A Novel Function of eIF2α Kinases as Inducers of the Phosphoinositide-3 Kinase Signaling Pathway

Phosphoinositide-3 kinase (PI3K) plays an important role in signal transduction in response to a wide range of cellular stimuli involved in cellular processes that promote cell proliferation and survival. Phosphorylation of the α subunit of the eukaryotic translation initiation factor eIF2 at Ser51 takes place in response to various types of environmental stress and is essential for regulation of translation initiation. Herein, we show that a conditionally active form of the eIF2α kinase PKR acts upstream of PI3K and turns on the Akt/PKB-FRAP/mTOR pathway leading to S6 and 4E-BP1 phosphorylation. Also, induction of PI3K signaling antagonizes the apoptotic and protein synthesis inhibitory effects of the conditionally active PKR. Furthermore, induction of the PI3K pathway is impaired in PKR−/− or PERK−/− mouse embryonic fibroblasts (MEFs) in response to various stimuli that activate each eIF2α kinase. Mechanistically, PI3K signaling activation is indirect and requires the inhibition of protein synthesis by eIF2α phosphorylation as demonstrated by the inactivation of endogenous eIF2α by small interfering RNA or utilization of MEFs bearing the eIF2α Ser51Ala mutation. Our data reveal a novel property of eIF2α kinases as activators of PI3K signaling and cell survival.

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A critical review of translation initiation factor eIF2α kinases in plants - regulating protein synthesis during stress

Functional Plant Biology ◽

10.1071/fp12116 ◽

2012 ◽

Vol 39 (9) ◽

pp. 717 ◽

Cited By ~ 14

Author(s):

Tracey M. Immanuel ◽

David R. Greenwood ◽

Robin M. MacDiarmid

Keyword(s):

Protein Synthesis ◽

Translation Initiation ◽

Current Knowledge ◽

Mrna Translation ◽

Translation Initiation Factor ◽

Initiation Factor ◽

Α Subunit ◽

Eif2α Phosphorylation ◽

Phosphorylation Mechanism ◽

Eif2α Kinase

Eukaryotic cells must cope with environmental stress. One type of general stress response is the downregulation of protein synthesis in order to conserve cellular resources. Protein synthesis is mainly regulated at the level of mRNA translation initiation and when the α subunit of eukaryotic translation initiation factor 2 (eIF2) is phosphorylated, protein synthesis is downregulated. Although eIF2 has the same translation initiation function in all eukaryotes, it is not known whether plants downregulate protein synthesis via eIF2α phosphorylation. Similarly, although there is evidence that plants possess eIF2α kinases, it is not known whether they operate in a similar manner to the well characterised mammalian and yeast eIF2α kinases. Two types of eIF2α kinases have been reported in plants, yet the full understanding of the plant eIF2α phosphorylation mechanism is still lacking. Here we review the current knowledge of the eIF2α phosphorylation mechanism within plants and discuss plant eIF2α, plant eIF2α kinase GCN2 and the data supporting and contradicting the hypothesis that a functional orthologue for the eIF2α kinase PKR, is present and functional in plants.

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Caspase-mediated cleavage of eukaryotic translation initiation factor subunit 2α

Biochemical Journal ◽

10.1042/bj3420065 ◽

1999 ◽

Vol 342 (1) ◽

pp. 65-70 ◽

Cited By ~ 31

Author(s):

Shinya SATOH ◽

Makoto HIJIKATA ◽

Hiroshi HANDA ◽

Kunitada SHIMOTOHNO

Keyword(s):

Protein Synthesis ◽

Translation Initiation ◽

Caspase 3 ◽

Translation Initiation Factor ◽

Initiation Factor ◽

Eukaryotic Translation Initiation Factor ◽

Α Subunit ◽

Eukaryotic Translation ◽

Initiation Of Translation ◽

Eukaryotic Translation Initiation

Eukaryotic translation initiation factor 2α (eIF-2α), a target molecule of the interferon-inducible double-stranded-RNA-dependent protein kinase (PKR), was cleaved in apoptotic Saos-2 cells on treatment with poly(I)˙poly(C) or tumour necrosis factor α. This cleavage occurred with a time course similar to that of poly(ADP-ribose) polymerase, a well-known caspase substrate. In addition, eIF-2α was cleaved by recombinant active caspase-3 in vitro. By site-directed mutagenesis, the cleavage site was mapped to an Ala-Glu-Val-Asp300 ↓ Gly301 sequence located in the C-terminal portion of eIF-2α. PKR phosphorylates eIF-2α on Ser51, resulting in the suppression of protein synthesis. PKR-mediated translational suppression was repressed when the C-terminally cleaved product of eIF-2α was overexpressed in Saos-2 cells, even though PKR can phosphorylate this cleaved product. These results suggest that caspase-3 or related protease(s) can modulate the efficiency of protein synthesis by cleaving the α subunit of eIF-2, a key component in the initiation of translation.

