Akt determines cell fate through inhibition of the PERK-eIF2α phosphorylation pathway.

Metazoans respond to various forms of environmental stress by inducing the phosphorylation of the α subunit of eukaryotic translation initiation factor 2 (eIF2α) at serine-51, a modification that leads to global inhibition of mRNA translation. We demonstrate induction of the phosphorylation of eIF2α in mammalian cells after either pharmacological inhibition of the phosphoinositide 3-kinase (PI3K)-Akt pathway or genetic or small interfering RNA-mediated ablation of Akt. This increase in the extent of eIF2α phosphorylation also occurred in Drosophila cells and depended on the endoplasmic reticulum (ER)-resident protein kinase PERK, which was inhibited by Akt-dependent phosphorylation at threonine-799. The activity of PERK and the abundance of phosphorylated eIF2α (eIF2αP) were reduced in mouse mammary gland tumors that contained activated Akt, as well as in cells exposed to ER stress or oxidative stress. In unstressed cells, the PERK-eIF2αP pathway mediated survival and facilitated adaptation to the deleterious effects of the inactivation of PI3K or Akt. Inactivation of the PERK-eIF2αP pathway increased the susceptibility of tumor cells to death by pharmacological inhibitors of PI3K or Akt. Thus, we suggest that the PERK-eIF2αP pathway provides a link between Akt signaling and translational control, which has implications for tumor formation and treatment.

Tumor suppression by PTEN requires the activation of the PKR-eIF2alpha phosphorylation pathway.

Inhibition of protein synthesis by phosphorylation of the alpha subunit of eukaryotic translation initiation factor 2 (eIF2) at Ser(51) occurs as a result of the activation of a family of kinases in response to various forms of stress. Although some consequences of eIF2alpha phosphorylation are cytoprotective, phosphorylation of eIF2alpha by RNA-dependent protein kinase (PKR) is largely proapoptotic and tumor suppressing. Phosphatase and tensin homolog deleted from chromosome 10 (PTEN) is a tumor suppressor protein that is mutated or deleted in various human cancers, with functions that are mediated through phosphatase-dependent and -independent pathways. Here, we demonstrate that the eIF2alpha phosphorylation pathway is downstream of PTEN. Inactivation of PTEN in human melanoma cells reduced eIF2alpha phosphorylation, whereas reconstitution of PTEN-null human glioblastoma or prostate cancer cells with either wild-type PTEN or phosphatase-defective mutants of PTEN induced PKR activity and eIF2alpha phosphorylation. The antiproliferative and proapoptotic effects of PTEN were compromised in mouse embryonic fibroblasts that lacked PKR or contained a phosphorylation-defective variant of eIF2alpha. Induction of the pathway leading to phosphorylation of eIF2alpha required an intact PDZ-binding motif in PTEN. These findings establish a link between tumor suppression by PTEN and inhibition of protein synthesis that is independent of PTEN's effects on phosphoinositide 3'-kinase signaling.

A novel function of eIF2alpha kinases as inducers of the phosphoinositide-3 kinase signaling pathway.

Phosphoinositide-3 kinase (PI3K) plays an important role in signal transduction in response to a wide range of cellular stimuli involved in cellular processes that promote cell proliferation and survival. Phosphorylation of the alpha subunit of the eukaryotic translation initiation factor eIF2 at Ser51 takes place in response to various types of environmental stress and is essential for regulation of translation initiation. Herein, we show that a conditionally active form of the eIF2alpha kinase PKR acts upstream of PI3K and turns on the Akt/PKB-FRAP/mTOR pathway leading to S6 and 4E-BP1 phosphorylation. Also, induction of PI3K signaling antagonizes the apoptotic and protein synthesis inhibitory effects of the conditionally active PKR. Furthermore, induction of the PI3K pathway is impaired in PKR(-/-) or PERK(-/-) mouse embryonic fibroblasts (MEFs) in response to various stimuli that activate each eIF2alpha kinase. Mechanistically, PI3K signaling activation is indirect and requires the inhibition of protein synthesis by eIF2alpha phosphorylation as demonstrated by the inactivation of endogenous eIF2alpha by small interfering RNA or utilization of MEFs bearing the eIF2alpha Ser51Ala mutation. Our data reveal a novel property of eIF2alpha kinases as activators of PI3K signaling and cell survival.