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AntiAging Cancer Drug - Ageing: A midlife longevity drug?
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Dowload the PDF here
Matt Kaeberlein1 & Brian K. Kennedy1
Nature 460, 331-332 (16 July 2009)
Abstract
The small molecule rapamycin, already approved for clinical use for various human disorders, has been found to significantly increase lifespan in mice. Is this a step towards an anti-ageing drug for people?
Anti-ageing drugs - compounds that slow the hands of time and allow humans to live far beyond their natural span - have long been fertile ground for science-fiction writers. More recently, however, the possibility that such compounds might exist, and might perhaps even be within reach, has gained scientific credibility. In this issue (page 392), Harrison et al.1 provide evidence that pharmacological intervention in the ageing process is feasible in mammals. They report that dietary supplementation with rapamycin - a compound known to be linked to lifespan in invertebrates - significantly increases the lifespan of mice.
The US National Institute on Aging's Interventions Testing Program (ITP) was designed to test compounds of interest for effects on ageing in mice1. Anyone from the scientific community can nominate a compound for consideration by the ITP, and selected compounds are tested in parallel longevity studies at laboratories at three sites, providing built-in triplicate replication and high statistical power. Several compounds have already been tested. Of these, rapamycin is the first to robustly increase lifespan across all three centres and in both male and female mice.
As is often the case in science, this study benefited from a fortuitous accident. Early on, the ITP researchers realized that simply adding rapamycin to feed failed to maintain high levels of the drug, so a specially formulated feed was developed in which rapamycin is encapsulated for timed release in the intestine. It took more than a year to develop the special feed, which meant that mice in the first cohort to receive rapamycin were 600 days old when supplementation was initiated. As Harrison et al.1 note, this is "roughly the equivalent of a 60-year-old person". Amazingly, both the median and maximum lifespan of these middle-aged mice were significantly increased by rapamycin supplementation. For instance, rapamycin increased maximum lifespan (defined by the 90th survival percentile) from 1,094 days to 1,245 days for female mice and from 1,078 days to 1,179 days for male mice. This translates into a striking increase in life expectancy at the time rapamycin supplementation was initiated of about 38% for females and 28% for males. In an ongoing study, mice are being fed rapamycin beginning at 270 days, with a significant increase in survival also being apparent in this cohort.
Rapamycin was first identified as a natural product of the bacterium Streptomyces hygroscopicus in soil samples from Easter Island - famous for its impressive rock-carved human statues (Fig. 1). The compound was selected for inclusion in the ITP on the basis of its known property as an inhibitor of the kinase enzyme, target of rapamycin (TOR). TOR signalling has previously been linked to the ageing process in invertebrates2, 3, 4, 5, 6, but until now it had remained an open question as to whether TOR signalling also has a central role in mammalian ageing. The findings of Harrison et al.1 make TOR the first protein that has been shown to modulate lifespan in each of the four model organisms most commonly used to study ageing: yeast, worms, flies and mice7.
How does TOR activity influence ageing? Among other functions, TOR promotes translation of messenger RNA into protein by the ribosome, and inhibits a pathway that degrades cellular products in lysosomal vesicles (autophagy) - both of which have been implicated in ageing in invertebrate species7. Regulation of mRNA translation by TOR, in particular, has emerged as a lifespan-determining pathway that is highly conserved between yeast and the nematode worm Caenorhabditis elegans. In both species, mutations in targets of TOR, such as ribosomal S6 kinase (an enzyme involved in protein translation), several translation-initiation factors and multiple ribosomal proteins, increase lifespan8. In addition, TOR influences cell growth, cell-cycle progression, mitochondrial metabolism and insulin-like signalling. Untangling the relative contributions of each of these processes to the lifespan extension in mice conferred by rapamycin is likely to stimulate much interest during the next few years.
