Cytokines induce endoplasmic reticulum stress in human, rat and mouse beta cells via different mechanisms

Interferon-α mediates human beta cell HLA class I overexpression, endoplasmic reticulum stress and apoptosis, three hallmarks of early human type 1 diabetes

AIMS/HYPOTHESIS

Three hallmarks of the pancreatic islets in early human type 1 diabetes are overexpression of HLA class I, endoplasmic reticulum (ER) stress and beta cell apoptosis. The mediators of these phenomena remain to be defined. The type I interferon IFNα is expressed in human islets from type 1 diabetes patients and mediates HLA class I overexpression. We presently evaluated the mechanisms involved in IFNα-induced HLA class I expression in human beta cells and determined whether this cytokine contributes to ER stress and apoptosis.

METHODS

IFNα-induced inflammation, ER stress and apoptosis were evaluated by RT-PCR, western blot, immunofluorescence and nuclear dyes, and proteins involved in type I interferon signalling were inhibited by small interfering RNAs. All experiments were performed in human islets or human EndoC-βH1 cells.

RESULTS

IFNα upregulates HLA class I, inflammation and ER stress markers in human beta cells via activation of the candidate gene TYK2, and the transcription factors signal transducer and activator of transcription 2 and IFN regulatory factor 9. Furthermore, it acts synergistically with IL-1β to induce beta cell apoptosis.

CONCLUSIONS/INTERPRETATION

The innate immune effects induced by IFNα may induce and amplify the adaptive immune response against human beta cells, indicating that IFNα has a central role in the early phases of diabetes.

TRPV4 regulates insulin mRNA expression and INS-1E cell death via ERK1/2 and NO-dependent mechanisms

TRPV4 is a Ca²⁺-permeable, nonselective cation channel. Recently, TRPV4 was implicated in controlling peripheral insulin sensitivity, insulin secretion and apoptosis of pancreatic beta cells. Here, we characterize the role and potential mechanisms of TRPV4 in regulating insulin mRNA expression and cell death in insulin producing INS-1E cells and rat pancreatic islets. TRPV4 protein production was downregulated by siRNA. Intracellular calcium level was measured using Fluo-3 AM. Gene expression was studied by real-time PCR. Phosphorylation of extracellular signal-regulated kinase (ERK1 and ERK2) was detected by Western blot. Nitric oxide (NO) production was assessed by chemiluminescent reaction. Reactive oxygen species (ROS) level was analysed using a fluorogenic dye (DCFDA). Cell death was evaluated by determination of cytoplasmic histone-associated DNA fragments. Downregulation of TRPV4 neither affected insulin mRNA expression nor INS-1E cell growth. By contrast, pharmacological TRPV4 activation by 100nmol/l GSK1016790A increased Ca²⁺ levels in INS-1E cells and enhanced insulin mRNA expression after 1 and 3h, whereas a suppression of insulin mRNA expression was detected after 24h incubation. GSK1016790A increased ERK1/2 phosphorylation and NO production but not ROS production. Pharmacological blockade of ERK1/2 attenuated GSK1016790A-induced insulin mRNA expression. Inhibition of NO synthesis by l-NAME failed to affect insulin mRNA expression in GSK1016790A treated INS-1E cells. Furthermore, inhibition of NO production attenuated GSK1016790A-induced INS-1E cell death. In pancreatic islets, 100nmol/l GSK1016790A increased insulin mRNA levels after 3h without inducing cytotoxicity after 24h. In conclusion, TRPV4 differently regulates insulin mRNA expression in INS-1E cells via ERK1/2 and NO-dependent mechanisms.

