Sepsis increases the expression and activity of the transcription factor Forkhead Box O 1 (FOXO1) in skeletal muscle by a glucocorticoid-dependent mechanism

Resveratrol prevents dexamethasone-induced expression of the muscle atrophy-related ubiquitin ligases atrogin-1 and MuRF1 in cultured myotubes through a SIRT1-dependent mechanism

Resveratrol (3,5,4'-trihydroxystilbene) has been ascribed multiple beneficial biological effects but the influence of resveratrol on glucocorticoid-induced muscle atrophy is not known. We examined the effects of resveratrol on dexamethasone-induced atrogin-1 and MuRF1 expression, FOXO1 acetylation, protein degradation and atrophy in cultured L6 myotubes. In addition, the role of the deacetylase SIRT1 in the effects of resveratrol was determined by transfecting myotubes with SIRT1 siRNA. The catabolic effects of dexamethasone were prevented by resveratrol and the protective effects of resveratrol on dexamethasone-induced atrogin-1 and MuRF1 expression were abolished in myotubes transfected with SIRT1 siRNA. Results suggest that resveratrol can prevent glucocorticoid-induced muscle wasting and that this effect is at least in part SIRT1-dependent.

Recent progress toward understanding the molecular mechanisms that regulate skeletal muscle mass

The maintenance of muscle mass is critical for health and issues associated with the quality of life. Over the last decade, extensive progress has been made with regard to our understanding of the molecules that regulate skeletal muscle mass. Not surprisingly, many of these molecules are intimately involved in the regulation of protein synthesis and protein degradation [e.g. the mammalian target of rapamycin (mTOR), eukaryotic initiation factor 2B (eIF2B), eukaryotic initiation factor 3f (eIF3f) and the forkhead box O (FoxO) transcription factors]. It is also becoming apparent that molecules which sense, or control, the energetic status of the cell play a key role in the regulation of muscle mass [e.g. AMP-activated protein kinase (AMPK) and peroxisome proliferator-activated receptor gamma coactivator-1 α (PGC1α)]. In this review we will attempt to summarize the current knowledge of how these molecules regulate skeletal muscle mass.

Inhibition of FoxO transcriptional activity prevents muscle fiber atrophy during cachexia and induces hypertrophy

Cachexia is characterized by inexorable muscle wasting that significantly affects patient prognosis and increases mortality. Therefore, understanding the molecular basis of this muscle wasting is of significant importance. Recent work showed that components of the forkhead box O (FoxO) pathway are increased in skeletal muscle during cachexia. In the current study, we tested the physiological significance of FoxO activation in the progression of muscle atrophy associated with cachexia. FoxO-DNA binding dependent transcription was blocked in the muscles of mice through injection of a dominant negative (DN) FoxO expression plasmid prior to inoculation with Lewis lung carcinoma cells or the induction of sepsis. Expression of DN FoxO inhibited the increased mRNA levels of atrogin-1, MuRF1, cathepsin L, and/or Bnip3 and inhibited muscle fiber atrophy during cancer cachexia and sepsis. Interestingly, during control conditions, expression of DN FoxO decreased myostatin expression, increased MyoD expression and satellite cell proliferation, and induced fiber hypertrophy, which required de novo protein synthesis. Collectively, these data show that FoxO-DNA binding-dependent transcription is necessary for normal muscle fiber atrophy during cancer cachexia and sepsis, and further suggest that basal levels of FoxO play an important role during normal conditions to depress satellite cell activation and limit muscle growth.

Muscle plasticity and β₂-adrenergic receptors: adaptive responses of β₂-adrenergic receptor expression to muscle hypertrophy and atrophy

We discuss the functional roles of β₂-adrenergic receptors in skeletal muscle hypertrophy and atrophy as well as the adaptive responses of β₂-adrenergic receptor expression to anabolic and catabolic conditions. β₂-Adrenergic receptor stimulation using anabolic drugs increases muscle mass by promoting muscle protein synthesis and/or attenuating protein degradation. These effects are prevented by the downregulation of the receptor. Endurance training improves oxidative performance partly by increasing β₂-adrenergic receptor density in exercise-recruited slow-twitch muscles. However, excessive stimulation of β₂-adrenergic receptors negates their beneficial effects. Although the preventive effects of β₂-adrenergic receptor stimulation on atrophy induced by muscle disuse and catabolic hormones or drugs are observed, these catabolic conditions decrease β₂-adrenergic receptor expression in slow-twitch muscles. These findings present evidence against the use of β₂-adrenergic agonists in therapy for muscle wasting and weakness. Thus, β₂-adrenergic receptors in the skeletal muscles play an important physiological role in the regulation of protein and energy balance.

