Diabetes as a disease of endoplasmic reticulum stress

DIABETES/METABOLISM RESEARCH AND REVIEWS REVIEW ARTICLE Diabetes Metab Res Rev 2010; 26: 611–621. Published online 3 October 2010 in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/dmrr.1132

Diabetes as a disease of endoplasmic reticulum stress

Sally E. Thomas Lucy E. Dalton Marie-Louise Daly Elke Malzer Stefan J. Marciniak* Department of Medicine, Cambridge Institute for Medical Research, University of Cambridge, Wellcome Trust/MRC Building, Cambridge, UK *Correspondence to: Stefan J. Marciniak, Department of Medicine, Cambridge Institute for Medical Research, University of Cambridge, Wellcome Trust/MRC Building, Hills Road, Cambridge CB0 2XY, UK. E-mail: [email protected]

Summary Endoplasmic reticulum (ER) stress is an integral part of life for all professional secretory cells, but it has been studied to greatest depth in the pancreatic β-cell. This reflects both the crucial role played by ER stress in the pathogenesis of diabetes and also the exquisite vulnerability of these cells to ER dysfunction. The adaptive cellular response to ER stress, the unfolded protein response, comprises mechanisms to both regulate new protein translation and a transcriptional program to allow adaptation to the stress. The core of this response is a triad of stress-sensing proteins: protein kinase R-like endoplasmic reticulum kinase (PERK), inositol-requiring enzyme 1 (IRE1) and activating transcription factor 6. All three regulate portions of the transcriptional unfolded protein response, while PERK also attenuates protein synthesis during ER stress and IRE1 interacts directly with the c-Jun amino-terminal kinase stress kinase pathway. In this review we shall discuss these processes in detail, with emphasis given to their impact on diabetes and how recent findings indicate that ER stress may be responsible for the loss of β-cell mass in the disease. Copyright  2010 John Wiley & Sons, Ltd. Keywords

ER stress; diabetes; UPR; PERK

Introduction Protein folding is deceptively complex and often ignored until its failure causes disease [1]. Secretory proteins pose a particular problem by being folded in a compartment separate from their site of translation. They are synthesized by ribosomes in the cytosol and co-translationally enter the endoplasmic reticulum (ER) lumen. If proteins enter the ER faster than they can be folded, there is the risk they might aggregate. It is this threat of aggregation that has been termed ‘ER stress’ and the cell’s homeostatic mechanisms have been called the unfolded protein response (UPR). This response is triggered by three ER membrane proteins, IRE1, PERK and activating transcription factor 6 (ATF6), each of which controls a separate arm of the UPR. In this review, we shall discuss the role of each arm in turn and evaluate the evidence suggesting their importance to the pathogenesis of diabetes.

Oxidative folding of insulin

Received: 21 April 2010 Revised: 17 August 2010 Accepted: 6 September 2010

Copyright  2010 John Wiley & Sons, Ltd.

When proinsulin enters the ER lumen, it interacts with many chaperones, such as immunoglobulin binding protein (BiP), a major Hsp70 family member. Early on, proinsulin is oxidized to form three intra-molecular disulphide bonds. The oxidizing potential to create these bonds is generated by the flavoenzyme ER oxidase 1 (ERO1) [2–4]. In mammals this enzyme exists as two isoforms, ERO1α and ERO1β, both of which

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are transcriptional targets of the UPR [5,6]. The ERO1β isoform appears to be expressed at especially high levels in pancreatic islets [7]. ERO1 forms intermolecular disulphide bonds with protein disulphide isomerase, which are then imparted to insulin by the formation of disulphide-bonded enzyme intermediates [8]. This process is rapid and initially indiscriminating, resulting in non-native bonds that must be re-arranged [9]. This process, referred to as disulphide bond reshuffling, requires further rounds of protein disulphide isomerase interaction. Creating disulphide bonds imposes an oxidative stress upon the cell and the degree of stress correlates with the number of disulphide bonds formed [10]. Insulin, with its three disulphide bonds per molecule, is likely to impose a significant oxidative burden to the β-cell (Figure 1). But, even in health, a proportion of new proteins may fail to fold appropriately. These terminally misfolded proteins are returned to the cytosol, ubiquitinated and degraded by the proteosome, a process called ER-associated degradation [11].

PERK An immediate response to ER stress is attenuation of protein synthesis to reduce the load of ER client proteins [12–14]. This is mediated by the ER-resident kinase PERK, which phosphorylates eukaryotic translation initiation factor 2 alpha (eIF2α), part of the complex that recruits the initiator methionyl-tRNA to the ribosome [15]. The complex requires GTP for its activity and hydrolyses this to GDP during each initiation cycle. Since GDP is bound more tightly than GTP, a guanine nucleotide exchange factor called eIF2B is needed to reactivate the complex [16]. Phosphorylation of eIF2α greatly increases its affinity for eIF2B, turning it into an eIF2B inhibitor and preventing further GTP exchange. Since eIF2α can

Figure 1. Endoplasmic reticulum (ER) stress. Newly synthesized protein enters the ER. Some client proteins, including insulin, require disulphide bond formation. The oxidizing potential is generated by the enzyme ER oxidase 1. Reactive oxygen species are generated as a by-product. Correctly folded proteins exit the ER in secretory vesicles, while misfolded proteins are targeted for proteasomal degradation; a process termed ER-associated degradation. When an excess of misfolded proteins overwhelms the degradation machinery, they are retained within the ER and trigger ER stress signalling Copyright  2010 John Wiley & Sons, Ltd.

Figure 2. Endoplasmic reticulum stress signalling in diabetes. (A) Misfolded proteins in the endoplasmic reticulum trigger three signalling molecules. PERK phosphorylates eIF2α leading both to inhibition of new protein translation and to induction of the activating transcription factor 4. IRE1 initiates splicing of the mRNA of the transcription factor XBP-1. Activating transcription factor 6 is cleaved within the Golgi apparatus to produce an active transcription factor fragment. Together activating transcription factor 4, XBP-1 and activating transcription factor 6 transactivate genes of the unfolded protein response. (B) When activated by ER stress, PERK phosphorylates eIF2α. This inhibits protein synthesis relieving the load of proteins entering the ER and activates ATF4 translation, which in turn activates CHOP to induce GADD34. Dephosphorylation of eIF2α by GADD34/PP1 holoenzyme completes a negative feedback cycle. (C) Recruitment of TNF receptor-associated factor 2 to activated IRE1 initiates a kinase cascade that ultimately leads to phosphorylation of insulin receptor substrate 1 on serine residues. This impairs signalling by the insulin receptor and thus causes insulin resistance

be phosphorylated by other stress-sensitive kinases, this process is called the integrated stress response [17] (Figure 2A and B). The importance of PERK to β-cell function is highlighted by a rare human syndrome. Wolcott and Rallison first described two siblings who developed early-onset Diabetes Metab Res Rev 2010; 26: 611–621. DOI: 10.1002/dmrr

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diabetes mellitus and defects of bone development [18]. Subsequent studies mapped the mutation of the Wolcott–Rallison syndrome to 2p12 where candidate gene analysis identified mutations in the PERK gene [19]. Deletion of PERK recapitulates the phenotype in mice [20]. Affected pups are born with islets capable of normal insulin secretion. Indeed, when isolated PERK−/− islets from neonates are cultured in hyperglycaemic conditions, their insulin secretion exceeds that of wild-type controls [20]. Nonetheless, during the first weeks of life, PERK−/− animals develop diabetes through β-cell death. This suggests that PERK is essential for β-cells to cope with physiological ER stress arising from day-to-day insulin synthesis. However, in PERK−/− animals and patients with Wolcott–Rallison syndrome, new proteins continue to enter the ER regardless of its ability to fold them. This manifests as the accumulation of misfolded products, excessive stress signalling and ultimately cell death [21]. The β-cell’s sensitivity to PERK deficiency manifests most strikingly during proliferation and differentiation and so may have relevance to maintenance of β-cell mass (see below) [22]. Regulation of PERK involves interaction between its ER luminal domain and the chaperone BiP [23,24]. In unstressed conditions, BiP maintains PERK as inactive monomers, but during ER stress, unfolded proteins sequester BiP, freeing PERK to oligomerize and transautophosphorylate [23–25]. This stimulates its kinase activity and increases its affinity for eIF2α [26]. In vitro studies of recombinant PERK have shown that following trans-autophosphorylation, kinase dimers of PERK’s domain remain tightly associated [26]. This persisting association in vitro contrasts with the rapid dissociation of PERK seen in vivo following the relief of ER stress, indicating that factors which inactivate PERK exist (unpublished observations, SJM). Once identified, inhibition of these factors may allow pharmacological modulation of PERK. Protein translation rates can be uncoupled from PERK by mutating the phosphorylation site of eIF2α from serine to alanine [27]. This eIF2α S51A mutant functions normally in the eIF2 complex but is insensitive to the eIF2α kinases. Homozygous eIF2α S51A animals are born with severe β-cell deficiency and die within 24 h. In contrast, eIF2α S51A heterozygotes appear normal but are sensitive to nutrient excess [28]. When fed a high-fat diet they are prone to develop obesity because of a failure to increase energy expenditure appropriately. They also suffer failure of glucose homeostasis with hyperinsulinaemia and hyperleptinaemia. Isolated islets from these animals show increased basal insulin secretion but reduced stimulated insulin secretion. Eventually, prolonged high-fat feeding leads to distension of the ER by misfolded proteins (including proinsulin) and the development of frank diabetes. This demonstrates that eIF2αsignalling plays an important role in helping β-cells survive prolonged increased demand, and, moreover, shows that subtle perturbations of eIF2α phosphorylation can have detrimental effects on peripheral tissue insulin sensitivity. Copyright  2010 John Wiley & Sons, Ltd.

