Despite technological innovations in dialytic treatment have led to extraordinary advances in the care of patients with chronic kidney disease (CKD), mortality and morbidity persist to be high in this patients’ population. Although even minor renal dysfunction is an independent predictor of adverse cardiovascular prognosis, malnutrition and cachexia become clinically manifest at an advanced stage, early before or during the dialytic stage. In patients with end-stage renal disease (ESRD), malnutrition is strongly associated with cardiovascular mortality . Uremia-induced alterations in protein metabolism and gastrointestinal tract function can result in poor nutritional status, which in turn increases the risks of cardiovascular disease (CVD) and infection. The classical risk factors for CVD are over-represented in the CKD population, which almost consists of patients affected by hypertension and diabetes. However, the excess of risk related to CKD may be due in part to a higher prevalence of nontraditional risk factors peculiar to CKD which per se may promote endothelial dysfunction and/or atherogenesis. 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 in CKD.
THE HANDLING OF S-ADENOSYLHOMOCYSTEINE BY THE HUMAN KIDNEY
Methylation processes play a major role in the epigenetic regulation of protein expression, and changes in human DNA methylation patterns are an important feature of many diseases, including atherosclerosis and cancer . S-Adenosylhomocysteine (SAH) accumulates in blood in uremia. As SAH is a potent feedback inhibitor of most methyltransferases, this compound plays an essential role in the control of transmethylation rates. To explore the sites and mechanisms underlying the regulation of circulating SAH levels, the arterio-venous differences of SAH were measured across the kidney, splanchnic bed and lung in patients undergoing venous catheterizations . The lungs did not remove or add any circulating SAH, whereas the liver released it into the hepatic veins. The kidney extracted 40% of arterial SAH and the SAH arterio-venous difference across the kidney was directly related to its arterial levels. Thus, the kidney plays a major role in maintaining SAH levels and may, indirectly, control tissue transmethylation reactions (see  for review).
ALTERED METABOLISM OF ASYMMETRIC DIMETHYLARGININE
The generation of nitric oxide from L-arginine plays a central regulatory reaction in the endothelium. Asymmetric dimethylarginine (ADMA) is an endogenous inhibitor of nitric oxide synthase which causes endothelial dysfunction and aggravates experimental atherosclerosis. ADMA is elevated in the plasma of patients with CKD and is an independent marker of progression of renal dysfunction, vascular complications and death. Lu et al. [5▪▪] recruited 298 consecutive CKD patients scheduled to undergo coronary angiography and measured plasma ADMA and estimated glomerular filtration rate (eGFR). The plasma ADMA levels correlated significantly with eGFR. During the median follow-up period of 2.7 years, multivariate Cox analysis revealed that an increase of 0.1 μmol/l in plasma ADMA level was associated with a 37% increased risk of all-cause deaths, nonfatal myocardial infarctions, and strokes. In another study, Luksha et al.  showed that arterial flow-mediated dilatation is attenuated and nitric oxide contribution to flow is lacking in ESRD patients. The mechanisms underlying the increase in plasma ADMA in CKD patients are not clearly understood, and include loss of renal clearance and decreased metabolism by liver dimethylarginine dimethylaminohydrolase 1 and 2 (DDAH). A better understanding of the biochemical pathways that regulate ADMA homeostasis in patients with CKD can provide a rationale for the development of targeted therapies. Pharmacological modulation of DDAH is now proposed to lower ADMA concentrations, regulating the production of nitric oxide .
Patients with proteinuria exhibit reduced very low density lipoprotein (VLDL) apoB-100 removal as well as increased albumin and fibrinogen secretion rates. In addition, CKD per se is associated with reduced HDL and low-density lipoprotein (LDL) and an increase in plasma triglycerides (see  for review). These changes inhibit vascular relaxation and are associated with CVD. In addition to reducing HDL levels, CKD may impair HDL structure and function. Holzer et al. [9▪▪] used mass spectrometry and biochemical analysis to investigate proteome and lipid composition of HDL in hemodialysis patients. They found an increased amount of acute-phase protein, serum amyloid A1, albumin, lipoprotein-associated phospholipase A2 and apoC-II in HDL from uremic patients, whereas the HDL content of phospholipid was reduced. These changes may inhibit HDL cardioprotective properties in uremia. To identify the mechanisms underlying the increase of apolipoprotein C III (apoC-III), a competitive inhibitor of lipoprotein lipase, Ooi et al. [10▪] investigated the kinetics of plasma apoC-III in patients with moderate CKD. The elevated plasma apoC-III concentration in patients with CKD was chiefly a consequence of impaired catabolism of apoC-III. In addition, they observed that elevated apoC-III concentration was associated with elevated VLDL cholesterol and VLDL triglyceride concentrations. These data indicate that altered plasma apoC-III metabolism is a feature of dyslipidemia in CKD which might affect CVD risk already in patients with moderate disease.
