Introduction
Humanity has long dreamed of the possibility of extending lifespan, from the
search for the mythical philosopher’s stone to Ponce de Leon’s quest
for the Fountain of Youth. While no such fantastical solution exists, recent
advances in aging research have brought us closer to finding treatments that may aid
in slowing the aging process and attenuating age-related diseases. In this review,
we will discuss small molecules that recent evidence suggests may be capable of
improving health and extending lifespan in mammals, focusing particularly on
resveratrol, rapamycin, and metformin.
Resveratrol
One of the most frequently mentioned potential anti-aging compounds is
resveratrol, a polyphenol found in a variety of plant sources, including berries,
peanuts, and red wine [1]. In 2003, the
laboratories of Howitz KT et al. [2] showed
using an in vitro assay that resveratrol and several
structurally-related compounds activate SIRT1, an NAD-dependent protein deacetylase
that is homologous to the product of the yeast longevity gene Silent Information
Regulator 2 (SIR2) [2].
Furthermore, resveratrol was subsequently shown to extend the lifespan of yeast, and
to confer stress-resistance to cultured mammalian cells. Later studies expanded
these findings to show resveratrol-dependent lifespan extension in
Caenorhabditis elegans, Drosophila melanogaster [3], and a short-lived species of
vertebrate fish
[4].
The initial trial of resveratrol’s effects on mammalian lifespan was
quite encouraging; resveratrol improved insulin sensitivity, transcriptional
profiles, and longevity in obese mice consuming a high-fat diet [5]. However, resveratrol
appears to primarily
protect from negative consequences of a high-fat diet, rather than ameliorating the
underlying aging process, as two separate trials of dietary resveratrol, conducted
by independent groups using multiple doses, failed to find any effect of resveratrol
on lifespan of mice fed a standard lab diet [6,7]. Resveratrol has often been
associated with the “French Paradox”, the observation that red wine
consumption may ameliorate the deleterious effects of a high fat diet. While it is
tempting to make this link, red wine contains many potentially beneficial molecules,
and the concentration of resveratrol alone is likely too low to account for its
benefits [1]. Nevertheless, resveratrol
continues to show significant promise as a potential therapeutic, suppressing many
forms of cancer, and improving insulin sensitivity, endurance, motor coordination,
vascular tone, bone strength, and resistance to ischemic injuries in mice [1,5, 8–10].
There have been a number of significant controversies concerning the ability
of resveratrol to extend lifespan in lower organisms, and its mechanism of action
in
mammals [11]. In both yeast and
Drosophila, failure to replicate the original observation of
lifespan extension has been reported [12,13]. However, lifespan
extension has been reproduced in both organisms by the same groups that made the
initial reports, and has also been reported independently [14,15]. A recent study
in Drosophila has found that the effect of resveratrol on lifespan
is dependent on the nutrient composition of the diet, suggesting that subtle
differences in experimental conditions may be contributing to the discrepancies
between findings from different labs [15].
While there is general agreement that resveratrol extends lifespan in C.
elegans [16– 18], Bass et al. reported that the effect was
variable and unrelated to the presence of Sir2 [12]. In contrast, Viswanathan et al.
concurred with the original report
that lifespan extension by resveratrol was robust and entirely dependent on Sir2
[19]. Therefore, a number of unresolved
issues concerning the effects of resveratrol and their dependence on Sir2 remain to
be clarified in lower organisms.
In mammals, resveratrol influences multiple direct targets, including
cyclooxygenases [8], cytochrome P450 enzymes
[20,21] the estrogen [22] and aryl
hydrocarbon receptors [23] and quinone
reductase 2 [24] and can indirectly activate
the AMP-activated protein kinase (AMPK) [5,25] and Nrf2/Keap1 signaling
pathways [26]. In addition, the biochemical
evidence for direct activation of SIRT1 by resveratrol has been challenged by
several groups, since the effect is dependent on the use of fluorescent substrates
[13,27]. On the other hand, it is quite clear that many of the effects of
resveratrol in cultured cells are dependent on the presence of SIRT1 [28–47] and the
limited evidence that is available supports the same
conclusion in vivo [48].
