Proinflammatory cytokines induce rapid, NO-independent apoptosis, expression of chemotactic mediators and interleukin-32 secretion in human pluripotent stem cell-derived beta cells

AIMS/HYPOTHESIS

The aim of this study was to examine the effects of proinflammatory cytokines on cells of different developmental stages during the generation of stem cell-derived beta cells (SC-beta cells) from human pluripotent stem cells (hPSCs). We wanted to find out to what extent human SC-beta cells are suitable as an experimental cellular model and, with regard to a possible therapeutic use, whether SC-beta cells have a comparable vulnerability to cytokines as bona fide beta cells.

METHODS

hPSCs were differentiated towards pancreatic organoids (SC-organoids) using a 3D production protocol. SC-beta cells and non-insulin-producing cells were separated by FACS and differential gene expression profiles of purified human SC-beta cells, progenitor stages and the human beta cell line EndoC-βH1, as a reference, were determined after 24 h incubation with the proinflammatory cytokines IL-1β, TNF-α and IFN-γ via a transcriptome microarray. Furthermore, we investigated apoptosis based on caspase cleavage, the generation of reactive oxygen species and activation of mitogen-activated protein-kinase (MAPK) stress-signalling pathways.

RESULTS

A 24 h exposure of SC-beta cells to proinflammatory cytokines resulted in significant activation of caspase 3/7 and apoptosis via the extrinsic and intrinsic apoptosis signalling pathways. At this time point, SC-beta cells showed a markedly higher sensitivity towards proinflammatory cytokines than non-insulin-producing cells and EndoC-βH1 cells. Furthermore, we were able to demonstrate the generation of reactive oxygen species and rule out the involvement of NO-mediated stress. A transient activation of stress-signalling pathways p38 mitogen-activated protein kinases (p38) and c-Jun N-terminal kinase (JNK) was already observed after 10 min of cytokine exposure. The transcriptome analysis revealed that the cellular response to proinflammatory cytokines increased with the degree of differentiation of the cells. Cytokines induced the expression of multiple inflammatory mediators including IL-32, CXCL9 and CXCL10 in SC-beta cells and in non-insulin-producing cells.

CONCLUSIONS/INTERPRETATION

Our results indicate that human SC-beta cells respond to proinflammatory cytokines very similarly to human islets. Due to the fast and fulminant cellular response of SC-beta cells, we conclude that SC-beta cells represent a suitable model for diabetes research. In light of the immaturity of SC-beta cells, they may be an attractive model for developmentally young beta cells as they are, for example, present in patients with early-onset type 1 diabetes. The secretion of chemotactic signals may promote communication between SC-beta cells and immune cells, and non-insulin-producing cells possibly participate in the overall immune response and are thus capable of amplifying the immune response and further stimulating inflammation. We demonstrated that cytokine-treated SC-organoids secrete IL-32, which is considered a promising candidate for type 1 diabetes onset. This underlines the need to ensure the survival of SC-beta cells in an autoimmune environment such as that found in type 1 diabetes.

To Be or Not to Be: The Divergent Action and Metabolism of Sphingosine-1 Phosphate in Pancreatic Beta-Cells in Response to Cytokines and Fatty Acids

Sphingosine-1 phosphate (S1P) is a bioactive sphingolipid with multiple functions conveyed by the activation of cell surface receptors and/or intracellular mediators. A growing body of evidence indicates its important role in pancreatic insulin-secreting beta-cells that are necessary for maintenance of glucose homeostasis. The dysfunction and/or death of beta-cells lead to diabetes development. Diabetes is a serious public health burden with incidence growing rapidly in recent decades. The two major types of diabetes are the autoimmune-mediated type 1 diabetes (T1DM) and the metabolic stress-related type 2 diabetes (T2DM). Despite many differences in the development, both types of diabetes are characterized by chronic hyperglycemia and inflammation. The inflammatory component of diabetes remains under-characterized. Recent years have brought new insights into the possible mechanism involved in the increased inflammatory response, suggesting that environmental factors such as a westernized diet may participate in this process. Dietary lipids, particularly palmitate, are substrates for the biosynthesis of bioactive sphingolipids. Disturbed serum sphingolipid profiles were observed in both T1DM and T2DM patients. Many polymorphisms were identified in genes encoding enzymes of the sphingolipid pathway, including sphingosine kinase 2 (SK2), the S1P generating enzyme which is highly expressed in beta-cells. Proinflammatory cytokines and free fatty acids have been shown to modulate the expression and activity of S1P-generating and S1P-catabolizing enzymes. In this review, the similarities and differences in the action of extracellular and intracellular S1P in beta-cells exposed to cytokines or free fatty acids will be identified and the outlook for future research will be discussed.

