Type 2 allergic immune responses are typically characterized by eosinophilic inflammation accompanied by increases in type 2 cytokines such as interleukin (IL)-4, IL-5, and IL-13 in affected tissues and blood. These responses are closely associated with various allergic diseases, including allergic rhinitis (AR), a hallmark of which is the presence of serum antigen-specific immunoglobulin E (IgE) indicative of adaptive immunity involving type 2 helper T cells (TH
2 cells) [52
The allergic response begins at the interface between the external environment and the epithelium and subsequently involves diverse cell types at all levels of innate and adaptive immunity. During these processes, cells produce large amounts of secretory proteins to defend themselves against endogenous and exogenous threats and/or to efficiently communicate with other cell types to generate an organized immune response. Thus, proper functioning of the ER and maintenance of protein homeostasis is critical for these cells. In these processes, the influence of ER stress is not restricted to protein folding, but instead can intersect with immunity at many levels, thereby leading to complex chronic inflammatory diseases such as allergy. Before T cells involved in various allergic disease endotypes can be activated, the allergen must be recognized by antigen-presenting dendritic cells (DCs) and presented to T cells in draining lymph nodes. Epithelial cells are known to be key modulators in controlling DC activation through release of cytokines and endogenous associated molecules. Given the role of epithelial cells as the first line of defense, diverse coexistent environmental insults may also converge on epithelial-DC interactions [53
]. For example, inflamed airway epithelial cells exhibit overt signs of ER stress, and bronchial epithelial XBP-1 has been reported to mediate inflammation-induced ER/Ca2+
store expansion, which amplifies Ca2+
-dependent secretion of cytokines [54
]. Thus, ER stress and the UPR pathway may be central players in the regulation of epithelial-DC interactions, which are crucial for the initiation and amplification of allergic response in the respiratory tract. In addition, XBP1 plays a pivotal role in the terminal differentiation of B cells into highly secretory plasma cells by mediating expansion of the ER and synthesis of proteins required for antibody production and secretion [56
]. Moreover, the IRE1α-XBP1 arm of the UPR is thought to be important in the early developmental stage of B cells and terminal differentiation of effector CD8+
T cells [55
]. Notably, considering that XBP-1 mRNA in naïve B cells is uniquely induced by IL-4, this arm of the UPR may be particularly important in the type-2 allergic response [58
]. Additionally, differentiation of eosinophils—a subset of granulocytes—from myeloid cell progenitors is uniquely dependent on the IRE1α-XBP1 pathway, and deletion of XBP1 results in massive defects in eosinophil maturation [59
]. Moreover, macrophages are known to connect cell surface innate toll-like receptor (TLR) signaling with intracellular IRE1α-XBP1 pathway-mediated secretion of pro-inflammatory cytokines, thereby linking the ER and UPR pathway to innate effector function in macrophages [60
]. Concurrent with this, TLR signaling suppresses the ATF4-CHOP-mediated cellular apoptotic pathway to effectively coordinate the innate function of macrophages [61
]. summarizes reports investigating the relationship between AR and ER stress. A recent in vitro study expanded on these observations in a primary cultured human nasal epithelial cell model of allergic rhinitis caused by the house dust mite (HDM). These researchers used HDM as an allergen at a concentration sufficient to induce an allergic reaction in primary cultured human nasal epithelial cells isolated from two non-asthmatic subjects. Cells from both subjects exhibited increases in p-IRE, as well as increases in the ER chaperone BiP, GRP94, and ER protein 57 (ERp57) 72 hours after exposure to HDM. Expression of the ER stress transducer ATF6α and the downstream transcriptional effector CHOP were also increased after HDM exposure. Allergen exposure was accompanied by activation of caspase-3 to varying degrees in primary human nasal epithelial cells. The authors of this study suggested that HDM induces ER stress in airway epithelial cells and that ATF6α and ERp57 play a significant role in the development of cardinal features of allergic airways disease, such as AR [62
Table 2. Studies assessing the association between rhinologic diseases and ER stress.
