Roles of mitogen-activated protein kinase signal-integrating kinases 1 and 2 in oxidant-mediated eIF4E phosphorylation.

Oxidative stress alters cellular metabolic processes including protein synthesis. The eukaryotic initiation factor, eIF4E, acts in the rate-limiting steps of initiation and promotes nuclear export. Phosphorylation of eIF4E by mitogen activated protein kinase signal-integrating kinases 1 and 2 (Mnk) influences the affinity of eIF4E for the 5'-mRNA cap and fosters nuclear export activity. Although phosphorylation of eIF4E on Ser209 is observed following oxidant exposure, the contribution of Mnk isoforms and the significance of phosphorylation remain elusive. Using a Mnk inhibitor and fibroblasts derived from Mnk knockout mice, we demonstrate that that H2O2 enhances eIF4E phosphorylation in cells containing Mnk1. In contrast, cells containing only Mnk2 show little change or a decrease in eIF4E phosphorylation in response to H2O2. H2O2 also shifted eIF4GI protein from the nucleus to the cytoplasm suggesting that the increases in eIF4E phosphorylation may reflect enhanced substrate availability to cytoplasmic Mnk1. In Mnk1(+/+) cells, H2O2 also enhanced eIF4E phosphorylation in the nucleus to a greater degree than in the cytoplasm, an effect not observed in cells containing Mnk2. In response to H2O2, all MEFs showed increased eIF4E:4E-BP1 and 4E-BP2:eIF4E binding and reduced eIF4E:eIF4GI binding. We also observed a dramatic increase in the amount of Mnk1 associated with eIF4E following affinity chromatography. These changes coincided with a smaller reduction in global protein synthesis in response to H2O2 in the DKO cells. These findings suggest that changes in eIF4GI distribution may enhance eIF4E phosphorylation and that the presence of either Mnk1 or 2 or any degree of eIF4E phosphorylation negatively regulates global protein synthesis in response to oxidant stress.