Establishment of 2-mercaptoethanol-dependent differentiated insulin-secreting cell lines

A CRISPR/Cas9 genome editing pipeline in the EndoC-βH1 cell line to study genes implicated in beta cell function

Type 2 diabetes (T2D) is a global pandemic with a strong genetic component, but most causal genes influencing the disease risk remain unknown. It is clear, however, that the pancreatic beta cell is central to T2D pathogenesis. In vitro gene-knockout (KO) models to study T2D risk genes have so far focused on rodent beta cells. However, there are important structural and functional differences between rodent and human beta cell lines. With that in mind, we have developed a robust pipeline to create a stable CRISPR/Cas9 KO in an authentic human beta cell line (EndoC-βH1). The KO pipeline consists of a dual lentiviral sgRNA strategy and we targeted three genes ( INS, IDE, PAM) as a proof of concept. We achieved a significant reduction in mRNA levels and complete protein depletion of all target genes. Using this dual sgRNA strategy, up to 94 kb DNA were cut out of the target genes and the editing efficiency of each sgRNA exceeded >87.5%. Sequencing of off-targets showed no unspecific editing. Most importantly, the pipeline did not affect the glucose-responsive insulin secretion of the cells. Interestingly, comparison of KO cell lines for NEUROD1 and SLC30A8 with siRNA-mediated knockdown (KD) approaches demonstrate phenotypic differences. NEUROD1-KO cells were not viable and displayed elevated markers for ER stress and apoptosis. NEUROD1-KD, however, only had a modest elevation, by 34%, in the pro-apoptotic transcription factor CHOP and a gene expression profile indicative of chronic ER stress without evidence of elevated cell death. On the other hand, SLC30A8-KO cells demonstrated no reduction in K ATP channel gene expression in contrast to siRNA silencing. Overall, this strategy to efficiently create stable KO in the human beta cell line EndoC-βH1 will allow for a better understanding of genes involved in beta cell dysfunction, their underlying functional mechanisms and T2D pathogenesis.

G-protein-coupled receptor 40 agonist GW9508 potentiates glucose-stimulated insulin secretion through activation of protein kinase Cα and ε in INS-1 cells

OBJECTIVE

The mechanism by which G-protein-coupled receptor 40 (GPR40) signaling amplifies glucose-stimulated insulin secretion through activation of protein kinase C (PKC) is unknown. We examined whether a GPR40 agonist, GW9508, could stimulate conventional and novel isoforms of PKC at two glucose concentrations (3 mM and 20 mM) in INS-1D cells.

METHODS

Using epifluorescence microscopy, we monitored relative changes in the cytosolic fluorescence intensity of Fura2 as a marker of change in intracellular Ca2+ ([Ca2+]i) and relative increases in green fluorescent protein (GFP)-tagged myristoylated alanine-rich C kinase substrate (MARCKS-GFP) as a marker of PKC activation in response to GW9508 at 3 mM and 20 mM glucose. To assess the activation of the two PKC isoforms, relative increases in membrane fluorescence intensity of PKCα-GFP and PKCε-GFP were measured by total internal reflection fluorescence microscopy. Specific inhibitors of each PKC isotype were constructed and synthesized as peptide fusions with the third α-helix of the homeodomain of Antennapedia.

RESULTS

At 3 mM glucose, GW9508 induced sustained MARCKS-GFP translocation to the cytosol, irrespective of changes in [Ca2+]i. At 20 mM glucose, GW9508 induced sustained MARCKS-GFP translocation but also transient translocation that followed sharp increases in [Ca2+]i. Although PKCα translocation was rarely observed, PKCε translocation to the plasma membrane was sustained by GW9508 at 3 mM glucose. At 20 mM glucose, GW9508 induced transient translocation of PKCα and sustained translocation as well as transient translocation of PKCε. While the inhibitors (75 μM) of each PKC isotype reduced GW9508-potentiated, glucose-stimulated insulin secretion in INS-1D cells, the PKCε inhibitor had a more potent effect.

CONCLUSION

GW9508 activated PKCε but not PKCα at a substimulatory concentration of glucose. Both PKC isotypes were activated at a stimulatory concentration of glucose and contributed to glucose-stimulated insulin secretion in insulin-producing cells.

Understanding glycaemic control and current approaches for screening antidiabetic natural products from evidence-based medicinal plants

Type 2 Diabetes Mellitus has reached epidemic proportions as a result of over-nutrition and increasingly sedentary lifestyles. Current therapies, although effective, are not without limitations. These limitations, the alarming increase in the prevalence of diabetes, and the soaring cost of managing diabetes and its complications underscores an urgent need for safer, more efficient and affordable alternative treatments. Over 1200 plant species are reported in ethnomedicine for treating diabetes and these represents an important and promising source for the identification of novel antidiabetic compounds. Evaluating medicinal plants for desirable bioactivity goes hand-in-hand with methods in analytical biochemistry for separating and identifying lead compounds. This review aims to provide a comprehensive summary of current methods used in antidiabetic plant research to form a useful resource for researchers beginning in the field. The review summarises the current understanding of blood glucose regulation and the general mechanisms of action of current antidiabetic medications, and combines knowledge on common experimental approaches for screening plant extracts for antidiabetic activity and currently available analytical methods and technologies for the separation and identification of bioactive natural products. Common in vivo animal models, in vitro models, in silico methods and biochemical assays used for testing the antidiabetic effects of plants are discussed with a particular emphasis on in vitro methods such as cell-based bioassays for screening insulin secretagogues and insulinomimetics. Enzyme inhibition assays and molecular docking are also highlighted. The role of metabolomics, metabolite profiling, and dereplication of data for the high-throughput discovery of novel antidiabetic agents is reviewed. Finally, this review also summarises sample preparation techniques such as liquid-liquid extraction, solid phase extraction, and supercritical fluid extraction, and the critical function of nuclear magnetic resonance and high resolution liquid chromatography-mass spectrometry for the dereplication, putative identification and structure elucidation of natural compounds from evidence-based medicinal plants.

The supply chain of human pancreatic β cell lines

Patients with type 1 or type 2 diabetes have an insufficiency in their functional β cell mass. To advance diabetes treatment and to work toward a cure, a better understanding of how to protect the pancreatic β cells against autoimmune or metabolic assaults (e.g., obesity, gestation) will be required. Over the past decades, β cell protection has been extensively investigated in rodents both in vivo and in vitro using isolated islets or rodent β cell lines. Transferring these rodent data to humans has long been challenging, at least partly for technical reasons: primary human islet preparations were scarce and functional human β cell lines were lacking. In 2011, we described a robust protocol of targeted oncogenesis in human fetal pancreas and produced the first functional human β cell line, and in subsequent years additional lines with specific traits. These cell lines are currently used by more than 150 academic and industrial laboratories worldwide. In this Review, we first explain how we developed the human β cell lines and why we think we succeeded where others, despite major efforts, did not. Next, we discuss the use of such functional human β cell lines and share some perspectives on their use to advance diabetes research.

CREB Promotes Beta Cell Gene Expression by Targeting Its Coactivators to Tissue-Specific Enhancers

CREB mediates effects of cyclic AMP on cellular gene expression. Ubiquitous CREB target genes are induced following recruitment of CREB and its coactivators to promoter proximal binding sites. We found that CREB stimulates the expression of pancreatic beta cell-specific genes by targeting CBP/p300 to promoter-distal enhancer regions. Subsequent increases in histone acetylation facilitate recruitment of the coactivators CRTC2 and BRD4, leading to release of RNA polymerase II over the target gene body. Indeed, CREB-induced hyperacetylation of chromatin over superenhancers promoted beta cell-restricted gene expression, which is sensitive to inhibitors of CBP/p300 and BRD4 activity. Neurod1 appears critical in establishing nucleosome-free regions for recruitment of CREB to beta cell-specific enhancers. Deletion of a CREB-Neurod1-bound enhancer within the Lrrc10b-Syt7 superenhancer disrupted the expression of both genes and decreased beta cell function. Our results demonstrate how cross talk between signal-dependent and lineage-determining factors promotes the expression of cell-type-specific gene programs in response to extracellular cues.

Regulation of cAMP accumulation and activity by distinct phosphodiesterase subtypes in INS-1 cells and human pancreatic β-cells

Pancreatic β-cells express multiple phosphodiesterase (PDE) subtypes, but the specific roles for each in β-cell function, particularly in humans, is not clear. We evaluated the cellular role of PDE1, PDE3, and PDE4 activity in the rat insulinoma cell line INS-1 and in primary human β-cells using subtype-selective PDE inhibitors. Using a genetically encoded, FRET-based cAMP sensor, we found that the PDE1 inhibitor 8MM-IBMX, elevated cAMP levels in the absence of glucose to a greater extent than either the PDE3 inhibitor cilostamide or the PDE4 inhibitor rolipram. In 18 mM glucose, PDE1 inhibition elevated cAMP levels to a greater extent than PDE3 inhibition in INS-1 cells, while PDE4 inhibition was without effect. Inhibition of PDE1 or PDE4, but not PDE3, potentiated glucose-stimulated insulin secretion in INS-1 cells. PDE1 inhibition, but not PDE3 or PDE4 inhibition, reduced palmitate-induced caspase-3/7 activation, and enhanced CREB phosphorylation in INS-1 cells. In human β-cells, only PDE3 or PDE4 inhibition increased cAMP levels in 1.7 mM glucose, but PDE1, PDE3, or PDE4 inhibition potentiated cAMP levels in 16.7 mM glucose. Inhibition of PDE1 or PDE4 increased cAMP levels to a greater extent in 16.7 mM glucose than in 1.7 mM glucose in human β-cells. In contrast, elevation of cAMP levels by PDE3 inhibition was not different at these glucose concentrations. PDE1 inhibition also potentiated insulin secretion from human islets, suggesting that the role of PDE1 may be conserved between INS-1 cells and human pancreatic β-cells. Our results suggest that inhibition of PDE1 may be a useful strategy to potentiate glucose-stimulated insulin secretion, and to protect β-cells from the toxic effects of excess fatty acids.

