Nrf2 plays a critical role in the metabolic response during and after spaceflight

Thirty years of NRF2: advances and therapeutic challenges

Over the last 30 years, NRF2 has evolved from being recognized as a transcription factor primarily involved in redox balance and detoxification to a well-appreciated master regulator of cellular proteostasis, metabolism and iron homeostasis. NRF2 plays a pivotal role in diverse pathologies, including cancer, and metabolic, inflammatory and neurodegenerative disorders. It exhibits a Janus-faced duality, safeguarding cellular integrity in normal cells against environmental insults to prevent disease onset, whereas in certain cancers, constitutively elevated NRF2 levels provide a tumour survival advantage, promoting progression, therapy resistance and metastasis. Advances in understanding the mechanistic regulation of NRF2 and its roles in human pathology have propelled the investigation of NRF2-targeted therapeutic strategies. This Review dissects the mechanistic intricacies of NRF2 signalling, its cross-talk with biological processes and its far-reaching implications for health and disease, highlighting key discoveries that have shaped innovative therapeutic approaches targeting NRF2.

Crosstalk between degradation and bioenergetics: how autophagy and endolysosomal processes regulate energy production

Cells undergo metabolic reprogramming to adapt to changes in nutrient availability, cellular activity, and transitions in cell states. The balance between glycolysis and mitochondrial respiration is crucial for energy production, and metabolic reprogramming stipulates a shift in such balance to optimize both bioenergetic efficiency and anabolic requirements. Failure in switching bioenergetic dependence can lead to maladaptation and pathogenesis. While cellular degradation is known to recycle precursor molecules for anabolism, its potential role in regulating energy production remains less explored. The bioenergetic switch between glycolysis and mitochondrial respiration involves transcription factors and organelle homeostasis, which are both regulated by the cellular degradation pathways. A growing body of studies has demonstrated that both stem cells and differentiated cells exhibit bioenergetic switch upon perturbations of autophagic activity or endolysosomal processes. Here, we highlighted the current understanding of the interplay between degradation processes, specifically autophagy and endolysosomes, transcription factors, endolysosomal signaling, and mitochondrial homeostasis in shaping cellular bioenergetics. This review aims to summarize the relationship between degradation processes and bioenergetics, providing a foundation for future research to unveil deeper mechanistic insights into bioenergetic regulation.

Model organisms for investigating the functional involvement of NRF2 in non-communicable diseases

Non-communicable chronic diseases (NCDs) are most commonly characterized by age-related loss of homeostasis and/or by cumulative exposures to environmental factors, which lead to low-grade sustained generation of reactive oxygen species (ROS), chronic inflammation and metabolic imbalance. Nuclear factor erythroid 2-like 2 (NRF2) is a basic leucine-zipper transcription factor that regulates the cellular redox homeostasis. NRF2 controls the expression of more than 250 human genes that share in their regulatory regions a cis-acting enhancer termed the antioxidant response element (ARE). The products of these genes participate in numerous functions including biotransformation and redox homeostasis, lipid and iron metabolism, inflammation, proteostasis, as well as mitochondrial dynamics and energetics. Thus, it is possible that a single pharmacological NRF2 modulator might mitigate the effect of the main hallmarks of NCDs, including oxidative, proteostatic, inflammatory and/or metabolic stress. Research on model organisms has provided tremendous knowledge of the molecular mechanisms by which NRF2 affects NCDs pathogenesis. This review is a comprehensive summary of the most commonly used model organisms of NCDs in which NRF2 has been genetically or pharmacologically modulated, paving the way for drug development to combat NCDs. We discuss the validity and use of these models and identify future challenges.

