Endoplasmic reticulum stress, obesity and diabetes

The endoplasmic reticulum (ER) stress response, also commonly known as the unfolded protein response (UPR), is an adaptive response used to align ER functional capacity with demand. It is activated in various tissues under conditions related to obesity and type 2 diabetes. Hypothalamic ER stress contributes to inflammation and leptin/insulin resistance. Hepatic ER stress contributes to the development of steatosis and insulin resistance, and components of the UPR regulate liver lipid metabolism. ER stress in enlarged fat tissues induces inflammation and modifies adipokine secretion, and saturated fats cause ER stress in muscle. Finally, prolonged ER stress impairs insulin synthesis and causes pancreatic β cell apoptosis. In this review, we discuss ways in which ER stress operates as a common molecular pathway in the pathogenesis of obesity and diabetes.

Oleate-induced beta cell dysfunction and apoptosis: a proteomic approach to glucolipotoxicity by an unsaturated fatty acid

High levels of fatty acids contribute to loss of functional beta cell mass in type 2 diabetes, in particular in combination with high glucose levels. The aim of this study was to elucidate the role of the unsaturated free fatty acid oleate in glucolipotoxicity and to unravel the molecular pathways involved. INS-1E cells were exposed to 0.5 mM oleate, combined or not with 25 mM glucose, for 24 h. Protein profiling of INS-1E cells was done by 2D-DIGE, covering pH ranges 4-7 and 6-9 (n = 4). Identification of differentially expressed proteins (P < 0.05) was based on MALDI-TOF analysis using Peptide Mass Fingerprint (PMF) and fragmentation (MS/MS) of the most intense peaks of PMF and proteomic results were confirmed by functional assays. Oleate impaired glucose-stimulated insulin secretion and decreased insulin content. 2D-DIGE analysis revealed 53 and 54 differentially expressed proteins for oleate and the combination of oleate and high glucose, respectively. Exposure to oleate down-regulated chaperones, hampered insulin processing and ubiquitin-related proteasomal degradation, and induced perturbations in vesicle transport and budding. In combination with high glucose, shunting of excess amounts of glucose toward reactive oxygen species production worsened beta cell death. The present findings provide new insights in oleate-induced beta cell dysfunction and identify target proteins for preservation of functional beta cell mass in type 2 diabetes.

Ubiquitin fold modifier 1 (UFM1) and its target UFBP1 protect pancreatic beta cells from ER stress-induced apoptosis

UFM1 is a member of the ubiquitin like protein family. While the enzymatic cascade of UFM1 conjugation has been elucidated in recent years, the biological function remains largely unknown. In this report we demonstrate that the recently identified C20orf116, which we name UFM1-binding protein 1 containing a PCI domain (UFBP1), and CDK5RAP3 interact with UFM1. Components of the UFM1 conjugation pathway (UFM1, UFBP1, UFL1 and CDK5RAP3) are highly expressed in pancreatic islets of Langerhans and some other secretory tissues. Co-localization of UFM1 with UFBP1 in the endoplasmic reticulum (ER) depends on UFBP1. We demonstrate that ER stress, which is common in secretory cells, induces expression of Ufm1, Ufbp1 and Ufl1 in the beta-cell line INS-1E. siRNA-mediated Ufm1 or Ufbp1 knockdown enhances apoptosis upon ER stress. Silencing the E3 enzyme UFL1, results in similar outcomes, suggesting that UFM1-UFBP1 conjugation is required to prevent ER stress-induced apoptosis. Together, our data suggest that UFM1-UFBP1 participate in preventing ER stress-induced apoptosis in protein secretory cells.

The transcription factor B-cell lymphoma (BCL)-6 modulates pancreatic {beta}-cell inflammatory responses

Type 1 diabetes is a chronic autoimmune disease with a strong inflammatory component. We have previously shown that expression of the transcriptional repressor B-cell lymphoma (BCL)-6 is very low in pancreatic β-cells, which may favor prolonged proinflammatory responses after exposure to the cytokines IL-1β and interferon γ. Here we investigated whether cytokine-induced inflammation and apoptosis can be prevented in β-cells by BCL-6 expression using plasmid, prolactin, and adenoviral approaches. The induction of mild or abundant BCL-6 expression in β-cells by prolactin or an adenoviral BCL-6 expression construct, respectively, reduced cytokine-induced inflammatory responses in a dose-dependent manner through inhibition of nuclear factor-κB activation. BCL-6 decreased Fas and inducible nitric oxide synthase expression and nitric oxide production, but it inhibited the expression of the antiapoptotic proteins Bcl-2 and JunB while increasing the expression of the proapoptotic death protein 5. The net result of these opposite effects was an augmentation of β-cell apoptosis. In conclusion, BCL-6 expression tones down the unrestrained cytokine-induced proinflammatory response of β-cells but it also favors gene networks leading to apoptosis. This suggests that cytokine-induced proinflammatory and proapoptotic signals can be dissociated in β-cells. Further understanding of these pathways may open new possibilities to improve β-cell survival in early type 1 diabetes or after transplantation.

