Renocardiac Effects of p-Cresyl Sulfate Administration in Acute Kidney Injury Induced by Unilateral Ischemia and Reperfusion Injury In Vivo

The precise mechanisms underlying the cardiovascular complications due to acute kidney injury (AKI) and the retention of uremic toxins like p-cresyl sulfate (PCS) remain incompletely understood. The objective of this study was to evaluate the renocardiac effects of PCS administration in animals subjected to AKI induced by ischemia and reperfusion (IR) injury. C57BL6 mice were subjected to distinct protocols: (i) administration with PCS (20, 40, or 60 mg/L/day) for 15 days and (ii) AKI due to unilateral IR injury associated with PCS administration for 15 days. The 20 mg/L dose of PCS led to a decrease in renal mass, an increase in the gene expression of Cystatin C and kidney injury molecule 1 (KIM-1), and a decrease in the α-actin in the heart. During AKI, PCS increased the renal injury biomarkers compared to control; however, it did not exacerbate these markers. Furthermore, PCS did not enhance the cardiac hypertrophy observed after 15 days of IR. An increase, but not potentialized, in the cardiac levels of interleukin (IL)-1β and IL-6 in the IR group treated with PCS, as well as in the injured kidney, was also noticed. In short, PCS administration did not intensify kidney injury, inflammation, and cardiac outcomes after AKI.

4-Ethylphenol-fluxes, metabolism and excretion of a gut microbiome derived neuromodulator implicated in autism

Gut-microbiome-derived metabolites, such as 4-Ethylphenol [4EP], have been shown to modulate neurological health and function. Although the source of such metabolites is becoming better understood, knowledge gaps remain as to the mechanisms by which they enter host circulation, how they are transported in the body, how they are metabolised and excreted, and the way they exert their effects. High blood concentrations of host-modified 4EP, 4-ethylphenol sulfate [4EPS], are associated with an anxiety phenotype in autistic individuals. We have reviewed the existing literature and discuss mechanisms that are proposed to contribute influx from the gut microbiome, metabolism, and excretion of 4EP. We note that increased intestinal permeability is common in autistic individuals, potentially explaining increased flux of 4EP and/or 4EPS across the gut epithelium and the Blood Brain Barrier [BBB]. Similarly, kidney dysfunction, another complication observed in autistic individuals, impacts clearance of 4EP and its derivatives from circulation. Evidence indicates that accumulation of 4EPS in the brain of mice affects connectivity between subregions, particularly those linked to anxiety. However, we found no data on the presence or quantity of 4EP and/or 4EPS in human brains, irrespective of neurological status, likely due to challenges sampling this organ. We argue that the penetrative ability of 4EP is dependent on its form at the BBB and its physicochemical similarity to endogenous metabolites with dedicated active transport mechanisms across the BBB. We conclude that future research should focus on physical (e.g., ingestion of sorbents) or metabolic mechanisms (e.g., conversion to 4EP-glucuronide) that are capable of being used as interventions to reduce the flux of 4EP from the gut into the body, increase the efflux of 4EP and/or 4EPS from the brain, or increase excretion from the kidneys as a means of addressing the neurological impacts of 4EP.

Microbial metabolite p-cresol inhibits gut hormone expression and regulates small intestinal transit in mice

p-cresol is a metabolite produced by microbial metabolism of aromatic amino acid tyrosine. p-cresol and its conjugated forms, p-cresyl sulfate and p-cresyl glucuronide, are uremic toxins that correlate positively with chronic kidney disease and diabetes pathogenesis. However, how p-cresol affects gut hormones is unclear. Here, we expose immortalized GLUTag cells to increasing concentrations of p-cresol and found that p-cresol inhibited Gcg expression and reduced glucagon-like peptide-1 (GLP-1) secretion in vitro. In mice, administration of p-cresol in the drinking water for 2 weeks reduced the transcript levels of Gcg and other gut hormones in the colon; however, it did not affect either fasting or glucose-induced plasma GLP-1 levels. Furthermore, it did not affect glucose tolerance but promoted faster small intestinal transit in mice. Overall, our data suggest that microbial metabolite p-cresol suppresses transcript levels of gut hormones and regulates small intestinal transit in mice.

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.

3-Carboxy-4-methyl-5-propyl-2-furanpropanoic acid (CMPF) induces cell death through ferroptosis and acts as a trigger of apoptosis in kidney cells

Ferroptosis is a cell death mechanism characterized by intracellular iron accumulation and lipid peroxidation. Effects of uremic toxins on ferroptosis in the kidney are not well understood. We investigated whether protein-bound uremic toxins induce ferroptosis, resulting in cell death, using the bilateral ureteral obstruction (BUO) mouse model and kidney cells. In BUO mice, we observed elevated lipid peroxidation, increased iron concentration, and decreased glutathione peroxidase 4 (GPX4) expression. Levels of transferrin receptor 1 and system Xc-, which are involved in iron transport and storage, were also elevated, while those of ferritin heavy and light chains (FHC and FLC) were reduced. Treatment of HK-2 and NRK49F kidney cells with CMPF decreased GSH levels and the expression of GPX4, FHC, and FLC, and increased levels of ROS, lipid peroxidation, and intracellular iron concentration. CMPF-induced and erastin-induced decreases in GPX4 levels and increases in Bax and cytochrome C levels were counteracted by ferrostatin-1 pretreatment. However, GPX4 mRNA levels, protein abundance, or promoter activity were not restored by Z-VAD-FMK, a multi-caspase inhibitor. These results suggest that ferroptosis induced by CMPF treatment induces apoptosis, and inhibition of ferroptosis reduces apoptosis, suggesting that ferroptosis plays a role in triggering cell death by apoptosis.

Rapid and sustainable HPLC method for the determination of uremic toxins in human plasma samples

Protein-bound uremic toxins, mainly indoxyl sulfate (3-INDS), p-cresol sulfate (pCS), and indole-3-acetic acid (3-IAA) but also phenol (Pol) and p-cresol (pC), are progressively accumulated during chronic kidney disease (CKD). Their accurate measurement in biomatrices is demanded for timely diagnosis and adoption of appropriate therapeutic measures. Multianalyte methods allowing the establishment of a uremic metabolite profile are still missing. Hence, the aim of this work was to develop a rapid and sensitive method based on high-performance liquid chromatography with fluorescence detection for the simultaneous quantification of Pol, 3-IAA, pC, 3-INDS, and pCS in human plasma. Separation was attained in 12 min, using a monolithic C18 column and isocratic elution with acetonitrile and phosphate buffer containing an ion-pairing reagent, at a flow rate of 2 mL min^-1. Standards were prepared in plasma and quantification was performed using the background subtraction approach. LOQ values were ≤ 0.2 µg mL^-1 for all analytes except for pCS (LOQ of 2 µg mL^-1). The method proved to be accurate (93.5-112%) and precise (CV ≤ 14.3%). The multianalyte application of the method, associated to a reduced sample volume (50 µL), a less toxic internal standard (eugenol) in comparison to the previously applied 2,6-dimethylphenol and 4-ethylphenol, and a green extraction solvent (ethanol), resulted in the AGREE score of 0.62 which is in line with the recent trend of green and sustainable analytical chemistry. The validated method was successfully applied to the analysis of plasma samples from control subjects exhibiting normal levels of uremic toxins and CKD patients presenting significantly higher levels of 3-IAA, pC, 3-INDS, and pCS that can be further investigated as biomarkers of disease progression.

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.

