Maslinic acid activates mTORC1 and human TGR5 and induces skeletal muscle hypertrophy

ABSTRACT Maslinic acid, a naturally occurring pentacyclic triterpene in more than 30 plants (including olives), reportedly increases human muscle mass and muscle strength; however, the underlying molecular mechanism remains unknown. C57BL/6J mice were fed a standard diet or supplemented with 0.27% maslinic acid for 4 weeks, and their skeletal muscle mass was measured. Mice that consumed maslinic acid displayed significant increases in gastrocnemius and soleus muscle mass. Cultured mouse-C2C12 skeletal muscle cells were treated with mammalian target of rapamycin complex 1 (mTORC1) or protein kinase b (Akt) inhibitor, and protein synthesis was quantified. Maslinic acid accelerated protein synthesis via mTORC1 activation independent of Akt. Furthermore, maslinic acid activated human Takeda G protein-coupled receptor 5 (TGR5) more strongly than mouse TGR5, augmenting the expression of several genes related to muscular hypertrophy. Maslinic acid activated mTORC1 and human TGR5, implying its contribution to human muscular hypertrophy through these effects.

Phytoecdysteroids Do Not Have Anabolic Effects in Skeletal Muscle in Sedentary Aging Mice

Skeletal muscle mass and strength are lost with aging. Phytoecdysteroids, in particular 20-hydroxyecdysone (20E), increase protein synthesis in C2C12 skeletal muscle cells and muscle strength in young rats. The objective of this study was to determine whether an extract from Ajuga turkestanica (ATE), enriched in phytoecdysteroids, and 20E affect skeletal muscle mass and fiber size, fiber type, activation of the PI3K–Akt signaling pathway, and the mRNA levels of MAFbx, MuRF-1, and myostatin in sedentary aging mice. Aging male C57BL/6 mice (20 months old) received ATE, 20E, or vehicle (CT) once per day for 28 days or a single acute dose. Treatment did not alter body, muscle, or organ mass; fiber cross-sectional area; or fiber type in the triceps brachii or plantaris muscles. Likewise, protein synthesis signaling markers (i.e., phosphorylation of AktSer473 and p70S6kThr389) measured after either 28 days or acutely were unchanged. Neither ATE nor 20E treatment for 28 days affected the mRNA levels of MAFbx, MuRF-1, and myostatin. In conclusion, these data indicate that phytoecdysteroid treatment does not alter muscle mass or fiber type, nor does it activate protein synthesis signaling in the skeletal muscle of sedentary aging mice.

Zinc Modulates Several Transcription-Factor Regulated Pathways in Mouse Skeletal Muscle Cells

Zinc is an essential metal ion involved in many biological processes. Studies have shown that zinc can activate several molecules in the insulin signalling pathway and the concomitant uptake of glucose in skeletal muscle cells. However, there is limited information on other potential pathways that zinc can activate in skeletal muscle. Accordingly, this study aimed to identify other zinc-activating pathways in skeletal muscle cells to further delineate the role of this metal ion in cellular processes. Mouse C2C12 skeletal muscle cells were treated with insulin (10 nM), zinc (20 µM), and the zinc chelator TPEN (various concentrations) over 60 min. Western blots were performed for the zinc-activation of pAkt, pErk, and pCreb. A Cignal 45-Reporter Array that targets 45 signalling pathways was utilised to test the ability of zinc to activate pathways that have not yet been described. Zinc and insulin activated pAkt over 60 min as expected. Moreover, the treatment of C2C12 skeletal muscle cells with TPEN reduced the ability of zinc to activate pAkt and pErk. Zinc also activated several associated novel transcription factor pathways including Nrf1/Nrf2, ATF6, CREB, EGR1, STAT1, AP-1, PPAR, and TCF/LEF, and pCREB protein over 120 min of zinc treatment. These studies have shown that zinc’s activity extends beyond that of insulin signalling and plays a role in modulating novel transcription factor activated pathways. Further studies to determine the exact role of zinc in the activation of transcription factor pathways will provide novel insights into this metal ion actions.

Effect of Quercetin on Dexamethasone-Induced C2C12 Skeletal Muscle Cell Injury

Glucocorticoids are widely used anti-inflammatory drugs in clinical settings. However, they can induce skeletal muscle atrophy by reducing fiber cross-sectional area and myofibrillar protein content. Studies have proven that antioxidants can improve glucocorticoid-induced skeletal muscle atrophy. Quercetin is a potent antioxidant flavonoid widely distributed in fruits and vegetables and has shown protective effects against dexamethasone-induced skeletal muscle atrophy. In this study, we demonstrated that dexamethasone significantly inhibited cell growth and induced cell apoptosis by stimulating hydroxyl free radical production in C2C12 skeletal muscle cells. Our results evidenced that quercetin increased C2C12 skeletal cell viability and exerted antiapoptotic effects on dexamethasone-treated C2C12 cells by regulating mitochondrial membrane potential (ΔΨm) and reducing oxidative species. Quercetin can protect against dexamethasone-induced muscle atrophy by regulating the Bax/Bcl-2 ratio at the protein level and abnormal ΔΨm, which leads to the suppression of apoptosis.

EXPRESSION OF IL6 mRNA IS INDEPENDENT WITH EXPRESSION OF TLRS mRNA IN LIPOPOLYSACCHARIDE-TREATED MYOTUBES

Obesity-related metabolic disorders such as insulin resistance and type 2 diabetes are raising as critical health problems of the modern world. Obesity induces an increased plasma level of lipopolysaccharide (LPS) that contributing to system chronic inflammation. This response has been shown as a marked factor linking obesity and its related metabolic disorders. In the present study, we use LPS to treat C2C12 skeletal muscle cells and observe the expression of several inflammatory makers. Our results show that the expression of inflammatory cytokine IL6 mRNA is strongly induced in the LPS-treated C2C12 cells compared to the control cells. Surprisingly, the expression of the upper controller of inflammation TLR2 mRNA in the LPS-treated C2C12 cells is similar to that in the control cells. Consistent with this expression mRNA level of TLR4, another upper regulator of inflammation does not differ between the two group cells. Taken together, our current data suggest that the LPS induced expression of IL6 mRNA in C2C12 cells is not dependent on the regulation of neither TLR2 nor TLR4