Targeting of Nuclear Factor Erythroid 2‐Related Factor 2 (NRF2) Signaling Pathway in Age‐Related Eye Diseases: Molecular Mechanisms and Opportunities for therapy

ABSTRACT

Age‐related eye diseases (AREDs), such as diabetic retinopathy (DR), cataract, glaucoma, age‐related macular degeneration (AMD), presbyopia, and represent the primary sources of vision impairment globally. Numerous investigations have indicated that the onset and progression of these conditions are significantly linked to oxidative stress (OS) affecting the eye. The Keap1‐Nrf2‐ARE signaling pathway is a well‐established mechanism that defends the body against OS and inflammation. This pathway is also implicated in the advancement of AREDs. However, the contribution of Nrf2 is likely to be disease‐context dependent, and in multifactorial conditions such as glaucoma and dry eye disease, OS should be viewed as one interacting mechanism among several pathogenic processes rather than the sole or dominant driver. Nuclear factor (erythroid‐derived 2)‐like 2 (Nrf2) serves as a key modulator of numerous life processes, playing a critical role in antioxidant mechanisms, anti‐inflammatory activities, antifibrotic responses, and cancer development. This review emphasizes the possible role of Nrf2 in the onset and progression of AREDs. Additionally, it explores various Nrf2 activators, encompassing noncoding RNAs and external substances, that regulate Nrf2 expression via distinct pathways within ocular disease models and eye cells, thereby safeguarding them from harmful alterations. However, most evidence supporting Nrf2‐targeted interventions in age‐related eye diseases remains preclinical, and important issues including disease‐specific ocular delivery, target engagement in human tissues, long‐term safety, and the potential risks of chronic Nrf2 activation must be addressed before clinical translation can be considered feasible. Consequently, Nrf2 may represent a significant target for the safeguarding of ocular cells against assorted stressors and the prevention of ocular ailments.

Age‐related eye diseases (AREDs), such as diabetic retinopathy (DR), cataract, glaucoma, age‐related macular degeneration (AMD), presbyopia, and represent the primary sources of vision impairment globally. Numerous investigations have indicated that the onset and progression of these conditions are significantly linked to oxidative stress (OS) affecting the eye. The Keap1‐Nrf2‐ARE signaling pathway is a well‐established mechanism that defends the body against OS and inflammation. This pathway is also implicated in the advancement of AREDs. Nuclear factor (erythroid‐derived 2)‐like 2 (Nrf2) serves as a key modulator of numerous life processes, playing a critical role in antioxidant mechanisms, anti‐inflammatory activities, antifibrotic responses, and cancer development. This review emphasizes the possible role of Nrf2 in the onset and progression of AREDs. Additionally, it explores various Nrf2 activators, encompassing noncoding RNAs and external substances, that regulate Nrf2 expression via distinct pathways within ocular disease models and eye cells, thereby safeguarding them from harmful alterations. Consequently, Nrf2 may represent a significant target for the safeguarding of ocular cells against assorted stressors and the prevention of ocular ailments.

1. Introduction

The predominant global contributors to blindness include age‐related eye diseases (AREDs) such as diabetic retinopathy (DR), presbyopia, cataracts, age‐related macular degeneration (AMD), dry eyes, and glaucoma [1]. Although the female‐to‐male ratio differs across these eye diseases, women constitute a majority among individuals who are blind or visually impaired; approximately two out of three blind persons are female. This gender disparity may be partly attributed to the greater longevity of women [2]. Both genetic and environmental factors influence the onset of these disorders. Age‐related vision loss is mostly caused by structural and functional alterations in the lens and retina [3]. In contrast, hyperglycemia mainly impairs cell metabolism and causes irreversible changes in stable macromolecules in various eye compartments. The buildup of advanced glycation end products (AGEs), which are the poisonous byproducts of the nonenzymatic interaction between lipids, proteins, nucleic acids, and reducing sugars, is a common pathogenic factor implicated in both cases [4].

The endogenous formation of AGEs is augmented under conditions of impaired glucose utilization, insulin resistance, and elevated oxidative stress (OS); these elements are linked to alterations in the intracellular and extracellular environments of the vascular endothelium, as well as disruptions in the structural integrity of essential organs [4]. Several studies have demonstrated that OS in the eye plays a significant role in the onset and progression of degenerative eye lesions [5, 6]. Typically, OS follows an imbalance between the production and removal of reactive oxygen species (ROS) caused by the body's biological defense mechanisms. In exchange, OS boosts ROS production, perpetuating a damaged cycle. The damage caused by ROS to DNA, lipids, proteins, and proteins has been observed and researched in corneal disorders [7], cataract [8], retinopathies [9], glaucoma [10], and so forth.

The clear definition of nuclear factor (erythroid‐derived 2)‐like 2 (Nrf2) signaling pathways that control OS responses has been among the most inspiring discoveries on OS in recent years. By regulating the expression of several genes, Nrf2 serves as a vital controller of defensive antioxidant and anti‐inflammatory pathways, including genes encoding antioxidant enzymes, as well as a range of genes linked to diverse processes such as inflammatory responses, tumor formation, and metastasis, as well as tissue remodeling and fibrosis [11, 12, 13]. Nrf2 has been identified as having a mechanistic role in a variety of systemic diseases, including tumors, degenerative illnesses, cardiovascular, and cerebrovascular conditions, respiratory infections, and, notably, eye diseases, due to its antioxidative properties [14]. In this review, we will discuss the role of Nrf2 in AREDs pathogenesis and therapeutic targeting of this signaling pathway as a new strategy. Although OS is a recurring theme across many AREDs, it does not explain the full pathogenesis of all AREDs to the same extent. In disorders such as glaucoma and dry eye disease, mechanical, vascular, inflammatory, neurosensory, metabolic, and tissue‐specific factors interact with OS in a complex pathogenic network. Therefore, Nrf2 should be interpreted primarily as an important stress‐response and cytoprotective pathway within this broader framework, and its contribution is likely to differ according to disease type, stage, and cellular context.

Importantly, the current evidence base supporting Nrf2 as a therapeutic target in AREDs is heterogeneous in both quality and translational relevance. While many studies support a protective role of Nrf2 against OS and inflammation, much of the available evidence derives from cell‐based systems and animal models that reproduce only selected aspects of human ocular disease. In addition, changes in Nrf2 expression or downstream antioxidant markers do not necessarily establish a causal role in disease initiation or progression.

Therefore, beyond summarizing published findings, this review also considers inconsistencies across studies, the relative strength of mechanistic evidence, and the translational limitations that currently constrain clinical application.

2. Nrf2 Signaling Pathway and OS

The Keap1‐Nrf2‐ARE (Kelch‐like ECH‐Aasociating protein 1—nuclear factor erythroid 2 related factor 2—antioxidant response element) pathway is essential for controlling a thorough and protective antioxidant response. The transcription factor Nrf2 is increased during OS. It initiates a chain of events that ultimately protects the cell from oxidative harm. Nrf2 begins the transcription of antioxidant enzymes by interacting with the antioxidant response element (ARE) located within the promoter regions of its target genes [15]. Under circumstances free of OS, Kelch‐like ECH‐associated protein 1 (Keap1) retains Nrf2 in the cytosol, so promoting its proteasomal degradation as shown in Figure 1. Keap1 undergoes a conformational shift in the presence of ROS that lets Nrf2 travel to the nucleus, bind to the ARE region, and begin transcription of target genes.

Figure 1.

Possible molecular mechanisms of OS‐induced Nrf2 activation. Keap1 keeps Nrf2 sequestered in the cytosol under nonstresses circumstances, where it facilitates proteasomal breakdown of Nrf2. Cysteine residues in Keap1 are oxidized under oxidative‐stressed conditions, resulting in a disulfide link. Oxidized Keap1 separates from Nrf2, enabling Nrf2 to move into the nucleus, bind to the ARE region, and start transcription of its target genes. HO‐1, heme oxygenase‐1; NQO1, NAD(P)H quinone oxidoreductase 1.

