Phosphorylation of eukaryotic translation initiation factor 4E is critical for growth

Drosophila RNA polymerase III repressor Maf1 controls body size and developmental timing by modulating tRNAiMet synthesis and systemic insulin signaling

The target-of-rapamycin pathway couples nutrient availability with tissue and organismal growth in metazoans. The key effectors underlying this growth are, however, unclear. Here we show that Maf1, a repressor of RNA polymerase III-dependent tRNA transcription, is an important mediator of nutrient-dependent growth in Drosophila. We find nutrients promote tRNA synthesis during larval development by inhibiting Maf1. Genetic inhibition of Maf1 accelerates development and increases body size. These phenotypes are due to a non-cell-autonomous effect of Maf1 inhibition in the fat body, the main larval endocrine organ. Inhibiting Maf1 in the fat body increases growth by promoting the expression of brain-derived insulin-like peptides and consequently enhanced systemic insulin signaling. Remarkably, the effects of Maf1 inhibition are reproduced in flies carrying one extra copy of the initiator methionine tRNA, tRNA(i)(Met). These findings suggest the stimulation of tRNA(i)(Met) synthesis via inhibition of dMaf1 is limiting for nutrition-dependent growth during development.

The translation initiation factor eIF4E regulates the sex-specific expression of the master switch gene Sxl in Drosophila melanogaster

In female fruit flies, Sex-lethal (Sxl) turns off the X chromosome dosage compensation system by a mechanism involving a combination of alternative splicing and translational repression of the male specific lethal-2 (msl-2) mRNA. A genetic screen identified the translation initiation factor eif4e as a gene that acts together with Sxl to repress expression of the Msl-2 protein. However, eif4e is not required for Sxl mediated repression of msl-2 mRNA translation. Instead, eif4e functions as a co-factor in Sxl-dependent female-specific alternative splicing of msl-2 and also Sxl pre-mRNAs. Like other factors required for Sxl regulation of splicing, eif4e shows maternal-effect female-lethal interactions with Sxl. This female lethality can be enhanced by mutations in other co-factors that promote female-specific splicing and is caused by a failure to properly activate the Sxl-positive autoregulatory feedback loop in early embryos. In this feedback loop Sxl proteins promote their own synthesis by directing the female-specific alternative splicing of Sxl-Pm pre-mRNAs. Analysis of pre-mRNA splicing when eif4e activity is compromised demonstrates that Sxl-dependent female-specific splicing of both Sxl-Pm and msl-2 pre-mRNAs requires eif4e activity. Consistent with a direct involvement in Sxl-dependent alternative splicing, eIF4E is associated with unspliced Sxl-Pm pre-mRNAs and is found in complexes that contain early acting splicing factors—the U1/U2 snRNP protein Sans-fils (Snf), the U1 snRNP protein U1-70k, U2AF38, U2AF50, and the Wilms' Tumor 1 Associated Protein Fl(2)d—that have been directly implicated in Sxl splicing regulation.

Gene expression in eukaryotes is a complex process that occurs in several discrete steps. Some of those steps are separated into different sub-cellular compartments and thus might be expected to occur independently of one another and involve entirely distinct factors. For example pre-mRNA splicing takes place in the nucleus where it is coupled with transcription, while mRNA translation requires export to the cytoplasm and ribosome loading. We describe studies on the fruit fly Drosophila which indicate that a cytoplasmic translation initiation factor, the cap binding protein eIF4E, plays a key role in alternative splicing in the nucleus. When eIF4E activity is compromised, we observe defects in sex-specific splicing of pre-mRNAs that are regulated by the sex determination master switch gene Sex-lethal. Our data argue that eIF4E likely plays a direct role in the regulation of alternative splicing by Sex-lethal.

TOR signaling never gets old: aging, longevity and TORC1 activity

The target of rapamycin (TOR) signal transduction network monitors intra- and extracellular conditions that favor cell growth. Research during the last decade has revealed a modular structure of the TOR signaling network. Each signaling module senses a particular set of signals from the cellular milieu and exerts regulatory control towards TOR activity. The TOR pathway responds to growth factor signals, nutrient availability, and cellular stresses like hypoxia and energy stress. The signaling modules and their molecular components constituting the TOR network are remarkably conserved in both sequence and function across species. In yeast, roundworms, flies, and mice, the TOR pathway has been shown to regulate lifespan. Correspondingly, genetic, dietary or pharmacological manipulation of individual signaling modules as well as TOR activity itself extends lifespan in these model organisms. We discuss the potential impact of manipulating TOR activity for human health and lifespan.

Therapeutic inhibition of MAP kinase interacting kinase blocks eukaryotic initiation factor 4E phosphorylation and suppresses outgrowth of experimental lung metastases

Activation of the translation initiation factor 4E (eIF4E) promotes malignant transformation and metastasis. Signaling through the AKT-mTOR pathway activates eIF4E by phosphorylating the inhibitory 4E binding proteins (4E-BP). This liberates eIF4E and allows binding to eIF4G. eIF4E can then be phosphorylated at serine 209 by the MAPK-interacting kinases (Mnk), which also interact with eIF4G. Although dispensable for normal development, Mnk function and eIF4E phosphorylation promote cellular proliferation and survival and are critical for malignant transformation. Accordingly, Mnk inhibition may serve as an attractive cancer therapy. We now report the identification of a potent, selective and orally bioavailable Mnk inhibitor that effectively blocks 4E phosphorylation both in vitro and in vivo. In cultured cancer cell lines, Mnk inhibitor treatment induces apoptosis and suppresses proliferation and soft agar colonization. Importantly, a single, orally administered dose of this Mnk inhibitor substantially suppresses eIF4E phosphorylation for at least 4 hours in human xenograft tumor tissue and mouse liver tissue. Moreover, oral dosing with the Mnk inhibitor significantly suppresses outgrowth of experimental B16 melanoma pulmonary metastases as well as growth of subcutaneous HCT116 colon carcinoma xenograft tumors, without affecting body weight. These findings offer the first description of a novel, orally bioavailable MNK inhibitor and the first preclinical proof-of-concept that MNK inhibition may provide a tractable cancer therapeutic approach.

