Crosstalk between oxidative stress and ocular diseases

Oxidative Stress (OS) is defi ned as an imbalance between the generation of oxidants and the potential of antioxidant defenses of cells, as well as the ability of damage repair mechanisms to favor an excess of oxidants. This unbalanced state can cause tissue damage or lead to the production of toxic species to all cell components [1-5]. The oxidants include mainly the molecules with unpaired electrons (free radicals): superoxide anion radical (O2 -), hydroxyl radical (HO), peroxyl radical, non-radical oxygen species, such as hydrogen peroxide (H2O2), an electronically excited form of molecular oxygen, termed singlet oxygen (O2), nitric oxide radical (NO2 ) and peroxynitrite (ONOO) commonly known as Reactive Oxygen Species (ROS) [3,6-8]. OS may lead to disturbance in the normal oxidation-reduction state of a cell resulting in adaptationresolving oxidant tone (eustress) or damage of the cellular components (distress). Related literature indicates that OS is involved in several human diseases, such as infl ammation, diabetes, arteriosclerosis, autoimmune disorders, skin diseases, hypertension, cancer, eye diseases, infectious diseases, among others [3,8-15]. Over the past decades, the OS theory has strongly stimulated interest in the role of antioxidant defense mechanisms in removing ROS, i.e. antioxidant enzymes and non-enzymatic antioxidants [13]. Concerns have been raised about the benefi ts of the regular physical activity of moderate intensity as the factor linked with adaptation of cells to OS and the ability to enhance the cell antioxidant system[16]. According to the World Health Organization report [17], about 347 million people were suffering from visual impairment worldwide in 2018. Among them, 46 million faced blindness and 300 millions suffered from moderate to severe vision impairments. The knowledge of the role of OS in the etiology of eye diseases, including cancer, mechanisms responsible for the Abstract


Introduction
Oxidative Stress (OS) is defi ned as an imbalance between the generation of oxidants and the potential of antioxidant defenses of cells, as well as the ability of damage repair mechanisms to favor an excess of oxidants. This unbalanced state can cause tissue damage or lead to the production of toxic species to all cell components [1][2][3][4][5]. The oxidants include mainly the molecules with unpaired electrons (free radicals): superoxide anion radical (O 2  -), hydroxyl radical (HO), peroxyl radical, non-radical oxygen species, such as hydrogen peroxide (H 2 O 2 ), an electronically excited form of molecular oxygen, termed singlet oxygen ( 1 O 2 ), nitric oxide radical (NO 2  ) and peroxynitrite (ONOO  ) commonly known as Reactive Oxygen Species (ROS) [3,[6][7][8]. OS may lead to disturbance in the normal oxidation-reduction state of a cell resulting in adaptationresolving oxidant tone (eustress) or damage of the cellular

The generation mechanisms of reactive oxygen
In aerobic cells, free radicals derived from molecular oxygen represent the most important group of ROS formed during the natural biologic processes in living cells. They are mainly produced by an electron transfer reaction through multiple pathways [18]. For example, the end ogenous sources of the sup eroxide radical formation include: i) mitochondrial electron transport chain; ii) cytochrome P-450 metabolism; iii) autoxidation of catecholamines and hemoglobin; iv) oxidation of reduced nicotinamide adenine dinucleotide phosphate (NADPH) by NADPH oxidase; v) oxidation of xanthine or hypoxanthine by xanthine oxidase; vi) reduction of molecular oxygen by NO synthase (NOS) isoforms (nNOS or eNOS) under arginine or tetrahydrobiopterin defi ciency; vii) peroxisome metabolism; viii) phagocyte cells (neutrophils, macrophages, eosinophils), and iv) UV-light irradiation of tryptophan, eumelanin and pheomelanin. It is worth adding that during aerobic respiration, 1-5% of total oxygen consumed undergoes reduction to superoxide anion radical [19]. In turn, NO  arises from L-arginine with the participation of one of the three NOS isoforms. The superoxide radical is highly reactive towards NO  forming a very toxic oxidant ONOO‾ (the rate constant -7.0 x 109 M-1s-1) [20], although the species is poorly reactive to biomolecules [14]. At physiological pH, superoxide radical undergoes the dismutation reaction catalyzed by superoxide dismutase (SOD) enzyme, forming H 2 O 2 . In contrast to superoxide radical, hydrogen peroxide can cross cell membranes and form highly reactive hydroxyl radicals via the Fenton reaction [21]: (Me = Fe, Cu or Co free transition ion) and via the Haber-Weiss reaction: Reaction (2) may be catalyzed by free translation metal ions and the iron complexes in hemoglobin, myoglobin, transferrin and lactoferine [22]. Reactions (1) and (2) are commonly considered as the major sources of hydroxyl radical in vivo.
