Chop deletion reduces oxidative stress, improves beta cell function, and promotes cell survival in multiple mouse models of diabetes

Endoplasmic reticulum stress in nonalcoholic fatty liver disease

The underlying causes of nonalcoholic fatty liver disease are unclear, although recent evidence has implicated the endoplasmic reticulum in both the development of steatosis and progression to nonalcoholic steatohepatitis. Disruption of endoplasmic reticulum homeostasis, often termed ER stress, has been observed in liver and adipose tissue of humans with nonalcoholic fatty liver disease and/or obesity. Importantly, the signaling pathway activated by disruption of endoplasmic reticulum homeostasis, the unfolded protein response, has been linked to lipid and membrane biosynthesis, insulin action, inflammation, and apoptosis. Therefore, understanding the mechanisms that disrupt endoplasmic reticulum homeostasis in nonalcoholic fatty liver disease and the role of the unfolded protein response in the broader context of chronic, metabolic diseases have become topics of intense investigation. The present review examines the endoplasmic reticulum and the unfolded protein response in the context of nonalcoholic fatty liver disease.

Protein-folding homeostasis in the endoplasmic reticulum and nutritional regulation

The flux of newly synthesized proteins entering the endoplasmic reticulum (ER) is under negative regulation by the ER-localized PKR-like ER kinase (PERK). PERK is activated by unfolded protein stress in the ER lumen and inhibits new protein synthesis by the phosphorylation of translation initiation factor eIF2α. This homeostatic mechanism, shared by all animal cells, has proven to be especially important to the well-being of professional secretory cells, notably the endocrine pancreas. PERK, its downstream effectors, and the allied branches of the unfolded protein response intersect broadly with signaling pathways that regulate nutrient assimilation, and ER stress and the response to it have been implicated in the development of the metabolic syndrome accompanying obesity in mammals. Here we review our current understanding of the cell biology underlying these relationships.

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.

Death protein 5 and p53-upregulated modulator of apoptosis mediate the endoplasmic reticulum stress-mitochondrial dialog triggering lipotoxic rodent and human β-cell apoptosis

Environmental factors such as diets rich in saturated fats contribute to dysfunction and death of pancreatic β-cells in diabetes. Endoplasmic reticulum (ER) stress is elicited in β-cells by saturated fatty acids. Here we show that palmitate-induced β-cell apoptosis is mediated by the intrinsic mitochondrial pathway. By microarray analysis, we identified a palmitate-triggered ER stress gene expression signature and the induction of the BH3-only proteins death protein 5 (DP5) and p53-upregulated modulator of apoptosis (PUMA). Knockdown of either protein reduced cytochrome c release, caspase-3 activation, and apoptosis in rat and human β-cells. DP5 induction depends on inositol-requiring enzyme 1 (IRE1)-dependent c-Jun NH₂-terminal kinase and PKR-like ER kinase (PERK)-induced activating transcription factor (ATF3) binding to its promoter. PUMA expression is also PERK/ATF3-dependent, through tribbles 3 (TRB3)-regulated AKT inhibition and FoxO3a activation. DP5(-/-) mice are protected from high fat diet-induced loss of glucose tolerance and have twofold greater pancreatic β-cell mass. This study elucidates the crosstalk between lipotoxic ER stress and the mitochondrial pathway of apoptosis that causes β-cell death in diabetes.

Mechanisms of modified LDL-induced pericyte loss and retinal injury in diabetic retinopathy

AIMS/HYPOTHESIS

In previous studies we have shown that extravasated, modified LDL is associated with pericyte loss, an early feature of diabetic retinopathy (DR). Here we sought to determine detailed mechanisms of this LDL-induced pericyte loss.

METHODS

Human retinal capillary pericytes (HRCP) were exposed to 'highly-oxidised glycated' LDL (HOG-LDL) (a model of extravasated and modified LDL) and to 4-hydroxynonenal or 7-ketocholesterol (components of oxidised LDL), or to native LDL for 1 to 24 h with or without 1 h of pretreatment with inhibitors of the following: (1) the scavenger receptor (polyinosinic acid); (2) oxidative stress (N-acetyl cysteine); (3) endoplasmic reticulum (ER) stress (4-phenyl butyric acid); and (4) mitochondrial dysfunction (cyclosporin A). Oxidative stress, ER stress, mitochondrial dysfunction, apoptosis and autophagy were assessed using techniques including western blotting, immunofluorescence, RT-PCR, flow cytometry and TUNEL assay. To assess the relevance of the results in vivo, immunohistochemistry was used to detect the ER stress chaperon, 78 kDa glucose-regulated protein, and the ER sensor, activating transcription factor 6, in retinas from a mouse model of DR that mimics exposure of the retina to elevated glucose and elevated LDL levels, and in retinas from human participants with and without diabetes and DR.

RESULTS

Compared with native LDL, HOG-LDL activated oxidative and ER stress in HRCP, resulting in mitochondrial dysfunction, apoptosis and autophagy. In a mouse model of diabetes and hyperlipidaemia (vs mouse models of either condition alone), retinal ER stress was enhanced. ER stress was also enhanced in diabetic human retina and correlated with the severity of DR.

CONCLUSIONS/INTERPRETATION

Cell culture, animal, and human data suggest that oxidative stress and ER stress are induced by modified LDL, and are implicated in pericyte loss in DR.

Future detection and monitoring of diabetes may entail analysis of both β-cell function and volume: how markers of β-cell loss may assist

Disease heterogeneity is as major issue in Type II Diabetes Mellitus (T2DM), and this patient inter-variability might not be sufficiently reflected by measurements of glycated haemoglobin (HbA1c).Β-cell dysfunction and β-cell death are initiating factors in development of T2DM. In fact, β-cells are known vanish prior to the development of T2DM, and autopsy of overt T2DM patients have shown a 60% reduction in β-cell mass.As the decline in β-cell function and mass have been proven to be pathological traits in T2DM, methods for evaluating β-cell loss is becoming of more interest. However, evaluation of β-cell death or loss is currently invasive and unattainable for the vast majority of diabetes patients. Serological markers, reflecting β-cell loss would be advantageous to detect and monitor progression of T2DM. Biomarkers with such capacities could be neo-epitopes of proteins with high β-cell specificity containing post translational modifications. Such tools may segregate T2DM patients into more appropriate treatment groups, based on their β-cell status, which is currently not possible. Presently individuals presenting with adequately elevated levels of both insulin and glucose are classified as T2DM patients, while an important subdivision of those is pending, namely those patients with sufficient β-cell capacity and those without. This may warrant two very different treatment options and patient care paths.Serological biomarkers reflecting β-cell health status may also assist development of new drugs for T2DM and aid physicians in better characterization of individual patients and tailor individual treatments and patient care protocols.

Activating transcription factor 6 mediates oxidized LDL-induced cholesterol accumulation and apoptosis in macrophages by up-regulating CHOP expression

AIM

This study was to explore whether activating transcription factor 6 (ATF6), an important sensor to endoplasmic reticulum (ER) stress, would mediate oxidized low-density lipoprotein (ox-LDL)- induced cholesterol accumulation and apoptosis in cultured macrophages and the underlying molecular mechanisms.

