Indoxyl sulphate and kidney disease: Causes, consequences and interventions

Traditional Chinese medicine improved diabetic kidney disease through targeting gut microbiota

CONTEXT

Diabetic kidney disease (DKD) affects nearly 40% of diabetic patients, often leading to end-stage renal disease that requires renal replacement therapies, such as dialysis and transplantation. The gut microbiota, an integral aspect of human evolution, plays a crucial role in this condition. Traditional Chinese medicine (TCM) has shown promising outcomes in ameliorating DKD by addressing the gut microbiota.

OBJECTIVE

This review elucidates the modifications in gut microbiota observed in DKD and explores the impact of TCM interventions on correcting microbial dysregulation.

METHODS

We searched relevant articles from databases including Web of Science, PubMed, ScienceDirect, Wiley, and Springer Nature. The following keywords were used: diabetic kidney disease, diabetic nephropathy, gut microbiota, natural product, TCM, Chinese herbal medicine, and Chinese medicinal herbs. Rigorous criteria were applied to identify high-quality studies on TCM interventions against DKD.

RESULTS

Dysregulation of the gut microbiota, including Lactobacillus, Streptococcus, and Clostridium, has been observed in individuals with DKD. Key indicators of microbial dysregulation include increased uremic solutes and decreased short-chain fatty acids. Various TCM therapies, such as formulas, tablets, granules, capsules, and decoctions, exhibit unique advantages in regulating the disordered microbiota to treat DKD.

CONCLUSION

This review highlights the importance of targeting the gut-kidney axis to regulate microbial disorders, their metabolites, and associated signaling pathways in DKD. The Qing-Re-Xiao-Zheng formula, the Shenyan Kangfu tablet, the Huangkui capsule, and the Bekhogainsam decoction are potential candidates to address the gut-kidney axis. TCM interventions offer a significant therapeutic approach by targeting microbial dysregulation in patients with DKD.

Kidney Aging and Chronic Kidney Disease

The process of aging inevitably leads to an increase in age-related comorbidities, including chronic kidney disease (CKD). In many aspects, CKD can be considered a state of accelerated and premature aging. Aging kidney and CKD have numerous common characteristic features, ranging from pathological presentation and clinical manifestation to underlying mechanisms. The shared mechanisms underlying the process of kidney aging and the development of CKD include the increase in cellular senescence, the decrease in autophagy, mitochondrial dysfunction, and the alterations of epigenetic regulation, suggesting the existence of potential therapeutic targets that are applicable to both conditions. In this review, we provide a comprehensive overview of the common characteristics between aging kidney and CKD, encompassing morphological changes, functional alterations, and recent advancements in understanding the underlying mechanisms. Moreover, we discuss potential therapeutic strategies for targeting senescent cells in both the aging process and CKD.

Dietary Phosphorus Levels Influence Protein-Derived Uremic Toxin Production in Nephrectomized Male Rats

Gut microbiota-derived uremic toxins (UT) accumulate in patients with chronic kidney disease (CKD). Dietary phosphorus and protein restriction are common in CKD treatment, but the relationship between dietary phosphorus, a key nutrient for the gut microbiota, and protein-derived UT is poorly studied. Thus, we explored the relationship between dietary phosphorus and serum UT in CKD rats. For this exploratory study, we used serum samples from a larger study on the effects of dietary phosphorus on intestinal phosphorus absorption in nephrectomized (Nx, n = 22) or sham-operated (sham, n = 18) male Sprague Dawley rats. Rats were randomized to diet treatment groups of low or high phosphorus (0.1% or 1.2% w/w, respectively) for 1 week, with serum trimethylamine oxide (TMAO), indoxyl sulfate (IS), and p-cresol sulfate (pCS) analyzed by LC-MS. Nx rats had significantly higher levels of serum TMAO, IS, and pCS compared to sham rats (all p < 0.0001). IS showed a significant interaction between diet and CKD status, where serum IS was higher with the high-phosphorus diet in both Nx and sham rats, but to a greater extent in the Nx rats. Serum TMAO (p = 0.24) and pCS (p = 0.34) were not affected by dietary phosphorus levels. High dietary phosphorus intake for 1 week results in higher serum IS in both Nx and sham rats. The results of this exploratory study indicate that reducing dietary phosphorus intake in CKD may have beneficial effects on UT accumulation.

Specific gut microbiome and metabolome changes in patients with continuous ambulatory peritoneal dialysis and comparison between patients with different dialysis vintages

Background

In recent years, the role of gut microbiota and derived metabolites in renal disease has attracted more attention. It has been established that the gut microbiota is a potential target for medical interventions in renal disease including chronic kidney disease (CKD), acute kidney injury (AKI) and renal calculus. Emerging evidence has related dialysis treatment to the microbial composition and function of the intestines, and there are many reports related to HD, but few studies have been related to PD. Previous studies have found that PD patients have intestinal flora disturbances, so we speculate that intestinal flora and its metabolites may be the regulatory factors in long-term therapy of PD. And as far as we know, there have been no studies characterized the gut microbiota in PD patients of different dialysis vintages.

Methods

It is a cross-sectional study based on clinical data and biological samples of 72 patients with CAPD, 13 patients with ESRD and 13 healthy volunteers. The intestinal microecological characteristics of CAPD patients were comprehensively evaluated by combining the intestinal microflora structure, enterotoxin and receptor (serum LPS and LBP), intestinal barrier function index (serum D-Lactate), intestinal uremic toxin (serum IS, PCS, TMAO), fecal SCFAs and other multi-dimensional and multi-omics studies. Furthermore, the changes of intestinal microecology in CAPD patients of different dialysis vintages (≥ 3 and < 12 months, ≥ 12 and < 24 months, ≥ 24 and < 60 months, ≥ 60 months) were further explored, and the correlations between intestinal microecology indicators and some clinical indicators were analyzed. Fecal and serum samples were collected from PD patients (PD group, n = 72), ESRD patients (ESRD group, n = 13) and healthy volunteers (Normal group, n = 13). Fecal samples were subjected to microbiome (16S rDNA) and SCFA (GC-MS) analyses. Serum samples were subjected to LPS, LBP, D-lactate, IS, PCS, and TMAO (ELISA) analyses.

Results

The diversity and richness of intestinal flora in CAPD patients were lower than those in healthy people and ESRD patients, and the microflora structure was different. Anaerobes of Blautia and facultative anaerobes and aerobic bacteria with Bacilli and Lactobacillales those in Firmicutes are the main intestinal flora in CAPD patients. The abundance of Bacteroidaceae, Bacteroides, Faecalibacterium and other dominant bacteria in the intestinal tract of CAPD patients decreased. Proteobacteria, Enterobacteriaceae and Escherichia-Shigella increased their colonization (LDA > 4). In CAPD patients of different dialysis vintages, there was no significant change in the diversity and richness of microflora, and the microflora structure of PDC group was significantly different from that of PDD, which the abnormal expansion of enterobacter group was more prominent in PDC and the abundance of Bacteroides group was relatively higher in PDD. Intestinal barrier damage, intestinal uremic toxin accumulation and short-chain fatty acid reduction were observed in CAPD patients, such as the serum level of D-Lactate, PCS and TMAO were significantly higher than that in the Normal group (P < 0.05),and the fecal levels of BA and CA were significantly lower (P < 0.05). The intestinal microecological disorder of PDC group, while that of PDD group showed a better trend. Such as the PDC group had a significantly higher serum level of LPS, D-Lactate and TMAO (P < 0.01), and significantly lower serum level of LBP (P < 0.01), and lower fecal levels of AA and BA (P > 0.05) than the PDD group.

Conclusion

The intestinal microecology and metabolic system of CAPD patients had changes compared with healthy people and ESRD non-dialysis patients, and there were differences in CAPD patients with different dialysis vintages. PD patients on dialysis for more than 60 months showed a better trend in the intestinal microecology than patients with 24∼36 months, which suggested that the intestinal microecology of PD patients had a certain ability of self-regulation and remodeling under the management of standardized system and it is necessary to strengthen the monitoring of the intestinal status and the occurrence of related complications in PD patients on dialysis of 24∼36 months of dialysis vintage. It is initially considered that the mechanism of intestinal microecology is a potential target for intervention in the diagnosis and treatment of CAPD and incorporating intestinal microecosystem monitoring into the long-term management of CAPD patients is a new strategy.

