Zinc and zinc transporter regulation in pancreatic islets and the potential role of zinc in islet transplantation

Metal ion content of internal organs in the calorically restricted Wistar rat

BACKGROUND

Despite the agreed principle that access to food is a human right, undernourishment and metal ion deficiencies are public health problems worldwide, exacerbated in impoverished or war-affected areas. It is known that maternal malnutrition causes growth retardation and affects behavioral and cognitive development of the newborn. Here we ask whether severe caloric restriction leads per se to disrupted metal accumulation in different organs of the Wistar rat.

METHODS

Inductively coupled plasma optical emission spectroscopy was used to determine the concentration of multiple elements in the small and large intestine, heart, lung, liver, kidney, pancreas, spleen, brain, spinal cord, and three skeletal muscles from control and calorically restricted Wistar rats. The caloric restriction protocol was initiated from the mothers prior to mating and continued throughout gestation, lactation, and post-weaning up to sixty days of age.

RESULTS

Both sexes were analyzed but dimorphism was rare. The pancreas was the most affected organ presenting a higher concentration of all the elements analyzed. Copper concentration decreased in the kidney and increased in the liver. Each skeletal muscle responded to the treatment differentially: Extensor Digitorum Longus accumulated calcium and manganese, gastrocnemius decreased copper and manganese, whereas soleus decreased iron concentrations. Differences were also observed in the concentration of elements between organs independently of treatment: The soleus muscle presents a higher concentration of Zn compared to the other muscles and the rest of the organs. Notably, the spinal cord showed large accumulations of calcium and half the concentration of zinc compared to brain. X-ray fluorescence imaging suggests that the extra calcium is attributable to the presence of ossifications whereas the latter finding is attributable to the low abundance of zinc synapses in the spinal cord.

CONCLUSION

Severe caloric restriction did not lead to systemic metal deficiencies but caused instead specific metal responses in few organs.

Recurrent hypoglycemia dampens functional regulation mediated via Neurexin-1, Neuroligin-2 and Mint-1 docking proteins: Intensified complications during diabetes

Glycemic regulation is important for maintaining critical physiological functions. Extreme variation in levels of circulating glucose are known to affect insulin secretion. Elevated insulin levels result in lowering of circulating glycemic levels culminating into hypoglycemia. Recurrence of hypoglycemia are often noted owing to fasting conditions, untimely meals, irregular dietary consumption, or as a side-effect of disease pathophysiology. Such events of hypoglycemia threaten to hamper the patterns of insulin secretion in diabetic condition. Insulin vesicle docking is a prerequisite phase which ensures anchoring of the vesicles to the β-cell membrane in order to expel the insulin cargo. Neurexin and Neuroligin are the marker docking proteins which assists in the tethering of the insulin granules to the secretory membrane. However, these cell adhesion molecules indirectly affect the glycemic levels by regulating insulin secretion. The effect of extreme levels of glycemic fluctuations on these molecules, and how it affects the docking machinery remains obscure. Our current study demonstrates down-regulated expression of Neurexin-1, Neuroligin-2 and Mint-1 molecules during hyperglycemia, hypoglycemia and diabetic hypoglycemia in rodents as well as for an in-vitro system using MIN6 cell-line. Studies with fluorescently labelled insulin revealed presence of lessened functional insulin secretory granules, concomitant with the alterations in morphology and as a result of hypoglycemia in control and diabetic condition which was found to be further deteriorating. Our studies indicate towards a feeble vesicular anchorage, which may partly be responsible for dwindled insulin secretion during diabetes. However, hypoglycemia poses as a potent diabetic complication in further deteriorating the docking machinery. To the best of our knowledge this is the first report which demonstrates the effect of hypoglycemic events in affecting insulin secretion by weakening insulin vesicular anchorage in normal and diabetic individuals.

Zinc Homeostasis Alters Zinc Transporter Protein Expression in Vascular Endothelial and Smooth Muscle Cells

INTRODUCTION

Zinc is an important essential micronutrient with anti-oxidative and anti-inflammatory properties in humans. The role of zinc in signalling has been characterized in the nervous, endocrine, gastrointestinal, renal and reproductive systems. Relatively little is known regarding its role in the vascular system, but the role of zinc homeostasis in augmenting vascular health and vasorelaxation is emerging. Zinc transport proteins are integral to the protective function of zinc, but knowledge of their expression in vascular endothelial and smooth muscle cells is lacking.

