SMAD3 augments FoxO3-induced MuRF-1 promoter activity in a DNA-binding-dependent manner

YTHDF2 governs muscle size through a targeted modulation of proteostasis

The regulation of proteostasis is fundamental for maintenance of muscle mass and function. Activation of the TGF-β pathway drives wasting and premature aging by favoring the proteasomal degradation of structural muscle proteins. Yet, how this critical post-translational mechanism is kept in check to preserve muscle health remains unclear. Here, we reveal the molecular link between the post-transcriptional regulation of m6A-modified mRNA and the modulation of SMAD-dependent TGF-β signaling. We show that the m6A-binding protein YTHDF2 is essential to determining postnatal muscle size. Indeed, muscle-specific genetic deletion of YTHDF2 impairs skeletal muscle growth and abrogates the response to hypertrophic stimuli. We report that YTHDF2 controls the mRNA stability of the ubiquitin ligase ASB2 with consequences on anti-growth gene program activation through SMAD3. Our study identifies a post-transcriptional to post-translational mechanism for the coordination of gene expression in muscle.

Therapeutic Consequences of Targeting the IGF-1/PI3K/AKT/FOXO3 Axis in Sarcopenia: A Narrative Review

The high prevalence of sarcopenia in an aging population has an underestimated impact on quality of life by increasing the risk of falls and subsequent hospitalization. Unfortunately, the application of the major established key therapeutic-physical activity-is challenging in the immobile and injured sarcopenic patient. Consequently, novel therapeutic directions are needed. The transcription factor Forkhead-Box-Protein O3 (FOXO3) may be an option, as it and its targets have been observed to be more highly expressed in sarcopenic muscle. In such catabolic situations, Foxo3 induces the expression of two muscle specific ubiquitin ligases (Atrogin-1 and Murf-1) via the PI3K/AKT pathway. In this review, we particularly evaluate the potential of Foxo3-targeted gene therapy. Foxo3 knockdown has been shown to lead to increased muscle cross sectional area, through both the AKT-dependent and -independent pathways and the reduced impact on the two major downstream targets Atrogin-1 and Murf-1. Moreover, a Foxo3 reduction suppresses apoptosis, activates satellite cells, and initiates their differentiation into muscle cells. While this indicates a critical role in muscle regeneration, this mechanism might exhaust the stem cell pool, limiting its clinical applicability. As systemic Foxo3 knockdown has also been associated with risks of inflammation and cancer progression, a muscle-specific approach would be necessary. In this review, we summarize the current knowledge on Foxo3 and conceptualize a specific and targeted therapy that may circumvent the drawbacks of systemic Foxo3 knockdown. This approach presumably would limit the side effects and enable an activity-independent positive impact on skeletal muscle.

Resistance exercise alleviates dexamethasone-induced muscle atrophy via Sestrin2/MSTN pathway in C57BL/6J mice

AIM

It has long been recognized that resistance exercise can substantially increase skeletal muscle mass and strength, but whether it can protect against glucocorticoid-induced muscle atrophy and its potential mechanism is yet to be determined. This study aimed to investigate the protective effects of resistance exercise in dexamethasone-induced muscle atrophy and elucidate the possible function of exercise-induced protein Sestrin2 in this process.

METHODS

Eight-week-old male C57BL/6J mice carried out the incremental mouse ladder exercise for 11 weeks. Two weeks before the end of the intervention, mice were daily intraperitoneally injected with dexamethasone. Body composition, muscle mass, and exercise performance were examined to evaluate muscle atrophy. In vitro, C2C12 cells were used for RT-qPCR, Western Blot, and immunofluorescence experiments to elucidate the potential mechanism.

RESULTS

Our results showed that long-term resistance exercise is an effective intervention for dexamethasone-induced muscle atrophy. We also found that Sestrin2 plays a vital role in dexamethasone-induced muscle atrophy. In both animal (P = .0006) and cell models (P = .0266), dexamethasone intervention significantly reduced the protein expression of Sestrin2, which was increased (P = .0112) by resistance exercise. Inversely, overexpression of Sestrin2 improved (P < .0001) dexamethasone-induced myotube cell atrophy by reducing the activation of the ubiquitin-proteasome pathway via inhibiting Forkhead box O3 (FoxO3a) and myostatin (MSTN)/small mother against decapentaplegic (Smad) signaling pathways.

CONCLUSION

Taken together, our results indicated that Sestrin2 may serve as an effective molecule that mimics the protective effect of resistance exercise on dexamethasone-induced muscle atrophy.

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.

FoxO3 normalizes Smad3-induced arterial smooth muscle cell growth

Transition of arterial smooth muscle (ASM) from a quiescent, contractile state to a growth-promoting state is a hallmark of cardiovascular disease (CVD), a leading cause of death and disability in the United States and worldwide. While many individual signals have been identified as important mechanisms in this phenotypic conversion, the combined impact of the transcription factors Smad3 and FoxO3 in ASM growth is not known. The purpose of this study was to determine that a coordinated, phosphorylation-specific relationship exists between Smad3 and FoxO3 in the control of ASM cell growth. Using a rat in vivo arterial injury model and rat primary ASM cell lysates and fractions, validated low and high serum in vitro models of respective quiescent and growth states, and adenoviral (Ad-) gene delivery for overexpression (OE) of individual and combined Smad3 and/or FoxO3, we hypothesized that FoxO3 can moderate Smad3-induced ASM cell growth. Key findings revealed unique cellular distribution of Smad3 and FoxO3 under growth conditions, with induction of both nuclear and cytosolic Smad3 yet primarily cytosolic FoxO3; Ad-Smad3 OE leading to cytosolic and nuclear expression of phosphorylated and total Smad3, with almost complete reversal of each with Ad-FoxO3 co-infection in quiescent and growth conditions; Ad-FoxO3 OE leading to enhanced cytosolic expression of phosphorylated and total FoxO3, both reduced with Ad-Smad3 co-infection in quiescent and growth conditions; Ad-FoxO3 inducing expression and activity of the ubiquitin ligase MuRF-1, which was reversed with concomitant Ad-Smad3 OE; and combined Smad3/FoxO3 OE reversing both the pro-growth impact of singular Smad3 and the cytostatic impact of singular FoxO3. A primary takeaway from these observations is the capacity of FoxO3 to reverse growth-promoting effects of Smad3 in ASM cells. Additional findings lend support for reciprocal antagonism of Smad3 on FoxO3-induced cytostasis, and these effects are dependent upon discrete phosphorylation states and cellular localization and involve MuRF-1 in the control of ASM cell growth. Lastly, results showing capacity of FoxO3 to normalize Smad3-induced ASM cell growth largely support our hypothesis, and overall findings provide evidence for utility of Smad3 and/or FoxO3 as potential therapeutic targets against abnormal ASM growth in the context of CVD.

