H. Yoshida, Department of Biophysics, Graduate School of Science, Kyoto University, Kitashirakawa-Oiwakecho, Sakyo-ku, Kyoto 606-8502, Japan Fax: +81 75 753 3718 Tel: +81 75 753 4201 E-mail: email@example.com
Proteins synthesized in the endoplasmic reticulum (ER) are properly folded with the assistance of ER chaperones. Malfolded proteins are disposed of by ER-associated protein degradation (ERAD). When the amount of unfolded protein exceeds the folding capacity of the ER, human cells activate a defense mechanism called the ER stress response, which induces expression of ER chaperones and ERAD components and transiently attenuates protein synthesis to decrease the burden on the ER. It has been revealed that three independent response pathways separately regulate induction of the expression of chaperones, ERAD components, and translational attenuation. A malfunction of the ER stress response caused by aging, genetic mutations, or environmental factors can result in various diseases such as diabetes, inflammation, and neurodegenerative disorders including Alzheimer's disease, Parkinson's disease, and bipolar disorder, which are collectively known as ‘conformational diseases’. In this review, I will summarize recent progress in this field. Molecules that regulate the ER stress response would be potential candidates for drug targets in various conformational diseases.
degradation in the endoplasmic reticulum protein 1
Der1-like protein 1
degradation in the endoplasmic reticulum protein 10
death receptor 5
ER degradation enhancing α−mannosidase-like protein
α-subunit of eukaryotic translational initiation factor 2
ER protein 72
ER stress response element
FK506-binding protein 13
growth arrest and DNA damage
human ER-associated dnaJ
human inhibitor of apoptosis 2
HMG-CoA reductase degradation protein 1
heat shock protein
inhibitor of apoptosis
insulin-dependent diabetes mellitus
inositol requirement 1
Kelch-like Ech-associated protein 1
basic leucine zipper protein
noninsulin-dependent diabetes mellitus
neutrophil NADPH oxidase factor
nuclear protein localization 4
nuclear respiratory factor
oxygen-regulated protein 150
58 kDa-inhibitor of protein kinase
the nuclear form of ATF6 protein
protein disulfide isomerase
PRKR-like endoplasmic reticulum kinase
double stranded RNA-dependent protein kinase
proteolipid protein 1
p53 up-regulated modulator of apoptosis
the spliced form of XBP1 protein
the unspliced form of XBP1 protein
regulated intramembrane proteolysis
regulator of sE
site 1 protease
site 2 protease
stress-activated protein kinase
suppressor of lin12-like
sterol response element-binding protein
T cell death-associated gene 51
tumor necrosis factor
tumor necrosis factor receptor 1
TNF receptor-associated factor 2
Tribbles homolog 3
ubiquitin conjugase 6
ubiquitin conjugase 7
ubiquitin-activating enzyme 1
ubiquitin-activating enzyme 2G2
UBX domain-containing protein 2
ubiquitin C-terminal esterase L1
ubiquitin fusion degradation protein 1
unfolded protein response element
Wolfram syndrome 1
x-box binding protein 1
inhibitor of apoptosis, x-linked
XTP3-transactivated gene B
The endoplasmic reticulum (ER) is an organelle where secretory or membrane proteins are synthesized. Nascent proteins are folded with the assistance of molecular chaperones and folding enzymes located in the ER (collectively called ER chaperones), and only correctly folded proteins are transported to the Golgi apparatus (Fig. 1). Unfolded or malfolded proteins are retained in the ER, retrotranslocated to the cytoplasm by the machinery of ER-associated degradation (ERAD), and degraded by the proteasome. ER chaperones and ERAD components are constitutively expressed in the ER to deal with nascent proteins. When cells synthesize secretory proteins in amounts that exceed the capacity of the folding apparatus and ERAD machinery, unfolded proteins are accumulated in the ER. Unfolded proteins expose hydrophobic amino-acid residues that should be located inside the protein and tend to form protein aggregates. Protein aggregates are so toxic that they induce apoptotic cell death and cause ‘conformational diseases’ such as neurodegenerative disorders and diabetes mellitus. To alleviate such a stressful situation (ER stress), eukaryotic cells activate a series of self-defense mechanisms referred to collectively as the ER stress response or unfolded protein response [1–4].
The mammalian ER stress response consists of four mechanisms. The first is attenuation of protein synthesis, which prevents any further accumulation of unfolded proteins. The second is the transcriptional induction of ER chaperone genes to increase folding capacity, and the third is the transcriptional induction of ERAD component genes to increase ERAD ability. The fourth is the induction of apoptosis to safely dispose of cells injured by ER stress to ensure the survival of the organism.
In this article, I will describe the basics of the mammalian ER stress response that are essential to understanding conformational diseases. I will review hot topics such as ERAD, regulated intramembrane proteolysis (RIP) and cytoplasmic splicing, and briefly summarize the ER stress-related diseases.
ER stress-inducing chemicals
Chemicals such as tunicamycin, thapsigargin, and dithiothreitol are usually used to evoke ER stress in cultured cells or animals for experimental purposes. I will briefly summarize the ER stress-inducing chemicals below.
The first group of ER stressors comprises glycosylation inhibitors. Most of the proteins synthesized in the ER are N-glycosylated, and the N-glycosylation is often essential for protein folding. Thus, chemicals that disturb N-glycosylation have the potential to induce ER stress. Tunicamycin is an antibiotic produced by Streptomyces lysosuperificus that inhibits N-glycosylation by preventing UDP-GlcNAc–dolichol phosphate GlcNAc-phosphate transferase activity [5,6]. 2-Deoxy-d-glucose is also used to inhibit N-glycosylation , but is less efficient than tunicamycin.
Another class of ER stressors is Ca2+ metabolism disruptors. As the concentration of Ca2+ ion in the ER is kept at a high level and ER chaperones such as BiP require Ca2+ ions, chemicals that perturb Ca2+ metabolism in the ER induce ER stress. Ca2+ ionophores such as A23187 and the Ca2+ pump inhibitor, thapsigargin, are often used to evoke ER stress [5,8].
The third category of ER stressors is reducing agents. As the lumen of the ER is highly oxidative, proteins synthesized there can form intermolecular or intramolecular disulfide bonds between their cysteine residues. As the formation of disulfide bonds is important for the folding of secretory proteins, reducing agents that disrupt disulfide bonds evoke ER stress. Dithiothreitol and 2-mercaptoethanol are often used to this end [9,10].
Hypoxia is also known to induce ER stress, although the underlying mechanism is unknown. It is speculated that a decrease in glucose concentration induced by hypoxia (because hypoxia induces glycolytic enzymes to sustain ATP production and then cells consume glucose) inhibits N-glycosylation, leading to ER stress .
ER chaperones include molecular chaperones and folding enzymes located in the ER, which are responsible for the folding of nascent proteins [4,12]. They are also involved in the unfolding of malfolded proteins in ERAD. In this section, I will review mammalian ER chaperones, focusing on recent discoveries.
