Calorie restriction extends lifespan and produces a metabolic profile desirable for treating diseases of ageing such as type 2 diabetes. SIRT1, an NAD+-dependent deacetylase, is a principal modulator of pathways downstream of calorie restriction that produce beneficial effects on glucose homeostasis and insulin sensitivity. Resveratrol, a polyphenolic SIRT1 activator, mimics the anti-ageing effects of calorie restriction in lower organisms and in mice fed a high-fat diet ameliorates insulin resistance, increases mitochondrial content, and prolongs survival. Here we describe the identification and characterization of small molecule activators of SIRT1 that are structurally unrelated to, and 1,000-fold more potent than, resveratrol. These compounds bind to the SIRT1 enzyme-peptide substrate complex at an allosteric site amino-terminal to the catalytic domain and lower the Michaelis constant for acetylated substrates. In diet-induced obese and genetically obese mice, these compounds improve insulin sensitivity, lower plasma glucose, and increase mitochondrial capacity. In Zucker fa/fa rats, hyperinsulinaemic-euglycaemic clamp studies demonstrate that SIRT1 activators improve whole-body glucose homeostasis and insulin sensitivity in adipose tissue, skeletal muscle and liver. Thus, SIRT1 activation is a promising new therapeutic approach for treating diseases of ageing such as type 2 diabetes.

Insulin resistance is a major metabolic feature of obesity and is a key factor in the etiology of a number of diseases, including type 2 diabetes. In this review, we discuss potential mechanisms by which brief nutrient excess and obesity lead to insulin resistance and propose that these mechanisms of action are different but interrelated. We discuss how pathways that "sense" nutrients within skeletal muscle are readily able to regulate insulin action. We then discuss how obesity leads to insulin resistance via a complex interplay among systemic fatty acid excess, microhypoxia in adipose tissue, ER stress, and inflammation. In particular, we focus on the hypothesis that the macrophage is an important cell type in the propagation of inflammation and induction of insulin resistance in obesity. Overall, we provide our integrative perspective regarding how nutrients and obesity interact to regulate insulin sensitivity.

Summary Adipose tissue hypoxia and inflammation has been causally implicated in obesity-induced insulin resistance. Here we report that early in the course of high fat diet (HFD) feeding and obesity, adipocyte respiration becomes uncoupled, leading to increased oxygen consumption and a state of relative adipocyte hypoxia. These events are sufficient to trigger HIF-1α induction, setting off the chronic adipose tissue inflammatory response characteristic of obesity. At the molecular level, these events involve saturated fatty acid stimulation of the adenine nucleotide translocase 2 (ANT2), an inner mitochondrial membrane protein, which leads to the uncoupled respiratory state. Genetic or pharmacologic inhibition of either ANT2 or HIF-1α can prevent or reverse these pathophysiologic events, restoring a state of insulin sensitivity and glucose tolerance. These results reveal the sequential series of events in obesity-induced inflammation and insulin resistance.

During early fasting, increases in skeletal muscle proteolysis liberate free amino acids for hepatic gluconeogenesis in response to pancreatic glucagon. Hepatic glucose output diminishes during the late protein-sparing phase of fasting, when ketone body production by the liver supplies compensatory fuel for glucose-dependent tissues 1–4. Glucagon stimulates the gluconeogenic program by triggering the dephosphorylation and nuclear translocation of the CREB regulated transcription coactivator 2 (CRTC2; also known as TORC2), while parallel decreases in insulin signaling augment gluconeogenic gene expression through the de-phosphorylation and nuclear shuttling of Forkhead Box O1 (FOXO1) 5–7. Here we show that a fasting-inducible switch, consisting of the histone acetyl-transferase (HAT) P300 and the nutrient-sensing deacetylase Sirtuin 1 (SIRT1), maintains energy balance through the sequential induction of CRTC2 and FOXO1. Following glucagon induction, CRTC2 stimulated gluconeogenic gene expression through an association with P300, which we show here is also activated by de-phosphorylation at Ser89 during fasting. In turn, P300 increased hepatic CRTC2 activity by acetylating it at Lys628, a site that also targets CRTC2 for degradation following its ubiquitination by the E3 ligase Constitutive Photomorphogenic Protein (COP1) 8. Glucagon effects were attenuated during late fasting, when CRTC2 was down-regulated due to SIRT1-mediated deacetylation and when FOXO1 supported expression of the gluconeogenic program. Disrupting SIRT1 activity, by liver-specific knockout of the SIRT1 gene or by administration of SIRT1 antagonist, increased CRTC2 activity and glucose output, while exposure to SIRT1 agonists reduced them. In view of the reciprocal activation of FOXO1 and its coactivator peroxisome proliferator activated receptor gamma coactivator 1 alpha (PGC-1α) by SIRT1 activators 9–12, our results illustrate how the exchange of two gluconeogenic regulators during fasting maintains energy balance.