Endoplasmic reticulum stress promotes sepsis-induced muscle atrophy via activation of STAT3 and Smad3

Endoplasmic reticulum (ER) stress is involved in skeletal muscle atrophy in various conditions, but the role of ER stress in sepsis-induced muscle atrophy is not well understood. In this study, we conducted experiments in wild-type (WT) mice and C/EBP homologous protein knockout (CHOP KO) mice to explore the role and mechanism of ER stress in sepsis-induced muscle atrophy. Cecal ligation and puncture (CLP) was used to establish a mouse model of sepsis. In WT mice, the body weight, muscle mass, and cross-sectional area of muscle fibers in CLP group both decreased significantly compared with sham group, which revealed that sepsis-induced dramatic muscle atrophy. Additionally, sepsis activated the ubiquitin-proteasome system (UPS), accompanied by the activation of ER stress. In vitro, inhibition of ER stress suppressed the activity of E3 ubiquitin ligases and alleviated the myotube atrophy. In vivo, CHOP KO also reduced the expression of E3 ubiquitin ligases and UPS-mediated protein degradation, and significantly attenuated sepsis-induced muscle atrophy. Deletion of CHOP also decreased the phosphorylation of signal transducer and activator of transcription 3 (STAT3) and Smad3, and inhibition of STAT3 and Smad3 partly reduced proteolysis caused by ER stress in vitro. These findings confirm that ER stress activates UPS-mediated proteolysis and promotes sepsis-induced muscle atrophy, which is partly achieved by activating STAT3 and Smad3.

Astragaloside IV alleviates sepsis-induced muscle atrophy by inhibiting the TGF-β1/Smad signaling pathway

BACKGROUND

Muscle atrophy occurs in patients with sepsis and increases mortality and disability. Remission of muscle atrophy may improve the quality of life in patients with sepsis. Astragaloside IV (ASIV) has been shown to have excellent anti-inflammatory and anti-fibrotic effects and to reduce organ damage caused by sepsis. However, the effect of ASIV on sepsis-induced muscle atrophy has not been reported. Therefore, this study explored the pharmacological effects and mechanisms of ASIV in sepsis-induced muscle atrophy.

METHODS

Cecal ligation and puncture (CLP) was used to establish a mouse model of sepsis and lipopolysaccharide (LPS)-stimulated C2C12 myotubes. After administration of ASIV, the body weight, tibialis anterior (TA) and gastrocnemius muscle weight and fiber cross-sectional area of the mice were measured. The diameter of myotubes was observed by immunofluorescence staining. ELISA was used to assess inflammatory factors in plasma and cell culture supernatants. RT-PCR and Western blotting were used to detect the expression of MuRF1, Atrogin-1 and TGF-β1/Smad signaling pathway components in TA and C2C12 myotubes.

RESULTS

Our study found that ASIV reduced serum inflammatory factors and improved survival in septic mice. ASIV alleviated muscle mass reduction, myofiber cross-sectional area reduction, and C2C12 myotube atrophy by inhibiting the expression of the E3 ubiquitin ligases MuRF1 and atrogin-1. In addition, we observed that ASIV inhibited TGF-β1/Smad signaling. Inhibition of the TGF-β1/Smad signaling pathway partly blocked the anti-muscle atrophy effect of ASIV.

CONCLUSION

ASIV can alleviate sepsis-induced muscle atrophy, which may be related to the inhibition of the TGF-β1/Smad signaling pathway.

Forkhead Box O Signaling Pathway in Skeletal Muscle Atrophy

Skeletal muscle atrophy is the consequence of protein degradation exceeding protein synthesis because of disease, aging, and physical inactivity. Patients with skeletal muscle atrophy have decreased muscle mass and fiber cross-sectional area, and experience reduced survival quality and motor function. The forkhead box O (FOXO) signaling pathway plays an important role in the pathogenesis of skeletal muscle atrophy by regulating E3 ubiquitin ligases and some autophagy factors. However, the mechanism of FOXO signaling pathway leading to skeletal muscle atrophy is still unclear. The development of treatment strategies for skeletal muscle atrophy has been a thorny clinical problem. FOXO-targeted therapy to treat skeletal muscle atrophy is a promising approach, and an increasing number of relevant studies have been reported. This article reviews the mechanism and therapeutic targets of the FOXO signaling pathway mediating skeletal muscle atrophy, and provides ideas for the clinical treatment of this condition.

Ubiquitin E3 ligase Atrogin-1 protein is regulated via the rapamycin-sensitive mTOR-S6K1 signaling pathway in C2C12 muscle cells

Atrogin-1 and MuRF1 are highly expressed in multiple conditions of skeletal muscle atrophy. The PI3K/Akt/FoxO signaling pathway is well known to regulate Atrogin-1 and MuRF1 gene expressions. However, Akt activation also activates the mammalian target of rapamycin complex 1 (mTORC1) which induces skeletal muscle hypertrophy. Whether mTORC1-dependent signaling has a role in regulating Atrogin-1 and/or MuRF1 gene and protein expression is currently unclear. In this study, we showed that activation of insulin-mediated Akt signaling suppresses both Atrogin-1 and MuRF1 protein contents and that inhibition of Akt increases both Atrogin-1 and MuRF1 protein contents in C2C12 myotubes. Interestingly, inhibition of mTORC1 using a specific mTORC1 inhibitor, rapamycin, increased Atrogin-1, but not MuRF1, protein content. Furthermore, activation of AMP-activated protein kinase (AMPK), a negative regulator of the mTORC1 signaling pathway, also showed distinct time-dependent changes between Atrogin-1 and MuRF1 protein contents, suggesting differential regulatory mechanisms between Atrogin-1 and MuRF1 protein content. To further explore the downstream of mTORC1 signaling, we employed a specific S6K1 inhibitor, PF-4708671. We found that Atrogin-1 protein content was dose-dependently increased with PF-4708671 treatment, whereas MuRF1 protein content was decreased at 50 μM of PF-4708671 treatment. However, MuRF1 protein content was unexpectedly increased when treated with PF-4708671 for a longer period. Overall, our results indicate that Atrogin-1 and MuRF1 protein contents are regulated by different mechanisms, the downstream of Akt, and that Atrogin-1 protein content can be regulated by rapamycin-sensitive mTOR-S6K1 dependent signaling pathway.

