Amino Acid and protein kinetics in renal failure: an integrated approach

Metabolomics profiling distinctively identified end-stage renal disease patients from chronic kidney disease patients

Chronic kidney disease (CKD) is a serious public health problem characterized by progressive kidney function loss leading to end-stage renal disease (ESRD) that demands dialysis or kidney transplantation. Early detection can prevent or delay progression to ESRD. The study aimed to gain new insights into the perturbed biochemical reactions and to identify novel distinct biomarkers between ESRD and CKD. Serum samples of 32 patients with ESRD (n = 13) and CKD (n = 19) were analyzed using chemical isotope labeling liquid chromatography-mass spectrometry metabolomics approach. A total of 193 metabolites were significantly altered in ESRD compared to CKD and were mainly involved in aminoacyl-tRNA biosynthesis, branched-chain amino acid (BCAA) biosynthesis, taurine metabolism, and tryptophan metabolism. Three kynurenine derivatives, namely, 2-aminobenzoic acid, xanthurenic acid, and hydroxypicolinic acid were upregulated in ESRD compared to CKD due to the significant decrease in glomerular filtration rate with the progression of CKD to ESRD. N-Hydroxy-isoleucine, 2-aminobenzoic acid, and picolinic acid yielded AUC > 0.99 when analyzed using Receiver Operating Characteristic (ROC) analysis. Our findings suggest that inhibiting the kynurenine pathway might be a promising target to delay CKD progression and that metabolites with high discriminative ability might serve as potential prognostic biomarkers to monitor the progression of CKD to ESRD or used in combination with current markers to indicate the status of kidney damage better.

The Impact of Chronic Kidney Disease on Nutritional Status and Its Possible Relation with Oral Diseases

Several studies have demonstrated a strong relation between periodontal diseases and chronic kidney disease (CKD). The main mechanisms at the base of this link are malnutrition, vitamin dysregulation, especially of B-group vitamins and of C and D vitamins, oxidative stress, metabolic acidosis and low-grade inflammation. In particular, in hemodialysis (HD) adult patients, an impairment of nutritional status has been observed, induced not only by the HD procedures themselves, but also due to numerous CKD-related comorbidities. The alteration of nutritional assessment induces systemic manifestations that have repercussions on oral health, like oral microbiota dysbiosis, slow healing of wounds related to hypovitaminosis C, and an alteration of the supporting bone structures of the oral cavity related to metabolic acidosis and vitamin D deficiency. Low-grade inflammation has been observed to characterize periodontal diseases locally and, in a systemic manner, CKD contributes to the amplification of the pathological process, bidirectionally. Therefore, CKD and oral disease patients should be managed by a multidisciplinary professional team that can evaluate the possible co-presence of these two pathological conditions, that negatively influence each other, and set up therapeutic strategies to treat them. Once these patients have been identified, they should be included in a follow-up program, characterized by periodic checks in order to manage these pathological conditions.

The value of a low-protein diet and ketoanalogues of essential amino acids in the сontrol of protein carbamylation and toxic effects of urea in chronic kidney disease

Chronic kidney disease (CKD) is characterized by high mortality from cardiovascular diseases, the development of which is facilitated by traditional risk factors (typical for the general population) and by nontraditional ones (specific to patients with CKD) as well. These factors include also uremic toxins, for which a causal relationship has been established with specific pathological processes in patients with CKD, comprising the development of vascular dysfunction and accelerated progression of atherosclerosis. Urea has long been considered not as a uremic toxin, but as a marker of metabolic imbalance or dialysis efficiency (Kt/V) in CKD patients. In recent years, more and more publications have appeared on the study of the toxic effects of urea with the development of toxic-uremic complications and the phenotype of premature aging, common in CKD. It was found that an increase in urea levels in uremic syndrome causes damage to the intestinal epithelial barrier with translocation of bacterial toxins into the bloodstream and the development of systemic inflammation, provokes apoptosis of vascular smooth muscle cells, as well as endothelial dysfunction, which directly contributes to the development of cardiovascular complications. The indirect effects of increased urea levels are associated with carbamylation reactions, when isocyanic acid (a product of urea catabolism) changes the structure and function of proteins in the body. Carbamylation of proteins in CKD patients is associated with the development of renal fibrosis, atherosclerosis and anemia. Thus, urea is now regarded as an important negative agent in the pathogenesis of complications in CKD. Studies on a low-protein diet with using ketoanalogues of essential amino acids to minimize the accumulation of urea and other uremic toxins demonstrate the clinical benefit of such an intervention in slowing the progression of CKD and the development of cardiovascular complications.