Binding protein (BiP)/glucose-regulated protein (GRP)78 is a well-known ER chaperone that belongs to the heat shock protein (HSP)70 family. BiP binds to the hydrophobic region of unfolded proteins via a substrate-binding domain and facilitates folding through conformational change evoked by the hydrolysis of ATP by the ATPase domain. Oxygen-regulated protein (ORP)150/GRP170 is an ER chaperone belonging to the HSP110 family (a HSP70 subfamily), and facilitates protein folding via a mechanism similar to that for BiP. It was originally identified as a protein expressed in response to hypoxia. ER dnaJ (ERdj)1, ERdj3/human ER-asociated dnaJ (HEDJ), ERdj4, ERdj5, SEC63, and p58IPK are ER chaperones belonging to the HSP40 family, and modulate the functions of BiP by regulating its ATPase activity as a cochaperone. BiP-associated protein (BAP), which is a member of the GrpE family, also modulates the functions of BiP by enhancing nucleotide exchange. GRP94 is an ER chaperone belonging to the HSP90 family, and facilitates folding through the hydrolysis of ATP. FKBP13 is a peptidyl-prolyl isomerase belonging to the FKBP family. These ER chaperones are involved in the general folding process of secretory proteins.
Calnexin and calreticulin are ER chaperones specifically involved in the folding of glycoprotein. High-mannose type oligosaccharide is attached en bloc to most proteins synthesized in the ER, and then trimmed sequentially (Fig. 2). When two glucose residues are trimmed by glucosidase I or II and the protein contains only one glucose residue, calnexin and calreticulin bind and fold the client protein. When the last glucose residue is trimmed by glucosidase II, the client is released from calnexin and calreticulin, and binds to UDP-glucose–glycoprotein glucosyltransferase. If the protein is folded, it is released from the enzyme and transported to the Golgi apparatus. If it is not folded, UDP-glucose–glycoprotein glucosyltransferase attaches one glucose residue and returns it to calnexin and calreticulin. This folding process is called the calnexin cycle . Calnexin and calreticulin share a similar molecular structure and function, although they are transmembrane and luminal proteins, respectively.
Numerous folding enzymes are involved in the formation of disulfide bonds in the ER, such as protein disulfide isomerase (PDI), ERp72, ERp61, GRP58/ERp57, ERp44, ERp29, and PDI-P5. These folding enzymes oxidize cysteine residues of nascent proteins and help proteins to form correct disulfide bonds. Reduced folding enzymes are reoxidized by ER oxidoreductin (ERO1), which can use molecular oxygen as a terminal electron acceptor .
Unfolded or malfolded proteins are trapped by the ERAD machinery and transported to the cytoplasm [15–17]. Retrotranslocated proteins are ubiquitinated and degraded by the proteasome in the cytosol. Thus, the process of ERAD can be divided into four steps, recognition, retrotranslocation, ubiquitination, and degradation (Fig. 3). As ERAD is one of the hottest topics in the study of ER stress, I will summarize our current understanding of mammalian ERAD systems.
During the calnexin cycle, the oligosaccharide of nascent polypeptides contains nine mannose residues. When one mannose residue is trimmed by α-mannosidase I, nascent polypeptides with eight mannose residues are released from calnexin or calreticulin and bind to ER degradation-enhancing α-mannosidase-like protein (EDEM) (Fig. 2), which discriminates unfolded proteins from folded proteins [18–22]. There are three genes for EDEM, and both EDEM1 and EDEM2 are involved in ERAD. EDEM1 is an ER membrane protein, whereas EDEM2 and EDEM3 are luminal proteins [23–25]. All EDEMs contain the mannosidase-like domain, which may be responsible for recognition of mannose residues.
Osteosarcoma 9 (OS9) and XTP3-transactivated gene B (XTP3B) are other ERAD components responsible for the recognition of unfolded proteins [26–28]. OS9 specifically binds to unfolded glycoproteins containing eight (or five) mannose residues. OS9 also binds to unglycosylated unfolded proteins, suggesting that it plays a critical role in the recognition of both glycosylated and unglycosylated proteins. OS9 and XTP3B  contain the mannose-6-phosphate receptor-like domain, which may be critical to the recognition of mannose residues.
Nascent glycoproteins recognized by EDEM and OS9 as malfolded are destined for the retrotranslocation machinery [30,31]. Before their retrotranslocation, nascent proteins associate with PDI and BiP to cleave disulfide bonds and to unfold the partially folded structure, respectively [32–34]. Although unfolded ER proteins were previously speculated to be retrotranslocated through the translocon containing Sec61, the molecular structure of the retrotranslocation machinery remains elusive. Derlin-1 is a mammalian homolog of yeast Der1, and thought to be a critical component of the machinery. Derlin-1 may form a retrotranslocation channel in the ER membrane and associates with p97 through an adaptor protein, valocin-containing protein (VCP)-interacting membrane protein 1 (VIMP1) . Derlin-2 and Derlin-3, other Der1 homologs, are also involved in ERAD [35–37], although the exact underlying mechanism is still unclear.
p97/cdc48/VCP is a cytosolic AAA-ATPase and recruits unfolded ER proteins to the cytosol [38,39]. Ubiquitin fusion degradation protein 1 (Ufd1) and nuclear protein localization 4 (Npl4) bind to p97 as a cofactor and help p97 to extract unfolded proteins. The polypeptide portion of unfolded proteins interacts with p97, whereas the polyubiquitin chains attached to them are recognized by both p97 and Ufd1 and may activate the ATPase activity of p97 [40–42].
Retrotranslocated (or retrotranslocating) proteins are ubiquitinated by the E1–E2–E3 ubiquitin system. Ubiquitin is first conjugated to enzyme E2 by enzyme E1, and then transferred to ERAD substrates by enzyme E3. HMG-CoA reductase degradation protein 1 (HRD1), gp78, and TEB4/Doa10 are membrane-anchored E3 ligases involved in ERAD [43–46], whereas ubiquitin conjugase (UBC)6 and UBE2G2/UBC7 are E2 conjugase involved in ERAD. UBE1 is an E1 ubiquitin-activating enzyme that is ubiquitously involved in protein degradation by the proteasome. HRD1 shows a preference for substrates that contain misfolded luminal domains, whereas Doa10 prefers transmembrane proteins containing misfolded cytosolic domains (Doa10 also ubiquitinates cytosolic proteins). These two distinctive ERAD systems are called ERAD-L (luminal ERAD) and ERAD-C (cytosolic ERAD) [47,48]. EDEM and OS9 are thought to specifically recognize ERAD-L substrates. Actually, they form distinct ubiquitin–ligase complexes: the HRD1 complex contains HRD1, OS9, HRD3, Derlin-1, USA1, UBX2 and p97, whereas the Doa10 complex consists of Doa10, UBX2 and p97 [49–51]. Substrates containing misfolded transmembrane domains skip the interaction to OS9 and HRD3, and directly associate with the HRD1 complex, which is called the ERAD-M pathway .
