Phosphorylation of eukaryotic initiation factor 4E (eIF4E) at Ser209 is not required for protein synthesis in vitro and in vivo

eIF4E orchestrates mRNA processing, RNA export and translation to modify specific protein production

The eukaryotic translation initiation factor eIF4E acts as a multifunctional factor that simultaneously influences mRNA processing, export, and translation in many organisms. Its multifactorial effects are derived from its capacity to bind to the methyl-7-guanosine cap on the 5'end of mRNAs and thus can act as a cap chaperone for transcripts in the nucleus and cytoplasm. In this review, we describe the multifactorial roles of eIF4E in major mRNA-processing events including capping, splicing, cleavage and polyadenylation, nuclear export and translation. We discuss the evidence that eIF4E acts at two levels to generate widescale changes to processing, export and ultimately the protein produced. First, eIF4E alters the production of components of the mRNA processing machinery, supporting a widescale reprogramming of multiple mRNA processing events. In this way, eIF4E can modulate mRNA processing without physically interacting with target transcripts. Second, eIF4E also physically interacts with both capped mRNAs and components of the RNA processing or translation machineries. Further, specific mRNAs are sensitive to eIF4E only in particular mRNA processing events. This selectivity is governed by the presence of cis-acting elements within mRNAs known as USER codes that recruit relevant co-factors engaging the appropriate machinery. In all, we describe the molecular bases for eIF4E's multifactorial function and relevant regulatory pathways, discuss the basis for selectivity, present a compendium of ~80 eIF4E-interacting factors which play roles in these activities and provide an overview of the relevance of its functions to its oncogenic potential. Finally, we summarize early-stage clinical studies targeting eIF4E in cancer.

Newly Synthesized Anticancer Purine Derivatives Inhibiting p-EIF4E Using Surface-Modified Lipid Nanovesicles

Translation of mRNA is one of the processes adopted by cancer cells to maintain survival via phosphorylated (p)-eIF4E overexpression. Once p-eIF4E binds to the cap structure of mRNA, it advocates a nonstop translation process. In this regard, 15 new-based GMP analogs were synthesized to target eIF4E and restrain its binding to cap mRNA. The compounds were tested against three types of cancer cell lines: Caco-2, HepG-2, MCF-7, and normal kidney cells (Vero cells). Most of the compounds showed high potency against breast cancer cells (MCF-7), characterized by the highest cancer type for overexpression of p-eIF4E. Compound 4b was found to be the most active against three cell lines, colon (Caco-2), hepatic (HepG-2), and breast (MCF-7), with positive IC50 values of 31.40, 27.15, and 21.71 μM, respectively. Then, chitosan-coated niosomes loaded with compound 4b (Cs/4b-NSs) were developed (as kinetically enhanced molecules) to improve the anticancer effects further. The prepared Cs/4b-NSs showed pronounced cytotoxicity compared to the free 4b against Caco2, Hepg2, and MCF-7 with IC50 values of 16.15, 26.66, and 6.90 μM, respectively. Then, the expression of both the phosphorylated and nonphosphorylated western blot techniques was conducted on MCF-7 cells treated with the most active compounds (based on the obtained IC50 values) to determine the total protein expression of both eIF4E and p-eIF4e. Interestingly, the selected most active compounds displayed 35.8-40.7% inhibition of p-eIF4E expression when evaluated on MCF-7 compared to Ribavirin (positive control). CS/4b-NSs showed the best inhibition (40.7%). The findings of the present joint in silico molecular docking, simulation dynamic studies, and experimental investigation suggest the potential use of niosomal nanovesicles as a promising nanocarrier for the targeted delivery of the newly synthesized compound 4b to eukaryotic initiation factor 4E. These outcomes support the possible use of Cs/4b-NSs in targeted cancer therapy.

Regulation of Eukaryotic Translation Initiation Factor 4E as a Potential Anticancer Strategy

Eukaryotic translation initiation factors (eIFs) are highly expressed in cancer cells, especially eIF4E, the central regulatory node driving cancer cell growth and a potential target for anticancer drugs. eIF4E-targeting strategies primarily focus on inhibiting eIF4E synthesis, interfering with eIF4E/eIF4G interactions, and targeting eIF4E phosphorylation and peptide inhibitors. Although some small-molecule inhibitors are in clinical trials, no eIF4E inhibitors are available for clinical use. We provide an overview of the regulatory mechanisms of eIF4E and summarize the progress in developing and discovering eIF4E inhibitor strategies. We propose that interference with eIF4E/eIF4G interactions will provide a new perspective for the design of eIF4E inhibitors and may be a preferred strategy.

Cancer cells hijack RNA processing to rewrite the message

Typically, cancer is thought to arise due to DNA mutations, dysregulated transcription and/or aberrant signalling. Recently, it has become clear that dysregulated mRNA processing, mRNA export and translation also contribute to malignancy. RNA processing events result in major modifications to the physical nature of mRNAs such as the addition of the methyl-7-guanosine cap, the removal of introns and the addition of polyA tails. mRNA processing is a critical determinant for the protein-coding capacity of mRNAs since these physical changes impact the efficiency by which a given transcript can be exported to the cytoplasm and translated into protein. While many of these mRNA metabolism steps were considered constitutive housekeeping activities, they are now known to be highly regulated with combinatorial and multiplicative impacts i.e. one event will influence the capacity to undergo others. Furthermore, alternative splicing and/or cleavage and polyadenylation can produce transcripts with alternative messages and new functionalities. The coordinated processing of groups of functionally related RNAs can potently re-wire signalling pathways, modulate survival pathways and even re-structure the cell. As postulated by the RNA regulon model, combinatorial regulation of these groups is achieved by the presence of shared cis-acting elements (known as USER codes) which recruit machinery for processing, export or translation. In all, dysregulated RNA metabolism in cancer gives rise to an altered proteome that in turn elicits biological responses related to malignancy. Studies of these events in cancer revealed new mechanisms underpinning malignancies and unearthed novel therapeutic opportunities. In all, cancer cells coopt RNA processing, export and translation to support their oncogenic activity.

Phosphorylation of eIF4E in the stroma drives the production and spatial organisation of collagen type I in the mammary gland

The extracellular matrix (ECM) plays critical roles in breast cancer development. Whether ECM composition is regulated by the phosphorylation of eIF4E on serine 209, an event required for tumorigenesis, has not been explored. Herein, we used proteomics and mouse modelling to investigate the impact of mutating serine 209 to alanine on eIF4E (i.e., S209A) on mammary gland (MG) ECM. The proteomic data have been deposited to the ProteomeXchange Consortium via the PRIDE partner repository with the dataset identifier PXD028953. We discovered that S209A knock-in mice, expressing a non-phosphorylatable form of eIF4E, have less collagen-I deposition in native and tumor-bearing MGs, leading to altered tumor cell invasion. Additionally, phospho-eIF4E-deficiency impacts collagen topology; fibers at the tumor-stroma boundary in phospho-eIF4E-deficient mice run parallel to the tumor edge but radiate outwards in wild-type mice. Finally, a phospho-eIF4E-deficient tumor microenvironment resists anti-PD-1 therapy-induced collagen deposition, correlating with an increased anti-tumor response to immunotherapy. Clinically, we showed that collagen-I and phospho-eIF4E are positively correlated in human breast cancer samples, and that stromal phospho-eIF4E expression is influenced by tumor proximity. Together, our work defines the importance of phosphorylation of eIF4E on S209 as a regulator of MG collagen architecture in the tumor microenvironment, thereby positioning phospho-eIF4E as a therapeutic target to augment response to therapy.

The Role of mRNA Translational Control in Tumor Immune Escape and Immunotherapy Resistance

Tremendous advances have been made in cancer immunotherapy over the last decade. Among the different steps of gene expression, translation of mRNA is emerging as an essential player in both cancer and immunity. Changes in mRNA translation are both rapid and adaptive, and translational reprogramming is known to be necessary for sustaining cancer cell proliferation. However, the role of mRNA translation in shaping an immune microenvironment permissive to tumors has not been extensively studied. Recent studies on immunotherapy approaches have indicated critical roles of mRNA translation in regulating the expression of immune checkpoint proteins, tuning the secretion of inflammation-associated factors, modulating the differentiation of immune cells in the tumor microenvironment, and promoting cancer resistance to immunotherapies. Careful consideration of the role of mRNA translation in the tumor-immune ecosystem could suggest more effective therapeutic strategies and may eventually change the current paradigm of cancer immunotherapy. In this review, we discuss recent advances in understanding the relationship between mRNA translation and tumor-associated immunity, the potential mechanisms of immunotherapy resistance in cancers linked to translational reprogramming, and therapeutic perspectives and potential challenges of modulating translational regulation in cancer immunotherapy.

