Activation of caspase-3 is an initial step triggering accelerated muscle proteolysis in catabolic conditions

Diabetes-induced muscle wasting: molecular mechanisms and promising therapeutic targets

Diabetes has become a serious public health crisis, presenting significant challenges to individuals worldwide. As the largest organ in the human body, skeletal muscle is a significant target of this chronic disease, yet muscle wasting as a complication of diabetes is still not fully understood and effective treatment methods have yet to be developed. Here, we discuss the targets involved in inducing muscle wasting under diabetic conditions, both validated targets and emerging targets. Diabetes-induced skeletal muscle wasting is known to involve changes in various signaling molecules and pathways, such as protein degradation pathways, protein synthesis pathways, mitochondrial function, and oxidative stress inflammation. Recent studies have shown that some of these present potential as promising therapeutic targets, including the neuregulin 1/epidermal growth factor receptor family, advanced glycation end-products, irisin, ferroptosis, growth differentiation factor 15 and more. This study's investigation and discussion of such pathways and their potential applications provides a theoretical basis for the development of clinical treatments for diabetes-induced muscle wasting and a foundation for continued focus on this disease.

From amino-acid to disease: the effects of oxidation on actin-myosin interactions in muscle

Actin-myosin interactions form the basis of the force-producing contraction cycle within the sarcomere, serving as the primary mechanism for muscle contraction. Post-translational modifications, such as oxidation, have a considerable impact on the mechanics of these interactions. Considering their widespread occurrence, the explicit contributions of these modifications to muscle function remain an active field of research. In this review, we aim to provide a comprehensive overview of the basic mechanics of the actin-myosin complex and elucidate the extent to which oxidation influences the contractile cycle and various mechanical characteristics of this complex at the single-molecule, myofibrillar and whole-muscle levels. We place particular focus on amino acids shown to be vulnerable to oxidation in actin, myosin, and some of their binding partners. Additionally, we highlight the differences between in vitro environments, where oxidation is controlled and limited to actin and myosin and myofibrillar or whole muscle environments, to foster a better understanding of oxidative modification in muscle. Thus, this review seeks to encompass a broad range of studies, aiming to lay out the multi layered effects of oxidation in in vitro and in vivo environments, with brief mention of clinical muscular disorders associated with oxidative stress.

Clinical features and molecular mechanism of muscle wasting in end stage renal disease

Muscle wasting in end-stage renal disease (ESRD) is an escalating issue due to the increasing global prevalence of ESRD and its significant clinical impact, including a close association with elevated mortality risk. The phenomenon of muscle wasting in ESRD, which exceeds the rate of muscle loss observed in the normal aging process, arises from multifactorial processes. This review paper aims to provide a comprehensive understanding of muscle wasting in ESRD, covering its epidemiology, underlying molecular mechanisms, and current and emerging therapeutic interventions. It delves into the assessment techniques for muscle mass and function, before exploring the intricate metabolic and molecular pathways that lead to muscle atrophy in ESRD patients. We further discuss various strategies to mitigate muscle wasting, including nutritional, pharmacological, exercise, and physical modalities intervention. This review seeks to provide a solid foundation for future research in this area, fostering a deeper understanding of muscle wasting in ESRD, and paving the way for the development of novel strategies to improve patient outcomes. [BMB Reports 2023; 56(8): 426-438].

Cholesterol: An Important Determinant of Muscle Atrophy in Astronauts

Since cholesterol is not routinely measured in astronauts before and after their return from space, there is no data on the role of blood cholesterol level in muscle atrophy and microgravity. Since the first moon landing, aerospace medicine became outdated and has not pushed boundaries like its rocket engineering counterpart. Since the 2019 astronaut twin study, there has yet to be another scientific breakthrough for aerospace medicine. Microgravity-induced muscle atrophy is the most known consequence of spaceflight. Yet, so far, there is no therapeutic solution to prevent it or any real efforts in understanding it on a cellular or molecular level. The most obvious reason to this unprecedented level of research is due to the small cohort of astronauts. With the establishment of private space industries and exponential recruitment of astronauts, there is more reason to push forward spaceflight-related health guidelines and ensure the safety of the brave humans who risk their lives for the progression of mankind. Spaceflight is considered the most challenging job and the failure to prevent injury or harm should be considered reckless negligence by the institutions that actively prevented sophistication of aerospace medicine. In this critical review, role of cholesterol is analyzed across the NASA-established parameters of microgravity-induced muscle atrophy with a focus on potential therapeutic targets for research.