However, there are a lot of other E3 ligases involved in the ERAD, and they preferentially recognize distinct ERAD substrates. FBX2 (F-box only protein 2) is another E3 ligase that specifically recognizes N-glycosylated proteins located in the cytosol [52,53]. Parkin is an E3 involved in Parkinson's disease (see below). In the case of cystic fibrosis transmembrane conductance regulator, its folding status is sequentially monitored by the two E3 ligase complexes, such as the RMA1 complex and the CHIP (C-terminus of Hsc70-interacting protein) complex .
Molecules other than E1–E2–E3 enzymes are also involved in ubiquitination. UBX2 binds to both p97 and E3 ligases such as HRD1 and Doa10 to recruit E3 to p97 , whereas gp78 directly associates with p97 . The ubiquitin-domain protein, Herp (homocysteine-induced endoplasmic reticulum protein), associates with a complex containing HRD1, p97, Derlin-1, and VCP-interacting membrane protein [57,58].
Retrotranslocated and ubiquitinated proteins are deglycosylated by peptide–N-glycanase before their degradation by the proteasome, because bulky glycan chains may hamper the entrance of substrates into the proteasome pore. As peptide–N-glycanase is associated with Derlin-1, it is possible that deglycosylation occurs coretrotranslocationally . Deglycosylated substrates are then delivered to the proteasome. Dsk2 and Rad23 facilitate this delivery of ERAD substrates .
Response pathways for ER stress
The mammalian ER stress response has four mechanisms: (1) translational attenuation; the enhanced expression of (2) ER chaperones and (3) ERAD components; (4) induction of apoptosis. These four responses are regulated by the regulatory pathways as described below (Fig. 4).
PERK is a type I transmembrane protein located in the ER, which senses the accumulation of unfolded proteins in the ER lumen [61–63]. The luminal portion of PERK is involved in sensing unfolded proteins, whereas the cytoplasmic portion contains a kinase domain. In the absence of ER stress, BiP binds to the luminal domain of PERK and keeps it from being activated (Figs 4 and 5A). In response to ER stress, BiP is released from PERK, and PERK is activated through oligomerization and trans-phosphorylation . Activated PERK phosphorylates and inactivates the α-subunit of eukaryotic translational initiation factor 2 (eIF2α), leading to translational attenuation. The phosphorylation of PERK is transient as the protein is dephosphorylated by specific phosphatases such as CReP (constitutive repressor of eIF2α phosphorylation), protein phosphatase 2C-GADD34, and p58IPK. CReP is constitutively expressed, whereas the expression of GADD34 and p58IPK is induced on ER stress by PERK and activating transcription factor (ATF)6 pathways, respectively.
Interestingly, translation of the transcription factor ATF4 is up-regulated by eIF2α-mediated translational attenuation. There are several small ORFs in the 5′-UTR of ATF4 mRNA (Fig. 5B). The ribosome first binds to a 5′-cap structure, slides on the ATF4 mRNA, and then starts translation at the small ORFs with unphosphorylated (active) eIF2α. As the ribosome is released from the ATF4 mRNA upon the termination of translation at the stop codon of small ORFs, the ATF4 ORF cannot be translated in the absence of ER stress. In contrast, as phosphorylated (inactive) eIF2α cannot start translation, the probability that the ribosome reaches the ATF4 ORF is increased in the presence of ER stress. Thus, the translation of ATF4 is remarkably enhanced in response to ER stress. The targets of ATF4 include CHOP (C/EBP homology protein), a transcription factor involved in the induction of apoptosis, and proteins involved in amino-acid metabolism such as asparagine synthetase or those involved in resistance to oxidative stress .
eIF2α is also phosphorylated by other kinases, such as dsRNA-dependent protein kinase (PKR), GCN2 (general control of amino-acid synthesis 2) and heme-regulated translational inhibitor. These kinases are activated by viral infections, amino-acid starvation, and heme deficiency, respectively, indicating that translational attenuation and ATF4 induction is induced by not only ER stress but also these physiological situations. Thus, the cellular response mediated by the phosphorylation of eIF2α is called the integrated stress response and is essential for cell survival .
There is another sensor molecule, ATF6, on the ER membrane [67–70]. ATF6 is a type II transmembrane protein, the luminal domain of which is responsible for the sensing of unfolded proteins. The cytoplasmic portion of ATF6 has a DNA-binding domain containing the basic-leucine zipper motif (bZIP) and a transcriptional activation domain. In the absence of ER stress, BiP binds to the luminal domain of ATF6 and hinders the Golgi-localization signal, leading to inhibition of ATF6 translocation (Fig. 5A) [71–75]. In response to the accumulation of unfolded proteins, BiP dissociates from ATF6, and ATF6 is moved to the Golgi apparatus by vesicular transport (Fig. 4). In the Golgi apparatus, ATF6 is sequentially cleaved by a pair of processing proteases called site 1 protease (S1P) and site 2 protease (S2P), and the resultant cytoplasmic portion of ATF6 [pATF6(N)] translocates into the nucleus. In the nucleus, pATF6(N) binds to a cis-acting element, the ER stress response element (ERSE), and activates the transcription of ER chaperone genes such as BiP, GRP94 and calreticulin . The consensus sequence of the ER stress response element is CCAAT-(N9)-CCACG, and ATF6 binds to the CCACG portion, whereas a general transcription factor, NF-Y (nuclear factor Y), binds to the CCAAT portion.
The cleavage of ATF6 is unique, especially as the second cleavage by S2P occurs in the transmembrane region . This process is called regulated intramembrane proteolysis (RIP), which is well conserved from bacteria to mammals (Fig. 6). The most characterized substrate of RIP is sterol response element-binding protein (SREBP) . SREBP is a transcription factor that is located in the ER membrane like ATF6. Upon a deficiency of sterol, SREBP is transported to the Golgi apparatus, cleaved by S1P and S2P, and activates the transcription of genes involved in the biosynthesis of sterol. Thus, the activation of ATF6 and SREBP is mainly regulated at the level of vesicular transport. The regulation of the transport of SREBP has been well characterized, and regulatory components such as the sensor-escort protein SCAP (SREBP cleavage-activating protein) and the anchor protein INSIG (insulin-induced gene 1) have been identified .
There are two genes for ATF6, called ATF6α and ATF6β, which have a similar function and are ubiquitously expressed [68,77]. Recently, several bZIP transcription factors located in the ER and regulated by RIP have been reported. cAMP response element-binding protein H (CREBH) is specifically expressed in liver, and processed by S1P and S2P in response to ER stress . CREBH activates the transcription of acute-phase response genes involved in acute inflammatory responses. OASIS (old astrocyte specifically induced substance) is also cleaved by S1P and S2P in response to ER stress in astrocytes and activates the transcription of BiP . A spermatid-specific transcription factor, Tisp40 (transcript induced in spermiogenesis 40), is also severed by S1p and S2P and activates the transcription of EDEM . These tissue-specific ATF6-like molecules may contribute to the ER stress response.
