Indoxyl sulfate-induced endothelial dysfunction in patients with chronic kidney disease via an induction of oxidative stress

Indoxyl sulfate promotes apoptosis in cultured osteoblast cells

Background

Indoxyl sulfate (IS), an organic anion uremic toxin, promotes the progression of renal dysfunction. Some studies have suggested that IS inhibits osteoclast differentiation and suppresses parathyroid hormone (PTH)-stimulated intracellular cAMP production, decreases PTH receptor expression, and induces oxidative stress in primary mouse calvaria osteoblast cell culture. However, the direct effects of IS on osteoblast apoptosis have not been fully evaluated. Hence, we investigated whether IS acts as a bone toxin by studying whether IS induces apoptosis and inhibits differentiation in the cultured osteoblast cell line MC3T3-E1.

Methods

We assessed the direct effect of IS on osteoblast differentiation and apoptosis in the MC3T3-E1 cell line. We examined caspase-3/7 activity, apoptosis-related proteins, free radical production, alkaline phosphatase activity, and mRNA expression of type 1 collagen and osteonectin. Furthermore, we investigated the uptake of IS via organic anion transport (OAT).

Results

We found that IS increased caspase activity and induced apoptosis. Production of free radicals increased depending on the concentration of IS. Furthermore, IS inhibited the expression of mRNA type 1 collagen and osteonectin and alkaline phosphatase activity. The expression of OAT, which is known to mediate the cellular uptake of IS, was detected in in the MC3T3-E1 cell line. The inhibition of OAT improved cell viability and suppressed the production of reactive oxygen species. These results suggest that IS is transported in MC3T3-E1 cells via OAT, which causes oxidative stress to inhibit osteoblast differentiation.

Conclusions

IS acts as a bone toxin by inhibiting osteoblast differentiation and inducing apoptosis.

Ligustrazine attenuates elevated levels of indoxyl sulfate, kidney injury molecule-1 and clusterin in rats exposed to cadmium

In this study, we aimed at evaluating the effect of ligustrazine, a major constituent of Ligusticum wallichii from traditional Chinese medicine, on Cd-induced changes in nephrotoxicity indices. Rats were divided into four experimental groups: control; ligustrazine; Cd and ligustrazine+Cd. Cd treated alone group showed significant decreases (P<0.05) in body weight, renal levels of superoxide dismutase (SOD) and glutathione reductase (GR); and significant increases (P<0.05) in urine volume (24h), pH values, serum blood urea nitrogen (BUN), serum uric acid, kidney malondialdehyde (MDA), urinary total protein, urinary glucose, urinary lactate dehydrogenase (LDH) and urinary alkaline phosphatase (ALP). Apart from indoxyl sulfate (a uremic toxin), two newly accepted nephrotoxicity biomarkers including kidney injury molecule-1 (kim-1) and clusterin were also found to be increased. Nonetheless, all these effects induced by Cd were reversed upon treatment by ligustrazine although it failed in decreasing the concentrations of Cd in kidney and urine. Histopathological studies in Cd-treated rats exhibited renal tubule damage, which was also ameliorated by ligustrazine pretreatment. These results suggest that ligustrazine exhibits protective effects on Cd-induced nephrotoxicity. Additionally, this study also demonstrates Cd exposure induces elevated levels of indoxyl sulfate in serum and kidney, and clusterin in urine.

Serum concentrations of p-cresyl sulfate and indoxyl sulfate, but not inflammatory markers, increase in incident peritoneal dialysis patients in parallel with loss of residual renal function

BACKGROUND

High serum concentrations of the protein-bound uremic retention solutes p-cresyl sulfate (PCS) and indoxyl sulfate (IndS) and inflammation are associated with increased cardiovascular morbidity and mortality in chronic kidney disease. Renal clearance contributes to up to 80% of the total clearance of PCS and IndS in peritoneal dialysis (PD) patients. Cross-sectional studies evaluating the impact of residual renal function (RRF) on serum concentrations of PCS, IndS, and circulating inflammatory markers have yielded conflicting results. ♢

METHODS

To clarify this issue, we carried out a prospective observational cohort study in incident PD patients (n = 35; 19 men; mean age: 55 ± 17 years). Midday blood samples were collected and analyzed for total serum PCS, IndS, C-reactive protein, and high-sensitivity interleukin 6. Peritoneal and renal clearances were calculated from urine and dialysate collections, and RRF was calculated as the mean of renal urea nitrogen and creatinine clearances. Patients were assessed 1, 6, 12, and 24 months after PD start. Differences between time points were analyzed using linear mixed models (LMMs). ♢

RESULTS

Residual renal function declined significantly over time (LMM p < 0.0001). Peritoneal clearances of both toxins tended to increase, but did not compensate for the declining renal clearances. Serum concentrations of PCS and IndS increased significantly over time (LMM p = 0.01; p = 0.0009). In contrast, total mass removal of both toxins remained stable. Circulating inflammatory markers did not change over time. ♢

CONCLUSIONS

Our data indicate that serum concentrations of PCS and IndS, but not inflammatory markers, increase in incident PD patients in parallel with loss of RRF.

