Cellular signaling of amino acids towards mTORC1 activation in impaired human leucine catabolism.
The regulation of cell growth and protein biosynthesis is triggered by the mammalian target of rapamycin complex 1 (mTORC1) responding to amino acids, especially leucine. The molecular mechanisms linking leucine to mTORC1 activation are not well understood. We analyzed whether the free intracellular leucine availability, a metabolite of leucine catabolism or the process of leucine oxidation activates mTORC1 signaling. We further investigated whether mTORC1 signaling is subject to altered regulation in disturbed leucine metabolism. Human fibroblasts with deficiencies in leucine catabolic steps and those from healthy control subjects were utilized. In all cells, leucine-induced mTORC1 signaling was significantly related to leucine pool size and leucine repletion capacity. The leucine/glutamine antiporter SLC7A5/SLC3A2 and the amino acid sensor MAP4K3 were identified as crucial determinants of signaling leucine availability to downstream targets. In cells with defective leucine catabolism, mTORC1 signaling towards phosphorylation of ribosomal protein S6 kinase 1 (S6K1) was significantly increased, whereas transcriptional down-regulation of MAP4K3 upon reduced amino acid supply was abrogated. Remarkably, these effects were observed irrespective of the localization of the enzymatic blockage. Our data provide evidence that mechanisms determining intracellular leucine availability and the amino acid sensor MAP4K3 are key upstream modulators of nutrient-sensitive mTORC1 signaling, whereas specific leucine metabolites or leucine oxidation rates do not play a role. In human fibroblasts deficient in leucine catabolic steps, we observed regulation consistent with sustaining a more efficient MAP4K3 and mTOR-S6K1 signaling. Such regulatory circuit might serve to protect cells against detrimental consequences of reduced nutrient utilization in human conditions associated with disturbed leucine metabolism.
Research Center, Dr. von Hauner Children's Hospital, Ludwig-Maximilians-Universität München, 80337 Munich, Germany.
Sonja C
SC
Schriever
Manuel J
MJ
Deutsch
Jerzy
J
Adamski
Adelbert A
AA
Roscher
Regina
R
Ensenauer
D017920:Antiporters / Q000235:genetics / Q000378:metabolism; D049109:Cell Proliferation; D002648:Child; D002675:Child, Preschool; D015536:Down-Regulation; D005347:Fibroblasts / Q000166:cytology / Q000378:metabolism* / Q000473:pathology; D027282:Fusion Regulatory Protein 1, Heavy Chain / Q000235:genetics / Q000378:metabolism; D005786:Gene Expression Regulation; D006801:Humans; D007223:Infant; D027283:Large Neutral Amino Acid-Transporter 1 / Q000235:genetics / Q000378:metabolism; D007930:Leucine / Q000378:metabolism*; D000076222:Mechanistic Target of Rapamycin Complex 1; D046912:Multiprotein Complexes / Q000235:genetics / Q000378:metabolism*; D010766:Phosphorylation; D014176:Protein Biosynthesis; D017346:Protein Serine-Threonine Kinases / Q000235:genetics / Q000378:metabolism; D038762:Ribosomal Protein S6 Kinases, 70-kDa / Q000235:genetics / Q000378:metabolism; D015398:Signal Transduction; D058570:TOR Serine-Threonine Kinases / Q000235:genetics / Q000378:metabolism*; D014162:Transfection
D017920:Antiporters; D027282:Fusion Regulatory Protein 1, Heavy Chain; D027283:Large Neutral Amino Acid-Transporter 1; D046912:Multiprotein Complexes; C545989:SLC3A2 protein, human; C543044:MAP4K3 protein, human; D000076222:Mechanistic Target of Rapamycin Complex 1; D017346:Protein Serine-Threonine Kinases; D038762:Ribosomal Protein S6 Kinases, 70-kDa; D058570:TOR Serine-Threonine Kinases; C448901:ribosomal protein S6 kinase, 70kD, polypeptide 1; D007930:Leucine