Acute nutrient regulation of the unfolded protein response and integrated stress response in cultured rat pancreatic islets

ER stress as a sentinel mechanism for ER Ca2+ homeostasis

Endoplasmic reticulum (ER) stress is triggered upon the interference with oxidative protein folding that aims to produce fully folded, disulfide-bonded and glycosylated proteins, which are then competent to exit the ER. Many of the enzymes catalyzing this process require the binding of Ca2+ ions, including the chaperones BiP/GRP78, calnexin and calreticulin. The induction of ER stress with a variety of drugs interferes with chaperone Ca2+ binding, increases cytosolic Ca2+through the opening of ER Ca2+ channels, and activates store-operated Ca2+ entry (SOCE). Posttranslational modifications (PTMs) of the ER Ca2+ handling proteins through ER stress-dependent phosphorylation or oxidation control these mechanisms, as demonstrated in the case of the sarco/endoplasmic reticulum ATPase (SERCA), inositol 1,4,5 trisphosphate receptors (IP3Rs) or stromal interaction molecule 1 (STIM1). Their aim is to restore ER Ca2+ homeostasis but also to increase Ca2+ transfer from the ER to mitochondria during ER stress. This latter function boosts ER bioenergetics, but also triggers apoptosis if ER Ca2+ signaling persists. ER Ca2+ toolkit oxidative modifications upon ER stress can occur within the ER lumen or in the adjacent cytosol. Enzymes involved in this redox control include ER oxidoreductin 1 (ERO1) or the thioredoxin-family protein disulfide isomerases (PDI) and ERp57. A tight, but adaptive connection between ER Ca2+ content, ER stress and mitochondrial readouts allows for the proper functioning of many tissues, including skeletal muscle, the liver, and the pancreas, where ER stress either maintains or compromises their function, depending on its extent and context. Upon mutation of key regulators of ER Ca2+ signaling, diseases such as muscular defects (e.g., from mutated selenoprotein N, SEPN1/SELENON), or diabetes (e.g., from mutated PERK) are the result.

Independent activation of CREB3L2 by glucose fills a regulatory gap in mouse β-cells by co-ordinating insulin biosynthesis with secretory granule formation

OBJECTIVE

Although individual steps have been characterized, there is little understanding of the overall process whereby glucose co-ordinates the biosynthesis of insulin with its export out of the endoplasmic reticulum (ER) and incorporation into insulin secretory granules (ISGs). Here we investigate a role for the transcription factor CREB3L2 in this context.

METHODS

MIN6 cells and mouse islets were analysed by immunoblotting after treatment with glucose, fatty acids, thapsigargin and various inhibitors. Knockdown of CREB3L2 was achieved using si or sh constructs by transfection, or viral delivery. In vivo metabolic phenotyping was conducted after deletion of CREB3L2 in β-cells of adult mice using Ins1-CreER+. Islets were isolated for RNAseq and assays of glucose-stimulated insulin secretion (GSIS). Trafficking was monitored in islet monolayers using a GFP-tagged proinsulin construct that allows for synchronised release from the ER.

RESULTS

With a Km ≈3.5 mM, glucose rapidly (T1/2 0.9 h) increased full length (FL) CREB3L2 followed by a slower rise (T1/2 2.5 h) in its transcriptionally-active cleavage product, P60 CREB3L2. Glucose stimulation repressed the ER stress marker, CHOP, and this was partially reverted by knockdown of CREB3L2. Activation of CREB3L2 by glucose was not due to ER stress, however, but a combination of O-GlcNAcylation, which impaired proteasomal degradation of FL-CREB3L2, and mTORC1 stimulation, which enhanced its conversion to P60. cAMP generation also activated CREB3L2, but independently of glucose. Deletion of CREB3L2 inhibited GSIS ex vivo and, following a high-fat diet (HFD), impaired glucose tolerance and insulin secretion in vivo. RNAseq revealed that CREB3L2 regulated genes controlling trafficking to-and-from the Golgi, as well as a broader cohort associated with β-cell compensation during a HFD. Although post-Golgi trafficking appeared intact, knockdown of CREB3L2 impaired the generation of both nascent ISGs and proinsulin condensates in the Golgi, implying a defect in ER export of proinsulin and/or its processing in the Golgi.

CONCLUSION

The stimulation of CREB3L2 by glucose defines a novel, rapid and direct mechanism for co-ordinating the synthesis, packaging and storage of insulin, thereby minimizing ER overload and optimizing β-cell function under conditions of high secretory demand. Upregulation of CREB3L2 also potentially contributes to the benefits of GLP1 agonism and might in itself constitute a novel means of treating β-cell failure.

Nutrient-dependent regulation of β-cell proinsulin content

Insulin is made from proinsulin, but the extent to which fasting/feeding controls the homeostatically regulated proinsulin pool in pancreatic β-cells remains largely unknown. Here, we first examined β-cell lines (INS1E and Min6, which proliferate slowly and are routinely fed fresh medium every 2-3 days) and found that the proinsulin pool size responds to each feeding within 1 to 2 h, affected both by the quantity of fresh nutrients and the frequency with which they are provided. We observed no effect of nutrient feeding on the overall rate of proinsulin turnover as quantified from cycloheximide-chase experiments. We show that nutrient feeding is primarily linked to rapid dephosphorylation of translation initiation factor eIF2α, presaging increased proinsulin levels (and thereafter, insulin levels), followed by its rephosphorylation during the ensuing hours that correspond to a fall in proinsulin levels. The decline of proinsulin levels is blunted by the integrated stress response inhibitor, ISRIB, or by inhibition of eIF2α rephosphorylation with a general control nonderepressible 2 (not PERK) kinase inhibitor. In addition, we demonstrate that amino acids contribute importantly to the proinsulin pool; mass spectrometry shows that β-cells avidly consume extracellular glutamine, serine, and cysteine. Finally, we show that in both rodent and human pancreatic islets, fresh nutrient availability dynamically increases preproinsulin, which can be quantified without pulse-labeling. Thus, the proinsulin available for insulin biosynthesis is rhythmically controlled by fasting/feeding cycles.

A highly sensitive strategy for monitoring real-time proliferation of targeted cell types in vivo

Cell proliferation processes play pivotal roles in timely adaptation to many biological situations. Herein, we establish a highly sensitive and simple strategy by which time-series showing the proliferation of a targeted cell type can be quantitatively monitored in vivo in the same individuals. We generate mice expressing a secreted type of luciferase only in cells producing Cre under the control of the Ki67 promoter. Crossing these with tissue-specific Cre-expressing mice allows us to monitor the proliferation time course of pancreatic β-cells, which are few in number and weakly proliferative, by measuring plasma luciferase activity. Physiological time courses, during obesity development, pregnancy and juvenile growth, as well as diurnal variation, of β-cell proliferation, are clearly detected. Moreover, this strategy can be utilized for highly sensitive ex vivo screening for proliferative factors for targeted cells. Thus, these technologies may contribute to advancements in broad areas of biological and medical research.

