PersPeCTives

focus on ageing

Perspectives opinion

Cancer and ageing: convergent and divergent mechanisms Manuel Serrano and Maria A. Blasco

Abstract | Cancer and ageing are both fuelled by the accumulation of cellular damage. Consequently, those mechanisms that protect cells from damage simultaneously provide protection against cancer and ageing. By contrast, cancer and longevity require a durable cell proliferation potential and, therefore, those mechanisms that limit indefinite proliferation provide cancer protection but favour ageing. The overall balance between these convergent and divergent mechanisms guarantees fitness and a cancer-free life until late adulthood for most individuals. In accordance with the evolutionary theory of the ‘disposable soma’1, anti-ageing and anti-cancer mechanisms have adapted their respective strengths to the natural lifespan of each species and, in this manner, these mechanisms ensure that most individuals are ageing-free and cancer-free for as long as they are useful or beneficial to their species. By comparing short-lived mammals, such as mice, with long-lived ones, such as humans, it becomes clear that humans must have more stringent cancer protection and antiageing mechanisms than mice. It is therefore important that anti-ageing and anti-cancer mechanisms must evolve in parallel and accommodate the natural lifespan of the species. The co-evolution of cancer and ageing protection seems to have deeper roots than just the parallel adaptation of two independent processes. Indeed, recent research has unveiled convergent mechanisms that simultaneously provide cancer resistance and ageing resistance, thus coupling their co-evolution. These convergent mechanisms act on common causes of cancer and ageing, most notably on the generation and accumulation of cellular damage. It is well established that cellular damage is at the origin of both cancer and ageing. Accordingly, those mechanisms that prevent cellular damage impinge on these two processes and provide anti-cancer and anti-ageing protection. Among these mechanisms are those that

improve the efficiency of energy consumption, therefore decreasing the generation of reactive oxygen species (ROS), which are considered to be a main source of endo genous damage. In addition, p53 is a master sensor of damage that triggers repair and defence responses. As discussed below, there is evidence indicating that those mechanisms or interventions that decrease ROS or improve p53 activity converge in providing protection against cancer and ageing.

...anti-ageing and anti-cancer mechanisms must evolve in parallel and accommodate the natural lifespan of the species. However, other ‘divergent’ mechanisms have also been discovered that have opp osing effects on cancer and ageing; specifically, protecting from cancer but promoting ageing. These mechanisms include telomere shortening and the derepression of the INK4a/ARF locus. Their purpose is to prevent excessive cellular proliferation, and this produces conflicting effects on cancer and ageing: while cancer protection benefits from these safeguard mechanisms, longterm regeneration and longevity become limited. As we argue below, divergent mechanisms could be mainly designed to prevent cancer, rather than to promote ageing.

nature reviews | molecular cell biology

Here, we briefly discuss the workings of the convergent and divergent mechanisms of cancer and ageing with a particular emphasis on the data that have been obtained using genetically manipulated mice. We should mention that many of the cancer protection mechanisms that we discuss here achieve protection by eliminating damaged (hence, potentially neoplastic) cells from the proliferative pool, either through cell death or through cell senescence. Current data indicate that senescent cells can be efficiently cleared from the organism2 and, therefore, it should not be assumed that senescence-inducing mechanisms are necessarily pro-ageing. Along these lines, apoptosis-inducing mechanisms may or may not be deleterious for the organism. Conceivably, elimination of damaged cells, either by senescence or apoptosis, can be anti-ageing or pro-ageing depending on the magnitude of these responses and the regeneration capacity of the damaged tissue. For a detailed discussion on the role of cellular senescence in cancer and ageing, the reader is referred to recent reviews by others (see the Review by Campisi and d’Adda di Fagagna in this issue) and by us3. Convergent mechanisms There is a general consensus that the accumulation of cellular damage is the initiating event of both cancer and ageing. Tumorigenesis is fuelled by the accumulation of genetic and epigenetic damage. Similarly, ageing occurs, at least in part, because of the accumulation of macromolecular damage, which initially affects cellular proteins, lipids and DNA, but eventually impairs tissue regeneration. According to this, those mechanisms that protect cells from damage could, in principle, protect from cancer and from ageing simultaneously. In this regard, it is important to keep in mind the general observation that long-lived organisms are, in general terms, more resistant to stress4.

