Xanthine oxidase inhibition attenuates skeletal muscle signaling following acute exercise but does not impair mitochondrial adaptations to endurance training

Antioxidant enzymes and Nrf2/Keap1 in human skeletal muscle: Influence of age, sex, adiposity and aerobic fitness

Ageing, a sedentary lifestyle, and obesity are associated with increased oxidative stress, while regular exercise is associated with an increased antioxidant capacity in trained skeletal muscles. Whether a higher aerobic fitness is associated with increased expression of antioxidant enzymes and their regulatory factors in skeletal muscle remains unknown. Although oestrogens could promote a higher antioxidant capacity in females, it remains unknown whether a sex dimorphism exists in humans regarding the antioxidant capacity of skeletal muscle. Thus, the aim was to determine the protein expression levels of the antioxidant enzymes SOD1, SOD2, catalase and glutathione reductase (GR) and their regulatory factors Nrf2 and Keap1 in 189 volunteers (120 males and 69 females) to establish whether sex differences exist and how age, VO2max and adiposity influence these. For this purpose, vastus lateralis muscle biopsies were obtained in all participants under resting and unstressed conditions. No significant sex differences in Nrf2, Keap1, SOD1, SOD2, catalase and GR protein expression levels were observed after accounting for VO2max, age and adiposity differences. Multiple regression analysis indicates that the VO2max in mL.kg LLM-1.min-1can be predicted from the levels of SOD2, Total Nrf2 and Keap1 (R = 0.58, P < 0.001), with SOD2 being the main predictor explaining 28 % of variance in VO2max, while Nrf2 and Keap1 explained each around 3 % of the variance. SOD1 protein expression increased with ageing in the whole group after accounting for differences in VO2max and body fat percentage. Overweight and obesity were associated with increased pSer40-Nrf2, pSer40-Nrf2/Total Nrf2 ratio and SOD1 protein expression levels after accounting for differences in age and VO2max. Overall, at the population level, higher aerobic fitness is associated with increased basal expression of muscle antioxidant enzymes, which may explain some of the benefits of regular exercise.

Discordant skeletal muscle gene and protein responses to exercise

The ability of skeletal muscle to adapt to repeated contractile stimuli is one of the most intriguing aspects of physiology. The molecular bases underpinning these adaptations involve increased protein activity and/or expression, mediated by an array of pre- and post-transcriptional processes, as well as translational and post-translational control. A longstanding dogma assumes a direct relationship between exercise-induced increases in mRNA levels and subsequent changes in the abundance of the proteins they encode. Drawing on the results of recent studies, we dissect and question the common assumption of a direct relationship between changes in the skeletal muscle transcriptome and proteome induced by repeated muscle contractions (e.g., exercise).

Determinants of the maximal functional reserve during repeated supramaximal exercise by humans: The roles of Nrf2/Keap1, antioxidant proteins, muscle phenotype and oxygenation

When high-intensity exercise is performed until exhaustion a "functional reserve" (FR) or capacity to produce power at the same level or higher than reached at exhaustion exists at task failure, which could be related to reactive oxygen and nitrogen species (RONS)-sensing and counteracting mechanisms. Nonetheless, the magnitude of this FR remains unknown. Repeated bouts of supramaximal exercise at 120% of VO2max interspaced with 20s recovery periods with full ischaemia were used to determine the maximal FR. Then, we determined which muscle phenotypic features could account for the variability in functional reserve in humans. Exercise performance, cardiorespiratory variables, oxygen deficit, and brain and muscle oxygenation (near-infrared spectroscopy) were measured, and resting muscle biopsies were obtained from 43 young healthy adults (30 males). Males and females had similar aerobic (VO2max per kg of lower extremities lean mass (LLM): 166.7 ± 17.1 and 166.1 ± 15.6 ml kg LLM-1.min-1, P = 0.84) and anaerobic fitness (similar performance in the Wingate test and maximal accumulated oxygen deficit when normalized to LLM). The maximal FR was similar in males and females when normalized to LLM (1.84 ± 0.50 and 2.05 ± 0.59 kJ kg LLM-1, in males and females, respectively, P = 0.218). This FR depends on an obligatory component relying on a reserve in glycolytic capacity and a putative component generated by oxidative phosphorylation. The aerobic component depends on brain oxygenation and phenotypic features of the skeletal muscles implicated in calcium handling (SERCA1 and 2 protein expression), oxygen transport and diffusion (myoglobin) and redox regulation (Keap1). The glycolytic component can be predicted by the protein expression levels of pSer40-Nrf2, the maximal accumulated oxygen deficit and the protein expression levels of SOD1. Thus, an increased capacity to modulate the expression of antioxidant proteins involved in RONS handling and calcium homeostasis may be critical for performance during high-intensity exercise in humans.

Transient changes to metabolic homeostasis initiate mitochondrial adaptation to endurance exercise

Endurance exercise is well established to increase mitochondrial content and function in skeletal muscle, a process termed mitochondrial biogenesis. Current understanding is that exercise initiates skeletal muscle mitochondrial remodeling via modulation of cellular nutrient, energetic and contractile stress pathways. These subtle changes in the cellular milieu are sensed by numerous transduction pathways that serve to initiate and coordinate an increase in mitochondrial gene transcription and translation. The result of these acute signaling events is the promotion of growth and assembly of mitochondria, coupled to a greater capacity for aerobic ATP provision in skeletal muscle. The aim of this review is to highlight the acute metabolic events induced by endurance exercise and the subsequent molecular pathways that sense this transient change in cellular homeostasis to drive mitochondrial adaptation and remodeling.

Localized Heat Therapy Improves Mitochondrial Respiratory Capacity but Not Fatty Acid Oxidation

AIM: Mild heat stress can improve mitochondrial respiratory capacity in skeletal muscle. However, long-term heat interventions are scarce, and the effects of heat therapy need to be understood in the context of the adaptations which follow the more complex combination of stimuli from exercise training. The purpose of this work was to compare the effects of 6 weeks of localized heat therapy on human skeletal muscle mitochondria to single-leg interval training. METHODS: Thirty-five subjects were assigned to receive sham therapy, short-wave diathermy heat therapy, or single-leg interval exercise training, localized to the quadriceps muscles of the right leg. All interventions took place 3 times per week. Muscle biopsies were performed at baseline, and after 3 and 6 weeks of intervention. Mitochondrial respiratory capacity was assessed on permeabilized muscle fibers via high-resolution respirometry. RESULTS: The primary finding of this work was that heat therapy and exercise training significantly improved mitochondrial respiratory capacity by 24.8 ± 6.2% and 27.9 ± 8.7%, respectively (p < 0.05). Fatty acid oxidation and citrate synthase activity were also increased following exercise training by 29.5 ± 6.8% and 19.0 ± 7.4%, respectively (p < 0.05). However, contrary to our hypothesis, heat therapy did not increase fatty acid oxidation or citrate synthase activity. CONCLUSION: Six weeks of muscle-localized heat therapy significantly improves mitochondrial respiratory capacity, comparable to exercise training. However, unlike exercise, heat does not improve fatty acid oxidation capacity.

MitoQ supplementation augments acute exercise-induced increases in muscle PGC1α mRNA and improves training-induced increases in peak power independent of mitochondrial content and function in untrained middle-aged men

The role of mitochondrial ROS in signalling muscle adaptations to exercise training has not been explored in detail. We investigated the effect of supplementation with the mitochondria-targeted antioxidant MitoQ on a) the skeletal muscle mitochondrial and antioxidant gene transcriptional response to acute high-intensity exercise and b) skeletal muscle mitochondrial content and function following exercise training. In a randomised, double-blind, placebo-controlled, parallel design study, 23 untrained men (age: 44 ± 7 years, VO_2peak: 39.6 ± 7.9 ml/kg/min) were randomised to receive either MitoQ (20 mg/d) or a placebo for 10 days before completing a bout of high-intensity interval exercise (cycle ergometer, 10 × 60 s at VO_2peak workload with 75 s rest). Blood samples and vastus lateralis muscle biopsies were collected before exercise and immediately and 3 h after exercise. Participants then completed high-intensity interval training (HIIT; 3 sessions per week for 3 weeks) and another blood sample and muscle biopsy were collected. There was no effect of acute exercise or MitoQ on systemic (plasma protein carbonyls and reduced glutathione) or skeletal muscle (mtDNA damage and 4-HNE) oxidative stress biomarkers. Acute exercise-induced increases in skeletal muscle peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC1-α) mRNA expression were augmented in the MitoQ group. Despite this, training-induced increases in skeletal muscle mitochondrial content were similar between groups. HIIT-induced increases in VO_2peak and 20 km time trial performance were also similar between groups while training-induced increases in peak power achieved during the VO_2peak test were augmented in the MitoQ group. These data suggest that training-induced increases in peak power are enhanced following MitoQ supplementation, which may be related to the augmentation of skeletal muscle PGC1α expression following acute exercise. However, these effects do not appear to be related to an effect of MitoQ supplementation on exercise-induced oxidative stress or training-induced mitochondrial biogenesis in skeletal muscle.

