Background Sarcopenia with chronic kidney disease (CKD) progression is associated with life prognosis. Oxidative stress has attracted interest as a trigger for causing CKD‐related muscular atrophy. Advanced oxidation protein products (AOPPs), a uraemic toxin, are known to increase oxidative stress. However, the role of AOPPs on CKD‐induced muscle atrophy remains unclear. Methods In a retrospective case–control clinical study, we evaluated the relationship between serum AOPPs levels and muscle strength in haemodialysis patients with sarcopenia (n = 26, mean age ± SEM: 78.5 ± 1.4 years for male patients; n = 22, mean age ± SEM: 79.1 ± 1.5 for female patients), pre‐sarcopenia (n = 12, mean age ± SEM: 73.8 ± 2.0 years for male patients; n = 4, mean age ± SEM: 74.3 ± 4.1 for female patients) or without sarcopenia (n = 12, mean age ± SEM: 71.3 ± 1.6 years for male patients; n = 7, mean age ± SEM: 77.7 ± 1.6 for female ). The molecular mechanism responsible for the AOPPs‐induced muscle atrophy was investigated by using 5/6‐nephrectomized CKD mice, AOPPs‐overloaded mice, and C2C12 mouse myoblast cells. Results The haemodialysis patients with sarcopenia showed higher serum AOPPs levels as compared with the patients without sarcopenia. The serum AOPPs levels showed a negative correlation with grip strength (P < 0.01 for male patients, P < 0.01 for female patients) and skeletal muscle index (P < 0.01 for male patients). Serum AOPPs levels showed a positive correlation with cysteinylated albumin (Cys‐albumin), a marker of oxidative stress (r2 = 0.398, P < 0.01). In the gastrocnemius of CKD mice, muscle AOPPs levels were also increased, and it showed a positive correlation with atrogin‐1 (r2 = 0.538, P < 0.01) and myostatin expression (r2 = 0.421, P < 0.05), but a negative correlation with PGC‐1α expression (r2 = 0.405, P < 0.05). Using C2C12 cells, AOPPs increased atrogin‐1 and myostatin expression through the production of reactive oxygen species via CD36/NADPH oxidase pathway, and decreased myotube formation. AOPPs also induced mitochondrial dysfunction. In the AOPPs‐overloaded mice showed that decreasing running time and hanging time accompanied by increasing AOPPs levels and decreasing cross‐sectional area in gastrocnemius. Conclusions Advanced oxidation protein products contribute to CKD‐induced sarcopenia, suggesting that AOPPs or its downstream signalling pathway could be a therapeutic target for the treatment of CKD‐induced sarcopenia. Serum AOPPs or Cys‐albumin levels could be a new diagnostic marker for sarcopenia in CKD.

Activation of mTORC1 (mechanistic target of rapamycin complex 1) in renal tissue has been reported in chronic kidney disease (CKD)-induced renal fibrosis. However, the molecular mechanisms responsible for activating mTORC1 in CKD pathology are not well understood. The purpose of this study was to identify the uremic toxin involved in mTORC1-induced renal fibrosis. Among the seven protein-bound uremic toxins, only indoxyl sulfate (IS) caused significant activation of mTORC1 in human kidney 2 cells (HK-2 cells). This IS-induced mTORC1 activation was inhibited in the presence of an organic anion transporter inhibitor, a NADPH oxidase inhibitor, and an antioxidant. IS also induced epithelial–mesenchymal transition of tubular epithelial cells (HK-2 cells), differentiation of fibroblasts into myofibroblasts (NRK-49F cells), and inflammatory response of macrophages (THP-1 cells), which are associated with renal fibrosis, and these effects were inhibited in the presence of rapamycin (mTORC1 inhibitor). In in vivo experiments, IS overload was found to activate mTORC1 in the mouse kidney. The administration of AST-120 or rapamycin targeted to IS or mTORC1 ameliorated renal fibrosis in Adenine-induced CKD mice. The findings reported herein indicate that IS activates mTORC1, which then contributes to renal fibrosis. Therapeutic interventions targeting IS and mTORC1 could be effective against renal fibrosis in CKD.

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Mannose receptors that are expressed on macrophages and fibroblasts in cancer stroma are promising therapeutic targets for cancer treatment. Albumin can be used as a drug carrier in chemotherapeutics due to its accumulation in the tumor tissue by the enhanced permeability and retention effects. A mannosylated albumin was recently developed as a new drug carrier targeting cells that express mannose receptors such as macrophages and fibroblasts in cancer stroma. The mannosylated albumin is specifically distributed to hepatic macrophages in vivo, leading to an extremely short residence time in the blood. Here, a dual‐modified albumin, i.e., mannosylated and polyethylene glycosylated (PEGylated) is reported, to improve its blood circulating time and stromal cell targeting. The product efficiently delivers paclitaxel to stromal cells in a mouse melanoma model, thus resulting in the disruption of stromal cells and suppressed tumor growth, which is seven times stronger than that for PEGylated albumin. The findings suggest that the dual‐modified albumin has the potential to provide maximal therapeutic efficacies of chemotherapeutics for the treatment of intractable cancer.

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