Mechanisms involved in follistatin-induced hypertrophy and increased insulin action in skeletal muscle

Molecular insights of exercise therapy in disease prevention and treatment

Despite substantial evidence emphasizing the pleiotropic benefits of exercise for the prevention and treatment of various diseases, the underlying biological mechanisms have not been fully elucidated. Several exercise benefits have been attributed to signaling molecules that are released in response to exercise by different tissues such as skeletal muscle, cardiac muscle, adipose, and liver tissue. These signaling molecules, which are collectively termed exerkines, form a heterogenous group of bioactive substances, mediating inter-organ crosstalk as well as structural and functional tissue adaption. Numerous scientific endeavors have focused on identifying and characterizing new biological mediators with such properties. Additionally, some investigations have focused on the molecular targets of exerkines and the cellular signaling cascades that trigger adaption processes. A detailed understanding of the tissue-specific downstream effects of exerkines is crucial to harness the health-related benefits mediated by exercise and improve targeted exercise programs in health and disease. Herein, we review the current in vivo evidence on exerkine-induced signal transduction across multiple target tissues and highlight the preventive and therapeutic value of exerkine signaling in various diseases. By emphasizing different aspects of exerkine research, we provide a comprehensive overview of (i) the molecular underpinnings of exerkine secretion, (ii) the receptor-dependent and receptor-independent signaling cascades mediating tissue adaption, and (iii) the clinical implications of these mechanisms in disease prevention and treatment.

Potential Utility of Urinary Follistatin as a Non-Invasive Indicator of Acute Tubular Damage in Patients with Acute Kidney Injury

Activin A is known to impede tubular repair following renal ischemia, whereas exogenous follistatin, an activin A antagonist, has been shown to ameliorate kidney damage in rats. Despite these findings, the precise role of endogenous follistatin in the kidney has yet to be elucidated. In this study, we investigated the localization of follistatin in the normal human kidney and its potential utility as a marker for acute kidney injury (AKI). In a total of 118 AKI patients and 16 healthy adults, follistatin levels in serum and urine were quantified using ELISA, and correlations with clinical parameters were analyzed. Follistatin-producing cells were positive for Na-Cl co-transporter and uromodulin, but negative for aquaporin 1 and aquaporin 2. Unlike healthy adults, urinary follistatin significantly increased in AKI patients, correlating positively with AKI severity. Urinary follistatin levels were notably higher in patients needing renal replacement therapy. Significant correlations were observed with urinary protein, α1 microglobulin, and urinary NGAL, but not with urinary KIM-1, urinary L-FABP, urinary NAG, urinary β2 microglobulin, or serum creatinine. Interestingly, no correlation between urinary and serum follistatin levels was identified, indicating a renal origin for urinary follistatin. In conclusion, follistatin, produced by distal tubules, is detectable in the urine of AKI patients, suggesting its potential as a valuable marker for monitoring acute tubular damage severity in AKI.

Sex-specific increases in myostatin and SMAD3 contribute to obesity-related insulin resistance in human skeletal muscle and primary human myotubes

The purpose of the present study was to determine the effects of obesity and biological sex on myostatin expression in humans and to examine the direct effects of myostatin, SMAD2, and SMAD3 on insulin signaling in primary human skeletal muscle cells (HSkMCs). For cohort 1, 15 lean [body mass index (BMI): 22.1 ± 0.5 kg/m2; n = 8 males; n = 7 females] and 14 obese (BMI: 40.6 ± 1.4 kg/m2; n = 7 males; n = 7 females) individuals underwent skeletal muscle biopsies and an oral glucose tolerance test. For cohort 2, 14 young lean (BMI: 22.4 ± 1.9 kg/m2; n = 6 males; n = 8 females) and 14 obese (BMI: 39.3 ± 7.9 kg/m2; n = 6 males; n = 8 females) individuals underwent muscle biopsies for primary HSkMC experiments. Plasma mature myostatin (P = 0.041), skeletal muscle precursor myostatin (P = 0.048), and skeletal muscle SMAD3 (P = 0.029) were elevated in obese females compared to lean females, and plasma mature myostatin (r = 0.58, P = 0.029) and skeletal muscle SMAD3 (r = 0.56, P = 0.037) were associated with insulin resistance in females but not males. Twenty-four hours of myostatin treatment impaired insulin signaling in primary HSkMCs derived from females (P < 0.024) but not males. Overexpression of SMAD3, but not SMAD2, impaired insulin-stimulated AS160 phosphorylation in HSkMCs derived from lean females (-27%, P = 0.040), whereas silencing SMAD3 improved insulin-stimulated AS160 phosphorylation and insulin-stimulated glucose uptake (25%, P < 0.014) in HSkMCs derived from obese females. These results suggest for the first time that myostatin-induced impairments in skeletal muscle insulin signaling are sex specific and that increased body fat in females is associated with detrimental elevations in myostatin and SMAD3, which contribute to obesity-related insulin resistance.NEW & NOTEWORTHY Obesity is considered a main risk factor for the development of insulin resistance and type 2 diabetes. The present study utilizes in vivo and in vitro experiments in human skeletal muscle to demonstrate for the first time that females are inherently more susceptible to myostatin-induced insulin resistance, which is further enhanced with obesity due to increased myostatin and SMAD3 expression.

Nuclear Nicotinamide Adenine Dinucleotide Deficiency by Nmnat1 Deletion Impaired Hepatic Insulin Signaling, Mitochondrial Function, and Hepatokine Expression in Mice Fed a High-Fat Diet

Metabolic syndrome (MetS) is a worldwide challenge that is closely associated with obesity, nonalcoholic liver disease, insulin resistance, and type 2 diabetes. Boosting nicotinamide adenine dinucleotide (NAD+) presents great potential in preventing MetS. However, the function of nuclear NAD+ in the development of MetS remains poorly understood. In this study, hepatocyte-specific Nmnat1 knockout mice were used to determine a possible link between nuclear NAD+ and high-fat diet (HFD)-induced MetS. We found that Nmnat1 knockout significantly reduced hepatic nuclear NAD+ levels but did not exacerbate HFD-induced obesity and hepatic triglycerides accumulation. Interestingly, loss of Nmnat1 caused insulin resistance. Further analysis revealed that Nmnat1 deletion promoted gluconeogenesis but inhibited glycogen synthesis in the liver. Moreover, Nmnat1 deficiency induced mitochondrial dysfunction by decreasing mitochondrial DNA (mtDNA)-encoded complexes Ⅰ and Ⅳ, suppressing mtDNA replication and mtRNA transcription and reducing mtDNA copy number. In addition, Nmnat1 depletion affected the expression of hepatokines in the liver, particularly downregulating the expression of follistatin. These findings highlight the importance of nuclear NAD+ in maintaining insulin sensitivity and provide insights into the mechanisms underlying HFD-induced insulin resistance.

The effects of TRX suspension training on sarcopenic biomarkers and functional abilities in elderlies with sarcopenia: a controlled clinical trial

BACKGROUND

Sarcopenia is an age-related progressive loss of muscle mass and strength that can be modulated by resistance training. This study aimed to investigate the effects of TRX Suspension Training (TST) on serum levels of neuromuscular and growth factors and functional indices in elderly men with sarcopenia, an age-related condition characterized by progressive muscle mass and strength loss.

