Early elimination of uremic toxin ameliorates AKI-to-CKD transition.

Acute kidney injury (AKI)-related fibrosis is emerging as a major driver of chronic kidney disease (CKD) development. Aberrant kidney recovery after AKI is multifactorial and still poorly understood. The accumulation of indoxyl sulfate (IS), a protein-bound uremic toxin, has been identified as a detrimental factor of renal fibrosis. However, the mechanisms underlying IS-related aberrant kidney recovery after AKI is still unknown. The present study aims to elucidate the effects of IS on tubular damage and its involvement in the pathogenesis of AKI-to-CKD transition. Our results showed that serum IS started to accumulate associated with the downregulation of tubular organic anion transporter but not observed in the small-molecule uremic toxins of the unilateral ischemia-reperfusion injury (UIRI) without a contralateral nephrectomy model. Serum IS is positively correlated with renal fibrosis and binding immunoglobulin protein (BiP) and CAAT/enhancer-binding protein (C/EBP) homologous protein (CHOP) expression induction in the UIRI with a contralateral nephrectomy model (UIRI+Nx). To evaluate the effects of IS in the AKI-to-CKD transition, we administered indole, a precursor of IS, at the early stage of UIRI. Our results demonstrated IS potentiates renal fibrosis, senescence-associated secretory phenotype (SASP), and activation of endoplasmic reticulum (ER) stress, which is attenuated by synergistic AST-120 administration. Furthermore, we clearly demonstrated that IS exposure potentiated hypoxia-reperfusion (H/R) induced G2/M cell cycle arrest, epithelial-mesenchymal transition (EMT) and aggravated ER stress induction in vitro. Finally, the ER chemical chaperon, 4-phenylbutyric acid (4-PBA), successfully reversed the above-mentioned AKI-to-CKD transition. Taken together, early IS elimination in the early stage of AKI is likely to be a useful strategy in the prevention and/or treatment of the AKI-to-CKD transition.

Paricalcitol attenuates indoxyl sulfate-induced apoptosis through the inhibition of MAPK, Akt, and NF-kB activation in HK-2 cells.

BACKGROUND/AIMS: Indoxyl sulfate (IS) is a uremic toxin and an important causative factor in the progression of chronic kidney disease. Recently, paricalcitol (19-nor-1,25-dihydroxyvitamin D2) was shown to exhibit protective effects in kidney injury. Here, we investigated the effects of paricalcitol treatment on IS-induced renal tubular injury. METHODS: The fluorescent dye 2',7'-dichlorofluorescein diacetate was used to measure intracellular reactive oxygen species (ROS) following IS administration in human renal proximal tubular epithelial (HK-2) cells. The effects of IS on cell viability were determined using MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) assays and levels of apoptosis-related proteins (Bcl-2-associated protein X [Bax] and B-cell lymphoma 2 [Bcl-2]), nuclear factor-κB (NF- κB) p65, and phosphorylation of mitogen-activated protein kinase (MAPK) and protein kinase B (Akt) were determined by semiquantitative immunoblotting. The promoter activity of NF-κB was measured by luciferase assays and apoptosis was determined by f low cytometry of cells stained with f luorescein isothiocyanate-conjugated Annexin V protein. RESULTS: IS treatment increased ROS production, decreased cell viability and induced apoptosis in HK-2 cells. IS treatment increased the expression of apoptosis-related protein Bax, decreased Bcl-2 expression, and activated phosphorylation of MAPK, NF-κB p65, and Akt. In contrast, paricalcitol treatment decreased Bax expression, increased Bcl-2 expression, and inhibited phosphorylation of MAPK, NF-κB p65, and Akt in HK-2 cells. NF-κB promoter activity was increased following IS, administration and was counteracted by pretreatment with paricalcitol. Additionally, flow cytometry analysis revealed that IS-induced apoptosis was attenuated by paricalcitol treatment, which resulted in decreased numbers of fluorescein isothiocyanate-conjugated Annexin V positive cells. CONCLUSION: Treatment with paricalcitol inhibited IS-induced apoptosis by regulating MAPK, NF-κB, and Akt signaling pathway in HK-2 cells.

