Beta-cell defence and repair mechanisms in human pancreatic islets

Use of human islets to understand islet biology and diabetes: progress, challenges and suggestions

Over the last two decades, improved access to human islets and the development of human islet distribution networks have enabled the use of millions of human islets in hundreds of scientific research projects, leading to a dramatic increase in our understanding of human islet biology. Here we discuss recent scientific advances as well as methodological and experimental challenges that impact human islet quality, experimental outcomes and the reporting of human islets used in scientific publications. In a survey of over 200 scientific publications with human islet experimentation, we found that the reporting of critical information was quite variable, sometimes obscure, and often failed to adequately outline the experiments and results using human islets. As the complexity of human islet research grows, we propose that members of the human islet research ecosystem work together to develop procedures and approaches for accessible and transparent collecting and reporting of crucial human islet characteristics and, through this, enhance collaboration, reproducibility and rigour, leading to further advances in our understanding of human islet biology.

Characterization of Apoptosis, Autophagy and Oxidative Stress in Pancreatic Islets Cells and Intestinal Epithelial Cells Isolated from Equine Metabolic Syndrome (EMS) Horses

Endocrine disorders are becoming an increasing problem in both human and veterinary medicine. In recent years, more and more horses worldwide have been suffering from equine metabolic syndrome (EMS). This metabolic disorder is characterized by pathological obesity, hyperinsulinaemia, hyperglycaemia and insulin resistance. Although metabolic disorders, including diabetes, have been extensively studied, there are still no data on the molecular effects of EMS in horses. Thus, the aim of this study was to evaluate apoptosis, oxidative stress, autophagy and microRNA (miR) expression in multipotent intestinal epithelial stem cells (IECs) and pancreatic islets (PIs) isolated post mortem form healthy and EMS diagnosed horses. Our group was the first to describe how EMS affects IEC and PI aging and senescence. First, we evaluated isolation and culture protocol for these cells and subsequently established their metabolic status in vitro. Both IECs and PIs isolated from EMS horses were characterized by increased apoptosis and senescence. Moreover, they accumulated elevated levels of reactive oxygen species (ROS). Here we have observed that autophagy/mitophagy may be a protective mechanism which allows those cells to maintain their physiological function, clear protein aggregates and remove damaged organelles. Furthermore, it may play a crucial role in reducing endoplasmic reticulum (ER) stress. This protective mechanism may help to overcome the harmful effects of ROS and provide building blocks for protein and ATP synthesis.

The Role of Oxidative Stress and Hypoxia in Pancreatic Beta-Cell Dysfunction in Diabetes Mellitus

SIGNIFICANCE

Metabolic syndrome is a frequent precursor of type 2 diabetes mellitus (T2D), a disease that currently affects ∼8% of the adult population worldwide. Pancreatic beta-cell dysfunction and loss are central to the disease process, although understanding of the underlying molecular mechanisms is still fragmentary. Recent Advances: Oversupply of nutrients, including glucose and fatty acids, and the subsequent overstimulation of beta cells, are believed to be an important contributor to insulin secretory failure in T2D. Hypoxia has also recently been implicated in beta-cell damage. Accumulating evidence points to a role for oxidative stress in both processes. Although the production of reactive oxygen species (ROS) results from enhanced mitochondrial respiration during stimulation with glucose and other fuels, the expression of antioxidant defense genes is unusually low (or disallowed) in beta cells.

CRITICAL ISSUES

Not all subjects with metabolic syndrome and hyperglycemia go on to develop full-blown diabetes, implying an important role in disease risk for gene-environment interactions. Possession of common risk alleles at the SLC30A8 locus, encoding the beta-cell granule zinc transporter ZnT8, may affect cytosolic Zn2+ concentrations and thus susceptibility to hypoxia and oxidative stress.