Cancer Cell Lines Are Useful Model Systems for Medical Research

Cell lines are in vitro model systems that are widely used in different fields of medical research, especially basic cancer research and drug discovery. Their usefulness is primarily linked to their ability to provide an indefinite source of biological material for experimental purposes. Under the right conditions and with appropriate controls, authenticated cancer cell lines retain most of the genetic properties of the cancer of origin. During the last few years, comparing genomic data of most cancer cell lines has corroborated this statement and those that were observed studying the tumoral tissue equivalents included in the The Cancer Genome Atlas (TCGA) database. We are at the disposal of comprehensive open access cell line datasets describing their molecular and cellular alterations at an unprecedented level of accuracy. This aspect, in association with the possibility of setting up accurate culture conditions that mimic the in vivo microenvironment (e.g., three-dimensional (3D) coculture), has strengthened the importance of cancer cell lines for continuing to sustain medical research fields. However, it is important to consider that the appropriate use of cell lines needs to follow established guidelines for guaranteed data reproducibility and quality, and to prevent the occurrence of detrimental events (i.e., those that are linked to cross-contamination and mycoplasma contamination).

Intestinal lysozyme liberates Nod1 ligands from microbes to direct insulin trafficking in pancreatic beta cells

Long-range communication between intestinal symbiotic bacteria and extra-intestinal organs can occur through circulating bacterial signal molecules, through neural circuits, or through cytokines or hormones from host cells. Here we report that Nod1 ligands derived from intestinal bacteria act as signal molecules and directly modulate insulin trafficking in pancreatic beta cells. The cytosolic peptidoglycan receptor Nod1 and its downstream adapter Rip2 are required for insulin trafficking in beta cells in a cell-autonomous manner. Mechanistically, upon recognizing cognate ligands, Nod1 and Rip2 localize to insulin vesicles, recruiting Rab1a to direct insulin trafficking through the cytoplasm. Importantly, intestinal lysozyme liberates Nod1 ligands into the circulation, thus enabling long-range communication between intestinal microbes and islets. The intestine-islet crosstalk bridged by Nod1 ligands modulates host glucose tolerance. Our study defines a new type of inter-organ communication based on circulating bacterial signal molecules, which has broad implications for understanding the mutualistic relationship between microbes and host.

The Selective Rat Toxicant Norbormide Blocks KATP Channels in Smooth Muscle Cells But Not in Insulin-Secreting Cells

Norbormide is a toxicant selective for rats to which it induces a widespread vasoconstriction. In a recent paper, we hypothesized a role of ATP-sensitive potassium (K_ATP) channels in norbormide-induced vasoconstriction. The current study was undertaken to verify this hypothesis by comparing the effects of norbormide with those of glibenclamide, a known K_ATP channel blocker. The whole-cell patch-clamp method was used to record K_ATP currents in myocytes freshly isolated from the rat and mouse caudal artery and from the rat gastric fundus, as well as in insulin-secreting pancreatic beta cells (INS-1 cells). Smooth muscle contractile function was assessed on either rat caudal artery rings or gastric fundus strips. Molecular modeling and docking simulation to K_ATP channel proteins were investigated in silico. Both norbormide (a racemic mixture of endo and exo isomers) and glibenclamide inhibited K_ATP currents in rat and mouse caudal artery myocytes, as well as in gastric fundus smooth muscle cells. In rat INS-1 cells, only glibenclamide blocked K_ATP channels, whereas norbormide was ineffective. The inhibitory effect of norbormide in rat caudal artery myocytes was not stereo-specific as both the endo isomers (active as vasoconstrictor) and the exo isomers (inactive as vasoconstrictor) had similar inhibitory activity. In rat caudal artery rings, norbormide-induced contraction was partially reverted by the K_ATP channel opener pinacidil. Computational approaches indicated the SUR subunit of K_ATP channels as the binding site for norbormide. K_ATP channel inhibition may play a role in norbormide-induced vasoconstriction, but does not explain the species selectivity, tissue selectivity, and stereoselectivity of its constricting activity. The lack of effect in INS-1 cells suggests a potential selectivity of norbormide for smooth muscle K_ATP channels.

The zinc transporter Zip14 (SLC39a14) affects Beta-cell Function: Proteomics, Gene expression, and Insulin secretion studies in INS-1E cells

Insulin secretion from pancreatic beta-cells is dependent on zinc ions as essential components of insulin crystals, zinc transporters are thus involved in the insulin secretory process. Zip14 (SLC39a14) is a zinc importing protein that has an important role in glucose homeostasis. Zip14 knockout mice display hyperinsulinemia and impaired insulin secretion in high glucose conditions. Endocrine roles for Zip14 have been established in adipocytes and hepatocytes, but not yet confirmed in beta-cells. In this study, we investigated the role of Zip14 in the INS-1E beta-cell line. Zip14 mRNA was upregulated during high glucose stimulation and Zip14 silencing led to increased intracellular insulin content. Large-scale proteomics showed that Zip14 silencing down-regulated ribosomal mitochondrial proteins, many metal-binding proteins, and others involved in oxidative phosphorylation and insulin secretion. Furthermore, proliferation marker Mki67 was down-regulated in Zip14 siRNA-treated cells. In conclusion, Zip14 gene expression is glucose sensitive and silencing of Zip14 directly affects insulin processing in INS-1E beta-cells. A link between Zip14 and ribosomal mitochondrial proteins suggests altered mitochondrial RNA translation, which could disturb mitochondrial function and thereby insulin secretion. This highlights a role for Zip14 in beta-cell functioning and suggests Zip14 as a future pharmacological target in the treatment of beta-cell dysfunction.

Acute exposure to 3‑deoxyglucosone at high glucose levels impairs insulin secretion from β‑cells by downregulating the sweet taste receptor signaling pathway

Sweet taste receptors (STRs) expressed on β‑cells stimulate insulin secretion in response to an increase in the circulating level of glucose, maintaining glucose homeostasis. 3‑Deoxyglucosone (3DG), a highly reactive α‑dicarbonyl compound, has been previously described as an independent factor associate with the development of prediabetes. In our previous study, pathological plasma levels of 3DG were induced in normal rats with a single intravenous injection of 50 mg/kg 3DG, and an acute rise in circulating 3DG induced glucose intolerance by impairing the function of pancreatic β‑cells. The present study aimed to investigate whether the deleterious effects of pathological plasma levels of 3DG on β‑cell function and insulin secretion were associated with STRs. INS‑1 cells, an in vitro model to study rat β‑cells, were treated with various concentrations of 3DG (1.85, 30.84 and 61.68 mM) or lactisole (5 mM). Pancreatic islets were collected from rats 2 h after a single intravenous injection of 50 mg/kg 3DG + 0.5 g/kg glucose. The insulin concentration was measured by ELISA. The protein expression levels of components of the STR signaling pathways were determined by western blot analysis. Treatment with 3DG and 25.5 mM glucose for 1 h significantly reduced insulin secretion by INS‑1 cells, which was consistent with the phenotype observed in INS‑1 cells treated with the STR inhibitor lactisole. Accordingly, islets isolated from rats treated with 3DG exhibited a significant reduction in insulin secretion following treatment with 25.5 mM glucose. Furthermore, acute exposure of INS‑1 cells to 3DG following treatment with 25.5 mM glucose for 1 h significantly reduced the protein expression level of the STR subunit taste 1 receptor member 3 and its downstream factors, transient receptor potential cation channel subfamily M member 5 and glucose transporter 2. Notably, islet tissues collected from rats treated with 3DG exhibited a similar downregulation of these factors. The present results suggested that acute exposure to pathologically relevant levels of 3DG in presence of high physiological levels of glucose decreased insulin secretion from β‑cells by, at least in part, downregulating the STR signaling pathway.

