How Obesity Triggers Cellular Stress Through Unfolded Protein Response and NRF2 Signaling

Obesity is often described as a disorder of excess fat accumulation, but a new review argues that this familiar definition misses the biological turmoil unfolding inside stressed cells. The condition is increasingly understood as a systemic disease in which overnutrition disrupts cellular quality-control systems, intensifies inflammation and generates damaging levels of reactive oxygen species (ROS). These changes can undermine the function of tissues involved in metabolism, including adipose tissue, liver, muscle and the pancreas. At the center of the review is nuclear factor-erythroid 2-related factor 2, better known as NRF2, a transcription factor that coordinates one of the body’s most important antioxidant and cytoprotective programs.

Published in the International Journal of Obesity, the review by Ş. Özgür, A. Medoro, S. Davinelli and colleagues examines how obesity-associated oxidative stress is connected to endoplasmic reticulum (ER) stress and the unfolded protein response (UPR). Rather than presenting these pathways as isolated reactions, the authors describe them as parts of an interconnected cellular network. The review focuses particularly on the two-way relationship between NRF2 and the UPR, suggesting that this molecular axis could become an important target for preventing or treating obesity-related metabolic dysfunction. The work is a synthesis of existing evidence rather than a report of a newly tested drug or clinical intervention.

The biological problem begins with the metabolic pressure created by chronic energy excess. When cells receive more nutrients than they can safely process or store, mitochondrial activity, lipid metabolism and other biochemical reactions can increase the production of ROS. These chemically reactive molecules include compounds such as superoxide and hydrogen peroxide. At controlled levels, ROS can act as signaling molecules, helping cells adapt to changes in their environment. When production overwhelms antioxidant defenses, however, oxidative stress can damage proteins, lipids and DNA, alter signaling pathways and interfere with the operation of mitochondria and other organelles.

The endoplasmic reticulum is particularly vulnerable to this metabolic overload. This organelle is responsible for producing, folding and processing many proteins, and it also participates in lipid synthesis and calcium storage. Excess nutrients, altered lipid composition and oxidative conditions can cause newly synthesized proteins to fold incorrectly or accumulate. Cells respond by activating the UPR, a coordinated defense system designed to restore protein-folding capacity and re-establish internal balance. The UPR reduces the production of new proteins, increases the machinery available for folding and promotes the removal of defective proteins. If the stress is too severe or persists for too long, the same pathways can promote inflammation, disrupt metabolism and trigger programmed cell death.

The review highlights three major branches of the UPR, represented by the sensors PERK, IRE1 and ATF6. These proteins detect disturbances in the ER and initiate distinct but overlapping responses. PERK can reduce general protein synthesis while activating the transcription factor ATF4, which helps cells adapt to stress but can also contribute to cell death if stimulation becomes prolonged. IRE1 processes messenger RNA encoding XBP1, generating a transcription factor that supports protein folding and ER-associated degradation. ATF6 travels to the Golgi apparatus, where it is cleaved into an active form that enters the nucleus and stimulates genes involved in ER recovery. Together, these pathways influence inflammation, lipid handling, insulin signaling and cellular survival in obesity.

NRF2 provides a complementary defense system. Under resting conditions, NRF2 is continuously targeted for degradation by KEAP1, a sensor protein that links NRF2 to the ubiquitin-proteasome system. Oxidative or electrophilic stress can modify reactive cysteine residues in KEAP1, weakening its ability to eliminate NRF2. The stabilized NRF2 then accumulates, enters the nucleus and binds antioxidant response elements in DNA. This activates genes encoding enzymes and proteins that neutralize ROS, recycle antioxidant molecules and protect cells from toxic damage. Among the NRF2-regulated systems are glutathione synthesis and recycling pathways, NADPH-generating reactions, detoxification enzymes and proteins involved in maintaining redox balance.

NRF2 and the UPR do not operate independently. Evidence discussed in the review indicates that ER stress can influence NRF2 activity through pathways linked to PERK and ATF4, while NRF2 can help limit the oxidative burden that fuels further ER dysfunction. This creates a feedback system with the potential to either restore balance or amplify disease. A moderate, temporary response may protect adipocytes and other metabolic cells by improving antioxidant capacity and protein quality control. In contrast, chronic activation can become maladaptive, particularly when prolonged inflammation, insulin resistance and nutrient excess continue to stimulate the same pathways.

This dual nature may help explain why NRF2 has produced complex results in obesity research. In several experimental models, increasing NRF2 activity has reduced oxidative damage, improved insulin sensitivity, limited inflammation and protected tissues from metabolic stress. NRF2-dependent defenses may also influence the behavior of adipose tissue, including the balance between energy-storing white fat and metabolically active beige or brown fat. Yet the review emphasizes that NRF2 is not universally beneficial in every context. Persistent or excessive NRF2 signaling can alter lipid metabolism, support survival of damaged cells or produce tissue-specific effects that differ according to disease stage, organ and genetic background. Any therapy aimed at this pathway would therefore need to control its intensity, duration and location.

The therapeutic possibilities described by the authors include small molecules that modify KEAP1, compounds that stimulate NRF2 transcriptional activity, dietary or pharmacological antioxidants and interventions that reduce ER stress. Strategies aimed at UPR components could also influence NRF2 indirectly, while NRF2 activation might help interrupt the cycle linking ROS accumulation to protein misfolding. However, translating these concepts into safe treatments remains challenging. Obesity is not driven by one molecular defect, and antioxidant pathways are deeply involved in normal immune responses, metabolism and cellular signaling. Broad, permanent activation could carry unintended consequences, making selective NRF2 modulators or tissue-targeted approaches more attractive than indiscriminate stimulation.

The review ultimately presents obesity as a disease of interconnected cellular stresses rather than a problem of energy storage alone. Excess nutrition can provoke ROS production, ER dysfunction, inflammation and impaired insulin signaling, while the NRF2-UPR network determines whether cells adapt or deteriorate. Mapping this network more precisely could help identify biomarkers that reveal when antioxidant defenses are protective and when they have become dysfunctional. The authors argue that future research should clarify how NRF2 behaves in different organs, how its activity changes across the stages of obesity and which combinations of metabolic, anti-inflammatory and ER-directed therapies can provide lasting benefit. For now, the NRF2-UPR axis offers a promising framework for understanding why obesity can progressively damage the cellular systems meant to protect the body.