Free Radical Biology and Medicine ∎ (∎∎∎∎) ∎∎∎–∎∎∎

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Exercise training as a drug to treat age associated frailty

Jose Viña a,b,c,d, Andrea Salvador-Pascual a,b,c,d, Francisco Jose Tarazona-Santabalbina b,c,Leocadio Rodriguez-Mañas d, Mari Carmen Gomez-Cabrera a,b,c,d,n

a Department of Physiology, University of Valencia, Investigación Hospital Clínico Universitario/INCLIVA, Spainb Hospital Universitario de la Ribera, Alzira, Valencia, Spainc School of Nursing, Catholic University of Valencia San Vicente Mártir, Spaind Servicio de Geriatría, Hospital Universitario de Getafe, Red Temática de Investigación Cooperativa en Envejecimiento y Fragilidad (RETICEF), Instituto deSalud Carlos III, Spain

a r t i c l e i n f o

Article history:Received 5 January 2016Received in revised form16 March 2016Accepted 24 March 2016

Keywords:Multicomponent exercisemTORPGC-1αOxidative stressAntioxidantReactive oxygen speciesROS signallingAging

x.doi.org/10.1016/j.freeradbiomed.2016.03.02449/& 2016 Published by Elsevier Inc.

espondence to: Department of Physiology, Fa15, Valencia 46010, Spain.ail address: carmen.gomez@uv.es (M.C. Gome

e cite this article as: J. Viña, et al., Exrg/10.1016/j.freeradbiomed.2016.03.0

a b s t r a c t

Exercise causes an increase in the production of free radicals [1]. As a result of a hormetic mechanismantioxidant enzymes are synthesised and the cells are protected against further oxidative stress. Thus,exercise can be considered as an antioxidant [2]. Age-associated frailty is a major medical and socialconcern as it can easily lead to dependency.

In this review we describe that oxidative stress is associated with frailty and the mechanism by whichexercise prevents age-associated frailty. We propose that individually tailored multicomponent exerciseprogrammes are one of the best ways to prevent and to treat age-associated frailty.

& 2016 Published by Elsevier Inc.

1. Oxidative stress in exercise

Knowledge of the occurrence of free radicals in biological ma-terials dates back to the 50's when Commoner and co-workers [3]reported that these radicals occurred in living matter. It was not,however, until the 80's when the first electron paramagnetic re-sonance measurements of free radicals during tetanic contractionwere reported [4]. A critical paper in this field was published byDavies, Quintanila, Brooks, and Packer. These pioneers proposedthat free radicals are produced in exercise in vivo [1]. This workwas of outstanding importance in starting the whole field of freeradical biology in exercise. Moreover, in that paper, it was firstsuggested that radicals could be signals to stimulate mitochon-driogenesis associated with exercise. So two major ideas emerged,the first was that exercise could cause oxidative stress but, veryimportantly, the second was that radicals could act as signals topromote adaptation to exercise. Later on, we reported that exercisecould only cause oxidative stress when it was exhaustive [5].Therefore we proposed that exhaustion and not exercise was the

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ercise training as a drug to t24i

source of oxidative stress and eventually damage associated withstrenuous exercise. Over the next years, the idea that radicals actas signals became entrenched in biological thought and we finallyproposed that exercise itself could be considered as an antioxidant,provided it is moderate, because it causes an upregulation of an-tioxidant enzymes that seriously increases the capacity of tissuesto detoxify free radical species [2].

The whole field of exercise-induced oxidative stress was thor-oughly reviewed by Powers and Jackson [6]. It is important, in acontext of this review paper, to note that the idea that exercisetraining and hence adaptation to exercise would increase the de-fence of cells against stress had occurred during the 90's and early2000s. For instance, Powers et al. reported that exercise trainingcould increase superoxide dismutase in myocardium [7]. More-over, Reid and co-workers [8] reported that free radicals are im-portant in the development of muscle force and promotion ofcontractility in the unfatigued muscle.

Thus, during the last two decades, the idea that radicals act assignals to promote muscle contractility and increase muscle masshas been accepted by the scientific community. On the other side,the fact that inactivity can cause muscle atrophy is also very im-portant. And the new ideas were that this atrophy could be due tothe activation of ubiquitin ligases, many of which would be up-regulated by the very presence of free radicals [9,10].

reat age associated frailty, Free Radic. Biol. Med. (2016), http://dx.