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Two regulatory subunits direct protein phosphatase 1 to dephosphorylate eIF2α. These are growth arrest and DNA damage gene 34 (GADD34) and constitutive regulator of eIF2α phosphorylation (CReP) [29,30]. Both GADD34 and CReP were identified in genetic screens for repressors of UPR signalling. GADD34 is normally expressed at very low levels, but is induced during the UPR [30,31]. In contrast, CReP is expressed constitutively [29]. GADD34 induction is necessary for the survival of ER stress in cultured cells, by allowing them to synthesize target proteins of the UPR [31] (Figure 2B). During PERK-mediated translation attenuation, some mRNAs, including that of activating transcription factor 4, are translated paradoxically more efficiently [32–36]. This allows PERK (and other integrated stress response kinases) to induce many genes [17]. Among these, CCAAT/enhancer-binding protein homologous protein (CHOP) is a transcription factor that has been implicated in mediating the lethal effects of PERK signalling [37–39]. Its mechanism remained obscure until we determined that GADD34 itself is a direct target of CHOP [6]. CHOP deletion had been shown to delay the onset of diabetes in mice harbouring the Akita mutation (C96Y) in the insulin 2 gene [40]. Unlike humans, mice possess two genes encoding for insulin, but these are functionally redundant [41]. However, the Akita mutation is inherited as a semi-dominant trait, suggesting a toxic gain-offunction [42,43]. The mutation causes substitution of a conserved cysteine residue required for the formation of an intra-molecular disulphide bond [21,44]. The mutant insulin fails to be secreted but is instead degraded by ERassociated degradation [42,45]. In addition, non-mutant proinsulin becomes trapped in complexes with misfolded Akita proinsulin impairing the secretion of wild-type protein [46]. This toxic effect of misfolded proinsulin is not restricted to mice, as recently a human mutation homologous to that in the Akita mouse has been identified as a rare cause of neonatal diabetes [47]. Now, multiple human insulin mutations and at least one further spontaneous mouse mutation (Munich) have been described, which cause diabetes by proinsulin misfolding and UPR activation [47,48]. However, the protective effects of CHOP deletion are not restricted to insulin mutations. It also protects against cytokine-induced β-cell death, in which nitric oxide triggers ER stress by the depletion of ER calcium stores [49]. Attempts to identify transcriptional targets of CHOP that link it with activation of cell death yielded surprising results. Rather than activating a set of pro-apoptotic genes, CHOP was shown to induce genes that defend ongoing protein secretion during ER stress, one of which is GADD34 [6]. To appreciate why, under some circumstances, GADD34 induction might be toxic, one should consider that during stress a cell is challenged with balancing the need to protect its ER from excess load, against the need to fulfil its secretory demands. It is likely that, on occasion, the organism may benefit from continued protein secretion even at the expense of impairing the viability of some cells. The apparent protective effect of CHOP deletion would reflect a re-setting of Diabetes Metab Res Rev 2010; 26: 611–621. DOI: 10.1002/dmrr

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this balance in favour of reduced ER load. The prediction of such a model is that GADD34 mutation would have a similar protective effect, and this indeed is the case [6]. Mice in which the GADD34 gene has been inactivated are outwardly healthy and, when treated with toxins which cause ER stress, suffer less tissue damage. Given that GADD34 enhances the toxicity of ER stress and that animals remain healthy despite GADD34 inactivation, might GADD34 inhibition be of therapeutic benefit in diseases of ER stress? A screen for small molecules that ameliorate the toxicity of ER stress identified salubrinal, a compound that prevented eIF2α dephosphorylation in herpes simplex virus-infected cells [50]. It is unclear if salubrinal acts directly or selectively against eIF2α phosphatases, but it might represent the prototype for a new class of GADD34 inhibitor. Undoubtedly, any therapeutic inhibition of the eIF2α phosphatases would require careful titration. In primary β-cell cultures, salubrinal itself can cause toxicity through eIF2α hyperphosphorylation leading to apoptosis [51]. The regulation of the eIF2α phosphatases also deserves further study. GADD34 has been shown to be the substrate of protein kinases that affect its toxicity in other settings and it has been shown to bind multiple other proteins [52–59]. It is interesting, therefore, that protein synthesis in β-cells appears to be enhanced by increased glucose levels via a rapid dephosphorylation of eIF2α and that salubrinal treatment was able to blunt this response [60]. Conversely, new agents that enhance insulin biosynthesis may function by augmenting the translational recovery caused by GADD34. Incretins are a family of hormones that stimulate insulin secretion in response to nutrient entry into the gut [61]. Their actions are mediated, at least in part, by the glucagon-like peptide 1 receptor. Exendin-4 is an agonist at this receptor and has been shown to enhance insulin synthesis and secretion, whilst promoting β-cell survival [62–64]. Remarkably, these actions appear to involve induction of activating transcription factor 4 and increased GADD34 expression [65]. The increased levels of activating transcription factor 4 and early reversal of translational attenuation appear to allow both increased insulin synthesis, while promoting β-cell survival during ER stress. An independent study showed exendin-4 to promote cellular survival by reducing levels of ER stress [66]. These encouraging findings suggest that through their effects on the ER, glucagon-like peptide 1 agonists may prove useful in treating type 2 diabetes by augmenting insulin secretion and opposing the loss of β-cell mass. Another mechanism to regulate protein synthesis involves the assembly of proteins onto the 5
mRNA cap [67]. First, eIF4E binds to the 5
cap and then is bound by eIF4G, allowing ribosome recruitment, and eIF4A to unwind RNA secondary structure. This assembly can be disrupted by other factors that bind to eIF4E and displace 4G (4E-binding proteins or 4E-BPs). It is now known that 4E-BP1 is a target of the UPR [68]. During ER stress 4E-BP1 levels increase in β-cells and attenuate protein synthesis long after eIF2α has been dephosphorylated. Importantly, it has been shown in vivo Copyright  2010 John Wiley & Sons, Ltd.

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that this mechanism protects β-cells from death during ER stress. Protein misfolding is not the only danger resulting from elevated ER protein flux; there is also the oxidative burden it imposes. It is therefore unsurprising that UPR targets include antioxidants genes [17]. However, the UPR not only acts to limit ER stress by reducing protein synthesis but it also helps adapt the cell to the increased demand. Accordingly, not only is the ability to buffer reactive oxygen species enhanced but so too is the capacity to perform oxidative protein folding. For this reason, ERO1α is a UPR target gene under the control of CHOP [6]. Once again, the apparent protective effect of CHOP deletion is mediated by preventing increased ER protein synthetic capacity. ERO1α fails to be upregulated in CHOP−/− cells, which consequently experience lower reactive oxygen species levels. Subsequent studies confirmed that CHOP deletion reduced oxidative damage and improved β-cell survival in models of diabetes [69]. Of note, CHOP deletion increased β-cell mass and prevented glucose intolerance both in eIF2α S51A mice fed a high-fat diet and in leptin receptor-deficient mice. This was mediated by increased β-cell proliferation and by reduced apoptosis. When mice were generated that expressed the eIF2α S51A mutant selectively in β-cells, they developed diabetes due to uncontrolled proinsiulin synthesis and increased oxidative stress [70]. The phenotype was ameliorated by the inclusion of antioxidants to their diet. This may be particularly relevant to the β-cell, as antioxidant levels appear to be especially low in the islet compared to that in other tissues [71,72]. It has also been shown that treating β-cells with antioxidants increases their ability to withstand high glucose levels and continue to secrete insulin [73–75]. The particular vulnerability of β-cells to oxidant stress may explain the toxicity of deleting the NCB5OR gene, which encodes an ER-localized reductase [76]. Although the enzyme is ubiquitously expressed, the main phenotype of the knockout animal is β-cell death. This does not involve induction of ER stress. Instead, reactive oxygen species remain unbuffered and cause oxidative damage. ERO1α may therefore represent a target for therapeutic intervention in diabetes, and it is encouraging that in Caenorhabditis elegans treated with tunicamycin (a glycosylation inhibitor that induces ER stress) knockdown of ero-1 with RNAi extends the animals’ lives [6]. Consistent with this, CHOP−/−;NCB5OR−/− mice have delayed onset of hyperglycaemia compared with CHOP+/+;NCB5OR−/− animals [77].

