Zinc Uptake Transporter Research Articles

A Zinc Uptake Transporter ZIP1-II Is Involved in Zinc Accumulation in the Hepatopancreas of Pacific Oyster Crassostrea gigas.

The Pacific oyster Crassostrea gigas is known to have an exceptional ability to accumulate zinc, which endows it with robust resistance to pathogens and makes it an excellent source of dietary zinc. ZIP1 has been identified as an important zinc uptake protein in other species, but its role in oysters remains unclear. In the present study, a ZIP1 homologue (CgZIP1-II) of the Zrt/Irt-like protein (ZIP) family was identified in C. gigas. The mRNA transcripts of CgZIP1-II were constitutively expressed in examined tissues of C. gigas, with higher levels in the hepatopancreas and gill. After zinc exposure, the mRNA transcripts of CgZIP1-II in the hepatopancreas showed a significant decline from 12 h to 14 d, while those in the gill significantly decreased at 72 h, followed by a recovery to basal level at 7 to 14 d. Immunocytochemical analysis revealed that the CgZIP1-II protein was mainly located at the plasma membrane of oyster hemocytes. Compared to the control cells, overexpression of CgZIP1-II in the transfected HEK293 cells resulted in a 2.44-fold (p < 0.05) increase in zinc content after incubation with 100 μM zinc for 24 h. Inhibition of endogenous CgZIP1-II expression with siRNAs led to a 42% reduction in zinc content in the hepatopancreas of oysters. Similarly, in vivo blocking of CgZIP1-II with anti-CgZIP1-II antibody caused a 43% decrease in zinc content in the hepatopancreas. These results collectively indicated that CgZIP1-II functioned as a zinc uptake transporter in C. gigas and played a certain role in zinc accumulation.

Zur and zinc increase expression of E. coli ribosomal protein L31 through RNA-mediated repression of the repressor L31p.

Bacteria can adapt in response to numerous stress conditions. One such stress condition is zinc depletion. The zinc-sensing transcription factor Zur regulates the way numerous bacterial species respond to severe changes in zinc availability. Under zinc sufficient conditions, Zn-loaded Zur (Zn2-Zur) is well-known to repress transcription of genes encoding zinc uptake transporters and paralogues of a few ribosomal proteins. Here, we report the discovery and mechanistic basis for the ability of Zur to up-regulate expression of the ribosomal protein L31 in response to zinc in E. coli. Through genetic mutations and reporter gene assays, we find that Zur achieves the up-regulation of L31 through a double repression cascade by which Zur first represses the transcription of L31p, a zinc-lacking paralogue of L31, which in turn represses the translation of L31. Mutational analyses show that translational repression by L31p requires an RNA hairpin structure within the l31 mRNA and involves the N-terminus of the L31p protein. This work uncovers a new genetic network that allows bacteria to respond to host-induced nutrient limiting conditions through a sophisticated ribosomal protein switching mechanism.

Decreased SLC39A1 (Solute carrier family 39 member 1) expression predicts unfavorable prognosis in patients with early-stage hepatocellular carcinoma

ABSTRACT Solute carrier family 39, member 1 (SLC39A1) is a member of the zinc-iron permease family and located to the cell membrane, acting as a zinc uptake transporter. However, the clinical impacts of SLC39A1 in early-stage hepatocellular carcinoma (EHCC) have not been defined. In this research, we compared the differential expression of SLC39A1 in EHCC and normal tissues based on tissue microarray, and the clinical significance of SLC39A1 in EHCC was evaluated as well. Compared with adjacent tissues, SLC39A1 was remarkably decreased in paired EHCC tissues. Besides, decreased SLC39A1 expression was significantly associated with several clinic-pathological features and serum biochemical indicators. Furthermore, Kaplan-Meier analysis exhibited that both overall survival (OS) and relapse-free survival (RFS) of patients with low expression of SLC39A1 were notably poorer than that of patients with high expression. Moreover, Cox regression analyses revealed that low expression of SLC39A1 was an independent prognostic factor for OS in patients with EHCC. Subgroup analysis also revealed beneficiary populations benefiting from the prognostic evaluation using SLC39A1 expression. Collectively, we summarized that downregulated expression of SLC39A1 is a worse prognostic factor for patients with EHCC, which can be used as a promising diagnostic and prognostic biomarker for EHCC.