Immunometabolism in type 2 diabetes mellitus: tissue-specific interactions

The immune system is frequently described in the context of its protective function against infections and its role in the development of autoimmunity. For more than a decade, the interactions between the immune system and metabolic processes have been reported, in effect creating a new research field, termed immunometabolism. Accumulating evidence supports the hypothesis that the development of metabolic diseases may be linked to inflammation, and reflects, in some cases, the activation of immune responses. As such, immunometabolism is defined by 1) inflammation as a driver of disease development and/or 2) metabolic processes stimulating cellular differentiation of the immune components. In this review, the main factors capable of altering the immuno-metabolic communication leading to the development and establishment of obesity and diabetes are comprehensively presented. Tissue-specific immune responses suggested to impair metabolic processes are described, with an emphasis on the adipose tissue, gut, muscle, liver, and pancreas.

MCPIP1 regulates the sensitivity of pancreatic beta-cells to cytokine toxicity

The autoimmune-mediated beta-cell death in type 1 diabetes (T1DM) is associated with local inflammation (insulitis). We examined the role of MCPIP1 (monocyte chemotactic protein–induced protein 1), a novel cytokine-induced antiinflammatory protein, in this process. Basal MCPIP1 expression was lower in rat vs. human islets and beta-cells. Proinflammatory cytokines stimulated MCPIP1 expression in rat and human islets and in insulin-secreting cells. Moderate overexpression of MCPIP1 protected insulin-secreting INS1E cells against cytokine toxicity by a mechanism dependent on the presence of the PIN/DUB domain in MCPIP1. It also reduced cytokine-induced Chop and C/ebpβ expression and maintained MCL-1 expression. The shRNA-mediated suppression of MCPIP1 led to the potentiation of cytokine-mediated NFκB activation and cytokine toxicity in human EndoC-βH1 beta-cells. MCPIP1 expression was very high in infiltrated beta-cells before and after diabetes manifestation in the LEW.1AR1-iddm rat model of human T1DM. The extremely high expression of MCPIP1 in clonal beta-cells was associated with a failure of the regulatory feedback-loop mechanism, ER stress induction and high cytokine toxicity. In conclusion, our data indicate that the expression level of MCPIP1 affects the susceptibility of insulin-secreting cells to cytokines and regulates the mechanism of beta-cell death in T1DM.

Overexpression of sphingosine-1-phosphate lyase protects insulin-secreting cells against cytokine toxicity

Increasing evidence suggests a crucial role of inflammation in cytokine-mediated β-cell dysfunction and death in type 1 diabetes mellitus, although the mechanisms are incompletely understood. Sphingosine 1-phosphate (S1P) is a multifunctional bioactive sphingolipid involved in the development of many autoimmune and inflammatory diseases. Here, we investigated the role of intracellular S1P in insulin-secreting INS1E cells by genetically manipulating the S1P-metabolizing enzyme S1P lyase (SPL). The expression of spl was down-regulated by cytokines in INS1E cells and rat islets. Overexpression of SPL protected against cytokine toxicity. Interestingly, the SPL overexpression did not suppress the cytokine-induced NFκB-iNOS-NO pathway but attenuated calcium leakage from endoplasmic reticulum (ER) stores as manifested by lower cytosolic calcium levels, higher expression of the ER protein Sec61a, decreased dephosphorylation of Bcl-2-associated death promoter (Bad) protein, and weaker caspase-3 activation in cytokine-treated (IL-1β, TNFα, and IFNγ) cells. This coincided with reduced cytokine-mediated ER stress, indicated by measurements of CCAAT/enhancer-binding protein homologous protein (chop) and immunoglobulin heavy chain binding protein (bip) levels. Moreover, cytokine-treated SPL-overexpressing cells exhibited increased expression of prohibitin 2 (Phb2), involved in the regulation of mitochondrial assembly and respiration. SPL-overexpressing cells were partially protected against cytokine-mediated ATP reduction and inhibition of glucose-induced insulin secretion. siRNA-mediated spl suppression resulted in effects opposite to those observed for SPL overexpression. Knockdown of phb2 partially reversed beneficial effects of SPL overexpression. In conclusion, the relatively low endogenous Spl expression level in insulin-secreting cells contributes to their extraordinary vulnerability to proinflammatory cytokine toxicity and may therefore represent a promising target for β-cell protection in type 1 diabetes mellitus.