Inflammatory stress of pancreatic beta cells drives release of extracellular heat-shock protein 90α

A major obstacle in predicting and preventing the development of autoimmune type 1 diabetes (T1D) in at-risk individuals is the lack of well-established early biomarkers indicative of ongoing beta cell stress during the pre-clinical phase of disease. Recently, serum levels of the α cytoplasmic isoform of heat-shock protein 90 (hsp90) were shown to be elevated in individuals with new-onset T1D. We therefore hypothesized that hsp90α could be released from beta cells in response to cellular stress and inflammation associated with the earliest stages of T1D. Here, human beta cell lines and cadaveric islets released hsp90α in response to stress induced by treatment with a combination of pro-inflammatory cytokines including interleukin-1β, tumour necrosis factor-α and interferon-γ. Mechanistically, hsp90α release was found to be driven by cytokine-induced endoplasmic reticulum stress mediated by c-Jun N-terminal kinase (JNK), a pathway that can eventually lead to beta cell apoptosis. Cytokine-induced beta cell hsp90α release and JNK activation were significantly reduced by pre-treating cells with the endoplasmic reticulum stress-mitigating chemical chaperone tauroursodeoxycholic acid. The hsp90α release by cells may therefore be a sensitive indicator of stress during inflammation and a useful tool in assessing therapeutic mitigation of cytokine-induced cell damage linked to autoimmunity.

Neuron-enriched RNA-binding Proteins Regulate Pancreatic Beta Cell Function and Survival

Pancreatic beta cell failure is the central event leading to diabetes. Beta cells share many phenotypic traits with neurons, and proper beta cell function relies on the activation of several neuron-like transcription programs. Regulation of gene expression by alternative splicing plays a pivotal role in brain, where it affects neuronal development, function, and disease. The role of alternative splicing in beta cells remains unclear, but recent data indicate that splicing alterations modulated by both inflammation and susceptibility genes for diabetes contribute to beta cell dysfunction and death. Here we used RNA sequencing to compare the expression of splicing-regulatory RNA-binding proteins in human islets, brain, and other human tissues, and we identified a cluster of splicing regulators that are expressed in both beta cells and brain. Four of them, namely Elavl4, Nova2, Rbox1, and Rbfox2, were selected for subsequent functional studies in insulin-producing rat INS-1E, human EndoC-βH1 cells, and in primary rat beta cells. Silencing of Elavl4 and Nova2 increased beta cell apoptosis, whereas silencing of Rbfox1 and Rbfox2 increased insulin content and secretion. Interestingly, Rbfox1 silencing modulates the splicing of the actin-remodeling protein gelsolin, increasing gelsolin expression and leading to faster glucose-induced actin depolymerization and increased insulin release. Taken together, these findings indicate that beta cells share common splicing regulators and programs with neurons. These splicing regulators play key roles in insulin release and beta cell survival, and their dysfunction may contribute to the loss of functional beta cell mass in diabetes.

Novel Treatment of Chronic Graft-Versus-Host Disease in Mice Using the ER Stress Reducer 4-Phenylbutyric Acid

Chronic graft-versus-host disease (cGVHD) is a notorious complication of allogeneic hematopoietic stem cell transplantation and causes disabling systemic inflammation and fibrosis. In this novel study, we focused on a relationship between endoplasmic reticulum (ER) stress and cGVHD, and aimed to create effective treatment of cGVHD. A series of experiments were conducted using a mouse model of cGVHD. Our data suggested (1) that ER stress was elevated in organs affected by cGVHD and (2) that 4-phenylbutyric acid (PBA) could reduce cGVHD-induced ER stress and thereby alleviate systemic inflammation and fibrosis. Because fibroblasts are thought to be implicated in cGVHD-elicited fibrosis and because macrophages are reported to play a role in the development of cGVHD, we investigated cGVHD-triggered ER stress in fibroblasts and macrophages. Our investigation demonstrated (1) that indicators for ER stress and activation markers for fibroblasts were elevated in cGVHD-affected lacrimal gland fibroblasts and (2) that they could be reduced by PBA. Our work also indicated that splenic macrophages from PBA-dosed mice exhibited the lower levels of ER stress and M2 macrophage markers than those from cGVHD-affected mice. Collectively, this study suggests that the reduction of ER stress utilizing PBA can be a clinically translatable method to treat systemic cGVHD.

