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(Last modification: 18. May 2010)

 

Sirtuins: arrangement of pages

 

Sirtuin Activators

Most of this page was extracted from an excellent review: Alcaín and Villalba, 2009). A few references and comments were added.

 

Resveratrol

 

Notes

- Resveratrol is the main topic in another page in this website (more...), and therefore there is no need here to discuss its structure and biosynthesis.

- There are many more publications on resveratrol effects found in in vitro studies, and they implicate significant roles in many critical metabolic processes, but here I will focus mostly only on those studies dealing with complete organisms: that is (or probably should be) one of the major concerns for people taking resveratrol, or are planning to do so. 

 

Key results

   A publication by Howitz et al. (2003) certainly was a key event in the development of the resveratrol story: they studied the capacity of small natural molecules to stimulate sirtuin activities. The screen with recombinant human SIRT1 led to the identification of several molecules, and resveratrol turned to be the most active one. Others, but with lower activity, were another stilbene (piceatannol), two chalcones (butein, isoliquiritigenin), and two flavonols (fisetin, quercetin) (see Fig. 1). The yeast Sir2 recombinant sirtuin was also tested in vitro. Somewhat disappointingly, the effect of resveratrol with yeast Sir2 in vitro was a stimulation of only about twofold, much less than with the human SIRT1 (but our interest is of course in people, not yeast!).

   These assays measured the deacetylation of acetylated peptides conjugated to a nonphysiological fluorophore. There are good reasons question whether the effects of these natural products were really caused by direct affects on the SIRT1 protein: more....

     The in vivo effects with yeast cells looked much better: butein, fisetin, and resveratrol increased average life span by 31%, 55%, and 70%, respectively. Whatever the limitations of these results are, many people were hypnotized by the 70% increase of life span, and quickly started to calculate what resveratrol supplementation could mean to human life extension.

    Follow-up in the next years demonstrated similar life extension effects by resveratrol in the fruit fly (Drosophila melanogaster) and the nematode Caenorhabiditis elegans (Wood et al., 2004; Viswanathan et al., 2005). However, this was disputed for both organisms in at least two publications (Bass et al., 2007; Pearson et al., 2008). One study even showed that resveratrol extended the lifespan of a fish (Valenzano et al., 2006).

 

Fig. 1.

Natural SIRT1 activators, and links to pages in this website (Resveratrol, Quercetin, Isoliquiritigenin = 6'-Deoxychalcone) or in Wikipedia (Piceatannol, Fisetin; both pages are not very informative, however). 

 

Natural SIRT1 activators

 

    The next important advance was the demonstration that resveratrol improved health and survival of mammals (mice) on a high-calorie diet (Baur et al., 2006). The significance is in the term 'high-calorie' diet, and the implications for people: the number of overweight or even obese people is steadily increasing, apparently reaching the staggering number of 2.1 billion world-wide, with a pretty high percentage in the Western countries. This is considered one of the major risk factors for many diseases, e.g. for cardiovascular disease and type 2 diabetes mellitus, and many more; these are often summarized as 'metabolic syndrome'. The study showed that resveratrol prevented the effects of excess calorie intake, that it shifted the physiology towards the state of mice on a standard diet, and that it led to a significant increase of survival. Comparable  conclusions on the effects of resveratrol were reached by Lagouge et al., 2006.

    One of the often discussed concerns about these experiments is the resveratrol dosis: to the layman it looks awfully high in the Baur et al. (2006) experiments, but laymen are often not aware of the fact that amounts per mice kilogram cannot simply be extrapolated to humans (more...). More recent work (Barger et al., 2008a) indicated that much lower concentrations of resveratrol are likely to be sufficient to obtain many of the beneficial effects, and that even short-term consumption can mimic at least partially the effects of a long-term calorie restricted diet (Barger et al., 2008b).

