title: TNF induction of atrogin-1/MAFbx mRNA depends on Foxo4 expression but not AKT-Foxo1/3 signaling.
abstract: Murine models of starvation-induced muscle atrophy demonstrate that reduced protein kinase B (AKT) function upregulates the atrophy-related gene atrogin-1/MAFbx (atrogin). The mechanism involves release of inhibition of Forkhead transcription factors, namely Foxo1 and Foxo3. Elevated atrogin mRNA also corresponds with elevated TNF in inflammatory catabolic states, including cancer and chronic heart failure. Exogenous tumor necrosis factor (TNF) increases atrogin mRNA in vivo and in vitro. We used TNF-treated C2C12 myotubes to test the hypothesis that AKT-Foxo1/3 signaling mediates TNF regulation of atrogin mRNA. Here we confirm that exposure to TNF increases atrogin mRNA (+125%). We also confirm that canonical AKT-mediated regulation of atrogin is active in C2C12 myotubes. Inhibition of phosphoinositol-3 kinase (PI3K)/AKT signaling with wortmannin reduces AKT phosphorylation (-87%) and increases atrogin mRNA (+340%). Activation with insulin-like growth factor (IGF) increases AKT phosphorylation (+126%) and reduces atrogin mRNA (-15%). Although AKT regulation is intact, our data suggest it does not mediate TNF effects on atrogin. TNF increases AKT phosphorylation (+50%) and stimulation of AKT with IGF does not prevent TNF induction of atrogin mRNA. Nor does TNF appear to signal through Foxo1/3 proteins. TNF has no effect on Foxo1/3 mRNA or Foxo1/3 nuclear localization. Instead, TNF increases nuclear Foxo4 protein (+55%). Small interfering RNA oligos targeted to two distinct regions of Foxo4 mRNA reduce the TNF-induced increase in atrogin mRNA (-34% and -32%). We conclude that TNF increases atrogin mRNA independent of AKT via Foxo4. These results suggest a mechanism by which inflammatory catabolic states may persist in the presence of adequate growth factors and nutrition.
authors.affiliation: Department of Physiology, University of Kentucky, Lexington, Kentucky 40536-0298, USA. jennifer.moylan@uky.edu
authors.forename: Jennifer S
authors.initials: JS
authors.lastname: Moylan
authors.forename: Jeffrey D
authors.initials: JD
authors.lastname: Smith
authors.forename: Melissa A
authors.initials: MA
authors.lastname: Chambers
authors.forename: Thomas J
authors.initials: TJ
authors.lastname: McLoughlin
authors.forename: Michael B
authors.initials: MB
authors.lastname: Reid
mesh_terms: D000818:Animals; D018797:Cell Cycle Proteins; D002460:Cell Line; D000071161:Forkhead Box Protein O1; D000071316:Forkhead Box Protein O3; D051858:Forkhead Transcription Factors / Q000235:genetics / Q000378:metabolism*; D005786:Gene Expression Regulation; D051379:Mice; D009124:Muscle Proteins / Q000235:genetics / Q000378:metabolism*; D032446:Myoblasts / Q000187:drug effects / Q000378:metabolism; D020033:Protein Isoforms; D051057:Proto-Oncogene Proteins c-akt / Q000378:metabolism*; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D044843:SKP Cullin F-Box Protein Ligases / Q000235:genetics / Q000378:metabolism*; D015398:Signal Transduction; D013002:Somatomedins / Q000378:metabolism; D014409:Tumor Necrosis Factor-alpha / Q000494:pharmacology*
chemical_list: D018797:Cell Cycle Proteins; D000071161:Forkhead Box Protein O1; D000071316:Forkhead Box Protein O3; D051858:Forkhead Transcription Factors; C474033:FoxO3 protein, mouse; C525394:FoxO4 protein, mouse; C467200:Foxo1 protein, mouse; D009124:Muscle Proteins; D020033:Protein Isoforms; D012333:RNA, Messenger; D013002:Somatomedins; D014409:Tumor Necrosis Factor-alpha; C486691:Fbxo32 protein, mouse; D044843:SKP Cullin F-Box Protein Ligases; D051057:Proto-Oncogene Proteins c-akt