Dysfunctions, molecular mechanisms, and therapeutic strategies of pancreatic β-cells in diabetes

Pancreatic beta-cell death has been established as a critical mediator in the progression of type 1 and type 2 diabetes mellitus. Beta-cell death is associated with exacerbating hyperglycemia and insulin resistance and paves the way for the progression of DM and its complications. Apoptosis has been considered the primary mechanism of beta-cell death in diabetes. However, recent pieces of evidence have implicated the substantial involvement of several other novel modes of cell death, including autophagy, pyroptosis, necroptosis, and ferroptosis. These distinct mechanisms are characterized by their unique biochemical features and often precipitate damage through the induction of cellular stressors, including endoplasmic reticulum stress, oxidative stress, and inflammation. Experimental studies were identified from PubMed literature on different modes of beta cell death during the onset of diabetes mellitus. This review summarizes current knowledge on the crucial pathways implicated in pancreatic beta cell death. The article also focuses on applying natural compounds as potential treatment strategies in inhibiting these cell death pathways.

Beta cell endoplasmic reticulum stress drives diabetes in the KINGS mouse without causing mass beta cell loss

AIMS

Beta cell endoplasmic reticulum (ER) stress can cause cellular death and dysfunction and has been implicated in the pathogenesis of diabetes. Animal models of beta cell ER stress are critical in further understanding this and for testing novel diabetes therapeutics. The KINGS mouse is a model of beta cell ER stress driven by a heterozygous mutation in Ins2. In this study, we investigated how beta cell ER stress in the KINGS mouse drives diabetes.

METHODS

We investigated whether the unfolded protein response (UPR) was activated in islets isolated from male and female KINGS mice and whether this impacted beta cell mass and turnover.

RESULTS

Whilst the UPR was up-regulated in KINGS islets, with increased protein expression of markers of all three UPR arms, this was not associated with a mass loss of beta cells; beta cell apoptosis rates did not increase until after the development of overt diabetes, and did not lead to substantial changes in beta cell mass.

CONCLUSION

We propose that the KINGS mouse represents a model where beta cell maladaptive UPR signalling drives diabetes development without causing mass beta cell loss.

Destabilization of β Cell FIT2 by saturated fatty acids alter lipid droplet numbers and contribute to ER stress and diabetes

SignificanceWith obesity on the rise, there is a growing appreciation for intracellular lipid droplet (LD) regulation. Here, we show how saturated fatty acids (SFAs) reduce fat storage-inducing transmembrane protein 2 (FIT2)-facilitated, pancreatic β cell LD biogenesis, which in turn induces β cell dysfunction and death, leading to diabetes. This mechanism involves direct acylation of FIT2 cysteine residues, which then marks the FIT2 protein for endoplasmic reticulum (ER)-associated degradation. Loss of β cell FIT2 and LDs reduces insulin secretion, increases intracellular ceramides, stimulates ER stress, and exacerbates diet-induced diabetes in mice. While palmitate and stearate degrade FIT2, unsaturated fatty acids such as palmitoleate and oleate do not, results of which extend to nutrition and diabetes.

Ischemic brain injury in diabetes and endoplasmic reticulum stress

Diabetes is a widespread disease characterized by high blood glucose levels due to abnormal insulin activity, production, or both. Chronic diabetes causes many secondary complications including cardiovascular disease: a life-threatening complication. Cerebral ischemia-related mortality, morbidity, and the extent of brain injury are high in diabetes. However, the mechanism of increase in ischemic brain injury during diabetes is not well understood. Multiple mechanisms mediate diabetic hyperglycemia and hypoglycemia-induced increase in ischemic brain injury. Endoplasmic reticulum (ER) stress mediates both brain injury as well as brain protection after ischemia-reperfusion injury. The pathways of ER stress are modulated during diabetes. Free radical generation and mitochondrial dysfunction, two of the prominent mechanisms that mediate diabetic increase in ischemic brain injury, are known to stimulate the pathways of ER stress. Increased ischemic brain injury in diabetes is accompanied by a further increase in the activation of ER stress. As there are many metabolic changes associated with diabetes, differential activation of the pathways of ER stress may mediate pronounced ischemic brain injury in subjects suffering from diabetes. We presently discuss the literature on the significance of ER stress in mediating increased ischemia-reperfusion injury in diabetes.

Chemical Chaperone PBA Attenuates ER Stress and Upregulates SOCS3 Expression as a Regulator of Leptin Signaling

Endoplasmic reticulum (ER) is very sensitive to the nutritional and energy states of the cells. Disruption of ER homeostasis leads to the accumulation of unfolded/misfolded proteins in the ER lumen, which is defined as ER stress. ER stress triggers the unfolded protein response (UPR). It is suggested that chronic ER stress is associated with obesity and leptin resistance. We investigated the role of ER stress and the effect of the ER stress inhibitor phenylbutyric acid (PBA) of ER stress, in obesity, as well as their impact on leptin signaling. This study involved twenty-four lean and twenty-four leptin-deficient (ob/ob) mice divided into PBA- and vehicle-treated groups. Pancreatic islets were isolated, incubated with leptin for 48 h, and assayed for the expression of CHOP and XBP1s (UPR signaling indicators) and SOCS3 (regulator of leptin signaling) by RT-qPCR. The expression levels of XBP1s and CHOP were markedly increased in the ob/ob controls compared to other groups with and without leptin treatment. No significant differences in the XBP1s and CHOP expression levels were found between the PBA-treated ob/ob and lean mice. SOCS3 expression was significantly upregulated in the PBA-treated ob/ob mice compared to the ob/ob controls after leptin treatment; but no significant difference in the SOCS3 expression was found between the PBA-treated ob/ob and lean mice with and without leptin treatment. Our findings suggested that ER stress plays an important role in the pathology of obesity, while PBA reduces ER stress and may potentially ameliorate leptin signaling.

Potential Roles of Endoplasmic Reticulum Stress and Cellular Proteins Implicated in Diabesity

The role of the endoplasmic reticulum (ER) has evolved from protein synthesis, processing, and other secretory pathways to forming a foundation for lipid biosynthesis and other metabolic functions. Maintaining ER homeostasis is essential for normal cellular function and survival. An imbalance in the ER implied stressful conditions such as metabolic distress, which activates a protective process called unfolded protein response (UPR). This response is activated through some canonical branches of ER stress, i.e., the protein kinase RNA-like endoplasmic reticulum kinase (PERK), inositol-requiring enzyme 1α (IRE1α), and activating transcription factor 6 (ATF6). Therefore, chronic hyperglycemia, hyperinsulinemia, increased proinflammatory cytokines, and free fatty acids (FFAs) found in diabesity (a pathophysiological link between obesity and diabetes) could lead to ER stress. However, limited data exist regarding ER stress and its association with diabesity, particularly the implicated proteins and molecular mechanisms. Thus, this review highlights the role of ER stress in relation to some proteins involved in diabesity pathogenesis and provides insight into possible pathways that could serve as novel targets for therapeutic intervention.

A New Hypothesis for Type 1 Diabetes Risk: The At-Risk Allele at rs3842753 Associates With Increased Beta-cell INS Messenger RNA in a Meta-Analysis of Single-Cell RNA-Sequencing Data

OBJECTIVES

Type 1 diabetes is characterized by the autoimmune destruction of insulin-secreting beta cells. Genetic variants upstream at the insulin (INS) locus contribute to ∼10% of type 1 diabetes heritable risk. Previous studies showed an association between rs3842753 C/C genotype and type 1 diabetes susceptibility, but the molecular mechanisms remain unclear. To date, no large-scale studies have looked at the effect of genetic variation at rs3842753 on INS mRNA at the single-cell level.

METHODS

We aligned all human islet single-cell RNA sequencing data sets available to us in year 2020 to the reference genome GRCh38.98 and genotyped rs3842753, integrating 2,315 β cells and 1,223 β-like cells from 13 A/A protected donors, 23 A/C heterozygous donors and 35 C/C at-risk donors, including adults without diabetes and with type 2 diabetes.

