The translation regulatory subunit eIF3f controls the kinase-dependent mTOR signaling required for muscle differentiation and hypertrophy in mouse

The Alleviative Effect of Sodium Butyrate on Dexamethasone-Induced Skeletal Muscle Atrophy

Skeletal muscle mass is significantly negatively regulated by glucocorticoids. Following glucocorticoid administration, the balance between protein synthesis and breakdown in skeletal muscle is disrupted, shifting towards a predominance of catabolic metabolism. Short-chain fatty acids like sodium butyrate have been found to regulate inflammatory reactions and successively activate signaling pathways. The preventive benefits of sodium butyrate against dexamethasone-induced skeletal muscle atrophy and myotube atrophy models were examined in this work, and the underlying mechanism was clarified. A total of 32 6-week-old C57BL/6 inbred male mice were randomly assigned to one of four groups and treated with dexamethasone to induce muscle atrophy and sodium butyrate. We found that sodium succinate alleviated dexamethasone-induced myotube atrophy in the myotube atrophy model by lowering the gene expression of two E3 ubiquitin ligases, Atrogin-1 and MURF1, and activating the AKT/mTOR signaling pathway. Pertussis toxin reversed this effect, indicating that G protein-coupled receptors were involved in sodium butyrate's action as a mediator. Additionally, pre-treatment with sodium butyrate lowered weight and muscle mass loss in a mouse model of skeletal muscle atrophy, dramatically decreased the MURF1 gene expression and decreased the nuclear translocation of the glucocorticoid receptor. In conclusion, this study shows that sodium butyrate inhibits the expression of atrophy genes, thus preventing the breakdown of proteins and the loss of muscle mass, while also inhibiting weight loss, in animal models.

Role of GLP‑1 receptor agonists in sepsis and their therapeutic potential in sepsis‑induced muscle atrophy (Review)

Sepsis‑induced myopathy (SIM) is a common complication in intensive care units, which is often associated with adverse outcomes, primarily manifested as skeletal muscle weakness and atrophy. Currently, the management of SIM focuses on prevention strategies, as effective therapeutic options remain elusive. Glucagon‑like peptide‑1 (GLP‑1) receptor agonists (GLP‑1RAs) have garnered attention as hypoglycemic and weight‑loss agents, with an increasing body of research focusing on the extrapancreatic effects of GLP‑1. In preclinical settings, GLP‑1RAs exert protective effects against sepsis‑related multiple organ dysfunction through anti‑inflammatory and antioxidant mechanisms. Based on the existing research, we hypothesized that GLP‑1RAs may serve potential protective roles in the repair and regeneration of skeletal muscle affected by sepsis. The present review aimed to explore the relationship between GLP‑1RAs and sepsis, as well as their impact on muscle atrophy‑related myopathy. Furthermore, the potential mechanisms and therapeutic benefits of GLP‑1RAs are discussed in the context of muscle atrophy induced by sepsis.

Muscle aging and sarcopenia: The pathology, etiology, and most promising therapeutic targets

Sarcopenia is a progressive muscle wasting disorder that severely impacts the quality of life of elderly individuals. Although the natural aging process primarily causes sarcopenia, it can develop in response to other conditions. Because muscle function is influenced by numerous changes that occur with age, the etiology of sarcopenia remains unclear. However, recent characterizations of the aging muscle transcriptional landscape, signaling pathway disruptions, fiber and extracellular matrix compositions, systemic metabolomic and inflammatory responses, mitochondrial function, and neurological inputs offer insights and hope for future treatments. This review will discuss age-related changes in healthy muscle and our current understanding of how this can deteriorate into sarcopenia. As our elderly population continues to grow, we must understand sarcopenia and find treatments that allow individuals to maintain independence and dignity throughout an extended lifespan.

Deubiquitinases in skeletal muscle-the underappreciated side of the ubiquitination coin

Ubiquitination is a posttranslational modification that plays important roles in regulating protein stability, function, localization, and protein-protein interactions. Proteins are ubiquitinated via a process involving specific E1 activating enzymes, E2 conjugating enzymes, and E3 ligases. Simultaneously, protein ubiquitination is opposed by deubiquitinating enzymes (DUBs). DUB-mediated deubiquitination can change protein function or fate and recycle ubiquitin to maintain the free ubiquitin pool. Approximately 100 DUBs have been identified in the mammalian genome, and characterized into seven classes [ubiquitin-specific protease (USP), ovarian tumor proteases (OTU), ubiquitin C-terminal hydrolase (UCH), Machado-Josephin disease (MJD), JAB1/MPN/Mov34 metalloprotease (JAMM), Ub-containing novel DUB family (MINDY), and zinc finger containing ubiquitin peptidase (ZUP) classes]. Of these 100 DUBs, there has only been relatively limited investigation of 20 specifically in skeletal muscle cells, in vitro or in vivo, using overexpression, knockdown, and knockout models. To date, evidence indicates roles for individual DUBs in regulating aspects of myogenesis, protein turnover, muscle mass, and muscle metabolism. However, the exact mechanism by which these DUBs act (i.e., the specific targets of these DUBs and the type of ubiquitin chains they target) is still largely unknown, underscoring how little we know about DUBs in skeletal muscle. This review endeavors to comprehensively summarize the current state of knowledge of the function of DUBs in skeletal muscle and highlight the opportunities for gaining a greater understanding through further research into this important area of skeletal muscle and ubiquitin biology.

Integrative effects of resistance training and endurance training on mitochondrial remodeling in skeletal muscle

Resistance training activates mammalian target of rapamycin (mTOR) pathway of hypertrophy for strength gain, while endurance training increases peroxisome proliferator-activated receptor γ coactivator 1α (PGC-1α) pathway of mitochondrial biogenesis benefiting oxidative phosphorylation. The conventional view suggests that resistance training-induced hypertrophy signaling interferes with endurance training-induced mitochondrial remodeling. However, this idea has been challenged because acute leg press and knee extension in humans enhance both muscle hypertrophy and mitochondrial remodeling signals. Thus, we first examined the muscle mitochondrial remodeling and hypertrophy signals with endurance training and resistance training, respectively. In addition, we discussed the influence of resistance training on muscle mitochondria, demonstrating that the PGC-1α-mediated muscle mitochondrial adaptation and hypertrophy occur simultaneously. The second aim was to discuss the integrative effects of concurrent training, which consists of endurance and resistance training sessions on mitochondrial remodeling. The study found that the resistance training component does not reduce muscle mitochondrial remodeling signals in concurrent training. On the contrary, concurrent training has the potential to amplify skeletal muscle mitochondrial biogenesis compared to a single exercise model. Concurrent training involving differential sequences of resistance and endurance training may result in varied mitochondrial biogenesis signals, which should be linked to the pre-activation of mTOR or PGC-1α signaling. Our review proposed a mechanism for mTOR signaling that promotes PGC-1α signaling through unidentified pathways. This mechanism may be account for the superior muscle mitochondrial remodeling change following the concurrent training. Our review suggested an interaction between resistance training and endurance training in skeletal muscle mitochondrial adaptation.

Calorie restriction and rapamycin distinctly restore non-canonical ORF translation in the muscles of aging mice

Loss of protein homeostasis is one of the hallmarks of aging. As such, interventions that restore proteostasis should slow down the aging process and improve healthspan. Two of the most broadly used anti-aging interventions that are effective in organisms from yeast to mammals are calorie restriction (CR) and rapamycin (RM) treatment. To identify the regulatory mechanisms by which these interventions improve the protein homeostasis, we carried out ribosome footprinting in the muscle of mice aged under standard conditions, or under long-term treatment with CR or RM. We found that the treatments distinctly impact the non-canonical translation, RM primarily remodeling the translation of upstream open reading frames (uORFs), while CR restores stop codon readthrough and the translation of downstream ORFs. Proteomics analysis revealed the expression of numerous non-canonical ORFs at the protein level. The corresponding peptides may provide entry points for therapies aiming to maintain muscle function and extend health span.

Integrating miRNA and full-length transcriptome profiling to elucidate the mechanism of muscle growth in Muscovy ducks reveals key roles for miR-301a-3p/ANKRD1

BACKGROUND

The popularity of Muscovy ducks is attributed not only to their conformation traits but also to their slightly higher content of breast and leg meat, as well as their stronger-tasting meat compared to that of typical domestic ducks. However, there is a lack of comprehensive systematic research on the development of breast muscle in Muscovy ducks. In addition, since the number of skeletal muscle myofibers is established during the embryonic period, this study conducted a full-length transcriptome sequencing and microRNA sequencing of the breast muscle. Muscovy ducks at four developmental stages, namely Embryonic Day 21 (E21), Embryonic Day 27 (E27), Hatching Day (D0), and Post-hatching Day 7 (D7), were used to isolate total RNA for analysis.

