Endoplasmic reticulum: nutrientsensor in physiology and pathologyJozsef Mandl1,2, Tamas Meszaros1,2, Gabor Banhegyi1,2, Laszlo Hunyady3 andMiklos Csala1,2

1 Department of Medical Chemistry, Molecular Biology and Pathobiochemistry, Semmelweis University, H-1094 Budapest, Hungary2 MTA-SE Pathobiochemistry Research Group, H-1094 Budapest, Hungary3 Department of Physiology, Semmelweis University, H-1094 Budapest, Hungary

Review

The endoplasmic reticulum (ER) is a metabolic organelleand an ideal nutrient sensor. In response to hypoglyce-mia, hyperglycemia or fatty acid overload, the ER triggersthe unfolded protein response, which represses proteinsynthesis, alters insulin responsiveness and favorsapoptosis. In addition, the ER affects steroid hormoneactivation and autophagy. The primary aim of theseresponses is to adjust the metabolism to environmentalchanges. Failure of the ER to adapt to changes in nutrientavailability can result in a pathological transition in ERfunctions, as observed in cases of obesity-related dis-eases. This review highlights the recent evidence thatthe ER has a prominent role in cellular adaptation, as wellas in the pathomechanism of type 2 diabetes.

ER: a metabolic hub ideal for nutrient sensingAdaptation to environmental challenges, such as the chan-ging balance between electron donor nutrients and elec-tron acceptor oxygen, is an essential ability of livingorganisms. Inflammation, for example, increases the nutri-ent and oxygen requirement of cells in higher organismsand demandsmobilization of resources. For an appropriateresponse, the cell requires the development of suitablesensors and signaling mechanisms. The endoplasmic reti-culum (ER) participates in all branches of metabolism,linking nutrient sensing to cellular signaling throughthe unfolded protein response and by influencing steroidhormone action. The primary purpose of these ERresponses is clearly to restore the metabolic balance; how-ever, failure to adapt can lead to pathological conditions,such as obesity and type 2 diabetes. This review focuses onthe contribution of the ER to physiological adaptation tochanges in nutrient supply. Interestingly, disturbances insuch ER adaptations contribute to b-cell dysfunction andinsulin resistance in obesity-related diseases, such as themetabolic syndrome and type 2 diabetes.

ER: a metabolic organelleThe ER is an integral contributor to cellular metabolismthat participates in virtually all anabolic and catabolicbranches, such as protein synthesis and degradation, glu-coneogenesis, glycogen synthesis and breakdown, mem-brane lipid synthesis and recycling, fat storage, andhormone and drug metabolism. Considering that the ERcan initiate the unfolded protein response and control local

Corresponding author: Mandl, J. (jozsef.mandl@eok.sote.hu)

194 1043-2760/$ – see front matter � 2009 Els

glucocorticoid activation and, hence, greatly affect insulinproduction and insulin action, this organelle can beregarded as an ideal site for nutrient sensing at the sub-cellular level.

The metabolic versatility of the ER is based on a com-bination of a large multi-domain membrane and a uniqueluminal compartment of carefully controlled milieu. TheER membrane is a continuous protein-rich lipid bilayernetwork that provides a platform for enzymatic reactions ofhydrophobic substances (e.g. cholesterol, phospholipids,triglycerides or xenobiotics) and maintains the specialmicroenvironment of the ER lumen. The membrane andluminal contents of the ER are distributed into othercellular organelles or secreted from the cell by vesiculartransport. The traffic of small water-soluble molecules andinorganic ions across the ER membrane is mediated byselective transporters, many of which have been charac-terized but not yet identified [1]. Because of the selectivepermeability of the ER membrane, the luminal compart-ment of the ER is characteristically different the cytosol.For example, they have different calcium ion concentrationand redox conditions.

The most important metabolic functions of the ER

The large membrane surface and the separated luminalcompartment of the ER are essential for synthesis andprocessing of proteins and lipids. Several enzymes involvedin cholesterol, phosphoglyceride and triglyceride metab-olism are localized in the ER membrane. Carbohydratemetabolism also occurs in the ER lumen, as evidenced bythe presence of the membrane-bound glucose-6-phospha-tase enzyme, which is responsible for hepatic glucoseproduction, and by the presence of hexose-6-phosphatedehydrogenase (H6PDH), which catalyzes the NADP-de-pendent oxidation of glucose-6-phosphate [2].

The rough ER is specialized in synthesizing and proces-sing secretory proteins and proteins of the plasma mem-brane, lysosomes, Golgi and ER itself. In the rough ER,nascent polypeptide chains undergo a complex array of post-translational modifications supervised by ER luminal cha-perones, lectines and foldases [3]. Sophisticated quality-control mechanisms ensure that only properly folded ma-ture proteins are exported by vesicular transport to thesecretory pathway. The misfolded ones are retained forcorrection or retro-translocated to the cytosol for ubiquityla-tionanddegradation (ER-associateddegradation, orERAD)

evier Ltd. All rights reserved. doi:10.1016/j.tem.2009.01.003 Available online 6 April 2009

Box 2. Steroid metabolism in the ER

The endocrine system has a major role in multicellular animals,

controlling and coordinating the responses to environmental

effects. Several enzymes regulating these responses, either by

producing or by eliminating hormones, are located in the ER [69].

This system plays a fundamental part in coordinating intermediary

metabolism (i.e. the internal environment) with exogenous stimuli

(i.e. the external environment).