Cap and cap-binding proteins in the control of gene expression

The 5' mRNA cap structure is essential for efficient gene expression from yeast to human. It plays a critical role in all aspects of the life cycle of an mRNA molecule. Capping occurs co-transcriptionally on the nascent pre-mRNA as it emerges from the RNA exit channel of RNA polymerase II. The cap structure protects mRNAs from degradation by exonucleases and promotes transcription, polyadenylation, splicing, and nuclear export of mRNA and U-rich, capped snRNAs. In addition, the cap structure is required for the optimal translation of the vast majority of cellular mRNAs, and it also plays a prominent role in the expression of eukaryotic, viral, and parasite mRNAs. Cap-binding proteins specifically bind to the cap structure and mediate its functions in the cell. Two major cellular cap-binding proteins have been described to date: eukaryotic translation initiation factor 4E (eIF4E) in the cytoplasm and nuclear cap binding complex (nCBC), a nuclear complex consisting of a cap-binding subunit cap-binding protein 20 (CBP 20) and an auxiliary protein cap-binding protein 80 (CBP 80). nCBC plays an important role in various aspects of nuclear mRNA metabolism such as pre-mRNA splicing and nuclear export, whereas eIF4E acts primarily as a facilitator of mRNA translation. In this review, we highlight recent findings on the role of the cap structure and cap-binding proteins in the regulation of gene expression. We also describe emerging regulatory pathways that control mRNA capping and cap-binding proteins in the cell.

Diminution of eIF4E activity suppresses parkin mutant phenotypes

Mutations in the human parkin (PARK2) gene cause autosomal recessive-juvenile Parkinson's disease (AR-JP). In Drosophila melanogaster, mutant parkin alleles display a broad range of phenotypic alterations, including female infertility. Here we report that reducing the level of eukaryotic translation initiation factor 4E (eIF4E) activity specifically rescues the female sterile phenotypes associated with the parkin(P23) mutant allele. Additional defects, including reduction of pupal viability and body size, are also entirely recovered in both male and female flies of the abovementioned genotype. We further show that a null eIF4E-binding protein (4E-BP) allele counteracts the in vivo effects produced, in a parkin(P23) mutant background, by the reduction of functional eIF4E copy number. Moreover, Parkin and eIF4E interact in vitro and co-localize at the posterior end of developing oocytes. Finally, we show that eIF4E is over-expressed in parkin(P23) mutant ovaries as compared to wild-types. Taken together, our data are consistent with the idea that Parkin and eIF4E act in a common pathway, likely modulating cap-dependent translation initiation events.

Specific roles of Target of rapamycin in the control of stem cells and their progeny in the Drosophila ovary

Stem cells depend on intrinsic and local factors to maintain their identity and activity, but they also sense and respond to changing external conditions. We previously showed that germline stem cells (GSCs) and follicle stem cells (FSCs) in the Drosophila ovary respond to diet via insulin signals. Insulin signals directly modulate the GSC cell cycle at the G2 phase, but additional unknown dietary mediators control both G1 and G2. Target of rapamycin, or TOR, is part of a highly conserved nutrient-sensing pathway affecting growth, proliferation, survival and fertility. Here, we show that optimal TOR activity maintains GSCs but does not play a major role in FSC maintenance, suggesting differential regulation of GSCs versus FSCs. TOR promotes GSC proliferation via G2 but independently of insulin signaling, and TOR is required for the proliferation, growth and survival of differentiating germ cells. We also report that TOR controls the proliferation of FSCs but not of their differentiating progeny. Instead, TOR controls follicle cell number by promoting survival, independently of either the apoptotic or autophagic pathways. These results uncover specific TOR functions in the control of stem cells versus their differentiating progeny, and reveal parallels between Drosophila and mammalian follicle growth.

Protein synthesis and the antagonistic pleiotropy hypothesis of aging

Growth and somatic maintenance are thought to be antagonistic piciotropic traits, but the molecular basis for this tradeoff is poorly understood. Here it is proposed that changes in protein synthesis mediate the tradeoffs that take place upon genetic and environmental manipulation in various model systems including yeast, worms, flies and mice. This hypothesis is supported by evidence that inhibition of the TOR (target of rapamycin) pathway and various translation factors that inhibit protein synthesis lead to slowing of growth and development but extend lifespan. Furthermore, dietary restriction (DR) that leads to antagonistic changes in growth and lifespan, also mediates this change by inhibiting protein synthesis. Direct screens to identify genes that extend lifespan from a subset of genes that are essential for growth and development have also uncovered a number of genes involved in protein synthesis. Given the conserved mechanisms of protein synthesis across species, I discuss potential mechanisms that mediate the lifespan extension by inhibition of protein synthesis that are likely to be important for aging and age-related disorders in humans.

4E-BP extends lifespan upon dietary restriction by enhancing mitochondrial activity in Drosophila

Dietary restriction (DR) extends lifespan in multiple species. To examine the mechanisms of lifespan extension upon DR, we assayed genome-wide translational changes in Drosophila. A number of nuclear encoded mitochondrial genes, including those in Complex I and IV of the electron transport chain, showed increased ribosomal loading and enhanced overall activity upon DR. We found that various mitochondrial genes possessed shorter and less structured 5'UTRs, which were important for their enhanced mRNA translation. The translational repressor 4E-BP, the eukaryotic translation initiation factor 4E binding protein, was upregulated upon DR and mediated DR dependent changes in mitochondrial activity and lifespan extension. Inhibition of individual mitochondrial subunits from Complex I and IV diminished the lifespan extension obtained upon DR, reflecting the importance of enhanced mitochondrial function during DR. Our results imply that translational regulation of nuclear-encoded mitochondrial gene expression by 4E-BP plays an important role in lifespan extension upon DR. For a video summary of this article, see the PaperFlick file with the Supplemental Data available online.

Pdk1 activity controls proliferation, survival, and growth of developing pancreatic cells

The formation of adequate masses of endocrine and exocrine pancreatic tissues during embryogenesis is essential to ensure proper nutrition and glucose homeostasis at postnatal stages. We generated mice with pancreas-specific ablation of the 3-phosphoinositide-dependent protein kinase 1 (Pdk1) to investigate how signaling downstream of the phosphatidylinositol-3-OH kinase (PI3K) pathway controls pancreas development. Pdk1-conditional knock-out mice were born with conspicuous pancreas hypoplasia, and within a few weeks, they developed severe hyperglycemia. Our detailed characterization of the mutant embryonic pancreas also revealed distinct temporal, cell type-specific requirements of Pdk1 activity in the control of cell proliferation, cell survival, and cell size during pancreas development. These results thus uncover Pdk1 as a novel, crucial regulator of pancreatic growth during embryogenesis. In addition, we provide evidence that Pdk1 activity is required differently in mature pancreatic cell types, since compensatory proliferation and possible mTORC2 activation occurred in exocrine cells but not in beta cells of the Pdk1-deficient postnatal pancreas.

Enhancing mTOR-targeted cancer therapy

BACKGROUND

The mammalian target of rapamycin (mTOR) has emerged as an attractive cancer therapeutic target. Accordingly, several mTOR inhibitors (e.g., rapamycin and its analogs; rapalogs) are currently being tested in many cancer clinical trials. Despite the encouraging results showing that some rapalogs improved overall survival among patients with metastatic renal-cell carcinoma, the single-agent activity of rapalogs in most other tumor-types has been modest, at best.

