The uremic solute indoxyl sulfate induces oxidative stress in endothelial cells

Non-targeted metabolite profiling reveals changes in oxidative stress, tryptophan and lipid metabolisms in fearful dogs

Background

Anxieties, such as shyness, noise phobia and separation anxiety, are common but poorly understood behavioural problems in domestic dogs, Canis familiaris. Although studies have demonstrated genetic and environmental contributions to anxiety pathogenesis, better understanding of the molecular underpinnings is needed to improve diagnostics, management and treatment plans. As a part of our ongoing canine anxiety genetics efforts, this study aimed to pilot a metabolomics approach in fearful and non-fearful dogs to identify candidate biomarkers for more objective phenotyping purposes and to refer to potential underlying biological problem.

Methods

We collected whole blood samples from 10 fearful and 10 non-fearful Great Danes and performed a liquid chromatography combined with mass spectrometry (LC–MS)-based non-targeted metabolite profiling.

Results

Non-targeted metabolomics analysis detected six 932 metabolite entities in four analytical modes [RP and HILIC; ESI(−) and ESI(+)], of which 239 differed statistically between the test groups. We identified changes in 13 metabolites (fold change ranging from 1.28 to 2.85) between fearful and non-fearful dogs, including hypoxanthine, indoxylsulfate and several phospholipids. These molecules are involved in oxidative stress, tryptophan and lipid metabolisms.

Conclusions

We identified significant alterations in the metabolism of fearful dogs, and some of these changes appear relevant to anxiety also in other species. This pilot study demonstrates the feasibility of the non-targeted metabolomics and warrants a larger replication study to confirm the role of the identified biomarkers and pathways in canine anxiety.

Crucial Role of the Aryl Hydrocarbon Receptor (AhR) in Indoxyl Sulfate-Induced Vascular Inflammation

AIM

The aryl hydrocarbon receptor (AhR), a ligand-inducible transcription factor mediating toxic effects of dioxins and uremic toxins, has recently emerged as a pathophysiological regulator of immune-inflammatory conditions. Indoxyl sulfate, a uremic toxin, is associated with cardiovascular disease in patients with chronic kidney disease and has been shown to be a ligand for AhR. The aim of this study was to investigate the potential role of AhR in indoxyl sulfate-induced leukocyte-endothelial interactions.

METHODS

Endothelial cell-specific AhR knockout (eAhR KO) mice were produced by crossing AhR floxed mice with Tie2 Cre mice. Indoxyl sulfate was administered for 2 weeks, followed by injection of TNF-α. Leukocyte recruitment to the femoral artery was assessed by intravital microscopy. Vascular endothelial cells were transfected with siRNA specific to AhR (siAhR) and treated with indoxyl sulfate, followed by stimulation with TNF-α.

RESULTS

Indoxyl sulfate dramatically enhanced TNF-α-induced leukocyte recruitment to the vascular wall in control animals but not in eAhR KO mice. In endothelial cells, siAhR significantly reduced indoxyl sulfate-enhanced leukocyte adhesion as well as E-selectin expression, whereas the activation of JNK and nuclear factor-κB was not affected. A luciferase assay revealed that the region between －153 and －146 bps in the E-selectin promoter was responsible for indoxyl sulfate activity via AhR. Mutational analysis of this region revealed that activator protein-1 (AP-1) is responsible for indoxyl sulfate-triggered E-selectin expression via AhR.

CONCLUSION

AhR mediates indoxyl sulfate-enhanced leukocyte-endothelial interactions through AP-1 transcriptional activity, which may constitute a new mechanism of vascular inflammation in patients with renal disease.

Challenges and opportunities for stem cell therapy in patients with chronic kidney disease

Chronic kidney disease (CKD) is a global health care burden affecting billions of individuals worldwide. The kidney has limited regenerative capacity from chronic insults, and for the most common causes of CKD, no effective treatment exists to prevent progression to end-stage kidney failure. Therefore, novel interventions, such as regenerative cell-based therapies, need to be developed for CKD. Given the risk of allosensitization, autologous transplantation of cells to boost regenerative potential is preferred. Therefore, verification of cell function and vitality in CKD patients is imperative. Two cell types have been most commonly applied in regenerative medicine. Endothelial progenitor cells contribute to neovasculogenesis primarily through paracrine angiogenic activity and partly by differentiation into mature endothelial cells in situ. Mesenchymal stem cells also exert paracrine effects, including proangiogenic, anti-inflammatory, and antifibrotic activity. However, in CKD, multiple factors may contribute to reduced cell function, including older age, coexisting cardiovascular disease, diabetes, chronic inflammatory states, and uremia, which may limit the effectiveness of an autologous cell-based therapy approach. This Review highlights current knowledge on stem and progenitor cell function and vitality, aspects of the uremic milieu that may serve as a barrier to therapy, and novel methods to improve stem cell function for potential transplantation.

Indoxyl sulfate, not p-cresyl sulfate, is associated with cognitive impairment in early-stage chronic kidney disease

BACKGROUNDS

Patients with chronic kidney disease (CKD) more commonly experience cognitive impairment, but the etiologies are not clear. Uremic toxins such as p-cresyl sulfate (PCS) and indoxyl sulfate (IS) have been shown to increase the risks of cardiovascular diseases and mortality; however, no study has investigated the associations of PCS and IS with cognitive function in patients with CKD.

METHODS

Patients with CKD aged ≥50 years and age- and sex-matched non-CKD comparison subjects were recruited. CKD stage was defined according to the National Kidney Foundation guidelines. Cognitive function was evaluated using comprehensive neuropsychological tests. The associations between uremic toxins and cognitive function domains were examined using multiple linear regression analysis. The interaction between uremic toxins and CKD stages on cognitive functions were also examined.

RESULTS

In total, 199 patients with CKD and 84 comparison subjects completed the study. The patients with CKD had poorer cognitive function and higher serum PCS and IS levels. A higher serum IS level was associated with poor executive function (β=-0.31, P=0.003) only in stage 3 CKD patients after adjustment for age, sex and educational level. Serum PCS level was not associated with cognitive function in patients with CKD.

CONCLUSIONS

Our study showed that a higher serum IS level was associated with poor executive function in the early stage of CKD. It would be worthwhile to investigate the effect of IS removal in early-stage CKD on the prevention of cognitive impairment in future studies.

Effects of glutamine against oxidative stress in the metabolome of rats—new insight

Glutamine exerts potential functions against the harmful effects of oxidative stress on animals.

