For example, an impressive study carried out by Nir Barzilai’s group showed that removal of white adipose tissues (WAT)
from rats can mimic the positive effects of DR, suggesting that WAT might mediate the DR response, or that DR regulates WAT.14 However, although DR-increased lifespan was discovered over 80 years ago, the detailed mechanisms underlying DR effects are still elusive. Sir2 and its homologue sirtuins became more attractive to the global aging community when a series of publications demonstrated that sirtuins are pivotal in the regulation of longevity in lower metazoans. Similar to the findings in the yeast Saccharomyces cerevisiae, increasing the Inhibitors,research,lifescience,medical AVL-301 research buy activity of sirtuins from Caenorhabditis elegans15 and Drosophila melanogaster,16 using either genetic17 or chemical means,18 also extends their lifespan by at least 15%. Inhibitors,research,lifescience,medical Therefore, it was suggested that the role of sirtuins in regulating lifespan is evolutionarily conserved, and understanding its regulation at the molecular level thus has great therapeutic opportunities. How are sirtuins regulated? To date we know of multiple layers of sirtuin regulation. Initially, an intense debate took place on this matter. At first, it was suggested that NAD+ levels regulate yeast Sir2 activity.19 However, in Inhibitors,research,lifescience,medical yeast, exact measurements showed that cellular
concentrations of NAD+ are around 4 mM. Thus, given that the Km of Sir2 for NAD+ is around 50 μM, a 10-fold change in NAD+ concentration was required to
affect Sir2 activity.20 Therefore, the Sinclair group suggested Inhibitors,research,lifescience,medical that sirtuin activity is regulated by nicotinamide (NAM), one of the products of its NAD+ dependent deacetylase enzymatic activity.21 On the other hand, it was recognized that fluctuations in NAD levels cannot provide a reasonable model for sirtuin regulation, and the Guarente group suggested that NADH, Inhibitors,research,lifescience,medical which exists in the micromolar range in the cell, inhibits Sir2 enzymatic activity.22 A recent study showed that yeast Sir2 activity can be regulated by both NADH and NAM: With high DR (0.1% glucose), clearance of NAM regulates Sir2 activity, whereas with low DR (0.5% glucose) a reduced NADH/NAD ratio regulates Sir2 activity.23 What controls endogenous NAM levels? In yeast, PNC1 enzyme converts NAM, a sirtuin inhibitor, into nicotinic acid (NA), which does not inhibit Sir2. This NA is later used by the NAD salvage ADP ribosylation factor pathway to generate NAD+. A sophisticated study by Anderson et al. demonstrates that PNC1 levels are regulated by nutrient availability.21 Under DR conditions, PNC1 levels and activity increase, resulting in increased Sir2 activity. This model was expanded for other mild stressed conditions such as increased osmolarity and heat, in which PNC1-dependent clearance of NAM under various stresses increases yeast lifespan via Sir2 up-regulation.