RESULTS

A total of 68,161 genes and 472 mature microRNAs were identified. In order to uncover deeper insights into the regulation of mRNA by miRNAs, we conducted an integration of the differentially expressed miRNAs (known as DEMs) with the differentially expressed genes (referred to as DEGs) across various developmental stages. This integration allowed us to make predictions regarding the interactions between miRNAs and mRNA. Through this analysis, we identified a total of 274 DEGs that may serve as potential targets for the 68 DEMs. In the predicted miRNA‒mRNA interaction networks, let-7b, miR-133a-3p, miR-301a-3p, and miR-338-3p were the hub miRNAs. In addition, multiple DEMs also showed predicted target relationships with the DEGs associated with skeletal system development. These identified DEGs and DEMs as well as their predicted interaction networks involved in the regulation of energy homeostasis and muscle development were most likely to play critical roles in facilitating the embryo-to-hatchling transition. A candidate miRNA, miR-301a-3p, exhibited increased expression during the differentiation of satellite cells and was downregulated in the breast muscle tissues of Muscovy ducks at E21 compared to E27. A dual-luciferase reporter assay suggested that the ANKRD1 gene, which encodes a transcription factor, is a direct target of miR-301a-3p.

CONCLUSIONS

miR-301a-3p suppressed the posttranscriptional activity of ANKRD1, which is an activator of satellite cell proliferation, as determined with gain- and loss-of-function experiments. miR-301a-3p functions as an inducer of myogenesis by targeting the ANKRD1 gene in Muscovy ducks. These results provide novel insights into the early developmental process of black Muscovy breast muscles and will improve understanding of the underlying molecular mechanisms.

Investigating Eukaryotic Elongation Factor 2 Kinase/Eukaryotic Translation Elongation Factor 2 Pathway Regulation and Its Role in Protein Synthesis Impairment during Disuse-Induced Skeletal Muscle Atrophy

The principal mechanism underlying the reduced rate of protein synthesis in atrophied skeletal muscle is largely unknown. Eukaryotic elongation factor 2 kinase (eEF2k) impairs the ability of eukaryotic translation elongation factor 2 (eEF2) to bind to the ribosome via T56 phosphorylation. Perturbations in the eEF2k/eEF2 pathway during various stages of disuse muscle atrophy have been investigated utilizing a rat hind limb suspension (HS) model. Two distinct components of eEF2k/eEF2 pathway misregulation were demonstrated, observing a significant (P < 0.01) increase in eEF2k mRNA expression as early as 1-day HS and in eEF2k protein level after 3-day HS. We set out to determine whether eEF2k activation is a Ca²⁺-dependent process with involvement of Cav1.1. The ratio of T56-phosphorylated/total eEF2 was robustly elevated after 3-day HS, which was completely reversed by BAPTA-AM and decreased by 1.7-fold (P < 0.05) by nifedipine. Transfection of C2C12 with pCMV-eEF2k and administration with small molecules were used to modulate eEF2k and eEF2 activity. More important, pharmacologic enhancement of eEF2 phosphorylation induced phosphorylated ribosomal protein S6 kinase (T389) up-regulation and restoration of global protein synthesis in the HS rats. Taken together, the eEF2k/eEF2 pathway is up-regulated during disuse muscle atrophy involving calcium-dependent activation of eEF2k partly via Cav1.1. The study provides evidence, in vitro and in vivo, of the eEF2k/eEF2 pathway impact on ribosomal protein S6 kinase activity as well as protein expression of key atrophy biomarkers, muscle atrophy F-box/atrogin-1 and muscle RING finger-1.

Re-organization of nucleolar architecture in myogenic differentiation

Myogenesis, the process of muscle differentiation, requires an extensive remodeling of the cellular transcriptome and proteome. Whereas the transcriptional program underpinning myogenesis is well characterized, the required adaptation in protein synthesis is incompletely understood. Enhanced protein synthesis necessitates ribosome biogenesis at the nucleolus. Nucleolar size and activity are inextricably linked with altered gene expression. Here, we report changes in nucleolar morphology and function during myogenic differentiation. Immunofluorescence analysis revealed alterations in nucleolar morphology that were dependent on the cellular state - proliferative or quiescent myogenic progenitors (myoblasts or reserve cells) contained multiple small nucleoli with a characteristic spherical shape, whereas multinucleated myotubes typically contained one large, often irregularly shaped nucleolus. These morphological alterations are consistent with changes to nucleolar phase separation properties. Re-organization of the nucleolar structure was correlated with enhanced rRNA production and protein translation. Inhibition of mTOR signaling with rapamycin perturbed nucleolar re-organization. Conversely, hyperactivated mTOR enhanced alterations in nucleolar morphology. These findings support the idea that there is an mTOR dependent re-organization of nucleolar structure during myogenesis, enhancing our understanding of myogenesis and possibly facilitating new approaches to therapeutic interventions in muscle pathologies.

Gaining insight into the role of FoxO1 in the progression of disuse-induced skeletal muscle atrophy

Expression of FoxO transcription factors increases during certain forms of atrophy. In a dephosphorylated state, FoxOs participate in ubiquitin-mediated proteasomal degradation through the transcriptional activation of E3-ubiquitin ligases such as MAFbx/atrogin-1 and MuRF1. There is exhaustive research demonstrating that FoxO3a is sufficient to induce MAFbx/atrogin-1 and MuRF-1 expressions. In contrast, the data are conflicting on the requirement of FoxO1 signaling in the activation of the E3-ubiquitin ligases. Moreover, no reports currently exist on the particular role of FoxO1 in the molecular mechanisms involved in the progression of physiological muscle wasting. Here, we have applied the most extensively used rodent model of microgravity/functional unloading to stimulate disuse-induced skeletal muscle atrophy such as rat hindlimb suspension (HS). We showed that inhibition of FoxO1 activity by a selective inhibitor AS1842856 completely reversed an increase in expression of MuRF-1, but not MAFbx/atrogin-1, observed upon HS. Furthermore, we demonstrated that FoxO1 induced upregulation of another E3-ubiquitin-ligase of a MuRF protein family MuRF-2 in skeletal muscle subjected to disuse. Prevention of the MuRF increase upon HS impeded upregulation of transcript expression of a negative regulator of NFATc1 pathway calsarcin-2, which was associated with a partial reversion of MyHC-IId/x and MyHC-IIb mRNA expressions. Importantly, FoxO1 inhibition induced a marked increase in p70S6k phosphorylation, an important stage in the initiation of protein translation, concomitant with the restoration of global protein synthesis in the skeletal muscle of the HS rats. Examination of eIF3f expression and the eEF2k/eEF2 pathway, other factors controlling translation initiation and elongation respectively, did not reveal any impact of FoxO1 on their activity. Lastly, we observed a decrease in transcript levels of Sesn3, but not Sesn1 and Sesn2, upon disuse, which was completely reversed by FoxO1 inhibition. These data demonstrate that FoxO1 signaling contributes to the development of disuse-induced skeletal muscle atrophy, including slow to fast MyHC isoform shift, mostly through upregulation of MuRF-1 and MuRF-2 expression. Furthermore, FoxO1 inhibition is required to recover Sesn3 mRNA expression in atrophic conditions, which likely contributes to the enhanced p70S6k activity and restoration of the protein synthesis rate.

