Stimulation of glycogen synthesis by insulin requires S6 kinase and phosphatidylinositol-3-kinase in HTC-IR cells.
In order to study the role of phosphatidylinositol-3-kinase (PI3K), PKB, FRAP, S6 kinase, and MAP kinase in insulin-stimulated glycogen synthesis, we used a specific inhibitor of PI3K, LY294002, the immunosuppressant inhibitor of FRAP, rapamycin, and the inhibitor of MAPK kinase (MEK)/MAPK, PD98059, in rat HTC hepatoma cells overexpressing human insulin receptors. The PI3K inhibitor LY294002 completely blocks insulin-stimulated glycogen synthesis by inhibiting glycogen synthase, PKB (Akt-1), and FRAP (RAFT) autophosphorylation, as well as p70 S6 kinase activation, whereas insulin receptor substrates tyrosine phosphorylation and MEK activity were not affected. However, rapamycin only partially blocks insulin-stimulated glycogen synthesis by partial inhibition of glycogen synthase, whereas it completely blocks S6 kinase activation and FRAP autophosphorylation, but does not affect either PKB autophosphorylation, MEK activity, or insulin receptor tyrosine phosphorylation. Insulin-stimulated glycogen synthesis and glycogen synthase were not affected by the MEK/MAPK inhibitor PD98059. These data suggest that the PI3K, and not the MAPK pathway plays an important role in the insulin-stimulated glycogen synthesis in the hepatocyte, partly mediated by FRAP and S6 kinase activation. However, the inhibition of FRAP and S6 kinase activation is not sufficient to block insulin-stimulated glycogen synthesis, suggesting an important role of a branching pathway upstream of S6 kinase and downstream of PI3K, which is probably mediated by PKB in the signaling of the insulin receptor in hepatoma HTC cells.
Sánchez-Margalet
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Departamento de Bioquímica Médica y Biología Molecular, Facultad de Medicina, Universidad de Sevilla; Unidad de Investigación del Hospital Universitario Virgen Macarena, Sevilla, Spain. vsanchez@cica.es
D000818:Animals; D002867:Chromones / Q000494:pharmacology; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005419:Flavonoids / Q000494:pharmacology; D006003:Glycogen / Q000096:biosynthesis*; D006006:Glycogen Synthase / Q000378:metabolism; D006801:Humans; D020104:Immunophilins / Q000378:metabolism; D007328:Insulin / Q000494:pharmacology*; D055504:Insulin Receptor Substrate Proteins; D007527:Isoenzymes / Q000378:metabolism; D020928:Mitogen-Activated Protein Kinases / Q000378:metabolism; D009025:Morpholines / Q000494:pharmacology; D019869:Phosphatidylinositol 3-Kinases / Q000737:chemistry / Q000502:physiology*; D010750:Phosphoproteins / Q000378:metabolism; D010766:Phosphorylation / Q000187:drug effects; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism; D011518:Proto-Oncogene Proteins; D051057:Proto-Oncogene Proteins c-akt; D051381:Rats; D019893:Ribosomal Protein S6 Kinases / Q000502:physiology*; D020123:Sirolimus / Q000494:pharmacology; D022021:Tacrolimus Binding Proteins; D014407:Tumor Cells, Cultured; D014443:Tyrosine / Q000378:metabolism
D002867:Chromones; D004791:Enzyme Inhibitors; D005419:Flavonoids; D006003:Glycogen; D006006:Glycogen Synthase; D020104:Immunophilins; D007328:Insulin; D055504:Insulin Receptor Substrate Proteins; D007527:Isoenzymes; D020928:Mitogen-Activated Protein Kinases; D009025:Morpholines; D019869:Phosphatidylinositol 3-Kinases; D010750:Phosphoproteins; D017346:Protein Serine-Threonine Kinases; D011518:Proto-Oncogene Proteins; D051057:Proto-Oncogene Proteins c-akt; D019893:Ribosomal Protein S6 Kinases; D020123:Sirolimus; D022021:Tacrolimus Binding Proteins; D014443:Tyrosine; C526292:IRS1 protein, human; C526296:Irs1 protein, rat; C085911:2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one; C494918:AKT1 protein, human; C494920:Akt1 protein, rat; C093973:2-(2-amino-3-methoxyphenyl)-4H-1-benzopyran-4-one