Exercise Modulates Oxidative Stress and Inflammation in Aging and Cardiovascular Diseases

Despite the wealth of epidemiological and experimental studies indicating the protective role of regular physical activity/exercise training against the sequels of aging and cardiovascular diseases, the molecular transducers of exercise/physical activity benefits are not fully identified but should be further investigated in more integrative and innovative approaches, as they bear the potential for transformative discoveries of novel therapeutic targets. As aging and cardiovascular diseases are associated with a chronic state of oxidative stress and inflammation mediated via complex and interconnected pathways, we will focus in this review on the antioxidant and anti-inflammatory actions of exercise, mainly exerted on adipose tissue, skeletal muscles, immune system, and cardiovascular system by modulating anti-inflammatory/proinflammatory cytokines profile, redox-sensitive transcription factors such as nuclear factor kappa B, activator protein-1, and peroxisome proliferator-activated receptor gamma coactivator 1-alpha, antioxidant and prooxidant enzymes, and repair proteins such as heat shock proteins, proteasome complex, oxoguanine DNA glycosylase, uracil DNA glycosylase, and telomerase. It is important to note that the effects of exercise vary depending on the type, intensity, frequency, and duration of exercise as well as on the individual's characteristics; therefore, the development of personalized exercise programs is essential.


Oxidative Stress and Inflammation in Aging and Cardiovascular Diseases
Aging is associated with oxidative stress that is mainly attributed to defective mitochondria, resulting from reduction in cytochrome C oxidase (complex IV) activity [23,122,123] and peroxidative damage of mitochondrial membrane [124]. Hence, a greater number of electrons are generated that can escape from the mitochondria to create a long trail of reactive oxygen species (ROS) [125,126], leading to further mitochondrial dysfunction and ROS generation and creating a vicious cycle of oxidative damage [127]. Age-associated increases in ROS production occur in skeletal muscles [128] and other organs such as the heart, liver, brain, and kidney [23,126,129,130]. Reduced protein synthesis limits antioxidant defense mechanisms and repair capacity in aged individuals, which further contributes to the state of oxidative stress. The free radical theory of aging hypothesizes that oxidative stress damages macromolecules, including lipids, proteins, and nucleic acids, overwhelming cellular antioxidant defense and repair mechanisms, leading to progressive deleterious changes over time [131,132]. Indeed, oxidatively damaged proteins [133], nucleic acids [134,135], and lipids [113,[136][137][138] are abundant in various organs and tissues such as kidney, liver, heart, arteries, skeletal muscles, and plasma in aged subjects.
Aging is also accompanied with a state of chronic inflammation that is mainly attributed to sarcopenia and adiposity. Sarcopenia, defined as age-associated progressive loss of muscle mass and strength [139,140], increases the incidence of muscle injury [18], which increases the infiltration of immune cells into injured muscles. Activated immune cells and injured muscles release proinflammatory mediators and reactive oxygen and nitrogen species (RONS) via lipoxygenase, NADPH oxidase, xanthine oxidase, and inducible nitric oxide synthase (iNOS) [141][142][143][144][145][146][147] leading to oxidative stress.
Sarcopenia can also lead to reduced physical activity and increased adiposity. Adiposity induces a state of low-grade but chronic inflammation through the release of a multitude of proinflammatory cytokines including tumor necrosis factor-alpha (TNF-), interleukin-6 (IL-6), and interleukin-1 beta (IL-1 ) [148][149][150]. Indeed, aging is associated with increased levels of TNF-, IL-6, and interleukin-1 receptor agonist (IL-1ra) and systemic inflammatory biomarkers such as C-reactive protein (CRP) as well as higher count of inflammatory cells such as neutrophil and monocytes [151][152][153]. Hence, aging is associated with oxidative stress and inflammation.
There are also other proteins such as thioredoxininteracting protein (TXNIP) linking oxidative stress and inflammation. Under resting conditions, TXNIP is bound to thioredoxin (TRX) via a disulphide bound, keeping it in an inactive form. Increased levels of ROS generation cause the dissociation of TXNIP from TRX, leaving it free to scavenge ROS and allowing TXNIP to stimulate the inflammatory cytokine IL-1 [174,175]. In agreement with this is the observation that antioxidant supplementation blocked the antiinflammatory effect of exercise by reducing IL-6 production [176,177].
transcription factors such as NF-B and AP-1 resulting in the generation of large quantities of these proinflammatory mediators and ROS ( Figure 1). Indeed, aging is associated with adverse health conditions such as atherosclerosis, metabolic syndrome, sarcopenia, arthritis, and chronic obstructive pulmonary disease that are characterized by elevated levels of both oxidative stress and inflammatory markers [178].
Not surprisingly, ROS can also induce proteins such as heat shock proteins (HSPs), HSP70 in particular [179], and heme oxygenase 1 (HO-1) [180] that can protect cells and tissues from the deleterious effects of inflammation. However, the balance of antioxidant/anti-inflammatory to oxidant/inflammatory proteins is tilted towards the latter during the aging process.