METHODS

Nineteen sarcopenic elderly men (age = 74.87 ± 4.58 years) were randomly assigned into two groups, the TST group (n = 10) and the control group (n = 9). Serum concentrations of regulatory muscle markers, anthropometric and body composition indices, and functional tests were evaluated at baseline and after 8 weeks. The training protocol consisted of eight weeks of TRX exercises, with three weekly sessions.

RESULTS

After 8 weeks of training, growth factors such as Follistatin (FST) (P = 0.001), 22 kDa C-terminal agrin fragment (CAF) (P = 0.031), and growth differentiation factor 15 (GDF15) (P = 0.049) increased significantly in the training group in comparison to the control group and Myostatin (MSTN) (P = 0.002) had a significant decrease. However, there was no significant difference in ASMM/m2 (P = 0.527), SMM/m2 (P = 0.621), or Body fat mass (P = 0.433) within or between groups. In addition, the TRX Suspension Training had a significant effect on the functional tests and improved gait speed (P = 0.037), chair stand (P = 0.016), and TUG (P = 0.016) as well as Handgrip strength (P = 0.035).

CONCLUSION

Our findings highlight the efficacy of TRX Suspension Training in enhancing the serum levels of muscle growth factors and functional capacities among elderly individuals with sarcopenia. Therefore, considering the ongoing COVID-19 pandemic, this protocol can prove beneficial for this demographic group.

TRIAL REGISTRY

Iranian Registry of Clinical Trials identifier: IRCT20230727058944N1, prospectively registered 20-09-2023, https://en.irct.ir/trial/71635.

A Genome-Wide Association Study for Resistance to Tropical Theileriosis in Two Bovine Portuguese Autochthonous Breeds

The control of Tropical Theileriosis, a tick-borne disease with a strong impact on cattle breeding, can be facilitated using marker-assisted selection in breeding programs. Genome-wide association studies (GWAS) using high-density arrays are extremely important for the ongoing process of identifying genomic variants associated with resistance to Theileria annulata infection. In this work, single-nucleotide polymorphisms (SNPs) were analyzed in the Portuguese autochthonous cattle breeds Alentejana and Mertolenga. In total, 24 SNPs suggestive of significance (p ≤ 10-4) were identified for Alentejana cattle and 20 SNPs were identified for Mertolenga cattle. The genomic regions around these SNPs were further investigated for annotated genes and quantitative trait loci (QTLs) previously described by other authors. Regarding the Alentejana breed, the MAP3K1, CMTM7, SSFA2, and ATG13 genes are located near suggestive SNPs and appear as candidate genes for resistance to Tropical Theileriosis, considering its action in the immune response and resistance to other diseases. On the other hand, in the Mertolenga breed, the UOX gene is also a candidate gene due to its apparent link to the pathogenesis of the disease. These results may represent a first step toward the possibility of including genetic markers for resistance to Tropical Theileriosis in current breed selection programs.

Follistatin and follistatin-like 3 in metabolic disorders

Follistatin (FST) is a glycoprotein which main role is antagonizing activity of transforming growth factor β superfamily members. Folistatin-related proteins such as follistatin-like 3 (FSTL3) also reveal these properties. The exact function of them has still not been established, but it can be bound to the pathogenesis of metabolic disorders. So far, there were performed a few studies about their role in type 2 diabetes, obesity or gestational diabetes and even less in type 1 diabetes. The outcomes are contradictory and do not allow to draw exact conclusions. In this article we summarize the available information about connections between follistatin, as well as follistatin-like 3, and metabolic disorders. We also emphasize the strong need of performing further research to explain their exact role, especially in the pathogenesis of diabetes and obesity.

Serum Follistatin Levels are Independently Associated with Exercise Tolerance in Patients with Obesity

OBJECTIVE

Follistatin (FST)/myostatin (MST) myokine-signaling axis is important for muscle metabolism and pathogenesis of obesity. FST, mainly secreted by skeletal muscle and liver, inhibits MST and affects skeletal muscle synthesis. This study aimed to identify the characteristics of myokines and independent predictors of serum FST levels in patients with obesity.

METHODS

In this retrospective cross-sectional study, 226 patients (mean age, 46.6 years; men, 35.4%) with obesity who initially visited an outpatient clinic between June 2014 and September 2020, were included and classified into obesity (25.0 ≤ body mass index (BMI) < 35.0 kg/m2) and severe obesity (BMI ≥35 kg/m2) groups based on the guidelines of the Japan Society for the Study of Obesity. Body composition was measured using bioelectrical impedance analysis and computed tomography. Muscle strength, exercise tolerance, metabolic parameters, and myokines were measured, including serum levels of FST, MST, irisin, and brain-derived neurotrophic factor.

RESULTS

Serum FST levels were significantly higher in the severe obesity group than in the obesity group (median: 768.4 vs. 895.1 pg/mL, P = 0.020). However, the levels of other myokines showed no significant differences between the groups. In Model 1, which included factors that significantly correlated with FST levels, stepwise multivariate regression analysis revealed peak oxygen uptake (VO2) as an independent predictor of FST levels based on the significance of the univariate analysis. Additionally, Model 2 was analyzed by adding myokine level to Model 1, revealing that peak VO2, MST, and irisin levels were independent predictors of FST levels.

CONCLUSION

Serum FST levels were higher in patients in the severe obesity group compared to those in the obesity group. There was an independent association between low exercise tolerance and elevated serum FST levels.

Mechanisms of mechanical overload-induced skeletal muscle hypertrophy: current understanding and future directions

Mechanisms underlying mechanical overload-induced skeletal muscle hypertrophy have been extensively researched since the landmark report by Morpurgo (1897) of "work-induced hypertrophy" in dogs that were treadmill trained. Much of the preclinical rodent and human resistance training research to date supports that involved mechanisms include enhanced mammalian/mechanistic target of rapamycin complex 1 (mTORC1) signaling, an expansion in translational capacity through ribosome biogenesis, increased satellite cell abundance and myonuclear accretion, and postexercise elevations in muscle protein synthesis rates. However, several lines of past and emerging evidence suggest that additional mechanisms that feed into or are independent of these processes are also involved. This review first provides a historical account of how mechanistic research into skeletal muscle hypertrophy has progressed. A comprehensive list of mechanisms associated with skeletal muscle hypertrophy is then outlined, and areas of disagreement involving these mechanisms are presented. Finally, future research directions involving many of the discussed mechanisms are proposed.