This pathway serves as a fundamental mechanism for combating OS within the organism. In typical physiological conditions, Nrf2 interacts with Keap1 and is rendered inactive through Keap1‐mediated ubiquitination, maintaining its levels low. In response to OS, the cysteine residue in Keap1 undergoes modifications that inhibit its ubiquitin activity. Consequently, Nrf2 migrates to the nucleus, where it associates with ARE and facilitates the activation of various genes encoding antioxidant proteins [16]. Modulating this pathway has been broadly acknowledged as a novel pharmacological target for addressing numerous diseases. Recently, the significance of this pathway in the progression of AREDs has also attracted increasing interest from researchers. With its regulatory molecules interacting with proteins, the Nrf2 signaling route in cells carries out key antioxidant and anti‐inflammatory activities. Under basal conditions, Nrf2 is sequestered in the cytoplasm by Keap1, which facilitates its ubiquitination through the Cullin (Cul3)‐based E3 ligase complex and targets Nrf2 for proteasome‐dependent degradation [17].

Keap1 undergoes conformational changes that enable the transport of Nrf2 to the nucleus, where it binds to ARE regions, as soon as cellular OS happens, particularly as a result of exposure to reactive species such as superoxide anion (O2•−), hydrogen peroxide (H₂O₂), and hydroxyl radical (•OH). P62, a marker linked to cell autophagy, may alternatively cause the separation of Nrf2 from the cytoplasmic Nrf2‐Keap1‐Cul3 complex. Another mechanism is facilitated by the E3 ligase adaptor beta‐transducin repeat‐containing protein (β‐TrCP) and glycogen synthase kinase 3 (GSK‐3). GSK‐3α and β stay inactive under typical circumstances. However, active GSK‐3 phosphorylates Nrf2 in its Neh6 domain in the absence of receptor signaling [18]. Certain substances, particularly exogenous agents such as terpenoids, flavonoids, polyphenols and noncoding ribonucleic acids (RNAs), have been identified as activators or inducers of Nrf2. These substances might be essential in safeguarding ocular cells from oxidative harm, inflammation, and fibrosis [19].

3. Nrf2 Signaling in the Retina: Normal Condition Versus Dissed Condition

The retina is an intricate, stratified arrangement made up of various cellular types. In mammals, it is formed by five primary neuronal cell types along with glial cells organized into five separate layers (Figure 2). The outermost layer of the retina is known as the retinal pigment epithelium (RPE), which works collaboratively with photoreceptors [20, 21].

Figure 2.

(A) The relationship between aging and NRF2. A key factor in the “hallmarks of aging” that contribute to the onset of several age‐related disorders is the age‐related drop in NRF2 activity; (B) the telomeres are protected from OS by NRF2. Oxidative damage is reduced by NRF2‐mediated transcription of PRDX1 and other cellular antioxidant defense mechanisms, which helps to protect the telomeres. 8‐oxoG, 8‐oxoguanine; ARE, antioxidant response elements; CAT, catalase; GPX, glutathione peroxidase; GST, glutathione S‐transferase; GTP, guanosine‐5′‐triphosphate; PRDX, peroxiredoxin; sMAF, small MAF proteins; SOD, Cu–Zn superoxide dismutase.

Two categories of photoreceptors exist within the retina: cones and rods, both located in the outer nuclear layer (ONL). Cones facilitate vision during the day because they are less sensitive to light than rods and are predominantly found in the fovea, where brighter light penetrates. The quantity of rods surpasses that of cones by approximately 20 times and they are located beyond the fovea. The cell body of the horizontal cell primarily resides in the inner nuclear layer (INL) of the retina, with its axon extending into the outer plexiform layer (OPL). Horizontal cells are responsible for regulating synaptic communication between photoreceptors and bipolar cells [22].

The bipolar cells found within the INL can be categorized into two types: cone bipolar cells and rod bipolar cells, depending on their role in processing signals from cone and rod photoreceptor cells, respectively. Amacrine cells inhabit both the INL and the ganglion cell layer (GCL). An amacrine cell extends its axon into the inner plexiform layer (IPL), where it establishes synapses with bipolar and ganglion cells. The most internal layer of the retina is the GCL, predominantly consisting of ganglion cells, which serve as the terminal output neurons that convert visual signals received from bipolar and amacrine cells into impulses transmitted to the optic nerve [23].

Within this retinal microenvironment, Nrf2 serves as a key endogenous defense pathway that helps maintain redox homeostasis under both physiological and stress conditions.

Nrf2 is a critical modulator of retinal OS. The retina is a metabolically active tissue with significant oxygen demand, which can result in greater ROS generation. The retina has a crucial antioxidant defense mechanism to guard against damage caused by ROS. Nrf2 is crucial for controlling this system, especially when it is triggered by factors like inflammation, aging, and sun exposure. Nrf2 is present in several retinal cell kinds, such as ganglion, Müller, and RPE cells, in a typical retina. Nrf2 aids in the prevention of ROS buildup in the retina and the preservation of its normal function by activating the expression of several antioxidative enzymes, including glutathione (GSH)‐related enzymes, superoxide dismutase (SOD), and heme oxygenase‐1 (HO‐1) [24].

AMD is a widespread condition that usually affects people over 55 years old and is marked by gradual vision loss. The development of drusen, yellowish extracellular deposits made up of proteins and lipids that can be seen using optical coherence tomography (OCT), is a clinical sign of AMD. Excessive ROS production is a recognized risk factor for AMD, along with genetic and environmental variables [25].

DR, a frequent complication of diabetes mellitus, initially presents with hyperglycemia and impairment of the blood‐retinal barrier (BRB). The elevated glucose levels in the retina cause nicotinamide adenine dinucleotide phosphate (NADPH) to be used, which results in a higher need for NADPH and a decrease in the production of antioxidants like GSH. This leads to a buildup of ROS and OS, which hastens the development of DR [26]. The maintenance of redox homeostasis in the retina is primarily regulated by Nrf2 signaling. These disorders are linked to an imbalanced redox system, which can lead to the buildup of ROS. The decreased Nrf2 expression in AMD may contribute to the disease's imbalanced redox system. In DR, the DNA‐binding ability is diminished, which leads to the decreased activity of Nrf2. Additionally, with respect to the nuclear factor kappa B (NF‐κB) pathway, disruption of the Nrf2 pathway raises the generation of proinflammatory cytokines, which aids in the development of DR [27].

4. OS and Nrf2 in AREDs

OS frequently targets the eye. It is constantly exposed to a variety of oxidative environments, including ionizing radiation, photo‐oxidation, smoke, and different types of contaminants. The retina is a very well‐oxygenated and perfused tissue, mainly due to its significant metabolic activity. Additionally, it contains a higher concentration of polyunsaturated fatty acids than other tissues in the human body [28]. The interplay of these elements makes it susceptible to harmful effects from oxidizing agents like ROS. Consequently, OS has been linked to numerous ocular conditions, particularly AMD, cataract, DR, and glaucoma [15]. The contemporary comprehension of the functions of oxidation and the Keap1‐Nrf2‐ARE signaling pathway in these disorders is summarized in this review. Representative preclinical and translational studies related to Nrf2‐targeted approaches in age‐related eye diseases are summarized in Table 1.

Table 1.

Representative preclinical and translational studies of Nrf2‐related interventions in age‐related eye diseases.

