aging is characterized by a progressive decline in cardiac function including stroke volume , cardiac output , blood flow , and oxygen consumption ( beere et al . , 1999 ;
an increase in susceptibility to inflammation , oxidative stress , and disease ( higashi et al .
, 2012 ; sastre et al . , 2000 ; zhang and herman , 2002 ) .
impaired cardiac function , increased inflammatory cytokines , and elevated oxidative stress are commonalities between aging and cardiovascular disease . a significant reduction in the number of myocytes
is now believed to contribute directly to impaired contractile function , cardiomyopathy , heart disease , and heart failure ( favaloro et al .
indeed , the typical 70 year old man has a 30% reduction in the number of cardiac myocytes compared with young adults ( higami and shimokawa , 2000 ; olivetti et al . , 1991 ) .
loss of myocytes with aging occurs through necrosis ( i.e. , accidental cell death ) and apoptosis ( i.e. , programmed cell death ) , two distinct mechanisms leading to cell death ( konstantinidis et al . , 2012 ) .
necrosis results from cellular injury seen with infection or inflammatory disease , and is characterized by cellular swelling and rupturing with inflammation ( carraro and franceschi , 1997 ; mughal et al .
, 2012 ; phaneuf and leeuwenburgh , 2001 ; zhang and herman , 2002 ) .
cell rupture with necrosis results in the release of stress and inflammatory proteins and substrates .
in contrast , apoptosis is highly regulated cell death without injury and inflammation , and is characterized by cell shrinking / blebbing and condensation of the nucleus ( carraro and franceschi , 1997 ; mughal et al . , 2012 ; pollack and leeuwenburgh , 2001 ; zhang and herman , 2002 ) .
apoptosis is a highly regulated , programmed means of cell death or elimination that plays an essential role in governing development , growth , and repair ( hengartner , 2000 ; jacobson et al . , 1997 ;
a dramatic increase in the rate of apoptosis has been reported with aging in the rat left ventricle , while the rate of necrosis remained constant ( higami and shimokawa , 2000 ; kajstura et al . , 1996 ; konstantinidis et al . , 2012 ; liu et al . , 2012 ) .
phaneuf and leeuwenburgh ( 2002 ) described age - related disruption of bcl-2 family signaling in the rat heart .
progressive apoptosis from age in post - mitotic tissues such as the heart is dire , as lost myocytes are not replaced .
unfortunately , the mechanisms responsible for apoptotic signaling and apoptosis in the aging heart remain unclear and very limited .
in contrast , exercise training can reduce the risk of injury , oxidative stress , and inflammatory signaling from mechanical and oxidant perturbations , as long as overtraining does not occur ( fielding and evans , 1997 ; hammeren et al .
also , exercise training improves cardiovascular capacity and reduces cardiovascular disease risk in both young adults and the elderly ( booth et al . , 2002 ; gremeaux et al . , 2012 ; lawler et al . ,
exercise has the potential to reduce apoptosis through upregulation of protective stress - sensitive proteins including nuclear factor kappab ( nf - kb ) , insulin - like growth factor ( igf-1 ) , and heat shock proteins ( hsp90 and hsp70 ) ( frost and lang , 2003 ; milne and noble , 2002 ; morton et al , 2009 ; naito et al . , 2001 ) .
however , the mechanisms by which exercise training improves heart function and cardiovascular disease risk profile are not well delineated .
moreover , the ability of exercise training to modulate bcl-2 family apoptotic signaling and apoptosis in the aging heart has not been clearly explored so far . therefore , in the next section , an overview of aging and apoptosis in the heart will be provided , followed by a brief review of the literature supporting the effects of exercise in the aging heart .
apoptosis is regulated by genetic programming and plays an essential role in governing development , growth , and disease ( hengartner , 2000 ; zhang and herman , 2002 ) .
one of the most visible examples of cell death that occurs in normal animal development is the loss of the tadpole s tail ( jacobson et al . , 1997 ) .
however , excessive apoptosis or dysregulation of apoptosis results in dysfunction and disease ( zhang and herman , 2002 ) .
