Aspirin Inhibits Serine Phosphorylation of Insulin Receptor Substrate 1 in Tumor Necrosis Factor-treated Cells through Targeting Multiple Serine Kinases

The hypoglycemic effects of high dose salicylates in the treatment of diabetes were documented before the advent of insulin. However, the molecular mechanisms by which salicylates exert these anti-diabetic effects are not well understood. In this study, we analyzed the effects of aspirin (acetylsalicylic acid) on serine phosphorylation of insulin receptor substrate I (IRS-1) in cells treated with tumor necrosis factor (TNF)-α. Phosphorylation of IRS-1 at Ser³⁰⁷, Ser²⁶⁷, and Ser⁶¹² was monitored by immunoblotting with phospho-specific IRS-1 anti-bodies. In 3T3-L1 and Hep G2 cells, phosphorylation of IRS-1 at Ser³⁰⁷ in response to TNF-α treatment correlated with phosphorylation of JNK, c-Jun, and degradation of IκBα. Moreover, phosphorylation of IRS-1 at Ser ³⁰⁷ in embryo fibroblasts derived from either JNK or IKK knockout mice was reduced when compared with that in the wild-type controls. Taken together, these data suggest that serine phosphorylation of IRS-1 in response to TNF-α is mediated, in part, by JNK and IKK. Interestingly, aspirin treatment inhibited the phosphorylation of IRS-1 at Ser³⁰⁷ as well as the phosphorylation of JNK, c-Jun, and degradation of IκBα. Furthermore, other serine kinases including Akt, extracellular regulated kinase, mammalian target of rapamycin, and PKCζ were also activated by TNF-α (as assessed by phospho-specific antibodies). Phosphorylation of IRS-1 at Ser²⁶⁷ and Ser⁶¹² correlated with the activation of these kinases. Phosphorylation of Akt and the mammalian target of rapamycin (but not extracellular regulated kinase or PKCζ) in response to TNF-α was inhibited by aspirin treatment. Finally, aspirin rescued insulin-induced glucose uptake in 3T3-L1 adipocytes pretreated with TNF-α. We conclude that aspirin may enhance insulin sensitivity by protecting IRS proteins from serine phosphorylation catalyzed by multiple kinases.