Type of samples	Drugs/diets	Results	References
AMD
RPE of young (2 months) and old (15 months) mice	Sodium iodate	The senescent RPE exhibited elevated expressions of the Nrf2 target genes NQO1, GCLM, and HO1 in comparison to the RPE of younger mice under nonstressed conditions, indicating an age‐dependent augmentation of basal OS	[29]
Human RPE cell line ARPE‐19	NIH1 (HO1 inducer)	The higher viability of siNrf2 cells under H2O2 stress may be attributable to the fact that a 24‐h pretreatment with NIH1 greatly enhances HO1 protein levels. NIH1 pretreatment's cytoprotective benefits are not eliminated by Nrf2 silencing, which implies that NIH1 may function via an Nrf2‐independent mechanism and/or on the remaining Nrf2 content that is still present in siNrf2 cells	[30]
Cataract
C57BL/6J mice	High/low glycemic diets, regular chow ad libitum, or regular chow with 30% caloric restriction	Male mice on high or low glycemic diets experienced similar cataracts, which were also seen in 21‐month‐old male and female Nrf2−/− mice fed ad libitum or a 30% calorie restriction. The retention of nuclei throughout the cortical region of the lens and significant disruption to fiber cell architecture were shown by histological examination of 18‐month cataractous lenses	[31]
Nrf2‐/‐ mice	Glutathione, vitamin C	Although interventional trials have not shown success in preventing cataracts and some studies suggest that vitamin C supplementation by itself may have harmful effects, the exogenous antioxidant ascorbate/vitamin C plays a role in controlling the redox environment of the lens, with research indicating that diets high in vitamin C can protect against a variety of age‐related cataracts. In addition, the activity of the transcription factor Nrf2, which activates a group of antioxidant response genes, is critical for preserving lens transparency during aging.	[32]
Forty‐five Lens epithelial cells (LECs) and lens cortex samples from Aaataract ARC patients	Biliverdin reductase A (BVRA)	This research validated that BVRA expedites the transition through the G1 phase and protects against H2O2‐induced premature senescence in LECs. In conclusion, BVRA safeguards LECs from OS and cellular senescence in ARC through the conversion of BV to BR, the enhancement of HO‐1 expression, and the activation of the ERK/Nrf2 signaling pathway.	[33]
DR
C57BL/6 mice	Geniposide	GEN exhibited the ability to shield Müller cells and mice from hyperglycemia‐induced OS and inflammation, primarily relying on the Nrf2 signaling pathway mediated by GLP‐1R	[34]
Müller cells	Sulforaphane (SFN)	In the retina of STZ rats, the findings demonstrated that SFN dramatically decreased the production of pro‐inflammatory cytokines (TNF‐α, IL‐6, and IL‐1β) and increased the activity of antioxidant enzymes (GSH, SOD, and CAT). Additionally, in the damaged retina, SFN boosted the nuclear accumulation of Nrf2 and raised the levels of two key downstream antioxidants of Nrf2, HO‐1 and NQO1	[35]
Mice (STZ)	Fenofibrate	Additionally, fenofibrate decreased ROS production in diabetic retinas and enhanced the expression of Nrf2 and its target genes NQO‐1 and HO‐1. In addition, fenofibrate treatment eliminated the significant rise in retinal expression of NLRP3, Caspase‐1 p20, IL‐1β p17, and ICAM‐1 in vehicle‐treated diabetic mice.	[36]
Diabetic mouse model (STZ)	Resveratrol (RSV)	RSV may decrease OS within the retina and suppress retinal ganglion cell apoptosis by modulating the Nrf2/HO‐1 pathway, thereby mitigating the detrimental effects of excessive glucose on the retina	[37]
Healthy male SD rats (6 weeks old, body weight 220~240 g)	High‐fat and high‐glucose diet	Model rats exhibited notably increased levels of blood FBG, TG, TC, and LDL‐C, along with reduced levels of HDL‐C. Model rats exhibited elevated levels of ROS, increased caspase‐3 activity, and enhanced apoptosis. In comparison to the control group, model rats exhibited increased MDA levels and decreased activity of SOD, GPx, and T‐AOC in their aqueous humor, accompanied by reduced Keap1 and enhanced Nrf2 expression in the retina.	[38]
Glaucoma
Thirty‐two female SD rats (6–8 weeks, 200–250 g)	Eye drops metformine	Metformin's antifibrotic potential was significantly diminished by the competition of organic cation transporters (OCTs) and the suppression of the Nrf2/AMP‐activated protein kinase (AMPK). Overall, this experiment suggests that metformin enters the cells of HConFs via OCTs, which can prevent the formation of scar tissue from filtering blebs in SD rats of GFS by activating the AMPK/Nrf2 axis and downregulating inflammatory and profibrogenic markers.	[39]
Dry eye
Thirty female C57BL/6N mice (6–8 weeks of age)	Astaxanthin	Under high‐stress conditions in hCEC cells, AST can safeguard mice against BAC‐induced DED, mitigate the progression of DED, and reduce the damage resulting from OS. Through the activation of the Keap1‐Nrf2/HO‐1 signaling pathway, AST may exert a protective role against DED. Based on our investigation, AST may represent an effective therapeutic approach for DED, as it offers innovative insights into strategies for tackling the condition.	[40]
C57BL/6J male mice (64, 6 weeks old	Filtered air room (PM free)/PM2.5 room	PM2.5 elicits an inflammatory response linked to DED in corneal epithelial cells. Cells were investigated to clarify their mechanisms: ROS produced by mitochondrial dysfunction in corneal epithelial cells after exposure to PM2.5 inhibited the expression of the anti‐inflammatory protein Nrf2, leading to the activation of the inflammatory protein NF‐κB P65 and its downstream molecules, thereby inducing an inflammatory response.	[41]
Six to eight weeks old female C57BL/6 mice	Salidroside	The protective effects of salidroside were demonstrated in this study using in vitro and in vivo models of dry eye disease. Salidroside stimulates autophagy through activation of the AMPK‐Sirt1 signaling pathway, which in turn promotes the nuclear translocation of Nrf2 and ultimately reduces OS. The treatment of clinical dry eye disease may benefit from the approach suggested by our study.	[42]
Presbyopia
Forty‐five Guinea pigs aged three weeks weighing 120 g~150 g	Tert‐butylhydroquinone (TBHQ,)	The findings from RT‐PCR and Western blotting indicated that TBHQ is capable of activating Nrf2, inducing the degradation of KEAP1, and increasing the expression of the antioxidant enzyme SOD. The regulation of the KEAP1‐ Nrf2 pathway along with its downstream SOD expression may modify the antioxidant capacity of the retina and affect the progression of myopia.	[43]
Nine‐week‐old of 18 C57Black/6 (C57BL/6) mice and Sprague–Dawley (SD) rats	Oral α‐glucosyl‐hesperidin (G‐Hsd)	To determine if oral consumption of α‐glucosyl‐hesperidin (G‐Hsd), which exhibits enhanced water solubility compared to hesperetin, may postpone the emergence of presbyopia. G‐Hsd treatment preserved lens elasticity, enhanced the mRNA expression of antioxidant enzymes such as glutathione reductase and thioredoxin reductase 1 in both plasma and lens, and mitigated the onset of premature cataract symptoms in selenite‐induced cataract rat lenses. Consequently, the anti‐presbyopic properties of G‐Hsd were ascribed, at least partially, to its antioxidant characteristics. G‐Hsd signifies the inaugural oral therapeutic agent exhibiting properties advantageous in mitigating presbyopia and/or cataracts.	[44]

At a general level, the evidence linking Nrf2 dysregulation to AREDs should be interpreted with caution. In many studies, altered Nrf2 signaling is documented alongside oxidative damage, inflammation, or cellular dysfunction, but such observations remain largely correlative unless supported by knockout models, gene‐silencing experiments, or rescue studies with clear mechanistic validation. Moreover, preclinical studies often emphasize molecular and histological readouts rather than long‐term visual or functional outcomes, which limits direct extrapolation to clinical benefit. For this reason, the therapeutic promise of Nrf2 activation should be weighed against the need for stronger causal evidence and more disease‐relevant translational models.

The following disease‐specific sections integrate OS mechanisms with Nrf2‐related changes rather than discussing them as fully separate processes.

4.1. AMD

Millions of people around the world are afflicted by AMD, which is one of the major causes of blindness in the world. Dry AMD, also known as non‐neovascular AMD, makes up around 80%–85% of all cases and has a typically better visual outlook. The remaining 15%–20% of patients are affected by neovascular AMD (“wet” AMD), which is responsible for around 80% of the serious vision loss caused by AMD [45, 46]. Drusen are extracellular buildup of waste material that develop between the Bruch's membrane and the RPE. Drusen's existence is considered the clinical hallmark of AMD. Even if they first do not cause visual loss, drusen deposits increase the likelihood of getting the more severe forms of AMD [47]. These findings imply that protein oxidative changes may play a role in the development of drusen. The buildup of lipofuscin in RPE cells is another distinguishing feature of AMD. Lipofuscin, also known as the “age pigment,” is commonly seen as a sign of aging [48].

Lipofuscin granules present in the RPE are auto fluorescent aggregates that result from the sustained accumulation of nondegradable end products derived from the phagocytosis of photoreceptor outer segments throughout an individual's lifespan. Age‐related accumulation of lipofuscin can lead to RPE damage, which is consequently linked to the pathogenesis of AMD [49].

