Skeletal muscle atrophy, referred to as sarcopenia, is often observed in chronic kidney disease (CKD) patients, especially in patients who are undergoing hemodialysis. The purpose of this study was to determine whether uremic toxins are involved in CKD-related skeletal muscle atrophy. Among six protein-bound uremic toxins, indole containing compounds, indoxyl sulfate (IS) significantly inhibited proliferation and myotube formation in C2C12 myoblast cells. IS increased the factors related to skeletal muscle breakdown, such as reactive oxygen species (ROS) and inflammatory cytokines (TNF-α, IL-6 and TGF-β1) in C2C12 cells. IS also enhanced the production of muscle atrophy-related genes, myostatin and atrogin-1. These effects induced by IS were suppressed in the presence of an antioxidant or inhibitors of the organic anion transporter and aryl hydrocarbon receptor. The administered IS was distributed to skeletal muscle and induced superoxide production in half-nephrectomized (1/2 Nx) mice. The chronic administration of IS significantly reduced the body weights accompanied by skeletal muscle weight loss. Similar to the in vitro data, IS induced the expression of myostatin and atrogin-1 in addition to increasing the production of inflammatory cytokines by enhancing oxidative stress in skeletal muscle. These data suggest that IS has the potential to accelerate skeletal muscle atrophy by inducing oxidative stress-mediated myostatin and atrogin-1 expression.

BackgroundChronic kidney disease (CKD) patients experience skeletal muscle wasting and decreased exercise endurance. Our previous study demonstrated that indoxyl sulfate (IS), a uremic toxin, accelerates skeletal muscle atrophy. The purpose of this study was to examine the issue of whether IS causes mitochondria dysfunction and IS‐targeted intervention using AST‐120, which inhibits IS accumulation, or mitochondria‐targeted intervention using L‐carnitine or teneligliptin, a dipeptidyl peptidase‐4 inhibitor which retains mitochondria function and alleviates skeletal muscle atrophy and muscle endurance in chronic kidney disease mice.MethodsThe in vitro effect of IS on mitochondrial status was evaluated using mouse myofibroblast cells (C2C12 cell). The mice were divided into sham or 5/6‐nephrectomized (CKD) mice group. Chronic kidney disease mice were also randomly assigned to non‐treatment group and AST‐120, L‐carnitine, or teneligliptin treatment groups.ResultsIn C2C12 cells, IS induced mitochondrial dysfunction by decreasing the expression of PGC‐1α and inducing autophagy in addition to decreasing mitochondrial membrane potential. Co‐incubation with an anti‐oxidant, ascorbic acid, L‐carnitine, or teneligliptine restored the values to their original state. In CKD mice, the body and skeletal muscle weights were decreased compared with sham mice. Compared with sham mice, the expression of interleukin‐6 and atrophy‐related factors such as myostatin and atrogin‐1 was increased in the skeletal muscle of CKD mice, whereas muscular Akt phosphorylation was decreased. In addition, a reduced exercise capacity was observed for the CKD mice, which was accompanied by a decreased expression of muscular PCG‐1α and increased muscular autophagy, as reflected by decreased mitochondria‐rich type I fibres. An AST‐120 treatment significantly restored these changes including skeletal muscle weight observed in CKD mice to the sham levels accompanied by a reduction in IS levels. An L‐carnitine or teneligliptin treatment also restored them to the sham levels without changing IS level.ConclusionsOur results indicate that IS induces mitochondrial dysfunction in skeletal muscle cells and provides a potential therapeutic strategy such as IS‐targeted and mitochondria‐targeted interventions for treating CKD‐induced muscle atrophy and decreased exercise endurance.

Recent research findings indicate that the posttranslational modification of human serum albumin (HSA) such as oxidation, glycation, truncation, dimerization, and carbamylation is related to certain types of diseases. We report herein on a simple and rapid analytical method, using an electrospray ionization time-of-flight mass spectrometry technique, that allows posttranslational modifications of HSA to be quantitatively and qualitatively evaluated with a high degree of sensitivity. In patients with chronic liver disease, chronic renal disease, and diabetes mellitus, an increase in the level of oxidized cysteine-34 (Cys-34) of HSA accompanied by a decrease in the level of reduced Cys-34 was observed. The redox status of Cys-34 was correlated with ligand binding and the antioxidative functions of HSA. Available evidence indicates that monitoring the redox state of Cys-34 not only could be a useful marker for evaluating the progression of disease and its complications but also would permit therapeutic efficacy to be predicted. The redox state of Cys-34 was also used as an index of the quality of HSA preparations. These data suggest that monitoring the posttranslational modifications of HSA can be important, because the function of HSA is related not only to its serum concentration but also to the preservation of its structural integrity under disease conditions.

Rhabdomyolysis-associated acute kidney injury (AKI) is a serious life-threatening condition. As such, more effective strategies are needed for its prevention. Thioredoxin-1 (Trx), a redox-active and macrophage migration inhibitory factor (MIF) modulating protein, has a short retention time in the blood. We examined the renoprotective effect of long acting Trx that was genetically fused with human serum albumin (HSA-Trx) against glycerol-induced AKI. An intravenous HSA-Trx pre-treatment attenuated the glycerol-induced decline in renal function, compared to a PBS, HSA or Trx alone. HSA-Trx caused a reduction in the tubular injuries and in the number of apoptosis-positive tubular cells. Renal superoxide, 8-hydroxy deoxyguanosine, nitrotyrosine and the plasma Cys34-cysteinylated albumin were clearly suppressed by the HSA-Trx treatment. Prior to decreasing TNF-α and IL-6, HSA-Trx suppressed an increase of plasma MIF level. In LLC-PK1 cells, HSA-Trx decreased the level of reactive oxygen species and lactate dehydrogenase release induced by myoglobin. HSA-Trx treatment resulted in a threefold increase in the survival of lethal glycerol-treated mice. The post-administration of HSA-Trx at 1 and 3 hr after glycerol injection exerted a significant renoprotective effect. These results suggest HSA-Trx has potential for use in the treatment of rhabdomyolysis-associated AKI via its extended effects of modulating oxidative stress and MIF.

Hyperuricemia occurs with increasing frequency among patients with hyperparathyroidism. However, the molecular mechanism by which the serum parathyroid hormone (PTH) affects serum urate levels remains unknown. This was studied in uremic rats with secondary hyperparathyroidism where serum urate levels were found to be increased and urate excretion in the intestine and kidney decreased, presumably due to down-regulation of the expression of the urate exporter ABCG2 in intestinal and renal epithelial membranes. These effects were prevented by administration of the calcimimetic cinacalcet, a PTH suppressor, suggesting that PTH may down-regulate ABCG2 expression. This was directly tested in intestinal Caco-2 cells where the expression of ABCG2 on the plasma membrane was down-regulated by PTH (1-34) while its mRNA level remained unchanged. Interestingly, an inactive PTH derivative (13-34) had no effect, suggesting that a posttranscriptional regulatory system acts through the PTH receptor to regulate ABCG2 plasma membrane expression. As found in an animal study, additional clinical investigations showed that treatment with cinacalcet resulted in significant reductions in serum urate levels together with decreases in PTH levels in patients with secondary hyperparathyroidism undergoing dialysis. Thus, PTH down-regulates ABCG2 expression on the plasma membrane to suppress intestinal and renal urate excretion, and the effects of PTH can be prevented by cinacalcet treatment.