Therefore, key questions that remain to be resolved in mammals are whether SIRT1
activation by resveratrol occurs through a direct or indirect mechanism, and what
the relative importance of this pathway is compared to other effects of the molecule
that might contribute to its health benefits.
Notably, AMPK was recently shown to be required for many of the benefits of
resveratrol in mice [49] and was previously
shown to be required for lifespan extension by resveratrol in worms [18]. Mouse models
lacking the catalytic
subunits of AMPK fail to show increased insulin sensitivity, improved glucose
tolerance, or enhanced mitochondrial biogenesis when treated with resveratrol, in
contrast to wild-type animals [49].
Activation of AMPK alone appears to be sufficient to extend C.
elegans lifespan [50] and is a
possible explanation for the increased lifespan of mice lacking S6K1 [51]. Although
SIRT1 can activate AMPK via
deacetylation and activation of the upstream kinase LKB1 [52] activation of AMPK by
resveratrol can occur independently
from SIRT1 [53]. Thus, it remains unclear
whether SIRT1 mediates the AMPK-dependent effects reported by Um et al.
Unfortunately, the developmental and metabolic abnormalities in SIRT1 null mice,
which are small and infrequently survive postnatally, have made it difficult to
perform parallel studies to determine whether metabolic effects of resveratrol are
similarly dependent on SIRT1 itself [54].
Interestingly, AMPK can also act upstream of SIRT1 by increasing production
of its cosubstrate, nicotinamide adenine dinucleotide (NAD) [55] and Canto et al.
have proposed that this is the major
mechanism contributing to SIRT1 activation following resveratrol treatment
in vivo [56]. One
possible mechanism for SIRT1-independent AMPK activation by resveratrol is direct
inhibition of mitochondrial oxidative phosphorylation, causing a drop in
intracellular ATP levels, and consequently, a rise in AMP [57,58]. However, it
remains to be seen if the concentrations of resveratrol achieved in
vivo are sufficient to elicit this effect. During the preparation of
this manuscript, Park et al. reported an alternative mechanism by which resveratrol
can activate AMPK. Their study identified resveratrol as an inhibitor of
cAMP-specific phosphodiesterases (PDEs), and described a multi-step mechanism by
which increased cAMP leads to activation of AMPK via phosphorylation by CAMKK [59].
In addition, they showed that a specific
inhibitor of PDE4, rolipram, is sufficient to reproduce many of the salient effects
of resveratrol, including increases in glucose tolerance, endurance, and energy
expenditure. Intriguingly, increased cAMP was also recently reported to lead to
enhanced activity of SIRT1 through a separate, PKA-dependent mechanism [60]. Therefore,
the ability of resveratrol to
increase intracellular cAMP signaling provides a plausible upstream mechanism for
the induction AMPK- and SIRT1-depdendent benefits (Figure 1).
A significant piece of evidence arguing in favor of sirtuins mediating key
protective effects of resveratrol in mice is a series of studies involving SRT1720,
a novel synthetic activator of SIRT1 [61].
Intriguingly, SRT1720 extends the survival and healthspan of mice fed a high-fat
diet, just as resveratrol does [62]. In
addition, this molecule has been shown to improve insulin sensitivity and endurance,
and induces a transcriptional profile that is very similar to the effect of
resveratrol [62–64]. Unlike resveratrol, SRT1720 does not have any acute effect
on AMPK activity. However, long-term (> 20 weeks) treatment in
vivo does result in modest AMPK activation [63]. Whether this is a direct result of
increased SIRT1
activity, or instead indicates additional mechanisms of action for SRT1720 remains
to be seen. Notably, SIRT1 activation by SRT1720, like activation by resveratrol,
is
substrate-dependent [65] and it has been
suggested that inhibition of the acetyltransferase p300, rather than activation of
SIRT1, may underlie some of SRT1720’s effects [66]. However, SRT1720 has been shown
to increase SIRT1 activity
in vitro using a substrate that contains only natural amino
acids, supporting the model that its effects on SIRT1 activity in
vivo are direct [67]. Therefore,
structurally unrelated compounds that activate SIRT1 in vitro have
similar protective effects in vivo, but further study will be
required to reach a consensus concerning their in vivo mechanism(s)
of action.
