Anabolic deficits and divergent unfolded protein response underlie skeletal and cardiac muscle growth impairments in the Yoshida hepatoma tumor model of cancer cachexia.

Cancer cachexia manifests as whole body wasting, however, the precise mechanisms governing the alterations in skeletal muscle and cardiac anabolism have yet to be fully elucidated. In this study, we explored changes in anabolic processes in both skeletal and cardiac muscles in the Yoshida AH-130 ascites hepatoma model of cancer cachexia. AH-130 tumor-bearing rats experienced significant losses in body weight, skeletal muscle, and heart mass. Skeletal and cardiac muscle loss was associated with decreased ribosomal (r)RNA, and hypophosphorylation of the eukaryotic factor 4E binding protein 1. Endoplasmic reticulum stress was evident by higher activating transcription factor mRNA in skeletal muscle and growth arrest and DNA damage-inducible protein (GADD)34 mRNA in both skeletal and cardiac muscles. Tumors provoked an increase in tissue expression of interferon-γ in the heart, while an increase in interleukin-1ß mRNA was apparent in both skeletal and cardiac muscles. We conclude that compromised skeletal muscle and heart mass in the Yoshida AH-130 ascites hepatoma model involves a marked reduction translational capacity and efficiency. Furthermore, our observations suggest that endoplasmic reticulum stress and tissue production of pro-inflammatory factors may play a role in the development of skeletal and cardiac muscle wasting.

Relative rDNA copy number is not associated with resistance training-induced skeletal muscle hypertrophy and does not affect myotube anabolism in vitro.

Ribosomal DNA (rDNA) copies exist across multiple chromosomes, and interindividual variation in copy number is speculated to influence the hypertrophic response to resistance training. Thus, we examined if rDNA copy number was associated with resistance training-induced skeletal muscle hypertrophy. Participants (n = 53 male, 21 ± 1 yr old; n = 29 female, 21 ± 2 yr old) performed 10-12 wk of full-body resistance training. Hypertrophy outcomes were determined, as was relative rDNA copy number from preintervention vastus lateralis (VL) biopsies. Pre- and postintervention VL biopsy total RNA was assayed in all participants, and mRNA/rRNA markers of ribosome content and biogenesis were also assayed in the 29 female participants before training, 24 h following training bout 1, and in the basal state after 10 wk of training. Across all participants, no significant associations were evident between relative rDNA copy number and training-induced changes in whole body lean mass (r = -0.034, P = 0.764), vastus lateralis thickness (r = 0.093, P = 0.408), mean myofiber cross-sectional area (r = -0.128, P = 0.259), or changes in muscle RNA concentrations (r = 0.026, P = 0.818), and these trends were similar when examining each gender. However, all Pol-I regulon mRNAs as well as 45S pre-rRNA, 28S rRNA, and 18S rRNA increased 24 h following the first training bout in female participants. Follow-up studies using LHCN-M2 myotubes demonstrated that a reduction in relative rDNA copy number induced by bisphenol A did not significantly affect insulin-like-growth factor-induced myotube hypertrophy. These findings suggest that relative rDNA copy number is not associated with myofiber hypertrophy.NEW & NOTEWORTHY We examined ribosomal DNA (rDNA) copy numbers in men and women who resistance trained for 10-12 wk and found no significant associations with skeletal muscle hypertrophy outcomes. These data, along with in vitro data in immortalized human myotubes whereby rDNA copy number was reduced, provide strong evidence that relative rDNA copy number is not associated with anabolism.

Metastatic or xenograft colorectal cancer models induce divergent anabolic deficits and expression of pro-inflammatory effectors of muscle wasting in a tumor-type-dependent manner.

