Introduction
At first glance, growth and aging appear
to be opposites. Growth is energy-driven synthesis of macromolecules from
simple nutrients, an increase of order and a decrease of entropy. Aging is
decay, a loss of order and a rise of entropy. Seemingly, growth and aging are
mutually exclusive. Forever proliferating cells, such as legendary hydras, do
not show signs of aging. In contrast, when an organism ceases to grow, aging
follows. However, manipulations that decrease growth also decrease aging and
prolong life span. For example, calorie restriction (reduced nutrient intake)
reduces growth and increases longevity in diverse species from yeast to mice.
Rapamycin, which inhibits growth in yeast, decelerates yeast aging. Inactivation
of the growth-promoting insulin/IGF-1 signaling pathway increases life span,
from worms to mice. Why do growth-inhibiting conditions slow down aging? Are
growth and aging mechanistically similar? As
we discuss here, growth and aging may not be opposites but rather a
continuation of one another driven by the same molecular pathway. Aging and
growth may be linked in a way that growth produces aging. In
other words, excessive growth is a driving force for aging. The molecular
pathway that drives both growth and aging appears to be the evolutionarily
conserved TOR (target of rapamycin) pathway.
The TOR pathway
TOR (Target Of Rapamycin), as its name indicates, was
originally discovered, in yeast, as the target of the antifungal drug rapamycin.
Rapamycin is a natural secondary metabolite produced by soil bacteria to
inhibit growth of fungal competitors. Thus, it is a mirror image of penicillin
that is produced by fungi to inhibit bacterial growth. Remarkably, TOR is
structurally and functionally conserved from yeast to human (including worms,
flies, plants and mice) as an essential, central controller of cell growth [1].
TOR in mammals (mTOR) controls cell growth and metabolism in response to
nutrients (e.g., amino acids), growth factors (e.g., insulin, IGF-1, PDGF), and
cellular energy status (ATP). Nutrients are the dominant TOR input as high
levels of amino acids can compensate for an absence of the other mTOR inputs
but not vice versa [2], and only nutrients activate TOR in unicellular
organisms. The growth factor signaling pathway, grafted onto the more
ancestral nutrient sensitive TOR pathway, co-evolved with multicellularity.
TOR activates cell growth by positively and negatively regulating several
anabolic and catabolic processes, respectively, that collectively determine mass
accumulation. The anabolic processes include transcription, protein
synthesis, ribosome biogenesis, nutrient transport, and mitochondrial
metabolism. Conversely, TOR negatively regulates catabolic processes such as
mRNA degradation, ubiquitin-dependent proteolysis, and autophagy. TOR is an
atypical serine/threonine kinase that is found in two functionally and
structurally distinct multiprotein complexes, TORC1 and TORC2 (mTORC1 and
mTORC2 in mammals), each of which signals via a different set of effector
pathways. TORC1 is rapamycin sensitive whereas TORC2 is rapamycin
insensitive. The best-characterized phosphorylation substrates of mTOR are S6K
and 4E-BP1 via which mTORC1 controls translation, and Akt/PKB via which mTORC2
controls cell survival and other processes [3]. Like TOR itself, the two TOR
complexes and the overall architecture of the TOR signaling network appear to
be conserved from yeast to human [1,4]. TOR and many of the processes it
controls have also been shown to play a role in aging (in addition to growth)
in a wide variety of organisms, as described below.
Aging in budding yeast
Budding yeast S. cerevisiae is a particularly
useful model system to study aging because it can be used to study both
replicative aging and chronological aging. Replicative aging is measured by
the number of daughter cells (divisions) a mother cell can produce.
Chronological aging, also referred to as postmitotic aging, is measured by the
length of time a non-dividing cell can survive. Inhibition of TORC1 signaling
in yeast extends both replicative [5] and chronological [6] life span. Thus,
TOR appears to promote aging regardless of physiological context (mitotic or
post-mitotic cells).
In yeast, the link between growth and
aging has been known sinse the 1950's is particularly spectacular. Yeast cells
grow larger as they grow older [7,8]. The yeast S. cerevisiae
typically divides asymmetrically to give a large mother cell and a smaller
daughter cell. As mother cells become old, they enlarge and produce daughter
cells that are larger than daughters derived from young mother cells. Like
large mothers, large daughter cells have shorter replicative life span [8].
The use of unicellular yeast to study aging is
revealing also because here the cell is the organism. Therefore, it is a model
for cell aging and organismal longevity. As we discuss below, TOR also
controls longevity in multicellular organisms.
