Regulation of signaling pathways downstream of IGF-I/insulin by androgen in skeletal muscle of glucocorticoid-treated rats

Acute In Vitro and In Vivo Effects of Dexamethasone in the Vocal Folds: a Pilot Study

OBJECTIVES/HYPOTHESIS

Glucocorticoids (GC)s are commonly employed to treat vocal fold (VF) pathologies. However, VF atrophy has been associated with intracordal GC injections. Dexamethasone-induced skeletal muscle atrophy is well-documented in other tissues and believed to be mediated by increased muscle proteolysis via upregulation of Muscle Ring Finger (MuRF)-1 and Atrogin-1. Mechanisms of dexamethasone-mediated VF atrophy have not been described. This pilot study employed in vitro and in vivo models to investigate the effects of dexamethasone on VF epithelium, thyroarytenoid (TA) muscle, and TA-derived myoblasts. We hypothesized that dexamethasone will increase atrophy-associated gene expression in TA muscle and myoblasts and decrease TA muscle fiber size and epithelial thickness.

STUDY DESIGN

In vitro, pre-clinical.

METHODS

TA myoblasts were isolated from a female Sprague-Dawley rat and treated with 1 μM dexamethasone for 24-h. In vivo, 15 New Zealand white rabbits were randomly assigned to three treatment groups: (1) bilateral intracordal injection of 40 μL dexamethasone (10 mg/ml; n = 5), (2) volume-matched saline (n = 5), and (3) untreated controls (n = 5). Larynges were harvested 7-days post-injection. Across in vivo and in vitro experimentation, MuRF-1 and Atrogin-1 mRNA expression were measured via RT-qPCR. TA muscle fiber cross-sectional area (CSA) and epithelial thickness were also quantified in vivo.

RESULTS

Dexamethasone increased MuRF-1 gene expression in TA myoblasts. Dexamethasone injection, however, did not alter atrophy-associated gene expression, TA CSA, or epithelial thickness in vivo.

CONCLUSION

Dexamethasone increased atrogene expression in TA myoblasts, providing foundational insight into GC induced atrophic gene transcription. Repeated dexamethasone injections may be required to elicit atrophy in vivo.

LEVEL OF EVIDENCE

NA Laryngoscope, 133:2264-2270, 2023.

The importance of biological sex in cardiac cachexia

Cardiac cachexia is a catabolic muscle wasting syndrome observed in approximately 1 in 10 heart failure patients. Increased skeletal muscle atrophy leads to frailty and limits mobility which impacts quality of life, exacerbates clinical care, and is associated with higher rates of mortality. Heart failure is known to exhibit a wide range of prevalence and severity when examined across individuals of different ages and with co-morbidities related to diabetes, renal failure and pulmonary dysfunction. It is also recognized that men and women exhibit striking differences in the pathophysiology of heart failure as well as skeletal muscle homeostasis. Given that both skeletal muscle and heart failure physiology are in-part sex dependent, the diagnosis and treatment of cachexia in heart failure patients may depend on a comprehensive examination of how these organs interact. In this review we explore the potential for sex-specific differences in cardiac cachexia. We summarize advantages and disadvantages of clinical methods used to measure muscle mass and function and provide alternative measurements that should be considered in preclinical studies. Additionally, we summarize sex-dependent effects on muscle wasting in preclinical models of heart failure, disuse, and cancer. Lastly, we discuss the endocrine function of the heart and outline unanswered questions that could directly impact patient care.

A narrative review of skeletal muscle atrophy in critically ill children: pathogenesis and chronic sequelae

