Nrf2: The Guardian of Cellular Harmony – Unveiling Its Role in Cell Biology and Senescence

Abstract

Nrf2, a key cellular regulator, plays a complex and multifaceted role in both protecting healthy cells and potentially promoting disease progression. This chapter delves into the intricate mechanisms by which Nrf2 exerts its protective effects, including combating carcinogens, maintaining cellular integrity, and inducing controlled cell death under severe stress. However, the chapter also explores the “dark side” of Nrf2, where its activity in cancer cells can contribute to chemoresistance, adaptation, and growth, hindering effective treatment. The chapter further investigates current research avenues for harnessing Nrf2’s potential for therapeutic benefit. Strategies for both activation and inhibition are explored, highlighting the importance of context-dependent effects, balancing protection and potential harm, and minimizing off-target effects. Promising new directions, such as developing tissue-specific modulators, identifying predictive biomarkers, and combining Nrf2 modulators with other therapeutic approaches, are also discussed. By understanding Nrf2’s complex and context-dependent roles, we can pave the way for the development of safer and more effective therapeutic strategies that leverage its beneficial effects while mitigating its potential drawbacks in various diseases.

1. Introduction

“In the intricate symphony of cellular biology, Nrf2 emerges as a central conductor orchestrating the delicate balance between cellular protection and senescence. This chapter delves into the multifaceted role of Nrf2, the guardian molecule, in maintaining cellular harmony. From its pivotal function in combating oxidative stress to its intricate involvement in regulating cellular senescence, the intricate dance of Nrf2 within the cellular landscape is unveiled, offering insights into its profound significance in both health and disease.”

1.1 Overview of Nrf2 and its significance in cell biology

The transcription factor Nuclear factor-E2-related factor 2 (Nrf2) stands at the crossroads of cellular health, aging, and disease. Its role in maintaining redox balance, preventing senescence, and safeguarding against oxidative stress underscores its significance in cell biology [1]. Further research into Nrf2’s regulatory mechanisms will pave the way for innovative therapies aimed at promoting healthy aging. Aging is an inevitable process that affects all living organisms.

1.1.1 Nrf2: a guardian of cellular redox balance

Under normal conditions, Nrf2 remains sequestered in the cytoplasm by its inhibitor, Keap1. However, when cells encounter oxidative stress or electrophilic insults, Nrf2 dissociates from Keap1 and translocates to the nucleus [2]. There, it binds to antioxidant response elements (AREs) in the promoter regions of target genes, activating their transcription.

1. Target Genes: Nrf2 induces the expression of various genes, including those encoding antioxidant enzymes and phase II detoxification enzymes.

2. Cellular Senescence: Premature senescence is associated with decreased Nrf2 levels, emphasizing its importance in aging processes [3].

1.1.2 Nrf2 and age-related diseases

The significance of Nrf2 extends beyond cellular senescence:

1. Protection Against Oxidative Stress: Nrf2 activation enhances the cellular antioxidant defense system implicated in various age-related diseases (e.g., neurodegenerative disorders, cardiovascular diseases, and cancer).

2. Inflammation and Immunity: Nrf2 also influences immune cell function, contributing to tissue repair and homeostasis [4, 5].

3. Metabolism and Autophagy: Dysregulation of Nrf2 can lead to metabolic imbalances and impaired autophagic processes [2, 3].

1.2 The battle against oxidative stress: exploring cellular defense mechanisms

Oxidative stress can lead to cellular damage, DNA mutations, and the development of various diseases, including cancer. However, living organisms have evolved intricate defense mechanisms to counteract the detrimental effects of oxidative stress [6]. Non-enzymatic antioxidants such as vitamins C and E, glutathione, Carotenoids, and Flavonoids neutralize ROS directly, preventing oxidative damage to biomolecules [5]. The synergistic action of these antioxidant systems helps to mitigate the harmful effects of oxidative stress and protect cellular components from damage.

1.2.1 DNA repair pathways

Oxidative stress can induce DNA damage, leading to mutations and genomic instability. To counteract this threat, cells employ a variety of DNA repair pathways to maintain genomic integrity. Base excision repair (BER), nucleotide excision repair (NER), and mismatch repair (MMR) pathways are activated in response to oxidative DNA damage, repairing lesions and preventing the accumulation of mutations [3, 7].

1.2.2 Cellular signaling pathways

Cellular signaling pathways play a crucial role in orchestrating the cellular response to oxidative stress. Moreover, the activation of stress-responsive kinases such as mitogen-activated protein kinases (MAPKs) and phosphoinositide 3-kinase (PI3K)/Akt pathway regulates cellular processes ensuring cell survival under adverse conditions [8].

