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Journal of Clinical Gerontology & Geriatrics
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Review article
Senile cataracts and oxidative stress
Min-Chieh Ho, MDa, Yi-Jie Peng, MDa, Shih-Jen Chen, MD, PhDb'c, Shih-Hwa Chiou, MD, PhD
a Department of Ophthalmology, Buddhist Tzu Chi General Hospital, Taipei Branch, Taipei, Taiwan b Department of Ophthalmology, Taipei Veterans General Hospital, Taipei, Taiwan c Institute of Clinical Medicine, School of Medicine, National Yang-Ming University, Taipei, Taiwan
d Department of Medical Research and Education, Taipei Veterans General Hospital, No. 201, Section 2, Shih-Pai Road, Taipei 11217, Taiwan
b,c,d, *
ARTICLE INFO
Article history: Received 5 February 2010 Received in revised form 27 July 2010 Accepted 29 July 2010
Keywords:
Antioxidant enzymes Cataracts Glutathione Oxidative stress
ABSTRACT
In numerous epidemiological and animal models, it can be inferred that oxidative stress is a key factor in cataract formation. Production of reactive oxygen species and reduction of endogenous antioxidants both contribute to cataract formation. In the cataractogenous process, lens proteins lose sulfhydryl groups and become thiolated or cross-linked by disulfide bonds. The resultant high molecular weight aggregates become insoluble and affect lens transparency. All these are consequences of changes in the redox state. A mixed protein-thiol and protein-protein disulfide bond precedes the morphological changes of cataract. Normally, sustained high levels of reduced glutathione provide a protective effect, while depletion of glutathione causes damage to epithelial cells and fiber cells. UV rays in the ambient environment evoke reactive oxygen species formation and also contribute to cataracts. The reduction in free UV filters and increase in their binding to lens proteins make the lens more predisposed to UV damage and oxidation. In the aqueous humor of cataract lenses, there is a decrease in antioxidant enzymes and increase in nitric oxide, which demonstrates the relationship between oxidative stress and cataracts. Though surgical intervention is the standard treatment for cataracts, experimental medical therapies for cataracts are under extensive investigation. Carnosine, a pro-drug of carnosine-N-acetylcarnosine, bendazac, ascorbic acid, and aldose reductase inhibitors are under therapeutic evaluation, and prevention of cataract formation may be possible in the future.
Copyright © 2010, Asia Pacific League of Clinical Gerontology and Geriatrics. Published by Elsevier Taiwan
LLC. All rights reserved.
1. Introduction
Cataract (lens opacification) is a major contributing factor of blindness. An estimate from the World Health Organization is that the current global prevalence of blindness is 0.57% (range, 0.2—1.0%), with more than 82% of all blindness occurring in individuals aged 50 and older. Cataract accounts for 47.8% of the roughly 37 million blind people in the world.1 While the main treatment for cataract is surgical intervention,2 it is associated with certain risks and subsequent suboptimal outcomes. Endoph-thalmitis is the most devastating complication following cataract surgery and usually results in severe vision impairment. Several studies have demonstrated that pseudophakic patients have a fourfold cumulative risk of retinal detachment for up to 20 years after cataract surgery.3 The incidence of pseudophakic posterior capsular opacification remains high,4 while laser capsulotomy is associated with a 3.9-fold increased risk of retinal detachment.5
* Corresponding author. Department of Medical Research and Education, Taipei Veterans General Hospital, No. 201, Section 2, Shih-Pai Road, Taipei 11217, Taiwan. E-mail address: shchiou@vghtpe.gov.tw (S.-H. Chiou).
Cataractogenesis is influenced by multiple risk factors, such as aging, diabetes mellitus, drugs, trauma, toxins, genetics and other ocular diseases.6-8 Among the various causes, oxidative stress is considered to play a key role in the molecular mechanism of cataract formation.9-11 In this article, we review the current understanding of oxidative stress in the process of cataract formation and examine the correlation between the proposed mechanisms and prospective therapeutic agents for cataract prevention.
2. Oxidative stress and cataract formation
The main composition of the lens is protein, also known as crystalline, that is generated from migrating fiber cells from anterior cubodial epithelia during embryogenesis. During migration, nuclei and mitochondria are lost and render the fiber cells susceptible to damage because of the absence of turnover process.12 There is also simultaneous loss of repair systems of the mitochondria that can restore the oxidative damage by utilizing glutathione (GSH), thio-redoxin (Trx), NADPH and FADH2 as electron donors, and maintain the proteins in stable redox status.
