Regulation of REDD1 by insulin-like growth factor-I in skeletal muscle and myotubes

Preventive effect of fermented whey protein mediated by Lactobacillus gasseri IM13 via the PI3K/AKT/FOXO pathway in muscle atrophy

This study investigated the preventive effects of whey protein fermented with Lactobacillus gasseri IM13 (F-WP) against dexamethasone (DEX)-induced muscle atrophy. C2C12 muscle cells were treated with F-WP followed by DEX treatment. Dexamethasone treatment inhibited myotube formation and the expression of myogenic regulatory factors; however, pretreatment with F-WP attenuated DEX-induced damage. The F-WP significantly activated the phosphorylation of the IGF-1/PI3K/AKT pathway and improved muscle homeostasis suppressed by DEX. Moreover, F-WP alleviated the phosphorylation of mTOR, S6K1, and 4E-BP1 and enhanced muscle protein synthesis. Muscle-specific ubiquitin ligases and autophagy lysosomes, which were activated by the dephosphorylation of FOXO3a by DEX treatment, were significantly attenuated by F-WP pretreatment of myotubes. For peptidomic analysis, F-WP was fractionated using preparative HPLC (prep-HPLC), and the AA sequences of 11 peptides were identified using MALDI-TOF/MS/MS. In conclusion, fermentation of whey protein by the specific probiotic strain IM13 produced bioactive peptides with high antioxidant and anti-sarcopenic-sarcopenic effects, which markedly enhanced myogenesis and muscle protein synthesis while diminishing muscle protein degradation compared with intact whey protein.

The stress-responsive protein REDD1 and its pathophysiological functions

Regulated in development and DNA damage-response 1 (REDD1) is a stress-induced protein that controls various cellular functions, including metabolism, oxidative stress, autophagy, and cell fate, and contributes to the pathogenesis of metabolic and inflammatory disorders, neurodegeneration, and cancer. REDD1 usually exerts deleterious effects, including tumorigenesis, metabolic inflammation, neurodegeneration, and muscle dystrophy; however, it also exhibits protective functions by regulating multiple intrinsic cell activities through either an mTORC1-dependent or -independent mechanism. REDD1 typically regulates mTORC1 signaling, NF-κB activation, and cellular pro-oxidant or antioxidant activity by interacting with 14-3-3 proteins, IκBα, and thioredoxin-interacting protein or 75 kDa glucose-regulated protein, respectively. The diverse functions of REDD1 depend on cell type, cellular context, interaction partners, and cellular localization (e.g., mitochondria, endomembrane, or cytosol). Therefore, comprehensively understanding the molecular mechanisms and biological roles of REDD1 under pathophysiological conditions is of utmost importance. In this review, based on the published literature, we highlight and discuss the molecular mechanisms underlying the REDD1 expression and its actions, biological functions, and pathophysiological roles.

Maslinic Acid Attenuates Denervation-Induced Loss of Skeletal Muscle Mass and Strength

Maslinic acid (MA) is a pentacyclic triterpene abundant in olive peels. MA reportedly increases skeletal muscle mass and strength in older adults; however, the underlying mechanism is unknown. This study aimed to investigate the effects of MA on denervated muscle atrophy and strength and to explore the underlying molecular mechanism. Mice were fed either a control diet or a 0.27% MA diet. One week after intervention, the sciatic nerves of both legs were cut to induce muscle atrophy. Mice were examined 14 days after denervation. MA prevented the denervation-induced reduction in gastrocnemius muscle mass and skeletal muscle strength. Microarray gene expression profiling in gastrocnemius muscle demonstrated several potential mechanisms for muscle maintenance. Gene set enrichment analysis (GSEA) revealed different enriched biological processes, such as myogenesis, PI3/AKT/mTOR signaling, TNFα signaling via NF-κB, and TGF-β signaling in MA-treated mice. In addition, qPCR data showed that MA induced Igf1 expression and suppressed the expressions of Atrogin-1, Murf1 and Tgfb. Altogether, our results suggest the potential of MA as a new therapeutic and preventive dietary ingredient for muscular atrophy and strength.

Is REDD1 a metabolic double agent? Lessons from physiology and pathology

The Akt/mechanistic target of rapamycin (mTOR) signaling pathway governs macromolecule synthesis, cell growth, and metabolism in response to nutrients and growth factors. Regulated in development and DNA damage response (REDD)1 is a conserved and ubiquitous protein, which is transiently induced in response to multiple stimuli. Acting like an endogenous inhibitor of the Akt/mTOR signaling pathway, REDD1 protein has been shown to regulate cell growth, mitochondrial function, oxidative stress, and apoptosis. Recent studies also indicate that timely REDD1 expression limits Akt/mTOR-dependent synthesis processes to spare energy during metabolic stresses, avoiding energy collapse and detrimental consequences. In contrast to this beneficial role for metabolic adaptation, REDD1 chronic expression appears involved in the pathogenesis of several diseases. Indeed, REDD1 expression is found as an early biomarker in many pathologies including inflammatory diseases, cancer, neurodegenerative disorders, depression, diabetes, and obesity. Moreover, prolonged REDD1 expression is associated with cell apoptosis, excessive reactive oxygen species (ROS) production, and inflammation activation leading to tissue damage. In this review, we decipher several mechanisms that make REDD1 a likely metabolic double agent depending on its duration of expression in different physiological and pathological contexts. We also discuss the role played by REDD1 in the cross talk between the Akt/mTOR signaling pathway and the energetic metabolism.

