Leucine amplifies the effects of metformin on insulin sensitivity and glycemic control in diet-induced obese mice

Background and objective. The Sirt1/AMPK signaling pathway is a key sensor of energy status and regulates glucose and lipid metabolism. Leucine (Leu) activates Sirt1 by lowering its Km for MAD and potentiates other sirtuin/AMPK-activators, resulting in improvement of insulin sensitivity. Since metformin (Met) converges on this pathway, we hypothesized that leucine would amplify its gluco-regulatory effects. Materials and methods. The effects of Leu (24 g/kg diet) + Met (0.05-0.5 g/kg diet) combinations were compared to standard therapeutic Met (1.5 g/kg diet; similar to 300 mg/kg BW) on glycemic control in high fat diet induced insulin resistant mice for 6 weeks. The effects of Leu on Met stimulation of Sirt1 and AMPK activities were further evaluated in adipocytes. Results. Sub-therapeutic levels of Met combined with Leu resulted in increases in Sirt1 activity and in tissue P-AMPK/AMPK ratio and corresponding dose-responsive improvements in fasting and post-prandial glucose, in glucose response to an insulin tolerance test and in the area under the curve in glucose tolerance tests. Changes were evident within 7 days of treatment and sustained throughout the 6-week study duration. The Leu + Met (0.25 g/kg)-combinations produced a comparable effect to a standard therapeutic Met dose, while the Leu + Met (0.5 g/kg diet) resulted in greater improvements. Since resveratrol also synergizes with leucine to augment sirtuin signaling and insulin sensitivity, we tested the addition of resveratrol to Leu-Met and found no additional benefit. Conclusion These data demonstrate that adding Leu to Met enables a dose reduction of 66% with improved efficacy and of 83% with comparable efficacy to standard metformin in diet-induced obese mice; and addition of resveratrol does not provide further benefit. (C) 2015 Elsevier Inc. All rights reserved.