apoptosis is distinct from necrosis ( carraro and franceschi , 1997 ; fiers et al . , 1998 ; konstantinidis et al . ,
accidental cell death  is the pathological process which is caused by injury , infection or inflammatory diseases such as high blood pressure and heart attacks ( hofmann , 1999 ) .
programmed cell death  is highly regulated cell death involving dna fragmentation , blebbing , and dismantling of the cell without inflammation ( favaloro et al . , 2012 ; jacobson et al . , 1997 ; lemasters et al . , 1999 )
apoptotic signaling induces apoptosis through caspase - dependent pathways ( budihardjo et al . , 1999 ;
1 ) . they include ( a ) cytokines / fas - driven pathways ( hofmann , 1999 ) , ( b ) mitochondrial - driven pathways ( mignotte and vayssiere , 1998 ) , and ( c ) endoplasmic reticulum ( er)/ca - driven pathways ( pollack and leeuwenburgh , 2001 ) .
binding of cytokines ( e.g. , tnf- , il-1 ) or fas to their cell membrane receptors initiate apoptotic pathway through the recruitment of adaptor proteins ( e.g. , fas - associated death domain ; fadd ) .
the resulting death - inducing signaling complex activates caspase-8 , which subsequently activates caspase-3 ( hofmann , 1999 ) .
endoplasmic reticulum ( er)-mediated apoptotic signaling pathway is initiated by the involvement of capase-12 , which also subsequently activates caspase-3 ( pollack and leeuwenburgh , 2001 ) .
mitochondrial - mediated pathways including the bcl-2 family is the best characterized , and bcl-2 signaling is believed critical in regulating apoptosis with aging ( green and reed , 1998 ; hengartner , 2000 ; konstantinidis et al . , 2012 ; mignotte and vayssiere , 1998 ; pollack and leeuwenburgh , 2001 ) . in other words , mitochondria are important sites of programmed cell death ( favaloro et al . , 2012 ; remmen and richardson , 2001 ) .
a critical factor in mitochondrial - mediated apoptosis is that deficiency of survival stimuli such as growth factors increases mitochondrial permeability .
mitochondrial membrane permeability transition pore opening is regulated by a number of pro - and anti - apoptotic proteins such as bcl-2 family of proteins ( green and reed , 1998 ; hengartner , 2000 ; mignotte and vayssiere , 1998 ) .
group iii such as bid also initiates apoptosis . in other words , group i functions as gatekeepers at the outer surface of mitochondria , whereas group ii and iii function as gatecrashers .
therefore , cell death or survival depends on the ratio of the pro- and anti - apoptotic proteins expressed .
for example , high levels of bcl-2 relative to bax promote survival , whereas the reverse ratio promotes myonuclear cell death ( hengartner , 2000 ; pawlowski and kraft , 2000 ; primeau et al . , 2002 ) .
thus the ratio of anti - apoptotic / pro - apoptotic proteins in the bcl-2 family functions as an upstream modulator of apoptosis in the mitochondrial - mediated apoptotic pathways .
the proteins of bcl-2 family can regulate apoptosis by controlling permeability transition pore ( ptp ) opening and the release of cytochrome c from mitochondria ( mignotte and vayssiere , 1998 ) .
caspases are a group of cysteine dependent aspartate - specific proteases ( favaloro et al .
fourteen caspases have now been identified in mammals , leading to apoptotic cell death ( kumar , 1999 ) .
caspases play a pivotal role in apoptotic pathways and interact with the non - caspase apoptotic pathways including endonuclease g ( endo g ) and apoptosis inducing factor ( aif ) ( marzetti et al .
cytochrome c associates with apaf-1 ( apoptotic protease - activating factor 1 ) and pro - cas pase-9 to form the apoptosome , which activates caspase-9 and caspase-3 , resulting in apoptosis through dna fragmentation ( favaloro et al . , 2012 ; kumar et al . , 2002 ;
aging is characterized by a general decline of physiological function ( dinenno et al . , 2001 ; liu et al . , 2012 ) .
it has been shown that aging is associated with increased bax protein as well as enhanced dna fragmentation in brains ( higami and shimokawa , 2000 ) .
aging - induced apoptosis may contribute to a 30% loss of cardiac myocytes outside changes noted in cardiac diseases ( higami and shimokawa , 2000 ; liu et al .