We investigated the impact of tumor burden on muscle wasting in metastatic (m) and xenograft (x) models of colorectal cancer (CRC). Male Nod SCID γ and CD2F1 mice were injected subcutaneously or intrasplenically with HCT116 or C26 tumor cells, respectively. CRC tumors resulted in significant muscle wasting regardless of tumor type or model, although muscle loss was exacerbated in mHCT116 hosts. The mHCT116 model decreased ribosomal (r)RNA content and rDNA transcription, whereas the mC26 model showed no loss of rRNA and the upregulation of rDNA transcription. The xHCT116 model reduced mTOR, RPS6, and 4E-BP1 phosphorylation, whereas the mHCT116 model had a similar effect on RPS6 and 4E-BP1 without altering mTOR phosphorylation. The C26 models caused a reduction in 4E-BP1 phosphorylation independent of mTOR. Muscle interleukin (IL)-6 mRNA was elevated in all models except xHCT116, and the NOD-, LRR-, and pyrin domain-containing protein 3 (NLRP3) mRNA was induced only in the mC26 model. IL-1ß mRNA increased in all groups with greater expression in metastatic relative to the xenograft model regardless of tumor types. Our findings indicate that HCT116 tumor burden results in more drastic muscle wasting and anabolic deficits, whereas C26 tumor burden causes similar muscle wasting but exhibits a divergent proinflammatory phenotype. These results highlight potentially important divergence in the pathogenesis of muscle wasting among preclinical models of CRC and demonstrate that tumor burden plays a role in determining anabolic deficits and the expression of proinflammatory effectors of muscle wasting in a tumor-type-dependent manner.NEW & NOTEWORTHY We provide evidence demonstrating that colorectal tumor burden plays a role in determining anabolic deficits and the expression of proinflammatory effectors of muscle wasting in a tumor-type-dependent manner.

LP07 and LLC preclinical models of lung cancer induce divergent anabolic deficits and expression of pro-inflammatory effectors of muscle wasting.

Preclinical models have been instrumental to elucidate the mechanisms underlying muscle wasting in lung cancer (LC). We investigated anabolic deficits and the expression of proinflammatory effectors of muscle wasting in the LP07 and Lewis lung carcinoma (LLC) tumor models. Tumor growth resulted in significant weakness in LP07 but not in LLC mice despite similar reductions in gastrocnemius muscle mass in both models. The LP07 tumors caused a reduction in ribosomal (r)RNA and a decrease in rRNA gene (rDNA) transcription elongation, whereas no changes in ribosomal capacity were evident in LLC tumor-bearing mice. Expression of RNA Polymerase I (Pol I) elongation-associated subunits Polr2f, PAF53, and Znrd1 mRNAs was significantly elevated in the LP07 model, whereas Pol I elongation-related factors FACT and Spt4/5 mRNAs were elevated in the LLC mice. Reductions in RPS6 and 4E-BP1 phosphorylation were similar in both models but were independent of mTOR phosphorylation in LP07 mice. Muscle inflammation was also tumor-specific, IL-6 and TNF-α mRNA increased with LLC tumors, and upregulation of NLRP3 mRNA was independent of tumor type. In summary, although both models caused muscle wasting, only the LP07 model displayed muscle weakness with reductions in ribosomal capacity. Intracellular signaling diverged at the mTOR level with similar reductions in RPS6 and 4E-BP1 phosphorylation regardless of tumor type. The increase in proinflammatory factors was more pronounced in the LLC model. Our results demonstrate novel divergent anabolic deficits and expression of proinflammatory effectors of muscle wasting in the LP07 and LLC preclinical models of lung cancer.NEW & NOTEWORTHY We provide novel data demonstrating significant divergence in anabolic deficits and the expression of proinflammatory effectors of muscle wasting consequent to different lung-derived tumors.

Chemotherapy agents reduce protein synthesis and ribosomal capacity in myotubes independent of oxidative stress.

