Читать книгу Anti-Aging Therapeutics Volume XIII - A4M American Academy - Страница 55

Beyond reduction in the incidence of degenerative and life-threatening diseases, the best-known means of extending lifespan is caloric restriction. This phenomenon has been known for many years, at least on an experimental basis. In order to achieve the effect, a reduction in caloric intake of at least 30-40% from ad libitum consumption is required to produce the change in metabolism that characterizes the effect. Prolongation of natural lifespan in the range of 30% or more, along with reduced incidence of diseases and markers of aging can be achieved in rhesus monkeys. It is believed to be an evolutionary adaptation to periods of famine or environmental stress, and is conserved from primitive organisms to mammals. However, dietary restriction of this magnitude is severe and impractical, so deciphering the underlying mechanisms and means of harnessing these processes has been one of the primary goals of anti-aging research.

Leonard Guarente is credited with the discovery of the epigenetic functions of sirtuins, now known to be the mediators of the caloric restriction phenomenon.²¹ Sirtuin enzymes are highly conserved evolutionarily, with homologs in all eukaryotic organisms. Sirtuins are histone deacetylases (HDAC’s), whose function is gene activation or silencing, the term deriving from genes known as “silent information regulators.” The sirtuins involved in the caloric restriction effect, known as type III HDAC’s, are nicotinamide adenine dinucleotide (NAD)-dependent, which is how the nutritional environment is sensed. This results in fundamental alterations in mitochondrial biogenesis, primarily in adipose tissue. (“Sirtuin” refers to the enzyme, the abbreviation “Sir” or Sirt” refers to the corresponding gene.)

Figure 4. Sirtuin “family tree”

In addition to promoting longevity, sirtuins mediate a range of healthspan-related effects through actions as diverse as improving insulin sensitivity, neuroendocrine modulation, reducing inflammation, and up-regulation of favorable adipokines. Sirtuins are involved in Alzheimer’s disease, telomere function, and expression of genes related to aging. Targeted activation of sirtuins has become an active frontier of biomedical research.

The tantalizing prospect of sirtuin activation for lifespan extension without caloric restriction appeared in 2003 with a publication claiming an increase in replicative lifespan in Saccharomyces cerevisiae (brewer’s yeast) via activation of sirtuins by resveratrol.²² Using chemical libraries to search for a sirtuin activator, the research group hit upon wine phenolics as a class and resveratrol in particular as the only category with significant activity. A fluorescence assay was used to link sirtuin activation to the resveratrol trigger. Given that sirtuins are evolutionarily conserved and the effect appeared specific, the same process should be repeatable in more complex organisms, and considerable media attention was devoted to the story.

The original findings were quickly followed up with reports of resveratrol activating sirtuins and extending lifespan up the phylogenetic ladder. The effect was seen in the roundworm Caenorhabditis elegans and the fruitfly Drosophila, and the homologous SIR genes identified (Sir1 and dSir2, respectively.) An important next step up the ladder is vertebrates, with a convenient model being a short-lived type of fish called Nothobranchius, with a similar response.

In mice however the lifespan extension phenomenon could not be duplicated, though several other favorable results were observed. Specifically, resveratrol countered the effect of a toxic high fat diet, leading to normalization of liver function and metabolic parameters, and prevented the premature death normally associated with murine obesity. Exercise tolerance was increased with a resveratrol-enhanced diet, and in primates early signs are that degenerative diseases are lessened. However, the hope for lifespan extension with resveratrol supplementation began to be questioned.

An early challenge came from failure to replicate the original results from yeast cells in other labs. The effect was noted to be substrate-specific and culture strain-specific. The fluorescence assay was questioned as producing possibly artifactual results, in that sirtuins could be made to fluoresce by resveratrol without being active. The improvements in the mice on the high fat diet were explained as a simple, if potent antioxidant effect to counter the hepatotoxicity of the specific fatty diet. Furthermore, roundworms have been shown to live longer in a bath of the antioxidant enzyme superoxide dismutase (SOD), indicating that for some primitive organisms life can be extended without sirtuin activation. Despite these contradictory findings, research labs around the world began to work on resveratrol and resveratrol derivatives, with the hope of finding variants with greater efficacy and specific clinical applications.

Additionally, clinical trials were instituted on several fronts, including Alzheimer’s disease (NCT00678431, recruiting as of January 2010, and NCT00743743, not yet recruiting); diabetes (NCT01158417, recruiting); melanoma (NCT00721877, closed); colon cancer (NCT00920803, closed, and NCT00256334, recruiting), and others. In November 2010, however, GlaxoSmithKline announced the abrupt suspension of a clinical trial with the promising resveratrol derivative SRT501, for adjunctive therapy in multiple myeloma, due to an unexpected high incidence of renal failure. Though this may have been a statistical aberration and related to the underlying disease or primary therapy rather than SRT501, the company elected to discontinue all development of the drug.

As the study of sirtuin physiology has advanced, it has become clear that resveratrol is not a direct activator. Amgen and Pfizer were independently evaluating resveratrol-based sirtuin activators, and both published peer-reviewed papers announcing that resveratrol and derivatives were inactive.²³ This confirmed earlier questions about the assay and variability of results, but left open the question of how the lifespan increase was produced.

An answer comes from the fact that resveratrol is an activator or inhibitor of at least 15 different enzymes. Another clue is that lifespan extension has been achieved in mice with the diabetes drug metformin and the immune suppressant rapamycin. Metformin, like resveratrol is a potentiator of AMP kinase, which is in turn a sirtuin activator.²⁴ It is therefore possible that this indirect effect is responsible but that metformin is more active or less subject to variations of substrate medium. There is also evidence that metformin has similar degenerative disease-fighting capacity as resveratrol appears to, but may have better bioavailability. Clinical use of metformin remains restricted to treatment of type 2 diabetes, however, with off-label use for health maintenance unproven.

A more direct caloric restriction mimetic may be via mammalian target of rapamycin (mTOR), a protein kinase involved in cell growth regulation, proliferation, and survival. Upstream signals including nutrient status, oxidative stress, redox status, insulin, AMP kinase, and growth factors all influence mTOR activity, which functions at an epigenetic level. Rapamycin is a bacterial protein with immune suppressant and antiproliferative properties, and is clinically used in organ transplantation. Rapamycin has been shown to extend lifespan in mice by up to 38%, and anti-cancer and anti-Alzheimer’s properties are being explored. (Both rapamycin and AMP kinase are mTOR inhibitors.)

The use of rapamycin as a caloric restriction mimetic is highly impractical, given its immune suppressive properties, but does pave a more direct path. Interestingly, resveratrol, by activating AMP kinase which then competitively binds mTOR, functions at an even further upstream level through the same channels. Analogs of rapamycin (called rapalogs) are being developed with targeted specificity for mTOR binding, though none appear to be at the clinical testing stage.²⁵ Epigallocatechin gallate (EGCG), from green tea, as well as caffeine and curcumin are also mTOR inhibitors. These common properties may explain the similar anticancer effects, as mTOR function may be dysregulated in cancer.

Figure 5. Lifespan extension pathways