Oxidative Stress in ESRD Patients on Dialysis and the Risk of Cardiovascular Diseases

Chronic kidney disease is highly prevalent worldwide. The decline of renal function is associated with inadequate removal of a variety of uremic toxins that exert detrimental effects on cells functioning, thus affecting the cardiovascular system. The occurrence of cardiovascular aberrations in CKD is related to the impact of traditional risk factors and non-traditional CKD-associated risk factors, including anemia; inflammation; oxidative stress; the presence of some uremic toxins; and factors related to the type, frequency of dialysis and the composition of dialysis fluid. Cardiovascular diseases are the most frequent cause for the deaths of patients with all stages of renal failure. The kidney is one of the vital sources of antioxidant enzymes, therefore, the impairment of this organ is associated with decreased levels of these enzymes as well as increased levels of pro-oxidants. Uremic toxins have been shown to play a vital role in the onset of oxidative stress. Hemodialysis itself also enhances oxidative stress. Elevated oxidative stress has been demonstrated to be strictly related to kidney and cardiac damage as it aggravates kidney dysfunction and induces cardiac hypertrophy. Antioxidant therapies may prove to be beneficial since they can decrease oxidative stress, reduce uremic cardiovascular toxicity and improve survival.


Introduction
Chronic kidney disease (CKD) is highly prevalent worldwide (between 11 and 13%), and it is most frequent in developed countries in Europe, USA, Canada, and Australia [1,2]. CKD progresses as GFR decreases and this process results from the deterioration of kidney function, which greatly influences body homeostasis and leads to biological and clinical dysfunctions, including the disturbances in cellular energetic metabolism, protein malnutrition, change in nitrogen input/output, insulin resistance, and significant increase in the synthesis of inflammation/oxidative stress mediators [2]. Finally, it progresses to end-stage renal disease (ESRD) and ends up with the necessity for renal replacement therapy (hemodialysis or peritoneal dialysis) or renal transplantation [3]. The decline of renal function is associated with inadequate removal of a variety of uremic toxins that should be excreted by the kidney. Due to the fact these substances are biologically active, they are both the cause and consequence of CKD [4]. According to studies, uremic toxins exert a detrimental effect on cells involved in the functioning of myocardium and vessels, including smooth muscle cells, endothelial cells (ECs), and platelets leucocytes, thus affecting the cardiovascular system [5]. Cardiovascular diseases (CAD) are the most frequent cause of death for patients with all stages of renal failure, and they are present in >50% of patients undergoing dialysis [4].

Oxidative Stress
The kidney is one of the vital sources of antioxidant enzymes, including glutathione peroxidases, and therefore, the impairment of this organ in the course of CKD is associated with decreased levels of these enzymes as well as increased levels of pro-oxidants [36]. The interplay between oxidants and antioxidants controls crucial pathways and cell metabolism [37,38]. In healthy conditions, reactive species are removed by natural endogenous defense mechanisms. However, in some diseases the defense mechanisms are impaired and/or the production of reactive species is so enhanced that it results in oxidative stress. Numerous studies confirmed the impairment of antioxidant systems (e.g., reduced activity of glutathione peroxidase and copper, zinc superoxide dismutase, and paraoxonase) in patients with CKD [39,40].
