Isolation of islets of Langerhans from rodent pancreas

β-Cell-selective regulation of gene expression by nitric oxide

Nitric oxide is produced at low micromolar levels following the induction of inducible nitric oxide synthase (iNOS) and is responsible for mediating the inhibitory actions of cytokines on glucose-stimulated insulin secretion by islets of Langerhans. It is through the inhibition of mitochondrial oxidative metabolism, specifically aconitase and complex 4 of the electron transport chain, that nitric oxide inhibits insulin secretion. Nitric oxide also attenuates protein synthesis, induces DNA damage, activates DNA repair pathways, and stimulates stress responses (unfolded protein and heat shock) in β-cells. In this report, the time- and concentration-dependent effects of nitric oxide on the expression of six genes known to participate in the response of β-cells to this free radical were examined. The genes included Gadd45α (DNA repair), Puma (apoptosis), Hmox1 (antioxidant defense), Hsp70 (heat shock), Chop (UPR), and Ppargc1α (mitochondrial biogenesis). We show that nitric oxide stimulates β-cell gene expression in a narrow concentration range of ∼0.5-1 µM or levels corresponding to iNOS-derived nitric oxide. At concentrations greater than 1 µM, nitric oxide fails to stimulate gene expression in β-cells, and this is associated with the inhibition of mitochondrial oxidative metabolism. This narrow concentration range of responses is β-cell selective, as the actions of nitric oxide in non-β-cells (α-cells, mouse embryonic fibroblasts, and macrophages) are concentration dependent. Our findings suggest that β-cells respond to a narrow concentration range of nitric oxide that is consistent with the levels produced following iNOS induction, and that these concentration-dependent actions are selective for insulin-containing cells.

Zinc-chelating BET bromodomain inhibitors equally target islet endocrine cell types

Inhibition of the bromodomain and extraterminal domain (BET) protein family is a potential strategy to prevent and treat diabetes; however, the clinical use of BET bromodomain inhibitors (BETis) is associated with adverse effects. Here, we explore a strategy for targeting BETis to β cells by exploiting the high-zinc (Zn2+) concentration in β cells relative to other cell types. We report the synthesis of a novel, Zn2+-chelating derivative of the pan-BETi (+)-JQ1, (+)-JQ1-DPA, in which (+)-JQ1 was conjugated to dipicolyl amine (DPA). As controls, we synthesized (+)-JQ1-DBA, a non-Zn2+-chelating derivative, and (-)-JQ1-DPA, an inactive enantiomer that chelates Zn2+. Molecular modeling and biophysical assays showed that (+)-JQ1-DPA and (+)-JQ1-DBA retain potent binding to BET bromodomains in vitro. Cellular assays demonstrated (+)-JQ1-DPA attenuated NF-ĸB target gene expression in β cells stimulated with the proinflammatory cytokine interleukin 1β. To assess β-cell selectivity, we isolated islets from a mouse model that expresses green fluorescent protein in insulin-positive β cells and mTomato in insulin-negative cells (non-β cells). Surprisingly, Zn2+ chelation did not confer β-cell selectivity as (+)-JQ1-DPA was equally effective in both β and α cells; however, (+)-JQ1-DPA was less effective in macrophages, a nonendocrine islet cell type. Intriguingly, the non-Zn2+-chelating derivative (+)-JQ1-DBA displayed the opposite selectivity, with greater effect in macrophages compared with (+)-JQ1-DPA, suggesting potential as a macrophage-targeting molecule. These findings suggest that Zn2+-chelating small molecules confer endocrine cell selectivity rather than β-cell selectivity in pancreatic islets and provide valuable insights and techniques to assess Zn2+ chelation as an approach to selectively target small molecules to pancreatic β cells.NEW & NOTEWORTHY Inhibition of BET bromodomains is a novel potential strategy to prevent and treat diabetes mellitus. However, BET inhibitors have negative side effects. We synthesized a BET inhibitor expected to exploit the high zinc concentration in β cells to accumulate in β cells. We show our inhibitor targeted pancreatic endocrine cells; however, it was less effective in immune cells. A control inhibitor showed the opposite effect. These findings help us understand how to target specific cells in diabetes treatment.

Deletion of Thioredoxin Reductase Disrupts Redox Homeostasis and Impairs β-Cell Function

Reactive oxygen species (ROS) have been implicated as mediators of pancreatic β-cell damage. While β-cells are thought to be vulnerable to oxidative damage, we have shown, using inhibitors and acute depletion, that thioredoxin reductase, thioredoxin, and peroxiredoxins are the primary mediators of antioxidant defense in β-cells. However, the role of this antioxidant cycle in maintaining redox homeostasis and β-cell survival in vivo remains unclear. Here, we generated mice with a β-cell specific knockout of thioredoxin reductase 1 (Txnrd1^fl/fl; Ins1^Cre/+ , βKO). Despite blunted glucose-stimulated insulin secretion, knockout mice maintain normal whole-body glucose homeostasis. Unlike pancreatic islets with acute Txnrd1 inhibition, βKO islets do not demonstrate increased sensitivity to ROS. RNA-sequencing analysis revealed that Txnrd1-deficient β-cells have increased expression of nuclear factor erythroid 2-related factor 2 (Nrf2)-regulated genes, and altered expression of genes involved in heme and glutathione metabolism, suggesting an adaptive response. Txnrd1-deficient β-cells also have decreased expression of factors controlling β-cell function and identity which may explain the mild functional impairment. Together, these results suggest that Txnrd1-knockout β-cells compensate for loss of this essential antioxidant pathway by increasing expression of Nrf2-regulated antioxidant genes, allowing for protection from excess ROS at the expense of normal β-cell function and identity.