Type II cytochrome P450 monooxygenase enzymes are located in

the ER and receive electrons from NADPH via P450 oxidoreductase,

in some cases assisted by cytochrome b5. The human genome

encodes 50 type II cytochrome P450 enzymes, including 20 enzymes

involved in the biosynthesis of steroids and other lipids, 15 enzymes

involved in the metabolism of xenobiotic agents and drugs, and 15

enzymes with unidentified function [70]. The type II enzymes

involved in steroid metabolism include: 17a-hydroxylase/17,20 lyase

(CYP17), which is essential for the biosynthesis of glucocorticoids

and sex steroids; 21-hydroxylase (CYP21A2), which catalyzes the 21-

hydroxylation of glucocorticoids and mineralocorticoids; and ar-

omatase (CYP19), which converts androgens to estrogens.

Hydroxysteroid dehydrogenases (HSDs) catalyze bidirectional

reactions in assay systems, but they have a strong directional

preference in cells. Because HSDs interconvert potent and inactive

steroid metabolites, the presence and activity of these enzymes

determines organ responses to steroids. One of these enzymes, 11b-

HSD type 1 (11bHSD1), which converts inactive cortisone to active

cortisol through its reductase activity, can increase glucocorticoid

effects in target tissues. It has been demonstrated that the reductase

activity of 11bHSD1 depends on NADPH synthesized in the ER by

H6PDH [71]. In obese humans, 11bHSD1 is highly expressed in

adipose tissue. In mice, overexpression of 11bHSD1 in adipose or

liver causes obesity and insulin resistance, respectively, whereas

mice lacking 11bHSD1 are resistant to diet-induced obesity and are

insulin sensitive [71,72]. Therefore, 11bHSD1 and other components

of the glucocorticoid-metabolizing machinery connecting nutrient

sensing to hormone activation in the ER are important drug targets

for the treatment of the metabolic syndrome and type 2 diabetes.

Box 1. ER stress – an implement of adaptation

The ER is equipped with an adaptation system for maintaining the

balance between the demand and capacity of protein folding.

Perturbations in cellular energy level, redox state, oxygen and

nutrient supply or Ca2+ concentration (e.g. hypoxia, oxidative stress

and hypoglycemia) reduce the folding capacity of the ER and result

in the accumulation of unfolded proteins, resulting in a state known

as ER stress. To cope with the detrimental consequences of ER

stress, a complex protective strategy – the unfolded protein

response (UPR) – has evolved (reviewed in Refs [4,5]).

ER-associated degradation (ERAD)I ensues when the UPR cannot

cope with the aberrant protein overload. The misfolded proteins are

retro-translocated into the cytosol through chaperone-based machin-

ery and degraded by the proteasome, a multicatalytic 26S proteinase

complex that is enriched at the ER membrane. When abnormal

proteins accumulate to very high levels and/or the ERADI is

compromised, misfolded proteins aggregate on the luminal side of

the ER membrane and induce ERADII. This process involves

autophagy and the engulfment of elongated ER-membrane-bearing

protein aggregates and their ultimate degradation by autolysosomes.

ER stress signaling switches from pro-survival to pro-apoptotic

when protein aggregation is persistent and stress cannot be

resolved by UPR or ERAD. Apoptosis is initiated by the proapoptotic

transcription factor, C/EBP homologous protein, and activation of c-

Jun N-terminal kinase by pancreatic-ER-kinase-like ER kinase and

inositol-requiring enzyme 1, respectively. Collectively, these

changes activate the pro-apoptotic B-cell lymphoma/leukemia 2

proteins and the caspase cascade.

In addition, the UPR activates sterol regulatory element-binding

protein, increasing ER-membrane synthesis and releasing proapop-

totic proteins such as Bax and Bad from the ER, which stimulates the

mitochondrial permeability transition, further enhancing the apop-

totic response. ER stress has been observed in a variety of

pathological conditions; hence, it is an intensively studied organelle

for drug development.

Review Trends in Endocrinology and Metabolism Vol.20 No.4

(Box 1). For more information on the unfolded proteinresponse (UPR), see Refs [4,5] and Figure 1. Protein foldingand chaperone functions are sensitive to alterations in theluminal calcium level [6].

In addition to its functions in the intermediary metab-olism, theERalso contributes considerably to oxidation andconjugation of endobiotics and xenobiotics. CytochromeP450 monooxygenase and UDP-glucuronosyltransferaseenzymes catalyze the main biotransformation reactionsand are integral proteins of the ER membrane [7]. Steroidand thyroid sulfoconjugates [8] and various glucuronides [9]can be deconjugated by enzymatic hydrolysis in the luminalcompartment. In addition, steroid dehydrogenase enzymesassociated with the ER membrane activate or inactivatesteroid hormones. For instance, two isoforms (type 1 and 2)of 11b-hydroxysteroid dehydrogenase (11bHSD) that con-vert cortisone into the active glucocorticoid cortisol, and viceversa, face the luminal and the cytosolic side of the mem-brane, respectively [10]. This topology makes local gluco-corticoid activity dependent on the redox conditions oneither side of the ER membrane and, hence, connects theER metabolism to the signaling network (for more details,see below).

Redox conditions in the ER

Oxidative protein folding includes the transfer of electronsfrom the cysteinyl thiol groups to oxygen by local oxido-reductases. The result is an oxidized thiol–disulfide redoxsystem in the ER lumen, also reflected by a luminal [gluta-

thione (GSH)]:[glutathione disulfide (GSSG)] ratio morethan 30 times lower than that of the cytosol [11]. The ERmembrane is impermeable to GSSG and to pyridine nucleo-tides (i.e. NAD(P)(H) [1]), whereas GSH is slowly trans-ported. The luminal thiol–disulfide and pyridine nucleotideredox systems are not only isolated physically from theircytosolic counterparts but also separated chemically fromone another. The absence of enzymes that catalyze redoxreactions between glutathione and NAD(P)(H) enables thesimultaneous existence of a dominantly reduced pyridinenucleotide pool in the thiol-oxidizing environment of thelumen [12]. The local production of NADPH is catalyzed byH6PDH using the substrate glucose-6-phosphate that istransported into the lumen by glucose-6-phosphate trans-porter (G6PT) [2]. Type111bHSDconsumesNADPHfor thereduction of cortisone (Box 2).

