Insulin resistance without elevated mammalian target of rapamycin complex 1 activity in muscles of mice fed a high-fat diet

Department of Exercise Science, Skidmore College, Saratoga Springs, New York Submitted 28 May 2009 ; accepted in final form 1 September 2009 The mammalian target of rapamycin complex 1 (mTORC1) appears to mediate the development of insulin resistance in cultured cells. We studied in vivo insulin action and mTORC1 signaling in skeletal muscles of mice fed a normal chow [control (CON)] diet or a high-fat diet (HFD) for 16 wk. We assessed in vivo insulin action by measuring glucose tolerance (GT), insulin tolerance (IT), and insulin-assisted GT (IAGT). Although GT was not altered, the HFD significantly reduced IT and IAGT. Acute treatment with rapamycin, a highly specific inhibitor of mTORC1, did not improve GT, IT, or IAGT in mice fed the CON diet or the HFD. Phosphorylation of S6 kinase (S6K) on Thr 389 , a surrogate measure of mTORC1 kinase activity, was assessed in skeletal muscles of mice 15 min after an intraperitoneal injection of insulin or saline. In the basal state and after insulin stimulation, phosphorylation of S6K on Thr 389 was similar in muscles of mice fed the HFD and mice fed the CON diet, indicating that mTORC1 activity is not elevated. Furthermore, phosphorylation of insulin receptor substrate 1 on Ser 636 , a site phosphorylated by mTORC1, was similar in muscles of mice fed the HFD and mice fed the CON diet. Taken together, these findings indicate that in vivo insulin resistance can occur without an increase in mTORC1 activity in skeletal muscle and that inhibition of mTORC1 with rapamycin does not improve insulin action. signal transduction; skeletal muscle Address for reprint requests and other correspondence: T. H. Reynolds IV, Dept. of Exercise Science, Skidmore College, 815 North Broadway, Saratoga Springs, NY 12866 (e-mail: treynold{at}skidmore.edu ).