COPD Results in a Reduction in UCP3 Long mRNA and UCP3 Protein Content in Types I and IIa Skeletal Muscle Fibers

Muscle Oxidative Capacity Is Reduced in Both Upper and Lower Limbs in COPD

INTRODUCTION

Skeletal muscle atrophy, weakness, mitochondrial loss, and dysfunction are characteristics of chronic obstructive pulmonary disease (COPD). It remains unclear whether muscle dysfunction occurs in both upper and lower limbs, because findings are inconsistent in the few studies where upper and lower limb muscle performance properties were compared within an individual. This study determined whether muscle oxidative capacity is low in upper and lower limbs of COPD patients compared with controls.

METHODS

Oxidative capacity of the forearm and medial gastrocnemius was measured using near-infrared spectroscopy to determine the muscle O2 consumption recovery rate constant (k, min) in 20 COPD (Global Initiative for Chronic Obstructive Lung Disease (GOLD) 2/3/4, n = 7/7/6) and 20 smokers with normal spirometry (CON). Muscle k is linearly proportional to oxidative capacity. Steps per day and vector magnitude units per minute (VMU·min) were assessed using triaxial accelerometry. Differences between group and limb were assessed by two-way ANOVA.

RESULTS

There was a significant main effect of group (F = 11.2, ηp = 0.13, P = 0.001): k was lower in both upper and lower limb muscles in COPD (1.01 ± 0.17 and 1.05 ± 0.24 min) compared with CON (1.29 ± 0.49 and 1.54 ± 0.60 min). There was no effect on k of limb (F = 1.8, ηp = 0.02, P = 0.18) or group-limb interaction (P = 0.35). (VMU·min) was significantly lower in COPD (-38%; P = 0.042). Steps per day did not differ between COPD (4738 ± 3194) and CON (6372 ± 2107; P = 0.286), although the difference exceeded a clinically important threshold (>600-1100 steps per day).

CONCLUSIONS

Compared with CON, muscle oxidative capacity was lower in COPD in both upper (-20%) and lower (-30%) limbs. These data suggest that mitochondrial loss in COPD is not isolated to locomotor muscles.

DNA methylation assessment from human slow- and fast-twitch skeletal muscle fibers

A new application of the reduced representation bisulfite sequencing method was developed using low-DNA input to investigate the epigenetic profile of human slow- and fast-twitch skeletal muscle fibers. Successful library construction was completed with as little as 15 ng of DNA, and high-quality sequencing data were obtained with 32 ng of DNA. Analysis identified 143,160 differentially methylated CpG sites across 14,046 genes. In both fiber types, selected genes predominantly expressed in slow or fast fibers were hypomethylated, which was supported by the RNA-sequencing analysis. These are the first fiber type-specific methylation data from human skeletal muscle and provide a unique platform for future research.NEW & NOTEWORTHY This study validates a low-DNA input reduced representation bisulfite sequencing method for human muscle biopsy samples to investigate the methylation patterns at a fiber type-specific level. These are the first fiber type-specific methylation data reported from human skeletal muscle and thus provide initial insight into basal state differences in myosin heavy chain I and IIa muscle fibers among young, healthy men.

Mitochondrial dysfunction in inflammatory responses and cellular senescence: pathogenesis and pharmacological targets for chronic lung diseases

Mitochondria are dynamic organelles, which couple the various cellular processes that regulate metabolism, cell proliferation and survival. Environmental stress can cause mitochondrial dysfunction and dynamic changes including reduced mitochondrial biogenesis, oxidative phosphorylation and ATP production, as well as mitophagy impairment, which leads to increased ROS, inflammatory responses and cellular senescence. Oxidative stress, inflammation and cellular senescence all have important roles in the pathogenesis of chronic lung diseases, such as chronic obstructive pulmonary disease, pulmonary fibrosis and bronchopulmonary dysplasia. In this review, we discuss the current state on how mitochondrial dysfunction affects inflammatory responses and cellular senescence, the mechanisms of mitochondrial dysfunction underlying the pathogenesis of chronic lung diseases and the potential of mitochondrial transfer and replacement as treatments for these diseases.

