Lysosomal proteolysis in skeletal muscle

Cisplatin triggers atrophy of skeletal C2C12 myotubes via impairment of Akt signalling pathway and subsequent increment activity of proteasome and autophagy systems

Cisplatin (cisPt) is an antineoplastic drug which causes an array of adverse effects on different organs and tissues, including skeletal muscle. In this work we show that cisPt behaves as a potent trigger to activate protein hypercatabolism in skeletal C2C12 myotubes. Within 24h of 50 μM cisPt administration, C2C12 myotubes displayed unchanged cell viability but showed a subset of hallmark signs typically recognized during atrophy, including severe reduction in body size, repression of Akt phosphorylation, transcriptional up-regulation of atrophy-related genes, such as atrogin-1, gabarap, beclin-1 and bnip-3, and loss of myogenic markers. As a consequence, proteasomal activity and formation of autophagosomes were remarkably increased in cisPt-treated myotubes, but forced stimulation of Akt pathway, as obtained through insulin administration or delivery of a constitutively activated Akt form, was sufficient to counter the cisPt-induced protein breakdown, leading to rescue of atrophic size. Overall, these results indicate that cisPt induces atrophy of C2C12 myotubes via activation of proteasome and autophagy systems, suggesting that the Akt pathway represents one sensitive target of cisPt molecular action in skeletal muscle.

The muscle fiber type-fiber size paradox: hypertrophy or oxidative metabolism?

An inverse relationship exists between striated muscle fiber size and its oxidative capacity. This relationship implies that muscle fibers, which are triggered to simultaneously increase their mass/strength (hypertrophy) and fatigue resistance (oxidative capacity), increase these properties (strength or fatigue resistance) to a lesser extent compared to fibers increasing either of these alone. Muscle fiber size and oxidative capacity are determined by the balance between myofibrillar protein synthesis, mitochondrial biosynthesis and degradation. New experimental data and an inventory of critical stimuli and state of activation of the signaling pathways involved in regulating contractile and metabolic protein turnover reveal: (1) higher capacity for protein synthesis in high compared to low oxidative fibers; (2) competition between signaling pathways for synthesis of myofibrillar proteins and proteins associated with oxidative metabolism; i.e., increased mitochondrial biogenesis via AMP-activated protein kinase attenuates the rate of protein synthesis; (3) relatively higher expression levels of E3-ligases and proteasome-mediated protein degradation in high oxidative fibers. These observations could explain the fiber type-fiber size paradox that despite the high capacity for protein synthesis in high oxidative fibers, these fibers remain relatively small. However, it remains challenging to understand the mechanisms by which contractile activity, mechanical loading, cellular energy status and cellular oxygen tension affect regulation of fiber size. Therefore, one needs to know the relative contribution of the signaling pathways to protein turnover in high and low oxidative fibers. The outcome and ideas presented are relevant to optimizing treatment and training in the fields of sports, cardiology, oncology, pulmonology and rehabilitation medicine.

Toll-like receptor 4 mediates lipopolysaccharide-induced muscle catabolism via coordinate activation of ubiquitin-proteasome and autophagy-lysosome pathways

Cachectic muscle wasting is a frequent complication of many inflammatory conditions, due primarily to excessive muscle catabolism. However, the pathogenesis and intervention strategies against it remain to be established. Here, we tested the hypothesis that Toll-like receptor 4 (TLR4) is a master regulator of inflammatory muscle catabolism. We demonstrate that TLR4 activation by lipopolysaccharide (LPS) induces C2C12 myotube atrophy via up-regulating autophagosome formation and the expression of ubiquitin ligase atrogin-1/MAFbx and MuRF1. TLR4-mediated activation of p38 MAPK is necessary and sufficient for the up-regulation of atrogin1/MAFbx and autophagosomes, resulting in myotube atrophy. Similarly, LPS up-regulates muscle autophagosome formation and ubiquitin ligase expression in mice. Importantly, autophagy inhibitor 3-methyladenine completely abolishes LPS-induced muscle proteolysis, while proteasome inhibitor lactacystin partially blocks it. Furthermore, TLR4 knockout or p38 MAPK inhibition abolishes LPS-induced muscle proteolysis. Thus, TLR4 mediates LPS-induced muscle catabolism via coordinate activation of the ubiquitin-proteasome and the autophagy-lysosomal pathways.

Mechanical ventilation-induced diaphragm disuse in humans triggers autophagy

RATIONALE

Controlled mechanical ventilation (CMV) results in atrophy of the human diaphragm. The autophagy-lysosome pathway (ALP) contributes to skeletal muscle proteolysis, but its contribution to diaphragmatic protein degradation in mechanically ventilated patients is unknown.

OBJECTIVES

To evaluate the autophagy pathway responses to CMV in the diaphragm and limb muscles of humans and to identify the roles of FOXO transcription factors in these responses.

METHODS

Muscle biopsies were obtained from nine control subjects and nine brain-dead organ donors. Subjects were mechanically ventilated for 2 to 4 hours and 15 to 276 hours, respectively. Activation of the ubiquitin-proteasome system was detected by measuring mRNA expressions of Atrogin-1, MURF1, and protein expressions of UBC2, UBC4, and the α subunits of the 20S proteasome (MCP231). Activation of the ALP was detected by electron microscopy and by measuring the expressions of several autophagy-related genes. Total carbonyl content and HNE-protein adduct formation were measured to assess oxidative stress. Total AKT, phosphorylated and total FOXO1, and FOXO3A protein levels were also measured.