N-terminal BET bromodomain inhibitors disrupt a BRD4-p65 interaction and reduce inducible nitric oxide synthase transcription in pancreatic β-cells

Chronic inflammation of pancreatic islets is a key driver of β-cell damage that can lead to autoreactivity and the eventual onset of autoimmune diabetes (T1D). In the islet, elevated levels of proinflammatory cytokines induce the transcription of the inducible nitric oxide synthase (iNOS) gene, NOS2, ultimately resulting in increased nitric oxide (NO). Excessive or prolonged exposure to NO causes β-cell dysfunction and failure associated with defects in mitochondrial respiration. Recent studies showed that inhibition of the bromodomain and extraterminal domain (BET) family of proteins, a druggable class of epigenetic reader proteins, prevents the onset and progression of T1D in the non-obese diabetic mouse model. We hypothesized that BET proteins co-activate transcription of cytokine-induced inflammatory gene targets in β-cells and that selective, chemotherapeutic inhibition of BET bromodomains could reduce such transcription. Here, we investigated the ability of BET bromodomain small molecule inhibitors to reduce the β-cell response to the proinflammatory cytokine interleukin 1 beta (IL-1β). BET bromodomain inhibition attenuated IL-1β-induced transcription of the inflammatory mediator NOS2 and consequent iNOS protein and NO production. Reduced NOS2 transcription is consistent with inhibition of NF-κB facilitated by disrupting the interaction of a single BET family member, BRD4, with the NF-κB subunit, p65. Using recently reported selective inhibitors of the first and second BET bromodomains, inhibition of only the first bromodomain was necessary to reduce the interaction of BRD4 with p65 in β-cells. Moreover, inhibition of the first bromodomain was sufficient to mitigate IL-1β-driven decreases in mitochondrial oxygen consumption rates and β-cell viability. By identifying a role for the interaction between BRD4 and p65 in controlling the response of β-cells to proinflammatory cytokines, we provide mechanistic information on how BET bromodomain inhibition can decrease inflammation. These studies also support the potential therapeutic application of more selective BET bromodomain inhibitors in attenuating β-cell inflammation.

Cytokine and Nitric Oxide-Dependent Gene Regulation in Islet Endocrine and Nonendocrine Cells

While exposure to inflammatory cytokines is thought to contribute to pancreatic β-cell damage during diabetes, primarily because cytokine-induced nitric oxide impairs β-cell function and causes cell death with prolonged exposure, we hypothesize that there is a physiological role for cytokine signaling that protects β-cells from a number of environmental stresses. This hypothesis is derived from the knowledge that β-cells are essential for survival even though they have a limited capacity to replicate, yet they are exposed to high cytokine levels during infection as most of the pancreatic blood flow is directed to islets. Here, mouse islets were subjected to single-cell RNA sequencing following 18-h cytokine exposure. Treatment with IL-1β and IFN-γ stimulates expression of inducible nitric oxide synthase (iNOS) mRNA and antiviral and immune-associated genes as well as repression of islet identity factors in a subset of β- and non-β-endocrine cells in a nitric oxide-independent manner. Nitric oxide-dependent expression of genes encoding heat shock proteins was observed in both β- and non-β-endocrine cells. Interestingly, cells with high expression of heat shock proteins failed to increase antiviral and immune-associated gene expression, suggesting that nitric oxide may be an internal "off switch" to prevent the negative effects of prolonged cytokine signaling in islet endocrine cells. We found no evidence for pro-apoptotic gene expression following 18-h cytokine exposure. Our findings suggest that the primary functions of cytokines and nitric oxide are to protect islet endocrine cells from damage, and only when regulation of cytokine signaling is lost does irreversible damage occur.

Single-cell RNA sequencing of mouse islets exposed to proinflammatory cytokines

Exposure to proinflammatory cytokines is believed to contribute to pancreatic β-cell damage during diabetes development. Although some cytokine-mediated changes in islet gene expression are known, the heterogeneity of the response is not well-understood. After 6-h treatment with IL-1β and IFN-γ alone or together, mouse islets were subjected to single-cell RNA sequencing. Treatment with both cytokines together led to expression of inducible nitric oxide synthase mRNA (Nos2) and antiviral and immune-associated genes in a subset of β-cells. Interestingly, IL-1β alone activated antiviral genes. Subsets of δ- and α-cells expressed Nos2 and exhibited similar gene expression changes as β-cells, including increased expression of antiviral genes and repression of identity genes. Finally, cytokine responsiveness was inversely correlated with expression of genes encoding heat shock proteins. Our findings show that all islet endocrine cell types respond to cytokines, IL-1β induces the expression of protective genes, and cellular stress gene expression is associated with inhibition of cytokine signaling.

Regulation of ATR-dependent DNA damage response by nitric oxide

We have shown that nitric oxide limits ataxia-telangiectasia mutated signaling by inhibiting mitochondrial oxidative metabolism in a β-cell selective manner. In this study, we examined the actions of nitric oxide on a second DNA damage response transducer kinase, ataxia-telangiectasia and Rad3-related protein (ATR). In β-cells and non-β-cells, nitric oxide activates ATR signaling by inhibiting ribonucleotide reductase; however, when produced at inducible nitric oxide synthase-derived (low micromolar) levels, nitric oxide impairs ATR signaling in a β-cell selective manner. The inhibitory actions of nitric oxide are associated with impaired mitochondrial oxidative metabolism and lack of glycolytic compensation that result in a decrease in β-cell ATP. Like nitric oxide, inhibitors of mitochondrial respiration reduce ATP levels and limit ATR signaling in a β-cell selective manner. When non-β-cells are forced to utilize mitochondrial oxidative metabolism for ATP generation, their response is more like β-cells, as nitric oxide and inhibitors of mitochondrial respiration attenuate ATR signaling. These studies support a dual role for nitric oxide in regulating ATR signaling. Nitric oxide activates ATR in all cell types examined by inhibiting ribonucleotide reductase, and in a β-cell selective manner, inducible nitric oxide synthase-derived levels of nitric oxide limit ATR signaling by attenuating mitochondrial oxidative metabolism and depleting ATP.

Inhibition of mitochondrial oxidative metabolism attenuates EMCV replication and protects β-cells from virally mediated lysis

Viral infection is one environmental factor that may contribute to the initiation of pancreatic β-cell destruction during the development of autoimmune diabetes. Picornaviruses, such as encephalomyocarditis virus (EMCV), induce a pro-inflammatory response in islets leading to local production of cytokines, such as IL-1, by resident islet leukocytes. Furthermore, IL-1 is known to stimulate β-cell expression of iNOS and production of the free radical nitric oxide. The purpose of this study was to determine whether nitric oxide contributes to the β-cell response to viral infection. We show that nitric oxide protects β-cells against virally mediated lysis by limiting EMCV replication. This protection requires low micromolar, or iNOS-derived, levels of nitric oxide. At these concentrations nitric oxide inhibits the Krebs enzyme aconitase and complex IV of the electron transport chain. Like nitric oxide, pharmacological inhibition of mitochondrial oxidative metabolism attenuates EMCV-mediated β-cell lysis by inhibiting viral replication. These findings provide novel evidence that cytokine signaling in β-cells functions to limit viral replication and subsequent β-cell lysis by attenuating mitochondrial oxidative metabolism in a nitric oxide-dependent manner.