Toxic ties: Unraveling the complex relationship between endocrine disrupting chemicals and chronic kidney disease

Environmental pollution is inherently linked to several metabolic diseases and high mortality. The kidney is more susceptible to environmental pollutants compared to other organs as it is involved in concentrating and filtering most of these toxins. Few epidemiological studies revealed the intrinsic relationship between exposure to Endocrine Disrupting Chemicals (EDCs) and CKD development. Though EDCs have the potential to cause severe pathologies, the specific molecular mechanisms by which they accelerate the progression of CKD remain elusive. In particular, our understanding of how pollutants affect the progression of chronic kidney disease (CKD) through the gut-kidney axis is currently limited. EDCs modulate the composition and function of the gut microbial community and favor the colonization of harmful gut pathogens. This alteration leads to an overproduction of uremic toxin and membrane vesicles. These vesicles carry several inflammatory molecules that exacerbate inflammation and renal tissue damage and aggravate the progression of CKD. Several experimental studies have revealed potential pathways by which uremic toxin further aggravates CKD. These include the induction of membrane vesicle production in host cells, which can trigger inflammatory pathways and insulin resistance. Reciprocally, CKD can also modulate gut bacterial composition that might further aggravate CKD condition. Thus, EDCs pose a significant threat to kidney health and the global CKD burden. Understanding this complicated issue necessitates multidisciplinary initiatives such as strict environmental controls, public awareness, and the development of novel therapeutic strategies targeting EDCs.

The AKI-to-CKD Transition: The Role of Uremic Toxins

After acute kidney injury (AKI), renal function continues to deteriorate in some patients. In a pro-inflammatory and profibrotic environment, the proximal tubules are subject to maladaptive repair. In the AKI-to-CKD transition, impaired recovery from AKI reduces tubular and glomerular filtration and leads to chronic kidney disease (CKD). Reduced kidney secretion capacity is characterized by the plasma accumulation of biologically active molecules, referred to as uremic toxins (UTs). These toxins have a role in the development of neurological, cardiovascular, bone, and renal complications of CKD. However, UTs might also cause CKD as well as be the consequence. Recent studies have shown that these molecules accumulate early in AKI and contribute to the establishment of this pro-inflammatory and profibrotic environment in the kidney. The objective of the present work was to review the mechanisms of UT toxicity that potentially contribute to the AKI-to-CKD transition in each renal compartment.

Indoxyl sulfate in atherosclerosis

Atherosclerosis (AS), a chronic vascular inflammatory disease, has become a main focus of attention worldwide for its chronic progressing disease course and serious complications in the later period. Nevertheless, explanations for the exact molecular mechanisms of AS initiation and development remain to be an unsolved problem. The classic pathogenesis theories, such as lipid percolation and deposition, endothelium injury, inflammation and immune damage, provide the foundation for discovering the new key molecules or signaling mechanisms. Recently, indoxyl sulfate (IS), one of non-free uremia toxins, has been noticeable for its multiple atherogenic effects. IS exists at high concentration in plasma for its great albumin binding rate. Patients with uremia have markedly elevated serum levels of IS due both to the deterioration of renal function and to the high binding affinity of IS to albumin. Nowadays, elevated incidence of circulatory disease among patients with renal dysfunction indicates correlation of uremic toxins with cardiovascular damage. In this review, the atherogenic effects of IS and the underlying mechanisms are summarized with emphasis on several key pathological events associated with AS developments, such as vascular endothelium dysfunction, arterial medial lesions, vascular oxidative stress, excessive inflammatory responses, calcification, thrombosis and foam cell formation. Although recent studies have proved the great correlation between IS and AS, deciphering cellular and pathophysiological signaling by confirming key factors involved in IS-mediated atherosclerosis development may enable identification of novel therapeutic targets.

Astragaloside IV attenuates indoxyl sulfate-induced injury of renal tubular epithelial cells by inhibiting the aryl hydrocarbon receptor pathway

ETHNOPHARMACOLOGICAL RELEVANCE

Astragalus membranaceus Fisch. ex Bunge has long been used to treat chronic kidney disease (CKD) in China. However, the mechanism of action requires further study. Indoxyl sulfate accumulation is the key cause of CKD progression. The aryl hydrocarbon receptor (AhR) plays an essential role in the renal tubular injury induced by indoxyl sulfate (IS).

AIM

We explored the effects of Astragaloside IV (AS-IV), a minor component of the flowering perennial Astragalus membranaceus Fisch. ex Bunge, on AhR activity during IS-induced injury of renal tubular epithelial cells.

METHODS

C57BL/6 mice fed a 0.2% adenine diet (adenine + IS) and intraperitoneally injected with IS were used to study the protective effects of AS-IV, and specifically the effect on the AhR. In addition, apoptosis (annexin/PI), oxidative stress and the AhR pathway were investigated in IS-stimulated HK-2 cells treated with AS-IV. The binding of AS-IV to the AhR was assessed in a molecular docking analysis. AhR knockdown using AhR siRNA allowed determination of the effects of AS-IV in IS-stimulated HK-2 cells.

RESULTS

AS-IV inhibited tubulointerstitial injury in adenine + IS mice. While AS-IV did not reduce serum IS levels, it did inhibit AhR expression in the kidney. In IS-stimulated HK-2 cells, AS-IV also dramatically reduced apoptosis, decreased oxidative stress responses and inhibited the expression of the AhR pathway. The molecular docking analysis showed surface binding of AS-IV to the AhR. Following AhR knockdown in HK-2 cells, IS-induced apoptosis was reduced and could not be further reduced by AS-IV.

CONCLUSION

By targeting the AhR, AS-IV may alleviate IS-induced renal tubular injury, thus offering a novel therapeutic approach to the treatment of chronic renal failure.

Intracellular Antioxidant and Anti-Inflammatory Effects and Bioactive Profiles of Coffee Cascara and Black Tea Kombucha Beverages

Kombucha is a functional beverage obtained through fermentation of sweetened Camellia sinensis infusion by a symbiotic culture of bacteria and yeasts that exerts many beneficial biological effects, mostly related to its antioxidant and anti-inflammatory effects. Alternative raw materials have been used to create new kombucha or kombucha-like products. Coffee is the most important food commodity worldwide and generates large amounts of by-products during harvest and post-harvest processing. The main coffee by-product is the dried fruit skin and pulp, popularly known as cascara. To date, no studies have evaluated the potential bioactivity of coffee cascara kombucha. In this study, we aimed to measure and compare the effects of infusions and kombuchas made with arabica coffee cascaras (n = 2) and black tea leaves (n = 1), fermented for 0, 3, 6, and 9 days on the intracellular production of Reactive Oxygen Species (ROS) and Nitric Oxide (NO) in model cells. Oxidative stress was induced in HK-2 cells with indoxyl sulfate (IS) and high glucose (G). Inflammation was induced with lipopolysaccharide (LPS) in RAW 264.7 macrophage. The contents of phenolic compounds, caffeine, and other physicochemical parameters were evaluated. To the best of our knowledge, this is the first study providing information on the bioactive profile and on the potential biological effects of coffee cascara kombucha. Fermentation caused the release of bound phenolic compounds from the infusions, especially total chlorogenic acids, with an average increase from 5.4 to 10.7 mg/100 mL (98%) and 2.6-3.4 mg/100 mL (30%) in coffee cascara and black tea kombucha, respectively, up to day 9. All evaluated beverages reduced (p < 0.0001) similarly the intracellular ROS (41% reduction, on average) and uric acid (10-55%) concentrations in HK-2 model cells, reversing the induced oxidative stress. All beverages also reduced (p < 0.0001, 81-90%) NO formation in LPS-induced macrophages, exhibiting an anti-inflammatory effect. These potential health benefits may be mostly attributed to polyphenols and caffeine, whose contents were comparable in all beverages. Coffee cascara showed similar potential to C. sinensis to produce healthy beverages and support sustainable coffee production.

