Ferroptosis in eye diseases: a systematic review

Abstract

Ferroptosis is a type of iron-dependent cell death that differs from apoptosis, necroptosis, autophagy, and other forms of cell death. It is mainly characterized by the accumulation of intracellular lipid peroxides, redox imbalance, and reduced levels of glutathione and glutathione peroxidase 4. Studies have demonstrated that ferroptosis plays an important regulatory role in the occurrence and development of neurodegenerative diseases, stroke, traumatic brain injury, and ischemia-reperfusion injuries. Multiple mechanisms, such as iron metabolism, ferritinophagy, p53, and p62/Keap1/Nrf2, as well as the combination of FSP1/CoQ/NADPH and hepcidin/FPN-1 can alter the vulnerability to ferroptosis. Nevertheless, there has been limited research on the development and management of ferroptosis in the realm of eye disorders, with most studies focusing on retinal conditions such as age-related macular degeneration and retinitis pigmentosa. This review offers a thorough examination of the disruption of iron homeostasis in eye disorders, investigating the underlying mechanisms. We anticipate that the occurrence of ferroptotic cell death will not only establish a fresh field of study in eye diseases, but also present a promising therapeutic target for treating these diseases.

Introduction

Cell death is essential for normal development, the maintenance of homeostasis, and the prevention of many diseases. Ferroptosis is a newly identified form of controlled cell death that relies on iron and was identified in 2012 by Dr. Brent R. Stockwell’s laboratory [1]. Previous studies have shown that ferroptosis exhibits distinct characteristics in terms of cellular architecture, biochemical properties, and regulatory mechanisms compared to other forms of regulated cell death, including apoptosis, necroptosis, and autophagy [2–8]. It is possible to stop cells from dying of ferroptosis with iron-chelating and anti-lipid peroxidation chemicals, but not with caspase inhibitors [9]. In addition, ferroptosis can be visually differentiated from other forms of cell death by the presence of intracellular lipid peroxide accumulation, redox imbalance, mitochondrial membrane shrinkage, and increased density [10]. Various biological processes, such as iron metabolism, ferritinophagy, cell adhesion, VDAC2/3, glutathione peroxidase 4 (GPx-4), heat shock protein beta-1 (HSPB-1), nuclear factor E2-related factor 2 (Nrf 2), nuclear receptor coactivator 4 (NCOA4), NADPH oxidase (NOX), p53, and SLC7A11, could alter the susceptibility to ferroptosis [2]. Thus, by studying the morphology, biochemistry, and key regulators of ferroptosis in disease tissues, a new area of research and potentially innovative therapeutic targets can be established.

Recent studies have shown that ferroptosis has a significant regulatory function in the onset and progression of various diseases, such as neurodegenerative diseases, stroke, traumatic brain injury, ischemia-reperfusion-associated kidney, heart, and liver injuries, as well as liver fibrosis and acute kidney injuries [3]. Ferroptosis has been a prominent field of study, attracting considerable attention and interest in the advancement of therapeutic approaches and the improvement of prognoses for these associated disorders. In addition, the research of iron balance disruption and ferroptosis is also gaining increasing attention as a potential underlying component for eye disorders. This review provides a comprehensive overview of the most recent advancements in the understanding of iron homeostasis and ferroptosis in various eye disorders. We believe that ferroptosis presents a novel field for the research of certain eye disorders and may provide a new therapeutic direction for these eye diseases.

Iron homeostasis overview

Iron is a crucial element in cellular metabolism, and when there is too much iron or it is not regulated properly, it can contribute to the development of several disorders [11]. Wong et al. [12] and He et al. [13] have provided a comprehensive analysis of iron homeostasis in recent reviews on iron toxicity. Ferritin, which is capable of storing intracellular iron, is a peptide consisting of H-ferritin (21 kDa) and L-ferritin (19.5 kDa) [14]. H-ferritin possesses a ferroxidase enzyme that enables it to efficiently convert ferrous iron into ferric iron. Iron is required for the execution of the citric acid cycle, the generation of adenosine triphosphate (ATP), and as a vital constituent of the rate-limiting enzyme in DNA synthesis [13]. Nevertheless, the presence of excessive iron within the cells can be harmful as it leads to the generation of reactive oxygen species (ROS) through the Fenton and Haber-Weiss reactions. This reaction involves the oxidation of ferrous iron (Fe²⁺) to ferric iron (Fe³⁺) by hydrogen peroxide (H₂O₂). As a result, a hydroxyl ion (OH-) and the hazardous hydroxyl radical (OH•) are generated. These radicals have the potential to induce oxidative harm to lipids, DNA, and proteins. Cellular lipid peroxidation has the potential to trigger ferroptosis [15, 16]. Several prior investigations have suggested that iron has a role in the process of human aging. Timmers et al. [17] and Daghlas et al. [18] have documented a correlation between genes involved in haem metabolism and life expectancy. Hence, maintaining a state of equilibrium in iron levels is crucial to ensure sufficient iron supply for cellular processes while preventing the harmful effects of excessive iron.

Cellular iron homeostasis is regulated by an iron regulatory protein (IRP) which modulates the expression of associated proteins and oversees processes such as iron uptake, storage, release, and utilization [19]. When intracellular labile iron levels are depleted, IRP binds to iron-responsive elements (IREs) on messenger RNAs (mRNAs) to regulate it. The IRPs attach to the IRE located at the 5′ end of ferritin mRNA and impede the process of translation. Insufficient iron levels in the body result in the IRP binding to the IRE located at the 3′ end of transferrin mRNA. This inhibits the degradation of mRNA and elevates the protein concentration [13]. Consequently, the levels of iron are carefully controlled, and disruption of iron balance is being increasingly studied as a potential factor in the development of many diseases.