Oxidative stress is defined as the imbalance between the production of pro-oxidants and antioxidant defense mechanisms based on reactive oxygen species degradation. Reactive species generation and their cellular localization are usually in equilibrium with the availability of antioxidant enzymes including cystolic catalase (CAT), superoxide dismutase (SOD), and glutathione peroxidase (Gpx). The normal functioning of cells requires appropriate levels of both elements. In oxidative stress, the production of reactive oxygen species (ROS) exceeds the scavenging capacity of antioxidant systems [3,40]. The enhancement of oxidative stress has been demonstrated already in the early stages of CKD [41][42][43]. Numerous studies indicated that oxidative stress was significantly increased in patients Antioxidants 2020, 9,1079 4 of 24 with advanced renal impairment, but this state is exacerbated by hemodialysis [2,7,9,44]. Some authors suggested on the basis of in vivo studies that increased oxidative damage was the result of diminished levels of these enzymes rather than enhanced ROS production [45,46]. However, according to others, oxidative stress is associated with the production of highly reactive intermediates during inflammation; on the other hand, also reactive oxygen species (ROS) are able to stimulate pro-inflammatory mediators, such as NF-κB, thus promoting inflammatory response [41]. Inflammatory cells have been confirmed to be a source of free radicals, such as reactive oxygen and nitrogen species [47]. The upregulation of inflammatory markers observed in CKD patients (including platelet-derived growth factor and tumor necrosis factor-α) results in NADPH oxidase activation and subsequent generation of intracellular O 2 • and H 2 O 2 [48,49]. The presence of aggravated inflammatory state in CKD can stimulate the activation/recruitment of polymorphonuclear neutrophils and monocytes, which leads to stimulation of myeloperoxidase (MPO) and enhanced ROS production [50,51]. Uremic toxins have been shown to play a vital role in the onset of oxidative stress. Martinon et al. [52] demonstrated that uremic toxins promoted the development of inflammatory state and oxidative stress via priming acute inflammatory polymorphonuclear lymphocytes, stimulating interleukin (IL)-1β and IL-8. In turn, Sakamaki et al. [53] suggested that they stimulated the innate immune response through CD8+ cells. Stockler-Pinto et al. [54] revealed that indoxyl sulphate-related ROS production primarily resulted from the activation of nicotinamide adenine dinucleotide phosphate (NADPH) oxidase. Also, the synthesis of uric acid can aggravate oxidative stress via the activity of xanthine oxidoreductase, which generates reactive oxygen species [55]. However, some other reports suggest that in the presence of specific components, in various physiochemical circumstances and in different compartments of the human body, uric acid may play an anti-oxidant role in vivo [56]. Uric acid poses strong reducing and antioxidant properties, however, its elevated levels in CKD patients are believed to pose potential risk factors for CVD [57]. Under conditions of oxidative stress, high concentrations of uric acid have been shown to act as a pro-oxidant, particularly when antioxidant systems are impaired. However, it was also found to be cleared by HD as evidenced by a reduction in uric acid levels compared to pre-HD state [58].
The hemodialysis itself also enhances the oxidative stress due to the fact that antioxidant systems, particularly those of low or very-low molecular weight, are filtered during the procedure, and both the dialysis membrane and dialysate can activate leukocytes, leading to the aggravation of inflammation and enhanced ROS production [3]. Increased oxidative stress occurring in HD patients depends on many factors including aging, impairment of the residual renal function and subsequent uremic state, as well as the HD procedure itself [58]. During the initiation of the dialysis process, the membrane and dialysate induce inflammation and promote an important increase in ROS production. Post-dialysis, the levels of oxLDL have been shown to be elevated. However, post-dialysis, the activity of XOD and 8-OHdG levels are considerably diminished, which suggests that markers of oxidative stress are efficiently filtered during the dialysis process [3]. Also, markers of antioxidant defense decreased after HD [3]. Also, Liakopoulos et al. [59] stated that excessive oxidative stress in HD patients was related with the loss of antioxidants during the procedure and the accumulation of oxidative products. The level of oxidative stress was shown to be higher in ESRD patients on peritoneal dialysis (PD) compared to non-dialyzed uremic patients, however, it is lower in comparison to HD patients [59,60]. This observation was confirmed by Chen et al. [61] who demonstrated higher resting levels of superoxide anion in the whole blood after each HD session. Moreover, Granata et al. [62] revealed that patients with CKD and those undergoing hemodialysis show impaired mitochondrial respiration. The aggravation of oxidative stress can also be associated with the impaired activation of nuclear factor erythroid 2-related factor 2 (Nrf2), which is responsible for the regulation of genes encoding detoxifying and antioxidant proteins and enzymes (e.g., CAT, NAD(P)H dehydrogenase [quinone] 1 (NQO1), SOD [63]. Oxidative stress in patients with advanced stages of CKD can also be exacerbated by iron therapy, which is frequently used to treat anemia [36]. This phenomenon is associated with the fact that the administration of intravenous iron and the supersaturation of iron sequestration proteins (e.g., ferritin and transferrin) may result in the formation of free iron showing oxidative properties.
Decreased NO production, the formation ofreactive nitrogen and oxygen species are responsible for the detrimental effects related to oxidative stress

Nitric Oxide (NO)
The kidney is an important source of L-arginine, which is a precursor for nitric oxide (NO); therefore, the decrease in its mass may result in diminished production of L-arginine and NO activity [47]. Due to the fact that nitric oxide is essential for vascular endothelial cell function, its decreased bioavailability is associated with endothelial dysfunction observed in hypertension, diabetes mellitus, atherosclerosis, and CKD [4]. Asymmetric dimethylarginine (ADMA), which is an endogenous amino acid resembling L-arginine, inhibits endothelial nitric oxide synthase, thus impairing NO synthesis. Elevated ADMA concentrations have been reported in ESRD. Moreover, Ravani et al. [64] suggested that elevated ADMA levels were a strong independent risk factor for the progression of CKD and patient mortality. O 2 − and H 2 O 2 are the precursors used for the production of even more powerful oxidants. The first of them shows the affinity towards free radical NO and their reaction results in the formation of peroxynitrite (ONOO − ). The effects of both ONOO − and hydroxyl (OH − ) involve extensive nitrosative and oxidative modifications to proteins, lipids and nucleic acids [41].