Attenuation of indoxyl sulfate-induced cell damage by cinchonidine-a Cinchona alkaloid-through the downregulation of p53 signaling pathway by promoting MDM2 cytoplasmic-nuclear shuttling in endothelial cells

Renocardiac syndromes are a critical concern among patients with chronic kidney disease (CKD). High level of indoxyl sulfate (IS), a protein-bound uremic toxin, in plasma is known to promote the pathogenesis of cardiovascular diseases by impairing endothelial function. However, the therapeutic effects of the adsorbent of indole, a precursor of IS, on renocardiac syndromes is still debated. Therefore, novel therapeutic approaches should be developed to treat IS-associated endothelial dysfunction. In the present study, we have found that cinchonidine, a major Cinchona alkaloid, exhibited superior cell-protective effects among the 131 test compounds in IS-stimulated human umbilical vein endothelial cells (HUVECs). IS-induced cell death, cellular senescence, and impairment of tube formation in HUVECs were substantially reversed after treatment with cinchonidine. Despite the cinchonidine did not alter reactive oxygen species formation, cellular uptake of IS and OAT3 activity, RNA-Seq analysis showed that the cinchonidine treatment downregulated p53-modulated gene expression and substantially reversed IS-caused G0/G1 cell cycle arrest. Although the mRNA levels of p53 were not considerably downregulated by cinchonidine in IS-treated HUVECs, the treatment of cinchonidine promoted the degradation of p53 and the cytoplasmic-nuclear shuttling of MDM2. Cinchonidine exhibited cell-protective effects against the IS-induced cell death, cellular senescence, and impairment of vasculogenic activity in HUVECs through the downregulation of p53 signaling pathway. Collectively, cinchonidine may be a potential cell-protective agent to rescue IS-induced endothelial cell damage.

Sample preparation and chromatographic methods for the determination of protein-bound uremic retention solutes in human biological samples: An overview

Protein-bound uremic retention solutes, such as indole-3-acetic acid, indoxyl sulfate, p-cresol and p-cresol sulfate, are associated with the development of several pathologies, namely renal, cardiovascular, and bone toxicities, due to their potential accumulation in the human body, thus requiring analytical methods for monitoring and evaluation. The present review addresses conventional and advanced sample treatment procedures for sample handling and the chromatographic analytical methods developed for quantification of these compounds in different biological fluids, with particular focus on plasma, serum, and urine. The sample preparation and chromatographic methods coupled to different detection systems are critically discussed, focusing on the different steps involved for sample treatment, namely elimination of interfering compounds present in the sample matrix, and the evaluation of their environmental impact through the AGREEprep tool. There is a clear trend for the application of liquid-chromatography coupled to tandem mass spectrometry, which requires protein precipitation, solid-phase extraction and/or dilution prior to analysis of biological samples. Furthermore, from a sustainability point of view, miniaturized methods resorting to microplate devices are highly recommended.

Identification of metabolic fingerprints in severe obstructive sleep apnea using gas chromatography-Mass spectrometry

Objective: Obstructive sleep apnea (OSA) is considered a major sleep-related breathing problem with an increasing prevalence rate. Retrospective studies have revealed the risk of various comorbidities associated with increased severity of OSA. This study aims to identify novel metabolic biomarkers associated with severe OSA. Methods: In total, 50 cases of OSA patients (49.74 ± 11.87 years) and 30 controls (39.20 ± 3.29 years) were included in the study. According to the polysomnography reports and questionnaire-based assessment, only patients with an apnea-hypopnea index (AHI >30 events/hour) exceeding the threshold representing severe OSA patients were considered for metabolite analysis. Plasma metabolites were analyzed using gas chromatography-mass spectrometry (GC-MS). Results: A total of 92 metabolites were identified in the OSA group compared with the control group after metabolic profiling. Metabolites and their correlated metabolic pathways were significantly altered in OSA patients with respect to controls. The fold-change analysis revealed markers of chronic kidney disease, cardiovascular risk, and oxidative stress-like indoxyl sulfate, 5-hydroxytryptamine, and 5-aminolevulenic acid, respectively, which were significantly upregulated in OSA patients. Conclusion: Identifying these metabolic signatures paves the way to monitor comorbid disease progression due to OSA. Results of this study suggest that blood plasma-based biomarkers may have the potential for disease management.

Multi-Omics Strategies for Investigating the Microbiome in Toxicology Research

Microbial communities on and within the host contact environmental pollutants, toxic compounds, and other xenobiotic compounds. These communities of bacteria, fungi, viruses, and archaea possess diverse metabolic potential to catabolize compounds and produce new metabolites. Microbes alter chemical disposition thus making the microbiome a natural subject of interest for toxicology. Sequencing and metabolomics technologies permit the study of microbiomes altered by acute or long-term exposure to xenobiotics. These investigations have already contributed to and are helping to re-interpret traditional understandings of toxicology. The purpose of this review is to provide a survey of the current methods used to characterize microbes within the context of toxicology. This will include discussion of commonly used techniques for conducting omic-based experiments, their respective strengths and deficiencies, and how forward-looking techniques may address present shortcomings. Finally, a perspective will be provided regarding common assumptions that currently impede microbiome studies from producing causal explanations of toxicologic mechanisms.

A robust, accurate, sensitive LC-MS/MS method to measure indoxyl sulfate, validated for plasma and kidney cells

Proximal tubular damage is an important prognostic determinant in various chronic kidney diseases (CKDs). Currently available diagnostic methods do not allow for early disease detection and are neither efficient. Indoxyl sulfate (IS) is an endogenous metabolite and protein-bound uremic toxin that is eliminated via renal secretion, but accumulates in plasma during tubular dysfunction. Therefore, it may be suitable as a tubular function marker. To evaluate this, a fast bioanalytical method was developed and validated for IS in various species and a kidney cell line using LC-MS/MS. An isotope-labeled IS potassium salt as an internal standard and acetonitrile (ACN) as a protein precipitant were used for sample pretreatment. The analyte was separated on a Polaris 3 C18-A column by gradient elution using 0.1% formic acid in water and ACN, and detected by negative electrospray ionization in selected reaction monitoring mode. The within-day (≤ 4.0%) and between-day (≤ 4.3%) precisions and accuracies (97.7 to 107.3%) were within the acceptable range. The analyte showed sufficient stability at all conditions investigated. Finally, applying this assay, significantly higher plasma and lower urine concentrations of IS were observed in mice with diabetic nephropathy with tubular damage, which encourages validation toward its use as a biomarker.

Does Mediterranean Adequacy Index Correlate with Cardiovascular Events in Patients with Advanced Chronic Kidney Disease? An Exploratory Study

The Mediterranean Diet (MD) is a healthy dietary pattern, demonstrated to reduce the risk of cancer, diabetes, cardiovascular and neurodegenerative diseases, and early death. The Mediterranean Adequacy Index (MAI) is used to measure adherence to the MD in perspective studies in the general population and correlates with cardiovascular events. The aim of this study was to calculate the MAI among patients with advanced chronic kidney disease (CKD) and correlate it with traditional uremic, microbiota-derived, and proatherogenic toxins as well as nutritional status, quality of life, and cardiovascular events. A total of 60 adult patients with advanced CKD were enrolled and their MAI was calculated. According to the median value, patients were divided into lower (l-MAI, <1.80) and higher (h-MAI, ≥1.80) MAI groups. Biochemical parameters, microbiota-derived and proatherogenic toxins (p-Cresyl sulphate, Indoxyl-sulphate, and Lipoprotein-associated phospholipase A2), nutritional status, quality of life, and cardiovascular events that occurred in the previous three years were recorded. The mean value of the MAI was 2.78 ± 2.86. The MAI was significantly higher in foreigners (median (IQR) 6.38 (8.98) vs. 1.74 (1.67), p < 0.001) and diabetic patients. The l-MAI and h-MAI groups had similar routinary blood, p-Cresyl-sulphate, Indoxyl-sulphate, and Lp-PLA2 as well as nutritional status and quality of life parameters. The MAI was not associated with previous cardiovascular events and did not correlate with cardiovascular events in CKD patients. New and nephro-tailored indexes are warranted to evaluate nutritional therapy in CKD patients.