Resveratrol is currently being investigated in human clinical trials as an
anti-cancer and anti-diabetic therapy, and is also marketed as a nutritional
supplement with a variety of claims related mainly to weight loss and increased
energy [68]. Although only a small fraction
of the ongoing studies have been published in peer-reviewed journals to date, there
is evidence that resveratrol can increase cerebral blood flow, improve insulin
sensitivity, decrease inflammation, and suppress cardiovascular risk factors in
humans [69–73]. A direct extrapolation from mouse studies, based on body
weight, would suggest that humans would require a large quantity of resveratrol to
obtain similar benefits, well above the ~1g/day at which significant
gastrointestinal side effects have been reported [74]. As has been pointed out, however,
scaling by body weight is not an
accurate method for determining dosing across species [75]. The wisdom of this assertion
was recently highlighted by
the finding that 150 mg/day resveratrol in humans (~2 mg/kg/day) achieved
equivalent or higher serum levels of resveratrol than 400 mg/kg/day in mice [73].
Subjects in the study exhibited reduced
metabolic rates, improvements in glucose and lipid metabolism, and reduced
inflammation, leading the authors to conclude that the effects of resveratrol
resembled those of calorie restriction. While a number of safety trials in healthy
humans have not raised any major concerns, it is noteworthy that a high-dose (5
g/day) trial of a resveratrol-based drug, SRT501, in multiple myeloma patients was
halted due to kidney complications [76]. It
was suggested that this effect was secondary to dehydration due to diarrhea, which
generally does not occur at doses below 1 g/day. Therefore, the existing data in
humans suggest that resveratrol supplementation may lead to improvements in health,
and is likely to be safe. However, it will be of great importance to design future
trials of resveratrol to detect adverse effects as well as potential benefits, and
to continue to perform well-controlled studies to determine how much of the promise
of this drug in mice will translate to human patients.
Rapamycin
Rapamycin is an inhibitor of the mTOR (mechanistic Target Of Rapamycin)
signaling pathway, which is found in most eukaryotes, including yeast, worms, flies,
plants, mice, and humans. mTOR integrates inputs from nutrients and growth factors,
including amino acids, glucose and insulin, to regulate many outputs involved in
growth and proliferation [77]. Indeed, mTOR
is so centrally positioned that it may be accurately described as a master regulator
of cell metabolism.
mTOR is found in two distinct protein complexes: mTORC1, which regulates
numerous cellular processes related to growth and differentiation, and mTORC2, which
plays a regulatory role in the insulin signaling cascade, among other functions.
Genetic attenuation of mTORC1 signaling is sufficient to promote longevity in
diverse organisms, including S. cerevisiae, C. elegans, and
D. melanogaster [78–80]. mTORC1 is the
canonical target of rapamycin, whereas mTORC2 is not acutely sensitive to the drug.
Rapamycin is FDA-approved as an immunosuppressant for transplant surgery, and is
also being investigated for its anti-tumor properties. Interestingly, the
immunological effects of rapamycin are proving more complex than initially supposed;
the drug even enhances immunity under some conditions [81]. Studies conducted over
the last few years have shown that
rapamycin treatment can extend lifespan in model organisms, including yeast and
flies [82–84] and that rapamycin treatment can even extend the lifespan
of mice. In a study conducted by the National Institute on Aging Interventions
Testing Program, rapamycin was found to extend the average and maximal lifespan of
both male and female mice, even when treatment was initiated at 20 months of age
[85]. A follow-up study demonstrated
similar effects when rapamycin was begun at 9 months of age [7]. It has been suggested
that rapamycin may extend rodent
lifespan via an anti-tumor mechanism, however, the available data do not support a
dramatic change in the range of cancers or other lethal or non-lethal illnesses
found in the mice at the time of death. Instead, it is believed that rapamycin acts
via an anti-aging mechanism, as suggested by the extension of both median and
maximal lifespan and the delayed appearance of age-associated pathologies. These
results strongly implicate the mTORC1 pathway in the regulation of mammalian
longevity, and suggest that pharmacologic inhibition of mTOR signaling explains, or
at least contributes to, lifespan extension by rapamycin.