Inhibition of placental mTOR signaling provides a link between placental malaria and reduced birthweight

BACKGROUND

Placental Plasmodium falciparum malaria can trigger intervillositis, a local inflammatory response more strongly associated with low birthweight than placental malaria infection alone. Fetal growth (and therefore birthweight) is dependent on placental amino acid transport, which is impaired in placental malaria-associated intervillositis. Here, we tested the hypothesis that mechanistic target of rapamycin (mTOR) signaling, a pathway known to regulate amino acid transport, is inhibited in placental malaria-associated intervillositis, contributing to lower birthweight.

METHODS

We determined the link between intervillositis, mTOR signaling activity, and amino acid uptake in tissue biopsies from both uninfected placentas and malaria-infected placentas with and without intervillositis, and in an in vitro model using primary human trophoblast (PHT) cells.

RESULTS

We demonstrated that (1) placental mTOR activity is lower in cases of placental malaria with intervillositis, (2) placental mTOR activity is negatively correlated with the degree of inflammation, and (3) inhibition of placental mTOR activity is associated with reduced placental amino acid uptake and lower birthweight. In PHT cells, we showed that (1) inhibition of mTOR signaling is a mechanistic link between placental malaria-associated intervillositis and decreased amino acid uptake and (2) constitutive mTOR activation partially restores amino acid uptake.

CONCLUSIONS

Our data support the concept that inhibition of placental mTOR signaling constitutes a mechanistic link between placental malaria-associated intervillositis and decreased amino acid uptake, which may contribute to lower birthweight. Restoring placental mTOR signaling in placental malaria may increase birthweight and improve neonatal survival, representing a new potential therapeutic approach.

MicroRNAs miR-23a-3p, miR-23b-3p, and miR-149-5p Regulate the Expression of Proapoptotic BH3-Only Proteins DP5 and PUMA in Human Pancreatic β-Cells

Type 1 diabetes (T1D) is an autoimmune disease leading to β-cell destruction. MicroRNAs (miRNAs) are small noncoding RNAs that control gene expression and organ formation. They participate in the pathogenesis of several autoimmune diseases, but the nature of miRNAs contributing to β-cell death in T1D and their target genes remain to be clarified. We performed an miRNA expression profile on human islet preparations exposed to the cytokines IL-1β plus IFN-γ. Confirmation of miRNA and target gene modification in human β-cells was performed by real-time quantitative PCR. Single-stranded miRNAs inhibitors were used to block selected endogenous miRNAs. Cell death was measured by Hoechst/propidium iodide staining and activation of caspase-3. Fifty-seven miRNAs were detected as modulated by cytokines. Three of them, namely miR-23a-3p, miR-23b-3p, and miR-149-5p, were downregulated by cytokines and selected for further studies. These miRNAs were found to regulate the expression of the proapoptotic Bcl-2 proteins DP5 and PUMA and consequent human β-cell apoptosis. These results identify a novel cross talk between a key family of miRNAs and proapoptotic Bcl-2 proteins in human pancreatic β-cells, broadening our understanding of cytokine-induced β-cell apoptosis in early T1D.

Targeting Extracellular Cyclophilin A Reduces Neuroinflammation and Extends Survival in a Mouse Model of Amyotrophic Lateral Sclerosis