RESULTS

INS expression mean and variance were significantly higher in single β cells from females compared with males. On comparing across β cells and β-like cells, we found that rs3842753 C‒containing cells (either homozygous or heterozygous) had the highest INS expression. We also found that β cells with the rs3842753 C allele had significantly higher endoplasmic reticulum stress marker gene expression compared with the A/A homozygous genotype.

CONCLUSIONS

These findings support the emerging concept that inherited risk of type 1 diabetes may be associated with inborn, persistent elevated insulin production, which may lead to β-cell endoplasmic reticulum stress and fragility.

Androgens impair β-cell function in a mouse model of polycystic ovary syndrome by activating endoplasmic reticulum stress

BACKGROUND

Androgens excess results in endoplasmic reticulum (ER) stress, which is an important cause of β cells dysfunction. Here, we investigated the molecular regulation of androgens excess, ER stress, and β-cell function in polycystic ovary syndrome (PCOS).

METHODS

PCOS mouse model was established by injection of DHEA. Primary cultured mouse islets were used to detect testosterone (TE)-induced ER stress. The response of ER stress, apoptosis, and hyperinsulinemia were analyzed in INS-1 cells with or without TE exposure. Androgen receptor (AR) antagonist and ER stress inhibitor treatment was performed to evaluate the role of TE in ER stress and proinsulin secretion of PCOS mice.

RESULTS

PCOS mice had higher ER stress in islets. TE exposure induced ER stress and apoptosis significantly through sustaining insulin overexpression in β cells, which in turn impaired proinsulin maturation and secretion. Blocking this process could significantly relieve ER stress and apoptosis and improve insulin homeostasis.

CONCLUSION

ER stress activated by androgens excess in PCOS contributes to β cell dysfunction and hyperinsulinemia.

Biosynthetic Activity Differs Between Islet Cell Types and in Beta Cells Is Modulated by Glucose and Not by Secretion

A correct biosynthetic activity is thought to be essential for the long-term function and survival of islet cells in culture and possibly also after islet transplantation. Compared to the secretory activity, biosynthetic activity has been poorly studied in pancreatic islet cells. Here we aimed to assess biosynthetic activity at the single cell level to investigate if protein synthesis is dependent on secretagogues and increased as a consequence of hormonal secretion. Biosynthetic activity in rat islet cells was studied at the single cell level using O-propargyl-puromycin (OPP) that incorporates into newly translated proteins and chemically ligates to a fluorescent dye by "click" reaction. Heterogeneous biosynthetic activity was observed between the four islet cell types, with delta cells showing the higher relative protein biosynthesis. Beta cells protein biosynthesis was increased in response to glucose while 3-isobutyl-1-methylxanthine and phorbol-12-myristate-13-acetate, 2 drugs known to stimulate insulin secretion, had no similar effect on protein biosynthesis. However, after several hours of secretion, protein biosynthesis remained high even when cells were challenged to basal conditions. These results suggest that mechanisms regulating secretion and biosynthesis in islet cells are different, with glucose directly triggering beta cells protein biosynthesis, independently of insulin secretion. Furthermore, this OPP labeling approach is a promising method to identify newly synthesized proteins under various physiological and pathological conditions.

Pathological β-Cell Endoplasmic Reticulum Stress in Type 2 Diabetes: Current Evidence

The notion that in diabetes pancreatic β-cells express endoplasmic reticulum (ER) stress markers indicative of increased unfolded protein response (UPR) signaling is no longer in doubt. However, what remains controversial is whether this increase in ER stress response actually contributes importantly to the β-cell failure of type 2 diabetes (akin to 'terminal UPR'), or whether it represents a coping mechanism that represents the best attempt of β-cells to adapt to changes in metabolic demands as presented by disease progression. Here an intercontinental group of experts review evidence for the role of ER stress in monogenic and type 2 diabetes in an attempt to reconcile these disparate views. Current evidence implies that pancreatic β-cells require a regulated UPR for their development, function and survival, as well as to maintain cellular homeostasis in response to protein misfolding stress. Prolonged ER stress signaling, however, can be detrimental to β-cells, highlighting the importance of "optimal" UPR for ER homeostasis, β-cell function and survival.

Developmental Programming and Glucolipotoxicity: Insights on Beta Cell Inflammation and Diabetes

Stimuli or insults during critical developmental transitions induce alterations in progeny anatomy, physiology, and metabolism that may be transient, sometimes reversible, but often durable, which defines programming. Glucolipotoxicity is the combined, synergistic, deleterious effect of simultaneously elevated glucose (chronic hyperglycemia) and saturated fatty acids (derived from high-fat diet overconsumption and subsequent metabolism) that are harmful to organs, micro-organs, and cells. Glucolipotoxicity induces beta cell death, dysfunction, and failure through endoplasmic reticulum and oxidative stress and inflammation. In beta cells, the misfolding of pro/insulin proteins beyond the cellular threshold triggers the unfolded protein response and endoplasmic reticulum stress. Consequentially there is incomplete and inadequate pro/insulin biosynthesis and impaired insulin secretion. Cellular stress triggers cellular inflammation, where immune cells migrate to, infiltrate, and amplify in beta cells, leading to beta cell inflammation. Endoplasmic reticulum stress reciprocally induces beta cell inflammation, whereas beta cell inflammation can self-activate and further exacerbate its inflammation. These metabolic sequelae reflect the vicious cycle of beta cell stress and inflammation in the pathophysiology of diabetes.

Pancreatic β-cells in type 1 and type 2 diabetes mellitus: different pathways to failure

Loss of functional β-cell mass is the key mechanism leading to the two main forms of diabetes mellitus - type 1 diabetes mellitus (T1DM) and type 2 diabetes mellitus (T2DM). Understanding the mechanisms behind β-cell failure is critical to prevent or revert disease. Basic pathogenic differences exist in the two forms of diabetes mellitus; T1DM is immune mediated and T2DM is mediated by metabolic mechanisms. These mechanisms differentially affect early β-cell dysfunction and eventual fate. Over the past decade, major advances have been made in the field, mostly delivered by studies on β-cells in human disease. These advances include studies of islet morphology and human β-cell gene expression in T1DM and T2DM, the identification and characterization of the role of T1DM and T2DM candidate genes at the β-cell level and the endoplasmic reticulum stress signalling that contributes to β-cell failure in T1DM (mostly IRE1 driven) and T2DM (mostly PERK-eIF2α dependent). Here, we review these new findings, focusing on studies performed on human β-cells or on samples obtained from patients with diabetes mellitus.