Age Related Changes in Muscle Mass and Force Generation in the Triple Transgenic (3xTgAD) Mouse Model of Alzheimer's Disease

Emerging evidence suggests that patients with Alzheimer's disease (AD) may show accelerated sarcopenia phenotypes. To investigate whether pathological changes associated with neuronal death and cognitive dysfunction also occur in peripheral motor neurons and muscle as a function of age, we used the triple transgenic mouse model of AD (3xTgAD mice) that carries transgenes for mutant forms of APP, Tau, and presenilin proteins that are associated with AD pathology. We measured changes in motor neurons and skeletal muscle function and metabolism in young (2 to 4 month) female control and 3xTgAD mice and in older (18-20 month) control and 3xTgAD female mice. In older 3xTgAD mice, we observed a number of sarcopenia-related phenotypes, including significantly fragmented and denervated neuromuscular junctions (NMJs) associated with a 17% reduction in sciatic nerve induced vs. direct muscle stimulation induced contractile force production, and a 30% decrease in gastrocnemius muscle mass. On the contrary, none of these outcomes were found in young 3xTgAD mice. We also measured an accumulation of amyloid-β (Aβ) in both skeletal muscle and neuronal tissue in old 3xTgAD mice that may potentially contribute to muscle atrophy and NMJ disruption in the older 3xTgAD mice. Furthermore, the TGF-β mediated atrophy signaling pathway is activated in old 3xTgAD mice and is a potential contributing factor in the muscle atrophy that occurs in this group. Perhaps surprisingly, mitochondrial oxygen consumption and reactive oxygen species (ROS) production are not elevated in skeletal muscle from old 3xTgAD mice. Together, these results provide new insights into the effect of AD pathological mechanisms on peripheral changes in skeletal muscle.

Out of Control: The Role of the Ubiquitin Proteasome System in Skeletal Muscle during Inflammation

The majority of critically ill intensive care unit (ICU) patients with severe sepsis develop ICU-acquired weakness (ICUAW) characterized by loss of muscle mass, reduction in myofiber size and decreased muscle strength leading to persisting physical impairment. This phenotype results from a dysregulated protein homeostasis with increased protein degradation and decreased protein synthesis, eventually causing a decrease in muscle structural proteins. The ubiquitin proteasome system (UPS) is the predominant protein-degrading system in muscle that is activated during diverse muscle atrophy conditions, e.g., inflammation. The specificity of UPS-mediated protein degradation is assured by E3 ubiquitin ligases, such as atrogin-1 and MuRF1, which target structural and contractile proteins, proteins involved in energy metabolism and transcription factors for UPS-dependent degradation. Although the regulation of activity and function of E3 ubiquitin ligases in inflammation-induced muscle atrophy is well perceived, the contribution of the proteasome to muscle atrophy during inflammation is still elusive. During inflammation, a shift from standard- to immunoproteasome was described; however, to which extent this contributes to muscle wasting and whether this changes targeting of specific muscular proteins is not well described. This review summarizes the function of the main proinflammatory cytokines and acute phase response proteins and their signaling pathways in inflammation-induced muscle atrophy with a focus on UPS-mediated protein degradation in muscle during sepsis. The regulation and target-specificity of the main E3 ubiquitin ligases in muscle atrophy and their mode of action on myofibrillar proteins will be reported. The function of the standard- and immunoproteasome in inflammation-induced muscle atrophy will be described and the effects of proteasome-inhibitors as treatment strategies will be discussed.

Ubiquitin Ligases at the Heart of Skeletal Muscle Atrophy Control

Skeletal muscle loss is a detrimental side-effect of numerous chronic diseases that dramatically increases mortality and morbidity. The alteration of protein homeostasis is generally due to increased protein breakdown while, protein synthesis may also be down-regulated. The ubiquitin proteasome system (UPS) is a master regulator of skeletal muscle that impacts muscle contractile properties and metabolism through multiple levers like signaling pathways, contractile apparatus degradation, etc. Among the different actors of the UPS, the E3 ubiquitin ligases specifically target key proteins for either degradation or activity modulation, thus controlling both pro-anabolic or pro-catabolic factors. The atrogenes MuRF1/TRIM63 and MAFbx/Atrogin-1 encode for key E3 ligases that target contractile proteins and key actors of protein synthesis respectively. However, several other E3 ligases are involved upstream in the atrophy program, from signal transduction control to modulation of energy balance. Controlling E3 ligases activity is thus a tempting approach for preserving muscle mass. While indirect modulation of E3 ligases may prove beneficial in some situations of muscle atrophy, some drugs directly inhibiting their activity have started to appear. This review summarizes the main signaling pathways involved in muscle atrophy and the E3 ligases implicated, but also the molecules potentially usable for future therapies.

Sarcopenia - Molecular mechanisms and open questions

Sarcopenia represents a muscle-wasting syndrome characterized by progressive and generalized degenerative loss of skeletal muscle mass, quality, and strength occurring during normal aging. Sarcopenia patients are mainly suffering from the loss in muscle strength and are faced with mobility disorders reducing their quality of life and are, therefore, at higher risk for morbidity (falls, bone fracture, metabolic diseases) and mortality. Several molecular mechanisms have been described as causes for sarcopenia that refer to very different levels of muscle physiology. These mechanisms cover e. g. function of hormones (e. g. IGF-1 and Insulin), muscle fiber composition and neuromuscular drive, myo-satellite cell potential to differentiate and proliferate, inflammatory pathways as well as intracellular mechanisms in the processes of proteostasis and mitochondrial function. In this review, we describe sarcopenia as a muscle-wasting syndrome distinct from other atrophic diseases and summarize the current view on molecular causes of sarcopenia development as well as open questions provoking further research efforts for establishing efficient lifestyle and therapeutic interventions.

