Regulation of Protein Synthesis by Hypoxia via Activation of the Endoplasmic Reticulum Kinase PERK and Phosphorylation of the Translation Initiation Factor eIF2α

Abstract

Hypoxia profoundly influences tumor development and response to therapy. While progress has been made in identifying individual gene products whose synthesis is altered under hypoxia, little is known about the mechanism by which hypoxia induces a global downregulation of protein synthesis. A critical step in the regulation of protein synthesis in response to stress is the phosphorylation of translation initiation factor eIF2α on Ser51, which leads to inhibition of new protein synthesis. Here we report that exposure of human diploid fibroblasts and transformed cells to hypoxia led to phosphorylation of eIF2α, a modification that was readily reversed upon reoxygenation. Expression of a transdominant, nonphosphorylatable mutant allele of eIF2α attenuated the repression of protein synthesis under hypoxia. The endoplasmic reticulum (ER)-resident eIF2α kinase PERK was hyperphosphorylated upon hypoxic stress, and overexpression of wild-type PERK increased the levels of hypoxia-induced phosphorylation of eIF2α. Cells stably expressing a dominant-negative PERK allele and mouse embryonic fibroblasts with a homozygous deletion of PERK exhibited attenuated phosphorylation of eIF2α and reduced inhibition of protein synthesis in response to hypoxia. PERK^−/− mouse embryo fibroblasts failed to phosphorylate eIF2α and exhibited lower survival after prolonged exposure to hypoxia than did wild-type fibroblasts. These results indicate that adaptation of cells to hypoxic stress requires activation of PERK and phosphorylation of eIF2α and suggest that the mechanism of hypoxia-induced translational attenuation may be linked to ER stress and the unfolded-protein response.

We thank Heather P. Harding and David Ron, Skirball Institute, NYU School of Medicine, for the generous gift of PERK^+/+ and PERK^−/− MEFs and the expression plasmids for wild-type and K618A mutant PERK. We also thank Randy Johnson, UCSD, for the gift of the HIF-1α^+/+ and HIF-1α^−/− MEFs. We are grateful to Amato Giaccia of Stanford University for helpful and stimulating discussions relating to this project.

This work was supported by U.S. Public Health Service grant CA94214 from the NCI, National Institutes of Health, to C.K. and by funds from the Department of Radiation Oncology, Wake Forest University School of Medicine.