Sensitivity of myofibrillar proteins to glucocorticoid-induced muscle proteolysis

A critical discussion on the relationship between E3 ubiquitin ligases, protein degradation, and skeletal muscle wasting: it's not that simple

Ubiquitination is an important post-translational modification (PTM) for protein substrates, whereby ubiquitin is added to proteins through the coordinated activity of activating (E1), ubiquitin-conjugating (E2), and ubiquitin ligase (E3) enzymes. The E3s provide key functions in the recognition of specific protein substrates to be ubiquitinated and aid in determining their proteolytic or nonproteolytic fates, which has led to their study as indicators of altered cellular processes. MuRF1 and MAFbx/Atrogin-1 were two of the first E3 ubiquitin ligases identified as being upregulated in a range of different skeletal muscle atrophy models. Since their discovery, the expression of these E3 ubiquitin ligases has often been studied as a surrogate measure of changes to bulk protein degradation rates. However, emerging evidence has highlighted the dynamic and complex regulation of the ubiquitin proteasome system (UPS) in skeletal muscle and demonstrated that protein ubiquitination is not necessarily equivalent to protein degradation. These observations highlight the potential challenges of quantifying E3 ubiquitin ligases as markers of protein degradation rates or ubiquitin proteasome system (UPS) activation. This perspective examines the usefulness of monitoring E3 ubiquitin ligases for determining specific or bulk protein degradation rates in the settings of skeletal muscle atrophy. Specific questions that remain unanswered within the skeletal muscle atrophy field are also identified, to encourage the pursuit of new research that will be critical in moving forward our understanding of the molecular mechanisms that govern protein function and degradation in muscle.

Effects of the Combination of β-Hydroxy-β-Methyl Butyrate and R(+) Lipoic Acid in a Cellular Model of Sarcopenia

: Sarcopenia is a clinical problem associated with several pathological and non-pathological conditions. The aim of the present research is the evaluation of the pharmacological profile of the leucine metabolite β-hydroxy-β-methyl butyrate (HMB) associated with the natural R(+) stereoisomer of lipoic acid (R(+)LA) in a cellular model of muscle wasting. The C2C12 cell line is used as myoblasts or is differentiated in myotubes, sarcopenia is induced by dexamethasone (DEX). A Bonferroni significant difference procedure is used for a post hoc comparison. DEX toxicity (0.01-300 µM concentration range) is evaluated in myoblasts to measure cell viability and caspase 3 activation after 24 h and 48 h; cell incubation with 1 µM DEX for 48 h is chosen as optimal treatment for decreasing cell viability and increasing caspase 3 activity. R(+)LA or HMB significantly prevents DEX-induced cell mortality; the efficacy is improved when 100 µM R(+)LA is combined with 1 mM HMB. Regarding myoblasts, this combination significantly reduces DEX-evoked O2- production and protein oxidative damage. During the early phase of myotube formation, the mixture preserves the number of myogenin-positive cells, whereas it completely prevents the DEX-dependent damage in a later phase of myotube differentiation (7 days), as evaluated by cell diameter and percentage of multinucleated cells. R(+)LA in association with HMB is suggested for sarcopenia therapy.

Effects of heat stress on animal physiology, metabolism, and meat quality: A review

Heat stress is one of the most stressful events in the life of livestock with harmful consequences for animal health, productivity and product quality. Ruminants, pigs and poultry are susceptible to heat stress due to their rapid metabolic rate and growth, high level of production, and species-specific characteristics such as rumen fermentation, sweating impairment, and skin insulation. Acute heat stress immediately before slaughter stimulates muscle glycogenolysis and can result in pale, soft and exudative (PSE) meat characterized by low water holding capacity (WHC). By contrast, animals subjected to chronic heat stress, have reduced muscle glycogen stores resulting in dark, firm and dry (DFD) meat with high ultimate pH and high WHC. Furthermore, heat stress leads to oxidative stress, lipid and protein oxidation, and reduced shelf life and food safety due to bacterial growth and shedding. This review discusses the scientific evidence regarding the effects of heat stress on livestock physiology and metabolism, and their consequences for meat quality and safety.

Corticosterone-Mediated Body Weight Loss Is an Important Catabolic Process for Poststroke Immunity and Survival

Background and Purpose- Stroke-induced acute severe body weight (BW) loss is associated with a high rate of mortality during a critical poststroke period. Several interventions to reduce weight loss, however, have not been successful. Currently, the biological significance of this extraordinary catabolic process is not well understood. Spleen-derived monocytes/macrophages (MMs) are the major immune cells recruited to the injured brain. The trafficking of MMs has been shown to be important for tissue repair and recovery. The purpose of the study is to investigate whether the BW reduction is essential for MM-mediated immune response for mice to survive and whether a corticosterone-mediated catabolic event underlies the processes. Methods- C57BL/6 male mice (12-week-old) were subjected to transient middle cerebral artery occlusion. BW, total MMs, and their Ly-6Chigh and Ly-6Clow subsets were determined in the spleen, blood, and the brain in poststroke mice. Poststroke survival rate and MM subsets were determined in mice with adrenalectomy, sham-adrenalectomy, and adrenalectomy mice supplemented with corticosterone. Results- Stroke reduced BW with a maximum reduction at day 3 poststroke (17.2±5.2%). The reduction at day 3 was positively linked to injury severity and selective depletion of MMs, but no other types of immune cells, in the spleen. Notably, the splenic MM depletion was significantly greater in mice with severe BW reduction (≥18% at day 3). In the blood, stroke depleted circulating MMs to a similar degree in animals with moderate and severe BW loss. Ly-6C+ monocyte infiltration in the poststroke brain was greater in mice with severe BW loss. Blocking the catabolic process by adrenalectomy significantly increased poststroke mortality, but the mortality was partially rescued by corticosterone supplement in adrenalectomy mice. Conclusions- Stroke-induced BW loss facilitates MM-mediated immune response, and the adrenal corticosterone-mediated catabolic process is necessary for poststroke survival. Visual Overview- An online visual overview is available for this article.