Peroxiredoxin 1 plays a primary role in protecting pancreatic β-cells from hydrogen peroxide and peroxynitrite

Both reactive nitrogen and oxygen species (RNS and ROS), such as nitric oxide, peroxynitrite, and hydrogen peroxide, have been implicated as mediators of pancreatic β-cell damage during the pathogenesis of autoimmune diabetes. While β-cells are thought to be vulnerable to oxidative damage due to reportedly low levels of antioxidant enzymes, such as catalase and glutathione peroxidase, we have shown that they use thioredoxin reductase to detoxify hydrogen peroxide. Thioredoxin reductase is an enzyme that participates in the peroxiredoxin antioxidant cycle. Peroxiredoxins are expressed in β-cells and, when overexpressed, protect against oxidative stress, but the endogenous roles of peroxiredoxins in the protection of β-cells from oxidative damage are unclear. Here, using either glucose oxidase or menadione to continuously deliver hydrogen peroxide, or the combination of dipropylenetriamine NONOate and menadione to continuously deliver peroxynitrite, we tested the hypothesis that β-cells use peroxiredoxins to detoxify both of these reactive species. Either pharmacological peroxiredoxin inhibition with conoidin A or specific depletion of cytoplasmic peroxiredoxin 1 (Prdx1) using siRNAs sensitizes INS 832/13 cells and rat islets to DNA damage and death induced by hydrogen peroxide or peroxynitrite. Interestingly, depletion of peroxiredoxin 2 (Prdx2) had no effect. Together, these results suggest that β-cells use cytoplasmic Prdx1 as a primary defense mechanism against both ROS and RNS.

The location of sensing determines the pancreatic β-cell response to the viral mimetic dsRNA

Viral infection is an environmental trigger that has been suggested to initiate pancreatic β-cell damage, leading to the development of autoimmune diabetes. Viruses potently activate the immune system and can damage β cells by either directly infecting them or stimulating the production of secondary effector molecules (such as proinflammatory cytokines) during bystander activation. However, how and where β cells recognize viruses is unclear, and the antiviral responses that are initiated following virus recognition are incompletely understood. In this study, we show that the β-cell response to dsRNA, a viral replication intermediate known to activate antiviral responses, is determined by the cellular location of sensing (intracellular versus extracellular) and differs from the cellular response to cytokine treatment. Using biochemical and immunological methods, we show that β cells selectively respond to intracellular dsRNA by expressing type I interferons (IFNs) and inducing apoptosis, but that they do not respond to extracellular dsRNA. These responses differ from the activities of cytokines on β cells, which are mediated by inducible nitric oxide synthase expression and β-cell production of nitric oxide. These findings provide evidence that the antiviral activities of type I IFN production and apoptosis are elicited in β cells via the recognition of intracellular viral replication intermediates and that β cells lack the capacity to respond to extracellular viral intermediates known to activate innate immune responses.

The Role of Metabolic Flexibility in the Regulation of the DNA Damage Response by Nitric Oxide

In this report, we show that nitric oxide suppresses DNA damage response (DDR) signaling in the pancreatic β-cell line INS 832/13 and rat islets by inhibiting intermediary metabolism. Nitric oxide is known to inhibit complex IV of the electron transport chain and aconitase of the Krebs cycle. Non-β cells compensate by increasing glycolytic metabolism to maintain ATP levels; however, β cells lack this metabolic flexibility, resulting in a nitric oxide-dependent decrease in ATP and NAD⁺ Like nitric oxide, mitochondrial toxins inhibit DDR signaling in β cells by a mechanism that is associated with a decrease in ATP. Non-β cells compensate for the effects of mitochondrial toxins with an adaptive shift to glycolytic ATP generation that allows for DDR signaling. Forcing non-β cells to derive ATP via mitochondrial respiration (replacing glucose with galactose in the medium) and glucose deprivation sensitizes these cells to nitric oxide-mediated inhibition of DDR signaling. These findings indicate that metabolic flexibility is necessary to maintain DDR signaling under conditions in which mitochondrial oxidative metabolism is inhibited and support the inhibition of oxidative metabolism (decreased ATP) as one protective mechanism by which nitric oxide attenuates DDR-dependent β-cell apoptosis.

Pancreatic β-cells detoxify H2O2 through the peroxiredoxin/thioredoxin antioxidant system

Oxidative stress is thought to promote pancreatic β-cell dysfunction and contribute to both type 1 and type 2 diabetes. Reactive oxygen species (ROS), such as superoxide and hydrogen peroxide, are mediators of oxidative stress that arise largely from electron leakage during oxidative phosphorylation. Reports that β-cells express low levels of antioxidant enzymes, including catalase and GSH peroxidases, have supported a model in which β-cells are ill-equipped to detoxify ROS. This hypothesis seems at odds with the essential role of β-cells in the control of metabolic homeostasis and organismal survival through exquisite coupling of oxidative phosphorylation, a prominent ROS-producing pathway, to insulin secretion. Using glucose oxidase to deliver H₂O₂ continuously over time and Amplex Red to measure extracellular H₂O₂ concentration, we found here that β-cells can remove micromolar levels of this oxidant. This detoxification pathway utilizes the peroxiredoxin/thioredoxin antioxidant system, as selective chemical inhibition or siRNA-mediated depletion of thioredoxin reductase sensitized β-cells to continuously generated H₂O₂ In contrast, when delivered as a bolus, H₂O₂ induced the DNA damage response, depleted cellular energy stores, and decreased β-cell viability independently of thioredoxin reductase inhibition. These findings show that β-cells have the capacity to detoxify micromolar levels of H₂O₂ through a thioredoxin reductase-dependent mechanism and are not as sensitive to oxidative damage as previously thought.

Role of Protein Phosphatase 1 and Inhibitor of Protein Phosphatase 1 in Nitric Oxide-Dependent Inhibition of the DNA Damage Response in Pancreatic β-Cells

Nitric oxide is produced at micromolar levels by pancreatic β-cells during exposure to proinflammatory cytokines. While classically viewed as damaging, nitric oxide also activates pathways that promote β-cell survival. We have shown that nitric oxide, in a cell type-selective manner, inhibits the DNA damage response (DDR) and, in doing so, protects β-cells from DNA damage-induced apoptosis. This study explores potential mechanisms by which nitric oxide inhibits DDR signaling. We show that inhibition of DDR signaling (measured by γH2AX formation and the phosphorylation of KAP1) is selective for nitric oxide, as other forms of reactive oxygen/nitrogen species do not impair DDR signaling. The kinetics and broad range of DDR substrates that are inhibited suggest that protein phosphatase activation may be one mechanism by which nitric oxide attenuates DDR signaling in β-cells. While protein phosphatase 1 (PP1) is a primary regulator of DDR signaling and an inhibitor of PP1 (IPP1) is selectively expressed only in β-cells, disruption of either IPP1 or PP1 does not modify the inhibitory actions of nitric oxide on DDR signaling in β-cells. These findings support a PP1-independent mechanism by which nitric oxide selectively impairs DDR signaling and protects β-cells from DNA damage-induced apoptosis.

