Contrarian research findings: newly-identified aging villain substances; calorie restriction longevity is not due to calorie restriction

Recent publications have surfaced substances that may contribute to aging including favorites used to extend longevity.  Also, it has cast light on longevity due to calorie restriction suggesting that what counts is not the calories as much as the food medium, at least in primitive species.  This blog entry reviews some of the recently-fingered life-shortening substances and research publications that implicate them and suggests a new way to look at longevity due to calorie restriction. The list of putative life-shortening substances contains some surprises! 

Vinegar – shortens life by burning up older cells 

Yes, the first substance is plain old acetic acid, not vinegar put on salads but acetic acid produced in yeast cells due to an excess of glucose in the medium.  The results may or may not be partially applicable to humans.  The 2010 publication Lessons on longevity from budding yeast tells the story.   “In chronologically ageing yeast, damage accumulates in non-dividing cells. In the external medium, ethanol initially accumulates and is converted to acetic acid, which induces an apoptosis-like response and cell death. Inside the chronologically ageing cell, damaged mitochondria and oxidized proteins also accumulate and probably contribute to chronological senescence.”  — “Yeast cells are typically aged in synthetic medium containing 2% glucose. Under these conditions, cells ferment pyruvate to ethanol. After glucose depletion, ethanol is metabolized leading to the production of acetic acid, which is toxic to yeast cells and induces an apoptosis-like response that limits CLS (chronological lifespan).   Under conditions of dietary restriction, the glucose concentration of the growth medium is reduced to 0.5% or lower, resulting in direct use of pyruvate by mitochondrial respiration, decreased acetic acid production and increased CLS.”  In other words, dietary restriction provides less-access to glucose leading to less production of acetic acid leading to less acid damage to cells and longer chronological lifespans.   

In further detail: “A new perspective on the conjecture that oxidative stress limits CLS has been offered by the finding that acetic acid is a primary molecular factor limiting the lifespan of yeast cells under these standard conditions6. As cells proceed to use ethanol as a secondary carbon source, acetic acid and other organic acids are secreted into the extracellular milieu, leading to acidification of the growth medium. Buffering the ageing culture to a higher pH or removing acetic acid from the expired medium is sufficient to extend CLS6. Transferring cells to water, rather than allowing them to age in expired medium, has also been shown to increase CLS, demonstrating that extracellular factors limit CLS9. Transferring post-mitotic yeast to water containing physiologically relevant concentrations of acetic acid, but not other acids, suppresses this lifespan extension, indicating that acetic acid is both necessary and sufficient to cause chronological ageing6. These findings are consistent with evidence that ageing yeast cells undergo an apoptosis-like process induced by acetic acid8, 10, 11 and with the observation that addition of ethanol to the medium can shorten CLS12.”The applicability of this result to humans is uncertain.  “How much of what we learn about ageing in yeast is relevant to people has become an important question. We do not yet know the answer, but the evidence so far suggests that although some aspects of ageing in yeast are specific to this organism, many of the most important features have been evolutionarily conserved in invertebrate species and rodents. — The identification of acetic acid as a limiting factor for chronological survival under standard conditions has led to questions regarding the validity of this system as a model for human ageing6, 7. Although it seems unlikely that acetic-acid-induced apoptosis has an important role in human ageing, the chronological ageing model may still provide a reasonable description of some aspects of ageing in people. For example, there is evidence that acetic acid increases the production of reactive oxygen species and causes mitochondrial dysfunction in yeast, suggesting that although the factor inducing chronological senescence (acetic acid) may be specific to yeast, the resulting damage and cellular responses may be shared7(ref)”

Glucose – shortens life

Although acetic acid is the killer of old yeast cells according to the above-described research, it is excess glucose in the medium that kicks off the problem in the first place.  A reader of this blog, eric25001, in his comment in this blog Thoughts on C3H8O3 put me onto a 2009 paper implicating glucose in aging: Tor1/Sch9-Regulated Carbon Source Substitution Is as Effective as Calorie Restriction in Life Span Extension.  The paper says that some mutant forms of yeast convert glucose to glycerol, and that these forms of yeast live as long as if they were subject to calorie restriction.  “We investigated the global gene expression changes and identified genes involved in the metabolism of various kinds of carbon sources that are associated with longevity in the single cell organism, the baker’s yeast. Although glucose and ethanol are common carbon sources for growth, they also have detrimental pro-aging effects in yeast. Long-lived yeast mutants actively utilize available glucose and ethanol and produce glycerol, which does not adversely affect the yeast life span extension. Our finding suggest that this “carbon source substitution” observed in long-lived yeast creates an environment mimicking calorie restriction, which together with the direct regulation of stress resistance systems optimizes life span extension.”  “Glycerol, unlike glucose or ethanol, did not adversely affect the life span extension induced by calorie restriction or starvation, suggesting that carbon source substitution may represent an alternative to calorie restriction as a strategy to delay aging.”  So there we have it.  It is not only the calories in calorie restriction that can delay aging, but also simply substituting glycerol for glucose.   