Recovery of muscle mass and muscle oxidative phenotype following disuse does not require GSK-3 inactivation

BACKGROUND

Physical inactivity contributes to muscle wasting and reductions in mitochondrial oxidative phenotype (OXPHEN), reducing physical performance and quality of life during aging and in chronic disease. Previously, it was shown that inactivation of glycogen synthase kinase (GSK)-3β stimulates muscle protein accretion, myogenesis, and mitochondrial biogenesis. Additionally, GSK-3β is inactivated during recovery of disuse-induced muscle atrophy.

AIM

Therefore, we hypothesize that GSK-3 inhibition is required for reloading-induced recovery of skeletal muscle mass and OXPHEN.

METHODS

Wild-type (WT) and whole-body constitutively active (C.A.) Ser^21/9 GSK-3α/β knock-in mice were subjected to a 14-day hind-limb suspension/14-day reloading protocol. Soleus muscle mass, fiber cross-sectional area (CSA), OXPHEN (abundance of sub-units of oxidative phosphorylation (OXPHOS) complexes and fiber-type composition), as well as expression levels of their main regulators (respectively protein synthesis/degradation, myogenesis and peroxisome proliferator-activated receptor-γ co-activator-1α (PGC-1α) signaling) were monitored.

RESULTS

Subtle but consistent differences suggesting suppression of protein turnover signaling and decreased expression of several OXPHOS sub-units and PGC-1α signaling constituents were observed at baseline in C.A. GSK-3 versus WT mice. Although soleus mass recovery during reloading occurred more rapidly in C.A. GSK-3 mice, this was not accompanied by a parallel increased CSA. The OXPHEN response to reloading was not distinct between C.A. GSK-3 and WT mice. No consistent or significant differences in reloading-induced changes in the regulatory steps of protein turnover, myogenesis or muscle OXPHEN were observed in C.A. GSK-3 compared to WT muscle.

CONCLUSION

This study indicates that GSK-3 inactivation is dispensable for reloading-induced recovery of muscle mass and OXPHEN.

Skeletal Muscle Response to Deflazacort, Dexamethasone and Methylprednisolone

Glucocorticoids represent some of the most prescribed drugs that are widely used in the treatment of neuromuscular diseases, but their usage leads to side effects such as muscle atrophy. However, different synthetic glucocorticoids can lead to different muscle effects, depending upon its chemical formulation. Here, we intended to demonstrate the muscle histologic and molecular effects of administering different glucocorticoids in equivalency and different dosages. Methods: Seventy male Wistar rats distributed into seven groups received different glucocorticoids in equivalency for ten days or saline solution. The study groups were: Control group (CT) saline solution; dexamethasone (DX) 1.25 or 2.5 mg/kg/day; methylprednisolone (MP) 6.7 or 13.3mg/kg/day; and deflazacort (DC) 10 or 20 mg/kg/day. At the end of the study, the animals were euthanized, and the tibialis anterior and gastrocnemius muscles were collected for metachromatic ATPase (Cross-sectional area (CSA) measurement), Western blotting (protein expression of IGF-1 and Ras/Raf/MEK/ERK pathways) and RT-PCR (MYOSTATIN, MuRF-1, Atrogin-1, REDD-1, REDD-2, MYOD, MYOG and IRS1/2 genes expression) experiments. Results: Muscle atrophy occurred preferentially in type 2B fibers in all glucocorticoid treated groups. DC on 10 mg/kg/day was less harmful to type 2B fibers CSA than other doses and types of synthetic glucocorticoids. In type 1 fibers CSA, lower doses of DC and DX were more harmful than high doses. DX had a greater effect on the IGF-1 pathway than other glucocorticoids. MP more significantly affected P-ERK1/2 expression, muscle fiber switching (fast-to-slow), and expression of REDD1 and MyoD genes than other glucocorticoids. Compared to DX and MP, DC had less of an effect on the expression of atrogenes (MURF-1 and Atrogin-1) despite increased MYOSTATIN and decreased IRS-2 genes expression. Conclusions: Different glucocorticoids appears to cause muscle atrophy affecting secondarily different signaling mechanisms. MP is more likely to affect body/muscles mass, MEK/ERK pathway and fiber type transition, DX the IGF-1 pathway and IRS1/2 expression. DC had the smallest effect on muscle atrophic response possibly due a delayed timing on atrogenes response.

Acute treadmill exercise discriminately improves the skeletal muscle insulin-stimulated growth signaling responses in mice lacking REDD1

A loss of the regulated in development and DNA damage 1 (REDD1) hyperactivates mechanistic Target of Rapamycin Complex 1 (mTORC1) reducing insulin-stimulated insulin signaling, which could provide insight into mechanisms of insulin resistance. Although aerobic exercise acutely inhibits mTORC1 signaling, improvements in insulin-stimulated signaling are exhibited. The goal of this study was to determine if a single bout of treadmill exercise was sufficient to improve insulin signaling in mice lacking REDD1. REDD1 wildtype (WT) and REDD1 knockout (KO) mice were acutely exercised on a treadmill (30 min, 20 m/min, 5% grade). A within animal noninsulin-to-insulin-stimulated percent change in skeletal muscle insulin-stimulated kinases (IRS-1, ERK1/2, Akt), growth signaling activation (4E-BP1, S6K1), and markers of growth repression (REDD1, AMPK, FOXO1/3A) was examined, following no exercise control or an acute bout of exercise. Unlike REDD1 KO mice, REDD1 WT mice exhibited an increase (P < 0.05) in REDD1 following treadmill exercise. However, both REDD1 WT and KO mice exhibited an increase (P < 0.05) AMPK phosphorylation, and a subsequent reduction (P < 0.05) in mTORC1 signaling after the exercise bout versus nonexercising WT or KO mice. Exercise increased (P < 0.05) the noninsulin-to-insulin-stimulated percent change phosphorylation of mTORC1, ERK1/2, IRS-1, and Akt on S473 in REDD1 KO mice when compared to nonexercised KO mice. However, there was no change in the noninsulin-to-insulin-stimulated percent change activation of Akt on T308 and FOXO1/3A in the KO when compared to WT or KO mouse muscle after exercise. Our data show that a bout of treadmill exercise discriminately improves insulin-stimulated signaling in the absence of REDD1.