Neuroinflammation is a major hallmark of amyotrophic lateral sclerosis (ALS), which is currently untreatable. Several anti-inflammatory compounds have been evaluated in patients and in animal models of ALS, but have been proven disappointing in part because effective targets have not yet been identified. Cyclophilin A, also known as peptidylprolyl cis-/trans-isomerase A (PPIA), as a foldase is beneficial intracellularly, but extracellularly has detrimental functions. We found that extracellular PPIA is a mediator of neuroinflammation in ALS. It is a major inducer of matrix metalloproteinase 9 and is selectively toxic for motor neurons. High levels of PPIA were found in the CSF of SOD1^G93A mice and rats and sporadic ALS patients, suggesting that our findings may be relevant for familial and sporadic cases. A specific inhibitor of extracellular PPIA, MM218, given at symptom onset, rescued motor neurons and extended survival in the SOD1^G93A mouse model of familial ALS by 11 d. The treatment resulted in the polarization of glia toward a prohealing phenotype associated with reduced NF-κB activation, proinflammatory markers, endoplasmic reticulum stress, and insoluble phosphorylated TDP-43. Our results indicates that extracellular PPIA is a promising druggable target for ALS and support further studies to develop a therapy to arrest or slow the progression of the disease in patients.SIGNIFICANCE STATEMENT We provide evidence that extracellular cyclophilin A, also known as peptidylprolyl cis-/trans-isomerase A (PPIA), is a mediator of the neuroinflammatory reaction in amyotrophic lateral sclerosis (ALS) and is toxic for motor neurons. Supporting this, a specific extracellular PPIA inhibitor reduced neuroinflammation, rescued motor neurons, and extended survival in the SOD1^G93A mouse model of familial ALS. Our findings suggest selective pharmacological inhibition of extracellular PPIA as a novel therapeutic strategy, not only for SOD1-linked ALS, but possibly also for sporadic ALS. This approach aims to address the neuroinflammatory reaction that is a major hallmark of ALS. However, given the complexity of the disease, a combination of therapeutic approaches may be necessary.

JNK at the crossroad of obesity, insulin resistance, and cell stress response

BACKGROUND

The cJun-N-terminal-kinase (JNK) plays a central role in the cell stress response, with outcomes ranging from cell death to cell proliferation and survival, depending on the specific context. JNK is also one of the most investigated signal transducers in obesity and insulin resistance, and studies have identified new molecular mechanisms linking obesity and insulin resistance. Emerging evidence indicates that whereas JNK1 and JNK2 isoforms promote the development of obesity and insulin resistance, JNK3 activity protects from excessive adiposity. Furthermore, current evidence indicates that JNK activity within specific cell types may, in specific stages of disease progression, promote cell tolerance to the stress associated with obesity and type-2 diabetes.

SCOPE OF REVIEW

This review provides an overview of the current literature on the role of JNK in the progression from obesity to insulin resistance, NAFLD, type-2 diabetes, and diabetes complications.

MAJOR CONCLUSION

Whereas current evidence indicates that JNK1/2 inhibition may improve insulin sensitivity in obesity, the role of JNK in the progression from insulin resistance to diabetes, and its complications is largely unresolved. A better understanding of the role of JNK in the stress response to obesity and type-2 diabetes, and the development of isoform-specific inhibitors with specific tissue distribution will be necessary to exploit JNK as possible drug target for the treatment of type-2 diabetes.

β-cell differentiation status in type 2 diabetes

Type 2 diabetes (T2D) affects 415 million people worldwide and is characterized by chronic hyperglycaemia and insulin resistance, progressing to insufficient insulin production, as a result of β-cell failure. Over time, chronic hyperglycaemia can ultimately lead to loss of β-cell function, leaving patients insulin-dependent. Until recently the loss of β-cell mass seen in T2D was considered to be the result of increased rates of apoptosis; however, it has been proposed that apoptosis alone cannot account for the extent of β-cell mass loss seen in the disease, and that a loss of function may also occur as a result of changes in β-cell differentiation status. In the present review, we consider current knowledge of determinants of β-cell fate in the context of understanding its relevance to disease process in T2D, and also the impact of a diabetogenic environment (hyperglycaemia, hypoxia, inflammation and dyslipidaemia) on the expression of genes involved in maintenance of β-cell identity. We describe current knowledge of the impact of the diabetic microenvironment on gene regulatory processes such alternative splicing, the expression of disallowed genes and epigenetic modifications. Elucidating the molecular mechanisms that underpin changes to β-cell differentiation status and the concomitant β-cell failure offers potential treatment targets for the future management of patients with T2D.