MEASUREMENTS AND MAIN RESULTS

Prolonged CMV triggered activation of the ALP as measured by the appearance of autophagosomes in the diaphragm and increased expressions of autophagy-related genes, as compared with controls. Induction of autophagy was associated with increased protein oxidation and enhanced expression of the FOXO1 gene, but not the FOXO3A gene. CMV also triggered the inhibition of both AKT expression and FOXO1 phosphorylation.

CONCLUSIONS

We propose that prolonged CMV causes diaphragm disuse, which, in turn, leads to activation of the ALP through oxidative stress and the induction of the FOXO1 transcription factor.

Branched-chain amino acids protect against dexamethasone-induced soleus muscle atrophy in rats

We investigated the utility of branched-chain amino acids (BCAA) in dexamethasone-induced muscle atrophy. Dexamethasone (600 microg/kg, intraperitoneally) and/or BCAA (600 mg/kg, orally) were administered for 5 days in rats, and the effect of BCAA on dexamethasone-induced muscle atrophy was evaluated. Dexamethasone decreased total protein concentration of rat soleus muscles. Concomitant administration of BCAA reversed the decrease. Dexamethasone decreased mean cross-sectional area of soleus muscle fibers, which was reversed by BCAA. Dexamethasone increased atrogin-1 expression, which has been reported to play a pivotal role in muscle atrophy. The increased expression of atrogin-1 mRNA was significantly attenuated by BCAA. Furthermore, dexamethasone-induced conversion from microtubule-associated protein 1 light chain 3 (LC3)-I to LC3-II, which is an indicator of autophagy, was blocked by BCAA. These findings suggest that BCAA decreased protein breakdown to prevent muscle atrophy. BCAA administration appears to be useful for prevention of steroid myopathy.

The cathepsin L gene is a direct target of FOXO1 in skeletal muscle

FOXO1 (forkhead box O1), a forkhead-type transcription factor whose gene expression is up-regulated in the skeletal muscle during starvation, appears to be a key molecule of energy metabolism and skeletal muscle atrophy. Cathepsin L, a lysosomal proteinase whose expression is also up-regulated in the skeletal muscle during starvation, is induced in transgenic mice overexpressing FOXO1 relative to wild-type littermates. In the present study, we conducted in vivo and in vitro experiments focusing on FOXO1 regulation of Ctsl (cathepsin L gene; CTSL1 in humans) expression in the skeletal muscle. During fasting and refeeding of C57BL/6 mice, Ctsl was regulated in parallel with FOXO1 in the skeletal muscle. Fasting-induced Ctsl expression was attenuated in transgenic mice overexpressing a dominant-negative form of FOXO1 or in skeletal-muscle-specific Foxo1-knockout mice relative to respective wild-type controls. Using C2C12 mouse myoblasts overexpressing a constitutively active form of FOXO1, we showed that FOXO1 induces Ctsl expression. Moreover, we found FOXO1-binding sites in both the mouse Ctsl and human CTSL1 promoters. The luciferase reporter analysis revealed that the mouse Ctsl and human CTSL1 promoters are activated by FOXO1, which is abolished by mutations in the consensus FOXO1-binding sites. Gel mobility-shift and chromatin immunoprecipiation assays showed that FOXO1 is recruited and binds to the Ctsl promoter. The present study provides in vivo and in vitro evidence that Ctsl is a direct target of FOXO1 in the skeletal muscle, thereby suggesting a role for the FOXO1/cathepsin L pathway in fasting-induced skeletal muscle metabolic change and atrophy.

Increasing intravenous glucose load in the presence of normoglycemia: effect on outcome and metabolism in critically ill rabbits

OBJECTIVES

Endocrine disturbances and a feeding-resistant wasting syndrome, characterized by a negative protein balance, promote delayed recovery and poor outcome of critical illness. Parenteral nutrition alone cannot counteract the hypercatabolic state, possibly in part as a result of aggravation of the hyperglycemic response to illness. In critically ill rabbits, we investigated the impact of varying amounts of intravenous glucose while maintaining normoglycemia on mortality, organ damage, and markers of catabolism/anabolism.

DESIGN

Prospective, randomized laboratory investigation.

SETTING

University animal and molecular laboratory.

SUBJECTS

Three-month-old male rabbits.

INTERVENTIONS

Critically ill rabbits were randomized into a fasting group, a standard parenteral nutrition group, and two groups receiving either intermediate or high additional physiological amounts of intravenous glucose while maintained normoglycemic with insulin. These groups were compared with a hyperglycemic group and healthy rabbits. Protein and lipid load was equal for all fed groups.

MEASUREMENTS AND MAIN RESULTS

Varying intravenous glucose load did not affect mortality or organ damage provided hyperglycemia was prevented. Fasted critically ill rabbits lost weight, which was attenuated by increasing intravenous glucose load. As compared with healthy rabbits, mRNA expression and/or activity of several ubiquitin-proteasome pathway components, cathepsin-L and calpain-1, was elevated in skeletal muscle of fasted critically ill rabbits. Intravenous feeding was able to counteract this response. Excessive glucose load and/or hyperglycemia, however, reduced the protective effect of feeding. Genes investigated in the diaphragm and myocardium revealed roughly a similar response. Except in the normoglycemic group with intermediate glucose load, circulating thyroid hormone and insulin-like growth factor-1 levels decreased, most pronounced in hyperglycemic rabbits.

CONCLUSIONS

Increasing intravenous glucose infusion within the physiological range, while maintaining normoglycemia, was safe for organ function and survival of critically ill rabbits. Concomitantly, it reduced the catabolic responses as compared with fasting. Whether this has a beneficial effect on muscle function and mass remains to be investigated.

