Apoptosis and myostatin mRNA are upregulated in the skeletal muscle of patients with chronic kidney disease

Myostatin Activates the Ubiquitin-Proteasome and Autophagy-Lysosome Systems Contributing to Muscle Wasting in Chronic Kidney Disease

Our evidence demonstrated that CKD upregulated the expression of myostatin, TNF-α, and p-IkBa and downregulated the phosphorylation of PI3K, Akt, and FoxO3a, which were also associated with protein degradation and muscle atrophy. The autophagosome formation and protein expression of autophagy-related genes were increased in muscle of CKD rats. The mRNA level and protein expression of MAFbx and MuRF-1 were also upregulated in CKD rats, as well as proteasome activity of 26S. Moreover, activation of myostatin elicited by TNF-α induces C2C12 myotube atrophy via upregulating the expression of autophagy-related genes, including MAFbx and MuRF1 and proteasome subunits. Inactivation of FoxO3a triggered by PI3K inhibitor LY294002 prevented the myostatin-induced increase of expression of MuRF1, MAFbx, and LC3-II protein in C2C12 myotubes. The findings were further consolidated by using siRNA interference and overexpression of myostatin. Additionally, expression of myostatin was activated by TNF-α via a NF-κB dependent pathway in C2C12 myotubes, while inhibition of NF-κB activity suppressed myostatin and improved myotube atrophy. Collectively, myostatin mediated CKD-induced muscle catabolism via coordinate activation of the autophagy and the ubiquitin-proteasome systems.

Fiber type-specific response of skeletal muscle satellite cells to high-intensity resistance training in dialysis patients

INTRODUCTION

The aim of this study was to assess the effect of high-intensity resistance training on satellite cell (SC) and myonuclear number in the muscle of patients undergoing dialysis.

METHODS

Patients (n = 21) underwent a 16-week control period, followed by 16 weeks of resistance training 3 times weekly. SC and myonuclear number were determined by immunohistochemistry of vastus lateralis muscle biopsy cross-sections. Knee extension torque was tested in a dynamometer.

RESULTS

During training, SCs/type I fibers increased by 15%, whereas SCs/type II fibers remained unchanged. Myonuclear content of type II, but not type I, fibers increased with training. Before the control period, the SC content of type II fibers was lower than that of type I fibers, whereas contents were comparable when normalized to fiber area. Torque increased after training.

CONCLUSIONS

Increased myonuclear content of type II muscle fibers of dialysis patients who perform resistance training suggests that SC dysfunction is not the limiting factor for muscle growth.

Insulin sensitivity of muscle protein metabolism is altered in patients with chronic kidney disease and metabolic acidosis

An emergent hypothesis is that a resistance to the anabolic drive by insulin may contribute to loss of strength and muscle mass in patients with chronic kidney disease (CKD). We tested whether insulin resistance extends to protein metabolism using the forearm perfusion method with arterial insulin infusion in 7 patients with CKD and metabolic acidosis (bicarbonate 19 mmol/l) and 7 control individuals. Forearm glucose balance and protein turnover (²H-phenylalanine kinetics) were measured basally and in response to insulin infused at different rates for 2 h to increase local forearm plasma insulin concentration by approximately 20 and 50 μU/ml. In response to insulin, forearm glucose uptake was significantly increased to a lesser extent (−40%) in patients with CKD than controls. In addition, whereas in the controls net muscle protein balance and protein degradation were decreased by both insulin infusion rates, in patients with CKD net protein balance and protein degradation were sensitive to the high (0.035 mU/kg per min) but not the low (0.01 mU/kg per min) insulin infusion. Besides blunting muscle glucose uptake, CKD and acidosis interfere with the normal suppression of protein degradation in response to a moderate rise in plasma insulin. Thus, alteration of protein metabolism by insulin may lead to changes in body tissue composition which may become clinically evident in conditions characterized by low insulinemia.

Bone is Not Alone: the Effects of Skeletal Muscle Dysfunction in Chronic Kidney Disease

Chronic kidney disease (CKD) is associated with a decline in muscle mass, strength, and function, collectively called "sarcopenia." Sarcopenia is associated with hospitalizations and mortality in CKD and is therefore important to understand and characterize. While the focus of skeletal health in CKD has traditionally focused on bone and mineral aberrations, it is now recognized that sarcopenia must also play a role in poor musculoskeletal health in this population. In this paper, we present an overview of skeletal muscle changes in CKD, including defects in skeletal muscle catabolism and anabolism in uremic tissue. There are many gaps in knowledge in this field that should be the focus for future research to unravel pathogenesis and therapies for musculoskeletal health in CKD.