IRE1 The most ancient arm of the UPR is regulated by IRE1 [76]. This sensor, like PERK, is a type I transmembrane protein that is activated when BiP dissociates during ER stress [23,24,77–79]. However, unlike PERK, its autophosphorylation does not cause kinase substrate recruitment. Instead, it stimulates IRE1’s Diabetes Metab Res Rev 2010; 26: 611–621. DOI: 10.1002/dmrr

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endoribonuclease (RNase) activity [80]. This initiates splicing of X box binding protein-1 (XBP-1) mRNA [81–83]. Most mRNA splicing occurs in the nucleus, but that of XBP-1 takes place at the ER membrane in response to ER stress. The XBP-1 primary transcript is inefficiently translated and the resulting protein is inactive. However, excision by IRE1 of 26 ribonucleotides causes a frame shift, allowing efficient translation of the full-length transcription factor [84]. This links IRE1 activation to an expression program including genes important in protein folding and ER-associated degradation [83,85,86]. In this way, physiological IRE1 activation improves cell survival during ER stress by adapting the ER for increased synthetic activity, and IRE1–XBP-1 target genes are essential for the differentiation of many secretory cell types [87–93]. In an attempt to generate an IRE1 mutant that could be inhibited by the ATP analogue 1 NM-PP1, Papa and colleagues enlarged IRE1’s ATP-binding pocket by mutating a bulky leucine to glycine (L745G) [94]. Remarkably, rather than inhibiting IRE1 activity, binding of 1 NM-PP1 promoted XBP-1 mRNA splicing by this mutant, presumably by inducing a similar conformational change as occurs during trans-autophosphorylation. Using this system, it was shown that selective IRE1 activation ameliorates the toxicity of ER stress in disease models. This raises the possibility that small molecules might be developed to cause similar activation of IRE1, allowing pharmacological induction of XBP-1’s protective transcriptional program [95,96]. Recent work has indicated a novel role for IRE1 in helping to adjust the rate of proinsulin synthesis to match plasma glucose levels [97]. Islet cell IRE1 is phosphorylated in response to elevated plasma glucose. Remarkably, this glucose response appears not to require the normal dissociation of BiP nor lead to XBP-1 mRNA splicing. Instead, initially it promotes proinsulin synthesis, but in the longer term (days) suppresses insulin expression. The latter response might involve a recently described function of IRE1. A subset of genes, whose mRNA levels normally fall during ER stress, was found to be stabilized in IRE1−/− cells [98]. This was independent of XBP-1 splicing and resulted from degradation of these mRNAs by IRE1. This subset of mRNAs includes many secreted proteins, suggesting the process acts to rid the ER of unwanted client proteins. Prolonged IRE1 signalling causes cell death [99]. A clue to the mechanism came from early observations that ER dysfunction can trigger the c-Jun amino-terminal kinase (JNK) [100]. JNK is a member of the mitogenactivated protein kinase (MAPK) family and serves to integrate many signals involved in cell survival, growth and differentiation. It is necessary for triggering apoptosis in response to tumour necrosis factor (TNF) receptor activation and to ultraviolet irradiation. In the case of TNF receptor ligation, receptor phosphorylation promotes the assembly of scaffolding proteins and effectors at the plasma membrane, which mediate signals to the cytoplasm. Intriguingly, the scaffolding protein TNF receptor-associated factor 2 has been shown also to bind Copyright  2010 John Wiley & Sons, Ltd.

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autophosphorylated IRE1 during ER stress [101]. Since IRE1 expression is necessary for ER stress-induced JNK signalling, it appears that a process analogous to that occurring at the plasma membrane during TNF receptor activation may also occur at the ER membrane when IRE1 is activated [101,102]. MAPK cascades classically involve the sequential action of three protein kinases: an MAPK kinase kinase phosphorylates and activates an MAPK kinase, which then activates an MAPK. In the case of TNF, the MAPK kinase kinase is apoptosis signal-regulating kinase 1 (ASK1), which interacts directly with TNF receptor-associated factor 2, ultimately to activate JNK [103]. Activation of ASK1 in response to TNF triggers cell death by mitochondria-dependent caspases [104–106]. In ASK1−/− cells, there is loss of JNK activation during ER stress, suggesting a model in which ASK1 acts as the MAPK kinase kinase linking IRE1 activation and TNF receptor-associated factor 2 recruitment to JNK activation [102]. It remains unclear, however, to what extent this pathway impacts on classical IRE1–XBP-1 signalling. In mice, caspase 12 (which is inactive in most humans [107]) is believed to mediate ER stress-induced death signals through interaction with IRE1 independent of its RNase activity [108]. In contrast, ASK1-interacting protein appears necessary for JNK activation by IRE1, but when deleted also impairs XBP-1 induction, suggesting a possible dependence of XBP-1 splicing on efficient ASK1 signalling [109]. This paradoxical relationship between the pro-survival IRE1–XBP-1 axis and proapoptotic pathways does not stop there. Bax and Bak are core components of the apoptotic machinery involved in pore formation at the mitochondrial membrane during apoptosis. Cells deficient in these proteins are resistant to cell death in response to diverse stimuli including ER stress [110,111]. However, animals with a combined deletion of Bax and Bak targeted to the liver show impaired IRE1 signalling and more extensive tissue damage during ER stress [112]. Binding of insulin triggers insulin receptor tyrosine autophosphorylation and the recruitment of proteins that propagate its signals into the cell. Insulin receptor substrate 1 (IRS1) is a multi-site docking protein targeted in this way to the insulin receptor, but is also a target of JNK [113]. When phosphorylated on tyrosine residues by the insulin receptor, IRS1 is able to bind to multiple targets leading to their activation. However, when phosphorylated on serine residues by JNK, IRS1 signalling is blocked. It has been noted that obesity-induced ER stress causes JNK activation and impaired insulin sensitivity peripheral tissues [114] (Figure 2C). This effect requires IRE1 expression. Interestingly, heterozygous XBP-1−/+ mice, show hypersensitivity to ER stress in exocrine tissues [87] and when fed a high-fat diet also develop peripheral insulin resistance [114]. It seems likely that peripheral tissue ER stress in obesity leads to JNK activation by IRE1, and subsequent phosphorylation of IRS1 on serine residues by JNK causing insulin resistance. Genetic support for this comes from experiments with the ER chaperone oxygen-regulated protein 150. When Diabetes Metab Res Rev 2010; 26: 611–621. DOI: 10.1002/dmrr

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oxygen-regulated protein 150 was knocked down either in the entire animal or specifically in the liver, this led to impaired IRS1-dependent insulin signalling and glucose intolerance [115]. Conversely, overexpression of oxygenregulated protein 150 protected obese mice from diabetes [115,116]. This may provide a novel therapeutic target, since two small molecular ‘chemical chaperones’, 4-phenyl butyric acid and taurine-conjugated ursodeoxycholic acid, relieve ER stress in vivo and improve peripheral insulin sensitivity in obese diabetic mice [117]. There are at least three plausible explanations for how obesity may induce ER stress, which are not mutually exclusive. Some cell types respond to nutrient excess by increasing ER protein synthesis thus increasing ER client load, notably the adipocyte [118]. Obesity increases peripheral tissue inflammation, which can also cause ER stress [119,120]. However, perhaps most interestingly, some circulating lipids directly induce ER stress. Certain fatty acids are ‘lipotoxic’ to some cell types, especially β-cells [121,122]. Saturated free fatty acids in particular trigger ER stress and cell death [123–126]. For example, palmitate (a saturated fatty acid) induces ER stress and apoptosis, while oleate (a mono-saturate) does not [122,123,126–129]. It may be the ratio of saturated to unsaturated fatty acids that is toxic, since co-administration of palmitate and oleate is less noxious than palmitate alone [127]. This may be amenable to therapeutic intervention, either by increased ingestion of unsaturated fatty acids or by promoting desaturation. The enzyme stearoyl-coenzyme A desaturase catalyses the rate-limiting step in mono-unsaturated lipid synthesis and its upregulation protects β-cells from lipotoxicity [124]. It is not clear why saturated fatty acids induce ER stress, which, in contrast to cytokine-induced ER stress, involves neither nuclear factor-κB nor nitric oxide signalling [122]. It may be relevant that saturated lipids are rapidly incorporated into microsomal membranes and this is associated with gross changes in ER morphology [130]. In cultured MIN6 insulinoma cells, palmitate, but not oleate, impairs traffic between the ER and Golgi without affecting ER protein folding [131]. This could promote accumulation of proteins in the ER and increase the risk of aggregation. Also, palmitate treatment depletes ER calcium in MIN6 and INS-1 insulinoma cells, which may induce ER stress since BiP requires calcium to function [132,133]. The interaction between IRE1 and insulin signalling may yet prove even more complex. Protein tyrosine phosphatase-1B (PTP-1B) is an ER-localized enzyme that has been shown to regulate negatively insulin signalling and is therefore a target of interest in the field of diabetes drug design [134]. Mice deficient in PTP-1B are protected from developing diabetes [135] and inhibitors of PTP-1B enhance insulin signalling [136,137]. It has been shown that cells lacking PTP-1B are deficient in IRE1 signalling, both XBP-1 splicing and JNK activation [138], but it is unclear how PTP-1B deletion causes these effects. IRE1 is not known to undergo tyrosine phosphorylation, but tyrosine autophosphorylation was Copyright  2010 John Wiley & Sons, Ltd.

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only recently detected in PERK [25]. Moreover, one could speculate that PTP-1B might regulate IRE1 indirectly. Recently, the p85 regulatory subunit of phosphoinositide 3-kinase was shown to be necessary for JNK activation during ER stress [139], and, since phosphoinositide 3-kinase activity is sensitive to PTP-1B through its actions on the insulin receptor and IRS1 [140], this might enable interaction between phosphoinositide 3-kinase signalling and ER stress-dependent JNK activation. Such a pathway, of course, would require further detailed characterization, but may constitute a novel therapeutic target.

