TNF alpha receptor genotype influences smoking-induced muscle-fibre-type shift and atrophy in mice

Do dietary supplements prevent loss of muscle mass and strength during muscle disuse? A systematic review and meta-analysis of randomized controlled trials

Objective

We performed a systematic review and meta-analysis of existing randomized controlled trials (RCTs) to assess whether dietary supplements can prevent loss of muscle mass and strength during muscle disuse.

Methods

We searched the following databases: PubMed, Embase, Cochrane, Scopus, Web of Science, and CINAHL for RCTs assessing the effect of dietary supplements on disuse muscular atrophy without language and time restrictions. Muscle strength and leg lean mass were used as the primary outcome indicators. Muscle cross-sectional area (CSA), muscle fiber type distribution, peak aerobic capacity and muscle volume were used as secondary outcome indicators. The risk of bias was assessed using the Cochrane Collaboration's Risk of Bias tool. Heterogeneity was tested using the I² statistic index. Mean and standard deviation of outcome indicators were extracted from the intervention and control groups to calculate effect sizes and 95% confidence intervals, with the significance level set at P < 0.05.

Results

Twenty RCTs were included with a total of 339 subjects. The results showed that dietary supplements had no effect on muscle strength, CSA, muscle fiber type distribution, peak aerobic capacity or muscle volume. But dietary supplements have a protective effect on the lean mass of the legs.

Conclusion

Dietary supplements can improve lean leg mass, but did not show a tendency to have an effect on muscle strength, CSA, muscle fiber type distribution, peak aerobic capacity or muscle volume during muscle disuse.

Systematic review registration

https://www.crd.york.ac.uk/PROSPERO/#recordDetails, identifier: CRD42022370230.

Main Pathogenic Mechanisms and Recent Advances in COPD Peripheral Skeletal Muscle Wasting

Chronic obstructive pulmonary disease (COPD) is a worldwide prevalent respiratory disease mainly caused by tobacco smoke exposure. COPD is now considered as a systemic disease with several comorbidities. Among them, skeletal muscle dysfunction affects around 20% of COPD patients and is associated with higher morbidity and mortality. Although the histological alterations are well characterized, including myofiber atrophy, a decreased proportion of slow-twitch myofibers, and a decreased capillarization and oxidative phosphorylation capacity, the molecular basis for muscle atrophy is complex and remains partly unknown. Major difficulties lie in patient heterogeneity, accessing patients' samples, and complex multifactorial process including extrinsic mechanisms, such as tobacco smoke or disuse, and intrinsic mechanisms, such as oxidative stress, hypoxia, or systemic inflammation. Muscle wasting is also a highly dynamic process whose investigation is hampered by the differential protein regulation according to the stage of atrophy. In this review, we report and discuss recent data regarding the molecular alterations in COPD leading to impaired muscle mass, including inflammation, hypoxia and hypercapnia, mitochondrial dysfunction, diverse metabolic changes such as oxidative and nitrosative stress and genetic and epigenetic modifications, all leading to an impaired anabolic/catabolic balance in the myocyte. We recapitulate data concerning skeletal muscle dysfunction obtained in the different rodent models of COPD. Finally, we propose several pathways that should be investigated in COPD skeletal muscle dysfunction in the future.

Impacts of Cigarette Smoke (CS) on Muscle Derangement in Rodents-A Systematic Review

Cigarette smoke (CS) is the major risk factor for chronic obstructive pulmonary disease (COPD) and can induce systemic manifestations, such as skeletal muscle derangement. However, inconsistent findings of muscle derangement were reported in previous studies. The aim of the present study was to consolidate the available evidence and assess the impact of CS on muscle derangement in rodents. A comprehensive literature search of five electronic databases identified ten articles for final analysis. Results showed that the diaphragm, rectus femoris, soleus, and gastrocnemius exhibited significant oxidative to glycolytic fiber conversions upon CS exposure. In contrast, the extensor digitorum longus (EDL), plantaris, and tibialis did not exhibit a similar fiber-type conversion after CS exposure. Hindlimb muscles, including the quadriceps, soleus, gastrocnemius, and EDL, showed significant reductions in the CSA of the muscle fibers in the CS group when compared to the control group. Changes in inflammatory cytokines, exercise capacity, and functional outcomes induced by CS have also been evaluated. CS could induce a shift from oxidative fibers to glycolytic fibers in high-oxidative muscles such as the diaphragm, rectus femoris, and soleus, and cause muscle atrophy, as reflected by a reduction in the CSA of hindlimb muscles such as the quadriceps, soleus, gastrocnemius, and EDL.