A host-gut microbial amino acid co-metabolite, p-cresol glucuronide, promotes blood-brain barrier integrity in vivo

The sequential activity of gut microbial and host processes can exert a powerful modulatory influence on dietary components, as exemplified by the metabolism of the amino acids tyrosine and phenylalanine to p-cresol by gut microbes, and then to p-cresol glucuronide (pCG) by host enzymes. Although such glucuronide conjugates are classically thought to be biologically inert, there is accumulating evidence that this may not always be the case. We investigated the activity of pCG, studying its interactions with the cerebral vasculature and the brain in vitro and in vivo. Male C57Bl/6 J mice were used to assess blood-brain barrier (BBB) permeability and whole-brain transcriptomic changes in response to pCG treatment. Effects were then further explored using the human cerebromicrovascular endothelial cell line hCMEC/D3, assessing paracellular permeability, transendothelial electrical resistance and barrier protein expression. Mice exposed to pCG showed reduced BBB permeability and significant changes in whole-brain transcriptome expression. Surprisingly, treatment of hCMEC/D3 cells with pCG had no notable effects until co-administered with bacterial lipopolysaccharide, at which point it was able to prevent the permeabilizing effects of endotoxin. Further analysis suggested that pCG acts as an antagonist at the principal lipopolysaccharide receptor TLR4. The amino acid phase II metabolic product pCG is biologically active at the BBB, antagonizing the effects of constitutively circulating lipopolysaccharide. These data add to the growing literature showing glucuronide conjugates to be more than merely metabolic waste products and highlight the complexity of gut microbe to host communication pathways underlying the gut-brain axis.

The Role of Gut-Derived, Protein-Bound Uremic Toxins in the Cardiovascular Complications of Acute Kidney Injury

Acute kidney injury (AKI) is a frequent disease encountered in the hospital, with a higher incidence in intensive care units. Despite progress in renal replacement therapy, AKI is still associated with early and late complications, especially cardiovascular events and mortality. The role of gut-derived protein-bound uremic toxins (PBUTs) in vascular and cardiac dysfunction has been extensively studied during chronic kidney disease (CKD), in particular, that of indoxyl sulfate (IS), para-cresyl sulfate (PCS), and indole-3-acetic acid (IAA), resulting in both experimental and clinical evidence. PBUTs, which accumulate when the excretory function of the kidneys is impaired, have a deleterious effect on and cause damage to cardiovascular tissues. However, the link between PBUTs and the cardiovascular complications of AKI and the pathophysiological mechanisms potentially involved are unclear. This review aims to summarize available data concerning the participation of PBUTs in the early and late cardiovascular complications of AKI.

High-volume hemodiafiltration decreases the pre-dialysis concentrations of indoxyl sulfate and p-cresyl sulfate compared to hemodialysis: a post-hoc analysis from the HDFit randomized controlled trial

BACKGROUND

Although high-volume online hemodiafiltration has been associated with higher clearance and lower pre-dialysis concentration of middle molecular weight toxins compared to hemodialysis, its effect on protein-bound uremic toxins has shown inconclusive results. In this study, we investigated whether hemodiafiltration impacts pre-dialysis plasma levels of the toxins indoxyl sulfate, p-cresyl sulfate, and indole-3-acetic acid compared to high-flux hemodialysis.

METHODS

This is a post-hoc analysis of the multicenter, randomized controlled trial HDFit (ClinicalTrials.gov: NCT02787161). Uremic toxins were determined by high performance liquid chromatography at baseline, 3, and 6 months. Mean differences in monthly changes of pre-dialysis uremic toxin concentrations between hemodiafiltration and high-flux hemodialysis were analyzed using linear mixed-effect models.

RESULTS

One hundred ninety-three patients (mean age 53 years old, 71% males) were analyzed. There were no differences between groups regarding clinical and biochemical characteristics at baseline or duration of dialysis session and blood flows throughout the follow-up. Mean differences in rates of change (μM/month, [confidence interval CI]) in high-flux hemodialysis vs. hemodiafiltration were 2.4 [0.3 to 4.56], 3.94 [- 1.54 to 9.41] and 0.06 [- 0.6 to 0.5] for indoxyl sulfate, p-cresyl sulfate and indole-3-acetic acid, respectively. In the exploratory analysis, these differences in high-flux hemodialysis vs. hemodiafiltration subgroup with convective volume > 27.5 L were 2.86 [0.43 to 5.28], 7.43 [0.7 to 14.16] and - 0.19 [- 0.88 to 0.50].

CONCLUSION

These exploratory findings suggest that hemodiafiltration is more effective in reducing indoxyl sulfate as compared to standard high-flux hemodialysis, and also that this effect was extended to p-cresyl sulfate in patients achieving higher convective volumes.

Can resveratrol supplementation reduce uremic toxins plasma levels from the gut microbiota in non-dialyzed chronic kidney disease patients?

BACKGROUND

Uremic toxins such as indoxyl sulfate (IS), p-cresyl sulfate (pCS), and indole-3-acetic acid (IAA) produced by the gut microbiota are recognized as risk factors for many comorbidities, including cardiovascular diseases. Chronic kidney disease (CKD) patients have an accumulation of these toxins and nutritional strategies have been proposed to mitigate gut dysbiosis and, consequently, reduce these toxins. This study aimed to evaluate the effects of resveratrol supplementation on the plasma levels of IS, pCS, and IAA in non-dialyzed CKD patients.

METHODS

In this placebo-controlled crossover study, twenty non-dialyzed patients were randomly divided into two groups: they received either one capsule/day containing 500 mg of trans-resveratrol (63 ± 7.5 years, glomerular filtration ratio (GFR): 34 ± 14 mL/min, body mass index (BMI): 26.8 ± 5.6 kg/m²) or a placebo containing 500 mg wheat flour (62 ± 8.4 years, GFR: 34 ± 13 mL/min, BMI: 28.6 ± 4.4 kg/m²) during four weeks. After eight weeks of washout (no supplementation), another four weeks of supplementation with crossover was initiated. IS, IAA, and pCS plasma levels were quantified by the Reverse Phase High-Efficiency Liquid Chromatography method with fluorescent detection. The mRNA expression of nuclear factor erythroid 2-related factor 2 (Nrf2) and nuclear factor kappa B (NF-κB) in peripheral blood mononuclear cells was evaluated by polymerase chain reaction. C-reactive protein (CRP) plasma levels were also evaluated.

RESULTS

As expected, the uremic toxins levels were negatively correlated with GFR, but no effect of trans-resveratrol supplementation was found on levels of IS, IAA, and pCS. There was a positive correlation between IS and Nrf2 (r = 0.24, p = 0.03) and also between IS and CRP (r = 0.21, p = 0.05).

CONCLUSION

Supplementation with trans-resveratrol did not reduce the plasma levels of IS, pCS, and IAA in non-dialyzed CKD patients. The interactions among uremic toxins and anti- and pro-inflammatory pathways deserve more studies.

Classification of Uremic Toxins and Their Role in Kidney Failure

Advances in our understanding of uremic retention solutes, and improvements in hemodialysis membranes and other techniques designed to remove uremic retention solutes, offer opportunities to readdress the definition and classification of uremic toxins. A consensus conference was held to develop recommendations for an updated definition and classification scheme on the basis of a holistic approach that incorporates physicochemical characteristics and dialytic removal patterns of uremic retention solutes and their linkage to clinical symptoms and outcomes. The major focus is on the removal of uremic retention solutes by hemodialysis. The identification of representative biomarkers for different classes of uremic retention solutes and their correlation to clinical symptoms and outcomes may facilitate personalized and targeted dialysis prescriptions to improve quality of life, morbidity, and mortality. Recommendations for areas of future research were also formulated, aimed at improving understanding of uremic solutes and improving outcomes in patients with CKD.