Reduction of circulating sphingosine-1-phosphate worsens mdx soleus muscle dystrophic phenotype

NEW FINDINGS

What is the central question of the study? What are the consequences of reducing circulating sphingosine-1-phosphate (S1P) for muscle physiology in the murine model of Duchenne muscular dystrophy (DMD)? What is the main result and its importance? Reduction of the circulating S1P level in mdx mice aggravates the dystrophic phenotype, as seen by an increase in fibre atrophy, fibrosis and loss of specific force, suggesting that S1P signalling is a potential therapeutic target in DMD. Although further studies are needed, plasma S1P levels have the intriguing possibility of being used as a biomarker for disease severity, an important issue in DMD.

ABSTRACT

Sphingosine-1-phosphate (S1P) is an important regulator of skeletal muscle properties. The dystrophin-deficient mdx mouse possesses low levels of S1P (∼50%) compared with wild type. Increased S1P availability was demonstrated to ameliorate the dystrophic phenotype in Drosophila and in mdx mice. Here, we analysed the effects produced by further reduction of S1P availability on the mass, force and regenerative capacity of dystrophic mdx soleus. Circulating S1P was neutralized by a specific anti-S1P antibody (S1P-Ab) known to lower the extracellular concentration of this signalling lipid. The S1P-Ab was administered intraperitoneally in adult mdx mice every 2 days for the duration of experiments. Soleus muscle properties were analysed 7 or 14 days after the first injection. The decreased availability of circulating S1P after the 14 day treatment reduced mdx soleus fibre cross-sectional area (-16%, P < 0.05), an effect that was associated with an increase in markers of proteolytic (MuRF1 and atrogin-1) and autophagic (p62 and LC3-II/LC3-I ratio) pathways. Moreover, an increase of fibrosis was also observed (+26%, P < 0.05). Notably, the treatment also caused a reduction of specific tetanic tension (-29%, P < 0.05). The mdx soleus regenerative capacity was only slightly influenced by reduced S1P. In conclusion, neutralization of circulating S1P reduces the mass and specific force and increases fibrosis of mdx soleus muscle, thus worsening the dystrophic phenotype. The results confirm that active, functional S1P signalling might counteract the progression of soleus mdx pathology and validate the pathway as a potential therapeutic target for muscular dystrophies.

MuRF1/TRIM63, Master Regulator of Muscle Mass

The E3 ubiquitin ligase MuRF1/TRIM63 was identified 20 years ago and suspected to play important roles during skeletal muscle atrophy. Since then, numerous studies have been conducted to decipher the roles, molecular mechanisms and regulation of this enzyme. This revealed that MuRF1 is an important player in the skeletal muscle atrophy process occurring during catabolic states, making MuRF1 a prime candidate for pharmacological treatments against muscle wasting. Indeed, muscle wasting is an associated event of several diseases (e.g., cancer, sepsis, diabetes, renal failure, etc.) and negatively impacts the prognosis of patients, which has stimulated the search for MuRF1 inhibitory molecules. However, studies on MuRF1 cardiac functions revealed that MuRF1 is also cardioprotective, revealing a yin and yang role of MuRF1, being detrimental in skeletal muscle and beneficial in the heart. This review discusses data obtained on MuRF1, both in skeletal and cardiac muscles, over the past 20 years, regarding the structure, the regulation, the location and the different functions identified, and the first inhibitors reported, and aim to draw the picture of what is known about MuRF1. The review also discusses important MuRF1 characteristics to consider for the design of future drugs to maintain skeletal muscle mass in patients with different pathologies.

Edward F. Adolph Distinguished Lecture. Skeletal muscle atrophy: Multiple pathways leading to a common outcome

Skeletal muscle atrophy continues to be a serious consequence of many diseases and conditions for which there is no treatment. Our understanding of the mechanisms regulating skeletal muscle mass has improved considerably over the past two decades. For many years it was known that skeletal muscle atrophy resulted from an imbalance between protein synthesis and protein breakdown, with the net balance shifting toward protein breakdown. However, the molecular and cellular mechanisms underlying the increased breakdown of myofibrils was unknown. Over the past two decades, numerous reports have identified novel genes and signaling pathways that are upregulated and activated in response to stimuli such as disuse, inflammation, metabolic stress, starvation and others that induce muscle atrophy. This review summarizes the discovery efforts performed in the identification of several pathways involved in the regulation of skeletal muscle mass: the mammalian target of rapamycin (mTORC1) and the ubiquitin proteasome pathway and the E3 ligases, MuRF1 and MAFbx. While muscle atrophy is a common outcome of many diseases, it is doubtful that a single gene or pathway initiates or mediates the breakdown of myofibrils. Interestingly, however, is the observation that upregulation of the E3 ligases, MuRF1 and MAFbx, is a common feature of many divergent atrophy conditions. The challenge for the field of muscle biology is to understand how all of the various molecules, transcription factors, and signaling pathways interact to produce muscle atrophy and to identify the critical factors for intervention.

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.

Role of FoxO3a as a negative regulator of the cardiac myofibroblast conversion induced by TGF-β1

Cardiac fibroblasts (CFs) are necessary to maintain extracellular matrix (ECM) homeostasis in the heart. Normally, CFs are quiescent and secrete small amounts of ECM components, whereas, in pathological conditions, they differentiate into more active cells called cardiac myofibroblasts (CMF). CMF conversion is characteristic of cardiac fibrotic diseases, such as heart failure and diabetic cardiomyopathy. TGF-β1 is a key protein involved in CMF conversion. SMADs are nuclear factor proteins activated by TGF-β1 that need other proteins, such as forkhead box type O (FoxO) family members, to promote CMF conversion. FoxO1, a member of this family protein, is necessary for TGF-β1-induced CMF conversion, whereas the role of FoxO3a, another FoxO family member, is unknown. FoxO3a plays an important role in many fibrotic processes in the kidney and lung. However, the participation of FoxO3a in the conversion of CFs into CMF is not clear. In this paper, we demonstrate that TGF-β1 decreases the activation and expression of FoxO3a in CFs. FoxO3a regulation by TGF-β1 requires activated SMAD3, ERK1/2 and Akt. Furthermore, we show that FoxO1 is crucial in the FoxO3a regulation induced by TGF-β1, as shown by overexpressed FoxO1 enhancing and silenced FoxO1 suppressing the effects of TGF-β1 on FoxO3a. Finally, the regulation of TGF-β1-induced CMF conversion was enhanced by FoxO3a silencing and suppressed by inhibited FoxO3a degradation. Considering these collective findings, we suggest that FoxO3a acts as a negative regulator of the CMF conversion that is induced by TGF-β1.