Hydroxyl radical is a particularly strong oxidant that reacts rapidly and non-specifi cally with all biomolecules, including the DNA bases. It is well documented that oxidized DNA is inclined to alterations in transcriptions and genetic mutations by conformational changes of its structure, including single strand and double strand breaks, infl uences in hydrogen bonding, and decrease in fi delity DNA and/or RNA polymerase [23]. In aqueous medium, the superoxide radical occurs in the equilibrium state with hydroperoxy radical (HOO  ). The HO  and HOO  radicals easily extract a hydrogen atom from unsaturated fatty acids. The process is followed by formation of a lipid radical (R  ) and further on a lipid peroxyl radical (ROO  ) after an addition of molecular oxygen. The latter radical is unstable, and abstracts a hydrogen atom from neighboring molecules or other lipid molecules, nucleic acids, forming lipid hydroperoxide (ROOH) and an alkyl radical (R  ). The hydroperoxide as unstable molecule undergoes spontaneous metal catalyzed decomposition, generating alkoxy radical (RO  ) and HO  .
The lipid peroxidation reaction is a chain reaction resulting in the production of a large number of free radicals, toxic aldehydes, polymerization and cross-linking reactions [24].
Additionally, the combination of two molecules of ROOH leads to the formation of a very strong oxidant 1 O 2 , similar to the reaction of ROOH with superoxide radical [25]: The high toxicity of aldehydes has long been recognized. Aldehydes can react with amino groups of proteins, free amino acids, nucleic acids, and with the thiol (-SH) groups of proteins, resulting in cell damage [26]. The second source of ROS includes actions of the physical factors (UV, -rays and X-rays r adiation, electromagnetic fi elds exposure) and environmental persistent chemicals exposure (xenobiotics, aromatic amines, ozone and organochlorines), and pathogens (reviewed previously by Klaunig, et al.) [9] (Figure 1). The increasing literature dealing with the role of ROS in cells has provided strong evidence of their participation in cell signaling processes, i.e. a communication between cells, response to extracellular signals and the expression of a number of genes [27].
Reactive oxygen species, particularly H 2 O 2 , have been recognized as the second messengers in signal transduction, and several transcription factors were reported to be modulated by these species: the nuclear factor (NF-B), signal transducer and activator of transcription STAT3, activator protein-1 (AP-1), Mitogen-Activated Protein Kinase (MAPK) pathway, nuclear factor p53, hypoxia-inducible factor (HIF-1), the nuclear factor of activated T cells family (NFAT), and others [13]. The transductional factors can induce gene expression, cell growth and differentiation.
For example, AP-1 is important for cell growth and differentiation. NF-B regulates genes involved in infl ammation, cell proliferation, and survival. Their activity is found to be dependent on the presence of transition metal ion [28]. Another aspect related to ROS, especially HO  and 1 O 2 , is their stimulating effect on the generation of NF-B, the factor which, among other activities, activates protein kinase C (PKC) -the enzyme which stimulates the formation of proinfl ammatory cytokines, e.g. interleukin-1 (IL-1), interleukin-6 (IL-6), tumor necrosis factor- (TNF-) and interferon. These cytokines have been reported to produce ROS in non-phagocytic cells [3].
It is generally recognized that ROS infl uence many stages in the cell signaling processes responsible for cell division, differentiation and apoptosis. It has been proven that ROS as a product of normal cellular metabolism play a dual role-benefi cial and harmful in biological systems.