Reactive Oxygen Species (ROS)
Reactive species produced in normal physiological state are inactivated by enzyme systems (e.g., glutathione) as well as other antioxidants (called scavengers) [41]. However, the excessive amount of ROS cannot be neutralized by scavenger systems, and therefore, they cause oxidative damage to proteins, nucleic acids and lipids; impair cellular activity; and hinder enzymatic activity [41]. In the kidneys, ROS are primarily synthesized by the mitochondrial respiratory chain and by enzymes such as NADPH oxidase (NOX) [41]. According to studies, NOX isoforms are vital players in the aggravation of oxidative stress, which results in the worsening of vascular function and promoting fibrosis [65,66]. To a lesser extent, ROS are produced by endoplasmic reticulum, peroxisomes and lysosomes [67]. Nox4 belonging to NADPH oxidase family is expressed in smooth muscle cells, vascular endothelial, as well renal proximal tubules, which explains why renal impairment may influence its expression or activity [68,69]. Also, pro-oxidant enzymes such as xanthine oxidase (XOD), in which activity has been shown to be considerably increased in uremia, is an additional possible source of ROS in CKD [3,70].
Reactive oxygen species (ROS) mainly include (O 2 •− ), the hydroxyl radical ( • OH) and hydrogen peroxide (H 2 O 2 ) [47]. In healthy metabolic cells, their production is counteracted by mitochondrial or cystolic catalase (CAT) or thiol peroxidases, which catalyze H 2 O 2 reduction into water and O 2 . Mitochondria comprise also other antioxidants, including manganese-SOD (Mn-SOD) and Gpx, which neutralize formed ROS. Mn-SOD converts O 2 •− to H 2 O 2 , which in the next step is decomposed by CAT and Gpx [71]. In peroxisomes, the stabilization of O 2 •− is related to the activity of copper/zinc-SOD (Cu/Zn-SOD) [72,73]. Also, glutathione homeostasis (Gpx, glutaredoxins, glutathione-S-transferase, peroxiredoxins and thioredoxins) is vital for maintaining cellular redox balance [47,74]. Xanthine oxidase (catalyzes the oxidation of hypoxanthine to uric acid, releasing in consequence ROS (O 2 • , • OH, and H 2 O 2 ) are by-products [51]. The formed uric acid accelerates CKD progression to renal failure and enhances the risk of cardiovascular events [75]. Reactive oxygen species are highly reactive and thus damage the variety of cellular structures and functional pathways [47]. Cellular H 2 O 2 is rather stable, however, it still has potential to interact with numerous substances and cause destruction. Ferrous iron (Fe 2+ ) can interact with H 2 O 2 , resulting in its cleavage and formation of the most reactive • OH form [76]. Protein tyrosine phosphatases have been shown to be major targets for oxidant signaling due to the fact that they are greatly susceptible to oxidative modification of amino acid residue of cysteine [77].
The presence of oxidative stress and the impairment of antioxidative defense mechanisms in patients with CKD/ESRD have been confirmed in numerous studies. Some products of oxidative metabolism, including advanced glycation end products (such as pentosidine or advanced oxidation protein products), have been demonstrated to accumulate in renal failure [78,79]. Kinugasa E [80] demonstrated increased circulating levels of oxidative stress markers, including advanced glycation end products (AGEs), malondialdehyde (MDA) and 8-hydroxyde-oxyguanosine in blood and/or tissue in CKD patients. Advanced glycation end products (AGEs) acting via a specific receptor (RAGE) activate MAP kinase transduction pathway and in consequence lead to an increase in the level of pro-inflammatory cytokines, enzymes and adhesion molecules [81,82]. Colombo et al. [83] confirmed the existence of a relationship between uremia and oxidative stress, which was assessed on the basis of severe protein oxidative damage (including plasma advanced oxidation protein products) in end-stage renal disease (ESRD) patients on maintenance hemodialysis (HD).