The uremic toxin indoxyl sulphate enhances macrophage response to LPS

Indoxyl sulphate (IS) is a protein-bound uremic toxin that results from the metabolism of dietary tryptophan normally excreted by kidney through the proximal tubules. Thus the toxin accumulates in the blood of patients with impaired renal function such as in chronic kidney disease (CKD). High IS serum levels in patients with CKD suggest its involvement in CKD progression and in the onset of complications. Its presence in plasma is also a powerful predictor of overall and cardiovascular morbidity/mortality. IS is a well known nephrovascular toxin but very little is known regarding its effects on the immune system and in particular during inflammation. In this study we examined the effect of IS on macrophage activation in response to lipopolysaccharide from E. coli (LPS), a gram negative bacterial endotoxin associated with inflammation and septic shock. To simulate the uremic condition, J774A.1 macrophages were incubated with IS at concentrations observed in uremic patients (1000-62.5 µM) both alone and during LPS challenge. IS alone induced release of reactive oxygen species (ROS), through a mechanism involving pro- and anti-oxidant systems, and alteration in intracellular calcium homeostasis. When added to J774A.1 macrophages in presence of LPS, IS significantly increased the nitric oxide (NO) release, inducible nitric oxide synthase (iNOS) and cycloxygenase-2 (COX-2) expression. IS pre-treatment was also associated with an increase in tumor necrosis factor-α (TNF-α) and interleukin-6 (IL-6) production by macrophages stimulated with LPS. Mechanistic studies revealed that IS increased LPS-induced NF-kB nuclear translocation, ROS release and altered calcium concentrations, mainly because of mitochondrial calcium overloading. Moreover also in primary mouse peritoneal macrophages IS enhances the inflammatory response to LPS increasing ROS, NO, iNOS, COX-2, TNF-α, IL-6 and NF-kB levels. This study provides evidences that IS stimulates macrophage function and enhances inflammatory reasponse associated with LPS, thus contributing to altered immune response dysfunctions observed in CKD.

Targeting protein-bound uremic toxins in chronic kidney disease

INTRODUCTION

Protein-bound uremic toxins such as indoxyl sulfate cannot be removed efficiently by hemodialysis. These protein-bound uremic toxins have emerged as important risk factors for the progression of chronic kidney disease (CKD) as well as cardiovascular disease (CVD).

AREAS COVERED

Indoxyl sulfate shows toxic effects on a variety of cells such as renal proximal tubular cells, glomerular mesangial cells, vascular smooth muscle cells, vascular endothelial cells, cardiomyocytes, cardiac fibroblasts, monocytes, osteoblasts and osteoclasts. This review overviews the cellular toxicity of indoxyl sulfate, its molecular mechanism and its role in the progression of CKD and CVD. Further, this review summarizes the clinical effects of AST-120 and the other strategies to reduce serum levels of indoxyl sulfate.

EXPERT OPINION

Protein-bound uremic toxins such as indoxyl sulfate have emerged as target molecules for therapeutic intervention of not only CKD but also CVD. An oral sorbent AST-120 reduces serum level of indoxyl sulfate by adsorbing indole in the intestine. The modulation of intestinal bacteria by prebiotics/probiotics might be effective in reducing the production of indole in the intestine followed by reduced serum levels of indoxyl sulfate. An alternative approach might be antagonist which can counteract indoxyl sulfate-induced cellular effects and signaling pathways.

Indoxyl sulfate down-regulates SLCO4C1 transporter through up-regulation of GATA3

The accumulated uremic toxins inhibit the expression of various renal transporters and this inhibition may further reduce renal function and subsequently cause the accumulation of uremic toxins. However, the precise mechanism of the nephrotoxicity of uremic toxins on renal transport has been poorly understood. Here we report that indoxyl sulfate, one of the potent uremic toxins, directly suppresses the renal-specific organic anion transporter SLCO4C1 expression through a transcription factor GATA3. The promoter region of SLCO4C1 gene has several GATA motifs, and indoxyl sulfate up-regulated GATA3 mRNA and subsequently down-regulated SLCO4C1 mRNA. Overexpression of GATA3 significantly reduced SLCO4C1 expression, and silencing of GATA3 increased SLCO4C1 expression vice versa. Administration of indoxyl sulfate in rats reduced renal expression of slco4c1 and under this condition, plasma level of guanidinosuccinate, one of the preferable substrates of slco4c1, was significantly increased without changing plasma creatinine. Furthermore, in 5/6 nephrectomized rats, treatment with oral adsorbent AST-120 significantly decreased plasma indoxyl sulfate level and conversely increased the expression of slco4c1, following the reduction of plasma level of guanidinosuccinate. These data suggest that the removal of indoxyl sulfate and blocking its signal pathway may help to restore the SLCO4C1-mediated renal excretion of uremic toxins in CKD.