The Microbiome and Uremic Solutes

Uremic retention solutes, especially the protein-bound compounds, are toxic metabolites, difficult to eliminate with progressive renal functional decline. They are of particular interest because these uremic solutes are responsible for the pathogenesis of cardiovascular and chronic kidney diseases. Evidence suggests that the relation between uremic toxins, the microbiome, and its host is altered in patients with chronic kidney disease, with the colon’s motility, epithelial integrity, and absorptive properties also playing an important role. Studies found an alteration of the microbiota composition with differences in species proportion, diversity, and function. Since uremic toxins precursors are generated by the microbiota, multiple therapeutic options are currently being explored to address dysbiosis. While an oral adsorbent can decrease the transport of bacterial metabolites from the intestinal lumen to the blood, dietary measures, supplements (prebiotics, probiotics, and synbiotics), and antibiotics aim to target directly the gut microbiota composition. Innovative approaches, such as the modulation of bacterial enzymes, open new perspectives to decrease the plasma level of uremic toxins.

Sex Differences in Fecal Microbiota Correlation With Physiological and Biochemical Indices Associated With End-Stage Renal Disease Caused by Immunoglobulin a Nephropathy or Diabetes

This study investigated the sex-specific differences in the correlation between intestinal microbiota and end-stage renal disease. Here, we compared the differences in the gut microbiota of male and female healthy controls (HC) and patients with end-stage renal disease (ESRD) caused by immunoglobulin A (IgA) nephropathy (ESRD-IgAN) or type-2 diabetes mellitus (ESRD-T2DM) using high-throughput sequencing of the 16S rRNA gene. We also analyzed the correlation between gut microbiota and clinical immune indicators. We assigned 8, 10, 5, 7, 11, and 20 volunteers to female HC, ESRD-IgAN, and ESRD-T2DM, and male HC, ESRD-IgAN, and ESRD-T2DM, respectively. The results showed sex-specific differences in both physiological and biochemical indices and intestinal microbiota composition, as well as the correlation between them. The correlations between physiological and biochemical indices in men were significantly lower than those in women, especially for indices related to immunity, blood glucose, and cardiac color sonography. Urine output, lymphocyte ratio, serum albumin, blood calcium, dialysis status, serum urea nitrogen, urine protein, and diabetes significantly correlated with male fecal microbiota composition, whereas only creatinine and 2-h post-prandial blood glucose significantly correlated with female fecal microbiota composition. The top 50 dominant operational taxonomic units showed a stronger correlation with physiological and biochemical indices in samples obtained from females than from males. These differences highlight sex-specific differences in the effectiveness of ESRD prevention and treatments via regulating intestinal microbiota.

Serum indoxyl sulphate and its relation to albumin and α1-acid glycoprotein as a potential biomarkers of maternal intestinal metabolism during pregnancy and postpartum

BACKGROUND

Serum indoxyl sulfate (IS) levels depend on the production of indole in the gut. The biological effects of IS in the vascular bed could be confirmed by changes in the levels of individual serum proteins during normal pregnancy and in the postpartum period as compared with non-pregnant controls. Albumin (Alb) and α1-acid glycoprotein (AGP, orosomucoid) are the most abundant serum carrier proteins with potential interrelationships with serum levels of IS.

METHODS

Serum levels of IS, Alb and AGP were measured in 84 pregnant women in the first, second and third trimester of pregnancy and in the postpartum period, as well as in non-pregnant controls (n = 20), using ultra-performance liquid chromatography (UPLC) coupled to mass spectrometry (IS), colorimetric assay (Alb) and immunoturbidimetric assay (AGP).

RESULTS

The postpartum serum levels [mg/L] of IS were lower (p = 0.027) than in the second trimester (mean±SD: 0.85±0.39 vs 0.58±0.32). There were no differences in the IS to ALB ratio calculated in the three trimesters of pregnancy, the postpartum period, and in the non-pregnant controls. The IS/AGP ratio increased from the first to the second trimester (p = 0.039), and decreased in the postpartum period (p<0.05), when it was lower than in the second and third trimester.

CONCLUSIONS

The variability of the serum IS/AGP ratio during pregnancy and in the postpartum period may reflect shared involvement in the regulation of their intravascular relationships. The link between serum levels of IS derived from the gut and AGP could serve a potential biomarkers of maternal intestinal metabolism during pregnancy and postpartum.

Diverse roles of microbial indole compounds in eukaryotic systems

Indole and its derivatives are widespread across different life forms, functioning as signalling molecules in prokaryotes and with more diverse roles in eukaryotes. A majority of indoles found in the environment are attributed to bacterial enzymes converting tryptophan into indole and its derivatives. The involvement of indoles among lower organisms as an interspecies and intraspecies signal is well known, with many reports showing that inter‐kingdom interactions involving microbial indole compounds are equally important as they influence defence systems and even the behaviour of higher organisms. This review summarizes recent advances in our understanding of the functional properties of indole and indole derivatives in diverse eukaryotes. Furthermore, we discuss current perspectives on the role of microbial indoles in human diseases such as diabetes, obesity, atherosclerosis, and cancers. Deciphering the function of indoles as biomarkers of metabolic state will facilitate the formulation of diet‐based treatments and open unique therapeutic opportunities.

Uremic Toxins and Their Relation with Oxidative Stress Induced in Patients with CKD

The presence of toxins is believed to be a major factor in the development of uremia in patients with chronic kidney disease (CKD) and end-stage renal disease (ESRD). Uremic toxins have been divided into 3 groups: small substances dissolved in water, medium molecules: peptides and low molecular weight proteins, and protein-bound toxins. One of the earliest known toxins is urea, the concentration of which was considered negligible in CKD patients. However, subsequent studies have shown that it can lead to increased production of reactive oxygen species (ROS), and induce insulin resistance in vitro and in vivo, as well as cause carbamylation of proteins, peptides, and amino acids. Other uremic toxins and their participation in the damage caused by oxidative stress to biological material are also presented. Macromolecules and molecules modified as a result of carbamylation, oxidative stress, and their adducts with uremic toxins, may lead to cardiovascular diseases, and increased risk of mortality in patients with CKD.

Structural component changes of erythrocytes caused by oxidative stress generated by indoxyl sulfate

Indoxyl sulfate (IS) belongs to groups of uremic toxins binding to proteins. This compound may contribute to the generation of oxidative stress in chronic kidney disease (CKD) patients. We hypothesized that a high concentration of IS in the blood may induce structural changes of erythrocyte components and thus may contribute to CKD progression. In the present study, we evaluated the influence of IS on hemolysate and membrane proteins' conformational state, lipid membrane fluidity, and internal viscosity in erythrocytes. We examined thiols, carbonyl groups, peroxides, and TBARS levels in erythrocyte incubated with IS. The treatment of erythrocytes with IS led to increase in lipid membrane fluidity, decrease in the internal viscosity of the cells and the motion of the spin labels attached to hemolysate proteins. We did not observe conformational changes in plasma membrane proteins; however, in the plasma membranes of erythrocytes incubated with IS, a decrease in the content of thiol groups and increase in the carbonyls levels and peroxides and TBARS in comparison with the control was observed. The obtained results indicate that IS induces the oxidative damage of erythrocyte components. This may be an important factor that affects the functional properties of erythrocytes in CKD patients.

Longitudinal wastewater sampling in buildings reveals temporal dynamics of metabolites

Direct sampling of building wastewater has the potential to enable "precision public health" observations and interventions. Temporal sampling offers additional dynamic information that can be used to increase the informational content of individual metabolic "features", but few studies have focused on high-resolution sampling. Here, we sampled three spatially close buildings, revealing individual metabolomics features, retention time (rt) and mass-to-charge ratio (mz) pairs, that often possess similar stationary statistical properties, as expected from aggregate sampling. However, the temporal profiles of features-providing orthogonal information to physicochemical properties-illustrate that many possess different feature temporal dynamics (fTDs) across buildings, with large and unpredictable single day deviations from the mean. Internal to a building, numerous and seemingly unrelated features, with mz and rt differences up to hundreds of Daltons and seconds, display highly correlated fTDs, suggesting non-obvious feature relationships. Data-driven building classification achieves high sensitivity and specificity, and extracts building-identifying features found to possess unique dynamics. Analysis of fTDs from many short-duration samples allows for tailored community monitoring with applicability in public health studies.

