Sirtuins and the hepatic regulation of energy homeostasis

Authors

  • Puneet Puri M.D.,

    1. Division of Gastroenterology, Hepatology, and Nutrition, Department of Internal Medicine, Virginia Commonwealth University, School of Medicine, Richmond, VA
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  • Arun J. Sanyal M.D.

    1. Division of Gastroenterology, Hepatology, and Nutrition, Department of Internal Medicine, Virginia Commonwealth University, School of Medicine, Richmond, VA
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  • Potential conflict of interest: Dr. Sanyal received grants from Sanofi-Aventis, Salix, Intercept, and Roche.

Nie Y, Erion DM, Yuan Z, Dietrich M, Shulman GI, Horvath TL, et al. STAT3 inhibition of gluconeogenesis is downregulated by SirT1. Nat Cell Biol 2009;11:492–500. (Reprinted with permission.)

Abstract

The fasting-activated longevity protein sirtuin 1 (SirT1) promotes gluconeogenesis in part, by increasing transcription of the key gluconeogenic genes pepck1 and g6pase, through deacetylating PGC-1α and FOXO1. In contrast, signal transducer and activator of transcription 3 (STAT3) inhibits glucose production by suppressing expression of these genes. It is not known whether the inhibition of gluconeogenesis by STAT3 is controlled by metabolic regulation. Here we show that STAT3 phosphorylation and function in the liver were tightly regulated by the nutritional status of an animal, through SirT1-mediated deacetylation of key STAT3 lysine sites. The importance of the SirT1-STAT3 pathway in the regulation of gluconeogenesis was verified in STAT3-deficient mice in which the dynamic regulation of gluconeogenic genes by nutritional status was disrupted. Our results reveal a new nutrient sensing pathway through which SirT1 suppresses the inhibitory effect of STAT3, while activating the stimulatory effect of PGC-1α and FOXO1 on gluconeogenesis, thus ensuring maximal activation of gluconeogenic gene transcription. The connection between acetylation and phosphorylation of STAT3 implies that STAT3 may have an important role in other cellular processes that involve SirT1.

Comment

The availability of energy to meet the metabolic needs for normal cell function is a fundamental requirement for life. The principal sources of such energy come from the oxidation of carbohydrates and fatty acids that release energy which is captured in the form of adenosine triphosphate (ATP), the metabolic currency used by cells for energy. Although many tissues can utilize both forms of energy sources, several tissues—e.g., the brain—are critically dependent on glucose as the primary metabolic substrate for energy generation. Elaborate mechanisms have thus developed to store excess energy on the one hand and to be able to provide a steady source of glucose to these tissues on the other. The liver and the pancreatic hormones insulin and glucagon play a central role in maintaining this balance.

Under fasting conditions, α-pancreatic cells release glucagon and induce hepatic nuclear translocation of transducer of regulated CREBP (cyclic adenosine monophosphate responsive element binding protein) activity 2 (TORC2).1 TORC2 then binds to and activates the transcription factor CREBP which stimulates gluconeogenesis and fatty acid oxidation through the induction of peroxisome proliferator-activated receptor gamma-coactivator-1α (PGC-1α). PGC-1α then binds to forkhead box protein O1 (FoxO1) and/or hepatocyte nuclear factor 4α (HNF4α) and induces transcription of phosphoenolpyruvate carboxykinase (PEPCK) and glucose-6-phosphatase (G6Pase). Under fed conditions, insulin is released and inhibits gluconeogenesis via the activation of Akt, which phosphorylates and inhibits several components of the gluconeogenic pathway including peroxisome proliferator-activated receptor gamma coactivator-alpha (PGC1-α) and FoxO1.

Sirtuin 1 (SirT1) is one of the seven mammalian homologs (sirtuins SirT1 through SirT7) of the yeast protein silent information regulator 2 (Sir2), a conserved oxidized nicotinamide adenine dinucleotide (NAD+)-dependent protein deacetylase and has emerged as another important regulator of gluconeogenesis (Fig. 1). Sirtuins are class III histone deacetylases and are distinct from other classes in that their activity depends on NAD+. SirT1 exerts its functions through deacetylation of target proteins, such as histones, transcription factors, and coregulators. Sirtuins are considered to provide a molecular link between cellular metabolic status, as expressed by NAD+/NADH levels, and adaptive transcriptional responses. Although Sir2 has been linked to longevity and caloric restriction in yeast, worms, and flies, such a role for SirT1 in higher species remains unknown. Recent data have, however, implicated a central role for SirT1 in the regulation of glucose homeostasis.

Figure 1.

Regulation of gluconeogenesis by SirT1. Fasting induces SirT1 which is mediated by increased pyruvate/NAD+ as a result of glycolysis. SirT1 deacetylates transcriptional coactivator PGC-1α and up-regulates transcriptional factors FoxO1 and HNF-4α. These transcriptional factors promote gluconeogenesis by activating PEPCK and G6Pase. FoxO1 is generally under negative control of insulin which decreases gluconeogenesis. Similarly, STAT3 inhibits glucose production by suppressing expression of these genes. In a newly discovered pathway, SirT1 suppresses the key transcriptional regulator STAT3, further promoting gluconeogenesis and simultaneously down-regulating glycolysis. Abbreviations: FoxO1, forkhead box protein O1; G6Pase, glucose-6-phosphatase; HNF-4α, hepatocyte nuclear factor 4α; PEPCK, phosphoenolpyruvate carboxykinase; PGC-1α, peroxisome proliferator-activated receptor gamma-coactivator-1α; SirT1, silent information regulator (sirtuin 1); STAT3, signal transducer and activator of transcription 3.