Luman/LZIP/CREB3 can be cut by S1P and S2P and activates the transcription of EDEM through a cis-acting element, unfolded protein response element (UPRE), although ER stress cannot induce Luman RIP [80–82]. CREB4 is transported to the Golgi apparatus in response to ER stress, is cleaved by S1P and S2P, and activates the transcription of BiP, although cleavage is not observed upon ER stress . These ATF6-like molecules, which are insensitive to ER stress, might be activated in situations other than ER stress and activate transcription of ER chaperones.
The third sensor molecule in the ER membrane is IRE1 (inositol requirement 1) [84–86]. The luminal domain of IRE1 is similar to that of PERK and involved in the sensing of unfolded proteins, whereas the cytoplasmic domain contains a kinase domain and an RNase domain. There are two genes for IRE1, IRE1α and IRE1β. Upon ER stress, BiP suppression of IRE1 activation is released, and IRE1 is activated through dimerization and transphosphorylation (Figs 4 and 5A) . Activated IRE1α converts XBP1 (x-box binding protein 1) pre-mRNA into mature mRNA by an unconventional splicing mechanism [69,87]. As the DNA-binding domain and the activation domain are encoded in ORFs in XBP1 pre-mRNA, a protein translated from pre-mRNA [pXBP1(U)] cannot activate transcription. In contrast, a protein translated from mature mRNA [pXBP1(S)] activates the transcription of ERAD component genes such as EDEM, HRD1, Derlin-2, and Derlin-3 through a cis-acting element, unfolded protein response element, as these two ORFs are joined in mature mRNA [37,88,89]. pXBP1(S) also induces the expression of proteins involved in lipid synthesis and ER biogenesis, as well as the expression of ER chaperones such as BiP, p58IPK, ERdj4, PDI-P5 and HEDJ [90,91]. Thus, XBP1 is essential to the function of cells that produce large amounts of secretory proteins such as pancreatic β-cells, hepatocytes, and antibody-producing plasma cells [92–95].
The splicing of XBP1 pre-mRNA by IRE1α is quite different from conventional mRNA splicing (Fig. 7A) . Conventional splicing is catalyzed by the spliceosome, and the consensus sequence at the exon–intron border is GU-AG or AU-AC (Chambon's rule). The splicing reaction is sequential: the 5′ site is cleaved first, then the 3′ site after a lariat structure is formed. In contrast, unconventional splicing of XBP1 pre-mRNA is catalyzed by IRE1α and RNA ligase, and there is a pair of stem–loop structures at the exon–intron border instead of GU-AG or AU-AC. Moreover, the splicing reaction is not sequential but random.
The most important difference between conventional and unconventional splicing is where the reaction occurs (Fig. 7B). Conventional splicing (nuclear splicing) takes place in the nucleus, whereas unconventional splicing (cytoplasmic splicing) occurs in the cytoplasm. The biological significance of cytoplasmic splicing is that pre-mRNA used for translation in the cytoplasm can be spliced when it is necessary to change the nature of the protein translated from the mRNA, in response to extracellular or intracellular signaling. In contrast, as nuclear splicing cannot splice mRNA exported to the cytoplasm, it is necessary for pre-mRNA to be transcribed de novo and spliced. Thus, cytoplasmic splicing would be a very rapid, versatile, and energy-efficient mechanism with minimal waste as compared with conventional mRNA splicing. Recently, it was found that pXBP1(U) encoded in XBP1 pre-mRNA is a negative feedback regulator of pXBP1(S). Thus, in the case of XBP1, pre-mRNA and mature mRNA encode negative and positive regulators, respectively, and their expression is switched by cytoplasmic splicing in response to the situation in the ER .
IRE1β is specifically expressed in epithelial cells of the gastrointestinal tract, and thought to cleave rRNA to attenuate translation in response to ER stress . When IRE1β–/– mice were exposed to an inducer of inflammatory bowel disease, they actually developed colitis, possibly because of the enhanced ER stress .
Recently, the crystal structure of the luminal domain of IRE1α was solved . The luminal domain is similar in structure to the peptide-binding domain of major histocompatibility complexes, suggesting the interesting possibility that it directly senses ER stress by directly binding unfolded proteins.
The accumulation of unfolded proteins in the ER is toxic to cells. Thus, if the PERK, ATF6, and IRE1 pathways cannot suppress ER stress, an apoptotic pathway is triggered to ensure survival of the organism as a last line of defense. A number of pathways have been reported to be involved in ER stress-induced apoptosis, and the full induction of apoptosis seems to require the concomitant activation of several death pathways, although there remain many arguments over ER stress-induced apoptosis [99–105]. In this section, I will briefly summarize the known death pathways, focusing on recent progress (Fig. 8).
The most characterized pathway is the CHOP pathway. CHOP/GADD153 (growth arrest and DNA damage 153) is a transcription factor, the expression of which is induced by the ATF6 and PERK pathways upon ER stress [70,106,107]. CHOP–/– cells exhibit less programmed cell death when faced with ER stress , suggesting that the CHOP pathway is a major regulator of ER stress-induced apoptosis. As for the target genes of CHOP, CHOP activates the transcription of GADD34, ERO1, DR5 (death receptor 5), and carbonic anhydrase VI, which seem to be responsible for apoptosis. GADD34 associated with protein phosphatase 2C enhances dephosphorylation of eIF2α and promotes ER client protein biosynthesis , whereas ERO1, which encodes an ER oxidase, makes the ER a more hyper-oxidizing environment . DR5, which encodes a cell surface death receptor, may activate caspase cascades . Carbonic anhydrase VI may change the cellular pH, affecting various cellular processes [112,113]. However, the exact signaling mechanism from CHOP to apoptosis is still unclear.
The second apoptotic pathway is the IRE1–TRAF2–ASK1 pathway. The cytoplasmic part of IRE1 binds to an adaptor protein, TRAF2 (tumor necrosis factor receptor-associated factor 2), which couples plasma membrane death receptor to Jun kinase (JNK) and stress-activated protein kinase (SAPK) . IRE1 and TRAF2 form a complex with a mitogen-activated protein kinase kinase kinase, ASK1 (apoptosis signal-regulating kinase 1), and this IRE1–TRAF2–ASK1 complex is responsible for the phosphorylation and activation of JNK . Actually, IRE1–/– cells as well as ASK1–/– cells are impaired in the activation of JNK and apoptosis by ER stress. In contrast, TRAF2–/– cells are more susceptible to apoptosis triggered by ER stress, which might be inconsistent with the above model . TRAF2 also associates with caspase-12 and regulates its activation . IRE1–TRAF2 activates the transcriptional repressor ATF3 as well, leading to the activation of apoptosis . These suggest that the IRE1–TRAF2–ASK1 pathway is a major regulator of ER stress-induced apoptosis. TNFR1 (tumor necrosis factor receptor 1), a receptor for TNF-induced cell death, associates with IRE1α upon ER stress, and the activation of JNK by ER stress is impaired in TNFR1–/– cells. This suggests that TNFR1 mediates the ER stress-induced activation of JNK , possibly forming a complex with IRE1α, TRAF2, and ASK1. The expression of TNFα is up-regulated by the IRE1 pathway during ER stress [120,121], which may contribute to the activation of TNFR1.