The downstream mechanism by which mTORC1 inhibition extends lifespan is not
yet clear. Rapamycin is an inhibitor of mTORC1-dependent translation, and one theory
of aging suggests that decreased translation can extend lifespan by reducing the
burden on the protein folding machinery, leading to improved protein quality.
Indeed, genetic depletion of ribosomal proteins, as well as inhibition of
translation initiation factors, can similarly extend lifespan in yeast and worms
[86,87]. Moreover, deletion of the mTORC1 substrate S6 kinase 1 (S6K1),
which plays a key role in the control of protein translation, is sufficient to
confer increased lifespan in female mice [51]. However, from a quantitative standpoint,
while rapamycin does decrease
translation [88] the effect is mild
in vivo, and the more salient effect may be a shift in the type
of mRNA that is translated [89]. Moreover,
the S6K-null mouse has no overt change in total translation in skeletal muscle,
providing additional evidence that decreased translation per se may
not be how mTOR inhibition promotes longevity [90]. In yeast, interfering with components
of the 60S, but not the 40S
subunit of the ribosome extends lifespan, and this correlates with increased
translation of a specific mRNA encoding the transcription factor Gcn4, which is,
itself, sufficient to extend life [87].
Whether a similar process might be occurring in mammals is not yet known. Clearly,
much remains to be understood about the consequences of long-term mTORC1 inhibition
in vivo.
Another possibility is that inhibition of mTORC1 may promote lifespan by
inducing autophagy, the process responsible for the normal degradation and renewal
of cellular components and organelles. A number of interventions that induce
autophagy have been shown to extend the lifespans of model organisms, and the role
of mTORC1 in suppressing autophagy is well-established [91,92]. Indeed,
rapamycin treatment induces autophagy in vivo, reducing levels of
amyloidbeta and rescuing cognitive defects in a mouse model of Alzheimer’s
disease [93]. Therefore, induction of
autophagy appears to account for at least some of the beneficial effects of
rapamcyin, and other mTORC1-specific kinase inhibitors may have similar protective
effects [92]. In addition, screens of
FDA-approved compounds for regulators of autophagy may be of significant use in
identifying molecules that could inhibit mTORC1 signaling in humans. In fact, one
such screen has already identified four mTORC1-specific inhibitors - perhexiline,
niclosamide, rottlerin and amiodarone (all likely acting through indirect
mechanisms) - that activate autophagy [94].
In support of the potential importance of autophagy in preventing age-related
decline, the restoration of autophagy to youthful levels in aged liver has been
shown to reverse functional deficits [95].
Induction of autophagy may therefore have benefits in the treatment of age-related
diseases that go well beyond it’s reported effects in Alzheimer’s
disease models.
Interestingly, resveratrol also activates autophagy in at least some cell
types [96–98] and in a cellular model of Parkinson’s disease,
resveratrol provides a protective effect via increased autophagy through and AMPK
and SIRT1 dependent mechanism [99]. However,
resveratrol can also inhibit autophagy in a number of settings both in
vitro and in vivo [100,101]. Although the mechanism
accounting for these discrepancies is not yet clear, it may be that resveratrol
stimulates opposing pathways, since the activation of autophagy is thought to
proceed via SIRT1 [99], while inhibition
appears to be due to suppression of signaling through S6K1 [100]. Therefore, the balance
of resveratrol’s effects
on these two enzymes may determine whether it activates or inhibits autophagy in a
given cell type.
Caloric restriction (CR), a reduction in energy intake in the absence of
malnutrition, extends mammalian lifespan [102,103] and decreases mTORC1
signaling in multiple tissues including liver, mammary tissues, and epithelia [104–106].
In yeast, replicative lifespan extension induced by rapamycin or
interference with the TOR pathway is not additive with CR (glucose restriction),
suggesting a common mechanism [78]. One of
the hallmarks of CR in mammals, including humans, is a reduction in blood glucose
and insulin levels, and increased insulin sensitivity [107]. Many long-lived mouse
models, including the S6K null
mouse, share these phenotypes, which have been postulated to contribute to lifespan
extension [108,109] although mice lacking the insulin receptor substrate
proteins IRS1 or IRS2 prove this is not a universal rule [110,111]. Conversely,
mice fed a high-fat diet become glucose intolerant, insulin-resistant, and have a
decreased lifespan [5]. These correlations
make it somewhat surprising that rodents treated with rapamycin, despite extended
lifespan, exhibit impaired glucose tolerance and insulin resistance, caused in part
by increased hepatic gluconeogenesis [112,113]. In fact, humans
treated clinically with rapamycin as an immunosuppressant have decreased insulin
sensitivity and an increased incidence of type 2 diabetes [114,115]. These
findings suggest that either rapamycin and CR work through distinct mechanisms, or
that lifespan extension by CR is not directly related to improvements in insulin
sensitivity.