Regulation of protein and mRNA expression of the mTORC1 repressor REDD1 in response to leucine and serum

Expression of the mTORC1 repressor, Regulated in DNA Damage and Development 1 (REDD1), is elevated in skeletal muscle during various catabolic conditions including fasting, hindlimb immobilization, and sepsis. Conversely, REDD1 expression is suppressed by anabolic stimuli such as resistance exercise or nutrient consumption following a fast. Though it is known that nutrient consumption reduces REDD1 expression, it is largely unknown how nutrients and hormones individually contribute to the reduction in REDD1 expression. Therefore, the purpose of the present study was to determine how nutrients and hormones individually regulate REDD1 expression. HeLa cells were deprived of leucine or serum for 10 h, after which either leucine or serum was reintroduced to cell culture medium for 60 min. Re-supplementation of either leucine or serum resulted in a reduction in REDD1 protein levels by 34.8±5.8% and 54.1±3.4%, respectively, compared to the deprived conditions. Re-supplementation of leucine or serum to deprived cells also led to a reduction in REDD1 mRNA content by 49.1±2.7% and 65.0±1.4%, respectively, compared to the deprived conditions. Interestingly, rates of REDD1 protein degradation were unaffected by either leucine or serum re-supplementation, as assessed in cells treated with cycloheximide to block protein synthesis. Likewise, addition of leucine- or serum to cells treated with Actinomycin D to inhibit gene transcription failed to alter the rate of REDD1 mRNA degradation. The data indicate that the leucine or serum-induced suppression of REDD1 expression occurs independent of changes in the rate of degradation of either the REDD1 protein or mRNA. Thus, the leucine- or serum-induced suppression likely occurs through alternative mechanism(s) such as reduced REDD1 gene transcription and/or mRNA translation.

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Deprivation of leucine or serum induces REDD1 mRNA and protein expression.

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Re-supplementation of leucine or serum reduces REDD1 mRNA and protein expression.

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Nutrient deplete or replete conditions do not affect the degradation rate of REDD1.

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REDD1 expression is controlled through altered rates of transcription.

Emerging role for regulated in development and DNA damage 1 (REDD1) in the regulation of skeletal muscle metabolism

Since its discovery, the protein regulated in development and DNA damage 1 (REDD1) has been implicated in the cellular response to various stressors. Most notably, its role as a repressor of signaling through the central metabolic regulator, the mechanistic target of rapamycin in complex 1 (mTORC1) has gained considerable attention. Not surprisingly, changes in REDD1 mRNA and protein have been observed in skeletal muscle under various physiological conditions (e.g., nutrient consumption and resistance exercise) and pathological conditions (e.g., sepsis, alcoholism, diabetes, obesity) suggesting a role for REDD1 in regulating mTORC1-dependent skeletal muscle protein metabolism. Our understanding of the causative role of REDD1 in skeletal muscle metabolism is increasing mostly due to the availability of genetically modified mice in which the REDD1 gene is disrupted. Results from such studies provide support for an important role for REDD1 in the regulation of mTORC1 as well as reveal unexplored functions of this protein in relation to other aspects of skeletal muscle metabolism. The goal of this work is to provide a comprehensive review of the role of REDD1 (and its paralog REDD2) in skeletal muscle during both physiological and pathological conditions.

Aging Reduces the Activation of the mTORC1 Pathway after Resistance Exercise and Protein Intake in Human Skeletal Muscle: Potential Role of REDD1 and Impaired Anabolic Sensitivity

This study was designed to better understand the molecular mechanisms involved in the anabolic resistance observed in elderly people. Nine young (22 ± 0.1 years) and 10 older (69 ± 1.7 years) volunteers performed a one-leg extension exercise consisting of 10 × 10 repetitions at 70% of their 3-RM, immediately after which they ingested 30 g of whey protein. Muscle biopsies were taken from the vastus lateralis at rest in the fasted state and 30 min after protein ingestion in the non-exercised (Pro) and exercised (Pro+ex) legs. Plasma insulin levels were determined at the same time points. No age difference was measured in fasting insulin levels but the older subjects had a 50% higher concentration than the young subjects in the fed state (p < 0.05). While no difference was observed in the fasted state, in response to exercise and protein ingestion, the phosphorylation state of PKB (p < 0.05 in Pro and Pro+ex) and S6K1 (p = 0.059 in Pro; p = 0.066 in Pro+ex) was lower in the older subjects compared with the young subjects. After Pro+ex, REDD1 expression tended to be higher (p = 0.087) in the older group while AMPK phosphorylation was not modified by any condition. In conclusion, we show that the activation of the mTORC1 pathway is reduced in skeletal muscle of older subjects after resistance exercise and protein ingestion compared with young subjects, which could be partially due to an increased expression of REDD1 and an impaired anabolic sensitivity.

