title: Differential effects of PDCD4 depletion on protein synthesis in myoblast and myotubes.
abstract: BACKGROUND
  Reduced muscle mass is a hallmark of metabolic diseases like diabetes and cancer. The mammalian (mechanistic) target of rapamycin complex 1/S6 kinase 1 (mTORC1/S6K1) pathway is critical to the regulation of muscle protein synthesis and mass but its mechanism of action is not completely understood.
  
  
  RESULTS
  Using L6 myotubes, we characterized the regulation of programmed cell death 4 (PDCD4), a recently described substrate of S6K1. The abundance, but not Ser67 phosphorylation, of PDCD4 was sensitive to amino acid and serum deprivation: values in starved cells were 4.5X of control (P < 0.001). Refeeding had opposite effects. Growth factors, compared to amino acids, appeared more critical in regulating PDCD4 abundance. Furthermore, inhibition of mTORC1 or the proteasome prevented the refeeding-associated decrease in PDCD4 abundance. Amino acid and serum deprivation significantly increased PDCD4 binding to eIF4A (P < 0.05); this was reversed during refeeding. PDCD4 depletion by RNA interference had no significant effect on phenylalanine incorporation into myotube mixed proteins in control cells but further suppressed (30%) this measure in nutrient-deprived cells (P < 0.0005). This was not observed in myoblasts. In starved myotubes, PDCD4 depletion further reduced the association of eIF4G with eIF4E.
  
  
  CONCLUSIONS
  Our data suggest that in myotubes, PDCD4 abundance is sensitive to nutritional manipulation in an mTORC1 and proteasome depended manner. Furthermore, the role of PDCD4 in regulating protein synthesis appears dependent on the developmental state of the cell.
authors.forename: Dhanshri
authors.initials: D
authors.lastname: Kakade
authors.forename: Nushaba
authors.initials: N
authors.lastname: Islam
authors.forename: Naomi
authors.initials: N
authors.lastname: Maeda
authors.affiliation: School of Kinesiology & Health Science and Muscle Health Research Centre, York University, 4700 Keele Street, Toronto, Ontario M3J 1P3, Canada. oadegoke@yorku.ca.
authors.forename: Olasunkanmi A J
authors.initials: OA
authors.lastname: Adegoke
mesh_terms: D000818:Animals; D051017:Apoptosis Regulatory Proteins / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism*; D002478:Cells, Cultured; D003470:Culture Media / Q000494:pharmacology; D039601:Eukaryotic Initiation Factor-4A / Q000378:metabolism; D039561:Eukaryotic Initiation Factor-4E / Q000378:metabolism; D039603:Eukaryotic Initiation Factor-4G / Q000378:metabolism; D036341:Intercellular Signaling Peptides and Proteins / Q000494:pharmacology; D000076222:Mechanistic Target of Rapamycin Complex 1; D046912:Multiprotein Complexes / Q000037:antagonists & inhibitors / Q000378:metabolism; D018485:Muscle Fibers, Skeletal / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D032446:Myoblasts / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D010649:Phenylalanine / Q000378:metabolism; D010766:Phosphorylation; D046988:Proteasome Endopeptidase Complex / Q000737:chemistry / Q000378:metabolism; D034622:RNA Interference; D051381:Rats; D058570:TOR Serine-Threonine Kinases / Q000037:antagonists & inhibitors / Q000378:metabolism
chemical_list: D051017:Apoptosis Regulatory Proteins; D003470:Culture Media; D039561:Eukaryotic Initiation Factor-4E; D039603:Eukaryotic Initiation Factor-4G; D036341:Intercellular Signaling Peptides and Proteins; D046912:Multiprotein Complexes; C467042:Pdcd4 protein, rat; D010649:Phenylalanine; D000076222:Mechanistic Target of Rapamycin Complex 1; D058570:TOR Serine-Threonine Kinases; D039601:Eukaryotic Initiation Factor-4A; D046988:Proteasome Endopeptidase Complex