PERK and GCN2 contribute to eIF2α phosphorylation and cell cycle arrest after activation of the unfolded protein response pathway

Exposure of cells to endoplasmic reticulum (ER) stress leads to activation of PKR-like ER kinase (PERK), eukaryotic translation initiation factor 2 alpha (eIF2 alpha) phosphorylation, repression of cyclin D1 translation, and subsequent cell cycle arrest in G, phase. However, whether PERK is solely responsible for regulating cyclin D1 accumulation after unfolded 14 protein response pathway (UPR) activation has not been assessed. Herein, we demonstrate that repression of cyclin D1 translation after UPR activation occurs independently of PERK, but it remains dependent on eIF2a phosphorylation. Although phosphorylation of eIF2a in PERK-/- fibroblasts is attenuated in comparison with wild-type fibroblasts, it is not eliminated. The residual eIF2a phosphorylation correlates with the kinetics of cyclin D1 loss, suggesting that another eIF2a kinase functions in the absence of PERK. In cells harboring targeted deletion of both PERK and GCN2, cyclin D1 loss is attenuated, suggesting GCN2 functions as the redundant kinase. Consistent with these results, cyclin D1 translation is also stabilized in cells expressing a nonphosphorylatable allele of eIF2 alpha; in contrast, repression of global protein translation still occurs in these cells, highlighting a high degree of specificity in transcripts targeted for translation inhibition by phosphorylated aeIF2 alpha. Our results demonstrate that PERK and GCN2 function to cooperatively regulate eIF2a phosphorylation and cyclin D1 translation after UPR activation.