Hemodialysis-Nutritional Flaws in Diagnosis and Prescriptions. Could Amino Acid Losses be the Sharpest "Sword of Damocles"?

This review aims to highlight the strengths and weaknesses emerging from diagnostic evaluations and prescriptions in an intent to prevent progression over time of malnutrition and/or protein-energy wasting (PEW) in hemodialysis (HD) patients. In particular, indications of the most effective pathway to follow in diagnosing a state of malnutrition are provided based on a range of appropriate chemical-clinical, anthropometric and instrumental analyses and monitoring of the nutritional status of HD patients. Finally, based on the findings of recent studies, therapeutic options to be adopted for the purpose of preventing or slowing down malnutrition have been reviewed, with particular focus on protein-calorie intake, the role of oral and/or intravenous supplements and efficacy of some classes of amino acids. A new determining factor that may lead inexorably to PEW in hemodialysis patients is represented by severe amino acid loss during hemodialysis sessions, for which mandatory compensation should be introduced.

Urea, a true uremic toxin: the empire strikes back

Blood levels of urea rise with progressive decline in kidney function. Older studies examining acute urea infusion suggested that urea was well-tolerated at levels 8-10× above normal values. More recent in vitro and in vivo work argue the opposite and demonstrate both direct and indirect toxicities of urea, which probably promote the premature aging phenotype that is pervasive in chronic kidney disease (CKD). Elevated urea at concentrations typically encountered in uremic patients induces disintegration of the gut epithelial barrier, leading to translocation of bacterial toxins into the bloodstream and systemic inflammation. Urea induces apoptosis of vascular smooth muscle cells as well as endothelial dysfunction, thus directly promoting cardiovascular disease. Further, urea stimulates oxidative stress and dysfunction in adipocytes, leading to insulin resistance. Finally, there are widespread indirect effects of elevated urea as a result of the carbamylation reaction, where isocyanic acid (a product of urea catabolism) alters the structure and function of proteins in the body. Carbamylation has been linked with renal fibrosis, atherosclerosis and anaemia. In summary, urea is a re-emerging Dark Force in CKD pathophysiology. Trials examining low protein diet to minimize accumulation of urea and other toxins suggest a clinical benefit in terms of slowing progression of CKD.

[Role of protein carbamylation in chronic kidney disease complications]

Carbamylation corresponds to the non-enzymatic binding of isocyanic acid, mainly derived from urea decomposition, on amino groups of proteins, and participates in their molecular aging. This process is increased during chronic kidney disease (CKD) because of hyperuremia, and in other pathologies like atherosclerosis, where isocyanic may be formed from thiocyanate by myeloperoxidase in atheroma plates. Carbamylation triggers structural and functional modifications of proteins, thus impairing their biological roles and their interactions with cells. Much experimental evidence in vitro has shown the potential deleterious effects of carbamylated proteins on cell and tissue functions. Carbamylation-derived products (CDPs), and especially their major component homocitrulline, accumulate in organism in long half-life proteins, and may participate in the development of different complications of CKD, especially cardiovascular diseases, renal fibrosis, or nutritional and metabolic troubles. Recent clinical studies have confirmed the link between serum protein carbamylation and morbi-mortality in patients suffering from CKD or undergoing hemodialysis. Some CDPs could be used as biomarkers in these pathologies.

The Effects of Parenteral Amino Acid Therapy on Protein Carbamylation in Maintenance Hemodialysis Patients

OBJECTIVE

Protein carbamylation is a urea-driven post-translational protein modification associated with mortality in dialysis patients. Free amino acids (AAs) are competitive inhibitors of protein carbamylation and animal studies suggest increasing AA concentrations reduces carbamylation burden. We hypothesized that AA therapy in maintenance hemodialysis patients would reduce carbamylation, carrying the potential to improve clinical outcomes.

DESIGN

Prospective pilot clinical trial (NCT1612429).

SETTING

The study was conducted from March 2013 to March 2014 in outpatient dialysis facilities in the Boston metropolitan area.

SUBJECTS AND INTERVENTION

We enrolled 23 consecutively consenting hemodialysis subjects, infusing the first 12 individuals with 250 cc of AAs 3 times per week postdialysis over 8 weeks. The remaining 11 subjects served as controls.

MAIN OUTCOME MEASURE

Change in carbamylated albumin (C-Alb), a measure of total body carbamylation burden, between baseline and 8 weeks was the primary outcome.