Effects of proinflammatory cytokines on lacrimal gland myoepithelial cells contraction

In the lacrimal gland, myoepithelial cells (MEC) express muscle contractile proteins such as alpha smooth muscle actin (SMA) and calponin and therefore can contract to help expel lacrimal fluid. In a previous study, we demonstrated that lacrimal gland MEC express the oxytocin receptor (OXTR) and they contract under oxytocin (OXT) stimulation. Using NOD and MRL/lpr mice (animal models of Sjogren's syndrome), we reported a decrease in SMA and calponin protein levels plus a decline in acini contraction after stimulation with OXT. It is known that proinflammatory cytokines, such as interleukin-1β (IL-1β), tumor necrosis factor alpha (TNF-α) or interferon gamma (IFN-γ), can affect OXTR expression and signaling capacity and inhibit MEC contraction. The aim of the current study was to investigate if proinflammatory cytokines are implicated in the loss of MEC contractile ability. Thus, lacrimal gland MEC from a SMA-GFP transgenic mouse were treated with IL-1β (10 ng/ml) for a total of 7 days. At days 0, 2, 4 and 7, GFP intensity, cell size/area, contractile proteins amounts and MEC contraction were assessed. At day 0, control and treated cells showed no differences in GFP intensity and cell size. GFP intensity started to decrease in treated MEC at day 2 (20%; p=0.02), continuing after day 4 (25%; p=0.007) and 7 (30%; p=0.0001). Mean cell area was also reduced at day 2 (34%; p=0.0005), and after 4 (51%; p<0.0001) and 7 days (30%; p=0.0015). The contraction assay at day 2 showed a 70% decrease of contraction in treated MEC (p<0.0001), 73% (p<0.0001) at day 4 and 82% (p=0.0015) at day 7 when compared to control. Levels of contractile proteins were measured on day 7 showing a decrease in SMA and calponin amount in treated MEC compared with the control group (around 30%; p=0.0016 and p=0.0206; respectively). Similar results were observed when TNF-α and IFN-γ were added along with IL-1β. Taken together the present data and those from our previous studies with Sjogren's syndrome mouse models, they strongly suggest that proinflammatory cytokines affect lacrimal gland MEC contractile ability that may account for the reduced tear secretion associated with Sjogren's syndrome dry eye disease.

Mechanisms Regulating Muscle Protein Synthesis in CKD

BACKGROUND

CKD induces loss of muscle proteins partly by suppressing muscle protein synthesis. Muscles of mice with CKD have increased expression of nucleolar protein 66 (NO66), as do muscle biopsy specimens from patients with CKD or those undergoing hemodialysis. Inflammation stimulates NO66 expression and changes in NF-κB mediate the response.

METHODS

Subtotal nephrectomy created a mouse model of CKD with BUN >80 mg/dl. Crossing NO66^flox/flox with MCK-Cre mice bred muscle-specific NO66 (MCK-NO66) knockout mice. Experiments assessed the effect of removing NO66.

RESULTS

Muscle-specific NO66 knockout in mice blocks CKD-induced loss of muscle mass and improves protein synthesis. NO66 suppression of ribosomal biogenesis via demethylase activity is the mechanism behind these responses. In muscle cells, expression of NO66, but not of demethylase-dead mutant NO66, decreased H3K4me3 and H3K36me3 and suppressed pre-rRNA expression. Knocking out NO66 increased the enrichment of H3K4me3 and H3K36me3 on ribosomal DNA. In primary muscle cells and in muscles of mice without NO66, ribosomal RNA, pre-rRNA, and protein synthesis all increased.

CONCLUSIONS

CKD suppresses muscle protein synthesis via epigenetic mechanisms that NO66 mediates. Blocking NO66 could suggest strategies that counter CKD-induced abnormal muscle protein catabolism.

MicroRNA-23a and MicroRNA-27a Mimic Exercise by Ameliorating CKD-Induced Muscle Atrophy

Muscle atrophy is a frequent complication of CKD, and exercise can attenuate the process. This study investigated the role of microRNA-23a (miR-23a) and miR-27a in the regulation of muscle mass in mice with CKD. These miRs are located in a gene cluster that is regulated by the transcription factor NFAT. CKD mice expressed less miR-23a in muscle than controls, and resistance exercise (muscle overload) increased the levels of miR-23a and miR-27a in CKD mice. Injection of an adeno-associated virus encoding the miR-23a/27a/24-2 precursor RNA into the tibialis anterior muscles of normal and CKD mice led to increases in mature miR-23a and miR-27a but not miR-24-2 in the muscles of both cohorts. Overexpression of miR-23a/miR-27a in CKD mice attenuated muscle loss, improved grip strength, increased the phosphorylation of Akt and FoxO1, and decreased the activation of phosphatase and tensin homolog (PTEN) and FoxO1 and the expression of TRIM63/MuRF1 and FBXO32/atrogin-1 proteins. Provision of miR-23a/miR-27a also reduced myostatin expression and downstream SMAD-2/3 signaling, decreased activation of caspase-3 and -7, and increased the expression of markers of muscle regeneration. Lastly, in silico miR target analysis and luciferase reporter assays in primary satellite cells identified PTEN and caspase-7 as targets of miR-23a and FoxO1 as a target of miR-27a in muscle. These findings provide new insights about the roles of the miR-23a/27a-24-2 cluster in CKD-induced muscle atrophy in mice and suggest a mechanism by which exercise helps to maintain muscle mass.

