Nrf2 regulates the alternative first exons of CD36 in macrophages through specific antioxidant response elements

Molecular Targets of Oxidative Stress: Focus on Nuclear Factor Erythroid 2-Related Factor 2 Function in Leukemia and Other Cancers

Nuclear factor erythroid 2-related factor 2 (Nrf2) is a transcription factor that plays a central role in regulating cellular responses to oxidative stress. It governs the expression of a broad range of genes involved in antioxidant defense, detoxification, metabolism, and other cytoprotective pathways. In normal cells, the transient activation of Nrf2 serves as a protective mechanism to maintain redox homeostasis. However, the persistent or aberrant activation of Nrf2 in cancer cells has been implicated in tumor progression, metabolic reprogramming, and resistance to chemotherapy and radiotherapy. These dual roles underscore the complexity of Nrf2 signaling and its potential as a therapeutic target. A deeper understanding of Nrf2 regulation in both normal and malignant contexts is essential for the development of effective Nrf2-targeted therapies. This review provides a comprehensive overview of Nrf2 regulation and function, highlighting its unique features in cancer biology, particularly its role in metabolic adaptation and drug resistance. Special attention is given to the current knowledge of Nrf2's involvement in leukemia and emerging strategies for its therapeutic modulation.

Adverse Effects of Nrf2 in Different Organs and the Related Diseases

Significance: Under normal physiological conditions, Nrf2 undergoes ubiquitination and subsequent proteasome degradation to maintain its basal activity. Oxidative stress can trigger Nrf2 activation, prompting its translocation to the nucleus where it functions as a transcription factor, activating various antioxidant pathways, and conferring antioxidant properties. Recent Advances: While extensive research has shown Nrf2's protective role in various diseases, emerging evidence suggests that Nrf2 activation can also produce harmful effects. Critical Issues: This review examines the pathological contexts in which Nrf2 assumes different roles, emphasizing the mechanisms and conditions that result in adverse outcomes. Future Directions: Persistent Nrf2 activation may have deleterious consequences, necessitating further investigation into the specific conditions and mechanisms through which Nrf2 exerts its harmful effects. Antioxid. Redox Signal. 00, 000-000.

Anticancer Effect of C19-Position Substituted Geldanamycin Derivatives Targeting NRF2-NQO1-activated Esophageal Squamous Cell Carcinoma

In esophageal squamous cell carcinoma, genetic activation of NRF2 increases resistance to chemotherapy and radiotherapy, which results in a significantly worse prognosis for patients. Therefore NRF2-activated cancers create an urgent clinical need to identify new therapeutic options. In this context, we previously identified the geldanamycin family of HSP90 inhibitors, which includes 17DMAG, to be synthetic lethal with NRF2 activity. As the first-generation of geldanamycin-derivative drugs were withdrawn from clinical trials due to hepatotoxicity, we designed second-generation compounds with C19-substituted structures in order to inhibit glutathione conjugation-mediated hepatotoxicity. In this study, using a variety of in vitro and in vivo cancer models, we found that C19-substituted 17DMAG compounds maintain their enhanced toxicity profile and synthetic lethal interaction with NRF2-NQO1-activated cancer cells. Importantly, using a xenograft mouse tumor model, we found that C19-substituted 17DMAG displayed significant anticancer efficacy against NRF2-NQO1-activated cancer cells without causing hepatotoxicity. These results clearly demonstrate the improved clinical potential for this new class of HSP90 inhibitor anticancer drugs, and suggest that patients with NRF2-NQO1-activated esophageal carcinoma may benefit from this novel therapeutic approach.

Sensor systems of KEAP1 uniquely detecting oxidative and electrophilic stresses separately In vivo

In the KEAP1-NRF2 stress response system, KEAP1 acts as a sensor for oxidative and electrophilic stresses through formation of S-S bond and C-S bond, respectively. Of the many questions left related to the sensor activity, following three appear important; whether these KEAP1 sensor systems are operating in vivo, whether oxidative and electrophilic stresses are sensed by the similar or distinct systems, and how KEAP1 equips highly sensitive mechanisms detecting oxidative and electrophilic stresses in vivo. To address these questions, we conducted a series of analyses utilizing KEAP1-cysteine substitution mutant mice, conditional selenocysteine-tRNA (Trsp) knockout mice, and human cohort whole genome sequence (WGS) data. Firstly, the Trsp-knockout provokes severe deficiency of selenoproteins and compensatory activation of NRF2. However, mice lacking homozygously a pair of critical oxidative stress sensor cysteine residues of KEAP1 fail to activate NRF2 in the Trsp-knockout livers. Secondly, this study provides evidence for the differential utilization of KEAP1 sensors for oxidative and electrophilic stresses in vivo. Thirdly, theoretical calculations show that the KEAP1 dimer equips quite sensitive sensor machinery in which modification of a single molecule of KEAP1 within the dimer is sufficient to affect the activity. WGS examinations of rare variants identified seven non-synonymous variants in the oxidative stress sensors in human KEAP1, while no variant was found in electrophilic sensor cysteine residues, supporting the fail-safe nature of the KEAP1 oxidative stress sensor activity. These results provide valuable information for our understanding how mammals respond to oxidative and electrophilic stresses efficiently.

Nrf2 as a regulator of energy metabolism and mitochondrial function

Nuclear factor erythroid-2-related factor 2 (Nrf2) is essential for the control of cellular redox homeostasis. When activated, Nrf2 elicits cytoprotective effects through the expression of several genes encoding antioxidant and detoxifying enzymes. Nrf2 can also improve antioxidant defense via the pentose phosphate pathway by increasing NADPH availability to regenerate glutathione. Microarray and genome-wide localization analyses have identified many Nrf2 target genes beyond those linked to its redox-regulatory capacity. Nrf2 regulates several intermediary metabolic pathways and is involved in cancer cell metabolic reprogramming, contributing to malignant phenotypes. Nrf2 also modulates substrate utilization for mitochondrial respiration. Here we review the experimental evidence supporting the essential role of Nrf2 in the regulation of energy metabolism and mitochondrial function.

Nrf2 Connects Cellular Autophagy and Vascular Senescence in Atherosclerosis: A Mini-Review

Nuclear factor erythroid 2-related factor 2 (Nrf2), a transcriptional factor that maintains intracellular redox equilibrium, modulates the expression of antioxidant genes, scavenger receptors, and cholesterol efflux transporters, all of which contribute significantly to foam cell development and plaque formation. Nrf2 has recently emerged as a key regulator that connects autophagy and vascular senescence in atherosclerosis. Autophagy, a cellular mechanism involved in the breakdown and recycling of damaged proteins and organelles, and cellular senescence, a state of irreversible growth arrest, are both processes implicated in the pathogenesis of atherosclerosis. The intricate interplay of these processes has received increasing attention, shedding light on their cumulative role in driving the development of atherosclerosis. Recent studies have revealed that Nrf2 plays a critical role in mediating autophagy and senescence in atherosclerosis progression. Nrf2 activation promotes autophagy, which increases lipid clearance and prevents the development of foam cells. Meanwhile, the activation of Nrf2 also inhibits cellular senescence by regulating the expression of senescence markers to preserve cellular homeostasis and function and delay the progression of atherosclerosis. This review provides an overview of the molecular mechanisms through which Nrf2 connects cellular autophagy and vascular senescence in atherosclerosis. Understanding these mechanisms can provide insights into potential therapeutic strategies targeting Nrf2 to modulate cellular autophagy and vascular senescence, thereby preventing the progression of atherosclerosis.

The effect and mechanism of combined exposure of MC-LR and NaNO2 on liver lipid metabolism

Microcystin-LR (MC-LR) and sodium nitrite (NaNO2) co-exist in the environment and are hepatotoxic. The liver has the function of lipid metabolism, but the impacts and mechanisms of MC-LR and NaNO2 on liver lipid metabolism are unclear. Therefore, we established a chronic exposure model of Balb/c mice and used LO2 cells for in vitro verification to investigate the effects and mechanisms of liver lipid metabolism caused by MC-LR and NaNO2. The results showed that after 6 months of exposure to MC-LR and NaNO2, the lipid droplets content was increased, and the activities of alanine aminotransferase (ALT) and aspartate aminotransferase (AST) were raised in the liver (P < 0.05). Moreover, MC-LR and NaNO2 synergistically induced hepatic oxidative stress by decreasing total superoxide dismutase (T-SOD) activity and glutathione (GSH) levels and increasing malondialdehyde (MDA) content levels. In addition, the levels of Nrf2, HO-1, NQO1 and P-AMPK was decreased and Keap1 was increased in the Nrf2/HO-1 pathway. The key factors of lipid metabolism, SREBP-1c, FASN and ACC, were up-regulated in the liver. More importantly, there was a combined effect on lipid deposition of MC-LR and NaNO2 co-exposure. In vitro experiments, MC-LR and NaNO2-induced lipid deposition and changes in lipid metabolism-related changes were mitigated after activation of the Nrf2/HO-1 signaling pathway by the Nrf2 activator tertiary butylhydroquinone (TBHQ). Additionally, TBHQ alleviated the rise of reactive oxygen species (ROS) in LO2 cells induced by MC-LR and NaNO2. Overall, our findings indicated that MC-LR and NaNO2 can cause abnormal liver lipid metabolism, and the combined effects were observed after MC-LR and NaNO2 co-exposure. The Nrf2/HO-1 signal pathway may be a potential target for prevention and control of liver toxicity caused by MC-LR and NaNO2.