Xanthine Oxidoreductase Activity in Marathon Runners: Potential Implications for Marathon-Induced Acute Kidney Injury

Excess activation of circulating xanthine oxidoreductase (XOR) may contribute to the pathogenesis of widespread remote organ injury, including kidney injury. The purpose of this study was to determine the acute impact of marathon running on plasma XOR activity and to examine whether plasma XOR activity is associated with marathon-induced elevations in biomarkers of acute kidney injury (AKI). Twenty-three young men (aged 20-25 years) who participated in the 38th Tsukuba Marathon were included. Blood and urine samples were collected before, immediately, 2 h (only blood sample), and 24 h after a full marathon run. Plasma XOR activity was evaluated using a highly sensitive assay utilizing a combination of [13C2,15N2] xanthine and liquid chromatography-triple quadrupole mass spectrometry. The levels of several AKI biomarkers, such as serum creatinine and urinary liver-type fatty acid-binding protein (L-FABP) were measured in each participant. Marathon running caused a transient elevation in plasma XOR activity and levels of purine degradation products (hypoxanthine, xanthine, and uric acid) as well as serum creatinine, urinary albumin, and urinary L-FABP levels. Immediately after the marathon, individual relative changes in plasma XOR activity were independently correlated with corresponding changes in serum creatinine and urinary L-FABP levels. In addition, the magnitude of marathon-induced elevation in plasma XOR activity and levels of purine degradation products were higher in individuals who developed AKI. These findings collectively suggest that marathon running substantially influences the purine metabolism pathway including XOR activity. Moreover, activated circulating XOR can be partly associated with elevated biomarkers of AKI after marathon running.

Exercise-Stimulated ROS Sensitive Signaling Pathways in Skeletal Muscle

Physical exercise represents a major challenge to whole-body homeostasis, provoking acute and adaptative responses at the cellular and systemic levels. Different sources of reactive oxygen species (ROS) have been described in skeletal muscle (e.g., NADPH oxidases, xanthine oxidase, and mitochondria) and are closely related to the physiological changes induced by physical exercise through the modulation of several signaling pathways. Many signaling pathways that are regulated by exercise-induced ROS generation, such as adenosine monophosphate-activated protein kinase (AMPK), mitogen activated protein kinase (MAPK), nuclear respiratory factor2 (NRF2), and PGC-1α are involved in skeletal muscle responses to physical exercise, such as increased glucose uptake, mitochondriogenesis, and hypertrophy, among others. Most of these adaptations are blunted by antioxidants, revealing the crucial role played by ROS during and after physical exercise. When ROS generation is either insufficient or exacerbated, ROS-mediated signaling is disrupted, as well as physical exercise adaptations. Thus, an understanding the limit between "ROS that can promote beneficial effects" and "ROS that can promote harmful effects" is a challenging question in exercise biology. The identification of new mediators that cause reductive stress and thereby disrupt exercise-stimulated ROS signaling is a trending on this topic and are covered in this current review.

Differences between Professional and Amateur Cyclists in Endogenous Antioxidant System Profile

Currently, no studies have examined the differences in endogenous antioxidant enzymes in professional and amateur cyclists and how these can influence sports performance. The aim of this study was to identify differences in endogenous antioxidants enzymes and hemogram between competitive levels of cycling and to see if differences found in these parameters could explain differences in performance. A comparative trial was carried out with 11 professional (PRO) and 15 amateur (AMA) cyclists. All cyclists performed an endogenous antioxidants analysis in the fasted state (visit 1) and an incremental test until exhaustion (visit 2). Higher values in catalase (CAT), oxidized glutathione (GSSG) and GSSG/GSH ratio and lower values in superoxide dismutase (SOD) were found in PRO compared to AMA (p < 0.05). Furthermore, an inverse correlation was found between power produced at ventilation thresholds 1 and 2 and GSSG/GSH (r = −0.657 and r = −0.635; p < 0.05, respectively) in PRO. Therefore, there is no well-defined endogenous antioxidant enzyme profile between the two competitive levels of cyclists. However, there was a relationship between GSSG/GSH ratio levels and moderate and submaximal exercise performance in the PRO cohort.

The Effect of Acute Aerobic Exercise on Redox Homeostasis and Mitochondrial Function of Rat White Adipose Tissue

Physical exercise is characterized by an increase in physical and metabolic demand in face of physical stress. It is reported that a single exercise session induces physiological responses through redox signaling to increase cellular function and energy support in diverse organs. However, little is known about the effect of a single bout of exercise on the redox homeostasis and cytoprotective gene expression of white adipose tissue (WAT). Thus, we aimed at evaluating the effects of acute aerobic exercise on WAT redox homeostasis, mitochondrial metabolism, and cytoprotective genic response. Male Wistar rats were submitted to a single moderate-high running session (treadmill) and were divided into five groups: control (CTRL, without exercise), and euthanized immediately (0 h), 30 min, 1 hour, or 2 hours after the end of the exercise session. NADPH oxidase activity was higher in 0 h and 30 min groups when compared to CTRL group. Extramitochondrial ROS production was higher in 0 h group in comparison to CTRL and 2 h groups. Mitochondrial respiration in phosphorylative state increased in 0 h group when compared to CTRL, 30 min, 1, and 2 h groups. On the other hand, mitochondrial ATP production was lower in 0 h in comparison to 30 min group, increasing in 1 and 2 h groups when compared to CTRL and 0 h groups. CAT activity was lower in all exercised groups when compared to CTRL. Regarding oxidative stress biomarkers, we observed a decrease in reduced thiol content in 0 h group compared to CTRL and 2 h groups, and higher levels of protein carbonylation in 0 and 30 min groups in comparison to the other groups. The levels returned to basal condition in 2 h group. Furthermore, aerobic exercise increased NRF2, GPX2, HMOX1, SOD1, and CAT mRNA levels. Taken together, our results suggest that one session of aerobic exercise can induce a transient prooxidative state in WAT, followed by an increase in antioxidant and cytoprotective gene expression.

Healthful aging mediated by inhibition of oxidative stress

The progressive increase in lifespan over the past century carries with it some adversity related to the accompanying burden of debilitating diseases prevalent in the older population. This review focuses on oxidative stress as a major mechanism limiting longevity in general, and healthful aging, in particular. Accordingly, the first goal of this review is to discuss the role of oxidative stress in limiting longevity, and compare healthful aging and its mechanisms in different longevity models. Secondly, we discuss common signaling pathways involved in protection against oxidative stress in aging and in the associated diseases of aging, e.g., neurological, cardiovascular and metabolic diseases, and cancer. Much of the literature has focused on murine models of longevity, which will be discussed first, followed by a comparison with human models of longevity and their relationship to oxidative stress protection. Finally, we discuss the extent to which the different longevity models exhibit the healthful aging features through physiological protective mechanisms related to exercise tolerance and increased β-adrenergic signaling and also protection against diabetes and other metabolic diseases, obesity, cancer, neurological diseases, aging-induced cardiomyopathy, cardiac stress and osteoporosis.

What Should I Eat before Exercise? Pre-Exercise Nutrition and the Response to Endurance Exercise: Current Prospective and Future Directions

The primary variables influencing the adaptive response to a bout of endurance training are exercise duration and exercise intensity. However, altering the availability of nutrients before and during exercise can also impact the training response by modulating the exercise stimulus and/or the physiological and molecular responses to the exercise-induced perturbations. The purpose of this review is to highlight the current knowledge of the influence of pre-exercise nutrition ingestion on the metabolic, physiological, and performance responses to endurance training and suggest directions for future research. Acutely, carbohydrate ingestion reduces fat oxidation, but there is little evidence showing enhanced fat burning capacity following long-term fasted-state training. Performance is improved following pre-exercise carbohydrate ingestion for longer but not shorter duration exercise, while training-induced performance improvements following nutrition strategies that modulate carbohydrate availability vary based on the type of nutrition protocol used. Contrasting findings related to the influence of acute carbohydrate ingestion on mitochondrial signaling may be related to the amount of carbohydrate consumed and the intensity of exercise. This review can help to guide athletes, coaches, and nutritionists in personalizing pre-exercise nutrition strategies, and for designing research studies to further elucidate the role of nutrition in endurance training adaptations.

Effects of Dietary Supplements on Adaptations to Endurance Training

Endurance training leads to a variety of adaptations at the cellular and systemic levels that serve to minimise disruptions in whole-body homeostasis caused by exercise. These adaptations are differentially affected by training volume, training intensity, and training status, as well as by nutritional choices that can enhance or impair the response to training. A variety of supplements have been studied in the context of acute performance enhancement, but the effects of continued supplementation concurrent to endurance training programs are less well characterised. For example, supplements such as sodium bicarbonate and beta-alanine can improve endurance performance and possibly training adaptations during endurance training by affecting buffering capacity and/or allowing an increased training intensity, while antioxidants such as vitamin C and vitamin E may impair training adaptations by blunting cellular signalling but appear to have little effect on performance outcomes. Additionally, limited data suggest the potential for dietary nitrate (in the form of beetroot juice), creatine, and possibly caffeine, to further enhance endurance training adaptation. Therefore, the objective of this review is to examine the impact of dietary supplements on metabolic and physiological adaptations to endurance training.