An official American Thoracic Society/European Respiratory Society statement: update on limb muscle dysfunction in chronic obstructive pulmonary disease

BACKGROUND

Limb muscle dysfunction is prevalent in chronic obstructive pulmonary disease (COPD) and it has important clinical implications, such as reduced exercise tolerance, quality of life, and even survival. Since the previous American Thoracic Society/European Respiratory Society (ATS/ERS) statement on limb muscle dysfunction, important progress has been made on the characterization of this problem and on our understanding of its pathophysiology and clinical implications.

PURPOSE

The purpose of this document is to update the 1999 ATS/ERS statement on limb muscle dysfunction in COPD.

METHODS

An interdisciplinary committee of experts from the ATS and ERS Pulmonary Rehabilitation and Clinical Problems assemblies determined that the scope of this document should be limited to limb muscles. Committee members conducted focused reviews of the literature on several topics. A librarian also performed a literature search. An ATS methodologist provided advice to the committee, ensuring that the methodological approach was consistent with ATS standards.

RESULTS

We identified important advances in our understanding of the extent and nature of the structural alterations in limb muscles in patients with COPD. Since the last update, landmark studies were published on the mechanisms of development of limb muscle dysfunction in COPD and on the treatment of this condition. We now have a better understanding of the clinical implications of limb muscle dysfunction. Although exercise training is the most potent intervention to address this condition, other therapies, such as neuromuscular electrical stimulation, are emerging. Assessment of limb muscle function can identify patients who are at increased risk of poor clinical outcomes, such as exercise intolerance and premature mortality.

CONCLUSIONS

Limb muscle dysfunction is a key systemic consequence of COPD. However, there are still important gaps in our knowledge about the mechanisms of development of this problem. Strategies for early detection and specific treatments for this condition are also needed.

Striated muscle activator of Rho signalling (STARS) is a PGC-1α/oestrogen-related receptor-α target gene and is upregulated in human skeletal muscle after endurance exercise

The striated muscle activator of Rho signalling (STARS) is an actin-binding protein specifically expressed in cardiac, skeletal and smooth muscle. STARS has been suggested to provide an important link between the transduction of external stress signals to intracellular signalling pathways controlling genes involved in the maintenance of muscle function. The aims of this study were firstly, to establish if STARS, as well as members of its downstream signalling pathway, are upregulated following acute endurance cycling exercise; and secondly, to determine if STARS is a transcriptional target of peroxisome proliferator-activated receptor gamma co-activator 1-α (PGC-1α) and oestrogen-related receptor-α (ERRα). When measured 3 h post-exercise, STARS mRNA and protein levels as well as MRTF-A and serum response factor (SRF) nuclear protein content, were significantly increased by 140, 40, 40 and 40%, respectively. Known SRF target genes, carnitine palmitoyltransferase-1β (CPT-1β) and jun B proto-oncogene (JUNB), as well as the exercise-responsive genes PGC-1α mRNA and ERRα were increased by 2.3-, 1.8-, 4.5- and 2.7-fold, 3 h post-exercise. Infection of C2C12 myotubes with an adenovirus-expressing human PGC-1α resulted in a 3-fold increase in Stars mRNA, a response that was abolished following the suppression of endogenous ERRα. Over-expression of PGC-1α also increased Cpt-1β, Cox4 and Vegf mRNA by 6.2-, 2.0- and 2.0-fold, respectively. Suppression of endogenous STARS reduced basal Cpt-1β levels by 8.2-fold and inhibited the PGC-1α-induced increase in Cpt-1β mRNA. Our results show for the first time that the STARS signalling pathway is upregulated in response to acute endurance exercise. Additionally, we show in C2C12 myotubes that the STARS gene is a PGC-1α/ERRα transcriptional target. Furthermore, our results suggest a novel role of STARS in the co-ordination of PGC-1α-induced upregulation of the fat oxidative gene, CPT-1β.