Apigenin inhibits indoxyl sulfate-induced endoplasmic reticulum stress and anti-proliferative pathways, CHOP and IL-6/p21, in human renal proximal tubular cells.

OBJECTIVE: Indoxyl sulfate (IS) has been reported to induce endoplasmic reticulum (ER) stress in tubular cells and to inhibit the cell proliferation via ER stress and ERK/IL-6/p21 pathways. This study has investigated the effect of apigenin on IS-induced ER stress in immortalized human renal proximal tubular HK-2 cells. MATERIALS AND METHODS: Human Kidney 2 (HK-2) cells were treated with IS (5 mM) in the absence or presence of apigenin (10 µM) or salubrinal (20 µM) for indicated times under the serum-free condition. Cell viability was evaluated by MTT assay. The levels of protein expression and phosphorylation were evaluated by Western blot analysis. RESULTS: In HK-2 cells, apigenin completely inhibited IS-induced ER stress, as indicated by decreased expression of CHOP, ATF4 and GRP78, although the phosphorylated level of eIF2α did not decrease. IS-induced expression levels of IL-6 and p21 proteins were also inhibited by apigenin, with no significant changes in ERK activation. The suppression of cell proliferation by IS was abolished by salubrinal, an ER stress inhibitor, but not by apigenin. Apigenin inhibited the phosphorylation of Akt and GSK-3ß in IS-treated HK-2 cells. The phosphorylation of GSK-3ß, which was inhibited by apigenin, resulted in hypo-phosphorylation of retinoblastoma (Rb) protein, which was associated with the decrease in cyclin D1 expression. CONCLUSIONS: These results suggest that apigenin may inhibit IS-induced ER stress and expression of IL-6 and p21 proteins in HK-2 cells. It is most likely that apigenin, together with its inhibitory effect on ER stress, may also suppress the cell growth by inducing the loss of Rb phosphorylation, which was associated with the decrease in cyclin D1 expression by GSK-3ß activation through the inhibition of PI3K/Akt pathway.

Indoxyl sulfate: a candidate target for the prevention and treatment of cardiovascular disease in chronic kidney disease.

The awareness that chronic kidney disease (CKD) is a condition of dramatically increased cardiovascular risk has prompted an intense research activity, aimed at identifying factors that are specifically involved in the development of cardiovascular complications of CKD and that can be delayed or reduced by novel pharmacological approaches. This may be the case with indoxyl sulfate (IS). IS is an endogenous molecule derived from indole, a product of protein metabolism by intestinal bacteria, which acts via the aryl hydrocarbon receptor (AhR). IS accumulates early in CKD and exerts proinflammatory and other detrimental effects on the cardiovascular system, in particular promoting atherosclerosis and atherosclerosis-related arterial remodeling. Furthermore, IS also contributes to renal damage, thereby fueling a vicious circle. Dialysis is poorly effective in removing IS, but its levels can be lowered by preventing the bacterial generation of indole or by absorbing this latter within the intestine. More intriguing, although still theoretical, is the possibility of inhibiting the action of IS at the cell level, by antagonizing the binding to AhR or IS intracellular signaling. Therefore, IS targeting might become an option for reducing the cardiovascular burden of CKD.

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.

Oral adsorbent AST-120 potentiates the effect of erythropoietin-stimulating agents on Stage 5 chronic kidney disease patients: a randomized crossover study.