Exercise Training: Modulation of Oxidative Stress and Inflammation
Exercise and regular physical activity counteract the deleterious effects of aging, not only by combating sarcopenia, obesity, and mitochondrial dysfunction, the major triggers of oxidative stress and inflammation in aging, but also by exerting additional antioxidant and anti-inflammatory actions as illustrated in Figure 2.

Effect of Exercise Training on Adiposity.
Adipose tissue, particularly visceral fat depots, and the macrophages trapped within fat depots are able to release proinflammatory cytokines such as IL-6 and TNF- [148-150, 181, 182]. Physical activity and exercise training increase energy expenditure and reduce body fat, particularly visceral fat, with/without weight loss [35,183,184], and therefore reduced production and release of IL-6 and TNF- [185][186][187][188][189]. Exercise training increased gene expression of PGC-1 alpha, a master regulator of mitochondrial biogenesis, in rat adipose tissue [190], leading to increased energy expenditure particularly in the visceral area. Exercise training inhibited the infiltration of the inflammatory phenotype M1 macrophages into adipose tissue, while also favoring the switch of macrophages to the less inflammatory phenotype M2 in obese mice [191]. Exercise training/physical activity also induces the release of adiponectin from adipose tissues [192][193][194][195][196][197]; adiponectin exerts antiapoptotic, anti-inflammatory, and antioxidative activities [198,199].

Effect of Exercise Training on Skeletal Muscles.
Physical activity/exercise increases nutritive blood supply to and removes waste from skeletal muscles, while also upregulating the expression of the anabolic myokine IL-15 [195-197, 200, 201]. Most importantly, physical activity/exercise stimulates mitochondrial biogenesis [202] and oxidative capacity [203] that provide energy for the synthesis of new proteins. Thus, physical activity/exercise improves muscle mass and strength and renders them less vulnerable to acute injury [204], therefore suppressing triggers of inflammation and oxidative damage [205][206][207][208][209].
Physical activity/exercise also induces the release of several myokines from skeletal muscle such as IL-6 [210][211][212], which suppresses IL-1 and TNF- [213] and triggers the release of many anti-inflammatory cytokines such as IL-1 receptor antagonist (IL-1ra) and IL-10, in addition to cortisol [214,215]. In turn, IL-10 inhibits the synthesis of some proinflammatory cytokines such as TNF-and IL-1 [200]. Exercise also reduces TNF-and IL-1 production in skeletal muscles [212,216,217].

Effect of Exercise Training on Mitochondrial Aging.
Exercise mitigates mitochondrial aging and interrupts the vicious cycle of oxidative damage by stimulating mitochondrial biogenesis [202] and enhancing mitochondrial oxidative capacity [203,223,224]. Excellent reviews on this topic are available [225,226].
It is worth noting that some interventional and randomized controlled trials studies did not detect a significant effect of regular exercise on systemic inflammatory biomarkers in adults [243,[272][273][274] or in aged adults [275][276][277][278] as shown in Table 1. A meta-analysis found only five randomized controlled trials that examined the effects of regular aerobic exercise (of at least 4-week duration) in adults and concluded that aerobic exercise did not reduce CRP levels [279]. It is likely that these discrepancies may be attributed to the smaller sample size used in the clinical trials examined.
On the other hand, the effects of resistance exercise on inflammatory mediators are mostly negative [280][281][282], although Brooks et al. [196] reported that 16 weeks of resistance training reduced CRP and increased adiponectin levels in older diabetic patients. The effects of physical activity and different exercise programs on inflammatory mediators in the elderly are detailed in Table 1. Clearly, the effects of exercise depend on the type (aerobic/resistance), intensity (mild/moderate/intense/exhaustive), and frequency (sessions per day/week/month) of exercise and also on the subject's characteristic (age, sex, endurance capacity, and health condition).

