Skeletal muscle hypertrophy rewires glucose metabolism: An experimental investigation and systematic review

BACKGROUND

Proliferating cancer cells shift their metabolism towards glycolysis, even in the presence of oxygen, to especially generate glycolytic intermediates as substrates for anabolic reactions. We hypothesize that a similar metabolic remodelling occurs during skeletal muscle hypertrophy.

METHODS

We used mass spectrometry in hypertrophying C2C12 myotubes in vitro and plantaris mouse muscle in vivo and assessed metabolomic changes and the incorporation of the [U-13C6]glucose tracer. We performed enzyme inhibition of the key serine synthesis pathway enzyme phosphoglycerate dehydrogenase (Phgdh) for further mechanistic analysis and conducted a systematic review to align any changes in metabolomics during muscle growth with published findings. Finally, the UK Biobank was used to link the findings to population level.

RESULTS

The metabolomics analysis in myotubes revealed insulin-like growth factor-1 (IGF-1)-induced altered metabolite concentrations in anabolic pathways such as pentose phosphate (ribose-5-phosphate/ribulose-5-phosphate: +40%; P = 0.01) and serine synthesis pathway (serine: -36.8%; P = 0.009). Like the hypertrophy stimulation with IGF-1 in myotubes in vitro, the concentration of the dipeptide l-carnosine was decreased by 26.6% (P = 0.001) during skeletal muscle growth in vivo. However, phosphorylated sugar (glucose-6-phosphate, fructose-6-phosphate or glucose-1-phosphate) decreased by 32.2% (P = 0.004) in the overloaded muscle in vivo while increasing in the IGF-1-stimulated myotubes in vitro. The systematic review revealed that 10 metabolites linked to muscle hypertrophy were directly associated with glycolysis and its interconnected anabolic pathways. We demonstrated that labelled carbon from [U-13C6]glucose is increasingly incorporated by ~13% (P = 0.001) into the non-essential amino acids in hypertrophying myotubes, which is accompanied by an increased depletion of media serine (P = 0.006). The inhibition of Phgdh suppressed muscle protein synthesis in growing myotubes by 58.1% (P < 0.001), highlighting the importance of the serine synthesis pathway for maintaining muscle size. Utilizing data from the UK Biobank (n = 450 243), we then discerned genetic variations linked to the serine synthesis pathway (PHGDH and PSPH) and to its downstream enzyme (SHMT1), revealing their association with appendicular lean mass in humans (P < 5.0e-8).

CONCLUSIONS

Understanding the mechanisms that regulate skeletal muscle mass will help in developing effective treatments for muscle weakness. Our results provide evidence for the metabolic rewiring of glycolytic intermediates into anabolic pathways during muscle growth, such as in serine synthesis.

Interaction of the C2C12 myotube contractions and glucose availability on transcriptome and extracellular vesicle microRNAs

Exercise-like electrical pulse stimulation (EL-EPS) of myotubes mimics many key physiological changes induced by in vivo exercise. Besides enabling intracellular research, EL-EPS allows to study secreted factors, including muscle-specific microRNAs (myomiRs) carried in extracellular vesicles (EVs). These factors can participate in contraction-induced intercellular cross talk and may mediate the health benefits of exercise. However, the current knowledge of these responses, especially under variable nutritional conditions, is limited. We investigated the effects of EL-EPS on C2C12 myotube transcriptome in high- and low-glucose conditions by messenger RNA sequencing, while the expression of EV-carried miRNAs was analyzed by small RNA sequencing and RT-qPCR. We show that higher glucose availability augmented contraction-induced transcriptional changes and that the majority of the differentially expressed genes were upregulated. Furthermore, based on the pathway analyses, processes related to contractility and cytokine/inflammatory responses were upregulated. In addition, we report that EL-EPS increased packing of miR-1-3p into EVs independent of glucose availability. Together our findings suggest that in vitro EL-EPS is a usable tool not only to study contraction-induced intracellular mechanisms but also extracellular responses. The distinct transcriptional changes observed under variable nutritional conditions emphasize the importance of careful consideration of media composition in future exercise-mimicking studies.NEW & NOTEWORTHY The present study examined for the first time the effects of exercise-like electrical pulse stimulation administered under distinct nutritional conditions on 1) the transcriptome of the C2C12 myotubes and 2) their media containing extracellular vesicle-carried microRNAs. We report that higher glucose availability augmented transcriptional responses related especially to contractility and cytokine/inflammatory pathways. Agreeing with in vivo studies, we show that the packing of exercise-responsive miR-1-3p was increased in the extracellular vesicles in response to myotube contractions.

Serine synthesis pathway enzyme PHGDH is critical for muscle cell biomass, anabolic metabolism, and mTORC1 signaling

Cells use glycolytic intermediates for anabolism, e.g., via the serine synthesis and pentose phosphate pathways. However, we still understand poorly how these metabolic pathways contribute to skeletal muscle cell biomass generation. The first aim of this study was therefore to identify enzymes that limit protein synthesis, myotube size, and proliferation in skeletal muscle cells. We inhibited key enzymes of glycolysis, the pentose phosphate pathway, and the serine synthesis pathway to evaluate their importance in C2C12 myotube protein synthesis. Based on the results of this first screen, we then focused on the serine synthesis pathway enzyme phosphoglycerate dehydrogenase (PHGDH). We used two different PHGDH inhibitors and mouse C2C12 and human primary muscle cells to study the importance and function of PHGDH. Both myoblasts and myotubes incorporated glucose-derived carbon into proteins, RNA, and lipids, and we showed that PHGDH is essential in these processes. PHGDH inhibition decreased protein synthesis, myotube size, and myoblast proliferation without cytotoxic effects. The decreased protein synthesis in response to PHGDH inhibition appears to occur mainly mechanistic target of rapamycin complex 1 (mTORC1)-dependently, as was evident from experiments with insulin-like growth factor 1 and rapamycin. Further metabolomics analyses revealed that PHGDH inhibition accelerated glycolysis and altered amino acid, nucleotide, and lipid metabolism. Finally, we found that supplementing an antioxidant and redox modulator, N-acetylcysteine, partially rescued the decreased protein synthesis and mTORC1 signaling during PHGDH inhibition. The data suggest that PHGDH activity is critical for skeletal muscle cell biomass generation from glucose and that it regulates protein synthesis and mTORC1 signaling.NEW & NOTEWORTHY The use of glycolytic intermediates for anabolism was demonstrated in both myoblasts and myotubes, which incorporate glucose-derived carbon into proteins, RNA, and lipids. We identify phosphoglycerate dehydrogenase (PHGDH) as a critical enzyme in those processes and also for muscle cell hypertrophy, proliferation, protein synthesis, and mTORC1 signaling. Our results thus suggest that PHGDH in skeletal muscle is more than just a serine-synthesizing enzyme.

Effects of fat loss and low energy availability on the serum cardiometabolic profile of physique athletes

Low energy availability (LEA) is a health concern for athletes, although it may paradoxically lead to improved cardiometabolic health in the general population. We investigated the associations between LEA, body composition, and serum cardiometabolic profile in 23 physique athletes (DIET) and 21 controls (CONT) during a 5-month pre-competition diet (MID), followed by 1 week of increased energy availability (COMP) and a 5-month weight regain period (POST). Quantification of 250 serum metabolome variables was conducted by NMR spectroscopy, body composition by dual-energy x-ray absorptiometry, dietary intake by food diaries, and exercise levels by training logs. Body fat percentage decreased from 19.5 ± 7.0% to 8.3 ± 5.3% (p < 0.001) in DIET through increased exercise levels and decreased energy intake, while CONT maintained those constant. In MID, DIET had increased (FDR < 0.01) HDL cholesterol, HDL particle size and number, and decreased (FDR < 0.05) VLDL lipids, serum triglycerides, and low-grade inflammation (glycoprotein acetyls) compared to baseline and CONT. The changes were associated with reduced android fat mass (-78 ± 13%) and energy intake (-28 ± 10%). In COMP, most of the metabolic changes found in MID persisted, except for altered triglycerides in all lipoprotein classes. After weight regain in POST, serum metabolome, body composition, energy intake, and exercise levels had reverted to baseline levels. In conclusion, fat loss and LEA may have beneficial yet transient effects on the serum cardiometabolic profile of lean individuals. Especially the HDL lipidome and lipoprotein triglycerides offer potential novel biomarkers for detecting LEA in athletes.

Age But Not Menopausal Status Is Linked to Lower Resting Energy Expenditure

CONTEXT

It remains uncertain whether aging before late adulthood and menopause are associated with fat-free mass and fat mass-adjusted resting energy expenditure (REEadj).

OBJECTIVES

We investigated whether REEadj differs between middle-aged and younger women and between middle-aged women with different menopausal statuses. We repeated the age group comparison between middle-aged mothers and their daughters to partially control for genotype. We also explored whether serum estradiol and FSH concentrations explain REEadj in midlife.

METHODS

We divided 120 women, including 16 mother-daughter pairs, into age groups; group I (n = 26) consisted of participants aged 17 to 21, group II (n = 35) of those aged 22 to 38, and group III (n = 59) of those aged 41 to 58 years. The women in group III were further categorized as pre- or perimenopausal (n = 19), postmenopausal (n = 30), or postmenopausal hormone therapy users (n = 10). REE was assessed using indirect calorimetry, body composition using dual-energy X-ray absorptiometry, and hormones using immunoassays.

RESULTS

The REEadj of group I was 126 kcal/day [95% confidence interval (CI): 93-160] higher than that of group III, and the REEadj of group II was 88 kcal/day (95% CI: 49-127) higher. Furthermore, daughters had a 100 kcal/day (95% CI: 63-138 kcal/day) higher REEadj than their middle-aged mothers (all P < .001). In group III, REEadj was not lower in postmenopausal women and did not vary by sex hormone concentrations.

CONCLUSIONS

We demonstrated that REEadj declines with age in women before late adulthood, also when controlling partially for genetic background, and that menopause may not contribute to this decline.