Alterations in the thiol–disulfide redoxstatecan interferewith luminal protein folding and can trigger the UPR. Theredox shift in the NADP–NADPH pool affects the function-ing of the G6PT–H6PD–11bHSD1 triad, which has animportant role in the pre-receptorial glucocorticoid acti-vation in various cell types. Therefore, redox conditionsarekey elements of themetabolic sensor functions of theER.

Nutrient sensing in the ERThe metabolic and nutritional conditions of the cell aremirrored by the levels of selected intermediates (such ascholesterol, fatty acids or glucose-6-phosphate), as well as

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Figure 1. The unfolded protein response. Upon accumulation of unfolded and/or misfolded proteins in the ER lumen, BiP (binding protein) chaperone dissociates from the

luminal domain of the three ER transmembrane stress receptors – pancreatic-ER-kinase-like ER kinase (PERK), activating transcription factor 6 (ATF6) and inositol-requiring

enzyme 1 (IRE1) – enabling their activation. ATF6 transits to the Golgi, where it is cleaved and its cytosolic fragment migrates to the nucleus. Both IRE1 and PERK are

oligomerized and autophosphorylated. Phosphorylation of eukaryotic initiation factor 2a (eIF2a) by PERK blocks protein synthesis and enables translation of ATF4

transcription factor, which translocates to the nucleus and induces the transcription of genes required to restore ER homeostasis. Active ATF6 is also a transcription factor

and regulates the expression of ER chaperones and X-box-binding protein 1 (XBP1). XBP1 mRNA spliced by IRE1 codes for the active transcription factor XBP1 s, which

controls the transcription of chaperones and ERAD components. This concerted action of the UPR pathways restores the folding capacity of the ER by blocking further build-

up of client proteins, enhancing its folding capacity and eliminating terminally misfolded proteins. Unresolved ER stress activates apoptotic mechanisms. Under these

conditions, Bak and Bax, ER-membrane-resident proteins, undergo conformational alteration to permit Ca2+ efflux. The elevated cytoplasmic Ca2+ activates calpain, which

cleaves and activates procaspase-12 (Pc12) to Caspase-12 (C12). The Ca2+ efflux and Bak and Bax release also favors activation of mitochondria-dependent apoptosis. C/EBP

homologous protein (CHOP), one of the UPR downstream effectors, inhibits the expression of Bcl-2 to promote apoptosis. IRE1 activation also leads to the activation of c-

Jun N-terminal kinase (JNK), a mediator of inflammation and apoptosis.

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by the redox state of the thiol–disulfide and pyridinenucleotide systems. Growing evidence indicates that theER senses alterations of these factors and, in response,activates endogenous signaling pathways to adjust themetabolic activities to balance intake and requirement;however, these same endogenous signaling pathways canalso contribute to the development of metabolic diseases.

Cholesterol sensing

The rate-limiting step of cholesterol synthesis is catalyzedby 3-hydroxy-3-methyl-glutaryl-CoA reductase (HMGR),an enzyme anchored in the ER membrane [13]. Theenzyme itself is a cholesterol sensor, and its ubiquitylationand consequent proteasomal degradation is enhanced byhigh cholesterol content of the lipid bilayer [14–16]. In

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addition, expression of the HMGR gene is regulated bycholesterol levels through the ER-resident sterol regulat-ory element-binding proteins (SREBP1a and -2) [17]. Lowsterol levels favor the transit of SREBPs to the Golgi,where proteases liberate the active transcription factors(nSREBPs) to upregulate cholesterol synthesis.

Triglyceride sensing

Another member of the SREBP family, SREBP1c, seems toplay a key part in fatty acid and triglyceride synthesis [18].Expression of the ER-membrane-embedded precursorSREBP1c is stimulated by insulin. Moreover, activationof this precursor by limited proteolysis after translocationto the Golgi is also insulin dependent. Therefore, whereasSREBP1a and SREBP2 are chiefly regulated by sterol

Box 3. Autophagy and nutrient sensing

By definition, autophagy refers to any cellular degradative pathway

that involves the delivery of cytoplasmic cargo to the lysosome. So

far, three types of autophagy have been described: macroauto-

phagy, microautophagy and chaperone-mediated autophagy

(CMA). Each differs in their mode of cargo delivery and physiolo-

gical functions. Both micro- and macroautophagy have the capacity

to engulf large structures through selective and non-selective

mechanisms, whereas CMA degrades only specific peptide motifs

of soluble proteins. Macroautophagy (hereafter referred to as

autophagy) is the major long-lived protein- and organelle-degrading

machinery of eukaryotic cells. It is a delicately regulated multi-step

procedure that involves the encapsulation of cytosol and/or

organelle to form the phagosome, a double-membraned vesicle.

This is followed by fusion of the phagosome with a lysosome to

form an autolysosome, where the degradation of entrapped

material and inner membrane takes place. Presently, 31 autop-

hagy-related gene (Atg) proteins have been identified in yeast that

are essential for autophagy execution, many of which are evolutio-

narily conserved.

Autophagy operates at low basal levels to perform housekeeping

functions, such as elimination of defective long-lived proteins and

organelles, and can be upregulated by various stimuli. The

classically studied triggers for autophagy are nutrient withdrawal,

low energy status and growth-factor deprivation. According to

genetic and biochemical studies, these signals converge on the

central nutrient sensor and autophagy repressor target of rapamycin

and decrease its serine/threonine protein kinase activity to initiate

autophagy [73]. During metabolic stress conditions, autophagy

provides amino acids for stress protein synthesis and helps

maintain cellular ATP production. Besides providing amino acids

under emergency conditions, autophagosomal activity is also

cardinal in protection against aging, neurodegenerative diseases

and infection. Recent publications indicate its crucial role in protein

quality control, as well [74]. The role of autophagy in cancer is

controversial; various autophagy-related-gene defects have been

identified in numerous tumors, implying a tumor-suppressor

function; however, in vitro treatment of tumor cell lines with

anticancer drugs is usually more effective in ATG-gene silenced

cells [75]. This illustrates that although the autophagy-related data

have expanded tremendously in the past decade, there is still much

to learn about the function and operation of autophagy in health and

disease.

