Regulation of REDD1 by Insulin-Like Growth Factor-I in Skeletal Muscle and Myotubes

Insulin-like growth factor-I (IGF-I) is a major anabolic hormone for skeletal muscle and a potent stimulus for protein synthesis and translation initiation. Recent studies suggest that translation can be inhibited by over expression of the mammalian target of rapamycin (mTOR) repressor REDD 1. The purpose of the present study was to determine whether IGF-I alters the expression of REDD 1 and whether this is associated with a concomitant change in protein synthesis in vitro. Subcutaneous injection of IGF-I or intravenous delivery of insulin for 3-4h increased REDD 1 mRNA in skeletal muscle 7-10-fold. A threefold increase in REDD 1 was observed when C2C12 myotubes were treated with IGF-I-REDD 1 protein continued to be expressed for up to 24 h after addition of IGF-I to cells. Withdrawal of IGF-I from myotubes lead to a rapid loss of REDD 1 protein content. IGF-I-induced REDD 1 mRNA and protein expression were prevented by inhibitors of transcription and translation. IGF-I had an additive effect with dexamethasone (Dex) on REDD 1 protein content in myotubes. The PI3K inhibitor LY294002 blocked IGF-I but not Dex induced REDD 1. IGF-I also stimulated REDD 1 promoter activity. Although REDD1 protein was elevated 5-6 It after addition of IGF-I to myotubes, protein synthesis measured during this I h window was paradoxically greater in myotubes expressing more REDD 1. In contrast to the IGF-I induced increase in REDD 1 mRNA, REDD2 mRNA was decreased by IGF-I We conclude that IGF-I stimulates REDD 1 expression in skeletal muscle and myotubes but under these conditions the REDD 1 response is not sufficient to repress protein synthesis. J. Cell. Biochem. 108: 1192-1202, 2009. (C) 2009 Wiley-Liss, Inc.