Pre-, pro-, and synbiotics: do they have a role in reducing uremic toxins? A systematic review and meta-analysis

Objective. This paper assessed the effectiveness of pre-, pro-, and synbiotics on reducing two protein-bound uremic toxins, p-cresyl sulphate (PCS) and indoxyl sulphate (IS). Methods. English language studies reporting serum, urinary, or fecal PCS and/or IS (or their precursors) following pre-, pro-, or synbiotic interventions (>1 day) in human adults were included. Population estimates of differences in the outcomes between the pre- and the postintervention were estimated for subgroups of studies using four meta-analyses. Quality was determined using the GRADE approach. Results. 19 studies met the inclusion criteria, 14 in healthy adults and five in haemodialysis patients. Eight studies investigated prebiotics, six probiotics, one synbiotics, one both pre- and probiotics, and three studies trialled all three interventions. The quality of the studies ranged from moderate to very low. 12 studies were included in the meta-analyses with all four meta-analyses reporting statistically significant reductions in IS and PCS with pre- and probiotic therapy. Conclusion. There is a limited but supportive evidence for the effectiveness of pre- and probiotics on reducing PCS and IS in the chronic kidney disease population. Further studies are needed to provide more definitive findings before routine clinical use can be recommended.

Searching for uremic toxins

Treatment of uremia by hemodialysis has become widespread over the last 40 years and has improved substantially over that time. However, people treated with this modality continue to suffer from multiple disabilities. Retention of organic solutes, especially those poorly removed by hemodialysis, likely contributes to these disabilities. Certain classes of solutes are removed less well than urea by hemodialysis and by the normal kidney. These include protein-bound solutes, relatively large solutes, sequestered compounds, and substances removed at rates higher than urea by the normal kidney. Several strategies could be used to discover the solutes responsible for residual morbidities in standardly dialyzed people. Rather than continue to focus only on urea removal as an index for dialysis adequacy, finding additional approaches for removing toxic solutes with characteristics different from urea (and the similar small solutes it represents) is a desirable and feasible goal.

Indoxyl sulfate predicts cardiovascular disease and renal function deterioration in advanced chronic kidney disease

BACKGROUND AND AIMS

Indoxyl sulfate, a protein-bound uremic toxin, was found to be accumulated in kidney tissues with a reduction in renal function. This, in turn, not only leads to kidney fibrosis and endothelial dysfunction but also to adverse clinical effects. We investigated the adverse effects of indoxyl sulfate on clinical outcomes in a study involving human subjects.

METHODS

Seventy pre-dialysis patients were enrolled from a single medical center. Serum indoxyl sulfate and biochemistry data were measured concurrently. Clinical outcomes including dialysis event, cardiovascular event and all-cause mortality were recorded during a 36-month follow-up.

RESULTS

Multivariate Cox regression analysis showed that age (HR: 0.95, p = 0.05), serum creatinine (HR: 1.29, p = 0.04) and indoxyl sulfate (HR: 1.06, p = 0.02) were independently associated with dialysis event; age (HR: 1.16, p = 0.01), serum phosphate (HR: 3.03, p = 0.05) and indoxyl sulfate level (HR: 1.11, p = 0.04) reached significant correlation with cardiovascular events after adjusting for other confounding factors. Kaplan-Meier analysis revealed that indoxyl sulfate level was significantly associated with cardiovascular and dialysis event (log rank p <0.01, log rank p = 0.01, respectively). In addition, serum indoxyl sulfate concentration was significantly increased in patients with dialysis and cardiovascular event (p <0.01, p <0.01, respectively).

CONCLUSIONS

Our results suggest that serum indoxyl sulfate level was a valuable marker in predicting cardiovascular disease and renal function decline in patients with advanced chronic kidney disease.