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supply, SREBP1c is affected by caloric intake via insulinand glucagon action andmediates certain metabolic effectsof insulin [19]. In accordance with this, the expression ofSREBP1c decreases in adipose tissue in obesity, whichmight be due to insulin resistance of these subjects [20].These findings demonstrate the importance of lipid sensingin the ER in metabolic regulation.

Thiol-redox-based sensing in the ER

Alterations of redox conditions in the ER lumen alsotrigger signaling pathways via molecular events thatcan, hence, be referred to as redox sensing. The luminalchaperone BiP and the ER stress transducer proteins aresensitive to thiol–disulfide redox alterations, which linksredox sensing with UPR activation. Dynamic redox-sensi-tive thiols modulate the probability that the ryanodinereceptor calcium channel will open; this is important,considering this channel has been postulated to be a majortransmembrane redox sensor in the sarcoplasmic reticu-lum [21,22]. Similar redox-based regulation of the inositol1,4,5-trisphosphate receptor has been also demonstrated[23,24].

Pyridine nucleotides and redox sensing in the ER

Little is known about sensing and signaling of the redoxstate of luminal pyridine nucleotides. However, studies inH6PD-knockout mice revealed that pyridine nucleotideredox shift causes ER stress and can activate the UPR[25]. The reduced and oxidized pyridine nucleotides arepotential ligands of ER chaperones, and because the func-tioning of ER chaperones is, indeed, affected by their redoxstate, protein folding can also be affected under certainredox conditions [26–28].

Moreover, the prereceptorial metabolism of steroid hor-mones greatly depends on the luminal [NADPH]:[NADP+]ratio and presumably on the cytosolic [NADH]:[NAD+]ratio, as has been revealed in the case of glucocorticoids[10,29]. This provides another potential mechanism ofredox-based nutrient sensing in the ER. Because boththe luminal and the cytosolic NAD(P)H generationdepends on glucose (or glucose-6-phosphate) availability,it is not surprising that exogenous glucose supply has beenreported to stimulate cortisol production from cortisone incell culture [30].

Sensing amino acid availability

The coordinated cellular response triggered by amino aciddeprivation includes a global reduction of protein syn-thesis. At the same time, the production of stress proteinsincreases, and the demanded amino acids are primarilygenerated by autophagy (Box 1). The role of the ER insensing and adapting to amino acid deprivation has beenrevealed recently. Amino acid starvation causes therelease of Bcl-2, to induce autophagy, whereas amino acidabundance strengthens Beclin1–Bcl-2 binding in the ER, toinhibit autophagy [31]. For further detail on autophagy,see Box 3. c-Jun N-terminal kinase (JNK)1 is known toregulate ER-resident Bcl-2 by phosphorylation, and thephosphorylated antiapoptotic protein is unable to seques-ter and inhibit BH3-only proapoptotic proteins [32]. Appar-ently, starvation-induced Bcl-2 phosphorylation has the

same effect on its interaction with Beclin1 and leads toautophagy [33]. Therefore, JNK1-mediated Bcl-2 phos-phorylation induces both pro-survival and pro-death sig-nals, but the factors that determine the outcome remain tobe identified.

The ER is involved in both insulin production and

insulin action

Recent findings indicate the role of the ER in the respon-siveness of pancreatic b cells to alterations in nutrientsupply. Decreased or increased glucose levels were shownto trigger different stress responses, including ER stress, inisolated pancreatic islets and insulinoma cells [34,35].Induction of ER stress is an integral element of the phys-iological b-cell response to acute nutrient abundance. Infact, both high glucose and high fatty acid supply induceER stress in b cells [36,37]. A nearly fivefold increase intotal protein synthesis, �50% of which is proinsulin pro-duction, challenges the folding capacity of the ER in theseglucose-stimulated b cells, and the activated UPR recoversthe balance by adjusting the mass of the organelle and theamount of chaperones and foldases. The UPR is needed tomaintain the appropriate environment in the luminal

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Figure 2. The role of ER stress in physiological adaptation and in the

pathomechanism of metabolic syndrome and type 2 diabetes. (a) In a

physiological fed state, insulin synthesis and secretion is stimulated in

pancreatic b cells through an increased nutrient (mainly glucose) supply. The

enhanced protein load challenges the folding capacity of the ER and causes ER

stress. The initiated UPR helps maintain or recover the balance of capacity and

demand; therefore, it is a necessary and appropriate adaptive mechanism that

supports the survival of b cells and appropriate insulin secretion. The metabolic

pathways leading to glucose utilization are activated in hepatocytes and

adipocytes, which store the nutrients as glycogen and fat, respectively. (b)

Frequent or sustained nutrient (mainly glucose and fatty acid) abundance

characteristic in obesity induces excessive and destructive ER stress in b cells,

hindering insulin production and reducing b-cell mass through enhanced

apoptosis. ER stress also develops in hepatocytes and adipocytes and

contributes to insulin resistance by interfering with the insulin-signaling

pathway. In addition, it increases the glucose-producing capacity of hepatocytes

and supports fat deposition and the growth of adipose tissue by stimulating

preadipocyte differentiation, aggravating the complex metabolic disorder,

potentially leading to type 2 diabetes.

