Zinc fluxes during acute and chronic exposure of INS-1E cells to increasing glucose levels

The efficiency of insulin production and its content in insulin-expressing model β-cells correlate with their Zn2+ levels

Insulin is produced and stored inside the pancreatic β-cell secretory granules, where it is assumed to form Zn2+-stabilized oligomers. However, the actual storage forms of this hormone and the impact of zinc ions on insulin production in vivo are not known. Our initial X-ray fluorescence experiment on granules from native Langerhans islets and insulinoma-derived INS-1E cells revealed a considerable difference in the zinc content. This led our further investigation to evaluate the impact of the intra-granular Zn2+ levels on the production and storage of insulin in different model β-cells. Here, we systematically compared zinc and insulin contents in the permanent INS-1E and BRIN-BD11 β-cells and in the native rat pancreatic islets by flow cytometry, confocal microscopy, immunoblotting, specific messenger RNA (mRNA) and total insulin analysis. These studies revealed an impaired insulin production in the permanent β-cell lines with the diminished intracellular zinc content. The drop in insulin and Zn2+ levels was paralleled by a lower expression of ZnT8 zinc transporter mRNA and hampered proinsulin processing/folding in both permanent cell lines. To summarize, we showed that the disruption of zinc homeostasis in the model β-cells correlated with their impaired insulin and ZnT8 production. This indicates a need for in-depth fundamental research about the role of zinc in insulin production and storage.

Elemental Analysis of Aging Human Pituitary Glands Implicates Mercury as a Contributor to the Somatopause

Background: Growth hormone levels often decline on aging, and this "somatopause" is associated with muscle and bone loss, visceral adiposity and impaired cardiovascular function. Mercury has been detected in human pituitary glands, so to see if mercury could play a part in the somatopause we measured the proportion of people at different ages who had mercury in their anterior pituitary cells. Materials and methods: Paraffin sections of pituitary glands taken at autopsy from 94 people between the ages of 2 and 99 years were stained for inorganic mercury using autometallography. Pituitary mercury content was classified as none, low (<30% of cells) or high (>30% of cells) in increasing two-decade age groups. Autometallography combined with immunohistochemistry determined which hormone-producing cells contained mercury. Laser ablation-inductively coupled plasma-mass spectrometry was used to confirm the presence of mercury. Results: The proportion of people with low-content pituitary mercury remained between 33 and 42% at all ages. The proportion of people with high-content mercury increased with increasing age, from 0% of people in the 2-20 year group to a peak of 50% of people in the 61-80 years group, followed by a fall to 35% of people in the 81-99 years group. Mercury, when present, was found always in somatotrophs, occasionally in corticotrophs, rarely in thyrotrophs and gonadotrophs, and never in lactotrophs. Laser ablation-inductively coupled plasma-mass spectrometry detected mercury in regions of pituitaries that stained with autometallography. Conclusions: The proportion of people with mercury in their anterior pituitary cells, mostly somatotrophs, increases with aging, suggesting that mercury toxicity could be one factor contributing to the decline in growth hormone levels found in advancing age.

Prolonged stimulation of insulin release from MIN6 cells causes zinc depletion and loss of β-cell markers

Zinc is integral for the normal function of pancreatic β-cells in glycaemic control. Large amounts of zinc are secreted from β-cells following insulin exocytosis and regulated replenishment is required, which is thought to be mediated by the ZIP family of zinc importer proteins. Within Type 2 Diabetic patients, β-cells are stressed through prolonged stimulation by hyperglycaemia and this is thought to be a major factor contributing to loss of β-cell identity and mass. However, the consequences for the β-cell zinc status remain largely unexplored. We used inductively coupled plasma mass spectrometry (ICP-MS) to show that 24 h treatment of MIN6 cells with potassium chloride, mimicking hyperglycaemic stimulation, reduces the total cellular zinc content 2.8-fold, and qPCR to show an increase in mRNA expression for metallothioneins (Mt1 and Mt2) following 4 and 24 h of stimulation, suggestive of an early rise in cytosolic zinc. To determine which ZIP paralogues may be responsible for zinc replenishment, we used immunocytochemistry, Western blot and qPCR to demonstrate initial ZIP1 protein upregulation proceeded by downregulation of mRNA coding for ZIP1, ZIP6, ZIP7 and ZIP14. To assign a biological significance to the decreased total cellular zinc content, we assessed expression of key β-cell markers to show downregulation of mRNA for MafA, Mnx-1, Nkx2.2 and Pax6. Our data suggest hyperglycaemia-induced zinc depletion may contribute to loss of β-cell markers and promote β-cell dedifferentiation through disrupting expression of key transcription factors.

