Complementary yet divergent effects of exercise and an exercise mimetic on microbiome in high-fat diet-induced obesity

Exercise is beneficial for obesity, partially through increased mitochondrial activity and raised nicotinamide adenine dinucleotide (NAD), a coenzyme critical for mitochondrial function and metabolism. Recent work has shown that increasing the availability of NAD through pharmacological means improves metabolic health in rodent models of diet-induced obesity and that the effect of these supplements when administered orally may be modulated by the gut microbiome. The gut microbiome is altered by both diet and exercise and is thought to contribute to some aspects of high-fat diet-induced metabolic dysfunction. We examined the independent and combined effects of treadmill exercise and nicotinamide mononucleotide (NMN) supplementation on the gut microbiome of female C57Bl6/J mice chronically fed a high-fat diet. We showed that 8 wk of treadmill exercise, oral-administered NMN, or combined therapy exert unique effects on gut microbiome composition without changing bacterial species richness. Exercise and NMN exerted additive effects on microbiota composition, and NMN partially or fully restored predicted microbial functions, specifically carbohydrate and lipid metabolism, to control levels. Further research is warranted to better understand the mechanisms underpinning the interactions between exercise and oral NAD+ precursor supplementation on gut microbiome.NEW & NOTEWORTHY Exercise and NAD+ precursor supplementation exerted additive and independent effects on gut microbiota composition and inferred function in female mice with diet-induced obesity. Notably, combining exercise and oral nicotinamide mononucleotide supplementation restored inferred microbial functions to control levels, indicating that this combination may improve high-fat diet-induced alterations to microbial metabolism.

Tau suppresses microtubule-regulated pancreatic insulin secretion

Tau protein is implicated in the pathogenesis of Alzheimer's disease (AD) and other tauopathies, but its physiological function is in debate. Mostly explored in the brain, tau is also expressed in the pancreas. We further explored the mechanism of tau's involvement in the regulation of glucose-stimulated insulin secretion (GSIS) in islet β-cells, and established a potential relationship between type 2 diabetes mellitus (T2DM) and AD. We demonstrate that pancreatic tau is crucial for insulin secretion regulation and glucose homeostasis. Tau levels were found to be elevated in β-islet cells of patients with T2DM, and loss of tau enhanced insulin secretion in cell lines, drosophila, and mice. Pharmacological or genetic suppression of tau in the db/db diabetic mouse model normalized glucose levels by promoting insulin secretion and was recapitulated by pharmacological inhibition of microtubule assembly. Clinical studies further showed that serum tau protein was positively correlated with blood glucose levels in healthy controls, which was lost in AD. These findings present tau as a common therapeutic target between AD and T2DM.

Concurrent betaine administration enhances exercise-induced improvements to glucose handling in obese mice

BACKGROUND AND AIMS

Betaine supplementation has been shown to enhance hepatic lipid metabolism in obese mice and improve exercise performance in healthy populations. We examined effects of betaine supplementation, alone or in combination with treadmill exercise, on the metabolic consequences of high fat diet (HFD)-induced obesity in mice.

METHODS AND RESULTS

Male C57BL/6 J mice were fed chow or HFD. After 15 weeks, HFD mice were split into: HFD, HFD with betaine (1.5% w/v), HFD with treadmill exercise, and HFD with both betaine and exercise (15 m/min for 45min, 6 days/week; n = 12/group) for 10 weeks. Compared to HFD mice, body weight was significantly reduced in exercise and exercise-betaine mice, but not in mice given betaine alone. Similarly, adiposity was reduced by exercise but not by betaine alone. HFD-induced glucose intolerance was slightly improved by exercise, but not with betaine alone. Significantly greater benefits were observed in exercise-betaine mice, compared to exercise alone, such that GTT-outcomes were similar to controls. This was associated with reduced insulin levels during ipGTT, suggesting enhanced insulin sensitivity. Modest benefits were observed in fatty acid metabolism genes in skeletal muscle, whilst limited effects were observed in the liver. HFD-induced increases in hepatic Mpc1 (mitochondrial pyruvate carrier 1) were normalized by all treatments, suggesting potential links to altered glucose metabolism.

CONCLUSIONS

Our data show that drinking 1.5% betaine was sufficient to augment metabolic benefits of exercise in obese mice. These processes appear to be facilitated by altered glucose metabolism, with limited effects on hepatic lipid metabolism.

XBP1 maintains beta cell identity, represses beta-to-alpha cell transdifferentiation and protects against diabetic beta cell failure during metabolic stress in mice

AIMS/HYPOTHESIS

Pancreatic beta cell dedifferentiation, transdifferentiation into other islet cells and apoptosis have been implicated in beta cell failure in type 2 diabetes, although the mechanisms are poorly defined. The endoplasmic reticulum stress response factor X-box binding protein 1 (XBP1) is a major regulator of the unfolded protein response. XBP1 expression is reduced in islets of people with type 2 diabetes, but its role in adult differentiated beta cells is unclear. Here, we assessed the effects of Xbp1 deletion in adult beta cells and tested whether XBP1-mediated unfolded protein response makes a necessary contribution to beta cell compensation in insulin resistance states.

METHODS

Mice with inducible beta cell-specific Xbp1 deletion were studied under normal (chow diet) or metabolic stress (high-fat diet or obesity) conditions. Glucose tolerance, insulin secretion, islet gene expression, alpha cell mass, beta cell mass and apoptosis were assessed. Lineage tracing was used to determine beta cell fate.

RESULTS

Deletion of Xbp1 in adult mouse beta cells led to beta cell dedifferentiation, beta-to-alpha cell transdifferentiation and increased alpha cell mass. Cell lineage-specific analyses revealed that Xbp1 deletion deactivated beta cell identity genes (insulin, Pdx1, Nkx6.1, Beta2, Foxo1) and derepressed beta cell dedifferentiation (Aldh1a3) and alpha cell (glucagon, Arx, Irx2) genes. Xbp1 deletion in beta cells of obese ob/ob or high-fat diet-fed mice triggered diabetes and worsened glucose intolerance by disrupting insulin secretory capacity. Furthermore, Xbp1 deletion increased beta cell apoptosis under metabolic stress conditions by attenuating the antioxidant response.

CONCLUSIONS/INTERPRETATION

These findings indicate that XBP1 maintains beta cell identity, represses beta-to-alpha cell transdifferentiation and is required for beta cell compensation and prevention of diabetes in insulin resistance states.

Iron chelation increases beige fat differentiation and metabolic activity, preventing and treating obesity

Beige and brown fat consume glucose and lipids to produce heat, using uncoupling protein 1 (UCP1). It is thought that full activation of brown adipose tissue (BAT) may increase total daily energy expenditure by 20%. Humans normally have more beige and potentially beige-able fat than brown fat. Strategies to increase beige fat differentiation and activation may be useful for the treatment of obesity and diabetes. Mice were fed chow or high-fat diet (HFD) with or without the iron chelator deferasirox. Animals fed HFD + deferasirox were markedly lighter than their HFD controls with increased energy expenditure (12% increase over 24 h, p < 0.001). Inguinal fat from HFD + deferasirox mice showed increased beige fat quantity with greater Ucp1 and Prdm16 expression. Inguinal adipose tissue explants were studied in a Seahorse bioanalyser and energy expenditure was significantly increased. Deferasirox was also effective in established obesity and in ob/ob mice, indicating that intact leptin signalling is not needed for efficacy. These studies identify iron chelation as a strategy to preferentially activate beige fat. Whether activating brown/beige fat is effective in humans is unproven. However, depleting iron to low-normal levels is a potential therapeutic strategy to improve obesity and related metabolic disorders, and human studies may be warranted.

Neuropeptide Y1 receptor antagonism protects β-cells and improves glycemic control in type 2 diabetes

OBJECTIVES

Loss of functional β-cell mass is a key factor contributing to poor glycemic control in advanced type 2 diabetes (T2D). We have previously reported that the inhibition of the neuropeptide Y1 receptor improves the islet transplantation outcome in type 1 diabetes (T1D). The aim of this study was to identify the pathophysiological role of the neuropeptide Y (NPY) system in human T2D and further evaluate the therapeutic potential of using the Y1 receptor antagonist BIBO3304 to improve β-cell function and survival in T2D.

METHODS

The gene expression of the NPY system in human islets from nondiabetic subjects and subjects with T2D was determined and correlated with the stimulation index. The glucose-lowering and β-cell-protective effects of BIBO3304, a selective orally bioavailable Y1 receptor antagonist, in high-fat diet (HFD)/multiple low-dose streptozotocin (STZ)-induced and genetically obese (db/db) T2D mouse models were assessed.

RESULTS

In this study, we identified a more than 2-fold increase in NPY1R and its ligand, NPY mRNA expression in human islets from subjects with T2D, which was significantly associated with reduced insulin secretion. Consistently, the pharmacological inhibition of Y1 receptors by BIBO3304 significantly protected β cells from dysfunction and death under multiple diabetogenic conditions in islets. In a preclinical study, we demonstrated that the inhibition of Y1 receptors by BIBO3304 led to reduced adiposity and enhanced insulin action in the skeletal muscle. Importantly, the Y1 receptor antagonist BIBO3304 treatment also improved β-cell function and preserved functional β-cell mass, thereby resulting in better glycemic control in both HFD/multiple low-dose STZ-induced and db/db T2D mice.

CONCLUSIONS

Our results revealed a novel causal link between increased islet NPY-Y1 receptor gene expression and β-cell dysfunction and failure in human T2D, contributing to the understanding of the pathophysiology of T2D. Furthermore, our results demonstrate that the inhibition of the Y1 receptor by BIBO3304 represents a potential β-cell-protective therapy for improving functional β-cell mass and glycemic control in T2D.

