title: Protein breakdown in muscle from burned rats is blocked by insulin-like growth factor i and glycogen synthase kinase-3beta inhibitors.
abstract: We reported previously that IGF-I inhibits burn-induced muscle proteolysis. Recent studies suggest that activation of the phosphotidylinositol 3-kinase (PI3K)/Akt signaling pathway with downstream phosphorylation of Forkhead box O transcription factors is an important mechanism of IGF-I-induced anabolic effects in skeletal muscle. The potential roles of other mechanisms in the anabolic effects of IGF-I are less well understood. In this study we tested the roles of mammalian target of rapamycin and glycogen synthase kinase-3beta (GSK-3beta) phosphorylation as well as MAPK- and calcineurin-dependent signaling pathways in the anticatabolic effects of IGF-I by incubating extensor digitorum longus muscles from burned rats in the presence of IGF-I and specific signaling pathway inhibitors. Surprisingly, the PI3K inhibitors LY294002 and wortmannin reduced basal protein breakdown. No additional inhibition by IGF-I was noticed in the presence of LY294002 or wortmannin. Inhibition of proteolysis by IGF-I was associated with phosphorylation (inactivation) of GSK-3beta. In addition, the GSK-3beta inhibitors, lithium chloride and thiadiazolidinone-8, reduced protein breakdown in a similar fashion as IGF-I. Lithium chloride, but not thiadiazolidinone-8, increased the levels of phosphorylated Foxo 1 in incubated muscles from burned rats. Inhibitors of mammalian target of rapamycin, MAPK, and calcineurin did not prevent the IGF-I-induced inhibition of muscle proteolysis. Our results suggest that IGF-I inhibits protein breakdown at least in part through a PI3K/Akt/GSK3beta-dependent mechanism. Additional experiments showed that similar mechanisms were responsible for the effect of IGF-I in muscle from nonburned rats. Taken together with recent reports in the literature, the present results suggest that IGF-I inhibits protein breakdown in skeletal muscle by multiple mechanisms, including PI3K/Akt-mediated inactivation of GSK-3beta and Foxo transcription factors.
authors.affiliation: Shriners Hospital for Children, 3229 Burnet Avenue, Cincinnati, Ohio 45229, USA. cfang@shrinenet.org
authors.forename: Cheng-Hui
authors.initials: CH
authors.lastname: Fang
authors.forename: Bing-Guo
authors.initials: BG
authors.lastname: Li
authors.forename: J Howard
authors.initials: JH
authors.lastname: James
authors.forename: Jy-Kung
authors.initials: JK
authors.lastname: King
authors.forename: Amy R
authors.initials: AR
authors.lastname: Evenson
authors.forename: Glenn D
authors.initials: GD
authors.lastname: Warden
authors.forename: Per-Olof
authors.initials: PO
authors.lastname: Hasselgren
mesh_terms: D000818:Animals; D002056:Burns / Q000378:metabolism*; D004268:DNA-Binding Proteins / Q000378:metabolism; D004789:Enzyme Activation / Q000502:physiology; D004791:Enzyme Inhibitors / Q000494:pharmacology*; D051858:Forkhead Transcription Factors; D038362:Glycogen Synthase Kinase 3 / Q000037:antagonists & inhibitors* / Q000378:metabolism; D000071679:Glycogen Synthase Kinase 3 beta; D006614:Hindlimb; D007334:Insulin-Like Growth Factor I / Q000494:pharmacology*; D018021:Lithium Chloride / Q000494:pharmacology; D008297:Male; D009124:Muscle Proteins / Q000378:metabolism*; D018482:Muscle, Skeletal / Q000378:metabolism; D009419:Nerve Tissue Proteins / Q000378:metabolism; D010447:Peptide Hydrolases / Q000187:drug effects / Q000378:metabolism*; D019869:Phosphatidylinositol 3-Kinases / Q000502:physiology; D000081082:Phosphoinositide-3 Kinase Inhibitors; D010766:Phosphorylation / Q000187:drug effects; D017346:Protein Serine-Threonine Kinases / Q000502:physiology; D011518:Proto-Oncogene Proteins / Q000502:physiology; D051057:Proto-Oncogene Proteins c-akt; D051381:Rats; D017207:Rats, Sprague-Dawley; D013830:Thiadiazoles / Q000494:pharmacology; D014034:Toes; D000071161:Forkhead Box Protein O1
chemical_list: D004268:DNA-Binding Proteins; D004791:Enzyme Inhibitors; D051858:Forkhead Transcription Factors; D038362:Glycogen Synthase Kinase 3; D000071679:Glycogen Synthase Kinase 3 beta; D007334:Insulin-Like Growth Factor I; D018021:Lithium Chloride; D009124:Muscle Proteins; D009419:Nerve Tissue Proteins; D010447:Peptide Hydrolases; D019869:Phosphatidylinositol 3-Kinases; D000081082:Phosphoinositide-3 Kinase Inhibitors; D017346:Protein Serine-Threonine Kinases; D011518:Proto-Oncogene Proteins; D051057:Proto-Oncogene Proteins c-akt; D013830:Thiadiazoles; C494356:4-benzyl-2-methyl-1,2,4-thiadiazolidine-3,5-dione; C074639:Foxo1 protein, rat; D000071161:Forkhead Box Protein O1; C494920:Akt1 protein, rat; C000605508:Gsk3b protein, rat