Aberrant REDD1-mTORC1 responses to insulin in skeletal muscle from Type 2 diabetics

The objective of this study was to establish whether alterations in the REDD1-mTOR axis underlie skeletal muscle insensitivity to insulin in Type 2 diabetic (T2D), obese individuals. Vastus lateralis muscle biopsies were obtained from lean, control and obese, T2D subjects under basal and after a 2-h hyperinsulinemic (40 mU·m(-2)·min(-1))-euglycemic (5 mM) clamp. Muscle lysates were examined for total REDD1, and phosphorylated Akt, S6 kinase 1 (S6K1), 4E-BP1, ERK1/2, and MEK1/2 via Western blot analysis. Under basal conditions [(-) insulin], T2D muscle exhibited higher S6K1 and ERK1/2 and lower 4E-BP1 phosphorylation (P < 0.05), as well as elevations in blood cortisol, glucose, insulin, glycosylated hemoglobin (P < 0.05) vs. lean controls. Following insulin infusion, whole body glucose disposal rates (GDR; mg/kg/min) were lower (P < 0.05) in the T2D vs. the control group. The basal-to-insulin percent change in REDD1 expression was higher (P < 0.05) in muscle from the T2D vs. the control group. Whereas, the basal-to-insulin percent change in muscle Akt, S6K1, ERK1/2, and MEK1/2 phosphorylation was significantly lower (P < 0.05) in the T2D vs. the control group. Findings from this study propose a REDD1-regulated mechanism in T2D skeletal muscle that may contribute to whole body insulin resistance and may be a target to improve insulin action in insulin-resistant individuals.

Muscle-specific GSK-3β ablation accelerates regeneration of disuse-atrophied skeletal muscle

Muscle wasting impairs physical performance, increases mortality and reduces medical intervention efficacy in chronic diseases and cancer. Developing proficient intervention strategies requires improved understanding of the molecular mechanisms governing muscle mass wasting and recovery. Involvement of muscle protein- and myonuclear turnover during recovery from muscle atrophy has received limited attention. The insulin-like growth factor (IGF)-I signaling pathway has been implicated in muscle mass regulation. As glycogen synthase kinase 3 (GSK-3) is inhibited by IGF-I signaling, we hypothesized that muscle-specific GSK-3β deletion facilitates the recovery of disuse-atrophied skeletal muscle. Wild-type mice and mice lacking muscle GSK-3β (MGSK-3β KO) were subjected to a hindlimb suspension model of reversible disuse-induced muscle atrophy and followed during recovery. Indices of muscle mass, protein synthesis and proteolysis, and post-natal myogenesis which contribute to myonuclear accretion, were monitored during the reloading of atrophied muscle. Early muscle mass recovery occurred more rapidly in MGSK-3β KO muscle. Reloading-associated changes in muscle protein turnover were not affected by GSK-3β ablation. However, coherent effects were observed in the extent and kinetics of satellite cell activation, proliferation and myogenic differentiation observed during reloading, suggestive of increased myonuclear accretion in regenerating skeletal muscle lacking GSK-3β. This study demonstrates that muscle mass recovery and post-natal myogenesis from disuse-atrophy are accelerated in the absence of GSK-3β.

Lack of REDD1 reduces whole body glucose and insulin tolerance, and impairs skeletal muscle insulin signaling

A lack of the REDD1 promotes dysregulated growth signaling, though little has been established with respect to the metabolic role of REDD1. Therefore, the goal of this study was to determine the role of REDD1 on glucose and insulin tolerance, as well as insulin stimulated growth signaling pathway activation in skeletal muscle. First, intraperitoneal (IP) injection of glucose or insulin were administered to REDD1 wildtype (WT) versus knockout (KO) mice to examine changes in blood glucose over time. Next, alterations in skeletal muscle insulin (IRS-1, Akt, ERK 1/2) and growth (4E-BP1, S6K1, REDD1) signaling intermediates were determined before and after IP insulin treatment (10min). REDD1 KO mice were both glucose and insulin intolerant when compared to WT mice, evident by higher circulating blood glucose concentrations and a greater area under the curve following IP injections of glucose or insulin. While the REDD1 KO exhibited significant though blunted insulin-stimulated increases (p<0.05) in Akt S473 and T308 phosphorylation versus the WT mice, acute insulin treatment has no effect (p<0.05) on REDD1 KO skeletal muscle 4E-BP1 T37/46, S6K1 T389, IRS-1 Y1222, and ERK 1/2 T202/Y204 phosphorylation versus the WT mice. Collectively, these novel data suggest that REDD1 has a more distinct role in whole body and skeletal muscle metabolism and insulin action than previously thought.