, 2012 ; olivetti et al . , 1991 ) . myocyte cell death including both apoptosis and necrosis increased with aging in the heart of fischer-344 rats , mediating ventricular dysfunction and failure ( kajstura et al . , 1996 ) .
1998 ) also reported that expression of bcl-2 and bax proteins both increased in the heart of elderly fischer-344 rats under physiological conditions .
moreover , our research group demonstrated that aging increased pro - apoptotic bax , cleaved caspase-3 , and dna fragmentation in rat skeletal muscle ( song et al .
( 1998 ) showed that there was an increase in cardiomyocyte apoptosis with age from fischer-344 rats without significant changes in bax and bcl-2 protein levels with age .
these findings suggest that age - related changes in the expression of bcl-2 family proteins ( bcl-2 and bax ) are still debatable .
progressive apoptosis with advancing age in post - mitotic tissues including heart , skeletal muscle , and brain is dire , because lost myocytes are not replaced .
phaneuf and leeuwenburgh ( 2002 ) showed markers of apoptosis in hearts ( left and right ventricles ) of 6- , 16- , and 24-month - old male fischer-344 rats .
cytosolic cyto - chrome c was significantly increased in the 16- and 24-month - old animals compared to the 6-month - old animals , indicating increased mitochondrial permeability transition pore ( ptp ) opening and activation of the apoptosome .
also , bcl-2 levels were decreased with aging , whereas mitochondrial bax levels remained unchanged .
display reduced pro - apoptotic protein levels , cytochrome c contents , caspase levels , and dna fragmentation with aging .
this dysregulation of apoptotic signaling from aging is postulated as a mechanism contributing to cancer risk ( suh et al . , 2002 ) .
for example , it has been reported that caspase-3 and caspase-9 protein levels and activities were lower in colon mucosa in old ( 2224 months ) fischer-344 rats compared to their young ( 45 months ) and middle - age ( 1214 months ) counterparts ( xiao et al . , 2001 ) .
these changes were accompanied by a reduced number of apoptotic cells in the colonic mucosa of 1214 and 2224 month old rats compared with in 46 month old rats .
in addition , the levels of pro - apoptotic bak were decreased by about 50% , whereas anti - apoptotic bcl - xl levels were increased by about 50% in 22 month old rats compared to 4 or 13 month old rats ( xiao et al . , 2001 ) .
( 2002 ) also showed that apoptosis - induced dna fragmentation was dramatically upregulated in the liver of young ( 2 months ) female fischer rats but not in the old ( 26 months ) counterparts .
additionally , the levels of cytochrome c in livers from old ( 26 months ) fischer-344 rats were significantly lower than those from young ( 6 months ) counterparts ( zhang et al . , 2002 ) .
these opposite results could be tissue specific and possible characteristics of mitotic but not post - mitotic tissues .
the heart may undergo an increase in apoptosis during ischemia - reperfusion , heart failure , doxorubicin treatment , and aging ( childs et al . , 2002 ;
it has been reported that a number of cardiomyopathies are associated with mitochondrial dna damage which leads to defects in electron transport chain and the upregulation of ros ( anderson et al , 2011 ; mignotte and vayssiere , 1998 ) , resulting in the apoptosis of cardiac myocytes ( li et al . , 2012 ;
mughal et al . , 2012 ; von harsdorf et al . , 1999 ) .
heart is an energetically - demanding tissue that requires a continuous production of atp via mitochondrial respiration ( li et al . , 2012 ; primeau et al . ,
it was also shown that tumor necrosis factor- ( tnf- ) increased apoptosis via inducible nitric oxide synthase ( inos ) expression and nitric oxide ( no ) in cardiac myocytes ( ing et al . , 1999 ;
the aging heart is characterized by a decrease in the total number of myocytes resulting in reactive hypertrophy of the remaining cells and increased fibrosis ( kwak et al . , 2006 , 2011 ; olivetti et al .
2002 ) , with direct relevance to heart disease and heart attacks ( quindry et al . ,
 protects the heart from the deleterious effects of longer periods of ischemia and reperfusion ( murry et al . , 1986 ; taylor and starnes , 2003 ) . the mechanisms of protection against apoptosis due to preconditioning may be mediated by nf - kb , which inhibits the susceptibility to apoptosis and can promote upregulation of anti - apoptotic bcl-2 ( das et al .