Zinc and its transporters, pancreatic β-cells, and insulin metabolism

Zinc is an essential trace metal for life. Two families of zinc transporters, SLC30A (ZNT) and SLC39A (ZIP) are required for maintaining cellular zinc homeostasis. ZNTs function to decrease cytoplasmic zinc concentrations whereas ZIPs do the opposite. Expression of zinc transporters can be tissue/cell-type specific or ubiquitous. Zinc transporters that are limited in tissue/cell distributions usually perform specialized tasks to satisfy biological processes in a given cell. For example, ZNT8 is mainly expressed in β-cells and functions to deliver zinc into granules for insulin maturation and secretion. Many other zinc transporters are also expressed in β-cells. Defects in these zinc transporters have been associated with abnormalities in insulin synthesis, maturation, and secretion and subsequent glucose metabolism. This review focuses on the specific roles of zinc and its transporters in insulin metabolism and describes the current knowledge of the function of zinc transporters in β-cell health in animal knockout mouse models with respect to diabetes development in humans.

Increased level of exogenous zinc induces cytotoxicity and up-regulates the expression of the ZnT-1 zinc transporter gene in pancreatic cancer cells. J Nutr Biochem. 2011;22:79-88. [PMID: 20392624 DOI: 10.1016/j.jnutbio.2009.12.001]

A balance between zinc uptake by ZIP (SLC39) and efflux of zinc from the cytoplasm into subcellular organelles and out of the cell by ZnT (SLC30) transporters is crucial for zinc homeostasis. It is not clear whether normal and cancerous pancreatic cells respond differently to increased extracellular zinc concentrations. Use of flow cytometry-based methods revealed that treatment with as little as 0.01 mM zinc induced significant cytotoxicity in two human ductal adenocarcinoma cell lines. In contrast, normal human pancreatic islet cells tolerated as high as 0.5 mM zinc. Insulinoma cell lines of mouse and rat origin also succumbed to high concentrations of zinc. Exposure to elevated zinc concentrations enhanced the numbers of carcinoma but not primary islet cells staining with the cell-permeable zinc-specific fluorescent dye, FluoZin-3, indicating increased zinc influx in transformed cells. Mitochondrial membrane depolarization, superoxide generation, decreased antioxidant thiols, intracellular acidosis and activation of intracellular caspases characterized zinc-induced carcinoma cell death. Only the antioxidant glutathione but not inhibitors of enzymes implicated in apoptosis or necrosis prevented zinc-induced cytotoxicity in insulinoma cells. Immunoblotting revealed that zinc treatment increased the ubiquitination of proteins in cancer cells. Importantly, zinc treatment up-regulated the expression of ZnT-1 gene in a rat insulinoma cell line and in two human ductal adenocarcinoma cell lines. These results indicate that the exposure of pancreatic cancer cells to elevated extracellular zinc concentrations can lead to cytotoxic cell death characterized by increased protein ubiquitination and up-regulation of the zinc transporter ZnT-1 gene expression.

Subtle modification of 2,2-dipicolylamine lowers the affinity and improves the turn-on of Zn(II)-selective fluorescent sensors

The spectroscopic and proton- and Zn(II)-binding properties of two new members of the Zinpyr family of fluorescent sensors are reported. In ZP1B and ZP3B, a (2-picolyl)(4-picolyl)amine (2,4-DPA) moiety is installed in place of the di(2-picolyl)amine (2,2-DPA) ligand used in the parent sensors ZP1 and ZP3. This modification has the benefit of both lowering the proton-induced turn-on at physiological pH levels and altering the Zn(II) affinity so as to detect only the most concentrated stores of this ion in biological samples. Comparison of the proton affinities of all four probes, as determined by potentiometric titrations, contributes to our understanding of the solution properties of this family of sensors.

