New Insight Into the Redox Properties of Uremic Solute Indoxyl Sulfate as a Pro- and Anti-Oxidant

Dose-dependent exposure to indoxyl sulfate alters AHR signaling, sirtuins gene expression, oxidative DNA damage, and bone mineral status in rats

Indoxyl sulfate (IS), an agonist of aryl hydrocarbon receptors (AhR), can accumulate in patients with chronic kidney disease, but its direct effect on bone is not clear. The present study investigated the effect of chronic exposure to low (100 mg/kg b.w.; 100 IS) and high (200 mg/kg b.w.; 200 IS) dose of IS on bone AhR pathway, sirtuins (SIRTs) expression, oxidative DNA damage and bone mineral status in Wistar rats. The accumulation of IS was observed only in trabecular bone tissue in both doses. The differences were observed in the bone parameters, depending on the applied IS dose. The exposure to 100 IS increased AhR repressor (AhRR)-CYP1A2 gene expression, which was associated with SIRT-1, SIRT-3 and SIRT-7 expression. At the low dose group, the oxidative DNA damage marker was unchanged in the bone samples, and it was inversely related to the abovementioned SIRTs expression. In contrast, the exposure to 200 IS reduced the expression of AhRR, CYP1A, SIRT-3 and SIRT-7 genes compared to 100 IS. The level of oxidative DNA damage was higher in trabecular bone in 200 IS group. Femoral bone mineral density was decreased, and inverse relations were noticed between the level of trabecular oxidative DNA damage and parameters of bone mineral status. In conclusion, IS modulates AhR-depending signaling affecting SIRTs expression, oxidative DNA damage and bone mineral status in a dose dependent manner.

Indoxyl-Sulfate-Induced Redox Imbalance in Chronic Kidney Disease

The accumulation of the uremic toxin indoxyl sulfate (IS) induces target organ damage in chronic kidney disease (CKD) patients, and causes complications including cardiovascular diseases, renal osteodystrophy, muscle wasting, and anemia. IS stimulates reactive oxygen species (ROS) production in CKD, which impairs glomerular filtration by a direct cytotoxic effect on the mesangial cells. IS further reduces antioxidant capacity in renal proximal tubular cells and contributes to tubulointerstitial injury. IS-induced ROS formation triggers the switching of vascular smooth muscular cells to the osteoblastic phenotype, which induces cardiovascular risk. Low-turnover bone disease seen in early CKD relies on the inhibitory effects of IS on osteoblast viability and differentiation, and osteoblastic signaling via the parathyroid hormone. Excessive ROS and inflammatory cytokine releases caused by IS directly inhibit myocyte growth in muscle wasting via myokines’ effects. Moreover, IS triggers eryptosis via ROS-mediated oxidative stress, and elevates hepcidin levels in order to prevent iron flux in circulation in renal anemia. Thus, IS-induced oxidative stress underlies the mechanisms in CKD-related complications. This review summarizes the underlying mechanisms of how IS mediates oxidative stress in the pathogenesis of CKD’s complications. Furthermore, we also discuss the potential role of oral AST-120 in attenuating IS-mediated oxidative stress after gastrointestinal adsorption of the IS precursor indole.

Oxidative Storm Induced by Tryptophan Metabolites: Missing Link between Atherosclerosis and Chronic Kidney Disease

Chronic kidney disease (CKD) occurrence is rising all over the world. Its presence is associated with an increased risk of premature death from cardiovascular disease (CVD). Several explanations of this link have been put forward. It is known that in renal failure, an array of metabolites cannot be excreted, and they accumulate in the organism. Among them, some are metabolites of tryptophan (TRP), such as indoxyl sulfate and kynurenine. Scientists have become interested in them in the context of inducing vascular damage in the course of chronic kidney impairment. Experimental evidence suggests the involvement of TRP metabolites in the progression of chronic kidney disease and atherosclerosis separately and point to oxidative stress generation as one of the main mechanisms that is responsible for worsening those states. Since it is known that blood levels of those metabolites increase significantly in renal failure and that they generate reactive oxygen species (ROS), which lead to endothelial injury, it is reasonable to suspect that products of TRP metabolism are the missing link in frequently occurring atherosclerosis in CKD patients. This review focuses on reports that shed a light on TRP metabolites as contributing factors to vascular damage in the progression of impaired kidney function.