TOR signalling has also received attention for its role as a possible mediator of dietary restriction. This is defined as a reduction in nutrient availability without malnutrition, and has long been known to increase lifespan in species ranging from yeast to rodents7. TOR activity is reduced by dietary restriction, and genetic studies in invertebrate models have linked the inhibition of TOR to increased longevity by dietary restriction7. For example, a recent study in yeast showed that TOR inhibition increases the amounts of a nutritionally responsive transcriptional activator Gcn4, and demonstrated that this is required for full lifespan extension from dietary restriction9. Similarly, autophagy must be induced for lifespan to be extended by dietary restriction in C. elegans10.
On the basis of these studies, it is tempting to speculate that rapamycin may be functioning as a dietary-restriction mimetic - a small molecule that provides the benefits of dietary restriction without requiring a reduction in food intake. Like dietary restriction, TOR inhibition not only increases lifespan, but also confers protection in invertebrate and rodent models against age-associated disorders, including cardiovascular dysfunction, diet-induced obesity and cancer7. Cancer inhibition in particular is a hallmark of dietary restriction in rodents, and rapamycin analogues are already used clinically as a treatment for certain forms of cancer.
Despite these links, Harrison et al.1 do not strongly favour the idea that rapamycin is mimicking dietary restriction in mice. This is based on their data that rapamycin extends lifespan without reducing body weight, and when treatment is initiated during middle age (late-life onset of dietary restriction has shown inconsistent effects on longevity in previous studies). It is worth pointing out, however, that a true dietary-restriction mimetic may not reduce body weight if it mimics the signalling events (and downstream responses) associated with dietary restriction without changing food consumption. Also, dietary restriction has not yet been extensively characterized in mice of the genetically diverse background used by Harrison et al., so it is difficult to predict whether dietary restriction in these animals would have effects similar to rapamycin. Thus, although it is premature to say for certain that rapamycin is functioning as a dietary-restriction mimetic in mice, the known role of TOR in the nutrient response, and the genetic relationship between TOR signalling and dietary restriction in invertebrates, make this a reasonable possibility.
Is this the first step towards an anti-ageing drug for people? Certainly, healthy individuals should not consider taking rapamycin to slow ageing - the potential immunosuppressive effects of this compound alone are sufficient to caution against this. On the basis of animal models, however, it is interesting to consider that rapamycin - or more sophisticated strategies to inhibit TOR signalling - might prove useful in combating many age-associated disorders. Also, as relevant downstream targets of TOR are better characterized, it may be possible to develop pharmacological strategies that provide the health and longevity benefits without unwanted side effects. So, although extending human lifespan with a pill remains the purview of science-fiction writers for now, the results of Harrison et al.1 provide a reason for optimism that, even during middle age, there's still time to change the road you're on.
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Cancer Drug Delays Aging in Mice: "Rapamycin seems to reduce mid-life mortality risk when started at 270 days of age, but additional data are needed to provide an accurate estimate of effect size, and to evaluate effects on maximal longevity."
By Brandon Keim http://www.wired.com
"Rapamycin may extend lifespan in old, genetically heterogeneous mice through a combination of anti-neoplastic effects25, 26 and effects on cellular stress resistance and response to nutrient dynamics27, 28, 29. The increase in both median and maximum lifespan seen in rapamycin-fed mice is consistent with the hypothesis that inhibiting the mTORC1 pathway retards mammalian ageing, but is not compelling proof that ageing rates are altered; this would require testing whether the intervention decelerates age-dependent changes in multiple organs, cell types and intracellular and extracellular processes14. Comparing the effects of rapamycin treatment and other models of decelerated ageing will help narrow the list of possible mechanisms for longevity extension.....It is especially noteworthy that rapamycin feeding can extend mouse lifespan even when started late in life; in terms of the percentage of the maximal lifespan, a 600-day-old mouse is roughly the equivalent of a 60-year-old person14. An effective anti-ageing intervention that could be initiated later than the midpoint of the lifespan could prove to be especially relevant to clinical situations, in which the efficacy of anti-ageing interventions would be particularly difficult to test in younger volunteers. Our data justify special attention to the role of the TOR pathway in control of ageing in mammals and in the pathogenesis of late-life illnesses."
In a potentially landmark study on the biology of aging and how to delay it, a drug gave elderly mice the human equivalent of thirteen extra years of life.