Caspases are well-known pro-apoptotic components, and caspases 2, 3, 4, 7, 9 and 12 are reported to be involved in ER stress-induced cell death [122–131]. Caspase-12 is associated with the ER membrane, and activated by ER stress, possibly by calpain . Then caspase-12 activates caspase-9, which in turn activates caspase-3 , leading to cell death. Caspase-12–/– mice are resistant to ER stress-induced apoptosis but sensitive to other death stimuli, suggesting that caspase-12 is a regulator specific to ER stress-induced apoptosis . The activation of caspase-12 by ER stress is observed in models of various diseases such as Alzheimer's disease, polyglutamine disease, ischemia, and viral infection [135–139], suggesting that ER stress-induced apoptosis is closely involved in these diseases (see below). However, the involvement of caspase-12 in apoptosis of human cells is still open to question, as the human caspase-12 gene contains several mutations critical to function . It is possible that an unidentified caspase other than caspase-12 is responsible in human cells.
Bcl2 family proteins are well-known components of the programmed cell death machinery, and some key components are involved in ER stress-induced apoptosis. In general, pro-apoptotic members of the Bcl2 family seem to be recruited to the ER surface and to activate caspase-12, whereas the anti-apoptotic members inhibit this recruitment, although the exact relationship between these factors is still unclear. So I will briefly describe our current understanding of Bcl2 family proteins. The anti-apoptotic factor Bcl2 is down-regulated by the transcription factor CHOP upon ER stress, which leads to enhanced oxidant injury and apoptosis . Overexpression of Bcl2 inhibits the activation of caspase-12 and apoptosis during ER stress . The pro-apoptotic factor, BAD (Bcl2 antagonist of cell death), is dephosphorylated and activated in response to ER stress , whereas other pro-apoptotic factors, Bax (Bcl2-associated X protein) and Bak (Bcl-2 homologous antagonist/killer), are present in the ER membrane as well as the mitochondrial membrane [144,145]. During ER stress, Bax and Bak oligomerize and activate caspase-12. Interestingly, Bax and Bak associate with IRE1α and modulate IRE1α function during ER stress . Bax and Bak are required for most forms of apoptosis . The transcription of PUMA (p53 up-regulated modulator of apoptosis) and NOXA (neutrophil NADPH oxidase factor), pro-apoptotic members of the BH3 (homology domain-3) domain-only family, is up-regulated by p53 during ER stress, and PUMA–/– cells and NOXA–/– cells are resistant to ER stress-induced apoptosis [148,149]. Another pro-apoptotic component, Bim (Bcl2-interacting mediator of cell death), translocates from the dynein-rich compartment to the ER membrane and activates caspase-12 in response to ER stress, whereas an anti-apoptotic factor, Bcl-xL (Bcl-2-like 1), binds to Bim and inhibits its translocation . Bim-knockdown cells are resistant to ER stress. The ER-localized anti-apoptotic factor BI-1 (Bax inhibitor-1) inhibits the activation of Bax during ER stress, and BI-1–/– mice are sensitive to ER stress, whereas mice overexpressing BI-1 are resistant . BIK (Bcl2-interacting killer) is an ER-localized pro-apoptotic component which enhances the recruitment of BAX and BAK to the ER . Bap31 (B cell receptor-associated protein 31) is a pro-apoptotic factor that is cleaved and activated upon ER stress, its cleavage being dependent on calnexin . In calnexin-deficient cells, the cleavage of Bap31 and ER stress-induced apoptosis are inhibited.
The inhibitor of apoptosis (IAP) family has also been reported to be involved in ER stress-induced apoptosis. Human inhibitor of apoptosis 2 (HIAP2) is an IAP that inhibits caspase-3 and caspase-7. Expression of HIAP2 is induced upon ER stress at the level of translation: caspases activated by ER stress cleave eukaryotic initiation factor, p97/DAP5/NAT1, and the cleavage product specifically activates the HIAP2 internal ribosome entry site, leading to enhanced translation of HIAP2 . Transcription of IAP-2 and XIAP (inhibitor of apoptosis, X-linked), two other IAPs, is up-regulated during ER stress, and cells in which these IAPs have been knocked down are sensitive to ER stress-induced apoptosis . Cells overexpressing XIAP or HIAP1 are resistant to ER stress [122,155]. These results suggest involvement of IAP proteins in ER stress-induced apoptosis.
c-Abl (Abelson murine leukemia viral oncogene homolog 1) is a protein tyrosine kinase distributed in the nucleus and cytoplasm, and c-Abl activated by a death signal induces phosphorylation and activation of pro-apoptotic JNK and SAPK. Interestingly, c-Abl is also located on the ER surface, and translocates to the mitochondria upon ER stress, where it induces the release of cytochrome c. c-Abl–/– cells are resistant to ER stress-induced apoptosis. A c-Abl-interacting protein, Aph2 (anterior phalynx defective 2), is also located on the ER and shows pro-apoptotic activity , suggesting that c-Abl forms a distinct pathway leading to ER stress-induced apoptosis.
PKR is a dsRNA-dependent protein kinase which is activated upon viral infection, and its phosphorylation in the nucleus is up-regulated in response to ER stress. Interestingly, PKR-knockdown cells or PKR mutant cells are resistant to ER stress-induced apoptosis, suggesting that PKR is a pro-apoptotic factor during ER stress .
TDAG51 (T-cell death-associated gene 51) is a member of the pleckstrin homology-related domain family, and its transcription is induced by ER stress through the PERK pathway . Overexpression of TDAG51 induces apoptosis, suggesting that TDAG51 is involved in ER stress-induced apoptosis and the development of atherosclerosis (see below).
Nuclear respiratory factor (NRF)1 and NRF2 are transcription factors that regulate the oxidative stress response. NRF2 is distributed in the cytoplasm through its association with the microtubule-associated protein Keap1 (Kelch-like Ech-associated protein 1). Upon ER stress, PERK phosphorylates NRF2 and dissociates it from Keap1, leading to the nuclear recruitment of NRF2 . Remarkably, NRF2–/– cells are sensitive to ER stress-induced apoptosis, whereas NRF1 is located in the ER membrane, and translocates to the nucleus upon ER stress , suggesting that these proteins are involved in ER stress-specific apoptosis.
ATF6 is also involved in the apoptotic process during myogenesis. In differentiating myoblast, the ATF6 pathway is activated, and expression of BiP and CHOP is up-regulated, which may activate caspase-12. Moreover, AEBSF [4-(2-aminoethyl)benzenesulfonyl fluoride hydrochloride], an inhibitor of ATF6 activation, blocks apoptosis, suggesting that the ATF6 pathway contributes to apoptosis during myogenesis .