Muscle wasting is now recognized as a growing, debilitating problem in critically ill adults, resulting in long-term deficits in function and an impaired quality of life. Ultrasonography has demonstrated decreases in skeletal muscle size during pediatric critical illness, although variations exist. However, muscle protein turnover patterns during pediatric critical illness are unclear. Understanding muscle protein turnover during critical illness is important in guiding interventions to reduce muscle wasting. The aim of this review was to explore the possible protein synthesis and breakdown patterns in pediatric critical illness. Muscle protein turnover studies in critically ill children are lacking, with the exception of those with burn injuries. Children with burn injuries demonstrate an elevation in both muscle protein breakdown (MPB) and synthesis during critical illness. Extrapolations from animal models and whole-body protein turnover studies in children suggest that children may be more dependent on anabolic factors (e.g., nutrition and growth factors), and may experience greater muscle degradation in response to insults than adults. Yet, children, particularly the younger ones, are more responsive to anabolic agents, suggesting modifiable muscle wasting during critical illness. There is a lack of evidence for muscle wasting in critically ill children and its correlation with outcomes, possibly due to current available methods to study muscle protein turnover in children-most of which are invasive or tedious. In summary, children may experience muscle wasting during critical illness, which may be more reversible by the appropriate anabolic agents than adults. Age appears an important determinant of skeletal muscle turnover. Less invasive methods to study muscle protein turnover and associations with long-term outcome would strengthen the evidence for muscle wasting in critically ill children.

Is Mitochondrial Oxidative Stress the Key Contributor to Diaphragm Atrophy and Dysfunction in Critically Ill Patients?

Diaphragm dysfunction is prevalent in the progress of respiratory dysfunction in various critical illnesses. Respiratory muscle weakness may result in insufficient ventilation, coughing reflection suppression, pulmonary infection, and difficulty in weaning off respirators. All of these further induce respiratory dysfunction and even threaten the patients' survival. The potential mechanisms of diaphragm atrophy and dysfunction include impairment of myofiber protein anabolism, enhancement of myofiber protein degradation, release of inflammatory mediators, imbalance of metabolic hormones, myonuclear apoptosis, autophagy, and oxidative stress. Among these contributors, mitochondrial oxidative stress is strongly implicated to play a key role in the process as it modulates diaphragm protein synthesis and degradation, induces protein oxidation and functional alteration, enhances apoptosis and autophagy, reduces mitochondrial energy supply, and is regulated by inflammatory cytokines via related signaling molecules. This review aims to provide a concise overview of pathological mechanisms of diaphragmatic dysfunction in critically ill patients, with special emphasis on the role and modulating mechanisms of mitochondrial oxidative stress.

Skeletal muscle atrogenes: From rodent models to human pathologies

Skeletal muscle atrophy is a common side effect of most human diseases. Muscle loss is not only detrimental for the quality of life but it also dramatically impairs physiological processes of the organism and decreases the efficiency of medical treatments. While hypothesized for years, the existence of an atrophying programme common to all pathologies is still incompletely solved despite the discovery of several actors and key regulators of muscle atrophy. More than a decade ago, the discovery of a set of genes, whose expression at the mRNA levels were similarly altered in different catabolic situations, opened the way of a new concept: the presence of atrogenes, i.e. atrophy-related genes. Importantly, the atrogenes are referred as such on the basis of their mRNA content in atrophying muscles, the regulation at the protein level being sometimes more complicate to elucidate. It should be noticed that the atrogenes are markers of atrophy and that their implication as active inducers of atrophy is still an open question for most of them. While the atrogene family has grown over the years, it has mostly been incremented based on data coming from rodent models. Whether the rodent atrogenes are valid for humans still remain to be established. An "atrogene" was originally defined as a gene systematically up- or down-regulated in several catabolic situations. Even if recent works often restrict this notion to the up-regulation of a limited number of proteolytic enzymes, it is important to keep in mind the big picture view. In this review, we provide an update of the validated and potential rodent atrogenes and the metabolic pathways they belong, and based on recent work, their relevance in human physio-pathological situations. We also propose a more precise definition of the atrogenes that integrates rapid recovery when catabolic stimuli are stopped or replaced by anabolic ones.

Sex Modulates Lactobacillus johnsonii N6.2 and Phytophenol Effectiveness in Reducing High Fat Diet Induced mTOR Activation in Sprague-Dawley Rats

