title: The rapid activation of protein synthesis by growth hormone requires signaling through mTOR.
abstract: An important function of growth hormone (GH) is to promote cell and tissue growth, and a key component of these effects is the stimulation of protein synthesis. In this study, we demonstrate that, in H4IIE hepatoma cells, GH acutely activated protein synthesis through signaling via the mammalian target of rapamycin (mTOR) and specifically through the rapamycin-sensitive mTOR complex 1 (mTORC1). GH treatment enhanced the phosphorylation of two targets of mTOR signaling, 4E-BP1 and ribosomal protein S6. Phosphorylation of S6 and 4E-BP1 was maximal at 30-45 min and 10-20 min after GH stimulation, respectively. Both proteins modulate components of the translational machinery. The GH-induced phosphorylation of 4E-BP1 led to its dissociation from eIF4E and increased binding of eIF4E to eIF4G to form (active) eIF4F complexes. The ability of GH to stimulate the phosphorylation of S6 and 4E-BP1 was blocked by rapamycin. GH also led to the dephosphorylation of a third translational component linked to mTORC1, the elongation factor eEF2. Its regulation followed complex biphasic kinetics, both phases of which required mTOR signaling. GH rapidly activated both the MAP kinase (ERK) and PI 3-kinase pathways. Signaling through PI 3-kinase alone was, however, sufficient to activate the downstream mTORC1 pathway. Consistent with this, GH increased the phosphorylation of TSC2, an upstream regulator of mTORC1, at sites that are targets for Akt/PKB. Finally, the activation of overall protein synthesis by GH in H4IIE cells was essentially completely inhibited by wortmannin or rapamycin. These results demonstrate for the first time that mTORC1 plays a major role in the rapid activation of protein synthesis by GH.
authors.affiliation: Institute of Food Nutrition and Human Health, Massey University, and Metabolism and Microbial Genomics Section, AgResearch Limited, Palmerston North, New Zealand.
authors.forename: Amanda A
authors.initials: AA
authors.lastname: Hayashi
authors.forename: Christopher G
authors.initials: CG
authors.lastname: Proud
mesh_terms: D000818:Animals; D002352:Carrier Proteins / Q000378:metabolism; D045744:Cell Line, Tumor; D039561:Eukaryotic Initiation Factor-4E / Q000378:metabolism; D039562:Eukaryotic Initiation Factor-4F / Q000096:biosynthesis; D039603:Eukaryotic Initiation Factor-4G / Q000378:metabolism; D013006:Growth Hormone / Q000037:antagonists & inhibitors / Q000494:pharmacology*; D047908:Intracellular Signaling Peptides and Proteins; D007700:Kinetics; D020652:Peptide Elongation Factor 2 / Q000378:metabolism; D019869:Phosphatidylinositol 3-Kinases / Q000378:metabolism; D010750:Phosphoproteins / Q000378:metabolism; D010766:Phosphorylation / Q000187:drug effects; D014176:Protein Biosynthesis / Q000187:drug effects*; D051381:Rats; D011994:Recombinant Proteins / Q000494:pharmacology; D038601:Ribosomal Protein S6 / Q000378:metabolism; D015398:Signal Transduction / Q000187:drug effects / Q000502:physiology; D020123:Sirolimus / Q000494:pharmacology; D013997:Time Factors; D014157:Transcription Factors / Q000378:metabolism*
chemical_list: D002352:Carrier Proteins; D039561:Eukaryotic Initiation Factor-4E; D039562:Eukaryotic Initiation Factor-4F; D039603:Eukaryotic Initiation Factor-4G; D013006:Growth Hormone; D047908:Intracellular Signaling Peptides and Proteins; D020652:Peptide Elongation Factor 2; D019869:Phosphatidylinositol 3-Kinases; D010750:Phosphoproteins; D011994:Recombinant Proteins; D038601:Ribosomal Protein S6; D020123:Sirolimus; D014157:Transcription Factors; C519510:Crtc1 protein, rat; C499526:Eif4ebp1 protein, rat