Paradoxically, GH and IGF-1 levels decline during normal aging, as well as in mouse models of premature aging (Schumacher et al., 2008). Thus, a decreased IIS is a common characteristic of both physiological and accelerated aging, whereas a constitutively decreased IIS extends longevity. These apparently contradictory observations can be accommodated under a unifying model by which IIS downmodulation reflects a defensive response aimed at minimizing cell growth and metabolism in the context of systemic damage (Garinis et al., 2008). According to this view, organisms with a constitutively decreased IIS can survive longer because they have lower rates of cell growth and metabolism and, hence, lower rates of cellular damage. Along the same lines, physiologically or pathologically aged organisms decrease IIS in an attempt to extend their lifespan. However, and this is a concept that will recur in the following sections, defensive responses against aging may have the risk of eventually becoming deleterious and aggravating aging. Thus, extremely low levels of IIS signaling are incompatible with life, as exemplified by mouse null mutations in the PI3K or AKT kinases that are embryonic lethal (Renner and Carnero, 2009). Also, there are cases of progeroid mice with very low levels of IGF-1, in which supplementation of IGF-1 can ameliorate premature aging (Mariño et al., 2010).

Other Nutrient-Sensing Systems: mTOR, AMPK, and Sirtuins

In addition to the IIS pathway that participates in glucose sensing, three additional related and interconnected nutrient-sensing systems are the focus of intense investigation: mTOR, for the sensing of high amino acid concentrations; AMPK, which senses low-energy states by detecting high AMP levels; and sirtuins, which sense low-energy states by detecting high NAD+ levels (Houtkooper et al., 2010) (Figure 4A).
The mTOR kinase is part of two multiprotein complexes, mTORC1 and mTORC2, that regulate essentially all aspects of anabolic metabolism (Laplante and Sabatini, 2012). Genetic downregulation of mTORC1 activity in yeast, worms, and flies extends longevity and attenuates further longevity benefits from DR, suggesting that mTOR inhibition phenocopies DR (Johnson et al., 2013). In mice, treatment with rapamycin also extends longevity in what is considered to be the most robust chemical intervention to increase lifespan in mammals (Harrison et al., 2009). Genetically modified mice with low levels of mTORC1 activity but normal levels of mTORC2 have increased lifespan (Lamming et al., 2012), and mice deficient in S6K1 (a main mTORC1 substrate) are also long-lived (Selman et al., 2009). Therefore, the downregulation of mTORC1/S6K1 appears as the critical mediator of mammalian longevity in relation to mTOR. Moreover, mTOR activity increases during aging in mouse hypothalamic neurons, contributing to age-related obesity, which is reversed by direct infusion of rapamycin to the hypothalamus (Yang et al., 2012). These observations, together with those involving the IIS pathway, indicate that intense trophic and anabolic activity, signaled through the IIS or the mTORC1 pathways, are major accelerators of aging. Although inhibition of TOR activity clearly has beneficial effects during aging, it also has undesirable side effects, such as impaired wound healing, insulin resistance, cataracts, and testicular degeneration in mice (Wilkinson et al., 2012). It will thus be important to understand the mechanisms involved in order to determine the extent to which beneficial and damaging effects of TOR inhibition can be separated from each other.
The other two nutrient sensors, AMPK and sirtuins, act in the opposite direction to IIS and mTOR, meaning that they signal nutrient scarcity and catabolism instead of nutrient abundance and anabolism. Accordingly, their upregulation favors healthy aging. AMPK activation has multiple effects on metabolism and, remarkably, shuts off mTORC1 (Alers et al., 2012). There is evidence indicating that AMPK activation may mediate lifespan extension following metformin administration to worms and mice (Anisimov et al., 2011; Mair et al., 2011; Onken and Driscoll, 2010). The role of sirtuins in lifespan regulation has been discussed above (see “Epigenetic Alterations”). In addition, SIRT1 can deacetylate and activate the PPARγ coactivator 1α (PGC-1α) (Rodgers et al., 2005). PGC-1α orchestrates a complex metabolic response that includes mitochondriogenesis, enhanced antioxidant defenses, and improved fatty acid oxidation (Fernandez-Marcos and Auwerx, 2011). Moreover, SIRT1 and AMPK can engage in a positive feedback loop, thus connecting both sensors of low-energy states into a unified response (Price et al., 2012).

Overview

Collectively, current available evidence strongly supports the idea that anabolic signaling accelerates aging and decreased nutrient signaling extends longevity (Fontana et al., 2010). Further, a pharmacological manipulation that mimics a state of limited nutrient availability, such as rapamycin, can extend longevity in mice (Harrison et al., 2009).

Mitochondrial Dysfunction

As cells and organisms age, the efficacy of the respiratory chain tends to diminish, thus increasing electron leakage and reducing ATP generation (Green et al., 2011) (Figure 4B). The relation between mitochondrial dysfunction and aging has been long suspected, but dissecting its details remains as a major challenge for aging research.

