Double-stranded RNA-dependent protein kinase phosphorylation of the α-subunit of eukaryotic translation initiation factor 2 mediates apoptosis

As the molecular processes of complex cell stress signaling pathways are defined, the subsequent challenge is to elucidate how each individual event influences the final biological outcome. Phosphorylation of the translation initiation factor 2 (eIF2 alpha) at Ser(51) is a molecular signal that inhibits translation in response to activation of any of four diverse eIF2 alpha stress kinases. We used gene targeting to replace the wild-type Ser51 allele with an Ala in the eIF2 alpha gene to test the hypothesis that translational control through eIF2 alpha phosphorylation is a central death stimulus in eukaryotic cells. Homozygous eIF2 alpha mutant mouse embryo fibroblasts were resistant to the apoptotic effects of dsRNA, tumor necrosis factor-alpha, and serum deprivation. TNF alpha treatment induced eIF2 alpha phosphorylation and activation of caspase 3 primarily through the dsRNA-activated eIF2 alpha kinase PKR. In addition, expression of a phospho-mimetic Ser51 to Asp mutant eIF2 alpha-activated caspase 3, indicating that eIF2 alpha phosphorylation is sufficient to induce apoptosis. The proapoptotic effects of PKR-mediated eIF2 alpha phosphorylation contrast with the anti-apoptotic response upon activation of the PKR-related endoplasmic reticulum eIF2 alpha kinase, PERK. Therefore, divergent fates of death and survival can be mediated through phosphorylation at the same site within eIF2 alpha. We propose that eIF2 alpha phosphorylation is fundamentally a death signal, yet it may promote either death or survival, depending upon coincident signaling events.