Serum Glucocorticoid-Regulated Kinase 1 Blocks CKD-Induced Muscle Wasting Via Inactivation of FoxO3a and Smad2/3

Muscle proteolysis in CKD is stimulated when the ubiquitin-proteasome system is activated. Serum glucocorticoid-regulated kinase 1 (SGK-1) is involved in skeletal muscle homeostasis, but the role of this protein in CKD-induced muscle wasting is unknown. We found that, compared with muscles from healthy controls, muscles from patients and mice with CKD express low levels of SGK-1. In mice, SGK-1-knockout (SGK-1-KO) induced muscle loss that correlated with increased expression of ubiquitin E3 ligases known to facilitate protein degradation by the ubiquitin-proteasome, and CKD substantially aggravated this response. SGK-1-KO also altered the phosphorylation levels of transcription factors FoxO3a and Smad2/3. In C2C12 muscle cells, expression of dominant negative FoxO3a or knockdown of Smad2/3 suppressed the upregulation of E3 ligases induced by loss of SGK-1. Additionally, SGK-1 overexpression increased the level of phosphorylated N-myc downstream-regulated gene 1 protein, which directly interacted with and suppressed the phosphorylation of Smad2/3. Overexpression of SGK-1 in wild-type mice with CKD had similar effects on the phosphorylation of FoxO3a and Smad2/3 and prevented CKD-induced muscle atrophy. Finally, mechanical stretch of C2C12 muscle cells or treadmill running of wild-type mice with CKD stimulated SGK-1 production, and treadmill running inhibited proteolysis in muscle. These protective responses were absent in SGK-1-KO mice. Thus, SGK-1 could be a mechanical sensor that mediates exercise-induced improvement in muscle wasting stimulated by CKD.

Role of Inflammation in Muscle Homeostasis and Myogenesis

Skeletal muscle mass is subject to rapid changes according to growth stimuli inducing both hypertrophy, through increased protein synthesis, and hyperplasia, activating the myogenic program. Muscle wasting, characteristic of several pathological states associated with local or systemic inflammation, has been for long considered to rely on the alteration of myofiber intracellular pathways regulated by both hormones and cytokines, eventually leading to impaired anabolism and increased protein breakdown. However, there are increasing evidences that even alterations of the myogenic/regenerative program play a role in the onset of muscle wasting, even though the precise mechanisms involved are far from being fully elucidated. The comprehension of the links potentially occurring between impaired myogenesis and increased catabolism would allow the definition of effective strategies aimed at counteracting muscle wasting. The first part of this review gives an overview of skeletal muscle intracellular pathways determining fiber size, while the second part considers the cells and the regulatory pathways involved in the myogenic program. In both parts are discussed the evidences supporting the role of inflammation in impairing muscle homeostasis and myogenesis, potentially determining muscle atrophy.

CKD Stimulates Muscle Protein Loss Via Rho-associated Protein Kinase 1 Activation

In patients with CKD, muscle wasting is common and is associated with morbidity and mortality. Mechanisms leading to loss of muscle proteins include insulin resistance, which suppresses Akt activity and thus stimulates protein degradation via the ubiquitin-proteasome system. However, the specific factors controlling CKD-induced suppression of Akt activity in muscle remain undefined. In mice with CKD, the reduction in Akt activity in muscle exceeded the decrease in upstream insulin receptor substrate-1-associated phosphatidylinositol 3-kinase activity, suggesting that CKD activates other pathways that suppress Akt. Furthermore, a CKD-induced increase uncovered caspase-3 activity in muscle in these mice. In C2C12 muscle cells, activated caspase-3 cleaves and activates Rho-associated protein kinase 1 (ROCK1), which enhances the activity of phosphatase and tensin homolog (PTEN) and reduces Akt activity. Notably, constitutive activation of ROCK1 also led to increased caspase-3 activity in vitro. In mice with either global ROCK1 knockout or muscle-specific PTEN knockout, CKD-associated muscle proteolysis was blunted. These results suggest ROCK1 activation in CKD and perhaps in other catabolic conditions can promote loss of muscle protein via a negative feedback loop.

Effects of short-term energy deficit on muscle protein breakdown and intramuscular proteolysis in normal-weight young adults