Metabolic syndrome (MetS) is the underlying cause of some devastating diseases, including type 2 diabetes and cardiovascular disease. These diseases have been associated with over-activation of the mechanistic Target of Rapamycin (mTOR) pathway. This study utilizes a high fat diet (HFD) to induce MetS and to dissect the effects of a beneficial bacterium, L. johnsonii N6.2, and natural phenolics on mTOR complex 1 (mTORC1) expression compared to a reduced energy density diet (REDD). HFD significantly elevated MetS markers in males, as noted through an increase in weight, glucose levels, and triglyceride levels. Treatments were effective in reducing mTORC1-activating phosphorylation of pAKT-T308 and pAKT-S473 (p = 0.0012 and 0.0049, respectively) in HFD-fed females, with the combined treatments of L. johnsonii and phytophenols reducing phosphorylation below REDD-fed control levels, and significantly below HFD-fed control levels. Meanwhile, diet was the significant factor influencing male mTORC1-activating phosphorylation (p < 0.0001), as treatments were only effective in reducing phosphorylation in REDD-fed animals. Downstream analysis of mTORC1 activated genes phosphogluconate dehydrogenase (pgd) and phosphofructose kinase (pfk) followed this similar trend, enforcing the significant effect sex has on a treatments’ ability to modulate diet induced abnormalities. Analyzing mTORC1 stimulators such as insulin, inflammatory cytokines, and tryptophan, revealed no significant differences among groups. These results indicate that the effects observed on mTORC1 are a direct consequence of the treatments, and not exerted indirectly via the modulation of stimuli. This study highlights the potential use of commensal microorganisms and natural compounds in reducing the onset of metabolic diseases through mTORC1.

Sex Differences in Autophagy Contribute to Female Vulnerability in Alzheimer's Disease

Alzheimer's disease (AD) is the most common form of dementia, with over 5. 4 million cases in the US alone (Alzheimer's Association, 2016). Clinically, AD is defined by the presence of plaques composed of Aβ and neurofibrillary pathology composed of the microtubule associated protein tau. Another key feature is the dysregulation of autophagy at key steps in the pathway. In AD, disrupted autophagy contributes to disease progression through the failure to clear pathological protein aggregates, insulin resistance, and its role in the synthesis of Aβ. Like many psychiatric and neurodegenerative diseases, the risk of developing AD, and disease course are dependent on the sex of the patient. One potential mechanism through which these differences occur, is the effects of sex hormones on autophagy. In women, the loss of hormones with menopause presents both a risk factor for developing AD, and an obvious example of where sex differences in AD can stem from. However, because AD pathology can begin decades before menopause, this does not provide the full answer. We propose that sex-based differences in autophagy regulation during the lifespan contribute to the increased risk of AD, and greater severity of pathology seen in women.

The role of sex steroid hormones in the pathophysiology and treatment of sarcopenia

Sex steroids influence the maintenance and growth of muscles. Decline in androgens, estrogens and progesterone by aging leads to the loss of muscular function and mass, sarcopenia. These steroid hormones can interact with different signaling pathways through their receptors. To date, sex steroid hormone receptors and their exact roles are not completely defined in skeletal and smooth muscles. Although numerous studies focused on the effects of sex steroid hormones on different types of cells, still many unexplained molecular mechanisms in both skeletal and smooth muscle cells remain to be investigated. In this paper, many different molecular mechanisms that are activated or inhibited by sex steroids and those that influence the growth, proliferation, and differentiation of skeletal and smooth muscle cells are reviewed. Also, the similarities of cellular and molecular pathways of androgens, estrogens and progesterone in both skeletal and smooth muscle cells are highlighted. The reviewed signaling pathways and participating molecules can be targeted in the future development of novel therapeutics.

Regulation of mTORC1 by growth factors, energy status, amino acids and mechanical stimuli at a glance

The mechanistic/mammalian target of rapamycin complex 1 (mTORC1) plays a pivotal role in the regulation of skeletal muscle protein synthesis. Activation of the complex leads to phosphorylation of two important sets of substrates, namely eIF4E binding proteins and ribosomal S6 kinases. Phosphorylation of these substrates then leads to an increase in protein synthesis, mainly by enhancing translation initiation. mTORC1 activity is regulated by several inputs, such as growth factors, energy status, amino acids and mechanical stimuli. Research in this field is rapidly evolving and unraveling how these inputs regulate the complex. Therefore this review attempts to provide a brief and up-to-date narrative on the regulation of this marvelous protein complex. Additionally, some sports supplements which have been shown to regulate mTORC1 activity are discussed.

Effects of tail suspension on serum testosterone and molecular targets regulating muscle mass

INTRODUCTION

The contribution of reduced testosterone levels to tail suspension (TS)-induced muscle atrophy remains equivocal. The molecular mechanism by which testosterone regulates muscle mass during TS has not been investigated.