Though the drug is an immune system suppressant that almost certainly won't have the same effect in humans, the study provides compelling evidence that pharmacologically slowing the process of aging itself may be possible.
"It's unlikely that the life extension came from merely postponing a few specific diseases," said Jackson Laboratory gerontologist David Harrison, a leader of one of three research teams who conducted the experiment separately. "And the treatment didn't start until the mice were the equivalent of a 60-year-old human. No other intervention has been so effective starting late in life."
The drug, called rapamycin, was originally found in bacteria from Easter Island. It showed promise for slowing cell growth, leading to its development as an immunosuppressant used to treat cancer and reduce organ transplant rejection. That ability also made rapamycin a candidate for research into the mechanisms of aging.
Gerontology itself is a youthful field, its progress having been slowed by aging's daunting complexity and a tendency among scientists and doctors to consider diseases as entirely separate, rather than as manifestations of a common origin. But in recent years, that thinking has started to change. From diabetes to cancer to dementia, many diseases become steadily more likely with advancing age. Their common risk profile hints at common origins.
Enticing research also comes from studies of caloric restriction. When animals are given the bare essentials of nutrition, biological mechanisms are triggered that lengthen their lives and slow disease development. Researchers have investigated the biology behind this effect, hoping to find non-dietary ways of getting the same results.
One promising class of compounds target sirtuins, a family of genes that are activated by caloric restriction, and regulate the function of mitochondria - cellular power generators whose dysfunction has been linked to many diseases of aging. Among these compounds is resveratrol, which has displayed powerful protective effects in animals, and is now being tested as a diabetes drug in people.
But at least in mice, even resveratrol hasn't proven as powerful as rapamycin, which targets a gene that's central to the regulation of cell growth and development. And though the latest findings, published Wednesday in Nature, represent an early stage of research - following on yeast and roundworm and fruit fly tests, and very far from applicable to humans - they're still strong.
"There's no obvious way to turn this into a lifespan extension for humans," said David Sinclair, a Harvard gerontologist not involved in this study. "But it's clearly a milestone in the field, to be able to use one small molecule to have such a big effect in an animal. Twenty years ago, if you suggested that one small molecule could slow down aging, people would have said it was impossible."
Harrison, Sinclair and other researchers all warned any benefits rapamycin might have in humans would be offset by its immune system-suppressing effects; unlike lab mice, humans don't live in sterile, rigorously controlled environments.
But the new study shows that rapamycin is still a useful tool for investigating aging. Results were replicated independently in three different laboratories. Mice were already old when they started treatment. They were also genetically diverse, ensuring that mice would die in many different ways, and requiring any potential anti-aging drug to have a truly broad effect.
"If you're going to make a claim that you're actually postponing aging, you really need to be showing that it has beneficial effects on total lifespan in a wide variety of genotypes," said Harrison. "You have to limit your claim to what you've done. The experiment here was designed to allow us to make claims about basic factors of aging."
More than 1,900 mice, their total genetic variation roughly comparable to that found in humans, were fed rapamycin. Treatment starting when they were around 20 months old, a stage comparable to early old age in people.
The average date at which 90 percent of the mice were dead - a convenient metric for quantifying lifespan - rose from 1,078 days to 1,179 days in males, and from 1,094 days to 1,245 days in females. In proportional terms, old age lasted one-quarter longer than expected for males, and two-fifths longer for females.
When tested on nine-month-old mice, rapamycin had little effect. "It's possible that for some agents, the most beneficial effect will only start late in life," said Harrison.
Steven Austad, a University of Texas gerontologist who has been skeptical about resveratrol's apparent longevity-enhancing effects, called the results "particularly significant." He said the multi-center study design gave them "instant credibility."
"If they had asked me, I would have suggested to forget even doing this experiment as the animals were too old to show any effect," said Austad. "Boy, was I wrong about that."
How rapamycin works remains only hazily understood. Its target - a gene called mTOR, short for mammalian target of rapomycin - produces an enzyme necessary for triggering a cascade of cellular signals involved in regulating cell growth, breakdown and mitochondrial function.
