Rapamycin


This isn’t exactly news, but it’s news to me: Rapamycin has an orally administered derivative, Everolimus, already in use as an anti-cancer and anti-rejection drug. (The two compounds are almost identical; Everolimus has one additional hydroxyethyl group on the protuberant cyclohexane ring, and apparently that’s enough to make the unwieldy rapamycin molecule orally bioavailable.)

This might be good news if it turns out that the longevity-enhancing qualities of rapamycin end up generalizing to humans: If you need to maintain constant levels of a chronically administered drug, t’s way easier to use timed-release oral capsules than injections. Also, as millions of diabetics will tell you, it’s just nice not to have to shoot up.

But the drug itself might be bad news, especially if it is taken over long periods of time: mTOR, the target of rapamycin, appears to be necessary for reconsolidation of long-term memory in mammals; inhibition of mTOR is efficacious enough at blocking fear memories that it’s discussed as a strategy for treating PTSD. The role of mTOR in memory appears to be general, i.e., in memories other than fearful ones, and it is evolutionarily ancient: TOR is important for long-term potentiation in the sea slug Aplysia, beloved model of scholars of learning and memory.

So, as I’ve commented before, I have this fear that rapamycin (or a derivative) will turn out to be a bona fide longevity enhancement drug, but one whose chronic use erodes long-term memory, which does defeat the purpose to some extent.

Then again, blood-brain barrier is an issue here: even though Everolimus can survive the stomach and pass through the gut into the blood, that doesn’t mean it will make it into the brain efficiently. Then again again, if the drug has a long half-life once it’s inside the brain, it might still accumulate there if one had to take it every day for the rest of one’s life.

On the happy side, if you find this possibility traumatic, the rapamycin will take care of that for you.

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We know that exercise is good for us, and increasingly we’re understanding how it works at the molecular and cellular level: Physical activity boosts levels of heat shock proteins, which help cells resist stress; it also improves mitochondrial function in a manner reminiscent of calorie restriction (CR). Our knowledge is sophisticated enough that we can identify and develop small-molecule exercise mimetics and drugs that improve exercise tolerance.

Overall, then, exercise and its molecular/cellular consequences are consistent with longevity assurance pathways and life extension interventions. However, there are complications emerging.

One of the results of exercise is increased activity of anabolic pathways, especially in muscle. Building up tissues require new protein synthesis, and new protein synthesis requires activity of the TOR pathway. TOR is increasingly thought to be a pro-aging or gerontogenic pathway: rapamycin, a drug that inhibits TOR, blocks senescence and extends lifespan in mice (we already knew that TOR inhibition increased longevity in worms and yeast).

Until recently, we’d believed that exercise modulated TOR in the “right” direction for longevity assurance (i.e., down). For instance, AMPK, a target of exercise mimetics, appears to downregulate TOR signaling.

But it would appear that the above result, obtained using exercise mimetics, may not be generally applicable to all exercise — in particular, it does not extend to a specific regimen of exercise designed to stimulate anabolism and muscle growth. In blood flow restriction (BFR) exercise, resistance training is combined with pressure cuffs that significantly decrease blood flow to the exercising muscle; it increases protein synthesis in muscle cells and activates the TOR pathway. Now, Fry et al. have shown that in older men (who don’t increase muscle mass in response to ordinary resistance training), BFR activates TOR.

Superficially, this would seem to represent a contradiction: a lifespan-extending intervention (exercise) activates a lifespan-shortening biochemical signaling pathway (TOR). How might this seeming paradox be resolved?

  • TOR activity in the muscle might be irrelevant to lifespan control. Testing this hypothesis is a special case of a broader question, which is the determination of the key tissues responsible for the lifespan extension by rapamycin. This will probably require tissue-specific conditional knockdowns of either TOR or downstream pathways (e.g., S6K), and will take a while.
  • Not all exercise is lifespan-extending. Perhaps exercise regimens specifically optimized to stimulate anabolism might be gerontogenic, while those that create acute stress and activate hormetic pathways might extend lifespan.

It’s also worth mentioning that BFR exercise may be uniquely bad vis-a-vis longevity control. In worms, one of the targets of TOR is HIF-1, the hypoxia inducible factor. HIF-1 is a gerontogene: knocking it down extends longevity, so its wildtype function must shorten lifespan. I wonder whether the blood flow restriction in BFR exercise might create low-grade hypoxia in the muscle tissue, inducing HIF-1 activity and incurring some gerontogenic effect. It certainly wouldn’t be the first time that an intervention that helped older men increase muscle mass ended up being bad for them in the long run (e.g., hGH).