Consequences of Oxidative Stress
Aggravated oxidative stress has been reported to be involved in the pathomechanisms of several diseases, including cardiovascular disease and chronic kidney disease. Elevated oxidative stress has been demonstrated to be strictly related to kidney and cardiac damage as it aggravates kidney dysfunction and induces cardiac hypertrophy, which is an independent risk factor for heart failure (HF) [43,51]. Oxidative stress has been shown to affect upstream transcriptional gene regulation. Numerous studies provided evidence that proliferator-activated receptors (PPARs), which play key roles in the transcriptional regulation of cell cycle progression, cell differentiation, glucose homeostasis, lipid metabolism, and inflammation, are altered in CKD and CVD [84][85][86]. Oxidative stress results in the damage of nucleic acids, including the modifications of bases (especially guanine in DNA) and covalent crosslinks, leading to single-and double-strand breaks. The oxidation of guanine is associated with the formation of oxidized products including 8-hydroxy-20-deoxyguanosine (8-OH-dG), which are highly prevalent in chronic and degenerative diseases, including CKD [87].
Oxidative stress is responsible for progressive renal damage, which in consequence may lead to renal ischemia, glomeruli damage, cell death and apoptosis, and further worsening of the severe inflammatory processes [41,88]. Fujii et al. correlated oxidative stress with glomerular abnormalities, including glomerular hypertrophy and mesangial proliferation, observed in the course of diabetic nephropathy [89]. It is also an infamous factor responsible for cardiac damage, such as hypertrophy, fibrosis, apoptosis, and remodeling [90]. Numerous mechanisms via which oxidation products promote vascular injury have been suggested [91][92][93][94]. NADPH oxidases, which are the major sources of ROS, participate in the pathogenesis of cardiac remodeling via its impact on redox-sensitive signal transduction [43]. Numerous studies confirmed that both the expression and the activity of NADPH oxidase were elevated in the myocardium of patients with ischemic and non-ischemic heart failure [95][96][97]. Moreover, higher activation of NADPH oxidase was involved in fibrosis and cardiac hypertrophy [98,99]. Uremic toxin, indoxyl sulphate, which promotes the production of ROS through the stimulation of NADPH oxidase or NADPH-like oxidase, has been shown to be involved in vascular disease, as it promotes vascular smooth muscle cell proliferation and vascular calcification; in addition, it is associated with higher mortality observed in CKD patients [100][101][102]. Moreover, this toxin also reduces levels of total glutathione in endothelial cells [43,103].
Oxidative stress-induced endothelial dysfunction and subsequent reduction in NO bioavailability promote the development of atherosclerosis. Peroxynitrite generated from NO is involved in numerous unfavorable vascular actions. The inactivation and the deficiency of NO resulting also from the actions of reactive species decrease the protection of kidney function, which is related to NO-dependent increase in renal blood flow, stimulation of pressure natriuresis, regulation of tubuloglomerular function, and maintenance of fluid and electrolyte homeostasis [41,104].
Oxidative stress is also associated with the formation of oxidized low density lipoprotein (ox-LDL), which play a crucial role in the pathogenesis of atherosclerosis [105]. The accumulation of oxidized low-density lipoproteins in arterial intima is the initial step of atherosclerotic process development [13]. Also, advanced glycation end products, in which production is enhanced in renal failure, exert atherogenic effects [13].
The link between cardiovascular disease and CKD may also involve the actions of the functional mitochondrial angiotensin system, which is regulated by oxidative stress [106]. Angiotensin type II receptors co-localized with angiotensin on the inner mitochondrial membrane of human mononuclear cells were shown to control mitochondrial NO production and respiration. The activation of the renin-angiotensin system (RAAS) in the course of renal impairment is involved in the process of left ventricular (LV) remodeling [107]. Ang II induces vasoconstriction and aldosterone release and it mediates hemodynamic alterations, which in consequence, lead to cardiac and vascular remodeling [108]. Furthermore, both angiotensin II and aldosterone actions involve the activation of mitogen-activated protein kinases (MAPKs), as well as c-Src and Ki-ras2A pathways engaged in the development of inflammation, in the production of O 2 and H 2 O 2 , endothelial dysfunction, as well as hypertrophic growth [109,110]. Higher O 2 levels are associated with enhanced protein kinase C (PKC) activity and NOS uncoupling, as well as consequent loss of vasodilation [51]. ONOO-associated loss of vasodilation and subsequent endothelial dysfunction play a vital role in the development of hypertension and further contributes to hypertrophic remodeling [111]. In CKD patients, oxidative stress leads also to left ventricular hypertrophy (LVH). The role of oxidative stress in the development of cardiac remodeling and heart failure has been summarized at Figure 1.