Age-related changes in central effects of corticotropin-releasing factor (CRF) suggest a role for this mediator in aging anorexia and cachexia

Hypothalamic corticotropin-releasing factor (CRF) lays downstream to catabolic melanocortins and at least partly mediates their catabolic effects. Age-related changes in the melanocortin system (weak responsiveness in middle-aged and a strong one in old rats) have been shown to contribute to middle-aged obesity and later to aging anorexia and cachexia of old age groups. We hypothesized that catabolic (anorexigenic and hypermetabolic) CRF effects vary with aging similarly to those of melanocortins. Thus, we aimed to test whether age-related variations of CRF effects may also contribute to middle-aged obesity and aging anorexia leading to weight loss of old age groups. Food intake, body weight, core temperature, heart rate, and activity were recorded in male Wistar rats of young, middle-aged, aging, and old age groups (from 3 to 24 months) during a 7-day intracerebroventricular CRF infusion (0.2 μg/μl/h) in a biotelemetric system. In addition, CRF gene expression was also assessed by quantitative RT-PCR in the paraventricular nucleus (PVN) of intact animals of the same age groups. The infusion suppressed body weight in the young, aging, and old rats, but not in middle-aged animals. Weak anorexigenic and hypermetabolic effects were detected in the young, whereas strong anorexia (without hypermetabolism) developed in the oldest age groups in which post mortem analysis showed also a reduction of retroperitoneal fat mass. CRF gene expression in the PVN increased with aging. Our results support the potential contribution of age-related changes in CRF effects to aging anorexia and cachexia. The role of the peptide in middle-aged obesity cannot be confirmed.

Cerebral Oedema, Blood-Brain Barrier Breakdown and the Decrease in Na(+),K(+)-ATPase Activity in the Cerebral Cortex and Hippocampus are Prevented by Dexamethasone in an Animal Model of Maple Syrup Urine Disease

Maple syrup urine disease (MSUD) is a rare metabolic disorder associated with acute and chronic brain dysfunction. This condition has been shown to lead to macroscopic cerebral alterations that are visible on imaging studies. Cerebral oedema is widely considered to be detrimental for MSUD patients; however, the mechanisms involved are still poorly understood. Therefore, we investigated whether acute administration of branched-chain amino acids (BCAA) causes cerebral oedema, modifies the Na(+),K(+)-ATPase activity, affects the permeability of the blood-brain barrier (BBB) and alters the levels of cytokines in the hippocampus and cerebral cortex of 10-day-old rats. Additionally, we investigated the influence of concomitant administration of dexamethasone on the alterations caused by BCAA. Our results showed that the animals submitted to the model of MSUD exhibited an increase in the brain water content, both in the cerebral cortex and in the hippocampus. By investigating the mechanism of cerebral oedema, we discovered an association between H-BCAA and the Na(+),K(+)-ATPase activity and the permeability of the BBB to small molecules. Moreover, the H-BCAA administration increases Il-1β, IL-6 and TNF-α levels in the hippocampus and cerebral cortex, whereas IL-10 levels were decreased in the hippocampus. Interestingly, we showed that the administration of dexamethasone successfully reduced cerebral oedema, preventing the inhibition of Na(+),K(+)-ATPase activity, BBB breakdown and the increase in the cytokines levels. In conclusion, these findings suggest that dexamethasone can improve the acute cerebral oedema and brain injury associated with high levels of BCAA, either through a direct effect on brain capillary Na(+),K(+)-ATPase or through a generalized effect on the permeability of the BBB to all compounds.

Compensatory growth of C2C12 myotubes induced by the combined effect of lysine sufficiency and modulation of IGF-I and glucocorticoid levels

We have reported that a change from a lysine-deficient diet to a lysine-sufficient diet induced compensatory growth in rats and pigs. The aim of the present study was to determine whether compensatory growth of C2C12 myotubes occurs only by sufficiency of lysine or also by the synergic effect of sufficiency of lysine and modulation of the levels of insulin-like growth factor-I (IGF-I) and glucocorticoid in a medium. The results provide the first evidence of compensatory growth of C2C12 myotubes induced by sufficiency of a single amino acid in combination with modulation of the levels of IGF-I and glucocorticoid.

Leucine and HMB differentially modulate proteasome system in skeletal muscle under different sarcopenic conditions

In the present study we have compared the effects of leucine supplementation and its metabolite β-hydroxy-β-methyl butyrate (HMB) on the ubiquitin-proteasome system and the PI3K/Akt pathway during two distinct atrophic conditions, hindlimb immobilization and dexamethasone treatment. Leucine supplementation was able to minimize the reduction in rat soleus mass driven by immobilization. On the other hand, leucine supplementation was unable to provide protection against soleus mass loss in dexamethasone treated rats. Interestingly, HMB supplementation was unable to provide protection against mass loss in all treatments. While solely fiber type I cross sectional area (CSA) was protected in immobilized soleus of leucine-supplemented rats, none of the fiber types were protected by leucine supplementation in rats under dexamethasone treatment. In addition and in line with muscle mass results, HMB treatment did not attenuate CSA decrease in all fiber types against either immobilization or dexamethasone treatment. While leucine supplementation was able to minimize increased expression of both Mafbx/Atrogin and MuRF1 in immobilized rats, leucine was only able to minimize Mafbx/Atrogin in dexamethasone treated rats. In contrast, HMB was unable to restrain the increase in those atrogenes in immobilized rats, but in dexamethasone treated rats, HMB minimized increased expression of Mafbx/Atrogin. The amount of ubiquitinated proteins, as expected, was increased in immobilized and dexamethasone treated rats and only leucine was able to block this increase in immobilized rats but not in dexamethasone treated rats. Leucine supplementation maintained soleus tetanic peak force in immobilized rats at normal level. On the other hand, HMB treatment failed to maintain tetanic peak force regardless of treatment. The present data suggested that the anti-atrophic effects of leucine are not mediated by its metabolite HMB.

Muscle wasting from kidney failure-a model for catabolic conditions

PURPOSE

Muscle atrophy is a frequent complication of chronic kidney disease (CKD) and is associated with increased morbidity and mortality. The processes causing loss of muscle mass are also present in several catabolic conditions. Understanding the pathogenesis of CKD-induced muscle loss could lead to therapeutic interventions that prevent muscle wasting in CKD and potentially, other catabolic conditions.