Some evidence suggests that mTOR's pathway shares many functions and genes with the sirtuin pathway targeted by resveratrol. The extent of the overlap is unclear, but both appear to be involved in processes affected by caloric restriction.
"There are only a handful of really crucial pathways that control lifespan," said Sinclair. "These pathways all talk to each other. You can think of them not as separate, but as part of a larger network of pathways that are communicating and working in concert."
Future studies on rapamycin and mTOR will work backwards from the general effect to specific mechanisms, said Harrison. He mentioned immune response, cellular breakdown and glucose metabolism as immediate areas of research focus, and in his own laboratory plans to study rapamycin's effects on stem cells.
For now, what explains the long lives of the mice remains an open research question. "The bottom line is, we can guess, but we don't know," said Harrison.
See Also:
· New Longevity Drugs Poised to Tackle Diseases of Aging
· Caloric Restriction Comes in a Pill
· Pharmaceutical Fountain of Youth Could Cost Pennies
· Anti-Aging Drugs Could Change the Nature of Death
Citations: "Rapamycin fed late in life extends lifespan in genetically heterogeneous mice." By David E. Harrison, Randy Strong, Zelton Dave Sharp, James F. Nelson, Clinton M. Astle, Kevin Flurkey, Nancy L. Nadon, J. Erby Wilkinson, Krystyna Frenkel, Christy S. Carter, Marco Pahor, Martin A. Javors, Elizabeth Fernandez & Richard A. Miller. Nature, Vol. 460, No. 7252
"A midlife longevity drug?" By Matt Kaeberlein and Brian K. Kennedy. Nature, Vol. 460, No. 7252
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Letter
Nature 460, 392-395 (16 July 2009)
Rapamycin fed late in life extends lifespan in genetically heterogeneous mice
Correspondence to: David E. Harrison1,11 Correspondence and requests for materials should be addressed to D.E.H. (Email: david.harrison@jax.org).
Abstract
Inhibition of the TOR signalling pathway by genetic or pharmacological intervention extends lifespan in invertebrates, including yeast, nematodes and fruitflies1, 2, 3, 4, 5; however, whether inhibition of mTOR signalling can extend lifespan in a mammalian species was unknown. Here we report that rapamycin, an inhibitor of the mTOR pathway, extends median and maximal lifespan of both male and female mice when fed beginning at 600 days of age. On the basis of age at 90% mortality, rapamycin led to an increase of 14% for females and 9% for males. The effect was seen at three independent test sites in genetically heterogeneous mice, chosen to avoid genotype-specific effects on disease susceptibility. Disease patterns of rapamycin-treated mice did not differ from those of control mice. In a separate study, rapamycin fed to mice beginning at 270 days of age also increased survival in both males and females, based on an interim analysis conducted near the median survival point. Rapamycin may extend lifespan by postponing death from cancer, by retarding mechanisms of ageing, or both. To our knowledge, these are the first results to demonstrate a role for mTOR signalling in the regulation of mammalian lifespan, as well as pharmacological extension of lifespan in both genders. These findings have implications for further development of interventions targeting mTOR for the treatment and prevention of age-related diseases.
Because incidences of most diseases rise rapidly with age6, interventions that delay ageing would greatly benefit health7, 8. So far, dietary additives that delay ageing and increase lifespan in rodent models have shown only weak effects9, 10, 11. Before clinical studies are considered, anti-ageing interventions must be repeatable and effective in many mouse genotypes, and not merely postpone strain-specific diseases12, 13, 14.
The National Institute on Aging Interventions Testing Program (ITP) evaluates agents that may delay ageing and increase lifespan in genetically heterogeneous mice15, 16, 17. Agents are chosen as summarized at http://www.nia.nih.gov/ResearchInformation/ScientificResources/InterventionsTestingProgram.htm. Studies are simultaneously replicated at three test sites: The Jackson Laboratory (TJL), the University of Michigan (UM), and the University of Texas Health Science Center (UT). BALB/cByJ x C57BL/6J F1 (CB6F1) females and C3H/HeJ x DBA/2J F1 (C3D2F1) males are supplied to each site by The Jackson Laboratory, and mated to produce genetically heterogeneous populations in which each animal is genetically unique, but a full sibling of all other mice in the population18. Sufficient mice are used to provide 80% power to detect a 10% increase (or decrease) in mean lifespan with respect to unmanipulated controls of the same sex, even if data from one of the three test sites were to be unavailable. Here we report that dietary encapsulated rapamycin increases mouse survival, including survival to the last decile, a measure of maximal lifespan.