2. Nrf2 pathway activation

When Nrf2 is activated, it enters the nucleus and turns on several hundred genes, known collectively as “survival genes.” NRF2 activation decreases overall inflammation and oxidation of the body with a hormetic effect. Nutrients known to activate Nrf2 include sulforaphane from broccoli seed extract, curcumin from the turmeric plant, and carotenoids such as zeaxanthin and lutein.

Basal Regulation: Under normal physiological conditions, Keap1 targets Nrf2 for ubiquitination and subsequent degradation by the proteasome, maintaining low basal levels of Nrf2 activity [9].

Activation by Oxidative Stress: When cells encounter oxidative stress, this disrupts the interaction between Nrf2 and Keap1, leading to the stabilization and accumulation of Nrf2 in the cytoplasm.

Nuclear Translocation: It forms heterodimers with small Maf proteins and binds to antioxidant response elements (AREs) present in the promoter regions of target genes.

Transcriptional Activation: Binding of Nrf2 to AREs induces the transcription of a battery of cytoprotective genes encoding antioxidant enzymes and restores cellular redox homeostasis [9].

Pharmacological Activation: Pharmacological activation of the Nrf2 pathway has emerged as a potential therapeutic strategy for combating oxidative stress-related diseases. Nrf2 activators, including natural compounds (e.g., sulforaphane, curcumin) and synthetic compounds (e.g., bardoxolone methyl), enhance Nrf2 activity and promote cytoprotection [10, 11].

2.1 Nrf2 activation mechanisms

2.1.1 Keap1-mediated regulation

Under normal conditions, Nrf2 is sequestered in the cytoplasm by its inhibitor, Keap1, a substrate adaptor protein for a Cullin 3 (CUL3)-based E3 ubiquitin ligase [12]. This interaction promotes the ubiquitination and subsequent proteasomal degradation of NRF2, keeping its levels low under normal conditions. KEAP1 contains reactive cysteine residues that act as sensors for oxidative stress [13, 14]. When these cysteines are modified by reactive oxygen species (ROS) or electrophiles, KEAP1 undergoes a conformational change that prevents it from promoting the degradation of NRF2.

Post-Translational Modifications:

• Phosphorylation, acetylation, and ubiquitination modulate Nrf2 activity.

• These modifications affect Nrf2 stability, nuclear translocation, and DNA binding.

Coordinated Signaling Pathways [15, 16]:

• Nrf2 interacts with multiple signaling pathways, including PI3K/Akt, MAPK, and mTOR.

• These interactions fine-tune Nrf2 activity and influence cellular fate.

3. Nrf2 and antioxidant response

3.1 Role of Nrf2 in regulating antioxidant genes

Defective Nrf2 activation or downregulation of its target genes may compromise cellular antioxidant capacity and render cells more susceptible to oxidative damage and inflammatory insults [17].

Antioxidant genes regulated by Nrf2 [18, 19]

Antioxidant genes regulated by Nrf2 encompass a wide range of cytoprotective enzymes and proteins, including but not limited:

• Phase II Detoxification Enzymes: These enzymes facilitate the conjugation and elimination of electrophilic compounds and xenobiotics, thereby reducing cellular exposure to potentially harmful agents.

• Antioxidant Enzymes & Redox Regulators: Notably, genes encoding thioredoxin (TXN), thioredoxin reductase (TXNRD), and glutaredoxin (GLRX) are among the targets of Nrf2-mediated transcriptional regulation. These proteins play crucial roles in modulating cellular redox state and redox-sensitive signaling cascades.

• Stress Response Proteins: In addition to classical antioxidant enzymes, Nrf2 regulates the expression of stress response proteins that confer cellular resilience against diverse insults as heat shock proteins (HSPs).

3.2 Impact on cellular homeostasis

• Regulation of Iron Homeostasis: NRF2 is a potential modulator for orchestrating iron homeostasis and redox balance in cancer cells [20].

• Dysfunction and Disease: Studies in genetically modified mice demonstrate that increased oxidative stress due to NRF2 deficiency impairs endothelial function, reducing functional hyperemia in the brain [21].

• Regulation of Stem Cell State and Function: While NRF2 activation by tert-butylhydroquinone (t-BHQ) or sulforaphane (SFN) can suppress differentiation, it displays cell type-specific and/or stage-dependent impact on stem cell biology in response to various environmental cues [13, 21].

  For adult stem cells (ASCs), NRF2 helps maintain their self-renewal, quiescence, and regenerative capacity while protecting against ASC depletion in response to stress and aging. NRF2 controls the proliferation of self-renewing embryonic stem cells (ESCs), three germ layer differentiation and cellular reprogramming NRF2-based molecular programs may govern stem cell state and function with age, and implications of this for age-related pathologies [22].

• Stress Response: NRF2 serves as a key mediator of the cellular stress response, coordinating adaptive responses to various stressors, including oxidative stress, electrophilic stress, and proteotoxic stress [23, 24].