The effects of short-term energy deficit (ED) on direct measures of muscle proteolysis and the intracellular mechanisms by which muscle proteins are degraded at rest and following aerobic exercise are not well described. This study evaluated the effects of a short-term diet-induced ED, on muscle fractional breakdown rate (FBR), intramuscular 26S proteasome activity, caspase-3 activation, and PSMA2 and MAFbx expression at rest, in the postabsorptive state, and following a single bout of moderate aerobic exercise (45 min at 65% peak oxygen uptake). Six men and 4 women participated in two 10-day diet interventions: weight maintenance (WM) followed by ED (80% estimated energy requirements). Dietary protein (1.5 g·kg−1·day−1) intake was constant for WM and ED. Mixed muscle FBR, proteasome activity, and intracellular proteolytic factor expression were measured using stable isotope methodology, fluorescent enzyme activity assays, and Western blotting, respectively. Overall, FBR and caspase-3 activation increased 60% and 11%, respectively, in response to ED (P < 0.05), but were not influenced by exercise. During ED, 26S proteasome α-subunit PSMA2 expression was 25% higher (P < 0.05) after exercise compared with rest. Exercise did not influence PSMA2 expression during WM, and MAFbx expression and 26S proteasome activity were not affected by ED or exercise. These data illustrate the effects of short-term, moderate ED on muscle protein degradation. In the context of skeletal muscle integrity during weight loss interventions, this work demonstrates a need for further investigations aimed at mitigating muscle loss associated with energy deficit imposed for intentional reduction of total body weight.

Decreased rate of protein synthesis, caspase-3 activity, and ubiquitin–proteasome proteolysis in soleus muscles from growing rats fed a low-protein, high-carbohydrate diet

The aim of this study was to investigate the changes in the rates of both protein synthesis and breakdown, and the activation of intracellular effectors that control these processes in soleus muscles from growing rats fed a low-protein, high-carbohydrate (LPHC) diet for 15 days. The mass and the protein content, as well as the rate of protein synthesis, were decreased in the soleus from LPHC-fed rats. The availability of amino acids was diminished, since the levels of various essential amino acids were decreased in the plasma of LPHC-fed rats. Overall rate of proteolysis was also decreased, explained by reductions in the mRNA levels of atrogin-1 and MuRF-1, ubiquitin conjugates, proteasome activity, and in the activity of caspase-3. Soleus muscles from LPHC-fed rats showed increased insulin sensitivity, with increased levels of insulin receptor and phosphorylation levels of AKT, which probably explains the inhibition of both the caspase-3 activity and the ubiquitin–proteasome system. The fall of muscle proteolysis seems to represent an adaptive response that contributes to spare proteins in a condition of diminished availability of dietary amino acids. Furthermore, the decreased rate of protein synthesis may be the driving factor to the lower muscle mass gain in growing rats fed the LPHC diet.

Muscle wasting in hemodialysis patients: new therapeutic strategies for resolving an old problem

Muscle wasting has long been recognized as a major clinical problem in hemodialysis (HD) patients. In addition to its impact on quality of life, muscle wasting has been proven to be associated with increased mortality rates. Identification of the molecular mechanisms underlying muscle wasting in HD patients provides opportunities to resolve this clinical problem. Several signaling pathways and humeral factors have been reported to be involved in the pathogenic mechanisms of muscle wasting in HD patients, including ubiquitin-proteasome system, caspase-3, insulin/insulin-like growth factor-1 (IGF-1) signaling, endogenous glucocorticoids, metabolic acidosis, inflammation, and sex hormones. Targeting the aforementioned crucial signaling and molecules to suppress protein degradation and augment muscle strength has been extensively investigated in HD patients. In addition to exercise training, administration of megestrol acetate has been proven to be effective in improving anorexia and muscle wasting in HD patients. Correction of metabolic acidosis through sodium bicarbonate supplements can decrease muscle protein degradation and hormone therapy with nandrolone decanoate has been reported to increase muscle mass. Although thiazolidinedione has been shown to improve insulin sensitivity, its role in the treatment of muscle wasting remains unclear. This review paper focuses on the molecular pathways and potential new therapeutic approaches to muscle wasting in HD patients.

Insulin resistance and muscle metabolism in chronic kidney disease

Insulin resistance is a common finding in chronic kidney disease (CKD) and is manifested by mild fasting hyperglycemia and abnormal glucose tolerance testing. Circulating levels of glucocorticoids are high. In muscle, changes in the insulin signaling pathway occur. An increase in the regulatory p85 subunit of Class I phosphatidylinositol 3-Kinase enzyme leads to decreased activation of the downstream effector protein kinase B (Akt). Mechanisms promoting muscle proteolysis and atrophy are unleashed. The link of Akt to the ubiquitin proteasome pathway, a major degradation pathway in muscle, is discussed. Another factor associated with insulin resistance in CKD is angiotensin II (Ang II) which appears to induce its intracellular effects through inflammatory cytokines or reactive oxygen species. Skeletal muscle ATP is depleted and the ability of AMP-activated protein kinase (AMPK) to replenish energy stores is blocked. How this can be reversed is discussed. Interleukin-6 (IL-6) levels are elevated in CKD and impair insulin signaling at the level of IRS-1. With exercise, IL-6 levels are reduced; glucose uptake and utilization are increased. For patients with CKD, exercise may improve insulin signaling and build up muscle. Treatment strategies for preventing muscle atrophy are discussed.

