Activation of mammalian target of rapamycin induces lipid accumulation in the diaphragm of ventilated rats and hypoxia-treated C2C12 cells

Astragalus membranaceus Enhances Myotube Hypertrophy through PI3K-Mediated Akt/mTOR Signaling Phosphorylation

Astragalus membranaceus (AM) is classified as a high-class traditional herbal medicine, which has strengthened vitality and multifunctional pharmacological activities, but limited empirical evidence is available to support its effects in muscular hypertrophy. It evokes skeletal muscle hypertrophy by increasing anabolic pathway, which is essential to prevent sarcopenia in elderly population. In this study, we examined the effects of AM on skeletal muscle hypertrophy by focusing on the molecular mechanism. We employed an in vitro model to investigate whether AM-treated skeletal muscle, as represented by myotube C2C12 cells, was hypertrophic, and to further investigate the efficacy of AM-activated phosphorylation of PI3K/Akt/mTOR signaling that must occur prior to myotube hypertrophy. The results showed that the myotubes formed larger multinucleated myotubes with increased diameter and thickness (1.16-fold relative to control group, p < 0.05). Administration of PI3K and mTOR inhibitors abolished AM-induced muscular hypertrophy. Moreover, AM-induced PI3K-mediated myotube hypertrophy was accompanied by the activation of Akt and mTOR signaling. We concluded that the AM is a nutritional activator to enhance muscular hypertrophy by increasing PI3K/Akt/mTOR signaling phosphorylation. As the AM is effective in myotube hypertrophy, AM and its derivatives may be promising candidates for ergogenic aid to prevent sarcopenia.

Differential protein metabolism and regeneration in hypertrophic diaphragm and atrophic gastrocnemius muscles in hibernating Daurian ground squirrels

NEW FINDINGS

What is the central question of this study? The aim was to investigate whether diaphragm hypertrophy and gastrocnemius atrophy during hibernation of Daurian ground squirrels involve differential regulation of protein metabolism and regeneration. What is the main finding and its importance? We clarified the differences in protein metabolism and muscle regenerative potential in the diaphragm and gastrocnemius of hibernating ground squirrels, reflecting the different adaptability of muscles.

ABSTRACT

Are differences in the regulation of protein metabolism and regeneration involved in the different phenotypic adaptation mechanisms of muscle hypertrophy and atrophy in hibernators? Two fast-type muscles (diaphragm and gastrocnemius) in summer active and hibernating Daurian ground squirrels were selected to detect changes in cross-sectional area (CSA) and protein expression indicative of protein synthesis metabolism (protein expression of P-Akt, P-mTORC1, P-S6K1 and P-4E-BP1), protein degradation metabolism (MuRF1, atrogin-1, calpain-1, calpain-2, calpastatin, desmin, troponin T, Beclin1 and LC3-II) and muscle regeneration (MyoD, myogenin and myostatin). In the hibernation group compared with the summer active group, the CSA of the diaphragm muscle increased significantly by 26.1%, whereas the CSA of the gastrocnemius muscle decreased significantly by 20.4%. Our study also indicated that increased protein synthesis, decreased protein degradation and increased muscle regenerative potential contributed to diaphragm muscle hypertrophy, whereas decreased protein synthesis, increased protein degradation and decreased muscle regenerative potential contributed to gastrocnemius muscle atrophy. In conclusion, the differences in muscle regeneration and regulatory pattern of protein metabolism might contribute to the different adaptive changes observed in the diaphragm and gastrocnemius muscles of ground squirrels.

Kindlin-2 regulates the differentiation of 3T3-L1 preadipocytes: implications for wound healing

Background

Adipose tissue has been proven to play a crucial role in wound healing, while kindlin-2, an integrin-associated protein, has been shown to regulate cell adhesion, migration, and differentiation. This study aimed to explore its involvement in the cell differentiation of 3T3-L1 preadipocytes and its role in wound healing.

Methods

Cell adhesion, Cell Counting Kit-8 (CCK-8), Transwell, and in vitro wound healing assays, along with adipogenic and osteogenic differentiation induction were performed in 3T3-L1 preadipocytes in which kindlin-2 was knocked down or overexpressed. In vivo, kindlin-2 (+/−) transgenic mice were constructed, and wound healing was analyzed by immunohistochemistry (IHC) in a mouse dorsal wound model. Real-time polymerase chain reaction (RT-PCR) and western blotting were performed to analyze the expression of adipokines and adipogenic markers in mouse wound tissues. Adipogenic differentiation induction of adipose tissue stromal vascular fraction (SVF) were performed, and the expression of adipogenic markers in SVF was detected by western blotting. The target signaling pathway highly related to adipogenic differentiation was explored by computational biology and verified by western blotting.

Results

Knockdown of kindlin-2 was found to inhibit the adhesion, migration, and adipogenic differentiation of 3T3-L1 preadipocytes while promoting their osteogenic differentiation. In contrast, kindlin-2 overexpression resulted in increased adhesion, migration, and adipogenic differentiation of 3T3-L1 preadipocytes while reducing osteogenic differentiation. In vivo, downregulation of kindlin-2 inhibited adipogenesis in kindlin-2 transgenic mice, resulting in delayed wound healing by inhibiting inflammation, angiogenesis, collagen remodeling, and wound contraction. Mechanistically, we found that kindlin-2 could regulate adipogenic differentiation through PI3K/AKT/mTOR signaling pathway.

Conclusions

Our study revealed the essential role that kindlin-2 has in the differentiation and wound healing of 3T3-L1 preadipocytes, which offers a theoretical basis for further research and a novel strategy for wound healing.

Ventilator-induced diaphragm dysfunction in critical illness

IMPACT STATEMENT

Mechanical ventilation (MV) is life-saving for patients with acute respiratory failure but also causes difficult liberation of patients from ventilator due to rapid decrease of diaphragm muscle endurance and strength, which is termed ventilator-induced diaphragmatic damage (VIDD). Numerous studies have revealed that VIDD could increase extubation failure, ICU stay, ICU mortality, and healthcare expenditures. However, the mechanisms of VIDD, potentially involving a multistep process including muscle atrophy, oxidative loads, structural damage, and muscle fiber remodeling, are not fully elucidated. Further research is necessary to unravel mechanistic framework for understanding the molecular mechanisms underlying VIDD, especially mitochondrial dysfunction and increased mitochondrial oxidative stress, and develop better MV strategies, rehabilitative programs, and pharmacologic agents to translate this knowledge into clinical benefits.