Ferroptosis mechanism

Ferroptosis is a type of cellular death triggered by the buildup of iron and lipid ROS within the cell. The primary features of ferroptosis include (1) the acceleration of cell death through the accumulation of intracellular ROS and iron overload; (2) the depletion of GSH, and GPx-4, and the occurrence of lipid peroxidation; and (3) the suppression of cell death through the use of scavengers that target lipid ROS and iron chelators [11, 20].

The initiation of ferroptosis is caused by intracellular oxidation. Transferrin promotes the transport of Fe³⁺ from outside the cell to the inside by specifically detecting and attaching to transferrin receptor 1 (TFR1) found on the cell membrane. Through the action of an iron oxide reductase called six-transmembrane epithelial antigen of the prostate 3 (STEAP3), Fe³⁺ is converted to Fe²⁺ and then stored in the endosome [21]. During cellular stress, a significant quantity of Fe²⁺ is released. This Fe²⁺ can then undergo a reaction with hydrogen peroxide, which is produced from the continuous metabolism of the mitochondria. This reaction is known as the Fenton reaction [2]. The Fenton reaction produces hydroxyl free radicals, which trigger a complex chain reaction involving ROS that carries lipids. This process results in the generation of a large amount of ROS [22, 23]. Oxidative stress occurs when there is an imbalance between the production and elimination of ROS, resulting in the creation of free radicals that have the potential to harm DNA, proteins, and lipids. This process is regulated by GPx-4 [24] or ferroptosis suppressor protein 1 (FSP1) [25, 26]. GPx-4 is an important antioxidant enzyme that reduces lipid hydroperoxides (L-OOH) to non-reactive lipid alcohols (L-OH) [27], protecting cell membranes from lipid peroxidation damage. When GPx-4 function is inhibited or its expression level decreases, lipid hydroperoxides within the cell cannot be effectively eliminated, leading to their accumulation. This accumulation triggers severe oxidative stress, ultimately causing cell membrane destruction and cell death. Small molecule inhibitors like Ras Selective Lethal 3 (RSL3) specifically inhibit GPx-4 activity, effectively inducing ferroptosis [24]. These findings highlight the central role of GPx-4 in regulating ferroptosis and provide new therapeutic targets to modulate this form of cell death.

During ferroptosis, the expression level of ChaC glutathione-specific gamma-glutamylcyclotransferase 1 (CHAC1) is significantly upregulated [28]. CHAC1 is an enzyme capable of degrading glutathione (GSH), a crucial intracellular antioxidant that neutralizes free radicals and peroxides, protecting cells from oxidative damage. When CHAC1 expression is upregulated, the degradation rate of GSH increases, leading to a marked decrease in intracellular GSH levels. The decline in GSH levels weakens the cell’s antioxidant defense capacity, allowing lipid peroxides to accumulate and further drive the process of ferroptosis. The upregulation of CHAC1 is not only a cellular stress response to oxidative stress but also an important regulatory mechanism in ferroptosis, reflecting the critical role of the imbalance in the intracellular antioxidant system in this process.

Glycerophospholipids, which are acylated with at least one polyunsaturated fatty acid (PUFA) chain, serve as the primary constituents of cell membranes. PUFAs are extremely vulnerable to peroxidation. Excessive oxidation of PUFAs leads to the production of peroxidized phospholipids. The process of lipid peroxidation and the generation of hydroxyl radicals can result in harm to cell membranes, DNA, and proteins [29, 30]. GPx-4 is the sole enzyme responsible for inhibiting harmful lipid peroxidation, promoting the conversion of GSH, and playing a helpful role in eliminating lipid ROS [31]. Consequently, the suppression of GPx-4 can induce ferroptosis [24, 31, 32]. Acyl-CoA synthetase long-chain family member 4 (ACSL4) and lysophosphatidylcholine acyltransferase 3 (LPCAT3) are involved in the generation and regulation of phosphatidylethanolamine (PE), a phospholipid that plays a crucial role in activating PUFAs and altering the transmembrane properties of PUFAs. PE is also the primary phospholipid responsible for triggering ferroptosis in cells. Studies have shown that when ACSL4 and LPCAT3 genes are deleted or their functions are inhibited, GPx-4 inhibitor-induced ferroptosis is significantly blocked [33]. This finding indicates that the presence of highly oxidizable PUFAs in the cell membrane is a necessary condition for ferroptosis, and the remodeling of membrane lipids plays a crucial role in regulating this process.

System Xc- is a cell membrane amino acid antiporter system composed of SLC7A11 and SLC3A2, responsible for transporting cystine (Cys2) into cells while transporting glutamate (Glu) out [34]. In the mechanism of ferroptosis, the inhibition of system Xc- is an important step [35]. Inhibiting system Xc- leads to a decrease in intracellular cystine levels, reducing the synthesis of GSH. GSH, which is an antioxidant required for GPx-4 function, decreased levels directly weaken the cell’s antioxidant capacity, promoting the accumulation of lipid peroxides. This process can be triggered by the chemical molecule erastin [1], which acts directly on system Xc- to inhibit its function, ultimately leading to ferroptosis. The inhibition of system Xc- not only reveals the importance of the intracellular antioxidant system in ferroptosis but also provides new strategies for treating diseases related to ferroptosis. Figure 1.

Fig. 1. This figure summarizes the intracellular iron metabolism and ferroptosis pathways.