Control of the eIF4E activity: structural insights and pharmacological implications

The central role of eukaryotic translation initiation factor 4E (eIF4E) in controlling mRNA translation has been clearly assessed in the last decades. eIF4E function is essential for numerous physiological processes, such as protein synthesis, cellular growth and differentiation; dysregulation of its activity has been linked to ageing, cancer onset and progression and neurodevelopmental disorders, such as autism spectrum disorder (ASD) and Fragile X Syndrome (FXS). The interaction between eIF4E and the eukaryotic initiation factor 4G (eIF4G) is crucial for the assembly of the translational machinery, the initial step of mRNA translation. A well-characterized group of proteins, named 4E-binding proteins (4E-BPs), inhibits the eIF4E–eIF4G interaction by competing for the same binding site on the eIF4E surface. 4E-BPs and eIF4G share a single canonical motif for the interaction with a conserved hydrophobic patch of eIF4E. However, a second non-canonical and not conserved binding motif was recently detected for eIF4G and several 4E-BPs. Here, we review the structural features of the interaction between eIF4E and its molecular partners eIF4G and 4E-BPs, focusing on the implications of the recent structural and biochemical evidence for the development of new therapeutic strategies. The design of novel eIF4E-targeting molecules that inhibit translation might provide new avenues for the treatment of several conditions.

Antivirals against the Chikungunya Virus

Chikungunya virus (CHIKV) is a mosquito-transmitted alphavirus that has re-emerged in recent decades, causing large-scale epidemics in many parts of the world. CHIKV infection leads to a febrile disease known as chikungunya fever (CHIKF), which is characterised by severe joint pain and myalgia. As many patients develop a painful chronic stage and neither antiviral drugs nor vaccines are available, the development of a potent CHIKV inhibiting drug is crucial for CHIKF treatment. A comprehensive summary of current antiviral research and development of small-molecule inhibitor against CHIKV is presented in this review. We highlight different approaches used for the identification of such compounds and further discuss the identification and application of promising viral and host targets.

MNK Inhibition Sensitizes KRAS-Mutant Colorectal Cancer to mTORC1 Inhibition by Reducing eIF4E Phosphorylation and c-MYC Expression

KRAS-mutant colorectal cancers are resistant to therapeutics, presenting a significant problem for ∼40% of cases. Rapalogs, which inhibit mTORC1 and thus protein synthesis, are significantly less potent in KRAS-mutant colorectal cancer. Using Kras-mutant mouse models and mouse- and patient-derived organoids, we demonstrate that KRAS with G12D mutation fundamentally rewires translation to increase both bulk and mRNA-specific translation initiation. This occurs via the MNK/eIF4E pathway culminating in sustained expression of c-MYC. By genetic and small-molecule targeting of this pathway, we acutely sensitize KRASG12D models to rapamycin via suppression of c-MYC. We show that 45% of colorectal cancers have high signaling through mTORC1 and the MNKs, with this signature correlating with a 3.5-year shorter cancer-specific survival in a subset of patients. This work provides a c-MYC-dependent cotargeting strategy with remarkable potency in multiple Kras-mutant mouse models and metastatic human organoids and identifies a patient population that may benefit from its clinical application. SIGNIFICANCE: KRAS mutation and elevated c-MYC are widespread in many tumors but remain predominantly untargetable. We find that mutant KRAS modulates translation, culminating in increased expression of c-MYC. We describe an effective strategy targeting mTORC1 and MNK in KRAS-mutant mouse and human models, pathways that are also commonly co-upregulated in colorectal cancer.This article is highlighted in the In This Issue feature, p. 995.

Pharmacological Manipulation of Translation as a Therapeutic Target for Chronic Pain

Dysfunction in regulation of mRNA translation is an increasingly recognized characteristic of many diseases and disorders, including cancer, diabetes, autoimmunity, neurodegeneration, and chronic pain. Approximately 50 million adults in the United States experience chronic pain. This economic burden is greater than annual costs associated with heart disease, cancer, and diabetes combined. Treatment options for chronic pain are inadequately efficacious and riddled with adverse side effects. There is thus an urgent unmet need for novel approaches to treating chronic pain. Sensitization of neurons along the nociceptive pathway causes chronic pain states driving symptoms that include spontaneous pain and mechanical and thermal hypersensitivity. More than a decade of preclinical research demonstrates that translational mechanisms regulate the changes in gene expression that are required for ongoing sensitization of nociceptive sensory neurons. This review will describe how key translation regulation signaling pathways, including the integrated stress response, mammalian target of rapamycin, AMP-activated protein kinase (AMPK), and mitogen-activated protein kinase-interacting kinases, impact the translation of different subsets of mRNAs. We then place these mechanisms of translation regulation in the context of chronic pain states, evaluate currently available therapies, and examine the potential for developing novel drugs. Considering the large body of evidence now published in this area, we propose that pharmacologically manipulating specific aspects of the translational machinery may reverse key neuronal phenotypic changes causing different chronic pain conditions. Therapeutics targeting these pathways could eventually be first-line drugs used to treat chronic pain disorders. SIGNIFICANCE STATEMENT: Translational mechanisms regulating protein synthesis underlie phenotypic changes in the sensory nervous system that drive chronic pain states. This review highlights regulatory mechanisms that control translation initiation and how to exploit them in treating persistent pain conditions. We explore the role of mammalian/mechanistic target of rapamycin and mitogen-activated protein kinase-interacting kinase inhibitors and AMPK activators in alleviating pain hypersensitivity. Modulation of eukaryotic initiation factor 2α phosphorylation is also discussed as a potential therapy. Targeting specific translation regulation mechanisms may reverse changes in neuronal hyperexcitability associated with painful conditions.

Contribution of miRNAs, tRNAs and tRFs to Aberrant Signaling and Translation Deregulation in Lung Cancer

Simple Summary

The profiles of miRNAs, tRNA-derived fragments and tRNAs from lung cancer biopsy specimens indicate involvement of gene networks that modulate signaling and translation initiation. The current study highlights the important role of several regulatory small non-coding RNAs in aberrant signaling and translation deregulation in lung cancer.

Abstract

Transcriptomics profiles of miRNAs, tRNAs or tRFs are used as biomarkers, after separate examination of several cancer cell lines, blood samples or biopsies. However, the possible contribution of all three profiles on oncogenic signaling and translation as a net regulatory effect, is under investigation. The present analysis of miRNAs and tRFs from lung cancer biopsies indicated putative targets, which belong to gene networks involved in cell proliferation, transcription and translation regulation. In addition, we observed differential expression of specific tRNAs along with several tRNA-related genes with possible involvement in carcinogenesis. Transfection of lung adenocarcinoma cells with two identified tRFs and subsequent NGS analysis indicated gene targets that mediate signaling and translation regulation. Broader analysis of all major signaling and translation factors in several biopsy specimens revealed a crosstalk between the PI3K/AKT and MAPK pathways and downstream activation of eIF4E and eEF2. Subsequent polysome profile analysis and 48S pre-initiation reconstitution experiments showed increased global translation rates and indicated that aberrant expression patterns of translation initiation factors could contribute to elevated protein synthesis. Overall, our results outline the modulatory effects that possibly correlate the expression of important regulatory non-coding RNAs with aberrant signaling and translation deregulation in lung cancer.

Alzheimer's disease-related dysregulation of mRNA translation causes key pathological features with ageing

Alzheimer's disease (AD) is characterised by Aβ and tau pathology as well as synaptic degeneration, which correlates best with cognitive impairment. Previous work suggested that this pathological complexity may result from changes in mRNA translation. Here, we studied whether mRNA translation and its underlying signalling are altered in an early model of AD, and whether modelling this deficiency in mice causes pathological features with ageing. Using an unbiased screen, we show that exposure of primary neurons to nanomolar amounts of Aβ increases FMRP-regulated protein synthesis. This selective regulation of mRNA translation is dependent on a signalling cascade involving MAPK-interacting kinase 1 (Mnk1) and the eukaryotic initiation factor 4E (eIF4E), and ultimately results in reduction of CYFIP2, an FMRP-binding protein. Modelling this CYFIP2 reduction in mice, we find age-dependent Aβ accumulation in the thalamus, development of tau pathology in entorhinal cortex and hippocampus, as well as gliosis and synapse loss in the hippocampus, together with deficits in memory formation. Therefore, we conclude that early stages of AD involve increased translation of specific CYFIP2/FMRP-regulated transcripts. Since reducing endogenous CYFIP2 expression is sufficient to cause key features of AD with ageing in mice, we suggest that prolonged activation of this pathway is a primary step toward AD pathology, highlighting a novel direction for therapeutic targeting.

Phosphorylation of eukaryotic initiation factor 4E is dispensable for skeletal muscle hypertrophy

The eukaryotic initiation factor 4E (eIF4E) is a major mRNA cap-binding protein that has a central role in translation initiation. Ser209 is the single phosphorylation site within eIF4E and modulates its activity in response to MAPK pathway activation. It has been reported that phosphorylation of eIF4E at Ser209 promotes translation of key mRNAs, such as cyclin D1, that regulate ribosome biogenesis. We hypothesized that phosphorylation at Ser209 is required for skeletal muscle growth in response to a hypertrophic stimulus by promoting ribosome biogenesis. To test this hypothesis, wild-type (WT) and eIF4E knocked-in (KI) mice were subjected to synergist ablation to induce muscle hypertrophy of the plantaris muscle as the result of mechanical overload; in the KI mouse, Ser209 of eIF4E was replaced with a nonphosphorylatable alanine. Contrary to our hypothesis, we observed no difference in the magnitude of hypertrophy between WT and KI groups in response to 14 days of mechanical overload induced by synergist ablation. Similarly, the increases in cyclin D1 protein levels, ribosome biogenesis, and translational capacity did not differ between WT and KI groups. Based on these findings, we conclude that phosphorylation of eIF4E at Ser209 is dispensable for skeletal muscle hypertrophy in response to mechanical overload.