Molecular Transducer of the Anti-Inflammatory Effects of Exercise
Training. The signaling pathways underlying the anti-inflammatory effects of exercise are complex and not completely understood. In addition to the effects of exercise on adipose tissue, skeletal muscles, and mitochondrial biogenesis mentioned above, exercise exerts additional antiinflammatory actions on the immune system, repair mechanisms, and vasculature.
Effects of Exercise on the Immune System. Regular exercise downregulates the innate immune response and activates the adaptive immune system with consequent suppression of inflammation. Exercise modulates the immune system by reducing the number of inflammatory CD14+CD16+ monocytes [250], increasing the number of CD4CD25 regulatory T cells [264,283], shifting blood macrophages towards the less inflammatory phenotype M2 [284], increasing the dominance of the anti-inflammatory Type 2 helper T cell over proinflammatory Type 1 helper T cell [265,[284][285][286], and reducing monocyte chemoattractant protein-1 (MCP-1) [188] and toll-like receptor-4 (TLR4) expression on monocyte surfaces [239,263,275]. On the other hand, ET increases the production of transforming growth factor beta (TGF 1) [254,264] from regulatory T cells.
Exercise also stimulates the sympathetic nervous system and the hypothalamic-pituitary-adrenal axis to increase serum glucocorticoid levels [287] with subsequent inhibition of the immune system [288].
Effects of Exercise on Vascular Endothelial Cells. By increasing shear stress on vascular endothelial cells, exercise modulates key players in the inflammatory process such as ICAM-1, NF-B, MAPK, and COX-2 [300,301]. Voluntary wheel running of aged mice for 10-14 weeks reduced the activation of NF-B in the aorta [302]. Subjecting aortic endothelial cells to in vitro shear stress for 4 h reduced the expression of vascular cell adhesion molecule-1 (VCAM-1) [303]. Treadmill training for 1-3 weeks reduced the expression of intercellular Direct association between adiponectin and physical activity [195] 6 Oxidative Medicine and Cellular Longevity Oxidative Medicine and Cellular Longevity 7 Increased level after training [196] 8 Oxidative Medicine and Cellular Longevity   cell adhesion molecule-1 (ICAM-1) in response to cerebral ischemia in rats [304]. The mechanisms underlying the antiinflammatory actions of exercise are summarized in Figure 3.
In elderly people, regular exercise reduced serum/plasma levels of myeloperoxidase, a marker of inflammation and oxidative stress [205], and thiobarbituric-reactive acid substances, a marker of lipid peroxidation [317]. Lower levels of nitrotyrosine [133] and thiobarbituric-reactive acid substances [318] were found in the more physically active elderly people. However, de Gonzalo-Calvo et al. [319] reported that although regular exercise increased protein carbonyl content and lipid peroxidation levels in the plasma and erythrocytes of long term trained elderly men, their overall health condition was markedly improved. Another clinical study showed that 8 weeks of walking exercise did not significantly change low density lipoprotein (LDL) oxidation or nitration in the elderly [320].

Molecular Transducer for the Antioxidant Effects of Exercise Training.
As discussed above, exercise exerts prominent anti-inflammatory actions, thus suppresses major sources of ROS and RNS generation, and produces indirect antioxidant effects. Exercise also upregulates the antioxidant defense mechanisms and repair proteins in the body via redox-sensitive transcription factors, mainly NF-B, activator protein-1 (AP-1) and peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1 ), and by increasing laminar shear stress on vascular endothelial cells.
The metabolic demands of skeletal muscles increase during exercise; the body responds by increasing oxygen uptake and blood flow to the muscles and other body organs. The increased metabolic rate results in greater ROS production in skeletal muscles [128,321] and in other organs as well [129,322]. Sources other than the electron transport chain enzymes in the mitochondria, such as xanthine oxidase [323][324][325] and NADPH oxidase [128,326], contribute to ROS generation during exercise. This transient increase in ROS levels activates NF-B, AP-1, and PGC-1 signaling.
Effects of Exercise on PGC-1 Signaling. Exercise stimulates mitochondrial biogenesis [202] and ameliorates the ageassociated decline in mitochondrial oxidative capacity in skeletal muscles [223] and other organs [23,190,340] via PGC-1 signaling [190,341,342]. PGC-1 is a redox-sensitive transcription factor that is activated by 5 -AMP-activated protein kinase (AMPK) [329,[343][344][345] to trigger the transcription of nuclear respiratory factor 1 (NRF-1) and expression of mitochondrial transcription factor A (mtTFA), a key regulator of mitochondrial DNA replication [346]. PGC-1 also increases the expression of antioxidant proteins such as glutathione peroxidase (GPX) and SOD-2 [347]. Safdar et al. [348] report that exercise reversed most of the multisystem pathology and premature mortality in mice which were genetically modified to accumulate mitochondrial mutations. The effects of exercise on AMPK and PGC-1 were preserved in the hippocampus of aging rats. However, results from Derbré et al. [341] suggest a blunted effect of exercise response in PGC-1 and NRF-1 in skeletal muscles of aged rats.