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compartment of the ER for proper proinsulin folding incases of hyperglycemia, insulin resistance or high-fat diet[5]. For example, hypoglycemia induces ER stress andUPRin liver cells [38]. Therefore, increased expression of glu-cose-6-phosphatase [39], which leads to enhanced glucoseoutput and elevated blood glucose levels, is a unique con-sequence of chronic pathophysiological ER stress in hep-atocytes.

Altered functions of the ER in pancreatic b cells,adipocytes and hepatocytes in pathological conditionsThe normal functioning of the ER is based on a balancednutrient supply. When too many or too few nutrients areprovided, the organelle initiates intracellular signalingcascades or hormonal effects that normally regulate cel-lular metabolism; however, these cascades have also beenimplicated in the development or aggravation of metabolicdiseases, such as obesity, metabolic syndrome or diabetes(Figure 2).

ER stress and UPR: central elements of b-cell

dysfunction

High glucose [37] and fattyacid [36,40,41] supply reportedlyinduce ER stress in b cells, thereby playing a part in thedevelopment of type 2 diabetes. Sustained or excessive ERstress reduces insulin expression [42] and increases apop-tosis [43], ultimately decreasing b-cell mass. It has beenproposed that ER stress suppresses insulin biosynthesis atthe transcriptional level through JNK activation [42].

The role of ER stress in b-cell dysfunction has beendemonstrated in animal models. Defective disulfide-bridgeformation leading to misfolding of proinsulin induces ERstress [44].Mutations leading to an impaired disulfide-bondformation in proinsulin have been shown to induce diabetesin mice [45]. Moreover, it has been reported that similarmutations in the human insulin gene can cause permanentneonatal diabetes [46]. Typical UPR-related events havebeen detected in b cells of type 2 diabetic patients [43].Moreover, C/EBP homologous protein (CHOP)-mediatedapoptosis seems to be a fundamental contributing factorto b-cell failure in this disease [5]. The formation of amyloidfrom islet amyloid polypeptide (IAPP)also contributes toERdysfunction in b cells, and a role for IAPP in ER-derived(CHOP-mediated) b-cell apoptosis has been demonstratedinhuman type 2diabetes [45]. In summary,ERstress seemsto be crucial not only in physiological regulation but also inb-cell failure in human type 2 diabetes. Onlymodest signs ofER stress can be detected in b cells from type 2 diabeticpatients, but these cells are more sensitive to ER-stressinducers in vitro [47]. The complexity of the system isapparent because on the one hand, reduced UPR owing tomutations in the PERK–eIF2a-signaling pathway causesdiabetes inmice (a condition referred to asWolcott-Rallisonsyndrome in humans [5]); on the other hand, deletion ofCHOP, an ER-related proapoptotic protein, promotes b-cellsurvival and improves glycemic control in animal models ofdiabetes [48].

Gluco- and lipotoxicity

Because destructive ER stress can be triggered by repeti-tive or sustained hyperglycemia and/or hyper-fatty acid-

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emia, the reduction of b-cell mass in type 2 diabetes mightbe due to gluco- and lipotoxicity [49,50]. The activation ofSREBP1c, a transcription factor that controls fatty acidand triglyceride synthesis, owing to hyperglycemia andhyperlipidemia was also suggested to contribute to thereduction in insulin-secreting capacity and b-cell mass(i.e. glucolipotoxicity) [51].

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Hyperglycemia is sensed in b cells through an acceler-ated oxidative and ATP-generating metabolism that isaccompanied by enhanced reactive oxygen species (ROS)production, contributing to glucotoxicity [52]. The b cellsare particularly sensitive to oxidative insults because oflow expression levels of antioxidant enzymes (e.g. catalaseand glutathione peroxidase) [5,52]. Hyperglycemia-induced proinsulin synthesis also stimulates ROS pro-duction because of excessive oxidase activities convertingprotein thiols to disulfides [5]. It was shown in Zuckerobese diabetic rats that hyperglycemia-induced oxidativestress induced general protein ubiquitylation and led tostorage of proteins in cytoplasmic aggregates that wereeventually cleared by autophagy [53]. This illustrates thatthe stress caused by nutrient imbalance and redox disturb-ances triggers alternativemechanisms beside theUPR andapoptosis.

ER stress and the UPR: contributors to insulin resistance

The ER stress response was first detected in liver andadipose tissue in a mouse model of obesity [54], and ERstress markers have been found recently in adipose tissuein obese patients [55]. Excessive caloric intake stimulatesthe synthesis and storage of triglycerides in adipose tissue.The increased adipose tissue mass is a result of bothhypertrophy and hyperplasia initiated by the buildup ofgrowing fat droplets in adipocytes. Paracrine mediators ofpreadipocyte differentiation, such as tumor necrosis factoralpha (TNF-a) and insulin-like growth factor 1, arereleased by the swelling fat cells [56]. The enhanced tri-glyceride synthesis causes ER stress and provokes certainelements of the UPR. Among others, CHOP induction wasshown and implicated in the reduction of adiponectinproduction in mouse models of obesity [57].

ER stress and the UPR interfere with insulin signalingto induce insulin resistance. The activation of JNK in theER stress response leads to inhibitory serine phosphoryl-ation of insulin-receptor substrate-1 (IRS-1), thereby redu-cing the insulin responsiveness of the cells [54,58,59]. Thisobservation provides a possible link between obesity andtype 2 diabetes because the enlargement of fat stores inadipose tissue is associated with elevated glucose and freefatty acid levels in the circulation, which causes a threat ofa vicious cycle because hyperglycemia and hyper-fattyacidemia induce ER stress, and ER stress can furtherincrease insulin resistance [60].

