**4.3. ER stress regulates cytokine production**

Cytokine production is an important inflammatory process in response to insults of pathogens, mutated self-antigens or tissue damage. ER stress is interconnected with the induction of inflammatory cytokines through multiple mechanisms including reactive oxygen species (ROS), NFκB and JNK (Figure 4). ROS are defined as highly reactive small molecules with unpaired electrons. They are important mediators of inflammatory response., Oxidative stress, caused by the accumulation of ROS, was confirmed to be as‐ sociated with ER stress (77). For example, the disulphide bond formation during the process of protein folding requires oxidizing condition (78). Therefore, increased protein folding load may lead to oxidative stress. The PERK axis of UPR is able to activate anti‐ oxidant pathway by promoting ATF4 and nuclear factor-erythroid-derived 2-related fac‐ tor 2 (NRF2) (79;80). Therefore, deficiency of PERK markedly increases ROS accumulation in response to toxic chemicals (79;81). The IRE1 axis of UPR can activate NFκB, a key regulator in inflammation, by recruiting IκB kinase (82). As a result, loss of IRE1 reduces the activation of NFκB activation and production of TNF-α (82). In addi‐ tion, the IRE1 axis can also activate JNK, and subsequently induce the expression of in‐ flammatory genes by activating activator protein 1 (AP1) (83). ATF6, the third axis of UPR signaling, can also activate NFκB pathway and induce inflammatory response. Therefore, suppression of ATF6 reduces NFκB activation caused by BiP degradation (84).

regulated during the differentiation of B cells (67). Mice with a deficiency of IRE1 in hematopoietic cells have a defective differentiation of pro-B cells towards pre-B cells (68). XBP-1, an IRE1 downstream molecule, is also involved in the differentiation of B cell and antibody production by mature B cells. It was found that the engagement of B-cell receptor induces ubiquitin-mediated degradation of BCL-6, a repressor for B-lymphocyte-induced maturation protein 1 (69), while B-lymphocyte-induced maturation protein 1 negatively reg‐ ulates the expression of B-cell-lineage-specific activator protein (70), a repressor for XBP-1 (71). In line with these results, B lymphocytes deficient in B-lymphocyte-induced maturation protein 1 failed to express XBP-1 in response to LPS stimulation (72). The expression of XBP-1 is rapidly up-regulated when B cells differentiate into plasma cells. Furthermore, XBP-1is able to initiate plasma cell differentiation when introduced into B-lineage cells. XBP-1-deficient lymphoid chimeras have a defective B-cell-dependent immune response due to the absence of immunoglobulin and plasma cells (30). In addition to IRE1/XBP-1 axis, ATF6 axis may also implicated in the differentiation of B cells, as increased ATF6 cleavage is found in differentiating B cells (67). However, PERK axis does not seem to be involved in

Activation of T lymphocyte, another important adaptive immune cell, seems also involves UPR. TCR engagement, the first T cell activation signal, induces the expression of ER chap‐ erons including BiP and GRP94. Inhibition of protein kinase C, a serine/threonine protein kinase downstream of TCR signaling, suppresses the activation of ER stress response in‐ duced by T cell activation (74). IRE1/XBP-1 axis regulates the differentiation of effector CD8+

promotes XBP-1 mRNA transcription, while TCR ligation induces the splicing of XBP-1

than IRE1/XBP-1, CHOP is also involved in the functionality of T cells. A recent report sug‐ gests GTPase of the immunity-associated protein 5 (Gimap5) mutation in BioBreeding dia‐ betes-prone rat, a model for type 1 diabetes, leads to ER stress and thus induces spontaneous apoptosis of T cells. Inhibition of CHOP protects Gimap5-/- T cells from ER

Cytokine production is an important inflammatory process in response to insults of pathogens, mutated self-antigens or tissue damage. ER stress is interconnected with the induction of inflammatory cytokines through multiple mechanisms including reactive oxygen species (ROS), NFκB and JNK (Figure 4). ROS are defined as highly reactive small molecules with unpaired electrons. They are important mediators of inflammatory response., Oxidative stress, caused by the accumulation of ROS, was confirmed to be as‐ sociated with ER stress (77). For example, the disulphide bond formation during the process of protein folding requires oxidizing condition (78). Therefore, increased protein folding load may lead to oxidative stress. The PERK axis of UPR is able to activate anti‐ oxidant pathway by promoting ATF4 and nuclear factor-erythroid-derived 2-related fac‐

T cell during acute infection. IL-2

T cell is reduced by suppression of XBP-1 (75). Other

the B-cell differentiation and maturation (68;73).

mRNA. The differentiation of CD8+

stress-induced apoptosis (76).