NOX2 deficiency exacerbates diet-induced obesity and impairs molecular training adaptations in skeletal muscle

The production of reactive oxygen species (ROS) by NADPH oxidase (NOX) 2 has been linked to both insulin resistance and exercise training adaptations in skeletal muscle. This study explores the previously unexamined role of NOX2 in the interplay between diet-induced insulin resistance and exercise training (ET). Using a mouse model that harbors a point mutation in the essential NOX2 regulatory subunit, p47phox (Ncf1*), we investigated the impact of this mutation on various metabolic adaptations. Wild-type (WT) and Ncf1* mice were assigned to three groups: chow diet, 60% energy fat diet (HFD), and HFD with access to running wheels (HFD + E). After a 16-week intervention, a comprehensive phenotypic assessment was performed, including body composition, glucose tolerance, energy intake, muscle insulin signaling, redox-related proteins, and mitochondrial adaptations. The results revealed that NOX2 deficiency exacerbated the impact of HFD on body weight, body composition, and glucose intolerance. Moreover, in Ncf1* mice, ET did not improve glucose tolerance or increase muscle cross-sectional area. ET normalized body fat independently of genotype. The lack of NOX2 activity during ET reduced several metabolic adaptations in skeletal muscle, including insulin signaling and expression of Hexokinase II and oxidative phosphorylation complexes. In conclusion, these findings suggest that NOX2 mediates key beneficial effects of exercise training in the context of diet-induced obesity.

The effects of resistance training on myostatin and follistatin in adults: A systematic review and meta-analysis

INTRODUCTION AND AIM

Myostatin and follistatin are the main hormones for regulating muscle mass, and previous research suggests they are modulated by resistance training. We therefore performed a systematic review and meta-analysis to investigate the impact of resistance training on circulating myostatin and follistatin in adults.

METHODS

A search was conducted in PubMed and Web of science from inception until October 2022 to identify original studies investigating the effects of resistance training compared with controls that did not exercise. Standardized mean differences and 95% confidence intervals (CIs) were calculated using random effects models.

RESULTS

A total 26 randomized studies, including 36 interventions, and involving 768 participants (aged ∼18 to 82 years), were included in the meta-analysis. Resistance training effectively decreased myostatin [-1.31 (95% CI -1.74 to -0.88, p=0.001, 26 studies] and increased follistatin [2.04 (95% CI: 1.51 to 2.52), p=0.001, 14 studies]. Subgroup analyses revealed a significant decrease in myostatin and increase in follistatin regardless of age.

CONCLUSION

Resistance training in adults is effective for reducing myostatin and increasing follistatin which may contribute to the beneficial effects of resistance training on muscle mass and metabolic outcomes.

Effects of Creatine Supplementation on the Myostatin Pathway and Myosin Heavy Chain Isoforms in Different Skeletal Muscles of Resistance-Trained Rats

Creatine has been used to maximize resistance training effects on skeletal muscles, including muscle hypertrophy and fiber type changes. This study aimed to evaluate the impact of creatine supplementation on the myostatin pathway and myosin heavy chain (MyHC) isoforms in the slow- and fast-twitch muscles of resistance-trained rats. Twenty-eight male Wistar rats were divided into four groups: a sedentary control (Cc), sedentary creatine supplementation (Cr), resistance training (Tc), and resistance training combined with creatine supplementation (Tcr). Cc and Tc received standard commercial chow; Cr and Tcr received a 2% creatine-supplemented diet. Tc and Tcr performed a resistance training protocol on a ladder for 12 weeks. Morphology, MyHC isoforms, myostatin, follistatin, and ActRIIB protein expressions were analyzed in soleus and white gastrocnemius portion samples. The results were analyzed using two-way ANOVA and Tukey's test. Tc and Tcr exhibited higher performance than their control counterparts. Resistance training increased the ratio between muscle and body weight, the cross-sectional area, as well as the interstitial collagen fraction. Resistance training alone increased MyHC IIx and follistatin while reducing myostatin (p < 0.001) and ActRIIB (p = 0.040) expressions in the gastrocnemius. Resistance training induced skeletal muscle hypertrophy and interstitial remodeling, which are more evident in the gastrocnemius muscle. The effects were not impacted by creatine supplementation.

Hepatic follistatin increases basal metabolic rate and attenuates diet-induced obesity during hepatic insulin resistance

OBJECTIVE

Body weight change and obesity follow the variance of excess energy input balanced against tightly controlled EE (energy expenditure). Since insulin resistance can reduce energy storage, we investigated whether genetic disruption of hepatic insulin signaling reduced adipose mass with increased EE.

METHODS

Insulin signaling was disrupted by genetic inactivation of Irs1 (Insulin receptor substrate 1) and Irs2 in hepatocytes of LDKO mice (Irs1L/L·Irs2L/L·CreAlb), creating a state of complete hepatic insulin resistance. We inactivated FoxO1 or the FoxO1-regulated hepatokine Fst (Follistatin) in the liver of LDKO mice by intercrossing LDKO mice with FoxO1L/L or FstL/L mice. We used DEXA (dual-energy X-ray absorptiometry) to assess total lean mass, fat mass and fat percentage, and metabolic cages to measure EE (energy expenditure) and estimate basal metabolic rate (BMR). High-fat diet was used to induce obesity.

RESULTS

Hepatic disruption of Irs1 and Irs2 (LDKO mice) attenuated HFD (high-fat diet)-induced obesity and increased whole-body EE in a FoxO1-dependent manner. Hepatic disruption of the FoxO1-regulated hepatokine Fst normalized EE in LDKO mice and restored adipose mass during HFD consumption; moreover, hepatic Fst disruption alone increased fat mass accumulation, whereas hepatic overexpression of Fst reduced HFD-induced obesity. Excess circulating Fst in overexpressing mice neutralized Mstn (Myostatin), activating mTORC1-promoted pathways of nutrient uptake and EE in skeletal muscle. Similar to Fst overexpression, direct activation of muscle mTORC1 also reduced adipose mass.

CONCLUSIONS

Thus, complete hepatic insulin resistance in LDKO mice fed a HFD revealed Fst-mediated communication between the liver and muscle, which might go unnoticed during ordinary hepatic insulin resistance as a mechanism to increase muscle EE and constrain obesity.

Postprandial secretion of follistatin after gastric bypass surgery and sleeve gastrectomy

Follistatin is secreted from the liver and may regulate muscle growth and insulin sensitivity. Protein intake stimulates follistatin secretion, which may be mediated by increased glucagon in the context of low insulin concentrations. We investigated circulating follistatin after mixed-meals in two cohorts of patients who were part of previously published studies and had undergone bariatric surgery with either simultaneous assessment of amino acid absorption or administration of the GLP-1 receptor antagonist exendin-(9-39), which increased glucagon concentrations and impaired insulin secretion. Study 1 comprised obese matched subjects with previous Roux-en-Y gastric bypass (RYGB) or sleeve gastrectomy (SG) surgery and unoperated controls who underwent 6-hour mixed-meal tests with intravenous and oral tracers including intrinsically labelled caseinate in the meal. Study 2 comprised obese subjects with previous RYGB who underwent two 5-hour mixed-meal tests with concomitant exendin-(9-39) or saline infusion. In study 1, the secretion of follistatin as well as the amino acid absorption was accelerated after RYGB compared with SG and controls, but the glucagon-to-C-peptide ratios did not differ between the groups. In study 2, exendin-(9-39) administration increased postprandial glucagon concentrations and lowered insulin secretion, whereas the concentration of follistatin was unchanged. In conclusion, postprandial follistatin secretion is accelerated in patients after RYGB which might be explained by an accelerated protein absorption rate rather than the glucagon-to-insulin ratio.