Normal and pathologic concentrations of uremic toxins

An updated review of the existing knowledge regarding uremic toxins facilitates the design of experimental studies. We performed a literature search and found 621 articles about uremic toxicity published after a 2003 review of this topic. Eighty-seven records provided serum or blood measurements of one or more solutes in patients with CKD. These records described 32 previously known uremic toxins and 56 newly reported solutes. The articles most frequently reported concentrations of β2-microglobulin, indoxyl sulfate, homocysteine, uric acid, and parathyroid hormone. We found most solutes (59%) in only one report. Compared with previous results, more recent articles reported higher uremic concentrations of many solutes, including carboxymethyllysine, cystatin C, and parathyroid hormone. However, five solutes had uremic concentrations less than 10% of the originally reported values. Furthermore, the uremic concentrations of four solutes did not exceed their respective normal concentrations, although they had been previously described as uremic retention solutes. In summary, this review extends the classification of uremic retention solutes and their normal and uremic concentrations, and it should aid the design of experiments to study the biologic effects of these solutes in CKD.

p-Cresol sulfate and indoxyl sulfate induce similar cellular inflammatory gene expressions in cultured proximal renal tubular cells

BACKGROUND

p-Cresol sulfate (PCS) and indoxyl sulfate (IS) have important roles in the kidney injury. The aim of this study was to determine the inflammatory response to PCS and IS.

METHODS

Cultured mouse proximal renal tubular cells were treated with PCS or IS and analyzed by polymerase chain reaction array with an inflammation and immune panel. Gene annotation enrichment and functional annotation clustering were analyzed with the Database for Annotation, Visualization, and Integrated Discovery (DAVID). Functional networks of the target genes were analyzed with the algorithm GeneMANIA.

RESULTS

PCS and IS increased the expression of inflammation associated genes. Sixteen upregulated gene clusters of cells treated with PCS or IS were found. The major cytokines in the functional networks generated by PCS or IS treatment were Tgfb1, Fasl, Il6/15, Il15, Csf1/3 and Cxcl10. The major intracellular signal triggered by PCS or IS included Stats, Smads, Nfkb2, Ikbkb, Bcl2 and Bax. In both PCS- and IS-treated cells, Col4a5, Cxc10, Fasl, Stat1 and Ikbkb were the target genes in the predicted molecular functional networks connected to Tgfb1.

CONCLUSIONS

PCS and IS stimulate significant cellular inflammation. Similar immune and cellular inflammatory responses were induced by PCS or IS on cultured proximal renal tubular cells.

Indoxyl sulfate-induced epithelial-to-mesenchymal transition and apoptosis of renal tubular cells as novel mechanisms of progression of renal disease

Indoxyl sulfate (IS), one of the uremic toxins, is regarded to have a substantial role in the progression of chronic kidney disease (CKD). Epithelial-to-mesenchymal transition (EMT) and apoptosis of renal tubular cells are known to be the critical mechanisms of the development and aggravation of CKD. We investigated the effect of IS on EMT and apoptosis in renal proximal tubular cells, NRK-52E cells. IS significantly inhibited cell proliferation and induced cell migration with a morphological transition from cuboidal epithelial cells to spindle-shaped scattered fibroblast-like cells. IS downregulated the expressions of zonula occluden-1 and E-cadherin, whereas upregulated α-SMA expression at 48 h, which was blocked by a pretreatment of the organic anion transporter, probenecid. IS also induced apoptosis of NRK cells from a concentration of 25 μg/ml with an activation of ERK1/2 and p38 MAP kinase (MAPK). Pretreatment of ERK1/2 or p38 MAPK inhibitors, PD98059 or SB203580, resulted in no significant effect on IS-induced EMT, whereas it ameliorated IS-induced apoptosis of NRK cells. These findings suggested phenotypic transition and apoptosis as potential mechanisms of IS-induced renal damage and the differential role of MAPK activation in IS-induced EMT and apoptosis of renal tubular cells.

Uremic toxins induce kidney fibrosis by activating intrarenal renin-angiotensin-aldosterone system associated epithelial-to-mesenchymal transition

Background

Uremic toxins are considered to have a determinant pathological role in the progression of chronic kidney disease. The aim of this study was to define the putative pathological roles of the renal renin–angiotensin–aldosterone system (RAAS) and renal tubular epithelial-to-mesenchymal transition (EMT) in kidney fibrosis induced by (indoxyl sulfate) IS and (p-cresol sulfate) PCS.

Methods

Mouse proximal renal tubular cells (PKSV-PRs) treated with IS or PCS were used. Half-nephrectomized B-6 mice were treated with IS or PCS for 4 weeks. In the losartan treatment study, the study animal was administrated with IS+losartan or PCS+losartan for 4 weeks.

Results

IS and PCS significantly activated the intrarenal RAAS by increasing renin, angiotensinogen, and angiotensin 1 (AT1) receptor expression, and decreasing AT2 receptor expression in vitro and in vivo. IS and PCS significantly increased transforming growth factor-β1 (TGF-β1) expression and activated the TGF-β pathway by increasing Smad2/Smad2-P, Smad3/Smad3-P, and Smad4 expression. The expression of the EMT-associated transcription factor Snail was increased by IS and PCS treatment. IS and PCS induced the phenotype of EMT-like transition in renal tubules by increasing the expression of fibronectin and α-smooth muscle actin and decreasing the expression of E-cadherin. Losartan significantly attenuated the expression of TGF-β1 and Snail, and decreased kidney fibrosis induced by IS and PCS in vivo.

