FKHR (FOXO1a) is required for myotube fusion of primary mouse myoblasts.
Activation of the transcription factor FKHR (Forkhead in human rhabdomyosarcoma, FOXO1a) in various established cell lines induces cell cycle arrest followed by apoptosis. These effects are inhibited through activation of the phosphatidylinositol 3-kinase/Akt pathway, resulting in FKHR phosphorylation and its export from the nucleus, thus blocking its pro-apoptotic activity. Here we report that FKHR regulates fusion of differentiating primary myoblasts. We demonstrate that FKHR is localized in the cytoplasm of proliferating myoblasts, yet translocates to the nucleus by a phosphorylation-independent pathway following serum starvation, a condition that induces myoblast differentiation. FKHR phosphorylation during terminal differentiation appears to downregulate its fusion activity, as a dominant-active non-phosphorylatable FKHR mutant dramatically augments the rate and extent of myotube fusion. However, this FKHR mutant exerts its effects only after other events initiated the differentiation pro cess. Conversely, enforced expression of a dominant-negative FKHR mutant blocks myotube formation whereas wild-type FKHR has no effect. We conclude that in addition to the role of FoxO proteins in regulating cell cycle progress and apoptosis, FKHR controls the rate of myotube fusion during myogenic differentiation.
Bois
Philippe R J
PR
St Jude Children's Research Hospital, Department of Genetics, 332 North Lauderdale, Memphis, TN 38105, USA.
Grosveld
Gerard C
GC
D021581:Active Transport, Cell Nucleus / Q000502:physiology*; D000818:Animals; D018797:Cell Cycle Proteins; D002454:Cell Differentiation / Q000502:physiology; D002458:Cell Fractionation; D002459:Cell Fusion; D002478:Cells, Cultured; D004268:DNA-Binding Proteins / Q000235:genetics / Q000378:metabolism*; D000071161:Forkhead Box Protein O1; D051858:Forkhead Transcription Factors; D020869:Gene Expression Profiling; D018507:Gene Expression Regulation, Developmental; D017930:Genes, Reporter; D006801:Humans; D051379:Mice; D008810:Mice, Inbred C57BL; D032446:Myoblasts / Q000166:cytology / Q000502:physiology*; D019869:Phosphatidylinositol 3-Kinases / Q000378:metabolism; D017346:Protein Serine-Threonine Kinases; D011518:Proto-Oncogene Proteins / Q000378:metabolism; D051057:Proto-Oncogene Proteins c-akt; D014157:Transcription Factors / Q000235:genetics / Q000378:metabolism*
D018797:Cell Cycle Proteins; D004268:DNA-Binding Proteins; D000071161:Forkhead Box Protein O1; D051858:Forkhead Transcription Factors; D019869:Phosphatidylinositol 3-Kinases; D017346:Protein Serine-Threonine Kinases; D011518:Proto-Oncogene Proteins; D051057:Proto-Oncogene Proteins c-akt; D014157:Transcription Factors; C085126:FOXO1 protein, human; C108803:FOXO4 protein, human; C467200:Foxo1 protein, mouse; C494918:AKT1 protein, human