METHODS

Effects of TS on serum testosterone levels, muscle mass, and expression of muscle atrophy- and hypertrophy-inducing targets were measured in soleus (SOL) and extensor digitorum longus (EDL) muscles after testosterone administration during 1, 5, and 14 days of TS in male mice.

RESULTS

TS produced an increase followed by a transient drop in testosterone levels. Muscle atrophy was associated with downregulation of Igf1 and upregulation of Mstn, Redd1, Atrogin-1, and MuRF1 mRNA with clear differences in Igf1, Mstn, and MAFbx/Atrogin-1 gene expression between SOL and EDL. Testosterone supplementation did not affect muscle mass or protein expression levels during TS. Conclusions The known anabolic effects of testosterone are not sufficient to ameliorate loss of muscle mass during TS.

Insulin down-regulates the expression of ubiquitin E3 ligases partially by inhibiting the activity and expression of AMP-activated protein kinase in L6 myotubes

We conclude that insulin inhibits AMPK through Akt phosphorylation in L6 myotubes, which may serve as a possible signalling pathway for the down-regulation of protein degradation. Besides, decreased expression of AMPK α2 may partially participate in inhibiting the activity of AMPK.

While insulin is an anabolic hormone, AMP-activated protein kinase (AMPK) is not only a key energy regulator, but it can also control substrate metabolism directly by inducing skeletal muscle protein degradation. The hypothesis of the present study was that insulin inhibits AMPK and thus down-regulates the expression of the ubiquitin E3 ligases, muscle atrophy F-box (MAFbx) and muscle RING finger 1 (MuRF1) in skeletal muscle cells. Differentiated L6 myotubes were treated with 5-aminoimidazole-4-carboxamide-1-β-4-ribofuranoside (AICAR) and/or compound C to stimulate and/or block AMPK respectively. These treatments were also conducted in the presence or absence of insulin and the cells were analysed by western blot and quantitative real-time PCR. In addition, nuleotide levels were determined using HPLC. The activation of AMPK with AICAR enhanced the mRNA levels of MAFbx and MuRF1. Insulin reduced the phosphorylation and activity AMPK, which was accompanied by reduced MAFbx and MuRF1 mRNA levels. Using a protein kinase B (PKB/Akt) inhibitor, we found that insulin regulates AMPK through the activation of Akt. Furthermore, insulin down-regulated AMPK α2 mRNA. We conclude that insulin inhibits AMPK through Akt phosphorylation in L6 myotubes, which may serve as a possible signalling pathway for the down-regulation of protein degradation. In addition, decreased expression of AMPK α2 may partially participate in inhibiting the activity of AMPK.

Skeletal muscle tissue engineering: methods to form skeletal myotubes and their applications

Skeletal muscle tissue engineering (SMTE) aims to repair or regenerate defective skeletal muscle tissue lost by traumatic injury, tumor ablation, or muscular disease. However, two decades after the introduction of SMTE, the engineering of functional skeletal muscle in the laboratory still remains a great challenge, and numerous techniques for growing functional muscle tissues are constantly being developed. This article reviews the recent findings regarding the methodology and various technical aspects of SMTE, including cell alignment and differentiation. We describe the structure and organization of muscle and discuss the methods for myoblast alignment cultured in vitro. To better understand muscle formation and to enhance the engineering of skeletal muscle, we also address the molecular basics of myogenesis and discuss different methods to induce myoblast differentiation into myotubes. We then provide an overview of different coculture systems involving skeletal muscle cells, and highlight major applications of engineered skeletal muscle tissues. Finally, potential challenges and future research directions for SMTE are outlined.

Skeletal muscle atrophy and the E3 ubiquitin ligases MuRF1 and MAFbx/atrogin-1

Muscle RING finger 1 (MuRF1) and muscle atrophy F-box (MAFbx)/atrogin-1 were identified more than 10 years ago as two muscle-specific E3 ubiquitin ligases that are increased transcriptionally in skeletal muscle under atrophy-inducing conditions, making them excellent markers of muscle atrophy. In the past 10 years much has been published about MuRF1 and MAFbx with respect to their mRNA expression patterns under atrophy-inducing conditions, their transcriptional regulation, and their putative substrates. However, much remains to be learned about the physiological role of both genes in the regulation of mass and other cellular functions in striated muscle. Although both MuRF1 and MAFbx are enriched in skeletal, cardiac, and smooth muscle, this review will focus on the current understanding of MuRF1 and MAFbx in skeletal muscle, highlighting the critical questions that remain to be answered.