Role of Zip1 in the regulation of NPY expression by MLT to promote fracture healing in rats.

Our previous study documented that melatonin (MLT) induced the osteogenic differentiation of mesenchymal stem cells (MSCs) and promoted the healing of femoral fractures in rats via the neuropeptide Y (NPY)/neuropeptide Y1 receptor (NPY1R) signaling pathway. MLT treatment upregulated the expression of the zinc uptake transporter zinc transporter 1 (Zip1) in nerve cells. Prior research demonstrated that oral zinc upregulated NPY expression. MSCs were isolated from rat bone marrow and identified using flow cytometry in our study. The results showed that MLT treatment upregulated NPY and NPY1R levels in MSCs with osteogenic differentiation, which was accompanied by upregulated Zip1 expression. However, the MLT-induced osteogenic differentiation of MSCs was reversed after interference of Zip1 expression. It was confirmed by the decreased alkaline phosphatase (ALP) level; downregulated activities of type I collagen α1 chain (COL1A1), osteocalcin (OCN), runt-related transcription factor 2 (Runx2) and ALP; and reduced mineralized nodule formation. MLT promoted fracture healing in rats with femoral fracture, which was accompanied by increased expression of NPY and NPY1R and significantly increased expression of Zip1. In contrast, the silencing of Zip1 expression reversed MLT-mediated fracture healing. In summary, Zip1 participated in the regulation of the NPY/NPY1R signaling pathway via MLT to promote the osteogenic differentiation of MSCs and fracture healing.

Insulin Secretion by β-Cell-Like Cells Derived from Pulp Stem Cells Depends on Augmented Cytosolic Zinc Levels than GABA Levels

Background: Stem cells harvested from human exfoliated deciduous teeth (SHED) are pluripotent and can be differentiated into insulin-secreting β-cells, i.e., SHED β-cells. Previously, we showed that zinc upregulates insulin secretion from SHED β-cells, potentially providing an extra source for insulin. Rationale: In this study, we determined the role of ionotropic γ-aminobutyric acid A (GABAA) receptor in zinc-enhanced insulin secretion from SHED β-cells. Autocrine/paracrine activation of GABAA receptors by GABA elevates calcium influx in pancreatic β-cells, in which intracellular chloride is maintained at high levels. Method and Findings: Differentiating SHED into SHED β-cells resulted in an increase in the expression of GABAA receptor subunits and Zrt-/irt-like protein3 (ZIP3), a zinc uptake transporter. Zinc pretreatment elevated the insulin gene transcription, whereas knockdown of ZIP3 reduced levels of intracellular zinc, and concomitantly reduced insulin secretion by SHED β-cells. Zinc-pretreated SHED β-cells exhibited a GABA-induced increase in Ca2+ influx, detected with a ratiometric calcium-sensitive dye, suggesting zinc-mediated regulation of GABAA receptors. Conclusion: Our results indicate that elevated levels of zinc and GABAA receptors are indispensable for efficient insulin secretion by SHED β-cells. These findings suggest an opportunity for using SHED β-cells for treating diabetes.

A Potential Zinc Treatment for Patients with Terminal Advanced Laryngeal or Tongue Carcinoma: Clioquinol Zinc Ionophore