FUTURE DIRECTIONS

Loss of normal beta-cell function, rather than total mass, is increasingly considered to be the major driver for impaired insulin secretion in diabetes. Better understanding of the role of oxidative changes, its modulation by genes involved in disease risk, and effects on beta-cell identity may facilitate the development of new therapeutic strategies to this disease. Antioxid. Redox Signal. 26, 501-518.

Species-Related Differences in the Proteome of Rat and Human Pancreatic Beta Cells

The core proteomes of human and rat pancreatic beta cells were compared by label-free LC-MS/MS: this resulted in quantification of relative molar abundances of 707 proteins belonging to functional pathways of intermediary metabolism, protein synthesis, and cytoskeleton. Relative molar abundances were conserved both within and between pathways enabling the selection of a housekeeping network for geometric normalization and the analysis of potentially relevant differential expressions. Human beta cells differed from rat beta cells in their lower level of enzymes involved in glucose sensing (MDH1, PC, and ACLY) and upregulation of lysosomal enzymes. Human cells also expressed more heat shock proteins and radical scavenging systems: apart from SOD2, they expressed high levels of H2O2-scavenger peroxiredoxin 3 (PRDX3), confirmed by microarray, Western blotting, and microscopy. Besides conferring lower susceptibility to oxidative stress to human cells PRDX3 might also play a role in physiological redox regulation as, in rat, its expression was restricted to a beta cell subset with higher metabolic glucose responsiveness. In conclusion, although their core proteomic architecture is conserved, human and rat beta cells differ in their molar expression of key enzymes involved in glucose sensing and redox control.

The transcription factor C/EBP delta has anti-apoptotic and anti-inflammatory roles in pancreatic beta cells

In the course of Type 1 diabetes pro-inflammatory cytokines (e.g., IL-1β, IFN-γ and TNF-α) produced by islet-infiltrating immune cells modify expression of key gene networks in β-cells, leading to local inflammation and β-cell apoptosis. Most known cytokine-induced transcription factors have pro-apoptotic effects, and little is known regarding “protective” transcription factors. To this end, we presently evaluated the role of the transcription factor CCAAT/enhancer binding protein delta (C/EBPδ) on β-cell apoptosis and production of inflammatory mediators in the rat insulinoma INS-1E cells, in purified primary rat β-cells and in human islets. C/EBPδ is expressed and up-regulated in response to the cytokines IL-1β and IFN-γ in rat β-cells and human islets. Small interfering RNA-mediated C/EBPδ silencing exacerbated IL-1β+IFN-γ-induced caspase 9 and 3 cleavage and apoptosis in these cells. C/EBPδ deficiency increased the up-regulation of the transcription factor CHOP in response to cytokines, enhancing expression of the pro-apoptotic Bcl-2 family member BIM. Interfering with C/EBPδ and CHOP or C/EBPδ and BIM in double knockdown approaches abrogated the exacerbating effects of C/EBPδ deficiency on cytokine-induced β-cell apoptosis, while C/EBPδ overexpression inhibited BIM expression and partially protected β-cells against IL-1β+IFN-γ-induced apoptosis. Furthermore, C/EBPδ silencing boosted cytokine-induced production of the chemokines CXCL1, 9, 10 and CCL20 in β-cells by hampering IRF-1 up-regulation and increasing STAT1 activation in response to cytokines. These observations identify a novel function of C/EBPδ as a modulatory transcription factor that inhibits the pro-apoptotic and pro-inflammatory gene networks activated by cytokines in pancreatic β-cells.