Convergence through improved metabolic efficiency. The availability of nutrients has important effects on the cellular metabolism and, ultimately, on the efficiency of the mitochondria. It is well known that under low nutrient conditions, ADP levels are higher volume 8 | september 2007 | 715

© 2007 Nature Publishing Group

Perspectives Low GH/IGF1, calorie restriction

Metabolism

Antioxidant defences

ROS

p53 pathway (including DNA repair and stability)

Damaged cells

Tissue dysfunction

Oncogenic proliferation

Ageing

Cancer

Figure 1 | Convergent mechanisms of cancer Nature Reviews | Molecular Cellfor Biology and ageing. A main source of damage cells originates from the cellular metabolism through the production of reactive oxygen species (ROS), which ultimately cause macromolecular damage, including DNA damage. This endogenous damage is thought to fuel the process of ageing as well as cancer. Those mechanisms that diminish the generation of endogenous damage, including decreased growth hormone (GH) and insulin-like growth factor-1 (IGF1) signalling, calorie restriction, antioxidant strategies and signalling by the tumour suppressor p53, simultaneously protect cells from ageing and from cancer.

and ADP influx to mitochondria drives the efficient performance of the electron transport chain. By contrast, an excess of nutrients translates to a low energy demand, which causes a suboptimal performance of the mitochondria and, consequently, a higher release of ROS5. The impact of the growth hormone (GH) and the insulinlike growth factor-1 (IGF1) pathways on ageing has been known for some time and has been thoroughly analysed6 (see also the Review by Russell and Kahn in this issue). The main mechanism that underlies the anti-ageing effect of low GH/IGF1 activity is likely to be an improved and more efficient mitochondrial respiration, with the ensuing beneficial effect of decreasing the generation of ROS (see below)6. Another important factor associated with a low GH/IGF1 activity is amelioration of the ageing-associated ‘metabolic syndrome’, which is now considered to be a main cause of morbidity in ageing individuals and is associated, among other things, with insulin resistance and high circulating levels of glucose and lipids in the blood7. Together, the improved metabolic efficiency (and reduced ROS production) and the diminished metabolic syndrome could account for the anti-ageing activity of low GH/IGF1 activity. In support of this,

mice with deficient insulin signalling have an extended lifespan8 and, remarkably, mice that are subjected to standard anti-diabetic pharmaceutical regimes also display delayed ageing and an increased maximum lifespan9. The above concept of improving the meta bolic efficiency (which translates into lower endogenous ROS and longevity) also applies to calorie restriction (CR) and to the protein deacetylase SIRT1 (Ref. 7). CR is well known because it delays ageing in all of the species in which it has been tested, and the NADdependent deacetylase SIRT1 is considered to be a main sensor of CR10 and a mediator of CR‑dependent increased longevity11. This protein senses the availability of nutrients by mechanisms that are still under investigation but that include the relative abundance of NAD necessary for the enzymatic activity of SIRT1, as well as transcriptional activation of the SIRT1 gene by a complex between the class O forkhead box transcription factor FOXO3 and p53 (REF. 12). Upon nutrient scarcity, SIRT1 implements a metabolic programme that is, in part, mediated by the association of SIRT1 with the transcription factor PGC1α, the main result of which is an improved metabolic efficiency and, hence, reduced generation of ROS7,13,14. Resveratrol is a compound that can be obtained through the diet and shows remarkable anti-ageing activities, although at concentrations that are much higher than those present in food. Resveratrol activates the catalytic activity of SIRT1 and extends the lifespan of yeast, worms, flies and fish11, as well as that of mice that are fed on a high-fat hypercaloric diet15,16. Overexpression of the secreted protein KLOTHO has also been reported to delay ageing in mice; it appears to do so by downregulation of the IGF1–insulin pathway17. The FOXO family of transcription factors deserves to be mentioned because of its possible role in mediating longevity and cancer protection in response to metabolic clues. FOXO proteins are activated by various stress stimuli through a mechanism that involves the Jun kinase pathway, as well as their association and deacetylation by SIRT1. By contrast, FOXOs are inactivated by growth factors, including insulin, through the AKT kinase18. FOXO proteins have many transcriptional targets that include gluconeogenic and antioxidant enzymes18. The FOXO homologue in worms and flies is sufficient to provide lifespan extension18 and it makes perfect sense that it could at least partly mediate the anti-ageing effects of low insulin signalling in mammals (although this has not yet been proven).