Nutrient availability determines the NAD+ and pyruvate levels within hepatocytes. In the fasted state, increased glycolytic activity leads to increased pyruvate and NAD+ levels. SirT1 activation by these signals deacetylates PGC-1α thereby up-regulating PEPCK and G6Pase and increasing gluconeogenesis.2 Simultaneously, glycolytic pathways are down-regulated. These effects of PGC-1α depend on SirT1 and cannot be reproduced in the absence of SirT1.3

The current study by Nie et al.4 provides further evidence of additional complexity in the ways SirT1 regulates gluconeogenesis by interacting with the key transcriptional regulator signal transducer and activator of transcription 3 (STAT3). STAT3 inhibits PEPCK and G6Pase by inhibition of PGC1α and potentially by independent mechanisms as well.5 These investigators demonstrated that SirT1 deacetylates key lysine sites in STAT3, thereby inhibiting the suppressive effect of STAT3 phosphorylation on gluconeogenic pathways. Thus, SirT1 enhances the glyconeogenic response by disinhibition of STAT3 effects on the one hand and promoting the activity of PGC-1α and FoxO1 on the other.

These data have potentially important implications for the development of type 2 diabetes mellitus (T2DM) and nonalcoholic fatty liver disease. An important pathophysiologic hallmark of T2DM is fasting hyperglycemia which largely reflects hepatic glucose output due mainly to gluconeogenesis.6 If SirT1 is indeed a critical player in driving hepatic gluconeogenesis, this could represent a druggable target for therapy of T2DM. Indeed, in an animal model of T2DM, inhibition of hepatic SirT1 by antisense oligonucleotides lowered both fasting glucose concentration and hepatic glucose output.7 This was accompanied by decreased PEPCK and G6Pase expression and increased acetylation of PGC-1α as well as STAT3. These data further corroborate the findings of Nie et al. and support a central role for SirT1 in the pathogenesis of T2DM.

There are, however, other mechanisms which may counter some of the prodiabetogenic effects of SirT1. SirT1 represses the transcription of protein tyrosine phosphatase-1, an important regulator of the phosphorylation status of the insulin receptor substrate-1 (IRS-1). Thus, more prolonged IRS-1 phosphorylation is expected upon exposure to insulin when SirT1 activity is increased. Indeed, it has been shown that after exposure to insulin, downstream Akt activity is diminished when SirT1 has been knocked down compared to normal cells. Conversely, SirT1 activation was associated with improved insulin signaling, increased GLUT4 translocation to cell surface, and consequently, enhanced glucose uptake in peripheral tissues.

The situation is further complicated by the effects of SirT1 on other pathophysiologic processes relevant to the genesis of T2DM. Impaired insulin-mediated suppression of lipolysis is an important pathophysiologic hallmark of the insulin-resistant state that drives the development of T2DM. There are conflicting reports of the effects of SirT1 on the expression of adiponectin, a key insulin-sensitizing hormone, in adipose tissue.8, 9 Increased peripheral lipolysis is associated with impaired glucose clearance by striated muscle and increased fasting fatty acid levels. The pancreas responds by increasing insulin release. The hyperinsulinemia associated with insulin resistance may also be modulated by SirT1. SirT1 overexpression causes membrane depolarization and Ca2+-dependent exocytosis of insulin.10 Eventual pancreatic β cell failure from prolonged synthetic demand and uncorrected endoplasmic reticulum stress eventually lead to β cell apoptosis and a fall in insulin levels, which usually precede development of overt T2DM. SirT1 may modulate this process by activation of NeuroD and MafA via deacetylation of FoxO1; these factors preserve β cell survival and insulin secretion in vivo.11

Based on these data, it is difficult to predict the consequence of generalized SirT1 suppression in the insulin-resistant state. In addition to the various prodiabetogenic and antidiabetogenic effects of SirT1, SirT1 also inhibits fatty acid synthesis and increases hepatic cholesterol disposal by activating the orphan nuclear receptor liver X receptor (LXR) which increases the conversion of cholesterol to bile acids via cytochrome P450 7a1.12 SirT1 suppression may potentially worsen hepatic steatosis and impair cholesterol clearance and thereby contribute worsening liver injury if the subject also has nonalcoholic steatohepatitis. The situation is further complicated by the observation that although lipogenic genes are not suppressed normally in the fasting state in sirT1 knockout mice, they also fail to gain weight and develop a fatty liver when given a high-fat diet.13

The interactions between STAT3 and SirT1 may have other consequences that are germane for the development of obesity, T2DM, and nonalcoholic steatohepatitis. Leptin signaling is partly mediated via phosphorylation of STAT3.14 Conceivably, if the phosphorylation of STAT3 and its downstream activities are affected by the acetylation status of STAT3, then SirT1 may turn out to be an important modulator of the metabolic actions of leptin in hepatocytes and the profibrogenic effects of leptin in promoting disease progression in chronic liver disease, especially nonalcoholic steatohepatitis. SirT1 is also prominently expressed in the hypothalamus, where it modulates sex hormone homeostasis.15 It is possible that increased SirT1 activity further impairs the anorexogenic properties of leptin in obese subjects and further drives eating habits and obesity. The current study opens the door and provides a rationale for studies to evaluate these possibilities.

In summary, SirT1 has important metabolic effects through complex transcriptional regulation of key genes. Both enhancing and suppressing SirT1 can produce beneficial effects which also depend on the nutritional status, whether fasting or fed. The current study provides new insights in to the interactions of SirT1 with transcriptional factors and may potentially explain some of the variable effects of SirT1 in various pathways. Such information is likely to be important in understanding the impact of SirT1 on the development of T2DM and nonalcoholic steatohepatitis and leveraging this information into development of novel therapeutic targets for these conditions.

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