TOR and aging from worm to mammals
Inhibition of TOR signaling enhances life span in
worms, flies and possibly mammals. The nematode C. elegans contains a
constant number of post-mitotic cells and lives about twenty days. The
first demonstration that TOR controls life span was by Vellai et al [9] who
showed that knocking down TOR in -C. elegans more than doubles this
worm's normal life span. They examined specifically the role of TOR in aging
based on the prior knowledge that both TOR and life span are regulated by
nutrients and insulin. Indeed, a large body of earlier, groundbreaking work
showing that calorie restriction [10,11] or down regulation of insulin/IGF-1
signaling [12,13] extends life span is consistent with the observation that
down regulation of TOR also extends life span. Interestingly, inhibition of
TOR starting either during development or on the first day of adulthood gives
comparable life span extension, suggesting that TOR controls longevity mainly,
if not exclusively, during adulthood [9]. Jia et al. [14] subsequently
demonstrated that worms deficient in raptor, a TORC1-specific subunit, also
have an extended life span, indicating that TOR promotes aging via at least
TORC1, if not via both TORCs.
In Drosophila, TOR is required for growth
during larval development, and for increases in cellular growth caused by
growth factor signaling and nutrient availability [15]. Genetic inhibition of
the Drosophila TOR pathway, either upstream or downstream of TOR,
extends life span [16,17]. Furthermore, reducing the function of Drosophila
TOR results in decreased lipid stores and glucose levels, and prevents
metabolic syndrome [18]. The life span extension is observed upon down
regulation of TOR signaling in the fat body [17], underscoring the importance
of fat in aging [19-21,12]. Downregulating TOR signaling in the fat body not
only extends the life of the fly, it also reduces the size of the entire
organism [22], providing yet another link between growth and aging. The
apparently central role of TOR signaling in the fly fat body in regulating life
span may be recapitulated by mammalian TOR in adipose tissue. In mice,
decreased insulin/IGF-1 signaling in adipose tissue, and consequently less
active downstream mTOR signaling, increases life span [23,20]. Furthermore,
adult-onset growth hormone or IGF-1 deficiency increases life span in rodents
[24]. Also, mice deficient for mTORC1 or the mTORC1 effector S6K are protected
against age- and diet-induced obesity [25,26]. Rapamycin is currently being
tested for its ability to extend lifespan in mice in the National Interventions
Testing Program.
Finally, calorie restriction, in which nutrient intake
is restricted to 60-70% that of voluntary levels, increases life span in most
species including mammals [10-12]. Although anti-aging mechanisms of CR are
still disputed, one of the mechanisms is likely inactivation of the TOR
pathway. Indeed, taking into account that i) inhibition of TOR extends life
span and ii) nutrients activate TOR, the mechanism of how restriction of
nutrients can increase life span seems apparent.
Downstream of TOR
How does TOR promote aging in response to nutrients?
In other words, which of the many TORC1-controlled processes that are either up
regulated or down regulated upon nutrient deprivation (TORC1 inactivation)
leads to longer life? Recent evidence suggests that TORC1 controls aging via
several of its downstream processes, including autophagy, ribosome biogenesis
and protein synthesis, transcription, and mitochondrial activity. Indeed,
there is a remarkable correlation between TOR-controlled processes and
processes in aging. It is also important to note that these processes
constitute the normal program via which TOR controls cell growth, suggesting
that TOR control of aging is an extension or continuation of its control of
growth.
TORC1 inhibits autophagy, a process of bulk
degradation of proteins and organelles by lysosomes [27]. Autophagy is
inhibited in aging and age-related diseases [28]. Restoration of autophagy
depletes mitochondria with deleterious mtDNA mutations but spares their normal
counterparts [29]. Furthermore, autophagy is essential for life span extension
at least in worms [30]. This suggests that TORC1 promotes aging in part via
inhibition of autophagy.
TORC1 activates ribosome biogenesis and protein
synthesis. Recent studies show that inhibition of ribosome biogenesis and
global protein synthesis extends life span [31-34]. Reducing the levels of
ribosomal proteins and translation initiation factors extends life span in both
yeast and worms. Thus, this is consistent with the notion that TORC1 may
promote aging via activation of ribosome biogenesis and protein synthesis.
TORC1 in yeast negatively regulates the
stress-activated transcription factors GIS1 and MSN2/4. Both transcription
factors are required for life span extension upon down regulation of TOR [35,36]. A longevity-related gene up regulated by MSN2/4 upon TOR inhibition is the
nicotinamidase gene PCN1. Interestingly, nicotinamidase converts
nicotinamide to NAD+ which in turn activates SIR2, suggesting that
TOR and sirtuins are part of the same longevity pathway [35]. Furthermore, as
discussed below, TOR negatively regulates mitochondrial gene expression to
limit life span [37].