Increased uptake of oxLDL does not exert lipotoxic effects in insulin-secreting cells

Modified lipoproteins can negatively affect beta cell function and survival. However, the mechanisms behind interactions of modified lipoproteins with beta cells - and in particular, relationships to increased uptake - are only partly clarified. By over-expressing the scavenger receptor CD36 (Tet-on), we increased the uptake of fluorescent low-density modified lipoprotein (oxLDL) into insulin-secreting INS-1 cells. The magnitude of uptake followed the degree of CD36 over-expression. CD36 over-expression increased concomitant efflux of 3H-cholesterol in proportion to the cellular contents of 3H-cholesterol. Exposure to concentrations of oxLDL from 20 to 100 µg/mL dose-dependently increased toxicity (evaluated by MTT) as well as apoptosis. However, the increased uptake of oxLDL due to CD36 over-expression did not exert additive effects on oxLDL toxicity - neither on viability, nor on glucose-induced insulin release and cellular content. Reciprocally, blocking CD36 receptors by Sulfo-N-Succinimidyl Oleate decreased the uptake of oxLDL but did not diminish the toxicity. Pancreatic islets of CD36-/- mice displayed reduced uptake of 3H-cholesterol-labeled oxLDL vs wild type but similar toxicity to oxLDL. OxLDL was found to increase the expression of CD36 in islets and INS-1 cells. In summary, given the experimental conditions, our results indicate that (1) increased uptake of oxLDL is not responsible for toxicity of oxLDL, (2) increased efflux of the cholesterol moiety of oxLDL counterbalances, at least in part, increased uptake and (3) oxLDL participates in the regulation of CD36 in pancreatic islets and in INS-1 cells.

Menaquinone-4 Amplified Glucose-Stimulated Insulin Secretion in Isolated Mouse Pancreatic Islets and INS-1 Rat Insulinoma Cells

Vitamin K2 is indispensable for blood coagulation and bone metabolism. Menaquinone-4 (MK-4) is the predominant homolog of vitamin K2, which is present in large amounts in the pancreas, although its function is unclear. Meanwhile, β-cell dysfunction following insulin secretion has been found to decrease in patients with type 2 diabetes mellitus. To elucidate the physiological function of MK-4 in pancreatic β-cells, we studied the effects of MK-4 treatment on isolated mouse pancreatic islets and rat INS-1 cells. Glucose-stimulated insulin secretion significantly increased in isolated islets and INS-1 cells treated with MK-4. It was further clarified that MK-4 enhanced cAMP levels, accompanied by the regulation of the exchange protein directly activated by the cAMP 2 (Epac2)-dependent pathway but not the protein kinase A (PKA)-dependent pathway. A novel function of MK-4 on glucose-stimulated insulin secretion was found, suggesting that MK-4 might act as a potent amplifier of the incretin effect. This study therefore presents a novel potential therapeutic approach for impaired insulinotropic effects.

Glucose Responsiveness of β-Cells Depends on Fatty Acids

Glucose-stimulated insulin secretion (GSIS) is the gold standard for β-cell function. Both experimental and clinical diabetology, i. e., preceding transplantation of isolated human islets, depend on functional testing. However, multiple factors influence GSIS rendering the comparison of different in vitro tests of glucose responsiveness difficult. This study examined the influence of bovine serum albumin (BSA)-coupled fatty acids on GSIS. Isolated islet preparations of human donors and of 12-months old mice displayed impaired GSIS in the presence of 0.5% FFA-free BSA compared to 0.5% BSA (fraction V, not deprived from fatty acids). In aged INS-1E cells, i. e. at a high passage number, GSIS became highly sensitive to FFA-free BSA. Readdition of 30 µM palmitate or 30 µM oleate to FFA-free BSA did not rescue GSIS, while the addition of 100 µM palmitate and the raise of extracellular Ca²⁺from 1.3 to 2.6 mM improved glucose responsiveness. A high concentration of palmitate (600 µM), which fully activates FFA1, largely restored insulin secretion. The FFA1-agonist TUG-469 also increased insulin secretion but to a lesser extent than palmitate. Glucose- and TUG-induced Ca²⁺oscillations were impaired in glucose-unresponsive, i. e., aged INS-1E cells. These results suggest that fatty acid deprivation (FFA-free BSA) impairs GSIS mainly through an effect on Ca²⁺sensitivity.

Cytokine-induced translocation of GRP78 to the plasma membrane triggers a pro-apoptotic feedback loop in pancreatic beta cells

The 78-kDa glucose-regulated protein (GRP78) is an ubiquitously expressed endoplasmic reticulum chaperone, with a central role in maintaining protein homeostasis. Recently, an alternative role for GRP78 under stress conditions has been proposed, with stress-induced extracellular secretion and translocation of GRP78 to the cell surface where it acts as a multifunctional signaling receptor. Here we demonstrate translocation of GRP78 to the surface of human EndoC-βH1 cells and primary human islets upon cytokine exposure, in analogy to observations in rodent INS-1E and MIN6 beta cell lines. We show that GRP78 is shuttled via the anterograde secretory pathway, through the Golgi complex and secretory granules, and identify the DNAJ homolog subfamily C member 3 (DNAJC3) as a GRP78-interacting protein that facilitates its membrane translocation. Evaluation of downstream signaling pathways, using N- and C-terminal anti-GRP78 blocking antibodies, demonstrates that both GRP78 signaling domains initiate pro-apoptotic signaling cascades in beta cells. Extracellular GRP78 itself is identified as a ligand for cell surface GRP78 (sGRP78), increasing caspase 3/7 activity and cell death upon binding, which is accompanied by enhanced Chop and Bax mRNA expression. These results suggest that inflammatory cytokines induce a self-destructive pro-apoptotic feedback loop through the secretion and membrane translocation of GRP78. This proapoptotic function distinguishes the role of sGRP78 in beta cells from its reported anti-apoptotic and proliferative role in cancer cells, opening the road for the use of compounds that block sGRP78 as potential beta cell-preserving therapies in type 1 diabetes.

Metabolic signatures suggest o-phosphocholine to UDP-N-acetylglucosamine ratio as a potential biomarker for high-glucose and/or palmitate exposure in pancreatic β-cells

INTRODUCTION

Chronic exposure to high-glucose and free fatty acids (FFA) alone/or in combination; and the resulting gluco-, lipo- and glucolipo-toxic conditions, respectively, have been known to induce dysfunction and apoptosis of β-cells in Diabetes. The molecular mechanisms and the development of biomarkers that can be used to predict similarities and differences behind these conditions would help in easier and earlier diagnosis of Diabetes.

OBJECTIVES

This study aims to use metabolomics to gain insight into the mechanisms by which β-cells respond to excess-nutrient stress and identify associated biomarkers.

METHODS

INS-1E cells were cultured in high-glucose, palmitate alone/or in combination for 24 h to mimic gluco-, lipo- and glucolipo-toxic conditions, respectively. Biochemical and cellular experiments were performed to confirm the establishment of these conditions. To gain molecular insights, abundant metabolites were identified and quantified using ¹H-NMR.

RESULTS

No loss of cellular viability was observed in high-glucose while exposure to FFA alone/in combination with high-glucose was associated with increased ROS levels, membrane damage, lipid accumulation, and DNA double-strand breaks. Forty-nine abundant metabolites were identified and quantified using ¹H-NMR. Chemometric pair-wise analysis in glucotoxic and lipotoxic conditions, when compared with glucolipotoxic conditions, revealed partial overlap in the dysregulated metabolites; however, the dysregulation was more significant under glucolipotoxic conditions.

CONCLUSION

The current study compared gluco-, lipo- and glucolipotoxic conditions in parallel and elucidated differences in metabolic pathways that play major roles in Diabetes. o-phosphocholine and UDP-N-acetylglucosamine were identified as common dysregulated metabolites and their ratio was proposed as a potential biomarker for these conditions.

Modelling the endocrine pancreas in health and disease

Diabetes mellitus is a multifactorial disease affecting increasing numbers of patients worldwide. Progression to insulin-dependent diabetes mellitus is characterized by the loss or dysfunction of pancreatic β-cells, but the pathomechanisms underlying β-cell failure in type 1 diabetes mellitus and type 2 diabetes mellitus are still poorly defined. Regeneration of β-cell mass from residual islet cells or replacement by β-like cells derived from stem cells holds great promise to stop or reverse disease progression. However, the development of new treatment options is hampered by our limited understanding of human pancreas organogenesis due to the restricted access to primary tissues. Therefore, the challenge is to translate results obtained from preclinical model systems to humans, which requires comparative modelling of β-cell biology in health and disease. Here, we discuss diverse modelling systems across different species that provide spatial and temporal resolution of cellular and molecular mechanisms to understand the evolutionary conserved genotype-phenotype relationship and translate them to humans. In addition, we summarize the latest knowledge on organoids, stem cell differentiation platforms, primary micro-islets and pseudo-islets, bioengineering and microfluidic systems for studying human pancreas development and homeostasis ex vivo. These new modelling systems and platforms have opened novel avenues for exploring the developmental trajectory, physiology, biology and pathology of the human pancreas.

