Valter D. Longo

Extending healthy life span--from yeast to humans.

Eat Less, Live Long Studies in several model organisms have shown that dietary restriction without malnutrition, or manipulation of nutrient-sensing pathways through mutations or drugs, can increase life span and reduce age-related disease. Fontana et al. (p. 321) review the ways in which nutrient-sensing pathways are central to aging. Studies of yeast, worms, rodents, and primates show that these pathways are conserved during evolution. Although data on the effects of dietary restriction in primates are very limited, in humans, the protective effects of dietary restriction against cancer, cardiovascular disease, and diabetes must be judged against potentially negative long-term effects. More work is needed to determine whether dietary restriction and the modulation of anti-aging pathways through drugs can extend life span and reduce pathologies in humans. When the food intake of organisms such as yeast and rodents is reduced (dietary restriction), they live longer than organisms fed a normal diet. A similar effect is seen when the activity of nutrient-sensing pathways is reduced by mutations or chemical inhibitors. In rodents, both dietary restriction and decreased nutrient-sensing pathway activity can lower the incidence of age-related loss of function and disease, including tumors and neurodegeneration. Dietary restriction also increases life span and protects against diabetes, cancer, and cardiovascular disease in rhesus monkeys, and in humans it causes changes that protect against these age-related pathologies. Tumors and diabetes are also uncommon in humans with mutations in the growth hormone receptor, and natural genetic variants in nutrient-sensing pathways are associated with increased human life span. Dietary restriction and reduced activity of nutrient-sensing pathways may thus slow aging by similar mechanisms, which have been conserved during evolution. We discuss these findings and their potential application to prevention of age-related disease and promotion of healthy aging in humans, and the challenge of possible negative side effects.

Sirtuins in Aging and Age-Related Disease

Sirtuins have been the focus of intense scrutiny since the discovery of Sir2 as a yeast longevity factor. Functioning as either deacetylases or ADP ribosylases, Sirtuins are regulated by the cofactor NAD and thus may serve as sensors of the metabolic state of the cell and organism. Here we examine the roles of Sirtuins in diverse eukaryotic species, with special emphasis on their links to aging and age-related diseases including cancer, diabetes, and neurodegenerative disorders.

The Chronological Life Span of Saccharomyces cerevisiae

Simple model systems have played an important role in the discovery of fundamental mechanisms of aging. Studies in yeast, worms and fruit flies have resulted in the identification of proteins and signalling pathways that regulate stress resistance and longevity. New findings indicate that these pathways may have evolved to prevent damage and postpone aging during periods of starvation and may be conserved from yeast to mammals. We will review the yeast S. cerevisiae model system with emphasis on the chronological life span as a model system to study aging and the regulation of stress resistance in eukaryotes.

Growth hormone receptor deficiency is associated with a major reduction in pro-aging signaling, cancer, and diabetes in humans.

