Dialysis modality-dependent changes in serum metabolites: accumulation of inosine and hypoxanthine in patients on haemodialysis

Optimized metabolomic approach to identify uremic solutes in plasma of stage 3-4 chronic kidney disease patients

Background

Chronic kidney disease (CKD) is characterized by the progressive accumulation of various potential toxic solutes. Furthermore, uremic plasma is a complex mixture hampering accurate determination of uremic toxin levels and the identification of novel uremic solutes.

Methods

In this study, we applied ¹H-nuclear magnetic resonance (NMR) spectroscopy, following three distinct deproteinization strategies, to determine differences in the plasma metabolic status of stage 3–4 CKD patients and healthy controls. Moreover, the human renal proximal tubule cell line (ciPTEC) was used to study the influence of newly indentified uremic solutes on renal phenotype and functionality.

Results

Protein removal via ultrafiltration and acetonitrile precipitation are complementary techniques and both are required to obtain a clear metabolome profile. This new approach, revealed that a total of 14 metabolites were elevated in uremic plasma. In addition to confirming the retention of several previously identified uremic toxins, including p-cresyl sulphate, two novel uremic retentions solutes were detected, namely dimethyl sulphone (DMSO₂) and 2-hydroxyisobutyric acid (2-HIBA). Our results show that these metabolites accumulate in non-dialysis CKD patients from 9±7 µM (control) to 51±29 µM and from 7 (0–9) µM (control) to 32±15 µM, respectively. Furthermore, exposure of ciPTEC to clinically relevant concentrations of both solutes resulted in an increased protein expression of the mesenchymal marker vimentin with more than 10% (p<0.05). Moreover, the loss of epithelial characteristics significantly correlated with a loss of glucuronidation activity (Pearson r = −0.63; p<0.05). In addition, both solutes did not affect cell viability nor mitochondrial activity.

Conclusions

This study demonstrates the importance of sample preparation techniques in the identification of uremic retention solutes using ¹H-NMR spectroscopy, and provide insight into the negative impact of DMSO₂ and 2-HIBA on ciPTEC, which could aid in understanding the progressive nature of renal disease.

The role of xanthine oxidase in hemodialysis-induced oxidative injury: relationship with nutritional status

The role of xanthine oxidase (XOD) in patients undergoing chronic hemodialysis treatment (HD) is poorly understood. Geriatric nutritional risk index (GNRI) ≤ 90 could be linked with malnutrition-inflammation complex syndrome. This study measured XOD, myeloperoxidase (MPO), superoxide dismutase (SOD), lipid hydroperoxides, total free thiol groups, and advanced oxidation protein products (AOPP) in 50 HD patients before commencing (pre-HD) and immediately after completion of HD session (post-HD) and in 22 healthy controls. Pre-HD serum hydroperoxides, AOPP, XOD, and SOD were higher and total thiol groups were lower in patients than in controls (P < 0.05, resp.). Compared to baseline values, serum MPO activity was increased irrespective of GNRI status. Serum XOD activity was increasing during HD treatment in the group with GNRI ≤ 90 (P = 0.030) whilst decreasing in the group with GNRI > 90 (P = 0.002). In a multiple regression analysis, post-HD serum XOD activity was independently associated with GNRI ≤ 90 (β  ± SE: 0.398 ± 0.151; P = 0.012) and HD vintage (β  ± SE: −0.349 ± 0.139; P = 0.016). These results indicate that an upregulated XOD may be implicated in HD-induced oxidative injury contributing to accelerated protein damage in patients with GNRI ≤ 90.