Altered nutrient response of mTORC1 as a result of changes in REDD1 expression: effect of obesity vs. REDD1 deficiency

Although aberrant mTORC1 signaling has been well established in models of obesity, little is known about its repressor, REDD1. Therefore, the initial goal of this study was to determine the role of REDD1 on mTORC1 in obese skeletal muscle. REDD1 expression (protein and message) and mTORC1 signaling (S6K1, 4E-BP1, raptor-mTOR association, Rheb GTP) were examined in lean vs. ob/ob and REDD1 wild-type (WT) vs. knockout (KO) mice, under conditions of altered nutrient intake [fasted and fed or diet-induced obesity (10% vs. 60% fat diet)]. Despite higher (P < 0.05) S6K1 and 4E-BP1 phosphorylation, two models of obesity (ob/ob and diet-induced) displayed elevated (P < 0.05) skeletal muscle REDD1 expression compared with lean or low-fat-fed mouse muscle under fasted conditions. The ob/ob mice displayed elevated REDD1 expression (P < 0.05) that coincided with aberrant mTORC1 signaling (hyperactive S6K1, low raptor-mTOR binding, elevated Rheb GTP; P < 0.05) under fasted conditions, compared with the lean, which persisted in a dysregulated fashion under fed conditions. REDD1 KO mice gained limited body mass on a high-fat diet, although S6K1 and 4E-BP1 phosphorylation remained elevated (P < 0.05) in both the low-fat and high-fat-fed KO vs. WT mice. Similarly, the REDD1 KO mouse muscle displayed blunted mTORC1 signaling responses (S6K1 and 4E-BP1, raptor-mTOR binding) and circulating insulin under fed conditions vs. the robust responses (P < 0.05) in the WT fed mouse muscle. These studies suggest that REDD1 in skeletal muscle may serve to limit hyperactive mTORC1, which promotes aberrant mTORC1 signaling responses during altered nutrient states.

The effects of omega-3 fatty acid supplementation on dexamethasone-induced muscle atrophy

Corticosteroids cause muscle atrophy by acting on proteasomal and lysosomal systems and by affecting pathways related to muscular trophysm, such as the IGF-1/PI-3k/Akt/mTOR. Omega-3 fatty acid (n-3) has been used beneficially to attenuate muscle atrophy linked to sepsis and cachexia; however, its effect on dexamethasone-induced muscle atrophy has not been evaluated. Objectives. We evaluated whether n-3 supplementation could mitigate the development of dexamethasone-induced muscle atrophy. Methods. Two groups of Wistar rats were orally supplemented with n-3 or vehicle solution for 40 days. In the last 10 days, dexamethasone, or saline solution, was administrated establishing four groups: control, dexamethasone, n-3, and dexamethasone + n-3. The cross-sectional areas of muscle fibers, gene expression (MyoD, Myogenin, MuRF-1, and Atrogin-1), and protein expression (Akt, GSK3β, FOXO3a, and mTOR) were assessed. Results. Dexamethasone induced a significant loss in body and muscle weight, atrophy in type 2B fibers, and decreased expression of P-Akt, P-GSK3β, and P-FOXO3a. N-3 supplementation did not attenuate the negative effects of dexamethasone on skeletal muscle; instead, it caused atrophy in type 1, 2A, reduced the expression of Myogenin, and increased the expression of Atrogin-1. Conclusion. Food supplements containing n-3 are usually healthful, but they may potentiate some of the side effects of glucocorticoids.

Myostatin induces DNA damage in skeletal muscle of streptozotocin-induced type 1 diabetic mice

One of the features of uncontrolled type 1 diabetes is oxidative stress that induces DNA damage and cell death. Skeletal muscle atrophy is also considerable in type 1 diabetes, however, the signaling mechanisms that induce oxidative stress culminating in muscle atrophy are not fully known. Here, we show that in Streptozotocin-induced diabetic wild type mice, hypo-phosphorylation of Akt, resulted in activation of Foxa2 transcription factor in the muscle. Foxa2 transcriptionally up-regulated Myostatin, contributing to exaggerated oxidative stress leading to DNA damage via p63/REDD1 pathway in skeletal muscle of Streptozotocin-treated wild type mice. In Myostatin(-/-) mice however, Streptozotocin treatment did not reduce Akt phosphorylation despite reduced IRS-1 signaling. Moreover, Foxa2 levels remained unaltered in Myostatin(-/-) mice, while levels of p63/REDD1 were higher compared with wild type mice. Consistent with these results, relatively less DNA damage and muscle atrophy was observed in Myostatin(-/-) muscle in response to Streptozotocin treatment. Taken together, our results for the first time show the role of Foxa2 in Myostatin regulation in skeletal muscle in diabetic mice. Altogether, these results demonstrate the mechanism by which Myostatin contributes to DNA damage in skeletal muscle of the diabetic mice that would lead to myofiber degeneration.

Plasma membrane translocation of REDD1 governed by GPCRs contributes to mTORC1 activation

The mTORC1 kinase promotes cell growth in response to growth factors by activation of receptor tyrosine kinase. It is regulated by the cellular energy level and the availability of nutrients. mTORC1 activity is also inhibited by cellular stresses through overexpression of REDD1 (regulated in development and DNA damage responses). We report the identification of REDD1 in a fluorescent live-imaging screen aimed at discovering new proteins implicated in G-protein-coupled receptor signaling, based on translocation criteria. Using a sensitive and quantitative plasma membrane localization assay based on bioluminescent resonance energy transfer, we further show that a panel of endogenously expressed GPCRs, through a Ca(2+)/calmodulin pathway, triggers plasma membrane translocation of REDD1 but not of its homolog REDD2. REDD1 and REDD2 share a conserved mTORC1-inhibitory motif characterized at the functional and structural level and differ most in their N-termini. We show that the N-terminus of REDD1 and its mTORC1-inhibitory motif participate in the GPCR-evoked dynamic interaction of REDD1 with the plasma membrane. We further identify REDD1 as a novel effector in GPCR signaling. We show that fast activation of mTORC1 by GPCRs correlates with fast and maximal translocation of REDD1 to the plasma membrane. Overexpression of functional REDD1 leads to a reduction of mTORC1 activation by GPCRs. By contrast, depletion of endogenous REDD1 protein unleashes mTORC1 activity. Thus, translocation to the plasma membrane appears to be an inactivation mechanism of REDD1 by GPCRs, which probably act by sequestering its functional mTORC1-inhibitory motif that is necessary for plasma membrane targeting.