NAD+ repletion with niacin counteracts cancer cachexia

Cachexia is a debilitating wasting syndrome and highly prevalent comorbidity in cancer patients. It manifests especially with energy and mitochondrial metabolism aberrations that promote tissue wasting. We recently identified nicotinamide adenine dinucleotide (NAD⁺) loss to associate with muscle mitochondrial dysfunction in cancer hosts. In this study we confirm that depletion of NAD⁺ and downregulation of Nrk2, an NAD⁺ biosynthetic enzyme, are common features of severe cachexia in different mouse models. Testing NAD⁺ repletion therapy in cachectic mice reveals that NAD⁺ precursor, vitamin B3 niacin, efficiently corrects tissue NAD⁺ levels, improves mitochondrial metabolism and ameliorates cancer- and chemotherapy-induced cachexia. In a clinical setting, we show that muscle NRK2 is downregulated in cancer patients. The low expression of NRK2 correlates with metabolic abnormalities underscoring the significance of NAD⁺ in the pathophysiology of human cancer cachexia. Overall, our results propose NAD⁺ metabolism as a therapy target for cachectic cancer patients.

Mimicking exercise in vitro - effects of myotube contractions and mechanical stretch on omics

The number of studies using skeletal muscle (SkM) cell culture models to study exercise in vitro are rapidly expanding. Progressively, more comprehensive analysis methods, such as different omics approaches including transcriptomics, proteomics and metabolomics have been used to examine the intra- and extracellular molecular responses to exercise mimicking stimuli in cultured myotubes. Among other techniques, exercise-like electrical pulse stimulation (EL-EPS) and mechanical stretch of SkM cells are the two most commonly used methods to mimic exercise in vitro. In this mini-review we focus on these two approaches and their effects on the omics of myotubes and/or cell culture media. Furthermore, besides traditional two-dimensional (2D) methods, the use of three-dimensional (3D) SkM approaches are increasing in the field of in vitro exercise mimicry. Our aim with this mini-review is to provide the reader with an up-to-date overview of the 2D and 3D models and the use of omics approaches to study the molecular response to exercise in vitro.

Effects of Long-Term Physical Activity and BCAA Availability on the Subcellular Associations between Intramyocellular Lipids, Perilipins and PGC-1α

Cellular skeletal muscle lipid metabolism is of paramount importance for metabolic health, specifically through its connection to branched-chain amino acids (BCAA) metabolism and through its modulation by exercise. In this study, we aimed at better understanding intramyocellular lipids (IMCL) and their related key proteins in response to physical activity and BCAA deprivation. By means of confocal microscopy, we examined IMCL and the lipid droplet coating proteins PLIN2 and PLIN5 in human twin pairs discordant for physical activity. Additionally, in order to study IMCLs, PLINs and their association to peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α) in cytosolic and nuclear pools, we mimicked exercise-induced contractions in C2C12 myotubes by electrical pulse stimulation (EPS), with or without BCAA deprivation. The life-long physically active twins displayed an increased IMCL signal in type I fibers when compared to their inactive twin pair. Moreover, the inactive twins showed a decreased association between PLIN2 and IMCL. Similarly, in the C2C12 cell line, PLIN2 dissociated from IMCL when myotubes were deprived of BCAA, especially when contracting. In addition, in myotubes, EPS led to an increase in nuclear PLIN5 signal and its associations with IMCL and PGC-1α. This study demonstrates how physical activity and BCAA availability affects IMCL and their associated proteins, providing further and novel evidence for the link between the BCAA, energy and lipid metabolisms.

Weight loss induces changes in adaptive thermogenesis in female and male physique athletes

Physique athletes lose substantial weight preparing for competitions, potentially altering systemic metabolism. We investigated sex differences in body composition, resting energy expenditure (REE), and appetite-regulating and thyroid hormone changes during a competition preparation among drug-free physique athletes. The participants were female (10 competing (COMP) and 10 nondieting controls (CTRL)) and male (13 COMP and 10 CTRL) physique athletes. COMP were tested before they started their diet 23 weeks before competing (PRE), during their diet one week before competing (MID), and 23 weeks after competing (POST), whereas CTRL were tested at similar intervals but did not diet. Measurements included body composition by dual-energy X-ray absorptiometry, muscle size, and subcutaneous fat thickness (SFA) by ultrasound, REE by indirect calorimetry, circulating ghrelin, leptin T3, and T4 hormone analysis. Fat mass (FM) and SFA decreased in both sexes (p < 0.001), while males (p < 0.001) lost more lean mass (LM) than females (p < 0.05). Weight loss, decreased energy intake, and increased aerobic exercise (p < 0.05) led to decreased LM- and FM-adjusted REE (p < 0.05), reflecting metabolic adaptation. Absolute leptin levels decreased in both sexes (p < 0.001) but more among females (p < 0.001) due to higher baseline leptin levels. These changes occurred with similar decreases in T3 (p < 0.001) and resting heart rate (p < 0.01) in both sexes. CTRL, who were former or upcoming physique athletes, showed no systematic changes in any measured variables. In conclusion, while dieting, female and male physique athletes experience REE and hormonal changes leading to adaptive thermogenesis. However, responses seemed temporary as they returned toward baseline after the recovery phase. ClinicalTrials.gov (NCT04392752).

Higher glucose availability augments the metabolic responses of the C2C12 myotubes to exercise-like electrical pulse stimulation

The application of exercise-like electrical pulse simulation (EL-EPS) has become a widely used exercise mimetic in vitro. EL-EPS produces similar physiological responses as in vivo exercise, while less is known about the detailed metabolic effects. Routinely, the C2C12 myotubes are cultured in high-glucose medium (4.5 g/L), which may alter EL-EPS responses. In this study, we evaluate the metabolic effects of EL-EPS under the high- and low-glucose (1.0 g/L) conditions to understand how substrate availability affects the myotube response to EL-EPS. The C2C12 myotube, media, and cell-free media metabolites were analyzed using untargeted nuclear magnetic resonance (NMR)-based metabolomics. Furthermore, translational and metabolic changes and possible exerkine effects were analyzed. EL-EPS enhanced substrate utilization as well as production and secretion of lactate, acetate, 3-hydroxybutyrate, and branched-chain fatty acids (BCFAs). The increase in BCFAs correlated with branched-chain amino acids (BCAAs) and BCFAs were strongly decreased when myotubes were cultured without BCAAs suggesting the action of acyl-CoA thioesterases on BCAA catabolites. Notably, not all EL-EPS responses were augmented by high glucose because EL-EPS increased phosphorylated c-Jun N-terminal kinase and interleukin-6 secretion independent of glucose availability. Administration of acetate and EL-EPS conditioned media on HepG2 hepatocytes had no adverse effects on lipolysis or triacylglycerol content. Our results demonstrate that unlike in cell-free media, the C2C12 myotube and media metabolites were affected by EL-EPS, particularly under high-glucose condition suggesting that media composition should be considered in future EL-EPS studies. Furthermore, acetate and BCFAs were identified as putative exerkines warranting more research.NEW & NOTEWORTHY The present study examined for the first time the metabolome of 1) C2C12 myotubes, 2) their growth media, and 3) cell-free media after exercise-like electrical pulse stimulation under distinct nutritional loads. We report that myotubes grown under high-glucose conditions had greater responsiveness to EL-EPS when compared with lower glucose availability conditions and increased media content of acetate and branched-chain fatty acids suggests they might act as putative exerkines warranting further research.

Muscle follistatin gene delivery increases muscle protein synthesis independent of periodical physical inactivity and fasting

Blocking of myostatin and activins effectively counteracts muscle atrophy. However, the potential interaction with physical inactivity and fasting in the regulation of muscle protein synthesis is poorly understood. We used blockade of myostatin and activins by recombinant adeno-associated virus (rAAV)-mediated follistatin (FS288) overexpression in mouse tibialis anterior muscle. To investigate the effects on muscle protein synthesis, muscles were collected 7 days after rAAV-injection in the nighttime or in the daytime representing high and low levels of activity and feeding, respectively, or after overnight fasting, refeeding, or ad libitum feeding. Muscle protein synthesis was increased by FS288 independent of the time of the day or the feeding status. However, the activation of mTORC1 signaling by FS288 was attenuated in the daytime and by overnight fasting. FS288 also increased the amount of mTOR colocalized with lysosomes, but did not alter their localization toward the sarcolemma. This study shows that FS288 gene delivery increases muscle protein synthesis largely independent of diurnal fluctuations in physical activity and food intake or feeding status, overriding the physiological signals. This is important for eg cachectic and sarcopenic patients with reduced physical activity and appetite. The FS288-induced increase in mTORC1 signaling and protein synthesis may be in part driven by increased amount of mTOR colocalized with lysosomes, but not by their localization toward sarcolemma.

Mitochondrial bioenergetic pathways in blood leukocyte transcriptome decrease after intensive weight loss but are rescued following weight regain in female physique athletes

Prolonged periods of energy deficit leading to weight loss induce metabolic adaptations resulting in reduced energy expenditure, but the mechanisms for energy conservation are incompletely understood. We examined 42 healthy athletic females (age 27.5 ± 4.0 years, body mass index 23.4 ± 1.7 kg/m² ) who volunteered into either a group dieting for physique competition (n = 25) or a control group (n = 17). The diet group substantially reduced their energy intake and moderately increased exercise levels to induce loss of fat mass that was regained during a voluntary weight regain period. The control group maintained their typical lifestyle habits and body mass as instructed. From the diet group, fasting blood samples were drawn at baseline (PRE), after 4- to 5-month weight loss (PRE-MID), and after 4- to 5-month weight regain (MID-POST) as well as from the control group at similar intervals. Blood was analyzed to determine leukocyte transcriptome by RNA-Sequencing and serum metabolome by nuclear magnetic resonance (NMR) platform. The intensive weight loss period induced several metabolic adaptations, including a prominent suppression of transcriptomic signature for mitochondrial OXPHOS and ribosome biogenesis. The upstream regulator analysis suggested that this reprogramming of cellular energy metabolism may be mediated via AMPK/PGC1-α signaling and mTOR/eIF2 signaling-dependent pathways. Our findings show for the first time that prolonged energy deprivation induced modulation of mitochondrial metabolism can be observed through minimally invasive measures of leukocyte transcriptome and serum metabolome at systemic level, suggesting that adaptation to energy deficit is broader in humans than previously thought.

