Analysis of global mRNA expression in human skeletal muscle during recovery from endurance exercise

Immunization of mice with T cell-dependent antigens promotes IL-6 and TNF-α production in muscle cells

IL-6 and TNF-alpha are proinflammatory cytokines involved in various inflammatory or non-inflammatory disorders characterized by muscle wasting. While infiltrating leukocytes are known to be the major source of these cytokines, it is unclear whether muscle cells also contribute to local inflammation. In this study, we first showed that cultured muscle cells and naive mouse muscle tissue were capable of producing IL-6 and TNF-alpha. We demonstrated an increased expression of IL-6 and TNF-alpha on muscle sections of C57BL/6J mice immunized with acetylcholine receptor (AChR) in the complete Freund's adjuvant (CFA) or with CFA only. In the presence of IL-6 or TNF-alpha, cultured AChR-expressing mouse (G-8) and human (TE671) skeletal muscle cells showed significantly decreased alpha-bungarotoxin-binding sites as measured by cellular ELISA. Moreover, IL-6- or TNF-alpha-treated muscle cells displayed a considerable increase in apoptotic cell ratios. Collectively, this study suggests a direct role for these two cytokines in muscle cell destruction and a possibility of muscle cell damage via an autocrine fashion.

Interaction of contractile activity and training history on mRNA abundance in skeletal muscle from trained athletes

Skeletal muscle displays enormous plasticity to respond to contractile activity with muscle from strength- (ST) and endurance-trained (ET) athletes representing diverse states of the adaptation continuum. Training adaptation can be viewed as the accumulation of specific proteins. Hence, the altered gene expression that allows for changes in protein concentration is of major importance for any training adaptation. Accordingly, the aim of the present study was to quantify acute subcellular responses in muscle to habitual and unfamiliar exercise. After 24-h diet/exercise control, 13 male subjects (7 ST and 6 ET) performed a random order of either resistance (8 x 5 maximal leg extensions) or endurance exercise (1 h of cycling at 70% peak O2 uptake). Muscle biopsies were taken from vastus lateralis at rest and 3 h after exercise. Gene expression was analyzed using real-time PCR with changes normalized relative to preexercise values. After cycling exercise, peroxisome proliferator-activated receptor-gamma coactivator-1alpha (ET approximately 8.5-fold, ST approximately 10-fold, P < 0.001), pyruvate dehydrogenase kinase-4 (PDK-4; ET approximately 26-fold, ST approximately 39-fold), vascular endothelial growth factor (VEGF; ET approximately 4.5-fold, ST approximately 4-fold), and muscle atrophy F-box protein (MAFbx) (ET approximately 2-fold, ST approximately 0.4-fold) mRNA increased in both groups, whereas MyoD (approximately 3-fold), myogenin (approximately 0.9-fold), and myostatin (approximately 2-fold) mRNA increased in ET but not in ST (P < 0.05). After resistance exercise PDK-4 (approximately 7-fold, P < 0.01) and MyoD (approximately 0.7-fold) increased, whereas MAFbx (approximately 0.7-fold) and myostatin (approximately 0.6-fold) decreased in ET but not in ST. We conclude that prior training history can modify the acute gene responses in skeletal muscle to subsequent exercise.

PPAR delta: a dagger in the heart of the metabolic syndrome

Obesity is a growing threat to global health by virtue of its association with insulin resistance, glucose intolerance, hypertension, and dyslipidemia, collectively known as the metabolic syndrome or syndrome X. The nuclear receptors PPARalpha and PPARgamma are therapeutic targets for hypertriglyceridemia and insulin resistance, respectively, and drugs that modulate these receptors are currently in clinical use. More recent work on the less-described PPAR isotype PPARdelta has uncovered a dual benefit for both hypertriglyceridemia and insulin resistance, highlighting the broad potential of PPARdelta in the treatment of metabolic disease. PPARdelta enhances fatty acid catabolism and energy uncoupling in adipose tissue and muscle, and it suppresses macrophage-derived inflammation. Its combined activities in these and other tissues make it a multifaceted therapeutic target for the metabolic syndrome with the potential to control weight gain, enhance physical endurance, improve insulin sensitivity, and ameliorate atherosclerosis.

The NR4A subgroup: immediate early response genes with pleiotropic physiological roles

The nuclear hormone receptor (NR) superfamily includes the orphan NR4A subgroup, comprised of Nur77 (NR4A1), Nurr1 (NR4A2) and NOR-1 (NR4A3). These NRs are classified as early response genes, are induced by a diverse range of signals, including fatty acids, stress, growth factors, cytokines, peptide hormones, phorbol esters, neurotransmitters, and physical stimuli (for example magnetic fields, shear stress). The ability to sense and rapidly respond to changes in the cellular environment thus appears to be a hallmark of this subfamily. The members of the NR4A subgroup are well conserved in the DNA binding domain (~91-95%) and the C-terminal ligand-binding domain (~60%), but are divergent in the N-terminal AB region. These receptors bind as monomers, homodimers and heterodimers with RXRs (to mediate retinoid signaling) to different permutations of the canonical NR binding motif. The NR4A subgroup activates gene expression in a constitutive ligand-independent manner. NR4A-mediated trans-activation (LBD) involves unusually active N-terminal AF-1 domains that mediate coactivator recruitment. Moreover, the NR4A receptors encode atypical LBDs and AF-2 domains. For example, the LBDs contain no cavity due to bulky hydrophobic residue side chains, and lack the classical coactivator-binding cleft constituted by helices 3, 4 and 12. However, a hydrophobic patch exists between helices 11 and 12, that encodes a novel cofactor interface that modulates transcriptional activity. In line with the pleiotropic physiological stimuli that induce the NR4A subgroup, these orphan NRs have been implicated in cell cycle regulation (and apoptosis), neurological disease, steroidogenesis, inflammation, carcinogenesis and atherogenesis.

Oligonucleotide microarray expression profiling: Human skeletal muscle phenotype and aerobic exercise training

Regular aerobic exercise reduces risk of cardiovascular disease far more effectively than any pharmaceutical agent. The precise mechanisms contributing to these health benefits are unknown. Currently, much of our knowledge regarding the molecular regulators of skeletal muscle phenotype remodeling in response to muscle activity is derived from rodent models. Over the past five years large scale gene analysis has emerged as a promising research strategy for studying complex processes in human tissue. This review will principally discuss the application of large scale gene expression profiling to study the molecular responses to longitudinal aerobic exercise training studies in humans. The focus is largely on the Affymetrix technology platform, as this can be most easily compared, in a quantitative manner, across laboratories. Indeed, there are compelling reasons to adopt a common standard to obtain maximum synergy across complex, expensive and invasive human studies. Direct comparisons between array data sets can be made, and these should be considered novel 'experiments', often providing great insight into disease mechanisms. Weaknesses in existing human studies are identified and future objectives are discussed.