Altered mitochondrial regulation in quadriceps muscles of patients with COPD

Evidence exists for locomotor muscle impairment in patients with chronic obstructive pulmonary disease (COPD), including fiber type alterations and reduced mitochondrial oxidative capacity. In this study high-resolution respirometry was used to quantify oxygen flux in permeabilized fibres from biopsies of the vastus lateralis muscle in patients with COPD and compared to healthy control subjects. The main findings of this study were that (i) routine state 2 respiration was higher in COPD; (ii) state 3 respiration in the presence of ADP was similar in both groups with substrate supply of electrons to complex I (COPD 38·28 ± 3·58 versus control 42·85 ± 3·10 pmol s(-1) mg tissue(-1) ), but O(2) flux with addition of succinate was lower in COPD patients (COPD 63·72 ± 6·33 versus control 95·73 ± 6·53 pmol s(-1) mg tissue(-1) ); (iii) excess capacity of cytochrome c oxidase in COPD patients was only ~50% that of control subjects. These results indicate that quadriceps muscle mitochondrial function is altered in patients with COPD. The regulatory mechanisms underlying these functional abnormalities remain to be uncovered.

Structural and functional changes of peripheral muscles in chronic obstructive pulmonary disease patients

PURPOSE OF REVIEW

The purpose of this review is to identify new advances in our understanding of skeletal muscle dysfunction in patients with chronic obstructive pulmonary disease (COPD).

RECENT FINDINGS

Recent studies have confirmed the relevance of muscle dysfunction as an independent prognosis factor in COPD. Animal studies have shed light on the molecular mechanisms governing skeletal muscle hypertrophy/atrophy. Recent evidence in patients with COPD highlighted the contribution of protein breakdown and mitochondrial dysfunction as pathogenic mechanisms leading to muscle dysfunction in these patients.

SUMMARY

COPD is a debilitating disease impacting negatively on health status and the functional capacity of patients. COPD goes beyond the lungs and incurs significant systemic effects among which muscle dysfunction/wasting is one of the most important. Muscle dysfunction is a prominent contributor to exercise limitation, healthcare utilization and an independent predictor of morbidity and mortality. Gaining more insight into the molecular mechanisms leading to muscle dysfunction/wasting is key for the development of new and tailored therapeutic strategies to tackle skeletal muscle dysfunction/wasting in COPD patients.

Skeletal muscle dysfunction in patients with chronic obstructive pulmonary disease

Chronic obstructive pulmonary disease (COPD) is a debilitating disease characterized by inflammation-induced airflow limitation and parenchymal destruction. In addition to pulmonary manifestations, patients with COPD develop systemic problems, including skeletal muscle and other organ-specific dysfunctions, nutritional abnormalities, weight loss, and adverse psychological responses. Patients with COPD often complain of dyspnea on exertion, reduced exercise capacity, and develop a progressive decline in lung function with increasing age. These symptoms have been attributed to increases in the work of breathing and in impairments in gas exchange that result from airflow limitation and dynamic hyperinflation. However, there is mounting evidence to suggest that skeletal muscle dysfunction, independent of lung function, contributes significantly to reduced exercise capacity and poor quality of life in these patients. Limb and ventilatory skeletal muscle dysfunction in COPD patients has been attributed to a myriad of factors, including the presence of low grade systemic inflammatory processes, nutritional depletion, corticosteroid medications, chronic inactivity, age, hypoxemia, smoking, oxidative and nitrosative stresses, protein degradation and changes in vascular density. This review briefly summarizes the contribution of these factors to overall skeletal muscle dysfunction in patients with COPD, with particular attention paid to the latest advances in the field.

