title: S6 kinase inactivation impairs growth and translational target phosphorylation in muscle cells maintaining proper regulation of protein turnover.
abstract: A defect in protein turnover underlies multiple forms of cell atrophy. Since S6 kinase (S6K)-deficient cells are small and display a blunted response to nutrient and growth factor availability, we have hypothesized that mutant cell atrophy may be triggered by a change in global protein synthesis. By using mouse genetics and pharmacological inhibitors targeting the mammalian target of rapamycin (mTOR)/S6K pathway, here we evaluate the control of translational target phosphorylation and protein turnover by the mTOR/S6K pathway in skeletal muscle and liver tissues. The phosphorylation of ribosomal protein S6 (rpS6), eukaryotic initiation factor-4B (eIF4B), and eukaryotic elongation factor-2 (eEF2) is predominantly regulated by mTOR in muscle cells. Conversely, in liver, the MAPK and phosphatidylinositol 3-kinase pathways also play an important role, suggesting a tissue-specific control. S6K deletion in muscle mimics the effect of the mTOR inhibitor rapamycin on rpS6 and eIF4B phosphorylation without affecting eEF2 phosphorylation. To gain insight on the functional consequences of these modifications, methionine incorporation and polysomal distribution were assessed in muscle cells. Rates and rapamycin sensitivity of global translation initiation are not altered in S6K-deficient muscle cells. In addition, two major pathways of protein degradation, autophagy and expression of the muscle-specific atrophy-related E3 ubiquitin ligases, are not affected by S6K deletion. Our results do not support a role for global translational control in the growth defect due to S6K deletion, suggesting specific modes of growth control and translational target regulation downstream of mTOR.
authors.affiliation: INSERM, U845, Paris, France.
authors.forename: Virginie
authors.initials: V
authors.lastname: Mieulet
authors.forename: Mila
authors.initials: M
authors.lastname: Roceri
authors.forename: Catherine
authors.initials: C
authors.lastname: Espeillac
authors.forename: Athanassia
authors.initials: A
authors.lastname: Sotiropoulos
authors.forename: Mickael
authors.initials: M
authors.lastname: Ohanna
authors.forename: Viola
authors.initials: V
authors.lastname: Oorschot
authors.forename: Judith
authors.initials: J
authors.lastname: Klumperman
authors.forename: Marco
authors.initials: M
authors.lastname: Sandri
authors.forename: Mario
authors.initials: M
authors.lastname: Pende
mesh_terms: D000818:Animals; D001343:Autophagy; D017871:Calcium-Calmodulin-Dependent Protein Kinases / Q000378:metabolism; D002478:Cells, Cultured; D054736:Elongation Factor 2 Kinase; D039642:Eukaryotic Initiation Factors / Q000378:metabolism; D022781:Hepatocytes / Q000201:enzymology / Q000378:metabolism / Q000473:pathology; D007328:Insulin / Q000378:metabolism; D007930:Leucine / Q000378:metabolism; D008099:Liver / Q000187:drug effects / Q000201:enzymology / Q000254:growth & development / Q000378:metabolism* / Q000473:pathology; D008297:Male; D051379:Mice; D018345:Mice, Knockout; D020928:Mitogen-Activated Protein Kinases / Q000378:metabolism; D024510:Muscle Development; D018485:Muscle Fibers, Skeletal / Q000201:enzymology / Q000378:metabolism* / Q000473:pathology; D018482:Muscle, Skeletal / Q000187:drug effects / Q000201:enzymology / Q000254:growth & development / Q000378:metabolism* / Q000473:pathology; D009133:Muscular Atrophy / Q000201:enzymology / Q000235:genetics / Q000378:metabolism* / Q000473:pathology; D019869:Phosphatidylinositol 3-Kinases / Q000378:metabolism; D010766:Phosphorylation; D014176:Protein Biosynthesis / Q000187:drug effects; D011494:Protein Kinases / Q000378:metabolism*; D038601:Ribosomal Protein S6 / Q000378:metabolism; D038744:Ribosomal Protein S6 Kinases, 90-kDa / Q000172:deficiency / Q000235:genetics / Q000378:metabolism*; D015398:Signal Transduction; D020123:Sirolimus / Q000494:pharmacology; D058570:TOR Serine-Threonine Kinases; D044767:Ubiquitin-Protein Ligases / Q000378:metabolism
chemical_list: D017871:Calcium-Calmodulin-Dependent Protein Kinases; D054736:Elongation Factor 2 Kinase; D039642:Eukaryotic Initiation Factors; D007328:Insulin; D007930:Leucine; D020928:Mitogen-Activated Protein Kinases; D019869:Phosphatidylinositol 3-Kinases; D011494:Protein Kinases; D038601:Ribosomal Protein S6; D038744:Ribosomal Protein S6 Kinases, 90-kDa; D020123:Sirolimus; D058570:TOR Serine-Threonine Kinases; D044767:Ubiquitin-Protein Ligases; C031799:eIF-4B; C448579:ribosomal protein S6, mouse; C546843:mTOR protein, mouse; C512964:Rps6ka1 protein, mouse; C069330:ribosomal protein S6 kinase, 90kDa, polypeptide 3; C517690:Eef2k protein, mouse