An article on this research based in part on an interview with the author Glucose To Glycerol Conversion Regulated By Long-lived Yeast Provides Equivalent Anti-Aging Effects To Calorie Restriction provides more graphic detail.  “–baker’s yeast cells maintained on a glycerol diet live twice as long as normal — as long as yeast cells on a severe caloric-restriction diet. They are also more resistant to cell damage. — “If you add glycerol, or restrict caloric intake, you obtain the same effect,” said senior author Valter Longo. “It’s as good as calorie restriction, yet cells can take it up and utilize it to generate energy or for the synthesis of cellular components.”  — Longo and colleagues Min Wei and Paola Fabrizio introduced a glycerol diet after discovering that genetically engineered long-lived yeast cells that survive up to 5-fold longer than normal have increased levels of the genes that produce glycerol. In fact, they convert virtually all the glucose and ethanol into glycerol. Notably, these cells have a reduced activity in the TOR1/SCH9 pathway, which is also believed to extend life span in organisms ranging from worms to mice.”

The 2009 paper Pro-Aging Effects of Glucose Signaling through a G Protein-Coupled Glucose Receptor in Fission Yeast implicates glucose in yeast aging even more deeply.  “Glucose sensed by the cells activates signaling pathways that, in yeast, favor the metabolic machinery that makes energy (glycolysis) and cell growth. The sensing of glucose also reduces stress resistance and the ability to live long. Does glucose provoke a pro-aging effect as a result of its metabolic activity or by activating signaling pathways? Here we addressed this question by studying the role of a glucose-signaling pathway in the life span of the fission yeast S. pombe. Genetic inactivation of the glucose-signaling pathway prolonged life span in this yeast, while its constitutive activation shortened it and blocked the longevity effects of calorie restriction. The pro-aging effects of glucose signaling correlated with a decrease in mitochondrial respiration and an increase in reactive oxygen species production. Moreover, a strain without glucose metabolism is still sensitive to detrimental effects of glucose due to signaling. Our work shows that glucose signaling through the glucose receptor GIT3 constitutes the main cause responsible for the pro-aging effects of glucose in fission yeast.”

Antioxidants – are they also evil?

Yes, at least one 2010 research publication suggests that taking antioxidants interferes with the hormetic effects of ROS on mitochondria, and therefore may be life-shortening.  No kidding, the suggestion is that antioxidants may be bad for you.  The publication is How increased oxidative stress promotes longevity and metabolic health: The concept of mitochondrial hormesis (mitohormesis).  “Recent evidence suggests that calorie restriction and specifically reduced glucose metabolism induces mitochondrial metabolism to extend life span in various model organisms, including Saccharomyces cerevisiae, Drosophila melanogaster, Caenorhabditis elegans and possibly mice. In conflict with Harman’s free radical theory of aging (FRTA), these effects may be due to increased formation of reactive oxygen species (ROS) within the mitochondria causing an adaptive response that culminates in subsequently increased stress resistance assumed to ultimately cause a long-term reduction of oxidative stress. This type of retrograde response has been named mitochondrial hormesis or mitohormesis, and may in addition be applicable to the health-promoting effects of physical exercise in humans and, hypothetically, impaired insulin/IGF-1-signaling in model organisms. Consistently, abrogation of this mitochondrial ROS signal by antioxidants impairs the lifespan-extending and health-promoting capabilities of glucose restriction and physical exercise, respectively. In summary, the findings discussed in this review indicate that ROS are essential signaling molecules which are required to promote health and longevity. Hence, the concept of mitohormesis provides a common mechanistic denominator for the physiological effects of physical exercise, reduced calorie uptake, glucose restriction, and possibly beyond.” (Italics are mine)