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In summary, several anti-ageing genetic manipulations (lowered activity of GH, IGF1 and the insulin receptor, and increased activity of FOXO, SIRT1 and KLOTHO) and anti-ageing interventions (anti-diabetics, CR and resveratrol) seem to share in common the ability to improve metabolic efficacy, thus reducing the rate of ROS generation (FIG. 1). The impact of the above-mentioned manipulations on cancer resistance is still under investigation and further studies are needed, but there is sufficient evidence to indicate that improving metabolic efficacy provides cancer protection19. In particular, there is convincing experimental evidence for the cancer protection activity of antidiabetics, CR and resveratrol, all of which can prevent or delay various cancer types in mice9,11,20. Remarkably, in the case of antidiabetics, there is preliminary evidence in humans that indicates a cancer protection effect21. Finally, recent evidence has shown that the three closely related FOXO proteins FOXO1, FOXO3 and FOXO4 have potent cancer protection activity22.

...improving the efficacy of cellular metabolism results in the decreased generation of endogenous damage, which may provide protection against both ageing and cancer. In conclusion, although the emerging picture is still incomplete, it compellingly suggests that improving the efficacy of cellular metabolism results in the decreased generation of endogenous damage, which may provide protection against both ageing and cancer (FIG. 1). Convergence through antioxidant defences. The connection between oxidative damage and ageing has been a topic of intense investigation23,24 (see the Opinion article by Pelicci and colleagues in this issue). ROS are generated throughout life and cause continuous damage to the macromolecular components of cells, including proteins, lipids and DNA. Importantly, the rate of ROS production increases with ageing, partly due to several ageing-associated alterations in the electron transport chain5. This in turn produces further mitochondrial damage, thus creating a pernicious cycle, the final result of which is a progressive increase in ROS and ROS-derived damage with ageing23,24. www.nature.com/reviews/molcellbio


focu s rosn i neg Pe p eacgtei v s Despite the substantial effort that has been put into understanding the connection between oxidative damage and ageing, there are only a few examples of genetic manipulations in mice that directly decrease oxidative damage and result in extended lifespan25. The most remarkable examples are mice that are deficient in p66Shc and mice that overexpress catalase. p66Shc is a multifunctional protein that localizes in the intermembrane space of mitochondria where it acts as a redox protein. It can interact with cytochrome c and receive electrons that are subsequently transferred directly to oxygen, thus generating ROS26. Catalase, by contrast, eliminates H2O2 (one of the most damaging forms of ROS) and mice that overexpress catalase in the mitochondria display delayed ageing27 (although this observation needs further confirmation25). In addition, mice that overexpress human thioredoxin also have an extended median and maximum lifespan28. Finally, it should be mentioned that the administration of antioxidants in the diet has been successful in delaying ageing in worms and flies, but not in mice29. Altogether, there is sufficient evidence to implicate oxidative damage as one probable cause of ageing (FIG. 1). Cancer resistance has not yet been reported in the above-mentioned ‘anti oxidant’ mice with increased longevity, although their increased lifespan implies that, at least, there is no increase in cancer susceptibility. However, the administration of antioxidants in the diet — particularly N‑acetylcysteine — is a potent anti-cancer intervention in mice, and has been shown to provide protection against carcinogeninduced lung cancers30 and against lymph omas that were produced by the absence of p53 or the ataxia-telangiectasia mutated (ATM) kinase31,32. Finally, it should be mentioned that the effects of resveratrol on ageing and cancer protection could be mediated in part through its known antioxidant activity11. In summary, the current available evidence suggests that antioxidant protection may simultaneously delay ageing and protect from cancer (FIG. 1). Convergence through p53. p53 is the quintessential tumour suppressor, the activity of which is lost in nearly half of all human cancers. Perhaps the best way to describe the function of this protein is as a master integrator of cellular stress with the capacity to orchestrate a wide range of defensive responses33. The p53 protein is normally inactive, partly as a result of its rapid degradation by the specific ubiquitin ligase MDM2.

Box 1 | Mouse models to address the impact of p53 on ageing Super-p53 mice Super-ARF mice