Preadipocyte differentiation is a fundamental require-ment for the development of obesity. The enhancement oflocal glucocorticoid production is an important event inpreadipocyte differentiation. The capacity of the ER toconvert cortisone to active cortisol is enhanced duringpreadipocyte differentiation by a remarkable inductionof 11bHSD1 [61]. It has been demonstrated that the redoxstate of the ER luminal pyridine nucleotides is a key factorof pre-receptorial hormone activation [62]. Because lumi-nal NADPH is generated at the expense of glucose 6-phosphate, it is plausible that hyperglycemia (i.e. anincreased supply of glucose 6-phosphate) favors preadipo-cyte differentiation by means of increased local cortisolproduction. It has been demonstrated recently that adiposetissue of obese mice is hypoxic [57], which might also

contribute to metabolic dysregulation and even to the de-velopment of ER stress [63].

The metabolic disorder is aggravated through ER dys-function in hepatocytes. Obesity-induced ER stress notonly causes insulin resistance through JNK-dependentIRS-1 phosphorylation but also leads to increased hepaticglucose production through the induction of glucose-6-phosphatase [39]. The enhancement of gluconeogenesiscan be expected to favor local cortisol production by theG6PT–H6PD–11bHSD1 triad, owing to the increased glu-cose-6-phosphate supply, and the metabolic effects of cor-tisol can worsen insulin resistance.

The involvement of chronic inflammation in obesity andinsulin resistance is indicated by the activation of certaininflammatory signaling pathways. Several observationshave been published on the role of TNF-a, a cytokine linkedto JNK and nuclear factor kappa B (NFkB) activation[64,65]. TNF-a upregulates genes involved in the UPRand oxidative stress signaling in adipose tissue. IncreasedROS formation is also involved in TNF-a-induced insulinresistance [66]. The production of ROS, the release ofluminal calcium and the activation of JNK and NF-kBrepresent strong connections between signaling pathwaysof the UPR and inflammation.

Concluding remarks and future perspectivesTheER is capable of sensing keymetabolic parameters andintegrating intra- and extracellular signals to support acoordinated cell response. Several aspects of the interfer-ence between carbohydrate, lipid and protein metabolismhave been outlined here. However, there are additionalER-related metabolic interactions of remarkable biologicalimportance and health impact. Common transcription fac-tors are involved in the control of drug-metabolizingenzymes and enzymes of membrane lipidmetabolism. Thisphenomenon, referred to as ‘xenobiotic sensing’, is cur-rently attracting growing scientific interest.

The links between calcium and redox homeostasis of theER and themitochondrion enable the integration of inflam-matory, metabolic and ER stress responses [67]. Severalobservations support the role of mitochondrial dysfunctionin lipid accumulation, altered redox balance and insulinresistance developing in adipocytes and hepatocytes [68].It is tempting to suggest that the effects of ER stress onmitochondrial function also contribute to this mechanism.The role of nutrient and redox sensing, as well as of therelated ER stress, in the development of obesity, metabolicsyndrome, atherosclerosis and type 2 diabetes offers anexample of a general biological phenomenon: how a phys-iological regulation contributes to the pathomechanism ofdiseases.

AcknowledgementsThanks are due to the Szentagothai Janos Knowledge Center and theJanos Bolyai Research Scholarship of the Hungarian Academy ofSciences.

References1 Csala, M. et al. (2007) Transport and transporters in the endoplasmic

reticulum. Biochim. Biophys. Acta 1768, 1325–13412 Csala, M. et al. (2006) Endoplasmic reticulum: a metabolic

compartment. FEBS Lett. 580, 2160–2165

199

Review Trends in Endocrinology and Metabolism Vol.20 No.4

3 van Anken, E. and Braakman, I. (2005) Versatility of the endoplasmicreticulum protein folding factory. Crit. Rev. Biochem. Mol. Biol. 40,191–228

4 Ron, D. and Walter, P. (2007) Signal integration in the endoplasmicreticulumunfolded protein response.Nat. Rev.Mol. Cell Biol. 8, 519–529

5 Scheuner, D. and Kaufman, R.J. (2008) The unfolded protein response:a pathway that links insulin demand with b-cell failure and diabetes.Endocr. Rev. 29, 317–333

6 Michalak, M. et al. (2002) Ca2+ signaling and calcium bindingchaperones of the endoplasmic reticulum. Cell Calcium 32, 269–278

7 Iyanagi, T. (2007) Molecular mechanism of phase I and phase II drug-metabolizing enzymes: implications for detoxification. Int. Rev. Cytol.260, 35–112

8 Ghosh, D. (2007) Human sulfatases: a structural perspective tocatalysis. Cell. Mol. Life Sci. 64, 2013–2022

9 Csala, M. et al. (2000) Beta-glucuronidase latency in isolated murinehepatocytes. Biochem. Pharmacol. 59, 801–805

10 Odermatt, A. et al. (2006) Why is 11b-hydroxysteroid dehydrogenasetype 1 facing the endoplasmic reticulum lumen? Physiologicalrelevance of the membrane topology of 11b-HSD1. Mol. Cell.Endocrinol. 248, 15–23

11 Hwang, C. et al. (1992) Oxidized redox state of glutathione in theendoplasmic reticulum. Science 257, 1496–1502

12 Piccirella, S. et al. (2006) Uncoupled redox systems in the lumen of theendoplasmic reticulum. Pyridine nucleotides stay reduced in anoxidative environment. J. Biol. Chem 281, 4671–4677

13 Liscum, L. et al. (1983) 3-Hydroxy-3-methylglutaryl-CoA reductase: atransmembrane glycoprotein of the endoplasmic reticulum with N-linked ‘‘high-mannose’’ oligosaccharides. Proc. Natl. Acad. Sci. U. S. A.80, 7165–7169

14 DeBose-Boyd, R.A. (2008) Feedback regulation of cholesterol synthesis:sterol-accelerated ubiquitination and degradation of HMG CoAreductase. Cell Res. 18, 609–621