BACKGROUND: Indoxyl sulfate (IS) suppresses erythropoietin (EPO) activity and exerts renal damage. The oral adsorbent AST-120 reduces IS load and has antioxidant and renoprotective properties; however, its roles in the treatment of anemia remain unclear in chronic kidney disease (CKD) patients. METHODS: Fifty-one Stage 5 predialysis CKD patients with hemoglobin <10 g/dL were randomly assigned to receive two period treatments with AST-120 plus once-monthly administration of continuous EPO receptor activator (CERA, A) and CERA alone (B), with a 4-week washout period in between. Mean changes of serum creatinine, estimated glomerular filtration rate (eGFR) and hemoglobin levels from the baseline were compared between two treatments. RESULTS: The baseline and postintervention mean creatinine levels were 5.48 and 5.36 mg/dL in the Treatment A, and 5.14 mg/dL and 5.61 g/dL in the Treatment B group, respectively (treatment effect P = 0.025, period effect P = 0.467, carryover effect P = 0.384). The baseline and postintervention mean hemoglobin levels were 9.27 and 10.47 g/dL in the Treatment A, and 9.63 g/dL and 9.54 g/dL in the Treatment B group, respectively (treatment effect P = 0.039, period effect P = 0.001, carryover effect P = 0.060). Use of AST-120 significantly reduced IS and p-cresyl sulfate (PCS) levels. Hierarchical regression showed that eGFR was an independent predictor for hemoglobin after adjustment of serum free IS and PCS levels (B = 0.049, P = 0.005). CONCLUSIONS: Use of adjuvant AST-120 may improve renal function and hemoglobin levels than use of CERA alone in late-stage CKD patients. The change of eGFR might play an intermediate role between serum IS/PCS and improve hemoglobin levels. The finding offered insight into novel therapeutic strategies of anemia for late-stage CKD patients.

Indoxyl sulfate-induced activation of (pro)renin receptor is involved in expression of TGF-ß1 and α-smooth muscle actin in proximal tubular cells.

Activation of (pro)renin receptor (PRR) is involved in the progression of chronic kidney disease. However, the role of indoxyl sulfate, a uremic toxin, in the activation of PRR is not clear. The present study aimed to clarify the role of indoxyl sulfate in activation of PRR, in relation to renal expression of fibrotic genes. Renal expression of PRR and renin/prorenin was up-regulated in chronic kidney disease rats compared with normal rats, whereas AST-120 suppressed these expression by reducing serum levels of indoxyl sulfate. Furthermore, administration of indoxyl sulfate to normotensive and hypertensive rats increased renal expression of PRR and renin/prorenin. Indoxyl sulfate induced expression of PRR and prorenin in cultured human proximal tubular cells (HK-2 cells). Indoxyl sulfate-induced PRR expression was inhibited by small interfering RNAs of signal transducer and activator of transcription 3 (Stat3) and nuclear factor-κB p65 in proximal tubular cells. N-acetylcysteine, an antioxidant, and diphenyleneiodonium, an inhibitor of nicotinamide adenine dinucleotide phosphate oxidase, suppressed indoxyl sulfate-induced PRR expression in proximal tubular cells. N-acetylcysteine prevented indoxyl sulfate-induced phosphorylation of Stat3 in proximal tubular cells. PRR small interfering RNA inhibited indoxyl sulfate-induced expression of TGF-ß1 and α-smooth muscle actin in proximal tubular cells. Taken together, indoxyl sulfate-induced up-regulation of prorenin expression and activation of PRR through production of reactive oxygen species and activation of Stat3 and nuclear factor-κB play an important role in the expression of TGF-ß1 and α-smooth muscle actin in proximal tubular cells. Thus, indoxyl sulfate-induced activation of prorenin/PRR might be involved in renal fibrosis.

Amelioration of uremic toxin indoxyl sulfate-induced endothelial cell dysfunction by Klotho protein.