Role of carboxylesterase and arylacetamide deacetylase in drug metabolism, physiology, and pathology

Carboxylesterases (CES1 and CES2) and arylacetamide deacetylase (AADAC), which are expressed primarily in the liver and/or gastrointestinal tract, hydrolyze drugs containing ester and amide bonds in their chemical structure. These enzymes often catalyze the conversion of prodrugs, including the COVID-19 drugs remdesivir and molnupiravir, to their pharmacologically active forms. Information on the substrate specificity and inhibitory properties of these enzymes, which would be useful for drug development and toxicity avoidance, has accumulated. Recently,in vitroandin vivostudies have shown that these enzymes are involved not only in drug hydrolysis but also in lipid metabolism. CES1 and CES2 are capable of hydrolyzing triacylglycerol, and the deletion of their orthologous genes in mice has been associated with impaired lipid metabolism and hepatic steatosis. Adeno-associated virus-mediated human CES overexpression decreases hepatic triacylglycerol levels and increases fatty acid oxidation in mice. It has also been shown that overexpression of CES enzymes or AADAC in cultured cells suppresses the intracellular accumulation of triacylglycerol. Recent reports indicate that AADAC can be up- or downregulated in tumors of various organs, and its varied expression is associated with poor prognosis in patients with cancer. Thus, CES and AADAC not only determine drug efficacy and toxicity but are also involved in pathophysiology. This review summarizes recent findings on the roles of CES and AADAC in drug metabolism, physiology, and pathology.

Nrf2 inhibition regulates intracellular lipid accumulation in mouse insulinoma cells and improves insulin secretory function

High amount of fat in the pancreas is linked to poor functioning of β-cells and raises the risk of type 2 diabetes. Here we report the putative role of a circulatory glycoprotein Fetuin-A, a known obesity marker, in promoting lipid accumulation in β-cells and its association with Fatty acid translocase/CD36 for lipid storage culminate in β-cell dysfunction. Additionally, this work reveals regulation of CD36 via Nrf2, a key regulator of oxidative stress, and reduction of lipid accumulation by suppression of Nrf2 that restores β-cell function. Palmitate (0.50 mM) and Fetuin-A (100 μg/mL) exposure showed high levels of intracellular lipid in MIN6 (mouse insulinoma cells) with a concomitant decrease in insulin secretion. This also increased the expression of important lipogenic factors, like CD36, PGC1α, PPARγ, and SREBP1. Flow cytometry analysis of CD36 membrane localization has been corroborated with an increased accumulation of lipids as indicated by Oil-Red-O staining. Immunoblotting and immunofluorescence of Nrf2 indicated its high expression in palmitate-fetuin-A incubation and translocation in the nucleus. Suppression of Nrf2 by siRNA showed a reduced expression of lipogenic genes, ablation of lipid droplets, decrease in the number of apoptotic cells, and restoration of insulin secretion with a corresponding increase of Pdx1, BETA2, and Ins1 gene expression. Our study thus suggested an important aspect of lipid accumulation in the pancreatic β-cells contributing to β-cell dysfunction and demonstrated the role of Fetuin-A in CD36 expression, with a possible way of restoring β-cell function by targeting Nrf2.

Transcription Factor NRF2 in Shaping Myeloid Cell Differentiation and Function

NFE2-related factor 2 (NRF2) is a master transcription factor (TF) that coordinates key cellular homeostatic processes including antioxidative responses, autophagy, proteostasis, and metabolism. The emerging evidence underscores its significant role in modulating inflammatory and immune processes. This chapter delves into the role of NRF2 in myeloid cell differentiation and function and its implication in myeloid cell-driven diseases. In macrophages, NRF2 modulates cytokine production, phagocytosis, pathogen clearance, and metabolic adaptations. In dendritic cells (DCs), it affects maturation, cytokine production, and antigen presentation capabilities, while in neutrophils, NRF2 is involved in activation, migration, cytokine production, and NETosis. The discussion extends to how NRF2's regulatory actions pertain to a wide array of diseases, such as sepsis, various infectious diseases, cancer, wound healing, atherosclerosis, hemolytic conditions, pulmonary disorders, hemorrhagic events, and autoimmune diseases. The activation of NRF2 typically reduces inflammation, thereby modifying disease outcomes. This highlights the therapeutic potential of NRF2 modulation in treating myeloid cell-driven pathologies.

CD36 maintains lipid homeostasis via selective uptake of monounsaturated fatty acids during matrix detachment and tumor progression

A high-fat diet (HFD) promotes metastasis through increased uptake of saturated fatty acids (SFAs). The fatty acid transporter CD36 has been implicated in this process, but a detailed understanding of CD36 function is lacking. During matrix detachment, endoplasmic reticulum (ER) stress reduces SCD1 protein, resulting in increased lipid saturation. Subsequently, CD36 is induced in a p38- and AMPK-dependent manner to promote preferential uptake of monounsaturated fatty acids (MUFAs), thereby maintaining a balance between SFAs and MUFAs. In attached cells, CD36 palmitoylation is required for MUFA uptake and protection from palmitate-induced lipotoxicity. In breast cancer mouse models, CD36-deficiency induced ER stress while diminishing the pro-metastatic effect of HFD, and only a palmitoylation-proficient CD36 rescued this effect. Finally, AMPK-deficient tumors have reduced CD36 expression and are metastatically impaired, but ectopic CD36 expression restores their metastatic potential. Our results suggest that, rather than facilitating HFD-driven tumorigenesis, CD36 plays a supportive role by preventing SFA-induced lipotoxicity.

NRF2 Deficiency Attenuates Diabetic Kidney Disease in Db/Db Mice via Down-Regulation of Angiotensinogen, SGLT2, CD36, and FABP4 Expression and Lipid Accumulation in Renal Proximal Tubular Cells

The role(s) of nuclear factor erythroid 2-related factor 2 (NRF2) in diabetic kidney disease (DKD) is/are controversial. We hypothesized that Nrf2 deficiency in type 2 diabetes (T2D) db/db mice (db/dbNrf2 knockout (KO)) attenuates DKD progression through the down-regulation of angiotensinogen (AGT), sodium-glucose cotransporter-2 (SGLT2), scavenger receptor CD36, and fatty -acid-binding protein 4 (FABP4), and lipid accumulation in renal proximal tubular cells (RPTCs). Db/dbNrf2 KO mice were studied at 16 weeks of age. Human RPTCs (HK2) with NRF2 KO via CRISPR-Cas9 genome editing and kidneys from patients with or without T2D were examined. Compared with db/db mice, db/dbNrf2 KO mice had lower systolic blood pressure, fasting blood glucose, kidney hypertrophy, glomerular filtration rate, urinary albumin/creatinine ratio, tubular lipid droplet accumulation, and decreased expression of AGT, SGLT2, CD36, and FABP4 in RPTCs. Male and female mice had similar results. NRF2 KO attenuated the stimulatory effect of the Nrf2 activator, oltipraz, on AGT, SGLT2, and CD36 expression and high-glucose/free fatty acid (FFA)-stimulated lipid accumulation in HK2. Kidneys from T2D patients exhibited markedly higher levels of CD36 and FABP4 in RPTCs than kidneys from non-diabetic patients. These data suggest that NRF2 exacerbates DKD through the stimulation of AGT, SGLT2, CD36, and FABP4 expression and lipid accumulation in RPTCs of T2D.

Nrf2 in TIME: The Emerging Role of Nuclear Factor Erythroid 2-Related Factor 2 in the Tumor Immune Microenvironment

Nuclear factor erythroid 2-related factor 2 (Nrf2) mediates the cellular antioxidant response, allowing adaptation and survival under conditions of oxidative, electrophilic and inflammatory stress, and has a role in metabolism, inflammation and immunity. Activation of Nrf2 provides broad and long-lasting cytoprotection, and is often hijacked by cancer cells, allowing their survival under unfavorable conditions. Moreover, Nrf2 activation in established human tumors is associated with resistance to chemo-, radio-, and immunotherapies. In addition to cancer cells, Nrf2 activation can also occur in tumor-associated macrophages (TAMs) and facilitate an anti-inflammatory, immunosuppressive tumor immune microenvironment (TIME). Several cancer cell-derived metabolites, such as itaconate, L-kynurenine, lactic acid and hyaluronic acid, play an important role in modulating the TIME and tumor-TAMs crosstalk, and have been shown to activate Nrf2. The effects of Nrf2 in TIME are context-depended, and involve multiple mechanisms, including suppression of pro-inflammatory cytokines, increased expression of programmed cell death ligand 1 (PD-L1), macrophage colony-stimulating factor (M-CSF) and kynureninase, accelerated catabolism of cytotoxic labile heme, and facilitating the metabolic adaptation of TAMs. This understanding presents both challenges and opportunities for strategic targeting of Nrf2 in cancer.