METHODOLOGY

Human coronary artery endothelial cells and pulmonary artery smooth muscle cells were assessed for gene expression (RT-PCR) of SLC39A (ZIP), SLC30A (ZnT) and metallothionein (MT) families of Zn transporters and storage proteins. Protein expression (fluorescence confocal microscopy) was then analysed for the proteins of interest that changed mRNA expression: ZIP2, ZIP12, ZnT1, ZnT2 and MT1/2.

RESULTS

Endothelial and smooth muscle cell mRNA expression of ZnT1, ZnT2 and MT1 was significantly downregulated by low and high Zn conditions, while ZIP2 and ZIP12 expression was induced by Zn depletion with the Zn chelator, TPEN. Changes in gene expression were consistent with protein expression levels for ZIP2, ZIP12 and MT1, where ZIP2 was localized to intracellular bodies and ZIP12 to lamellipodia.

CONCLUSION

Vascular endothelial and smooth muscle cells actively regulate specific Zn transport and metallothionein gene and protein expressions to achieve Zn homeostasis.

Zinc oxide nanoparticles modulate the gene expression of ZnT1 and ZIP8 to manipulate zinc homeostasis and stress-induced cytotoxicity in human neuroblastoma SH-SY5Y cells

Zinc ions (Zn²⁺) are important messenger molecules involved in various physiological functions. To maintain the homeostasis of cytosolic Zn²⁺ concentration ([Zn²⁺]_c), Zrt/Irt-related proteins (ZIPs) and Zn²⁺ transporters (ZnTs) are the two families of proteins responsible for decreasing and increasing the [Zn²⁺]_c, respectively, by fluxing Zn²⁺ across the membranes of the cell and intracellular compartments in opposite directions. Most studies focus on the cytotoxicity incurred by a high concentration of [Zn²⁺]_c and less investigate the [Zn²⁺]_c at physiological levels. Zinc oxide-nanoparticle (ZnO-NP) is blood brain barrier-permeable and elevates the [Zn²⁺]_c to different levels according to the concentrations of ZnO-NP applied. In this study, we mildly elevated the [Zn²⁺]_c by ZnO-NP at concentrations below 1 μg/ml, which had little cytotoxicity, in cultured human neuroblastoma SH-SY5Y cells and characterized the importance of Zn²⁺ transporters in 6-hydroxy dopamine (6-OHDA)-induced cell death. The results show that ZnO-NP at low concentrations elevated the [Zn²⁺]_c transiently in 6 hr, then declined gradually to a basal level in 24 hr. Knocking down the expression levels of ZnT₁ (located mostly at the plasma membrane) and ZIP₈ (present in endosomes and lysosomes) increased and decreased the ZnO-NP-induced elevation of [Zn²⁺]_c, respectively. ZnO-NP treatment reduced the basal levels of reactive oxygen species and Bax/Bcl-2 mRNA ratios; in addition, ZnO-NP decreased the 6-OHDA-induced ROS production, p53 expression, and cell death. These results show that ZnO-NP-induced mild elevation in [Zn²⁺]_c activates beneficial effects in reducing the 6-OHDA-induced cytotoxic effects. Therefore, brain-delivery of ZnO-NP can be regarded as a potential therapy for neurodegenerative diseases.

Zinc deficiency alters the susceptibility of pancreatic beta cells (INS-1) to arsenic exposure

Pancreatic beta cells produce and release insulin, a hormone that regulates blood glucose levels, and their dysfunction contributes to the development of diabetes mellitus. Zinc deficiency and inorganic arsenic exposure both independently associate with the development of diabetes, although the effects of their combination on pancreatic beta cell health and function remain unknown. We hypothesized zinc deficiency increases the toxicity associated with arsenic exposure, causing an increased susceptibility to DNA damage and disruption of insulin production. Zinc deficiency decreased cell proliferation by 30% in pancreatic INS-1 rat insulinoma cells. Arsenic exposure (0, 50 or 500 ppb exposures) significantly decreased cell proliferation, and increased mRNA levels of genes involved in stress response (Mt1, Mt2, Hmox1) and DNA damage (p53, Ogg1). When co-exposed to both zinc deficiency and arsenic, zinc deficiency attenuated this response to arsenic, decreasing the expression of Mt1, Hmox1, and Ogg1, and significantly increasing DNA double-strand breaks 2.9-fold. Arsenic exposure decreased insulin expression, but co-exposure did not decrease insulin levels beyond the arsenic alone condition, but did result in a further 33% decline in cell proliferation at the 500 ppb arsenic dose, and a significant increase in beta cell apoptosis. These results suggest zinc deficiency and arsenic, both independently and in combination, adversely affect pancreatic beta cell health and both factors should be considered in the evaluation of health outcomes for susceptible populations.

