Indoxyl sulfate downregulates expression of Mas receptor via OAT3/AhR/Stat3 pathway in proximal tubular cells

Natural antagonistic flavones for AhR inhibit indoxyl sulfate-induced inflammatory gene expression in vitro and renal pathological damages in vivo

Background

Uremic toxin indoxyl sulfate (IS) induces vascular inflammation, a crucial event in renal failure, and vascular complications in patients with chronic kidney disease (CKD). In endothelial cells, IS increases the production of inflammatory cytokines partially via the activation of the aryl hydrocarbon receptor (AhR), and several food flavonoids have been reported to act as antagonists of AhR.

Objective

This study aimed to investigate whether antagonistic flavonoids can attenuate IS-induced inflammatory responses in vascular endothelial cells in vitro and renal failure in vivo.

Design

Human umbilical vein endothelial cells (HUVECs) pretreated with the flavones apigenin, chrysin, or luteolin were stimulated with IS. Expression levels of genes involved in AhR signaling, inflammatory cytokine production, and reactive oxygen species (ROS) production were analyzed. Uninephrectomized mice were orally administered chrysin and received daily intraperitoneal injections of IS for 4 weeks.

Results

In HUVECs, IS upregulated the mRNA expression of AhR-targeted genes (CYP1A1 and AhRR), and genes involved in inflammation (NOX4, MCP-1, IL-6, and COX2) and monocyte invasion/adhesion (ICAM1). All three flavones attenuated the IS-induced increase in the expression of these mRNAs. They also suppressed the IS-induced nuclear translocation of AhR and intracellular ROS production. Furthermore, IS-induced phosphorylation of the signal transducer and activator of transcription 3 (STAT3) was inhibited by treatment with these flavones. The results of in-vivo experiments showed that administration with chrysin attenuated the elevation of blood urea nitrogen levels and AhR-target gene expression and the pathological impairment of renal tissues in mice, regardless of higher serum levels of IS.

Conclusions

Natural food flavones antagonizing AhR exerted protective effects against IS-induced inflammation through the inhibition of the AhR-STAT3 pathway in HUVECs. Moreover, chrysin ameliorated IS-induced renal dysfunction in a mouse model of CKD. These flavonoids could be a therapeutic strategy for vascular inflammation in CKD.

Role of the Microbiome in Gut-Heart-Kidney Cross Talk

Homeostasis is a prerequisite for health. When homeostasis becomes disrupted, dysfunction occurs. This is especially the case for the gut microbiota, which under normal conditions lives in symbiosis with the host. As there are as many microbial cells in and on our body as human cells, it is unlikely they would not contribute to health or disease. The gut bacterial metabolism generates numerous beneficial metabolites but also uremic toxins and their precursors, which are transported into the circulation. Barrier function in the intestine, the heart, and the kidneys regulates metabolite transport and concentration and plays a role in inter-organ and inter-organism communication via small molecules. This communication is analyzed from the perspective of the remote sensing and signaling theory, which emphasizes the role of a large network of multispecific, oligospecific, and monospecific transporters and enzymes in regulating small-molecule homeostasis. The theory provides a systems biology framework for understanding organ cross talk and microbe-host communication involving metabolites, signaling molecules, nutrients, antioxidants, and uremic toxins. This remote small-molecule communication is critical for maintenance of homeostasis along the gut-heart-kidney axis and for responding to homeostatic perturbations. Chronic kidney disease is characterized by gut dysbiosis and accumulation of toxic metabolites. This slowly impacts the body, affecting the cardiovascular system and contributing to the progression of kidney dysfunction, which in its turn influences the gut microbiota. Preserving gut homeostasis and barrier functions or restoring gut dysbiosis and dysfunction could be a minimally invasive way to improve patient outcomes and quality of life in many diseases, including cardiovascular and kidney disease.

Indoxyl Sulfate Promotes Metastatic Characteristics of Ovarian Cancer Cells via Aryl Hydrocarbon Receptor-Mediated Downregulation of the Mas Receptor

Although platinum-combination chemotherapy shows a high response rate at the primary site, epithelial ovarian cancer (EOC) treatment remains challenging because of tumor recurrence and metastasis. Recent studies have revealed that chemotherapy paradoxically promotes cancer cell survival, proliferation, and metastasis, although the reason for this remains unclear. The underlying molecular mechanisms that contribute to chemotherapy-induced metastasis need to be elucidated to establish effective therapeutic strategies. Acute kidney injury is a known side effect of cisplatin treatment, and kidney dysfunction results in the accumulation of uremic toxins in the serum. The present study aimed to investigate whether indoxyl sulfate (IS), a representative uremic toxin, affects the pathophysiology of EOC. In this study, IS reduced the expression of Mas receptor (MasR) in cultured human EOC cells. Both knockdown of the aryl hydrocarbon receptor (AhR), which is an intracellular IS receptor, and inhibition of AhR function suppressed IS-mediated downregulation of MasR in SK-OV-3 cells. IS induced the phosphorylation of signal transducer and activator of transcription 3 (STAT3) in an AhR-dependent manner. Inhibition of the STAT3 pathway or reactive oxygen species production suppressed the IS-mediated reduction of MasR. IS stimulated cell migration and invasion of SK-OV-3 cells in an AhR-dependent manner. Cisplatin-nephropathy model mice exhibited elevated levels of serum IS accompanied by elevated levels of blood urea nitrogen and serum creatinine. Furthermore, intraperitoneal administration of IS in mice promoted tumor growth and metastasis. Finally, we found that the MasR agonist Ang-(1-7) suppressed the IS-mediated effects on cell proliferation, migration, and invasion of SK-OV-3 cells. However, the knockdown of MasR expression by specific small interfering RNA in the absence of IS resulted in only minimal promotion of cell migration and invasion. These findings demonstrate that IS promotes malignancy in ovarian cancer via AhR-mediated downregulation of MasR function, whereas Ang-(1-7) attenuates this effect, thereby suggesting that Ang-(1-7) could provide a future treatment strategy for this cancer type.

AST-120 to Target Protein-Bound Uremic Toxins Improves Cardiac Output and Kidney Oxygenation in Experimental Chronic Kidney Disease

INTRODUCTION

Chronic kidney disease (CKD) is a global health problem with increasing incidence which is closely associated with cardiac dysfunction. In CKD, uremic toxins accumulate as kidney function declines. Additionally, high salt intake is a growing health issue worldwide which can exacerbate kidney disease. In this study, we investigated the effect of reducing plasma levels of protein-bound uremic toxins in a rat model of CKD, challenged with high salt intake and compared the effects to those of conventional treatment using an angiotensin-converting enzyme inhibitor (ACEI).