Oxidative stress and beta-cell dysfunction

Diabetes mellitus type 1 and 2 (T1DM and T2DM) are complex multifactorial diseases. Loss of beta-cell function caused by reduced secretory capacity and enhanced apoptosis is a key event in the pathogenesis of both diabetes types. Oxidative stress induced by reactive oxygen and nitrogen species is critically involved in the impairment of beta-cell function during the development of diabetes. Because of their low antioxidant capacity, beta-cells are extremely sensitive towards oxidative stress. In beta-cells, important targets for an oxidant insult are cell metabolism and K(ATP) channels. The oxidant-evoked alterations of K(ATP) channel activity seem to be critical for oxidant-induced dysfunction because genetic ablation of K(ATP) channels attenuates the effects of oxidative stress on beta-cell function. Besides the effects on metabolism, interference of oxidants with mitochondria induces key events in apoptosis. Consequently, increasing antioxidant defence is a promising strategy to delay beta cell failure in (pre)-diabetic patients or during islet transplantation. Knock-out of K(ATP) channels has beneficial effects on oxidant-induced inhibition of insulin secretion and cell death. Interestingly, these effects can be mimicked by sulfonylureas that have been used in the treatment of T2DM for many years. Loss of functional K(ATP) channels leads to up-regulation of antioxidant enzymes, a process that depends on cytosolic Ca(2+). These observations are of great importance for clinical intervention because they show a possibility to protect beta-cells at an early stage before dramatic changes of the secretory capacity and loss of cell mass become manifest and lead to glucose intolerance or even overt diabetes.

Protection against cytokine toxicity through endoplasmic reticulum and mitochondrial stress prevention by prostacyclin synthase overexpression in insulin-producing cells

Proinflammatory cytokines play a crucial role in the pathogenesis of type 1 diabetes mellitus. One of the cytokine-regulated pathways mediating inflammation in this autoimmune disease is the arachidonic acid metabolism pathway, comprising both the induction of cyclooxygenases and the production of different prostaglandins. Cytokine toxicity is mediated in many cell types, including pancreatic beta cells through this pathway. Interestingly, some cell types have been shown to be insensitive to such toxicity, and this correlated with a high expression of prostacyclin synthase (PGIS). Using insulin-producing RINm5F cells as a model for pancreatic beta cells, PGIS was overexpressed and exhibited a large protective effect against cytokine toxicity. This protective effect of PGIS against cytokine toxicity correlated with a decreased activation of the transcription factor NFkappaB and the inducible NO synthase promoter as well as a reduced inducible NO synthase protein expression and nitrite production. A reduction in the cytokine-stimulated endoplasmic reticulum and mitochondrial stress was also found in the PGIS-overexpressing cells. Moreover, cytokine-induced caspase-3 activation and reduction of glucose oxidation and cell proliferation were suppressed. Thus, PGIS overexpression apparently protects insulin-producing cells against cytokine toxicity via suppression of endoplasmic reticulum and mitochondrial stress-mediated cell death pathways.

MnTMPyP, a metalloporphyrin-based superoxide dismutase/catalase mimetic, protects INS-1 cells and human pancreatic islets from an in vitro oxidative challenge

AIMS

Pancreatic islets can be lost early following allotransplantation from oxidative stress. Antioxidant enzyme overexpression could confer a beneficial effect on islets exposed to reactive oxygen species (ROS) and nitrogen species. Here, we tested the effect of MnTMPyP, a superoxide dismutase/catalase mimetic.

METHODS

INS-1 insulin-secreting cells or human islets were cultured with MnTMPyP and exposed to a superoxide donor (the hypoxanthine/xanthine oxidase (HX/XO) system), a nitric oxide donor [3-morpholinosydnonimine (SIN-1)] or menadione. Viability of INS-1 cells was assessed by WST-1 colorimetric assay and FACS analysis (Live/Dead test). ROS production was determined using fluorescent probes. Islet viability was estimated by WST-1 assay and endocrine function by static incubation.

RESULTS

Following MnTMPyP treatment, ROS production in INS-1 cells was reduced by 4- to 20-fold upon HX/XO challenge and up to 2-fold upon SIN-1 stress. This phenomenon correlated with higher viability measured by WST-1 or Live/Dead test. MnTMPyP preserved islet viability upon exposure to SIN-1 or menadione but not upon an HX/XO challenge. Similarly, decrease in insulin secretion tended to be less pronounced in MnTMPyP-treated islets than in control islet when exposed to SIN-1, but no changes were noticed during an HX/XO stress.