Glutathione: A Samsonian life-sustaining small molecule that protects against oxidative stress, ageing and damaging inflammation

Many local and systemic diseases especially diseases that are leading causes of death globally like chronic obstructive pulmonary disease, atherosclerosis with ischemic heart disease and stroke, cancer and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) causing coronavirus disease 19 (COVID-19), involve both, (1) oxidative stress with excessive production of reactive oxygen species (ROS) that lower glutathione (GSH) levels, and (2) inflammation. The GSH tripeptide (γ- L-glutamyl-L-cysteinyl-glycine), the most abundant water-soluble non-protein thiol in the cell (1-10 mM) is fundamental for life by (a) sustaining the adequate redox cell signaling needed to maintain physiologic levels of oxidative stress fundamental to control life processes, and (b) limiting excessive oxidative stress that causes cell and tissue damage. GSH activity is facilitated by activation of the Kelch-like ECH-associated protein 1 (Keap1)-Nuclear factor erythroid 2-related factor 2 (Nrf2)-antioxidant response element (ARE) redox regulator pathway, releasing Nrf2 that regulates expression of genes controlling antioxidant, inflammatory and immune system responses. GSH exists in the thiol-reduced (>98% of total GSH) and disulfide-oxidized (GSSG) forms, and the concentrations of GSH and GSSG and their molar ratio are indicators of the functionality of the cell. GSH depletion may play a central role in inflammatory diseases and COVID-19 pathophysiology, host immune response and disease severity and mortality. Therapies enhancing GSH could become a cornerstone to reduce severity and fatal outcomes of inflammatory diseases and COVID-19 and increasing GSH levels may prevent and subdue these diseases. The life value of GSH makes for a paramount research field in biology and medicine and may be key against systemic inflammation and SARS-CoV-2 infection and COVID-19 disease. In this review, we emphasize on (1) GSH depletion as a fundamental risk factor for diseases like chronic obstructive pulmonary disease and atherosclerosis (ischemic heart disease and stroke), (2) importance of oxidative stress and antioxidants in SARS-CoV-2 infection and COVID-19 disease, (3) significance of GSH to counteract persistent damaging inflammation, inflammaging and early (premature) inflammaging associated with cell and tissue damage caused by excessive oxidative stress and lack of adequate antioxidant defenses in younger individuals, and (4) new therapies that include antioxidant defenses restoration.

Nrf2 activation reprograms macrophage intermediary metabolism and suppresses the type I interferon response

To overcome oxidative, inflammatory, and metabolic stress, cells have evolved cytoprotective protein networks controlled by nuclear factor-erythroid 2 p45-related factor 2 (Nrf2) and its negative regulator, Kelch-like ECH associated protein 1 (Keap1). Here, using high-resolution mass spectrometry we characterize the proteomes of macrophages with altered Nrf2 status revealing significant differences among the genotypes in metabolism and redox homeostasis, which were validated with respirometry and metabolomics. Nrf2 affected the proteome following lipopolysaccharide (LPS) stimulation, with alterations in redox, carbohydrate and lipid metabolism, and innate immunity. Notably, Nrf2 activation promoted mitochondrial fusion. The Keap1 inhibitor, 4-octyl itaconate remodeled the inflammatory macrophage proteome, increasing redox and suppressing type I interferon (IFN) response. Similarly, pharmacologic or genetic Nrf2 activation inhibited the transcription of IFN-β and its downstream effector IFIT2 during LPS stimulation. These data suggest that Nrf2 activation facilitates metabolic reprogramming and mitochondrial adaptation, and finetunes the innate immune response in macrophages.

Nuclear Factor Erythroid 2-Related Factor 2 Depletion Sensitizes Pancreatic Cancer Cells to Gemcitabine via Aldehyde Dehydrogenase 3a1 Repression

As the central regulator of the oxidative stress response, nuclear factor erythroid 2-related factor 2 (Nrf2) is attracting great interest as a therapeutic target for various cancers, and the possible clinical applications of novel Nrf2 inhibitors have been explored in Nrf2-activated cancers. In the present study, we specifically investigated halofuginone, which is derived from a natural plant alkaloid. We found that halofuginone administration decreased the number of pancreatic intraepithelial neoplasias in pancreas-specific Kras and p53 mutant (KPC) mice. In Nrf2-activated pancreatic cancer cell lines established from KPC mice, halofuginone rapidly depleted Nrf2 in Nrf2-activated cancer cells. Both in vitro and in vivo, it sensitized Nrf2-activated pancreatic cancer cells to gemcitabine, which is the first-line chemotherapy in clinical practice. In our mechanistic study, we found that halofuginone downregulated aldehyde dehydrogenase 3a1 (ALDH3A1) in mouse pancreatic cancer cells. The Nrf2 inducer diethyl maleate upregulated ALDH3A1, and knockdown of Aldh3a1 sensitized Nrf2-activated cancer cells to gemcitabine, strongly suggesting that ALDH3A1 is regulated by Nrf2 and that it contributes to gemcitabine resistance. The current study demonstrated the therapeutic benefits of halofuginone in Nrf2-activated pancreatic cancers. SIGNIFICANCE STATEMENT: We identified nuclear factor erythroid 2-related factor 2 (Nrf2) and its downstream target aldehyde dehydrogenase 3a1 (ALDH3A1) as novel therapeutic targets in pancreatic cancer. They negatively affect the efficacy of a conventional chemotherapeutic agent, gemcitabine. We confirmed that Nrf2 plays a pivotal role in the induction of ALDH3A1.

Nrf2 expression in pancreatic stellate cells promotes progression of cancer

It was previously identified that systemic Nrf2 deletion attenuates pancreatic cancer progression in a mutant K-ras/p53-expressing mouse model (KPC mouse). In this study, the type of cell that is responsible for the retarded cancer progression was elucidated. Human pancreatic cancers were first examined, and elevated expression of NRF2-target gene products in α-smooth muscle actin-positive cells was found, suggesting that pancreatic stellate cells (PSCs) are involved in this process. Closer examination of primary cultured PSCs from Nrf2-deleted mice revealed that the cells were less proliferative and retained a lower migration capacity. The conditioned medium of Nrf2-deleted PSCs exhibited reduced growth-stimulating effects in pancreatic cancer cells. KPC mouse-derived pancreatic cancer cells coinjected with wild-type PSCs developed significantly larger subcutaneous tumors in immunodeficient mice than those coinjected with Nrf2-deleted PSCs. These results demonstrate that Nrf2 actively contributes to the function of PSCs to sustain KPC cancer progression, thus, suggesting that Nrf2 inhibition in PSCs may be therapeutically important in pancreatic cancer.NEW & NOTEWORTHY This study identified that Nrf2 contributes to PSC activation. Nrf2 deletion in PSCs resulted in attenuation of cancer-promoting role. Nrf2 in PSCs could be an attractive therapeutic target in pancreatic cancer.

Liver-specific Nrf2 deficiency accelerates ethanol-induced lethality and hepatic injury in vivo

Alcoholic liver disease (ALD) is a major cause of morbidity and mortality from liver disorders. Various mechanisms, including oxidative stress and impaired lipid metabolism, have been implicated in the pathogenesis of ALD. Our previous studies showed that nuclear factor erythroid-derived 2-like 2 (Nrf2) is a master regulator of adaptive antioxidant response and lipid metabolism by using a liver-specific Nrf2 knockout (Nrf2(L)-KO) mouse model. In the current study, an ALD model was developed by a Lieber-DeCarli liquid-based ethanol diet given to this Nrf2(L)-KO mouse strain. We found that Nrf2(L)-KO mice were quite sensitive to lethality from 6.3% ethanol diet. We thus decreased the ethanol concentration to 4.2% to obtain tissues to analyze the role of hepatic Nrf2 in the development of ALD. We found that mild hepatic steatosis occurred with both liquid control and 4.2% ethanol diet feeding, which contain 35% fat. Both the fatty acid β-oxidation marker peroxisome proliferators-activated receptor α (PPARα), and lipogenesis regulator PPARγ were reduced with ethanol feeding in Nrf2(L)-KO mice, compared to Nrf2 floxed control mice (Nrf2-LoxP). However, Nrf2(L)-KO livers showed more cell injury than the livers of Nrf2-LoxP mice. Consistent with these data, there was increased proportion of apoptotic cells in the liver of ethanol-fed Nrf2(L)-KO mice comparing Nrf2-LoxP controls. Mechanistically, Nrf2 mediated expression of ethanol detoxification enzymes, such as alcohol dehydrogenase 1 and aldehyde dehydrogenase1a1, likely contributed to the sensitivity to ethanol toxicity. In conclusion, hepatic Nrf2 is critical to the development of ALD, particularly the morbidity and liver injury.