Conclusion

Activating the renal RAAS/TGF-β pathway has an important pathological role in chronic kidney injury caused by IS and PCS. IS and PCS may increase Snail expression and induce EMT-like transition.

New insights into uremia-induced alterations in metabolic pathways

PURPOSE OF REVIEW

This article summarizes recent studies on uremia-induced alterations in metabolism, with particular emphasis on the application of emerging metabolomics technologies.

RECENT FINDINGS

The plasma metabolome is estimated to include more than 4000 distinct metabolites. Because these metabolites can vary dramatically in size and polarity and are distributed across several orders of magnitude in relative abundance, no single analytical method is capable of comprehensive metabolomic profiling. Instead, a variety of analytical techniques, including targeted and nontargeted liquid chromatography-mass spectrometry, have been employed for metabolomic analysis of human plasma. Recent efforts to apply this technology to study uremia have reinforced the common view that end-stage renal disease is a state of generalized small molecule excess. However, the identification of precursor depletion and downstream metabolite excess - for example, with tryptophan and downstream kynurenine metabolites, with low molecular weight triglycerides and dicarboxylic acids, and with phosphatidylcholines, choline, and trimethylamine-N-oxide - suggest that uremia may directly modulate these metabolic pathways. Metabolomic studies have also begun to expand some of these findings to individuals with chronic kidney disease and in model systems.

SUMMARY

Uremia is associated with diverse, but incompletely understood metabolic disturbances. Metabolomic approaches permit higher resolution phenotyping of these disturbances, but significant efforts will be required to understand the functional significance of select findings.

Dietary phosphate restriction ameliorates endothelial dysfunction in adenine-induced kidney disease rats

Hyperphosphatemia causes endothelial dysfunction as well as vascular calcification. Management of serum phosphate level by dietary phosphate restriction or phosphate binders is considered to be beneficial to prevent chronic kidney disease patients from cardiovascular disease, but it has been unclear whether keeping lower serum phosphate level can ameliorate endothelial dysfunction. In this study we investigated whether low-phosphate diet can ameliorate endothelial dysfunction in adenine-induced kidney disease rats, one of useful animal model of chronic kidney disease. Administration of 0.75% adenine-containing diet for 21 days induced renal failure with hyperphosphatemia, and impaired acetylcholine-dependent vasodilation of thoracic aortic ring in rats. Then adenine-induced kidney disease rats were treated with either control diet (1% phosphate) or low-phosphate diet (0.2% phosphate) for 16 days. Low-phosphate diet ameliorated not only hyperphosphatemia but also the impaired vasodilation of aorta. In addition, the activatory phosphorylation of endothelial nitric oxide synthase at serine 1177 and Akt at serine 473 in the aorta were inhibited by in adenine-induced kidney disease rats. The inhibited phosphorylations were improved by the low-phosphate diet treatment. Thus, dietary phosphate restriction can improve aortic endothelial dysfunction in chronic kidney disease with hyperphosphatemia by increase in the activatory phosphorylations of endothelial nitric oxide synthase and Akt.

Effect of an Oral Adsorbent, AST-120, on Dialysis Initiation and Survival in Patients with Chronic Kidney Disease

The oral adsorbent AST-120 has the potential to delay dialysis initiation and improve survival of patients on dialysis. We evaluated the effect of AST-120 on dialysis initiation and its potential to improve survival in patients with chronic kidney disease. The present retrospective pair-matched study included 560 patients, grouped according to whether or not they received AST-120 before dialysis (AST-120 and non-AST-120 groups). The cumulative dialysis initiation free rate and survival rate were compared by the Kaplan-Meier method. Multivariate analysis was used to determine the impact of AST-120 on dialysis initiation. Our results showed significant differences in the 12- and 24-month dialysis initiation free rate (P < 0.001), although no significant difference was observed in the survival rate between the two groups. In conclusion, AST-120 delays dialysis initiation in chronic kidney disease (CKD) patients but has no effect on survival. AST-120 is an effective therapy for delaying the progression of CKD.