OBJECTIVE

To review the current understanding of the mTOR axis and discuss potential strategies to enhance mTOR-targeted cancer therapy.

METHODS

Preclinical and clinical data in peer-reviewed reports on the novel biological and therapeutic parts of the mTOR axis are discussed.

CONCLUSION

The mTOR axis involves complex regulatory networks. Inhibition of the mTOR axis with a rapalog induces feedback activation of several survival signaling pathways such as Akt activation, which, in turn, blunt rapalogs' anticancer efficacy. Thus, blockage or prevention of the activation of these survival signaling pathways may enhance mTOR-targeted cancer therapy.

A quassinoid 6alpha-tigloyloxychaparrinone inhibits hypoxia-inducible factor-1 pathway by inhibition of eukaryotic translation initiation factor 4E phosphorylation

Hypoxia-inducible factor-1 (HIF-1) is the central mediator of cellular responses to low oxygen and vital to many aspects of cancer biology. In a search for HIF-1 inhibitors, we identified a quassinoid 6alpha-tigloyloxychaparrinone (TCN) as an inhibitor of HIF-1 activation from Ailantus altissima. We here demonstrated the effect of TCN on HIF-1 activation induced by hypoxia or CoCl2. TCN showed the potent inhibitory activity against HIF-1 activation induced by hypoxia in various human cancer cell lines. This compound markedly decreased the hypoxia-induced accumulation of HIF-1alpha protein dose-dependently, whereas it did not affect the expressions of HIF-1beta and topoisomerase-I. Furthermore, TCN prevented hypoxia-induced expression of HIF-1 target genes for vascular endothelial growth factor (VEGF) and erythropoietin. Further analysis revealed that TCN strongly inhibited HIF-1alpha protein synthesis, without affecting the expression level of HIF-1alpha mRNA or degradation of HIF-1alpha protein. Moreover, the levels of phosphorylation of extracellular signal-regulated kinase-1/2 (ERK1/2), mitogen-activated protein (MAP) kinase-interacting protein kinase-1 (MNK1) and eukaryotic initiation factor 4E (eIF4E) were significantly suppressed by the treatment of TCN, without changing the total levels of these proteins. Our data suggested that TCN may exhibit anticancer activity by inhibiting HIF-1alpha translation through the inhibition of eIF4E phosphorylation pathway and thus provide a novel mechanism for the anticancer activity of quassinoids. TCN could be a new HIF-1-targeted anticancer agent and be effective on mammalian target of rapamycin (mTOR)-targeted cancer therapy, in which mTOR inhibition increases eIF4E phosphorylation.

Regulation of neurogenesis and epidermal growth factor receptor signaling by the insulin receptor/target of rapamycin pathway in Drosophila

Determining how growth and differentiation are coordinated is key to understanding normal development, as well as disease states such as cancer, where that control is lost. We have previously shown that growth and neuronal differentiation are coordinated by the insulin receptor/target of rapamycin (TOR) kinase (InR/TOR) pathway. Here we show that the control of growth and differentiation diverge downstream of TOR. TOR regulates growth by controlling the activity of S6 kinase (S6K) and eIF4E. Loss of s6k delays differentiation, and is epistatic to the loss of tsc2, indicating that S6K acts downstream or in parallel to TOR in differentiation as in growth. However, loss of eIF4E inhibits growth but does not affect the timing of differentiation. We also show, for the first time in Drosophila, that there is crosstalk between the InR/TOR pathway and epidermal growth factor receptor (EGFR) signaling. InR/TOR signaling regulates the expression of several EGFR pathway components including pointedP2 (pntP2). In addition, reduction of EGFR signaling levels phenocopies inhibition of the InR/TOR pathway in the regulation of differentiation. Together these data suggest that InR/TOR signaling regulates the timing of differentiation through modulation of EGFR target genes in developing photoreceptors.

Dissecting eIF4E action in tumorigenesis

Genetically engineered mouse models are powerful tools for studying cancer genes and validating targets for cancer therapy. We previously used a mouse lymphoma model to demonstrate that the translation initiation factor eIF4E is a potent oncogene in vivo. Using the same model, we now show that the oncogenic activity of eIF4E correlates with its ability to activate translation and become phosphorylated on Ser 209. Furthermore, constitutively activated MNK1, an eIF4E Ser 209 kinase, promotes tumorigenesis in a manner similar to eIF4E, and a dominant-negative MNK mutant inhibits the in vivo proliferation of tumor cells driven by mutations that deregulate translation. Phosphorylated eIF4E promotes tumorigenesis primarily by suppressing apoptosis and, accordingly, the anti-apoptotic protein Mcl-1 is one target of both phospho-eIF4E and MNK1 that contributes to tumor formation. Our results provide insight into how eIF4E contributes to tumorigenesis and pinpoint a level of translational control that may be suitable for therapeutic intervention.

Loss of MNK function sensitizes fibroblasts to serum-withdrawal induced apoptosis

Map kinase-interacting protein kinases 1 and 2 (MNK1, MNK2) function downstream of p38 and ERK MAP kinases, but there are large gaps in our knowledge of how MNKs are regulated and function. Mice deleted of both genes are apparently normal, suggesting that MNKs function in adaptive pathways during stress. Here, we show that mouse embryo fibroblasts (MEFs) obtained from mnk1 (-/-)/mnk2 (-/-) as well as mnk1 (-/-) and mnk2 (-/-) mice are sensitized to caspase-3 activation upon withdrawal of serum in comparison to wild-type cells. Caspase-3 cleavage occurs with all cells in the panel, but most rapidly and robustly in cells derived from mice lacking both MNK genes. Treatment of wild-type MEFs in the panel with a compound (CGP57380) that inhibits MNK1 and MNK2 sensitizes wild-type cells for serum-withdrawal induced apoptosis, suggesting that sensitization is due to loss of MNK function and not to a secondary event. Reintroduction of wild-type MNK1 in the double knockout MEFs results in decreased sensitivity to serum withdrawal that is not observed for wild-type MNK2, or the kinase dead variant. Our work identifies MNKs as kinases involved in anti-apoptotic signaling in response to serum withdrawal.