4. Nrf2 in detoxification processes

4.1 Nrf2: orchestrator of cellular defenses through phase II enzyme induction

Nrf2 sits at the helm of the detoxification pathway, acting as a master transcription factor for Antioxidant Response Elements (AREs) to initiate the transcription and subsequent production of these detoxifying enzymes, mounting a coordinated cellular response to the encountered stress [25].

Phase II enzyme arsenal: neutralizing the threats [26]

Phase II enzymes comprise a diverse array of biochemical warriors, each specializing in neutralizing specific toxins and harmful molecules. Key players include:

4.1.1 Glutathione S-transferases (GSTs)

Mechanism:

• The “tagging” process with GSH involves a nucleophilic attack by the thiol group of GSH on the electrophilic center of the toxin. This forms a conjugate, making the toxin less reactive and more water-soluble.

  • GSTM1: Detoxifies polycyclic aromatic hydrocarbons (PAHs) found in cigarette smoke and pollution.

  • GSTT1: Protects against aflatoxins, carcinogens present in some food products.

  • GSTP1: Plays a role in metabolizing various drugs and pesticides.

Diversity:

• Genetic variations like single nucleotide polymorphisms (SNPs) can influence individual GST activity and detoxification capacity [27].

Induction:

• Nrf2 binding to AREs in GST gene promoters leads to increased mRNA and protein expression.

• Dietary factors like cruciferous vegetables can also boost GST activity through Nrf2 activation.

4.1.2 NAD(P)H: quinone oxidoreductase 1 (NQO1)

Mechanism:

• NQO1 catalyzes a two-step reduction process using NAD(P)H as a cofactor.

• First, it converts the quinone to a semiquinone radical.

• Second, it further reduces the semiquinone to a less harmful hydroquinone.

Induction [28]:

• Nrf2 is a potent activator of NQO1 expression, offering potential therapeutic strategies in cancer prevention and treatment.

• Dietary compounds like curcumin and sulforaphane can activate NQO1 through Nrf2 pathways.

4.1.3 UDP-glucuronosyltransferases (UGTs)

Mechanism:

• UGTs use UDP-glucuronic acid as a donor molecule to attach glucuronic acid to various compounds through an O-glycosidic bond.

• This conjugation process increases the molecule’s size and polarity, rendering it inactive and water-soluble, facilitating renal or biliary excretion.

Individual Variability:

• Several genetic polymorphisms exist in UGT genes, leading to individual differences in enzyme activity.

• For example, the UGT2B7*2 allele reduces enzyme activity, potentially impacting drug response and contributing to adverse effects.

4.1.4 Heme oxygenase-1 (HO-1)

Mechanism:

• HO-1 catalyzes the degradation of heme, the iron-containing prosthetic group of hemoglobin, into Biliverdin, Iron, and Carbon monoxide

• By reducing iron release and generating beneficial metabolites, HO-1 protects against various inflammatory diseases like atherosclerosis, neurodegenerative disorders, and autoimmune diseases [29, 30].

4.2 NRF2: nature’s guardian against xenobiotic and electrophilic threats

4.2.1 Xenobiotics

Nature of the Threats [31]:

• Definition: Foreign chemicals not naturally produced by or intended for biological organisms.

• Sources:

  • Environmental pollutants (e.g., pesticides, air pollutants)

  • Dietary toxins (e.g., mycotoxins, aflatoxins)

  • Pharmaceuticals and medications

Mechanisms of Toxicity [32]:

• Direct Damage: Interfere with essential cellular processes or disrupt macromolecular structures (e.g., DNA, proteins).

• Oxidative Stress: Generate free radicals, leading to cellular damage and inflammation.

• Epigenetic Alterations: Can modify gene expression patterns, potentially contributing to disease development.

4.2.2 Electrophilic stressors

Definition: Molecules possessing an electron-deficient center, readily attracting electrons from other molecules.

Mechanisms of Stress [33]:

• Covalent Binding: React with cellular proteins, DNA, and lipids, causing functional disruptions and potential mutations.

• Oxidative Stress: Can trigger the generation of free radicals, leading to cellular damage and inflammation.

• Disruption of Cellular Signaling: Interfere with essential cell communication pathways.

Interplay of Xenobiotics and Electrophilic Stressors [34, 35]:

• Xenobiotics can be metabolized into electrophilic intermediates, amplifying their toxic effects.

• Electrophilic stressors can potentiate the toxicity of xenobiotics by increasing their reactivity or promoting free radical formation.

5. Nrf2 and cellular metabolism

Understanding the interactions between Nrf2 and cellular metabolism provides insights into the pathophysiology of metabolic disorders and offers opportunities for therapeutic interventions aimed at restoring metabolic homeostasis and improving health outcomes.

