GDF15 Protects Insulin-Producing Beta Cells against Pro-Inflammatory Cytokines and Metabolic Stress via Increased Deamination of Intracellular Adenosine.

It has been proposed that antidiabetic drugs, such as metformin and imatinib, at least in part, promote improved glucose tolerance in type 2 diabetic patients via increased production of the inflammatory cytokine GDF15. This is supported by studies, performed in rodent cell lines and mouse models, in which the addition or production of GDF15 improved beta-cell function and survival. The aim of the present study was to determine whether human beta cells produce GDF15 in response to antidiabetic drugs and, if so, to further elucidate the mechanisms by which GDF15 modulates the function and survival of such cells. The effects and expression of GDF15 were analyzed in human insulin-producing EndoC-betaH1 cells and human islets. We observed that alpha and beta cells exhibit considerable heterogeneity in GDF15 immuno-positivity. The predominant form of GDF15 present in islet and EndoC-betaH1 cells was pro-GDF15. Imatinib, but not metformin, increased pro-GDF15 levels in EndoC-betaH1 cells. Under basal conditions, exogenous GDF15 increased human islet oxygen consumption rates. In EndoC-betaH1 cells and human islets, exogenous GDF15 partially ameliorated cytokine- or palmitate + high-glucose-induced loss of function and viability. GDF15-induced cell survival was paralleled by increased inosine levels, suggesting a more efficient disposal of intracellular adenosine. Knockdown of adenosine deaminase, the enzyme that converts adenosine to inosine, resulted in lowered inosine levels and loss of protection against cytokine- or palmitate + high-glucose-induced cell death. It is concluded that imatinib-induced GDF15 production may protect human beta cells partially against inflammatory and metabolic stress. Furthermore, it is possible that the GDF15-mediated activation of adenosine deaminase and the increased disposal of intracellular adenosine participate in protection against beta-cell death.

The selective NOX4 inhibitor GLX7013159 decreases blood glucose concentrations and human beta-cell apoptotic rates in diabetic NMRI nu/nu mice transplanted with human islets.

NADPH oxidase 4 (NOX4) inhibition has been reported to mitigate diabetes-induced beta-cell dysfunction and improve survival in vitro, as well as counteract high-fat diet-induced glucose intolerance in mice. We investigated the antidiabetic effects of the selective NOX4 inhibitor GLX7013159 in vivo in athymic diabetic mice transplanted with human islets over a period of 4 weeks. The GLX7013159-treated mice achieved lower blood glucose and water consumption throughout the treatment period. Furthermore, GLX7013159 treatment resulted in improved insulin and c-peptide levels, better insulin secretion capacity, as well as in greatly reduced apoptotic rates of the insulin-positive human cells, measured as colocalization of insulin and cleaved caspase-3. We conclude that the antidiabetic effects of NOX4 inhibition by GLX7013159 are observed also during a prolonged study period in vivo and are likely to be due to an improved survival and function of the human beta-cells.

CLEC11A improves insulin secretion and promotes cell proliferation in human beta-cells.

Beta-cell dysfunction is a hallmark of disease progression in patients with diabetes. Research has been focused on maintaining and restoring beta-cell function during diabetes development. The aims of this study were to explore the expression of C-type lectin domain containing 11A (CLEC11A), a secreted sulphated glycoprotein, in human islets and to evaluate the effects of CLEC11A on beta-cell function and proliferation in vitro. To test these hypotheses, human islets and human EndoC-ßH1 cell line were used in this study. We identified that CLEC11A was expressed in beta-cells and alpha-cells in human islets but not in EndoC-ßH1 cells, whereas the receptor of CLEC11A called integrin subunit alpha 11 was found in both human islets and EndoC-ßH1 cells. Long-term treatment with exogenous recombinant human CLEC11A (rhCLEC11A) accentuated glucose-stimulated insulin secretion, insulin content, and proliferation from human islets and EndoC-ßH1 cells, which was partially due to the accentuated expression levels of transcription factors MAFA and PDX1. However, the impaired beta-cell function and reduced mRNA expression of INS and MAFA in EndoC-ßH1 cells that were caused by chronic palmitate exposure could only be partially improved by the introduction of rhCLEC11A. Based on these results, we conclude that rhCLEC11A promotes insulin secretion, insulin content, and proliferation in human beta-cells, which are associated with the accentuated expression levels of transcription factors MAFA and PDX1. CLEC11A, therefore, may provide a novel therapeutic target for maintaining beta-cell function in patients with diabetes.