Loss of FoxOs in muscle reveals sex-based differences in insulin sensitivity but mitigates diet-induced obesity

OBJECTIVE

Gender influences obesity-related complications, including diabetes. Females are more protected from insulin resistance after diet-induced obesity, which may be related to fat accumulation and muscle insulin sensitivity. FoxOs regulate muscle atrophy and are targets of insulin action, but their role in muscle insulin sensitivity and mitochondrial metabolism is unknown.

METHODS

We measured muscle insulin signaling, mitochondrial energetics, and metabolic responses to a high-fat diet (HFD) in male and female muscle-specific FoxO1/3/4 triple knock-out (TKO) mice.

RESULTS

In male TKO muscle, insulin-stimulated AKT activation was decreased. AKT2 protein and mRNA levels were reduced and insulin receptor protein and IRS-2 mRNA decreased. These changes contributed to decreased insulin-stimulated glucose uptake in glycolytic muscle in males. In contrast, female TKOs maintain normal insulin-mediated AKT phosphorylation, normal AKT2 levels, and normal glucose uptake in glycolytic muscle. When challenged with a HFD, fat gain was attenuated in both male and female TKO mice, and associated with decreased glucose levels, improved glucose homeostasis, and reduced muscle triglyceride accumulation. Furthermore, female TKO mice showed increased energy expenditure, relative to controls, due to increased lean mass and maintenance of mitochondrial function in muscle.

CONCLUSIONS

FoxO deletion in muscle uncovers sexually dimorphic regulation of AKT2, which impairs insulin signaling in male mice, but not females. However, loss of FoxOs in muscle from both males and females also leads to muscle hypertrophy and increases in metabolic rate. These factors mitigate fat gain and attenuate metabolic abnormalities in response to a HFD.

FOXO3a from the Nucleus to the Mitochondria: A Round Trip in Cellular Stress Response

Cellular stress response is a universal mechanism that ensures the survival or negative selection of cells in challenging conditions. The transcription factor Forkhead box protein O3 (FOXO3a) is a core regulator of cellular homeostasis, stress response, and longevity since it can modulate a variety of stress responses upon nutrient shortage, oxidative stress, hypoxia, heat shock, and DNA damage. FOXO3a activity is regulated by post-translational modifications that drive its shuttling between different cellular compartments, thereby determining its inactivation (cytoplasm) or activation (nucleus and mitochondria). Depending on the stress stimulus and subcellular context, activated FOXO3a can induce specific sets of nuclear genes, including cell cycle inhibitors, pro-apoptotic genes, reactive oxygen species (ROS) scavengers, autophagy effectors, gluconeogenic enzymes, and others. On the other hand, upon glucose restriction, 5′-AMP-activated protein kinase (AMPK) and mitogen activated protein kinase kinase (MEK)/extracellular signal-regulated kinase (ERK) -dependent FOXO3a mitochondrial translocation allows the transcription of oxidative phosphorylation (OXPHOS) genes, restoring cellular ATP levels, while in cancer cells, mitochondrial FOXO3a mediates survival upon genotoxic stress induced by chemotherapy. Interestingly, these target genes and their related pathways are diverse and sometimes antagonistic, suggesting that FOXO3a is an adaptable player in the dynamic homeostasis of normal and stressed cells. In this review, we describe the multiple roles of FOXO3a in cellular stress response, with a focus on both its nuclear and mitochondrial functions.

Protection of dilator function of coronary arteries from homocysteine by tetramethylpyrazine: Role of ER stress in modulation of BKCa channels

OBJECTIVES

We recently reported the involvement of ER stress-mediated BK_Ca channel inhibition in homocysteine-induced coronary dilator dysfunction. In another study, we demonstrated that tetramethylpyrazine (TMP), an active ingredient of the Chinese herb Chuanxiong, possesses potent anti-ER stress capacity. The present study investigated whether TMP protects BK_Ca channels from homocysteine-induced inhibition and whether suppression of ER stress is a mechanism contributing to the protection. Furthermore, we explored the signaling transduction involved in TMP-conferred protection on BK_Ca channels.

METHODS

BK_Ca channel-mediated relaxation was studied in porcine small coronary arteries. Expressions of BK_Ca channel subunits, ER stress molecules, and E3 ubiquitin ligases, as well as BK_Ca ubiquitination were determined in porcine coronary arterial smooth muscle cells (PCASMCs). Whole-cell BK_Ca currents were recorded.

RESULTS

Exposure of PCASMCs to homocysteine or the chemical ER stressor tunicamycin increased the expression of ER stress molecules, which was significantly inhibited by TMP. Suppression of ER stress by TMP preserved the BK_Ca β1 protein level and restored the BK_Ca current in PCASMCs, concomitant with an improved BK_Ca-mediated dilatation in coronary arteries. TMP attenuated homocysteine-induced BK_Ca β1 protein ubiquitination, in which inhibition of ER stress-mediated FoxO3a activation and FoxO3a-dependent atrogin-1 and Murf-1 was involved.

CONCLUSIONS

Reversal of BK_Ca channel inhibition via suppressing ER stress-mediated loss of β1 subunits contributes to the protective effect of TMP against homocysteine on coronary dilator function. Inhibition of FoxO3a-dependent ubiquitin ligases is involved in TMP-conferred normalization of BK_Ca β1 protein level. These results provide new mechanistic insights into the cardiovascular benefits of TMP.

Muscle Wasting Diseases: Novel Targets and Treatments

Adequate skeletal muscle plasticity is an essential element for our well-being, and compromised muscle function can drastically affect quality of life, morbidity, and mortality. Surprisingly, however, skeletal muscle remains one of the most under-medicated organs. Interventions in muscle diseases are scarce, not only in neuromuscular dystrophies, but also in highly prevalent secondary wasting pathologies such as sarcopenia and cachexia. Even in other diseases that exhibit a well-established risk correlation of muscle dysfunction due to a sedentary lifestyle, such as type 2 diabetes or cardiovascular pathologies, current treatments are mostly targeted on non-muscle tissues. In recent years, a renewed focus on skeletal muscle has led to the discovery of various novel drug targets and the design of new pharmacological approaches. This review provides an overview of the current knowledge of the key mechanisms involved in muscle wasting conditions and novel pharmacological avenues that could ameliorate muscle diseases.