Central Suppression of the GH/IGF Axis and Abrogation of Exercise-Related mTORC1/2 Activation in the Muscle of Phenotype-Selected Male Marathon Mice (DUhTP)

The somatotropic axis is required for a number of biological processes, including growth, metabolism, and aging. Due to its central effects on growth and metabolism and with respect to its positive effects on muscle mass, regulation of the GH/IGF-system during endurance exercise is of particular interest. In order to study the control of gene expression and adaptation related to physical performance, we used a non-inbred mouse model, phenotype-selected for high running performance (DUhTP). Gene expression of the GH/IGF-system and related signaling cascades were studied in the pituitary gland and muscle of sedentary males of marathon and unselected control mice. In addition, the effects of three weeks of endurance exercise were assessed in both genetic groups. In pituitary glands from DUhTP mice, reduced expression of Pou1f1 (p = 0.002) was accompanied by non-significant reductions of Gh mRNA (p = 0.066). In addition, mRNA expression of Ghsr and Sstr2 were significantly reduced in the pituitary glands from DUhTP mice (p ≤ 0.05). Central downregulation of Pou1f1 expression was accompanied by reduced serum concentrations of IGF1 and coordinated downregulation of multiple GH/IGF-signaling compounds in muscle (e.g., Ghr, Igf1, Igf1r, Igf2r, Irs1, Irs2, Akt3, Gskb, Pik3ca/b/a2, Pten, Rictor, Rptor, Tsc1, Mtor; p ≤ 0.05). In response to exercise, the expression of Igfbp3, Igfbp 4, and Igfbp 6 and Stc2 mRNA was increased in the muscle of DUhTP mice (p ≤ 0.05). Training-induced specific activation of AKT, S6K, and p38 MAPK was found in muscles from control mice but not in DUhTP mice (p ≤ 0.05), indicating a lack of mTORC1 and mTORC2 activation in marathon mice in response to physical exercise. While hormone-dependent mTORC1 and mTORC2 pathways in marathon mice were repressed, robust increases of Ragulator complex compounds (p ≤ 0.001) and elevated sirtuin 2 to 6 mRNA expression were observed in the DUhTP marathon mouse model (p ≤ 0.05). Activation of AMPK was not observed under the experimental conditions of the present study. Our results describe coordinated downregulation of the somatotropic pathway in long-term selected marathon mice (DUhTP), possibly via the pituitary gland and muscle interaction. Our results, for the first time, demonstrate that GH/IGF effects are repressed in a context of superior running performance in mice.

Temporal Proteomic Profiling During Differentiation of Normal and Dystrophin-Deficient Human Muscle Cells

BACKGROUND

Myogenesis is a dynamic process involving temporal changes in the expression of many genes. Lack of dystrophin protein such as in Duchenne muscular dystrophy might alter the natural course of gene expression dynamics during myogenesis.

OBJECTIVE

To gain insight into the dynamic temporal changes in protein expression during differentiation of normal and dystrophin deficient myoblasts to myotubes.

METHOD

A super SILAC spike-in strategy in combination and LC-MS/MS was used for temporal proteome profiling of normal and dystrophin deficient myoblasts during differentiation. The acquired data was analyzed using Proteome Discoverer 2.2. and data clustering using R to define significant temporal changes in protein expression.

RESULTS

sFour major temporal protein clusters that showed sequential dynamic expression profiles during myogenesis of normal myoblasts were identified. Clusters 1 and 2, consisting mainly of proteins involved mRNA splicing and processing expression, were elevated at days 0 and 0.5 of differentiation then gradually decreased by day 7 of differentiation, then remained lower thereafter. Cluster 3 consisted of proteins involved contractile muscle and actomyosin organization. They increased in their expression reaching maximum at day 7 of differentiation then stabilized thereafter. Cluster 4 consisting of proteins involved in skeletal muscle development glucogenesis and extracellular remodeling had a lower expression during myoblast stage then gradually increased in their expression to reach a maximum at days 11-15 of differentiation. Lack of dystrophin expression in DMD muscle myoblast caused major alteration in temporal expression of proteins involved in cell adhesion, cytoskeleton, and organelle organization as well as the ubiquitination machinery.

CONCLUSION

Time series proteome profiling using super SILAC strategy is a powerful method to assess temporal changes in protein expression during myogenesis and to define the downstream consequences of lack of dystrophin on these temporal protein expressions. Key alterations were identified in dystrophin deficient myoblast differentiation compared to normal myoblasts. These alterations could be an attractive therapeutic target.

Repeated bouts of resistance exercise in rats alter mechanistic target of rapamycin complex 1 activity and ribosomal capacity but not muscle protein synthesis

NEW FINDINGS

What is the central question of this study? Is muscle protein synthesis (MPS) additionally activated following exercise when ribosomal capacity is increased after repeated bouts of resistance exercise (RE)? What is the main finding and its importance? Skeletal muscles with increased ribosome content through repeated RE bouts showed sufficient activation of MPS with lower mechanistic target of rapamycin complex 1 signalling. Thus, repeated bouts of RE possibly change the translational capacity and efficiency to optimize translation activation following RE.

ABSTRACT

Resistance exercise (RE) activates ribosome biogenesis and increases ribosome content in skeletal muscles. However, it is unclear whether the increase in ribosome content subsequently causes an increase in RE-induced activation of muscle protein synthesis (MPS). Thus, this study aimed to investigate the relationship between ribosome content and MPS after exercise using a rat RE model. Male Sprague-Dawley rats were categorized into three groups (n = 6 for each group): sedentary (SED) and RE trained with one bout (1B) or three bouts (3B). The RE stimulus was applied to the right gastrocnemius muscle by transcutaneous electrical stimulation under isoflurane anaesthesia. The 3B group underwent stimulation every other day. Our results revealed that 6 h after the last bout of RE, muscles in the 3B group showed an increase in total RNA and 18S+28S rRNA content per muscle weight compared with the SED and 1B groups. In both the 1B and 3B groups, MPS, estimated by puromycin incorporation in proteins, was higher than that in the SED group 6 h after exercise; however, no significant difference was observed between the 1B and 3B groups. In the 1B and 3B groups, phosphorylated p70S6K at Thr-389 increased, indicating mechanistic target of rapamycin complex 1 (mTORC1) activity. p70S6K phosphorylation level was lower in the 3B group than in the 1B group. Finally, protein synthesis per ribosome (indicator of translation efficiency) was lower in the 3B group than in the 1B group. Thus, three bouts of RE changed the ribosome content and mTORC1 activation, but not the degree of RE-induced global MPS activation.

Molecular Regulation of Skeletal Muscle Growth and Organelle Biosynthesis: Practical Recommendations for Exercise Training

The regulation of skeletal muscle mass and organelle homeostasis is dependent on the capacity of cells to produce proteins and to recycle cytosolic portions. In this investigation, the mechanisms involved in skeletal muscle mass regulation-especially those associated with proteosynthesis and with the production of new organelles-are presented. Thus, the critical roles of mammalian/mechanistic target of rapamycin complex 1 (mTORC1) pathway and its regulators are reviewed. In addition, the importance of ribosome biogenesis, satellite cells involvement, myonuclear accretion, and some major epigenetic modifications related to protein synthesis are discussed. Furthermore, several studies conducted on the topic of exercise training have recognized the central role of both endurance and resistance exercise to reorganize sarcomeric proteins and to improve the capacity of cells to build efficient organelles. The molecular mechanisms underlying these adaptations to exercise training are presented throughout this review and practical recommendations for exercise prescription are provided. A better understanding of the aforementioned cellular pathways is essential for both healthy and sick people to avoid inefficient prescriptions and to improve muscle function with emergent strategies (e.g., hypoxic training). Finally, current limitations in the literature and further perspectives, notably on epigenetic mechanisms, are provided to encourage additional investigations on this topic.

Ubiquitin Proteasome System Activity is Suppressed by Curcumin following Exercise-Induced Muscle Damage in Human Skeletal Muscle

PURPOSE

Curcumin is a polyphenolic compound that is suggested to dysregulate the ubiquitin-proteasome system (UPS). This study investigated the effects of curcumin supplementation on markers of UPS activity in response to muscle damage.

METHODS

Twenty-three recreationally active male and females between the ages of 18-30 were randomized into a curcumin (CUR) or placebo (PLA) group. Both groups ingested 2 g of their respective supplement and 20 mg of piperine for 11 consecutive days. Following 8 consecutive days of supplementation, participants performed a 45-minute eccentrically-biased treadmill protocol at 60% VO2max. Muscle biopsies and delayed onset muscle soreness (DOMS) assessments were performed 30 minutes prior and 3, 24, 48, and 72 hours following exercise. Skeletal muscle ubiquitin, MAFbx/Atrogin-1, ubiquitin specific peptidase 19 (USP19), and chymotrypsin-like protease concentrations were measured using ELISA. A 3-way repeated measures ANOVA with pairwise comparisons was conducted with significance set at p ≤ 0.05.

RESULTS

Compared to baseline, DOMS for both groups was significantly increased (p < 0.05) at all time points except 72 hours following exercise. No significant differences were found for USP19 between groups. Ubiquitin (p=.016) and MAFbx/Atrogin-1 (p=.006) were significantly lower for CUR compared to PLA. Additionally, MAFbx/Atrogin-1 was significantly greater for females (p=.013) compared to males. In males, curcumin resulted in significant reductions (p = .049) in chymotrypsin-like protease (p = .049).

CONCLUSION

While elevations in UPS activity were not observed in response to muscle damage, curcumin supplementation in humans does appear to dysregulate basal UPS activity in the presence of exercise-induced muscle damage.