Muscle NAD+ depletion and Serpina3n as molecular determinants of murine cancer cachexia-the effects of blocking myostatin and activins

Objective

Cancer cachexia and muscle loss are associated with increased morbidity and mortality. In preclinical animal models, blocking activin receptor (ACVR) ligands has improved survival and prevented muscle wasting in cancer cachexia without an effect on tumour growth. However, the underlying mechanisms are poorly understood. This study aimed to identify cancer cachexia and soluble ACVR (sACVR) administration-evoked changes in muscle proteome.

Methods

Healthy and C26 tumour-bearing (TB) mice were treated with recombinant sACVR. The sACVR or PBS control were administered either prior to the tumour formation or by continued administration before and after tumour formation. Muscles were analysed by quantitative proteomics with further examination of mitochondria and nicotinamide adenine dinucleotide (NAD⁺) metabolism. To complement the first prophylactic experiment, sACVR (or PBS) was injected as a treatment after tumour cell inoculation.

Results

Muscle proteomics in TB cachectic mice revealed downregulated signatures for mitochondrial oxidative phosphorylation (OXPHOS) and increased acute phase response (APR). These were accompanied by muscle NAD⁺ deficiency, alterations in NAD⁺ biosynthesis including downregulation of nicotinamide riboside kinase 2 (Nrk2), and decreased muscle protein synthesis. The disturbances in NAD⁺ metabolism and protein synthesis were rescued by treatment with sACVR. Across the whole proteome and APR, in particular, Serpina3n represented the most upregulated protein and the strongest predictor of cachexia. However, the increase in Serpina3n expression was associated with increased inflammation rather than decreased muscle mass and/or protein synthesis.

Conclusions

We present evidence implicating disturbed muscle mitochondrial OXPHOS proteome and NAD⁺ homeostasis in experimental cancer cachexia. Treatment of TB mice with a blocker of activin receptor ligands restores depleted muscle NAD⁺ and Nrk2, as well as decreased muscle protein synthesis. These results indicate putative new treatment therapies for cachexia and that although acute phase protein Serpina3n may serve as a predictor of cachexia, it more likely reflects a condition of elevated inflammation.

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Cachectic muscle proteome shows decreased OXPHOS and increased acute phase response.

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Cancer cachexia is characterized by lowered muscle Nrk2 expression and NAD⁺ levels.

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Blocking activin receptor 2B ligands rescues muscle NAD⁺ homeostasis in cachexia.

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Blocking activin receptor 2B ligands prevents affected protein synthesis in cachexia.

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Serpina3n predicts cachexia and cancer-induced APR independently from muscle atrophy.

Systemic blockade of ACVR2B ligands attenuates muscle wasting in ischemic heart failure without compromising cardiac function

Signaling through activin receptors regulates skeletal muscle mass and activin receptor 2B (ACVR2B) ligands are also suggested to participate in myocardial infarction (MI) pathology in the heart. In this study, we determined the effect of systemic blockade of ACVR2B ligands on cardiac function in experimental MI, and defined its efficacy to revert muscle wasting in ischemic heart failure (HF). Mice were treated with soluble ACVR2B decoy receptor (ACVR2B-Fc) to study its effect on post-MI cardiac remodeling and on later HF. Cardiac function was determined with echocardiography, and myocardium analyzed with histological and biochemical methods for hypertrophy and fibrosis. Pharmacological blockade of ACVR2B ligands did not rescue the heart from ischemic injury or alleviate post-MI remodeling and ischemic HF. Collectively, ACVR2B-Fc did not affect cardiomyocyte hypertrophy, fibrosis, angiogenesis, nor factors associated with cardiac regeneration except modification of certain genes involved in metabolism or cell growth/survival. ACVR2B-Fc, however, was able to reduce skeletal muscle wasting in chronic ischemic HF, accompanied by reduced LC3II as a marker of autophagy and increased mTOR signaling and Cited4 expression as markers of physiological hypertrophy in quadriceps muscle. Our results ascertain pharmacological blockade of ACVR2B ligands as a possible therapy for skeletal muscle wasting in ischemic HF. Pharmacological blockade of ACVR2B ligands preserved myofiber size in ischemic HF, but did not compromise cardiac function nor exacerbate cardiac remodeling after ischemic injury.

Differentiation of Murine C2C12 Myoblasts Strongly Reduces the Effects of Myostatin on Intracellular Signaling

Alongside in vivo models, a simpler and more mechanistic approach is required to study the effects of myostatin on skeletal muscle because myostatin is an important negative regulator of muscle size. In this study, myostatin was administered to murine (C2C12) and human (CHQ) myoblasts and myotubes. Canonical and noncanonical signaling downstream to myostatin, related ligands, and their receptor were analyzed. The effects of tumorkines were analyzed after coculture of C2C12 and colon cancer-C26 cells. The effects of myostatin on canonical and noncanonical signaling were strongly reduced in C2C12 cells after differentiation. This may be explained by increased follistatin, an endogenous blocker of myostatin and altered expression of activin receptor ligands. In contrast, CHQ cells were equally responsive to myostatin, and follistatin remained unaltered. Both myostatin administration and the coculture stimulated pathways associated with inflammation, especially in C2C12 cells. In conclusion, the effects of myostatin on intracellular signaling may be cell line- or organism-specific, and C2C12 myotubes seem to be a nonoptimal in vitro model for investigating the effects of myostatin on canonical and noncanonical signaling in skeletal muscle. This may be due to altered expression of activin receptor ligands and their regulators during muscle cell differentiation.

Sprint and Strength Training Modulates Autophagy and Proteostasis in Aging Sprinters

PURPOSE

Exercise and aging may modulate muscle protein homeostasis and autophagy, but few studies examine highly trained middle-age or older individuals. This study elucidated the effects of a new long-term training stimulus on markers of muscle autophagy and unfolded protein response (UPR) and on sprint running performance in masters sprinters.

METHODS

Thirty-two male competitive sprinters (age 40-76 yr) were randomly divided into experimental (EX) and control (CTRL) groups. The EX training program was a combination of heavy and explosive strength and sprint exercises aimed at improving sprint performance. Fifteen and thirteen participants completed the 20-wk intervention period in EX and CTRL, respectively. The latter were told to continue their routine exercises. Key protein markers were analyzed by Western blotting from vastus lateralis (VL) muscle biopsies. The muscle thickness of VL was analyzed by ultrasonography and sprint performance by a 60-m running test.

RESULTS

EX induced improvement in 60-m sprint performance when compared with controls (time-group, P = 0.003) without changes in VL muscle thickness. Content of lipidated microtubule-associated protein 1A/1B-light chain 3 (LC3-II) increased in EX (P = 0.022), suggesting increased autophagosome content. In addition, an autophagosome clearance marker sequestosome 1 (p62) decreased in EX (P = 0.006). Markers of UPR selectively modulated with decreases (e.g., ATF4, P = 0.003) and increases (e.g., EIF2α, P = 0.019) observed in EX.

CONCLUSIONS

These findings suggest that a new intensive training stimulus that combines strength training with sprint training may increase muscle autophagosome content in a basal state without any evidence of impaired autophagosome clearance in masters sprinters. Simultaneously, the combined training may have a selective effect on the content of UPR signaling components.

Blocking Activin Receptor Ligands Is Not Sufficient to Rescue Cancer-Associated Gut Microbiota-A Role for Gut Microbial Flagellin in Colorectal Cancer and Cachexia?

Colorectal cancer (CRC) and cachexia are associated with the gut microbiota and microbial surface molecules. We characterized the CRC-associated microbiota and investigated whether cachexia affects the microbiota composition. Further, we examined the possible relationship between the microbial surface molecule flagellin and CRC. CRC cells (C26) were inoculated into mice. Activin receptor (ACVR) ligands were blocked, either before tumor formation or before and after, to increase muscle mass and prevent muscle loss. The effects of flagellin on C26-cells were studied in vitro. The occurrence of similar phenomena were studied in murine and human tumors. Cancer modulated the gut microbiota without consistent effects of blocking the ACVR ligands. However, continued treatment for muscle loss modified the association between microbiota and weight loss. Several abundant microbial taxa in cancer were flagellated. Exposure of C26-cells to flagellin increased IL6 and CCL2/MCP-1 mRNA and IL6 excretion. Murine C26 tumors expressed more IL6 and CCL2/MCP-1 mRNA than C26-cells, and human CRC tumors expressed more CCL2/MCP-1 than healthy colon sites. Additionally, flagellin decreased caspase-1 activity and the production of reactive oxygen species, and increased cytotoxicity in C26-cells. Conditioned media from flagellin-treated C26-cells deteriorated C2C12-myotubes and decreased their number. In conclusion, cancer increased flagellated microbes that may promote CRC survival and cachexia by inducing inflammatory proteins such as MCP-1. Cancer-associated gut microbiota could not be rescued by blocking ACVR ligands.

Molecular Pathways Mediating Immunosuppression in Response to Prolonged Intensive Physical Training, Low-Energy Availability, and Intensive Weight Loss

Exercise and exercise-induced weight loss have a beneficial effect on overall health, including positive effects on molecular pathways associated with immune function, especially in overweight individuals. The main aim of our study was to assess how energy deprivation (i.e., “semi-starvation”) leading to substantial fat mass loss affects the immune system and immunosuppression in previously normal weight individuals. Thus, to address this hypothesis, we applied a high-throughput systems biology approach to better characterize potential key pathways associated with immune system modulation during intensive weight loss and subsequent weight regain. We examined 42 healthy female physique athletes (age 27.5 ± 4.0 years, body mass index 23.4 ± 1.7 kg/m²) volunteered into either a diet group (n = 25) or a control group (n = 17). For the diet group, the energy intake was reduced and exercise levels were increased to induce loss of fat mass that was subsequently regained during a recovery period. The control group was instructed to maintain their typical lifestyle, exercise levels, and energy intake at a constant level. For quantification of systems biology markers, fasting blood samples were drawn at three time points: baseline (PRE), at the end of the weight loss period (MID 21.1 ± 3.1 weeks after PRE), and at the end of the weight regain period (POST 18.4 ± 2.9 weeks after MID). In contrast to the control group, the diet group showed significant (false discovery rate <0.05) alteration of all measured immune function parameters—white blood cells (WBCs), immunoglobulin G glycome, leukocyte transcriptome, and cytokine profile. Integrative omics suggested effects on multiple levels of immune system as dysregulated hematopoiesis, suppressed immune cell proliferation, attenuated systemic inflammation, and loss of immune cell function by reduced antibody and chemokine secretion was implied after intense weight loss. During the weight regain period, the majority of the measured immune system parameters returned back to the baseline. In summary, this study elucidated a number of molecular pathways presumably explaining immunosuppression in individuals going through prolonged periods of intense training with low-energy availability. Our findings also reinforce the perception that the way in which weight loss is achieved (i.e., dietary restriction, exercise, or both) has a distinct effect on how the immune system is modulated.