Aging and uremia: Is there cellular and molecular crossover?

Many observers have noted that the morphological changes that occur in chronic kidney disease (CKD) patients resemble those seen in the geriatric population, with strikingly similar morbidity and mortality profiles and rates of frailty in the two groups, and shared characteristics at a pathophysiological level especially in respect to the changes seen in their vascular and immune systems. However, whilst much has been documented about the shared physical characteristics of aging and uremia, the molecular and cellular similarities between the two have received less attention. In order to bridge this perceived gap we have reviewed published research concerning the common molecular processes seen in aging subjects and CKD patients, with specific attention to altered proteostasis, mitochondrial dysfunction, post-translational protein modification, and senescence and telomere attrition. We have also sought to illustrate how the cell death and survival pathways apoptosis, necroptosis and autophagy are closely interrelated, and how an understanding of these overlapping pathways is helpful in order to appreciate the shared molecular basis behind the pathophysiology of aging and uremia. This analysis revealed many common molecular characteristics and showed similar patterns of cellular dysfunction. We conclude that the accelerated aging seen in patients with CKD is underpinned at the molecular level, and that a greater understanding of these molecular processes might eventually lead to new much needed therapeutic strategies of benefit to patients with renal disease.

The effects of an ActRIIb receptor Fc fusion protein ligand trap in juvenile simian immunodeficiency virus-infected rhesus macaques

There are no approved therapies for muscle wasting in children infected with human immunodeficiency virus (HIV), which portends poor disease outcomes. To determine whether a soluble ActRIIb receptor Fc fusion protein (ActRIIB.Fc), a ligand trap for TGF-β/activin family members including myostatin, can prevent or restore loss of lean body mass and body weight in simian immunodeficiency virus (SIV)-infected juvenile rhesus macaques (Macaca mulatta). Fourteen pair-housed, juvenile male rhesus macaques were inoculated with SIVmac239 and, 4 wk postinoculation (WPI) treated with intramuscular injections of 10 mg ⋅ kg(-1) ⋅ wk(-1) ActRIIB.Fc or saline placebo. Body weight, lean body mass, SIV titers, and somatometric measurements were assessed monthly for 16 wk. Age-matched SIV-infected rhesus macaques were injected with saline. Intervention groups did not differ at baseline. Gains in lean mass were significantly greater in the ActRIIB.Fc group than in the placebo group (P < 0.001). Administration of ActRIIB.Fc was associated with greater gains in body weight (P = 0.01) and upper arm circumference than placebo. Serum CD4(+) T-lymphocyte counts and SIV copy numbers did not differ between groups. Administration of ActRIIB.Fc was associated with higher muscle expression of myostatin than placebo. ActRIIB.Fc effectively blocked and reversed loss of body weight, lean mass, and fat mass in juvenile SIV-infected rhesus macaques.

Akt1-mediated fast/glycolytic skeletal muscle growth attenuates renal damage in experimental kidney disease

Muscle wasting is frequently observed in patients with kidney disease, and low muscle strength is associated with poor outcomes in these patients. However, little is known about the effects of skeletal muscle growth per se on kidney diseases. In this study, we utilized a skeletal muscle-specific, inducible Akt1 transgenic (Akt1 TG) mouse model that promotes the growth of functional skeletal muscle independent of exercise to investigate the effects of muscle growth on kidney diseases. Seven days after Akt1 activation in skeletal muscle, renal injury was induced by unilateral ureteral obstruction (UUO) in Akt1 TG and wild-type (WT) control mice. The expression of atrogin-1, an atrophy-inducing gene in skeletal muscle, was upregulated 7 days after UUO in WT mice but not in Akt1 TG mice. UUO-induced renal interstitial fibrosis, tubular injury, apoptosis, and increased expression of inflammatory, fibrosis-related, and adhesion molecule genes were significantly diminished in Akt1 TG mice compared with WT mice. An increase in the activating phosphorylation of eNOS in the kidney accompanied the attenuation of renal damage by myogenic Akt1 activation. Treatment with the NOS inhibitor L-NAME abolished the protective effect of skeletal muscle Akt activation on obstructive kidney disease. In conclusion, Akt1-mediated muscle growth reduces renal damage in a model of obstructive kidney disease. This improvement appears to be mediated by an increase in eNOS signaling in the kidney. Our data support the concept that loss of muscle mass during kidney disease can contribute to renal failure, and maintaining muscle mass may improve clinical outcome.