The combination of smoking with vitamin D deficiency impairs skeletal muscle fiber hypertrophy in response to overload in mice

Vitamin D deficiency, which is highly prevalent in the general population, exerts similar deleterious effects on skeletal muscles to those induced by cigarette smoking. We examined whether cigarette smoke (CS) exposure and/or vitamin D deficiency impairs the skeletal muscle hypertrophic response to overload. Male C57Bl/6JolaH mice on a normal or vitamin D-deficient diet were exposed to CS or room air for 18 wk. Six weeks after initiation of smoke or air exposure, sham surgery or denervation of the agonists of the left plantaris muscle was performed. The right leg served as internal control. Twelve weeks later, the hypertrophic response was assessed. CS exposure instigated loss of body and muscle mass, and increased lung inflammatory cell infiltration (P < 0.05), independently of diet. Maximal exercise capacity, whole body strength, in situ plantaris muscle force, and key markers of hypertrophic signaling (Akt, 4EBP1, and FoxO1) were not significantly affected by smoking or diet. The increase in plantaris muscle fiber cross-sectional area in response to overload was attenuated in vitamin D-deficient CS-exposed mice (smoking × diet interaction for hypertrophy, P = 0.03). In situ fatigue resistance was elevated in hypertrophied plantaris, irrespective of vitamin D deficiency and/or CS exposure. In conclusion, our data show that CS exposure or vitamin D deficiency alone did not attenuate the hypertrophic response of overloaded plantaris muscles, but this hypertrophic response was weakened when both conditions were combined. These data suggest that current smokers who also present with vitamin D deficiency may be less likely to respond to a training program.NEW & NOTEWORTHY Plantaris hypertrophy caused by compensatory overload after denervation of the soleus and gastrocnemius muscles showed increased mass and fiber dimensions, but to a lesser extent when vitamin D deficiency was combined with cigarette smoking. Fatigue resistance was elevated in hypertrophied plantaris, irrespective of diet or smoking, whereas physical fitness, hypertrophic markers, and in situ plantaris force were similar. These data showed that the hypertrophic response to overload is attenuated when both conditions are combined.

Two Weeks of Smoking Cessation Reverse Cigarette Smoke-Induced Skeletal Muscle Atrophy and Mitochondrial Dysfunction in Mice

INTRODUCTION

Apart from its adverse effects on the respiratory system, cigarette smoking also induces skeletal muscle atrophy and dysfunction. Whether short-term smoking cessation can restore muscle mass and function is unknown. We, therefore, studied the impact of 1- and 2-week smoking cessation on skeletal muscles in a mouse model.

METHODS

Male mice were divided into four groups: Air-exposed (14 weeks); cigarette smoke (CS)-exposed (14 weeks); CS-exposed (13 weeks) followed by 1-week cessation; CS-exposed (12 weeks) followed by 2 weeks cessation to examine exercise capacity, physical activity levels, body composition, muscle function, capillarization, mitochondrial function and protein expression in the soleus, plantaris, and diaphragm muscles.

RESULTS

CS-induced loss of body and muscle mass was significantly improved within 1 week of cessation due to increased lean and fat mass. Mitochondrial respiration and protein levels of the respiratory complexes in the soleus were lower in CS-exposed mice, but similar to control values after 2 weeks of cessation. Exposing isolated soleus muscles to CS extracts reduced mitochondrial respiration that was reversed after removing the extract. While physical activity was reduced in all groups, exercise capacity, limb muscle force, fatigue resistance, fiber size and capillarization, and diaphragm cytoplasmic HIF-1α were unaltered by CS-exposure. However, CS-induced diaphragm atrophy and increased capillary density were not seen after 2 weeks of smoking cessation.