Activating transcription factor 6 The third arm of the UPR is governed by a class of stress sensor unrelated to either IRE1 or PERK, but shares functional similarity with the sterol regulatory elementbinding proteins [141–143]. The prototypical member of this family is a transcription factor, ATF6, originally identified as a ligand of the ER stress-responsive element found in the promoters of many UPR genes [141]. It is a type II transmembrane protein retained in the ER through interaction with BiP [144]. However, unlike PERK and IRE1, when ATF6 is released from BiP during ER stress, rather than oligomerize, it traffics to the Golgi [143,144]. In a reaction analogous to the proteolytic processing of the sterol regulatory element-binding proteins, ATF6 is cleaved by site-1 protease and site-2 protease, which liberate a soluble cytosolic transcription factor free to migrate into the nucleus [145]. There are several related paralogues of ATF6 (luman [146–148], OASIS [149], CREBH [150] and CREB4 [151]), some of which are tissue specific, e.g. OASIS is found only in astrocytes while CREBH is restricted to the liver. At least in the case of CREBH, this allows a tissue-specific response to ER stress, characterized by enhanced acute phase response protein synthesis in by the liver.

ER stress and β-cell mass ER stress and the maintenance of β-cell mass appear to be linked. Pancreatic duodenal homeobox 1 (Pdx1 = maturity-onset diabetes of the young, type 4 (MODY4)) is a transcription factor involved in early pancreas formation and mature β-cell survival [152]. In humans, heterozygous mutations in PDX1 are associated with type 2 diabetes [153] and in mice, it is required for islet cell compensation for insulin resistance by allowing enhanced insulin secretion or β-cell mass expansion [154,155]. Recently, it was shown that Pdx1+/− mice showed increased ER stress and knockdown of Pdx1 in cultured insulinoma MIN6 cells increased apoptosis in response to ER stress [156]. This appeared to be mediated through impaired expression of many ER-related genes in Pdx1deficient cells. Diabetes Metab Res Rev 2010; 26: 611–621. DOI: 10.1002/dmrr

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The turnover of β-cells in the pancreas and the mechanism by which these cells replenish has been hotly debated. Initially, two models were proposed, namely ‘β-cell self-duplication’ whereby β-cells could replicate, and ‘neogenesis’ by which new insulin-secretory cells arise from other cell types. Despite some evidence that β-cells can arise from duct epithelial [157,158], acinar [159,160] or pluripotent islet stem cells [161], there is now good evidence that β-cell self-duplication is the major mechanism. In an elegant experiment, insulin-secreting cells in adult mice were lineage tagged by the expression of human placental phosphatase and over a 2-year period the majority of new β-cells expressed this marker [162]. This clearly demonstrated that the progenitor cell for most, if not all, β-cells in the adult mouse is itself an insulin-secreting cell. Moreover, in a separate study when 80% of β-cells were killed by diphtheria toxin, animals recovered glucose homeostasis by increasing β-cell mass and lineage analysis confirmed that these new cells were derived from the surviving β-cell population [163]. Even in humans, replication of β-cells has been observed [164]. This raises an important question: could factors that affect the cell cycle, e.g. ER stress, affect the ability of patients to maintain their β-cell mass? Solid tumours deficient in PERK grow poorly in hypoxic conditions thus limiting their size, suggesting a role for the integrated stress response in enabling tissues to match cell proliferation with oxygen and nutrient supply [165]. Recently, PERK−/− islets have been shown to develop fewer and smaller insulinomas when made to express SV40 large T antigen owing to reduced β-cell proliferation and reduced tumour vascularity [166]. Regulating the rate of protein translation in response to ambient nutrient levels is an important factor in determining cell growth and helps to integrate signalling from other stress-sensitive kinases such as AMPK and mTOR [167]. PERK activation has been shown to cause loss of cyclin D1 resulting in G1 cell cycle arrest during ER stress, which may reflect inhibition of cyclin D1 translation or its increased proteasomal degradation [168–170]. However, growth arrest is only attenuated, not abolished in PERK−/− cells, suggesting the existence of PERK and cyclin D1-independent regulation of the cell cycle [171]. In addition, islets from neonatal PERK−/− mice show a deficit of gene products involved in G2-M cell cycle transition [22]. A number of reports have identified changes in p53 phosphorylation and its subcellular localization during ER stress, which may mediate some of these effects [171–175]. Impaired ribosomal biogenesis leads to G1 cell cycle arrest through an inhibitory interaction between ribosomal proteins rpL5, rpL11 and rpL23 with MDM2, an E3 ligase of p53 [176–180]. Recently, ribosomal RNA synthesis has been shown to be repressed rapidly upon eIF2α phosphorylation by PERK [181] and ER stress has been shown to promote rpL5, rpL11 and rpL23 interaction with MDM2 causing p53 stabilization [171]. However, ER stress has also been shown to induce glycogen synthase kinase-3β activity in a PERK-dependent manner causing p53 destabilization Copyright  2010 John Wiley & Sons, Ltd.

[172,175]. It is therefore essential that further studies address the complex interaction between ER stress signalling and cell cycle regulation.

Conclusion The last 10 years have seen great strides in our understanding of the cellular response to ER stress. It is clear that the pancreatic β-cell is exquisitely sensitive to perturbations of ER function, most likely due to the large swings in protein flux through its secretory pathway and the significant oxidative stress imposed by the synthesis of insulin. Armed with this knowledge, it should soon be possible to generate novel strategies for treating directly the very molecular pathology of diabetes.

Acknowledgements SET is funded by a PhD studentship from Diabetes UK. SJM is an MRC Clinician Scientist (G0601840).

Conflict of interest None to declare.

References 1. Marciniak SJ, Ron D. Endoplasmic reticulum signalling in disease. Physiol Rev 2006; 86: 1133–1149. 2. Pollard MG, Travers KJ, Weissman JS. Ero1p: a novel and ubiquitous protein with an essential role in oxidative protein folding in the endoplasmic reticulum. Mol Cell 1998; 1: 171–182. 3. Frand AR, Kaiser CA. The ERO1 gene of yeast is required for oxidation of protein dithiols in the endoplasmic reticulum. Mol Cell 1998; 1: 161–170. 4. Cabibbo A, Pagani M, Fabbri M, et al. ERO1-L, a human protein that favors disulfide bond formation in the endoplasmic reticulum. J Biol Chem 2000; 275: 4827–4833. 5. Pagani M, Fabbri M, Benedetti C, et al. Endoplasmic reticulum oxidoreductin 1-lbeta (ERO1-Lbeta), a human gene induced in the course of the unfolded protein response. J Biol Chem 2000; 275: 23685–23692. 6. Marciniak SJ, Yun CY, Oyadomari S, et al. CHOP induces death by promoting protein synthesis and oxidation in the stressed endoplasmic reticulum. Genes Dev 2004; 18: 3066–3077. 7. Dias-Gunasekara S, Gubbens J, van Lith M, et al. Tissuespecific expression and dimerization of the endoplasmic reticulum oxidoreductase Ero1beta. J Biol Chem 2005; 280: 33066–33075. 8. Mezghrani A, Fassio A, Benham A, Simmen T, Braakman I, Sitia R. Manipulation of oxidative protein folding and PDI redox state in mammalian cells. EMBO J 2001; 20: 6288–6296. 9. Jansens A, van Duijn E, Braakman I. Coordinated nonvectorial folding in a newly synthesized multidomain protein. Science 2002; 298: 2401–2403. 10. Haynes CM, Titus EA, Cooper AA. Degradation of misfolded proteins prevents ER-derived oxidative stress and cell death. Mol Cell 2004; 15: 767–776. 11. Brodsky JL. The protective and destructive roles played by molecular chaperones during ERAD (endoplasmic-reticulumassociated degradation). Biochem J 2007; 404: 353–363. Diabetes Metab Res Rev 2010; 26: 611–621. DOI: 10.1002/dmrr