METHODS

In rats, the right kidney and 2/3 of the left kidney were surgically removed (5/6 nephrectomy). Animals were fed a normal-salt diet and randomized to either no treatment (control) or chronic treatment with either the oral absorbent AST-120 to reduce plasma levels of protein-bound uremic toxins or the ACEI enalapril to inhibit angiotensin II signaling for 5 weeks. Following treatment, kidney function was measured before and after a week of high salt intake. Cardiac output and markers of oxidative stress were measured at the end of the study period.

RESULTS

Treatment with AST-120 resulted in decreased levels of the uremic toxin indoxyl sulfate, improved cardiac output (mL/min: AST-120 44.9 ± 5.4 compared to control 26.6 ± 2.0; p < 0.05), and decreased urinary oxidative stress. ACEI reduced oxidative stress in kidney tissue and improved the glomerular filtration rate in response to high salt intake (mL/min: ACEI 1.5 ± 0.1; compared to control 1.1 ± 0.1; p < 0.05). Both interventions improved intrarenal oxygen availability (mm Hg: AST-120 42.8 ± 0.8; ACEI 43.2 ± 1.9; compared to control 33.4 ± 1.3; p < 0.05).

CONCLUSION

AST-120 administered to reduce plasma levels of uremic toxins, such as indoxyl sulfate, has potential beneficial effects on both cardiac and kidney function. Targeting uremic toxins and angiotensin II signaling simultaneously could be an efficient strategy to target both cardiac and kidney dysfunction in CKD, to further slow progression of disease in patients with CKD.

Extensive Summary of the Important Roles of Indole Propionic Acid, a Gut Microbial Metabolite in Host Health and Disease

Increasing evidence suggests that metabolites produced by the gut microbiota play a crucial role in host-microbe interactions. Dietary tryptophan ingested by the host enters the gut, where indole-like metabolites such as indole propionic acid (IPA) are produced under deamination by commensal bacteria. Here, we summarize the IPA-producing bacteria, dietary patterns on IPA content, and functional roles of IPA in various diseases. IPA can not only stimulate the expression of tight junction (TJ) proteins to enhance gut barrier function and inhibit the penetration of toxic factors, but also modulate the immune system to exert anti-inflammatory and antioxidant effects to synergistically regulate body physiology. Moreover, IPA can act on target organs through blood circulation to form the gut-organ axis, which helps maintain systemic homeostasis. IPA shows great potential for the diagnosis and treatment of various clinical diseases, such as NAFLD, Alzheimer's disease, and breast cancer. However, the therapeutic effect of IPA depends on dose, target organ, or time. In future studies, further work should be performed to explore the effects and mechanisms of IPA on host health and disease to further improve the existing treatment program.

Aryl Hydrocarbon Receptor in Oxidative Stress as a Double Agent and Its Biological and Therapeutic Significance

The aryl hydrocarbon receptor (AhR) has long been implicated in the induction of a battery of genes involved in the metabolism of xenobiotics and endogenous compounds. AhR is a ligand-activated transcription factor necessary for the launch of transcriptional responses important in health and disease. In past decades, evidence has accumulated that AhR is associated with the cellular response to oxidative stress, and this property of AhR must be taken into account during investigations into a mechanism of action of xenobiotics that is able to activate AhR or that is susceptible to metabolic activation by enzymes encoded by the genes that are under the control of AhR. In this review, we examine various mechanisms by which AhR takes part in the oxidative-stress response, including antioxidant and prooxidant enzymes and cytochrome P450. We also show that AhR, as a participant in the redox balance and as a modulator of redox signals, is being increasingly studied as a target for a new class of therapeutic compounds and as an explanation for the pathogenesis of some disorders.

Aryl Hydrocarbon Receptor Mechanisms Affecting Chronic Kidney Disease

The aryl hydrocarbon receptor (AHR) is a basic helix-loop-helix transcription factor that binds diverse endogenous and xenobiotic ligands, which regulate AHR stability, transcriptional activity, and cell signaling. AHR activity is strongly implicated throughout the course of chronic kidney disease (CKD). Many diverse organic molecules bind and activate AHR and these ligands are reported to either promote glomerular and tubular damage or protect against kidney injury. AHR crosstalk with estrogen, peroxisome proliferator-activated receptor-γ, and NF-κB pathways may contribute to the diversity of AHR responses during the various forms and stages of CKD. The roles of AHR in kidney fibrosis, metabolism and the renin angiotensin system are described to offer insight into CKD pathogenesis and therapies.

Toxic Effects of Indoxyl Sulfate on Osteoclastogenesis and Osteoblastogenesis

Uremic toxins, such as indoxyl sulfate (IS) and kynurenine, accumulate in the blood in the event of kidney failure and contribute to further bone damage. To maintain the homeostasis of the skeletal system, bone remodeling is a persistent process of bone formation and bone resorption that depends on a dynamic balance of osteoblasts and osteoclasts. The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that regulates the toxic effects of uremic toxins. IS is an endogenous AhR ligand and is metabolized from tryptophan. In osteoclastogenesis, IS affects the expression of the osteoclast precursor nuclear factor of activated T cells, cytoplasmic 1 (NFATc1) through AhR signaling. It is possible to increase osteoclast differentiation with short-term and low-dose IS exposure and to decrease differentiation with long-term and/or high-dose IS exposure. Coincidentally, during osteoblastogenesis, through the AhR signaling pathway, IS inhibits the phosphorylation of ERK, and p38 reduces the expression of the transcription factor 2 (Runx2), disturbing osteoblastogenesis. The AhR antagonist resveratrol has a protective effect on the IS/AhR pathway. Therefore, it is necessary to understand the multifaceted role of AhR in CKD, as knowledge of these transcription signals could provide a safe and effective method to prevent and treat CKD mineral bone disease.

