title: Phosphatase control of 4E-BP1 phosphorylation state is central for glycolytic regulation of retinal protein synthesis.
abstract: Control of protein synthesis in insulin-responsive tissues has been well characterized, but relatively little is known about how this process is regulated in nervous tissues. The retina exhibits a relatively high protein synthesis rate, coinciding with high basal Akt and metabolic activities, with the majority of retinal ATP being derived from aerobic glycolysis. We examined the dependency of retinal protein synthesis on the Akt-mTOR signaling and glycolysis using ex vivo rat retinas. Akt inhibitors significantly reduced retinal protein synthesis but did not affect glycolytic lactate production. Surprisingly, the glycolytic inhibitor 2-deoxyglucose (2-DG) markedly inhibited Akt1 and Akt3 activities, as well as protein synthesis. The effects of 2-DG, and 2-fluorodeoxyglucose (2-FDG) on retinal protein synthesis correlated with inhibition of lactate production and diminished ATP content, with all these effects reversed by provision of d-mannose. 2-DG treatment was not associated with increased AMPK, eEF2, or eIF2α phosphorylation; instead, it caused rapid dephosphorylation of 4E-BP1. 2-DG reduced total mTOR activity by 25%, but surprisingly, it did not reduce mTORC1 activity, as indicated by unaltered raptor-associated mTOR autophosphorylation and ribosomal protein S6 phosphorylation. Dephosphorylation of 4E-BP1 was largely prevented by inhibition of PP1/PP2A phosphatases with okadaic acid and calyculin A, and inhibition of PPM1 phosphatases with cadmium. Thus, inhibition of retinal glycolysis diminished Akt and protein synthesis coinciding with accelerated dephosphorylation of 4E-BP1 independently of mTORC1. These results demonstrate a novel mechanism regulating protein synthesis in the retina involving an mTORC1-independent and phosphatase-dependent regulation of 4E-BP1.
authors.affiliation: Kellogg Eye Center, Department of Ophthalmology and Visual Sciences, University of Michigan, Ann Arbor, Michigan; and Department of Molecular and Integrative Physiology, University of Michigan, Ann Arbor, Michigan.
authors.forename: Thomas W
authors.initials: TW
authors.lastname: Gardner
authors.affiliation: Kellogg Eye Center, Department of Ophthalmology and Visual Sciences, University of Michigan, Ann Arbor, Michigan; and.
authors.forename: Steven F
authors.initials: SF
authors.lastname: Abcouwer
authors.affiliation: Kellogg Eye Center, Department of Ophthalmology and Visual Sciences, University of Michigan, Ann Arbor, Michigan; and.
authors.forename: Mandy K
authors.initials: MK
authors.lastname: Losiewicz
authors.affiliation: Kellogg Eye Center, Department of Ophthalmology and Visual Sciences, University of Michigan, Ann Arbor, Michigan; and patricef@umich.edu.
authors.forename: Patrice E
authors.initials: PE
authors.lastname: Fort
mesh_terms: D055372:AMP-Activated Protein Kinases / Q000378:metabolism; D000818:Animals; D002352:Carrier Proteins / Q000378:metabolism*; D015852:Eukaryotic Initiation Factor-2 / Q000378:metabolism; D006019:Glycolysis; D047908:Intracellular Signaling Peptides and Proteins; D019344:Lactic Acid / Q000378:metabolism; D008297:Male; D000076222:Mechanistic Target of Rapamycin Complex 1; D046912:Multiprotein Complexes / Q000378:metabolism; D020652:Peptide Elongation Factor 2 / Q000378:metabolism; D010750:Phosphoproteins / Q000378:metabolism*; D010766:Phosphorylation; D014176:Protein Biosynthesis; D054645:Protein Phosphatase 1 / Q000378:metabolism; D054648:Protein Phosphatase 2 / Q000378:metabolism; D051057:Proto-Oncogene Proteins c-akt / Q000378:metabolism*; D051381:Rats; D012160:Retina / Q000378:metabolism*; D038601:Ribosomal Protein S6 / Q000378:metabolism; D015398:Signal Transduction; D058570:TOR Serine-Threonine Kinases / Q000378:metabolism*
chemical_list: D002352:Carrier Proteins; C499526:Eif4ebp1 protein, rat; D015852:Eukaryotic Initiation Factor-2; D047908:Intracellular Signaling Peptides and Proteins; D046912:Multiprotein Complexes; D020652:Peptide Elongation Factor 2; D010750:Phosphoproteins; D038601:Ribosomal Protein S6; D019344:Lactic Acid; C546845:mTOR protein, rat; C494920:Akt1 protein, rat; C111490:Akt3 protein, rat; D000076222:Mechanistic Target of Rapamycin Complex 1; D051057:Proto-Oncogene Proteins c-akt; D058570:TOR Serine-Threonine Kinases; D055372:AMP-Activated Protein Kinases; D054645:Protein Phosphatase 1; D054648:Protein Phosphatase 2
keywords: Akt/mTOR pathway; glycolysis; protein phosphatases; retinal protein synthesis