618 12. Harding H, Zhang Y, Ron D. Translation and protein folding are coupled by an endoplasmic reticulum resident kinase. Nature 1999; 397: 271–274. 13. Sood R, Porter AC, Ma K, Quilliam LA, Wek RC. Pancreatic eukaryotic initiation factor-2alpha kinase (PEK) homologues in humans, Drosophila melanogaster and Caenorhabditis elegans that mediate translational control in response to endoplasmic reticulum stress. Biochem J 2000; 346(Pt 2): 281–293. 14. Harding H, Zhang Y, Bertolotti A, Zeng H, Ron D. Perk is essential for translational regulation and cell survival during the unfolded protein response. Mol Cell 2000; 5: 897–904. 15. Harding HP, Calfon M, Urano F, Novoa I, Ron D. Transcriptional and translational control in the mammalian unfolded protein response. Annu Rev Cell Dev Biol 2002; 18: 575–599. 16. Clemens M. Protein kinases that phosphorylate eIF2 and eIF2B, their role in eukaryotic cell translational control. In Translational Control, Hershey J, Mathews M, Sonenberg N (eds). Cold Spring Harbor Laboratory Press: Cold Spring Harbor, 1996; 139–172. 17. Harding H, Zhang Y, Zeng H, et al. An integrated stress response regulates amino acid metabolism and resistance to oxidative stress. Mol Cell 2003; 11: 619–633. 18. Wolcott C, Rallison M. Infancy-onset diabetes mellitus and multiple epiphyseal dysplasia. J Pediatrics 1972; 80: 292–297. 19. Delepine M, Nicolino M, Barrett T, Golamaully M, Lathrop GM, Julier C. EIF2AK3, encoding translation initiation factor 2-alpha kinase 3, is mutated in patients with WolcottRallison syndrome. Nat Genet 2000; 25: 406–409. 20. Harding H, Zeng H, Zhang Y, et al. Diabetes mellitus and excocrine pancreatic dysfunction in Perk−/− mice reveals a role for translational control in survival of secretory cells. Mol Cell 2001; 7: 1153–1163. 21. Liu M, Li Y, Cavener D, Arvan P. Proinsulin disulfide maturation and misfolding in the endoplasmic reticulum. J Biol Chem 2005; 280: 13209–13212. 22. Zhang W, Feng D, Li Y, Iida K, McGrath B, Cavener DR. PERK EIF2AK3 control of pancreatic beta cell differentiation and proliferation is required for postnatal glucose homeostasis. Cell Metab 2006; 4: 491–497. 23. Liu CY, Xu Z, Kaufman RJ. Structure and intermolecular interactions of the luminal dimerization domain of human IRE1alpha. J Biol Chem 2003; 278: 17680–17687. 24. Bertolotti A, Zhang Y, Hendershot L, Harding H, Ron D. Dynamic interaction of BiP and the ER stress transducers in the unfolded protein response. Nat Cell Biol 2000; 2: 326–332. 25. Su Q, Wang S, Gao HQ, et al. Modulation of the eukaryotic initiation factor 2 alpha-subunit kinase PERK by tyrosine phosphorylation. J Biol Chem 2008; 283: 469–475. 26. Marciniak SJ, Garcia-Bonilla L, Hu J, Harding HP, Ron D. Activation-dependent substrate recruitment by the eukaryotic translation initiation factor 2 kinase PERK. J Cell Biol 2006; 172: 201–209. 27. Scheuner D, Song B, McEwen E, et al. Translational control is required for the unfolded protein response and in-vivo glucose homeostasis. Mol Cell 2001; 7: 1165–1176. 28. Scheuner D, Mierde DV, Song B, et al. Control of mRNA translation preserves endoplasmic reticulum function in beta cells and maintains glucose homeostasis. Nat Med 2005; 11: 757–764. 29. Jousse C, Oyadomari S, Novoa I, et al. Inhibition of a constitutive translation initiation factor 2a phosphatase, CReP, promotes survival of stressed cells. J Cell Biol 2003; 163: 767–775. 30. Novoa I, Zeng H, Harding H, Ron D. Feedback inhibition of the unfolded protein response by GADD34-mediated dephosphorylation of eIF2a. J Cell Biol 2001; 153: 1011–1022. 31. Novoa I, Zhang Y, Zeng H, Jungreis R, Harding HP, Ron D. Stress-induced gene expression requires programmed recovery from translational repression. EMBO J 2003; 22: 1180–1187. 32. Lu PD, Harding HP, Ron D. Translation re-initiation at alternative open reading frames regulates gene expression in an integrated stress response. J Cell Biol 2004; 167: 27–33. 33. Mueller PP, Jackson BM, Miller PF, Hinnebusch AG. The first and fourth upstream open reading frames in GCN4 mRNA have similar initiation efficiencies but respond differently in translational control to change in length and sequence. Mol Cell Biol 1988; 8: 5439–5447. Copyright  2010 John Wiley & Sons, Ltd.

S. E. Thomas et al. 34. Tzamarias D, Roussou I, Thireos G. Coupling of GCN4 mRNA translational activation with decreased rates of polypeptide chain initiation. Cell 1989; 57: 947–954. 35. Hinnebusch AG. Gene-specific translational control of the yeast GCN4 gene by phosphorylation of eukaryotic initiation factor 2. Mol Microbiol 1993; 10: 215–223. 36. Vattem KM, Wek RC. Reinitiation involving upstream ORFs regulates ATF4 mRNA translation in mammalian cells. Proc Natl Acad Sci U S A 2004; 101: 11269–11274. 37. Wang X-Z, Lawson B, Brewer J, et al. Signals from the stressed endoplasmic reticulum induce C/EBP homologous protein (CHOP/GADD153). Mol Cell Biol 1996; 16: 4273–4280. 38. Zinszner H, Kuroda M, Wang X, et al. CHOP is implicated in programmed cell death in response to impaired function of the endoplasmic reticulum. Genes Dev 1998; 12: 982–995. 39. Jousse C, Bruhat A, Harding HP, Ferrara M, Ron D, Fafournoux P. Amino acid limitation regulates CHOP expression through a specific pathway independent of the unfolded protein response. FEBS Lett 1999; 448: 211–216. 40. Oyadomari S, Koizumi A, Takeda K, et al. Targeted disruption of the Chop gene delays endoplasmic reticulum stress-mediated diabetes. J Clin Invest 2002; 109: 525–532. 41. Leroux L, Desbois P, Lamotte L, et al. Compensatory responses in mice carrying a null mutation for Ins1 or Ins2. Diabetes 2001; 50(Suppl. 1): S150–S153. 42. Wang J, Takeuchi T, Tanaka S, et al. A mutation in the insulin 2 gene induces diabetes with severe pancreatic beta-cell dysfunction in the Mody mouse. J Clin Invest 1999; 103: 27–37. 43. Izumi T, Yokota-Hashimoto H, Zhao S, Wang J, Halban PA, Takeuchi T. Dominant negative pathogenesis by mutant proinsulin in the Akita diabetic mouse. Diabetes 2003; 52: 409–416. 44. Nozaki J, Kubota H, Yoshida H, et al. The endoplasmic reticulum stress response is stimulated through the continuous activation of transcription factors ATF6 and XBP1 in Ins2+/Akita pancreatic beta cells. Genes Cells 2004; 9: 261–270. 45. Allen JR, Nguyen LX, Sargent KE, Lipson KL, Hackett A, Urano F. High ER stress in beta-cells stimulates intracellular degradation of misfolded insulin. Biochem Biophys Res Commun 2004; 324: 166–170. 46. Liu M, Hodish I, Rhodes CJ, Arvan P. Proinsulin maturation, misfolding, and proteotoxicity. Proc Natl Acad Sci U S A 2007; 104: 15841–15846. 47. Stoy J, Edghill EL, Flanagan SE, et al. Insulin gene mutations as a cause of permanent neonatal diabetes. Proc Natl Acad Sci U S A 2007; 104: 15040–15044. 48. Herbach N, Rathkolb B, Kemter E, et al. Dominant-negative effects of a novel mutated Ins2 allele causes early-onset diabetes and severe beta-cell loss in Munich Ins2C95S mutant mice. Diabetes 2007; 56: 1268–1276. 49. Oyadomari S, Takeda K, Takiguchi M, et al. Nitric oxideinduced apoptosis in pancreatic b-cells is mediated by the endoplasmic reticulum stress pathway. Proc Natl Acad Sci U S A 2001; 98: 10845–10850. 50. Boyce M, Bryant KF, Jousse C, et al. A selective inhibitor of eIF2a dephosphorylation protects cells from ER stress. Science 2005; 307: 935–939. 51. Cnop M, Ladriere L, Hekerman P, et al. Selective inhibition of eukaryotic translation initiation factor 2 alpha dephosphorylation potentiates fatty acid-induced endoplasmic reticulum stress and causes pancreatic beta-cell dysfunction and apoptosis. J Biol Chem 2007; 282: 3989–3997. 52. Adler HT, Chinery R, Wu DY, et al. Leukemic HRX fusion proteins inhibit GADD34-induced apoptosis and associate with the GADD34 and hSNF5/INI1 proteins. Mol Cell Biol 1999; 19: 7050–7060. 53. Hasegawa T, Isobe K. Evidence for the interaction between Translin and GADD34 in mammalian cells. Biochim Biophys Acta 1999; 1428: 161–168. 54. Hasegawa T, Xiao H, Hamajima F, Isobe K. Interaction between DNA-damage protein GADD34 and a new member of the Hsp40 family of heat shock proteins that is induced by a DNA-damaging reagent. Biochem J 2000; 352: 795–800. 55. Hasegawa T, Yagi A, Isobe K. Interaction between GADD34 and kinesin superfamily, KIF3A. Biochem Biophys Res Commun 2000; 267: 593–596. Diabetes Metab Res Rev 2010; 26: 611–621. DOI: 10.1002/dmrr

