The extension in lifespan achieved by eating fewer calories depends on activation of sirtuins, a family of NAD-dependent protein deacetylases that control the expression of key metabolic pathway genes. But the sirtuins may regulate metabolism in a more direct manner as well, according to a report in the June 21 PNAS online. John Denu and colleagues at the University of Wisconsin Medical School in Madison show that two mammalian sirtuins, SIRT1 and SIRT3, directly deacetylate and activate the metabolic enzyme Acetyl-CoA synthetase. The significance to aging is unclear as yet, but the results show that the regulation of energy pathways by sirtuins occurs at multiple levels.
The finding is of interest to those studying diseases of aging and may be of particular interest to those studying Alzheimer disease (AD) given that calorie restriction (CR) reduces Aβ production and amyloid pathology in mice (Wang et al., 2005; Patel et al., 2005). Coincidentally, another recent report from Giulio Pasinetti’s laboratory at the Mt. Sinai School of Medicine, New York, shows that neuronal SIRT1 has a hand in that process, too, by enhancing the non-amyloidogenic α-secretase cleavage of the amyloid precursor protein. In other aging news, Korean researchers report that a kinase inhibitor, CGK733, may delay cellular senescence.
First, the acetate story. The enzyme AcetylCoA synthetase (AceCS) metabolically activates acetate by conjugating it to CoA, after which the AcetylCoA (AcCoA) can be used as an acetyl donor or building block for fatty acids, or burned for energy in mitochondria. Starting from the observation that sirtuin deacetylates and regulates an AceCS in bacteria, authors William Hallows, Susan Lee and Denu wondered whether mammalian AceCSs might do the same. They showed that mouse cytosolic AcetylCoA synthetase, AceCS1, became acetylated on a catalytic residue in cultured cells, and that expression of SIRT1 in cells dramatically reduced the amount of the acetylated protein. Of all the mammalian sirtuins, 2-7, only SIRT1 (the sirtuin associated with life extension by CR) caused the deacetylation of AceCS1 in cells.
Sirtuin-catalyzed deacetylation activated AceCS1 in the test tube, and also when expressed in cells. Acetylated recombinant AceCS1 was inactive, but incubation with SIRT1 rendered it fully active. In cells, overexpression of AceCS1 led to a 60 percent increase in the incorporation of acetate into fatty acids. Coexpression with SIRT1, but not SIRT2, led to an additional increase, more than doubling the rate of fatty acid synthesis compared to vector alone.
In vitro assays with purified recombinant sirtuins revealed that SIRT3, which did not deacetylate AceCS1 in cells, was as potent at activating the enzyme as SIRT1. In cells, SIRT3 is located in mitochondria, while both AceCS1 and SIRT1 are cytosolic. Mammals do have a mitochondrial AcetylCoA synthetase, AceCS2, and the researchers showed that SIRT3 deacetylates AceCS2 in vitro, raising the possibility that SIRT3/AceCS2 could be the mitochondrial counterpart to the SIRT1/AceCS1 pair.
Both SIRT1 and SIRT3 have been implicated in regulating lifespan—SIRT1 is required for life extension by calorie restriction in animal models, while SIRT3 gene variants have been linked to long life in the elderly (Rose et al., 2003; Bellizzi et al., 2005). Though acetate metabolism is impaired in older people (Skutches et al., 1979), its role in aging and the significance of sirtuin regulation remain to be seen.
It is clear that at the same time a semi-starvation regimen is staving off death, it delays aging and the onset of aging-related diseases. The reduction of Aβ production and accumulation in mice as a result of calorie restriction is mediated by SIRT1 in neurons, according to Pasinetti and colleagues’ new study. In press in the Journal of Biological Chemistry since June 2, their paper shows that SIRT1 decreases Aβ generation and increases non-amyloidogenic amyloid precursor protein processing by α-secretase. SIRT1 acts by inhibiting ROCK1 kinase expression, a condition that has been previously shown to increase α-secretase (see ARF related news story). The results further support the idea that sirtuin activators, like the red wine ingredient resveratrol, might be useful for slowing the accumulation of Aβ (Chen et al., 2005; Marambaud et al., 2005; ARF related news story).
Finally, Korean researchers are reporting a new way around the aging that occurs at the cellular level—replicative senescence. In an online publication in Nature Chemical Biology, Tae Kook Kim and colleagues at the Korea Advanced Institute of Science and Technology in Daejeon describe a high-throughput, cell-based screen for compounds that reverse cell senescence. For a model, they induced senescence by expression of a dominant negative form of telomeric repeat factor-2 (TRF2) protein, a protein essential for telomere formation.
The screen turned up a small molecule that could reversibly kick-start replication of normal senescent cells. Using a novel magnetic nanoprobe capture technology, the investigators identified the target of the compound (a kinase inhibitor) as the ataxia telangiectasia mutated (ATM) protein, and to a lesser extent ATM- and Rad3-related protein (ATR), both checkpoint proteins that regulate p53 and cell replication in response to DNA damage. The compound was selective for ATM and ATR kinase activities—it did not inhibit the related PI3K, or downstream pathways like AKT activation.
Loss of ATM function in humans results in ataxia telangiectasia, where cells show accelerated senescence, yet treatment with CGK733 had the opposite effect of delaying senescence. The authors suggest that the compound may “fine-tune” the elevated kinase activity of ATM in senescent cells so that it is below the threshold for inducing senescence but still adequate for the other functions including telomere maintenance, control of ROS and DNA damage, and DNA replication.—Pat McCaffrey