Metabolic functions of glucocorticoid receptor in skeletal muscle

Glucocorticoids (GCs) exert key metabolic influences on skeletal muscle. GCs increase protein degradation and decrease protein synthesis. The released amino acids are mobilized from skeletal muscle to liver, where they serve as substrates for hepatic gluconeogenesis. This metabolic response is critical for mammals' survival under stressful conditions, such as fasting and starvation. GCs suppress insulin-stimulated glucose uptake and utilization and glycogen synthesis, and play a permissive role for catecholamine-induced glycogenolysis, thus preserving the level of circulating glucose, the major energy source for the brain. However, chronic or excess exposure of GCs can induce muscle atrophy and insulin resistance. GCs convey their signal mainly through the intracellular glucocorticoid receptor (GR). While GR can act through different mechanisms, one of its major actions is to regulate the transcription of its primary target genes through genomic glucocorticoid response elements (GREs) by directly binding to DNA or tethering onto other DNA-binding transcription factors. These GR primary targets trigger physiological and pathological responses of GCs. Much progress has been made to understand how GCs regulate protein and glucose metabolism. In this review, we will discuss how GR primary target genes confer metabolic functions of GCs, and the mechanisms governing the transcriptional regulation of these targets. Comprehending these processes not only contributes to the fundamental understanding of mammalian physiology, but also will provide invaluable insight for improved GC therapeutics.

Testosterone regulation of Akt/mTORC1/FoxO3a signaling in skeletal muscle

Low endogenous testosterone production, known as hypogonadism is commonly associated with conditions inducing muscle wasting. Akt signaling can control skeletal muscle mass through mTOR regulation of protein synthesis and FoxO regulation of protein degradation, and this pathway has been previously identified as a target of androgen signaling. However, the testosterone sensitivity of Akt/mTOR signaling requires further understanding in order to grasp the significance of varied testosterone levels seen with wasting disease on muscle protein turnover regulation. Therefore, the purpose of this study is to determine the effect of androgen availability on muscle Akt/mTORC1/FoxO3a regulation in skeletal muscle and cultured C(2)C(12) myotubes. C57BL/6 mice were either castrated for 42 days or castrated and treated with the nandrolone decanoate (ND) (6 mg/kg bw/wk). Testosterone loss (TL) significantly decreased volitional grip strength, body weight, and gastrocnemius (GAS) muscle mass, and ND reversed these changes. Related to muscle mass regulation, TL decreased muscle IGF-1 mRNA, the rate of myofibrillar protein synthesis, Akt phosphorylation, and the phosphorylation of Akt targets, GSK3β, PRAS40 and FoxO3a. TL induced expression of FoxO transcriptional targets, MuRF1, atrogin1 and REDD1. Muscle AMPK and raptor phosphorylation, mTOR inhibitors, were not altered by low testosterone. ND restored IGF-1 expression and Akt/mTORC1 signaling while repressing expression of FoxO transcriptional targets. Testosterone (T) sensitivity of Akt/mTORC1 signaling was examined in C(2)C(12) myotubes, and mTOR phosphorylation was induced independent of Akt activation at low T concentrations, while a higher T concentration was required to activate Akt signaling. Interestingly, low concentration T was sufficient to amplify myotube mTOR and Akt signaling after 24 h of T withdrawal, demonstrating the potential in cultured myotubes for a T initiated positive feedback mechanism to amplify Akt/mTOR signaling. In summary, androgen withdrawal decreases muscle myofibrillar protein synthesis through Akt/mTORC1 signaling, which is independent of AMPK activation, and readily reversible by anabolic steroid administration. Acute Akt activation in C(2)C(12) myotubes is sensitive to a high concentration of testosterone, and low concentrations of testosterone can activate mTOR signaling independent of Akt.