Indoxyl sulfate induces intestinal barrier injury through IRF1-DRP1 axis-mediated mitophagy impairment

Rationale: The dysfunctional gut-kidney axis forms a vicious circle, which eventually becomes a catalyst for the progression of chronic kidney disease (CKD) and occurrence of related complications. However, the pathogenic factors of CKD-associated intestinal dysfunction and its mechanism remain elusive. Methods: We first identified the protein-bound uremic toxin indoxyl sulfate (IS) as a possible contributor to intestinal barrier injury. Transepithelial electrical resistance, permeability assay and transmission electron microscopy were carried out to evaluate the damaging effect of IS on intestinal barrier in intestinal epithelial cells, IS-injected mice and CKD mice. In vitro and in vivo experiments were performed to investigate the role of IS in intestinal barrier injury and the underlying mechanism. Finally, CKD mice treated with AST-120 (an oral adsorbent for IS) and gene knockout mice were used to verify the mechanism and to explore possible interventions for IS-induced intestinal barrier injury. Results: Transepithelial electrical resistance and the expressions of tight junction-related genes were significantly suppressed by IS in intestinal epithelial cells. In vitro experiments demonstrated that IS inhibited the expression of dynamin-related protein 1 (DRP1) and mitophagic flux, whereas DRP1 overexpression attenuated IS-induced mitophagic inhibition and intestinal epithelial cell damage. Furthermore, IS suppressed DRP1 by upregulating the expression of interferon regulatory factor 1 (IRF1), and IRF1 could directly bind to the promoter region of DRP1. Additionally, the decreased expression of DRP1 and autophagosome-encapsulated mitochondria were observed in the intestinal tissues of CKD patients. Administration of AST-120 or genetic knockout of IRF1 attenuated IS-induced DRP1 reduction, mitophagic impairment and intestinal barrier injury in mice. Conclusions: These findings suggest that reducing IS accumulation or targeting the IRF1-DRP1 axis may be a promising therapeutic strategy for alleviating CKD-associated intestinal dysfunction.

Effects of the oral adsorbent AST-120 on fecal p-cresol and indole levels and on the gut microbiota composition

In chronic kidney disease, elevated levels of circulating uremic toxins are associated with a variety of symptoms and organ dysfunction. Indoxyl sulfate (IS) and p-cresyl sulfate (pCS) are microbiota-derived metabolites and representative uremic toxins. We have previously shown that the oral adsorbent AST-120 profoundly reduced pCS compared to IS in adenine-induced renal failure in mice. However, the mechanisms of the different attenuation effects of AST-120 between IS and pCS are unclear. To clarify the difference of AST-120 on IS and pCS, we investigated the levels of fecal indole and p-cresol, the respective precursors of IS and pCS, and examined the influence on the gut microbiota. Although fecal indole was detected in all groups analyzed, fecal p-cresol was not detected in AST-120 treatment groups. In genus level, a total of 23 organisms were significantly changed by renal failure or AST-120 treatment. Especially, AST-120 reduced the abundance of Erysipelotrichaceae uncultured and Clostridium sensu stricto 1, which have a gene involved in p-cresol production. Our findings suggest that, in addition to the adsorption of the uremic toxin precursors, AST-120 affects the abundance of some gut microbiota in normal and renal failure conditions, thereby explaining the different attenuation effects on IS and pCS.

Phosphate acts directly on the calcium-sensing receptor to stimulate parathyroid hormone secretion

Extracellular phosphate regulates its own renal excretion by eliciting concentration-dependent secretion of parathyroid hormone (PTH). However, the phosphate-sensing mechanism remains unknown and requires elucidation for understanding the aetiology of secondary hyperparathyroidism in chronic kidney disease (CKD). The calcium-sensing receptor (CaSR) is the main controller of PTH secretion and here we show that raising phosphate concentration within the pathophysiologic range for CKD significantly inhibits CaSR activity via non-competitive antagonism. Mutation of residue R62 in anion binding site-1 abolishes phosphate-induced inhibition of CaSR. Further, pathophysiologic phosphate concentrations elicit rapid and reversible increases in PTH secretion from freshly-isolated human parathyroid cells consistent with a receptor-mediated action. The same effect is seen in wild-type murine parathyroid glands, but not in CaSR knockout glands. By sensing moderate changes in extracellular phosphate concentration, the CaSR represents a phosphate sensor in the parathyroid gland, explaining the stimulatory effect of phosphate on PTH secretion.

Elevated inorganic phosphate levels promote excessive parathyroid hormone secretion, which contributes to the aetiology of secondary hyperparathyroidism. Here, the authors show that phosphate directly inhibits the calcium-sensing receptor, the main regulator of parathyroid hormone secretion.

Indoxyl Sulfate Stimulates Angiogenesis by Regulating Reactive Oxygen Species Production via CYP1B1

Indoxyl sulfate (IS) is an accumulative protein-bound uremic toxin found in patients with kidney disease. It is reported that IS impairs the vascular endothelium, but a comprehensive overview of all mechanisms active in IS-injury currently remains lacking. Here we performed RNA sequencing in human umbilical vein endothelial cells (HUVECs) after IS or control medium treatment and identified 1293 genes that were affected in a IS-induced response. Gene enrichment analysis highlighted pathways involved in altered vascular formation and cell metabolism. We confirmed these transcriptome profiles at the functional level by demonstrating decreased viability and increased cell senescence in response to IS treatment. In line with the additional pathways highlighted by the transcriptome analysis, we further could demonstrate that IS exposure of HUVECs promoted tubule formation as shown by the increase in total tubule length in a 3D HUVECs/pericytes co-culture assay. Notably, the pro-angiogenic response of IS and increased ROS production were abolished when CYP1B1, one of the main target genes that was highly upregulated by IS, was silenced. This observation indicates IS-induced ROS in endothelial cells is CYP1B1-dependent. Taken together, our findings demonstrate that IS promotes angiogenesis and CYP1B1 is an important factor in IS-activated angiogenic response.

Indoxyl sulfate-induced TNF-α is regulated by crosstalk between the aryl hydrocarbon receptor, NF-κB, and SOCS2 in human macrophages

Indoxyl sulfate (IS) is a uremic toxin associated with increased prevalence of cardiovascular diseases (CVDs) in patients with chronic kidney disease. Despite the crucial role of uremia-related immune dysfunction, a majority of studies attempting to elucidate its pathogenic role in CVD have focused on IS-mediated endothelial dysfunction. Thus, we investigated the underlying molecular mechanisms involved in IS-induced production of TNF-α, a major cardiotoxic cytokine, by human macrophages. We found that crosstalk between the aryl hydrocarbon receptor (AhR), NF-κB, and the suppressor of cytokine signaling (SOCS)2 is important for TNF-α production in IS-stimulated human macrophages. IS-activated AhR rapidly associates with the p65 NF-κB subunit, resulting in mutual inhibition of AhR and NF-κB and inhibition of TNF-α production at an early time point. Later, this repression of TNF-α production is alleviated when SOCS2, a negative modulator of NF-κB, is directly induced by IS-activated AhR. In addition, once free of inhibition, activated AhR induces TNF-α expression by interacting with AhR binding sites in the TNF-α gene. Lastly, we confirmed decreased AhR and increased SOCS2 expression in monocytes of patients with end-stage renal disease, indicating the activation of AhR. Taken together, our results suggest that IS-induced TNF-α production in macrophages is regulated through a complicated mechanism involving interaction of AhR, NF-κB, and SOCS2.-Kim, H. Y., Yoo, T.-H., Cho, J.-Y., Kim, H. C., Lee, W.-W. Indoxyl sulfate-induced TNF-α is regulated by crosstalk between the aryl hydrocarbon receptor, NF-κB, and SOCS2 in human macrophages.