Laryngeal and tongue squamous cell carcinomas account for an estimated 37,000 cases and 7,000 deaths/year in the U.S. The localized lesions are generally treated by surgical resection, and/or radiation therapy. When progression to metastases occurs, a systemic chemotherapy is necessary. An efficacious medication has not been available; largely due to the absence of information regarding the development and progression of those malignancies. Consequently, potential targets for successful chemotherapy are not identified. Since decreased zinc has been implicated in the development and treatment of all other carcinomas, it likely exists in laryngeal and tongue squamous cell carcinomas. This report demonstrates that zinc is decreased in these malignancies when compared to the normal cells; and is likely due to the downregulation of their functional ZIP-family zinc-uptake transporter. The reason is that the higher zinc level in the normal cells is cytotoxic in the malignant cells. Therefore, a chemotherapy that restores the higher zinc levels will be cytotoxic in the malignant cells. To achieve this, we employed clioquinol zinc ionophore to facilitate the uptake and accumulation of zinc. The treatment inhibited proliferation of HN4 laryngeal carcinoma cells and HN6 tongue carcinoma cells. That is consistent with other reports that collectively demonstrate that all carcinomas are “ZIP-deficient/decreased zinc” malignancies; and can be terminated by a treatment, such as clioquinol, to facilitate the uptake of cytotoxic levels of zinc. It is now necessary to establish the efficacy of the clioquinol treatment for human laryngeal and tongue tumors under in vivo conditions in xenograft implants in animals; and ultimately in human clinical trials. A case report of the first successful treatment of a patient with terminal advanced prostate carcinoma had included clioquinol. It is reasonable to expect that clioquinol treatment could be the first chemotherapy for terminating tongue and laryngeal squamous cell carcinomas.

Zinc: The Wonder Drug for the Treatment of Carcinomas.

Evidence is evolving that support the relationship that all carcinomas exhibit the following important relationships: The malignant cells exhibit a significant decreased zinc compared to the normal cells. The higher zinc levels that exist in the normal cells are cytotoxic in the malignant cells. The decrease in zinc is due to the down regulation of the ZIP-family zinc uptake transporter. These cells are as "ZIP-deficient/decreased zinc" malignancies. This provides a target for a chemotherapy that can restore the high zinc levels that will manifest cytotoxic effects in the malignant cells. In order to achieve this, a vehicle that facilitates the uptake and accumulation of zinc in the ZIP-deficient cells is required. The zinc ionophore, clioquinol, exhibits the properties that will provide these requirements. This is demonstrated by the treatment of a patient with 3% Clioquinol Cream, which successfully suppressed the progression of androgen-dependent prostate cancer. This treatment should also be efficacious for pancreatic cancer, liver cancer, breast cancer, thyroid cancer, kidney cancer, stomach cancer, gall bladder cancer, and lung cancer; which are carcinomas that exhibit decreased zinc. Thus, it is appropriate to describe that "Zinc is the wonder drug for the treatment of carcinomas".

Biphasic unbinding of a metalloregulator from DNA for transcription (de)repression in Live Bacteria.

Microorganisms use zinc-sensing regulators to alter gene expression in response to changes in the availability of zinc, an essential micronutrient. Under zinc-replete conditions, the Fur-family metalloregulator Zur binds to DNA tightly in its metallated repressor form to Zur box operator sites, repressing the transcription of zinc uptake transporters. Derepression comes from unbinding of the regulator, which, under zinc-starvation conditions, exists in its metal-deficient non-repressor forms having no significant affinity with Zur box. While the mechanism of transcription repression by Zur is well-studied, little is known on how derepression by Zur could be facilitated. Using single-molecule/single-cell measurements, we find that in live Escherichia coli cells, Zur's unbinding rate from DNA is sensitive to Zur protein concentration in a first-of-its-kind biphasic manner, initially impeded and then facilitated with increasing Zur concentration. These results challenge conventional models of protein unbinding being unimolecular processes and independent of protein concentration. The facilitated unbinding component likely occurs via a ternary complex formation mechanism. The impeded unbinding component likely results from Zur oligomerization on chromosome involving inter-protein salt-bridges. Unexpectedly, a non-repressor form of Zur is found to bind chromosome tightly, likely at non-consensus sequence sites. These unusual behaviors could provide functional advantages in Zur's facile switching between repression and derepression.

A role for the Drosophila zinc transporter Zip88E in protecting against dietary zinc toxicity.