Rapamycin reduces function of the rapamycin target kinase TOR and has anti-neoplastic activities; genetic inhibition of TOR extends lifespan in short-lived model organisms. In male and female mice at each of three collaborating research sites, median and maximum lifespan were extended by feeding encapsulated rapamycin starting at 600 days of age (Fig. 1). We analysed the data set as of 1 February 2009, with 2% (38 of 1,901) of mice still alive. For data pooled across sites, a log-rank test rejected the null hypothesis that treatment and control groups did not differ (P < 0.0001); mice fed rapamycin were longer lived than controls (P < 0.0001) in both males and females. Expressed as mean lifespan, the effect sizes were 9% for males and 13% for females in the pooled data set. Expressed as life expectancy at 600 days (the age of first exposure to rapamycin), the effect sizes were 28% for males and 38% for females. Mice treated with other agents (enalapril and CAPE (caffeic acid phenethyl ester)) evaluated in parallel did not differ from controls at the doses used (Supplementary Fig. 1).
Rapamycin-fed and control mice were then compared separately for each combination of site and gender. Rapamycin had a consistent benefit, compared with controls, with P values ranging from 0.03 to 0.0001 (Fig. 2).
Female mice at all three sites had improved survival after rapamycin feeding (Fig. 2). Mean lifespan increases for females were 15%, 16% and 7% (TJL, UM and UT, respectively), and life expectancy at 600 days increased by 45%, 48% and 22% for females at the three sites. Median lifespan estimates of control females were consistent across sites (881-895 days), and were similar to values noted in Cohort 2004, which ranged from 858 to 909 days15. Thus, the improvement in survival seen in the rapamycin-fed females is not an artefact of low survival for the control females.
Male mice at all three sites also had improved survival after rapamycin feeding (Fig. 2). Mean lifespan increases for males were 5%, 8% and 15% (TJL, UM and UT, respectively), and male life expectancy at 600 days increased by 16%, 23% and 52%. Interpretation is complicated by differences among sites in survival of control males, and because mice assigned to the rapamycin-fed group at UT and perhaps at UM had lower mortality before 600 days than controls. Control mice at UT and UM differed from those fed rapamycin not only in exposure to rapamycin from 600 days of age but also in specific formulation of the mouse chows (all based on the NIH-31 standard) used between weaning and 600 days. We thus cannot rule out the possibility that improved survival among males in the rapamycin group, at UT and at UM, might reflect differences in nutritional or health status between control and rapamycin groups before 600 days, rather than solely the effects of rapamycin. Notably, the significant benefits of rapamycin on male (and female) survival at TJL could not have been affected by diet before drug administration, because at TJL both control and rapamycin-fed mice received the same chow (Purina 5LG6) throughout this period.
Maximum lifespan was increased by rapamycin feeding. Table 1 shows the ages at the 90th percentile for control and rapamycin-treated mice, along with the 95% upper confidence bound for the controls. For each site and sex, the 90th percentile age for rapamycin-treated mice is higher than the upper limit for the corresponding control group, showing that rapamycin increases the age for 90th percentile survival.
To determine whether increases in maximal lifespan due to rapamycin feeding are statistically significant, we compared the proportion of living mice in each group after 90% had died in the joint life table19 (details in Supplementary Table 1). Summing across the three sites, 4.8% of the female control mice were alive at these ages, compared with 21.5% of the rapamycin-treated females (P < 0.0001). For males, the corresponding values were 5.9% of controls and 20.2% of rapamycin-treated mice (P < 0.0001). The site-specific calculations documented a significant effect on females at both TJL (P = 0.0006) and UM (P = 0.0001); for males, we noted a significant effect at both TJL (P = 0.008) and UT (P = 0.0001), with a marginal effect at UM (P = 0.07). Rapamycin feeding initiated at 600 days of age thus leads to a significant increase in maximal lifespan.