Reactive Oxygen Species

The mitochondrial free radical theory of aging proposes that the progressive mitochondrial dysfunction that occurs with aging results in increased production of ROS, which in turn causes further mitochondrial deterioration and global cellular damage (Harman, 1965). Multiple data support a role for ROS in aging, but we focus here on the developments of the last 5 years, which have forced an intense re-evaluation of the mitochondrial free radical theory of aging (Hekimi et al., 2011). Of particular impact has been the unexpected observation that increased ROS may prolong lifespan in yeast and C. elegans (Doonan et al., 2008; Mesquita et al., 2010; Van Raamsdonk and Hekimi, 2009). Similarly unexpected, genetic manipulations in mice that increase mitochondrial ROS and oxidative damage do not accelerate aging (Van Remmen et al., 2003; Zhang et al., 2009), mice with increased antioxidant defenses do not present an extended lifespan (Pérez et al., 2009), and, finally, genetic manipulations that impair mitochondrial function but do not increase ROS accelerate aging (Edgar et al., 2009; Hiona et al., 2010; Kujoth et al., 2005; Trifunovic et al., 2004; Vermulst et al., 2008). These and other data have paved the way to a reconsideration of the role of ROS in aging (Ristow and Schmeisser, 2011). Indeed, parallel and separate to the work on the damaging effects of ROS, the field of intracellular signaling has accumulated solid evidence for the role of ROS in triggering proliferation and survival in response to physiological signals and stress conditions (Sena and Chandel, 2012). The two lines of evidence can be harmonized if ROS is regarded as a stress-elicited survival signal conceptually similar to AMP or NAD+ (see “Deregulated Nutrient Sensing”). In this sense, the primary effect of ROS will be the activation of compensatory homeostatic responses. As chronological age advances, cellular stress and damage increase and the levels of ROS increase in parallel in an attempt to maintain survival. Beyond a certain threshold, ROS levels betray their original homeostatic purpose and eventually aggravate, rather than alleviate, the age-associated damage (Hekimi et al., 2011). This new conceptual framework may accommodate apparently conflicting evidence regarding the positive, negative, or neutral effects of ROS on aging.

Mitochondrial Integrity and Biogenesis

Dysfunctional mitochondria can contribute to aging independently of ROS, as exemplified by studies with mice that are deficient in DNA polymerase γ (Edgar et al., 2009; Hiona et al., 2010) (see “Genomic Instability”). This could happen through a number of mechanisms; for example, mitochondrial deficiencies may affect apoptotic signaling by increasing the propensity of mitochondria to permeabilize in response to stress (Kroemer et al., 2007) and trigger inflammatory reactions by favoring ROS-mediated and/or permeabilization-facilitated activation of inflammasomes (Green et al., 2011). Also, mitochondrial dysfunction may directly impact cellular signaling and interorganellar crosstalk by affecting the interface between the outer mitochondrial membrane and the endoplasmic reticulum (Raffaello and Rizzuto, 2011).
The reduced efficiency of mitochondrial bioenergetics with aging may result from multiple converging mechanisms, including reduced biogenesis of mitochondria—for instance, as a consequence of telomere attrition in telomerase-deficient mice, with subsequent p53-mediated repression of PGC-1α and PGC-1β (Sahin and DePinho, 2012). This mitochondrial decline also occurs during physiological aging in wild-type mice and can be partially reversed by telomerase activation (Bernardes de Jesus et al., 2012). SIRT1 modulates mitochondrial biogenesis through a process involving the transcriptional coactivator PGC-1α (Rodgers et al., 2005) and the removal of damaged mitochondria by autophagy (Lee et al., 2008). SIRT3, which is the main mitochondrial deacetylase (Lombard et al., 2007), targets many enzymes involved in energy metabolism, including components of the respiratory chain, tricarboxylic acid cycle, ketogenesis, and fatty acid β-oxidation (Giralt and Villarroya, 2012). SIRT3 may also directly control the rate of ROS production by deacetylating manganese superoxide dismutase, a major mitochondrial antioxidant enzyme (Qiu et al., 2010; Tao et al., 2010). Collectively, these results support the idea that telomeres and sirtuins may control mitochondrial function and thus play a protective role against age-associated diseases.
Other mechanisms causing defective bioenergetics include accumulation of mutations and deletions in mtDNA, oxidation of mitochondrial proteins, destabilization of the macromolecular organization of respiratory chain (super)complexes, changes in the lipid composition of mitochondrial membranes, alterations in mitochondrial dynamics resulting from imbalance of fission and fusion events, and defective quality control by mitophagy, an organelle-specific form of macroautophagy that targets deficient mitochondria for proteolytic degradation (Wang and Klionsky, 2011). The combination of increased damage and reduced turnover in mitochondria, due to lower biogenesis and reduced clearance, may contribute to the aging process (Figure 4B).
Interestingly, endurance training and alternate-day fasting may improve healthspan through their capacity to avoid mitochondrial degeneration (Castello et al., 2011; Safdar et al., 2011). It is tempting to speculate that these beneficial effects are mediated, at least in part, through the induction of autophagy, for which both endurance training and fasting constitute potent triggers (Rubinsztein et al., 2011). However, autophagy induction is probably not the sole mechanism through which a healthy lifestyle can retard aging, as depending on the precise DR regime, additional longevity pathways can be activated (Kenyon, 2010).

