Editors' ChoiceCell Biology

PERKing Up the ER Stress Response

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Sci. Signal.  29 May 2012:
Vol. 5, Issue 226, pp. ec151
DOI: 10.1126/scisignal.2003259

Endoplasmic reticulum (ER) stress triggers activation of pancreatic ER kinase (PERK), an ER-localized transmembrane kinase that phosphorylates eukaryotic initiation factor 2α (eIF2α) to reduce protein synthesis. PERK has also been implicated in activation of phosphatidylinositol 3-kinase (PI3K) and the kinase Akt. Bobrovnikova-Marjon et al. report that PERK can also act as a lipid kinase and that this activity—not phosphorylation of eIF2α—is required for the ER stress response. Tunicamycin is an inhibitor of protein glycosylation and triggers ER stress. Tunicamycin-induced ER stress elicited phosphorylation of Akt in wild-type mouse embryonic fibroblasts (MEFs), in MEFs bearing a nonphosphorylatable mutant version of eIF2α, and in MEFs in which the catalytic activity of PI3K was pharmacologically inhibited. In response to the ER stress–inducing compound thapsigargin, which inhibits the ER calcium pump and thus disrupts ER calcium homeostasis, the abundance of phosphatidic acid (PA) increased in wild-type MEFs and in MEFs bearing the nonphosphorylatable version of eIF2α, but not in PERK–/– MEFs. Experiments using pharmacological inhibitors of DAG kinase, phospholipase D, PI3K, mammalian target of rapamycin complex 1 (mTORC1), and Akt indicated that none of these kinases were required for the increase in PA abundance due to ER stress, suggesting that PERK activity was directly responsible for PA generation under ER stress conditions. Indeed, the wild-type catalytic domain of PERK, but not a kinase-dead version, exhibited lipid kinase activity in vitro and, compared with other lipids, PERK preferentially phosphorylated diacylglycerol to produce PA. The lipid kinase activity of PERK also played a role in mTOR and Ras signaling because ER stress–induced activation of these pathways was lost in PERK–/– MEFs, but addition of PA to the culture medium restored activation of both pathways. The p85α regulatory subunit of PI3K was required for ER stress–induced phosphorylation of Akt, and p85α immunoprecipitated with PERK in MEF extracts. PERK was required for maximal Akt phosphorylation during differentiation of MEFs into adipocytes in vitro. Furthermore, PERK immunoprecipitated with p85α during mid- and late stages of differentiation, during which it also exhibited increased lipid kinase activity as compared with early stages of differentiation. Experiments in MEFs also indicated that PERK played a role in insulin signaling, suggesting that the generation of PA by PERK could be physiologically important in multiple signaling contexts.

E. Bobrovnikova-Marjon, D. Pytel, M. J. Riese, L. P. Vaites, N. Singh, G. A. Koretzky, E. S. Witze, J. A. Diehl, PERK utilizes intrinsic lipid kinase activity to generate phosphatidic acid, mediate AICT activation and promote adipocyte differentiation. Mol. Cell. Biol. 32, 2268–2278 (2012). [Abstract] [Full Text]