Antioxidants 2020, 9, x FOR PEER REVIEW 7 of 24 activation of the renin-angiotensin system (RAAS) in the course of renal impairment is involved in the process of left ventricular (LV) remodeling [107]. Ang II induces vasoconstriction and aldosterone release and it mediates hemodynamic alterations, which in consequence, lead to cardiac and vascular remodeling [108]. Furthermore, both angiotensin II and aldosterone actions involve the activation of mitogen-activated protein kinases (MAPKs), as well as c-Src and Ki-ras2A pathways engaged in the development of inflammation, in the production of O2 and H2O2, endothelial dysfunction, as well as hypertrophic growth [109,110]. Higher O2 levels are associated with enhanced protein kinase C (PKC) activity and NOS uncoupling, as well as consequent loss of vasodilation [51]. ONOO-associated loss of vasodilation and subsequent endothelial dysfunction play a vital role in the development of hypertension and further contributes to hypertrophic remodeling [111]. In CKD patients, oxidative stress leads also to left ventricular hypertrophy (LVH). The role of oxidative stress in the development of cardiac remodeling and heart failure has been summarized at Figure 1. According to studies, oxidative stress as well as excessive ROS production are important factors mediating osteochondrogenic transdifferentiation of vascular smooth muscle cells (VSMCs) and enhanced vascular calcification [112]. The development of vascular calcifications (VC), which occurs commonly in CKD patients, exerts a direct impact on vessel functions and CVD development. Considerable leukocyte infiltration and the presence of IL-1β and MMP-1 have confirmed that human calcified areas in aortic valves lead to accelerated atherosclerosis, as well as higher rates of cardiovascular and all-cause mortality [113][114][115][116]. Oxidative stress has been demonstrated to contribute to the phenotype switch of vascular smooth muscle cells (VSMCs) even in early CKD [116]. Huang et al. [116] provided evidence for the existence of a kinetic relationship between oxidative stress and vascular calcification and osteoblastic transition. They observed that serum derived from patients with early stage CKD directly induced osteoblastic transition of primary rat VSMCs and calcium deposition in VSCMs, but it did not affect serum phosphorus level. In in vitro studies, hydrogen peroxide (H2O2) and xanthine/xanthine oxidase, which generates superoxide anion, were shown to boost osteochondrogenic transdifferentiation of VSMCs [112,117,118]. Intensified calcification in the presence of H2O2 was associated with higher expression of osteogenic markers, such as osteocalcin (OCN), runt-related transcription factor 2 (Runx2) and alkaline phosphatase (ALP), and lower expression of the contractile VSMCs phenotype markers, such as smooth muscle αactin (α-SMA) and SM-22α [118]. It has been suggested that Msx2 is an important factor involved not only in transcriptional programming of osteoblastic lineage development but also in BMP-2- According to studies, oxidative stress as well as excessive ROS production are important factors mediating osteochondrogenic transdifferentiation of vascular smooth muscle cells (VSMCs) and enhanced vascular calcification [112]. The development of vascular calcifications (VC), which occurs commonly in CKD patients, exerts a direct impact on vessel functions and CVD development. Considerable leukocyte infiltration and the presence of IL-1β and MMP-1 have confirmed that human calcified areas in aortic valves lead to accelerated atherosclerosis, as well as higher rates of cardiovascular and all-cause mortality [113][114][115][116]. Oxidative stress has been demonstrated to contribute to the phenotype switch of vascular smooth muscle cells (VSMCs) even in early CKD [116]. Huang et al. [116] provided evidence for the existence of a kinetic relationship between oxidative stress and vascular calcification and osteoblastic transition. They observed that serum derived from patients with early stage CKD directly induced osteoblastic transition of primary rat VSMCs and calcium deposition in VSCMs, but it did not affect serum phosphorus level. In in vitro studies, hydrogen peroxide (H 2 O 2 ) and xanthine/xanthine oxidase, which generates superoxide anion, were shown to boost osteochondrogenic transdifferentiation of VSMCs [112,117,118]. Intensified calcification in the presence of H 2 O 2 was associated with higher expression of osteogenic markers, such as osteocalcin (OCN), runt-related transcription factor 2 (Runx2) and alkaline phosphatase (ALP), and lower expression of the contractile VSMCs phenotype markers, such as smooth muscle α-actin (α-SMA) and SM-22α [118]. It has been suggested that Msx2 is an important factor involved not only in transcriptional programming of osteoblastic lineage development but also in BMP-2-mediated vascular