Fe³⁺ is internalized into the cell via TFR1. The STEAP3 protein assists in the reduction of Fe³⁺ to Fe²⁺ within endosomes. SLC40A1 is the cellular iron exporter whose activity is negatively regulated by the hormone hepcidin. Within the cell, iron is safely stored in ferritin, composed of FTH1/FTL subunits. Iron can be sequestered into ferritin via PCBP1/2 and released through the action of NCOA4 (ferritinophagy). Fe²⁺ participates in the Fenton reaction, generating ROS, and this process is regulated by the p62-keap1-nrf2 pathway. GSH involves cystine uptake via system Xc-, which is then reduced to cysteine. GSH is oxidized to GSSG during the reduction of hydrogen peroxide and lipid hydroperoxides, which is catalyzed by GPx-4. PUFAs are vulnerable to peroxidation, and this process is catalyzed by LOXs. ACSL4 esterifies PUFAs to PL-PUFA, making them susceptible to peroxidation. The accumulation of lipid peroxides leads to membrane damage and ferroptosis. p53 has a repressive effect on system Xc-.

Biochemical Regulation of Ferroptosis

GPx-4 is essential for safeguarding cells against oxidative harm [36]. The primary role of GPx-4, in conjunction with the upstream function of system Xc− and the resulting generation of GSH (the GPx-4 co-factor), is to reduce complex hydroperoxides, such as phospholipid hydroperoxides and cholesterol hydroperoxides, while simultaneously preventing the lipid peroxidation cascade. Cellular consumption of GSH reduces the function of the GPx-4 enzyme, enhances the accumulation of L-ROS, and triggers ferroptosis [37]. Therefore, the system Xc−/GSH/GPx-4 axis is the primary focus for triggering the process of ferroptosis [1, 23, 24].

Nrf2 plays a crucial role in regulating the antioxidant response to defend against cellular oxidative damage [38]. Keap1, a protein linked with Kelch-like ECH-associated protein 1, plays a role in maintaining low levels of Nrf 2 under normal physiological conditions. Keap1 does this by promoting the ubiquitination and subsequent destruction of Nrf2 through the proteasome. In contrast, when exposed to oxidative stress, the Nrf2 protein triggers a complex series of steps to activate specific antioxidant genes. The activation of Nrf2 is closely linked to the prevention of cell death and is responsible for regulating the transcription of nearly all genes involved in ferroptosis, such as those related to NADPH regeneration, which is essential for GPx-4 action, and genes involved in GSH control [39]. Nrf2 can additionally control the expression of genes involved in iron metabolism and inhibit ferroptosis by suppressing cellular iron absorption and the limitation of free iron availability. Nrf2 also regulates the proteins FTL/FTH1 and SLC40A1, which control the release of iron from the cell [40]. The regulation of these processes is carried out by p62, which is an autophagy receptor and a multifunctional protein. p62 directly suppresses Keap1, while concurrently promoting Nrf2 activation and regulating the process of ferroptosis. According to Lee and Sun et al. [41, 42], Nrf2 also controls lipid metabolism through a ligand-mediated transcription factor called peroxisome proliferator-activated receptor gamma (PPARγ). Hence, the p62-Keap1-Nrf2 pathway plays a role in regulating cellular iron and ROS metabolism.

NCOA4 functions as a receptor for transporting FTH1 within the autophagosome. It promotes the delivery of FTH1 to the lysosome, where it is broken down, releasing iron for systemic physiological needs. This process is known as selective ferritinophagy. Prior research has demonstrated a correlation between ferroptosis and autophagy, suggesting that proteins responsible for regulating ferroptosis may also play a role in regulating autophagy [43, 44]. Overstimulation of intracellular autophagy results in the accumulation of intracellular free iron and lipid peroxides, which in turn facilitates the onset of ferroptosis. NCOA4 binds to FTH1 and facilitates the autophagic breakdown of ferritin. Hou et al. [45] found that the upregulation of NCOA4 by gene transfection in pancreatic cancer cells suppressed the expression of FIH1 and enhanced the occurrence of erastin-induced ferroptosis in their research. Furthermore, the research conducted by Yoshida et al. [46] obtained the same conclusions.

P53 acts as an antitumor protein by performing many roles such as promoting programmed cell death, preventing cell death caused by injury or disease, and facilitating the degradation of cellular components [32]. The regulation of cell ferroptosis produced by p53 involves inhibiting the expression of SLC7A11, promoting the expression of SAT1, and promoting the expression of glutaminase 2 (GLS2) [47]. The gene SLC7A11 encodes the subunit xCT/SCL7A11 of system Xc-. The binding process decreases the expression of SLC7A11 and block system Xc-, which can hinder the absorption of cystine and the production of GSH, resulting in the increase of ROS and the occurrence of ferroptosis.

FSP1 is a very effective resistance factor and functions as a strong inhibitor of ferroptosis. Bersuker et al. [15] and Doll et al. [25] discovered that FSP1 successfully hindered the GSH and GPx-4 signaling pathways that are associated with ferroptosis. FSP1 acts as an oxidoreductase, similar to NADPH-dependent coenzyme Q, by reducing coenzyme Q10 (CoQ10) and producing a lipophilic radical-trapping antioxidant (RTA) that stops the spread of lipid peroxides [23]. The FSP1/CoQ/NADPH/CoQ/NADPH pathway offers a novel strategy for the design of pharmaceuticals aimed at inhibiting ferroptosis.