Summary Chronic, low-grade inflammation, particularly in adipose tissue, is an important modulator of obesity-induced insulin resistance and the toll-like receptor 4 (Tlr4) is a key initiator of inflammatory responses in macrophages. We performed bone marrow transplantation (BMT) of Tlr4lps-del or control C57Bl/10J bone marrow cells into irradiated wild type C57Bl6 recipient mice to generate hematopoietic cell specific Tlr4 deletion mutant (BMT-Tlr4-/-) and control (BMT-wt) mice. When mice were fed a high-fat diet (HFD) for 16 weeks, BMT-wt mice developed obesity, hyperinsulinemia, glucose intolerance and insulin resistance. In contrast, BMT-Tlr4-/- mice became obese, but did not develop fasting hyperinsulinemia, and had improved hepatic and skeletal muscle insulin sensitivity during euglycemic clamp studies compared to HFD BMT-wt mice. The HFD BMT-Tlr4-/- mice showed markedly reduced adipose tissue inflammatory markers and macrophage content compared to HFD BMT-wt mice. In summary, our results indicate that Tlr4 signaling in hematopoietic-derived cells is important for the development of hepatic and adipose tissue insulin resistance in obese mice.

Fatty acid oversupply is a key mediator of skeletal muscle insulin resistance in obesity, primarily via accumulation of fatty acid metabolites and activation of proinflammatory pathways. Herein, we demonstrate that fatty acid-induced insulin resistance in humans is completely prevented the day after 1 session of endurance exercise. Because skeletal muscle is the primary site for systemic glucose disposal and is highly susceptible to impaired insulin action by elevated fatty acid availability, we obtained skeletal muscle samples to investigate possible mechanisms mediating this protective effect of exercise. Prevention of fatty acid-induced insulin resistance after exercise accompanied enhanced skeletal muscle protein expression of key lipogenic enzymes and an increase in muscle triglyceride synthesis. Partitioning more fatty acids toward triglyceride synthesis within muscle reduced the accumulation of fatty acid metabolites and suppressed the proinflammatory response in skeletal muscle, as evidenced by decreased phosphorylation and activation of JNK and increased abundance of inhibitor of NF-κB α (IκB-α) and IκB-β. We believe this is the first study to demonstrate that 1 session of exercise completely reverses fatty acid-induced insulin resistance in humans. Reversal of insulin resistance accompanied enhanced lipogenic capacity within skeletal muscle, reduced accumulation of highly bioactive fatty acid metabolites, and suppressed activation of proinflammatory pathways known to impair insulin action. IntroductionExcessive fatty acid mobilization found in abdominal obesity is a key mediator of many obesity-related metabolic complications, including insulin resistance (1-3). A high rate of fatty acid availability and subsequent uptake by skeletal muscle can augment intramuscular triglyceride (IMTG) accumulation (1, 3), and there is a strong correlation between IMTG concentration and the severity of insulin resistance in obese subjects and individuals with type 2 diabetes (4, 5). Recent studies, however, clarify that IMTG accumulation likely does not directly affect insulin action (6-9) but instead appears to be a biologically inert reservoir for fatty acids that can serve as a marker for high rates of fatty acid flux into muscle. Alternatively, it is now apparent that highly bioactive fatty acid metabolites, such as diacylglycerol and ceramide (and not IMTG), play a pivotal role in mediating fatty acid-induced insulin resistance in muscle (10-12). Considering the negative effects of diacylglycerol and ceramide on insulin action, we propose what we believe to be a novel hypothesis that, under conditions of high fatty acid flux, enhancing the partitioning of fatty acids entering the muscle toward IMTG synthesis will reduce the formation and accumulation of more damaging fatty acid metabolites within the cell, thereby improving insulin action (8).