Metabolomics in chronic kidney disease

Chronic kidney disease (CKD) represents a major challenge to public healthcare. Traditional clinical biomarkers of renal function (blood urea nitrogen and serum creatinine) are not sensitive or specific enough and only increase significantly after the presence of substantial CKD. Therefore, more sensitive biomarkers of CKD are needed. CKD-specific biomarkers at an early disease stage and early diagnosis of specific renal diseases would enable improved therapeutic treatment and reduced the personal and financial burdens. The goal of metabolomics is to identify non-targeted, global small-molecule metabolite profiles of complex samples, such as biofluids and tissues. This method offers the potential for a holistic approach to clinical medicine, as well as improvements in disease diagnoses and the understanding of pathological mechanisms. This review article presents an overview of the recent developments in the field of metabolomics, followed by an in-depth discussion of its application to the study of CKD (primary, chronic glomerulonephritis such as IgA nephropathy; secondary, chronic renal injury such as diabetic nephropathy; chronic renal failure including end-stage kidney disease with and without undergoing replacement therapies, etc), including metabolomic analytical technologies, chemometrics, and metabolomics in experimental and clinical research. We describe the current status of the identification of metabolic biomarkers in CKD. Several markers have been confirmed across multiple studies to detect CKD earlier than traditional clinical chemical and histopathological methods. The application of metabolomics in CKD studies provides researchers the opportunity to gain new insights into metabolic profiling and pathophysiological mechanisms. Particular challenges in the field are presented and placed within the context of future applications of metabolomic approaches to the studies of CKD.

Clinical determinants and mortality predictability of stearoyl-CoA desaturase-1 activity indices in dialysis patients

BACKGROUND

Stearoyl-CoA desaturase-1 (SCD-1) converts dietary saturated fatty acids to monounsaturated fatty acids. Elevated SCD-1 activity thus signifies impaired fatty acid metabolism and excess saturated fat intake. In the general population, increased SCD-1 activity is associated with cardiovascular disease and mortality. The determinants and implications of SCD-1 activity in dialysis patients are unknown.

SUBJECTS

A total of 222 dialysis patients (39% women) with prospective follow-up, median age of 57 years and an average of 12 months of dialysis.

DESIGN

Fatty acid compositions in plasma phospholipids and free fatty acids (FFAs) were assessed by gas-liquid chromatography. SCD-1 activity indices were calculated as the product-to-precursor fatty acid ratio (palmitoleic acid/palmitic acid) in each fraction to reflect SCD-1 activities in the liver and adipose tissue.

RESULTS

Median hepatic and adipose tissue SCD-1 activity indices were 0.016 and 0.150, respectively. In multivariate analyses, SCD-1 was positively associated with age, female sex and serum interleukin-6 level. During 18.4 (interquartile range 5.5-37.3) months of follow-up, there were 61 deaths and 115 kidney transplants. The cut-off level for high SCD-1 indices was determined by receiver operating characteristic curve analyses. In fully adjusted competing risk models, patients with high SCD-1 indices in both phospholipids and FFAs had more than twofold increased mortality risk before kidney transplantation [hazard ratio (HR) 2.29, 95% confidence interval (CI) 1.28-4.11 and HR 2.36, 95% CI 1.38-4.03, respectively], compared with patients with low SCD-1 indices.

CONCLUSIONS

Both hepatic and adipose tissue SCD-1 activity indices independently predict mortality in dialysis patients. Further studies are warranted to determine whether reducing SCD-1 activity by dietary intervention (limiting saturated fat) could improve survival in dialysis patients.

Advanced kidney failure and hyperuricemia

Metabolic end products accumulate in kidney failure, including uric acid (UA), a terminal product of purine catabolism. Hyperuricemia (HUA) can cause gout and has been increasingly linked with cardiovascular (CV) morbidity and mortality, outcomes that are highly prevalent in patients with kidney disease. Serum UA levels rise as glomerular filtration declines, whereas the frequency of gouty attacks declines and the incidence of CV death rises precipitously. Herein, we review the kinetics of UA metabolism in CKD and dialysis and discuss the possible mechanisms of gout mitigation in kidney failure and the potential contribution of hyperuricemic milieu to CV outcomes in patients with kidney disease.