Impaired hypertrophy in myoblasts is improved with testosterone administration

We investigated the ability of testosterone (T) to restore differentiation in multiple population doubled (PD) murine myoblasts, previously shown to have a reduced differentiation in monolayer and bioengineered skeletal muscle cultures vs. their parental controls (CON) (Sharples et al., 2011, 2012 [7,26]). Cells were exposed to low serum conditions in the presence or absence of T (100nM)±PI3K inhibitor (LY294002) for 72h and 7 days (early and late muscle differentiation respectively). Morphological analyses were performed to determine myotube number, diameter (μm) and myonuclear accretion as indices of differentiation and myotube hypertrophy. Changes in gene expression for myogenin, mTOR and myostatin were also performed. Myotube diameter in CON and PD cells increased from 17.32±2.56μm to 21.02±1.89μm and 14.58±2.66μm to 18.29±3.08μm (P≤0.05) respectively after 72h of T exposure. The increase was comparable in both PD (+25%) and CON cells (+21%) suggesting a similar intrinsic ability to respond to exogenous T administration. T treatment also significantly increased myonuclear accretion (% of myotubes expressing 5+ nuclei) in both cell types after 7 days exposure (P≤0.05). Addition of PI3K inhibitor (LY294002) in the presence of T attenuated these effects in myotube morphology (in both cell types) suggesting a role for the PI3K pathway in T stimulated hypertrophy. Finally, PD myoblasts showed reduced responsiveness to T stimulated mRNA expression of mTOR vs. CON cells and T also reduced myostatin expression in PD myoblasts only. The present study demonstrates testosterone administration improves hypertrophy in myoblasts that basally display impaired differentiation and hypertrophic capacity vs. their parental controls, the action of testosterone in this model was mediated by PI3K/Akt pathway.

Skeletal muscle glycoprotein 130's role in Lewis lung carcinoma-induced cachexia

Chronic inflammation is associated with cachexia-induced skeletal muscle mass loss in cancer. Levels of IL-6 cytokine family members are increased during cancer-related cachexia and induce intracellular signaling through glycoprotein130 (gp130). Although muscle STAT3 and circulating IL-6 are implicated in cancer-induced muscle wasting, there is limited understanding of muscle gp130's role in this process. Therefore, we investigated the role of skeletal muscle gp130 (skm-gp130) in cancer-induced alterations in the regulation of muscle protein turnover. Lewis lung carcinoma (LLC) cells were injected into 8-wk-old skm-gp130-knockout (KO) mice or wild-type mice. Skeletal muscle loss was attenuated by 16% in gp130-KO mice, which coincided with attenuated LLC-induced phosphorylation of muscle STAT3, p38, and FOXO3. gp130 KO did not restore mTOR inhibition or alter AMP-activated protein kinase (AMPK) expression. The induction of atrogin expression and p38 phosphorylation in C2C12 myotubes exposed to LLC-treated medium was attenuated by gp130 inhibition, but mTOR inhibition was not restored. STAT signaling inhibition in LLC-treated myotubes did not attenuate the induction of p38 or AMPK phosphorylation. We concluded that, during LLC-induced cachexia, skm-gp130 regulates muscle mass signaling through STAT3 and p38 for the activation of FOXO3 and atrogin, but does not directly regulate the suppression of mTOR.

Effect of acute environmental hypoxia on protein metabolism in human skeletal muscle

Hypoxia-induced muscle wasting has been observed in several environmental and pathological conditions. However, the molecular mechanisms behind this loss of muscle mass are far from being completely elucidated, certainly in vivo. When studying the regulation of muscle mass by environmental hypoxia, many confounding factors have to be taken into account, such as decreased protein ingestion, sleep deprivation or reduced physical activity, which make difficult to know whether hypoxia per se causes a reduction in muscle mass.

AIM

We hypothesized that acute exposure to normobaric hypoxia (11% O2 ) would repress the activation of the mTOR pathway usually observed after a meal and would activate the proteolytic pathways in skeletal muscle.

METHODS

Fifteen subjects were exposed passively for 4 h to normoxic and hypoxic conditions in a random order after consumption of a light breakfast. A muscle biopsy and a blood sample were taken before, after 1 and 4 h of exposure.

RESULTS

After 4 h, plasma insulin concentration and the phosphorylation state of PKB and S6K1 in skeletal muscle were higher in hypoxia than in normoxia (P < 0.05). At the same time, Redd1 mRNA level was upregulated (P < 0.05), whilst MAFbx mRNA decreased (P < 0.05) in hypoxia compared with normoxia. Proteasome, cathepsin L and calpain activities were not altered by environmental hypoxia.

CONCLUSION

Contrary to our hypothesis and despite an increase in the mRNA level of Redd1, an inhibitor of the mTORC1 pathway, short-term acute environmental hypoxia induced a higher response of PKB and S6K1 to a meal, which may be due to increased plasma insulin concentration.