Cardiac proteasome activity in muscle ring finger-1 null mice at rest and following synthetic glucocorticoid treatment

Muscle ring finger-1 (MuRF1) is a muscle-specific E3 ubiquitin ligase that has been implicated in the regulation of cardiac mass through its control of the ubiquitin proteasome system. While it has been suggested that MuRF1 is required for cardiac atrophy, a resting cardiac phenotype has not been reported in mice with a null deletion [knockout (KO)] of MuRF1. Here, we report that MuRF1 KO mice have significantly larger hearts than age-matched wild-type (WT) littermates at ≥ 6 mo of age and that loss of cardiac mass can occur in the absence of MuRF1. The objective of this study was to determine whether changes in proteasome activity were responsible for the cardiac phenotypes observed in MuRF1 KO mice. Cardiac function, architecture, and proteasome activity were analyzed at rest and following 28 days of dexamethasone (Dex) treatment in 6-mo-old WT and MuRF1 KO mice. Echocardiography demonstrated normal cardiac function in the enlarged hearts in MURF1 KO mice. At rest, heart mass and cardiomyocyte diameter were significantly greater in MuRF1 KO than in WT mice. The increase in cardiac size in MuRF1 KO mice was related to a decrease in proteasome activity and an increase in Akt signaling relative to WT mice. Dex treatment induced a significant loss of cardiac mass in MuRF1 KO, but not WT, mice. Furthermore, Dex treatment resulted in an increase in proteasome activity in KO, but a decrease in WT, mice. In contrast, Akt/mammalian target of rapamycin signaling decreased in MuRF1 KO mice and increased in WT mice in response to Dex treatment. These findings demonstrate that MuRF1 plays an important role in regulating cardiac size through alterations in protein turnover and that MuRF1 is not required to induce cardiac atrophy.

Differential influence of peripheral and systemic sex steroids on skeletal muscle quality in pre- and postmenopausal women

Aging is associated with gradual decline of skeletal muscle strength and mass often leading to diminished muscle quality. This phenomenon is known as sarcopenia and affects about 30% of the over 60-year-old population. Androgens act as anabolic agents regulating muscle mass and improving muscle performance. The role of female sex steroids as well as the ability of skeletal muscle tissue to locally produce sex steroids has been less extensively studied. We show that despite the extensive systemic deficit of sex steroid hormones in postmenopausal compared to premenopausal women, the hormone content of skeletal muscle does not follow the same trend. In contrast to the systemic levels, muscle tissue of post- and premenopausal women had similar concentrations of dehydroepiandrosterone and androstenedione, while the concentrations of estradiol and testosterone were significantly higher in muscle of the postmenopausal women. The presence of steroidogenetic enzymes in muscle tissue indicates that the elevated postmenopausal steroid levels in skeletal muscle are because of local steroidogenesis. The circulating sex steroids were associated with better muscle quality while the muscle concentrations reflected the amount of infiltrated fat within muscle tissue. We conclude that systemically delivered and peripherally produced sex steroids have distinct roles in the regulation of neuromuscular characteristics during aging.

Corticosteroid effects on ventilator-induced diaphragm dysfunction in anesthetized rats depend on the dose administered

BACKGROUND

High dose of corticosteroids has been previously shown to protect against controlled mechanical ventilation (CMV)-induced diaphragmatic dysfunction while inhibiting calpain activation. Because literature suggests that the calpain inhibiting effect of corticosteroid depends on the dose administered, we determined whether lower doses of corticosteroids would also provide protection of the diaphragm during CMV. This may be important for patients undergoing mechanical ventilation and receiving corticosteroids.

METHODS

Rats were assigned to controls or to 24 hours of CMV while being treated at the start of mechanical ventilation with a single intramuscular administration of either saline, or 5 mg/kg (low MP) or 30 mg/kg (high MP) of methylprednisolone.

RESULTS

Diaphragmatic force was decreased after CMV and this was exacerbated in the low MP group while high MP rescued this diaphragmatic dysfunction. Atrophy was more severe in the low MP group than after CMV while no atrophy was observed in the high MP group. A significant and similar increase in calpain activity was observed in both the low MP and CMV groups whereas the high dose prevented calpain activation. Expression of calpastatin, the endogenous inhibitor of calpain, was decreased in the CMV and low MP groups but its level was preserved to controls in the high MP group. Caspase-3 activity increased in all CMV groups but to a lesser extent in the low and high MP groups. The 20S proteasome activity was increased in CMV only.