It has been documented that exposure to sunlight, UV radiation, cigarette smoke, polymorphisms in the complement system, a diet high in fat, and insufficient consumption of antioxidant‐rich foods can increase the likelihood of individuals developing AMD [50]. Donor eyes have demonstrated increased oxidative alterations of protein sulfhydryl groups, DNA, lipids, and other molecules, and numerous studies have linked AMD to persistent OS. Due to the retina's high metabolic rate compared to other tissues, ROS are released as a byproduct of metabolism. The retina is more susceptible to ROS buildup due to its high concentration of polyunsaturated fatty acids and rapid metabolism. Additionally, the macula is more vulnerable to endogenous oxygen and OS because it receives a large quantity of blood [51].

For instance, during phagocytosis, live RPE cells produce more catalase (CAT) to selectively remove H₂O₂, hence reducing ROS formation [15]. For this reason, a vital equilibrium between ROS formation and antioxidants is necessary for the smooth operation of RPE cells. With advancing age, the expression and efficacy of antioxidant enzymes diminish, resulting in a prevailing OS environment within the cell. Excessive cell death, which may lead to retinal degeneration, arises when the concentration of ROS exceeds the ability of antioxidant enzymes to mitigate the burden. Despite the abundance of antioxidant enzymes in the retina, aging causes the tissue to become more susceptible to a variety of external OS, such as cigarette smoke and UV light. With each puff, cigarette smoke is expected to release around 1015 free radicals and contribute a large number of pro‐oxidants [52].

Moreover, research indicates that smoking directly affects RPE in a manner dependent on dosage and is associated with geographic atrophy, which is a characteristic feature of the incurable dry form of AMD [15]. In conclusion, photoreceptor damage and death speed up as a result of aging, chronic OS from UV radiation or cigarette smoke, and decreased antioxidant enzyme activity, resulting in increased inflammation and drusen deposition. Since mitochondria are the main organelles that generate free radicals and ROS as a metabolic byproduct, mitochondrial malfunction is frequently associated with AMD progression. This is important because mitochondria are essential for adenosine triphosphate (ATP) synthesis and cell survival.

Beyond these oxidative mechanisms, growing evidence suggests that impaired or dysregulated Nrf2 signaling may further weaken antioxidant defense in AMD. The decreased Nrf2 expression in AMD may contribute to the disease's imbalanced redox system.

Lenox et al. concluded that the enhanced activation of the unfolded protein response (UPR) in senescent retinas results in a significant reduction of Nrf2 and its subsequent protein HO‐1. Consequently, the impaired antioxidant function is accompanied by an elevation of proinflammatory markers such as regulated upon activation, normal T cell expressed and secreted (RANTES or CCL5), indicating the substantial influence of aging on age‐related retinal diseases, including AMD and diabetic retinopathy [43, 53]. Sachdeva and colleagues research revealed that the RPE of aging mice has more Nrf2, HO‐1, and NAD(P)H: quinone oxidoreductase 1 (NQO1). But the Nrf2 pathway is unable to activate when exposed to sodium iodate. In addition to elevated superoxide buildup and malondialdehyde (MDA), aged RPE also exhibit common indicators of OS. The only way to partially restore Nrf2 signaling in aged RPE was through a conditional knockout of Keap1, the Nrf2 inhibitor [29]. Changes in Nrf2 have been linked to an increased chance of developing AMD. A specific mutation of Nrf2 at position 25129 A > C, associated with an increased risk of AMD, was identified through DNA analysis of peripheral blood lymphocytes obtained from individuals with both wet and dry forms of AMD. Possessing an A/C genotype diminished the probability of developing AMD, whereas having a C/C genotype elevated the risk of developing dry AMD. The C/C genotype proved particularly detrimental when combined with advanced age, poor dietary practices, tobacco use, and a notable family history. Furthermore, polymorphisms associated with HO‐1 (HMOX1) and HO‐2 (HMOX2), both recognized as downstream targets of Nrf2 and involved in converting harmful heme catabolism to the antioxidant biliverdin, have been shown to increase the risk of AMD in certain individuals [54]. Likewise, Synoweic and colleagues have demonstrated that the G/A genotype transition of the HMOX2 gene correlates with a heightened risk of developing dry AMD, while the A/G genotype exhibits a protective effect. The 19th nucleotide position of the HMOX1 gene is associated with a decreased risk of AMD. The G/C transversion genotype at the 19th position of the HMOX1 gene elucidates the transition from the dry to the wet form of AMD. The 19G>C‐HMOX1 and −42 + 1444A>G‐HMOX2 mutations are generally regarded as contributing to the advancement of AMD in affected individuals. The diminished protein levels of HO‐1 and HO‐2 in individuals with exudative AMD indicate the existence of mechanisms targeting their degradation. Notably, both HO‐1 and HO‐2 displayed a significantly increased concentration within the lysosome relative to their usual cytoplasmic location, which suggests an enhanced turnover rate of HO‐1 and HO‐2 in the oxidatively stressed retinal milieu. The reverse was true for young, healthy people, who had significantly greater concentrations of HO‐1 and HO‐2 in their cytoplasm. Remarkably, additional subsequent downstream Nrf2 phase II antioxidant, glutathione peroxidase (GPx), displayed no significant activity in the RPE of AMD patients [55].

Taken together, the AMD literature supports an important association between impaired antioxidant defense and altered Nrf2 signaling, but the strength of evidence varies across study types. Experimental systems such as sodium iodate injury models, cultured RPE cells, and aging mice are useful for studying oxidative injury, yet they do not fully recapitulate the chronic, multifactorial, and heterogeneous nature of human AMD. In addition, apparently divergent findings should be interpreted carefully: some studies report elevated basal expression of Nrf2‐related genes in aged tissue, whereas others describe impaired inducibility of the pathway under oxidative challenge. Rather than being mutually exclusive, these findings may indicate a compensatory increase in baseline stress signaling accompanied by reduced adaptive reserve in aging RPE. Human genetic and tissue studies strengthen the biological plausibility of Nrf2 involvement, but they remain insufficient to prove that Nrf2 dysfunction is a primary driver rather than a secondary consequence of disease progression.

4.2. Cataracts

Cataracts constitute a type of impaired vision that arises from the opacification of the lens. As a consequence of the obstruction in the light pathway, individuals experience visual impairment and blurriness. It represents the most prevalent contributor to vision impairment among individuals aged 40 and above globally, encompassing approximately 20.5 million Americans [56]. People over the age of 80 have a 50% chance of getting cataracts at some point in their lives. Three primary categories of cataracts exist: nuclear cataracts, cortical cataracts, and subcapsular cataracts. Typically, subcapsular cataracts involve the deposition of granular or fibrillary material and the disintegration of the lens fibers. The cortical region is where the radial or wedge‐shaped opacification of cortical cataracts is most pronounced. The most common kind of cataract is the nuclear cataract, which is distinguished by the progressive yellowing of the lens core. Each type of cataract displays unique associated risk factors in addition to various mechanisms [57]. Nevertheless, both aging and OS, exemplified by UV irradiation, serve as the fundamental common denominators. Nevertheless, the utilization of corticosteroids, the presence of diabetes, and lifestyle factors including malnutrition, sunlight exposure, tobacco use, and alcohol intake can elevate the probability of cataract development [58]. Presently, the sole effective intervention for cataracts is surgical, yet the swift aging of the population necessitates the development of alternative treatments for the prevention and management of this condition [59].

The reduction of GSH levels associated with aging, particularly in the lens nucleus, can result in protein oxidation, which may subsequently lead to protein mixed disulfides with glutathione (PSSG), protein aggregation, decreased crystallin, anomalies in protein solubility, and a general yellowing of the lens [60]. The decrease in protein sulfhydryl groups is theorized to be an essential mechanism involved in the advancement of nuclear cataracts. Over 90% of cysteine residues and 50% of methionine residues are either eliminated or subjected to oxidation [61]. Wei et al. were the first to propose that large quantities of protein‐mixed disulfides (PSSP) are the primary cause of protein aggregation in the lens. Furthermore, it was shown that the progressive yellowing is associated with a rising concentration of PSSP and PSSG within the lens nucleus.