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Resistance to Vesicular Stomatitis Virus Infection Requires a Functional Cross Talk between the Eukaryotic Translation Initiation Factor 2α Kinases PERK and PKR

Journal of Virology ◽

10.1128/jvi.78.23.12747-12761.2004 ◽

2004 ◽

Vol 78 (23) ◽

pp. 12747-12761 ◽

Cited By ~ 61

Author(s):

Dionissios Baltzis ◽

Li-Ke Qu ◽

Stavroula Papadopoulou ◽

Jaime D. Blais ◽

John C. Bell ◽

...

Keyword(s):

Protein Synthesis ◽

Virus Infection ◽

Vesicular Stomatitis Virus ◽

Translation Initiation ◽

Translation Initiation Factor ◽

Initiation Factor ◽

Eukaryotic Translation ◽

Eukaryotic Translation Initiation ◽

Vesicular Stomatitis ◽

Eif2α Kinase

ABSTRACT Phosphorylation of the alpha (α) subunit of the eukaryotic translation initiation factor 2 (eIF2) leads to the inhibition of protein synthesis in response to diverse stress conditions, including viral infection. The eIF2α kinase PKR has been shown to play an essential role against vesicular stomatitis virus (VSV) infection. We demonstrate here that another eIF2α kinase, the endoplasmic reticulum-resident protein kinase PERK, contributes to cellular resistance to VSV infection. We demonstrate that mouse embryonic fibroblasts (MEFs) from PERK−/− mice are more susceptible to VSV-mediated apoptosis than PERK+/+ MEFs. The higher replication capacity of VSV in PERK−/− MEFs results from their inability to attenuate viral protein synthesis due to an impaired eIF2α phosphorylation. We also show that VSV-infected PERK−/− MEFs are unable to fully activate PKR, suggesting a cross talk between the two eIF2α kinases in virus-infected cells. These findings further implicate PERK in virus infection, and provide evidence that the antiviral and antiapoptotic roles of PERK are mediated, at least in part, via the activation of PKR.

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Acute attenuation of translation initiation and protein synthesis by glucocorticoids in skeletal muscle

AJP Endocrinology and Metabolism ◽

10.1152/ajpendo.2000.278.1.e76 ◽

2000 ◽

Vol 278 (1) ◽

pp. E76-E82 ◽

Cited By ~ 74

Author(s):

O. Jameel Shah ◽

Scot R. Kimball ◽

Leonard S. Jefferson

Keyword(s):

Skeletal Muscle ◽

Protein Synthesis ◽

Translation Initiation ◽

Intraperitoneal Administration ◽

Translation Initiation Factor ◽

Initiation Factor ◽

Nucleotide Exchange ◽

Α Subunit ◽

Initiation Factors

Glucocorticoids are diabetogenic factors that not only antagonize the action of insulin in target tissues but also render these tissues catabolic. Therefore, in rats, we endeavored to characterize the effects in skeletal muscle of glucocorticoids on translation initiation, a regulated process that, in part, governs overall protein synthesis through the modulated activities of eukaryotic initiation factors (eIFs). Four hours after intraperitoneal administration of dexamethasone (100 μg/100 g body wt), protein synthesis in skeletal muscle was reduced to 59% of the value recorded in untreated control animals. Furthermore, translation initiation factor eIF4E preferred association with its endogenous inhibitor 4E-BP1 rather than eIF4G. Dexamethasone treatment resulted in dephosphorylation of both 4E-BP1 and the 40S ribosomal protein S6 kinase concomitant with enhanced phosphorylation of eIF4E. Moreover, the guanine nucleotide exchange activity of eIF2B was unaffected as was phosphorylation of the α-subunit of eIF2. Hence glucocorticoids negatively modulate the activation of a subset of the protein synthetic machinery, thereby contributing to the catabolic properties of this class of hormones in vivo.

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Defects in Translational Regulation Mediated by the α Subunit of Eukaryotic Initiation Factor 2 Inhibit Antiviral Activity and Facilitate the Malignant Transformation of Human Fibroblasts

Molecular and Cellular Biology ◽

10.1128/mcb.24.5.2025-2040.2004 ◽

2004 ◽

Vol 24 (5) ◽

pp. 2025-2040 ◽

Cited By ~ 42

Author(s):

Darren J. Perkins ◽

Glen N. Barber

Keyword(s):