Sensitivity profile of the human EndoC-βH1 beta cell line to proinflammatory cytokines

AIMS/HYPOTHESIS

The aim of this study was to perform a detailed analysis of cytokine toxicity in the new human EndoC-βH1 beta cell line.

METHODS

The expression profile of the antioxidative enzymes in the new human EndoC-βH1 beta cells was characterised and compared with that of primary beta cells in the human pancreas. The effects of proinflammatory cytokines on reactive oxygen species formation, insulin secretory responsiveness and apoptosis of EndoC-βH1 beta cells were determined.

RESULTS

EndoC-βH1 beta cells were sensitive to the toxic action of proinflammatory cytokines. Glucose-dependent stimulation of insulin secretion and an increase in the ATP/ADP ratio was abolished by proinflammatory cytokines without induction of IL-1β expression. Cytokine-mediated caspase-3 activation was accompanied by reactive oxygen species formation and developed more slowly than in rodent beta cells. Cytokines transiently increased the expression of unfolded protein response genes, without inducing endoplasmic reticulum stress-marker genes. Cytokine-mediated NFκB activation was too weak to induce inducible nitric oxide synthase expression. The resultant lack of nitric oxide generation in EndoC-βH1 cells, in contrast to rodent beta cells, makes these cells dependent on exogenously generated nitric oxide, which is released from infiltrating immune cells in human type 1 diabetes, for full expression of proinflammatory cytokine toxicity.

CONCLUSIONS/INTERPRETATION

EndoC-βH1 beta cells are characterised by an imbalance between H2O2-generating and -inactivating enzymes, and react to cytokine exposure in a similar manner to primary human beta cells. They are a suitable beta cell surrogate for cytokine-toxicity studies.

Physiological characterization of the human EndoC-βH1 β-cell line

In the new human EndoC-βH1 β-cell line, a detailed analysis of the physiological characteristics was performed. This new human β-cell line expressed all target structures on the gene and protein level, which are crucial for physiological function and insulin secretion induced by glucose and other secretagogues. Glucose influx measurements revealed an excellent uptake capacity of EndoC-βH1 β-cells by the Glut1 and Glut2 glucose transporters. A high expression level of glucokinase enabled efficient glucose phosphorylation, increasing the ATP/ADP ratio along with stimulation of insulin secretion in the physiological glucose concentration range. The EC50 value of glucose for insulin secretion was 10.3 mM. Mannoheptulose, a specific glucokinase inhibitor, blocked glucose-induced insulin secretion (GSIS). The nutrient insulin secretagogues l-leucine and 2-ketoisocaproate also stimulated insulin secretion, with a potentiating effect of l-glutamine. The Kir 6.2 potassium channel blocker glibenclamide and Bay K 8644, an opener of the voltage-sensitive Ca(2+) channel significantly potentiated GSIS. Potentiation of GSIS by IBMX and forskolin went along with a strong stimulation of cAMP generation. In conclusion, the new human EndoC-βH1 β-cell line fully mirrors the analogous physiological characteristics of primary mouse, rat and human β-cells. Thus, this new human EndoC-βH1 β-cell line is very well suited for physiological β-cell studies.

Unveiling a common mechanism of apoptosis in β-cells and neurons in Friedreich's ataxia

Friedreich's ataxia (FRDA) is a neurodegenerative disorder associated with cardiomyopathy and diabetes. Effective therapies for FRDA are an urgent unmet need; there are currently no options to prevent or treat this orphan disease. FRDA is caused by reduced expression of the mitochondrial protein frataxin. We have previously demonstrated that pancreatic β-cell dysfunction and death cause diabetes in FRDA. This is secondary to mitochondrial dysfunction and apoptosis but the underlying molecular mechanisms are not known. Here we show that β-cell demise in frataxin deficiency is the consequence of oxidative stress-mediated activation of the intrinsic pathway of apoptosis. The pro-apoptotic Bcl-2 family members Bad, DP5 and Bim are the key mediators of frataxin deficiency-induced β-cell death. Importantly, the intrinsic pathway of apoptosis is also activated in FRDA patients' induced pluripotent stem cell-derived neurons. Interestingly, cAMP induction normalizes mitochondrial oxidative status and fully prevents activation of the intrinsic pathway of apoptosis in frataxin-deficient β-cells and neurons. This preclinical study suggests that incretin analogs hold potential to prevent/delay both diabetes and neurodegeneration in FRDA.