Effect of renal impairment on atherosclerosis: only partially mediated by homocysteine

BACKGROUND

Cardiovascular risk and plasma total homocysteine (tHcy) are high in patients with renal failure. High tHcy may account for a substantial part of the increased risk. We assessed mediation by tHcy of the association of estimated glomerular filtration rate (eGFR CKD/EPI) with carotid total plaque area (TPA) and carotid stenosis.

METHODS

TPA and carotid stenosis were measured by ultrasound. Multiple linear regression was used to assess the effects of eGFR and/or tHcy after adjustment for age, sex, systolic blood pressure (SBP), smoking, LDL, HDL and weight.

RESULTS

Complete data were available for 1967 patients. eGFR decreased, and TPA and total stenosis increased linearly with age. After adjustment [age, sex, SBP, smoking (in pack years), low-density lipoprotein (LDL), high-density lipoprotein (HDL) and weight], eGFR and tHcy were independently associated with TPA (P < 0.01), but when both were added to the model, their significance was attenuated (P = 0.06 for eGFR, 0.03 for tHcy). Mediation analysis showed that tHcy seems to contribute to a significant mediation of the association of eGFR with TPA but not stenosis; after adjustment for the set of risk factors listed above, tHcy still demonstrated significant mediation on TPA (P = 0.03), but not on stenosis (P = 0.16).

CONCLUSIONS

tHcy accounts for a significant part, but not all of the effect of renal impairment on atherosclerosis. Other uremic toxins including metabolic products of the intestinal microbiome may explain residual effects of renal failure on atherosclerosis. Therapeutic approaches arising from that hypothesis are discussed.

Role of Indoxyl Sulfate as a Predisposing Factor for Atrial Fibrillation in Renal Dysfunction

Background

Renal dysfunction is a major risk factor for atrial fibrillation (AF). The uremic toxin indoxyl sulfate may contribute to the progression of cardiac fibrosis and AF substrate in renal dysfunction.

Methods and Results

Male Sprague–Dawley rats were assigned randomly to the following groups: 5/6 nephrectomy (5/6Nx) with vehicle, 5/6Nx with AST‐120, sham procedure with vehicle, and sham procedure with AST‐120. Vehicle and AST‐120 were administered for 4 weeks. Serum levels of IS were significantly increased in 5/6Nx groups. Expression of malondialdehyde, an indicator of oxidative stress, was upregulated in the left atrium of 5/6Nx groups and was accompanied by an increase in expression of NADPH oxidase 2 and 4. Monocyte‐mediated inflammatory signals such as CD68, monocyte chemoattractant protein 1, and vascular cell adhesion molecule 1 were also upregulated in 5/6Nx groups. Interstitial fibrosis was promoted heterogeneously, and expression of profibrotic indicators such as transforming growth factor β1, α‐smooth muscle actin, and collagen type 1 was upregulated in left atrium tissue of 5/6Nx groups. In cultured atrial fibroblasts, incubation with IS upregulated expression of the markers of oxidative stress, inflammation, and profibrotic factors. These results suggest the direct effects of IS on the progression of AF substrate. AF was consistently and invariably induced by atrial extrastimuli in 5/6Nx groups in electrophysiological experiments. AST‐120 treatment significantly alleviated renal dysfunction–induced oxidative stress, inflammation, and atrial fibrosis and, consequently, attenuated AF inducibility.

Conclusions

Indoxyl sulfate facilitates atrial fibrosis and AF and thus is a novel therapeutic target for prevention of renal dysfunction–induced AF.

Serum Indoxyl Sulfate Associates with Postangioplasty Thrombosis of Dialysis Grafts

Hemodialysis vascular accesses are prone to recurrent stenosis and thrombosis after endovascular interventions.In vitro data suggest that indoxyl sulfate, a protein-bound uremic toxin, may induce vascular dysfunction and thrombosis. However, there is no clinical evidence regarding the role of indoxyl sulfate in hemodialysis vascular access. From January 2010 to June 2013, we prospectively enrolled patients undergoing angioplasty for dialysis access dysfunction. Patients were stratified into tertiles by baseline serum indoxyl sulfate levels. Study participants received clinical follow-up at 6-month intervals until June 2014. Primary end points were restenosis, thrombosis, and failure of vascular access. Median follow-up duration was 32 months. Of the 306 patients enrolled, 262 (86%) had symptomatic restenosis, 153 (50%) had access thrombosis, and 25 (8%) had access failure. In patients with graft access, free indoxyl sulfate tertiles showed a negative association with thrombosis-free patency (thrombosis-free patency rates of 54%, 38%, and 26% for low, middle, and high tertiles, respectively;P=0.001). Patients with graft thrombosis had higher free and total indoxyl sulfate levels. Using multivariate Cox regression analysis, graft thrombosis was independently predicted by absolute levels of free indoxyl sulfate (hazard ratio=1.14;P=0.01) and free indoxyl sulfate tertiles (high versus low, hazard ratio=2.41;P=0.001). Results of this study provide translational evidence that serum indoxyl sulfate is a novel risk factor for dialysis graft thrombosis after endovascular interventions.

Indoxyl sulfate exacerbates low bone turnover induced by parathyroidectomy in young adult rats

Low-turnover bone disease is one of the bone abnormalities observed in patients with chronic kidney disease (CKD) and is recognized to be associated with low serum parathyroid hormone (PTH) level and skeletal resistance to PTH. Indoxyl sulfate (IS) is a representative uremic toxin that accumulates in the blood as renal dysfunction progresses in CKD patients. A recent in vitro study using an osteoblastic cell culture system suggests that IS has an important role in the pathogenesis of low bone turnover through induction of skeletal resistance to PTH. However, the effects of IS on the progression of low bone turnover have not been elucidated. In the present study, we produced rats with low bone turnover by performing parathyroidectomy (PTX) and fed these rats a diet containing indole, a precursor of IS, to elevate blood IS level from indole metabolism. Bone metabolism was evaluated by measuring histomorphometric parameters of secondary spongiosa of the femur. Histomorphometric analyses revealed significant decreases in both bone formation-related parameters and bone resorption-related parameters in PTX rats. In indole-treated PTX rats, further decreases in bone formation-related parameters were observed. In addition, serum alkaline phosphatase activity, a bone formation marker, and bone mineral density of the tibia tended to decrease in indole-treated PTX rats. These findings strongly suggest that IS exacerbates low bone turnover through inhibition of bone formation by mechanisms unrelated to skeletal resistance to PTH.

Deleterious vascular effects of indoxyl sulfate and reversal by oral adsorbent AST-120

BACKGROUND

In chronic kidney disease (CKD), blood vessels are permanently exposed to uremic toxins such as indoxyl sulfate (IS). We hypothesized that IS could alter vascular tone and that reducing its serum concentration could be beneficial.

