Inhibition of MAP/ERK kinase prevents IGF-I-induced hypertrophy in rat muscles.
Insulin-like growth factor-I (IGF-I) has been shown to stimulate a hypertrophy response in skeletal muscles in vivo. In vitro studies have delineated two primary intracellular pathways that appear to mediate the effects of IGF-I in skeletal muscle: the Ras-ERK pathway and the phosphoinositide-3 kinase pathway. In vitro, the Ras pathway appears to regulate the mitogenic effects of IGF-I signaling, whereas the phosphoinositide-3 kinase pathway is associated with cellular differentiation. On the basis of the results from in vitro studies, we hypothesized that the coinfusion of both IGF-I and an inhibitor of the Ras pathway would result in some increase in muscle protein but an inhibition of cell proliferation. Our results show that 14 days of coinfusion of MAPK/ERK kinase inhibitor PD-098059 (PD) limited the phosphorylation of ERK and prevented IGF-I induced increases in protein (18%, P < 0.05 vs. 7%, not significant) or myofibrillar protein (23%, P < 0.01 vs. 5%, not significant). However, there were similar increases in indicators of cell proliferation (e.g., total DNA, 50 and 52%, P < 0.001) in both the IGF- and IGF+PD-infused muscles. The most notable impact on IGF-I signaling was a significant blunting of IGF-I induced increase in S6K1 phosphorylation by PD-98059 coinfusion ( approximately 5-fold, P < 0.001 vs. 3-fold, P < 0.01). These results suggest that there are interactions between the various pathways down stream of the IGF-I receptor that may behave differently in vivo than in myogenic cell lines in vitro.
Haddad
Fadia
F
Department of Physiology and Biophysics, University of California, Irvine, CA 92697-4560, USA.
Adams
Gregory R
GR
D000222:Adaptation, Physiological / Q000502:physiology; D000818:Animals; D002455:Cell Division; D019938:Cyclin D1 / Q000235:genetics; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005260:Female; D005419:Flavonoids / Q000494:pharmacology; D006984:Hypertrophy; D018974:Insulin-Like Growth Factor Binding Protein 4 / Q000235:genetics; D018975:Insulin-Like Growth Factor Binding Protein 5 / Q000235:genetics; D007334:Insulin-Like Growth Factor I / Q000494:pharmacology*; D020935:MAP Kinase Signaling System / Q000187:drug effects / Q000502:physiology*; D018485:Muscle Fibers, Skeletal / Q000166:cytology / Q000187:drug effects / Q000502:physiology; D018482:Muscle, Skeletal / Q000166:cytology / Q000187:drug effects / Q000502:physiology*; D012333:RNA, Messenger / Q000032:analysis; D051381:Rats; D017207:Rats, Sprague-Dawley
D004791:Enzyme Inhibitors; D005419:Flavonoids; D018974:Insulin-Like Growth Factor Binding Protein 4; D018975:Insulin-Like Growth Factor Binding Protein 5; D012333:RNA, Messenger; D019938:Cyclin D1; D007334:Insulin-Like Growth Factor I; C093973:2-(2-amino-3-methoxyphenyl)-4H-1-benzopyran-4-one