Progesterone induces apoptosis of insulin-secreting cells: insights into the molecular mechanism

Progesterone has been associated with the development of gestational diabetes (GD) due to the enhancement of insulin resistance. As β-cell apoptosis participates in type 1 and type 2 diabetes pathophysiology, we proposed the hypothesis that progesterone might contribute to the development of GD through a mechanism that also involves β-cell death. To address this question, RINm5F insulin-producing cells were incubated with progesterone (25-100 μM), in the presence or absence of α-tocopherol (40 μM). After 24 or 48 h, membrane integrity and DNA fragmentation were analyzed by flow cytometry. Caspase activity was used to identify the mode of cell death. The involvement of endoplasmic reticulum stress in the action of progesterone was investigated by western blotting. Oxidative stress was measured by 2',7'-dichlorofluorescein diacetate (DCFDA) oxidation. Isolated rat islets were used in similar experiments in order to confirm the effect of progesterone in primary β-cells. Incubation of RINm5F cells with progesterone increased the number of cells with loss of membrane integrity and DNA fragmentation. Progesterone induced generation of reactive species. Pre-incubation with α-tocopherol attenuated progesterone-induced apoptosis. Western blot analyses revealed increased expression of CREB2 and CHOP in progesterone-treated cells. Progesterone caused apoptotic death of rat islet cells and enhanced generation of reactive species. Our results show that progesterone can be toxic to pancreatic β-cells through an oxidative-stress-dependent mechanism that induces apoptosis. This effect may contribute to the development of GD during pregnancy, particularly under conditions that require administration of pharmacological doses of this hormone.

IL-1β hampers glucose-stimulated insulin secretion in Cohen diabetic rat islets through mitochondrial cytochrome c oxidase inhibition by nitric oxide

A high-sucrose, low-copper-diet (HSD) induces inhibition of glucose-sensitive rats (CDs) but not Cohen diabetes-resistant rats (CDr). Copper-supplemented HSD increased activity of the copper-dependent mitochondrial respiratory chain enzyme cytochrome c oxidase (COX) and reversed hyperglycemia. This study examined the mechanism by which interleukin-1β modulates GSIS and the role of COX in this process. We measured COX activity, ATP content, GSIS, iNOS expression, and nitrite production with and without IL-1β, N(ω)-nitro-l-arginine, copper, or potassium cyanide in isolated islets of CDs and CDr fed different diets. We found reduced COX activity, ATP content, and GSIS in isolated islets of CDs rats fed a regular diet. These were severely reduced following HSD and were restored to regular diet levels on copper-supplemented HSD (P < 0.01 vs. CDr islets). Potassium cyanide chemically reduced COX activity, decreasing GSIS and thus reinforcing the link between islet COX activity and GSIS. Interleukin-1β (2.5 U/ml) reduced GSIS and COX activity in CDs islets. Exposure to 10 U/ml interleukin-1β decreased GSIS and COX activity in both CDs and CDr islets, inducing a similar nitrite production. Nevertheless, the effect on GSIS was more marked in CDs islets. A significant iNOS expression was detected in CDs on the HSD diet, which was reduced by copper supplementation. N(ω)-nitro-l-arginine and copper prevented the deleterious effect of interleukin-1β on COX activity and GSIS. We conclude that reduced islet COX activity renders vulnerability to GSIS inhibition on low-copper HSD through two interrelated pathways: 1) by further reducing the activity of COX that is essential for β-cell ATP-production and insulin secretion and 2) by inducing the expression of iNOS and nitric oxide-mediated COX inhibition. We suggest that islet COX activity must be maintained above a critical threshold to sustain adequate GSIS with exposure to low-copper HSD.

Overexpression of the antioxidant enzyme catalase does not interfere with the glucose responsiveness of insulin-secreting INS-1E cells and rat islets

AIMS/HYPOTHESIS

Hydrogen peroxide (H2O2)-inactivating enzymes such as catalase are produced in extraordinarily low levels in beta cells. Whether this low expression might be related to a signalling function of H2O2 within the beta cell is unknown. A high level of H2O2-inactivating enzymes could potentially be incompatible with glucose-induced insulin secretion. Therefore the effect of catalase overexpression on mitochondrial function and physiological insulin secretion was studied in insulin-secreting INS-1E and primary islet cells.