SLC30A3 responds to glucose- and zinc variations in beta-cells and is critical for insulin production and in vivo glucose-metabolism during beta-cell stress

BACKGROUND

Ion transporters of the Slc30A- (ZnT-) family regulate zinc fluxes into sub-cellular compartments. beta-cells depend on zinc for both insulin crystallization and regulation of cell mass.

METHODOLOGY/PRINCIPAL FINDINGS

This study examined: the effect of glucose and zinc chelation on ZnT gene and protein levels and apoptosis in beta-cells and pancreatic islets, the effects of ZnT-3 knock-down on insulin secretion in a beta-cell line and ZnT-3 knock-out on glucose metabolism in mice during streptozotocin-induced beta-cell stress. In INS-1E cells 2 mM glucose down-regulated ZnT-3 and up-regulated ZnT-5 expression relative to 5 mM. 16 mM glucose increased ZnT-3 and decreased ZnT-8 expression. Zinc chelation by DEDTC lowered INS-1E insulin content and insulin expression. Furthermore, zinc depletion increased ZnT-3- and decreased ZnT-8 gene expression whereas the amount of ZnT-3 protein in the cells was decreased. Zinc depletion and high glucose induced apoptosis and necrosis in INS-1E cells. The most responsive zinc transporter, ZnT-3, was investigated further; by immunohistochemistry and western blotting ZnT-3 was demonstrated in INS-1E cells. 44% knock-down of ZnT-3 by siRNA transfection in INS-1E cells decreased insulin expression and secretion. Streptozotocin-treated mice had higher glucose levels after ZnT-3 knock-out, particularly in overt diabetic animals.

CONCLUSION/SIGNIFICANCE

Zinc transporting proteins in beta-cells respond to variations in glucose and zinc levels. ZnT-3, which is pivotal in the development of cellular changes as also seen in type 2 diabetes (e.g. amyloidosis in Alzheimer's disease) but not previously described in beta-cells, is present in this cell type, up-regulated by glucose in a concentration dependent manner and up-regulated by zinc depletion which by contrast decreased ZnT-3 protein levels. Knock-down of the ZnT-3 gene lowers insulin secretion in vitro and affects in vivo glucose metabolism after streptozotocin treatment.

Down-regulation of ZnT8 expression in INS-1 rat pancreatic beta cells reduces insulin content and glucose-inducible insulin secretion

The SLC30A8 gene codes for a pancreatic beta-cell-expressed zinc transporter, ZnT8. A polymorphism in the SLC30A8 gene is associated with susceptibility to type 2 diabetes, although the molecular mechanism through which this phenotype is manifest is incompletely understood. Such polymorphisms may exert their effect via impacting expression level of the gene product. We used an shRNA-mediated approach to reproducibly downregulate ZnT8 mRNA expression by >90% in the INS-1 pancreatic beta cell line. The ZnT8-downregulated cells exhibited diminished uptake of exogenous zinc, as determined using the zinc-sensitive reporter dye, zinquin. ZnT8-downregulated cells showed reduced insulin content and decreased insulin secretion (expressed as percent of total insulin content) in response to hyperglycemic stimulus, as determined by insulin immunoassay. ZnT8-depleted cells also showed fewer dense-core vesicles via electron microscopy. These data indicate that reduced ZnT8 expression in cultured pancreatic beta cells gives rise to a reduced insulin response to hyperglycemia. In addition, although we provide no direct evidence, these data suggest that an SLC30A8 expression-level polymorphism could affect insulin secretion and the glycemic response in vivo.