Effects of Exercise on Vascular Endothelial Cells.
To meet the increasing metabolic demands of the body during exercise, perfusion of skeletal muscles and other tissues increases, subjecting vascular endothelial cells to higher levels of laminar shear stress. Increased laminar shear stress modulates gene expression and activity of SOD [349][350][351] possibly via NF-B and MAPK signaling [300,301].

Effects of Exercise on Antioxidant and Prooxidant Enzymes
Expression and Activity. The NF-B, AP-1, PGC-1 , and shear stress signaling cascades converge to upregulate antioxidant defense mechanisms to counteract and interrupt the vicious cycle of inflammation and oxidative stress associated with aging and cardiovascular diseases. The most intensely studied antioxidant enzymes in laboratory animals and in humans are SOD, catalase, GPx, glutathione transferase (GST), and glutathione reductase (GSR). The effects of exercise on antioxidant enzymes are summarized in Table 3.
(1) SOD-1. Just increasing in vitro shear stress is sufficient to upregulate the gene and protein expression of SOD-1 in human endothelial cells [349][350][351]. A study by Ennezat et al. [361] showed that regular exercise for 12 weeks increased gene expression of SOD-1 in skeletal muscles of congestive heart failure patients. Increases in protein expression of SOD-1 were observed after exercise in skeletal muscles [222,362], heart [295,363], brain [307], and arteries [194,364,365] of several experimental animals, importantly of aged animals [295]. However, other studies reported decreased or no change in expression of SOD-1 following long term exercise in aged animals [222,366] and adult animals [367].
(3) SOD-3. Higher activity was found in more physically active older men [133] and increased protein expression was observed in men after a bout of acute exercise but not endurance training [373]. However, increased protein levels were detected in experimental animals after endurance exercise [366,367].
(5) GPx. Athletes had higher activity than untrained subjects [352]. Also, physically active elders had higher activity than less active individuals in their erythrocytes [381]. Long term endurance exercise increased GPx activity in healthy adults [354,355] and upregulated gene expression in congestive heart failure patients [361]. In experimental animals, increased activity was observed in heart [136,356,382,383], liver [126,136,356,372,377,382], lung [356,377], kidney [136], brain [377,378], testes [377], and skeletal muscles [311,313,371,[382][383][384], particularly in aged animals [126,313,356,377,378,384]. Other investigators reported reduction [380] and no change [292,293,298] in GPx activity following endurance exercise. Increased GPx gene and protein expression following long term endurance exercise were also reported [307,372].  Protein expression increased in young rats but decreased in old rats [222] Males, young (3-5 months) and aged (24-27 months) Skeletal muscles and heart Exhausting treadmill running Increased activity in skeletal muscles and heart of young rats and hearts of old rats [358] Males, aged 24 months Protein and activity increased [365] 14 Oxidative Medicine and Cellular Longevity  Male, normotensive and hypertensive (11-12 weeks) Liver, kidney, skeletal muscles, and heart Treadmill running for 10 weeks Reduced activity in all tissues in hypertensive and normotensive rats [380] Mice Males and females, aged 28, 52, and 78 weeks Brain, heart, liver, and kidney Long term moderate-intensity treadmill exercise Increased activity in all issues at 52but not 78-week-old mice [23] Females, 3 months Liver, heart, skeletal muscle, and salivary gland Treadmill for a total of 8 weeks Increased activity [136] Pigs Females Aortic endothelial cells Exercise training for 16-19 weeks No change in protein level [365] 16 Oxidative Medicine and Cellular Longevity  Reduced expression and activity [360] Males, diabetic and young Aorta Treadmill exercise for 10 weeks Decreased protein expression of gp91(phox) [194] 18  (6) GSR. Increased activity was reported in humans [355] as well as in rats' brain, liver, lung, muscles, heart, muscle, and testes [311,377,382,383]. Other studies reported that exercise training produced no change in GPx activity [292,298].
(7) GST. Increased activity was reported in liver and salivary glands of mice after 8 weeks of treadmill training [136].
(8) Thioredoxin Reductase 1. Exercise increased thioredoxin reductase 1 (TrxR1), one of the thioredoxin system enzymes with direct and indirect antioxidant effects, in peripheral blood mononuclear cells in humans [205,385].
