Isabel Novoa

Regulated Translation Initiation Controls Stress-Induced Gene Expression in Mammalian Cells

Protein kinases that phosphorylate the alpha subunit of eukaryotic initiation factor 2 (eIF2alpha) are activated in stressed cells and negatively regulate protein synthesis. Phenotypic analysis of targeted mutations in murine cells reveals a novel role for eIF2alpha kinases in regulating gene expression in the unfolded protein response (UPR) and in amino acid starved cells. When activated by their cognate upstream stress signals, the mammalian eIF2 kinases PERK and GCN2 repress translation of most mRNAs but selectively increase translation of Activating Transcription Factor 4 (ATF4), resulting in the induction of the downstream gene CHOP (GADD153). This is the first example of a mammalian signaling pathway homologous to the well studied yeast general control response in which eIF2alpha phosphorylation activates genes involved in amino acid biosynthesis. Mammalian cells thus utilize an ancient pathway to regulate gene expression in response to diverse stress signals.

An Integrated Stress Response Regulates Amino Acid Metabolism and Resistance to Oxidative Stress

Eukaryotic cells respond to unfolded proteins in their endoplasmic reticulum (ER stress), amino acid starvation, or oxidants by phosphorylating the alpha subunit of translation initiation factor 2 (eIF2alpha). This adaptation inhibits general protein synthesis while promoting translation and expression of the transcription factor ATF4. Atf4(-/-) cells are impaired in expressing genes involved in amino acid import, glutathione biosynthesis, and resistance to oxidative stress. Perk(-/-) cells, lacking an upstream ER stress-activated eIF2alpha kinase that activates Atf4, accumulate endogenous peroxides during ER stress, whereas interference with the ER oxidase ERO1 abrogates such accumulation. A signaling pathway initiated by eIF2alpha phosphorylation protects cells against metabolic consequences of ER oxidation by promoting the linked processes of amino acid sufficiency and resistance to oxidative stress.

Stress‐induced gene expression requires programmed recovery from translational repression

Active repression of protein synthesis protects cells against protein malfolding during endoplasmic reticulum stress, nutrient deprivation and oxidative stress. However, long‐term adaptation to these conditions requires synthesis of new stress‐induced proteins. Phosphorylation of the α‐subunit of translation initiation factor 2 (eIF2α) represses translation in diverse stressful conditions. GADD34 is a stress‐inducible regulatory subunit of a holophosphatase complex that dephosphorylates eIF2α, and has been hypothesized to play a role in translational recovery. Here, we report that GADD34 expression correlated temporally with eIF2α dephosphorylation late in the stress response. Inactivation of both alleles of GADD34 prevented eIF2α dephosphorylation and blocked the recovery of protein synthesis, normally observed late in the stress response. Furthermore, defective recovery of protein synthesis markedly impaired translation of stress‐induced proteins and interfered with programmed activation of stress‐induced genes in the GADD34 mutant cells. These observations indicate that GADD34 controls a programmed shift from translational repression to stress‐induced gene expression, and reconciles the apparent contradiction between the translational and transcriptional arms of cellular stress responses.

Increased sensitivity to dextran sodium sulfate colitis in IRE1β-deficient mice

The epithelial cells of the gastrointestinal tract are exposed to toxins and infectious agents that can adversely affect protein folding in the endoplasmic reticulum (ER) and cause ER stress. The IRE1 genes are implicated in sensing and responding to ER stress signals. We found that epithelial cells of the gastrointestinal tract express IRE1beta, a specific isoform of IRE1. BiP protein, a marker of ER stress, was elevated in the colonic mucosa of IRE1beta(-/-) mice, and, when exposed to dextran sodium sulfate (DSS) to induce inflammatory bowel disease, mutant mice developed colitis 3-5 days earlier than did wild-type or IRE1beta(+/-) mice. The inflammation marker ICAM-1 was also expressed earlier in the colonic mucosa of DSS-treated IRE1beta(-/-) mice, indicating that the mutation had its impact early in the inflammatory process, before the onset of mucosal ulceration. These findings are consistent with a model whereby perturbations in ER function, which are normally mitigated by the activity of IRE1beta, participate in the development of colitis.

Cytoprotection by pre‐emptive conditional phosphorylation of translation initiation factor 2

Transient phosphorylation of the α‐subunit of translation initiation factor 2 (eIF2α) represses translation and activates select gene expression under diverse stressful conditions. Defects in the eIF2α phosphorylation‐dependent integrated stress response impair resistance to accumulation of malfolded proteins in the endoplasmic reticulum (ER stress), to oxidative stress and to nutrient deprivations. To study the hypothesized protective role of eIF2α phosphorylation in isolation of parallel stress signaling pathways, we fused the kinase domain of pancreatic endoplasmic reticulum kinase (PERK), an ER stress‐inducible eIF2α kinase that is normally activated by dimerization, to a protein module that binds a small dimerizer molecule. The activity of this artificial eIF2α kinase, Fv2E‐PERK, is subordinate to the dimerizer and is uncoupled from upstream stress signaling. Fv2E‐PERK activation enhanced the expression of numerous stress‐induced genes and protected cells from the lethal effects of oxidants, peroxynitrite donors and ER stress. Our findings indicate that eIF2α phosphorylation can initiate signaling in a cytoprotective gene expression pathway independently of other parallel stress‐induced signals and that activation of this pathway can single‐handedly promote a stress‐resistant preconditioned state.

Control of PERK eIF2α kinase activity by the endoplasmic reticulum stress-induced molecular chaperone P58IPK

P58IPK is an Hsp40 family member known to inhibit the interferon (IFN)-induced, double-stranded RNA-activated, eukaryotic initiation factor 2α (eIF2α) protein kinase R (PKR) by binding to its kinase domain. We find that the stress of unfolded proteins in the endoplasmic reticulum (ER) activates P58IPK gene transcription through an ER stress-response element in its promoter region. P58IPK interacts with and inhibits the PKR-like ER-localized eIF2α kinase PERK, which is normally activated during the ER-stress response to protect cells from ER stress by attenuating protein synthesis and reducing ER client protein load. Levels of phosphorylated eIF2α were lower in ER-stressed P58IPK-overexpressing cells and were enhanced in P58IPK mutant cells. In the ER-stress response, PKR-like ER kinase (PERK)-mediated translational repression is transient and is followed by translational recovery and enhanced expression of genes that increase the capacity of the ER to process client proteins. The absence of P58IPK resulted in increased expression levels of two ER stress-inducible genes, BiP and Chop, consistent with the enhanced eIF2α phosphorylation in the P58IPK deletion cells. Our studies suggest that P58IPK induction during the ER-stress response represses PERK activity and plays a functional role in the expression of downstream markers of PERK activity in the later phase of the ER-stress response.