619

Diabetes and ER Stress 56. Grishin AV, Azhipa O, Semenov I, Corey SJ. Interaction between growth arrest-DNA damage protein 34 and Src kinase Lyn negatively regulates genotoxic apoptosis. Proc Natl Acad Sci U S A 2001; 98: 10172–10177. 57. Wu DY, Tkachuck DC, Roberson RS, Schubach WH. The human SNF5/INI1 protein facilitates the function of the growth arrest and DNA damage-inducible protein (GADD34) and modulates GADD34-bound protein phosphatase-1 activity. J Biol Chem 2002; 277: 27706–27715. 58. Hung WJ, Roberson RS, Taft J, Wu DY. Human BAG-1 proteins bind to the cellular stress response protein GADD34 and interfere with GADD34 functions. Mol Cell Biol 2003; 23: 3477–3486. 59. Shi W, Sun C, He B, et al. GADD34-PP1c recruited by Smad7 dephosphorylates TGF{beta} type I receptor. J Cell Biol 2004; 164: 291–300. 60. Vander Mierde D, Scheuner D, Quintens R, et al. Glucose activates a protein phosphatase-1-mediated signaling pathway to enhance overall translation in pancreatic beta-cells. Endocrinology 2007; 148: 609–617. 61. Drucker DJ. The biology of incretin hormones. Cell Metab 2006; 3: 153–165. 62. Drucker DJ. Glucagon-like peptide-1 and the islet beta-cell: augmentation of cell proliferation and inhibition of apoptosis. Endocrinology 2003; 144: 5145–5148. 63. Kim JG, Baggio LL, Bridon DP, et al. Development and characterization of a glucagon-like peptide 1-albumin conjugate: the ability to activate the glucagon-like peptide 1 receptor in vivo. Diabetes 2003; 52: 751–759. 64. Li Y, Hansotia T, Yusta B, Ris F, Halban PA, Drucker DJ. Glucagon-like peptide-1 receptor signaling modulates beta cell apoptosis. J Biol Chem 2003; 278: 471–478. 65. Yusta B, Baggio LL, Estall JL, et al. GLP-1 receptor activation improves beta cell function and survival following induction of endoplasmic reticulum stress. Cell Metab 2006; 4: 391–406. 66. Tsunekawa S, Yamamoto N, Tsukamoto K, et al. Protection of pancreatic beta-cells by exendin-4 may involve the reduction of endoplasmic reticulum stress; in vivo and in vitro studies. J Endocrinol 2007; 193: 65–74. 67. Richter JD, Sonenberg N. Regulation of cap-dependent translation by eIF4E inhibitory proteins. Nature 2005; 433: 477–480. 68. Yamaguchi S, Ishihara H, Yamada T, et al. ATF4-mediated induction of 4E-BP1 contributes to pancreatic beta cell survival under endoplasmic reticulum stress. Cell Metab 2008; 7: 269–276. 69. Song B, Scheuner D, Ron D, Pennathur S, Kaufman RJ. Chop deletion reduces oxidative stress, improves beta cell function, and promotes cell survival in multiple mouse models of diabetes. J Clin Invest 2008; 118: 3378–3389. 70. Back SH, Scheuner D, Han J, et al. Translation attenuation through eIF2alpha phosphorylation prevents oxidative stress and maintains the differentiated state in beta cells. Cell Metab 2009; 10: 13–26. 71. Sigfrid LA, Cunningham JM, Beeharry N, et al. Antioxidant enzyme activity and mRNA expression in the islets of Langerhans from the BB/S rat model of type 1 diabetes and an insulin-producing cell line. J Mol Med 2004; 82: 325–335. 72. Lenzen S, Drinkgern J, Tiedge M. Low antioxidant enzyme gene expression in pancreatic islets compared with various other mouse tissues. Free Radic Biol Med 1996; 20: 463–466. 73. Tanaka Y, Gleason CE, Tran PO, Harmon JS, Robertson RP. Prevention of glucose toxicity in HIT-T15 cells and Zucker diabetic fatty rats by antioxidants. Proc Natl Acad Sci U S A 1999; 96: 10857–10862. 74. Tanaka Y, Tran PO, Harmon J, Robertson RP. A role for glutathione peroxidase in protecting pancreatic beta cells against oxidative stress in a model of glucose toxicity. Proc Natl Acad Sci U S A 2002; 99: 12363–12368. 75. Kaneto H, Kajimoto Y, Miyagawa J, et al. Beneficial effects of antioxidants in diabetes: possible protection of pancreatic betacells against glucose toxicity. Diabetes 1999; 48: 2398–2406. 76. Cox JS, Shamu CE, Walter P. Transcriptional induction of genes encoding endoplasmic reticulum resident proteins requires a transmembrane protein kinase. Cell 1993; 73: 1197–1206. 77. Okamura K, Kimata Y, Higashio H, Tsuru A, Kohno K. Dissociation of Kar2p/BiP from an ER sensory molecule, Copyright  2010 John Wiley & Sons, Ltd.

78. 79. 80.

81. 82.

83. 84.

85. 86. 87. 88.

89.

90.

91.

92. 93. 94. 95.

96. 97.

98. 99. 100.

Ire1p, triggers the unfolded protein response in yeast. Biochem Biophys Res Commun 2000; 279: 445–450. Kimata Y, Kimata YI, Shimizu Y, et al. Genetic evidence for a role of BiP/Kar2 that regulates Ire1 in response to accumulation of unfolded proteins. Mol Biol Cell 2003; 14: 2559–2569. Bertolotti A, Ron D. Alterations in an IRE1-RNA complex in the mammalian unfolded protein response. J Cell Sci 2001; 114: 3207–3212. Lee KP, Dey M, Neculai D, Cao C, Dever TE, Sicheri F. Structure of the dual enzyme Ire1 reveals the basis for catalysis and regulation in nonconventional RNA splicing. Cell 2008; 132: 89–100. Shen X, Ellis RE, Lee K, et al. Complementary signaling pathways regulate the unfolded protein response and are required for C. elegans development. Cell 2001; 107: 893–903. Yoshida H, Matsui T, Yamamoto A, Okada T, Mori K. XBP1 mRNA is induced by ATF6 and spliced by IRE1 in response to ER stress to produce a highly active transcription factor. Cell 2001; 107: 881–891. Calfon M, Zeng H, Urano F, et al. IRE1 couples endoplasmic reticulum load to secretory capacity by processing the XBP-1 mRNA. Nature 2002; 415: 92–96. Lee K, Tirasophon W, Shen X, et al. IRE1-mediated unconventional mRNA splicing and S2P-mediated ATF6 cleavage merge to regulate XBP1 in signaling the unfolded protein response. Genes Dev 2002; 16: 452–466. Yoshida H, Matsui T, Hosokawa N, Kaufman RJ, Nagata K, Mori K. A time-dependent phase shift in the mammalian unfolded protein response. Dev Cell 2003; 4: 265–271. Lee AH, Iwakoshi NN, Glimcher LH. XBP-1 regulates a subset of endoplasmic reticulum resident chaperone genes in the unfolded protein response. Mol Cell Biol 2003; 23: 7448–7459. Lee AH, Chu GC, Iwakoshi NN, Glimcher LH. XBP-1 is required for biogenesis of cellular secretory machinery of exocrine glands. EMBO J 2005; 24: 4368–4380. Shaffer AL, Shapiro-Shelef M, Iwakoshi NN, et al. XBP1, downstream of Blimp-1, expands the secretory apparatus and other organelles, and increases protein synthesis in plasma cell differentiation. Immunity 2004; 21: 81–93. Sriburi R, Jackowski S, Mori K, Brewer JW. XBP1: a link between the unfolded protein response, lipid biosynthesis and biogenesis of the endoplasmic reticulum. J Cell Biol 2004; 167: 35–41. Iwakoshi NN, Lee AH, Glimcher LH. The X-box binding protein1 transcription factor is required for plasma cell differentiation and the unfolded protein response. Immunol Rev 2003; 194: 29–38. Iwakoshi NN, Lee AH, Vallabhajosyula P, Otipoby KL, Rajewsky K, Glimcher LH. Plasma cell differentiation and the unfolded protein response intersect at the transcription factor XBP-1. Nat Immunol 2003; 4: 321–329. Reimold AM, Etkin A, Clauss I, et al. An essential role in liver development for transcription factor XBP-1. Genes Dev 2000; 14: 152–157. Reimold AM, Iwakoshi NN, Manis J, et al. Plasma cell differentiation requires the transcription factor XBP-1. Nature 2001; 412: 300–307. Papa FR, Zhang C, Shokat K, Walter P. Bypassing a kinase activity with an ATP-competitive drug. Science 2003; 302: 1533–1537. Han D, Upton JP, Hagen A, Callahan J, Oakes SA, Papa FR. A kinase inhibitor activates the IRE1alpha RNase to confer cytoprotection against ER stress. Biochem Biophys Res Commun 2008; 365: 777–783. Lin JH, Li H, Yasumura D, et al. IRE1 signaling affects cell fate during the unfolded protein response. Science 2007; 318: 944–949. Lipson KL, Fonseca SG, Ishigaki S, et al. Regulation of insulin biosynthesis in pancreatic beta cells by an endoplasmic reticulum-resident protein kinase IRE1. Cell Metab 2006; 4: 245–254. Hollien J, Weissman JS. Decay of endoplasmic reticulumlocalized mRNAs during the unfolded protein response. Science 2006; 313: 104–107. Iwawaki T, Hosoda A, Okuda T, et al. Translational control by the ER transmembrane kinase/ribonuclease IRE1 under ER stress. Nat Cell Biol 2001; 3: 158–164. Li WW, Alexandre S, Cao X, Lee AS. Transactivation of the grp78 promoter by Ca2+ depletion. A comparative analysis Diabetes Metab Res Rev 2010; 26: 611–621. DOI: 10.1002/dmrr

620

101.

102.

103. 104. 105. 106.

107. 108.

109. 110. 111.

112. 113. 114. 115. 116. 117. 118. 119.

120.

121. 122.