DESIGN

We studied acute and longer-term effects of IS and AST-120, an oral charcoal adsorbent, on vascular reactivity, endothelium integrity and expression of adhesion molecules VCAM-1 and ICAM-1 in aortic rings of normal and uremic wild type (WT) mice in vitro, and the cardiovascular effects of AST-120 in both WT and apoE-/- mice with CKD in vivo.

RESULTS

In vitro, 1.0 mM IS acutely reduced vascular relaxation (64% for IS 1.0 mM vs. 80% for control, p < 0.05). The effect was more marked after 4 days exposure (39% for IS 1.0 mM 4 days; p < 0.001, prolonged vs. acute exposure), and was associated with endothelial cell loss and upregulation of ICAM-1/VCAM-1 expression. In vitro, AST-120 restored normal vascular function and prevented IS induced endothelial cell loss and ICAM-1/VCAM-1 upregulation. In vivo, AST-120 treatment of CKD mice (1) improved vascular relaxation (72% vs. 48% maximal relaxation in treated vs. untreated mice, p < 0.001), (2) reduced aortic VCAM-1 and ICAM-1 expression, (3) decreased aorta systolic expansion rate (9 ± 3% CKD vs. 14 ± 3% CKD + AST-120, p < 0.02), and (4) prevented the increase in pulse wave velocity (3.56 ± 0.17 m/s CKD vs. 3.10 ± 0.08 m/s CKD + AST-120, p < 0.006). Similar changes were observed in apoE-/- mice.

CONCLUSION

IS appears to be an important contributor to the vascular dysfunction associated with CKD. AST-120 treatment ameliorates this dysfunction, possibly via a decrease in serum IS concentration.

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.

Cellular and Molecular Mechanisms of Chronic Kidney Disease with Diabetes Mellitus and Cardiovascular Diseases as Its Comorbidities

Chronic kidney disease (CKD), diabetes mellitus (DM), and cardiovascular diseases (CVD) are complex disorders of partly unknown genesis and mostly known progression factors. CVD and DM are the risk factors of CKD and are strongly intertwined since DM can lead to both CKD and/or CVD, and CVD can lead to kidney disease. In recent years, our knowledge of CKD, DM, and CVD has been expanded and several important experimental, clinical, and epidemiological associations have been reported. The tight cellular and molecular interactions between the renal, diabetic, and cardiovascular systems in acute or chronic disease settings are becoming increasingly evident. However, the (patho-) physiological basis of the interactions of CKD, DM, and CVD with involvement of multiple endogenous and environmental factors is highly complex and our knowledge is still at its infancy. Not only single pathways and mediators of progression of these diseases have to be considered in these processes but also the mutual interactions of these factors are essential. The recent advances in proteomics and integrative analysis technologies have allowed rapid progress in analyzing complex disorders and clearly show the opportunity for new efficient and specific therapies. More than a dozen pathways have been identified so far, including hyperactivity of the renin–angiotensin (RAS)–aldosterone system, osmotic sodium retention, endothelial dysfunction, dyslipidemia, RAS/RAF/extracellular-signal-regulated kinase pathway, modification of the purinergic system, phosphatidylinositol 3-kinase (PI 3-kinase)-dependent signaling pathways, and inflammation, all leading to histomorphological alterations of the kidney and vessels of diabetic and non-diabetic patients. Since a better understanding of the common cellular and molecular mechanisms of these diseases may be a key to successful identification of new therapeutic targets, we review in this paper the current literature about cellular and molecular mechanisms of CKD.

Indoxyl sulfate suppresses hepatic fetuin-A expression via the aryl hydrocarbon receptor in HepG2 cells

BACKGROUND

Fetuin-A is a liver-derived circulating protein that has potent calcification-inhibitory activity. Uraemic patients exhibit decreased serum fetuin-A levels, increased vascular calcification and elevated cardiovascular mortality. Because the mechanisms for fetuin-A deficiency are unknown, we hypothesized that some uraemic toxins suppressed hepatic fetuin-A production, which resulted in accelerated vascular calcification and poor outcome. Among these potential candidates, indoxyl sulfate (IS) has highly toxic properties.

METHODS

We examined the direct effects of IS on hepatic fetuin-A expression using the human hepatoma HepG2 cell line.

RESULTS

IS, but not p-cresyl sulfate, suppressed the mRNA and protein expression of fetuin-A in a dose- and time-dependent manner. As reported previously, IS stimulated p38 MAPK phosphorylation and reactive oxygen species (ROS) production, although the knockdown of p38 and inhibition of ROS generation had no effect on IS-induced fetuin-A suppression. Then, because IS is a potent endogenous ligand of the aryl hydrocarbon receptor (AhR), we assessed whether IS suppresses fetuin-A production via AhR. The knockdown of AhR prevented IS-induced fetuin-A suppression. However, some attention should be paid to no effect of IS on fetuin-A expression in mouse and human primary cultured hepatocytes.

CONCLUSIONS

These findings suggest that IS could suppress hepatic fetuin-A expression by activating AhR, suggesting a relationship between uraemia and fetuin-A deficiency.

Renal systems biology of patients with systemic inflammatory response syndrome

A systems biology approach was used to comprehensively examine the impact of renal disease and hemodialysis (HD) on patient response during critical illness. To achieve this we examined the metabolome, proteome, and transcriptome of 150 patients with critical illness, stratified by renal function. Quantification of plasma metabolites indicated greater change as renal function declined, with the greatest derangements in patients receiving chronic HD. Specifically, 6 uremic retention molecules, 17 other protein catabolites, 7 modified nucleosides, and 7 pentose phosphate sugars increased as renal function declined, consistent with decreased excretion or increased catabolism of amino acids and ribonucleotides. Similarly, the proteome showed increased levels of low-molecular weight proteins and acute phase reactants. The transcriptome revealed a broad-based decrease in mRNA levels among patients on HD. Systems integration revealed an unrecognized association between plasma RNASE1 and several RNA catabolites and modified nucleosides. Further, allantoin, N1-methyl-4-pyridone-3-carboxamide, and n-acetylaspartate were inversely correlated with the majority of significantly down-regulated genes. Thus, renal function broadly affected the plasma metabolome, proteome, and peripheral blood transcriptome during critical illness; changes not effectively mitigated by hemodialysis. These studies allude to several novel mechanisms whereby renal dysfunction contributes to critical illness.

Impact of Indoxyl Sulfate, a Uremic Toxin, on Non-Culprit Coronary Plaque Composition Assessed on Integrated Backscatter Intravascular Ultrasound

BACKGROUND

Uremic toxin has emerged as an important determinant of cardiovascular risk. The aim of this study was to examine the relationship between serum uremic toxin and coronary plaque composition on integrated backscatter intravascular ultrasound (IB-IVUS).