Zinc fluxes and zinc transporter genes in chronic diseases

The group IIb metal zinc (Zn) is an essential dietary component that can be found in protein rich foods such as meat, seafood and legumes. Thousands of genes encoding Zn binding proteins were identified, especially after the completion of genome projects, an indication that a great number of biological processes are Zn dependent. Imbalance in Zn homeostasis was found to be associated with several chronic diseases such as asthma, diabetes and Alzheimer's disease. As it is now evident for most nutrients, body Zn status results from the interaction between diet and genotype. Zn ions cross biological membranes with the aid of specialized membrane proteins, belonging to the ZRT/IRT-related Proteins (ZIP) and zinc transporters (ZnT) families. The ZIPs are encoded by the Slc39A gene family and are responsible for uptake of the metal, ZnTs are encoded by the Slc30A genes and are involved in intracellular traffic and/or excretion. Both ZnTs and Zips exhibit unique tissue-specific expression, differential responsiveness to dietary Zn deficiency and excess, as well as to physiological stimuli via hormones and cytokines. Intracellular Zn concentration is buffered by metallothioneins (MTs), a class of cytosolic protein with high affinity for metals. Scattered information is available on the role of proteins responsible for regulating Zn fluxes in the onset and progression of chronic diseases. This paper reviews reports that link Zn transporter genes, their allelic variants and/or expression profiles in the context of specific diseases. Further investigation in this direction is very important, since Zn imbalance can result not only from insufficient dietary intake, but also from impaired activity of proteins that regulate Zn metabolism, thus contributing to multifactorial diseases.

In vivo expression and functional characterization of the zinc transporter ZnT8 in glucose-induced insulin secretion

Insulin-secreting pancreatic beta cells are exceptionally rich in zinc. In these cells, zinc is required for zinc-insulin crystallization within secretory vesicles. Secreted zinc has also been proposed to be a paracrine and autocrine modulator of glucagon and insulin secretion in pancreatic alpha and beta cells, respectively. However, little is known about the molecular mechanisms underlying zinc accumulation in insulin-containing vesicles. We previously identified a pancreas-specific zinc transporter, ZnT-8, which colocalized with insulin in cultured beta cells. In this paper we studied its localization in human pancreatic islet cells, and its effect on cellular zinc content and insulin secretion. In human pancreatic islet cells, ZnT-8 was exclusively expressed in insulin-producing beta cells, and colocalized with insulin in these cells. ZnT-8 overexpression stimulated zinc accumulation and increased total intracellular zinc in insulin-secreting INS-1E cells. Furthermore, ZnT-8-overexpressing cells display enhanced glucose-stimulated insulin secretion compared with control cells, only for a high glucose challenge, i.e. >10 mM glucose. Altogether, these data strongly suggest that the zinc transporter ZnT-8 is a key protein for both zinc accumulation and regulation of insulin secretion in pancreatic beta cells.

Silver enhancement of quantum dots resulting from (1) metabolism of toxic metals in animals and humans, (2) in vivo, in vitro and immersion created zinc–sulphur/zinc–selenium nanocrystals, (3) metal ions liberated from metal implants and particles

Autometallographic (AMG) silver enhancement is a potent histochemical tool for tracing a variety of metal containing nanocrystals, e.g. pure gold and silver nanoclusters and quantum dots of silver, mercury, bismuth or zinc, with sulphur and/or selenium. These nanocrystals can be created in many different ways, e.g. (1) by manufacturing colloidal gold or silver particles, (2) by treating an organism in vivo with sulphide or selenide ions, (3) as the result of a metabolic decomposition of bismuth-, mercury- or silver-containing macromolecules in cell organelles, or (4) as the end product of histochemical processing of tissue sections. Such nano-sized AMG nanocrystals can then be silver-amplified several times of magnitude by being exposed to an AMG developer, i.e. a normal photographic developer enriched with silver ions. The present monograph attempts to provide a review of the autometallographic silver amplification techniques known today and their use in biology. After achieving a stronghold in histochemistry by Timm's introduction of the "silver-sulphide staining" in 1958, the AMG technique has evolved and expanded into several different areas of research, including immunocytochemistry, tracing of enzymes at LM and EM levels, blot staining, retrograde axonal tracing of zinc-enriched (ZEN) neurons, counterstaining of semithin sections, enhancement of histochemical reaction products, marking of phagocytotic cells, staining of myelin, tracing of gold ions released from gold implants, and visualization of capillaries. General technical comments, protocols for the current AMG methods and a summary of the most significant scientific results obtained by this wide variety of AMG histochemical approaches are included in the present article.