Mitochondrial pyruvate and fatty acid flux modulate MICU1-dependent control of MCU activity

The tricarboxylic acid (TCA) cycle converts the end products of glycolysis and fatty acid β-oxidation into the reducing equivalents NADH and FADH2 Although mitochondrial matrix uptake of Ca2+ enhances ATP production, it remains unclear whether deprivation of mitochondrial TCA substrates alters mitochondrial Ca2+ flux. We investigated the effect of TCA cycle substrates on MCU-mediated mitochondrial matrix uptake of Ca2+, mitochondrial bioenergetics, and autophagic flux. Inhibition of glycolysis, mitochondrial pyruvate transport, or mitochondrial fatty acid transport triggered expression of the MCU gatekeeper MICU1 but not the MCU core subunit. Knockdown of mitochondrial pyruvate carrier (MPC) isoforms or expression of the dominant negative mutant MPC1R97W resulted in increased MICU1 protein abundance and inhibition of MCU-mediated mitochondrial matrix uptake of Ca2+ We also found that genetic ablation of MPC1 in hepatocytes and mouse embryonic fibroblasts resulted in reduced resting matrix Ca2+, likely because of increased MICU1 expression, but resulted in changes in mitochondrial morphology. TCA cycle substrate-dependent MICU1 expression was mediated by the transcription factor early growth response 1 (EGR1). Blocking mitochondrial pyruvate or fatty acid flux was linked to increased autophagy marker abundance. These studies reveal a mechanism that controls the MCU-mediated Ca2+ flux machinery and that depends on TCA cycle substrate availability. This mechanism generates a metabolic homeostatic circuit that protects cells from bioenergetic crisis and mitochondrial Ca2+ overload during periods of nutrient stress.

HDL inhibits endoplasmic reticulum stress-induced apoptosis of pancreatic β-cells in vitro by activation of Smoothened

Loss of pancreatic β-cell mass and function as a result of sustained ER stress is a core step in the pathogenesis of diabetes mellitus type 2. The complex control of β-cells and insulin production involves hedgehog (Hh) signaling pathways as well as cholesterol-mediated effects. In fact, data from studies in humans and animal models suggest that HDL protects against the development of diabetes through inhibition of ER stress and β-cell apoptosis. We investigated the mechanism by which HDL inhibits ER stress and apoptosis induced by thapsigargin, a sarco/ER Ca²⁺-ATPase inhibitor, in β-cells of a rat insulinoma cell line, INS1e. We further explored effects on the Hh signaling receptor Smoothened (SMO) with pharmacologic agonists and inhibitors. Interference with sterol synthesis or efflux enhanced β-cell apoptosis and abrogated the anti-apoptotic activity of HDL. During ER stress, HDL facilitated the efflux of specific oxysterols, including 24-hydroxycholesterol (OHC). Supplementation of reconstituted HDL with 24-OHC enhanced and, in cells lacking ABCG1 or the 24-OHC synthesizing enzyme CYP46A1, restored the protective activity of HDL. Inhibition of SMO countered the beneficial effects of HDL and also LDL, and SMO agonists decreased β-cell apoptosis in the absence of ABCG1 or CYP46A1. The translocation of the SMO-activated transcription factor glioma-associated oncogene GLI-1 was inhibited by ER stress but restored by both HDL and 24-OHC. In conclusion, the protective effect of HDL to counter ER stress and β-cell death involves the transport, generation, and mobilization of oxysterols for activation of the Hh signaling receptor SMO.

Impact of nutrient overload on metabolic homeostasis

Nutrient overload occurs worldwide as a consequence of the modern diet pattern and the physical inactivity that sometimes accompanies it. Cells initiate multiple protective mechanisms to adapt to elevated intracellular metabolites and restore metabolic homeostasis, but irreversible injury to the cells can occur in the event of prolonged nutrient overload. Many studies have advanced the understanding of the different detrimental effects of nutrient overload; however, few reports have made connections and given the full picture of the impact of nutrient overload on cellular metabolism. In this review, detailed changes in metabolic and energy homeostasis caused by chronic nutrient overload, as well as their associations with the development of metabolic disorders, are discussed. Overnutrition-induced changes in key organelles and sensors rewire cellular bioenergetic pathways and facilitate the shift of the metabolic state toward biosynthesis, thereby leading to the onset of various metabolic disorders, which are essentially the downstream manifestations of a misbalanced metabolic equilibrium. Based on these mechanisms, potential therapeutic targets for metabolic disorders and new research directions are proposed.

Phlda3 regulates beta cell survival during stress

The loss of functional beta cell mass characterises all forms of diabetes. Beta cells are highly susceptible to stress, including cytokine, endoplasmic reticulum (ER) and oxidative stress. This study examined the role of pleckstrin homology-like, domain family A, member 3 (Phlda3) in beta cell survival under stress conditions and the regulatory basis. We found that the mRNA levels of Phlda3 were markedly upregulated in vivo in the islets of diabetic humans and mice. In vitro, exposure of MIN6 cells or islets to cytokines, palmitate, thapsigargin or ribose upregulated Phlda3 mRNA and protein levels, concurrent with the induction of ER stress (Ddit3 and Trb3) and antioxidant (Hmox1) genes. Furthermore, H₂O₂ treatment markedly increased PHLDA3 immunostaining in human islets. Phlda3 expression was differentially regulated by adaptive (Xbp1) and apoptotic (Ddit3) unfolded protein response (UPR) mediators. siRNA-mediated knockdown of Xbp1 inhibited the induction of Phlda3 by cytokines and palmitate, whereas knockdown of Ddit3 upregulated Phlda3. Moreover, knockdown of Phlda3 potentiated cytokine-induced apoptosis in association with upregulation of inflammatory genes (iNos, IL1β and IκBα) and NFκB phosphorylation and downregulation of antioxidant (Gpx1 and Srxn1) and adaptive UPR (Xbp1, Hspa5 and Fkbp11) genes. Knockdown of Phlda3 also potentiated apoptosis under oxidative stress conditions induced by ribose treatment. These findings suggest that Phlda3 is crucial for beta cell survival under stress conditions. Phlda3 regulates the cytokine, oxidative and ER stress responses in beta cells via the repression of inflammatory gene expression and the maintenance of antioxidant and adaptive UPR gene expression. Phlda3 may promote beta cell survival in diabetes.

eIF5A inhibition influences T cell dynamics in the pancreatic microenvironment of the humanized mouse model of Type 1 Diabetes

We have developed a transgenic mouse model of Type 1 Diabetes (T1D) in which human GAD65 is expressed in pancreatic β-cells, and human MHC-II is expressed on antigen presenting cells. Induced GAD65 antigen presentation activates T-cells, which initiates the downstream events leading to diabetes. In our humanized mice, we have shown downregulation of eukaryotic translation initiation factor 5 A (elF5A), expressed only in actively dividing mammalian cells. In-vivo inhibition of elF5A hypusination by deoxyhypusine synthase (DHS) inhibitor "GC7" was studied; DHS inhibitor alters the pathophysiology in our mouse model by catalyzing the crucial hypusination and the rate-limiting step of elF5A activation. In our mouse model, we have shown that inhibition of eIF5A resets the pro-inflammatory bias in the pancreatic microenvironment. There was: (a) reduction of Th1/Th17 response, (b) an increase in Treg numbers, (c) debase in IL17 and IL21 cytokines levels in serum, (d) lowering of anti-GAD65 antibodies, and (e) ablation of the ER stress that improved functionality of the β-cells, but minimal effect on the cytotoxic CD8 T-cell (CTL) mediated response. Conclusively, immune modulation, in the case of T1D, may help to manipulate inflammatory responses, decreasing disease severity, and may help manage T1D in early stages of disease. Our study also demonstrates that without manipulating the CTLs mediated response extensively, it is difficult to treat T1D.