METHODS AND RESULTS

IB-IVUS was performed in 47 patients with planned treatment for angina pectoris. Non-culprit intermediate plaque analyzed in this study had to be >5 mm apart from the intervention site. 3-D IB-IVUS analysis was performed to determine percent lipid volume (LV) and fibrous volume (FV). We also measured serum uremic toxins (indoxyl sulfate [IS], asymmetric dimethylarginine [ADMA], and p-cresol [PC]). Glomerular filtration rate correlated with IS (r=-0.329, P=0.04), but did not correlate with ADMA or PC. Percent LV correlated with IS (r=0.365, P=0.02), but did not correlate with ADMA or PC. Percent FV also correlated with IS (r=-0.356, P=0.03), but did not correlate with ADMA or PC. On multivariate regression, only IS was associated with percent LV (r=0.359, P=0.04) and percent FV (r=-0.305, P=0.04) independently of potentially confounding coronary risk factors.

CONCLUSIONS

Among the uremic toxins, serum IS might be a novel useful biomarker to detect and monitor lipid-rich coronary plaque on IB imaging.

Indoxyl sulfate-induced oxidative stress, mitochondrial dysfunction, and impaired biogenesis are partly protected by vitamin C and N-acetylcysteine

Indoxyl sulfate (IS) contributes to oxidative stress and endothelial dysfunction in chronic kidney disease patients. However, the role of mitochondria in IS-induced oxidative stress is not very clear. In this study, we examined whether mitochondria play a pivotal role in modulating the effects of antioxidants during IS treatment. In the context of human umbilical vein endothelial cells, we found that IS had a dose-dependent antiproliferative effect. In addition, we used flow cytometry to demonstrate that the level of reactive oxygen species increased in a dose-dependent manner after treatment with IS. High doses of IS also corresponded to increased mitochondrial depolarization and decreased mitochondrial DNA copy number and mitochondrial mass. However, these effects could be reversed by the addition of antioxidants, namely, vitamin C and N-acetylcysteine. Thus, our results suggest that IS-induced oxidative stress and antiproliferative effect can be attributed to mitochondrial dysfunction and impaired biogenesis and that these processes can be protected by treatment with antioxidants.

Role of altered intestinal microbiota in systemic inflammation and cardiovascular disease in chronic kidney disease

The normal intestinal microbiota plays a major role in the maintenance of health and disease prevention. In fact, the alteration of the intestinal microbiota has been shown to contribute to the pathogenesis of several pathological conditions, including obesity and insulin resistance, among others. Recent studies have revealed profound alterations of the gut microbial flora in patients and animals with chronic kidney disease (CKD). Alterations in the composition of the microbiome in CKD may contribute to the systemic inflammation and accumulation of gut-derived uremic toxins, which play a central role in the pathogenesis of accelerated cardiovascular disease and numerous other CKD-associated complications. This review is intended to provide a concise description of the potential role of the CKD-associated changes in the gut microbiome and its potential role the pathogenesis of inflammation and uremic toxicity. In addition, the potential efficacy of pre- and pro-biotics in the restoration of the microbiome is briefly described.

Effect of uraemia on endothelial cell damage is mediated by the integrin linked kinase pathway

KEY POINTS

Patients with chronic kidney disease have a higher risk of developing cardiovascular diseases than the general population. Their vascular endothelium is dysfunctional, among other things, because it is permanently exposed to uraemic toxins, several of which have poor clearance by conventional dialysis. Recent studies have demonstrated the important role of integrin-linked kinase (ILK) in the maintenance of endothelial integrity and in this study we investigate the involvement of ILK in the mechanism underlying vascular endothelial damage that occurs in uraemia. For the first time, we demonstrate the implication of ILK in the protection against endothelial cell damage (inhibition of proliferation, toxicity, oxidative stress and programed cell death) induced by uraemic serum from chronic kidney disease patients and uraemic toxins. This molecular mechanism may have clinical relevance because it highlights the importance of maintaining high levels of ILK activity to help preserve endothelial integrity, at least in early stages of chronic kidney disease.

ABSTRACT

Patients with chronic kidney disease (CKD) have a higher risk of developing cardiovascular diseases. Their vascular endothelium is dysfunctional, among other things, because it is permanently exposed to uraemic toxins, several of which, mostly protein-bound compounds such as indoxyl sulfate (IS) and p-cresyl sulphate, having poor clearance by conventional dialysis, induce endothelial toxicity. However, the molecular mechanism by which uraemic toxins regulate early stages of endothelial dysfunction remains unclear. Recent studies have demonstrated the important role of integrin-linked kinase (ILK) in the maintenance of endothelial integrity. In this study, we investigate the involvement of ILK in the mechanism underlying vascular endothelial damage that occurs in uraemia. First, we show that incubation of EA.hy926 cells with human uraemic serum from CKD patients upregulates ILK activity. This ILK activation also occurs when the cells are exposed to IS (25-100 μg ml(-1)), p-cresol (10-100 μg ml(-1)) or both combined, compared to human serum control. Next, we observed that high doses of both toxins together induce a slight decrease in cell proliferation and increase apoptosis and reactive oxygen species production. Interestingly, these toxic effects displayed a strong increase when the ILK protein is knocked down by small interfering RNA, even at low doses of uraemic toxins. Abrogation of AKT has demonstrated the ILK/AKT signalling pathway involved in these processes. This study has demonstrated the implication of ILK in the protection against endothelial cell damage induced by uraemic toxins, a molecular mechanism that could play a protective role in the early stages of endothelial dysfunction observed in uraemic patients.

In-vitro evidence of enhanced breast cancer resistance protein-mediated intestinal urate secretion by uremic toxins in Caco-2 cells

OBJECTIVES

It has been reported that intestinal urate excretion is increased at chronic kidney disease (CKD) state. In this report, whether uremic toxins are involved in the upregulation of intestinal breast cancer resistance protein (BCRP), an intestinal urate exporter, was examined.

METHODS

Uremic toxins that were increased at least 15-fold at CKD state were selected for investigation. Caco-2 cells were exposed to these uremic toxins at clinically relevant concentrations. mRNA was quantified by real-time PCR, and flow cytometry was utilized to measure BCRP protein and function in Caco-2 cells. Transcellular secretory transport of [(14) C]urate was determined utilizing Transwell studies after uremic toxin exposure.