Muscle and serum metabolomes are dysregulated in colon-26 tumor-bearing mice despite amelioration of cachexia with activin receptor type 2B ligand blockade

Cancer-associated cachexia reduces survival, which has been attenuated by blocking the activin receptor type 2B (ACVR2B) ligands in mice. The purpose of this study was to unravel the underlying physiology and novel cachexia biomarkers by use of the colon-26 (C26) carcinoma model of cancer cachexia. Male BALB/c mice were subcutaneously inoculated with C26 cancer cells or vehicle control. Tumor-bearing mice were treated with vehicle (C26+PBS) or soluble ACVR2B either before (C26+sACVR/b) or before and after (C26+sACVR/c) tumor formation. Skeletal muscle and serum metabolomics analysis was conducted by gas chromatography-mass spectrometry. Cancer altered various biologically functional groups representing 1) amino acids, 2) energy sources, and 3) nucleotide-related intermediates. Muscle metabolomics revealed increased content of free phenylalanine in cancer that strongly correlated with the loss of body mass within the last 2 days of the experiment. This correlation was also detected in serum. Decreased ribosomal RNA content and phosphorylation of a marker of pyrimidine synthesis revealed changes in nucleotide metabolism in cancer. Overall, the effect of the experimental C26 cancer predominated over blocking ACVR2B ligands in both muscle and serum. However, the level of methyl phosphate, which was decreased in muscle in cancer, was restored by sACVR2B-Fc treatment. In conclusion, experimental cancer affected muscle and blood metabolomes mostly independently of blocking ACVR2B ligands. Of the affected metabolites, we have identified free phenylalanine as a promising biomarker of muscle atrophy or cachexia. Finally, the decreased capacity for pyrimidine nucleotide and protein synthesis in tumor-bearing mice opens up new avenues in cachexia research.

Resistance Training Induces Antiatherogenic Effects on Metabolomic Pathways

INTRODUCTION

Arising evidence suggests that resistance training has the potential to induce beneficial modulation of biomarker profile. To date, however, only immediate responses to resistance training have been investigated using high-throughput metabolomics whereas the effects of chronic resistance training on biomarker profile have not been studied in detail.

METHODS

A total of 86 recreationally active healthy men without previous systematic resistance training background were allocated into (i) a resistance training (RT) group (n = 68; age, 33 ± 7 yr; body mass index, 28 ± 3 kg·m) and (ii) a non-RT group (n = 18; age, 31 ± 4 yr; body mass index, 27 ± 3 kg·m). Blood samples were collected at baseline (PRE), after 4 wk (POST-4wk), and after 16 wk of resistance training intervention (POST-16wk), as well as baseline and after the non-RT period (20-24 wk). Nuclear magnetic resonance-metabolome platform was used to determine metabolomic responses to chronic resistance training.

RESULTS

Overall, the resistance training intervention resulted in favorable alterations (P < 0.05) in body composition with increased levels of lean mass (~2.8%), decreased levels of android (~9.6%), and total fat mass (~7.5%). These changes in body composition were accompanied by antiatherogenic alterations in serum metabolome profile (false discovery rate < 0.05) as reductions in non-high-density lipoprotein cholesterol (e.g., free cholesterol, remnant cholesterol, intermediate-density lipoprotein cholesterols, low-density lipoprotein cholesterols) and related apolipoprotein B, and increments in conjugated linoleic fatty acids levels were observed. Individuals with the poorest baseline status (i.e., body composition, metabolome profile) benefitted the most from the resistance training intervention.

CONCLUSIONS

In conclusion, resistance training improves cardiometabolic risk factors and serum metabolome even in previously healthy young men. Thus, suggesting attenuated risk for future cardiovascular disease.

Systemic Blockade of ACVR2B Ligands Protects Myocardium from Acute Ischemia-Reperfusion Injury

Activin A and myostatin, members of the transforming growth factor (TGF)-β superfamily of secreted factors, are potent negative regulators of muscle growth, but their contribution to myocardial ischemia-reperfusion (IR) injury is not known. The aim of this study was to investigate if activin 2B (ACVR2B) receptor ligands contribute to myocardial IR injury. Mice were treated with soluble ACVR2B decoy receptor (ACVR2B-Fc) and subjected to myocardial ischemia followed by reperfusion for 6 or 24 h. Systemic blockade of ACVR2B ligands by ACVR2B-Fc was protective against cardiac IR injury, as evidenced by reduced infarcted area, apoptosis, and autophagy and better preserved LV systolic function following IR. ACVR2B-Fc modified cardiac metabolism, LV mitochondrial respiration, as well as cardiac phenotype toward physiological hypertrophy. Similar to its protective role in IR injury in vivo, ACVR2B-Fc antagonized SMAD2 signaling and cell death in cardiomyocytes that were subjected to hypoxic stress. ACVR2B ligand myostatin was found to exacerbate hypoxic stress. In addition to acute cardioprotection in ischemia, ACVR2B-Fc provided beneficial effects on cardiac function in prolonged cardiac stress in cardiotoxicity model. By blocking myostatin, ACVR2B-Fc potentially reduces cardiomyocyte death and modifies cardiomyocyte metabolism for hypoxic conditions to protect the heart from IR injury.

Activin Receptor Ligand Blocking and Cancer Have Distinct Effects on Protein and Redox Homeostasis in Skeletal Muscle and Liver

Muscle wasting in cancer cachexia can be alleviated by blocking activin receptor type 2 (ACVR2) ligands through changes in protein synthesis/degradation. These changes in cellular and protein metabolism may alter protein homeostasis. First, we elucidated the acute (1–2 days) and 2-week effects of blocking ACVR2 ligands by soluble activin receptor 2B (sACVR2B-Fc) on unfolded protein response (UPR), heat shock proteins (HSPs) and redox balance in a healthy mouse skeletal muscle. Second, we examined UPR, autophagy and redox balance with or without sACVR2B-Fc administration in muscle and liver of C26 tumor-bearing mice. The indicators of UPR and HSPs were not altered 1–2 days after a single sACVR2B-Fc administration in healthy muscles, but protein carbonyls increased (p < 0.05). Two weeks of sACVR2B-Fc administration increased muscle size, which was accompanied by increased UPR markers: GRP78 (p < 0.05), phosphorylated eIF2α (p < 0.01) and HSP47 (p < 0.01). Additionally, protein carbonyls and reduced form of glutathione increased (GSH) (p < 0.05). On the other hand, C26 cancer cachexia manifested decreased UPR markers (p-eIF2α, HSP47, p-JNK; p < 0.05) and antioxidant GSH (p < 0.001) in muscle, whereas the ratio of oxidized to reduced glutathione increased (GSSG/GSH; p < 0.001). Administration of sACVR2B-Fc prevented the decline in GSH and increased some of the UPR indicators in tumor-bearing mice. Additionally, autophagy markers LC3II/I (p < 0.05), Beclin-1 (p < 0.01), and P62 (p < 0.05) increased in the skeletal muscle of tumor-bearing mice. Finally, indicators of UPR, PERK, p-eIF2α and GRP78, increased (p < 0.05), whereas ATF4 was strongly decreased (p < 0.01) in the liver of tumor-bearing mice while sACVR2B-Fc had no effect. Muscle GSH and many of the altered UPR indicators correlated with tumor mass, fat mass and body mass loss. In conclusion, experimental cancer cachexia is accompanied by distinct and tissue-specific changes in proteostasis. Muscle hypertrophy induced by blocking ACVR2B ligands may be accompanied by the induction of UPR and increased protein carbonyls but blocking ACVR2B ligands may upregulate antioxidant protection.

Moderate exercise in mice improves cancer plus chemotherapy-induced muscle wasting and mitochondrial alterations

Cancer cachexia is a multifactorial syndrome characterized by anorexia, body wasting, and muscle and adipose tissue loss, impairing patient's tolerance to anticancer treatments and survival. The aim of the present study was to compare the effects induced in mice by tumor growth alone (C26) or in combination with chemotherapy [C26 oxaliplatin and 5-fluorouracil (oxfu)] and to evaluate the potential of moderate exercise. Oxfu administration to C26 mice exacerbated muscle wasting and triggered autophagy or mitophagy, decreased protein synthesis, and induced mitochondrial alterations. Exercise in C26 oxfu mice counteracted the loss of muscle mass and strength, partially rescuing autophagy and mitochondrial function. Nevertheless, exercise worsened survival in C26 oxfu mice in late stages of cachexia. In summary, chemotherapy further impinges on cancer-induced alterations, worsening muscle wasting. An ideal multifactorial and early intervention to prevent cancer cachexia could take advantage of exercise, improving patient's energy metabolism, mobility, and quality of life.-Ballarò, R., Beltrà, M., De Lucia, S., Pin, F., Ranjbar, K., Hulmi, J. J., Costelli, P., Penna, F. Moderate exercise in mice improves cancer plus chemotherapy-induced muscle wasting and mitochondrial alterations.

