Genetic down-regulation of a major nutrient-sensing pathway, TOR (target of rapamycin) signalling, can improve health and extend lifespan in evolutionarily distant organisms such as yeast and mammals. Recently, it has been demonstrated that treatment with a pharmacological inhibitor of the TOR pathway, rapamycin, can replicate those findings and improve aging in a variety of model organisms. The proposed underlying anti-aging mechanisms are down-regulated translation, increased autophagy, altered metabolism and increased stress resistance.
- target of rapamycin (TOR)
Despite the complexity of the aging process, single gene mutations suffice to delay the onset of age-related phenotypes in a variety of model organisms, from yeast to mammals. Importantly, these long-lived mutants are healthier and protected from a broad spectrum of aging-related diseases. Remarkably, the processes mediating lifespan extension are evolutionary conserved, suggesting that discoveries from model organisms could be applied to ameliorate aging and prevent diseases in humans.
The most robust and well-studied improvements in aging are caused by dietary restriction, a reduction of nutrients without malnutrition, and also by genetic down-regulation of nutrient-sensing pathways, namely TOR (target-of-rapamycin) and IIS (insulin/insulin-like growth factor signalling) [1,2]. However, since both dietary restriction and genetic inhibition of IIS and TOR signalling are difficult to translate directly into human studies, aging research has recently focused on pharmacological approaches to target these pathways.
The present review focuses on the TOR pathway and its pharmacological inhibitor rapamycin, with the aim of outlining downstream processes implicated in aging.
The TOR pathway
The TOR pathway is an evolutionarily conserved nutrient-sensing pathway that adjusts metabolism and growth to amino acid availability, growth factors, energy status and stress [3,4]. The central component of the TOR pathway, the TOR kinase, forms two functionally different complexes: TORC (TOR complex) 1 and TORC2. When activated, TORC1 promotes growth by positively regulating S6K (S6 kinase)  and favours cap-dependent translation through inhibitory phosphorylation of 4E-BP (eukaryotic initiation factor 4E-binding protein) . Under starvation conditions, TORC1 is responsible for up-regulating autophagy, a process that generates energy by degrading portions of the cytoplasm [7,8]. Less is known about TORC2, whose principal outputs are to regulate the actin cytoskeleton and to phosphorylate and activate the pro-survival Akt kinase of the IIS pathway [9,10]. Once activated, Akt phosphorylates and inhibits PRAS40 (proline-rich Akt substrate of 40 kDa) and TSC2 (tuberous sclerosis protein 2), the latter leading to activation of the GTPase Rheb, thus causing TOR signalling up-regulation. In contrast with TORC2, TORC1 acts as an inhibitor of IIS through negative feedback of S6K on IRS (insulin receptor substrate) . Owing to the intricate wiring between IIS and TOR signalling, these pathways should be regarded as a network (summarized in Figure 1).
Genetic and pharmacological down-regulation of the TOR pathway
Genes in the TOR pathway that are implicated in longevity
Genetic down-regulation of TOR signalling delays aging in evolutionarily distant organisms from yeast to mammals . For example, both chronological and replicative lifespans are extended in the yeast tor1Δ mutant (reviewed in ). Nematode worms mutated in TOR (let-363) or S6K (rsks-1)  are long-lived, as are fruitflies overexpressing either a dominant-negative form of S6K or negative regulators of the TOR pathway, TSC1 (tuberous sclerosis complex 1) or TSC2 . Importantly, mice carrying a deletion in S6K1 have lifespan extension and amelioration of health, including improved glucose homoeostasis, immune cell profile and motor activities . Overall, TOR signalling plays an important and robust role in modulating aging.
Pharmacological inhibition of the TOR pathway by rapamycin
As an alternative to genetic interventions, the TORC1 branch of the TOR pathway can be successfully inhibited pharmacologically by rapamycin . As the name suggests, it was the search for the target of this anti-fungal and immunosuppressant agent that led to the discovery of the TOR pathway . Rapamycin and its analogues are already approved for human use as immunosuppressants and for advanced renal carcinoma [16,17].
Importantly, rapamycin treatment reproduces the longevity effects of the genetic TOR pathway mutants, extending lifespan in yeast  and in fruitflies . Excitingly, 600-day-old mice treated with rapamycin also had lifespan extension . Therefore pharmacological interventions initiated even in middle age can be beneficial for aging. However, despite its anti-cancer properties, no decrease in tumour incidence was observed for the rapamycin-treated mice in this study, although exposure to rapamycin at an earlier age or a different dosage might possibly have shown a more pronounced anti-cancer outcome.