Protective effects of Chinese Fenggang zinc selenium tea on metabolic syndrome in high-sucrose-high-fat diet-induced obese rats

The protective effect of zinc selenium tea against metabolic syndrome (MetS) was tested by using a high-sucrose-high-fat diet (HSHFD)-induced MetS model. Fifty Sprague–Dawley rats were randomly divided into five groups: normal diet (C-group), HSHFD (CH-group), HSHFD + green tea (0.24 g/kg/day) (TH-group), HSHFD + low-dose zinc selenium organic tea (0.24 g/kg/day) (ZTHL-group), and HSHFD + high-dose zinc selenium organic tea (1.20 g/kg/day) (ZTHH-group). After 8 weeks, compared to both the C-group and CH-group, the hepatosomatic index (HI) was significantly reduced in the ZTHL-group (p < 0.05). Fasting blood glucose (FBG) levels were highest in the TH-group, followed by the CH-group, then the ZTHL-group, then the ZTHH-group, and finally the C-group. Compared with the CH-group, the serum total cholesterol (TC) and low density lipid-cholesterol (LDL-C) concentrations were significantly lower in the ZTHH-group (p < 0.05). Significant decreases in serum alanine aminotransferase (ALT), aspartate aminotransferase (AST), total bile acids (TBA), alkaline phosphatase (ALP), and direct bilirubin (DBIL) levels were observed in ZTHL-group versus the CH-group (p < 0.05). Serum alpha-L-fucosidase (AFU) levels in the ZTHH-group were lower than in the CH-group (P < 0.01). Histopathological examination of the liver and fat biopsies illustrates that the liver cells showed a decrease in the extent of necrosis and dropsy in the ZTHL-group and ZTHH-group versus the CH-group. Zinc selenium tea showed a protection effect against hepatic damage.

Zinc Supplementation Improves Glucose Homeostasis in High Fat-Fed Mice by Enhancing Pancreatic β-Cell Function

Zinc is an essential component of the insulin granule and it possibly modulates insulin secretion and signaling. Since insulin resistance is a hallmark in the development of type 2 diabetes mellitus, this study aimed at investigating if zinc supplementation is able to improve glucose tolerance and β-cell function in a model of insulin resistance. Male C57BL/6 mice were distributed in four groups according to the diet: normal fat (NF); normal fat supplemented with ZnCl₂ (NFZ); high-fat (HF); and, high-fat chow supplemented with ZnCl₂ (HFZ). Intraperitoneal glucose (ipGTT) and insulin (ipITT) tolerance, glycemia, insulinemia, HOMA-IR, and HOMA-β were determined after 15 weeks in each diet. Glucose-stimulated insulin secretion (GSIS) was investigated in isolated islets. The insulin effect on glucose uptake, metabolism, and signaling was investigated in soleus muscle. ZnCl₂ did not affect body mass or insulin sensitivity as assessed by ipITT, HOMA-IR, muscle glucose metabolism, and Akt and GSK3-β phosphorylation. However, glucose tolerance, HOMA-β, and GSIS were significantly improved by ZnCl₂ supplementation. Therefore, ZnCl₂ supplementation improves glucose homeostasis in high fat-fed mice by a mechanism that enhances β-cell function, rather than whole-body or muscle insulin sensitivity.

Expression of the ZIP/SLC39A transporters in β-cells: a systematic review and integration of multiple datasets

BACKGROUND

Pancreatic β-cells require a constant supply of zinc to maintain normal insulin secretory function. Following co-exocytosis with insulin, zinc is replenished via the Zrt- and Irt-like (ZIP; SLC39A) family of transporters. However the ZIP paralogues of particular importance for zinc uptake, and associations with β-cell function and Type 2 Diabetes remain largely unexplored. We retrieved and statistically analysed publically available microarray and RNA-seq datasets to perform a systematic review on the expression of β-cell SLC39A paralogues. We complemented results with experimental data on expression profiling of human islets and mouse β-cell derived MIN6 cells, and compared transcriptomic and proteomic sequence conservation between human, mouse and rat.