2210-8335/$ — see front matter Copyright © 2010, Asia Pacific League of Clinical Gerontology and Geriatrics. Published by Elsevier Taiwan LLC. All rights reserved. doi:10.1016/j.jcgg.2010.10.006
It is generally accepted that oxidation is a key feature of cataract formation. Free radicals, including numerous reactive oxygen species (ROS) such as superoxide anion radical (-O2~), H2O2, and hydroxyl free radical ( OH), may lead to structural damage of the crystalline lens and contribute to cataract formation. ROS may be generated exogenously after UV light and ionizing radiation exposure, or endogenously as a result of normal metabolism through the enzymatic reaction of lipoxygenases, cytochrome P450, NADPH, and mitochondrial electron transport in various cellular compartments (mitochondria, peroxisomes, cytoplasm). Oxidative stress occurs when the level of pro-oxidants surpass the level of antiox-idants. Thus, the level of ROS should be finely regulated, otherwise damage to the mitochondria and overproduction of ROS may occur.
Antioxidant enzymes work with reducing systems and protein repair systems to protect against ROS-induced damage. In the mitochondria matrix, MnSOD (SOD2) converts the superoxide anion, which is generated by the electron transport chain, into hydrogen peroxide. By regulating the level of SOD2, lens epithelial cells can overcome oxidative stress.13 CuZnSOD (SOD1), present in the mitochondria intermembrane space, also effectively transforms superoxide anion to hydrogen peroxide in a similar manner.14 Elevated levels of SOD1 protects the lens from H2O2-mediated
damage.15
Hydrogen peroxide is a relatively stable ROS species with moderate destructive potency. Enzymes including catalase (CAT), GSH peroxidase and the peroxiredoxins cooperate to reduce the potential for damage. Among ROS, H2O2 is the most stable oxygen species present in the aqueous humor. H2O2 can also diffuse into the interior lens. Lenticular epithelial cells have antioxidant defences (including CAT, GSH, and GSH peroxidase), which are capable of removing aqueous-derived H2O2. The normal concentration of H2O2 in aqueous humor and lens is 25—30 mM, but there can be a threefold
increase in cataract patients
RO S-induced destruction at
different targets within the crystalline lens, such as proteins or lipids, is believed to underlie the pathogenesis of cataracts.18
GSH is abundant in the lens and can be synthesized by the lens.19 The GSH repair system consists of reduced GSH and its oxidized form (GSSG). GSH is maintained in its reduced form rather than its oxidized form by GSH reductase enzymes in the presence of NADPH, and the formation of mixed disulfide is prevented. Reduced GSH acts as an electron donor for GSH peroxidase while reducing H2O2 to H2O and O2.20 The level of oxidized GSH rises significantly once cataract develops. In experimentally-induced cataract, decrease in GSH level is also a typical finding. Furthermore, studies have shown that reduced GSH prevents protein sulfhydryl groups from intramolecular and intermolecular disulfide cross-linking. Therefore, GSH can reduce high molecular weight protein aggregation in the lens and prevent subsequent light scattering and cataract deterioration. Hence, GSH is considered to be a crucial factor involved in cataract formation. The protective effect of GSH is considered to be concentration-dependent. GSH above 1 mM effectively inhibits hydroxyl radical formation, whereas a concentration below 1 mM accelerates its production.21,22
Trx, another electron donor, is a small thiol protein with active-site dithiol, which reduces protein disulfide actively. Oxidized Trx is further reduced by Trx reductase in a NADPH-dependent manner. The production of NADPH increases under oxidative stress, making it important in redox control.23
To correlate these biochemical facts to lens morphology, the lens barrier may be of most importance. The barrier at the cortex/nucleus interface impedes the flow of molecules such as antioxidants into the nucleus and thus predisposes the lens center to oxidative damage. Furthermore, the unstable molecules stay much longer in the central zone and cause more structural oxidative damage in the central lens.24,25
Many antioxidants, such as vitamins C and E and the caroten-oids, are considered to work as ROS scavengers. In animal models, antioxidant levels have been found to correlate with cataract progression. However, studies on the protective effects of antioxi-dant supplementation in human lens have been limited by few participants, high dropout, coeffectiveness of previous nutritional supplements, and dietary differences.
3. Photooxidative stress
The lens that works together with the cornea to focus radiation on the retina is constantly exposed to ambient radiation including UVA, UVB, and visible light, which lead to photochemical insult to the eye. As already mentioned, there is no turnover of proteins in the lens throughout life, so the lens is more vulnerable to photochemical insult. The differences in cataract risk in epidemiological assessments, particularly in geographic studies, indicate an association between cataract and UV light exposure.