The alteration in Cu/Zn superoxide dismutase and adhesion molecules concentrations in diabetic patients with chronic kidney disease: the effect of dialysis treatment

AIMS

The study investigated the effect of chronic kidney disease (CKD) and renal replacement therapy on diabetes mellitus-mediated alterations in antioxidative defence and in soluble adhesion molecules (sCAMs).

METHODS

We assessed plasma Cu/Zn superoxide dismutase (Cu/Zn SOD) and sCAMs (sICAM-1, sVCAM-1) concentrations by ELISA in diabetic and non-diabetic patients on hemodialysis (HD), continuous ambulatory peritoneal dialysis (CAPD) and on conservative treatment.

RESULTS

All studied parameters were higher in diabetic patients when compared to non-diabetics CKD patients. Cu/Zn SOD and sVCAM-1 were lower in diabetic patients on HD and on CAPD relative to their non-diabetic counterparts. In contrary, sICAM-1 level was grown up in dialyzed diabetic patients relative to the non-diabetic patients. The positive association was between Cu/Zn SOD and sVCAM-1 in each studied group. The inverse relationship was between glycemia and these parameters in CAPD, whereas the positive correlation was observed between glycemia and sICAM-1.

CONCLUSIONS

The coexistence of diabetes mellitus is the important factor for the elevation both antioxidant defence as well as sCAMs in CKD patients. The inception of dialysis treatment can affect Cu/Zn SOD and sVCAM-1 but not sICAM-1 levels in diabetics. Moreover, these data suggest that both antioxidative defence and sCAMs could be regulated in different ways by glycemia.

Hyperuricemia at 1 year after renal transplantation, its prevalence, associated factors, and graft survival

BACKGROUND

The present study investigated the prevalence and predictors for the development of hyperuricemia within 1 year after transplantation and their associations with genetic polymorphisms and graft outcome in patients taking tacrolimus and mycophenolate mofetil.

METHODS

One hundred twenty-one renal allograft recipients transplanted between January 2001 and March 2009 were studied. Patients with serum uric acid concentrations above 7.0 mg/dL within 1 year after transplantation were defined as having hyperuricemia, and all were treated with allopurinol. Genetic polymorphisms of nitric oxide synthase, angiotensin-converting enzyme, methylenetetrahydrofolate reductase, and 3 uric acid transporters were examined.

RESULTS

At 1 year after transplantation, 46 (38%) recipients developed hyperuricemia. Male gender, higher body mass index, long-term pretransplantation dialysis, and hypertension were associated with the development of hyperuricemia. The estimated glomerular filtration rate (eGFR) at 1 year after transplantation was lower in the patients with hyperuricemia than in those without. There were no differences in graft survival between the two groups. The pharmacokinetics of tacrolimus and mycophenolic acid and 6 polymorphisms were not associated with hyperuricemia. In the multivariate analysis, male gender, long-term pretransplantation dialysis (>36 months), and eGFR (<60 mL/min) were independently associated with the development of hyperuricemia.

CONCLUSION

The incidence of hyperuricemia in our cohort was 38%. Male gender and long-term pretransplantation dialysis were predictors for the development of hyperuricemia. The eGFR was lower in patients with hyperuricemia, but graft survival did not differ between the patients with hyperuricemia treated with alloprinol and those without hyperuricemia. We could not define the significance of the pharmacokinetics of immunosuppressants and genetic risk factors for hyperuricemia.