METHODS

Intracellular lipid droplets and total cholesterol levels were assayed by oil red O staining and enzymatic colorimetry, respectively. Cell viability and apoptosis were determined using MTT assay and AnnexinV-FITC apoptosis detection kit, respectively. The nuclear translocation of ATF6 in cells was detected by immunofluorescence analysis. Protein and mRNA levels were examined by Western blot analysis and real time-PCR, respectively. ATF6 siRNA was transfected to RAW264.7 cells by lipofectamin.

RESULTS

Exposure of cells to ox-LDL induced glucose-regulated protein 78 (GRP78). C/EBP homologous protein (CHOP), a key-signaling component of ER stress-induced apoptosis, was up-regulated in ox-LDL-treated cells. ATF6, a factor that positively regulates CHOP expression, was activated by ox-LDL in a concentration- and time- dependent manner. The role of the ATF6-mediated ER stress pathway was further confirmed through the siRNA-mediated knockdown of ATF6, which attenuated ox-LDL-induced upregulation of CHOP, cholesterol accumulation and apoptosis in macrophages. In addition, the phosphorylation of double-stranded RNA-activated protein kinase-like endoplasmic reticulum kinase (PERK), another factor that positively regulates CHOP expression, was induced in the presence of ox-LDL, and PERK-specific siRNA also inhibited the ox-LDL-induced upregulation of CHOP and apoptosis in RAW264.7 cells.

CONCLUSION

These results demonstrate that ER stress-related proteins, particularly ATF6 and its downstream molecule CHOP, are involved in ox-LDL-induced cholesterol accumulation and apoptosis in macrophages.

Deletion of p47phox attenuates the progression of diabetic nephropathy and reduces the severity of diabetes in the Akita mouse

AIMS/HYPOTHESIS

Reactive oxygen species (ROS) contribute to diabetes-induced glomerular injury and endoplasmic reticulum (ER) stress-induced beta cell dysfunction, but the source of ROS has not been fully elucidated. Our aim was to determine whether p47(phox)-dependent activation of NADPH oxidase is responsible for hyperglycaemia-induced glomerular injury in the Akita mouse, a model of type 1 diabetes mellitus resulting from ER stress-induced beta cell dysfunction.

METHODS

We examined the effect of deleting p47 (phox) (also known as Ncf1), the gene for the NADPH oxidase subunit, on diabetic nephropathy in the Akita mouse (Ins2 (WT/C96Y)) by studying four groups of mice: (1) non-diabetic mice (Ins2 (WT/WT)/p47 (phox+/+)); (2) non-diabetic p47 (phox)-null mice (Ins2 (WT/WT)/p47 (phox-/-)); (3) diabetic mice: (Ins2 (WT/C96Y)/p47 (phox+/+)); and (4) diabetic p47 (phox)-null mice (Ins2 (WT/C96Y)/p47 (phox-/-)). We measured the urinary albumin excretion rate, oxidative stress, mesangial matrix expansion, and plasma and pancreatic insulin concentrations in 16-week-old mice; we also measured glucose tolerance and insulin sensitivity, islet and glomerular NADPH oxidase activity and subunit expression, and pro-fibrotic gene expression in 8-week-old mice. In addition, we measured NADPH oxidase activity, subunit expression and pro-fibrotic gene expression in high glucose-treated murine mesangial cells.

RESULTS

Deletion of p47 (phox) reduced kidney hypertrophy, oxidative stress and mesangial matrix expansion, and also reduced hyperglycaemia by increasing pancreatic and circulating insulin concentrations. p47 (phox-/-) mice exhibited improved glucose tolerance, but modestly decreased insulin sensitivity. Deletion of p47 (phox) attenuated high glucose-induced activation of NADPH oxidase and pro-fibrotic gene expression in glomeruli and mesangial cells.

CONCLUSIONS/INTERPRETATION

Deletion of p47 (phox) attenuates diabetes-induced glomerular injury and beta cell dysfunction in the Akita mouse.

Endoplasmic reticulum stress, pancreatic β-cell degeneration, and diabetes

Overwhelming of protein folding in the endoplasmic reticulum (ER)--referred to as "ER stress"--activates a set of intracellular signaling pathways termed the unfolded protein response (UPR). Beneficial outputs of the UPR promote adaptation in cells experiencing manageably low levels of ER stress. However, if ER stress reaches critically high levels, the UPR uses destructive outputs to trigger programmed cell death. Genetic mutations in various UPR components cause inherited syndromes of diabetes mellitus in both rodents and humans, implicating the UPR in the proper functioning and survival of pancreatic islet β cells. Markers of chronically elevated ER stress, terminal UPR signaling, and apoptosis are evident in β cells in these rare disorders; these markers are similarly present in islets of human patients with common forms of diabetes. These findings promise to enhance our molecular understanding of human diabetes significantly and may lead to new and effective therapies.

Expression profiling and biochemical analysis suggest stress response as a potential mechanism inhibiting proliferation of polyamine-depleted cells

Polyamines are small organic polycations that are absolutely required for cell growth and proliferation; yet the basis for this requirement is mostly unknown. Here, we combined a genome-wide expression profiling with biochemical analysis to reveal the molecular basis for inhibited proliferation of polyamine-depleted cells. Transcriptional responses accompanying growth arrest establishment in polyamine-depleted cells or growth resumption following polyamine replenishment were monitored and compared. Changes in the expression of genes related to various fundamental cellular processes were established. Analysis of mirror-symmetric expression patterns around the G(1)-arrest point identified a set of genes representing a stress-response signature. Indeed, complementary biochemical analysis demonstrated activation of the PKR-like endoplasmic reticulum kinase arm of the unfolded protein response and of the stress-induced p38 MAPK. These changes were accompanied by induction of key growth-inhibitory factors such as p21 and Gadd45a and reduced expression of various cyclins, most profoundly cyclin D1, setting the basis for the halted proliferation. However, although the induced stress response could arrest growth, polyamine depletion also inhibited proliferation of PKR-like endoplasmic reticulum kinase and p38α-deficient cells and of cells harboring a nonphosphorylatable mutant eIF2α (S51A), suggesting that additional yet unidentified mechanisms might inhibit proliferation of polyamine-depleted cells. Despite lengthy persistence of the stress and activation of apoptotic signaling, polyamine-depleted cells remained viable, apparently due to induced expression of protective genes and development of autophagy.

The impact of the unfolded protein response on human disease

A central function of the endoplasmic reticulum (ER) is to coordinate protein biosynthetic and secretory activities in the cell. Alterations in ER homeostasis cause accumulation of misfolded/unfolded proteins in the ER. To maintain ER homeostasis, eukaryotic cells have evolved the unfolded protein response (UPR), an essential adaptive intracellular signaling pathway that responds to metabolic, oxidative stress, and inflammatory response pathways. The UPR has been implicated in a variety of diseases including metabolic disease, neurodegenerative disease, inflammatory disease, and cancer. Signaling components of the UPR are emerging as potential targets for intervention and treatment of human disease.

