- Endoplasmic reticulum (ER) stress arises when the protein‑folding capacity of the ER becomes insufficient to manage the load of newly synthesised, secretory or membrane‑bound proteins. Because the ER is responsible for folding, modifying and trafficking nearly one‑third of the cellular proteome, its functional integrity is essential for cellular homeostasis. Perturbations such as oxidative stress, calcium imbalance, viral infection, metabolic overload, lipid dysregulation or increased secretory demand can disrupt ER function, leading to the accumulation of misfolded or unfolded proteins. This imbalance triggers a highly conserved signalling network designed to restore proteostasis and protect the cell.
- ER stress is detected by three transmembrane sensors—IRE1, PERK and ATF6—each of which initiates a distinct branch of the unfolded protein response (UPR). Under normal conditions, these sensors remain inactive through association with the ER chaperone BiP/GRP78. When misfolded proteins accumulate, BiP is recruited away from the sensors, allowing them to activate signalling cascades that adjust transcription, translation and degradation to rebalance ER function.
- IRE1 is the most ancient ER stress sensor. Upon activation, it oligomerises and initiates an unconventional splicing reaction that produces the transcription factor XBP1s. XBP1s upregulates genes involved in protein folding, ER‑associated degradation (ERAD), lipid biosynthesis and secretory pathway expansion. IRE1 also engages regulated IRE1‑dependent decay (RIDD), selectively degrading mRNAs to reduce ER load. These mechanisms collectively enhance ER capacity and promote survival under moderate stress.
- PERK activation leads to phosphorylation of eIF2α, transiently reducing global protein synthesis and decreasing the influx of nascent polypeptides into the ER. This translational pause is essential for preventing further accumulation of misfolded proteins. Despite global translation attenuation, specific mRNAs such as ATF4 are preferentially translated. ATF4 induces genes involved in amino‑acid metabolism, antioxidant defence and autophagy, linking ER stress to broader cellular stress pathways including the oxidative stress response and autophagy.
- ATF6 activation involves transport to the Golgi, where it is cleaved to release a transcription factor that upregulates chaperones, ERAD components and lipid‑biosynthesis genes. ATF6 enhances folding capacity and supports ER membrane expansion, complementing the functions of IRE1 and PERK. Together, the three UPR branches form a multilayered network that restores ER proteostasis.
- ER stress does not occur in isolation; it intersects with multiple cellular stress pathways. The heat‑shock response increases cytosolic chaperone availability when misfolded proteins accumulate. The proteostasis network coordinates folding, repair and degradation across cellular compartments. Mitochondria communicate with the ER through calcium signalling and ROS production, influencing both stress severity and recovery. Crosstalk between organelles ensures coordinated adaptation to stress.
- When ER stress is prolonged or severe, the UPR shifts from adaptive to apoptotic signalling. Persistent PERK activation induces CHOP, a transcription factor that promotes cell death by downregulating anti‑apoptotic proteins and sensitising mitochondria to stress. This ensures that irreparably damaged cells do not compromise tissue function or become malignant. In multicellular organisms, ER‑stress‑induced apoptosis is essential for maintaining tissue integrity.
- Dysregulation of ER stress contributes to numerous diseases. Chronic ER stress underlies neurodegenerative disorders such as Alzheimer’s, Parkinson’s and ALS, where misfolded proteins overwhelm folding capacity. In metabolic diseases such as diabetes and fatty‑liver disease, ER stress impairs insulin production and lipid homeostasis. In cancer, tumour cells often exploit the UPR to tolerate high secretory demand and proteotoxic stress, making ER stress pathways attractive therapeutic targets.
- In summary, ER stress reflects a disruption of protein‑folding homeostasis within the endoplasmic reticulum. Through coordinated activation of IRE1, PERK and ATF6, cells initiate the unfolded protein response to expand folding capacity, reduce protein load and restore homeostasis. When recovery fails, ER stress triggers apoptosis to protect organismal integrity. Its central role in proteostasis, metabolism and disease makes ER stress a major focus of modern cell biology.