Resistance exercise induced mTORC1 signaling is not impaired by subsequent endurance exercise in human skeletal muscle

The current dogma is that the muscle adaptation to resistance exercise is blunted when combined with endurance exercise. The suggested mechanism (based on rodent experiments) is that activation of adenosine monophosphate-activated protein kinase (AMPK) during endurance exercise impairs muscle growth through inhibition of the mechanistic target of rapamycin complex 1 (mTORC1). The purpose of this study was to investigate potential interference of endurance training on the signaling pathway of resistance training [mTORC1 phosphorylation of ribosomal protein S6 kinase 1 (S6K1)] in human muscle. Ten healthy and moderately trained male subjects performed on two separate occasions either acute high-intensity and high-volume resistance exercise (leg press, R) or R followed by 30 min of cycling (RE). Muscle biopsies were collected before and 1 and 3 h post resistance exercise. Phosphorylation of mTOR (Ser²⁴⁴⁸) increased 2-fold (P < 0.05) and that of S6K1 (Thr³⁸⁹) 14-fold (P < 0.05), with no difference between R and RE. Phosphorylation of eukaryotic elongation factor 2 (eEF2, Thr⁵⁶) was reduced ~70% during recovery in both trials (P < 0.05). An interesting finding was that phosphorylation of AMPK (Thr¹⁷²) and acetyl-CoA carboxylase (ACC, Ser⁷⁹) decreased ~30% and ~50%, respectively, 3 h postexercise (P < 0.05). Proliferator-activated receptor-γ coactivator-1 (PGC-1α) mRNA increased more after RE (6.5-fold) than after R (4-fold) (RE vs. R: P < 0.01) and was the only gene expressed differently between trials. These data show that the signaling of muscle growth through the mTORC1-S6K1 axis after heavy resistance exercise is not inhibited by subsequent endurance exercise. It is also suggested that prior activation of mTORC1 signaling may repress subsequent phosphorylation of AMPK.

Regulated in development and DNA damage responses -1 (REDD1) protein contributes to insulin signaling pathway in adipocytes

REDD1 (Regulated in development and DNA damage response 1) is a hypoxia and stress response gene and is a negative regulator of mTORC1. Since mTORC1 is involved in the negative feedback loop of insulin signaling, we have studied the role of REDD1 on insulin signaling pathway and its regulation by insulin. In human and murine adipocytes, insulin transiently stimulates REDD1 expression through a MEK dependent pathway. In HEK-293 cells, expression of a constitutive active form of MEK stabilizes REDD1 and protects REDD1 from proteasomal degradation mediated by CUL4A-DDB1 ubiquitin ligase complex. In 3T3-L1 adipocytes, silencing of REDD1 with siRNA induces an increase of mTORC1 activity as well as an inhibition of insulin signaling pathway and lipogenesis. Rapamycin, a mTORC1 inhibitor, restores the insulin signaling after downregulation of REDD1 expression. This observation suggests that REDD1 positively regulates insulin signaling through the inhibition of mTORC1 activity. In conclusion, our results demonstrate that insulin increases REDD1 expression, and that REDD1 participates in the biological response to insulin.

Synergistic stimulation of myogenesis by glucocorticoid and IGF-I signaling

Muscle wasting is associated with poor prognosis in chronic obstructive pulmonary disease (COPD). Exercise stimulates muscle recovery, but its efficacy is variable, depending on the clinical condition and medical treatment. Systemic glucocorticoids, commonly administered in high doses during acute disease exacerbations or as maintenance treatment in end-stage disease, are known to contribute to muscle wasting. As muscle mass recovery involves insulin-like growth factor (IGF)-I signaling, which can be stimulated by anabolic steroids, the impact of glucocorticoids and the effect of simultaneous IGF-I stimulation by anabolic steroids on muscle recovery and growth were investigated. The effects of, and interactions between, glucocorticoid and IGF-I signaling on skeletal muscle growth were assessed in differentiating C2C12 myocytes. As proof of principle, we performed a post hoc analysis stratifying patients by glucocorticoid use of a clinical trial investigating the efficacy of anabolic steroid supplementation on muscle recovery in muscle-wasted patients with COPD. Glucocorticoids strongly impaired protein synthesis signaling, myotube formation, and muscle-specific protein expression. In contrast, in the presence of glucocorticoids, IGF-I synergistically stimulated myotube fusion and myofibrillar protein expression, which corresponded with restored protein synthesis signaling by IGF-I and increased transcriptional activation of muscle-specific genes by glucocorticoids. In COPD patients on maintenance glucocorticoid treatment, the clinical trial also revealed an enhanced effect of anabolic steroids on muscle mass and respiratory muscle strength. In conclusion, synergistic effects of anabolic steroids and glucocorticoids on muscle recovery may be caused by relief of the glucocorticoid-imposed blockade on protein synthesis signaling, allowing effective translation of glucocorticoid-induced accumulation of muscle-specific gene transcripts.

Multifaceted role of insulin-like growth factors and mammalian target of rapamycin in skeletal muscle

This review describes the current literature on the interaction between insulin-like growth factors, endocrine hormones, and branched-chain amino acids on muscle physiology in healthy young individuals and during select pathologic conditions. Emphasis is placed on the mechanism by which physical and hormonal signals are transduced at the cellular level to either grow or atrophy skeletal muscle. The key role of the mammalian target of rapamycin and its ability to respond to hypertrophic and atrophic signals informs our understanding how a combination of physical, nutritional, and pharmacologic therapies may be used in tandem to prevent or ameliorate reductions in muscle mass.