Metabolic stress-induced human beta-cell death is mediated by increased intracellular levels of adenosine.

Introduction: High intracellular concentrations of adenosine and 2'-deoxyadenosine have been suggested to be an important mediator of cell death. The aim of the present study was to characterize adenosine-induced death in insulin-producing beta-cells, at control and high glucose + palmitate-induced stress conditions. Methods: Human insulin-producing EndoC-betaH1 cells were treated with adenosine, 2'-deoxyadenosine, inosine and high glucose + sodium palmitate, and death rates using flow cytometry were studied. Results: We observed that adenosine and the non-receptor-activating analogue 2-deoxyadenosine, but not the adenosine deamination product inosine, promoted beta-cell apoptosis at concentrations exceeding maximal adenosine-receptor stimulating concentrations. Both adenosine and inosine were efficiently taken up by EndoC-betaH1 cells, and inosine counteracted the cell death promoting effect of adenosine by competing with adenosine for uptake. Both adenosine and 2'-deoxyadenosine promptly reduced insulin-stimulated production of plasma membrane PI(3,4,5)P3, an effect that was reversed upon wash out of adenosine. In line with this, adenosine, but not inosine, rapidly diminished Akt phosphorylation. Both pharmacological Bax inhibition and Akt activation blocked adenosine-induced beta-cell apoptosis, indicating that adenosine/2'-deoxyadenosine inhibits the PI3K/Akt/BAD anti-apoptotic pathway. High glucose + palmitate-induced cell death was paralleled by increased intracellular adenosine and inosine levels. Overexpression of adenosine deaminase-1 (ADA1) in EndoC-betaH1 cells, which increased Akt phosphorylation, prevented both adenosine-induced apoptosis and high glucose + palmitate-induced necrosis. ADA2 overexpression not only failed to protect against adenosine and high glucose + palmitate-activated cell death, but instead potentiated the apoptosis-stimulating effect of adenosine. In line with this, ADA1 overexpression increased inosine production from adenosine-exposed cells, whereas ADA2 did not. Knockdown of ADA1 resulted in increased cell death rates in response to both adenosine and high glucose + palmitate. Inhibition of miR-30e-3p binding to the ADA1 mRNA 3'-UTR promoted the opposite effects on cell death rates and reduced intracellular adenosine contents. Discussion: It is concluded that intracellular adenosine/2'-deoxyadenosine regulates negatively the PI3K pathway and is therefore an important mediator of beta-cell apoptosis. Adenosine levels are controlled, at least in part, by ADA1, and strategies to upregulate ADA1 activity, during conditions of metabolic stress, could be useful in attempts to preserve beta-cell mass in diabetes.

Are off-target effects of imatinib the key to improving beta-cell function in diabetes?

The small tyrosine kinase (TK) inhibitor imatinib mesylate (Gleevec, STI571) protects against both type 1 and type 2 diabetes, but as it inhibits many TKs and other proteins, it is not clear by which mechanisms it acts. This present review will focus on the possibility that imatinib acts, at least in part, by improving beta-cell function and survival via off-target effects on beta-cell signaling/metabolic flow events. Particular attention will be given to the possibility that imatinib and other TK inhibitors function as inhibitors of mitochondrial respiration. A better understanding of how imatinib counteracts diabetes will possibly help to clarify the pathogenic role of beta-cell signaling events and mitochondrial function, and hopefully leading to improved treatment of the disease.

Pharmacological Inhibition of NOX4 Improves Mitochondrial Function and Survival in Human Beta-Cells.