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The relatively recent interest in age-associated frailty, which isin a very high proportion the product of loss of muscle force aswell as a loss of motor coordination, is relevant to the generalscope that radicals can be important in determining musclefunction. The interaction between frailty, exercise, and oxidativestress will be developed in the coming paragraphs of this review.

2. Free radicals and ageing – a brief summary

The free radical theory of ageing was first postulated in ageneral way by Rebecca Gerschman, an Argentinian investigatorwho was working in the 50's in the United States. She proposedthat age-associated damage could be similar to radiation toxicity[11]. This paved the way for a more general postulation on theproper theory of ageing by Denham Harman who in 1956, pro-posed that “ageing and the degenerative diseases associated withit are attributed basically to the deleterious side attacks of freeradicals on cell constituents and the connecting tissues” [12]. Thiswas a hallmark in ageing research. The free radical theory ofageing has been subjected to critical tests and modifications onmany occasions, and especially around the turn of the century.Many experiments have been performed that supported the the-ory but also very many supported the idea that the theory is nolonger valid as it does not explain some experiments in whichdamage caused by free radicals is not always associated withageing [13]. Many refinements of this theory have been postulated.One of the major experimental corollaries from this theory is thatantioxidants should delay ageing and promote wellbeing. Thisproved to be incorrect. Meta-analysis very carefully performed onlarge numbers of persons who take antioxidants has shown thatthese do not protect against age-associated diseases and in factthat they do not prolong life [14–18]. Moreover, evidence from ourown laboratory showed that antioxidants could prevent the onsetof mitochondriogenesis associated with physical exercise andwould be inadequate for training in young individuals and animals[19–21]. Although controversial [22,23], these findings were laterconfirmed and extended by Michael Ristow's group who showedthat these antioxidants would not only impair the effects on theefficacy of training (as we had reported) but that they would alsoprevent some of the health effects associated with physical ex-ercise [24]. These researchers coined the term mitohormesis [25].To reconcile the pros and cons of the free radical theory of agingwe proposed a modified, revised version of the theory: the cellsignalling disruption theory of ageing. We suggested that, in aging,free radicals lose the capacity to be effective signals to control cellmetabolism. This results in a lowering of homoeostatic capacity inthe cell and therefore a lower “resilience” of the cells to toleratestresses eventually resulting in lower capacity of the old organismto resist such stresses [13].

More recently, as will be apparent in the next paragraph, wehave come up with the idea that probably, free radicals are moreinvolved in the development of age-associated frailty than inageing itself. There is a big difficulty in differentiating, at least inmammals, ageing with some measure of frailty. This is especiallytrue at the very last stages of life. Our experiments, which will bedescribed in the next section of this review, reveal that it is likelythat oxidative stress is more associated with frailty than withageing.

3. Oxidative stress and frailty

Frailty can be defined as a geriatric syndrome caused by adisorder of several interrelated physiological systems [26]. A mainoutcome of frailty is that it may lead to disability. The latter is a

Please cite this article as: J. Viña, et al., Exercise training as a drug to tdoi.org/10.1016/j.freeradbiomed.2016.03.024i

major source of distress to both the personal life of elderly peopleand to society as a whole. Disability causes a major burden on theeconomic programmes of nations and it is likely that the problemwill become more and more difficult to solve.

In their article on “Frailty in the clinical setting” Rodriguez-Mañas and Fried state that the aim of health care has changedsubstantially because after centuries of trying to live longer, thetime for living better has come [27].