Mitochondria as a Source and a Target for Uremic Toxins

Elucidation of molecular and cellular mechanisms of the uremic syndrome is a very challenging task. More than 130 substances are now considered to be "uremic toxins" and represent a very diverse group of molecules. The toxicity of these molecules affects many cellular processes, and expectably, some of them are able to disrupt mitochondrial functioning. However, mitochondria can be the source of uremic toxins as well, as the mitochondrion can be the site of complete synthesis of the toxin, whereas in some scenarios only some enzymes of the pathway of toxin synthesis are localized here. In this review, we discuss the role of mitochondria as both the target and source of pathological processes and toxic compounds during uremia. Our analysis revealed about 30 toxins closely related to mitochondria. Moreover, since mitochondria are key regulators of cellular redox homeostasis, their functioning might directly affect the production of uremic toxins, especially those that are products of oxidation or peroxidation of cellular components, such as aldehydes, advanced glycation end-products, advanced lipoxidation end-products, and reactive carbonyl species. Additionally, as a number of metabolic products can be degraded in the mitochondria, mitochondrial dysfunction would therefore be expected to cause accumulation of such toxins in the organism. Alternatively, many uremic toxins (both made with the participation of mitochondria, and originated from other sources including exogenous) are damaging to mitochondrial components, especially respiratory complexes. As a result, a positive feedback loop emerges, leading to the amplification of the accumulation of uremic solutes. Therefore, uremia leads to the appearance of mitochondria-damaging compounds, and consecutive mitochondrial damage causes a further rise of uremic toxins, whose synthesis is associated with mitochondria. All this makes mitochondrion an important player in the pathogenesis of uremia and draws attention to the possibility of reducing the pathological consequences of uremia by protecting mitochondria and reducing their role in the production of uremic toxins.

Diet, gut microbiome and indoxyl sulphate in chronic kidney disease patients

Emerging evidence suggests that intestinal dysbiosis plays an important role in host inflammation locally and systemically. Such pathological condition is even more prevailing in patients with chronic kidney disease (CKD). Of note, indoxyl sulphate (IS), a gut-derived uremic toxin, is notorious for its pro-inflammatory feature in CKD patients. IS accumulates in the body as the urinary excretion of uremic toxins is impaired, and further worsens the kidney function in a vicious cycle to CKD. Dietary restriction in vegetables, fruits and yogurt leads to the predominance of indole-producing intestinal microbial flora and further exaggerates the accumulation of IS in CKD patients. Recently, interventional studies have shown that circulating IS can be reduced by dietary prebiotic and/or probiotic supplements. However, further randomized controlled trials are warranted to examine whether such beneficial effect of dietary prebiotic/probiotic supplements could be extrapolated to better hard outcomes in CKD population. In this review, we would also like to emphasize the importance of achieving sufficient intake of dietary fibre by proper vegetable pre-treatment and accurate fruit selection, instead of directly avoiding these potassium-rich yet fibre-rich and base-producing foods.

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.

Gut microbiota and inflammation in chronic kidney disease and their roles in the development of cardiovascular disease

The health and proper functioning of the cardiovascular and renal systems largely depend on crosstalk in the gut-kidney-heart/vessel triangle. Recent evidence suggests that the gut microbiota has an integral function in this crosstalk. Mounting evidence indicates that the development of chronic kidney and cardiovascular diseases follows chronic inflammatory processes that are affected by the gut microbiota via various immune, metabolic, endocrine, and neurologic pathways. Additionally, deterioration of the function of the cardiovascular and renal systems has been reported to disrupt the original gut microbiota composition, further contributing to the advancement of chronic cardiovascular and renal diseases. Considering the interaction between the gut microbiota and the renal and cardiovascular systems, we can infer that interventions for the gut microbiota through diet and possibly some medications can prevent/stop the vicious cycle between the gut microbiota and the cardiovascular/renal systems, leading to a decrease in chronic cardiovascular and renal diseases.

Impacts of Indoxyl Sulfate and p-Cresol Sulfate on Chronic Kidney Disease and Mitigating Effects of AST-120

Uremic toxins, such as indoxyl sulfate (IS) and p-cresol, or p-cresyl sulfate (PCS), are markedly accumulated in the organs of chronic kidney disease (CKD) patients. These toxins can induce inflammatory reactions and enhance oxidative stress, prompting glomerular sclerosis and interstitial fibrosis, to aggravate the decline of renal function. Consequently, uremic toxins play an important role in the worsening of renal and cardiovascular functions. Furthermore, they destroy the quantity and quality of bone. Oral sorbent AST-120 reduces serum levels of uremic toxins in CKD patients by adsorbing the precursors of IS and PCS generated by amino acid metabolism in the intestine. Accordingly, AST-120 decreases the serum IS levels and reduces the production of reactive oxygen species by endothelial cells, to impede the subsequent oxidative stress. This slows the progression of cardiovascular and renal diseases and improves bone metabolism in CKD patients. Although large-scale studies showed no obvious benefits from adding AST-120 to the standard therapy for CKD patients, subsequent sporadic studies may support its use. This article summarizes the mechanisms of the uremic toxins, IS, and PCS, and discusses the multiple effects of AST-120 in CKD patients.

Indoxyl Sulfate Generates Free Radicals, Decreases Antioxidant Defense, and Leads to Damage to Mononuclear Blood Cells

Indoxyl sulfate (IS) is a uremic toxin that has been associated with inflammation and oxidative stress as well as with the progression of chronic kidney disease (CKD). IS is a protein metabolite that is concentrated in the serum of CKD patients. IS is a well-known uremic toxin, but there are very few reports on the effect of IS on cells including mononuclear cells (MNCs). We hypothesized that a high concentration of IS in CKD patients may induce changes in redox balance in the in vitro cells exposed. In the present study, we investigated the effect of IS on free radical production, antioxidant capacity, and protein damage in the mononuclear blood cells. As already determined, the concentrations (0.2 or 1 mM) of IS used in this study do not affect the survival rate of MNCs. For both the concentrations of IS, there was an increase in superoxide and nitric oxide and a release of other reactive oxygen species (ROS) inside the cells, as measured using fluorescent probes. However, an increase in other ROS as indicated by H₂DCF-DA was found only for 1 mM of IS. Moreover, a decrease in the non-enzymatic antioxidant capacity and an increase in the superoxide dismutase activity after incubation of the cells with IS were observed. Furthermore, we found an increase in the levels of carbonyl compounds and peroxides in the cells treated with both the concentrations of IS. The obtained results show that IS induces oxidative stress and a decrease in antioxidant defense in cells leading to lipid and protein damage.

The crosstalk of gut microbiota and chronic kidney disease: role of inflammation, proteinuria, hypertension, and diabetes mellitus

Chronic kidney disease (CKD) has been shown to result in profound changes in the composition and functions of the gut microbial flora which by disrupting intestinal epithelial barrier and generating toxic by-products contributes to systemic inflammation and the associated complications. On the other hand, emerging evidence points to the role of the gut microbiota in the development and progression of CKD by provoking inflammation, proteinuria, hypertension, and diabetes. These observations demonstrate the causal interconnection between the gut microbial dysbiosis and CKD. The gut microbiota closely interacts with the inflammatory, renal, cardiovascular, and endocrine systems via metabolic, humoral, and neural signaling pathways, events which can lead to chronic systemic inflammation, proteinuria, hypertension, diabetes, and kidney disease. Given the established role of the gut microbiota in the development and progression of CKD and its complications, favorable modification of the composition and function of the gut microbiome represents an appealing therapeutic target for prevention and treatment of CKD. This review provides an overview of the role of the gut microbial dysbiosis in the pathogenesis of the common causes of CKD including hypertension, diabetes, and proteinuria as well as progression of CKD.

Indoxyl Sulfate Induces Apoptosis and Hypertrophy in Human Kidney Proximal Tubular Cells

Indoxyl sulfate (IS) is a protein-bound uremic toxin that accumulates in patients with declining kidney function. Although generally thought of as a consequence of declining kidney function, emerging evidence demonstrates direct cytotoxic role of IS on endothelial cells and cardiomyocytes, largely through the expression of pro-inflammatory and pro-fibrotic factors. The direct toxicity of IS on human kidney proximal tubular epithelial cells (PTECs) remains a matter of debate. The current study explored the effect of IS on primary cultures of human PTECs and HK-2, an immortalized human PTEC line. Pathologically relevant concentrations of IS induced apoptosis and increased the expression of the proapoptotic molecule Bax in both cell types. IS impaired mitochondrial metabolic activity and induced cellular hypertrophy. Furthermore, statistically significant upregulation of pro-fibrotic (transforming growth factor-β, fibronectin) and pro-inflammatory molecules (interleukin-6, interleukin-8, and tumor necrosis factor-α) in response to IS was observed. Albumin had no influence on the toxicity of IS. The results of this study suggest that IS directly induced a pro-inflammatory and pro-fibrotic phenotype in proximal tubular cells. In light of the associated apoptosis, hypertrophy, and metabolic dysfunction, this study demonstrates that IS may play a role in the progression of chronic kidney disease.