HIV gene expression deactivates redox-sensitive stress response program in mouse tubular cells both in vitro and in vivo

Human immunodeficiency virus (HIV)-1 has been reported to cause tubular cell injury both in in vivo and in vitro studies. In the present study, we evaluated the role of oxidative stress in the induction of apoptosis in HIV gene expressing mouse tubular cells in in vivo (Tg26, a transgenic mouse model of HIV-associated nephropathy) and in vitro (tubular cells were transduced with pNL4-3: ΔG/P-GFP, VSV.G psueudo typed virus) studies. Although Tg26 mice showed enhanced tubular cell reactive oxygen species (ROS) generation and apoptosis, renal tissue did not display a robust antioxidant response in the form of enhanced free radical scavenger (MnSOD/catalase) expression. Tg26 mice not only showed enhanced tubular cell expression of phospho-p66ShcA but also displayed nuclear Foxo3a translocation to the cytoplasm. These findings indicated deactivation of tubular cell Foxo3A-dependent redox-sensitive stress response program (RSSRP) in Tg26 mice. In in vitro studies, NL4-3 (pNL4-3: ΔG/P-GFP, VSV.G pseudotyped virus)-transduced mouse proximal tubular cells (NL4-3/MPTEC) displayed enhanced phosphorylation of p66ShcA. NL4-3/MPTECs also displayed greater (P < 0.01) ROS generation when compared with empty vector-transduced tubular cells; however, both diminution of p66ShcA and N-acetyl cysteine attenuated NL4-3-induced tubular cell ROS generation as well as apoptosis. In addition, both antioxidants and free radical scavengers partially inhibited HIV-induced tubular cell apoptosis. NL4-3/MPTEC displayed deactivation of RSSRP in the form of enhanced phosphorylation of Foxo3A and attenuated expression of superoxide dismutase (SOD) and catalase. Since both SOD and catalase were able to provide protection against HIV-1-induced tubular cell apoptosis, it suggests that HIV-1-induced proapoptotic effect may be a consequence of the deactivated RSSRP.

Secreted klotho and chronic kidney disease

Soluble Klotho (sKl) in the circulation can be generated directly by alterative splicing of the Klotho transcript or the extracellular domain of membrane Klotho can be released from membrane-anchored Klotho on the cell surface. Unlike membrane Klotho which functions as a coreceptor for fibroblast growth factor-23 (FGF23), sKl, acts as hormonal factor and plays important roles in anti-aging, anti-oxidation, modulation of ion transport, and Wnt signaling. Emerging evidence reveals that Klotho deficiency is an early biomarker for chronic kidney diseases as well as a pathogenic factor. Klotho deficiency is associated with progression and chronic complications in chronic kidney disease including vascular calcification, cardiac hypertrophy, and secondary hyperparathyroidism. In multiple experimental models, replacement of sKl, or manipulated up-regulation of endogenous Klotho protect the kidney from renal insults, preserve kidney function, and suppress renal fibrosis, in chronic kidney disease. Klotho is a highly promising candidate on the horizon as an early biomarker, and as a novel therapeutic agent for chronic kidney disease.

Vascular incompetence in dialysis patients--protein-bound uremic toxins and endothelial dysfunction

Patients with chronic kidney disease (CKD) have a much higher risk of cardiovascular diseases than the general population. Endothelial dysfunction, which participates in accelerated atherosclerosis, is a hallmark of CKD. Patients with CKD display impaired endothelium-dependent vasodilatation, elevated soluble biomarkers of endothelial dysfunction, and increased oxidative stress. They also present an imbalance between circulating endothelial populations reflecting endothelial injury (endothelial microparticles and circulating endothelial cells) and repair (endothelial progenitor cells). Endothelial damage induced by a uremic environment suggests an involvement of uremia-specific factors. Several uremic toxins, mostly protein-bound, have been shown to have specific endothelial toxicity: ADMA, homocysteine, AGEs, and more recently, p-cresyl sulfate and indoxyl sulfate. These toxins, all poorly removed by hemodialysis therapies, share mechanisms of endothelial toxicity: they promote pro-oxidant and pro-inflammatory response and inhibit endothelial repair. This article (i) reviews the evidence for endothelial dysfunction in CKD, (ii) specifies the involvement of protein-bound uremic toxins in this dysfunction, and (iii) discusses therapeutic strategies for lowering uremic toxin concentrations or for countering the effects of uremic toxins on the endothelium.