KEY FINDINGS

Indoxyl sulfate (IS) treatment alone resulted in ∼ 3-fold increase in BCRP mRNA in Caco-2 cells. Membrane protein expression of BCRP in Caco-2 cells also was increased by 1.8-fold after treatment with IS. Intracellular accumulation of pheophorbide A, a selective BCRP substrate, was decreased by 22% after IS treatment for 3 days. Consistent with these findings, transcellular secretory transport of urate across Caco-2 cell monolayers was increased by 22%.

CONCLUSION

Intestinal urate secretion may be increased at CKD state partially by upregulation of intestinal BCRP by uremic toxins such as IS.

Uremic toxins, oxidative stress, and renal fibrosis: an interwined complex

The prevalence of end-stage renal diseases is currently on the rise globally, and finding the way to curb this tide is urgently needed. Tubulointerstitial fibrosis is a common pathway for essentially all the nephropathy categories known to date, and the manifestations of renal fibrosis include excessive deposition of extracellular matrix with distortion of renal microstructures and functional deterioration. Uremic toxins have been gradually found to play an important role in the development of progressive renal fibrosis, with protein-bound indoxyl sulfate, p-cresol, and p-cresyl sulfate receiving the most attention. However, the contribution of oxidative stress among the pathogenesis of uremic toxins and renal fibrosis has not been evaluated much until recently. In this review, we will discuss about the nature and sources of oxidative stress in the kidney and how uremic toxins use oxidative stress to orchestrate the processes of renal fibrosis.

N-acetylcysteine as a potential strategy to attenuate the oxidative stress induced by uremic serum in the vascular system

AIMS

Chronic kidney disease (CKD) progression is accompanied by systemic oxidative stress, which contributes to an increase in the risk of cardiovascular diseases (CVDs). N-acetylcysteine (NAC) is among the most studied antioxidants, but its therapeutic benefits in CKD-associated CVDs remain controversial. Here, we investigated whether NAC could inhibit the oxidative stress induced by uremia in vitro and in vivo.

MAIN METHODS

Endothelial and smooth muscle cells were challenged with human uremic or non-uremic sera, and the effects of a pre-treatment with 2mM NAC were evaluated. Reactive oxygen species (ROS) production, protein oxidation and total glutathione/glutathione disulfide (tGSH/GSSG) ratios were measured. Five-sixths nephrectomized or sham-operated rats were orally treated (in the drinking water) with 60 mg/kg/day NAC or not treated for 53 days. Plasma cysteine/cystine reduction potential Eh(Cyss/2Cys) was determined as a novel marker of the systemic oxidative stress.

KEY FINDINGS

NAC inhibited all the determined oxidative stress parameters, likely by increasing the tGSH/GSSG ratio, in both cell lines exposed to uremic serum. Orally administered NAC attenuated the systemic oxidative stress in uremic rats.

SIGNIFICANCE

The present results indicate that NAC, by preventing GSH depletion in vascular cells exposed to uremic serum and by attenuating the systemic oxidative stress during CKD progression, emerges as a potential strategy to prevent the oxidative stress induced by uremic toxicity in the vascular system.

New methods and technologies for measuring uremic toxins and quantifying dialysis adequacy

This publication reviews the currently available methods to identify uremic retention solutes, to determine their biological relevance and to quantify their removal. The analytical methods for the detection of uremic solutes have improved continuously, allowing the identification of several previously unknown solutes. Progress has been accelerated by the development of comprehensive strategies such as genomics, proteomics and the latest "omics" area, metabolomics. Those methodologies will be further refined in future. Once the concentration of solutes of interest is known based on targeted analysis, their biological relevance can be studied by means of in vitro, ex vivo, or animal models, provided those are representative for the key complications of the uremic syndrome. For this to come to pass, rigid protocols should be applied, e.g., aiming at free solute concentrations conform those found in uremia. Subsequently, the decrease in concentration of relevant solutes should be pursued by nondialysis (e.g., by influencing nutritional intake or intestinal generation, using sorbents, modifying metabolism, or preserving renal function) and dialysis methods. Optimal dialysis strategies can be sought by studying solute kinetics during dialysis. Clinical studies are necessary to assess the correct impact of those optimized strategies on outcomes. Although longitudinal studies of solute concentration and surrogate outcome studies are first steps in suggesting the usefulness of a given approach, ultimately hard outcome randomized controlled trials are needed to endorse evidence-based therapeutic choices. The nonspecificity of dialysis removal is however a handicap limiting the chances to provide proof of concept that a given solute or group of solutes has definite biological impact.

p-Cresyl sulfate, a uremic toxin, causes vascular endothelial and smooth muscle cell damages by inducing oxidative stress

The major cause of death in patients with chronic kidney disease (CKD) is cardiovascular disease. Here, p-Cresyl sulfate (PCS), a uremic toxin, is considered to be a risk factor for cardiovascular disease in CKD. However, our understanding of the vascular toxicity induced by PCS and its mechanism is incomplete. The purpose of this study was to determine whether PCS enhances the production of reactive oxygen species (ROS) in vascular endothelial and smooth muscle cells, resulting in cytotoxicity. PCS exhibited pro-oxidant properties in human umbilical vein endothelial cells (HUVEC) and aortic smooth muscle cells (HASMC) by enhancing NADPH oxidase expression. PCS also up-regulates the mRNA levels and the protein secretion of monocyte chemotactic protein-1 (MCP-1) in HUVEC. In HASMC, PCS increased the mRNA levels of alkaline phosphatase (ALP), osteopontin (OPN), core-binding factor alpha 1, and ALP activity. The knockdown of Nox4, a subunit of NADPH oxidase, suppressed the cell toxicity induced by PCS. The vascular damage induced by PCS was largely suppressed in the presence of probenecid, an inhibitor of organic anion transporters (OAT). In PCS-overloaded 5/6-nephrectomized rats, plasma MCP-1 levels, OPN expression, and ALP activity of the aortic arch were increased, accompanied by the induction of Nox4 expression. Collectively, the vascular toxicity of PCS can be attributed to its intracellular accumulation via OAT, which results in an enhanced NADPH oxidase expression and increased ROS production. In conclusion, we found for the first time that PCS could play an important role in the development of cardiovascular disease by inducing vascular toxicity in the CKD condition.