The benefi cial role includes the ROS control of physiological functions, including participation in the cellular reduction/ oxidation reactions and protection of cells against OS. The damaging role of ROS appears when their production exceeds the cell's antioxidant defense system capacity. There are several consequences of un-balanced production of ROS, like direct DNA oxidation, considered as an action responsible for the initiation of mutagenesis, carcinogenesis, and also ageing. The next consequence is lipid peroxidation resulting in disruption of structural and functional role and/or formation of the hydroperoxides, having the ability to affect signaling pathways. Another outcome of OS is oxidation of proteins and their amino acids (residues) that may lead to the generation of disulfi de linkages between proteins, protein-SH groups and low molecular weight thiols, mainly glutathione (GSH), compounds containing carbonyl groups, and protein hydroperoxides. For example, the aldehydes malondialdehyde (MDA) and 4-hydroxy-2-nonenal are the most commonly detected end-products of lipid peroxidation or polyunsaturated fatty acid residues of phospholipids caused by ROS. These biomarkers of oxidative damage of lipids and fatty acids have been found to be mutagenic and carcinogenic, owing to their reaction with lysine, histidine and cysteine of the cellular protein's residues [1]. The next sensitive biomarker of the human diseases with participation of OS is the product of DNA damage 8-hydroxy-2'-deoxyguanosine (8-oxo-dG) resulting from hydroxylation of the C-8 position of guanine. Another determinant of the proteins oxidative damage associated with human diseases is production of mixed disulphides between the -SH groups of their amino acid residues and GSH and also a higher concentration of protein containing carbonyl groups (for exhaustive details, the reader is referred to a paper of Dalle-Donne, et al. [29]). Due to multiple mechanisms by which ROS may alter signaling processes, the activity of growth factors, enzymes or transcription factors may be modifi ed that, in turn, modulates gene expression or induces apoptosis. The permanent modifi cation of genes consists of the fi rst step involved in mutagenesis, carcinogenesis and ageing [30].

Antioxidants
The level of cellular ROS is kept under control due to the presence of a number of antioxidants operating in hydrophilic and hydrophobic environment of the cell. Antioxidants are molecules able to maintain good cellular functions owing to the ability of stabilization or deactivation of the ROS, thereby protecting the cells against auto-oxidative damage. Rahman [31] has reported that "An ideal antioxidant should be readily absorbed and quench free radicals, and chelate redox metals at physiological relevant levels. It should also work in both aqueous and/or membrane domains and effect gene expression in a positive way". The water-soluble antioxidants (hydrophilic) prevent the cell cytosol and the blood plasma  vitamin D, proteins, free amino acids, and peptides [32].
Compounds containing thiol group are essential for protection against ROS mediated oxidation of biomolecules. They act by scavenging free radicals and are a substrate for several enzymes.
Glutathione disulfi de (GSSG) (GSH in the oxidized state) is accumulated inside the cell under OS conditions. The ratio of GSH/GSSG is often used as a measure of OS in an organism [33]. In healthy cells, more than 90% of the total glutathione is present in the reduced form. Valko, et al. [3]. listed several main preventive actions of this antioxidant against oxidation: an ability to act as a cofactor for several detoxifying enzymes e.g. GPx, participation in transport of amino acids through the plasma membrane, regeneration of vitamins E and C back to their antioxidant state, and reduction of the tocopherol radical.
