Sirt1 protects against high-fat diet-induced metabolic damage

Abstract

The identification of new pharmacological approaches to effectively prevent, treat, and cure the metabolic syndrome is of crucial importance. Excessive exposure to dietary lipids causes inflammatory responses, deranges the homeostasis of cellular metabolism, and is believed to constitute a key initiator of the metabolic syndrome. Mammalian Sirt1 is a protein deacetylase that has been involved in resveratrol-mediated protection from high-fat diet-induced metabolic damage, but direct proof for the implication of Sirt1 has remained elusive. Here, we report that mice with moderate overexpression of Sirt1 under the control of its natural promoter exhibit fat mass gain similar to wild-type controls when exposed to a high-fat diet. Higher energy expenditure appears to be compensated by a parallel increase in food intake. Interestingly, transgenic Sirt1 mice under a high-fat diet show lower lipid-induced inflammation along with better glucose tolerance, and are almost entirely protected from hepatic steatosis. We present data indicating that such beneficial effects of Sirt1 are due to at least two mechanisms: induction of antioxidant proteins MnSOD and Nrf1, possibly via stimulation of PGC1alpha, and lower activation of proinflammatory cytokines, such as TNFalpha and IL-6, via down-modulation of NFkappaB activity. Together, these results provide direct proof of the protective potential of Sirt1 against the metabolic consequences of chronic exposure to a high-fat diet.

Fig. 1.

Generation of a BAC-based transgenic Sirt1 mouse line. (A) Scheme of the Bacterial Artificial Chromosome (BAC) used to generate the Sirt1-tg mouse strain. (B) Levels of Sirt1 mRNA (Upper) and Sirt1 protein (Lower) in mouse embryo fibroblasts. (C) Levels of Sirt1 protein in the indicated tissues.

Fig. 2.

Energy homeostasis of transgenic Sirt1 mice. (A) Body weight of wild type (WT) or transgenic (tg) Sirt1 mice during exposure to a standard chow diet (SD) or a high-fat diet (HFD). (B) Food intake. (C) Fat mass in WT and tg mice after 19 weeks of exposure to SD or HFD. (D) Lean mass. (E and F) Energy expenditure per kg of body weight after 8 weeks of SD or HFD: 5-day measurements in SD mice (E Left) or HFD mice (E Right), and total mean values (F). n = 7–8 per group; means ± SEM; *, P < 0.05.

Fig. 3.

Transgenic Sirt1 mice are protected from HFD-induced hepatosteatosis. (A) Oil red staining lipid droplets in frozen liver sections (3 per group) from both wild-type (WT) or transgenic (tg) mice on a standard diet (SD) or after high-fat diet (HFD) exposure for 19 weeks. (Scale bars, 50 μm.) (B) Gene expression analysis (real-time PCR) of hepatic Sirt1 and the transcription factor SREBP1c. n = 7–8 per group; means ± SEM; *, P < 0.05; ***, P < 0.001.

Fig. 4.

Transgenic Sirt1 mice are protected from HFD-induced hepatic glucose intolerance. (A) Glucose tolerance test (GTT) after 7 weeks of diet exposure by using an i.p. dose of 2 g of glucose per kg of body weight. (B) Pyruvate tolerance test (PTT) after 16 weeks of diet exposure by using 2 g of pyruvate per kg of body weight. (C) Insulin tolerance test (ITT) after 13 weeks of diet exposure by using 0.75 IU/kg insulin. Curves of glucose levels (Left) and values of the area under the curve (Right). n = 7–8 per group; means ± SEM; *, P < 0.05; **, P < 0.01; ***, P < 0.001.

Fig. 5.

Transgenic Sirt1 mice are protected from hepatic inflammation. (A) Gene expression analyses (real-time PCR) of the proinflammatory cytokines IL-6 and TNFα in livers of WT and tg mice on SD or HFD (19 weeks) (n = 7–8 per group; means ± SEM; *, P < 0.05). (B) Kaplan–Meier plot of survival curves of WT and tg mice on SD after injection of lipopolysaccharide (LPS) (n = 5 per group). (C) Luciferase reporter gene assay of WT and tg mouse embryonic fibroblasts after transfection with a pNFκB-Luc reporter vector and stimulation with 10 ng/ml of TNFα. Luminescence was measured 6 h after TNFα incubation. (n = 6–8 per group; means ± SEM; *, P < 0.05). (D) Gene expression analyses (real-time PCR) of the antioxidant proteins MnSOD and Nrf1 in livers of WT and tg mice on SD or HFD (19 weeks) (n = 7–8 per group; means ± SEM; *, P < 0.05; **, P < 0.01). (E) Model of Sirt1 effects on known molecular pathways linking dietary lipids, hepatosteatosis, and hepatic glucose intolerance.