   The application of resveratrol (or other substances with even higher activity, see below) for treating the many problems of the metabolic syndrome was one of the central themes in many recent studies (see Alcain and Villalba, 2009 for a more detailed discussion, citation at end of this page). In this context, these authors  state that 'modifying the dietary habits of populations will be extremely difficult'. It is therefore probably only realistic (although certainly not the optimal solution) to look for dietary supplements that mimic the effects of a low calorie diet. Clinical studies on the effects of resveratrol on type 2 diabetes and other diseases are supposed to be completed soon (November 2009). The review cited above also gives information on some companies, their resveratrol preparations (e.g. SRT501, Longevinex, Resvida), and numbers of the patents.

 

There are a few concerns with the application of resveratrol to humans

  • Bioavailability
       Resveratrol is considered as a pharmacologically safe drug: it can be taken up to 5 g/day for 7 days without noticeable side effects (Elliott and Jirousek, 2008; see also Williams et al., 2009), but little seems to be known about possible side effects in long-term applications.
       The in vitro experiments with enzymes or tissues indicate that about 10 to 200 µM resveratrol are necessary to elicit the desired activation of SIRT1. However, this concentration is hardly reached in the blood plasma even after application of very high doses of resveratrol. Most of it is found very fast as glucuronide and sulfate derivatives, and these appear to reach much higher plasma levels than the resveratrol itself. It looks as if this should be investigated further, in particular in their possible role of the derivatives in resveratrol effects.
        A few simple stilbene derivatives were tested for improved stability and bioavailability (Yang et al, 2007), but it seems not clear whether they will be of practical use.

  • Tissue specificity
       Taking resveratrol as supplement in whatever form means that it principally should be available to all organs and cells, but there is basically no evidence for that. Also, usually it is assumed tacitly that the effects of sirtuins are the same in all cells/tissues, e.g. in mediating the good effects of calorie restriction. Recent results, however, indicate that SIRT1 regulation by calorie restriction is tissue specific, and not the same in all cell-types (Chen et al., 2008). Can one predict with some reliability the consequences of a system-wide activation of sirtuins? It may be possible that the results for systemic SIRT1 stimulation by resveratrol are more complex than understood so far. One example of the complexity of sirtuin regulation, and complex effects of resveratrol and the sirtuin inhibitor sirtinol is in this publication: Pfister et al., 2008.

  • Down-regulation of p53
       This protein plays an important role in controlling growth of cells. Acetylation of p53 leads to its activation and either growth arrest or apoptosis (programmed cell death); this is a major factor in controlling tumor cells. It is acetylated in the active form. SIRT1 binds to p53 and deacetylates it, i.e. it downregulates its activity and delays or inhibits apoptosis (Vaziri et al., 2001; van Leeuwen and Lain, 2009). This is good for the survival of normal cells, but might also lead to increased survival and growth of tumor cells. In such cases, resveratrol could have quite undesirable effects as tumor promoter, and the application of SIRT1 inhibitors might be indicated instead (more...).

  • Validity of the tests for sirtuin activators (and inhibitors)
           Most if not all of these assays used as substrates acetylated peptides conjugated to a nonphysiological fluorophore. Later results showed, however, that the fluorophore played an unexpected role that it should not have: no activity was found in its absence of the fluorophore, i.e. with only the acetylated peptides in the assays. Therefore it was questioned whether the effects of these natural products were really caused by direct affects on the sirtuin protein, and it was suggested that the focus solely on sirtuins might be to narrow (Borra et al., 2005; Kaeberlein et al., 2005). A more recent publication confirmed the claim that resveratrol is not a direct activator of SIRT1 enzyme activity (Beher et al., 2009).
       A publication in 2008 (Kahyo et al., 2008) actually showed that the use of a different acetylated peptide as SIRT1 substrate led to quite different results: in their assay system, resveratrol and fisetin showed only marginal activation of SIRT1 (about 1.3-fold), and only piceatannol had a significant activating effect (about 3-fold). The other supposed activators actually behaved as inhibitors of SIRT1: Butein, isoliquiritigenin, and quercetin reduced the activities to 0.04, 0.32, and 0.38-fold, respectively. 