A roadmap of the generation of neoantigens as targets of the immune system in type 1 diabetes

Type 1 diabetes (T1D) is an autoimmune disease characterized by the selective destruction of the insulin-producing beta cells. Beta cell dysfunction caused by an inflammatory microenvironment is believed to trigger the peripheral activation of CD4 and CD8 autoreactive T cells. This review will compile post-transcriptional and post-translational modifications (PTM) involved in the generation of beta cell neoantigens and proposes a reconstruction of the sequence of events connecting environmental changes and autoimmunity.

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.

Aberrant Accumulation of the Diabetes Autoantigen GAD65 in Golgi Membranes in Conditions of ER Stress and Autoimmunity

Pancreatic islet β-cells are particularly susceptible to endoplasmic reticulum (ER) stress, which is implicated in β-cell dysfunction and loss during the pathogenesis of type 1 diabetes (T1D). The peripheral membrane protein GAD65 is an autoantigen in human T1D. GAD65 synthesizes γ-aminobutyric acid, an important autocrine and paracrine signaling molecule and a survival factor in islets. We show that ER stress in primary β-cells perturbs the palmitoylation cycle controlling GAD65 endomembrane distribution, resulting in aberrant accumulation of the palmitoylated form in trans-Golgi membranes. The palmitoylated form has heightened immunogenicity, exhibiting increased uptake by antigen-presenting cells and T-cell stimulation compared with the nonpalmitoylated form. Similar accumulation of GAD65 in Golgi membranes is observed in human β-cells in pancreatic sections from GAD65 autoantibody-positive individuals who have not yet progressed to clinical onset of T1D and from patients with T1D with residual β-cell mass and ongoing T-cell infiltration of islets. We propose that aberrant accumulation of immunogenic GAD65 in Golgi membranes facilitates inappropriate presentation to the immune system after release from stressed and/or damaged β-cells, triggering autoimmunity.

Mass production of functional human pancreatic β-cells: why and how?

Diabetes (either type 1 or type 2) is due to insufficient functional β-cell mass. Research has, therefore, aimed to discover new ways to maintain or increase either β-cell mass or function. For this purpose, rodents have mainly been used as model systems and a large number of discoveries have been made. Meanwhile, although we have learned that rodent models represent powerful systems to model β-cell development, function and destruction, we realize that there are limitations when attempting to transfer the data to what is occurring in humans. Indeed, while human β-cells share many similarities with rodent β-cells, they also differ on a number of important parameters. In this context, developing ways to study human β-cell development, function and death represents an important challenge. This review will describe recent data on the development and use of convenient sources of human β-cells that should be useful tools to discover new ways to modulate functional β-cell mass in humans.

Endoplasmic reticulum stress and the unfolded protein response in pancreatic islet inflammation

Insulin-secreting pancreatic β-cells are extremely dependent on their endoplasmic reticulum (ER) to cope with the oscillatory requirement of secreted insulin to maintain normoglycemia. Insulin translation and folding rely greatly on the unfolded protein response (UPR), an array of three main signaling pathways designed to maintain ER homeostasis and limit ER stress. However, prolonged or excessive UPR activation triggers alternative molecular pathways that can lead to β-cell dysfunction and apoptosis. An increasing number of studies suggest a role of these pro-apoptotic UPR pathways in the downfall of β-cells observed in diabetic patients. Particularly, the past few years highlighted a cross talk between the UPR and inflammation in the context of both type 1 (T1D) and type 2 diabetes (T2D). In this article, we describe the recent advances in research regarding the interplay between ER stress, the UPR, and inflammation in the context of β-cell apoptosis leading to diabetes.