Previous studies have reported beneficial effects of NADPH oxidase 4 (NOX4) inhibition on beta-cell survival in vitro and in vivo. The mechanisms by which NOX4 inhibition protects insulin producing cells are, however, not known. The aim of the present study was to investigate the effects of a pharmacological NOX4 inhibitor (GLX7013114) on human islet and EndoC-ßH1 cell mitochondrial function, and to correlate such effects with survival in islets of different size, activity, and glucose-stimulated insulin release responsiveness. We found that maximal oxygen consumption rates, but not the rates of acidification and proton leak, were increased in islets after acute NOX4 inhibition. In EndoC-ßH1 cells, NOX4 inhibition increased the mitochondrial membrane potential, as estimated by JC-1 fluorescence; mitochondrial reactive oxygen species (ROS) production, as estimated by MitoSOX fluorescence; and the ATP/ADP ratio, as assessed by a bioluminescent assay. Moreover, the insulin release from EndoC-ßH1 cells at a high glucose concentration increased with NOX4 inhibition. These findings were paralleled by NOX4 inhibition-induced protection against human islet cell death when challenged with high glucose and sodium palmitate. The NOX4 inhibitor protected equally well islets of different size, activity, and glucose responsiveness. We conclude that pharmacological alleviation of NOX4-induced inhibition of beta-cell mitochondria leads to increased, and not decreased, mitochondrial ROS, and this was associated with protection against cell death occurring in different types of heterogeneous islets. Thus, NOX4 inhibition or modulation may be a therapeutic strategy in type 2 diabetes that targets all types of islets.

Imatinib protects against human beta-cell death via inhibition of mitochondrial respiration and activation of AMPK.

The protein tyrosine kinase inhibitor imatinib is used in the treatment of various malignancies but may also promote beneficial effects in the treatment of diabetes. The aim of the present investigation was to characterize the mechanisms by which imatinib protects insulin producing cells. Treatment of non-obese diabetic (NOD) mice with imatinib resulted in increased beta-cell AMP-activated kinase (AMPK) phosphorylation. Imatinib activated AMPK also in vitro, resulting in decreased ribosomal protein S6 phosphorylation and protection against islet amyloid polypeptide (IAPP)-aggregation, thioredoxin interacting protein (TXNIP) up-regulation and beta-cell death. 5-Aminoimidazole-4-carboxamide ribonucleotide (AICAR) mimicked and compound C counteracted the effect of imatinib on beta-cell survival. Imatinib-induced AMPK activation was preceded by reduced glucose/pyruvate-dependent respiration, increased glycolysis rates, and a lowered ATP/AMP ratio. Imatinib augmented the fractional oxidation of fatty acids/malate, possibly via a direct interaction with the beta-oxidation enzyme enoyl coenzyme A hydratase, short chain, 1, mitochondrial (ECHS1). In non-beta cells, imatinib reduced respiratory chain complex I and II-mediated respiration and acyl-CoA carboxylase (ACC) phosphorylation, suggesting that mitochondrial effects of imatinib are not beta-cell specific. In conclusion, tyrosine kinase inhibitors modestly inhibit mitochondrial respiration, leading to AMPK activation and TXNIP down-regulation, which in turn protects against beta-cell death.

ZBED6 counteracts high-fat diet-induced glucose intolerance by maintaining beta cell area and reducing excess mitochondrial activation.