Causes and cures for endoplasmic reticulum stress in lipotoxic β-cell dysfunction

Pancreatic β-cell dysfunction is central to the pathogenesis of type 2 diabetes, and the loss of functional β-cell mass in type 2 diabetes is at least in part secondary to increased β-cell apoptosis. Accumulating evidence suggests that endoplasmic reticulum (ER) stress is present in β-cells in type 2 diabetes. Free fatty acids (FFAs) cause ER stress and are putative mediators of β-cell dysfunction and death. In this review, we discuss the molecular mechanisms underlying ER stress induced by saturated and unsaturated FFAs. Oleate and palmitate trigger ER stress through ER Ca(2+) depletion and build-up of unfolded proteins in the secretory pathway. Saturated and unsaturated FFAs elicit a differential signal transduction in the three branches of the ER stress response, resulting in different survival/apoptosis outcomes. The protection of β-cells against FFAs through the interference with ER stress signalling has opened novel therapeutic perspectives for type 2 diabetes. Chemical chaperones, salubrinal and glucagon-like peptide-1 (GLP-1) analogues have been used to protect β-cells from lipotoxic ER stress. Importantly, the pro- and antiapoptotic effects of these compounds are cell and context dependent.

Palmitate induces a pro-inflammatory response in human pancreatic islets that mimics CCL2 expression by beta cells in type 2 diabetes

AIMS/HYPOTHESIS

Beta cell failure is a crucial component in the pathogenesis of type 2 diabetes. One of the proposed mechanisms of beta cell failure is local inflammation, but the presence of pancreatic islet inflammation in type 2 diabetes and the mechanisms involved remain under debate.

METHODS

Chemokine and cytokine expression was studied by microarray analysis of laser-capture microdissected islets from pancreases obtained from ten non-diabetic and ten type 2 diabetic donors, and by real-time PCR of human islets exposed to oleate or palmitate at 6 or 28 mmol/l glucose. The cellular source of the chemokines was analysed by immunofluorescence of pancreatic sections from individuals without diabetes and with type 2 diabetes.

RESULTS

Microarray analysis of laser-capture microdissected beta cells showed increased chemokine and cytokine expression in type 2 diabetes compared with non-diabetic controls. The inflammatory response in type 2 diabetes was mimicked by exposure of non-diabetic human islets to palmitate, but not to oleate or high glucose, leading to the induction of IL-1beta, TNF-alpha, IL-6, IL-8, chemokine (C-X-C motif) ligand 1 (CXCL1) and chemokine (C-C motif) ligand 2 (CCL2). Interference with IL-1beta signalling abolished palmitate-induced cytokine and chemokine expression but failed to prevent lipotoxic human islet cell death. Palmitate activated nuclear factor kappaB (NF-kappaB) in human pancreatic beta and non-beta cells, and chemically induced endoplasmic reticulum stress caused cytokine expression and NF-kappaB activation similar to that occurring with palmitate.

CONCLUSIONS/INTERPRETATION

Saturated-fatty-acid-induced NF-kappaB activation and endoplasmic reticulum stress may contribute to IL-1beta production and mild islet inflammation in type 2 diabetes. This inflammatory process does not contribute to lipotoxicity ex vivo, but may lead to local chemokine release.

Enhanced signaling downstream of ribonucleic Acid-activated protein kinase-like endoplasmic reticulum kinase potentiates lipotoxic endoplasmic reticulum stress in human islets

BACKGROUND

Free fatty acids cause pancreatic beta-cell apoptosis and may contribute to beta-cell loss in type 2 diabetes via the induction of endoplasmic reticulum (ER) stress. Eukaryotic translation initiation factor 2alpha (eIF2alpha) phosphorylation is an adaptive response to ER stress, and reductions in eIF2alpha phosphorylation trigger beta-cell failure. Salubrinal inhibits eIF2alpha dephosphorylation and has been proposed as a novel therapy for diabetes.

OBJECTIVE

The objective of the study was to examine whether salubrinal modulates human islet susceptibility to lipotoxicity.