Simulated microgravity predisposes kidney to injury through promoting intrarenal artery remodeling

Spaceflight-induced multi-organ dysfunction affects the health of astronauts and the safety of in-orbit flight. However, the effect of microgravity on the kidney and the underlying mechanisms are unknown. In the current study, we used a hindlimb unweighting (HU) animal model to simulate microgravity and employed histological analysis, ischemia-reperfusion experiments, renal ultrasonography, bioinformatics analysis, isometric force measurement, and other molecular experimental settings to evaluate the effects of microgravity on the kidneys and the underlying mechanisms involved in this transition. Relative to controls, 31-day hindlimb unweighting had no obvious effect on serum creatinine and urea nitrogen levels as well as on renal injury scores; however, animals in the HU group showed an impaired renal recovery to ischemia-reperfusion injury. Ultrasonography showed that renal resistive index was increased, which indicated an altered renal hemodynamics induced by simulated microgravity. The enhanced vasoconstriction mediated by the Rho-kinase pathway and impaired vasodilation mediated by NO-eNOS of the intrarenal artery contributed to the altered renal hemodynamics. Simulated microgravity predisposes the kidney to ischemia-reperfusion injury. The altered renal hemodynamics induced by renal arterial remodeling may account for the increased renal injury susceptibility, providing a target for early intervention in kidney dysfunction under microgravity.

Targeting Nrf2 signaling in dry eye

Dry eye, the most common ocular surface disease, can cause ocular surface tissue damage and discomfort symptoms and seriously affect people's quality of life. The etiology of dry eye is diverse, and its pathogenesis is complex. The oxidative stress reaction is considered to be among the important factors in the pathogenesis of dry eye. Therefore, activating the antioxidant system has a potential therapeutic effect on dry eye. Nuclear factor erythroid 2-related factor 2 (Nrf2) signaling pathway is considered the most important antioxidant pathway in the body. The activation of the Nrf2 signaling pathway and its interaction with other pathways are important mechanisms to prevent the occurrence and development of dry eye. This review describes the structure and function of Nrf2, summarizes the changes in the oxidative stress response in dry eye, focuses on the potential mechanism of the Nrf2 signaling pathway in the treatment of dry eye, and, finally, summarizes the drugs that activate the Nrf2 signaling pathway in the treatment of dry eye.

Upregulation of Amy1 in the salivary glands of mice exposed to a lunar gravity environment using the multiple artificial gravity research system

Introduction: Space is a unique environment characterized by isolation from community life and exposure to circadian misalignment, microgravity, and space radiation. These multiple differences from those experienced on the earth may cause systemic and local tissue stress. Autonomic nerves, including sympathetic and parasympathetic nerves, regulate functions in multiple organs. Saliva is secreted from the salivary gland, which is regulated by autonomic nerves, and plays several important roles in the oral cavity and digestive processes. The balance of the autonomic nervous system in the seromucous glands, such as the submandibular glands, precisely controls serous and mucous saliva. Psychological stress, radiation damage, and other triggers can cause an imbalance in salivary secretion systems. A previous study reported that amylase is a stress marker in behavioral medicine and space flight crews; however, the detailed mechanisms underlying amylase regulation in the space environment are still unknown. Methods: In this study, we aimed to elucidate how lunar gravity (1/6 g) changes mRNA expression patterns in the salivary gland. Using a multiple artificial gravity research system during space flight in the International Space Station, we studied the effects of two different gravitational levels, lunar and Earth gravity, on the submandibular glands of mice. All mice survived, returned to Earth from space, and their submandibular glands were collected 2 days after landing. Results: We found that lunar gravity induced the expression of the salivary amylase gene Amy1; however, no increase in Aqp5 and Ano1, which regulate water secretion, was observed. In addition, genes involved in the exocrine system, such as vesicle-associated membrane protein 8 (Vamp8) and small G proteins, including Rap1 and Rab families, were upregulated under lunar gravity. Conclusion: These results imply that lunar gravity upregulates salivary amylase secretion via Rap/Rab signaling and exocytosis via Vamp8. Our study highlights Amy1 as a potential candidate marker for stress regulation in salivary glands in the lunar gravity environment.

