Impaired insulin-induced site-specific phosphorylation of TBC1 domain family, member 4 (TBC1D4) in skeletal muscle of type 2 diabetes patients is restored by endurance exercise-training

Abstract

Aims/hypothesis: Insulin-mediated glucose disposal rates (Rd) are reduced in type 2 diabetic patients, a process in which intrinsic signalling defects are thought to be involved. Phosphorylation of TBC1 domain family, member 4 (TBC1D4) is at present the most distal insulin receptor signalling event linked to glucose transport. In this study, we examined insulin action on site-specific phosphorylation of TBC1D4 and the effect of exercise training on insulin action and signalling to TBC1D4 in skeletal muscle from type 2 diabetic patients. Methods: During a 3 h euglycaemic-hyperinsulinaemic (80 mU min -1 m-2) clamp, we obtained M. vastus lateralis biopsies from 13 obese type 2 diabetic and 13 obese, nondiabetic control individuals before and after 10 weeks of endurance exercise-training. Results: Before training, reductions in insulin-stimulated Rd, together with impaired insulin-stimulated glycogen synthase fractional velocity, Akt Thr308 phosphorylation and phosphorylation of TBC1D4 at Ser318, Ser 588 and Ser751 were observed in skeletal muscle from diabetic patients. Interestingly, exercise-training normalised insulin-induced TBC1D4 phosphorylation in diabetic patients. This happened independently of increased TBC1D4 protein content, but exercise-training did not normalise Akt phosphorylation in diabetic patients. In both groups, training-induced improvements in insulin-stimulated Rd (∼20%) were associated with increased muscle protein content of Akt, TBC1D4, α2-AMP-activated kinase (AMPK), glycogen synthase, hexokinase II and GLUT4 (20-75%). Conclusions/interpretation: Impaired insulin-induced site-specific TBC1D4 phosphorylation may contribute to skeletal muscle insulin resistance in type 2 diabetes. The mechanisms by which exercise-training improves insulin sensitivity in type 2 diabetes may involve augmented signalling of TBC1D4 and increased skeletal muscle content of key insulin signalling and effector proteins, e.g., Akt, TBC1D4, AMPK, glycogen synthase, GLUT4 and hexokinase II. © Springer-Verlag 2010.