Curcumin ameliorates cadmium-induced nephrotoxicity in Sprague-Dawley rats

Chronic exposure to cadmium (Cd) causes remarkable damage to the kidneys, a target organ of accumulated Cd after oral administration. The aim of the present study was to investigate the protective effect of curcumin against Cd-induced nephrotoxicity. Sprague-Dawley male rats were divided into the following four treatment groups: control, curcumin (50 mg/kg, oral), CdCl₂, (25 mg/kg, oral), and pre-treatment with curcumin (50 mg/kg) 1 h prior to the administration of CdCl₂ (25 mg/kg, oral) for 7 days. At 24 h after the final treatment, the animals were killed, and the biomarkers associated with nephrotoxicity were measured. Our data indicated that blood urea nitrogen (BUN) and serum creatinine (sCr) levels were significantly reduced by curcumin pre-treatment in CdCl₂-treated animals. Histopathological studies showed hydropic swelling and hypertrophy of the proximal tubular cells in the renal cortex after Cd treatment. Pretreatment with curcumin ameliorated the histological alterations induced by Cd. The urinary excretion of kidney injury molecule-1 (Kim-1), osteopontin (OPN), tissue inhibitor of metalloproteinases 1 (TIMP-1), neutrophil gelatinase-associated lipocalin (NGAL), and netrin-1 significantly reduced by curcumin treatment compared to that in the CdCl₂-treated group. The administration of curcumin provided a significant protective effect against Cd-induced nephrotoxicity.

Impact of the Oral Adsorbent AST-120 on Organ-Specific Accumulation of Uremic Toxins: LC-MS/MS and MS Imaging Techniques

Elevated circulating uremic toxins are associated with a variety of symptoms and organ dysfunction observed in patients with chronic kidney disease (CKD). Indoxyl sulfate (IS) and p-cresyl sulfate (PCS) are representative uremic toxins that exert various harmful effects. We recently showed that IS induces metabolic alteration in skeletal muscle and causes sarcopenia in mice. However, whether organ-specific accumulation of IS and PCS is associated with tissue dysfunction is still unclear. We investigated the accumulation of IS and PCS using liquid chromatography/tandem mass spectrometry in various tissues from mice with adenine-induced CKD. IS and PCS accumulated in all 15 organs analyzed, including kidney, skeletal muscle, and brain. We also visualized the tissue accumulation of IS and PCS with immunohistochemistry and mass spectrometry imaging techniques. The oral adsorbent AST-120 prevented some tissue accumulation of IS and PCS. In skeletal muscle, reduced accumulation following AST-120 treatment resulted in the amelioration of renal failure-associated muscle atrophy. We conclude that uremic toxins can accumulate in various organs and that AST-120 may be useful in treating or preventing organ dysfunction in CKD, possibly by reducing tissue accumulation of uremic toxins.

Indoxyl Sulfate Upregulates Liver P-Glycoprotein Expression and Activity through Aryl Hydrocarbon Receptor Signaling

In patients with CKD, not only renal but also, nonrenal clearance of drugs is altered. Uremic toxins could modify the expression and/or activity of drug transporters in the liver. We tested whether the uremic toxin indoxyl sulfate (IS), an endogenous ligand of the transcription factor aryl hydrocarbon receptor, could change the expression of the following liver transporters involved in drug clearance: SLC10A1, SLC22A1, SLC22A7, SLC47A1, SLCO1B1, SLCO1B3, SLCO2B1, ABCB1, ABCB11, ABCC2, ABCC3, ABCC4, ABCC6, and ABCG2 We showed that IS increases the expression and activity of the efflux transporter P-glycoprotein (P-gp) encoded by ABCB1 in human hepatoma cells (HepG2) without modifying the expression of the other transporters. This effect depended on the aryl hydrocarbon receptor pathway. Presence of human albumin at physiologic concentration in the culture medium did not abolish the effect of IS. In two mouse models of CKD, the decline in renal function associated with the accumulation of IS in serum and the specific upregulation of Abcb1a in the liver. Additionally, among 109 heart or kidney transplant recipients with CKD, those with higher serum levels of IS needed higher doses of cyclosporin, a P-gp substrate, to obtain the cyclosporin target blood concentration. This need associated with serum levels of IS independent of renal function. These findings suggest that increased activity of P-gp could be responsible for increased hepatic cyclosporin clearance. Altogether, these results suggest that uremic toxins, such as IS, through effects on drug transporters, may modify the nonrenal clearance of drugs in patients with CKD.

Pathophysiology and therapeutics of premature ageing in chronic kidney disease, with a focus on glycative stress

Chronic kidney disease (CKD) is a major concern in public health. The pathology of CKD includes premature ageing in the kidney and vessels, which results in a high risk of cardiovascular events and end-stage renal disease. Many factors are involved in premature ageing in CKD, including hormonal imbalance, glycative stress, nitrogenous metabolites, and oxidative stress. Of these, the most important role in premature ageing in CKD is played by glycative stress, namely a massive and unfavourable glycation state, since the kidney is responsible for the clearance of advanced glycation endproducts (AGEs). In an animal model, overexpression of glyoxalase I (GLO-1), a detoxifier of AGEs, has been found to alleviate premature ageing in the kidney and vessels. Both lifestyle changes and drug therapy have shown promise in overcoming premature ageing. Promising drug therapies include a GLO-1 activator and an absorbent against glycotoxin and nitrogenous metabolites.

Microvesicles Derived from Indoxyl Sulfate Treated Endothelial Cells Induce Endothelial Progenitor Cells Dysfunction

Cardiovascular disease is a major cause of mortality in chronic kidney disease patients. Indoxyl sulfate (IS) is a typical protein-bound uremic toxin that cannot be effectively cleared by conventional dialysis. Increased IS is associated with the progression of chronic kidney disease and development of cardiovascular disease. After endothelial activation by IS, cells release endothelial microvesicles (EMV) that can induce endothelial dysfunction. We developed an in vitro model of endothelial damage mediated by IS to evaluate the functional effect of EMV on the endothelial repair process developed by endothelial progenitor cells (EPCs). EMV derived from IS-treated endothelial cells were isolated by ultracentrifugation and characterized for miRNAs content. The effects of EMV on healthy EPCs in culture were studied. We observed that IS activates endothelial cells and the generated microvesicles (IsEMV) can modulate the classic endothelial roles of progenitor cells as colony forming units and form new vessels in vitro. Moreover, 23 miRNAs were contained in IsEMV including four (miR-181a-5p, miR-4454, miR-150-5p, and hsa-let-7i-5p) that were upregulated in IsEMV compared with control endothelial microvesicles. Other authors have found that miR-181a-5p, miR-4454, and miR-150-5p are involved in promoting inflammation, apoptosis, and cellular senescence. Interestingly, we observed an increase in NFκB and p53, and a decrease in IκBα in EPCs treated with IsEMV. Our data suggest that IS is capable of inducing endothelial vesiculation with different membrane characteristics, miRNAs and other molecules, which makes maintaining of vascular homeostasis of EPCs not fully functional. These specific characteristics of EMV could be used as novel biomarkers for diagnosis and prognosis of vascular disease.

Role of uremic toxin indoxyl sulfate in the progression of cardiovascular disease

The prevalence of cardiovascular disease (CVD) among patients with chronic kidney disease (CKD) is relatively high. Deterioration of renal function in CKD leads to accumulation of indoxyl sulfate, a tryptophan metabolite produced by gut microbiota. It is acknowledged that indoxyl sulfate is capable to stimulate oxidative stress, which in turn contributes to the progression of vascular disorders and its resultant coronary artery disease. Recent research have demonstrated the adverse effects of indoxyl sulfate on the heart, together with the acceleration of vascular dysfunction, suggesting that indoxyl sulfate might contribute to high prevalence of CVD in CKD. The present mini review has focused on the potential mechanisms by which indoxyl sulfate exerts this pro-oxidant effects on the cardiovascular system. The action of indoxyl sulfate are related to multiple NADPH oxidase-mediated redox signaling pathways, which have been implicated in the pathophysiology of different forms of CVD, including chronic heart failure, arrhythmia, atherosclerotic vascular disease and coronary calcification. Future therapeutic options are discussed, including modulating gut microbial flora and blocking responsible pathophysiologic pathways.