RESULTS

The treated and control groups had similar clinical characteristics and similar baseline C-Alb levels (mean ± SE 9.5 ± 2.4 and 9.3 ± 1.3 mmol/mol, respectively; P = .61). The treated arm showed a significant reduction in C-Alb compared with controls at 4 weeks (8.4% reduction in the treated arm vs. 4.3% increase in controls; P = .03) and the effect was greater by 8 weeks (15% reduction in the treated vs. 1% decrease in controls; P = .01).

CONCLUSION

In this pilot study, AA therapy appeared safe and effective at reducing C-Alb levels in hemodialysis patients compared with no treatment. The impact of reduced protein carbamylation on clinical outcomes should be further investigated.

Protein carbamylation in kidney disease: pathogenesis and clinical implications

Carbamylation describes a nonenzymatic posttranslational protein modification mediated by cyanate, a dissociation product of urea. When kidney function declines and urea accumulates, the burden of carbamylation naturally increases. Free amino acids may protect proteins from carbamylation, and protein carbamylation has been shown to increase in uremic patients with amino acid deficiencies. Carbamylation reactions are capable of altering the structure and functional properties of certain proteins and have been implicated directly in the underlying mechanisms of various disease conditions. A broad range of studies has demonstrated how the irreversible binding of urea-derived cyanate to proteins in the human body causes inappropriate cellular responses leading to adverse outcomes such as accelerated atherosclerosis and inflammation. Given carbamylation's relationship to urea and the evidence that it contributes to disease pathogenesis, measurements of carbamylated proteins may serve as useful quantitative biomarkers of time-averaged urea concentrations while also offering risk assessment in patients with kidney disease. Moreover, the link between carbamylated proteins and disease pathophysiology creates an enticing therapeutic target for reducing the rate of carbamylation. This article reviews the biochemistry of the carbamylation reaction, its role in specific diseases, and the potential diagnostic and therapeutic implications of these findings based on recent advances.

Effect of kidney failure and hemodialysis on protein and amino acid metabolism

PURPOSE OF REVIEW

Despite technological innovations in renal replacement therapy, mortality is still high in patients with end-stage renal disease. This increase in mortality is not only limited to dialysis patients, but also includes all stages of chronic kidney disease (CKD) and is mainly because of cardiovascular disease. Protein-energy wasting becomes clinically manifest at an advanced CKD stage, early before or during the dialytic stage, and increases the morbidity and mortality in this patients' population. The purpose of this article is to review the recent observations on alterations of amino acid and protein metabolism which cause wasting and increase cardiovascular risk.

RECENT FINDINGS

Recent studies have consistently increased our understanding of mechanisms causing wasting and vascular disease in CKD patients. These include changes in amino acid and lipoprotein metabolism potentially leading to alterations of biology and function of the vascular wall, anorexia and endocrine dysfunction, altered muscle intracellular signaling through the insulin receptor substrate/phosphatidylinositol 3-kinase/Akt pathway, and defective myocyte regeneration. These mechanisms may trigger wasting through an increase in protein degradation and/or acceleration of apoptotic processes in skeletal muscle and may be accelerated by hemodialysis, leading to progression of vascular disease and wasting.

SUMMARY

The new understanding holds promise for new treatments which can prevent/treat vascular diseases and wasting in CKD patients.

Leucine-stimulated mTOR signaling is partly attenuated in skeletal muscle of chronically uremic rats

The branched-chain amino acid leucine stimulates muscle protein synthesis in part by directly activating the mTOR signaling pathway. Furthermore, leucine, if given in conjunction with resistance exercise, enhances the exercise-induced mTOR signaling and protein synthesis. Here we tested whether leucine can activate the mTOR anabolic signaling pathway in uremia and whether it can enhance work overload (WO)-induced signaling through this pathway. Chronic kidney disease (CKD) and control rats were studied after 7 days of surgically induced unilateral plantaris muscle WO and a single leucine or saline load. In the basal state, 4E-BP1 phosphorylation was modestly depressed in non-WO muscle of CKD rats, whereas rpS6 phosphorylation was nearly completely suppressed. After oral leucine mTOR, S6K1 and rpS6 phosphorylation increased similarly in both groups, whereas the phospho-4E-BP1 response was modestly attenuated in CKD. WO alone activated the mTOR signaling pathway in control and CKD rats. In WO CKD, muscle leucine augmented mTOR and 4E-BP1 phosphorylation, but its effect on S6K1 phosphorylation was attenuated. Taken together, this study has established that the chronic uremic state impairs basal signaling through the mTOR anabolic pathway, an abnormality that may contribute to muscle wasting. However, despite this abnormality, leucine can stimulate this signaling pathway in CKD, although its effectiveness is partially attenuated, including in skeletal muscle undergoing sustained WO. Thus, although there is some resistance to leucine in CKD, the data suggest a potential role for leucine-rich supplements in the management of uremic muscle wasting.