Two Toxic Lipid Aldehydes, 4-hydroxy-2-hexenal (4-HHE) and 4-hydroxy-2-nonenal (4-HNE), Accumulate in Patients with Chronic Kidney Disease

Lipid aldehydes originating from the peroxidation of n-3 and n-6 polyunsaturated fatty acids are increased in hemodialysis (HD) patients, a process already known to promote oxidative stress. However, data are lacking for patients with chronic kidney disease (CKD) before the initiation of HD. We prospectively evaluated the changes of plasma concentrations of two major lipid aldehydes, 4-HHE and 4-HNE, according to the decrease of glomerular filtration rate (GFR) in 40 CKD and 13 non-CKD participants. GFR was measured by inulin or iohexol clearance. Thus, 4-hydroxy-2-nonenal (4-HNE) and 4-hydroxy-2-hexenal (4-HHE) were quantitated in plasma by gas chromatography coupled with mass spectrometry and their covalent adducts on proteins were quantified by immunoblotting. On the one hand, 4-HHE plasma concentration increased from CKD stage I–II to CKD stage IV–V compared to non-CKD patients (4.5-fold higher in CKD IV–V, p < 0.005). On the other hand, 4-HNE concentration only increased in CKD stage IV–V patients (6.2-fold, p < 0.005). The amount of covalent adducts of 4-HHE on plasma protein was 9.5-fold higher in CKD patients than in controls (p < 0.005), while no difference was observed for 4-HNE protein adducts. Plasma concentrations of 4-HNE and 4-HHE are increased in CKD IV–V patients before the initiation of hemodialysis.

Sarcopenia in Chronic Kidney Disease: Factors, Mechanisms, and Therapeutic Interventions

Chronic kidney disease (CKD), a chronic catabolic condition, is characterized by muscle wasting and decreased muscle endurance. Many insights into the molecular mechanisms of muscle wasting in CKD have been obtained. A persistent imbalance between protein degradation and synthesis in muscle causes muscle wasting. During muscle wasting, high levels of reactive oxygen species (ROS) and inflammatory cytokines are detected in muscle. These increased ROS and inflammatory cytokine levels induce the expression of myostatin. The myostatin binding to its receptor activin A receptor type IIB stimulates the expression of atrogenes such as atrogin-1 and muscle ring factor 1, members of the muscle-specific ubiquitin ligase family. Impaired mitochondrial function also contributes to reducing muscle endurance. The increased protein-bound uremic toxin, parathyroid hormone, glucocorticoid, and angiotensin II levels that are observed in CKD all have a negative effect on muscle mass and endurance. Among the protein-bound uremic toxins, indoxyl sulfate, an indole-containing compound has the potential to induce muscle atrophy by stimulating ROS-mediated myostatin and atrogenes expression. Indoxyl sulfate also impairs mitochondrial function. Some potential therapeutic approaches based on the muscle wasting mechanisms in CKD are currently in the testing stages.

Altered microbiome in chronic kidney disease: systemic effects of gut-derived uremic toxins

In chronic kidney disease (CKD), influx of urea and other retained toxins exerts a change in the gut microbiome. There is decreased number of beneficial bacteria that produce short-chain fatty acids, an essential nutrient for the colonic epithelium, concurrent with an increase in bacteria that produce uremic toxins such as indoxyl sulphate, p-cresyl sulphate, and trimethylamine-N-oxide (TMAO). Due to intestinal wall inflammation and degradation of intercellular tight junctions, gut-derived uremic toxins translocate into the bloodstream and exert systemic effects. In this review, we discuss the evidence supporting a role for gut-derived uremic toxins in promoting multiorgan dysfunction via inflammatory, oxidative stress, and apoptosis pathways. End-organ effects include vascular calcification, kidney fibrosis, anemia, impaired immune system, adipocyte dysfunction with insulin resistance, and low turnover bone disease. Higher blood levels of gut-derived uremic toxins are associated with increased cardiovascular events and mortality in the CKD population. Clinical trials that have examined interventions to trap toxic products or reverse gut microbial dysbiosis via oral activated charcoal AST-120, prebiotics and probiotics have not shown impact on cardiovascular or survival outcomes but were limited by sample size and short trials. In summary, the gut microbiome is a major contributor to adverse cardiovascular outcomes and progression of CKD.