Protein Synthesis ◽

Translation Initiation ◽

T Antigen ◽

Translation Initiation Factor ◽

Initiation Factor ◽

Eukaryotic Initiation Factor ◽

Α Subunit ◽

Eukaryotic Initiation Factor 2 ◽

Initiation Factor 2

ABSTRACT Suppression of protein synthesis through phosphorylation of the translation initiation factor α subunit of eukaryotic initiation factor 2 (eIF2α) is known to occur in response to many forms of cellular stress. To further study this, we have developed novel cell lines that inducibly express FLAG-tagged versions of either the phosphomimetic eIF2α variant, eIF2α-S51D, or the phosphorylation-insensitive eIF2α-S51A. These variants showed authentic subcellular localization, were incorporated into endogenous ternary complexes, and were able to modulate overall rates of protein synthesis as well as influence cell division. However, phosphorylation of eIF2α failed to induce cell death or sensitize cells to killing by proapoptotic stimuli, though it was able to inhibit viral replication, confirming the role of eIF2α in host defense. Further, although the eIF2α-S51A variant has been shown to transform NIH 3T3 cells, it was unable to transform the murine fibroblast 3T3 L1 cell line. To therefore clarify this issue, we explored the role of eIF2α in growth control and demonstrated that the eIF2α-S51A variant is capable of collaborating with hTERT and the simian virus 40 large T antigen in the transformation of primary human kidney cells. Thus, dysregulation of translation initiation is indeed sufficient to cooperate with defined oncogenic elements and participate in the tumorigenesis of human tissue.

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Correction of eIF2-dependent defects in brain protein synthesis, synaptic plasticity, and memory in mouse models of Alzheimer’s disease

Science Signaling ◽

10.1126/scisignal.abc5429 ◽

2021 ◽

Vol 14 (668) ◽

pp. eabc5429

Author(s):

Mauricio M. Oliveira ◽

Mychael V. Lourenco ◽

Francesco Longo ◽

Nicole P. Kasica ◽

Wenzhong Yang ◽

...

Keyword(s):

Alzheimer’S Disease ◽

Alzheimer's Disease ◽

Protein Synthesis ◽

Synaptic Plasticity ◽

Translation Initiation ◽

Mrna Translation ◽

Translation Initiation Factor ◽

Initiation Factor ◽

Postmortem Brain Tissue ◽

Phosphorylated Eif2α

Neuronal protein synthesis is essential for long-term memory consolidation, and its dysregulation is implicated in various neurodegenerative disorders, including Alzheimer’s disease (AD). Cellular stress triggers the activation of protein kinases that converge on the phosphorylation of eukaryotic translation initiation factor 2α (eIF2α), which attenuates mRNA translation. This translational inhibition is one aspect of the integrated stress response (ISR). We found that postmortem brain tissue from AD patients showed increased phosphorylation of eIF2α and reduced abundance of eIF2B, another key component of the translation initiation complex. Systemic administration of the small-molecule compound ISRIB (which blocks the ISR downstream of phosphorylated eIF2α) rescued protein synthesis in the hippocampus, measures of synaptic plasticity, and performance on memory-associated behavior tests in wild-type mice cotreated with salubrinal (which inhibits translation by inducing eIF2α phosphorylation) and in both β-amyloid-treated and transgenic AD model mice. Thus, attenuating the ISR downstream of phosphorylated eIF2α may restore hippocampal protein synthesis and delay cognitive decline in AD patients.

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Circadian clock control of eIF2α phosphorylation is necessary for rhythmic translation initiation

Proceedings of the National Academy of Sciences ◽

10.1073/pnas.1918459117 ◽

2020 ◽

Vol 117 (20) ◽

pp. 10935-10945 ◽

Cited By ~ 1

Author(s):

Shanta Karki ◽

Kathrina Castillo ◽

Zhaolan Ding ◽

Olivia Kerr ◽

Teresa M. Lamb ◽

...

Keyword(s):

Circadian Clock ◽

Translation Initiation ◽

Translation Initiation Factor ◽

Initiation Factor ◽

Nutrient Metabolism ◽

Eif2α Phosphorylation ◽

Eif2α Kinase ◽

The Impact

The circadian clock in eukaryotes controls transcriptional and posttranscriptional events, including regulation of the levels and phosphorylation state of translation factors. However, the mechanisms underlying clock control of translation initiation, and the impact of this potential regulation on rhythmic protein synthesis, were not known. We show that inhibitory phosphorylation of eIF2α (P-eIF2α), a conserved translation initiation factor, is clock controlled in Neurospora crassa, peaking during the subjective day. Cycling P-eIF2α levels required rhythmic activation of the eIF2α kinase CPC-3 (the homolog of yeast and mammalian GCN2), and rhythmic activation of CPC-3 was abolished under conditions in which the levels of charged tRNAs were altered. Clock-controlled accumulation of P-eIF2α led to reduced translation during the day in vitro and was necessary for the rhythmic synthesis of select proteins in vivo. Finally, loss of rhythmic P-eIF2α levels led to reduced linear growth rates, supporting the idea that partitioning translation to specific times of day provides a growth advantage to the organism. Together, these results reveal a fundamental mechanism by which the clock regulates rhythmic protein production, and provide key insights into how rhythmic translation, cellular energy, stress, and nutrient metabolism are linked through the levels of charged versus uncharged tRNAs.

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