Nrf2 Activation Sensitizes K-Ras Mutant Pancreatic Cancer Cells to Glutaminase Inhibition

Pancreatic cancer remains intractable owing to the lack of effective therapy for unresectable cases. Activating mutations of K-ras are frequently found in pancreatic cancers, but these have not yet been targeted by cancer therapies. The Keap1-Nrf2 system plays a crucial role in mediating the oxidative stress response, which also contributes to cancer progression. Nrf2 activation reprograms the metabolic profile to promote the proliferation of cancer cells. A recent report suggested that K-ras- and Nrf2-active lung cancer cells are sensitive to glutamine depletion. This finding led to the recognition of glutaminase inhibitors as novel anticancer agents. In the current study, we used murine pancreatic cancer tissues driven by mutant K-ras and p53 to establish cell lines expressing constitutively activated Nrf2. Genetic or pharmacological Nrf2 activation in cells via Keap1 deletion or Nrf2 activation sensitized cells to glutaminase inhibition. This phenomenon was confirmed to be dependent on K-ras activation in human pancreatic cancer cell lines harboring mutant K-ras, i.e., Panc-1 and MiaPaCa-2 in response to DEM pretreatment. This phenomenon was not observed in BxPC3 cells harboring wildtype K-ras. These results indicate the possibility of employing Nrf2 activation and glutaminase inhibition as novel therapeutic interventions for K-ras mutant pancreatic cancers.

Identification of Dominant Transcripts in Oxidative Stress Response by a Full-Length Transcriptome Analysis

Our body responds to environmental stress by changing the expression levels of a series of cytoprotective enzymes/proteins through multilayered regulatory mechanisms, including the KEAP1-NRF2 system. While NRF2 upregulates the expression of many cytoprotective genes, there are fundamental limitations in short-read RNA sequencing (RNA-Seq), resulting in confusion regarding interpreting the effectiveness of cytoprotective gene induction at the transcript level. To precisely delineate isoform usage in the stress response, we conducted independent full-length transcriptome profiling (isoform sequencing; Iso-Seq) analyses of lymphoblastoid cells from three volunteers under normal and electrophilic stress-induced conditions. We first determined the first exon usage in KEAP1 and NFE2L2 (encoding NRF2) and found the presence of transcript diversity. We then examined changes in isoform usage of NRF2 target genes under stress conditions and identified a few isoforms dominantly expressed in the majority of NRF2 target genes. The expression levels of isoforms determined by Iso-Seq analyses showed striking differences from those determined by short-read RNA-Seq; the latter could be misleading concerning the abundance of transcripts. These results support that transcript usage is tightly regulated to produce functional proteins under electrophilic stress. Our present study strongly argues that there are important benefits that can be achieved by long-read transcriptome sequencing.

NRF2 and the Ambiguous Consequences of Its Activation during Initiation and the Subsequent Stages of Tumourigenesis

NF-E2 p45-related factor 2 (NRF2, encoded in the human by NFE2L2) mediates short-term adaptation to thiol-reactive stressors. In normal cells, activation of NRF2 by a thiol-reactive stressor helps prevent, for a limited period of time, the initiation of cancer by chemical carcinogens through induction of genes encoding drug-metabolising enzymes. However, in many tumour types, NRF2 is permanently upregulated. In such cases, its overexpressed target genes support the promotion and progression of cancer by suppressing oxidative stress, because they constitutively increase the capacity to scavenge reactive oxygen species (ROS), and they support cell proliferation by increasing ribonucleotide synthesis, serine biosynthesis and autophagy. Herein, we describe cancer chemoprevention and the discovery of the essential role played by NRF2 in orchestrating protection against chemical carcinogenesis. We similarly describe the discoveries of somatic mutations in NFE2L2 and the gene encoding the principal NRF2 repressor, Kelch-like ECH-associated protein 1 (KEAP1) along with that encoding a component of the E3 ubiquitin-ligase complex Cullin 3 (CUL3), which result in permanent activation of NRF2, and the recognition that such mutations occur frequently in many types of cancer. Notably, mutations in NFE2L2, KEAP1 and CUL3 that cause persistent upregulation of NRF2 often co-exist with mutations that activate KRAS and the PI3K-PKB/Akt pathway, suggesting NRF2 supports growth of tumours in which KRAS or PKB/Akt are hyperactive. Besides somatic mutations, NRF2 activation in human tumours can occur by other means, such as alternative splicing that results in a NRF2 protein which lacks the KEAP1-binding domain or overexpression of other KEAP1-binding partners that compete with NRF2. Lastly, as NRF2 upregulation is associated with resistance to cancer chemotherapy and radiotherapy, we describe strategies that might be employed to suppress growth and overcome drug resistance in tumours with overactive NRF2.

Nrf2 Mediates Metabolic Reprogramming in Non-Small Cell Lung Cancer

Nuclear factor erythroid-2–related factor-2 (NFE2L2/Nrf2) is a transcription factor that regulates the expression of antioxidant genes. Both Kelch-like ECH-associated protein 1 (Keap1) mutations and Nrf2 mutations contribute to the activation of Nrf2 in non-small cell lung cancer (NSCLC). Nrf2 activity is associated with poor prognosis in NSCLC. Metabolic reprogramming represents a cancer hallmark. Increasing studies reveal that Nrf2 activation promotes metabolic reprogramming in cancer. In this review, we discuss the underlying mechanisms of Nrf2-mediated metabolic reprogramming and elucidate its role in NSCLC. Inhibition of Nrf2 can alter metabolic processes, thus suppress tumor growth, prevent metastasis, and increase sensitivity to chemotherapy in NSCLC. In conclusion, Nrf2 may serve as a therapeutic target for the treatment of NSCLC.

SIRT3 deficiency exacerbates fatty liver by attenuating the HIF1α-LIPIN 1 pathway and increasing CD36 through Nrf2

Background

Deficiency of mitochondrial sirtuin 3 (SIRT3), a NAD⁺-dependent protein deacetylase that maintains redox status and lipid homeostasis, contributes to hepatic steatosis. In this study, we investigated additional mechanisms that might play a role in aggravating hepatic steatosis in Sirt3-deficient mice fed a high-fat diet (HFD).

Methods

Studies were conducted in wild-type (WT) and Sirt3^−/− mice fed a standard diet or a HFD and in SIRT3-knockdown human Huh-7 hepatoma cells.

Results

Sirt3^−/− mice fed a HFD presented exacerbated hepatic steatosis that was accompanied by decreased expression and DNA-binding activity of peroxisome proliferator-activated receptor (PPAR) α and of several of its target genes involved in fatty acid oxidation, compared to WT mice fed the HFD. Interestingly, Sirt3 deficiency in liver and its knockdown in Huh-7 cells resulted in upregulation of the nuclear levels of LIPIN1, a PPARα co-activator, and of the protein that controls its levels and localization, hypoxia-inducible factor 1α (HIF-1α). These changes were prevented by lipid exposure through a mechanism that might involve a decrease in succinate levels. Finally, Sirt3^−/− mice fed the HFD showed increased levels of some proteins involved in lipid uptake, such as CD36 and the VLDL receptor. The upregulation in CD36 was confirmed in Huh-7 cells treated with a SIRT3 inhibitor or transfected with SIRT3 siRNA and incubated with palmitate, an effect that was prevented by the Nrf2 inhibitor ML385.

Conclusion

These findings demonstrate new mechanisms by which Sirt3 deficiency contributes to hepatic steatosis.

Molecular Mechanism of Cellular Oxidative Stress Sensing by Keap1

The Keap1-Nrf2 system plays a central role in the oxidative stress response; however, the identity of the reactive oxygen species sensor within Keap1 remains poorly understood. Here, we show that a Keap1 mutant lacking 11 cysteine residues retains the ability to target Nrf2 for degradation, but it is unable to respond to cysteine-reactive Nrf2 inducers. Of the 11 mutated cysteine residues, we find that 4 (Cys226/613/622/624) are important for sensing hydrogen peroxide. Our analyses of multiple mutant mice lines, complemented by MEFs expressing a series of Keap1 mutants, reveal that Keap1 uses the cysteine residues redundantly to set up an elaborate fail-safe mechanism in which specific combinations of these four cysteine residues can form a disulfide bond to sense hydrogen peroxide. This sensing mechanism is distinct from that used for electrophilic Nrf2 inducers, demonstrating that Keap1 is equipped with multiple cysteine-based sensors to detect various endogenous and exogenous stresses.

NRF2, a Transcription Factor for Stress Response and Beyond

Nuclear factor erythroid 2-related factor 2 (NRF2) is a transcription factor that regulates the cellular defense against toxic and oxidative insults through the expression of genes involved in oxidative stress response and drug detoxification. NRF2 activation renders cells resistant to chemical carcinogens and inflammatory challenges. In addition to antioxidant responses, NRF2 is involved in many other cellular processes, including metabolism and inflammation, and its functions are beyond the originally envisioned. NRF2 activity is tightly regulated through a complex transcriptional and post-translational network that enables it to orchestrate the cell’s response and adaptation to various pathological stressors for the homeostasis maintenance. Elevated or decreased NRF2 activity by pharmacological and genetic manipulations of NRF2 activation is associated with many metabolism- or inflammation-related diseases. Emerging evidence shows that NRF2 lies at the center of a complex regulatory network and establishes NRF2 as a truly pleiotropic transcription factor. Here we summarize the complex regulatory network of NRF2 activity and its roles in metabolic reprogramming, unfolded protein response, proteostasis, autophagy, mitochondrial biogenesis, inflammation, and immunity.