5.1 Influence of Nrf2 on metabolic pathways

• Fatty Acid Metabolism: Nrf2 regulates fatty acid metabolism by modulating the expression of genes involved in fatty acid oxidation (FAO) such as carnitine palmitoyltransferase 1 (CPT1) and acyl-CoA oxidase (ACOX), leading to increased utilization of fatty acids as an energy source [36, 37].

• Glucose Homeostasis: Activation of Nrf2 enhances glucose uptake and utilization by upregulating the expression of glucose transporters (e.g., GLUT1, GLUT3) and glycolytic enzymes (e.g., hexokinase, phosphofructokinase) [38].

• Redox Regulation of Metabolism: Nrf2 influences cellular metabolism through its antioxidant and redox regulatory functions. By enhancing antioxidant defense mechanisms, Nrf2 protects cells from oxidative damage and maintains redox balance, which is essential for proper metabolic function [36, 39].

• Metabolic Diseases: Dysregulation of Nrf2 signaling has been implicated in the pathogenesis of metabolic diseases, including obesity, type 2 diabetes, and metabolic syndrome. Impaired Nrf2 activation disrupts metabolic homeostasis, leading to metabolic dysfunction, insulin resistance, and inflammation [21, 36].

5.2 Influence of Nrf2 on mitochondrial function

Nrf2 plays a pivotal role in protecting mitochondria from oxidative stress and promoting their optimal function through multiple mechanisms:

5.2.1 Antioxidant response

Nrf2 activation upregulates antioxidant genes like superoxide dismutase (SOD) and catalase, neutralizing ROS and protecting mitochondrial membranes.

5.2.2 Mitochondrial biogenesis

Nrf2, the cellular defense chief, plays a crucial role in orchestrating this vital process, ensuring proper mitochondrial function and overall health [40]:

• Transcriptional Activation: Nrf2 binds to specific DNA sequences called Antioxidant Response Elements (AREs) in genes involved in mitochondrial biogenesis, such as PGC-1α. This activation leads to increased mRNA production for these crucial proteins.

• Mitochondrial DNA Replication: Nrf2 indirectly promotes mitochondrial DNA replication by upregulating genes involved in this process, ensuring new mitochondria have their own genetic machinery.

• Mitochondrial Import: Nrf2 regulates the import of proteins and other components necessary for mitochondrial assembly from the cytoplasm, ensuring proper construction [41].

Interplay with Other Pathways:

Nrf2 does not work in isolation. It interacts with other cellular signaling pathways to fine-tune biogenesis:

• AMPK: This energy sensor can activate Nrf2, promoting biogenesis in response to increased energy demands.

• Sirtuins: These longevity proteins can activate Nrf2, contributing to biogenesis and overall mitochondrial health [42].

5.2.3 Mitophagy

Mitophagy is a vital cellular process involved in the removal and recycling of damaged or dysfunctional mitochondria. Mitophagy plays a crucial role in maintaining mitochondrial quality control by eliminating damaged or dysfunctional mitochondria. Dysregulation of mitophagy has been implicated in various diseases, including neurodegenerative disorders, metabolic diseases, and aging, highlighting its importance in maintaining cellular homeostasis [43].

Nrf2: The Maestro of Mitophagy:

Nrf2 plays a critical role in orchestrating mitophagy through several mechanisms [44]:

• Transcriptional Activation: Nrf2 binds to AREs in genes involved in mitophagy pathways, such as Parkin, PINK1, and BNIP3, leading to increased production of these key proteins.

• Regulation of Post-Translational Modifications: Nrf2 can influence the phosphorylation and ubiquitination of proteins involved in mitophagy, facilitating their interaction and efficient targeting of damaged mitochondria.

• Mitochondrial Quality Control: Nrf2 promotes the expression of genes that monitor mitochondrial health, allowing for early detection of damage and triggering mitophagy when necessary.

5.2.4 Calcium homeostasis

Nrf2 emerges as a crucial player in maintaining calcium homeostasis, influencing intracellular and extracellular calcium levels and impacting various physiological processes [45]:

• Regulation of Calcium Channels: Nrf2 can modulate the expression and activity of calcium channels in various cell types, influencing calcium influx and intracellular levels [45].

• Endoplasmic Reticulum (ER) Stress Response: Nrf2 activation contributes to the unfolded protein response (UPR) in the ER, a stress response pathway that can influence calcium signaling and homeostasis [46].

• Mitochondrial Calcium Handling: Nrf2 indirectly impacts mitochondrial calcium uptake and release, potentially influencing energy production and cell function [43].

Impact on Health and Disease:

Understanding Nrf2’s role in calcium homeostasis sheds light on various health aspects:

• Neurodegenerative Diseases: Dysregulation of calcium signaling and Nrf2 dysfunction are implicated in Alzheimer’s and Parkinson’s diseases.

• Cardiovascular Diseases: Nrf2 may influence calcium handling in heart muscle cells, potentially impacting cardiac function and arrhythmia risk.