Cation-Independent Mannose 6-Phosphate Receptor Deficiency Enhances β-Cell Susceptibility to Palmitate

Palmitate attenuates insulin secretion and reduces the viability of insulin-producing cells. Previous studies identified the aberrant palmitoylation or mispalmitoylation of proteins as one mechanism by which palmitate causes β-cell damage. In this report, we identify a role for lysosomal protein degradation as a mechanism by which β cells defend themselves against excess palmitate. The cation-independent mannose 6-phosphate receptor (CI-MPR) is responsible for the trafficking of mannose 6-phosphate-tagged proteins to lysosomes via Golgi sorting and from extracellular locations through endocytosis. RINm5F cells, which are highly sensitive to palmitate, lack CI-MPR. The reconstitution of CI-MPR expression attenuates the induction of endoplasmic reticulum (ER) stress and the toxic effects of palmitate on RINm5F cell viability. INS832/13 cells express CI-MPR and are resistant to the palmitate-mediated loss of cell viability. The reduction of CI-MPR expression increases the sensitivity of INS832/13 cells to the toxic effects of palmitate treatment. The inhibition of lysosomal acid hydrolase activity by weak base treatment of islets under glucolipotoxic conditions causes islet degeneration that is prevented by the inhibition of protein palmitoylation. These findings indicate that defects in lysosomal function lead to the enhanced sensitivity of insulin-producing cells to palmitate and support a role for normal lysosomal function in the protection of β cells from excess palmitate.

A Protective Role for NKG2D-H60a Interaction via Homotypic T Cell Contact in Nonobese Diabetic Autoimmune Diabetes Pathogenesis

The NK group 2 member D (NKG2D) immune receptor is implicated in both human and mouse autoimmune diabetes. However, the significance of NKG2D in diabetes pathogenesis has been unclear due to conflicting reports as to the importance of this receptor in the NOD mouse model. In this study we demonstrate that NKG2D expression affects NOD diabetes development by at least two previously undescribed, and opposing, mechanisms. First, we demonstrate that the NKG2D ligand H60a is induced on activated NOD T cells, and that NKG2D-H60a interaction during CD8⁺ T cell differentiation into CTLs generally decreases the subsequent CTL effector cytokine response. This corresponds to an increase in diabetes development in NKG2D-deficient compared with wild-type NOD mice under microbiota-depleted conditions. Second, we demonstrate that NKG2D promotes NOD diabetes development through interaction with the microbiota. Together these findings reveal a previously undescribed role for NKG2D ligand expression by activated T cells in CTL development. Further, they demonstrate that NKG2D has both diabetogenic and antidiabetogenic roles in NOD diabetes development.

Pancreatic mitochondrial complex I exhibits aberrant hyperactivity in diabetes

It is well established that NADH/NAD+ redox balance is heavily perturbed in diabetes, and the NADH/NAD+ redox imbalance is a major source of oxidative stress in diabetic tissues. In mitochondria, complex I is the only site for NADH oxidation and NAD+ regeneration and is also a major site for production of mitochondrial reactive oxygen species (ROS). Yet how complex I responds to the NADH/NAD+ redox imbalance and any potential consequences of such response in diabetic pancreas have not been investigated. We report here that pancreatic mitochondrial complex I showed aberrant hyperactivity in either type 1 or type 2 diabetes. Further studies focusing on streptozotocin (STZ)-induced diabetes indicate that complex I hyperactivity could be attenuated by metformin. Moreover, complex I hyperactivity was accompanied by increased activities of complexes II to IV, but not complex V, suggesting that overflow of NADH via complex I in diabetes could be diverted to ROS production. Indeed in diabetic pancreas, ROS production and oxidative stress increased and mitochondrial ATP production decreased, which can be attributed to impaired pancreatic mitochondrial membrane potential that is responsible for increased cell death. Additionally, cellular defense systems such as glucose 6-phosphate dehydrogenase, sirtuin 3, and NQO1 were found to be compromised in diabetic pancreas. Our findings point to the direction that complex I aberrant hyperactivity in pancreas could be a major source of oxidative stress and β cell failure in diabetes. Therefore, inhibiting pancreatic complex I hyperactivity and attenuating its ROS production by various means in diabetes might serve as a promising approach for anti-diabetic therapies.

Introducing a New Experimental Islet Transplantation Model using Biomimetic Hydrogel and a Simple High Yield Islet Isolation Technique

BACKGROUND

Islet transplantation could be an ideal alternative treatment to insulin therapy for type 1 diabetes Mellitus (T1DM). This clinical and experimental field requires a model that covers problems such as requiring a large number of functional and viable islets, the optimal transplantation site, and the prevention of islet dispersion. Hence, the methods of choice for isolation of functional islets and transplantation are crucial.

METHODS

The present study has introduced an experimental model that overcomes some critical issues in islet transplantation, including in situ pancreas perfusion by digestive enzymes through common bile duct. In comparison with conventional methods, we inflated the pancreas in Petri dishes with only 1 ml collagenase type XI solution, which was followed by hand-picking isolation or Ficoll gradient separation to purify the islets. Then we used a hydrogel composite in which the islets were embedded and transplanted into the peritoneal cavity of the streptozotocin-induced diabetic C57BL/6 mice.

RESULTS

As compared to the yield of the classical methods, in our modified technique, the mean yield of isolation was about 130-200 viable islets/mouse pancreas. In vitro glucose-mediated insulin secretion assay indicated an appropriate response in isolated islets. In addition, data from in vivo experiments revealed that the allograft remarkably maintained blood glucose levels under 400 mg/dl and hydrogel composite prevents the passage of immune cells.

CONCLUSION

In the model presented here, the rapid islet isolation technique and the application of biomimetic hydrogel wrapping of islets could facilitate islet transplantation procedures.