Hypoxia reduces ER-to-Golgi protein trafficking and increases cell death by inhibiting the adaptive unfolded protein response in mouse beta cells

AIMS/HYPOTHESIS

Hypoxia may contribute to beta cell failure in type 2 diabetes and islet transplantation. The adaptive unfolded protein response (UPR) is required for endoplasmic reticulum (ER) homeostasis. Here we investigated whether or not hypoxia regulates the UPR in beta cells and the role the adaptive UPR plays during hypoxic stress.

METHODS

Mouse islets and MIN6 cells were exposed to various oxygen (O2) tensions. DNA-damage inducible transcript 3 (DDIT3), hypoxia-inducible transcription factor (HIF)1α and HSPA5 were knocked down using small interfering (si)RNA; Hspa5 was also overexpressed. db/db mice were used.

RESULTS

Hypoxia-response genes were upregulated in vivo in the islets of diabetic, but not prediabetic, db/db mice. In isolated mouse islets and MIN6 cells, O2 deprivation (1-5% vs 20%; 4-24 h) markedly reduced the expression of adaptive UPR genes, including Hspa5, Hsp90b1, Fkbp11 and spliced Xbp1. Coatomer protein complex genes (Copa, Cope, Copg [also known as Copg1], Copz1 and Copz2) and ER-to-Golgi protein trafficking were also reduced, whereas apoptotic genes (Ddit3, Atf3 and Trb3 [also known as Trib3]), c-Jun N-terminal kinase (JNK) phosphorylation and cell death were increased. Inhibition of JNK, but not HIF1α, restored adaptive UPR gene expression and ER-to-Golgi protein trafficking while protecting against apoptotic genes and cell death following hypoxia. DDIT3 knockdown delayed the loss of the adaptive UPR and partially protected against hypoxia-induced cell death. The latter response was prevented by HSPA5 knockdown. Finally, Hspa5 overexpression significantly protected against hypoxia-induced cell death.

CONCLUSIONS/INTERPRETATION

Hypoxia inhibits the adaptive UPR in beta cells via JNK and DDIT3 activation, but independently of HIF1α. Downregulation of the adaptive UPR contributes to reduced ER-to-Golgi protein trafficking and increased beta cell death during hypoxic stress.

ER stress and the decline and fall of pancreatic beta cells in type 1 diabetes

Components of the unfolded protein response (UPR) modulate beta cell inflammation and death in early type 1 diabetes (T1D). The UPR is a mechanism by which cells react to the accumulation of misfolded proteins in the endoplasmic reticulum (ER). It aims to restore cellular homeostasis, but in case of chronic or overwhelming ER stress the persistent activation of the UPR triggers apoptosis, contributing to the loss of beta cells in both T1D and type 2 diabetes. It remains to be determined how and why the transition from 'physiological' to 'pathological' UPR takes place. A key component of the UPR is the ER transmembrane protein IRE1α (inositol-requiring enzyme 1α). IRE1α activity is modulated by both intra-ER signals and by the formation of protein complexes at its cytosolic domain. The amplitude and duration of IRE1α signaling is critical for the transition between the adaptive and cell death programs, with particular relevance for the activation of the pro-apoptotic c-Jun N-terminal kinase (JNK) in beta cells. In the present review we discuss the available information on IRE1α-regulating proteins in beta cells and their downstream targets, and the important differences observed between cytokine-induced UPR in human and rodent beta cells.