SIRT1 is a prominent member of a family of NAD؉ -dependent enzymes and affects a variety of cellular functions ranging from gene silencing, regulation of the cell cycle and apoptosis, to energy homeostasis. In mature adipocytes, SIRT1 triggers lipolysis and loss of fat content. However, the potential effects of SIRT1 on insulin signaling pathways are poorly understood. To assess this, we used RNA interference to knock down SIRT1 in 3T3-L1 adipocytes. SIRT1 depletion inhibited insulin-stimulated glucose uptake and GLUT4 translocation. This was accompanied by increased phosphorylation of JNK and serine phosphorylation of insulin receptor substrate 1 (IRS-1), along with inhibition of insulin signaling steps, such as tyrosine phosphorylation of IRS-1, and phosphorylation of Akt and ERK. In contrast, treatment of cells with specific small molecule SIRT1 activators led to an increase in glucose uptake and insulin signaling as well as a decrease in serine phosphorylation of IRS-1. Moreover, gene expression profiles showed that SIRT1 expression was inversely related to inflammatory gene expression. Finally, we show that treatment of 3T3-L1 adipocytes with a SIRT1 activator attenuated tumor necrosis factor alpha-induced insulin resistance. Taken together, these data indicate that SIRT1 is a positive regulator of insulin signaling at least partially through the anti-inflammatory actions in 3T3-L1 adipocytes.

padhyay G, Olefsky JM. SIRT1 inhibits inflammatory pathways in macrophages and modulates insulin sensitivity. Am J Physiol Endocrinol Metab 298: E419 -E428, 2010. First published December 8, 2009; doi:10.1152/ajpendo.00417.2009.-Chronic inflammation is an important etiology underlying obesity-related disorders such as insulin resistance and type 2 diabetes, and recent findings indicate that the macrophage can be the initiating cell type responsible for this chronic inflammatory state. The mammalian silent information regulator 2 homolog SIRT1 modulates several physiological processes important for life span, and a potential role of SIRT1 in the regulation of insulin sensitivity has been shown. However, with respect to inflammation, the role of SIRT1 in regulating the proinflammatory pathway within macrophages is poorly understood. Here, we show that knockdown of SIRT1 in the mouse macrophage RAW264.7 cell line and in intraperitoneal macrophages broadly activates the JNK and IKK inflammatory pathways and increases LPS-stimulated TNF␣ secretion. Moreover, gene expression profiles reveal that SIRT1 knockdown leads to an increase in inflammatory gene expression. We also demonstrate that SIRT1 activators inhibit LPS-stimulated inflammatory pathways, as well as secretion of TNF␣, in a SIRT1-dependent manner in RAW264.7 cells and in primary intraperitoneal macrophages. Treatment of Zucker fatty rats with a SIRT1 activator leads to greatly improved glucose tolerance, reduced hyperinsulinemia, and enhanced systemic insulin sensitivity during glucose clamp studies. These in vivo insulin-sensitizing effects were accompanied by a reduction in tissue inflammation markers and a decrease in the adipose tissue macrophage proinflammatory state, fully consistent with the in vitro effects of SIRT1 in macrophages. In conclusion, these results define a novel role for SIRT1 as an important regulator of macrophage inflammatory responses in the context of insulin resistance and raise the possibility that targeting of SIRT1 might be a useful strategy for treating the inflammatory component of metabolic diseases. macrophage; insulin resistance FOR MANY YEARS, IT HAS BEEN KNOWN that caloric restriction extends life span over a wide range of species, including mammals (27). Silent information regulator 2 (Sir2) is a NADdependent deacetylase that is one of the components connecting the metabolic effects of caloric restriction to longevity in yeast, worms, and flies (7). Mammals express 7 homologs of yeast Sir2, identified as the SIRTUIN family, SIRT1-7 (7). SIRT1 has the closest homology to Sir2, and recent data suggest that activation of SIRT1 may be, at least partially, responsible for the extension of life span in mammals (4, 5, 7).