Keap1 deletion accelerates mutant K-ras/p53-driven cholangiocarcinoma

The activation of the Kelch-like ECH-associated protein 1 (Keap1)-NF-E2-related factor 2 (Nrf2) pathway contributes to cancer progression in addition to oxidative stress responses. Loss-of-function Keap1 mutations were reported to activate Nrf2, leading to cancer progression. We examined the effects of Keap1 deletion in a cholangiocarcinoma mouse model using a mutant K-ras/p53 mouse. Introduction of the Keap1 deletion into liver-specific mutant K-ras/p53 expression resulted in the formation of invasive cholangiocarcinoma. Comprehensive analyses of the gene expression profiles identified broad upregulation of Nrf2-target genes such as Nqo1 and Gstm1 in the Keap1-deleted mutant K-ras/p53 expressing livers, accompanied by upregulation of cholangiocyte-related genes. Among these genes, the transcriptional factor Sox9 was highly expressed in the dysplastic bile duct. The Keap-Nrf2-Sox9 axis might serve as a novel therapeutic target for cholangiocarcinoma.NEW & NOTEWORTHY The Keap1-Nrf2 system has a wide variety of effects in addition to the oxidative stress response in cancer cells. Addition of the liver-specific Keap1 deletion to mice harboring mutant K-ras and p53 accelerated cholangiocarcinoma formation, together with the hallmarks of Nrf2 activation. This process involved the expansion of Sox9-positive cells, indicating increased differentiation toward the cholangiocyte phenotype.

BACH1 Promotes Pancreatic Cancer Metastasis by Repressing Epithelial Genes and Enhancing Epithelial-Mesenchymal Transition

Pancreatic ductal adenocarcinoma (PDAC) is among the cancers with the poorest prognoses due to its highly malignant features. BTB and CNC homology 1 (BACH1) has been implicated in RAS-driven tumor formation. We focused on the role of BACH1 in PDAC, more than 90% of which have KRAS mutation. Knockdown of BACH1 in PDAC cell lines reduced cell migration and invasion, in part, by increasing E-cadherin expression, whereas its overexpression showed opposite effects. BACH1 directly repressed the expression of FOXA1 that is known to activate the expression of CDH1 encoding E-cadherin and to inhibit epithelial-to-mesenchymal transition. BACH1 also directly repressed the expression of genes important for epithelial cell adhesion including CLDN3 and CLDN4. In a mouse orthotopic implantation model, BACH1 was required for the high metastatic ability of AsPC-1 cells. IHC analysis of clinical specimens with a newly developed anti-BACH1 mAb revealed that high expression of BACH1 is a poor prognostic factor. These results suggest that the gene regulatory network of BACH1 and downstream genes including CDH1 contribute to the malignant features of PDAC by regulating epithelial-to-mesenchymal transition. SIGNIFICANCE: Greater understanding of the gene regulatory network involved in epithelial-to-mesenchymal transition of pancreatic cancer cells will provide novel therapeutic targets and diagnostic markers.

Direct and Specific Functional Evaluation of the Nrf2 and MafG Heterodimer by Introducing a Tethered Dimer into Small Maf-Deficient Cells

A group of cytoprotective genes is regulated by heterodimers composed of the cap'n'collar (CNC) family member Nrf2 and one of the small Maf (sMaf) proteins (MafF, MafG, or MafK) through the antioxidant response element (ARE, also referred to as the CNC-sMaf binding element [CsMBE]). Many lines of evidence support this model; however, a direct and specific evaluation of the Nrf2-sMaf heterodimer remains to be executed. To address this issue, we constructed a tethered Nrf2-MafG (T-N2G) heterodimer using a flexible linker peptide. We then introduced the T-N2G construct into cells lacking all three sMaf proteins to specifically evaluate the function of the tethered heterodimer without interference from other endogenous CNC-sMaf heterodimers or sMaf homodimers. In response to an Nrf2 activator, diethyl maleate, the T-N2G protein can widely activate the target genes of Nrf2 but not those of Nrf1, such as proteasome subunit genes. Genome-wide binding analysis showed that the T-N2G protein preferentially bound to the CsMBE motifs in the regulatory regions of the Nrf2 target genes. These results provide direct evidence that the Nrf2-MafG heterodimer acts as a transcriptional activator of Nrf2-dependent genes and show that this assay system will be a powerful tool to specifically examine the function of other CNC-sMaf heterodimers.

Lithium chloride promoted hematoma resolution after intracerebral hemorrhage through GSK-3β-mediated pathways-dependent microglia phagocytosis and M2-phenotype differentiation, angiogenesis and neurogenesis in a rat model

Some neuroprotective agents have been used clinically to address the resulting various adverse effects after intracerebral hemorrhage (ICH). Particularly, effectively removing the hematoma is of practical significance to exert neuroprotective effects following ICH. However, such agents are still in need of development. Lithium chloride (LiCl) has shown neuroprotective effects through glycogen synthase kinase-3β (GSK-3β) inhibition in a variety of central nervous system diseases. However, the impact of LiCl on hematoma clearance and the potential molecular mechanisms have not been reported. We hypothesize that LiCl may exert neuroprotective roles after ICH, partly through promoting hematoma resolution. In this study, male Sprague-Dawley rats were subjected to ICH followed by intraperitoneal injection of LiCl (60 mg/kg). The hematoma volumes of ipsilateral hemisphere were determined using Drabkin's method. The sensorimotor deficits were evaluated by neurobehavioral tests. The expressions of target molecules involved in the process of hematoma clearance were assayed using immunofluorescence and Western blot. Our results showed that animals treated with LiCl presented significantly reduced hematoma volume after ICH, which was coupled with enhanced microglia phagocytosis and its differentiation into M2-phenotype within the first 7 days and up-regulated angiogenesis and neurogenesis in the next 7 days. Meanwhile, GSK-3β was inhibited by LiCl and β-catenin became stabilized, which was followed by up-regulation of nuclear factor erythroid 2-related factor 2 and CD36 from days 3 to 7, and increase of vascular endothelial growth factor and brain-derived neurotrophic factor from days 7 to 14. These data suggest that LiCl promotes hematoma resolution via enhancing microglia phagocytosis and M2-phenotype differentiation in the early stage (< 7 days) and angiogenesis and neurogenesis in the chronic phase (days 7-14), thus eventually improving the functional outcomes of ICH rats.

Organic dust, causing both oxidative stress and Nrf2 activation, is phagocytized by bronchial epithelial cells

Inhalation of organic dust (OD) from swine confinement facilities leads to pulmonary inflammation, airway hyperresponsiveness, and oxidative stress. In mice, pretreatment with a hydroxyl radical scavenger prevents airway inflammation and airway hyperresponsiveness (AHR) induced by OD exposure. We sought to determine a mechanism by which OD could induce oxidative stress in bronchial epithelial cells. Human bronchial epithelial cells (BEAS-2B or NHBE) were treated with various concentrations of OD, followed by evaluation of intracellular oxidative stress using 2',7'-dichlorofluorescein diacetate (DCFDA). After stimulation with OD, gene expression of antioxidant genes was assessed by real-time quantitative PCR followed by quantification of Nrf2 nuclear translocation using a luciferase reporter assay. Phagocytic markers (CD36 and CD68) were analyzed by FACS. Cells were treated with an actin inhibitor, cytochalasin D, before OD exposure and evaluated for Nrf2 nuclear translocation and DCFDA. Mice were pretreated with sulforaphane, the Nrf2 activator, before OD exposure and evaluated for pulmonary inflammation and airway reactivity. OD induced a time- and concentration-dependent increase in DCFDA. mRNA expression levels of Nrf2-dependent genes and Nrf2 nuclear translocation were increased after OD exposure. OD exposure increased the expression of CD68 and CD36. Cytochalasin D prevented oxidative stress and Nrf2 nuclear translocation after OD. Pretreatment with sulforaphane prevented OD-induced inflammation and AHR while increasing the uptake of OD in bronchial epithelial cells. Bronchial epithelial cells can phagocytose OD, resulting in an increase in endogenous oxidative stress. Nrf2-dependent mechanisms mediate the antioxidant response to OD.

Nrf2 Involvement in Chemical-Induced Skin Innate Immunity

Exposure to certain chemicals disturbs skin homeostasis. In particular, protein-reactive chemical contact sensitizers trigger an inflammatory immune response resulting in eczema and allergic contact dermatitis. Chemical sensitizers activate innate immune cells which orchestrate the skin immune response. This involves oxidative and inflammatory pathways. In parallel, the Nrf2/Keap1 pathway, a major ubiquitous regulator of cellular oxidative and electrophilic stress is activated in the different skin innate immune cells including epidermal Langerhans cells and dermal dendritic cells, but also in keratinocytes. In this context, Nrf2 shows a strong protective capacity through the downregulation of both the oxidative stress and inflammatory pathways. In this review we highlight the important role of Nrf2 in the control of the innate immune response of the skin to chemical sensitizers.

NRF2 and the Hallmarks of Cancer

The transcription factor NRF2 is the master regulator of the cellular antioxidant response. Though recognized originally as a target of chemopreventive compounds that help prevent cancer and other maladies, accumulating evidence has established the NRF2 pathway as a driver of cancer progression, metastasis, and resistance to therapy. Recent studies have identified new functions for NRF2 in the regulation of metabolism and other essential cellular functions, establishing NRF2 as a truly pleiotropic transcription factor. In this review, we explore the roles of NRF2 in the hallmarks of cancer, indicating both tumor suppressive and tumor-promoting effects.