Aryl Hydrocarbon Receptor and Cysteine Redox Dynamics Underlie (Mal)adaptive Mechanisms to Chronic Intermittent Hypoxia in Kidney Cortex

We hypothesized that an interplay between aryl hydrocarbon receptor (AhR) and cysteine-related thiolome at the kidney cortex underlies the mechanisms of (mal)adaptation to chronic intermittent hypoxia (CIH), promoting arterial hypertension (HTN). Using a rat model of CIH-HTN, we investigated the impact of short-term (1 and 7 days), mid-term (14 and 21 days, pre-HTN), and long-term intermittent hypoxia (IH) (up to 60 days, established HTN) on CYP1A1 protein level (a sensitive hallmark of AhR activation) and cysteine-related thiol pools. We found that acute and chronic IH had opposite effects on CYP1A1 and the thiolome. While short-term IH decreased CYP1A1 and increased protein-S-thiolation, long-term IH increased CYP1A1 and free oxidized cysteine. In addition, an in vitro administration of cystine, but not cysteine, to human endothelial cells increased Cyp1a1 expression, supporting cystine as a putative AhR activator. This study supports CYP1A1 as a biomarker of obstructive sleep apnea (OSA) severity and oxidized pools of cysteine as risk indicator of OSA-HTN. This work contributes to a better understanding of the mechanisms underlying the phenotype of OSA-HTN, mimicked by this model, which is in line with precision medicine challenges in OSA.

Aryl Hydrocarbon Receptor and Its Diverse Ligands and Functions: An Exposome Receptor

The aryl hydrocarbon receptor (AhR) is a transcriptional factor that regulates multiple functions following its activation by a variety of ligands, including xenobiotics, natural products, microbiome metabolites, and endogenous molecules. Because of this diversity, the AhR constitutes an exposome receptor. One of its main functions is to regulate several lines of defense against chemical insults and bacterial infections. Indeed, in addition to its well-established detoxication function, it has several functions at physiological barriers, and it plays a critical role in immunomodulation. The AhR is also involved in the development of several organs and their homeostatic maintenance. Its activity depends on the type of ligand and on the time frame of the receptor activation, which can be either sustained or transient, leading in some cases to opposite modes of regulations as illustrated in the regulation of different cancer pathways. The development of selective modulators and their pharmacological characterization are important areas of research. Expected final online publication date for the Annual Review of Pharmacology and Toxicology, Volume 62 is January 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Renal and non-renal response of ABC and SLC transporters in chronic kidney disease

INTRODUCTION

The solute carrier (SLC) and the ATP-binding cassette (ABC) transporter superfamilies play essential roles in the disposition of small molecules (endogenous metabolites, uremic toxins, drugs) in the blood, kidney, liver, intestine, and other organs. In chronic kidney disease (CKD), the loss of renal function is associated with altered function of remote organs. As renal function declines, many molecules accumulate in the plasma. Many studies now support the view that ABC and SLC transporters as well as drug metabolizing enzymes (DMEs) in renal and non-renal tissues are directly or indirectly affected by the presence of various types of uremic toxins, including those derived from the gut microbiome; this can lead to aberrant inter-organ communication.

AREAS COVERED

Here, the expression, localization and/or function of various SLC and ABC transporters as well as DMEs in the kidney and other organs are discussed in the context of CKD and systemic pathophysiology.

EXPERT OPINION

According to the Remote Sensing and Signaling Theory (RSST), a transporter and DME-centric network that optimizes local and systemic metabolism maintains homeostasis in the steady state and resets homeostasis following perturbations due to renal dysfunction. The implications of this view for pharmacotherapy of CKD are also discussed.

The role of AMP-activated protein kinase α1-mediated endoplasmic reticulum stress in alleviating the toxic effect of uremic toxin indoxyl sulfate on vascular endothelial cells by Klotho

Recently, the Klotho protein (Klotho) has received substantial attention as protective factor against cardiovascular complications of chronic kidney disease (CKD). However, the direct effect and mechanism of Klotho on endothelial cells injury are not well-known. In this study, we incubated human vein umbilical endothelial cells (HUVECs) with uremic toxin indoxyl sulfate (IS) to mimic CKD internal environment and investigated the direct effect of Klotho on the HUVECs injury induced by IS and to explore the mechanism in this process. We found IS inhibited cell viability, increased endoplasmic reticulum stress, and mediated apoptosis of HUVECs. Treatment with Klotho significantly attenuated IS-induced above effects. Furthermore, Klotho alleviated the IS toxic effect on HUVECs via promoting AMP-activated protein kinase (AMPK) α1 phosphorylation instead of directly upregulating AMPKα1, which could be partly blocked by AMPK pathway inhibitor-Compound C. In addition, Klotho also inhibited intercellular adhesion molecule-1 (ICAM-1) and vascular cell adhesion molecule-1 (VCAM-1) expression induced by IS. Altogether, these results indicated that Klotho can protect HUVECs from IS-induced injury by alleviating AMPKα1-mediated endoplasmic reticulum stress.

2020 update on the renin-angiotensin-aldosterone system in pediatric kidney disease and its interactions with coronavirus

The last decade was crucial for our understanding of the renin-angiotensin-aldosterone system (RAAS) as a two-axis, counter-regulatory system, divided into the classical axis, formed by angiotensin-converting enzyme (ACE), angiotensin II (Ang II), and the angiotensin type 1 receptor (AT1R), and the alternative axis comprising angiotensin-converting enzyme 2 (ACE2), angiotensin-(1-7) (Ang-(1-7)), and the Mas receptor. Breakthrough discoveries also took place, with other RAAS endopeptides being described, including alamandine and angiotensin A. In this review, we characterize the two RAAS axes and the role of their components in pediatric kidney diseases, including childhood hypertension (HTN), pediatric glomerular diseases, congenital abnormalities of the kidney and urinary tract (CAKUT), and chronic kidney disease (CKD). We also present recent findings on potential interactions between the novel coronavirus, SARS-CoV-2, and components of the RAAS, as well as potential implications of coronavirus disease 2019 (COVID-19) for pediatric kidney diseases.

The Aryl Hydrocarbon Receptor in Chronic Kidney Disease: Friend or Foe?

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that promotes cell responses to small molecules derived from the diet, microorganisms, metabolism and pollutants. The AhR signal regulates many basic cellular processes, including cell cycle progression, adhesion, migration, apoptosis and cell proliferation. Many studies have shown that AhR is associated with chronic kidney disease (CKD) and its complications. This article reviews the current knowledge about the role of AhR in CKD, showing that AhR mediates CKD complications, including cardiovascular disease, anemia, bone disorders, cognitive dysfunction and malnutrition, and that it influences drug metabolism in individuals with CKD. AhR enhances the intestinal barrier function to reduce the harmful effects of uremic toxins. Therefore, understanding the complex roles of AhR during CKD is important to be able to target this transcription factor safely and effectively for CKD prevention and treatment.

