Phosphorylation of Irs1 at SER-522 inhibits insulin signaling.

Multisite phosphorylation of Irs1 on serine and threonine residues regulates insulin signaling that can contribute to insulin resistance. We identified by mass spectrometry the phosphorylation of Ser522 in rat Irs1 ( S522(Irs1)). The functional effects of this phosphorylation site were investigated in cultured cells using a sequencespecific phosphoserine antibody. Insulin stimulated the phosphorylation of S522(Irs1) in L6 myoblasts and myotubes. S522(Irs1) phosphorylation was inhibited by wortmannin, whereas PD98059, rapamycin, or glucose-starvation had no effect. Reducing Akt expression with small interfering RNA inhibited insulin-stimulated phosphorylation of S522(Irs1), suggesting the involvement of the phosphatidylinositol 3-kinase3 -> Akt cascade. A S522(Irs1)-> A522(Irs1) substitution increased insulinstimulated tyrosine phosphorylation of Irs1 and signaling, whereas a S522(Irs1)-> E522(Irs1) substitution reduced insulin-stimulated Irs1 tyrosine phosphorylation. Together, these results suggest the phosphatidylinositol 3-kinase -> Akt cascade can inhibit insulin signaling through the phosphorylation of S522(Irs1).