Translating from cancer to the brain: regulation of protein synthesis by eIF4F

Formation of eukaryotic initiation factor 4F (eIF4F) is widely considered to be the rate-limiting step in cap-dependent translation initiation. Components of eIF4F are often up-regulated in various cancers, and much work has been done to elucidate the role of each of the translation initiation factors in cancer cell growth and survival. In fact, many of the basic mechanisms describing how eIF4F is assembled and how it functions to regulate translation initiation were first investigated in cancer cell lines. These same eIF4F translational control pathways also are relevant for neuronal signaling that underlies long-lasting synaptic plasticity and memory, and in neurological diseases where eIF4F and its upstream regulators are dysregulated. Although eIF4F is important in cancer and for brain function, there is not always a clear path to use the results of studies performed in cancer models to inform one of the roles that the same translation factors have in neuronal signaling. Issues arise when extrapolating from cell lines to tissue, and differences are likely to exist in how eIF4F and its upstream regulatory pathways are expressed in the diverse neuronal subtypes found in the brain. This review focuses on summarizing the role of eIF4F and its accessory proteins in cancer, and how this information has been utilized to investigate neuronal signaling, synaptic function, and animal behavior. Certain aspects of eIF4F regulation are consistent across cancer and neuroscience, whereas some results are more complicated to interpret, likely due to differences in the complexity of the brain, its billions of neurons and synapses, and its diverse cell types.

The Role of the Eukaryotic Translation Initiation Factor 4E (eIF4E) in Neuropsychiatric Disorders

Protein synthesis in eukaryotic cells is a complex, multi-step and tightly regulated process. Translation initiation, the rate limiting step in protein synthesis, is dependent on the activity of eukaryotic translation Initiation Factor 4E (eIF4E). eIF4E is the cap-binding protein which, in synergy with proteins such as the helicase eIF4A and the scaffolding protein eIF4G, binds to mRNA, allowing the recruitment of ribosomes and translation initiation. The function of eIF4E is tightly regulated in cells under normal physiological conditions and can be controlled by post-translational modifications, such as phosphorylation, and by the binding of inhibitory proteins, including eIF4E binding proteins (4E-BPs) and CYFIP1. Recent studies have highlighted the importance of eIF4E in normal or aberrant function of the nervous system. In this mini-review, we will highlight the role of eIF4E function and regulation in the pathophysiology of neurodevelopmental and neuropsychiatric disorders.

Translation acrobatics: how cancer cells exploit alternate modes of translational initiation

Recent work has brought to light many different mechanisms of translation initiation that function in cells in parallel to canonical cap-dependent initiation. This has important implications for cancer. Canonical cap-dependent translation initiation is inhibited by many stresses such as hypoxia, nutrient limitation, proteotoxic stress, or genotoxic stress. Since cancer cells are often exposed to these stresses, they rely on alternate modes of translation initiation for protein synthesis and cell growth. Cancer mutations are now being identified in components of the translation machinery and in cis-regulatory elements of mRNAs, which both control translation of cancer-relevant genes. In this review, we provide an overview on the various modes of non-canonical translation initiation, such as leaky scanning, translation re-initiation, ribosome shunting, IRES-dependent translation, and m⁶A-dependent translation, and then discuss the influence of stress on these different modes of translation. Finally, we present examples of how these modes of translation are dysregulated in cancer cells, allowing them to grow, to proliferate, and to survive, thereby highlighting the importance of translational control in cancer.

Repurposed FDA-Approved drug sorafenib reduces replication of Venezuelan equine encephalitis virus and other alphaviruses

The New World alphaviruses -Venezuelan, eastern, and western equine encephalitis viruses (VEEV, EEEV, and WEEV respectively) - cause a febrile disease that is often lethal in equines and children and leads to long-term neurological sequelae in survivors. Endemic to the Americas, epizootic outbreaks of the three viruses occur sporadically in the continental United States. All three viruses aerosolize readily, replicate to high titers in cell culture, and have low infectious doses. Additionally, there are no FDA-approved vaccines or therapeutics for human use. To address the therapeutic gap, a high throughput assay utilizing a luciferase reporter virus, TC83-luc, was performed to screen a library of commercially available, FDA-approved drugs for antiviral activity. From a group of twenty compounds found to significantly decrease luminescence, the carcinoma therapeutic sorafenib inhibited replication of VEEV-TC83 and TrD in vitro. Additionally, sorafenib inhibited replication of EEEV and two Old World alphaviruses, Sindbis virus and chikungunya virus, at 8 and 16 h post-infection. Sorafenib caused no toxicity in Vero cells, and coupled with a low EC₅₀ value, yielded a selectivity index of >19. Mechanism of actions studies suggest that sorafenib inhibited viral translation through dephosphorylation of several key proteins, including eIF4E and p70S6K, leading to a reduction in viral protein production and overall viral replication.

Loss of eIF4E Phosphorylation Engenders Depression-like Behaviors via Selective mRNA Translation

The MAPK/ERK (mitogen-activated protein kinases/extracellular signal-regulated kinase) pathway is a cardinal regulator of synaptic plasticity, learning, and memory in the hippocampus. One of major endpoints of this signaling cascade is the 5′ mRNA cap binding protein eIF4E (eukaryotic Initiation Factor 4E), which is phosphorylated on Ser 209 by MNK (MAPK-interacting protein kinases) and controls mRNA translation. The precise role of phospho-eIF4E in the brain is yet to be determined. Herein, we demonstrate that ablation of eIF4E phosphorylation in male mice (4Eki mice) does not impair long-term spatial or contextual fear memory, or the late phase of LTP. Using unbiased translational profiling in mouse brain, we show that phospho-eIF4E differentially regulates the translation of a subset of mRNAs linked to inflammation, the extracellular matrix, pituitary hormones, and the serotonin pathway. Consequently, 4Eki male mice display exaggerated inflammatory responses and reduced levels of serotonin, concomitant with depression and anxiety-like behaviors. Remarkably, eIF4E phosphorylation is required for the chronic antidepressant action of the selective serotonin reuptake inhibitor fluoxetine. Finally, we propose a novel phospho-eIF4E-dependent translational control mechanism in the brain, via the GAIT complex (gamma IFN activated inhibitor of translation). In summary, our work proposes a novel translational control mechanism involved in the regulation of inflammation and depression, which could be exploited to design novel therapeutics.

SIGNIFICANCE STATEMENT We demonstrate that downstream of the MAPK (mitogen-activated protein kinase) pathway, eukaryotic Initiation Factor 4E (eIF4E) Ser209 phosphorylation is not required for classical forms of hippocampal LTP and memory. We reveal a novel role for eIF4E phosphorylation in inflammatory responses and depression-like behaviors. eIF4E phosphorylation is required for the chronic action of antidepressants, such as fluoxetine in mice. These phenotypes are accompanied by selective translation of extracellular matrix, pituitary hormones, and serotonin pathway genes, in eIF4E phospho-mutant mice. We also describe a previously unidentified translational control mechanism in the brain, whereby eIF4E phosphorylation is required for inhibiting the translation of gamma IFN activated inhibitor of translation element-containing mRNAs. These findings can be used to design novel therapeutics for depression.

Systems Biology of Immunomodulation for Post-Stroke Neuroplasticity: Multimodal Implications of Pharmacotherapy and Neurorehabilitation

AIMS

Recent studies indicate that anti-inflammatory drugs, act as a double-edged sword, not only exacerbating secondary brain injury but also contributing to neurological recovery after stroke. Our aim is to explore whether there is a beneficial role for neuroprotection and functional recovery using anti-inflammatory drug along with neurorehabilitation therapy using transcranial direct current stimulation (tDCS) and repetitive transcranial magnetic stimulation (rTMS), so as to improve functional recovery after ischemic stroke.

METHODS

We develop a computational systems biology approach from preclinical data, using ordinary differential equations, to study the behavior of both phenotypes of microglia, such as M1 type (pro-inflammatory) vis-à-vis M2 type (anti-inflammatory) under anti-inflammatory drug action (minocycline). We explore whether pharmacological treatment along with cerebral stimulation using tDCS and rTMS is beneficial or not. We utilize the systems pathway analysis of minocycline in nuclear factor kappa beta (NF-κB) signaling and neurorehabilitation therapy using tDCS and rTMS that act through brain-derived neurotrophic factor (BDNF) and tropomyosin-related kinase B (TrkB) signaling pathways.