MAJOR FINDINGS

Insulin or IGF-1 resistance caused by CKD, acidosis, inflammation, glucocorticoids or cancer causes defects in insulin-stimulated intracellular signaling that suppresses IRS-1 activity leading to decreased phosphorylation of Akt (p-Akt). A low p-Akt activates caspase-3 which provides muscle proteins substrates of the ubiquitin-proteasome system (UPS). A low p-Akt also leads to decreased phosphorylation of forkhead transcription factors which enter the nucleus to stimulate the expression of atrogin-1/MAFbx and MuRF1, E3 ubiquitin ligases that can be associated with proteolysis of muscle cells by the UPS. Caspase-3 also stimulates proteasome-dependent proteolysis in muscle.

SUMMARY

In CKD, diabetes, inflammatory conditions or in response to acidosis or excess glucocorticoids, insulin resistance develops, initiating reduced IRS-1/PI3K/Akt signaling. In CKD, this reduces p-Akt which stimulates muscle proteolysis by activating caspase-3 and the UPS. Second, caspase-3 cleaves actomyosin yielding substrates for the UPS and increased proteasome-mediated proteolysis. Third, p-Akt down-regulation suppresses myogenesis in CKD. Fourth, exercise in CKD stimulates insulin/IGF-1 signaling to reduce muscle atrophy. Lastly, there is evidence that microRNAs influence insulin signaling providing a potential opportunity to design therapeutic interventions. This article is part of a Directed Issue entitled: Molecular basis of muscle wasting.

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.

Transcriptional response of skeletal muscle to a low-protein gestation diet in porcine offspring accumulates in growth- and cell cycle-regulating pathways

Inadequate maternal protein supply during gestation represents an environmental factor that affects physiological signaling pathways with long-term consequences for growth, function, and structure of various tissues. Hypothesizing that the offspring's transcriptome is persistently altered by maternal diets, we used a porcine model to monitor the longitudinal expression changes in muscle to identify pathways relevant to fetal initiation of postnatal growth and development. German Landrace gilts were fed isoenergetic gestational diets containing 6.5% (LP) or 12.1% protein. The longissimus dorsi samples were collected from offspring at 94 days postconception (dpc) and 1, 28, and 188 days postnatum (dpn) for expression profiling. At 94 dpc, 1 dpn, and 28 dpn relatively few transcripts (<130) showed an altered abundance between the dietary groups. In fact, at 94 dpc genes of G2/M checkpoint regulation and mitotic roles of Polo-like kinases showed lowered transcript abundance in LP. At 188 dpn 677 transcripts were altered including those related to oxidative phosphorylation, citrate cycle, fatty acid metabolism (higher abundance in LP) and cell cycle regulation (lower abundance in LP). Correspondingly, transcriptional alterations during pre and postnatal development differed considerably among dietary groups, particularly for genes related to cell cycle regulation (G1/S and G2/M checkpoint regulation; cyclines), growth factor signaling (GH, IGF1, mTOR, RAN, VEGF, INSR), lipid metabolism, energy metabolism, and nucleic acid metabolism. In skeletal muscle, fetal programming related to maternal LP diets disturbed gene expression in growth-related pathways into adulthood. Diet-dependent gene expression may hamper proper development, thereby affecting signaling pathways related to energy utilization.

Muscle wasting in animal models of severe illness

Muscle wasting is a serious complication of various clinical conditions that significantly worsens the prognosis of the illnesses. Clinically relevant models of muscle wasting are essential for understanding its pathogenesis and for selective preclinical testing of potential therapeutic agents. The data presented here indicate that muscle wasting has been well characterized in rat models of sepsis (endotoxaemia, and caecal ligation and puncture), in rat models of chronic renal failure (partial nephrectomy), in animal models of intensive care unit patients (corticosteroid treatment combined with peripheral denervation or with administration of neuromuscular blocking drugs) and in murine and rat models of cancer (tumour cell transplantation). There is a need to explore genetically engineered mouse models of cancer. The degree of protein degradation in skeletal muscle is not well characterized in animal models of liver cirrhosis, chronic heart failure and chronic obstructive pulmonary disease. The major difficulties with all models are standardization and high variation in disease progression and a lack of reflection of clinical reality in some of the models. The translation of the information obtained by using these models to clinical practice may be problematic.

In utero glucocorticoid exposure reduces fetal skeletal muscle mass in rats independent of effects on maternal nutrition

Maternal stress and undernutrition can occur together and expose the fetus to high glucocorticoid (GLC) levels during this vulnerable period. To determine the consequences of GLC exposure on fetal skeletal muscle independently of maternal food intake, groups of timed-pregnant Sprague-Dawley rats (n = 7/group) were studied: ad libitum food intake (control, CON); ad libitum food intake with 1 mg dexamethasone/l drinking water from embryonic day (ED)13 to ED21 (DEX); pair-fed (PF) to DEX from ED13 to ED21. On ED22, dams were injected with [(3)H]phenylalanine for measurements of fetal leg muscle and diaphragm fractional protein synthesis rates (FSR). Fetal muscles were analyzed for protein and RNA contents, [(3)H]phenylalanine incorporation, and MuRF1 and atrogin-1 (MAFbx) mRNA expression. Fetal liver tyrosine aminotransferase (TAT) expression was quantified to assess fetal exposure to GLCs. DEX treatment reduced maternal food intake by 13% (P < 0.001) and significantly reduced placental mass relative to CON and PF dams. Liver TAT expression was elevated only in DEX fetuses (P < 0.01). DEX muscle protein masses were 56% and 70% than those of CON (P < 0.01) and PF (P < 0.05) fetuses, respectively; PF muscles were 80% of CON (P < 0.01). Muscle FSR decreased by 35% in DEX fetuses (P < 0.001) but were not different between PF and CON. Only atrogin-1 expression was increased in DEX fetus muscles. We conclude that high maternal GLC levels and inadequate maternal food intake impair fetal skeletal muscle growth, most likely through different mechanisms. When combined, the effects of decreased maternal intake and maternal GLC intake on fetal muscle growth are additive.

