Regulation of both glycogen synthase and PHAS-I by insulin in rat skeletal muscle involves mitogen-activated protein kinase-independent and rapamycin-sensitive pathways.
Incubating rat diaphragm muscles with insulin increased the glycogen synthase activity ratio (minus glucose 6-phosphate/plus glucose 6-phosphate) by approximately 2-fold. Insulin increased the activities of mitogen-activated protein (MAP) kinase and the Mr = 90,000 isoform of ribosomal protein S6 kinase (Rsk) by approximately 1.5-2.0-fold. Epidermal growth factor (EGF) was more effective than insulin in increasing MAP kinase and Rsk activity, but in contrast to insulin, EGF did not affect glycogen synthase activity. The activation of both MAP kinase and Rsk by insulin was abolished by incubating muscles with the MAP kinase kinase (MEK) inhibitor, PD 098059; however, the MEK inhibitor did not significantly reduce the effect of insulin on activating glycogen synthase. Incubating muscles with concentrations of rapamycin that inhibited activation of p70S6K abolished the activation of glycogen synthase. Insulin also increased the phosphorylation of PHAS-I (phosphorylated heat- and acid-stable protein) and promoted the dissociation of the PHAS-I*eIF-4E complex. Increasing MAP kinase activity with EGF did not mimic the effect of insulin on PHAS-I phosphorylation, and the effect of insulin on increasing MAP kinase could be abolished with the MEK inhibitor without decreasing the effect of insulin on PHAS-I. The effects of insulin on PHAS-I were attenuated by rapamycin. Thus, activation of the MAP kinase/Rsk signaling pathway appears to be neither necessary nor sufficient for insulin action on glycogen synthase and PHAS-I in rat skeletal muscle. The results indicate that the effects of insulin on increasing the synthesis of glycogen and protein in skeletal muscle, two of the most important actions of the hormone, involve a rapamycin-sensitive mechanism that may include elements of the p70S6K signaling pathway.
Azpiazu
I
I
Department of Molecular Biology and Pharmacology, Washington University School of Medicine, St Louis, Missouri 63110, USA.
Saltiel
A R
AR
DePaoli-Roach
A A
AA
Lawrence
J C
JC
D000595:Amino Acid Sequence; D000818:Animals; D017871:Calcium-Calmodulin-Dependent Protein Kinases / Q000276:immunology / Q000378:metabolism*; D002352:Carrier Proteins; D003964:Diaphragm; D004791:Enzyme Inhibitors / Q000494:pharmacology; D004815:Epidermal Growth Factor / Q000494:pharmacology; D005419:Flavonoids / Q000494:pharmacology; D006006:Glycogen Synthase / Q000096:biosynthesis*; D006706:Homeostasis; D007106:Immune Sera; D066298:In Vitro Techniques; D007328:Insulin / Q000494:pharmacology*; D047908:Intracellular Signaling Peptides and Proteins; D007700:Kinetics; D008297:Male; D048052:Mitogen-Activated Protein Kinase 3; D020929:Mitogen-Activated Protein Kinase Kinases; D020928:Mitogen-Activated Protein Kinases; D008969:Molecular Sequence Data; D018482:Muscle, Skeletal / Q000187:drug effects / Q000378:metabolism*; D010455:Peptides / Q000138:chemical synthesis / Q000276:immunology; D010750:Phosphoproteins / Q000378:metabolism*; D011090:Polyenes / Q000494:pharmacology*; D047428:Protein Kinase Inhibitors; D011494:Protein Kinases / Q000378:metabolism; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley; D019893:Ribosomal Protein S6 Kinases; D020123:Sirolimus; D013997:Time Factors
D002352:Carrier Proteins; C499526:Eif4ebp1 protein, rat; D004791:Enzyme Inhibitors; D005419:Flavonoids; D007106:Immune Sera; D007328:Insulin; D047908:Intracellular Signaling Peptides and Proteins; D010455:Peptides; D010750:Phosphoproteins; D011090:Polyenes; D047428:Protein Kinase Inhibitors; D004815:Epidermal Growth Factor; D006006:Glycogen Synthase; D011494:Protein Kinases; D017346:Protein Serine-Threonine Kinases; D019893:Ribosomal Protein S6 Kinases; D017871:Calcium-Calmodulin-Dependent Protein Kinases; D048052:Mitogen-Activated Protein Kinase 3; D020928:Mitogen-Activated Protein Kinases; D020929:Mitogen-Activated Protein Kinase Kinases; C093973:2-(2-amino-3-methoxyphenyl)-4H-1-benzopyran-4-one; D020123:Sirolimus