ER stress pathway

The ER is a large membrane-enclosed organelle present in all eukaryotic cells. It is responsible for folding membranes and secreted proteins, synthesizing lipids and sterols, and storing free calcium. The ER also serves as a quality-control organelle, assisting in maintaining protein homeostasis. ER stress follows physiological stresses such as an increased secretory load and pathological pressures like the presence of mutated proteins that cannot fold correctly in the ER. During ER stress, the balance between the demand for protein folding and the ER’s capacity to perform this function is disturbed¹. Eukaryotic cells have developed various signal transduction pathways to detect and respond to ER stress, collectively known as the unfolded protein response (UPR). The primary regulators of the UPR located in the ER consist of a group of transmembrane proteins, which include inositol-requiring protein 1 (IRE1), PKR-like endoplasmic reticulum kinase (PERK), and activating transcription factor 6 (ATF6). These proteins have domains that extend into the ER lumen, which detect ER stress, paired with cytosolic effector domains, enabling them to detect and respond to endoplasmic reticulum stress effectively².

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IRE1 is a single-spanning transmembrane protein with protein kinase and ribonuclease activities. It regulates the most phylogenetically conserved UPR signaling pathway. IRE1α is present in all mammalian cells, while IRE1β is only expressed in intestinal epithelial cells. When IRE1 is activated, it dimerizes and/or oligomerizes, driving the trans-phosphorylation of positive regulatory sites within its protein kinase domain. This process requires adenosine nucleotides as cofactors to activate its nuclease activity. Once the nuclease function of IRE1 is activated, an intron from the mRNA that encodes a UPR-specific transcription factor known as XBP1 (X-box binding protein 1) is removed. This process converts the full-length XBP1 (XBP1u) into its truncated or spliced form (XBP1s), which is then transported to the nucleus to facilitate gene translation.

When unfolded proteins bind to PERK, they induce conformational changes that lead to self-multimerization and self-phosphorylation. This process inactivates eukaryotic translation initiation factor 2α (eIF2α) via phosphorylation. As a result, translation is inhibited, protein synthesis is reduced, and the overall protein load is decreased.

ATF6 is a protein with a single-pass type 2 transmembrane protein with a large luminal domain in the ER that is constitutively expressed in most cells. It features a cytosolic NH2-terminal domain that functions as a transcription factor belonging to the basic-leucine-zipper (bZip) family. Under conditions of ER stress, ATF6 is cleaved by Site-1 and Site-2 proteases, generating a cytosolic fragment known as ATF6f, which directly regulates the transcription of XBP.

Various cellular mechanisms can lead to endoplasmic reticulum stress, and these can be associated with diseases such as cancer, neurodegenerative disorders such as Alzheimer’s and Parkinson’s disease, and diabetes. One of the key hallmarks of cancer, genetic instability, and the accumulation of mutations can result in the continuous activation of the ER stress response pathway, promoting cell growth, proliferation, differentiation, and migration. Furthermore, the uncontrolled and rapid development of cancer cells requires high protein production rates, significantly impacting the ER systems³.

A range of small molecules that efficiently activate ER stress through various mechanisms has been identified. For instance, stressors like tunicamycin and 2-deoxyglucose affect the N-linked glycosylation of proteins, while dithiothreitol inhibits the formation of protein disulfide bonds. Conversely, Brefeldin A disrupts the transport of proteins from the ER to the Golgi apparatus, leading to a rapid and reversible inhibition of protein secretion. Compounds like thapsigargin and cyclopiazonic acid target the Sarco/ER Ca²⁺ ATPase (SERCA), reducing ER Ca²⁺ levels and impairing the cell's ability to fold proteins, leading to ER stress. In contrast, other molecules that help resolve ER stress have been identified. These include several small molecules, peptides, and proteostasis regulators. For example, 4‐phenylbutyric acid (4‐PBA) reduces the accumulation of misfolded proteins in the ER, while tauroursodeoxycholic acid (TUDCA) is an endogenous bile acid able to resolve ER stress in pancreatic islet cells. TUDCA is the taurine conjugate of ursodeoxycholic acid (UDCA), an FDA‐approved drug for primary biliary cirrhosis that can alleviate ER stress. The exact mechanism by which proteostasis modulators function remains unclear.

Our ER stress pathway poster overviews the complex signaling networks involved in physiological and pathological endoplasmic reticulum stress.

References

1. Lin, J. H., Walter, P. & Yen, T. S. Endoplasmic reticulum stress in disease pathogenesis.  Annu. Rev. Pathol.  3, 399–425 (2008).

2. Chen, X., Shi, C., He, M. et al. Endoplasmic reticulum stress: molecular mechanism and therapeutic targets.  Sig. Transduct. Target Ther.  8, 352 (2023).

3. Almanza, A. et al. Endoplasmic reticulum stress signaling - from basic mechanisms to clinical applications.  FEBS J.  286, 241–278 (2019).