Muscle sparing in muscle RING finger 1 null mice: response to synthetic glucocorticoids

Skeletal muscle atrophy occurs under a variety of conditions and can result from alterations in both protein synthesis and protein degradation. The muscle-specific E3 ubiquitin ligases, MuRF1 and MAFbx, are excellent markers of muscle atrophy and increase under divergent atrophy-inducing conditions such as denervation and glucocorticoid treatment. While deletion of MuRF1 or MAFbx has been reported to spare muscle mass following 14 days of denervation, their role in other atrophy-inducing conditions is unclear. The goal of this study was to determine whether deletion of MuRF1 or MAFbx attenuates muscle atrophy after 2 weeks of treatment with the synthetic glucocorticoid dexamethasone (DEX). The response of the triceps surae (TS) and tibialis anterior (TA) muscles to 14 days of DEX treatment (3 mg kg(-1) day(-1)) was examined in 4 month-old male and female wild type (WT) and MuRF1 or MAFbx knock out (KO) mice. Following 14 days of DEX treatment, muscle wet weight was significantly decreased in the TS and TA of WT mice. Comparison of WT and KO mice following DEX treatment revealed significant sparing of mass in both sexes of the MuRF1 KO mice, but no muscle sparing in MAFbx KO mice. Further analysis of the MuRF1 KO mice showed significant sparing of fibre cross-sectional area and tension output in the gastrocnemius (GA) after DEX treatment. Muscle sparing in the MuRF1 KO mice was related to maintenance of protein synthesis, with no observed increases in protein degradation in either WT or MuRF1 KO mice. These results demonstrate that MuRF1 and MAFbx do not function similarly under all atrophy models, and that the primary role of MuRF1 may extend beyond controlling protein degradation via the ubiquitin proteasome system.

Limited and excess protein intake of pregnant gilts differently affects body composition and cellularity of skeletal muscle and subcutaneous adipose tissue of newborn and weanling piglets

AIM

This study investigated whether dietary protein intake less (50%) or greater (250%) than requirements throughout gestation differently affects offspring body composition and cellular properties of skeletal muscle and subcutaneous adipose tissue (SCAT).

METHODS

Primiparous gilts were fed iso-energetic diets containing adequate (22 AP), high (21 HP), or low (19 LP) protein contents. Newborn (n = 166) and weanling piglets cross-fostered to sows fed a standard diet (day 28; n = 83) were examined by morphological, biochemical, histological, and molecular analyses of the body, SCAT, and semitendinosus, longissimus, biceps femoris muscles.

RESULTS

Lowered birth weight (BW) in response to the HP and LP diets (p < 0.01) resulted from decreases in all body constituents in LP, and mainly from reduced body fat in HP piglets (p < 0.05). In the light BW class within litters, HP piglets exhibited a greater percentage of muscle tissue (p < 0.05) than LP piglets. Less SCAT mass in HP and LP piglets resulted from reduced (p < 0.05) number, but not the size of adipocytes. The LP diet adversely affected myogenesis and muscular differentiation derived from less (p < 0.01) primary and secondary myofibers, lower creatine kinase activity (p < 0.05), less IGF2 mRNA (p < 0.10), and greater expression of the embryonic myosin heavy chain isoform (p < 0.01). Catch-up growth of LP but not HP pigs until day 28 increased body fat (p = 0.01). Despite compensated muscle growth in LP piglets, the deficit in myofiber number remained.

CONCLUSION

Poor intrauterine environment by limited and excess protein supply retards fetal growth, but only limited protein supply impairs myogenesis, persistently restricts muscle growth potential, and favors obesity at infancy.

Muscle atrophy in chronic kidney disease results from abnormalities in insulin signaling

Muscle atrophy is a significant consequence of chronic kidney disease that increases a patient's risk of mortality and decreases their quality of life. The loss of lean body mass results, in part, from an increase in the rate of muscle protein degradation. In this review, the proteolytic systems that are activated during chronic kidney disease and the key insulin signaling pathways that regulate the protein degradative processes are described.

REDD1 is a major target of testosterone action in preventing dexamethasone-induced muscle loss

Glucocorticoids are a well-recognized and common cause of muscle atrophy that can be prevented by testosterone. However, the molecular mechanisms underlying such protection have not been described. Thus, the global effects of testosterone on dexamethasone-induced changes in gene expression were evaluated in rat gastrocnemius muscle using DNA microarrays. Gene expression was analyzed after 7-d administration of dexamethasone, dexamethasone plus testosterone, or vehicle. Dexamethasone changed expression of 876 probe sets by at least 2-fold. Among these, 474 probe sets were changed by at least 2-fold in the opposite direction in the dexamethasone plus testosterone group (genes in opposition). Major biological themes represented by genes in opposition included IGF-I signaling, myogenesis and muscle development, and cell cycle progression. Testosterone completely prevented the 22-fold increase in expression of the mammalian target of rapamycin (mTOR) inhibitor regulated in development and DNA damage responses 1 (REDD1), and attenuated dexamethasone induced increased expression of eIF4E binding protein 1, Forkhead box O1, and the p85 regulatory subunit of the IGF-I receptor but prevented decreased expression of IRS-1. Testosterone attenuated increases in REDD1 protein in skeletal muscle and L6 myoblasts and prevented dephosphorylation of p70S6 kinase at the mTOR-dependent site Thr389 in L6 myoblast cells. Effects of testosterone on REDD1 mRNA levels occurred within 1 h, required the androgen receptor, were blocked by bicalutamide, and were due to inhibition of transcriptional activation of REDD1 by dexamethasone. These data suggest that testosterone blocks dexamethasone-induced changes in expression of REDD1 and other genes that collectively would otherwise down-regulate mTOR activity and hence also down-regulate protein synthesis.