STUDY DESIGN

Human islets were treated with oleate or palmitate, alone or in combination with salubrinal, and examined for apoptosis, ultrastructure, and gene expression.

RESULTS

Salubrinal enhanced signaling downstream of eIF2alpha and markedly induced the proapoptotic transcription factor CCAAT/enhancer-binding protein homologous protein, but it did not induce the inositol requiring-1alpha or activating transcription factor 6 ER stress pathways. Salubrinal potentiated the deleterious effects of oleate and palmitate in human islets. This proapoptotic effect involved ER dilation and mitochondrial rounding and fragmentation.

CONCLUSIONS

Excessive eIF2alpha phosphorylation is poorly tolerated by human islets and exacerbates fatty acid-induced apoptosis through ER and mitochondrial mechanisms. This should be taken into consideration when designing approaches to pharmacologically modulate the beta-cell ER stress response in type 2 diabetes.

ER stress in pancreatic beta cells: the thin red line between adaptation and failure

Secretory cells, such as pancreatic beta cells, face the challenge of increasing protein synthesis severalfold during acute or chronic stimulation. This poses a burden on the endoplasmic reticulum (ER), the organelle where proinsulin synthesis and folding takes place. Thus, beta cells use various adaptive mechanisms to adjust the functional capacity of the ER to the prevailing demand. These check-and-balance mechanisms are collectively known as the unfolded protein response (UPR). It remains unclear how UPR signaling is ultimately regulated and what delineates the boundaries between a physiological and a pathological response. New discoveries point to the divergent effects of acute and chronic metabolic fluxes and chemical ER stressors on the formation of complexes among UPR transducers, scaffold proteins, and phosphatases. These and other findings provide a first glimpse on how different signals trigger diverging UPR outcomes.

The long lifespan and low turnover of human islet beta cells estimated by mathematical modelling of lipofuscin accumulation

AIMS/HYPOTHESIS

Defects in pancreatic beta cell turnover are implicated in the pathogenesis of type 2 diabetes by genetic markers for diabetes. Decreased beta cell neogenesis could contribute to diabetes. The longevity and turnover of human beta cells is unknown; in rodents <1 year old, a half-life of 30 days is estimated. Intracellular lipofuscin body (LB) accumulation is a hallmark of ageing in neurons. To estimate the lifespan of human beta cells, we measured beta cell LB accumulation in individuals aged 1-81 years.

METHODS

LB content was determined by electron microscopical morphometry in sections of beta cells from human (non-diabetic, n = 45; type 2 diabetic, n = 10) and non-human primates (n = 10; 5-30 years) and from 15 mice aged 10-99 weeks. Total cellular LB content was estimated by three-dimensional (3D) mathematical modelling.

RESULTS

LB area proportion was significantly correlated with age in human and non-human primates. The proportion of human LB-positive beta cells was significantly related to age, with no apparent differences in type 2 diabetes or obesity. LB content was low in human insulinomas (n = 5) and alpha cells and in mouse beta cells (LB content in mouse <10% human). Using 3D electron microscopy and 3D mathematical modelling, the LB-positive human beta cells (representing aged cells) increased from >or=90% (<10 years) to >or=97% (>20 years) and remained constant thereafter.

CONCLUSIONS/INTERPRETATION

Human beta cells, unlike those of young rodents, are long-lived. LB proportions in type 2 diabetes and obesity suggest that little adaptive change occurs in the adult human beta cell population, which is largely established by age 20 years.

Glucagon-like peptide-1 agonists protect pancreatic beta-cells from lipotoxic endoplasmic reticulum stress through upregulation of BiP and JunB

OBJECTIVE

Chronic exposure of pancreatic beta-cells to saturated free fatty acids (FFAs) causes endoplasmic reticulum (ER) stress and apoptosis and may contribute to beta-cell loss in type 2 diabetes. Here, we evaluated the molecular mechanisms involved in the protection of beta-cells from lipotoxic ER stress by glucagon-like peptide (GLP)-1 agonists utilized in the treatment of type 2 diabetes.

RESEARCH DESIGN AND METHODS

INS-1E or fluorescence-activated cell sorter-purified primary rat beta-cells were exposed to oleate or palmitate with or without the GLP-1 agonist exendin-4 or forskolin. Cyclopiazonic acid was used as a synthetic ER stressor, while the activating transcription factor 4-C/EBP homologous protein branch was selectively activated with salubrinal. The ER stress signaling pathways modulated by GLP-1 agonists were studied by real-time PCR and Western blot. Knockdown by RNA interference was used to identify mediators of the antiapoptotic GLP-1 effects in the ER stress response and downstream mitochondrial cell death mechanisms.