RESULTS

We demarcate the role of neuroinflammation and immunomodulation in post-stroke recovery, under minocycline activated-microglia and neuroprotection together with improved neurogenesis, synaptogenesis, and functional recovery under the action of rTMS or tDCS. We elucidate the feasibility of utilizing rTMS/tDCS to increase neuroprotection across the reperfusion stage during minocycline administration. We delineate that the signaling pathways of minocycline by modulation of inflammatory genes in NF-κB and proteins activated by tDCS and rTMS through BDNF, TrkB, and calmodulin kinase (CaMK) signaling. Utilizing systems biology approach, we show that the activation pathways for pharmacotherapy (minocycline) and neurorehabilitation (rTMS applied to ipsilesional cortex and tDCS) results into increased neuronal and synaptic activity that commonly occur through activation of N-methyl-d-aspartate receptors. We construe that considerable additive neuroprotection effect would be obtained and delayed reperfusion injury can be remedied, if one uses multimodal intervention of minocycline together with tDCS and rTMS.

CONCLUSION

Additive beneficial effect is, thus, noticed for pharmacotherapy along with neurorehabilitation therapy, by maneuvering the dynamics of immunomodulation using anti-inflammatory drug and cerebral stimulation for augmenting the functional recovery after stroke, which may engender clinical applicability for enhancing plasticity, rehabilitation, and neurorestoration.

Inhibition of Mitogen-activated Protein Kinase (MAPK)-interacting Kinase (MNK) Preferentially Affects Translation of mRNAs Containing Both a 5'-Terminal Cap and Hairpin

The MAPK-interacting kinases 1 and 2 (MNK1 and MNK2) are activated by extracellular signal-regulated kinases 1 and 2 (ERK1/2) or p38 in response to cellular stress and extracellular stimuli that include growth factors, cytokines, and hormones. Modulation of MNK activity affects translation of mRNAs involved in the cell cycle, cancer progression, and cell survival. However, the mechanism by which MNK selectively affects translation of these mRNAs is not understood. MNK binds eukaryotic translation initiation factor 4G (eIF4G) and phosphorylates the cap-binding protein eIF4E. Using a cell-free translation system from rabbit reticulocytes programmed with mRNAs containing different 5'-ends, we show that an MNK inhibitor, CGP57380, affects translation of only those mRNAs that contain both a cap and a hairpin in the 5'-UTR. Similarly, a C-terminal fragment of human eIF4G-1, eIF4G(1357-1600), which prevents binding of MNK to intact eIF4G, reduces eIF4E phosphorylation and inhibits translation of only capped and hairpin-containing mRNAs. Analysis of proteins bound to m(7)GTP-Sepharose reveals that both CGP and eIF4G(1357-1600) decrease binding of eIF4E to eIF4G. These data suggest that MNK stimulates translation only of mRNAs containing both a cap and 5'-terminal RNA duplex via eIF4E phosphorylation, thereby enhancing the coupled cap-binding and RNA-unwinding activities of eIF4F.

Weighing up the possibilities: Controlling translation by ubiquitylation and sumoylation

Regulation of protein synthesis is of fundamental importance to cells. It has a critical role in the control of gene expression, and consequently cell growth and proliferation. The importance of this control is supported by the fact that protein synthesis is frequently upregulated in tumor cells. The major point at which regulation occurs is the initiation stage. Initiation of translation involves the interaction of several proteins to form the eIF4F complex, the recognition of the mRNA by this complex, and the subsequent recruitment of the 40S ribosomal subunit to the mRNA. This results in the formation of the 48S complex that then scans the mRNA for the start codon, engages the methionyl-tRNA and eventually forms the mature 80S ribosome which is elongation-competent. Formation of the 48S complex is regulated by the availability of individual initiation factors and through specific protein-protein interactions. Both of these events can be regulated by post-translational modification by ubiquitin or Ubls (ubiquitin-like modifiers) such as SUMO or ISG15. We provide here a summary of translation initiation factors that are modified by ubiquitin or Ubls and, where they have been studied in detail, describe the role of these modifications and their effects on regulating protein synthesis.

The effect of dietary protein levels on the expression of genes coding for four selected protein translation initiation factors in muscle tissue of Wujin pig

The objective of this study was to investigate the regulatory mechanism underlying the increased muscle protein accumulation in pigs while were fed a high protein diet. The eukaryotic initiation factors (eIFs) have been reported to involve in muscle protein synthesis. We investigated the mRNA and protein expression levels of eIF2B1, 4A1, 4B and 4E in Wujin pigs fed either a high protein (HP: 18%) or a low protein (LP: 14%) diet at 30, 60 or 100 kg body weight, based on real-time PCR and western blotting analyses. Our results indicated that the expression levels of eIF2B1 mRNA and protein were increased by HP diet at all body weight. The HP diet showed higher mRNA and protein levels of eIF4B gene at 60 and 100 kg. The protein expression of eIF4E phosphorylation was increased by HP diet only at 30 kg. These data suggested that the HP diet promoted porcine muscle protein accumulation mainly by up-regulating eIF2B1, 4B and 4E rather than 4A1 expression along the growth stages.

Mnk1 and 2 are dispensable for T cell development and activation but important for the pathogenesis of experimental autoimmune encephalomyelitis

T cell development and activation are usually accompanied by expansion and production of numerous proteins that require active translation. The eukaryotic translation initiation factor 4E (eIF4E) binds to the 5' cap structure of mRNA and is critical for cap-dependent translational initiation. It has been hypothesized that MAPK-interacting kinase 1 and 2 (Mnk1/2) promote cap-dependent translation by phosphorylating eIF4E at serine 209 (S209). Pharmacologic studies using inhibitors have suggested that Mnk1/2 have important roles in T cells. However, genetic evidence supporting such conclusions is lacking. Moreover, the signaling pathways that regulate Mnk1/2 in T cells remain unclear. We demonstrate that TCR engagement activates Mnk1/2 in primary T cells. Such activation is dependent on Ras-Erk1/2 signaling and is inhibited by diacylglycerol kinases α and ζ. Mnk1/2 double deficiency in mice abolishes TCR-induced eIF4E S209 phosphorylation, indicating their absolute requirement for eIF4E S209 phosphorylation. However, Mnk1/2 double deficiency does not affect the development of conventional αβ T cells, regulatory T cells, or NKT cells. Furthermore, T cell activation, in vivo primary and memory CD8 T cell responses to microbial infection, and NKT cell cytokine production were not obviously altered by Mnk1/2 deficiency. Although Mnk1/2 deficiency causes decreased IL-17 and IFN-γ production by CD4 T cells following immunization of mice with myelin oligodendrocyte glycoprotein peptide in complete Freund's adjuvant, correlating with milder experimental autoimmune encephalomyelitis scores, it does not affect Th cell differentiation in vitro. Together, these data suggest that Mnk1/2 has a minimal role in T cell development and activation but may regulate non-T cell lineages to control Th1 and Th17 differentiation in vivo.

An efficient factor-depleted mammalian in vitro translation system

Much of the regulation of gene expression occurs at the level of protein synthesis. In addition to the canonical translation factors, a multitude of proteins and microRNAs (miRNAs) act as regulatory trans-acting factors. Mechanistic analysis of translational control benefits from functional cell-free systems that can be depleted of the responsible regulatory factors. Although antisense oligonucleotides facilitate the functional sequestration of the regulatory RNAs, immunodepletion of protein factors is technically challenging. Here we describe a simple and robust alternative protocol for the preparation of factor-depleted in vitro translation system derived from HeLa cells. The procedure relies on RNA interference-mediated knockdown of the factor of interest prior to extract preparation, and it overcomes problems with the availability and specificity of antibodies, as well as with the co-depletion of proteins associated with the factor under study. The complete procedure can normally be conducted within 1 week and carried out in parallel for multiple (candidate) factors.

Natural occurrence and physiological role of a truncated eIF4E in the porcine endometrium during implantation

The present study is the first report providing evidence for a physiological role of a truncated form of the mRNA cap-binding protein eIF4E1 (eukaryotic initiation factor 4E1). Our initial observation was that eIF4E, which mediates the mRNA cap function by recruiting the eIF4F complex (composed of eIF4E, 4G and 4A), occurs in two forms in porcine endometrial tissue in a strictly temporally restricted fashion. The ubiquitous prototypical 25 kDa form of eIF4E was found in ovariectomized and cyclic animals. A new stable 23 kDa variant, however, is predominant during early pregnancy at the time of implantation. Northern blotting, cDNA sequence analysis, in vitro protease assays and MS showed that the 23 kDa form does not belong to a new class of eIF4E proteins. It represents a proteolytically processed variant of eIF4E1, lacking not more than 21 amino acids at the N-terminus. Steroid replacements indicated that progesterone in combination with 17β-oestradiol induced the formation of the 23 kDa eIF4E. Modified cell-free translation systems mimicking the situation in the endometrium revealed that, besides eIF4E, eIF4G was also truncated, but not eIF4A or PABP [poly(A)-binding protein]. The 23 kDa form of eIF4E reduced the repressive function of 4E-BP1 (eIF4E-binding protein 1) and the truncated eIF4G lacked the PABP-binding site. Thus we suggest that the truncated eIF4E provides an alternative regulation mechanism by an altered dynamic of eIF4E/4E-BP1 binding under conditions where 4E-BP1 is hypophosphorylated. Together with the impaired eIF4G–PABP interaction, the modified translational initiation might particularly regulate protein synthesis during conceptus attachment at the time of implantation.