Normal ageing

MDM2-depleted mice

Increased p53 activity but normal regulation

Super-ARF/p53 mice

Delayed ageing

Aberrant activation of p53 due to lack of regulation by MDM2

Accelerated ageing

Aberrant activation of p53 due to persistent stress or damage

Accelerated ageing

‘m’ mice p44 tg mice

BRCA1∆11 mice ZMPSTEnull mice

When p53 activity is enhanced while maintaining its basic regulation, p53 provides cancer Reviews | Molecular Cell Biology protection without negatively affecting ageing. This is the caseNature for mice with extra gene copies of 38 40 p53 (super‑p53) , or with extra gene copies of ARF (super-ARF) , or decreased activity of MDM2 (Mdm2puro/∆7–12, where the puro allele is a low-expression allele of the wild-type protein and the ∆7‑12 allele is a null allele)41. All of these mice display increased cancer protection together with normal ageing (see figure). In the case of super‑p53 mice, telomere-driven ageing obtained in a telomerase-null background was also normal39. Importantly, compound super-ARF/p53 mice display lower age-associated damage and have an increased average lifespan42. The latter demonstrates that the ability of the ARF/p53 module to decrease endogenous damage (through the antioxidant targets of p53) and to eliminate damaged cells, either by apoptosis or senescence, can delay ageing. A completely different situation exists when p53 loses its normal regulation. This has been achieved in two mouse models that carry truncations of the N‑terminal region of p53, which is crucial for the interaction with MDM2. This is the case for p53+/m (‘m’) mice; in heterozygosis, they carry a deletion of ~500 kb that includes approximately 24 genes upstream of p53, as well as the first six exons of the p53 gene43,83. A second example is provided by transgenic mice that overexpress a natural short isoform of p53 that initiates at exon 4 (p44) (Ref. 44). In these two mouse models, p53 has increased stability and increased transcriptional activity43,44. These mice display enhanced tumour suppression43 but show accelerated ageing43,44 (see figure). Conceivably, unscheduled p53 activity in these mice may result in premature exhaustion of the regenerative capacity of tissues. A conceptually similar situation occurs when mice are subjected to permanent damage at levels considerably higher than those occurring under normal physiological conditions. In these cases, the persistence of the damage mediated by p53 results in accelerated ageing. For example, mice deficient in BRCA1 (a protein involved in the maintenance of genomic stability84) are embryonic lethal, but this lethality is rescued by the absence of p53 (Ref. 85). Similarly, in the mouse model for Hutchinson–Gilford progeria (in which mice are deficient for the protease ZMPSTE24), mice have an aberrant nuclear architecture that results in persistent DNA damage and chromosomal instability leading to premature ageing mediated by p53 (Refs 86,87) (see figure).

A multitude of stresses converge on p53 through complex (and only partially understood) signalling pathways that stabilize and modify p53 (Ref. 34). The tumour suppressor ARF is also relevant in this context; ARF, which is encoded by the INK4a/ARF locus (see below), binds and inhibits MDM2, thereby stabilizing p53 (Refs 35,36). Upon stimulation by stress, p53 gains full transcriptional competence and activates a wide range of transcriptional targets. The list of genes that are activated by p53 is long and is still under investigation. Together, the p53 transcriptional targets implement a cellular response (repair, transient arrest, senescence


or apoptosis) that will depend on the type and intensity of the stress as well as on the cellular context37. This is exemplified by the antioxidant and pro-oxidant targets of p53: stimulation of p53 by low-intensity stresses is sufficient to activate antioxidant targets and does not compromise cell viability. Instead, high-intensity stimulation of p53 activates pro-oxidant targets that participate in triggering cell death31. Tumorigenesis can be regarded as a gradual process by which cells acquire the ability to survive and proliferate under stress. Stresses include a lack of nutrients and oxygen, attack by immune cells, lack volume 8 | september 2007 | 717


Perspectives Mitogenic signalling

Telomere shortening

Lifelong stem-cell proliferation

Oncogenic signalling

Aberrant proliferation INK4a

Longevity

Cancer

Figure 2 | Divergent mechanisms of cancer and Nature Reviews | Molecular Cell Biology ageing. Cells possess two main autonomous systems to limit their proliferative potential: telomere shortening and upregulation of the cyclindependent kinase inhibitor INK4a. Both systems impact on the proliferative potential of stem cells, particularly at old age, and on the aberrant proliferation stimulated by oncogenic signalling. This limit, while beneficial to prevent the development of cancer, may be detrimental for tissue regeneration at an advanced age and, therefore, may promote ageing.