The Uremic Toxin Indoxyl Sulfate Accelerates Thrombotic Response after Vascular Injury in Animal Models

Chronic kidney disease (CKD) patients are at high risk for thrombotic events. Indoxyl sulfate (IS) is one of the most potent uremic toxins that accumulates during CKD. Even though IS is associated with an increased risk for cardiovascular disease, its impact on thrombotic events still remains not fully understood. The purpose of the study was to evaluate the direct effect of IS on thrombotic process. We examined the impact of acute exposure to IS on thrombus development induced by electric current in Wistar rats, intravital thrombus formation after laser-induced injury in the mice endothelium, coagulation profile, clot formation dynamics, platelet aggregations, and erythrocyte osmotic resistance. IS doses: 10, 30 and 100 mg/kg body weight (b.w.) increased weight of thrombus induced by electric current in dose-dependent manner (p < 0.001). Furthermore, two highest IS doses increased laser-induced thrombus formation observed via confocal system (increase in fluorescence intensity and total thrombus area (p < 0.01)). Only the highest IS dose decreased clotting time (p < 0.01) and increased maximum clot firmness (p < 0.05). IS did not affect blood morphology parameters and erythrocyte osmotic resistance, but augmented collagen-induced aggregation. Obtained data indicate that IS creates prothrombotic state and contributes to more stable thrombus formation. Thus, we concluded that IS may be one of crucial uremic factors promoting thrombotic events in CKD patients.

Nitration of indoxyl sulfate facilitates its cytotoxicity in human renal proximal tubular cells via expression of heme oxygenase-1

Peroxynitrite, the reaction product of superoxide [Formula: see text] and nitric oxide (NO), nitrates tyrosine residues, unsaturated fatty acids, cyclic guanosine monophosphate and other phenolics. We report herein that indoxyl sulfate (IS) is also nitrated by peroxynitrite in vitro and forms 2-nitro-IS, as determined from spectral characteristics and (1)H-NMR. IS is one of the very important uremic toxins that accelerate the progression of chronic kidney disease via various mechanisms. However, cell viability experiments with human proximal tubular cells show that the cytotoxicity of 2-nitro-IS is several-fold higher than that of IS. The explanation for this finding seems to be that 2-nitro-IS induces a much more pronounced generation of intracellular reactive oxygen species (ROS) than IS. Results with inhibitors revealed that an organic anion transporter, several intracellular enzymes and nonprotein-bound iron ions are reasons for this finding. Most importantly, however, as detected by immunofluorescence and Western blotting, 2-nitro-IS induces the expression of heme oxygenase-1 and thereby the formation of ROS; most probably through the Fenton reaction. The final result of the increased amounts of ROS is death of the kidney cells. Thus, nitration of uremic toxins by peroxynitrite may help us to understand the initiation and progress of chronic kidney diseases.

High-density lipoprotein: structural and functional changes under uremic conditions and the therapeutic consequences

High-density lipoprotein (HDL) has attracted interest as a therapeutic target in cardiovascular diseases in recent years. Although many functional mechanisms of the vascular protective effects of HDL have been identified, increasing the HDL plasma level has not been successful in all patient cohorts with increased cardiovascular risk. The composition of the HDL particle is very complex and includes diverse lipids and proteins that can be modified in disease conditions. In patients with chronic kidney disease (CKD), the accumulation of uremic toxins, high oxidative stress, and chronic micro-inflammatory conditions contribute to changes in the HDL composition and may also account for protein/lipid modifications. These conditions are associated with a decreased protective function of HDL. Therefore, the HDL quantity and the functional quality of the particle must be considered. This review summarizes the current knowledge of dyslipidemia in CKD patients, the effects of lipid-modulating therapy, and the structural modifications of HDL that are associated with dysfunction.

Podocyte injury caused by indoxyl sulfate, a uremic toxin and aryl-hydrocarbon receptor ligand

