The First Molecule Able To Activate Anti-Aging Target Sirtuin 6

Lombard, David B., and Richard A. Miller. “Ageing: Sorting out the sirtuins.” Nature 483.7388 (2012): 166-167.

Sirtuins are a class of proteins that gained celebrity status as the supposed targets of the life-extension compound derived from red wine, resveratrol. However some more recent publications have casted doubt on the mechanism by which resveratrol lengthens lifespan. In fact, even the relationship between the sirtuins and aging has been called into question.

Yeast has five sirtuin genes (the best known and first one discovered was Sir-2), the roundworm C. elegans has four sirtuin genes (Sir-2.1 to Sir-2.4), fruit flies have five sirtuin genes (including dSir2), and mammals like ourselves have seven sirtuin genes (Sirt-1 to Sirt-7). The different sirtuin genes within an organism have differences in their function and localization within the cell. Hence it is important to specify which sirtuin gene one is talking about. Sirt-1 and the most closely related sirtuin genes (homologs) Sir2 (yeast), dSir2 (fruit flies), and Sir2.1 (roundworm) have received the most attention in the aging research field.

Placing extra copies of Sir2 into the genome of yeast extends lifespan (Longo et al., 2012). This lifespan extension is correlated with a decreased accumulation of circular DNA fragments inside the nucleus of the aging yeast cell. Interestingly, such circular fragments do not accumulate in the nucleus of other species and may thus represent a yeast-specific mechanism of aging (a so called private mechanism of aging). Several studies have suggested that placing extra copies of the dSir2/Sir2.1 gene in the genome of fruit flies or C. elegans also extends its lifespan but a more recent large study failed to find lifespan extension and concluded that the original results were caused by experimental artifacts (Burnett et al., 2011).

Sirt 6 and mice

Male mice that overexpress Sirt-6 have longer lifespans and are protected from obesity-induced metabolic abnormalities. Conversely, mice that lack Sirt-6 only live 4-5 weeks and show aging-associated phenotypes such as an abnormal low level of a certain type of white blood cell (= lymphopenia), loss of fat under the skin (= subcutaneous fat), and severely low levels of blood sugar (= hypoglycemia) (Nakagawa and Guarente, 2011; Tasselli et al., 2016).

In a new paper published in the prestigious journal Angewandte Chemie International Edition researchers succeeded in developing the first class of synthetic molecules that activates the SIRT-6 protein. While it was previously found that fatty acids could weakly activate SIRT-6, no SIRT-6 activators were known that exhibited ‘drug-like properties’. This discovery could potentially lead to a new class of anti-aging drugs.

Main reference

You W et al. (2017). Structural basis of sirtuin 6 activation by synthetic small molecules. Angew Chem Int Ed 56: 1007-1011.  

References

Burnett C et al. (2011). Absence of effects of Sir2 overexpression on lifespan in C. elegans and Drosophila. Nature 477: 482-485.

Longo VD et al. (2012). Replicative and chronological aging in Saccharomyces cerevisiae. Cell Metab 16(1): 18-31. 

Nakagawa T, Guarente L (2011). Sirtuins at a glance. J Cell Sci 124: 833-838. 

Tasselli L et al. (2016). SIRT6: Novel mechanisms and links to aging and disease. Trends Endocrinol Metab [Epub ahead of print].

Glycation Inhibitors Extend Lifespan In Yeast

Credit: Oregon State University

Glycation is the non-regulated reaction of sugars with proteins leading to a loss of function of these proteins. Glycation is increased in people who have chronic high blood sugar concentrations such as in poorly managed diabetes patients. Diabetics have a higher risk for cardiovascular disease, kidney failure, nerve damage, and loss of eyesight. While other mechanisms besides glycation exist that explain the link between diabetes and these health problems it seems likely that glycation is at least partially responsible. Hence inhibitors that reduce the rate of glycation could have a large therapeutic potential in alleviating the complications of diabetes and ‘normal’ aging.

Testing 3 glycation inhibitors

A team of researchers from the Indian CSIR-National Chemical Laboratory and the Academy of Scientific and Innovative research (AcSIR) have investigated the effect of glycation inhibitors on the lifespan of baker’s yeast. The team used three glycation inhibitors: (i) the gold-standard inhibitor aminoguanidine, (ii) the antidiabetic drug metformin, and (iii) the powerful glycation inhibitor hydralazine. Metformin and aminoguanidine share structural similarity. To determine if the lifespan extension was caused by a decrease in glycation or because the structural similarity causes both to hit the same non-glycation-related target, a third inhibitor of glycation, hydralazine, was tested. Hydralazine does not share any structural similarity with either metformin or aminoguanidine.

Aminoguanidine extended the chronological lifespan of non-calorie restricted yeast by 45-50% while metformin only extended lifespan by 20-25%. The lifespan extension by aminoguanidine is comparable to that of calorie restriction in yeast. Interestingly, aminoguanidine is a more powerful glycation inhibitor hence if lifespan extension was caused by the inhibition of glycation then we would indeed expect that aminoguanidine has a stronger effect on lifespan. The powerful glycation inhibitor hydralazine extended lifespan by 50-55%. These data suggest that the inhibition of glycation might be (partially) responsible for the lifespan extending effect of these three compounds. All three glycation inhibitors were tested to verify that they indeed reduced glycation in yeast.

Image credit: Sven Bulterijs. Based on data from Kazi et al., 2017.

Next the researchers determined the abundances of all proteins present in the cell through a technique known as SWATH mass spectrometry. Yeast grown in high glucose conditions (2%, non-calorie restriction) have a change (increase or decrease) in the levels of 359 proteins compared to calorie restricted yeast. They found that aminoguanidine changed the expression of these proteins back to a state more similar to calorie restriction. Similar results were obtained with metformin and hydralazine but hydralazine was more efficient than metformin suggesting that the anti-glycation properties may be responsible for the change in protein expression. Many of the proteins that are downregulated by high glucose but rescued by the addition of a glycation inhibitor are involved in mitochondrial respiration.

Glycation inhibitors also affect mitochondrial respiration

Because glycation inhibitors were found to influence mitochondrial respiration the researchers decided to investigate the effect on reactive oxygen species (ROS). Glycation has previously been linked to a higher production of ROS in cells. In the current study the authors showed that all three glycation inhibitors reduced the ROS production. Next, the cells were treated with an inhibitor of complex III of the electron transport chain, antimycin A. Antimycin A reduced cell viability but this was rescued by all three glycation inhibitors. This experiment confirms that mitochondrial respiration plays a role in the lifespan extension by glycation inhibitors.

Kazi RS et al. (2017). Glycation inhibitors extend yeast chronological lifespan by reducing advanced glycation end products and by back regulation of proteins involved in mitochondrial respiration. J Proteomics [Epub ahead of print]