Silencing carboxylesterase 1 in human THP-1 macrophages perturbs genes regulated by PPARγ/RXR and RAR/RXR: down-regulation of CYP27A1-LXRα signaling

Macrophage foam cells store excess cholesterol as cholesteryl esters, which need to be hydrolyzed for cholesterol efflux. We recently reported that silencing expression of carboxylesterase 1 (CES1) in human THP-1 macrophages [CES1KD (THP-1 cells with CES1 expression knocked down) macrophages] reduced cholesterol uptake and decreased expression of CD36 and scavenger receptor-A in cells loaded with acetylated low-density lipoprotein (acLDL). Here, we report that CES1KD macrophages exhibit reduced transcription of cytochrome P45027A1 (CYP27A1) in nonloaded and acLDL-loaded cells. Moreover, levels of CYP27A1 protein and its enzymatic product, 27-hydroxycholesterol, were markedly reduced in CES1KD macrophages. Transcription of LXRα (liver X receptor α) and ABCA1 (ATP-binding cassette transporter A1) was also decreased in acLDL-loaded CES1KD macrophages, suggesting reduced signaling through PPARγ-CYP27A1-LXRα. Consistent with this, treatment of CES1KD macrophages with agonists for PPARγ, RAR, and/or RAR/RXR partially restored transcription of CYP27A1 and LXRα, and repaired cholesterol influx. Conversely, treatment of control macrophages with antagonists for PPARγ and/or RXR decreased transcription of CYP27A1 and LXRα Pharmacologic inhibition of CES1 in both wild-type THP-1 cells and primary human macrophages also decreased CYP27A1 transcription. CES1 silencing did not affect transcript levels of PPARγ and RXR in acLDL-loaded macrophages, whereas it did reduce the catabolism of the endocannabinoid 2-arachidonoylglycerol. Finally, the gene expression profile of CES1KD macrophages was similar to that of PPARγ knockdown cells following acLDL exposures, further suggesting a mechanistic link between CES1 and PPARγ. These results are consistent with a model in which abrogation of CES1 function attenuates the CYP27A1-LXRα-ABCA1 signaling axis by depleting endogenous ligands for the nuclear receptors PPARγ, RAR, and/or RXR that regulate cholesterol homeostasis.

Nrf2 promotes mutant K-ras/p53-driven pancreatic carcinogenesis

The Keap1-Nrf2 system contributes to the maintenance of homeostasis by regulating oxidative stress responses in normal tissues and organs, and is exploited in various cancers for proliferation, survival and acquisition of therapy resistance. Pancreatic cancer remains one of the intractable cancers, despite the improved clinical outcomes of other types of cancer, due to its invasive and refractory nature to therapeutic intervention. The current study aimed to clarify the contribution of Nrf2 to pancreatic carcinogenesis using a pancreas-specific mutant K-ras and p53 (KPC) mouse model. Deletion of Nrf2 in KPC mice (KPCN) decreased the formation of precancerous lesions as well as the development of invasive pancreatic cancer. The pancreatic tumor-derived cancer cell lines from KPCN mouse showed decreased expression of glutathione S-transferases (GST), UDP glucuronosyltransferases (UGT) and ABC transporters. Along with these biochemical changes, cell lines from KPCN mice revealed increased sensitivity to oxidative stress and chemotherapeutic agent. The current study revealed that Nrf2 contributes to pancreatic carcinogenesis in a way distinct from the chemoresistance of lung and esophagus, and that Nrf2 could be a novel therapeutic target of pancreatic cancer.

15-Deoxy-Δ12,14-Prostaglandin J2 Exerts Proresolving Effects Through Nuclear Factor E2-Related Factor 2-Induced Expression of CD36 and Heme Oxygenase-1

AIMS

15-Deoxy-Δ^12,14-prostaglandin J₂ (15d-PGJ₂) has been shown to rescue cells from inflammatory insults and to participate in the resolution of acute inflammation. In this study, we investigated molecular mechanisms underlying proresolving effects of 15d-PGJ₂.

RESULTS

15d-PGJ₂ injected into the peritoneum of mice facilitated the resolution of zymosan A-induced peritonitis. 15d-PGJ₂ administration reduced the number of total leukocytes and attenuated polymorphonuclear leukocyte infiltration. Furthermore, 15d-PGJ₂ increased the proportion of macrophages engulfing apoptotic neutrophils, a process called efferocytosis. In addition, when the thioglycollate-elicited mouse peritoneal macrophages were stimulated with 15d-PGJ₂, their efferocytic activity was amplified. In another experiment, RAW264.7 murine macrophages exposed to 15d-PGJ₂ conducted phagocytic clearance of apoptotic cells to a greater extent than the control cells. Under these conditions, expression of CD36 and heme oxygenase-1 (HO-1) was enhanced along with increased accumulation of the nuclear factor E2-related factor 2 (Nrf2) in the nucleus. Knockdown of Nrf2 abolished 15d-PGJ₂-induced expression of CD36 and HO-1, and silencing of CD36 and HO-1 attenuated 15d-PGJ₂-induced efferocytosis. Moreover, peritoneal macrophages isolated from Nrf2-null mice failed to upregulate 15d-PGJ₂-induced expression of CD36 and HO-1 and to mediate efferocytosis. Unlike 15d-PGJ₂, its nonelectrophilic analog 9,10-dihydro-15d-PGJ₂ lacking the α,β-unsaturated carbonyl group could not induce CD36 expression and efferocytosis.

INNOVATION

15d-PGJ₂, as one of the terminal products of cyclooxygenase-2, exerts proresolving effects through induction of efferocytosis. The results of this study suggest that 15d-PGJ₂ possesses a therapeutic value in the management of inflammatory disorders.

CONCLUSION

15d-PGJ₂ facilitates resolution of inflammation by inducing Nrf2-induced expression of CD36 and HO-1 in macrophages. Antioxid. Redox Signal. 27, 1412-1431.

NRF2 Activation Impairs Quiescence and Bone Marrow Reconstitution Capacity of Hematopoietic Stem Cells

Tissue stem cells are maintained in quiescence under physiological conditions but proliferate and differentiate to replenish mature cells under stressed conditions. The KEAP1-NRF2 system plays an essential role in stress response and cytoprotection against redox disturbance. To clarify the role of the KEAP1-NRF2 system in tissue stem cells, we focused on hematopoiesis in this study and used Keap1-deficient mice to examine the effects of persistent activation of NRF2 on long-term hematopoietic stem cells (LT-HSCs). We found that persistent activation of NRF2 due to Keap1 deficiency did not change the number of LT-HSCs but reduced their quiescence in steady-state hematopoiesis. During hematopoietic regeneration after bone marrow (BM) transplantation, persistent activation of NRF2 reduced the BM reconstitution capacity of LT-HSCs, suggesting that NRF2 reduces the quiescence of LT-HSCs and promotes their differentiation, leading to eventual exhaustion. Transient activation of NRF2 by an electrophilic reagent also promotes the entry of LT-HSCs into the cell cycle. Taken together, our findings show that NRF2 drives the cell cycle entry and differentiation of LT-HSCs at the expense of their quiescence and maintenance, an effect that appears to be beneficial for prompt recovery from blood loss. We propose that the appropriate control of NRF2 activity by KEAP1 is essential for maintaining HSCs and guarantees their stress-induced regenerative response.

Systemic Activation of NRF2 Alleviates Lethal Autoimmune Inflammation in Scurfy Mice

The transcription factor NRF2 (nuclear factor [erythroid-derived 2]-like 2) plays crucial roles in the defense mechanisms against oxidative stress and mediates anti-inflammatory actions under various pathological conditions. Recent studies showed that the dysfunction of regulatory T cells (Tregs) is directly linked to the initiation and progression of various autoimmune diseases. To determine the Treg-independent impact of NRF2 activation on autoimmune inflammation, we examined scurfy (Sf) mice, which are deficient in Tregs and succumb to severe multiorgan inflammation by 4 weeks of age. We found that systemic activation of NRF2 by Keap1 (Kelch-like ECH-associated protein 1) knockdown ameliorated tissue inflammation and lethality in Sf mice. Activated T cells and their cytokine production were accordingly decreased by Keap1 knockdown. In contrast, NRF2 activation through cell lineage-specific Keap1 disruption (i.e., in T cells, myeloid cells, and dendritic cells) achieved only partial or no improvement in the inflammatory status of Sf mice. Our results indicate that systemic activation of NRF2 suppresses effector T cell activities independently of Tregs and that NRF2 activation in multiple cell lineages appears to be required for sufficient anti-inflammatory effects. This study emphasizes the possible therapeutic application of NRF2 inducers in autoimmune diseases that are accompanied by Treg dysfunction.