Antioxidant supplements and endurance exercise: Current evidence and mechanistic insights

Antioxidant supplements are commonly consumed by endurance athletes to minimize exercise-induced oxidative stress, with the intention of enhancing recovery and improving performance. There are numerous commercially available nutritional supplements that are targeted to athletes and health enthusiasts that allegedly possess antioxidant properties. However, most of these compounds are poorly investigated with respect to their in vivo redox activity and efficacy in humans. Therefore, this review will firstly provide a background to endurance exercise-related redox signalling and the subsequent adaptations in skeletal muscle and vascular function. The review will then discuss commonly available compounds with purported antioxidant effects for use by athletes. N-acetyl cysteine may be of benefit over the days prior to an endurance event; while chronic intake of combined 1000 mg vitamin C + vitamin E is not recommended during periods of heavy training associated with adaptations in skeletal muscle. Melatonin, vitamin E and α-lipoic acid appear effective at decreasing markers of exercise-induced oxidative stress. However, evidence on their effects on endurance performance are either lacking or not supportive. Catechins, anthocyanins, coenzyme Q10 and vitamin C may improve vascular function, however, evidence is either limited to specific sub-populations and/or does not translate to improved performance. Finally, additional research should clarify the potential benefits of curcumin in improving muscle recovery post intensive exercise; and the potential hampering effects of astaxanthin, selenium and vitamin A on skeletal muscle adaptations to endurance training. Overall, we highlight the lack of supportive evidence for most antioxidant compounds to recommend to athletes.

The effect of high intensity functional training on the oxidative status, muscle damage and performance of basketball players

BACKGROUND

The purpose of this study was to evaluate the effect of 10 weeks of two different in-season training programs on the oxidative status and muscle damage and performance of professional basketball players. We hypothesized that high intensity functional training (HIFT) induces more redox sensitive adaptations than common strength and conditioning training (CSCT).

METHODS

Twenty professional basketball players of Iran national women's basketball league (age 21.95±2.45, years of experience 7.15±1.7), were divided into two equal training groups; HIFT and CSCT (in average of 80-150 and 180-240 minutes per week respectively). Blood samples and performance tests including VO<inf>2max</inf>, basketball simulated performance (BEST), anaerobic power, agility and vertical jump were taken before and after training. Oxidative status and tissue damage were assessed through xanthine oxidase, total antioxidant capacity, pro-oxidant/antioxidant balance and creatine kinase. Data were analyzed through repeated measure mixed ANOVA.

RESULTS

BEST, average power and Fatigue Index significantly improved in HIFT group (α<0.05). VO<inf>2max</inf> and agility t-test improved significantly in both groups (α<0.05), with no significant difference between the two groups. Lateral agility and vertical jump did not change significantly in neither of the groups. No significant group × time interaction was observed in the biochemical factors. XO increased, TAC and CK decreased significantly in both groups (α<0.05), with no significant change in PAB in neither of the groups after training.

CONCLUSIONS

The findings of the present study showed no oxidative stress and tissue damage in none of the training groups, recommending the implication of more time-efficient HIFT method into the in-season training of team sports.

Redox basis of exercise physiology

Redox reactions control fundamental processes of human biology. Therefore, it is safe to assume that the responses and adaptations to exercise are, at least in part, mediated by redox reactions. In this review, we are trying to show that redox reactions are the basis of exercise physiology by outlining the redox signaling pathways that regulate four characteristic acute exercise-induced responses (muscle contractile function, glucose uptake, blood flow and bioenergetics) and four chronic exercise-induced adaptations (mitochondrial biogenesis, muscle hypertrophy, angiogenesis and redox homeostasis). Based on our analysis, we argue that redox regulation should be acknowledged as central to exercise physiology.

The Emerging Roles of Nicotinamide Adenine Dinucleotide Phosphate Oxidase 2 in Skeletal Muscle Redox Signaling and Metabolism

Significance: Skeletal muscle is a crucial tissue to whole-body locomotion and metabolic health. Reactive oxygen species (ROS) have emerged as intracellular messengers participating in both physiological and pathological adaptations in skeletal muscle. A complex interplay between ROS-producing enzymes and antioxidant networks exists in different subcellular compartments of mature skeletal muscle. Recent evidence suggests that nicotinamide adenine dinucleotide phosphate (NADPH) oxidases (NOXs) are a major source of contraction- and insulin-stimulated oxidants production, but they may paradoxically also contribute to muscle insulin resistance and atrophy. Recent Advances: Pharmacological and molecular biological tools, including redox-sensitive probes and transgenic mouse models, have generated novel insights into compartmentalized redox signaling and suggested that NOX2 contributes to redox control of skeletal muscle metabolism. Critical Issues: Major outstanding questions in skeletal muscle include where NOX2 activation occurs under different conditions in health and disease, how NOX2 activation is regulated, how superoxide/hydrogen peroxide generated by NOX2 reaches the cytosol, what the signaling mediators are downstream of NOX2, and the role of NOX2 for different physiological and pathophysiological processes. Future Directions: Future research should utilize and expand the current redox-signaling toolbox to clarify the NOX2-dependent mechanisms in skeletal muscle and determine whether the proposed functions of NOX2 in cells and animal models are conserved into humans.

Reactive oxygen species-dependent regulation of pyruvate dehydrogenase kinase-4 in white adipose tissue

Reactive oxygen species (ROS) are important signaling molecules mediating the exercise-induced adaptations in skeletal muscle. Acute exercise also drives the expression of genes involved in reesterification and glyceroneogenesis in white adipose tissue (WAT), but whether ROS play any role in this effect has not been explored. We speculated that exercise-induced ROS would regulate acute exercise-induced responses in WAT. To address this question, we utilized various models to alter redox signaling in WAT. We examined basal and exercise-induced gene expression in a genetically modified mouse model of reduced mitochondrial ROS emission [mitochondrial catalase overexpression (MCAT)]. Additionally, H₂O₂, various antioxidants, and the β3-adrenergic receptor agonist CL316243 were used to assess gene expression in white adipose tissue culture. MCAT mice have reduced ROS emission from WAT, enlarged WAT depots and adipocytes, and greater pyruvate dehydrogenase kinase-4 (Pdk4) gene expression. In WAT culture, H₂O₂ reduced glyceroneogenic gene expression. In wild-type mice, acute exercise induced dramatic but transient increases in Pdk4 and phosphoenolpyruvate carboxykinase (Pck1) mRNA in both subcutaneous inguinal WAT and epididymal WAT depots, which was almost completely absent in MCAT mice. Furthermore, the induction of Pdk4 and Pck1 in WAT culture by CL316243 was markedly reduced in the presence of antioxidants N-acetyl-cysteine or vitamin E. Genetic and nutritional approaches that attenuate redox signaling prevent exercise- and β-agonist-induced gene expression within WAT. Combined, these data suggest that ROS represent important mediators of gene expression within WAT.

Acute HIIE elicits similar changes in human skeletal muscle mitochondrial H2O2 release, respiration, and cell signaling as endurance exercise even with less work

It remains unclear whether high-intensity interval exercise (HIIE) elicits distinct molecular responses to traditional endurance exercise relative to the total work performed. We aimed to investigate the influence of exercise intensity on acute perturbations to skeletal muscle mitochondrial function (respiration and reactive oxygen species) and metabolic and redox signaling responses. In a randomized, repeated measures crossover design, eight recreationally active individuals (24 ± 5 yr; V̇o2peak: 48 ± 11 ml·kg−1·min−1) undertook continuous moderate-intensity [CMIE: 30 min, 50% peak power output (PPO)], high-intensity interval (HIIE: 5 × 4 min, 75% PPO, work matched to CMIE), and low-volume sprint interval (SIE: 4 × 30 s) exercise, ≥7 days apart. Each session included muscle biopsies at baseline, immediately, and 3 h postexercise for high-resolution mitochondrial respirometry ( Jo2) and H2O2 emission ( Jh2o2) and gene and protein expression analysis. Immediately postexercise and irrespective of protocol, Jo2 increased during complex I + II leak/state 4 respiration but Jh2o2 decreased ( P < 0.05). AMP-activated protein kinase and acetyl co-A carboxylase phosphorylation increased ~1.5 and 2.5-fold respectively, while thioredoxin-reductase-1 protein abundance was ~35% lower after CMIE vs. SIE ( P < 0.05). At 3 h postexercise, regardless of protocol, Jo2 was lower during both ADP-stimulated state 3 OXPHOS and uncoupled respiration ( P < 0.05) but Jh2o2 trended higher ( P < 0.08) and PPARGC1A mRNA increased ~13-fold, and peroxiredoxin-1 protein decreased ~35%. In conclusion, intermittent exercise performed at high intensities has similar dynamic effects on muscle mitochondrial function compared with endurance exercise, irrespective of whether total workload is matched. This suggests exercise prescription can accommodate individual preferences while generating comparable molecular signals known to promote beneficial metabolic adaptations.