METHODS

INS-1E and rat islet cells were lentivirally transduced to overexpress catalase in the cytosol (CytoCat) or in mitochondria (MitoCat). Cell viability and caspase-3 activation were assessed after cytokine incubation and hypoxia. Insulin secretion was quantified and expression of the gene encoding the mitochondrial uncoupling protein 2 (Ucp2) was measured in parallel to mitochondrial membrane potential and reactive oxygen species (ROS) formation.

RESULTS

The ability to secret insulin in a glucose-dependent manner was not suppressed by catalase overexpression, although the glucose-dependent increase in the mitochondrial membrane potential was attenuated in MitoCat cells along with an increased Ucp2 expression and reduced mitochondrial ROS formation. In addition, MitoCat overexpressing cells were significantly more resistant against pro-inflammatory cytokines and hypoxia than CytoCat and control cells.

CONCLUSIONS/INTERPRETATION

The results demonstrate that an improved antioxidative defence status of insulin-secreting cells allowing efficient H2O2 inactivation is not incompatible with proper insulin secretory responsiveness to glucose stimulation and provide no support for a signalling role of H2O2 in insulin-secreting cells. Interestingly, the results also document for the first time that the decreased ROS formation with increasing glucose concentrations is of mitochondrial origin.

Limited GADD45α expression and function in IL-1β toxicity towards insulin-producing cells

Growth arrest and DNA damage-inducible (GADD) 45 proteins are regulators of cell death and survival. The proinflammatory cytokine IL-1β strongly increases the level of the transcript encoding GADD45α in rat insulin-producing INS-1E cells. The activation of Gadd45α gene is clearly dependent on JNK and NF-κB activation and the synthesis of the secondary mediator nitric oxide (NO). Interestingly, the observed twelve-fold increase in the GADD45α-coding transcript level is not followed by increased expression of GADD45α at the protein level. An analysis of IL-1β toxicity in INS-1E cells overexpressing GADD45α revealed no correlation between the GADD45α protein level and the sensitivity to IL-1β toxicity. These findings suggest that the potential engagement of GADD45α in IL-1β toxicity towards beta cells is limited to the effects induced by the basal expression level of this protein.

Is there a role for neuronal nitric oxide synthase (nNOS) in cytokine toxicity to pancreatic beta cells?

Nitric oxide (NO), produced by the action of the inducible NO synthase, plays a crucial role in cytokine toxicity to pancreatic beta cells during type 1 diabetes development. It was the aim of this study to analyze the role of the neuronal NOS (nNOS) in proinflammatory cytokine-mediated beta cell toxicity. Expression of different isoforms of nitric oxide synthase in insulin-secreting INS1E cells and rat islets was analyzed by quantitative real-time PCR and Western blotting. The expression of nNOS in insulin-secreting INS1E cells was similar to that found in rat brain, while two other isoforms, namely the endothelial eNOS and inducible iNOS were not expressed in untreated cells. IL-1β alone or in combination with TNF-α and/or IFNγ induced iNOS but not eNOS expression. In contrast, nNOS expression was strongly decreased by the mixture of the three proinflammatory cytokines (IL-1β, TNF-α and IFNγ) both on the gene and protein level in INS1E cells and rat islet cells. The effects of cytokines on glucose-induced insulin-secretion followed the pattern of nNOS expression reduction and, on the other hand, of the iNOS induction. The data indicate that a low level of nitric oxide originating from the constitutive expression of nNOS in pancreatic beta cells is not deleterious. In particular since proinflammatory cytokines reduce this expression. This nNOS suppression can compensate for NO generation by low concentrations of IL-1β through iNOS induction. Thus, this basal nNOS expression level in pancreatic beta cells represents a protective element against cytokine toxicity.