RESULTS

The 14 ZIP paralogues have 73-98% amino sequence conservation between human and rodents. We identified 18 datasets for β-cell SLC39A analysis, which compared relative expression to non-β-cells, and expression in response to PDX-1 activity, cytokines, glucose and type 2 diabetic status. Published expression data demonstrate enrichment of transcripts for ZIP7 and ZIP9 transporters within rodent β-cells and of ZIP6, ZIP7 and ZIP14 within human β-cells, with ZIP1 most differentially expressed in response to cytokines and PDX-1 within rodent, and ZIP6 in response to diabetic status in human and glucose in rat. Our qPCR expression profiling data indicate that SLC39A6, -9, -13, and - 14 are the highest expressed paralogues in human β-cells and Slc39a6 and -7 in MIN6 cells.

CONCLUSIONS

Our systematic review, expression profiling and sequence alignment reveal similarities and potentially important differences in ZIP complements between human and rodent β-cells. We identify ZIP6, ZIP7, ZIP9, ZIP13 and ZIP14 in human and rodent and ZIP1 in rodent as potentially biologically important for β-cell zinc trafficking. We propose ZIP6 and ZIP7 are key functional orthologues in human and rodent β-cells and highlight these zinc importers as important targets for exploring associations between zinc status and normal physiology of β-cells and their decline in Type 2 Diabetes.

Label-Free Dynamic Mass Redistribution and Bio-Impedance Methods for Drug Discovery

Label-free biosensors are increasingly employed in drug discovery. Cell-based biosensors provide valuable insights into the biological consequences of exposing cells and tissues to chemical agents and the underlying molecular mechanisms associated with these effects. Optical biosensors based on the detection of dynamic mass redistribution (DMR) and impedance biosensors using cellular dielectric spectroscopy (CDS) capture changes of the cytoskeleton of living cells in real time. Because signal transduction correlates with changes in cell morphology, DMR and CDS biosensors are exquisitely suited for recording integrated cell responses in an unbiased, yet pathway-specific manner without the use of labels that may interfere with cell function. Described in this unit are several experimental approaches utilizing optical label-free system capturing dynamic mass redistribution (DMR) in living cells (Epic System) and an impedance-based CDS technology (CellKey). In addition, potential pitfalls associated with these assays and alternative approaches for overcoming such technical challenges are discussed. © 2017 by John Wiley & Sons, Inc.

β Cells that Resist Immunological Attack Develop during Progression of Autoimmune Diabetes in NOD Mice

Type 1 diabetes (T1D) is a chronic autoimmune disease that involves immune-mediated destruction of β cells. How β cells respond to immune attack is unknown. We identified a population of β cells during the progression of T1D in non-obese diabetic (NOD) mice that survives immune attack. This population develops from normal β cells confronted with islet infiltrates. Pathways involving cell movement, growth and proliferation, immune responses, and cell death and survival are activated in these cells. There is reduced expression of β cell identity genes and diabetes antigens and increased immune inhibitory markers and stemness genes. This new subpopulation is resistant to killing when diabetes is precipitated with cyclophosphamide. Human β cells show similar changes when cultured with immune cells. These changes may account for the chronicity of the disease and the long-term survival of β cells in some patients.

Type 1 diabetes in Sardinia: facts and hypotheses in the context of worldwide epidemiological data

Type 1 diabetes (T1D) results from an autoimmune destruction of insulin-producing beta cells that requires lifelong insulin treatment. While significant advances have been achieved in treatment, prevention of complications and quality of life in diabetic people, the identification of environmental triggers of the disease is far more complex. The island of Sardinia has the second highest incidence of T1D in the world (45/100,000), right after Finland (64.2/100,000). The genetic background as well as the environment of the island's inhabitants makes it an ideal region for investigating environmental, immunological and genetic factors related to the etiopathogenesis of T1D. Several epidemiological studies, conducted over the years, have shown that exposures to important known environmental risk factors have changed over time, including nutritional factors, pollution, chemicals, toxins and infectious diseases in early life. These environmental risk factors might be involved in T1D pathogenesis, as they might initiate autoimmunity or accelerate and precipitate an already ongoing beta cell destruction. In terms of environmental factors, Sardinia is also particular in terms of the incidence of infection with Mycobacterium avium paratuberculosis (MAP) that recent studies have linked to T1D in the Sardinian population. Furthermore, the unique geochemical profile of Sardinia, with its particular density of heavy metals, leads to the assumption that exposure of the Sardinian population to heavy metals could also affect T1D incidence. These factors lead us to hypothesize that T1D incidence in Sardinia may be affected by the exposure to multifactorial agents, such as MAP, common viruses and heavy metals.

