Arisa Ochi

Cbl-b Is a Critical Regulator of Macrophage Activation Associated With Obesity-Induced Insulin Resistance in Mice

We previously reported the potential involvement of casitas B-cell lymphoma-b (Cbl-b) in aging-related murine insulin resistance. Because obesity also induces macrophage recruitment into adipose tissue, we elucidated here the role of Cbl-b in obesity-related insulin resistance. Cbl-b+/+ and Cbl-b−/− mice were fed a high-fat diet (HFD) and then examined for obesity-related changes in insulin signaling. The HFD caused recruitment of macrophages into adipose tissue and increased inflammatory reaction in Cbl-b−/− compared with Cbl-b+/+ mice. Peritoneal macrophages from Cbl-b−/− mice and Cbl-b–overexpressing RAW264.7 macrophages were used to examine the direct effect of saturated fatty acids (FAs) on macrophage activation. In macrophages, Cbl-b suppressed saturated FA-induced Toll-like receptor 4 (TLR4) signaling by ubiquitination and degradation of TLR4. The physiological role of Cbl-b in vivo was also examined by bone marrow transplantation and Eritoran, a TLR4 antagonist. Hematopoietic cell-specific depletion of the Cbl-b gene induced disturbed responses on insulin and glucose tolerance tests. Blockade of TLR4 signaling by Eritoran reduced fasting blood glucose and serum interleukin-6 levels in obese Cbl-b−/− mice. These results suggest that Cbl-b deficiency could exaggerate HFD-induced insulin resistance through saturated FA-mediated macrophage activation. Therefore, inhibition of TLR4 signaling is an attractive therapeutic strategy for treatment of obesity-related insulin resistance.

Prevention of skeletal muscle atrophy in vitro using anti-ubiquitination oligopeptide carried by atelocollagen

Skeletal muscle atrophy occurs when the rate of protein degradation exceeds that of protein synthesis in various catabolic conditions, such as fasting, disuse, aging, and chronic diseases. Insulin-like growth factor-1 (IGF-1) signaling stimulates muscle growth and suppresses muscle protein breakdown. In atrophied muscles, ubiquitin ligase, Cbl-b, increases and stimulates the ubiquitination and degradation of IRS-1, an intermediate in IGF-1 signaling pathway, resulting in IGF-1 resistance. In this study, we evaluated the efficacy of atelocollagen (ATCOL)-transported anti-ubiquitination oligopeptide (Cblin: Cbl-b inhibitor) (consisting of tyrosine phosphorylation domain of IRS-1) in starved C2C12 myotubes. The amount of IRS-1 protein was lower in starved versus unstarved myotubes. The Cblin-ATCOL complex inhibited IRS-1 degradation in a concentration-dependent manner. Myotubes incubated with Cblin-ATCOL complex showed significant resistance to starvation-induced atrophy (p<0.01). Furthermore, the Cblin-ATCOL complex significantly inhibited any decrease in Akt phosphorylation (p<0.01) and localization of FOXO3a to the nucleus in starved myotubes. These results suggest that Cblin prevented starvation-induced C2C12 myotube atrophy by maintaining the IGF-1/Akt/FOXO signaling. Therefore, attachment of anti-ubiquitination oligopeptide, Cblin, to ATCOL enhances its delivery to myotubes and could be a potentially effective strategy in the treatment of atrophic myopathies.

N-myristoylated ubiquitin ligase Cbl-b inhibitor prevents on glucocorticoid-induced atrophy in mouse skeletal muscle.

A DGpYMP peptide mimetic of tyrosine(608)-phosphorylated insulin receptor substrate-1 (IRS-1), named Cblin, was previously shown to significantly inhibit Cbl-b-mediated IRS-1 ubiquitination. In the present study, we developed N-myristoylated Cblin and investigated whether it was effective in preventing glucocorticoid-induced muscle atrophy. Using HEK293 cells overexpressing Cbl-b, IRS-1 and ubiquitin, we showed that the 50% inhibitory concentrations of Cbl-b-mediated IRS-1 ubiquitination by N-myristoylated Cblin and Cblin were 30 and 120 μM, respectively. Regarding the DEX-induced atrophy of C2C12 myotubes, N-myristoylated Cblin was more effective than Cblin for inhibiting the DEX-induced decreases in C2C12 myotube diameter and IRS-1 degradation. The inhibitory efficacy of N-myristoylated Cblin on IRS-1 ubiquitination in C2C12 myotubes was approximately fourfold larger than that of Cblin. Furthermore, N-myristoylation increased the incorporation of Cblin into HEK293 cells approximately 10-folds. Finally, we demonstrated that N-myristoylated Cblin prevented the wet weight loss, IRS-1 degradation, and MAFbx/atrogin-1 and MuRF-1 expression in gastrocnemius muscle of DEX-treated mice approximately fourfold more effectively than Cblin. Taken together, these results suggest that N-myristoylated Cblin prevents DEX-induced skeletal muscle atrophy in vitro and in vivo, and that N-myristoylated Cblin more effectively prevents muscle atrophy than unmodified Cblin.