Peptidyl-prolyl cis-trans isomerase NIMA interacting 1 regulates skeletal muscle fusion through structural modification of Smad3 in the linker region

Myoblast fusion is critical for muscle growth, regeneration, and repair. We previously reported that the enzyme peptidyl‐prolyl cis–trans isomerase NIMA interacting 1 (Pin1) is involved in osteoclast fusion. The objective of this study was to investigate the possibility that Pin1 also inhibits myoblast fusion. Here, we show the increased number of nuclei in the Pin1^+/− mice muscle fiber compared to that in wild‐type mice. Moreover, we show that low dose of the Pin1 inhibitor dipentamethylene thiuram monosulfide treatment caused enhanced fusion in C2C12 cells. The R‐Smads are well‐known mediators of muscle hypertrophy and hyperplasia as well as being substrates of Pin1. We found that Pin1 is crucial for maintaining the stability of Smad3 (homologues of the Drosophila protein, mothers against decapentaplegic (Mad) and the Caenorhabditis elegans protein Sma). Our results show that serine 204 within Smad3 is the key Pin1‐binding site during inhibition of myoblast fusion and that both the transforming growth factor‐β receptor and extracellular signal‐regulated kinase (ERK)‐mediated phosphorylation are required for the interaction of Pin1 with Smad3. These findings suggest that a precise level of Pin1 activity is essential for regulating myoblast fusion during myogenesis and muscle regeneration.

MYOD and HAND transcription factors have conserved recognition sites in mTOR promoter: insights from in silico analysis

mTOR regulates multiple cellular processes that are critical for proper maintenance of cell growth and development. However, mechanisms and factors responsible for transcriptional regulation of mTOR are partially known. To identify different transcription factor binding sites in promoter region of mTOR, we performed in silico phylogenetic foot printing analysis of diverse set of human orthologs. Phylogenetic tree for the orthologs was generated to establish the evolutionary relationships among them. Conserved binding sites among the species were predicted by tool MEME. The predicted conserved sites were further analyzed for binding of transcription factors by MatInspector program. Predicted TFs were then integrated with known physical interactions and coexpression data to decipher the important transcriptional regulators of mTOR signaling. Our study suggests that motifs AGGCGGG (+ 15 to + 21) and GGCGGC (+ 60 to + 65) are highly conserved across the species and are recognition sequence for HAND and MYOD transcription factors, respectively. Also these two transcription factors show direct physical interaction in protein-protein interaction map, indicating their regulatory role on expression of mTOR for control of myogenesis. Our study provides novel clues on differential regulation of mTOR under diverse environmental conditions.

Cyclic Nucleotide-Directed Protein Kinases in Cardiovascular Inflammation and Growth

Cardiovascular disease (CVD), including myocardial infarction (MI) and peripheral or coronary artery disease (PAD, CAD), remains the number one killer of individuals in the United States and worldwide, accounting for nearly 18 million (>30%) global deaths annually. Despite considerable basic science and clinical investigation aimed at identifying key etiologic components of and potential therapeutic targets for CVD, the number of individuals afflicted with these dreaded diseases continues to rise. Of the many biochemical, molecular, and cellular elements and processes characterized to date that have potential to control foundational facets of CVD, the multifaceted cyclic nucleotide pathways continue to be of primary basic science and clinical interest. Cyclic adenosine monophosphate (cyclic AMP) and cyclic guanosine monophosphate (cyclic GMP) and their plethora of downstream protein kinase effectors serve ubiquitous roles not only in cardiovascular homeostasis but also in the pathogenesis of CVD. Already a major target for clinical pharmacotherapy for CVD as well as other pathologies, novel and potentially clinically appealing actions of cyclic nucleotides and their downstream targets are still being discovered. With this in mind, this review article focuses on our current state of knowledge of the cyclic nucleotide-driven serine (Ser)/threonine (Thr) protein kinases in CVD with particular emphasis on cyclic AMP-dependent protein kinase (PKA) and cyclic GMP-dependent protein kinase (PKG). Attention is given to the regulatory interactions of these kinases with inflammatory components including interleukin 6 signals, with G protein-coupled receptor and growth factor signals, and with growth and synthetic transcriptional platforms underlying CVD pathogenesis. This article concludes with a brief discussion of potential future directions and highlights the importance for continued basic science and clinical study of cyclic nucleotide-directed protein kinases as emerging and crucial controllers of cardiac and vascular disease pathologies.

Palmitate and oleate co-treatment increases myocellular protein content via impaired protein degradation

OBJECTIVES

Protein balance is a crucial determinant of myocellular size and function. The effects of fatty acids on myocellular protein balance remain controversial. The aim of this study was to determine the direct effects of a mixed-species fatty acid environment on myocellular protein synthesis and degradation.

METHODS

C2C12 myotubes were cultured in media containing equimolar (250 μM) palmitic acid and oleate (PO) or bovine serum albumin control for ≤72 h. Myocellular protein balance was determined via incorporation (synthesis) or release (degradation) of ³H-tyrosine after 24, 48, and 72 h of treatment. Expression of major proteolytic genes was measured by reverse transcription polymerase chain reaction.

RESULTS

PO significantly increased myocellular protein content at 24, 48, and 72 h. Basal myocellular protein synthesis was unchanged by PO. However, PO significantly decreased basal rate of protein degradation at 24 h and this effect persisted throughout 72 h of treatment. Expression of the proteolytic genes Atrogin-1 (MAFbx), MuRF-1, LC3, and ATG4 B, was reduced during the 72 h PO.

CONCLUSIONS

A mixed-species fatty acid environment increases myocellular protein content by decreasing the rate of protein degradation, which may be regulated at the level of gene transcription.

Molecular Cloning and Motif Identification of the Sheep Musclin Gene Promoter

Musclin is a bioactive factor that functions in regulating the muscle growth and metabolism. To investigate the transcriptional regulatory mechanism of the gene, the 1.4 kb musclin promoter in sheep was cloned (GenBank accession: JX966391) and the sequence was analyzed to predict the motifs associated with muscle growth. Next the enhanced green fluorescent protein (EGFP) was selected as the reporter gene and various wild-type and motif-mutant vectors were constructed. The transcriptional regulatory activities were compared by observing the fluorescence strength and detecting the EGFP mRNA expression in C2C12 myoblasts transfected with the vectors. The results showed that the different lengths of promoters could drive the transcription of EGFP and the mutation of some motifs up- or downregulated the activity of the promoter. Furthermore, the electrophoresis mobility shift assay showed that these motifs regulated the musclin gene transcription through binding to the corresponding transcriptional factors in sheep muscle tissue.

