Insulin treatment promotes tyrosine phosphorylation of PKR and inhibits polyIC induced PKR threonine phosphorylation

Tyrosine phosphorylation of insulin receptor beta (IR beta) in insulin treated HepG2 cells is inversely correlated to ser(51) phosphorylation in the alpha-subunit of eukaryotic initiation factor 2 (eIF2 alpha) that regulates protein synthesis. Insulin stimulates interaction between IR beta and PKR, double stranded RNA-dependent protein kinase, also known as EIF2AK2, and phosphorylation of tyrosine residues in PKR, as analyzed by immunoprecipitation and pull down assays using anti-IR beta and anti-phosphotyrosine antibodies, recombinant IR beta and immunopurified PKR. Further polyIC or synthetic double stranded RNA-induced threonine phosphorylation or activation of immunopurified and cellular PKR is suppressed in the presence of insulin treated purified IR beta and cell extracts. Acute, but not chronic, insulin treatment enhances tyrosine phosphorylation of IR beta, its interaction with PKR and tyrosine phosphorylation of PKR. In contrast, lipopolysaccharide that stimulates threonine phosphorylation of PKR and eIF2 alpha phosphorylation and AG 1024, an inhibitor of the tyrosine kinase activity of IR beta, reduces PKR association with the receptor, IR beta in HepG2 cells. These findings therefore may suggest that tyrosine phosphorylated PKR plays a role in the regulation of insulin induced protein synthesis and in maintaining insulin sensitivity, whereas, suppression of polyIC-mediated threonine phosphorylation of PKR by insulin compromises its ability to fight against virus infection in host cells. (C) 2015 Elsevier Inc. All rights reserved.