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Skeletal muscle is a highly pliable tissue and various adaptations such as muscle hypertrophy or atrophy are induced by overloading or disuse, respectively. However, the combined effect of overloading and disuse on the quantitative adaptation of skeletal muscle is unknown. Thus, the aim of this study was to investigate the effects of the combined stimuli of overloading and disuse on mouse skeletal muscle mass and the expression of regulatory factors for muscle protein anabolism or catabolism. Male mice from the Institute Cancer Research were subjected to denervation concomitant with unilateral functional overload or functional overload concomitant with unilateral denervation. Disuse and functional overload were induced by sciatic nerve transection (denervation) and synergist ablation, respectively, and the plantaris muscle was harvested 14 days after the operation. Our results showed that denervation attenuated functional overload‐induced muscle hypertrophy and functional overload partially ameliorated the denervation‐induced muscle atrophy. P70S6K phosphorylation, an indicator of mechanistic target of rapamycin complex 1 (mTORC1) activation, was not increased by unilateral functional overload in denervated muscles or by unilateral denervation in functional overloaded muscles. Denervation did not affect the increase of LC3‐II, a marker of autophagy activation, and ubiquitinated protein expression upon unilateral functional overload. Also, functional overload did not affect ubiquitinated protein expression during unilateral denervation. Thus, our findings suggest that functional overload‐induced muscle hypertrophy or denervation‐induced muscle atrophy was attenuated by the combined stimuli of overload and denervation. [ABSTRACT FROM AUTHOR]

Lactate is a metabolic product of glycolysis and has recently been shown to act as a signaling molecule that induces adaptations in oxidative metabolism. In this study, we investigated whether lactate administration enhanced muscle hypertrophy and protein synthesis responses during resistance exercise in animal models. We used male ICR mice (7–8 weeks old) were used for chronic (mechanical overload induced by synergist ablation: [OL]) and acute (high‐intensity muscle contraction by electrical stimulation: [ES]) resistance exercise models. The animals were intraperitoneally administrated a single dose of sodium lactate (1 g/kg of body weight) in the ES study, and once a day for 14 consecutive days in the OL study. Two weeks of mechanical overload increased plantaris muscle wet weight (main effect of OL: p < 0.05) and fiber cross‐sectional area (main effect of OL: p < 0.05), but those were not affected by lactate administration. Following the acute resistance exercise by ES, protein synthesis and phosphorylation of p70 S6 kinase and ribosomal protein S6, which are downstream molecules in the anabolic signaling cascade, were increased (main effect of ES: p < 0.05), but lactate administration had no effect. This study demonstrated that exogenous lactate administration has little effect on the muscle hypertrophic response during resistance exercise using acute ES and chronic OL models. Our results do not support the hypothesis that elevated blood lactate concentration induces protein synthesis responses in skeletal muscle. [ABSTRACT FROM AUTHOR]

Adipose tissue has been classified into white adipose tissue (WAT), brown adipose tissue (BAT), and beige adipose tissue the latter of which is produced as WAT changes into BAT due to exposure to cold temperature or exercise. In response to these stimulations, WAT produces heat by increasing mitochondrial contents and the expression of uncoupling protein 1 (UCP1), thus facilitating browning. Exercise is known to be one of the triggers for WAT browning, but the effects of exercise intensity on the browning of WAT remain to be unclear. Therefore, in this study, we aimed to examine the effects of high‐ or low‐intensity exercises on the browning of WAT. Mice performed high‐ or low‐intensity running on a treadmill running 3 days a week for four weeks. As per our findings, it was determined that four weeks of running did not significantly reduce inguinal WAT (iWAT) wet weight but did significantly reduce adipocytes size, regardless of exercise intensity. The protein expression level of UCP1 was significantly increased in iWAT by high‐intensity running. In addition, the expression of oxidative phosphorylation proteins (OXPHOS) in iWAT was significantly increased by high‐intensity running. These results demonstrated that high‐intensity exercise might be effective for increasing mitochondrial contents and heat production capacity in iWAT. Furthermore, we found that high‐intensity running increased the protein expression level of fibroblast growth factor 21 (FGF21) in skeletal muscle compared with that in low intensity running. We have also examined the relationship between browning of WAT and the expression of FGF21 in skeletal muscle and found a positive correlation between the protein expression of UCP1 in iWAT and the protein expression of FGF21 in gastrocnemius muscle. In conclusion, we suggest that high‐intensity exercise is effective for the browning of WAT and the increase of FGF21 in skeletal muscle. [ABSTRACT FROM AUTHOR]