Stimuli and sensors that initiate skeletal muscle hypertrophy following resistance exercise

One of the most striking adaptations to exercise is the skeletal muscle hypertrophy that occurs in response to resistance exercise. A large body of work shows that a mammalian target of rapamycin complex 1 (mTORC1)-mediated increase of muscle protein synthesis is the key, but not sole, mechanism by which resistance exercise causes muscle hypertrophy. While much of the hypertrophy signaling cascade has been identified, the initiating, resistance exercise-induced and hypertrophy-stimulating stimuli have remained elusive. For the purpose of this review, we define an initiating, resistance exercise-induced and hypertrophy-stimulating signal as "hypertrophy stimulus," and the sensor of such a signal as "hypertrophy sensor." In this review we discuss our current knowledge of specific mechanical stimuli, damage/injury-associated and metabolic stress-associated triggers, as potential hypertrophy stimuli. Mechanical signals are the prime hypertrophy stimuli candidates, and a filamin-C-BAG3-dependent regulation of mTORC1, Hippo, and autophagy signaling is a plausible albeit still incompletely characterized hypertrophy sensor. Other candidate mechanosensing mechanisms are nuclear deformation-initiated signaling or several mechanisms related to costameres, which are the functional equivalents of focal adhesions in other cells. While exercise-induced muscle damage is probably not essential for hypertrophy, it is still unclear whether and how such muscle damage could augment a hypertrophic response. Interventions that combine blood flow restriction and especially low load resistance exercise suggest that resistance exercise-regulated metabolites could be hypertrophy stimuli, but this is based on indirect evidence and metabolite candidates are poorly characterized.

Treating cachexia using soluble ACVR2B improves survival, alters mTOR localization, and attenuates liver and spleen responses

BACKGROUND

Cancer cachexia increases morbidity and mortality, and blocking of activin receptor ligands has improved survival in experimental cancer. However, the underlying mechanisms have not yet been fully uncovered.

METHODS

The effects of blocking activin receptor type 2 (ACVR2) ligands on both muscle and non-muscle tissues were investigated in a preclinical model of cancer cachexia using a recombinant soluble ACVR2B (sACVR2B-Fc). Treatment with sACVR2B-Fc was applied either only before the tumour formation or with continued treatment both before and after tumour formation. The potential roles of muscle and non-muscle tissues in cancer cachexia were investigated in order to understand the possible mechanisms of improved survival mediated by ACVR2 ligand blocking.

RESULTS

Blocking of ACVR2 ligands improved survival in tumour-bearing mice only when the mice were treated both before and after the tumour formation. This occurred without effects on tumour growth, production of pro-inflammatory cytokines or the level of physical activity. ACVR2 ligand blocking was associated with increased muscle (limb and diaphragm) mass and attenuation of both hepatic protein synthesis and splenomegaly. Especially, the effects on the liver and the spleen were observed independent of the treatment protocol. The prevention of splenomegaly by sACVR2B-Fc was not explained by decreased markers of myeloid-derived suppressor cells. Decreased tibialis anterior, diaphragm, and heart protein synthesis were observed in cachectic mice. This was associated with decreased mechanistic target of rapamycin (mTOR) colocalization with late-endosomes/lysosomes, which correlated with cachexia and reduced muscle protein synthesis.

CONCLUSIONS

The prolonged survival with continued ACVR2 ligand blocking could potentially be attributed in part to the maintenance of limb and respiratory muscle mass, but many observed non-muscle effects suggest that the effect may be more complex than previously thought. Our novel finding showing decreased mTOR localization in skeletal muscle with lysosomes/late-endosomes in cancer opens up new research questions and possible treatment options for cachexia.

Prevention of chemotherapy-induced cachexia by ACVR2B ligand blocking has different effects on heart and skeletal muscle

BACKGROUND

Toxicity of chemotherapy on skeletal muscles and the heart may significantly contribute to cancer cachexia, mortality, and decreased quality of life. Doxorubicin (DOX) is an effective cytostatic agent, which unfortunately has toxic effects on many healthy tissues. Blocking of activin receptor type IIB (ACVR2B) ligands is an often used strategy to prevent skeletal muscle loss, but its effects on the heart are relatively unknown.

METHODS

The effects of DOX treatment with or without pre-treatment with soluble ACVR2B-Fc (sACVR2B-Fc) were investigated. The mice were randomly assigned into one of the three groups: (1) vehicle (PBS)-treated controls, (2) DOX-treated mice (DOX), and (3) DOX-treated mice administered with sACVR2B-Fc during the experiment (DOX + sACVR2B-Fc). DOX was administered with a cumulative dose of 24 mg/kg during 2 weeks to investigate cachexia outcome in the heart and skeletal muscle. To understand similarities and differences between skeletal and cardiac muscles in their responses to chemotherapy, the tissues were collected 20 h after a single DOX (15 mg/kg) injection and analysed with genome-wide transcriptomics and mRNA and protein analyses. The combination group was pre-treated with sACVR2B-Fc 48 h before DOX administration. Major findings were also studied in mice receiving only sACVR2B-Fc.

RESULTS

The DOX treatment induced similar (~10%) wasting in skeletal muscle and the heart. However, transcriptional changes in response to DOX were much greater in skeletal muscle. Pathway analysis and unbiased transcription factor analysis showed that p53-p21-REDD1 is the main common pathway activated by DOX in both skeletal and cardiac muscles. These changes were attenuated by blocking ACVR2B ligands especially in skeletal muscle. Tceal7 (3-fold to 5-fold increase), transferrin receptor (1.5-fold increase), and Ccl21 (0.6-fold to 0.9-fold decrease) were identified as novel genes responsive to blocking ACVR2B ligands. Overall, at the transcriptome level, ACVR2B ligand blocking had only minor influence in the heart while it had marked effects in skeletal muscle. The same was also true for the effects on tissue wasting. This may be explained in part by about 18-fold higher gene expression of myostatin in skeletal muscle compared with the heart.

CONCLUSIONS

Cardiac and skeletal muscles display similar atrophy after DOX treatment, but the mechanisms for this may differ between the tissues. The present results suggest that p53-p21-REDD1 signalling is the main common DOX-activated pathway in these tissues and that blocking activin receptor ligands attenuates this response, especially in skeletal muscle supporting the overall stronger effects of this treatment in skeletal muscles.

Morphological, molecular and hormonal adaptations to early morning versus afternoon resistance training

It has been clearly established that maximal force and power is lower in the morning compared to noon or afternoon hours. This morning neuromuscular deficit can be diminished by regularly training in the morning hours. However, there is limited and contradictory information upon hypertrophic adaptations to time-of-day-specific resistance training. Moreover, no cellular or molecular mechanisms related to muscle hypertrophy adaptation have been studied with this respect. Therefore, the present study examined effects of the time-of-day-specific resistance training on muscle hypertrophy, phosphorylation of selected proteins, hormonal concentrations and neuromuscular performance. Twenty five previously untrained males were randomly divided into a morning group (n = 11, age 23 ± 2 yrs), afternoon group (n = 7, 24 ± 4 yrs) and control group (n = 7, 24 ± 3 yrs). Both the morning and afternoon group underwent hypertrophy-type of resistance training with 22 training sessions over an 11-week period performed between 07:30-08:30 h and 16:00-17:00 h, respectively. Isometric MVC was tested before and immediately after an acute loading exclusively during their training times before and after the training period. Before acute loadings, resting blood samples were drawn and analysed for plasma testosterone and cortisol. At each testing occasion, muscle biopsies from m. vastus lateralis were obtained before and 60 min after the acute loading. Muscle specimens were analysed for muscle fibre cross-sectional areas (CSA) and for phosphorylated p70S6K, rpS6, p38MAPK, Erk1/2, and eEF2. In addition, the right quadriceps femoris was scanned with MRI before and after the training period. The control group underwent the same testing, except for MRI, between 11:00 h and 13:00 h but did not train. Voluntary muscle strength increased significantly in both the morning and afternoon training group by 16.9% and 15.2 %, respectively. Also muscle hypertrophy occurred by 8.8% and 11.9% (MRI, p < 0.001) and at muscle fibre CSA level by 21% and 18% (p < 0.01) in the morning and afternoon group, respectively. No significant changes were found in controls within these parameters. Both pre- and post-training acute loadings induced a significant (p < 0.001) reduction in muscle strength in all groups, not affected by time of day or training. The post-loading phosphorylation of p70S6Thr421/Ser424 increased independent of the time of day in the pre-training condition, whereas it was significantly increased in the morning group only after the training period (p < 0.05). Phosphorylation of rpS6 and p38MAPK increased acutely both before and after training in a time-of-day independent manner (p < 0.05 at all occasions). Phosphorylation of p70S6Thr389, eEF2 and Erk1/2 did not change at any time point. No statistically significant correlations were found between changes in muscle fibre CSA, MRI and cell signalling data. Resting testosterone was not statistically different among groups at any time point. Resting cortisol declined significantly from pre- to post-training in all three groups (p < 0.05). In conclusion, similar levels of muscle strength and hypertrophy could be achieved regardless of time of the day in previously untrained men. However, at the level of skeletal muscle signalling, the extent of adaptation in some parameters may be time of day dependent.

Treatment with soluble activin type IIB-receptor improves bone mass and strength in a mouse model of Duchenne muscular dystrophy

BACKGROUND

Inhibition of activin/myostatin pathway has emerged as a novel approach to increase muscle mass and bone strength. Duchenne muscular dystrophy (DMD) is a neuromuscular disorder that leads to progressive muscle degeneration and also high incidence of fractures. The aim of our study was to test whether inhibition of activin receptor IIB ligands with or without exercise could improve bone strength in the mdx mouse model for DMD.

METHODS

Thirty-two mdx mice were divided to running and non-running groups and to receive either PBS control or soluble activin type IIB-receptor (ActRIIB-Fc) once weekly for 7 weeks.

RESULTS

Treatment of mdx mice with ActRIIB-Fc resulted in significantly increased body and muscle weights in both sedentary and exercising mice. Femoral μCT analysis showed increased bone volume and trabecular number (BV/TV +80%, Tb.N +70%, P < 0.05) in both ActRIIB-Fc treated groups. Running also resulted in increased bone volume and trabecular number in PBS-treated mice. However, there was no significant difference in trabecular bone structure or volumetric bone mineral density between the ActRIIB-Fc and ActRIIB-Fc-R indicating that running did not further improve bone structure in ActRIIB-Fc-treated mice. ActRIIB-Fc increased bone mass also in vertebrae (BV/TV +20%, Tb.N +30%, P < 0.05) but the effects were more modest. The number of osteoclasts was decreased in histological analysis and the expression of several osteoblast marker genes was increased in ActRIIB-Fc treated mice suggesting decreased bone resorption and increased bone formation in these mice. Increased bone mass in femurs translated into enhanced bone strength in biomechanical testing as the maximum force and stiffness were significantly elevated in ActRIIB-Fc-treated mice.

