SirT1 gain of function increases energy efficiency and prevents diabetes in mice

Abstract

In yeast, worms, and flies, an extra copy of the gene encoding the Sirtuin Sir2 increases metabolic efficiency, as does administration of polyphenols like resveratrol, thought to act through Sirtuins. But evidence that Sirtuin gain of function results in increased metabolic efficiency in mammals is limited. We generated transgenic mice with moderate overexpression of SirT1, designed to mimic the Sirtuin gain of function that improves metabolism in C. elegans. These mice exhibit normal insulin sensitivity but decreased food intake and locomotor activity, resulting in decreased energy expenditure. However, in various models of insulin resistance and diabetes, SirT1 transgenics display improved glucose tolerance due to decreased hepatic glucose production and increased adiponectin levels, without changes in body weight or composition. We conclude that SirT1 gain of function primes the organism for metabolic adaptation to insulin resistance, increasing hepatic insulin sensitivity and decreasing whole-body energy requirements. These findings have important implications for Sirtuin-based therapies in humans.

Figure 1. Expression Levels and Tissue Distribution of Sirt1 in SirBacO Mice

(A) Western blot of SirT1 in mouse tissues. (B) Immunohistochemistry of SirT1 in mouse brain; DMH: dorsomedial hypothalamus; VMH: ventromedial hypothalamus; ARC: arcuate nucleus. (C) SirT1 expression in SirBACO::db/db mice and controls.

Figure 2. Metabolic effects of SirT1 overexpression

(A) Intraperitoneal glucose tolerance tests and (B) Insulin tolerance tests in four-month-old mice on standard diet (n= 9–11 each). (C) Oxygen consumption (VO₂), (D) respiratory quotient (RQ), and (E) locomotor activity in eight-week-old mice over 24-hr (line chart) and mean 24-hr values (bar graphs) in WT (full bars) and SirBACO mice (empty bars) (n= 7–8 each). (F) Body weight of chow-fed, four-month-old mice (n= 10–11 each). (G) 24-hr food intake (n= 9–11). (H) Body temperature in fed and 24-hr-fasted mice (n=9–12 each). *= P<0.05.

Figure 3. SirT1 increases insulin sensitivity in obese mice

(A) Fasting glucose and (B) insulin levels in SirBACO and WT mice in three conditions: 11-month-old on standard diet (aged), five-month-old on HFD, or eight-week-old on db/db background. (C) Glucose tolerance in four-month-old SirBACO mice on HFD. (D) Body weight in SirBACO (empty symbols) and WT mice (filled symbols) fed regular chow (n=13–14 each), HFD (n=12–16 each) or bred with db/db (n= 9–14 each). (E) Fat mass, (F) indirect calorimetry, and (G) locomotor activity in db/db (full bars), high fat-fed WT (full bars) and SirBACO::db/db or HFD-fed SirBACO mice (empty bars) (n= 5–8 each). *= P<0.05, **= P<0.01.

Figure 4. SirT1 decreases HGP and regulates adiponectin

(A) Glucose infusion (GIR) and disappearance rates (Rd) and (B) Insulin suppression of hepatic glucose production (HGP) in high fat-fed mice (n= 5–11 each). (C) Plasma adiponectin (n=11–13) and (D) WAT mRNA levels (Acrp30) in 16-week-old mice (eight-week-old in the SirBACO::db/db cross). (E) Plasma adiponectin isoform distribution. (F) AMPK phosphorylation in WAT and liver. (G) mRNA levels of adiponectin target genes (n= 4–5 each). (H) FoxO1 acetylation in hepatocytes and (I) Pgc1α acetylation in gastrocnemius muscle. (J–K) Regulation ofAcrp30 mRNA levels in 3T3-L1. *= P<0.05, **= P<0.01. (L) Gene expression in primary mouse hepatocytes (n=4 each).