USP19-deubiquitinating enzyme regulates levels of major myofibrillar proteins in L6 muscle cells

The ubiquitin-proteasome system plays an important role in the degradation of myofibrillar proteins that occurs in muscle wasting. Many studies have demonstrated the importance of enzymes mediating conjugation of ubiquitin. However, little is known about the role of deubiquitinating enzymes. We previously showed that the USP19-deubiquitinating enzyme is induced in atrophying skeletal muscle (Combaret L, Adegoke OA, Bedard N, Baracos V, Attaix D, Wing SS. Am J Physiol Endocrinol Metab 288: E693-E700, 2005). To further explore the role of USP19, we used small interfering RNA (siRNA) in L6 muscle cells. Lowering USP19 by 70-90% in myotubes resulted in a 20% decrease in the rate of proteolysis and an 18% decrease in the rate of protein synthesis, with no net change in protein content. Despite the decrease in overall synthesis, there were approximately 1.5-fold increases in protein levels of myosin heavy chain (MHC), actin, and troponin T and a approximately 2.5-fold increase in tropomyosin. USP19 depletion also increased MHC and tropomyosin mRNA levels, suggesting that this effect is due to increased transcription. Consistent with this, USP19 depletion increased myogenin protein and mRNA levels approximately twofold. Lowering myogenin using siRNA prevented the increase in MHC and tropomyosin upon USP19 depletion, indicating that myogenin mediated the increase in myofibrillar proteins. Dexamethasone treatment lowered MHC and increased USP19. Depletion of USP19 reversed the dexamethasone suppression of MHC. These studies demonstrate that USP19 modulates transcription of major myofibrillar proteins and indicate that the ubiquitin system not only mediates the increased protein breakdown but is also involved in the decreased protein synthesis in atrophying skeletal muscle.

Endogenous glucocorticoids and impaired insulin signaling are both required to stimulate muscle wasting under pathophysiological conditions in mice

Muscle wasting is associated with a number of pathophysiologic conditions, including metabolic acidosis, diabetes, sepsis, and high angiotensin II levels. Under these conditions, activation of muscle protein degradation requires endogenous glucocorticoids. As the mechanism(s) underlying this dependence on glucocorticoids have not been identified, we analyzed the effects of glucocorticoids on muscle wasting in a mouse model of acute diabetes. Adrenalectomized, acutely diabetic mice given a physiologic dose of glucocorticoids exhibited decreased IRS-1-associated PI3K activity in muscle and progressive muscle atrophy. These responses were related to increased association of PI3K with the glucocorticoid receptor (GR). In mice with muscle-specific GR deletion (referred to as MGRKO mice), acute diabetes minimally suppressed IRS-1-associated PI3K activity in muscle and did not cause muscle atrophy. However, when a physiologic dose of glucocorticoids was given to mice with muscle-specific IR deletion, muscle protein degradation was accelerated. Fluorescence resonance energy transfer and an in vitro competition assay revealed that activated GRs competed for PI3K, reducing its association with IRS-1. Reexpression of WT GRs or those with a mutation in the nuclear localization signal in the muscle of MGRKO mice indicated that competition for PI3K was a prominent mechanism underlying reduced IRS-1-associated PI3K activity. This nongenomic influence of the GR contributes to activation of muscle protein degradation. We therefore conclude that stimulation of muscle proteolysis requires 2 events, increased glucocorticoid levels and impaired insulin signaling.

GHRP-2, a GHS-R agonist, directly acts on myocytes to attenuate the dexamethasone-induced expressions of muscle-specific ubiquitin ligases, Atrogin-1 and MuRF1

Recent reports suggest that Atrogin-1 and MuRF1, E3 ubiquitin ligases, play a pivotal role in muscle atrophy. In the present study, effect of Growth Hormone Releasing Peptide-2 (GHRP-2), a GH secretagogue receptor (GHS-R) agonist, on the expressions of Atrogin-1 and MuRF1 in vivo rat muscles was examined. Dexamethasone administration increased Atrogin-1 mRNA level in rat soleus muscle. The increased mRNA level of Atrogin-1 was significantly attenuated by GHRP-2. In addition, GHRP-2 decreased MuRF1 mRNA level irrespective of the presence of dexamethasone. Although IGF-I is a well-known protective factor for muscle atrophy, GHRP-2 did not influence plasma IGF-I levels and IGF-I mRNA levels in muscles. To clarify a direct effect of GHRP-2, differentiated C2C12 myocytes were used. Ten micrometer dexamethasone increased both Atrogin-1 and MuRF1 mRNA levels in C2C12 cells. GHRP-2 attenuated dexamethasone-induced expression of them dose-dependently and decreased the basal level of MuRF1 mRNA. The suppressive effect on the expressions of Atrogin-1 and MuRF1 by GHRP-2 was blocked by [D-Lys(3)]-GHRP-6, a GHS-R1a blocker, suggesting the effect of GHRP-2 was mediated through GHS-R1a. Taken together, GHRP-2 directly attenuates Atrogin-1 and MuRF1 mRNA levels through ghrelin receptors in myocytes.

Regulation of airway smooth muscle alpha-actin expression by glucocorticoids

Airway smooth muscle hypertrophy appears to be present in severe asthma. However, the effect of corticosteroids on airway smooth muscle cell size or contractile protein expression has not been studied. We examined the effects of dexamethasone, fluticasone, and salmeterol on contractile protein expression in transforming growth factor (TGF)-beta-treated primary bronchial smooth muscle cells. Dexamethasone and fluticasone, but not salmeterol, each reduced expression of alpha-smooth muscle actin and the short isoform of myosin light chain kinase. Steady-state alpha-actin mRNA level and stability were unchanged, consistent with posttranscriptional control. Fluticasone significantly decreased alpha-actin protein synthesis following treatment with the transcriptional inhibitor actinomycin D, indicative of an inhibitory effect on mRNA translation. Fluticasone also significantly increased alpha-actin protein turnover. Finally, fluticasone reduced TGF-beta-induced incorporation of alpha-actin into filamentous actin, cell length, and cell shortening in response to ACh and KCl. We conclude that glucocorticoids reduce human airway smooth muscle alpha-smooth muscle actin expression and incorporation into contractile filaments, as well as contractile function, in part by attenuation of mRNA translation and enhancement of protein degradation.