RESULTS

Exendin-4 and forskolin protected beta-cells against FFAs via the induction of the ER chaperone BiP and the antiapoptotic protein JunB that mediate beta-cell survival under lipotoxic conditions. On the other hand, exendin-4 and forskolin protected against synthetic ER stressors by inactivating caspase 12 and upregulating Bcl-2 and X-chromosome-linked inhibitor of apoptosis protein that inhibit mitochondrial apoptosis.

CONCLUSIONS

These observations suggest that GLP-1 agonists increase in a context-dependent way the beta-cell defense mechanisms against different pathways involved in ER stress-induced apoptosis. The identification of the pathways modulated by GLP-1 agonists allows for targeted approaches to alleviate beta-cell ER stress in diabetes.

Neonatal hemochromatosis and Martinez-Frias syndrome of intestinal atresia and diabetes mellitus in a consanguineous newborn

Neonatal hemochromatosis is a heterogeneous disorder of iron metabolism characterized by hepatic failure and marked iron accumulation in liver and extrahepatic tissues. Autosomal recessive transmission is found in most cases. Neonatal hemochromatosis shares cellular features with the adult disease but is clinically and genetically distinct, the causal gene(s) being presently unknown. We report on a newborn from consanguineous parents who presented with multiple congenital anomalies and neonatal hemochromatosis. The syndrome consisted of intra-uterine growth retardation, intestinal atresia, gallbladder aplasia and diabetes mellitus, and fitted with the diagnosis of Martinez-Frias syndrome, a very rare autosomal recessive phenotype, the gene of which remains to be identified. We suggest that neonatal hemochromatosis may be part of the Martinez-Frias syndrome. Molecular analyses in this and other reported patients with the Martinez-Frias syndrome should shed light on gut development and iron metabolism.

Functional genomic analysis of frataxin deficiency reveals tissue-specific alterations and identifies the PPARgamma pathway as a therapeutic target in Friedreich's ataxia

Friedreich's ataxia (FRDA), the most common inherited ataxia, is characterized by focal neurodegeneration, diabetes mellitus and life-threatening cardiomyopathy. Frataxin, which is significantly reduced in patients with this recessive disorder, is a mitochondrial iron-binding protein, but how its deficiency leads to neurodegeneration and metabolic derangements is not known. We performed microarray analysis of heart and skeletal muscle in a mouse model of frataxin deficiency, and found molecular evidence of increased lipogenesis in skeletal muscle, and alteration of fiber-type composition in heart, consistent with insulin resistance and cardiomyopathy, respectively. Since the peroxisome proliferator-activated receptor gamma (PPARgamma) pathway is known to regulate both processes, we hypothesized that dysregulation of this pathway could play a key role in frataxin deficiency. We confirmed this by showing a coordinate dysregulation of the PPARgamma coactivator Pgc1a and transcription factor Srebp1 in cellular and animal models of frataxin deficiency, and in cells from FRDA patients, who have marked insulin resistance. Finally, we show that genetic modulation of the PPARgamma pathway affects frataxin levels in vitro, supporting PPARgamma as a novel therapeutic target in FRDA.

Transient oxidative stress damages mitochondrial machinery inducing persistent beta-cell dysfunction

Transient exposure of beta-cells to oxidative stress interrupts the transduction of signals normally coupling glucose metabolism to insulin secretion. We investigated putative persistence of effects induced by one transient oxidative stress (200 microm H(2)O(2), 10 min) on insulin secreting cells following recovery periods of days and weeks. Three days after oxidative stress INS-1E cells and rat islets exhibited persistent dysfunction. In particular, the secretory response to 15 mm glucose was reduced by 40% in INS-1E cells stressed 3 days before compared with naïve cells. Compared with non-stressed INS-1E cells, we observed reduced oxygen consumption (-43%) and impaired glucose-induced ATP generation (-46%). These parameters correlated with increased mitochondrial reactive oxygen species formation (+60%) accompanied with down-regulation of subunits of the respiratory chain and decreased expression of genes responsible for mitochondrial biogenesis (TFAM, -24%; PGC-1alpha, -67%). Three weeks after single oxidative stress, both mitochondrial respiration and secretory responses were recovered. Moreover, such recovered INS-1E cells exhibited partial resistance to a second transient oxidative stress and up-regulation of UCP2 (+78%) compared with naïve cells. In conclusion, one acute oxidative stress induces beta-cell dysfunction lasting over days, explained by persistent damages in mitochondrial components.