AIMS/HYPOTHESIS: ZBED6 (zinc finger, BED-type containing 6) is known to regulate muscle mass by suppression of Igf2 gene transcription. In insulin-producing cell lines, ZBED6 maintains proliferative capacity at the expense of differentiation and beta cell function. The aim was to study the impact of Zbed6 knockout on beta cell function and glucose tolerance in C57BL/6 mice. METHODS: Beta cell area and proliferation were determined in Zbed6 knockout mice using immunohistochemical analysis. Muscle and fat distribution were assessed using micro-computed tomography. Islet gene expression was assessed by RNA sequencing. Effects of a high-fat diet were analysed by glucose tolerance and insulin tolerance tests. ZBED6 was overexpressed in EndoC-ßH1 cells and human islet cells using an adenoviral vector. Beta cell cell-cycle analysis, insulin release and mitochondrial function were studied in vitro using propidium iodide staining and flow cytometry, ELISA, the Seahorse technique, and the fluorescent probes JC-1 and MitoSox. RESULTS: Islets from Zbed6 knockout mice showed lowered expression of the cell cycle gene Pttg1, decreased beta cell proliferation and decreased beta cell area, which occurred independently from ZBED6 effects on Igf2 gene expression. Zbed6 knockout mice, but not wild-type mice, developed glucose intolerance when given a high-fat diet. The high-fat diet Zbed6 knockout islets displayed upregulated expression of oxidative phosphorylation genes and genes associated with beta cell differentiation. In vitro, ZBED6 overexpression resulted in increased EndoC-ßH1 cell proliferation and a reduced glucose-stimulated insulin release in human islets. ZBED6 also reduced mitochondrial JC-1 J-aggregate formation, mitochondrial oxygen consumption rates (OCR) and mitochondrial reactive oxygen species (ROS) production, both at basal and palmitate + high glucose-stimulated conditions. ZBED6-induced inhibition of OCR was not rescued by IGF2 addition. ZBED6 reduced levels of the mitochondrial regulator PPAR-γ related coactivator 1 protein (PRC) and bound its promoter/enhancer region. Knockdown of PRC resulted in a lowered OCR. CONCLUSIONS/INTERPRETATION: It is concluded that ZBED6 is required for normal beta cell replication and also limits excessive beta cell mitochondrial activation in response to an increased functional demand. ZBED6 may act, at least in part, by restricting PRC-mediated mitochondrial activation/ROS production, which may lead to protection against beta cell dysfunction and glucose intolerance in vivo.

The importance of the ZBED6-IGF2 axis for metabolic regulation in mouse myoblast cells.

The transcription factor ZBED6 acts as a repressor of Igf2 and affects directly or indirectly the transcriptional regulation of thousands of genes. Here, we use gene editing in mouse C2C12 myoblasts and show that ZBED6 regulates Igf2 exclusively through its binding site 5'-GGCTCG-3' in intron 1 of Igf2. Deletion of this motif (Igf2ΔGGCT ) or complete ablation of Zbed6 leads to ~20-fold upregulation of the IGF2 protein. Quantitative proteomics revealed an activation of Ras signaling pathway in both Zbed6-/- and Igf2ΔGGCT myoblasts, and a significant enrichment of mitochondrial membrane proteins among proteins showing altered expression in Zbed6-/- myoblasts. Both Zbed6-/- and Igf2ΔGGCT myoblasts showed a faster growth rate and developed myotube hypertrophy. These cells exhibited an increased O2 consumption rate, due to IGF2 upregulation. Transcriptome analysis revealed ~30% overlap between differentially expressed genes in Zbed6-/- and Igf2ΔGGCT myotubes, with an enrichment of upregulated genes involved in muscle development. In contrast, ZBED6-overexpression in myoblasts led to cell apoptosis, cell cycle arrest, reduced mitochondrial activities, and ceased myoblast differentiation. The similarities in growth and differentiation phenotypes observed in Zbed6-/- and Igf2ΔGGCT myoblasts demonstrates that ZBED6 affects mitochondrial activity and myogenesis largely through its regulation of IGF2 expression. This study adds new insights how the ZBED6-Igf2 axis affects muscle metabolism.

The protein synthesis inhibitor brusatol normalizes high-fat diet-induced glucose intolerance in male C57BL/6 mice: role of translation factor eIF5A hypusination.