Dietary strategies for gut-derived protein-bound uremic toxins and cardio-metabolic risk factors in chronic kidney disease: A focus on dietary fibers

Chronic kidney disease (CKD) is associated with altered composition and function of gut microbiota. The cause of gut dysbiosis in CKD is multifactorial and encompasses the following: uremic state, metabolic acidosis, slow colonic transit, dietary restrictions of plant-based fiber-rich foods, and pharmacological therapies. Dietary restriction of potassium-rich fruits and vegetables, which are common sources of fermentable dietary fibers, inhibits the conversion of dietary fibers to short-chain fatty acids (SCFA), which are the primary nutrient source for the symbiotic gut microbiota. Reduced consumption of fermentable dietary fibers limits the population of SCFA-forming bacteria and causes dysbiosis of gut microbiota. Gut dysbiosis induces colonic fermentation of protein and formation of gut-derived uremic toxins. In this review, we discuss the roles and benefits of dietary fiber on gut-derived protein-bound uremic toxins and plant-based dietary patterns that could be recommended to decrease uremic toxin formation in CKD patients. Recent studies have indicated that dietary fiber supplementation may be useful to decrease gut-derived uremic toxin formation and slow CKD progression. However, research on associations between adherence of healthy dietary patterns and gut-derived uremic toxins formation in patients with CKD is lacking.

Simple Determination of p-Cresol in Plasma Samples Using Fluorescence Spectroscopy Technique

The development of simple, fast, cheap and reliable analytical methods for tracing biological indicators is demanded through clinical investigations. Herein, we developed, for the first time, a cheap and specific method for the extraction and quantification of p-cresol (pC) in real plasma samples of chronic kidney disease (CKD). Plasma samples were prepared by hydrolyzing in an acidic medium to convert pCS (p-cresol sulfate) and p-Cresol glucuronide (pCG) to pC. Next, proteins of plasma samples were precipitated and then pC was extracted by acetonitrile (ACN) and saturated NaCl (as salting-out agent). Finally, fluorescence emissions were measured at λ_ex/λ_em = 280/310 nm. The specificity of the method was checked by testing various possible interfering agents. The obtained results revealed a specific determination of pC. Under optimal conditions, a linear range was detected from 0.5 to 30 µg/mL of pC with a lower limit of detection (LLOQ) of 0.5 µg/mL. The reliability of the method was checked by calculating the repeatability, selectivity, and accuracy of the developed method for pC determination in plasma samples. The application of the developed method was investigated for the detection of pC in a number of CKD patients. Due to the simplicity and selectivity, the developed method could be applied for routine analysis of pC concentrations in the plasma samples of CKD patients. In addition, the developed method showed great potential for developing a point-of-care testing (POCT) device.

Is there an association between the plasma levels of uremic toxins from gut microbiota and anemia in patients on hemodialysis?

BACKGROUND

Anemia is one of the most frequent complications in patients with chronic kidney disease (CKD). Despite being multifactorial, the relative or absolute deficiency of erythropoietin production is the leading cause. Recent studies have shown that uremic toxins produced by the gut microbiota also may play a role in the genesis of anemia in these patients.

OBJECTIVE

To evaluate the possible association between uremic toxins plasma levels and anemia in patients with CKD on hemodialysis (HD).

METHODS

This cross-sectional study evaluated one hundred fifty-four patients (53.2% men, 51.2 ± 11.2 years, hemoglobin (Hb) levels of 11.2 ± 1.6 g/dL). Biochemical variables such as urea, creatinine, hemoglobin, hematocrit, were measured according to standard methods and uremic toxins such as indoxyl sulfate (IS), indole-3-acetic acid (IAA), p-cresyl sulfate (p-CS) plasma levels were measured by reverse-phase high-performance liquid chromatography (RP-HPLC).

RESULTS

The levels of uremic toxins such as IS, IAA, p-CS were increased in all patients. However, no correlation was found between uremic toxins plasma levels and anemia parameters. Only patients with Hb < 11 g/dL presented a negative correlation between hematocrit and IAA plasma levels.

CONCLUSION

There is no strong evidence that uremic toxins produced by the gut microbiota may be associated with anemia in patients with CKD on HD.

The complexity of sleep disorders in dialysis patients

Background

Dialysis patients experience a high burden of physical and emotional symptoms directly affecting their sleep and quality of life. In this study, objective and subjective measurements to quantify sleep were performed, compared with those of healthy controls, and associated with burden of comorbidity and uraemic toxicity.

Methods

A total of 64 dialysis patients were included—10 peritoneal dialysis, 42 in-centre daytime haemodialysis (HD) and 12 in-centre nocturnal HD patients—as well as one-to-one age- and gender-matched healthy controls. Assumed and actual sleep time, sleep efficiency and fragmentation index were measured by actigraphy for at least two consecutive nights. Patients and controls also completed Insomnia Severity Index (ISI) and Pittsburgh Sleep Quality Index (PSQI) questionnaires. The patients’ blood was sampled to determine concentrations of a representative series of uraemic toxins and the Davies–Stoke comorbidity index was derived from medical records.

Results

Apart from the assumed sleep time, all objectively and subjectively measured sleep parameters were worse in the dialysis group compared with the healthy controls. No differences were seen in any of the measured sleep parameters among the different dialysis groups. None of the objectively measured sleep parameters were associated with ISI or PSQI scores in dialysis patients, while sleep times were related to the subjective scores in the healthy cohort. Objectively assessed sleep parameters were associated to neither the uraemic toxicity load nor the Davies–Stoke score.

Conclusions

Independent of the modality, dialysis patients have sleep quality much worse than age- and gender-matched healthy controls. The objectively measured sleep parameters could not be associated to the subjective score, uraemic toxicity or comorbidity score, highlighting the need for objective measurements of sleep and clinical guidelines to aid patient management.

Effects of the L-tyrosine-derived bacterial metabolite p-cresol on colonic and peripheral cells

Specific families of bacteria present within the intestinal luminal content produce p-cresol from L-tyrosine. Although the hosts do not synthesize p-cresol, they can metabolize this compound within their colonic mucosa and liver leading to the production of co-metabolites including p-cresyl sulfate (p-CS) and p-cresyl glucuronide (p-CG). p-Cresol and its co-metabolites are recovered in the circulation mainly conjugated to albumin, but also in their free forms that are excreted in the urine. An increased dietary protein intake raises the amount of p-cresol recovered in the feces and urine, while fecal excretion of p-cresol is diminished by a diet containing undigestible polysaccharides. p-Cresol in excess is genotoxic for colonocytes. In addition, in these cells, this bacterial metabolite decreases mitochondrial oxygen consumption, while increasing the anion superoxide production. In chronic kidney disease (CKD), marked accumulation of p-cresol and p-CS in plasma is measured, and in renal tubular cells, p-cresol and p-CS increase oxidative stress, affect mitochondrial function, and lead to cell death, strongly suggesting that these 2 compounds act as uremic toxins that aggravate CKD progression. p-Cresol and p-CS are also suspected to play a role in the CKD-associated adverse cardiovascular events, since they affect endothelial cell proliferation and migration, decrease the capacity of endothelial wound repair, and increase the senescence of endothelial cells. Finally, the fact that concentration of p-cresol is transiently increased in young autistic children biological fluids, and that intraperitoneal injection of p-cresol in animal models induces some behavioral characteristics observed in the autism spectrum disorders (ASD), raise the view that p-cresol may possibly represent one of the components involved in ASD etiology. Further pre-clinical and clinical studies are obviously needed to determine if the lowering of p-cresol and/or p-CS circulating concentrations, by dietary and/or pharmacological means, would allow, by itself or in combination with other interventions, to improve CKD progression and associated cardiovascular outcomes, as well as some neurological outcomes in children with an early diagnosis of autism.