PTPN2, a candidate gene for type 1 diabetes, modulates interferon-gamma-induced pancreatic beta-cell apoptosis

OBJECTIVE

The pathogenesis of type 1 diabetes has a strong genetic component. Genome-wide association scans recently identified novel susceptibility genes including the phosphatases PTPN22 and PTPN2. We hypothesized that PTPN2 plays a direct role in beta-cell demise and assessed PTPN2 expression in human islets and rat primary and clonal beta-cells, besides evaluating its role in cytokine-induced signaling and beta-cell apoptosis.

RESEARCH DESIGN AND METHODS

PTPN2 mRNA and protein expression was evaluated by real-time PCR and Western blot. Small interfering (si)RNAs were used to inhibit the expression of PTPN2 and downstream STAT1 in beta-cells, allowing the assessment of cell death after cytokine treatment.

RESULTS

PTPN2 mRNA and protein are expressed in human islets and rat beta-cells and upregulated by cytokines. Transfection with PTPN2 siRNAs inhibited basal- and cytokine-induced PTPN2 expression in rat beta-cells and dispersed human islets cells. Decreased PTPN2 expression exacerbated interleukin (IL)-1beta + interferon (IFN)-gamma-induced beta-cell apoptosis and turned IFN-gamma alone into a proapoptotic signal. Inhibition of PTPN2 amplified IFN-gamma-induced STAT1 phosphorylation, whereas double knockdown of both PTPN2 and STAT1 protected beta-cells against cytokine-induced apoptosis, suggesting that STAT1 hyperactivation is responsible for the aggravation of cytokine-induced beta-cell death in PTPN2-deficient cells.

CONCLUSIONS

We identified a functional role for the type 1 diabetes candidate gene PTPN2 in modulating IFN-gamma signal transduction at the beta-cell level. PTPN2 regulates cytokine-induced apoptosis and may thereby contribute to the pathogenesis of type 1 diabetes.

Initiation and execution of lipotoxic ER stress in pancreatic beta-cells

Free fatty acids (FFA) cause apoptosis of pancreatic beta-cells and might contribute to beta-cell loss in type 2 diabetes via the induction of endoplasmic reticulum (ER) stress. We studied here the molecular mechanisms implicated in FFA-induced ER stress initiation and apoptosis in INS-1E cells, FACS-purified primary beta-cells and human islets exposed to oleate and/or palmitate. Treatment with saturated and/or unsaturated FFA led to differential ER stress signaling. Palmitate induced more apoptosis and markedly activated the IRE1, PERK and ATF6 pathways, owing to a sustained depletion of ER Ca(2+) stores, whereas the unsaturated FFA oleate led to milder PERK and IRE1 activation and comparable ATF6 signaling. Non-metabolizable methyl-FFA analogs induced neither ER stress nor beta-cell apoptosis. The FFA-induced ER stress response was not modified by high glucose concentrations, suggesting that ER stress in primary beta-cells is primarily lipotoxic, and not glucolipotoxic. Palmitate, but not oleate, activated JNK. JNK inhibitors reduced palmitate-mediated AP-1 activation and apoptosis. Blocking the transcription factor CHOP delayed palmitate-induced beta-cell apoptosis. In conclusion, saturated FFA induce ER stress via ER Ca(2+) depletion. The IRE1 and resulting JNK activation contribute to beta-cell apoptosis. PERK activation by palmitate also contributes to beta-cell apoptosis via CHOP.

The role for endoplasmic reticulum stress in diabetes mellitus

Accumulating evidence suggests that endoplasmic reticulum (ER) stress plays a role in the pathogenesis of diabetes, contributing to pancreatic beta-cell loss and insulin resistance. Components of the unfolded protein response (UPR) play a dual role in beta-cells, acting as beneficial regulators under physiological conditions or as triggers of beta-cell dysfunction and apoptosis under situations of chronic stress. Novel findings suggest that "what makes a beta-cell a beta-cell", i.e., its enormous capacity to synthesize and secrete insulin, is also its Achilles heel, rendering it vulnerable to chronic high glucose and fatty acid exposure, agents that contribute to beta-cell failure in type 2 diabetes. In this review, we address the transition from physiology to pathology, namely how and why the physiological UPR evolves to a proapoptotic ER stress response and which defenses are triggered by beta-cells against these challenges. ER stress may also link obesity and insulin resistance in type 2 diabetes. High fat feeding and obesity induce ER stress in liver, which suppresses insulin signaling via c-Jun N-terminal kinase activation. In vitro data suggest that ER stress may also contribute to cytokine-induced beta-cell death. Thus, the cytokines IL-1beta and interferon-gamma, putative mediators of beta-cell loss in type 1 diabetes, induce severe ER stress through, respectively, NO-mediated depletion of ER calcium and inhibition of ER chaperones, thus hampering beta-cell defenses and amplifying the proapoptotic pathways. A better understanding of the pathways regulating ER stress in beta-cells may be instrumental for the design of novel therapies to prevent beta-cell loss in diabetes.