1998 ) . indeed , nf - kb dna binding activity progressively increased as a function of the pre- conditioning of ischemia ( maulik et al . ,
2000 ) , and bcl-2 levels were decreased in the ischemic / reperfused heart , but increased in the preconditioned myocardium ( maulik et al . , 2000 ) . in addition , overexpression of anti - apoptotic bcl-2 was effective in reducing myocardial reperfusion injury and improving heart function ( brocheriou et al . , 2000 ) .
indeed , the balance between bax / bcl-2 is postulated as a critically important factor in the increased rate of apoptosis in cardiac myocytes ( condorelli et al . , 1999 ;
for example , condorelli et al . ( 1999 ) indicated that left ventricular hypertrophy and left ventricular dysfunction in a rat model of chronic pressure overload were accompanied by upregulated pro - apoptotic bax and bax / bcl-2 ratio , leading to cardiomyocyte apoptosis .
( 2000 ) also found that apoptosis predominated in cardiomyocytes after reoxygenation through a mitochondrial apoptotic pathway , and that bcl-2 prevented reoxygenation - induced apoptosis by inhibiting the release of cytochrome c from mitochondria .
in addition , kirshenbaum and moissac ( 1997 ) concluded that the anti - apoptotic bcl-2 protein can prevent apoptosis of ventricular myocytes .
1997 ) showed that cardiomyocyte apoptosis dramatically increased in the heart of patients with cardiac failure , although the level of bcl-2 was about two times higher than that of the normal hearts , while the level of bax remained unchanged .
saraste el al . ( 1999 ) also showed that the number of cardiomyocytes undergoing apoptosis was significantly increased in failing human hearts compared with control myocardium , and that the expression of bcl-2 was increased in failing hearts .
enhanced expression of bcl-2 in the failing heart suggested that compensatory mechanisms were activated to prevent apoptosis ( saraste et al . , 1999 ) .
the expression of bcl-2 protein was induced in human cardiac myocytes at the acute stage of infarction , but the expression of bax protein was overexpressed at the old stage , which was related to myocyte death in human hearts ( misao et al . , 1996 ) .
our research group recently indicated that the bax / bcl-2 ratio was significantly upregulated with age and markedly reduced by exercise training in the left ventricles ( kwak et al . , 2006 ) .
these data imply that aging heart is more susceptible to apoptosis than young heart , and that chronic exercise training exerts anti - apoptotic action in the aging heart ( soufi et al . , 2008 ) .
these data are consistent with previous reports showing that aging is associated with increased bax protein as well as decreased bcl-2 ( higami and shimokawa , 2000 ; liu et al . , 1998 ) , indicating that mitochondrial bcl-2 family signaling is an important site of regulating apoptosis with aging and exercise training . based on our data ( kwak et al . , 2006 ) , the increased bax / bcl-2 ratio with aging heart was directly related to increases in downstream caspase signaling including caspase-9 and cleaved caspase-3 , which promote dna fragmentation and cell death ( e.g. , increased tunel - positive nuclei ) .
similarly , there were consistent reports that caspase-9 and caspase-3 activities were increased in aging rat liver ( zhang and herman , 2002 ) .
in contrast , exercise training markedly decreased caspase-9 and cleaved caspase-3 levels as well as the bax / bcl-2 ratio in the aging heart , which presumably led to the decreased dna fragmentation .
these results are consistent with the hypotheses that aging increases bcl-2 family pro - apoptotic signaling in the heart , and exercise training in the aging heart results in amelioration of age - induced changes in the mitochondrial - mediated apoptotic pathways .
in particular , cleaved caspase-3 showed similar patterns to dna fragmentation in the effect of aging and exercise training , suggesting that caspase-3 may be an essential hallmark of apoptosis ( fig .
aging is generally characterized by a decline of cardiac function and an upregulation of oxidative stress in the heart ( beere et al . , 1999 ;
, 2001 ; sastre et al . , 2000 ; soufi et al . , 2008 ) , which are major contributors to cell death through mitochondrial dysfunction ( dirks and leeuwenburgh , 2002 ; sastre et al . , 2000 ) .
previous work has demonstrated that aging promotes the susceptibility to apoptosis in rat heart ( higami and shimokawa , 2000 ; kajstura et al .
