SUMMARY Hepatic metabolic derangements are key components in the development of fatty liver, insulin resistance, and atherosclerosis. SIRT1, a NAD+-dependent protein deacetylase, is an important regulator of energy homeostasis in response to nutrient availability. Here we demonstrate that hepatic SIRT1 regulates lipid homeostasis by positively regulating PPARα, a nuclear receptor that mediates the adaptive response to fasting and starvation. Hepatocyte-specific deletion of SIRT1 impairs PPARα signaling and decreases fatty acid β-oxidation, whereas overexpression of SIRT1 induces the expression of PPARα targets. SIRT1 interacts with PPARα and is required to activate PPARα co-activator PGC-1α. When challenged with a high-fat diet, liver-specific SIRT1 knockout mice develop hepatic steatosis, hepatic inflammation, and endoplasmic reticulum stress. Taken together, our data indicate that SIRT1 plays a vital role in the regulation of hepatic lipid homeostasis, and that pharmacological activation of SIRT1 may be important for the prevention of obesity-associated metabolic diseases.

Macrophage activation and infiltration into resident tissues is known to mediate local inflammation and is a hallmark feature of metabolic syndrome. Members of the sirtuin family of proteins regulate numerous physiological processes, including those involved in nutrient regulation and the promotion of longevity. However, the important role that SIRT1, the leading sirtuin family member, plays in immune response remains unclear. In this study, we demonstrate that SIRT1 modulates the acetylation status of the RelA/p65 subunit of NF-B and thus plays a pivotal role in regulating the inflammatory, immune, and apoptotic responses in mammals. Using a myeloid cell-specific SIRT1 knockout (Mac-SIRT1 KO) mouse model, we show that ablation of SIRT1 in macrophages renders NF-B hyperacetylated, resulting in increased transcriptional activation of proinflammatory target genes. Consistent with increased proinflammatory gene expression, Mac-SIRT1 KO mice challenged with a high-fat diet display high levels of activated macrophages in liver and adipose tissue, predisposing the animals to development of systemic insulin resistance and metabolic derangement. In summary, we report that SIRT1, in macrophages, functions to inhibit NF-B-mediated transcription, implying that myeloid cell-specific modulation of this sirtuin may be beneficial in the treatment of inflammation and its associated diseases.Chronic inflammation is increasingly recognized as a causal factor leading to the development of obesity, insulin resistance, and type 2 diabetes (15, 31). This low-grade inflammatory state is in part mediated by macrophages, key sentinels of the innate immune system. Macrophages quiescently monitor the tissue milieu for signs of infection or damage (13,25). Upon stimulation, macrophages infiltrate resident tissue, perpetuating local inflammation and contributing to the development of insulin resistance and metabolic derangements (17,37,43). The nuclear factor kappa B (NF-B) transcription factor signaling pathway is a key mediator of immune response in macrophages (5, 7). NF-B is composed of a heterodimer of p50 and RelA/ p65 subunits. In unstimulated cells, NF-B resides in the cytoplasm bound to its inhibitory proteins, which are members of the inhibitor of B (IB) family. Stimulation of cells by environmental factors, including dietary fatty acids, liberates NF-B, allowing it to translocate to the nucleus, where it mediates gene transcription (12). Under environmental stresses, such as those surrounding obesity-like conditions, this chain of events is believed to ultimately lead to insulin resistance, setting in motion the vicious cycle of the metabolic syndrome.Sirtuins are highly conserved NAD ϩ -dependent deacetylases that target histones, transcription factors, coregulators, and other key regulators to adapt gene expression and metabolism to the cellular energy state (16,22,32). SIRT1, the leading family member, has been reported to promote longevity in species ranging from yeast to flies (1)(2)(3)6). It is believed that these life-exte...

