REGULATION OF BOTH GLYCOGEN-SYNTHASE AND PHAS-I BY INSULIN IN RAT SKELETAL-MUSCLE INVOLVES MITOGEN-ACTIVATED PROTEIN KINASE-INDEPENDENT ANDRAPAMYCIN-SENSITIVE PATHWAYS

Incubating rat diaphragm muscles with insulin increased the glycogen s ynthase activity ratio (minus glucose 6-phosphate/plus glucose 6-phosp hate) by approximately 2-fold. Insulin increased the activities of mit ogen-activated protein (MAP) kinase and the M(r) = 90,000 isoform of r ibosomal protein S6 kinase (Rsk) by approximately 1.5-2.0-fold. Epider mal growth factor (EGF) was more effective than insulin in increasing MAP kinase and Rsk activity, but in contrast to insulin, EGF did not a ffect glycogen synthase activity, The activation of both MAP kinase an d Rsk by insulin was abolished by incubating muscles with the MAP kina se kinase (MEK) inhibitor, PD 098059; however, the MEK inhibitor did n ot significantly reduce the effect of insulin on activating glycogen s ynthase. Incubating muscles with concentrations of rapamycin that inhi bited activation of p70(S6K) abolished the activation of glycogen synt hase, Insulin also increased the phosphorylation of PHAS-I (phosphoryl ated heat- and acid-stable protein) and promoted the dissociation of t he PHAS-I . eIF-4E complex. Increasing MAP kinase activity with EGF di d not mimic the effect of insulin on PHAS-I phosphorylation, and the e ffect 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, activ ation of the MAP kinase/Rsk signaling pathway appears to be neither ne cessary nor sufficient for insulin action on glycogen synthase and PHA S-I in rat skeletal muscle. The results indicate that the effects of i nsulin on increasing the synthesis of glycogen and protein in skeletal muscle, two of the most important actions of the hormone, involve a r apamycin-sensitive mechanism that may include elements of the p70(S6K) signaling pathway.