Developmental Programming and Reprogramming of Hypertension and Kidney Disease: Impact of Tryptophan Metabolism

The concept that hypertension and chronic kidney disease (CKD) originate in early life has emerged recently. During pregnancy, tryptophan is crucial for maternal protein synthesis and fetal development. On one hand, impaired tryptophan metabolic pathway in pregnancy impacts fetal programming, resulting in the developmental programming of hypertension and kidney disease in adult offspring. On the other hand, tryptophan-related interventions might serve as reprogramming strategies to prevent a disease from occurring. In the present review, we aim to summarize (1) the three major tryptophan metabolic pathways, (2) the impact of tryptophan metabolism in pregnancy, (3) the interplay occurring between tryptophan metabolites and gut microbiota on the production of uremic toxins, (4) the role of tryptophan-derived metabolites-induced hypertension and CKD of developmental origin, (5) the therapeutic options in pregnancy that could aid in reprogramming adverse effects to protect offspring against hypertension and CKD, and (6) possible mechanisms linking tryptophan metabolism to developmental programming of hypertension and kidney disease.

Both aldosterone and spironolactone can modulate the intracellular ACE/ANG II/AT1 and ACE2/ANG (1-7)/MAS receptor axes in human mesangial cells

The kidney is an important target of the renin‐ANG‐aldosterone system (RAAS). To date, several studies have demonstrated the existence of a local RAAS in various tissues, including the renal tissue. The mineralocorticoid aldosterone is known to play a critical role in the classical RAAS; however, its effect on mesangial cells (MCs) remains to be elucidated. Based on this, our aim was to investigate whether aldosterone stimulation can modulate the intracellular RAAS of immortalized human MCs by evaluating ANG‐converting enzyme (ACE)/ANG II/ANG II receptor type 1 (AT1) and ANG‐converting enzyme 2 (ACE2)/ANG (1‐7)/MAS receptor axes. To realise this, protein expression, enzyme activity, and immunofluorescence were performed under aldosterone stimulation and in the presence of the mineralocorticoid receptor (MR) antagonist spironolactone (SPI). We observed that high doses of aldosterone increase ACE activity. The effect of aldosterone on the catalytic activity of ACE was completely abolished with the pretreatment of SPI suggesting that the aldosterone‐induced cell injuries through ANG II release were attenuated. Aldosterone treatment also decreased the expression of MAS receptor, but did not alter the expression or the catalytic activity of ACE 2 and ANG (1‐7) levels. Spironolactone modulated the localization of ANG II and AT1 receptor and decreased ANG (1‐7) and MAS receptor levels. Our data suggest that both aldosterone and the MR receptor antagonist can modulate both of these axes and that spironolactone can protect MCs from the damage induced by aldosterone.

Renin-angiotensin system molecules are associated with subclinical atherosclerosis and disease activity in rheumatoid arthritis

OBJECTIVES

To compare serum levels of RAS components in women with RA versus healthy females and to investigate the association between these molecules and subclinical atherosclerosis.

METHODS

A cross-sectional study involving female RA patients without ischemic CVD. Disease activity was assessed using the DAS28 and the CDAI. IMT of the common carotid artery was evaluated by ultrasonography. Serum levels of Ang II, Ang-(1-7), ACE and ACE2 were determined by enzyme immunoassay.

RESULTS

Fifty women with RA, mean 48.2 (7.3) years, were compared to 30 healthy women, paired by age. RA patients had higher plasma levels of Ang II (p < .01), Ang-(1-7) (p < .01), and ACE (p < .01) than controls. The ratios of ACE to ACE2 were higher in RA patients, whereas Ang II/Ang-(1-7) ratios were lower in RA patients. The presence of hypertension and the treatment with ACE inhibitors did not significantly modify serum levels of Ang II, Ang-(1-7), ACE and ACE2 in patients with RA. Seven RA patients had altered IMT, and eight patients exhibited atherosclerotic plaque. There was a negative correlation between ACE2 levels and IMT (p = .041). IMT positively correlated with age (p = .022), disease duration (p = .012) and overall Framingham risk score (p = .008). Ang II concentrations positively correlated with DAS28 (p = .034) and CDAI (p = .040).

CONCLUSION

Patients with RA had an activation of the RAS, suggesting an association with disease activity and cardiovascular risk. Rheumatological key messages Imbalance of both RAS axes may be associated with cardiovascular risk and disease activity in rheumatoid arthritis. Ultrasonography of the carotid arteries can identify early, subclinical atherosclerotic disease in rheumatoid arthritis patients. Angiotensin-converting enzyme inhibition or angiotensin 1 receptor blockade may be beneficial for rheumatoid arthritis patients.

Serum metabolic profile characteristics of offspring rats before and after birth caused by prenatal caffeine exposure

Epidemiological investigations have confirmed that prenatal caffeine intake could increase the incidence rate of intrauterine growth retardation (IUGR) and multiple diseases after birth. Based on liquid chromatography-mass spectrometry, we analyzed serum metabolic profiles of offspring rats before and after birth in IUGR model induced by prenatal caffeine exposure (PCE). We discovered that differential metabolites in PCE fetuses mainly manifested as amino acids and lipid metabolism. In adulthood, PCE offspring showed less and inconsistent types of differential metabolites compared to those in utero, which still exhibited gender differences. The main differential metabolites induced by PCE, including phospholipids, platelet-activating factor, arachidonic acid, bile acid, sphingosine-1-phosphoric acid, indoxyl sulfuric acid, and cortexolone, may participate in the pathological and physiological processes of organ toxicities. This study demonstrated the short- and long-term developmental toxicity and gender differences of caffeine, providing new ideas for exploring the early warning and drug intervention targets of IUGR offspring.

Aryl hydrocarbon receptor activation mediates kidney disease and renal cell carcinoma

The aryl hydrocarbon receptor (AhR) is a well-known ligand-activated cytoplasmic transcription factor that contributes to cellular responses against environmental toxins and carcinogens. AhR is activated by a range of structurally diverse compounds from the environment, microbiome, natural products, and host metabolism, suggesting that AhR possesses a rather promiscuous ligand binding site. Increasing studies have indicated that AhR can be activated by a variety of endogenous ligands and induce the expression of a battery of genes. AhR regulates a variety of physiopathological events, including cell proliferation, differentiation, apoptosis, adhesion and migration. These new roles have expanded our understanding of the AhR signalling pathways and endogenous metabolites interacting with AhR under homeostatic and pathological conditions. Recent studies have demonstrated that AhR is linked to cardiovascular disease (CVD), chronic kidney disease (CKD) and renal cell carcinoma (RCC). In this review, we summarize gut microbiota-derived ligands inducing AhR activity in patients with CKD, CVD, diabetic nephropathy and RCC that may provide a new diagnostic and prognostic approach for complex renal damage. We further highlight polyphenols from natural products as AhR agonists or antagonists that regulate AhR activity. A better understanding of structurally diverse polyphenols and AhR biological activities would allow us to illuminate their molecular mechanism and discover potential therapeutic strategies targeting AhR activation.