Blood glutamate scavenging: insight into neuroprotection

Brain insults are characterized by a multitude of complex processes, of which glutamate release plays a major role. Deleterious excess of glutamate in the brain's extracellular fluids stimulates glutamate receptors, which in turn lead to cell swelling, apoptosis, and neuronal death. These exacerbate neurological outcome. Approaches aimed at antagonizing the astrocytic and glial glutamate receptors have failed to demonstrate clinical benefit. Alternatively, eliminating excess glutamate from brain interstitial fluids by making use of the naturally occurring brain-to-blood glutamate efflux has been shown to be effective in various animal studies. This is facilitated by gradient driven transport across brain capillary endothelial glutamate transporters. Blood glutamate scavengers enhance this naturally occurring mechanism by reducing the blood glutamate concentration, thus increasing the rate at which excess glutamate is cleared. Blood glutamate scavenging is achieved by several mechanisms including: catalyzation of the enzymatic process involved in glutamate metabolism, redistribution of glutamate into tissue, and acute stress response. Regardless of the mechanism involved, decreased blood glutamate concentration is associated with improved neurological outcome. This review focuses on the physiological, mechanistic and clinical roles of blood glutamate scavenging, particularly in the context of acute and chronic CNS injury. We discuss the details of brain-to-blood glutamate efflux, auto-regulation mechanisms of blood glutamate, natural and exogenous blood glutamate scavenging systems, and redistribution of glutamate. We then propose different applied methodologies to reduce blood and brain glutamate concentrations and discuss the neuroprotective role of blood glutamate scavenging.

Proof-of-principle study to detect metabolic changes in peritoneal dialysis effluent in patients who develop encapsulating peritoneal sclerosis

BACKGROUND

Prolonged peritoneal dialysis (PD) therapy can result in the development of encapsulating peritoneal sclerosis (EPS), characterized by extensive sclerosis of the peritoneum with bowel adhesions often causing obstruction.

METHODS

As a proof-of-principle study, holistic profiling of endogenous metabolites has been applied in a prospective collection of PD effluent collected in multiple UK renal centres over 6 years in order to investigate metabolic differences in PD effluent between PD therapy patients who later developed clinically defined EPS (n = 11) and controls, who were matched for PD vintage, age and gender (n = 11).

RESULTS

'Fit-for-purpose' analytical methods employing gas chromatography-mass spectrometry (MS), direct injection MS and quality control samples were developed and validated. These methods were applied in a proof-of-principle study to define metabolic differences in PD effluent related to subsequent development of EPS. Changes in amino acids, amines and derivatives, short-chain fatty acids and derivatives and sugars were observed prior to EPS developing, and changes in the metabolomic profiles could be detected.

CONCLUSION

There is potential for applying metabolic profiles to identify patients at risk of developing EPS although long-term prospective studies with larger patient cohorts are required.

Urinary metabonomics study on biochemical changes in an experimental model of chronic renal failure by adenine based on UPLC Q-TOF/MS

BACKGROUND

Chronic renal failure (CRF) is a serious clinical symptom, occurring as the end result of all kinds of chronic kidney disease and its pathophysiological mechanism is not yet well understood. We investigated the metabolic profiling of urine samples from CRF model rats to find potential disease biomarkers and research pathology of CRF.

METHODS

An animal model of CRF was produced by adenine. Metabolic profiling of the urine was performed by using ultra performance liquid chromatography coupled with quadrupole time-of-flight mass spectrometry (UPLC Q-TOF/MS). Acquired data were subjected to principal component analysis (PCA) for differentiating the CRF and the normal control groups. Potential biomarkers were screened by using S-plot and were identified by the accurate mass, isotopic pattern and MS(E) fragments information obtained from UPLC Q-TOF/MS analysis.

RESULTS

12 metabolites in urine were identified as potential biomarkers. Adenine-induced CRF rats were characterized by the increase of phytosphingosine, adrenosterone, tryptophan, 2,8-dihydroxyadenine, creatinine, and dihydrosphingosine together with the decrease of N-acetylleucine, 3-O-methyldopa, ethyl-N2-acetyl-L-argininate, dopamine, phenylalanine and kynurenic acid in urine. The altered metabolites demonstrated perturbations of amino acids metabolism, phospholipids metabolism and creatinine metabolism in CRF rats.

CONCLUSION

This work shows that metabonomics method is a valuable tool in CRF mechanism study and assists in clinical diagnosis of CRF.