Insulin secretory granules labelled with phogrin-fluorescent proteins show alterations in size, mobility and responsiveness to glucose stimulation in living β-cells

The intracellular life of insulin secretory granules (ISGs) from biogenesis to secretion depends on their structural (e.g. size) and dynamic (e.g. diffusivity, mode of motion) properties. Thus, it would be useful to have rapid and robust measurements of such parameters in living β-cells. To provide such measurements, we have developed a fast spatiotemporal fluctuation spectroscopy. We calculate an imaging-derived Mean Squared Displacement (iMSD), which simultaneously provides the size, average diffusivity, and anomalous coefficient of ISGs, without the need to extract individual trajectories. Clustering of structural and dynamic quantities in a multidimensional parametric space defines the ISGs' properties for different conditions. First, we create a reference using INS-1E cells expressing proinsulin fused to a fluorescent protein (FP) under basal culture conditions and validate our analysis by testing well-established stimuli, such as glucose intake, cytoskeleton disruption, or cholesterol overload. After, we investigate the effect of FP-tagged ISG protein markers on the structural and dynamic properties of the granule. While iMSD analysis produces similar results for most of the lumenal markers, the transmembrane marker phogrin-FP shows a clearly altered result. Phogrin overexpression induces a substantial granule enlargement and higher mobility, together with a partial de-polymerization of the actin cytoskeleton, and reduced cell responsiveness to glucose stimulation. Our data suggest a more careful interpretation of many previous ISG-based reports in living β-cells. The presented data pave the way to high-throughput cell-based screening of ISG structure and dynamics under various physiological and pathological conditions.

Zinc-Chelating Small Molecules Preferentially Accumulate and Function within Pancreatic β Cells

Diabetes is a hyperglycemic condition characterized by pancreatic β-cell dysfunction and depletion. Whereas methods for monitoring β-cell function in vivo exist, methods to deliver therapeutics to β cells are lacking. We leveraged the rare ability of β cells to concentrate zinc to preferentially trap zinc-binding molecules within β cells, resulting in β-cell-targeted compound delivery. We determined that zinc-rich β cells and islets preferentially accumulated TSQ (6-methoxy-8-p-toluenesulfonamido-quinoline) in a zinc-dependent manner compared with exocrine pancreas. Next, we asked whether appending a zinc-chelating moiety onto a β-cell replication-inducing compound was sufficient to confer preferential β-cell accumulation and activity. Indeed, the hybrid compound preferentially accumulated within rodent and human islets in a zinc-dependent manner and increased the selectivity of replication-promoting activity toward β cells. These data resolve the fundamental question of whether intracellular accumulation of zinc-chelating compounds is influenced by zinc content. Furthermore, application of this principle yielded a proof-of-concept method for β-cell-targeted drug delivery and bioactivity.

Eeyarestatin Compounds Selectively Enhance Sec61-Mediated Ca2+ Leakage from the Endoplasmic Reticulum

Eeyarestatin 1 (ES1) inhibits p97-dependent protein degradation, Sec61-dependent protein translocation into the endoplasmic reticulum (ER), and vesicular transport within the endomembrane system. Here, we show that ES1 impairs Ca²⁺ homeostasis by enhancing the Ca²⁺ leakage from mammalian ER. A comparison of various ES1 analogs suggested that the 5-nitrofuran (5-NF) ring of ES1 is crucial for this effect. Accordingly, the analog ES24, which conserves the 5-NF domain of ES1, selectively inhibited protein translocation into the ER, displayed the highest potency on ER Ca²⁺ leakage of ES1 analogs studied and induced Ca²⁺-dependent cell death. Using small interfering RNA-mediated knockdown of Sec61α, we identified Sec61 complexes as the targets that mediate the gain of Ca²⁺ leakage induced by ES1 and ES24. By interacting with the lateral gate of Sec61α, ES1 and ES24 likely capture Sec61 complexes in a Ca²⁺-permeable, open state, in which Sec61 complexes allow Ca²⁺ leakage but are translocation incompetent.

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ES1, ES2, and ES24 deplete Ca²⁺ in ER

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ESR35 and ES47 do not affect cellular Ca²⁺ homeostasis

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The most potent eeyarestatin, ES24, comprises only the 5-nitrofuran domain

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ES1 and ES24 target Sec61 complexes in ER

Coxsackievirus B Tailors the Unfolded Protein Response to Favour Viral Amplification in Pancreatic β Cells

Type 1 diabetes (T1D) is an autoimmune disease characterized by islet inflammation and progressive pancreatic β cell destruction. The disease is triggered by a combination of genetic and environmental factors, but the mechanisms leading to the triggering of early innate and late adaptive immunity and consequent progressive pancreatic β cell death remain unclear. The insulin-producing β cells are active secretory cells and are thus particularly sensitive to endoplasmic reticulum (ER) stress. ER stress plays an important role in the pathologic pathway leading to autoimmunity, islet inflammation, and β cell death. We show here that group B coxsackievirus (CVB) infection, a putative causative factor for T1D, induces a partial ER stress in rat and human β cells. The activation of the PERK/ATF4/CHOP branch is blunted while the IRE1α branch leads to increased spliced XBP1 expression and c-Jun N-terminal kinase (JNK) activation. Interestingly, JNK1 activation is essential for CVB amplification in both human and rat β cells. Furthermore, a chemically induced ER stress preceding viral infection increases viral replication, in a process dependent on IRE1α activation. Our findings show that CVB tailors the unfolded protein response in β cells to support their replication, preferentially triggering the pro-viral IRE1α/XBP1s/JNK1 pathway while blocking the pro-apoptotic PERK/ATF4/CHOP pathway.

Glycogen Synthase Kinase 3 Beta Controls Presenilin-1-Mediated Endoplasmic Reticulum Ca²⁺ Leak Directed to Mitochondria in Pancreatic Islets and β-Cells

Background/Aims

In pancreatic β-cells, the intracellular Ca²⁺ homeostasis is an essential regulator of the cells’ major functions. The endoplasmic reticulum (ER) as interactive intracellular Ca²⁺ store balances cellular Ca²⁺. In this study basal ER Ca²⁺ homeostasis was evaluated in order to reveal potential β-cell-specificity of ER Ca²⁺ handling and its consequences for mitochondrial Ca²⁺, ATP and respiration.

Methods

The two pancreatic cell lines INS-1 and MIN-6, freshly isolated pancreatic islets, and the two non-pancreatic cell lines HeLA and EA.hy926 were used. Cytosolic, ER and mitochondrial Ca²⁺ and ATP measurements were performed using single cell fluorescence microscopy and respective (genetically-encoded) sensors/dyes. Mitochondrial respiration was monitored by respirometry. GSK3β activity was measured with ELISA.

Results

An atypical ER Ca²⁺ leak was observed exclusively in pancreatic islets and β-cells. This continuous ER Ca²⁺ efflux is directed to mitochondria and increases basal respiration and organellar ATP levels, is established by GSK3β-mediated phosphorylation of presenilin-1, and is prevented by either knockdown of presenilin-1 or an inhibition/knockdown of GSK3β. Expression of a presenlin-1 mutant that mimics GSK3β-mediated phosphorylation established a β-cell-like ER Ca²⁺ leak in HeLa and EA.hy926 cells. The ER Ca²⁺ loss in β-cells was compensated at steady state by Ca²⁺ entry that is linked to the activity of TRPC3.

Conclusion

Pancreatic β-cells establish a cell-specific ER Ca²⁺ leak that is under the control of GSK3β and directed to mitochondria, thus, reflecting a cell-specific intracellular Ca²⁺ handling for basal mitochondrial activity.

The transcription factor STAT6 plays a critical role in promoting beta cell viability and is depleted in islets of individuals with type 1 diabetes

AIMS/HYPOTHESIS

In type 1 diabetes, selective beta cell loss occurs within the inflamed milieu of insulitic islets. This milieu is generated via the enhanced secretion of proinflammatory cytokines and by the loss of anti-inflammatory molecules such as IL-4 and IL-13. While the actions of proinflammatory cytokines have been well-studied in beta cells, the effects of their anti-inflammatory counterparts have received relatively little attention and we have addressed this.

METHODS

Clonal beta cells, isolated human islets and pancreas sections from control individuals and those with type 1 diabetes were employed. Gene expression was measured using targeted gene arrays and by quantitative RT-PCR. Protein expression was monitored in cell extracts by western blotting and in tissue sections by immunocytochemistry. Target proteins were knocked down selectively with interference RNA.

RESULTS

Cytoprotection achieved with IL-4 and IL-13 is mediated by the early activation of signal transducer and activator of transcription 6 (STAT6) in beta cells, leading to the upregulation of anti-apoptotic proteins, including myeloid leukaemia-1 (MCL-1) and B cell lymphoma-extra large (BCLXL). We also report the induction of signal regulatory protein-α (SIRPα), and find that knockdown of SIRPα is associated with reduced beta cell viability. These anti-apoptotic proteins and their attendant cytoprotective effects are lost following siRNA-mediated knockdown of STAT6 in beta cells. Importantly, analysis of human pancreas sections revealed that STAT6 is markedly depleted in the beta cells of individuals with type 1 diabetes, implying the loss of cytoprotective responses.

CONCLUSIONS/INTERPRETATION

Selective loss of STAT6 may contribute to beta cell demise during the progression of type 1 diabetes.