Monitoring gene expression in muscle tissue of macaca fascicularis under the influence of testosterone and SARM

The focus of this study was to evaluate data on the gene expression profiles induced by testosterone and a selective androgen receptor modulator (SARM, TAP Pharmaceutical Products Inc., Lake Forest, IL, USA) in androgen sensitive muscle tissue to obtain a better understanding on the molecular mechanisms of action and to identify biomarkers for SARM function in primate organs. A total of 24 male cyomolgus monkeys were divided into four groups: testosterone group, SARM1 group, SARM10 group, and control group, each consisting of six animals. The testosterone group was treated i.m. with 3.0 mg/kg Testostoviron

p38 MAPK links oxidative stress to autophagy-related gene expression in cachectic muscle wasting

Oxidative stress is a primary trigger of cachectic muscle wasting, but the signaling pathway(s) that links it to the muscle wasting processes remains to be defined. Here, we report that activation of p38 mitogen-activated protein kinase (MAPK) (phosphorylation) and increased oxidative stress (trans-4-hydroxy-2-nonenal protein modification) in skeletal muscle occur as early as 8 h after lipopolysaccharide (1 mg/kg) and 24 h after dexamethasone (25 mg/kg) injection (intraperitoneal) in mice, concurrent with upregulation of autophagy-related genes, Atg6, Atg7, and Atg12. Treating cultured C2C12 myotubes with oxidant hydrogen peroxide (4 h) resulted in increased p38 phosphorylation and reduced FoxO3 phosphorylation along with induced Atg7 mRNA expression without activation of NF-kappaB or FoxO3a transcriptional activities. Furthermore, inhibition of p38alpha/beta by SB202190 blocked hydrogen peroxide-induced atrophy with diminished upregulation of Atg7 and atrogenes [muscle atrophy F-box protein (MAFbx/Atrogin-1), muscle ring finger protein 1 (MuRF-1), and Nedd4]. These findings provide direct evidence for p38alpha/beta MAPK in mediating oxidative stress-induced autophagy-related genes, suggesting that p38alpha/beta MAPK regulates both the ubiquitin-proteasome and the autophagy-lysosome systems in muscle wasting.

Gene expression and muscle fiber function in a porcine ICU model

Skeletal muscle wasting and impaired muscle function in response to mechanical ventilation and immobilization in intensive care unit (ICU) patients are clinically challenging partly due to 1) the poorly understood intricate cellular and molecular networks and 2) the unavailability of an animal model mimicking this condition. By employing a unique porcine model mimicking the conditions in the ICU with long-term mechanical ventilation and immobilization, we have analyzed the expression profile of skeletal muscle biopsies taken at three time points during a 5-day period. Among the differentially regulated transcripts, extracellular matrix, energy metabolism, sarcomeric and LIM protein mRNA levels were downregulated, while ubiquitin proteasome system, cathepsins, oxidative stress responsive genes and heat shock proteins (HSP) mRNAs were upregulated. Despite 5 days of immobilization and mechanical ventilation single muscle fiber cross-sectional areas as well as the maximum force generating capacity at the single muscle fiber level were preserved. It is proposed that HSP induction in skeletal muscle is an inherent, primary, but temporary protective mechanism against protein degradation. To our knowledge, this is the first study that isolates the effect of immobilization and mechanical ventilation in an ICU condition from various other cofactors.

Valosin containing protein associated inclusion body myopathy: abnormal vacuolization, autophagy and cell fusion in myoblasts

Inclusion body myopathy associated with Paget's disease and frontotemporal dementia (IBMPFD) is caused by mutations in the valosin containing protein (VCP) gene. The disease is associated with progressive proximal muscle weakness, inclusions and vacuoles in muscle fibers, malfunction in the bone remodeling process resulting in Paget's disease, and premature frontotemporal dementia. VCP is involved in several cellular processes related to the endoplasmic reticulum associated degradation of proteins. To understand the pathological mechanisms underlying the myopathy in IBMPFD, we have studied the cellular consequences of VCP mutations in human primary myoblasts. Our results revealed that patients' myoblasts accumulate large vacuoles. Lysosomal membrane proteins Lamp1 and Lamp2 show increased molecular weights in patients' myoblasts due to differential N-glycosylation. Additionally, mutant myoblasts show increased autophagy when cultured in the absence of nutrients, as well as defective cell fusion and increased apoptosis. Our results elucidate that VCP mutations result in disturbances in several cellular processes, which will help us in the understanding of the pathological mechanisms resulting in muscle weakness and other features of VCP associated disease.

Sarcolipin and ubiquitin carboxy-terminal hydrolase 1 mRNAs are over-expressed in skeletal muscles of alpha-tocopherol deficient mice