MANF Is Required for the Postnatal Expansion and Maintenance of Pancreatic β-Cell Mass in Mice

Global lack of mesencephalic astrocyte-derived neurotropic factor (MANF) leads to progressive postnatal loss of β-cell mass and insulin-dependent diabetes in mice. Similar to Manf^-/- mice, embryonic ablation of MANF specifically from the pancreas results in diabetes. In this study, we assessed the importance of MANF for the postnatal expansion of pancreatic β-cell mass and for adult β-cell maintenance in mice. Detailed analysis of Pdx-1Cre^+/- ::Manf^fl/fl mice revealed mosaic MANF expression in postnatal pancreata and a significant correlation between the number of MANF-positive β-cells and β-cell mass in individual mice. In vitro, recombinant MANF induced β-cell proliferation in islets from aged mice and protected from hyperglycemia-induced endoplasmic reticulum (ER) stress. Consequently, excision of MANF from β-cells of adult MIP-1Cre^ERT::Manf^fl/fl mice resulted in reduced β-cell mass and diabetes caused largely by β-cell ER stress and apoptosis, possibly accompanied by β-cell dedifferentiation and reduced rates of β-cell proliferation. Thus, MANF expression in adult mouse β-cells is needed for their maintenance in vivo. We also revealed a mechanistic link between ER stress and inflammatory signaling pathways leading to β-cell death in the absence of MANF. Hence, MANF might be a potential target for regenerative therapy in diabetes.

Glucose-Induced β-Cell Dysfunction In Vivo: Evidence for a Causal Role of C-jun N-terminal Kinase Pathway

Prolonged elevation of glucose can adversely affect β-cell function. Oxidative stress, which has been implicated in glucose-induced β-cell dysfunction, can activate c-jun N-terminal kinase (JNK). However, whether JNK is causal in glucose-induced β-cell dysfunction in vivo is unclear. Therefore, we aimed at investigating the causal role of JNK activation in in vivo models of glucose-induced β-cell dysfunction. Glucose-induced β-cell dysfunction was investigated in the presence or absence of JNK inhibition. JNK inhibition was achieved using either (i) the JNK-specific inhibitor SP600125 or (ii) JNK-1-null mice. (i) Rats or mice were infused intravenously with saline or glucose with or without SP600125. (ii) JNK-1 null mice and their littermate wild-type controls were infused intravenously with saline or glucose. Following the glucose infusion periods in rats and mice, β-cell function was assessed in isolated islets or in vivo using hyperglycemic clamps. Forty-eight-hour hyperglycemia at ~20 mM in rats or 96-hour hyperglycemia at ~13 mM in mice impaired β-cell function in isolated islets and in vivo. Inhibition of JNK using either SP600125 or JNK-1-null mice prevented glucose-induced β-cell dysfunction in isolated islets and in vivo. Islets of JNK-1-null mice exposed to hyperglycemia in vivo showed an increase in Pdx-1 and insulin 2 mRNA, whereas islets of wild-type mice did not. Together, these data show that JNK pathway is involved in glucose-induced β-cell dysfunction in vivo and is thus a potential therapeutic target for type 2 diabetes.

Emerging Roles for Mesencephalic Astrocyte-Derived Neurotrophic Factor (MANF) in Pancreatic Beta Cells and Diabetes

Mesencephalic astrocyte-derived neurotrophic factor (MANF) was originally identified as a secreted trophic factor for dopamine neurons in vitro. It protects and restores damaged cells in rodent models of Parkinson's disease, brain and heart ischemia, spinocerebellar ataxia and retina in vivo. However, its exact mechanism of action is not known. MANF is widely expressed in most human and mouse organs with high levels in secretory tissues. Intracellularly, MANF localizes to the endoplasmic reticulum (ER) and ER stress increases it's expression in cells and tissues. Furthermore, increased MANF levels has been detected in the sera of young children with newly diagnosed Type 1 (T1D) diabetes and Type 2 (T2D) diabetic patients. ER stress is caused by the accumulation of misfolded and aggregated proteins in the ER. It activates a cellular defense mechanism, the unfolded protein response (UPR), a signaling cascade trying to restore ER homeostasis. However, if prolonged, unresolved ER stress leads to apoptosis. Unresolved ER stress contributes to the progressive death of pancreatic insulin-producing beta cells in both T1D and T2D. Diabetes mellitus is characterized by hyperglycemia, caused by the inability of the beta cells to maintain sufficient levels of circulating insulin. The current medications, insulin and antidiabetic drugs, alleviate diabetic symptoms but cannot reconstitute physiological insulin secretion which increases the risk of devastating vascular complications of the disease. Thus, one of the main strategies in improving current diabetes therapy is to define and validate novel approaches to protect beta cells from stress as well as activate their regeneration. Embryonic deletion of the Manf gene in mice led to gradual postnatal development of insulin-deficient diabetes caused by reduced beta cell proliferation and increased beta cell death due to increased and sustained ER stress. In vitro, recombinant MANF partly protected mouse and human beta cells from ER stress-induced beta cell death and potentiated mouse and human beta cell proliferation. Importantly, in vivo overexpression of MANF in the pancreas of T1D mice led to increased beta cell proliferation and decreased beta cell death, suggesting that MANF could be a new therapeutic candidate for beta cell protection and regeneration in diabetes.

β-Cell Autophagy in Diabetes Pathogenesis

Nearly 100 years have passed since Frederick Banting and Charles Best first discovered and purified insulin. Their discovery and subsequent improvements revolutionized the treatment of diabetes, and the field continues to move at an ever-faster pace with respect to unique treatments for both type 1 and type 2 diabetes. Despite these advances, we still do not fully understand how apoptosis of the insulin-producing β-cells is triggered, presenting a challenge in the development of preventative measures. In recent years, the process of autophagy has generated substantial interest in this realm due to discoveries highlighting its clear role in the maintenance of cellular homeostasis. As a result, the number of studies focused on islet and β-cell autophagy has increased substantially in recent years. In this review, we will discuss what is currently known regarding the role of β-cell autophagy in type 1 and type 2 diabetes pathogenesis, with an emphasis on new and exciting developments over the past 5 years. Further, we will discuss how these discoveries might be translated into unique treatments in the coming years.

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.

Stress-Induced MicroRNA-708 Impairs β-Cell Function and Growth

The pancreatic β-cell transcriptome is highly sensitive to external signals such as glucose oscillations and stress cues. MicroRNAs (miRNAs) have emerged as key factors in gene expression regulation. Here, we aimed to identify miRNAs that are modulated by glucose in mouse pancreatic islets. We identified miR-708 as the most upregulated miRNA in islets cultured at low glucose concentrations, a setting that triggers a strong stress response. miR-708 was also potently upregulated by triggering endoplasmic reticulum (ER) stress with thapsigargin and in islets of ob/ob mice. Low-glucose induction of miR-708 was blocked by treatment with the chemical chaperone 4-phenylbutyrate, uncovering the involvement of ER stress in this response. An integrative analysis identified neuronatin (Nnat) as a potential glucose-regulated target of miR-708. Indeed, Nnat expression was inversely correlated with miR-708 in islets cultured at different glucose concentrations and in ob/ob mouse islets and was reduced after miR-708 overexpression. Consistent with the role of Nnat in the secretory function of β-cells, miR-708 overexpression impaired glucose-stimulated insulin secretion (GSIS), which was recovered by NNAT overexpression. Moreover, miR-708 inhibition recovered GSIS in islets cultured at low glucose. Finally, miR-708 overexpression suppressed β-cell proliferation and induced β-cell apoptosis. Collectively, our results provide a novel mechanism of glucose regulation of β-cell function and growth by repressing stress-induced miR-708.