Vitamin D Increases Glucose Stimulated Insulin Secretion from Insulin Producing Beta Cells (INS1E)

BACKGROUND

Vitamin D affects the pancreatic beta cell function and in vitro studies have shown that vitamin D may influence insulin secretion, apoptosis, and gene regulation. However, the outcomes have differed and there has been uncertainty regarding the effect of different vitamin D metabolites on insulin secretion.

OBJECTIVES

We hypothesized that vitamin D could increase insulin secretion in insulin producing beta cells and investigated the effect of 25(OH) vitamin D and 1,25(OH)₂ vitamin D on insulin secretion.

METHODS

The study was conducted in INS1E cells, an established insulinoma cell line from rat. The cells were divided into three groups; a control group, a group with 1,25(OH)₂ vitamin D enriched medium (10 nM), and a group with 25(OH) vitamin D (10 nM) supplemented medium. After 72 hours of treatment, the cells underwent glucose stimulation at different concentrations (0, 5, 11, and 22 mM) for 60 minutes.

RESULTS

INS1E cells treated with 1,25(OH)₂ vitamin D showed a trend towards increased insulin secretion at all glucose concentrations compared to control cells and at 22 mM glucose, the difference was significant (18.40 +/- 1.97 vs 12.90 +/- 2.22 nmol/L, P < 0.05). However, pretreatment with 25(OH) vitamin D did not show any significant increase in insulin secretion compared to cells without vitamin D treatment. There was no difference in insulin secretion in cells not stimulated with glucose.

CONCLUSIONS

Treatment with 1,25(OH)₂ vitamin D combined with high levels of glucose increased insulin secretion in INS1E cells, whereas 25(OH) vitamin D had no effect. This suggests that glucose stimulated insulin secretion in INS1E beta cells appears to be related to the type of vitamin D metabolite treatment.

Class IIa HDACs do not influence beta-cell function under normal or high glucose conditions

Inhibiting Class IIa Histone Deacetylase (HDAC) function is a promising approach to therapeutically enhance skeletal and cardiac muscle metabolic health in several chronic diseases including type 2 diabetes. However, the importance of Class IIa HDACs in the beta-cell remains unknown. As beta-cell function is vital to maintaining glycaemia it is essential that the importance of Class IIa HDACs in the beta-cell is determined. Here we used the INS-1E cell line cultured in normal glucose (11.1 mM) or hyperglycaemic (20 mM) conditions for 48 hrs to represent cells in a normal and diabetic environment respectively. Cells cultured in high glucose showed significantly reduced insulin secretory function and increased apoptotic signalling compared to cells cultured in normal glucose. Class IIa HDACS, HDAC-4 and -5, were not regulated at the transcript or protein level under normal or hyperglycaemic conditions suggesting that they may not play a role in beta-cell dysfunction. Furthermore, overexpression of wild-type HDAC-4 and -5 or dominant negative HDAC-4 and -5 did not alter insulin secretion, insulin mRNA expression or apoptotic signalling under normal or hyperglycaemic conditions. This suggests that Class IIa Histone Deacetylases do not play an important physiological role in the beta-cell under normal or diabetic conditions. Thus, Class IIa Histone Deacetylase inhibitors are not likely to have a detrimental effect on beta-cells supporting the use of these inhibitors to treat metabolic diseases such as type 2 diabetes.

Polysaccharides from Portulaca oleracea L. regulated insulin secretion in INS-1 cells through voltage-gated Na+ channel

The present study was undertaken to determine the involvement of voltage-gated Na⁺ channel (VGSC) and other mechanism related to insulin secretion in polysaccharides from Portulaca oleracea L. (POP)-induced secretion of insulin from insulin-secreting β-cell line cells (INS-1) cells. Our results showed that the concentration of insulin both in culture medium and inside INS-1 cells were increased under the existing of different concentration of glucose by POP or TTX, respectively. However, the effect POP on insulin secretion and production were blocked by TTX, a VGSC blocker. Meanwhile, POP improved the mitochondrial membrane potential (Δψm), increased adenosine triphosphate (ATP) production, depolarized cell membrane potential (MP) and increased intracellular Ca²⁺ levels ([Ca²⁺]_i). Furthermore, POP treatment increased the expression level of Nav_1.3 and decreased the expression level of Nav_1.7. TTX treatment decreased the expression level of Nav_1.3 and Nav_1.7. On the other hand, POP also elevated the survival of INS-1 cells. These results suggested that POP induced-secretion/production of insulin in INS-1 cells were mediated by VGSC through its change of function and subunits expression and subsequent VGSC- dependent events such as change of intracellular Ca²⁺ releasing, ATP metabolism, cell membrane and mitochondrial membrane potential, and also improvement of INS-1 cell survival. Meanwhile, our data indicated the potentiality of developing POP to be a drug for diabetes treatment and VGSC as a therapeutic target in diabetes treatment is valuable to be investigated further.

Pancreatic β-cell tRNA hypomethylation and fragmentation link TRMT10A deficiency with diabetes

Transfer RNAs (tRNAs) are non-coding RNA molecules essential for protein synthesis. Post-transcriptionally they are heavily modified to improve their function, folding and stability. Intronic polymorphisms in CDKAL1, a tRNA methylthiotransferase, are associated with increased type 2 diabetes risk. Loss-of-function mutations in TRMT10A, a tRNA methyltransferase, are a monogenic cause of early onset diabetes and microcephaly. Here we confirm the role of TRMT10A as a guanosine 9 tRNA methyltransferase, and identify tRNAGln and tRNAiMeth as two of its targets. Using RNA interference and induced pluripotent stem cell-derived pancreatic β-like cells from healthy controls and TRMT10A-deficient patients we demonstrate that TRMT10A deficiency induces oxidative stress and triggers the intrinsic pathway of apoptosis in β-cells. We show that tRNA guanosine 9 hypomethylation leads to tRNAGln fragmentation and that 5'-tRNAGln fragments mediate TRMT10A deficiency-induced β-cell death. This study unmasks tRNA hypomethylation and fragmentation as a hitherto unknown mechanism of pancreatic β-cell demise relevant to monogenic and polygenic forms of diabetes.

Kalirin/Trio Rho GDP/GTP exchange factors regulate proinsulin and insulin secretion

Key features for progression to pancreatic β-cell failure and disease are loss of glucose responsiveness and an increased ratio of secreted proinsulin to insulin. Proinsulin and insulin are stored in secretory granules (SGs) and the fine-tuning of hormone output requires signal mediated recruitment of select SG populations according to intracellular location and age. The GTPase Rac1 coordinates multiple signaling pathways that specify SG release and Rac1 activity is controlled in part by GDP/GTP exchange factors (GEFs). To explore the function of two large multidomain GEFs, Kalirin and Trio in β-cells, we manipulated their Rac1-specific GEF1 domain activity by using small molecule inhibitors and by genetically ablating Kalirin. We examined age related secretory granule behavior employing radiolabeling protocols. Loss of Kalirin/Trio function attenuated radioactive proinsulin release by reducing constitutive-like secretion and exocytosis of 2-hour old granules. At later chase times or at steady state, Kalirin/Trio manipulations decreased glucose stimulated insulin output. Finally, use of a Rac1 FRET biosensor with cultured β-cell lines, demonstrated that Kalirin/Trio GEF1 activity was required for normal rearrangement of Rac1 to the plasma membrane in response to glucose. Rac1 activation can be evoked by both glucose metabolism and signaling through the incretin glucagon-like peptide 1 (GLP-1) receptor. GLP-1 addition restored Rac1 localization/activity and insulin secretion in the absence of Kalirin, thereby assigning Kalirin's participation to stimulatory glucose signaling.

Inhibition of TBK1/IKKε Promotes Regeneration of Pancreatic β-cells

β-cell proliferation induction is a promising therapeutic strategy to restore β-cell mass. By screening small molecules in a transgenic zebrafish model of type 1 diabetes, we identified inhibitors of non-canonical IκB kinases (IKKs), TANK-binding kinase 1 (TBK1) and IκB kinase ε (IKKε), as enhancers of β-cell regeneration. The most potent β-cell regeneration enhancer was a cinnamic acid derivative (E)-3-(3-phenylbenzo[c]isoxazol-5-yl)acrylic acid (PIAA), which, acting through the cAMP-dependent protein kinase A (PKA), stimulated β-cell-specific proliferation by increasing cyclic AMP (cAMP) levels and mechanistic target of rapamycin (mTOR) activity. A combination of PIAA and cilostamide, an inhibitor of β-cell-enriched cAMP hydrolyzing enzyme phosphodiesterase (PDE) 3, enhanced β-cell proliferation, whereas overexpression of PDE3 blunted the mitogenic effect of PIAA in zebrafish. PIAA augmented proliferation of INS-1β-cells and β-cells in mammalian islets including human islets with elevation in cAMP levels and insulin secretion. PIAA improved glycemic control in streptozotocin (STZ)-induced diabetic mice with increases in β-cell proliferation, β-cell area, and insulin content in the pancreas. Collectively, these data reveal an evolutionarily conserved and critical role of TBK1/IKKε suppression in expanding functional β-cell mass.