FPN-1 (SLC40A1) is a transmembrane protein that belongs to the extensive solute carrier gene family. It has been regarded as the exclusive cellular transporter of iron [48]. The expression of FPN-1 is controlled at the transcriptional level by many stimuli, including exposure to haem, hypoxia, or inflammatory mediators. Hepcidin and FPN-1 play a crucial role in maintaining the balance of iron levels in the body [49]. Hepcidin suppresses the release of iron from macrophages and intestinal mucosal cells into the circulation. Increased levels of hepcidin in the circulation can cause the breakdown of FPN-1, resulting in the accumulation of iron in cells. In addition, Jiang et al. [8] found that the inhibition of FPN-1 in neuroblastoma cells could enhance the process of erastin-induced ferroptosis by promoting the accumulation of ROS. Therefore, regulators like hepcidin and FPN-1 are crucial in maintaining iron balance and facilitating ferroptosis.

Ferroptosis and eye diseases

Presently, numerous studies have demonstrated a strong correlation between ferroptosis and various systemic diseases. Ferroptosis has emerged as a prominent area of research, garnering significant attention and interest in the development of therapeutic interventions and the enhancement of prognoses for these related diseases [7]. Consequently, there is also a growing interest in investigating the disturbance of iron balance and ferroptosis as a possible underlying factor for eye diseases. Allison et al. [50] investigated the data regarding iron’s probable contribution to the onset of eye disorders. The iron lines seen in the cornea during normal aging, as well as in conditions such as keratoconus and pterygium, were discussed. However, the unregulated presence of free iron has a harmful effect on cells and tissues, promoting the progression of ocular disease and resulting in substantial negative outcomes. This review provides a comprehensive overview of the most recent advancements in the understanding of iron homeostasis and ferroptosis in various eye disorders, including corneal disorders, age-related cataracts, glaucoma, retinal pigment epithelial-associated eye diseases, diabetic retinopathy, retinal ischemia-reperfusion injury, retinoblastoma, and retinitis pigmentosa.

Corneal disorders

The probable involvement of iron in corneal illnesses is supported by the presence of iron lines in the cornea, including the Hudson-Stähli line in the normal aging cornea [51, 52], Fleischer’s ring in keratoconus [53, 54], and Stocker’s line in pterygium [55, 56]. Corneal iron lines, which result from an excess accumulation of iron, can be observed in the basal epithelial cells of the cornea during slit-lamp examination. Ferritin’s storage of iron reduces its toxicity. Nevertheless, the excess iron can lead to tissue injury by generating ROS and subsequently oxidizing biomolecules [57]. The topic of oxidative stress and the antioxidant system has been extensively examined in corneal diseases [58]. GPx-4 transforms potentially harmful oxidative damage, sustains redox balance in corneal epithelial cells, and enhances the process of wound healing [59]. The deficit of GPx-4 resulted in a notable rise in cytotoxicity when compared to the suppression of other antioxidant enzymes. Sakai et al. [60] showed that GPx-4 has an impact on the cytotoxicity, lethality, cell activity, and wound healing of human corneal epithelial cells. In their study, lactate dehydrogenase (LDH) activity was significantly increased in human corneal epithelial cells transfected with GPx-4-specific knockdown siRNA, and the significantly increased cell death rate and cytotoxicity caused by GPx-4 inhibition were reversed by ferroptosis inhibitors α-tocopherol and ferrostatin-1. Based on their findings, it can be inferred that the antioxidant enzyme GPx-4 has a significant impact on maintaining the balance of oxidation, promoting cell survival, and facilitating wound healing in corneal epithelial cells. Additionally, the inhibition of ferroptosis serves as a protective mechanism for corneal epithelial cells.

Fuchs’ endothelial corneal dystrophy (FECD) is the primary cause of endogenous degeneration of the corneal endothelium. It is characterized by a noticeable reduction in the density of corneal endothelial cells (CEnC), along with abnormal CEnC morphology exhibiting polymegathism and pleomorphism of the cells [61]. These changes typically result in stromal edema and impaired visual acuity. Prior research has demonstrated that oxidative stress significantly contributes to the long-term degenerative process of the corneal endothelium and the programmed cell death of corneal endothelial cells observed in FECD [62–64]. Maya et al. [65] aimed to examine the process by which Nrf2 is controlled in normal and FECD CEnCs when exposed to oxidative stress. The study examined the impact of tert-butyl hydroperoxide (tBHP) on oxidative stress, as well as the protein and mRNA levels of Nrf2, DJ-1, p53, and Keap1. The findings indicate a considerable decrease in the Nrf2 protein stabilizer DJ-1 in FECD CEnCs compared to normal cells. On the other hand, the Nrf2 protein repressor Keap1 remained stable initially but showed an increase under conditions of oxidative stress. The expression of Nrf2 is reduced in FECD. Lovatt et al. [66] showed that the redox sensor peroxiredoxin 1 (PRDX1) is specifically absent in CEnCs from patients with FECD. The presence of PRDX1 is crucial in preventing damage to CEnCs caused by lipid peroxidation, which could potentially trigger ferroptosis. Furthermore, they determined that the compound known as ferrostatin-1, which inhibits the process of ferroptosis, effectively prevents lipid peroxidation and cell death in CEnCs. In addition, they present evidence that Nrf2 also controls lipid peroxidation in CEnCs. Therefore, we believe that ferroptosis might be present in the CEnCs of FECD patients. Stopping ferroptosis in CEnCs could be very helpful in treating FECD patients.