ER-resident antioxidative GPx7 and GPx8 enzyme isoforms protect insulin-secreting INS-1E β-cells against lipotoxicity by improving the ER antioxidative capacity

Increased circulating levels of saturated fatty acids (FFAs) and glucose are considered to be major mediators of β-cell dysfunction and death in T2DM. Although it has been proposed that endoplasmic reticulum (ER) and oxidative stress play a crucial role in gluco/lipotoxicity, their interplay and relative contribution to β-cell dysfunction and apoptosis has not been fully elucidated. In addition it is still unclear how palmitate - the physiologically most abundant long-chain saturated FFA - elicits ER stress and which immediate signals commit β-cells to apoptosis. To study the underlying mechanisms of palmitate-mediated ER stress and β-cell toxicity, we exploited the observation that the recently described ER-resident GPx7 and GPx8 are not expressed in rat β-cells. Expression of GPx7 or GPx8 attenuated FFAs-mediated H₂O₂ generation, ER stress, and apoptosis induction. These results could be confirmed by a H₂O₂-specific inactivating ER catalase, indicating that accumulation of H₂O₂ in the ER lumen is critical in FFA-induced ER stress. Furthermore, neither the expression of GPx7 nor of GPx8 increased insulin content or facilitated disulfide bond formation in insulin-secreting INS-1E cells. Hence, reduction of H₂O₂ by ER-GPx isoforms is not rate-limiting in oxidative protein folding in rat β-cells. These data suggest that FFA-mediated ER stress is partially dependent on oxidative stress and selective expression of GPx7 or GPx8 improves the ER antioxidative capacity of rat β-cells without compromising insulin production and the oxidative protein folding machinery.

Mitochondria-Associated Membranes Response to Nutrient Availability and Role in Metabolic Diseases

Metabolic diseases are associated with nutrient excess and metabolic inflexibility. Mitochondria and endoplasmic reticulum are important organelles and nutrient sensors, and their dysfunction has been extensively and independently implicated in metabolic diseases. Both organelles interact at sites known as mitochondria-associated membranes (MAMs), in order to exchange metabolites and calcium. Recent evidence indicates that MAM could be a hub of hepatic insulin signaling and nutrient sensing. In this review, we discuss the roles organelle function and communication play in the cell's adaptation to nutrient availability, in both physiology and metabolic diseases. We highlight how dynamic regulation of MAM affects mitochondria physiology and adaptation of cellular metabolism to nutrient availability, and how chronic MAM disruption participates in the metabolic inflexibility associated with metabolic disorders.

Culture at low glucose up-regulates mitochondrial function in pancreatic β cells with accompanying effects on viability

We tested whether exposure of β cells at reduced glucose leads to mitochondrial adaptions and whether such adaptions modulate effects of hypoxia. Rat islets, human islets and INS-1 832/13 cells were pre-cultured short term at half standard glucose concentrations (5.5 mM for rat islets and cells, 2.75 mM for human islets) without overtly negative effects on subsequently measured function (insulin secretion and cellular insulin contents) or on viability. Culture at half standard glucose upregulated complex I and tended to upregulate complex II in islets and INS-1 cells alike. An increased release of lactate dehydrogenase that followed exposure to hypoxia was attenuated in rat islets which had been pre-cultured at half standard glucose. In INS-1 cells exposure to half standard glucose attenuated hypoxia-induced effects on several viability parameters (MTT, cell number and incremental apoptotic DNA). Thus culture at reduced glucose of pancreatic islets and clonal β cells leads to mitochondrial adaptions which possibly lessen the negative impact of hypoxia on β cell viability. These findings appear relevant in the search for optimization of pre-transplant conditions in a clinical setting.

Endoplasmic Reticulum Stress Links Oxidative Stress to Impaired Pancreatic Beta-Cell Function Caused by Human Oxidized LDL

Elevated plasma concentration of the pro-atherogenic oxidized low density lipoprotein cholesterol (LDL) triggers adverse effects in pancreatic beta-cells and is associated with type 2 diabetes. Here, we investigated whether the endoplasmic reticulum (ER) stress is a key player coupling oxidative stress to beta-cell dysfunction and death elicited by human oxidized LDL. We found that human oxidized LDL activates ER stress as evidenced by the activation of the inositol requiring 1α, and the elevated expression of both DDIT3 (also called CHOP) and DNAJC3 (also called P58IPK) ER stress markers in isolated human islets and the mouse insulin secreting MIN6 cells. Silencing of Chop and inhibition of ER stress markers by the chemical chaperone phenyl butyric acid (PBA) prevented cell death caused by oxidized LDL. Finally, we found that oxidative stress accounts for activation of ER stress markers induced by oxidized LDL. Induction of Chop/CHOP and p58IPK/P58IPK by oxidized LDL was mimicked by hydrogen peroxide and was blocked by co-treatment with the N-acetylcystein antioxidant. As a conclusion, the harmful effects of oxidized LDL in beta-cells requires ER stress activation in a manner that involves oxidative stress. This mechanism may account for impaired beta-cell function in diabetes and can be reversed by antioxidant treatment.

IRS1 deficiency protects β-cells against ER stress-induced apoptosis by modulating sXBP-1 stability and protein translation

Endoplasmic reticulum (ER) stress is among several pathological features that underlie β-cell failure in the development of type 1 and type 2 diabetes. Adaptor proteins in the insulin/insulin-like-growth factor-1 signaling pathways, such as insulin receptor substrate-1 (IRS1) and IRS2, differentially impact β-cell survival but the underlying mechanisms remain unclear. Here we report that β-cells deficient in IRS1 (IRS1KO) are resistant, while IRS2 deficiency (IRS2KO) makes them susceptible to ER stress-mediated apoptosis. IRS1KOs exhibited low nuclear accumulation of spliced XBP-1 due to its poor stability, in contrast to elevated accumulation in IRS2KO. The reduced nuclear accumulation in IRS1KO was due to protein instability of Xbp1 secondary to proteasomal degradation. IRS1KO also demonstrated an attenuation in their general translation status in response to ER stress revealed by polyribosomal profiling. Phosphorylation of eEF2 was dramatically increased in IRS1KO enabling the β-cells to adapt to ER stress by blocking translation. Furthermore, significantly high ER calcium (Ca²⁺) was detected in IRS1KO β-cells even upon induction of ER stress. These observations suggest that IRS1 could be a therapeutic target for β-cell protection against ER stress-mediated cell death by modulating XBP-1 stability, protein synthesis, and Ca²⁺ storage in the ER.

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

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

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

AIMS/HYPOTHESIS

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

METHODS

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

RESULTS

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

CONCLUSIONS/INTERPRETATION

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

Oxidative and endoplasmic reticulum stress in β-cell dysfunction in diabetes

The inability of pancreatic β-cells to make sufficient insulin to control blood sugar is a central feature of the aetiology of most forms of diabetes. In this review we focus on the deleterious effects of oxidative stress and endoplasmic reticulum (ER) stress on β-cell insulin biosynthesis and secretion and on inflammatory signalling and apoptosis with a particular emphasis on type 2 diabetes (T2D). We argue that oxidative stress and ER stress are closely entwined phenomena fundamentally involved in β-cell dysfunction by direct effects on insulin biosynthesis and due to consequences of the ER stress-induced unfolded protein response. We summarise evidence that, although these phenomenon can be driven by intrinsic β-cell defects in rare forms of diabetes, in T2D β-cell stress is driven by a range of local environmental factors including increased drivers of insulin biosynthesis, glucolipotoxicity and inflammatory cytokines. We describe our recent findings that a range of inflammatory cytokines contribute to β-cell stress in diabetes and our discovery that interleukin 22 protects β-cells from oxidative stress regardless of the environmental triggers and can correct much of diabetes pathophysiology in animal models. Finally we summarise evidence that β-cell dysfunction is reversible in T2D and discuss therapeutic opportunities for relieving oxidative and ER stress and restoring glycaemic control.