CONCLUSION

In male mice, 2 weeks of smoking cessation reversed smoking-induced mitochondrial dysfunction, limb muscle mass loss, and diaphragm muscle atrophy, highlighting immediate benefits of cessation on skeletal muscles.

IMPLICATIONS

Our study demonstrates that CS-induced skeletal muscle mitochondrial dysfunction and atrophy are significantly improved by 2 weeks of cessation in male mice. We show for the first time that smoking cessation as short as 1 to 2 weeks is associated with immediate beneficial effects on skeletal muscle structure and function with the diaphragm being particularly sensitive to CS-exposure and cessation. This could help motivate smokers to quit smoking as early as possible. The knowledge that smoking cessation has potential positive extrapulmonary effects is particularly relevant for patients referred to rehabilitation programs and those admitted to hospitals suffering from acute or chronic muscle deterioration yet struggling with smoking cessation.

Trichostatin A inhibits skeletal muscle atrophy induced by cigarette smoke exposure in mice

AIMS

It is well known that cigarette smoke (CS) is the main risk factor for chronic obstructive pulmonary disease (COPD) accompanied by skeletal muscle atrophy. Histone deacetylases (HDACs) that remove acetyl groups from target proteins are necessary for the muscle atrophy associated with skeletal muscle disuse. However, the role of HDACs and trichostatin A (TSA), a HDAC inhibitor, in skeletal muscle atrophy caused by CS exposure remains poorly understood.

MAIN METHODS

Female mice were exposed to CS twice daily for 40 days and TSA injected intraperitoneally into CS-exposed mice on alternate days. Skeletal muscles were weighed and gastrocnemius (Gas) muscle histomorphology examined by hematoxylin and eosin staining. Histone deacetylases 1 and 2 (HDAC1/2), and markers of ubiquitin degradation, muscle differentiation, apoptosis, pyroptosis, and the cytoskeletal proteins were assessed by western blot and immunohistochemistry in Gas.

KEYFINDINGS

CS exposure decreased body and skeletal muscle weights and triggered an increase in the percentage of fiber with centralized nuclei in Gas. HDAC1/2 proteins were upregulated in the Gas of mice exposed to CS, while TSA effectively inhibited HDAC1/2 protein levels and attenuated the loss of body weight and skeletal muscle wet weight induced by CS exposure. Markers for ubiquitin degradation, muscle differentiation, cytoskeletal proteins, apoptosis and pyroptosis were all upregulated following CS exposure and effectively restored by TSA.

SIGNIFICANCE

TSA may inhibit skeletal muscle atrophy and histomorphological alterations induced by CS exposure by downregulating markers of ubiquitin degradation, muscle fiber differentiation, cytoskeletal proteins, apoptosis and pyroptosis via HDAC1/2 inhibition.

Interaction between Physical Activity and Smoking on Lung, Muscle, and Bone in Mice

Physical inactivity is an important contributor to skeletal muscle weakness, osteoporosis, and weight loss in chronic obstructive pulmonary disease. However, the effects of physical inactivity, in interaction with smoking, on lung, muscle, and bone are poorly understood. To address this issue, male mice were randomly assigned to an active (daily running), moderately inactive (space restriction), or extremely inactive group (space restriction followed by hindlimb suspension to mimic bed rest) during 24 weeks and simultaneously exposed to either cigarette smoke or room air. The effects of different physical activity levels and smoking status and their respective interaction were examined on lung function, body composition, in vitro limb muscle function, and bone parameters. Smoking caused emphysema, reduced food intake with subsequent loss of body weight, and fat, lean, and muscle mass, but increased trabecular bone volume. Smoking induced muscle fiber atrophy, which did not result in force impairment. Moderate inactivity only affected lung volumes and compliance, whereas extreme inactivity increased lung inflammation, lowered body and fat mass, induced fiber atrophy with soleus muscle dysfunction, and reduced exercise capacity and all bone parameters. When combined with smoking, extreme inactivity also aggravated lung inflammation and emphysema, and accelerated body and muscle weight loss. This study shows that extreme inactivity, especially when imposed by absolute rest, accelerates lung damage and inflammation. When combined with smoking, extreme inactivity is deleterious for muscle bulk, bone, and lungs. These data highlight that the consequences of physical inactivity during the course of chronic obstructive pulmonary disease should not be neglected.