Melatonin ameliorates simulated-microgravity-induced mitochondrial dysfunction and lipid metabolism dysregulation in hepatocytes

The liver is an essential multifunctional organ, which constantly communicates with nearly all tissues. It has raised the concern that microgravity exposure can lead to liver dysfunction and metabolic syndromes. However, molecular mechanisms and intervention measures of the adverse effects of microgravity on hepatocytes are limited. In this study, we utilized the random positioning machine culture system to investigate the adverse effects on hepatocytes under simulated microgravity (SMG). Our results showed that SMG impaired hepatocyte viability, causing cell cycle arrest and apoptosis. Compared to normal gravity, it also triggered lipid accumulation, elevated triglyceride (TG) and ROS levels, and impaired mitochondria function in hepatocytes. Furthermore, RNA sequencing results showed that SMG upregulated genes implicated in lipid metabolisms, including PPARγ, PLIN2, CD36, FABPs, etc. Importantly, all these defects can be suppressed by melatonin, a potent antioxidant secreted by the pineal gland, suggesting its potential use of therapeutic intervention.

Spaceflight-Induced Gene Expression Profiles in the Mouse Brain Are Attenuated by Treatment with the Antioxidant BuOE

The demands of deep space pose a health risk to the central nervous system that has long been a concern when sending humans to space. While little is known about how spaceflight affects transcription spatially in the brain, a greater understanding of this process has the potential to aid strategies that mitigate the effects of spaceflight on the brain. Therefore, we performed GeoMx Digital Spatial Profiling of mouse brains subjected to either spaceflight or grounded controls. Four brain regions were selected: Cortex, Frontal Cortex, Corunu Ammonis I, and Dentate Gyrus. Antioxidants have emerged as a potential means of attenuating the effects of spaceflight, so we treated a subset of the mice with a superoxide dismutase mimic, MnTnBuOE-2-PyP 5+ (BuOE). Our analysis revealed hundreds of differentially expressed genes due to spaceflight in each of the four brain regions. Both common and region-specific transcriptomic responses were observed. Metabolic pathways and pathways sensitive to oxidative stress were enriched in the four brain regions due to spaceflight. These findings enhance our understanding of brain regional variation in susceptibility to spaceflight conditions. BuOE reduced the transcriptomic effects of spaceflight at a large number of genes, suggesting that this compound may attenuate oxidative stress-induced brain damage caused by the spaceflight environment.

Nrf2 alleviates spaceflight-induced immunosuppression and thrombotic microangiopathy in mice

Spaceflight-related stresses impact health via various body systems, including the haematopoietic and immune systems, with effects ranging from moderate alterations of homoeostasis to serious illness. Oxidative stress appears to be involved in these changes, and the transcription factor Nrf2, which regulates expression of a set of cytoprotective and antioxidative stress response genes, has been implicated in the response to spaceflight-induced stresses. Here, we show through analyses of mice from the MHU-3 project, in which Nrf2-knockout mice travelled in space for 31 days, that mice lacking Nrf2 suffer more seriously from spaceflight-induced immunosuppression than wild-type mice. We discovered that a one-month spaceflight-triggered the expression of tissue inflammatory marker genes in wild-type mice, an effect that was even more pronounced in the absence of Nrf2. Concomitant with induction of inflammatory conditions, the consumption of coagulation-fibrinolytic factors and platelets was elevated by spaceflight and further accelerated by Nrf2 deficiency. These results highlight that Nrf2 mitigates spaceflight-induced inflammation, subsequent immunosuppression, and thrombotic microangiopathy. These observations reveal a new strategy to relieve health problems encountered during spaceflight.

PM2.5 Increases Systemic Inflammatory Cells and Associated Disease Risks by Inducing NRF2-Dependent Myeloid-Biased Hematopoiesis in Adult Male Mice

Although PM_2.5 (fine particles with aerodynamic diameter <2.5 μm) exposure shows the potential to impact normal hematopoiesis, the detailed alterations in systemic hematopoiesis and the underlying mechanisms remain unclear. For hematopoiesis under steady-state or stress conditions, nuclear factor erythroid 2-related factor 2 (NRF2) is essential for regulating hematopoietic processes to maintain blood homeostasis. Herein, we characterized changes in the populations of hematopoietic stem progenitor cells and committed hematopoietic progenitors in the lungs and bone marrow (BM) of wild-type and Nrf2^-/- C57BL/6J male mice. PM_2.5-induced NRF2-dependent biased hematopoiesis toward myeloid lineage in the lungs and BM generates excessive numbers of various inflammatory immune cells, including neutrophils, monocytes, and platelets. The increased population of these immune cells in the lungs, BM, and peripheral blood has been associated with observed pulmonary fibrosis and high disease risks in an NRF2-dependent manner. Therefore, although NRF2 is a protective factor against stressors, upon PM_2.5 exposure, NRF2 is involved in stress myelopoiesis and enhanced PM_2.5 toxicity in pulmonary injury, even leading to systemic inflammation.