calcification through the activation of Wnt catenin signaling and β-catenin-induced activation of Pit1, a type III sodium-dependent phosphate cotransporter [119][120][121]. Cai et al. found that WNT/β-catenin signaling directly elicited osteogenic transdifferentiation and calcification of VSMCs though the modulation of Runx2 gene expression [122]. Huang et al. [116] suggested that the development of vascular calcification could be partly mediated by upregulation of NOX1 as well as ERK kinases as downstream events of NOX1-induced VC. Oxidative stress also indirectly may stimulate vascular calcification. It has been demonstrated that lipid oxidation products present in oxidized low-density lipoprotein rise the activity of ALP and promote calcification of vascular cells, which in consequence may lead to atherosclerosis-associated intimal calcification [123]. Moreover, it seems that oxidative stress in uremia enhances the formation of advanced oxidation protein products (AOPP), but at the same time, the accumulation of AOPP may pose a trigger for enhanced oxidative stress, which gives a positive feedback loop of elevated and maintained oxidative stress in uremic patients [124]. The results of in vitro study revealed that AOPP could directly stimulate osteoblast differentiation and calcification of smooth muscle cells [124]. You et al. [124] demonstrated that AOPP rose the calcium level in human aortic smooth muscle cells (HASMCs) (probably inducing their calcification) and considerably enhanced protein levels and mRNA expression of osteopontin (OPN), which may suggest that AOPP could promote osteoblast differentiation of HASMCs. Moreover, AOPP up-regulated mRNA expression of a transcription factor CBF-α1, which had earlier been found to increase the expression of osteoblast-specific genes, e.g., osteocalcin and alkaline phosphatase [125,126]. Finally, they were shown to considerably lower the expression of SM-α-actin expression [124]. According to in vitro studies, advanced oxidation products can not only trigger the oxidative burst of human monocyte and neutrophil but also induce enhanced production of oxidants by leukocytes [124,127,128]. It has been suggested that the mechanism of AOPP-stimulated smooth muscle cells differentiation may involve the activation of extracellular signal-regulated kinase (ERK), which is a part of MAPK pathway [124]. ERK is able to induce the osteoblast-related gene expression by extracellular matrix-integrin receptor interaction, bone morphogenetic protein 2 (BMP-2) and growth factors, thus leading to osteoblast differentiation [129][130][131]. Moreover, MAPK increases the expression of osteocalcin and AOPP-induced calcium deposition, which results also in the calcification of HASMC [130,132]. The role of oxidative stress in the development of atherosclerosis and other adverse consequences has been presented at Figure 2. muscle cells differentiation may involve the activation of extracellular signal-regulated kinase (ERK), which is a part of MAPK pathway [124]. ERK is able to induce the osteoblast-related gene expression by extracellular matrix-integrin receptor interaction, bone morphogenetic protein 2 (BMP-2) and growth factors, thus leading to osteoblast differentiation [129][130][131]. Moreover, MAPK increases the expression of osteocalcin and AOPP-induced calcium deposition, which results also in the calcification of HASMC [130,132]. The role of oxidative stress in the development of atherosclerosis and other adverse consequences has been presented at Figure 2.  Finally, oxidative stress, especially the exposure to H 2 O 2 , has been demonstrated to alter membrane properties of red blood cells (RBC) and accelerate RBC removal in the spleen [51,133]. The increased susceptibility of RBCs to oxidative damage along with the higher risk of ROS production in iron deficiency anemia in CKD create a vicious cycle of enhanced RBC death, anemia and oxidative stress severity [51,133,134]. The lowering of hemoglobin content in iron deficiency-related anemia is associated with the decrease in partial pressure of oxygen, and this hypoxia-resembling state aggravates oxidative stress via auto-oxidation of hemoglobin to met-hemoglobin (metHb) with accompanying generation of O2 [135,136]. Iron deficiency also affects the expression of iron-containing endogenous antioxidant proteins e.g., peroxidase and catalase, as well as concentration of selenium, thus decreasing the activity of selenium-dependent enzyme GPx [137][138][139]. Therefore, it seems that timely intravenous iron replacement and the administration of antioxidants in clinical setting could improve CKD patients' quality of life and decrease the risk of morbidity [140].
Numerous studies have confirmed that oxidative stress-related oxidation of fatty acid end-products (malondialdehyde) and serum albumin is associated with higher mortality in hemodialysis [141].