C/EBPβ regulates dexamethasone-induced muscle cell atrophy and expression of atrogin-1 and MuRF1

Muscle wasting in catabolic patients is in part mediated by glucocorticoids and is associated with increased expression and activity of the transcription factor C/EBPβ. It is not known, however, if C/EBPβ is causally linked to glucocorticoid-induced muscle atrophy. We used dexamethasone-treated L6 myoblasts and myotubes to test the role of C/EBPβ in glucocorticoid-induced expression of the muscle-specific ubiquitin ligases atrogin-1 and MuRF1, protein degradation, and muscle atrophy by transfecting cells with C/EBPβ siRNA. In myoblasts, silencing C/EBPβ expression with siRNA inhibited dexamethasone-induced increase in protein degradation, atrogin-1 and MuRF1 expression, and muscle cell atrophy. Similar effects of C/EBPβ siRNA were seen in myotubes except that the dexamethasone-induced increase in MuRF1 expression was not affected by C/EBPβ siRNA in myotubes. In additional experiments, overexpressing C/EBPβ did not influence atrogin-1 or MuRF1 expression in myoblasts or myotubes. Taken together, our observations suggest that glucocorticoid-induced muscle wasting is at least in part regulated by C/EBPβ. Increased C/EBPβ expression alone, however, is not sufficient to upregulate atrogin-1 and MuRF1 expression.

mTor signaling in skeletal muscle during sepsis and inflammation: where does it all go wrong?

The mammalian target of rapamycin (mTOR) is an evolutionarily conserved protein kinase that exquisitely regulates protein metabolism in skeletal muscle. mTOR integrates input from amino acids, growth factors, and intracellular cues to make or break muscle protein. mTOR accomplishes this task by stimulating the phosphorylation of substrates that control protein translation while simultaneously inhibiting proteasomal and autophagic protein degradation. In a metabolic twist of fate, sepsis induces muscle atrophy in part by the aberrant regulation of mTOR. In this review, we track the steps of normal mTOR signaling in muscle and examine where they go astray in sepsis and inflammation.

Activated protein synthesis and suppressed protein breakdown signaling in skeletal muscle of critically ill patients

Background

Skeletal muscle mass is controlled by myostatin and Akt-dependent signaling on mammalian target of rapamycin (mTOR), glycogen synthase kinase 3β (GSK3β) and forkhead box O (FoxO) pathways, but it is unknown how these pathways are regulated in critically ill human muscle. To describe factors involved in muscle mass regulation, we investigated the phosphorylation and expression of key factors in these protein synthesis and breakdown signaling pathways in thigh skeletal muscle of critically ill intensive care unit (ICU) patients compared with healthy controls.

Methodology/Principal Findings

ICU patients were systemically inflamed, moderately hyperglycemic, received insulin therapy, and showed a tendency to lower plasma branched chain amino acids compared with controls. Using Western blotting we measured Akt, GSK3β, mTOR, ribosomal protein S6 kinase (S6k), eukaryotic translation initiation factor 4E binding protein 1 (4E-BP1), and muscle ring finger protein 1 (MuRF1); and by RT-PCR we determined mRNA expression of, among others, insulin-like growth factor 1 (IGF-1), FoxO 1, 3 and 4, atrogin1, MuRF1, interleukin-6 (IL-6), tumor necrosis factor α (TNF-α) and myostatin. Unexpectedly, in critically ill ICU patients Akt-mTOR-S6k signaling was substantially higher compared with controls. FoxO1 mRNA was higher in patients, whereas FoxO3, atrogin1 and myostatin mRNAs and MuRF1 protein were lower compared with controls. A moderate correlation (r² = 0.36, p<0.05) between insulin infusion dose and phosphorylated Akt was demonstrated.