SmD3 regulates intronic noncoding RNA biogenesis

Accumulation of excess lipid in nonadipose tissues is associated with oxidative stress and organ dysfunction and plays an important role in diabetic complications. To elucidate molecular events critical for lipotoxicity, we used retroviral promoter trap mutagenesis to generate mutant Chinese hamster ovary cell lines resistant to lipotoxic and oxidative stress. A previous report of a mutant from this screen demonstrated that under lipotoxic conditions, small nucleolar RNAs (snoRNAs) in the rpL13a gene accumulate in the cytosol and serve as critical mediators of lipotoxic cell death. We now report a novel, independent mutant in which a single provirus disrupted one allele of the gene encoding the spliceosomal protein SmD3, creating a model of haploinsufficiency. We show that snoRNA expression and the abundance of snoRNA-containing intron lariats are decreased in SmD3 mutant cells, even though haploinsufficiency of SmD3 supports pre-mRNA splicing. The mechanism through which SmD3 regulates the expression of intronic snoRNAs likely involves effects of SmD3 on the levels of small nuclear RNAs (snRNAs) U4 and U5. Our data implicate SmD3 as a critical determinant in the processing of intronic noncoding RNAs in general and as an upstream mediator of metabolic stress response pathways through the regulation of snoRNA expression.

Quercetin protects macrophages from oxidized low-density lipoprotein-induced apoptosis by inhibiting the endoplasmic reticulum stress-C/EBP homologous protein pathway

Quercetin (QUE), a member of the bioflavonoid family, has been proposed to have antioxidative, anti-inflammatory and antihypertensive properties. This study was designed to investigate the protective effect of QUE on oxidized low-density lipoprotein (ox-LDL)-induced cytotoxicity in RAW264.7 macrophages and specifically the endoplasmic reticulum (ER) stress-C/EBP homologous protein (CHOP) pathway-mediated apoptosis. Our results showed that treatment with QUE (20, 40 and 80 μmol/L) significantly attenuated ox-LDL-induced cholesterol accumulation in macrophages and foam cell formation in a dose-dependent manner. Similar to tunicamycin (TM), a classical ER stress inducer, ox-LDL reduced cell viability and induced apoptosis in RAW264.7 macrophages. The cytotoxic effects of ox-LDL and TM were significantly inhibited by QUE treatment. Interestingly, we found that QUE also significantly suppressed the ox-LDL- and TM-induced activation of ER stress signaling events, including the phosphorylation of inositol-requiring enzyme 1 (IRE1), translocation of activating transcription factor 6 (ATF6) from the cytoplasm to the nucleus and upregulation of X-box-binding protein 1. In addition, exposure of RAW264.7 macrophages to ox-LDL or TM resulted in a significant increase in the expression of CHOP, a transcription factor regulated by IRE1 and ATF6 under conditions of ER stress, as well as a decrease in Bcl-2 transcript and protein concentrations. QUE blocked these effects in a dose-dependent manner. These data indicate that QUE can protect RAW264.7 cells from ox-LDL-induced apoptosis and that the mechanism at least partially involves its ability to inhibit the ER stress-CHOP signaling pathway.

Cellular inhibitor of apoptosis protein-1 (cIAP1) plays a critical role in β-cell survival under endoplasmic reticulum stress: promoting ubiquitination and degradation of C/EBP homologous protein (CHOP)

Lipotoxicity in pancreatic β-cells, arising from excess free fatty acid-induced endoplasmic reticulum (ER) stress response, has been recognized as a key pathogenic factor causing loss of β-cell mass and contributing to type 2 diabetes. However, how the adaptive ER stress response causes cell death remains enigmatic. We report herein a critical role of cellular inhibitor of apoptosis protein-1 (cIAP1) in controlling β-cell survival under ER stress. While both palmitate and palmitoleate induced an overt ER stress response, lipotoxicity was only observed in β-cells exposed to palmitate but not palmitoleate. Interestingly, cells treated with palmitoleate exerted a sustainable level of cIAP1, whereas the protein quickly degraded following palmitate treatment. Enforced overexpression of cIAP1 prevented palmitate-induced cell death. In contrast, siRNA-mediated knockdown of cIAP1 in β-cells or knock-out of cIap1 in mouse embryonic fibroblasts not only increased palmitate-induced apoptosis, but also committed cells to death in response to the nontoxic palmitoleate treatment. Of importance, we found that cIAP1 functions as an E3 ubiquitin ligase promoting ubiquitination and degradation of C/EBP homologous protein (CHOP), a key mediator of ER stress-induced cell death. These findings define a novel mechanism for β-cell survival under ER stress and help to identify targets for therapeutic intervention against lipotoxicity in β-cells.

Lack of Wdr13 gene in mice leads to enhanced pancreatic beta cell proliferation, hyperinsulinemia and mild obesity

WD-repeat proteins are very diverse, yet these are structurally related proteins that participate in a wide range of cellular functions. WDR13, a member of this family, is conserved from fishes to humans and localizes into the nucleus. To understand the in vivo function(s) of Wdr13 gene, we have created and characterized a mutant mouse strain lacking this gene. The mutant mice had higher serum insulin levels and increased pancreatic islet mass as a result of enhanced beta cell proliferation. While a known cell cycle inhibitor, p21, was downregulated in the mutant islets, over expression of WDR13 in the pancreatic beta cell line (MIN6) resulted in upregulation of p21, accompanied by retardation of cell proliferation. We suggest that WDR13 is a novel negative regulator of the pancreatic beta cell proliferation. Given the higher insulin levels and better glucose clearance in Wdr13 gene deficient mice, we propose that this protein may be a potential candidate drug target for ameliorating impaired glucose metabolism in diabetes.

Role of endoplasmic reticulum stress in metabolic disease and other disorders

Perturbations in the normal functions of the endoplasmic reticulum (ER) trigger a signaling network that coordinates adaptive and apoptotic responses. There is accumulating evidence implicating prolonged ER stress in the development and progression of many diseases, including neurodegeneration, atherosclerosis, type 2 diabetes, liver disease, and cancer. With the improved understanding of the underlying molecular mechanisms, therapeutic interventions that target the ER stress response would be potential strategies to treat various diseases driven by prolonged ER stress.

Eukaryotic initiation factor 2 phosphorylation and translational control in metabolism

Regulation of mRNA translation is a rapid and effective means to couple changes in the cellular environment with global rates of protein synthesis. In response to stresses, such as nutrient deprivation and accumulation of misfolded proteins in the endoplasmic reticulum, phosphorylation of the α subunit of eukaryotic initiation factor 2 (eIF2α~P) reduces general translation initiation while facilitating the preferential translation of select transcripts, such as that encoding activating transcription factor 4 (ATF4), a transcriptional activator of genes subject to the integrated stress response (ISR). In this review, we highlight the translational control processes regulated by nutritional stress, with an emphasis on the events triggered by eIF2α~P, and describe the family of eukaryotic initiation factor 2 kinases and the mechanisms by which each sense different stresses. We then address 3 questions. First, what are the mechanisms by which eIF2α~P confers preferential translation on select mRNA and what are the consequences of the gene expression induced by the ISR? Second, what are the molecular processes by which certain stresses can differentially activate eIF2α~P and ATF4 expression? The third question we address is what are the modes of cross-regulation between the ISR and other stress response pathways, such as the unfolded protein response and mammalian target of rapamycin, and how do these regulatory schemes provide for gene expression programs that are tailored for specific stresses? This review highlights recent advances in each of these areas of research, emphasizing how eIF2α~P and the ISR can affect metabolic health and disease.