3-Hydroxykynurenine (3OHKyn), present as a UV filter in the human lens, reacts with lens proteins and affects lens coloration.26 After continual exposure to 300—400 nm UV light, loss of 3-hydroxykynurenine glucoside (3HKG, glucosided form of 3OHKyn) and subsequent yellowing of the lens is noted. Other compounds of 3OHKyn, such as oxidized xanthurenic acid and xanthommatin, are also potentially cataractogenic molecules. In the lens compartment where the antioxidant (e.g., GSH) is depleted, 3OHKyn can act alternatively as an antioxidant.27 Since the attachment of 3OHKyn to lens proteins results in lens coloration, increased photon and UV light absorption may be a potential causative cofactor in cataract development.
a-Crystallin, as a major protein family in the lens, is responsible for the maintenance of lens transparency. It has been demonstrated that exposure to UV induces cross-linking of a-, b- and g-crystallins, which lead to conformational and solubility change, resulting in light scattering in cataracts. Nevertheless, the a-crystallins themselves can act as chaperones against photodamage by UV irradiation.28 The lens epithelium also has anti-photooxidative enzymes, which prevent the lens from being damaged by photooxidation induced by UV.29 The human lens membrane contains high levels of cholesterol, plasmalogen, and dihydrosphingomyelin; UVB irradiation induces perox-idation of membrane lipids, and leads to formation of hydroxyl and hydroperoxyl lipids. Membrane repair can be compromised by reduction of protein synthesis after UV irradiation.16,30
4. Non-enzymatic glycation
Non-enzymatic glycation is a condensation reaction between amino groups in proteins and reducing sugars. The glycated product, known as early glycation product, may undergo further reaction by oxidative or non-oxidative pathway. Under oxidative conditions, the early glycation product does not undergo further reaction. Under non-oxidative conditions, the early glycation product can react with many amino groups to give rise to brown and cross-linked products called advanced glycation end products.31—33
The lens derives 80% of its metabolic energy from anaerobic glycosis, with the remaining 15% and 5% from the hexose monophosphate shunt and Krebs cycle, respectively. It is hypothesized that increased intracellular glucose may overflow the glycolysis pathway, resulting in activation of aldose reductase. Thereafter, the cellular osmotic pressure increase and influx of water into the cell membrane leads to lens swelling and opacification.34 In vitro, gly-cation partially unfolds lens proteins and exposes buried sulfhy-dryls, leading to disulfide formation and high molecular weight aggregation. Further, cortical proteins are more susceptible to
glycation than are nuclear proteins.35—37 Another consequence of partial unfolding is exposure of hydrophobic groups, which may promote hydrophobic interaction. This mechanism may also contribute to high molecular weight aggregation and eventual protein insolubilization. In addition to the lens protein, glycation also influences the lenticular membrane. Cross-linking of the intrinsic proteins in the membrane affects membrane rigidity and permeability, and may also lead to membrane opacity.38
5. Parameters in aqueous humor
Clinically, epidemiological surveys provide an idea of the potential risk factors of cataract, while animal models and in vitro studies make the hypothesis and mechanism more convincing. Some studies on oxidative stress markers in aqueous humor have been proposed to correlate the aqueous humor micro-condition with cataractogenesis in the human lens. ROS is difficult to detect because of its instability and rapid reactions; hence, measurement of antioxidant enzymes in aqueous humor is a reliable surrogate.
Among these, superoxide dismutase (SOD), a major antioxidant enzyme, which decomposes superoxide into hydrogen peroxide, and CAT, which breaks down hydrogen peroxide into water, are the two major enzymes discussed. These antioxidant enzymes in the aqueous humor protect the lens protein from ROS-induced damage.39 SOD activities in aqueous humor was typically low in previous reports, but Sawada et al. found that in more advanced cataract, there was a significant increase in the level of SOD activity and total protein in aqueous humor.40 The increase was proportional to cataract severity but not to patient age. They mainly detected the activities of SOD1 and SOD2 in cells from the cornea, ciliary epithelium and lens, not extracellular SOD. It may be inferred that advanced cataract is associated with molecules leaking through the lens capsule.