Targeting eIF3f Suppresses the Growth of Prostate Cancer Cells by Inhibiting Akt Signaling

Background

Eukaryotic initiation factor 3 (eIF3) is the largest translation initiation factor, and oncogenic roles have been discovered for its subunits, including the f subunit (ie, eIF3f), in various human cancers. However, the roles of eIF3f in the development and progression of prostate cancer (PCa) have not been reported.

Materials and Methods

We performed in silico analysis to screen the expression of eIF3 subunits. Relevant shRNAs were used to knock down eIF3 subunits in 22Rv1 cells and cell proliferation was analyzed. eIF3f expression in PCa specimens was confirmed by immunohistochemistry. eIF3f knockdown was established to evaluate the effects of eIF3f on cell proliferation in vitro and in vivo. RNA‐seq, bioinformatics analysis and Western blotting were applied to explore the molecular details underlying the biological function of eIF3f in PCa cells. shRNA-resistant eIF3f and myristoylated-Akt were used to rescue the effects of eIF3f disturbance on PCa cells.

Results

Functional analyses confirmed that eIF3f is essential for PCa proliferation. Notably, the expression of eIF3f was found to be elevated in human PCa tissues as well as in PCa cell lines. eIF3f silencing significantly suppressed the growth of PCa cells, both in vitro and in vivo. eIF3f expression was positively correlated with Akt signaling activity in RNA-seq profiles and published prostate cohorts. Knockdown of eIF3f markedly reduced the levels of phosphorylated Akt in PCa cells. Exogenous expression of shRNA-resistant eIF3f in eIF3f knockdown cells restored Akt phosphorylation levels and cell growth. Importantly, rescue experiments revealed that ectopic expression of myristoylated-Akt partially alleviated the suppressive effects of eIF3f disturbance with respect to the growth of PCa cells.

Conclusion

These results suggested that eIF3f has an oncogenic role in PCa, mediated at least partially through the regulation of Akt signaling, and that eIF3f represents a potential target for the inhibition of PCa growth and progression.

Exploring the Interface between Inflammatory and Therapeutic Glucocorticoid Induced Bone and Muscle Loss

Due to their potent immunomodulatory anti-inflammatory properties, synthetic glucocorticoids (GCs) are widely utilized in the treatment of chronic inflammatory disease. In this review, we examine our current understanding of how chronic inflammation and commonly used therapeutic GCs interact to regulate bone and muscle metabolism. Whilst both inflammation and therapeutic GCs directly promote systemic osteoporosis and muscle wasting, the mechanisms whereby they achieve this are distinct. Importantly, their interactions in vivo are greatly complicated secondary to the directly opposing actions of GCs on a wide array of pro-inflammatory signalling pathways that underpin catabolic and anti-anabolic metabolism. Several clinical studies have attempted to address the net effects of therapeutic glucocorticoids on inflammatory bone loss and muscle wasting using a range of approaches. These have yielded a wide array of results further complicated by the nature of inflammatory disease, underlying the disease management and regimen of GC therapy. Here, we report the latest findings related to these pathway interactions and explore the latest insights from murine models of disease aimed at modelling these processes and delineating the contribution of pre-receptor steroid metabolism. Understanding these processes remains paramount in the effective management of patients with chronic inflammatory disease.

Natural constituents from food sources: potential therapeutic agents against muscle wasting

Skeletal muscle wasting is highly correlated with not only reduced quality of life but also higher morbidity and mortality. Although an increasing number of patients are suffering from various kinds of muscle atrophy and weakness, there is still no effective therapy available, and skeletal muscle is considered as an under-medicated organ. Food provided not only essential macronutrients but also functional substances involved in the modulation of the physiological systems of our body. Natural constituents from commonly consumed dietary plants, either extracts or compounds, have attracted more and more attention to be developed as agents for preventing and treating muscle wasting due to their safety and effectiveness, as well as structural diversity. This review provides an overview of the mechanistic aspects of muscle wasting, and summarizes the extracts and compounds from food sources as potential therapeutic agents against muscle wasting.

If my muscle could talk: Myokines as a biomarker of frailty

Frailty is a potentially reversible state of increased vulnerability to negative health-related outcomes that occurs as a result of multisystem biological impairment and environmental aspects. Given the relevance of this condition in both clinics and research, biomarkers of frailty have been actively sought after. Although several candidate biomarkers of frailty have been identified, none of them has yet been incorporated in the assessment or monitoring of the condition. Over the last years, increasing research interest has been focused on myokines, a set of cytokines, small proteins and proteoglycan peptides that are synthetized, expressed and released by skeletal myocytes in response to muscular contractions. Myokines may act in autocrine, paracrine, and endocrine manner and regulate several processes associated with physical frailty, including muscle wasting, dynapenia, and slowness. This review discusses the rationale to support the use of myokines as biomarkers of frailty in older adults.

Recent Data on Cellular Component Turnover: Focus on Adaptations to Physical Exercise

Significant progress has expanded our knowledge of the signaling pathways coordinating muscle protein turnover during various conditions including exercise. In this manuscript, the multiple mechanisms that govern the turnover of cellular components are reviewed, and their overall roles in adaptations to exercise training are discussed. Recent studies have highlighted the central role of the energy sensor (AMP)-activated protein kinase (AMPK), forkhead box class O subfamily protein (FOXO) transcription factors and the kinase mechanistic (or mammalian) target of rapamycin complex (MTOR) in the regulation of autophagy for organelle maintenance during exercise. A new cellular trafficking involving the lysosome was also revealed for full activation of MTOR and protein synthesis during recovery. Other emerging candidates have been found to be relevant in organelle turnover, especially Parkin and the mitochondrial E3 ubiquitin protein ligase (Mul1) pathways for mitochondrial turnover, and the glycerolipids diacylglycerol (DAG) for protein translation and FOXO regulation. Recent experiments with autophagy and mitophagy flux assessment have also provided important insights concerning mitochondrial turnover during ageing and chronic exercise. However, data in humans are often controversial and further investigations are needed to clarify the involvement of autophagy in exercise performed with additional stresses, such as hypoxia, and to understand the influence of exercise modality. Improving our knowledge of these pathways should help develop therapeutic ways to counteract muscle disorders in pathological conditions.

Consecutive bouts of electrical stimulation-induced contractions alter ribosome biogenesis in rat skeletal muscle

Ribosome biogenesis has been implicated in resistance exercise training (RET)-induced skeletal muscle hypertrophy. However, it is unclear how increasing bouts of RET affects ribosome content and biogenesis. This was investigated in the present study using simulated RET where rat skeletal muscle is subjected to increasing bouts of electrical stimulation. Sprague-Dawley rats were randomly assigned to the following seven groups: sedentary for 5 days (SED) or 6 wk (SED_6w), resistance-exercise trained with 1 bout (1B), 2 bouts (2B), 3 bouts (3B), 6 bouts (6B), and 18 bouts (18B). RET was simulated on the right gastrocnemius muscle by transcutaneous electric stimulation under isoflurane anesthesia, and a RET bout was given 3 times a week. Rats in 1B, 2B, and 3B groups showed increased 45S precursor (pre-) rRNA and 18S+28S rRNA content per muscle weight and elevated mRNA levels of c- myc and upstream binding factor (UBF). Increases in phosphorylated UBF and total cyclin D1 protein level were observed 48 h after RET; the former increased as a function of RET duration. In 3B, 6B, and 18B groups, the 18S+28S rRNA content per muscle weight was kept unchanged, and 45S pre-rRNA, cyclin D1, and phosphorylated UBF levels in 18B were lower than those in 3B. These results suggest that RET activates ribosome biogenesis and increases ribosome content through modulation of UBF and cyclin D1 activity at its early phase. Additional bouts of RET may not lead to a further increase in ribosome content per muscle weight through possibly the attenuation of transcription process.

NEW & NOTEWORTHY Ribosome biogenesis has been implicated in resistance exercise training-induced skeletal muscle hypertrophy. However, it remains unclear how this is influenced by the volume of repeated bouts of resistance exercise training. Using resistance exercise training model with rat skeletal muscle, we provide evidence that ribosome biogenesis is stimulated by the initial few bouts of resistance exercise training with no additional effect of further increase in the exercise bout.