The naturally occurring quassinoid compound brusatol improves the survival of insulin-producing cells when exposed to the proinflammatory cytokines IL-1ß and IFN-γ in vitro. The aim of the present study was to investigate whether brusatol also promotes beneficial effects in mice fed a high-fat diet (HFD), and if so, to study the mechanisms by which brusatol acts. In vivo, we observed that the impaired glucose tolerance of HFD-fed male C57BL/6 mice was counteracted by a 2 wk treatment with brusatol. Brusatol treatment improved both ß-cell function and peripheral insulin sensitivity of HFD-fed mice. In vitro, brusatol inhibited ß-cell total protein and proinsulin biosynthesis, with an ED50 of ∼40 nM. In line with this, brusatol blocked cytokine-induced iNOS protein expression via inhibition of iNOS mRNA translation. Brusatol may have affected protein synthesis, at least in part, via inhibition of eukaryotic initiation factor 5A (eIF5A) hypusination, as eIF5A spermidine association and hypusination in RIN-5AH cells was reduced in a dose- and time-dependent manner. The eIF5A hypusination inhibitor GC7 promoted a similar effect. Both brusatol and GC7 protected rat RIN-5AH cells against cytokine-induced cell death. Brusatol reduced eIF5A hypusination and cytokine-induced cell death in EndoC-ßH1 cells as well. Finally, hypusinated eIF5A was reduced in vivo by brusatol in islet endocrine and endothelial islet cells of mice fed an HFD. The results of the present study suggest that brusatol improves glucose intolerance in mice fed an HFD, possibly by inhibiting protein biosynthesis and eIF5A hypusination.-Turpaev, K., Krizhanovskii, C., Wang, X., Sargsyan, E., Bergsten, P., Welsh, N. The protein synthesis inhibitor brusatol normalizes high-fat diet-induced glucose intolerance in male C57BL/6 mice: role of translation factor eIF5A hypusination.

ZBED6 negatively regulates insulin production, neuronal differentiation, and cell aggregation in MIN6 cells.

Zinc finger BED domain containing protein 6 ( Zbed6) has evolved from a domesticated DNA transposon and encodes a transcription factor unique to placental mammals. The aim of the present study was to investigate further the role of ZBED6 in insulin-producing cells, using mouse MIN6 cells, and to evaluate the effects of Zbed6 knockdown on basal ß-cell functions, such as morphology, transcriptional regulation, insulin content, and release. Zbed6-silenced cells and controls were characterized with a range of methods, including RNA sequencing, chromatin immunoprecipitation sequencing, insulin content and release, subplasma membrane Ca2+ measurements, cAMP determination, and morphologic studies. More than 700 genes showed differential expression in response to Zbed6 knockdown, which was paralleled by increased capacity to generate cAMP, as well as by augmented subplasmalemmal calcium concentration and insulin secretion in response to glucose stimulation. We identified >4000 putative ZBED6-binding sites in the MIN6 genome, with an enrichment of ZBED6 sites at upregulated genes, such as the ß-cell transcription factors v-maf musculoaponeurotic fibrosarcoma oncogene homolog A and Nk6 homeobox 1. We also observed altered morphology/growth patterns, as indicated by increased cell clustering, and in the appearance of axon-like Neurofilament, medium polypeptide and tubulin ß 3, class III-positive protrusions. We conclude that ZBED6 acts as a transcriptional regulator in MIN6 cells and that its activity suppresses insulin production, cell aggregation, and neuronal-like differentiation.-Wang, X., Jiang, L., Wallerman, O., Younis, S., Yu, Q., Klaesson, A., Tengholm, A., Welsh, N., Andersson, L. ZBED6 negatively regulates insulin production, neuronal differentiation, and cell aggregation in MIN6 cells.

The novel NADPH oxidase 4 selective inhibitor GLX7013114 counteracts human islet cell death in vitro.

It has been proposed that pancreatic beta-cell dysfunction in type 2 diabetes is promoted by oxidative stress caused by NADPH oxidase (Nox) over-activity. The aim of the present study was to evaluate the efficacy of novel Nox inhibitors as protective agents against cytokine- or high glucose + palmitate-induced human beta-cell death. The Nox2 protein was present mainly in the cytoplasm and was induced by cytokines. Nox4 protein immunoreactivity, with some nuclear accumulation, was observed in human islet cells, and was not affected by islet culture in the presence of cytokines or high glucose + palmitate. Nox inhibitors with partial or no isoform selectivity (DPI, dapsone, GLX351322, and GLX481372) all reduced ROS production of human islet cells exposed to high glucose + palmitate. This was paralleled by improved viability and reduced caspase 3 activation. The Nox1 selective inhibitor ML171 failed to reduce human islet cell death in response to both cytokines and high glucose + palmitate. The selective Nox2 inhibitor Phox-I2 also failed to protect against cytokines, but protected partially against high glucose + palmitate-induced cellular death. The highly selective Nox4 inhibitor GLX7013114 protected islet cells against both cytokines and high glucose + palmitate. However, as no osmotic control for high glucose was used, we cannot exclude the possibility that the high glucose effect was due to osmosis. It is concluded that Nox4 may participate in stress-induced islet cell death in human islets in vitro. We propose that Nox4 mediates pro-apoptotic effects in intact islets under stressful conditions and that selective Nox4-inhibition may be a therapeutic strategy in type 2 diabetes.