While acute treatment with rapamycin specifically inhibits mTORC1 signaling,
we have found that chronic rapamycin exposure over the course of 24–48 hours
can inhibit mTORC2 in some cultured cell lines (Figure
2) possibly by preventing the assembly of new complexes [116]. We have recently demonstrated
that this
also occurs in vivo in each of the tissues that we have tested
(liver, skeletal muscle, and white adipose), at the same doses that extend lifespan,
within two weeks of treatment [117]. We have
found that mTORC2 disruption is a major cause of insulin resistance induced by
chronic rapamycin treatment in vivo. Therefore, at least some
detrimental effects of rapamycin might be separable from mTORC1-dependent lifespan
extension. This was exemplified by our recent finding that female mice heterozygous
for both mTOR and mLST8 have reduced activity of mTORC1, but not mTORC2, and have
increased longevity with normal glucose tolerance [117]. This strongly suggests that
specific inhibitors of mTORC1
signaling may provide some of the benefits of rapamycin with respect to health and
longevity, while avoiding sideeffects caused by inhibition of mTORC2. Interestingly,
a number of the FDA-approved compounds identified by Balgi et al. [94] as inducers
of autophagy that act via
inhibition of mTORC1 do not appear to inhibit signaling to mTORC2, and thus may be
interesting candidates to pursue in this regard.
The effect of mTOR and mLST8 heterozygosity on longevity was not observed in
males, even though rapamycin extends life in both genders. Interestingly, rapamycin
treatment has a stronger effect on lifespan in females than males [7]. Moreover, lifespan
extension resulting from
deletion of the mTORC1 substrate S6K1 is also specific to females [51] raising the
possibility that lifespan
extension by rapamycin in males is due to a separate mechanism. Soukas et al.
recently showed that mTORC2 disruption confers lifespan extension in C.
elegans fed a nutrient-rich diet, although it has the opposite effect
under standard conditions [118]. In
addition, inhibition of mTORC2 may contribute to the tumor-suppressive effects of
rapamycin in humans [119]. Therefore,
despite its negative effects on metabolism, mTORC2 disruption might also contribute
to the overall improvement in longevity in rapamycin-treated mice. Treatment of
humans with rapamycin has various other side effects, including altered testosterone
and luteinizing hormone levels and, in males, decreased sperm production [120]. Gaining
a more mechanistic understanding
of the beneficial and detrimental effects of rapamycin should be a high priority for
the aging research community, given its success at extending life in mice and its
possible unsuitability for sustained use in healthy humans.
Metformin
Metformin is an oral anti-diabetic drug that has been FDA approved since
1995, and is the consensus choice of the American Diabetes Association and the
European Association for the Study of Diabetes as the initial pharmacologic therapy
for hyperglycemia in type 2 diabetes [121].
Treatment with metformin lowers blood glucose levels, inhibits lipolysis and
decreases circulating free fatty acids, while producing few undesired side effects
[122]. Although metformin may have a
protective effect on β-cells, it does not directly affect insulin secretion
[123]. Instead, metformin treatment
results in increased insulin sensitivity in the liver and muscle, resulting in
decreased hepatic gluconeogenesis and increased peripheral utilization of glucose
[122]. Metformin is also effective in
preventing high-risk individuals from developing type-2- diabetes [124]. Long-term
follow-up of patients from
studies including the UK Prospective Diabetes Study has shown that the treatment of
diabetic patients with metformin decreased mortality from all causes, including
diabetes-related mortality, cancer, and myocardial infarction [125,126].