The protective arm of the renin-angiotensin system may counteract the intense inflammatory process in fetuses with posterior urethral valves

OBJECTIVE

Posterior urethral valve is the most common lower urinary tract obstruction in male children. A high percentage of patients with posterior urethral valve evolve to end-stage renal disease. Previous studies showed that cytokines, chemokines, and components of the renin-angiotensin system contribute to the renal damage in obstructive uropathies. The authors recently found that urine samples from fetuses with posterior urethral valve have increased levels of inflammatory molecules. The aim of this study was to measure renin-angiotensin system molecules and to investigate their correlation with previously detected inflammatory markers in the same urine samples of fetuses with posterior urethral valve.

METHODS

Urine samples from 24 fetuses with posterior urethral valve were collected and compared to those from 22 healthy male newborns at the same gestational age (controls). Renin-angiotensin system components levels were measured by enzyme-linked immunosorbent assay.

RESULTS

Fetuses with posterior urethral valve presented increased urinary levels of angiotensin (Ang) I, Ang-(1-7) and angiotensin-converting enzyme 2 in comparison with controls. ACE levels were significantly reduced and Ang II levels were similar in fetuses with posterior urethral valve in comparison with controls.

CONCLUSIONS

Increased urinary levels of angiotensin-converting enzyme 2 and of Ang-(1-7) in fetuses with posterior urethral valve could represent a regulatory response to the intense inflammatory process triggered by posterior urethral valve.

Molecular mechanisms of protein-bound uremic toxin-mediated cardiac, renal and vascular effects: underpinning intracellular targets for cardiorenal syndrome therapy

Cardiorenal syndrome (CRS) remains a global health burden with a lack of definitive and effective treatment. Protein-bound uremic toxin (PBUT) overload has been identified as a non-traditional risk factor for cardiac, renal and vascular dysfunction due to significant albumin-binding properties, rendering these solutes non-dialyzable upon the state of irreversible kidney dysfunction. Although limited, experimental studies have investigated possible mechanisms in PBUT-mediated cardiac, renal and vascular effects. The ultimate aim is to identify relevant and efficacious targets that may translate beneficial outcomes in disease models and eventually in the clinic. This review will expand on detailed knowledge on mechanisms involved in detrimental effects of PBUT, specifically affecting the heart, kidney and vasculature, and explore potential effective intracellular targets to abolish their effects in CRS initiation and/or progression.

Skatole regulates intestinal epithelial cellular functions through activating aryl hydrocarbon receptors and p38

Intestinal bacteria produce skatole (3-methylindole) from tryptophan in dietary proteins and ingesting large quantities of animal protein is associated with increased fecal skatole concentrations. Although possibly associated with disrupted intestinal homeostasis, the influence of skatole on intestinal epithelial cellular function has not been characterized in detail. The present study aimed to determine whether skatole induces intestinal epithelial cell (IEC) dysfunction. We found that skatole dose-dependently caused IEC death and time-dependently induced IEC apoptosis. Since skatole directly interacts with aryl hydrocarbon receptors (AhR), we investigated whether these receptors influence the skatole-induced death of IEC. In addition to increased AhR transcriptional activity induced by skatole, the AhR antagonist CH223191 partially suppressed of skatole-induced IEC death. Extracellular signal-related kinase (ERK), p38 and c-Jun N-terminal kinase (JNK) are mitogen-activated protein kinases (MAPK) induced by skatole. None of them were repressed by CH223191, whereas the p38 inhibitor SB203580 promoted skatole-induced IEC death. These findings together indicated that skatole induces both AhR-dependent activation pathways and the AhR-independent activation of p38, consequently regulating the amount of IEC death. Accumulating evidence indicates that consuming large amounts of animal protein is associated with the pathogenesis and progression of inflammatory bowel diseases (IBD). Thus, intestinal skatole production induced by large amounts of dietary animal protein might be associated via IEC death with intestinal pathologies such as IBD.

Evidence for a role of angiotensin converting enzyme 2 in proteinuria of idiopathic nephrotic syndrome

Introduction: Renin angiotensin system (RAS) plays a role in idiopathic nephrotic syndrome (INS). Most studies investigated only the classical RAS axis. Therefore, the aims of the present study were to evaluate urinary levels of RAS molecules related to classical and to counter-regulatory axes in pediatric patients with INS, to compare the measurements with levels in healthy controls and to search for associations with inflammatory molecules, proteinuria and disease treatment. Subjects and methods: This cross-sectional study included 31 patients with INS and 19 healthy controls, matched for age and sex. Patients and controls were submitted to urine collection for measurement of RAS molecules [Ang II, Ang-(1-7), ACE and ACE2] by enzyme immunoassay and cytokines by Cytometric Bead Array. Findings in INS patients were compared according to proteinuria: absent (<150 mg/dl, n = 15) and present (≥150 mg/dl, n = 16). Results: In comparison to controls, INS patients had increased Ang II, Ang-(1-7) and ACE, levels while ACE2 was reduced. INS patients with proteinuria had lower levels of ACE2 than those without proteinuria. ACE2 levels were negatively correlated with 24-h-proteinuria. Urinary concentrations of MCP-1/CCL2 were significantly higher in INS patients, positively correlated with Ang II and negatively with Ang-(1-7). ACE2 concentrations were negatively correlated with IP-10/CXCL-10 levels, which, in turn, were positively correlated with 24-h-proteinuria. Conclusion: INS patients exhibited changes in RAS molecules and in chemokines. Proteinuria was associated with low levels of ACE2 and high levels of inflammatory molecules.