ER, endoplasmic reticulum; AR, allergic rhinitis; HDM, house dust mite; p-IRE, phosphorylation of inositol-requiring enzyme; GRP, glucose-regulated protein; BiP, immunoglobulin heavy chain-binding protein; ERp, endoplasmic reticulum protein; ATF, activating transcription factor; CHOP, C/EBP-homologous protein; CRSwNP, chronic rhinosinusitis with nasal polyp; p-eIFα, phospho-eukaryotic initiation factor-α; SEB, S. aureus enterotoxin B; ROS, reactive oxygen species; EP, eosinophilic polyp; NEP, non-eosinophilic polyp; p47phox, neutrophil cytosol factor 1 (NCF1); NP, nasal polyp; IT, inferior turbinate of the nose; PNEC, primary nasal epithelial cells; RT-PCR, reverse transcription-polymerase chain reaction; XBP1, X-box-binding protein 1; TGF, tumor growth factor; EMT, epithelial-mesenchymal transition; α-SMA, alpha-smooth muscle actin; 4-PBA, 4-phenylbutylic acid; PP2, Src and RIP2 kinase inhibitor; OSA, obstructive sleep apnea; CIH, chronic intermittent hypoxia; PERK, endoplasmic reticulum kinase; Ad, adiponectin; LVF, left ventricular function; TUNEL, terminal deoxynucleotidyl transferase dUTP nick-end labeling; Bax, BCL2-associated X; PAG, DL-propargylglycine; HIT, high intensity training; HIF-1α, hypoxia inducible factor-1α; TUDCA, tauroursodeoxycholic acid; ELISA, enzyme-linked immunosorbent assay; JNK, c-Jun N-terminal kinase; qPCR, quantitative polymerase chain reaction; Sal, salubrinal; ERO-1, endoplasmic reticulum oxidoreductin-1; CPAP, continuous positive airway pressure.
As previously mentioned, a significant portion of allergic disease, such as AR, is associated with the TH
2 immune response. However, in a recent study, only 50% of patients with asthma were associated with a type 2 immune response. Therefore, research on non-type 2 immune response has been actively conducted recently [72
]. Generally, both type 17 (TH
17) and type 1 (TH
1) responses are often associated with neutrophilic inflammation of airways. In recent study, corticosteroid-insensitive severe asthma possesses mixed type 17/type 1 immune response in the background of variable type 2 immunity [72
]. In other study, IFN-γ has been implicated in bronchial asthma pathogenesis through TH
2-independent IFN-γ/mast cell axis as well as its classical effects on TH
2 cells [73
]. In addition, LPS-induced ER stress leads to the increased expression of IL-17 in airway epithelium, thereby further potentiating ER stress and NF-κB activation via forming a positive feedback loop in airway epithelial cells [74
]. These data suggest that ER stress and UPR pathways may play a role in non-type 2 neutrophilic allergic response. In summary, the role of ER stress in the development of AR is related to Type 2 allergic immune responses, similar to that in asthma, and some are also related to non-Th2 immune responses.
Intermittent Hypoxia and Obstructive Sleep Apnea
Obstructive sleep apnea (OSA) is a disorder in which all or a part of the upper respiratory tract, such as the nasal-oral-pharyngeal-laryngeal region, is partially or completely closed during sleep, resulting in repetitive hypopnea or apnea. This repetitive obstruction of the respiratory tract results in decreased oxygen levels in the blood, frequent sleep fragmentation and various additional complications, including cardiovascular impairments and dementia. In particular, intermittent hypoxia (IH), which occurs repetitively as part of this process, is closely associated with ER stress, a relationship that has been the subject of study. One such study investigated the preventive effect of adiponectin (Ad)-mediated suppression of ER stress on chronic IH (CIH) injury in the rat myocardium. Ad, a circulating cytokine derived from white adipose tissue and cardiomyocytes, has been suggested to exert cardioprotective effects by virtue of its anti-inflammatory, anti-atherogenic, anti-hypertensive, and insulin-sensitizing properties. It has also been reported that Ad levels in plasma are decreased in OSA patients compared with healthy controls. Hence, the authors of this study investigated whether ER stress is mechanistically related to the occurrence of repetitive CIH and whether Ad exerts protective effects on cardiac function in rats. They found that left ventricular function (LVF) was improved in CIH model rats after addition of Ad (CIH + Ad group). ER stress was shown to act through CHOP and BiP, which were significantly increased in the CIH-only group, to affect this process. IRE1, XBP-1, PERK, eIF2α, and ATF6 were also increased in the CIH-only group compared with normal controls—increases that were mitigated by addition of Ad. It was further shown that the percentage of apoptotic cells and the prevalence of cleaved caspase-3, -9, and -12 were significantly higher in the CIH-only group than in normal control CIH + Ad groups. Taken together, these observations suggest that cardiac function is depressed by ER stress-induced apoptosis and protected by Ad-mediated inhibition of ER stress [65
]. Moreover, another study reported that DL-propargylglycine (PAG), a known effective inhibitor of cystathionine γ-lyase (CSE)-synthesized hydrogen sulfide (H2
S), protects against myocardial injury in the context of ER stress-induced cardiac dysfunction in the CIH state in rats. Endogenous H2
S plays an important role in maintaining cardiovascular functions, and exogenous H2
S exerts protective effects against myocardial injury induced by various cardiovascular diseases. Predictably, inhibiting the generation of endogenous H2
S has opposite effects. Among the findings of this study was that pretreatment with PAG significantly improved cardiac function in CIH model rats (CIH + PAG group) compared with the CIH-only group. Moreover, levels of oxidative stress, ER stress and apoptosis were significantly lower in the CIH + PAG group compared with the CIH-only group [66
]. In another study in rats that addressed cardiovascular morbidity, protein expression of the proapoptotic ER stress markers, BiP, p-PERK, ATF4 and CHOP, were increased in CIH. Increases in cleaved caspase-3, an apoptosis marker, have also been shown to cause myocardial apoptosis—an IH-associated proapoptotic alteration that correlates with infarct size. In addition, application of high-intensity training in rats was reported to reduce IH-associated proapoptotic ER stress and infarct size [67
]. Similarly, ER stress markers and hypoxia inducible factor-1α (HIF-1α) were increased in CIH mice, effects that were also correlated with increased infarct size. HIF-1α is known to be a factor involved in the life or death of cardiomyocites, similar to ER stress, and IH has been reported to not increase the size of infarction in mice genetically deficient in HIF-1α. Additionally, administration of tauroursodeoxycholic acid (TUDCA), a known inhibitor of ER stress, suppressed the enlargement of infarct size in these mice [68
]. Taken together, these observations suggest that ER stress is a significant detrimental factor in the essential pathology of patients with OSA, the cardiovascular morbidity associated with CIH. Therefore, if the ER stress response is properly regulated in OSA patients or the factors that suppress ER stress are appropriately managed, the prediction is that cardiovascular morbidity will be decreased.