Gastric cancer does not affect the expression of atrophy-related genes in human skeletal muscle

INTRODUCTION

We evaluated the gene expression levels of atrogin-1, MuRF1, myostatin, follistatin, activin A, and inhibin alpha in skeletal muscle samples of patients with gastric cancer and controls.

METHODS

We studied 38 cancer patients and 12 controls who underwent surgery for gastric adenocarcinoma and benign abdominal diseases, respectively. A biopsy specimen was obtained from the rectus abdominis muscle from all participants. The relative gene expression of atrogin-1, MuRF1, myostatin, follistatin, activin A, and inhibin alpha was determined by quantitative real-time polymerase chain reaction analysis.

RESULTS

Atrogin-1 and MuRF1 mRNA expression was similar between cancer patients and controls and was unaffected by the disease stage or the severity of body weight loss. Transcript levels of myostatin and follistatin did not differ between cases and controls and were similar across disease stages and categories of weight loss. Finally, no differences were detected in activin A and inhibin alpha gene expression between cancer patients and controls.

CONCLUSIONS

In skeletal muscle, the gene expression of atrogin-1, MuRF1, myostatin, follistatin, activin A, and inhibin alpha is not affected by the presence of cancer. The expression of atrophy-related genes is unaffected by the disease stage and the degree of weight loss.

Role of microRNAs in skeletal muscle hypertrophy

Skeletal muscle comprises approximately 40% of body weight, and is important for locomotion, as well as for metabolic homeostasis. Adult skeletal muscle mass is maintained by a fine balance between muscle protein synthesis and degradation. In response to cytokines, nutrients, and mechanical stimuli, skeletal muscle mass is increased (hypertrophy), whereas skeletal muscle mass is decreased (atrophy) in a variety of conditions, including cancer cachexia, starvation, immobilization, aging, and neuromuscular disorders. Recent studies have determined two important signaling pathways involved in skeletal muscle mass. The insulin-like growth factor-1 (IGF-1)/Akt pathway increases skeletal muscle mass via stimulation of protein synthesis and inhibition of protein degradation. By contrast, myostatin signaling negatively regulates skeletal muscle mass by reducing protein synthesis. In addition, the discovery of microRNAs as novel regulators of gene expression has provided new insights into a multitude of biological processes, especially in skeletal muscle physiology. We summarize here the current knowledge of microRNAs in the regulation of skeletal muscle hypertrophy, focusing on the IGF-1/Akt pathway and myostatin signaling.

Effects of peritoneal dialysis on protein metabolism

Protein-energy wasting is relatively common in renal patients treated with haemodialysis or peritoneal dialysis (PD) and is associated with worse outcome. In this article, we review the current state of our knowledge regarding the effects of PD on protein metabolism and the possible interactions between PD-induced changes in protein turnover and the uraemia-induced alterations in protein metabolism. Available evidence shows that PD induces a new state in muscle protein dynamics, which is characterized by decreased turnover rates and a reduced efficiency of protein turnover, a condition which may be harmful in stress conditions, when nutrient intake is diminished or during superimposed catabolic illnesses. There is a need to develop more effective treatments to enhance the nutritional status of PD patients. New approaches include the use of amino acid/keto acids-containing supplements combined with physical exercise, incremental doses of intraperitoneal amino acids, vitamin D and myostatin antagonism for malnourished patients refractory to standard nutritional therapy.