Cereal and nonfat milk support muscle recovery following exercise

BACKGROUND

This study compared the effects of ingesting cereal and nonfat milk (Cereal) and a carbohydrate-electrolyte sports drink (Drink) immediately following endurance exercise on muscle glycogen synthesis and the phosphorylation state of proteins controlling protein synthesis: Akt, mTOR, rpS6 and eIF4E.

METHODS

Trained cyclists or triathletes (8 male: 28.0 +/- 1.6 yrs, 1.8 +/- 0.0 m, 75.4 +/- 3.2 kg, 61.0 +/- 1.6 ml O2*kg-1*min-1; 4 female: 25.3 +/- 1.7 yrs, 1.7 +/- 0.0 m, 66.9 +/- 4.6 kg, 46.4 +/- 1.2 mlO2*kg-1*min-1) completed two randomly-ordered trials serving as their own controls. After 2 hours of cycling at 60-65% VO2MAX, a biopsy from the vastus lateralis was obtained (Post0), then subjects consumed either Drink (78.5 g carbohydrate) or Cereal (77 g carbohydrate, 19.5 g protein and 2.7 g fat). Blood was drawn before and at the end of exercise, and at 15, 30 and 60 minutes after treatment. A second biopsy was taken 60 minutes after supplementation (Post60). Differences within and between treatments were tested using repeated measures ANOVA.

RESULTS

At Post60, blood glucose was similar between treatments (Drink 6.1 +/- 0.3, Cereal 5.6 +/- 0.2 mmol/L, p < .05), but after Cereal, plasma insulin was significantly higher (Drink 123.1 +/- 11.8, Cereal 191.0 +/- 12.3 pmol/L, p < .05), and plasma lactate significantly lower (Drink 1.4 +/- 0.1, Cereal 1.00 +/- 0.1 mmol/L, p < .05). Except for higher phosphorylation of mTOR after Cereal, glycogen and muscle proteins were not statistically different between treatments. Significant Post0 to Post60 changes occurred in glycogen (Drink 52.4 +/- 7.0 to 58.6 +/- 6.9, Cereal 58.7 +/- 9.6 to 66.0 +/- 10.0 mumol/g, p < .05) and rpS6 (Drink 17.9 +/- 2.5 to 35.2 +/- 4.9, Cereal 18.6 +/- 2.2 to 35.4 +/- 4.4 %Std, p < .05) for each treatment, but only Cereal significantly affected glycogen synthase (Drink 66.6 +/- 6.9 to 64.9 +/- 6.9, Cereal 61.1 +/- 8.0 to 54.2 +/- 7.2%Std, p < .05), Akt (Drink 57.9 +/- 3.2 to 55.7 +/- 3.1, Cereal 53.2 +/- 4.1 to 60.5 +/- 3.7 %Std, p < .05) and mTOR (Drink 28.7 +/- 4.4 to 35.4 +/- 4.5, Cereal 23.0 +/- 3.1 to 42.2 +/- 2.5 %Std, p < .05). eIF4E was unchanged after both treatments.

CONCLUSION

These results suggest that Cereal is as good as a commercially-available sports drink in initiating post-exercise muscle recovery.

Stimulation of muscle protein synthesis by somatotropin in pigs is independent of the somatotropin-induced increase in circulating insulin

Chronic treatment of growing pigs with porcine somatotropin (pST) promotes protein synthesis and doubles postprandial levels of insulin, a hormone that stimulates translation initiation. This study aimed to determine whether the pST-induced increase in skeletal muscle protein synthesis was mediated through an insulin-induced stimulation of translation initiation. After 7-10 days of pST (150 microg x kg(-1) x day(-1)) or control saline treatment, pancreatic glucose-amino acid clamps were performed in overnight-fasted pigs to reproduce 1) fasted (5 microU/ml), 2) fed control (25 microU/ml), and 3) fed pST-treated (50 microU/ml) insulin levels while glucose and amino acids were maintained at baseline fasting levels. Fractional protein synthesis rates and indexes of translation initiation were examined in skeletal muscle. Effectiveness of pST treatment was confirmed by reduced urea nitrogen and elevated insulin-like growth factor I levels in plasma. Skeletal muscle protein synthesis was independently increased by both insulin and pST. Insulin increased the phosphorylation of protein kinase B and the downstream effectors of the mammalian target of rapamycin, ribosomal protein S6 kinase, and eukaryotic initiation factor (eIF)4E-binding protein-1 (4E-BP1). Furthermore, insulin reduced inactive 4E-BP1.eIF4E complex association and increased active eIF4E.eIF4G complex formation, indicating enhanced eIF4F complex assembly. However, pST treatment did not alter translation initiation factor activation. We conclude that the pST-induced stimulation of skeletal muscle protein synthesis in growing pigs is independent of the insulin-associated activation of translation initiation.

CCL5-mediated T-cell chemotaxis involves the initiation of mRNA translation through mTOR/4E-BP1

The multistep, coordinated process of T-cell chemotaxis requires chemokines, and their chemokine receptors, to invoke signaling events to direct cell migration. Here, we examined the role for CCL5-mediated initiation of mRNA translation in CD4(+) T-cell chemotaxis. Using rapamycin, an inhibitor of mTOR, our data show the importance of mTOR in CCL5-mediated T-cell migration. Cycloheximide, but not actinomycin D, significantly reduced chemotaxis, suggesting a possible role for mRNA translation in T-cell migration. CCL5 induced phosphorylation/activation of mTOR, p70 S6K1, and ribosomal protein S6. In addition, CCL5 induced PI-3'K-, phospholipase D (PLD)-, and mTOR-dependent phosphorylation and deactivation of the transcriptional repressor 4E-BP1, which resulted in its dissociation from the eukaryotic initiation factor-4E (eIF4E). Subsequently, eIF4E associated with scaffold protein eIF4G, forming the eIF4F translation initiation complex. Indeed, CCL5 initiated active translation of mRNA, shown by the increased presence of high-molecular-weight polysomes that were significantly reduced by rapamycin treatment. Notably, CCL5 induced protein translation of cyclin D1 and MMP-9, known mediators of migration. Taken together, we describe a novel mechanism by which CCL5 influences translation of rapamycin-sensitive mRNAs and "primes" CD4(+) T cells for efficient chemotaxis.

Approaches for analyzing the differential activities and functions of eIF4E family members

The translational initiation factor eIF4E binds to the m(7)G-containing cap of mRNA and participates in recruitment of mRNA to ribosomes for protein synthesis. eIF4E also functions in nucleocytoplasmic transport of mRNA, sequestration of mRNA in a nontranslatable state, and stabilization of mRNA against decay in the cytosol. Multiple eIF4E family members have been identified in a wide range of organisms that includes plants, flies, mammals, frogs, birds, nematodes, fish, and various protists. This chapter reviews methods that have been applied to learn the biochemical properties and physiological functions that differentiate eIF4E family members within a given organism. Much has been learned to date about approaches to discover new eIF4E family members, their in vitro properties (cap binding, stimulation of cell-free translation systems), tissue and developmental expression patterns, protein-binding partners, and their effects on the translation or repression of specific subsets of mRNA. Despite these advances, new eIF4E family members continue to be found and new physiological roles discovered.

Control of protein translation by phosphorylation of the mRNA 5′-cap-binding complex

Initiation of mRNA translation is a key regulatory step in the control of gene expression. Microarray analysis indicates that total mRNA levels do not always reflect protein levels, since mRNA association with polyribosomes is necessary for protein synthesis. Phosphorylation of translation initiation factors offers a cost-effective and rapid way to adapt to physiological and environmental changes, and there is increasing evidence that many of these factors are subject to multiple regulatory phosphorylation events. The present article focuses on the nature of reversible phosphorylation and the function of the 5′-cap-binding complex in plants.

Functional analysis of individual binding activities of the scaffold protein eIF4G

Eukaryotic initiation factor (eIF) 4G is an integral member of the translation initiation machinery. The molecule serves as a scaffold for several other initiation factors, including eIF4E, eIF4AI, the eIF3 complex, and poly(A)-binding protein (PABP). Previous work indicates that complexes between these proteins exhibit enhanced mRNA cap-binding and RNA helicase activities relative to the respective individual proteins, eIF4E and eIF4A. The eIF4G-PABP interaction has been implicated in enhancing the formation of 48 S and 80 S initiation complexes and ribosome recycling through mRNA circularization. The eIF3-eIF4GI interaction is believed to forge the link between the 40 S subunit and the mRNA. Here we have investigated the behavior in vitro and in intact cells of eIF4GIf molecules lacking either the PABP-binding site, the eIF3-binding site, the middle domain eIF4A-binding site, or the C-terminal segment that includes the second eIF4A-binding site. Although in some cases the mutant forms were recruited more slowly, all of these eIF4G variants could form complexes with eIF4E, enter 48 S complexes and polysomes in vivo and in vitro, and partially rescue translation in cells targeted with eIF4GI short interfering RNA. In the reticulocyte lysate, eIF4G unable to interact directly with PABP showed little impairment in its ability to support translation, whereas loss of either of the eIF4A-binding sites or the eIF3-binding site resulted in a marked decrease in activity. We conclude that there is considerable redundancy in the mechanisms forming initiation complexes in mammalian cells, such that many individual interactions have regulatory rather than essential roles.

