Regulation of amino acid-sensitive TOR signaling by leucine analogues in adipocytes

J Cell Biochem. 2000 Mar;77(2):234-51. doi: 10.1002/(sici)1097-4644(20000501)77:2<234::aid-jcb7>3.0.co;2-i.

Abstract

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.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Adaptor Proteins, Signal Transducing
  • Adipocytes / drug effects*
  • Adipocytes / metabolism*
  • Amino Acids / pharmacology*
  • Animals
  • Binding Sites
  • CHO Cells
  • Carrier Proteins*
  • Cricetinae
  • Humans
  • In Vitro Techniques
  • Intracellular Signaling Peptides and Proteins
  • Jurkat Cells
  • Leucine / analogs & derivatives*
  • Leucine / chemistry
  • Leucine / pharmacology*
  • Male
  • Phosphoproteins / metabolism
  • Phosphorylation
  • Phosphotransferases (Alcohol Group Acceptor) / metabolism*
  • Protein Kinases*
  • Rats
  • Rats, Sprague-Dawley
  • Signal Transduction / drug effects
  • Sirolimus / pharmacology
  • Structure-Activity Relationship
  • TOR Serine-Threonine Kinases

Substances

  • Adaptor Proteins, Signal Transducing
  • Amino Acids
  • Carrier Proteins
  • EIF4EBP1 protein, human
  • Eif4ebp1 protein, rat
  • Intracellular Signaling Peptides and Proteins
  • Phosphoproteins
  • Protein Kinases
  • Phosphotransferases (Alcohol Group Acceptor)
  • MTOR protein, human
  • TOR Serine-Threonine Kinases
  • mTOR protein, rat
  • Leucine
  • Sirolimus