• Bone Health: Nrf2 might indirectly affect bone formation and resorption through its influence on calcium homeostasis and oxidative stress response [47].

6. Nrf2 in cell proliferation and differentiation

Nrf2, the master regulator of cellular defense, plays a complex and multifaceted role in both cell proliferation and differentiation.

6.1 Nrf2’s involvement in cell growth and development

Promoting Cell Growth [48]:

• Antioxidant and Anti-inflammatory Effects: By countering oxidative stress and inflammation, Nrf2 creates a favorable environment for cell growth, especially in tissues with high renewal rates like skin and intestine.

• Metabolic Regulation: Nrf2 can influence energy metabolism and nutrient availability, indirectly impacting cell growth under certain conditions.

• Interaction with Growth Factors: Nrf2 may interact with specific growth factor signaling pathways, potentially promoting cell proliferation in response to growth signals.

Modulating Cell Differentiation:

• Transcriptional Regulation: Nrf2 can regulate genes involved in differentiation pathways, either promoting or inhibiting it depending on the context.

• Cell-Type Specificity: The influence of Nrf2 on differentiation varies significantly between different cell types. For example, it can promote differentiation in embryonic stem cells but inhibit it in muscle cells.

• Oxidative Stress Response: Under high oxidative stress, Nrf2 activation may prioritize survival and stress response over differentiation, temporarily delaying the process [41].

6.2 Crosstalk between Nrf2 and other signaling pathways

Mechanisms of Crosstalk:

• Direct Protein-Protein Interactions: Nrf2 can physically interact with proteins in other pathways, influencing their activity and localization.

• Transcriptional Regulation: Nrf2 can modulate the expression of genes involved in other pathways, and vice versa.

• Post-Translational Modifications: Phosphorylation, ubiquitination, and other modifications can influence the interaction and activity of proteins in both pathways.

Overview of key signaling pathways that interact with Nrf2:

PI3K/Akt Pathway: The phosphoinositide 3-kinase (PI3K)/Akt pathway plays a crucial role in cell survival, proliferation, and metabolism. Activation of Akt can promote Nrf2 activation by phosphorylating Nrf2 or its upstream regulators, leading to increased Nrf2 stability and nuclear translocation. Conversely, Nrf2 activation can enhance Akt signaling by suppressing phosphatase and tensin homolog (PTEN) expression, thereby amplifying cell survival pathways in response to oxidative stress [49].

MAPK Pathway: The mitogen-activated protein kinase (MAPK) signaling pathway, including extracellular signal-regulated kinase (ERK), c-Jun N-terminal kinase (JNK), and p38 MAPK, regulates various cellular processes such as proliferation, differentiation, and stress responses. Crosstalk between MAPK and Nrf2 pathways occurs at multiple levels, with MAPKs influencing Nrf2 activity through phosphorylation-mediated modulation of Nrf2 stability, subcellular localization, and transcriptional activity. Conversely, Nrf2 activation can attenuate MAPK signaling by inhibiting inflammatory gene expression and ROS production [46, 50].

NF-κB Pathway: Crosstalk between Nrf2 and NF-κB pathways occurs bidirectionally, with Nrf2 activation inhibiting NF-κB signaling by suppressing pro-inflammatory gene expression and NF-κB activation. Conversely, NF-κB activation can repress Nrf2 activity by inducing the expression of Keap1 or promoting Nrf2 degradation, thus modulating the balance between pro-inflammatory and antioxidant responses [51].

HIF-1α Pathway: The hypoxia-inducible factor 1-alpha (HIF-1α) pathway regulates cellular responses to hypoxia and oxygen availability. Coordination between Nrf2 and HIF-1α pathways ensures cellular adaptation and survival under fluctuating oxygen and nutrient levels [52].

AMPK Pathway: The adenosine monophosphate-activated protein kinase activation can stimulate Nrf2 signaling by phosphorylating Nrf2 or its regulators, leading to increased Nrf2 activity and antioxidant gene expression. Conversely, Nrf2 activation can enhance AMPK signaling by promoting mitochondrial biogenesis and fatty acid oxidation, thus coordinating cellular responses to energy stress and oxidative stress [53].

7. Nrf2 and cell senescence

7.1 Unveiling the mysteries of Nrf2 and the aging cell

Nrf2, the master regulator of cellular defense, stands at the crossroads of aging and cellular health. Its influence on the aging process is intricate and multifaceted, offering both potential benefits and challenges.

The Double-Edged Sword:

• Cancer Risk: While Nrf2 protects healthy cells, its overactivation in cancer cells can promote their survival and growth. This highlights the need for a nuanced understanding of Nrf2’s role in different contexts [54].

• Hormonal Decline: Estrogen and other hormones can activate Nrf2, and their decline with age might contribute to reduced Nrf2 activity and accelerated aging [55].