Age-related cataracts

Age-related cataracts are lens opacifications caused by the gradual accumulation of crystallin in the lens. While the exact cause of cataracts is not fully known, there is growing evidence indicating that the gradual accumulation of ROS and disturbance of redox balance may contribute to the development of cataracts [67–70]. According to West et al. [71, 72], sunlight exposure and cigarette smoking have been found to result in elevated oxidative damage and reduced antioxidant levels in the lens of cataracts. Giblin et al. [73] discovered that the cataractous lens saw a decrease in the production of GSH by enzymatic synthesis, leading to a reduction in the amount of GSH present in the lens. This loss was particularly significant when combined with an overexpression of oxidation. The lack of glutathione makes the nucleus more susceptible to specific forms of oxidative damage. Zigler et al. [74] stated that iron played a role in the development of oxygen-free radicals in the cause of cataracts. In summary, the presence of iron redox, reduced deposition of GSH, and altered redox homeostasis in the lens of cataracts leads to a decrease in GPx-4 activity and an increase in lipid oxidation. This is closely related to the physical and molecular features associated with ferroptosis. The physiological response of cells in in vitro lens epithelial cells and ex vivo lens epithelial tissue was investigated in a previous study using system Xc- inhibitors (erastin) and GPx-4 inhibitors (RSL3) [75]. The findings indicate that lens epithelial cells exhibit a high susceptibility to ferroptosis triggered by erastin and RSL3. Nevertheless, the precise function and governing mechanisms of ferroptosis in the development of age-related cataracts are not yet fully understood.

Glaucoma

Glaucoma is a sight-threatening condition characterized by damage to the optic nerve and a deficiency of visual field. It is mostly caused by the gradual loss of retinal ganglion cells (RGCs). Glaucoma’s etiology involves several risk factors, including elevated intraocular pressure (IOP), fluctuations in IOP, vascular dysregulation, glutamate neurotoxicity, inadequate nutritional supply, increased ROS, and inflammatory responses [76, 77]. Klocker et al. [78] and Cheah et al. [79] discovered that oxidative stress, glutamate neurotoxicity, and oxidative damage may play a role in the death of RGCs. Furthermore, Moreno et al. [80] conducted an experiment on rats with glaucoma and found that the antioxidant defenses of the retina, such as SOD, catalase, and GSH, were reduced compared to the control retinas.

Iron is a plentiful metal found in the retina. It serves as a co-factor for important retinal enzymes that play a role in many processes, such as phototransduction cascade, photoreceptor outer segment disc biogenesis, and energy production [81]. Multiple investigations have indicated a potential connection between disrupted iron balance and iron-related proteins in glaucoma, although the exact nature of this interaction remains unclear [82–84]. Farkas et al. [85, 86] discovered elevated concentrations of the iron-associated proteins transferrin and ferritin in both glaucoma patients and cynomolgus monkeys. These findings indicate that the presence of iron-induced oxidative stress is involved in the development of glaucoma. Excessive depletion of iron in the retina could potentially impair normal vision function, as iron is necessary for many enzymatic activities involved in phototransduction. Iron poisoning resulted in a notable elevation of indicators for necrotic and ferroptotic cell death in the RGC layer [87]. Available evidence indicates that dysregulation of iron and its toxic accumulation contribute to the death of RGC through a process known as ferroptosis [87]. Frataxin (FXN) is a protein that acts as a facilitator for the transport of iron into the mitochondria of cells. A sudden rise in IOP leads to an increase in the production of endogenous retinal FXN. Increased expression of FXN in Müller cells provides protection to RGCs against acute ischemia-reperfusion injury by preserving mitochondrial function and enhancing the antioxidant response [88, 89]. Ferric iron chelators, such as DFO and deferasirox (DFX), effectively decreased the death of RGC caused by NMDA and reduced the accumulation of Fe²⁺ and lipid peroxidation. This protective effect helps shield RGCs from excitoneurotoxicity or IOP problems by lowering oxidative stress [59, 90]. These results indicate that the glutamate receptor has a vital function in the transfer of iron, and inhibiting this receptor could be a possible strategy for reducing ferroptosis in retinal illnesses such as glaucoma.

Another potential cause of injury to RGCs could be the malfunctioning of mitochondria or the disruption of transport via axons and glial Müller cells [87]. Given the significant function of mitochondria in iron metabolism, it is hypothesized that the death of RGCs is linked to ferroptosis. In addition, Schiavi et al. [91] discovered that the dysregulation of iron metabolism during the damage process of RGCs is linked to autophagy and mitophagy. The disruption of iron metabolism was believed to be linked to the degradation of ferritin through NCOA4 [92]. Based on the above studies, we speculate that abnormal intracellular iron metabolism, enhanced oxidative stress, and the occurrence of ferroptosis may be the underlying causes of glaucoma and RGC layer damage. However, the question of excess iron supply in RGCs and its distribution, transport, and effects on mitochondria in glaucoma patients remains to be addressed.

Retinal diseases

The retina is a layer in the posterior segment of the eye that contains cells called photoreceptors (PR). Its main function is to transduce light into neural impulses and transmit them to the brain. The retina is primarily composed of two layers: the neural layer and the pigment epithelial layer [50]. The retinal neurons comprise PR, bipolar cells, ganglion cells, horizontal cells, and amacrine cells. The retinal pigment epithelium (RPE) is a single layer of pigment cells that lies between the neural retina and the choroid. Oxygen is the metabolite in the retina that is most limited in terms of its availability. The choroid supplies oxygen to the outer retina, while the retinal circulation gives oxygen to the inner retina [81]. Retinal PR possesses a dense concentration of mitochondria and exhibits exceptionally elevated metabolic activity and oxygen consumption. Excessive oxygen levels are harmful to the outer layer of the retina. Cones experience oxidative damage when exposed to high levels of oxygen over an extended period of time [93].