Fatiguing contractions increase protein S-glutathionylation occupancy in mouse skeletal muscle

Protein S-glutathionylation is an important reversible post-translational modification implicated in redox signaling. Oxidative modifications to protein thiols can alter the activity of metabolic enzymes, transcription factors, kinases, phosphatases, and the function of contractile proteins. However, the extent to which muscle contraction induces oxidative modifications in redox sensitive thiols is not known. The purpose of this study was to determine the targets of S-glutathionylation redox signaling following fatiguing contractions. Anesthetized adult male CB6F1 (BALB/cBy × C57BL/6) mice were subjected to acute fatiguing contractions for 15 min using in vivo stimulations. The right (stimulated) and left (unstimulated) gastrocnemius muscleswere collected 60 min after the last stimulation and processed for redox proteomics assay of S-glutathionylation. Using selective reduction with a glutaredoxin enzyme cocktail and resin-assisted enrichment technique, we quantified the levels of site-specific protein S-glutathionylation at rest and following fatiguing contractions. Redox proteomics revealed over 2200 sites of S-glutathionylation modifications, of which 1290 were significantly increased after fatiguing contractions. Muscle contraction leads to the greatest increase in S-glutathionylation in the mitochondria (1.03%) and the smallest increase in the nucleus (0.47%). Regulatory cysteines were significantly S-glutathionylated on mitochondrial complex I and II, GAPDH, MDH1, ACO2, and mitochondrial complex V among others. Similarly, S-glutathionylation of RYR1, SERCA1, titin, and troponin I2 are known to regulate muscle contractility and were significantly S-glutathionylated after just 15 min of fatiguing contractions. The largest fold changes (> 1.6) in the S-glutathionylated proteome after fatigue occurred on signaling proteins such as 14-3-3 protein gamma and MAP2K4, as well as proteins like SERCA1, and NDUV2 of mitochondrial complex I, at previously unknown glutathionylation sites. These findings highlight the important role of redox control over muscle physiology, metabolism, and the exercise adaptive response. This study lays the groundwork for future investigation into the altered exercise adaptation associated with chronic conditions, such as sarcopenia.

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A single bout of fatiguing contractions increase muscle protein S-glutathionylation.

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Mitochondrial proteins are sensitive to oxidative modifications following fatigue.

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The glutathionylated proteome includes cysteines of known functional importance.

Nutritional Ketosis and Mitohormesis: Potential Implications for Mitochondrial Function and Human Health

Impaired mitochondrial function often results in excessive production of reactive oxygen species (ROS) and is involved in the etiology of many chronic diseases, including cardiovascular disease, diabetes, neurodegenerative disorders, and cancer. Moderate levels of mitochondrial ROS, however, can protect against chronic disease by inducing upregulation of mitochondrial capacity and endogenous antioxidant defense. This phenomenon, referred to as mitohormesis, is induced through increased reliance on mitochondrial respiration, which can occur through diet or exercise. Nutritional ketosis is a safe and physiological metabolic state induced through a ketogenic diet low in carbohydrate and moderate in protein. Such a diet increases reliance on mitochondrial respiration and may, therefore, induce mitohormesis. Furthermore, the ketone β-hydroxybutyrate (BHB), which is elevated during nutritional ketosis to levels no greater than those resulting from fasting, acts as a signaling molecule in addition to its traditionally known role as an energy substrate. BHB signaling induces adaptations similar to mitohormesis, thereby expanding the potential benefit of nutritional ketosis beyond carbohydrate restriction. This review describes the evidence supporting enhancement of mitochondrial function and endogenous antioxidant defense in response to nutritional ketosis, as well as the potential mechanisms leading to these adaptations.

Exercise and Mitochondrial Dynamics: Keeping in Shape with ROS and AMPK

Exercise is a robust stimulus for mitochondrial adaptations in skeletal muscle which consequently plays a central role in enhancing metabolic health. Despite this, the precise molecular events that underpin these beneficial effects remain elusive. In this review, we discuss molecular signals generated during exercise leading to altered mitochondrial morphology and dynamics. In particular, we focus on the interdependence between reactive oxygen species (ROS) and redox homeostasis, the sensing of cellular bioenergetic status via 5’ adenosine monophosphate (AMP)-activated protein kinase (AMPK), and the regulation of mitochondrial fission and fusion. Precisely how exercise regulates the network of these responses and their effects on mitochondrial dynamics is not fully understood at present. We highlight the limitations that exist with the techniques currently available, and discuss novel molecular tools to potentially advance the fields of redox biology and mitochondrial bioenergetics. Ultimately, a greater understanding of these processes may lead to novel mitochondria-targeted therapeutic strategies to augment or mimic exercise in order to attenuate or reverse pathophysiology.

Exercise training with concomitant nitric oxide synthase inhibition improved anxiogenic behavior, spatial cognition, and BDNF/P70S6 kinase activation in 20-month-old rats

This study aimed to investigate the effect of exercise and nitric oxide synthase (NOS) inhibition on memory, anxiety, and protein levels of brain-derived neurotrophic factor (BDNF) and P70S6 kinase (P70S6K). Twenty-month-old rats were divided into 6 groups: a control group, 2 groups treated with l-nitro-arginine methyl ester (L-NAME) (25 or 100 mg/kg) for 63 days, 2 groups treated with L-NAME (25 or 100 mg/kg) for 63 days plus 2 months of exercise, and 1 group treated with exercise. Behavioral tests were conducted to determine the anxiolytic and memory-improving role of exercise and NOS inhibition. BDNF, P70S6K, and cleaved caspase-3 protein levels in the hippocampus and prefrontal cortex were evaluated by Western blotting. Exercise and L-NAME (25 mg/kg) or their combination had an anxiolytic effect and improved spatial memory in old rats compared with the control or exercised group, respectively. Exercise and treatment with a low dose of L-NAME (25 mg/kg) each increased BDNF and P70S6K in the hippocampus and prefrontal cortex compared with levels in control rats. In comparison with exercise alone, co-treatment with exercise and a low dose of L-NAME (25 mg/kg) also increased BDNF and P70S6K in the hippocampus. The neuronal level of cleaved caspase-3 was reduced in the L-NAME (25 mg/kg) + exercise group compared with the exercised group. The L-NAME (100 mg/kg) + exercise treatment had no positive behavioral or molecular effects compared with exercise alone. The protective role of NOS inhibition and aerobic exercise against aging is probably modulated via BDNF and P70S6K in the brain.

Moderate Exercise Allows for shorter Recovery Time in Critical Limb Ischemia

Whether and how moderate exercise might allow for accelerated limb recovery in chronic critical limb ischemia (CLI) remains to be determined. Chronic CLI was surgically induced in mice, and the effect of moderate exercise (training five times per week over a 3-week period) was investigated. Tissue damages and functional scores were assessed on the 4th, 6th, 10th, 20th, and 30th day after surgery. Mice were sacrificed 48 h after the last exercise session in order to assess muscle structure, mitochondrial respiration, calcium retention capacity, oxidative stress and transcript levels of genes encoding proteins controlling mitochondrial functions (PGC1α, PGC1β, NRF1) and anti-oxidant defenses markers (SOD1, SOD2, catalase). CLI resulted in tissue damages and impaired functional scores. Mitochondrial respiration and calcium retention capacity were decreased in the ischemic limb of the non-exercised group (V_max = 7.11 ± 1.14 vs. 9.86 ± 0.86 mmol 02/min/g dw, p < 0.001; CRC = 7.01 ± 0.97 vs. 11.96 ± 0.92 microM/mg dw, p < 0.001, respectively). Moderate exercise reduced tissue damages, improved functional scores, and restored mitochondrial respiration and calcium retention capacity in the ischemic limb (V_max = 9.75 ± 1.00 vs. 9.82 ± 0.68 mmol 02/min/g dw; CRC = 11.36 ± 1.33 vs. 12.01 ± 1.24 microM/mg dw, respectively). Exercise also enhanced the transcript levels of PGC1α, PGC1β, NRF1, as well as SOD1, SOD2, and catalase. Moderate exercise restores mitochondrial respiration and calcium retention capacity, and it has beneficial functional effects in chronic CLI, likely by stimulating reactive oxygen species-induced biogenesis and anti-oxidant defenses. These data support further development of exercise therapy even in advanced peripheral arterial disease.

Hormetic Property of Ginseng Steroids on Anti-Oxidant Status against Exercise Challenge in Rat Skeletal Muscle

Background: Existing literature on anti-oxidant capacity of ginseng has been inconsistent due to variance in the profile of ginseng steroids (Ginsenosides) that is because of differences in seasons and species. Methods: We used various doses of ginseng steroids to determine its effect on oxidative stress and anti-oxidant capacity of rat skeletal muscle against exercise. Results: Under non-exercise conditions, we found increased thiobarbituric acid reactive substance (TBARS) levels and decreased reduced/oxidized glutathione ratio (GSH/GSSG) in rat skeletal muscle as dose increases (p < 0.05), which indicates the pro-oxidant property of ginseng steroids at baseline. Intriguingly, exhaustive exercise-induced increased TBARS and decreased GSH/GSSG ratio were attenuated with low and medium doses of ginseng steroids (20 and 40 mg per kg), but not with high dose (120 mg per kg). At rest, anti-oxidant enzyme activities, including catalase (CAT), glutathione reductase (GR) and glutathione S-transferase (GST) were increased above vehicle-treated level, but not with the high dose, suggesting a hormetic dose-response of ginseng steroids. Conclusion: The results of this study provide an explanation for the inconsistent findings on anti-oxidative property among previous ginseng studies. For optimizing the anti-oxidant outcome, ginseng supplementation at high dose should be avoided.