CONCLUSIONS

MnTMPyP was able to improve the viability of INS-1 cells and human islets exposed to oxidative challenges in vitro. Protection of INS-1 cells could be as high as 90%. This agent is therefore potentially attractive in situations involving the overproduction of ROS, such as islet transplantation.

Evidence of expression of endotoxin receptors CD14, toll-like receptors TLR4 and TLR2 and associated molecule MD-2 and of sensitivity to endotoxin (LPS) in islet beta cells

CD14, a GPI-linked membrane protein, is a component of the lipopolysaccharide (LPS) receptor complex, one of the pattern-recognizing receptors (PRR) expressed by myeloid lineage cells. Here we report that CD14, the functionally linked toll-like receptor molecules, TLR2 and TLR4, and the associated molecule MD-2 are expressed in endocrine cells of the human pancreatic islets. CD14 expression in human pancreatic islets was determined by immunofluorescence staining of tissue sections and primary cultures, and confirmed by flow cytometry of dispersed normal islets and SV40-transformed islet cells (HP62). The latter cells synthesized and secreted CD14 in response to lipopolysaccharide (LPS) in a time- and dose-dependent manner. Reverse transcription polymerase chain reaction (RT-PCR)-Southern was positive for CD14, TLR2, TLR4 and MD-2 in human pancreas, purified islets and HP62 cells. In vitro experiments using rat islets (also positive for CD14 by RT-PCR) and HP62 cells showed that LPS regulates glucose-dependent insulin secretion and induces inflammatory cytokines [interleukin (IL)-1alpha, IL-6 and tumour necrosis factor (TNF)-alpha]. The functional expression of CD14 and associated molecules in islet beta cells adds a new pathway that islet cells may follow to adjust their function to endotoxaemia situations and become vulnerable to the inflammatory events that occur during diabetogenic insulitis.

Protection of insulin-secreting INS-1 cells against oxidative stress through adenoviral-mediated glutathione peroxidase overexpression

OBJECTIVES

A large fraction of an islet graft can be lost early following allotransplantation from various non specific mechanisms including oxidative stress. Overexpression of antioxidant enzymes could confer a beneficial effect on islets exposed to reactive oxygen and nitrogen species. We examined the viability of beta cells driven to overexpress glutathione peroxidase (GPx) and exposed to a superoxide donor (hypoxanthine/xanthine oxidase HX/XO) and a nitric oxide donor (3-morpholinosydnonimine SIN-1).

METHODS

Cultured INS-1 rat-derived insulin-secreting cells were transfected by an E1-deleted adenovirus carrying GPx cDNA (AdGPx). Additional experiments were performed with an adenovector carrying Cu/Zn superoxide dismutase cDNA (AdSOD). Cellular viability was tested by the WST-1 colorimetric assay and functionality by static incubation.

RESULTS

AdGPx increased GPx activity within 48 hours from 0 (untransfected cells) to 60 +/- 11 U/g (cells transfected at an MOI of 25: 1). GPx overexpression significantly reduced cytotoxicity induced by HX/XO from 10.81 +/- 1.41 to 5.42 +/- 2.62% at 10 mU/ml and from 61.19 +/- 4.17 to 52.9 +/- 4.39% at 20 mU/ml (p=0.0002, transfected cells vs control cells). Doses of SIN-1 from 600 to 1000 micromol/l resulted in cytotoxicity ranging from 17.66 +/- 3.48 to 45.97 +/- 6.48% in control cells and from 5.65 +/- 1.37 to 35.80 +/- 5.59% in AdGPx transfected cells (p=0.015). The combination of AdGPx and AdSOD did not exhibit any synergistic cytoprotective effect. Control cells exposed to a HX/XO stress exhibited a reduction in glucose-theophylline stimulated insulin secretion by half, while stressed GPx overexpressing-cells maintained the same insulin secretion level than non-stressed cells.