Mechanistic/mammalian target of rapamycin (mTOR) is a central factor of protein synthesis signaling and plays an important role in the resistance training‐induced increase in skeletal muscle mass and subsequent skeletal muscle hypertrophy response. In particular, mTOR complex 1 (mTORC1) promotes protein synthesis in ribosomes by activating the downstream effectors, p70S6K and 4EBP1, in skeletal muscle and is highly sensitive to rapamycin, an mTOR inhibitor. Recently, resistance training has also been shown to affect mitochondrial dynamics, which is coupled with mitochondrial function. In skeletal muscle, mitochondria dynamically change their morphology through repeated fusion and fission, which may be key for controlling the quality of skeletal muscle. However, how the mechanisms of mitochondrial dynamics function during hypertrophy in skeletal muscle remains unclear. The aim of this study was to examine the impact of mTOR inhibition on mitochondrial dynamics during skeletal muscle hypertrophy. Consistent with previous studies, functional overload by synergist (gastrocnemius and soleus) ablation‐induced progressive hypertrophy (increase in protein synthesis and fiber cross‐sectional area) of the plantaris muscle was observed in mice. Moreover, these hypertrophic responses were significantly inhibited by rapamycin administration. Fourteen days of functional overload increased levels of MFN2 and OPA1, which regulate mitochondrial fusion, whereas this enhancement was inhibited by rapamycin administration. Additionally, overload decreased the levels of DRP1, which regulates mitochondrial fission and oxidative phosphorylation, regardless of rapamycin administration. These observations suggest that the relative reduction in mitochondrial function or content is complemented by enhancement of mitochondrial fusion and that this complementary response may be regulated by mTORC1. [ABSTRACT FROM AUTHOR]

Athletes train to improve strength and endurance to demonstrate maximum performance during competitions. Training methods vary but most focus on strength, endurance, or both. Concurrent training is a combination of two different modes of training. In this study, we combined resistance exercise (RE) and high‐intensity interval exercise (HIIE) to investigate the influence of the order of the concurrent training on signal molecules on hypertrophy and glycolysis in the skeletal muscle. The phosphorylation levels of mechanistic target of rapamycin (mTOR) signals, p70 S6 kinase (p70S6 K), ribosomal protein S6 (S6), and glycogen synthase kinase beta (GSK‐3β) were significantly increased in the HIIE first group compared with the control group. The combined training course did not affect the glycogen content and expression levels of proteins concerning glycolytic and metabolic capacity, suggesting that a combination of HIIE and RE on the same day, with HIIE prior to RE, improves hypertrophy response and glycolysis enhancement. [ABSTRACT FROM AUTHOR]

For many ball games, both resistance and endurance training are necessary to improve muscle strength and endurance capacity. Endurance training has been reported to inhibit muscle strength and hypertrophy, but some studies have reported that endurance exercise (EE) does not inhibit the effects of resistance exercise. Here, we examined the effect of short‐ or long‐duration EE on mouse skeletal muscle hypertrophy induced by functional overload (OL) at the molecular level. Plantaris muscle hypertrophy was induced by OL with synergist ablation in mice. Body mass was reduced with endurance training, but EE duration (30 or 90 min) had no effect. The ratio of plantaris muscle weight to body weight was higher in the OL and EE for 30 min (OL+EE30) and OL and EE for 90 min (OL+EE90) groups compared with the OL group. Expression of mechanistic target of rapamycin signaling proteins, which is related to protein synthesis and hypertrophy, was increased in the OL+EE30 group. Expression of Forkhead box‐containing protein O1, which is related to protein breakdown and atrophy, remained unchanged. However, microtubule‐associated protein 1 light chain 3, a known marker of autophagy, and MAFbx, which is related to protein breakdown, were significantly increased in the OL+EE90 group. Furthermore, markers of oxidative stress, ubiquitin and 4‐hydroxynonenal were also significantly increased in the OL+EE90 group compared with other groups. In conclusion, EE duration did not affect body mass and plantaris mass and did not interfere with mechanistic target of rapamycin signaling, but it did increase ubiquitinated proteins and oxidative stress. It is therefore necessary to consider training durations for EE when combining endurance and resistance training. [ABSTRACT FROM AUTHOR]