Glucose-induced beta cell dysfunction in vivo in rats: link between oxidative stress and endoplasmic reticulum stress

AIMS/HYPOTHESIS

Endoplasmic reticulum (ER) stress has been implicated in glucose-induced beta cell dysfunction. However, its causal role has not been established in vivo. Our objective was to determine the causal role of ER stress and its link to oxidative stress in glucose-induced beta cell dysfunction in vivo.

METHODS

Healthy Wistar rats were infused i.v. with glucose for 48 h to achieve 20 mmol/l hyperglycaemia with or without the co-infusion of the superoxide dismutase mimetic tempol (TPO), or the chemical chaperones 4-phenylbutyrate (PBA) or tauroursodeoxycholic acid (TUDCA). This was followed by assessment of beta cell function and measurement of ER stress markers and superoxide in islets.

RESULTS

Glucose infusion for 48 h increased mitochondrial superoxide and ER stress markers and impaired beta cell function. Co-infusion of TPO, which we previously found to reduce mitochondrial superoxide and prevent glucose-induced beta cell dysfunction, reduced ER stress markers. Similar to findings with TPO, co-infusion of PBA, which decreases mitochondrial superoxide, prevented glucose-induced beta cell dysfunction in isolated islets. TUDCA was also effective. Also similar to findings with TPO, PBA prevented beta cell dysfunction during hyperglycaemic clamps in vivo and after hyperglycaemia (15 mmol/l) for 96 h.

CONCLUSIONS/INTERPRETATION

Here, we causally implicate ER stress in hyperglycaemia-induced beta cell dysfunction in vivo. We show that: (1) there is a positive feedback cycle between oxidative stress and ER stress in glucose-induced beta cell dysfunction, which involves mitochondrial superoxide; and (2) this cycle can be interrupted by superoxide dismutase mimetics as well as chemical chaperones, which are of potential interest to preserve beta cell function in type 2 diabetes.

Deletion of C/EBP homologous protein (Chop) in C57Bl/6 mice dissociates obesity from insulin resistance

AIMS/HYPOTHESIS

Endoplasmic reticulum (ER) stress has been implicated in the development of type 2 diabetes, via effects on obesity, insulin resistance and pancreatic beta cell health. C/EBP homologous protein (CHOP) is induced by ER stress and has a central role in apoptotic execution pathways triggered by ER stress. The aim of this study was to characterise the role of CHOP in obesity and insulin resistance.

METHODS

Metabolic studies were performed in Chop ( -/- ) and wild-type C57Bl/6 mice, and included euglycaemic-hyperinsulinaemic clamps and indirect calorimetry. The inflammatory state of liver and adipose tissue was determined by quantitative RT-PCR, immunohistology and macrophage cultures. Viability and absence of ER stress in islets of Langerhans was determined by electron microscopy, islet culture and quantitative RT-PCR.

RESULTS

Systemic deletion of Chop induced abdominal obesity and hepatic steatosis. Despite marked obesity, Chop ( -/- ) mice had preserved normal glucose tolerance and insulin sensitivity. This discrepancy was accompanied by lower levels of pro-inflammatory cytokines and less infiltration of immune cells into fat and liver.

CONCLUSIONS/INTERPRETATION

These observations suggest that insulin resistance is not induced by fat accumulation per se, but rather by the inflammation induced by ectopic fat. CHOP may play a key role in the crosstalk between excessive fat deposition and induction of inflammation-mediated insulin resistance.

RNase L controls terminal adipocyte differentiation, lipids storage and insulin sensitivity via CHOP10 mRNA regulation

Adipose tissue structure is altered during obesity, leading to deregulation of whole-body metabolism. Its function depends on its structure, in particular adipocytes number and differentiation stage. To better understand the mechanisms regulating adipogenesis, we have investigated the role of an endoribonuclease, endoribonuclease L (RNase L), using wild-type and RNase L-knockout mouse embryonic fibroblasts (RNase L(-/-)-MEFs). Here, we identify C/EBP homologous protein 10 (CHOP10), a dominant negative member of the CCAAT/enhancer-binding protein family, as a specific RNase L target. We show that RNase L is associated with CHOP10 mRNA and regulates its stability. CHOP10 expression is conserved in RNase L(-/-)-MEFs, maintaining preadipocyte state while impairing their terminal differentiation. RNase L(-/-)-MEFs have decreased lipids storage capacity, insulin sensitivity and glucose uptake. Expression of ectopic RNase L in RNase L(-/-)-MEFs triggers CHOP10 mRNA instability, allowing increased lipids storage, insulin response and glucose uptake. Similarly, downregulation of CHOP10 mRNA with CHOP10 siRNA in RNase L(-/-)-MEFs improves their differentiation in adipocyte. In vivo, aged RNase L(-)/(-) mice present an expanded adipose tissue, which, however, is unable to correctly store lipids, illustrated by ectopic lipids storage in the liver and in the kidney. These findings highlight RNase L as an essential regulator of adipogenesis via the regulation of CHOP10 mRNA.

Endoplasmic reticulum stress and type 2 diabetes

Given the functional importance of the endoplasmic reticulum (ER), an organelle that performs folding, modification, and trafficking of secretory and membrane proteins to the Golgi compartment, the maintenance of ER homeostasis in insulin-secreting β-cells is very important. When ER homeostasis is disrupted, the ER generates adaptive signaling pathways, called the unfolded protein response (UPR), to maintain homeostasis of this organelle. However, if homeostasis fails to be restored, the ER initiates death signaling pathways. New observations suggest that both chronic hyperglycemia and hyperlipidemia, known as important causative factors of type 2 diabetes (T2D), disrupt ER homeostasis to induce unresolvable UPR activation and β-cell death. This review examines how the UPR pathways, induced by high glucose and free fatty acids (FFAs), interact to disrupt ER function and cause β-cell dysfunction and death.

