Insulin-like growth factor I-mediated skeletal muscle hypertrophy is characterized by increased mTOR-p70S6K signaling without increased Akt phosphorylation.
BACKGROUND
Insulin-like growth factor I (IGF-I) is an anabolic hormone that is known to induce skeletal muscle hypertrophy. However, the signaling pathways mediating IGF-I's hypertrophic effect in vivo are unknown.


METHODS
The phosphorylation of 46 proteins was investigated by Kinetworks proteomic analysis in the gastrocnemius muscle of transgenic mice overexpressing IGF-I myosin light chain/muscle specific IGF-I (MLC/mlgf-I) and wild-type littermates.


RESULTS
In the hypertrophic muscle of MLC/mlgf-I mice, we observed increased phosphorylation of phosphoinositide-dependent protein kinase 1 (PDK1; 53% increase), the mammalian target of rapamycin (mTOR; 112% increase), and p70 S6 kinase (p70S6K) (254% increase) but no significant change in Akt phosphorylation (4% decrease). Furthermore, we found reduced phosphorylation of MAP kinase kinase 1 and 2 (MEK1/2) (60% decrease) and of mitogen-activated protein kinase kinases 3 and 6 (MKK3/6) (50% decrease) in muscle from transgenic mice, suggesting that the hypertrophic and mitogenic effects of IGF-I are mediated via distinct signaling pathways in skeletal muscle and that inhibition of the mitogen-activated protein (MAP) kinase pathway may be required for the IGF-I-induced hypertrophic effect. Single-fiber analysis revealed a trend toward a higher percentage of the fast twitch fibers (IIb and IIx) in the transgenic mice.


CONCLUSIONS
Persistent overexpression of IGF-I in mice skeletal muscle results in hypertrophy, which is likely mediated via the mTOR/p70S6K pathway, potentially via an Akt-independent signaling pathway.
Song
Yao-Hua
YH
Department of Medicine, Section of Cardiology, Tulane University School of Medicine, New Orleans, LA 70112-2699, USA.
Godard
Michael
M
Li
Yangxin
Y
Richmond
Scott R
SR
Rosenthal
Nadia
N
Delafontaine
Patrick
P
D000818:Animals; D015870:Gene Expression; D006984:Hypertrophy; D007334:Insulin-Like Growth Factor I / Q000235:genetics / Q000378:metabolism*; D051379:Mice; D008822:Mice, Transgenic; D018482:Muscle, Skeletal / Q000378:metabolism* / Q000473:pathology*; D010750:Phosphoproteins / Q000378:metabolism; D020033:Protein Isoforms / Q000235:genetics / Q000378:metabolism; D011494:Protein Kinases / Q000378:metabolism; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism; D011518:Proto-Oncogene Proteins / Q000378:metabolism; D051057:Proto-Oncogene Proteins c-akt; D051381:Rats; D038762:Ribosomal Protein S6 Kinases, 70-kDa / Q000378:metabolism; D015398:Signal Transduction; D058570:TOR Serine-Threonine Kinases
D010750:Phosphoproteins; D020033:Protein Isoforms; D011518:Proto-Oncogene Proteins; D007334:Insulin-Like Growth Factor I; D011494:Protein Kinases; C546843:mTOR protein, mouse; C546845:mTOR protein, rat; C494920:Akt1 protein, rat; D017346:Protein Serine-Threonine Kinases; D051057:Proto-Oncogene Proteins c-akt; D038762:Ribosomal Protein S6 Kinases, 70-kDa; D058570:TOR Serine-Threonine Kinases