**3. ER stress and ER stress response**

## **3.1 Overview of ER stress response cascade**

The ER is an organelle involved in synthesis of secretory proteins and membrane proteins. In the ER, unfolded proteins, immediately after translation and entrance into the organelle, are matured through modification, such as folding, formation of disulfide bonds, and initiation of glycosylation. Under ER stress, in which unfolded or misfolded proteins accumulate in the ER due to increased protein synthesis or chaperone dysfunction, various kinds of ER stress response are induced, and some of them are cytoprotective and others are cytotoxic [12].

Among the numerous molecules involved in ER stress response, also called UPR (unfolded protein response), BiP (binding immunoglobulin protein), also known as GRP78 (glucose-regulated protein 78), is a chaperone with an ATPase domain, which plays pivotal roles in ER stress response mainly via interaction with ERdjs (ER-localized DnaJ family members) [13]. BiP binds to unfolded or misfolded proteins in the ER and promotes folding by consuming ATP. Moreover, BiP binds to ER stress sensor molecules, IRE1-alpha (inositol-requiring enzyme 1 alpha), ATF6 (activating transcription factor 6), and PERK (PKR-like endoplasmic reticulum kinase), and prevent them from activation. Under ER stress, however, BiP is mainly engaged in increased unfolded or misfolded proteins and dissociates from the ER stress sensors, resulting in phosphorylation of IRE1-alpha and PERK, as well as cleavage of ATF6 followed by nuclear translocation to the nucleus [12, 14].

It is well known that phosphorylated IRE1-alpha splices Xbp1 (X-box binding protein 1) mRNA [15]. XBP-1 s protein induces chaperones, including BiP and XBP-1 s itself, as a transcription factor by binding to motifs called ERSEs (ER stress response elements). XBP-1 s also promotes ERAD (ER-associated degradation) by binding to motifs called UPRs (UPR response elements), and attenuates translation via mRNA degradation. nATF6 (nuclear ATF6) also works as a transcription factor to induce chaperones by binding to ERSEs. Phosphorylated PERK phosphorylates eIF2 alpha (eukaryotic initiation factor-2 alpha), which suppresses translation and lowers protein loading. Overall, these responses are protective against ER stress, by suppressing protein synthesis, inducing chaperones, and promoting protein degradation.

Sustained ER stress is, however, known to induce rather cytotoxic responses. Phosphorylated IRE1-alpha activates JNK, resulting in inflammation, oxidative stress and apoptosis. Phosphorylated eIF-2 alpha also induces CHOP (CCAATenhancer-binding protein homologous protein), a transcription factor involved in apoptosis, as well as oxidative stress and inflammation, via another transcription factor, ATF4 [12, 14].

It is difficult to detect unfolded/misfolded proteins in the ER of mammals directly, but activation of the upstream ER stress sensors is considered to be a good marker to reflect ER stress. Activation or expression of the downstream molecules involved in ER stress response are also frequently used as ER stress markers, but we are going to discuss the discrepancy between the upstream sensors and the downstream effectors, which we call ER stress response failure, in subsections below (**Figure 2**).

**Figure 2.** *Schematic description of the cascade of ER stress response.*