Inhibition of mammalian target of rapamycin induces phosphatidylinositol 3-kinase-dependent and Mnk-mediated eukaryotic translation initiation factor 4E phosphorylation

The initiation factor eukaryotic translation initiation factor 4E (eIF4E) plays a critical role in initiating translation of mRNAs, including those encoding oncogenic proteins. Therefore, eIF4E is considered a survival protein involved in cell cycle progression, cell transformation, and apoptotic resistance. Phosphorylation of eIF4E (usually at Ser209) increases its binding affinity for the cap of mRNA and may also favor its entry into initiation complexes. Mammalian target of rapamycin (mTOR) inhibitors suppress cap-dependent translation through inhibition of the phosphorylation of eIF4E-binding protein 1. Paradoxically, we have shown that inhibition of mTOR signaling increases eIF4E phosphorylation in human cancer cells. In this study, we focused on revealing the mechanism by which mTOR inhibition increases eIF4E phosphorylation. Silencing of either mTOR or raptor could mimic mTOR inhibitors' effects to increase eIF4E phosphorylation. Moreover, knockdown of mTOR, but not rictor or p70S6K, abrogated rapamycin's ability to increase eIF4E phosphorylation. These results indicate that mTOR inhibitor-induced eIF4E phosphorylation is secondary to mTOR/raptor inhibition and independent of p70S6K. Importantly, mTOR inhibitors lost their ability to increase eIF4E phosphorylation only in cells where both Mnk1 and Mnk2 were knocked out, indicating that mTOR inhibitors increase eIF4E phosphorylation through a Mnk-dependent mechanism. Given that mTOR inhibitors failed to increase Mnk and eIF4E phosphorylation in phosphatidylinositol 3-kinase (PI3K)-deficient cells, we conclude that mTOR inhibition increases eIF4E phosphorylation through a PI3K-dependent and Mnk-mediated mechanism. In addition, we also suggest an effective therapeutic strategy for enhancing mTOR-targeted cancer therapy by cotargeting mTOR signaling and Mnk/eIF4E phosphorylation.

Ras Transformation of RIE-1 Cells Activates Cap-Independent Translation of Ornithine Decarboxylase: Regulation by the Raf/MEK/ERK and Phosphatidylinositol 3-Kinase Pathways

Ornithine decarboxylase (ODC) is the first and generally rate-limiting enzyme in polyamine biosynthesis. Deregulation of ODC is critical for oncogenic growth, and ODC is a target of Ras. These experiments examine translational regulation of ODC in RIE-1 cells, comparing untransformed cells with those transformed by an activated Ras12V mutant. Analysis of the ODC 5' untranslated region (5'UTR) revealed four splice variants with the presence or absence of two intronic sequences. All four 5'UTR species were found in both cell lines; however, variants containing intronic sequences were more abundant in Ras-transformed cells. All splice variants support internal ribosome entry site (IRES)-mediated translation, and IRES activity is markedly elevated in cells transformed by Ras. Inhibition of Ras effector targets indicated that the ODC IRES element is regulated by the phosphorylation status of the translation factor eIF4E. Dephosphorylation of eIF4E by inhibition of mitogen-activated protein/extracellular signal-regulated kinase (ERK) kinase (MEK) or the eIF4E kinase Mnk1/2 increases ODC IRES activity in both cell lines. When both the Raf/MEK/ERK and phosphatidylinositol 3-kinase/mammalian target of rapamycin pathways are inhibited in normal cells, ODC IRES activity is very low and cells arrest in G(1). When these pathways are inhibited in Ras-transformed cells, cell cycle arrest does not occur and ODC IRES activity increases, helping to maintain high ODC activity.

Roles of mitogen-activated protein kinase signal-integrating kinases 1 and 2 in oxidant-mediated eIF4E phosphorylation

Oxidative stress alters cellular metabolic processes including protein synthesis. The eukaryotic initiation factor, eIF4E, acts in the rate-limiting steps of initiation and promotes nuclear export. Phosphorylation of eIF4E by mitogen activated protein kinase signal-integrating kinases 1 and 2 (Mnk) influences the affinity of eIF4E for the 5'-mRNA cap and fosters nuclear export activity. Although phosphorylation of eIF4E on Ser209 is observed following oxidant exposure, the contribution of Mnk isoforms and the significance of phosphorylation remain elusive. Using a Mnk inhibitor and fibroblasts derived from Mnk knockout mice, we demonstrate that that H2O2 enhances eIF4E phosphorylation in cells containing Mnk1. In contrast, cells containing only Mnk2 show little change or a decrease in eIF4E phosphorylation in response to H2O2. H2O2 also shifted eIF4GI protein from the nucleus to the cytoplasm suggesting that the increases in eIF4E phosphorylation may reflect enhanced substrate availability to cytoplasmic Mnk1. In Mnk1(+/+) cells, H2O2 also enhanced eIF4E phosphorylation in the nucleus to a greater degree than in the cytoplasm, an effect not observed in cells containing Mnk2. In response to H2O2, all MEFs showed increased eIF4E:4E-BP1 and 4E-BP2:eIF4E binding and reduced eIF4E:eIF4GI binding. We also observed a dramatic increase in the amount of Mnk1 associated with eIF4E following affinity chromatography. These changes coincided with a smaller reduction in global protein synthesis in response to H2O2 in the DKO cells. These findings suggest that changes in eIF4GI distribution may enhance eIF4E phosphorylation and that the presence of either Mnk1 or 2 or any degree of eIF4E phosphorylation negatively regulates global protein synthesis in response to oxidant stress.

The structure of eukaryotic translation initiation factor-4E from wheat reveals a novel disulfide bond

Eukaryotic translation initiation factor-4E (eIF4E) recognizes and binds the m(7) guanosine nucleotide at the 5' end of eukaryotic messenger RNAs; this protein-RNA interaction is an essential step in the initiation of protein synthesis. The structure of eIF4E from wheat (Triticum aestivum) was investigated using a combination of x-ray crystallography and nuclear magnetic resonance (NMR) methods. The overall fold of the crystallized protein was similar to eIF4E from other species, with eight beta-strands, three alpha-helices, and three extended loops. Surprisingly, the wild-type protein did not crystallize with m(7)GTP in its binding site, despite the ligand being present in solution; conformational changes in the cap-binding loops created a large cavity at the usual cap-binding site. The eIF4E crystallized in a dimeric form with one of the cap-binding loops of one monomer inserted into the cavity of the other. The protein also contained an intramolecular disulfide bridge between two cysteines (Cys) that are conserved only in plants. A Cys-to-serine mutant of wheat eIF4E, which lacked the ability to form the disulfide, crystallized with m(7)GDP in its binding pocket, with a structure similar to that of the eIF4E-cap complex of other species. NMR spectroscopy was used to show that the Cys that form the disulfide in the crystal are reduced in solution but can be induced to form the disulfide under oxidizing conditions. The observation that the disulfide-forming Cys are conserved in plants raises the possibility that their oxidation state may have a role in regulating protein function. NMR provided evidence that in oxidized eIF4E, the loop that is open in the ligand-free crystal dimer is relatively flexible in solution. An NMR-based binding assay showed that the reduced wheat eIF4E, the oxidized form with the disulfide, and the Cys-to-serine mutant protein each bind m(7)GTP in a similar and labile manner, with dissociation rates in the range of 20 to 100 s(-1).