Cytokines and Pancreatic β-Cell Apoptosis

The discovery 30 years ago that inflammatory cytokines cause a concentration, activity, and time-dependent bimodal response in pancreatic β-cell function and viability has been a game-changer in the fields of research directed at understanding inflammatory regulation of β-cell function and survival and the causes of β-cell failure and destruction in diabetes. Having until then been confined to the use of pathophysiologically irrelevant β-cell toxic chemicals as a model of β-cell death, researchers could now mimic endocrine and paracrine effects of the cytokine response in vitro by titrating concentrations in the low to the high picomolar-femtomolar range and vary exposure time for up to 14-16h to reproduce the acute regulatory effects of systemic inflammation on β-cell secretory responses, with a shift to inhibition at high picomolar concentrations or more than 16h of exposure to illustrate adverse effects of local, chronic islet inflammation. Since then, numerous studies have clarified how these bimodal responses depend on discrete signaling pathways. Most interest has been devoted to the proapoptotic response dependent upon mainly nuclear factor κ B and mitogen-activated protein kinase activation, leading to gene expressional changes, endoplasmic reticulum stress, and triggering of mitochondrial dysfunction. Preclinical studies have shown preventive effects of cytokine antagonism in animal models of diabetes, and clinical trials demonstrating proof of concept are emerging. The full clinical potential of anticytokine therapies has yet to be shown by testing the incremental effects of appropriate dosing, timing, and combinations of treatments. Due to the considerable translational importance of enhancing the precision, specificity, and safety of antiinflammatory treatments of diabetes, we review here the cellular, preclinical, and clinical evidence of which of the death pathways recently proposed in the Nomenclature Committee on Cell Death 2012 Recommendations are activated by inflammatory cytokines in the pancreatic β-cell to guide the identification of antidiabetic targets. Although there are still scarce human data, the cellular and preclinical studies point to the caspase-dependent intrinsic apoptosis pathway as the prime effector of inflammatory β-cell apoptosis.

Interferon-γ-Induced Unfolded Protein Response in Conjunctival Goblet Cells as a Cause of Mucin Deficiency in Sjögren Syndrome

Goblet cells (GCs) are specialized secretory cells that produce mucins and a variety of other proteins. Significant conjunctival GC loss occurs in both experimental dry eye models and patients with keratoconjunctivitis sicca due to the induction of interferon (IFN)-γ. With the use of a primary murine culture model, we found that GCs are highly sensitive to IFN-γ with significantly reduced proliferation and altered structure with low concentrations. GC cultures treated with IFN-γ have increased gene expression of Muc2 and Muc5AC but do not express these mucin glycoproteins. We hypothesized that IFN-γ induces endoplasmic reticulum stress and the unfolded protein response (UPR) in GCs. Cultures treated with IFN-γ increased expression of UPR-associated genes and proteins. Increased GRP78 and sXBP1 expression was found in experimental dry eye and Sjögren syndrome models and was GC specific. Increased GRP78 was also found in the conjunctiva of patients with Sjögren syndrome at the gene and protein levels. Treatment with dexamethasone inhibited expression of UPR-associated genes and increased mucin production. These results indicate that induction of UPR by IFN-γ is an important cause of GC-associated mucin deficiency observed in aqueous-deficient dry eye. Therapies to block the effects of IFN-γ on the metabolically active endoplasmic reticulum in these cells might enhance synthesis and secretion of the protective GC mucins on the ocular surface.

Ubiquitin D Regulates IRE1α/c-Jun N-terminal Kinase (JNK) Protein-dependent Apoptosis in Pancreatic Beta Cells

Pro-inflammatory cytokines contribute to pancreatic beta cell apoptosis in type 1 diabetes at least in part by inducing endoplasmic reticulum (ER) stress and the consequent unfolded protein response (UPR). It remains to be determined what causes the transition from "physiological" to "apoptotic" UPR, but accumulating evidence indicates that signaling by the ER transmembrane protein IRE1α is critical for this transition. IRE1α activation is regulated by both intra-ER and cytosolic cues. We evaluated the role for the presently discovered cytokine-induced and IRE1α-interacting protein ubiquitin D (UBD) on the regulation of IRE1α and its downstream targets. UBD was identified by use of a MAPPIT (mammalian protein-protein interaction trap)-based IRE1α interactome screen followed by comparison against functional genomic analysis of human and rodent beta cells exposed to pro-inflammatory cytokines. Knockdown of UBD in human and rodent beta cells and detailed signal transduction studies indicated that UBD modulates cytokine-induced UPR/IRE1α activation and apoptosis. UBD expression is induced by the pro-inflammatory cytokines interleukin (IL)-1β and interferon (IFN)-γ in rat and human pancreatic beta cells, and it is also up-regulated in beta cells of inflamed islets from non-obese diabetic mice. UBD interacts with IRE1α in human and rodent beta cells, modulating IRE1α-dependent activation of JNK and cytokine-induced apoptosis. Our data suggest that UBD provides a negative feedback on cytokine-induced activation of the IRE1α/JNK pro-apoptotic pathway in cytokine-exposed beta cells.