S. E. Thomas et al. with A23187 and the endoplasmic reticulum Ca(2+)-ATPase inhibitor thapsigargin. J Biol Chem 1993; 268: 12003–12009. Urano F, Wang X, Bertolotti A, et al. Coupling of stress in the endoplasmic reticulum to activation of JNK protein kinases by transmembrane protein kinase IRE1. Science 2000; 287: 664–666. Nishitoh H, Matsuzawa A, Tobiume K, et al. ASK1 is essential for endoplasmic reticulum stress-induced neuronal cell death triggered by expanded polyglutamine repeats. Genes Dev 2002; 16: 1345–1355. Nishitoh H, Saitoh M, Mochida Y, et al. ASK1 is essential for JNK/SAPK activation by TRAF2. Mol Cell 1998; 2: 389–395. Ichijo H, Nishida E, Irie K, et al. Induction of apoptosis by ASK1, a mammalian MAPKKK that activates SAPK/JNK and p38 signaling pathways. Science 1997; 275: 90–94. Saitoh M, Nishitoh H, Fujii M, et al. Mammalian thioredoxin is a direct inhibitor of apoptosis signal-regulating kinase (ASK) 1. EMBO J 1998; 17: 2596–2606. Hatai T, Matsuzawa A, Inoshita S, et al. Execution of apoptosis signal-regulating kinase 1 (ASK1)-induced apoptosis by the mitochondria-dependent caspase activation. J Biol Chem 2000; 275: 26576–26581. Fischer H, Koenig U, Eckhart L, Tschachler E. Human caspase 12 has acquired deleterious mutations. Biochem Biophys Res Commun 2002; 293: 722–726. Yoneda T, Imaizumi K, Oono K, et al. Activation of caspase12, an endoplastic reticulum (ER) resident caspase, through tumor necrosis factor receptor-associated factor 2- dependent mechanism in response to the ER stress. J Biol Chem 2001; 276: 13935–13940. Luo D, He Y, Zhang H, et al. AIP1 is critical in transducing IRE1-mediated endoplasmic reticulum stress response. J Biol Chem 2008; 283: 11905–11912. Wei MC, Zong WX, Cheng EH, et al. Proapoptotic BAX and BAK: a requisite gateway to mitochondrial dysfunction and death. Science 2001; 292: 727–730. Tonnesen MF, Grunnet LG, Friberg J, et al. Inhibition of nuclear factor-kappaB or Bax prevents endoplasmic reticulum stress but not nitric oxide-mediated apoptosis in INS-1E cells. Endocrinology 2009; 150: 4094–4103. Hetz C, Bernasconi P, Fisher J, et al. Proapoptotic BAX and BAK modulate the unfolded protein response by a direct interaction with IRE1alpha. Science 2006; 312: 572–576. Thirone AC, Huang C, Klip A. Tissue-specific roles of IRS proteins in insulin signaling and glucose transport. Trends Endocrinol Metab 2006; 17: 72–78. Ozcan U, Cao Q, Yilmaz E, et al. Endoplasmic reticulum stress links obesity, insulin action, and type 2 diabetes. Science 2004; 306: 457–461. Nakatani Y, Kaneto H, Kawamori D, et al. Involvement of endoplasmic reticulum stress in insulin resistance and diabetes. J Biol Chem 2005; 280: 847–851. Ozawa K, Miyazaki M, Matsuhisa M, et al. The endoplasmic reticulum chaperone improves insulin resistance in type 2 diabetes. Diabetes 2005; 54: 657–663. Ozcan U, Yilmaz E, Ozcan L, et al. Chemical chaperones reduce ER stress and restore glucose homeostasis in a mouse model of type 2 diabetes. Science 2006; 313: 1137–1140. Gregor MF, Hotamisligil GS. Thematic review series: adipocyte biology. Adipocyte stress: the endoplasmic reticulum and metabolic disease. J Lipid Res 2007; 48: 1905–1914. Xue X, Piao JH, Nakajima A, et al. Tumor necrosis factor alpha (TNFalpha) induces the unfolded protein response (UPR) in a reactive oxygen species (ROS)-dependent fashion, and the UPR counteracts ROS accumulation by TNFalpha. J Biol Chem 2005; 280: 33917–33925. Cardozo AK, Ortis F, Storling J, et al. Cytokines downregulate the sarcoendoplasmic reticulum pump Ca2+ ATPase 2b and deplete endoplasmic reticulum Ca2+, leading to induction of endoplasmic reticulum stress in pancreatic beta-cells. Diabetes 2005; 54: 452–461. El-Assaad W, Buteau J, Peyot ML, et al. Saturated fatty acids synergize with elevated glucose to cause pancreatic beta-cell death. Endocrinology 2003; 144: 4154–4163. Kharroubi I, Ladriere L, Cardozo AK, Dogusan Z, Cnop M, Eizirik DL. Free fatty acids and cytokines induce pancreatic beta-cell apoptosis by different mechanisms: role of nuclear factor-kappaB and endoplasmic reticulum stress. Endocrinology 2004; 145: 5087–5096.

Copyright  2010 John Wiley & Sons, Ltd.

123. Laybutt DR, Preston AM, Akerfeldt MC, et al. Endoplasmic reticulum stress contributes to beta cell apoptosis in type 2 diabetes. Diabetologia 2007; 50: 752–763. 124. Wei Y, Wang D, Pagliassotti MJ. Saturated fatty acid-mediated endoplasmic reticulum stress and apoptosis are augmented by trans-10, cis-12-conjugated linoleic acid in liver cells. Mol Cell Biochem 2007; 303: 105–113. 125. Wang D, Wei Y, Pagliassotti MJ. Saturated fatty acids promote endoplasmic reticulum stress and liver injury in rats with hepatic steatosis. Endocrinology 2006; 147: 943–951. 126. Karaskov E, Scott C, Zhang L, Teodoro T, Ravazzola M, Volchuk A. Chronic palmitate but not oleate exposure induces endoplasmic reticulum stress which may contribute to INS1 pancreatic {beta}-cell apoptosis. Endocrinology 2006; 147: 3398–3407. 127. Guo W, Wong S, Xie W, Lei T, Luo Z. Palmitate modulates intracellular signaling, induces endoplasmic reticulum stress, and causes apoptosis in mouse 3T3-L1 and rat primary preadipocytes. Am J Physiol Endocrinol Metab 2007; 293: E576–E586. 128. Wei Y, Wang D, Topczewski F, Pagliassotti MJ. Saturated fatty acids induce endoplasmic reticulum stress and apoptosis independently of ceramide in liver cells. Am J Physiol Endocrinol Metab 2006; 291: E275–E281. 129. Moffitt JH, Fielding BA, Evershed R, Berstan R, Currie JM, Clark A. Adverse physicochemical properties of tripalmitin in beta cells lead to morphological changes and lipotoxicity in vitro. Diabetologia 2005; 48: 1819–1829. 130. Borradaile NM, Han X, Harp JD, Gale SE, Ory DS, Schaffer JE. Disruption of endoplasmic reticulum structure and integrity in lipotoxic cell death. J Lipid Res 2006; 47: 2726–2737. 131. Preston AM, Gurisik E, Bartley C, Laybutt DR, Biden TJ. Reduced endoplasmic reticulum (ER)-to-Golgi protein trafficking contributes to ER stress in lipotoxic mouse beta cells by promoting protein overload. Diabetologia 2009; 52: 2369–2373. 132. Gwiazda KS, Yang TL, Lin Y, Johnson JD. Effects of palmitate on ER and cytosolic Ca2+ homeostasis in beta-cells. Am J Physiol Endocrinol Metab 2009; 296: E690–E701. 133. Cunha DA, Hekerman P, Ladriere L, et al. Initiation and execution of lipotoxic ER stress in pancreatic beta-cells. J Cell Sci 2008; 121: 2308–2318. 134. Kasibhatla B, Wos J, Peters KG. Targeting protein tyrosine phosphatase to enhance insulin action for the potential treatment of diabetes. Curr Opin Investig Drugs 2007; 8: 805–813. 135. Elchebly M, Payette P, Michaliszyn E, et al. Increased insulin sensitivity and obesity resistance in mice lacking the protein tyrosine phosphatase-1B gene. Science 1999; 283: 1544–1548. 136. Umezawa K, Kawakami M, Watanabe T. Molecular design and biological activities of protein-tyrosine phosphatase inhibitors. Pharmacol Ther 2003; 99: 15–24. 137. Xie L, Lee SY, Andersen JN, et al. Cellular effects of small molecule PTP1B inhibitors on insulin signaling. Biochemistry 2003; 42: 12792–12804. 138. Gu F, Nguyen DT, Stuible M, Dube N, Tremblay ML, Chevet E. Protein-tyrosine phosphatase 1B potentiates IRE1 signaling during endoplasmic reticulum stress. J Biol Chem 2004; 279: 49689–49693. 139. Taniguchi CM, Aleman JO, Ueki K, et al. The p85alpha regulatory subunit of phosphoinositide 3-kinase potentiates c-Jun N-terminal kinase-mediated insulin resistance. Mol Cell Biol 2007; 27: 2830–2840. 140. Venable CL, Frevert EU, Kim YB, et al. Overexpression of protein-tyrosine phosphatase-1B in adipocytes inhibits insulinstimulated phosphoinositide 3-kinase activity without altering glucose transport or Akt/protein kinase B activation. J Biol Chem 2000; 275: 18318–18326. 141. Li M, Baumeister P, Roy B, et al. ATF6 as a transcription activator of the endoplasmic reticulum stress element: thapsigargin stress-induced changes and synergistic interactions with NF-Y and YY1. Mol Cell Biol 2000; 20: 5096–5106. 142. Haze K, Yoshida H, Yanagi H, Yura T, Mori K. Mammalian transcription factor ATF6 is synthesized as a transmembrane protein and activated by proteolysis in response to endoplasmic reticulum stress. Mol Biol Cell 1999; 10: 3787–3799. 143. Chen X, Shen J, Prywes R. The luminal domain of ATF6 senses endoplasmic reticulum (ER) stress and causes translocation Diabetes Metab Res Rev 2010; 26: 611–621. DOI: 10.1002/dmrr

Diabetes and ER Stress

144.

