SirT1 Regulates Energy Metabolism and Response to Caloric Restriction in Mice Gino Boily1, Erin L. Seifert2, Lisa Bevilacqua2, Xiao Hong He 1 , Guillaume Sabourin1, Carmen Estey2, Cynthia Moffat1, Sean Crawford1, Sarah Saliba1, Karen Jardine1, Jian Xuan2, Meredith Evans2, Mary-Ellen Harper2, Michael W. McBurney1,2* 1 Center for Cancer Therapeutics, Ottawa Health Research Institute, Department of Medicine, University of Ottawa, Ottawa, Ontario, Canada, 2 Department of Biochemistry, Microbiology and Immunology, University of Ottawa, Ottawa, Ontario, Canada Abstract The yeast sir2 gene and its orthologues in Drosophila and C. elegans have well-established roles in lifespan determination and response to caloric restriction. We have studied mice carrying two alleles for SirT1, the mammalian orthologue of sir2, and found that these animals inefficiently utilize ingested food. These mice are hypermetabolic, contain inefficient liver mitochondria, and have elevated rates of lipid oxidation. When challenged with a 40% reduction in caloric intake, normal mice maintained their metabolic rate and increased their physical activity while the metabolic rate of SirT1-null mice dropped and their activity did not increase. Moreover, CR did not extend lifespan of SirT1-null mice. Thus, SirT1 is an important regulator of energy metabolism and, like its orthologues from simpler eukaryotes, the SirT1 protein appears to be required for a normal response to caloric restriction. Citation: Boily G, Seifert EL, Bevilacqua L, He XH, Sabourin G, et al. (2008) SirT1 Regulates Energy Metabolism and Response to Caloric Restriction in Mice. PLoS ONE 3(3): e1759. doi:10.1371/journal.pone.0001759 Editor: Edathara Abraham, University of Arkansas, United States of America Received January 15, 2008; Accepted February 6, 2008; Published March 12, 2008 Copyright: 2008 Boily et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: This project was funded by the Canadian Institutes of Health Research (CIHR; MM and MEH). GB and ES are recipients of fellowships from the Fonds de Recherche en Sante´ du Que´bec (FRSQ) and the Canadian Diabetes Association (CDA), respectively. Competing Interests: The authors have declared that no competing interests exist. * E-mail: mmcburney@ohri.ca Introduction Silent information regulator 2 (sir2) is a yeast gene encoding a nicotinamide adenine dinucleotide (NAD+)-dependent histone deacetylase [1,2]. Homologues of this protein are found through- out prokaryotes and eukaryotes [3]. In yeast, worms and flies, an extra copy of the sir2 gene or its orthologue increases lifespan by 18% to 50% [4–6] whereas a deletion of the gene, in yeast, reduces lifespan [4]. Mammals have 7 homologues of the Sir2 protein, Sirtuins 1-7 (SirT1-7) [7,8]. SirT1 is the family member sharing the most homology to Sir2 [8] and is considered to be its orthologue. It is not known whether SirT1 is involved in aging although it is frequently assumed to share this role with its sir2 orthologues. SirT1 can deacetylate histones [1,9] as well as a wide variety of nuclear and cytoplasmic proteins (reviewed in [10]). We [11] and others [12] have created mice carrying targeted mutations in the SirT1 gene. On the 129/J inbred background, SirT1-null mice die shortly after birth; however, on an outbred genetic background, most SirT1-null mice survive to adulthood and some can reach 24 months of age. SirT1-null mice look grossly normal but are small, sterile, have craniofacial abnormal- ities, and develop an eyelid inflammatory condition [11]. Several lines of evidence suggest that SirT1 plays a role in energy metabolism. The dependence on NAD+ as a cofactor for catalysis is thought to link SirT1 activity to the energetic state of the cell [1,2]. In mammals, glucose metabolism is regulated by insulin. SirT1 is known to promote insulin expression [13] and secretion [14,15] in pancreatic b-cells and to modulate the expression [16] and secretion [17] of adiponectin, a hormone that enhances insulin sensitivity. The metabolism of glucose, fatty acids and cholesterol is modulated in various cell types by the effects of SirT1 on known regulators of metabolic enzymes such as PPARc [18] and PGC-1a [19–21]. Calorie restriction (CR) has been known for decades to extend lifespan of virtually all organisms from yeast to mammals. Sir2 is required for CR to mediate lifespan extension in yeast and flies [6,22]. We set out to determine if SirT1 is required for CR to mediate its physiological response in mammals by studying the energy metabolism of SirT1 null-mice under ad libitum (AL) or CR diets. We report below that SirT1-null mice inefficiently utilize food, are hypermetabolic and that their liver mitochondria are functionally altered. Importantly, we present data suggesting that SirT1 is required for in vivo response to CR. Results Energy balance in SirT1- mice We have previously created mice that do not synthesize the SirT1 protein [11]. Early in their life (2–4 months old), SirT1-null mice are about 25% smaller than their normal littermates (Figure 1A). As they get older, this difference increases to 30% at 5–8 months and almost to 40% at 13–20 months (Figure 1A). Despite this difference in size, SirT1-null mice eat similar or only slightly less food than controls (Figure 1B). When daily food intake is normalized to body weight (BWT), it is evident that the SirT1- mice are hyperphagic (Figure 1C). This difference in food intake is not caused by inefficient food absorption by the digestive PLoS ONE | www.plosone.org 1 2008 | Volume 3 | Issue 3 | e1759