Transcriptome analysis of islets from diabetes-resistant and diabetes-prone obese mice reveals novel gene regulatory networks involved in beta-cell compensation and failure

The mechanisms underpinning beta-cell compensation for obesity-associated insulin resistance and beta-cell failure in type 2 diabetes remain poorly understood. We used a large-scale strategy to determine the time-dependent transcriptomic changes in islets of diabetes-prone db/db and diabetes-resistant ob/ob mice at 6 and 16 weeks of age. Differentially expressed genes were subjected to cluster, gene ontology, pathway and gene set enrichment analyses. A distinctive gene expression pattern was observed in 16 week db/db islets in comparison to the other groups with alterations in transcriptional regulators of islet cell identity, upregulation of glucose/lipid metabolism, and various stress response genes, and downregulation of specific amino acid transport and metabolism genes. In contrast, ob/ob islets displayed a coordinated downregulation of metabolic and stress response genes at 6 weeks of age, suggestive of a preemptive reconfiguration in these islets to lower the threshold of metabolic activation in response to increased insulin demand thereby preserving beta-cell function and preventing cellular stress. In addition, amino acid transport and metabolism genes were upregulated in ob/ob islets, suggesting an important role of glutamate metabolism in beta-cell compensation. Gene set enrichment analysis of differentially expressed genes identified the enrichment of binding motifs for transcription factors, FOXO4, NFATC1, and MAZ. siRNA-mediated knockdown of these genes in MIN6 cells altered cell death, insulin secretion, and stress gene expression. In conclusion, these data revealed novel gene regulatory networks involved in beta-cell compensation and failure. Preemptive metabolic reconfiguration in diabetes-resistant islets may dampen metabolic activation and cellular stress during obesity.

Heparan sulfate proteoglycans in beta cells provide a critical link between endoplasmic reticulum stress, oxidative stress and type 2 diabetes

Heparan sulfate proteoglycans (HSPGs) consist of a core protein with side chains of the glycosaminoglycan heparan sulfate (HS). We have previously identified (i) the HSPGs syndecan-1 (SDC1), and collagen type XVIII (COL18) inside mouse and human islet beta cells, and (ii) a critical role for HS in beta cell survival and protection from reactive oxygen species (ROS). The objective of this study was to investigate whether endoplasmic reticulum (ER) stress contributes to oxidative stress and type 2 diabetes (T2D) by depleting beta cell HSPGs/HS. A rapid loss of intra-islet/beta cell HSPGs, HS and heparanase (HPSE, an HS-degrading enzyme) accompanied upregulation of islet ER stress gene expression in both young T2D-prone db/db and Akita Ins2^WT/C96Y mice. In MIN6 beta cells, HSPGs, HS and HPSE were reduced following treatment with pharmacological inducers of ER stress (thapsigargin or tunicamycin). Treatment of young db/db mice with Tauroursodeoxycholic acid (TUDCA), a chemical protein folding chaperone that relieves ER stress, improved glycemic control and increased intra-islet HSPG/HS. In vitro, HS replacement with heparin (a highly sulfated HS analogue) significantly increased the survival of wild-type and db/db beta cells and restored their resistance to hydrogen peroxide-induced death. We conclude that ER stress inhibits the synthesis/maturation of HSPG core proteins which are essential for HS assembly, thereby exacerbating oxidative stress and promoting beta cell failure. Diminished intracellular HSPGs/HS represent a previously unrecognized critical link bridging ER stress, oxidative stress and beta cell failure in T2D.

Peripheral-specific Y1 receptor antagonism increases thermogenesis and protects against diet-induced obesity

Obesity is caused by an imbalance between food intake and energy expenditure (EE). Here we identify a conserved pathway that links signalling through peripheral Y1 receptors (Y1R) to the control of EE. Selective antagonism of peripheral Y1R, via the non-brain penetrable antagonist BIBO3304, leads to a significant reduction in body weight gain due to enhanced EE thereby reducing fat mass. Specifically thermogenesis in brown adipose tissue (BAT) due to elevated UCP1 is enhanced accompanied by extensive browning of white adipose tissue both in mice and humans. Importantly, selective ablation of Y1R from adipocytes protects against diet-induced obesity. Furthermore, peripheral specific Y1R antagonism also improves glucose homeostasis mainly driven by dynamic changes in Akt activity in BAT. Together, these data suggest that selective peripheral only Y1R antagonism via BIBO3304, or a functional analogue, could be developed as a safer and more effective treatment option to mitigate diet-induced obesity.

The Role of Fatty Acid Signaling in Islet Beta-Cell Adaptation to Normal Pregnancy

BACKGROUND

Maintenance of a normal fetal nutrient supply requires major adaptations in maternal metabolic physiology, including of the islet beta-cell. The role of lipid signaling processes in the mechanisms of islet beta-cell adaptation to pregnancy has been minimally investigated.

OBJECTIVE

To determine the effects of pregnancy on islet fatty acid (FA) metabolic partitioning and FA augmentation of glucose-stimulated insulin secretion (GSIS).

METHODS

Age matched virgin, early pregnant (gestational day-11, G11) and late pregnant (G19) Sprague-Dawley rats were studied. Fasted and fed state biochemistry, oral glucose tolerance tests (OGTT), and fasted and post-OGTT liver glycogen, were determined to assess in vivo metabolic characteristics. In isolated islets, FA (BSA-bound palmitate 0.25 mmol/l) augmentation of GSIS, FA partitioning into esterification and oxidation processes using metabolic tracer techniques, lipolysis by glycerol release, triacylglycerols (TG) content, and the expression of key beta-cell genes were determined.

RESULTS

Plasma glucose in pregnancy was lower, including during the OGTT (glucose area under the curve 0-120 min (AUC_0-120); 655±24 versus 849±13 mmol.l^-1.min; G19 vs virgin; P<0.0001), with plasma insulin concentrations equivalent to those of virgin rats (insulin AUC_0-120; 97±7 versus 83±7 ng.ml^-1.min; G19 vs virgin; not significant). Liver glycogen was depleted in fasted G19 rats with full recovery after oral glucose. Serum TG increased during pregnancy (4.4±0.4, 6.7±0.5; 17.1±1.5 mmol/l; virgin, G11, G19, P<0.0001), and islet TG content decreased (147±42, 172±27, 73±13 ng/µg protein; virgin, G11, G19; P<0.01). GSIS in isolated islets was increased in G19 compared to virgin rats, and this effect was augmented in the presence of FA. FA esterification into phospholipids, monoacylglycerols and TG were increased, whereas FA oxidation was reduced, in islets of pregnant compared to virgin rats, with variable effects on lipolysis dependent on gestational age. Expression of Ppargc1a, a key regulator of mitochondrial metabolism, was reduced by 51% in G11 and 64% in G19 pregnant rat islets compared to virgin rat islets (P<0.001).

CONCLUSION

A lowered set-point for islet and hepatic glucose homeostasis in the pregnant rat has been confirmed. Islet adaptation to pregnancy includes increased FA esterification, reduced FA oxidation, and enhanced FA augmentation of glucose-stimulated insulin secretion.

β Cell Hypoxia-Inducible Factor-1α Is Required for the Prevention of Type 1 Diabetes

The development of autoimmune disease type 1 diabetes (T1D) is determined by both genetic background and environmental factors. Environmental triggers include RNA viruses, particularly coxsackie-virus (CV), but how they induceT1D is not understood. Here, we demonstrate that deletion of the transcription factor hypoxia-inducible factor-1α (HIF-1α) from β cells increases the susceptibility of non-obese diabetic (NOD) mice to environmentally triggered T1D from coxsackieviruses and the β cell toxin streptozotocin. Similarly, knockdown of HIF-1α in human islets leads to a poorer response to coxsackievirus infection. Studies in coxsackievirus-infected islets demonstrate that lack of HIF-1α leads to impaired viral clearance, increased viral load, inflammation, pancreatitis, and loss of β cell mass. These findings show an important role for β cells and, specifically, lack of β cell HIF-1α in the development of T1D. These data suggest new strategies for the prevention of T1D.

Lalwani et al. describe a role for β cell hypoxia-inducible factor-1α (HIF1a) in determining whether β cell injury is followed by resolution and normal function or ongoing injury, autoimmunity, and type 1 diabetes.

A fluorescent timer reporter enables sorting of insulin secretory granules by age

Within the pancreatic β-cells, insulin secretory granules (SGs) exist in functionally distinct pools, displaying variations in motility as well as docking and fusion capability. Current therapies that increase insulin secretion do not consider the existence of these distinct SG pools. Accordingly, these approaches are effective only for a short period, with a worsening of glycemia associated with continued decline in β-cell function. Insulin granule age is underappreciated as a determinant for why an insulin granule is selected for secretion and may explain why newly synthesized insulin is preferentially secreted from β-cells. Here, using a novel fluorescent timer protein, we aimed to investigate the preferential secretion model of insulin secretion and identify how granule aging is affected by variation in the β-cell environment, such as hyperglycemia. We demonstrate the use of a fluorescent timer construct, syncollin-dsRedE5TIMER, which changes its fluorescence from green to red over 18 h, in both microscopy and fluorescence-assisted organelle-sorting techniques. We confirm that the SG-targeting construct localizes to insulin granules in β-cells and does not interfere with normal insulin SG behavior. We visualize insulin SG aging behavior in MIN6 and INS1 β-cell lines and in primary C57BL/6J mouse and nondiabetic human islet cells. Finally, we separated young and old insulin SGs, revealing that preferential secretion of younger granules occurs in glucose-stimulated insulin secretion. We also show that SG population age is modulated by the β-cell environment in vivo in the db/db mouse islets and ex vivo in C57BL/6J islets exposed to different glucose environments.