of proper attachment to other cells or to the extracellular matrix, DNA damage, oncogenic signals, aberrant metabolism, and so on. p53 reacts to all of these stresses; first, through transcriptional programmes that try to re-establish homeostasis, and then, if the stress persists, by preventing the propagation of the damaged cells (either by cell death or by cell senescence)37. An emerging topic of potential relevance for ageing protection is the transcriptional programme that is maintained by basal p53 (‘basal’ refers to the activity of p53 that results from the normal endogenous levels of stress). This basal activity of p53 appears to include a group of target genes (SCO2, SENS1 and SENS2, AIF, TIGAR, ALDH4 and others), the final, combined effect of which is to improve the efficiency of mitochondrial respiration and to decrease the generation of ROS37. According to this concept, p53 not only provides protection to aberrant stresses, such as during tumorigenesis, but also to the endogenous damage that occurs in everyday life (FIG. 1). Given the central role of p53, it is not surprising that its manipulation in mouse models is yielding a complex picture. The available mouse models with increased p53 activity can be grouped into two categories. First are those models in which p53 retains its normal regulatory controls; that is, p53 still depends on the presence of stress signals to gain full activity, including the ability to trigger apoptosis and senescence38–42. And second are those models in which the amino-acid sequence of p53 has been modified to relieve part of

its normal regulation, notably deletion of its N‑terminal region, which is required for interaction with MDM2 (that is, p53 activity becomes partly independent of stress signals)43,44. Both groups of mouse models share the phenotype of being more resistant to cancer but they have contrasting ageing phenotypes. In the case of mice with higher levels of normally regulated p53, ageing is not accelerated, whereas in the case of mice with deregulated p53 activity, ageing is accelerated (see BOX 1 for a more thorough discussion). Here, we focus on those mouse models in which p53 levels have been modified without affecting its regulation. We have shown that mice with an extra gene copy of normally regulated p53 age normally38. But detailed analysis of their aged tissues in a telomerase-deficient background indicated that these mice can eliminate telomere-damaged cells more efficiently39. In a continuation of these studies, we recently combined an extra gene copy of p53 and an extra gene copy of ARF in the same mouse strain (named superARF/p53 mice). Importantly, analysis of their lifespan indicated that, apart from the benefit of a lower incidence of cancer, these super-ARF/p53 mice displayed an extended average lifespan. Indeed, these mice showed lower basal levels of ageing-associated damage, probably due to a higher level of the antioxidant transcriptional targets of p53 (Ref. 42) (see also the Research Highlight on page 676 of this issue). In fact, if ageing results from the accumulation of damage and p53 is the main defence against damage, then it is not so surprising that p53 (together with ARF) could have anti-ageing activity.

Given the central role of p53, it is not surprising that its manipulation in mouse models is yielding a complex picture. An interesting finding along these lines has been made in Caenorhabditis elegans, in which it has been observed that various longevity mutations (including decreased insulin/IGF1 activity, decreased mitochondrial respiration or decreased food intake) also provide protection against tumours due to a higher activity of p53 (Ref. 45). The two observations, namely, that p53 can provide anti-ageing activity42 and that longevity mutations can provide p53-dependent cancer protection45, suggest that longevity and cancer resistance are intimately linked and at least partly coordinated by p53.


Finally, it should be mentioned that those systems that maintain genomic stability can also be considered to be convergent mechanisms in the sense that they diminish DNA damage and, therefore, prevent both cancer and ageing. There are multiple examples of mice with DNA instability that are cancerprone and have accelerated ageing. For a detailed discussion, the reader is referred to recent reviews3,46 (see also BOX 1 regarding the implication of p53 in the accelerated ageing phenotype of mice with rampant genomic instability). Divergent mechanisms Above, we have described mechanisms that diminish cellular damage and, therefore, by acting on a common cause of cancer and ageing, simultaneously provide protection against both processes. However, there are cancer protection mechanisms that operate by limiting the proliferative potential of cells and, consequently, may have pro-ageing effects. Cells have developed mechanisms that keep a ‘memory’ of the proliferative history of cells and that prevent cell proliferation beyond a certain point. These are potent tumoursuppression mechanisms, which may, however, contribute to ageing. In old individuals, cells (in particular stem and progenitor cells) have accumulated a long history of proliferation and may succumb to the same mechanisms that prevent indefinite proliferation in cancer cells. Therefore, those mechanisms that prevent indefinite proliferation have divergent effects on cancer and ageing.