Zinc absorption in animals is thought to be regulated in a local, cell autonomous manner with intestinal cells responding to dietary zinc content. The Drosophila zinc transporter Zip88E shows strong sequence similarity to Zips 42C.1, 42C.2 and 89B as well as mammalian Zips 1, 2 and 3, suggesting that it may act in concert with the apically-localised Drosophila zinc uptake transporters to facilitate dietary zinc absorption by importing ions into the midgut enterocytes. However, the functional characterisation of Zip88E presented here indicates that Zip88E may instead play a role in detecting and responding to zinc toxicity. Larvae homozygous for a null Zip88E allele are viable yet display heightened sensitivity to elevated levels of dietary zinc. This decreased zinc tolerance is accompanied by an overall decrease in Metallothionein B transcription throughout the larval midgut. A Zip88E reporter gene is expressed only in the salivary glands, a handful of enteroendocrine cells at the boundary between the anterior and middle midgut regions, and in two parallel strips of sensory cell projections connecting to the larval ventral ganglion. Zip88E expression solely in this restricted subset of cells is sufficient to rescue the Zip88E mutant phenotype. Together, our data suggest that Zip88E may be functioning in a small subset of cells to detect excessive zinc levels and induce a systemic response to reduce dietary zinc absorption and hence protect against toxicity.

Zinc Up-Regulates Insulin Secretion from β Cell-Like Cells Derived from Stem Cells from Human Exfoliated Deciduous Tooth (SHED).

Stem cells from human exfoliated deciduous tooth (SHED) offer several advantages over other stem cell sources. Using SHED, we examined the roles of zinc and the zinc uptake transporter ZIP8 (Zrt- and irt-like protein 8) while inducing SHED into insulin secreting β cell-like stem cells (i.e., SHED-β cells). We observed that ZIP8 expression increased as SHED differentiated into SHED-β cells, and that zinc supplementation at day 10 increased the levels of most pancreatic β cell markers—particularly Insulin and glucose transporter 2 (GLUT2). We confirmed that SHED-β cells produce insulin successfully. In addition, we note that zinc supplementation significantly increases insulin secretion with a significant elevation of ZIP8 transporters in SHED-β cells. We conclude that SHED can be converted into insulin-secreting β cell-like cells as zinc concentration in the cytosol is elevated. Insulin production by SHED-β cells can be regulated via modulation of zinc concentration in the media as ZIP8 expression in the SHED-β cells increases.

A short-term zinc-deficient diet decreases bone formation through down-regulated BMP2 in rat bone.

We investigated the effects of a short-term dietary zinc deficiency on bone metabolism. Zinc deficiency increased the mRNA expression of zinc uptake transporters such as Zip1, Zip13, and Zip14 in bone. However, zinc deficiency might not maintain zinc storage in bone, resulting in a decrease in bone formation through downregulation of the expression levels of osteoblastogenesis-related genes.

Transient Intermittent Hypoxia Exposure Disrupts Neonatal Bone Strength.

A brief intermittent hypoxia (IH, ambient O2 levels alternating between room air and 12% O2) for 1 h immediately after birth resulted in pancreatic islet dysfunction associated with zinc deficiency as previously reported. We hypothesized that IH exposure modulates zinc homeostasis in bone as well, which leads to increased bone fragility. To test this hypothesis, we used neonatal rats and human osteoblasts (HObs). To examine IH influences on osteoblasts devoid of neural influences, we quantified amounts of alkaline phosphatase and mineralization in IH-treated HObs. Bones harvested from IH-treated animals showed significantly reduced hardness and elasticity. The IH group also showed discretely decreased levels of alkaline phosphatase and mineralization amounts. The IH group showed a decreased expression of ZIP8 or Zrt and Irt-like protein 8 (a zinc uptake transporter), Runx2 (or Runt-related transcription factor 2, a master protein in bone formation), Collagen-1 (a major protein comprising the extracellular matrix of the bone), osteocalcin, and zinc content. When zinc was eliminated from the media containing HObs using a zinc chelate and added later with zinc sulfate, Runx2, ZIP8, and osteocalcin expression decreased first, and recovered with zinc supplementation. Adenovirus-mediated ZIP8 over-expression in osteoblasts increased mineralization significantly as well. We conclude that IH impairs zinc homeostasis in bones and osteoblasts, and that such disturbances decrease bone strength, which can be recovered by zinc supplementation.

Zinc Ionophore (Clioquinol) Inhibition of Human ZIP1-Deficient Prostate Tumor Growth in the Mouse Ectopic Xenograft Model: A Zinc Approach for the Efficacious Treatment of Prostate Cancer.