15 Doolman, R. et al. (2004) Ubiquitin is conjugated by membraneubiquitin ligase to three sites, including the N terminus, intransmembrane region of mammalian 3-hydroxy-3-methylglutarylcoenzyme A reductase: implications for sterol-regulated enzymedegradation. J. Biol. Chem. 279, 38184–38193

16 Song, B.L. et al. (2005) Gp78, a membrane-anchored ubiquitin ligase,associates with Insig-1 and couples sterol-regulated ubiquitination todegradation of HMG CoA reductase. Mol. Cell 19, 829–840

17 Raghow, R. et al. (2008) SREBPs: the crossroads of physiological andpathological lipid homeostasis. Trends Endocrinol. Metab. 19, 65–73

18 Jump, D.B. et al. (2005) Fatty acid regulation of hepatic genetranscription. J. Nutr. 135, 2503–2506

19 Foufelle, F. and Ferre, P. (2002) New perspectives in the regulation ofhepatic glycolytic and lipogenic genes by insulin and glucose: a role forthe transcription factor sterol regulatory element binding protein-1c.Biochem. J. 366, 377–391

20 Kolehmainen, M. et al. (2001) Sterol regulatory element bindingprotein 1c (SREBP-1c) expression in human obesity. Obes. Res. 9,706–712

21 Feng, W. et al. (2000) Transmembrane redox sensor of ryanodinereceptor complex. J. Biol. Chem. 275, 35902–35907

22 Zable, A.C. et al. (1997) Glutathione modulates ryanodine receptorfrom skeletal muscle sarcoplasmic reticulum. Evidence for redoxregulation of the Ca2+ release mechanism. J. Biol. Chem 272, 7069–

707723 Kaplin, A.I. et al. (1994) Purified reconstituted inositol 1,4,5-

trisphosphate receptors. Thiol reagents act directly on receptorprotein. J. Biol. Chem 269, 28972–28978

24 Higo, T. et al. (2005) Subtype-specific and ER lumenal environment-dependent regulation of inositol 1,4,5-trisphosphate receptor type 1 byERp44. Cell 120, 85–98

25 Lavery, G.G. et al. (2008) Deletion of hexose-6-phosphatedehydrogenase activates the unfolded protein response pathway andinduces skeletal myopathy. J. Biol. Chem. 283, 8453–8461

26 Callebaut, I. et al. (1994) Redox mechanism for the chaperone activityof heat shock proteins HSPs 60, 70 and 90 as suggested by hydrophobiccluster analysis: hypothesis. Comptes Rendus de l’Academie desSciences 317, 721–729

27 Papp, E. et al. (2003) Molecular chaperones, stress proteins and redoxhomeostasis. Biofactors 17, 249–257

200

28 Papp, E. et al. (2006) Changes of endoplasmic reticulum chaperonecomplexes, redox state, and impaired protein disulfide reductaseactivity in misfolding a1-antitrypsin transgenic mice. FASEB J. 20,1018–1020

29 Lavery, G.G. et al. (2006) Hexose-6-phosphate dehydrogenase knock-out mice lack 11 beta-hydroxysteroid dehydrogenase type 1-mediatedglucocorticoid generation. J. Biol. Chem. 281, 6546–6551

30 Dzyakanchuk, A.A. et al. (2009) 11b-Hydroxysteroid dehydrogenase 1reductase activity is dependent on a high ratio of NADPH/NADP+ and isstimulated by extracellular glucose.Mol. Cell. Endocrinol. 301, 137–141

31 Pattingre, S. et al. (2005) Bcl-2 antiapoptotic proteins inhibit Beclin 1-dependent autophagy. Cell 122, 927–939

32 Bassik, M.C. et al. (2004) Phosphorylation of BCL-2 regulates ER Ca2+

homeostasis and apoptosis. EMBO J. 23, 1207–121633 Wei, Y. et al. (2008) JNK1-mediated phosphorylation of Bcl-2 regulates

starvation-induced autophagy. Mol. Cell 30, 678–68834 Elouil, H. et al. (2007) Acute nutrient regulation of the unfolded protein

response and integrated stress response in cultured rat pancreaticislets. Diabetologia 50, 1442–1452

35 Greenman, I.C. et al. (2007) Distinct glucose-dependent stressresponses revealed by translational profiling in pancreatic b-cells. J.Endocrinol. 192, 179–187

36 Cunha, D.A. et al. (2008) Initiation and execution of lipotoxic ER stressin pancreatic beta-cells. J. Cell Sci. 121, 2308–2318

37 Hou, Z.Q. et al. (2008) Involvement of chronic stresses in rat islet andINS-1 cell glucotoxicity induced by intermittent high glucose.Mol. Cell.Endocrinol. 291, 71–78

38 Gonzales, J.C. et al. (2008) Chemical induction of the unfolded proteinresponse in the liver increases glucose production and is activatedduring insulin-induced hypoglycaemia in rats. Diabetologia 51, 1920–

192939 Ji, C. and Kaplowitz, N. (2006) ER stress: can the liver cope? J. Hepatol.

45, 321–33340 Pirot, P. et al. (2007) Transcriptional regulation of the endoplasmic

reticulum stress gene chop in pancreatic insulin-producing cells.Diabetes 56, 1069–1077

41 Kharroubi, I. et al. (2004) Free fatty acids and cytokines inducepancreatic beta-cell apoptosis by different mechanisms: role ofnuclear factor-kappaB and endoplasmic reticulum stress.Endocrinology 145, 5087–5096

42 Kaneto, H. et al. (2006) Role of oxidative stress, endoplasmic reticulumstress, and c-Jun N-terminal kinase in pancreatic beta-cell dysfunctionand insulin resistance. Int. J. Biochem. Cell Biol. 38, 782–793

43 Laybutt, D.R. et al. (2007) Endoplasmic reticulum stress contributes tobeta cell apoptosis in type 2 diabetes. Diabetologia 50, 752–763