The effect of exercise-intensity on skeletal muscle stress kinase and insulin protein signaling

Background

Stress and mitogen activated protein kinase (SAPK) signaling play an important role in glucose homeostasis and the physiological adaptation to exercise. However, the effects of acute high-intensity interval exercise (HIIE) and sprint interval exercise (SIE) on activation of these signaling pathways are unclear.

Methods

Eight young and recreationally active adults performed a single cycling session of HIIE (5 x 4 minutes at 75% W_max), SIE (4 x 30 second Wingate sprints), and continuous moderate-intensity exercise work-matched to HIIE (CMIE; 30 minutes at 50% of W_max), separated by a minimum of 1 week. Skeletal muscle SAPK and insulin protein signaling were measured immediately, and 3 hours after exercise.

Results

SIE elicited greater skeletal muscle NF-κB p65 phosphorylation immediately after exercise (SIE: ~40%; HIIE: ~4%; CMIE; ~13%; p < 0.05) compared to HIIE and CMIE. AS160^Ser588 phosphorylation decreased immediately after HIIE (~-27%; p < 0.05), and decreased to the greatest extent immediately after SIE (~-60%; p < 0.05). Skeletal muscle JNK (~42%; p < 0.05) and p38 MAPK (~171%; p < 0.05) phosphorylation increased, and skeletal muscle Akt^Ser473 phosphorylation (~-32%; p < 0.05) decreased, to a similar extent immediately after all exercise protocols. AS160^Ser588 phosphorylation was similar to baseline three hours after SIE (~-12%; p > 0.05), remained lower 3 hours after HIIE (~-34%; p < 0.05), and decreased 3 hours after CMIE (~-33%; p < 0.05).

Conclusion

Despite consisting of less total work than CMIE and HIIE, SIE proved to be an effective stimulus for the activation of stress protein kinase signaling pathways linked to exercise-mediated adaptation of skeletal muscle. Furthermore, post-exercise AS160^Ser588 phosphorylation decreased in an exercise-intensity and post-exercise time-course dependent manner.

Exercise and Glycemic Control: Focus on Redox Homeostasis and Redox-Sensitive Protein Signaling

Physical inactivity, excess energy consumption, and obesity are associated with elevated systemic oxidative stress and the sustained activation of redox-sensitive stress-activated protein kinase (SAPK) and mitogen-activated protein kinase signaling pathways. Sustained SAPK activation leads to aberrant insulin signaling, impaired glycemic control, and the development and progression of cardiometabolic disease. Paradoxically, acute exercise transiently increases oxidative stress and SAPK signaling, yet postexercise glycemic control and skeletal muscle function are enhanced. Furthermore, regular exercise leads to the upregulation of antioxidant defense, which likely assists in the mitigation of chronic oxidative stress-associated disease. In this review, we explore the complex spatiotemporal interplay between exercise, oxidative stress, and glycemic control, and highlight exercise-induced reactive oxygen species and redox-sensitive protein signaling as important regulators of glucose homeostasis.

Do antioxidant supplements interfere with skeletal muscle adaptation to exercise training?

A popular belief is that reactive oxygen species (ROS) and reactive nitrogen species (RNS) produced during exercise by the mitochondria and other subcellular compartments ubiquitously cause skeletal muscle damage, fatigue and impair recovery. However, the importance of ROS and RNS as signals in the cellular adaptation process to stress is now evident. In an effort to combat the perceived deleterious effects of ROS and RNS it has become common practice for active individuals to ingest supplements with antioxidant properties, but interfering with ROS/RNS signalling in skeletal muscle during acute exercise may blunt favourable adaptation. There is building evidence that antioxidant supplementation can attenuate endurance training-induced and ROS/RNS-mediated enhancements in antioxidant capacity, mitochondrial biogenesis, cellular defence mechanisms and insulin sensitivity. However, this is not a universal finding, potentially indicating that there is redundancy in the mechanisms controlling skeletal muscle adaptation to exercise, meaning that in some circumstances the negative impact of antioxidants on acute exercise response can be overcome by training. Antioxidant supplementation has been more consistently reported to have deleterious effects on the response to overload stress and high-intensity training, suggesting that remodelling of skeletal muscle following resistance and high-intensity exercise is more dependent on ROS/RNS signalling. Importantly there is no convincing evidence to suggest that antioxidant supplementation enhances exercise-training adaptions. Overall, ROS/RNS are likely to exhibit a non-linear (hormetic) pattern on exercise adaptations, where physiological doses are beneficial and high exposure (which would seldom be achieved during normal exercise training) may be detrimental.

Exercise-induced skeletal muscle signaling pathways and human athletic performance

Skeletal muscle is a highly malleable tissue capable of altering its phenotype in response to external stimuli including exercise. This response is determined by the mode, (endurance- versus resistance-based), volume, intensity and frequency of exercise performed with the magnitude of this response-adaptation the basis for enhanced physical work capacity. However, training-induced adaptations in skeletal muscle are variable and unpredictable between individuals. With the recent application of molecular techniques to exercise biology, there has been a greater understanding of the multiplicity and complexity of cellular networks involved in exercise responses. This review summarizes the molecular and cellular events mediating adaptation processes in skeletal muscle in response to exercise. We discuss established and novel cell signaling proteins mediating key physiological responses associated with enhanced exercise performance and the capacity for reactive oxygen and nitrogen species to modulate training adaptation responses. We also examine the molecular bases underpinning heterogeneous responses to resistance and endurance exercise and the dissociation between molecular 'markers' of training adaptation and subsequent exercise performance.

Muscle redox signalling pathways in exercise. Role of antioxidants

Recent research highlights the importance of redox signalling pathway activation by contraction-induced reactive oxygen species (ROS) and nitric oxide (NO) in normal exercise-related cellular and molecular adaptations in skeletal muscle. In this review, we discuss some potentially important redox signalling pathways in skeletal muscle that are involved in acute and chronic responses to contraction and exercise. Specifically, we discuss redox signalling implicated in skeletal muscle contraction force, mitochondrial biogenesis and antioxidant enzyme induction, glucose uptake and muscle hypertrophy. Furthermore, we review evidence investigating the impact of major exogenous antioxidants on these acute and chronic responses to exercise. Redox signalling pathways involved in adaptive responses in skeletal muscle to exercise are not clearly elucidated at present, and further research is required to better define important signalling pathways involved. Evidence of beneficial or detrimental effects of specific antioxidant compounds on exercise adaptations in muscle is similarly limited, particularly in human subjects. Future research is required to not only investigate effects of specific antioxidant compounds on skeletal muscle exercise adaptations, but also to better establish mechanisms of action of specific antioxidants in vivo. Although we feel it remains somewhat premature to make clear recommendations in relation to application of specific antioxidant compounds in different exercise settings, a bulk of evidence suggests that N-acetylcysteine (NAC) is ergogenic through its effects on maintenance of muscle force production during sustained fatiguing events. Nevertheless, a current lack of evidence from studies using performance tests representative of athletic competition and a potential for adverse effects with high doses (>70mg/kg body mass) warrants caution in its use for performance enhancement. In addition, evidence implicates high dose vitamin C (1g/day) and E (≥260 IU/day) supplementation in impairments to some skeletal muscle cellular adaptations to chronic exercise training. Thus, determining the utility of antioxidant supplementation in athletes likely requires a consideration of training and competition periodization cycles of athletes in addition to type, dose and duration of antioxidant supplementation.

Exercise-induced ROS in heat shock proteins response

Cells have evolved multiple and sophisticated stress response mechanisms aiming to prevent macromolecular (including proteins, lipids, and nucleic acids) damage and to maintain or re-establish cellular homeostasis. Heat shock proteins (HSPs) are among the most highly conserved, ubiquitous, and abundant proteins in all organisms. Originally discovered more than 50 years ago through heat shock stress, they display multiple, remarkable roles inside and outside cells under a variety of stresses, including also oxidative stress and radiation, recognizing unfolded or misfolded proteins and facilitating their restructuring. Exercise consists in a combination of physiological stresses, such as metabolic disturbances, changes in circulating levels of hormones, increased temperature, induction of mild to severe inflammatory state, increased production of reactive oxygen and nitrogen species (ROS and RNS). As a consequence, exercise is one of the main stimuli associated with a robust increase in different HSPs in several tissues, which appears to be also fundamental in facilitating the cellular remodeling processes related to the training regime. Among all factors involved in the exercise-related modulation of HSPs level, the ROS production in the contracting muscle or in other tissues represents one of the most attracting, but still under discussion, mechanism. Following exhaustive or damaging muscle exercise, major oxidative damage to proteins and lipids is likely involved in HSP expression, together with mechanically induced damage to muscle proteins and the inflammatory response occurring several days into the recovery period. Instead, the transient and reversible oxidation of proteins by physiological concentrations of ROS seems to be involved in the activation of stress response following non-damaging muscle exercise. This review aims to provide a critical update on the role of HSPs response in exercise-induced adaptation or damage in humans, focusing on experimental results where the link between redox homeostasis and HSPs expression by exercise has been addressed. Further, with the support of in vivo and in vitro studies, we discuss the putative molecular mechanisms underlying the ROS-mediated modulation of HSP expression and/or activity during exercise.