Consistent with this oxidative model, several studies suggest that impaired Nrf2 signaling and altered Keap1 regulation may reduce the antioxidant capacity of the lens during aging and cataract formation.

According to von Otter and colleagues, the onset of cataracts can be predicted by specific mutations in the Nrf2 gene, but they do not always increase the risk of developing cataracts. In one study, 489 European cataract cases were examined for haplotypes and single nucleotide polymorphism at the Nrf2 and Keap1 gene loci. One Nrf2 haplotype, GAAAA, was found to be significantly correlated with the beginning of cataracts 4 years prior, indicating that it could predict the development of cataracts, While none of the identified single nucleotide polymorphisms in the Nrf2 or Keap1 genes showed a predisposition for cataract development, the presence of the haplotype allele GAAGAGGC in the Nrf2 gene delayed the need for cataract surgery by an additional 4 years [62]. In research conducted by Gao and colleagues, Nrf2 expression was found to be at its lowest in the lens epithelium of individuals aged 65–80 years, while Keap1 expression was notably elevated. The levels of mRNA yielded comparably consistent results. An analysis of Keap1 gene methylation in individuals aged 45–90 years with healthy and cataract lenses revealed a progressive demethylation trend with advancing age, increasing from 16% in the 45–65‐year‐old demographic to 39% in the 65–80‐year‐old group. This is analogous to the 42% demethylation observed in the lenses of individuals aged 65–80 years [63]. Often accompanied by a rise in the transcriptional activation of the Keap1 gene, demethylation may explain why people's Nrf2 pathway antioxidant proteins drop as they age and develop cataracts. Keap1 demethylation has been supported in diabetic cataractous lenses by other studies, and protein oxidation combined with aging can raise the risk of cataracts [64]. In a study of 21 diabetic cataractous lenses, Palsamy et al. discovered that the fragment‐1 of the cataractous lens had 20 CpG dinucleotides, while the fragment‐2 of the clear lens had 43 CpG dinucleotides. In contrast to the cataractous lens, where only 12% of the cytosine residues were methylated, 64% of the cytosine residues in the clear lens were methylated. The clear lens has greater methylation, which indicates less transcription of Keap1 genes. As their study demonstrated, greater Keap1 methylation in human lens epithelial cell lines was associated with increased ROS levels and cell death. In a recent study conducted by the same team, it was discovered that methylglyoxal, a key component of high‐sugar diets that lead to diabetic complications, can upregulate the demethylation enzyme TET1 as well as block DNA methyltransferases and Nrf2 [65]. By modifying the arginine and lysine residues of lens proteins via protein aggregation, methylglyoxal promotes the production of AGEs compounds. Due to increased proteasomal degradation of Nrf2, more UPR, and ultimately more endoplasmic reticulum (ER) stress, Keap1 demethylation causes more proteasomal destruction. The well‐known antiseizure drug valproic acid has also been connected by this group to an increased risk of cataracts in epilepsy patients and the demethylation of Keap1 [66]. The role of Nrf2 pathway genes in the development of cataracts is currently limited by research. Chandra and colleagues discovered a link between the null genotype of the glutathione S‐transferase mu 1 (GSTM1) gene and the onset of cataracts in a cohort of 131 cataract patients from northern India. Additionally, it was found that the glutathione S‐transferase theta 1 (GSTT1) gene was 20% less common in cataractous lenses [67].

Overall, the evidence implicating Nrf2 in cataractogenesis is suggestive but not yet definitive. Much of the current literature is based on genetic association studies, epigenetic analyses, and lens epithelial cell models, which collectively support a link between disturbed redox regulation and lens opacity. However, these approaches do not fully establish whether impaired Nrf2 signaling is a causal driver of cataract formation or a downstream consequence of aging, hyperglycemia, and cumulative oxidative damage. In addition, although methylation changes in Keap1 and altered Nrf2 expression are mechanistically intriguing, the clinical significance of these findings remains uncertain because prospective human studies and intervention trials directly targeting this pathway are still limited. Thus, the translational gap between molecular observations and clinically meaningful cataract prevention remains substantial.

4.3. DR

The primary contributor to the incidence of adult blindness is DR, the most common retinal vascular disease. By 2050, it is anticipated that 15 million individuals will experience the ramifications of diabetes [68]. DR is expected to soon be the major cause of visual impairment everywhere.

Hyperglycemia induces an excessive generation of mitochondrial ROS and leads to OS. Elevated levels of mitochondrial ROS initiate the poly‐ADP‐ribose polymerase (PARP) pathway, resulting in a reduction in glyceraldehyde 3‐phosphate dehydrogenase (GAPDH) activity. The subsequent decrease in GAPDH levels leads to the overactivation of four traditional mechanisms induced by hyperglycemia, namely, the polyol pathway, the protein kinase C (PKC) pathway, the AGEs pathway, and the hexosamine pathway. The inhibition of GADPH activity can be averted through the action of MnSOD [69].

These hyperglycemia‐driven oxidative abnormalities are closely linked to defective Nrf2 signaling, which appears to compromise retinal antioxidant defense in DR. In DR, the DNA‐binding ability of Nrf2 is diminished, which contributes to reduced pathway activity. In addition, disruption of the Nrf2 pathway may increase proinflammatory cytokine production through interactions with NF‐κB‐related signaling.

The diverse pathophysiology of DR relies heavily on the Nrf2‐Keap1 system. But high blood sugar levels in diabetes reduce the protection provided by Nrf2. In diabetic mice lacking Nrf2, retinal superoxide was found to be considerably higher than in wild‐type mice after 5 weeks of diabetes, as shown by a study supporting this conclusion [15]. Furthermore, diabetes lowers the concentration of the intracellular antioxidant GSH. Additionally, the enzymes involved in the glutathione redox cycle, GPx and glutathione reductase, are affected. Glutamate cysteine ligase is the rate‐limiting enzyme in the synthesis of GSH, which is a crucial component of the defense system against OS. It is widely accepted that Nrf2 is a crucial transcription factor in controlling the catalytic subunit of glutamate cysteine ligase (GCLC). A quantitative investigation of the Nrf2‐GCLC pathway in isolated retinal endothelial cells (RECs), which are the location of histopathological changes associated with diabetic retinopathy, revealed that the nuclear expression of Nrf2 and its DNA binding activity were decreased by 50% to 60% in RECs exposed to high glucose levels [70]. In diabetic rats, the interaction between Nrf2 and GCLC was reduced by 90%, which led to a marked drop in GCLC expression. This shows that higher glucose levels inhibit Nrf2 activity and cause less expression of the antioxidant gene GCLC. Studies of human donor eyes afflicted by DR revealed lower GCLC concentrations as compared to age‐matched nondiabetic control groups. Additional evidence that disruptions in Nrf2‐GCLC signaling are a key factor in the onset of DR comes from the finding that diabetic Nrf2 knockout mice had much lower retinal GSH levels than wild‐type diabetic mice [71]. The inverse correlation between glucose levels and GCLC activity is clearly demonstrated by these findings, but the precise mechanism by which diabetes affects the Nrf2‐mediated control of GSH synthesis is still unknown. This discovery is noteworthy as it delineates the pathway commencing with hyperglycemia and culminating in an impaired antioxidant defense system, especially within the retina. Low retinal GSH, which leads to an inadequate antioxidant defense system, paves the way for the onset of DR. Immunofluorescence investigations demonstrated that elevated glucose levels promoted cytosolic expression of Nrf2 as well as greater cytosolic localization of Nrf2 and Keap1 [70, 71]. In comparison to healthy control rats, diabetic rats have demonstrated elevated mRNA and protein levels of Keap1. To validate these in vivo observations, Keap1‐siRNA was introduced into RECs, preventing a glucose‐induced drop in nuclear Nrf2 buildup [72]. These findings show that the hyperglycemia condition promotes the expression of Keap1, which in turn causes more cytosolic sequestration of Nrf2. This research further implies that silencing Keap1 may have a role in facilitating the translocation of Nrf2 to the nucleus and the start of antioxidant gene transcription. The findings regarding the Nrf2‐Keap1 system in diabetes mellitus suggest that the elevated OS induced by the diabetic milieu inhibits Nrf2 from entering the nucleus and enhancing transcriptional mechanisms. Diabetes‐induced posttranslational or epigenetic alterations of retinal proteins, such as Keap1, are another explanation for the change in the way Nrf2 binds to Keap1 in diabetes. Indeed, retinal proteins have been shown to undergo ribosylation, nitration, and other posttranslational alterations in diabetes [73].