The second thiol antioxidant thioredoxin possesses two -SH groups and the oxidoreductase activity in its reduced form. The next thiol antioxidant -lipoic acid is also a disulphide and occurs in the cellular membranes and the cytosol of eukaryotic and prokaryotic cells. All thiol-containing antioxidants are powerful scavenging oxygen radicals and 1 O 2 , decompose H 2 O 2 , and lipid peroxides. Their antioxidant potential is due to the sulfur atom: Further, melatonin -an indoleamine neurohormone that is synthesized in the pineal gland and is also acquired in diet, exhibits an effi cient scavenging of ROS. The hormone is able to inhibit the metal ion-catalyzed oxidation [34]. Many researches have been carried out so far on the infl uence of nutrients on ROS production and removal. It has been found that high intake of proteins and animal fat can stimulate the generation of ROS, which, in turn, triggers lipid peroxidation [35]. Conversely, the diet rich in vegetables and fruits prevents against OS [36]. Additionally, the research has documented that products of natural origin play an important preventive role against cancer development [37].  [42]. The health and medicinal benefi ts of tea drinking is known since ancient times. This is due to the fact that green tea is a rich source of fl avonoids and anthocyanins. Recent molecular studies have revealed that green tea polyphenols protect against ocular disorders and cancer [43,44]  for example cyclooxygenase (COX-2), AP-1 or NF- [15].
Unfortunately, some antioxidants can act synergistically with other antioxidants and exert pro-oxidant action. Similarly, the pro-oxidant behavior of carotenoids was observed at their high concentration [45]. Recent literature fi ndings have showed that molecular hydrogen (H 2 ) exhibits antioxidative activity and exerts anti-infl ammatory and anti-allergic activities of high potency [46]. The protective and therapeutic applications of H 2 have been found in 38 diseases (citation for Ohta [47]), including ocular diseases. H 2 is the only antioxidant that crosses the blood-brain and blood-ocular barriers. It quickly penetrates through tissue due to its small molecular size and effectively removes ROS, mainly hydroxyl radicals and peroxynitrite. Apart from its antioxidant effects, H 2 also displays anti-infl ammatory, antiapoptotic, cytoprotective and mitohormetic properties ( Figure 2) [48].

Oxidative stress in eye diseases
Numerous researches have demonstrated that OS has been implicated in the pathogenesis of several ocular conditions and diseases, such as cataract, retrolental fi broplasia, glaucoma, membrane and to produce HO  radicals (reactions 1 and 2), its unbalanced concentration has been reported to be deleterious to the lens [54].
It is a commonly considered hypothesis that OS is a key factor in cataract formation. The disease is characterized by damage of crystalline proteins, nucleic acids, lipids, and polysaccharides with consequent opacity of the eye lens [51]. POAG patients had a lower TAS in the blood and higher levels of SOD, GPX, and CAT in the aqueous humor [58]. As support for this, it is reviewed that antioxidant such as polyphenols, can contribute, not only theoretically, to neuroprotection but which are also able to counteract the metabolic pathways that lead to glaucomatous damage [59].
Other ocular disease in which OS has been involved is diabetic retinopathy (DR). The worldwide prevalence of DR has been estimated to be 35%, and the prevalence of visionthreatening DR (VTDR) has been estimated to be 10% [60].
As reviewed by these authors, there is a relationship between  and ageing [49]. The eye is one of the major targets of ROS attack because it is environmentally exposed to UV radiation, irritants, high pressure of molecular oxygen, pollutants, industrial smoke, driving fumes, temperature, and wind. These OS has also been identifi ed to play a role also in retinitis  accumulates within the cycle of life and its rate increases with age [78]. This process has been observed in dry eye disease as a change in the quality and quantity of the tear fl uid with age, and is closely related to earlier discussed eye diseases. Today, it is still unclear whether OS is an initiator of eye diseases, but it is a general agreement that ROS participate in the propagation of the cell and tissue damages being involved in biochemical, physical, molecular and also pathological cellular events ( Figure 3).
OS is involved in infl ammation and a variety of cancers, including eye. Chronic infl ammation can increase cell proliferation and differentiation, limit apoptosis and form new blood vessels [79]. Cancer as a multistage and multistep process involves multiple molecular and cellular reactions that transform a normal cell to a malignant neoplastic cell [7,79]. and cancer [69].