; olivetti et al . , 1991 ; soufi et al . , 2008 ) . while aging heart was vulnerable to apoptosis , exercise training was effective in diminishing apoptosis ( dna fragmentation ) in the aging heart , which indicated the beneficial effect of long - term exercise training on apoptosis in the aging heart ( soufi et al . , 2008 ) .
interestingly we found that even if apoptosis was dramatically increased with aging , there was no significant changes in heart - to - body weight ratio between young sedentary rats and old sedentary rats ( kwak et al . , 2006 ) because the aging process of the heart in humans and animals is characterized by a significant loss of myocytes and reactive hypertrophy of the remaining cells ( olivetti et al .
thus , the loss of cardiac myoctyes does nt necessarily cause a reduction in mass .
in addition , with aging there is an increase in heart connective tissue , increasing wall stiffness / thickness , and losing elasticity of the heart ( anversa et al .
, 1990 ; kwak et al . , 2011 ; lakatta and sollott , 2002 ) .
however , cardiac myocytes that hypertrophy often have poor contractile function due to increased left ventricle wall thickness ( lakatta and sollott , 2002 ) . based upon current findings regarding exercise and apoptosis in the aging heart , further investigations as future directions are required to determine the effects of exercise training on the upstream cell protective mechanisms in the aging heart ( fig .
, exercise training may promote cell - survival proteins including mn isoform of superoxide dismutase ( mnsod ) , nf - kb , extracellular receptor kinase ( erk ) , igf-1/akt pathway , and heat shock proteins ( hsps ) in heart ( higashi et al .
, 2012 ; powers et al . , 2002 ; soufi et al . , 2008 ; starnes et al .
, 2003 ; taylor and starnes , 2003 ) , which may be potential upstream regulators of age - induced apoptosis in the aging heart .
in summary , aging resulted in increases in mitochondrial - mediated apoptotic pathways including pro - apoptotic protein levels such as bax , bax / bcl-2 ratio , caspase-9 , and cleaved caspase-3 and apoptosis . however , endurance exercise training reversed the elevation of apoptotic signaling and apoptosis , suggesting that exercise training protects the heart against apoptosis
. however , further research is necessary to determine the cellular and molecular mechanisms by which exercise training protects against aging - induced apoptosis in the heart .
 aging is characterized by a progressive decline in cardiac function . a critical contributor to the age - related impairment in cardiac function 
 is the loss of cardiac myocytes through  apoptosis  , or programmed cell death . 
 structural remodeling in the heart with advancing age includes ( a ) loss of cardiomyocytes , ( b ) reactive hypertrophy of the remaining cardiomyocytes , and ( c ) increased connective tissue and altered geometry . 
 the loss of cardiomyocytes with aging occurs through apoptosis . particularly , mitochondrial - mediated apoptotic pathway is the best characterized and believed critical in regulating apoptosis with aging , suggesting that mitochondria are very important sites of programmed cell death . 
 it has been also reported that mitochondrial dysfunction , oxidative stress , and impaired stress response contribute to age - induced mechanical remodeling as well as apoptosis . 
 in contrast , exercise training not only improves cardiac function , but also reduces the risk of heart disease . 
 we recently found that aging increased mitochondrial - mediated apoptotic signaling and apoptosis in the left ventricle , while chronic exercise training was effective in diminishing mitochondrial - mediated apoptotic signaling pathways in the aging heart , as indicated by lower dna fragmentation , terminal deoxynucleotidyl transferase - mediated dutp nick end labeling ( tunel)-positive staining , and caspase-3 cleavage , when compared with left ventricles from the age - matched sedentary group . in this review , we will provide a comprehensive update regarding the effects of aging and exercise training on apoptosis in the heart . 
INTRODUCTION
APOPTOSIS AND MITOCHONDRIAL CONTROL
AGING AND APOPTOSIS
APOPTOSIS AND HEART
EXERCISE AND APOPTOSIS IN THE AGING HEART
CONCLUSIONS