145. 146.

147. 148.

149. 150. 151. 152.

153. 154.

155.

156. 157. 158. 159. 160.

161.

162. 163.

of ATF6 from the ER to the Golgi. J Biol Chem 2002; 277: 13045–13052. Shen J, Chen X, Hendershot L, Prywes R. ER stress regulation of ATF6 localization by dissociation of BiP/GRP78 and unmasking of Golgi localization signals. Dev Cell 2002; 3: 99–111. Ye J, Rawson RB, Komuro R, et al. ER stress induces cleavage of membrane-bound ATF6 by the same proteases that process SREBPs. Mol Cell 2000; 6: 1355–1364. Raggo C, Rapin N, Stirling J, et al. Luman, the cellular counterpart of herpes simplex virus VP16, is processed by regulated intramembrane proteolysis. Mol Cell Biol 2002; 22: 5639–5649. Lu R, Misra V. Potential role for luman, the cellular homologue of herpes simplex virus VP16 (alpha gene trans-inducing factor), in herpesvirus latency. J Virol 2000; 74: 934–943. DenBoer LM, Hardy-Smith PW, Hogan MR, Cockram GP, Audas TE, Lu R. Luman is capable of binding and activating transcription from the unfolded protein response element. Biochem Biophys Res Commun 2005; 331: 113–119. Kondo S, Murakami T, Tatsumi K, et al. OASIS, a CREB/ATFfamily member, modulates UPR signalling in astrocytes. Nat Cell Biol 2005; 7: 186–194. Zhang K, Shen X, Wu J, et al. Endoplasmic reticulum stress activates cleavage of CREBH to induce a systemic inflammatory response. Cell 2006; 124: 587–599. Stirling J, O’Hare P. CREB4, a transmembrane bZip transcription factor and potential new substrate for regulation and cleavage by S1P. Mol Biol Cell 2006; 17: 413–426. Babu DA, Deering TG, Mirmira RG. A feat of metabolic proportions: Pdx1 orchestrates islet development and function in the maintenance of glucose homeostasis. Mol Genet Metab 2007; 92: 43–55. Stoffers DA, Ferrer J, Clarke WL, Habener JF. Early-onset typeII diabetes mellitus (MODY4) linked to IPF1. Nat Genet 1997; 17: 138–139. Kulkarni RN, Jhala US, Winnay JN, Krajewski S, Montminy M, Kahn CR. PDX-1 haploinsufficiency limits the compensatory islet hyperplasia that occurs in response to insulin resistance. J Clin Invest 2004; 114: 828–836. Brissova M, Blaha M, Spear C, et al. Reduced PDX-1 expression impairs islet response to insulin resistance and worsens glucose homeostasis. Am J Physiol Endocrinol Metab 2005; 288: E707–E714. Sachdeva MM, Claiborn KC, Khoo C, et al. Pdx1 (MODY4) regulates pancreatic beta cell susceptibility to ER stress. Proc Natl Acad Sci U S A 2009; 106: 19090–19095. Bonner-Weir S, Toschi E, Inada A, et al. The pancreatic ductal epithelium serves as a potential pool of progenitor cells. Pediatr Diabetes 2004; 5(Suppl. 2): 16–22. Gu D, Lee MS, Krahl T, Sarvetnick N. Transitional cells in the regenerating pancreas. Development 1994; 120: 1873–1881. Lipsett M, Finegood DT. Beta-cell neogenesis during prolonged hyperglycemia in rats. Diabetes 2002; 51: 1834–1841. Minami K, Okuno M, Miyawaki K, et al. Lineage tracing and characterization of insulin-secreting cells generated from adult pancreatic acinar cells. Proc Natl Acad Sci U S A 2005; 102: 15116–15121. Zulewski H, Abraham EJ, Gerlach MJ, et al. Multipotential nestin-positive stem cells isolated from adult pancreatic islets differentiate ex vivo into pancreatic endocrine, exocrine, and hepatic phenotypes. Diabetes 2001; 50: 521–533. Dor Y, Brown J, Martinez OI, Melton DA. Adult pancreatic beta-cells are formed by self-duplication rather than stem-cell differentiation. Nature 2004; 429: 41–46. Nir T, Melton DA, Dor Y. Recovery from diabetes in mice by beta cell regeneration. J Clin Invest 2007; 117: 2553–2561.

Copyright  2010 John Wiley & Sons, Ltd.

621 164. Meier JJ, Lin JC, Butler AE, Galasso R, Martinez DS, Butler PC. Direct evidence of attempted beta cell regeneration in an 89-year-old patient with recent-onset type 1 diabetes. Diabetologia 2006; 49: 1838–1844. 165. Bi M, Naczki C, Koritzinsky M, et al. ER stress-regulated translation increases tolerance to extreme hypoxia and promotes tumor growth. EMBO J 2005; 24: 3470–3481. 166. Gupta S, McGrath B, Cavener DR. PERK regulates the proliferation and development of insulin-secreting beta-cell tumors in the endocrine pancreas of mice. PLoS ONE 2009; 4: e8008. 167. Liu L, Cash TP, Jones RG, Keith B, Thompson CB, Simon MC. Hypoxia-induced energy stress regulates mRNA translation and cell growth. Mol Cell 2006; 21: 521–531. 168. Brewer JW, Hendershot LM, Sherr CJ, Diehl JA. Mammalian unfolded protein response inhibits cyclin D1 translation and cell-cycle progression. Proc Natl Acad Sci U S A 1999; 96: 8505–8510. 169. Brewer JW, Diehl JA. PERK mediates cell-cycle exit during the mammalian unfolded protein response. Proc Natl Acad Sci U S A 2000; 97: 12625–12630. 170. Raven JF, Baltzis D, Wang S, et al. PKR and PKR-like endoplasmic reticulum kinase induce the proteasomedependent degradation of cyclin D1 via a mechanism requiring eukaryotic initiation factor 2alpha phosphorylation. J Biol Chem 2008; 283: 3097–3108. 171. Zhang F, Hamanaka RB, Bobrovnikova-Marjon E, et al. Ribosomal stress couples the unfolded protein response to p53-dependent cell cycle arrest. J Biol Chem 2006; 281: 30036–30045. 172. Qu L, Huang S, Baltzis D, et al. Endoplasmic reticulum stress induces p53 cytoplasmic localization and prevents p53dependent apoptosis by a pathway involving glycogen synthase kinase-3beta. Genes Dev 2004; 18: 261–277. 173. Pluquet O, Qu LK, Baltzis D, Koromilas AE. Endoplasmic reticulum stress accelerates p53 degradation by the cooperative actions of Hdm2 and glycogen synthase kinase 3beta. Mol Cell Biol 2005; 25: 9392–9405. 174. Li J, Lee B, Lee AS. Endoplasmic reticulum stress-induced apoptosis: multiple pathways and activation of p53-upregulated modulator of apoptosis (PUMA) and NOXA by p53. J Biol Chem 2006; 281: 7260–7270. 175. Baltzis D, Pluquet O, Papadakis AI, Kazemi S, Qu LK, Koromilas AE. The eIF2alpha kinases PERK and PKR activate glycogen synthase kinase 3 to promote the proteasomal degradation of p53. J Biol Chem 2007; 282: 31675–31687. 176. Lohrum MA, Ludwig RL, Kubbutat MH, Hanlon M, Vousden KH. Regulation of HDM2 activity by the ribosomal protein L11. Cancer Cell 2003; 3: 577–587. 177. Zhang Y, Wolf GW, Bhat K, et al. Ribosomal protein L11 negatively regulates oncoprotein MDM2 and mediates a p53dependent ribosomal-stress checkpoint pathway. Mol Cell Biol 2003; 23: 8902–8912. 178. Dai MS, Lu H. Inhibition of MDM2-mediated p53 ubiquitination and degradation by ribosomal protein L5. J Biol Chem 2004; 279: 44475–44482. 179. Dai MS, Zeng SX, Jin Y, Sun XX, David L, Lu H. Ribosomal protein L23 activates p53 by inhibiting MDM2 function in response to ribosomal perturbation but not to translation inhibition. Mol Cell Biol 2004; 24: 7654–7668. 180. Jin A, Itahana K, O’Keefe K, Zhang Y. Inhibition of HDM2 and activation of p53 by ribosomal protein L23. Mol Cell Biol 2004; 24: 7669–7680. 181. DuRose JB, Scheuner D, Kaufman RJ, Rothblum LI, Niwa M. Phosphorylation of eukaryotic translation initiation factor 2alpha coordinates rRNA transcription and translation inhibition during endoplasmic reticulum stress. Mol Cell Biol 2009; 29: 4295–4307.

Diabetes Metab Res Rev 2010; 26: 611–621. DOI: 10.1002/dmrr