Signalling to translation: how signal transduction pathways control the protein synthetic machinery

Recent advances in our understanding of both the regulation of components of the translational machinery and the upstream signalling pathways that modulate them have provided important new insights into the mechanisms by which hormones, growth factors, nutrients and cellular energy status control protein synthesis in mammalian cells. The importance of proper control of mRNA translation is strikingly illustrated by the fact that defects in this process or its control are implicated in a number of disease states, such as cancer, tissue hypertrophy and neurodegeneration. Signalling pathways such as those involving mTOR (mammalian target of rapamycin) and mitogen-activated protein kinases modulate the phosphorylation of translation factors, the activities of the protein kinases that act upon them and the association of RNA-binding proteins with specific mRNAs. These effects contribute both to the overall control of protein synthesis (which is linked to cell growth) and to the modulation of the translation or stability of specific mRNAs. However, important questions remain about both the contributions of individual regulatory events to the control of general protein synthesis and the mechanisms by which the translation of specific mRNAs is controlled.

Investigating Translation Initiation Using Drosophila Molecular Genetics

Genetic tools enable insights into how translation controls development of a multicellular organism. Different genetic approaches offer the ability to manipulate the Drosophila genome in very precise ways, thereby allowing the investigation of how translation factors work in the context of a whole organism. We present here an overview of selected techniques used to identify genes involved in translation initiation, and quantitative methods to characterize phenotypes caused by mutations in genes encoding translation initiation or regulatory factors.

A novel function of Drosophila eIF4A as a negative regulator of Dpp/BMP signalling that mediates SMAD degradation

Signalling by the TGF-beta superfamily member and BMP orthologue Decapentaplegic (Dpp) is crucial for multiple developmental programmes and has to be tightly regulated. Here, we demonstrate that the Drosophila Dpp pathway is negatively regulated by eukaryotic translation initiation factor 4A (eIF4A), which mediates activation-dependent degradation of the Dpp signalling components Mad and Medea. eIF4A mutants exhibit increased Dpp signalling and accumulation of Mad and phospho-Mad. Overexpression of eIF4A decreases Dpp signalling and causes loss of Mad and phospho-Mad. Furthermore, eIF4A physically associates with Mad and Medea, and promotes their degradation following activation of Dpp signalling in a translation-independent manner. Finally, we show that eIF4A acts synergistically with, but independently of, the ubiquitin ligase DSmurf, indicating that a dual system controls SMAD degradation. Thus, in addition to being an obligatory component of the cap-dependent translation initiation complex, eIF4A has a novel function as a specific inhibitor of Dpp signalling that mediates the degradation of SMAD homologues.

Structural and mechanistic insights into hepatitis C viral translation initiation

Hepatitis C virus uses an internal ribosome entry site (IRES) to control viral protein synthesis by directly recruiting ribosomes to the translation-start site in the viral mRNA. Structural insights coupled with biochemical studies have revealed that the IRES substitutes for the activities of translation-initiation factors by binding and inducing conformational changes in the 40S ribosomal subunit. Direct interactions of the IRES with initiation factor eIF3 are also crucial for efficient translation initiation, providing clues to the role of eIF3 in protein synthesis.

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.

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.

The Drosophila Par domain protein I gene, Pdp1, is a regulator of larval growth, mitosis and endoreplication

PDP1 is a basic leucine zipper (bZip) transcription factor that is expressed at high levels in the muscle, epidermis, gut and fat body of the developing Drosophila embryo. We have identified three mutant alleles of Pdp1, each having a similar phenotype. Here, we describe in detail the Pdp1 mutant allele, Pdp1(p205), which is null for both Pdp1 RNA and protein. Interestingly, homozygous Pdp1(p205) embryos develop normally, hatch and become viable larvae. Analyses of Pdp1 null mutant embryos reveal that the overall muscle pattern is normal as is the patterning of the gut and fat body. Pdp1(p205) larvae also appear to have normal muscle and gut function and respond to ecdysone. These larvae, however, are severely growth delayed and arrested. Furthermore, although Pdp1 null larvae live a normal life span, they do not form pupae and thus do not give rise to eclosed flies. The stunted growth of Pdp1(p205) larvae is accompanied by defects in mitosis and endoreplication similar to that associated with nutritional deprivation. The cellular defects resulting from the Pdp1(p205) mutation are not cell autonomous. Moreover, PDP1 expression is sensitive to nutritional conditions, suggesting a link between nutrition, PDP1 isotype expression and growth. These results indicate that Pdp1 has a critical role in coordinating growth and DNA replication.

Sodium arsenite-induced inhibition of eukaryotic translation initiation factor 4E (eIF4E) results in cytotoxicity and cell death

Exposure to arsenic (As) is a risk factor for the development of diabetes, vascular diseases and cancer. Several theories have been proposed to account for the mechanisms potentially responsible for As toxicity and carcinogenesis. Currently, we have investigated whether the eukaryotic translation initiation factor 4E (eIF4E), the mRNA cap binding and rate limiting factor required for translation, is a target for As-induced cytotoxicity and cell death. We have also investigated the potential cellular mechanisms underlying the As-induced de-regulation of expression of eIF4E that are most likely responsible for the cytotoxicity and cell death induced by As. Exposure of four different human cell lines - HCT15 (colorectal adenocarcinoma), PLC/PR/5 (hepatocellular carcinoma), HeLa (cervical adenocarcinoma) and Chang (likely derived from HeLa cells) to sodium arsenite (NaAsO2) for time intervals up to 24 h resulted in a concentration-dependent cytotoxicity and cell death. All the NaAsO2-treated cells exhibited significant inhibition of eIF4E gene (protein). The potential involvement of eIF4E gene expression in the NaAsO2-induced cytotoxicity and cell death was investigated by silencing the cellular expression of the eIF4E gene by employing a small interfering RNA (SiRNA) specifically targeting the eIF4E gene's expression. The SiRNA-mediated silencing of eIF4E gene expression also resulted in significant cytotoxicity and cell death suggesting that the toxicity noticed among the NaAsO2-treated cells was probably due to the chemically induced inhibition of eIF4E gene expression. The potential involvement of inhibition of eIF4E gene expression in the NaAsO2-induced cytotoxicity and cell death was further investigated by employing transgenic cell lines overexpressing the eIF4E gene. Overexpression of the eIF4E gene in the Chinese hamster ovary cell line was protective against the NaAsO2-induced cytotoxicity and cell death. Additional studies conducted to understand the potential mechanisms responsible for NaAsO2-induced inhibition of eIF4E gene expression demonstrated that exposure to NaAsO2 resulted in transcriptional down-regulation of the eIF4E gene only in HCT-15 and HeLa cells, while in the NaAsO2-treated and PLC/PR/5 and Chang cells, the eIF4E mRNA expression level was comparable to those of the corresponding control cells. Cellular levels of ubiquitin and the process of ubiquitination were significantly higher in the NaAsO2-treated cells compared with the control cells. Immunoprecipitation of lysates obtained from the NaAsO2-treated cells and the subsequent western blot analysis of the immunoprecipitated protein(s) using the eIF4E antibody detected the presence of eIF4E protein in the immunoprecipitate suggesting possible ubiquitination of eIF4E protein in the NaAsO2-treated cells. Pre-exposure of the NaAsO2-treated cells to proteasome inhibitors blocked the inhibition of eIF4E gene expression as well as the resulting cytotoxicity and cell death. Furthermore, exposure of cells to NaAsO2 resulted in a significant inhibition of expression of the cell cycle and growth regulating gene, cyclin D1. Whether or not the inhibition of cyclin D1 in the NaAsO2-treated cells is mediated through the inhibition of eIF4E was tested by silencing the expression of eIF4E gene in the cells. Transfection of cells with SiRNA specifically targeting eIF4E gene expression resulted in a significant inhibition of cyclin D1 gene suggesting that the observed inhibition of cyclin D1 gene in the NaAsO2-treated cells is most likely mediated through inhibition of eIF4E gene. Taken together, our results indicate that the exposure of cells to NaAsO2 resulted in cytotoxicity and cell death, at least in part, due to the inhibition of eIF4E gene expression leading to diminished cellular levels of critical genes such as cyclin D1.