The regulation of cellular protein synthesis is a critical determinant of skeletal muscle growth and hypertrophy in response to an increased workload such as resistance exercise. The mechanistic target of rapamycin complex 1 (mTORC1) and its upstream protein kinase Akt1 have been implicated as a central signaling pathway that regulates protein synthesis in the skeletal muscle; however, the precise molecular regulation of mTORC1 activity is largely unknown. This study employed germline Akt1 knockout (KO) mice to examine whether upstream Akt1 regulation is necessary for the acute activation of mTORC1 signaling in the plantaris muscle following mechanical overload. The phosphorylation states of S6 kinase 1, ribosomal protein S6, and eukaryotic translation initiation factor 4E‐binding protein 1 which show the functional activity of mTORC1 signaling, were significantly increased in the skeletal muscle of both wildtype and Akt1 KO mice following an acute bout (3 and 12 hr) of mechanical overload. Akt1 deficiency did not affect load‐induced alteration of insulin‐like growth factor‐1 (IGF‐1)/IGF receptor mRNA expression. Also, no effect of Akt1 deficiency was observed on the overload‐induced increase in the gene expressions of pax7 and myogenic regulatory factor of myogenin. These observations show that the upstream IGF‐1/Akt1 regulation is dispensable for the acute activation of mTORC1 signaling and regulation of satellite cells in response to mechanical overload. [ABSTRACT FROM AUTHOR]

Denervation‐induced muscle atrophy increases signaling through both protein degradation and synthesis pathways. Renalase is a flavin adenine dinucleotide‐dependent amine oxidase that inhibits apoptosis and inflammation and promotes cell survival. This study aimed to elucidate the effect of renalase on denervation‐induced muscle atrophy. We used 7‐week‐old renalase knock‐out (KO) mice (a model of denervation‐induced muscle atrophy) and wild‐type (WT) mice (KO: n = 6, weight = 20–26 g; WT: n = 5, weight = 19–23 g). After their left legs were denervated, these mice were killed 1 week later. KO mice had lighter muscle weight than the WT mice. We observed an increase in molecular signaling through protein degradation pathway as well as oxidative stress in denervated muscles compared with that in sham‐operated muscles in both WT and KO mice. Additionally, we also observed the main effect of renalase in WT and KO mice. Mitochondrial oxidative phosphorylation protein content was lower in denervated muscles than in sham‐operated muscles in both WT and KO mice. However, a significant difference was noted in the reaction with Akt and p70S6K (components of the protein synthesis pathway) between WT and KO mice. In conclusion, mice with renalase deficiency demonstrated an attenuation of denervation‐induced muscle atrophy. This might be related to catecholamines because signaling through the protein synthesis pathway was increased following denervation in renalase KO mice compared with that in WT mice, despite showing no change in signaling through protein degradation pathways. [ABSTRACT FROM AUTHOR]

Concurrent training involves a combination of two different modes of training. In this study, we conducted an experiment by combining resistance and endurance training. The purpose of this study was to investigate the influence of the order of concurrent training on signal molecules in skeletal muscle. The phosphorylation levels of p70 S6 kinase, S6 ribosomal protein, and 4E‐binding protein 1, which are related to hypertrophy signaling, increased significantly in the resistance–endurance order group as compared with in control group not the endurance–resistance order group. The gene expressions related to metabolism were not changed by the order of concurrent training. The mitochondrial respiratory chain complex was evaluated by western blot. Although both groups of concurrent training showed a significant increase in MTCO1, UQCRC2, and ATP5A protein levels, we could not detect a difference based on the order of concurrent training. In conclusion, a concurrent training approach involving resistance training before endurance training on the same day is an effective way to activate both mTOR signaling and mitochondria biogenesis. [ABSTRACT FROM AUTHOR]