204 Type 1 Diabetes

T cell. IRE1/XBP-1 pathway is activated in effector CD8+

**4.3. ER stress regulates cytokine production**

**Figure 4. UPR-mediated inflammatory signaling.** UPR regulates inflammation through a variety of mechanisms in‐ volving ROS, JNK, and NFκB. PERK promotes ATF4 and NRF2, which then suppress ROS production by activating anti‐ oxidant pathway. Upon activation, IRE1/TRAF2 complex recruits IKK (IκB Kinase), leading to the phosphorylation of IκBα and subsequent activation of NFκB. IRE1/TRAF2 can also activate JNK, followed by the activation of AP1. XBP-1 induced by IRE1 can also induce the expression of various genes implicated inflammation. Furthermore, cleaved ATF6 can promote inflammation via activating NFκB.

ER stress regulates the expression of cytokines, while cytokines in turn may also induce ER stress via pathways including inducible nitric oxide synthase (iNOS) and JNK. JNK pathway is activated by IL-1β. Suppression of JNK by its inhibitor SP600125 can protectβ cells from IL-1β-induced apoptosis (85). Inflammatory cytokines induce iNOS expression in β cells and produce copious amount of nitric oxygen (86).Nitric oxygen is an important mediator of βcell death in type 1 diabetes. Excessive nitric oxygencan induce DNA damage, which leads to β cell apoptosis through p53 pathway or necrosis through poly (ADP-ribose) polymerase pathway (87). In addition, nitric oxygencan also deplete ER Ca2+ stores by activating Ca2+ channels or inhibiting Ca2+ pumps (88-90). Depletion of Ca2+ then leads to the activation of CHOP and induces ER stress and apoptosis of β cells (91;92).

**5. The role of ER stress in β cell destruction**

**5.1. The involvement of ER stress in β cell destruction**

immune responses during the process of type 1 diabetes.

Increasing evidence suggests an important role of ER stress in autoimmune-mediated β cell destruction (99;100). It was noted that β cell loss is the direct causing factor for insufficient insulin secretion in type 1 diabetes patients. Pancreatic β cells have a very well-developed ER to fulfill their biological function for secreting insulin and other glycoproteins, causing the high sensitivity of β cells to ER stress and the subsequent UPR. Severe or long-term ER stress would direct β cells undergoing apoptosis (99). As described earlier, all the three path‐ ways of ER stress are important in the execution of β cell function and involved in the auto‐

Endoplasmic Reticulum (ER) Stress in the Pathogenesis of Type 1 Diabetes

http://dx.doi.org/10.5772/52644

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Pro-inflammatory cytokines are believed as the major mediators contributing to ER stress in β cell mediated by autoimmune response. Autoreactive immune cells infiltrated in pancreas produce pro-inflammatory cytokines, the primary causing factor for β cell death in type 1 diabetes(101). Autoreactive macrophages and T-lymphocytes present in the pancreatic islets in the early stage of type 1 diabetes and secrete massive pro-inflammatory cytokines includ‐ ing IL-1β, IFN-γ and TNF-α. Pro-inflammatory cytokines have been confirmed as strong in‐ ducers of ER stress in pancreatic β cells. Insult of β cells with IL-1β and IFN-γ was reported to induce the expression of death protein 5, a protein involved in the cytokine-induced ER stress and β cell death (102). Suppression of death protein 5 by siRNA provides protection for β cells against pro-inflammatory cytokine-induced ER stress (102). In addition, stimula‐ tion of β cells with IL-1β and IFN-γ can decrease the expression of sarcoendoplasmic reticu‐ lum pump Ca2+ ATPase 2b, leading to subsequent depletion of Ca2+ in the ER (103). It has been well demonstrated that altered ER Ca2+ concentration induces the accumulation of un‐ folded proteins in ER associated with the induction of UPR and ER stress in β cells (104). Reactive oxygen species such as nitric oxygen produced during inflammation are believed to play a critical role in ER stress-induced β cell death. Excessive nitric oxygen production

during insulitis induces β cell apoptosis in a CHOP-dependent manner (91).