BRICHOS domain of Bri2 inhibits islet amyloid polypeptide (IAPP) fibril formation and toxicity in human beta cells.

Aggregation of islet amyloid polypeptide (IAPP) into amyloid fibrils in islets of Langerhans is associated with type 2 diabetes, and formation of toxic IAPP species is believed to contribute to the loss of insulin-producing beta cells. The BRICHOS domain of integral membrane protein 2B (Bri2), a transmembrane protein expressed in several peripheral tissues and in the brain, has recently been shown to prevent fibril formation and toxicity of Aß42, an amyloid-forming peptide in Alzheimer disease. In this study, we demonstrate expression of Bri2 in human islets and in the human beta-cell line EndoC-ßH1. Bri2 colocalizes with IAPP intracellularly and is present in amyloid deposits in patients with type 2 diabetes. The BRICHOS domain of Bri2 effectively inhibits fibril formation in vitro and instead redirects IAPP into formation of amorphous aggregates. Reduction of endogenous Bri2 in EndoC-ßH1 cells with siRNA increases sensitivity to metabolic stress leading to cell death while a concomitant overexpression of Bri2 BRICHOS is protective. Also, coexpression of IAPP and Bri2 BRICHOS in lateral ventral neurons of Drosophila melanogaster results in an increased cell survival. IAPP is considered to be the most amyloidogenic peptide known, and described findings identify Bri2, or in particular its BRICHOS domain, as an important potential endogenous inhibitor of IAPP aggregation and toxicity, with the potential to be a possible target for the treatment of type 2 diabetes.

Addition of exogenous sodium palmitate increases the IAPP/insulin mRNA ratio via GPR40 in human EndoC-ßH1 cells.

BACKGROUND: Enhanced IAPP production may contribute to islet amyloid formation in type 2 diabetes. The objective of this study was to determine the effects of the saturated fatty acid palmitate on IAPP levels in human ß-cells. METHODS: EndoC-ßH1 cells and human islets were cultured in the presence of sodium palmitate. Effects on IAPP/insulin mRNA expression and secretion were determined using real-time qPCR/ELISA. Pharmacological activators and/or inhibitors and RNAi were used to determine the underlying mechanisms. RESULTS: We observed that EndoC-ßH1 cells exposed to palmitate for 72 h displayed decreased expression of Pdx-1 and MafA and increased expression of thioredoxin-interacting protein (TXNIP), reduced insulin mRNA expression and glucose-induced insulin secretion, as well as increased IAPP mRNA expression and secretion. Further, these effects were independent of fatty acid oxidation, but abolished in response to GPR40 inhibition/downregulation. In human islets both a high glucose concentration and palmitate promoted increased IAPP mRNA levels, resulting in an augmented IAPP/insulin mRNA ratio. This was paralleled by elevated IAPP/insulin protein secretion and content ratios. CONCLUSIONS: Addition of exogenous palmitate to human ß-cells increased the IAPP/insulin expression ratio, an effect contributed to by activation of GPR40. These findings may be pertinent to our understanding of the islet amyloid formation process.

EndoC-ßH1 cells display increased sensitivity to sodium palmitate when cultured in DMEM/F12 medium.