Ecuadorians who have a genetic mutation in the growth hormone receptor almost never die of cancer or diabetes complications, possibly because of high resistance to oxidative damage and low circulating insulin. Clues to a Cancer- and Diabetes-Free Life In the 1958 film Live Fast and Die Young, two reckless sisters threaten to burn out early. Similarly, one theory of aging predicts that a faster metabolism leads to a shorter life. Does this trade-off also apply to age-related disease? A new study by Guevara-Aguirre et al. offers clues that address this seminal question. The authors’ findings stem from studies of a unique group of Ecuadorian people who have a mutation in the growth hormone receptor (GHR) gene and a resulting insulin-like growth factor–1 (IGF-1) deficiency, which stunts their growth. These descendants of Spanish conversos, Jews who converted to Christianity to avoid the Inquisition, almost never get diabetes or cancer as a result, the authors postulate, of the privileged metabolic status that arises from their altered hormonal state. Relative to controls, these subjects show lower insulin concentrations and higher insulin sensitivity, and when stressed, their cells tend to self-destruct rather than accumulate mutations and DNA damage—all features that are known to promote cell protection in model organisms. For 22 years, this group of 99 related Ecuadorians—most of whom are homozygous for an A-to-G splice site mutation at position 180 in exon 6 of the GHR gene—has been monitored extensively, so that their health details are well documented. From this reservoir of data, plus information about the diseases of family members as well as causes of death of those relatives who have died, the authors deciphered that the Ecuadorian subjects who carried the GHR mutation had an abnormally low incidence of cancer and diabetes. The group showed only one case of nonlethal cancer and no cases of diabetes, whereas the controls—unaffected relatives—developed cancer (17%) and diabetes (5%) at rates similar to those of the Ecuadorian population as a whole. To illuminate the underlying reason for the subjects’ freedom from these diseases, the authors focused on the components carried in their blood. In experiments on cultured human epithelial cells, Guevara-Aguirre et al. found that low concentrations of one of these, IGF-1, was responsible for preventing oxidative DNA damage when the cells were exposed to the oxidizing agent H2O2 and for promoting cell death when stress-related DNA damage did occur, a checkpoint that averts cancer-promoting behavior by abnormal cells. Analysis of the participating cell signaling pathways identified activation of the transcription factor FoxO under conditions of low IGF-1 as a likely mediator of these effects. Further, the lower blood insulin concentrations and higher insulin sensitivity in these subjects likely account for the absence of diabetes in this population. Although it is difficult to prove that alterations in IGF-1 amounts are responsible for the cancer- and diabetes-free lives of these Ecuadorian people, genetic work from several model organisms suggests that this is so. In yeast, mutations in genes that encode components of a growth-promoting pathway protect against age-dependent genomic instability, and mutations in the insulin/IGF-1–like signaling pathway increase life span and reduce abnormal cellular proliferation in worms. Mice with defects in GH and IGF-1 live exceptionally long lives, with delayed appearance of age-dependent mutations and cancer. The Ecuadorians do not live longer-than-normal lives compared with their compatriots, but rather die in due course from causes of death other than cancer and diabetes complications. Thus, the metabolic inverse of “live fast and die young”—a slowed metabolism yields a longer life—is not supported by the current findings. But a life free from two dreaded diseases may be considered a desirable trade-off. Mutations in growth signaling pathways extend life span, as well as protect against age-dependent DNA damage in yeast and decrease insulin resistance and cancer in mice. To test their effect in humans, we monitored for 22 years Ecuadorian individuals who carry mutations in the growth hormone receptor (GHR) gene that lead to severe GHR and IGF-1 (insulin-like growth factor–1) deficiencies. We combined this information with surveys to identify the cause and age of death for individuals in this community who died before this period. The individuals with GHR deficiency exhibited only one nonlethal malignancy and no cases of diabetes, in contrast to a prevalence of 17% for cancer and 5% for diabetes in control subjects. A possible explanation for the very low incidence of cancer was suggested by in vitro studies: Serum from subjects with GHR deficiency reduced DNA breaks but increased apoptosis in human mammary epithelial cells treated with hydrogen peroxide. Serum from GHR-deficient subjects also caused reduced expression of RAS, PKA (protein kinase A), and TOR (target of rapamycin) and up-regulation of SOD2 (superoxide dismutase 2) in treated cells, changes that promote cellular protection and life-span extension in model organisms. We also observed reduced insulin concentrations (1.4 μU/ml versus 4.4 μU/ml in unaffected relatives) and a very low HOMA-IR (homeostatic model assessment–insulin resistance) index (0.34 versus 0.96 in unaffected relatives) in individuals with GHR deficiency, indicating higher insulin sensitivity, which could explain the absence of diabetes in these subjects. These results provide evidence for a role of evolutionarily conserved pathways in the control of aging and disease burden in humans.

Sir2 Blocks Extreme Life-Span Extension

Sir2 is a conserved deacetylase that modulates life span in yeast, worms, and flies and stress response in mammals. In yeast, Sir2 is required for maintaining replicative life span, and increasing Sir2 dosage can delay replicative aging. We address the role of Sir2 in regulating chronological life span in yeast. Lack of Sir2 along with calorie restriction and/or mutations in the yeast AKT homolog, Sch9, or Ras pathways causes a dramatic chronological life-span extension. Inactivation of Sir2 causes uptake and catabolism of ethanol and upregulation of many stress-resistance and sporulation genes. These changes while sufficient to extend chronological life span in wild-type yeast require severe calorie restriction or additional mutations to extend life span of sir2Delta mutants. Our results demonstrate that effects of SIR2 on chronological life span are opposite to replicatve life span and suggest that the relevant activities of Sir2-like deacetylases may also be complex in higher eukaryotes.