In addition, rapamycin is effective in the prevention of various neurodegenerative diseases, showing protection and reducing pathology in fruitfly and mouse models of Parkinson's, Huntington's and Alzheimer's disease (reviewed in ). There is also evidence that rapamycin can restore haemopoietic stem cell function  and rescue cellular senescence caused by the DNA-damaging agent doxorubicin . Overall, this suggests that rapamycin could potentially be envisaged as a treatment to improve human health, the main drawback being its side effects such as hyperlipidaemia, although this is reversible, treatable and dose-dependent .
An additional cautionary note is rapamycin's effect on the immune system. Despite being frequently considered a potent immunosuppressant used in organ transplant patients, recent reports describe rapamycin as a modulator of the immune system. This is due to its opposing roles on different immune cells: rapamycin inhibits T-cell proliferation, but promotes pro-inflammatory responses in monocytes, macrophages and peripheral myeloid dendritic cells, the latter shown to protect against bacterial infections [24,25]. On the other hand, the susceptibility to viral infection increases upon rapamycin treatment, because of its negative effect on genes encoding interferons . A further complex effect of rapamycin on immunity is its enhancement of immune memory .
The immune response is clearly implicated in aging , and rapamycin modulates the immune system by affecting both innate and adaptive immune response in mammals. However, in Drosophila at least, the main anti-aging mechanism of rapamycin seems to be independent of immunity, requiring alterations in autophagy and translation . It should be noted that studies with laboratory mice are performed in a pathogen-free environment, and most mouse strains are devoid of severe cardiovascular problems, which may be caused by hyperlipidaemia in humans. Therefore it remains to be established whether the side effects of rapamycin represent a valid obstacle for its usage in human aging studies, where long-term administration would be needed. It may be that very low rapamycin doses could be effective in ameliorating health and preventing age-related disease, while minimizing side effects. Alternatively, other drugs, for instance targeting either both TORC1 and TORC2, or a specific downstream effector of the TOR pathway, might prove more effective for health improvement .
TOR signalling processes that slow aging
Alteration of TOR signalling results in changes to many cellular processes, such as translation, ribosomal biogenesis, autophagy, mitochondrial activity, lipid biosynthesis and glycolysis [3,29]. Because of these numerous outputs, the mechanisms responsible for TOR-mediated lifespan extension await elucidation. Separating the anti-aging processes from the negative side effects of TOR down-regulation may provide valuable drug targets for more robust health improvements compared with inhibition of the entire TOR pathway. Some of the potential anti-aging processes linked to the TOR pathway are described below.
Down-regulation of translation
Regulation of translation is among the best described functions of the TOR pathway, exerted mainly by TORC1 through activating phosphorylation of S6K and inhibitory phosphorylation of 4E-BP . Lowering protein synthesis in yeast, nematode worms and fruitflies promotes longevity: mutants having decreased expression of translation initiation factors and ribosomal proteins, as well as S6K mutants, are all long-lived [11–13,30]. Lowering global translation may be beneficial, by creating less burden for the protein repair and degradation machinery, leading to improved protein quality and fewer protein aberrations. Alternatively or additionally, TOR inhibition may result in the preferential translation of a specific set of proteins compared with translation under favourable growth conditions, and these proteins may be responsible for longevity. For instance, 4E-BP activation favours translation of mRNAs that do not possess complex secondary structures in their 5′-UTRs (5′-untranslated regions) [6,31]. In Drosophila under dietary restriction when 4E-BP is derepressed, there is enrichment in mRNAs of nuclear-encoded mitochondrial genes, such as those for the electron transport chain complexes, which are implicated in longevity . Furthermore, in yeast, depletion of the 60S ribosomal subunit leads to the reduction in protein synthesis and extension of lifespan by specifically promoting translation of the mRNA encoding the nutrient-responsive transcription factor Gcn4 (general control non-derepressible 4), via a mechanism of short upstream ORFs (open reading frames) . Overall, complete understanding of how decreased translation governs lifespan extension and improved health will be crucial for the unravelling the aging process.