ResearchBlogging.orgFry, C., Glynn, E., Drummond, M., Timmerman, K., Fujita, S., Abe, T., Dhanani, S., Volpi, E., & Rasmussen, B. (2010). Blood flow restriction exercise stimulates mTORC1 signaling and muscle protein synthesis in older men Journal of Applied Physiology DOI: 10.1152/japplphysiol.01266.2009

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A review I should have cited yesterday addresses the promise and challenges of the two most prominent natural-product candidates for longevity-enhancing therapeutics. The author is prominent biogerontologist and all-around bright feller Matt Kaeberlein (see here for earlier posts on his group’s work).

Resveratrol and rapamycin: are they anti-aging drugs?

Studies of the basic biology of aging have advanced to the point where anti-aging interventions, identified from experiments in model organisms, are beginning to be tested in people. Resveratrol and rapamycin, two compounds that target conserved longevity pathways and may mimic some aspects of dietary restriction, represent the first such interventions. Both compounds have been reported to slow aging in yeast and invertebrate species, and rapamycin has also recently been found to increase life span in rodents. In addition, both compounds also show impressive effects in rodent models of age-associated diseases. Clinical trials are underway to assess whether resveratrol is useful as an anti-cancer treatment, and rapamycin is already approved for use in human patients. Compounds such as these, identified from longevity studies in model organisms, hold great promise as therapies to target multiple age-related diseases by modulating the molecular causes of aging.

Note that resveratrol has been taking a bit of a thrashing of late, with recently released studies calling into question its ability to directly activate sirtuins. Briefly, the critics posit that the early data may have been misinterpreted due to artifacts in a fluorescence-based system used to detect protein-drug interactions — but check comment #32 on that post for David Sinclair’s personal response on this issue.

ResearchBlogging.orgKaeberlein, M. (2010). Resveratrol and rapamycin: are they anti-aging drugs? BioEssays, 32 (2), 96-99 DOI: 10.1002/bies.200900171

One of 2009’s most significant breakthroughs in biogerontology (or in any field; q.v. Science, WIRED) last year was the announcement that the macrolide drug rapamycin can extend longevity in mice.

More specifically, rapamycin can accomplish this when administered to adult, wildtype mice. In other words, no genetic modification or early-life intervention is necessary, making rapamycin one of the first compounds that meets the criteria for an anti-aging drug that could be used for people who are already alive and well down the road toward aging themselves.

The lifespan extension achieved is modest (~10%), but this is more impressive in light of the fact that the mice were quite old at the time treatment began, and the study used only a single dose rate. Future studies will undoubtedly seek to optimize the dose and regimen with the goal of achieving greater enhancement of lifespan.

How does it work? As the saying goes, further study is required, and at multiple levels.

• Organism: It is possible that rapamycin acts by delaying the onset of cancer, frankly slowing the aging process, or a combination of both. (This issue could be addressed by using genetically engineered mouse strains that exhibit very little cancer.)

• Tissue: Rapamycin might decelerate cellular senescence, which could fight aging in two ways: by maintaining cells in a division-competent state (and thereby increasing the pool of cells available to regenerate tissues), and by ameliorating the damaging effects of deleterious inflammatory secretion by senescent cells. This is complicated by the fact that senescence is itself a tumor-suppressor pathway; in the absence of data to the contrary, one might have expected the drug to have a modest oncogenic effect, but that doesn’t seem to be the case in the mouse studies. (It’s worth mentioning that the author of that first senescence study prognosticated the efficacy of rapamycin as an anti-aging drug several years ago).

• Cell: With respect to cellular and molecular mechanisms, all eyes are on the TOR pathway (“target of rapamycin”; the protein is inhibited by rapamycin) . The TOR kinase, which has been implicated in lifespan control in smaller organisms, regulates translation by modulating the activity of ribosomal proteins and elongation factors. Deleting the S6 kinase gene (a target of TOR; eliminating S6K is like selectively turning off a specific arm of the TOR pathway) extends lifespan in rodents – consistent with the idea that TOR exerts its effects on aging by controlling translation.

There’s a good deal left to discover about the rapamycin’s effects on aging in general — and regarding the specific mechanistic relationship between translational control, senescence, and organismal aging — but I have it on good authority that there’s a great deal of effort being exerted in that direction. Watch this space for future developments.

If you’re interested in reading more, there’s a nice post on the issue over at Fight Aging!

Oh, I almost forgot – impending pun alert – in the “cruel irony” department, rapamycin may inhibit the formation, consolidation and preservation of long-term memory; it’s even been proposed as a treatment for PTSD. (To make a very long story short, protein translation is required for establishment and maintenance of memories.) It’s not yet clear whether the doses of rapamycin that extend lifespan will have an effect on memory, but it’s clearly crucial to figure that out. It would be a damn shame to live an extra ten or twenty years at the cost of slowly forgetting one’s past. I’ll be following that emerging story with interest.

More elsewhere:

The TOR (“target of rapamycin”) protein is a master regulator of cell growth, governing connect nutrient sensing, protein synthesis, and proliferation. It has become increasingly clear that the TOR pathway plays an essential role in longevity determination — specifically, higher TOR activity is associated with more rapid aging and shorter lifespan.