Resveratrol long-term treatment differentiates INS-1E beta-cell towards improved glucose response and insulin secretion

The clonal INS-1E beta-cell line has proven to be instrumental for numerous studies investigating the mechanisms of glucose-stimulated insulin secretion. The composition of its culture medium has not changed over the years, although some compounds have been recently highlighted for their effects on tissue differentiation. The present study investigated the effects of long-term treatment of INS-1E cells with 1 μM resveratrol on glucose-stimulated insulin secretion, testing an extended glucose dose response. The data demonstrate that chronic exposure to low-dose resveratrol expands the range of the glucose dose response of INS-1E cells beyond 15 mM glucose. We also assessed whether such beneficial effects could be retained after resveratrol withdrawal from the culture medium. This was not the case as INS-1E cells deprived of resveratrol returned to the phenotype of naïve cells, i.e., exhibiting a plateau phase at 15 mM glucose. Of note, although resveratrol has antioxidant properties, it cannot substitute for β-mercaptoethanol normally present in the medium of INS-1E cells as a reducing agent. In conclusion, the addition of resveratrol as a standard component of the culture medium of INS-1E cells improves glucose-stimulated insulin secretion.

Evaluating the effects of IADHFL on inhibiting DPP-IV activity and expression in Caco-2 cells and contributing to the amount of insulin released from INS-1 cells in vitro

Dipeptidyl peptidase-IV (DPP-IV) is a serine exo-peptidase that can inactivate incretins by removing N-terminal dipeptides. Currently, inhibiting the DPP-IV activity is a common treatment for type 2 diabetes (T2D). The goal of this study is to investigate whether IADHFL, a novel DPP-IV inhibitory peptide identified from bighead carp (Hypophthalmichthys nobilis), has the potential to modulate T2D. IADHFL remained stable after simulated gastrointestinal digestion and significantly decreased the activity and expression of both soluble and membrane-bound DPP-IV after 24 h and 48 h of treatment. Intact peptide absorption was observed, but a percentage of the peptide was degraded while passing through a monolayer of Caco-2 cells. In addition, a double-layered cell model showed that the peptide could increase insulin secretion from INS-1 cells after glucose treatments of 2.8 mM and 16.7 mM. Finally, IADHFL could regulate the expression levels of genes associated with insulin secretion and T2D in INS-1 cells.

Silencing of the FTO gene inhibits insulin secretion: An in vitro study using GRINCH cells

Expression of fat mass and obesity-associated gene (FTO) and ADP-ribosylation factor-like 15 (ARL15) in human islets is inversely correlated with HbA_1c. However, their impact on insulin secretion is still ambiguous. Here in, we investigated the role of FTO and ARL15 using GRINCH (Glucose-Responsive Insulin-secreting C-peptide-modified Human proinsulin) clonal rat β-cells. GRINCH cells have inserted GFP into the human C-peptide insulin gene. Hence, secreted CpepGFP served to monitor insulin secretion. mRNA silencing of FTO in GRINCH cells showed a significant reduction in glucose but not depolarization-stimulated insulin secretion, whereas ARL15 silencing had no effect. A significant down-regulation of insulin mRNA was observed in FTO knockdown cells. Type-2 Diabetic islets revealed a reduced expression of FTO mRNA. In conclusion, our data suggest that fluorescent CpepGFP released from GRINCH cells may serve as a convenient marker for insulin secretion. Silencing of FTO expression, but not ARL15, inhibits insulin secretion by affecting metabolic signaling.

Distribution and density of melatonin receptors in human main pancreatic islet cell types

Recent investigations of our group established that melatonin modulates hormone secretion of pancreatic islets via melatonin receptor types MT1 and MT2. Expression of MT1 and MT2 has been shown in mouse, rat, and human pancreatic islets as well as in the β-, α-, and δ-cell lines INS-1, αTC1.9, and QGP-1. In view of these earlier investigations, this study was performed to analyze in detail the distribution and density of melatonin receptors on the main islet cell types in human pancreatic tissue obtained from nondiabetic and type 2 diabetic patients. Immunohistochemical analysis established the presence of MT1 and MT2 in β-, α-, and δ-cells, but notably, with differences in receptor density. In general, the lowest MT1 and MT2 receptor density was measured in α-cells compared to the 2 other cell types. In type 2 diabetic islets, MT1 and MT2 receptor density was increased in δ-cells compared to normoglycemic controls. In human islets in batch culture of a nondiabetic donor, an increase of somatostatin secretion was observed under melatonin treatment while in islets of a type 2 diabetic donor, an inhibitory influence could be observed, especially in the presence of 5.5 mmol/L glucose. These data suggest the following: i) cell-type-specific density of MT1 and MT2 receptors in human pancreatic islets, which should be considered in context of the hormone secretion of islets, ii) the influence of diabetes on density of MT1 and MT2 as well as iii) the differential impact of melatonin on somatostatin secretion of nondiabetic and type 2 diabetic islets.

Neuronatin regulates pancreatic β cell insulin content and secretion

Neuronatin (Nnat) is an imprinted gene implicated in human obesity and widely expressed in neuroendocrine and metabolic tissues in a hormone- and nutrient-sensitive manner. However, its molecular and cellular functions and precise role in organismal physiology remain only partly defined. Here we demonstrate that mice lacking Nnat globally or specifically in β cells display impaired glucose-stimulated insulin secretion leading to defective glucose handling under conditions of nutrient excess. In contrast, we report no evidence for any feeding or body weight phenotypes in global Nnat-null mice. At the molecular level neuronatin augments insulin signal peptide cleavage by binding to the signal peptidase complex and facilitates translocation of the nascent preprohormone. Loss of neuronatin expression in β cells therefore reduces insulin content and blunts glucose-stimulated insulin secretion. Nnat expression, in turn, is glucose-regulated. This mechanism therefore represents a novel site of nutrient-sensitive control of β cell function and whole-animal glucose homeostasis. These data also suggest a potential wider role for Nnat in the regulation of metabolism through the modulation of peptide processing events.

Adipose tissue derived-factors impaired pancreatic β-cell function in diabetes

Inflammatory factors produced and secreted by adipose tissue, in particular peri-pancreatic adipose tissue (P-WAT), may influence pancreatic β-cell dysfunction. Using the ZDF Rat model of diabetes, we show the presence of infiltrating macrophage (ED1 staining) on pancreatic tissue and P-WAT in the pre-diabetes stage of the disease. Then, when the T2D is installed, infiltrating cells decreased. Meanwhile, the P-WAT conditioned-medium composition, in terms of inflammatory factors, varies during the onset of the T2D. Using chemiarray technology, we observed an over expression of CXCL-1, -2, -3, CCL-3/MIP-1α and CXCL-5/LIX and TIMP-1 in the 9 weeks old obese ZDF pre-diabetic rat model. Surprisingly, the expression profile of these factors decreased when animals become diabetic (12 weeks obese ZDF rats). The expression of these inflammatory proteins is highly associated with inflammatory infiltrate. P-WAT conditioned-medium from pre-diabetes rats stimulates insulin secretion, cellular proliferation and apoptosis of INS-1 cells. However, inhibition of conditioned-medium chemokines acting via CXCR2 receptor do not change cellular proliferation apoptosis and insulin secretion of INS-1 cells induced by P-WAT conditioned-medium. Taken together, these results show that among the secreted chemokines, increased expression of CXCL-1, -2, -3 and CXCL-5/LIX in P-WAT conditioned-medium is concomitant with the onset of the T2D but do not exerted a direct effect on pancreatic β-cell dysfunction.

High-glucose 3D INS-1 cell model combined with a microfluidic circular concentration gradient generator for high throughput screening of drugs against type 2 diabetes

In vitro models for screening of drugs against type 2 diabetes are crucial for the pharmaceutical industry. This paper presents a new approach for integration of a three-dimensionally-cultured insulinoma cell line (INS-1 cell) incubated in a high concentration of glucose as a new model. In this model, INS-1 cells tended to aggregate in the 3D gel (basement membrane extractant, BME), in a similar way to 3D in vivo cell culture models. The proliferation of INS-1 cells in BME was initially promoted and then suppressed by the high concentration of glucose, and the function of insulin secretion also was initially enhanced and then inhibited by the high concentration of glucose. These phenomena were similar to hyperglycemia symptoms, proving the validity of the proposed model. This model can help find the drugs that stimulate insulin secretion. Then, we identified the difference between the new model and the traditional two-dimensional model in terms of cell morphology, inhibition rate of cell proliferation, and insulin secretion. Simultaneously, we developed a circular drug concentration gradient generator based on microfluidic technology. We integrated the high-glucose 3D INS-1 cell model and the circular concentration gradient generator in the same microdevice, tested the utility of this microdevice in the field of drug screening with glipizide as a model drug, and found that the microdevice was more sensitive than the traditional device to screen the anti-diabetic drugs.