Glucocorticoid receptor signaling represses the antioxidant response by inhibiting histone acetylation mediated by the transcriptional activator NRF2

NRF2 (nuclear factor erythroid 2-related factor 2) is a key transcriptional activator that mediates the inducible expression of antioxidant genes. NRF2 is normally ubiquitinated by KEAP1 (Kelch-like ECH-associated protein 1) and subsequently degraded by proteasomes. Inactivation of KEAP1 by oxidative stress or electrophilic chemicals allows NRF2 to activate transcription through binding to antioxidant response elements (AREs) and recruiting histone acetyltransferase CBP (CREB-binding protein). Whereas KEAP1-dependent regulation is a major determinant of NRF2 activity, NRF2-mediated transcriptional activation varies from context to context, suggesting that other intracellular signaling cascades may impact NRF2 function. To identify a signaling pathway that modifies NRF2 activity, we immunoprecipitated endogenous NRF2 and its interacting proteins from mouse liver and identified glucocorticoid receptor (GR) as a novel NRF2-binding partner. We found that glucocorticoids, dexamethasone and betamethasone, antagonize diethyl maleate-induced activation of NRF2 target genes in a GR-dependent manner. Dexamethasone treatment enhanced GR recruitment to AREs without affecting chromatin binding of NRF2, resulting in the inhibition of CBP recruitment and histone acetylation at AREs. This repressive effect was canceled by the addition of histone deacetylase inhibitors. Thus, GR signaling decreases NRF2 transcriptional activation through reducing the NRF2-dependent histone acetylation. Consistent with these observations, GR signaling blocked NRF2-mediated cytoprotection from oxidative stress. This study suggests that an impaired antioxidant response by NRF2 and a resulting decrease in cellular antioxidant capacity account for the side effects of glucocorticoids, providing a novel viewpoint for the pathogenesis of hypercorticosteroidism.

Halofuginone enhances the chemo-sensitivity of cancer cells by suppressing NRF2 accumulation

The KEAP1-NRF2 system regulates the cellular defence against oxidative and xenobiotic stresses. NRF2 is a transcription factor that activates the expression of cytoprotective genes encoding antioxidative, detoxifying and metabolic enzymes as well as transporters. Under normal conditions, KEAP1 represses NRF2 activity by degrading the NRF2 protein. When cells are exposed to stresses, KEAP1 stops promoting NRF2 degradation, and NRF2 rapidly accumulates and activates the transcription of target genes. Constitutive accumulation of NRF2 via a variety of mechanisms that disrupt KEAP1-mediated NRF2 degradation has been observed in various cancer types. Constitutive NRF2 accumulation confers cancer cells with a proliferative advantage as well as resistance to anti-cancer drugs and radiotherapies. To suppress the chemo- and radio-resistance of cancer cells caused by NRF2 accumulation, we conducted high-throughput chemical library screening for NRF2 inhibitors and identified febrifugine derivatives. We found that application of the less-toxic derivative halofuginone in a low dose range rapidly reduced NRF2 protein levels. Halofuginone induced a cellular amino acid starvation response that repressed global protein synthesis and rapidly depleted NRF2. Halofuginone treatment ameliorated the resistance of NRF2-addicted cancer cells to anti-cancer drugs both in vitro and in vivo. These results provide preclinical proof-of-concept evidence for halofuginone as an NRF2 inhibitor applicable to treatment of chemo- and radio-resistant forms of cancer.

Modulation of cAMP levels by high-fat diet and curcumin and regulatory effects on CD36/FAT scavenger receptor/fatty acids transporter gene expression

Curcumin, a polyphenol from turmeric (Curcuma longa), reduces inflammation, atherosclerosis, and obesity in several animal studies. In Ldlr^-/- mice fed a high-fat diet (HFD), curcumin reduces plasma lipid levels, therefore contributing to a lower accumulation of lipids and to reduced expression of fatty acid transport proteins (CD36/FAT, FABP4/aP2) in peritoneal macrophages. In this study, we analyzed the molecular mechanisms by which curcumin (500, 1000, 1500 mg/kg diet, for 4 months) may influence plasma and tissue lipid levels in Ldlr^-/- mice fed an HFD. In liver, HFD significantly suppressed cAMP levels, and curcumin restored almost normal levels. Similar trends were observed in adipose tissues, but not in brain, skeletal muscle, spleen, and kidney. Treatment with curcumin increased phosphorylation of CREB in liver, what may play a role in regulatory effects of curcumin in lipid homeostasis. In cell lines, curcumin increased the level of cAMP, activated the transcription factor CREB and the human CD36 promoter via a sequence containing a consensus CREB response element. Regulatory effects of HFD and Cur on gene expression were observed in liver, less in skeletal muscle and not in brain. Since the cAMP/protein kinase A (PKA)/CREB pathway plays an important role in lipid homeostasis, energy expenditure, and thermogenesis by increasing lipolysis and fatty acid β-oxidation, an increase in cAMP levels induced by curcumin may contribute to its hypolipidemic and anti-atherosclerotic effects. © 2016 BioFactors, 43(1):42-53, 2017.

Absolute Amounts and Status of the Nrf2-Keap1-Cul3 Complex within Cells

The transcription factor Nrf2 (NF-E2-related-factor 2) is essential for the oxidative and electrophilic stress responses. Keap1 (Kelch-like-ECH-associated-protein 1), an adaptor for a cullin-3 (Cul3)-based ubiquitin ligase, regulates Nrf2 activity through proteasomal degradation, and acts as a sensor for oxidative and electrophilic stresses. The Keap1-Cul3 complex is a critical regulator of the cellular Nrf2 level, and yet quantitative information regarding their endogenous intracellular concentrations in homeostatic conditions and during stress responses is unknown. We analyzed the absolute amounts of the Nrf2, Keap1, and Cul3 proteins in five murine cell lines by comparison with serial dilutions of purified recombinant protein standards in combination with quantitative immunoblot analyses. In the basal state, the amount of Nrf2 was maintained at lower levels than those of Keap1 and Cul3 proteins, whereas the electrophilic agent diethylmaleate dramatically increased Nrf2 to a level greater than that of Keap1 and Cul3, resulting in the accumulation of Nrf2 in the nucleus. In contrast, Keap1 and Cul3 did not display any changes in their abundance, subcellular localization, or interaction in response to electrophilic stimuli. Our results demonstrate that the regulation of the Nrf2 protein level during stress responses is mediated by the activity but not the composition of the Nrf2-Keap1-Cul3 complex.

Nrf2 suppresses macrophage inflammatory response by blocking proinflammatory cytokine transcription

Nrf2 (NF-E2-related factor-2) transcription factor regulates oxidative/xenobiotic stress response and also represses inflammation. However, the mechanisms how Nrf2 alleviates inflammation are still unclear. Here, we demonstrate that Nrf2 interferes with lipopolysaccharide-induced transcriptional upregulation of proinflammatory cytokines, including IL-6 and IL-1β. Chromatin immunoprecipitation (ChIP)-seq and ChIP-qPCR analyses revealed that Nrf2 binds to the proximity of these genes in macrophages and inhibits RNA Pol II recruitment. Further, we found that Nrf2-mediated inhibition is independent of the Nrf2-binding motif and reactive oxygen species level. Murine inflammatory models further demonstrated that Nrf2 interferes with IL6 induction and inflammatory phenotypes in vivo. Thus, contrary to the widely accepted view that Nrf2 suppresses inflammation through redox control, we demonstrate here that Nrf2 opposes transcriptional upregulation of proinflammatory cytokine genes. This study identifies Nrf2 as the upstream regulator of cytokine production and establishes a molecular basis for an Nrf2-mediated anti-inflammation approach.

Nrf2 is a transcriptional activator of oxidative stress response genes. Here the authors show that Nrf2 binds to promoters of proinflammatory genes and interferes with their transcriptional upregulation in LPS-stimulated macrophages independently of its role in regulation of reactive oxygen species.

Generation of a New Model Rat: Nrf2 Knockout Rats Are Sensitive to Aflatoxin B1 Toxicity

THE TRANSCRIPTION FACTOR NRF2: (NF-E2-related-factor 2) REGULATES A BATTERY OF ANTIOXIDATIVE STRESS-RESPONSE GENES AND DETOXICATION GENES, AND NRF2 KNOCKOUT LINES OF MICE HAVE BEEN CONTRIBUTING CRITICALLY TO THE CLARIFICATION OF ROLES THAT NRF2 PLAYS FOR CELL PROTECTION HOWEVER, THERE ARE APPARENT LIMITATIONS IN USE OF THE MOUSE MODELS FOR INSTANCE, RATS EXHIBIT MORE SUITABLE FEATURES FOR TOXICOLOGICAL OR PHYSIOLOGICAL EXAMINATIONS THAN MICE IN THIS STUDY, WE GENERATED 2 LINES OF NRF2 KNOCKOUT RATS BY USING A GENOME EDITING TECHNOLOGY; 1 LINE HARBORS A 7-BP DELETION Δ7 AND THE OTHER LINE HARBORS A 1-BP INSERTION +1 IN THE NRF2 GENE IN THE LIVERS OF RATS HOMOZYGOUSLY DELETING THE NRF2 GENE, AN ACTIVATOR OF NRF2 SIGNALING, CDDO-IM, COULD NOT INDUCE EXPRESSION OF REPRESENTATIVE NRF2 TARGET GENES TO EXAMINE ALTERED TOXICOLOGICAL RESPONSE, WE TREATED THE NRF2 KNOCKOUT RATS WITH AFLATOXIN B1 AFB1, A CARCINOGENIC MYCOTOXIN THAT ELICITS GENE MUTATIONS THROUGH BINDING OF ITS METABOLITES TO DNA AND FOR WHICH THE RAT HAS BEEN PROPOSED AS A REASONABLE SURROGATE FOR HUMAN TOXICITY INDEED, IN THE NRF2 KNOCKOUT RAT LIVERS THE ENZYMES OF THE AFB1 DETOXICATION PATHWAY WERE SIGNIFICANTLY DOWNREGULATED SINGLE DOSE ADMINISTRATION OF AFB1 INCREASED HEPATOTOXICITY AND BINDING OF AFB1-N7-GUANINE TO HEPATIC DNA IN NRF2 KNOCKOUT RATS COMPARED WITH WILD-TYPE NRF2 KNOCKOUT RATS REPEATEDLY TREATED WITH AFB1 WERE PRONE TO LETHALITY AND CDDO-IM WAS NO LONGER PROTECTIVE THESE RESULTS DEMONSTRATE THAT NRF2 KNOCKOUT RATS ARE QUITE SENSITIVE TO AFB1 TOXICITIES AND THIS RAT GENOTYPE EMERGES AS A NEW MODEL ANIMAL IN TOXICOLOGY.

