FRAP-dependent serine phosphorylation of IRS-1 inhibits IRS-1 tyrosine phosphorylation

We have previously shown that interferon-α (IFNα)-dependent tyrosine phosphorylation of insulin receptor substrate-1 (IRS-1) is impaired by serine phosphorylation of IRS-1 due to the reduced ability of serine phosphorylated IRS-1 to serve as a substrate for Janus kinase I (JAK1). Here we report that FKBP12-rapamycin-associated protein (FRAP) is a physiologic IRS-1 kinase that blocks IFNα signaling by serine phosphorylating IRS-1. We found that both FRAP and insulin-activated p70 S6 kinase (p70^s6k) serine phosphorylated IRS-1 between residues 511 and 772 (IRS-1^511-772). Importantly, only FRAP-dependent IRS-1^511-772 serine phosphorylation inhibited by 50% subsequent JAK1-dependent tyrosine phosphorylation of IRS-1. Furthermore, treatment of U266 cells with the FRAP inhibitor rapamycin increased IFNα-dependent tyrosine phosphorylation by twofold while reducing constitutive IRS-1 serine phosphorylation within S/T-P motifs by 80%. Taken together, these data indicate that FRAP, but not p70^s6k, is a likely physiologic IRS-1 serine kinase that negatively regulates JAK1-dependent IRS-1 tyrosine phosphorylation and suggests that FRAP may modulate IRS-dependent cytokine signaling.