The molecular mechanism(s) underlying the actions of metformin have been
difficult to pin down [127]. A number of
antidiabetic effects have been reported, including activation of the insulin
receptor [128] and stimulation of the
incretin axis [129] but most attention has
focused on metformin’s role as an activator of AMPK [130]. As a central regulator
of energy balance within the
cell, activated AMPK induces a complex series of changes that result in an overall
decrease in anabolic processes and enhancement of catabolic processes to restore ATP
levels. While it is undisputed that metformin activates AMPK in
vivo, metformin does not activate AMPK in vitro,
demonstrating that the mechanism of action is indirect. Accumulating evidence
suggests that AMPK activation during metformin treatment may, in fact, be secondary
to direct energetic stress, caused by inhibition of complex I of the mitochondrial
respiratory chain and a subsequent fall in the ADP:ATP ratio with an accompanying
rise in AMP [131,132]. Moreover, inhibition of hepatic gluconeogenesis, which
is perhaps the most clinically relevant effect of metformin, appears to be mediated
by energetic stress through an AMPK-independent mechanism [133]. Therefore, AMPK activation
may well be a bystander in
many of metformin’s effects, rather than a central player in mediating
improvements in metabolism. Interestingly, inhibition of complex I is not observed
when metformin is supplied directly to isolated mitochondria, but the effect is
restored when metformin is supplied in lysosomal form, suggesting that a
membrane-mediated event is required [134].
Elucidating this mechanism, and the downstream events that lead to suppression of
hepatic glucose output, will be major challenges for the field in the coming years.
(Figure 3)
Importantly, treatment with metformin inhibits the mammalian target of
rapamycin (mTOR) signaling pathway, resulting in decreased phosphorylation of the
mTOR complex I (mTORC1) substrates S6K1 and 4E-BP1 and decreased translation [135].
These effects of metformin on mTORC1
activity were presumed to result entirely from activation of AMPK, since
AMPK-dependent phosphorylation activates TSC1/2, a repressor of mTORC1 activity
[136], and inhibits raptor, a component
of mTORC1 [137]. However, it was recently
demonstrated that metformin can regulate mTORC1 independently from AMPK via
induction of the mTORC1 inhibitor REDD1 [138], and separately, via regulation of the
Rag GTPases [139]. The Rag GTPases regulate the
localization of mTORC1 in response to amino acids, and are required for mTORC1
activity [140,141]. There is also evidence that metformin can regulate
autophagy, either through inhibition of mTORC1 signaling or via an AMPK-dependent
pathway. Metformin has been shown to activate autophagy in cardiac cells [142] as
well as in melanoma [143]. Together, these results suggest that the
effects of rapamycin and metformin may be partially overlapping.
It has also been proposed that metformin can suppress gluconeogenesis
through altering the balance of acetylation on key transcriptional regulators, which
is mediated largely by SIRT1 and GCN5 [144].
Here again, an AMPK-dependent mechanism is likely to contribute, since AMPK
activation enhances expression of nicotinamide phosphoribosyltransferase (Nampt),
and thereby increases the availability of NAD, a cosubstrate for SIRT1 [55]. However,
increases in GCN5 mRNA, and GCN5
and SIRT1 protein levels were all found to be independent of AMPK in this study
[144].
Based on its ability to reduce circulating glucose, insulin, and IGF-1, and
to disrupt signaling from the latter hormones to mTORC1, metformin has been
suggested as a potential CR mimetic and anti-aging compound. Transcriptional
profiling supports a significant overlap between the effects of metformin and CR
[145] and metformin extends both the
lifespan and healthspan of the nematode C. elegans [146]. In C. elegans, the
effects of metformin on lifespan are independent of the insulin signaling pathway,
but are dependent on AMPK and LKB1, as well as the oxidative stress transcription
factor Skn-1/Nrf2 [146]. Skn-1 [147] and AMPK [148,149] are also
required for certain CR regimens to extend the lifespan of C.
elegans, suggesting that CR and metformin may extend lifespan by
similar mechanisms in this organism. Interestingly, Skn-1/Nrf2 has also been
implicated in the lifespan extension induced by inhibition of translation, raising
the possibility that it might also be active during inhibition of the TOR signaling
pathway [150].
With respect to mammals, metformin was shown to extend the lifespan of
short-lived tumor-prone HER2/neu mice [151].
