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MUSCLE CELL BIOLOGY AND CELL MOTILITY
1University of California, Irvine, California 92697-4560; and 2University of Colorado, Boulder, Colorado 80304
Submitted 4 February 2003 ; accepted in final form 22 May 2003
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 
140-160%
increase (P < 0.0001) in IGF-I pre-mRNA (a transcriptional
marker). IGF-I mRNA expression also increased by 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 5- and 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.
muscle fiber; hypertrophy; functional overload; transcription factor; myogenic regulatory factor; pre-messenger ribonucleic acid; myotube
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