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 REDD1. The purpose of the present study was to determine whether IGF-I alters the expression of REDD1 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 3-4 h increased REDD1 mRNA in skeletal muscle 7-10-fold. A threefold increase in REDD1 was observed when C2C12 myotubes were treated with IGF-I. REDD1 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 REDD1 protein content. IGF-I-induced REDD1 mRNA and protein expression were prevented by inhibitors of transcription and translation. IGF-I had an additive effect with dexamethasone (Dex) on REDD1 protein content in myotubes. The PI3K inhibitor LY294002 blocked IGF-I but not Dex induced REDD1. IGF-I also stimulated REDD1 promoter activity. Although REDD1 protein was elevated 5-6 h after addition of IGF-I to myotubes, protein synthesis measured during this 1 h window was paradoxically greater in myotubes expressing more REDD1. In contrast to the IGF-I induced increase in REDD1 mRNA, REDD2 mRNA was decreased by IGF-I. We conclude that IGF-I stimulates REDD1 expression in skeletal muscle and myotubes but under these conditions the REDD1 response is not sufficient to repress protein synthesis.
Department of Cellular and Molecular Physiology, Pennsylvania State University College of Medicine, Hershey, Pennsylvania 17033, USA. rfrost@psu.edu
Robert A
RA
Frost
Danuta
D
Huber
Anne
A
Pruznak
Charles H
CH
Lang
D048868:Adaptor Proteins, Signal Transducing; D000818:Animals; D002460:Cell Line; D002867:Chromones / Q000494:pharmacology; D004268:DNA-Binding Proteins; D003907:Dexamethasone / Q000494:pharmacology; D004791:Enzyme Inhibitors / Q000494:pharmacology; D006801:Humans; D007328:Insulin / Q000378:metabolism / Q000494:pharmacology; D007334:Insulin-Like Growth Factor I / Q000378:metabolism* / Q000494:pharmacology; D051379:Mice; D009025:Morpholines / Q000494:pharmacology; D018485:Muscle Fibers, Skeletal / Q000187:drug effects / Q000378:metabolism*; D018482:Muscle, Skeletal / Q000187:drug effects / Q000378:metabolism*; D019869:Phosphatidylinositol 3-Kinases / Q000378:metabolism; D000081082:Phosphoinositide-3 Kinase Inhibitors; D014176:Protein Biosynthesis / Q000187:drug effects; D011506:Proteins / Q000235:genetics / Q000378:metabolism; D051381:Rats; D012097:Repressor Proteins / Q000235:genetics / Q000378:metabolism; D014157:Transcription Factors; D015533:Transcriptional Activation / Q000187:drug effects; D015854:Up-Regulation
D048868:Adaptor Proteins, Signal Transducing; D002867:Chromones; D004268:DNA-Binding Proteins; D003907:Dexamethasone; D004791:Enzyme Inhibitors; D007328:Insulin; D007334:Insulin-Like Growth Factor I; D009025:Morpholines; D019869:Phosphatidylinositol 3-Kinases; D000081082:Phosphoinositide-3 Kinase Inhibitors; D011506:Proteins; D012097:Repressor Proteins; D014157:Transcription Factors; C000632146:Ddit4 protein, rat; C450979:Ddit4l protein, mouse; C085911:2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one