Molecular mechanism of sarcopenia and cachexia: recent research advances

Skeletal muscle provides a fundamental basis for human function, enabling locomotion and respiration. Muscle loss occurs as a consequence of several chronic diseases (cachexia) and normal aging (sarcopenia). Although many negative regulators (atrogin-1, muscle ring finger-1, nuclear factor-kappaB (NF-κB), myostatin, etc.) have been proposed to enhance protein degradation during both sarcopenia and cachexia, the adaptation of these mediators markedly differs within both conditions. Sarcopenia and cachectic muscles have been demonstrated to be abundant in myostatin-linked molecules. The ubiquitin-proteasome system (UPS) is activated during rapid atrophy model (cancer cachexia), but few mediators of the UPS change during sarcopenia. NF-κB signaling is activated in cachectic, but not in sarcopenic, muscle. Recent studies have indicated the age-related defect of autophagy signaling in skeletal muscle, whereas autophagic activation occurs in cachectic muscle. This review provides recent research advances dealing with molecular mediators modulating muscle mass in both sarcopenia and cachexia.

Vitamin D deficiency impairs skeletal muscle function in a smoking mouse model

Chronic obstructive pulmonary disease (COPD) is associated with skeletal muscle dysfunction. Vitamin D plays an important role in muscle strength and performance in healthy individuals. Vitamin D deficiency is highly prevalent in COPD, but its role in skeletal muscle dysfunction remains unclear. We examined the time-course effect of vitamin D deficiency on limb muscle function in mice with normal or deficient vitamin D serum levels exposed to air or cigarette smoke for 6, 12 or 18 weeks. The synergy of smoking and vitamin D deficiency increased lung inflammation and lung compliance from 6 weeks on with highest emphysema scores observed at 18 weeks. Smoking reduced body and muscle mass of the soleus and extensor digitorum longus (EDL), but did not affect contractility, despite type II atrophy. Vitamin D deficiency did not alter muscle mass but reduced muscle force over time, downregulated vitamin D receptor expression, and increased muscle lipid peroxidation but did not alter actin and myosin expression, fiber dimensions or twitch relaxation time. The combined effect of smoking and vitamin D deficiency did not further deteriorate muscle function but worsened soleus mass loss and EDL fiber atrophy at 18 weeks. We conclude that the synergy of smoking and vitamin D deficiency in contrast to its effect on lung disease, had different, independent but important noxious effects on skeletal muscles in a mouse model of mild COPD.

Targeting oxidant-dependent mechanisms for the treatment of COPD and its comorbidities

Chronic obstructive pulmonary disease (COPD) is an incurable global health burden and is characterised by progressive airflow limitation and loss of lung function. In addition to the pulmonary impact of the disease, COPD patients often develop comorbid diseases such as cardiovascular disease, skeletal muscle wasting, lung cancer and osteoporosis. One key feature of COPD, yet often underappreciated, is the contribution of oxidative stress in the onset and development of the disease. Patients experience an increased burden of oxidative stress due to the combined effects of excess reactive oxygen species (ROS) and nitrogen species (RNS) generation, antioxidant depletion and reduced antioxidant enzyme activity. Currently, there is a lack of effective treatments for COPD, and an even greater lack of research regarding interventions that treat both COPD and its comorbidities. Due to the involvement of oxidative stress in the pathogenesis of COPD and many of its comorbidities, a unique therapeutic opportunity arises where the treatment of a multitude of diseases may be possible with only one therapeutic target. In this review, oxidative stress and the roles of ROS/RNS in the context of COPD and comorbid cardiovascular disease, skeletal muscle wasting, lung cancer, and osteoporosis are discussed and the potential for therapeutic benefit of anti-oxidative treatment in these conditions is outlined. Because of the unique interplay between oxidative stress and these diseases, oxidative stress represents a novel target for the treatment of COPD and its comorbidities.

Systemic inflammation in chronic obstructive pulmonary disease and lung cancer: common driver of pulmonary cachexia?

