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This Article Full Text Full Text (PDF) All Versions of this Article: 297/4/C1019    most recent 00169.2009v1 Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in Web of Science Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Google Scholar Articles by Shanely, R. A. Articles by Booth, F. W. PubMed PubMed Citation Articles by Shanely, R. A. Articles by Booth, F. W.

MUSCLE CELL BIOLOGY AND CELL MOTILITY

IGF-I activates the mouse type IIb myosin heavy chain gene

Departments of ¹Biomedical Sciences, ²Medical Pharmacology and Physiology, and ³Biochemistry, ⁴Dalton Cardiovascular Institute, ⁵Health Activity Center, and ⁶Health and Activity Rehabilitation Research and Training Center, University of Missouri-Columbia, Columbia, Missouri; and ⁷Department of Health, Leisure, and Exercise Science, Appalachian State University, Boone, North Carolina

Submitted 20 April 2009 ; accepted in final form 30 July 2009

IGF-I increases skeletal muscle mass, but whether IGF-I increases type IIb myosin heavy chain (MyHC) transcriptional activity is not known. C₂C₁₂ myotubes were cultured with or without IGF-I to determine whether IGF-I increases type IIb MyHC promoter activity, and if so, what region of the promoter might IGF-I signaling regulate. At differentiation days 3 and 4, IGF-I increased type IIb MyHC mRNA and mouse 3.0-kb type IIb MyHC promoter activity. Deletion construct studies identified a potential IGF-I-responsive region between 1.25 and 1.2 kb of the type IIb MyHC promoter, which contained an exact 6-bp T-cell factor/lymphoid enhancer factor (Tcf/Lef) binding site at position –1206 to –1201. Site-specific mutation of the putative Tcf/Lef binding site reduced IGF-I-induced 1.3-kb type IIb MyHC promoter activity. To identify potential IGF-I signaling molecules, the phosphatidylinositol 3-kinase (PI3K) inhibitors wortmannin and LY-294002 were both found to markedly attenuate IGF-I activation of the 1.3-kb type IIb MyHC promoter. Downstream signaling of IGF-I can phosphorylate and inactivate GSK-3β, thereby enhancing β-catenin protein. The GSK-3β inhibitor, LiCl, dramatically enhanced IGF-I induction of the 1.3-kb type IIb MyHC promoter, and constitutively active GSK-3β attenuated IGF-I-induced 1.3-kb type IIb MyHC promoter activity. Finally, IGF-I increased nuclear β-catenin protein, and small interfering RNA knockdown of β-catenin attenuated IGF-I-induced 1.3-kb type IIb MyHC promoter activity and type IIb MyHC mRNA. In summary, IGF-I stimulation of C₂C₁₂ myotubes increases mouse type IIb MyHC promoter activity, likely through signaling of PI3K, GSK-3β, β-catenin, and a Tcf/Lef binding site at –1,206 to –1,201 bp in the promoter.

insulin-like growth factor I; C₂C₁₂ myocytes; β-catenin