Thus, research into frailty has become a major issue in basicbiological research as well as in clinical practice [27]. The Eur-opean Union has launched a major effort to promote research totreat frailty (JA-02-2015 Prevention of Frailty). Even though clin-ical interest in frailty has grown in recent years [27,28], to ourknowledge, research in experimental animal models of frailty isvery scarce. Only three models have been reported in the literature[29–31]. Thus, a major shortcoming in research into the biologicalbasis of frailty is that we still lack a convenient animal model tostudy it. We will describe later on a model of frailty based on in-activity. As mentioned earlier, the free radical theory of agingstated that free radical associated damage could cause ageing [12].Facts have disproven the general statement of the theory althoughrestricted ones still hold true [32]. We asked whether frailty couldbe associated with oxidative stress. To that end, we measuredoxidative stress in a population of elderly people (65–85) anddetermined lipid peroxidation (malondialdehyde levels) and pro-tein oxidation [33]. We came to the rather surprising conclusionthat oxidative stress is not strictly associated with ageing, but withfrailty. Indeed, in the elderly persons from the Toledo cohort, wefound that indices of oxidative stress were associated with thefrailty status and not with the age of the persons [33]. Of course,persons in their eighties have more signs of oxidative stress in theblood plasma than persons in their twenties. But if we restrictourselves to the geriatric age (65 years five or more) we do notfind that oxidative stress is associated with age but rather with thefrailty status. Vigorous old persons show fewer signs of oxidativestress than frail relatively young ones (i.e., around the age of 65).Only one publication had described the occurrence of oxidativestress associated with frailty [34], but the association of oxidativestress with frailty and not with age had not been described prior toour study [33]. The mechanisms by which frailty leads to oxidativestress are not fully understood. However, since the lack of frailty isusually associated with the capacity of performing exercise, thepossibility remains that the defective regulation of mitochon-driogenesis and of mitochondrial functions in general may be apossible explanation for the oxidative stress due to frailty. This willbe described in the section below.

4. Exercise induced mitochondriogenesis is lost in ageing

The role of mitochondria in the process of ageing was under-pinned by the work of Jaime Miquel. This researcher publishedthat mitochondria are both origin and targets of free radicals thatcould cause damage in ageing [35]. In fact, we were first to provethat mitochondria were really involved in ageing by performingstudies of mitochondria inside cells using both flow cytometry anda metabolic approach [36]. The doubt was whether mitochondriafrom old animals or persons were more fragile than those fromyoung ones and whether mitochondrial dysfunction in ageing wasderived from the fact that these organelles were damaged as theywere being isolated from the organ. We showed that mitochondriawere indeed damaged within cells of old animals [36]. But inter-estingly, the pioneer work of Davies and co-workers already in-dicated that free radicals could act as signals to stimulate mi-tochondriogenesis [1]. This is an important adaptation to physicalexercise and one that is critical to maintain normal cell function.

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Since mitochondriogenesis is stimulated by physical exercise inthe young [37], we were wondering whether this adaptation couldbe hampered in the old.

PGC-1α is an important co-activator to promote mitochon-driogenesis [38]. We studied the role of PGC-1α in mitochon-driogenesis in young animals, in old ones, and in animals thatwere depleted of this co-activator (PGC-1α-KO). The main con-clusion we reached is that old animals subjected to exercise weremore similar to animals depleted of PGC-1α than to young ones,i.e., that the old animal behaved as if they were PGC-1α-KO [39].So we have traced the lack of effectiveness of exercise in pro-moting mitochondriogenesis to a low activation of PGC-1α [39].We further realised that p38, which is an activator of PGC-1α, wasconstantly stimulated in ageing. In this sense, p38 (that signalsoxidative stress) was not activated by exercise in ageing, but ratherwas at a constant hyperactivation and that the signalling functionslost effectiveness. Thus, the old animals behave as if p38 beingconstantly activated does not promote PGC-1α actions and hencemitochondrial biogenesis is impaired. All this bears importance onthe maintenance of normal muscle function. Lack of energy inmuscle (because of low mitochondrial function) results in lowerproteins synthesis and eventually in sarcopaenia. Sarcopaenia it-self usually leads to loss of muscle strength and to the develop-ment of frailty [40] (see Fig. 1).

5. mTOR at a crossroad between aerobic and anaerobic exercise

Mammalian target of rapamycin (mTOR) was discovered byDavid Sabatini over twenty years ago [41]. It has come up as acritical protein controlling energy and protein metabolism in cells.The general view is that mTOR is sensitive to activation by anabolichormones [42]. For instance, growth hormone (whose release isstimulated by exercise) activates the hepatic production of IGF-1,which is considered as a mediator of growth hormone action andwhich travels bloodborne to muscle, binding to the IGF-1 receptorwhich in turn, intracellularly, activates AKT and eventually mTOR.