Aims - Human pancreatic islets are known to die in response to the free fatty acid of sodium palmitate when cultured in vitro. This is in contrast to EndoC-ßH1 cells, which in our hands are not sensitive to the cell death-inducing effects sodium palmitate, making these cells seemingly unsuitable for lipotoxicity studies. However, the EndoC-ßH1 cells are routinely cultured in a nutrient mixture based on Dulbecco's Modified Eagle Medium (DMEM), which may not be the optimal choice for studies dealing with lipotoxicity. The aim of the present investigation was to define culture conditions that render EndoC-ßH1 cells sensitive to toxic effects of sodium palmitate. Methods - EndoC-ßH1 cells were cultured at standard conditions in either DMEM or DMEM/F12 culture medium. Cell death was analyzed using propidium iodide staining and flow cytometry. Insulin release and content was quantified using a human insulin ELISA. Results - We presently observe that substitution of DMEM for a DMEM/Ham's F12 mixture (50%/50% vol/vol) renders the cells sensitive to the apoptotic effects of sodium palmitate and sodium palmitate + high glucose leading to an increased cell death. Supplementation of the DMEM culture medium with linoleic acid partially mimicked the effect of DMEM/F12. Culture of EndoC-ßH1 cells in DMEM/F12 resulted also in increased proliferation, ROS production and insulin contents, but markers for metabolic stress, autophagy or amyloid deposits were unaffected. Conclusions - The culture conditions for EndoC-ßH1 cells can be modified so these cells display signs of lipotoxicity in response to sodium palmitate.

PTB and TIAR binding to insulin mRNA 3'- and 5'UTRs; implications for insulin biosynthesis and messenger stability.

OBJECTIVES: Insulin expression is highly controlled on the posttranscriptional level. The RNA binding proteins (RBPs) responsible for this result are still largely unknown. METHODS AND RESULTS: To identify RBPs that bind to insulin mRNA we performed mass spectrometry analysis on proteins that bound synthetic oligonucloetides mimicing the 5'- and the 3'-untranslated regions (UTRs) of rat and human insulin mRNA in vitro. We observed that the RBPs heterogeneous nuclear ribonucleoprotein (hnRNP) U, polypyrimidine tract binding protein (PTB), hnRNP L and T-cell restricted intracellular antigen 1-related protein (TIA-1-related protein; TIAR) bind to insulin mRNA sequences, and that the in vitro binding affinity of these RBPs changed when INS-1 cells were exposed to glucose, 3-isobutyl-1-methylxanthine (IBMX) or nitric oxide. High glucose exposure resulted in a modest increase in PTB and TIAR binding to an insulin mRNA sequence. The inducer of nitrosative stress DETAnonoate increased markedly hnRNP U and TIAR mRNA binding. An increased PTB to TIAR binding ratio in vitro correlated with higher insulin mRNA levels and insulin biosynthesis rates in INS-1 cells. To further investigate the importance of RNA-binding proteins for insulin mRNA stability, we decreased INS-1 and EndoC-ßH1 cell levels of PTB and TIAR by RNAi. In both cell lines, decreased levels of PTB resulted in lowered insulin mRNA levels while decreased levels of TIAR resulted in increased insulin mRNA levels. Thapsigargin-induced stress granule formation was associated with a redistribution of TIAR from the cytosol to stress granules. CONCLUSIONS: These experiments indicate that alterations in insulin mRNA stability and translation correlate with differential RBP binding. We propose that the balance between PTB on one hand and TIAR on the other participates in the control of insulin mRNA stability and utilization for insulin biosynthesis.

Aromatic malononitriles stimulate the resistance of insulin-producing beta-cells to oxidants and inflammatory cytokines.

We presently report that treatment with tyrphostin AG-126 (2-(3-hydroxy-4-nitrobenzylidene)malononitrile) and ten other aromatic malononitrile compounds (AMN) improves the resistance of insulin-producing ßTC6, RIN-5AH, and MIN6 cells to oxidative stress and pro-inflammatory cytokines. On the molecular level AMN compounds promote nuclear accumulation of the Nrf2 transcription factor and expression of the cytoprotective genes heme ogygenase 1 (HO-1) and NAD(P)H/quinone oxidoreductase 1 (NQO1), inhibit cytokine-dependent inducible nitric oxide synthase (iNOS) induction, suppress intracellular production of reactive oxygen species in ßTC6 and counteract to impairments of glucose-stimulated insulin secretion induced by pro-inflammatory cytokines in MIN6 cells. Nrf2 up-regulation and HO-1 induction by AG-126 are attenuated at the presence of siRNA against Nrf2 and brusatol, an inhibitor of the Nrf2 signaling pathway. Our present results indicate that in respect of inhibition of IL-1ß-dependent iNOS induction, ßTC6 cells are more sensitive to EMK 1071 (2-((5-methylthiophen-2-yl)methylene)malononitrile) and EMK 31 (2-(4-hydroxy-3-methoxybenzylidene)malononitrile) as compared to other analyzed AMN compounds. We suggest that the ability of AMN compounds to inhibit iNOS induction and other cytokine-induced transcriptional events might be a tool to achieve improved ß-cell survival and functionality.