TUDCA, which is known to suppress ER stress, is also reported to prevent liver damage in mice exposed to CIH. In particular, TUDCA inhibited the dissociation between GRP78 and PERK, resulting in reduced ER stress-mediated cell death [69
]. Specifically, administration of TUDCA attenuated pathological changes in the liver, reduced serum alanine aminotransferase and aspartate aminotransferase levels, suppressed ROS activity, decreased TNF-α and IL-1β levels, and inhibited hepatocyte apoptosis induced by CIH. TUDCA also inhibited CIH-induced ER stress in the liver, as evidenced by decreased expression of BiP, UPR transducers, and ER proapoptotic proteins. Taken together, the results of this latter study describe a liver-protective effect of TUDCA in CIH model mice, an effect that is mediated, at least in part, by inhibition of ER stress [69
Another similar animal study reported the mechanism of ER stress in CIH-induced brain damage in rats. In this study, overall cognitive function was decreased in CIH model rats, and BiP expression was increased in rats with hippocampal and prefrontal lesions. These data indicate that CIH induces ER stress in the hippocampus and prefrontal cortex. CIH also induced a time-dependent, progressive increase in p-PERK levels in the hippocampus and prefrontal cortex, and downregulated p-eIF2α, a downstream target of PERK, indicating that the PERK signaling pathway is sensitive to CIH-induced ER stress in the hippocampus and prefrontal cortex. The authors of this study further found that, following CIH, p-IRE1 levels were increased, but there were no increases in spliced XBP-1 or EDEM (ER-associated degradation enhancing α-mannosidase-like protein), a downstream target of XBP-1. In addition, they demonstrated that XBP-1 mRNA splicing, activated via the IRE-1 pathway, was not significantly increased after CIH. However, they did find that ATF4 and CHOP were significantly upregulated over time after CIH, and that levels of p-JNK/JNK, apoptotic signaling kinase 1 (ASK1), and TNF receptor-associated factor 2 (TRAF2) were significantly increased. Furthermore, Sal induced a significant increase in eIF2α phosphorylation, but significantly downregulated CHOP after CIH (CIH + Sal group) compared with the CIH-only group. Sal also significantly reduced the number of apoptotic cells after CIH injury, as revealed by TUNEL staining. Collectively, these observations indicate that Sal inhibits CIH-induced neuronal apoptosis in the hippocampus and prefrontal cortex [70
A final additional study analyzed the correlation between ER stress and the therapeutic effects of continuous positive airway pressure (CPAP) and weight loss, which are typical treatment methods for obese OSA patients. In this study, obese patients were categorized into three groups, as follows: (1) those who showed good adherence to weight loss and CPAP treatment; (2) those who showed poor adherence to treatments, and (3) those who were obese but did not have OSA. The good-adherence group showed significantly decreased expression of ATF4, CHOP, and endoplasmic reticulum oxidoreductin-1 (ERO-1) mRNAs in their adipose tissue compared with the other groups. On the basis of these results, the authors suggested that effective CPAP therapy improves obesity-associated ER stress markers in obese patients with OSA [71
Taken together, these observations suggest that CIH—the main pathological mechanism of OSA—induces ER stress that impacts the heart, brain, and liver to increase OSA-related morbidity. Given that the known ER stress inhibitors Sal and TUDCA effectively suppressed the activation process or incidence of ER stress, the use of such agents to manage the process appropriately holds promise as a new therapeutic approach to OSA.