title: Signal transducer and activator of transcription 3 (STAT3) mediates amino acid inhibition of insulin signaling through serine 727 phosphorylation.
abstract: Nutrient overload is associated with the development of obesity, insulin resistance, and type II diabetes. High plasma concentrations of amino acids have been found to correlate with insulin resistance. At the cellular level, excess amino acids impair insulin signaling, the mechanisms of which are not fully understood. Here, we report that STAT3 plays a key role in amino acid dampening of insulin signaling in hepatic cells. Excess amino acids inhibited insulin-stimulated Akt phosphorylation and glycogen synthesis in mouse primary hepatocytes as well as in human hepatocarcinoma HepG2 cells. STAT3 knockdown protected insulin sensitivity from inhibition by amino acids. Amino acids stimulated the phosphorylation of STAT3 at Ser(727), but not Tyr(705). Replacement of the endogenous STAT3 with wild-type, but not S727A, recombinant STAT3 restored the ability of amino acids to inhibit insulin signaling, suggesting that Ser(727) phosphorylation was critical for STAT3-mediated amino acid effect. Furthermore, overexpression of STAT3-S727D was sufficient to inhibit insulin signaling in the absence of excess amino acids. Our results also indicated that mammalian target of rapamycin was likely responsible for the phosphorylation of STAT3 at Ser(727) in response to excess amino acids. Finally, we found that STAT3 activity and the expression of its target gene socs3, known to be involved in insulin resistance, were both stimulated by excess amino acids and inhibited by rapamycin. In conclusion, our study reveals STAT3 as a novel mediator of nutrient signals and identifies a Ser(727) phosphorylation-dependent and Tyr(705) phosphorylation-independent STAT3 activation mechanism in the modulation of insulin signaling.
authors.affiliation: Department of Cell and Developmental Biology, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA. jiechen@illinois.edu
authors.forename: Jeong-Ho
authors.initials: JH
authors.lastname: Kim
authors.forename: Mee-Sup
authors.initials: MS
authors.lastname: Yoon
authors.forename: Jie
authors.initials: J
authors.lastname: Chen
mesh_terms: D019943:Amino Acid Substitution; D000596:Amino Acids / Q000378:metabolism* / Q000494:pharmacology; D000818:Animals; D003924:Diabetes Mellitus, Type 2; D005786:Gene Expression Regulation / Q000187:drug effects / Q000235:genetics; D055785:Gene Knockdown Techniques; D056945:Hep G2 Cells; D022781:Hepatocytes / Q000378:metabolism*; D006801:Humans; D007328:Insulin / Q000378:metabolism* / Q000494:pharmacology; D007333:Insulin Resistance / Q000235:genetics; D020125:Mutation, Missense; D010766:Phosphorylation / Q000187:drug effects / Q000235:genetics; D050796:STAT3 Transcription Factor / Q000235:genetics / Q000378:metabolism*; D015398:Signal Transduction; D000071223:Suppressor of Cytokine Signaling 3 Protein; D050826:Suppressor of Cytokine Signaling Proteins / Q000096:biosynthesis / Q000235:genetics
chemical_list: D000596:Amino Acids; D007328:Insulin; C106644:SOCS3 protein, human; D050796:STAT3 Transcription Factor; C494086:STAT3 protein, human; C491411:Socs3 protein, mouse; C494087:Stat3 protein, mouse; D000071223:Suppressor of Cytokine Signaling 3 Protein; D050826:Suppressor of Cytokine Signaling Proteins