The transcriptome of ataxic muscles from alpha-tocopherol transfer protein deficient (ATTP-KO), 23-month old, mice was compared with that of their normal littermates. Genes encoding sarcolipin (sln) and ubiquitin carboxyl-terminal hydrolase (uchl1) were over-expressed (> or =10-fold) in ataxic muscles. SLN is a 3.2 kDa membrane protein that binds to sarcoplasmic reticulum calcium ATPase, regulates Ca(+ +) transport and muscle relaxation-contraction cycles. UCHL1 is a 24.8 kDa member of proteosome proteins; it is over-expressed in myofibrillar myopathy and is associated with neurodegenerative diseases. Furthermore, six additional transcripts, three encoding thin-filament proteins and three encoding Ca(+ +) sensing proteins that participate in contraction-relaxation cycle, and eight transcripts that encode members of lysosomal proteins were also over-expressed in ataxic muscles. These observations suggest that chronic alpha-tocopherol (AT) deficiency activates critical genes of muscle contractility and protein degradation pathways, simultaneously. The magnitude of induction of sln and uchl1 was lower in asymptomatic, 8-month old, ATTP-KO mice and in 8-month old mice fed an AT-depleted diet. These studies suggest sln and uchl1 genes as novel targets of AT deficiency and may offer molecular correlates of well documented descriptions of neuromuscular dysfunctions in AT-deficient rodents. Since the neuromuscular deficits of ATTP-KO mice appear to be similar to those of patients with ATTP mutations, it is suggested that over-expression of sln and uchl1 may also contribute to AT-sensitive ataxia in humans.

Powerful signals for weak muscles

The aim of the present review is to summarise, evaluate and critique the different mechanisms involved in anabolic growth of skeletal muscle and the catabolic processes involved in cancer cachexia and sarcopenia of ageing. This is highly relevant, since they represent targets for future promising clinical investigations. Sarcopenia is an inevitable process associated with a gradual reduction in muscle mass and strength, associated with a reduction in motor unit number and atrophy of muscle fibres, especially the fast type IIa fibres. The loss of muscle mass with ageing is clinically important because it leads to diminished functional ability and associated complications. Cachexia is widely recognised as severe and rapid wasting accompanying disease states such as cancer or immunodeficiency disease. One of the main characteristics of cancer cachexia is asthenia or lack of strength, which is directly related to the muscle loss. Indeed, apart from the speed of loss, muscle wasting during cancer and ageing share many common metabolic pathways and mediators. In healthy young individuals, muscles maintain their mass and function because of a balance between protein synthesis and protein degradation associated with rates of anabolic and catabolic processes, respectively. Muscles grow (hypertrophy) when protein synthesis exceeds protein degradation. Conversely, muscles shrink (atrophy) when protein degradation dominates. These processes are not occurring independently of each other, but are finely coordinated by a web of intricate signalling networks. Such signalling networks are in charge of executing environmental and cellular cues that ultimately determine whether muscle proteins are synthesised or degraded. Increasing our understanding for the pathways involved in hypertrophy and atrophy and particularly the interaction of these pathways is essential in designing therapeutic strategies for both prevention and treatment of muscle wasting conditions with age and with disease.

Molecular events and signalling pathways involved in skeletal muscle disuse-induced atrophy and the impact of countermeasures

Disuse-induced skeletal muscle atrophy occurs following chronic periods of inactivity such as those involving prolonged bed rest, trauma and microgravity environments. Deconditioning of skeletal muscle is mainly characterized by a loss of muscle mass, decreased fibre cross-sectional area, reduced force, increased fatigability, increased insulin resistance and transitions in fibre types. A description of the role of specific transcriptional mechanisms contributing to muscle atrophy by altering gene expression during muscle disuse has recently emerged and focused primarily on short period of inactivity. A better understanding of the transduction pathways involved in activation of proteolytic and apoptotic pathways continues to represent a major objective, together with the study of potential cross-talks in these cellular events. In parallel, evaluation of the impact of countermeasures at the cellular and molecular levels in short- and long-term disuse experimentations or microgravity environments should undoubtedly and synergistically increase our basic knowledge in attempts to identify new physical, pharmacological and nutritional targets to counteract muscle atrophy. These investigations are important as skeletal muscle atrophy remains an important neuromuscular challenge with impact in clinical and social settings affecting a variety of conditions such as those seen in aging, cancer cachexia, muscle pathologies and long-term space exploration.

Alterations of protein turnover underlying disuse atrophy in human skeletal muscle

Unloading-induced atrophy is a relatively uncomplicated form of muscle loss, dependent almost solely on the loss of mechanical input, whereas in disease states associated with inflammation (cancer cachexia, AIDS, burns, sepsis, and uremia), there is a procatabolic hormonal and cytokine environment. It is therefore predictable that muscle loss mainly due to disuse alone would be governed by mechanisms somewhat differently from those in inflammatory states. We suggest that in vivo measurements made in human subjects using arterial-venous balance, tracer dilution, and tracer incorporation are dynamic and thus robust by comparison with static measurements of mRNA abundance and protein expression and/or phosphorylation in human muscle. In addition, measurements made with cultured cells or in animal models, all of which have often been used to infer alterations of protein turnover, appear to be different from results obtained in immobilized human muscle in vivo. In vivo measurements of human muscle protein turnover in disuse show that the primary variable that changes facilitating the loss of muscle mass is protein synthesis, which is reduced in both the postabsorptive and postprandial states; muscle proteolysis itself appears not to be elevated. The depressed postprandial protein synthetic response (a phenomenon we term "anabolic resistance") may even be accompanied by a diminished suppression of proteolysis. We therefore propose that most of the loss of muscle mass during disuse atrophy can be accounted for by a depression in the rate of protein synthesis. Thus the normal diurnal fasted-to-fed cycle of protein balance is disrupted and, by default, proteolysis becomes dominant but is not enhanced.

Nutrition and muscle catabolism in maintenance hemodialysis: does feeding make muscle cells selective self-eaters?

