FOXO1 Regulates the Expression of 4E-BP1 and Inhibits mTOR Signaling in Mammalian Skeletal Muscle

• Published in: The Journal of biological chemistry Vol. 282; no. 29; pp. 21176 - 21186
• Main Authors: Southgate, Robert J., Neill, Bronwyn, Prelovsek, Oja, El-Osta, Assam, Kamei, Yasutomi, Miura, Shinji, Ezaki, Osamu, McLoughlin, Thomas J., Zhang, Wenwei, Unterman, Terry G., Febbraio, Mark A.
• Format: Journal Article
• Language: English
• Published: United States Elsevier Inc 20-07-2007; American Society for Biochemistry and Molecular Biology
• Subjects: Adaptor Proteins, Signal Transducing; Animals; Binding Sites; Carrier Proteins - chemistry; Cell Cycle Proteins; Eukaryotic Initiation Factors; Forkhead Box Protein O1; Forkhead Transcription Factors - chemistry; Forkhead Transcription Factors - physiology; Gene Expression Regulation; Mice; Mice, Transgenic; Models, Biological; Muscle, Skeletal - metabolism; Phenotype; Phosphoproteins - chemistry; Phosphorylation; Protein Binding; Protein Kinases - metabolism; Protein Kinases - physiology; RNA, Messenger - metabolism; Signal Transduction; TOR Serine-Threonine Kinases
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1074/jbc.M702039200

The mammalian target of rapamycin (mTOR) is regulated by growth factors to promote protein synthesis. In mammalian skeletal muscle, the Forkhead-O1 transcription factor (FOXO1) promotes catabolism by activating ubiquitin-protein ligases. Using C2C12 mouse myoblasts that stably express inducible FOXO1-ER fusion proteins and transgenic mice that specifically overexpress constitutively active FOXO1 in skeletal muscle (FOXO++/+), we show that FOXO1 inhibits mTOR signaling and protein synthesis. Activation of constitutively active FOXO1 induced the expression of eukaryotic initiation factor 4E-binding protein 1 (4E-BP1) mRNA via binding to the promoter. This resulted in an increased total 4E-BP1 abundance and a reduced 4E-BP1 (Thr-37/46) phosphorylation. The reduction in 4E-BP1 phosphorylation was associated with a reduction in the abundance of Raptor and mTOR proteins, Raptor-associated mTOR, reduced phosphorylation of the downstream protein p70S6 kinase, and attenuated incorporation of [14C]phenylalanine into protein. The FOXO++/+ mice, characterized by severe skeletal muscle atrophy, displayed similar patterns of mRNA expression and protein abundance to those observed in the constitutively active FOXO1 C2C12 myotubes. These data suggest that FOXO1 may be an important therapeutic target for human diseases where anabolism is impaired.