Translational Repression Mediates Activation of Nuclear Factor Kappa B by Phosphorylated Translation Initiation Factor 2

TLDR

Stress‑induced phosphorylation of eIF2α attenuates translation and initiates the integrated stress response, and this phosphorylation is also required for NF‑κB activation during ER stress. Activation of PERK to phosphorylate eIF2α, independent of stress, is sufficient to trigger NF‑κB DNA binding and reporter activity, primarily by repressing IκBα translation. Phosphorylated eIF2α reduces IκBα protein levels without altering its phosphorylation or stability, demonstrating that translational repression of IκBα directly drives NF‑κB activation during ER stress.

Abstract

Numerous stressful conditions activate kinases that phosphorylate the alpha subunit of translation initiation factor 2 (eIF2α), thus attenuating mRNA translation and activating a gene expression program known as the integrated stress response. It has been noted that conditions associated with eIF2α phosphorylation, notably accumulation of unfolded proteins in the endoplasmic reticulum (ER), or ER stress, are also associated with activation of nuclear factor kappa B (NF-κB) and that eIF2α phosphorylation is required for NF-κB activation by ER stress. We have used a pharmacologically activable version of pancreatic ER kinase (PERK, an ER stress-responsive eIF2α kinase) to uncouple eIF2α phosphorylation from stress and found that phosphorylation of eIF2α is both necessary and sufficient to activate both NF-κB DNA binding and an NF-κB reporter gene. eIF2α phosphorylation-dependent NF-κB activation correlated with decreased levels of the inhibitor IκBα protein. Unlike canonical signaling pathways that promote IκBα phosphorylation and degradation, eIF2α phosphorylation did not increase phosphorylated IκBα levels or affect the stability of the protein. Pulse-chase labeling experiments indicate instead that repression of IκBα translation plays an important role in NF-κB activation in cells experiencing high levels of eIF2α phosphorylation. These studies suggest a direct role for eIF2α phosphorylation-dependent translational control in activating NF-κB during ER stress.