Cell signaling in response to an array of diverse stress stimuli converges on the phosphorylation of the α-subunit of eukaryotic initiation factor 2 (eIF2). Phosphorylation of eIF2α on serine 51 results in a severe decline in de novo protein synthesis and is an important strategy in the cell’s armory against stressful insults including viral infection, the accumulation of misfolded proteins, and starvation. The phosphorylation of eIF2α is carried out by a family of four kinases, PERK (PKR-like ER kinase), PKR (protein kinase double-stranded RNA-dependent), GCN2 (general control non-derepressible-2), and HRI (heme-regulated inhibitor). Each primarily responds to a distinct type of stress or stresses. Thus, while significant sequence similarity exists between the eIF2α kinases in their kinase domains, underlying their common role in phosphorylating eIF2α, additional unique features determine the regulation of these four proteins, that is, what signals activate them. This review will describe the structure of each eIF2α kinase and discuss how this is linked to their activation and function. In parallel to the general translational attenuation elicited by eIF2α kinase activation the translation of stress-induced mRNAs, most notably activating transcription factor 4 (ATF4) is enhanced and these set in motion cascades of gene expression constituting the integrated stress response (ISR), which seek to remediate stress and restore homeostasis. Depending on the cellular context and concurrent signaling pathways active, however, translational attenuation can also facilitate apoptosis. Accordingly, the role of the kinases in determining cell fate will also be discussed.

The eIF2α kinases: their structures and functions

The endoplasmic reticulum (ER) is a membranous intracellular organelle and the first compartment of the secretory pathway As such, the ER contributes to the production and folding of approximately one-third of cellular proteins, and is thus inextricably linked to the maintenance of cellular homeostasis and the fine balance between health and disease Specific ER stress signalling pathways, collectively known as the unfolded protein response (UPR), are required for maintaining ER homeostasis The UPR is triggered when ER protein folding capacity is overwhelmed by cellular demand and the UPR initially aims to restore ER homeostasis and normal cellular functions However, if this fails, then the UPR triggers cell death In this review, we provide a UPR signalling-centric view of ER functions, from the ER's discovery to the latest advancements in the understanding of ER and UPR biology Our review provides a synthesis of intracellular ER signalling revolving around proteostasis and the UPR, its impact on other organelles and cellular behaviour, its multifaceted and dynamic response to stress and its role in physiology, before finally exploring the potential exploitation of this knowledge to tackle unresolved biological questions and address unmet biomedical needs Thus, we provide an integrated and global view of existing literature on ER signalling pathways and their use for therapeutic purposes

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Endoplasmic Reticulum Stress signalling - from basic mechanisms to clinical applications

Upstream open reading frames (uORFs) are major gene expression regulatory elements. In many eukaryotic mRNAs, one or more uORFs precede the initiation codon of the main coding region. Indeed, several studies have revealed that almost half of human transcripts present uORFs. Very interesting examples have shown that these uORFs can impact gene expression of the downstream main ORF by triggering mRNA decay or by regulating translation. Also, evidence from recent genetic and bioinformatic studies implicates disturbed uORF-mediated translational control in the etiology of many human diseases, including malignancies, metabolic or neurologic disorders, and inherited syndromes. In this review, we will briefly present the mechanisms through which uORFs regulate gene expression and how they can impact on the organism's response to different cell stress conditions. Then, we will emphasize the importance of these structures by illustrating, with specific examples, how disturbed uORF-mediated translational control can be involved in the etiology of human diseases, giving special importance to genotype-phenotype correlations. Identifying and studying more cases of uORF-altering mutations will help us to understand and establish genotype-phenotype associations, leading to advancements in diagnosis, prognosis, and treatment of many human disorders.

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Gene expression regulation by upstream open reading frames and human disease.

Eukaryotic Initiation Factor 2 Phosphorylation and Translational Control in Metabolism

http://www.jbc.org/content/286/13/10939.full.pdf

Phosphorylation of eIF2 Facilitates Ribosomal Bypass of an Inhibitory Upstream ORF to Enhance CHOP Translation

Cells respond to environmental stress by inducing translation of a subset of mRNAs important for survival or apoptosis. CHOP, a downstream transcriptional target of stress-induced ATF4, is also regulated translationally in a uORF-dependent manner under stress. Low concentration of anisomycin induces CHOP expression at both transcriptional and translational levels. To study specifically the translational aspect of CHOP expression, and further clarify the regulatory mechanisms underlying stress-induced translation initiation, we developed a CMV promoter-regulated, uORFchop-driven reporter platform. Here we show that anisomycin-induced CHOP expression depends on phosphorylated eIF4E/S209 and eIF2α/S51. Contrary to phospho-eIF2α/S51, phospho-eIF4E/S209 is not involved in thapsigargin-induced CHOP expression. Studies using various kinase inhibitors and mutants uncovered that both the p38MAPK-Mnk and mTOR signaling pathways contribute to stress-responsive reporter and CHOP expression. We also demonstrated that anisomycin-induced translation is tightly regulated by partner binding preference of eIF4E. Furthermore, mutating the uORF sequence abolished the anisomycin-induced association of chop mRNA with phospho-eIF4E and polysomes, thus demonstrating the significance of this cis-regulatory element in conferring on the transcript a stress-responsive translational inducibility. Strikingly, although insulin treatment activated ERK-Mnk and mTOR pathways, and consequently eIF4E/S209 phosphorylation, it failed to induce phospho-eIF2α/S51 and reporter translation, thus pinpointing a crucial determinant in stress-responsive translation.

Jin-Shin Chen

Papers

Differential regulation of CHOP translation by phosphorylated eIF4E under stress conditions