PDIA6 regulates insulin secretion by selectively inhibiting the RIDD activity of IRE1

Protein disulfide isomerase A6 (PDIA6) interacts with protein kinase RNA-like endoplasmic reticulum kinase (PERK) and inositol requiring enzyme (IRE)-1 and inhibits their unfolded protein response signaling. In this study, shRNA silencing of PDIA6 expression in insulin-producing mouse cells reduced insulin production (5-fold) and, consequently, glucose-stimulated insulin secretion (3-4-fold). This inhibition of insulin release was independent of the PDIA6-PERK interaction or PERK activity. Acute inhibition of PERK did not change the short-term response of β cells to glucose. Rather, PDIA6 affected insulin secretion by modulating one of the activities of IRE1. At 11 mM glucose and lower, the regulated IRE1-dependent decay (RIDD) of the mRNA activity of IRE1 was activated, but not its X-box binding protein (XBP)-1 splicing activity. In the absence of PDIA6, RIDD activity toward insulin transcripts was enhanced up to 4-fold, as shown by molecular assays in cultured cells and the use of a fluorescent reporter in intact islets. Such physiologic activation of IRE1 by glucose contrasted with IRE1 activation by chemical stress, when both IRE1 activities were induced. Thus, whereas the stimulus determines the quality of IRE1 signaling, PDIA6 attenuates multiple enzymatic activities of IRE1, maintaining its signaling within a physiologically tolerable range.

Glucokinase activation is beneficial or toxic to cultured rat pancreatic islets depending on the prevailing glucose concentration

In rat pancreatic islets, β-cell gene expression, survival, and subsequent acute glucose stimulation of insulin secretion (GSIS) are optimally preserved by prolonged culture at 10 mM glucose (G10) and markedly altered by culture at G5 or G30. Here, we tested whether pharmacological glucokinase (GK) activation prevents these alterations during culture or improves GSIS after culture. Rat pancreatic islets were cultured 1-7 days at G5, G10, or G30 with or without 3 μM of the GK activator Ro 28-0450 (Ro). After culture, β-cell apoptosis and islet gene mRNA levels were measured, and the acute glucose-induced increase in NAD(P)H autofluorescence, intracellular calcium concentration, and insulin secretion were tested in the absence or presence of Ro. Prolonged culture of rat islets at G5 or G30 instead of G10 triggered β-cell apoptosis and reduced their glucose responsiveness. Addition of Ro during culture differently affected β-cell survival and glucose responsiveness depending on the glucose concentration during culture: it was beneficial to β-cell survival and function at G5, detrimental at G10, and ineffective at G30. In contrast, acute GK activation with Ro increased the glucose sensitivity of islets cultured at G10 but failed at restoring β-cell glucose responsiveness after culture at G5 or G30. We conclude that pharmacological GK activation prevents the alteration of β-cell survival and function by long-term culture at G5 but mimics glucotoxicity when added to G10. The complex effects of glucose on the β-cell phenotype result from changes in glucose metabolism and not from an effect of glucose per se.

Insulin demand regulates β cell number via the unfolded protein response

Although stem cell populations mediate regeneration of rapid turnover tissues, such as skin, blood, and gut, a stem cell reservoir has not been identified for some slower turnover tissues, such as the pancreatic islet. Despite lacking identifiable stem cells, murine pancreatic β cell number expands in response to an increase in insulin demand. Lineage tracing shows that new β cells are generated from proliferation of mature, differentiated β cells; however, the mechanism by which these mature cells sense systemic insulin demand and initiate a proliferative response remains unknown. Here, we identified the β cell unfolded protein response (UPR), which senses insulin production, as a regulator of β cell proliferation. Using genetic and physiologic models, we determined that among the population of β cells, those with an active UPR are more likely to proliferate. Moreover, subthreshold endoplasmic reticulum stress (ER stress) drove insulin demand-induced β cell proliferation, through activation of ATF6. We also confirmed that the UPR regulates proliferation of human β cells, suggesting that therapeutic UPR modulation has potential to expand β cell mass in people at risk for diabetes. Together, this work defines a stem cell-independent model of tissue homeostasis, in which differentiated secretory cells use the UPR sensor to adapt organ size to meet demand.

Molecular Events Linking Oxidative Stress and Inflammation to Insulin Resistance and β-Cell Dysfunction

The prevalence of diabetes mellitus (DM) is increasing worldwide, a consequence of the alarming rise in obesity and metabolic syndrome (MetS). Oxidative stress and inflammation are key physiological and pathological events linking obesity, insulin resistance, and the progression of type 2 DM (T2DM). Unresolved inflammation alongside a “glucolipotoxic” environment of the pancreatic islets, in insulin resistant pathologies, enhances the infiltration of immune cells which through secretory activity cause dysfunction of insulin-secreting β-cells and ultimately cell death. Recent molecular investigations have revealed that mechanisms responsible for insulin resistance associated with T2DM are detected in conditions such as obesity and MetS, including impaired insulin receptor (IR) signalling in insulin responsive tissues, oxidative stress, and endoplasmic reticulum (ER) stress. The aim of the present review is to describe the evidence linking oxidative stress and inflammation with impairment of insulin secretion and action, which result in the progression of T2DM and other conditions associated with metabolic dysregulation.

Endoplasmic reticulum stress and Nrf2 repression in circulating cells of type 2 diabetic patients without the recommended glycemic goals

Endoplasmic reticulum (ER) stress plays a role in the pathogenesis of type 2 diabetes mellitus (T2DM), with activation of the unfolded protein response (UPR) and ER apoptosis in β-cells. The aim of the study is investigating the role of the prolonged glycemic, inflammatory, and oxidative impairment as possible UPR and ER apoptosis inductors in triggering the ER stress response and the protective nuclear erythroid-related factor 2 (Nrf2)/antioxidant-related element (ARE) activation in peripheral blood mononuclear cells (PBMC) of T2DM patients without glycemic target. Oxidative stress markers (oxidation product of phospholipid 1-palmitoyl-2-arachidonoyl-sn-glycero-3-phosphorylcholine [oxPAPC], and malondialdehyde [MDA]), the UPR and ER apoptosis, the activation of the pro-inflammatory nuclear factor-kappa B (NF-kB) with its inhibitory protein inhibitor-kBα, and the expression of the protective Nrf2 and heme oxygenase-1 (HO-1) were evaluated in PBMC of 15 T2DM patients and 15 healthy controls (C). OxPAPC concentrations (in PBMC and plasma), MDA levels (in plasma), the expressions of the glucose-regulated protein 78 kDa (or BiP) as representative of UPR, and of the CCAAT/enhancer-binding protein homologous protein as representative of ER apoptosis were significantly higher (p < 0.01) in T2DM with respect to C. IkBα expression was significantly lower (p < 0.01) in T2DM as well as Nrf2 and HO-1. In vitro experiments demonstrated that hyperglycemic conditions, if prolonged, were NF-kB inductors, without a corresponding Nrf2/ARE response. In PBMC of T2DM without glycemic target achievement, there is an activation of the UPR and of the ER apoptosis, which may be related to the chronic exposure to hyperglycemia, to the augmented inflammation, and to the augmented oxidative stress, without a corresponding Nrf2/ARE defense activation.