Aryl Hydrocarbon Receptor Promotes IL-10 Expression in Inflammatory Macrophages Through Src-STAT3 Signaling Pathway

The aryl hydrocarbon receptor (AhR) is an important immune regulator with a role in inflammatory response. However, the role of AhR in IL-10 production by inflammatory macrophages is currently unknown. In this study, we investigated LPS-induced IL-10 expression in macrophages from AhR-KO mice and AhR-overexpressing RAW264.7 cells. AhR was highly expressed after LPS stimulation through NF-κB pathway. Loss of AhR resulted in reduced IL-10 expression in LPS-induced macrophages. Moreover, the IL-10 expression was elevated in LPS-induced AhR-overexpressing RAW264.7 cells. Maximal IL-10 expression was dependent on an AhR non-genomic pathway closely related to Src and STAT3. Furthermore, AhR-associated Src activity was responsible for tyrosine phosphorylation of STAT3 and IL-10 expression by inflammatory macrophages. Adoptive transfer of AhR-expressing macrophages protected mice against LPS-induced peritonitis associated with high IL-10 production. In conclusion, we identified the AhR-Src-STAT3-IL-10 signaling pathway as a critical pathway in the immune regulation of inflammatory macrophages, It suggests that AhR may be a potential therapeutic target in immune response.

Impact of Altered Intestinal Microbiota on Chronic Kidney Disease Progression

In chronic kidney disease (CKD), accumulation of uremic toxins is associated with an increased risk of CKD progression. Some uremic toxins result from nutrient processing by gut microbiota, yielding precursors of uremic toxins or uremic toxins themselves, such as trimethylamine N-Oxide (TMAO), p-cresyl sulphate, indoxyl sulphate and indole-3 acetic acid. Increased intake of some nutrients may modify the gut microbiota, increasing the number of bacteria that process them to yield uremic toxins. Circulating levels of nutrient-derived uremic toxins are associated to increased risk of CKD progression. This offers the opportunity for therapeutic intervention by either modifying the diet, modifying the microbiota, decreasing uremic toxin production by microbiota, increasing toxin excretion or targeting specific uremic toxins. We now review the link between nutrients, microbiota and uremic toxin with CKD progression. Specific focus will be placed on the generation specific uremic toxins with nephrotoxic potential, the decreased availability of bacteria-derived metabolites with nephroprotective potential, such as vitamin K and butyrate and the cellular and molecular mechanisms linking these toxins and protective factors to kidney diseases. This information provides a conceptual framework that allows the development of novel therapeutic approaches.

Monocytic angiotensin-converting enzyme 2 relates to atherosclerosis in patients with chronic kidney disease

Background: Increased levels of monocytic angiotensin-converting enzyme (ACE) found in haemodialysis (HD) patients may directly participate in the pathogenesis of atherosclerosis. We demonstrated recently that uremia triggers the development of highly pro-atherogenic monocytes via an angiotensin II (AngII)–dependent mechanism. Opposing actions of the AngII-degrading ACE2 remain largely unknown. We examined the status of both ACEs and related receptors in circulating leukocytes of HD, not-dialyzed CKD and healthy individuals. Furthermore, we tested the possible impact of monocytic ACEs on atherogenesis and behaviour of the cells under conditions mimicking chronic renal failure.

Methods: Expression of ACE, ACE2, AT1R, AT2R and MASR was investigated on circulating leukocytes from 71 HD (62 ± 14 years), 24 CKD stage 3–5 (74 ± 10 years) patients and 37 healthy control subjects (53 ± 6 years) and isolated healthy monocytes treated with normal and uremic serum. Analyses of ACE, ACE2, ICAM-1, VCAM-1, MCSF and endothelial adhesion were tested on ACE-overexpressing THP-1 monocytes treated with captopril or losartan. ACE2-overexpressing monocytes were subjected to transmigration and adhesion assays and investigated for MCP-1, ICAM-1, VCAM-1, MCSF, AT1R and AT2R expression.

Results: The ACE mRNA level was significantly increased in HD and CKD stage 3–5 leukocytes. Correspondingly, ACE2 was downregulated and AngII as well as MAS receptor expression was upregulated in these cells. Healthy monocytes preconditioned with uremic serum reflected the same expressional regulation of ACE/ACE2, MAS and AngII receptors as those observed in HD and CKD stage 3–5 leukocytes. Overexpression of monocytic ACE dramatically decreased levels of ACE2 and induced a pro-atherogenic phenotype, partly reversed by AngII-modifying treatments, leading to an increase in ACE2. Overexpression of ACE2 in monocytes led to reduced endothelial adhesion, transmigration and downregulation of adhesion-related molecules.

Conclusions: HD and not-dialyzed CKD stage 3–5 patients show enhanced ACE and decreased ACE2 expression on monocytes. This constellation renders the cells endothelial adhesive and likely supports the development of atherosclerosis.

Prebiotic prevents impaired kidney and renal Oat3 functions in obese rats

Obesity is health issue worldwide, which can lead to kidney dysfunction. Prebiotics are non-digestible foods that have beneficial effects on health. This study aimed to investigate the effects of xylooligosaccharide (XOS) on renal function, renal organic anion transporter 3 (Oat3) and the mechanisms involved. High-fat diet was provided for 12 weeks in male Wistar rats. After that, the rats were divided into normal diet (ND); normal diet treated with XOS (NDX); high-fat diet (HF) and high-fat diet treated with XOS (HFX). XOS was given daily at a dose of 1000 mg for 12 weeks. At week 24, HF rats showed a significant increase in obesity and insulin resistance associated with podocyte injury, increased microalbuminuria, decreased creatinine clearance and impaired Oat3 function. These alterations were improved by XOS supplementation. Renal MDA level and the expression of AT1R, NOX4, p67^phox, 4-HNE, phosphorylated PKCα and ERK1/2 were significantly decreased after XOS treatment. In addition, Nrf2-Keap1 pathway, SOD2 and GCLC expression as well as renal apoptosis were also significantly reduced by XOS. These data suggest that XOS could indirectly restore renal function and Oat3 function via the reduction of oxidative stress and apoptosis through the modulating of AT1R-PKCα-NOXs activation in obese insulin-resistant rats. These attenuations were instigated by the improvement of obesity, hyperlipidemia and insulin resistance.