Anthocyanins from purple corn activate free fatty acid-receptor 1 and glucokinase enhancing in vitro insulin secretion and hepatic glucose uptake

The objective of this study was to evaluate the ability of anthocyanins (ANC) present in purple corn to enhance insulin secretion and hepatic glucose uptake in pancreatic cells and hepatocytes, through activation of the free fatty acid receptor-1 (FFAR1) and glucokinase (GK), respectively. Using a dual-layer cell culture with Caco-2 cells, INS-1E or HepG2 cells were treated with an anthocyanin-rich extract from the pericarp of purple corn (PCW), as well as pure ANC cyanidin-3-O-glucoside (C3G), peonidin-3-O-glucoside, pelargonidin-3-O-glucoside. Delphinidin-3-O-glucoside (D3G) was used for comparative purposes. Semipurified C3G (C3G-P) and condensed forms (CF-P) isolated from PCW were also used. At 100 μM, the pure ANC enhanced glucose-stimulated insulin secretion (GSIS) in INS-1E cells ranging from 18% to 40% (p<0.05) compared to untreated cells. PCW increased GSIS by 51%. D3G was the most effective anthocyanin activating FFAR1 (EC50: 196.6 μM). PCW had activating potential on FFAR1 (EC50: 77 μg/mL). PCW, as well as C3G and D3G increased the expression of FFAR1, PLC, and phosphorylation of PKD, related to the FFAR1-dependent insulin secretory pathway. The treatment with 100 μM of P3G and C3G increased (p<0.05) glucose uptake in HepG2 cells by 19% and 31%. PCW increased the glucose uptake in HepG2 cells by 48%. It was determined that CF-P was the most effective for activating GK (EC50: 39.9 μM) and the PCW extracts had an efficacy of EC50: 44 μg/mL. The ANC in purple corn also reduced AMPK phosphorylation and PEPCK expression in HepG2 cells, known to be related to reduction in gluconeogenesis. It is demonstrated for the first time that dietary ANC can enhance the activity of novel biomarkers FFAR1 and GK and potentially ameliorate type-2 diabetes comorbidities.

Prolactin protects against cytokine-induced beta-cell death by NFκB and JNK inhibition

Type 1 diabetes is caused by an autoimmune assault that induces progressive beta-cell dysfunction and dead. Pro-inflammatory cytokines, such as interleukin 1 beta (IL1B), tumor necrosis factor (TNF) and interferon gamma (IFNG) contribute for beta-cell death, which involves the activation of the nuclear factor kappa B (NFκB) and c- Jun N-terminal kinase (JNK). Prolactin (PRL), a physiological mediator for beta-cell proliferation, was shown to protect beta cells against cytokines pro-apoptotic effects. We presently investigated the mechanisms involved in the protective effects of prolactin against cytokine-induced beta-cell death. The findings obtained indicate that STAT3 activation is involved in the anti-apoptotic role of PRL in rat beta cells. PRL prevents the activation of JNK via AKT and promotes a shift from expression of pro- to anti-apoptotic proteins downstream of the JNK cascade. Furthermore, PRL partially prevents the activation of NFκB and the transcription of its target genes IkBa, Fas, Mcp1, A20 and Cxcl10 and also decreases NO production. On the other hand, the pro-survival effects of PRL do not involve modulation of cytokine-induced endoplasmic reticulum stress. These results suggest that the beneficial effects of PRL in beta cells involve augmentation of anti-apoptotic mechanisms and, at the same time, reduction of pro-apoptotic effectors, rendering beta cells better prepared to deal with inflammatory insults. The better understanding of the pro-survival mechanisms modulated by PRL in beta cells can provide tools to prevent cell demise during an autoimmune attack or following islet transplantation.

Increased toxicity of amylin (Islet Amyloid Polypeptide) in beta cells induced by photochemical internalization

BACKGROUND

Amylin and oligomers formed from amylin are implicated in demise of beta cells in type 2 diabetes. However, whether putative toxicity is exerted intra or extracellularly is unclear. Use of photochemical internalization (PCI) technique may give clues for impact of intracellular toxicity.

AIM

(a) To optimize the concentration and exposure set up of the photosensitizing compound meso-disulfonated tetraphenyl chlorin TPCS_2a (Amphinex^®) for use in insulin producing beta cells and (b) to utilize the photosensitizing technique to probe for intracellular effects in beta cells by amylin.

MATERIALS AND METHODS

The titration of TPCS_2a and blue light exposure was evaluated by MTT assay. The insulin producing INS-1 832/13 beta cells were incubated with the photosensitizing agent TPCS_2a prior to exposure of amylin. Viability and function were further evaluated by standard biochemical techniques.

RESULTS

A protocol was developed for use in INS-1 832/13 cells in which the optimal concentration of TPCS_2a was found to be 4ng/ml. Using this protocol human amylin (10 μM, 8 h) in combination with TPCS_2a (4 ng/ml, 18 h) and blue light exposure (60 s) exerted toxic effects above those by TPCS_2a and illumination alone as measured by MTT (15 ± 3.6%, n = 6, p < 0.007) for effect of amylin exposure. On the other hand, rat amylin (which does not form oligomers) had no effect. Insulin secretion was non-significantly reduced by the combination of human amylin with TPCS_2a and illumination compared to TPCS_2a and illumination alone. Cellular insulin content was not affected, nor were measured parameters of apoptosis and necrosis.

CONCLUSION

PCI technology could be a useful tool to induce endosomal rupture in clonal beta cells. The present results using PCI are compatible with intracellular negative effects following exposure to amylin.

Distinct roles of systemic and local actions of insulin on pancreatic β-cells

OBJECTIVE

Pancreatic β-cell mass and function are critical in glucose homeostasis. Their regulatory mechanisms have been studied principally under experimental conditions of reduced β-cell numbers, such as β-cell ablation and partial pancreatectomy. In the present study, we generated an opposite mouse model with an excessive amount of ectopic β-cells, and analyzed its consequence on β-cell mass and survival.

METHODS

Mice underwent sub-renal transplantation (SRT) of pseudo-islets generated from a pancreatic β-cell line MIN6 or intra-pancreatic transplantation (IPT) of MIN6 cells, and morphological and functional changes of their endocrine pancreata were analyzed. Cellular fate of pancreatic β-cells after transplantation was traced using RipCre:Rosa26-tdTomato mice. By using MIN6 cells, we evaluated the roles of extracellular glucose, membrane potential, and insulin signaling on β-cell survival.

RESULTS

SRT mice developed severe, progressive hypoglycemia associated with marked reduction in insulin-positive (Ins+) cell mass and apparent increase in apoptotic Ins+ cells. In in vitro experiments of MIN6 cells, insulin signaling blockade potently induced cell death, suggesting that local insulin action is required for β-cell survival. In fact, IPT (i.e. transplantation close to endogenous β-cells) resulted in fewer apoptotic Ins+ cells compared with those induced by SRT. On the other hand, β-cell mass was decreased in proportion to the decrease in blood glucose levels in both SRT and IPT mice, suggesting a contribution of hypoglycemia induced by systemic hyperinsulinemia.

CONCLUSION

Insulin plays distinct roles in β-cell survival and β-cell mass regulation through its local and systemic actions on β-cells, respectively.

Knocking down Insulin Receptor in Pancreatic Beta Cell lines with Lentiviral-Small Hairpin RNA Reduces Glucose-Stimulated Insulin Secretion via Decreasing the Gene Expression of Insulin, GLUT2 and Pdx1

Type 2 diabetes (T2D) is a metabolic disorder characterized by beta cell dysfunction and insulin resistance in fat, muscle and liver cells. Recent studies have shown that the development of insulin resistance in pancreatic beta cell lines may contribute to beta cell dysfunction in T2D. However, there still is a lack of detailed investigations regarding the mechanisms by which insulin deficiency may contribute in diabetes. In this study, we firstly established a stable insulin receptor knockdown cell line in pancreatic beta cells INS-1 (InsRβKD cells) using anti InsRβ small hairpin RNA (InsRβ-shRNA) encoded by lentiviral vectors. The resultant InsRβKD cells demonstrated a significantly reduced expression of InsRβ as determined by real-time PCR and Western blotting analyses. Upon removing glucose from the medium, these cells exhibited a significant decrease in insulin gene expression and protein secretion in response to 20 mM glucose stimulation. In accordance with this insulin reduction, the glucose uptake efficiency as indicated by a ³[H]-2-deoxy-d-glucose assay also decreased. Furthermore, InsRβKD cells showed a dramatic decrease in glucose transporter 2 (GLUT2, encoded by SLC2A2) and pancreatic duodenal homeobox (Pdx1) mRNA expression compared to the controls. These data collectively suggest that pancreatic beta cell insulin resistance contributes to the development of beta cell dysfunction by impairing pancreatic beta cell glucose sensation through the Pdx1- GLUT2 pathway. InsRβKD cells provide a good model to further investigate the mechanism of β-cell dysfunction in T2D.