See also a comment in the general page on Sirtuins: more... , and remarks in the page on sirtuin inhibitors (more...)

 

Synthetic sirtuin activators

 

  The poor bioavailability of resveratrol, its rapid metabolization, and its capacity to interact with multiple targets are not properties desirable in a drug ('dirty' molecules for the pharmaceutical companies). Therefore it is of considerable interest to find other molecules with the same effect, but higher bioavailabiltiy, specificity, and potency. Several companies looked for that. One found several quinoxaline derivatives havíng such effects (Nayagam et al. 2006), but I will not go into detail here, because there are apparently no detailed studies in animals yet.

   Another company (Sirtris Pharmaceuticals) is apparently much further. The screening identified a number of promising compounds (Milne et al., 2007), and some of them are shown in Fig. 2. In particular SRT1720 appeared to be 1000-fold more active than SRT501 (the resveratrol formulation of the company) in in vitro assays, and the experiments with mice also were quite successful. Recent work (Feige et al., 2008; Smith et al., 2009) demonstrated that not only resveratrol, but also SRT1720 produced effects like calorie restricted diets. Modifications of the backbone were also tested (Vu et al., 2009), and novel SRT1 activators were published (Fig. 3) (Bemis et al., 2009). Human trials are apparently already under way (October 2009), and it will be exciting to see the outcome.

 

Fig. 2.

Synthetic SIRT1 activators.

Synthetic SIRT1 activators

 

 

Fig. 3.

Recently published novel SIRT1 activators (Bemis et al., 2009)

Recently published novel SIRT1 activators (2009)

 

 

Conclusions
   I will not attempt to list and discuss all the publications dealing with the many roles of sirtuins and the many effects observed with resveratrol in cell-free systems, with tissue cultures, and in animal model systems: that is just too much, and should be the privilege of more qualified authors.
   Nevertheless, it is quite clear that there is pretty good evidence that resveratrol and other compounds can mimic the effects of a low calorie diet. However, it is also clear that there are some concerns that should be considered before taking resveratrol: more...

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The source for most of the information summarized in this page

  • Alcain, F. J., Villalba, J. M., 2009a. Sirtuin activators. Expert Opinion on Therapeutic Patents 19, 403-414.
    Background: Sirtuin 1 - 7 (SIRT1 - 7) are deacetylases that are dependent on NAD+ for their activity. SIRT1 down-regulates p53 activity, increasing lifespan, cell survival, and neuroprotection; it also deacetylates peroxisome proliferator-activated receptor-gamma and its coactivator 1α, promoting fat mobilization, increasing mitochondrial size and number, and positively regulating insulin secretion. Sirtuins link nutrient availability and energy metabolism. Calorie restriction, which increases lifespan and is beneficial in age-related disorders, activates sirtuin. Major efforts are thus focused to developing sirtuin activators.
    Objective: After discussing the potential involvement of sirtuins in pathophysiological processes, this review looks at new, synthetic sirtuin activators.
    Conclusions: To date, resveratrol is the most potent natural compound able to activate SIRT1, mimicking the positive effect of calorie restriction. Resveratrol might help in the treatment or prevention of obesity and in preventing the aging-related decline in heart function and neuronal loss. As resveratrol has low bioavailability and interacts with multiple molecular targets, the development of new molecules with better bioavailability and targeting sirtuin at lower concentrations is a promising field of the medicinal chemistry. New SIRT1 activators that are up to 1000 times more effective than resveratrol have recently been identified. These improve the response to insulin and increase the number and activity of mitochondria in obese mice. Human trials with a formulation of resveratrol with improved bioavailability and with a synthetic SIRT1 activator are in progress.
    (Note: In Fig. 3, the structures of fisetin and quercetin lack an H-atom at the single bonded oxygen in the 3-position. Also, although it is formally correct to call them flavones, in the nomenclature used by most plant scientists they are called 'flavonols' because of the -OH group in the 3-position. In Fig. 6, the three O-CH3 groups at the 'top' aromatic ring are missing in SRT1460).

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