Regulation of c-jun mRNA expression in adult cardiocytes by MAP kinase interacting kinase-1 (MNK1)

Hypertrophic growth of adult myocardium is associated with increased expression of the early response gene c-jun. The purpose of this study was to determine whether eukaryotic initiation factor (elF) 4E (eIF4E) regulates translational efficiency of c-jun mRNA as measured by flux into polysomes. Adult feline cardiomyocytes in primary culture were treated with 0.2 microM 12-O-tetradecanoylphorbol 13-acetate (TPA), and c-jun mRNA was quantified in total, monosome, and polysome fractions by real-time polymerase chain reaction. After 1 h, TPA increased total c-jun mRNA by 10.5-fold. The corresponding flux into polysomes was significantly lower (5-fold). Adenoviral-mediated overexpression of either eIF4E or a nonphosphorylatable mutant (S209/A) did not affect total c-jun mRNA or its flux between monosomes and polysomes. Similar results were obtained following overexpression of the eIF4E kinase Mnk1. Thus, translational efficiency of c-jun mRNA was not affected by changes in activity or amount of eIF4E. In contrast, a kinase-deficient Mnk1 mutant significantly reduced total c-jun mRNA from 9.8-fold to 6.0-fold while flux between monosomes and polysomes remained constant. The decrease in total c-jun mRNA resulted from increased decay of c-jun mRNA incorporated into the polysomes. We conclude that Mnk1 activity stabilizes c-jun mRNA in polysomes independent of eIF4E phosphorylation.

Caliciviruses differ in their functional requirements for eIF4F components

Two classes of viruses, namely members of the Potyviridae and Caliciviridae, use a novel mechanism for the initiation of protein synthesis that involves the interaction of translation initiation factors with a viral protein covalently linked to the viral RNA, known as VPg. The calicivirus VPg proteins can interact directly with the initiation factors eIF4E and eIF3. Translation initiation on feline calicivirus (FCV) RNA requires eIF4E because it is inhibited by recombinant 4E-BP1. However, to date, there have been no functional studies carried out with respect to norovirus translation initiation, because of a lack of a suitable source of VPg-linked viral RNA. We have now used the recently identified murine norovirus (MNV) as a model system for norovirus translation and have extended our previous studies with FCV RNA to examine the role of the other eIF4F components in translation initiation. We now demonstrate that, as with FCV, MNV VPg interacts directly with eIF4E, although, unlike FCV RNA, translation of MNV RNA is not sensitive to 4E-BP1, eIF4E depletion, or foot-and-mouth disease virus Lb protease-mediated cleavage of eIF4G. We also demonstrate that both FCV and MNV RNA translation require the RNA helicase component of the eIF4F complex, namely eIF4A, because translation was sensitive (albeit to different degrees) to a dominant negative form and to a small molecule inhibitor of eIF4A (hippuristanol). These results suggest that calicivirus RNAs differ with respect to their requirements for the components of the eIF4F translation initiation complex.

Stopped-flow kinetic analysis of eIF4E and phosphorylated eIF4E binding to cap analogs and capped oligoribonucleotides: evidence for a one-step binding mechanism

Recruitment of eukaryotic mRNA to the 48 S initiation complex is rate-limiting for protein synthesis under normal conditions. Binding of the 5' -terminal cap structure of mRNA to eIF4E is a critical event during this process. Mammalian eIF4E is phosphorylated at Ser-209 by Mnk1 and Mnk2 kinases. We investigated the interaction of both eIF4E and phosphorylated eIF4E (eIF4E(P)) with cap analogs and capped oligoribonucleotides by stopped-flow kinetics. For m(7)GpppG, the rate constant of association, k(on), was dependent on ionic strength, decreasing progressively up to 350 mm KCl, but the rate constant of dissociation, k(off), was independent of ionic strength. Phosphorylation of eIF4E decreased k(on) by 2.1-2.3-fold at 50-100 mm KCl but had progressively less effect at higher ionic strengths, being negligible at 350 mm. Contrary to published evidence, eIF4E phosphorylation had no effect on k(off). Several observations supported a simple one-step binding mechanism, in contrast to published reports of a two-step mechanism. The kinetic function that best fit the data changed from single- to double-exponential as the eIF4E concentration was increased. However, measuring k(off) for dissociation of a pre-formed eIF4E.m(7)GpppG complex suggested that the double-exponential kinetics were caused by dissociation of eIF4E dimers, not a two-step mechanism. Addition of a 12-nucleotide chain to the cap structure increased affinity at high ionic strength for both eIF4E (24-fold) and eIF4E(P) (7-fold), primarily due to a decrease in k(off). This suggests that additional stabilizing interactions between capped oligoribonucleotides and eIF4E, which do not occur with cap analogs alone, act to slow dissociation.

Mnk is a negative regulator of cap-dependent translation in Aplysia neurons

To investigate the mechanisms underlying regulation of eukaryotic initiation factor 4E (eIF4E) phosphorylation in Aplysia neurons, we have cloned the Aplysia homolog of the vertebrate eIF4E kinases, Mnk1 and -2. Aplysia Mnk shares many conserved regions with vertebrate Mnk, including putative eukaryotic initiation factor 4G binding regions, activation loop phosphorylation sites, and a carboxy-terminal anchoring site for MAP kinases. As expected, purified Aplysia Mnk phosphorylated Aplysia eIF4E at a conserved carboxy-terminal serine and over-expression of Aplysia Mnk in sensory neurons led to increased phosphorylation of endogenous eIF4E. Over-expression of Aplysia Mnk led to strong decreases in cap-dependent translation, while generally sparing internal ribosomal entry site (IRES)-dependent translation. However, decreases in cap-dependent translation seen after expression of Aplysia Mnk could only be partly explained by increases in eIF4E phosphorylation. In Aplysia sensory neurons, phosphorylation of eIF4E is reduced during intermediate memory formation. However, we found that this physiological regulation of eIF4E phosphorylation was independent of changes in Aplysia Mnk phosphorylation. We propose that changes in eIF4E phosphorylation in Aplysia neurons are a consequence of changes in cap-dependent translation that are independent of regulation of Aplysia Mnk.

Acute treatment with TNF-alpha attenuates insulin-stimulated protein synthesis in cultures of C2C12 myotubes through a MEK1-sensitive mechanism

Insulin and TNF-alpha exert opposing effects on skeletal muscle protein synthesis that are mediated in part by the rapamycin-sensitive mammalian target of rapamycin (mTOR) pathway and the PD-98059-sensitive, extracellular signal-regulated kinase (ERK)1/2 pathway. The present study examined the separate and combined effects of insulin (INS), TNF, PD-98059, or dnMEK1 adenovirus on the translational control of protein synthesis in C(2)C(12) myotubes. Cultures were treated with INS, TNF, PD-98059, dnMEK1, or a combination of INS + TNF with PD-98059 or dnMEK1. INS stimulated protein synthesis, enhanced eIF4E.eIF4G association, and eIF4G phosphorylation and repressed eIF4E.4E-BP1 association vs. control. INS also promoted phosphorylation of ERK1/2, S6K1, and 4E-BP1 and dephosphorylation of eIF4E. TNF alone did not have an effect on protein synthesis (vs. control), eIF4E.eIF4G association, or the phosphorylation of eIF4G, S6K1, or 4E-BP1, although it transiently increased ERK1/2 and eIF4E phosphorylation. When myotubes were treated with TNF + INS, the cytokine blocked the insulin-induced stimulation of protein synthesis. This appeared to be due to an attenuation of insulin-stimulated eIF4E.eIF4G association, because other stimulatory effects of INS, e.g., phosphorylation of ERK1/2, 4E-BP1, S6K1, eIF4G, and eIF4E and eIF4E.4E-BP1 association, were unaffected. Finally, treatment of myotubes with PD-98059 or dnMEK1 adenovirus before TNF + INS addition resulted in a derepression of protein synthesis and the association of eIF4G with eIF4E. These findings suggest that TNF abrogates insulin-induced stimulation of protein synthesis in myotubes through a decrease in eIF4F complex assembly independently of S6K1 and 4E-BP1 signaling and dependently on a MEK1-sensitive signaling pathway.