12/15-Lipoxygenase signaling in the endoplasmic reticulum stress response

Central obesity is associated with chronic inflammation, insulin resistance, β-cell dysfunction, and endoplasmic reticulum (ER) stress. The 12/15-lipoxygenase enzyme (12/15-LO) promotes inflammation and insulin resistance in adipose and peripheral tissues. Given that obesity is associated with ER stress and 12/15-LO is expressed in adipose tissue, we determined whether 12/15-LO could mediate ER stress signals. Addition of 12/15-LO lipid products 12(S)-HETE and 12(S)-HPETE to differentiated 3T3-L1 adipocytes induced expression and activation of ER stress markers, including BiP, XBP-1, p-PERK, and p-IRE1α. The ER stress inducer, tunicamycin, upregulated ER stress markers in adipocytes with concomitant 12/15-LO activation. Addition of a 12/15-LO inhibitor, CDC, to tunicamycin-treated adipocytes attenuated the ER stress response. Furthermore, 12/15-LO-deficient adipocytes exhibited significantly decreased tunicamycin-induced ER stress. 12/15-LO action involves upregulation of interleukin-12 (IL-12) expression. Tunicamycin significantly upregulated IL-12p40 expression in adipocytes, and IL-12 addition increased ER stress gene expression; conversely, LSF, an IL-12 signaling inhibitor, and an IL-12p40-neutralizing antibody attenuated tunicamycin-induced ER stress. Isolated adipocytes and liver from 12/15-LO-deficient mice fed a high-fat diet revealed a decrease in spliced XBP-1 expression compared with wild-type C57BL/6 mice on a high-fat diet. Furthermore, pancreatic islets from 12/15-LO-deficient mice showed reduced high-fat diet-induced ER stress genes compared with wild-type mice. These data suggest that 12/15-LO activity participates in ER stress in adipocytes, pancreatic islets, and liver. Therefore, reduction of 12/15-LO activity or expression could provide a new therapeutic target to reduce ER stress and downstream inflammation linked to obesity.

CDK5 and MEKK1 mediate pro-apoptotic signalling following endoplasmic reticulum stress in an autosomal dominant retinitis pigmentosa model

Chronic stress in the endoplasmic reticulum (ER) underlies many degenerative and metabolic diseases involving apoptosis of vital cells. A well-established example is autosomal dominant retinitis pigmentosa (ADRP), an age-related retinal degenerative disease caused by mutant rhodopsins. Similar mutant alleles of Drosophila Rhodopsin-1 also impose stress on the ER and cause age-related retinal degeneration in that organism. Well-characterized signalling responses to ER stress, referred to as the unfolded protein response (UPR), induce various ER quality control genes that can suppress such retinal degeneration. However, how cells activate cell death programs after chronic ER stress remains poorly understood. Here, we report the identification of a signalling pathway mediated by cdk5 and mekk1 required for ER-stress-induced apoptosis. Inactivation of these genes specifically suppressed apoptosis, without affecting other protective branches of the UPR. CDK5 phosphorylates MEKK1, and together, they activate the JNK pathway for apoptosis. Moreover, disruption of this pathway can delay the course of age-related retinal degeneration in a Drosophila model of ADRP. These findings establish a previously unrecognized branch of ER-stress response signalling involved in degenerative diseases.

Loss of both ABCA1 and ABCG1 results in increased disturbances in islet sterol homeostasis, inflammation, and impaired β-cell function

Cellular cholesterol homeostasis is important for normal β-cell function. Disruption of cholesterol transport by decreased function of the ATP-binding cassette (ABC) transporter ABCA1 results in impaired insulin secretion. Mice lacking β-cell ABCA1 have increased islet expression of ABCG1, another cholesterol transporter implicated in β-cell function. To determine whether ABCA1 and ABCG1 have complementary roles in β-cells, mice lacking ABCG1 and β-cell ABCA1 were generated and glucose tolerance, islet sterol levels, and β-cell function were assessed. Lack of both ABCG1 and β-cell ABCA1 resulted in increased fasting glucose levels and a greater impairment in glucose tolerance compared with either ABCG1 deletion or loss of ABCA1 in β-cells alone. In addition, glucose-stimulated insulin secretion was decreased and sterol accumulation increased in islets lacking both transporters compared with those isolated from knockout mice with each gene alone. Combined deficiency of ABCA1 and ABCG1 also resulted in significant islet inflammation as indicated by increased expression of interleukin-1β and macrophage infiltration. Thus, lack of both ABCA1 and ABCG1 induces greater defects in β-cell function than deficiency of either transporter individually. These data suggest that ABCA1 and ABCG1 each make complimentary and important contributions to β-cell function by maintaining islet cholesterol homeostasis in vivo.

Stress management at the ER: regulators of ER stress-induced apoptosis

The endoplasmic reticulum (ER) is an elaborate cellular organelle essential for cell function and survival. Conditions that interfere with ER function lead to the accumulation and aggregation of unfolded proteins which are detected by ER transmembrane receptors that initiate the unfolded protein response (UPR) to restore normal ER function. If the ER stress is prolonged, or the adaptive response fails, apoptotic cell death ensues. Many studies have focused on how this failure initiates apoptosis, particularly because ER stress-induced apoptosis is implicated in the pathophysiology of several neurodegenerative and cardiovascular diseases. In this review we aim to shed light on the proteins that are not core components of the UPR signaling pathway but which can influence the course of the ER stress response by regulating the switch from the adaptive phase to apoptosis.

Diet supplementation with green tea extract epigallocatechin gallate prevents progression to glucose intolerance in db/db mice

BACKGROUND

Green tea was suggested as a therapeutic agent for the treatment of diabetes more than 70 years ago, but the mechanisms behind its antidiabetic effect remains elusive. In this work, we address this issue by feeding a green tea extract (TEAVIGO™) with a high content of epigallocatechin gallate (EGCG) or the thiazolidinedione PPAR-γ agonist rosiglitazone, as positive control, to db/db mice, an animal model for diabetes.

METHODS

Young (7 week-old) db/db mice were randomized and assigned to receive diets supplemented with or without EGCG or rosiglitazone for 10 weeks. Fasting blood glucose, body weight and food intake was measured along the treatment. Glucose and insulin levels were determined during an oral glucose tolerance test after 10 weeks of treatment. Pancreata were sampled at the end of the study for blinded histomorphometric analysis. Islets were isolated and their mRNA expression analyzed by quantitative RT-PCR.

RESULTS

The results show that, in db/db mice, EGCG improves glucose tolerance and increases glucose-stimulated insulin secretion. EGCG supplementation reduces the number of pathologically changed islets of Langerhans, increases the number and the size of islets, and heightens pancreatic endocrine area. These effects occurred in parallel with a reduction in islet endoplasmic reticulum stress markers, possibly linked to the antioxidative capacity of EGCG.

CONCLUSIONS

This study shows that the green tea extract EGCG markedly preserves islet structure and enhances glucose tolerance in genetically diabetic mice. Dietary supplementation with EGCG could potentially contribute to nutritional strategies for the prevention and treatment of type 2 diabetes.

Differential effects of unfolded protein response pathways on axon injury-induced death of retinal ganglion cells

Loss of retinal ganglion cells (RGCs) accounts for visual function deficits after optic nerve injury, but how axonal insults lead to neuronal death remains elusive. By using an optic nerve crush model that results in the death of the majority of RGCs, we demonstrate that axotomy induces differential activation of distinct pathways of the unfolded protein response in axotomized RGCs. Optic nerve injury provokes a sustained CCAAT/enhancer binding homologous protein (CHOP) upregulation, and deletion of CHOP promotes RGC survival. In contrast, IRE/XBP-1 is only transiently activated, and forced XBP-1 activation dramatically protects RGCs from axon injury-induced death. Importantly, such differential activations of CHOP and XBP-1 and their distinct effects on neuronal cell death are also observed in RGCs with other types of axonal insults, such as vincristine treatment and intraocular pressure elevation, suggesting a new protective strategy for neurodegeneration associated with axonal damage.