PURPOSE OF REVIEW

In this article, a putative role of systemic inflammation as a driver of pulmonary cachexia induced by either chronic obstructive pulmonary disease or nonsmall cell lung cancer is reviewed. Gaps in current translational research approaches are discussed and alternative strategies are proposed to provide new insights.

RECENT FINDINGS

Activation of the ubiquitin proteasome system has generally been considered a cause of pulmonary cachexia, but current animal models lack specificity and evidence is lacking in nonsmall cell lung cancer and conflicting in chronic obstructive pulmonary disease patients. Recent studies have shown activation of the autophagy-lysosome pathway in both nonsmall cell lung cancer and chronic obstructive pulmonary disease. Myonuclear loss, as a consequence of increased apoptotic events in myofibers, has been suggested in cancer-cachexia-associated muscle atrophy. Plasma transfer on myotube cultures can be used to detect early inflammatory signals in patients and presence of atrophy-inducing activity within the circulation.

SUMMARY

Comparative clinical research between nonsmall cell lung cancer and chronic obstructive pulmonary disease in different disease stages is useful to unravel disease-specific versus common denominators of pulmonary cachexia.

Time course of cigarette smoke-induced changes of systemic inflammation and muscle structure

It has become more evident that long-term cigarette smoking (LTCS) has an important extrapulmonary toxicity. The aim of the study was to investigate the time-dependent effects of cigarette smoke exposure on exercise capacity, markers of systemic inflammation, and skeletal muscle structure. c57bl/6j-mice were either exposed to mainstream cigarette smoke for 6 h/day, 5 days/wk [smoke-exposed (SE) group] or assigned to the control, unexposed group (Con group). SE group mice were exposed for 8, 16, 24, and 32 wk to smoke and unexposed Con mice were used as age-matched controls. Exercise capacity was investigated by spiroergometry. Systemic inflammatory status was analyzed by flow cytometry and multiplexed fluorescent immunoassay. For analysis of muscle tissue, histological techniques and microarray analysis were used. Mice of the SE group exhibited a lower increase of body mass and a decrease of V̇o2 max (P < 0.05). An increase of lymphocyte CD62, ICAM, and VCAM expression was found in SE mice (P < 0.05). A biphasic trend of protein up- and downregulation was observed in markers of systemic inflammation, tissue deterioration, and allergic reactions such as C-reactive protein (CRP), eotaxin, haptoglobin, macrophage colony-stimulating factor-1 (M-CSF-1), and macrophage inflammatory protein-1γ (MIP-1γ). Thereby, the expression of several chemotactic proteins in plasma correlated with their expression in muscle. A time-dependent decrease of muscle mass, oxidative type-I fibers, and muscle cross-sectional area was found (P < 0.05). Microarray analysis revealed a SE-induced upregulation of several pathways of metabolic processes and tissue degradation. Taken together it was found that the loss of exercise capacity and systemic inflammation are early events of SE, which might induce muscular atrophy and loss of oxidative muscle capacity.

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.

Animal models of chronic obstructive pulmonary disease

INTRODUCTION

Chronic obstructive pulmonary disease (COPD) is a leading global cause of mortality and chronic morbidity. Inhalation of cigarette smoke is the principal risk factor for development of this disease. COPD is a progressive disease that is typically characterised by chronic pulmonary inflammation, mucus hypersecretion, airway remodelling and emphysema that collectively reduce lung function. There are currently no therapies that effectively halt or reverse disease progression. It is hoped that the development of animal models that develop the hallmark features of COPD, in a short time frame, will aid in the identifying and testing of new therapeutic approaches.

AREAS COVERED

The authors review the recent developments in mouse models of chronic cigarette smoke-induced COPD as well as the principal findings. Furthermore, the authors discuss the use of mouse models to understand the pathogenesis and the contribution of infectious exacerbations. They also discuss the investigations of the systemic co-morbidities of COPD (pulmonary hypertension, cachexia and osteoporosis).

EXPERT OPINION

Recent advances in the field mark a point where animal models recapitulate the pathologies of COPD patients in a short time frame. They also reveal novel insights into the pathogenesis and potential treatment of this debilitating disease.