Adipose tissue adipokines and lipokines: Functions and regulatory mechanism in skeletal muscle development and homeostasis

Skeletal muscle plays important roles in normal biological activities and whole-body energy homeostasis in humans. The growth and development of skeletal muscle also directly influence meat production and meat quality in animal production. Therefore, regulating the development and homeostasis of skeletal muscle is crucial for human health and animal production. Adipose tissue, which includes white adipose tissue (WAT) and brown adipose tissue (BAT), not only functions as an energy reserve but also has attracted substantial attention because of its role as an endocrine organ. The novel signalling molecules known as "adipokines" and "lipokines" that are secreted by adipose tissue were identified through the secretomic technique, which broadened our understanding of the previously unknown crosstalk between adipose tissue and skeletal muscle. In this review, we summarize and discuss the secretory role of adipose tissues, both WAT and BAT, as well as the regulatory roles of various adipokines and lipokines in skeletal muscle development and homeostasis. We suggest that adipokines and lipokines have potential as drug candidates for the treatment of skeletal muscle dysfunction and related metabolic diseases and as promising nutrients for improving animal production.

Diabetes as a potential compounding factor in COVID-19-mediated male subfertility

Recent work indicates that male fertility is compromised by SARS-CoV-2 infection. Direct effects derive from the presence of viral entry receptors (ACE2 and/or CD147) on the surface of testicular cells, such as spermatocytes, Sertoli cells, and Leydig cells. Indirect effects on testis and concentrations of male reproductive hormones derive from (1) virus-stimulated inflammation; (2) viral-induced diabetes, and (3) an interaction between diabetes and inflammation that exacerbates the deleterious effect of each perturbation. Reproductive hormones affected include testosterone, luteinizing hormone, and follicle-stimulating hormone. Reduction of male fertility is also observed with other viral infections, but the global pandemic of COVID-19 makes demographic and public health implications of reduced male fertility of major concern, especially if it occurs in the absence of serious symptoms that would otherwise encourage vaccination. Clinical documentation of COVID-19-associated male subfertility is now warranted to obtain quantitative relationships between infection severity and subfertility; mechanistic studies using animal models may reveal ways to mitigate the problem. In the meantime, the possibility of subfertility due to COVID-19 should enter considerations of vaccine hesitancy by reproductive-age males.

MMP12 knockout prevents weight and muscle loss in tumor-bearing mice

BACKGROUND

Colorectal cancer is a malignant gastrointestinal cancer, in which some advanced patients would develop cancer cachexia (CAC). CAC is defined as a multi-factorial syndrome characterized by weight loss and muscle loss (with or without fat mass), leading to progressive dysfunction, thereby increasing morbidity and mortality. Apc^Min/+ mice develop spontaneous intestinal adenoma, which provides an established model of colorectal cancer for CAC study. Upon studying the Apc^Min/+ mouse model, we observed a marked decrease in weight gain beginning around week 15. Such a reduction in weight gain was rescued when Apc^Min/+ mice were crossed with MMP12^-/- mice, indicating that MMP12 has a role in age-related Apc^Min/+-associated weight loss. As a control, the weight of MMP12^-/- mice on a weekly basis, their weight were not significantly different from those of WT mice.

METHODS

Apc^Min/+; MMP12^-/- mice were obtained by crossing Apc^Min/+ mice with MMP12 knockout (MMP12 ^-/-) mice. Histological scores were assessed using hematoxylin-eosin (H&E) staining. MMP12 expression was confirmed by immunohistochemistry and immunofluorescence staining. ELISA, protein microarrays and quantitative Polymerase Chain Reaction (qPCR) were used to investigate whether tumor could up-regulate IL-6. Cell-based assays and western blot were used to verify the regulatory relationship between IL-6 and MMP12. Fluorescence intensity was measured to determine whether MMP12 is associated with insulin and insulin-like growth factor 1 (IGF-1) in vitro. MMP12 inhibitors were used to explore whether MMP12 could affect the body weight of Apc^Min/+ mice.

RESULTS

MMP12 knockout led to weight gain and expansion of muscle fiber cross-sectional area (all mice had C57BL/6 background) in Apc^Min/+ mice, while inhibiting MMP12 could suppress weight loss in Apc^Min/+ mice. MMP12 was up-regulated in muscle tissues and peritoneal macrophages of Apc^Min/+ mice. IL-6 in tumor cells and colorectal cancer patients is up-regulation. IL-6 stimulated MMP12 secretion of macrophage.

CONCLUSIONS

MMP12 is essential for controlling body weight of Apc ^Min/+ mice. Our study shows that it exists the crosstalk between cancer cells and macrophages in muscle tissues that tumor cells secrete IL-6 inducing macrophages to up-regulate MMP12. This study may provide a new perspective of MMP12 in the treatment for weight loss induced by CAC.

Genes controlling skeletal muscle glucose uptake and their regulation by endurance and resistance exercise

Exercise improves the insulin sensitivity of glucose uptake in skeletal muscle. Due to that, exercise has become a cornerstone treatment for type 2 diabetes mellitus (T2DM). The mechanisms by which exercise improves skeletal muscle insulin sensitivity are, however, incompletely understood. We conducted a systematic review to identify all genes whose gain or loss of function alters skeletal muscle glucose uptake. We subsequently cross-referenced these genes with recently generated data sets on exercise-induced gene expression and signaling. Our search revealed 176 muscle glucose-uptake genes, meaning that their genetic manipulation altered glucose uptake in skeletal muscle. Notably, exercise regulates the expression or phosphorylation of more than 50% of the glucose-uptake genes or their protein products. This included many genes that previously have not been associated with exercise-induced insulin sensitivity. Interestingly, endurance and resistance exercise triggered some common but mostly unique changes in expression and phosphorylation of glucose-uptake genes or their protein products. Collectively, our work provides a resource of potentially new molecular effectors that play a role in the incompletely understood regulation of muscle insulin sensitivity by exercise.

Skeletal muscle-targeted delivery of Fgf6 protects mice from diet-induced obesity and insulin resistance

Obesity, a major health care issue, is characterized by metabolic abnormalities in multiple tissues, including the skeletal muscle. Although dysregulation of skeletal muscle metabolism can strongly influence the homeostasis of systemic energy, the underlying mechanism remains unclear. We found promoter hypermethylation and decreased gene expression of fibroblast growth factor 6 (FGF6) in the skeletal muscle of individuals with obesity using high-throughput sequencing. Reduced binding of the cyclic AMP responsive element binding protein-1 (CREB1) to the hypermethylated cyclic AMP response element, which is a regulatory element upstream of the transcription initiation site, partially contributed to the downregulation of FGF6 in patients with obesity. Overexpression of Fgf6 in mouse skeletal muscle stimulated protein synthesis, activating the mammalian target of rapamycin pathway, and prevented the increase in weight and the development of insulin resistance in high-fat diet–fed mice. Thus, our findings highlight the role played by Fgf6 in regulating skeletal muscle hypertrophy and whole-body metabolism, indicating its potential in strategies aimed at preventing and treating metabolic diseases.