Indoxyl sulfate induces myotube atrophy by ROS-ERK and JNK-MAFbx cascades

Accumulations of uremic toxins has been widely recognized as the major trigger of skeletal muscle loss in chronic kidney disease (CKD), which is defined as uremic sarcopenia. Current study was aimed to examine the effects of representative uremic toxin, indoxyl sulfate (IS), on C2C12 myotubes. The incubation of IS (from 0.1 mM to 1.2 mM) exerted the reduction in myotube diameter without cell survival impairment. Elevated oxidative stress and mitogen-activated protein kinase (MAPKs) phosphorylation were observed after IS stimulation for 1 and 24 h. After N-acetylcysteine (NAC) treatment as antioxidants, the recovery in IS-induced decrease myotube diameter and ERK phosphorylation was observed. This findings were implicit the transduction of p-ERK in IS-induced ROS toxicity. Moreover, the increase of LC3β was found closely with IS treatment in C2C12 myotubes. The reverse effect of NAC on LC3β expression revealed the ROS-responsibility in autophagy regulation of CKD myopathy. The evaluation of IS-treated proteasome system showed increased phospho-myosin light chain, along with the upregulation of muscle atrophy F-box (MAFbx) mRNA and protein. This alteration in MAFbx was also identified in nephrectomy-induced CKD model. Besides, the inhibition of p-JNK was capable to attenuate IS-induced upward change in MAFbx protein expression. These findings indicated that IS-mediated myotube atrophy may manipulate through ROS-ERK axis and JNK-MAFbx regulation in C2C12 cells.

Estrogen receptor α promotes protein synthesis by fine-tuning the expression of the eukaryotic translation initiation factor 3 subunit f (eIF3f)

Approximately two thirds of all breast cancer cases are estrogen receptor (ER)-positive. The treatment of this breast cancer subtype with endocrine therapies is effective in the adjuvant and recurrent settings. However, their effectiveness is compromised by the emergence of intrinsic or acquired resistance. Thus, identification of new molecular targets can significantly contribute to the development of novel therapeutic strategies. In recent years, many studies have implicated aberrant levels of translation initiation factors in cancer etiology and provided evidence that identifies these factors as promising therapeutic targets. Accordingly, we observed reduced levels of the eIF3 subunit eIF3f in ER-positive breast cancer cells compared with ER-negative cells, and determined that low eIF3f levels are required for proper proliferation and survival of ER-positive MCF7 cells. The expression of eIF3f is tightly controlled by ERα at the transcriptional (genomic pathway) and translational (nongenomic pathway) level. Specifically, estrogen-bound ERα represses transcription of the EIF3F gene, while promoting eIF3f mRNA translation. To regulate translation, estrogen activates the mTORC1 pathway, which enhances the binding of eIF3 to the eIF4F complex and, consequently, the assembly of the 48S preinitiation complexes and protein synthesis. We observed preferential translation of mRNAs with highly structured 5'-UTRs that usually encode factors involved in cell proliferation and survival (e.g. cyclin D1 and survivin). Our results underscore the importance of estrogen-ERα-mediated control of eIF3f expression for the proliferation and survival of ER-positive breast cancer cells. These findings may provide rationale for the development of new therapies to treat ER-positive breast cancer.

Quantifying the contribution of recessive coding variation to developmental disorders

We estimated the genome-wide contribution of recessive coding variation in 6040 families from the Deciphering Developmental Disorders study. The proportion of cases attributable to recessive coding variants was 3.6% in patients of European ancestry, compared with 50% explained by de novo coding mutations. It was higher (31%) in patients with Pakistani ancestry, owing to elevated autozygosity. Half of this recessive burden is attributable to known genes. We identified two genes not previously associated with recessive developmental disorders, KDM5B and EIF3F, and functionally validated them with mouse and cellular models. Our results suggest that recessive coding variants account for a small fraction of currently undiagnosed nonconsanguineous individuals, and that the role of noncoding variants, incomplete penetrance, and polygenic mechanisms need further exploration.

Cellular fatty acid level regulates the effect of tolylfluanid on mitochondrial dysfunction and insulin sensitivity in C2C12 skeletal myotubes

Previous research suggests that the endocrine disrupting chemical tolylfluanid (TF) may promote metabolic dysfunction and insulin resistance in humans. The potential impact of TF on skeletal muscle metabolism has yet to be fully investigated. The purpose of this study was to determine whether TF can promote insulin resistance and metabolic dysfunction in mammalian skeletal muscle cells. C2C12 murine skeletal myotubes were exposed to 1 ppm TF for 24 h. To examine the potential effect of cellular fatty acid levels on TF-dependent regulation of mitochondrial metabolism and insulin signaling, we treated skeletal myotubes with 0.25 mM or 1.0 mM oleic acid (OA) during TF exposure trials. Tolylfluanid (1-10 ppm) reduced lipid accumulation by approximately 20% in 0.25 and 1.0 mM OA treated cells. The addition of 0.25 mM OA completely inhibited the TF-dependent reduction in maximal mitochondrial oxygen consumption rate (OCR) while 1.0 mM OA exacerbated the TF-dependent reduction in mitochondrial OCR. Exposing skeletal myotubes to 1 ppm TF promoted an 80% reduction in mitochondrial membrane potential, which was completely inhibited by 0.25 mM OA and partially inhibited by1.0 mM OA. The addition of 0.25 mM OA promoted a TF-dependent increase in insulin-dependent P-Akt (Ser473). In contrast, the addition of 1.0 mM OA promoted a significant reduction in insulin-dependent P-Akt (Ser473). Further, the addition of 1 ppm TF significantly reduced insulin-dependent mTORC1 activity regardless of OA concentration. Finally, TF significantly reduced insulin-dependent protein synthesis in the 1 mM OA treated cells only. Our results demonstrate that the effect of 1 ppm TF on mitochondrial function and insulin-dependent protein synthesis in skeletal myotubes was largely dependent upon cellular fatty acid levels.

Decreased abundance of eIF4F subunits predisposes low-birth-weight neonatal pigs to reduced muscle hypertrophy

Muscle hypertrophy is limited in low-birth-weight (LBWT) neonates, suggesting a reduction in protein synthesis and increased protein degradation. Sixteen pairs of 1-d old normal-birth-weight (NBWT) and LBWT littermates (n = 16) were euthanized and the longissimus dorsi (LD) was sampled for protein abundance and kinase phosphorylation profiles measures. Eukaryotic initiation factor (eIF) 4E and eIF4G abundance, and assembly of the active eIF4E-eIF4G complex was less for LBWT than for NBWT pig muscles. Similarly, eIF3f abundance was reduced in muscle of LBWT compared with NBWT pig and was associated with diminished ribosomal protein S6 kinase 1 (S6K1) phosphorylation. This decrease was linked to a lower phosphorylation of programmed cell death protein 4 (PDCD4) in LBWT pig muscle. By contrast, PDCD4 abundance was greater in muscle of LBWT than NBWT group, suggesting lower release and availability of eIF4A from PDCD4-eIF4A complex. Moreover, protein abundance of eIF4A was lower in LBWT muscle, which is expected to further impair the formation of eIF4F translation initiation complex. Microtubule associated light chain 3 (LC3) II to total LC3 ratio was greater in LBWT LD lysates yet P62 abundance was similar between the two groups suggesting no difference in autophagy. Muscle atrophy F-box (atrogin-1) abundance was less in LBWT LD lysates, suggesting decreased degradation through the ubiquitin-proteasome system. In conclusion, limited eIF4F subunit abundance and downregulated translation initiation are plausible mechanisms for diminished muscle growth in LBWT compared with NBWT neonatal pigs.

G protein-dependent signaling triggers a β-arrestin-scaffolded p70S6K/ rpS6 module that controls 5'TOP mRNA translation

Many interaction partners of β-arrestins intervene in the control of mRNA translation. However, how β-arrestins regulate this cellular process has been poorly explored. In this study, we show that β-arrestins constitutively assemble a p70S6K/ribosomal protein S6 (rpS6) complex in HEK293 cells and in primary Sertoli cells of the testis. We demonstrate that this interaction is direct, and experimentally validate the interaction interface between β-arrestin 1 and p70S6K predicted by our docking algorithm. Like most GPCRs, the biological function of follicle-stimulating hormone receptor (FSHR) is transduced by G proteins and β-arrestins. Upon follicle-stimulating hormone (FSH) stimulation, activation of G protein-dependent signaling enhances p70S6K activity within the β-arrestin/p70S6K/rpS6 preassembled complex, which is not recruited to the FSHR. In agreement, FSH-induced rpS6 phosphorylation within the β-arrestin scaffold was decreased in cells depleted of Gα_s. Integration of the cooperative action of β-arrestin and G proteins led to the translation of 5' oligopyrimidine track mRNA with high efficacy within minutes of FSH input. Hence, this work highlights new relationships between G proteins and β-arrestins when acting cooperatively on a common signaling pathway, contrasting with their previously shown parallel action on the ERK MAP kinase pathway. In addition, this study provides insights into how GPCR can exert trophic effects in the cell.-Tréfier, A., Musnier, A., Landomiel, F., Bourquard, T., Boulo, T., Ayoub, M. A., León, K., Bruneau, G., Chevalier, M., Durand, G., Blache, M.-C., Inoue, A., Fontaine, J., Gauthier, C., Tesseraud, S., Reiter, E., Poupon, A., Crépieux, P. G protein-dependent signaling triggers a β-arrestin-scaffolded p70S6K/ rpS6 module that controls 5'TOP mRNA translation.

