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 3β (GSK-3β). One of the targets of GSK-3β 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-3β phosphorylation, in programmed cell death. We demonstrate that expression of eIF2B mutants lacking the GSK-3β phosphorylation or priming sites is sufficient to protect both Rat-1 and PC12 cells from apoptosis induced by overexpression of GSK-3β, 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-3β 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.

We are grateful to C. G. Proud for the eIF2B phosphospecific antibody, to T. Herbert for eIF2B cDNA, to C. E. Rudd for the dominant-negative PI 3-kinase plasmid, to K. S. Kosik for the human tau expression plasmid, to E. Drouin and C. Powell for help with the flow cytometer, to A. Visegrády for assistance with confocal microscopy, to M. Jurk for helpful discussions, and to U. Hansen and K. McCall for comments on the manuscript.

This work was supported by NIH grant RO1 CA18689.