Efforts to build muscle by increased protein feeding in hemodialysis patients have been thwarted by parallel increases in both muscle protein synthesis and degradation. The evidence suggests that muscle cells replace older proteins in response to feeding rather than using new proteins to drive muscle cell hypertrophy. This review presents the hypothesis that protein feeding provides an opportunity for muscle to accelerate proteolysis of proteins that have been damaged by oxidation, nitrosylation, and/or glycosylation and to replace damaged mitochondria that contribute to oxidative stress. Increases in proteolysis with feeding are driven by insulin resistance and the increased oxidative stress of mitochondrial respiration. Oxidized proteins and organelles are excellent substrates for degradation by the proteasome, macroautophagy, and chaperone-mediated autophagy: these systems of proteolysis seem to be activated by oxydatiative stress. Replacement of oxidized and other damaged proteins may be a benefit of protein feeding in hemodialysis, but alternative strategies, including exercise, will be required to build muscle.

Cathepsin Y Expression is Up-Regulated in Liver and Spleen of the Rats Growing under a Low Protein Diet

Background/Aims

We have previously isolated a novel lysosomal cysteine protease, cathepsin Y, which produces a kinin-potenciating octapeptide from rat plasma. The present study aimed to elucidate the influence of protein-restriction on cathepsin Y mRNA expression in tissues of growing rat.

Methods

Weaning rats were fed either 20% casein (normal-protein, NP) or 5% casein (low-protein, LP) diet for 12 weeks, from post-weaning period to maturity. Cathepsin Y mRNA expression in tissues was examined using RT-PCR and compared with that of cathepsin L, a general marker of muscle atrophy.

Results

Body, liver and spleen weights and diet intake of LP were significantly lower than those of NP. Cathepsin Y expression in LP was increased in liver (3.70 fold vs. NP) and spleen (1.28 fold) at early periods of protein-restriction, whereas that of cathepsin L was increased in liver and pectoral muscle through all intake periods. However, significant differences were detected in neither cathepsin Y expression in pectoral muscle nor cathepsin L expression in spleen throughout intake periods.

Conclusions

A significant up-regulation of cathepsin Y was observed in spleen and liver, suggesting that it may associate with immunodepression in rats under protein-malnutrition during the early post-weaning period.

Cardiac ankyrin repeat protein is a marker of skeletal muscle pathological remodelling

In an attempt to identify potential therapeutic targets for the correction of muscle wasting, the gene expression of several pivotal proteins involved in protein metabolism was investigated in experimental atrophy induced by transient or definitive denervation, as well as in four animal models of muscular dystrophies (deficient for calpain 3, dysferlin, alpha-sarcoglycan and dystrophin, respectively). The results showed that: (a) the components of the ubiquitin-proteasome pathway are upregulated during the very early phases of atrophy but do not greatly increase in the muscular dystrophy models; (b) forkhead box protein O1 mRNA expression is augmented in the muscles of a limb girdle muscular dystrophy 2A murine model; and (c) the expression of cardiac ankyrin repeat protein (CARP), a regulator of transcription factors, appears to be persistently upregulated in every condition, suggesting that CARP could be a hub protein participating in common pathological molecular pathway(s). Interestingly, the mRNA level of a cell cycle inhibitor known to be upregulated by CARP in other tissues, p21(WAF1/CIP1), is consistently increased whenever CARP is upregulated. CARP overexpression in muscle fibres fails to affect their calibre, indicating that CARP per se cannot initiate atrophy. However, a switch towards fast-twitch fibres is observed, suggesting that CARP plays a role in skeletal muscle plasticity. The observation that p21(WAF1/CIP1) is upregulated, put in perspective with the effects of CARP on the fibre type, fits well with the idea that the mechanisms at stake might be required to oppose muscle remodelling in skeletal muscle.

Expression of atrophy mRNA relates to tendon tear size in supraspinatus muscle

Skeletal muscle atrophy and fatty infiltration develop after tendon tearing. The extent of atrophy serves as one prognostic factor for the outcome of surgical repair of rotator cuff tendon tears. We asked whether mRNA of genes involved in regulation of degradative processes leading to muscle atrophy, ie, FOXOs, MSTN, calpains, cathepsins, and transcripts of the ubiquitin-proteasome pathway, are overexpressed in the supraspinatus muscle in patients with and without rotator cuff tears. We evaluated biopsy specimens collected during surgery of 53 consecutive patients with different sizes of rotator cuff tendon tears and six without tears. The levels of corresponding gene transcripts in total RNA extracts were assessed by semiquantitative reverse transcriptase-polymerase chain reaction (RT-PCR) analysis. Supraspinatus muscle atrophy was assessed by MRI. The area of muscle tissue (or atrophy), decreased (increased) with increasing tendon tear size. The transcripts of CAPN1, UBE2B, and UBE3A were upregulated more than twofold in massive rotator cuff tears as opposed to smaller tears or patients without tears. These atrophy gene products may be involved in cellular processes that impair functional recovery of affected muscles after surgical rotator cuff repair. However, the damaging effects of gene products in their respective proteolytic processes on muscle structures and proteins remains to be investigated.

Skeletal muscle proteolysis in aging

PURPOSE OF REVIEW

To understand age-related changes in proteolysis and apoptosis in skeletal muscle in relation to oxidative stress and mitochondrial alterations.

RECENT FINDINGS

During aging, a progressive loss of muscle mass (sarcopenia) has been described in both human and rodents. Sarcopenia is attributable to an imbalance between protein synthesis and degradation or between apoptosis and regeneration processes or both. Major age-dependent alterations in muscle proteolysis are a lack of responsiveness of the ubiquitin-proteasome-dependent proteolytic pathway to anabolic and catabolic stimuli and alterations in the regulation of autophagy. In addition, increased oxidative stress leads to the accumulation of damaged proteins, which are not properly eliminated, aggregate, and in turn impair proteolytic activities. Finally, the mitochondria-associated apoptotic pathway may be activated. These age-induced changes may contribute to sarcopenia and decreased ability of old individuals to recover from stress.

