Oxidative stress mechanisms as potential therapeutic targets in chronic kidney disease

Introduction

Chronic kidney disease (CKD) is a leading health problem worldwide, especially in developed countries. According to the data published by the Centres for Disease Control and Prevention in 2021, more than 1 in 7, i.e. 15% of US adults (about 37 million people), are estimated to have CKD [1]. According to official data, there were 4,335,349 cases of CKD in Poland in 2017 and there were 649.2–752.1 million people with CKD in the world. Thus, the global prevalence of CKD was estimated as 9.1% of the world’s population [2].
CKD is diagnosed on the basis of commonly accepted guidelines of the National Kidney Foundation (NKF) Kidney Disease Outcome Quality Initiative (KDOQI) Group. According to the former KDOQI guidelines released in 2002, CKD was defined as kidney damage for ≥ 3 months, as defined by structural or functional abnormalities of the kidney, with or without decreased GFR, manifested by either pathological abnormalities or markers of kidney damage, including abnormalities in the composition of the blood or urine, or abnormalities in imaging tests. In the above-mentioned guidelines, another diagnostic criterion for the diagnosis of CKD was GFR < 60 ml/min/1.73 m2 for ≥ 3 months, with or without kidney damage [3]. In line with KDOQI 2012 guidelines, which are still in force, CKD is defined as abnormalities of kidney structure or function, present for > 3 months, with implications for health. The detailed criteria for CKD involve the GFR decrease < 60 ml/min/1.73 m2 with/or the presence of one or more markers of kidney damage (albuminuria, urine sediment abnormalities, electrolyte and other abnormalities due to tubular disorders, abnormalities detected by histology, structural abnormalities detected by imaging, history of kidney transplantation) present for > 3 months [4]. The KDIGO Consensus Conference recently convened in June 2019, held on as the unchained the criteria for CKD (markers of kidney damage or GFR < 60 ml/min per 1.73 m2 for > 3 months). The updated guidelines recommend that ascertainment of CKD, its severity, and prognosis should be based on the cause of disease, level of GFR (6 categories), and level of albuminuria (3 categories), collectively known as the CGA classification, rather than on GFR alone. Moreover, the albuminuria and GFR categories have been grouped into 4 risk categories (usually portrayed as a “heat map”) according to their associations with risks for various outcomes (all-cause and cardiovascular mortality, kidney failure requiring replacement therapy, AKI and CKD progression) [5].
The clinical presentation of CKD in the stage of kidney failure is very rich and involves systemic symptoms and developing complications originating from the following: skin (e.g. pallor, itching), cardiovascular (hypertension, pericarditis, heart failure), respiratory (pulmonary congestion and oedema), digestive (gastroduodenitis, peptic ulcer disease), nervous (impaired intellectual and emotional functions, sleep disorders, polyneuropathy), and endocrine disorders (fertility disorders, secondary hyperparathyroidism, hyperphosphataemia, osteodystrophy) and general symptoms (hypothermia, electrolyte disturbances, immune disorders). The main aetiological factors of CKD are hypertensive nephropathy and diabetic nephropathy as well as diseases primarily affecting the kidneys and urinary tract, such as: chronic glomerulonephritis, tubulointerstitial nephritis, polycystic kidney disease, ischaemic nephropathy, and obstructive uropathy. The rare causes involve systemic connective tissue disease, sarcoidosis, or amyloidosis. The pathophysiology of CKD is complex, related to inflammatory, immune, and metabolic disturbances. Briefly, CKD pathogenesis involves gradual, irreversible structural loss of the nephrons, with hyperfiltration, and pressure overload of the remaining nephrons. These dysfunctions, along with the ongoing inflammation, finally lead to glomerular hypertrophy and sclerosis, and progressive fibrosis of the kidney interstitial tissue [6, 7].