Divergence through telomere shortening. The progressive telomere shortening that is experienced by most somatic cells, including stem and progenitor cells, constitutes the best-understood mechanism by which cells keep a proliferative memory47. Telomeres are nucleoprotein structures that protect the ends of chromosomes from unscheduled DNA repair and degradation48. In vertebrates, telomeres consist of 5′-TTAGGG-3′ repeats that are bound by a specialized multiprotein complex, known as shelterin48. Proper telo mere regulation requires a minimum length of 5′-TTAGGG-3′ repeats, the integrity of the shelterin complex and the presence of constitutive heterochromatin48,49. In the absence of compensatory mechanisms, telomeres are lost with each cell-division cycle due to the incapacity of the replication machinery to copy the terminal sequences of linear templates50, a problem that is conceivably aggravated by DNAdegrading activities that may operate on telomeres51,52. Telomere loss is compensated www.nature.com/reviews/molcellbio


focu s rosn i neg Pe p eacgtei v s for by telomerase, a reverse transcriptase that adds telomeric repeats de novo after each cell division53. However, adult somatic tissues, including stem cells, do not have sufficient telomerase activity to counteract telomere shortening with ageing50 and, consequently, aged organisms accumulate telomere-derived chromosomal damage54. In most cases, tumours have shorter telomeres than the normal surrounding tissue, which is in agreement with the notion that telomeres shorten during the extensive proliferation that precedes the formation of a full-blown cancer55. Indeed, activation of the telomerase gene (which is silent in normal cells) occurs in the majority of cancers and, in this manner, most cancer cells are able to stably maintain their short telomeres, or even to elongate them55. Genetic manipulation of mice has shown that the expression of telo merase in postnatal life by means of transgenesis increases the incidence of cancer, probably by hyperactivating stem cells56–58. These pieces of evidence favour a scenario in which telomere shortening imposes a potent barrier to the proliferation of tumour cells. Accordingly, only those cells that overcome this barrier, generally by activating the normally silent telomerase gene (although occasionally through telomeric recombination59), are able to generate a malignant cancer. Genetic proof of this model has been obtained in mice in which the telomereshortening barrier is more difficult to overcome due to the genetic absence of telomerase. These telomerase-deficient mice display a general increase in cancer protection, as shown in various experimental tumour systems47. It is worth mentioning two relevant exceptions that occur when telomerase deficiency is combined with the absence of p53 (Ref. 60) or with overexpression of the shelterin component TRF2 (Ref. 61) (which recruits the nuclease XPF to the telomeres, inducing their degradation)52,62. The absence of p53 or overexpression of TRF2 results in rampant genetic instability of telomerasedeficient cells, which fuels the development of cancer60,61. Apart from these two exceptions (p53 deficiency and TRF2 over expression), telomere shortening — through activation of p53 — prevents the indefinite proliferation of cancer cells and constitutes one of the most important tumoursuppressive mechanisms (FIG. 2). Because telomeres are essential for chromosomal stability, telomere loss at an advanced age could be a cause of organismal ageing. Indeed, telomere length is predictive of age-related pathologies and longevity in humans47,63,64. In mice, even the first genera-

Box 2 | A telomere-based model for cancer and ageing Young or adult (normal telomeres) Niche

Tissue renewal

Time

Mutations Tumorigenic (high telomerase levels)

Old (short telomeres)

Niche

Niche

Aged tissue

Tumour

In young or adult organisms, stem cell (blue rounded cells) niches efficiently repopulate tissues as Molecular Cell Biology needed: they exit from the niche, proliferate and differentiate Nature (squareReviews orange |cells; see figure, top). In old organisms, stem cells have insufficient telomerase activity to maintain telomere integrity, stem-cell telomeres are too short and, consequently, the mobilization of stem cells and tissue regeneration are suboptimal (see figure, bottom left). Decreased stem-cell mobilization conceivably reduces the probability of accumulating abnormal cells in tissues, thus providing a mechanism for cancer protection. The ultimate consequence of impaired mobilization, however, will be organ failure due to tissue degeneration. If stem cells express aberrantly high levels of telomerase (as could conceivably occur due to a mutation or an epigenetic error that activates telomerase expression), stem cells mobilize more efficiently than normal. This effect of telomerase could occur through telomere-independent mechanisms, which are still poorly characterized57,58. Under these conditions of higher mobilization, tissue fitness would be maintained for a longer time, therefore increasing lifespan. However, the probabilities of initiating a tumour are higher (see figure, bottom right). These three situations have been demonstrated in genetically engineered mice that either lack or overexpress telomerase57,58,88. Finally, a prediction from this model is that stem cells will have the longest telomeres within a given tissue, although a formal demonstration of this is still pending.