Soy Glycinin Contains a Functional Inhibitory Sequence against Muscle-Atrophy-Associated Ubiquitin Ligase Cbl-b

Background. Unloading stress induces skeletal muscle atrophy. We have reported that Cbl-b ubiquitin ligase is a master regulator of unloading-associated muscle atrophy. The present study was designed to elucidate whether dietary soy glycinin protein prevents denervation-mediated muscle atrophy, based on the presence of inhibitory peptides against Cbl-b ubiquitin ligase in soy glycinin protein. Methods. Mice were fed either 20% casein diet, 20% soy protein isolate diet, 10% glycinin diet containing 10% casein, or 20% glycinin diet. One week later, the right sciatic nerve was cut. The wet weight, cross sectional area (CSA), IGF-1 signaling, and atrogene expression in hindlimb muscles were examined at 1, 3, 3.5, or 4 days after denervation. Results. 20% soy glycinin diet significantly prevented denervation-induced decreases in muscle wet weight and myofiber CSA. Furthermore, dietary soy protein inhibited denervation-induced ubiquitination and degradation of IRS-1 in tibialis anterior muscle. Dietary soy glycinin partially suppressed the denervation-mediated expression of atrogenes, such as MAFbx/atrogin-1 and MuRF-1, through the protection of IGF-1 signaling estimated by phosphorylation of Akt-1. Conclusions. Soy glycinin contains a functional inhibitory sequence against muscle-atrophy-associated ubiquitin ligase Cbl-b. Dietary soy glycinin protein significantly prevented muscle atrophy after denervation in mice.

Effects of dietary soy protein on skeletal muscle volume and strength in humans with various physical activities

BACKGROUND In recent years, the number of bedridden people is rapidly increasing due to aging or lack of exercise in Japan. This problem is becoming more serious, since there is no countermeasure against it. In the present study, we designed to investigate whether dietary proteins, especially soy, had beneficial effects on skeletal muscle in 59 volunteers with various physical activities. METHODS We subjected 59 volunteers with various physical activities to meal intervention examination. Persons with low and high physical activities were divided into two dietary groups, the casein diet group and the soy diet group. They ate daily meals supplemented with 7.8 g of powdered casein or soy protein isolate every day for 30 days. Bedridden patients in hospitals were further divided into three dietary groups: the no supplementation diet group, the casein diet group and the soy diet group. They were also subjected to a blood test, a urinalysis, magnetic resonance imaging analysis and muscle strength test of the knee before and after the meal intervention study. RESULTS Thirty-day soy protein supplementation significantly increased skeletal muscle volume in participants with low physical activity, compared with 30-day casein protein supplementation. Both casein and soy protein supplementation increased the volume of quadriceps femoris muscle in bedridden patients. Consistently, soy protein significantly increased their extension power of the knee, compared with casein protein. Although casein protein increased skeletal muscle volume more than soy protein in bedridden patients, their muscle strength changes by soy protein supplementation were bigger than those by casein protein supplementation. CONCLUSIONS The supplementation of soy protein would be one of the effective foods which prevent the skeletal muscle atrophy caused by immobilization or unloading.

Novel Interplay Between Smad1 and Smad3 Phosphorylation via AGE Regulates the Progression of Diabetic Nephropathy

Diabetic nephropathy (DN) is the major cause of end-stage renal failure and is associated with increased morbidity and mortality compared with other causes of renal diseases. We previously found that Smad1 plays a critical role in the development of DN both in vitro and in vivo. However, functional interaction between Smad1 and Smad3 signaling in DN is unclear. Here, we addressed the molecular interplay between Smad1 and Smad3 signaling under a diabetic condition by using Smad3-knockout diabetic mice. Extracellular matrix (ECM) protein overexpression and Smad1 activation were observed in the glomeruli of db/db mice but were suppressed in the glomeruli of Smad3+/−; db/db mice. Smad3 activation enhanced the phosphorylation of Smad1 C-terminal domain but decreased the phosphorylation of linker domain, thus regulating Smad1 activation in advanced glycation end product-treated mesangial cells (MCs). However, forced phosphorylation of the Smad1 linker domain did not affect Smad3 activation in MCs. Phosphorylation of the Smad1 linker domain increased in Smad3+/−; db/db mice and probucol-treated db/db mice, which was consistent with the attenuation of ECM overproduction. These results indicate that Smad3 expression and activation or probucol treatment alters Smad1 phosphorylation, thus suggesting new molecular mechanisms underlying DN development and progression.

Structural analysis of the TKB domain of ubiquitin ligase Cbl-b complexed with its small inhibitory peptide, Cblin.

Cbl-b is a RING-type ubiquitin ligase. Previously, we showed that Cbl-b-mediated ubiquitination and proteosomal degradation of IRS-1 contribute to muscle atrophy caused by unloading stress. The phospho-pentapeptide DGpYMP (Cblin) mimics Tyr612-phosphorylated IRS-1 and inhibits the Cbl-b-mediated ubiquitination and degradation of IRS-1 in vitro and in vivo. In this study, we confirmed the direct interaction between Cblin and the TKB domain of Cbl-b using NMR. Moreover, we showed that the shortened tripeptide GpYM also binds to the TKB domain. To elucidate the inhibitory mechanism of Cblin, we solved the crystal structure of the TKB-Cblin complex at a resolution of 2.5 Å. The pY in Cblin inserts into a positively charged pocket in the TKB domain via hydrogen-bond networks and hydrophobic interactions. Within this complex, the Cblin structure closely resembles the TKB-bound form of another substrate-derived phosphopeptide, Zap-70-derived phosphopeptide. These peptides lack the conserved intrapeptidyl hydrogen bond between pY and a conserved residue involved in TKB-domain binding. Instead of the conserved interaction, these peptides specifically interact with the TKB domain. Based on this binding mode of Cblin to the TKB domain, we can design drugs against unloading-mediated muscle atrophy.