Sex differences in adult rat insulin and glucose responses to arginine: programming effects of neonatal separation, hypoxia, and hypothermia

Acute neonatal hypoxia, a common stressor, causes a spontaneous decrease in body temperature which may be protective. There is consensus that hypothermia should be prevented during acute hypoxia in the human neonate; however, this may be an additional stress with negative consequences. We hypothesize that maintaining body temperature during hypoxia in the first week of postnatal life alters the subsequent insulin, glucose, and glucagon secretion in adult rats. Rat pups were separated from their dam daily from postnatal days (PD) 2-6 for the following 90 min experimental treatments: (1) normoxic separation (control), (2) hypoxia (8% O2) allowing spontaneous hypothermia, (3) normoxic hypothermia with external cold, and (4) exposure to 8% O2 while maintaining body temperature using external heat. An additional normoxic non-separated control group was performed to determine if separation per se changed the adult phenotype. Plasma insulin, glucose, and glucagon responses to arginine stimulation were evaluated from PD105 to PD133. Maternal separation (compared to non-separated neonates) had more pronounced effects on the adult response to arginine compared to the hypoxic, hypothermic, and hypoxic-isothermic neonatal treatments. Adult males exposed to neonatal maternal separation had augmented insulin and glucose responses to arginine compared to unseparated controls. Additionally, neonatal treatment had a significant effect on body weight gain; adults exposed to neonatal maternal separation were significantly heavier. Female adults had significantly smaller insulin and glucose responses to arginine regardless of neonatal treatment. Neonatal maternal separation during the first week of life significantly altered adult beta-cell function in a sexually dimorphic manner.

Nitric Oxide Suppresses β-Cell Apoptosis by Inhibiting the DNA Damage Response

Nitric oxide, produced in pancreatic β cells in response to proinflammatory cytokines, plays a dual role in the regulation of β-cell fate. While nitric oxide induces cellular damage and impairs β-cell function, it also promotes β-cell survival through activation of protective pathways that promote β-cell recovery. In this study, we identify a novel mechanism in which nitric oxide prevents β-cell apoptosis by attenuating the DNA damage response (DDR). Nitric oxide suppresses activation of the DDR (as measured by γH2AX formation and the phosphorylation of KAP1 and p53) in response to multiple genotoxic agents, including camptothecin, H2O2, and nitric oxide itself, despite the presence of DNA damage. While camptothecin and H2O2 both induce DDR activation, nitric oxide suppresses only camptothecin-induced apoptosis and not H2O2-induced necrosis. The ability of nitric oxide to suppress the DDR appears to be selective for pancreatic β cells, as nitric oxide fails to inhibit DDR signaling in macrophages, hepatocytes, and fibroblasts, three additional cell types examined. While originally described as the damaging agent responsible for cytokine-induced β-cell death, these studies identify a novel role for nitric oxide as a protective molecule that promotes β-cell survival by suppressing DDR signaling and attenuating DNA damage-induced apoptosis.

Neuronostatin acts via GPR107 to increase cAMP-independent PKA phosphorylation and proglucagon mRNA accumulation in pancreatic α-cells

Neuronostatin (NST) is a recently described peptide that is produced from the somatostatin preprohormone in pancreatic δ-cells. NST has been shown to increase glucagon secretion from primary rat pancreatic islets in low-glucose conditions. Here, we demonstrate that NST increases proglucagon message in α-cells and identify a potential mechanism for NST's cellular activities, including the phosphorylation of PKA following activation of the G protein-coupled receptor, GPR107. GPR107 is abundantly expressed in the pancreas, particularly, in rodent and human α-cells. Compromise of GPR107 in pancreatic α-cells results in failure of NST to increase PKA phosphorylation and proglucagon mRNA levels. We also demonstrate colocalization of GPR107 and NST on both mouse and human pancreatic α-cells. Taken together with our group's observation that NST infusion in conscious rats impairs glucose clearance in response to a glucose challenge and that plasma levels of the peptide are elevated in the fasted compared with the fed or fasted-refed state, these studies support the hypothesis that endogenous NST regulates islet cell function by interacting with GPR107 and initiating signaling in glucagon-producing α-cells.

Distinct differences in the responses of the human pancreatic β-cell line EndoC-βH1 and human islets to proinflammatory cytokines

While insulinoma cells have been developed and proven to be extremely useful in studies focused on mechanisms controlling β-cell function and viability, translating findings to human β-cells has proven difficult because of the limited access to human islets and the absence of suitable insulinoma cell lines of human origin. Recently, a human β-cell line, EndoC-βH1, has been derived from human fetal pancreatic buds. The purpose of this study was to determine whether human EndoC-βH1 cells respond to cytokines in a fashion comparable to human islets. Unlike most rodent-derived insulinoma cell lines that respond to cytokines in a manner consistent with rodent islets, EndoC-βH1 cells fail to respond to a combination of cytokines (IL-1, IFN-γ, and TNF) in a manner consistent with human islets. Nitric oxide, produced following inducible nitric oxide synthase (iNOS) expression, is a major mediator of cytokine-induced human islet cell damage. We show that EndoC-βH1 cells fail to express iNOS or produce nitric oxide in response to this combination of cytokines. Inhibitors of iNOS prevent cytokine-induced loss of human islet cell viability; however, they do not prevent cytokine-induced EndoC-βH1 cell death. Stressed human islets or human islets expressing heat shock protein 70 (HSP70) are resistant to cytokines, and, much like stressed human islets, EndoC-βH1 cells express HSP70 under basal conditions. Elevated basal expression of HSP70 in EndoC-βH1 cells is consistent with the lack of iNOS expression in response to cytokine treatment. While expressing HSP70, EndoC-βH1 cells fail to respond to endoplasmic reticulum stress activators, such as thapsigargin. These findings indicate that EndoC-βH1 cells do not faithfully recapitulate the response of human islets to cytokines. Therefore, caution should be exercised when making conclusions regarding the actions of cytokines on human islets when using this human-derived insulinoma cell line.

Loss of NADPH oxidase-derived superoxide skews macrophage phenotypes to delay type 1 diabetes

Macrophages are early islet-infiltrating cells seen in type 1 diabetes (T1D). While proinflammatory M1 macrophages induce T1D, M2 macrophages have been shown to delay this autoimmune disease in nonobese diabetic (NOD) mice, but the environmental cues that govern macrophage polarization and differentiation remain unresolved. We previously demonstrated the importance of reactive oxygen species (ROS) in T1D, as NOD mice deficient in NADPH oxidase (NOX)-derived superoxide (Ncf1(m1J)) were protected against T1D partly because of blunted Toll-like receptor-dependent macrophage responses. We provide evidence that NOX-derived ROS contribute to macrophage differentiation in T1D. During spontaneous diabetes progression, T1D-resistant NOD.Ncf1(m1J) islet-resident macrophages displayed a dampened M1 and increased M2 phenotype. The transfer of diabetogenic T cells into NOX-deficient NOD.Rag.Ncf1(m1J) recipients resulted in decreased TNF-α(+) and IL-1β(+) islet-infiltrating M1 macrophages and a concomitant enhancement in arginase-1(+) M2 macrophages. Mechanistic analysis of superoxide-deficient bone marrow-derived macrophages revealed a marked diminution in a proinflammatory M1 phenotype due to decreased P-STAT1 (Y701) and interferon regulatory factor 5 compared with NOD mice. We have therefore defined a novel mechanistic link between NOX-derived ROS and macrophage phenotypes, and implicated superoxide as an important factor in macrophage differentiation. Thus, targeting macrophage redox status may represent a promising therapy in halting human T1D.