Chemotherapeutic agents (CAs) are first-line antineoplastic treatments against a wide variety of cancers. Despite their effectiveness in halting tumor progression, side effects associated with CAs promote muscle loss by incompletely understood mechanisms. To address this problem, we first identified how oxidative stress impairs protein synthesis in C2C12 myotubes. Transient elevations in reactive oxygen species (ROS) resulted in protein synthesis deficits and reduced ribosomal (r)RNA levels. Oxidative stress did not reduce rRNA gene (rDNA) transcription, but it caused an increase in rRNA and protein oxidation. To determine whether CAs affect protein synthesis independent of oxidative stress, we exposed myotubes to Paclitaxel (PTX), Doxorubicin (DXR), or Marizomib (Mzb) at doses that did result in elevated ROS levels (sub-ROS). Exposure to CAs reduced protein synthesis and rRNA levels, but unlike oxidative stress, sub-ROS exposures impaired rDNA transcription. These results indicate that although oxidative stress disrupts protein synthesis by compromising ribosomal quantity and quality, CAs at sub-ROS doses compromise protein synthesis and ribosomal capacity, at least in part, by reducing rDNA transcription. Therefore, CAs negatively impact protein synthesis by causing oxidative stress in addition to directly reducing the ribosomal capacity of myotubes in a ROS-independent manner.

Reduced rDNA transcription diminishes skeletal muscle ribosomal capacity and protein synthesis in cancer cachexia.

Muscle wasting in cancer is associated with deficits in protein synthesis, yet, the mechanisms underlying this anabolic impairment remain poorly understood. The capacity for protein synthesis is mainly determined by the abundance of muscle ribosomes, which is in turn regulated by transcription of the ribosomal (r)RNA genes (rDNA). In this study, we investigated whether muscle loss in a preclinical model of ovarian cancer is associated with a reduction in ribosomal capacity and was a consequence of impaired rDNA transcription. Tumor bearing resulted in a significant loss in gastrocnemius muscle weight and protein synthesis capacity, and was consistent with a significant reduction in rDNA transcription and ribosomal capacity. Despite the induction of the ribophagy receptor NUFIP1 mRNA and the loss of NUFIP1 protein, in vitro studies revealed that while inhibition of autophagy rescued NUFIP1, it did not prevent the loss of rRNA. Electrophoretic analysis of rRNA fragmentation from both in vivo and in vitro models showed no evidence of endonucleolytic cleavage, suggesting that rRNA degradation may not play a major role in modulating muscle ribosome abundance. Our results indicate that in this model of ovarian cancer-induced cachexia, the ability of skeletal muscle to synthesize protein is compromised by a reduction in rDNA transcription and consequently a lower ribosomal capacity. Thus, impaired ribosomal production appears to play a key role in the anabolic deficits associated with muscle wasting in cancer cachexia.

mTOR signaling regulates myotube hypertrophy by modulating protein synthesis, rDNA transcription, and chromatin remodeling.

Ribosome production is an early event during skeletal muscle hypertrophy and precedes muscle protein accretion. Signaling via mTOR is crucial for ribosome production and hypertrophy; however, the mechanisms by which it regulates these processes remain to be identified. Herein, we investigated the activation of mTOR signaling in hypertrophying myotubes and determined that mTOR coordinates various aspects of gene expression important for ribosome production. First, inhibition of translation with cycloheximide had a more potent effect on protein synthesis than rapamycin indicating that mTOR function during hypertrophy is not on general, but rather on specific protein synthesis. Second, blocking Pol II transcription had a similar effect as Rapamycin and, unexpectedly, revealed the necessity of Pol II transcription for Pol I transcription, suggesting that mTOR may regulate ribosome production also by controlling Class II genes at the transcriptional level. Third, Pol I activity is essential for rDNA transcription and, surprisingly, for protein synthesis as selective Pol I inhibition blunted rDNA transcription, protein synthesis, and the hypertrophic response of myotubes. Finally, mTOR has nuclear localization in muscle, which is not sensitive to rapamycin. Inhibition of mTOR signaling by rapamycin disrupted mTOR-rDNA promoter interaction and resulted in altered histone marks indicative of repressed transcription and formation of higher-order chromatin structure. Thus mTOR signaling appears to regulate muscle hypertrophy by affecting protein synthesis, Class I and II gene expression, and chromatin remodeling.