Activation of Akt and eIF4E survival pathways by rapamycin-mediated mammalian target of rapamycin inhibition

The mammalian target of rapamycin (mTOR) has emerged as an important cancer therapeutic target. Rapamycin and its derivatives that specifically inhibit mTOR are now being actively evaluated in clinical trials. Recently, the inhibition of mTOR has been shown to reverse Akt-dependent prostate intraepithelial neoplasia. However, many cancer cells are resistant to rapamycin and its derivatives. The mechanism of this resistance remains a subject of major therapeutic significance. Here we report that the inhibition of mTOR by rapamycin triggers the activation of two survival signaling pathways that may contribute to drug resistance. Treatment of human lung cancer cells with rapamycin suppressed the phosphorylation of p70S6 kinase and 4E-BP1, indicating an inhibition of mTOR signaling. Paradoxically, rapamycin also concurrently increased the phosphorylation of both Akt and eIF4E. The rapamycin-induced phosphorylation of Akt and eIF4E was suppressed by the phosphatidylinositol-3 kinase (PI3K) inhibitor LY294002, suggesting the requirement of PI3K in this process. The activated Akt and eIF4E seem to attenuate rapamycin's growth-inhibitory effects, serving as a negative feedback mechanism. In support of this model, rapamycin combined with LY294002 exhibited enhanced inhibitory effects on the growth and colony formation of cancer cells. Thus, our study provides a mechanistic basis for enhancing mTOR-targeted cancer therapy by combining an mTOR inhibitor with a PI3K or Akt inhibitor.

Binding analyses for the interaction between plant virus genome-linked protein (VPg) and plant translational initiation factors

The turnip mosaic virus (TuMV) genome-linked protein (VPg) and Arabidopsis thaliana translation initiation factors were expressed and purified in order to investigate their binding properties and kinetics. Affinity chromatography on m(7)GTP-sepharose showed that bound A. thaliana eIF(iso)4E was eluted with crude TuMV VPg. Further column studies with purified VPg and other A. thaliana eIF4E isoforms showed that VPg preferentially bound eIF(iso)4E. Structural data implicate Trp-46 and Trp-92 in eIF(iso)4E in cap recognition. When Trp-46 or Trp-92 were changed to Leu, eIF(iso)4E lost the ability to form a complex with both VPg and m(7)GTP-sepharose. This suggests that the VPg-binding site is located in or near the cap-recognition pocket on eIF(iso)4E. Affinity constants for the interactions with eIF(iso)4E of VPg and capped RNA oligomer were determined using surface plasmon resonance (SPR). The K(D) values showed that the binging affinity of VPg for eIF(iso)4E is stronger than that of capped RNA. This suggests that viral VPg can interfere with formation of a translational initiation complex on host plant cellular mRNA by sequestering eIF(iso)4E. Further experiments with affinity chromatography showed that VPg forms a ternary complex with eIF(iso)4E and eIF(iso)4G. Thus, VPg may participate in viral translational initiation by functioning as an alternative cap-like structure.

Regulation of the translation initiation factor eIF4F by multiple mechanisms in human cytomegalovirus-infected cells

As a viral opportunistic pathogen associated with serious disease among the immunocompromised and congenital defects in newborns, human cytomegalovirus (HCMV) must engage the translational machinery within its host cell to synthesize the viral proteins required for its productive growth. However, unlike many viruses, HCMV does not suppress the translation of host polypeptides. Here, we examine how HCMV regulates the cellular cap recognition complex eIF4F, a critical component of the cellular translation initiation apparatus that recruits the 40S ribosome to the 5' end of the mRNA. This study establishes that the cap binding protein eIF4E, together with the translational repressor 4E-BP1, are both phosphorylated early in the productive viral growth cycle and that the activity of the cellular eIF4E kinase, mnk, is critical for efficient viral replication. Furthermore, HCMV replication also induces an increase in the overall abundance of eIF4F components and promotes assembly of eIF4F complexes. Notably, increasing the abundance of select eIF4F core components and associated factors alters the ratio of active eIF4F complexes in relation to the 4E-BP1 translational repressor, illustrating a new strategy through which members of the herpesvirus family enhance eIF4F activity during their replicative cycle.