Determinants of resting energy expenditure in hemodialysis patients, and comparison with healthy subjects

OBJECTIVE

This study evaluated the factors that affect the resting energy expenditure (REE) and nutritional status of hemodialysis patients, and to assess any differences with healthy subjects.

DESIGN

This was an observational case-control study.

SETTING

This study took place at the Hemodialysis Units of the Laikon General Hospital and 401 General Military Hospital (Athens, Greece).

PATIENTS

Twenty-five patients undergoing hemodialysis and 23 controls were recruited. Controls were healthy volunteers with no history of diseases or medication use. Patients with active infectious or inflammatory disease were excluded.

INTERVENTION

We measured REE using indirect calorimetry. Body composition as measured by Bioelectrical Impedance Analysis (BIA), anthropometry, and biochemical parameters were assessed in both groups.

MAIN OUTCOME MEASURE

Adjusted or unadjusted for muscle mass, REE was compared between the two groups. Multiple linear regression analysis was used to study independent determinants of REE.

RESULTS

Patients had no clinical signs of malnutrition. Resting energy expenditure was not different between the two groups, but REE adjusted for muscle mass was significanlty higher in hemodialysis patients. There were no differences in body composition, with the exception of muscle mass. Patients had lower muscle mass than controls, as determined by duration of disease. The only significant determinant of REE was muscle mass.

CONCLUSION

Patients had a higher REE adjusted for muscle mass than did controls, implying that they might be at a catabolic stage, and at risk of malnutrition. These findings indicate the need for determining body composition and for nutritional assessment and support in hemodialysis patients, even without indications of malnutrition.

Emerging cardiovascular risk factors that account for a significant portion of attributable mortality risk in chronic kidney disease

Chronic kidney disease (CKD) increases cardiovascular risk and mortality. However, traditional cardiovascular risk factors do not adequately account for the substantial increase in mortality observed in CKD. The aim of this study was to examine the relative contributions of novel cardiovascular risk factors to the risk between CKD and mortality. The study population included 4,680 consecutive new patients from a tertiary care preventive cardiology program from 1996 to 2005. Estimated glomerular filtration rate was calculated using the Modification of Diet in Renal Disease (MDRD) method. Baseline levels of traditional (low-density lipoprotein cholesterol, high-density lipoprotein cholesterol, hypertension, triglycerides, total cholesterol, and fasting glucose) and emerging (apolipoproteins A-I and B, lipoprotein[a], fibrinogen, homocysteine, and high-sensitivity C-reactive protein) risk factors were examined. All-cause mortality was obtained from the Social Security Death Index. There were 278 deaths over a median follow-up period of 22 months. CKD (estimated glomerular filtration rate <or=60 ml/min/1.73 m(2)) was strongly associated with mortality after adjusting for traditional cardiovascular risk factors (hazard ratio 2.31, 95% confidence interval 1.77 to 3.11, p<0.001) and with the addition of propensity score (hazard ratio 2.33, 95% confidence interval 1.75 to 3.10, p<0.001). Of all the traditional and emerging risk factors monitored, only the addition of homocysteine and fibrinogen significantly attenuated the association between CKD and mortality (adjusted hazard ratio 1.73, 95% confidence interval 1.23 to 2.34, p<0.001), explaining 38% of the attributable mortality risk from CKD. A significant interaction (p=0.004) between homocysteine and estimated glomerular filtration rate was observed whereby the annual mortality rate in subjects with CKD with homocysteine <10 micromol/L (the bottom tertile) was similar to those with normal renal function (1% per year), whereas homocysteine levels >or=12.5 micromol/L (the top tertile) were associated with a sevenfold greater mortality risk. In conclusion, homocysteine and fibrinogen levels explain nearly 40% of the attributable mortality risk from CKD.