High CO2 levels cause skeletal muscle atrophy via AMP-activated kinase (AMPK), FoxO3a protein, and muscle-specific Ring finger protein 1 (MuRF1).

Patients with chronic obstructive pulmonary disease, acute lung injury, and critical care illness may develop hypercapnia. Many of these patients often have muscle dysfunction which increases morbidity and impairs their quality of life. Here, we investigated whether hypercapnia leads to skeletal muscle atrophy. Mice exposed to high CO2 had decreased skeletal muscle wet weight, fiber diameter, and strength. Cultured myotubes exposed to high CO2 had reduced fiber diameter, protein/DNA ratios, and anabolic capacity. High CO2 induced the expression of MuRF1 in vivo and in vitro, whereas MuRF1(-/-) mice exposed to high CO2 did not develop muscle atrophy. AMP-activated kinase (AMPK), a metabolic sensor, was activated in myotubes exposed to high CO2, and loss-of-function studies showed that the AMPKα2 isoform is necessary for muscle-specific ring finger protein 1 (MuRF1) up-regulation and myofiber size reduction. High CO2 induced AMPKα2 activation, triggering the phosphorylation and nuclear translocation of FoxO3a, and leading to an increase in MuRF1 expression and myotube atrophy. Accordingly, we provide evidence that high CO2 activates skeletal muscle atrophy via AMPKα2-FoxO3a-MuRF1, which is of biological and potentially clinical significance in patients with lung diseases and hypercapnia.

Effects of immunosuppressive treatment on interleukin-15 and interleukin-15 receptor α expression in muscle tissue of patients with polymyositis or dermatomyositis.

OBJECTIVES: To investigate the expression of interleukin (IL)-15 and IL-15 receptor α (IL-15Rα) in muscle tissue from patients with polymyositis or dermatomyositis before and after conventional immunosuppressive (IS) treatment. METHODS: Muscle biopsies from 17 patients before and after conventional IS treatment and seven healthy individuals were investigated by immunohistochemistry using antibodies against IL-15 and IL-15Rα. Quantification was performed by computerised image analysis. Cellular localisation of IL-15 was determined by double immunofluorescence. Clinical outcome was measured by the functional index and serum creatine kinase. Human myotubes were cultured and IL-15 staining was performed by immunocytochemistry. RESULTS: IL-15 was observed in mononuclear inflammatory cells of muscle tissue while IL-15Rα was localised to mononuclear inflammatory cells, capillaries and large vessels. Double staining showed localisation of IL-15 to CD163+ macrophages. A significantly larger number of IL-15 and IL-15Rα-positive cells were seen in muscle tissue of patients compared with healthy individuals. Baseline IL-15 expression correlated negatively with improvement in muscle function. After conventional IS treatment, a significantly lower number of IL-15 and IL-15Rα-positive cells was found. However, compared with controls, eight of 17 patients still had more IL-15-positive cells and less muscle function improvement was shown in this group of patients, both in short-term and long-term observations. Human differentiated myotubes were negative for IL-15 staining. CONCLUSIONS: IL-15 and its receptor are expressed in the muscle tissue of patients with myositis and IL-15 expression is correlated with improvement in muscle function. IL-15 may play a role in the pathogenesis of myositis and could be a biological treatment target, at least in a subgroup of patients with polymyositis or dermatomyositis.

Resistance exercise training influences skeletal muscle immune activation: a microarray analysis.