Transcription factor FoxO1, the dominant mediator of muscle wasting in chronic kidney disease, is inhibited by microRNA-486

Chronic kidney disease (CKD) accelerates muscle protein degradation by stimulating the ubiquitin proteasome system through activation of the E3 ligases, Atrogin-1/MaFbx and MuRF-1. Forkhead transcription factors (FoxO) can control the expression of these E3 ligases, but the contribution of individual FoxOs to muscle wasting is unclear. To study this we created mice with a muscle-specific FoxO1 deletion. The absence of FoxO1 blocked 70% of the increase in E3 ligases induction by CKD as well as the proteolysis and loss of muscle mass. Thus, FoxO1 has a role in controlling ubiquitin proteasome system-related proteolysis. Since microRNA (miR)-486 reportedly dampens FoxO1 expression and its activity, we transfected a miR-486 mimic into primary cultures of myotubes and found this blocked dexamethasone-stimulated protein degradation without influencing protein synthesis. It also decreased FoxO1 protein translation and increased FoxO1 phosphorylation by down-regulation of PTEN phosphatase, a negative regulator of p-Akt. To test its efficacy in vivo, we electroporated miR-486 into muscles and found expression of the E3 ligases was suppressed and muscle mass increased despite CKD. Thus, FoxO1 is a dominant mediator of CKD-induced muscle wasting and miR-486 coordinately decreases FoxO1 and PTEN to protect against this catabolic response.

Smad3 induces atrogin-1, inhibits mTOR and protein synthesis, and promotes muscle atrophy in vivo

Myostatin, a member of the TGF superfamily, is sufficient to induce skeletal muscle atrophy. Myostatin-induced atrophy is associated with increases in E3-ligase atrogin-1 expression and protein degradation and decreases in Akt/mechanistic target of rapamycin (mTOR) signaling and protein synthesis. Myostatin signaling activates the transcription factor Smad3 (Small Mothers Against Decapentaplegic), which has been shown to be necessary for myostatin-induced atrogin-1 expression and atrophy; however, it is not known whether Smad3 is sufficient to induce these events or whether Smad3 simply plays a permissive role. Thus, the aim of this study was to address these questions with an in vivo model. To accomplish this goal, in vivo transfection of plasmid DNA was used to create transient transgenic mouse skeletal muscles, and our results show for the first time that Smad3 expression is sufficient to stimulate atrogin-1 promoter activity, inhibit Akt/mTOR signaling and protein synthesis, and induce muscle fiber atrophy. Moreover, we propose that Akt/mTOR signaling is inhibited by a Smad3-induced decrease in microRNA-29 (miR-29) expression and a subsequent increase in the translation of phosphatase and tensin homolog (PTEN) mRNA. Smad3 is also sufficient to inhibit peroxisome proliferator-activated receptor-γ coactivator-1α (PGC1α) promoter activity and to increase FoxO (Forkhead Box Protein, Subclass O)-mediated signaling and the promoter activity of plasminogen activator inhibitor 1 (PAI-1). Combined, this study provides the first evidence that Smad3 is sufficient to regulate many of the events associated with myostatin-induced atrophy and therefore suggests that Smad3 signaling may be a viable target for therapies aimed at preventing myostatin-induced muscle atrophy.

Stat3 activation links a C/EBPδ to myostatin pathway to stimulate loss of muscle mass

Catabolic conditions like chronic kidney disease (CKD) cause loss of muscle mass by unclear mechanisms. In muscle biopsies from CKD patients, we found activated Stat3 (p-Stat3) and hypothesized that p-Stat3 initiates muscle wasting. We created mice with muscle-specific knockout (KO) that prevents activation of Stat3. In these mice, losses of body and muscle weights were suppressed in models with CKD or acute diabetes. A small-molecule that inhibits Stat3 activation produced similar responses, suggesting a potential for translation strategies. Using CCAAT/enhancer-binding protein δ (C/EBPδ) KO mice and C2C12 myotubes with knockdown of C/EBPδ or myostatin, we determined that p-Stat3 initiates muscle wasting via C/EBPδ, stimulating myostatin, a negative muscle growth regulator. C/EBPδ KO also improved survival of CKD mice. We verified that p-Stat3, C/EBPδ, and myostatin were increased in muscles of CKD patients. The pathway from p-Stat3 to C/EBPδ to myostatin and muscle wasting could identify therapeutic targets that prevent muscle wasting.

Muscle wasting from kidney failure-a model for catabolic conditions

PURPOSE

Muscle atrophy is a frequent complication of chronic kidney disease (CKD) and is associated with increased morbidity and mortality. The processes causing loss of muscle mass are also present in several catabolic conditions. Understanding the pathogenesis of CKD-induced muscle loss could lead to therapeutic interventions that prevent muscle wasting in CKD and potentially, other catabolic conditions.