Targeting interleukin-1 for reversing fat browning and muscle wasting in infantile nephropathic cystinosis

BACKGROUND

Ctns-/- mice, a mouse model of infantile nephropathic cystinosis, exhibit hypermetabolism with adipose tissue browning and profound muscle wasting. Inflammatory cytokines such as interleukin (IL)-1 trigger inflammatory cascades and may be an important cause for cachexia. We employed genetic and pharmacological approaches to investigate the effects of IL-1 blockade in Ctns-/- mice.

METHODS

We generated Ctns-/- Il1β-/- mice, and we treated Ctns-/- and wild-type control mice with IL-1 receptor antagonist, anakinra (2.5 mg/kg/day, IP) or saline as vehicle for 6 weeks. In each of these mouse lines, we characterized the cachexia phenotype consisting of anorexia, loss of weight, fat mass and lean mass, elevation of metabolic rate, and reduced in vivo muscle function (rotarod activity and grip strength). We quantitated energy homeostasis by measuring the protein content of uncoupling proteins (UCPs) and adenosine triphosphate in adipose tissue and skeletal muscle. We measured skeletal muscle fiber area and intramuscular fatty infiltration. We also studied expression of molecules regulating adipose tissue browning and muscle mass metabolism. Finally, we evaluated the impact of anakinra on the muscle transcriptome in Ctns-/- mice.

RESULTS

Skeletal muscle expression of IL-1β was significantly elevated in Ctns-/- mice relative to wild-type control mice. Cachexia was completely normalized in Ctns-/- Il1β-/- mice relative to Ctns-/- mice. We showed that anakinra attenuated the cachexia phenotype in Ctns-/- mice. Anakinra normalized UCPs and adenosine triphosphate content of adipose tissue and muscle in Ctns-/- mice. Anakinra attenuated aberrant expression of beige adipose cell biomarkers (UCP-1, CD137, Tmem26, and Tbx1) and molecules implicated in adipocyte tissue browning (Cox2/Pgf2α, Tlr2, Myd88, and Traf6) in inguinal white adipose tissue in Ctns-/- mice. Moreover, anakinra normalized gastrocnemius weight and fiber size and attenuated muscle fat infiltration in Ctns-/- mice. This was accompanied by correction of the increased muscle wasting signalling pathways (increased protein content of ERK1/2, JNK, p38 MAPK, and nuclear factor-κB p65 and mRNA expression of Atrogin-1 and Myostatin) and the decreased myogenesis process (decreased mRNA expression of MyoD and Myogenin) in the gastrocnemius muscle of Ctns-/- mice. Previously, we identified the top 20 differentially expressed skeletal muscle genes in Ctns-/- mice by RNAseq. Aberrant expression of these 20 genes have been implicated in muscle wasting, increased energy expenditure, and lipolysis. We showed that anakinra attenuated 12 of those top 20 differentially expressed muscle genes in Ctns-/- mice.

CONCLUSIONS

Anakinra may provide a targeted novel therapy for patients with infantile nephropathic cystinosis.

The Role of the Skeletal Muscle Secretome in Mediating Endurance and Resistance Training Adaptations

Exercise, in the form of endurance or resistance training, leads to specific molecular and cellular adaptions not only in skeletal muscles, but also in many other organs such as the brain, liver, fat or bone. In addition to direct effects of exercise on these organs, the production and release of a plethora of different signaling molecules from skeletal muscle are a centerpiece of systemic plasticity. Most studies have so far focused on the regulation and function of such myokines in acute exercise bouts. In contrast, the secretome of long-term training adaptation remains less well understood, and the contribution of non-myokine factors, including metabolites, enzymes, microRNAs or mitochondrial DNA transported in extracellular vesicles or by other means, is underappreciated. In this review, we therefore provide an overview on the current knowledge of endurance and resistance exercise-induced factors of the skeletal muscle secretome that mediate muscular and systemic adaptations to long-term training. Targeting these factors and leveraging their functions could not only have broad implications for athletic performance, but also for the prevention and therapy in diseased and elderly populations.

A Leptin Receptor Antagonist Attenuates Adipose Tissue Browning and Muscle Wasting in Infantile Nephropathic Cystinosis-Associated Cachexia

Mice lacking the functional cystinosin gene (Ctns-/-), a model of infantile nephropathic cystinosis (INC), exhibit the cachexia phenotype with adipose tissue browning and muscle wasting. Elevated leptin signaling is an important cause of chronic kidney disease-associated cachexia. The pegylated leptin receptor antagonist (PLA) binds to but does not activate the leptin receptor. We tested the efficacy of this PLA in Ctns-/- mice. We treated 12-month-old Ctns-/- mice and control mice with PLA (7 mg/kg/day, IP) or saline as a vehicle for 28 days. PLA normalized food intake and weight gain, increased fat and lean mass, decreased metabolic rate and improved muscle function. It also attenuated perturbations of energy homeostasis in adipose tissue and muscle in Ctns-/- mice. PLA attenuated adipose tissue browning in Ctns-/- mice. PLA increased gastrocnemius weight and fiber size as well as attenuated muscle fat infiltration in Ctns-/- mice. This was accompanied by correcting the increased expression of muscle wasting signaling while promoting the decreased expression of myogenesis in gastrocnemius of Ctns-/- mice. PLA attenuated aberrant expressed muscle genes that have been associated with muscle atrophy, increased energy expenditure and lipolysis in Ctns-/- mice. Leptin antagonism may represent a viable therapeutic strategy for adipose tissue browning and muscle wasting in INC.

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.

HMEJ-mediated site-specific integration of a myostatin inhibitor increases skeletal muscle mass in porcine

As a robust antagonist of myostatin (MSTN), follistatin (FST) is an important regulator of skeletal muscle development, and the delivery of FST to muscle tissue represents a potential therapeutic strategy for muscular dystrophies. The N terminus and FSI domain of FST are the functional domains for MSTN binding. Here, we aimed to achieve site-specific integration of FSI-I-I, including the signal peptide, N terminus, and three FSI domains, into the last codon of the porcine MSTN gene using a homology-mediated end joining (HMEJ)-based strategy mediated by CRISPR-Cas9. Based on somatic cell nuclear transfer (SCNT) technology, we successfully obtained FSI-I-I knockin pigs. H&E staining of longissimus dorsi and gastrocnemius cross-sections showed larger myofiber sizes in FSI-I-I knockin pigs than in controls. Moreover, the Smad and Erk pathways were inhibited, whereas the PI3k/Akt pathway was activated in FSI-I-I knockin pigs. In addition, the levels of MyoD, Myf5, and MyoG transcription were upregulated while that of MRF4 was downregulated in FSI-I-I knockin pigs. These results indicate that the FSI-I-I gene mediates skeletal muscle hypertrophy through an MSTN-related signaling pathway and the expression of myogenic regulatory factors. Overall, FSI-I-I knockin pigs with hypertrophic muscle tissue hold great promise as a therapeutic model for human muscular dystrophies.