Life Span Extension by Calorie Restriction Depends on Rim15 and Transcription Factors Downstream of Ras/PKA, Tor, and Sch9

Calorie restriction (CR), the only non-genetic intervention known to slow aging and extend life span in organisms ranging from yeast to mice, has been linked to the down-regulation of Tor, Akt, and Ras signaling. In this study, we demonstrate that the serine/threonine kinase Rim15 is required for yeast chronological life span extension caused by deficiencies in Ras2, Tor1, and Sch9, and by calorie restriction. Deletion of stress resistance transcription factors Gis1 and Msn2/4, which are positively regulated by Rim15, also caused a major although not complete reversion of the effect of calorie restriction on life span. The deletion of both RAS2 and the Akt and S6 kinase homolog SCH9 in combination with calorie restriction caused a remarkable 10-fold life span extension, which, surprisingly, was only partially reversed by the lack of Rim15. These results indicate that the Ras/cAMP/PKA/Rim15/Msn2/4 and the Tor/Sch9/Rim15/Gis1 pathways are major mediators of the calorie restriction-dependent stress resistance and life span extension, although additional mediators are involved. Notably, the anti-aging effect caused by the inactivation of both pathways is much more potent than that caused by CR.

Superoxide is a mediator of an altruistic aging program in Saccharomyces cerevisiae

Aging is believed to be a nonadaptive process that escapes the force of natural selection. Here, we challenge this dogma by showing that yeast laboratory strains and strains isolated from grapes undergo an age- and pH-dependent death with features of mammalian programmed cell death (apoptosis). After 90–99% of the population dies, a small mutant subpopulation uses the nutrients released by dead cells to grow. This adaptive regrowth is inversely correlated with protection against superoxide toxicity and life span and is associated with elevated age-dependent release of nutrients and increased mutation frequency. Computational simulations confirm that premature aging together with a relatively high mutation frequency can result in a major advantage in adaptation to changing environments. These results suggest that under conditions that model natural environments, yeast organisms undergo an altruistic and premature aging and death program, mediated in part by superoxide. The role of similar pathways in the regulation of longevity in organisms ranging from yeast to mice raises the possibility that mammals may also undergo programmed aging.

Fasting: Molecular Mechanisms and Clinical Applications

Fasting has been practiced for millennia, but, only recently, studies have shed light on its role in adaptive cellular responses that reduce oxidative damage and inflammation, optimize energy metabolism, and bolster cellular protection. In lower eukaryotes, chronic fasting extends longevity, in part, by reprogramming metabolic and stress resistance pathways. In rodents intermittent or periodic fasting protects against diabetes, cancers, heart disease, and neurodegeneration, while in humans it helps reduce obesity, hypertension, asthma, and rheumatoid arthritis. Thus, fasting has the potential to delay aging and help prevent and treat diseases while minimizing the side effects caused by chronic dietary interventions.

SirT1 inhibition reduces IGF-I/IRS-2/Ras/ERK1/2 signaling and protects neurons

Sirtuins are known to protect cells and extend life span, but our previous studies indicated that S. cerevisiae Sir2 can also increase stress sensitivity and limit life-span extension. Here we provide evidence for a role of the mammalian Sir2 ortholog SirT1 in the sensitization of neurons to oxidative damage. SirT1 inhibition increased acetylation and decreased phosphorylation of IRS-2; it also reduced activation of the Ras/ERK1/2 pathway, suggesting that SirT1 may enhance IGF-I signaling in part by deacetylating IRS-2. Either the inhibition of SirT1 or of Ras/ERK1/2 was associated with resistance to oxidative damage. Markers of oxidized proteins and lipids were reduced in the brain of old SirT1-deficient mice, but the life span of the homozygote knockout mice was reduced under both normal and calorie-restricted conditions. These results are consistent with findings in S. cerevisiae and other model systems, suggesting that mammalian sirtuins can play both protective and proaging roles.

Low Protein Intake Is Associated with a Major Reduction in IGF-1, Cancer, and Overall Mortality in the 65 and Younger but Not Older Population

Mice and humans with growth hormone receptor/IGF-1 deficiencies display major reductions in age-related diseases. Because protein restriction reduces GHR-IGF-1 activity, we examined links between protein intake and mortality. Respondents aged 50-65 reporting high protein intake had a 75% increase in overall mortality and a 4-fold increase in cancer death risk during the following 18 years. These associations were either abolished or attenuated if the proteins were plant derived. Conversely, high protein intake was associated with reduced cancer and overall mortality in respondents over 65, but a 5-fold increase in diabetes mortality across all ages. Mouse studies confirmed the effect of high protein intake and GHR-IGF-1 signaling on the incidence and progression of breast and melanoma tumors, but also the detrimental effects of a low protein diet in the very old. These results suggest that low protein intake during middle age followed by moderate to high protein consumption in old adults may optimize healthspan and longevity.