The lung inflammation and skeletal muscle wasting induced by subchronic cigarette smoke exposure are not altered by a high-fat diet in mice

Obesity and cigarette smoking independently constitute major preventable causes of morbidity and mortality and obesity is known to worsen lung inflammation in asthma. Paradoxically, higher body mass index (BMI) is associated with reduced mortality in smoking induced COPD whereas low BMI increases mortality risk. To date, no study has investigated the effect of a dietary-induced obesity and cigarette smoke exposure on the lung inflammation and loss of skeletal muscle mass in mice. Male BALB/c mice were exposed to 4 cigarettes/day, 6 days/week for 7 weeks, or sham handled. Mice consumed either standard laboratory chow (3.5 kcal/g, 12% fat) or a high fat diet (HFD, 4.3 kcal/g, 32% fat). Mice exposed to cigarette smoke for 7 weeks had significantly more inflammatory cells in the BALF (P<0.05) and the mRNA expression of pro-inflammatory cytokines and chemokines was significantly increased (P<0.05); HFD had no effect on these parameters. Sham- and smoke-exposed mice consuming the HFD were significantly heavier than chow fed animals (12 and 13%, respectively; P<0.05). Conversely, chow and HFD fed mice exposed to cigarette smoke weighed 16 and 15% less, respectively, compared to sham animals (P<0.05). The skeletal muscles (soleus, tibialis anterior and gastrocnemius) of cigarette smoke-exposed mice weighed significantly less than sham-exposed mice (P<0.05) and the HFD had no protective effect. For the first time we report that cigarette smoke exposure significantly decreased insulin-like growth factor-1 (IGF-1) mRNA expression in the gastrocnemius and tibialis anterior and IGF-1 protein in the gastrocnemius (P<0.05). We have also shown that cigarette smoke exposure reduced circulating IGF-1 levels. IL-6 mRNA expression was significantly elevated in all three skeletal muscles of chow fed smoke-exposed mice (P<0.05). In conclusion, these findings suggest that a down-regulation in local IGF-1 may be responsible for the loss of skeletal muscle mass following cigarette smoke exposure in mice.

Alterations in skeletal muscle cell homeostasis in a mouse model of cigarette smoke exposure

BACKGROUND

Skeletal muscle dysfunction is common in chronic obstructive pulmonary disease (COPD), a disease mainly caused by chronic cigarette use. An important proportion of patients with COPD have decreased muscle mass, suggesting that chronic cigarette smoke exposure may interfere with skeletal muscle cellular equilibrium. Therefore, the main objective of this study was to investigate the kinetic of the effects that cigarette smoke exposure has on skeletal muscle cell signaling involved in protein homeostasis and to assess the reversibility of these effects.

METHODS

A mouse model of cigarette smoke exposure was used to assess skeletal muscle changes. BALB/c mice were exposed to cigarette smoke or room air for 8 weeks, 24 weeks or 24 weeks followed by 60 days of cessation. The gastrocnemius and soleus muscles were collected and the activation state of key mediators involved in protein synthesis and degradation was assessed.

RESULTS

Gastrocnemius and soleus were smaller in mice exposed to cigarette smoke for 8 and 24 weeks compared to room air exposed animals. Pro-degradation proteins were induced at the mRNA level after 8 and 24 weeks. Twenty-four weeks of cigarette smoke exposure induced pro-degradation proteins and reduced Akt phosphorylation and glycogen synthase kinase-3β quantity. A 60-day smoking cessation period reversed the cell signaling alterations induced by cigarette smoke exposure.

CONCLUSIONS

Repeated cigarette smoke exposure induces reversible muscle signaling alterations that are dependent on the duration of the cigarette smoke exposure. These results highlights a beneficial aspect associated with smoking cessation.

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.