Nanospaces between endoplasmic reticulum and mitochondria as control centres of pancreatic β-cell metabolism and survival

Nanometre-scale spaces between organelles represent focused nodes for signal transduction and the control of cellular decisions. The endoplasmic reticulum (ER) and the mitochondria form dynamic quasi-synaptic interaction nanodomains in all cell types examined, but the functional role of these junctions in cellular metabolism and cell survival remains to be fully understood. In this paper, we review recent evidence that ER Ca(2+) channels, such as the RyR and IP(3)R, can signal specifically across this nanodomain to the adjacent mitochondria to pace basal metabolism, with focus on the pancreatic β-cell. Blocking these signals in the basal state leads to a form of programmed cell death associated with reduced ATP and the induction of calpain-10 and hypoxia-inducible factors. On the other hand, the hyperactivity of this signalling domain plays a deleterious role during classical forms of apoptosis. Thus, the nanospace between ER and mitochondria represents a critical rheostat controlling both metabolism and programmed cell death. Many aspects of the mechanisms underlying this control system remain to be uncovered, and new nanotechnologies are required understand these domains at a molecular level.

C/EBP homologous protein-10 (CHOP-10) limits postnatal neovascularization through control of endothelial nitric oxide synthase gene expression

BACKGROUND

C/EBP homologous protein-10 (CHOP-10) is a novel developmentally regulated nuclear protein that emerges as a critical transcriptional integrator among pathways regulating differentiation, proliferation, and survival. In the present study, we analyzed the role of CHOP-10 in postnatal neovascularization.

METHODS AND RESULTS

Ischemia was induced by right femoral artery ligation in wild-type and CHOP-10(-/-) mice. In capillary structure of skeletal muscle, CHOP-10 mRNA and protein levels were upregulated by ischemia and diabetes mellitus. Angiographic score, capillary density, and foot perfusion were increased in CHOP-10(-/-) mice compared with wild-type mice. This effect was associated with a reduction in apoptosis and an upregulation of endothelial nitric oxide synthase (eNOS) levels in ischemic legs of CHOP-10(-/-) mice compared with wild-type mice. In agreement with these results, eNOS mRNA and protein levels were significantly upregulated in CHOP-10 short interfering RNA-transfected human endothelial cells, whereas overexpression of CHOP-10 inhibited basal transcriptional activation of the eNOS promoter. Using a chromatin immunoprecipitation assay, we also showed that CHOP-10 was bound to the eNOS promoter. Interestingly, enhanced postischemic neovascularization in CHOP-10(-/-) mice was fully blunted in CHOP-10/eNOS double-knockout animals. Finally, we showed that induction of diabetes mellitus is associated with a marked upregulation of CHOP-10 that substantially inhibited postischemic neovascularization.

CONCLUSIONS

This study identifies CHOP-10 as an important transcription factor modulating vessel formation and maturation.

Endoplasmic reticulum stress and inflammation: mechanisms and implications in diabetic retinopathy

The endoplasmic reticulum (ER) is the primary cellular compartment where proteins are synthesized and modified before they can be transported to their destination. Dysfunction of the ER impairs protein homeostasis and leads to the accumulation of misfolded/unfolded proteins in the ER, or ER stress. While it has long been recognized that ER stress is a major cause of conformational disorders, such as Alzheimer's disease, Huntington's disease, certain types of cancer, and type 2 diabetes, recent evidence suggests that ER stress is also implicated in many chronic inflammatory diseases. These diseases include irritable bowel syndrome, atherosclerosis, diabetic complications, and many others. Diabetic retinopathy is a common microvascular complication of diabetes, characterized by chronic inflammation, progressive damage to retinal vascular and neuronal cells, vascular leakage, and abnormal blood vessel growth (neovascularization). In this review, we discuss the role and mechanisms of ER stress in retinal inflammation and vascular damage in diabetic retinopathy.

Intermittent but not constant high glucose induces ER stress and inflammation in human retinal pericytes

Diabetic retinopathy is a chronic inflammatory disease characterized by vascular damage and neuronal degeneration. Previously we reported that activated retinal pericytes secret high levels of pro-inflammatory cytokines, such as macrophage chemoattractant protein 1 (MCP-1), and may play a pivotal role in macrophage recruitment and inflammatory retinal damage. However, the mechanism underlying diabetes-induced pericyte inflammation remains poorly understood. In the present study, we evaluated the effects of constant and intermittent high glucose on inflammatory cytokine production in human retinal pericytes (HRP) and explored the role of endoplasmic reticulum (ER) stress in pericyte inflammation. We found that intermittent high glucose, but not constant high glucose, increases MCP-1 secretion and expression of activating transcription factor 4 (ATF4) and C/EBP homologous protein (CHOP), key mediators of ER stress-associated inflammation and cell death. Inhibition of ER stress by chemical chaperones successfully prevented glucose fluctuation-induced ATF4/CHOP activation and inflammatory cytokine production. Our results suggest that activation of ER stress by glucose fluctuation may play a causal role in pericyte injury and inflammation in diabetic retinopathy.

Endoplasmic Reticulum Stress and Unfolded Protein Response Pathways: Potential for Treating Age-related Retinal Degeneration

Accumulation of misfolded proteins in the endoplasmic reticulum (ER) and their aggregation impair normal cellular function and can be toxic, leading to cell death. Prolonged expression of misfolded proteins triggers ER stress, which initiates a cascade of reactions called the unfolded protein response (UPR). Protein misfolding is the basis for a variety of disorders known as ER storage or conformational diseases. There are an increasing number of eye disorders associated with misfolded proteins and pathologic ER responses, including retinitis pigmentosa (RP). Herein we review the basic cellular and molecular biology of UPR with focus on pathways that could be potential targets for treating retinal degenerative diseases.

Processing and stability of type IIc sodium-dependent phosphate cotransporter mutations in patients with hereditary hypophosphatemic rickets with hypercalciuria

Mutations in the apically located Na(+)-dependent phosphate (NaPi) cotransporter, SLC34A3 (NaPi-IIc), are a cause of hereditary hypophosphatemic rickets with hypercalciuria (HHRH). We have characterized the impact of several HHRH mutations on the processing and stability of human NaPi-IIc. Mutations S138F, G196R, R468W, R564C, and c.228delC in human NaPi-IIc significantly decreased the levels of NaPi cotransport activities in Xenopus oocytes. In S138F and R564C mutant proteins, this reduction is a result of a decrease in the V(max) for P(i), but not the K(m). G196R, R468W, and c.228delC mutants were not localized to oocyte membranes. In opossum kidney (OK) cells, cell surface labeling, microscopic confocal imaging, and pulse-chase experiments showed that G196R and R468W mutations resulted in an absence of cell surface expression owing to endoplasmic reticulum (ER) retention. G196R and R468W mutants could be partially stabilized by low temperature. In blue native-polyacrylamide gel electrophoresis analysis, G196R and R468W mutants were either denatured or present in an aggregation complex. In contrast, S138F and R564C mutants were trafficked to the cell surface, but more rapidly degraded than WT protein. The c.228delC mutant did not affect endogenous NaPi uptake in OK cells. Thus, G196R and R468W mutations cause ER retention, while S138F and R564C mutations stimulate degradation of human NaPi-IIc in renal epithelial cells. Together, these data suggest that the NaPi-IIc mutants in HHRH show defective processing and stability.