Unique cistrome defined as CsMBE is strictly required for Nrf2-sMaf heterodimer function in cytoprotection

Nrf2-small Maf (sMaf) heterodimer is essential for the inducible expression of cytoprotective genes upon exposure to oxidative and xenobiotic stresses. While the Nrf2-sMaf heterodimer recognizes DNA sequences referred to as the antioxidant/electrophile responsive element (ARE/EpRE), we here define these DNA sequences collectively as CNC-sMaf binding element (CsMBE). In contrast, large and small Maf proteins are able to form homodimers that recognize the Maf recognition element (MARE). CsMBE and MARE share a conserved core sequence but they differ in the 5'-adjacent nucleotide neighboring the core. Because of the high similarity between the CsMBE and MARE sequences, it has been unclear how many target binding sites and target genes are shared by the Nrf2-sMaf heterodimers and Maf homodimers. To address this issue, we introduced a substitution mutation of alanine to tyrosine at position 502 in Nrf2, which rendered the DNA-binding domain structure of Nrf2 similar to Maf, and generated knock-in mice expressing the Nrf2(A502Y) mutant. Our chromatin immunoprecipitation-sequencing analyses showed that binding sites of Nrf2(A502Y)-sMaf were dramatically changed from CsMBE to MARE in vivo. Intriguingly, however, one-quarter of the Nrf2(A502Y)-sMaf binding sites also bound Nrf2-sMaf commonly and vice versa. RNA-sequencing analyses revealed that Nrf2(A502Y)-sMaf failed to induce expression of major cytoprotective genes upon stress stimulation, which increased the sensitivity of Nrf2(A502Y) mutant mice to acute acetaminophen toxicity. These results demonstrate that the unique cistrome defined as CsMBE is strictly required for the Nrf2-sMaf heterodimer function in cytoprotection and that the roles played by CsMBE differ sharply from those of MARE.

NRF2 Is a Key Target for Prevention of Noise-Induced Hearing Loss by Reducing Oxidative Damage of Cochlea

Noise-induced hearing loss (NIHL) is one of the most common sensorineural hearing deficits. Recent studies have demonstrated that the pathogenesis of NIHL is closely related to ischemia-reperfusion injury of cochlea, which is caused by blood flow decrease and free radical production due to excessive noise. This suggests that protecting the cochlea from oxidative stress is an effective therapeutic approach for NIHL. NRF2 is a transcriptional activator playing an essential role in the defense mechanism against oxidative stress. To clarify the contribution of NRF2 to cochlear protection, we examined Nrf2^–/– mice for susceptibility to NIHL. Threshold shifts of the auditory brainstem response at 7 days post-exposure were significantly larger in Nrf2^–/– mice than wild-type mice. Treatment with CDDO-Im, a potent NRF2-activating drug, before but not after the noise exposure preserved the integrity of hair cells and improved post-exposure hearing levels in wild-type mice, but not in Nrf2^–/– mice. Therefore, NRF2 activation is effective for NIHL prevention. Consistently, a human NRF2 SNP was significantly associated with impaired sensorineural hearing levels in a cohort subjected to occupational noise exposure. Thus, high NRF2 activity is advantageous for cochlear protection from noise-induced injury, and NRF2 is a promising target for NIHL prevention.

The Mediator Subunit MED16 Transduces NRF2-Activating Signals into Antioxidant Gene Expression

The KEAP1-NRF2 system plays a central role in cytoprotection. NRF2 is stabilized in response to electrophiles and activates transcription of antioxidant genes. Although robust induction of NRF2 target genes confers resistance to oxidative insults, how NRF2 triggers transcriptional activation after binding to DNA has not been elucidated. To decipher the molecular mechanisms underlying NRF2-dependent transcriptional activation, we purified the NRF2 nuclear protein complex and identified the Mediator subunits as NRF2 cofactors. Among them, MED16 directly associated with NRF2. Disruption of Med16 significantly attenuated the electrophile-induced expression of NRF2 target genes but did not affect hypoxia-induced gene expression, suggesting a specific requirement for MED16 in NRF2-dependent transcription. Importantly, we found that 75% of NRF2-activated genes exhibited blunted inductions by electrophiles in Med16-deficient cells compared to wild-type cells, which strongly argues that MED16 is a major contributor supporting NRF2-dependent transcriptional activation. NRF2-dependent phosphorylation of the RNA polymerase II C-terminal domain was absent in Med16-deficient cells, suggesting that MED16 serves as a conduit to transmit NRF2-activating signals to RNA polymerase II. MED16 indeed turned out to be essential for cytoprotection against oxidative insults. Thus, the KEAP1-NRF2-MED16 axis has emerged as a new regulatory pathway mediating the antioxidant response through the robust activation of NRF2 target genes.

Characterizations of Three Major Cysteine Sensors of Keap1 in Stress Response

The Keap1-Nrf2 system plays a central role in cytoprotection against electrophilic/oxidative stresses. Although Cys151, Cys273, and Cys288 of Keap1 are major sensor cysteine residues for detecting these stresses, it has not been technically feasible to evaluate the functionality of Cys273 or Cys288, since Keap1 mutants that harbor substitutions in these residues and maintain the ability to repress Nrf2 accumulation do not exist. To overcome this problem, we systematically introduced amino acid substitutions into Cys273/Cys288 and finally identified Cys273Trp and Cys288Glu mutations that do not affect Keap1's ability to repress Nrf2 accumulation. Utilizing these Keap1 mutants, we generated stable murine embryonic fibroblast (MEF) cell lines and knock-in mouse lines. Our analyses with the MEFs and peritoneal macrophages from the knock-in mice revealed that three major cysteine residues, Cys151, Cys273, and Cys288, individually and/or redundantly act as sensors. Based on the functional necessity of these three cysteine residues, we categorized chemical inducers of Nrf2 into four classes. Class I and II utilizes Cys151 and Cys288, respectively, while class III requires all three residues (Cys151/Cys273/Cys288), while class IV inducers function independently of all three of these cysteine residues. This study thus demonstrates that Keap1 utilizes multiple cysteine residues specifically and/or collaboratively as sensors for the detection of a wide range of environmental stresses.

The emerging role of Nrf2 in mitochondrial function

The transcription factor NF-E2 p45-related factor 2 (Nrf2; gene name NFE2L2) allows adaptation and survival under conditions of stress by regulating the gene expression of diverse networks of cytoprotective proteins, including antioxidant, anti-inflammatory, and detoxification enzymes as well as proteins that assist in the repair or removal of damaged macromolecules. Nrf2 has a crucial role in the maintenance of cellular redox homeostasis by regulating the biosynthesis, utilization, and regeneration of glutathione, thioredoxin, and NADPH and by controlling the production of reactive oxygen species by mitochondria and NADPH oxidase. Under homeostatic conditions, Nrf2 affects the mitochondrial membrane potential, fatty acid oxidation, availability of substrates (NADH and FADH2/succinate) for respiration, and ATP synthesis. Under conditions of stress or growth factor stimulation, activation of Nrf2 counteracts the increased reactive oxygen species production in mitochondria via transcriptional upregulation of uncoupling protein 3 and influences mitochondrial biogenesis by maintaining the levels of nuclear respiratory factor 1 and peroxisome proliferator-activated receptor γ coactivator 1α, as well as by promoting purine nucleotide biosynthesis. Pharmacological Nrf2 activators, such as the naturally occurring isothiocyanate sulforaphane, inhibit oxidant-mediated opening of the mitochondrial permeability transition pore and mitochondrial swelling. Curiously, a synthetic 1,4-diphenyl-1,2,3-triazole compound, originally designed as an Nrf2 activator, was found to promote mitophagy, thereby contributing to the overall mitochondrial homeostasis. Thus, Nrf2 is a prominent player in supporting the structural and functional integrity of the mitochondria, and this role is particularly crucial under conditions of stress.