Muscle fibre type shifting in the vastus lateralis of patients with COPD is associated with disease severity: a systematic review and meta-analysis

BACKGROUND

Skeletal muscle dysfunction is a common feature in chronic obstructive pulmonary disease (COPD) which is associated with intrinsic muscular abnormalities. One of the most consistently reported alterations is a shift from fibre type I to II in the vastus lateralis of these patients. Surprisingly, the relationship between this shift and the severity and phenotype of COPD remains unclear. A study was conducted to determine whether vastus lateralis muscle fibre type proportions are associated with COPD disease severity and to provide reference values for the proportions of fibre types in the vastus lateralis in COPD.

METHODS

A systematic review and a meta-analysis were conducted in which muscle fibre type data and markers of disease severity were collected from the literature.

RESULTS

The forced expiratory volume in 1 s (FEV(1)), the ratio of FEV(1) to forced vital capacity (FVC) and body mass index were positively associated with the proportion of type I fibres in COPD. A proportion of 51% for vastus lateralis fibre type I and 13% for fibre type IIX were calculated from the combined data as normal values for patients with typical GOLD stage 3-4 COPD aged 60-70 years. Based on these reference values, a proportion of fibre type I <27% and of fibre type IIX >29% were defined as pathologically abnormal.

CONCLUSIONS

This review sheds new light on the relationship between skeletal muscle abnormalities and important hallmarks of the disease in severe COPD, and identifies absence of data in GOLD stages 1-2. This review also provides reference values on fibre type composition for diagnostic purposes in COPD.

UCP3 in muscle wasting, a role in modulating lipotoxicity?

UCP3 has been postulated to function in the defense against lipid-induced oxidative muscle damage (lipotoxicity). We explored this hypothesis during cachexia in rats (zymosan-induced sepsis), a condition characterized by increased oxidative stress and supply of fatty acids to the muscle. Muscle UCP3 protein content was increased 2, 6 and 11 days after zymosan injection. Plasma FFA levels were increased at day 2, but dropped below control levels on days 6 and 11. Muscular levels of the lipid peroxidation byproduct 4-hydroxy-2-nonenal (4-HNE) were increased at days 6 and 11 in zymosan-treated rats, supporting a role for UCP3 in modulating lipotoxicity during cachexia.

Exercise training restores uncoupling protein-3 content in limb muscles of patients with chronic obstructive pulmonary disease

Oxidative capacity and uncoupling protein-3 (UCP3) content are reduced in limb muscles of patients with chronic obstructive pulmonary disease (COPD). It has been hypothesized that the physiological role of UCP3 is to protect mitochondria against lipotoxicity in cases where fatty acid influx exceeds the capacity to oxidize them. Exercise training improves oxidative capacity and reduces UCP3 protein content in healthy subjects, but the response of UCP3 to training in COPD is unknown. We studied the effect of exercise training on UCP3 content in limb muscles of COPD patients. For this, seven healthy age-matched subjects and thirteen patients with COPD were studied. All patients were admitted to an 8-wk exercise training intervention. Exercise capacity was assessed by means of an incremental cycle ergometry test. Biopsies were taken from the vastus lateralis in which UCP3 and lipid peroxidation levels were determined by Western blotting. Citrate synthase and 3-hydroxyacyl-CoA dehydrogenase (HAD; an enzyme involved in fatty acid oxidation) were measured as indexes of muscle oxidative capacity. UCP3 in COPD was approximately 50% lower compared with healthy age-matched controls. In COPD, training induced upregulation of UCP3 [from 67.7 (SD 41.8) to 113.8 (SD 104.2) arbitrary units (AU), P = 0.062], especially in the patients who showed no increase in HAD activity [from 80.9 (SD 52.6) to 167.9 (SD 109.1) AU, P = 0.028], whereas lipid peroxidation levels remained unaltered. We conclude that exercise-training can restore muscle UCP3 protein level in COPD, and the nature of this response complies with the hypothesis that UCP3 may protect against lipotoxicity.