Amino-acid-based peritoneal dialysis solution improves amino-acid transport into skeletal muscle

Abnormalities of amino-acid (AA) and protein metabolism are known to occur in chronic kidney disease (CKD). Protein malnutrition may contribute to impaired prognosis of dialysis patients. A crucial step in protein metabolism is AA transport into the cells. We compared the effects of an AA-containing peritoneal dialysis (PD) solution to glucose-based solutions on skeletal muscle AA uptake. Thirteen nondiabetic PD patients were studied twice in a random order and in a crossover manner both in the fasting state and during euglycemic insulin stimulation using [(11)C]methylaminoisobutyrate ([(11)C]MeAIB) and positron emission tomography (PET). Before both PET study days, patients had been using either glucose-based PD solutions only or one daily bag of AA solution in addition to glucose-based PD solutions for at least 6 weeks. Skeletal muscle AA uptake was calculated with graphical analysis. AA-containing PD solution increased plasma AA concentrations from 2.18+/-0.34 to 3.08+/-0.55 mmol l(-1) in the fasting state (P=0.0002) and from 1.88+/-0.15 to 2.42+/-0.30 mmol l(-1) during insulin stimulation (P<0.0001). As compared to PD treatment using glucose-based solutions only, skeletal muscle AA uptake was significantly higher during treatment containing AA solution both in the fasting state (15.2+/-5.8 vs 20.0+/-5.6 micromol kg(-1) min(-1), respectively, P=0.0057) and during insulin stimulation (16.8+/-4.5 vs 21.1+/-4.9 micromol kg(-1) min(-1), respectively, P=0.0046). In conclusion, PD treatment with an AA-containing PD solution is associated with a significant increase in skeletal muscle AA uptake both in the fasting state and during insulin stimulation.

Homocysteine and asymmetric dimethylarginine (ADMA): biochemically linked but differently related to vascular disease in chronic kidney disease

Asymmetric dimethylarginine (ADMA), an endogenous inhibitor of nitric oxide synthase, is formed by methylation of arginine residues in proteins and released after proteolysis. In this reaction, S-adenosylmethionine is methyldonor and S-adenosylhomocysteine the demethylated product. ADMA and homocysteine are thus biochemically linked. Both plasma homocysteine and ADMA concentrations are increased in patients with renal dysfunction, probably as a result of an impairment in their metabolic, but not urinary, clearance. Hyperhomocysteinemia has been associated with an increased risk of cardiovascular disease in end-stage renal disease, especially in patients without malnutrition and inflammation. Also, plasma ADMA levels have been associated with cardiovascular disease in renal failure patients. Both homocysteine and ADMA are thought to mediate their adverse vascular effects by impairing endothelial, nitric oxide-dependent function resulting in decreased vasodilatation, increased smooth muscle cell proliferation, platelet dysfunction and increased monocyte adhesion. At the same time, it has been shown that the correlation between plasma ADMA and homocysteine is weak and that, in renal patients, the association of plasma ADMA carotid intima-media thickness, cardiovascular events and overall mortality is independent of homocysteine. This indicates that the negative vascular effects of ADMA and homocysteine have a different etiology. Treatment with folic acid substantially lowers homocysteine, but not ADMA concentration. So far, homocysteine-lowering therapy has not been very successful in decreasing cardiovascular disease. In patients with renal failure, ADMA reduction may be an interesting new goal in the prevention of cardiovascular disease.

Albumin and whole-body protein synthesis respond differently to intraperitoneal and oral amino acids

Patients with peritoneal dialysis are at risk for malnutrition and hypoalbuminemia, which are indicators of poor outcome. Recently, it was shown that dialysis solutions containing amino acids (AAs) and glucose improve protein anabolism in peritoneal dialysis patients. We determined if the same solutions could increase the fractional synthesis rate of albumin along with whole-body protein synthesis. Changes in the fractional albumin synthetic rate reflect acute change in hepatic albumin synthesis. A random-order cross-over study compared the effects of Nutrineal (AA source) plus Physioneal (glucose) dialysate with Physioneal alone dialysate. Eight patients in the overnight fasting state were compared to 12 patients in the daytime-fed state. Fractional albumin synthetic rate and whole-body protein synthesis were determined simultaneously using a primed-continuous infusion of L-[1-(13)C]-leucine. Fractional albumin synthesis on AAs plus glucose dialysis did not differ significantly from that on glucose alone in the fasting or the fed state. Protein intake by itself (fed versus fasting) failed to induce a significant increase in the fractional synthetic rate of albumin. Conversely, the oral protein brought about a significant stimulation of whole-body protein synthesis. Our findings show that the supply of AAs has different effects on whole-body protein synthesis and the fractional synthetic rate of albumin.