According to some authors, the decrease in antioxidant defense in hemodialysis results in enhanced all-cause and cardiovascular mortality in these patients [142][143][144]. Sangeetha Lakshmi et al. [58] revealed that the concentration of malondialdehyde (biomarker of oxidative stress) was considerably increased in patients with CKD and accompanying cardiovascular disease compared to patients with CKD but without cardiovascular disease. Substantial elevation of serum malondialdehyde levels observed in HD patients suffering from CVD, compared with those without CVD, indicated an association between oxidative stress and the development of atherosclerosis in these patients [9]. Juretic et al. [141] observed reduced PON in those patients with uremia who were at higher risk of cardiovascular disease, compared to persons with normal kidney function. It has been suggested that the loss of PON activity may increase the risk for oxidative stress and cardiovascular disease patients with chronic kidney disease, despite the lack of correlation with oxidized LDL [40]. Finally, Russa et al. [2] observed higher values of both oxidative stress and antioxidant barrier in hemodialysis patients with previous acute myocardial infarction compared to patients without cardiovascular events.
Enhanced risk of mortality risk might be mitigated by diminishing of oxidative stress, for example through the use of less aggressive types of dialysis (e.g., peritoneal) or antioxidant therapies [3]. Table 1 presents the results of selected articles concerning adverse impact of oxidative stress.

Treatment Aiming to Decrease the Risk Resulting from Oxidative Stress
According to studies, antioxidant therapies may prove to be beneficial since they can decrease oxidative stress, reduce uremic cardiovascular toxicity and improve survival [104]. Endogenous or dietary antioxidants have also been suggested to exert protective effects against inflammation and kidney damage in patients with CKD [41]. The application of mitochondrial-targeted antioxidant therapy resulted in the improvement of cardiac hypertrophy and diastolic dysfunction through the diminution of oxidative stress, which confirms the role of oxidative stress in the progression of heart failure [145]. Antioxidant supplementation with vitamins A, C, and E; β-carotene; or N-acetyl cysteine (NAC) seems to be beneficial in decreasing cardiovascular risk in hemodialysis patents [3,146]. Vitamin E is a powerful antioxidant exerting anti-inflammatory properties; it has been shown to interfere with cell membrane lipid peroxidation [147]. Observational clinical studies have shown that the intake of vitamin E (more than 100 IU/day), which inhibits oxLDL formation by hindering lipid peroxidation, reduced the rate of coronary events in hemodialysis [41,148,149]. Randomized placebo-controlled Secondary Prevention with Antioxidants of Cardiovascular Disease in End-stage Renal Disease (SPACE) trial revealed that in hemodialysis patients, the supplementation of alpha-tocopherol (800 IU) decreased cardiovascular disease endpoints and resulted in a substantial improvement of cardiovascular complications (myocardial infarction) [149]. Antioxidant therapy with DL-α-tocopherol has been revealed to improve left ventricular hypertrophy (LVH) and to decrease adverse changes within the myocardium in experimental CKD [150].
Vitamin C plays a significant antioxidative role as it can reduce ROS levels, thus providing protection against kidney oxidative damage and helping to maintain vascular and endothelial function [151]. Wang et al. [152] demonstrated that vitamin C (ascorbic acid) diminished oxidative damage, inflammation and renal injury in ischemia nephrotoxic acute kidney injury and rhabdomyolysis-induced renal injury. Deicher et al. observed deficiency of vitamin C (non-enzymatic antioxidant) in hemodialysis patients, which was associated with dietary restrictions and/or its loss during dialysis. Study of HD patients demonstrated that low plasma vitamin C levels predicted fatal and major non-fatal adverse cardiovascular events in this group [153]. Some studies indicate that patients with chronic kidney disease and ESRD patients should be administered a limited dose of daily vitamin C supplement of 75 mg for females and 90 mg for males [154]. However, Jankowska et al. [155] suggested that the supplementation of vitamin C might lead to oxalate accumulation and subsequent elevation in oxidative stress, and therefore, antioxidants administration may not always be the best alternative.
In turn, vitamin D is vital not only for the homeostasis of calcium/phosphorus and skeletal health but also for renal functioning. The deficiency of this vitamin is frequently observed in CKD and ESRD and has been shown to contribute to the deterioration of renal function and increased morbidity and mortality in patients with CKD [56]. Some studies have demonstrated that the intake of vitamin D can reduce kidney injury by suppressing inflammation, fibrosis and apoptosis, via hindering multiple pathways crucial in kidney injury, including renin-angiotensin-aldosterone system (RAAS), NFκ-B, Wnt/β-catenin, and TGF-β/Smad signaling pathways [156][157][158].
