Tryptophanylation of Insulin Receptor by WARS Attenuates Insulin Signaling

Abstract Background: Increased circulating tryptophan has been linked to insulin resistance and development of type 2 diabetes (T2D). Although the indispensable roles of tryptophan in behavioral health and immunity, which are mediated by its catabolized metabolites, are well documented, whether and how tryptophan directly affects insulin sensitivity remains largely unknown.Methods: The correlation of excessive tryptophan and insulin resistance was verified in tryptophan-rich chow fed mice. The effects of tryptophan and WARS to insulin signaling and glucose uptake were demonstrated in the HPA-v cells, HepG2 cells and HeLa cells. The tryptophanylation site was screened by single-site directed mutagenesis of all lysine residues in the b subunit of IR, and verified using an in vitro tryptophanylation assay. Finally, the detryptophanylation enzyme was identified and its role in insulin sensitivity was addressed by immunoblotting analysis. Results: Mice fed with tryptophan-rich chow developed insulin resistance. Tryptophan can tryptophanylate insulin receptor (IR) and attenuate insulin signaling. Excessive tryptophan promotes tryptophanyl-tRNA synthetase (WARS) to tryptophanylate lysine 1209 of IR (W-K1209), which induces insulin resistance by inhibiting the insulin-stimulated phosphorylation of IR, AKT and AS160. SIRT1, but not other sirtuins, detryptophanylates IRW-K1209 to increase the insulin sensitivity.Conclusions: Our results unveiled a novel mechanism of how tryptophan was sensed and tryptophanylated to IR by WARS, thus impaired insulin signaling. Meanwhile, our data indicated that WARS might be a target for insulin sensitivity control.