Fig. 1. Role of oxidative stress in age-associated frailty.Oxidative stress causes frailty in a direct way and through the disruption of themitochondriogenic cell signalling pathway. P38 MAPK is continuously activated inaging and losses its signalling function. Lack of energy in muscle (because of lowmitochondrial function) results in lower proteins synthesis and eventually in sar-copaenia. This loss of muscle mass is related with the development of frailty.

Please cite this article as: J. Viña, et al., Exercise training as a drug to tdoi.org/10.1016/j.freeradbiomed.2016.03.024i

Activation of the latter results in a phosphorylation of ribosomalprotein S6 Kinase (p70S6K) and direct activation of proteinsynthesis [43] (see Fig. 2). The pathway we have just outlinedprovides a link between growth hormone and protein synthesis.However, if oxidative stress occurs, p38 will be activated and thisMAPKinase will phosphorylate (and thus stimulate) FoxO3 [44].The latter inhibits mTOR, but activates ubiquitin ligases, especiallyMAFbx [45]. This leads to degradation of myosin via the ubiquitinproteasome pathway. FoxO3 in turn inhibits mTOR [45]. Thereforeoxidative stress leads to an increase in proteolysis and inhibition ofprotein synthesis (because it deactivates mTOR) (see Fig. 2). Fi-nally, aerobic exercise has been well documented to activate AMPkinase (an enzyme that senses the energy status of cells andwhose activity is increased by lowering of the ATP/AMP ratio [46]).AMPK phosphorylates and activates PGC-1α, the master activatorof mitochondriogenesis [38,47] and phosphorylates and inhibitsmTOR [42]. Therefore we are faced here with two different path-ways, one leading to protein synthesis (activation of mTOR byanabolic signals) and the other leading to mitochondriogenesis(activation of AMPK) [42]. Oxidative stress, in the classical sense ofthe term, by activation of p38, promotes the degradation of pro-teins and therefore is a contributing factor to sarcopaenia [10]. It iswell known that immobilisation leads to oxidative stress [48] (seeFig. 2).

The general picture that we find here is that mTOR is at acrossroads between myogenesis (activating it) and mitochon-driogenesis (inhibiting it) and this raises the possibility that mo-lecular mechanisms of adaptation induced by endurance (leadingto an increase in mitochondriogenesis through the activation ofPGC-1α) and resistance training (leading to an increase in hyper-trophy through the activation of mTOR) are distinct, with eachmode of exercise activating and/or repressing specific subsets ofgenes and cellular signalling pathways [49]. This phenomenon isknown, in sports science, as the “interference effect” [42] and itpredicts that simultaneously training for both endurance andstrength results in a compromised adaptation compared withtraining for either exercise modality alone [42].

However, the picture is less clear because an important paperby Puigserver [50] the discoverer of PGC-1α, shows that mTORalso could activate mitochondrial oxidative function via a YY1–PGC-1α transcriptional complex. So mTOR might bypass theblocking of mitochondriogenesis via direct binding to the YingYang 1 (YY1) transcription factor [50].

Thus, in terms of the role of oxidative stress in muscle phy-siology, one should consider mTOR as a critical crossroads proteinwhose activity tends to increase protein synthesis (as a result ofanaerobic exercise) and not necessarily to lower mitochon-driogenesis (as a result of aerobic exercise). Modulation of mTORmay be a target for the pharmacological treatment of frailty.

6. Multicomponent exercise training as an estrategy to reversefrailty

Frailty is a major concern in clinical medicine because it is themain determinant of longevity and quality of life in the elderlypopulation [28]. Importantly, frailty is reversible, especially if di-agnosed early in the process [27]. Physical frailty can potentially betreated with nutritional (protein-calorie, vitamin D supplementa-tion) [51], and pharmacological interventions [52]. However, it isconsidered that one of the most prominent interventions to delayand treat frailty is physical exercise [53–57]. It has been recentlyfound that aerobic exercise training has the potential to alteranabolic and catabolic pathways in the skeletal muscle thusleading to a prevention of the age-associated loss of muscle massand cachexia [58]. But recommendations on the appropriate

reat age associated frailty, Free Radic. Biol. Med. (2016), http://dx.

