Because the number of senescent cells increases with aging, it has been widely assumed that senescence contributes to aging. However, this view undervalues what conceivably is the primary purpose of senescence, which is to prevent the propagation of damaged cells and to trigger their demise by the immune system. Therefore, it is possible that senescence is a beneficial compensatory response that contributes to rid tissues from damaged and potentially oncogenic cells. This cellular checkpoint, however, requires an efficient cell replacement system that involves clearance of senescent cells and mobilization of progenitors to re-establish cell numbers. In aged organisms, this turnover system may become inefficient or may exhaust the regenerative capacity of progenitor cells, eventually resulting in the accumulation of senescent cells that may aggravate the damage and contribute to aging (Figure 5A).
In recent years, it has been appreciated that senescent cells manifest dramatic alterations in their secretome, which is particularly enriched in proinflammatory cytokines and matrix metalloproteinases and is referred to as the “senescence-associated secretory phenotype” (Kuilman et al., 2010; Rodier and Campisi, 2011). This proinflammatory secretome may contribute to aging (see “Intercellular Communication”).

The INK4a/ARF Locus and p53

In addition to DNA damage, excessive mitogenic signaling is the other stress most robustly associated to senescence. A recent account listed more than 50 oncogenic or mitogenic alterations that are able to induce senescence (Gorgoulis and Halazonetis, 2010). The number of mechanisms that implement senescence in response to this variety of oncogenic insults has also grown, but the originally reported p16INK4a/Rb and p19ARF/p53 pathways remain, in general, the most important ones (Serrano et al., 1997). The relevance of these pathways for aging becomes even more striking when considering that the levels of p16INK4a (and, to a lower extent, also p19ARF) correlate with the chronological age of essentially all tissues analyzed both in mice and humans (Krishnamurthy et al., 2004; Ressler et al., 2006). We are not aware of any other gene or protein whose expression is so robustly correlated with chronological aging across tissues, across species, and with a range of variation that, on average, is one order of magnitude between young and old tissues. Both p16INK4a and p19ARF are encoded by the same genetic locus, the INK4a/ARF locus. A recent meta-analysis of more than 300 genome-wide association studies (GWAS) identified the INK4a/ARF locus as the genomic locus that is genetically linked to the highest number of age-associated pathologies, including several types of cardiovascular diseases, diabetes, glaucoma, and Alzheimer’s disease (Jeck et al., 2012). These observations place the INK4a/ARF locus as the best documented gene that controls human aging and aging-associated pathologies. A critical pending issue is to determine whether the disease-associated INK4a/ARF alleles present gain or loss of function.
The critical role of p16INK4a and p53 in the induction of cell senescence has favored the hypothesis that p16INK4a-induced and p53-induced senescence contribute to physiological aging. According to this view, the pro-aging activity of p16INK4a and p53 would be a tolerable toll compared to their benefits in tumor suppression. In support of this, mutant mice with premature aging due to extensive and persistent damage present dramatic levels of senescence, and their progeroid phenotypes are ameliorated by elimination of p16INK4a or p53. This is the case with mice deficient in BRCA1 (Cao et al., 2003), with a mouse model of HGPS (Varela et al., 2005), and with mice with defective chromosomal stability due to a hypomorphic mutation of BubR1 (Baker et al., 2011). However, other evidences suggest a more complicated picture. In contrast to their anticipated pro-aging role, mice with a mild and systemic increase in p16INK4a, p19ARF, or p53 tumor suppressors exhibit extended longevity, which cannot be accounted for by their lower cancer incidence (Matheu et al., 2007, 2009). Also, elimination of p53 aggravates the phenotypes of some progeroid mutant mice (Begus-Nahrmann et al., 2009; Murga et al., 2009; Ruzankina et al., 2009). Again, as discussed above for senescence, the activation of p53 and INK4a/ARF can be regarded as a beneficial compensatory response aimed at avoiding the propagation of damaged cells and its consequences on aging and cancer. However, when damage is pervasive, the regenerative capacity of tissues can be exhausted or saturated, and under these extreme conditions, the p53 and INK4a/ARF responses can become deleterious and accelerate aging.

Overview

We propose that cellular senescence is a beneficial compensatory response to damage that becomes deleterious and accelerates aging when tissues exhaust their regenerative capacity. Given these complexities, it is not possible to give a simple answer to the question of whether cell senescence fulfills the third ideal criteria for the definition of a hallmark. A moderate enhancement of the senescence-inducing tumor suppressor pathways may extend longevity (Matheu et al., 2007, 2009), and at the same time, elimination of senescent cells in an experimental progeria model delays age-related pathologies (Baker et al., 2011). Therefore, two interventions that are conceptually opposite are able to extend healthspan.