MECHANISM OF WASTING IN EXPERIMENTAL UREMIA AND IN CHRONIC KIDNEY DISEASE PATIENTS: ANOREXIA
Anorexia and endocrine dysfunction independently play an important role in wasting (see  for review) and could be responsible for alterations of the vascular wall. Suneja et al. [12▪] described a combination of hormonal abnormalities involved in the regulation of energy homeostasis (high leptin, peptideYY, and neuropeptideY with suppressed ghrelin), which might predispose dialysis patients to adverse vascular events.
SPLANCHNIC PROTEIN TURNOVER IN METABOLIC ACIDOSIS
Metabolic acidosis is common in CKD patients and causes wasting  by increasing protein catabolism. Its effects on splanchnic protein turnover and energy expenditure have never been studied in humans. Tessari et al. [14▪▪] investigated the effects of experimentally induced, chronic metabolic acidosis (mean HCO3 = 17 mmol/l) on splanchnic protein dynamics and oxygen consumption in humans by using a leucine tracer and mass-balance technique. On the basis of plasma measurements, patients with metabolic acidosis showed reduced protein synthesis rates and a negative splanchnic leucine balance. However, based on measurements in whole blood, splanchnic protein degradation and synthesis did not differ significantly between acidotic and control individuals. These data show that moderate acidosis reduces splanchnic protein turnover and results in a negative leucine balance, an effect that apparently is offset by the contribution of blood cells to protein dynamics. As an additional finding, the authors observed that protein synthesis itself accounts for a major proportion (∼67%) of splanchnic energy expenditure. The same study provides an estimate of the energy cost for each gram of protein synthesis (∼24.1 kJ) in splanchnic organs. This figure is remarkably similar to 21.5 kJ/g protein reported by early studies in animals .
INSULIN RESISTANCE AND DEFECTIVE IRS/PI3K/AKT SIGNALING IN SKELETAL MUSCLE
An acceleration of muscle protein breakdown, which depends on the upregulation of the ubiquitin–proteasome system, is a feature of animal models of atrophy [16▪]. Bailey et al.  recently identified a series of abnormal postreceptor signaling changes in the insulin/IGF-1 pathway in the muscle of rats with CKD which lead to decreased phosphorylation of the downstream effector Akt (pAkt). The low pAkt activity has been shown to stimulate the expression of specific E3 ubiquitin-conjugating enzymes, atrogin-1/MAFbx and MuRF1. Further, a decrease in muscle PI3K activity could activate Bax leading to stimulation of caspase-3 activity and increase protein degradation. It is interesting that these catabolic responses are partially abolished by the activity of the X chromosome-linked inhibitor of apoptosis protein XIAP [18▪], suggesting that apoptosis and enhanced protein degradation share common mechanisms in the regulation of muscle physiology.
Resistance to growth hormone (GH)/IGF-1 causes growth retardation in uremic children. Recently, Chen et al. [19▪▪] observed decreased GH-stimulated JAK2 transducers and activators of transcription 5 (STAT5) phosphorylation and nuclear translocation in uremic rodents. In addition, they observed that the binding of available phospho-STAT5b to DNA is decreased, thus contributing to impaired IGF-1 gene expression in the liver. It is interesting that endotoxin-induced inflammation markedly increases the resistance to GH-mediated STAT5b signaling, and further decreases STAT5b binding to DNA and IGF-1 gene expression. These perturbations appear to be related to increased cytokine expression . Overall, these observations indicate that resistance to GH-induced IGF-1 expression in uremia arises because of defects in STAT5b phosphorylation and its impaired binding to DNA, and that these processes are further aggravated by inflammation. These observations are important, as sepsis-induced inflammation is relatively common in uremia and causes wasting.