Characterizations of Human UDP-Glucuronosyltransferase Enzymes in the Conjugation of p-Cresol

p-Cresol is a uremic toxin that is formed by intestinal microbiota and extensively conjugated by first-pass metabolism. p-Cresol glucuronide exerts various forms of cellular toxicity in vitro and is accumulated in the plasma of subjects with kidney disease, where associations with adverse cardiovascular and renal outcomes are evident. The objective of this study was to determine the contributions of human UDP-glucuronosyltransferase (UGT) enzymes in the formation of p-cresol glucuronide. Utilizing commonly expressed hepatic or renal human recombinant UGTs (ie, hrUGT1A1, 1A3, 1A4, 1A6, 1A7, 1A8, 1A9, 1A10, 2B4, 2B7, 2B10, 2B15, and 2B17), hrUGT1A6 and hrUGT1A9 exhibited the highest catalytic activities in the generation of p-cresol glucuronide. The kinetics of p-cresol glucuronide formation in hrUGT1A6 and pooled human liver microsomes were best described by the Hill equation and in hrUGT1A9 and pooled human kidney microsomes by substrate inhibition. Using inhibitory and selective UGT inhibitors (ie, acetaminophen or amentoflavone for UGT1A6 and niflumic acid for UGT1A9), UGT1A6 was identified the predominant enzyme responsible for p-cresol glucuronide production in pooled human liver (78.4%-81.3% contribution) and kidney (54.3%-62.9%) microsomes, whereas UGT1A9 provided minor contributions (2.8% and 35.5%, respectively). The relative contributions of UGT1A6 (72.6 ± 11.3%, mean ± SD) and UGT1A9 (5.7 ± 4.1%) in individual human liver microsomes from 12 adult donors were highly variable, where an inverse association (R = -.784, p = .003) between UGT1A6 contribution and UGT1A9 probe substrate activity (ie, mycophenolic acid) was evident. Our novel findings provide valuable tools for conducting further mechanistic studies and for designing clinical interventions to mitigate the toxicities associated with p-cresol glucuronide.

Uremic toxins levels from the gut microbiota seem not to be altered by physical exercise in hemodialysis patients

PURPOSE

Regular physical exercise may result in many benefits to patients with chronic kidney disease (CKD) on hemodialysis (HD), including gut microbiota modulation and solute removal. The study aimed to evaluate the effects of two programs of intradialytic exercises on uremic toxins plasma levels in HD patients.

METHODS

In experiment 1, twenty HD patients [12 men, 44.1 ± 8.9 years, BMI of 23.4 ± 2.4 kg/m²] were randomized into two groups: Aerobic exercise group (AEG, n = 11) that performed aerobic exercise on an adapted exercise bike three times a week for three months (36 sessions) and Control group (CG, n = 9). In experiment 2, twenty-six HD patients [19 men, 47.6 ± 11.0 years, BMI of 25.9 ± 3.6 kg/m²] were randomized into Resistance exercise group (REG, n = 14) that performed a resistance exercise program (using elastic bands and ankle cuffs with both lower limbs) monitored three times a week, during six months (72 sessions) and CG (n = 12). P-cresyl sulfate (p-CS), indoxyl sulfate (IS), and indol-3-acetic acid (IAA) plasma levels were determined by high-performance liquid chromatography (HPLC) with fluorescent detection.

RESULTS

The uremic toxins plasma levels did not reduce in both exercise programs, aerobic exercise (IS: 32.7 ± 14.0 vs 33.0 ± 15.4 mg/L, p = 0.86; p-CS: 59.9 ± 39.3 vs 60.0 ± 41.2 mg/L, p = 0.99; IAA: 2233 [1488-2848] vs 2227 [1275-2824] µg/L, p = 0.72) and resistance exercise (IS: 28.3 ± 11.3 vs 29.1 ± 9.7 mg/L, p = 0.77; p-CS: 31.4 ± 21.3 vs 34.2 ± 19.8 mg/L, p = 0.63; IAA: 1628 [1330-3530] vs 2000 [971-3085] µg/L, p = 0.35) in HD patients.

CONCLUSION

According to our findings, physical exercise does not appear to alter the levels of uremic toxins produced by the gut microbiota in HD patients.

Effects of p-Cresol on Oxidative Stress, Glutathione Depletion, and Necrosis in HepaRG Cells: Comparisons to Other Uremic Toxins and the Role of p-Cresol Glucuronide Formation

The toxicological effects of p-cresol have primarily been attributed to its metabolism products; however, very little human data are available in the key organ (i.e., liver) responsible for the generation of these metabolites. Experiments were conducted in HepaRG cells utilizing the following markers of cellular toxicity: 2′-7′-dichlorofluorescein (DCF; oxidative stress) formation, total cellular glutathione (GSH) concentration, and lactate dehydrogenase (LDH; cellular necrosis) release. Concentrations of p-cresol, p-cresol sulfate, and p-cresol glucuronide were determined using validated assays. p-Cresol exposure resulted in concentration- and time-dependent changes in DCF (EC₅₀ = 0.64 ± 0.37 mM at 24 h of exposure) formation, GSH (EC₅₀ = 1.00 ± 0.07 mM) concentration, and LDH (EC₅₀ = 0.85 ± 0.14 mM) release at toxicologically relevant conditions. p-Cresol was also relatively more toxic than 3-carboxy-4-methyl-5-propyl-2-furanpropanoic acid, indole-3-acetic acid, indoxyl sulfate, kynurenic acid, and hippuric acid on all markers. Although the exogenous administration of p-cresol sulfate and p-cresol glucuronide generated high intracellular concentrations of these metabolites, both metabolites were less toxic compared to p-cresol at equal-molar conditions. Moreover, p-cresol glucuronide was the predominant metabolite generated in situ from p-cresol exposure. Selective attenuation of glucuronidation (without affecting p-cresol sulfate formation, while increasing p-cresol accumulation) using independent chemical inhibitors (i.e., 0.75 mM l-borneol, 75 µM amentoflavone, or 100 µM diclofenac) consistently resulted in further increases in LDH release associated with p-cresol exposure (by 28.3 ± 5.3%, 30.0 ± 8.2% or 27.3 ± 6.8%, respectively, compared to p-cresol treatment). These novel data indicated that p-cresol was a relatively potent toxicant, and that glucuronidation was unlikely to be associated with the manifestation of its toxic effects in HepaRG cells.

Free p-cresyl sulfate shows the highest association with cardiovascular outcome in chronic kidney disease

Background

Several protein-bound uraemic toxins (PBUTs) have been associated with cardiovascular (CV) and all-cause mortality in chronic kidney disease (CKD) but the degree to which this is the case per individual PBUT and the pathophysiological mechanism have only partially been unraveled.

Methods

We compared the prognostic value of both total and free concentrations of five PBUTs [p-cresyl sulfate (pCS), p-cresyl glucuronide, indoxyl sulfate, indole acetic acid and hippuric acid] in a cohort of 523 patients with non-dialysis CKD Stages G1–G5. Patients were followed prospectively for the occurrence of a fatal or non-fatal CV event as the primary endpoint and a number of other major complications as secondary endpoints. In addition, association with and the prognostic value of nine markers of endothelial activation/damage was compared.

Results

After a median follow-up of 5.5 years, 149 patients developed the primary endpoint. In multivariate Cox regression models adjusted for age, sex, systolic blood pressure, diabetes mellitus and estimated glomerular filtration rate, and corrected for multiple testing, only free pCS was associated with the primary endpoint {hazard ratio [HR]1.39 [95% confidence interval (CI) 1.14–1.71]; P = 0.0014}. Free pCS also correlated with a disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13 (r = −0.114, P &lt; 0.05), angiopoietin-2 (ANGPT2) (r = 0.194, P &lt; 0.001), matrix metallopeptidase 7 (MMP-7; (r = 0.238, P &lt; 0.001) and syndecan 1 (r = 0.235, P &lt; 0.001). Of these markers of endothelial activation/damage, ANGPT2 [HR 1.46 (95% CI 1.25–1.70); P &lt; 0.0001] and MMP-7 [HR 1.31 (95% CI 1.08–1.59); P = 0.0056] were also predictive of the primary outcome.

Conclusions

Among PBUTs, free pCS shows the highest association with CV outcome in non-dialysed patients with CKD. Two markers of endothelial activation/damage that were significantly correlated with free pCS, ANGPT2 and MMP-7 were also associated with CV outcome. The hypothesis that free pCS exerts its CV toxic effects by an adverse effect on endothelial function deserves further exploration.