Enhancement of homocysteine toxicity to insulin-secreting BRIN-BD11 cells in combination with alloxan

Previous studies have shown that homocysteine (HC) has a detrimental impact on insulin secretion and pancreatic beta cell function. The aim of the present study was to determine the role of reactive oxygen species (ROS) in the in vitro toxic effects of HC on insulin secretion and function of BRIN-BD11 insulin-secreting cells. In this study, insulin secretion from BRIN-BD11 cells was determined radioimmunologically, cell viability by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide assay and glucokinase activity by a glucose phosphorylation assay following culture with HC plus alloxan (Alx). Treatment with HC resulted in concentration-dependent inhibition of insulin secretion induced by glucose and other insulinotropic agents. HC in combination with Alx resulted in a more pronounced decline in insulin secretion, including that induced by 20  mM alanine, by 43% (P<0.001) and 30  mM KCl by 60% (P<0.001), compared with control culture. The glucokinase phosphorylating capacity in cells cultured with HC plus Alx was significantly lower, compared with control cells. The cells also displayed a significant 84% (P<0.001) decline in cell viability. Prolonged, 72-h culture of insulin-secreting cells with HC followed by 18-h culture without HC did not result in full restoration of beta cell responses to insulinotropic agents. In vitro oxygen consumption was enhanced by a combination of Alx with HC. The study arrived at the conclusion that HC generates ROS in a redox-cycling reaction with Alx that explains the decline in viability of insulin-secreting cells, leading to reduced glucokinase phosphorylating ability, diminished insulin secretory responsiveness and cell death.

Mechanism of prostacyclin-induced potentiation of glucose-induced insulin secretion

Arachidonic acid metabolites are crucial mediators of inflammation in diabetes. Although eicosanoids are established modulators of pancreatic β-cell function, the role of prostacyclin (prostaglandin I2) is unknown. Therefore, this study aimed to analyze the role of prostacyclin in β-cell function. Prostacyclin synthase (PGIS) was weakly expressed in rat islet cells but nevertheless significantly increased by incubation with 30 mM glucose, especially in non-β-cells. PGIS was overexpressed in INS1E cells, and the regulation of insulin secretion was analyzed. PGIS overexpression strongly potentiated glucose-induced insulin secretion along with increased insulin content and ATP production. Importantly, overexpression of PGIS potentiated only nutrient-induced insulin secretion. The effect of PGIS overexpression was mediated by prostacyclin released from insulin-secreting cells and dependent on prostacyclin receptor (IP receptor) activation, with concomitant cAMP production. The cAMP-mediated potentiation of glucose-induced insulin secretion by prostacyclin was independent of the protein kinase A pathway but strongly attenuated by the knockdown of the exchange protein directly activated by cAMP 2 (Epac2), pointing to a crucial role for Epac2 in this process. Thus, prostacyclin is a powerful potentiator of glucose-induced insulin secretion. It improves the secretory capacity by inducing insulin biosynthesis and probably by stimulating exocytosis. Our findings open a new therapeutical perspective for an improved treatment of type 2 diabetes.

Induction of the intrinsic apoptosis pathway in insulin-secreting cells is dependent on oxidative damage of mitochondria but independent of caspase-12 activation

Pro-inflammatory cytokine-mediated beta cell apoptosis is activated through multiple signaling pathways involving mitochondria and endoplasmic reticulum. Activation of organelle-specific caspases has been implicated in the progression and execution of cell death. This study was therefore performed to elucidate the effects of pro-inflammatory cytokines on a possible cross-talk between the compartment-specific caspases 9 and 12 and their differential contribution to beta cell apoptosis. Moreover, the occurrence of ROS-mediated mitochondrial damage in response to beta cell toxic cytokines has been quantified. ER-specific caspase-12 was strongly activated in response to pro-inflammatory cytokines; however, its inhibition did not abolish cytokine-induced mitochondrial caspase-9 activation and loss of cell viability. In addition, there was a significant induction of oxidative mitochondrial DNA damage and elevated cardiolipin peroxidation in insulin-producing RINm5F cells and rat islet cells. Overexpression of the H(2)O(2) detoxifying enzyme catalase effectively reduced the observed cytokine-induced oxidative damage of mitochondrial structures. Taken together, the results strongly indicate that mitochondrial caspase-9 is not a downstream substrate of ER-specific caspase-12 and that pro-inflammatory cytokines cause apoptotic beta cell death through activation of caspase-9 primarily by hydroxyl radical-mediated mitochondrial damage.

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.