Epigenetics of cancer-associated muscle catabolism

Cancer patients are commonly affected by cachexia, a wasting process involving muscle and fat. Specifically, loss of the muscle compartment has been associated with poor prognosis and suboptimal response to therapy. Nutritional support has been ineffective in treating this process leading to investigations into the underlying molecular processes governing muscle catabolism. In this commentary, we discuss the molecular mechanisms of cancer-associated muscle metabolism and the epigenetic processes responsible for the muscle wasting phenotype. Ultimately, we highlight how the epigenome may serve as a promising therapeutic target in reversing cancer-associated muscle catabolism.

Osteolytic Breast Cancer Causes Skeletal Muscle Weakness in an Immunocompetent Syngeneic Mouse Model

Muscle weakness and cachexia are significant paraneoplastic syndromes of many advanced cancers. Osteolytic bone metastases are common in advanced breast cancer and are a major contributor to decreased survival, performance, and quality of life for patients. Pathologic fracture caused by osteolytic cancer in bone (OCIB) leads to a significant (32%) increased risk of death compared to patients without fracture. Since muscle weakness is linked to risk of falls which are a major cause of fracture, we have investigated skeletal muscle response to OCIB. Here, we show that a syngeneic mouse model of OCIB (4T1 mammary tumor cells) leads to cachexia and skeletal muscle weakness associated with oxidation of the ryanodine receptor and calcium (Ca²⁺) release channel (RyR1). Muscle atrophy follows known pathways via both myostatin signaling and expression of muscle-specific ubiquitin ligases, atrogin-1 and MuRF1. We have identified a mechanism for skeletal muscle weakness due to increased oxidative stress on RyR1 via NAPDH oxidases [NADPH oxidase 2 (Nox2) and NADPH oxidase 4 (Nox4)]. In addition, SMAD3 phosphorylation is higher in muscle from tumor-bearing mice, a critical step in the intracellular signaling pathway that transmits TGFβ signaling to the nucleus. This is the first time that skeletal muscle weakness has been described in a syngeneic model of OCIB and represents a unique model system in which to study cachexia and changes in skeletal muscle.

Popdc1/Bves Functions in the Preservation of Cardiomyocyte Viability While Affecting Rac1 Activity and Bnip3 Expression

The Popeye domain containing1, also called Bves (Popdc1/Bves), is a transmembrane protein that functions in muscle regeneration, heart rate regulation, hypoxia tolerance, and ischemia preconditioning. The expression of Popdc1/Bves is elevated in cardiomyocytes maintained in serum free defined medium. We hypothesized that Popdc1/Bves is important for cardiomyocyte survival under the stress of serum deprivation and investigated the mechanisms involved. A deficit in Popdc1/Bves, achieved by siRNA-mediated gene silencing, results in cardiomyocyte injury and death, upregulation of the pro-apoptotic protein Bcl-2/adenovirus E1B 19-kDa interacting protein3 (Bnip3), as well as reduction in Rac1-GTPase activity and in Akt phosphorylation. Combined Popdc1/Bves and Bnip3 silencing attenuated cell injury and prevented Bnip3 upregulation induced by the silencing of Popdc1/Bves alone. Chromatin immunoprecipitation indicated an increased binding of the transcription factor FoxO3 to the Bnip3 promoter although augmentation of FoxO3 in the nuclei was not detected. By contrast, the transcription factor NFκB was excluded from the nuclei of Popdc1/Bves deficient cardiomyocytes and exhibited decreased binding to the Bnip3 promoter. The data indicates that Popdc1/Bves plays a role in the preservation of cardiomyocyte viability under serum deficiency through the alteration of Rac1 activity and the regulation of Bnip3 expression by FoxO3 and NFκB transcription factors pointing to Popdc1/Bves as a potential target to enhance heart protection. J. Cell. Biochem. 118: 1505-1517, 2017. © 2016 Wiley Periodicals, Inc.

PGC-1α over-expression suppresses the skeletal muscle atrophy and myofiber-type composition during hindlimb unloading

Disuse leads to severe muscle atrophy and a slow-to-fast myofiber-type transition. PGC-1α (Peroxisome proliferator-activated receptor γ coactivator 1α) is documented to play an important role in muscle atrophy and slow-twitch myofiber determination. Transcription of atrophy-related Atrogin-1 by FoxO3 can be reduced by PGC-1α. While Smad3 augments FoxO3-induced Atrogin-1 and MuRF1 promoter activity. So PGC-1α, as a transcription co-activator, may regulate hindlimb unloading (HU)-induced myofiber-type transition and muscle atrophy through Smad3. Our results showed that transgenic PGC-1α mice resisted HU-induced muscle loss, atrophy-related genes expression, and slow-to-fast myofiber_-type transition. Furthermore, over-expression of PGC-1α resisted the increase in pSmad3 during muscle atrophy in vivo and in vitro. And, PGC-1α over-expression inhibited the expression of atrogenes via suppressing the phosphorylation of Smad3 in vitro. Thus, PGC-1α is effective in regulating myofiber-type transition during HU, and it alleviates skeletal muscle atrophy partially through suppressing the activation of Smad3.

Prediction and Validation of Transcription Factors Modulating the Expression of Sestrin3 Gene Using an Integrated Computational and Experimental Approach

SESN3 has been implicated in multiple biological processes including protection against oxidative stress, regulation of glucose and lipid metabolism. However, little is known about the factors and mechanisms controlling its gene expression at the transcriptional level. We performed in silico phylogenetic footprinting analysis of 5 kb upstream regions of a diverse set of human SESN3 orthologs for the identification of high confidence conserved binding motifs (BMo). We further analyzed the predicted BMo by a motif comparison tool to identify the TFs likely to bind these discovered motifs. Predicted TFs were then integrated with experimentally known protein-protein interactions and experimentally validated to delineate the important transcriptional regulators of SESN3. Our study revealed high confidence set of BMos (integrated with DNase I hypersensitivity sites) in the upstream regulatory regions of SESN3 that could be bound by transcription factors from multiple families including FOXOs, SMADs, SOXs, TCFs and HNF4A. TF-TF network analysis established hubs of interaction that include SMAD3, TCF3, SMAD2, HDAC2, SOX2, TAL1 and TCF12 as well as the likely protein complexes formed between them. We show using ChIP-PCR as well as over-expression and knock out studies that FOXO3 and SOX2 transcriptionally regulate the expression of SESN3 gene. Our findings provide an important roadmap to further our understanding on the regulation of SESN3.