SUMMARY

Alterations in proteasome-dependent or lysosomal proteolysis, increased oxidative stress, mitochondrial dysfunction, and apoptosis presumably contribute to the development of sarcopenia.

Daily variations in dietary lysine content alter the expression of genes related to proteolysis in chicken pectoralis major muscle

Amino acids are known to be anabolic factors that affect protein metabolism, but the response of animals to daily amino acid changes is little understood. We aimed to test the effects of feeding birds with alternations of diets varying in lysine content on the expression of genes related to proteolysis in chicken muscle. Cyclic feeding programs with 2 diets, each given for 24 h during 48-h cycles, were carried out from 10 d of age. Three programs were used: 1) control treatment with continuous distribution of a complete diet containing standard medium lysine level (ML; 11.9 g/kg); 2) alternation of diets with high (HL) and low (LL) lysine levels; 3) alternation of ML and LL diets, where LL = 70%, ML = 100%, HL = 130% of standard lysine level. The Pectoralis major muscles were sampled after 2 wk of cyclic feeding. Measurements included the expression patterns of 6 genes involved in proteolysis, and mammalian target of rapamycin and Forkhead box-O transcription factor (FoxO) signaling. Cathepsin B, m-calpain, and E3 ubiquitin ligases Muscle Ring Finger-1 and Muscle Atrophy F box were significantly overexpressed in chickens transiently fed the LL diet, whereas the mRNA levels of 20S proteasome C2 subunit and ubiquitin remained unchanged. Modifications of E3 ubiquitin ligase expression can be partly explained by significant changes in FoxO phosphorylation with cyclic dietary treatments. Our results suggest timing-sensitive regulation of proteolysis in chicken muscle according to dietary treatment and a high metabolism capacity to compensate for changes in amino acid supply, which might be used for nutritional purposes.

Positive and negative modulation of peptidases by pro-inflammatory cytokines

The capacity of pro-inflammatory cytokines to modulate proteolysis was analyzed by liquid chromatography using human fibroblasts as cell model and enzyme source, and the immunodominant epitope gp100(280-288) (YLEPGPVTA) as substrate. The measurements made after fibroblast pre-incubation with either IL-1, TNF, or IL-6 plus its soluble receptors have been compared with those made with un-stimulated fibroblasts. The results obtained suggest an uneven association of cytokine treatment with substrate degradation, and with a prevailingly positive - but also negative - association with release of smaller peptides and free amino acids. Data obtained by separately measuring these two groups of by-products indicate that, after IL-1 cell pre-treatment, the velocity of formation of both groups of by-products increased, resulting in a net increase of substrate degradation. After TNF and IL-6 pre-treatment, the increase of one group was compensated by a decrease of the other group; specifically, the compensation was only partial for TNF, and overall substrate hydrolysis increased. In the case of IL-6, the increase of free amino acids was almost exactly compensated by a reduction of peptidic by-products, resulting in a negligible increase of substrate hydrolysis. In addition, the existence of reaction time-related modifications in the apparent velocity of substrate degradation and formation of by-products, allows hypothesizing different effects of cytokines on the enzymes degrading the substrate with different time constants. Taken together, these data can be interpreted as indicating different, positive and negative, effects of the three cytokines on the individual enzymes expressed by fibroblasts and capable of degrading peptidic substrates.

Atrophy and programmed cell death of skeletal muscle

Striated skeletal is subject to nonlethal cycles of atrophy in response to a variety of physiological and pathological stimuli, including: starvation, disuse, denervation and inflammation. These cells can also undergo cell death in response to appropriate developmental signals or specific pathological insults. Most of the insights gained into the control of vertebrate skeletal muscle atrophy and death have resulted from experimental interventions rather than natural processes. In contrast, the intersegmental muscles (ISMs) of moths are giant cells that initiate sequential and distinct programs of atrophy and death at the end of metamorphosis as a normal component of development. This model has provided fundamental information about the control, biochemistry, molecular biology and anatomy of naturally occurring atrophy and death in vivo. The ISMs have provided a good complement to studies in vertebrates and may provide insights into clinically relevant disorders.

Epigenetic drugs in the treatment of skeletal muscle atrophy

PURPOSE OF REVIEW

A dynamic network of anabolic and catabolic pathways regulates skeletal muscle mass in adult organisms. Muscle atrophy is the detrimental outcome of an imbalance of this network. The purpose of this review is to provide a critical evaluation of different forms of muscle atrophy from a mechanistic and therapeutic point of view.

RECENT FINDINGS

The identification and molecular characterization of distinct pathways implicated in the pathogenesis of muscle atrophy have revealed potential targets for therapeutic interventions. However, an effective application of these therapies requires a better understanding of the relative contribution of these pathways to the development of muscle atrophy in distinct pathological conditions.

SUMMARY

We propose that the decline in anabolic signals ('passive atrophy') and activation of catabolic pathways ('active atrophy') contribute differently to the pathogenesis of muscle atrophy associated with distinct diseases or unfavorable conditions. Interestingly, these pathways might converge on common transcriptional effectors, suggesting that an optimal intervention should be directed to targets at the chromatin level. We provide the rationale for the use of epigenetic drugs such as deacetylase inhibitors, which target multiple signaling pathways implicated in the pathogenesis of muscle atrophy.