Can curcumin supplementation reduce plasma levels of gut-derived uremic toxins in hemodialysis patients? A pilot randomized, double-blind, controlled study

BACKGROUND

Gut dysbiosis is common in patients with chronic kidney disease (CKD) and is closely related to inflammatory processes. Some nutritional strategies, such as bioactive compounds present in curcumin, have been proposed as an option to modulate the gut microbiota and decrease the production of uremic toxins such as indoxyl sulfate (IS), p-cresyl sulfate (pCS) and indole-3 acetic acid (IAA).

OBJECTIVE

To evaluate the effects of curcumin supplementation on uremic toxins plasma levels produced by gut microbiota in patients with CKD on hemodialysis (HD).

METHODS

Randomized, double-blind trial in 28 patients [53.6 ± 13.4 years, fourteen men, BMI 26.7 ± 3.7 kg/m², dialysis vintage 37.5 (12-193) months]. Fourteen patients were randomly allocated to the curcumin group and received 100 mL of orange juice with 12 g carrot and 2.5 g of turmeric and 14 patients to the control group who received the same juice but without turmeric three times per week after HD sessions for three months. IS, pCS, IAA plasma levels were measured by reverse-phase high-performance liquid chromatography RESULTS: After three months of supplementation, the curcumin group showed a significant decrease in pCS plasma levels [from 32.4 (22.1-45.9) to 25.2 (17.9-37.9) mg/L, p = 0.009], which did not occur in the control group. No statistical difference was observed in IS and IAA levels in both groups.

CONCLUSION

The oral supplementation of curcumin for three months seems to reduce p-CS plasma levels in HD patients, suggesting a gut microbiota modulation.

Pilot Study of Probiotic Supplementation on Uremic Toxicity and Inflammatory Cytokines in Chronic Kidney Patients

Background:

Bacterial metabolism contributes to the generation of uremic toxins in patients with chronic kidney disease (CKD). It has been investigated the use of probiotics in the reduction of uremic toxins intestinal production.

Objective:

The aim of this pilot study was to evaluate the effect of probiotic supplementation on reducing the production of uremic toxins and the inflammatory profile of CKD patients.

Methods:

We performed a randomized, blind, placebo-controlled, crossover study on patients with CKD stages 3 and 4. The intervention was a probiotic formulation composed of Lactobacillus acidophilus strains given orally three times a day for 3 months. Changes in uremic toxins (p-Cresylsulfate and Indoxyl Sulfate) and serum inflammatory cytokines were the primary endpoints.

Results:

Of the 44 patients randomized, 25 completed the study (mean age 51 ± 9.34, 64% female, mean eGFR 36 ± 14.26 mL/min/1.73m², mean BMI 28.5 ± 5.75 kg/m²). At 3 months, there were no significant changes in any of the studied biomarkers including p-cresylsulfate (p = 0.57), Indoxyl sulfate (p = 0.08) and interleukin-6 (p = 0.55).

Conclusion:

Lactobacillus acidophilus strains given as probiotic were not able to reduce serum levels of uremic toxins and biomarkers of inflammation in CKD patients in stage 3 and 4.

Gut-Derived Metabolites and Their Role in Immune Dysfunction in Chronic Kidney Disease

Several of the uremic toxins, which are difficult to remove by dialysis, originate from the gut bacterial metabolism. This opens opportunities for novel targets trying to decrease circulating levels of these toxins and their pathophysiological effects. The current review focuses on immunomodulatory effects of these toxins both at their side of origin and in the circulation. In the gut end products of the bacterial metabolism such as p-cresol, trimethylamine and H₂S affect the intestinal barrier structure and function while in the circulation the related uremic toxins stimulate cells of the immune system. Both conditions contribute to the pro-inflammatory status of patients with chronic kidney disease (CKD). Generation and/or absorption of these toxin precursors could be targeted to decrease plasma levels of their respective uremic toxins and to reduce micro-inflammation in CKD.

Composite Uremic Load and Physical Performance in Hemodialysis Patients: A Cross-Sectional Study

Impaired physical performance is common in patients on hemodialysis (HD) and is associated with poor prognosis. A patient relevant marker of adequacy of dialysis is lacking. Previous studies evaluated uremic toxicity by assessing the impact of different uremic toxins separately. However, such an approach is most likely not reflective of true uremic toxicity. Therefore, this cross-sectional study aimed to examine if the uremic syndrome, estimated as one composite of different uremic toxins (facilitated by ridge regression method) to reflect the kinetic behavior during dialysis, is associated with physical performance in patients on HD. Levels of p-cresyl glucuronide and sulfate, indole-acetic acid, indoxyl sulfate, uric acid, hippuric acid, and 3-carboxy-4-methyl-5-propyl-2-furanpropionic acid were assessed and associated by ridge regression to muscle strength, functional exercise capacity, and measures of balance and coordination. 75 HD patients were included (mean age 68 years, 57% male). The composite of different uremic toxins (i.e., uremic load) explained 22% of the variance in handgrip strength. Although there was an association between full body muscle strength and the composite uremic load independent of nutritional status, age and gender, the predictive power of composite uremic load for muscle weakness is limited. Single uremic toxins as well as composite uremic load were not associated with exercise capacity, coordination, and balance, indicating that the degree of uremia does not predict physical performance in patients on HD.

Gut microbiota generation of protein-bound uremic toxins and related metabolites is not altered at different stages of chronic kidney disease

Chronic kidney disease (CKD) is characterized by accumulation of protein-bound uremic toxins such as p-cresyl sulfate, p-cresyl glucuronide, indoxyl sulfate and indole-3-acetic acid, which originate in the gut. Intestinal bacteria metabolize aromatic amino acids into p-cresol and indole, (further conjugated in the colon mucosa and liver) and indole-3-acetic acid. Here we measured fecal, plasma and urine metabolite concentrations; the contribution of gut bacterial generation to plasma protein-bound uremic toxins accumulation; and influx into the gut of circulating protein-bound uremic toxins at different stages of CKD. Feces, blood and urine were collected from 14 control individuals and 141 patients with CKD. Solutes were quantified by ultra-high performance liquid chromatography. To assess the rate of bacterial generation of p-cresol, indole and indole-3-acetic acid, fecal samples were cultured ex vivo. With CKD progression, an increase in protein-bound uremic toxins levels was observed in plasma, whereas the levels of these toxins and their precursors remained the same in feces and urine. Anaerobic culture of fecal samples showed no difference in ex vivo p-cresol, indole and indole-3-acetic acid generation. Therefore, differences in plasma protein-bound uremic toxins levels between different CKD stages cannot be explained by differences in bacterial generation rates in the gut, suggesting retention due to impaired kidney function as the main contributor to their increased plasma levels. Thus, as fractional clearance decreased with the progression of CKD, tubular clearance appeared to be more affected than the glomerular filtration rate, and there was no net increase in protein-bound uremic toxins influx into the gut lumen with increased plasma levels.