Metallothionein 1 negatively regulates glucose-stimulated insulin secretion and is differentially expressed in conditions of beta cell compensation and failure in mice and humans

AIMS/HYPOTHESIS

The mechanisms responsible for beta cell compensation in obesity and for beta cell failure in type 2 diabetes are poorly defined. The mRNA levels of several metallothionein (MT) genes are upregulated in islets from individuals with type 2 diabetes, but their role in beta cells is not clear. Here we examined: (1) the temporal changes of islet Mt1 and Mt2 gene expression in mouse models of beta cell compensation and failure; and (2) the role of Mt1 and Mt2 in beta cell function and glucose homeostasis in mice.

METHODS

Mt1 and Mt2 expression was assessed in islets from: (1) control lean (chow diet-fed) and diet-induced obese (high-fat diet-fed for 6 weeks) mice; (2) mouse models of diabetes (db/db mice) at 6 weeks old (prediabetes) and 16 weeks old (after diabetes onset) and age-matched db/+ (control) mice; and (3) obese non-diabetic ob/ob mice (16-week-old) and age-matched ob/+ (control) mice. MT1E, MT1X and MT2A expression was assessed in islets from humans with and without type 2 diabetes. Mt1-Mt2 double-knockout (KO) mice, transgenic mice overexpressing Mt1 under the control of its natural promoter (Tg-Mt1) and corresponding control mice were also studied. In MIN6 cells, MT1 and MT2 were inhibited by small interfering RNAs. mRNA levels were assessed by real-time RT-PCR, plasma insulin and islet MT levels by ELISA, glucose tolerance by i.p. glucose tolerance tests and overnight fasting-1 h refeeding tests, insulin tolerance by i.p. insulin tolerance tests, insulin secretion by RIA, cytosolic free Ca²⁺ concentration with Fura-2 leakage resistant (Fura-2 LR), cytosolic free Zn²⁺ concentration with Fluozin-3, and NAD(P)H by autofluorescence.

RESULTS

Mt1 and Mt2 mRNA levels were reduced in islets of murine models of beta cell compensation, whereas they were increased in diabetic db/db mice. In humans, MT1X mRNA levels were significantly upregulated in islets from individuals with type 2 diabetes in comparison with non-diabetic donors, while MT1E and MT2A mRNA levels were unchanged. Ex vivo, islet Mt1 and Mt2 mRNA and MT1 and MT2 protein levels were downregulated after culture with glucose at 10-30 mmol/l vs 2-5 mmol/l, in association with increased insulin secretion. In human islets, mRNA levels of MT1E, MT1X and MT2A were downregulated by stimulation with physiological and supraphysiological levels of glucose. In comparison with wild-type (WT) mice, Mt1-Mt2 double-KO mice displayed improved glucose tolerance in association with increased insulin levels and enhanced insulin release from isolated islets. In contrast, isolated islets from Tg-Mt1 mice displayed impaired glucose-stimulated insulin secretion (GSIS). In both Mt1-Mt2 double-KO and Tg-Mt1 models, the changes in GSIS occurred despite similar islet insulin content, rises in cytosolic free Ca²⁺ concentration and NAD(P)H levels, or intracellular Zn²⁺ concentration vs WT mice. In MIN6 cells, knockdown of MT1 but not MT2 potentiated GSIS, suggesting that Mt1 rather than Mt2 affects beta cell function.

CONCLUSIONS/INTERPRETATION

These findings implicate Mt1 as a negative regulator of insulin secretion. The downregulation of Mt1 is associated with beta cell compensation in obesity, whereas increased Mt1 accompanies beta cell failure and type 2 diabetes.

Macrophage alterations in islets of obese mice linked to beta cell disruption in diabetes

AIMS/HYPOTHESIS

Mild islet inflammation has been suggested as a contributing factor to beta cell failure in type 2 diabetes. Macrophage levels are elevated in the islets of humans and mice with type 2 diabetes, but their effects on beta cells are not understood. Our goal was to examine the gene expression changes in islet-associated macrophages in obesity models with opposing disposition to diabetes development and to assess their potential contribution to beta cell (mal)adaptation.

METHODS

Islets were isolated from lean control mice, obese diabetes-prone db/db mice and obese diabetes-resistant ob/ob mice. Macrophages were sorted using flow cytometry. Islets were treated ex vivo with clodronate-containing liposomes to deplete macrophages. Gene expression was assessed by real-time RT-PCR.

RESULTS

Macrophage levels were increased in islets from db/db mice but not in islets from ob/ob mice compared with lean control mice. Macrophages from db/db and ob/ob islets displayed distinct changes in gene expression compared with control islet macrophages, suggesting differential shifts in functional state. Macrophages from db/db islets displayed increased expression of interferon regulatory factor 5 (Irf5), IL-1 receptor antagonist (Il1rn) and mannose receptor C-type 1 (Mrc1), whereas macrophages from ob/ob islets showed elevated levels of transforming growth factor beta 1 (Tgfb1) and reduced IL-1β (Il1b). Clodronate-liposome treatment of islets depleted macrophages, as evidenced by reduced mRNA expression of Cd11b (also known as Itgam) and F4/80 (also known as Adgre1) compared with PBS-liposome-treated islets. The depletion of macrophages in db/db islets increased the expression of genes related to beta cell identity. The mRNA levels of islet-associated transcription factors (Mafa and Pdx1), glucose transporter (Glut2 [also known as Slc2a2]), ATP-sensitive K⁺ channel (Kcnj11), incretin receptor (Gipr) and adaptive unfolded protein response (UPR) genes (Xbp1, Hspa5, Pdia4 and Fkbp11) were increased in db/db islets after macrophage depletion, whereas the mRNA levels of the deleterious UPR effector, Ddit3, were reduced. In contrast, depletion of macrophages in islets of ob/ob mice did not affect beta cell identity gene expression.

CONCLUSIONS/INTERPRETATION

The findings of this study suggest that distinct alterations in islet macrophages of obese mice are critically important for the disruption of beta cell gene expression in diabetes.

Deletion distal to the PAX6 coding region reveals a novel basis for familial cosegregation of aniridia and diabetes mellitus

AIMS

Analyze cosegregation of aniridia and diabetes to identify genetic criteria for detection and early treatment of diabetes-susceptible aniridia patients.

METHODS

We assessed a two-generation family: three individuals with aniridia, two previously diagnosed as type 2 diabetes. One individual with aniridia, with unknown diabetes status, was evaluated by oral glucose tolerance test. Genetic analysis of aniridia-associated genes was performed on all available family members. Candidate genes were functionally tested by gene silencing in MIN6 pancreatic β-cells.

RESULTS

A 25 year old male with aniridia had a diabetic oral glucose tolerance test despite a normal fasting blood glucose. A 484-630 kb deletion ∼120 kb distal to PAIRED BOX 6 (PAX6) showed dominant cosegregation with aniridia and diabetes in all affected family members. The deleted region contains regulatory elements for PAX6 expression and four additional coding regions. Knockdown of two of the deleted genes (Dnajc24 or Immp1l) with Pax6 impaired glucose-stimulated insulin secretion.

CONCLUSIONS

We demonstrate dominant cosegregation of diabetes and aniridia with a deletion distal to PAX6, which is clinically distinct from the mild glucose intolerance previously reported with PAX6 coding mutations. Asymptomatic aniridia individuals appear at risk of diabetes (and its complications) and could benefit from earlier diagnosis and treatment.

Contribution of the Long Noncoding RNA H19 to β-Cell Mass Expansion in Neonatal and Adult Rodents

Pancreatic β-cell expansion throughout the neonatal period is essential to generate the appropriate mass of insulin-secreting cells required to maintain blood glucose homeostasis later in life. Hence, defects in this process can predispose to diabetes development during adulthood. Global profiling of transcripts in pancreatic islets of newborn and adult rats revealed that the transcription factor E2F1 controls expression of the long noncoding RNA H19, which is profoundly downregulated during the postnatal period. H19 silencing decreased β-cell expansion in newborns, whereas its re-expression promoted proliferation of β-cells in adults via a mechanism involving the microRNA let-7 and the activation of Akt. The offspring of rats fed a low-protein diet during gestation and lactation display a small β-cell mass and an increased risk of developing diabetes during adulthood. We found that the islets of newborn rats born to dams fed a low-protein diet express lower levels of H19 than those born to dams that did not eat a low-protein diet. Moreover, we observed that H19 expression increases in islets of obese mice under conditions of increased insulin demand. Our data suggest that the long noncoding RNA H19 plays an important role in postnatal β-cell mass expansion in rats and contributes to the mechanisms compensating for insulin resistance in obesity.

Y1 receptor deficiency in β-cells leads to increased adiposity and impaired glucose metabolism

Insulin secretion from pancreatic β-cells is critical for maintaining glucose homeostasis and deregulation of circulating insulin levels is associated with the development of metabolic diseases. While many factors have been implicated in the stimulation of insulin secretion, the mechanisms that subsequently reduce insulin secretion remain largely unexplored. Here we demonstrate that mice with β-cell specific ablation of the Y1 receptor exhibit significantly upregulated serum insulin levels associated with increased body weight and adiposity. Interestingly, when challenged with a high fat diet these β-cell specific Y1-deficient mice also develop hyperglycaemia and impaired glucose tolerance. This is most likely due to enhanced hepatic lipid synthesis, resulting in an increase of lipid accumulation in the liver. Together, our study demonstrates that Y1 receptor signaling negatively regulates insulin release, and pharmacological inhibition of Y1 receptor signalling for the treatment of non-insulin dependent diabetes should be taken into careful consideration.