Influence of corticosteroids on myonuclear domain size in the rat diaphragm muscle

Skeletal muscle fibers are multinucleated. Each myonucleus regulates gene products and protein expression in only a restricted portion of the muscle fiber, the myonuclear domain (MND). In the rat diaphragm muscle (DIAm), corticosteroid (CoS) treatment causes atrophy of fibers containing myosin heavy chain (MHC): MHC2X and/or MHC2B. We hypothesized that DIAm fiber MND size is maintained during CoS-induced atrophy. Adult male rats received methylprednisolone for 11 days at 1 (CoS-Low, n = 8) or 8 mg·kg−1·day−1 (CoS-High, n = 8). Age-matched (CTL-AgeM, n = 8), sham-operated (SHAM-AgeM, n = 8), and weight-matched (CTL-WtM, n = 8) animals served as controls. In single DIAm fibers, cross-sectional area (CSA), MND size, and MHC expression were determined. Fiber CSA and MND size were similar in CTL-AgeM and SHAM-AgeM groups. Only fibers containing MHCslow or MHC2A displayed smaller CSA in CTL-WtM than in CTL-AgeM and SHAM-AgeM groups, and MND size was reduced in all fibers. Thus fibers containing MHCslow and MHC2A maintain the number of myonuclei, whereas MHC2X or MHC2B fibers show loss of myonuclei during normal muscle growth. Both CoS groups displayed smaller CSA and MND size than CTL-AgeM and SHAM-AgeM groups. However, compared with CTL-WtM DIAm fibers, only fibers containing MHC2X or MHC2B displayed reduced CSA and MND size after CoS treatment. Thus little, if any, loss of myonuclei was associated with CoS-induced atrophy of MHC2X or MHC2B DIAm fibers. In summary, MND size does not appear to be regulated during CoS-induced DIAm atrophy.

IGF-I stimulates muscle growth by suppressing protein breakdown and expression of atrophy-related ubiquitin ligases, atrogin-1 and MuRF1

Muscle atrophy results primarily from accelerated protein degradation and is associated with increased expression of two muscle-specific ubiquitin ligases (E3s): atrogin-1 and muscle ring finger 1 (MuRF1). Glucocorticoids are essential for many types of muscle atrophy, and their effects are opposite to those of insulin-like growth factor I (IGF-I) and insulin, which promote growth. In myotubes, dexamethasone (Dex) inhibited growth and enhanced breakdown of long-lived cell proteins, especially myofibrillar proteins (as measured by 3-methylhistidine release), while also increasing atrogin-1 and MuRF1 mRNA. Conversely, IGF-I suppressed protein degradation and prevented the Dex-induced increase in proteolysis. IGF-I rapidly reduced atrogin-1 expression within 1 h by blocking mRNA synthesis without affecting mRNA degradation, whereas IGF-I decreased MuRF1 mRNA slowly. IGF-I and insulin also blocked Dex induction of these E3s and several other atrophy-related genes ("atrogenes"). Changes in overall proteolysis with Dex and IGF-I correlated tightly with changes in atrogin-1 mRNA content, but not with changes in MuRF1 mRNA. IGF-I activates the phosphatidylinositol 3-kinase (PI3K)-Akt pathway, and inhibition of this pathway [but not the calcineurin-nuclear factor of activated T cell (NFAT) or the MEK-ERK pathway] increased proteolysis and atrogin-1 mRNA expression. Thus an important component of growth stimulation by IGF-I, through the PI3K-Akt pathway, is its ability to rapidly suppress transcription of the atrophy-related E3 atrogin-1 and other atrogenes and degradation of myofibrillar proteins.

Dexamethasone inhibits the stimulation of muscle protein synthesis and PHAS-I and p70 S6-kinase phosphorylation

Glucocorticoids inhibit protein synthesis in muscle. In contrast, insulin and amino acids exert anabolic actions that arise in part from their ability to phosphorylate ribosomal p70 S6-kinase (p70(S6k)) and eukaryotic initiation factor (eIF)4E binding protein (BP)1 (PHAS-I), proteins that regulate translation initiation. Whether glucocorticoids interfere with this action was examined by giving rats either dexamethasone (DEX, 300 microg. kg(-1). day(-1), n = 10) or saline (n = 10) for 5 days. We then measured the phosphorylation of PHAS-I and p70(S6k) in rectus muscle biopsies taken before and at the end of a 180-min infusion of either insulin (10 mU. min(-1). kg(-1) euglycemic insulin clamp, n = 5 for both DEX- and saline-treated groups) or a balanced amino acid mixture (n = 5 for each group also). Protein synthesis was also measured during the infusion period. The results were that DEX-treated rats had higher fasting insulin, slower glucose disposal, less lean body mass, and decreased protein synthetic rates during insulin or amino acid infusion (P < 0.05 each). DEX did not affect basal PHAS-I or p70(S6k) phosphorylation but blocked insulin-stimulated phosphorylation of PHAS-I- and amino acid-stimulated phosphorylation of both PHAS-I and p70(S6k) (P < 0.01, for each). DEX also increased muscle PHAS-I concentration. These effects can, in part, explain glucocorticoid-induced muscle wasting.

Nutrition and muscle loss in humans during spaceflight

The protein loss in humans during spaceflight is partly due to a normal adaptive response to a decreased work load on the muscles involved in weight bearing. The process is mediated by changes in prostaglandin release, secondary to the decrease in tension on the affected muscles. On missions, where there is a high level of physical demands on the astronauts, there tends to be an energy deficit, which adds to the muscle protein loss and depletes the body fat reserves. While the adaptive response is a normal part of homeostasis, the additional protein loss from an energy deficit can, in the long run, have a negative effect on health and capability of humans to live and work in space and afterward return to Earth.