Iron, which is widely and unevenly distributed in the retina, is also necessary for retinal function. It is a transition metal that can change its oxidation state from Fe²⁺ to Fe³⁺ involving an electron transfer. Proton-induced X-ray emission revealed the most significant levels of iron in the choroid, RPE, and the inner segments of the PR cells. Immunohistochemical techniques have identified the presence of the TFR in various layers of the eye, including the ganglion cell layer, inner nuclear layer, outer plexiform layer, inner segments of PR, RPE, and the choroid [94]. PR cells rely heavily on iron-containing enzymes, such as fatty acid desaturase, to produce lipids that are essential for creating new disc membranes [95]. RPE65 is an iron-containing protein located in the microsomal membrane of the RPE. It plays a crucial role in the visual cycle by accelerating the conversion of all-trans-retinyl ester to 11-cis-retinol. The isomerohydrolase activity of RPE65 is dependent on iron [96]. Hypoxia-inducible factor (HIF) proteins, which act as regulators of oxygen homeostasis, depend on iron for their activity and regulate genes involved in iron metabolism [97]. Iron and oxygen are intricately interconnected in retinal metabolism under both healthy and diseased circumstances.

The iron concentration also fluctuates throughout the process of retinal development and aging. The regulation of iron levels in the retina is not influenced by systemic regulation. Instead, the retina itself produces the key proteins responsible for maintaining iron balance. The presence of an excessive amount of iron in the RPE can lead to harmful effects. This occurs when iron catalyzes the Fenton reaction, resulting in the accumulation of ROS and the occurrence of ferroptosis [98]. Recent studies have demonstrated the involvement of ferroptosis in the development of retinal disorders, including age-related macular degeneration, diabetic retinopathy, retinal ischemia-reperfusion injury, retinoblastoma, and retinitis pigmentosa.

Age-related macular degeneration

The deterioration of RPE is a significant factor in retinal disorders such as retinal degeneration (RD) and age-related macular degeneration (AMD) [99]. AMD is the primary cause of permanent vision loss in developed nations among individuals aged 65 and older [100]. Age is widely considered the most consistent risk factor. The pathological aging of the macula can lead to two types of age-related macular degeneration (AMD): dry or non-neovascular AMD and wet or neovascular AMD. Both types involve disruption of the RPE. The development of AMD is caused by multiple mechanisms, including hereditary and environmental influences. It involves in dysregulation in the angiogenic, oxidative stress, lipid, inflammatory, and complement pathways [101].

Oxidative stress and free radical damage are thought to be the main causes of RPE cell damage and AMD progression. Continual exposure to solar irradiation, high oxygen consumption, and high concentrations of PUFAs in the PR outer segments increase the production of ROS in the retina [102, 103], especially in non-neovascular AMD, which accounts for most AMD cases. ROS overproduction due to chronic oxidative stress can excessively deplete the antioxidation capability of GSH and GSH peroxidase, leading to damage to carbohydrates, membrane lipids, proteins, and nucleic acids.

Retinal iron accumulation is a characteristic feature of AMD. In AMD, iron is present in the PR, RPE, and Bruch’s membrane [12, 104, 105]. Lipid peroxidation in aged retinal vessels leads to the death of retinal endothelial cells [106]. Genetic mutations in the gene that codes for mitochondrial ferritin, a protein that stores iron and is found especially in retinal mitochondria, can lead to decreased defense against oxidative stress caused by mitochondrial iron. This, in turn, can contribute to the development of AMD [107]. Intracellular iron accumulation triggers genomic disintegration, promoting aging by inducing DNA damage and blocking DNA repair [108].

As an antioxidant in RPE cell, GSH is crucial for protecting RPE cells against oxidative damage, and depletion of GSH could result in cell death. Sun et al. [109] found that RPE cell death reduced by GSH was inhibited by ferroptosis inhibitors more effectively than by apoptosis or necrosis inhibitors in an in vitro model of AMD. Therefore, GSH dysfunction may be implicated in the pathogenesis of AMD patients.

According to current evidence for disturbed iron and redox homeostasis in the aging retina, we speculate that the pathogenesis of AMD may be related to abnormal iron ion metabolism, lipid peroxidation, and the resulting ferroptosis.

Totsuka et al. [110] found that Fe²⁺ accumulation, lipid peroxidation, and GSH depletion in tBHP-treated RPE cells can be markedly reversed by ferroptosis inhibitors (ferrostatin-1) and iron chelators (DFO), indicating that tBH-induced RPE cell death is, at least in part, due to ferroptosis. Tang et al. [111] demonstrated that the regulation of ferroptosis by haem oxygenase-1 (HO-1) is an important pathological process underlying RPE cell degeneration in the sodium iodate-induced oxidative stress model, and ferroptosis of RPE cells can be significantly blocked by knockout of HO-1. Studies on age-related eye disease have demonstrated that dietary supplementation of nutrients with lipid antioxidant properties (e.g., lutein and zeaxanthin, zinc, vitamin C, and E) can reduce the risk of AMD progression [112, 113].

In the pathological mechanism of AMD, numerous mechanisms regulate the process of ferroptosis. Under conditions of oxidative stress, Keap1 undergoes a conformational modification and releases Nrf2 for translocation to the nucleus, where it binds to AREs. Nrf2 deficiency increases susceptibility to oxidative stress in all tissues, including the RPE [114]. RPE cells generate vascular endothelial growth factor (VEGF) in neovascular AMD when they are under conditions of oxidative stress. Iron mediates succinate receptor-G-protein-coupled receptor 91 (GPR91) signaling in retinal RPE cells and stimulates the expression and secretion of VEGF. Paeng et al. [115] and Arjamaa et al. [116] evaluated the protein expression level of SLC7A11 and VEGF during disease progression and assessed the role of SLC7A11 in the laser-induced neovascular AMD model. They found that ferroptosis in laser-induced neovascular AMD was accompanied by an increase in the content of Fe²⁺ and the expression of GPx-4 in the isolated RPE/choroid complexes, and they confirmed the inhibitory function of SLC7A11. These results suggest that SLC7A11 can possibly represent a target for therapeutic intervention in patients with neovascular AMD.