Contribution of the uremic milieu to an increased pro-inflammatory monocytic phenotype in chronic kidney disease

Intermediate (CD14⁺⁺CD16⁺) monocytes have important pro-inflammatory and atherogenic features and are increased in patients with chronic kidney disease (CKD). The present study aims to elucidate the role of the uremic milieu and of platelet activation in monocyte differentiation. Monocyte subtypes were analyzed in CKD patients (n = 193) and healthy controls (n = 27). Blood from healthy controls (Ctrl; n = 8) and hemodialysis patients (HD; n = 8) was centrifuged, and plasma (pl) was exchanged between Ctrl and HD (Ctrlcells/HDpl and HDcells/Ctrlpl) or reconstituted as original (Ctrlsham and HDsham) and incubated for 24 h (T24). Monocyte differentiation and platelet aggregation to monocytes (MPA) was assessed by flow cytometry. Especially, a higher proportion of CD14⁺⁺CD16⁺ monocytes was found in hemodialysis (HD) patients (p < 0.01). In plasma exchange experiments, Ctrl cells/HD pl T24 showed an increased percentage of CD14⁺⁺CD16⁺ monocytes versus Ctrl sham (33.7% ± 15 vs. 15.7% ± 9.6; P < 0.005), comparable to the level of CD14⁺⁺CD16⁺ monocytes in the HD sham condition. The percentage of CD14⁺⁺CD16⁺ monocytes was lowered by suspending HD cells in Ctrl pl (18.4% ± 7.8 vs. 36.7% ± 15 in HD sham; P < 0.005) reaching the level of the Ctrl sham condition (15.7% ± 9.6). A mixture of uremic sulfates increased CD14⁺⁺CD16⁺ monocytes compared to control (19.8 ± 9.6% vs. 15.8 ± 10.9%; P < 0.05), paralleled by a rise MPA. Blocking MPA by abciximab, a potential therapeutic strategy, or anti-CD62P did not inhibit differentiation towards the CD14⁺⁺CD16⁺ monocytes. In conclusion, in the present cohort, CD14⁺⁺CD16⁺ monocytes are especially increased in HD patients and this can at least in part be attributed to the presence of the uremic milieu, with uremic sulfates inducing a reversible shift towards pro-inflammatory CD14⁺⁺CD16⁺ monocytes.

Could resistant starch supplementation improve inflammatory and oxidative stress biomarkers and uremic toxins levels in hemodialysis patients? A pilot randomized controlled trial

An imbalance of gut microbiota is considered a new cardiovascular risk factor for chronic kidney disease (CKD) patients, since it is directly associated with increased uremic toxin production, inflammation and oxidative stress. Strategies such as prebiotic supplementation have been suggested to mitigate these complications. We hypothesized that prebiotic-resistant starch could ameliorate uremic toxins levels, oxidative stress, and inflammatory states in hemodialysis (HD) patients. This pilot study evaluated 31 HD patients assigned to either resistant starch (16 g of resistant starch Hi-Maize® 260) or placebo (manioc flour) supplementation, which they received for 4 weeks on alternate days through cookies on dialysis days and powder in a sachet on non-dialysis days. Levels of interleukin (IL)-6, high-sensitive C-reactive protein, thiobarbituric acid reactive substances plasma (TBARS), protein carbonylation, indoxyl sulfate (IS) and p-cresyl sulfate were measured. Anthropometric and biochemical parameters, as well as, food intake were also evaluated. As expected, resistant starch group increased fiber intake (p > 0.01), in addition the prebiotic supplementation reduced IL-6 (p = 0.01), TBARS (p > 0.01), and IS (p > 0.01) plasma levels. No significant differences were evident in the placebo group. Prebiotic-resistant starch supplementation seems to be a promising nutritional strategy to improve inflammation, oxidative stress and to reduce IS plasma levels in CKD patients on HD.

Alpha-lipoic acid attenuates p-cresyl sulfate-induced renal tubular injury through suppression of apoptosis and autophagy in human proximal tubular epithelial cells

The p-cresyl sulfate accumulates in kidney disease and may be involved in renal injury. α-Lipoic acid (α-LA) acts as an antioxidant in cell injury. We investigated the effects of α-LA treatment on p-cresyl sulfate-induced renal tubular injury. p-Cresyl sulfate induced cell death, and increased Bax/Bcl-2, cleaved caspase-3, Beclin-1, and LC3BII/LC3BI in human renal proximal tubular epithelial (HK-2) cells, which was counteracted by α-LA treatment. p-Cresyl sulfate-induced apoptosis was reduced by autophagy inhibitor 3-methyladenine, and p-cresyl sulfate induced autophagy was reduced by pan-caspase inhibitor Z-VAD-FMK. Moreover, p-cresyl sulfate treatment increased the expression of ER stress proteins and decreased the expression of baculoviral IAP repeat-containing proteins 6; these effects were prevented by α-LA treatment. Apoptosis and autophagy were associated with the phosphorylation of mitogen-activated protein kinase and nuclear translocation of the nuclear factor-κB p65 subunit. Pretreatment inhibitors of p38 and JNK, and knockdown of ATF4 gene reduced apoptosis- and autophagy-related protein expressions in p-cresyl sulfate treated HK-2 cells. These results demonstrate that α-lipoic acid attenuated p-cresyl sulfate-induced cell death by suppression of apoptosis and autophagy via regulation of ER stress in HK-2 cells.

Association between Protein-Bound Uremic Toxins and Asymptomatic Cardiac Dysfunction in Patients with Chronic Kidney Disease

Although the relationship between protein-bound uremic toxins (PBUTs) and cardiac structure and cardiac mortality in chronic kidney disease (CKD) has been studied in the past, the association between cardiac dysfunction and PBUTs has not yet been studied. We therefore evaluated the association between impaired peak cardiac performance and the serum free and total concentrations of potentially cardiotoxic PBUTs. In a cross-sectional study of 56 male CKD patients (stages 2–5 (pre-dialysis)) who were asymptomatic with no known cardiac diseases or diabetes we measured peak cardiac power (CPO_max), aerobic exercise capacity (VO_2max), and echocardiographic parameters of cardiac morphology and evaluated their association with PBUTs. The serum total and free concentrations of indoxyl sulfate (IXS), p-cresyl sulfate (PCS), p-cresyl glucuronide, indole acetic acid, and hippuric acid showed significant negative correlation with CPO_max and VO_2max. IXS and PCS were independently associated with CPO_max and VO_2max even after controlling for eGFR. No correlation between left ventricular mass index (LVMI) and PBUTs was seen. The present study for the first time has demonstrated the association between subclinical cardiac dysfunction in CKD and serum levels of a panel of PBUTs. Further studies are required to evaluate the mechanism of cardiotoxicity of the individual uremic toxins.

Contribution of uraemic toxins to the vascular fibrosis associated with chronic kidney disease

BACKGROUND

Patients with chronic kidney disease present with an accumulation of uraemic toxins, which have been identified as pathogenic agents associated with cardiovascular mortality, which is very high is this patient group. A phenomenon common to the progressive renal dysfunction and associated vascular damage, is the abnormal accumulation of extracellular matrix (ECM) proteins in the renal or vascular structures.

OBJECTIVE

To determine the contribution of uraemia or the uraemic toxins to the production of cytokinins and ECM in aortas of uraemic animals or human aortic smooth muscle cells (HASMCs).

MATERIALS AND METHODS

Mice were used with uraemia induced by a diet rich in adenine (0.2%) for 2, 4 or 6 weeks. Kidney function was evaluated by means of urine volume, plasma levels of creatinine, urea, fractional excretion of sodium, and vascular damage using histology, as well as protein expression using RT-qPCR. The HASMCs were incubated in vitro with uraemic toxins: p-cresol 10-100 (μg/ml) and indoxyl-sulphate25-100 (μg/ml) alone or simultaneously. The protein expression was evaluated using Western blot and confocal microscopy.

RESULTS

The administration of adenine produced progressive kidney damage in the mice, thickening of the aortic wall, and increasing the expression of TGF-β1 and ECM proteins. The toxins at high doses and combined also induced the expression of TGF-β1 and ECM proteins by the HASMCs.

CONCLUSIONS

The uraemia produced by an adenine rich diet or high doses of uraemic toxins induced the abnormal deposit of ECM proteins in the vascular wall or its production by HASMCs. The understanding of the mechanisms that underlie this pathophysiological process may be useful in the prevention of cardiovascular damage associated with the progress of chronic kidney disease, a disease, at the moment that is irreversible and occasional silent until its diagnosis in advanced stages.