Up-regulation of autophagy
Autophagy is up-regulated when nutrients are low and signalling through the TOR pathway diminishes. Autophagy constitutes a major cellular mechanism that enables survival under starvation, accomplished by lysosome-mediated degradation of a portion of the cytoplasm captured within the autophagosome. Thereby the cell is replenished with building blocks such as amino acids and sources of energy to produce ATP. In addition, autophagy removes damaged cellular components and is the only known mechanism for degrading damaged mitochondria. Entire bacteria can also be engulfed, thus contributing to cellular immunity . Autophagy is necessary for the lifespan-extension of daf-2 (abnormal dauer formation 2) insulin-receptor mutant nematode worms , of TOR pathway mutants and by dietary restriction  as well as for rapamycin-treated fruitflies . Although up-regulation of autophagy is important for longevity, presumably through the removal of damage and generation of energy, increased autophagy in nematode worms does not seem to be sufficient for lifespan extension, because a mutation in the forkhead transcription factor daf-16 [abnormal dauer formation 16; FOXO (forkhead box O)], the main downstream effector of the IIS pathway, blocks daf-2 longevity, but does not reduce autophagy levels . In contrast, in fruitflies, up-regulation of autophagy by overexpression of the autophagy protein Atg8 is beneficial for lifespan and it also decreases levels of protein damage . Lack of more extensive genetic evidence that an increase in autophagy alone has anti-aging properties might be because autophagy is also a degradative process associated with cell death. Hence, although small increases could be beneficial, extensive enhancement of autophagy may be detrimental. It should be mentioned that drugs that increase autophagy, such as rapamycin and spermidine, promote longevity .
Importantly, autophagy is proposed as a potential target in treatments of neurodegeneration and cancer . Increased autophagy is protective against neurodegenerative diseases by the removal of toxic aggregates. However, the situation with cancer is complex, and whether autophagy should be enhanced or inhibited depends on the tumour stage and type . In addition, autophagy has been implicated in a variety of cellular processes, such as lipid metabolism genomic DNA stability, degradation of ribosomes, presentation of antigens to T-lymphocytes and programmed cell death (reviewed in ). Thus autophagy plays a critical, but complex, role among anti-aging processes downstream of TORC1.
Effects of the TOR pathway on metabolism
The TORC1 branch of the pathway positively regulates glycolysis, the pentose phosphate pathway and lipid metabolism by acting on HIF-1 (hypoxia-inducible factor 1) and SREBP (sterol-regulatory-element-binding protein) [29,36]. Interestingly, lipid metabolism is important for longevity in Caenorhabditis elegans, where germline stem cells promote lifespan extension by up-regulating a specific lipase in the intestine, which then leads to lipid hydrolysis and longevity . In contrast, cancer cells depend on lipid biosynthesis, and its inhibition by down-regulation of fatty acid synthase reduces their tumorigenic phenotype in vitro and in vivo .
Whole-body knockout mice for any component of TORC1 or TORC2 are embryonic lethal. Therefore most progress in understanding the role of TOR signalling in metabolism has been made with tissue-specific knockout mice, for instance targeting raptor (regulatory associated protein of TOR), which is a specific and essential component of TORC1. Interestingly, raptor knockout can either improve or damage glucose homoeostasis and mitochondrial function, depending on whether it is deleted in adipose tissue or muscles respectively (reviewed in ). Presumably, this tissue specificity is partly a consequence of differences in raptor-interacting proteins present in those tissues. Whole-body knockouts of TORC1 downstream targets are also informative, such as mice carrying deletions in the TORC1 negative effectors 4E-BP1 and 4E-BP2, causing insulin resistance and augmented diet-induced obesity . On the other hand, S6K1-null mice are long-lived, lean, resistant to diet-induced obesity, and have improved glucose tolerance and insulin sensitivity in old age [15,39,41].
Mitochondria have long been considered one of the important determinants of the aging process, because they generate energy but can also induce damage via ROS (reactive oxygen species) production . As the TOR pathway regulates both mitochondrial activity  and removal of damaged mitochondria via autophagy , this may underlie some of its anti-aging mechanisms.
Collectively, which energy store is used and how this energy is extracted seem to contribute towards health and longevity, with low TOR signalling providing the healthiest balance.
Long-lived mutants are commonly resistant to various stresses, such as starvation, oxidative stress and xenobiotics, and this is believed to contribute to their health and longevity [43,44]. The TOR pathway integrates various stresses, such as hypoxia signals from the transcription factors HIF-1, REDD (regulated in development and DNA damage responses) 1 and 2, and cellular energy levels from AMPK (AMP-activated protein kinase) . The output of the TOR pathway upon a given stress depends on the cell type, the amount of stress and cell environment (reviewed in [4,45]).