The primary aim of this investigation was to evaluate the effect of training on the immune activation in skeletal muscle in response to an acute bout of resistance exercise (RE). Seven young healthy men and women underwent a 12-wk supervised progressive unilateral arm RE training program. One week after the last training session, subjects performed an acute bout of bilateral RE in which the trained and the untrained arm exercised at the same relative intensity. Muscle biopsies were obtained 4 h postexercise from the biceps brachii of both arms and assessed for global transcriptom using Affymetrix U133 plus 2.0 microarrays. Significantly regulated biological processes and gene groups were analyzed using a logistic regression-based method following differential (trained vs. untrained) gene expression testing via an intensity-based Bayesian moderated t-test. The results from the present study suggest that training blunts the transcriptional upregulation of immune activation by minimizing expression of genes involved in monocyte recruitment and enhancing gene expression involved in macrophage anti-inflammatory polarization. Additionally, our data suggest that training blunts the transcriptional upregulation of the stress response and the downregulation of glucose metabolism, mitochondrial structure, and oxidative phosphorylation, and it enhances the transcriptional upregulation of the extracellular matrix and cytoskeleton development and organization and the downregulation of gene transcription and muscle contraction. This study provides novel insight into the molecular processes involved in the adaptive response of skeletal muscle following RE training and the cellular and molecular events implicating the protective role of training on muscle stress and damage inflicted by acute mechanical loading.

The gene expression profile in the synovium as a predictor of the clinical response to infliximab treatment in rheumatoid arthritis.

BACKGROUND: Although the use of TNF inhibitors has fundamentally changed the way rheumatoid arthritis (RA) is treated, not all patients respond well. It is desirable to facilitate the identification of responding and non-responding patients prior to treatment, not only to avoid unnecessary treatment but also for financial reasons. In this work we have investigated the transcriptional profile of synovial tissue sampled from RA patients before anti-TNF treatment with the aim to identify biomarkers predictive of response. METHODOLOGY/PRINCIPAL FINDINGS: Synovial tissue samples were obtained by arthroscopy from 62 RA patients before the initiation of infliximab treatment. RNA was extracted and gene expression profiling was performed using an in-house spotted long oligonucleotide array covering 17972 unique genes. Tissue sections were also analyzed by immunohistochemistry to evaluate cell infiltrates. Response to infliximab treatment was assessed according to the EULAR response criteria. The presence of lymphocyte aggregates dominated the expression profiles and a significant overrepresentation of lymphocyte aggregates in good responding patients confounded the analyses. A statistical model was set up to control for the effect of aggregates, but no differences could be identified between responders and non-responders. Subsequently, the patients were split into lymphocyte aggregate positive- and negative patients. No statistically significant differences could be identified except for 38 transcripts associated with differences between good- and non-responders in aggregate positive patients. A profile was identified in these genes that indicated a higher level of metabolism in good responding patients, which indirectly can be connected to increased inflammation. CONCLUSIONS/SIGNIFICANCE: It is pivotal to account for the presence of lymphoid aggregates when studying gene expression patterns in rheumatoid synovial tissue. In spite of our original hypothesis, the data do not support the notion that microarray analysis of whole synovial biopsy specimens can be used in the context of personalized medicine to identify non-responders to anti-TNF therapy before the initiation of treatment.

Dysferlin deficiency shows compensatory induction of Rab27A/Slp2a that may contribute to inflammatory onset.

Mutations in the dysferlin gene cause limb girdle muscular dystrophy 2B (LGMD2B) and Miyoshi myopathy. Dysferlin-deficient cells show abnormalities in vesicular traffic and membrane repair although onset of symptoms is not commonly seen until the late teenage years and is often associated with subacute onset and marked muscle inflammation. To identify molecular networks specific to dysferlin-deficient muscle that might explain disease pathogenesis, muscle mRNA profiles from 10 mutation-positive LGMD2B/MM patients were compared with a disease control [LGMD2I; (n = 9)], and normal muscle samples (n = 11). Query of inflammatory pathways suggested LGMD2B-specific increases in co-stimulatory signaling between dendritic cells and T cells (CD86, CD28, and CTLA4), associated with localized expression of both versican and tenascin. LGMD2B muscle also showed an increase in vesicular trafficking pathway proteins not normally observed in muscle (synaptotagmin-like protein Slp2a/SYTL2 and the small GTPase Rab27A). We propose that Rab27A/Slp2a expression in LGMD2B muscle provides a compensatory vesicular trafficking pathway that is able to repair membrane damage in the absence of dysferlin. However, this same pathway may release endocytotic vesicle contents, resulting in an inflammatory microenvironment. As dysferlin deficiency has been shown to enhance phagocytosis by macrophages, together with our findings of abnormal myofiber endocytosis pathways and dendritic-T cell activation markers, these results suggest a model of immune and inflammatory network over-stimulation that may explain the subacute inflammatory presentation.