NADPH oxidase-2 mediates zinc deficiency-induced oxidative stress and kidney damage

Zn²⁺ deficiency (ZnD) is comorbid with chronic kidney disease and worsens kidney complications. Oxidative stress is implicated in the detrimental effects of ZnD. However, the sources of oxidative stress continue to be identified. Since NADPH oxidases (Nox) are the primary enzymes that contribute to renal reactive oxygen species generation, this study's objective was to determine the role of these enzymes in ZnD-induced oxidative stress. We hypothesized that ZnD promotes NADPH oxidase upregulation, resulting in oxidative stress and kidney damage. To test this hypothesis, wild-type mice were pair-fed a ZnD or Zn²⁺-adequate diet. To further investigate the effects of Zn²⁺ bioavailability on NADPH oxidase regulation, mouse tubular epithelial cells were exposed to the Zn²⁺ chelator N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine (TPEN) or vehicle followed by Zn²⁺ supplementation. We found that ZnD diet-fed mice develop microalbuminuria, electrolyte imbalance, and whole kidney hypertrophy. These markers of kidney damage are accompanied by elevated Nox2 expression and H₂O₂ levels. In mouse tubular epithelial cells, TPEN-induced ZnD stimulates H₂O₂ generation. In this in vitro model of ZnD, enhanced H₂O₂ generation is prevented by NADPH oxidase inhibition with diphenyleneiodonium. Specifically, TPEN promotes Nox2 expression and activation, which are reversed when intracellular Zn²⁺ levels are restored following Zn²⁺ supplementation. Finally, Nox2 knockdown by siRNA prevents TPEN-induced H₂O₂ generation and cellular hypertrophy in vitro. Together, these findings reveal that Nox2 is a Zn²⁺-regulated enzyme that mediates ZnD-induced oxidative stress and kidney hypertrophy. Understanding the specific mechanisms by which ZnD contributes to kidney damage may have an important impact on the treatment of chronic kidney disease.

Aluminum-doped zinc oxide nanoparticles attenuate the TSLP levels via suppressing caspase-1 in activated mast cells

Zinc oxide nanoparticles (ZO-NPs) are used as antimicrobials, anti-inflammatories, and to treat cancer. However, although ZO-NPs have excellent efficiency and specificity, their cytotoxicity is higher than that of micron-sized zinc oxide. Doping ZO-NPs with aluminum can improve therapeutic efficacy, but the biological effects and mechanisms involved have not been elucidated. Here, we reported the efficacy of aluminum-doped ZO-NP (AZO) on thymic stromal lymphopoietin (TSLP) production and caspase-1 activation in human mast cell line, HMC-1 cells. AZO significantly reduced TSLP levels as well as interleukin (IL)-6, IL-8, and tumor necrosis factor (TNF)-α without inducing cytotoxicity. Furthermore, AZO more effectively reduced TSLP, IL-6, IL-8, and TNF-α levels than ZO-NP. The levels of inflammatory cytokine mRNA were also reduced by AZO treatment. AZO blocked production of IL-1β and activations of caspase-1 and nuclear factor-κB by inhibiting IκB kinase β and receptor interacting protein 2. In addition, AZO attenuated phosphorylation of mitogen-activated protein kinases, such as extracellular signal-regulated kinase, c-Jun N-terminal kinases, and p38. These findings provide evidence that AZO improves anti-inflammatory properties and offer a safe and effective potential treatment option.