Conclusions/Significance

We present for the first time muscle protein turnover signaling in critically ill ICU patients, and we show signaling pathway activity towards a stimulation of muscle protein synthesis and a somewhat inhibited proteolysis.

Calpain activity is increased in skeletal muscle from gastric cancer patients with no or minimal weight loss

The influence of cancer on skeletal muscle calpain expression and activity in humans is poorly understood. We tested the hypothesis that calpain activity is increased in skeletal muscle from gastric cancer patients with no or <5% weight loss. Muscle biopsies were obtained from rectus abdominis muscle in 15 patients who underwent surgery for gastric cancer and had <5% weight loss and also in 15 control patients. Calpain activity was determined using a calpain-specific substrate in the absence or presence of calcium. The expression of μ- and m-calpain, calpastatin, atrogin-1, and MuRF1 was determined by real-time polymerase chain reaction. Calpain activity was increased by 70% in cancer patients compared with controls. There were no differences in mRNA levels for μ- and m-calpain, calpastatin, atrogin-1, or MuRF1 between control and cancer patients. Calpain activity may be increased in muscle from gastric cancer patients even before changes in molecular markers of muscle wasting and significant weight loss occur.

Sepsis downregulates myostatin mRNA levels without altering myostatin protein levels in skeletal muscle

Myostatin is a negative regulator of muscle mass and has been reported to be upregulated in several conditions characterized by muscle atrophy. The influence of sepsis on myostatin expression and activity is poorly understood. Here, we tested the hypothesis that sepsis upregulates the expression and downstream signaling of myostatin in skeletal muscle. Because sepsis-induced muscle wasting is at least in part regulated by glucocorticoids, we also determined the influence of glucocorticoids on myostatin expression. Sepsis was induced in rats by cecal ligation and puncture and control rats were sham-operated. In other experiments, rats were injected intraperitoneally with dexamethasone (10 mg/kg) or corresponding volume of vehicle. Surprisingly, myostatin mRNA levels were reduced and myostatin protein levels were unchanged in muscles from septic rats. Muscle levels of activin A, follistatin, and total and phosphorylated Smad2 (p-Smad2) were not influenced by sepsis, suggesting that myostatin downstream signaling was not altered during sepsis. Interestingly, total and p-Smad3 levels were increased in septic muscle, possibly reflecting altered signaling through pathways other than myostatin. Similar to sepsis, treatment of rats with dexamethasone reduced myostatin mRNA levels and did not alter myostatin protein levels. Fasting, an additional condition characterized by muscle wasting, reduced myostatin mRNA and activin A protein levels, increased myostatin protein, and did not influence follistatin and p-Smad2 levels. Of note, total and p-Smad3 levels were reduced in muscle during fasting. The results suggest that sepsis and glucocorticoids do not upregulate the expression and activity of myostatin in skeletal muscle. The role of myostatin may vary between different conditions characterized by muscle wasting. Downstream signaling through Smad2 and 3 is probably regulated not only by myostatin but by other mechanisms as well.

Corticosteroids and muscle wasting: role of transcription factors, nuclear cofactors, and hyperacetylation

PURPOSE OF REVIEW

The purpose of this review is to discuss novel insight into mechanisms of glucocorticoid-regulated muscle wasting, in particular the role of transcription factors and nuclear cofactors. In addition, novel strategies that may become useful in the treatment or prevention of glucocorticoid-induced muscle wasting are reviewed.

RECENT FINDINGS

Studies suggest that glucocorticoid-induced upregulation of the transcription factors Forkhead box O 1 and CCAAT/enhancer-binding protein beta and downregulation of MyoD and myogenin are involved in glucocorticoid-induced muscle wasting. In addition, glucocorticoid-induced hyperacetylation caused by increased expression of the nuclear cofactor p300 and its histone acetyl transferase activity and decreased expression and activity of histone deacetylases plays an important role in glucocorticoid-induced muscle proteolysis and wasting. Other mechanisms may also be involved in glucocorticoid-induced muscle wasting, including insulin resistance and store-operated calcium entry. Novel potential strategies to prevent or treat glucocorticoid-induced muscle wasting include the use of small molecule histone deacetylase activators, dissociated glucocorticoid receptor agonists, and 11beta-hydroxysteroid dehydrogenase type 1 inhibitors.