Indoxyl Sulfate Promotes Macrophage IL-1β Production by Activating Aryl Hydrocarbon Receptor/NF-κ/MAPK Cascades, but the NLRP3 inflammasome Was Not Activated

In chronic kidney disease (CKD) patients, accumulation of uremic toxins is associated with cardiovascular risk and mortality. One of the hallmarks of kidney disease-related cardiovascular disease is intravascular macrophage inflammation, but the mechanism of the reaction with these toxins is not completely understood. Macrophages differentiated from THP-1 cells were exposed to indoxyl sulfate (IS), a representative uremic toxin, and changes in inflammatory cytokine production and intracellular signaling molecules including interleukin (IL)-1, aryl hydrocarbon receptor (AhR), nuclear factor (NF)-κ, and mitogen-activated protein kinase (MAPK) cascades as well as the NLRP3 inflammasome were quantified by real-time PCR, Western blot analysis, and enzyme-linked immunosorbent assay. IS induced macrophage pro-IL-1β mRNA expression, although mature IL-1 was only slightly increased. IS increased AhR and the AhR-related mRNA expression; this change was suppressed by administration of proteasome inhibitor. IS promoted phosphorylation of NF-κB p65 and MAPK enzymes; the reaction and IL-1 expression were inhibited by BAY11-7082, an inhibitor of NF-κB. In contrast, IS decreased NLRP3 and did not change ASC, pro-caspase 1, or caspase-1 activation. IS-inducing inflammation in macrophages results from accelerating AhR-NF-κB/MAPK cascades, but the NLRP3 inflammasome was not activated. These reactions may restrict mature IL-1β production, which may explain sustained chronic inflammation in CKD patients.

Indole-3-propionic acid suppresses indoxyl sulfate-induced expression of fibrotic and inflammatory genes in proximal tubular cells

Indoxyl sulfate (IS) induces fibrosis and inflammation in kidneys via oxidative stress through the induction of transforming growth factor-β1 (TGF-β1) and monocyte chemotactic protein-1 (MCP-1). Furthermore, IS is a potent endogenous agonist for aryl hydrocarbon receptor (AHR), which regulates the transcription of genes such as cytochrome P450 (CYP) 1A1. Indole-3-propionic acid (IPA) is an antioxidant and has been reported to be neuroprotective. We determined whether IPA suppresses IS-induced expression of AHR, CYP1A1, TGF-β1, and MCP-1 in proximal tubular cells. The effects of IS on the expression of AHR, CYP1A1, TGF-β1, and MCP-1 were studied using normotensive rats and hypertensive rats. The effects of IPA on IS-induced expression of AHR, CYP1A1, TGF-β1, and MCP-1 were studied using proximal tubular cells (HK-2). Furthermore, the effects of IPA on IS-induced expression and phosphorylation of signal transducer and activator of transcription 3 (Stat3) were studied in HK-2 cells. Administration of IS induced the expression of AHR, CYP1A1, TGF-β1, and MCP-1 in the tubular cells of rat kidneys. IPA significantly suppressed IS-induced mRNA and protein expression of AHR, CYP1A1, TGF-β1, and MCP-1 in HK-2 cells. IPA suppressed the IS-induced expression and phosphorylation of Stat3 in HK-2 cells. Furthermore, knockdown of Stat3 inhibited the IS-induced mRNA and protein expression of AHR, CYP1A1, TGF-β1, and MCP-1 in HK-2 cells. In conclusion, IPA suppressed the IS-induced expression of AHR, CYP1A1, TGF-β1, and MCP-1 through suppression of Stat3 in proximal tubular cells. Thus, IPA suppresses IS-induced expression of fibrotic and inflammatory genes in proximal tubular cells.

Inhibition of indoxyl sulfate-induced intrarenal renin-angiotensin system activation: targeting the aryl hydrocarbon receptor

Indoxyl sulfate, a protein-bound uremic toxin, leads to CKD (chronic kidney disease) progression and its complications through the activation of AhR (aryl hydrocarbon receptor) and RAS (renin-angiotensin system). Inhibition of these pathways may slow the development of CKD and CKD-associated complications.

Indoxyl sulfate - the uremic toxin linking hemostatic system disturbances with the prevalence of cardiovascular disease in patients with chronic kidney disease

Background

During chronic kidney disease progression, kidney-specific risk factors for cardiovascular disease come into play. The present study investigated the impact of indoxyl sulfate, dietary tryptophan-derived uremic toxin, accumulated in the blood of patients with chronic kidney disease on hemostatic parameters, markers of inflammation, oxidative stress and monocyte to macrophage transition.

Methods

Fifty-one CKD patients not undergoing hemodialysis were enrolled in the study. Coagulation factors, fibrinolytic parameters, adhesion molecules, endothelial dysfunction markers, oxidative stress as well as inflammation markers were examined using immune-enzymatic method. Indoxyl sulfate levels were assessed using high-performance liquid chromatography. Biochemical parameters were determined by routine laboratory techniques using an automated analyzers. All assessed parameters were compared with controls and subjected to cross-sectional statistical analysis.

Results

Elevated concentrations of indoxyl sulfate, the vast majority of parameters affecting hemostasis, and markers of renal insufficiency conditions were observed. Part of hemostatic factors, namely tissue factor, von Willebrand factor, thrombomodulin, soluble urokinase-type plasminogen activator receptor, soluble intercellular adhesion molecule-1, soluble vascular cell adhesion protein were correlated with the fraction of indoxyl sulfate. A significant quantity of assessed parameters showed strong correlations with superoxide-dismutase, renal insufficiency rate, C-reactive protein, and neopterin. Levels of indoxyl sulfate were independently associated with markers of impaired endothelial function (thrombomodulin, adhesion molecules), oxidative stress (superoxide-dismutase) and monocytes activation determinant (neopterin), which indicate unconventional links between these systems and the role of indoxyl sulfate. Furthermore, parameters that correlated with the levels of indoxyl sulfate (von Willebrand factor, soluble urokinase-type plasminogen activator receptor, soluble intercellular adhesion molecule-1) were positively associated with the prevalence of cardiovascular disease in a CKD patients.

Conclusions

The study demonstrated that in conditions of chronic exposure to uremic toxins, indoxyl sulfate seems to be one of the “missing links” between impaired renal function and prevalence of cardiovascular events, especially hemostatic disorders. The main functions of the action appear to be altered monocytes activation, intensified inflammatory process, and augmented oxidative stress by this uremic toxin.