The non-canonical NF-κB pathway is induced by cytokines in pancreatic beta cells and contributes to cell death and proinflammatory responses in vitro

AIMS/HYPOTHESIS

Activation of the transcription factor nuclear factor (NF)-κB by proinflammatory cytokines plays an important role in beta cell demise in type 1 diabetes. Two main signalling pathways are known to activate NF-κB, namely the canonical and the non-canonical pathways. Up to now, studies on the role of NF-κB activation in beta cells have focused on the canonical pathway. The aim of this study was to investigate whether cytokines activate the non-canonical pathway in beta cells, how this pathway is regulated and the consequences of its activation on beta cell fate.

METHODS

NF-κB signalling was analysed by immunoblotting, promoter reporter assays and real-time RT-PCR, after knockdown or overexpression of key genes/proteins. INS-1E cells, FACS-purified rat beta cells and the human beta cell line EndoC-βH1 exposed to cytokines were used as models.

RESULTS

IL-1β plus IFN-γ induced stabilisation of NF-κB-inducing kinase and increased the expression and cleavage of p100 protein, culminating in the nuclear translocation of p52, the hallmark of the non-canonical signalling. This activation relied on different crosstalks between the canonical and non-canonical pathways, some of which were beta cell specific. Importantly, cytokine-mediated activation of the non-canonical pathway controlled the expression of 'late' NF-κB-dependent genes, regulating both pro-apoptotic and inflammatory responses, which are implicated in beta cell loss in early type 1 diabetes.

CONCLUSIONS/INTERPRETATION

The atypical activation of the non-canonical NF-κB pathway by proinflammatory cytokines constitutes a novel 'feed-forward' mechanism that contributes to the particularly pro-apoptotic effect of NF-κB in beta cells.

Molybdenum induces pancreatic β-cell dysfunction and apoptosis via interdependent of JNK and AMPK activation-regulated mitochondria-dependent and ER stress-triggered pathways

Molybdenum (Mo), a well-known toxic environmental and industrial pollutant, causes adverse health effects and diseases in humans and has received attention as a potential risk factor for DM. However, the roles of Mo in the mechanisms of the toxicological effects in pancreatic β-cells are mostly unclear. In this study, the results revealed dysfunction of insulin secretion and apoptosis in the pancreatic β-cell-derived RIN-m5F cells and the isolated mouse islets in response to Mo. These effects were accompanied by a mitochondria-dependent apoptotic signals including a decreased in the MMP, an increase in cytochrome c release, and the activation of caspase cascades and PARP. In addition, ER stress was triggered as indicated by several key molecules of the UPR. Furthermore, exposure to Mo induced the activation of ERK1/2, JNK, AMPKα, and GSK3-α/β. Pretreatment with specific pharmacological inhibitors (in RIN-m5F cells and isolated mouse islets) of JNK (SP600125) and AMPK (Compound C) or transfection with si-RNAs (in RIN-m5F cells) specific to JNK and AMPKα effectively prevented the Mo-induced apoptosis and related signals, but inhibitors of ERK1/2 and GSK3-α/β (PD98059 and LiCl, respectively) did not reverse the Mo-induced effects. Additionally, both the inhibitors and specific si-RNAs could suppress the Mo-induced phosphorylation of JNK and AMPKα each other. Taken together, these results suggest that Mo exerts its cytotoxicity on pancreatic β-cells by inducing dysfunction and apoptosis via interdependent JNK and AMPK activation downstream-regulated mitochondrial-dependent and ER stress-triggered apoptosis pathways.