Osteoglycin, a novel coordinator of bone and glucose homeostasis

Objective

The skeleton, which is strongly controlled by endocrine factors, has recently been shown to also play an active endocrine role itself, specifically influencing energy metabolism. However, much less is known about this role. Therefore, we sought to identify novel endocrine factors involved in the regulation of both bone mass and whole-body glucose homeostasis.

Methods

We used transcriptomic and proteomic analysis of Y1 receptor deficient osteoblasts combined with the generation of a novel osteoglycin deficient mouse model and performed comprehensive in vivo phenotype profiling, combined with osteoglycin administration in wildtype mice and human studies.

Results

Here we identify a novel role for osteoglycin, a secreted proteoglycan, in coordinating bone accretion with changes in energy balance. Using an osteoglycin knockout mouse model, we show that at a whole body level, osteoglycin acts to suppress bone formation and modulate whole body energy supplies by altering glucose uptake through changes in insulin secretion and sensitivity, as well as by altering food intake through central signaling. Examining humans following gastric surgery as a model of negative energy balance, we show that osteoglycin is associated with BMI and lean mass as well as changes in weight, BMI, and glucose levels.

Conclusions

Thus, we identify osteoglycin as a novel factor involved in the regulation of energy homeostasis and identify a role for it in facilitating the matching of bone acquisition to alterations in energy status.

•

Osteoglycin regulates insulin action, bone mass and food intake in mice.

•

Osteoglycin is associated with changes in weight, BMI and glucose in obese humans.

•

Osteoglycin is a downstream mediator of NPY signaling via osteoblastic Y1 receptors.

On-target action of anti-tropomyosin drugs regulates glucose metabolism

The development of novel small molecule inhibitors of the cancer-associated tropomyosin 3.1 (Tpm3.1) provides the ability to examine the metabolic function of specific actin filament populations. We have determined the ability of these anti-Tpm (ATM) compounds to regulate glucose metabolism in mice. Acute treatment (1 h) of wild-type (WT) mice with the compounds (TR100 and ATM1001) led to a decrease in glucose clearance due mainly to suppression of glucose-stimulated insulin secretion (GSIS) from the pancreatic islets. The impact of the drugs on GSIS was significantly less in Tpm3.1 knock out (KO) mice indicating that the drug action is on-target. Experiments in MIN6 β-cells indicated that the inhibition of GSIS by the drugs was due to disruption to the cortical actin cytoskeleton. The impact of the drugs on insulin-stimulated glucose uptake (ISGU) was also examined in skeletal muscle ex vivo. In the absence of drug, ISGU was decreased in KO compared to WT muscle, confirming a role of Tpm3.1 in glucose uptake. Both compounds suppressed ISGU in WT muscle, but in the KO muscle there was little impact of the drugs. Collectively, this data indicates that the ATM drugs affect glucose metabolism in vivo by inhibiting Tpm3.1's function with few off-target effects.

Evidence against a role for NLRP3-driven islet inflammation in db/db mice

Objectives

Type 2 diabetes (T2D) is associated with chronic, low grade inflammation. Activation of the NLRP3 inflammasome and secretion of its target interleukin-1β (IL-1β) have been implicated in pancreatic β cell failure in T2D. Specific targeting of the NLRP3 inflammasome to prevent pancreatic β cell death could allow for selective T2D treatment without compromising all IL-1β-associated immune responses. We hypothesized that treating a mouse model of T2D with MCC950, a compound that specifically inhibits NLRP3, would prevent pancreatic β cell death, thereby preventing the onset of T2D.

Methods

Diabetic db/db mice were treated with MCC950 via drinking water for 8 weeks from 6 to 14 weeks of age, a period over which they developed pancreatic β cell failure. We assessed metabolic parameters such as body composition, glucose tolerance, or insulin secretion over the course of the intervention.

Results

MCC950 was a potent inhibitor of NLRP3-induced IL-1β in vitro and was detected at high levels in the plasma of treated db/db mice. Treatment of pre-diabetic db/db mice with MCC950, however, did not prevent pancreatic dysfunction and full onset of the T2D pathology. When examining the NLRP3 pathway in the pancreas of db/db mice, we could not detect an activation of this pathway nor increased levels of its target IL-1β.

Conclusions

NLRP3 driven-pancreatic IL-1β inflammation does not play a key role in the pathogenesis of the db/db murine model of T2D.

•

Inhibition of NLRP3 via MCC950 in db/db mice did not improve glucose tolerance.

•

MCC950 treatment did not prevent beta cell loss of function.

•

Expression of IL1beta and NLRP3 does not appear increased in db/db islets.

•

We conclude against a role for NLRP3 in db/db pancreatic dysfunction.

Mechanisms of β-cell dedifferentiation in diabetes: recent findings and future research directions

Like all the cells of an organism, pancreatic β-cells originate from embryonic stem cells through a complex cellular process termed differentiation. Differentiation involves the coordinated and tightly controlled activation/repression of specific effectors and gene clusters in a time-dependent fashion thereby giving rise to particular morphological and functional cellular features. Interestingly, cellular differentiation is not a unidirectional process. Indeed, growing evidence suggests that under certain conditions, mature β-cells can lose, to various degrees, their differentiated phenotype and cellular identity and regress to a less differentiated or a precursor-like state. This concept is termed dedifferentiation and has been proposed, besides cell death, as a contributing factor to the loss of functional β-cell mass in diabetes. β-cell dedifferentiation involves: (1) the downregulation of β-cell-enriched genes, including key transcription factors, insulin, glucose metabolism genes, protein processing and secretory pathway genes; (2) the concomitant upregulation of genes suppressed or expressed at very low levels in normal β-cells, the β-cell forbidden genes; and (3) the likely upregulation of progenitor cell genes. These alterations lead to phenotypic reconfiguration of β-cells and ultimately defective insulin secretion. While the major role of glucotoxicity in β-cell dedifferentiation is well established, the precise mechanisms involved are still under investigation. This review highlights the identified molecular mechanisms implicated in β-cell dedifferentiation including oxidative stress, endoplasmic reticulum (ER) stress, inflammation and hypoxia. It discusses the role of Foxo1, Myc and inhibitor of differentiation proteins and underscores the emerging role of non-coding RNAs. Finally, it proposes a novel hypothesis of β-cell dedifferentiation as a potential adaptive mechanism to escape cell death under stress conditions.

Circular RNAs as novel regulators of β-cell functions in normal and disease conditions

OBJECTIVE

There is strong evidence for an involvement of different classes of non-coding RNAs, including microRNAs and long non-coding RNAs, in the regulation of β-cell activities and in diabetes development. Circular RNAs were recently discovered to constitute a substantial fraction of the mammalian transcriptome but the contribution of these non-coding RNAs in physiological and disease processes remains largely unknown. The goal of this study was to identify the circular RNAs expressed in pancreatic islets and to elucidate their possible role in the control of β-cells functions.

METHODS

We used a microarray approach to identify circular RNAs expressed in human islets and searched their orthologues in RNA sequencing data from mouse islets. We then measured the level of four selected circular RNAs in the islets of different Type 1 and Type 2 diabetes models and analyzed the role of these circular transcripts in the regulation of insulin secretion, β-cell proliferation, and apoptosis.

RESULTS

We identified thousands of circular RNAs expressed in human pancreatic islets, 497 of which were conserved in mouse islets. The level of two of these circular transcripts, circHIPK3 and ciRS-7/CDR1as, was found to be reduced in the islets of diabetic db/db mice. Mimicking this decrease in the islets of wild type animals resulted in impaired insulin secretion, reduced β-cell proliferation, and survival. ciRS-7/CDR1as has been previously proposed to function by blocking miR-7. Transcriptomic analysis revealed that circHIPK3 acts by sequestering a group of microRNAs, including miR-124-3p and miR-338-3p, and by regulating the expression of key β-cell genes, such as Slc2a2, Akt1, and Mtpn.

CONCLUSIONS

Our findings point to circular RNAs as novel regulators of β-cell activities and suggest an involvement of this novel class of non-coding RNAs in β-cell dysfunction under diabetic conditions.

Hyperactivation of the Insulin Signaling Pathway Improves Intracellular Proteostasis by Coordinately Up-regulating the Proteostatic Machinery in Adipocytes

Hyperinsulinemia, which is associated with aging and metabolic disease, may lead to defective protein homeostasis (proteostasis) due to hyperactivation of insulin-sensitive pathways such as protein synthesis. We investigated the effect of chronic hyperinsulinemia on proteostasis by generating a time-resolved map of insulin-regulated protein turnover in adipocytes using metabolic pulse-chase labeling and high resolution mass spectrometry. Hyperinsulinemia increased the synthesis of nearly half of all detected proteins and did not affect protein degradation despite suppressing autophagy. Unexpectedly, this marked elevation in protein synthesis was accompanied by enhanced protein stability and folding and not by markers of proteostasis stress such as protein carbonylation and aggregation. The improvement in proteostasis was attributed to a coordinate up-regulation of proteins in the global proteostasis network, including ribosomal, proteasomal, chaperone, and endoplasmic reticulum/mitochondrial unfolded protein response proteins. We conclude that defects associated with hyperactivation of the insulin signaling pathway are unlikely attributed to defective proteostasis because up-regulation of protein synthesis by insulin is accompanied by up-regulation of proteostatic machinery.

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.

A Reevaluation of the Role of the Unfolded Protein Response in Islet Dysfunction: Maladaptation or a Failure to Adapt?