TDP-43 toxicity is mediated by the unfolded protein response-unrelated induction of C/EBP homologous protein expression

Transactive response DNA-binding protein-43 (TDP-43) neuronal toxicity plays an essential role in the pathogenesis of amyotrophic lateral sclerosis and frontotemporal lobar degeneration with ubiquitin-positive inclusions. In our previous study, we showed that low-grade overexpression of TDP-43, which is thought to mimic the gain-of-function of TDP-43, caused neuronal death, mediated by the upregulation of Bim and the downregulation of Bcl-xL in vitro. In this study, we show that TDP-43 overexpression caused the upregulation of C/EBP-homologous protein (CHOP) and that disruption of the CHOP gene markedly attenuated TDP-43-induced cell death. These results indicate that increases in CHOP expression contribute to TDP-43-induced cell death. We also show that the TDP-43-induced upregulation of CHOP expression is mediated by both the upregulation of the mRNA level of CHOP and the attenuation of thedegradation of CHOP, which is independent on the PERK/eIF2α/ATF4 or other pathway related to the unfolded protein response (UPR) to endoplasmic reticulum stress. This study provides the first example of the CHOP-mediated cell death that is independent of the UPR. © 2011 Wiley Periodicals, Inc.

Intracellular pathways of pancreatic β-cell apoptosis in type 1 diabetes

BACKGROUND

Apoptosis of β cells is a feature of type 1 diabetes. It is also increasingly recognized in type 2 diabetes and islet graft rejection.

METHODS

We have studied the intracellular pathways that regulate β-cell apoptosis in type 1 and 2 diabetes. We have examined the role of Bid, a pro-apoptotic member of the Bcl-2 family, using islets from mice deficient in Bid. We also studied the Bcl-2 family molecules involved in killing by using high concentrations of reducing sugars such as glucose or ribose.

RESULTS

We found that Bid-deficient islets are protected from recombinant human perforin and granzyme B, as well as from Fas-mediated killing. This makes Bid a target for protection of β cells from multiple insults relevant to type 1 diabetes. In contrast to granzyme B and death receptor signalling, we found that islets lacking Bim or Puma were protected from glucose toxicity.

CONCLUSIONS

Our data indicate that different stimuli activate different initiator molecules in the Bcl-2-regulated pathway in β cells.

Silencing GADD153/CHOP gene expression protects against Alzheimer's disease-like pathology induced by 27-hydroxycholesterol in rabbit hippocampus

Endoplasmic reticulum (ER) stress is suggested to play a key role in the pathogenesis of neurodegenerative diseases including Alzheimer's disease (AD). Sustained ER stress leads to activation of the growth arrest and leucine zipper transcription factor, DNA damage inducible gene 153 (gadd153; also called CHOP). Activated gadd153 can generate oxidative damage and reactive oxygen species (ROS), increase β-amyloid (Aβ) levels, disturb iron homeostasis and induce inflammation as well as cell death, which are all pathological hallmarks of AD. Epidemiological and laboratory studies suggest that cholesterol dyshomeostasis contributes to the pathogenesis of AD. We have previously shown that the cholesterol oxidized metabolite 27-hydroxycholesterol (27-OHC) triggers AD-like pathology in organotypic slices. However, the extent to which gadd153 mediates 27-OHC effects has not been determined. We silenced gadd153 gene with siRNA and determined the effects of 27-OHC on AD hallmarks in organotypic slices from adult rabbit hippocampus. siRNA to gadd153 reduced 27-OHC-induced Aβ production by mechanisms involving reduction in levels of β-amyloid precursor protein (APP) and β-secretase (BACE1), the enzyme that initiates cleavage of APP to yield Aβ peptides. Additionally, 27-OHC-induced tau phosphorylation, ROS generation, TNF-α activation, and iron and apoptosis-regulatory protein levels alteration were also markedly reduced by siRNA to gadd153. These data suggest that ER stress-mediated gadd153 activation plays a central role in the triggering of AD pathological hallmarks that result from incubation of hippocampal slices with 27-OHC. Our results add important insights into cellular mechanisms that underlie the potential contribution of cholesterol metabolism in AD pathology, and suggest that preventing gadd153 activation protects against AD related to cholesterol oxidized products.

Guards and culprits in the endoplasmic reticulum: glucolipotoxicity and β-cell failure in type II diabetes

The endoplasmic reticulum (ER) is a cellular organelle responsible for multiple important cellular functions including the biosynthesis and folding of newly synthesized proteins destined for secretion, such as insulin. The ER participates in all branches of metabolism, linking nutrient sensing to cellular signaling. Many pathological and physiological factors perturb ER function and induce ER stress. ER stress triggers an adaptive signaling cascade, called the unfolded protein response (UPR), to relieve the stress. The failure of the UPR to resolve ER stress leads to pathological conditions such as β-cell dysfunction and death, and type II diabetes. However, much less is known about the fine details of the control and regulation of the ER response to hyperglycemia (glucotoxicity), hyperlipidemia (lipotoxicity), and the combination of both (glucolipotoxicity). This paper considers recent insights into how the response is regulated, which may provide clues into the mechanism of ER stress-mediated β-cell dysfunction and death during the progression of glucolipotoxicity-induced type II diabetes.

Endoplasmic reticulum stress in the β-cell pathogenesis of type 2 diabetes

Type 2 diabetes is a complex metabolic disorder characterized by high blood glucose in the context of insulin resistance and relative insulin deficiency by β-cell failure. Even if the mechanisms underlying the pathogenesis of β-cell failure are still under investigation, recent increasing genetic, experimental, and clinical evidence indicate that hyperactivation of the unfolded protein response (UPR) to counteract metabolic stresses is closely related to β-cell dysfunction and apoptosis. Signaling pathways of the UPR are “a double-edged sword” that can promote adaptation or apoptosis depending on the nature of the ER stress condition. In this paper, we summarized our current understanding of the mechanisms and components related to ER stress in the β-cell pathogenesis of type 2 diabetes.

Endoplasmic reticulum stress, obesity and diabetes

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

Involvement of endoplasmic stress protein C/EBP homologous protein in arteriosclerosis acceleration with augmented biological stress responses

BACKGROUND

The processes of arteriosclerosis, including atherosclerosis and vascular remodeling, are affected by interactions among numerous biological pathways such as responses to inflammation, oxidative stress, and endoplasmic reticulum stress. C/EBP homologous protein (CHOP), which is well known to induce cellular apoptosis in response to severe endoplasmic reticulum stress, is reportedly upregulated in plaque lesions.