Lipotoxic endoplasmic reticulum stress, β cell failure, and type 2 diabetes mellitus

Failure of the unfolded protein response (UPR) to maintain optimal folding of pro-insulin in the endoplasmic reticulum (ER) leads to unresolved ER stress and β cell death. This contributes not only to some rare forms of diabetes, but also to type 2 diabetes mellitus (T2DM). Many key findings, elaborated over the past decade, are based on the lipotoxicity model, entailing chronic exposure of β cells to elevated levels of fatty acids (FAs). Here, we update recent progress on how FAs initiate ER stress, particularly via disruption of protein trafficking, and how this leads to apoptosis. We also highlight differences in how β cells are impacted by the classic UPR, versus the more selective UPR that arises as part of a broader response to lipotoxicity.

An islet in distress: β cell failure in type 2 diabetes

Over 200 million people worldwide suffer from diabetes, a disorder of glucose homeostasis. The majority of these individuals are diagnosed with type 2 diabetes. It has traditionally been thought that tissue resistance to the action of insulin is the primary defect in type 2 diabetes. However, recent longitudinal and genome‐wide association studies have shown that insulin resistance is more likely to be a precondition, and that the failure of the pancreatic β cell to meet the increased insulin requirements is the triggering factor in the development of type 2 diabetes. A major emphasis in diabetes research has therefore shifted to understanding the causes of β cell failure. Collectively, these studies have implicated a complex network of triggers, which activate intersecting execution pathways leading to β cell dysfunction and death. In the present review, we discuss these triggers (glucotoxicity, lipotoxicity, amyloid and cytokines) with respect to the pathways they activate (oxidative stress, inflammation and endoplasmic reticulum stress) and propose a model for understanding β cell failure in type 2 diabetes. (J Diabetes Invest, doi: 10.1111/j.2040‐1124.2010.00021.x, 2010)

Inhibition of palmitate-induced GADD34 expression augments apoptosis in mouse insulinoma cells (MIN6)

Saturated fatty acids like palmitate induce endoplasmic reticulum (ER) stress in pancreatic beta-cells, an event linked to apoptotic loss of β-cells in type 2 diabetes. Sustained activation of the ER stress response leads to expression of growth arrest and DNA damage-inducible protein 34 (GADD34), a regulatory subunit of protein phosphatase 1. In the present study, we have used small interfering RNA in order to knockdown GADD34 expression in insulin-producing MIN6 cells prior to induction of ER stress by palmitate and evaluated its consequences on RNA-activated protein kinase-like ER-localized eIF2alpha kinase (PERK) signalling and apoptosis. Salubrinal, a specific inhibitor of eukaryotic initiation factor 2α (eIF2α) dephosphorylation, was used as a comparison. Salubrinal treatment augmented palmitate-induced ER stress and increased GADD34 levels. Both GADD34 knockdown and salubrinal treatment potentiated the cytotoxic effects of palmitate as evidenced by increased DNA fragmentation and activation of caspase 3, with the fundamental difference that the former did not involve enhanced levels of GADD34. The data from this study suggest that sustained activation of PERK signalling and eIF2α phosphorylation sensitizes insulin-producing MIN6 cells to lipoapoptosis independently of GADD34 expression levels.

Mitogen-activated protein kinases and protein phosphatase 5 mediate glucocorticoid-induced cytotoxicity in pancreatic islets and β-cells

Glucocorticoid excess is associated with glucose intolerance and diabetes. In addition to inducing insulin resistance, glucocorticoids impair β-cell function and cause β-cell apoptosis. In this study we show that dexamethasone activates mitogen-activated protein kinases (MAPKs) signaling in MIN6 β-cells, as evident by enhanced phosphorylation of p38 MAPK and c-Jun N-terminal kinase (JNK). In contrast, the integrated stress response pathway was inhibited by dexamethasone. A p38 MAPK inhibitor attenuated dexamethasone-induced apoptosis in β-cells and isolated islets and decreased glucocorticoid receptor phosphorylation at S220. In contrast, a JNK inhibitor augmented DNA fragmentation and dexamethasone-induced formation of cleaved caspase 3. We also show that inhibition of protein phosphatase 5 (PP5) augments apoptosis in dexamethasone-exposed islets and β-cells, with a concomitant activation of p38 MAPK. In conclusion, our data provide evidence that in islets and β-cells, p38 MAPK and JNK phosphorylation work in concert with PP5 to regulate the cytotoxic effects exerted by glucocorticoids.

Tribbles 3 mediates endoplasmic reticulum stress-induced insulin resistance in skeletal muscle

Endoplasmic Reticulum (ER) stress has been linked to insulin resistance in multiple tissues but the role of ER stress in skeletal muscle has not been explored. ER stress has also been reported to increase tribbles 3 (TRB3) expression in multiple cell lines. Here, we report that high fat feeding in mice, and obesity and type 2 diabetes in humans significantly increases TRB3 and ER stress markers in skeletal muscle. Overexpression of TRB3 in C2C12 myotubes and mouse tibialis anterior muscles significantly impairs insulin signaling. Incubation of C2C12 cells and mouse skeletal muscle with ER stressors thapsigargin and tunicamycin increases TRB3 and impairs insulin signaling and glucose uptake, effects reversed in cells overexpressing RNAi for TRB3 and in muscles from TRB3 knockout mice. Furthermore, TRB3 knockout mice are protected from high fat diet-induced insulin resistance in skeletal muscle. These data demonstrate that TRB3 mediates ER stress-induced insulin resistance in skeletal muscle.

Chronic high glucose and pyruvate levels differentially affect mitochondrial bioenergetics and fuel-stimulated insulin secretion from clonal INS-1 832/13 cells

Glucotoxicity in pancreatic β-cells is a well established pathogenetic process in type 2 diabetes. It has been suggested that metabolism-derived reactive oxygen species perturb the β-cell transcriptional machinery. Less is known about altered mitochondrial function in this condition. We used INS-1 832/13 cells cultured for 48 h in 2.8 mm glucose (low-G), 16.7 mm glucose (high-G), or 2.8 mm glucose plus 13.9 mm pyruvate (high-P) to identify metabolic perturbations. High-G cells showed decreased responsiveness, relative to low-G cells, with respect to mitochondrial membrane hyperpolarization, plasma membrane depolarization, and insulin secretion, when stimulated acutely with 16.7 mm glucose or 10 mm pyruvate. In contrast, high-P cells were functionally unimpaired, eliminating chronic provision of saturating mitochondrial substrate as a cause of glucotoxicity. Although cellular insulin content was depleted in high-G cells, relative to low-G and high-P cells, cellular functions were largely recovered following a further 24-h culture in low-G medium. After 2 h at 2.8 mm glucose, high-G cells did not retain increased levels of glycolytic or TCA cycle intermediates but nevertheless displayed increased glycolysis, increased respiration, and an increased mitochondrial proton leak relative to low-G and high-P cells. This notwithstanding, titration of low-G cells with low protonophore concentrations, monitoring respiration and insulin secretion in parallel, showed that the perturbed insulin secretion of high-G cells could not be accounted for by increased proton leak. The present study supports the idea that glucose-induced disturbances of stimulus-secretion coupling by extramitochondrial metabolism upstream of pyruvate, rather than exhaustion from metabolic overload, underlie glucotoxicity in insulin-producing cells.