Endoplasmic reticulum (ER) stress caused by perturbations in ER homeostasis activates an adaptive response termed the unfolded protein response (UPR) whose function is to resolve ER stress. If unsuccessful, the UPR initiates a proapoptotic program to eliminate the malfunctioning cells from the organism. It is the activation of this proapoptotic UPR in pancreatic β-cells that has been implicated in the onset of type 2 diabetes and thus, in this context, is considered a maladaptive response. However, there is growing evidence that β-cell death in type 2 diabetes may not be caused by a maladaptive UPR but by the inhibition of the adaptive UPR. In this review, we discuss the evidence for a role of the UPR in β-cell dysfunction and death in the development of type 2 diabetes and ask the following question: Is β-cell dysfunction the result of a maladaptive UPR or a failure of the UPR to adequately adapt? The answer to this question is critically important in defining potential therapeutic strategies for the treatment and prevention of type 2 diabetes. In addition, we discuss the potential role of the adaptive UPR in staving off type 2 diabetes by enhancing β-cell mass and function in response to insulin resistance.

A Syntenic Cross Species Aneuploidy Genetic Screen Links RCAN1 Expression to β-Cell Mitochondrial Dysfunction in Type 2 Diabetes

Type 2 diabetes (T2D) is a complex metabolic disease associated with obesity, insulin resistance and hypoinsulinemia due to pancreatic β-cell dysfunction. Reduced mitochondrial function is thought to be central to β-cell dysfunction. Mitochondrial dysfunction and reduced insulin secretion are also observed in β-cells of humans with the most common human genetic disorder, Down syndrome (DS, Trisomy 21). To identify regions of chromosome 21 that may be associated with perturbed glucose homeostasis we profiled the glycaemic status of different DS mouse models. The Ts65Dn and Dp16 DS mouse lines were hyperglycemic, while Tc1 and Ts1Rhr mice were not, providing us with a region of chromosome 21 containing genes that cause hyperglycemia. We then examined whether any of these genes were upregulated in a set of ~5,000 gene expression changes we had identified in a large gene expression analysis of human T2D β-cells. This approach produced a single gene, RCAN1, as a candidate gene linking hyperglycemia and functional changes in T2D β-cells. Further investigations demonstrated that RCAN1 methylation is reduced in human T2D islets at multiple sites, correlating with increased expression. RCAN1 protein expression was also increased in db/db mouse islets and in human and mouse islets exposed to high glucose. Mice overexpressing RCAN1 had reduced in vivo glucose-stimulated insulin secretion and their β-cells displayed mitochondrial dysfunction including hyperpolarised membrane potential, reduced oxidative phosphorylation and low ATP production. This lack of β-cell ATP had functional consequences by negatively affecting both glucose-stimulated membrane depolarisation and ATP-dependent insulin granule exocytosis. Thus, from amongst the myriad of gene expression changes occurring in T2D β-cells where we had little knowledge of which changes cause β-cell dysfunction, we applied a trisomy 21 screening approach which linked RCAN1 to β-cell mitochondrial dysfunction in T2D.

Genetic predisposition for beta cell fragility underlies type 1 and type 2 diabetes

Type 1 (T1D) and type 2 (T2D) diabetes share pathophysiological characteristics, yet mechanistic links have remained elusive. T1D results from autoimmune destruction of pancreatic beta cells, whereas beta cell failure in T2D is delayed and progressive. Here we find a new genetic component of diabetes susceptibility in T1D non-obese diabetic (NOD) mice, identifying immune-independent beta cell fragility. Genetic variation in Xrcc4 and Glis3 alters the response of NOD beta cells to unfolded protein stress, enhancing the apoptotic and senescent fates. The same transcriptional relationships were observed in human islets, demonstrating the role of beta cell fragility in genetic predisposition to diabetes.

The balance between adaptive and apoptotic unfolded protein responses regulates β-cell death under ER stress conditions through XBP1, CHOP and JNK

Endoplasmic reticulum (ER) stress and the subsequent unfolded protein response (UPR) have been implicated in β-cell death in type 1 and type 2 diabetes. However, the UPR is also a fundamental mechanism required for β-cell adaptation and survival. The mechanisms regulating the transition from adaptive to apoptotic UPR remain to be clarified. Here, we investigated the relationships between XBP1, CHOP and JNK in the transition from adaptive to apoptotic UPR and β-cell death in models of type 1 and type 2 diabetes. XBP1 inhibition potentiated cell death induced by pro-inflammatory cytokines or the saturated fatty acid palmitate in MIN6 β-cells. This response was prevented by CHOP inhibition. IRE1/XBP1 inhibition led to alterations in islets from diabetes-resistant ob/ob mice that resemble those found in diabetes, including increases in cell death and inflammation and antioxidant gene expression. Similarly, IRE1/XBP1 inhibition increased cell death in islets from NOD mice. On the other hand, JNK inhibition: 1) increased adaptive UPR and reduced cell death in islets from diabetic db/db mice, and 2) restored adaptive UPR while protecting against apoptotic UPR gene expression and β-cell death and dysfunction following cytokine exposure. These findings suggest that the balance between XBP1-mediated adaptive and CHOP-dependent apoptotic UPR is critically important for β-cell survival during ER stress. JNK activation regulates the transition from adaptive to apoptotic UPR, thus providing a mechanism for β-cell propensity to cell death rather than ER stress adaptation in type 1 and type 2 diabetes.

Pancreatic PYY Is Critical in the Control of Insulin Secretion and Glucose Homeostasis in Female Mice

Insulin secretion is tightly controlled through coordinated actions of a number of systemic and local factors. Peptide YY (PYY) is expressed in α-cells of the islet, but its role in control of islet function such as insulin release is not clear. In this study, we generated a transgenic mouse model (Pyy(tg/+)/Rip-Cre) overexpressing the Pyy gene under the control of the rat insulin 2 gene promoter and assessed the impact of islet-released PYY on β-cell function, insulin release, and glucose homeostasis in mice. Our results show that up-regulation of PYY in islet β-cells leads to an increase in serum insulin levels as well as improved glucose tolerance. Interestingly, PYY-overproducing mice show increased lean mass and reduced fat mass with no significant changes in food intake or body weight. Energy expenditure is also increased accompanied by increased respiratory exchange ratio. Mechanistically, the enhanced insulin levels and improved glucose tolerance are primarily due to increased β-cell mass and secretion. This is associated with alterations in the expression of genes important for β-cell proliferation and function as well as the maintenance of the β-cell phenotype. Taken together, these data demonstrate that pancreatic islet-derived PYY plays an important role in controlling glucose homeostasis through the modulation of β-cell mass and function.

Pancreas-Specific Sirt1-Deficiency in Mice Compromises Beta-Cell Function without Development of Hyperglycemia

AIMS/HYPOTHESIS

Sirtuin 1 (Sirt1) has been reported to be a critical positive regulator of glucose-stimulated insulin secretion in pancreatic beta-cells. The effects on islet cells and blood glucose levels when Sirt1 is deleted specifically in the pancreas are still unclear.

METHODS

This study examined islet glucose responsiveness, blood glucose levels, pancreatic islet histology and gene expression in Pdx1Cre; Sirt1ex4F/F mice that have loss of function and loss of expression of Sirt1 specifically in the pancreas.

RESULTS

We found that in the Pdx1Cre; Sirt1ex4F/F mice, the relative insulin positive area and the islet size distribution were unchanged. However, beta-cells were functionally impaired, presenting with lower glucose-stimulated insulin secretion. This defect was not due to a reduced expression of insulin but was associated with a decreased expression of the glucose transporter Slc2a2/Glut2 and of the Glucagon like peptide-1 receptor (Glp1r) as well as a marked down regulation of endoplasmic reticulum (ER) chaperones that participate in the Unfolded Protein Response (UPR) pathway. Counter intuitively, the Sirt1-deficient mice did not develop hyperglycemia. Pancreatic polypeptide (PP) cells were the only other islet cells affected, with reduced numbers in the Sirt1-deficient pancreas.

CONCLUSIONS/INTERPRETATION

This study provides new mechanistic insights showing that beta-cell function in Sirt1-deficient pancreas is affected due to altered glucose sensing and deregulation of the UPR pathway. Interestingly, we uncovered a context in which impaired beta-cell function is not accompanied by increased glycemia. This points to a unique compensatory mechanism. Given the reduction in PP, investigation of its role in the control of blood glucose is warranted.

Inhibitor of differentiation proteins protect against oxidative stress by regulating the antioxidant-mitochondrial response in mouse beta cells

AIMS/HYPOTHESIS

Oxidative stress is implicated in beta cell glucotoxicity in type 2 diabetes. Inhibitor of differentiation (ID) proteins are transcriptional regulators induced by hyperglycaemia in islets, but the mechanisms involved and their role in beta cells are not clear. Here we investigated whether or not oxidative stress regulates ID levels in beta cells and the role of ID proteins in beta cells during oxidative stress.

METHODS

MIN6 cells were cultured in H2O2 or ribose to induce oxidative stress. ID1, ID3 and small MAF proteins (MAFF, MAFG and MAFK) were inhibited using small interfering RNA. Isolated islets from Id1(-/-), Id3(-/-) and diabetic db/db mice were used.

RESULTS

ID1-4 expression was upregulated in vivo in the islets of diabetic db/db mice and stimulated in vitro by ribose and H2O2. Id1/3 inhibition reduced the expression of multiple antioxidant genes and potentiated oxidative stress-induced apoptosis. This finding was associated with increased levels of intracellular reactive oxygen species, altered mitochondrial morphology and reduced expression of Tfam, which encodes a mitochondrial transcription factor, and respiratory chain components. Id1/3 inhibition also reduced the expression of small MAF transcription factors (MafF, MafG and MafK), interacting partners of nuclear factor, erythroid 2-like 2 (NFE2L2), master regulator of the antioxidant response. Inhibition of small MAFs reduced the expression of antioxidant genes and potentiated oxidative stress-induced apoptosis, thus recapitulating the effects of Id1/3 inhibition.

