Regulation of amino acid-sensitive TOR signaling by leucine analogues in adipocytes.
In adipocytes, amino acids stimulate the target of rapamycin (TOR) signaling pathway leading to phosphorylation of the translational repressor, eIF-4E binding protein-I (4E-BP1), and ribosomal protein S6. L-leucine is the primary mediator of these effects. The structure-activity relationships of a putative L-leucine recognition site in adipocytes (LeuR(A)) that regulates TOR activity were analyzed by examining the effects of leucine analogues on the rapamycin-sensitive phosphorylation of the translational repressor, eIF-4E binding protein-I (4E-BP1), an index of TOR activity. Several amino acids that are structurally related to leucine strongly stimulated 4E-BP1 phosphorylation at concentrations greater than the EC(50) value for leucine. The order of potency was leucine > norleucine > threo-L-beta-hydroxyleucine approximately Ile > Met approximately Val. Other structural analogues of leucine, such as H-alpha-methyl-D/L-leucine, S-(-)-2-amino-4-pentenoic acid, and 3-amino-4-methylpentanoic acid, possessed only weak agonist activity. However, other leucine-related compounds that are known agonists, antagonists, or ligands of other leucine binding/recognition sites did not affect 4E-BP1 phosphorylation. We conclude from the data that small lipophilic modifications of the leucine R group and alpha-hydrogen may be tolerated for agonist activity; however, leucine analogues with a modified amino group, a modified carboxylic group, charged R groups, or bulkier aliphatic R groups do not seem to possess significant agonist activity. Furthermore, the leucine recognition site that regulates TOR signaling in adipocytes appears to be different from the following: (1) a leucine receptor that regulates macroautophagy in liver, (2) a leucine recognition site that regulates TOR signaling in H4IIE hepatocytes, (3) leucyl tRNA or leucyl tRNA synthetase, (4) the gabapentin-sensitive leucine transaminase, or (5) the system L-amino acid transporter.
Lynch
C J
CJ
Department of Cellular and Molecular Physiology, Pennsylvania State University College of Medicine, Hershey, Pennsylvania 17033, USA. clynch@psu.edu
Fox
H L
HL
Vary
T C
TC
Jefferson
L S
LS
Kimball
S R
SR
D048868:Adaptor Proteins, Signal Transducing; D017667:Adipocytes / Q000187:drug effects* / Q000378:metabolism*; D000596:Amino Acids / Q000494:pharmacology*; D000818:Animals; D001665:Binding Sites; D016466:CHO Cells; D002352:Carrier Proteins; D018797:Cell Cycle Proteins; D006224:Cricetinae; D006801:Humans; D066298:In Vitro Techniques; D047908:Intracellular Signaling Peptides and Proteins; D019169:Jurkat Cells; D007930:Leucine / Q000031:analogs & derivatives* / Q000737:chemistry / Q000494:pharmacology*; D008297:Male; D010750:Phosphoproteins / Q000378:metabolism; D010766:Phosphorylation; D017853:Phosphotransferases (Alcohol Group Acceptor) / Q000378:metabolism*; D011494:Protein Kinases; D051381:Rats; D017207:Rats, Sprague-Dawley; D015398:Signal Transduction / Q000187:drug effects; D020123:Sirolimus / Q000494:pharmacology; D013329:Structure-Activity Relationship; D058570:TOR Serine-Threonine Kinases
D048868:Adaptor Proteins, Signal Transducing; D000596:Amino Acids; D002352:Carrier Proteins; D018797:Cell Cycle Proteins; C087000:EIF4EBP1 protein, human; C499526:Eif4ebp1 protein, rat; D047908:Intracellular Signaling Peptides and Proteins; D010750:Phosphoproteins; D011494:Protein Kinases; D017853:Phosphotransferases (Alcohol Group Acceptor); C546842:MTOR protein, human; C546845:mTOR protein, rat; D058570:TOR Serine-Threonine Kinases; D007930:Leucine; D020123:Sirolimus