The endoplasmic reticulum (ER) is the site where proteins destined for the endo/exocytotic pathway are folded and processed. When ER homeostasis is disrupted, eukaryotic cells activate the unfolded protein response (UPR), a signaling pathway that helps restore protein folding capacity. The UPR involves three main pathways: IRE1, ATF6, and PERK. Each pathway responds to ER stress by activating different signaling cascades that regulate protein folding, secretion, and degradation. The UPR also plays a critical physiological role in various secretory cells, such as plasma cells, pancreatic β-cells, hepatocytes, and osteoblasts, where high levels of secretory protein synthesis require precise folding and processing. The UPR is essential for maintaining cellular homeostasis and preventing apoptosis under ER stress. Recent studies have shown that the UPR is involved in a wide range of physiological processes, including development, metabolism, and disease. The UPR can be activated by various stressors, including nutrient deprivation, oxidative stress, and misfolded proteins. The UPR has also been implicated in the pathogenesis of diseases such as diabetes, osteoporosis, and liver dysfunction. The UPR is regulated by a complex network of signaling pathways, including the PERK/eIF2α pathway, which is crucial for maintaining protein folding capacity and cell survival. The UPR is also involved in the regulation of gene expression, protein synthesis, and apoptosis. The UPR is a dynamic process that is tightly regulated to ensure proper cellular function and survival under ER stress. The study highlights the importance of the UPR in maintaining cellular homeostasis and its potential therapeutic applications in diseases associated with ER stress.The endoplasmic reticulum (ER) is the site where proteins destined for the endo/exocytotic pathway are folded and processed. When ER homeostasis is disrupted, eukaryotic cells activate the unfolded protein response (UPR), a signaling pathway that helps restore protein folding capacity. The UPR involves three main pathways: IRE1, ATF6, and PERK. Each pathway responds to ER stress by activating different signaling cascades that regulate protein folding, secretion, and degradation. The UPR also plays a critical physiological role in various secretory cells, such as plasma cells, pancreatic β-cells, hepatocytes, and osteoblasts, where high levels of secretory protein synthesis require precise folding and processing. The UPR is essential for maintaining cellular homeostasis and preventing apoptosis under ER stress. Recent studies have shown that the UPR is involved in a wide range of physiological processes, including development, metabolism, and disease. The UPR can be activated by various stressors, including nutrient deprivation, oxidative stress, and misfolded proteins. The UPR has also been implicated in the pathogenesis of diseases such as diabetes, osteoporosis, and liver dysfunction. The UPR is regulated by a complex network of signaling pathways, including the PERK/eIF2α pathway, which is crucial for maintaining protein folding capacity and cell survival. The UPR is also involved in the regulation of gene expression, protein synthesis, and apoptosis. The UPR is a dynamic process that is tightly regulated to ensure proper cellular function and survival under ER stress. The study highlights the importance of the UPR in maintaining cellular homeostasis and its potential therapeutic applications in diseases associated with ER stress.