Muscle protein metabolism during compensatory growth with changing dietary lysine levels from deficient to sufficient in growing rats

Livestock and laboratory animals show compensatory growth when they are fed ad libitum following a period of restriction feeding. Lysine is a major limiting essential amino acid in the diets both for humans and animals. We hypothesized that changing dietary lysine levels from deficient to sufficient induced compensatory growth in young rats. We elucidated the effect of lysine sufficiency on the dynamics of hormones, relevant to muscle protein synthesis and degradation, insulin-like growth factor-I (IGF-I) and corticosterone, and on the expression of proteolytic-related genes in skeletal muscle during compensatory growth. Lysine sufficiency where the dietary lysine level was increased from 0.46% to 1.30% after 2 wk of subjecting the rats to the lower lysine level induced 80% enhancement of growth rate of rats. During compensatory growth with the lysine sufficiency, fractional muscle protein synthesis rates were higher whereas fractional muscle protein degradation rates were lower than those of the control group (p<0.05). After lysine sufficiency, the expression of atrogin-1/MAFbx mRNA was decreased in gastrocnemius muscle (p<0.05). With the lysine sufficiency, serum IGF-I concentration increased (p<0.05) whereas serum corticosterone decreased (p<0.05). These findings suggest that compensatory growth with lysine sufficiency is due to a change of hormone levels before and after changing diets, resulting in incrementation of protein synthesis and suppression of protein degradation of skeletal muscle.

Various jobs of proteolytic enzymes in skeletal muscle during unloading: facts and speculations

Skeletal muscles, namely, postural muscles, as soleus, suffer from atrophy under disuse. Muscle atrophy development caused by unloading differs from that induced by denervation or other stimuli. Disuse atrophy is supposed to be the result of shift of protein synthesis/proteolysis balance towards protein degradation increase. Maintaining of the balance involves many systems of synthesis and proteolysis, whose activation leads to muscle adaptation to disuse rather than muscle degeneration. Here, we review recent data on activity of signaling systems involved in muscle atrophy development under unloading and muscle adaptation to the lack of support.

Decreased miR-29 suppresses myogenesis in CKD

The mechanisms underlying the muscle wasting that accompanies CKD are not well understood. Animal models suggest that impaired differentiation of muscle progenitor cells may contribute. Expression of the myogenesis-suppressing transcription factor Ying Yang-1 increases in muscle of animals with CKD, but the mechanism underlying this increased expression is unknown. Here, we examined a profile of microRNAs in muscles from mice with CKD and observed downregulation of both microRNA-29a (miR-29a) and miR-29b. Because miR-29 has a complementary sequence to the 3'-untranslated region of Ying Yang-1 mRNA, a decrease in miR-29 could increase Ying Yang-1. We used adenovirus-mediated gene transfer to express miR-29 in C2C12 myoblasts and measured its effect on both Ying Yang-1 and myoblast differentiation. An increase in miR-29 decreased the abundance of Ying Yang-1 and improved the differentiation of myoblasts into myotubes. Similarly, using myoblasts isolated from muscles of mice with CKD, an increase in miR-29 improved differentiation of muscle progenitor cells into myotubes. In conclusion, CKD suppresses miR-29 in muscle, which leads to higher expression of the transcription factor Ying Yang-1, thereby suppressing myogenesis. These data suggest a potential mechanism for the impaired muscle cell differentiation associated with CKD.

Proteolysis by the ubiquitin-proteasome system and kidney disease

Proteins in all cells turnover continuously such that rigorous control of proteolysis is required to govern levels of proteins with vastly different half-lives and actions including those regulating transcription, metabolic pathways, or the breakdown of muscle proteins to amino acids used in gluconeogenesis or the synthesis of new proteins. Critical cellular functions would be disrupted without precise regulation of protein degradation. Thus, it is surprising that the bulk of protein in all cells is degraded by the ATP-dependent, ubiquitin-proteasome system. The system achieves remarkable specificity by selective conjugation of ubiquitin (Ub) to a doomed protein in a process catalyzed by >1000 ubiquitin ligases that recognize individual substrate proteins. Because the pathogenesis of certain kidney diseases and their complications are linked to the function of the ubiquitin-proteasome system, understanding its mechanisms could lead to novel therapies.

Skeletal muscle growth defect in human growth hormone transgenic rat is accompanied by phenotypic changes in progenitor cells

Growth hormone (GH) is known to have a pivotal role in the maintenance of skeletal muscle mass. Sarcopenia, the loss of skeletal muscle mass, is a common phenomenon in aging, and it is widely accepted that sarcopenia is largely attributed to age-related decline in GH secretion. In the present study, we tested if human growth hormone transgenic rats (GH-TG rats) whose plasma GH levels are maintained relatively low could be an appropriate model for sarcopenia. Analyses of GH-TG rats revealed that they exhibit skeletal muscle growth defect as well as atrophy of myofibers. The number of myofibers in tibialis anterior muscle was comparable to that of WT rats, while the proportion of type I slow myofibers in tibialis anterior muscle was increased in GH-TG rats after 5 months. Neither increased expression of ubiquitin ligases, MuRF1 and MAFbx, nor indication of apoptotic cell death was observed. Notably, myogenic differentiation potential of skeletal muscle progenitor cells in GH-TG rats was lower than WT rats, and this was accompanied by increased adipogenic potential. These results indicate that GH-TG rats could be a useful model to elucidate the mechanism of sarcopenia induced by reduced GH action and raised the possibility that decreased GH action may cause an alteration of differentiation potential of skeletal muscle progenitor cells.