Phospholipase Cγ-Erk Axis in Vascular Endothelial Growth Factor-induced Eukaryotic Initiation Factor 4E Phosphorylation and Protein Synthesis in Renal Epithelial Cells

Vascular endothelial growth factor (VEGF) increases protein synthesis and induces hypertrophy in renal tubular epithelial cells (Senthil, D., Choudhury, G. G., McLaurin, C., and Kasinath, B. S. (2003) Kidney Int. 64, 468-479). We examined the role of Erk1/2 MAP kinase in protein synthesis induced by VEGF. VEGF stimulated Erk phosphorylation that was required for induction of protein synthesis. VEGF-induced Erk activation was not dependent on phosphoinositide (PI) 3-kinase activation but required sequential phosphorylation of type 2 VEGF receptor, PLCgamma and c-Src, as demonstrated by inhibitors SU1498, U73122, and PP1, respectively. c-Src phosphorylation was inhibited by U73122, indicating it was downstream of phospholipase (PL)Cgamma. Studies with PP1/2 showed that phosphorylation of c-Src was required for tyrosine phosphorylation of Raf-1, an upstream regulator of Erk. VEGF also stimulated phosphorylation of Pyk-2; VEGF-induced phosphorylation of Pyk2, c-Src and Raf-1 could be abolished by BAPTA/AM, demonstrating requirement for induction of intracellular calcium currents. We examined the downstream events following the phosphorylation of Erk. VEGF stimulated phosphorylation of Mnk1 and eIF4E and induced Mnk1 to shift from the cytoplasm to the nucleus upon phosphorylation. VEGF-induced phosphorylation of Mnk1 and eIF4E required phosphorylation of PLCgamma, c-Src, and Erk. Expression of dominant negative Mnk1 abrogated eIF4E phosphorylation and protein synthesis induced by VEGF. VEGF-stimulated protein synthesis could be blocked by inhibition of PLCgamma by a chemical inhibitor or expression of a dominant negative construct. Our data demonstrate that VEGF-stimulated protein synthesis is Erk-dependent and requires the activation of VEGF receptor 2, PLCgamma, c-Src, Raf, and Erk pathway. VEGF also stimulates Erk-dependent phosphorylation of Mnk1 and eIF4E, crucial events in the initiation phase of protein translation.

The Drosophila protein kinase LK6 is regulated by ERK and phosphorylates the eukaryotic initiation factor eIF4E in vivo

In Drosophila cells, phosphorylation of eIF4E (eukaryotic initiation factor 4E) is required for growth and development. In Drosophila melanogaster, LK6 is the closest homologue of mammalian Mnk1 and Mnk2 [MAPK (mitogen-activated protein kinase) signal-integrating kinases 1 and 2 respectively] that phosphorylate mammalian eIF4E. Mnk1 is activated by both mitogen- and stress-activated signalling pathways [ERK (extracellular-signal-regulated kinase) and p38 MAPK], whereas Mnk2 contains a MAPK-binding motif that is selective for ERKs. LK6 possesses a binding motif similar to that in Mnk2. In the present study, we show that LK6 can phosphorylate eIF4E at the physiological site. LK6 activity is increased by the ERK signalling pathway and not by the stress-activated p38 MAPK signalling pathway. Consistent with this, LK6 binds ERK in mammalian cells, and this requires an intact binding motif. LK6 can bind to eIF4G in mammalian cells, and expression of LK6 increases the phosphorylation of the endogenous eIF4E. In Drosophila S2 Schneider cells, LK6 binds the ERK homologue Rolled, but not the p38 MAPK homologue. LK6 phosphorylates Drosophila eIF4E in vitro. The phosphorylation of endogenous eIF4E in Drosophila cells is increased by activation of the ERK pathway but not by arsenite, an activator of p38 MAPK. RNA interference directed against LK6 significantly decreases eIF4E phosphorylation in Drosophila cells. These results show that LK6 binds to ERK and is activated by ERK signalling and it is responsible for phosphorylating eIF4E in Drosophila.

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.

A genetic screen for maternal-effect suppressors of decapentaplegic identifies the eukaryotic translation initiation factor 4A in Drosophila

The Dpp signaling pathway is essential for many developmental processes in Drosophila and its activity is tightly regulated. To identify additional regulators of Dpp signaling, we conducted a genetic screen for maternal-effect suppressors of dpp haplo-insufficiency. We screened approximately 7000 EMS-mutagenized genomes and isolated and mapped seven independent dominant suppressors of dpp, Su(dpp), which were recovered as second-site mutations that resulted in viable flies in trans-heterozygous with dpp(H46), a dpp null allele. Most of the Su(dpp) mutants exhibited increased cell numbers of the amnioserosa, a cell type specified by the Dpp pathway, suggesting that these mutations may augment Dpp signaling activity. Here we report the unexpected identification of one of the Su(dpp) mutations as an allele of the eukaryotic translation initiation factor 4A (eIF4A). We show that Su(dpp)(YE9) maps to eIF4A and that this allele is associated with a substitution, arginine 321 to histidine, at a well-conserved amino acid and behaves genetically as a dominant-negative mutation. This result provides an intriguing link between a component of the translation machinery and Dpp signaling.

Translational regulation during oogenesis and early development: the cap-poly(A) tail relationship

Metazoans rely on the regulated translation of select maternal mRNAs to control oocyte maturation and the initial stages of embryogenesis. These transcripts usually remain silent until their translation is temporally and spatially required during early development. Different translational regulatory mechanisms, varying from cytoplasmic polyadenylation to localization of maternal mRNAs, have evolved to assure coordinated initiation of development. A common feature of these mechanisms is that they share a few key trans-acting factors. Increasing evidence suggest that ubiquitous conserved mRNA-binding factors, including the eukaryotic translation initiation factor 4E (eIF4E) and the cytoplasmic polyadenylation element binding protein (CPEB), interact with cell-specific molecules to accomplish the correct level of translational activity necessary for normal development. Here we review how capping and polyadenylation of mRNAs modulate interaction with multiple regulatory factors, thus controlling translation during oogenesis and early development.

Genetic interactions of Drosophila melanogaster arrest reveal roles for translational repressor Bruno in accumulation of Gurken and activity of Delta

arrest mutants have pleiotropic phenotypes, ranging from an early arrest of oogenesis to irregular embryonic segmentation defects. One function of arrest is in translational repression of oskar mRNA; this biochemical activity is presumed to be involved in other functions of arrest. To identify genes that could provide insight into how arrest contributes to translational repression or that may be targets for arrest-dependent translational control, we screened deficiency mutants for dominant modification of the arrest phenotype. Only four of the many deficiencies tested, which cover approximately 30% of the genome, modified the starting phenotype. One enhancer, identified fortuitously, is the Star gene. Star interaction with arrest results in excess Gurken protein, supporting the model that gurken is a target of repression. Two modifiers were mapped to individual genes. One is Lk6, which encodes a protein kinase predicted to regulate the rate-limiting initiation factor eIF4E. The second is Delta. The interaction between arrest and Delta mimics the phenotype of homozygous Delta mutants, suggesting that arrest could positively control Delta activity. Indeed, arrest mutants have significantly reduced levels of Delta protein at the interface of germline and follicle cells.