Autocrine effect of Zn²⁺ on the glucose-stimulated insulin secretion

It is well known that zinc (Zn(2+)) is required for the process of insulin biosynthesis and the maturation of insulin secretory granules in pancreatic beta (β)-cells, and that changes in Zn(2+) levels in the pancreas have been found to be associated with diabetes. Glucose-stimulation causes a rapid co-secretion of Zn(2+) and insulin with similar kinetics. However, we do not know whether Zn(2+) regulates insulin availability and secretion. Here we investigated the effect of Zn(2+) on glucose-stimulated insulin secretion (GSIS) in isolated mouse pancreatic islets. Whereas Zn(2+) alone (control) had no effect on the basal secretion of insulin, it significantly inhibited GSIS. The application of CaEDTA, by removing the secreted Zn(2+) from the extracellular milieu of the islets, resulted in significantly increased GSIS, suggesting an overall inhibitory role of secreted Zn(2+) on GSIS. The inhibitory action of Zn(2+) was mostly mediated through the activities of KATP/Ca(2+) channels. Furthermore, during brief paired-pulse glucose-stimulated Zn(2+) secretion (GSZS), Zn(2+) secretion following the second pulse was significantly attenuated, probably by the secreted endogenous Zn(2+) after the first pulse. Such an inhibition on Zn(2+) secretion following the second pulse was completely reversed by Zn(2+) chelation, suggesting a negative feedback mechanism, in which the initial glucose-stimulated Zn(2+) release inhibits subsequent Zn(2+) secretion, subsequently inhibiting insulin co-secretion as well. Taken together, these data suggest a negative feedback mechanism on GSZS and GSIS by Zn(2+) secreted from β-cells, and the co-secreted Zn(2+) may act as an autocrine inhibitory modulator.

Type 1 diabetes epidemic in Finland is triggered by zinc-containing amorphous silica nanoparticles

Type 1 diabetes (T1D), an autoimmune disease, breaks out in some of the children who has genetic susceptibility to T1D. Besides genetic susceptibility some environmental factor(s) are required to trigger the disease. The incidence of T1D in Finland is highest in the world, so we must seek an environmental factor, that is typical for Finland and can declare many aspects of T1D epidemiology and biology. In the literature most popular trigger has been enterovirus infections. It is difficult however to find why enteroviruses would be in this role in Finland in contrary to neighbouring countries e.g. Sweden. Colloidal amorphous silica (ASi) is typical for Finnish environment in consequency of the geohistory of Finland, great part of Finland is an ancient lake and sea bottom. ASi concentrations in natural waters are high in April-June and in November, only traces can be found in the rest of months. Pure colloidal ASi is not a strong trigger for T1D, but ASi particle which has surface adsorbed tetrahedrally coordinated zinc (ASiZn) is probably the trigger which has kept it's secret up to date. Zn functions as address label which conducts the ASiZn particle to the beta cell, whose content of zinc is highest in the body. ASi particle adheres to membrane proteins distorting their tertiary structure revealing new epitopes. If the fetus has not met these epitopes at proper time during intrauterine development, the consequence is that the negative selection of lymphocytes in the thymus and bone marrow and fetal liver is not perfect. When a child later in postnatal life becomes predisposed to ASiZn particles the immune system reacts to these as to nonself proteins. As a consequence the insulin producing beta cells are destroyed. Many observations from diabetes research support the hypothesis, some to mentioned. 1. Three common autoantigens (ZnT8, ICA512/IA-2, GAD65) are membrane proteins whose function zinc regulates. 2. Geographical variation in Finland is convergent with surface water manganese concentrations. Manganese is the principal Zn scavenger and high manganese in water reduces ASiZn particle formation and the incidence of T1D. 3. The incidence of T1D depends of drinking water pH. The highest incidence can be found within water pH 6.2-6.9. Zn coordination changes from octahedral (unphysiologic) to tetrahedral (physiologic) at pH 6.56. In the text are presented five more supporting observations e.g. the similarity between the soils in Sardinia and Finland in respect to ASi.

Hypoxia lowers SLC30A8/ZnT8 expression and free cytosolic Zn2+ in pancreatic beta cells

AIMS/HYPOTHESIS

Hypoxic damage complicates islet isolation for transplantation and may contribute to beta cell failure in type 2 diabetes. Polymorphisms in the SLC30A8 gene, encoding the secretory granule zinc transporter 8 (ZnT8), influence type 2 diabetes risk, conceivably by modulating cytosolic Zn(2+) levels. We have therefore explored the role of ZnT8 and cytosolic Zn(2+) in the response to hypoxia of pancreatic islet cells.

METHODS

Human, mouse or rat islets were isolated and exposed to varying O2 tensions. Cytosolic free zinc was measured using the adenovirally expressed recombinant targeted zinc probe eCALWY4. Gene expression was measured using quantitative (q)RT-PCR, western (immuno-) blotting or immunocytochemistry. Beta cells were identified by insulin immunoreactivity.

