Transcriptional regulation of IGF-I expression in skeletal muscle.
The present study investigated the role of transcription in the regulation of insulin-like growth factor (IGF)-I expression in skeletal muscle. RT-PCR was used to determine endogenous expression of IGF-I pre-mRNA and mRNA in control (Con) and functionally overloaded (FO) rat plantaris. The transcriptional activities of five different-length IGF-I promoter fragments controlling transcription of a firefly luciferase (FLuc) reporter gene were tested in vitro by transfection of myoblasts or in vivo during FO by direct gene transfer into the plantaris. Increased endogenous IGF-I gene transcription during 7 days of plantaris FO was evidenced by an approximately 140-160% increase (P < 0.0001) in IGF-I pre-mRNA (a transcriptional marker). IGF-I mRNA expression also increased by approximately 90% (P < 0.0001), and it was correlated (R = 0.93; P < 0.0001) with the pre-mRNA increases. The three longest IGF-I exon 1 promoters induced reporter gene expression in proliferating C2C12 and L6E9 myoblasts. In differentiated L6E9 myotubes, promoter activity increased approximately two- to threefold over myoblasts. Overexpression of calcineurin and MyoD increased the activity of the -852/+192 promoter in C2C12 myotubes by approximately 5- and approximately 18-fold, respectively. However, FO did not induce these exogenous promoter fragments. Nevertheless, the present findings are consistent with the hypothesis that the IGF-I gene is transcriptionally regulated during muscle hypertrophy in vivo as evidenced by the induction of the endogenous IGF-I pre-mRNA during plantaris FO. The exon 1 promoter region of the IGF-I gene is sufficient to direct inducible expression in vitro; however, an in vivo response to FO may require elements outside the -852/+346 region of the exon 1 IGF-I promoter or features inherent to the endogenous IGF-I gene.
McCall
G E
GE
University of California, Irvine, CA 92697-4560, USA.
Allen
D L
DL
Haddad
F
F
Baldwin
K M
KM
D000818:Animals; D002478:Cells, Cultured; D005260:Female; D005347:Fibroblasts / Q000502:physiology; D018014:Gene Transfer Techniques; D007334:Insulin-Like Growth Factor I / Q000235:genetics* / Q000378:metabolism*; D032342:Muscle Cells / Q000502:physiology; D018482:Muscle, Skeletal / Q000166:cytology / Q000378:metabolism* / Q000502:physiology; D011401:Promoter Regions, Genetic / Q000502:physiology; D012322:RNA Precursors / Q000378:metabolism; D012333:RNA, Messenger / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley; D014158:Transcription, Genetic / Q000502:physiology*; D014162:Transfection
D012322:RNA Precursors; D012333:RNA, Messenger; D007334:Insulin-Like Growth Factor I