Dynamic effects of autophagy on arsenic trioxide-induced death of human leukemia cell line HL60 cells

AIM

To evaluate the contribution of an autophagic mechanism to the As2O3- induced death of human acute myeloid leukaemia cell line HL60 cells.

METHODS

The growth inhibition of HL60 cells induced by As2O3 was assessed with 3-(4,5- dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide colorimetric assay. The activation of autophagy was determined with monodansylcadaverine labeling and transmission electron microscope. The role of autophagy in the As2O3-induced death of HL60 cells was assessed using autophagic and lysosomal inhibitors. Immunofluorescence, flow cytometry, and Western blot analysis were used to study the apoptotic and autophagic mechanisms.

RESULTS

After treatment with As2O3, the proliferation of HL60 cells was significantly inhibited and the formation of autophagosomes increased. The blockade of autophagy maturation with the autophagy-specific inhibitor 3-methyladenine (3-MA) or the lysosome-neutralizing agent NH4Cl 1 h before As2O3 potentiated the As2O3-induced death of HL60 cells. In contrast, 3-MA attenuated As2O3-induced death when administered 30 min after As2O3. 3-MA and NH4Cl also inhibited As2O3-induced upregulation of microtubule-associated protein 1 light chain 3, the protein required for autophagy in mammalian cells. Following As2O3, lysosomes were activated as indicated by increased levels of cathepsins B and L. The apoptotic response of HL60 cells to As2O3 was suggested by the collapse of mitochondrial membrane potential, release of cytochrome c from mitochondria, and the activation of caspase-3. Pretreatment with 3-MA prior to As2O3 amplified these apoptotic signals, while posttreatment with 3-MA 30 min after As2O3 attenuated the apoptotic pathways.

CONCLUSION

Autophagy plays complex roles in the As2O3-induced death of HL60 cells; it inhibits As2O3-induced apoptosis in the initiation stage, but amplifies the As2O3-mediated apoptotic program if it is persistently activated.

FoxO3 controls dangerous proteolytic liaisons

FoxO3 regulates the transcription of critical components of the ubiquitin-proteasome system in muscle wasting. Two reports (Mammucari et al., 2007; Zhao et al., 2007) now implicate FoxO3 in the transcription of autophagy-related genes and provide the first direct evidence for a coordinated role of autophagy in muscle atrophy.

FoxO3 coordinately activates protein degradation by the autophagic/lysosomal and proteasomal pathways in atrophying muscle cells

Muscle atrophy occurs in many pathological states and results primarily from accelerated protein degradation and activation of the ubiquitin-proteasome pathway. However, the importance of lysosomes in muscle atrophy has received little attention. Activation of FoxO transcription factors is essential for the atrophy induced by denervation or fasting, and activated FoxO3 by itself causes marked atrophy of muscles and myotubes. Here, we report that FoxO3 does so by stimulating overall protein degradation and coordinately activating both lysosomal and proteasomal pathways. Surprisingly, in C2C12 myotubes, most of this increased proteolysis is mediated by lysosomes. Activated FoxO3 stimulates lysosomal proteolysis in muscle (and other cell types) by activating autophagy. FoxO3 also induces the expression of many autophagy-related genes, which are induced similarly in mouse muscles atrophying due to denervation or fasting. These studies indicate that decreased IGF-1-PI3K-Akt signaling activates autophagy not only through mTOR but also more slowly by a transcription-dependent mechanism involving FoxO3.

FoxO3 controls autophagy in skeletal muscle in vivo

Autophagy allows cell survival during starvation through the bulk degradation of proteins and organelles by lysosomal enzymes. However, the mechanisms responsible for the induction and regulation of the autophagy program are poorly understood. Here we show that the FoxO3 transcription factor, which plays a critical role in muscle atrophy, is necessary and sufficient for the induction of autophagy in skeletal muscle in vivo. Akt/PKB activation blocks FoxO3 activation and autophagy, and this effect is not prevented by rapamycin. FoxO3 controls the transcription of autophagy-related genes, including LC3 and Bnip3, and Bnip3 appears to mediate the effect of FoxO3 on autophagy. This effect is not prevented by proteasome inhibitors. Thus, FoxO3 controls the two major systems of protein breakdown in skeletal muscle, the ubiquitin-proteasomal and autophagic/lysosomal pathways, independently. These findings point to FoxO3 and Bnip3 as potential therapeutic targets in muscle wasting disorders and other degenerative and neoplastic diseases in which autophagy is involved.

Roles and potential therapeutic targets of the ubiquitin proteasome system in muscle wasting

Muscle wasting, characterized by the loss of protein mass in myofibers, is in most cases largely due to the activation of intracellular protein degradation by the ubiquitin proteasome system (UPS). During the last decade, mechanisms contributing to this activation have been unraveled and key mediators of this process identified. Even though much remains to be understood, the available information already suggests screens for new compounds inhibiting these mechanisms and highlights the potential for pharmaceutical drugs able to treat muscle wasting when it becomes deleterious. This review presents an overview of the main pathways contributing to UPS activation in muscle and describes the present state of efforts made to develop new strategies aimed at blocking or slowing muscle wasting.

Publication history: Republished from Current BioData's Targeted Proteins database (TPdb; ).