CONCLUSIONS/INTERPRETATION

Our study identifies IDs as a novel family of oxidative stress-responsive proteins in beta cells. IDs are crucial regulators of the adaptive antioxidant-mitochondrial response that promotes beta cell survival during oxidative stress through a novel link to the NFE2L2-small MAF pathway.

Identification of particular groups of microRNAs that positively or negatively impact on beta cell function in obese models of type 2 diabetes

AIMS/HYPOTHESIS

MicroRNAs are key regulators of gene expression involved in health and disease. The goal of our study was to investigate the global changes in beta cell microRNA expression occurring in two models of obesity-associated type 2 diabetes and to assess their potential contribution to the development of the disease.

METHODS

MicroRNA profiling of pancreatic islets isolated from prediabetic and diabetic db/db mice and from mice fed a high-fat diet was performed by microarray. The functional impact of the changes in microRNA expression was assessed by reproducing them in vitro in primary rat and human beta cells.

RESULTS

MicroRNAs differentially expressed in both models of obesity-associated type 2 diabetes fall into two distinct categories. A group including miR-132, miR-184 and miR-338-3p displays expression changes occurring long before the onset of diabetes. Functional studies indicate that these expression changes have positive effects on beta cell activities and mass. In contrast, modifications in the levels of miR-34a, miR-146a, miR-199a-3p, miR-203, miR-210 and miR-383 primarily occur in diabetic mice and result in increased beta cell apoptosis. These results indicate that obesity and insulin resistance trigger adaptations in the levels of particular microRNAs to allow sustained beta cell function, and that additional microRNA deregulation negatively impacting on insulin-secreting cells may cause beta cell demise and diabetes manifestation.

CONCLUSIONS/INTERPRETATION

We propose that maintenance of blood glucose homeostasis or progression toward glucose intolerance and type 2 diabetes may be determined by the balance between expression changes of particular microRNAs.

Influence of chronic hyperglycemia on the loss of the unfolded protein response in transplanted islets

Chronic hyperglycemia contributes to β-cell dysfunction in diabetes and with islet transplantation, but the mechanisms remain unclear. Recent studies demonstrate that the unfolded protein response (UPR) is critical for β-cell function. Here, we assessed the influence of hyperglycemia on UPR gene expression in transplanted islets. Streptozotocin-induced diabetic or control nondiabetic mice were transplanted under the kidney capsule with syngeneic islets either sufficient or not to normalize hyperglycemia. Twenty-one days after transplantation, islet grafts were excised and RT-PCR was used to assess gene expression. In islet grafts from diabetic mice, expression levels of many UPR genes of the IRE1/ATF6 pathways, which are important for adaptation to endoplasmic reticulum stress, were markedly reduced compared with that in islet grafts from control mice. UPR genes of the PERK pathway were also downregulated. The normalization of glycemia restored the changes in mRNA expression, suggesting that chronic hyperglycemia contributes to the downregulation of multiple arms of UPR gene expression. Similar correlations were observed between blood glucose and mRNA levels of transcription factors involved in the maintenance of β-cell phenotype and genes implicated in β-cell function, suggesting convergent regulation of UPR gene expression and β-cell differentiation by hyperglycemia. However, the normalization of glycemia was not accompanied by restoration of antioxidant or pro-inflammatory cytokine mRNA levels, which were increased in islet grafts from diabetic mice. These studies demonstrate that chronic hyperglycemia contributes to the downregulation of multiple arms of UPR gene expression in transplanted mouse islets. Failure of the adaptive UPR may contribute to β-cell dedifferentiation and dysfunction in diabetes.

Failure of the adaptive unfolded protein response in islets of obese mice is linked with abnormalities in β-cell gene expression and progression to diabetes

The normal β-cell response to obesity-associated insulin resistance is hypersecretion of insulin. Type 2 diabetes develops in subjects with β-cells that are susceptible to failure. Here, we investigated the time-dependent gene expression changes in islets of diabetes-prone db/db and diabetes-resistant ob/ob mice. The expressions of adaptive unfolded protein response (UPR) genes were progressively induced in islets of ob/ob mice, whereas they declined in diabetic db/db mice. Genes important for β-cell function and maintenance of the islet phenotype were reduced with time in db/db mice, whereas they were preserved in ob/ob mice. Inflammation and antioxidant genes displayed time-dependent upregulation in db/db islets but were unchanged in ob/ob islets. Treatment of db/db mouse islets with the chemical chaperone 4-phenylbutyric acid partially restored the changes in several β-cell function genes and transcription factors but did not affect inflammation or antioxidant gene expression. These data suggest that the maintenance (or suppression) of the adaptive UPR is associated with β-cell compensation (or failure) in obese mice. Inflammation, oxidative stress, and a progressive loss of β-cell differentiation accompany diabetes progression. The ability to maintain the adaptive UPR in islets may protect against the gene expression changes that underlie diabetes development in obese mice.

The molecular mechanisms of pancreatic β-cell glucotoxicity: recent findings and future research directions

It is well established that regular physiological stimulation by glucose plays a crucial role in the maintenance of the β-cell differentiated phenotype. In contrast, prolonged or repeated exposure to elevated glucose concentrations both in vitro and in vivo exerts deleterious or toxic effects on the β-cell phenotype, a concept termed as glucotoxicity. Evidence indicates that the latter may greatly contribute to the pathogenesis of type 2 diabetes. Through the activation of several mechanisms and signaling pathways, high glucose levels exert deleterious effects on β-cell function and survival and thereby, lead to the worsening of the disease over time. While the role of high glucose-induced β-cell overstimulation, oxidative stress, excessive Unfolded Protein Response (UPR) activation, and loss of differentiation in the alteration of the β-cell phenotype is well ascertained, at least in vitro and in animal models of type 2 diabetes, the role of other mechanisms such as inflammation, O-GlcNacylation, PKC activation, and amyloidogenesis requires further confirmation. On the other hand, protein glycation is an emerging mechanism that may play an important role in the glucotoxic deterioration of the β-cell phenotype. Finally, our recent evidence suggests that hypoxia may also be a new mechanism of β-cell glucotoxicity. Deciphering these molecular mechanisms of β-cell glucotoxicity is a mandatory first step toward the development of therapeutic strategies to protect β-cells and improve the functional β-cell mass in type 2 diabetes.

Cross-talk between the unfolded protein response and nuclear factor-κB signalling pathways regulates cytokine-mediated beta cell death in MIN6 cells and isolated mouse islets

AIMS/HYPOTHESIS

Pancreatic beta cell destruction in type 1 diabetes may be mediated by cytokines such as IL-1β, IFN-γ and TNF-α. Endoplasmic reticulum (ER) stress and nuclear factor-κB (NFκB) signalling are activated by cytokines, but their significance in beta cells remains unclear. Here, we investigated the role of cytokine-induced ER stress and NFκB signalling in beta cell destruction.

METHODS

Isolated mouse islets and MIN6 beta cells were incubated with IL-1β, IFN-γ and TNF-α. The chemical chaperone 4-phenylbutyric acid (PBA) was used to inhibit ER stress. Protein production and gene expression were assessed by western blot and real-time RT-PCR.

RESULTS

We found in beta cells that inhibition of cytokine-induced ER stress with PBA unexpectedly potentiated cell death and NFκB-regulated gene expression. These responses were dependent on NFκB activation and were associated with a prolonged decrease in the inhibitor of κB-α (IκBα) protein, resulting from increased IκBα protein degradation. Cytokine-mediated NFκB-regulated gene expression was also potentiated after pre-induction of ER stress with thapsigargin, but not tunicamycin. Both PBA and thapsigargin treatments led to preferential upregulation of ER degradation genes over ER-resident chaperones as part of the adaptive unfolded protein response (UPR). In contrast, tunicamycin activated a balanced adaptive UPR in association with the maintenance of Xbp1 splicing.

CONCLUSIONS/INTERPRETATION

These data suggest a novel mechanism by which cytokine-mediated ER stress interacts with NFκB signalling in beta cells, by regulating IκBα degradation. The cross-talk between the UPR and NFκB signalling pathways may be important in the regulation of cytokine-mediated beta cell death.

Mice deficient in GEM GTPase show abnormal glucose homeostasis due to defects in beta-cell calcium handling

Aims and Hypothesis

Glucose-stimulated insulin secretion from beta-cells is a tightly regulated process that requires calcium flux to trigger exocytosis of insulin-containing vesicles. Regulation of calcium handling in beta-cells remains incompletely understood. Gem, a member of the RGK (Rad/Gem/Kir) family regulates calcium channel handling in other cell types, and Gem over-expression inhibits insulin release in insulin-secreting Min6 cells. The aim of this study was to explore the role of Gem in insulin secretion. We hypothesised that Gem may regulate insulin secretion and thus affect glucose tolerance in vivo.

Methods

Gem-deficient mice were generated and their metabolic phenotype characterised by in vivo testing of glucose tolerance, insulin tolerance and insulin secretion. Calcium flux was measured in isolated islets.

Results

Gem-deficient mice were glucose intolerant and had impaired glucose stimulated insulin secretion. Furthermore, the islets of Gem-deficient mice exhibited decreased free calcium responses to glucose and the calcium oscillations seen upon glucose stimulation were smaller in amplitude and had a reduced frequency.

Conclusions

These results suggest that Gem plays an important role in normal beta-cell function by regulation of calcium signalling.