TRB3 (Tribbles homolog 3) is a human ortholog of Drosophila tribble, and its transcription is induced by ER stress through the PERK–ATF4–CHOP pathway. Interestingly, TRB3-knockdown cells are resistant to ER stress-induced apoptosis, suggesting that TRB3 is a pro-apoptotic factor during ER stress [163,164].
p53 is a transcription factor that induces growth arrest and apoptosis in response to various forms of cellular stress, such as DNA damage. There are several reports suggesting a connection between p53 and ER stress. Upon ER stress, p53 is phosphorylated by glycogen synthase kinase 3B, leading to the distribution of p53 in the cytoplasm and destabilization of p53, and attenuation of p53-dependent apoptosis [165–167]. Interestingly, Scotin and SCN3B (sodium channel subunit beta 3), p53-inducible pro-apoptotic proteins, are located in the ER [168,169]. Moreover, transcription of PUMA and NOXA, which are involved in ER stress-induced apoptosis, is up-regulated by p53 during ER stress , suggesting that the p53 pathway regulates the apoptotic pathway during ER stress.
Other components such as VCP and ALG-2 (apoptosis-linked-gene 2) , AIGP1 , elongation factor-1α, and NRADD (neurotrophin receptor associated death domain)  are also reported to be involved in ER stress-induced apoptosis. Their precise functions and working mechanisms are still to be clarified.
ER stress-related diseases
Unfolded or malfolded proteins readily form aggregates in the ER as well as the cytosol. Recent reports suggested that small aggregates are highly toxic, as they impair the ubiquitin proteasome pathway  and sequester transcription factors such as CREB-binding protein and TATA-binding protein [174,175], whereas large aggregates such as the aggresome and the inclusion body are cytoprotective . Interestingly, molecular chaperones such as HSP70 and TRiC (TCP1- ring complex) can suppress the formation as well as toxicity of protein aggregates [174,177–181]. The diseases caused by the malfolding of cellular proteins are collectively called ‘conformational diseases’ or ‘folding diseases’. As malfolded proteins and protein aggregates can evoke ER stress, it has been speculated that ER stress is involved in most conformational diseases, particularly Alzheimer's disease and Parkinson's disease, although it is still heavily disputed whether ER stress (or protein aggregates) is a major cause of these diseases. In this section, I will briefly summarize recent findings on how ER stress is involved in conformational diseases.
Neurons are thought to be sensitive to protein aggregates, and there are many reports that ER stress is involved in neurodegenerative diseases [182–184]. In fact, disruption of SIL1/BAP, a cochaperone of BiP, results in the accumulation of protein aggregates and neurodegeneration . Most of these diseases are caused by aging or genetic background, although several are assumed to be infectious, especially prion disease.
Alzheimer's disease is the most common neurodegenerative disease, and characterized by cerebral neuritic plaques of amyloid β-peptide [185–191]. Recent findings strongly suggest that one of the major causes of Alzheimer's disease is an accumulation of amyloid β-peptide, although tau also seems to be involved in the disease. Studies of patients with autosomal-dominant familial Alzheimer's disease have identified three genes responsible for the disease, amyloid precursor protein (APP), PS1 and PS2. The amyloid precursor protein encoded by APP is a transmembrane protein, the function of which is still unknown, whereas both PS1 and PS2 encode a protein called presenilin, which is an essential component of a protease called γ-secretase. APP is sequentially cleaved by a β-secretase called BACE (β-site amyloid β A4 precursor protein-cleaving enzyme 1) and γ-secretase, leading to the accumulation of amyloid β-peptide. Interestingly, cells expressing PS1 mutants show a waned ER stress response and are sensitive to ER stress . The activation of ATF6, IRE1, and PERK is also disturbed in these mutant cells . Moreover, proteins involved in ER stress-induced apoptosis, such as PKR and caspase-4, are involved in the onset of Alzheimer's disease [125,158]. Actually, the ER stress response is activated in patients with Alzheimer's disease [192,194,195], and polymorphism of SEL1, a component of ERAD, is linked to Alzheimer's disease . These findings strongly suggest a strong causal relationship between ER stress and Alzheimer's disease, and it is highly possible that ER stress invoked by the accumulation of amyloid β-peptide is one of the key mechanisms of Alzheimer's disease.
Parkinson's disease is the second most common neurodegenerative disease, which is characterized by a loss of dopaminergic neurons . Analyses of patients with familial Parkinson's disease have revealed three genes responsible for the disease, encoding α-synuclein, Parkin, and ubiquitin C-terminal esterase L1 (UCH-L1). α-synuclein is a cytoplasmic protein which forms aggregates, called Lewy bodies, characteristic of Parkinson's disease, but the link between α-synuclein and ER stress is unclear. In contrast, Parkin is a ubiquitin-protein ligase (E3) involved in ERAD . One of the substrates of ERAD ubiquitinated by Parkin is Pael receptor, a homolog of endothelin receptor type B . Interestingly, expression of Parkin is induced by ER stress, and neuronal cells overexpressing Parkin are resistant to ER stress . As for UCH-L1, it is an abundant protein in neurons and stabilizes a monomeric ubiquitin [201,202]. It has been shown that UCH-L1 ubiquitinates unfolded proteins and might be involved in ERAD . These findings strongly suggest the involvement of ER stress in Parkinson's disease. In addition, there are several other reports supporting the link between ER stress and Parkinson's disease. First, Parkinson's disease mimetics, such as 6-hydroxydopamine, specifically induce ER stress in neuronal cells [204,205]. Second, expression of ER chaperones such as PDI is up-regulated in the brain of Parkinson's disease patients, and PDI is accumulated in Lewy bodies . The incidence of sporadic Parkinson's disease increases with age, but it is still unclear whether the ER stress response wanes in patients with Parkinson's disease.
Polyglutamine (polyQ) diseases are neurodegenerative disorders caused by duplications of the CAG-repeat in certain genes, and include Huntington's disease, spinobulbar muscular atrophy (Kennedy disease), Machado-Joseph disease, dentatorubral-pallidoluysian atrophy (Haw River Syndrome), and spinocerebellar ataxia. Large polyQ stretches translated from the CAG-repeat form insoluble protein aggregates, which are toxic to cells. Although the exact mechanism behind the toxicity of polyQ remains to be clarified, one possibility is that polyQ aggregates sequester other normal proteins, such as transcription factors, which are indispensable to cell function . Another possibility is that the polyQ aggregate itself is toxic to cells. All known polyQ proteins causing neurodegenerative diseases are cytosolic, but they evoke ER stress, as polyQ proteins suppress the function of the proteasome, which is an essential component of ERAD [115,136,207]. Interestingly, p97, a component of the ERAD machinery, enhances degradation of polyQ proteins and suppresses polyQ protein-induced neurodegeneration . Judging from these findings, it is probable that ER stress is involved in the onset of polyQ diseases.
Pelizaeus-Merzbacher disease is a progressive neurodegenerative disorder characterized by a loss of coordination, motor abilities, and intellectual function . Currently, it is thought that the disease is caused by a mutation of the PLP1 gene which encodes a transmembrane proteolipid essential for the maintenance of myelin sheaths, as well as in oligodendrocyte development and axonal survival [210,211]. As a missense mutation causes a more severe phenotype than a null mutation, it is speculated that the missense mutant of PLP1 forms aggregates in the ER and evokes ER stress-induced cell death . Actually, expression of CHOP, BiP, ERp59, and ERp72 is up-regulated in the brains of mice expressing missense PLP and the brains of patients with Pelizaeus-Merzbacher disease.