• Cellular Senescence: Nrf2 can both promote and inhibit cellular senescence, a state of cell cycle arrest associated with aging. Understanding the specific mechanisms involved is crucial [1].

7.1.1 Unraveling the mechanisms: Nrf2’s fight against aging hallmarks

These mechanisms are interconnected, and Nrf2’s influence on these hallmarks of aging is likely complex and context-dependent.

7.1.1.1 DNA damage

• Antioxidant Response: Nrf2 upregulates genes encoding antioxidant enzymes like superoxide dismutase (SOD) and catalase, which neutralize free radicals that can damage DNA [56].

• DNA Repair Pathways: Nrf2 activates genes involved in DNA repair mechanisms like base excision repair and non-homologous end joining, facilitating the repair of damaged DNA strands [57].

• Modulation of Inflammatory Signaling: Nrf2 suppresses chronic inflammation, which can indirectly contribute to DNA damage through the generation of reactive species [56].

7.1.1.2 Telomere shortening

• Telomerase Regulation: While the direct role of Nrf2 in telomerase activity remains under investigation [58], some evidence suggests that it might indirectly promote telomerase expression, protecting telomeres from shortening [59, 60].

• DNA Repair Mechanisms: As aforementioned, Nrf2’s activation of DNA repair pathways can help maintain telomere integrity by repairing damage at the ends of chromosomes [57].

• Antioxidant Defense: By reducing oxidative stress, Nrf2 protects telomeres from free radical-induced damage, potentially slowing down their shortening [59, 61].

7.1.1.3 Protein misfolding

• Chaperone Induction: Nrf2 upregulates genes encoding molecular chaperones, proteins that help proteins fold correctly and prevent misfolding, a key contributor to protein aggregation and cellular dysfunction in aging [62].

• Proteasomal Activity: Nrf2 can enhance the activity of the proteasome, the cellular machinery responsible for degrading misfolded proteins, ensuring efficient protein turnover and preventing aggregation [63].

• Autophagy Activation: Nrf2 indirectly promotes autophagy, a cellular process that removes damaged proteins and organelles, further contributing to protein quality control [64].

7.1.2 The impact of aging on Nrf2 role in different organ systems

Nrf2 plays a crucial role in combating various aging hallmarks across different organ systems. However, its specific function and the impact of aging on its activity vary depending on the tissue:

Brain [65, 66]:

Aging is associated with increased oxidative stress and impaired antioxidant defense mechanisms in the brain, contributing to neurodegenerative diseases such as Alzheimer’s disease and Parkinson’s disease. Nrf2 plays a crucial role in protecting neurons from oxidative damage and promoting neuroprotection through the upregulation of antioxidant enzymes and phase II detoxification enzymes.

Liver:

The liver is a major site of metabolic and detoxification processes, making it susceptible to oxidative stress and age-related changes in Nrf2 signaling. Aging is associated with decreased Nrf2 activity and reduced capacity for detoxification and increased susceptibility to liver damage [67].

Kidneys:

Dysregulation of Nrf2 signaling in the kidneys has been implicated in the pathogenesis of age-related kidney diseases, including chronic kidney disease (CKD) and renal fibrosis [68].

Cardiovascular System:

Nrf2 plays a protective role in the cardiovascular system by upregulating antioxidant genes and inhibiting inflammatory pathways. However, aging-related decline in Nrf2 activity and expression may impair vascular function, promote endothelial dysfunction, and increase susceptibility to cardiovascular diseases [69].

Lungs:

The lungs are constantly exposed to environmental toxins and oxidative stress, making them vulnerable to age-related changes in Nrf2 signaling [70]. Aging-related decline in Nrf2 activity increases susceptibility to oxidative damage and respiratory diseases, such as chronic obstructive pulmonary disease (COPD) and pulmonary fibrosis [71].

Muscle:

Nrf2 helps maintain muscle function by promoting mitochondrial health and preventing protein misfolding. Aging can decrease Nrf2 activity in muscles, leading to sarcopenia (muscle loss) and decreased physical function [72].

Skin:

Nrf2 shields the skin from UV radiation and promotes wound healing. Aging reduces Nrf2 activity in the skin, contributing to wrinkles, age spots, and decreased wound healing capacity [73].

Immune System:

Nrf2 balances immune responses and prevents excessive inflammation. Aging can lead to dysregulation of Nrf2 in immune cells, contributing to autoimmune diseases and decreased immune function [74].

Bone:

Nrf2 regulates bone remodeling and protects against bone loss. Aging can decrease Nrf2 activity in bone cells, contributing to osteoporosis [75].

8. Nrf2 in Cancer biology

8.1 Dual role of Nrf2 in cancer as a protector and potential target

Nrf2, the cellular defense chief, plays a complex and paradoxical role in cancer. While it acts as a protector in healthy cells by shielding them from harmful stressors, it can emerge as a potential target in cancer cells, promoting their survival and resistance to therapy.