Prostate cancer remains the second leading cause of cancer deaths in males. This is mainly due to the absence of an available efficacious chemotherapy despite decades of research in pursuit of effective treatment approaches. A plausible target for the treatment is the established clinical relationship that the zinc levels in the malignant cells are markedly decreased compared to the normal epithelium in virtually all cases of prostate cancer, and at all stages malignancy. The decrease in zinc results from the downregulation of the functional zinc uptake transporter, ZIP1; which occurs during early development of prostate malignancy. This is an essential requirement for the development of malignancy to prevent the cytotoxic/tumor-suppressor effects of increased zinc on the premalignant and malignant cells. Thus prostate cancer is a ZIP1-deficient malignancy. This relationship provides the basis for a treatment regimen that will facilitate the uptake and accumulation of zinc into the premalignant and malignant cells. In this report we employed a zinc ionophore (clioquinol) approach in the treatment of mice with human ZIP1-deficient prostate tumors (ectopic xenograft model). Clioquinol treatment resulted in 85%inhibition of tumor growth due to the cytotoxic effects of zinc. Coupled with additional results from earlier studies, the compelling evidence provides a plausible approach for the effective treatment of human prostate cancer; including primary site malignancy, hormone-resistant cancer, and metastasis. Additionally, this approach might be effective in preventing the development of malignancy in individuals suspected of presenting with early development of malignancy. Clinical trials are now required in leading to the potential for an efficacious zinc-treatment approach, which is urgently needed for the treatment of prostate cancer.

A role for dZIP89B in Drosophila dietary zinc uptake reveals additional complexity in the zinc absorption process

Dietary zinc is the principal source of zinc in eukaryotes, with its uptake and distribution controlled by a complex network of numerous membrane-spanning transport proteins. Dietary absorption is achieved by members of the SLC39A (ZIP) gene family, which encode proteins that are generally responsible for the movement of zinc into the cytosol. ZIP4 is thought to be the primary mammalian zinc uptake gene in the small intestine, with mutations in this gene causing the zinc deficiency disease Acrodermatitis enteropathica. In Drosophila, dual knockdown of the major dietary zinc uptake genes dZIP42C.1 (dZIP1) and dZIP42C.2 (dZIP2) results in a severe sensitivity to zinc-deficient media. However, the symptoms associated with ZIP4 loss can be reversed by zinc supplementation and dZIP42C.1 and 2 knockdown has minimal effect under normal dietary conditions, suggesting that additional pathways for zinc absorption exist in both mammals and flies. This study provides evidence that dZIP89B is an ideal candidate for this role in Drosophila, encoding a low-affinity zinc uptake transporter active in the posterior midgut. Flies lacking dZIP89B, while viable and apparently healthy, show indications of low midgut zinc levels, including reduced metallothionein B expression and compensatory up-regulation of dZIP42C.1 and 2. Furthermore dZIP89B mutants display a dramatic resistance to toxic dietary zinc levels which is abrogated by midgut-specific restoration of dZIP89B activity. We postulate that dZIP89B works in concert with the closely related dZIP42C.1 and 2 to ensure optimal zinc absorption under a range of dietary conditions.

Evidence that Osteoblasts are Specialized Citrate-producing Cells that Provide the Citrate for Incorporation into the Structure of Bone.

Citrate is a major component of bone in all vertebrates, but its implications in bone have remained largely unknown. Recent studies identified that citrate is incorporated into the structure of the hydroxyapatite nanocrystal/collagen complex; and is essential for the important biomechanical properties of bone. This raises the important question, “What is the source of citrate for incorporation into bone?”; A question that heretofore had remained unresolved. Studies in this report were designed to determine the plausibility of our concept that the osteoblasts are specialized citrate-producing cells, which provide the citrate that is incorporated into the structure of bone; and that osteogenic differentiation of mesenchyme cells leads to the development of the citrate-producing osteoblasts. The results demonstrated that primary human osteoblasts exhibit the capability of citrate-production. Undifferentiated mesenchyme cells do not exhibit the capability of citrate production; and osteogenic differentiation results in citrate-producing osteoblasts. The up-regulation of zinc uptake transporter ZIP1 is essential for the manifestation of the citrate-producing capability of the osteoblasts. We determined that osteoblast transport of citrate from plasma is not a likely source of citrate in bone. Thus, this study establishes for the first time that the osteoblasts are specialized citrate-producing cells that provide the citrate for incorporation into the structure of bone; and that mesenchyme cell osteogenesis leads to differentiated citrate-producing osteoblasts. This is a new understanding; which must include the osteogenic development of citrate-producing osteoblasts, and the process of “citration” in concert with mineralization during bone formation. It also provides a new understanding of the role of bone in the homeostatic maintenance of plasma citrate concentration.