Detection of Pancreatic Cancer-Induced Cachexia Using a Fluorescent Myoblast Reporter System and Analysis of Metabolite Abundance

The dire effects of cancer-induced cachexia undermine treatment and contribute to decreased survival rates. Therapeutic options for this syndrome are limited, and therefore efforts to identify signs of precachexia in cancer patients are necessary for early intervention. The applications of molecular and functional imaging that would enable a whole-body "holistic" approach to this problem may lead to new insights and advances for diagnosis and treatment of this syndrome. Here we have developed a myoblast optical reporter system with the purpose of identifying early cachectic events. We generated a myoblast cell line expressing a dual tdTomato:GFP construct that was grafted onto the muscle of mice-bearing human pancreatic cancer xenografts to provide noninvasive live imaging of events associated with cancer-induced cachexia (i.e., weight loss). Real-time optical imaging detected a strong tdTomato fluorescent signal from skeletal muscle grafts in mice with weight losses of only 1.2% to 2.7% and tumor burdens of only approximately 79 to 170 mm(3). Weight loss in cachectic animals was also associated with a depletion of lipid, cholesterol, valine, and alanine levels, which may provide informative biomarkers of cachexia. Taken together, our findings demonstrate the utility of a reporter system that is capable of tracking tumor-induced weight loss, an early marker of cachexia. Future studies incorporating resected tissue from human pancreatic ductal adenocarcinoma into a reporter-carrying mouse may be able to provide a risk assessment of cachexia, with possible implications for therapeutic development.

Effects of ageing on expression of the muscle-specific E3 ubiquitin ligases and Akt-dependent regulation of Foxo transcription factors in skeletal muscle

Controversy exists as to whether the muscle-specific E3 ubiquitin ligases MAFbx and MuRF1 are transcriptionally upregulated in the process of sarcopenia. In the present study, we investigated the effects of ageing on mRNA/protein expression of muscle-specific E3 ubiquitin ligases and Akt/Foxo signalling in gastrocnemius muscles of female mice. Old mice exhibited a typical sarcopenic phenotype, characterized by loss of muscle mass and strength, decreased amount of myofibrillar proteins, incidence of aberrant muscle fibres, and genetic signature to sarcopenia. Activation levels of Akt were lower in adult and old mice than in young mice. Consequently, Akt-mediated phosphorylation levels of Foxo1 and Foxo3 proteins were decreased. Nuclear levels of Foxo1 and Foxo3 proteins showed an overall increasing trend in old mice. MAFbx mRNA expression was decreased in old mice relative to adult mice, whereas MuRF1 mRNA expression was less affected by ageing. At the protein level, MAFbx was less affected by ageing, whereas MuRF1 was increased in old mice relative to adult mice, with ubiquitin-protein conjugates being increased with ageing. In conclusion, we provided evidence for no mRNA upregulation of muscle-specific E3 ubiquitin ligases and disconnection between their expression and Akt/Foxo signalling in sarcopenic mice. Their different responsiveness to ageing may reflect different roles in sarcopenia.

AMP-activated protein kinase inhibits transforming growth factor-β-mediated vascular smooth muscle cell growth: implications for a Smad-3-dependent mechanism

Dysfunctional vascular growth is a major contributor to cardiovascular disease, the leading cause of morbidity and mortality worldwide. Growth factor-induced activation of vascular smooth muscle cells (VSMCs) results in a phenotypic switch from a quiescent, contractile state to a proliferative state foundational to vessel pathology. Transforming growth factor-β (TGF-β) is a multifunctional signaling protein capable of growth stimulation via Smad signaling. Although Smad signaling is well characterized in many tissues, its role in VSM growth disorders remains controversial. Recent data from our lab and others implicate the metabolic regulator AMP-activated protein kinase (AMPK) in VSM growth inhibition. We hypothesized that AMPK inhibits VSMC proliferation by reducing TGF-β-mediated growth in a Smad-dependent fashion. Treatment of rat VSMCs with the AMPK agonist AICAR significantly decreased TGF-β-mediated activation of synthetic Smad2 and Smad3 and increased inhibitory Smad7. Flow cytometry and automated cell counting revealed that AICAR reversed TGF-β-mediated cell cycle progression at 24 h and elevated cell numbers at 48 h. TGF-β/Smad signaling increased the G0/G1 inducers cyclin D1/cyclin-dependent kinase (CDK) 4 and cyclin E/CDK2; however, AICAR reversed these events while increasing cytostatic p21. The specific role of Smad3 in AMPK-mediated reversal of TGF-β-induced growth was then explored using adenovirus-mediated Smad3 overexpression (Ad-Smad3). Ad-Smad3 cells increased cell cycle progression and cell numbers compared with Ad-GFP control cells, and these were restored to basal levels with concomitant AICAR treatment. These findings support a novel AMPK target in TGF-β/Smad3 for VSMC growth control and support continued investigation of AMPK as a possible therapeutic target for reducing vascular growth disorders.

Skeletal muscle myotubes in severe obesity exhibit altered ubiquitin-proteasome and autophagic/lysosomal proteolytic flux

OBJECTIVE

Whole-body protein metabolism is dysregulated with obesity. The goal of the study was to determine whether activity and expression of major protein degradation pathways are compromised specifically in human skeletal muscle with obesity.

METHODS

Primary human skeletal muscle (HSkM) cell cultures were utilized since cellular mechanisms can be studied absent of hormones and contractile activity that could independently influence metabolism. HSkM from 10 lean women (BMI ≤ 26.0 kg/m(2) ) and 8 women with severe obesity (BMI ≥ 39.0) were examined basally and when stimulated to atrophy (serum and amino acid starvation).

RESULTS

HSkM from obese donors had a lower proportion of type I myosin heavy chain and slower flux through the autophagic/lysosomal pathway. During starvation, flux through the ubiquitin-proteasome system diverged according to obesity status, with a decrease in lean subjects and an increase in HSkM from subjects with obesity. HSkM in obesity also displayed elevated proteasome activity despite no difference in proteasome content. Atrophy-related gene expression and myotube area were similar in myotubes derived from individuals with and without obesity under basal and starved conditions.

CONCLUSIONS

Our data indicate that muscle cells in lean individuals and in those with severe obesity have innate differences in management of protein degradation, which may explain their metabolic differences.

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.