Indoxyl sulfate is a uremic toxin and a ligand of the aryl-hydrocarbon receptor (AhR), a transcriptional regulator. Elevated serum indoxyl sulfate levels may contribute to progressive kidney disease and associated vascular disease. We asked whether indoxyl sulfate injures podocytes in vivo and in vitro. Mice exposed to indoxyl sulfate for 8 w exhibited prominent tubulointerstitial lesions with vascular damage. Indoxyl sulfate-exposed mice with microalbuminuria showed ischemic changes, while more severely affected mice showed increased mesangial matrix, segmental solidification, and mesangiolysis. In normal mouse kidneys, AhR was predominantly localized to the podocyte nuclei. In mice exposed to indoxyl sulfate for 2 h, isolated glomeruli manifested increased Cyp1a1 expression, indicating AhR activation. After 8 w of indoxyl sulfate, podocytes showed foot process effacement, cytoplasmic vacuoles, and a focal granular and wrinkled pattern of podocin and synaptopodin expression. Furthermore, vimentin and AhR expression in the glomerulus was increased in the indoxyl sulfate-exposed glomeruli compared to controls. Glomerular expression of characteristic podocyte mRNAs was decreased, including Actn4, Cd2ap, Myh9, Nphs1, Nphs2, Podxl, Synpo, and Wt1. In vitro, immortalized-mouse podocytes exhibited AhR nuclear translocation beginning 30 min after 1 mM indoxyl sulfate exposure, and there was increased phospho-Rac1/Cdc42 at 2 h. After exposure to indoxyl sulfate for 24 h, mouse podocytes exhibited a pro-inflammatory phenotype, perturbed actin cytoskeleton, decreased expression of podocyte-specific genes, and decreased cell viability. In immortalized human podocytes, indoxyl sulfate treatment caused cell injury, decreased mRNA expression of podocyte-specific proteins, as well as integrins, collagens, cytoskeletal proteins, and bone morphogenetic proteins, and increased cytokine and chemokine expression. We propose that basal levels of AhR activity regulate podocyte function under normal conditions, and that increased activation of podocyte AhR by indoxyl sulfate contributes to progressive glomerular injury.

Differential effects of indoxyl sulfate and inorganic phosphate in a murine cerebral endothelial cell line (bEnd.3)

Endothelial dysfunction plays a key role in stroke in chronic kidney disease patients. To explore the underlying mechanisms, we evaluated the effects of two uremic toxins on cerebral endothelium function. bEnd.3 cells were exposed to indoxyl sulfate (IS) and inorganic phosphate (Pi). Nitric oxide (NO), reactive oxygen species (ROS) and O2•⁻ were measured using specific fluorophores. Peroxynitrite and eNOS uncoupling were evaluated using ebselen, a peroxide scavenger, and tetrahydrobiopterin (BH₄), respectively. Cell viability decreased after IS or Pi treatment (p < 0.01). Both toxins reduced NO production (IS, p < 0.05; Pi, p < 0.001) and induced ROS production (p < 0.001). IS and 2 mM Pi reduced O2•⁻ production (p < 0.001). Antioxidant pretreatment reduced ROS levels in both IS- and Pi-treated cells, but a more marked reduction of O2•⁻ production was observed in Pi-treated cells (p < 0.001). Ebselen reduced the ROS production induced by the two toxins (p < 0.001); suggesting a role of peroxynitrite in this process. BH₄ addition significantly reduced O2•⁻ and increased NO production in Pi-treated cells (p < 0.001), suggesting eNOS uncoupling, but had no effect in IS-treated cells. This study shows, for the first time, that IS and Pi induce cerebral endothelial dysfunction by decreasing NO levels due to enhanced oxidative stress. However, Pi appears to be more deleterious, as it also induces eNOS uncoupling.

The uremic toxin indoxyl sulfate acts as a pro- or antioxidant on LDL oxidation

Uremic toxins have been shown to play a role in chronic kidney disease (CKD) associated oxidative stress. Oxidative stress and inflammation have been associated with increased risk of cardiovascular disease in uraemia. The oxidative modification of LDL may play a role in early atherogenesis. Enhanced LDL oxidation has been found in uremic patients which may account for accelerated atherosclerosis observed in CKD. The uremic toxin indoxyl sulfate (IS) has been reported to exert oxidative and antioxidative activity. Thus, in the present study we have investigated the influence of IS on the atherogenic modifications of LDL exposed in vitro to different oxidising systems. The transition metal ion (Cu(2+)) and hemin/H2O2 induced lipid oxidation reactions monitored by conjugated diene formation, were inhibited by the presence of IS, which points to possible antioxidant effects by this uremic toxin. A protective effect of IS on LDL apoprotein modification by the exposure to the product of the myeloperoxidase/H2O2/Cl(-) system HOCl, was also observed as estimated by protein carbonyl formation. In contrast, a marked increase in conjugated dienes and lipid hydroperoxides was observed when lipid oxidation was initiated by the free radical generator AAPH in presence of IS. The GC-MS analysis revealed the formation of indole-2,3-dione and 6,12-dihydro-6,12-dioxo-indolo[2,1-b]quinazoline (tryptanthrin) in IS/AAPH reaction. A scheme for the generation of tryptanthrin from IS via indoxyl radicals is proposed, which may facilitate LDL lipid oxidation. Our observations add further insight in the Janus-faced properties of this important uremic toxin.

