A Novel Feedback Loop Regulates the Response to Endoplasmic Reticulum Stress via the Cooperation of Cytoplasmic Splicing and mRNA Translation

Abstract

The accumulation of unfolded proteins in the endoplasmic reticulum (ER) triggers transcriptional and translational reprogramming. This unfolded protein response (UPR) protects cells during transient stress and can lead to apoptosis during prolonged stress. Two key mediators of the UPR are PKR-like ER kinase (PERK), which phosphorylates the α subunit of eukaryotic translation initiation factor 2 (eIF2α), resulting in decreased protein synthesis, and the α subunit of inositol-requiring enzyme 1 (IRE1α), which initiates cytoplasmic splicing of the mRNA encoding the transcription factor X-box binding protein 1 (XBP1). XBP1 induces transcription of genes involved in protein quality control. This report describes cross talk between these two pathways: phosphorylation of eIF2α was required for maximal induction of spliced XBP1 (XBP1s) protein levels via a mechanism that involved stabilization of XBP1s mRNA. By using mouse embryo fibroblasts deficient in UPR signaling pathways, we demonstrate that stress-induced stabilization of XBP1s mRNA requires cytoplasmic splicing of the mRNA and inhibition of its translation. Because the XBP1s protein promotes transcription of its own gene, the UPR-induced mRNA stabilization is part of a positive feedback loop that induces XBP1s protein accumulation and transcription of target genes during stress. We propose a model in which eIF2α phosphorylation-mediated control of mRNA turnover is a molecular switch that regulates the stress response transcription program and the ER's capacity for protein folding during stress.

SUPPLEMENTAL MATERIAL

Supplemental material for this article may be found at http://dx.doi.org/10.1128/MCB.06665-11.

ACKNOWLEDGMENTS

This work was supported by grants DK060596 and DK053307 (to M.H.) and DK42304, HL52173, and HL057346 (to R.J.K.) from the National Institutes of Health.

We thank Michael Harris, Case Western Reserve University, for assistance with the studies evaluating the half-lives of the mRNAs and David Ron, University of Cambridge, for GADD34^−/− MEFs.

ADDENDUM

While the manuscript was under review, a report by the Gardner group (58) included XBP1 in the results of a microarray analysis of a genome-wide screen for NMD target mRNAs. The data showed that emetine and tunicamycin treatment of cells caused increased stability of XBP1 mRNA.