Effects of 16 Weeks of Resistance Training on Muscle Quality and Muscle Growth Factors in Older Adult Women with Sarcopenia: A Randomized Controlled Trial

This study examined the effects of resistance training on muscle quality, muscle growth factors, and functional fitness in older adult women with sarcopenia. Twenty-two older adult women aged over 65 with sarcopenia were randomly assigned to either resistance training (RT, n = 12) or non-exercise control group (CG, n = 10). The body weight-based and elastic band RT were performed three times a week, 60 min per session, for 16 weeks. Body composition and thigh muscle quality were estimated by dual-energy X-ray absorptiometry (DEXA) and computed tomography (CT), respectively. The muscle growth factors, including growth differentiation factor-8 (GDF-8), growth differentiation factor-15 (GDF-15), activin A, and follistatin, were analyzed via blood samples. Statistical analyses were performed using repeated measures multivariate analysis of variance (MANOVA), analysis of variance (ANOVA), and effect size (i.e., cohen’s d, partial eta square), and the significance level was set at 0.05. The RT group improved their functional fitness, grip strength, gait speed, and isometric muscle strength (p < 0.01, d > 0.99; large), while these variables did not change in the CG. An increase in intramuscular fat was only observed in the CG (p < 0.01, 1.06; large). Muscle growth factors such as follistatin were significantly increased in the RT (p < 0.05, 0.81; large), but other variables did not change following resistance training. Sixteen weeks of resistance training improved functional fitness and prevented age-related increases in intramuscular fat in the thigh area. However, there were only some changes in muscle growth factors, such as follistatin, suggesting that the effectiveness of resistance training on muscle growth factors is limited. Body weight-based and elastic band resistance training is an alternative training method for sarcopenia to minimize the age-related adverse effects on muscle function and quality.

p21-Activated Kinase 1 Is Permissive for the Skeletal Muscle Hypertrophy Induced by Myostatin Inhibition

Skeletal muscle, the most abundant tissue in the body, plays vital roles in locomotion and metabolism. Understanding the cellular processes that govern regulation of muscle mass and function represents an essential step in the development of therapeutic strategies for muscular disorders. Myostatin, a member of the TGF-β family, has been identified as a negative regulator of muscle development. Indeed, its inhibition induces an extensive skeletal muscle hypertrophy requiring the activation of Smad 1/5/8 and the Insulin/IGF-I signaling pathway, but whether other molecular mechanisms are involved in this process remains to be determined. Using transcriptomic data from various Myostatin inhibition models, we identified Pak1 as a potential mediator of Myostatin action on skeletal muscle mass. Our results show that muscle PAK1 levels are systematically increased in response to Myostatin inhibition, parallel to skeletal muscle mass, regardless of the Myostatin inhibition model. Using Pak1 knockout mice, we investigated the role of Pak1 in the skeletal muscle hypertrophy induced by different approaches of Myostatin inhibition. Our findings show that Pak1 deletion does not impede the skeletal muscle hypertrophy magnitude in response to Myostatin inhibition. Therefore, Pak1 is permissive for the skeletal muscle mass increase caused by Myostatin inhibition.

Follistatin secretion is enhanced by protein, but not glucose or fat ingestion, in obese persons independently of previous gastric bypass surgery

Follistatin is secreted from the liver and is involved in the regulation of muscle mass and insulin sensitivity via inhibition of activin A in humans. The secretion of follistatin seems to be stimulated by glucagon and inhibited by insulin, but only limited knowledge on the postprandial regulation of follistatin exists. Moreover, results on postoperative changes after Roux-en-Y gastric bypass (RYGB) are conflicting with reports of increased, unaltered, and lowered fasting concentrations of follistatin. In this study, we investigated postprandial follistatin and activin A concentrations after intake of isocaloric amounts of protein, fat, or glucose in subjects with obesity with and without previous RYGB to explore the regulation of follistatin by the individual macronutrients. Protein intake enhanced follistatin concentrations similarly in the two groups, whereas glucose and fat ingestion did not change postprandial follistatin concentrations. Concentrations of activin A were lower after protein intake compared with glucose intake in RYGB. Glucagon concentrations were also particularly enhanced by protein intake and tended to correlate with follistatin in RYGB. In conclusion, we demonstrated that protein intake, but not glucose or fat, is a strong stimulus for follistatin secretion in subjects with obesity and that this regulation is maintained after RYGB surgery.NEW & NOTEWORTHY Circulating follistatin and activin A were studied after intake of isocaloric protein, fat, or glucose drinks in subjects with obesity with and without previous Roux-en-Y gastric bypass (RYGB). Protein intake enhanced follistatin similarly in both groups, whereas glucose and fat ingestion did not change follistatin. Activin A was lower after protein compared with glucose in RYGB. The novel finding is that protein intake, but neither glucose nor fat, stimulates follistatin secretion independently of previous RYGB.

Plasma Activin A and Decorin in Exercised Purebred Arabian Horses - Preliminary Study

Development of the musculoskeletal system occurs, in part, through an impact on regulatory proteins, such as activin A and decorin. Activin A induces atrophic effects on skeletal muscle, and decorin regulates the formation of connective tissue proteins like collagen and elastin in tendons and ligaments. The aim of this study was to evaluate the influence of different intensities of exercise on blood plasma activin A and decorin concentrations in horses. Ten young purebred Arabian horses were subjected to routine training sessions on a racetrack, and another 11 adult horses of the same breed participated in endurance rides. Race horses were examined during gallop tests over a distance of 1200 m and endurance horses at distances from 20 to 80 km. Blood samples were collected at rest and after exercise to determine plasma concentrations of activin A, decorin and cortisol. Despite differences in the intensity of exercise, the plasma decorin concentration remained unchanged, and activin A tended to decrease in endurance horses only. The exercise-induced changes in plasma activin A concentrations correlated with the covered distance (r = -0.43; P < .05), but not with the changes in cortisol values. Further studies are needed to confirm the usefulness of plasma activin A as a potential indicator of a horse's endurance performance.

The many actions of insulin in skeletal muscle, the paramount tissue determining glycemia

As the principal tissue for insulin-stimulated glucose disposal, skeletal muscle is a primary driver of whole-body glycemic control. Skeletal muscle also uniquely responds to muscle contraction or exercise with increased sensitivity to subsequent insulin stimulation. Insulin's dominating control of glucose metabolism is orchestrated by complex and highly regulated signaling cascades that elicit diverse and unique effects on skeletal muscle. We discuss the discoveries that have led to our current understanding of how insulin promotes glucose uptake in muscle. We also touch upon insulin access to muscle, and insulin signaling toward glycogen, lipid, and protein metabolism. We draw from human and rodent studies in vivo, isolated muscle preparations, and muscle cell cultures to home in on the molecular, biophysical, and structural elements mediating these responses. Finally, we offer some perspective on molecular defects that potentially underlie the failure of muscle to take up glucose efficiently during obesity and type 2 diabetes.