Neuronostatin inhibits glucose-stimulated insulin secretion via direct action on the pancreatic α-cell

Neuronostatin is a recently described peptide hormone encoded by the somatostatin gene. We previously showed that intraperitoneal injection of neuronostatin into mice resulted in c-Jun accumulation in pancreatic islets in a pattern consistent with the activation of glucagon-producing α-cells. We therefore hypothesized that neuronostatin could influence glucose homeostasis via a direct effect on the α-cell. Neuronostatin enhanced low-glucose-induced glucagon release in isolated rat islets and in the immortalized α-cell line αTC1-9. Furthermore, incubation with neuronostatin led to an increase in transcription of glucagon mRNA, as determined by RT-PCR. Neuronostatin also inhibited glucose-stimulated insulin secretion from isolated islets. However, neuronostatin did not alter insulin release from the β-cell line INS 832/13, indicating that the effect of neuronostatin on insulin secretion may be secondary to a direct action on the α-cell. In agreement with our in vitro data, intra-arterial infusion of neuronostatin in male rats delayed glucose disposal and inhibited insulin release during a glucose challenge. These studies suggest that neuronostatin participates in maintaining glucose homeostasis through cell-cell interactions between α-cells and β-cells in the endocrine pancreas, leading to attenuation in insulin secretion.

Nitric oxide induces ataxia telangiectasia mutated (ATM) protein-dependent γH2AX protein formation in pancreatic β cells

In this study, the effects of cytokines on the activation of the DNA double strand break repair factors histone H2AX (H2AX) and ataxia telangiectasia mutated (ATM) were examined in pancreatic β cells. We show that cytokines stimulate H2AX phosphorylation (γH2AX formation) in rat islets and insulinoma cells in a nitric oxide- and ATM-dependent manner. In contrast to the well documented role of ATM in DNA repair, ATM does not appear to participate in the repair of nitric oxide-induced DNA damage. Instead, nitric oxide-induced γH2AX formation correlates temporally with the onset of irreversible DNA damage and the induction of apoptosis. Furthermore, inhibition of ATM attenuates cytokine-induced caspase activation. These findings show that the formation of DNA double strand breaks correlates with ATM activation, irreversible DNA damage, and ATM-dependent induction of apoptosis in cytokine-treated β cells.

Do β-cells generate peroxynitrite in response to cytokine treatment?

The purpose of this study was to determine the reactive species that is responsible for cytokine-mediated β-cell death. Inhibitors of inducible nitric oxide synthase prevent this death, and addition of exogenous nitric oxide using donors induces β-cell death. The reaction of nitric oxide with superoxide results in the generation of peroxynitrite, and this powerful oxidant has been suggested to be the mediator of β-cell death in response to cytokine treatment. Recently, coumarin-7-boronate has been developed as a probe for the selective detection of peroxynitrite. Using this reagent, we show that addition of the NADPH oxidase activator phorbol 12-myristate 13-acetate to nitric oxide-producing macrophages results in peroxynitrite generation. Using a similar approach, we demonstrate that cytokines fail to stimulate peroxynitrite generation by rat islets and insulinoma cells, either with or without phorbol 12-myristate 13-acetate treatment. When forced to produce superoxide using redox cyclers, this generation is associated with protection from nitric oxide toxicity. These findings indicate that: (i) nitric oxide is the likely mediator of the toxic effects of cytokines, (ii) β-cells do not produce peroxynitrite in response to cytokines, and (iii) when forced to produce superoxide, the scavenging of nitric oxide by superoxide is associated with protection of β-cells from nitric oxide-mediated toxicity.

Differential responses of pancreatic β-cells to ROS and RNS

Reactive oxygen species (ROS) and reactive nitrogen species (RNS) direct the activation of distinct signaling pathways that determine cell fate. In this study, the pathways activated and the mechanisms by which ROS and RNS control the viability of pancreatic β-cells were examined. Although both nitric oxide and hydrogen peroxide (H₂O₂) induce DNA damage, reduce cell viability, and activate AMPK, the mechanisms of AMPK activation and cell death induction differ between each reactive species. Nitric oxide activates the unfolded protein and heat shock responses and MAPK kinase signaling, whereas H₂O₂ stimulates p53 stabilization and poly(ADP-ribose) polymerase (PARP) activation but fails to induce the unfolded protein or heat shock responses or MAPK activation. The control of cell fate decisions is selective for the form of stress. H₂O₂-mediated reduction in β-cell viability is controlled by PARP, whereas cell death in response to nitric oxide is PARP independent but associated with the nuclear localization of GAPDH. These findings show that both ROS and RNS activate AMPK, induce DNA damage, and reduce cell viability; however, the pathways controlling the responses of β-cells are selective for the type of reactive species.

Reduction in ATP levels triggers immunoproteasome activation by the 11S (PA28) regulator during early antiviral response mediated by IFNβ in mouse pancreatic β-cells

Autoimmune destruction of insulin producing pancreatic β-cells is the hallmark of type I diabetes. One of the key molecules implicated in the disease onset is the immunoproteasome, a protease with multiple proteolytic sites that collaborates with the constitutive 19S and the inducible 11S (PA28) activators to produce immunogenic peptides for presentation by MHC class I molecules. Despite its importance, little is known about the function and regulation of the immunoproteasome in pancreatic β-cells. Of special interest to immunoproteasome activation in β-cells are the effects of IFNβ, a type I IFN secreted by virus-infected cells and implicated in type I diabetes onset, compared to IFNγ, the classic immunoproteasome inducer secreted by cells of the immune system. By qPCR analysis, we show that mouse insulinoma MIN6 cells and mouse islets accumulate the immune proteolytic β1_i, β2_i and β5_i, and 11S mRNAs upon exposure to IFNβ or IFNγ. Higher concentrations of IFNβ than IFNγ are needed for similar expression, but in each case the expression is transient, with maximal mRNA accumulation in 12 hours, and depends primarily on Interferon Regulatory Factor 1. IFNs do not alter expression of regular proteasome genes, and in the time frame of IFNβ-mediated response, the immune and regular proteolytic subunits co-exist in the 20S particles. In cell extracts with ATP, these particles have normal peptidase activities and degrade polyubiquitinated proteins with rates typical of the regular proteasome, implicating normal regulation by the 19S activator. However, ATP depletion rapidly stimulates the catalytic rates in a manner consistent with levels of the 11S activator. These findings suggest that stochastic combination of regular and immune proteolytic subunits may increase the probability with which unique immunogenic peptides are produced in pancreatic β-cells exposed to IFNβ, but primarily in cells with reduced ATP levels that stimulate the 11S participation in immunoproteasome function.