44 Herbach, N. et al. (2007) Dominant-negative effects of a novel mutatedIns2 allele causes early-onset diabetes and severe beta-cell loss inMunich Ins2C95S mutant mice. Diabetes 56, 1268–1276

45 Huang, C.J. et al. (2007) High expression rates of human islet amyloidpolypeptide induce endoplasmic reticulum stress mediated beta-cellapoptosis, a characteristic of humans with type 2 but not type 1diabetes. Diabetes 56, 2016–2027

46 Stoy, J. et al. (2007) Insulin gene mutations as a cause of permanentneonatal diabetes. Proc. Natl. Acad. Sci. U. S. A. 104, 15040–15044

47 Marchetti, P. et al. (2007) The endoplasmic reticulum in pancreaticbeta cells of type 2 diabetes patients. Diabetologia 50, 2486–2494

48 Song, B. et al. (2008) Chop deletion reduces oxidative stress, improvesbeta cell function, and promotes cell survival in multiple mouse modelsof diabetes. J. Clin. Invest. 118, 3378–3379

49 Kahn, S.E. et al. (2006) Mechanisms linking obesity to insulinresistance and type 2 diabetes. Nature 444, 840–846

50 Prentki, M. and Nolan, C.J. (2006) Islet beta cell failure in type 2diabetes. J. Clin. Invest. 116, 1802–1812

51 Wang, H. et al. (2005) ER stress and SREBP-1 activation are implicatedin beta-cell glucolipotoxicity. J. Cell Sci. 118, 3905–3915

52 Robertson, R.P. (2004) Chronic oxidative stress as a central mechanismfor glucose toxicity in pancreatic islet beta cells in diabetes. J. Biol.Chem. 279, 42351–42354

53 Kaniuk, N.A. et al. (2007) Ubiquitinated-protein aggregates form inpancreatic beta-cells during diabetes-induced oxidative stress and areregulated by autophagy. Diabetes 56, 930–939

54 Ozcan, U. et al. (2004) Endoplasmic reticulum stress links obesity,insulin action, and type 2 diabetes. Science 306, 457–461

Review Trends in Endocrinology and Metabolism Vol.20 No.4

55 Sharma, N.K. et al. (2008) Endoplasmic reticulum stress markers areassociated with obesity in non-diabetic subjects. J. Clin. Endocrinol.Metab. 93, 4532–4541

56 Hausman, D.B. et al. (2001) The biology of white adipocyteproliferation. Obes. Rev. 2, 239–254

57 Hosogai, N. et al. (2007) Adipose tissue hypoxia in obesity and itsimpact on adipocytokine dysregulation. Diabetes 56, 901–911

58 Aguirre, V. et al. (2000) The c-Jun NH2-terminal kinase promotesinsulin resistance during association with insulin receptorsubstrate-1 and phosphorylation of Ser307. J. Biol. Chem. 275, 9047–

905459 Hirosumi, J. et al. (2002) A central role for JNK in obesity and insulin

resistance. Nature 420, 333–33660 Gregor, M.F. and Hotamisligil, G.S. (2007) Thematic review series:

adipocyte biology. Adipocyte stress: the endoplasmic reticulum andmetabolic disease. J. Lipid Res 48, 1905–1914

61 Senesi, S. et al. (2008) Constant expression of hexose-6-phosphatedehydrogenase during differentiation of human adipose-derivedmesenchymal stem cells. J. Mol. Endocrinol. 41, 125–133

62 Marcolongo, P. et al. (2008) Metyrapone prevents cortisone-inducedpreadipocyte differentiation by depleting luminal NADPH of theendoplasmic reticulum. Biochem. Pharmacol. 76, 382–390

63 Koumenis, C. and Wouters, B.G. (2006) ‘‘Translating’’ tumor hypoxia:unfolded protein response (UPR)-dependent and UPR-independentpathways. Mol. Cancer Res. 4, 423–436

64 Cawthorn, W.P. and Sethi, J.K. (2008) TNF-a and adipocyte biology.FEBS Lett. 582, 117–131

65 Hotamisligil, G.S. (2006) Inflammation and metabolic disorders.Nature 444, 860–867

66 Houstis, N. et al. (2006) Reactive oxygen species have a causal role inmultiple forms of insulin resistance. Nature 440, 944–948

67 Zhang, K. and Kaufman, R.J. (2008) From endoplasmic-reticulumstress to the inflammatory response. Nature 454, 455–462

68 de Ferranti, S. and Mozaffarian, D. (2008) The perfect storm: obesity,adipocyte dysfunction, and metabolic consequences. Clin. Chem. 54,945–955

69 Nebert, D.W. (1991) Proposed role of drug-metabolizing enzymes:regulation of steady state levels of the ligands that effect growth,homeostasis, differentiation, and neuroendocrine functions. Mol.Endocrinol. 5, 1203–1214

70 Miller, W.L. (2005) Minireview: regulation of steroidogenesis byelectron transfer. Endocrinology 146, 2544–2550

71 White, P.C. et al. (2008) Physiological roles of 11 beta-hydroxysteroiddehydrogenase type 1 and hexose-6-phosphate dehydrogenase. Curr.Opin. Pediatr. 20, 453–457

72 Wamil, M. and Seckl, J.R. (2007) Inhibition of 11b-hydroxysteroiddehydrogenase type 1 as a promising therapeutic target. DrugDiscov. Today 12, 504–520

73 Lum, J.J. et al. (2005) Autophagy inmetazoans: cell survival in the landof plenty. Nat. Rev. Mol. Cell Biol. 6, 439–448

74 Kruse, K.B. et al. (2006) Autophagy: an ER protein quality controlprocess. Autophagy 2, 135–137

75 Levine, B. and Kroemer, G. (2008) Autophagy in the pathogenesis ofdisease. Cell 132, 27–42

201