Detailed mechanisms related to specifi c functions of ROS in carcinogenesis have been revised by different authors [9,10,15,69]  in the adnexal structures like the ducts and the eyelid. The most common type of primary intraocular cancer that starts in the eyeball is melanoma. Melanomas develop more frequently in white than black people. This may be due to higher sensitivity to sunlight. The suggested risk factors for eye melanoma include eye color, large number of moles on the skin, too much exposure to sunlight and exposure to UV radiation. Some researchers have observed that people with blue, green or grey eyes develop eye melanomas more frequently compared to those with brown eyes [81]. This type of cancer starts in the OS is implicated in the pathogenesis of cancer mainly through excessive production of ROS and defi ciency of antioxidant enzymes and non-enzymatic antioxidants, conducting to disruption of signaling pathways and gene expression [69]. For that reason, it is important to note that lutein and its isomer zeaxanthin, being members of the carotenoids group, are present in the lens and macula of retina and prevent against free radical damage. Their mode of action includes a screening of highly energetic blue light and scavenging/inhibiting of ROS arising during light irradiation [86]. Research fi ndings have shown that dietary intake of lutein signifi cantly reduces eye diseases, such as AMD and cataracts.
Mode of action of dietary antioxidants on several types of cancer has been recently listed by Valluru, et al. [83]. It is worth noting that both, ROS and antioxidants are implicated in cancer therapy and prevention [39]. Examples of the ROS application in cancer therapy are chemotherapy and radiation therapy [51,57,59,65,66]. Usage of ROS in cancer therapy involves the production of free radicals to cause damage of cancer cells and their necrosis. In turn, usage of antioxidants in chemotherapy helps protect normal tissues against damage from cytokines generated by ROS. Unfortunately, radiation therapy based on usage of X-and -rays to treat cancer includes ROS formation that kills tumor cells with simultaneous threat of the integrity and survival of the surrounding normal cells [87]. Usage of combinations of antioxidants with radiation or chemotherapy was reported to increase ROS levels within tumor, and also to reduce toxicity in the normal cells and to increase survival time [88]. In order to reduce the risk of free radical damage of surrounding normal cells during cancer chemotherapy and to improve the distribution of the drug in the exact target sites, bioactive nanoparticles (e.g. nanoparticles, nanofi bers, nanocapsules, nanotubes and others) have been projected as nanoplatforms for delivery of the drug [89].
Bio-oxidative therapy based on recognized molecular pathophysiology could produce adaptation of cells to OS. This action includes the increased generation of antioxidants (SOD, GSH) and the increase of the cells' resistance against the ROS driven reactions. These benefi ts are suggested to result from low level of ROS formed in systemic via through diverse modalities that is able to induce the expression of antioxidant enzymes and other protective systems [90]. The molecular mechanism involves up-regulation of the NF- transcription factor (for the mechanism of protection against OS on the NF- signaling way. The next important property of bioxidative therapy activity is an improvement in immune function by its positive effect on monocytes, neutrophils, lymphocytes and eosinophil and also can decrease the concentration of proinfl ammatory markers, such as IL-6, TNF-, and C-Reactive Protein (CRP) [91].
In a particular way, ozone by rectal insuffl ation application in retinitis pigmentosa has been evaluated in Cuba and demonstrated its effectiveness even with protective effects in several organs [92]. Another possible alternative is the regular practice of exercise, similar to bioxidative therapy could generate an increase in antioxidant response which reduced oxidant tone enough to reduce BMI. Physical exercise actions has e dual role: protection against the ROS damage by regular-moderate physical activity and damaging effect through mediation of OS by endurance exercise without adaptable physical training [52]. Otherwise, OS is related to the eye diseases as confi rmed by several animal and human studies, future research is encouraged in order to elucidate the detailed mechanisms of cellular processes linked to ROS and participating in the eye diseases etiology and prevention, to evaluate effi cacy and safety of particular modulated strategies, as well as the specifi c bio-oxidative therapy enhancing the antioxidant system and effecting adaptation of cells to OS. target the redox sensitive pathways and transcription factors contributing to adaptation and stress resistance. Due to common agreement that RP is caused by genetic bases, this could start from oxidative damage to DNA with consequent alteration of the gene expression. It is important to promote the healthy life style in order to prevent OS.

Confl icts of interest
The authors declare that there is no confl ict of interest regarding the publication of this paper.
This research did not receive any specifi c grant from funding agencies in the public, commercial, or not-for-profi t sectors.