MAJOR FINDINGS

Insulin or IGF-1 resistance caused by CKD, acidosis, inflammation, glucocorticoids or cancer causes defects in insulin-stimulated intracellular signaling that suppresses IRS-1 activity leading to decreased phosphorylation of Akt (p-Akt). A low p-Akt activates caspase-3 which provides muscle proteins substrates of the ubiquitin-proteasome system (UPS). A low p-Akt also leads to decreased phosphorylation of forkhead transcription factors which enter the nucleus to stimulate the expression of atrogin-1/MAFbx and MuRF1, E3 ubiquitin ligases that can be associated with proteolysis of muscle cells by the UPS. Caspase-3 also stimulates proteasome-dependent proteolysis in muscle.

SUMMARY

In CKD, diabetes, inflammatory conditions or in response to acidosis or excess glucocorticoids, insulin resistance develops, initiating reduced IRS-1/PI3K/Akt signaling. In CKD, this reduces p-Akt which stimulates muscle proteolysis by activating caspase-3 and the UPS. Second, caspase-3 cleaves actomyosin yielding substrates for the UPS and increased proteasome-mediated proteolysis. Third, p-Akt down-regulation suppresses myogenesis in CKD. Fourth, exercise in CKD stimulates insulin/IGF-1 signaling to reduce muscle atrophy. Lastly, there is evidence that microRNAs influence insulin signaling providing a potential opportunity to design therapeutic interventions. This article is part of a Directed Issue entitled: Molecular basis of muscle wasting.

Mechanisms stimulating muscle wasting in chronic kidney disease: the roles of the ubiquitin-proteasome system and myostatin

Catabolic conditions including chronic kidney disease (CKD), cancer, and diabetes cause muscle atrophy. The loss of muscle mass worsens the burden of disease because it is associated with increased morbidity and mortality. To avoid these problems or to develop treatment strategies, the mechanisms leading to muscle wasting must be identified. Specific mechanisms uncovered in CKD generally occur in other catabolic conditions. These include stimulation of protein degradation in muscle arising from activation of caspase-3 and the ubiquitin-proteasome system (UPS). These proteases act in a coordinated fashion with caspase-3 initially cleaving the complex structure of proteins in muscle, yielding fragments that are substrates that are degraded by the UPS. Fortunately, the UPS exhibits remarkable specificity for proteins to be degraded because it is the major intracellular proteolytic system. Without a high level of specificity cellular functions would be disrupted. The specificity is accomplished by complex reactions that depend on recognition of a protein substrate by specific E3 ubiquitin ligases. In muscle, the specific ligases are Atrogin-1 and MuRF-1, and their expression has characteristics of a biomarker of accelerated muscle proteolysis. Specific complications of CKD (metabolic acidosis, insulin resistance, inflammation, and angiotensin II) activate caspase-3 and the UPS through mechanisms that include glucocorticoids and impaired insulin or IGF-1 signaling. Mediators activate myostatin, which functions as a negative growth factor in muscle. In models of cancer or CKD, strategies that block myostatin prevent muscle wasting, suggesting that therapies that block myostatin could prevent muscle wasting in catabolic conditions.

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.

Protein-energy wasting and mortality in chronic kidney disease

Protein-energy wasting (PEW) is common in patients with chronic kidney disease (CKD) and is associated with an increased death risk from cardiovascular diseases. However, while even minor renal dysfunction is an independent predictor of adverse cardiovascular prognosis, PEW becomes clinically manifest at an advanced stage, early before or during the dialytic stage. Mechanisms causing loss of muscle protein and fat are complex and not always associated with anorexia, but are linked to several abnormalities that stimulate protein degradation and/or decrease protein synthesis. In addition, data from experimental CKD indicate that uremia specifically blunts the regenerative potential in skeletal muscle, by acting on muscle stem cells. In this discussion recent findings regarding the mechanisms responsible for malnutrition and the increase in cardiovascular risk in CKD patients are discussed. During the course of CKD, the loss of kidney excretory and metabolic functions proceed together with the activation of pathways of endothelial damage, inflammation, acidosis, alterations in insulin signaling and anorexia which are likely to orchestrate net protein catabolism and the PEW syndrome.