CONCLUSIONS

Adenoviral-induced overexpression of GPx enhances the resistance of a rat beta cell line to both reactive oxygen (ROS) and reactive nitrogen species (RNS) cytotoxicity. Transposition of these findings to human islet transplantation with a clinically-relevant procedure deserves further investigations.

High glucose causes apoptosis in cultured human pancreatic islets of Langerhans: a potential role for regulation of specific Bcl family genes toward an apoptotic cell death program

Type 2 diabetes is characterized by insulin resistance and inadequate insulin secretion. In the advanced stages of the disease, beta-cell dysfunction worsens and insulin therapy may be necessary to achieve satisfactory metabolic control. Studies in autopsies found decreased beta-cell mass in pancreas of people with type 2 diabetes. Apoptosis, a constitutive program of cell death modulated by the Bcl family genes, has been implicated in loss of beta-cells in animal models of type 2 diabetes. In this study, we compared the effect of 5 days' culture in high glucose concentration (16.7 mmol/l) versus normal glucose levels (5.5 mmol/l) or hyperosmolar control (mannitol 11 mmol/l plus glucose 5 mmol/l) on the survival of human pancreatic islets. Apoptosis, analyzed by flow cytometry and electron and immunofluorescence microscopy, was increased in islets cultured in high glucose (HG5) as compared with normal glucose (NG5) or hyperosmolar control (NG5+MAN5). We also analyzed by reverse transcriptase-polymerase chain reaction and Western blotting the expression of the Bcl family genes in human islets cultured in normal glucose or high glucose. The antiapoptotic gene Bcl-2 was unaffected by glucose change, whereas Bcl-xl was reduced upon treatment with HG5. On the other hand, proapoptotic genes Bad, Bid, and Bik were overexpressed in the islets maintained in HG5. To define the pancreatic localization of Bcl proteins, we performed confocal immunofluorescence analysis on human pancreas. Bad and Bid were specifically expressed in beta-cells, and Bid was also expressed, although at low levels, in the exocrine pancreas. Bik and Bcl-xl were expressed in other endocrine islet cells as well as in the exocrine pancreas. These data suggest that in human islets, high glucose may modulate the balance of proapoptotic and antiapoptotic Bcl proteins toward apoptosis, thus favoring beta-cell death.

Natural resistance of human beta cells toward nitric oxide is mediated by heat shock protein 70

Human beta cells exhibit increased resistance against nitric oxide (NO) radicals as compared with rodent islet cells. Here we tested whether endogenous heat shock protein 70 (hsp70) accounts for the resistance of human cells. Stable transfection of the human beta cell line CM with an antisense hsp70 mRNA-expressing plasmid (ashsp70) caused selective suppression (>95%) of spontaneously expressed hsp70 but not of hsc70 or GRP75 protein. ashsp70 transfection abolished the resistance of CM cells to the NO donors (Z)-1- (2-(2-aminoethyl)-N-(2-ammonioethyl)amino)diazen-1-ium -1,2-diolate and sodium nitroprusside and increased the proportions of necrotic cells 3-5-fold (p < 0.05) and of apoptotic cells about 2-fold (p < 0.01). Re-induction of hsp70 expression by heat shock re-established resistance to NO toxicity. hsp70 did not exert its protective effect at the level of membrane lipid integrity because radical induced lipid peroxidation appeared independent of hsp70 expression. However, after NO exposure only hsp70-deficient cells showed significantly decreased mitochondrial activity, by 40-80% (p < 0.01). These results suggest a key role of hsp70 in the natural resistance of human beta cells against NO induced injury, by preserving mitochondrial function. These findings provide important implications for the development of beta cell protective strategies in type 1 diabetes and islet transplantation.