Cytokine-mediated β-cell damage in PARP-1-deficient islets

Poly(ADP)-ribose polymerase (PARP) is an abundant nuclear protein that is activated by DNA damage; once active, it modifies nuclear proteins through attachment of poly(ADP)-ribose units derived from β-nicotinamide adenine dinucleotide (NAD(+)). In mice, the deletion of PARP-1 attenuates tissue injury in a number of animal models of human disease, including streptozotocin-induced diabetes. Also, inflammatory cell signaling and inflammatory gene expression are attenuated in macrophages isolated from endotoxin-treated PARP-1-deficient mice. In this study, the effects of PARP-1 deletion on cytokine-mediated β-cell damage and macrophage activation were evaluated. There are no defects in inflammatory mediator signaling or inflammatory gene expression in macrophages and islets isolated from PARP-1-deficient mice. While PARP-1 deficiency protects islets against cytokine-induced islet cell death as measured by biochemical assays of membrane polarization, the genetic absence of PARP-1 does not effect cytokine-induced inhibition of insulin secretion or cytokine-induced DNA damage in islets. While PARP-1 deficiency appears to provide protection from cell death, it fails to provide protection against the inhibitory actions of cytokines on insulin secretion or the damaging actions on islet DNA integrity.

RAE1ε ligand expressed on pancreatic islets recruits NKG2D receptor-expressing cytotoxic T cells independent of T cell receptor recognition

The mechanisms by which cytotoxic T lymphocytes (CTLs) enter and are retained in nonlymphoid tissue are not well characterized. With a transgenic mouse expressing the NKG2D ligand retinoic acid early transcript 1ε (RAE1ε) in β-islet cells of the pancreas, we found that RAE1 expression was sufficient to induce the recruitment of adoptively transferred CTLs to islets. This was dependent on NKG2D expression by the CTLs and independent of antigen recognition. Surprisingly, the recruitment of CTLs resulted in the subsequent recruitment of a large number of endogenous lymphocytes. Whereas transgenic mice did not develop diabetes, RAE1 expression was sufficient to induce insulitis in older, unmanipulated transgenic mice that was enhanced by viral infection and pancreatic inflammation. These results demonstrate that the expression of an NKG2D ligand in islets is sufficient to recruit CTLs regardless of their antigen specificity and to induce insulitis.

FoxO1 and SIRT1 regulate beta-cell responses to nitric oxide

For many cell types, including pancreatic β-cells, nitric oxide is a mediator of cell death; paradoxically, nitric oxide can also activate pathways that promote the repair of cellular damage. In this report, a role for FoxO1-dependent transcriptional activation and its regulation by SIRT1 in determining the cellular response to nitric oxide is provided. In response to nitric oxide, FoxO1 translocates from the cytoplasm to the nucleus and stimulates the expression of the DNA repair gene GADD45α, resulting in FoxO1-dependent DNA repair. FoxO1-dependent gene expression appears to be regulated by the NAD(+)-dependent deacetylase SIRT1. In response to SIRT1 inhibitors, the FoxO1-dependent protective actions of nitric oxide (GADD45α expression and DNA repair) are attenuated, and FoxO1 activates a proapoptotic program that includes PUMA (p53-up-regulated mediator of apoptosis) mRNA accumulation and caspase-3 cleavage. These findings support primary roles for FoxO1 and SIRT1 in regulating the cellular responses of β-cells to nitric oxide.

Homogeneous insulin and C-Peptide sensors for rapid assessment of insulin and C-peptide secretion by the islets

OBJECTIVE

Glucose-stimulated islet insulin or C-peptide secretion experiments are a fundamental tool for studying and assessing islet function. The goal of this work was to develop Ab-based fluorescent homogenous sensors that would allow rapid, inexpensive, near-instantaneous determinations of insulin and C-peptide levels in biological samples.

RESEARCH DESIGN AND METHODS

Our approach was to use molecular pincer design (Heyduk et al., Anal Chem 2008;80:5152-5159) in which a pair of antibodies recognizing nonoverlapping epitopes of the target are modified with short fluorochrome-labeled complementary oligonucleotides and are used to generate a fluorescence energy transfer (FRET) signal in the presence of insulin or C-peptide.

RESULTS

The sensors were capable of detecting insulin and C-peptide with high specificity and with picomolar concentration detection limits in times as short as 20 min. Insulin and C-peptide levels determined with the FRET sensors showed outstanding correlation with determinations performed under the same conditions with enzyme-linked immunosorbent assay. Most importantly, the sensors were capable of rapid and accurate determinations of insulin and C-peptide secreted from human or rodent islets, verifying their applicability for rapid assessment of islet function.

CONCLUSIONS

The homogeneous, rapid, and uncomplicated nature of insulin and C-peptide FRET sensors allows rapid assessment of β-cell function and could enable point-of-care determinations of insulin and C-peptide.

Facilitative glucose transporter 9 expression affects glucose sensing in pancreatic beta-cells

Facilitative glucose transporters (GLUTs) including GLUT9, accelerate the facilitative diffusion of glucose across the plasma membrane. Studies in GLUT2-deficient mice suggested the existence of another GLUT in the mammalian beta-cell responsible for glucose sensing. The objective of this study was to determine the expression and function of GLUT9 in murine and human beta-cells. mRNA and protein expression levels were determined for both isoforms of GLUT9 in murine and human isolated islets as well as insulinoma cell lines (MIN6). Immunohistochemistry and subcellular localization were performed to localize the protein within the cell. Small interfering RNA knockdown of GLUT9 was used to determine the effect of this transporter, in the presence of GLUT2, on cell metabolism and insulin secretion in MIN6 and INS cells. In this report we demonstrate that GLUT9a and GLUT9b are expressed in pancreatic islets and that this expression localizes to insulin-containing beta-cells. Subcellular localization studies indicate that mGLUT9b is found associated with the plasma membrane as well as in the high-density microsome fraction and low-density microsome fraction, whereas mGLUT9a appears to be located only in the high-density microsome and low-density microsome under basal conditions. Functionally GLUT9 appears to participate in the regulation of glucose-stimulated insulin secretion in addition to GLUT2. small interfering RNA knockdown of GLUT9 results in reduced cellular ATP levels that correlate with reductions in glucose-stimulated insulin secretion in MIN6 and INS cells. These studies confirm the expression of GLUT9a and GLUT9b in murine and human beta-cells and suggest that GLUT9 may participate in glucose-sensing in beta-cells.