tion that lack telomerase have a decreased lifespan39. Moreover, examination of telomere length in wild-type mice has shown telomeres shorten significantly at old age, both in Mus spretus65 and in the laboratory mouse, Mus musculus (I. Flores and M. A. B., unpublished observations). Finally, mice that overexpress telomerase and do not succumb to cancer (see above) show a modest increase in the maximum lifespan66.Together, there is strong


evidence for ageing-associated telomere shortening, both in humans and in mice, as well as for the ageing potential of telomere shortening in experimental settings. Regarding the mechanism of telomeredriven ageing, telomerase-deficient mice age prematurely due to multiple organ defects that are caused by impaired tissue regeneration associated with the reduced function ality of various stem-cell compartments47. volume 8 | september 2007 | 719


Perspectives

Divergent mechanisms

Convergent mechanisms Ageing Damage Cancer

Proliferation potential

Healthy young and adult life

Figure 3 | Balance between convergent and Reviewsof| Molecular Cellageing. Biology divergent Nature mechanisms cancer and Convergent mechanisms simultaneously prevent cancer and ageing, thereby facilitating the coevolution of cancer resistance and longevity. By contrast, divergent mechanisms seem mainly designed to prevent cancer but may, in the long term, promote ageing. During evolution, each species must fine-tune the contributions of the convergent and divergent mechanisms to ensure that cancer and ageing do not occur during the young and adult life of most individuals.

This suggests that short telomeres provoke ageing through their deleterious effects on stem-cell functionality (BOX 2). For example, short telomeres have been shown to impair the ability of epidermal stem cells to exit their quiescent state and to regenerate tissues, anticipating their pro-ageing effects57. The shorter lifespan and the regenerative defects of telomerase-deficient mice are reminiscent of human patients with dyskeratosis congenita syndrome, as well as of certain patients with aplastic anaemia50,67. Recent data indicate that telomere-driven ageing is mediated by the p53 transcriptional target p21Cip1 (Ref. 68) in a process that also involves the mismatch-repair protein PMS2 (Ref. 69). In summary, the accumulated evidence indicates that telomere shortening is a relevant component of physiological ageing, which is likely to be mediated by the impact of telomere length on stem cells (FIG. 2). We think that the absence of telomere maintenance after embryonic development is primarily designed as a tumour-suppressive mechanism, rather than as an active proageing mechanism. This is in line with the idea of the ‘disposable soma’ by which natural selection, rather than selecting for genes that actively promote ageing, simply neglects protection to individuals that have surpassed their window of usefulness for the species1. In protected artificial environments, however, individuals have a longer lifespan than in the wild and this ‘unnaturally’ long lifespan gives the opportunity for the occurrence of critical telomere shortening

and telomere-driven ageing. In this manner, the same mechanism that protects us from cancer may also contribute to ageing. Divergence through INK4a. The INK4a/ARF locus encodes two unrelated tumour suppressors, p16INK4a and ARF (p14ARF in humans and p19ARF in mice). Inactivation of this locus by complete deletion or by aberrant promoter methylation is extremely common in essentially all known types of malignancies. An overall incidence of inactivation of 30% makes this locus one of the most important cancer defences in mammalian organisms36. The mechanism of action of the INK4a/ARF locus is well understood. ARF stabilizes p53 by binding and inhibiting MDM2. In the case of p16INK4a, its tumour-suppressive activity is based on its ability to bind and inhibit the cyclin‑D-dependent kinases CDK4 and CDK6. This particular class of kinases, as well as D‑type cyclins, are known to have oncogenic potential and phosphorylate the retinoblastoma family of tumour suppressors (RB, p107 and p130), which in turn are main negative regulators of the cell cycle36.

...we have distinguished two types of mechanisms in the interplay between cancer and ageing. A key feature of the INK4a/ARF locus resides in the fact that it is transcriptionally silent during embryonic development and during most of the postnatal life until adulthood, but becomes active upon excessive mitogenic stimulation, for example, by oncogenic signalling in tumours70–74 as well as in old organisms75,76. In fact, the derepression of the INK4a/ARF locus during ageing is of remarkable magnitude and could well be among the most dramatic changes in gene expression associated with ageing. The INK4a/ARF locus is maintained as silent in normal, non-stressed cells by epigenetic mechanisms that appear to depend on Polycomb transcriptional repressor complexes77. Upon cell-culture stress or during ageing, the levels of the Polycomb components BMI1 and EZH2 decrease76,78 and this could conceivably explain the age-dependent derepression of the INK4a/ARF locus (FIG. 2). However, besides Polycomb, many other transcriptional regulators impinge on the activity of the INK4a/ARF locus77. The identification of the relevant factors that are responsible for the derepression of the INK4a/ARF locus during cancer and ageing remains a challenge for the near future.