The enhanced rate of protein synthesis in skeletal muscle cells results in a net increase in total protein content that leads to skeletal muscle growth/hypertrophy. The mitogen-activated protein kinase kinase (MEK)/extracellular signal-regulated kinase (ERK)-dependent regulation of the activity of mechanistic target of rapamycin (mTOR) and subsequent protein synthesis has been suggested as a regulatory mechanism; however, the exact molecular processes underlying such a regulation are poorly defined. The purpose of this study was to investigate regulatory mechanisms involved in the MEK/ERK-dependent pathway leading to mTORC1 activation in skeletal muscle cells. Treatment with phorbol-12-myristate-13-acetate (PMA), a potent agonist of protein kinase C (PKC) and its downstream effector in the MEK/ERK-dependent pathway, resulted in the activation of mTORC1 signaling and phosphorylation of the upstream regulator tuberous sclerosis 2 (TSC2) in C2C12 myoblasts. PMA-induced activation of mTORC1 signaling was partially prevented by treatment with U0126 (a selective inhibitor of MEK1/2) or BIX-02189 (a selective inhibitor of MEK5) and completely blocked with BIM-I (a selective inhibitor of upstream PKC). TSC2 phosphorylation at Ser664 (an ERK-dependent phosphorylation site) was prevented with U0126, and BIM-I treatment blocked PMA-induced phosphorylation of TSC2 at multiple residues (Ser664, Ser939, and Thr1462). Overexpression of Ras homolog enriched in brain (Rheb), a downstream target of TSC2, and an mTORC1 activator, was sufficient to activate mTORC1 signaling. We also identified that PMA-induced activation of mTORC1 signaling was significantly inhibited in the absence of Rheb with siRNA knockdown. These observations demonstrate that the PKC/MEK/ERK-dependent activation of mTORC1 is mediated through TSC2 phosphorylation and its downstream target Rheb in C2C12 myoblasts. [ABSTRACT FROM AUTHOR]

The purposes of our study were to examine (1) Rh B glycoprotein (Rhbg) and Rh C glycoprotein (Rhcg) expressions in mouse skeletal muscle; and (2) the effect of 6-week training on Rhbg and Rhcg expressions. The levels of Rhbg and Rhcg expressions were much higher in the soleus (Sol) than in the plantaris (Pla) or gastrocnemius (Gas). Immunofluorescence microscopy demonstrated that Rhbg colocalizes with dystrophin, a plasma membrane protein marker, whereas Rhcg colocalizes with CD31, a vascular endothelial cell marker. In a 6-week swim training study, we set up two different training groups. Endurance ( END) group underwent swim training without load for 30 min and exercise time was increased by 30 min every 2 weeks. High-intensity interval training ( HIIT) group underwent 10-12 sets of swim training at 20 sec per set and intervals of 10 sec, with a load of 10% body weight. After 6 weeks of training, all mice performed exhaustive swimming performance test ( PT), with 9% of body weight to exhaustion. HIIT group could significantly swim more and showed significantly lower blood ammonia level compared with control ( CON) group at immediately after PT. Rhbg and Rhcg levels did not change after 6 weeks in both END and HIIT groups. Our results indicate that ammonia transporters are more abundant in slow fiber than fast fiber muscles and 6 weeks swim training suppresses blood ammonia elevation induced by high-intensity exercise with performance improvement, although the levels of ammonia transporter proteins does not change. [ABSTRACT FROM AUTHOR]

Mammalian target of rapamycin (mTOR) pathway positively regulates the cell growth through ribosome biogenesis in many cell type. In general, myostatin is understood to repress skeletal muscle hypertrophy through inhibition of mTOR pathway and myogenesis. However, these relationships have not been clarified in skeletal muscle undergoing atrophy. Here, we observed a significant decrease of skeletal muscle mass at 2 weeks after denervation. Unexpectedly, however, mTOR pathway and the expression of genes related to myogenesis were markedly increased, and that of myostatin was decreased. However, de novo ribosomal RNA synthesis and the levels of ribosomal RNAs were dramatically decreased in denervated muscle. These results indicate that ribosome biogenesis is strongly controlled by factors other than the mTOR pathway in denervated atrophic muscle. Finally, we assessed rRNA transcription factors expression and observed that TAFIa was the only factor decreased. TAFIa might be a one of the limiting factor for rRNA synthesis in denervated muscle. J. Cell. Physiol. 227: 1569-1576, 2012. © 2011 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