Mitohormesis in exercise training

Hormesis is a process whereby exposure to a low dose of a potentially harmful stressor promotes adaptive changes to the cell that enables it to better tolerate subsequent stress. In recent years this concept has been applied specifically to the mitochondria (mitohormesis), suggesting that in response to a perturbation the mitochondria can initiate and transduce a signal to the nucleus that coordinates a transcriptional response resulting in both mitochondrial and non-mitochondrial adaptations that return and maintain cellular homeostasis. In this review we summarize the evidence that mitohormesis is a significant adaptive-response signaling pathway, and suggest that it plays a role in mediating exercise-induced adaptations. We discuss potential mitochondrial emitters of retrograde signals that may activate known exercise-sensitive transcription factors to modulate transcription responses to exercise, and draw on evidence from mitochondrial dysfunction animal models to support a role for mitohormesis in mitochondrial biogenesis. Studies directly linking mitohormesis to the exercise training response are lacking, however mounting evidence suggests numerous signals are emitted from the mitochondria during exercise and have the potential to induce a nuclear transcription response, with reactive oxygen species (ROS) being the primary candidate.

NOX2 Inhibition Impairs Early Muscle Gene Expression Induced by a Single Exercise Bout

Reactive oxygen species (ROS) participate as signaling molecules in response to exercise in skeletal muscle. However, the source of ROS and the molecular mechanisms involved in these phenomena are still not completely understood. The aim of this work was to study the role of skeletal muscle NADPH oxidase isoform 2 (NOX2) in the molecular response to physical exercise in skeletal muscle. BALB/c mice, pre-treated with a NOX2 inhibitor, apocynin, (3 mg/kg) or vehicle for 3 days, were swim-exercised for 60 min. Phospho–p47^phox levels were significantly upregulated by exercise in flexor digitorum brevis (FDB). Moreover, exercise significantly increased NOX2 complex assembly (p47^phox–gp91^phox interaction) demonstrated by both proximity ligation assay and co-immunoprecipitation. Exercise-induced NOX2 activation was completely inhibited by apocynin treatment. As expected, exercise increased the mRNA levels of manganese superoxide dismutase (MnSOD), glutathione peroxidase (GPx), citrate synthase (CS), mitochondrial transcription factor A (tfam) and interleukin-6 (IL-I6) in FDB muscles. Moreover, the apocynin treatment was associated to a reduced activation of p38 MAP kinase, ERK 1/2, and NF-κB signaling pathways after a single bout of exercise. Additionally, the increase in plasma IL-6 elicited by exercise was decreased in apocynin-treated mice compared with the exercised vehicle-group (p < 0.001). These results were corroborated using gp91-dstat in an in vitro exercise model. In conclusion, NOX2 inhibition by both apocynin and gp91dstat, alters the intracellular signaling to exercise and electrical stimuli in skeletal muscle, suggesting that NOX2 plays a critical role in molecular response to an acute exercise.

Role of ROS and RNS Sources in Physiological and Pathological Conditions

There is significant evidence that, in living systems, free radicals and other reactive oxygen and nitrogen species play a double role, because they can cause oxidative damage and tissue dysfunction and serve as molecular signals activating stress responses that are beneficial to the organism. Mitochondria have been thought to both play a major role in tissue oxidative damage and dysfunction and provide protection against excessive tissue dysfunction through several mechanisms, including stimulation of opening of permeability transition pores. Until recently, the functional significance of ROS sources different from mitochondria has received lesser attention. However, the most recent data, besides confirming the mitochondrial role in tissue oxidative stress and protection, show interplay between mitochondria and other ROS cellular sources, so that activation of one can lead to activation of other sources. Thus, it is currently accepted that in various conditions all cellular sources of ROS provide significant contribution to processes that oxidatively damage tissues and assure their survival, through mechanisms such as autophagy and apoptosis.

Nuclear factor erythroid-derived 2-like 2 (NFE2L2, Nrf2) mediates exercise-induced mitochondrial biogenesis and the anti-oxidant response in mice

KEY POINTS

Reactive oxygen species (ROS) and nitric oxide (NO) regulate exercise-induced nuclear factor erythroid 2-related factor 2 (NFE2L2) expression in skeletal muscle. NFE2L2 is required for acute exercise-induced increases in skeletal muscle mitochondrial biogenesis genes, such as nuclear respiratory factor 1 (NRF-1) and mitochondrial transcription factor A, and anti-oxidant genes, such as superoxide dismutase (SOD)1, SOD2 and catalase. Following exercise training mice with impaired NFE2L2 expression have reduced exercise performance, energy expenditure, mitochondrial volume and anti-oxidant activity. In muscle cells, ROS and NO can regulate mitochondrial biogenesis via a NFE2L2/NRF-1-dependent pathway.

ABSTRACT

Regular exercise induces adaptations to skeletal muscle, which can include mitochondrial biogenesis and enhanced anti-oxidant reserves. These adaptations and others are at least partly responsible for the improved health of physically active individuals. Reactive oxygen species (ROS) and nitric oxide (NO) are produced during exercise and may mediate the adaptive response to exercise in skeletal muscle. However, the mechanisms through which they act are unclear. In the present study, we aimed to determine the role of the redox-sensitive transcription factor nuclear factor erythroid-derived 2-like 2 (NFE2L2) in acute exercise- and training-induced mitochondrial biogenesis and the anti-oxidant response. We report that ROS and NO regulate acute exercise-induced expression of NFE2L2 in mouse skeletal muscle and muscle cells, and that deficiency in NFE2L2 prevents normal acute treadmill exercise-induced increases in mRNA of the mitochondrial biogenesis markers, nuclear respiratory factor 1 (NRF-1) and mitochondrial transcription factor A (mtTFA), and the anti-oxidants superoxide dismutase (SOD) 1 and 2, as well as catalase, in mouse gastrocnemius muscle. Furthermore, after 5 weeks of treadmill exercise training, mice deficient in NFE2L2 had reduced exercise capacity and whole body energy expenditure, as well as skeletal muscle mitochondrial mass and SOD activity, compared to wild-type littermates. In C2C12 myoblasts, acute treatment with exogenous H2 O2 (ROS)- and diethylenetriamine/NO adduct (NO donor) induced increases in mtTFA, which was prevented by small interfering RNA and short hairpin RNA knockdown of either NFE2L2 or NRF-1. Our results suggest that, during exercise, ROS and NO can act via NFE2L2 to functionally regulate skeletal muscle mitochondrial biogenesis and anti-oxidant defence gene expression.

Vitamin C and E supplementation prevents some of the cellular adaptations to endurance-training in humans

BACKGROUND

It is clear that reactive oxygen species (ROS) produced during skeletal muscle contraction have a regulatory role in skeletal muscle adaptation to endurance exercise. However, there is much controversy in the literature regarding whether attenuation of ROS by antioxidant supplementation can prevent these cellular adaptations. Therefore, the aim of this study was to determine whether vitamin C and E supplementation attenuates performance and cellular adaptations following acute endurance exercise and endurance training.

METHODS

A double-blinded, placebo-controlled randomized control trial was conducted in eleven healthy young males. Participants were matched for peak oxygen consumption (VO 2peak) and randomly allocated to placebo or antioxidant (vitamin C (2 × 500 mg/day) and E (400 IU/day)) groups. Following a four-week supplement loading period, participants completed acute exercise (10 × 4 min cycling at 90% VO 2peak, 2 min active recovery). Vastus lateralis muscle samples were collected pre-, immediately-post- and 3h-post-exercise. Participants then completed four weeks of training (3 days/week) using the aforementioned exercise protocol while continuing supplementation. Following exercise training, participants again completed an acute exercise bout with muscle biopsies.

RESULTS

Acute exercise tended to increase skeletal muscle oxidative stress as measured by oxidized glutathione (GSSG) (P=0.058) and F2-isoprostanes (P=0.056), with no significant effect of supplementation. Acute exercise significantly increased mRNA levels of peroxisome proliferator-activated receptor gamma coactivator 1α (PGC-1α), mitochondrial transcription factor A (TFAM) and PGC related coactivator (PRC), with no effect of supplementation. Following endurance training, supplementation did not prevent significantly increased VO 2peak, skeletal muscle levels of citrate synthase activity or mRNA or protein abundance of cytochrome oxidase subunit 4 (COX IV) (P<0.05). However, following training, vitamin C and E supplementation significantly attenuated increased skeletal muscle superoxide dismutase (SOD) activity and protein abundance of SOD2 and TFAM.

CONCLUSION

Following acute exercise, supplementation with vitamin C and E did not attenuate skeletal muscle oxidative stress or increased gene expression of mitochondrial biogenesis markers. However, supplementation attenuated some (SOD, TFAM) of the increased skeletal muscle adaptations following training in healthy young men.