Mitochondrial Morphology and Function of the Pancreatic β-Cells INS-1 Model upon Chronic Exposure to Sub-Lethal Cadmium Doses

The impact of chronic cadmium exposure and slow accumulation on the occurrence and development of diabetes is controversial for human populations. Islets of Langerhans play a prominent role in the etiology of the disease, including by their ability to secrete insulin. Conversion of glucose increase into insulin secretion involves mitochondria. A rat model of pancreatic β-cells was exposed to largely sub-lethal levels of cadmium cations applied for the longest possible time. Cadmium entered cells at concentrations far below those inducing cell death and accumulated by factors reaching several hundred folds the basal level. The mitochondria reorganized in response to the challenge by favoring fission as measured by increased circularity at cadmium levels already ten-fold below the median lethal dose. However, the energy charge and respiratory flux devoted to adenosine triphosphate synthesis were only affected at the onset of cellular death. The present data indicate that mitochondria participate in the adaptation of β-cells to even a moderate cadmium burden without losing functionality, but their impairment in the long run may contribute to cellular dysfunction, when viability and β-cells mass are affected as observed in diabetes.

Pseudo-hemorrhagic region formation in pancreatic neuroendocrine tumors is a result of blood vessel dilation followed by endothelial cell detachment

Aberrant blood vessel formation and hemorrhage may contribute to tumor progression and are potential targets in the treatment of several types of cancer. Pancreatic neuroendocrine tumors (PNETs) are highly vascularized, particularly when they are well-differentiated. However, the process of vascularization and endothelial cell detachment in PNETs is poorly understood. In the present study, 132 PNET clinical samples were examined and a special type of hemorrhagic region was observed in ~30% of the samples regardless of tumor subtype. These hemorrhagic regions were presented as blood-filled caverns with a smooth boundary and were unlined by endothelial cells. Based on the extensive endothelial cell detachment observed in the clinical samples, the formation process of these blood-filled caverns was hypothesized. Blood vessel dilation followed by detachment of endothelial cells from the surrounding tumor tissue was speculated. This was further supported using an INS-1 xenograft insulinoma model. As the formation process was distinct from the typical diffusive hemorrhage, it was named ‘pseudo-hemorrhage’. Furthermore, it was demonstrated that epithelial (E-) cadherin and β-catenin were overexpressed in tumor cells surrounding these pseudo-hemorrhagic regions. Therefore, even though no statistically significant association of pseudo-hemorrhage with clinical features (metastasis or disease recurrence) was identified, the high levels of E-cadherin and β-catenin expression may suggest that a number of features of normal islet cells are retained.

The neuroendocrine phenotype, genomic profile and therapeutic sensitivity of GEPNET cell lines

Experimental models of neuroendocrine tumour disease are scarce, and no comprehensive characterisation of existing gastroenteropancreatic neuroendocrine tumour (GEPNET) cell lines has been reported. In this study, we aimed to define the molecular characteristics and therapeutic sensitivity of these cell lines. We therefore performed immunophenotyping, copy number profiling, whole-exome sequencing and a large-scale inhibitor screening of seven GEPNET cell lines. Four cell lines, GOT1, P-STS, BON-1 and QGP-1, displayed a neuroendocrine phenotype while three others, KRJ-I, L-STS and H-STS, did not. Instead, these three cell lines were identified as lymphoblastoid. Characterisation of remaining authentic GEPNET cell lines by copy number profiling showed that GOT1, among other chromosomal alterations, harboured losses on chromosome 18 encompassing the SMAD4 gene, while P-STS had a loss on 11q. BON-1 had a homozygous loss of CDKN2A and CDKN2B, and QGP-1 harboured amplifications of MDM2 and HMGA2 Whole-exome sequencing revealed both disease-characteristic mutations (e.g. ATRX mutation in QGP-1) and, for patient tumours, rare genetic events (e.g. TP53 mutation in P-STS, BON-1 and QGP-1). A large-scale inhibitor screening showed that cell lines from pancreatic NETs to a greater extent, when compared to small intestinal NETs, were sensitive to inhibitors of MEK. Similarly, neuroendocrine NET cells originating from the small intestine were considerably more sensitive to a group of HDAC inhibitors. Taken together, our results provide a comprehensive characterisation of GEPNET cell lines, demonstrate their relevance as neuroendocrine tumour models and explore their therapeutic sensitivity to a broad range of inhibitors.

Cellular models for beta-cell function and diabetes gene therapy

Diabetes is characterized by the destruction and/or relative dysfunction of insulin-secreting beta-cells in the pancreatic islets of Langerhans. Consequently, considerable effort has been made to understand the physiological processes governing insulin production and secretion in these cells and to elucidate the mechanisms involved in their deterioration in the pathogenesis of diabetes. To date, considerable research has exploited clonal beta-cell lines derived from rodent insulinomas. Such cell lines have proven to be a great asset in diabetes research, in vitro drug testing, and studies of beta-cell physiology and provide a sustainable, and in many cases, more practical alternative to the use of animals or primary tissue. However, selection of the most appropriate rodent beta cell line is often challenging and no single cell line entirely recapitulates the properties of human beta-cells. The generation of stable human beta-cell lines would provide a much more suitable model for studies of human beta-cell physiology and pathology and could potentially be used as a readily available source of implantable insulin-releasing tissue for cell-based therapies of diabetes. In this review, we discuss the history, development, functional characteristics and use of available clonal rodent beta-cell lines, as well as reflecting on recent advances in the generation of human-derived beta-cell lines, their use in research studies and their potential for cell therapy of diabetes.

Androgen signaling expands β-cell mass in male rats and β-cell androgen receptor is degraded under high-glucose conditions

A deficient pancreatic β-cell mass increases the risk of type 2 diabetes mellitus. Here, we investigated the effects of testosterone on the development of pancreatic β-cell mass in male rats. The β-cell mass of male rats castrated at 6 wk of age was reduced to ~30% of that of control rats at 16 wk of age, and castration caused glucose intolerance. Loss of β-cell mass occurred because of decreases in islet density per pancreas and β-cell cluster size. Castration was negatively associated with the number of Ki-67-positive β-cells and positively associated with the number of TUNEL-positive β-cells. These β-cell changes could be prevented by testosterone treatment. In contrast, castration did not affect β-cell mass in male mice. Androgen receptor (AR) localized differently in mouse and rat β-cells. Testosterone enhanced the viability of INS-1 and INS-1 #6, which expresses high levels of AR, in rat β-cell lines. siRNA-mediated AR knockdown or AR antagonism with hydroxyflutamide attenuated this enhancement. Moreover, testosterone did not stimulate INS-1 β-cell viability under high d-glucose conditions. In INS-1 β-cells, d-glucose dose dependently (5.5-22.2 mM) downregulated AR protein levels both in the presence and absence of testosterone. The intracellular calcium chelator (BAPTA-AM) could prevent this decrease in AR expression. AR levels were also reduced by a calcium ionophore (A23187), but not by insulin, in the absence of the proteasome inhibitor MG132. Our results indicate that testosterone regulates β-cell mass, at least in part, by AR activation in the β-cells of male rats and that the β-cell AR is degraded under hyperglycemic conditions.

Reproducible insulin secretion from isolated rat pancreas preparations using an organ bath

Diabetes mellitus is a lifestyle-related disease that is characterized by inappropriate or diminished insulin secretion. Ex vivo pharmacological studies of hypoglycemic agents are often conducted using perfused pancreatic preparations. Pancreas preparations for organ bath experiments do not require cannulation and are therefore less complex than isolated perfused pancreas preparations. However, previous research has generated almost no data on insulin secretion from pancreas preparations using organ bath preparations. The purpose of this study was to investigate the applicability of isolated rat pancreas preparations using the organ bath technique in the quantitative analysis of insulin secretion from β-cells. We found that insulin secretion significantly declined during incubation in the organ bath, whereas it was maintained in the presence of 1 µM GLP-1. Conversely, amylase secretion exhibited a modest increase during incubation and was not altered in the presence of GLP-1. These results demonstrate that the pancreatic organ bath preparation is a sensitive and reproducible method for the ex vivo assessment of the pharmacological properties of hypoglycemic agents.

A type 2 diabetes-associated SNP in KCNQ1 (rs163184) modulates the binding activity of the locus for Sp3 and Lsd1/Kdm1a, potentially affecting CDKN1C expression

Although genome-wide association studies have shown that potassium voltage-gated channel subfamily Q member 1 (KCNQ1) is one of the genes that is most significantly associated with type 2 diabetes mellitus (T2DM), functionally annotating disease-associated single nucleotide polymorphisms (SNPs) remains a challenge. Recently, our group described a novel strategy to identify proteins that bind to SNP-containing loci in an allele-specific manner. The present study successfully applied this strategy to investigate rs163184, a T2DM susceptibility SNP located in the intronic region of KCNQ1. Comparative analysis of DNA-binding proteins revealed that the binding activities for the genomic region containing SNP rs163184 differed between alleles for several proteins, including Sp3 and Lsd1/Kdm1a. Sp3 preferentially bound to the non-risk rs163184 allele and stimulated transcriptional activity in an artificial promoter containing this region. Lsd1/Kdm1a was identified to be preferentially recruited to the non-risk allele of the rs163184 region and reduced Sp3-dependent transcriptional activity in the artificial promoter. In addition, expression of the nearby cyclin‑dependent kinase inhibitor 1C (CDKN1C) gene was revealed to be upregulated after SP3 knockdown in cells that possessed non-risk alleles. This suggests that CDKN1C is potentially one of the functional targets of SNP rs163184, which modulates the binding activity of the locus for Sp3 and Lsd1/Kdm1a.