Nitric oxide (NO), generated by NO synthases, and known as a detrimental free radical, has been found to regulate cell metabolism and to play an important role in the aging process. Non-enzymatic nitration of a-crystallin in the cataractous lens matrix may be a mechanism in lens protein damage.41—44 Furthermore, different nitrogen oxide species, including peroxynitrite, are highly reactive species that can oxidize proteins and non-protein sulfhy-dryl groups. Significantly higher levels of NO in the aqueous humor of patients with mature cataract compared to patients with cortical, nuclear or posterior capsular cataract have been found. In addition, NO levels in the aqueous humor increase with age and in patients with age-related cataract.45 These oxidative stress markers in the aqueous humor reveal a possible relationship with cataract. However, the precise mechanism is not fully understood. In the future, control of NO formation may be an alternative way to prevent cataract progression.
6. Therapeutic agents
Quinax has been widely used for prophylaxis of cataract formation. Oxidative stress, as the key to cataract formation, is related to a cascade of biochemical reactions as described above. Many enzymes and oxidative agents are involved in cataracto-genesis. Once the pathway is blocked, the progression of cataract formation may be controlled. Several medications are under investigation for cataract prevention or treatment.
6.1. Lipid peroxidation and carnosine
Lipid peroxidation (LPO), as an endogenous source of oxidative injury, leads to formation of reactive oxygen radicals and high
molecular weight protein aggregation of low solubility.46,47 A potential treatment strategy is to prevent activation of LPO and accumulation of damaging LPO products by exogenously administered antioxidants with a strong affinity to LPO. Carnosine was found to be the most potent lipid peroxidase mimetic with a powerful antioxidant property that can protect cell membranes from oxidative damage.48
The ophthalmic pro-drug carnosine-N-acetylcarnosine (NAC) has recently been developed for clinical use.49 Babizhayev et al. found that after 6 months of NAC treatment, 96% of the treated eyes showed improvement in lens clarity as based on slit-lamp images and retroillumination photographs. Significantly, NAC was found to be able to reverse lens opacity in canine eyes, and a "melting snow" phenomenon was described.46 Other studies also showed improvements in best-corrected visual acuity or lens opacity. Nevertheless, there remain some barriers to widespread adoption of this experimental therapy, such as the small number of participants, high dropout rate, too little progression to show convincing worsening of untreated cataracts, and insufficient baseline measurements to compare the efficacy of NAC. A larger trial is needed to justify the benefit of long-term NAC therapy.50
6.2. Irradiation cataract and bendazac
Bendazac has been demonstrated to have a protective effect against lens protein denaturation both in vitro and in vivo.51 Pan-dolfo et al. found that bendazac was especially effective in protecting against X-ray-induced cataracts in the rabbit lens with preservation of the antioxidant enzyme system.52 In addition, bendazac's main metabolite, 5-hydroxybendazac, was found to be an effective hydroxyl radical scavenger.53 Unfortunately, the reliability of these results on bendazac is limited because the present conclusions are based on studies of subjective criteria, high dropout rate, and small participant numbers. The results were concluded to be ambiguous.
6.3. Ascorbic acid
In the lens, ascorbic acid acts as a reductant and a free radical scavenger. Ascorbic acid was reported to be capable of preventing photooxidation-induced protein cross-linking.54 Likewise, aspirin works as an acetylating agent, which prevents protein denaturation within the lens.55 Even if ascorbic acid is proven to be beneficial in cataract prevention in case-controlled studies, other retrospective studies have not arrived at a favorable conclusion.56—59
6.4. Diabetic cataract and aldose reductase inhibitors
Aldose reductase is the key enzyme in the polyol pathway. Some drugs have been designed as aldose reductase inhibitors to prevent cataracts. Sorbinil, an experimental aldose reductase inhibitor, reduces polyol production and prevents NADPH from being oxidized. Sorbinil is also capable of maintaining GSH in the reduced state in experimental cataracts. However, sorbinil showed little beneficial effect on cataract formation and many studies were stopped due to adverse effects.60,61
7. Conclusion
The crystalline lens is constantly subjected to oxidative stress from free radicals, which can potentially lead to lens protein damage. The lens has several protective mechanisms such as the GSH system and Trx, which work as repair systems to protect the lens protein from oxidation and further cataractogenous
changes. The relationship between UV light and cataract was first discovered by epidemiological studies, while further surveys showed that 3OHKyn is a crucial mediator of UV light absorption. UV light-induced cross-linking of lens proteins also contributes to UV-related cataracts. Research is revealing that the levels of CAT, NO and proteins in the aqueous humor may be correlated with cataractogenesis in the human lens. Further investigation of cataractogenesis is of clinical value and may lead to the development of potential medical therapy as an alternative to surgery.
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