Role of Translation Initiation Factor 2B in Control of Cell Survival by the Phosphatidylinositol 3-Kinase/Akt/Glycogen Synthase Kinase 3β Signaling Pathway

Abstract

The phosphatidylinositol 3-kinase (PI 3-kinase)/Akt signaling pathway is an important mediator of growth factor-dependent survival of mammalian cells. A variety of targets of the Akt protein kinase have been implicated in cell survival, including the protein kinase glycogen synthase kinase 3␤ (GSK-3␤). One of the targets of GSK-3␤ is translation initiation factor 2B (eIF2B), linking global regulation of protein synthesis to PI 3-kinase/Akt signaling. Because of the central role of protein synthesis, we have investigated the involvement of eIF2B, which is inhibited as a result of GSK-3␤ phosphorylation, in programmed cell death. We demonstrate that expression of eIF2B mutants lacking the GSK-3␤ phosphorylation or priming sites is sufficient to protect both Rat-1 and PC12 cells from apoptosis induced by overexpression of GSK-3␤, inhibition of PI 3-kinase, or growth factor deprivation. Consistent with these effects on cell survival, expression of nonphosphorylatable eIF2B prevented inhibition of protein synthesis following treatment of cells with the PI 3-kinase inhibitor LY294002. Conversely, cycloheximide induced apoptosis of PC12 and Rat-1 cells, further indicating that protein synthesis was required for cell survival. Inhibition of translation resulting from treatment with cycloheximide led to the release of cytochrome c from mitochondria, similar to the effects of inhibition of PI 3-kinase. Expression of nonphosphorylatable eIF2B prevented cytochrome c release resulting from PI 3-kinase inhibition but did not affect cytochrome c release or apoptosis induced by cycloheximide. Regulation of translation resulting from phosphorylation of eIF2B by GSK-3␤ thus appears to contribute to the control of cell survival by the PI 3-kinase/Akt signaling pathway, acting upstream of mitochondrial cytochrome c release.

Central Role of Glycogen Synthase Kinase-3β in Endoplasmic Reticulum Stress-induced Caspase-3 Activation

“…Among these are several survivalpromoting transcription factors, such as cAMP response element-binding protein, of which inhibition by GSK3␤ (49) may contribute to facilitation of apoptosis. Also, activation of GSK3␤ inhibits protein synthesis (77), a logical response to ER stress to reduce the further production of proteins, and this action has been linked to the promotion of apoptosis by GSK3␤ (78). However, the one or more precise proapoptotic targets of GSK3␤ during ER stress remain to be identified.…”

Kallikrein/Kinin Protects against Myocardial Apoptosis after Ischemia/Reperfusion via Akt-Glycogen Synthase Kinase-3 and Akt-Bad·14-3-3 Signaling Pathways

“…Akt is a key effector of PI-3-kinase in the survival pathway against apoptosis. Phosphorylation of GSK-3␤ by Akt results in its inactivation (30,31). Inactivation of GSK-3␤ has been shown to be a negative regulator of cardiomyocyte hypertrophy induced by angiotensin II and endothelin-1 in vitro (29).…”

GSK-3 Represses Growth Factor-inducible Genes by Inhibiting NF-κB in Quiescent Cells

“…Shortly before transfection, 10 5 cells/ml were seeded on 60-mm plates in 4 ml of medium containing 10% fetal bovine serum. Transfection reactions containing 5 nM of siRNA, 20 l of HiPerfect (Qiagen), and 100 l of serum-free media were incubated for 10 min at room temperature and added dropwise onto the cells.…”

“…Growth factor stimulation and activation of PI 3-kinase/Akt signaling also results in inhibition of GSK-3, caused by phosphorylation by Akt (7). In addition to ␤-catenin, the targets of GSK-3 that have been implicated in the regulation of cell proliferation and survival include cyclin D1 (5), the Bcl-2 family member Mcl-1 (8), eukaryotic translation initiation factor 2B (9,10), and several transcription factors (1,2).…”