The endoplasmic reticulum in pancreatic beta cells of type 2 diabetes patients

AIMS/HYPOTHESIS

Pancreatic beta cells have highly developed endoplasmic reticulum (ER) due to their role in insulin secretion. Since ER stress has been associated with beta cell dysfunction, we studied several features of beta cell ER in human type 2 diabetes.

METHODS

Pancreatic samples and/or isolated islets from non-diabetic controls (ND) and type 2 diabetes patients were evaluated for insulin secretion, apoptosis (electron microscopy and ELISA), morphometric ER assessment (electron microscopy), and expression of ER stress markers in beta cell prepared by laser capture microdissection and in isolated islets.

RESULTS

Insulin release was lower and beta cell apoptosis higher in type 2 diabetes than ND islets. ER density volume was significantly increased in type 2 diabetes beta cells. Expression of alpha-mannosidase (also known as mannosidase, alpha, class 1A, member 1) and UDP-glucose glycoprotein glucosyl transferase like 2 (UGCGL2), assessed by microarray and/or real-time reverse transcriptase polymerase chain reaction (RT-PCR), differed between ND and type 2 diabetes beta cells. Expression of immunoglobulin heavy chain binding protein (BiP, also known as heat shock 70 kDa protein 5 [glucose-regulated protein, 78 kDa] [HSPA5]), X-box binding protein 1 (XBP-1, also known as XBP1) and C/EBP homologous protein (CHOP, also known as damage-inducible transcript 3 [DDIT3]) was not higher in type 2 diabetes beta cell or isolated islets cultured at 5.5 mmol/l glucose (microarray and real-time RT-PCR) than in ND samples. When islets were cultured for 24 h at 11.1 mmol/l glucose, there was induction of BiP and XBP-1 in type 2 diabetes islets but not in ND islets.

CONCLUSIONS/INTERPRETATION

Beta cell in type 2 diabetes showed modest signs of ER stress when studied in pancreatic samples or isolated islets maintained at physiological glucose concentration. However, exposure to increased glucose levels induced ER stress markers in type 2 diabetes islet cells, which therefore may be more susceptible to ER stress induced by metabolic perturbations.

Loss of PPAR gamma in immune cells impairs the ability of abscisic acid to improve insulin sensitivity by suppressing monocyte chemoattractant protein-1 expression and macrophage infiltration into white adipose tissue

Abscisic acid (ABA) is a natural phytohormone and peroxisome proliferator-activated receptor gamma (PPARgamma) agonist that significantly improves insulin sensitivity in db/db mice. Although it has become clear that obesity is associated with macrophage infiltration into white adipose tissue (WAT), the phenotype of adipose tissue macrophages (ATMs) and the mechanisms by which insulin-sensitizing compounds modulate their infiltration remain unknown. We used a loss-of-function approach to investigate whether ABA ameliorates insulin resistance through a mechanism dependent on immune cell PPARgamma. We characterized two phenotypically distinct ATM subsets in db/db mice based on their surface expression of F4/80. F4/80(hi) ATMs were more abundant and expressed greater concentrations of chemokine receptor (CCR) 2 and CCR5 when compared to F4/80(lo) ATMs. ABA significantly decreased CCR2(+) F4/80(hi) infiltration into WAT and suppressed monocyte chemoattractant protein-1 (MCP-1) expression in WAT and plasma. Furthermore, the deficiency of PPARgamma in immune cells, including macrophages, impaired the ability of ABA to suppress the infiltration of F4/80(hi) ATMs into WAT, to repress WAT MCP-1 expression and to improve glucose tolerance. We provide molecular evidence in vivo demonstrating that ABA improves insulin sensitivity and obesity-related inflammation by inhibiting MCP-1 expression and F4/80(hi) ATM infiltration through a PPARgamma-dependent mechanism.