Oxidative stress and low-grade inflammation in chronic kidney disease

One of the hallmarks of the pathogenesis of CKD resulting in its development, progression, and complications is low-grade ongoing inflammation that is inseparably linked to oxidative stress (OS), because reactive oxygen species (ROS) and reactive nitrogen species (RNS) are regarded as inflammatory mediators and there is a crosstalk between pathways of inflammation and OS. The ongoing inflammatory reaction is due to the presence of resident immune cells, including dendritic cells, macrophages, regulatory T lymphocytes, lymphocytes CD8, and NK, which are in close relationship with the renal parenchymal cells. Once activated by the events related to the baseline disease affecting the kidneys, these cells start the secretion of the inflammatory mediators initiating kidney response. In the course of CKD, the inflammatory-trigering factors involve metabolic acidosis development, intestinal dysbiosis and gut microbiota translocation into systemic circulation with subsequent triggering kidney inflammatory response, impaired kidney elimination of endogenous toxins (indoxyl sulphate, paracresyl sulphate, asymmetric dimethylarginine), external noxious stimuli, and dialysis-related factors that still sustain the response [8].
Thus, under a low-grade inflammation of the kidney, a chronic and persistent activation of the immune cells occurs, which results in consequent ROS/RNS production. Besides chronic inflammation, with subsequent activation of the myeloperoxidase of neutrophils and macrophages, there are also other mechanisms co-responsible for the increased oxidative stress in CKD. They involve mitochondrial sources of ROS: oxidative phosphorylation, uncoupling of the respiratory chain (misutilization of the electrons for heat and ROS production instead of ATP production), and cytosolic ones: deficiency of glucose-6-phosphate dehydrogenase, overactivity of NADPH oxidase, myeloperoxidase and xanthine oxidase, eNOS uncoupling due to restricted L-arginine availability, or the absence of cofactors (flavinmononucleotide, bihydrobiopterin, calmodulin, flavin adenine dinucleotide), along with the impairment and deficit of the antioxidant systems. The increased mitochondrial ROS/RNS generation is reported in CKD patients, especially in diabetic nephropathy [9]. Similarly, RAA system overactivation and angiotensin II action are factors enhancing oxidative stress because angiotensin II contributes to ROS production through NAPDH oxidase [10]. The next important issue is the endothelial nitric oxidase (eNOS) uncoupling in the kidney, induced by uremic toxins (e.g. asymmetric dimethylarginine). It leads to peroxynitrite overproduction, which, in turn, further inhibits eNOS activity. The reduced nitric oxide synthesis results in the tendency of vasoconstriction and consequent GFR reduction [11]. The other well-described mechanism favouring increased stress in the kidney is increased xanthine oxidase or myeloperoxidase activity [12].
The abovementioned mechanisms are accompanied by the decreased activity of kidney antioxidants. It was reported that the expression of kidney superoxide dismutase – a key enzyme responsible for the detoxification of free radicals – is decreased in CKD. Moreover, it was demonstrated along with insufficiency of the GSH antioxidant system and selenium deficiency [13]. The listed mechanisms are initiated by some of the external and internal factors, which are given in Figure 1. It is worth mentioning that renal replacement therapy with the haemodialysis process itself is an important source of OS because ROS/RNS are excreted by phagocytes settling on the surface of the dialysis membrane due to the bioincompatibility. Similarly, peritoneal dialysis session also triggers OS due to the bioincompatibility and high osmolality of dialysis solutions, low pH, and loss of water-soluble vitamins and trace elements in the filtrate. The supplementary OS-generating factor in CKD patients is dietary restrictions with reduced consumption of potassium-rich vegetables and fruits, which contributes to a reduced intake of antioxidants [9].