Uremia Impacts VE-Cadherin and ZO-1 Expression in Human Endothelial Cell-to-Cell Junctions

Endothelial dysfunction in uremia can result in cell-to-cell junction loss and increased permeability, contributing to cardiovascular diseases (CVD) development. This study evaluated the impact of the uremic milieu on endothelial morphology and cell junction’s proteins. We evaluated (i) serum levels of inflammatory biomarkers in a cohort of chronic kidney disease (CKD) patients and the expression of VE-cadherin and Zonula Occludens-1 (ZO-1) junction proteins on endothelial cells (ECs) of arteries removed from CKD patients during renal transplant; (ii) ECs morphology in vitro under different uremic conditions, and (iii) the impact of uremic toxins p-cresyl sulfate (PCS), indoxyl sulfate (IS), and inorganic phosphate (Pi) as well as of total uremic serum on VE-cadherin and ZO-1 gene and protein expression in cultured ECs. We found that the uremic arteries had lost their intact and continuous endothelial morphology, with a reduction in VE-cadherin and ZO-1 expression. In cultured ECs, both VE-cadherin and ZO-1 protein expression decreased, mainly after exposure to Pi and uremic serum groups. VE-cadherin mRNA expression was reduced while ZO-1 was increased after exposure to PCS, IS, Pi, and uremic serum. Our findings show that uremia alters cell-to-cell junctions leading to an increased endothelial damage. This gives a new perspective regarding the pathophysiological role of uremia in intercellular junctions and opens new avenues to improve cardiovascular outcomes in CKD patients.

Cardiotoxicity of Uremic Toxins: A Driver of Cardiorenal Syndrome

Cardiovascular disease (CVD) is highly prevalent in the setting of chronic kidney disease (CKD). Such coexistence of CVD and CKD—the so-called “cardiorenal or renocardiac syndrome”—contributes to exponentially increased risk of cardiovascular (CV) mortality. Uremic cardiomyopathy is a characteristic cardiac pathology commonly found in CKD. CKD patients are also predisposed to heart rhythm disorders especially atrial fibrillation. Traditional CV risk factors as well as known CKD-associated CV risk factors such as anemia are insufficient to explain CV complications in the CKD population. Accumulation of uremic retention solutes is a hallmark of impaired renal excretory function. Many of them have been considered inert solutes until their biological toxicity is unraveled and they become accepted as “uremic toxins”. Direct cardiotoxicity of uremic toxins has been increasingly demonstrated in recent years. This review offers a mechanistic insight into the pathological cardiac remodeling and dysfunction contributed by uremic toxins with a main focus on fibroblastic growth factor-23, an emerging toxin playing a central role in the chronic kidney disease–mineral bone disorder, and the two most investigated non-dialyzable protein-bound uremic toxins, indoxyl sulfate and p-cresyl sulfate. Potential therapeutic strategies that could address these toxins and their relevant mediated pathways since pre-dialysis stages are also discussed.

Synbiotic meal decreases uremic toxins in hemodialysis individuals: A placebo-controlled trial

Generation of uremic toxins p-cresylsulfate (p-CS), indoxyl sulfate (IS) and indole 3-acetic acid (IAA) in hemodialysis (HD) individuals may be associated with the gut flora and recognized markers of disease progression. This study investigated the effect of synbiotic meal on uremic toxins in HD individuals. We conducted randomized singleblind and placebo-controlled intervention study with 58 HD subjects (20F/38M, 63.1 ± 10.9-old) who were randomly allocated in synbiotic group (SG, 40 g of extruded sorghum plus 100 mL of unfermented probiotic milk) or control group (CG, 40 g of extruded corn plus 100 mL of pasteurized milk), during 7-wk Metabolic markers and uremic toxins, fecal concentration of short chain fatty acid and pH value was determined. The SG group had decreased serum p-CS and IS, as well as decreased urea concentration (p < .05) compared to CG. SG showed higher fecal butyric acid and lower pH compared to baseline and SC (p < .05). In addition, serum p-CS and fecal pH were positively correlated to urea concentration in SG participants at the endpoint. The consumption of the synbiotic meal during 7-wk reduced colonic pH, and reduced serum uremic (p-CS and IS) toxins and urea in HD subjects.

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.

Role of Uremic Toxins for Kidney, Cardiovascular, and Bone Dysfunction

With decreasing kidney function, cardiovascular disease (CVD) and mineral bone disorders frequently emerge in patients with chronic kidney disease (CKD). For these patients, in addition to the traditional risk factors, non-traditional CKD-specific risk factors are also associated with such diseases and conditions. One of these non-traditional risk factors is the accumulation of uremic toxins (UTs). In addition, the accumulation of UTs further deteriorates kidney function. Recently, a huge number of UTs have been identified. Although many experimental and clinical studies have reported associations between UTs and the progression of CKD, CVD, and bone disease, these relationships are very complex and have not been fully elucidated. Among the UTs, indoxyl sulfate, asymmetric dimethylarginine, and p-cresylsulfate have been of particular focus, up until now. In this review, we summarize the pathophysiological influences of these UTs on the kidney, cardiovascular system, and bone, and discuss the clinical data regarding the harmful effects of these UTs on diseases and conditions.

Determination of the binding properties of p-cresyl glucuronide to human serum albumin

p-Cresyl glucuronide (p-CG) is a by-product of tyrosine metabolism that accumulates in patients with end-stage renal disease. p-CG binding to human serum albumin in physiological conditions (37 °C, pH 7.40) was studied by ultrafiltration (MWCO 10 kDa) and data were analyzed assuming one binding site. The estimated value of the association constant was 2.77 × 10³ M^-1 and a maximal stoichiometry of 3.80 mol per mole. At a concentration relevant for end-stage renal patients, p-CG was 23% bound to albumin. Competition experiments, using fluorescent probes, demonstrated that p-CG did not bind to Sudlow's site I or site II. The p-CG did not interfere with the binding of p-cresyl-sulfate or indoxyl sulfate to serum albumin.

Does Low-Protein Diet Influence the Uremic Toxin Serum Levels From the Gut Microbiota in Nondialysis Chronic Kidney Disease Patients?

OBJECTIVES

To evaluate the effects of low-protein diet (LPD) on uremic toxins and the gut microbiota profile in nondialysis chronic kidney disease (CKD) patients.

DESIGN AND METHODS

Longitudinal study with 30 nondialysis CKD patients (stage 3-4) undergoing LPD for 6 months. Adherence to the diet was evaluated based on the calculation of protein equivalent of nitrogen appearance from the 24-hour urine analysis. Good adherence to LPD was considered when protein intake was from 90% to 110% of the prescribed amount (0.6 g/kg/day). Food intake was analyzed by the 24-hour recall method. The anthropometric, biochemical and lipid profile parameters were measured according to standard methods. Uremic toxin serum levels (indoxyl sulfate, p-cresyl sulfate, indole-3-acetic acid) were obtained by reversed-phase high-performance liquid chromatography (RP-HPLC). Fecal samples were collected to evaluate the gut microbiota profile through polymerase chain reaction and denaturing gradient gel electrophoresis. Statistical analysis was performed by the SPSS 23.0 program software.

RESULTS

Patients who adhered to the diet (n = 14) (0.7 ± 0.2 g/kg/day) presented an improvement in renal function (nonsignificant) and reduction in total and low-density lipoprotein cholesterol (183.9 ± 48.5-155.7 ± 37.2 mg/dL, P = .01; 99.4 ± 41.3-76.4 ± 33.2 mg/dL, P = .01, respectively). After 6 months of nutricional intervention, p-cresyl sulfate serum levels were reduced significantly in patients who adhered to the LPD (19.3 [9.6-24.7] to 15.5 [9.8-24.1] mg/L, P = .03), and in contrast, the levels were increased in patients who did not adhere (13.9 [8.0-24.8] to 24.3 [8.1-39.2] mg/L, P = .004). In addition, using the denaturing gradient gel electrophoresis technique, it was observed change in the intestinal microbiota profile after LPD intervention in both groups, and the number of bands was positively associated with protein intake (r = 0.44, P = .04).