The serine-threonine kinase MNK1 is post-translationally stabilized by PML-RARalpha and regulates differentiation of hematopoietic cells

Microarray analyses were performed to identify target genes that are shared by the acute myeloid leukemia (AML) translocation products PML-RARalpha, PLZF-RARalpha and AML1-ETO in inducibly transfected U937 cell lines. The cytoplasmic serine and threonine kinase MNK1 was identified as one of the target genes. At the protein level, MNK1 was significantly induced by each of the three fusion proteins. Protein half-life analyses showed that PML-RARalpha enhanced MNK1 protein stability in U937 cells and ATRA exposure decreased MNK1 half-life in NB4 cells. EIF4E, the main MNK1 substrate, plays a role in the pathogenesis of a variety of cancers. Upon MNK1 overexpression, eIF4E phosphorylation increased as a sign of functional activation. Interestingly, MNK1 protein expression decreased during myeloid differentiation. Inhibition of MNK1 activity by a specific inhibitor (CGP57380) enhanced differentiation of HL60 and 32D cells, further suggesting a role for MNK1 in the myeloid differentiation. In addition, kinase dead mutants of MNK1 significantly impaired proliferation of 32D cells. Immunohistochemistry of primary AML bone marrow biopsies showed strong cytoplasmic MNK1 expression in 25 of 99 AML specimens (25%). MNK1 expression was associated with high levels of c-myc expression. Taken together, we identified MNK1 as a target gene of several leukemogenic fusion proteins in AML. MNK1 plays a role in myeloid differentiation. These data suggest a role for MNK1 in the AML fusion protein-associated differentiation block.

Translational regulation of vascular endothelial growth factor expression in renal epithelial cells by angiotensin II

ANG II regulates growth factor expression in the kidney. We investigated whether ANG II regulated vascular endothelial growth factor (VEGF) synthesis in proximal tubular epithelial (MCT) cells. ANG II (1 nM) increased VEGF protein expression within 5 min, the effect lasting for 30 min. There was no change in VEGF mRNA levels or mRNA stability, and transcription inhibitors did not affect ANG II-induced VEGF expression. Regulation of VEGF translation was investigated. Polyribosomal analysis revealed selective enrichment of heavy ribosomes (polysomes) with VEGF mRNA transcripts compared with light ribosomes in ANG II-treated cells, although distribution of GAPDH was unaltered. In vitro translation of total RNA from polysomal fractions showed selective increase in VEGF protein synthesis in ANG II-treated cells. Preincubation with LY-294002, a PI 3-kinase inhibitor, or expression of dominant-negative Akt prevented ANG II-stimulated increase in VEGF translation. ANG II increased phosphorylation of eukaryotic initiation factor 4E and its binding protein 4E-BP1, critical events that regulate the initiation phase of protein translation. ANG II failed to increase VEGF mRNA translation in cells stably expressing the phosphorylation mutant of 4E-BP1. Our data illustrate that a rapid increase in VEGF protein expression by ANG II is regulated at the initiation phase of translation of VEGF mRNA in renal epithelial cells. Regulation of VEGF translation by ANG II represents a novel pathway of renal response to injury.

Signal transduction pathways leading to increased eIF4E phosphorylation caused by oxidative stress

Phosphorylation of eIF4E is associated with increased activity of the translational machinery. Oxidative stress of resident vascular cells and macrophages potently enhances eIF4E phosphorylation. Oxidative stress activates numerous intracellular signaling pathways, including MAP-family kinase pathways and pathways leading to S6 kinase activation. The activation of MAP-family kinase pathways leads to the activation of Mnk and hence eIF4E phosphorylation, whereas the S6 kinase pathway is not involved, based on insensitivity to its inhibitors rapamycin and wortmannin. Ca-dependent pathways have been implicated in eIF4E phosphorylation, but the oxidative stress response pathway targeting eIF4E does not appear to require their participation. The results suggest that the potent activation of ERK and p38 protein kinases is sufficient to account for the enhanced eIF4E phosphorylation. Either is independently sufficient to effect the change, as neither PD098059 (Erk pathway inhibitor) nor SB202190 (p38 pathway inhibitor) alone can block the response, but when combined the response is almost completely abrogated. Mnk activation by oxidative stress leading to enhanced eIF4E phosphorylation may play a role in promoting stress-induced hyperproliferative diseases, such as smooth muscle cell proliferation and hypertrophy in cardiovascular disease, as the synthesis of several key regulators of cell growth has been shown to be held in check by moderation of eIF4E activity.

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.

SKAR is a specific target of S6 kinase 1 in cell growth control

BACKGROUND

The mammalian target of rapamycin (mTOR) and phosphatidylinositol 3-kinase (PI3K) signaling pathways promote cell growth and cell cycle progression in response to nutritional, energy, and mitogenic cues. In mammalian cells, the ribosomal protein S6 kinases, S6K1 and S6K2, lie downstream of mTOR and PI3K, suggesting that translational control through the phosphorylation of S6 regulates cell growth. Interestingly, genetic experiments predict that a substrate that is specific to S6K1 but not S6K2 regulates cell growth.

RESULTS

Here we identify SKAR as a novel and specific binding partner and substrate of S6K1 but not S6K2. We find that serines 383 and 385 of human SKAR are insulin-stimulated and rapamycin-sensitive S6K1 phosphorylation sites. Quantitative mass spectrometry reveals that serine 383/385 phosphorylation is sensitive to RNA interference (RNAi)-mediated S6K1 reduction, but not S6K2 reduction. Furthermore, RNAi-mediated reduction of SKAR decreases cell size. SKAR is nuclear protein with homology to the Aly/REF family of RNA binding proteins, which has been proposed to couple transcription with pre-mRNA splicing and mRNA export.

CONCLUSIONS

We have identified a novel and specific target of S6K1, SKAR, which regulates cell growth. The homology of SKAR to the Aly/REF family links S6K1 with mRNA biogenesis in the control of cell growth.

Belle is a Drosophila DEAD-box protein required for viability and in the germ line

DEAD-box proteins are ATP-dependent RNA helicases that function in various stages of RNA processing and in RNP remodeling. Here, we report identification and characterization of the Drosophila protein Belle (Bel), which belongs to a highly conserved subfamily of DEAD-box proteins including yeast Ded1p, Xenopus An3, mouse PL10, human DDX3/DBX, and human DBY. Mutations in DBY are a frequent cause of male infertility in humans. Bel can substitute in vivo for Ded1p, an essential yeast translation factor, suggesting a requirement for Bel in translation initiation. Consistent with an essential cellular function, strong loss of function mutations in bel are recessive lethal with a larval growth defect phenotype. Hypomorphic bel mutants are male-sterile. Bel is also closely related to the Drosophila DEAD-box protein Vasa (Vas), a germ line-specific translational regulator. We find that Bel and Vas colocalize in nuage and at the oocyte posterior during oogenesis, and that bel function is required for female fertility. However, unlike Vas, Bel is not specifically enriched in embryonic pole cells. We conclude that the DEAD-box protein Bel has evolutionarily conserved roles in fertility and development.