eIF3: a factor for human health and disease

The eukaryotic initiation factor 3 (eIF3) is one of the most complex translation initiation factors in mammalian cells, consisting of several subunits (eIF3a to eIF3m). It is crucial in translation initiation and termination, and in ribosomal recycling. Accordingly, deregulated eIF3 expression is associated with different pathological conditions, including cancer. In this manuscript, we discuss the interactome and function of each subunit of the human eIF3 complex. Furthermore, we review how altered levels of eIF3 subunits correlate with neurodegenerative disorders and cancer onset and development; in addition, we evaluate how such misregulation may also trigger infection cascades. A deep understanding of the molecular mechanisms underlying eIF3 role in human disease is essential to develop new eIF3-targeted therapeutic approaches and thus, overcome such conditions.

Ginsenoside Rg1 prevents starvation-induced muscle protein degradation via regulation of AKT/mTOR/FoxO signaling in C2C12 myotubes

Skeletal muscle atrophy is often caused by catabolic conditions including fasting, disuse, aging and chronic diseases, such as chronic obstructive pulmonary disease. Atrophy occurs when the protein degradation rate exceeds the rate of protein synthesis. Therefore, maintaining a balance between the synthesis and degradation of protein in muscle cells is a major way to prevent skeletal muscle atrophy. Ginsenoside Rg1 (Rg1) is a primary active ingredient in Panax ginseng, which is considered to be one of the most valuable herbs in traditional Chinese medicine. In the current study, Rg1 was observed to inhibit the expression of MuRF-1 and atrogin-1 in C2C12 muscle cells in a starvation model. Rg1 also activated the phosphorylation of mammalian target of rapamycin (mTOR), protein kinase B (AKT), and forkhead transcription factor O, subtypes 1 and 3a. This phosphorylation was inhibited by LY294002, a phosphatidylinositol 3-kinase inhibitor. These data suggest that Rg1 may participate in the regulation of the balance between protein synthesis and degradation, and that the function of Rg1 is associated with the AKT/mTOR/FoxO signaling pathway.

Acute high-caffeine exposure increases autophagic flux and reduces protein synthesis in C2C12 skeletal myotubes

Caffeine is a highly catabolic dietary stimulant. High caffeine concentrations (1-10 mM) have previously been shown to inhibit protein synthesis and increase protein degradation in various mammalian cell lines. The purpose of this study was to examine the effect of short-term caffeine exposure on cell signaling pathways that regulate protein metabolism in mammalian skeletal muscle cells. Fully differentiated C2C12 skeletal myotubes either received vehicle (DMSO) or 5 mM caffeine for 6 h. Our analysis revealed that caffeine promoted a 40% increase in autolysosome formation and a 25% increase in autophagic flux. In contrast, caffeine treatment did not significantly increase the expression of the skeletal muscle specific ubiquitin ligases MAFbx and MuRF1 or 20S proteasome activity. Caffeine treatment significantly reduced mTORC1 signaling, total protein synthesis and myotube diameter in a CaMKKβ/AMPK-dependent manner. Further, caffeine promoted a CaMKII-dependent increase in myostatin mRNA expression that did not significantly contribute to the caffeine-dependent reduction in protein synthesis. Our results indicate that short-term caffeine exposure significantly reduced skeletal myotube diameter by increasing autophagic flux and promoting a CaMKKβ/AMPK-dependent reduction in protein synthesis.

Interaction of p190A RhoGAP with eIF3A and Other Translation Preinitiation Factors Suggests a Role in Protein Biosynthesis

The negative regulator of Rho family GTPases, p190A RhoGAP, is one of six mammalian proteins harboring so-called FF motifs. To explore the function of these and other p190A segments, we identified interacting proteins by tandem mass spectrometry. Here we report that endogenous human p190A, but not its 50% identical p190B paralog, associates with all 13 eIF3 subunits and several other translational preinitiation factors. The interaction involves the first FF motif of p190A and the winged helix/PCI domain of eIF3A, is enhanced by serum stimulation and reduced by phosphatase treatment. The p190A/eIF3A interaction is unaffected by mutating phosphorylated p190A-Tyr³⁰⁸, but disrupted by a S296A mutation, targeting the only other known phosphorylated residue in the first FF domain. The p190A-eIF3 complex is distinct from eIF3 complexes containing S6K1 or mammalian target of rapamycin (mTOR), and appears to represent an incomplete preinitiation complex lacking several subunits. Based on these findings we propose that p190A may affect protein translation by controlling the assembly of functional preinitiation complexes. Whether such a role helps to explain why, unique among the large family of RhoGAPs, p190A exhibits a significantly increased mutation rate in cancer remains to be determined.

Inflammatory Mechanisms Associated with Skeletal Muscle Sequelae after Stroke: Role of Physical Exercise

Inflammatory markers are increased systematically and locally (e.g., skeletal muscle) in stroke patients. Besides being associated with cardiovascular risk factors, proinflammatory cytokines seem to play a key role in muscle atrophy by regulating the pathways involved in this condition. As such, they may cause severe decrease in muscle strength and power, as well as impairment in cardiorespiratory fitness. On the other hand, physical exercise (PE) has been widely suggested as a powerful tool for treating stroke patients, since PE is able to regenerate, even if partially, physical and cognitive functions. However, the mechanisms underlying the beneficial effects of physical exercise in poststroke patients remain poorly understood. Thus, in this study we analyze the candidate mechanisms associated with muscle atrophy in stroke patients, as well as the modulatory effect of inflammation in this condition. Later, we suggest the two strongest anti-inflammatory candidate mechanisms, myokines and the cholinergic anti-inflammatory pathway, which may be activated by physical exercise and may contribute to a decrease in proinflammatory markers of poststroke patients.

eIF4A inactivates TORC1 in response to amino acid starvation

Amino acids regulate TOR complex 1 (TORC1) via two counteracting mechanisms, one activating and one inactivating. The presence of amino acids causes TORC1 recruitment to lysosomes where TORC1 is activated by binding Rheb. How the absence of amino acids inactivates TORC1 is less well understood. Amino acid starvation recruits the TSC1/TSC2 complex to the vicinity of TORC1 to inhibit Rheb; however, the upstream mechanisms regulating TSC2 are not known. We identify here the eIF4A-containing eIF4F translation initiation complex as an upstream regulator of TSC2 in response to amino acid withdrawal in Drosophila We find that TORC1 and translation preinitiation complexes bind each other. Cells lacking eIF4F components retain elevated TORC1 activity upon amino acid removal. This effect is specific for eIF4F and not a general consequence of blocked translation. This study identifies specific components of the translation machinery as important mediators of TORC1 inactivation upon amino acid removal.

Mechanical Signaling in the Pathophysiology of Critical Illness Myopathy

The complete loss of mechanical stimuli of skeletal muscles, i.e., the loss of external strain, related to weight bearing, and internal strain, related to the contraction of muscle cells, is uniquely observed in pharmacologically paralyzed or deeply sedated mechanically ventilated intensive care unit (ICU) patients. The preferential loss of myosin and myosin associated proteins in limb and trunk muscles is a significant characteristic of critical illness myopathy (CIM) which separates CIM from other types of acquired muscle weaknesses in ICU patients. Mechanical silencing is an important factor triggering CIM. Microgravity or ground based microgravity models form the basis of research on the effect of muscle unloading-reloading, but the mechanisms and effects may differ from the ICU conditions. In order to understand how mechanical tension regulates muscle mass, it is critical to know how muscles sense mechanical information and convert stimulus to intracellular biochemical actions and changes in gene expression, a process called cellular mechanotransduction. In adult skeletal muscles and muscle fibers, this process may differ, the same stimulus can cause divergent response and the same fiber type may undergo opposite changes in different muscles. Skeletal muscle contains multiple types of mechano-sensors and numerous structures that can be affected differently and hence respond differently in distinct muscles.