Diabetic retinopathy

Diabetic retinopathy (DR) is one of the microvascular complications of diabetes mellitus and is recognized as the primary cause of visual impairment in the working-age population [117]. Although previous studies have implicated pyroptosis and necrosis in the development of DR [118, 119], evidence from recent studies indicates that diabetic individuals may suffer oxidative stress and problems with their PR before they develop early vascular disease [120]. The retina is a highly metabolically active tissue that has a significant demand for oxygen. During periods of elevated glucose levels, the retina is more susceptible to injury compared to other organs due to the increased consumption of antioxidants, such as GSH [121]. At the same time, high blood glucose in diabetic patients leads to a disruption of the blood-brain barrier and the release of large amounts of Fe²⁺, which induces abundant mitochondrial ROS production [23]. Singh et al. [122] showed that high glucose-induced aberrant redox protein expression and oxidative stress in adult retinal pigment epithelial cell line-19 (ARPE-19) cause mitochondrial-lysosomal axis dysregulation, which activates ferritinophagy and ultimately contributes to the ferroptosis of these cells. Hence, oxidative stress and ferroptosis have been considered vital pathological mechanisms that cause many diabetic complications, including DR [123]. Yang et al. [124] also reported that ferroptosis is a main factor in the occurrence and development of DR.

Liu et al. [125] found that ferroptosis inhibitors, such as liproxstatin-1, glia maturation factor-β antibody, and lysosome activator, have a significant ability to protect retinal function in a rat model of diabetes. Zhu et al. [126] demonstrated that circ-PSNE1 exhibited elevated expression levels in ARPE-19 cells that were exposed to a high glucose concentration, replicating the conditions of diabetic retinopathy. Downregulation of circ-PSEN1 can modulate intracellular levels of GSH, malondialdehyde, and Fe²⁺, enhance cell viability, and inhibit ferroptosis. TRIM46 is a promising biomarker for the development of cancer and controls the growth of cancer cells through VEGF [127]. Treatment of retinal capillary endothelial cells (RCEC) with high glucose leads to upregulation of TRIM46 expression, resulting in reduced cellular resistance to high glucose-induced ferroptosis. TRIM46 interacts with GPx-4. Upregulating GPx-4 mitigates the consequences of TRIM46 overexpression and safeguards RCECs against ferroptosis [127]. Therefore, the regulatory function of ferroptosis could potentially offer a novel approach to the clinical management of RD.

Retinal ischemia-reperfusion injury

Retinal ischemia-reperfusion (IR) injury is involved in the pathological mechanisms of many eye diseases, including glaucoma, diabetic retinopathy, and retinal occlusion [100, 128]. Retinal IR is characterized by the progressive degeneration of RGCs. Dvoriantchikova et al. [129] found that not one but at least four types of programmed cell death, including necroptosis, pyroptosis, oxytosis/ferroptosis, and parthanatos, are simultaneously active in the IR retina. They found that a large amount of Steap3 enzymes in ischemic RGCs reduces Fe³⁺ to Fe²⁺, which may lead to the generation of a large number of Fe²⁺. In recent years, it has been found that ferroptosis inhibition can effectively ameliorate ischemic injury of the brain, heart, liver, and kidney [130], and dysfunction of the key ferroptosis-surveilling systems could hypersensitize mice to tubular necrosis during acute kidney injury [131]. Simultaneous inhibition of ferroptosis could significantly protect mice against bilateral renal IR injury. Qin et al. [132] demonstrated that ferroptosis is involved in the whole course of retinal IR in mice. Ferrostatin-1 exhibited marked protection of RGCs against IR both in mice and in primary cultured RGCs. This evidence indicates that ferroptosis plays a vital role in the process of retinal IR. On the other hand, recent work has shown that the inflammatory response of retinal IR, which increases vascular leakage and participates in the death of RGCs, is accompanied by manifestations of ferroptosis [133]. Some inflammatory mediators are involved in the ferroptosis process of retinal IR. However, the pathological mechanism by which inflammatory mediators regulate ferroptosis still needs further study.

Retinoblastoma

Retinoblastoma (RB), the predominant malignant tumor affecting the eyes throughout childhood, can be fatal if not treated. Most bilateral RB cases are caused by germline mutations of the RB1 gene, which are either inherited from an affected survivor (25%) or the result of a new germline mutation (75%) [134]. Cytogenetic studies have also indicated that approximately 65% of retinoblastomas exhibit a genetic gain of MDM4, which can suppress the p53 pathway in retinoblastomas [135]. Sorafenib is a strong inducer of ferroptosis, regulating RB proteins in hepatocellular carcinoma (HCC) to induce ferroptosis. Nie et al. [136] found that treating of patients with advanced liver cancer using sorafenib, which mediates RB protein, can significantly increase the sensitivity of liver cancer cells to iron and sensitize patients to the anticancer effects of sorafenib. Kuganesan et al. [137] sought to uncover the contribution of p53 and RB to the regulation of ferroptosis in their study. The findings showed that higher levels of p53 and the deletion of RB genes improved ferroptosis in various isogenic and inducible systems. Liu et al. [138] assessed the efficacy and safety of 4-octyl itaconate, a compound derived from the metabolite itaconate, in promoting ferritinophagy-dependent ferroptosis for treating multi-drug-resistant RB cells in experiments and in mice with transplanted tumors. They provide evidence that inducing autophagy-dependent ferroptosis is an effective strategy to eliminate drug-tolerant RB cells, and the anticancer potential of an itaconate derivative in RB cells relies on ferritinophagy-mediated ferroptosis. These findings may provide crucial insights into the mechanism by which autophagy-dependent ferroptosis induces the death of drug-resistant persistent cells during RB therapy. Therefore, a growing number of scientists have proposed that alterations in the p53 and RB1 genes play a role in the occurrence of ferroptosis.

