title: Inhibition of protein synthesis by activation of NMDA receptors in cultured retinal cells: a new mechanism for the regulation of nitric oxide production.
abstract: The synthesis of nitric oxide (NO) is limited by the intracellular availability of L-arginine. Here we show that stimulation of NMDA receptors promotes an increase of intracellular L-arginine which supports an increase in the production of NO. Although L-[3H]arginine uptake measured in cultured chick retina cells incubated in the presence of cycloheximide (CHX, a protein synthesis inhibitor) was inhibited approximately 75% at equilibrium, quantitative thin-layer chromatography analysis showed that free intracellular L-[3H]arginine was six times higher in CHX-treated than in control cultures. Extracellular L-[3H]citrulline levels increased threefold in CHX-treated groups, an effect blocked by NG-nitro-L-arginine, a NO synthase (NOS) inhibitor. NMDA promoted a 40% increase of free intracellular L-[3H]arginine in control cultures, an effect blocked by the NMDA antagonist 2-amino 5-phosphonovaleric acid. In parallel, NMDA promoted a reduction of 40-50% in the incorporation of 35[S]methionine or L-[3H]arginine into proteins. Western blot analysis revealed that NMDA stimulates the phosphorylation of eukaryotic elongation factor 2 (eEF2, a factor involved in protein translation), an effect inhibited by (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate (MK801). In conclusion, we have shown that the stimulation of NMDA receptors promotes an inhibition of protein synthesis and a consequent increase of an intracellular L-arginine pool available for the synthesis of NO. This effect seems to be mediated by activation of eEF2 kinase, a calcium/calmodulin-dependent enzyme which specifically phosphorylates and blocks eEF2. The results raise the possibility that NMDA receptor activation stimulates two different calmodulin-dependent enzymes (eEF2 kinase and NOS) reinforcing local NO production by increasing precursor availability together with NOS catalytic activity.
authors.affiliation: Program of Neuroimmunology, Institute of Biology, Federal Fluminense University, Niterói, RJ, Brazil.
authors.forename: Marcelo
authors.initials: M
authors.lastname: Cossenza
authors.forename: Daniel V
authors.initials: DV
authors.lastname: Cadilhe
authors.forename: Rodrigo N
authors.initials: RN
authors.lastname: Coutinho
authors.forename: Roberto
authors.initials: R
authors.lastname: Paes-de-Carvalho
mesh_terms: D000818:Animals; D001120:Arginine / Q000378:metabolism*; D017871:Calcium-Calmodulin-Dependent Protein Kinases / Q000187:drug effects / Q000378:metabolism; D002478:Cells, Cultured; D002642:Chick Embryo; D002956:Citrulline / Q000378:metabolism; D054736:Elongation Factor 2 Kinase; D004791:Enzyme Inhibitors / Q000494:pharmacology; D018690:Excitatory Amino Acid Agonists / Q000494:pharmacology; D018691:Excitatory Amino Acid Antagonists / Q000494:pharmacology; D016202:N-Methylaspartate / Q000494:pharmacology; D009419:Nerve Tissue Proteins / Q000096:biosynthesis*; D009457:Neuroglia / Q000187:drug effects / Q000378:metabolism; D009474:Neurons / Q000187:drug effects / Q000378:metabolism; D009569:Nitric Oxide / Q000096:biosynthesis*; D019001:Nitric Oxide Synthase / Q000037:antagonists & inhibitors / Q000378:metabolism; D019335:Nitroarginine / Q000494:pharmacology; D020652:Peptide Elongation Factor 2 / Q000187:drug effects / Q000378:metabolism; D010766:Phosphorylation / Q000187:drug effects; D011500:Protein Synthesis Inhibitors / Q000494:pharmacology; D016194:Receptors, N-Methyl-D-Aspartate / Q000187:drug effects / Q000378:metabolism*; D012160:Retina / Q000187:drug effects / Q000378:metabolism*; D015854:Up-Regulation / Q000187:drug effects / Q000502:physiology*
chemical_list: D004791:Enzyme Inhibitors; D018690:Excitatory Amino Acid Agonists; D018691:Excitatory Amino Acid Antagonists; D009419:Nerve Tissue Proteins; D020652:Peptide Elongation Factor 2; D011500:Protein Synthesis Inhibitors; D016194:Receptors, N-Methyl-D-Aspartate; D019335:Nitroarginine; D002956:Citrulline; D009569:Nitric Oxide; D016202:N-Methylaspartate; D001120:Arginine; D019001:Nitric Oxide Synthase; D017871:Calcium-Calmodulin-Dependent Protein Kinases; D054736:Elongation Factor 2 Kinase