CONCLUSIONS

Administration of 30 mg/kg methylprednisolone during CMV protected against CMV-induced diaphragm dysfunction while 5 mg/kg was more deleterious. The protective effect is due mainly to an inhibition of the calpain system through preservation of calpastatin levels and to a lesser extent to a caspase-3 inhibition.

Impact of viral-mediated IGF-I gene transfer on skeletal muscle following cast immobilization

Insulin-like growth factor I (IGF-I) is a potent myogenic factor that plays a critical role in muscle regeneration and muscle hypertrophy. The purpose of this study was to evaluate the effect of IGF-I overexpression on the recovery of muscle size and function during reloading/reambulation after a period of cast immobilization in predominantly fast twitch muscles. In addition, we investigated concomitant molecular responses in IGF-I receptor and binding proteins (BPs). Recombinant adeno-associated virus vector for IGF-I (rAAV-IGF-IA) was injected into the anterior compartment of one of the hindlimbs of young (3 wk) C57BL6 female mice. At 20 wk of age, both hindlimbs were cast immobilized in a shortened position for 2 wk to unload the tibialis anterior (TA) and extensor longus digitorum (EDL) muscles. The TA and EDL muscles were removed bilaterally after 2 wk of cast immobilization and after 1 and 3 wk of free cage reambulation. Increases in IGF-I mRNA and protein levels with IGF-I overexpression were associated with significant increases in muscle wet weight, fiber size, and tetanic force, although overexpression did not protect against cast immobilization-induced muscle atrophy. After 1 wk of reambulation, evidence of enhanced muscle regeneration was noted in IGF-I-overexpressing muscles with an increased prevalence of central nuclei, embryonic myosin, and Pax7 positive fibers. We also observed larger relative gains in muscle size (wet weight and fiber area), but not force, during the 3-wk reambulation period in hindlimb muscles overexpressing IGF-I compared with contralateral control legs. Changes in IGFBP-5 mRNA expression during cast immobilization and reambulation paralleled those of IGF-I, whereas IGFBP-3 expression changed inversely to IGFBP-5.

Effects of a novel selective androgen receptor modulator on dexamethasone-induced and hypogonadism-induced muscle atrophy

Glucocorticoids are the most widely used antiinflammatory drugs in the world. However, prolonged use of glucocorticoids results in undesirable side effects such as muscle wasting, osteoporosis, and diabetes. Skeletal muscle wasting, which currently has no approved therapy, is a debilitating condition resulting from either reduced muscle protein synthesis or increased degradation. The imbalance in protein synthesis could occur from increased expression and function of muscle-specific ubiquitin ligases, muscle atrophy F-box (MAFbx)/atrogin-1 and muscle ring finger 1 (MuRF1), or decreased function of the IGF-I and phosphatidylinositol-3 kinase/Akt kinase pathways. We examined the effects of a nonsteroidal tissue selective androgen receptor modulator (SARM) and testosterone on glucocorticoid-induced muscle atrophy and castration-induced muscle atrophy. The SARM and testosterone propionate blocked the dexamethasone-induced dephosphorylation of Akt and other proteins involved in protein synthesis, including Forkhead box O (FoxO). Dexamethasone caused a significant up-regulation in the expression of ubiquitin ligases, but testosterone propionate and SARM administration blocked this effect by phosphorylating FoxO. Castration induced rapid myopathy of the levator ani muscle, accompanied by up-regulation of MAFbx and MuRF1 and down-regulation of IGF-I, all of which was attenuated by a SARM. The results suggest that levator ani atrophy caused by hypogonadism may be the result of loss of IGF-I stimulation, whereas that caused by glucocorticoid treatment relies almost solely on up-regulation of MAFbx and MuRF1. Our studies provide the first evidence that glucocorticoid- and hypogonadism-induced muscle atrophy are mediated by distinct but overlapping mechanisms and that SARMs may provide a more effective and selective pharmacological approach to prevent glucocorticoid-induced muscle loss than steroidal androgen therapy.