The Measurement of Reversible Redox Dependent Post-translational Modifications and Their Regulation of Mitochondrial and Skeletal Muscle Function

Mitochondrial oxidative stress is a common feature of skeletal myopathies across multiple conditions; however, the mechanism by which it contributes to skeletal muscle dysfunction remains controversial. Oxidative damage to proteins, lipids, and DNA has received the most attention, yet an important role for reversible redox post-translational modifications (PTMs) in pathophysiology is emerging. The possibility that these PTMs can exert dynamic control of muscle function implicates them as a mechanism contributing to skeletal muscle dysfunction in chronic disease. Herein, we discuss the significance of thiol-based redox dependent modifications to mitochondrial, myofibrillar, and excitation-contraction (EC) coupling proteins with an emphasis on how these changes could alter skeletal muscle performance under chronically stressed conditions. A major barrier to a better mechanistic understanding of the role of reversible redox PTMs in muscle function is the technical challenges associated with accurately measuring the changes of site-specific redox PTMs. Here we will critically review current approaches with an emphasis on sample preparation artifacts, quantitation, and specificity. Despite these challenges, the ability to accurately quantify reversible redox PTMs is critical to understanding the mechanisms by which mitochondrial oxidative stress contributes to skeletal muscle dysfunction in chronic diseases.

Influence of vitamin C and vitamin E on redox signaling: Implications for exercise adaptations

The exogenous antioxidants vitamin C (ascorbate) and vitamin E (α-tocopherol) often blunt favorable cell signaling responses to exercise, suggesting that redox signaling contributes to exercise adaptations. Current theories posit that this antioxidant paradigm interferes with redox signaling by attenuating exercise-induced reactive oxygen species (ROS) and reactive nitrogen species (RNS) generation. The well-documented in vitro antioxidant actions of ascorbate and α-tocopherol and characterization of the type and source of the ROS/RNS produced during exercise theoretically enable identification of redox-dependent mechanisms responsible for the blunting of favorable cell signaling responses to exercise. This review aimed to apply this reasoning to determine how the aforementioned antioxidants might attenuate exercise-induced ROS/RNS production. The principal outcomes of this analysis are (1) neither antioxidant is likely to attenuate nitric oxide signaling either directly (reaction with nitric oxide) or indirectly (reaction with derivatives, e.g., peroxynitrite); (2) neither antioxidant reacts appreciably with hydrogen peroxide, a key effector of redox signaling; (3) ascorbate but not α-tocopherol has the capacity to attenuate exercise-induced superoxide generation; and (4) alternate mechanisms, namely pro-oxidant side reactions and/or reduction of bioactive oxidized macromolecule adducts, are unlikely to interfere with exercise-induced redox signaling. Out of all the possibilities considered, ascorbate-mediated suppression of superoxide generation with attendant implications for hydrogen peroxide signaling is arguably the most cogent explanation for blunting of favorable cell signaling responses to exercise. However, this mechanism is dependent on ascorbate accumulating at sites rich in NADPH oxidases, principal contributors to contraction-mediated superoxide generation, and outcompeting nitric oxide and superoxide dismutase isoforms. The major conclusions of this review are: (1) direct evidence for interference of ascorbate and α-tocopherol with exercise-induced ROS/RNS production is lacking; (2) theoretical analysis reveals that both antioxidants are unlikely to have a major impact on exercise-induced redox signaling; and (3) it is worth considering alternate redox-independent mechanisms.

Modulating exercise-induced hormesis: Does less equal more?

Hormesis encompasses the notion that low levels of stress stimulate or upregulate existing cellular and molecular pathways that improve the capacity of cells and organisms to withstand greater stress. This notion underlies much of what we know about how exercise conditions the body and induces long-term adaptations. During exercise, the body is exposed to various forms of stress, including thermal, metabolic, hypoxic, oxidative, and mechanical stress. These stressors activate biochemical messengers, which in turn activate various signaling pathways that regulate gene expression and adaptive responses. Historically, antioxidant supplements, nonsteroidal anti-inflammatory drugs, and cryotherapy have been favored to attenuate or counteract exercise-induced oxidative stress and inflammation. However, reactive oxygen species and inflammatory mediators are key signaling molecules in muscle, and such strategies may mitigate adaptations to exercise. Conversely, withholding dietary carbohydrate and restricting muscle blood flow during exercise may augment adaptations to exercise. In this review article, we combine, integrate, and apply knowledge about the fundamental mechanisms of exercise adaptation. We also critically evaluate the rationale for using interventions that target these mechanisms under the overarching concept of hormesis. There is currently insufficient evidence to establish whether these treatments exert dose-dependent effects on muscle adaptation. However, there appears to be some dissociation between the biochemical/molecular effects and functional/performance outcomes of some of these treatments. Although several of these treatments influence common kinases, transcription factors, and proteins, it remains to be determined if these interventions complement or negate each other, and whether such effects are strong enough to influence adaptations to exercise.

Acute exercise increases insulin sensitivity in adult sheep: a new preclinical model

In healthy humans and rodents, chronic and acute exercise improves subsequent insulin sensitivity of skeletal muscle. A large animal species with similar metabolic responses to exercise would permit longitudinal studies, including repeated biopsies of muscle and other tissues not possible in rodents, and enable study of interactions with insulin-resistant physiological states not feasible in humans. Therefore, we examined whether acute exercise increases insulin sensitivity in adult sheep. Insulin sensitivity was measured by hyperinsulinemic euglycemic clamp (HEC) in mature female sheep ( n = 7). Sheep were familiarized to treadmill walking and then performed an acute exercise bout (30 min, 8% slope, up to 4.4 km/h). A second HEC was conducted ∼18 h after the acute exercise. Musculus semimembranosus biopsies were obtained before and after each HEC. Glucose infusion rate during the HEC increased 40% ( P = 0.003) and insulin sensitivity (glucose infusion rate/plasma insulin concentration) increased 32% ( P = 0.028) after acute exercise. Activation of proximal insulin signaling in skeletal muscle after the HEC, measured as Ser473 phosphorylation of Akt, increased approximately five-fold in response to insulin ( P < 0.001) and was unaltered by acute exercise performed 18 h earlier. PGC1α and GLUT4 protein, glycogen content and citrate synthase activity in skeletal muscle did not change in response to insulin or exercise. In conclusion, improved insulin sensitivity and unchanged proximal insulin signaling on the day after acute exercise in sheep are consistent with responses in humans and rodents, suggesting that the sheep is an appropriate large-animal model in which to study responses to exercise.

Effects of allopurinol on exercise-induced muscle damage: new therapeutic approaches?

Intensive muscular activity can trigger oxidative stress, and free radicals may hence be generated by working skeletal muscle. The role of the enzyme xanthine oxidase as a generating source of free radicals is well documented and therefore is involved in the skeletal muscle damage as well as in the potential transient cardiovascular damage induced by high-intensity physical exercise. Allopurinol is a purine hypoxanthine-based structural analog and a well-known inhibitor of xanthine oxidase. The administration of the xanthine oxidase inhibitor allopurinol may hence be regarded as promising, safe, and an economic strategy to decrease transient skeletal muscle damage (as well as heart damage, when occurring) in top-level athletes when administered before a competition or a particularly high-intensity training session. Although continuous administration of allopurinol in high-level athletes is not recommended due to its possible role in hampering training-induced adaptations, the drug might be useful in non-athletes. Exertional rhabdomyolysis is the most common form of rhabdomyolysis and affects individuals participating in a type of intense exercise to which they are not accustomed. This condition can cause exercise-related myoglobinuria, thus increasing the risk of acute renal failure and is also associated with sickle cell trait. In this manuscript, we have reviewed the recent evidence about the effects of allopurinol on exercise-induced muscle damage. More research is needed to determine whether allopurinol may be useful for preventing not only exertional rhabdomyolysis and acute renal damage but also skeletal muscle wasting in critical illness as well as in immobilized, bedridden, sarcopenic or cachectic patients.

Altering the redox state of skeletal muscle by glutathione depletion increases the exercise-activation of PGC-1α

We investigated the relationship between markers of mitochondrial biogenesis, cell signaling, and antioxidant enzymes by depleting skeletal muscle glutathione with diethyl maleate (DEM) which resulted in a demonstrable increase in oxidative stress during exercise. Animals were divided into six groups: (1) sedentary control rats; (2) sedentary rats + DEM; (3) exercise control rats euthanized immediately after exercise; (4) exercise rats + DEM; (5) exercise control rats euthanized 4 h after exercise; and (6) exercise rats + DEM euthanized 4 h after exercise. Exercising animals ran on the treadmill at a 10% gradient at 20 m/min for the first 30 min. The speed was then increased every 10 min by 1.6 m/min until exhaustion. There was a reduction in total glutathione in the skeletal muscle of DEM treated animals compared to the control animals (P < 0.05). Within the control group, total glutathione was higher in the sedentary group compared to after exercise (P < 0.05). DEM treatment also significantly increased oxidative stress, as measured by increased plasma F₂–isoprostanes (P < 0.05). Exercising animals given DEM showed a significantly greater increase in peroxisome proliferator activated receptor γ coactivator‐1α (PGC–1α) mRNA compared to the control animals that were exercised (P < 0.05). This study provides novel evidence that by lowering the endogenous antioxidant glutathione in skeletal muscle and inducing oxidative stress through exercise, PGC‐1α gene expression was augmented. These findings further highlight the important role of exercise induced oxidative stress in the regulation of mitochondrial biogenesis.