Concerning aging, in C. elegans, both down-regulation and up-regulation of HIF-1 are implicated in longevity (reviewed in ) and overexpression of the catalytic subunit of the AMPK extends lifespan . Furthermore, recently discovered stress-responsive proteins, sestrins, keep TOR signalling in check by down-regulating it when it is in excess . Interestingly, sestrin-mediated inhibition of the TOR pathway, upon the presence of ROS and genotoxic stress, results in the delayed onset of various pathologies in Drosophila . In general, low TOR signalling facilitates damage repair, promotes stress resistance and extends lifespan .
TOR pathway and related anti-aging interventions, IIS and dietary restriction
Because of its role in regulating gene expression under low-nutrient conditions, the TOR pathway has emerged as the prime candidate to mediate extension of lifespan by dietary restriction. Supporting this hypothesis, the lifespan of TOR mutants in nematode worms  and fruitflies  cannot be extended further by dietary restriction. It should be noted that, rather than a cut in caloric intake, it has been suggested that lifespan extension by dietary restriction is mediated by a reduction in amino acids [48,49]. Recent compelling evidence reveals that amino acid levels are communicated to TORC1 by Rag GTPases , making them a potential druggable target that might mimic the benefits of dietary restriction without the difficulties of reduced food intake. In addition, the liver transcription profiles of long-lived S6K-null mice and mice under dietary restriction are strikingly similar . In this respect, it has been suggested that the TORC1 inhibitor rapamycin might act as a dietary restriction mimetic.
Besides dietary restriction and TOR signalling, IIS is another well-established evolutionarily conserved process that is implicated in aging from C. elegans to mice . Importantly, all these interventions are interconnected and, as mentioned above, IIS and TOR signalling cross-talk at several levels . Interestingly, the FOXO transcription factor is responsible for the longevity of the mutants with down-regulated IIS, but it is not required for the lifespan extension of TOR mutants . This argues that the TOR and IIS pathways have different anti-aging downstream targets. Nevertheless, double mutants affected in both TOR and IIS do not have additive effects on lifespan [13,18]. Perhaps carefully combining anti-aging downstream targets, thus avoiding negative outputs of these two nutrient-sensing pathways, may result in additive beneficial effects on health and longevity.
Remarkable progress has been made in aging research over the past decade, firmly demonstrating that the main factors responsible for modulating lifespan are the evolutionarily conserved nutrient-sensing TOR and IIS network. Recent pharmacological approaches to translate these findings and treat age-related diseases in humans have proven successful, and rapamycin is the first drug that improves longevity from yeast to mice. Anti-aging outputs of the TOR pathway include autophagy, translation, stress responses, mitochondrial regulation and metabolism. Since the TOR pathway plays an essential role in numerous key cellular processes, its deregulation is implicated in various diseases such as cancer, diabetes and neurodegeneration. Although these diseases have diverse aetiologies and current treatment strategies, lessening TOR signalling may ameliorate health in all instances by improving stress resistance and by carefully adjusting metabolism. Thus drugs targeting the TOR pathway may have broad-spectrum preventive effects for the different diseases of old age.
This work was supported by Age UK, The Wellcome Trust and The Max Planck Society.
We thank Helena Cochemé, Matthew Piper and Jorge Ivan Castillo Quan for a critical reading of the paper. We are grateful to Helena Cochemé for improving the Figure.
mTOR Signalling in Health and Disease: A Biochemical Society Focused Meeting held at Charles Darwin House, London, U.K., 11–12 November 2010. Organized and Edited by Ivan Gout (University College London, U.K.), Christopher Proud (Southampton, U.K.) and Michael Seckl (Imperial College London, U.K.).
Abbreviations: AMPK, AMP-activated protein kinase; daf-2, abnormal dauer formation 2; 4E-BP, eukaryotic initiation factor 4E-binding protein; FOXO, forkhead box O; HIF-1, hypoxia-inducible factor 1; IIS, insulin/insulin-like growth factor signalling; ROS, reactive oxygen species; S6K, S6 kinase; TOR, target of rapamycin; raptor, regulatory associated protein of TOR; TORC1, TOR complex 1; TORC2, TOR complex 2; TSC, tuberous sclerosis complex
- © The Authors Journal compilation © 2011 Biochemical Society