The effect of anabolic steroids and corticosteroids on healing of muscle contusion injury

The effect of an anabolic steroid (nandrolone decanoate, 20 mg/kg) and a corticosteroid (methylprednisolone acetate, 25 mg/kg) on healing muscle injured with a drop-mass technique in a reproducible muscle contusion injury model in the rat was studied. Healing was determined by measuring active contractile tension in each muscle and histologic analysis. At day 2, the corticosteroid group showed significant improvement in both twitch and tetanic strength relative to the controls. At day 7, this effect was reversed and the corticosteroid muscles were significantly weaker than the control muscles, but there was still no significant effect seen in the anabolic steroid group. At day 14, the corticosteroid muscles were totally degenerated, with disorganized muscle fiber architecture. The anabolic steroid muscles were significantly stronger in twitch, and a similar trend was seen in tetanus relative to control muscles. The results indicate that in an animal model corticosteroids may be beneficial in the short term, but they cause irreversible damage to healing muscle in the long term, including disordered fiber structure and a marked diminution in force-generating capacity. Anabolic steroids may aid in the healing of muscle contusion injury to speed the recovery of force-generating capacity. Although anabolic steroids are considered renegade drugs, they may have an ethical clinical application to aid healing in severe muscle contusion injury, and their use in the treatment of muscle injuries warrants further research.

Energy-ubiquitin-dependent muscle proteolysis during sepsis in rats is regulated by glucocorticoids

Recent studies suggest that sepsis-induced increase in muscle proteolysis mainly reflects energy-ubiquitin-dependent protein breakdown. We tested the hypothesis that glucocorticoids activate the energy-ubiquitin-dependent proteolytic pathway in skeletal muscle during sepsis. Rats underwent induction of sepsis by cecal ligation and puncture or were sham-operated and muscle protein breakdown rates were measured 16 h later. The glucocorticoid receptor antagonist RU 38486 or vehicle was administered to groups of septic and sham-operated rats. In other experiments, dexamethasone (2.5 or 10 mg/kg) was injected subcutaneously in normal rats. Total and myofibrillar proteolysis was determined in incubated extensor digitorum longus muscles as release of tyrosine and 3-methylhistidine, respectively. Energy-dependent proteolysis was determined in incubated muscles depleted of energy with 2-deoxyglucose and 2,4-dinitrophenol. Levels of muscle ubiquitin mRNA and free and conjugated ubiquitin were determined by Northern and Western blot, respectively. RU 38486 inhibited the sepsis-induced increase in total and myofibrillar energy-dependent protein breakdown rates and blunted the increase in ubiquitin mRNA levels and free ubiquitin. Some, but not all, sepsis-induced changes in ubiquitin protein conjugates were inhibited by RU 38486. Injection of dexamethasone in normal rats increased energy-dependent proteolysis and ubiquitin mRNA levels. The results suggest that glucocorticoids regulate the energy-ubiquitin-dependent proteolytic pathway in skeletal muscle during sepsis.

Sensitivity and protein turnover response to glucocorticoids are different in skeletal muscle from adult and old rats. Lack of regulation of the ubiquitin-proteasome proteolytic pathway in aging

We studied glucocorticoid-induced muscle wasting and subsequent recovery in adult (7-mo-old) and old (22-mo-old) rats, since the increased incidence of various disease states may result in glucocorticoids hypersecretion in aging. Adult and old rats received dexamethasone in their drinking water and were then allowed to recover. Muscle wasting occurred more rapidly in old rats and the recovery of muscle mass was impaired, suggesting that glucocorticoids may be involved in the emergence of muscle atrophy with advancing age. According to measurements in incubated epitrochlearis muscles, dexamethasone-induced muscle wasting mainly resulted from increased protein breakdown in the adult, but from depressed protein synthesis in the aged animal. Increased expression of cathepsin D, m-calpain, and ubiquitin was observed in the muscles from both dexamethasone-treated adult and old rats. By contrast, the disappearance of the stimulatory effect of glucocorticoids on protein break-down in aging occurred along with a loss of ability of steroids to enhance the expression of the 14-kD ubiquitin carrier protein E2, which is involved in protein substrates ubiquitinylation, and of subunits of the 20 S proteasome (the proteolytic core of the 26 S proteasome that degrades ubiquitin conjugates). Thus, if glucocorticoids play any role in the progressive muscle atrophy seen in aging, this is unlikely to result from an activation of the ubiquitin-proteasome proteolytic pathway.

Early hormonal changes affect the catabolic response to trauma

OBJECTIVE

The authors sought to determine how temporary insulin suppression might alter the catabolic effects of cortisol, glucagon, and epinephrine.

SUMMARY BACKGROUND DATA

The metabolic responses to injury include hypermetabolism, accelerated net skeletal muscle protein breakdown, glucose intolerance, and insulin resistance. These alterations are associated with increased stress hormone concentrations. Insulin elaboration is usually suppressed immediately after an injury but is abundant later during convalescence. An infusion of hydrocortisone, glucagon, and epinephrine increases both stress hormone concentrations and insulin levels. It induces many of the metabolic alterations seen in critically ill patients, but it does not affect net muscle breakdown.

METHODS

Seven healthy adults received a stress hormone infusion for 3 days in two separate studies. During one study they, also received an infusion of the somatostatin analogue, octreotide (0.005 micrograms/kg/min), to suppress insulin elaboration for the first 24 hours. During the other study (control), insulin was permitted to rise unchecked.

RESULTS

Stress hormone concentrations, hypermetabolism (+/- 20% above basal), and leukocytosis were similar during both study periods. When insulin elaboration was temporarily suppressed, whole-body nitrogen loss was increased during the first 48 hours, and the efflux of amino acids from the forearm after 72 hours of infusion was 60% greater than the control level.

CONCLUSIONS

Temporary insulin suppression during physiologic increases in stress hormone concentrations amplified whole-body nitrogen loss and led to the development of accelerated net skeletal muscle protein breakdown. Early hormonal changes after an injury may affect the development of later catabolic responses.