Long-term nose-only cigarette smoke exposure induces emphysema and mild skeletal muscle dysfunction in mice

Mouse models of chronic obstructive pulmonary disease (COPD) focus on airway inflammation and lung histology, but their use has been hampered by the lack of pulmonary function data in their assessment. Systemic effects such as muscle dysfunction are also poorly modeled in emphysematous mice. We aimed to develop a cigarette-smoke-induced emphysema mouse model in which serial lung function and muscular dysfunction could be assessed, allowing the disease to be monitored more appropriately. C57Bl6 mice were nose-only exposed to cigarette smoke or filtered air for 3–6 months. Lung function tests were repeated in the same mice after 3 and 6 months of cigarette smoke or air exposure and compared with lung histological changes. Contractile properties of skeletal muscles and muscle histology were also determined at similar time points in separate groups of mice. Serial lung function measurements documented hyperinflation after 3 and 6 months of cigarette smoke exposure, with a significant 31–37% increase in total lung capacity (TLC) and a significant 26–35% increase in compliance (Cchord) when compared with animals exposed to filtered air only (P<0.001 after 3 and after 6 months). These functional changes preceded the changes in mean linear intercept, which became only significant after 6 months of cigarette smoke exposure and which correlated very well with TLC (r=0.74, P=0.004) and Cchord (r=0.79, P=0.001). After 6 months of cigarette smoke exposure, a significant fiber-type shift from IIa to IIx/b was also observed in the soleus muscle (P<0.05), whereas a 20% reduction of force was present at high stimulation frequencies (80 Hz; P=0.09). The extensor digitorum longus (EDL) muscle was not affected by cigarette smoke exposure. These serial pulmonary function variables are sensitive outcomes to detect emphysema progression in a nose-only cigarette-smoke-exposed animal model of COPD. In this model, muscular changes became apparent only after 6 months, particularly in muscles with a mixed fiber-type composition.

Elastase-induced pulmonary emphysema: insights from experimental models

Several distinct stimuli can be used to reproduce histological and functional features of human emphysema, a leading cause of disability and death. Since cigarette smoke is the main cause of emphysema in humans, experimental researches have attempted to reproduce this situation. However, this is an expensive and cumbersome method of emphysema induction, and simpler, more efficacious alternatives have been sought. Among these approaches, elastolytic enzymes have been widely used to reproduce some characteristics of human cigarette smoke-induced disease, such as: augmentation of airspaces, inflammatory cell influx into the lungs, and systemic inflammation. Nevertheless, the use of elastase-induced emphysema models is still controversial, since the disease pathways involved in elastase induction may differ from those occurring in smoke-induced emphysema. This indicates that the choice of an emphysema model may impact the results of new therapies or drugs being tested. The aim of this review is to compare the mechanisms of disease induction in smoke and elastase emphysema models, to describe the differences among various elastase models, and to establish the advantages and disadvantages of elastase-induced emphysema models. More studies are required to shed light on the mechanisms of elastase-induced emphysema.

Extrapulmonary manifestations of chronic obstructive pulmonary disease in a mouse model of chronic cigarette smoke exposure

Cigarette smoking is the most commonly encountered risk factor for chronic obstructive pulmonary disease (COPD), reflected by irreversible airflow limitation, frequently associated with airspace enlargement and pulmonary inflammation. In addition, COPD has systemic consequences, including systemic inflammation, muscle wasting, and loss of muscle oxidative phenotype. However, the role of smoking in the development of these extrapulmonary manifestations remains rather unexplored. Mice were exposed to cigarette smoke or control air for 6 months. Subsequently, emphysema was assessed by morphometry of lung tissue, and blood cytokine and chemokine levels were determined by a multiplex assay. Soleus, plantaris, gastrocnemius, and tibialis muscles were dissected and weighed. Muscle fiber typing was performed based on I, IIA, IIB, and IIX myosin heavy-chain isoform composition. Lungs of the smoke-exposed animals showed pulmonary inflammation and emphysema. Moreover, circulating levels of primarily proinflammatory proteins, especially TNF-alpha, were elevated after smoke exposure. Despite an attenuated body weight gain, only the soleus showed a tendency toward lower muscle weight after smoke exposure. Oxidative fiber type IIA proportion was significantly reduced in the soleus. Muscle oxidative enzyme activity was slightly reduced after smoke exposure, being most prominent for citrate synthase in the soleus and tibialis. In this mouse model, chronic cigarette smoke exposure resulted in systemic features that closely resemble the early signs of the extrapulmonary manifestations observed in patients with COPD.