Environmental carbon monoxide level is associated with the level of high-sensitivity C-reactive protein in peritoneal dialysis patients

Inflammation is highly prevalent among peritoneal dialysis (PD) patients. High-sensitivity C-reactive protein (hs-CRP) is the most widely used inflammatory marker in clinical medicine and is correlated with mortality in PD patients. Air pollution is associated with systemic inflammation. The aim of this cross-sectional study was to assess the role of air pollutants and other clinical variables on hs-CRP values in PD patients.We recruited a total of 175 patients who had been undergoing continuous ambulatory PD or automated PD for at least 4 months and regularly followed up. Air pollution levels were recorded by a network of 27 monitoring stations near or in the patients' living areas throughout Taiwan. The 12-month average concentrations of particulate matter (PM) with an aerodynamic diameter of <10 and <2.5 μm (PM10 and PM2.5), sulfur dioxide (SO2), nitrogen dioxide (NO2), carbon monoxide (CO), and ozone (O3) were included.In stepwise linear regression, after adjustment for related factors, white blood cell count (β: 0.27, 95% confidence interval [CI] [0.71, 2.11]) and CO level (β: 0.17, 95% CI [2.5, 21.32]) were positively associated with hs-CRP and serum albumin levels (β: -0.25, 95% CI [-13.69, -3.96]) and normalized protein nitrogen appearance (β: -0.18, 95% CI [-17.7, -2.51]) was negatively associated with hs-CRP. However, serum indoxyl sulfate and p-cresyl sulfate levels were not significantly associated with hs-CRP (P > 0.05).In PD patients, the environmental CO level was positively correlated with hs-CRP level.

Association of indoxyl sulfate with heart failure among patients on hemodialysis

BACKGROUND AND OBJECTIVES

Indoxyl sulfate, a protein-bound uremic toxin, may be associated with cardiovascular events and mortality in patients with CKD. This study aimed to investigate the relationship between indoxyl sulfate and heart failure in patients on hemodialysis.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

Patients on hemodialysis for >6 months were enrolled within 6 months. Patients with congestive heart failure, angina pectoris, acute myocardial infarction, cerebral infarction, or cerebral hemorrhage within 3 months before the study or those <18 years old were excluded. The primary end point was first heart failure event during follow-up.

RESULTS

In total, 258 patients (145 men) with a mean age of 57.0 ± 14.6 years old were enrolled. Median plasma indoxyl sulfate level was used to categorize patients into two groups: the low-indoxyl sulfate group (indoxyl sulfate ≤ 2.35 μg/ml) and the high-indoxyl sulfate group (indoxyl sulfate >32.35 μg/ml). Then, patients were prospectively followed up for a median of 48.0 (interquartile range: 33.5-48.0) months. During follow-up, 68 patients experienced episodes of first heart failure. Kaplan-Meier analysis revealed the incidence of first heart failure event in the high-indoxyl sulfate group was significantly higher than in the low-indoxyl sulfate group (log rank P<0.001). Cox regression analysis showed indoxyl sulfate was significantly associated with first heart failure event (indoxyl sulfate as the continuous variable: hazard ratio, 1.02; 95% confidence interval [95% CI], 1.01 to 1.03; P=0.001; indoxyl sulfate as the dichotomous variable: hazard ratio, 3.49; 95% CI, 1.97 to 6.20; P<0.001). After adjustment for other confounding factors, the results remained significant (indoxyl sulfate as the continuous variable: hazard ratio, 1.04; 95% CI, 1.02 to 1.06; P<0.001; indoxyl sulfate as the dichotomous variable: hazard ratio, 5.31; 95% CI, 2.43 to 11.58; P<0.001).

CONCLUSIONS

Plasma indoxyl sulfate was associated with first heart failure event in patients on hemodialysis. Whether indoxyl sulfate is only a biomarker or involved in the pathogenesis of heart failure in hemodialysis warrants additional study.

Klotho Prevents NFκB Translocation and Protects Endothelial Cell From Senescence Induced by Uremia

In patients with renal disease, uremia raises oxidative stress and senescence in endothelial cells, which can lead to endothelial dysfunction and cardiovascular disease. Klotho protein is a β-glucuronidase capable of hydrolyzing steroid β-glucuronides. This protein is recognized as an antiaging gene, that modulate both stress-induced senescence and functional response. The aim of the study was to investigate how senescence and oxidative stress induced by uremia in endothelial cells affects Klotho expression and whether intra or extracellular Klotho has effects on the response of these cells. Senescence and oxidative stress was obtained by exposure to uremic serum. Telomere length, the enzyme β-galactosidase, and oxidative stress were studied by flow cytometry. Nuclear factor kappa B activity was determined by electrophoretic mobility shift assay. The expression of Klotho decreased with the uremia and preceded the manifestations of cell aging. Levels of intracellular Klotho decreases associated to endothelial senescence, and exogenous Klotho prevents cellular senescence by inhibiting the increase in oxidative stress induced by uremia and diminished the nuclear factor kappa B-DNA binding ability.

Alterations of intestinal barrier and microbiota in chronic kidney disease

Recent studies have highlighted the close relationship between the kidney and the gastrointestinal (GI) tract--frequently referred to as the kidney--gut axis--in patients with chronic kidney disease (CKD). In this regard, two important pathophysiological concepts have evolved: (i) production and accumulation of toxic end-products derived from increased bacterial fermentation of protein and other nitrogen-containing substances in the GI tract, (ii) translocation of endotoxins and live bacteria from gut lumen into the bloodstream, due to damage of the intestinal epithelial barrier and quantitative/qualitative alterations of the intestinal microbiota associated with the uraemic milieu. In both cases, these gut-centred alterations may have relevant systemic consequences in CKD patients, since they are able to trigger chronic inflammation, increase cardiovascular risk and worsen uraemic toxicity. The present review is thus focused on the kidney-gut axis in CKD, with special attention to the alterations of the intestinal barrier and the local microbiota (i.e. the collection of microorganisms living in a symbiotic coexistence with their host in the intestinal lumen) and their relationships to inflammation and uraemic toxicity in CKD. Moreover, we will summarize the most important clinical data suggesting the potential for nutritional modulation of gut-related inflammation and intestinal production of noxious by-products contributing to uraemic toxicity in CKD patients.

The cardiovascular effect of the uremic solute indole-3 acetic acid

In CKD, uremic solutes may induce endothelial dysfunction, inflammation, and oxidative stress, leading to increased cardiovascular risk. We investigated whether the uremic solute indole-3 acetic acid (IAA) predicts clinical outcomes in patients with CKD and has prooxidant and proinflammatory effects. We studied 120 patients with CKD. During the median study period of 966 days, 29 patients died and 35 experienced a major cardiovascular event. Kaplan-Meier analysis revealed that mortality and cardiovascular events were significantly higher in the higher IAA group (IAA>3.73 µM) than in the lower IAA group (IAA<3.73 µM). Multivariate Cox regression analysis demonstrated that serum IAA was a significant predictor of mortality and cardiovascular events after adjustments for age and sex; cholesterol, systolic BP, and smoking; C-reactive protein, phosphate, body mass index, and albumin; diastolic BP and history of cardiovascular disease; and uremic toxins p-cresyl sulfate and indoxyl sulfate. Notably, IAA level remained predictive of mortality when adjusted for CKD stage. IAA levels were positively correlated with markers of inflammation and oxidative stress: C-reactive protein and malondialdehyde, respectively. In cultured human endothelial cells, IAA activated an inflammatory nongenomic aryl hydrocarbon receptor (AhR)/p38MAPK/NF-κB pathway that induced the proinflammatory enzyme cyclooxygenase-2. Additionally, IAA increased production of endothelial reactive oxygen species. In conclusion, serum IAA may be an independent predictor of mortality and cardiovascular events in patients with CKD. In vitro, IAA induces endothelial inflammation and oxidative stress and activates an inflammatory AhR/p38MAPK/NF-κB pathway.