Effect of Various Exercise Regimens on Selected Exercise-Induced Cytokines in Healthy People

Different forms of physical activity—endurance, resistance or dynamic power—stimulate cytokine release from various tissues to the bloodstream. Receptors for exercise-induced cytokines are present in muscle tissue, adipose tissue, liver, brain, bones, cardiovascular system, immune system, pancreas, and skin. They have autocrine, paracrine and endocrine activities. Many of them regulate the myocyte growth and differentiation necessary for muscle hypertrophy and myogenesis. They also modify energy homeostasis, lipid, carbohydrate, and protein metabolism, regulate inflammation and exchange information (crosstalk) between remote organs. So far, interleukin 6 and irisin have been the best studied exercise-induced cytokines. However, many more can be grouped into myokines, hepatokines and adipomyokines. This review focuses on the less known exercise-induced cytokines such as myostatin, follistatin, decorin, brain-derived neurotrophic factor, fibroblast growth factor 21 and interleukin 15, and their relation to various forms of exercise, i.e., acute vs. chronic, regular training in healthy people.

Hepatokines as a Molecular Transducer of Exercise

Exercise has health benefits and prevents a range of chronic diseases caused by physiological and biological changes in the whole body. Generally, the metabolic regulation of skeletal muscle through exercise is known to have a protective effect on the pathogenesis of metabolic syndrome, non-alcoholic fatty liver disease (NAFLD), type 2 diabetes (T2D), and cardiovascular disease (CVD). Besides this, the importance of the liver as an endocrine organ is a hot research topic. Hepatocytes also secrete many hepatokines in response to nutritional conditions and/or physical activity. In particular, certain hepatokines play a major role in the regulation of whole-body metabolic homeostasis. In this review, we summarize the recent research findings on the exercise-mediated regulation of hepatokines, including fibroblast growth factor 21, fetuin-A, angiopoietin-like protein 4, and follistatin. These hepatokines serve as molecular transducers of the metabolic benefits of physical activity in chronic metabolic diseases, including NAFLD, T2D, and CVDs, in various tissues.

Molecular Mechanisms of Skeletal Muscle Hypertrophy

Skeletal muscle hypertrophy can be induced by hormones and growth factors acting directly as positive regulators of muscle growth or indirectly by neutralizing negative regulators, and by mechanical signals mediating the effect of resistance exercise. Muscle growth during hypertrophy is controlled at the translational level, through the stimulation of protein synthesis, and at the transcriptional level, through the activation of ribosomal RNAs and muscle-specific genes. mTORC1 has a central role in the regulation of both protein synthesis and ribosomal biogenesis. Several transcription factors and co-activators, including MEF2, SRF, PGC-1α4, and YAP promote the growth of the myofibers. Satellite cell proliferation and fusion is involved in some but not all muscle hypertrophy models.

TMEPAI/PMEPA1 Is a Positive Regulator of Skeletal Muscle Mass

Inhibition of myostatin- and activin-mediated SMAD2/3 signaling using ligand traps, such as soluble receptors, ligand-targeting propeptides and antibodies, or follistatin can increase skeletal muscle mass in healthy mice and ameliorate wasting in models of cancer cachexia and muscular dystrophy. However, clinical translation of these extracellular approaches targeting myostatin and activin has been hindered by the challenges of achieving efficacy without potential effects in other tissues. Toward the goal of developing tissue-specific myostatin/activin interventions, we explored the ability of transmembrane prostate androgen-induced (TMEPAI), an inhibitor of transforming growth factor-β (TGF-β1)-mediated SMAD2/3 signaling, to promote growth, and counter atrophy, in skeletal muscle. In this study, we show that TMEPAI can block activin A, activin B, myostatin and GDF-11 activity in vitro. To determine the physiological significance of TMEPAI, we employed Adeno-associated viral vector (AAV) delivery of a TMEPAI expression cassette to the muscles of healthy mice, which increased mass by as much as 30%, due to hypertrophy of muscle fibers. To demonstrate that TMEPAI mediates its effects via inhibition of the SMAD2/3 pathway, tibialis anterior (TA) muscles of mice were co-injected with AAV vectors expressing activin A and TMEPAI. In this setting, TMEPAI blocked skeletal muscle wasting driven by activin-induced phosphorylation of SMAD3. In a model of cancer cachexia associated with elevated circulating activin A, delivery of AAV:TMEPAI into TA muscles of mice bearing C26 colon tumors ameliorated the muscle atrophy normally associated with cancer progression. Collectively, the findings indicate that muscle-directed TMEPAI gene delivery can inactivate the activin/myostatin-SMAD3 pathway to positively regulate muscle mass in healthy settings and models of disease.

Exercise-Induced Increases in Insulin Sensitivity After Bariatric Surgery Are Mediated By Muscle Extracellular Matrix Remodeling

Exercise seems to enhance the beneficial effect of bariatric (Roux-en-Y gastric bypass [RYGB]) surgery on insulin resistance. We hypothesized that skeletal muscle extracellular matrix (ECM) remodeling may underlie these benefits. Women were randomized to either a combined aerobic and resistance exercise training program following RYGB (RYGB + ET) or standard of care (RYGB). Insulin sensitivity was assessed by oral glucose tolerance test. Muscle biopsy specimens were obtained at baseline and 3 and 9 months after surgery and subjected to comprehensive phenotyping, transcriptome profiling, molecular pathway identification, and validation in vitro. Exercise training improved insulin sensitivity beyond surgery alone (e.g., Matsuda index: RYGB 123% vs. RYGB + ET 325%; P ≤ 0.0001). ECM remodeling was reduced by surgery alone, with an additive benefit of surgery and exercise training (e.g., collagen I: RYGB -41% vs. RYGB + ET -76%; P ≤ 0.0001). Exercise and RYGB had an additive effect on enhancing insulin sensitivity, but surgery alone did not resolve insulin resistance and ECM remodeling. We identified candidates modulated by exercise training that may become therapeutic targets for treating insulin resistance, in particular, the transforming growth factor-β1/SMAD 2/3 pathway and its antagonist follistatin. Exercise-induced increases in insulin sensitivity after bariatric surgery are at least partially mediated by muscle ECM remodeling.

Circulating Follistatin and Activin A and Their Regulation by Insulin in Obesity and Type 2 Diabetes

BACKGROUND

Circulating follistatin (Fst) binds activin A and thereby regulates biological functions such as muscle growth and β-cell survival. However, Fst and activin A's implication in metabolic regulation is unclear.

OBJECTIVE

To investigate circulating Fst and activin A in obesity and type 2 diabetes (T2D) and determine their association with metabolic parameters. Further, to examine regulation of Fst and activin A by insulin and the influence of obesity and T2D hereon.

METHODS

Plasma Fst and activin A levels were analyzed in obese T2D patients (N = 10) closely matched to glucose-tolerant lean (N = 12) and obese (N = 10) individuals in the fasted state and following a 4-h hyperinsulinemic-euglycemic clamp (40 mU·m-2·min-1) combined with indirect calorimetry.

RESULTS

Circulating Fst was ~30% higher in patients with T2D compared with both lean and obese nondiabetic individuals (P < .001), while plasma activin A was unaltered. In the total cohort, fasting plasma Fst correlated positively with fasting plasma glucose, serum insulin and C-peptide levels, homeostasis model assessment of insulin resistance, and hepatic and adipose tissue insulin resistance after adjusting for age, gender and group (all r > 0.47; P < .05). However, in the individual groups these correlations only achieved significance in patients with T2D (not plasma glucose). Acute hyperinsulinemia at euglycemia reduced circulating Fst by ~30% (P < .001) and this response was intact in patients with T2D. Insulin inhibited FST expression in human hepatocytes after 2 h and even further after 48 h.