In addition, a study that looked at 50 control vs. 50 metformin-treated (100
mg/kg/day) female SHR outbred mice found a 91.9% increase in median
lifespan, a 37.8% increase in mean lifespan, and a 10.3% extension
in maximum lifespan [152]. Unfortunately,
the interpretation of this study with respect to aging is compromised by the tumor
susceptibility and short lifespan of SHR mice. In contrast, a recently completed
study on the metformin treatment of Fisher-344 rats (300 mg/kg/day) failed to show
increased lifespan [153]. The reason for the
disparity between this study and the study by Anisimov et al. [152] is unclear, but
could be related to the differences in
species (mice vs. rats), the higher dose of metformin used by Smith et al. [153],
or the tumor susceptibility of the
strains used by Anisimov et al. [152]. In a
mouse model of Huntington’s disease, it has been reported that while
treatment with a high dose of metformin (750 mg/kg/day) had no effect on survival,
a
lower dose (300 mg/kg/day) significantly extended lifespan [154]. A similar dose response
was seen in a recent C.
elegans study, in which a high dose of metformin (100mM) failed to
extend lifespan while a lower dose (50mM) succeeded [146]. The study by Smith et al.
[153] also had technical issues that make it difficult to
interpret the results. With respect to the biological activity of the metformin, the
treated rats in the Smith et al. [153] study
failed to displayed significantly altered glucose or insulin levels, which are
well-established consequences of metformin treatment. In addition, Smith et al.
[153] included a CR group as a control;
this group had a statistically insignificant increase in lifespan of only
8.7% on a 30% CR diet. Further, the CR rats had lower insulin levels
than the ad libitum controls at only one out of four time points. The authors of the
study propose that that the diet fed to these rats, NTP-2000, may explain some of
these discrepancies, but the failure of the CR diet to extend lifespan means that
the effects of metformin on longevity are not conclusive [153].
Given the potential of metformin treatment to recapitulate effects of
resveratrol, rapamycin, and CR, it will be extremely interesting to see whether it
can influence longevity in a long-lived strain of mice, such as C57BL/6. Unlike
resveratrol and rapamycin, the use of which in humans has been generally very
limited, metformin is regarded as a well-tolerated and affordable compound that is
currently being administered to millions of patients. Therefore, there is a major
opportunity to study the incidence of age-related diseases in patients who are
already taking metformin, which could provide valuable evidence as to its efficacy
for preventing age-related diseases prior to the initiation of lengthy and expensive
clinical trials.
Conclusion
Modulation of the rate of aging in mammals is achievable, as is routinely
demonstrated by studies of caloric restriction and genetic manipulation in rodent
models. Given the potential benefit in terms of amelioration of age-related
diseases, the search for pharmacologic means to mimic these effects should be a high
priority in biomedical research. As such, determining the mechanism by which
anti-aging compounds truly impact lifespan is of significant importance for
pharmaceutical development. While rapamycin, metformin, and resveratrol all exert
beneficial effects on health that could conceivably influence human longevity, the
molecular mechanisms by which they function remain unclear.
The compounds discussed in this review highlight the difficulties in
studying a complex phenotype such as aging. While some progress has been made, it
will take a concerted effort by many groups to assign definitive mechanisms and
refine approaches to translate the benefits seen in rodents to the human population.
Although this review has been focused on three specific compounds, there are many
other promising molecules whose mechanisms of action require further study, e.g.
spermidine [155] aspirin [156] l-deprenyl [157] oxaloacetate [158] and a variety of
antioxidant and anti-inflammatory compounds. In
particular, it will be important to design future studies that test combinations of
different approaches in order to determine whether a common downstream effect of
various treatments explains most changes in longevity, or if multiple factors
contribute to lifespan independently, and might have additive effects. Finally, as
is highlighted by the effects of resveratrol and metformin on murine lifespan, the
influence of factors such as diet and strain background must be carefully considered
when interpreting studies of longevity. While any molecule that measurably improves
health in aging mice potentially provides an exciting lead for the development of
human therapeutics, an intervention that truly slows the underlying aging process
should extend the maximum survival time of lean, healthy animals on a long-lived
strain background. Given the potential benefit to human health, identifying and
understanding genetic and pharmacologic interventions that pass this test should be
a major focus for biomedical research.