Impaired function of satellite cells is another possible link between impaired insulin/IGF-I signaling and wasting [21▪▪]. Isolated satellite cells from mice with CKD had less MyoD and suppressed myotube formation. In vivo, CKD delayed the regeneration of injured muscle and decreased MyoD and myogenin expression. These observations suggest that CKD impairs the proliferation and differentiation of satellite cells. Taken together, impaired IGF-1 signaling in CKD might lead not only to abnormal protein metabolism, but also to impaired satellite cells’ function in muscle. These signaling pathways may hold potential therapeutic targets to reduce CKD-related muscle wasting and need further studies in humans.
INFLAMMATION AND MYOKINES
Although myokines are considered to exert a beneficial anti-inflammatory effect in diabetes and atherosclerosis , circulating interleukin (IL)-6 is a strong positive predictor of mortality and morbidity in CKD patients . Studies from two different laboratories have documented an upregulation of IL-6 expression and release from skeletal muscle of CKD patients in the basal state  or during hemodialysis , suggesting that peripheral tissues are a major site for IL-6 export. The release of IL-6 from peripheral tissues is associated with increased muscle net protein catabolism, indicating that muscle IL-6 per se is either cause or consequence of a catabolic condition. Recently, Zhang et al. [26▪▪] in a rodent model of CKD demonstrated that myostatin is upregulated in skeletal muscle and that myostatin suppression decreased the levels of circulating inflammatory cytokines, including TNF-α and IL-6. Moreover, they observed that TNF-α stimulates myostatin expression, whereas myostatin stimulates IL-6 production. This study sheds light on the involvement of multiple catabolic signal transduction pathways which may promote wasting and impair muscle regeneration in uremia. Recently, Verzola et al. [27▪] observed an upregulation of myostatin gene expression in skeletal muscle of predialysis CKD patients. Myostatin was associated with IL-6 gene expression and upregulated phosphorylated JNK, suggesting that microinflammatory changes taking place in muscle cells and myostatin are also metabolically linked in humans.
ALTERED BRANCHED-CHAIN AMINO ACID METABOLISM
Reduced intake and increased muscle oxidation account for low branched-chain amino acid (BCAA) circulating pools in patients with CKD . It is of note that leucine cooperates with IGF-1 in stimulating the activation of myogenic satellite cells, which are responsible for muscle regeneration. The leucine-induced activation of satellite cells depends on the mammalian target of rapamycin (mTOR) signaling. Chen et al. [29▪] established that CKD impairs basal leucine-induced signaling through the mTOR pathway. However, despite this abnormality, leucine was able to partially stimulate this signaling pathway. Thus, even though there is some resistance to leucine in CKD, the data suggests a potential role for leucine-rich supplements in the management of uremic muscle wasting.
ACCELERATED APOPTOSIS IN SKELETAL MUSCLE
In the pathogenesis of muscle atrophy, the loss of skeletal myonuclei by apoptosis also needs to be considered . Failure in the satellite cells and/or the decrease in the availability or sensitivity to growth factors which are necessary to maintain muscle mass and satellite cell survival could cause sarcopenia and atrophy. In-vivo time-lapse microscopy [31,32] supports the concept that apoptosis in atrophy regards mainly nonmyocyte cells (satellite cells and stromal cells). In animal models, uremia impairs the proliferation and differentiation of satellite cells and delays the regeneration of injured muscle [21▪▪]. These events appear to follow CKD-induced impaired Akt phosphorylation. Recently, Verzola et al. [27▪] analyzed muscle apoptosis and myostatin mRNA and their related intracellular signals in rectus abdominis samples obtained from stage V CKD patients before the beginning of peritoneal dialysis. Apoptosis, determined both by antisingle-stranded DNA antibody and terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling assay (TUNEL), was three-fold to five-fold increased and was related to low pAkt. Even if only a few of apoptotic satellite cells were observed, it is possible that an imbalance between regeneration and loss processes takes place in the muscle of patients with CKD, as demonstrated recently in rodents with CKD [21▪▪]. Therefore, this issue needs to be specifically addressed in humans.