The action of ROS/RNS affects all of the kidney elements: renal microcirculation, glomerulus, renal tubules, and interstitial tissue. ROS, acting synergistically with inflammation mediators cause the remodeling of kidney tissues resulting in tubulointerstitial inflammation and fibrosis, tubular atrophy, glomerulosclerosis, and renal vasculopathy. It was shown that OS plays an important role in CKD progression, originating from diabetic nephropathy, glomerulonephritis, tubulointerstitial inflammation, or chronic renal allograft dysfunction. As mentioned above, oxidative stress impairs renal blood flow because it causes vasocontraction of the vascular smooth muscle cells. It is accompanied by impairment of the endothelial nitric oxide synthase activity, leading to the decrease of the vasoactive NO [13]. In kidney mesangium, the excessive ROS/RNS production activates the signalization pathway associated with protein kinase C (PKC), protein kinase B (PKB), and c-Jun-N-terminal kinase (JNK), which results in the phenotypic transformation into fibroblasts [14]. Moreover, an increased local synthesis of transforming growth factor  (TGF-), as previously mentioned, accounts for the increased collagen and fibronectin synthesis. These phenomena finally lead to glomerular fibrosis [13]. Moreover, in the glomeruli mesangial cell expansion, contraction of the glomerular tuft occurs, which also contributes to filtration barrier damage. ROS/RNS also directly affect the glomerular filtration barrier because these molecules contribute to autophagy and apoptosis of the podocytes, and the damage of the barrier is further intensified by the release of the other inflammatory mediators by neutrophils and monocytes/macrophages. The changes exclude functionally damaged glomeruli, so the remaining ones are hyperfiltrated, resulting in global glomerulosclerosis [13]. In the renal tubules, the proximal coins are the location with the most accentuated oxidative stress, because this part of the nephron is characterized by high oxygen consumption for ATP production, necessary for active pumps (e.g. 2Na/3K antiport) activity, but without the ability to synthetize oxido-protective glutathione. ROS/RNS can influence the expression of some of the active systems (e.g. Na/H antiport) thus causing decreased sodium reabsorption [13, 14]. In the other part of the nephron, ROS/RNS may act in the opposite manner, contributing to enhanced sodium reabsorption. In the ascending part of the Henle loop oxidative stress stimulates the 2Na/3K antiport and Na/K/2Cl co-transporter activity. In turn, at the level of distal tubules, the sodium channel sensitive to amiloride (ENaC) activity is increased [13]. Ultimately, taking all these facts together, it can be concluded that excessive oxidative stress favors sodium retention. Furthermore, similar to glomerulus, increased production of extracellular matrix, loss of the tubular transport properties, and phenotypic transformation to the fibroblasts takes place. Finally, fibrosis of the renal parenchyma occurs, which is responsible for the progression of kidney failure and acceleration of achieving the ERSD phase. Exposure of the renal tubular cells to aggravated oxidative stress leads to their senescence and apoptosis (as a result of oxidation and activation of caspase 3 and 8) or necrosis (due to the stimulation of the [ADP-rybose]-polymerase 1) [13, 14]. To sum up, the OS occurrence in CKD already starts in the early phase of CKD as a part of the inflammatory process, and leads to the acceleration of the disease and its progression to the ESRD stage. It is worth noting that early histopathological abnormalities in kidneys may occur without significant clinical presentation due to the high adaptability of the kidney, and once the adaptive threshold is reached, the symptoms of kidney injury rapidly progress. Furthermore, there are also systemic consequences of inflammatory and oxidative damage that take place in the kidneys. Many inflammatory compounds act as mediators of kidney-circulatory system cross-talk, which results in the reciprocal dysfunction and cardio-renal syndrome development. The common denominator of these diseases is OS-induced endothelial dysfunction and the increased risk of cardiovascular disease development. Thus, the process of atherosclerosis is exaggerated [15]. The other OS-related cardiovascular entity in CKD patients is the aggravation of hypertension and cardiac hypertrophy. Cardiac hypertrophy is intrinsically arrhythmogenic, it reduces the coronary flow reserve, increases cardiac oxygen consumption, and is strongly associated with diastolic dysfunction. Taking all these facts together, cardiac hypertrophy is a predictor of increased cardiovascular mortality in CKD patients [9]. CKD patients often present other comorbidities related to OS, like dyslipidaemia, diabetes mellitus, or vascular calcification. ROS are also genotoxic and therefore may contribute to the higher malignancy rate in CKD patients [15]. Lastly, OS in the course of CKD may also predispose to neurological disturbances (as a result of oxidation of myelin), anaemia (due to the decrease of the erythrocyte lifespan), and hormonal abnormalities (e.g. exacerbation of parathyroid-related bone disorders) [15].