Indoxyl sulphate and kidney disease: Causes, consequences and interventions

In the last decade, chronic kidney disease (CKD), defined as reduced renal function (glomerular filtration rate (GFR) < 60 mL/min per 1.73 m(2) ) and/or evidence of kidney damage (typically manifested as albuminuria) for at least 3 months, has become one of the fastest-growing public health concerns worldwide. CKD is characterized by reduced clearance and increased serum accumulation of metabolic waste products (uremic retention solutes). At least 152 uremic retention solutes have been reported. This review focuses on indoxyl sulphate (IS), a protein-bound, tryptophan-derived metabolite that is generated by intestinal micro-organisms (microbiota). Animal studies have demonstrated an association between IS accumulation and increased fibrosis, and oxidative stress. This has been mirrored by in vitro studies, many of which report cytotoxic effects in kidney proximal tubular cells following IS exposure. Clinical studies have associated IS accumulation with deleterious effects, such as kidney functional decline and adverse cardiovascular events, although causality has not been conclusively established. The aims of this review are to: (i) establish factors associated with increased serum accumulation of IS; (ii) report effects of IS accumulation in clinical studies; (iii) critique the reported effects of IS in the kidney, when administered both in vivo and in vitro; and (iv) summarize both established and hypothetical therapeutic options for reducing serum IS or antagonizing its reported downstream effects in the kidney.

Indoxyl Sulfate Downregulates Mas Receptor via Aryl Hydrocarbon Receptor/Nuclear Factor-kappa B, and Induces Cell Proliferation and Tissue Factor Expression in Vascular Smooth Muscle Cells

BACKGROUND/AIM

Angiotensin converting enzyme-related carboxypeptidase 2/angiotensin (Ang)-(1-7)/Mas receptor axis is protective in the development of chronic kidney disease and cardiovascular disease. This study is aimed at investigating whether indoxyl sulfate (IS) affects Mas receptor expression, cell proliferation and tissue factor expression in vascular smooth muscle cells, and if Ang-(1-7), an activator of Mas receptor, counteracts the IS-induced effects.

METHODS

IS was administered to normotensive and hypertensive rats. Human aortic smooth muscle cells (HASMCs) were cultured with IS.

RESULTS

IS reduced the expression of Mas receptor in the aorta of normotensive and hypertensive rats. IS downregulated the Mas receptor expression in a time- and dose-dependent manner in HASMCs. Knockdown of aryl hydrocarbon receptor (AhR) and nuclear factor-kappa B (NF-x03BA;B) inhibited IS-induced downregulation of Mas receptor. Further, IS stimulated cell proliferation and tissue factor expression in HASMCs. Ang-(1-7) attenuated IS-induced cell proliferation and tissue factor expression in HASMCs. Ang-(1-7) suppressed phosphorylation of extracellular signal-regulated kinase 1/2 (ERK1/2) and NF-x03BA;B in HASMCs.

CONCLUSION

IS downregulated the expression of Mas receptor via AhR/NF-x03BA;B, and induced cell proliferation and tissue factor expression in HASMCs. Ang-(1-7) inhibited IS-induced cell proliferation and tissue factor expression by suppressing the phosphorylation of ERK1/2 and NF-x03BA;B p65.

Increased Proinflammatory Cytokine Production and Decreased Cholesterol Efflux Due to Downregulation of ABCG1 in Macrophages Exposed to Indoxyl Sulfate

One of the possible causes of enhanced atherosclerosis in patients with chronic kidney disease (CKD) is the accumulation of uremic toxins. Since macrophage foam cell formation is a hallmark of atherosclerosis, we examined the direct effect of indoxyl sulfate (IS), a representative uremic toxin, on macrophage function. Macrophages differentiated from THP-1 cells were exposed to IS in vitro. IS decreased the cell viability of THP-1 derived macrophages but promoted the production of inflammatory cytokines (IL-1β, IS 1.0 mM: 101.8 ± 21.8 pg/mL vs. 0 mM: 7.0 ± 0.3 pg/mL, TNF-α, IS 1.0 mM: 96.6 ± 11.0 pg/mL vs. 0 mM: 15.1 ± 3.1 pg/mL) and reactive oxygen species. IS reduced macrophage cholesterol efflux (IS 0.5 mM: 30.3% ± 7.3% vs. 0 mM: 43.5% ± 1.6%) and decreased ATP-binding cassette transporter G1 expression. However, lipid uptake into cells was not enhanced. A liver X receptor (LXR) agonist, T0901317, improved IS-induced production of inflammatory cytokines as well as reduced cholesterol efflux. In conclusion, IS induced inflammatory reactions and reduced cholesterol efflux in macrophages. Both effects of IS were improved with activation of LXR. Direct interactions of uremic toxins with macrophages may be a major cause of atherosclerosis acceleration in patients with 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.

Uraemic toxins and new methods to control their accumulation: game changers for the concept of dialysis adequacy

The current concept of an adequate dialysis based only on the dialysis process itself is rather limited. We now have considerable knowledge of uraemic toxicity and improved tools for limiting uraemic toxin accumulation. It is time to make use of these. A broader concept of adequacy that focusses on uraemic toxicity is required. As discussed in the present review, adequacy could be achieved by many different methods in combination with, or instead of, dialysis. These include preservation of renal function, dietary intake, reducing uraemic toxin generation rate and intestinal absorption, isolated ultrafiltration and extracorporeal adsorption of key uraemic toxins. A better measure of the quality of dialysis treatment would quantify the uraemic state in the patient using levels of a panel of key uraemic toxins. Treatment would focus on controlling uraemic toxicity while reducing harm or inconvenience to the patient. Delivering more dialysis might not be the best way to achieve this.

The intestine and the kidneys: a bad marriage can be hazardous

The concept that the intestine and chronic kidney disease influence each other, emerged only recently. The problem is multifaceted and bidirectional. On one hand, the composition of the intestinal microbiota impacts uraemic retention solute production, resulting in the generation of essentially protein-bound uraemic toxins with strong biological impact such as vascular damage and progression of kidney failure. On the other hand, the uraemic status affects the composition of intestinal microbiota, the generation of uraemic retention solutes and their precursors and causes disturbances in the protective epithelial barrier of the intestine and the translocation of intestinal microbiota into the body. All these elements together contribute to the disruption of the metabolic equilibrium and homeostasis typical to uraemia. Several measures with putative impact on intestinal status have recently been tested for their influence on the generation or concentration of uraemic toxins. These include dietary measures, prebiotics, probiotics, synbiotics and intestinal sorbents. Unfortunately, the quality and the evidence base of many of these studies are debatable, especially in uraemia, and often results within one study or among studies are contradictory. Nevertheless, intestinal uraemic metabolite generation remains an interesting target to obtain in the future as an alternative or additive to dialysis to decrease uraemic toxin generation. In the present review, we aim to summarize (i) the role of the intestine in uraemia by producing uraemic toxins and by generating pathophysiologically relevant changes, (ii) the role of uraemia in modifying intestinal physiology and (iii) the therapeutic options that could help to modify these effects and the studies that have assessed the impact of these therapies.

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

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