Cellular energy status modulates translational control mechanisms in ischemic-reperfused rat hearts

Mechanisms regulating ischemia and reperfusion (I/R)-induced changes in mRNA translation in the heart are poorly defined, as are the factors that initiate these changes. Because cellular energy status affects mRNA translation under physiological conditions, it is plausible that I/R-induced changes in translation may in part be a result of altered cellular energy status. Therefore, the purpose of the studies described herein was to compare the effects of I/R with those of altered energy substrate availability on biomarkers of mRNA translation in the heart. Isolated adult rat hearts were perfused with glucose or a combination of glucose plus palmitate, and effects of I/R on various biomarkers of translation were subsequently analyzed. When compared with hearts perfused with glucose plus palmitate, hearts perfused with glucose alone exhibited increased phosphorylation of eukaryotic elongation factor (eEF)2, the alpha-subunit of eukaryotic initiation factor (eIF)2, and AMP-activated protein kinase (AMPK), and these hearts also exhibited enhanced association of eIF4E with eIF4E binding protein (4E-BP)1. Regardless of the energy substrate composition of the buffer, phosphorylation of eEF2 and AMPK was greater than control values after ischemia. Phosphorylation of eIF2alpha and eIF4E and the association of eIF4E with 4E-BP1 were also greater than control values after ischemia but only in hearts perfused with glucose plus palmitate. Reperfusion reversed the ischemia-induced increase in eEF2 phosphorylation in hearts perfused with glucose and reversed ischemia-induced changes in eIF4E, eEF2, and AMPK phosphorylation in hearts perfused with glucose plus palmitate. Because many ischemia-induced changes in mRNA translation are mimicked by the removal of a metabolic substrate under normal perfusion conditions, the results suggest that cellular energy status represents an important modulator of I/R-induced changes in mRNA translation.

Phosphorylation of the eukaryotic translation initiation factor eIF4E contributes to its transformation and mRNA transport activities

The eukaryotic translation initiation factor eIF4E is dysregulated in a wide variety of human cancers. In the cytoplasm, eIF4E acts in the rate-limiting step of translation initiation whereas in the nucleus, eIF4E forms nuclear bodies and promotes the nucleo-cytoplasmic export of a subset of growth-promoting mRNAs including cyclin D1. The only known post-translational modification of eIF4E is its phosphorylation at S209. Many studies have examined the role of phosphorylation on cap-dependent translation. However, no studies to date have explored the role of phosphorylation on the ability of eIF4E to transform cells. Using mutagenesis and separately a small molecular inhibitor of eIF4E phosphorylation, we show that eIF4E phosphorylation enhances both its mRNA transport function and its transformation activity in cell culture. Thus, phosphorylation of nuclear eIF4E seems to be an important step in control of the mRNA transport and thus the transforming properties of eIF4E.

Serum-deprivation stimulates cap-binding by PARN at the expense of eIF4E, consistent with the observed decrease in mRNA stability

PARN, a poly(A)-specific ribonuclease, binds the 5′ cap-structure of mRNA and initiates deadenylation-dependent decay. Eukaryotic initiation factor 4E (eIF4E) also binds to the cap structure, an interaction that is critical for initiating cap-dependent translation. The stability of various mRNA transcripts in human cell lines is reduced under conditions of serum starvation as determined by both functional and chemical half-lives. Serum starvation also leads to enhanced cap association by PARN. In contrast, the 5′ cap occupancy by eIF4E decreases under serum-deprivation, as does the translation of reporter transcripts. Further, we show that PARN is a phosphoprotein and that this modification can be modulated by serum status. Taken together, these data are consistent with a natural competition existing at the 5′ cap structure between PARN and eIF4E that may be regulated by changes in post-translational modifications. These phosphorylation-induced changes in the interplay of PARN and eIF4E may determine whether the mRNA is translated or decayed.

Diet-Dependent Effects of the Drosophila Mnk1/Mnk2 Homolog Lk6 on Growth via eIF4E

The control of cellular growth is tightly linked to the regulation of protein synthesis. A key function in translation initiation is fulfilled by the 5' cap binding eukaryotic initiation factor 4E (eIF4E), and dysregulation of eIF4E is associated with malignant transformation and tumorigenesis . In mammals, the activity of eIF4E is modulated by phosphorylation at Ser209 by mitogen-activated protein kinases (MAPK)-interacting kinases 1 and 2 (Mnk1 and Mnk2) , which themselves are activated by ERK and p38 MAPK in response to mitogens, cytokines or cellular stress . Whether phosphorylation of eIF4E at Ser209 exerts a positive or inhibitory effect on translation efficiency has remained controversial. Here we provide a genetic characterization of the Drosophila homolog of Mnk1/2, Lk6. Lk6 function is dispensable under a high protein diet, consistent with the recent finding that mice lacking both Mnk1 and Mnk2 are not growth-impaired . Interestingly, loss of Lk6 function causes a significant growth reduction when the amino acid content in the diet is reduced. Overexpression of Lk6 also results in growth inhibition in an eIF4E-dependent manner. We propose a model of eIF4E regulation that may reconcile the contradictory findings with regard to the role of phosphorylation by Mnk1/2.

Functional analysis of seven genes encoding eight translation initiation factor 4E (eIF4E) isoforms in Drosophila

The Drosophila genome-sequencing project has revealed a total of seven genes encoding eight eukaryotic initiation factor 4E (eIF4E) isoforms. Four of them (eIF4E-1,2, eIF4E-3, eIF4E-4 and eIF4E-5) share exon/intron structure in their carboxy-terminal part and form a cluster in the genome. All eIF4E isoforms bind to the cap (m7GpppN) structure. All of them, except eIF4E-6 and eIF4E-8 were able to interact with Drosophila eIF4G or eIF4E-binding protein (4E-BP). eIF4E-1, eIF4E-2, eIF4E-3, eIF4E-4 and eIF4E-7 rescued a yeast eIF4E-deficient mutant in vivo. Only eIF4E-1 mRNAs and, at a significantly lower level, eIF4E3 and eIF4E-8 are expressed in embryos and throughout the life cycle of the fly. The transcripts of the remaining isoforms were detected from the third instar larvae onwards. This indicates the cap-binding activity relies mostly on eIF4E-1 during embryogenesis. This agrees with the proteomic analysis of the eIF4F complex purified from embryos and with the rescue of l(3)67Af, an embryonic lethal mutant for the eIF4E-1,2 gene, by transgenic expression of eIF4E-1. Overexpression of eIF4E-1 in wild-type embryos and eye imaginal discs results in phenotypic defects in a dose-dependent manner.

Suppression of human Mnk1 by small interfering RNA increases the eukaryotic initiation factor 4F activity in HEK293T cells

Short-interfering RNAs (siRNAs) have proved to be a useful tool in studying gene function in plants, invertebrates and mammalian systems. Herein, we report the use of siRNAs for targeting the human MAP kinase-interacting kinase Mnk1 gene. This study demonstrates the efficacy of the designed siRNA in silencing Mnk1 in the human cell line HEK293T and shows that Mnk1 suppression decreases eukaryotic initiation factor 4E phosphorylation without causing any change in global protein synthesis rate and cell proliferation. Interestingly, suppression of Mnk1 results in a significant increase in eukaryotic initiation factor 4F complex formation after 72 h of transfection.

Cap-binding activity of an eIF4E homolog from Leishmania

All eukaryotic mRNAs possess a 5'-cap (m(7)GpppN) that is recognized by a family of cap-binding proteins. These participate in various processes, such as RNA transport and stabilization, as well as in assembly of the translation initiation complex. The 5'-cap of trypanosomatids is complex; in addition to 7-methyl guanosine, it includes unique modifications on the first four transcribed nucleotides, and is thus denoted cap-4. Here we analyze a cap-binding protein of Leishmania, in an attempt to understand the structural features that promote its binding to this unusual cap. LeishIF4E-1, a homolog of eIF4E, contains the conserved cap-binding pocket, similar to its mouse counterpart. The mouse eIF4E has a higher K(as) for all cap analogs tested, as compared with LeishIF4E-1. However, whereas the mouse eIF4E shows a fivefold higher affinity for m(7)GTP than for a chemically synthesized cap-4 structure, LeishIF4E-1 shows similar affinities for both ligands. A sequence alignment shows that LeishIF4E-1 lacks the region that parallels the C terminus in the murine eIF4E. Truncation of this region in the mouse protein reduces the difference that is observed between its binding to m(7)GTP and cap-4, prior to this deletion. We hypothesize that variations in the structure of LeishIF4E-1, possibly also the absence of a region that is homologous to the C terminus of the mouse protein, promote its ability to interact with the cap-4 structure. LeishIF4E-1 is distributed in the cytoplasm, but its function is not clear yet, because it cannot substitute the mammalian eIF4E in a rabbit reticulocyte in vitro translation system.