AMP-activated protein kinase attenuates nitric oxide-induced beta-cell death

During the initial autoimmune response in type 1 diabetes, islets are exposed to a damaging mix of pro-inflammatory molecules that stimulate the production of nitric oxide by beta-cells. Nitric oxide causes extensive but reversible cellular damage. In response to nitric oxide, the cell activates pathways for functional recovery and adaptation as well as pathways that direct beta-cell death. The molecular events that dictate cellular fate following nitric oxide-induced damage are currently unknown. In this study, we provide evidence that AMPK plays a primary role controlling the response of beta-cells to nitric oxide-induced damage. AMPK is transiently activated by nitric oxide in insulinoma cells and rat islets following IL-1 treatment or by the exogenous addition of nitric oxide. Active AMPK promotes the functional recovery of beta-cell oxidative metabolism and abrogates the induction of pathways that mediate cell death such as caspase-3 activation following exposure to nitric oxide. Overall, these data show that nitric oxide activates AMPK and that active AMPK suppresses apoptotic signaling allowing the beta-cell to recover from nitric oxide-mediated cellular stress.

Nitric oxides mediates a shift from early necrosis to late apoptosis in cytokine-treated β-cells that is associated with irreversible DNA damage

For many cell types, including pancreatic β-cells, nitric oxide is a mediator of cell death; however, it is paradoxical that for a given cell type nitric oxide can induce both necrosis and apoptosis. This report tests the hypothesis that cell death mediated by nitric oxide shifts from an early necrotic to a late apoptotic event. Central to this transition is the ability of β-cells to respond and repair nitric oxide-mediated damage. β-Cells have the ability to repair DNA that is damaged following 24-h incubation with IL-1; however, cytokine-induced DNA damage becomes irreversible following 36-h incubation. This irreversible DNA damage following 36-h incubation with IL-1 correlates with the activation of caspase-3 (cleavage and activity). The increase in caspase activity correlates with reductions in endogenous nitric oxide production, as nitric oxide is an inhibitor of caspase activity. In contrast, caspase cleavage or activation is not observed under conditions in which β-cells are capable of repairing damaged DNA (24-h incubation with cytokines). These findings provide evidence that β-cell death in response to cytokines shifts from an early necrotic process to apoptosis and that this shift is associated with irreversible DNA damage and caspase-3 activation.

Repair of nitric oxide-damaged DNA in beta-cells requires JNK-dependent GADD45alpha expression

Proinflammatory cytokines induce nitric oxide-dependent DNA damage and ultimately beta-cell death. Not only does nitric oxide cause beta-cell damage, it also activates a functional repair process. In this study, the mechanisms activated by nitric oxide that facilitate the repair of damaged beta-cell DNA are examined. JNK plays a central regulatory role because inhibition of this kinase attenuates the repair of nitric oxide-induced DNA damage. p53 is a logical target of JNK-dependent DNA repair; however, nitric oxide does not stimulate p53 activation or accumulation in beta-cells. Further, knockdown of basal p53 levels does not affect DNA repair. In contrast, expression of growth arrest and DNA damage (GADD) 45alpha, a DNA repair gene that can be regulated by p53-dependent and p53-independent pathways, is stimulated by nitric oxide in a JNK-dependent manner, and knockdown of GADD45alpha expression attenuates the repair of nitric oxide-induced beta-cell DNA damage. These findings show that beta-cells have the ability to repair nitric oxide-damaged DNA and that JNK and GADD45alpha mediate the p53-independent repair of this DNA damage.

Selective mtDNA mutation accumulation results in beta-cell apoptosis and diabetes development

To test the hypothesis that somatic mitochondrial (mt)DNA mutation accumulation predisposes mice to beta-cell loss and diabetes development, transgenic mice expressing a proofreading-deficient mtDNA polymerase-gamma under the control of the rat insulin-1 promoter were generated. At 6 wk of age, mtDNA mutations reached 0.01% (1.05 mutations/10,000 bp) in islets isolated from transgenic mice. This mutational burden is associated with impaired glucose tolerance and a diabetes prevalence of 52% in male transgenic mice. Female transgenic mice maintain slightly elevated fasting glucose levels, mild glucose intolerance, and a diabetes prevalence of 14%. Diabetes in transgenic animals is associated with insulin insufficiency that results from a significant reduction in beta-cell mass. Importantly, apoptosis of beta-cells is increased 7-fold in female and 11-fold in male transgenic mice compared with littermate controls. These results are consistent with a causative role of somatic mtDNA mutation accumulation in the loss of beta-cell mass and diabetes development.

Interleukin-1 stimulates beta-cell necrosis and release of the immunological adjuvant HMGB1

BACKGROUND

There are at least two phases of beta-cell death during the development of autoimmune diabetes: an initiation event that results in the release of beta-cell-specific antigens, and a second, antigen-driven event in which beta-cell death is mediated by the actions of T lymphocytes. In this report, the mechanisms by which the macrophage-derived cytokine interleukin (IL)-1 induces beta-cell death are examined. IL-1, known to inhibit glucose-induced insulin secretion by stimulating inducible nitric oxide synthase expression and increased production of nitric oxide by beta-cells, also induces beta-cell death.

METHODS AND FINDINGS

To ascertain the mechanisms of cell death, the effects of IL-1 and known activators of apoptosis on beta-cell viability were examined. While IL-1 stimulates beta-cell DNA damage, this cytokine fails to activate caspase-3 or to induce phosphatidylserine (PS) externalization; however, apoptosis inducers activate caspase-3 and the externalization of PS on beta-cells. In contrast, IL-1 stimulates the release of the immunological adjuvant high mobility group box 1 protein (HMGB1; a biochemical maker of necrosis) in a nitric oxide-dependent manner, while apoptosis inducers fail to stimulate HMGB1 release. The release of HMGB1 by beta-cells treated with IL-1 is not sensitive to caspase-3 inhibition, while inhibition of this caspase attenuates beta-cell death in response to known inducers of apoptosis.

CONCLUSIONS

These findings indicate that IL-1 induces beta-cell necrosis and support the hypothesis that macrophage-derived cytokines may participate in the initial stages of diabetes development by inducing beta-cell death by a mechanism that promotes antigen release (necrosis) and islet inflammation (HMGB1 release).