CONCLUSIONS

Our results indicate that treatment of mdx mice with the soluble ActRIIB-Fc results in a robust increase in bone mass, without any additive effect by voluntary running. Thus ActRIIB-Fc could be an attractive option in the treatment of musculoskeletal disorders.

The Effects of Intensive Weight Reduction on Body Composition and Serum Hormones in Female Fitness Competitors

Worries about the potential negative consequences of popular fat loss regimens for aesthetic purposes in normal weight females have been surfacing in the media. However, longitudinal studies investigating these kinds of diets are lacking. The purpose of the present study was to investigate the effects of a 4-month fat-loss diet in normal weight females competing in fitness-sport. In total 50 participants finished the study with 27 females (27.2 ± 4.1 years) dieting for a competition and 23 (27.7 ± 3.7 years) acting as weight-stable controls. The energy deficit of the diet group was achieved by reducing carbohydrate intake and increasing aerobic exercise while maintaining a high level of protein intake and resistance training in addition to moderate fat intake. The diet led to a ~12% decrease in body weight (P < 0.001) and a ~35-50% decrease in fat mass (DXA, bioimpedance, skinfolds, P < 0.001) whereas the control group maintained their body and fat mass (diet × group interaction P < 0.001). A small decrease in lean mass (bioimpedance and skinfolds) and in vastus lateralis muscle cross-sectional area (ultrasound) were observed in diet (P < 0.05), whereas other results were unaltered (DXA: lean mass, ultrasound: triceps brachii thickness). The hormonal system was altered during the diet with decreased serum concentrations of leptin, triiodothyronine (T3), testosterone (P < 0.001), and estradiol (P < 0.01) coinciding with an increased incidence of menstrual irregularities (P < 0.05). Body weight and all hormones except T3 and testosterone returned to baseline during a 3-4 month recovery period including increased energy intake and decreased levels aerobic exercise. This study shows for the first time that most of the hormonal changes after a 35-50% decrease in body fat in previously normal-weight females can recover within 3-4 months of increased energy intake.

Effects of muscular dystrophy, exercise and blocking activin receptor IIB ligands on the unfolded protein response and oxidative stress

Protein homeostasis in cells, proteostasis, is maintained through several integrated processes and pathways and its dysregulation may mediate pathology in many diseases including Duchenne muscular dystrophy (DMD). Oxidative stress, heat shock proteins, endoplasmic reticulum (ER) stress and its response, i.e. unfolded protein response (UPR), play key roles in proteostasis but their involvement in the pathology of DMD are largely unknown. Moreover, exercise and activin receptor IIB blocking are two strategies that may be beneficial to DMD muscle, but studies to examine their effects on these proteostasis pathways are lacking. Therefore, these pathways were examined in the muscle of mdx mice, a model of DMD, under basal conditions and in response to seven weeks of voluntary exercise and/or activin receptor IIB ligand blocking using soluble activin receptor-Fc (sAcvR2B-Fc) administration. In conjunction with reduced muscle strength, mdx muscle displayed greater levels of UPR/ER-pathway indicators including greater protein levels of IRE1α, PERK and Atf6b mRNA. Downstream to IRE1α and PERK, spliced Xbp1 mRNA and phosphorylation of eIF2α, were also increased. Most of the cytoplasmic and ER chaperones and mitochondrial UPR markers were unchanged in mdx muscle. Oxidized glutathione was greater in mdx and was associated with increases in lysine acetylated proteome and phosphorylated sirtuin 1. Exercise increased oxidative stress when performed independently or combined with sAcvR2B-Fc administration. Although neither exercise nor sAcvR2B-Fc administration imparted a clear effect on ER stress/UPR pathways or heat shock proteins, sAcvR2B-Fc administration increased protein expression levels of GRP78/BiP, a triggering factor for ER stress/UPR activation and TxNIP, a redox-regulator of ER stress-induced inflammation. In conclusion, the ER stress and UPR are increased in mdx muscle. However, these processes are not distinctly improved by voluntary exercise or blocking activin receptor IIB ligands and thus do not appear to be optimal therapeutic choices for improving proteostasis in DMD.

Systemic blockade of ACVR2B ligands prevents chemotherapy-induced muscle wasting by restoring muscle protein synthesis without affecting oxidative capacity or atrogenes

Doxorubicin is a widely used and effective chemotherapy drug. However, cardiac and skeletal muscle toxicity of doxorubicin limits its use. Inhibiting myostatin/activin signalling can prevent muscle atrophy, but its effects in chemotherapy-induced muscle wasting are unknown. In the present study we investigated the effects of doxorubicin administration alone or combined with activin receptor ligand pathway blockade by soluble activin receptor IIB (sACVR2B-Fc). Doxorubicin administration decreased body mass, muscle size and bone mineral density/content in mice. However, these effects were prevented by sACVR2B-Fc administration. Unlike in many other wasting situations, doxorubicin induced muscle atrophy without markedly increasing typical atrogenes or protein degradation pathways. Instead, doxorubicin decreased muscle protein synthesis which was completely restored by sACVR2B-Fc. Doxorubicin administration also resulted in impaired running performance without effects on skeletal muscle mitochondrial capacity/function or capillary density. Running performance and mitochondrial function were unaltered by sACVR2B-Fc administration. Tumour experiment using Lewis lung carcinoma cells demonstrated that sACVR2B-Fc decreased the cachectic effects of chemotherapy without affecting tumour growth. These results demonstrate that blocking ACVR2B signalling may be a promising strategy to counteract chemotherapy-induced muscle wasting without damage to skeletal muscle oxidative capacity or cancer treatment.

Heterogeneity in resistance training-induced muscle strength and mass responses in men and women of different ages

Physical activity recommendations for public health include typically muscle-strengthening activities for a minimum of 2 days a week. The range of inter-individual variation in responses to resistance training (RT) aiming to improve health and well-being requires to be investigated. The purpose of this study was to quantify high and low responders for RT-induced changes in muscle size and strength and to examine possible effects of age and sex on these responses. Previously collected data of untrained healthy men and women (age 19 to 78 years, n = 287 with 72 controls) were pooled for the present study. Muscle size and strength changed during RT are 4.8 ± 6.1 % (range from -11 to 30 %) and 21.1 ± 11.5 % (range from -8 to 60 %) compared to pre-RT, respectively. Age and sex did not affect to the RT responses. Fourteen percent and 12 % of the subjects were defined as high responders (>1 standard deviation (SD) from the group mean) for the RT-induced changes in muscle size and strength, respectively. When taking into account the results of non-training controls (upper 95 % CI), 29 and 7 % of the subjects were defined as low responders for the RT-induced changes in muscle size and strength, respectively. The muscle size and strength responses varied extensively between the subjects regardless of subject's age and sex. Whether these changes are associated with, e.g., functional capacity and metabolic health improvements due to RT requires further studies.

Body composition and power performance improved after weight reduction in male athletes without hampering hormonal balance

The aim of this study was to investigate the effects of a 4-week weight reduction period with high protein and reduced carbohydrate intake on body composition, explosive power, speed, serum hormones, and acid-base balance in male track and field jumpers and sprinters. Eight participants were assigned to a high weight reduction group (HWR; energy restriction 750 kcal·d) and 7 to a low weight reduction group (LWR; energy restriction 300 kcal·d). Energy and carbohydrate intake decreased significantly (p ≤ 0.05) only in HWR by 740 ± 330 kcal·d and 130 ± 29 g·d, respectively. Furthermore, total body mass and fat mass decreased (p ≤ 0.05) only in HWR by 2.2 ± 1.0 kg and 1.7 ± 1.6 kg, respectively. Fat-free mass (FFM), serum testosterone, cortisol, and sex hormone-binding globulin did not change significantly. Ca ion and pH decreased (p ≤ 0.05) only in HWR (3.1 ± 2.8% and 0.8 ± 0.8%, respectively), whereas (Equation is included in full-text article.)declined (p ≤ 0.05) in both groups by 19.3 ± 6.2% in HWR and by 13.1 ± 8.5% in LWR. The countermovement jump and 20-m sprint time improved consistently (p ≤ 0.05) only in HWR, by 2.6 ± 2.5 cm and 0.04 ± 0.04 seconds, respectively. Finally, athletes with a fat percentage of 10% or more at the baseline were able to preserve FFM. In conclusion, altered acid-base balance but improved weight-bearing power performance was observed without negative consequences on serum hormones and FFM after a 4-week weight reduction of 0.5 kg·wk achieved by reduced carbohydrate but maintained high protein intake.

The effects of whey protein with or without carbohydrates on resistance training adaptations

Background

Nutrition intake in the context of a resistance training (RT) bout may affect body composition and muscle strength. However, the individual and combined effects of whey protein and carbohydrates on long-term resistance training adaptations are poorly understood.

Methods

A four-week preparatory RT period was conducted in previously untrained males to standardize the training background of the subjects. Thereafter, the subjects were randomized into three groups: 30 g of whey proteins (n = 22), isocaloric carbohydrates (maltodextrin, n = 21), or protein + carbohydrates (n = 25). Within these groups, the subjects were further randomized into two whole-body 12-week RT regimens aiming either for muscle hypertrophy and maximal strength or muscle strength, hypertrophy and power. The post-exercise drink was always ingested immediately after the exercise bout, 2–3 times per week depending on the training period. Body composition (by DXA), quadriceps femoris muscle cross-sectional area (by panoramic ultrasound), maximal strength (by dynamic and isometric leg press) and serum lipids as basic markers of cardiovascular health, were analysed before and after the intervention.

Results

Twelve-week RT led to increased fat-free mass, muscle size and strength independent of post-exercise nutrient intake (P < 0.05). However, the whey protein group reduced more total and abdominal area fat when compared to the carbohydrate group independent of the type of RT (P < 0.05). Thus, a larger relative increase (per kg bodyweight) in fat-free mass was observed in the protein vs. carbohydrate group (P < 0.05) without significant differences to the combined group. No systematic effects of the interventions were found for serum lipids. The RT type did not have an effect on the adaptations in response to different supplementation paradigms.