Prion disease, also called transmissible spongiform encephalopathy, encompasses Creutzfeldt-Jakob disease, Gerstmann–Straussler–Scheinker syndrome, fatal familial insomnia, Kuru, Alpers syndrome, bovine spongiform encephalopathy, transmissible milk encephalopathy, chronic wasting disease, and scrapie . Characteristic features of the disease are a loss of motor control and dementia. The only gene to be identified so far as being responsible for prion disease is PrP, which encodes a protein anchored to the cell surface. There is no difference in amino-acid sequence between the normal protein PrPc and its pathological form PrPSc. PrPSc is rich in β-sheets, converts PrPc into PrPSc, and forms amyloid fibrils that are thought to be toxic to cells. Interestingly, in murine cells infected with PrPSc, the expression of ER chaperones such as GRP58 and GRP94 as well as caspase-12 is up-regulated . Moreover, the expression of these chaperones is considerably increased in patients with Creutzfeldt-Jakob disease. Finally, overexpression of GRP58 protects cells from PrPSc-induced cell death, whereas inhibition of GRP58 expression with small interfering RNA results in a severe phenotype . These findings strongly suggest that ER stress is involved in the pathology of prion disease.
Amyotrophic lateral sclerosis, also called Lou Gehrig's disease, is a progressive neuromuscular disease and shows characteristic pathological features such as a loss of motor neurons in the cerebral cortex and spinal cord. Analysis of patients with familial amyotrophic lateral sclerosis has revealed that superoxide dismutase-1 is responsible for the disease. Mutant superoxide dismutase-1 forms aggregates in the ER, evokes ER stress, induces the expression of BiP, and activates caspase-12, leading to neuronal cell death. These findings support the notion that ER stress induced by superoxide dismutase aggregates is a major cause of amyotrophic lateral sclerosis [216–218].
GM1 gangliosidosis is an autosomal recessive lysosomal storage disorder characterized by an accumulation of GM1 gangliosides in the brain. The disease is caused by a mutation of lysosomal β-galactosidase that converts GM1 gangliosidosides to GM2 gangliosidosides. The analysis of β-galactosidase-knockout mice has revealed that the accumulation of GM1 gangliosidosides evokes ER stress, induces the expression of BiP and CHOP, activates JNK2 and caspase-12, and causes the apoptosis of neurons . These findings suggest a strong correlation between ER stress and GM1 gangliosidosis.
Bipolar disorder is a class of mood disorders where the person experiences recurrent episodes of mania and depression . Genetic linkage studies suggest that genetic factors are involved in the disease. Microarray analyses of cells derived from twins discordant with respect to the disease revealed that the expression of XBP1 and BiP wanes in the affected twins. Recent reports showed that there are polymorphisms in the promoter regions of XBP1 and BiP that are common to patients [221–224], although this conclusion is still controversial . It is interesting that mood-stabilizing drugs, such as valproate and lithium, which are highly effective in the treatment of bipolar disorder can increase the expression of ER chaperones such as BiP, GRP94, and calreticulin . Further analyses are required to confirm that ER stress is involved in the onset of bipolar disorder.
Diabetes mellitus is a disease characterized by hyperglycemia caused by the impaired secretion or action of insulin [227–229]. Type I diabetes mellitus [insulin-dependent diabetes mellitus (IDDM)] results from selective destruction of insulin-secreting β-cells, whereas type II [noninsulin-dependent diabetes mellitus (NIDDM)] is characterized clinically by insulin resistance. As β-cells seem to suffer from ER stress caused by the production of a large amount of insulin, the link between ER stress and IDDM has been investigated thoroughly, especially the PERK pathway. PERK is responsible for an early infancy IDDM called Wolcott–Rallison syndrome , and PERK–/– mice show symptoms characteristic of IDDM [62,231]. Moreover, mice with a homozygous mutation at the eIF2α phosphorylation site (Ser51Ala) show similar pathological features [232,233], whereas the onset of diabetes is delayed in CHOP–/– mice . Interestingly, components of ER stress pathways other than the PERK pathway are also involved in IDDM. First, the expression of BiP, GRP94, ORP150 and HRD1 is up-regulated, and both ATF6 and XBP1 are activated in the Akita diabetes mouse model [235,236]. Second, p58IPK–/– mice also show symptoms resembling IDDM . Finally, the expression of WFS1, a gene responsible for the onset of Wolfman syndrome (juvenile diabetes), is induced by ER stress, and knockdown or knockout of WFS1 causes ER stress in pancreatic β-cells [238,239]. These findings strongly suggest that ER stress is deeply involved in the onset of IDDM.
There are several reports suggesting that ER stress is also involved in NIDDM. First, obesity, one of the causes of NIDDM, evokes ER stress, and XBP1–/– mice develop insulin resistance , although the underlying mechanism is unknown. Second, ectopic expression of ORP150 in β-cells improves insulin tolerance [240,241]. A possible explanation for this is that ORP150 expression may improve the folding capacity in the ER and protect cells from ER stress-induced apoptosis. Finally, a small nuclear polymorphism analysis revealed that a polymorphism in ATF6 is associated with NIDDM in Pima Indians .
Atherosclerosis is a disease wherein arteries harden and narrow because of the accumulation of fatty substances, cholesterol, cellular waste, Ca2+, and other substances in the arterial inner lining, leading to heart attack or stroke. One of the risk factors for atherosclerosis is the accumulation of homocysteine, which is an intermediate produced during the metabolism of sulfur amino acids [243,244]. Interestingly, homocysteine induces ER stress by an unknown mechanism, and then increases the expression of BiP, GRP94, CHOP, Herp, and TDAG51 [159,245,246]. The ER stress induced by homocysteine also increases cholesterol synthesis by activating the transcription factor SREBP. Both the induction of CHOP and TDAG51 (a member of the pleckstrin homology-related domain family) and the accumulation of cholesterol appear to promote the apoptosis of macrophages, leading to the deposition of macrophage debris in blood vessels that contributes to the development of the atherosclerosis observed in hyperhomocysteinemia [247–249]. Actually, CHOP–/– macrophages are less sensitive to cholesterol-induced apoptosis. These findings strongly support the notion that ER stress-induced apoptosis is one of the major factors that cause atherosclerosis.