METHODS AND RESULTS

We examined the effects of CHOP deficiency on 2 types of arteriosclerosis: cuff injury-induced neointimal formation and hypercholesterolemia-induced atherosclerosis. Cuff injury-induced neointimal formation was markedly inhibited in CHOP(-/-) mice with suppressed aortic expression of inflammatory factors and smooth muscle cell proliferation-related proteins. A CHOP deficiency also inhibited aortic plaque formation in hypercholesterolemic apolipoprotein E(-/-) mice with suppressed aortic expression of inflammatory factors and oxidative stress markers. Bone marrow transplantation experiments revealed that recipient CHOP deficiency significantly suppressed both cuff injury-induced neointimal formation and hypercholesterolemia-induced atherosclerotic plaque formation to a greater extent than donor CHOP deficiency, suggesting the importance of CHOP in vascular cells for arteriosclerosis progression. Furthermore, in our in vitro experiments, in not only macrophages but also endothelial and smooth muscle cell lines, endoplasmic reticulum stress inducers upregulated inflammation-, adhesion-, or smooth muscle cell proliferation-related proteins, whereas decreased CHOP expression remarkably suppressed endoplasmic reticulum stress-induced upregulation of these proteins.

CONCLUSIONS

In addition to the well-known signaling for apoptosis induction, CHOP may play important roles in augmenting potentially pathological biological stress responses. This noncanonical role of CHOP, especially that expressed in vascular cells, may contribute to the progression of vascular remodeling and atherosclerosis.

In vivo misfolding of proinsulin below the threshold of frank diabetes

OBJECTIVE

Endoplasmic reticulum (ER) stress has been described in pancreatic β-cells after onset of diabetes-a situation in which failing β-cells have exhausted available compensatory mechanisms. Herein we have compared two mouse models expressing equally small amounts of transgenic proinsulin in pancreatic β-cells.

RESEARCH DESIGN AND METHODS

In hProCpepGFP mice, human proinsulin (tagged with green fluorescent protein [GFP] within the connecting [C]-peptide) is folded in the ER, exported, converted to human insulin, and secreted. In hProC(A7)Y-CpepGFP mice, misfolding of transgenic mutant proinsulin causes its retention in the ER. Analysis of neonatal pancreas in both transgenic animals shows each β-cell stained positively for endogenous insulin and transgenic protein.

RESULTS

At this transgene expression level, most male hProC(A7)Y-CpepGFP mice do not develop frank diabetes, yet the misfolded proinsulin perturbs insulin production from endogenous proinsulin and activates ER stress response. In nondiabetic adult hProC(A7)Y-CpepGFP males, all β-cells continue to abundantly express transgene mRNA. Remarkably, however, a subset of β-cells in each islet becomes largely devoid of endogenous insulin, with some of these cells accumulating large quantities of misfolded mutant proinsulin, whereas another subset of β-cells has much less accumulated misfolded mutant proinsulin, with some of these cells containing abundant endogenous insulin.

CONCLUSIONS

The results indicate a source of pancreatic compensation before the development of diabetes caused by proinsulin misfolding with ER stress, i.e., the existence of an important subset of β-cells with relatively limited accumulation of misfolded proinsulin protein and maintenance of endogenous insulin production. Generation and maintenance of such a subset of β-cells may have implications in the avoidance of type 2 diabetes.

Endoplasmic reticulum stress and the unfolded protein response in nonalcoholic fatty liver disease

The underlying causes of nonalcoholic fatty liver disease (NAFLD) are unclear, although recent evidence has implicated the endoplasmic reticulum (ER) in both the development of steatosis and progression to nonalcoholic steatohepatitis. Disruption of ER homeostasis, often termed "ER stress," has been observed in liver and adipose tissue of humans with NAFLD and/or obesity. Importantly, the signaling pathway activated by disruption of ER homeostasis, the unfolded protein response, has been linked to lipid biosynthesis, insulin action, inflammation, and apoptosis. Therefore, understanding the mechanisms that disrupt ER homeostasis in NAFLD and the role of ER-mediated signaling have become topics of intense investigation. The present review will examine the ER and the unfolded protein response in the context of NAFLD.

Unravelling the story of protein misfolding in diabetes mellitus

Both environmental and genetic factors contribute to the development of diabetes mellitus and although monogenic disorders are rare, they offer unique insights into the fundamental biology underlying the disease. Mutations of the insulin gene or genes involved in the response to protein misfolding cause early onset diabetes. These have revealed an important role for endoplasmic reticulum stress in β-cell survival. This form of cellular stress occurs when secretory proteins fail to fold efficiently. Of all the professional secretory cells we possess, β-cells are the most sensitive to endoplasmic reticulum stress because of the large fluctuations in protein synthesis they face daily. Studies of endoplasmic reticulum stress signaling therefore offer the potential to identify new drug targets to treat diabetes.

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.

Endoplasmic reticulum stress and pancreatic β-cell death

In pancreatic β-cells, the endoplasmic reticulum (ER) is an important cellular compartment for insulin biosynthesis, which accounts for half of the total protein production in these cells. Protein flux through the ER must be carefully monitored to prevent dysregulation of ER homeostasis and stress. ER stress elicits a signaling cascade known as the unfolded protein response (UPR), which influences both life and death decisions in cells. β-cell loss is a pathological component of both type 1 and type 2 diabetes, and recent findings suggest that ER stress is involved. In this review, we address the transition from the physiological ER stress response to the pathological response, and explore the mechanisms of ER stress-mediated β-cell loss during the progression of diabetes.

The peroxide dilemma: opposing and mediating insulin action

Recent compelling data show that reactive oxygen species (ROS) not only are a harmful by-product of aerobic metabolism, but also are used as signaling molecules to regulate various cellular processes. In mammalian cells, ROS are produced transiently in response to many extracellular stimuli, including insulin, and specific inhibition of the ROS suppresses insulin-dependent signaling. Initially, this finding rationalized the concept of ROS acting as insulin mimetics. However, it is becoming evident that ROS are also causal to diabetes, a metabolic disorder characterized by insufficiency of secretion of, or receptor insensitivity to, endogenous insulin. This notion underlines a dual role for ROS in insulin signaling as both deleterious and beneficiary. Moreover, it strongly suggests that a delicate redox balance is required for insulin signaling to remain "healthy" for an organism.

Endoplasmic reticulum stress and glucose homeostasis

PURPOSE OF REVIEW

Balancing glucose homeostasis is crucial to maintain appropriate energy and metabolic state. Chronic hyperglycemia with insulin resistance and development of type II diabetes mellitus is a growing health and health-economic threat. The unfolded protein response (UPR) is a mechanism by which the endoplasmic reticulum copes with diverse physiological and pathophysiological stress stimuli. Unresolved and chronic endoplasmic reticulum stress are important features in the development of diabetes mellitus. Understanding how the UPR impacts glucose balance and what disrupts this balance is critical for development of future therapies.

RECENT FINDINGS

In pancreatic β-cells, evidence is growing that the single branches of the UPR work in concert to supply insulin in response to acute glucose availability. Chronic glucose stimulation disrupts these primarily adaptive changes into an overwhelming UPR, which leads to reduced insulin supply and β-cell mass due to apoptosis. In hepatocytes, the UPR interacts with key transcription factors to physiologically regulate glucose and lipid homeostasis. Prolonged endoplasmic reticulum stress disrupts these feedback loops and results in ongoing gluconeogenesis and steatosis.

SUMMARY

Unraveling the molecular networks underlying the adaptive and contra-adaptive roles of the UPR in glucose metabolism will identify novel therapeutic approaches in the battle against diabetes mellitus.