Conclusions

Post-exercise supplementation with whey proteins when compared to carbohydrates or combination of proteins and carbohydrates did not have a major effect on muscle size or strength when ingested two to three times a week. However, whey proteins may increase abdominal fat loss and relative fat-free mass adaptations in response to resistance training when compared to fast-acting carbohydrates.

PGC-1 isoforms and their target genes are expressed differently in human skeletal muscle following resistance and endurance exercise

The primary aim of the present study was to investigate the acute gene expression responses of PGC-1 isoforms and PGC-1α target genes related to mitochondrial biogenesis (cytochrome C), angiogenesis (VEGF-A), and muscle hypertrophy (myostatin), after a resistance or endurance exercise bout. In addition, the study aimed to elucidate whether the expression changes of studied transcripts were linked to phosphorylation of AMPK and MAPK p38. Nineteen physically active men were divided into resistance exercise (RE, n = 11) and endurance exercise (EE, n = 8) groups. RE group performed leg press exercise (10 × 10 RM, 50 min) and EE walked on a treadmill (∼80% HR_max, 50 min). Muscle biopsies were obtained from the vastus lateralis muscle before, 30 min, and 180 min after exercise. EE and RE significantly increased the gene expression of alternative promoter originated PGC-1α exon 1b- and 1bxs’-derived isoforms, whereas the proximal promoter originated exon 1a-derived transcripts were less inducible and were upregulated only after EE. Truncated PGC-1α transcripts were upregulated both after EE and RE. Neither RE nor EE affected the expression of PGC-1β. EE upregulated the expression of cytochrome C and VEGF-A, whereas RE upregulated VEGF-A and downregulated myostatin. Both EE and RE increased the levels of p-AMPK and p-MAPK p38, but these changes were not linked to the gene expression responses of PGC-1 isoforms. The present study comprehensively assayed PGC-1 transcripts in human skeletal muscle and showed exercise mode-specific responses thus improving the understanding of early signaling events in exercise-induced muscle adaptations.

Cannabinoid receptor 1 and acute resistance exercise--In vivo and in vitro studies in human skeletal muscle

AIM

This study aimed to determine whether Cannabinoid receptor 1 (CB1) is involved in mammalian target of rapamycin (mTOR) signaling and skeletal muscle protein synthesis.

METHODS

This study used human vastus lateralis skeletal muscle biopsies obtained before and after a resistance exercise (RE) bout in young men (n=18). The signaling mechanisms were studied in vitro in human myotubes. Protein expression was determined by Western blot and confocal microscopy, and gene expression by quantitative PCR. Protein synthesis was measured in vitro using puromycin-based SuNSET technique.

RESULTS

In human skeletal muscle, an anabolic stimulus in the form of RE down-regulated CB1 expression. The negative change in CB1 expression was associated with increased phosphorylation of mTOR signaling proteins. In vitro, CB1 antagonist AM251 induced phosphorylation of mTOR downstream targets, ribosomal protein S6 kinase (S6K1), S6 and eukaryotic initiation factor 4E binding protein (4E-BP1) in human myotubes. These effects were ERK1/2-dependent and insensitive to mTOR inhibitor, rapamycin. Compared to AM251 treatment alone, inhibition of ERK1/2 by UO126 in the presence of AM251 decreased phosphorylation of S6K1, S6 and and 4E-BP1 at Thr(37/46). AM251 increased protein synthesis in cultured human myotubes, which was not rapamycin-sensitive but was ERK1/2-dependent.

CONCLUSIONS

Our results indicate that RE down-regulates CB1 expression. Inhibition of CB1 signaling increases skeletal muscle anabolic signaling down-stream of mTOR and protein synthesis through ERK1/2. Our study may provide base for the development of CB1-blocking drugs to treat or prevent muscle wasting.

Myostatin/activin blocking combined with exercise reconditions skeletal muscle expression profile of mdx mice

Duchenne muscular dystrophy is characterized by muscle wasting and decreased aerobic metabolism. Exercise and blocking of myostatin/activin signaling may independently or combined counteract muscle wasting and dystrophies. The effects of myostatin/activin blocking using soluble activin receptor-Fc (sActRIIB-Fc) administration and wheel running were tested alone or in combination for 7 weeks in dystrophic mdx mice. Expression microarray analysis revealed decreased aerobic metabolism in the gastrocnemius muscle of mdx mice compared to healthy mice. This was not due to reduced home-cage physical activity, and was further downregulated upon sActRIIB-Fc treatment in enlarged muscles. However, exercise activated pathways of aerobic metabolism and counteracted the negative effects of sActRIIB-Fc. Exercise and sActRIIB-Fc synergistically increased expression of major urinary protein, but exercise blocked sActRIIB-Fc induced phosphorylation of STAT5 in gastrocnemius muscle. In conclusion, exercise alone or in combination with myostatin/activin blocking corrects aerobic gene expression profiles of dystrophic muscle toward healthy wild type mice profiles.

Effect of diet composition on acid-base balance in adolescents, young adults and elderly at rest and during exercise

BACKGROUND

Diets rich in animal protein and cereal grains and deficient in vegetables and fruits may cause low-grade metabolic acidosis, which may impact exercise and health. We hypothesized that (1) a normal-protein diet with high amount of vegetables and fruits (HV) induces more alkaline acid-base balance compared with a high-protein diet with no vegetables and fruits (HP) and (2) diet composition has a greater impact on acid-base balance in the elderly (ELD).

SUBJECTS/METHODS

In all, 12-15 (adolescents (ADO)), 25-35 (young adults (YAD)) and 60-75 (ELD)-year-old male and female subjects (n=88) followed a 7-day HV and a 7-day HP in a randomized order and at the end performed incremental cycle ergometer tests. We investigated the effect of diet composition and age on capillary (c-pH) and urine pH (u-pH), strong ion difference (SID), partial pressure of carbon dioxide (pCO2) and total concentration of weak acids (Atot). Linear regression analysis was used to examine the contribution of SID, pCO2 and Atot to c-pH.

RESULTS

In YAD and ELD, c-pH (P⩽0.038) and u-pH (P<0.001) were higher at rest after HV compared with HP. During cycling, c-pH was higher (P⩽0.034) after HV compared with HP at submaximal workloads in YAD and at 75% of VO2max (maximal oxygen consumption) in ELD. The contribution of SID, pCO2 and Atot to c-pH varied widely. Gender effects or changes in acid-base balance of ADO were not detected.

CONCLUSIONS

A high intake of vegetables and fruits increases blood and u-pH in YAD and ELD. ELD compared with younger persons may be more sensitive for the diet-induced acid-base changes.

Vitamin C and E supplementation alters protein signalling after a strength training session, but not muscle growth during 10 weeks of training

This study investigated the effects of vitamin C and E supplementation on acute responses and adaptations to strength training. Thirty-two recreationally strength-trained men and women were randomly allocated to receive a vitamin C and E supplement (1000 mg day(-1) and 235 mg day(-1), respectively), or a placebo, for 10 weeks. During this period the participants' training involved heavy-load resistance exercise four times per week. Muscle biopsies from m. vastus lateralis were collected, and 1 repetition maximum (1RM) and maximal isometric voluntary contraction force, body composition (dual-energy X-ray absorptiometry), and muscle cross-sectional area (magnetic resonance imaging) were measured before and after the intervention. Furthermore, the cellular responses to a single exercise session were assessed midway in the training period by measurements of muscle protein fractional synthetic rate and phosphorylation of several hypertrophic signalling proteins. Muscle biopsies were obtained from m. vastus lateralis twice before, and 100 and 150 min after, the exercise session (4 × 8RM, leg press and knee-extension). The supplementation did not affect the increase in muscle mass or the acute change in protein synthesis, but it hampered certain strength increases (biceps curl). Moreover, increased phosphorylation of p38 mitogen-activated protein kinase, Extracellular signal-regulated protein kinases 1 and 2 and p70S6 kinase after the exercise session was blunted by vitamin C and E supplementation. The total ubiquitination levels after the exercise session, however, were lower with vitamin C and E than placebo. We concluded that vitamin C and E supplementation interfered with the acute cellular response to heavy-load resistance exercise and demonstrated tentative long-term negative effects on adaptation to strength training.

Upregulation of activin-B and follistatin in pulmonary fibrosis - a translational study using human biopsies and a specific inhibitor in mouse fibrosis models

Background

Activins are members of the TGF-ß superfamily of growth factors. First, we identified by expression array screening that activin-B and follistatin are upregulated in human idiopathic pulmonary fibrosis (IPF). Next, we wanted to clarify their specific role in lung fibrosis formation.

Methods

We used specific antibodies for activin-A and -B subunits and follistatin to measure and localize their levels in idiopathic pulmonary fibrosis and control lung biopsies. To inhibit activin signaling, we used soluble activin type IIB receptor fused to the Fc portion of human IgG1 (sActRIIB-Fc) in two different mouse models of pulmonary fibrosis.

Results

Activin-B and follistatin mRNA levels were elevated in the human IPF lung. Immunoreactivity to activin-A, -B and follistatin localized predominantly to the hyperplastic, activated alveolar epithelium, but was also seen in inflammatory cells. Mice treated with sActRIIB-Fc showed increased skeletal muscle mass and a clear reduction in alveolar cell counts in bronchoalveolar lavage fluid, but no significant antifibrotic effect in the lung was observed.

Conclusions

The upregulation of activin-B and follistatin in IPF is a novel finding. Our results indicate that activin inhibition is not an efficient tool for antifibrotic therapy, but could be useful in reducing alveolar cellular response to injury. Activin-B and follistatin levels may be useful as biomarkers of IPF.

Lipid droplet-associated proteins in high-fat fed mice with the effects of voluntary running and diet change

OBJECTIVE

The relation between lipid accumulation and influence of exercise on insulin sensitivity is not straightforward. A proper balance between lipid droplet synthesis, lipolysis, and oxidative metabolism would ensure low local intramyocellular fatty acid levels, thereby possibly protecting against lipotoxicity-associated insulin resistance. This study investigated whether the accumulation of triglycerides and lipid droplets in response to high availability of fatty acids after high-fat feeding would parallel the abundance of intramyocellular perilipin proteins, especially PLIN5. The effects on these variables after diet change or voluntary running exercise intervention in skeletal muscle were also investigated.