Altered REDD1, myostatin, and Akt/mTOR/FoxO/MAPK signaling in streptozotocin-induced diabetic muscle atrophy

Type 1 diabetes, if poorly controlled, leads to skeletal muscle atrophy, decreasing the quality of life. We aimed to search highly responsive genes in diabetic muscle atrophy in a common diabetes model and to further characterize associated signaling pathways. Mice were killed 1, 3, or 5 wk after streptozotocin or control. Gene expression of calf muscles was analyzed using microarray and protein signaling with Western blotting. We identified translational repressor protein REDD1 (regulated in development and DNA damage responses) that increased seven- to eightfold and was associated with muscle atrophy in diabetes. The diabetes-induced increase in REDD1 was confirmed at the protein level. This result was accompanied by the increased gene expression of DNA damage/repair pathways and decreased expression in ATP production pathways. Concomitantly, increased phosphorylation of AMPK and dephosphorylation of the Akt/mTOR/S6K1/FoxO pathway of proteins were observed together with increased protein ubiquitination. These changes were especially evident during the first 3 wk, along with the strong decrease in muscle mass. Diabetes also induced an increase in myostatin protein and decreased MAPK signaling. These, together with decreased serum insulin and increased serum glucose, remained altered throughout the 5-wk period. In conclusion, diabetic myopathy induced by streptozotocin led to alteration of multiple signaling pathways. Of those, increased REDD1 and myostatin together with decreased Akt/mTOR/FoxO signaling are associated with diabetic muscle atrophy. The increased REDD1 and decreased Akt/mTOR/FoxO signaling followed a similar time course and thus may be explained, in part, by increased expression of genes in DNA damage/repair and possibly also decrease in ATP-production pathways.

mRNA expression signatures of human skeletal muscle atrophy identify a natural compound that increases muscle mass

Skeletal muscle atrophy is a common and debilitating condition that lacks a pharmacologic therapy. To develop a potential therapy, we identified 63 mRNAs that were regulated by fasting in both human and mouse muscle, and 29 mRNAs that were regulated by both fasting and spinal cord injury in human muscle. We used these two unbiased mRNA expression signatures of muscle atrophy to query the Connectivity Map, which singled out ursolic acid as a compound whose signature was opposite to those of atrophy-inducing stresses. A natural compound enriched in apples, ursolic acid reduced muscle atrophy and stimulated muscle hypertrophy in mice. It did so by enhancing skeletal muscle insulin/IGF-I signaling and inhibiting atrophy-associated skeletal muscle mRNA expression. Importantly, ursolic acid's effects on muscle were accompanied by reductions in adiposity, fasting blood glucose, and plasma cholesterol and triglycerides. These findings identify a potential therapy for muscle atrophy and perhaps other metabolic diseases.

Sepsis-induced alterations in protein-protein interactions within mTOR complex 1 and the modulating effect of leucine on muscle protein synthesis

Sepsis-induced muscle atrophy is produced in part by decreased protein synthesis mediated by inhibition of mTOR (mammalian target of rapamycin). The present study tests the hypothesis that alteration of specific protein-protein interactions within the mTORC1 (mTOR complex 1) contributes to the decreased mTOR activity observed after cecal ligation and puncture in rats. Sepsis decreased in vivo translational efficiency in gastrocnemius and reduced the phosphorylation of eukaryotic initiation factor (eIF) 4E-binding protein (BP) 1, S6 kinase (S6K) 1, and mTOR, compared with time-matched pair-fed controls. Sepsis decreased T246-phosphorylated PRAS40 (proline-rich Akt substrate 40) and reciprocally increased S792-phosphorylated raptor (regulatory associated protein of mTOR). Despite these phosphorylation changes, sepsis did not alter PRAS40 binding to raptor. The amount of the mTOR-raptor complex did not differ between groups. In contrast, the binding and retention of both 4E-BP1 and S6K1 to raptor were increased, and, conversely, the binding of raptor with eIF3 was decreased in sepsis. These changes in mTORC1 in the basal state were associated with enhanced 5'-AMP activated kinase activity. Acute in vivo leucine stimulation increased muscle protein synthesis in control, but not septic rats. This muscle leucine resistance was associated with coordinated changes in raptor-eIF3 binding and 4E-BP1 phosphorylation. Overall, our data suggest the sepsis-induced decrease in muscle protein synthesis may be mediated by the inability of 4E-BP1 and S6K1 to be phosphorylated and released from mTORC1 as well as the decreased recruitment of eIF3 necessary for a functional 48S complex. These data provide additional mechanistic insight into the molecular mechanisms by which sepsis impairs both basal protein synthesis and the anabolic response to the nutrient signal leucine in skeletal muscle.

Insulin induces REDD1 expression through hypoxia-inducible factor 1 activation in adipocytes

REDD1 (regulated in development and DNA damage responses) is essential for the inhibition of mTORC1 (mammalian target of rapamycin complex) signaling pathway in response to hypoxia. REDD1 expression is regulated by many stresses such as hypoxia, oxidative stress, and energy depletion. However, the regulation of REDD1 expression in response to insulin remains unknown. In the present study, we demonstrate that in murine and in human adipocytes, insulin stimulates REDD1 expression. Insulin-induced REDD1 expression occurs through phosphoinositide 3-kinase/mTOR-dependent pathways. Moreover, using echinomycin, a hypoxia-inducible factor 1 (HIF-1) inhibitor, and HIF-1alpha small interfering RNA, we demonstrate that insulin stimulates REDD1 expression only through the transcription factor HIF-1. In conclusion, our study shows that insulin stimulates REDD1 expression in adipocytes.