Effect of redox modulating NRF2 activators on chronic kidney disease

Chronic kidney disease (CKD) is featured by a progressive decline of kidney function and is mainly caused by chronic diseases such as diabetes mellitus and hypertension. CKD is a complex disease due to cardiovascular complications and high morbidity; however, there is no single treatment to improve kidney function in CKD patients. Since biological markers representing oxidative stress are significantly elevated in CKD patients, oxidative stress is receiving attention as a contributing factor to CKD pathology. Nuclear factor erythroid-2 related factor 2 (NRF2) is a predominant transcription factor that regulates the expression of a wide array of genes encoding antioxidant proteins, thiol molecules and their generating enzymes, detoxifying enzymes, and stress response proteins, all of which can counteract inflammatory and oxidative damages. There is considerable experimental evidence suggesting that NRF2 signaling plays a protective role in renal injuries that are caused by various pathologic conditions. In addition, impaired NRF2 activity and consequent target gene repression have been observed in CKD animals. Therefore, a pharmacological intervention activating NRF2 signaling can be beneficial in protecting against kidney dysfunction in CKD. This review article provides an overview of the role of NRF2 in experimental CKD models and describes current findings on the renoprotective effects of naturally occurring NRF2 activators, including sulforaphane, resveratrol, curcumin, and cinnamic aldehyde. These experimental results, coupled with recent clinical experiences with a synthetic triterpenoid, bardoxolone methyl, have brought a light of hope for ameliorating CKD progression by preventing oxidative stress and maintaining cellular redox homeostasis.

Plasma indoxyl sulfate and estimated glomerular filtration rate

BACKGROUND

Indoxyl sulfate (IS), a uremic toxin, has cardiovascular as well as uremic toxicity. We evaluated the prognostic value of blood IS level for long-term outcome.

METHODS AND RESULTS

This study followed 311 patients with coronary artery disease. Plasma IS level and estimated glomerular filtration rate (eGFR) were determined. The endpoint was a major adverse cardiac event (MACE). Median follow-up was 759 days. IS was significantly higher in patients with MACE than in those without (P<0.001). Patients were divided according to quartiles (Q) of plasma IS level (Q1, Q2, Q3, and Q4). On Kaplan-Meier analysis a significantly lower MACE-free rate was obtained for Q4 compared with the other quartiles (P<0.001). In patients with eGFR ≥90, 89-60, 59-30, 29-15, and <15 ml·min(-1)·1.73 m(-2), the percentage of patients in Q4 was 0%, 13%, 29%, 100%, and 100%, respectively. In patients with eGFR 89-60 ml·min(-1)·1.73 m(-2), there was no significant difference in MACE-free rate between Q4 and the other quartiles; in patients with eGFR 59-30 ml·min(-1)·1.73 m(-2), a significantly lower MACE-free rate was obtained for Q4 compared with the other quartiles (P=0.832 and P=0.015, respectively).

CONCLUSIONS

Plasma IS level is a significant predictor of MACE, especially in patients with eGFR 59-30 ml·min(-1)·1.73 m(-2).

Modulation of NADPH oxidase activity by known uraemic retention solutes

BACKGROUND

Uraemia and cardiovascular disease appear to be associated with an increased oxidative burden. One of the key players in the genesis of reactive oxygen species (ROS) is nicotinamide adenine dinucleotide phosphate (NADPH) oxidase. Based on initial experiments demonstrating a decreased inhibitory effect on NADPH oxidase activity in the presence of plasma from patients with CKD-5D after dialysis compared with before dialysis, we investigated the effect of 48 known and commercially available uraemic retention solutes on the enzymatic activity of NADPH oxidase.

METHODS

Mononuclear leucocytes isolated from buffy coats of healthy volunteers were isolated, lysed and incubated with NADH in the presence of plasma from healthy controls and patients with CKD-5D. Furthermore, the leucocytes were lysed and incubated in the presence of uraemic retention solute of interest and diphenyleneiodonium chloride (DPI), an inhibitor of NADPH oxidase. The effect on enzymatic activity of NADPH oxidase was quantified within an incubation time of 120 min.

RESULTS

Thirty-nine of the 48 uraemic retention solutes tested had a significant decreasing effect on NADPH oxidase activity. Oxalate has been characterized as the strongest inhibitor of NADPH oxidase (90% of DPI inhibition). Surprisingly, none of the uraemic retention solutes we investigated was found to increase NADPH oxidase activity. Furthermore, plasma from patients with CKD-5D before dialysis caused significantly higher inhibitory effect on NADPH oxidase activity compared with plasma from healthy subjects. However, this effect was significantly decreased in plasma from patients with CKD-5D after dialysis.

CONCLUSIONS

The results of this study show that uraemic retention solutes modulated the activity of the NADPH oxidase. The results of this study might be the basis for the development of inhibitors applicable as drug in the situation of increased oxidative stress.

Feasibility of detecting prostate cancer by ultraperformance liquid chromatography-mass spectrometry serum metabolomics

Prostate cancer (PCa) is the second leading cause of cancer-related mortality in men. The prevalent diagnosis method is based on the serum prostate-specific antigen (PSA) screening test, which suffers from low specificity, overdiagnosis, and overtreatment. In this work, untargeted metabolomic profiling of age-matched serum samples from prostate cancer patients and healthy individuals was performed using ultraperformance liquid chromatography coupled to high-resolution tandem mass spectrometry (UPLC-MS/MS) and machine learning methods. A metabolite-based in vitro diagnostic multivariate index assay (IVDMIA) was developed to predict the presence of PCa in serum samples with high classification sensitivity, specificity, and accuracy. A panel of 40 metabolic spectral features was found to be differential with 92.1% sensitivity, 94.3% specificity, and 93.0% accuracy. The performance of the IVDMIA was higher than the prevalent PSA test. Within the discriminant panel, 31 metabolites were identified by MS and MS/MS, with 10 further confirmed chromatographically by standards. Numerous discriminant metabolites were mapped in the steroid hormone biosynthesis pathway. The identification of fatty acids, amino acids, lysophospholipids, and bile acids provided further insights into the metabolic alterations associated with the disease. With additional work, the results presented here show great potential toward implementation in clinical settings.