Despite the strong correlative evidence between the derepression of the INK4a/ARF locus and ageing, genetic evidence is still lacking. Mice that contain an extra copy of the intact locus in the form of a large genomic transgene do not display premature ageing, whereas they are more strongly protected against cancer40,42. Taken at face value, these data suggest that the INK4a/ARF locus is a potent tumour suppressor but has a minor impact on physiological ageing. However, using mice that either lack or overexpress p16INK4a, it has been shown that p16INK4a limits the regenerative potential of stem cells79–81 (see also the Review by Sharpless and DePinho in this issue). This is highly suggestive of a pro-ageing activity for p16INK4a, although this remains to be demonstrated directly. In this regard, a complicating factor derives from the possibility that cell-cycle inhibitors may keep stem cell proliferation at a low pace and this could be beneficial by preventing the pre mature exhaustion of stem-cell pools, as has been demonstrated for p21Cip1 (Ref. 82). In summary, the INK4a/ARF locus undergoes a remarkable derepression during ageing, perhaps as a manifestation of the excessive or prolonged mitogenic stimulation that is accumulated by the stem cells (FIG. 2). By contrast, derepression of the INK4a/ARF locus during tumorigenesis provides potent cancer protection, as evidenced by the strong selective pressure to eliminate the locus in cancer. As we argued before for telomere shortening, and in agreement with the theory of the ‘disposable soma’, we believe that the main biological purpose of the INK4a/ARF locus is to provide cancer protection and not to actively enforce ageing in a genetically programmed manner. This does not exclude that the remarkable derepression of the locus at old ages could contribute to the process of ageing. A unifying view In summary, we have distinguished two types of mechanisms in the interplay between cancer and ageing. Convergent mechanisms diminish the amount of cellular damage and protect simultaneously against cancer and ageing (FIG. 3). It is tempting to speculate that these convergent mechanisms participate in the co-evolution of cancer protection and longevity. Divergent mechanisms appear to be designed mainly to prevent excessive proliferation, which is the basis of their anti-cancer activity. These divergent mechanisms do not normally limit the natural lifespan of animals, but in protected artificial environments may contribute to ageing. The net balance between www.nature.com/reviews/molcellbio


focu s rosn i neg Pe p eacgtei v s convergent and divergent mechanisms must ensure a healthy (cancer-free and ageingfree) lifespan for the majority of individuals during their young and adult life. The challenge for the future will be to manipulate these mechanisms so that the ongoing enterprise of a longer and healthier life can be continued. In the case of convergent mechanisms, their enhancement through hypothetical chemopreventive treatments should, in theory, provide antiageing and anti-cancer benefits. This is not as speculative as it may sound; for example, preliminary studies have shown that antidiabetic drugs have cancer-protective effects in humans21 and also increase longevity in rodents9. In the case of divergent mechanisms (telomere shortening and derepression of INK4a/ARF), any hypothetical treatment that is aimed at enhancing their cancerprotective effects (inhibition of telomerase or activation of INK4a/ARF) should be transitory and take into consideration its possible pro-ageing effects. Manuel Serrano and Maria A. Blasco are at the Spanish National Cancer Research Centre (CNIO), 3 Melchor Fernandez Almagro Street, Madrid E‑28029, Spain. Correspondence to M.S. e-mail: [email protected] doi:10.1038/nrm2242 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.

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Acknowledgements

Research at the laboratories of M.S. and M.A.B. is funded by the CNIO, the Spanish Ministry of Education and Science, the European Union (projects PROTEOMAGE and INTACT to M.S. and INTACT, TELOSENS, ZINCAGE, RISC-RAD and MOL CANCER MED to M.A.B.), and the Josef Steiner Cancer Research Award 2003 to M.A.B.

Competing interests statement

The authors declare no competing financial interests.

DATABASES Entrez Gene: http://www.ncbi.nlm.nih.gov/entrez/query. fcgi?db=gene INK4a OMIM: http://www.ncbi.nlm.nih.gov/entrez/query. fcgi?db=OMIM aplastic anaemia | dyskeratosis congenita syndrome UniProtKB: http://ca.expasy.org/sprot IGF1 | MDM2 | p53 | SIRT1

FURTHER INFORMATION Manuel Serrano’s homepage: http://www.cnio.es/ing/ grupos/plantillas/curriculum.asp?pag=58 Maria A. Blasco’s homepage: http://www.cnio.es/ing/ grupos/plantillas/curriculum.asp?pag=39

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