METHODS

During a 19-week experiment, C57BL/6J mice were studied in six different groups: low-fat diet sedentary, low-fat diet active, high-fat diet sedentary, high-fat diet active and two groups which were high-fat sedentary for nine weeks, after which divided into low-fat sedentary or low-fat active groups. Myocellular triglyceride concentration and perilipin protein expression levels were assessed.

RESULTS

We show that, concurrently with impaired insulin sensitivity, the expression level of PLIN5 and muscular triglyceride concentration increased dramatically after high-fat diet. These adaptations were reversible after the diet change intervention with no additional effect of exercise.

CONCLUSIONS

After high-fat diet, lipid droplets become larger providing more surface area for PLIN5. We suggest that PLIN5 is an important regulator of lipid droplet turnover in altered conditions of fatty acid supply and consumption. Imbalances in lipid droplet metabolism and turnover might lead to lipotoxicity-related insulin resistance.

VEGF-B-induced vascular growth leads to metabolic reprogramming and ischemia resistance in the heart

Angiogenic growth factors have recently been linked to tissue metabolism. We have used genetic gain- and loss-of function models to elucidate the effects and mechanisms of action of vascular endothelial growth factor-B (VEGF-B) in the heart. A cardiomyocyte-specific VEGF-B transgene induced an expanded coronary arterial tree and reprogramming of cardiomyocyte metabolism. This was associated with protection against myocardial infarction and preservation of mitochondrial complex I function upon ischemia-reperfusion. VEGF-B increased VEGF signals via VEGF receptor-2 to activate Erk1/2, which resulted in vascular growth. Akt and mTORC1 pathways were upregulated and AMPK downregulated, readjusting cardiomyocyte metabolic pathways to favor glucose oxidation and macromolecular biosynthesis. However, contrasting with a previous theory, there was no difference in fatty acid uptake by the heart between the VEGF-B transgenic, gene-targeted or wildtype rats. Importantly, we also show that VEGF-B expression is reduced in human heart disease. Our data indicate that VEGF-B could be used to increase the coronary vasculature and to reprogram myocardial metabolism to improve cardiac function in ischemic heart disease.

Subject Categories Cardiovascular System; Metabolism

See also: C Kupatt and R Hinkel (March 2014)

Effect of sodium bicarbonate and beta-alanine supplementation on maximal sprint swimming

BACKGROUND

This study examined the effect of simultaneous supplementation of extracellular buffer sodium bicarbonate (SB) and intracellular buffer beta-alanine (BA) on maximal sprint swimming.

METHODS

Thirteen competitive male swimmers completed 4 different treatments (placebo [PL], SB, BA + PL, and BA + SB) in a crossover procedure. PL or SB supplementation (0.3 g/kg body weight) was ingested 60 min before two maximal 100-m freestyle swims that were performed with a passive recovery of 12-min between each swim. Because of the known long washout period for carnosine, four weeks of BA supplementation (4.8 g per day) was started after the first week of PL or SB supplementation and performance testing.

RESULTS

The first maximal swims were similar, but the increase in time of the second versus the first 100-m swimming time was 1.5 s more (p < 0.05) in PL than in SB. Blood pH values were significantly (p < 0.05) greater in the SB and in the BA + SB groups compared to the PL and BA + PL values. There were no differences in peak blood lactate between the treatments.

CONCLUSION

Supplementing with SB prior to performing maximal sprint swimming with repetitions under 60 s improves performance. However, co-supplementation with SB and BA did not confer any added benefit on maximal swim performance.

Are skeletal muscle FNDC5 gene expression and irisin release regulated by exercise and related to health?

Recently, contradictory findings have been reported concerning the function of irisin and its precursor gene, skeletal muscle FNDC5, in energy homeostasis, and the associated regulatory role of exercise and PGC-1α. We therefore evaluated whether muscle FNDC5 mRNA and serum irisin are exercise responsive and whether PGC-1α expression is associated with FNDC5 expression. The male subjects in the study performed single exercises: (1) 1 h low-intensity aerobic exercise (AE) (middle-aged, n = 17), (2) a heavy-intensity resistance exercise (RE) bout (young n = 10, older n = 11) (27 vs. 62 years), (3) long-term 21 weeks endurance exercise (EE) training alone (twice a week, middle-aged, n = 9), or (4) combined EE and RE training (both twice a week, middle-aged, n = 9). Skeletal muscle mRNA expression was analysed by quantitative PCR and serum irisin by ELISA. No significant changes were observed in skeletal muscle PGC-1α, FNDC5 and serum irisin after AE, EE training or combined EE + RE training. However, a single RE bout increased PGC-1α by 4-fold in young and by 2-fold in older men, while FNDC5 mRNA only increased in young men post-RE, by 1.4-fold. Changes in PGC-1α or serum irisin were not consistently accompanied by changes in FNDC5. In conclusion, for the most part, neither longer-term nor single exercise markedly increases skeletal muscle FNDC5 expression or serum irisin. Therefore their changes in response to exercise are probably random and not consistent excluding the confirmation of any definitive link between exercise and FNDC5 expression and irisin release in humans. Moreover, irisin and FNDC5 were not associated with glucose tolerance and being overweight, or with metabolic disturbances, respectively. Finally, factor(s) other than PGC-1α and transcription may regulate FNDC5 expression.

Exercise restores decreased physical activity levels and increases markers of autophagy and oxidative capacity in myostatin/activin-blocked mdx mice

The importance of adequate levels of muscle size and function and physical activity is widely recognized. Myostatin/activin blocking increases skeletal muscle mass but may decrease muscle oxidative capacity and can thus be hypothesized to affect voluntary physical activity. Soluble activin receptor IIB (sActRIIB-Fc) was produced to block myostatin/activins. Modestly dystrophic mdx mice were injected with sActRIIB-Fc or PBS with or without voluntary wheel running exercise for 7 wk. Healthy mice served as controls. Running for 7 wk attenuated the sActRIIB-Fc-induced increase in body mass by decreasing fat mass. Running also enhanced/restored the markers of muscle oxidative capacity and autophagy in mdx mice to or above the levels of healthy mice. Voluntary running activity was decreased by sActRIIB-Fc during the first 3-4 wk correlating with increased body mass. Home cage physical activity of mice, quantified from the force plate signal, was decreased by sActRIIB-Fc the whole 7-wk treatment in sedentary mice. To understand what happens during the first weeks after sActRIIB-Fc administration, when mice are less active, healthy mice were injected with sActRIIB-Fc or PBS for 2 wk. During the sActRIIB-Fc-induced rapid 2-wk muscle growth period, oxidative capacity and autophagy were reduced, which may possibly explain the decreased running activity. These results show that increased muscle size and decreased markers of oxidative capacity and autophagy during the first weeks of myostatin/activin blocking are associated with decreased voluntary activity levels. Voluntary exercise in dystrophic mice enhances the markers of oxidative capacity and autophagy to or above the levels of healthy mice.

Muscle protein synthesis, mTORC1/MAPK/Hippo signaling, and capillary density are altered by blocking of myostatin and activins

Loss of muscle mass and function occurs in various diseases. Myostatin blocking can attenuate muscle loss, but downstream signaling is not well known. Therefore, to elucidate associated signaling pathways, we used the soluble activin receptor IIb (sActRIIB-Fc) to block myostatin and activins in mice. Within 2 wk, the treatment rapidly increased muscle size as expected but decreased capillary density per area. sActRIIB-Fc increased muscle protein synthesis 1-2 days after the treatment correlating with enhanced mTORC1 signaling (phosphorylated rpS6 and S6K1, r = 0.8). Concurrently, increased REDD1 and eIF2Bε protein contents and phosphorylation of 4E-BP1 and AMPK was observed. In contrast, proangiogenic MAPK signaling and VEGF-A protein decreased. Hippo signaling has been characterized recently as a regulator of organ size and an important regulator of myogenesis in vitro. The phosphorylation of YAP (Yes-associated protein), a readout of activated Hippo signaling, increased after short- and longer-term myostatin and activin blocking and in exercised muscle. Moreover, dystrophic mdx mice had elevated phosphorylated and especially total YAP protein content. These results show that the blocking of myostatin and activins induce rapid skeletal muscle growth. This is associated with increased protein synthesis and mTORC1 signaling but decreased capillary density and proangiogenic signaling. It is also shown for the first time that Hippo signaling is activated in skeletal muscle after myostatin blocking and exercise and also in dystrophic muscle. This suggests that Hippo signaling may have a role in skeletal muscle in various circumstances.

Combined effect of AAV-U7-induced dystrophin exon skipping and soluble activin Type IIB receptor in mdx mice

Adeno-associated virus (AAV)-U7-mediated skipping of dystrophin-exon-23 restores dystrophin expression and muscle function in the mdx mouse model of Duchenne muscular dystrophy. Soluble activin receptor IIB (sActRIIB-Fc) inhibits signaling of myostatin and homologous molecules and increases muscle mass and function of wild-type and mdx mice. We hypothesized that combined treatment with AAV-U7 and sActRIIB-Fc may synergistically improve mdx muscle function. Bioactivity of sActRIIB-Fc on skeletal muscle was first demonstrated in wild-type mice. In mdx mice we show that AAV-U7-mediated dystrophin restoration improved specific muscle force and resistance to eccentric contractions when applied alone. Treatment of mdx mice with sActRIIB-Fc increased body weight, muscle mass and myofiber size, but had little effect on muscle function. Combined treatment stimulated muscle growth comparable to the effect of sActRIIB-Fc alone and dystrophin rescue was similar to AAV-U7 alone. Moreover, combined treatment improved maximal tetanic force and the resistance to eccentric contraction to similar extent as AAV-U7 alone. In conclusion, combination of dystrophin exon skipping with sActRIIB-Fc brings together benefits of each treatment; however, we failed to evidence a clear synergistic effect on mdx muscle function.