Transcriptional regulation of the endoplasmic reticulum stress gene chop in pancreatic insulin-producing cells

Endoplasmic reticulum stress-mediated apoptosis may play an important role in the destruction of pancreatic beta-cells, thus contributing to the development of type 1 and type 2 diabetes. One of the regulators of endoplasmic reticulum stress-mediated cell death is the CCAAT/enhancer binding protein (C/EBP) homologous protein (Chop). We presently studied the molecular regulation of Chop expression in insulin-producing cells (INS-1E) in response to three pro-apoptotic and endoplasmic reticulum stress-inducing agents, namely the cytokines interleukin-1beta + interferon-gamma, the free fatty acid palmitate, and the sarcoendoplasmic reticulum pump Ca(2+) ATPase blocker cyclopiazonic acid (CPA). Detailed mutagenesis studies of the Chop promoter showed differential regulation of Chop transcription by CPA, cytokines, and palmitate. Whereas palmitate- and cytokine-induced Chop expression was mediated via a C/EBP-activating transcription factor (ATF) composite and AP-1 binding sites, CPA induction required the C/EBP-ATF site and the endoplasmic reticulum stress response element. Cytokines, palmitate, and CPA induced eIF2alpha phosphorylation in INS-1E cells leading to activation of the transcription factor ATF4. Chop transcription in response to cytokines and palmitate depends on the binding of ATF4 and AP-1 to the Chop promoter, but distinct AP-1 dimers were formed by cytokines and palmitate. These results suggest a differential response of beta-cells to diverse endoplasmic reticulum stress inducers, leading to a differential regulation of Chop transcription.

Progressive loss of beta-cell function leads to worsening glucose tolerance in first-degree relatives of subjects with type 2 diabetes

OBJECTIVE

The relative roles of insulin resistance and beta-cell dysfunction in the pathogenesis of impaired glucose tolerance (IGT) and type 2 diabetes are debated. First-degree relatives of individuals with type 2 diabetes are at increased risk of developing hyperglycemia.

RESEARCH DESIGN AND METHODS

We evaluated the evolution of insulin sensitivity, beta-cell function, glucose effectiveness, and glucose tolerance over 7 years in 33 nondiabetic, first-degree relatives of type 2 diabetic individuals using frequently sampled tolbutamide-modified intravenous and oral glucose tolerance tests.

RESULTS

Subjects gained weight, and their waist circumference increased (P < 0.05). Insulin sensitivity, the acute insulin response to glucose, and glucose effectiveness did not change significantly. However, when we accounted for the modulating effect of insulin sensitivity on insulin release, beta-cell function determined as the disposition index decreased by 22% (P < 0.05). This decrease was associated with declines in intravenous and oral glucose tolerance (P < 0.05 and P < 0.001, respectively). Of the subjects with normal glucose tolerance at the first assessment, we compared those who progressed to IGT with those who did not. The disposition index was 50% lower in the progressors than in the nonprogressors at follow-up (P < 0.05).

CONCLUSIONS

The decline in glucose tolerance over time in first-degree relatives of type 2 diabetic individuals is strongly related to the loss of beta-cell function. Thus, early interventions to slow the decline in beta-cell function should be considered in high-risk individuals.

Selective inhibition of eukaryotic translation initiation factor 2 alpha dephosphorylation potentiates fatty acid-induced endoplasmic reticulum stress and causes pancreatic beta-cell dysfunction and apoptosis

Free fatty acids cause pancreatic beta-cell apoptosis and may contribute to beta-cell loss in type 2 diabetes via the induction of endoplasmic reticulum stress. Reductions in eukaryotic translation initiation factor (eIF) 2alpha phosphorylation trigger beta-cell failure and diabetes. Salubrinal selectively inhibits eIF2alpha dephosphorylation, protects other cells against endoplasmic reticulum stress-mediated apoptosis, and has been proposed as a beta-cell protector. Unexpectedly, salubrinal induced apoptosis in primary beta-cells, and it potentiated the deleterious effects of oleate and palmitate. Salubrinal induced a marked eIF2alpha phosphorylation and potentiated the inhibitory effects of free fatty acids on protein synthesis and insulin release. The synergistic activation of the PERK-eIF2alpha branch of the endoplasmic reticulum stress response, but not of the IRE1 and activating transcription factor-6 pathways, led to a marked induction of activating transcription factor-4 and the pro-apoptotic transcription factor CHOP. Our findings demonstrate that excessive eIF2alpha phosphorylation is poorly tolerated by beta-cells and exacerbates free fatty acid-induced apoptosis. This modifies the present paradigm regarding the beneficial role of eIF2alpha phosphorylation in beta-cells and must be taken into consideration when designing therapies to protect beta-cells in type 2 diabetes.