The sterol regulatory element-binding protein (SREBP) transcription factor family is a critical regulator of lipid and sterol homeostasis in eukaryotes. In mammals, SREBPs are highly active in the fed state to promote the expression of lipogenic and cholesterogenic genes and facilitate fat storage. During fasting, SREBP-dependent lipid/cholesterol synthesis is rapidly diminished in the mouse liver; however, the mechanism has remained incompletely understood. Moreover, the evolutionary conservation of fasting regulation of SREBP-dependent programs of gene expression and control of lipid homeostasis has been unclear. We demonstrate here a conserved role for orthologs of the NAD + -dependent deacetylase SIRT1 in metazoans in down-regulation of SREBP orthologs during fasting, resulting in inhibition of lipid synthesis and fat storage. Our data reveal that SIRT1 can directly deacetylate SREBP, and modulation of SIRT1 activity results in changes in SREBP ubiquitination, protein stability, and target gene expression. In addition, chemical activators of SIRT1 inhibit SREBP target gene expression in vitro and in vivo, correlating with decreased hepatic lipid and cholesterol levels and attenuated liver steatosis in dietinduced and genetically obese mice. We conclude that SIRT1 orthologs play a critical role in controlling SREBPdependent gene regulation governing lipid/cholesterol homeostasis in metazoans in response to fasting cues. These findings may have important biomedical implications for the treatment of metabolic disorders associated with aberrant lipid/cholesterol homeostasis, including metabolic syndrome and atherosclerosis. Lipids and sterols play key roles in diverse biological processes in eukaryotes, such as membrane biosynthesis, intra-and extracellular signaling, and energy storage. In humans, aberrant lipid and cholesterol homeostasis has been linked to a number of diseases prevalent in the developed world, including metabolic syndrome-a constellation of conditions and diseases that includes obesity, insulin resistance, liver steatosis, and hypertension, as well as type 2 diabetes, cardiovascular disease, and cancers (Cornier et al. 2008). An improved understanding of the molecular mechanisms governing lipid/cholesterol homeostasis might lead to novel therapeutic strategies to ameliorate such diseases.Fasting (short-term food deprivation) produces a rapid metabolic shift from lipid/cholesterol synthesis and fat storage to mobilization of fat, and recent studies have suggested that fasting may improve conditions associated with metabolic syndrome (Varady and Hellerstein 2008;Fontana et al. 2010). There is thus keen interest in determining the mechanism of fasting-dependent regulation of lipid/cholesterol metabolism to facilitate the development

Background & Aims Sirtuin 1 (SIRT1), the most conserved mammalian NAD+-dependent protein deacetylase, is an important metabolic sensor in many tissues. However, little is known about its role in the small intestine, which absorbs and senses nutrients. We investigated the functions of intestinal Sirt1 in systemic bile acid and cholesterol metabolism in mice. Methods Sirt1 was specifically deleted from intestines of mice using the Flox-villin-Cre system (Sirt1 iKO mice). Intestinal and heptic tissues were collected, and bile acid absorption was analyzed using the everted gut sac experiment. Systemic bile acid metabolism was studied in Sirt1 iKO and Flox control mice placed on standard diets, diets containing 0.5% cholic acid or 1.25% cholesterol, or lithogenic diets. Results Sirt1 iKO mice had reduced intestinal Fxr signaling via Hnf1a compared with controls, which reduced expression of the bile acid transporter genes Asbt and Mcf2l (encodes Ost) and absorption of ileal bile acids. Sirt1 regulated Hnf1α–Fxr signaling partially through Dcoh2, which increases dimerization of Hnf1α. Sirt1 was found to deacetylate DCoH2, promoting its interaction with Hnf1α and inducing DNA binding by Hnf1α. Intestine-specific deletion of Sirt1 increased hepatic bile acid biosynthesis, reduced hepatic accumulation of bile acids, and protected animals from liver damage from high-bile acid diets. Conclusions Intestinal Sirt1, a key nutrient sensor, is required for ileal bile acid absorption and systemic bile acid homeostasis in mice. We delineated the mechanism of metabolic regulation of Hnf1α–Fxr signaling. Reagents designed to inhibit intestinal SIRT1 might be developed to treat bile acid-related diseases such as cholestasis.