The aim of this study was to investigate the effects of training and detraining on the monocarboxylate transporter (MCT) 1 and MCT4 levels in the gluteus medius muscle of Thoroughbred horses. Twelve Thoroughbred horses were used for the analysis. For 18 weeks, all the horses underwent high-intensity training (HIT), with running at 90-110% maximal oxygen consumption () for 3 min, 5 days week. Thereafter, the horses either underwent detraining for 6 weeks by either 3 min of moderate-intensity training (MIT) at 70%, 5 days week (HIT-MIT group) or stall rest (HIT-SR group). The horses underwent an incremental exercise test, was measured and resting muscle samples were obtained from the middle gluteus muscle at 0, 18 and 24 weeks. The content of MCT1 and MCT4 proteins increased after 18 weeks of HIT. At the end of this period, an increase was noted in the citrate synthase activity, while phosphofructokinase activity remained unchanged. After 6 weeks of detraining, all these indexes returned to the pretraining levels in the HIT-SR group. However, in the HIT-MIT group, the increase in the MCT1 protein content and citrate synthase activity was maintained after 6 weeks of MIT, while the MCT4 protein content decreased to the pretraining value. These results suggest that the content of MCT1 and MCT4 proteins increases after HIT in Thoroughbred horses. In addition, the increase in the MCT1 protein content and oxidative capacity induced by HIT can be maintained by MIT of 70%, but the increase in the MCT4 protein content cannot be maintained by MIT. [ABSTRACT FROM AUTHOR]

Oxidative stress and mitochondrial dysfunction are associated with the aging process. However, the role of nuclear factor erythroid 2 ‐related factor 2 (Nrf2) in skeletal muscle during aging remains to be clarified. In the current study, we assessed whether the lack of Nrf2, which is known as a master regulator of redox homeostasis, promotes age‐related mitochondrial dysfunction and muscle atrophy in skeletal muscle. Here, we demonstrated that mitochondrial 4‐hydroxynonenal and protein carbonyls, markers of oxidative stress, were robustly elevated in aged Nrf2 knockout (KO) mice because of the decreased expression of Nrf2‐target antioxidant genes. Mitochondrial respiration declined with aging; however, there was no difference between Nrf2 KO and age‐matched WT mice. Similarly, cytochrome c oxidase activity was lower in aged WT and Nrf2 KO mice compared with young WT mice. The expression of Mfn1 and Mfn2 mRNA was lower in aged Nrf2 KO muscle. Mitochondrial reactive oxygen species production per oxygen consumed was elevated in aged Nrf2 KO mice. There was no effect of Nrf2 KO on muscle mass normalized to body weight. These results suggest that Nrf2 deficiency exacerbates age‐related mitochondrial oxidative stress but does not affect the decline of respiratory function in skeletal muscle. We demonstrated that mitochondrial 4‐hydroxynonenal and protein carbonyls, markers of oxidative stress, were robustly elevated in aged Nrf2 knockout mice, due to the decreased expression of Nrf2‐target antioxidant genes. However, Nrf2 deficiency did not affect the decline of mitochondrial respiratory function in aged skeletal muscle. [ABSTRACT FROM AUTHOR]

The aim of this study was to investigate the effects of training and detraining on the monocarboxylate transporter (MCT) 1 and MCT4 levels in the gluteus medius muscle of Thoroughbred horses. Twelve Thoroughbred horses were used for the analysis. For 18 weeks, all the horses underwent high-intensity training (HIT), with running at 90-110% maximal oxygen consumption (VO2 max ) for 3 min, 5 days week(-1). Thereafter, the horses either underwent detraining for 6 weeks by either 3 min of moderate-intensity training (MIT) at 70% VO2 max, 5 days week(-1) (HIT-MIT group) or stall rest (HIT-SR group). The horses underwent an incremental exercise test, VO2 max was measured and resting muscle samples were obtained from the middle gluteus muscle at 0, 18 and 24 weeks. The content of MCT1 and MCT4 proteins increased after 18 weeks of HIT. At the end of this period, an increase was noted in the citrate synthase activity, while phosphofructokinase activity remained unchanged. After 6 weeks of detraining, all these indexes returned to the pretraining levels in the HIT-SR group. However, in the HIT-MIT group, the increase in the MCT1 protein content and citrate synthase activity was maintained after 6 weeks of MIT, while the MCT4 protein content decreased to the pretraining value. These results suggest that the content of MCT1 and MCT4 proteins increases after HIT in Thoroughbred horses. In addition, the increase in the MCT1 protein content and oxidative capacity induced by HIT can be maintained by MIT of 70% VO2 max, but the increase in the MCT4 protein content cannot be maintained by MIT.