Stem Cell Exhaustion

The decline in the regenerative potential of tissues is one of the most obvious characteristics of aging (Figure 5B). For example, hematopoiesis declines with age, resulting in a diminished production of adaptive immune cells—a process termed immunosenescence—and in an increased incidence of anemia and myeloid malignancies (Shaw et al., 2010). A similar functional attrition of stem cells has been found in essentially all adult stem cell compartments, including the mouse forebrain (Molofsky et al., 2006), the bone (Gruber et al., 2006), or the muscle fibers (Conboy and Rando, 2012). Studies on aged mice have revealed an overall decrease in cell-cycle activity of hematopoietic stem cells (HSCs), with old HSCs undergoing fewer cell divisions than young HSCs (Rossi et al., 2007). This correlates with the accumulation of DNA damage (Rossi et al., 2007) and with the overexpression of cell-cycle inhibitory proteins such as p16INK4a (Janzen et al., 2006). In fact, old INK4a−/− HSCs exhibit better engraftment capacity and increased cell-cycle activity compared with old wild-type HSCs (Janzen et al., 2006). Telomere shortening is also an important cause of stem cell decline with aging in multiple tissues (Flores et al., 2005; Sharpless and DePinho, 2007). These are just examples of a much larger picture in which stem cell decline emerges as the integrative consequence of multiple types of damage.
Although deficient proliferation of stem and progenitor cells is obviously detrimental for the long-term maintenance of the organism, an excessive proliferation of stem and progenitor cells can also be deleterious by accelerating the exhaustion of stem cell niches. The importance of stem cell quiescence for the long-term functionality of stem cells has been compellingly demonstrated in the case of Drosophila intestinal stem cells, in which excessive proliferation leads to exhaustion and premature aging (Rera et al., 2011). A similar situation is encountered in p21 null mice, which present premature exhaustion of HSCs and neural stem cells (Cheng et al., 2000; Kippin et al., 2005). In this regard, the induction of INK4a during aging (see “Cellular Senescence”) and the decrease of serum IGF-1 (see “Deregulated Nutrient Sensing”) may both reflect an attempt of the organism to preserve the quiescence of stem cells. Also, recent studies have shown that an increase in FGF2 signaling in the aged muscle stem cell niche results in the loss of quiescence and eventually in stem cell depletion and diminished regenerative capacity, whereas suppression of this signaling pathway rescues these defects (Chakkalakal et al., 2012). This opens the possibility of designing strategies aimed at inhibiting FGF2 signaling to reduce stem cell exhaustion during aging. Along these lines, DR has been reported to increase intestinal and muscle stem functions (Cerletti et al., 2012; Yilmaz et al., 2012).
An important debate regarding the decline in stem cell function is the relative role of cell-intrinsic pathways compared to cell-extrinsic ones (Conboy and Rando, 2012). Recent work has provided strong support for the latter. In particular, transplantation of muscle-derived stem cells from young mice to progeroid mice extends lifespan and improves degenerative changes of these animals even in tissues in which donor cells are not detected, suggesting that their therapeutic benefit may derive from systemic effects caused by secreted factors (Lavasani et al., 2012). Furthermore, parabiosis experiments have demonstrated that the decline in neural and muscle stem cell function in old mice can be reversed by systemic factors from young mice (Conboy et al., 2005; Villeda et al., 2011).
Pharmacological interventions are also being explored to improve stem cell function. In particular, mTORC1 inhibition with rapamycin, which can postpone aging by improving proteostasis (see “Loss of Proteostasis”) and by affecting energy sensing (see “Deregulated Nutrient Sensing”), may also improve stem cell function in the epidermis, in the hematopoietic system, and in the intestine (Castilho et al., 2009; Chen et al., 2009; Yilmaz et al., 2012). This also illustrates the difficulty of disentangling the mechanistic basis for the anti-aging activity of rapamycin and underscores the interconnectedness between the different hallmarks of aging discussed here. It is also worth mentioning that it is possible to rejuvenate human senescent cells by pharmacological inhibition of the GTPase CDC42, whose activity is increased in aged HSCs (Florian et al., 2012).

Overview

Stem cell exhaustion unfolds as the integrative consequence of multiple types of aging-associated damages and likely constitutes one of the ultimate culprits of tissue and organismal aging. Recent promising studies suggest that stem cell rejuvenation may reverse the aging phenotype at the organismal level (Rando and Chang, 2012).

Altered Intercellular Communication

Beyond cell-autonomous alterations, aging also involves changes at the level of intercellular communication, be it endocrine, neuroendocrine, or neuronal (Laplante and Sabatini, 2012; Rando and Chang, 2012; Russell and Kahn, 2007; Zhang et al., 2013) (Figure 5C). Thus, neurohormonal signaling (e.g., renin-angiotensin, adrenergic, insulin-IGF1 signaling) tends to be deregulated in aging as inflammatory reactions increase, immunosurveillance against pathogens and premalignant cells declines, and the composition of the peri- and extracellular environment changes.

Inflammation

A prominent aging-associated alteration in intercellular communication is “inflammaging,” i.e., a smoldering proinflammatory phenotype that accompanies aging in mammals (Salminen et al., 2012). Inflammaging may result from multiple causes, such as the accumulation of proinflammatory tissue damage, the failure of an ever more dysfunctional immune system to effectively clear pathogens and dysfunctional host cells, the propensity of senescent cells to secrete proinflammatory cytokines (see “Cellular Senescence”), the enhanced activation of the NF-κB transcription factor, or the occurrence of a defective autophagy response (Salminen et al., 2012). These alterations result in an enhanced activation of the NLRP3 inflammasome and other proinflammatory pathways, finally leading to increased production of IL-1β, tumor necrosis factor, and interferons (Green et al., 2011; Salminen et al., 2012). Inflammation is also involved in the pathogenesis of obesity and type 2 diabetes, two conditions that contribute to and correlate with aging in the human population (Barzilai et al., 2012). Likewise, defective inflammatory responses play a critical role in atherosclerosis (Tabas, 2010). The recent finding that age-associated inflammation inhibits epidermal stem cell function (Doles et al., 2012) further supports the intricate concatenation of different hallmarks that reinforces the aging process. Paralleling inflammaging, the function of the adaptive immune system declines (Deeks, 2011). This immunosenescence may aggravate the aging phenotype at the systemic level due to the failure of the immune system to clear infectious agents, infected cells, and cells on the verge of malignant transformation. Moreover, one of the functions of the immune system is to recognize and eliminate senescent cells (see “Stem Cell Exhaustion”), as well as hyperploid cells that accumulate in aging tissues and premalignant lesions (Davoli and de Lange, 2011; Senovilla et al., 2012).