In mammals, TOR interferes with stem cell functions, and TOR activity is downregulated by exercise. It has been proposed that TOR inhibitors might even be used as anti-aging drugs (and in fact we’re going to investigate some recent relevant tests of that idea, sometime next week). The relationship between TOR and lifespan holds true across great evolutionary distances: loss of TOR function (in conjunction with other mutations) can dramatically increase the chronological lifespan of yeast.

How does TOR control the rate of aging? In order to answer this question, we must look downstream, to proteins that are controlled by TOR. A recent study from the Kapahi lab (our neighbors at the Buck Institute for Age Research) investigated the role of one such TOR target: HIF-1 (“hypoxia inducible factor”; it is also involved in metabolism). The authors find that loss-of-function mutations in HIF-1 result in longer-lived C. elegans. Chen et al.:

HIF-1 Modulates Dietary Restriction-Mediated Lifespan Extension via IRE-1 in Caenorhabditis elegans

Dietary restriction (DR) extends lifespan in various species and also slows the onset of age-related diseases. Previous studies from flies and yeast have demonstrated that the target of rapamycin (TOR) pathway is essential for longevity phenotypes resulting from DR. TOR is a conserved protein kinase that regulates growth and metabolism in response to nutrients and growth factors. While some of the downstream targets of TOR have been implicated in regulating lifespan, it is still unclear whether additional targets of this pathway also modulate lifespan. It has been shown that the hypoxia inducible factor-1 (HIF-1) is one of the targets of the TOR pathway in mammalian cells. HIF-1 is a transcription factor complex that plays key roles in oxygen homeostasis, tumor formation, glucose metabolism, cell survival, and inflammatory response. Here, we describe a novel role for HIF-1 in modulating lifespan extension by DR in Caenorhabditis elegans. We find that HIF-1 deficiency results in extended lifespan, which overlaps with that by inhibition of the RSKS-1/S6 kinase, a key component of the TOR pathway. Using a modified DR method based on variation of bacterial food concentrations on solid agar plates, we find that HIF-1 modulates longevity in a nutrient-dependent manner. The hif-1 loss-of-function mutant extends lifespan under rich nutrient conditions but fails to show lifespan extension under DR. Conversely, a mutation in egl-9, which increases HIF-1 activity, diminishes the lifespan extension under DR. This deficiency is rescued by tissue-specific expression of egl-9 in specific neurons and muscles. Increased lifespan by hif-1 or DR is dependent on the endoplasmic reticulum (ER) stress regulator inositol-requiring protein-1 (IRE-1) and is associated with lower levels of ER stress. Therefore, our results demonstrate a tissue-specific role for HIF-1 in the lifespan extension by DR involving the IRE-1 ER stress pathway.

The mutants’ life extension was observed when the worms could eat ad libitum but not when they were dietarily restricted (DR), implying that the mechanism of the HIF-1 mutation is similar to that of DR. Conversely, activation of HIF-1 expression (by mutating EGL-9, which ubiquitinates HIF-1) decreases the lifespan extension due to DR. Taken together, the findings imply that downregulation of HIF-1 expression is both necessary and sufficient for DR-mediated longevity enhancement.

One more step down the rabbit hole, then: What are HIF-1 and DR doing? The authors find that lifespan extension requires the IRE1-gene, a principle mediator of the unfolded protein response (UPR). The UPR is activated when the endoplasmic reticulum (ER) is stressed — when protein folding is inefficient, or the secretory machinery is overloaded; the pathway returns the cell to homeostasis by inducing expression of genes that fold, sort, and process proteins in the ER (or degrade the proteins that can’t be saved). Perhaps lifespan extension requires increased ER capacity, or more efficient degradation of misfolded proteins?

On a closing note: Attentive readers will have recalled that not very long ago, we reported on a paper that appears to have reached the opposite conclusion — specifically, that high expression of HIF-1 (induced the same way as here, by mutation in the ubiquitin E3 ligase EGL-9) results in extended lifespan and decreased proteotoxicity. I don’t want to get in the middle of this controversy, except to point out that the systems were different in a number of ways, and that it is a formal possibility that a gene’s activity could be “tuned” such that either an increase or a decrease in expression could increase lifespan (implying that the wildtype expression levels are at a “sweet spot” of lower lifespan but presumably higher fitness, due to some sort of tradeoff between longevity and reproductive success). I am sure that the authors of both studies are working to reconcile the apparent contradiction. We’ll look forward to learning more as the story develops.

ResearchBlogging.orgChen, D., Thomas, E., & Kapahi, P. (2009). HIF-1 Modulates Dietary Restriction-Mediated Lifespan Extension via IRE-1 in Caenorhabditis elegans PLoS Genetics, 5 (5) DOI: 10.1371/journal.pgen.1000486

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