Comparative genomic analysis of slc39a12/ZIP12: insight into a zinc transporter required for vertebrate nervous system development.

The zinc transporter ZIP12, which is encoded by the gene slc39a12, has previously been shown to be important for neuronal differentiation in mouse Neuro-2a neuroblastoma cells and primary mouse neurons and necessary for neurulation during Xenopus tropicalis embryogenesis. However, relatively little is known about the biochemical properties, cellular regulation, or the physiological role of this gene. The hypothesis that ZIP12 is a zinc transporter important for nervous system function and development guided a comparative genetics approach to uncover the presence of ZIP12 in various genomes and identify conserved sequences and expression patterns associated with ZIP12. Ortholog detection of slc39a12 was conducted with reciprocal BLAST hits with the amino acid sequence of human ZIP12 in comparison to the human paralog ZIP4 and conserved local synteny between genomes. ZIP12 is present in the genomes of almost all vertebrates examined, from humans and other mammals to most teleost fish. However, ZIP12 appears to be absent from the zebrafish genome. The discrimination of ZIP12 compared to ZIP4 was unsuccessful or inconclusive in other invertebrate chordates and deuterostomes. Splice variation, due to the inclusion or exclusion of a conserved exon, is present in humans, rats, and cows and likely has biological significance. ZIP12 also possesses many putative di-leucine and tyrosine motifs often associated with intracellular trafficking, which may control cellular zinc uptake activity through the localization of ZIP12 within the cell. These findings highlight multiple aspects of ZIP12 at the biochemical, cellular, and physiological levels with likely biological significance. ZIP12 appears to have conserved function as a zinc uptake transporter in vertebrate nervous system development. Consequently, the role of ZIP12 may be an important link to reported congenital malformations in numerous animal models and humans that are caused by zinc deficiency.

The cytotoxic role of RREB1, ZIP3 zinc transporter, and zinc in human pancreatic adenocarcinoma

Pancreatic cancer (ductal adenocarcinoma) remains a deadly cancer with ~85% mortality, and a 5-year survival rate of ~6% or less for the past 30 years. The factors and events associated with the development of pancreatic cancer are poorly identified. As such, effective biomarkers for early detection of malignancy are lacking. Efficacious chemotherapy once the cancer is identified does not exist. Recent clinical studies have revealed that the zinc levels are consistently and markedly decreased in adenocarcinoma as compared with normal/benign pancreatic tissue. The decreased zinc is exhibited in well-differentiated malignancy and in progressing malignancy, and also exists throughout the development of PanIN. Concurrent with the decrease in zinc, RREB1 transcription factor and ZIP3 zinc uptake transporter are downregulated. Thus, a RREB1/ZIP3/Zinc transformation appears to be an early event in the development of pancreatic cancer. We propose that this transformation is necessary to prevent the accumulation of high cellular zinc levels, which result in cytotoxic effects on the developing malignant cells. This report now demonstrates that exposure of Panc1 cells to physiological concentrations of zinc that result in increased zinc uptake and accumulation also inhibits cell proliferation. The study further shows that ZIP3 is the important transporter required for the accumulation of zinc and its inhibition of proliferation. RREB1 is identified as the positive regulator of ZIP3 expression. Therefore, the pathway of RREB1/ZIP3/Zinc and its downregulation during oncogenesis exist to prevent the accumulation of cytotoxic levels of zinc during the development and progression of the malignant cells in pancreatic adenocarcinoma.