tein trafficking from ER to Golgi apparatus also causes ER stress in β cells (106).

In addition to cytokine-induced ER stress, defective protein processing and trafficking are also a direct cause of ER stress in β cell. For instance, mis-folding of insulin in β cells directly induces chronic ER stress as evidenced by the observations in Akita mice. The mutation of *Ins2* gene in Akita mouse disrupts a disulfide bond betweenα and β chain of proinsulin, leading to the mis-folding of the mutated insulin. This mutation therefore induces chronic ER stress in β cells and finally causes diabetes in Akita mouse (105). The inefficiency of pro‐

Hyperglycemia occurs only when β cells fail to compensate the increased demand for insu‐ lin. Therefore, β cells are usually "exhausted" in diabetic patients (87). The increased insulin demandrequires the remaining functional β cellsto increase insulin synthesis to compensate the decrease of β mass. The altered insulin synthesis causes ER stress in the β cells of pa‐ tients with type 1 diabetes. In later case, this compensation is beneficial for control of blood glucose homeostasisin a short term.However, the long term alterations of insulin synthesis

### **4.4. ER stress in the autoimmune process of type 1 diabetes**

Given the involvement of ER stress in both innate and adaptive immune systems, pathways of ER stress play a role in the autoimmune process of type 1 diabetes. For example, mice de‐ ficient in PERK, a molecule responsible for regulating UPR, are extremely susceptible to dia‐ betes. Although the exocrine and endocrine pancreas developed normally, the *null* mice display a progressive loss of β mass and insulin insufficiency postnatally (93) (93). A severe defect of β cell proliferation and differentiation was also found in *PERK null* mice, resulting in low pancreatic β mass and proinsulin trafficking defects (94). Consistent with those obser‐ vations in mice, some infant-onset diabetic cases in humans are confirmed to be associated with the mutations in PERK. For example, loss of *EIF2AK3* (the gene encodes PERK) devel‐ ops Wolcott-Rallison syndrome, an autosomal recessive disorder featured by early infancy insulin-dependency and multiple systemic manifestations including growth retardation, hepatic/renal dysfunction, mental retardation, and cardiovascular abnormalities (86;95). Similarly, disruption of UPR by mutating eIF2α, the downstream molecule of PERK signal‐ ing, enhances the sensitivity to ER stress-induced apoptosis and results in defective gluco‐ neogenesis. Mice carrying a homozygous Ser51Ala mutation for eIF2α show multiple defects in pancreatic β cells including the smaller core of insulin-secreting β cells and attenu‐ ated insulin secretion (41). Altogether, defects in PERK/eIF2α signaling render β cells highly vulnerable to ER stress in both humans and mice (87;96). In addition to PERK/eIF2α signal‐ ing, the other two pathways of ER stress, IRE1 and ATF6, are also implicated in the func‐ tionality of β cells. The activation of IRE1 signaling is involved in the insulin biosynthesis induced by hyperglycemia. Transient exposure to high glucose enhances IRE1α phosphory‐ lation without activation of XBP-1 and BiP dissociation. IRE1α activation induced by transi‐ ent exposure to high glucose induces insulin biosynthesis by up-regulating WFS1, a component involved in UPR and maintaining ER homeostasis (10;97). However, chronic ex‐ posure of β cells to high glucose may cause activation of IRE1 but with a different down‐ stream signaling, leading to the suppression of insulin biosynthesis (10). The activation of ATF6 induced by ER stress also suppressed the expression of insulin by up-regulating or‐ phan nuclear receptor small heterodimer partner (98).