Denying the longevity value of antioxidants may come across about as realistic as denying the Holocaust.  However, the point in the above-mentioned paper about the hormetic value of reactive oxygen species (ROS) is reinforced in the 2010 publication Living on the edge: stress and activation of stress responses promote lifespan extension.” Oxidative stress constitutes the basis of physio-pathological situations such as neurodegenerative diseases and aging. However, sublethal exposure to toxic molecules such as reactive oxygen species can induce cellular responses that result in stress fitness. Studies in Schizosaccharomyces pombe have recently showed that the Sty1 MAP kinase, known to be activated by hydrogen peroxide and other cellular stressors, plays a pivotal role in promoting fitness and longevity when it becomes activated by calorie restriction, a situation which induces oxidative metabolism and reactive oxygen species production. Activation of the MAP kinase by calorie restriction during logarithmic growth induces a transcriptional anti-stress response including genes essential to promote lifespan extension. Importantly enough, the lifespan promotion exerted by deletion of the pka1 or sck2 genes, inactivating the two main nutrient-responsive pathways, is dependent on the presence of a functional Sty1 stress pathway, since double mutants also lacking Sty1 or its main substrate Atf1 do not display extended viability. In this Research Perspective, we review these findings in relation to previous reports and extend important aspects of the original study. We propose that moderate stress levels that are not harmful for cells can make them stronger.”

Longer telomeres – shorten lifespans

Yes, that is right at least in budding yeast according to a 1998 paper Changes of telomere length cause reciprocal changes in the lifespan of mother cells in Saccharomyces cerevisiaes.  “Budding yeast cells divide asymmetrically, giving rise to a mother and its daughter. Mother cells have a limited division potential, called their lifespan, which ends in proliferation-arrest and lysis. In this report we mutate telomerase in Saccharomyces cerevisiae to shorten telomeres and show that, rather than shortening lifespan, this leads to a significant extension in lifespan. This extension requires the product of the SIR3 gene, an essential component of the silencing machinery which binds to telomeres. In contrast, longer telomeres in a genotypically wild-type strain lead to a decrease in lifespan. These findings suggest that the length of telomeres dictates the lifespan by regulating the amount of the silencing machinery available to nontelomeric locations in the yeast genome.”  The result may be valid for yeast and not for humans but it shows how tricky the relationship between telomere lengths and lifespan may be.

Methylglyoxal – a hidden enemy of aging?

Having just skewered two sacred cows of anti-aging (antioxidants and long telomeres), I turn finally to a relatively unfamiliar substance, methylglyoxal.  The 2010 paper Oxidative stress and aging: is methylglyoxal the hidden enemy? points the finger. “Aging is a multifactorial process that involves changes at the cellular, tissue, organ and the whole body levels resulting in decreased functioning, development of diseases, and ultimately death. Oxidative stress is believed to be a very important factor in causing aging and age-related diseases. Oxidative stress is caused by an imbalance between oxidants such as reactive oxygen species (ROS) and antioxidants. ROS are produced from the mitochondrial electron transport chain and many oxidative reactions. Methylglyoxal (MG) is a highly reactive dicarbonyl metabolite formed during glucose, protein and fatty acid metabolism. MG levels are elevated in hyperglycemia and other conditions. An excess of MG formation can increase ROS production and cause oxidative stress. MG reacts with proteins, DNA and other biomolecules, and is a major precursor of advanced glycation end products (AGEs). AGEs are also associated with the aging process and age-related diseases such as cardiovascular complications of diabetes, neurodegenerative diseases and connective tissue disorders. AGEs also increase oxidative stress. In this review we discuss the potential role of MG in the aging process through increasing oxidative stress besides causing AGEs formation. Specific and effective scavengers and crosslink breakers of MG and AGEs are being developed and can become potential treatments to slow the aging process and prevent many diseases.  Specific and effective scavengers and crosslink breakers of MG and AGEs are being developed and can become potential treatments to slow the aging process and prevent many diseases.”  I mention that several substances in the Tissue Glycation Firewall described in my treatise function to prevent the formation of AGEs or to break them up after they are formed. 

Discussion

This blog entry cites research publications that throw into question several “sacred cow” propositions of longevity science:

–        That glucose, the basic stuff cells use to get energy,  shortens lives,

–        That taking antioxidants confers longevity,

–        That longer telomeres are associated with longevity, and

–        That the benefits of calorie restricted diets are due to calorie restriction.

These observations are mostly based on studies of yeasts and, yet, some may to some extent be applicable also to humans.  As for me personally right now, my cells will go on using glucose; I am not ready to stop taking antioxidants; I have already decided that messing with telomere lengths via “telomerase activators”may have little to do with longevity; and I have not practiced calorie restriction and do not plan to though I still cling to the hope that resveratrol works as a calorie restriction mimetic.