8.1.1 Nrf2’s protective shield in healthy cells: A multifaceted defense

Nrf2 plays a crucial role in safeguarding healthy cells from various stressors and preventing cancer initiation.

Maintaining Cellular Integrity:

• Inflammation Modulation: Nrf2 suppresses chronic inflammation, which can contribute to cancer development by creating a pro-tumorigenic environment. Nrf2 does this by regulating the expression of inflammatory genes and promoting the resolution of inflammation [71].

• Protein Quality Control: Nrf2 activates genes encoding molecular chaperones, proteins that help proteins fold correctly and prevent misfolding. Misfolded proteins can accumulate and damage cells, potentially contributing to cancer development. Nrf2 also promotes proteasomal activity, the cellular machinery responsible for degrading damaged and misfolded proteins, ensuring efficient protein turnover and preventing protein aggregation [74].

• Autophagy Activation: Nrf2 indirectly promotes autophagy, a cellular process that removes damaged organelles and proteins. This helps maintain cellular health and prevents the accumulation of harmful components that could contribute to cancer development [60].

Cell Death Induction:

• Under Severe Stress: When cellular damage is beyond repair, Nrf2 can switch gears and trigger programmed cell death (apoptosis) in severely damaged cells. This prevents them from accumulating mutations and potentially becoming cancerous.

8.1.2 The dark side of Nrf2 in cancer cells

While Nrf2 acts as a guardian in healthy cells, its influence on cancer cells takes a sinister turn. The mechanisms by which Nrf2 becomes a potential target in cancer, promoting their survival and resistance to therapy [76, 77]:

Fueling Cancer Cell Survival:

• Antioxidant Shield: Nrf2 activates genes encoding antioxidant enzymes, originally meant to neutralize harmful free radicals. However, in cancer cells, these enzymes can also scavenge therapeutic drugs and reduce their effectiveness. For example, Nrf2 can induce enzymes that detoxify chemotherapeutic agents, rendering them less potent [78].

• Metabolic Adaptation: Nrf2 helps cancer cells adapt to stressful environments like nutrient deprivation or hypoxia (low oxygen). This allows them to survive and continue growing even under conditions that might otherwise kill them. Nrf2 achieves this by regulating genes involved in alternative metabolic pathways, enabling cancer cells to scavenge energy and survive in harsh environments [79].

• DNA Repair Enhancement: While Nrf2-mediated DNA repair is beneficial in healthy cells, it can become detrimental in cancer. Nrf2 can enhance DNA repair mechanisms in cancer cells, allowing them to repair damage caused by chemotherapy or radiation and continue proliferating. This contributes to tumor recurrence and treatment resistance [80].

Adaptation and Growth [81, 82]:

• Proliferation Signals: Nrf2 can interact with certain growth factor pathways in cancer cells, leading to increased proliferation and tumor growth. This highlights the context-dependent nature of Nrf2’s role, where it can switch from protective to pro-tumorigenic depending on the specific cellular context.

• Angiogenesis Promotion: Nrf2 can induce genes involved in angiogenesis (formation of new blood vessels), providing cancer cells with a vital supply of nutrients and oxygen for their continued growth and expansion.

• Metastasis Support: Nrf2 might contribute to metastasis by promoting epithelial-mesenchymal transition (EMT), a process where cancer cells acquire migratory and invasive properties, enabling them to spread to other organs.

Therapeutic Strategies: Balancing the Act [83, 84]:

• Harnessing Nrf2’s Protective Potential: In healthy tissues or early-stage cancers, low-dose Nrf2 activators might offer benefits by promoting DNA repair and preventing further carcinogenesis.

• Targeting Nrf2 in Established Cancers: Developing Nrf2 inhibitors or drugs that disrupt its interaction with specific cancer-promoting pathways is being explored to overcome chemoresistance and enhance treatment efficacy.

• Personalized Medicine: Understanding individual variations in Nrf2 activity and genetic makeup is crucial for tailoring therapeutic strategies to maximize benefits and minimize risks associated with targeting Nrf2 in cancer.

8.2 Overview of research on Nrf2 and chemoresistance

Research provided a rationale for targeting Nrf2 signaling as a therapeutic strategy to overcome chemoresistance and improve treatment outcomes in cancer patients [85].

8.2.1 Mechanisms of Nrf2-mediated chemoresistance

Nrf2-mediated metabolic reprogramming can also contribute to chemoresistance by altering cellular metabolism, promoting glycolysis, and enhancing cellular antioxidant capacity, thereby protecting cancer cells from chemotherapy-induced oxidative stress and apoptosis [86].