To test if the spectrum of lesions was altered by dietary rapamycin, complete necropsies were conducted on 31 control and 40 rapamycin-fed mice that were either found dead or killed when moribund (details in Supplementary Table 2). Although rapamycin postpones death, it did not change the distribution of presumptive causes of death.
A separate group of mice was used to evaluate the effects of encapsulated rapamycin initiated at 270 days of age (Fig. 3a). At the time of analysis, 51% of the females and 68% of the males had died, and a stratified log-rank test showed significantly lower mortality risk in the rapamycin-treated mice compared to controls, pooling across the three test sites (P = 0.0002 for males and P < 0.0001 for females). When each site was evaluated separately, the beneficial effect of rapamycin for females was significant at each site (P < 0.005); for males, the effect was significant (P < 0.025) at UM and UT, but not at TJL. Rapamycin seems to reduce mid-life mortality risk when started at 270 days of age, but additional data are needed to provide an accurate estimate of effect size, and to evaluate effects on maximal longevity.
To document biochemical effects of rapamycin at the dose used for the lifespan studies, we evaluated the phosphorylation status of ribosomal protein subunit S6 (rpS6)-a target substrate of S6 kinase 1 in the mTOR signalling pathway20-in visceral white adipose tissue (a sensitive indicator of mTOR inhibition by rapamycin treatment in vivo). Figure 3b shows that rapamycin feeding reduced the levels of phosphorylated rpS6 4-5-fold when fed from 270 to about 800 days of age. Blood levels of rapamycin in the treated mice were equivalent in males and females, between 60 and 70 ng ml-1.
Initial evidence that reduced TOR function can extend longevity came primarily from studies in yeast1, 2 and invertebrates3, 4, 5. Beneficial effects of diet restriction21 and dwarf mutations, both of which extend lifespan in rodents, may, to some degree, result from repression of the mTOR complex 1 (mTORC1) pathway22, 23. It is not yet known to what extent inhibition of mTOR will recapitulate other aspects of the phenotypes associated with diet restriction or dwarf mutations. Our demonstration that rapamycin feeding increases lifespan even when started late in life, as well as the absence of changes in body weight (data not shown), distinguishes our results from studies using diet restriction: in all cases diet restriction reduces body weight, and in most reports21, although not all24, diet restriction produces little, if any, benefit if started after about 550 days of age.
Rapamycin may extend lifespan in old, genetically heterogeneous mice through a combination of anti-neoplastic effects25, 26 and effects on cellular stress resistance and response to nutrient dynamics27, 28, 29. The increase in both median and maximum lifespan seen in rapamycin-fed mice is consistent with the hypothesis that inhibiting the mTORC1 pathway retards mammalian ageing, but is not compelling proof that ageing rates are altered; this would require testing whether the intervention decelerates age-dependent changes in multiple organs, cell types and intracellular and extracellular processes14. Comparing the effects of rapamycin treatment and other models of decelerated ageing will help narrow the list of possible mechanisms for longevity extension.
At the cellular level, mTORC1 helps to coordinate growth and survival responses induced by alterations in nutrient availability, energy status, growth factor stimuli and exposure to potentially lethal cell stresses27, 28, 29; this strategic position at the nexus of nutrient/stress sensing pathways may contribute to the importance of TOR function in regulating lifespan in invertebrates and in mammals as well. It is especially noteworthy that rapamycin feeding can extend mouse lifespan even when started late in life; in terms of the percentage of the maximal lifespan, a 600-day-old mouse is roughly the equivalent of a 60-year-old person14. An effective anti-ageing intervention that could be initiated later than the midpoint of the lifespan could prove to be especially relevant to clinical situations, in which the efficacy of anti-ageing interventions would be particularly difficult to test in younger volunteers. Our data justify special attention to the role of the TOR pathway in control of ageing in mammals and in the pathogenesis of late-life illnesses.
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