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

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

Key Role for the Organic Anion Transporters, OAT1 and OAT3, in the in vivo Handling of Uremic Toxins and Solutes

In vitro data indicates that the kidney proximal tubule (PT) transporters of uremic toxins and solutes (e.g., indoxyl sulfate, p-cresol sulfate, kynurenine, creatinine, urate) include two “drug” transporters of the organic anion transporter (OAT) family: OAT1 (SLC22A6, originally NKT) and OAT3 (SLC22A8). Here, we have examined new and prior metabolomics data from the Oat1KO and Oat3KO, as well as newly obtained metabolomics data from a “chemical double” knockout (Oat3KO plus probenecid). This gives a picture of the in vivo roles of OAT1 and OAT3 in the regulation of the uremic solutes and supports the centrality of these “drug” transporters in independently and synergistically regulating uremic metabolism. We demonstrate a key in vivo role for OAT1 and/or OAT3 in the handling of over 35 uremic toxins and solutes, including those derived from the gut microbiome (e.g., CMPF, phenylsulfate, indole-3-acetic acid). Although it is not clear whether trimethylamine-N-oxide (TMAO) is directly transported, the Oat3KO had elevated plasma levels of TMAO, which is associated with cardiovascular morbidity in chronic kidney disease (CKD). As described in the Remote Sensing and Signaling (RSS) Hypothesis, many of these molecules are involved in interorgan and interorganismal communication, suggesting that uremia is, at least in part, a disorder of RSS.

Nutrients Turned into Toxins: Microbiota Modulation of Nutrient Properties in Chronic Kidney Disease

In chronic kidney disease (CKD), accumulation of uremic toxins is associated with an increased risk of death. Some uremic toxins are ingested with the diet, such as phosphate and star fruit-derived caramboxin. Others result from nutrient processing by gut microbiota, yielding precursors of uremic toxins or uremic toxins themselves. These nutrients include l-carnitine, choline/phosphatidylcholine, tryptophan and tyrosine, which are also sold over-the-counter as nutritional supplements. Physicians and patients alike should be aware that, in CKD patients, the use of these supplements may lead to potentially toxic effects. Unfortunately, most patients with CKD are not aware of their condition. Some of the dietary components may modify the gut microbiota, increasing the number of bacteria that process them to yield uremic toxins, such as trimethylamine N-Oxide (TMAO), p-cresyl sulfate, indoxyl sulfate and indole-3 acetic acid. Circulating levels of nutrient-derived uremic toxins are associated to increased risk of death and cardiovascular disease and there is evidence that this association may be causal. Future developments may include maneuvers to modify gut processing or absorption of these nutrients or derivatives to improve CKD patient outcomes.

Uraemia: an unrecognized driver of central neurohumoral dysfunction in chronic kidney disease?

Chronic kidney disease (CKD) carries a large cardiovascular burden in part due to hypertension and neurohumoral dysfunction - manifesting as sympathetic overactivity, baroreflex dysfunction and chronically elevated circulating vasopressin. Alterations within the central nervous system (CNS) are necessary for the expression of neurohumoral dysfunction in CKD; however, the underlying mechanisms are poorly defined. Uraemic toxins are a diverse group of compounds that accumulate as a direct result of renal disease and drive dysfunction in multiple organs, including the brain. Intensive haemodialysis improves both sympathetic overactivity and cardiac baroreflex sensitivity in renal failure patients, indicating that uraemic toxins participate in the maintenance of autonomic dysfunction in CKD. In rodents exposed to uraemia, immediate early gene expression analysis suggests upregulated activity of not only pre-sympathetic but also vasopressin-secretory nuclei. We outline several potential mechanisms by which uraemia might drive neurohumoral dysfunction in CKD. These include superoxide-dependent effects on neural activity, depletion of nitric oxide and induction of low-grade systemic inflammation. Recent evidence has highlighted superoxide production as an intermediate for the depolarizing effect of some uraemic toxins on neuronal cells. We provide preliminary data indicating augmented superoxide production within the hypothalamic paraventricular nucleus in the Lewis polycystic kidney rat, which might be important for mediating the neurohumoral dysfunction exhibited in this CKD model. We speculate that the uraemic state might serve to sensitize the central actions of other sympathoexcitatory factors, including renal afferent nerve inputs to the CNS and angiotensin II, by way of recruiting convergent superoxide-dependent and pro-inflammatory pathways.

Metabolic alterations by indoxyl sulfate in skeletal muscle induce uremic sarcopenia in chronic kidney disease

Sarcopenia is associated with increased morbidity and mortality in chronic kidney disease (CKD). Pathogenic mechanism of skeletal muscle loss in CKD, which is defined as uremic sarcopenia, remains unclear. We found that causative pathological mechanism of uremic sarcopenia is metabolic alterations by uremic toxin indoxyl sulfate. Imaging mass spectrometry revealed indoxyl sulfate accumulated in muscle tissue of a mouse model of CKD. Comprehensive metabolomics revealed that indoxyl sulfate induces metabolic alterations such as upregulation of glycolysis, including pentose phosphate pathway acceleration as antioxidative stress response, via nuclear factor (erythroid-2-related factor)-2. The altered metabolic flow to excess antioxidative response resulted in downregulation of TCA cycle and its effected mitochondrial dysfunction and ATP shortage in muscle cells. In clinical research, a significant inverse association between plasma indoxyl sulfate and skeletal muscle mass in CKD patients was observed. Our results indicate that indoxyl sulfate is a pathogenic factor for sarcopenia in CKD.

Gut microbiota in renal physiology: focus on short-chain fatty acids and their receptors

A number of recent studies have begun to explore a new and exciting area: the interaction between the gut microbiome and renal physiology. In particular, multiple studies have focused on the role of microbially produced short chain fatty acids, which are generally thought to promote health. This review will focus on what is known to date regarding the influence of the microbiome on renal function, with emphasis on the cell biology, physiology, and clinical implications of short chain fatty acids and short chain fatty acid receptors. It is clear that microbe-host interactions are an exciting and ever-expanding field, which has implications for how we view diseases such as hypertension, acute kidney injury, and chronic kidney disease. However, it is important to recognize that although the potential promise of this area is extremely enticing, we are only the very edge of this new field.

Low-protein diets for chronic kidney disease patients: the Italian experience

BACKGROUND

Nutritional treatment has always represented a major feature of CKD management. Over the decades, the use of nutritional treatment in CKD patients has been marked by several goals. The first of these include the attainment of metabolic and fluid control together with the prevention and correction of signs, symptoms and complications of advanced CKD. The aim of this first stage is the prevention of malnutrition and a delay in the commencement of dialysis. Subsequently, nutritional manipulations have also been applied in association with other therapeutic interventions in an attempt to control several cardiovascular risk factors associated with CKD and to improve the patient's overall outcome. Over time and in reference to multiple aims, the modalities of nutritional treatment have been focused not only on protein intake but also on other nutrients.

DISCUSSION

This paper describes the pathophysiological basis and rationale of nutritional treatment in CKD and also provides a report on extensive experience in the field of renal diets in Italy, with special attention given to approaches in clinical practice and management. Italian nephrologists have a longstanding tradition in implementing low protein diets in the treatment of CKD patients, with the principle objective of alleviating uremic symptoms, improving nutritional status and also a possibility of slowing down the progression of CKD or delaying the start of dialysis. A renewed interest in this field is based on the aim of implementing a wider nutritional therapy other than only reducing the protein intake, paying careful attention to factors such as energy intake, the quality of proteins and phosphate and sodium intakes, making today's low-protein diet program much more ambitious than previous. The motivation was the reduction in progression of renal insufficiency through reduction of proteinuria, a better control of blood pressure values and also through correction of metabolic acidosis. One major goal of the flexible and innovative Italian approach to the low-protein diet in CKD patients is the improvement of patient adherence, a crucial factor in the successful implementation of a low-protein diet program.