Molecular Basis of the KEAP1-NRF2 Signaling Pathway

Transcription factor NRF2 (NF-E2-related factor 2) is a master regulator of cellular responses against environmental stresses. NRF2 induces expression of detoxification and antioxidant enzymes and suppresses inductions of pro-inflammatory cytokine genes. KEAP1 (Kelch-like ECH-associated protein 1) is an adaptor subunit of CULLIN 3 (CUL3)-based E3 ubiquitin ligase. KEAP1 regulates the activity of NRF2 and acts as a sensor for oxidative and electrophilic stresses. NRF2 has been found to be activated in many types of cancers with poor prognosis. Therapeutic strategies to control NRF2-overeactivated cancers have been considered not only by targeting cancer cells with NRF2 inhibitors or NRF2 synthetic lethal chemicals, but also by targeting host defense with NRF2 inducers. Understanding precise molecular mechanisms how the KEAP1-NRF2 system senses and regulates the cellular response is critical to overcome intractable NRF2-activated cancers.

The KEAP1-NRF2 System and Neurodegenerative Diseases

Significance: Central nervous system (CNS) diseases are disorders of the brain and/or spinal cord and include neurodegenerative diseases such as Alzheimer's disease, Parkinson's disease, and multiple sclerosis. Nuclear factor erythroid 2-related factor 2 (NRF2) is a transcription factor belonging to the cap-n-collar family that harbors a unique basic leucine zipper motif and plays as a master regulator of homeostatic responses. Recent Advances: Kelch-like ECH-associated protein 1 (KEAP1) is an adaptor of the Cullin3 (CUL3)-based ubiquitin E3 ligase that enhances the ubiquitylation of NRF2, which promotes the degradation of NRF2 to suppress its transcriptional activity in the absence of stress. Cysteine residues of KEAP1 are modified under stress conditions, and NRF2 degradation is attenuated, allowing it to accumulate and induce the expression of target genes. This regulatory system is referred to as the KEAP1-NRF2 system and plays a central role in protecting cells against various stresses. NRF2 also negatively regulates the expression of inflammatory cytokine and chemokine genes and suppresses pathological inflammation. As oxidative stress, inflammation, and proteostasis are known to contribute to neurodegenerative diseases, the KEAP1-NRF2 system is an attractive target for the treatment of these diseases. Critical Issues: In mouse models of neurodegenerative diseases, Nrf2 depletion exacerbates symptoms and enhances oxidative damage and inflammation in the CNS. In contrast, chemical or genetic NRF2 activation improves these symptoms. Indeed, the NRF2-activating chemical dimethyl fumarate is now widely used for the clinical treatment of MS. Future Directions: The KEAP1-NRF2 system is a promising therapeutic target for neurodegenerative diseases.

The KEAP1-NRF2 system and neurodegenerative diseases

Significance: Central nervous system (CNS) diseases are disorders of the brain and/or spinal cord and include neurodegenerative diseases such as Alzheimer's disease (AD), Parkinson's disease (PD), and multiple sclerosis (MS). NF-E2-related factor 2 (NRF2) is a transcription factor belonging to the cap-n-collar (CNC) family that harbors a unique basic leucine zipper motif and plays as a master regulator of homeostatic responses. Recent Advances: Kelch-like ECH-associated protein 1 (KEAP1) is an adaptor of the Cullin3 (CUL3)-based ubiquitin E3 ligase that enhances the ubiquitylation of NRF2, which promotes the degradation of NRF2 to suppress its transcriptional activity in the absence of stress. Cysteine residues of KEAP1 are modified under stress conditions, and NRF2 degradation is attenuated, allowing it to accumulate and induce the expression of target genes. This regulatory system is referred to as the KEAP1-NRF2 system and plays a central role in protecting cells against various stresses. NRF2 also negatively regulates the expression of inflammatory cytokine and chemokine genes and suppresses pathological inflammation. As oxidative stress, inflammation, and proteostasis are known to contribute to neurodegenerative diseases, the KEAP1-NRF2 system is an attractive target for the treatment of these diseases. Critical Issues: In mouse models of neurodegenerative diseases, Nrf2 depletion exacerbates symptoms and enhances oxidative damage and inflammation in the CNS. In contrast, chemical or genetic NRF2 activation improves these symptoms. Indeed, the NRF2-activating chemical dimethyl fumarate (DMF) is now widely used for the clinical treatment of MS. Future Directions: The KEAP1-NRF2 system is a promising therapeutic target for neurodegenerative diseases.