Among the AREDs discussed in this review, diabetic retinopathy is supported by comparatively stronger mechanistic evidence for Nrf2 involvement. Convergent findings from high‐glucose cell systems, diabetic animal models, Nrf2‐deficient mice, and Keap1‐silencing experiments support a functional contribution of this pathway to retinal redox imbalance. Nevertheless, important limitations remain. Commonly used models such as streptozotocin‐induced diabetes reproduce selected metabolic and vascular features of DR but do not fully mimic the chronic course and heterogeneity of human disease. Furthermore, many pharmacological agents reported to activate Nrf2 also affect inflammatory, mitochondrial, or metabolic pathways independently of Nrf2, making attribution of benefit to this pathway alone difficult. Therefore, although the evidence in DR is more mechanistically persuasive than in several other ocular disorders, it still falls short of confirming that Nrf2‐directed therapy will reliably improve long‐term clinical outcomes in patients.

4.4. Glaucoma

A group of diseases defined by retinopathy and optic neuropathy is called glaucoma. Increased intraocular pressure (IOP) brought on by decreased aqueous humor outflow is regarded as the major risk factor for glaucoma, even if the cause of the condition is still unknown. The visual loss associated with glaucoma, which is characterized by a unique pattern of visual field deficits, is caused by the apoptosis of retinal ganglion cells (RGCs), which appears to originate at the optic nerve head where the axons of the RGCs pass [74].

It should be emphasized that glaucoma is a multifactorial optic neuropathy rather than a disorder driven exclusively by OS. In addition to redox imbalance, elevated intraocular pressure, vascular dysregulation, mitochondrial dysfunction, axonal transport failure, excitotoxicity, and neuroinflammatory signaling all contribute to retinal ganglion cell injury and optic nerve degeneration. Accordingly, Nrf2 in glaucoma is best considered one protective and modulatory pathway within a broader pathogenic network, rather than a single central driver of disease onset and progression.

Oxidative DNA damage, evidenced by elevated levels of 8‐hydroxy‐2′‐deoxyguanosine, has been observed in trabecular meshwork tissues obtained from individuals diagnosed with glaucoma. Additional research revealed a correlation among oxidative damage to mitochondrial DNA, average intraocular pressure, and visual field deficits in individuals diagnosed with glaucoma [75]. OS may be seen in the retina as well as the TM. In the glaucomatous human retina and optic nerve head, there has been demonstrated an increase in the expression of proteins induced by hypoxic stress, such as hypoxia‐inducible factor‐1α and heat shock proteins [76].

Within this multifactorial setting, Nrf2 appears to function primarily as a protective stress‐response pathway rather than as a single dominant driver of glaucoma pathogenesis.

Animal models mimicking certain aspects of glaucoma pathology have shown the neuroprotective effect of the Nrf2 ARE pathway. The lack of Nrf2 makes the death of RGCs brought on by optic nerve damage worse. Nrf2 knockout mice show increased levels of OS compared to wild‐type control mice, together with neuronal and capillary deterioration [77].

The glaucoma‐related evidence also warrants careful interpretation. Most available studies rely on optic nerve injury, retinal ischemia/reperfusion, or other experimental paradigms that model acute neurodegenerative stress rather than the full spectrum of chronic human glaucoma. These models are valuable for demonstrating biological plausibility and for showing that Nrf2 deficiency can exacerbate oxidative injury, but they do not fully resolve how Nrf2 signaling behaves across different glaucoma subtypes or stages of disease. In addition, the relative contributions of intraocular pressure, vascular dysregulation, mitochondrial dysfunction, and neuroinflammation are difficult to disentangle in simplified models. As a result, the current literature supports a neuroprotective role for Nrf2 in experimental glaucoma, but the extent to which this can be translated into sustained clinical benefit remains uncertain.

4.5. Dry Eye

Dry eye is caused by a variety of factors, including inflammation, ocular surface injury, hyperosmolarity, tear film instability, and neuropathic paresthesia. The aforementioned variables are thought to raise the possibility of OS and inflammation of the ocular surface. Likewise, dry eye disease is a heterogeneous and multifactorial disorder in which OS represents only one component of the disease process. Tear film instability, hyperosmolarity, meibomian gland dysfunction, lacrimal gland impairment, epithelial barrier disruption, chronic inflammation, environmental exposure, and neurosensory abnormalities can all participate in disease initiation and persistence. In this setting, Nrf2 should be viewed as a contributory protective response pathway that may counter oxidative and inflammatory damage at the ocular surface, rather than as the sole mechanistic explanation for dry eye pathogenesis.

Furthermore, the conjunctival epithelial nitric oxide synthase levels of dry eye patients steadily increased along with the slow rise in irritation, a symptom of dry eye, and the same was true of inflammatory mediators (interleukin [IL‐1β], IL‐6, IL‐8, tumor necrosis factor‐α [TNF‐α], etc). These results point to the role of OS in the etiology of dry eye [78].

Against this oxidative and inflammatory background, Nrf2 has been proposed as a protective pathway that may help preserve ocular surface homeostasis and promote epithelial recovery.

Nrf2 knockout mice showed a shorter tear film break‐up time (TBUT) and more noticeable corneal fluorescein sodium staining than wild‐type animals in a smoke exposure model. Subsequent studies showed a marked rise in 4‐HNE and 8‐OHdG in the cornea and conjunctiva of Nrf2 knockout mice. Additionally, in a mouse model of dry eye, the use of esculetin (an Nrf2 agonist) greatly increased Nrf2 expression and upregulated downstream antioxidant genes such as HO‐1, NQO1 [NAD(P)H: NQO1], SOD1, and SOD2, thereby alleviating symptoms of dry eye. These results suggest that Nrf2 is essential for controlling the degree of OS in ocular surface tissue during dry eye and that it could be a potential therapeutic target for this disease [79].

Nrf2 promotes the corneal epithelial wound healing process by promoting cell migration. As a result, inducing Nrf2 activation might accomplish the goal of dry eye therapy by stimulating the growth of corneal epithelial cells. By suppressing persistent inflammation, pain, and encouraging the repair of corneal epithelial injury, Nrf2 can achieve the aim of dry eye treatment [80].

However, the dry eye literature is also characterized by important model‐related limitations. Common experimental systems, including smoke exposure and benzalkonium chloride‐induced injury, effectively reproduce OS and inflammatory damage at the ocular surface, but they do not encompass the full multifactorial and chronic nature of human dry eye disease. In clinical settings, dry eye is highly heterogeneous and may involve tear film instability, neurosensory abnormalities, gland dysfunction, inflammation, and environmental triggers to varying degrees. Therefore, although Nrf2 activation appears beneficial in preclinical models, further work is needed to determine which patient subgroups are most likely to benefit, what route of delivery is most appropriate, and whether long‐term activation of this pathway is both safe and effective in humans.

4.6. Presbyopia

After the age of 50, presbyopia, one of the most prevalent eye ailments, primarily affects adults. Patients lose the ability to concentrate on nearby objects because the lens becomes harder and there is a breakdown in accommodative change. Age‐associated alterations in the redox status of the lens, characterized by a reduction in the levels of reduced GSH and ascorbic acid (AsA), may contribute to the development of presbyopia and nuclear cataracts. Consequently, the regular consumption of antioxidants represents a valuable strategy for mitigating the onset of presbyopia and cataract development [44]. Compared with AMD, cataract, and DR, the direct mechanistic evidence linking Nrf2 dysregulation to presbyopia remains more limited.

Collectively, these disease‐specific observations suggest that Nrf2 contributes to multiple AREDs in a context‐dependent manner, thereby motivating interest in therapeutic modulation of this pathway.