RESULTS

Deprivation of O2 (1% vs 5% or 21%) for 24 h lowered free cytosolic Zn(2+) concentrations by ~40% (p < 0.05) and ~30% (p < 0.05) in mouse and human islet cells, respectively. Hypoxia similarly decreased SLC30A8 mRNA expression in islets, and immunoreactivity in beta cells. Implicating lowered ZnT8 levels in the hypoxia-induced fall in cytosolic Zn(2+), genetic ablation of Slc30a8 from mouse islets lowered cytosolic Zn(2+) by ~40% (p < 0.05) and decreased the induction of metallothionein (Mt1, Mt2) genes. Cell survival in the face of hypoxia was enhanced in small islets of older (>12 weeks) Slc30a8 null mice vs controls, but not younger animals.

CONCLUSIONS/INTERPRETATION

The response of pancreatic beta cells to hypoxia is characterised by decreased SLC30A8 expression and lowered cytosolic Zn(2+) concentrations. The dependence on ZnT8 of hypoxia-induced changes in cell survival may contribute to the actions of SLC30A8 variants on diabetes risk in humans.

Molecular pathogenesis of spondylocheirodysplastic Ehlers-Danlos syndrome caused by mutant ZIP13 proteins

The zinc transporter protein ZIP13 plays critical roles in bone, tooth, and connective tissue development, and its dysfunction is responsible for the spondylocheirodysplastic form of Ehlers-Danlos syndrome (SCD-EDS, OMIM 612350). Here, we report the molecular pathogenic mechanism of SCD-EDS caused by two different mutant ZIP13 proteins found in human patients: ZIP13(G64D), in which Gly at amino acid position 64 is replaced by Asp, and ZIP13(ΔFLA), which contains a deletion of Phe-Leu-Ala. We demonstrated that both the ZIP13(G64D) and ZIP13(ΔFLA) protein levels are decreased by degradation via the valosin-containing protein (VCP)-linked ubiquitin proteasome pathway. The inhibition of degradation pathways rescued the protein expression levels, resulting in improved intracellular Zn homeostasis. Our findings uncover the pathogenic mechanisms elicited by mutant ZIP13 proteins. Further elucidation of these degradation processes may lead to novel therapeutic targets for SCD-EDS.

Dietary zinc supplementation to the donor improves insulin secretion after islet transplantation in chemically induced diabetic rats

OBJECTIVES

Zinc (Zn) is related to insulin synthesis, storage, and secretion. This study demonstrates the effects of Zn supplementation in donor rats on the outcomes of islet transplantation.

METHODS

Donor rats received 3 different regimens of dietary Zn supplementation for 2 weeks before undergoing pancreas donation: a standard diet containing Zn at 50 ppm (control), 1 ppm (low-Zn group) or 1000 ppm (high-Zn group), respectively. Diabetic recipient rats underwent islet transplantation, and the blood glucose levels and insulin secretion were monitored for 7 days after transplantation.

RESULTS

The serum and pancreatic Zn levels at the time of donation were significantly lower in the low-Zn group (48.8 ± 25.5 µg/dL and 11.3 ± 1.9 µg/g) and higher in the high-Zn group (147.3 ± 17.6 µg/dL and 18.7 ± 2.2 µg/g) when compared with those observed in the controls (118.7 ± 7.9 µg/dL and 14.6 ± 2.0 µg/g) (P < 0.05). The blood glucose levels became re-elevated 2 days after transplantation in rats receiving islet grafts from the controls and the low-Zn groups. In contrast, in the rats that received islets from the high-Zn groups, these were maintained within a reference range (P < 0.01).

CONCLUSIONS

These data indicate that a Zn-rich diet for donor rats improves the function of islet grafts in chemically induced diabetic rats.

Zinc and insulin in pancreatic beta-cells

Zinc (Zn2+) is an essential element crucial for growth and development, and also plays a role in cell signaling for cellular processes like cell division and apoptosis. In the mammalian pancreas, Zn2+ is essential for the correct processing, storage, secretion, and action of insulin in beta (β)-cells. Insulin is stored inside secretory vesicles or granules, where two Zn2+ ions coordinate six insulin monomers to form the hexameric-structure on which maturated insulin crystals are based. The total Zn2+ content of the mammalian pancreas is among the highest in the body, and Zn2+ concentration reach millimolar levels in the interior of the dense-core granule. Changes in Zn2+ levels in the pancreas have been found to be associated with diabetes. Hence, the relationship between co-stored Zn2+ and insulin undoubtedly is critical to normal β-cell function. The advances in the field of Zn2+ biology over the last decade have facilitated our understanding of Zn2+ trafficking, its intracellular distribution and its storage. When exocytosis of insulin occurs, insulin granules fuse with the β-cell plasma membrane and release their contents, i.e., insulin as well as substantial amount of free Zn2+, into the extracellular space and the local circulation. Studies increasingly indicate that secreted Zn2+ has autocrine or paracrine signaling in β-cells or the neighboring cells. This review discusses the Zn2+ homeostasis in β-cells with emphasis on the potential signaling role of Zn2+ to islet biology.