Indoxyl sulfate (IS), a common kind of uremic toxin, is considered as a risk factor for aggravating endothelial function in CKD patients due to its oxidative activity. The anti-aging protein Klotho, which is produced by the kidneys and down-regulated in uremic conditions, has the ability to resist oxidative stress. Here, we carried out an in vitro study to investigate the deleterious effects of IS on endothelial cells and the protective role of Klotho protein. The cultured human umbilical vein endothelial cells (HUVECs) were incubated with IS in the presence or absence of Klotho protein. The release of reactive oxygen species (ROS) and the expression of monocyte chemoattractant protein-1 (MCP-1) were enhanced while the cell viability and production of nitric oxide (NO) were inhibited by IS in a concentration-dependent manner. Meanwhile, the phosphorylation of p38MAPK and the nuclear translocation of NF-κB were increased in HUVECs treated with IS. Pretreatment with Klotho protein resulted in remarkable increase of cell viability and decrease of ROS production in IS-treated HUVECs. Like ROS scavenger, N-acetyl-l-cysteine (NAC), Klotho protein could inhibit the IS-induced activations of p38MAPK and NF-κB. Moreover, Klotho protein could also attenuate IS-induced reduction of NO production and up-regulation of MCP-1 expression. These results suggest that IS can damage the functions of endothelial cells. Klotho protein has the ability to ameliorate the IS-induced endothelial dysfunction, which may be partly through inhibiting the ROS/p38MAPK and downstream NF-κB signaling pathways.

A nexus of progression of chronic kidney disease: tryptophan, profibrotic cytokines, and charcoal.

Fibrosis plays a major role in the pathogenesis of progressive chronic kidney disease (CKD). The inhibition of the renin-angiotensin system, which promotes fibrosis, has become the standard of care in the treatment of patients with CKD. The use of alternative agents capable of blocking the actions of profibrotic cytokines such as transforming growth factor-beta (TGF-ß) is also an important strategy that is in its early stages of development. An example of such a drug is AST-120, a charcoal compound that ultimately inhibits the synthesis of TGF-ß in the kidney. The inhibition is mediated by blocking the intestinal absorption of tryptophan-derived indole by AST-120. This reduces the hepatic conversion of indole to indoxyl sulfate (IS). IS stimulates the production of TGF-ß in the renal parenchyma, and lowering the level of IS with AST-120 appears to slow progression of CKD. The status of recent trials examining the safety and efficacy of AST-120 has been described, including a multicenter, randomized, placebo-controlled, phase III trial of approximately 2,000 subjects being conducted to gain approval of this drug by the U.S. Food and Drug Administration.

AST-120 ameliorates epithelial-to-mesenchymal transition and interstitial fibrosis in the kidneys of chronic kidney disease rats.

OBJECTIVE: Indoxyl sulfate (IS), a uremic toxin, is a risk factor for progression of chronic kidney disease (CKD). AST-120 reduces serum IS and delays the progression of CKD. This study aimed to examine whether AST-120 inhibits epithelial-to-mesenchymal transition (EMT) in the kidneys of CKD rats. METHODS: CKD rats were produced by 5/6 nephrectomy and were divided into 2 groups: (1) CKD rats and (2) AST-120-treated CKD rats at a dosage of 4 g/kg body weight/day. After 10 weeks, their kidneys were excised for histological and immunohistochemical analysis. EMT was evaluated by immunohistochemistry of zonula occludens (ZO-1), an epithelial marker, and alpha-smooth muscle actin (α-SMA), a mesenchymal marker. Interstitial fibrosis was evaluated by Masson's trichrome staining. RESULTS: CKD rats showed reduced expression of ZO-1 and enhanced expression of α-SMA as compared with normal rats. Administration of AST-120 to CKD rats increased expression of ZO-1 and decreased expression of α-SMA as compared with CKD rats. Further, CKD rats showed enhanced extent of interstitial fibrosis as compared with normal rats, and administration of AST-120 to CKD rats ameliorated interstitial fibrosis. CKD rats showed increased serum level of IS as compared with normal rats, whereas administration of AST-120 to CKD rats decreased both serum and urine levels of IS. CONCLUSION: We conclude that AST-120 ameliorated EMT and interstitial fibrosis in the kidneys of CKD rats, probably by alleviating IS overload on the kidneys.

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.