SUMMARY

An increased understanding of molecular mechanisms regulating glucocorticoid-induced muscle wasting will help develop new strategies to prevent and treat this debilitating condition.

BCATm deficiency ameliorates endotoxin-induced decrease in muscle protein synthesis and improves survival in septic mice

Endotoxin (LPS) and sepsis decrease mammalian target of rapamycin (mTOR) activity in skeletal muscle, thereby reducing protein synthesis. Our study tests the hypothesis that inhibition of branched-chain amino acid (BCAA) catabolism, which elevates circulating BCAA and stimulates mTOR, will blunt the LPS-induced decrease in muscle protein synthesis. Wild-type (WT) and mitochondrial branched-chain aminotransferase (BCATm) knockout mice were studied 4 h after Escherichia coli LPS or saline. Basal skeletal muscle protein synthesis was increased in knockout mice compared with WT, and this change was associated with increased eukaryotic initiation factor (eIF)-4E binding protein-1 (4E-BP1) phosphorylation, eIF4E.eIF4G binding, 4E-BP1.raptor binding, and eIF3.raptor binding without a change in the mTOR.raptor complex in muscle. LPS decreased muscle protein synthesis in WT mice, a change associated with decreased 4E-BP1 phosphorylation as well as decreased formation of eIF4E.eIF4G, 4E-BP1.raptor, and eIF3.raptor complexes. In BCATm knockout mice given LPS, muscle protein synthesis only decreased to values found in vehicle-treated WT control mice, and this ameliorated LPS effect was associated with a coordinate increase in 4E-BP1.raptor, eIF3.raptor, and 4E-BP1 phosphorylation. Additionally, the LPS-induced increase in muscle cytokines was blunted in BCATm knockout mice, compared with WT animals. In a separate study, 7-day survival and muscle mass were increased in BCATm knockout vs. WT mice after polymicrobial peritonitis. These data suggest that elevating blood BCAA is sufficient to ameliorate the catabolic effect of LPS on skeletal muscle protein synthesis via alterations in protein-protein interactions within mTOR complex-1, and this may provide a survival advantage in response to bacterial infection.

Sepsis and glucocorticoids upregulate p300 and downregulate HDAC6 expression and activity in skeletal muscle

Muscle wasting during sepsis is in part regulated by glucocorticoids. In recent studies, treatment of cultured muscle cells in vitro with dexamethasone upregulated expression and activity of p300, a histone acetyl transferase (HAT), and reduced expression and activity of the histone deacetylases-3 (HDAC3) and -6, changes that favor hyperacetylation. Here, we tested the hypothesis that sepsis and glucocorticoids regulate p300 and HDAC3 and -6 in skeletal muscle in vivo. Because sepsis-induced metabolic changes are particularly pronounced in white, fast-twitch skeletal muscle, most experiments were performed in extensor digitorum longus muscles. Sepsis in rats upregulated p300 mRNA and protein levels, stimulated HAT activity, and reduced HDAC6 expression and HDAC activity. The sepsis-induced changes in p300 and HDAC expression were prevented by the glucocorticoid receptor antagonist RU38486. Treatment of rats with dexamethasone increased expression of p300 and HAT activity, reduced expression of HDAC3 and -6, and inhibited HDAC activity. Finally, treatment with the HDAC inhibitor trichostatin A resulted in increased muscle proteolysis and expression of the ubiquitin ligase atrogin-1. Taken together, our results suggest for the first time that sepsis-induced muscle wasting may be regulated by glucocorticoid-dependent hyperacetylation caused by increased p300 and reduced HDAC expression and activity. The recent development of pharmacological HDAC activators may provide a novel avenue to prevent and treat muscle wasting in sepsis and other catabolic conditions.