EFFECTS OF HEMODIALYSIS ON PROTEIN METABOLISM
Hemodialysis has a two-edge sword effect on the nutritional state of patients with CKD. On the one hand, the recommended hemodialysis dose (corresponding to a GFR of about 12 ml/min) is sufficient enough to increase appetite and nutrient intake in anorectic patients with stage V CKD. On the other hand, hemodialysis-related factors can boost the risk of wasting and CVD. It appears more and more important to administer an ‘ideal’ treatment that can get as close as possible to the glomerular basement membrane, using biocompatible (which do not cause complement activation) dialysis membrane to effectively remove a wide range of substances. A further need is to eliminate the risk of endotoxin-contaminated dialysate. A hemodialysis session may impair protein metabolism in different ways: because of the associated losses of amino acids and because of the promotion of net protein catabolism by the process itself. About 6–12 g of amino acids are lost in the dialysate per hemodialysis session . However, amino acid loss could not account for changes in protein turnover observed in some studies , suggesting that other factors might be at play. Studies on the effects of hemodialysis on protein turnover have shown a dissociation between whole-body and muscle protein turnover (see [34,35] for review). Although whole-body protein turnover shows decreased protein synthesis after dialysis (an effect which may be explained by amino acid loss in the dialysate), protein breakdown is not increased. Muscle protein kinetics studies, however, identify enhanced muscle protein breakdown with inadequate compensatory increases in synthesis as the cause of the catabolism. A current hypothesis is that the microinflammatory state observed in a significant percentage of patients with CKD is boosted by hemodialysis, thus increasing the risk of cachexia and cardiovascular complications. Proinflammatory cytokines released from peripheral blood mononuclear cells and skeletal muscle during hemodialysis are likely to be the major factors in the sequence of catabolic events and may provide a link between wasting and cardiovascular events. An increase in C-reactive protein has been observed during a single dialysis session in 25% of patients and this increase was independently associated with a higher mortality risk . However, these findings could not be replicated in the study published by Meuwese et al. . During hemodialysis, direct correlations between IL-6 and synthesis rates of albumin and fibrinogen and muscle protein catabolism have been observed, suggesting that the cytokine activation by dialysis directly or indirectly promotes proteolysis  and blunts the suppressive effects of amino acid supplements on protein breakdown. Boivin et al.  measured caspase-3 activity in the skeletal muscle of hemodialysis patients before and after hemodialysis. Both caspase-3 activity and actin fragments (14 kDa) generated by caspase-3-mediated cleavage of actomyosin were higher in patients than in controls in the basal state, and increased further during hemodialysis. In addition, apoptosis, as well as IL-6 content in the soluble fraction of the skeletal muscle increased at the end of hemodialysis. Protein degradation was higher in ESRD patients during hemodialysis compared with prehemodialysis and control individuals. This study supports the hypothesis that the increase in IL-6 expression in skeletal muscle and amino acid depletion during hemodialysis result in overt activation of proteolytic and apoptotic pathways resulting in net protein loss and muscle atrophy.
The activation of the alternate pathway of complement may be an additional mechanism leading to malnutrition in dialysis patients . However, the mechanisms and signals responsible for the complement-dependent activation of the inflammatory response by hemodialysis are not fully elucidated. There is also a lack of controlled studies exploring the muscle proteolytic response to different filter materials. By a proteomic approach, Mares et al.  studied the processes that take place on dialysis membranes by eluting proteins adsorbed to the polysulfone dialyzer membranes. These proteins included ficolin-2 (an essential component of innate immunity that contributes to the immune recognition of pathogens), clusterin, complement C3c fragment, and apolipoprotein A1. In addition , the hemodialysis-induced decrease in plasma ficolin-2 was associated with leukopenia and C5a production, suggesting that ficolin-2 adsorption to polysulfone dialyzer initiates the lectin pathway of complement activation.
Several mechanisms which may predispose CKD patients to wasting have been recently identified in rodent models of uremia and, at least in part, in patients (Fig. 1). These include resistance to insulin/IGF-1 and altered PI3K regulation, as well as upregulation of myostatin and inflammation in skeletal muscle. These mechanisms may cause muscle atrophy by an enhancement of apoptotic processes and changes in protein degradation in skeletal muscle. A current hypothesis is that hemodialysis boosts the catabolic and atherogenic pathways which are already upregulated in the muscle of nondialyzed CKD patients. Mechanisms causing wasting in CKD overlap those already operating in aging and in comorbid conditions, such as diabetes and sepsis, to orchestrate the wasting syndrome and boost cardiovascular risk.
This study was supported by the grants from Fondazione Carige and the Baxter Clinical Evidence Council (CEC) Extramural Program.
Conflicts of interest
There are no conflicts of interest.
REFERENCES AND RECOMMENDED READING
Papers of particular interest, published within the annual period of review, have been highlighted as:
- ▪ of special interest
- ▪▪ of outstanding interest
Additional references related to this topic can also be found in the Current World Literature section in this issue (p. 96).
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