Correlation between Ribosome Biogenesis and the Magnitude of Hypertrophy in Overloaded Skeletal Muscle

External loads applied to skeletal muscle cause increases in the protein translation rate, which leads to muscle hypertrophy. Although some studies have demonstrated that increases in the capacity and efficiency of translation are involved in this process, it remains unclear how these two factors are related to the magnitude of muscle hypertrophy. The present study aimed to clarify the roles played by the capacity and efficiency of translation in muscle hypertrophy. We used an improved synergist ablation in which the magnitude of compensatory hypertrophy could be controlled by partial removal of synergist muscles. Male rats were assigned to four groups in which the plantaris muscle was unilaterally subjected to weak (WK), moderate (MO), middle (MI), and strong (ST) overloading by four types of synergist ablation. Fourteen days after surgery, the weight of the plantaris muscle per body weight increased by 8%, 22%, 32% and 45%, in the WK, MO, MI and ST groups, respectively. Five days after surgery, 18+28S rRNA content (an indicator of translational capacity) increased with increasing overload, with increases of 1.8-fold (MO), 2.2-fold (MI), and 2.5-fold (ST), respectively, relative to non-overloaded muscle (NL) in the WK group. rRNA content showed a strong correlation with relative muscle weight measured 14 days after surgery (r = 0.98). The phosphorylated form of p70S6K (a positive regulator of translational efficiency) showed a marked increase in the MO group, but no further increase was observed with further increase in overload (increases of 22.6-fold (MO), 17.4-fold (MI), and 18.2-fold (ST), respectively, relative to NL in the WK group). These results indicate that increases in ribosome biogenesis at the early phase of overloading are strongly dependent on the amount of overloading, and may play an important role in increasing the translational capacity for further gain of muscular size.

C/EBPβ-LAP*/LAP Expression Is Mediated by RSK/eIF4B-Dependent Signalling and Boosted by Increased Protein Stability in Models of Monocytic Differentiation

The transcription factor C/EBPβ plays a key role in monocytic differentiation and inflammation. Its small isoform LIP is associated with proliferation at early premonocytic developmental stages and regulated via mTOR-dependent signalling. During later stages of (pre)monocytic differentiation there is a considerable increase in the large C/EBPβ isoforms LAP*/LAP which inhibit proliferation thus supporting terminal differentiation. Here, we showed in different models of monocytic differentiation that this dramatic increase in the LAP*/LAP protein and LAP/LIP ratio was accompanied by an only modest/retarded mRNA increase suggesting an important role for (post)translational mechanisms. We found that LAP*/LAP formation was induced via MEK/RSK-dependent cascades, whereas mTOR/S6K1 were not involved. Remarkably, LAP*/LAP expression was dependent on phosphorylated eIF4B, an acceleratory protein of RNA helicase eIF4A. PKR inhibition reduced the expression of eIF4B and C/EBPβ in an eIF2α-independent manner. Furthermore, under our conditions a marked stabilisation of LAP*/LAP protein occurred, accompanied by reduced chymotrypsin-like proteasome/calpain activities and increased calpastatin levels. Our study elucidates new signalling pathways inducing LAP*/LAP expression and indicates new alternative PKR functions in monocytes. The switch from mTOR- to RSK-mediated signalling to orchestrate eIF4B-dependent LAP*/LAP translation, accompanied by increased protein stability but only small mRNA changes, may be a prototypical example for the regulation of protein expression during selected processes of differentiation/proliferation.

The eukaryotic translation initiation factor 3f (eIF3f) interacts physically with the alpha 1B-adrenergic receptor and stimulates adrenoceptor activity

Background

eIF3f is a multifunctional protein capable of interacting with proteins involved in different cellular processes, such as protein synthesis, DNA repair, and viral mRNA edition. In human cells, eIF3f is related to cell cycle and proliferation, and its deregulation compromises cell viability.

Results

We here report that, in native conditions, eIF3f physically interacts with the alpha 1B-adrenergic receptor, a plasma membrane protein considered as a proto-oncogene, and involved in vasoconstriction and cell proliferation. The complex formed by eIF3f and alpha 1B-ADR was found in human and mouse cell lines. Upon catecholamine stimulation, eIF3f promotes adrenoceptor activity in vitro, independently of the eIF3f proline- and alanine-rich N-terminal region.

Conclusions

The eIF3f/alpha adrenergic receptor interaction opens new insights regarding adrenoceptor-related transduction pathways and proliferation control in human cells. The eIf3f/alpha 1B-ADR complex is found in mammals and is not tissue specific.

Role of IGF-I in follistatin-induced skeletal muscle hypertrophy

Follistatin, a physiological inhibitor of myostatin, induces a dramatic increase in skeletal muscle mass, requiring the type 1 IGF-I receptor/Akt/mTOR pathway. The aim of the present study was to investigate the role of IGF-I and insulin, two ligands of the IGF-I receptor, in the follistatin hypertrophic action on skeletal muscle. In a first step, we showed that follistatin increases muscle mass while being associated with a downregulation of muscle IGF-I expression. In addition, follistatin retained its full hypertrophic effect toward muscle in hypophysectomized animals despite very low concentrations of circulating and muscle IGF-I. Furthermore, follistatin did not increase muscle sensitivity to IGF-I in stimulating phosphorylation of Akt but, surprisingly, decreased it once hypertrophy was present. Taken together, these observations indicate that increased muscle IGF-I production or sensitivity does not contribute to the muscle hypertrophy caused by follistatin. Unlike low IGF-I, low insulin, as obtained by streptozotocin injection, attenuated the hypertrophic action of follistatin on skeletal muscle. Moreover, the full anabolic response to follistatin was restored in this condition by insulin but also by IGF-I infusion. Therefore, follistatin-induced muscle hypertrophy requires the activation of the insulin/IGF-I pathway by either insulin or IGF-I. When insulin or IGF-I alone is missing, follistatin retains its full anabolic effect, but when both are deficient, as in streptozotocin-treated animals, follistatin fails to stimulate muscle growth.

mTOR kinase-dependent, but raptor-independent regulation of downstream signaling is important for cell cycle exit and myogenic differentiation

Myogenic differentiation in the C2C12 myoblast model system reflects a concerted and controlled activation of transcription and translation following the exit of cells from the cell cycle. Previously we have shown that the mTORC1 signaling inhibitor, RAD001, decreased protein synthesis rates, delayed C2C12 myoblast differentiation, decreased p70S6K activity but did not affect the hypermodification of 4E-BP1. Here we have further investigated the modification of 4E-BP1 during the early phase of differentiation as cells exit the cell cycle, using inhibitors to target mTOR kinase and siRNAs to ablate the expression of raptor and rictor. As predicted, inhibition of mTOR kinase activity prevented p70S6K, 4E-BP1 phosphorylation and was associated with an inhibition of myogenic differentiation. Surprisingly, extensive depletion of raptor did not affect p70S6K or 4E-BP1 phosphorylation, but promoted an increase in mTORC2 activity (as evidenced by increased Akt Ser473 phosphorylation). These data suggest that an mTOR kinase-dependent, but raptor-independent regulation of downstream signaling is important for myogenic differentiation.

Enteral delivery of proteins enhances the expression of proteins involved in the cytoskeleton and protein biosynthesis in human duodenal mucosa

BACKGROUND

Amino acids are well known to be key effectors of gut protein turnover. We recently reported that enteral delivery of proteins markedly stimulated global duodenal protein synthesis in carbohydrate-fed healthy humans, but specifically affected proteins remain unknown.

OBJECTIVE

We aimed to assess the influence of an enteral protein supply on the duodenal mucosal proteome in carbohydrate-fed humans.

DESIGN

Six healthy volunteers received for 5 h, on 2 occasions and in random order, either an enteral infusion of maltodextrins alone (0.25 g · kg⁻¹ · h⁻¹) mimicking the fed state or maltodextrins with a protein powder (0.14 g proteins · kg⁻¹ · h⁻¹). Endoscopic duodenal biopsy specimens were then collected and frozen until analysis. A 2-dimensional polyacrylamide gel electrophoresis-based comparative proteomics analysis was then performed, and differentially expressed proteins (at least ±1.5-fold change; Student's t test, P < 0.05) were identified by mass spectrometry. Protein expression changes were confirmed by Western blot analysis.

RESULTS

Thirty-two protein spots were differentially expressed after protein delivery compared with maltodextrins alone: 28 and 4 spots were up- or downregulated, respectively. Among the 22 identified proteins, 11 upregulated proteins were involved either in the cytoskeleton (ezrin, moesin, plastin 1, lamin B1, vimentin, and β-actin) or in protein biosynthesis (glutamyl-prolyl-transfer RNA synthetase, glutaminyl-transfer RNA synthetase, elongation factor 2, elongation factor 1δ, and eukaryotic translation and initiation factor 3 subunit f).