Longitudinal metabolomic profiles reveal sex-specific adjustments to long-duration spaceflight and return to Earth

Spaceflight entails a variety of environmental and psychological stressors that may have long-term physiological and genomic consequences. Metabolomics, an approach that investigates the terminal metabolic outputs of complex physiological alterations, considers the dynamic state of the human body and allows the identification and quantification of down-stream metabolites linked to up-stream physiological and genomic regulation by stress. Employing a metabolomics-based approach, this study investigated longitudinal metabolic perturbations of male (n = 40) and female (n = 11) astronauts on 4-6-month missions to the International Space Station (ISS). Proton nuclear magnetic resonance (1H-NMR) spectroscopy followed by univariate, multivariate and machine learning analyses were used on blood serum to examine sex-specific metabolic changes at various time points throughout the astronauts' missions, and the metabolic effects of long-duration space travel. Space travel resulted in sex-specific changes in energy metabolism, bone mineral and muscle regulation, immunity, as well as macromolecule maintenance and synthesis. Additionally, metabolic signatures suggest differential metabolic responses-especially during the recovery period-with females requiring more time to adjust to return to Earth. These findings provide insight into the perturbations in glucose and amino acid metabolism and macromolecule biosynthesis that result from the stressors of long-duration spaceflight. Metabolomic biomarkers may provide a viable approach to predicting and diagnosing health risks associated with prolonged space travel and other physiological challenges on Earth.

Nrf2 deficiency deteriorates diabetic kidney disease in Akita model mice

Oxidative stress is an essential component in the progression of diabetic kidney disease (DKD), and the transcription factor NF-E2-related factor-2 (Nrf2) plays critical roles in protecting the body against oxidative stress. To clarify the roles of Nrf2 in protecting against DKD, in this study we prepared compound mutant mice with diabetes and loss of antioxidative defense. Specifically, we prepared compound Ins2^Akita/+ (Akita) and Nrf2 knockout (Akita::Nrf2^-/-) or Akita and Nrf2 induction (Akita::Keap1^FA/FA) mutant mice. Eighteen-week-old Akita::Nrf2^-/- mice showed more severe diabetic symptoms than Akita mice. In the Akita::Nrf2^-/- mouse kidneys, the glomeruli showed distended capillary loops, suggesting enhanced mesangiolysis. Distal tubules showed dilation and an increase in 8-hydroxydeoxyguanosine-positive staining. In the Akita::Nrf2^-/- mouse kidneys, the expression of glutathione (GSH) synthesis-related genes was decreased, and the actual GSH level was decreased in matrix-assisted laser desorption/ionization mass spectrometry imaging analysis. Akita::Nrf2^-/- mice exhibited severe inflammation and enhancement of infiltrated macrophages in the kidney. To further examine the progression of DKD, we compared forty-week-old Akita mouse kidney compounds with Nrf2-knockout or Nrf2 mildly induced (Akita::Keap1^FA/FA) mice. Nrf2-knockout Akita (Akita::Nrf2^-/-) mice displayed severe medullary cast formation, but the formation was ameliorated in Akita::Keap1^FA/FA mice. Moreover, in Akita::Keap1^FA/FA mice, tubule injury and inflammation-related gene expression were significantly suppressed, which was evident in Akita::Nrf2^-/- mouse kidneys. These results demonstrate that Nrf2 contributes to the protection of the kidneys against DKD by suppressing oxidative stress and inflammation.