IGF-I stimulates protein synthesis in skeletal muscle through multiple signaling pathways during sepsis

Department of Cellular and Molecular Physiology, Penn State University College of Medicine, Hershey, Pennsylvania Submitted 10 May 2005 ; accepted in final form 5 September 2005 Chronic septic abscess formation causes an inhibition of protein synthesis in gastrocnemius not observed in rats with a sterile abscess. Inhibition is associated with an impaired mRNA translation initiation that can be ameliorated by elevating IGF-I but not insulin. The present study investigated the ability of IGF-I signaling to stimulate protein synthesis in gastrocnemius by accelerating mRNA translation initiation. Experiments were performed in perfused hindlimb preparations from rats 5 days after induction of a septic abscess. Protein synthesis in gastrocnemius from septic rats was accelerated twofold by the addition of IGF-I (10 nM) to perfusate. IGF-I increased the phosphorylation of translation repressor 4E-binding protein-1 (4E-BP1). Hyperphosphorylation of 4E-BP1 in response to IGF-I resulted in its dissociation from the inactive eukaryotic initiation factor (eIF) 4E·4E-BP1 complex. Assembly of the active eIF4F complex (as assessed by the association eIF4G with eIF4E) was increased twofold by IGF-I in the perfusate. In addition, phosphorylation of eIF4G and ribosomal protein S6 kinase-1 (S6K1) was also enhanced by IGF-I. Activation of mammalian target of rapamycin, an upstream kinase implicated in phosphorylating both 4E-BP1 and S6K1, was also observed. Thus the ability of IGF-I to accelerate protein synthesis during sepsis may be related to a stimulation of signaling to multiple steps in translation initiation with an ensuing increased phosphorylation of eIF4G, eIF4E availability, and S6K1 phosphorylation. messenger ribonuclease; translation initiation; eukaryotic initiation factors; eukaryotic initiation factor 4G; 4E-binding protein-1; eukaryotic initiation factor 4E; gastrocnemius; ribosomal protein S6 kinase-1 Address for reprint requests and other correspondence: T. C. Vary, Dept. of Cellular and Molecular Physiology, Rm. C4710, Penn State Univ. College of Medicine, H166, 500 Univ. Drive, Hershey, PA 17033 (e-mail: tvary{at}psu.edu )