Indoxyl sulfate, a representative uremic toxin, suppresses erythropoietin production in a HIF-dependent manner

Advanced chronic kidney disease (CKD) patients encounter anemia through insufficient erythropoietin (EPO) production by peritubular fibroblasts. Recent studies showed an increase in EPO production by pharmacological activation of hypoxia-inducible transcription factors (HIFs) in dialysis patients, suggesting that desensitization of the oxygen-sensing mechanism is responsible for the development of renal anemia. Our recent work demonstrated that indoxyl sulfate (IS), a uremic toxin, dysregulates oxygen metabolism in tubular cells. Here we provide evidence of an additional property that IS impairs oxygen sensing in EPO-producing cells. HepG2 cells were stimulated with cobalt chloride (CoCl(2)) or hypoxia under varying concentrations of IS. EPO mRNA was evaluated by quantitative PCR. Nuclear accumulation of HIF-α was evaluated by western blotting. Transcriptional activity of HIF was checked by hypoxia-responsive element (HRE)-luciferase reporter assay. The impact of IS was further evaluated in vivo by administering rats with indole, a metabolic precursor of IS, and subjecting them to CoCl(2) stimulation, in which renal EPO mRNA as well as plasma EPO levels were measured by quantitative PCR and enzyme-linked immunosorbent assay, respectively. Although IS induced cellular toxicity at relatively high concentrations (2.5 mM), EPO mRNA expression was significantly suppressed by IS at concentrations below cytotoxic ranges. In HepG2 cells, IS treatment decreased nuclear accumulation of HIF-α proteins and suppressed HRE-luciferase activity following hypoxia. Furthermore, administration of rats with indole suppressed renal EPO mRNA expression and plasma EPO levels, corroborating in vitro findings. Results of the present study provide a possible connection between a uremic toxin and the desensitization of the oxygen-sensing mechanism in EPO-producing cells, which may partly explain inadequate EPO production in hypoxic kidneys of CKD patients.

NF-κB plays an important role in indoxyl sulfate-induced cellular senescence, fibrotic gene expression, and inhibition of proliferation in proximal tubular cells

We previously demonstrated that indoxyl sulfate induces senescence and dysfunction of proximal tubular cells by activating p53 expression. However, little is known about the role of nuclear factor (NF)-κB in these processes. The present study examines whether activation (phosphorylation) of NF-κB by indoxyl sulfate promotes senescence and dysfunction in human proximal tubular cells (HK-2 cells). Indoxyl sulfate induced phosphorylation of NF-κB p65 on Ser-276, which was suppressed by N-acetylcysteine, an antioxidant. Furthermore, indoxyl sulfate induced NF-κB p65 expression. Inhibitors of NF-κB (pyrrolidine dithiocarbamate and isohelenin) and NF-κB p65 small interfering RNA (siRNA) suppressed indoxyl sulfate-induced senescence-associated β-galactosidase activity and expression of p53, transforming growth factor (TGF)-β1, and α-smoothe muscle actin (SMA). The induction of p53 expression and p53 promoter activity by indoxyl sulfate were inhibited by pifithrin-α, p-nitro, an inhibitor of p53, whereas p53-transfected cells showed enhanced p53 promoter activity. NF-κB inhibitors suppressed indoxyl sulfate-induced p21 expression, whereas NF-κB p65 siRNA enhanced its expression. NF-κB inhibitors partially alleviated indoxyl sulfate-induced inhibition of cellular proliferation. NF-κB p65 siRNA-transfected cells showed less proliferation in the presence of indoxyl sulfate than control cells. Phosphorylated NF-κB p65 was expressed and colocalized with p53, p21, β-galactosidase, TGF-β1, and α-SMA in the kidneys of chronic renal failure (CRF) rats. AST-120, which reduces serum indoxyl sulfate level, suppressed their expression in the CRF rat kidneys. Taken together, NF-κB plays an important role in indoxyl sulfate-induced cellular senescence, fibrotic gene expression, and inhibition of proliferation in proximal tubular cells. More notably, indoxyl sulfate accelerates proximal tubular cell senescence with progression of CRF through reactive oxygen species-NF-κB-p53 pathway.

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.

Serum free p-cresyl sulfate levels predict cardiovascular and all-cause mortality in elderly hemodialysis patients--a prospective cohort study

BACKGROUND

The mortality rate of elderly hemodialysis (HD) patients is high. Serum p-cresyl sulfate (PCS) and indoxyl sulfate (IS) are associated with cardiovascular (CV) disease and mortality in renal patients. The association between such biomarkers and mortality in elderly HD patients has a high clinical value but remains unclear.

METHODS

This prospective cohort study investigated the association of serum IS and PCS with all-cause and CV mortality in elderly HD patients. Multivariate Cox regression analysis was used to estimate the risk of all-cause and CV mortality in this prospective cohort.

RESULTS

Of 112 patients, 45 deaths (18 CV deaths) were identified after a mean follow-up of 33.2 months. The cumulative and CV survival of patients with lower free PCS was significantly better than high free PCS patients. In multivariate Cox regression analysis, serum free PCS was associated with all-cause and CV mortality after various adjustments, including age, gender and diabetes status (Model 1), albumin (Model 2), Ca × P product and intact parathyroid hormone (Model 3), hemoglobin and high-sensitivity C-reactive protein (Model 4) and hierarchically selected covariates (age, diabetes status and albumin, Model 5).