BCL-6: a possible missing link for anti-inflammatory PPAR-delta signalling in pancreatic beta cells

AIMS/HYPOTHESIS

Inflammatory mediators contribute to pancreatic beta cell death in type 1 diabetes. Beta cells respond to cytokine exposure by activating gene networks that alter cellular metabolism, induce chemokine release (thereby increasing insulitis), and cause apoptosis. We have previously shown by microarray analysis that exposure of INS-1E cells to IL-1beta + IFN-gamma induces the transcription factor peroxisome proliferator-activated receptor (Ppar)-delta and several of its target genes. PPAR-delta controls cellular lipid metabolism and is a major regulator of inflammatory responses. We therefore examined the role of PPAR-delta in cytokine-treated beta cells.

MATERIALS AND METHODS

Primary beta cells that had been purified by fluorescence-activated cell sorting and INS-1E cells were cultured in the presence of the cytokines TNF-alpha, IL-1beta, or IL-1beta + IFN-gamma, or the synthetic PPAR-delta agonist GW501516. Gene expression was analysed by real-time PCR. PPAR-delta, monocyte chemoattractant protein (MCP-1, now known as CCL2) promoter and NF-kappaB activity were determined by luciferase reporter assays.

RESULTS

Exposure of primary beta cells or INS-1E cells to cytokines induced Ppar-delta mRNA expression and PPAR-delta-dependent CD36, lipoprotein lipase, acyl CoA synthetase and adipophilin mRNAs. Cytokines and the PPAR-delta agonist GW501516 also activated a PPAR-delta response element reporter in beta cells. Unlike immune cells, neither INS-1E nor beta cells expressed the transcriptional repressor B-cell lymphoma-6 (BCL-6). As a consequence, PPAR-delta activation by GW501516 did not decrease cytokine-induced Mcp-1 promoter activation or mRNA expression, as reported for macrophages. Transient transfection with a BCL-6 expression vector markedly reduced Mcp-1 promoter and NF-kappaB activities in beta cells.

CONCLUSIONS/INTERPRETATION

Cytokines activate the PPAR-delta gene network in beta cells. This network does not, however, regulate the pro-inflammatory response to cytokines because beta cells lack constitutive BCL-6 expression. This may render beta cells particularly susceptible to propagating inflammation in type 1 diabetes.

Mechanisms of pancreatic beta-cell death in type 1 and type 2 diabetes: many differences, few similarities

Type 1 and type 2 diabetes are characterized by progressive beta-cell failure. Apoptosis is probably the main form of beta-cell death in both forms of the disease. It has been suggested that the mechanisms leading to nutrient- and cytokine-induced beta-cell death in type 2 and type 1 diabetes, respectively, share the activation of a final common pathway involving interleukin (IL)-1beta, nuclear factor (NF)-kappaB, and Fas. We review herein the similarities and differences between the mechanisms of beta-cell death in type 1 and type 2 diabetes. In the insulitis lesion in type 1 diabetes, invading immune cells produce cytokines, such as IL-1beta, tumor necrosis factor (TNF)-alpha, and interferon (IFN)-gamma. IL-1beta and/or TNF-alpha plus IFN-gamma induce beta-cell apoptosis via the activation of beta-cell gene networks under the control of the transcription factors NF-kappaB and STAT-1. NF-kappaB activation leads to production of nitric oxide (NO) and chemokines and depletion of endoplasmic reticulum (ER) calcium. The execution of beta-cell death occurs through activation of mitogen-activated protein kinases, via triggering of ER stress and by the release of mitochondrial death signals. Chronic exposure to elevated levels of glucose and free fatty acids (FFAs) causes beta-cell dysfunction and may induce beta-cell apoptosis in type 2 diabetes. Exposure to high glucose has dual effects, triggering initially "glucose hypersensitization" and later apoptosis, via different mechanisms. High glucose, however, does not induce or activate IL-1beta, NF-kappaB, or inducible nitric oxide synthase in rat or human beta-cells in vitro or in vivo in Psammomys obesus. FFAs may cause beta-cell apoptosis via ER stress, which is NF-kappaB and NO independent. Thus, cytokines and nutrients trigger beta-cell death by fundamentally different mechanisms, namely an NF-kappaB-dependent mechanism that culminates in caspase-3 activation for cytokines and an NF-kappaB-independent mechanism for nutrients. This argues against a unifying hypothesis for the mechanisms of beta-cell death in type 1 and type 2 diabetes and suggests that different approaches will be required to prevent beta-cell death in type 1 and type 2 diabetes.