Protection of insulin-producing cells against toxicity of dexamethasone by catalase overexpression

Pancreatic beta cells are very sensitive to reactive oxygen species (ROS) and this might play an important role in beta cell death in diabetes. Dexamethasone is a synthetic diabetogenic glucocorticoid, which impairs pancreatic beta cell function. Therefore we investigated the toxicity of dexamethasone in RINm5F insulin-producing cells and its dependence on the expression level of the antioxidant enzyme catalase, which inactivates hydrogen peroxide. This was correlated with oxidative stress and cell death. An increased generation of ROS was observed in dexamethasone-treated cells together with an increase in caspase-3 activity and apoptosis rate. Interestingly, exposure to dexamethasone increased the cytosolic superoxide dismutase Cu/ZnSOD protein expression and activity, whereas the mitochondrial MnSOD isoform was not affected by the glucocorticoid. Catalase overexpression in insulin-producing cells prevented all the cytotoxic effects of dexamethasone. In conclusion, dexamethasone-induced cell death in insulin-producing cells is ROS mediated. Increased levels of expression and activity of the Cu/ZnSOD might favor the generation of hydrogen peroxide in dexamethasone-treated cells. Increased ROS scavenging capacity in insulin-producing cells, through overexpression of catalase, prevents a deleterious increase in hydrogen peroxide generation and thus prevents dexamethasone-induced apoptosis.

Interaction between pro-inflammatory and anti-inflammatory cytokines in insulin-producing cells

Pro-inflammatory cytokines cause beta-cell dysfunction and death. The aim of this study was to investigate the interactions between different pro- and anti-inflammatory cytokines and their effects on apoptotic beta-cell death pathways. Insulin-producing RINm5F cells were exposed to different combinations of cytokines. Gene expression analyses of manganese superoxide dismutase (MnSOD) and inducible nitric oxide synthase (iNOS) were performed by real-time RT-PCR. Cell viability was measured by the MTT assay, NFkappaB activation using a SEAP reporter gene assay, protein expression by western blotting and caspase-3 activity using the DEVD cleavage method. IL-1beta, tumour necrosis factor alpha (TNFalpha) and a combination of all three pro-inflammatory cytokines increased while IFNgamma alone did not affect NFkappaB activity and iNOS gene and protein expression. Interestingly, the anti-inflammatory cytokines IL-4, IL-13 and IL-10 decreased IL-1beta-stimulated NFkappaB activation and iNOS expression. IL-1beta, TNFalpha and the pro-inflammatory cytokine combination also increased MnSOD gene and protein expression. But IL-4, IL-13 and IL-10 did not affect MnSOD expression and did not modulate IL-1beta-stimulated MnSOD expression. Caspase-3 activity was increased by IL-1beta and the pro-inflammatory cytokine combination, and to a lesser extent by TNFalpha. In contrast, IFNgamma had no effect on caspase-3 activity. IL-4, IL-13 and IL-10 decreased caspase-3 activity and increased viability of insulin-producing cells treated with pro-inflammatory cytokines. The anti-inflammatory cytokines counteracted the cytotoxic effects of pro-inflammatory cytokines in insulin-producing cells. This was achieved through the reduction of nitrosative stress. Thus, a balance between the anti-inflammatory and the pro-inflammatory cytokines is of crucial importance for the prevention of pancreatic beta-cell destruction.

Relation between triketone structure, generation of reactive oxygen species, and selective toxicity of the diabetogenic agent alloxan

The diabetogenic agent alloxan is a triketone that selectively destroys pancreatic beta cells. To investigate the importance of the triketone structure of alloxan for its cytotoxic potency, alloxan was compared with ninhydrin, also a triketone, and the amino derivative of alloxan uramil, which is not a triketone, because the 5-keto group of the alloxan is replaced by an amino group. Both compounds are cytotoxic but not diabetogenic. Ninhydrin was capable of generating cytotoxic reactive oxygen species (ROS) through redox cycling with dithiols, and uramil could also generate cytotoxic ROS. Both ninhydrin and uramil could not redox cycle with glutathione (GSH) and were not selectively toxic to beta cells; their structure does not allow selective cellular uptake via the GLUT2 glucose transporter. Thus, the results show that the 5-keto group in the pyrimidine ring structure of the triketone alloxan is crucially important for its ability to be selectively taken up into the beta cells via the specific glucose transporter GLUT2. The 5-keto group of the molecule enables redox cycling of alloxan through reaction with glutathione (GSH), thereby generating the cytotoxic ROS. Thus, the unique combination of these two properties confers on alloxan the beta cell-selective toxicity and diabetogenicity. Replacement of the 5-keto group by an amino group, as in uramil, abolishes selective beta cell toxicity because of the loss of the glucose analogue structure and the capability to generate ROS via redox cycling with GSH and cysteine.