A number of studies have attempted to elucidate mechanisms for the role of exercise‐induced reactive oxygen species in cell signalling and mitochondrial biogenesis including inhibiting reactive oxygen species production, either by enzymatic inhibitors such as the treatment of allopurinol, or through antioxidant supplementation. Our study is the first to investigate the relationship among mitochondrial biogenesis, cell signalling, and antioxidant enzymes by depleting skeletal muscle glutathione with diethyl maleate (DEM) which resulted in a demonstrable increase in oxidative stress during exercise. The major outcome of our study was that by reducing endogenous antioxidant glutathione content, there was impaired capacity for skeletal muscle to neutralize oxidative stress during exercise, resulting in greater PGC‐1α gene expression.

High-dose vitamin C supplementation increases skeletal muscle vitamin C concentration and SVCT2 transporter expression but does not alter redox status in healthy males

Antioxidant vitamin C (VC) supplementation is of potential clinical benefit to individuals with skeletal muscle oxidative stress. However, there is a paucity of data reporting on the bioavailability of high-dose oral VC in human skeletal muscle. We aimed to establish the time course of accumulation of VC in skeletal muscle and plasma during high-dose VC supplementation in healthy individuals. Concurrently we investigated the effects of VC supplementation on expression levels of the key skeletal muscle VC transporter sodium-dependent vitamin C transporter 2 (SVCT2) and intramuscular redox and mitochondrial measures. Eight healthy males completed a randomized placebo-controlled, crossover trial involving supplementation with ascorbic acid (2×500 mg/day) over 42 days. Participants underwent muscle and blood sampling on days 0, 1, 7, and 42 during each treatment. VC supplementation significantly increased skeletal muscle VC concentration after 7 days, which was maintained at 42 days (VC 3.0±0.2 (mean±SEM) to 3.9±0.4 mg/100 g wet weight (ww) versus placebo 3.1±0.3 to 2.9±0.2 mg/100 g ww, p=0.001). Plasma VC increased after 1 day, which was maintained at 42 days (VC 61.0±6.1 to 111.5±10.4 µmol/L versus placebo 60.7±5.3 to 59.2±4.8 µmol/L, p<0.001). VC supplementation significantly increased skeletal muscle SVCT2 protein expression (main treatment effect p=0.006) but did not alter skeletal muscle redox measures or citrate synthase activity. A main finding of our study was that 7 days of high-dose VC supplementation was required to significantly increase skeletal muscle vitamin C concentration in healthy males. Our findings implicate regular high-dose vitamin C supplementation as a means to safely increase skeletal muscle vitamin C concentration without impairing intramuscular ascorbic acid transport, antioxidant concentrations, or citrate synthase activity.

The "Goldilocks Zone" from a redox perspective-Adaptive vs. deleterious responses to oxidative stress in striated muscle

Consequences of oxidative stress may be beneficial or detrimental in physiological systems. An organ system's position on the “hormetic curve” is governed by the source and temporality of reactive oxygen species (ROS) production, proximity of ROS to moieties most susceptible to damage, and the capacity of the endogenous cellular ROS scavenging mechanisms. Most importantly, the resilience of the tissue (the capacity to recover from damage) is a decisive factor, and this is reflected in the disparate response to ROS in cardiac and skeletal muscle. In myocytes, a high oxidative capacity invariably results in a significant ROS burden which in homeostasis, is rapidly neutralized by the robust antioxidant network. The up-regulation of key pathways in the antioxidant network is a central component of the hormetic response to ROS. Despite such adaptations, persistent oxidative stress over an extended time-frame (e.g., months to years) inevitably leads to cumulative damages, maladaptation and ultimately the pathogenesis of chronic diseases. Indeed, persistent oxidative stress in heart and skeletal muscle has been repeatedly demonstrated to have causal roles in the etiology of heart disease and insulin resistance, respectively. Deciphering the mechanisms that underlie the divergence between adaptive and maladaptive responses to oxidative stress remains an active area of research for basic scientists and clinicians alike, as this would undoubtedly lead to novel therapeutic approaches. Here, we provide an overview of major types of ROS in striated muscle and the divergent adaptations that occur in response to them. Emphasis is placed on highlighting newly uncovered areas of research on this topic, with particular focus on the mitochondria, and the diverging roles that ROS play in muscle health (e.g., exercise or preconditioning) and disease (e.g., cardiomyopathy, ischemia, metabolic syndrome).

[Allopurinol and its role in the treatment of sarcopenia]

Xanthine oxidase (XO) is an enzyme that catalyzes the oxidation of hypoxanthine to xanthine and uric acid and plays an important role in purine catabolism. The purine analogue, allopurinol, is a well-known inhibitor of XO widely used in the clinical management of gout and conditions associated with hyperuricemia. More recent data indicate that allopurinol reduces oxidative stress and improves vascular function in several cardiometabolic diseases, prolongs exercise time in angina, and improves the efficiency of cardiac contractility in heart failure. XO also plays an important role in free radical generation during skeletal muscle contraction and thus, it has been related to the muscle damage associated to exhaustive exercise. Several research groups have shown the protective effect of allopurinol in the prevention of this type of damage. Based on this background, a critical overview is presented on the possible role of allopurinol in the treatment of sarcopenia, a geriatric syndrome characterized by progressive and generalized loss of skeletal muscle mass and strength with a risk of adverse outcomes, such as physical disability, poor quality of life and death.

The effect of green tea extract supplementation on exercise-induced oxidative stress parameters in male sprinters

Background

Although research suggests that antioxidant supplementation can protect against exercise-induced muscle damage and oxidative stress, also delayed post-exercise muscle recovery and hindered adaptation to training were reported in the supplemented athletes.

Purpose

The purpose of the study was to evaluate the effects of green tea extract (GTE) supplementation on selected blood markers of oxidative stress and muscle damage in sprinters during preparatory phase of their training cycle.

Methods

Sixteen sprinters participated in a double-blind, randomized, placebo (PL)-controlled crossover study, including two 4-week treatment periods with PL and GTE (980 mg polyphenols daily). The sprinters performed two repeated cycle sprint tests (RST; 4 × 15 s, with 1-min rest intervals), after PL and GTE supplementation. Blood was sampled before (at rest), 5 min after RST, and after the 24-h recovery. The activities of superoxide dismutase (SOD) and glutathione peroxidase were measured in erythrocytes, and total polyphenols, total antioxidant capacity (TAC), uric acid (UA), albumin (AL), malondialdehyde (MDA), and creatine kinase (CK) were determined in blood plasma.

Results

Repeated cycle sprint test performed after PL induced an increase in MDA, TAC, and SOD. Moreover, an increase in UA, AL, and CK was observed after RST irrespective of experimental conditions (PL, GTE). Supplementation with GTE caused an increase in total polyphenols and TAC at rest, and a decrease in MDA and SOD after RST. No significant changes in sprint performance were noted after GTE, as compared to PL.

Conclusions

Supplementation with GTE prevents oxidative stress induced by RST in sprinters. Furthermore, GTE supplementation does not seem to hinder training adaptation in antioxidant enzyme system. On the other hand, neither prevention of exercise-induced muscle damage, nor an improvement in sprint performance is noted after GTE administration.

Exercise-induced skeletal muscle remodeling and metabolic adaptation: redox signaling and role of autophagy

SIGNIFICANCE

Skeletal muscle is a highly plastic tissue. Exercise evokes signaling pathways that strongly modify myofiber metabolism and physiological and contractile properties of skeletal muscle. Regular physical activity is beneficial for health and is highly recommended for the prevention of several chronic conditions. In this review, we have focused our attention on the pathways that are known to mediate physical training-induced plasticity.

RECENT ADVANCES

An important role for redox signaling has recently been proposed in exercise-mediated muscle remodeling and peroxisome proliferator-activated receptor γ (PPARγ) coactivator-1α (PGC-1α) activation. Still more currently, autophagy has also been found to be involved in metabolic adaptation to exercise.

CRITICAL ISSUES

Both redox signaling and autophagy are processes with ambivalent effects; they can be detrimental and beneficial, depending on their delicate balance. As such, understanding their role in the chain of events induced by exercise and leading to skeletal muscle remodeling is a very complicated matter. Moreover, the study of the signaling induced by exercise is made even more difficult by the fact that exercise can be performed with several different modalities, with this having different repercussions on adaptation.

FUTURE DIRECTIONS

Unraveling the complexity of the molecular signaling triggered by exercise on skeletal muscle is crucial in order to define the therapeutic potentiality of physical training and to identify new pharmacological compounds that are able to reproduce some beneficial effects of exercise. In evaluating the effect of new "exercise mimetics," it will also be necessary to take into account the involvement of reactive oxygen species, reactive nitrogen species, and autophagy and their controversial effects.