Imatinib prevents beta cell death in vitro but does not improve islet transplantation outcome.

Introduction Improving islet transplantation outcome could not only bring benefits to individual patients but also widen the patient pool to which this life-changing treatment is available. Imatinib has previously been shown to protect beta cells from apoptosis in a variety of in vitro and in vivo models. The aim of this study was to investigate whether imatinib could be used to improve islet transplantation outcome. Methods Islets were isolated from C57Bl/6 mice and pre-cultured with imatinib prior to exposure to streptozotocin and cytokines in vitro. Cell viability and glucose-induced insulin secretion were measured. For transplantation experiments, islets were pre-cultured with imatinib for either 72 h or 24 h prior to transplantation into streptozotocin-diabetic C57Bl/6 mice. In one experimental series mice were also administered imatinib after islet transplantation. Results Imatinib partially protected islets from beta cell death in vitro. However, pre-culturing islets in imatinib or administering the drug to the mice in the days following islet transplantation did not improve blood glucose concentrations more than control-cultured islets. Conclusion Although imatinib protected against beta cell death from cytokines and streptozotocin in vitro, it did not significantly improve syngeneic islet transplantation outcome.

Knock-down of ZBED6 in insulin-producing cells promotes N-cadherin junctions between beta-cells and neural crest stem cells in vitro.

The role of the novel transcription factor ZBED6 for the adhesion/clustering of insulin-producing mouse MIN6 and ßTC6 cells was investigated. Zbed6-silencing in the insulin producing cells resulted in increased three-dimensional cell-cell clustering and decreased adhesion to mouse laminin and human laminin 511. This was paralleled by a weaker focal adhesion kinase phosphorylation at laminin binding sites. Zbed6-silenced cells expressed less E-cadherin and more N-cadherin at cell-to-cell junctions. A strong ZBED6-binding site close to the N-cadherin gene transcription start site was observed. Three-dimensional clustering in Zbed6-silenced cells was prevented by an N-cadherin neutralizing antibody and by N-cadherin knockdown. Co-culture of neural crest stem cells (NCSCs) with Zbed6-silenced cells, but not with control cells, stimulated the outgrowth of NCSC processes. The cell-to-cell junctions between NCSCs and ßTC6 cells stained more intensely for N-cadherin when Zbed6-silenced cells were co-cultured with NCSCs. We conclude that ZBED6 decreases the ratio between N- and E-cadherin. A lower N- to E-cadherin ratio may hamper the formation of three-dimensional beta-cell clusters and cell-to-cell junctions with NCSC, and instead promote efficient attachment to a laminin support and monolayer growth. Thus, by controlling beta-cell adhesion and cell-to-cell junctions, ZBED6 might play an important role in beta-cell differentiation, proliferation and survival.

Role of the AMP kinase in cytokine-induced human EndoC-ßH1 cell death.

The aim of the present investigation was to delineate cytokine-induced signaling and death using the EndoC-ßH1 cells as a model for primary human beta-cells. The cytokines IL-1ß and IFN-γ induced a rapid and transient activation of NF-κB, STAT-1, ERK, JNK and eIF-2α signaling. The EndoC-ßH1 cells died rapidly when exposed to IL-1ß + IFN-γ, and this occurred also in the presence of the actinomycin D. Inhibition of NF-κB and STAT-1 did not protect against cell death, nor did the cytokines activate iNOS expression. Instead, cytokines promoted a rapid decrease in EndoC-ßH1 cell respiration and ATP levels, and we observed protection by the AMPK activator AICAR against cytokine-induced cell death. It is concluded that EndoC-ßH1 cell death can be prevented by AMPK activation, which suggests a role for ATP depletion in cytokine-induced human beta-cell death.