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.

Hormone treatment and muscle anabolism during aging: androgens

Aging is associated with a gradual decline in circulating testosterone concentrations and decreased musculature in men. While testosterone administration is often considered when symptoms of hypogonadism are presented, the long-term effects of androgen use on muscle physiology are not yet fully understood. The definition of hypogonadism in men remains obscure but is generally indicated by total testosterone concentrations less than a threshold value of 300-500 ng/dL. Androgen replacement therapy is generally safe in men and women with low endogenous testosterone concentrations. The development of selective androgen receptor modulators (SARMs) may provide additional options in treatment of hypogonadism while lowering the potential of side effects often associated with long-term androgen use. Androgen administration, either alone or in combination with other treatments, can be successful in improving muscle mass by increasing protein anabolism and reducing protein catabolism in men and women. Further research is necessary to optimize the anabolic and anticatabolic properties of androgens for treatment and prevention of muscle loss in men and women.

Changes in the natural abundance of 13CO2/12CO2 in breath due to lipopolysacchride-induced acute phase response

The natural abundance of carbon-13 in blood proteins increases during the cachectic state and may be a biomarker for disease status. We hypothesized a corresponding drop in the relative abundance of 13C in breath CO2. Using the lipopolysacchride (LPS)-induced endotoxemia model of the acute cachectic state, we demonstrated that the acute phase response causes shifts in the stable isotopes of carbon in exhaled CO2 (13CO2/12CO2 delta value) shortly after administration of LPS while glucocorticoid treatment does not. Mice were injected with LPS and stable isotopes of blood amino acids and carbon in exhaled CO2 were monitored. An increase in the relative isotopic mass of serum alanine, proline and threonine was observed at 3 h after LPS injection. Breath delta values began dropping immediately after administration of LPS, and were 4-5 delta values lower than those of the control animals by 2.5 h after injection. A corresponding drop in delta value was not observed with dexamethasone treatment. Thus protein synthesis during the acute phase response probably caused the fractionation of stable isotopes observed in the plasma amino acids and in exhaled breath 13CO2 delta values. The exhaled breath 13CO2 delta value may be a valuable real-time biomarker of cachexia associated with an acute phase response due to endotoxemia.

Time course expression of Foxo transcription factors in skeletal muscle following corticosteroid administration

Increased expression of forkhead box O (Foxo) transcription factors were reported in cultured myotubes and mouse limb muscle with corticosteroid (CS) treatment. We previously reported that administration of CS to rats resulted in muscle fiber atrophy only by day 7. The aim of this study, therefore, was to evaluate the time-course changes in the expression of Foxo transcription factors and muscle-specific ubiquitin E3 ligases in rat limb muscle following CS administration. Triamcinolone (TRI; 1 mg x kg(-1) x day(-1) im) was administered for 1, 3, or 7 days. Control (CTL) rats were given saline. Muscle mRNA was analyzed by real-time RT-PCR. Compared with CTL, body weights of TRI-treated animals decreased by 3, 12, and 21% at days 1, 3, and 7, respectively. Muscle IGF-1 mRNA levels decreased by 33, 65, and 58% at days 1, 3, and 7 in TRI-treated rats compared with CTL. Levels of phosphorylated Akt were 28, 50, and 36% lower in TRI animals at these time points. Foxo1 mRNA increased progressively by 1.2-, 1.4-, and 2.5-fold at days 1, 3, and 7 in TRI animals. Similar changes were noted in the expression of Foxo3a mRNA (1.3-, 1.4-, and 2.6-fold increments). By contrast, Foxo4 mRNA was not significantly changed in TRI animals. With TRI, muscle atrophy F box/Atrogin-1 increased by 1.8-, 4.1-, and 7.5-fold at days 1, 3, and 7 compared with CTL rats. By contrast, muscle RING finger 1 increased only from day 7 (2.7-fold). Gradual reduction in IGF-I expression with TRI over the time series paralleled that of Akt. These findings are consistent with a progressive stimulus to muscle protein degradation and the need to process/remove disassembled muscle proteins via the ubiquitin-proteasome system. Elucidating the dynamic catabolic responses to CS challenge is important in understanding the mechanisms underlying muscle atrophy and therapeutic measures to offset this.