XIAP reduces muscle proteolysis induced by CKD

X-chromosome-linked inhibitor of apoptosis protein (XIAP) is an endogenous caspase inhibitor. Caspase-3 contributes to the muscle wasting associated with chronic kidney disease (CKD) and other systemic illnesses, but whether XIAP modulates muscle wasting in CKD is unknown. Here, overexpression of XIAP in cultured skeletal muscle cells decreased protein degradation induced by serum deprivation, suggesting that caspase-mediated proteolysis contributes to muscle atrophy. We generated transgenic mice that overexpress human XIAP specifically in skeletal muscle (mXIAP) and evaluated muscle protein degradation induced by CKD. mXIAP mice with normal kidney function exhibited mild skeletal muscle hypertrophy. Muscle weights of mXIAP mice with CKD (mXIAP-CKD) were indistinguishable from wild-type mice, suggesting that overexpression of XIAP in skeletal muscle protects from CKD-induced muscle atrophy. The rate of total protein degradation, proteasome chymotrypsin-like activity, and caspase-3-mediated actin cleavage all were lower in muscle isolated from mXIAP-CKD mice compared with wild-type CKD mice. Concomitant with the reduction in overall proteolysis, mRNA levels of ubiquitin, muscle-specific ring finger 1, and atrogin-1/muscle atrophy F-box were lower in mXIAP-CKD mice, suggesting that decreased expression of the ubiquitin-proteasome pathway components may contribute to the protein-sparing effects of XIAP. In summary, these results demonstrate that XIAP inhibits multiple aspects of protein degradation in skeletal muscle during CKD.

Comparison of proteolytic-related gene expression in the skeletal muscles of layer and broiler chickens

Previous studies have shown that the muscle protein degradation rates of broiler are lower than those of layer chickens. In this study, we assessed proteolytic-related gene expression in the pectoralis muscle of layer and broiler chickens. The mRNA levels of atrogin-1/MAFbx and proteasome C2 subunit, but not those of ubiquitin, m-calpain large subunit, cathepsin B, or caspase-3, were lower in the skeletal muscle of the broilers than in the layers at 7 and 14 d of age. We infer that the lower muscle protein degradation of broilers than of layers at least partly relates to lower mRNA expression of atrogin-1/MAFbx and proteasome C2 subunit in the skeletal muscle of broilers.

IL-6 and serum amyloid A synergy mediates angiotensin II-induced muscle wasting

Animal studies suggest that increased levels of circulating angiotensin II (AngII) could contribute to the loss of lean body mass in chronic kidney disease, but the mechanism by which this occurs is unclear. Here, AngII infusion increased circulating IL-6 and its hepatic production in wild-type mice, suggesting that AngII-induced inflammation may trigger muscle loss. AngII infusion also stimulated the suppressor of cytokine signaling (SOCS3) in muscle, which led to loss of insulin receptor substrate 1 (IRS-1), thereby impairing insulin/IGF-1 signaling and enhancing protein degradation. All of these responses to AngII were suppressed in IL-6-deficient mice. Recombinant human IL-6 (rhIL-6) treatment of cultured myotubes only minimally increased SOCS3, however, suggesting the contribution of other mediators. Because AngII increases hepatic serum amyloid A (SAA) expression in an IL-6-dependent manner, we treated wild-type mice with rhIL-6 and an SAA1-overexpressing adenovirus; the combination led to a significantly greater increase in SOCS3 and decrease in IRS-1 compared with either rhIL-6 or SAA1 alone. We observed similar effects on SOCS3 and IRS-1 when we treated cultured muscle myotubes with rhIL-6 and SAA1. Taken together, these results suggest an interorgan response to high levels of AngII: Hepatic production of IL-6 and SAA increases, and these mediators act synergistically to impair insulin/IGF-1 signaling, which promotes muscle proteolysis. Targeting the high levels of IL-6 and SAA in catabolic disorders might be a therapeutic approach to prevent muscle wasting.

Stressed out: the skeletal muscle ryanodine receptor as a target of stress

Over the past century, understanding the mechanisms underlying muscle fatigue and weakness has been the focus of much investigation. However, the dominant theory in the field, that lactic acidosis causes muscle fatigue, is unlikely to tell the whole story. Recently, dysregulation of sarcoplasmic reticulum (SR) Ca(2+) release has been associated with impaired muscle function induced by a wide range of stressors, from dystrophy to heart failure to muscle fatigue. Here, we address current understandings of the altered regulation of SR Ca(2+) release during chronic stress, focusing on the role of the SR Ca(2+) release channel known as the type 1 ryanodine receptor.

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; ).