CONCLUSIONS

Enteral delivery of proteins altered the duodenal mucosal proteome and mainly stimulated the expression of proteins involved in cytoskeleton and protein biosynthesis. These results suggest that protein supply may affect intestinal morphology by stimulating actin cytoskeleton remodeling.

Exercise and the Regulation of Skeletal Muscle Hypertrophy

Skeletal muscle is a critical organ serving as the primary site for postprandial glucose disposal and the generation of contractile force. The size of human skeletal muscle mass is dependent upon the temporal relationship between changes in muscle protein synthesis (MPS) and muscle protein breakdown. The aim of this chapter is to review our current understanding of how resistance exercise influences protein turnover with a specific emphasis on the molecular factors regulating MPS. We also will discuss recent data relating to the prescription of resistance exercise to maximize skeletal muscle hypertrophy. Finally, we evaluate the impact of age and periods of disuse on the loss of muscle mass and the controversy surround the etiology of muscle disuse atrophy.

Mechanisms Regulating Neuromuscular Junction Development and Function and Causes of Muscle Wasting

The neuromuscular junction is the chemical synapse between motor neurons and skeletal muscle fibers. It is designed to reliably convert the action potential from the presynaptic motor neuron into the contraction of the postsynaptic muscle fiber. Diseases that affect the neuromuscular junction may cause failure of this conversion and result in loss of ambulation and respiration. The loss of motor input also causes muscle wasting as muscle mass is constantly adapted to contractile needs by the balancing of protein synthesis and protein degradation. Finally, neuromuscular activity and muscle mass have a major impact on metabolic properties of the organisms. This review discusses the mechanisms involved in the development and maintenance of the neuromuscular junction, the consequences of and the mechanisms involved in its dysfunction, and its role in maintaining muscle mass during aging. As life expectancy is increasing, loss of muscle mass during aging, called sarcopenia, has emerged as a field of high medical need. Interestingly, aging is also accompanied by structural changes at the neuromuscular junction, suggesting that the mechanisms involved in neuromuscular junction maintenance might be disturbed during aging. In addition, there is now evidence that behavioral paradigms and signaling pathways that are involved in longevity also affect neuromuscular junction stability and sarcopenia.

MD11-mediated delivery of recombinant eIF3f induces melanoma and colorectal carcinoma cell death

The f subunit of the eukaryotic initiation factor 3 (eIF3f) is downregulated in several cancers and in particular in melanoma and pancreatic cancer cells. Its enforced expression by transient gene transfection negatively regulates cancer cell growth by activating apoptosis. With the aim to increase the intracellular level of eIF3f proteins and activate apoptosis in cancer cell lines, we developed a protein transfer system composed of a cell-penetrating peptide sequence fused to eIF3f protein sequence (MD₁₁-eIF3f). To determine whether exogenously administered eIF3f proteins were able to compensate the loss of endogenous eIF3f and induce cancer cell death, we analyzed the therapeutic action of MD₁₁-eIF3f in several tumor cells. We identified four cell lines respondent to eIF3f-treatment and we evaluated the antitumor properties of the recombinant proteins using dose- and time-dependent studies. Our results demonstrate that this protein delivery approach represents an innovative and powerful strategy for cancer treatment.

Mechanisms modulating skeletal muscle phenotype

Mammalian skeletal muscles are composed of a variety of highly specialized fibers whose selective recruitment allows muscles to fulfill their diverse functional tasks. In addition, skeletal muscle fibers can change their structural and functional properties to perform new tasks or respond to new conditions. The adaptive changes of muscle fibers can occur in response to variations in the pattern of neural stimulation, loading conditions, availability of substrates, and hormonal signals. The new conditions can be detected by multiple sensors, from membrane receptors for hormones and cytokines, to metabolic sensors, which detect high-energy phosphate concentration, oxygen and oxygen free radicals, to calcium binding proteins, which sense variations in intracellular calcium induced by nerve activity, to load sensors located in the sarcomeric and sarcolemmal cytoskeleton. These sensors trigger cascades of signaling pathways which may ultimately lead to changes in fiber size and fiber type. Changes in fiber size reflect an imbalance in protein turnover with either protein accumulation, leading to muscle hypertrophy, or protein loss, with consequent muscle atrophy. Changes in fiber type reflect a reprogramming of gene transcription leading to a remodeling of fiber contractile properties (slow-fast transitions) or metabolic profile (glycolytic-oxidative transitions). While myonuclei are in postmitotic state, satellite cells represent a reserve of new nuclei and can be involved in the adaptive response.

Myostatin and the skeletal muscle atrophy and hypertrophy signaling pathways

Myostatin, a member of the transforming growth factor-β superfamily, is a potent negative regulator of skeletal muscle growth and is conserved in many species, from rodents to humans. Myostatin inactivation can induce skeletal muscle hypertrophy, while its overexpression or systemic administration causes muscle atrophy. As it represents a potential target for stimulating muscle growth and/or preventing muscle wasting, myostatin regulation and functions in the control of muscle mass have been extensively studied. A wealth of data strongly suggests that alterations in skeletal muscle mass are associated with dysregulation in myostatin expression. Moreover, myostatin plays a central role in integrating/mediating anabolic and catabolic responses. Myostatin negatively regulates the activity of the Akt pathway, which promotes protein synthesis, and increases the activity of the ubiquitin-proteasome system to induce atrophy. Several new studies have brought new information on how myostatin may affect both ribosomal biogenesis and translation efficiency of specific mRNA subclasses. In addition, although myostatin has been identified as a modulator of the major catabolic pathways, including the ubiquitin-proteasome and the autophagy-lysosome systems, the underlying mechanisms are only partially understood. The goal of this review is to highlight outstanding questions about myostatin-mediated regulation of the anabolic and catabolic signaling pathways in skeletal muscle. Particular emphasis has been placed on (1) the cross-regulation between myostatin, the growth-promoting pathways and the proteolytic systems; (2) how myostatin inhibition leads to muscle hypertrophy; and (3) the regulation of translation by myostatin.

The ever-evolving role of mTOR in translation

Control of translation allows for the production of stoichiometric levels of each protein in the cell. Attaining such a level of fine-tuned regulation of protein production requires the coordinated temporal and spatial control of numerous cellular signalling cascades impinging on the various components of the translational machinery. Foremost among these is the mTOR signalling pathway. The mTOR pathway regulates both the initiation and elongation steps of protein synthesis through the phosphorylation of numerous translation factors, while simultaneously ensuring adequate folding of nascent polypeptides through co-translational degradation of misfolded proteins. Perhaps most remarkably, mTOR is also a key regulator of the synthesis of ribosomal proteins and translation factors themselves. Two seminal studies have recently shown in translatome analysis that the mTOR pathway preferentially regulates the translation of mRNAs encoding ribosomal proteins and translation factors. Therefore, the role of the mTOR pathway in the control of protein synthesis extends far beyond immediate translational control. By controlling ribosome production (and ultimately ribosome availability), mTOR is a master long-term controller of protein synthesis. Herein, we review the literature spanning the early discoveries of mTOR on translation to the latest advances in our understanding of how the mTOR pathway controls the synthesis of ribosomal proteins.

Ribosome biogenesis: emerging evidence for a central role in the regulation of skeletal muscle mass

The ribosome is a supramolecular ribonucleoprotein complex that functions at the heart of the translation machinery to convert mRNA into protein. Ribosome biogenesis is the primary determinant of translational capacity of the cell and accordingly has an essential role in the control of cell growth in eukaryotes. Cumulative evidence supports the hypothesis that ribosome biogenesis has an important role in the regulation of skeletal muscle mass. The purpose of this review is to, first, summarize the main mechanisms known to regulate ribosome biogenesis and, second, put forth the hypothesis that ribosome biogenesis is a central mechanism used by skeletal muscle to regulate protein synthesis and control skeletal muscle mass in response to anabolic and catabolic stimuli. The mTORC1 and Wnt/β-catenin/c-myc signaling pathways are discussed as the major pathways that work in concert with each of the three RNA polymerases (RNA Pol I, II, and III) in regulating ribosome biogenesis. Consistent with our hypothesis, activation of these two pathways has been shown to be associated with ribosome biogenesis during skeletal muscle hypertrophy. Although further study is required, the finding that ribosome biogenesis is altered under catabolic states, in particular during disuse atrophy, suggests that its activation represents a novel therapeutic target to reduce or prevent muscle atrophy. Lastly, the emerging field of ribosome specialization is discussed and its potential role in the regulation of gene expression during periods of skeletal muscle plasticity.