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The on-line version of this article (available at http://www.jbc.org) contains a supplemental figure. ![]()
1 To whom correspondence should be addressed. E-mail: cousins{at}ufl.edu.
2 The abbreviations used are: MT, metallothionein; ZnT, zinc transporter; TMD, transmembrane domain; IL, interleukin; LZT, LIV-1 subfamily of Zip zinc transporters; STAT, signal transducer and activator of transcription; iNOS, inducible nitric oxide synthase; CDF, cation diffusion facilitator; EMT, epithelial-mesenchymal transition. ![]()
3 J. P. Liuzzi, F. Aydemir, H. Nam, M. D. Knutson, and R. J. Cousins, unpublished observations. ![]()
4 R. J. Cousins and T. Beker Aydemir, unpublished results. ![]()
5 L. A. Lichten and R. J. Cousins, unpublished results. ![]()
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