Acute effects of corticosterone on tissue protein synthesis and insulin-sensitivity in rats in vivo

The effect of corticosterone treatment on the sensitivity of muscle protein synthesis to insulin infusion was assessed in post-absorptive young rats. To select the optimal time period for corticosterone treatment, protein synthesis was measured by injection of L-[2,6-3H]phenylalanine (1.5 mmol/kg body weight) 1, 4, 12 or 24 h after injection of corticosterone (5 mg/kg body wt.). Muscle protein synthesis was significantly decreased at 4 h and the effect was maximal by 12 h; liver protein synthesis was elevated at 12 h and 24 h. The dose-response of muscle protein synthesis to a 30 min infusion with 0-150 munits of insulin/h was then compared in rats pretreated with corticosterone (10 mg/100 g body wt.) or vehicle alone. When no insulin was infused, corticosterone inhibited protein synthesis in gastrocnemius muscle. High doses of insulin stimulated protein synthesis, but the inhibition by corticosterone was similar to that in the absence of insulin. At intermediate doses of insulin there was an increased requirement for insulin to elicit an equivalent response in muscle protein synthesis. Plantaris muscle responded in a manner similar to that of gastrocnemius, but neither soleus muscle nor liver responded significantly to insulin. These data suggest that corticosterone has two modes of action; one which is independent from and opposite to that of insulin, and a second which causes insulin-resistance through a decrease in sensitivity rather than a change in responsiveness.

Prostaglandin E2 does not regulate total or myofibrillar protein breakdown in incubated skeletal muscle from normal or septic rats

The role of prostaglandins in the regulation of muscle protein breakdown is controversial. We examined the influence of arachidonic acid (5 microM), prostaglandin E2 (PGE2) (2.8 microM) and the prostaglandin-synthesis inhibitor indomethacin (3 microM) on total and myofibrillar protein breakdown in rat extensor digitorum longus and soleus muscles incubated under different conditions in vitro. In other experiments, the effects of indomethacin, administered in vivo to septic rats (3 mg/kg, injected subcutaneously twice after induction of sepsis by caecal ligation and puncture) on plasma levels and muscle release of PGE2 and on total and myofibrillar protein breakdown rates were determined. Total and myofibrillar proteolysis was assessed by measuring production by incubated muscles of tyrosine and 3-methylhistidine respectively. Arachidonic acid or PGE2 added during incubation of muscles from normal rats did not affect total or myofibrillar protein degradation under a variety of different conditions in vitro. Indomethacin inhibited muscle PGE2 production by incubated muscles from septic rats, but did not lower proteolytic rates. Administration in vivo of indomethacin did not affect total or myofibrillar muscle protein breakdown, despite effective plasma levels of indomethacin with decreased plasma PGE2 levels and inhibition of muscle PGE2 release. The present results suggest that protein breakdown in skeletal muscle of normal or septic rats is not regulated by PGE2 or other prostaglandins.

Further evidence that accelerated muscle protein breakdown during sepsis is not mediated by prostaglandin E2

Prostaglandin E2 (PGE2) reportedly increases protein break-down in skeletal muscle. The role of PGE2 for accelerated muscle proteolysis during sepsis, however, is controversial. In this study, the effect of the prostaglandin synthesis inhibitor indomethacin on PGE2 release and protein breakdown in skeletal muscle from nonseptic and septic rats was evaluated. Sepsis was induced in male Sprague-Dawley rats (40-60 g) by cecal ligation and puncture (CLP). After 16 hours the extensor digitorum longus (EDL) and soleus (SOL) muscles were dissected with intact tendons and incubated in an oxygenated medium, and the release of tyrosine (protein breakdown) and PGE2 into the incubation medium was determined. Paired muscles were incubated in the absence or presence of indomethacin (3 mumol/L or 6 mumol/L). In some experiments the effect of indomethacin was investigated in the presence of different concentrations of insulin (1, 10, or 100 mU/mL) since previous reports suggested an interaction between insulin and prostaglandins on protein turnover in skeletal muscle. In other experiments muscles were incubated in a flaccid or stretched state, which is known to influence the metabolic response to different substances. Protein breakdown rate was 0.210 +/- 0.013 and 0.492 +/- 0.025 mumol Tyr/g X 2 hours in EDL from nonseptic and septic rats, respectively (p less than 0.01). The corresponding values for SOL were 0.480 +/- 0.037 and 0.712 +/- 0.039 mumol Tyr/g X 2 hours (p less than 0.01). Addition of indomethacin to the incubation medium reduced PGE2 release from 29.1 +/- 3.1 to 6.8 +/- 0.7 ng/g X 2 hours in nonseptic SOL and from 50.6 +/- 10.4 to 5.6 +/- 0.7 ng/g X 2 hours in septic SOL. Protein breakdown rate in SOL and EDL from sham-operated or septic rats was unaffected by indomethacin, both when muscles were incubated in a flaccid or stretched state, and when they were incubated in the presence or absence of insulin. The present results do not suggest a role of PGE2 for accelerated muscle proteolysis in the present experimental septic model.

Acute alterations in sodium flux in vitro lead to decreased myofibrillar protein breakdown in rat skeletal muscle

Myofibrillar protein breakdown was evaluated by measuring the release of N tau-methylhistidine by isolated rat skeletal muscles or perfused rat muscles in the presence of a variety of agents known to affect Na+ flux. Total cell proteolysis was evaluated simultaneously by measuring tyrosine release by muscles after the inhibition of protein synthesis with cycloheximide. Treatment of muscles with the Na+ ionophore monensin or inhibitors of Na+-K+ ATPase (ouabain, digoxin or vanadate) decreased N tau-methylhistidine release by muscles by 21-35%. A phorbol ester (phorbol 12-myristate 13-acetate) as well as a synthetic diacylglycerol known to activate protein kinase C and a Na+/H+ antiport also decreased N tau-methylhistidine release by muscles. Removal of extracellular Na+ blocked the ability of these agents to attenuate N tau-methylhistidine release by muscles, suggesting that their effectiveness required a change in Na+ flux. In contrast with N tau-methylhistidine release by muscles, these agents, except for monensin, did not effect the release of tyrosine, suggesting that they attenuate specifically the breakdown of myofibrillar proteins. Overall these results indicate a link between Na+ and the regulation of protein breakdown in rat skeletal muscle, whereby an influx of Na+ can result in a decrease in myofibrillar proteolysis. Left unresolved is whether phospholipid hydrolysis is involved in this scheme.