CONCLUSION

LPD seems be a good strategy to reduce the uremic toxins production by the gut microbiota in nondialysis CKD patients.

Biochemical and Clinical Impact of Organic Uremic Retention Solutes: A Comprehensive Update

In this narrative review, the biological/biochemical impact (toxicity) of a large array of known individual uremic retention solutes and groups of solutes is summarized. We classified these compounds along their physico-chemical characteristics as small water-soluble compounds or groups, protein bound compounds and middle molecules. All but one solute (glomerulopressin) affected at least one mechanism with the potential to contribute to the uremic syndrome. In general, several mechanisms were influenced for each individual solute or group of solutes, with some impacting up to 7 different biological systems of the 11 considered. The inflammatory, cardio-vascular and fibrogenic systems were those most frequently affected and they are one by one major actors in the high morbidity and mortality of CKD but also the mechanisms that have most frequently been studied. A scoring system was built with the intention to classify the reviewed compounds according to the experimental evidence of their toxicity (number of systems affected) and overall experimental and clinical evidence. Among the highest globally scoring solutes were 3 small water-soluble compounds [asymmetric dimethylarginine (ADMA); trimethylamine-N-oxide (TMAO); uric acid], 6 protein bound compounds or groups of protein bound compounds [advanced glycation end products (AGEs); p-cresyl sulfate; indoxyl sulfate; indole acetic acid; the kynurenines; phenyl acetic acid;] and 3 middle molecules [β₂-microglobulin; ghrelin; parathyroid hormone). In general, more experimental data were provided for the protein bound molecules but for almost half of them clinical evidence was missing in spite of robust experimental data. The picture emanating is one of a complex disorder, where multiple factors contribute to a multisystem complication profile, so that it seems of not much use to pursue a decrease of concentration of a single compound.

From bench to the hemodialysis clinic: protein-bound uremic toxins modulate NF-κB/Nrf2 expression

PURPOSE

Uremic toxins produced by gut microbiota (indoxyl sulfate-IS, p-cresyl sulfate-p-CS, and indole-3-acetic acid-IAA) accumulate in hemodialysis (HD) patients and exhibit potent inflammatory effects. However, the impact of these toxins on nuclear E2-related factor 2 (Nrf2) and nuclear factor-kappa B (NF-κB) expression in HD patients remains poorly defined. The aim of this study was to evaluate the association between uremic toxins and Nrf2/NF-κB expression in vitro (RAW 264.7 macrophage-like cells) and in peripheral blood mononuclear cells from HD patients.

METHODS

Uremic toxins, C-reactive protein (CRP), interleukin-6 (IL-6) and malondialdehyde (MDA) levels were measured in fifteen HD patients and nine healthy individuals. RAW 264.7 macrophage-like cells were incubated with IS, as a prototype of protein-bound uremic toxin. Nrf2 and NF-κB expressions were analyzed by RT-qPCR.

RESULTS

HD patients presented high levels of inflammatory markers, MDA and uremic toxins. In addition, they presented high NF-κB and low Nrf2 expression. Uremic toxins were positively correlated with NF-κB expression (IS, ρ = 0.58, p < 0.003; p-CS, ρ = 0.71, p < 0.001; IAA, ρ = 0.62, p < 0.001) and negatively with Nrf2 (IS, ρ = - 0.48, p = 0.01; p-CS, ρ = - 0.46, p < 0.02). Uremic toxins also exhibited positive correlations with CRP and MDA levels. Multivariate analysis revealed that p-CS is a determinant factor of NF-κB expression. In RAW 264.7 culture, NF-κB mRNA expression was stimulated by IS, while Nrf2 was downregulated.

CONCLUSIONS

Thus, uremic toxins may stimulate NF-κB mRNA and decrease Nrf2 expression in HD patients and, consequently, trigger inflammation and oxidative stress.

Spontaneous variability of pre-dialysis concentrations of uremic toxins over time in stable hemodialysis patients

Background and aim

Numerous outcome studies and interventional trials in hemodialysis (HD) patients are based on uremic toxin concentrations determined at one single or a limited number of time points. The reliability of these studies however entirely depends on how representative these cross-sectional concentrations are. We therefore investigated the variability of predialysis concentrations of uremic toxins over time.

Methods

Prospectively collected predialysis serum samples of the midweek session of week 0, 1, 2, 3, 4, 8, 12, and 16 were analyzed for a panel of uremic toxins in stable chronic HD patients (N = 18) while maintaining dialyzer type and dialysis mode during the study period.

Results

Concentrations of the analyzed uremic toxins varied substantially between individuals, but also within stable HD patients (intra-patient variability). For urea, creatinine, beta-2-microglobulin, and some protein-bound uremic toxins, Intra-class Correlation Coefficient (ICC) was higher than 0.7. However, for phosphorus, uric acid, symmetric and asymmetric dimethylarginine, and the protein-bound toxins hippuric acid and indoxyl sulfate, ICC values were below 0.7, implying a concentration variability within the individual patient even exceeding 65% of the observed inter-patient variability.

Conclusion

Intra-patient variability may affect the interpretation of the association between a single concentration of certain uremic toxins and outcomes. When performing future outcome and interventional studies with uremic toxins other than described here, one should quantify their intra-patient variability and take into account that for solutes with a large intra-patient variability associations could be missed.

Probiotic Supplementation in Chronic Kidney Disease: A Double-blind, Randomized, Placebo-controlled Trial

OBJECTIVE

The objective of the study was to evaluate the effects of probiotic supplementation on the gut microbiota profile and inflammatory markers in chronic kidney disease patients undergoing maintenance hemodialysis (HD).

DESIGN AND METHODS

This was a randomized, double-blind, placebo-controlled study. Forty-six HD patients were assigned to receive 1 of 2 treatments: probiotic (n = 23; Streptococcus thermophilus, Lactobacillus acidophilus e Bifidobacterialongum, 90 billion colony-forming units per day) or placebo (n = 23) daily for 3 months. Blood and feces were collected at baseline and after intervention. The inflammatory markers (C-reactive protein and interleukin-6) were analyzed by immunoenzymatic assay (enzyme-linked immunosorbent assay). Uremic toxins plasma levels (indoxyl sulfate, p-cresyl sulfate, and indole-3-acetic acid) were obtained by Reversed-Phase High-Performance Liquid Chromatography. Routine laboratory parameters were measured by standard techniques. Fecal pH was measured by the colorimetric method, and the gut microbiota profile was assessed by Denaturing Gradient Gel Electrophoresis analysis.

RESULTS

Sixteen patients remained in the probiotic group (11 men, 53.6 ± 11.0 year old, 25.3 ± 4.6 kg/m²) and 17 in the placebo group (10 men, 50.3 ± 8.5 year old, 25.2 ± 5.7 kg/m²). After probiotic supplementation there was a significant increase in serum urea (from 149.6 ± 34.2 mg/dL to 172.6 ± 45.0 mg/dL, P = .02), potassium (from 4.4 ± 0.4 mmol/L to 4.8 ± 0.4 mmol/L, P = .02), and indoxyl sulfate (from 31.2 ± 15.9 to 36.5 ± 15.0 mg/dL, P = .02). The fecal pH was reduced from 7.2 ± 0.8 to 6.5 ± 0.5 (P = .01). These parameters did not change significantly in placebo group. Changes in the percentage delta (Δ) between groups were exhibited with no statistical differences observed. The inflammatory markers and gut profile were not altered by supplementation.

CONCLUSIONS

Aprobiotic supplementation failed to reduce uremic toxins and inflammatory markers. Therefore, probiotic therapy should be chosen with caution in HD patients. Further studies addressing probiotic therapy in chronic kidney disease patients are needed.

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.