Inflammation is the first response of the immune system to infection. Although the mechanism of inflammation is complicated, ER stress is involved in some types of inflammation. In inflammation of the central nervous system, interferon-γ induces ER stress and apoptosis of oligodendrocytes . Interestingly, PERK+ mice show enhanced central nervous system hypomyelination and oligodendrocyte loss, suggesting that the PERK pathway has a protective role against interferon-γ-induced apoptosis. In the case of lipopolysaccharide-induced inflammation of the lungs, lipopolysaccharide induces ER stress and CHOP expression, leading to the apoptosis of lung cells . Diclofenac, a nonsteroidal anti-inflammatory drug, suppresses ER stress-induced apoptosis , suggesting that ER stress is one of the major mediators of inflammation. Nitric oxide (NO) is another substance involved in the induction of apoptosis during inflammation. Although NO-induced apoptosis has been generally considered to be mediated by either DNA damage or mitochondrial damage, NO also induces apoptosis mediated by ER stress and CHOP in pancreatic β-cells, microglial cells, and macrophages [103,229,234,253–257]. Recently, it was reported that proinflammatory cytokines and lipopolysaccharide evoke ER stress and induce the transcription and activation of CREBH in the liver . The transcription factor CREBH induces the expression of proteins involved in the acute inflammatory response such as serum amyloid P-component and C-reactive protein. As mentioned above, CREBH is structurally related to ATF6 and activated by a mechanism similar to ATF6. These findings suggest a strong link between ER stress and inflammation, although it should be clarified how ER stress is induced during inflammation.
ER stress is also involved in autoimmunity, and there are three reports supporting this notion. First, BiP associates with the clinically relevant autoantigen Ro52 (ribonucleoprotein autoantigen 52 kDa) [Sjoegren syndrome type A antigen (SS-A)], and is thought to be involved in autoimmunity and rheumatoid arthritis . Second, a microarray analysis using muscle tissue of patients with myositis revealed that the expression of BiP, CHOP, GADD45 and asparagine synthetase is induced in the patients' cells, suggesting that the ER stress response is responsible for the skeletal muscle damage and dysfunction in autoimmune myositis . Third, overexpression of synoviolin, an E3 ubiquitin ligase involved in ERAD, in mice causes arthropathy with synovial hyperplasia, whereas knockdown of synoviolin results in increased apoptosis of synovial cells and less sensitivity to collagen-induced arthritis .
As hypoxia induces ER stress as mentioned above, ER stress is an important cause of ischemia-related diseases. For instance, brain ischemia induces ER stress in neurons and activates the ATF6, IRE1 and PERK pathways , leading to the CHOP-mediated apoptosis of neurons . Ischemia also induces ER stress and the expression of ER chaperones in the heart, leading to degeneration of cardiomyocytes , suggesting that ER stress is involved in the development of ischemic heart disease (see below).
It has been reported that ER stress is involved in heart-related diseases. Pressure overload by transverse aortic constriction induces expression of ER chaperones and ER stress-induced apoptosis of cardiac myocytes, leading to cardiac hypertrophy . Interestingly, transgenic mice expressing a mutant KDEL receptor, a retrieval receptor for ER chaperones in the early secretory pathway, developed dilated cardiomyopathy due to a malfunction of the ER quality control machinery and increased ER stress . Moreover, up-regulation of ER chaperones protected cardiomyocytes from ER stress-induced apoptosis . These findings strongly suggest that the ER stress response is essential for homeostasis of cardiomyocytes.
Hepatocytes have a well-developed ER structure that is essential for the vigorous synthesis of secretory proteins, and it has been reported that ER stress is involved in liver-related diseases . For instance, alcohol is known to cause liver injury by various mechanisms including ER stress [268,269]. Although exactly how alcohol causes ER stress remains unclear; one possible explanation is that alcohol inhibits the activity of the key enzymes for sulfur amino-acid metabolism such as methionine synthetase and betaine-homocysteine methyltransferase, leading to the accumulation of a potent ER stress inducer, homocysteine [121,270]. ER stress evoked by alcohol activates transcription factors such as CHOP, leading to CHOP-induced apoptosis of hepatocytes. ER stress also activates another transcription factor, SREBP, which is responsible for the up-regulation of fatty acid synthesis, resulting in lipid-induced apoptosis [268,271].
ER stress is also involved in hepatocarcinogenesis [272–274]. In human hepatocellular carcinoma, the ATF6 and IRE1 pathways are activated, and expression of BiP is markedly increased, suggesting that the transformation of hepatocytes induces ER stress, and cells cope with the stress by activating the ER stress response pathways.
Various factors are known to induce renal injury by evoking ER stress. First, the analgesic and antipyretic drug paracetamol (acetaminophen) causes renal tubular injury, and ER stress-induced apoptosis is one of the mechanisms involved . Actually, paracetamol induces an ER stress response that includes the induction of CHOP and cleavage of caspase-12. Second, complement attack also induces ER stress and activates the PERK pathway, leading to glomerular epithelial cell injury . Third, excessive accumulation of secretory proteins induces podocyte injury by evoking ER stress . Fourth, overexpression of a kidney-specific serine protease inhibitor, megsin, induces ER stress and renal injury . Interestingly, prior induction of BiP expression protects renal epithelial cells from chemicals inducing renal injury . These findings suggest that ER stress is one of the major causes of chemically induced renal injury, and that the ER stress response is a defense mechanism against renal injury.
Infections caused by various viruses such as hepatitis B virus [280,281], hepatitis C virus , hepatitis D virus , flavivirus , Borna disease virus , murine leukemia virus  and Moloney murine leukemia virus  are known to induce ER stress, possibly because the vigorous synthesis of viral proteins makes the ER very busy. Upon viral infection, cells activate the ER stress response pathways to protect them from ER stress-induced apoptosis. ER stress induced by a viral infection is thought to cause various pathogenetic effects such as neurodegeneration, liver injury and carcinogenesis, depending on the cell types infected .
Hereditary tyrosinemia type I
Hereditary tyrosinemia type I is a metabolic disease affecting mainly the liver and renal functions, which is caused by a deficiency of fumarylacetate hydrolase which catalyzes the hydrolysis of fumarylacetate. In cells of patients with hereditary tyrosinemia type I, fumarylacetate is accumulated and evokes ER stress as well as mutagenic, cytostatic and apoptotic effects and chromosomal instability . In hamster lung cells, fumarylacetate induces an ER stress response that includes the induction of BiP and CHOP expression and the cleavage of caspase-12, suggesting that ER stress is involved in this disease.
Golgi stress response
The Golgi apparatus is another organelle responsible for the production of secretory proteins . Secretory proteins synthesized in the ER are transported to the Golgi apparatus and receive various modifications by enzymes located there, such as the modification of oligosaccharide chains and processing of peptide chains. Proteins properly processed in the Golgi apparatus are transported to the plasma membrane or secreted to the extracellular matrix. Thus, it is highly possible that the Golgi apparatus sends an emergency signal to the nucleus when the amount of client exceeds the capacity of its processing machinery, inducing the transcription of genes involved in Golgi apparatus function . Although the mechanism of this response pathway (the Golgi apparatus stress response pathway) has not yet been clarified, it is worth investigating Golgi apparatus stress as it is probably correlated with various diseases.
Conclusions and perspectives
Clarification of the basic mechanisms of ER stress response has led to the discovery of a close relationship between ER stress and various diseases. As research on ER stress has expanded explosively, I hope this small review will help to give researchers a comprehensive view and to further develop this field academically and clinically.
We thank Ms. Kaoru Miyagawa for secretarial assistance. This work was supported by the PRESTO-SORST program of the Japan Science and Technology Agency, and grants from the Ministry of Education, Culture, Sports, Science, and Technology of Japan (17026022, 17370061 and 18050013).