5. Therapeutic Potential, Clinical Feasibility, and Translational Challenges of Targeting the KEAP1‐ Nrf2‐ARE Pathway in AREDs

Numerous ailments, such as cancer, neurodegenerative disorders, diabetes, reproductive system illnesses, septic kidney injury, prion disease, and osteoarthritis, have been widely identified as new pharmacological targets for the treatment of the Keap1‐Nrf2‐ARE pathway, a significant defense mechanism against OS and inflammation [18, 81, 82]. Researchers have also focused on the function of regulating this pathway in the treatment of age‐related eye illnesses since it is the primary mechanism for combating OS. The amount of Nrf2 expression in glaucoma trabecular mesh (GTM) and HTM cells was assessed by Cheng et al. [83]. The experiments below have demonstrated that the over‐expression of Nrf2 might promote cell proliferation and prevent apoptosis in both GTM and HTM, despite the fact that Nrf2 was found to be downregulated in GTM cells as compared to HTM cells. Additionally, several studies have shown that lowering ROS production and increasing Nrf2 signaling protect RGCs, retinal epithelial cells, and lenses from OS [15]. Through modulation of the Keap1‐Nrf2‐ARE pathway, several compounds have shown protective or disease‐modifying effects in preclinical models of AREDs, including sulforaphane, Calcium Dobesilate (CaD), polyphenols, alpha lipoic acid (ALA), lutein/zeaxanthin isomers (L/Zi), probucol, and coumarins (Table 2). However, for most of these agents, the currently available evidence should be regarded as preclinical proof‐of‐concept rather than established clinical efficacy (Table 2).

Table 2.

Regulatory components of Nrf2 signaling pathway in age‐related eye diseases.

Sample type/age related eye disease	Function and mechanisms	References
Calcium dobesilate
Rat model/DR	CaD enhances the expression of Nrf2 and HO‐1 in the lens while suppressing Keap1 levels in rats with diabetic cataracts, resulting in a notable reduction in the severity of lens opacity in a D‐galactose‐induced cataract rat model	[84]
Diabetic db/db mice (a diabetic mouse model)/DR	CaD significantly reduced the levels of dihydroethidium (DHE) and MDA, two markers of OS, as well as the expression of interleukin (IL)‐6, IL‐8, nuclear factor (NF)‐κB, tumor necrosis factor (TNF)‐α, and other inflammatory mediators	[85]
Wistar rats/DR	CaD can suppress alterations in the expression and organization of tight junction proteins, as well as leukocyte adhesion, to mitigate excessive vascular permeability in the retina of diabetic patients. The protective role of CaD may be associated with the suppression of p38 mitogen‐activated protein kinase (MAPK) and NF‐κB activation by mitigating oxidative/nitrosative stress.	[86]
Sulforaphane
Six‐ to 8‐month‐old wild‐type and Nrf2 knockout mice	In the Nrf2 knockout mice with MnSOD knockdown, retinal thickness was greatly increased by treatment with SFN, but this was not the case in the wild‐type mice. SFN therapy decreased morphological indicators of RPE atrophy and deterioration in both genotypes, but the improvement was not proportionate to the functional restoration.	[87]
Male Sprague–Dawley rats (STZ)/DR	In vitro, sulforaphane (SFN) mitigated inflammation and OS damage induced by high glucose in Müller cells. Furthermore, SFN demonstrated antioxidant properties, enhanced the Nrf2 pathway, and suppressed the NLRP3 inflammasome in Müller cells.	[35]
Human ARPE‐19 cells—Male Sprague–Dawley rats	Sulforaphane enhanced the activity of phase II detoxification enzymes while simultaneously inhibiting pro‐inflammatory mediators. We temporarily elevated the intraocular pressure of rats to create a model of retinal ischemia‐reperfusion (IR) injury. Pretreatment with high‐dose sulforaphane mitigated the reduction in b‐wave amplitude observed in electroretinograms, suggesting that sulforaphane may effectively maintain retinal function. Electrophoretic mobility shift assays demonstrated that these effects stem from the enhancement of the Nrf2 pathway and the suppression of the NF‐κB pathway.	[88]
Human ARPE‐19	According to the findings, SFN's protective function is exerted under blue light exposure by sustaining the redox condition associated with Nrf2, increasing autophagy, and boosting the expression of SIRT1 and PGC‐1α	[89]
Human ARPE‐19	In this model of an inflamed epithelium, the administration of vitamin D and sulforaphane also resulted in a reduction in the production of vascular endothelial growth factor (VEGF), an essential angiogenic factor, and the restoration of indicators of epithelial integrity, while concurrently diminishing levels of ROS and inhibiting the associated expression of the pro‐inflammatory cytokines interleukin‐6 and interleukin‐8 induced by transforming growth factor‐beta (TGF‐β). Remarkably, the co‐stimulation of the two chemicals amplified all assessed biological outcomes, which were mediated by the ERK 1/2 pathway instead of the expression of the vitamin D receptor.	[90]
Polyphenol
Streptozotocin (STZ)‐induced diabetic rats/DR	By virtue of its antioxidant capacity to donate hydrogen atoms to free radicals, curcumin treatment reduced the compensatory activation of the Nrf2 pathway caused by OS. The effect of keeping the Nrf2 pathway homeostasis in diabetic rats was greater when curcumin was used in conjunction with insulin as compared to insulin alone.	[91]
Esculetin
Human corneal epithelial (HCE) cell line/Dry eye	Esculetin efficiently attenuated H2O2‐induced oxidative damage in HCE cells via its antioxidant properties, which were associated with the Nrf2 signaling pathway. Also, esculetin exhibited the significant effects on improving dry eye symptoms.	[92]
Human RPE cells (ARPE‐19)/AMD	In a dose‐dependent manner, esculetin treatment markedly reduced LPS‐induced cell death brought about by apoptosis and necrosis. Esculetin decreased the expression of LPS‐induced cytokines, VEGF, TNFR, and TRAIL, while LPS induced considerable inflammation with cytokine increase in cells.	[93]
Lutein
Tricolor guinea pigs/presbyopia	The expression of smooth muscle contractile proteins was enhanced by lutein. Additionally, lutein reduced Keap1 expression while simultaneously raising the expression of Nrf2, GPx2, NQO1, and HO‐1. A decline in Nrf2 activity caused a decrease in lutein's capacity to activate antioxidant enzymes in cells, which in turn lessened its inhibitory impact on cell senescence.	[94]
63 Wistar rats	Although retinal injury was brought about by exposure to LED light, therapy with L/Z, QC, and QCG, particularly a combination of L/Z and QCG, had protective effects on the retina. The manipulation of nuclear transcription factors and neuroplasticity markers in the retinal cells of the rats may be the cause of this.	[95]
Forty‐two male rats (age: 8 weeks)	The levels of retinal Rho, Rod‐arrestin (Sag), Gnat1, NCAM, GAP43, BDNF, NGF, IGF1, Nrf2, and HO‐1 were all significantly increased by the L/Zi treatment, as was the antioxidant capacity. In contrast, the administration of L/Zi lowered the levels of GFAP and NF‐κB.	[96]
Esculeoside A
Adult Wistar male rats (STZ)/DR	The retinas of T1DM rats were enhanced by ESA in the structure of ganglionic cells and in the number of cells in the inner nuclear and plexiform layers. In addition, it decreased the amounts of malondialdehyde (lipid peroxides), vascular endothelial growth factor, interleukin‐6, tumor necrosis factor‐α, Bax, and caspase‐3 in the retina. In the retinas of the control and diabetic rats, ESA increased the levels of total glutathione, superoxide dismutase, heme‐oxygenase‐1, and Bcl2, lowered the mRNA levels of REDD1, and raised the cytoplasmic and nuclear levels of Nrf2.	[97]

Despite these encouraging findings, several translational challenges should be emphasized. First, many compounds described as Nrf2 activators have pleiotropic effects and may influence multiple signaling pathways in parallel, making it difficult to conclude that the observed protection is exclusively Nrf2‐dependent. Second, most studies evaluate biochemical, histological, or short‐term cellular endpoints rather than clinically meaningful outcomes such as visual function, disease progression, or durable tissue preservation. Third, major pharmacological questions remain unresolved, including ocular bioavailability, optimal route of administration, tissue specificity, dose‐response relationships, and long‐term safety. Consequently, while Nrf2‐targeted therapy represents a promising strategy, current evidence is not yet sufficient to define it as a validated clinical approach for AREDs.