Potential future options for pharmacological mitigation of kidney damage induced by oxidative stress

Because a growing body of evidence clearly suggests a role of oxidative stress in the pathogenesis of CKD, the obvious pharmacological possibility of influencing the mechanisms of oxidative stress and its reduction is the use of agents characterised by antioxidant activity (e.g. vitamin E – -tocopherol, vitamin C – ascorbic acid, coenzyme Q10 or plant-derived compounds (e.g. quercetin, curcumin, resveratrol)). However, there are also approaches to adopt some agents, selectively targeting a specific molecular mechanism of inflammation and OS, such as allopurinol and other xanthine oxidase inhibitors, NADPH oxidase inhibitors, agents targeting and enhancing the activity of nuclear factor erythroid 2-related factor (Nrf2), protein kinase C inhibitors, or TGF- inhibitors [16]. The selected studies relating to the use of the abovementioned compounds are listed in Table 1 [17–38].
Although these antioxidant therapies seem promising, their use is controversial and none of these molecules are routinely used in clinical practice for treatment of CKD patients. There are certain limitations of antioxidant therapy, which affects their importance in therapy. The results of different studies are divergent. Mostly, the results of in vitro studies confirm the ability of some antioxidants to detoxify harmful oxidants. However, these pre-clinical studies are not always confirmed in clinical trials. A possible explanation of this is that in-vitro studies are isolated and without a holistic nature. Also, the pre-clinical in-vivo studies, mostly performed in rodents, sometimes show discrepancies in translation into CKD patients, perhaps due to the use of different experimental CKD models, which may not fully reflect the pathophysiological conditions in each CKD patient. It seems that clinical trials evaluating the efficacy and safety of the antioxidant therapy should cover the selected CKD patients with confirmed oxidative stress, because this cohort would fulfill an ideal criteria “intention to treat” study. Because there are some methodological differences between the conducted studies, the results remain inconclusive and require further investigation to unequivocally assess the clinical usefulness of antioxidant therapy in patients with CKD. Moreover, it is likely that multi-drug therapy, i.e. joint, simultaneous administration of several antioxidants, is required to modify numerous mechanisms that develop during CKD, e.g. decrease of lipid peroxidation (by vitamin E supplementation), glutathione redox improvement (by N-acetylcysteine administration), mitochondrial dysfunction (coenzyme Q10), a decrease in uremic toxin synthesis (by allopurinol), or attenuation of inflammatory mechanisms (by targeted therapy) [39]. Finally, the effectiveness of antioxidant therapy depends on the extent to which OS contributes to the pathogenesis of CKD in a given patient. Meanwhile, OS and inflammation coexist, being a cause and a consequence of each other, and are part of a comprehensive pathogenesis, including disorders not corrected by therapy aimed at reducing OS or inflammation. The above-mentioned limitation challenges the ability to apply the antioxidant strategies in routine clinical practice [40].
On a margin, it is worth mentioning that the drugs with an established pharmacotherapeutic position, which exert pleiotropic mechanisms of action and are used in the pharmacotherapy of various disorders, are also characterized by exerting an anti-inflammatory effect and the ability to reduce OS. Thus, these drugs may also play a nephroprotective role. -blockers, angiotensin-converting enzyme inhibitors, angioten- sin II AT1 receptor antagonists, direct renin inhibitors, and statins should be mentioned among these drugs.
To sum up, due to the complex nature of the oxidative stress integrally related to the inflammatory response, the antioxidant therapy and pharmacological therapy targeted to specific inflammatory and OS pathways seem to be rational and effective. However, it should be concluded that despite the involvement of antioxidant therapies, they have not become a standard therapeutic option in CKD, and none of the study molecules have yet been introduced into wide clinical, routine practice. Some reservations concerning both the pre-clinical and clinical research performed to date exist, e.g. low sample size, biodisponibility, and short follow-up. Thus, future prospective and comparative studies with long follow-up are needed.

Conflict of interest

The author declares no conflict of interest.

References