Overexpression of the Sirtuin Sirt1 Increases Insulin Sensitivity in Aging Mice.

Banks, Alexander; Kon, Ning; Knight, Collette; Rossetti, Luciano; Accili, Domenico; Gu, Wei

Sirtuins are enzymes with protein deacetylase or GDP-ribosylase activity. Gain-of-function of Sir2 alpha, the prototypical Sirtuin, increases lifespan in yeast and nematodes. Mammalian SirT1, the closest ortholog of Sir2 alpha, has been shown to possess deacetylase activity. Among its substrates are FoxO1 and PGC-1 alpha, two critical metabolic regulators. It has been suggested that SirT1 regulates mammalian metabolism, an observation supported by evidence that SirT1 activation by resveratrol increases insulin sensitivity. To understand the mechanism by which SirT1 affects mammalian metabolism, we generated transgenic mice overexpressing WT and catalytically inactive (H363Y) SirT1. In order to retain locus control elements, we used a BAC transgene to generate the animals. Among various founders, we selected those with moderate (2- to 3-fold) levels of overexpression, to avoid artifacts due to excess enzyme activity, and we introduced a HA tag to distinguish endogenous SirT1 from transgene-encoded SirT1. Expression patterns of HA-SirT1 were indistinguishable from those of the endogenous protein. Growth, body mass, body composition, pancreatic islet mass, fasting glucose, insulin, and free fatty acid levels of SirT1 transgenics were similar to WT controls. However, various metabolic measures indicate that these mice are more insulin-sensitive than their WT littermates. Glucose tolerance tests following 12 weeks of high fat feeding showed that SirT1 transgenics were more glucose tolerant than WT controls. Similar results were obtained in one year-old mice, indicating that SirT1 over-expression protects against age-induced deterioration of glucose tolerance. Moreover, hyperinsulinemic euglycemic clamps in one year-old mice showed increased insulin sensitivity. Consistent-with increased insulin sensitivity, we detected decreased FoxO1 acetylation in SirT1 mice. Acetylated FoxO1 cannot be regulated by insulin-mediated phosphorylation. Hence, we propose that the improvement in insulin sensitivity can be explained by decreased cycling of FoxO1 (and potentially PGC1 alpha) through the acetylation/deacetylation cycle that has been shown to regulate FoxO1-dependent transcription. The data provide a mechanism linking cell survival (aging) with metabolic regulation through Foxo proteins.

ENZYMES; INSULIN resistance; PROTEINS; METABOLISM; MICE

Diabetes, 2007, Vol 56, pA61

0012-1797

Academic Journal