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.

A correlation study between multiple sclerosis and type 1 diabetes incidences and geochemical data in Europe

Complex multifactorial disorders usually arise in individuals genetically at risk in the presence of permissive environmental factors. For many of these diseases, predisposing gene variants are partly known while the identification of the environmental component is much more difficult. This study aims to investigate whether there are correlations between the incidence of two complex traits, multiple sclerosis and type 1 diabetes, and some chemical elements and compounds present in soils and stream sediments in Europe. Data were obtained from the published literature and analyzed by calculating the mean values of each element and of disease incidence for each Country, respectively, 17 for multiple sclerosis and 21 for type 1 diabetes. Correlation matrices and regression analyses were used in order to compare incidence data and geochemical data. R correlation index and significance were evaluated. The analyses performed in this study have revealed significant positive correlations between barium and sodium oxide on one hand and multiple sclerosis and diabetes incidences on the other hand that may suggest interactions to be evaluated between silicon-rich lithologies and/or marine environments. The negative correlations shown by cobalt, chromium and nickel (typical of silicon-poor environment), which in this case can be interpreted as protective effects against the two diseases onset, make the split between favorable and protective environments even more obvious. In conclusion, if other studies will confirm the involvement of the above elements and compounds in the etiology of these pathologies, then it will be possible to plan strategies to reduce the spread of these serious pandemics.

Influence of vanadium-organic ligands treatment on selected metal levels in kidneys of STZ rats

The objective of the study was to investigate the effects of five organic vanadium complexes supplement and a small dose of insulin injection on V, Fe, Cu, Zn, Mn, Ca, and K level in the streptozotocin diabetic rat's kidney during a 5-week treatment with the tested complexes. In all groups of animals, metal level in the lyophilized kidney organs was investigated by means of the proton induced X-ray emission method. Tissue vanadium level was naturally higher in vanadium-treated rats. The maximum level of vanadium was observed in the kidney (x(mean) = 16.6 μg/g). The influence of vanadium administration on other metal level in rat's tissue was also investigated. Spectacular influence of vanadium action was observed on copper and zinc level in examined tissue.

Protective antioxidant and antiapoptotic effects of ZnCl2 in rat pancreatic islets cultured in low and high glucose concentrations

Aim/Hypothesis

Rat pancreatic islet cell apoptosis is minimal after prolonged culture in 10 mmol/l glucose (G10), largely increased in 5 mmol/l glucose (G5) and moderately increased in 30 mmol/l glucose (G30). This glucose-dependent asymmetric V-shaped profile is preceded by parallel changes in the mRNA levels of oxidative stress-response genes like Metallothionein 1a (Mt1a). In this study, we tested the effect of ZnCl₂, a potent inducer of Mt1a, on apoptosis, mitochondrial oxidative stress and alterations of glucose-induced insulin secretion (GSIS) induced by prolonged exposure to low and high vs. intermediate glucose concentrations.

Methods

Male Wistar rat islets were cultured in RPMI medium. Islet gene mRNA levels were measured by RTq-PCR. Apoptosis was quantified by measuring islet cytosolic histone-associated DNA fragments and the percentage of TUNEL-positive β-cells. Mitochondrial thiol oxidation was measured in rat islet cell clusters expressing “redox sensitive GFP” targeted to the mitochondria (mt-roGFP1). Insulin secretion was measured by RIA.

Results

As observed for Mt1a mRNA levels, β-cell apoptosis and loss of GSIS, culture in either G5 or G30 vs. G10 significantly increased mt-roGFP1 oxidation. While TPEN decreased Mt1a/2a mRNA induction by G5, addition of 50–100 µM ZnCl₂ to the culture medium strongly increased Mt1a/2a mRNA and protein levels, reduced early mt-roGFP oxidation and significantly decreased late β-cell apoptosis after prolonged culture in G5 or G30 vs. G10. It did not, however, prevent the loss of GSIS under these culture conditions.

Conclusion

ZnCl₂ reduces mitochondrial oxidative stress and improves rat β-cell survival during culture in the presence of low and high vs. intermediate glucose concentrations without improving their acute GSIS.