(9) NAD(P)H Oxidase. Reduced gene [386], protein expression [194,360,365,386,387], and activity [359,360] have been detected in humans [386] and experimental animals following endurance exercise. Also, the physically active elderly had lower NAD(P)H oxidase activity in their vascular endothelial cells compared with less active subjects [133]. Exercise-induced adaptation of antioxidant and prooxidant enzymes is highly isoform [296,307,370,372,373], tissue [311,362,371,377,378,383], age [23,138,222,297,311,366,384], time course [23,230,292,362], and exercise mode specific [352,373,388]. Exercise modulates the three SOD isoforms differently [296,362,365,372,373,389] as the promoter region of SOD-2 contains more ROS-sensitive binding sites [390]. Exercise-induced protein expression of SOD is time dependent; SOD-1 protein expression was increased in rat skeletal muscles 48 hours after exercise, whereas SOD-2 protein content was increased after 10 and 24 hours but not 48 hours [362].
Effects of Exercise on Repair Mechanisms. Exercise can also stimulate the proteasome complex, which is responsible for the degradation of oxidatively damaged proteins [308,[391][392][393], and therefore enhances the cellular repair processes. Exercise modulates the activity of DNA repair enzymes, particularly oxoguanine DNA glycosylase (OGG1) and uracil DNA glycosylase (UDG), and thus reduces the accumulation of nuclear 8-hydroxydeoxyguanosine (8-OHdG) and mutations in skeletal muscles [314,394,395] but not brains of aged rats [396].
Effects of Exercise on Telomeres. Telomeres are often regarded as "the guardians of the genome." Telomere dysfunction activates p53, leading to suppression of PGC-1 and PGC-1 promoters with consequent metabolic and organ failure [397]. Ten cross-sectional and longitudinal studies described a positive association of physical activity with telomere length in immune cells and skeletal muscles [398]. The leukocyte telomere was 200 nucleotides longer in people who exercise regularly, which roughly corresponds to a ten-year increase in longevity [399]. Exercise increases the activity of telomerase and induces the expression of telomere repeat-binding factor 2 and Ku70 in thoracic aorta and leukocytes from mice and humans [400]. However, other studies showed no association or inverted U relationship of physical activity with telomere length [398] warranting further investigation. The signaling pathways underlying the antioxidant actions of exercise are summarized in Figure 4.
Exercise training confers a myriad of physiological benefits in aging and cardiovascular diseases through its antioxidant and anti-inflammatory actions. The inflammatory actions of exercise are mainly exerted on adipose tissue (by reducing its mass and inflammatory environment), on the immune system (by shifting immune cells towards the less inflammatory phenotype, modulating the cytokines profile, and stimulating glucocorticoids), on skeletal muscles (by stimulating mitochondrial biogenesis, upregulating the anabolic myokine IL-15, anti-inflammatory cytokines, and repair proteins, improving muscle mass and strength, and reducing proinflammatory cytokines), and on the vasculature (by increasing laminar shear stress). It is likely that regular exercise exerts the most substantial anti-inflammatory effects in patients having high baseline inflammatory biomarkers, particularly when associated with visceral fat loss.
Exercise exerts antioxidant effects by suppressing inflammatory pathways and therefore inhibiting prominent sources of RONS generation. Importantly, exercise also activates redox-sensitive transcription factors, mainly NF-B and AP-1 and PGC-1 , leading to the enhancement of the antioxidant defense mechanisms by enhancing the expression and activities of SOD, catalase, GPx, GSR, GST, and TrxR1, while downregulating NADPH oxidase. Exercise also upregulates repair proteins such as HSPs, proteasome complex, OGG1, UDG, and telomerase. It is clear that the effects of exercise vary depending on the type, intensity, frequency, and duration of exercise, and also on the individual's age, sex, fitness level, health status, and endurance capacity. More integrative and innovative research approaches such as proteomics and metabolomics should be utilized to reveal the whole map of the molecular transducers of exercise benefits and risks not only at tissue/organ level but also at the whole organism level. This will allow the development of personalized exercise program and hold the promise for transformative discoveries of novel therapeutic targets.