CONCLUSIONS

Elevated circulating Fst, but not activin A, is strongly associated with measures of insulin resistance in patients with T2D. However, the ability of insulin to suppress circulating Fst is preserved in T2D.

A focused review of myokines as a potential contributor to muscle hypertrophy from resistance-based exercise

PURPOSE

Resistance exercise induces muscle growth and is an important treatment for age-related losses in muscle mass and strength. Myokines are hypothesized as a signal conveying physiological information to skeletal muscle, possibly to "fine-tune" other regulatory pathways. While myokines are released from skeletal muscle following contraction, their role in increasing muscle mass and strength in response to resistance exercise or training is not established. Recent research identified both local and systemic release of myokines after an acute bout of resistance exercise. However, it is not known whether myokines with putative anabolic function are mechanistically involved in producing muscle hypertrophy after resistance exercise. Further, nitric oxide (NO), an important mediator of muscle stem cell activation, upregulates the expression of certain myokine genes in skeletal muscle.

METHOD

In the systemic context of complex hypertrophic signaling, this review: (1) summarizes literature on several well-recognized, representative myokines with anabolic potential; (2) explores the potential mechanistic role of myokines in skeletal muscle hypertrophy; and (3) identifies future research required to advance our understanding of myokine anabolism specifically in skeletal muscle.

RESULT

This review establishes a link between myokines and NO production, and emphasizes the importance of considering systemic release of potential anabolic myokines during resistance exercise as complementary to other signals that promote hypertrophy.

CONCLUSION

Investigating adaptations to resistance exercise in aging opens a novel avenue of interdisciplinary research into myokines and NO metabolites during resistance exercise, with the longer-term goal to improve muscle health in daily living, aging, and rehabilitation.

Intravascular Follistatin gene delivery improves glycemic control in a mouse model of type 2 diabetes

Type 2 diabetes (T2D) manifests from inadequate glucose control due to insulin resistance, hypoinsulinemia, and deteriorating pancreatic β-cell function. The pro-inflammatory factor Activin has been implicated as a positive correlate of severity in T2D patients, and as a negative regulator of glucose uptake by skeletal muscle, and of pancreatic β-cell phenotype in mice. Accordingly, we sought to determine whether intervention with the Activin antagonist Follistatin can ameliorate the diabetic pathology. Here, we report that an intravenous Follistatin gene delivery intervention with tropism for striated muscle reduced the serum concentrations of Activin B and improved glycemic control in the db/db mouse model of T2D. Treatment reversed the hyperglycemic progression with a corresponding reduction in the percentage of glycated-hemoglobin to levels similar to lean, healthy mice. Follistatin gene delivery promoted insulinemia and abundance of insulin-positive pancreatic β-cells, even when treatment was administered to mice with advanced diabetes, supporting a mechanism for improved glycemic control associated with maintenance of functional β-cells. Our data demonstrate that single-dose intravascular Follistatin gene delivery can ameliorate the diabetic progression and improve prognostic markers of disease. These findings are consistent with other observations of Activin-mediated mechanisms exerting deleterious effects in models of obesity and diabetes, and suggest that interventions that attenuate Activin signaling could help further understanding of T2D and the development of novel T2D therapeutics.

Role of Metabolic Stress and Exercise in Regulating Fibro/Adipogenic Progenitors

Obesity is a major public health concern and is associated with decreased muscle quality (i.e., strength, metabolism). Muscle from obese adults is characterized by increases in fatty, fibrotic tissue that decreases the force producing capacity of muscle and impairs glucose disposal. Fibro/adipogenic progenitors (FAPs) are muscle resident, multipotent stromal cells that are responsible for muscle fibro/fatty tissue accumulation. Additionally, they are indirectly involved in muscle adaptation through their promotion of myogenic (muscle-forming) satellite cell proliferation and differentiation. In conditions similar to obesity that are characterized by chronic muscle degeneration, FAP dysfunction has been shown to be responsible for increased fibro/fatty tissue accumulation in skeletal muscle, and impaired satellite cell function. The role of metabolic stress in regulating FAP differentiation and paracrine function in skeletal muscle is just beginning to be unraveled. Thus, the present review aims to summarize the recent literature on the role of metabolic stress in regulating FAP differentiation and paracrine function in skeletal muscle, and the mechanisms responsible for these effects. Furthermore, we will review the role of physical activity in reversing or ameliorating the detrimental effects of obesity on FAP function.

Mechanisms involved in follistatin-induced hypertrophy and increased insulin action in skeletal muscle

BACKGROUND

Skeletal muscle wasting is often associated with insulin resistance. A major regulator of muscle mass is the transforming growth factor β (TGF-β) superfamily, including activin A, which causes atrophy. TGF-β superfamily ligands also negatively regulate insulin-sensitive proteins, but whether this pathway contributes to insulin action remains to be determined.

METHODS

To elucidate if TGF-β superfamily ligands regulate insulin action, we used an adeno-associated virus gene editing approach to overexpress an activin A inhibitor, follistatin (Fst288), in mouse muscle of lean and diet-induced obese mice. We determined basal and insulin-stimulated 2-deoxy-glucose uptake using isotopic tracers in vivo. Furthermore, to evaluate whether circulating Fst and activin A concentrations are associated with obesity, insulin resistance, and weight loss in humans, we analysed serum from morbidly obese subjects before, 1 week, and 1 year after Roux-en-Y gastric bypass (RYGB).

RESULTS

Fst288 muscle overexpression markedly increased in vivo insulin-stimulated (but not basal) glucose uptake (+75%, P < 0.05) and increased protein expression and intracellular insulin signalling of AKT, TBC1D4, PAK1, pyruvate dehydrogenase-E1α, and p70S6K, while decreasing TBC1D1 signaling (P < 0.05). Fst288 increased both basal and insulin-stimulated protein synthesis, but no correlation was observed between the Fst288-driven hypertrophy and the increase in insulin-stimulated glucose uptake. Importantly, Fst288 completely normalized muscle glucose uptake in insulin-resistant diet-induced obese mice. RYGB surgery doubled circulating Fst and reduced activin A (-24%, P < 0.05) concentration 1 week after surgery before any significant weight loss in morbidly obese normoglycemic patients, while major weight loss after 1 year did not further change the concentrations.

CONCLUSIONS

We here present evidence that Fst is a potent regulator of insulin action in muscle, and in addition to AKT and p70S6K, we identify TBC1D1, TBC1D4, pyruvate dehydrogenase-E1α, and PAK1 as Fst targets. Circulating Fst more than doubled post-RYGB surgery, a treatment that markedly improved insulin sensitivity, suggesting a role for Fst in regulating glycaemic control. These findings demonstrate the therapeutic potential of inhibiting TGF-β superfamily ligands to improve insulin action and Fst's relevance to muscle wasting-associated insulin-resistant conditions in mice and humans.