Quinotrierixin inhibited ER stress-induced XBP1 mRNA splicing through inhibition of protein synthesis

Quinotrierixin was isolated from microbes as an inhibitor of ER stress-induced XBP1 mRNA splicing, but its mode of action was unclear. We found that quinotrierixin is an inhibitor of protein synthesis, and that the required dose range of quinotrierixin to inhibit ER stress-induced XBP1 mRNA splicing was similar to that to inhibit protein synthesis. Furthermore, we also found that quinotrierixin inhibited the ER stress-induced increases of unfolded protein response-related genes such as GRP78, CHOP, EDEM, ERdj4, and p58(IPK). Thus, we showed that quinotrierixin inhibited the ER stress-induced unfolded protein response, possibly due to its inhibitory activity of protein synthesis.     

A quantitative method for detection of spliced X-box binding protein-1 (XBP1) mRNA as a measure of endoplasmic reticulum (ER) stress

Endoplasmic reticulum (ER) stress is increasingly recognized as an important mechanism in a wide range of diseases including cystic fibrosis, alpha-1 antitrypsin deficiency, Parkinson's and Alzheimer's disease. Therefore, there is an increased need for reliable and quantitative markers for detection of ER stress in human tissues and cells. Accumulation of unfolded or misfolded proteins in the endoplasmic reticulum can cause ER stress, which leads to the activation of the unfolded protein response (UPR). UPR signaling involves splicing of X-box binding protein-1 (XBP1) mRNA, which is frequently used as a marker for ER stress. In most studies, the splicing of the XBP1 mRNA is visualized by gel electrophoresis which is laborious and difficult to quantify. In the present study, we have developed and validated a quantitative real-time RT-PCR method to detect the spliced form of XBP1 mRNA.     

Overexpressed Derlin-1 inhibits ER expansion in the endothelial cells derived from human hepatic cavernous hemangioma

Proteins that are unfolded or misfolded in the endoplasmic reticulum (ER) must be targeted for refolding or degradation to maintain the homeostasis of the ER. Derlin-1 was reportedly implicated in the retro-translocation of misfolded proteins from the ER to the cytosol for degradation. In this report, we showed that Derlin-1 was down-regulated in the endothelial cells derived from human hepatic cavernous hemangioma (CHEC) compared with other tested cells. Electron microscopy analysis showed that ER was aberrantly enlarged in CHEC cells, but not in other tested cells. When overexpressed, Derlin-1 induced the dilated ER to return normal size. This ER dynamic was associated with the activation of unfolded protein response (UPR). In CHEC cells where Derlin-1 was down-regulated, increased expression of the immunoglobulin heavy chain-binding protein (Bip) and UPR-specific splicing of X-box DNAbinding protein 1 (XBP1) mRNA were detected, as compared with that in other tested cells, indicating that UPR was activated. After Derlin-1 overexpression, the extent of UPR activation diminished, as evidenced by decreased expression of Bip, reduced amount of the spliced form of XBP1 (XBP1s), and elevated expression of the unspliced form of XBP1 (XBP1u). Taken together, these findings provide another example of a single protein being able to affect ER dynamic in mammalian cells, and an insight into the possible molecular mechanism(s).     

Immobilization stress induces XBP1 splicing in the mouse brain

Cells activate the unfolded protein response (UPR) to cope with endoplasmic reticulum (ER) stress. In the present study, we investigated the possible involvement of psychological stress on UPR induction in the mouse brain. When mice were exposed to immobilization stress for 8?h, XBP1 mRNA splicing was significantly induced in the hippocampus, cortex, hypothalamus, cerebellum, and brain stem. On the other hand, we did not observe any increase in XBP1 splicing in the liver, suggesting that this effect is specific to the brain. Stress-induced XBP1 splicing was attenuated 2 days after immobilization stress. We did not observe increases in any other UPR genes, such as CHOP or GRP78, in mouse brains after immobilization stress. These findings indicate an important specific role of XBP1 in response to psychological stress in the mouse brain.     

IRE1α disruption causes histological abnormality of exocrine tissues, increase of blood glucose level, and decrease of serum immunoglobulin level

Accumulation of unfolded proteins in the endoplasmic reticulum (ER) causes ER stress. As a cellular adaptive response to ER stress, unfolded protein response (UPR) activates molecules for the quality control of ER proteins. One enzyme that plays an important role in UPR is Inositol Requiring Enzyme-1 (IRE1), which is highly conserved from yeast to humans. In particular, mammalian IRE1α activates X-box-binding protein 1 (XBP1) by unconventional splicing of XBP1 mRNA during ER stress. From analysis of knockout mice, both IRE1α and XBP1 have been shown to be essential for development and that XBP1 is necessary for the secretory machinery of exocrine glands, plasma cell differentiation, and hepatic lipogenesis. However, the essentiality of IRE1α in specific organs and tissues remains incompletely understood. Here, we analyzed the phenotype of IRE1α conditional knockout mice and found that IRE1α-deficient mice exhibit mild hypoinsulinemia, hyperglycemia, and a low-weight trend. Moreover, IRE1α disruption causes histological abnormality of the pancreatic acinar and salivary serous tissues and decrease of serum level of immunoglobulin produced in the plasma cells, but not dysfunction of liver. Comparison of this report with previous reports regarding XBP1 conditional knockout mice might provide some clues for the discovery of the novel functions of IRE1α and XBP1.