Albumin-binding capacity (ABiC) is reduced in patients with chronic kidney disease along with an accumulation of protein-bound uraemic toxins

BACKGROUND

Albumin is an important transport protein for non-water-soluble protein-bound drugs and uraemic toxins. Its transport capacity is reduced in patients with advanced chronic kidney disease (CKD) and unbound fractions of uraemic toxins are related to complications of CKD. We investigated whether this reduction could be quantified and how it correlated with the stages of CKD. Albumin-binding capacity (ABiC) is a dye-based method that quantifies the remaining binding capacity of one major binding site (site II) of the albumin molecule.

METHODS

Blood samples from 104 CKD patients were incubated with a binding site-specific fluorescent marker and the amount of unbound marker was determined by means of fluorescence detection after filtration. Measurements in a pooled human plasma were used for reference. Glomerular filtration rate and serum indoxyl sulphate (IS) levels were also determined.

RESULTS

Impairment of renal function was associated with a reduction in ABiC (mean ± SD: 118 ± 12; 111 ± 11; 99 ± 8 and 79 ± 9% for Stages 1/2, 3, 4 and 5, respectively; P < 0.001) and an increase in IS (3.9 ± 1.1; 6.2 ± 3.2; 16.3 ± 14.9 and 56.1 ± 28.6 μmol/L for Stages 1/2, 3, 4 and 5, respectively; P < 0.001). In dialysis patients, ABiC was lower in those with urine outputs <500 mL/day than in those with preserved urine output (73.7 ± 6.0 versus 83.8 ± 8.5%; P < 0.001).

CONCLUSION

Impaired albumin function in CKD patients can be quantified, is related to severity of kidney disease and is associated with an accumulation of uraemic albumin-bound retention solutes.

The uremic toxin indoxyl sulfate reflects cardio-renal risk and intestinal-renal relationship

Uremic syndrome and condition is primarily a result of kidney failure in which uremic toxins are accumulated. More and more attention is paid to possibilities for removal of uremic toxins, which not only means dialysis, but also takes into account special dietary considerations and treatments, which aim to absorb the toxins or reduce their production. These uremic toxins, which also increase the cardiovascular risks, play a major part in morbidity and mortality of patients suffering from chronic renal failure and those receiving renal replacement therapy. One of them is a member of the indol group, the indoxyl sulfate. This toxin is difficult to remove with dialysis and is an endogenous protein-bound uremic toxin. Today we know that indoxyl sulfate is a vascular-nephrotoxic agent, which is able to enhance progression of cardiovascular and renal diseases. It is of particular importance that because of its redox potency, this toxin causes oxidative stress and antioxidant effects at the same time and, on top of that, it is formed in the intestinal system. Its serum concentration depends on the nutrition and the tubular function and, therefore, it can also signal the progression of chronic renal failure independently of glomerular filtration rate. Successful removal of indoxyl sulfate reduces the morbidity and mortality and improves survival. Therefore, it could be a possible target or area to facilitate the reduction of uremia in chronic renal failure. The use of probiotics and prebiotics with oral adsorbents may prove to be a promising opportunity to reduce indoxyl sulfate accumulation. Orv. Hetil., 2011, 152, 1724–1730.

Protein-bound uremic toxins: new insight from clinical studies

The uremic syndrome is attributed to the progressive retention of a large number of compounds which, under normal conditions, are excreted by healthy kidneys. The compounds are called uremic toxins when they interact negatively with biological functions. The present review focuses on a specific class of molecules, namely the family of protein-bound uremic toxins. Recent experimental studies have shown that protein-bound toxins are involved not only in the progression of chronic kidney disease (CKD), but also in the generation and aggravation of cardiovascular disease. Two protein-bound uremic retention solutes, namely indoxyl sulfate and p-cresyl sulfate, have been shown to play a prominent role. However, although these two molecules belong to the same class of molecules, exert toxic effects on the cardiovascular system in experimental animals, and accumulate in the serum of patients with CKD they may have different clinical impacts in terms of cardiovascular disease and other complications. The principal aim of this review is to evaluate the effect of p-cresyl sulfate and indoxyl sulfate retention on CKD patient outcomes, based on recent clinical studies.