Resistance exercise, muscle loading/unloading and the control of muscle mass.

Muscle mass is determined by the difference between the rate of protein synthesis and degradation. If synthesis is greater than degradation, muscle mass will increase (hypertrophy) and when the reverse is true muscle mass will decrease (atrophy). Following resistance exercise/increased loading there is a transient increase in protein synthesis within muscle. This change in protein synthesis correlates with an increase in the activity of protein kinase B/Akt and mTOR (mammalian target of rapamycin). mTOR increases protein synthesis by increasing translation initiation and by inducing ribosomal biogenesis. By contrast, unloading or inactivity results in a decrease in protein synthesis and a significant increase in muscle protein breakdown. The decrease in synthesis is due in part to the inactivation of mTOR and therefore a decrease in translation initiation, but also to a decrease in the rate of translation elongation. The increase in degradation is the result of a co-ordinated response of the calpains, lysosomal proteases and the ATP-dependent ubiquitin-proteosome. Caspase 3 and the calpains act upstream of the ubiquitin-proteosome system to assist in the complete breakdown of the myofibrillar proteins. Two muscle specific E3 ubiquitin ligases, MuRF1 and MAFbx/atrogen-1, have been identified as key regulators of muscle atrophy. In this chapter, these pathways and how the balance between anabolism and catabolism is affected by loading and unloading will be discussed.

Loss of emerin at the nuclear envelope disrupts the Rb1/E2F and MyoD pathways during muscle regeneration.

Emery-Dreifuss muscular dystrophy (EDMD1) is caused by mutations in either the X-linked gene emerin (EMD) or the autosomal lamin A/C (LMNA) gene. Here, we describe the derivation of mice lacking emerin in an attempt to derive a mouse model for EDMD1. Although mice lacking emerin show no overt pathology, muscle regeneration in these mice revealed defects. A bioinformatic array analysis of regenerating Emd null muscle revealed abnormalities in cell-cycle parameters and delayed myogenic differentiation, which were associated with perturbations to transcriptional pathways regulated by the retinoblastoma (Rb1) and MyoD genes. Temporal activation of MyoD transcriptional targets was significantly delayed, whereas targets of the Rb1/E2F transcriptional repressor complex remained inappropriately active. The inappropriate modulation of Rb1/MyoD transcriptional targets was associated with up-regulation of Rb1, MyoD and their co-activators/repressors transcripts, suggesting a compensatory effort to overcome a molecular block to differentiation at the myoblast/myotube transition during regeneration. This compensation appeared to be effective for MyoD transcriptional targets, although was less effective for Rb1 targets. Analysis of Rb1 phosphorylation states showed prolonged hyper-phosphorylation at key developmental stages in Emd null myogenic cells, both in vivo and in vitro. We also analyzed the same pathways in Lmna null muscle, which shows extensive dystrophy. Surprisingly, Lmna null muscle did not show the same perturbations to Rb- and MyoD-dependent pathways. We did observe increased transcriptional expression of Lap2alpha and delayed expression of Rb1, which may regulate alternative transcriptional pathways in the Lmna null myoblasts. We suggest that the dominant LMNA mutations seen in many clinically disparate laminopathies may similarly alter Rb function, with regard to either the timing of exit from the cell cycle or terminal differentiation programs or both.

mTOR function in skeletal muscle hypertrophy: increased ribosomal RNA via cell cycle regulators.