TORC1 controls mitochondrial activity, but in
different ways depending on the organism. In yeast, TORC1 inhibits
mitochondrial respiration whereas in mammals (at least in muscle) it stimulates
respiration [37-42]. This divergence in regulation is probably related to the
fact that glucose, a nutrient sensed by TORC1, triggers anaerobic fermentation
in yeast. A similar glucose-dependent shift in respiration does not occur in
mammals. Consistent with the above, increased mitochondrial respiration
extends life span in yeast whereas in mammalian cells life span extension
correlates with reduced respiration [37,39,43]. However, the role of
mitochondria in life span extension remains elusive, particularly with the
recent demonstration that TORC1 in mammalian adipose tissue, like in yeast,
negatively controls respiration [26].
Accumulation of aggregation-prone proteins is involved
in neurodegeneration. TOR causes neurodegeneration in a Drosophila
tauopathy model [44]. The TOR pathway is involved in Alzheimer's disease by
increasing Tau protein synthesis [45]. Furthermore, rapamycin enhances
clearance of pathologic proteins and thereby reduces their toxicity [46].
As we discuss below, overactive TOR seems to be
involved in the hypertrophic phenotype of aging mammalian cells, thus linking
TOR mediated cell hypertrophy to organismal aging. In contrast, a replicative
limit has never been shown to be important in vivo [47]. It is a
hyperthropic, secretory phenotype of aging cells that can be linked to organismal
aging [48-50].
Hypertrophic phenotype of aging cells
If growth and aging are mechanistically linked, are
older cells larger? In yeast, old cells are large and cell size predicts
replicative life span [51,52]. This also appears to be the case for senescent
mammalian cells. An increase in cell size is a hallmark of senescent
fibroblasts [53]. Their cell volume is several fold greater compared with
proliferating cells. Cell size is progressively increased in cell culture as
cells progress toward senescence [54-56]. Furthermore, it was suggested 20
years ago that cell size is a marker of cell senescence [54,57]. Ironically,
TOR had not been discovered at that time and the significance of this
phenomenological observation was unclear. The notion that TOR is involved in
both growth and aging now provides a mechanistic explanation for an old
observation.
Cell growth is an increase in cell volume, or mass,
due to metabolic activity including synthesis of macromolecules (RNA, protein,
lipid) and organelles. If a cell grows without division, it becomes hypertrophic.
In other words, when the cell cycle is blocked in the presence of
growth-promoting signaling, then cells increase in size [56,58,59].
Thus, cell growth is counterbalanced by cell division
such that cells maintain a characteristic size. The simplest way to cause both
cell hypertrophy and cell senescence is to prevent cell division without
inhibiting cell growth. Inhibition of mTOR with rapamycin decreases the
hypertrophic cell phenotype caused by induction of the CDK inhibitor p21
[58-60].
All these observations suggest that mTOR signaling
plays a role in aging of single cells. How is this related to aging of
multicellular organisms? As discussed elsewhere [1,61], TOR-driven alterations
can be linked to metazoan aging and, in particular, diseases of aging such as
cancer, metabolic syndrome, atherosclerosis, hypertension and hypertrophic
heart.
Rapamycin in humans
Rapamycin is given to renal transplant patients
everyday for several years to prevent organ rejection. We view this as an
unintentional clinical trial of a potentially anti-aging drug. First, in such
patients, rapamycin unexpectedly turned out to prevent cancer [62-64] and even
cured some types of pre-existing tumors [65,66]. Second, 2 years after
transplantation, body-mass index was significantly lower in the rapamycin-based
treatment arm compared to cyclosporine, indicating that rapamycin prevents obesity
[67].
Rapamycin is safe enough to be used in
healthy volunteers to study its pharmacokinetics [68-70]. In healthy
volunteers, a single dose was not associated with side effects. In 11 healthy
men (29 years old, BMI 23 kg/m2), 6 mg of rapamycin decreased
S6K phosphorylation, preventing insulin resistance caused by nutrients. Thus,
the activity state of the mTOR pathway can modulate insulin sensitivity in
humans and mTOR inhibitors prevent nutrient-induced insulin resistance [70].
Why TOR?
Cell growth and division are the two most fundamental
features of life. Using simple compounds and energy, living organisms build
macromolecules according to their own plan, transforming non-self to self. Not
surprisingly, the growth-controlling TOR signaling pathway is conserved from
yeast to human. In unicellular organisms, it maximizes growth as long as
nutrients are available. However, life-promoting TOR signaling seems also to
contain seeds of death. Aging and its manifestations such as age-related
diseases appear with excessive growth-promoting signaling, when actual growth
is not longer possible. Aging is not programmed, of course, but is an aimless
continuation of the same process that drives developmental growth. Since aging
does not limit life span in the wild, switch-off of this ‘growth program'
cannot evolve. Growth should be robust and not be slowed down to avoid aging.
Furthermore, the aging-growth program cannot be switched off by an accidental
mutation, because such a mutation would be lethal or at least reduce fitness
during development. Yet, TOR can be inhibited pharmacologically.
M.N.H. acknowledges support from the Swiss National
Science Foundation and the Canton of Basel.