Insulin Production Hampered by Intermittent Hypoxia via Impaired Zinc Homeostasis

Without zinc, pancreatic beta cells cannot either assemble insulin molecules or precipitate insulin crystals; thus, a lack of zinc concentration in the beta cells would result in a decreased insulin production. ZIP8 is one of the zinc uptake transporters involved in zinc influx into the cytosol of beta cells. Thus, if ZIP8 is down-regulated, a decreased insulin production would result. We assumed that intermittent hypoxic exposure to the beta cells may result in a decreased production of insulin due to a lack of zinc. To test this hypothesis we harvested pancreatic islets from the rats conditioned under intermittent hypoxia (IH) (fluctuating between 20.5% and 10% every 4 min for 1 h) and compared the results with those from control animals and islets. We also compared their insulin and glucose homeostasis using glucose tolerance tests (GTT) after 3 weeks. GTT results show a significant delay (P<0.05) in recovery of the blood glucose level in IH treated pups. ZIP8 expression in the beta cell membrane was down-regulated. The zinc concentration in the cell as well as insulin production was significantly decreased in the islets harvested from IH animals. However, mRNA for insulin and C-peptide/insulin protein levels in the total cell lysates remained the same as those of controls. When we treated the beta cells using siRNA mediated ZIP8, we observed the commensurate results from the IH-treated islets. We conclude that a transient IH exposure could knockdown ZIP8 transporters at mRNA as well as protein levels in the beta cells, which would decrease the level of blood insulin. However, the transcriptional activity of insulin remains the same. We conclude that the precipitation process of insulin crystal may be disturbed by a lack of zinc in the cytosol that is modulated by mainly ZIP8 after IH exposure.

Extreme Population Differences in the Human Zinc Transporter ZIP4 (SLC39A4) Are Explained by Positive Selection in Sub-Saharan Africa

Extreme differences in allele frequency between West Africans and Eurasians were observed for a leucine-to-valine substitution (Leu372Val) in the human intestinal zinc uptake transporter, ZIP4, yet no further evidence was found for a selective sweep around the ZIP4 gene (SLC39A4). By interrogating allele frequencies in more than 100 diverse human populations and resequencing Neanderthal DNA, we confirmed the ancestral state of this locus and found a strong geographical gradient for the derived allele (Val372), with near fixation in West Africa. In extensive coalescent simulations, we show that the extreme differences in allele frequency, yet absence of a classical sweep signature, can be explained by the effect of a local recombination hotspot, together with directional selection favoring the Val372 allele in Sub-Saharan Africans. The possible functional effect of the Leu372Val substitution, together with two pathological mutations at the same codon (Leu372Pro and Leu372Arg) that cause acrodermatitis enteropathica (a disease phenotype characterized by extreme zinc deficiency), was investigated by transient overexpression of human ZIP4 protein in HeLa cells. Both acrodermatitis mutations cause absence of the ZIP4 transporter cell surface expression and nearly absent zinc uptake, while the Val372 variant displayed significantly reduced surface protein expression, reduced basal levels of intracellular zinc, and reduced zinc uptake in comparison with the Leu372 variant. We speculate that reduced zinc uptake by the ZIP4-derived Val372 isoform may act by starving certain pathogens of zinc, and hence may have been advantageous in Sub-Saharan Africa. Moreover, these functional results may indicate differences in zinc homeostasis among modern human populations with possible relevance for disease risk.

Zinc and zinc transporters in prostate carcinogenesis

The healthy human prostate accumulates the highest level of zinc of any soft tissue in the body. This unique property is retained in BPH, but is lost in prostatic malignancy, which implicates changes in zinc and its transporters in carcinogenesis. Indeed, zinc concentrations diminish early in the course of prostate carcinogenesis, preceding histopathological changes, and continue to decline during progression toward castration-resistant disease. Numerous studies suggest that increased zinc intake might protect against progression of prostatic malignancy. In spite of increased dietary intake, zinc accumulation might be limited by the diminished expression of zinc uptake transporters, resulting in decreased intratumoural zinc levels. This finding can explain the conflicting results of various epidemiological studies evaluating the role of zinc supplementation on primary and secondary prostate cancer prevention. Overall, more research into the mechanisms of zinc homeostasis are needed to fully understand its impact on prostate carcinogenesis. Only then can the potential of zinc and zinc transport proteins be harnessed in the diagnosis and treatment of men with prostate cancer.