Towards the control of intracellular protein turnover: mitochondrial Lon protease inhibitors versus proteasome inhibitors

Cellular protein homeostasis results from the combination of protein biogenesis processes and protein quality control mechanisms, which contribute to the functional state of cells under normal and stress conditions. Proteolysis constitutes the final step by which short-lived, misfolded and damaged intracellular proteins are eliminated. Protein turnover and oxidatively modified protein degradation are mainly achieved by the proteasome in the cytosol and nucleus of eukaryotic cells while several ATP-dependent proteases including the matrix protease Lon take part in the mitochondrial protein degradation. Moreover, Lon protease seems to play a major role in the elimination of oxidatively modified proteins in the mitochondrial matrix. Specific inhibitors are commonly used to assess cellular functions of proteolytic systems as well as to identify their protein substrates. Here, we present and discuss known proteasome and Lon protease inhibitors. To date, very few inhibitors of Lon have been described and no specific inhibitors of this protease are available. The current knowledge on both catalytic mechanisms and inhibitors of these two proteases is first described and attempts to define specific non-peptidic inhibitors of the human Lon protease are presented.

Cysteine cathepsins: cellular roadmap to different functions

Cysteine cathepsins belong to the papain-like family C1 of clan CA cysteine peptidases. These enzymes are ubiquitously expressed and exert their proteolytic activity mainly, but not exclusively within the compartments along the endocytic pathway. Moreover, cysteine cathepsins are active in pericellular environments as soluble enzymes or bound to cell surface receptors at the plasma membrane, and possibly even within secretory vesicles, the cytosol, mitochondria, and within the nuclei of eukaryotic cells. Proteolytic actions performed by cysteine cathepsins are essential in the maintenance of homeostasis and depend heavily upon their correct sorting and trafficking within cells. As a consequence, the numerous and diverse approaches to identification, qualitative and quantitative determination, and visualization of cysteine cathepsin functions in vitro, in situ, and in vivo cover the entire spectrum of biochemistry, molecular and cell biology. This review focuses upon the transport pathways directing cysteine cathepsins to their points of action and thus emphasizes the broader role and functionality of cysteine cathepsins in a number of specific cellular locales. Such understanding will provide a foundation for future research investigating the involvement of these peptidases with their substrates, inhibitors, and the intertwined proteolytic networks at the hubs of complex biological systems.

Leupeptin Inhibits Ventilator-induced Diaphragm Dysfunction in Rats

RATIONALE

Controlled mechanical ventilation (CMV) has been shown to result in elevated diaphragmatic proteolysis and atrophy together with diaphragmatic contractile dysfunction.

OBJECTIVES

To test whether administration of leupeptin, an inhibitor of lysosomal proteases and calpain, concomitantly with 24 hours of CMV, would protect the diaphragm from the deleterious effects of mechanical ventilation.

METHODS

Rats were assigned to either a control group or 24 hours of CMV; animals in the ventilation group received either a single intramuscular injection of saline or 15 mg/kg of the protease inhibitor, leupeptin.

MEASUREMENTS AND MAIN RESULTS

Compared with control animals, mechanical ventilation resulted in a significant reduction of the in vitro diaphragm-specific force production at all stimulation frequencies. Leupeptin completely prevented this reduction in force generation. Atrophy of type IIx/b fibers was present after CMV, but not after treatment with leupeptin. Cathepsin B and calpain activities were significantly higher after CMV compared with the other groups; this was abolished by treatment with leupeptin. Significant inverse correlations were found between diaphragmatic force generation and cathepsin B and calpain activity, and illustrate the deleterious role of proteolysis in diminishing diaphragmatic force production after prolonged CMV.

CONCLUSIONS

Administration of the protease inhibitor leupeptin concomitantly with mechanical ventilation completely prevented ventilation-induced diaphragmatic contractile dysfunction and atrophy.

Growth hormone modulates the degradative capacity of muscle nucleases but not of cathepsin D in post-weaning mice

We determined whether recombinant human growth hormone (rhGH) administration might modulate the enzyme degradative capacity of the muscle lysosomal system and influence muscle growth. Muscle cathepsin D, acid RNase and DNase II activities are determined in the gastrocnemius muscle of rhGH-treated post-weaning female BALB/c mice. Linear regressions were used to analyze the relationships of each enzyme with their respective substrate. GH induced a depletion-recovery response of muscle growth through a mechanism which is similar to catch-up growth. In these conditions, cathepsin D activity decreased with age in all animals (GH: 40%; saline: 79%), showing a substantial developmental decline that could reflect changes in the rate of protein breakdown. However, the degradative capacity of cathepsin D was paradoxically unmodified in rhGH-mice compared with saline mice (according to the enzyme vs. substrate linear regression slope), in spite of the increase in enzyme activity elicited by GH. This suggests that the muscle protein breakdown is not increased by GH-treatment in post-weaning mice. The enhancement of muscle protein deposition as indicated by the augmented muscle cell size (protein:DNA ratio) of rhGH-mice (increased 178% from 25 to 50 days) vs. saline, can be attributed to a higher muscle K(RNA). In contrast, acid RNase and DNase II activities directly participate in muscle RNA and DNA degradation. Both nucleases were inhibited by GH treatment (a decrease of 48% and 63%, respectively, vs. saline at 50 days). The decrease in RNase activity suggests an inverse relation between the rate of protein synthesis (high) and acid RNase activity (low), leading to spare muscle RNA for synthesizing protein during catch-up growth. Also, low DNase II activity could contribute to inhibiting of muscle DNA degradation, facilitating muscle growth. Thus, GH seems to act as a direct modulator of the degradative capacity of skeletal muscle nucleases but not of cathepsin D, influencing DNA and RNA degradation during the depletion-recovery response to GH of gastrocnemius muscle in female post-weaning mice.