Retinitis pigmentosa

Retinitis pigmentosa (RP) is a prevalent genetic eye disease characterized by the extensive loss of rod and cone cells, leading to retinal degeneration [139]. The primary symptoms noticed by individuals with RP are night blindness, progressive visual field loss, and reduced central vision [140, 141]. The pathophysiology of RP is caused by the degeneration of PR, originating at the outer edges of the retina and advancing towards the macula and fovea [142]. However, the underlying mechanisms are not completely elucidated. Researchers have recently discovered a potential link between the pathogenic process and the progression of RP and ferroptosis, following an extensive investigation into the latter. RP models have shown instances of iron metabolic failure, resulting in either iron accumulation or iron overload [143]. Multiple investigations have demonstrated that RP has various characteristics associated with ferroptosis, including the presence of GPx-4 and GSH, as well as lipid peroxidation [144, 145]. Upregulating the expression of GPx-4 and SOD1 provided significant protection to PR cells against retinal degeneration caused by oxidative stress. Deleon et al. [144] investigated iron homeostasis by examining the levels of retinal mRNA and protein expression of transferrin, TFR, and ceruloplasmin in chemokine receptor 2 (CCR2)-deficient animals during retinal degeneration. The study found that transferrin and ceruloplasmin mRNA levels increased by 2 to 12 times during retinal degeneration compared to controls of the same age. These data suggest that retinal degeneration is associated with altered iron homeostasis and oxidative injury.

Liu et al. [145] generated retinal degeneration models by inducing the death of RPE cells using sodium iodate, a widely used oxidant, and investigated the features of these cells in ARPE-19 cultures. The findings demonstrated that during the process of death, there was a significant rise in the amounts of labile iron within the cells, as well as an increase in ROS and lipid peroxides. These changes were similar to the main characteristics of ferroptosis. The ferroptosis inhibitors, DFO and ferrostatin-1, somewhat reduced the sodium iodate-induced cell death. Wang et al. [146] demonstrated that in a model of rapidly progressive RP, iron-chelating agents preserve visual acuity and substantially rescue cones by increasing the expressions of GSH and GPx-4. These findings suggest that ferroptosis is closely linked to the harmful process of RP, and chelation of labile iron could be an effective way to treat RP patients.

Conclusions and perspectives

Since ferroptosis was first named in 2012, an increasing amount of research has demonstrated that ferroptosis plays a vital role in specific types of pathophysiological processes, and the concept of ferroptosis-inducing therapy is becoming more established in the field of cancer treatments. Currently, researchers have discovered that ferroptosis plays a role in several blinding diseases, such as glaucoma, age-related macular degeneration, and retinitis pigmentosa. Overloaded iron and the production of ROS result in the degeneration of corneal epithelial cells, RGCs, RPEs, and PRs, which is induced by oxidative DNA damage, lipid peroxidation, and cellular membrane damage, namely the manifestation of ferroptosis. Iron chelation has been shown to protect cells from oxidative damage and cell death in various models.

This review presents a current and comprehensive summary of the mechanisms and regulation of ferroptosis, the disruption of iron balance in eye disorders, and explores current and potential future research in the field of ocular diseases. Disruption of iron homeostasis may serve as a trigger for numerous ocular disorders. For example, we recently reported iron metabolic disturbance in the cornea of high-myopia eyes [147]. Our results revealed that the expression of FTL and FTH1 was negatively associated with myopia development, while the expression of serotransferrin was positively related to myopia status. These findings suggest that iron metabolic proteins could serve as an essential modulator in the pathogenesis of myopia. Therefore, understanding the signaling pathways of iron homeostasis and ferroptosis in blinding diseases may help us find new treatments to deter disease progression. Nevertheless, there are still some problems with biological behavior, and it will take a long time before these findings can be applied to clinical cases.

Summary

What was known before

• Ferroptosis, a novel and recently recognized form of regulated cell death, is regulated by specific genes and is distinct from apoptosis, necroptosis, autophagy, and other forms of cell death.

• Ferroptosis plays an important regulatory role in the occurrence and development of many diseases.

What this study adds

• The perturbation of iron homeostasis and ferroptosis are increasingly being investigated as potential causes of eye disease.

• Understanding the signaling pathways of iron homeostasis and ferroptosis in blinding diseases may help us find new treatments to deter disease progression.

Author contributions

The article was written by WSS with help from LJ, ZYH, LY, and WY. The final manuscript was read and approved by all the authors.

Funding

This study was funded by the National Program on Key Research Project of China (No. 2022YFC2404502), the National Natural Science Foundation of China (No. 82271118), Xi ‘an Health Committee research projects (No. 2023yb14), and Shaanxi Province Natural Science Basic Research Project (No. 2024JC-YBMS-623).

Competing interests

The authors declare no competing interests.