Lipid-induced endoplasmic reticulum stress in liver cells results in two distinct outcomes: adaptation with enhanced insulin signaling or insulin resistance

Chronically elevated fatty acids contribute to insulin resistance through poorly defined mechanisms. Endoplasmic reticulum (ER) stress and the subsequent unfolded protein response (UPR) have been implicated in lipid-induced insulin resistance. However, the UPR is also a fundamental mechanism required for cell adaptation and survival. We aimed to distinguish the adaptive and deleterious effects of lipid-induced ER stress on hepatic insulin action. Exposure of human hepatoma HepG2 cells or mouse primary hepatocytes to the saturated fatty acid palmitate enhanced ER stress in a dose-dependent manner. Strikingly, exposure of HepG2 cells to prolonged mild ER stress activation induced by low levels of thapsigargin, tunicamycin, or palmitate augmented insulin-stimulated Akt phosphorylation. This chronic mild ER stress subsequently attenuated the acute stress response to high-level palmitate challenge. In contrast, exposure of HepG2 cells or hepatocytes to severe ER stress induced by high levels of palmitate was associated with reduced insulin-stimulated Akt phosphorylation and glycogen synthesis, as well as increased expression of glucose-6-phosphatase. Attenuation of ER stress using chemical chaperones (trimethylamine N-oxide or tauroursodeoxycholic acid) partially protected against the lipid-induced changes in insulin signaling. These findings in liver cells suggest that mild ER stress associated with chronic low-level palmitate exposure induces an adaptive UPR that enhances insulin signaling and protects against the effects of high-level palmitate. However, in the absence of chronic adaptation, severe ER stress induced by high-level palmitate exposure induces deleterious UPR signaling that contributes to insulin resistance and metabolic dysregulation.

Differing endoplasmic reticulum stress response to excess lipogenesis versus lipid oversupply in relation to hepatic steatosis and insulin resistance

Mitochondrial dysfunction and endoplasmic reticulum (ER) stress have been implicated in hepatic steatosis and insulin resistance. The present study investigated their roles in the development of hepatic steatosis and insulin resistance during de novo lipogenesis (DNL) compared to extrahepatic lipid oversupply. Male C57BL/6J mice were fed either a high fructose (HFru) or high fat (HFat) diet to induce DNL or lipid oversupply in/to the liver. Both HFru and HFat feeding increased hepatic triglyceride within 3 days (by 3.5 and 2.4 fold) and the steatosis remained persistent from 1 week onwards (p<0.01 vs Con). Glucose intolerance (iAUC increased by ∼60%) and blunted insulin-stimulated hepatic Akt and GSK3β phosphorylation (∼40–60%) were found in both feeding conditions (p<0.01 vs Con, assessed after 1 week). No impairment of mitochondrial function was found (oxidation capacity, expression of PGC1α, CPT1, respiratory complexes, enzymatic activity of citrate synthase & β-HAD). As expected, DNL was increased (∼60%) in HFru-fed mice and decreased (32%) in HFat-fed mice (all p<0.05). Interestingly, associated with the upregulated lipogenic enzymes (ACC, FAS and SCD1), two (PERK/eIF2α and IRE1/XBP1) of three ER stress pathways were significantly activated in HFru-fed mice. However, no significant ER stress was observed in HFat-fed mice during the development of hepatic steatosis. Our findings indicate that HFru and HFat diets can result in hepatic steatosis and insulin resistance without obvious mitochondrial defects via different lipid metabolic pathways. The fact that ER stress is apparent only with HFru feeding suggests that ER stress is involved in DNL per se rather than resulting from hepatic steatosis or insulin resistance.

Glucose-induced O₂ consumption activates hypoxia inducible factors 1 and 2 in rat insulin-secreting pancreatic beta-cells

Background

Glucose increases the expression of glycolytic enzymes and other hypoxia-response genes in pancreatic beta-cells. Here, we tested whether this effect results from the activation of Hypoxia-Inducible-factors (HIF) 1 and 2 in a hypoxia-dependent manner.

Methodology/Principal Findings

Isolated rat islets and insulin-secreting INS-1E cells were stimulated with nutrients at various pO₂ values or treated with the HIF activator CoCl₂. HIF-target gene mRNA levels and HIF subunit protein levels were measured by real-time RT-PCR, Western Blot and immunohistochemistry. The formation of pimonidazole-protein adducts was used as an indicator of hypoxia. In INS-1E and islet beta-cells, glucose concentration-dependently stimulated formation of pimonidazole-protein adducts, HIF1 and HIF2 nuclear expression and HIF-target gene mRNA levels to a lesser extent than CoCl₂ or a four-fold reduction in pO₂. Islets also showed signs of HIF activation in diabetic Lepr^db/db but not non-diabetic Lepr^db/+ mice. In vitro, these glucose effects were reproduced by nutrient secretagogues that bypass glycolysis, and were inhibited by a three-fold increase in pO₂ or by inhibitors of Ca²⁺ influx and insulin secretion. In INS-1E cells, small interfering RNA-mediated knockdown of Hif1α and Hif2α, alone or in combination, indicated that the stimulation of glycolytic enzyme mRNA levels depended on both HIF isoforms while the vasodilating peptide adrenomedullin was a HIF2-specific target gene.

Conclusions/Significance

Glucose-induced O₂ consumption creates an intracellular hypoxia that activates HIF1 and HIF2 in rat beta-cells, and this glucose effect contributes, together with the activation of other transcription factors, to the glucose stimulation of expression of some glycolytic enzymes and other hypoxia response genes.

Inhibition of Id1 augments insulin secretion and protects against high-fat diet-induced glucose intolerance

OBJECTIVE

The molecular mechanisms responsible for pancreatic β-cell dysfunction in type 2 diabetes remain unresolved. Increased expression of the helix-loop-helix protein Id1 has been found in islets of diabetic mice and in vitro models of β-cell dysfunction. Here, we investigated the role of Id1 in insulin secretion and glucose homeostasis.

RESEARCH DESIGN AND METHODS

Id1 knockout (Id1(-/-)) and wild-type mice were fed a chow or high-fat diet. Glucose tolerance, insulin tolerance, β-cell mass, insulin secretion, and islet gene expression were assessed. Small interfering RNA (siRNA) was used to silence Id1 in MIN6 cells, and responses to chronic palmitate treatment were assessed.

RESULTS

Id1(-/-) mice exhibited an improved response to glucose challenge and were almost completely protected against glucose intolerance induced by high-fat diet. This was associated with increased insulin levels and enhanced insulin release from isolated islets, whereas energy intake, body weight, fat pad weight, β-cell mass, and insulin action were unchanged. Islets from Id1(-/-) mice displayed reduced stress gene expression and were protected against high-fat diet-induced downregulation of β-cell gene expression (pancreatic duodenal homeobox-1, Beta2, Glut2, pyruvate carboxylase, and Gpr40). In MIN6 cells, siRNA-mediated inhibition of Id1 enhanced insulin secretion after chronic palmitate treatment and protected against palmitate-mediated loss of β-cell gene expression.

CONCLUSIONS

These findings implicate Id1 as a negative regulator of insulin secretion. Id1 expression plays an essential role in the etiology of glucose intolerance, insulin secretory dysfunction, and β-cell dedifferentiation under conditions of increased lipid supply.

Differential regulation of adaptive and apoptotic unfolded protein response signalling by cytokine-induced nitric oxide production in mouse pancreatic beta cells

AIMS/HYPOTHESIS

Pro-inflammatory cytokines such as IL-1β, IFN-γ and TNF-α may contribute to pancreatic beta cell destruction in type 1 diabetes. A mechanism requiring nitric oxide, which is generated by inducible nitric oxide synthase (iNOS), in cytokine-induced endoplasmic reticulum (ER) stress and apoptosis has been proposed. Here, we tested the role of nitric oxide in cytokine-induced ER stress and the subsequent unfolded protein response (UPR) in beta cells.

METHODS

Isolated islets from wild-type and iNos (also known as Nos2) knockout (iNos ( -/- )) mice, and MIN6 beta cells were incubated with IL-1β, IFN-γ and TNF-α for 24-48 h. N (G)-methyl-L: -arginine was used to inhibit nitric oxide production in MIN6 cells. Protein levels and gene expression were assessed by western blot and real-time RT-PCR.

RESULTS

In islets and MIN6 cells, inhibition of nitric oxide production had no effect on the generation of ER stress by cytokines, as evidenced by downregulation of Serca2b (also known as Atp2a2) mRNA and increased phosphorylation of PKR-like ER kinase, Jun N-terminal kinase (JNK) and eukaryotic translation initiation factor 2 α subunit. However, nitric oxide regulated the pattern of UPR signalling, which delineates the cellular decision to adapt to ER stress or to undergo apoptosis. Inhibition of nitric oxide production led to reduced expression of pro-apoptotic UPR markers, Chop (also known as Ddit3), Atf3 and Trib3. In contrast, adaptive UPR markers (chaperones, foldases and degradation enhancers) were increased. Further analysis of mouse islets showed that cytokine-induced Chop and Atf3 expression was also dependent on JNK activity.

CONCLUSIONS/INTERPRETATION

The mechanism by which cytokines induce ER stress in mouse beta cells is independent of nitric oxide production. However, nitric oxide may regulate the switch between adaptive and apoptotic UPR signalling.

Deletion of protein kinase Cδ in mice modulates stability of inflammatory genes and protects against cytokine-stimulated beta cell death in vitro and in vivo

AIMS/HYPOTHESIS

Proinflammatory cytokines contribute to beta cell destruction in type 1 diabetes, but the mechanisms are incompletely understood. The aim of the current study was to address the role of the protein kinase C (PKC) isoform PKCδ, a diverse regulator of cell death, in cytokine-stimulated apoptosis in primary beta cells.