Fig. 2. mTOR as crossroad between mitochondriogenesis and myogenesis. In anaerobic exercise the production of GH promotes the hepatic production of IGF-1 that inmuscle activates PI3K and AKT. This leads to mTOR activation and then, through the phosphorylation of p70S6K, protein synthesis is promoted. Note that, this effect does notoccur if mTOR is inhibited by FoxO3, which is activated by p38 MAPK and whose activation is produced when oxidative stress ensues. FoxO3 leads to the degradation ofmyosin via activation of MAFbx ubiquitin ligase. Aerobic exercise leads to the inactivation of mTOR because of the increased activity of AMPK, consequence of the lower ATP/AMP ratio that occurs in this kind of exercise. Also, AMPK activates PGC-1α and then mitochondriogenesis is promoted.

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design of an exercise protocol to maximise its beneficial effects ina population of frail individuals is still scarce [54,57]. As recentlycommented in The Lancet, the effects of the exercise interventionsare not conclusive and do not show convincing evidence of ef-fectiveness [27].

There are six systematic reviews [54,56,59–62] published spe-cifically on the benefits of exercise in frail older adults. In somecases clinical trials did not show convincing evidence of effec-tiveness [27,57,63], while in others the main conclusion is thatexercise can improve partial aspects of functional outcomes in thefrail population such as: sit-to-stand performance, balance, agility,and ambulation [53,57,64]. The lack of consistency among thestudies is due to the differences in the definition of frailty, trainingprotocols, characteristics of the inactive groups, and in the mainoutcomes assessed [57]. Thus, a definite conclusion has not yetbeen reached [57].

Our previous work shows that exercise acts as a drug [65] andthat tailored exercise programmes are needed especially in theelderly [66]. We have reviewed the continuous debate on howmuch, what type, how often, what intensity and how lengthyphysical activity should be to prevent and/or treat diseases or acomplex geriatric syndrome such as frailty [65–67]. There are noevidences available in the literature to pinpoint which kind ofexercise is more effective for frail individuals [57] but it seems that

Please cite this article as: J. Viña, et al., Exercise training as a drug to tdoi.org/10.1016/j.freeradbiomed.2016.03.024i

a multicomponent exercise intervention could be the best choice[68].

A multicomponent exercise programme is defined as a com-bined programme of endurance, strength, coordination, balance,and flexibility exercises, that have the potential to impact a varietyof functional performance measures [69]. We have previouslysuggested that simultaneously training for both endurance andstrength results in a compromised adaptation compared withtraining for either exercise modality alone (“interference effect”)[42]. Human skeletal muscle partially retains the capacity to re-spond to divergent contractile stimuli [70] and that combiningendurance and resistance exercise amplifies the adaptive signal-ling response of mitochondrial biogenesis compared with single-mode endurance exercise [71]. Thus, it seems that contrary to the“interference effect” hypothesis, concurrent training may be ben-eficial for the adaptations of muscle oxidative capacity andstrength.

We have recently found (data not published) that a tailoredmulticomponent exercise training intervention can reverse frailtyand improve physical function, cognitive, emotional, and socialnetwork determinations in community-dwelling frail individuals.These results in a significant decrease in the number of visits tothe primary care physician after the training programme.

Our major conclusion is that adherence to individually tailored

reat age associated frailty, Free Radic. Biol. Med. (2016), http://dx.

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multicomponent exercise programmes is an excellent way to delayand even treat age-associated frailty.

Conflict of interest

The authors declare that no conflict of interest exists.

Acknowledgements

We thank Mrs. Marilyn Noyes for her kind help in reviewingthe manuscript. This work was supported by Grants: IntegratedProject of Excellence PIE15/00013 (ISCIII. FEDER), ISCIII2012-RED-43-029 from the “Red Tematica de investigacion cooperativa enenvejecimiento y fragilidad” (RETICEF), PROMETEO2010/074 from“Conselleria, de Sanitat de la Generalitat Valenciana”, 35NEUROGentxGent from “Fundacio Gent Per Gent de la Comunitat Va-lenciana” and EU Funded CM1001 and FRAILOMIC-HEALTH.2012.2.1.1-2. This study has been co-financed by FEDERfunds from the European Union.

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