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.Insulin resistance is a major risk factor and a principal defect in type II diabetes. Nutrient overload in affluent societies has been associated with increased occurrence of metabolic syndrome (1, 2). High protein diets are associated with altered glucose metabolism and increased occurrence of type II diabetes (3, 4). Elevated plasma concentrations of amino acids have long been found in obesity and insulin-resistant states (5-8). Furthermore, amino acid infusion induces insulin resistance in healthy individuals (9). Most recently, it has been reported that branched-chain amino acids in diet contribute to insulin resistance in high fat diet-fed rats and that a similar consequence of such a dietary pattern may exist in human (10). Currently, a role of dietary proteins in the pathogenesis of insulin resistance has been well recognized (11) (14); among them the mammalian target of rapamycin (mTOR) has been shown to phosphorylate STAT3 in neuronal cells (15, 16) and IL-6-stimulated hepatocytes (17).As a negative feedback control, STATs induce the expression of SOCS proteins, which are characterized by their ability to down-regulate cytokine signaling (18). SOCSs also play an important role in the pathogenesis of insulin resistance by integrating cytokine signaling with insulin signaling (19). Overexpression of SOCS3 inhibited ...