Due to the fact that indoxyl sulphate stimulates oxidative stress and hastens the progression of CVD in CKD, the lowering of its concentration may prove beneficial in this group of patients. This suggestion was confirmed in several studies that demonstrated that the use of oral charcoal adsorbent, which decreases the levels of circulating uremic toxins, prevented histological and functional aggravation of CKD and suppressed oxidative stress and the advancement of cardiac damage in CKD [159][160][161]. In other studies, the decrease in heart and left ventricular volumes, cardiac fibrosis, as well as the attenuation of cardiac concentric change were observed in pre-dialysis CKD patients after AST-120 administration [162,163]. Taken together, these results suggest that the administration of AST-120 may become a useful option for improving cardiovascular health in CKD patients [43].
Also, melatonin (N-acetyl-5-methoxytryptamine) has been proven to be highly efficient in many disorders associated with oxidative stress and inflammation in experimental animals [164][165][166].
It is an endogenous neurohormone modulating sleep, immune function, circadian rhythm, and sexual behaviors, which exerts free radical scavenger, antioxidant and anti-inflammatory effects [165,167]. Due to the fact that it scavenges reactive oxygen and nitrogen species and enhances antioxidant defense systems, melatonin prevents tissue damage and hampers transcriptional factors of pro-inflammatory cytokines. Numerous studies have indicated that it indirectly decreases oxidative stress via stimulation of the expression and function of some antioxidant enzymes, enhancing the activities of antioxidative defense systems and glutathione as well as rising the efficacy of the mitochondrial electron transport chain [168][169][170]. CKD patients have been shown to have impaired night-time secretion of melatonin, which is further associated with higher stimulation of intrarenal renin-angiotensin system (RAS), leading to enhanced reactive oxygen species (ROS) production, sodium retention, inflammation, and fibrosis [171]. These pathologies accelerate the progression of CKD to end-stage renal disease (ESRD). The results of studies indicated that the supplementation with exogenous melatonin can reverse adverse changes, slow down the progression of kidney impairment, reduce blood pressure, and also help to maintain the bioavailability of nitric oxide by acting on melatonin receptor MT2 [172]. Studies on animal models indicated that prolonged administration of melatonin enhanced the expression of markers associated with decreased oxidative stress, inflammation and vasoprotection [173]. It has also been shown to improve cardiovascular function as well as renal, cardiac and cerebral damage [174].
The introduction of therapy based on the thiol-containing compound acetylcysteine has been demonstrated to diminish the toxic effects of ischemia reperfusion syndromes of the heart, kidney, liver, and lung and enabled the reduction in the risk of primary cardiovascular endpoint (fatal and non-fatal myocardial infarction) by 40% in hemodialysis patients [13]. Tepel et al. [13] revealed that after adjustment for age, baseline systolic and diastolic blood pressure, medications, smoking, and the duration of hemodialysis, in the study group treated with acetylcysteine, the survival related to the primary end point was higher compared with the control group. Moreover, they observed 30% decrease in cardiac events, 69% reduction in ischemic strokes, and 36% drop in peripheral vascular disease in the acetylcysteine group, however, the differences did not reach the level of statistical significance [13]. Due to the fact that acetylcysteine acts as a free-radical scavenger or as a reactive sulfhydryl compound, it enhances the reductive capacity of the cell ameliorating coronary and peripheral vascular function [175]. However, this antioxidant therapy proved ineffective in patients with heart failure without renal disease [176]. Therefore, it seems that the systemic oxidative stress in uraemic milieu plays a vital role in the development of cardiac disease in renal patients Some studies indicated that in patients undergoing maintenance, HD plasma concentrations of CoQ10 are reduced, which suggests that CoQ10 supplementation could represent a great antioxidant therapy for these patients [177]. Randomized, double-blind, placebo-controlled study carried out by Rivara et al. [178] demonstrated that administration of CoQ10 (1200 mg daily) as an antioxidant therapy was safe and well tolerated in patients receiving MHD, and it resulted in a substantial, dose-dependent rise in plasma CoQ10 levels compared to placebo and considerably diminished plasma concentrations of F2-isoprostanes, which are considered a robust plasma marker of oxidative stress.

Conclusions
Elevated cardiovascular morbidity and mortality in patients with end-stage renal failure remains to be a challenge in medicine. Numerous studies indicate that oxidative stress may play an important role in the development and progression of cardiovascular disease. However, antioxidant therapies seem to exert beneficial effects as they decrease cardiovascular risk and they bring hope for less cardiovascular complications in this group of patients.

Funding:
No external funding was received in relation to this article.

Conflicts of Interest:
The authors declare no conflict of interest.