METHODS

Islets isolated from wild-type or Prkcd(-/-) mice were treated with IL-1β, TNF-α and IFNγ and assayed for apoptosis, nitric oxide (NO) generation and insulin secretion. Activation of signalling pathways, apoptosis and endoplasmic reticulum (ER) stress were determined by immunoblotting. Stabilisation of mRNA transcripts was measured by RT-PCR following transcriptional arrest. Mice were injected with multiple low doses of streptozotocin (MLD-STZ) and fasting blood glucose monitored.

RESULTS

Deletion of Prkcd inhibited apoptosis and NO generation in islets stimulated ex vivo with cytokines. It also delayed the onset of hyperglycaemia in MLD-STZ-treated mice. Activation of ERK, p38, JNK, AKT1, the ER stress markers DDIT3 and phospho-EIF2α and the intrinsic apoptotic markers BCL2 and MCL1 was not different between genotypes. However, deletion of Prkcd destabilised mRNA transcripts for Nos2, and for multiple components of the toll-like receptor 2 (TLR2) signalling complex, which resulted in disrupted TLR2 signalling.

CONCLUSIONS/INTERPRETATION

Loss of PKCδ partially protects against hyperglycaemia in the MLD-STZ model in vivo, and against cytokine-mediated apoptosis in vitro. This is accompanied by reduced NO generation and destabilisation of Nos2 and components of the TLR2 signalling pathway. The results highlight a mechanism for regulating proinflammatory gene expression in beta cells independently of transcription.

Bip overexpression, but not CHOP inhibition, attenuates fatty-acid-induced endoplasmic reticulum stress and apoptosis in HepG2 liver cells

AIMS

In this study we investigated whether attenuation of endoplasmic reticulum stress (ER stress) could protect HepG2 cells from free fatty acid (FFA)-induced apoptosis.

MAIN METHODS

Human liver cell line HepG2 cells were exposed to Sodium Palmitate (Pa) or Sodium Oleate (Ol). Apoptosis and ER stress of HepG2 cells were analyzed with flow cytometry, real-time RT-PCR and Western Blotting. An expression plasmid encoding for the ER chaperone immunoglobulin heavy chain-binding protein (Bip) was transfected into HepG2 cells to attenuate ER stress. Small interfering RNA siCHOP was used to knockdown the expression of C/EBP Homologous Protein (CHOP) in HepG2.

KEY FINDINGS

Pa led to cytotoxicity and apoptosis in HepG2 cells in a dose-dependent pattern and also induced ER stress indicated by increased phosphorylation of eIF2α, upregulation of IRE1α and CHOP. Bip expression levels were slightly down regulated after Pa treatment. The unsaturated fatty acid, Ol, induced neither apoptosis nor ER stress in HepG2 cells. Overexpression of Bip attenuated Pa-induced ER stress and led to a significant reduction in Pa-mediated apoptosis, indicating a requirement of ER stress for lipotoxicity in liver cells. siRNA-mediated reduction of CHOP did not protect against Pa-induced apoptosis.

SIGNIFICANCE

While ER stress makes a necessary contribution to palmitate cytotoxicity, inhibition of CHOP alone is not sufficient to prevent palmitate-induced apoptosis. Our findings could advance the detailed understanding on the mechanism of high fatty acid (FFA)-induced apoptosis.

Hypoxia-inducible factor-1alpha regulates beta cell function in mouse and human islets

Hypoxia-inducible factor-1alpha (HIF-1alpha) is a transcription factor that regulates cellular stress responses. While the levels of HIF-1alpha protein are tightly regulated, recent studies suggest that it can be active under normoxic conditions. We hypothesized that HIF-1alpha is required for normal beta cell function and reserve and that dysregulation may contribute to the pathogenesis of type 2 diabetes (T2D). Here we show that HIF-1alpha protein is present at low levels in mouse and human normoxic beta cells and islets. Decreased levels of HIF-1alpha impaired glucose-stimulated ATP generation and beta cell function. C57BL/6 mice with beta cell-specific Hif1a disruption (referred to herein as beta-Hif1a-null mice) exhibited glucose intolerance, beta cell dysfunction, and developed severe glucose intolerance on a high-fat diet. Increasing HIF-1alpha levels by inhibiting its degradation through iron chelation markedly improved insulin secretion and glucose tolerance in control mice fed a high-fat diet but not in beta-Hif1a-null mice. Increasing HIF-1alpha levels markedly increased expression of ARNT and other genes in human T2D islets and improved their function. Further analysis indicated that HIF-1alpha was bound to the Arnt promoter in a mouse beta cell line, suggesting direct regulation. Taken together, these findings suggest an important role for HIF-1alpha in beta cell reserve and regulation of ARNT expression and demonstrate that HIF-1alpha is a potential therapeutic target for the beta cell dysfunction of T2D.

Reduced endoplasmic reticulum (ER)-to-Golgi protein trafficking contributes to ER stress in lipotoxic mouse beta cells by promoting protein overload

AIMS/HYPOTHESIS

Saturated fatty acids augment endoplasmic reticulum (ER) stress in pancreatic beta cells and this is implicated in the loss of beta cell mass that accompanies type 2 diabetes. However, the mechanisms underlying the induction of ER stress are unclear. Our aim was to establish whether saturated fatty acids cause defects in ER-to-Golgi protein trafficking, which may thereby contribute to ER stress via protein overload.

METHODS

Cells of the mouse insulinoma cell line MIN6 were transfected with temperature-sensitive vesicular stomatitis virus G protein (VSVG) tagged with green fluorescent protein to quantify the rate of ER-to-Golgi protein trafficking. I14 antibody, which detects only correctly folded VSVG, was employed to probe the folding environment of the ER. ER stress markers were monitored by western blotting.

RESULTS

Pretreatment with palmitate, but not oleate, significantly reduced the rate of ER-to-Golgi protein trafficking assessed using VSVG. This was not secondary to ER stress, since thapsigargin, which compromises chaperone function by depletion of ER calcium, markedly inhibited VSVG folding and promoted strong ER stress but only slightly reduced protein trafficking. Blockade of ER-to-Golgi protein trafficking with brefeldin A (BFA) was sufficient to trigger ER stress, but neither BFA nor palmitate compromised VSVG folding.

CONCLUSIONS/INTERPRETATION

Reductions in ER-to-Golgi protein trafficking potentially contribute to ER stress during lipoapoptosis. In this case ER stress would be triggered by protein overload, rather than a disruption of the protein-folding capacity of the ER.

p38 MAPK is a major regulator of MafA protein stability under oxidative stress

Mammalian MafA/RIPE3b1 is an important glucose-responsive transcription factor that regulates function, maturation, and survival of beta-cells. Increased expression of MafA results in improved glucose-stimulated insulin secretion and beta-cell function. Because MafA is a highly phosphorylated protein, we examined whether regulating activity of protein kinases can increase MafA expression by enhancing its stability. We demonstrate that MafA protein stability in MIN6 cells and isolated mouse islets is regulated by both p38 MAPK and glycogen synthase kinase 3. Inhibiting p38 MAPK enhanced MafA stability in cells grown under both low and high concentrations of glucose. We also show that the N-terminal domain of MafA plays a major role in p38 MAPK-mediated degradation; simultaneous mutation of both threonines 57 and 134 into alanines in MafA was sufficient to prevent this degradation. Under oxidative stress, a condition detrimental to beta-cell function, a decrease in MafA stability was associated with a concomitant increase in active p38 MAPK. Interestingly, inhibiting p38 MAPK but not glycogen synthase kinase 3 prevented oxidative stress-dependent degradation of MafA. These results suggest that the p38 MAPK pathway may represent a common mechanism for regulating MafA levels under oxidative stress and basal and stimulatory glucose concentrations. Therefore, preventing p38 MAPK-mediated degradation of MafA represents a novel approach to improve beta-cell function.

Cytokine-induced beta-cell death is independent of endoplasmic reticulum stress signaling

OBJECTIVE

Cytokines contribute to beta-cell destruction in type 1 diabetes. Endoplasmic reticulum (ER) stress-mediated apoptosis has been proposed as a mechanism for beta-cell death. We tested whether ER stress was necessary for cytokine-induced beta-cell death and also whether ER stress gene activation was present in beta-cells of the NOD mouse model of type 1 diabetes.

RESEARCH DESIGN AND METHODS

INS-1 beta-cells or rat islets were treated with the chemical chaperone phenyl butyric acid (PBA) and exposed or not to interleukin (IL)-1beta and gamma-interferon (IFN-gamma). Small interfering RNA (siRNA) was used to silence C/EBP homologous protein (CHOP) expression in INS-1 beta-cells. Additionally, the role of ER stress in lipid-induced cell death was assessed.

RESULTS

Cytokines and palmitate triggered ER stress in beta-cells as evidenced by increased phosphorylation of PKR-like ER kinase (PERK), eukaryotic initiation factor (EIF)2alpha, and Jun NH(2)-terminal kinase (JNK) and increased expression of activating transcription factor (ATF)4 and CHOP. PBA treatment attenuated ER stress, but JNK phosphorylation was reduced only in response to palmitate, not in response to cytokines. PBA had no effect on cytokine-induced cell death but was associated with protection against palmitate-induced cell death. Similarly, siRNA-mediated reduction in CHOP expression protected against palmitate- but not against cytokine-induced cell death. In NOD islets, mRNA levels of several ER stress genes were reduced (ATF4, BiP [binding protein], GRP94 [glucose regulated protein 94], p58, and XBP-1 [X-box binding protein 1] splicing) or unchanged (CHOP and Edem1 [ER degradation enhancer, mannosidase alpha-like 1]).

CONCLUSIONS

While both cytokines and palmitate can induce ER stress, our results suggest that, in contrast to lipoapoptosis, the PERK-ATF4-CHOP ER stress-signaling pathway is not necessary for cytokine-induced beta-cell death.