CONCLUSION

Serum free PCS levels may help in predicting risk of all-cause and CV mortality in elderly HD patients beyond traditional and uremia related risk factors.

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.

Determination of uremic solutes in biological fluids of chronic kidney disease patients by HPLC assay

During chronic kidney disease (CKD), solutes called uremic solutes, accumulate in blood and tissues of patients. We developed an HPLC method for the simultaneous determination of several uremic solutes of clinical interest in biological fluids: phenol (Pol), indole-3-acetic acid (3-IAA), p-cresol (p-C), indoxyl sulfate (3-INDS) and p-cresol sulfate (p-CS). These solutes were separated by ion-pairing HPLC using an isocratic flow and quantified with a fluorescence detection. The mean serum concentrations of 3-IAA, 3-INDS and p-CS were 2.12, 1.03 and 13.03 μM respectively in healthy subjects, 3.21, 17.45 and 73.47 μM in non hemodialyzed stage 3-5 CKD patients and 5.9, 81.04 and 120.54 μM in hemodialyzed patients (stage 5D). We found no Pol and no p-C in any population. The limits of quantification for 3-IAA, 3-INDS, and p-CS were 0.83, 0.72, and 3.2 μM respectively. The within-day CVs were between 1.23 and 3.12% for 3-IAA, 0.98 and 2% for 3-INDS, and 1.25 and 3.01% for p-CS. The between-day CVs were between 1.78 and 5.48% for 3-IAA, 1.45 and 4.54% for 3-INDS, and 1.19 and 6.36% for p-CS. This HPLC method permits the simultaneous and quick quantification of several uremic solutes for daily analysis of large numbers of samples.

Nitric oxide counters the inhibitory effects of uremic toxin indoxyl sulfate on endothelial cells by governing ERK MAP kinase and myosin light chain activation

Uremic toxins such as indoxyl sulfate (IS) accumulate at a high level in end stage renal disease (ESRD) and can exhibit significant systemic endothelial toxicity leading to accelerated cardiovascular events. The precise molecular mechanisms by which IS causes endothelial dysfunction are unknown. We tested the hypothesis that IS negatively influences properties of endothelial cells, such as migration and tube formation, by depleting nitric oxide (NO) bioavailability, and that an NO donor can reverse these inhibitory effects. IS inhibited human umbilical vein endothelial cell (HUVEC) migration and formation of tubes on matrigel. Mechanistically, IS inhibited VEGF-induced NO release from HUVECs. An NO donor, SNAP, reversed IS-mediated inhibition of HUVEC migration as well as tube-formation. IS inhibited ERK 1/2 MAP kinase activity in a dose-dependent manner, but this was preserved by SNAP. Inhibition of ERK 1/2 with a pharmacological inhibitor (U0126) decreased HUVEC migration and tube formation; these effects too were prevented by SNAP. Further, IS stimulated activation of myosin light chain (MLC), potentially stimulating endothelial contractility, while SNAP decreased MLC activation. Thus, we conclude that the negative effects of IS on endothelial cells are prevented, to a major extent, by NO, via its divergent actions on ERK MAP kinase and MLC.

Increased levels of total P-Cresylsulphate and indoxyl sulphate are associated with coronary artery disease in patients with diabetic nephropathy

BACKGROUND

Indoxyl sulphate (IS) and p-cresylsulphate (PCS) are uremic toxins with similar protein-binding, dialytic clearance, and proinflammatory features. Few studies have evaluated the possible associations between these solutes and coronary artery disease (CAD) in type 2 diabetes (T2D) patients.

METHODS

A hospital-based case control study was performed. A total of 209 T2D patients were divided into two groups based on the presence/absence of significant CAD (≥50% luminal reduction). Serum total PCS and IS levels were measured using the Ultra Performance LC System. The relationship between total PCS and IS levels were investigated. Coronary calcium scores and the modified Gensini score were analyzed.

RESULTS

Serum total PCS and IS levels were significantly higher in patients with both T2D and significant CAD, than in non-diabetic control subjects and T2D patients without CAD (all p < 0.05). Logistic regression analysis revealed independent and significant associations between the two solutes and CAD status. Serum total PCS, IS, and numbers of diseased vessels were elevated in groups with estimated glomerular filtration rate (eGFR) of 60-89 ml/min/1.73 m2 and below. Also, serum total PCS and IS levels were significantly associated with eGFR, coronary calcium scores, Gensini score, adipocytokines (adiponectin, visfatin, and leptin), and total white blood cell count.

CONCLUSIONS

Serum total PCS and IS levels were elevated in patients with T2D and CAD. These increases were associated with renal function deterioration, inflammation, and coronary atherosclerosis.