From a clinical perspective, feasibility will depend not only on whether Nrf2 can be activated, but also on whether it can be modulated at the appropriate magnitude, in the relevant ocular compartment, and for a sufficient yet safe duration. This is particularly important because AREDs differ substantially in anatomy, accessibility, rate of progression, and therapeutic goals. Topical delivery may be more realistic for ocular surface disorders, whereas posterior segment diseases such as AMD and DR may require intravitreal, periocular, sustained‐release, or vector‐based approaches to achieve therapeutically meaningful exposure in the retina or retinal pigment epithelium. In addition, clinical benefit will need to be demonstrated by functional and disease‐relevant endpoints, including visual acuity, retinal structure, progression to advanced disease, or patient‐reported symptom improvement, rather than by antioxidant biomarkers alone. Thus, clinical feasibility should be discussed in terms of disease‐specific delivery, pharmacokinetics, target engagement, and measurable functional benefit.

Major barriers to translation also remain. These include the limited predictive value of commonly used cell and animal models, heterogeneity among patients, incomplete understanding of which cell populations should be targeted in each disease, and the lack of validated biomarkers that reliably reflect intraocular Nrf2 activation in humans. Moreover, many candidate Nrf2 activators exert pleiotropic effects, making it difficult to separate true Nrf2‐dependent benefit from broader anti‐inflammatory or metabolic actions. Therefore, the path to translation will require rigorous pharmacological characterization, improved disease‐relevant models, biomarker development, and carefully designed early‐phase clinical studies that can establish both biological activity and clinical relevance.

5.1. Novel Therapeutic of Nrf2 Signaling Pathway in AREDs: Noncoding RNAs

Important elements in the control of gene expression and the processes of disease development are non‐coding RNAs (ncRNAs), which include microRNAs (miRNAs), long non‐coding RNAs (lncRNAs), and circular RNAs (circRNAs). MicroRNAs represent a category of small noncoding RNAs (19–25 nucleotides) that regulate a wide range of cellular functions by suppressing the transcription or translation of their target genes. Conversely, lncRNAs are RNA molecules exceeding 200 nucleotides in length that possess little to no protein‐coding capacity; however, through several processes, they may control transcription in cis or trans, arrange nuclear domains, and affect RNA or protein synthesis. In recent times, noncoding RNAs have emerged as a prominent subject in scientific inquiry, including research focused on the eye. Noncoding RNAs offer a compelling avenue for mitigating OS in the diagnosis and prognosis of ocular pathologies [98, 99].

In contrast to numerous other tissues, the eye offers distinctive characteristics that facilitate the delivery of miRNA mimics or antagomirs. Non‐coding RNAs encapsulated within appropriate carriers can be directly administered into the vitreous humor; subsequently, the RNA could be conveyed to the retina and ultimately access the RPE. The ability of RPE cells to phagocytose extraneous substances is a significant characteristic in this phenomenon. Consequently, the vehicles may consist of double‐membrane encapsulated extracellular vesicles, approximately 30–100 nm in size, which are generated within donor cells and encapsulate the introduced miRNAs into vesicles; these vesicles are subsequently released from the cells via exocytosis and extracted for utilization. Another strategy might involve the transfer of microRNAs via artificial nanoparticles ranging in size from 1 to 100 nm. Although there is limited evidence of pathogenic associations between lncRNAs, particularly circRNAs, and AMD, more substantial data are available regarding miRNAs. Nevertheless, numerous ncRNAs influence mitochondrial function and the antioxidant stress response, both of which are significant in the pathogenesis of AMD [98]. Furthermore, the manipulation of the ncRNAs discussed herein may prove beneficial in addressing other pathologies linked to mitochondrial dysfunction or deficiencies in the antioxidant response. Emerging evidence indicates that lncRNAs, such as LINC00167, ZNF503‐AS1, and MEG3, are essential in the pathogenesis of AMD. These lncRNAs demonstrate either a protective role in the advancement of AMD or contribute to adverse effects on the retina, culminating in AMD pathology [100]. Liang and colleagues suggest that circZNF292, which is a downregulated circRNA observed in the anterior lens capsule of individuals with age‐related cataract (ARC), may participate in the mitigation of oxidative damage and apoptosis of lens epithelial cells through the sequestration of miR‐23b‐3p, thereby presenting a potential therapeutic target for the prevention and treatment of ARC [101].

An additional issue that deserves explicit consideration is the safety profile of sustained Nrf2 activation. In most ocular models, activation of Nrf2 is associated with reduced OS, dampened inflammation, and improved cellular survival. However, chronic or excessive activation may not always be desirable. Prolonged enhancement of cytoprotective signaling could disturb physiological redox homeostasis, blunt adaptive stress responses, alter normal cell turnover, or permit the persistence of damaged cells that would otherwise undergo clearance. These concerns are particularly relevant in AREDs, in which treatment may require repeated or long‐term administration. Moreover, evidence from nonocular systems suggests that persistent Nrf2 activation can, under some circumstances, contribute to maladaptive or even protumorigenic programs. For this reason, future therapeutic strategies should focus on context‐specific and appropriately controlled modulation of Nrf2, with particular attention to therapeutic window, treatment duration, ocular tissue specificity, and local versus systemic exposure, rather than assuming that stronger or longer activation is uniformly beneficial.

6. Conclusion and Perspective

Researchers have also discovered the negative side of Nrf2, which may be a driving force in the development of cancer, even though it has several antistress functions. Nrf2 is activated by cancer‐related mutations. Additionally, Nrf2 interacts with the ARE gene of Kruppel‐like factor 9 (Klf9) and enhances Klf9 expression when ROS exceed a critical threshold. Subsequently, Klf9 suppresses Trx reductase two expression, which amplifies the ROS cascade and eventually causes cell death. Cells with aberrant phosphatase and tensin homolog (PTEN) have elevated levels of Nrf2 and phosphoinositide 3‐kinase/protein kinase B signaling pathway (PI3K/AKT) signaling, which leads to increased proliferation and tumor formation. It is therefore necessary to establish the line between the positive and possibly negative consequences of activating Nrf2. As a transcription factor responsible for maintaining cellular homeostasis, Nrf2 exhibits a notable anti‐OS effect. Furthermore, scholarly investigation into the domain of Nrf2 has notably intensified in recent years, particularly in relation to phytochemical constituents. Numerous Nrf2 activators are currently being evaluated in clinical trials for a range of chronic conditions. Increasing evidence indicates that Nrf2 contributes to stress adaptation and disease modulation in ocular disorders; however, in several AREDs it should be regarded as one component of a broader multifactorial pathogenic landscape rather than a singular primary driver. Consequently, Nrf2 may represent a promising target for safeguarding ocular cells against diverse stresses and mitigating the development of ocular diseases. Overall, the key take‐home message is that Nrf2 is a promising but context‐dependent therapeutic target in AREDs, and its eventual clinical value will depend on precise, disease‐specific, and well‐controlled activation rather than indiscriminate pathway stimulation.

In summary, the available literature supports Nrf2 as a biologically plausible and potentially valuable therapeutic target in AREDs, but the evidence is still weighted toward preclinical research. Future studies should prioritize disease‐relevant models, standardized functional endpoints, validation in human tissues, and rigorous assessment of long‐term safety. Specific future priorities include the development of reliable ocular biomarkers of Nrf2 target engagement, improved patient stratification according to disease subtype, stage, and OS burden, and determination of the optimal timing and duration of Nrf2 activation required to achieve benefit without inducing maladaptive effects. These priorities will be essential for designing biomarker‐guided and clinically translatable trials of Nrf2‐targeted therapies in ophthalmology.

Particular attention should also be paid to distinguishing correlation from causation, clarifying inconsistencies across studies, and identifying the contexts in which Nrf2 activation is protective versus potentially maladaptive. Addressing these issues will be essential for moving the field from broad preclinical promise toward biomarker‐guided, disease‐specific, and clinically actionable Nrf2‐based interventions in routine ophthalmic practice.

Author Contributions

Fu XinYu: conceptualization, writing – review and editing, validation. Luo Yi: writing – original draft. Zhang Liu: writing – review and editing.

Ethics Statement

The authors have nothing to report.

Consent

The authors have nothing to report.

Conflicts of Interest

The authors declare no conflicts of interest.

Data Availability Statement

Data sharing not applicable to this article as no data sets were generated or analyzed during the current study.