Identification and molecular characterization of Mnk1b, a splice variant of human MAP kinase-interacting kinase Mnk1

In this paper, we report the identification and molecular characterization of a splice variant of human Mnk1 which has been named as Mnk1b. Human Mnk1b mRNA is homologous to human Mnk1 mRNA but lacking a region corresponding to exon 19, which causes a change in the reading frame generating a stop codon. The resulting protein lacks the last 89 amino acids at the C-terminal region that are replaced by 12 amino acids with an entirely new sequence. The C-terminal end in Mnk1 corresponds to the extracellular signal-regulated kinase (ERK1/2) binding site. Although Mnk1b lacks this domain and, consequently, is not phosphorylated by ERK1/2, it is able, however, to phosphorylate eIF4E in vitro and in vivo in a mitogen-activated protein kinases-independent manner. This result suggests that Mnk1b may play a key role in regulating protein translation in the absence of stimuli. Interestingly, a significant population of cells shows Mnk1b within the nucleus whereas Mnk1 is always detected in the cytoplasm. This fact may be explained because Mnk1b maintains the nuclear localization signal (NLS) but lacks the nuclear export sequence (NES).

Influence of electric charge variation at residues 209 and 159 on the interaction of eIF4E with the mRNA 5' terminus

Eukaryotic translation initiation factor 4E (eIF4E) is essential for efficient protein synthesis in cap-dependent translation. The protein specifically binds the cap structure at the mRNA 5' terminus and facilitates the assembly of the mRNA with other initiation factors and the 40S ribosomal subunit. Phosphorylation of eIF4E is implicated in the regulation of the initiation step of translation. However, the molecular mechanism of this regulation still remains unclear. To address this problem, we have determined the binding affinities of eIF4E specifically mutated at position 209 or 159 for a series of novel mono- and dinucleotide cap analogues by a fluorometric time-synchronized titration method. A 1.5-3-fold reduction in the affinity of cap for the S209E mutant and a 1-2-fold increase in the affinity of cap for the S209K mutant, depending on the negative charge of phosphate chains, indicate that phosphorylation at Ser209 creates electrostatic repulsion between the protein and the negatively charged cap structure. The inhibition of the ability to bind cap analogues by the K159A mutant and its phosphorylated counterpart shows significant participation of Lys159 in the binding of the capped mRNA. Both structural modifications, phosphorylation and the replacement of lysine with alanine, result in an increase in the negative Gibbs free energy of association that is proportional to the length of the cap phosphate chain and additive, i.e., equal to the sum of the individual destabilizing changes of DeltaG degrees. The possible implication of these results for the mechanism of control of eIF4E by phosphorylation, especially for the "clamping model", is discussed.

Mnk2 and Mnk1 are essential for constitutive and inducible phosphorylation of eukaryotic initiation factor 4E but not for cell growth or development

Mnk1 and Mnk2 are protein kinases that are directly phosphorylated and activated by extracellular signal-regulated kinase (ERK) or p38 mitogen-activated protein (MAP) kinases and implicated in the regulation of protein synthesis through their phosphorylation of eukaryotic translation initiation factor 4E (eIF4E) at Ser209. To investigate their physiological functions, we generated mice lacking the Mnk1 or Mnk2 gene or both; the resulting KO mice were viable, fertile, and developed normally. In embryonic fibroblasts prepared from Mnk1-Mnk2 DKO mice, eIF4E was not detectably phosphorylated at Ser209, even when the ERK and/or p38 MAP kinases were activated. Analysis of embryonic fibroblasts from single KO mice revealed that Mnk1 is responsible for the inducible phosphorylation of eIF4E in response to MAP kinase activation, whereas Mnk2 mainly contributes to eIF4E's basal, constitutive phosphorylation. Lipopolysaccharide (LPS)- or insulin-induced upregulation of eIF4E phosphorylation in the spleen, liver, or skeletal muscle was abolished in Mnk1(-/-) mice, whereas the basal eIF4E phosphorylation levels were decreased in Mnk2(-/-) mice. In Mnk1-Mnk2 DKO mice, no phosphorylated eIF4E was detected in any tissue studied, even after LPS or insulin injection. However, neither general protein synthesis nor cap-dependent translation, as assayed by a bicistronic reporter assay system, was affected in Mnk-deficient embryonic fibroblasts, despite the absence of phosphorylated eIF4E. Thus, Mnk1 and Mnk2 are exclusive eIF4E kinases both in cultured fibroblasts and adult tissues, and they regulate inducible and constitutive eIF4E phosphorylation, respectively. These results strongly suggest that eIF4E phosphorylation at Ser209 is not essential for cell growth during development.

Indinavir impairs protein synthesis and phosphorylations of MAPKs in mouse C2C12 myocytes

Anti-retroviral therapy promotes clinical, immunologic, and virologic improvement in human immunodeficiency virus-infected patients. Whereas this therapy adversely affects carbohydrate and lipid metabolism, the effects of anti-retroviral drugs on muscle protein synthesis and degradation have not been reported. To examine these processes, we treated C2C12 myocytes with increasing concentrations of the protease inhibitor indinavir for 1 or 2 days. Treatment of myocytes with a therapeutic concentration of indinavir (20 microM) for 24 h decreased basal protein synthesis by 18%, whereas a 42% decline was observed after 48 h. A similar decrement, albeit quantitatively smaller, was detected with other protease inhibitors. Indinavir did not alter the rate of proteolysis. Likewise, indinavir did not impair the anabolic effect of insulin-like growth factor-I on protein synthesis. Mechanistically, indinavir decreased the phosphorylation of the S6 ribosomal protein (rpS6), and this reduction was associated with a decreased phosphorylation of p70S6 kinase and p90rsk as well as the upstream regulators ERK1/2 and MEK1/2. Indinavir also decreased the phosphorylation of Mnk1 and its upstream effectors, p38 MAPK and ERK1/2. Indinavir did not affect the phosphorylation of mTOR or 4E-BP1, but it did decrease the amount of the active eukaryotic initiation factor eIF4G-eIF4E complex. In conclusion, indinavir decreased protein synthesis in myocytes. This decrease was associated with the disruption of the ERK1/2 and p38 MAPK pathways and a reduction in both the level of functional eIF4F complex and rpS6 phosphorylation.

Phosphorylation of eIF4E by Mnk-1 enhances HSV-1 translation and replication in quiescent cells

Although the activity of the translation initiation factor eIF4F is regulated in part by translational repressors (4E-BPs) that prevent incorporation of eIF4E, the cap-binding protein, into the initiation complex, the contribution of eIF4E phosphorylation to translational control remains controversial. Here, we demonstrate that the herpes simplex virus-1 (HSV-1) ICP0 gene product, a multifunctional transactivator of viral gene expression with ubiquitin E3 ligase activity that is important for vegetative replication and reactivation of latent infections, is required to stimulate phosphorylation of eIF4E as well as 4E-BP1, and promote assembly of eIF4F complexes in infected cells. Furthermore, 4E-BP1 is degraded by the proteasome in an ICP0-dependent manner, establishing that the proteasome can control 4E-BP1 steady-state levels. Preventing eIF4E phosphorylation by inhibiting the eIF4E kinase mnk-1 dramatically reduced viral replication and the translation of viral polypeptides in quiescent cells, providing the first evidence that phosphorylation of eIF4E by mnk-1 is critical for viral protein synthesis and replication. Thus, in marked contrast to many viruses that inactivate eIF4F, HSV-1 stimulates eIF4F complex assembly in quiescent, differentiated cells; moreover, this is important for viral replication, and may be crucial for HSV-1 to initiate its productive growth cycle in resting cells, such as latently infected neurons.

Regulation of protein synthesis by eIF4E phosphorylation in adult cardiocytes: the consequence of secondary structure in the 5'-untranslated region of mRNA

In adult cardiocytes, eIF4E (eukaryotic initiation factor 4E) activity and protein synthesis are increased concomitantly in response to stimuli that induce hypertrophic growth. We tested the hypothesis that increases in eIF4E activity selectively improve the translational efficiency of mRNAs that have an excessive amount of secondary structure in the 5'-UTR (5'-untranslated region). The activity of eIF4E was modified in primary cultures of adult cardiocytes using adenoviral gene transfer to increase either the amount of eIF4E or the extent of endogenous eIF4E phosphorylation. Subsequently, the effects of eIF4E on translational efficiency were assayed following adenoviral-mediated expression of luciferase reporter mRNAs that were either 'stronger' (less structure in the 5'-UTR) or 'weaker' (more structure in the 5'-UTR) with respect to translational efficiency. The insertion of G+C-rich repeats into the 5'-UTR doubled the predicted amount of secondary structure and was sufficient to reduce translational efficiency of the reporter mRNA by 48+/-13%. Translational efficiency of the weaker reporter mRNA was not significantly improved by overexpression of wild-type eIF4E when compared with the stronger reporter mRNA. In contrast, overexpression of the eIF4E kinase Mnk1 [MAP (mitogen-activated protein) kinase signal-integrating kinase 1] was sufficient to increase the translational efficiency of either reporter mRNA, independent of the amount of secondary structure in their respective 5'-UTRs. The increases in translational efficiency produced by Mnk1 occurred in association with corresponding decreases in mRNA levels. These findings indicate that the positive effect of eIF4E phosphorylation on translational efficiency in adult cardiocytes is coupled with the stability of mRNA.