Myosin accumulation and striated muscle myopathy result from the loss of muscle RING finger 1 and 3

Maintenance of skeletal and cardiac muscle structure and function requires precise control of the synthesis, assembly, and turnover of contractile proteins of the sarcomere. Abnormalities in accumulation of sarcomere proteins are responsible for a variety of myopathies. However, the mechanisms that mediate turnover of these long-lived proteins remain poorly defined. We show that muscle RING finger 1 (MuRF1) and MuRF3 act as E3 ubiquitin ligases that cooperate with the E2 ubiquitin-conjugating enzymes UbcH5a, -b, and -c to mediate the degradation of beta/slow myosin heavy chain (beta/slow MHC) and MHCIIa via the ubiquitin proteasome system (UPS) in vivo and in vitro. Accordingly, mice deficient for MuRF1 and MuRF3 develop a skeletal muscle myopathy and hypertrophic cardiomyopathy characterized by subsarcolemmal MHC accumulation, myofiber fragmentation, and diminished muscle performance. These findings identify MuRF1 and MuRF3 as key E3 ubiquitin ligases for the UPS-dependent turnover of sarcomeric proteins and reveal a potential basis for myosin storage myopathies.

AMPK activation stimulates myofibrillar protein degradation and expression of atrophy-related ubiquitin ligases by increasing FOXO transcription factors in C2C12 myotubes

In skeletal muscle, AMP-activated protein kinase (AMPK) is a metabolic master switch regulating glucose and lipid metabolism. Recently, AMPK has been implicated in the control of protein synthesis in skeletal muscle, but the effect of AMPK activation on myofibrillar protein degradation has yet to be elucidated. The present study was designed to examine the effect of 5-aminoimidazole-4-carboxamide-1-beta-D-ribonucleoside (AICAR)-induced AMPK signaling on effector mechanisms of myofibrillar protein degradation and the expression of atrophy-related genes (atrogin-1/MAFbx, MuRF1, proteasome C2 subunit, calpains, cathepsin B, and caspase-3) in C2C12 myotubes. AICAR stimulated myofibrillar protein degradation (as measured by N(tau)-methylhistidine release), while also increasing the levels of atrogin-1/MAFbx and MuRF1 mRNA, but the expression of other atrophy-related genes was not enhanced by AICAR treatment in C2C12 myotubes. AICAR also stimulated the level of FOXO transcription factors mRNA and protein in C2C12 myotubes. These results indicate that activation of AMPK stimulates myofibrillar protein degradation through the expression of atrogin-1/MAFbx and MuRF1 by increasing FOXO transcription factors in skeletal muscles.

Activity of the Akt-dependent anabolic and catabolic pathways in muscle and liver samples in cancer-related cachexia

In animal models of cachexia, alterations in the phosphatidylinositol 3-kinase (PI3-K)/Akt pathway have been demonstrated in atrophying skeletal muscles. Therefore, we assessed the activity of proteins in this pathway in muscle and liver biopsies from 16 patients undergoing pancreatectomy for suspect of carcinoma. Patients were divided in a non-cachectic or cachectic group according to their weight loss before operation. Extracts of skeletal muscle and liver tissue from eight cachectic patients with pancreas carcinoma and eight non-cachectic patients were analysed by Western blotting using pan- and phospho-specific antibodies directed against eight important signal transduction proteins of the PI3-K/Akt pathway. Muscle samples from cachectic patients revealed significantly decreased levels of myosin heavy chain (-45%) and actin (-18%) in comparison to non-cachectic samples. Akt protein level was decreased by -55%. The abundance and/or phosphorylation of the transcription factors Foxo1 and Foxo3a were reduced by up to fourfold in muscle biopsies from cachectic patients. Various decreases of the phosphorylated forms of the protein kinases mTOR (-82%) and p70S6K (-39%) were found. In contrast to skeletal muscle, cachexia is associated with a significant increase in phosphorylated Akt level in the liver samples with a general activation of the PI3-K/Akt cascade. Our study demonstrates a cachexia-associated loss of Akt-dependent signalling in human skeletal muscle with decreased activity of regulators of protein synthesis and a disinhibition of protein degradation.

Development of a diagnostic method for detecting increased muscle protein degradation in patients with catabolic conditions

Muscle atrophy in catabolic illnesses is due largely to accelerated protein degradation. Unfortunately, methods for detecting accelerated muscle proteolysis are cumbersome. The goal of this study was to develop a method for detecting muscle protein breakdown and assess the effectiveness of anticatabolic therapy. In rodent models of catabolic conditions, it was found that accelerated muscle protein degradation is triggered by activation of caspase-3. Caspase-3 cleaves actomyosin/myofibrils to form substrates for the ubiquitin-proteasome system and leaves a characteristic 14-kD actin fragment in the insoluble fraction of a muscle lysate. Muscle biopsies were obtained from normal adults and three groups of patients: 14 who were undergoing hip arthroplasty, 28 hemodialysis patients who were participating in exercise programs, and seven severely burned patients. In muscle of patients who were undergoing hip arthroplasty, the 14-kD actin fragment level was correlated (r = 0.787, P < 0.01) with the fractional rate of protein degradation. In muscle of hemodialysis patients who were undergoing endurance exercise training, the 14-kD actin fragment decreased to values similar to levels in normal adults; strength training did not significantly decrease the actin fragment. Severely burned patients had increased muscle protein degradation and actin fragment levels, but the two measures were not significantly correlated. The experimental results suggest that the 14-kD actin fragment in muscle biopsies is increased in catabolic states and could be used in conjunction with other methods to detect and monitor changes in muscle proteolysis that occur in patients with mild or sustained increases in muscle proteolysis.