Relative importance of cellular uptake and reactive oxygen species for the toxicity of alloxan and dialuric acid to insulin-producing cells

The diabetogenic agent alloxan is selectively accumulated in insulin-producing cells through uptake via the GLUT2 glucose transporter in the plasma membrane. In the presence of intracellular thiols, especially glutathione, alloxan generates "reactive oxygen species" (ROS) in a cyclic reaction between this substance and its reduction product, dialuric acid. The cytotoxic action of alloxan is initiated by free radicals formed in this redox reaction. Autoxidation of dialuric acid generates superoxide radicals (O(2)(*-)) and hydrogen peroxide (H(2)O(2)), and finally hydroxyl radicals ((*)OH). Thus, while superoxide dismutase (SOD) only reduced the toxicity, catalase, in particular in the presence of SOD, provided complete protection of insulin-producing cells against the cytotoxic action of alloxan and dialuric acid due to H(2)O(2) destruction and the prevention of hydroxyl radical ((*)OH) formation, indicating that it is the hydroxyl radical ((*)OH) which is the ROS ultimately responsible for cell death. After selective accumulation in pancreatic beta cells, which are weakly protected against oxidative stress, the cytotoxic glucose analogue alloxan destroys these insulin-producing cells and causes a state of insulin-dependent diabetes mellitus through ROS-mediated toxicity in rodents and in other animal species, which express this glucose transporter isoform in their beta cells.

Importance of mitochondrial superoxide dismutase expression in insulin-producing cells for the toxicity of reactive oxygen species and proinflammatory cytokines

AIMS/HYPOTHESIS

Free radicals generated in mitochondria play a crucial role in the toxic effects of cytokines upon insulin-producing cells. This study therefore investigated the role of manganese superoxide dismutase (MnSOD) in cytokine-mediated toxicity in insulin-producing cells.

METHODS

MnSOD was either stably overexpressed (MnSODsense) or stably suppressed (MnSODantisense) in insulin-producing RINm5F cells. Cell viability was quantified after incubation with different chemical reactive oxygen species (ROS) generators and with cytokines (IL-1beta alone or a mixture of IL-1beta, TNF-alpha and IFN-gamma). Additionally, cell proliferation and endogenous MnSOD protein expression were determined after exposure to cytokines.

RESULTS

After incubation with hydrogen peroxide (H(2)O(2)) or hypoxanthine/xanthine oxidase no significant differences were observed in viability between control and MnSODsense or MnSODantisense clones. MnSOD overexpression reduced the viability of MnSODsense cells after exposure to the intracellular ROS generator menadione compared with control and MnSODantisense cells. MnSODsense cells also showed the highest susceptibility to cytokine toxicity with more than 75% loss of viability and a significant reduction of the proliferation rate after 72 h of incubation with a cytokine mixture. In comparison with control cells (67% viability loss), the reduction of viability in MnSODantisense cells was lower (50%), indicating a sensitising role of MnSOD in the progression of cytokine toxicity. The cell proliferation rate decreased in parallel to the reduction of cell viability. The MnSOD expression level after exposure to cytokines was also significantly lower in MnSODantisense cells than in control or MnSODsense cells.

CONCLUSIONS/INTERPRETATION

The increase of the mitochondrial imbalance between the superoxide- and the H(2)O(2)-inactivating enzyme activities corresponds with a greater susceptibility to cytokines. Thus optimal antioxidative strategies to protect insulin-producing cells against cytokine toxicity may comprise a combined overexpression of H(2)O(2)-inactivating enzymes or suppression of MnSOD activity.

Limited GADD45α expression and function in IL-1β toxicity towards insulin-producing cells

Growth arrest and DNA damage-inducible (GADD) 45 proteins are regulators of cell death and survival. The proinflammatory cytokine IL-1β strongly increases the level of the transcript encoding GADD45α in rat insulin-producing INS-1E cells. The activation of Gadd45α gene is clearly dependent on JNK and NF-κB activation and the synthesis of the secondary mediator nitric oxide (NO). Interestingly, the observed twelve-fold increase in the GADD45α-coding transcript level is not followed by increased expression of GADD45α at the protein level. An analysis of IL-1β toxicity in INS-1E cells overexpressing GADD45α revealed no correlation between the GADD45α protein level and the sensitivity to IL-1β toxicity. These findings suggest that the potential engagement of GADD45α in IL-1β toxicity towards beta cells is limited to the effects induced by the basal expression level of this protein.