Constitutive differences in antioxidant defense status distinguish alloxan-resistant and alloxan-susceptible mice

Alloxan (AL), a potent generator of superoxide and hydroxyl radicals, selectively destroys rodent pancreatic beta-cells. Alloxan-susceptible (ALS/Lt) and AL-resistant (ALR/Lt) are inbred mouse strains derived in Japan by inbreeding CD-1 (ICR) mice with concomitant selection for high or low sensitivity to a relatively low AL dose. The present study was undertaken to examine whether resistance was mediated by differences in either systemic or beta-cell antioxidant defense status. Superoxide dismutase (SOD), catalase (CAT), glutathione reductase (GR), and glutathione peroxidase (GPX) activities were determined in tissues of AL-untreated ALR/Lt and ALS/Lt male mice at 7 weeks of age. Specific activities of pancreatic SOD1, GR, and GPX were significantly increased in ALR/Lt mice compared with ALS/Lt mice. ALR/Lt mice further exhibited higher levels of glutathione in plasma, blood, pancreas, and liver combined with lower constitutive lipid peroxides in serum, liver, and pancreas. These results support the hypothesis that the selection process leading to the development of an AL-resistant mouse strain entailed accumulation of a gene or genes contributing to upregulated antioxidant status.

The Dual Role of Nitric Oxide in Islet beta-Cells

In pancreatic islets, nitric oxide (NO) produced on exposure to cytokines mediates beta-cell injury leading to diabetes mellitus. On the other hand, L-arginine-derived NO may participate in the signal transduction pathway of physiological insulin secretion. This review focuses on the dual role of NO as a mediator of physiological and pathophysiological processes in pancreatic islets.

Cytokines induce deoxyribonucleic acid strand breaks and apoptosis in human pancreatic islet cells

We have previously observed that a 6-day exposure of human pancreatic islets to a combination of cytokines (interleukin-1beta 50 U/ml + tumour necrosis factor-alpha 1000 U/ml + interferon-gamma 1000 U/ml) severely impairs beta-cell functions. In the present study, we examined whether this condition affects DNA integrity and viability of human islet cells. Cells were studied after 3, 6, and 9 days of cytokine treatment by both single cell gel electrophoresis (the "comet assay," a sensitive method for detection of DNA strand breaks) and by a cytotoxicity assay using the DNA binding dyes Hoechst 33342 and propidium iodide as indices for the number of viable, necrotic, and apoptotic cells. Cytokine treatment for 6 and 9 days resulted in a 50% increase in comet length (P < 0.01 vs. controls), indicating DNA strand breaks, as well as in a significant increase in the number of apoptotic cells (P < 0.02 vs. controls), but not in the number of necrotic cells. The arginine analogs N(G)-nitro-L-arginine and N(G)-monomethyl-L-arginine prevented nitric oxide formation by the cytokines but did not interfere with cytokine-induced DNA strand breaks and apoptosis. The present data suggest that prolonged (6-9 days) exposure of human pancreatic islets to a mixture of cytokines induces DNA strand breaks and cell death by apoptosis. These deleterious effects of cytokines appear to be independent of nitric oxide generation.

Sensitivity of human pancreatic islets to peroxynitrite-induced cell dysfunction and death

Nitric oxide and peroxynitrite (generated by the reaction of nitric oxide with the superoxide anion) may both be mediators of beta-cell damage in early insulin-dependent diabetes mellitus. We observed that acute exposure of primary cultured human pancreatic islets to peroxynitrite results in a significant decrease in glucose oxidation and islet retrieval. DNA strand breaks in single human and rat islet cells are detectable after acute peroxynitrite exposure, followed by a decrease in islet cell survival after 1 h and 24 h. Cell death appeared to occur via a toxic cell death mechanism (necrosis) rather than apoptosis, as suggested by vital staining and ultrastructural evidence of early membrane and organelle degradation, mitochondrial swelling and loss of matrix. This study demonstrates for the first time that cultured human pancreatic islets are susceptible to the noxious effects of peroxynitrite.