Estimation of removed uremic toxin indoxyl sulphate during hemodialysis by using optical data of the spent dialysate

The aim of this study was to explore the possibility to determine the amount of total removed Indoxyl Sulphate (TR_IS) during dialysis session, an optical method utilizing absorbance and fluorescence spectral data of the spent dialysate was used. Eight uremic patients from Linköping, Sweden and 10 from Tallinn, Estonia, were studied during dialysis treatments. Dialysate samples were taken during each treatment and analyzed at a laboratory. Fluorescence and absorbance spectra of the spent dialysate were measured with spectrofluorophotometer and spectrophotometer. The spectral values were transformed into IS concentration using multiple linear regression model from the total material noted as optical method (Opt). IS concentration was estimated using high-performance liquid chromatography (HPLC) method as a reference. TR_IS values were calculated. Achieved results were compared regarding mean values and SD and collated with the amount of total removed urea value (TR_Urea) for the same dialysis procedures. Mean TR value ± SD (mg) for urea was 28 947 ± 9 241; TR for IS was 151.4 ± 87.3 estimated by HPLC and 149.4 ± 84.9 estimated by Opt. The TR_IS values were not significantly different (p ≤ 0.05). This study indicates, that it is possible to estimate TR_IS using only spectral values of the spent dialysate and the parameter can be used for quantifying the elimination of protein bound uremic toxins during the dialysis procedure.

Hepatic sulfotransferase as a nephropreventing target by suppression of the uremic toxin indoxyl sulfate accumulation in ischemic acute kidney injury

Ischemia/reperfusion (I/R)-induced acute kidney injury (AKI) is evoked by diverse pathophysiological conditions and/or surgical procedures. Here, we evaluated the nephropreventive effect of sulfotransferase (SULT) inhibitors, quercetin, and resveratrol, which hamper hepatic indoxyl sulfate (IS) production. I/R of the kidney caused severe renal injury with marked accumulation of serum and renal IS and urinary excretion of kidney injury molecule-1. Oral administration of AST-120 resulted in a significant restoration of kidney injury, suggesting that uremic toxins, which can be suppressed or adsorbed by AST-120 in the intestine, contribute to the progression or development of I/R-induced AKI. Oral administration of resveratrol or quercetin, SULT inhibitors, suppressed IS accumulation, accompanied by significant amelioration of renal dysfunction. The expression of nuclear factor E2-related factor 2 (Nrf2) in the renal nuclear fractions was markedly elevated by renal I/R, but suppressed by treatment with SULT inhibitors. IS is primarily taken up by HK-2 cells derived from human proximal tubular cells via organic anion transporters, which then evokes activation of Nrf2, most likely due to intracellular oxidative stress. Renal basolateral organic anion transporters OAT1 and OAT3, which mediate renal tubular uptake of IS in basolateral membrane, were markedly downregulated by renal I/R, but restored by SULT inhibitors. Our results suggest that renal accumulation of IS in ischemic AKI induces oxidative stress and downregulation of organic anion transporters resulting in kidney damage, which could be restored to some extent by inhibiting hepatic SULT activity as a nephropreventive target.

Cardiorenal syndrome: acute kidney injury secondary to cardiovascular disease and role of protein-bound uraemic toxins

Cardiovascular disease (CVD) and kidney disease are closely interrelated. Disease of one organ can induce dysfunction of the other, ultimately leading to failure of both. Clinical awareness of synergistic adverse clinical outcomes in patients with coexisting CVD and kidney disease or 'cardiorenal syndrome (CRS)' has existed. Renal dysfunction, even mild, is a strong independent predictor for poor prognosis in CVD patients. Developing therapeutic interventions targeting acute kidney injury (AKI) has been limited due mainly to lack of effective tools to accurately detect AKI in a timely manner. Neutrophil gelatinase-associated lipocalin and kidney injury molecule-1 have been recently demonstrated to be potential candidate biomarkers in patients undergoing cardiac surgery. However, further validation of AKI biomarkers is needed in other CVD settings, especially acute decompensated heart failure and acute myocardial infarction where AKI commonly occurs. The other concern with regard to understanding the pathogenesis of renal complications in CVD is that mechanistically oriented studies have been relatively rare. Pre-clininal studies have shown that activation of renal inflammation-fibrosis processes, probably triggered by haemodynamic derangement, underlies CVD-associated renal dysfunction. On the other hand, it is postulated that there still are missing links in the heart-kidney connection in CRS patients who have significant renal dysfunction. At present, non-dialysable protein-bound uraemic toxins (PBUTs) appear to be the main focus in this regard. Evidence of the causal role of PBUTs in CRS has been increasingly demonstrated, mainly focusing on indoxyl sulfate (IS) and p-cresyl sulfate (pCS). Both IS and pCS are derived from colonic microbiotic metabolism of dietary amino acids, and hence the colon has become a target of treatment in addition to efforts to improve dialysis techniques for better removal of PBUTs. Novel therapy targeting the site of toxin production has led to new prospects in early intervention for predialysis patients with chronic kidney disease.

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 toxicity of indoxyl sulfate and p-cresyl sulfate: a systematic review

A growing number of publications supports a biologic effect of the protein-bound uremic retention solutes indoxyl sulfate and p-cresyl sulfate. However, the use of unrealistically high free concentrations of these compounds and/or inappropriately low albumin concentrations may blur the interpretation of these results. Here, we performed a systematic review, selecting only studies in which, depending on the albumin concentration, real or extrapolated free concentrations of indoxyl sulfate and p-cresyl sulfate remained in the uremic range. The 27 studies retrieved comprised in vitro and animal studies. A quality score was developed, giving 1 point for each of the following criteria: six or more experiments, confirmation by more than one experimental approach, neutralization of the biologic effect by counteractive reagents or antibodies, use of a real-life model, and use of dose-response analyses in vitro and/or animal studies. The overall average score was 3 of 5 points, with five studies scoring 5 of 5 points and six studies scoring 4 of 5 points, highlighting the superior quality of a substantial number of the retrieved studies. In the 11 highest scoring studies, most functional deteriorations were related to uremic cardiovascular disease and kidney damage. We conclude that our systematic approach allowed the retrieval of methodologically correct studies unbiased by erroneous conditions related to albumin binding. Our data seem to confirm the toxicity of indoxyl sulfate and p-cresyl sulfate and support their roles in vascular and renal disease progression.