title: Amino acid signaling through the mammalian target of rapamycin (mTOR) pathway: Role of glutamine and of cell shrinkage.
abstract: Mammalian target of rapamycin (mTOR) mediates a signaling pathway that couples amino acid availability to S6 kinase (S6K) activation, translational initiation and cell growth rate, participating to a versatile checkpoint that inspects the energy status of the cell. The pathway is activated by branched-chain amino acids (BCAA), leucine being the most effective, whereas amino acid dearth and ATP shortage lead to its deactivation. Glutamine- or amino acid-deprivation and hyperosmotic stress induce a fast cell shrinkage (with marked decrease of the intracellular water volume) associated to mTOR-dependent S6K1 dephosphorylation. Using cultured Jurkat cells, we have measured the changes of cell content and intracellular concentration of ATP, of relevant amino acids (BCAA) and of ninhydrin-positive substances (NPS, as measure of NH(2)-bearing organic osmolytes) under conditions that deactivate (leucine-deprivation, glutamine-deprivation, amino acid withdrawal, sorbitol-induced hyperosmotic stress) or reactivate a previously deactivated, mTOR-S6K1 pathway. We have also assessed the mitochondrial function by measurements of mitochondrial transmembrane potential in cells subjected to hypertonic stress. Our results indicate that diverse control signals converge on the mTOR-S6K1 signaling pathway. In the presence of adequate energy resources, the pathway senses the amino acid availability as inward transport of effective amino acids (as BCAA and especially leucine), but its activation occurs only in the presence of an extracellular amino acid complement, with glutamine as obligatory component, and does not tolerate decrements of cell water volume incapable of maintaining adequate intracellular physicochemical conditions.
authors.affiliation: Department of Experimental Medicine, Division of Molecular Pathology and Immunology, University of Parma, Parma, Italy.
authors.forename: Claudia
authors.initials: C
authors.lastname: Fumarola
authors.forename: Silvia
authors.initials: S
authors.lastname: La Monica
authors.forename: Guido G
authors.initials: GG
authors.lastname: Guidotti
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism; D000903:Antibiotics, Antineoplastic / Q000494:pharmacology; D048429:Cell Size; D005973:Glutamine / Q000378:metabolism* / Q000494:pharmacology; D006801:Humans; D019169:Jurkat Cells; D007930:Leucine / Q000378:metabolism / Q000494:pharmacology; D009997:Osmotic Pressure; D011494:Protein Kinases / Q000378:metabolism*; D038762:Ribosomal Protein S6 Kinases, 70-kDa / Q000378:metabolism; D015398:Signal Transduction / Q000502:physiology*; D020123:Sirolimus / Q000494:pharmacology; D058570:TOR Serine-Threonine Kinases; D014867:Water / Q000378:metabolism; D014882:Water-Electrolyte Balance / Q000502:physiology*
chemical_list: D000903:Antibiotics, Antineoplastic; D014867:Water; D005973:Glutamine; D000255:Adenosine Triphosphate; D011494:Protein Kinases; C546842:MTOR protein, human; D038762:Ribosomal Protein S6 Kinases, 70-kDa; D058570:TOR Serine-Threonine Kinases; C448901:ribosomal protein S6 kinase, 70kD, polypeptide 1; D007930:Leucine; D020123:Sirolimus