Mitohormesis

Mitochondrial dysfunctions during aging are also connected with hormesis, a concept on which a number of aging research lines have recently converged (Calabrese et al., 2011). According to this concept, mild toxic treatments trigger beneficial compensatory responses that surpass the repair of the triggering damage and actually produce an improvement in cellular fitness when compared to the starting predamage conditions. Thus, although severe mitochondrial dysfunction is pathogenic, mild respiratory deficiencies may increase lifespan, perhaps due to a hormetic response (Haigis and Yankner, 2010). Hormetic reactions may trigger a mitochondrial defensive response both in the same tissue in which mitochondria are defective and even in distant tissues, as shown in C. elegans (Durieux et al., 2011). There is compelling evidence that compounds such as metformin and resveratrol are mild mitochondrial poisons that induce a low-energy state characterized by increased AMP levels and activation of AMPK (Hawley et al., 2010). Importantly, metformin extends lifespan in C. elegans through the induction of a compensatory stress response mediated by AMPK and the master antioxidant regulator NRF2 (Onken and Driscoll, 2010). Recent studies have also shown that metformin retards aging in worms by impairing folate and methionine metabolism of their intestinal microbiome (Cabreiro et al., 2013). Regarding mammals, metformin can increase mouse lifespan when administered from early life (Anisimov et al., 2011). In the cases of resveratrol and the sirtuin activator SRT1720, there is convincing evidence that they protect from metabolic damage and improve mitochondrial respiration in a PGC-1α-dependent fashion (Baur et al., 2006; Feige et al., 2008; Lagouge et al., 2006; Minor et al., 2011), although resveratrol does not extend mouse lifespan under normal dietary conditions (Pearson et al., 2008; Strong et al., 2013). Further support for the role of PGC-1α in longevity comes from the observation that PGC-1α overexpression suffices to extend Drosophila lifespan in association with improved mitochondrial activity (Rera et al., 2011). Finally, mitochondrial uncoupling—either genetically through the overexpression of the uncoupling protein UCP1 or by administration of the chemical uncoupler 2-4-dinitrophenol—can increase lifespan in flies and mice (Caldeira da Silva et al., 2008; Fridell et al., 2009; Gates et al., 2007; Mookerjee et al., 2010).

Overview

Mitochondrial function has a profound impact on the aging process. Mitochondrial dysfunction can accelerate aging in mammals (Kujoth et al., 2005; Trifunovic et al., 2004; Vermulst et al., 2008), but it is less clear whether improving mitochondrial function, for example through mitohormesis, can extend lifespan in mammals, though suggestive evidence in this sense already exists.

Cellular Senescence

Cellular senescence can be defined as a stable arrest of the cell cycle coupled to stereotyped phenotypic changes (Campisi and d’Adda di Fagagna, 2007; Collado et al., 2007; Kuilman et al., 2010) (Figure 5A). This phenomenon was originally described by Hayflick in human fibroblasts serially passaged in culture (Hayflick and Moorhead, 1961). Today, we know that the senescence observed by Hayflick is caused by telomere shortening (Bodnar et al., 1998), but there are other aging-associated stimuli that trigger senescence independently of this telomeric process. Most notably, nontelomeric DNA damage and derepression of the INK4/ARF locus, both of which progressively occur with chronological aging, are also capable of inducing senescence (Collado et al., 2007). The accumulation of senescent cells in aged tissues has been often inferred using surrogate markers such as DNA damage. Some studies have directly used senescence-associated β-galactosidase (SABG) to identify senescence in tissues (Dimri et al., 1995). Of note, a detailed and parallel quantification of SABG and DNA damage in liver produced comparable quantitative data, yielding a total of ∼8% senescent cells in young mice and ∼17% in very old mice (Wang et al., 2009). Similar results were obtained in the skin, lung, and spleen, but no changes were observed in heart, skeletal muscle, and kidney (Wang et al., 2009). Based on these data, it is clear that cellular senescence is not a generalized property of all tissues in aged organisms. In the case of senescent tumor cells, there is good evidence that they are subjected to strict immune surveillance and are efficiently removed by phagocytosis (Hoenicke and Zender, 2012; Kang et al., 2011; Xue et al., 2007). Conceivably, the accumulation of senescent cells with aging can reflect an increase in the rate of generation of senescent cells and/or a decrease in their rate of clearance—for example, as a consequence of an attenuated immune response.