Chronic kidney disease causes defects in signaling through the insulin receptor substrate/phosphatidylinositol 3-kinase/Akt pathway: implications for muscle atrophy

Complications of chronic kidney disease (CKD) include depressed responses to insulin/IGF-1 and accelerated muscle proteolysis as a result of activation of caspase-3 and the ubiquitin-proteasome system. Experimentally, proteolysis in muscle cells occurs when there is suppression of phosphatidylinositol 3-kinase (PI3-K) activity. Postreceptor signaling through the insulin receptor substrate (IRS)/PI3-K/Akt pathway was evaluated in muscles of acidotic, CKD and pair-fed control rats under physiologic conditions and in response to a dose of insulin that quickly stimulated the pathway. Basal IRS-1-associated PI3-K activity was suppressed by CKD; IRS-2-associated PI3-K activity was increased. The basal level of activated Akt in CKD muscles also was low, indicating that the higher IRS-2-associated PI3-K activity did not compensate for the reduced IRS-1-associated PI3-K activity. Insulin treatment overcame this abnormality. The low IRS-1-associated PI3-K activity in muscle was not due to a decrease in IRS-1 protein, but there was a higher amount of the PI3-K p85 subunit protein without a concomitant increase in the p110 catalytic subunit, offering a potential explanation for the lower IRS-1-associated PI3-K activity. Eliminating the acidosis of CKD partially corrected the decrease in basal IRS-1-associated PI3-K activity and protein degradation in muscle. It is concluded that in CKD, acidosis and an increase in the PI3-K p85 subunit are mechanisms that contribute to suppression of PI3-K activity in muscle, and this leads to accelerated muscle proteolysis.

Increased caspase-3 expression and activity contribute to reduced CD3zeta expression in systemic lupus erythematosus T cells

T cells isolated from patients with systemic lupus erythematosus (SLE) express low levels of CD3zeta-chain, a critical molecule involved in TCR-mediated signaling, but the involved mechanisms are not fully understood. In this study we examined caspase-3 as a candidate for cleaving CD3zeta in SLE T cells. We demonstrate that SLE T cells display increased expression and activity of caspase-3. Treatment of SLE T cells with the caspase-3 inhibitor Z-Asp-Glu-Val-Asp-FMK reduced proteolysis of CD3zeta and enhanced its expression. In addition, Z-Asp-Glu-Val-Asp-FMK treatment increased the association of CD3zeta with lipid rafts and simultaneously reversed the abnormal lipid raft preclustering, heightened TCR-induced calcium responses, and reduced the expression of FcRgamma-chain exclusively in SLE T cells. We conclude that caspase-3 inhibitors can normalize SLE T cell function by limiting the excessive digestion of CD3zeta-chain and suggest that such molecules can be considered in the treatment of this disease.

Molecular mechanisms activating muscle protein degradation in chronic kidney disease and other catabolic conditions

Muscle atrophy is a prominent feature of chronic kidney disease (CKD) and is frequent in other catabolic conditions. Results from animal models of these conditions as well as patients indicate that atrophy is mainly owing to accelerated muscle proteolysis in the ubiquitin-proteasome (Ub-P'some) proteolytic system. The Ub-P'some system, however, rapidly degrades actin or myosin but cannot breakdown actomyosin or myofibrils. Consequently, another protease must initially cleave the complex structure of muscle. We identified caspase-3 as an initial and potentially rate-limiting proteolytic step that cleaves actomyosin/myofibrils to produce substrates degraded by the Ub-P'some system. In rodent models of CKD and other catabolic conditions, we find that caspase-3 is activated and cleaves actomyosin to actin, myosin and their fragments. This initial proteolytic step in muscle leaves a characteristic footprint, a 14-kDa actin band, providing a potential diagnostic tool to detect muscle catabolism. We also found that stimulation of caspase-3 activity depends on inhibition of IRS-1-associated phosphatidylinositol 3-kinase (PI3K) activity; inhibiting PI3K in muscle cells also leads to expression of a critical E3-ubiquitin-conjugating enzyme involved in muscle protein breakdown: atrogin-1/MAFbx. Thus, protein breakdown by caspase-3 and the ubiquitin-proteasome system in muscle are stimulated by the same signal: a low PI3K activity. These responses could yield therapeutic strategies to block muscle atrophy.