8.2.2 Clinical relevance and prognostic significance

Clinical studies have shown that patients with high Nrf2 expression or Nrf2 activation have lower response rates to chemotherapy, shorter progression-free survival, and reduced overall survival compared to patients with low Nrf2 expression [84].

8.2.3 Therapeutic targeting of Nrf2 signaling

Strategies to inhibit Nrf2 signaling have been explored as a means to overcome chemoresistance and enhance the efficacy of chemotherapy in cancer treatment.

Small molecule inhibitors of Nrf2 or its upstream regulators, such as Keap1 or glycogen synthase kinase 3β (GSK-3β), have been developed and investigated for their ability to sensitize cancer cells to chemotherapy and overcome drug resistance [87].

9. Therapeutic potential of Nrf2 modulation

Deeper scientific research into the molecular mechanisms of Nrf2 signaling and the development of Nrf2-targeted therapeutics may pave the way for novel treatments for oxidative stress-related diseases and contribute to improved health outcomes and quality of life.

9.1 Potential strategies for targeting Nrf2 in disease treatment

Nrf2’s complex role in both protecting healthy cells and potentially promoting disease progression presents a unique challenge for therapeutic development.

Activation Strategies: Unlocking Potential Benefits

Activating Nrf2 holds promise for treating various diseases by leveraging its protective and reparative functions in cells.

Natural Compounds:

• Dietary Sources: Fruits and vegetables rich in phytochemicals like sulforaphane (broccoli, Brussels sprouts), curcumin (turmeric), resveratrol (grapes, red wine), and epigallocatechin gallate (green tea) can activate Nrf2 through various mechanisms.

• Marine-Derived Compounds: Certain marine compounds like fucoidan (found in brown algae) and astaxanthin (microalgae) show promise in activating Nrf2 and offering potential benefits in neurodegenerative diseases and age-related decline. Ongoing research is exploring their efficacy and safety profiles.

Nrf2 Activators:

• Drug Development: Some activators like bardoxolone methyl and dimethyl fumarate are undergoing clinical trials for specific diseases like chronic kidney disease and multiple sclerosis.

• Specificity Considerations: Designing Nrf2 activators with tissue-specific or disease-specific activity is a major focus to minimize potential off-target effects.

Gene Therapy:

• Future Potential: In rare cases with severe Nrf2 dysfunction due to genetic mutations, future gene therapy approaches might explore delivering functional Nrf2 genes directly into cells.

9.2 Discussion on current challenges and future directions

Challenges and considerations:

9.2.1 Context-dependent effects

• Disease Specificity: Nrf2’s role can be vastly different depending on the disease. For instance, it might be protective in neurodegenerative diseases but contribute to chemoresistance in cancer.

• Disease Stage: Nrf2’s role can even change within the same disease depending on its stage. In early-stage cancer, it might offer some protection, while in advanced stages, it might promote tumor growth.

• Individual Genetic Variations: Genetic variations in Nrf2 itself or its regulatory pathways can significantly influence its activity and response to modulators.

9.2.2 Balancing protection and harm

• Preserving Nrf2’s Protective Roles: Inhibiting Nrf2 completely can disrupt its essential functions in healthy tissues, potentially increasing susceptibility to oxidative stress, inflammation, and other health problems.

• Understanding Off-Target Effects: Careful evaluation of potential off-target effects and long-term safety is essential before clinical application.

• Developing Tissue-Specific Modulators: Designing Nrf2 modulators with activity restricted to specific tissues or disease-relevant pathways holds promise for minimizing off-target effects and maximizing therapeutic benefits.

9.2.3 Off-target effects

• Direct Nrf2 Inhibition: Directly targeting Nrf2 protein can inadvertently affect its interactions with other proteins involved in various cellular processes, potentially leading to unforeseen side effects.

• Upstream and Downstream Effects: Targeting upstream regulators or downstream pathways of Nrf2 can also have unintended consequences due to the complex network of interactions within the cell.

• Preclinical Testing and Personalized Medicine: Rigorous preclinical testing and personalized medicine approaches that consider individual genetic variations and disease contexts can help mitigate off-target effects and identify patients who might benefit most from specific Nrf2 modulation strategies.

9.2.4 Combining Nrf2 modulators with other therapies

• Synergistic Effects: Combining Nrf2 modulators with other established therapies, such as chemotherapy or radiation for cancer, or immune checkpoint inhibitors, could offer synergistic effects and improve treatment outcomes.

• Overcoming Resistance: Combining Nrf2 modulators with strategies that target resistance mechanisms in diseases like cancer can help overcome current therapeutic challenges and improve patient survival.

• Tailored Combinations: Personalized combinations based on individual genetic profiles, disease stage, and other factors can optimize treatment efficacy and minimize potential side effects [1, 85].

Acknowledgments

The authors acknowledge the use of Grammarly and ProWritingAid tools for language polishing of the manuscript.