The purpose of this study was to identify the potential downstream functions associated with mammalian target of rapamycin (mTOR) signaling during myotube hypertrophy. Terminally differentiated myotubes were serum stimulated for 3, 6, 12, 24, and 48 h. This treatment resulted in significant myotube hypertrophy (protein/DNA) and increased RNA content (RNA/DNA) with no changes in DNA content or indices of cell proliferation. During myotube hypertrophy, the increase in RNA content was accompanied by an increase in tumor suppressor protein retinoblastoma (Rb) phosphorylation and a corresponding increase in the availability of the ribosomal DNA transcription factor upstream binding factor (UBF). Serum stimulation also induced an increase in cyclin D1 protein expression in the differentiated myotubes with a concomitant increase in cyclin D1-dependent cyclin-dependent kinase (CDK)-4 activity toward Rb. The increases in myotube hypertrophy and RNA content were blocked by rapamycin treatment, which also prevented the increase in cyclin D1 protein expression, CDK-4 activity, Rb phosphorylation, and the increase in UBF availability. Our findings demonstrate that activation of mTOR is necessary for myotube hypertrophy and suggest that the role of mTOR is in part to modulate cyclin D1-dependent CDK-4 activity in the regulation of Rb and ribosomal RNA synthesis. On the basis of these results, we propose that common molecular mechanisms contribute to the regulation of myotube hypertrophy and growth during the G1 phase of the cell cycle.

Molecular determinants of skeletal muscle mass: getting the "AKT" together.

Skeletal muscle is the most abundant tissue in the human body and its normal physiology plays a fundamental role in health and disease. During many disease states, a dramatic loss of skeletal muscle mass (atrophy) is observed. In contrast, physical exercise is capable of producing significant increases in muscle mass (hypertrophy). Maintenance of skeletal muscle mass is often viewed as the net result of the balance between two separate processes, namely protein synthesis and protein degradation. However, these two biochemical processes are not occurring independent of each other but they rather appear to be finely coordinated by a web of intricate signaling networks. Such signaling networks are in charge of executing environmental and cellular cues that will ultimate determine whether muscle proteins are synthesized or degraded. In this review, recent findings are discussed demonstrating that the AKT1/FOXOs/Atrogin-1(MAFbx)/MuRF1 signaling network plays an important role in the progression of skeletal muscle atrophy. These novel findings highlight an important mechanism that coordinates the activation of the protein synthesis machinery with the activation of a genetic program responsible for the degradation of muscle proteins during skeletal muscle atrophy.

Response of rat muscle to acute resistance exercise defined by transcriptional and translational profiling.

To further understand molecular mechanisms underlying skeletal muscle hypertrophy, expression profiles of translationally and transcriptionally regulated genes were characterized following an acute bout of maximally activated eccentric contractions. Experiments demonstrated that translational mechanisms contribute to acute gene expression changes following high resistance contractions with two candidate mRNAs, basic fibroblast growth factor (bFGF) and elongation factor-1 alpha (EF1alpha), targeted to the heavier polysomal fractions after a bout of contractions. Gene profiling was performed using Affymetrix Rat U34A GeneChips with either total RNA or polysomal RNA at one and six hours following contractions. There were 18 genes that changed expression at one hour and 70 genes that were different (60 genes increased:10 genes decreased)at six hours after contractions. The model from this profiling suggests that following high resistance contractions skeletal muscle shares a common growth profile with proliferating cells exposed to serum. This cluster of genes can be classified as "growth" genes and is commonly associated with progression of the cell cycle. However, a unique aspect was that there was induction of a cluster of tumour suppressor or antigrowth genes. We propose that this cluster of "antigrowth" genes is induced by the stress of contractile activity and may act to maintain skeletal muscle in the differentiated state. From the profiling results, further experiments determined that p53 levels increased in skeletal muscle at 6 h following contractions. This novel finding of p53 induction following exercise also demonstrates the power of expression profiling for identification of novel pathways involved in the response to muscle contraction.